A theory for coupled lithium insertion and viscoplastic flow in amorphous anode materials for Li-ion batteries

Abstract

Amorphous lithium metal alloys (LixM, with M=Si, Ge, Sn, …) are attractive anode materials for lithium-ion batteries owing to their high energy-storage capacity and safety characteristics. However, repeated insertion of lithium often leads to chemo-mechanical degradation of the alloy, which can severely reduce the battery capacity and cycle life. Better understanding of the chemo-mechanical response of lithium alloys is needed to guide the design of damage-resistant anode microstructures. In this work, we propose a constitutive theory that couples large, viscoplastic deformations to the insertion and extraction of lithium in amorphous electrode materials. The theory relies on the concept of Shear Transformation Zone as carrier of plastic flow in the amorphous material, and accounts for microstructural evolution via an internal “free volume” variable. The model is used to gain insight into several features of the plasticity of amorphous alloys during lithiation, including rate-dependency, pressure-dependency, and structural evolution. Model predictions are also compared to experimental data for amorphous silicon.