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Doctor of Philosophy (PhD)
Chemical and Materials Engineering
Dr. Yang-Tse Cheng
Replacing organic liquid electrolytes with nonflammable solid electrolytes can improve safety, offer higher volumetric and gravimetric energy densities, and lower the cost of lithium-ion batteries. However, today’s all-solid-state batteries suffer from low Li-ion conductivity in the electrolyte, slow Li-ion transport across the electrolyte/electrode interface, and slow solid-state Li-ion diffusion within the electrode. Defect chemistry is critical to understanding ionic conductivity and predicting the charge transport through heterogeneous solid interfaces. The goal of this dissertation is to analyze and improve solid state materials for energy storage applications by understanding their defect structure and transport properties.
I have investigated defect chemistry of cubic Li7La3Zr2O12 (c-LLZO), one of the most promising candidate solid electrolytes for all-solid-state lithium batteries. By combining conductivity measurements with defect modeling, I constructed a defect diagram of c-LLZO featuring the intrinsic formation of lithium vacancy-hole pairs. The findings provided insights into tailoring single-phase mixed lithium-ion/electron conducting materials for emerging ionic devices, i.e., composite cathodes requiring both fast electronic and ionic paths in solid-state batteries. I suggested that oxygen vacancies could increase the Li-ion conductivity by reducing the amount of electron holes bound with lithium vacancies.
Using a simpler but also attractive solid electrolyte Li2ZrO3 (LZO) as an example, I significantly improved Li-ion conductivity by creating extra oxygen vacancies via cation doping. In particular, Fe-doped LZO shows the highest Li-ion conductivity reported for the family of LZO compounds, reaching 3.3 mS/cm at 300 °C. This study brought attentions to the long-neglected oxygen vacancy defects in lithium-ion conductors and revealed their critical role in promoting Li-ion transport. More importantly, it established a novel defect engineering strategy for designing Li-oxide based solid electrolytes for all-solid-state batteries.
I surface-modified LiNi0.6Co0.2Mn0.2O2 cathode material with a LZO coating prepared under dry air and oxygen, and systematically investigated the effect of coating atmosphere on their transport properties and electrochemical behaviors. The LZO coating prepared in oxygen is largely amorphous. It not only provided surface protection against the electrolyte corrosion but also enabled faster lithium-ion transport. Additionally, oxygen atmosphere facilitated Zr diffusion from the surface coating to the bulk of LiNi0.6Co0.2Mn0.2O2, which stabilized the crystal structure and enhanced lithium ion diffusion. Consequently, LiNi0.6Co0.2Mn0.2O2 cathodes coated with Li2ZrO3 in oxygen achieved a significant improvement in high-voltage cycling stability and high-rate performance.
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This research was supported by US National Science Foundation Award 1355438 (Powering the Kentucky Bioeconomy for a Sustainable Future) and the Department of Chemical and Materials Engineering at the University of Kentucky.
Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
Zhan, Xiaowen, "DEFECT CHEMISTRY AND TRANSPORT PROPERTIES OF SOLID STATE MATERIALS FOR ENERGY STORAGE APPLICATIONS" (2018). Theses and Dissertations--Chemical and Materials Engineering. 88.