Insight on lithium metal anode interphasial chemistry: Reduction mechanism of cyclic ether solvent and SEI film formation

While the solid-electrolyte-interphase (SEI) originating from carbonate-based electrolytes has been extensively studied due to the success of Li-ion batteries, significantly less is known about the SEI formed in ether-based electrolytes, which have become increasingly important for many “beyond-Li ion” batteries, including lithium-sulfur and other lithium metal battery systems. Li dendrite growth and poor cycling efficiencies related to high rate and/or high capacity cycling of lithium are two of the primary factors limiting practical application of Li metal anodes. Similar to graphite in Li-ion batteries, these behaviors are inextricably linked to the mechanism for SEI formation, the resulting interphase chemistry, and the film stability during cycling—all of which require further understanding. Employing both computational and experimental means in this effort, we investigated the reduction chemistry of dimethoxyethane (DME) and 1,3-dioxolane (DOL) on the surface of metallic lithium. We determined that ether-based SEIs are layer-structured, with an outer organic/polymeric layer consisting of lithium oligoethoxides with C-C-O or O-C-O linkages and an inner layer of simple inorganic oxides (Li₂O). Remarkably, although Li⁺ is preferentially solvated by DME, it is the cyclic DOL that primarily contributes to the interphase chemistry. This selective electrochemical reduction of ether solvents is corroborated by precise calculation of transition state structures and energies, providing a valuable guide for future design and manipulation of Li anode interphasial chemistries.