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

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. 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.