Realizing Wide-Temperature Reversible Ca Metal Anodes through a Ca²⁺-Conducting Artificial Layer

Room-temperature Ca deposition/stripping is impeded by the formation of ionic insulating interfaces. Electrolyte optimization could partially enhance Ca reversibility by tailoring the interfaces, but the precise regulation of the composition remains challenging. Herein, we construct an ex situ artificial layer on Ca metal via a facile displacement reaction between metal halides and Ca. These Ca-driven spontaneous layers with precisely controlled interfacial chemistry consist of a Ca metal alloy phase and a calcium halide matrix for conducting Ca2+ and insulating the electrons, as revealed by theoretical and experimental investigations. In particular, the Ca31Sn20/CaBr2 interface enables Ca metal anodes to achieve low polarization and humid air stability over a wide temperature range from −25 to +50 °C. This proof-of-concept work provides an alternative approach to boost Ca2+ diffusivity through customized interfacial chemistry regulation.