Shuai Xu

Date of Award

1-1-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Xiaodong Hou

Abstract

The increasing demands on high energy density and long cycling life in electronics and energy storage systems require lithium-ion batteries (LIBs) with higher capacity anodes to replace the present commercial anode, graphite, which has a limited capacity of only 372 mAh g-1. Silicon-based materials (silicon and silicon suboxides) are among the most attractive alternative because of their high theoretical capacity but can hardly reach commercialization due to numerous technical and engineering barriers. The effort in this dissertation focuses on improving the electrochemical performance of SiO and Si anode materials, trying to pave a way to a greener and cleaner energy industry. A total of 5 projects (from Chapter Ⅱ to ChapterⅥ) are included in this dissertation as follows.The first chapter introduces the background and challenges in the development of anode materials for LIBs, followed by the motivation of my work in the years of pursuing the Ph.D. degree. In Chapter Ⅱ, low-cost coal-derived humic acid (HA) is used as a feedstock to synthesize in situ graphene-coated disproportionated SiO (D-SiO@G) anode with a facile method. HR-TEM and XRD confirm the well-coated graphene layers on a SiO surface. Scanning transmission X-ray microscopy and X-ray absorption near-edge structure spectra analyses indicate that the graphene coating effectively hinders the side-reactions between the electrolyte and SiO particles. As a result, the D-SiO@G anode presents an initial discharge capacity of 1937.6 mAh g−1 at 0.1 A g−1 and an initial coulombic efficiency of 78.2%. High reversible capacity (1023 mAh g−1 at 2.0 A g−1), excellent cycling performance (72.4% capacity retention after 500 cycles at 2.0 A g−1), and rate capability (774 mAh g−1 at 5 A g−1) results are substantial. Full coin cells assembled with LiFePO4 electrodes and D-SiO@G electrodes display impressive rate performance. These results indicate promising potential for practical use in high-performance lithium-ion batteries. In addition, the effect of particle size and carbon content on the electrochemical performance of the SiO@G anode is also analyzed. In Chapter Ⅲ, the microstructure and composition evolution in amorphous SiO and graphene-coated SiO is investigated using different heat-treatment conditions. X-ray absorption near-edge structure techniques are also employed to analyze the surface and bulk composition change during the initial lithiation process, supplemented by physical or chemical characterization and electrochemical testing. The results reveal the structural transition of SiO during heat treatment, from amorphous to disproportionated hierarchical structure, where the as-formed dielectric exterior SiO2 shell and interior SiO2 matrix severely polarize electrodes, hindering the lithiation process. Carbon coating on SiO effectively restricts the growth of the SiO2 shell and facilitates charge transfer, leading to improved electrochemical performance. A schematic model is proposed to reveal the relationship between the treatments, the resultant structural evolutions, and corresponding electrochemical behaviors. Chapter Ⅳ depicts the work of using lignite-derived coal tar pitch to develop high-performance SiO/C/Gr anode composites. Two synthesis routes using high softening point pitch and low softening point pitch with SiO anode are discussed separately. Benefiting from the LS-CTP coating, the SiO/G/C composite shows a long cycle life with 84.5% capacity retention after 500 cycles. The SiO/C composite synthesized by HS-CTP also shows a high reversible capacity (~1000 mAh g-1 at 2.0 A g-1) with a considerable cycle life (85% capacity retention after 200 cycles). In Chapter Ⅴ, SiO2 nanoparticles (nano-SiO2) and low-cost coal-derived HA are used as feedstock, by magnesiothermic reduction and spray drying, to synthesize micrometer-sized porous Si coated with graphitized carbon shell (mpSi@C). Our Si-C anodes feature micrometer-sized porous Si coated with a graphitized carbon shell. The hierarchical graphitized carbon shell and porous silicon structure relieve the mechanical stress of the Si phase upon cycling, which stabilizes the structure. This mpSi@C composite design allows for a high initial discharge capacity of 2199.9 mAh g-1 at 0.1 A g-1 and a cycling performance of 68% capacity retention after 100 cycles at 1.0 A g-1. The multipoint contact between the Si anode and C structure allows for a remarkable performance rate of 566.3 mAh g-1 at 5.0 A g-1. In Chapter Ⅵ, coal fly ash is used to replace SiO2 nanoparticles as a low-cost feedstock and structure template to in-situ synthesize hierarchical hollow porous Si@C composites. The hollow and porous structure allows the Si anode to expand and contract freely during lithiation and delithiation. The in-situ synthesized tough carbon coating acts as an effective electrolyte barrier and structure sustainer to keep the electrochemical performance composites from decaying during long-time cycling. As a result of this hierarchical design, the synthesized HP Si@C composite presents an initial discharge capacity of 2502.3 mAh g-1 at 0.05 A g-1 and initial coulombic efficiency (ICE) of 81.7%. Besides, the multipoint contact between the Si anode and C shell leads to excellent rate performance even with high mass loads.

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