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Modification of electrode materials for lithium ion batteries

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Modification of electrode materials for lithium ion batteries

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MODIFICATION OF ELECTRODE MATERIALS FOR LITHIUM ION
BATTERIES

By

Xin Fang

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)

December 2015

Copyright 2015 Xin Fang
ii

Epigraph

自强不息，厚德载物

iii

Dedication

Dedicated to my beloved family.
iv

Acknowledgements

I will forever remember my past five years as PhD student studying in University of
Southern California. I learned a lot from this valuable experience. During these years,
there are so many people who helped me in my study as well as other aspects of my life. I
would like to express my deep thanks to all of them for making this dissertation possible.
First of all, my sincere appreciation goes to my advisor, Prof. Chongwu Zhou, for his
guidance and support in my PhD research. I wouldn ’t have the current achievements
without him. The multidisciplinary research in his group has broadened my knowledge
and exposed me to multiple interesting research directions. His encouragement and
advice have influenced not only my PhD research, but also my career goal. The time and
effort I spent in his research group will be a valuable memory for the rest of my life.
I would also like to thank my dissertation committee members, Prof. Edward Goo,
Prof. Steve Cronin and Prof. Pin Wang for their helpful comments and suggestions, as
well as Prof. Steven Nutt and Prof. Andrew Armani for serving on my qualify exam
committee.
In addition, my sincere gratitude goes to Prof. Chunhua Chen, who is the advisor of
my undergraduate research in University of Science and Technology of China. He has
introduced me into this interesting research field of lithium ion batteries. His enthusiasm
and personality has greatly impacted my research and my life. His training and
mentorship is important help for my PhD study here at USC.
v

Moreover, I want to thank my lab mates, especially Dr. Mingyuan Ge and Dr.
Jiepeng Rong. We joined the battery team here in the same year, but they are more
experienced than me and I received a lot of help from them. Additionally, many thanks go
to my former and fellow graduate students and post-docs, Dr. Bilu Liu, Dr. Gang Liu, Dr.
Chuan Wang, Dr. Alexander Badmaev, Dr. Hsiao-Kang Chang, Dr. Anuj Madaria, Dr. Yi
Zhang, Dr. Jialu Zhang, Yue Fu, Dr. Yuchi Che, Dr. Jia Liu, Dr. Haitian Chen, Dr. Jing Xu,
Dr. Shelley Wang, Dr. Maoqing Yao, Dr. Nappadol Aroonyadet, Zhen Li, Jing Qiu, Xue
Lin, Dr. Luyao Zhang, Chenfei Shen, Anyi Zhang, Yihang Liu, Hui Gui, Ahmad Abbas,
Liang Chen, Yuqiang Ma, Xuan Cao, Yu Cao, Sen Cong, Fanqi Wu, Qinzhou Liu, Pyojae
Kim, Younghyun Na, Kuan-Teh Li, Ning Yang, Rebecca Lee, and Pattaramon
Vuttipittayamongkol.
Finally and most importantly, I would like to express my sincere gratitude to my
families, especially my parents. I would not have all these achievements without you.
Your endless love and support is the most important reason of where I am today.

vi

Table of Contents

Epigraph ............................................................................................................................ ii
Dedication ......................................................................................................................... iii
Acknowledgements .......................................................................................................... iv
List of Figures ................................................................................................................. viii
Abstract ........................................................................................................................... xiv
Chapter 1 Introduction ..................................................................................................1
1.1 Thermodynamic analysis ............................................................................................2
1.2 Terminologies in electrochemistry .............................................................................3
1.3 Characteristics of batteries .........................................................................................4
1.4 Lithium ion batteries ..................................................................................................5
Chapter 1 References ..................................................................................................10
Chapter 2 Coaxial Si/Anodized Titanium Oxide/Si Nanotube Arrays for
Lithium-ion Battery Anode .............................................................................................11
2.1 Introduction ..............................................................................................................11
2.2 Experimental Procedure ...........................................................................................13
2.3 Results and Discussion .............................................................................................14
2.3.1 Numerical Simulation Results .........................................................................14
2.3.2 Electrode Fabrication and Characterization ....................................................17
2.3.3 Electrochemical Measurements .........................................................................19
2.4 Conclusion ................................................................................................................25
Chapter 2 References ..................................................................................................26
Chapter 3 Graphene-Oxide-Coated LiNi
0.5
Mn
1.5
O
4
as High Voltage Cathode for
Lithium Ion Batteries with High Energy Density and Long Cycle Life .....................29
3.1 Introduction ..............................................................................................................29
3.2 Experimental Procedure ...........................................................................................32
3.3 Results and Discussion .............................................................................................33
3.4 Conclusion ................................................................................................................43
vii

Chapter 3 References ..................................................................................................44
Chapter 4 Ultrathin Surface Modification by Atomic Layer Deposition on High
Voltage Cathode LiNi
0.5
Mn
1.5
O
4
for Lithium-ion Batteries .........................................49
4.1 Introduction ..............................................................................................................49
4.2 Experimental Procedure ...........................................................................................52
4.3 Results and Discussion .............................................................................................53
4.4 Conclusion ................................................................................................................63
Chapter 4 References ..................................................................................................65
Chapter 5 Free-standing LiNi
0.5
Mn
1.5
O
4
/Carbon Nanofiber Network Film as
Light-weight and High-power Cathode for Lithium-ion Batteries ...........................69
5.1 Introduction ..............................................................................................................69
5.2 Experimental Procedure ...........................................................................................71
5.3 Results and Discussion .............................................................................................72
5.4 Conclusion ................................................................................................................85
Chapter 5 References ..................................................................................................86
Chapter 6 High-power Lithium-ion Batteries Based on Flexible and Light-weight
Cathode of LiNi
0.5
Mn
1.5
O
4
/Carbon Nanotube Film ....................................................88
6.1 Introduction ..............................................................................................................88
6.2 Experimental Procedure ...........................................................................................90
6.3 Results and Discussion .............................................................................................92
6.4 Conclusion ..............................................................................................................106
Chapter 6 References ................................................................................................107
Chapter 7 Conclusion and Future Directions ........................................................109
7.1 Conclusion ..............................................................................................................109
7.2 Future Directions ....................................................................................................111
Chapter 7 References ................................................................................................ 115
Bibliography .................................................................................................................116

viii

List of Figures

Figure 1.1 Schematic illustration of the first Li-ion battery (LiCoO
2
/Li
+
electrolyte / graphite)

7
Figure 1.2 V oltage versus capacity of several electrode materials

8
Figure 2.1 (a-f) Schematic diagram (a,c,e) and finite element modeling
(b,d,f) of CNT-Si core-shell structure (a,b), NW-Si core-shell
structure (c,d), and coaxial Si-ATO-Si nanotube structure (e,f).
(g) Schematic of fabrication process of coaxial Si-ATO-Si
nanotube structure.

15
Figure 2.2 Characterization of ATO and coaxial Si-ATO-Si nanotubes. (a)
SEM of as-prepared ATO nanotubes on a Ti substrate. (b) TEM
image of an as-prepared ATO nanotube. (c) Line scan profile
over an ATO nanotube (from point A to B in Figure 2c inset).
(d) TEM image of the cross-section view of a coaxial
Si-ATO-Si nanotube. (e) TEM image of the side view of a
coaxial Si-ATO-Si nanotube. (f) Line scan profile over an
coaxial Si-ATO-Si nanotube (from point A to B in Figure 2f
inset).

18
Figure 2.3 Electrochemical performance of a coaxial Si-ATO-Si nanotube
anode. (a) Typical cyclic voltammetry curve comparison of the
ATO scaffold and coaxial Si-ATO-Si nanotube, showing the
inert nature of ATO with Li
+
between 0.01V-1V v.s. Li/Li
+
. (b)
Galvanostatic charge-discharge voltage profile between
0.01V-1V vs. Li/Li
+
for the 1
st
, 2
nd
, and 50
th
cycle at 140 mA/g.
(c) Charge-discharge specific capacity and Coulombic
efficiency v.s. cycle number for 50 cycles at different rates
ranging from 140 mA/g to 1400 mA/g. Only the weight of Si is
considered for specific capacity calculation. (d) Discharge
specific capacity and Coulombic efficiency v.s. cycle number at
1400 mAh/g with voltage window between 0.01V-1V v.s.
Li/Li
+
, in which condition it takes around 1 hour to charge or
discharge the battery.

21
Figure 2.4 (a, b) SEM image of coaxial Si-ATO-Si nanotubes after 10
charge-discharge cycles and rinsed in acetonitrile and 1M HCl
24
ix

consecutively to remove residue electrolyte and SEI. (c) EDX
line scan profile of Si (blue), Ti (black) and O (red) over one
coaxial Si-ATO-Si nanotube (from point A to B in Figure 4c
inset).

Figure 3.1 X-ray diffraction pattern of as-synthesized LiNi
0.5
Mn
1.5
O
4
.

34
Figure 3.2 (a) SEM image of as-synthesized LiNi
0.5
Mn
1.5
O
4
. (b) SEM
image of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
. (c) TEM
image showing the surface of as-synthesized LiNi
0.5
Mn
1.5
O
4
.
(d) TEM image of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
. (e)
HRTEM image of as-synthesized LiNi
0.5
Mn
1.5
O
4
showing the
fine lattice. (f) HRTEM image of graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
showing the LiNi
0.5
Mn
1.5
O
4
lattice together with
the layered stacking of graphene oxide on the surface.

36
Figure 3.3 EDX elemental mapping of graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
. The upper left image shows the mapping area
under SEM.

37
Figure 3.4 (a) charge/discharge curves of pristine LiNi
0.5
Mn
1.5
O
4
(denoted
by LNMO) and graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
(denoted
by LNMO+GO) at the current of C/5. (b) CV curve of
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at a scan rate of 0.05
mV/s. (c) Comparison of cyclability of pristine LiNi
0.5
Mn
1.5
O
4

and graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at C/2. (d)
Discharge capacity of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4

and pristine LiNi
0.5
Mn
1.5
O
4
at different current rates, the charge
current was kept at C/2. (e) Long cycling result of
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at C/2.

40
Figure 3.5 AC impedance test result of (a) pristine LiNi
0.5
Mn
1.5
O
4

(denoted by LNMO) at the 5
th
cycle, (b) graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
(denoted by LNMO+GO) at the 5
th
cycle, (c)
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at the 50
th
cycle and (d)
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at the 60
th
cycle.

42
Figure 4.1 Characterization of as-synthesized LiNi
0.5
Mn
1.5
O
4
particles. (a)
SEM image of as-synthesized LiNi
0.5
Mn
1.5
O
4
particles. (b)
XRD pattern of the as-synthesized LiNi
0.5
Mn
1.5
O
4
particles.

53
x

Scheme 4.1 Schematic diagram showing ALD Al
2
O
3
coating on
pre-fabricated electrodes.

54
Figure 4.2 EDX elemental mapping of ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4

electrode (a) from the top and (b) from a cross-section. The
mapping area is shown on the upper left. High-resolution TEM
images showing (c) morphology of 30 cycles ALD Al
2
O
3

coated on LiNi
0.5
Mn
1.5
O
4
and (d) the crystalline nature of
LiNi
0.5
Mn
1.5
O
4
particles.
56
Figure 4.3 Comparison of the effect from different number of ALD cycles.
(a) room temperature cycling performance at C/2; (b) room
temperature performance at different current rate; (c) high
temperature (55º C) cycling performance at C/2.
57
Figure 4.4 Room temperature cycling test results. (a) Comparison of
cyclability of ALD Al
2
O
3
coated and bare LiNi
0.5
Mn
1.5
O
4
at
current rate of C/2 (1C = 140 mA g
-1
). Charge/discharge curves
of the (b) 5
th
,(c) 40
th
and (d) 100
th
cycle from both ALD Al
2
O
3

coated and bare LiNi
0.5
Mn
1.5
O
4
.

59
Figure 4.5 55 º C cycling test results. (a) Comparison of cyclability of ALD
Al
2
O
3
coated and bare LiNi
0.5
Mn
1.5
O
4
at current rate of C/2 (1C
= 140 mA g
-1
). Charge/discharge curves of the (b) 5
th
,(c) 40
th

and (d) 100
th
cycle from both ALD Al
2
O
3
coated and bare
LiNi
0.5
Mn
1.5
O
4
.
60
Figure 4.6 AC impedance test results of both ALD Al
2
O
3
coated and bare
LiNi
0.5
Mn
1.5
O
4
after cycling at 55 º C for 100 cycles.

62
Figure 4.7 Long cycling performance of ALD Al
2
O
3
coated
LiNi
0.5
Mn
1.5
O
4
at C/2.
63
Figure 5.1 Schematics and photos of LiNi
0.5
Mn
1.5
O
4
/ carbon nanofiber
(CNF) network electrodes. (a)-(c), schematic showing the
fabrication process: bottom CNF thin layer (a), main part of
LiNi
0.5
Mn
1.5
O
4
and CNF added(b), and final coverage with
CNF thin layer (c). (d) photo of LiNi
0.5
Mn
1.5
O
4
/ CNF
network after filtration. (e) photo of free-standing
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrode before assembling into
battery.

74
xi

Figure 5.2 (a) TEM and (b) HRTEM image of CNF; (c) SEM image of the
surface of LiNi
0.5
Mn
1.5
O
4
/ CNF network; (d-f) SEM images of
the inside of LiNi
0.5
Mn
1.5
O
4
/ CNF network for compositions of
CNF1/2 (d), CNF1/4(e) and CNF1/6(f).

76
Figure 5.3 (a) Comparison of cycling performance of
LiNi0.5Mn1.5O4/CNF network electrodes and conventional
electrodes at C/2 (1C = 140 mAh/g); (b) extended cycling
results from LiNi0.5Mn1.5O4/CNF network electrodes for up
to 500 cycles at C/2; (c) comparison of discharge capacity of
LiNi0.5Mn1.5O4/CNF network electrodes and conventional
electrodes from C/2 to 20C. Charging rate was kept at C/2.

78
Figure 5.4 Specific capacity of carbon nanofibers at 3.5-5 V against Li
metal.

79
Figure 5.5 Discharge curves of LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes
and conventional electrodes at 3C (a) and 5C (b). Cycling
performance of LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and
conventional electrodes at 3C (c) and 5C (d). Charging rate was
also 3C or 5C, correspondingly.

80
Figure 5.6 Discharge profiles of CNF1/2 (a), CNF1/4 (b), CNF1/6 (c) and
conventional electrodes (d) from C/2 to 20 C.

82
Figure 5.7 V oltage vs. mass current profiles of CNF1/2 (a), CNF1/4 (b),
CNF1/6 (c) and conventional electrodes (d) from 20% to 70%
depth of discharge.

82
Figure 5.8 Comparison of polarization resistance of batteries with
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and conventional
electrodes.

83
Figure 5.9 Comparison of LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and
conventional electrodes in terms of weight of the electrodes and
weight percentage of active material in the electrodes. The
calculation was based on active material loading of 3 mg/cm
2
.

84
Figure 6.1 (a) Schematic illustration of the fabrication process of the
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes. (b) Digital photo of the
LiNi
0.5
Mn
1.5
O
4
/ MWCNT network film to show the flexibility,
93
xii

inset is the photo taken after the film was cut into small
electrodes before assembling batteries. (c) SEM image of the
MWCNT network on the surface of the LiNi
0.5
Mn
1.5
O
4
/
MWCNT electrodes. (d) SEM image of the inside mixture of
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes where LiNi
0.5
Mn
1.5
O
4
particles are dispersed in MWCNT network. (e) High resolution
SEM image showing the LiNi
0.5
Mn
1.5
O
4
particles are connected
by MWCNTs in the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes.

Figure 6.2 SEM image taken at 45 degree tilt angle to show the cross
section of LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes

95
Figure 6.3 Charge/discharge curves of CNT30 (a) and CNT20 (b) samples
at C/2. (c) Specific capacity vs. cycle number over 100 cycles
for CNT30, CNT20 and conventional electrodes. (d) Specific
capacity of CNT30, CNT20 and conventional electrodes at
different current densities. (e) Capacity retention at different
current densities with respect to the capacity at 1C for CNT30,
CNT20 and conventional electrodes. (f) Polarization resistance
calculated for CNT30, CNT20 and conventional electrodes at
different depth of discharge.

96
Figure 6.4 Cycling performance for up to 500 cycles at C/2 97
Figure 6.5 Specific capacity of MWCNT network without LiNi
0.5
Mn
1.5
O
4

particles.

98
Figure 6.6 Discharge profiles of CNT30 (a), CNT20 (c) and conventional
electrodes (e) at different current densities from C/2 to 7C.
V oltage vs. current density profiles of CNT30 (b), CNT20 (d)
and conventional electrodes (f) at different depth of discharge
from 10% to 90%

100
Figure 6.7 Discharge curves of CNT30, CNT20 and conventional
electrodes at 5C (a) and 10C (b). Specific capacity vs. cycle
number over 100 cycles for CNT30 and CNT20 samples at 5C
(c) and 10C (d). (e) Total weight of CNT30, CNT20 and
conventional electrodes calculated based on 5 mg/cm
2
active
material loading. (f) Power density of CNT30, CNT20 and
conventional electrodes calculated based on the total weight of
electrode and voltage plateau at 10C.
102
xiii

Figure 6.8 (a) Open circuit voltage of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT
electrodes at different bending radius; inset is a photo showing
the test under bending. (b) Photos showing a blue LED is
powered up by the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes under
bending.

104
Figure 6.9 Open circuit voltage of LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes
recorded over 10 hours at bending radius of 1.6 cm (a), 2.1 cm
(b), 2.7 cm (c) and flat state (d).

