Dual Fluoride Salt-Based Hydrate-Melt Electrolyte: Advancing Aqueous Fluoride Shuttle Battery

Introduction Fluoride shuttle battery (FSB) is an attractive post lithium-ion battery (LIB) candidate owing to its high theoretical energy density due to the use of multi-electron conversion reactions on transition metal fluorides as high as 5000 Wh L −1 . However, its utilization of charge-discharge reactions at room temperature (RT) has been hindered by the scarcity to adopt adequately conductive electrolytes which enable reversible fluorination, and most of the reported FSBs employ solid-state electrolytes at operating high-temperatures 1 . Few reports on the liquid electrolytes suggested the employment of a saturated organic solution to control the metal fluoride dissolution for improving its cycling performance 2 . However, inorganic fluoride salts do not readily dissolve into organic solvents, and concentrated electrolytes were not obtained despite the addition of anion receptors, which hinders the fluoride conductivity 3 . Therefore, the use of organic solvents carries many demerits such as poor ionic conductivity, flammability, toxicity and so on. In this context, concentrated aqueous electrolytes could be a great substitute to address these issues owing to a) the toxic-free nature of water, b) the high solubility of inorganic fluoride salts in water, c) electrochemical stability enhancement at the electrodes due to the minimal free solvent molecules, d) facile room temperature transport properties 4 . Recently, a concentrated 35 mol kg -1 CsF electrolyte with a potential window of 3.1 V and ionic conductivity of around 50 mS cm –1 was reported 5 . However, there are still some challenges in terms of reversibility of metal fluoride electrodes. Here we report a fluoride hydrate melt with the highest F - concentration of 45.2 mol kg -1 , which corresponds to a F - :water molar ratio of 1:1.2. We used a eutectic mixture of CsF-RbF, inspired by our previous report on eutectic dual salt-based hydrate-melts for aqueous LIBs 4 . We studied the electrochemical properties of the CsF-RbF hydrate melt and applied it to an aqueous electrolyte for FSBs. Experiment CsF and RbF were mixed with water in several molar ratios to find the highest solubility for synthesizing hydrate-melts. Linear sweep voltammetry (LSV) tests were conducted using Pt metal, Pt mesh, and Ag/AgCl as the working, counter and reference electrodes, respectively. The ionic conductivity was evaluated by electrochemical impedance spectroscopy using symmetric cells with Pt electrodes. To explor