Date of Award

May 2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Chris Yuan

Committee Members

Junhong Chen, Deyang Qu, Benjamin Church, Arsenio Andrew Pacheco

Keywords

High Energy, High Rate, Lithium Ion Batteries, Nanomaterals, Oxygen Reduction Reaction, Stability

Abstract

Undoubtedly, one of the most pressing issues that world is dealing with today is global warming. It requires long-term potential actions for sustainable development to achieve solutions to address this kind of environmental problems that we are facing today. In this regard, renewable energy resources appear to be the one of the most efficient and effective solutions to reduce reliance on fossil fuels that cause release of excess amounts of carbon dioxide, the main contributor to global warming. As an alternative, electrochemical energy production has long been explored due to its intrinsic nature of more sustainable and more environmentally friendly. Lithium-ion batteries and fuel cells are two of the most studied systems for electrochemical energy storage and conversion. Although the energy storage and conversion mechanisms of lithium-ion batteries and fuel cells are different, there are “electrochemical similarities” of these two systems, all consisting of two electrodes in contact with an electrolyte solution. In order to improve the electrochemical performance of rechargeable lithium-ion batteries, various anode materials including Si and Sn have attracted tremendous interests to replace currently used graphite anode due to their high theoretical specific capacity. However, up to 400% volume expansion/contraction causes cracking and pulverization leading to repaid capacity fading during charge and discharge process. On the other side, the searching of advanced nanocatalysts with unprecedented catalytic efficiency at low-cost limps toward commercialization of highly efficient fuel cells and lithium-air batteries, whereas the pivotal challenges lie in the kinetically sluggish oxygen reduction reaction (ORR) at the cathode.

Nanostructured materials have offered new opportunities to design high capacity lithium-ion battery anodes and efficient catalysts to replace traditional noble metal such as Pt based materials. The objective of this study is to demonstrate high performance anode with superior rate capacity and long-cycle-life and effective nanocatalyst for oxygen reduction reaction with superior electrocatalytic activity and stability through rational design of novel nanomaterials. First, a three-dimensionally interconnected carbon nanotube/layered MoS2 nanohybrid network is reported with best-so-far rate capability and outstanding long cycle life. The monolayer and bilayer MoS2 ultrathin nanosheets with large surface to volume ratio facilitate fast Li ion transport further boosting high power capability, while incorporating high conductive CNT enhances the electronic conductivity and retains the structural integrity. The nanohybrid delivers discharge capacity as high as 512 mAh g-1 at 100 A g-1 and 1679 mAh g-1 over 425 cycles at 1 A g-1 with 96% discharge capacity retention of the initial cycle.

Then a novel 3D carbon coated Si NPs loaded on high conductive ultrathin graphene nanosheets was fabricated for potential use as anode material for high performance LIBs. The unique structural design of Si@C/NRGO has the combined merits of the carbon layer coating and graphene nanosheets which not only provides volume buffer and improve the conductivity but also separates the Si particles from direct exposure to electrolyte to form a stable SEI layer. Thus, the Si@C/NRGO nanohybrid demonstrates a superior electrochemical performance which is an ideal candidate for high performance LIBs. The nanohybrid delivers discharge/charge capacity of 3079 and 2522 mAh g-1 in the initial cycle at 100 mA g-1, corresponding to a Coulombic efficiency of 82%. A reversible capacity of 2312 mAh g-1 with an approximate Coulombic efficiency of 92% is retained when the current density increases to 1 A g-1 at the 3rd cycle. After 250 cycles, the nanohybrid still retains charge capacity of 1525 mAh g-1, close to a 60% capacity retention of the first cycle at 100 mA g-1. Moreover, the Si@C/NRGO nanohybrid demonstrates proficient cyclic stability with reversible capacities of 1932, 1507, and 1245 mAh g−1 for 2, 4, and 8 A g−1, respectively.

Subsequently, a three-dimensionally core-shell structured edge enriched Fe3C@C nanocrystals on graphene network is demonstrated with superior electrocatalytic activity and stability. The graphene nanosheets provide host and vital support for locally grown edge enriched Fe3C nanocrystals, which in-turn perform like separator/spacer to avoid the stacking of ultrathin graphene sheets, leading to a high surface area and super-stable Fe3C@C/rGO hybrid structure. The unique structural design of Fe3C@C/rGO nanohybrid with large surface area enables fast mass transport and a large number of active sites for catalytic reactions. The Fe3C@C/rGO nanohybrid exhibits excellent ORR catalytic activity with a high positive onset potential close to 1.0 V, a Tafel slope of 65 mV/decade, and excellent durability with only ~8% current density decay at 0.8 V after 20,000 seconds’ continuous operation, which is superior to that of a commercial Pt/C in an alkaline electrolyte.

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