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

Abstract Current electrolyte design for Li metal anodes emphasizes fluorination as the 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 can accurately quantify SEI compositional breakdown impedes rigorous scrutiny of other potentially key phases. Here we demonstrate a new quantitative titration approach to reveal Li 2 O 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, Li 2 O correlates most strongly with CE above other phases, reaching highest values when Li 2 O particles order along the SEI-electrolyte interface as revealed by cryo-TEM, demonstrating integrated chemical–structural function. The beneficial role of Li 2 O was exploited to create a first set of entirely fluorine-free electrolytes to breach > 99% CE, highlighting electrolyte/SEI oxygenation as an underexplored and powerful design strategy.