105
Figure 7.1 Charge-discharge curve of LiNi
0.5
Mn
1.5
O
4
-Si full cell.

112
Figure 7.2 Cycling performance of LiNi
0.5
Mn
1.5
O
4
-Si full cell.

112

xiv

Abstract:
In this dissertation, I discuss the modification of electrode materials for high
performance lithium ion batteries. Lithium ion batteries are widely used in many
applications in our daily life, especially as the power source for mobile electronics, due to
the advantages such as high energy density, no memory effect and long cycle life. Despite
the success of lithium ion batteries in the past years, higher demands have been raised by
the development of electric vehicles and new generation of electronic products such as
ultrathin and flexible devices. Further enhancement of energy/power density, cycle life
and other features is thus needed.
The major goal of my PhD research is to develop high energy lithium ion batteries. I
have used two methods to improve the energy of the batteries: (1) to improve the capacity
and (2) to improve the voltage. I first worked on nanostructured Silicon (Si) as anode
material for lithium ion batteries. Si has high theoretical capacity (4200mAh/g) which is
about ten times higher than traditional anode material graphite (372mAh/g). In this regard,
it is a promising candidate to be used for high energy lithium ion batteries. However, the
large volume change during charge and discharge process will lead to pulverization of the
electrode and thus the failure of the battery, which is the biggest problem hindering the
application of Si anode in lithium ion batteries. In my research, I developed Si/anodized
titanium oxide (ATO)/Si coaxial nanostructure to accommodate the volume change of Si
and the battery performance was improved significantly. After improving the capacity, I
xv

have also worked on high voltage cathode material LiNi
0.5
Mn
1.5
O
4
, which can work at
4.7V vs. Li metal. I used mildly oxidized graphene oxide and Al
2
O
3
as coating materials
to suppress the undesired reactions during high voltage cycling and thus improve the
cycle life of the batteries. After that, carbon nanofibers (CNF) and carbon nanotubes
(CNT) were used to develop CNT (CNF)/ LiNi
0.5
Mn
1.5
O
4
composite network films as
light weight electrodes, which reduced the total weight and improved the power density
of the batteries.
This dissertation is structured as follows. In Chapter 1 the basic concepts and
features of lithium ion batteries and related electrochemistry is discussed, which can
serve as the background for the following chapters. In Chapter 2, I present nanostructured
Si anode study using anodized Titanium oxide array as substrate and template. In Chapter
3, 4, 5 and 6, I present LiNi
0.5
Mn
1.5
O
4
high voltage cathode study. In Chapter 3 and 4, the
coating work of LiNi
0.5
Mn
1.5
O
4
is discussed, where mildly oxidized graphene oxide
coating and atomic layer deposited Al
2
O
3
coating can suppress the side reaction between
the surface of LiNi
0.5
Mn
1.5
O
4
and electrolyte. Chapter 5 and 6 present a new design of
electrode structure. CNT and CNF network with embedded LiNi
0.5
Mn
1.5
O
4
particles are
developed into free-standing electrodes, and results demonstrated ultra-light electrodes
with enhanced energy and power density. Finally, the conclusion is drawn and potential
future work is discussed in Chapter 7.
1

Chapter 1

Introduction

Batteries are now widely used in various applications in our daily life, including
mobile electronic devices, energy storage systems, transportation, defense, health care
and so on. With the rapid development of global economy, the demand for batteries is
increasing dramatically, especially in the fields of mobile telecommunication,
eco-friendly transportation and energy storage grids.
The first form of battery is believed to be the voltaic pile, invented by Italian scientist
Alessandro Volta in 1800. Since the invention of voltaic pile, various batteries have been
developed. The first widely used primary battery was the Leclanche cell, followed by
some new types such as zinc-air batteries and lithium primary batteries. The earliest type
of secondary batteries is lead-acid battery. After that, NiCd batteries emerged and later
were favored by NiMH batteries. In the early 1990s, lithium secondary batteries were
commercialized and soon dominated the market for portable electronic devices,
especially laptops and cellphones. [1]

2

1.1 Thermodynamic analysis
Thermodynamics provides the fundamental analysis of the relationship between cell
potential and the concentration of the active electrode materials. Let us consider the
reduction reaction:
O + n𝑒 −
↔ R
The relationship between the concentration of oxidized species [O], concentration of
reduced species [R], and free energy ∆G is given as
∆G = ∆𝐺 0
+ 𝑅𝑇𝑙𝑛 [R]
[O]

The relationship between the maximum potential between two electrodes, also called
open circuit potential or equilibrium potential, E, is given as
∆G = −𝑛𝐹𝐸
In the above equations, R is gas constant (8.3145 J mol
-1
K
-1
), F is Faraday’s constant
(96485.3 C mol
-1
) and n is the number of electrons exchanged. The standard electrode
potential, E
0
, is calculated for a reaction in the direction of reduction and when the
reactant and product both have unit activity
∆𝐺 0
= −𝑛𝐹 𝐸 0

Here we can see the standard electrode potential E
0
(V) is related to the standard
Gibbs free energy ∆G
0
(J mol
-1
). For all spontaneous reactions, ∆G
0
<0, hence the
standard electrode potential will be positive.
Through the above analysis, we can get the correlation between potential and
concentration of a cell reaction, which is known as Nernst Equation:
𝐸 = 𝐸 0
+
𝑅𝑇
𝑛𝐹
𝑙𝑛
[O]
[R]

3

This mathematical expression is one of the central equations in electrochemistry. [2]
1.2 Terminologies in Electrochemistry
The definitions here follow the instructions from International Union of Pure and
Applied Chemistry (IUPAC), and some terms have specific meaning in electrochemistry.
Anode: electrode where oxidation takes place.
Cathode: electrode where reduction take place.
Normal/Standard Hydrogen Electrode (NHE/SHE): the standard reference electrode
which has a potential of 0.000 V by definition; all standard potentials are referred to
NHE.
Standard Reduction Potential: the potential of the reduction half-reaction at the electrode,
with respect to the NHE.
Working electrode: electrode where the redox processes under study occur, typically
cathodes.
Reference electrode: electrode that can maintain a constant potential under changing
experimental conditions, typically anodes.
Counter electrode: electrode that helps pass the current flowing through the cell, typically
no processes of interest occur.
Electrochemical cell: device that involves the presence of faradaic currents as a result of
redox chemical reactions; it can be either a galvanic cell, when the reactions are
spontaneous, or an electrolytic cell, when the reactions are non-spontaneous.
4

Cell potential: the sum of electrical potentials within an electrochemical cell that also
accounts for all redox processes occurring at the electrodes
Battery: one or more galvanic cells. [2]
1.3 Characteristics of batteries
The open circuit voltage V
oc
is the difference in electric potential between the two
electrodes. However, during charge and discharge, the voltage V
dis
and V
ch
differ from
V
oc
due to an overpotential η caused by polarization:
𝑉 𝑑𝑖𝑠 = 𝑉 𝑜𝑐
− 𝜂 (𝑞 , 𝐼 𝑑𝑖𝑠 )
𝑉 𝑐 ℎ
= 𝑉 𝑜𝑐
+ 𝜂 (𝑞 , 𝐼 𝑐 ℎ
)
Here q represents state of charge. The total charge Q transferred per unit weight or
per unit volume is the capacity at a certain current I = 𝑑𝑞 𝑑𝑡 ⁄ .
Q(I) = ∫ 𝐼𝑑𝑡 ∆𝑡 0
= ∫ 𝑑𝑞 𝑄 0

The available energy stored in a battery depends on the discharge current I
dis
energy = ∫ 𝐼𝑉 (𝑡 )𝑑𝑡 ∆𝑡 0
= ∫ 𝑉 (𝑞 )𝑑𝑞 𝑄 0

The output power of a battery also depends on the discharge current I
dis
P(q) = V(q)𝐼 𝑑𝑖𝑠
The coulumbic efficiency of a cycle can be calculated by
coulumbic efficiency=
𝑄 𝑑𝑖𝑠 𝑄 𝑐 ℎ
× 100
The percent efficiency of a battery to store energy is
energy efficiency=
∫ 𝑉 𝑑𝑖𝑠 (𝑞 )𝑑𝑞 𝑄 𝑑𝑖𝑠 0
∫ 𝑉 𝑐 ℎ
(𝑞 )𝑑𝑞 𝑄 𝑐 ℎ
0
× 100
5

The cycle life of a battery is the number of cycles until the capacity is depleted, or
sometimes, until the capacity fades to 80% of its initial reversible value. The shelf life,
also called calendar life of a battery is the maximum admissible interval between
manufacturing and utilization in discharge, which is a reflection of the rate of
self-discharge of a battery.
One or some of the above mentioned characteristics may be of particular importance
according to different applications. For example, volumetric energy density is of great
interest in portable batteries that power hand-held or laptop devices. In thus conditions,
tap density of the active materials is often measured to show the packing density of the
particles. [3]
1.4 Lithium ion Batteries
Lithium is the lightest metal and has the lowest standard reduction potential, hence
would lead to high working voltage and specific energy of the batteries. Due to its high
chemical activity and interaction with protonic solvents, no much progress has been made
until 1960s, when batteries containing lithium and aprotic electrolyte were introduced.
The first generation of batteries containing lithium was actually primary batteries.
The electrolyte used were certain lithium salts such as lithium perchlorate LiClO
4
and
lithium tetrachloroaluminate LiAlCl
4
dissolved in solvents such as propylene carbonate
(PC), tetrahydrofuran (THF) and dimethyformamide (DMF). The cathode materials used
were quite a few inorganic compounds such as manganese dioxide MnO
2
and fluorinated
carbon CF
x
. The working process would be the dissolve of metallic lithium while the ions
intercalate into the cathode. A representative chemical reaction would be
𝑀𝑛𝑂 2
+ 𝑥 𝐿𝑖
+
+ 𝑥 𝑒 −
→ 𝐿𝑖
𝑥 𝑀𝑛 𝑂 2

6

This type of batteries achieved up to 600 Wh/kg specific energy and satisfactory
reliability. However, the high cost became a major limitation for them to be widely used.
[4]
The idea of making secondary batteries with lithium is thus developed. At the early
stage, metallic lithium was still used in the batteries. In 1976, Whittingham reported the
discovery of TiS
2
as cathode for lithium ion batteries.[5] This was regarded as the earliest
development of the concept “lithium ion battery” with lithium metal as anode. Although
thousands of cycles were achieved, safety was a severe problem that limited the
development of such lithium secondary batteries. The dendrites formed on lithium during
cycling can penetrate the separator and cause combustion or explosion of the batteries.
While people were struggling with the difficulties of overcoming this issue, the idea of
lithium ion batteries was demonstrated. In 1980, Goodenough reported LiCoO
2
cathode
which contains lithium ions and hence can be coupled with non-lithium anode.[6] At the
same time, graphite anode was developed by Bell Lab.[7] With LiCoO
2
as cathode and
graphite as anode materials, metallic lithium would not be necessary in the battery hence
the dendrite problem can be avoided. The basic chemical reactions are
𝑥𝐿𝑖
+
+ 𝐶 𝑛 + 𝑥𝑒
−
↔ 𝐿𝑖
𝑥 𝐶 𝑛
𝐿𝑖
𝑦 𝐶𝑜 𝑂 2
+ 𝑥𝐿𝑖
+
+ 𝑥𝑒
−
↔ 𝐿𝑖
𝑦 +𝑥 𝐶𝑜 𝑂 2

Figure 1.1 shows the working mechanism of these lithium ion batteries. [3] During
the charge/discharge process, lithium ions move across the separator and insert into the
electrode material layers, while electrons move through the outer circuit. Two years after
LiCoO
2
was discovered, Goodenough reported another important study which discussed
about spinel LiMn
2
O
4
cathode.[8] The discovery of lithium-containing cathode and
non-lithium anode finally leads to the real application of lithium ion batteries.
7

Figure 1.1 Schematic illustration of the first Li-ion battery (LiCoO
2
/Li
+
electrolyte/graphite).

Years later, Sony first commercialized the lithium ion batteries in 1990. Since that, rapid
progress in the lithium ion battery field has been made and lithium ion batteries have
almost dominated the market as power source for portable electronics.
With the increasing demand in the lithium ion battery field, especially to develop
lithium ion batteries for electric vehicles, the energy density of the existing batteries is no
longer sufficient. People are working on a variety of strategies in improving the
performance of the current lithium ion battery technology. Research on new electrode
materials has attracted a lot of attention. Till now, various materials have been studied,
including spinel structure cathode, olivine structure cathode, lithium rich layered cathode,
alloy anode and so on. Figure 1.2 lists a portion of the materials under study with respect
to their theoretical capacity and voltage vs. Li/Li
+
. [9]
8

Figure 1.2 Voltage versus capacity of several electrode materials.

To enhance the energy density of lithium ion batteries, there are mainly three
strategies, to increase the capacity, to increase the voltage and to reduce the weight of the
batteries. In present research, all these strategies are widely explored. High capacity and
high voltage electrode materials are receiving enormous attention. People are also trying
to modify the system design for lithium ion batteries to enhance both energy density and
battery life.

Figure 1.2 shows that the voltage of cathode materials can reach around 5 V.
Comparing with 4.1 V from current cathode material LiCoO
2
, the increase in voltage can
obviously enhance the energy of the batteries. With higher voltage from each battery, the
total number of batteries needed for a battery pack can be reduced, thus also reducing the
weight from packaging such as battery cases. Some high capacity anode materials not
shown here include Si and metal oxides such as SnO
2
. Recently Si has been highly
9

reported due to the highest known capacity of ~4000 mAh/g and low voltage similar to
graphite as anode material. With Si anode, the total capacity of the full battery can be
improved by almost 30%.
In my PhD research, both high capacity anode Si and high voltage cathode
LiNi
0.5
Mn
1.5
O
4
have been studied. The materials with surface or structural modification
demonstrated excellent cycling stability and current rate capability. In addition, new
electrode design of using carbon nanotube/nanofiber network as free-standing electrode
has been explored, where the total weight of an electrode gets highly reduced while the
power gets enhanced. With high capacity, high voltage and reduced electrode weight, we
are well prepared to combine all the favorable features into a high energy lithium ion full
battery and eventually develop generation power source and energy storage devices for
portable electronics and electric vehicles.

10

References
[1] Principles and Applications of Lithium Secondary Batteries, edited by J.K.Park, 2012
[2] Handbook of Electrochemsitry, edited by C.G.Zoski, 2007
[3] J.B.Goodenough and K.S.Park, The Li-Ion Rechargeable Battery: A Perspective, J.
Am. Chem. Soc., 2013, 135 (4), 1167–1176
[4] Fundamentals of Electrochemistry, second edition, edited by V.S.Bagotsky, 2006
[5] Whittingham, M. S. Electrical Energy-Storage and Intercalation Chemistry. Science
1976, 192, 1126-1127.
[6] Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Lixcoo2
"(Oless-Thanxless-Than-or-Equal-To1) - a New Cathode Material for Batteries of
High-Energy Density. Mater. Res. Bull. 1980, 15, 783-789.
[7] Basu, S. Rechargeable Battery, U.S.Patent 4304825
[8] Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Lithium Insertion
into Manganese Spinels. Mater. Res. Bull. 1983, 18, 461-472.
[9] J.B.Goodenough, and Y.Kim, Challenges for Rechargeable Li Batteries, Chem.
Mater., 2010, 22(3), 587–603

11

Chapter 2

Coaxial Si / Anodic Titanium Oxide /
Si Nanotube Arrays for Lithium-ion
Battery Anode

2.1 Introduction
Increasing efforts have been devoted to the development of lithium-ion batteries with
higher energy density, higher charging and discharging rate, and longer cycle life to meet
the requirement of ever growing portable electronic and next-generation electrical
vehicles industry [1-13]. Silicon has drawn particular attention as anode material for
lithium-ion batteries primarily because it has the highest known theoretical capacity
(3590 mAh/g for Li
15
Si
4
, and 4200 mAh/g for Li
22
Si
5
), which is nine times higher than
that of commercial graphite anodes and other oxide and nitride materials [14]. However,
its application is restricted by severe capacity fading caused by pulverization, which is
due to large volume expansion and contraction during lithiation and delithiation process
(Si + xLi+ + xe-↔LixSi (0≤x≤4.4)).

12

Thus far, it has been evidenced that nanostructured Si could be utilized to obtain
improved electrochemical performance over bulk Si and is considered as promising
candidates for high performance lithium-ion battery anodes, such as Si nanowires (NWs)
[15-17], carbon-Si core-shell NWs [18], carbon-Si nanocomposites [19,20], TiSi2/Si
nanonets [21], three-dimensional porous Si [22,23], and sealed Si nanotubes [24]. These
studies indicate that the key feature for electrode design is providing enough free space
around Si so that large volume expansion could be accommodated. In addition, Si
nanostructures grown directly on metallic current collector would enhance both power
density and energy density of lithium-ion battery by minimizing internal resistance and
the usage of non-active materials, such as carbon black and binder.
Here my labmates and I develop a coaxial silicon / anodic titanium oxide / silicon
(Si-ATO-Si) nanotube array electrode in which anodic titanium oxide (ATO) nanotubes
were rooted on titanium (Ti) current collector as the inert scaffold in the voltage window
0.01V-1V vs. Li/Li
+
, and the outer and inner Si shell worked as active material to store
Li
+
(as shown in Figure 1g). This novel coaxial design incorporated all the key features of
designing high performance lithium-ion battery anode. ATO scaffold provide a rough
surface for improved Si adhesion, and direct contact with the Ti substrate working as
current collector. More importantly, Si layers were coated on both outer and inner surface
of ATO nanotube array scaffold, leaving abundant space between nanotubes as well as
inside each tube, which allowed for better accommodation of volume expansion outward
and inward. We first used theoretical modeling to confirm our assumption that coaxial
Si-ATO-Si nanotube has superior mechanical stability compared with some other
Si-based nanostructures recently reported. Experimentally fabricating coaxial Si-ATO-Si
nanotube array electrodes and measuring electrochemical performance as lithium-ion
battery anode further validated the feasibility of our proposal. We have achieved high
first discharge (lithiation) capacity of 2824 mAh/g at the current density of 140 mA/g and
13

long-cycling test after that achieved stable cycling performance with capacity over 1500
mAh/g at 1400 mA/g current rate, which means less than 5% capacity degradation for
100 cycles. Excellent rate capability was also demonstrated.

2.2 Experimental Procedure

ATO nanotube array synthesis
The ATO nanotube array was directly formed on a Ti substrate by potentiostatic
anodization at 70 V vs a carbon counter electrode in diethylene glycol electrolyte (DEG)
containing 1% HF. The anodization was continuously carried out for 19 hours at room
temperature. After anodization, the ATO nanotube array on top of Ti substrate was rinsed
with ethanol and naturally dried in air.
Coaxial Si-ATO-Si nanotube synthesis
An amorphous Si layer, functioning as the Li storage media, was deposited on ATO
nanotube scaffold by chemical vapor deposition (CVD) of silane (2% SiH4 balanced in
Ar) at 530 oC for 10 min in a quartz tube furnace (1 inch diameter). The total chamber
pressure was 100 Torr. Thickness of Si coating could be easily controlled by reaction
time.

14

2.3 Results and Discussion
2.3.1 Numerical Simulation Results
Finite element modeling has been explored to numerically simulate stress evolution
in electrode materials to help us better understand the process of high-stress buildup [25].
The high-stress buildup may eventually lead to mechanical fracture and isolation of
electrode material from the metallic current collector, which was regarded as the key
point to solve capacity fading problem associated with many electrode materials, such as
Si and LiMn
2
O
4
. Here, we adapted a finite element model developed by Lu [25] to
calculate the stress distribution caused by volume change due to Li
+
intercalation. By
comparing the maximum stress existing in coaxial Si-ATO-Si nanotube array structure
we proposed here with other two types of high performance Si-based nanocomposites
recently reported [26-28], we found the coaxial Si-ATO-Si nanotube array structure we
proposed have the lowest maximum hydrostatic stress indicating it has lowest risk of
mechanical fracture and highest stability during charge/discharge cycling. The three
nanocomposites we studied are (1) carbon nanotube (CNT)-Si core-shell structure [26],
with 10nm CNT diameter (Figure 2.1a,b), (2) nanowire (NW)-Si core-shell structure,
with 100nm NW diameter (Figure 1c,d), where the inner NW can be TiC NW [27] or
copper NW [28], and (3) coaxial Si-ATO-Si nanotube structure, with 300nm and 340nm
as ATO nanotube inner diameter and outer diameter (Figure 2.1e,f). In our model, the
loading of Si was the same on unit length of CNT, NW and ATO nanotube, which was
calculated to be 174 nm and 136 nm thick Si layer on CNT and NW respectively, and 50
nm on both inner and outer surface of ATO nanotube. Considering the uniformity in the
longitude direction of all three nanocomposites, the calculation was carried out on
two-dimensional cross-section. ATO nanotubes do not participate in either the first
lithiation (from open-circuit voltage to 0.01 V) or the following lithiation/delithiation
15

between 0.01 V-1 V and therefore experience no volume change. Thus, in the numerical
simulation, ATO nanotubes are fixed in dimension.

