Date of Award

August 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Junhong Chen

Committee Members

Peter Hallac, Chris Yuan, Ying Li, Benjamin Church

Keywords

Anode, Chemical Vapor Deposition, Highly Branched Graphene Nanosheets, Lithium in Battery, Silicon Film, Substrate-bound Nanostructure

Abstract

Li-ion batteries (LIBs) are commercially dominant in electrochemical energy storage devices. Their wide applications as power sources, from portable electronics to electric vehicles/hybrid electric vehicles, are mainly attributed to their superior energy density based on unit volume and unit weight over other battery systems. Growing demands for lighter, longer-lasting, and more powerful devices spur the development of new electrode materials, because the performance of LIBs is highly dependent on the characteristics of electrodes. The ideal candidate for anode materials of LIBs should have high lithium storage capability and stable structure after repeated charge/discharge cycles. Since its first commercialization by Sony Corporation in 1991, graphite has been the most popular anode material in LIBs because of its abundance, decent reversible energy capacity, and good electrochemical stability within a wide potential range. However, current LIB technology using graphite-based anode has reached its performance limit due to the constraints by the theoretical specific capacity (372 mAh/g) of graphite. Silicon is one of the most promising anode materials for LIBs to replace conventional graphite anodes due to its highest theoretical capacity of 3,579 mAh/g. However, a substantial volume expansion/contraction of silicon during the lithiation/de-lithiation cycles typically leads to the loss of electrical contact between the silicon materials and the current collector, causing shortened battery life and poor battery performance. It is thus highly desirable to design new electrodes that can survive the large volume change over the extended cycle life without significant degradation.

The objective of this research is to explore new hybrid materials containing silicon thin films (active materials) and substrate-bound carbon nanostructures (less active supporting matrix) as an anode for LIBs. The resulting hybrid anode materials inherit both unique properties of the Si films (e.g., high lithium storage capacity) and those of the substrate-bound structure (e.g., strong adhesion to the current collector, good electrical conductivity). The direct growth of supporting matrix on a current collector without any additives results in strong adhesion to the substrate to ensure a stable cycle life. Instead of using catalysts to grow the supporting matrix, various carbon nanostructures including carbon nanofibers (CNFs) and highly branched graphene nanosheets (HBGNs) are grown directly on the current collector and integrated with Si films.

All the hybrid materials grown on the current collector are directly used for an anode without further processing. The binder- and conductive additive-free electrodes eliminate the deadweight loss, and thus increase the specific energy density in the battery system. The hybrid Si/HBGN exhibits a good cycle performance with proper control of the loading density and the thickness of the Si film. The void space in HBGNs plays a critical role in the electrochemical performance to accommodate the strain relief of the Si film. The hybrid Si/CNF provides a facile approach to produce Si- and carbon-based hybrid electrodes by modifying the current collector to favor Si film deposition. The poor capacity retention of Si electrodes is effectively addressed by combining the advantageous features of CNFs and Si film. The hybrid CNF/HBGN provides a continuous conduction pathway, which is expected to lead to a high charge carrier mobility. The hybrid CNF/HBGN as an anode material has a higher Li storage capability compared with CNF alone. The HBGNs with nanoporous cavities, large surface area, and edges of exposed graphene platelets provide more sites for Li-ion storage.

Based on this study, we conclude that the substrate-bound carbon nanostructures in the hybrid provide a strong mechanical support for long cycle life and a large surface area for a high loading density of active materials, while Si thin films offer a high Li storage capacity. By conducting systematic studies on the different types of substrate-bound carbon nanostructured electrodes, this study contributes to the development of novel anodes in LIBs. It is anticipated that this study will lead to an efficient route to fabricating high-performance binder- and conductive additive-free anode electrodes for next-generation LIBs.

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