Water-in-salt electrolytes have enabled the development of novel high-voltage aqueous lithium-ion batteries due to their wide electrochemical stability window. Here, we explore why analogous sodium electrolytes have struggled to reach the same level of electrochemical stability and compare solution structure and electrochemical stability for eleven sodium salts, selected among the major classes of salts proposed for highly concentrated electrolytes.^[1-3] We relate the water environment established for each anion to its position in the Hofmeister series, finding a strong correlation between the chaotropicity of the anion and the resulting electrochemical stability of the electrolyte. We discuss the importance of the solid-electrolyte interphase for stable battery performance, how it relates to the choice of anion, and further show that the search for suitable sodium salts is complicated by the fact that higher salt concentrations are needed than for their lithium equivalents.^[4] Reaching such a high concentration of 25 to 30 mol/kg with one or a combination of multiple sodium salts that have the desired properties remains a major challenge. We conclude that alternative approaches such as mixed water/organic solvent or dual-cation electrolytes should be pursued to enable the development of high-voltage aqueous sodium-ion batteries. We demonstrate the potential of the mixed-solvent approach by showing that the water solubility of NaTFSI can be increased from 8 to 30 mol/kg in the presence of ionic liquids. Such a ternary electrolyte, based on highly chaotropic anions, enables stable cycling of a 2 V-class sodium-ion battery based on the NaTi₂(PO₄)₃/Na₂Mn[Fe(CN)₆] electrode couple for 300 cycles at 1C with a Coulombic efficiency of >99.5%.^[5]

References:

^[1] Kühnel, R.-S.; Reber, D.; Battaglia, C., A High-Voltage Aqueous Electrolyte for Sodium-Ion Batteries. ACS Energy Lett. 2017, 2, 2005-2006.

^[2] Reber, D.; Kühnel, R.-S.; Battaglia, C., Suppressing Crystallization of Water-in-Salt Electrolytes by Asymmetric Anions Enables Low-Temperature Operation of High-Voltage Aqueous Batteries. ACS Materials Letters 2019, 1, 44-51.

^[3] Reber, D.; Takenaka, N.; Kühnel, R.-S., Yamada, A.; Battaglia, C., Impact of Anion Asymmetry on Local Structure and Supercooling Behavior of Water-in-Salt Electrolytes. J. Phys. Chem. Lett. 2020, 11, 4720

^[4] Reber, D.; Kühnel, R.-S.; Battaglia, C., Stability of aqueous electrolytes based on LiFSI and NaFSI. Electrochim. Acta 2019, 321, 134644

^[5] Reber, D.; Grissa, R.; Becker, M.; Kühnel, R.-S.; Battaglia, C., Anion Selection Criteria for Water-in-Salt Electrolytes. Adv. Energy Mater. 2021, 11, 2002913