Impact of electrochemical and mechanical interactions on lithium-ion battery performance investigated by operando dilatometry

The demand for higher energy densities in lithium-ion batteries leads to an increased utilization of the space within the confinements of the cell housing for the electrodes, resulting in increased electrochemical/mechanical interactions and stress inside the cells. In this study, the correlating effects of externally-induced mechanical stress and dilation of the electrodes on the performance of LIBs were investigated using an operando three-electrode dilation cell. The results demonstrate that most of the initial irreversible dilation in a graphite/LiNi0.33Co0.33Mn0.33O2 (NCM111) cell occurs during the initial lithium intercalation at the anode during the transition from stage 2 to 1, i.e. LiC12 to LiC6, due to SEI formation, particle rearrangement and graphene layer spacing. Moreover, high applied pressure, which leads to a reduction of the porosity inside the separator and therefore increased ionic transport resistance, tortuosity and overpotential, results in a hastened degradation mainly at graphite anode. These effects are more pronounced with an inhomogeneous pressure distribution and lithium plating is identified as the main cause of degradation. This dilation cell is a powerful tool for studying the electrode/active material expansion and electrochemical/mechanical effects for a homogeneous and inhomogeneous pressure distribution.