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Magnesium batteries could offer high energy density and safety due to the non-dendritic Mg metal anode. However, Mg²⁺ ingress into and diffusion within cathode materials are kinetically sluggish. It is therefore intriguing that recently organic cathodes were shown to deliver high energy and power even at room temperature. Herein we reveal that previous organic cathodes likely all operated on a MgCl-storage chemistry sustained by a large amount of electrolyte that significantly reduces cell energy. We then demonstrate Mg batteries featuring a Mg²⁺-storage chemistry using quinone polymer cathodes, chloride-free electrolytes, and a Mg metal anode. Under lean electrolyte conditions, the Mg²⁺-storing organic cathodes deliver the same energy while using ∼10% of the amount of electrolyte needed for the MgCl-based counterparts. The observed specific energy (up to 243 Whr kg⁻¹), power (up to 3.4 kW kg⁻¹ …