Aqueous interphase formed by CO₂ brings electrolytes back to salt-in-water regime

Super-concentrated water-in-salt electrolytes make high-voltage aqueous batteries possible, but at the expense of high cost and several adverse effects, including high viscosity, low conductivity and slow kinetics. Here, we observe a concentration-dependent association between CO₂ and TFSI anions in water that reaches maximum strength at 5 mol kg^-1 LiTFSI. This TFSI-CO₂ complex and its reduction chemistry allow us to decouple the interphasial responsibility of an aqueous electrolyte from its bulk properties, hence making high-voltage aqueous Li-ion batteries practical in dilute salt-in-water electrolytes. The CO₂/salt-in-water electrolyte not only inherits the wide electrochemical stability window and non-flammability from water-in-salt electrolytes but also successfully circumvents the numerous disadvantages induced by excessive salt. This work represents a deviation from the water-in-salt pathway that not only benefits the development of practical aqueous batteries, but also highlights how the complex interactions between electrolyte components can be used to manipulate interphasial chemistry.