Different Efforts but Similar Insights in Battery R&D: Electrochemical Impedance Spectroscopy <i>vs</i> Galvanostatic (Constant Current) Technique

Electrochemical impedance spectroscopy (EIS) using alternating currents is a widely established technique to investigate kinetic aspects of batteries and their components, though it requires an interruption of battery operation with extra measurement time and effort. In this work, EIS is compared with the conventional galvanostatic (constant current) technique, which is based on direct currents, being the standard operation mode of batteries. Data from constant current measurements not only are representing application conditions but also are automatically and continuously generated during routine charge/discharge processes, i.e., without extra measurement efforts, and do give kinetic insights via the characteristic overvoltage (= resistance-reasoned voltage rise/decrease), as well. In fact, distinguishing between even very similar values for ohmic (RΩ), charge transfer (Rct), and mass transport (Rmt) resistances can be done via analysis of overvoltage data from constant current measurements, as exemplarily demonstrated in symmetric Li||Li and LiNi0.6Mn0.2Co0.2O2 (NMC622)||Li cells with poly(ethylene oxide)-based solid polymer electrolyte, finally proving their validity. From a practical point of view, direct-current methods can be beneficial for R&D of kinetic aspects in batteries, as data is directly obtained and, thus, application-oriented