Date of Award

1-1-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Jian Xie

Second Advisor

Steven Son

Committee Member 1

Jian Xie

Committee Member 2

Steven Son

Committee Member 3

Jeffery Youngblood

Committee Member 4

Wenquan Lu

Committee Member 5

Andres Tovar

Abstract

Six parts consist of my dissertation work centered on the investigation of V2O5 and LiFePO4 as the cathode materials for Li-ion batteries. The first part is about the investigation of structure evolution of vanadium oxide (V2O5) nanocrystals during the Li+ ion intercalation and deintercalation processes using in operando high-energy x-ray diffraction (HEXRD) and in operando x-ray adsorption near edge spectroscopy (XANES). The HEXRD results clearly show that V2O5 undergoes phase transformations during the first Li+ ion intercalation (i.e. discharge) process. The analysis of the XANES data suggests that the average oxidation state of vanadium in fully charged V2O5 nanocrystals decreases to less than +5 after the first four cycles. The combined results of the unchanged crystal structure (HEXRD) and the decreased oxidation state (XANES) lead to the conclusion that some of the Li+ ions are trapped within the V2O5 framework and the V2O5 exists as Li0.18V2O5 instead of pure V2O5 after the first four cycles, while the trapped Li+ ion may increase the stability of V2O5 framework. The second part is about development of novel method of incorporating graphene sheets into V2O5 nanoribbons via the so-gel process. The graphene modified nanostructured V2O5 Hybrid has been developed with extraordinary electrochemical performance, 438 mAh/g, almost achieving the theoretical specific capacity, 443 mAh/g, with only 2% graphene in the composite. An in operando high energy synchrotron XRD revealed that such performance is the result of the enhanced thermal stability of the V2O5 xerogel. The graphene sheets help to preserve the V2O5 xerogel structure and keep the xerogel from collapsing by maintaining 0.3 water molecules per V2O5 (water molecules serve as a pillar between the V2O5 layers) during the annealing process. The AC impedance indicates that the electric conduction, vanadium redox reaction, and Li+ diffusion in the graphene modified nanostructured V2O5 hybrid have been greatly improved, resulting in a significant improvement on rate performance and cycle life. This method provides a new avenue to create nanostructured metal oxide/graphene hybrid with improved properties, as long as they can be synthesized via the sol-gel process or by a reaction in solutions.

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