Lithium‐Ion Transfer at Cathode‐Electrolyte Interface in Diluted Electrolytes Using Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) exhibits a superior performance in separating elementary processes based on different time constants of wide frequency scans without the destruction of electronic devices. Therefore, it was adapted for the evaluation of the interface between the electrode and the electrolyte in lithium‐ion batteries (LIBs). Using the cathode half‐cell of the LIBs, systematic EIS analyses were conducted for various electrolyte concentrations and rest periods at precisely controlled states of charge. Remarkable hysteresis of the charge transfer resistance between the charge and discharge processes was presented in the diluted electrolyte solutions; furthermore, the hysteresis increased with longer rest periods and remained even after 10 h. It is very likely that the hysteresis was caused by the evolution of the cathode–electrolyte interface, which would depend on the relative ratio of solvated species to free solvents based on the concentration of the electrolyte.