Revealing the Role of Lithium Carbonate at Lithium Metal Anodes Through Study of Gas-Reacted Interphases

A stable, ionically-conductive solid electrolyte interphase (SEI) is vital to lithium (Li) metal anodes, yet key properties of common SEI phases remain unknown. Among these, Li2CO3, central to foundational SEI models, has been difficult to probe given its metastability on Li. To address this, we adopted two approaches: (i) synthesis and study of model Li2CO3-based SEI and (ii) cycling and titration-based analysis of SEI from Li-Cu cells with or without additive CO2, with the aim of modulating Li2CO3 content natively. In (i), reductive instability of Li2CO3 led to co-formation of Li2O and Li2C2 in a multiphasic film with a Li+ conductivity (similar to 8 x 10-9 S cm-1) more than 4x higher than previously-measured Li2O or LiF films. Li2CO3 content in native interphases from (ii) was found to correlate with decreased inactive Li0 accumulation and improved Coulombic efficiency (CE) across diverse electrolytes having moderate CE. In high CE electrolytes, however, capacity losses become dominated by SEI formation rather than inactive Li0, and Li2CO3 enrichment had negligible impact. This work updates understanding of SEI Li2CO3 formed in modern electrolytes, reveals a leading mechanism by which Li2CO3 can boost CE despite its metastability, and indicates the potential and limitations of enriching this phase through electrolyte design.