Solid electrolyte interphase in water-in-salt electrolytes

The water-in-salt strategy successfully expands the electrochemical window of the aqueous electrolyte from 1.23 to ∼3.0 V, which can lead to a breakthrough in the energy output of the aqueous battery system while maintaining the advantage of high safety. The expanded electrochemical window of the water-in-salt electrolytes can be ascribed to the decreased water activity and the solid electrolyte interphase formed on the anode. The solid electrolyte interphase in the aqueous system is not fully understood, and the basic composition, the structure, and the formation mechanism are still cloaked in mystery. This perspective summarizes the published research with emphasis on the most possible formation mechanism and composition of the interphase layer in the aqueous system. Further understanding of the interphase as well as rounded assessment of the water-in-salt electrolyte in practical operating conditions is encouraged. The full understanding of the interface will guide the design of aqueous electrolytes and help to build novel aqueous batteries with high safety and high energy density.