Figure 2.1 (a-f) Schematic diagram (a,c,e) and finite element modeling (b,d,f) of CNT-Si core-shell
structure (a,b), NW-Si core-shell structure (c,d), and coaxial Si-ATO-Si nanotube structure (e,f). (g)
Schematic of fabrication process of coaxial Si-ATO-Si nanotube structure.

The elastic field was taken into consideration when solving the diffusion problem to
obtain the concentration and stress profile. The diffusion flux J is given by,
J = −D(∇c −
Ωc
RT
∇σ
h
) (1)

where c is concentration of lithium ions, D is diffusion coefficient, R is the gas constant,
T is the absolute temperature, σh is the hydrostatic stress, and Ω is the partial molar
16

volume. The two terms on the right side of the equation above take care of the effect of
Li+ concentration gradient and stress gradient due to Li+ insertion.
Applying mass conservation equation,
∂c
∂t
⁄ + ∇ ∙ J = 0, we could get hydrostatic
stress σh satisfying,
∂c
∂t
− ∇ ∙ [D (∇c −
Ωc
RT
∇σ
h
)] = 0 (2)
The hydrostatic stress could be calculated and mapped using FEMLAB (COMSOL
Multiphysics) by applying a constant current boundary condition
J ∙ n = i
n
/F (3)
Where n is the normal vector of the surface, in is the current density, and F is
Faraday’s constant.
The hydrostatic stress profiles of three structures in Figure 2.1a, c and e were plotted
in Figure 2.1b, d and f, respectively. The maximum hydrostatic stress existed in (1)
CNT-Si core-shell structure (Figure 2.1b), (2) NW-Si core-shell structure (Figure 2.1d),
and (3) coaxial Si-ATO-Si nanotube structure (Figure 2.1f) are 4.2, 4.1 and 3.4 GPa,
respectively. The corresponding composition is Li
15
Si
4
for all structures. The positions of
the maximum stress were located at the most inner layer of Si shell for all three cases.
The structures we proposed had lowest maximum hydrostatic stress, indicating it had
lowest risk of mechanical fracture and highest stability during charge/discharge cycling.
This may result from the difference in Si-layer thickness. Coaxial Si-ATO-Si nanotube
structure had the thinnest Si-layer thickness of 50 nm on both inner and outer side of
ATO tubes, as well as the smallest maximum stress of 3.4 GPa. In comparison, CNT-Si
core-shell structure had the thickest Si-layer thickness of 174 nm and the largest
maximum stress of 4.2 GPa. We stress that our ATO scaffolds provide a rather unique
17

advantage that silicon can be loaded both on inner and outer surface, and an inner pore
can still be maintained to provide space for silicon volume expansion. Those features
cannot be obtained with the CNT or NW scaffold structures.

2.3.2. Electrode Fabrication and Characterization
Coaxial Si-ATO-Si nanotube array electrode was experimentally fabricated by
employing a template approach using ATO nanotubes, as shown by the schematic in
Figure 2.1g. ATO scaffold was first prepared by anodization of Ti foil [29], and it
provided both mechanical support as well as charge transport path for the active
amorphous Si layer on both inner and outer surface. In addition, ATO nanotube does not
react with lithium in the voltage window 0.01V-1V vs. Li/Li
+
, which has been confirmed
by our cyclic voltammetry (CV) measurements (Figure 2.3a). We successfully got ATO
nanotubes with 100 nm to 300 nm in diameter and 1 µ m to 10 µ m in length by utilizing
different reaction time and voltage. Then, an amorphous Si layer, functioning as the Li
storage media, was conformally coated on the surface of ATO nanotube scaffold by
chemical vapor deposition (CVD). In this way, the weight ratio of the Si layers to the
ATO scaffold could be tuned by varying the thickness of Si coating, which could be
easily controlled by CVD time. Si layer needs to be thick enough to provide reasonable
loading and thinner than the fracture threshold to avoid losing structural integrity during
lithiation-delithiation cycling. Different Si layer thicknesses ranging from 20 nm to 80
nm and different ATO nanotube lengths were tested, and no obvious difference in terms
of electrochemical performance was observed.
18

Figure 2.2 Characterization of ATO and coaxial Si-ATO-Si nanotubes. (a) SEM of as-prepared ATO
nanotubes on a Ti substrate. (b) TEM image of an as-prepared ATO nanotube. (c) Line scan profile over an
ATO nanotube (from point A to B in Figure 2c inset). (d) TEM image of the cross-section view of a coaxial
Si-ATO-Si nanotube. (e) TEM image of the side view of a coaxial Si-ATO-Si nanotube. (f) Line scan
profile over an coaxial Si-ATO-Si nanotube (from point A to B in Figure 2f inset).

The morphology and composition of as-synthesized ATO nanotube array was first
characterized by scanning electron microscopy (SEM). Figure 2.2a shows that vertically
aligned ATO nanotube array with uniform diameter could be obtained. The morphology
of ATO nanotubes was further characterized by transmission electron microscopy (TEM),
as shown in Figure 2.2b. The ATO nanotubes have an inner radius (R
in
) of 150 nm and an
outer radius (R
out
) of 170 nm. The rough external and internal surface of ATO nanotubes,
intrinsically resulted from the anodization process, would be beneficial to enhance the
19

adherence of Si, which was proven to be a key factor to govern the electrochemical
performance of lithium-ion batteries [19,30-34]. The element distribution in ATO
nanotubes was analyzed by energy dispersive X-ray (EDX) spectroscopy line scan over
the cross-section of one nanotube (Figure 2.2c). The opposite trend of signal intensity
between oxygen (O) and Ti over scanning distance is because O signal is solely from
ATO, while Ti is from both ATO and Ti substrate beneath, which could explain why Ti
exhibits higher signal intensity over the void space in the center. Coaxial Si-ATO-Si
nanotubes were fabricated by Si deposition on ATO nanotubes by CVD. Coaxial
Si-ATO-Si nanotube fragments were generated by performing sonication on coaxial
Si-ATO-Si nanotube array and characterized by TEM, as shown in Figure 2.2d. Both
cross-section view (Figure 2.2d) and side view (Figure 2.2e) revealed a coaxial
Si-ATO-Si structure with 50 nm uniform conformal Si coating on both inner and outer
surface of ATO nanotubes. Si distribution was investigated by EDX (Figure 2.2f),
confirming that Si symmetrically distributed on both inner and outer surface of ATO
nanotubes. After 50 nm Si coated on ATO nanotubes, few-nanometers thick Si at the
surface will be oxidized to SiOx when exposed to air. Oxygen signals in EDX results
could be from both the thin layer of SiO
x
and ATO nanotube inside.

2.3.3 Electrochemical Measurements
After fabrication of coaxial Si-ATO-Si nanotube array on Ti substrate, we evaluated
its electrochemical performance. Figure 2.3a shows typical cyclic voltammetry (CV)
curves of the ATO nanotube array scaffold before and after Si deposition over the voltage
window of 0.01V-1V vs Li/Li
+
at a scan rate of 0.1 mV/s. ATO nanotubes with no Si
coating exhibited electrochemical double-layer capacitor behavior with no peak related to
reaction with lithium, which confirmed the inactive nature of ATO as a scaffold.
20

Although titanium oxide is a widely studied anode material reacting with lithium as
TiO
2
+ xLi
+
+ xe
-
↔LixTiO
2
at 1.7 V vs Li/Li
+
[35], we limited the voltage window
between 0.01V-1V vs Li/Li
+
for both CV and galvanostatic charge/discharge
measurements, so TiO
2
did not participate in lithiation/delithiation reaction with lithium
and hence ATO just functioned as inert scaffold in this study. After Si deposition, the
shape of CV curve changed dramatically, and signature peaks of Si-Li alloy/dealloy
reactions were observed. The peak at 0.19 V in the cathodic branch (lithiation)
corresponds to the conversion of amorphous Si to Li
x
Si. In the anodic branch
(delithiation), the two peaks at 0.35 V and 0.52 V are attributed to the delithiation of
Li
x
Si back to amorphous Si [36].
2032 type coin cells were assembled with coaxial Si-ATO-Si nanotube array grown
on Ti current collector as working electrode and lithium metal as the counter electrode to
investigate its electrochemical performance. No binder or carbon black additives were
employed. Teklon® polymer separator was used in our coin cells. 1.0 M LiPF
6
in 1:1
w/w ethylene carbonate/diethyl carbonate was used as electrolyte. Figure 2.3b shows the
voltage profile for the 1st, 2nd, and 50th cycle of galvanostatic charge/discharge
measurement at the same current rate of 140 mAh/g. For the first discharge (lithiation)
and charge (delithiation), specific capacity reached 3803 mAh/g and 2802 mAh/g
respectively, taking only Si mass into calculation. The Coulombic efficiency in the first
cycle was 73.7% and approached above 95% at all charge/discharge rates after the first
cycle. The limited Coulombic efficiency in the first cycle and the improved performance
thereafter could be resulted from the formation of solid electrolyte interphase (SEI) on
the electrode surface, which would consume Li
+
in an irreversible manner. In addition,
the silicon oxide (SiO
x
) formed on Si surface during its exposure to air could also
contribute to the restricted Coulombic efficiency in the first cycle. The reaction between
SiO
x
and lithium is partially reversible, and the reversibility depends on the x value
21

following the formula, SiO
x
+2x Li
+
↔ Si + x Li
2
O [37,38]. Both processes mentioned
above mainly happened in the first cycle and were then suppressed or slowed down, thus
resulting in the limited Coulombic efficiency in the first cycle and significant
improvement afterwards.

Figure 2.3 Electrochemical performance of a coaxial Si-ATO-Si nanotube anode. (a) Typical cyclic
voltammetry curve comparison of the ATO scaffold and coaxial Si-ATO-Si nanotube, showing the inert
nature of ATO with Li
+
between 0.01V-1V vs. Li/Li
+
. (b) Galvanostatic charge-discharge voltage profile
between 0.01V-1V vs. Li/Li
+
for the 1
st
, 2
nd
, and 50
th
cycle at 140 mA/g. (c) Charge-discharge specific
capacity and Coulombic efficiency vs. cycle number for 50 cycles at different rates ranging from 140 mA/g
to 1400 mA/g. Only the weight of Si is considered for specific capacity calculation. (d) Discharge specific
capacity and Coulombic efficiency vs. cycle number at 1400 mAh/g with voltage window between
0.01V-1V v.s. Li/Li
+
, in which condition it takes around 1 hour to charge or discharge the battery.

As shown in Figure 2.3c, the coaxial Si-ATO-Si nanotube array electrode exhibited
stable cycling performance at different current rates of 140, 280, 700, and 1400 mA/g.
The discharge capacities at each current rate were 2717, 2260, 1823, and 1480 mAh/g,
22

respectively. The discharge capacity at 1400mA/g, at which rate it took around 1 hour to
fully discharge/charge the battery, was higher than the theoretical capacity of graphite
electrode by a factor of four. The long cycle performance of coaxial Si-ATO-Si nanotube
array electrode was also explored by continuously charge and discharge at 1400 mA/g for
100 cycles after the first cycle (Figure 2.3d). The discharge capacity for the second and
the 101st cycle were 1624 and 1548 mAh/g respectively, corresponding to 4.7%
degradation for 100 cycles or less than 0.05% decay per cycle, indicating a superior
cycling stability of the electrode. The retaining capacity of 1548 mAh/g after 100
charge/discharge cycles was still more than 4 times higher than the theoretical capacity of
graphite. Besides the high gravimetric specific capacity achieved, volumetric specific
capacity is estimated to be around 2800 mAh/cm3, around 3.5 times that of graphite (800
mAh/cm3). The electrode also demonstrated favorable Coulombic efficiency, which
rapidly recovered to over 98% after the first cycle, and further increased to more than 99%
after 10 cycles.
The excellent electrochemical performance of the coaxial Si-ATO-Si nanotube array
electrode could be attributed to: (1) the ATO nanotube array provided an excellent
inactive, mechanically strong scaffold and survived charge/discharge cycling intact. (2)
The ATO nanotube scaffold provided direct contact with the Ti substrate working as
current collector. This design could enhance both power density and energy density of
lithium-ion battery by minimizing internal resistance and the usage of non-active
materials. (3) The rough surface and special geometry of ATO nanotubes not only
provided an ideal interface for enhanced adhesion between Si and ATO, but also behaved
as a superior host of Si with lower stress associated with lithiation. (4) Si could be loaded
on both the inner and outer surface of ATO scaffolds, and an inner pore can be
maintained to provide room for Si volume expansion. In addition to the justification by
simulation and battery performance above, these effects could be further confirmed as
23

following. Batteries with coaxial Si-ATO-Si nanotube array electrode were disassembled
after 10 charge/discharge cycles, and rinsed with acetonitrile and 1 M HCl consecutively
to remove residue electrolyte and SEI. The morphology and composition were
characterized with SEM and EDX. As we expected, coaxial Si-ATO-Si nanotube array
electrode also showed limited change in terms of morphology and composition after
cycling, which agreed with the stable cycling performance we observed. SEM images
(Figure 2.4 a, b) and EDX line scan profile of Si (blue), Ti (black), and O (red) (Figure
2.4c) confirmed the coaxial Si-ATO-Si nanotube array electrode survived
charge/discharge cycling intact.
24

Figure 2.4 (a, b) SEM image of coaxial Si-ATO-Si nanotubes after 10 charge-discharge cycles and rinsed
in acetonitrile and 1M HCl consecutively to remove residue electrolyte and SEI. (c) EDX line scan profile
of Si (blue), Ti (black) and O (red) over one coaxial Si-ATO-Si nanotube (from point A to B in Figure 4c
inset).

25

2.4 Conclusion
In summary, we successfully fabricated coaxial Si-ATO-Si nanotube array structures
and applied this novel structure to lithium-ion battery anode. The coaxial Si-ATO-Si
nanotube array demonstrated high specific capacity and excellent cycling performance.
After 100 cycles, the capacity still remained above 1500mAh/g and the capacity decay
was less than 0.05% per cycle. This excellent cycling stability was due to the unique
coaxial structure in which ATO provided a strong inert scaffold, a rough surface for Si
adhesion, and the low stress upon lithiation of the Si layer by maintaining an inner pore,
which was also proven by simulation. This novel structure thus can be a promising
candidate for anode material to improve lithium-ion battery performance.

26

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29

Chapter 3

Graphene-Oxide-Coated
LiNi
0.5
Mn
1.5
O
4
as High Voltage
Cathode for Lithium Ion Batteries
with High Energy Density and Long
Cycle Life

3.1 Introduction
Since Sony first commercialized lithium ion batteries in early 1990s, the market for
lithium ion batteries has been rapidly growing with the increasing demand for energy
storage systems. In spite of the great success of lithium ion battery technology developed
for portable electronic devices, higher requirements are raised by electric vehicles (EVs),
hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) in aspects
of higher energy/power density, better rate capability, longer cycle life, and lower
cost.[1-4] As the specific energy densities is calculated by ∫ V(q)dq/wt
Q
0
,[5] increasing
the working voltage of lithium ion battery has become one of the most important
strategies to enhance the energy density.

30

As the voltage of a lithium ion battery is mainly determined by the cathode material,
the research of cathode materials for new lithium ion batteries has become extremely
crucial and has gained enormous attention. LiNi
0.5
Mn
1.5
O
4
was studied when people were
developing the method of partially substituting Mn with other metal ions (Co
3+
, Ge
4+
,
Zn
2+
, Ni
2+
, Mg
2+
, Cr
3+
and Cu
2+
) to suppress the Jahn-Teller distortion in spinel LiMn
2
O
4

in the early 1990s.[6-9] LiNi
0.5
Mn
1.5
O
4
was first reported as a 3V cathode material by
Amine et al. in 1996,[10] and the 4.7V voltage plateau was discovered by Dahn et al. in
1997.[11] With a theoretical capacity of 146.7 mAh/g and a high working voltage of 4.7
V, LiNi
0.5
Mn
1.5
O
4
has 20% and 30% higher energy density than that of LiCoO
2
and
LiFePO
4
, respectively, thus becoming a potential candidate to be used in EVs in the
future.[12-17]
However, the conductivity of LiNi
0.5
Mn
1.5
O
4
is relatively low.[18] In addition, it is
difficult to maintain the electrochemical stability of the carbonate-based liquid electrolyte
at such high working voltage, and the interfacial side reaction between the high-voltage
charged LiNi
0.5
Mn
1.5
O
4
and the liquid electrolyte causes serious capacity fading during
cycling.[19, 20] There are usually a small amount of Mn
3+
ions existing in the crystal,
and the Mn
3+
ions incline to decompose into Mn
2+
and Mn
4+
. The Mn
2+
ions are reported
to have a tendency to dissolve into the electrolyte and further deposit on the surface of the
anode, and the deposition subsequently increases the impedance of the battery and causes
capacity fading.[21] A potential approach to overcome this problem is to modify the
surface of LiNi
0.5
Mn
1.5
O
4
with a thin layer of coating material, which is a strategy that
has been successfully used on a number of cathode and anode materials.[22-25]
Previously, the effect of some coating materials, such as Au,[26] Ag,[27] Bi
2
O
3
,[28]
BiOF,[20] ZnO,[19, 29] ZrO
2
,[30] ZrP
2
O
7
,[30] AlF
3
,[31] conductive carbon,[18] and
polyimide,[32] have been studied and showed improvement on the performance of
LiNi
0.5
Mn
1.5
O
4
to a certain degree. However, no long cycling result was reported, which
31

makes it difficult to judge the effect of coating in the long run. Among the coating
materials mentioned above, most of the metal oxide and metal fluoride coatings only
worked as a protection shell, but cannot enhance the electron conductivity of
LiNi
0.5
Mn
1.5
O
4
. It was found that Bi
2
O
3
converted to Bi metal during cycling, which can
help with fast electron transfer, but the cycling stability was not sufficiently good due to
the microstructural changes of Bi
2
O
3
during charge/discharge process.[28] Other groups
reported the direct coating of metals, such as Au and Ag; however, the improvement of
Ag coating was limited,[27] and Au even made the battery performance worse than
before.[26] Conductive carbon coating can be a good choice, since it can work as a
protection layer and also enhance the electron conductivity of LiNi
0.5
Mn
1.5
O
4
. Although
this strategy is effective on many other materials, it is difficult for LiNi
0.5
Mn
1.5
O
4
since
reduction atmosphere is needed for carbon source to carbonize at high temperature, while
LiNi
0.5
Mn
1.5
O
4
needs oxygen atmosphere to avoid too many oxygen vacancies in the
crystal. In the report of conductive carbon coating of LiNi
0.5
Mn
1.5
O
4
,[18] only slow
charge rate was shown and the coulomb efficiency is low. In this regard, it is important to
find an alternative coating material that can act as a protection layer and at the same time
enhance the conductivity.
Graphene and graphene oxide have been reported to improve cycling stability and
rate performance in lithium ion batteries and lithium sulfur batteries.[33-41] Graphene
and graphene oxide were reported to be a stable wrapping layer during charge/discharge
process. The conductivity and interaction with the active materials can be tuned via
controlling the degree of oxidation of graphene. However, to the best of our knowledge,
the effect of graphene oxide on the performance of LiNi
0.5
Mn
1.5
O
4
has not been reported,
in spite of the importance of LiNi
0.5
Mn
1.5
O
4
as a representative high voltage cathode
material. In this paper, we explored the potential of using mildly oxidized graphene oxide
as a coating layer for LiNi
0.5
Mn
1.5
O
4
cathode and got very promising results. The
32

batteries showed only 0.039% capacity decay per cycle for up to 1000 cycles. At high
charge/discharge rate of 5C, 7C and 10C (1C = 140 mA/g), 77%, 66% and 56% of the 1C
capacity can be retained, respectively. The results indicate that graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
showed excellent cycling stability and rate capability as a high voltage
cathode material and thus has great potential for high energy density and long life lithium
ion batteries.
3.2 Experimental Procedure
Material synthesis
To produce LiNi
0.5
Mn
1.5
O
4
, nickel acetate (Ni(Ac)
2
•4H
2
O) and manganese acetate
(Mn(Ac)
2
•4H
2
O) were mixed at a molar ratio of Ni : Mn = 1 : 3 and milled in a mortar.
The mixture was then heated to 500 º C with a heating rate of 3 º C / min and calcined at
500 º C for 5 hours. After cooling down naturally, lithium acetate (LiAc •2H
2
O) was
added to the mixture with a molar ratio of Li : Ni : Mn = 2.1 : 1 : 3 (5% excess Li source
was added in order to make up for the volatilization of Li during calcination), and the
mixture was heated to 500 º C for 5 hours once more. Then the mixture was milled and
sintered at 900 º C for 10 hours followed by annealing at 700 º C for 10 hours.
Graphene oxide was prepared using the modified Hummers’ method[42] and the
details can be found in literature.[43] The solid content in the graphene oxide solution
was 1mg/ml. Then the as-synthesized LiNi
0.5
Mn
1.5
O
4
powder was mixed with graphene
oxide/ethanol solution at room temperature with moderate stirring. The weight ratio of
graphene oxide and the as-synthesized LiNi
0.5
Mn
1.5
O
4
was 1:20. The mixture was
subsequently annealed at 90 º C overnight to get rid of the residue solvent that may exist
between the surface of LiNi
0.5
Mn
1.5
O
4
and the graphene oxide wrapping layer to enhance
the surface interaction.
33

