Understanding of Rate-Limiting Behavior and of Diffusion in Commercial LiCoO2 Electrodes Via Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy has been used to investigate rate-limiting conditions in commercial LiCoO2 cathodes. The insights gained may be extended to most lithium-ion battery systems. In general, two possible rate-limiting conditions may exist in a lithium-ion battery - i) diffusion limited, and ii) electron exchange-rate limited. In the former case, rate of diffusion (of ions) is slower than that of redox reactions occurring at active material surface - leading to a negative concentration gradient (of ions) toward the electrode surface for cathodic reactions. In the latter case, a positive concentration gradient persists toward the electrode surface for cathodic reactions.  Insights into electrical equivalent circuits derived via electrochemical impedance spectroscopy have led to novel ways of recognizing the rate-limiting behavior in LiCoO2 half-cell.