Optimizing Calcium Electrolytes by Solvent Manipulation for Calcium Batteries

Abstract Calcium is a highly attractive metal anode because of its high earth abundance and low reduction potential. However, the lack of calcium electrolytes for reversible calcium deposition significantly hampers the development of Ca rechargeable batteries. Herein, the calcium deposition/stripping behaviors of a calcium salt electrolyte, Ca[B(hfip) 4 ] 2 ([B(hfip) 4 ] − =tetrakis(hexafluoroisopropyloxy)borate) were systematically studied using different working electrodes (GC, Pt, Cu, and Al) and different solvents including tetrahydrofuran (THF), dimethoxyethane (DME), and diglyme (DGM). It was found that the Ca[B(hfip) 4 ] 2 /DGM electrolyte demonstrated the highest reversibility and stability in cyclic voltammetry and symmetric Ca/Ca half‐cell studies. The Ca[B(hfip) 4 ] 2 /DGM electrolyte was further employed to demonstrate a 3.4 V Ca battery using a FePO 4 cathode with a discharge capacity of 120 Ah/mg.