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SYNCHROTRON-BASED X-RAY CHARACTERIZATIONS OF HIGH CAPACITY ANODE MATERIALS IN LITHIUM-ION BATTERIES
thesisposted on 04.12.2020, 15:29 by Tianyi LiTianyi Li
Lithium-ion batteries have been widely used in current numerous devices include laptops, smartphones, medical devices, and electric vehicles. The high specific energy with low maintenance and limited self-discharge characteristics of lithium-ion batteries changed the way of communications and transportations during the last few decades. The current commercial anode material for lithium-ion batteries is graphite because of its well-defined layered structure and good cycling stability with a specific capacity of 372 mAh g-1. But the energy density of graphite is limited and will not likely satisfy the increasing market demand on high capacity batteries. Therefore, it is essential to develop and investigate alternative anode materials.
Currently, there is growing interest in using other group IV elements such as silicon (Si), germanium (Ge), tin (Sn) due to their high theoretical capacity. However, the intrinsic volume changes during (de)lithiation processes lead to mechanical pulverization and unstable solid electrolyte interface of the active materials, resulting in delamination and capacity fading issues. To address these problems, fundamental understandings related to the chemical and microstructural evolutions of high capacity anode materials are necessary and beneficial.
This dissertation is mainly focused on the chemical and morphological evolutions of Ge, Sn, and gallium liquid metal-based anode materials by using synchrotron-based characterizations including transmission X-ray microscopy (TXM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and computed nanotomography (NanoCT). The main objective is to investigate the morphological and chemical evolutions of the active materials. The impacts caused by particle size and shape variations and the interactions between different particles on the same electrode has been examined and interpreted by employing tin as the anode. Composites containing a high capacity anode and stress accommodating phase is a promising approach to withstand massive strain in the high capacity anode materials. The effect of selenium doping into germanium anode and the formation of the in-situ formed stress accommodation phase have been studied to explain the good cycling and rate capability of selenium-doped germanium comparing to the germanium anode itself. Pulverization and delamination are a major concern which limits the implementation of high capacity anode materials. Combining a liquid metal anode with a solid electrolyte provides a potential strategy to solve cracking problems due to its self-healing ability at room temperature. Cracks formed due to volume expansion can be recovered by the fluidity of
the liquid metal and the performance of the gallium tin alloy typed anode has been presented. Although the battery system involves a complex structure including electrodes, membrane, electrolyte, etc. The complicated architecture limits the understanding of the material itself. Therefore, it is in need to investigate the active material stand along without interferences caused by other factors. A novel designed single particle schematic has been implemented and interpreted herein which provides insights into the dynamic compositional and morphological characteristics of the selenium-doped germanium anodes under the operational condition that helps understand the outstanding reversibility and cycling performance.