Electrochemical measurements
CR2032 coin cells were assembled with Li metal as counter electrode. The weight
ratio in cathode is active material : poly(vinylidene fluoride) : carbon black = 8 : 1 : 1.
The loading of cathode active material was kept between 2~3 mg/cm2 for all batteries
tested in this paper. 1 M solution of LiPF6 in ethylene carbonate (EC) and dimethyl
carbonate (DMC) (1:1, w/w) was used as electrolyte. The batteries were cycled in the
voltage range of 3.5 V ~ 5 V. Cyclic voltammetry (CV) curves were tested with a scan
rate of 0.05 mV/s. Electrochemical impedance spectra (EIS) were collected with AC
voltage at 5 mV amplitude and frequency range of 100 kHz to 10 mHz.
3.3 Results and discussion
LiNi
0.5
Mn
1.5
O
4
can be prepared via a variety of methods, including solid state
reaction,[38, 44] co-precipitation,[45] molten salt method,[46] radiated polymer gel
method,[47] thermal polymerization,[48] sol-gel,[49] spray pyrolysis[50] and so on.
Among these methods, solid state reaction and co-precipitation are the most compatible
with large scale synthesis which is essential for industry application. Compared to
co-precipitation, solid state reaction does not involve waste water, so it is more
environmentally friendly. In this work, we used a modified solid state reaction method[38]
to synthesize LiNi
0.5
Mn
1.5
O
4
with potential to be adopted by industry. Figure 3.1 shows
the X-ray diffraction (XRD) pattern of the as-synthesized LiNi
0.5
Mn
1.5
O
4
. The pattern
corresponds to cubic structure of spinel LiNi
0.5
Mn
1.5
O
4
, which is in agreement with
literature.[51] According to previous study,[52] LiNi
0.5
Mn
1.5
O
4
has two space groups:
Fd3
̅
m and P4
3
32. In Fd3
̅
m space group, Mn
3+
exists due to O vacancy in the crystal,
while in P4
3
32 space group, all Mn ions are Mn
4+
.Even though LiNi
0.5
Mn
1.5
O
4
was
commonly used in literature[16] and also in this paper, the actual formula of
LiNi
0.5
Mn
1.5
O
4
with Fd3
̅
m space group should be LiNi
0.5
Mn
1.5
O
4-δ
. In our study, the
34

product obtained was Fd3
̅
m spinel, which can be seen from the charge-discharge curves
with the presence of a small Mn
3+
plateau. Although the disordered Fd3
̅
m spinel was
reported to give better performance,[13, 52] it encounters the problem of Mn
3+
ion
distortion and dissolution, which may get reduced and deposit on the surface of anode.
This will subsequently lead to increased impedance and capacity fading.

Figure 3.1 X-ray diffraction pattern of as-synthesized LiNi
0.5
Mn
1.5
O
4
.

To modify the surface of LiNi
0.5
Mn
1.5
O
4
and protect it from undesired reactions, we
coated the as-prepared LiNi
0.5
Mn
1.5
O
4
with mildly oxidized graphene oxide. Figure 3.2
shows the scanning electron microscopy (SEM) and transmission electron microscopy
(TEM) images of the as-synthesized LiNi
0.5
Mn
1.5
O
4
(Figure 3.2a, 3.2c, and 3.2e) and
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
(Figure 3.2b, 3.2d and 3.2f). Figure 3.2a and 3.2b
35

correspond to the pristine and graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
, respectively. The
size of the LiNi
0.5
Mn
1.5
O
4
particles is mostly around 500 nm. While the TEM image of
pristine LiNi
0.5
Mn
1.5
O
4
(Figure 3.2c) shows very clean surface, in comparison, it is
evident in Figure 3.2d that graphene oxide has been coated onto the outer surface of
LiNi
0.5
Mn
1.5
O
4
, with thickness about 5 nm. High resolution TEM (HRTEM) images
(Figure 23.2e and 3.2f) show the fine structure of LiNi
0.5
Mn
1.5
O
4
and graphene oxide
layer. In Figure 3.2f, the layered stacking of graphene oxide can be clearly seen together
with the LiNi
0.5
Mn
1.5
O
4
lattice, and there is no gap between graphene oxide and
LiNi
0.5
Mn
1.5
O
4
particle, indicating tight adhesion between graphene oxide and
LiNi
0.5
Mn
1.5
O
4
surface. Figure 3.3 shows the energy dispersive X-ray spectroscopy (EDX)
mapping of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
under SEM (the SEM image on the
top left side shows the mapping area). It is obvious that the mapping of element C, which
is from graphene oxide, has the same distribution as elements Ni, Mn and O. This
indicates that our graphene oxide is uniformly coated onto LiNi
0.5
Mn
1.5
O
4
particles.
36

Figure 3.2 (a) SEM image of as-synthesized LiNi
0.5
Mn
1.5
O
4
. (b) SEM image of graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
. (c) TEM image showing the surface of as-synthesized LiNi
0.5
Mn
1.5
O
4
. (d) TEM image of
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
. (e) HRTEM image of as-synthesized LiNi
0.5
Mn
1.5
O
4
showing the
fine lattice. (f) HRTEM image of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
showing the LiNi
0.5
Mn
1.5
O
4
lattice
together with the layered stacking of graphene oxide on the surface.
37

Figure 3.3 EDX elemental mapping of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
. The upper left image shows
the mapping area under SEM.

To evaluate the electrochemical performance of graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
, CR2032 coin cells were assembled with Li metal as counter electrode. As
a comparison, pristine LiNi
0.5
Mn
1.5
O
4
was also tested in the same condition. The results
are shown in Figure3.4. The charge/discharge curves in Figure 3.4a show the high
working voltage feature of LiNi
0.5
Mn
1.5
O
4
. Compared to other traditional cathode
materials, such as LiCoO
2
(3.9 V), LiMn
2
O
4
(4.1 V), and LiFePO
4
(3.5 V), a 4.7 V high
working voltage means that higher energy density can be achieved when LiNi
0.5
Mn
1.5
O
4

is used as cathode for lithium ion batteries. By comparing the charge/discharge curve of
pristine and graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
, it is obvious that after graphene oxide
coating, the voltage difference between charge and discharge plateaus gets smaller. This
indicates that graphene oxide coating can help to reduce the polarization and inner
resistance of the batteries. There is a small voltage plateau around 4.1 V, which can be
attributed to redox couple of Mn
3+
/Mn
4+
. The small plateau here confirms our analysis
38

that the Ni and Mn ions should be disorderly distributed in the crystal structure, and the
LiNi
0.5
Mn
1.5
O
4
particles we produced should possess Fd3
̅
m space group.
Cyclic voltammetry (CV) curves of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
shown in
Figure 3.4b are in good agreement with our charge/discharge curves. The main peaks are
in the high voltage range, while the small peaks at 4~4.1 V correspond to the existence of
Mn
3+
. The integrated area of the 4V peaks is much smaller than that of the 4.7 V peaks,
meaning that the main contribution of the total capacity and the total energy is from
Ni
2+
/Ni
4+
redox couple. There is no other peak in Figure 3.4b, indicating that graphene
oxide does not lead to extra redox reactions in the testing voltage range, and thus should
remain stable and does not contribute to the capacity.
Figure 3.4c shows the cycling performance of the pristine LiNi
0.5
Mn
1.5
O
4
(denoted
by LNMO) and graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
(denoted by LNMO+GO) in
comparison (the specific capacity of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
was
calculated using the total weight of graphene oxide and LiNi
0.5
Mn
1.5
O
4
). It is observed
that the coated LiNi
0.5
Mn
1.5
O
4
showed more stable cycling performance than the uncoated
LiNi
0.5
Mn
1.5
O
4
as cathode in lithium ion batteries. Results from current rate capability
tests are shown in Figure 3.4d. When discharged at large current density of 5C, 7C, and
10C, the graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
can still deliver 77%, 66%, and 56% of
the 1C capacity, respectively. At each stage of the current rate test, the batteries showed
excellent stability and no obvious degradation. When the current density went back to
C/2, the capacity recovered to the original value, indicating that large current density and
rapid lithiation/delithiation did not lead to any permanent damage to the crystal structure.
In comparison, once the batteries were tested under large current densities, especially
10C, it is evident that pristine LiNi
0.5
Mn
1.5
O
4
cannot get fully lithiated/delithiated due to
large inner resistance, so the capacity decreased immediately. In addition, the capacity
39

could not recover when the current density went back. This supports our analysis that
graphene oxide can improve the conductivity and reduce the inner resistance of the
cathode material. Long cycling result of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
is shown
in Figure 3.4e. After cycling at C/2 (1C = 140 mA/g) for 1000 cycles, the capacity
retention of the graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
is 61 %, meaning only 0.039%
capacity decay per cycle. We attribute this improvement to the increased conductivity
and the protection effect from mildly oxidized graphene oxide on the surface. The
remarkable cycling and current rate performance from the graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
cathode indicates a potential of application in next-generation lithium ion
batteries with high energy density, long cycle life, and good rate capability.
The performance of our graphene oxide coating compares favorably to other coatings
reported in literature. As mentioned before, metal oxide and metal fluoride coatings, such
as Al
2
O
3
and AlF
3
, can only work as a protection layer, but cannot enhance the
conductivity. On the other hand, Ag coating only led to limited improvement, while Au
coating even made the battery performance worse than before. Although carbon coating
showed the most promising result, only 92% capacity retention was achieved even with
the best coating ratio. In contrast, we have achieved a long cycle life of 1000 cycles with
our graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
. In addition, the first 100 cycles showed over
96% capacity retention.
40

Figure 3.4 (a) charge/discharge curves of pristine LiNi
0.5
Mn
1.5
O
4
(denoted by LNMO) and
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
(denoted by LNMO+GO) at the current of C/5. (b) CV curve of
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at a scan rate of 0.05 mV/s. (c) Comparison of cyclability of pristine
LiNi
0.5
Mn
1.5
O
4
and graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at C/2. (d) Discharge capacity of
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
and pristine LiNi
0.5
Mn
1.5
O
4
at different current rates, the charge
current was kept at C/2. (e) Long cycling result of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at C/2.

The promising performance caused by graphene oxide coating can be ascribed to
three reasons. First of all, graphene oxide coating enhanced the conductivity of
LiNi
0.5
Mn
1.5
O
4
. With continuous graphene oxide coating, the LiNi
0.5
Mn
1.5
O
4
particles
were interconnected with each other, and hence the conductivity was enhanced and the
inner resistance was reduced. The modified Hummers’ method we used yielded mildly
41

oxidized graphene oxide. It was reported to be low-defect,[43] which was important for
conductivity. In addition, the existence of oxygenated groups rendered the mildly
oxidized graphene oxide highly miscible with LiNi
0.5
Mn
1.5
O
4
particles when mixed in
ethanol, and this way the interaction of the graphene oxide and LiNi
0.5
Mn
1.5
O
4
particles
were enhanced and the wrapping was more effective. Second, Mn
3+
dissolution was
suppressed due to graphene oxide coating. It was reported in literature that Mn
3+
ions had
a tendency to undergo a disproportion reaction and dissolve into the electrolyte.[53-56]
This dissolution can cause a destruction of the cathode crystal and thus affected the
stability of the batteries. With the protection from graphene oxide coating, the dissolution
of Mn
3+
ions was suppressed and in this way the cycling stability was improved. Third,
graphene oxide coating helped to reduce side reactions between Ni
4+
ions and electrolyte.
Ni
4+
ions, formed at the charged state of the cathode, were reported to be active towards
the electrolyte and even raised safety concerns.[57] In addition, Ni ions were also found
to dissolve into the electrolyte and further deposit onto the surface of the anode.[21, 45]
Since neither Mn nor Ni had good Li ion conductivity, the deposited layer increased the
impedance of the batteries and caused capacity fading problem. It was also reported that
the thickness of the products of undesired reactions, such as LixPFyOz and ROCO
2
M (M
= metal) species, increased with cycling and also caused capacity fading.[58] Hence, a
protective surface layer like graphene oxide would be highly desired to improve the
performance.
42

Figure 3.5 AC impedance test result of (a) pristine LiNi
0.5
Mn
1.5
O
4
(denoted by LNMO) at the 5
th
cycle, (b)
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
(denoted by LNMO+GO) at the 5
th
cycle, (c) graphene-oxide-coated
LiNi
0.5
Mn
1.5
O
4
at the 50
th
cycle and (d) graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at the 60
th
cycle.

To confirm our analysis, electrochemical impedance spectra (EIS) were collected
with AC voltage at 5 mV amplitude and frequency range of 100 kHz to 10 mHz. Figure
3.5a and 3.5b show the AC impedance results for pristine LiNi
0.5
Mn
1.5
O
4
and
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
at the 5th cycle, respectively. It can be clearly seen
that the impedance of graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
is much smaller than the
uncoated sample. This result supports our analysis that graphene oxide can help to reduce
the impedance and thus improve the battery performance. We also further investigated
the impedance of the graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
after long cycling test and
the results are shown in Figure 3.5c and 3.5d. After 50 and 60 cycles, no obvious increase
in battery impedance was observed. Even after long cycling, the impedance of
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
was still smaller than pristine LiNi
0.5
Mn
1.5
O
4
, thus
43

demonstrating the advantage of graphene oxide coating in improving the battery
performance.
3.4 Conclusion
We successfully synthesized high voltage cathode LiNi
0.5
Mn
1.5
O
4
via a highly
scalable method of solid state reaction, and further modified the surface of
LiNi
0.5
Mn
1.5
O
4
by graphene oxide coating. The 4.7 V high working voltage gives the
lithium ion batteries 20% and 30% higher energy density than batteries using traditional
LiCoO
2
and LiFePO
4
, respectively. The continuous graphene oxide coating provided
better protection against interfacial side reactions between LiNi
0.5
Mn
1.5
O
4
surface and
electrolyte than previously reported discontinuous metal oxide coating. As a result, the
graphene-oxide-coated LiNi
0.5
Mn
1.5
O
4
showed remarkable performance as cathode for
high energy and long life lithium ion batteries. For up to 1000 cycles, the batteries
showed only 0.039% capacity decay per cycle. When discharged at large current of 5C,
7C, and 10C, the batteries can still deliver 77%, 66%, and 56% of the 1C capacity. AC
impedance test confirmed our analysis that graphene oxide coating helped to reduce the
impedance of the battery and hence improved the battery performance. The graphene
oxide coating reported in this work demonstrated a new method in enhancing the high
voltage cathode performance and showed promising result in developing high energy
density lithium ion batteries to be used for portable electronics, HEVs, PHEVs, and EVs
in the future.

44

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49

Chapter 4

Ultrathin surface modification by
atomic layer deposition on high
voltage cathode LiNi
0.5
Mn
1.5
O
4
for
lithium ion batteries

4.1 Introduction
In this chapter, I present the research about using Atomic Layer Deposition (ALD)
for Al
2
O
3
coating on high voltage cathode LiNi
0.5
Mn
1.5
O
4
. In Chapter 3, we have
introduced high voltage cathode LiNi
0.5
Mn
1.5
O
4
and graphene-oxide coating. The mildly
oxidized graphene oxide can improve both current rate performance and cycle life of the
batteries. Based on this work, we then moved towards improving high temperature
performance of the high voltage batteries.
The 4.7 V operating voltage of LiNi
0.5
Mn
1.5
O
4
was discovered by Dahn et al. in
1997.[1] The origin of the increased voltage was attributed to the increased binding
energy of Ni 3d electrons, which was found to be 0.5 eV higher than Mn 3d eg electrons.
Since the working potential is at the edge of the electrolyte window Eg,[2] side reactions
50

occur between the high voltage cathode surface and the electrolyte.[3-5] Also, LiPF
6

based electrolyte unavoidably contains HF, which will react with Li
x
MO
y
cathode.[6, 7]
The undesirable process compromises the life of the batteries.
Similar to what we mentioned in Chapter 3, surface modification has been reported
to be an effective method to suppress the side reactions and enhance the battery
performance.[3, 8-12] Various materials have been explored as coating layers to stabilize
the surface, such as Al
2
O
3
,[13, 14] Bi
2
O
3
,[13, 14] BiOF,[9] ZnO,[3] SiO
2
,[15] ZrO
2
,[10]
Li
3
PO
4
,[16] AlPO
4
,[13] ZrP
2
O
7
,[10] AlF
3
,[8] Au,[17] Ag,[18] polyimide,[11] conductive
carbon,[12] graphene oxide[19] and so on. Among those coating materials studied
previously, including the graphene oxide coating reported in Chapter 3, although all of
them can prevent the surface of LiNi
0.5
Mn
1.5
O
4
from being directly exposed to electrolyte,
metal oxides are believed to act as HF scavenger at the same time[20] thus being ideal in
improving the battery performance, especially at elevated temperature. As LiPF
6
based
electrolyte has become one of the most widely used electrolyte systems, it is beneficial to
have a protection layer against the unavoidable HF. However, traditional coating strategy
are usually based on solution chemistry,[3, 8, 10, 11, 13] which cannot control the
uniformity, conformity and thickness precisely. While using more coating precursors can
improve the coverage over the surface of bare LiNi
0.5
Mn
1.5
O
4
, it is difficult to avoid the
coating materials forming agglomerated particles. As many metal oxide materials do not
have good conductivity, especially Al
2
O
3
, agglomeration or increased thickness becomes
detrimental to the conductivity of the electrode. Moreover, for all the metal oxide
coatings mentioned above, no long cycling performance was reported, in spite of the fact
that cycle life is an important criterion in judging the battery performance.
ALD has emerged as an advanced technology to improve the performance of lithium
ion batteries.[21-38] For example, ALD coating has been reported for materials such as
51

LiCoO
2
,[30, 31] graphite,[22] and LiMn
2
O
4
.[35-38] However, to the best of our
knowledge, there has been no report on employing ALD in surface modification of
LiNi
0.5
Mn
1.5
O
4
. In this study, we report the first ultrathin Al
2
O
3
coating using ALD for
LiNi
0.5
Mn
1.5
O
4
, and we have achieved much improved performance in both room
temperature and elevated temperature (55 º C) cycling test. By coating LiNi
0.5
Mn
1.5
O
4

with ALD Al
2
O
3
, the following benefits can be obtained: 1) the coating thickness can be
precisely controlled to the scale of Å, which not only minimized the extra mass added to
the electrode, but also preserved the conductivity of the electrode, since electrons can
tunnel through Al
2
O
3
when it is below 4 nm;[39] 2) ALD provides superior coating
quality in terms of uniformity and conformity, in contrast, traditional solution chemistry
method easily leads to agglomerated Al
2
O
3
particles on the surface;[40] 3) ALD can be
applied to pre-fabricated electrodes, where all surface that may be exposed to the
electrolyte can be covered, but the electron and Li ion path between LiNi
0.5
Mn
1.5
O
4
,
carbon black and current collector is not blocked; 4) ALD needs very little precursor and
no solvent, thus no waste water is involved. With the above-mentioned advantages of
ALD, remarkable improvement of the battery performance was achieved. After 700
cycles, the ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
showed 71% capacity retention when the
bare LiNi
0.5
Mn
1.5
O
4
can only maintain 75% after 200 cycles. At an elevated temperature
of 55 º C, the ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
delivered 116 mAh/g at the 100
th
cycle,
in comparison, the capacity for bare LiNi
0.5
Mn
1.5
O
4
decreased to 98 mAh/g. This
promising result demonstrated the potential of employing ALD Al
2
O
3
coated
LiNi
0.5
Mn
1.5
O
4
for high energy and long life lithium ion batteries.

