Highly Reversible Calcium Plating and Stripping at Ambient Temperature Enabled by Ca(BH ₄ ) ₂ ·2THF and Boron Nitride Dual‐Additives Strategy

Calcium metal batteries offer a promising alternative to lithium‐ion batteries owing to calcium's natural abundance, cost‐effectiveness, low redox potential, and divalent charge. However, the practical implementation is hindered by the lack of an electrolyte that enables reversible calcium plating/stripping at ambient temperature and is compatible with high‐voltage cathodes. In this study, we present a dual‐additive strategy to enhance the performance of a Ca(BF 4 ) 2 ‐based electrolyte in an EC:PC (1:1, v/v) solvent system by incorporating Ca(BH 4 ) 2 ·2THF and boron nitride. The dual‐additive formulation promotes the formation of a conductive borate‐rich solid electrolyte interphase and facilitates Ca 2+ desolvation, enabling Ca plating with a low overpotential of 0.43 V (vs. Ca/ Ca 2+ , using an Ag quasi‐reference electrode calibrated by the Fc/Fc + redox couple) and a high Coulombic efficiency of 98%, while suppressing CaF 2 formation. XRD and SEM analyses confirm successful calcium plating on both Cu and Ni substrates, and linear sweep voltammetry reveals a wide electrochemical stability window with Al current collectors. A full cell using a NASICON‐type NaV 2 (PO 4 ) 3 cathode demonstrates stable room‐temperature cycling, delivering a discharge capacity of ∼100 mAh·g −1 and ∼85% efficiency. Although long‐term cycling is limited by cathode/electrolyte interphase formation, these findings represent a key step toward realizing high‐energy, room‐temperature calcium metal batteries.