Dipole Effect-Assisted Solvent Screening toward Better Calcium Fluoroborate Salt Electrolytes for High-Voltage Ca-Metal Batteries

Calcium fluoroborate salt electrolytes are promising for high-voltage Ca-metal batteries (CMBs), yet challenged by poor Ca-metal reversibility at room temperature, urgently necessitating formulation innovation. Here, a qualitative descriptor space is established, strongly correlating solvent dipole effect with Ca-metal reversibility in calcium fluoroborate salt electrolytes. It deduces the desirable solvents for better Ca-metal reversibility are of stronger local and proper overall dipole effects due to a higher degree of salt dissociation and more favorable desolvation and interphase kinetics. Under this guidance, the designed ester-based pure calcium fluoroborate electrolyte demonstrated unparalleled Ca-metal reversibility with initial Ca plating/stripping overpotentials <0.16 V (vs Ca/Ca 2+ ) stably cycling >500 h at 0.02 mA cm –2 and 0.02 mAh cm –2, and full cell capacity retention of 84.5% after 120 cycles at 50 mA g –1 and 1–4.2 V. This work indicates the feasibility of regulating solvent dipole effect to enhance the performance of high-voltage CMBs.