An “Ether‐In‐Water” Electrolyte Boosts Stable Interfacial Chemistry for Aqueous Lithium‐Ion Batteries

Abstract Aqueous batteries are promising devices for electrochemical energy storage because of their high ionic conductivity, safety, low cost, and environmental friendliness. However, their voltage output and energy density are limited by the failure to form a solid‐electrolyte interphase (SEI) that can expand the inherently narrow electrochemical window of water (1.23 V) imposed by hydrogen and oxygen evolution. Here, a novel (Li 4 (TEGDME)(H 2 O) 7 ) is proposed as a solvation electrolyte with stable interfacial chemistry. By introducing tetraethylene glycol dimethyl ether (TEGDME) into a concentrated aqueous electrolyte, a new carbonaceous component for both cathode−electrolyte interface and SEI formation is generated. In situ characterizations and ab initio molecular dynamics (AIMD) calculations reveal a bilayer hybrid interface composed of inorganic LiF and organic carbonaceous species reduced from Li + 2 (TFSI − ) and Li + 4 (TEGDME). Consequently, the interfacial films kinetically broaden the electrochemical stability window to 4.2 V, thus realizing a 2.5 V LiMn 2 O 4 −Li 4 Ti 5 O 12 full battery with an excellent energy density of 120 W h kg −1 for 500 cycles. The results provide an in‐depth, mechanistic understanding of a potential design of more effective interphases for next‐generation aqueous lithium‐ion batteries.