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Fluoride ion batteries (FIBs) hold promise as energy storage systems of high energy density and high safety. To proceed beyond the proof-of-concept stage, however, working batteries with appealing specifications are yet to be demonstrated. Here, we present a rational design on liquid electrolytes for FIBs. The core idea is to employ a primary solvent, which enables the high solubility of fluoride salt CsF, and a solvation regulator, which can continuously tune the solubility of CsF. We propose ethylene glycol (EG) and choline chloride (ChCl) as the primary solvent and solvation regulator, respectively. The strategy allows for a balanced solvation strength to the F– ions for avoiding thermodynamic barriers at the interfaces between the electrodes and the electrolyte. The optimized EG-ChCl-CsF electrolyte enables us to fabricate a pouch cell exhibiting the initial and reversible capacities of 525 and ∼250 mA h/g, respectively. The high-rate capability close to 1 C (i.e., 166 mA h/g at 500 mA/g) is also demonstrated.
Yu et al. (Fri,) studied this question.