Current electrolyte design for Li metal anodes emphasizes fluorination as the pre-eminent guiding principle for high Coulombic efficiency (CE) based largely on perceived benefits of LiF in the solid–electrolyte interphase (SEI). However, the lack of experimental techniques that accurately quantify SEI compositional breakdown impedes rigorous scrutiny of other potentially key phases. Here we demonstrate a quantitative titration approach to reveal Li₂O content in cycled Li anodes, enabling this previously titration-silent phase to be compared statistically with a wide range of other leading SEI constituents including LiF. Across diverse electrolytes, CE correlates most strongly with Li₂O above other phases, reaching its highest values when Li₂O particles order along the SEI, demonstrating integrated chemical–structural function. The beneficial role of Li₂O was exploited to create entirely fluorine-free electrolytes that breach >99% CE, highlighting electrolyte/SEI oxygenation as an underexplored design strategy.

当前锂金属负极的电解质设计强调氟化作为高库仑效率 (CE) 的首要指导原则，这在很大程度上基于 LiF 在固体电解质界面 (SEI) 中的优势。然而，缺乏准确量化 SEI 成分分解的实验技术阻碍了对其他潜在关键相的严格审查。在这里，我们展示了一种定量滴定方法来揭示循环锂阳极中的 Li ₂ O 含量，使这种先前滴定沉默的相能够与包括 LiF 在内的各种其他主要 SEI 成分进行统计比较。在不同的电解质中，CE 与 Li ₂ O的相关性高于其他相，当 Li ₂ O 颗粒沿着 SEI 排列时达到最高值，展示了集成的化学结构功能。利用 Li ₂ O的有益作用来制造完全无氟的电解质，其 CE 值超过 99%，这凸显了电解质/SEI 氧化是一种尚未充分探索的设计策略。