A CaI₂‐Based Electrolyte Enabled by Borate Ester Anion Receptors for Reversible Ca−Organic and Ca−Se Batteries

Passivating solid electrolyte interphases (SEIs) in Ca metal anodes constitute a long‐standing challenge, as they block Ca 2+ transport and inhibit reversible Ca deposition/stripping. Current solutions focus primarily on boron/aluminum‐based electrolytes to mitigate such interfacial issues by producing Ca 2+ ‐conductive species, yet the complex synthetic procedure of these salts restricts the widespread application. Moreover, whether any inorganic phases possess decent Ca 2+ conductivity within SEIs remains ambiguous. Herein, we report that a commercially available CaI 2 ‐dimethoxyethane electrolyte supports reversible Ca/Ca 2+ redox reactions via forming CaI 2 ‐involved SEI, inspired by our density functional theory calculations where CaI 2 species is predicted to possess the lowest Ca 2+ diffusion barrier among a range of inorganic phases. We further materialize this finding by introducing a serial of borate ester anion receptors, resulting in the formation of CaI 2 /borides hybrid SEIs with an enhanced Ca 2+ conductivity. Consequently, the resultant electrolytes realize a 7‐fold reduction in deposition/stripping overpotential compared to anion receptor‐free one, allowing for the construction of reversible Ca‐metal full cells with high‐capacity selenium and organic cathodes.