Electrolyte Optimization and Interphase Regulation for Significantly Enhanced Storage Capability in Ca‐Metal Batteries

Abstract Electrolyte solvent, salt, and additive directly determine many battery indicators such as voltage limit, safe reliability, and temperature limit. In Ca‐metal batteries, suitable electrolyte and interphase are necessary to achieve reversible calcium plating/stripping, which is challenging for F‐based salts at room temperature. Ion transport and charge transfer in calcium batteries with Ca(BF 4 ) 2 ‐NaPF 6 electrolyte are regulated for the first time by altering solvent and salt categories and ratios, achieving adjustable interphase species and microstructure, calcium deposition/dissolution overpotentials, and cycling stability. The optimized electrolyte of 0.1 m Ca(BF 4 ) 2 ‐0.9 m NaPF 6 in ethylene carbonate/diethyl carbonate/dimethyl carbonate/ethyl methyl carbonate (7:1:6:6, v/v) contributes to the most stable Ca plating/stripping cycling and the weakest overpotential, as well as Na/Ca‐based hybrid solid electrolyte interphases including proper amount of organic matter. Ca‐metal batteries with the optimized electrolyte achieve an ultrahigh reversible capacity of ≈ 290 mAh g −1 (at 200 mA g −1 and 1.2–4.5 V), almost four times more than that of counterpart ones ( ≈ 65 mAh g −1 ), equivalent to energy/power performance of ≈ 616 Wh kg −1 at 429 W kg −1 among the best ones for multivalent ion rechargeable metal batteries.