Performing accurate, linear and stable Electrochemical Impedance Spectroscopy of Li/SOCl 2 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/SOCl 2 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.

Impedance spectra obtained by fast Fourier transformation of the response to a multi-sine potential perturbation are shown to be consistent with the Kramers–Kronig relations, even for systems that are nonlinear and nonstationary. These results, observed for measurements on a Li/SOCl2 battery, were confirmed by numerical simulations. Consistency with the Kramers–Kronig relations was confirmed by use of the measurement model developed by Agrawal et al. and by a linear measurement model approach developed by Boukamp and implemented by Gamry. The present work demonstrates that application of the Kramers–Kronig relations to the results of multi-sine measurements cannot be used to determine whether the experimental system satisfies the conditions of linearity, causality and stability.

Non-linear EIS analysis is gaining wider attention and interest due to the deeper understanding that is provided especially when combined with linear EIS. The nonlinear part of the data can get corrupted due to a number of effects. One of these is the initial transient, which is the response right after excitation signal is applied before a steady-state is reached. In this study, we demonstrate this phenomenon through analyzing simplified Randles’ cells via Kirchoff’s laws. To get rid of the effects of the initial transient, instrument manufacturers typically discard some fraction of the response, the effectiveness of which, as demonstrated here has to be checked.

One of the major hurdles in the utilization of metallic lithium anodes is understanding the Li+ transfer kinetics through the solid electrolyte interface (SEI) in addition to Li oxidation. Electrochemical impedance spectroscopy (EIS) combined with temperature variation provides deeper comprehension and reveals kinetic parameters of individual processes separately. In this study, we report temperature-dependent EIS analysis of metallic Li anodes to shed light on the kinetics of anodic/interfacial processes at different states of charge and wide temperature ranges (−25 to 75 °C), utilizing lithium thionyl chloride (Li/SOCl2) and lithium manganese dioxide (Li/MnO2) primary batteries as model systems. We found in both batteries that the impedance of the SEI processes is highly temperature-dependent with non-Arrhenius behavior at temperatures greater than 35 °C. Conversely, the kinetics of the anodic process showed small temperature dependence that is explained by the Arrhenius equation throughout the temperature range studied. The results provide a deeper understanding of the underlying processes separately in metallic Li anodes under operando and real-time conditions.

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