AbstractHerein we report micro-alloying with the combination of Ca/In as a novel strategy to improve the anode performance for Mg-air batteries. Two micro-alloyed Mg–Ca–In anodes, i.e. Mg−0.1%Ca−0.2%In and Mg−0.2%Ca−0.4%In (wt%), are fabricated and evaluated in both configurations: half-cell and Mg-air full cell. Re-deposition of metallic In on anode surface during discharge is demonstrated. Anodic activation is then promoted by galvanic coupling between Mg and the re-deposited In, and film breakdown induced by In re-precipitation at the substrate/oxide film interface. Thus, the voltage and power density of Mg-air system are enhanced via adopting Mg–Ca–In anodes. Besides, wasteful-discharge of the Ca/In micro-alloyed anodes, which is related to the negative difference effect (NDE), is significantly suppressed. Anodic efficiency is consequently improved, reaching 80.2% at 5 mA cm−2 initial current density, and so is the service life of the Mg-air battery. Due to the enhanced voltage and anodic efficiency, Mg–Ca–In anodes enable Mg-air battery to exhibit outstanding energy density, e.g. 2259 Wh kg−1 at 5 mA cm−2. Mg−0.1%Ca−0.2%In anode possesses superior performance in terms of low wasteful-discharge, enhanced discharge activity and high anodic efficiency. Therefore, we recommend micro-alloyed Mg–Ca–In, like Mg−0.1%Ca−0.2%In, as excellent candidates for anode materials of primary aqueous Mg-air batteries.