Electrochemical impedance spectroscopy analysis with a symmetric cell for LiCoO₂ cathode degradation correlated with Co dissolution

Static degradation of LiCoO₂ cathodes is a problem that hinders accurate analysis using our developed separable symmetric cell. Therefore, in this study we investigate the static degradation of LiCoO₂ cathodes in separable symmetric cells by electrochemical impedance spectroscopy (EIS) and inductively coupled plasma analyses. EIS measurements of LiCoO₂ cathodes are conducted in various electrolytes, with different anions and with or without HF and/or Hli O. This allows us to determine the static degradation of LiCoO₂ cathodes relative to their increase of charge transfer resistance. The increase of the charge transfer resistance of the LiCoO₂ cathodes is attributed to cobalt dissolution from the active material of LiCoO₂. Cobalt dissolution from LiCoO₂ is revealed to occur even at low potential in the presence of HF, which is generated from LiPF₆ and H₂O. The results indicate that avoidance of HF generation is important for the analysis of lithium-ion battery electrodes by using the separable cell. These findings reveal the condition to achieve accurate analysis by EIS using the separable cell.