Linear and nonlinear electrochemical impedance spectroscopy studies of Li/SOCl2 batteries

Performing accurate, linear and stable Electrochemical Impedance Spectroscopy of Li/SOCl2 based batteries (and more generally, all primary batteries) is challenging due to the lack of a (well-defined) charging reaction. The difficulty stems from irreversible operation chemistry (compromising stable operation) and the inconsistent anode passivation of the cell. The very scarce literature examples lack proper measurement protocol and accurate EIS data for Li/SOCl2 that can be modeled. In this study, we demonstrate how these challenges can be overcome by performing Galvanostatic-EIS in discharge mode and investigate the details of how experimental parameters influence not only the measurement, but also the cell itself. We present linear and stable data that is compatible with the Kramers-Kronig relations in frequency ranges as wide as 10 kHz to 1 mHz for fully charged to fully discharged cells. Moreover, we utilize Harmonic Analysis to study the nonlinearities in the measurement and further show that the passivation of the anode is a major cause for the nonlinearities.