Aqueous sodium-ion batteries promise increased operational safety and lower cost compared to current state-of-the-art lithium-ion batteries based on organic electrolytes. For large-scale stationary systems, which find increasing application in the grid integration of electricity generated from intermittent renewable sources, these advantages of aqueous electrolyte batteries could translate into lower total-cost-of-ownership compared to organic electrolyte batteries. The major disadvantage of water as electrolyte solvent for batteries is its intrinsically narrow electrochemical stability window (thermodynamically only 1.23 V) limiting maximum cell voltage and consequently the battery’s energy density. We recently discovered an aqueous sodium-ion electrolyte system with a much enhanced electrochemical stability window. The wide stability window of 2.6 V for 35m aqueous NaFSI solutions broadens the choice of suitable active materials for aqueous sodium-ion batteries. We demonstrate stable cycling of a NaTi₂(PO₄)₃ anode and a Na₃(VOPO₄)₂F cathode in this aqueous electrolyte enabling the fabrication of high-voltage rechargeable aqueous sodium-ion batteries and discuss route to further increase the cell voltage.

R.-S. Kühnel, D. Reber, C. Battaglia, ACS Energy Letters 2017, 2, 2005