High‐Voltage Aqueous Na‐Ion Battery Enabled by Inert‐Cation‐Assisted Water‐in‐Salt Electrolyte

Abstract Water‐in‐salt (WiS) electrolytes provide a new pathway to widen the electrochemical window of aqueous electrolytes. However, their formulation strongly depends on the solubility of the chosen salts, imposing a stringent restriction on the number of possible WiS systems. This issue becomes more severe for aqueous Na‐ion batteries (ANIBs) owing to the relatively lower solubility of sodium salts compared to its alkaline cousins (Li, K, and Cs). A new class of the inert‐cation‐assisted WiS (IC‐WiS) electrolytes containing the tetraethylammonium (TEA + ) inert cation is reported. The Na IC‐WiS electrolyte at a superhigh concentration of 31 mol kg –1 exhibits a wide electrochemical window of 3.3 V, suppresses transition metal dissolution from the cathode, and ensures singular intercalation of Na into both cathode and anode electrodes during cycling, which is often problematic in mixed alkali cation systems such as K–Na and Li–Na. Owing to these unique advantages of the IC‐WiS electrolyte, the NaTiOPO 4 anode and Prussian blue analog Na 1.88 Mn[Fe(CN) 6 ] 0.97 ·1.35H 2 O cathode can be coupled to construct a full ANIB, delivering an average voltage of 1.74 V and a high energy density of 71 Wh kg −1 with a capacity retention of 90% after 200 cycles at 0.25C and of 76% over 800 cycles at 1C.