52

4.2 Experimental procedures
Material synthesis
LiNi
0.5
Mn
1.5
O
4
particles were synthesized via solid state reactions. Nickel acetate
(Ni(Ac)
2
•4H
2
O) and manganese acetate (Mn(Ac)
2
•4H
2
O) were mixed at a molar ratio of
Ni : Mn = 1 : 3 and milled in a mortar. After heating at 500 º C for 5 hours, lithium acetate
(LiAc •2H
2
O) was added to the mixture with a molar ratio of Li : Ni : Mn = 2.1 : 1 : 3 (5%
excess Li source was added in order to make up for the volatilization of Li during
calcination), and the mixture was heated to 500 º C for 5 hours once more. Then the
mixture was milled and sintered at 950 º C for 10 hours followed by annealing at 700 º C
for 10 hours.
ALD process
ALD was performed on pre-fabricated electrode with a home-made system. The
electrodes were prepared with traditional slurry casting method on Al current collector.
The weight ratio in the electrode was active material : poly(vinylidene fluoride) : carbon
black = 8 : 1 : 1. Trimethylaluminum (TMA) and H
2
O were used as precursors. ALD
were performed at the temperature of 90 º C and pressure of 6x10
-1
torr in a vacuum
chamber.
Electrochemical measurements
CR 2032 coin cells were assembled with Li metal as counter electrodes. 1.2 M
solution of LiPF
6
in ethylene carbonate (EC) and dimethyl carbonate (DMC) (3:7) was
used as electrolyte. The batteries were cycled in the voltage range of 3.5 V ~ 5 V at both
room temperature and elevated temperature of 55 º C. Electrochemical impedance spectra
(EIS) were collected with AC voltage at 5 mV amplitude and frequency range of 100 kHz
53

to 10 mHz. The batteries were fully charged and then rested to reach equilibrium before
the impedance test.
4.3 Results and discussion
Figure 4.1a shows the scanning electron microscopy (SEM) image of the
as-synthesized LiNi
0.5
Mn
1.5
O
4
particles. The size of the particles is mostly around 1 µm
and the shape is polyhedral. Figure 4.1b presents the X-ray diffraction (XRD) pattern of
the as-synthesized LiNi
0.5
Mn
1.5
O
4
particles. The peaks correspond to the typical cubic
structure of spinel LiNi
0.5
Mn
1.5
O
4
, which agrees with literature.[41]

Figure 4.1 Characterization of as-synthesized LiNi
0.5
Mn
1.5
O
4
particles. (a) SEM image of as-synthesized
LiNi
0.5
Mn
1.5
O
4
particles. (b) XRD pattern of the as-synthesized LiNi
0.5
Mn
1.5
O
4
particles.
According to previous studies,[42] LiNi
0.5
Mn
1.5
O
4
has two space groups: m 3 Fd and
P4
3
32. In both of these two phases, oxygen atoms form a cubic close packing, Mn and Ni
atoms occupy half of the octahedral sites and Li atoms occupy one eighth of the
tetrahedral sites. In the octahedral sites, Mn and Ni may be orderly or disorderly arranged
which correspond to P4
3
32 and m 3 Fd , respectively. It is difficult to distinguish the two
phases from XRD pattern, but we can see the existence of m 3 Fd spinel from our
charge/discharge curves, where clearly show the small Mn
3+
plateau at around 4.1 V, a
54

fingerprint of m 3 Fd LiNi
0.5
Mn
1.5
O
4
. We note that there are some oxygen vacancies in
the m 3 Fd spinel, so the actual chemical formula should be LiNi
0.5
Mn
1.5
O
4-δ
, although
LiNi
0.5
Mn
1.5
O
4
is commonly used in this paper and literature. It has been reported that
m 3 Fd spinel has better performance than P4
3
32 spinel.[42] However, Mn
3+
ions are
inclined to decompose into Mn
2+
and Mn
4+
, the former of which is easy to dissolve into
the electrolyte.[43] It is also reported that Ni
2+
has the tendency to dissolve as well.[43,
44] In this regard, a coating layer is highly desirable to protect the metal ions from
dissolution thus improving the performance of the batteries.

Scheme 4.1 Schematic diagram showing ALD Al
2
O
3
coating on pre-fabricated electrodes.
ALD was performed on pre-fabricated electrodes made from LiNi
0.5
Mn
1.5
O
4
, as
shown in Scheme 4.1. We note that the thickness of the Al
2
O
3
layer should be much
smaller than that of the electrode. To make the Al
2
O
3
layer visible, the schematic is not
drawn to scale. After coating the electrode with Al
2
O
3
, energy dispersive X-ray
spectroscopy (EDX) was used to confirm the successful coating by checking the
existence of Al on the electrode. Figure 4.2a shows the mapping of the surface from the
top of the electrode (mapping area is shown on the upper left). From the distribution of Al
we can confirm the uniformity and conformity of the deposited Al
2
O
3
layer. We believe it
is because LiNi
0.5
Mn
1.5
O
4
possesses hydroxyl-terminated surface, which is common for
55

metal oxides, and this surface feature is favored in ALD process.[22] This way ALD can
be directly conducted on LiNi
0.5
Mn
1.5
O
4
electrodes without any pre-treatment. Figure
4.2b displays the EDX mapping result on a cross-section of the LiNi
0.5
Mn
1.5
O
4
electrode
(mapping area is shown on the upper left). We intentionally avoided including the most
bottom part of the electrode to eliminate the inadvertent Al signal from the current
collector. The distribution of Al shows that the ALD precursors can diffuse through the
pores of the pre-fabricated electrodes, hence the surface of the pores inside the electrode
can also be covered. This phenomenon is in agreement with what has been shown with
ALD Al
2
O
3
coated natural graphite, where the ALD precursors deposited a conformal
Al
2
O
3
film in the torturous path of the entire electrode structure.[22] We believe this is
favorable in protecting the electrode from the undesired reactions, since any surface area
that would be in contact with the electrolyte can be covered in the ALD process. To show
the morphology of ALD Al
2
O
3
coating on LiNi
0.5
Mn
1.5
O
4
, high-resolution TEM images
are presented in Figure 4.2c and 4.2d. The images clearly show the Al
2
O
3
layer on the
outer surface of LiNi
0.5
Mn
1.5
O
4
(Figure 4.2c) and the crystalline nature of the
LiNi
0.5
Mn
1.5
O
4
particles (Figure 4.2d). In Figure 4.2c, the Al
2
O
3
layer comes from 30
cycles ALD coating. The thickness of 3-4 nm Al
2
O
3
corresponds well with the growth
rate of 0.12 nm per ALD cycle reported in literature.[30]
56

Figure 4.2 EDX elemental mapping of ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
electrode (a) from the top and (b)
from a cross-section. The mapping area is shown on the upper left. High-resolution TEM images showing
(c) morphology of 30 cycles ALD Al
2
O
3
coated on LiNi
0.5
Mn
1.5
O
4
and (d) the crystalline nature of
LiNi
0.5
Mn
1.5
O
4
particles.
To evaluate the effect of ALD Al
2
O
3
coating on the electrochemical performance,
2032 coin cells were assembled with metallic lithium as counter electrodes. The uncoated
LiNi
0.5
Mn
1.5
O
4
electrodes were also tested at the same condition as a comparison.
According to literature,[22, 30, 31, 33, 34] once the coating thickness exceeds a threshold
value, the electrochemical stability will be affected by the kinetics associated with ion
and electron transportation. The maximum allowed thickness can be different for
different materials. We have also studied the effect of thickness of oxide coating. With
our home-made ALD system, 3, 10 and 30 cycles of ALD Al
2
O
3
were coated on the
electrodes and the battery performance is shown in Figure 4.3. It is clear that 3and 10
cycles ALD coating showed better performance than 30 cycles at room temperature
57

(shown in Figure 4.3a and 4.3b). Further test at elevated temperature (55º C) of 3 and 10
cycles ALD coating demonstrated that 10 cycles ALD coating should be the optimized
thickness in our system, since 3 cycles ALD coating cannot greatly enhance the high
temperature performance (shown in Figure 4.3c). In this regard, 10 cycles ALD was
chosen as a representative for the analysis of ALD coated LiNi
0.5
Mn
1.5
O
4
in comparison
with bare LiNi
0.5
Mn
1.5
O
4
.

Figure 4.3 Comparison of the effect from different number of ALD cycles. (a) room temperature cycling
performance at C/2; (b) room temperature performance at different current rate; (c) high temperature (55º C)
cycling performance at C/2.

Figure 4.4a shows the results from cycling test of the LiNi
0.5
Mn
1.5
O
4
cathode with
and without Al
2
O
3
ALD. It is evident that the batteries showed better capacity retention
after Al
2
O
3
coating. After 200 cycles at the charging/discharging rate of C/2 (1C = 140
mA/g), ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
cathode maintained 91% of the original
58

capacity. In comparison, the bare LiNi
0.5
Mn
1.5
O
4
cathode can only deliver 75% of the
original capacity at the 200
th
cycle. Although the batteries showed very small difference
in capacity retention in the first 40 cycles, the improvement from ALD coating became
much more obvious with the extending cycling test. We believe this is because the thin
layer of Al
2
O
3
can help to form stable solid electrolyte interface (SEI) and protect the
cathode material from undesirable side reactions, such as HF etching and metal ions
dissolution. It has been reported that HF, generated by trace amount of moisture reacting
with LiPF
6
based electrolyte, continuously attacks the electrode active material and
causes capacity fading as cycling goes by.[40] This effect is not observed during the early
stage of cycling, but gradually turns obvious in extensively cycled batteries. Figure 4.4b,
4.4c and 4.4d represent the charge/discharge curves of the 5
th
, 40
th
and 100
th
cycle,
respectively. It is evident that the overpotential of the batteries with bare LiNi
0.5
Mn
1.5
O
4

increased significantly compared with that of the ALD Al
2
O
3
coated batteries. Also, the
decrease of capacity is more obvious in bare LiNi
0.5
Mn
1.5
O
4
batteries. This phenomenon
clearly shows the influence of the side reactions from electrolyte. As more byproducts
adhere to the surface of the electrode and separator, the resistance increases with cycling
thus leading to the increase of overpotential. In addition, Mn and Ni ions have been found
to dissolve into the electrolyte and get reduced and deposited on the surface of the
anode.[43, 44] As neither Mn nor Ni metal is good lithium ion conductor, this dissolution
of metal ions not only causes loss of capacity, but also increases the total resistance of the
batteries. In comparison, the ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
shows smaller
overpotantial and better capacity retention, which clearly demonstrates the merits from
the thin protection layer of Al
2
O
3
.
59

Figure 4.4 Room temperature cycling test results. (a) Comparison of cyclability of ALD Al
2
O
3
coated and
bare LiNi
0.5
Mn
1.5
O
4
at current rate of C/2 (1C = 140 mA g
-1
). Charge/discharge curves of the (b) 5
th
,(c) 40
th

and (d) 100
th
cycle from both ALD Al
2
O
3
coated and bare LiNi
0.5
Mn
1.5
O
4
.
To further assess the improvement from ALD Al
2
O
3
coating, the batteries were also
tested at an aggressive temperature of 55 º C, since lithium ion batteries need to operate at
a wide temperature range in practical applications. Figure 4.5a shows the results from
high temperature cycling test. At 55 º C, the ALD coated LiNi
0.5
Mn
1.5
O
4
performs
favorably over the bare LiNi
0.5
Mn
1.5
O
4
in terms of both capacity and stability. After
cycling at 55 ºC for 100 cycles, the ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
delivered 116
mAh/g, corresponding to 90% of the original capacity, but the bare LiNi
0.5
Mn
1.5
O
4
can
only maintain 98 mAh/g, which is less than 83% of the initial capacity. The
charge/discharge curves of the 5
th
, 40
th
and 100
th
cycle at 55 ºC are shown in Figure 4.5b,
4.5c and 4.5d, respectively. The difference in overpotential and capacity retention
between the Al
2
O
3
ALD coated LiNi
0.5
Mn
1.5
O
4
and bare LiNi
0.5
Mn
1.5
O
4
became
increasingly evident with the cycling test going on. Since the side reactions from
60

electrolyte became more pronounced at elevated temperature,[11] the degradation of the
bare LiNi
0.5
Mn
1.5
O
4
became more severe if comparing Figure 4.4d and 4.5d. In contrast,
the ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
exhibited better performance in suppressing the
undesirable effects. The results from high temperature test further demonstrated the
advantage of having the ALD Al
2
O
3
surface protection layer on LiNi
0.5
Mn
1.5
O
4

electrodes.

Figure 4.5 55 º C cycling test results. (a) Comparison of cyclability of ALD Al
2
O
3
coated and bare
LiNi
0.5
Mn
1.5
O
4
at current rate of C/2 (1C = 140 mA g
-1
). Charge/discharge curves of the (b) 5
th
,(c) 40
th
and
(d) 100
th
cycle from both ALD Al
2
O
3
coated and bare LiNi
0.5
Mn
1.5
O
4
.
The promising results from ALD coated LiNi
0.5
Mn
1.5
O
4
can be ascribed to the
following reasons. First, Al
2
O
3
can act as HF scavenger hence LiNi
0.5
Mn
1.5
O
4
is protected
from HF etching. As reported in Al
2
O
3
coated layered lithium transition metal oxide
cathode,[40] the amount of HF generated during cycling is quite suppressed by Al
2
O
3
.
This way the active cathode material is protected, especially at elevated temperature.
61

Second, the Al
2
O
3
layer prevented LiNi
0.5
Mn
1.5
O
4
from being directly exposed to
electrolyte, hence the dissolution of metal ions was suppressed. As both Mn and Ni ions
have been reported to dissolve into the electrolyte, which lead to an increase of
impedance, it is beneficial to cut off the direct exposure with the Al
2
O
3
layer. In addition,
due to the advantage of ALD, the coating layer can cover any surface that may be
exposed to electrolyte uniformly as mentioned before, however, the connection between
LiNi
0.5
Mn
1.5
O
4
other electrode materials, such as carbon black and PVDF, is not blocked.
Electrons and Li ions can travel from LiNi
0.5
Mn
1.5
O
4
to carbon black or Al current
collector without any added resistance. Another advantage of ALD is that it has atomic
thickness control, which cannot be achieved with traditional solution chemistry. Usually
the overall specific capacity is lowered as the coating layer is an extra weight added to
the electrodes,[40] but negligible weight is added in the ultrathin coating via ALD. ALD
process also eliminates the possibility that Al
2
O
3
forms agglomerated particles on the
surface of the cathode, which is shown with solution chemistry coating.[40] Since
electrons can tunnel through the Al
2
O
3
layer when it is less than 4 nm,[39] the precise
control of thickness plays an important role in preserving the conductivity of the
electrodes with negligible extra weight.
62

Figure 4.6 AC impedance test results of both ALD Al
2
O
3
coated and bare LiNi
0.5
Mn
1.5
O
4
after cycling at 55
º C for 100 cycles.

To confirm our analysis, electrochemical impedance spectra (EIS) of both bare
LiNi
0.5
Mn
1.5
O
4
and ALD coated LiNi
0.5
Mn
1.5
O
4
were collected after 100 cycles at 55 º C.
According to literature,[4] the first semicircle (at high frequency) represents the Li ion
diffusion through the surface layer and the second semicircle (at medium to low
frequency) represents charge transfer reaction. The bare LiNi
0.5
Mn
1.5
O
4
showed much
larger impedance in the first semicircle, meaning the Li ions encountered larger barrier
when diffuse through surface layer of bare LiNi
0.5
Mn
1.5
O
4
. This is an indication that the
ultrathin Al
2
O
3
layer can suppress the undesirable reactions during cycling, hence the
surface of the electrode was protected and the increase in surface impedance was also
suppressed. In addition, the similar charge transfer resistance between bare and ALD
coated LiNi
0.5
Mn
1.5
O
4
supports our analysis that this ultrathin Al
2
O
3
layer did not hinder
the electron transfer obviously.
63

Figure 4.7 Long cycling performance of ALD Al
2
O
3
coated LiNi
0.5
Mn
1.5
O
4
at C/2.
With all the benefits brought by ALD Al
2
O
3
coating mentioned previously, the
batteries showed superior long cycling performance. As shown in Figure 4.7, 71%
capacity retention was achieved after 700 cycles, which converts to only 0.04% capacity
decay per cycle. The remarkable cycling performance demonstrated the stability and
effectiveness of surface modification from ALD Al
2
O
3
coating, and also the potential of
employing this strategy in the application of high energy and long life lithium ion
batteries
4.4 Conclusion
In summary, we have successfully synthesized LiNi
0.5
Mn
1.5
O
4
with highly scalable
solid state reaction and coated ultrathin Al
2
O
3
layer via ALD on the electrodes. With the
advantage of uniformity, conformity and thickness control from ALD, the active material
LiNi
0.5
Mn
1.5
O
4
is protected from direct exposure to the liquid electrolyte and the
conductivity is reserved. The batteries with ALD Al
2
O
3
coating showed much improved
performance over bare LiNi
0.5
Mn
1.5
O
4
. The capacity retention after ALD coating is 71%
after 700 cycles. In comparison, the bare LiNi
0.5
Mn
1.5
O
4
can only maintain 75% of the
original capacity after 200 cycles. At elevated temperature of 55 º C, the ALD coated
LiNi
0.5
Mn
1.5
O
4
showed 91% capacity retention after 100 cycles while the bare
64

LiNi
0.5
Mn
1.5
O
4
showed less than 83%. Analysis of overpotential and impedance
confirmed that the improvement was brought by reducing undesirable reactions during
cycling, thus suppressing the increase in the barrier of Li ion and electron transportation.
The strategy of coating ultrathin Al
2
O
3
via ALD on high voltage cathode LiNi
0.5
Mn
1.5
O
4

leads to new opportunities in developing high energy density lithium ion batteries, and
the promising performance demonstrates the potential of employing the surface modified
high voltage cathode in the application of portable electronics, EVs, HEVs and PHEVs in
the future.

