Magnesium–oxygen (Mg–O2) batteries are considered promising candidates for next-generation energy storage systems due to their high specific energy, low cost and intrinsic safety. However, poor rechargeability remains a critical barrier, largely due to the lack of suitable electrolytes and undesired irreversible cathode reactions. Here we report a rechargeable non-aqueous Mg–O2 battery enabled by a tailored electrolyte featuring a cage-like Mg2+ solvation environment. We find that varying ether solvent chain lengths, alongside a tridentate chelating agent, can act coordinately to form a dynamic solvation microreactor. This solvation environment is consistent with the reversible formation of nanocrystalline magnesium peroxide at the cathode and remains compatible with reversible magnesium plating/stripping. The resulting Mg–O2 batteries achieve a high initial discharge voltage approaching 2.0 V, an ultralow overpotential of 0.35 V, a remarkable round-trip energy efficiency of 80% and over 450 stable cycles. These results highlight a solvation-structure-guided strategy for enabling rechargeable Mg–O2 batteries and potentially other multivalent metal–air systems.