Anion Selection Criteria for Water-in-Salt Electrolytes

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 struggled reach same level and compare solution structure for eleven salts, selected among major classes salts proposed highly concentrated electrolytes. [1-3] We relate water environment established each anion its position in Hofmeister series, finding a strong correlation between chaotropicity resulting electrolyte. discuss importance solid-electrolyte interphase stable battery performance, how it relates choice anion, further show that search suitable is complicated by fact higher salt concentrations are needed than lithium equivalents. [4] Reaching such high concentration 25 30 mol/kg with one or combination multiple desired properties remains challenge. conclude alternative approaches as mixed water/organic solvent dual-cation should be pursued enable sodium-ion batteries. demonstrate potential mixed-solvent approach showing solubility NaTFSI can increased from 8 presence ionic liquids. Such ternary electrolyte, based on chaotropic anions, enables cycling 2 V-class NaTi (PO 4 ) 3 /Na Mn[Fe(CN) 6 ] electrode couple 300 cycles at 1C Coulombic efficiency >99.5%. [5] References: [1] Kühnel, R.-S.; Reber, D.; Battaglia, C., A High-Voltage Aqueous Electrolyte Sodium-Ion Batteries. ACS Energy Lett. 2017 , 2005-2006. [2] Suppressing Crystallization Water-in-Salt Electrolytes Asymmetric Anions Enables Low-Temperature Operation Materials Letters 2019 1 44-51. [3] Takenaka, N.; R.-S., Yamada, A.; Impact Anion Asymmetry Local Structure Supercooling Behavior Electrolytes. J. Phys. Chem. 2020 11 4720 Stability LiFSI NaFSI. Electrochim. Acta 321 134644 Grissa, R.; Becker, M.; Selection Criteria Adv. Mater. 2021 2002913