Water-in-salt electrolytes have successfully expanded the electrochemical stability window of aqueous electrolytes beyond 2V .Further improvements in stability can be achieved by partially substituting water with either classical organic solvents or ionic liquids.H ere,w es tudy ternary electrolytes composed of LiTFSI, water,and imidazolium ionic liquids.We find that the LiTFSI solubility strongly increases from 21 mol kg À1 in water to up to 60 mol kg À1 in the presence of ionic liquid. The solution structure is investigated with Raman and NMR spectroscopyand the enhanced LiTFSI solubility is found to originate from ahydrotropic effect of the ionic liquids. The increased reductive stability of the ternary electrolytes enables stable cycling of an aqueous lithium-ion battery with an energy density of 150 Wh kg À1 on the active material level based on commercially relevant Li 4 Ti 5 O 12 and Li-Ni 0.8 Mn 0.1 Co 0.1 O 2 electrode materials.
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 NaTi2(PO4)3/Na2Mn[Fe(CN)6] electrode couple for 300 cycles at 1C with a Coulombic efficiency of >99.5%.
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