Electrolyte-Derived Chemical Activation Enables Reversible Cathode Chemistry in Magnesium Batteries

Rechargeable magnesium (Mg) metal batteries are promising high-energy-density alternatives to lithium-ion systems, yet sluggish cathode kinetics hinder their development. Departing from the conventional host-centric view, we demonstrate that reversibility in Mg batteries can be governed by electrolyte-derived reaction chemistry. Here, we harness trace water to impart controlled electrolyte reactivity, enabling electrolyte-derived Mg/Cl-containing species to participate in cathode activation. In tunnel-type MnO2, the regulated electrolyte induces lattice expansion and improves electrochemical reversibility. In organic cathodes, improved rechargeability is consistent with a stabilizing role of Cl– in anion-coupled redox. Because polar-solvent chemistry can irreversibly passivate Mg metal, a Sn/MgCl2 artificial interface suppresses parasitic anode reactions while preserving cathode activation. This strategy enables reversible electrochemistry across oxide, organic, and conversion cathodes, including α/γ-MnO2, methylphenothiazine, tetrathiafulvalene, and hybrid I2/O2 systems. These findings establish electrolyte-derived chemical activation as a guiding principle for regulating cathode reaction pathways in magnesium batteries.