Rechargeable multivalent metal (e.g., Ca, Mg, or Al) batteries are promising for large-scale energy storage due to their low cost. However, challenges like low reversibility, dendrite growth, sluggish ion kinetics, and poor electrode compatibility with non-aqueous electrolytes hinder their application. This study introduces aqueous multivalent-ion batteries using concentrated aqueous gel electrolytes, sulfur-containing anodes, and high-voltage metal oxide cathodes. This design achieves satisfactory specific energy, reversibility, and safety. A calcium-ion/sulfur|metal oxide full cell demonstrates a specific energy of 110 Wh kg⁻¹ and excellent cycling stability. Molecular dynamics and experiments show that side reactions are suppressed by reduced water activity and a protective inorganic solid electrolyte interphase. The strategy is also demonstrated for magnesium and aluminum-ion systems.
