Modeling crack growth during Li extraction and insertion within the second half cycle

During operation of a lithium ion cell electrode storage particles experience an inhomogeneous volume change due to local differences in the internal lithium concentration. The resulting mechanical stress can become large enough to provoke particle fracture, an aging mechanism considered to have a severe detrimental impact on the life time of lithium ion cells. In this work, we use a coupled model of mechanical stress, lithium diffusion and crack growth to study the problem of fracture in storage particles. The model was successfully applied to study crack growth during a single half cycle of lithium insertion or extraction in earlier investigations. It was demonstrated that, under specific circumstances, particle breakage may occur in a single half cycle. Here, we consider the second half cycle and examine under which conditions cracks that either remained stable or underwent growth in the first half cycle, can lead to particle fragmentation during the subsequent half cycle. From both numerical results and supportive analytic solutions, we find that growth of a through crack during Li insertion is a strong indicator for particle breakage, either in one or two half cycles. Such a relationship is not found for growth of a surface crack during Li extraction.