University of Southern California Dissertations and Theses

Nanostructure design of sulfur cathodes and lithium metal anodes for lithium-ion batteries

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Nanostructure design of sulfur cathodes and lithium metal anodes for lithium-ion batteries

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Abstract (if available)

Abstract Lithium-ion batteries (LIBs) have been successfully used in portable electronics and electric vehicles for many years. Although the demand for high energy density keeps increasing, the existing commercial LIB systems are getting close to their theoretical capacity limits. In order to push the energy density to an even higher level, lithium sulfur (Li-S) batteries attract a lot of attention due to their high energy density and low cost. In this thesis, I will discuss the current challenges we are facing in Li-S battery development and our proposed solutions to address those issues using nanostructure design. ❧ I will first present our accomplishment to enhance the S cathode performance. We have developed a scalable and inexpensive design for S cathode by capping a flexible gel polymer / carbon nanofiber (CNF) composite membrane onto a free-standing and binder-free CNF + Li₂S₆ cathode, thus forming a three-dimensional (3D) structural design. While the CNF network was used as the current collector and S holder to overcome the insulating nature and volume expansion of S, the composite membrane composed of a gel polymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and CNF additive was used as an interlayer to trap polysulfides and recycle remaining S species, leading to high specific capacity and long cycle life. This 3D structure enables excellent cyclability for 500 cycles at 0.5 C with a small capacity decay of 0.092% per cycle. Furthermore, outstanding cycle stability has also been achieved at even higher current densities (1.0 C - 2.0 C), indicating the great promise for practical applications of Li-S batteries. ❧ Next, I will talk about our achievement for stable Li metal anodes. We have demonstrated a distinctive design for dendrite-free deposition of Li by modifying the Cu current collector with a 3D CNF network. Due to the large surface area and high conductivity of the CNF network, Li metal can insert into and deposit onto CNF directly and no dendritic Li metal was observed, leaving a flat Li metal surface. With Li metal as the counter electrode for Li deposition, an average Coulombic efficiency of 99.9% was achieved for over 300 cycles, at a large current density of 1 mA/cm² and 2 mA/cm² with a high loading of 1 mAh/cm² of deposited Li. The gravimetric specific capacity we achieved was about 536 mAh/g, which is higher than the theoretical specific capacity of commercial anode materials. The scalable preparation method and impressive results achieved here demonstrated the potential of applying our design in the development of dendrite-free Li metal anodes in future. ❧ In addition, I will also present my research to further improve the performance of Li metal anodes. We have proposed a novel design of 3D interconnected graphene (IG) framework synthesized with the help of nickel (Ni) microspheres for stable Li metal anodes. The as-prepared IG framework consisted of multiple stacks of two-dimensional (2D) graphene layers and plenty of hollow graphene microspheres in between, and thus provided protective layers on the top to suppress lithium dendrites, sufficient surface area to reduce the effective current density, as well as ion channels for fast Li transport, which is confirmed by post-cycle morphology characterizations. When Li foil was used as the counter electrode for Li deposition, the assembled coin cell maintained an average Coulombic efficiency of more than 97.5% for 100 cycles at current density of 1 mA cm⁻² with a Li loading of 1 mAh cm⁻². Because the thickness of graphene is at atomic level, an IG current collector with much smaller mass can still provide enough void space for Li deposition. Therefore, the gravimetric specific capacity can be as large as 4,286 mAh/g when calculated based on the total mass of the IG current collectors. Furthermore, we achieved stable cycling for more than 300 hours at a current density of 1 mA cm⁻² with a Li loading of 2 mAh cm⁻² when assembled as symmetric cells. This strategy of vertically stacking 2D materials provides a novel approach towards dendrite-free Li metal anodes for the next-generation energy storage systems.

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Asset Metadata

Creator Zhang, Anyi (author)

Core Title Nanostructure design of sulfur cathodes and lithium metal anodes for lithium-ion batteries

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 11/12/2018

Defense Date 10/23/2018

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

Tag lithium metal anodes,lithium-ion batteries,nanostructure,OAI-PMH Harvest,sulfur cathodes

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Chongwu (committee chair), Armani, Andrea (committee member), Wu, Wei (committee member)

Creator Email anyizhan@usc.edu,zhanganyi88@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-106740

Unique identifier UC11675359

Identifier etd-ZhangAnyi-6965.pdf (filename),usctheses-c89-106740 (legacy record id)

Legacy Identifier etd-ZhangAnyi-6965.pdf

Dmrecord 106740

Document Type Dissertation

Format application/pdf (imt)

Rights Zhang, Anyi

Type texts

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...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA