UCLA

As a key passivation layer that governs battery operation, the solid electrolyte interphase (SEI) has long been credited for enabling high performing batteries or blamed for their eventual death. However, qualitative descriptions of the SEI often found in the literature (e.g., “conductive”, “passivating”) highlight our incomplete understanding, where even the most basic properties foundational to SEI function (e.g., ionic and electronic conductivity) remain difficult to determine. Here, we propose a methodology to quantify SEI conductivities using a separatorfree Cu|SEI|Li architecture that treats the SEI as a solid-state electrolyte. Although the non-ideal experimental setup leads to inconsistent and widespread SEI conductivity measurement, several improvement and failure analysis are discussed in this work. Perhaps most striking, we find that reversible Li metal deposition is possible in our separator-less Cu|SEI|Li cell with additional liquid electrolyte, demonstrating that the SEI might be able to function like a solid-state electrolyte. However, the excess liquid electrolyte in the Cu|SEI|Li cell makes the argument of using SEI as solid state electrolyte somewhat questionable since SEI solely function as a passivated film and the lithium ions only come from the additional liquid electrolyte. Indeed, using SEI as solid state electrolyte remain challenging, but several future improvements are also discussed in this work. Our work provides quantitative measurement for how “conductive” and “passivating” SEI films and further enrich our understanding of the SEI, not just as a passivation layer to enable battery operation, but as a functional structure that can potentially have important implications for solid-state batteries.