Diffusion Mediated Lithiation Stresses in Si Thin Film Electrodes

Stress due to Li insertion has been considered as a major driving force for the mechanical degradation of high capacity electrode materials in lithium ion batteries. The stress level and distribution in active materials are closely related to lithium ion diffusion and the resulting concentration gradients. In this study, we report in situ stress measurements on amorphous Si thin films to investigate Li diffusion and the mechanical response during electrochemical cycling. This configuration was used as a model system to understand how Li diffusion can impact the cyclic performance. The effect of diffusion limitations were studied by systematically varying the C-rate from C/20 to 5C. Experimentally obtained results were then compared to a finite strain model of stress, diffusion, plastic flow, and electrochemical reactions in a lithium half-cell. The calculated results indicate that slow Li diffusion in silicon leads to stress gradients and that high C rates can lead to local stresses and film cracking that are consistent with experimental observations.