A Heterogeneous Oxide Enables Reversible Calcium Electrodeposition for a Calcium Battery

Multivalent rechargeable metal batteries offer a safer, more sustainable, and higher energy density alternative to lithium-ion batteries, though several challenges remain. Recent demonstrations of room-temperature reversible electrodeposition and dissolution of Ca metal indicate that it is possible to stabilize metallic Ca anodes with spontaneously formed solid electrolyte interphases (SEIs). However, further progress toward the goal of an energy-efficient, long cycle-life Ca anode requires correlating interphase identity with electrode performance. In this work, we demonstrate that the SEI formed from calcium borohydride solvated in tetrahydrofuran, an electrolyte that supports reversible Ca deposition, is a compositionally and structurally heterogeneous oxide, sufficiently thin to support Ca2+ cation transmission while stabilizing Ca from corrosive loss during long-term electrolyte contact. The significant advance demonstrated here is that ionically nonconductive materials, like calcium oxide, can form cation-transmissive interphases in which conductivity can be tailored through control of heterogeneity, providing an approach for stabilizing reactive metal electrodes.