Physical Interpretations of Electrochemical Impedance Spectroscopy of Redox Active Electrodes for Electrical Energy Storage

This study aims to provide physical interpretations of electrochemical impedance spectroscopy (EIS) measurements for redox active electrodes in a three-electrode configuration. To do so, a physicochemical transport model was used accounting for (i) reversible redox reactions at the electrode/electrolyte interface, (ii) charge transport in the electrode, (iii) ion intercalation into the pseudocapacitive electrode, (iv) electric double layer formation, and (v) ion electrodiffusion in binary and symmetric electrolytes. Typical Nyquist plots generated by EIS of redox active electrodes were reproduced numerically for a wide range of electrode electrical conductivity, electrolyte thickness, redox reaction rate constant, and bias potential. The electrode, bulk electrolyte, charge transfer, and mass transfer resistances could be unequivocally identified from the Nyquist plots. The electrode and bulk electrolyte resistances were independent of the bias potential, while the sum of the charge and mass transfer resistances increased with increasing bias potential. Finally, these results and interpretation were confirmed experimentally for LiNi_0.6Co_0.2Mn_0.2O₂ and MoS₂ electrodes in organic electrolytes.