Suppressing Crystallization of Water-in-Salt Electrolytes by Asymmetric Anions Enables Low-Temperature Operation of High-Voltage Aqueous Batteries

The discovery of enhanced electrochemical stability for aqueous electrolytes with very high salt concentration has stimulated the development of high-voltage aqueous batteries. We show that a key factor limiting the applicability of these batteries is the tendency of highly concentrated electrolytes to crystallize near room temperature, leading to cell failure. Here we report the use of asymmetric anions as solution to suppress the crystallization of highly concentrated aqueous electrolytes. We demonstrate this approach with a ternary sodium-ion electrolyte that we employ in a 2 V class aqueous sodium-ion battery based on a NaTi2(PO4)3 anode and a Na3(VOPO4)2F cathode. This cell displays excellent cycling stability at 30 °C with capacity retention of 85% after 100 cycles at C/5 and 77% after 500 cycles at 1C. The cell reaches a specific energy of 64 Wh kg–1 on the basis of the active masses of both electrodes, twice as high as previously reported for this electrode couple. Further, the cell can be operated down to temperatures of at least −10 °C, with capacity retention of 74% after 500 cycles at C/5.