Robust interphase on both anode and cathode enables stable aqueous lithium-ion battery with coulombic efficiency exceeding 99%

Recent breakthroughs in “water-in-salt” electrolyte (WiSE) made highly-safe aqueous Li-ion batteries (LIBs) possible. However, long-term cycling stability of the WiSE-based LIBs is limited by the Coulombic efficiency (CE) of only 97–98% at low current density. Achieving a CE closer to unity (1.0) is thus highly desirable and also a critical challenge due to the unfavorable splitting of water at the electrolyte/electrode interfaces. Here we report a urea-containing-WiSE that enables the aqueous LIBs with a high CE of 99.1% at low rate of 0.15 C, higher than typical WiSE-based LIBs operated at similar low current density. Robust water-resistant interphases with organic-inorganic hybrid amorphous microstructure were grown on both anode and cathode, which are responsible for the high CE in low rate. Such advanced interfacial chemistry with functional additives may open the way for designing cheaper but more durable aqueous batteries.