Toward Stable Multivalent Metal Batteries: Understanding the Interfacial Chemistry for Magnesium and Calcium Metal Anodes

Metal anode batteries, particularly divalent systems including magnesium (Mg) and calcium (Ca), are promising candidates for next-generation energy storage due to their high natural abundance and superior theoretical energy densities. However, the electrode/electrolyte interphase remains the paramount factor limiting the reversibility and reaction kinetics of Mg and Ca metal deposition/dissolution. Unlike the ionic-conductive interphase demonstrated in monovalent metal (Li and Na) anode systems, conventional electrolytes often form ion-insulating layers on the surface of Mg and Ca anodes, putting a significant barrier for reversible stripping and plating. This review provides a systematic analysis of recent advancements in understanding how the passivation layers impact Mg²⁺ and Ca²⁺ transport and reduction, and how these interphases evolve during electrochemical cycling. We also scrutinize the chemical composition and microstructures of the interphases in relation to electrolyte formulations. By establishing a correlation among electrolyte chemistry, interphase properties, and electrochemical performance for Mg and Ca metal anodes, this work provides a roadmap for the rational design of effective interphases for viable Mg and Ca metal batteries.