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.

Automated summary

By observing the concentration-dependent association between CO2 and TFSI anions in water, researchers have developed a new CO2/salt-in-water electrolyte that inherits advantages of water-in-salt electrolytes while avoiding their drawbacks, making high-voltage aqueous Li-ion batteries practical in dilute salt-in-water electrolytes.