Updated electrochemical impedance model for understanding the interface of metallic lithium

High energy density rechargeable lithium metal battery technology is expected to dominate the next generation of rechargeable batteries, if the safety issues will be rectified. Its rise is predicted due to the demand for higher energy density due to electrification of vehicles and ever more powerful mobile electronic devices. As the achievable energy density by optimizing conventional lithium ion batteries, based on graphite negative and transition metal positive electrodes, is reaching its limits, new solutions are needed. As none of truly new chemistries is emerging to enable significant increase of energy density of the battery, the scientist turned recently back to metallic lithium, which has been discarded after discovery of lithium ion battery based on graphite intercalation, due to the safety concerns. Metallic lithium has highly negative potential (−3.04 V vs. SHE) as well as high specific capacity (3860 mAh g⁻¹), both of which can potentially double the energy density of already-existing Li-ion batteries.

Progress in implementing metallic lithium technology is impeded by safety issues related to dendrite growth; limited cycle-life due to the high reactivity of metallic lithium toward the conventional organic liquid electrolytes, where the generated unstable solid electrolyte interphase (SEI) and dead lithium formed during cycling lead to continuous and irreversible lithium  consumption;  and large volume changes during lithium plating and stripping. All three aspects are related to the metallic lithium interface and therefore electrochemical impedance spectroscopy (EIS) is an important tool to obtain physical and chemical information. However, it is complex technique, requiring data post-treatment by applying fitting models in order to extract numerical values of different parameters, which allow comparison and correlation between different samples.

Most of the EIS studies, reported in literature, have been performed using thick lithium foil, and therefore, severely simplified equivalent Randles circuit has been used. However, the thin lithium of 50 μm in thickness has shown two semicircles, instead of one, as is usual in EIS spectra of thick (>500 μm) lithium. Therefore, we have updated the EIS model to enable studying more complex metallic lithium impedance spectra and taking into consideration both the lithium ion migration and the charge-transfer processes that occur at the Li interface, but still allowing to use standard software, coming with commercial potentiostats, instead of using much more complex transmission-line model, which requires dedicated scripts. Further investigation identified that all three commercially-available lithium foils give distinctly different EIS spectra, even if the thickness of two (of the three) was the same. Based on this, we have designed the experiments and proved that the resulting differences in EIS spectra are determined by native lithium passivation layer, which most likely comes either from the lithium foil production specificities or from lithium foil storage after production. A three-electrode set-up allowed us also to separate contributions of both working and counter electrodes during rest and cycling. As a result, a direct correlation of the observed overpotential during cycling of in symmetric cells and the corresponding impedance was demonstrated, clarifying working and counter electrode contributions to the overall cell impedance spectra.

Surprisingly, we have found that the impedance, generated at each side of the cell during cycling of symmetric cells, is not identical and should not be treated as such. We confirmed the continuous localized self-repairing re-passivation process, taking place after exposing freshly deposited lithium to the electrolyte, and demonstrated that it is time and electrolyte dependent. In conclusion, this work demonstrates that EIS can be used as a powerful technique, providing crucial information regarding the interfacial reactivity of the lithium metal anode at rest and during cycling. The proposed updated model has been validated in various conditions and is shown to be reliable for fitting EIS spectra of lithium metal anodes, regardless of the type of lithium used.