Silicon (Si) has been considered as a promising anode material for lithium-ion batteries due to its high theoretical capacity (3750 mAh/g), low discharge voltage, abundancy, and low cost. However, electrochemical lithiation and delithiation of Si proceeds via solid-state amorphization and massive volume expansion/contraction, resulting in destructive consequences such as slow rate performance, irreversible capacity loss, and mechanical degradation. These problems significantly affect the capacity retention and cycle life and limits the wide application of Si anode. In this thesis, molecular dynamic (MD) simulations with reactive force field (ReaxFF) were performed to better understand and design optimized Si anodes with enhanced rate performance and minimized irreversible capacity loss. Furthermore, the transferability of ReaxFF to simulate SiO system was evaluate and the ground work was laid to design extensive training set of Li-Si system for machine-learning potentials development. There are two major discoveries based on the simulation work. First, to elucidate the rate-limiting factor upon lithiation of Si for improved rate performance, reactive MD simulations were performed in crystalline-Si and amorphous-Si at the atomic-scale. It was discovered that Si movement is the rate-limiting factor. It was also revealed that Li diffusivity increases with Li concentrations, opposite to many currently used intercalation compounds. Furthermore, the new finding highlighted that vacancies in Si can accelerate the lithiation process dramatically.Then, the irreversible atomic-scale structural changes upon delithiation was studied using a newly-developed reactive MD-based delithiation algorithm with well-controlled chemical potential gradient driving force and delithiation rate. During fast delithiation, a cage-like structure with high Si content was formed near the surface, thus trapping significant amount of Li atoms inside the Si-thin-film. Furthermore, delithiated amorphous LixSi (with no porosity and trapped Li) still has higher volume (lower density) than the equilibrium structures at the same Li concentration throughout the whole delithiation process regardless of the delithiation rates. The origin of the excess volume is the loss of directly bonded Si-Si pairs, which makes the subsequent re-lithiation proceed faster. These new insights lead to several recommendations, such as the delithiation rate and depth of charge, to avoid trapped Li and coating delamination in order to enhance the life of Si electrodes.
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