Anion Selection Criteria for Water‐in‐Salt Electrolytes

Abstract Water‐in‐salt electrolytes have enabled the development of novel high‐voltage aqueous lithium‐ion batteries. This study explores the reasons why analogous sodium electrolytes have struggled to reach the same level of electrochemical stability. Solution structure and electrochemical stability are compared for 11 sodium salts, selected among the major classes of salts proposed for highly concentrated electrolytes. The water environment established for each anion is related to its position in the Hofmeister series and a surprisingly strong correlation between the chaotropicity of the anion and the resulting electrochemical stability of the electrolyte is found. The search for suitable sodium salts is complicated by the fact that higher salt concentrations are needed than for their lithium equivalents. Reaching such a high concentration of > 25 mol kg −1 with one or a combination of multiple sodium salts that have the desired properties remains a major challenge. Hence, alternative approaches such as multisolvent systems should be explored. The water solubility of NaTFSI can be increased from 8 to 30 mol kg −1 in the presence of ionic liquids. Such a ternary electrolyte enables stable cycling of a 2 V class sodium‐ion battery based on the NaTi 2 (PO 4 ) 3 /Na 2 MnFe(CN) 6 electrode couple for 300 cycles at 1C with a Coulombic efficiency of > 99.5%.