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 Ca²⁺ transport and inhibit reversible Ca deposition/stripping. Current solutions focus primarily on boron/aluminum-based electrolytes to mitigate such interfacial issues by producing Ca²⁺-conductive species, yet the complex synthetic procedure of these salts restricts the widespread application. Moreover, whether any inorganic phases possess decent Ca²⁺ conductivity within SEIs remains ambiguous. Herein, we report that a commercially available CaI₂-dimethoxyethane electrolyte supports reversible Ca/Ca²⁺ redox reactions via forming CaI₂-involved SEI, inspired by our density functional theory calculations where CaI₂ species is predicted to possess the lowest Ca²⁺ 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 CaI₂/borides hybrid SEIs with an enhanced Ca²⁺ 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.