Decarbonization of long-haul transportation, i.e., ships and trains is among the toughest challenges toward eliminating greenhouse emissions, but metal-air batteries have extraordinary potential to meet this challenge. More specifically, Mg-air batteries have the potential for 30–40 times the energy of lithium-ion batteries at very high efficiency, and their Mg anode and molten salt materials are abundant in seawater. The two main criteria for these batteries are stability of the cathode material and removal of MgO product from the electrolyte through directional solidification. This talk will present experimental and modeling results for a novel molten salt magnesium-air battery with an MgCl₂–NaCl–KCl electrolyte operating at 420–620 °C. O²⁻ dissolves at the cathodes and Mg²⁺ at anodes. Experimental results show 1.9 V open-circuit voltage, which is the highest to date for an Mg-air battery. Modeling shows up to 1.17 W/cm² at 73% efficiency and 2.89 W/cm² at 36% efficiency. This work illustrates the proof of concept of Mg-air batteries and discusses the requirements for larger-scale cells.