High lithium oxide prevalence in the lithium solid–electrolyte interphase for high Coulombic efficiency

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.