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Diana Zapata Dominguez

Investigation of the cycling mechanisms in silicon and germanium-based Lithium-ion batteries by in-situ and operando X-ray scattering techniques

Published on 18 December 2020
Thesis presented December 18, 2020

Lithium-ion batteries (LiBs) are one of the best solutions for energy storage. Increasing the performance of LiBs demands the use of materials that can host higher quantities of lithium ions (high energy density). Germanium and silicon are promising active anode materials due to their high theoretical capacities (1623 and 3576 mAh/g, respectively) compared to the commercial graphite (372 mAh/g). However, Si and Ge experience significant volume expansion upon the alloying-dealloying reactions with lithium-ions, provoking mechanical deformation. Understanding the mechanisms during cycling is essential to provide information about the degradation processes.
There are different strategies to improve the cyclability and durability of these materials. Using nanostructures is one of them, as it allows mitigating the pulverization and the active compound degradation. Nevertheless, the use of active nanoparticles favors the formation of a solid electrolyte interface layer (SEI), inducing a decrease of the reversible capacity and consequently limiting the cyclability. An alternative approach is to use composite materials in which silicon is mixed with other active or inactive components. However, to date, the silicon amount is limited (less than ~20% of the anode), decreasing the anode capacity.
Ge has received less attention than Si. Although it is less accessible than silicon, it has appealing characteristics besides its high theoretical capacity, such as better electronic conductivity and Li diffusivity than Si. Therefore, mixing Ge with Si is interesting with respect to benchmark graphite to provide an increase in capacity while taking advantage of Ge stability.
This thesis aims at studying the (de)lithiation mechanisms in silicon and germanium-based negative electrodes, focusing on two types of systems: pure Ge, Si, SiGe-alloys nanoparticles, and a commercial-grade silicon-based composite. The structural evolution occurring upon (de)lithiation was probed mainly by operando X-ray scattering techniques, allowing to propose a detailed description of the lithiation mechanisms, as well as Li15(Si100-xGex)4, formation process, which support theoretical predictions on the physical properties of these materials during cycling. Besides, we explored the potentialities of synchrotron X-ray Raman scattering to gain insight into the Solid Electrolyte Interphase (SEI) composition, providing insights into the SEI evolution and its dependence on the state of charge. Our in-depth multi-techniques characterizations bring knowledge to design better Si-based anodes for high-density long-lasting batteries.

Silicon, Germanium, negative electrodes, X-ray scattering techniques, silicon composites, Li-ion battery

On-line thesis.