Diffusion-deformation theory for amorphous silicon anodes: The role of plastic deformation on electrochemical performance

Amorphous silicon (a-Si) is a promising material for anodes in Li-ion batteries due to its increased capacity relative to the current generation of graphite-based anode materials. However, the intercalation of lithium into a-Si induces very large elastic-plastic deformations, including volume changes of approximately 300%. We have formulated and numerically implemented a fully-coupled diffusion-deformation theory, which accounts for transient diffusion of lithium and accompanying large elastic-plastic deformations. The material parameters in the theory have been calibrated to experiments of galvanostatic cycling of a half-cell composed of an a-Si thin-film anode deposited on a quartz substrate, which have been reported in the literature. We show that our calibrated theory satisfactorily reproduces the mechanical response of such an anode - as measured by the changes in curvature of the substrate, as well as the electrochemical response - as measured by the voltage versus state-of-charge (SOC) response. We have applied our numerical simulation capability to model galvanostatic charging of hollow a-Si nanotubes whose exterior walls have been oxidized to prevent outward expansion; such anodes have been recently experimentally-realized in the literature. We show that the results from our numerical simulations are in good agreement with the experimentally-measured voltage versus SOC behavior at various charging rates (C-rates). Through our simulations, we have identified two major effects of plasticity on the electrochemical performance of a-Si anodes: First, for a given voltage cut-off, plasticity enables lithiation of the anode to a higher SOC. This is because plastic flow reduces the stresses generated in the material, and thus reduces the potential required to lithiate the material. Second, plastic deformation accounts for a significant percentage of the energy dissipated during the cycling of the anode at low C-rates. Hence, plasticity can have either (a) a beneficial effect, that is, a higher SOC for a given voltage cut-off; or (b) a detrimental effect, that is significant energy dissipation at low C-rates. (C) 2015 Elsevier Ltd. All rights reserved.