Water-in-salt electrolytes enable the operation of batteries at higher voltage than previously possible with conventional aqueous electrolytes.[1,2] High-voltage aqueous batteries are potential candidates for stationary electricity storage where low total cost of ownership is more important than high energy density.

To date, most water-in-salt studies have focused on enabling aqueous lithium-ion batteries. However, considering the high salt content of water-in-salt electrolytes, sodium-ion batteries appear particularly interesting for this approach as sodium salts are typically significantly less expensive than their lithium analogues, but typically exhibit insufficient solubility due to the lower charge density of Na⁺ compared to Li⁺.[3]

Recently, we discovered that NaFSI displays very high solubility in water, enabling a wide electrochemical stability window.[4] Although S-F bonds present in the FSI anion are known to be relatively weak and hence make FSI prone to hydrolysis, we demonstrate a 2 V class NaTi₂(PO₄)₃/Na₃(VOPO₄)₂F cell with an excellent capacity retention of 85% after 500 cycles at 1C. To the best of our knowledge, the specific energy of our cell of 65 Wh/kg (based on the active masses of both electrodes) is the highest reported to date for an aqueous sodium-ion battery. Using strategies to prevent electrolyte crystallization, we also demonstrate stable cycling at temperatures as low as -10 °C.

[1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science 350 (2015) 938.

[2] Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, A. Yamada, Nat. Energy 1 (2016) 16129.

[3] D. Reber, R.-S. Kühnel, C. Battaglia, Sustainable Energy Fuels 1 (2017) 2155.

[4] R.-S. Kühnel, D. Reber, C. Battaglia, ACS Energy Lett. 2 (2017) 2005.