Modeling the Mesoscopic Influence of SEI Mechanical Response to Electrode Swelling

Abstract

This work focuses on investigating the role that mesoscopic configuration plays in dictating the solid-electrolyte interface (SEI) mechanical durability on lithiation in silicon (Si) anodes. Due to the large expansion of Si upon lithiation, SEI formation and cohesion at the electrolyte interface of Si based anodes is a principle concern in performance of the overall cell, and enabling a more durable SEI would lead to the ability to employ higher Si percentages in anodes, increasing capacity. However, we are still building fundamental understanding of SEI formation and the resultant meso- and microscopic structure evolved due to this formation. This work seeks not to understand the formation itself, but instead to establish intuition about what type of resultant structure we should target as we learn to control the SEI formation process, by investigating the effect that non-homogenous SEI configurations have upon the overall SEI response when subjected to large strain due to the swelling of the underlying Si anode. Utilizing the material point method (MPM), a continuum mechanics approach akin to the finite element method, but particularly well suited for investigation of materials subjected to large deformation, we model the intrinsic swelling of Silicon under lithiation when coupled to a model SEI. This model SEI, in turn, is constructed of multiple different phases with mesoscopic variability and realistic material response parameterized from atomistic simulation of the mesoscopic phases including both crystalline and glassy inner SEI compounds and ordered/disordered outer SEI compounds derived from liquid carbonate decomposition products. Model mesostructures are constructed and fed into the MPM framework, from which an overall SEI mechanical response may be extracted via numerical simulation. By varying the mesostructures, sensitivity of the mechanical response to the mesostructural features is quantified, providing guidance to possible experimental approaches at improving the SEI capability of protecting the anode during multiple charge/discharge cycles.