65

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LiNi0.5Mn1.5O4 as high voltage cathode for lithium ion batteries with high energy
density and long cycle life. J. Mater. Chem. A 2013, 1, 4083-4088.
[20] Chen, Z. H.; Qin, Y.; Amine, K.; Sun, Y. K. Role of surface coating on cathode
materials for lithium-ion batteries. J. Mater. Chem. 2010, 20, 7606-7612.
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Deposition for Lithium-Ion Battery Studies. Adv. Mater. 2012, 24, 3589-3615.
[22] Jung, Y. S.; Cavanagh, A. S.; Riley, L. A.; Kang, S. H.; Dillon, A. C.; Groner, M. D.;
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Kim, J. K.; Lee, J. Fe3O4 Nanoparticles Confined in Mesocellular Carbon Foam for High
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Performance Anode Materials for Lithium-Ion Batteries. Adv. Funct. Mater. 2011, 21,
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54, 7471-7475.

69

Chapter 5

Free-standing LiNi
0.5
Mn
1.5
O
4
/carbon
nanofiber network film as
light-weight and high-power cathode
for lithium ion batteries

5.1 Introduction
In this chapter, a new design of electrode structure is presented. In the previous
chapters, I have talked about improving the energy of batteries based on our research on
high voltage cathode LNMO and high capacity anode Si. In this and next chapters, the
way of reducing total weight of electrodes are discussed.
As we mentioned before, with high working voltage of 4.7 V and theoretical capacity
of 146.7 mAh/g, LiNi
0.5
Mn
1.5
O
4
can provide 20% and 30% higher energy density than
traditional cathode materials LiCoO
2
and LiFePO
4
, respectively.[1-3] To further enhance
the energy density, reducing the weight of the batteries can be an important and essential
strategy.
Recently, new designs of battery electrodes have been reported, which replace or
even eliminate the use of binders or current collectors in conventional battery electrode
70

structure. [4-13] These designs have stimulated a new trend of developing high energy
density lithium ion batteries through light-weight electrodes. However, to the best of our
knowledge, there has been no report on further improving the energy density of lithium
ion batteries with a co-effect of high voltage cathode together with novel design of
electrode structure.
In this chapter, we are to discuss free-standing LiNi
0.5
Mn
1.5
O
4
/carbon nanofiber
(CNF) electrode without using any other conductive additives, polymer binders or metal
current collectors. Through this approach, we are able to reduce the weight and increase
the working voltage of the electrodes simultaneously, and hence result in a
complementary enhancement of the energy/power density of the electrode. The
electrodes consist of LiNi
0.5
Mn
1.5
O
4
particles synthesized via highly scalable solid state
reaction method, and CNFs which are mass produced and commercially available already.
The LiNi
0.5
Mn
1.5
O
4
particles are distributed in free-standing CNF network, which
provides direct access to electrolyte and also facilitates electron transfer.
Through our design of electrode structure, the following benefits can be achieved: 1)
the total weight of the electrodes is highly reduced, since the CNFs can act as binder,
conductive additive and current collector simultaneously, and no other electrochemically
inactive additives will be needed in the electrodes; 2) the conductivity of the electrodes is
greatly enhanced due to the good conductivity of the CNFs compared with the traditional
binders, which are usually insulating; 3) the continuous and porous CNF network
facilitates electrolyte infiltration in addition to electron transfer, and in this way the
LiNi
0.5
Mn
1.5
O
4
particles can access Li ions more efficiently; 4) our active material
LiNi
0.5
Mn
1.5
O
4
provides high working voltage while the CNF network renders the
electrode light-weight, and hence the energy density is enhanced by a combined effect of
increasing voltage and reducing weight simultaneously. In the meanwhile, the capacity is
71

not compromised at all since LiNi
0.5
Mn
1.5
O
4
offers similar capacity compared with other
traditional cathode materials such as LiCoO
2
.
5.2 Experimental procedures
Materials preparation
Solid state reaction was employed to prepare LiNi
0.5
Mn
1.5
O
4
particles. Nickel acetate
(Ni(Ac)
2
•4H
2
O) and manganese acetate (Mn(Ac)
2
•4H
2
O) were first mixed at a molar
ratio of Ni : Mn = 1 : 3 and hand-milled in a mortar. The mixture was then heated up to
500 º C with a heating rate of 3 º C/min. After 5 hours heat treatment, lithium acetate
(LiAc •2H
2
O) was added to the mixture with a molar ratio of Li : Ni : Mn = 2.1 : 1 : 3 (5%
excess lithium acetate was added in order to make up for the volatilization of Li during
calcination), and the mixture was heated to 500 º C for 5 hours again. After that, the
mixture was milled and sintered at 950 º C for 10 hours followed by annealing at 700 º C
for 10 hours.
CNFs were obtained from Pyrograf Products Inc. (Pyrograf-III, Carbon Nanofiber,
PR-24-XT-HHT, batch information: PS 1392 BOX 5 HT 183). The CNFs were produced
via chemical vapor deposition and heat-treated afterwards to graphitize the carbon
overcoat on the surface. According to the manufacturer,[14] the CNFs were highly
conductive after heat treatment. The average diameter was 100 nm and surface area was
35-45 m
2
/g. The CNFs were treated with nitric acid and sulfuric acid (3:1, v/v) at 90 º C
over night in order to break up bundles. The CNFs were then washed with DI water and
collected by filtration.
Free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes were prepared through
vacuum filtration. The weight ratio of CNFs and LiNi
0.5
Mn
1.5
O
4
particles were 1:1, 1:3
72

and 1:5, denoted by CNF1/2, CNF1/4 and CNF1/6, respectively. During filtration, a
small amount of CNFs was first added to the filtration system to obtain a thin layer of
CNF network at the bottom. Then LiNi
0.5
Mn
1.5
O
4
/ CNF mixture was added to form the
main part of the free-standing composite film. On top of the mixture, another CNF thin
layer was added again. This way, both the top and bottom surfaces were cover by CNF
network in order to prevent LiNi
0.5
Mn
1.5
O
4
particles from falling out. The weight ratios
here take into account all the CNFs used in the electrodes, including CNFs used for
surfaces and inside the electrodes. According to previous study about CNF-containing
composites,[15, 16] the percolation threshold is around or below 3 wt%. Therefore, our
CNF1/2, CNF1/4 and CNF1/6 samples are all above the percolation threshold, and the
CNF network should provide good electric conduction.
Electrochemical measurements
CR 2032 coin cells were assembled with Li metal as counter electrodes. 1.2 M
solution of LiPF
6
in ethylene carbonate (EC) and dimethyl carbonate (DMC) (3:7) was
used as electrolyte. The batteries were cycled in the voltage range of 3.5 V ~ 5 V. The
free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes were used directly as cathode in
batteries. As a control, conventional electrodes were also prepared by slurry-casting on
Al current collector with weight ratio LiNi
0.5
Mn
1.5
O
4
: poly(vinylidene fluoride) : carbon
black = 8 : 1 : 1. To make a fair comparison, the active material loading of all electrodes
was maintained between 2-3 mg/cm
2
.
5.3 Results and discussion
Our free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes were fabricated through
vacuum filtration. To ensure that the particles are trapped inside the CNF network instead
of easily falling out from the surface, the surface was covered by an extra thin layer of
73

CNF network. The process is shown as schematic diagram in Figure 5.1 (a)-(c) and also
described in detail in the experimental section. Briefly, a small amount of CNFs were
first filtrated to get a thin CNF layer on the bottom, as shown in Figure 5.1(a), and then
the main part of the electrode, LiNi
0.5
Mn
1.5
O
4
/ CNF mixture, was filtrated on top (shown
in Figure 5.1(b)). After that, a final thin layer of CNF network was added to cover the top
surface, as shown in Figure 5.1(c). Digital image of the LiNi
0.5
Mn
1.5
O
4
/ CNF network
after filtration is shown in Figure 5.1(d). The network can be easily peeled off from
filtration paper after drying and cut into small electrodes to assemble coin cells. Figure
5.1(e) shows the digital image of the free-standing electrode that is cut into coin cell size
and ready for assembling. Traditionally, metal foils are used in batteries as current
collectors, which work as a conductive support for electron transport. As both the active
cathode material and carbon black are particles, polymer binders such as poly(vinylidene
fluoride) (PVDF) are needed to connect the particles to the metal current collector. In our
new design of the LiNi
0.5
Mn
1.5
O
4
/CNF network electrodes, since electrodes are
free-standing and the CNF network is conductive, it is not necessary to attach the
LiNi
0.5
Mn
1.5
O
4
/CNF network onto a current collector. The CNFs are entangled and form
network configuration, so the LiNi
0.5
Mn
1.5
O
4
particles can be trapped inside and
connected by the CNF network, and thus no binder is needed. Also, as the CNFs are
conductive, conductive additives such as carbon black are not needed either. As a result,
the total weight of the electrode is highly reduced.

74

Figure 5.1 Schematics and photos of LiNi
0.5
Mn
1.5
O
4
/ carbon nanofiber (CNF) network electrodes. (a)-(c),
schematic showing the fabrication process: bottom CNF thin layer (a), main part of LiNi
0.5
Mn
1.5
O
4
and
CNF added(b), and final coverage with CNF thin layer (c). (d) photo of LiNi
0.5
Mn
1.5
O
4
/ CNF network
after filtration. (e) photo of free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network electrode before assembling into
battery
The whole electrode is made of LiNi
0.5
Mn
1.5
O
4
particles and CNFs exclusively.
LiNi
0.5
Mn
1.5
O
4
can be synthesized via a variety of methods, such as co-precipitation,[17]
solid state reaction,[18, 19] sol-gel method,[20] thermal polymerization,[21] molten salt
method,[22] and so on. Among these methods, co-precipitation and solid state reaction
are adopted by industry since they are most scalable and compatible with industrial
processing. In our work, we used modified solid state reaction to prepare LiNi
0.5
Mn
1.5
O
4
thus having the potential for large scale applications. Hollow CNFs used in this study are
mass-produced and purchased from Pyrograp Products Inc.. Transmission electron
microscope (TEM) images of the CNFs are shown in Figure 5.2a and 5.2b. Figure 5.2a
reflects the hollow nature of the CNFs and Figure 5.2b clearly shows the detailed
75

structure of the walls. The outer diameter of the CNFs is over 100 nm and the inner
diameter is 50-60 nm. Layered structure of the walls can be clearly seen in the high
resolution transmission electron microscope (HRTEM) image of Figure 5.2b. The
graphitic nature of the CNFs is beneficial in enhancing the conductivity of the electrodes
thus leading to the superior performance under large current density and fast
charge/discharge. Scanning electron microscope (SEM) image of the surface of the
free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes is shown in Figure 5.2c. It is
obvious that the surface is covered by CNF network and no LiNi
0.5
Mn
1.5
O
4
particle can
be seen. The CNFs are over 100 μm in length and are entangled with each other. This
approach allows us to fully utilize LiNi
0.5
Mn
1.5
O
4
particles without losing them during
battery assembling or cycling. Figure 5.2d, 5.2e and 5.2f reveal the interior of the
LiNi
0.5
Mn
1.5
O
4
/ CNF composite electrodes where CNFs take 1/2, 1/4 and 1/6 of the total
weight, respectively. It is evident from these SEM images that the LiNi
0.5
Mn
1.5
O
4
particles are evenly distributed in the CNF network. In this way, the CNF network can
provide electron pathway to the LiNi
0.5
Mn
1.5
O
4
particles and enhance the conductivity of
the whole electrode. In addition, the network is strong enough to accommodate large
loading of LiNi
0.5
Mn
1.5
O
4
particles inside, as in the case of CNF1/6 samples, the CNF
network can hold LiNi
0.5
Mn
1.5
O
4
particles that are five times the weight of CNFs.
76

Figure 5.2 (a) TEM and (b) HRTEM image of CNF; (c) SEM image of the surface of LiNi
0.5
Mn
1.5
O
4
/ CNF
network; (d-f) SEM images of the inside of LiNi
0.5
Mn
1.5
O
4
/ CNF network for compositions of CNF1/2 (d),
CNF1/4(e) and CNF1/6(f).

Figure 5.3 presents the battery test results from free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF
network electrodes with all three compositions. As a control, conventional electrodes
were also tested in the same condition. After 100 cycles at a regular current rate of C/2,
the capacity retention of CNF1/2, CNF1/4, CNF1/6 and conventional electrode is 98.3%,
77

95.7%, 94.2% and 93.9%, respectively. Results from extended cycles of CNF1/2,
CNF1/4 and CNF1/6 are shown in Figure 5.3b, which have demonstrated the cycling
stability for up to 500 cycles. Although the specific capacity of conventional electrodes is
higher than CNF1/6, it is obvious from Figure 5.3c that all free-standing LiNi
0.5
Mn
1.5
O
4
/
CNF network electrodes showed much superior performance under large current rates. It
is noteworthy that at current rate as large as 20C, the conventional electrodes showed
almost no capacity while the LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes can still deliver
99.3mAh/g (CNF1/2), 69.0 mAh/g (CNF1/4) and 65.3 mAh/g (CNF1/6). It is evident
from the results here CNF network provides much better conductivity to the whole
electrode than conventional PVDF and carbon black. The remarkable performance
demonstrated the advantages of employing CNF network in the electrode structure. We
note here pure CNF network without LiNi
0.5
Mn
1.5
O
4
was also tested in the voltage
window of 3.5-5 V with Li metal as counter electrode. All other conditions were kept the
same as battery tests for LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and conventional
electrodes. The result is shown in Figure 5.4. From Figure 5.4 it is clear that CNFs can
provide only around 20 mAh/g capacity in the voltage window of 3.5-5 V. In order to
eliminate this effect and make fair comparison among all electrodes, we have deducted
the capacity from CNFs when calculating the specific capacities.
78

Figure 5.3 (a) Comparison of cycling performance of LiNi
0.5
Mn
1.5
O
4
/CNF network electrodes and
conventional electrodes at C/2 (1C = 140 mAh/g); (b) extended cycling results from LiNi
0.5
Mn
1.5
O
4
/CNF
network electrodes for up to 500 cycles at C/2; (c) comparison of discharge capacity of
LiNi
0.5
Mn
1.5
O
4
/CNF network electrodes and conventional electrodes from C/2 to 20C. Charging rate was
kept at C/2.
79

0 20 40 60 80 100
0
20
40
60
80
100
120
140

CNF only
specific capacity (mAh/g)
cycle number

Figure 5.4 Specific capacity of carbon nanofibers at 3.5-5 V against Li metal.

To examine the cyclability of the free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network
electrodes at large current rate, the batteries were tested at 3C and 5C for 100 cycles
continuously. The results are shown in Figure 5.5. Figure 5.5a and 5.5b represent the
discharge curves at 3C and 5C, respectively. The CNF1/2 electrodes maintained a high
voltage plateau at 4.67 V at 3C, while the CNF1/4 and CNF1/6 electrodes provided 4.62
V. At 5C, CNF1/2 electrodes kept a working voltage of 4.64 V while the voltage of
CNF1/4 and CNF1/6 electrodes slightly decreased to 4.58 V and 4.56 V, respectively.
Comparing to the free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes, it is obvious
that conventional electrodes cannot maintain the working voltage during large current
cycling, and hence the energy and power provided by these batteries has been severely
reduced. The cyclability results in Figure 5.5c and 5.5d have proved that the free-standing
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes can sustain large current charge/discharge
continuously without compromising cycle life. However, it is not meaningful to discuss
80

the capacity of conventional electrodes since the high voltage plateau cannot be reserved.
The comparison here has revealed the advantages of free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF
network electrodes apparently: not only the total weight of the electrode is reduced, but
also the current rate capability and retention are enhanced, both leading to significant
improvement on energy/power density of the batteries.

Figure 5.5 Discharge curves of LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and conventional electrodes at 3C
(a) and 5C (b). Cycling performance of LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and conventional
electrodes at 3C (c) and 5C (d). Charging rate was also 3C or 5C, correspondingly.

It has been reported that polarization behavior plays an important role on the rate
capability of the batteries.[23, 24] To further study the effect from CNF network on the
rate capability of the electrodes, polarization resistance R
p
was calculated based on
methods presented in literature.[23, 24] The batteries were discharged at different current
rates to obtain discharge profiles shown in Figure 5.6. It is clear that the LiNi
0.5
Mn
1.5
O
4
/
CNF network electrodes maintained higher capacity and voltage than conventional
81

electrodes at large current densities. Comparing with LiNi
0.5
Mn
1.5
O
4
/ CNF network
electrodes, the conventional electrodes almost cannot deliver any capacity at high voltage
range under current densities larger than 5C. Further analysis was performed following
Reference 30 and 31 to determine the polarization resistance, which is defined as the
slope of E(I
m
) = f(I
m
), where E is the potential, I
m
is the mass current and f(I
m
) is the
correlation between potential and mass current. With the curves at different discharge
rates, voltage vs. mass current profile from different depth of discharge (DOD) can be
obtained, as shown in Figure 5.7. For example, the black curve in Figure 5.7a was
obtained by extracting the voltage values corresponding to 20% of DOD from curves in
Figure 5.6a with different current rates. According to the relation R
p
= V/I, polarization
resistance R
p
can be extracted from the slope of voltage vs. mass current curves. After
doing linear fit for each of the curves in Figure 5.7, R
p
values at different DOD were
obtained and plotted in Figure 5.8. It should be clarified that the R
p
values here represent
the polarization resistance of the whole battery instead of the cathode only. However,
since the anode Li metal, separator, electrolyte and battery case are identical in all
batteries, the difference in R
p
can be an index of the difference between free-standing
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and conventional electrodes.
82

Figure 5.6 Discharge profiles of CNF1/2 (a), CNF1/4 (b), CNF1/6 (c) and conventional electrodes (d) from
C/2 to 20 C.

Figure 5.7 Voltage vs. mass current profiles of CNF1/2 (a), CNF1/4 (b), CNF1/6 (c) and conventional
electrodes (d) from 20% to 70% depth of discharge.
83

Figure 5.8 Comparison of polarization resistance of batteries with LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes
and conventional electrodes.

