A Water-in-Salt Electrolyte for High-Voltage Aqueous Sodium-Ion Batteries

The inherent safety and potential low cost make rechargeable batteries based on aqueous electrolytes a promising option for large-scale energy storage for grid applications. 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 a highly-concentrated aqueous sodium bis(fluorosulfonyl)imide (NaFSI) (35 molal) electrolyte we recently reported a stability window of 2.6 V [1] and further developed a highly-concentrated ternary aqueous sodium-ion electrolyte which is robust against crystallization even at low temperatures and allows long term cycling of full-cells well below 0 °C. A 2 V class cell, based on this electrolyte, NaTi 2 (PO 4 ) 3 [2] and Na 3 (VOPO 4 ) 2 F [3], displays excellent cycling stability between 30 and -10 °C with an energy density of 64 Wh kg -1 , based on the active masses of both electrodes. This is almost twice as high as that of previously reported aqueous sodium-ion batteries [4]. At elevated temperatures however, the FSI-anion is susceptible to hydrolysis. As most so called water-in-salt electrolytes reported to date are based on fluorinated sulfonylimides we investigated LiFSI and NaFSI as model systems over a wide concentration and temperature range. We find that highly concentrated NaFSI electrolytes are significantly more stable than their lithium analogues. For 35m LiFSI solutions we observe complete degradation after storage at 60 °C for one week, whereas 35m NaFSI only exhibits a small decrease in pH. We suggest that FSI-destabilizing, strong anion-cation interactions in such highly concentrated electrolytes are cation dependent and relate the stability of the anion to the charge density of the cation. References: Kühnel, R. S.; Reber, D.; Battaglia, C., A High-Voltage Aqueous Electrolyte for Sodium-Ion Batteries. ACS Energy Lett. 2 , 2005 (2017). Li, Z.; Young, D.; Xiang, K.; Carter, W. C.; Chiang, Y.-M., Towards High Power High Energy Aqueous Sodium-Ion Batteries: The NaTi 2 (PO 4 ) 3 /Na 44 MnO 2 System. Adv. Energy Mater. 3 , 290 (2013). Qi, Y.; Mu, L.; Zhao, J.; Hu, Y. S.; Liu, H.; Dai, S., Superior Na-Storage Performance of Low-Temperature-Synthesized Na 3 (VO( 1-x )PO 4 ) 2 F(1+2x) (0≤x≤1) Nanoparticles for Na-Ion Batteries. Chem. Int. Ed. 54 , 9911 (2015). Kumar, P. R.; Jung, Y. H.; Wang, J. E.; Kim, D. K., Na 3 V 2 O 2 (PO 4 ) 2 F-MWCNT nanocomposites as a stable and high rate cathode for aqueous and non-aqueous sodium-ion batteries. Power Sources 324 , 421 (2016).