Highly Reversible Calcium Plating and Stripping at Ambient Temperature Enabled by Ca(BH₄)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(BF₄)₂-based electrolyte in an EC:PC (1:1, v/v) solvent system by incorporating Ca(BH₄)₂·2THF and boron nitride. The dual-additive formulation promotes the formation of a conductive borate-rich solid electrolyte interphase and facilitates Ca²⁺ desolvation, enabling Ca plating with a low overpotential of 0.43 V (vs. Ca/ Ca²⁺, using an Ag quasi-reference electrode calibrated by the Fc/Fc⁺ redox couple) and a high Coulombic efficiency of 98%, while suppressing CaF₂ 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 NaV₂(PO₄)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.