It is obvious from Figure 5.8 that conventional electrodes bear much larger
polarization resistance than the LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes. Among the
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes, CNF1/2 showed lowest R
p
values owing to the
largest CNF content. The R
p
values from CNF1/4 and CNF1/6 turned out to be close to
each other. We believe this is because the difference between CNF contents of 25% and
16.7% is not sufficiently significant to distinguish from the voltage plateau of Ni
2+
/Ni
3+
,
Ni
3+
/Ni
4+
redox couples. Instead, the larger capacity of CNF1/4 came from the domain
below the working voltage of Ni
2+
/Ni
3+
, Ni
3+
/Ni
4+
redox couples to the cut off voltage of
3.5 V. By judging the discharge profiles together with the R
p
values, we believe a
decrease of CNF content from 25% to 16.7% will not affect the energy/power delivered
from the high voltage region. However, if the full capacity needs to be utilized, a slightly
higher CNF content such as 25% would be beneficial in releasing the energy beyond the
84

high voltage region. All LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes showed much smaller
R
p
values than the conventional electrodes, indicating the favorable conductivity
enhancement from CNF network. Nevertheless, we note that the charge transfer
mechanism between CNF and LiNi
0.5
Mn
1.5
O
4
is not well understood and deserves further
study.
In addition to the polarization study, we also compared the total weight of the
different electrodes as well as the active material weight percentage (wt%) in the
electrodes, shown in Figure 5.9. Compared with conventional electrodes, the
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes can yield up to 55% reduction in total weight
and 2.2 times enhancement in the wt% of active material in the whole electrode. Since
these properties strongly affect the gravimetric energy/power density of the batteries, the
reduced weight and enhanced wt% of active material will lead to considerable
improvements in light-weight and high-power lithium ion batteries.

Figure 5.9 Comparison of LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes and conventional electrodes in terms of
weight of the electrodes and weight percentage of active material in the electrodes. The calculation was
based on active material loading of 3 mg/cm
2
.
85

5.4 Conclusion
In summary, we have developed free-standing LiNi
0.5
Mn
1.5
O
4
/ CNF network
electrode as high voltage cathode for lithium ion batteries. This new design of electrode
structure can reduce the total weight in addition to improving the working voltage of the
electrode, thus resulting in a further enhancement of energy density. The free-standing
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes showed excellent performance in fast
charge/discharge cycling test, which demonstrated their capability in sustaining large
current during battery operation. Moreover, the remarkable current rate capability also
enables us to develop high power lithium ion batteries with the free-standing
LiNi
0.5
Mn
1.5
O
4
/ CNF network electrodes. With highly scalable production of both
LiNi
0.5
Mn
1.5
O
4
and CNFs, the new method has great potential to promote the
development of light-weight and high-power lithium ion batteries for ultra-thin and
ultra-light electronic devices and even electric vehicles in the future.

86

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[8] Zhu, H. L.; Jia, Z.; Chen, Y. C.; Weadock, N.; Wan, J. Y.; Vaaland, O.; Han, X. G.;
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Mechanical Buffer and Electrolyte Reservoir. Nano Lett. 2013, 13, 3093-3100.
[9] Chen, X. Y.; Zhu, H. L.; Chen, Y. C.; Shang, Y. Y.; Cao, A. Y.; Hu, L. B.; Rubloff,
G. W. MWCNT/V2O5 Core/Shell Sponge for High Areal Capacity and Power Density
Li-Ion Cathodes. Acs Nano 2012, 6, 7948-7955.
[10] Liu, B.; Zhang, J.; Wang, X. F.; Chen, G.; Chen, D.; Zhou, C. W.; Shen, G. Z.
Hierarchical Three-Dimensional ZnCo2O4 Nanowire Arrays/Carbon Cloth Anodes for a
Novel Class of High-Performance Flexible Lithium-Ion Batteries. Nano Lett. 2012, 12,
3005-3011.
[11] Liu, B.; Wang, X. F.; Chen, H. T.; Wang, Z. R.; Chen, D.; Cheng, Y. B.; Zhou, C.
W.; Shen, G. Z. Hierarchical silicon nanowires-carbon textiles matrix as a binder-free
anode for high-performance advanced lithium-ion batteries. Sci. Rep-Uk 2013, 3.
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[12] Cheng, Q.; Song, Z. M.; Ma, T.; Smith, B. B.; Tang, R.; Yu, H. Y.; Jiang, H. Q.;
Chan, C. K. Folding Paper-Based Lithium-Ion Batteries for Higher Areal Energy
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conductivity of carbon nanofiber/semi-crystalline polymer composites. Acta Mater. 2008,
56, 2381-2388.
[17] Fang, X.; Ding, N.; Feng, X. Y.; Lu, Y.; Chen, C. H. Study of LiNi0.5Mn1.5O4
synthesized via a chloride-ammonia co-precipitation method: Electrochemical
performance, diffusion coefficient and capacity loss mechanism. Electrochim. Acta 2009,
54, 7471-7475.
[18] Fang, X.; Lu, Y.; Ding, N.; Feng, X. Y.; Liu, C.; Chen, C. H. Electrochemical
properties of nano- and micro-sized LiNi0.5Mn1.5O4 synthesized via thermal
decomposition of a ternary eutectic Li-Ni-Mn acetate. Electrochim. Acta 2010, 55,
832-837.
[19] Feng, X. Y.; Shen, C.; Fang, X.; Chen, C. H. Synthesis of LiNi0.5Mn1.5O4 by
solid-state reaction with improved electrochemical performance. J. Alloys Compd. 2011,
509, 3623-3626.
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[21] Zhong, G. B.; Wang, Y. Y.; Zhang, Z. C.; Chen, C. H. Effects of Al substitution for
Ni and Mn on the electrochemical properties of LiNi0.5Mn1.5O4. Electrochim. Acta
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[22] Kim, J. H.; Myung, S. T.; Sun, Y. K. Molten salt synthesis of LiNi0.5Mn1.5O4
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[24] Delacourt, C.; Laffont, L.; Bouchet, R.; Wurm, C.; Leriche, J. B.; Morcrette, M.;
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A913-A921.
88

Chapter 6

High-power lithium ion batteries
based on flexible and light-weight
cathode of LiNi
0.5
Mn
1.5
O
4
/carbon
nanotube film

6.1 Introduction
In order to keep pace with the development of new-generation portable electronics,
such as ultra-thin/ultra-light laptop computers and cellular phone, or bendable and
flexible displays, new requirements such as light-weight and/or flexibility have been
imposed on the batteries that work as power source for those devices, in addition to the
ever-increasing demand of high energy density and power density. To address the
challenges and requirements from new-generation electronics, innovation and
improvement in batteries are highly desired.[1]
89

In this regard, many groups including us have reported novel designs of electrode
structure.[2-18] For example, we discussed in Chapter 5 about the combination of
LiNi
0.5
Mn
1.5
O
4
with carbon nanofibers, which led to improved performance compared
with conventional slurry-casting electrodes.[18] Wu et al. studied traditional layered
cathode LiNi
0.5
Co
0.2
Mn
0.3
O
2
embedded in carbon nanotube (CNT) network, and showed
that the batteries performed better than those with CNTs simply added to the slurry.[17]
However, to the best of our knowledge, there are few reports on integrating both material
optimization and electrode design to achieve multiple enhancements and functionalities
simultaneously. In addition, many of the methods reported requires sophisticated
synthesis or fabrication process, which is difficult to scale up in the battery industry.
In this chapter, we present our research on flexible, light-weight, and yet high power
electrodes through the integration of high voltage cathode LiNi
0.5
Mn
1.5
O
4
and multiwall
carbon nanotubes (MWCNTs). The LiNi
0.5
Mn
1.5
O
4
particles are embedded in conductive
and free-standing MWCNT network. This way the use of binder, conductive additive and
metal current collector is totally eliminated. As a result, the total weight of the electrodes
is highly reduced.
Our LiNi
0.5
Mn
1.5
O
4
/MWCNT network electrode combines the advantages from both
material optimization and electrode structure design. In addition to the high voltage
provided by LiNi
0.5
Mn
1.5
O
4
, the MWCNTs provide highly conductive and yet
light-weight network, which is also free-standing and flexible. While facilitating the
electron transport in the electrodes, the porous MWCNT network also facilitates
electrolyte infiltration, and thus the LiNi
0.5
Mn
1.5
O
4
particles can access both electrons and
Li ions effectively. With the advantages from both high voltage cathode material
LiNi
0.5
Mn
1.5
O
4
and conductive MWCNT network structure, the LiNi
0.5
Mn
1.5
O
4
/
MWCNT electrodes can deliver over 80% capacity at up to 20C current rate. After 100
90

cycles at 10C charge/discharge, the high voltage plateau is still maintained and no
obvious capacity decay is observed. Since the total weight is also reduced, the power
density of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes is evidently enhanced comparing with
conventional electrodes. The combined effect from high voltage, high conductivity, light
weight and flexibility render the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes promising
candidate to work in high power batteries for new-generation ultralight/ultrathin and
flexible devices.
6.2 Experimental procedure
Materials preparation
LiNi
0.5
Mn
1.5
O
4
particles are synthesized via solid state reaction. Nickel acetate
(Ni(Ac)
2
•4H
2
O) and manganese acetate (Mn(Ac)
2
•4H
2
O) were first mixed and
hand-milled in a mortar at a molar ratio of Ni : Mn = 1 :3. The mixture was heated up to
500 º C for 5 hours with a heating rate of 3 ºC/min. Lithium acetate (LiAc•2H
2
O) was
then added to the mixture with a molar ratio of Li : Ni : Mn = 2.1 : 1 : 3 (5% excess
lithium acetate was added in order to make up for the volatilization of Li during
calcination). After that, the mixture was heated to 500 º C for 5 hours again. Finally, the
mixture was hand-milled once more and sintered at 950 º C for 10 hours followed by
annealing at 700 º C for 10 hours.
MWCNTs were prepared via fluidized bed catalytic chemical vapor deposition as
reported in literature.[19] The MWCNTs were washed with HF and H
2
O
2
twice to mildly
oxide the surface before use. The MWCNTs were then dispersed in
N-Methyl-2-pyrrolidone by shaking in a vortex mixer. About one quarter of the MWCNT
suspension was taken out to be used for top and bottom layer of the LiNi
0.5
Mn
1.5
O
4
/
91

MWCNT electrodes. LiNi
0.5
Mn
1.5
O
4
particles were added to the rest of the MWCNT
suspension and dispersed by shaking again.
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes were obtained by vacuum filtration. The
MWCNT suspension partially filtrated first to get a thin MWCNT layer before adding the
mixture of LiNi
0.5
Mn
1.5
O
4
/ MWCNT. The rest of MWCNT suspension was then added
on top to form another MWCNT layer on the surface. The film was then dried and can be
easily peeled off from the filtration paper.
Electrochemical test
Other than the pouch cells mentioned in the discussion, other measurements were
conducted with CR 2032 coin cells. Before assembling the coin cells, the LiNi
0.5
Mn
1.5
O
4
/
MWCNT film was cut into small pieces with 14 mm diameter. The electrolyte was 1.2 M
solution of LiPF
6
in ethylene carbonate (EC) and dimethyl carbonate (DMC) (3:7).
Conventional electrodes were prepared by slurry casting method on Al foil as control
samples. The weight ratio was LiNi
0.5
Mn
1.5
O
4
: poly(vinylidene fluoride) : carbon black =
8 : 1 : 1. The voltage window in the test was 3.5 V to 5 V and lithium metal was used as
counter electrode.
Pouch cells were assembled to test the self-discharge property of the LiNi
0.5
Mn
1.5
O
4
/
MWCNT film at bended state. The Cu foil connected to one end of the Ni tap was used
as back contact for anode, and the Al foil connected to one end of the Al tap was used as
back contact for cathode. The middle part of the taps was covered with a thermal tape
which was used to seal the pouch cell upon heating. The LiNi
0.5
Mn
1.5
O
4
/ MWCNT film
was put between the Al foil and the separator without using binder, to test the mechanical
and electrochemical stability of the film during bending. A very thin Li film was put
between the separator and the Cu foil to act as anode. The outer ends of the taps were
92

connect to test channels with wires, and the test method was the same as testing coin cells,
except that the pouch cells were tested at bended state.
6.3 Results and discussion
The fabrication process and structure of our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes are
schematically illustrated in Figure 6.1a. The MWCNTs and LiNi
0.5
Mn
1.5
O
4
particles are
dispersed in N-Methyl-2-pyrrolidone before vacuum filtration. During the filtration, a
thin layer of MWCNT film is filtrated first before the filtration of LiNi
0.5
Mn
1.5
O
4
/
MWCNT mixture. Another thin layer of MWCNT film is then filtrated on top of the
LiNi
0.5
Mn
1.5
O
4
/ MWCNT network. This way the top and bottom layers of our
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes are both covered by MWCNT films, so that the
LiNi
0.5
Mn
1.5
O
4
particles would not easily fall out from the surface. The as-obtained
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes can be easily peeled off from the filtration paper.
The photograph in Figure 6.1b shows that the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes are
flexible and robust, which can be bended back and forth. The LiNi
0.5
Mn
1.5
O
4
/ MWCNT
electrodes can also be cut into arbitrary shapes. The inset of Figure 6.1b shows the
flexibility of the electrodes when cut into a 14 mm diameter piece before assembling
batteries.
93

Figure 6.1 (a) Schematic illustration of the fabrication process of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes.
(b) Digital photo of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT network film to show the flexibility, inset is the photo
taken after the film was cut into small electrodes before assembling batteries. (c) SEM image of the
MWCNT network on the surface of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes. (d) SEM image of the inside
mixture of LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes where LiNi
0.5
Mn
1.5
O
4
particles are dispersed in MWCNT
network. (e) High resolution SEM image showing the LiNi
0.5
Mn
1.5
O
4
particles are connected by MWCNTs
in the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes.

Our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes are made from LiNi
0.5
Mn
1.5
O
4
particles and
MWCNTs exclusively, without any binders, additives or substrates. In our
94

LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes the LiNi
0.5
Mn
1.5
O
4
particles are synthesized via
solid state reaction, and the MWCNTs are produced via fluidized bed catalytic chemical
vapor deposition,[19] with brand name Flotube 9000, which can also be mass produced
in scale of 1000 tons. The advantage from scalable preparation methods enables our
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes to have the potential for large scale production in
the future. Details of the experimental procedure can be found in the experimental
section.
Figure 6.1c through 6.1e present the characterization of the LiNi
0.5
Mn
1.5
O
4
/
MWCNT electrodes by Scanning Electron Microscope (SEM). Figure 6.1c was taken on
the surface of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes, which was covered by a thin
layer of MWCNT film. It can be clearly seen that the MWCNTs can form a network
structure without large bundles. The voids in the network can allow electrolyte to
infiltrate into the electrodes, so the LiNi
0.5
Mn
1.5
O
4
particles have sufficient access to Li
ions during charge/discharge process. The inside of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT
mixture layer is shown in Figure 6.1d, where the LiNi
0.5
Mn
1.5
O
4
particles are loaded on
the bottom MWCNT film with high density. It’s important that the active material layer
is a mixture of LiNi
0.5
Mn
1.5
O
4
/ MWCNT, as compared to simply depositing the
LiNi
0.5
Mn
1.5
O
4
particles within two layers of MWCNT films. Here in our electrodes the
LiNi
0.5
Mn
1.5
O
4
particles are connected by the MWCNTs, which is obvious from the high
magnification SEM image shown in Figure 6.1e. This way the MWCNTs can provide
efficient pathways for electrons to transport inside the LiNi
0.5
Mn
1.5
O
4
/ MWCNT
electrodes, and thus the current rate capability can be significantly enhanced. A cross
section SEM image is shown in Figure 6.2, where the fiber-like MWCNTs and
LiNi
0.5
Mn
1.5
O
4
particles can be clearly seen.
95

Figure 6.2 SEM image taken at 45 degree tilt angle to show the cross section of LiNi
0.5
Mn
1.5
O
4
/
MWCNT electrodes

In our study, we tested LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes with two weight
percentages of MWCNTs, 30% and 20%, denoted by CNT30 and CNT20, respectively.
Here the weight of MWCNTs has included the MWCNTs from top and bottom layers. As
a control, conventional electrodes were also tested and the weight ratio was
LiNi
0.5
Mn
1.5
O
4
: poly(vinylidene fluoride) : carbon black = 8 : 1 : 1. The charge and
discharge curves of CNT30 and CNT20 at C/2 are presented in Figure 6.3a and 6.3b,
respectively. The curves show a typical high voltage plateau at 4.7 V, which is
considerably higher than the traditional cathode materials such as LiCoO
2
(4.1 V) and
LiFePO
4
(3.5 V). The small plateau at around 4.1 V reveals the existence of a small
amount of Mn
3+
ions in the crystal, which is helpful for structural reversibility during
cycling.[20]
96

Figure 6.3 Charge/discharge curves of CNT30 (a) and CNT20 (b) samples at C/2. (c) Specific capacity vs.
cycle number over 100 cycles for CNT30, CNT20 and conventional electrodes. (d) Specific capacity of
CNT30, CNT20 and conventional electrodes at different current densities. (e) Capacity retention at
different current densities with respect to the capacity at 1C for CNT30, CNT20 and conventional
electrodes. (f) Polarization resistance calculated for CNT30, CNT20 and conventional electrodes at
different depth of discharge.

Cycling test results for both CNT30 and CNT20 at C/2 (1C = 140 mAh/g) are shown
in Figure 2c, together with results from conventional electrodes as a control. After 100
97

cycles, the CNT30 samples can still deliver a high capacity of 135 mAh/g, and the
CNT20 samples can deliver 120 mAh/g. However, the LiNi
0.5
Mn
1.5
O
4
particles
synthesized from the same batch but made into electrodes with conventional
slurry-casting method, can only start with 118 mAh/g and maintain 110 mAh/g after 100
cycles under the same testing condition. More cycling data for up to 500 cycles from
CNT30 and CNT20 samples are shown in Figure 6.4, where around 87% of original
capacity is retained after 500 cycles. Both CNT30 and CNT20 samples performed better
than the conventional electrodes, which demonstrated that the LiNi
0.5
Mn
1.5
O
4
/ MWCNT
electrode structure has indeed facilitated the electron and Li ion transport, thus leading to
a better utilization of the LiNi
0.5
Mn
1.5
O
4
particles. In addition, the problem of adhesion
between binder and metal current collector is totally eliminated in our new structure,
which is also beneficial to the performance of the batteries.[21] We note that the capacity
of CNT30 and CNT20 samples here comes from LiNi
0.5
Mn
1.5
O
4
particles only. We have
subtracted the contribution from CNTs in order to make a fair comparison with the
control samples. (The capacity of CNTs in the voltage window of 3.5 – 5 V was tested
and shown in Figure 6.5.)

