Towards dry and contaminant free Ca(BF4)2-based electrolytes for Ca plating

Calcium-metal-anode based batteries have recently gained much attention owing to their higher theoretical capacity when compared to the commercial lithium-ion cells. However, electrodeposition of metallic calcium is challenging and currently there is only a few reported organic electrolytes allowing reversible plating/stripping, including Ca(BF₄)₂ in carbonate solvents. Many of the commercial salts are sold as hydrates, which is the case of Ca(BF₄)₂, however the drying of a divalent-metal cation organic electrolyte is not trivial. Herein, several procedures for drying BF₄⁻-based electrolytes are explored and discussed. It is shown that the tetrafluoroborate anion can easily get hydrolyzed during some drying protocols producing impurities, and thus, it is necessary to prepare the salt in anhydrous conditions to ensure low water and contaminant contents. Two different synthetic routes are presented as alternatives to the commercial hydrated salt.