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, Li 2 CO 3 , 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 Li 2 CO 3 -based SEI and (ii) cycling and titration-based analysis of SEI from Li–Cu cells with or without additive CO 2 , with the aim of modulating Li 2 CO 3 content natively. In (i), reductive instability of Li 2 CO 3 led to co-formation of Li 2 O and Li 2 C 2 in a multiphasic film with a Li + conductivity (∼8 × 10 −9 S cm −1 ) more than 4x higher than previously-measured Li 2 O or LiF films. Li 2 CO 3 content in native interphases from (ii) was found to correlate with decreased inactive Li 0 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 Li 0 , and Li 2 CO 3 enrichment had negligible impact. This work updates understanding of SEI Li 2 CO 3 formed in modern electrolytes, reveals a leading mechanism by which Li 2 CO 3 can boost CE despite its metastability, and indicates the potential and limitations of enriching this phase through electrolyte design.