Shape Influence of Active Material Micro-Structure on Diffusion and Contact Stress in Lithium-Ion Batteries

Electrochemical-mechanical modelling is a key issue to estimate the damage of active material, as direct measurements cannot be performed due to the particles nanoscale. The aim of this paper is to overcome the common assumptions of spherical and standalone particle, proposing a general approach that considers a parametrized particle shape and studying its influence on the mechanical stresses which arise in active material particles during battery operation. The shape considered is a set of ellipsoids with variable aspect ratio (elongation), which aims to approximate real active material particles. Active material particle is divided in two domains: non-contact domain and contact domain, whether contact with neighbouring particles affects stress distribution or not. Non-contact areas are affected by diffusion stress, caused by lithium concentration gradient inside particles. Contact areas are affected simultaneously by diffusion stress and contact stress, caused by contact with neighbouring particles as a result of particle expansion due to lithium insertion. A finite element model is developed in Ansys™APDL to perform the multi-physics computation in non-spherical domain. The finite element model is validated in the spherical case by analytical models of diffusion and contact available for simple geometry. Then, the shape factor is derived to describe how particle shape affects mechanical stress in non-contact and contact domains.