A CaI₂-based electrolyte enabled by borate ester anion receptors for reversible Ca-organic and Ca-Se batteries

Abstract

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. Graphical abstract: [Figure not available: see fulltext.]