Moving beyond 99.9% Coulombic efficiency for lithium anodes in liquid electrolytes

Irreversibility is one of the biggest obstacles in realizing practical Li-metal batteries. This Review surveys the trends in Coulombic efficiency (CE)-a reversibility indicator-reported for Li-metal batteries over the last five decades, abstracts key CE descriptors and analyses promising strategies to improve CE. As Li-ion battery costs decrease, energy density and thus driving range remains a roadblock for mass-market vehicle electrification. While Li-metal anodes help achieve Department of Energy targets of 500 Wh kg(-1) (750 Wh l(-1)), Li Coulombic efficiencies fall below the 99.95+% required for 1,000+ cycles. Here we examine historical electrolyte developments underlying increased Coulombic efficiency and discuss emerging frameworks that support rational strategies to move beyond 99.9%. While multiple electrolytes reach 98-99% Coulombic efficiency over subsets of cycles, achieving >99.9% Coulombic efficiency consistently throughout cycling is an as yet unmet challenge. We analyse important interplays between electrolyte, solid electrolyte interphase composition, plating-stripping kinetics and Li morphology, many of which are only recently being quantified experimentally at the Li interface, and which collectively determine Coulombic efficiency. We also discuss forward-looking strategies that, if mastered, represent new opportunities to refine understanding and support new record values of Coulombic efficiency in the coming years.