Figure 6.4 Cycling performance for up to 500 cycles at C/2
98

Figure 6.5 Specific capacity of MWCNT network without LiNi
0.5
Mn
1.5
O
4
particles
To reveal the advantages from CNT network in the electrodes, we have tested the
CNT30 and CNT20 samples at different current rates up to 20 C. The results are
presented in Figure 6.3d, in comparison with results from conventional electrodes. The
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes apparently performed much better than the
conventional electrodes, especially at large current rates. At 20 C charge/discharge rate,
the CNT30 and CNT20 samples can maintain a capacity of 107.9 mAh/g and 84.4 mAh/g,
respectively, while capacity from the conventional electrodes almost decreased to zero.
The capacity retention at each current rate (compared to the capacity at 1C) was
calculated and shown in Figure 6.3e. The conventional electrodes started to show an
obvious loss in capacity when the current increased to 5C, and further increasing the
current has led to more severe capacity degradation. However, the CNT30 and CNT20
samples still maintained 81% and 70% of the 1C capacity during 20C charge/discharge,
respectively. The comparison from capacity retention at large current density exhibits the
distinct superiority from the rational design in our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes.
Between our CNT30 and CNT20 electrodes, the difference is not very obvious until the
99

current density reached 15C, so we believe the conductivity from CNT20 samples should
be sufficient for applications in large current up to 10C. The lower capacity from CNT20
samples should be due to the low content of CNTs, where some LiNi
0.5
Mn
1.5
O
4
particles
are not in contact with CNTs and thus do not participate in the charge/discharge process.
We believe this can be tuned by a more uniform dispersion of LiNi
0.5
Mn
1.5
O
4
/ MWCNT
mixture in the future large scale applications.
To further study the origin of the excellent current rate performance of our
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes, we calculated the polarization resistance R
p
of both
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes and conventional electrodes following the method
reported in literature. [22, 23] Briefly, the correlation between potential and mass current
was extracted from three sets of voltage vs. capacity curves at different current densities
(shown in Figure 6.6a, c, e). The R
p
values at different depth of discharge (DOD) were
then obtained by doing linear fit of the potential vs. mass current relationship (shown in
Figure 6.6b, d, f). Detail of the data processing is described in supporting information.
The calculated R
p
is shown in Figure 6.3f. From the comparison we can see a significant
reduction of polarization resistance R
p
in the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes. Since
the polymer binder is usually not conductive, replacing it with CNT network can
effectively help to enhance the conductivity of the whole electrode. The reduced
polarization is an important reason why our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes can
maintain the high voltage plateau and high capacity at large current densities.
100

Figure 6.6 Discharge profiles of CNT30 (a), CNT20 (c) and conventional electrodes (e) at different current
densities from C/2 to 7C. Voltage vs. current density profiles of CNT30 (b), CNT20 (d) and conventional
electrodes (f) at different depth of discharge from 10% to 90%

To explore the cycling stability under large current charge/discharge process, we
tested the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes at 5C and 10C charge/discharge for 100
cycles continuously. The voltage plateau is shown in Figure 6.7a and 6.7b, where we can
observe an evident improvement in maintaining the high voltage plateau from the CNT30
101

and CNT20 samples, comparing with the conventional electrodes. At 5C, the CNT30 and
CNT20 samples can provide a voltage plateau at 4.60V and 4.58V, respectively.
However, the conventional electrodes can only offer around 4.1V due to the large
polarization. The voltage drop became more obvious when the current density increased
to 10C. The high voltage plateau of CNT30 and CNT20 samples remained at 4.58V and
4.51V, respectively, while the conventional electrode cannot even maintain a plateau.
Continuous cycling performance is presented in Figure 6.7c and 6.7d, where we can see
the stability of our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes under large current cycling test.
After 100 cycles, no obvious degradation is observed and a high capacity is still exhibited.
The voltage and capacity retention under large current and the excellent cycling stability
from the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes remarkably outperforms the conventional
electrodes. We noticed an increase in capacity from the CNT30 samples at 10C and we
believe this is due to the insufficient electrolyte infiltration into the CNT network during
the beginning cycles. Comparing with CNT20 samples, there should be larger surface
area from CNTs in the CNT30 samples, which tends to absorb more electrolyte to fully
wet the surface, and thus need more time to get the electrolyte fully infiltrated. As a result
of the large current and fast lithiation, the limited amount of electrolyte around the
surface of the particles was depleted of Li ions, leading to an insufficient lithiation. With
prolonged cycling, the electrolyte was fully infiltrated into the CNT network, and hence
the capacity got recovered with sufficient supply of Li ions from the electrolyte.
102

Figure 6.7 Discharge curves of CNT30, CNT20 and conventional electrodes at 5C (a) and 10C (b). Specific
capacity vs. cycle number over 100 cycles for CNT30 and CNT20 samples at 5C (c) and 10C (d). (e) Total
weight of CNT30, CNT20 and conventional electrodes calculated based on 5 mg/cm
2
active material
loading. (f) Power density of CNT30, CNT20 and conventional electrodes calculated based on the total
weight of electrode and voltage plateau at 10C.

In addition to the excellent capacity retention at large current rate, the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes are light-weight since the use of conductive additive, binder and
current collector is eliminated. The total weight of an electrode is calculated in Figure
103

6.7e, based on LiNi
0.5
Mn
1.5
O
4
particles loading of 5 mg/cm
2
, and the power density is
then calculated in Figure 3f considering the voltage provided at 10C. The power density
offered by the CNT20 electrodes is over 2 times larger than that from conventional
electrodes. We note that the power density may be compromised when other components
in the batteries are taken into consideration, and hence we only discuss the comparison
instead of an accurate number. Nevertheless, the benefits from voltage and capacity
retention of our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes, as well as the highly stable cycling
performance, would remain an advantage in high power batteries, and the reduced weight
would still be a significant feature in battery design. The overall performance from the
CNT30 and CNT20 samples has definitely demonstrated the potential of our
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes to be employed in high power batteries in the
future.
104

Figure 6.8 (a) Open circuit voltage of the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes at different bending radius;
inset is a photo showing the test under bending. (b) Photos showing a blue LED is powered up by the
LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes under bending.
105

Figure 6.9 Open circuit voltage of LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes recorded over 10 hours at bending
radius of 1.6 cm (a), 2.1 cm (b), 2.7 cm (c) and flat state (d).

Since our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes are flexible, pouch cells are
assembled and bent to different radius to show the flexibility. The open circuit voltage
(OCV) at different bending radius is shown in Figure 6.8a. The inset picture illustrates
the testing conditions where the batteries were attached to cylinders with different
diameters to keep the bending radius. The test was kept for 10 hours at each bending
radius to inspect the voltage decay and self-discharge ratio. The details of the test results
are shown in Figure 6.9. Comparing with the flat state OCV, the OCV at 1.6 cm bending
radius is only 0.026 V lower, which is within a reasonable fluctuation range. The stable
voltage at bent state exhibits favorable energy storage property to be a potential flexible
power source for electronic devices. One example is shown in Figure 6.8b. We
successfully employed the flexible battery to drive a blue light emitting diode (LED),
106

which requires more energy than red LEDs usually use. The results have demonstrated
that our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes can be a promising candidate for
high-power, flexible, and yet light-weight batteries in the future.
6.4 Conclusion
In conclusion, we have developed high-voltage, high-power, flexible, and
light-weight electrodes for lithium ion batteries by embedding LiNi
0.5
Mn
1.5
O
4
particles in
MWCNT network. The LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes can maintain 118 mAh/g
capacity and 4.6 V voltage during 10C charge/discharge. When the current density
increased to 20C, the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes can still deliver over 80% of
the capacity at 1C. The high voltage together with high capacity at large current density
indicates that the LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes can be a promising candidate for
high power lithium ion batteries. In addition, our LiNi
0.5
Mn
1.5
O
4
/ MWCNT electrodes
are light-weight and flexible, which is favorable for new generation ultrathin/ultralight or
flexible electronics. The excellent electrochemical performance, flexibility, and reduced
weight demonstrate the potential of employing our electrode design in batteries that work
as power source for flexible, light-weight and high-power devices in the future.

107

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109

Chapter 7

Conclusion and future directions

7.1 Conclusion
As a conclusion, this dissertation discusses the development of high energy lithium
ion batteries. According to literature, the energy density of a battery is calculated by the
following equation:
energy density = ∫
𝐼𝑉 (𝑡 )𝑑𝑡 𝑤𝑡
= ∫
𝑉 (𝑞 )𝑑𝑞 𝑤𝑡
𝑄 0
∆𝑡 0

In my PhD research, I used three methods to improve the energy density of lithium
ion batteries: improving the capacity, improving the voltage and reducing the weight.
Chapter 2 of this dissertation talks about using Si anode to improve the capacity of
lithium ion batteries. The Si/ATO/Si coaxial structure showed stable cycling performance
for 100 cycles with specific capacity around 1500 mAh/g. The high capacity
demonstrated the advantage of using nanostructure Si as anode and the potential of
improving the total capacity of lithium ion batteries by replacing graphite anode with
nanostructure Si.
110

Chapter 3 and Chapter 4 discussed the research on applying high voltage cathode
LiNi
0.5
Mn
1.5
O
4
in lithium ion batteries to improve the total energy. LiNi
0.5
Mn
1.5
O
4
can
provide 4.7V voltage vs. Li metal, while traditional cathode materials such as LiCoO2
and LiFePO
4
can only offer 4.1 V and 3.6 V, respectively. With surface coating to protect
LiNi
0.5
Mn
1.5
O
4
from side reactions with electrolyte, the cycling stability was obviously
improved. The coated LiNi
0.5
Mn
1.5
O
4
showed less than 0.4% capacity decay per cycle for
as long as 1000 cycles. Comparing mildly oxidized graphene oxide coating and ALD
Al
2
O
3
coating, the conductive coating can improve the current rate performance a lot,
while the Al
2
O
3
coating can obviously improve the high temperature cycling stability.
Chapter 5 and Chapter 6 demonstrated the high power and light weight electrodes
fabricated with carbon nanotube/nanofiber and LiNi
0.5
Mn
1.5
O
4
composite. By replacing
the conventional metal current collector with carbon nanotube/nanofiber network film,
the total weight of the electrodes can be highly reduced, and the current rate performance
was significantly improved. The energy and power density were enhanced by reducing
weight and increasing voltage and current rate. In addition, the carbon nanotube/
LiNi
0.5
Mn
1.5
O
4
composite film is flexible, which can power a blue LED while bended to
different angles.
In summary, this dissertation demonstrated three methods to improve the energy
density of lithium ion batteries. The experimental results proved the possibility of
developing high energy lithium ion batteries through high capacity anode Si, high voltage
cathode LiNi
0.5
Mn
1.5
O
4
and light weight carbon nanotube/nanofiber electrodes.

111

7.2 Future directions
Full cell with the combination of LiNi
0.5
Mn
1.5
O
4
and Si
As we have developed LiNi
0.5
Mn
1.5
O
4
as high voltage cathode and Si as high
capacity anode, we have the materials ready to combine high voltage and high capacity
into a full cell. This way we can combine the advantages from high capacity and high
voltage to achieve a potential improvement of 60-70% energy density.
In literature most of the reports about LiNi
0.5
Mn
1.5
O
4
full cell were about the
combination of LiNi
0.5
Mn
1.5
O
4
and Li
4
Ti
5
O
12
, where the property and features of the full
cell were studied and analyzed.[1-6] However, there’s very limited report about the full
cell study of LiNi
0.5
Mn
1.5
O
4
and Si. A full cell of LiNi
0.5
Mn
1.5
O
4
and amorphous Si thin
film was reported in 2013, where 74% capacity retention was achieved after 500
cycles.[7] However, the Si electrode was prelithiated to 10 mV vs. Li/Li
+
in a half cell
before being assembled into a full cell. This method is difficult to scale up. In addition,
since both the cathode and anode were at lithiated state when the full cell was assembled,
the full cell should be considered as over lithiated where it’s very easy to cause safety
problems such as lithium plating. In this regard, more detailed study of the full cell is
needed before we can achieve the combined improvement from both high voltage and
high capacity.
The most important issue to address would be the coulombic efficiency of Si anode,
especially for the first few cycles. Most of the reports about Si anode study focus on the
morphology and cycling performance of Si anode half cell, while the effects from
coulombic efficiency have been overlooked. However, when using Si anode in practice,
the low efficiency from first few cycles can lead to failure of the full cell since the total
amount of lithium source in cathode is limited. Prelithiation is reported as an effective
112

way to compensate the irreversible lithium loss in anode.[7-9] Both electrochemical
prelithiation and chemical prelithiation are reported. In our preliminary test, we used
electrochemical prelithiation to compensate for the first cycle efficiency of Si anode. The
charge-discharge curve is shown in Figure 7.1 and the cycling performance is shown in
Figure 7.2.

Figure 7.1 Charge-discharge curve of LiNi
0.5
Mn
1.5
O
4
-Si full cell.

Figure 7.2 Cycling performance of LiNi
0.5
Mn
1.5
O
4
-Si full cell.
113

The preliminary result proves that the integration of LiNi
0.5
Mn
1.5
O
4
cathode and Si
anode can provide high voltage full cell, but the cycling stability still needs further
improvement. The next step would be to work out controlled prelithiation of Si anode and
then the amount of LiNi
0.5
Mn
1.5
O
4
cathode for full cell integration can be determined by
testing battery performance with different cathode to anode ratios.
In addition to prelithiation control, another problem to solve for this integration
would be to determine the voltage window of the high voltage full cell. Since
over-charging will affect the battery’s cycle life and may cause serious safety problem,
selecting a proper voltage window is thus of high importance to the success of the full
cell. In previous research about LiNi
0.5
Mn
1.5
O
4
and Li
4
Ti
5
O
12
full cell, back to back half
cells were used to study the full cell parameters and properties.[2] This method can reveal
certain behavior of an electrode in a full cell configuration, such as the difference from
cathode or anode limited full cell. In our recent study, we applied this back to back half
cell configuration to study the voltage profile of LiCoO
2
and Si full cell. In our
preliminary study, we set the full cell voltage window to be 2 – 4.2 V and monitored the
voltage of both anode and cathode half cell. The results indicate that the cut-off voltage
increases with number of cycles. In the first charge the voltage of Si anode went down to
14 mV, which means the LiCoO
2
cathode was charged to 4.214 V. However, for the 10
th

cycle, the voltage of LiCoO
2
went up to 4.25 V at the end of charge process. This
indicates that the LiCoO
2
cathode may get over-charged with the continuous cycling
process. In the future, we can apply the back to back half cell test to LiNi
0.5
Mn
1.5
O
4
and
Si full cell study. The over-charge problem may need special attention for LiNi
0.5
Mn
1.5
O
4

and Si full cell, since the voltage is higher than traditional batteries already.
114

With both high capacity and high voltage, the LiNi
0.5
Mn
1.5
O
4
and Si full cell can be a
promising candidate as high energy power source for future portable electronic devices
and electric vehicles.

115

Reference
[1] Wu, H. M.; Belharouak, I.; Deng, H.; Abouimrane, A.; Sun, Y. K.; Amine, K.
Development of LiNi0.5Mn1.5O4/Li4Ti5O12 System with Long Cycle Life. J.
Electrochem. Soc. 2009, 156, A1047-A1050.
[2] Li, S. R.; Chen, C. H.; Xia, X.; Dahn, J. R. The Impact of Electrolyte Oxidation
Products in LiNi0.5Mn1.5O4/Li4Ti5O12 Cells. J. Electrochem. Soc. 2013, 160,
A1524-A1528.
[3] Li, S. R.; Chen, C. H.; Dahn, J. R. Studies of LiNi0.5Mn1.5O4 as a Positive
Electrode for Li-Ion Batteries: M3+ Doping (M = Al, Fe, Co and Cr), Electrolyte Salts
and LiNi0.5Mn1.5O4/Li4Ti5O12 Cells. J. Electrochem. Soc. 2013, 160, A2166-A2175.
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Electrolyte Additives for LiNi0.5Mn1.5O4/Li4Ti5O12 Li-Ion Cells. J. Electrochem. Soc.
2013, 160, A2014-A2020.
[5] Jung, H. G.; Jang, M. W.; Hassoun, J.; Sun, Y. K.; Scrosati, B. A high-rate long-life
Li4Ti5O12/Li[Ni0.45Co0.1Mn1.45]O-4 lithium-ion battery. Nat. Commun. 2011, 2.
[6] Xiang, H. F.; Zhang, X.; Jin, Q. Y.; Zhang, C. P.; Chen, C. H.; Ge, X. W. Effect of
capacity matchup in the LiNi0.5Mn1.5O4/Li4Ti5O12 cells. J. Power Sources 2008, 183,
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of high performance amorphous columnar silicon thin film anode, 5 V LiNi0.5Mn1.5O4
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Commun. 2013, 33, 31-34.
[8] Wang, Z. H.; Fu, Y. B.; Zhang, Z. C.; Yuan, S. W.; Amine, K.; Battaglia, V.; Liu, G.
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Metal Powder (SLMP). Nano Lett. 2013, 13, 4158-4163.

116

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Abstract (if available)

Abstract In this dissertation, I discuss the modification of electrode materials for high performance lithium ion batteries. Lithium ion batteries are widely used in many applications in our daily life, especially as the power source for mobile electronics, due to the advantages such as high energy density, no memory effect and long cycle life. Despite the success of lithium ion batteries in the past years, higher demands have been raised by the development of electric vehicles and new generation of electronic products such as ultrathin and flexible devices. Further enhancement of energy/power density, cycle life and other features is thus needed. ❧ The major goal of my PhD research is to develop high energy lithium ion batteries. I have used two methods to improve the energy of the batteries: (1) to improve the capacity and (2) to improve the voltage. I first worked on nanostructured Silicon (Si) as anode material for lithium ion batteries. Si has high theoretical capacity (4200mAh/g) which is about ten times higher than traditional anode material graphite (372mAh/g). In this regard, it is a promising candidate to be used for high energy lithium ion batteries. However, the large volume change during charge and discharge process will lead to pulverization of the electrode and thus the failure of the battery, which is the biggest problem hindering the application of Si anode in lithium ion batteries. In my research, I developed Si/anodized titanium oxide (ATO)/Si coaxial nanostructure to accommodate the volume change of Si and the battery performance was improved significantly. After improving the capacity, I have also worked on high voltage cathode material LiNi₀.₅Mn₁.₅O₄, which can work at 4.7V vs. Li metal. I used mildly oxidized graphene oxide and Al₂O₃ as coating materials to suppress the undesired reactions during high voltage cycling and thus improve the cycle life of the batteries. After that, carbon nanofibers (CNF) and carbon nanotubes (CNT) were used to develop CNT (CNF)/LiNi₀.₅Mn₁.₅O₄ composite network films as light weight electrodes, which reduced the total weight and improved the power density of the batteries. ❧ This dissertation is structured as follows. In Chapter 1 the basic concepts and features of lithium ion batteries and related electrochemistry is discussed, which can serve as the background for the following chapters. In Chapter 2, I present nanostructured Si anode study using anodized Titanium oxide array as substrate and template. In Chapter 3, 4, 5 and 6, I present LiNi₀.₅Mn₁.₅O₄ high voltage cathode study. In Chapter 3 and 4, the coating work of LiNi₀.₅Mn₁.₅O₄ is discussed, where mildly oxidized graphene oxide coating and atomic layer deposited Al₂O₃ coating can suppress the side reaction between the surface of LiNi₀.₅Mn₁.₅O₄ and electrolyte. Chapter 5 and 6 present a new design of electrode structure. CNT and CNF network with embedded LiNi₀.₅Mn₁.₅O₄ particles are developed into free-standing electrodes, and results demonstrated ultra-light electrodes with enhanced energy and power density. Finally, the conclusion is drawn and potential future work is discussed in Chapter 7.

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Asset Metadata

Creator Fang, Xin (author)

Core Title Modification of electrode materials for lithium ion batteries

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Publication Date 11/12/2015

Defense Date 10/05/2015

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag electrode,Energy,LiNi₀.₅Mn₁.₅O₄,lithium ion battery,OAI-PMH Harvest,silicon

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Chongwu (committee chair), Cronin, Stephen (committee member), Goo, Edward (committee member), Wang, Pin (committee member)

Creator Email xinfang@usc.edu,xinfang87@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-198904

Unique identifier UC11278989

Identifier etd-FangXin-4038.pdf (filename),usctheses-c40-198904 (legacy record id)

Legacy Identifier etd-FangXin-4038.pdf

Dmrecord 198904

Document Type Dissertation

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Rights Fang, Xin

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

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