Reversible Calcium Plating and Stripping at Room Temperature Using a Borate Salt

Interest has rekindled in reversible calcium plating and stripping, renewing hopes for the development of Ca-ion batteries. However, the development of an electrolyte that operates at room temperature and is stable to oxidation at practical potentials remains a significant barrier. Here we report the synthesis and crystal structure of a new fluorinated alkoxyborate Ca(B(Ohfip)4)2·4DME salt. Reversible plating and the dissolution of calcium from pure solutions of this salt in dimethoxyethane are demonstrated at 25 °C with capacities of 1 mAh cm–2 at a rate of 0.5 mA cm–2 over 30–40 cycles, with an anodic stability of >4.1 V vs Ca/Ca2+ (and up to 4.9 V in dimethyltriflamide). The dominant product is calcium, accompanied by CaF2 that forms by the reduction of the fluorinated anion. Whereas the cathodic stability requires improvement, this work shows that facile calcium plating and stripping at room temperature can be achieved using bulk electrodes.