Influence of the series resistance on the anomalous diffusion transients of mixed ionic-electronic conductors for energy storage applications

Mixed ionic-electronic conductors are standard components in a wide range of electrochemical energy storage devices, where charge transport is commonly governed by anomalous diffusion processes that deviate from classical Fickian behavior. In this study, we investigate the ubiquituous effect of the series resistance ( R s ), representing the cumulative resistive contributions of the electrolyte, current collectors, contacts, and/or interfacial effects, on the transient responses of electrochemical materials for energy storage. Our theoretical findings show that, although the impedance spectra exhibit an obvious shift with variations in R s , the time-domain analysis in potentiostatic mode reveals a pronounced modification of the current responses, as further corroborated by the admittance analysis. Specifically, it affects directly the initial transient response and the transition between temporal regimes from the fast finite-length Warburg-like diffusion to slow charging governed by capacitive dispersion. Neglecting R s in time-resolved experiments can therefore lead to significant errors in the estimation of diffusion parameters, which is an issue that we explicitly address in this work. The theoretical predictions are validated against experimental measurements performed on three electrochemical devices, based on mixed ionic-electronic conductors. Finally, we show that galvanostatic responses are, in contrast, insensitive to changes in R s . These results demonstrate that the proper consideration of R s is essential for accurately characterizing anomalous diffusion and capacitive processes and interpreting time-domain transients in electrochemical systems for energy storage.