The coupled lithium ion diffusion and stress in battery electrodes

The electrochemical processes in Li-ion batteries are rather complicated, making the insight and guidance from the modeling and simulations essential for the design and optimization of novel electrodes. In this study a continuum model that couples diffusion and mechanics of ion intercalation in electrodes is presented, based on the entropy and mechanical energy associated with the intercalation of ions. The chemical potential difference across the electrode–electrolyte interface is used as the activation energy under the Arrhenius type of kinetics, complemented by governing equation of mechanical equilibrium. The model is implemented into COMSOL finite element platform, and graphite electrodes are used as case studies. It is found that the geometry and topology of the electrodes have important effects on the kinetics as well as capacity of the electrodes, as they have substantial influence on the stress distribution induced by Li-ion intercalation. Higher compressive stress results in lower ion concentration and thus lower capacity, and it also reduces the diffusion rate. As a result, structured electrodes with higher surface area and relaxed stress state tend to have higher capacity and faster rate performance.