Due to their inherent safety, environmental friendliness, and potential low cost, rechargeable batteries based on aqueous electrolytes are being developed as a large-scale energy storage option for grid applications to enable wide spread integration of renewables. In this field cost and safety are more important than energy density, which is why water based systems are a promising candidate. So far, operational voltages of aqueous batteries have been too low to enable market penetration due to the narrow electrochemical stability window of water (~1.23 V). Using highly-concentrated aqueous sodium bis(fluorosulfonyl)imide (NaFSI) solutions (35m), we recently reported a stability window of 2.6 V on stainless steel current collectors¹. Meanwhile addressing the raw materials supply chain as important challenge for scaling, we developed an aqueous sodium-ion battery employing only non-critical raw materials, namely NaTi₂(PO₄)₃² and Na₃(VOPO₄)₂F³ on the anode and cathode side, respectively. The average discharge voltage is 1.5 V, whereas the 35-molal NaFSI aqueous electrolyte allowed us to use the full capacity of the high-voltage cathode material. Due to the enhanced oxidative stability of the electrolyte, this battery displays an energy density that is almost twice as high as that of previously reported aqueous sodium-ion batteries⁴.

1. Kühnel, R. S.; Reber, D.; Battaglia, C., A High-Voltage Aqueous Electrolyte for Sodium-Ion Batteries. ACS Energy Lett. 2017, 2 , 2005.
2. Li, Z.; Young, D.; Xiang, K.; Carter, W. C.; Chiang, Y.-M., Towards High Power High Energy Aqueous Sodium-Ion Batteries: The NaTi₂(PO₄)₃/Na_0.44MnO₂ System. Adv. Energy Mater. 2013, 3, 290.
3. Qi, Y.; Mu, L.; Zhao, J.; Hu, Y. S.; Liu, H.; Dai, S., Superior Na-Storage Performance of Low-Temperature-Synthesized Na₃(VO(_1-x)PO₄)₂F(1+2x) (0≤x≤1) Nanoparticles for Na-Ion Batteries. Angew. Chem. Int. Ed. 2015, 54, 9911.
4. Kumar, P. R.; Jung, Y. H.; Wang, J. E.; Kim, D. K., Na₃V₂O₂(PO₄)₂F-MWCNT nanocomposites as a stable and high rate cathode for aqueous and non-aqueous sodium-ion batteries. J. Power Sources 2016, 324, 421.