Failure analysis of thermally abused lithium-ion battery cell by microscopy, electrochemical impedance spectroscopy, and acoustic emission

The lithium-ion battery can be reasonably abused by using it at a higher temperature than the general recommendation of 60 °C depending on the situation, such as in a car in the summer season. In this investigation, the material damage mechanisms of an isothermally exposed lithium-ion battery containing LiPF₆ salt in the liquid electrolyte were investigated after exposure to 60 °C and 85 °C for up to 14 days, respectively. The chemical reaction product on the anode surface and microcrack in the cathode material were microscopically observed when exposed to 85 °C, while no such damage was observed for the battery exposed to 60 °C. These microscopic damages were in-situ monitored by applying electrochemical impedance spectroscopy (EIS) and acoustic emission (AE) technique, respectively. The possible mechanisms of these damages were discussed in relation to the role of the thermally decomposed LiPF₆ salt and resulting products. The highly accelerated capacity degradation after exposure to 85 °C was attributed to the observed microscopic material damages.