Aqueous sodium-ion batteries promise increased operational safety and lower manufacturing 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. Sodium-ion batteries are of particular interest, considering that the economically accessible lithium reserves might not be sufficient for a worldwide large-scale adoption of lithium-ion batteries for electric mobility and stationary applications, while the sodium reserves are much larger.
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 sodium bis(fluorosulfonyl)imide (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.
R.-S. Kühnel, D. Reber, C. Battaglia, ACS Energy Letters 2017, 2, 2005