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A water-in-salt electrolyte (WiSE) offers an electrochemical stability window much wider than typical aqueous electrolytes but still falls short in accommodating high-energy anode materials, mainly because of the enrichment of water molecules in the primary solvation sheath of Li+. Herein, we report a new strategy in which a non-Li cosalt was introduced to alter the Li+-solvation sheath structure. The presence of an asymmetric ammonium salt (Me3EtN·TFSI) in water increases the solubility of LiTFSI by two times, pushes the salt/water molar ratio from 0.37 in WiSE to an unprecedented value of 1.13, and significantly suppresses the water activity in both bulk electrolyte and the Li+-solvation sheath. This new 63 m (mol kgsolvent–1) aqueous electrolyte (42 m LiTFSI + 21 m Me3EtN·TFSI) offers a wide potential window of 3.25 V and supports a 2.5 V aqueous Li-ion battery (LiMn2O4//Li4Ti5O12) to deliver a high energy density of 145 Wh kg–1 stably over 150 cycles.
Chen et al. (2020) studied this question.