State of Charge Dependency of Graphitized-Carbon-Based Reactions in a Lithium-ion Secondary Cell Studied by Electrochemical Impedance Spectroscopy

Li-ion insertion/deinsertion reactions of the anode of a Li-ion secondary cell incorporating a reference electrode were analyzed by electrochemical impedance spectroscopy (EIS) at different temperatures and state of charges (SOCs). The cell uses graphitized carbon as anode electroactive material. Impedance spectra fittings were carried out using an equivalent circuit, so that the reaction kinetics could be evaluated accurately. The dependencies of the Li-ion conduction (Rsl1), Li-ion solvation/desolvation (Rsl2) and charge transfer (Rct) reactions on the SOC and anode potential were evaluated. The results were compared with a dQ/dE vs. E curve of graphite to analyze how the structural changes of graphite affect the Li-ion insertion process. The charge transfer process was found to be dependent on the SOC and anode potential. On the other hand, Li-ion conduction and Li-ion solvation/desolvation processes did not depend on the SOC and anode potential.