Analysis of Cathode Reactions of Lithium Ion Cells Using Dynamic Electrochemical Impedance

We used dynamic electrochemical impedance spectroscopy (DEIS), a method that superimposes an alternating current on a direct current during battery charging and discharging, to analyze a cathode half-cell of lithium ion batteries. Based on the analysis using instantaneous impedances derived from time-interpolation of DEIS, a comparison between the instantaneous impedances and stationary impedances that are generally measured at the equilibrium of a specific state of charge (SOC), when no direct current is flowing (SEIS), have revealed that the charge transfer resistances (Rct) obtained by DEIS during charge (extraction of lithium in a cathode) always have lower values than Rct by DEIS during discharge (insertion of lithium), while the results obtained from SEIS are almost the same during charging and discharging. Moreover, the variations of Rct vs SOC during charge depend on C-rate, whereas those during discharge are independent of the C-rate. These variations of Rct observed in charging and discharging during DEIS are attributed to the differences of rate constants for charge transfer. The underlying cause of the series of variations of Rct on DEIS can be explained using the equation of Frumkin-Butler-Volmer with Rct, which was derived previously from cyclic voltammetry as a well-known dynamic electrochemical method.