Synergistic proton and Mg²⁺/Ca²⁺ insertion boosting aqueous divalent ion batteries

Divalent ion batteries (Mg²⁺, Ca²⁺) are promising candidates for next-generation energy storage devices. However, divalent ions’ large radius and high charge density commonly lead to sluggish diffusion kinetics, resulting in an inferior capacity and rate capability. Herein, using an organic anode, 3,4,9,10-perylenetetracarboxylic diimide, as the model system, we report incorporating proton insertion with the metal ion to boost the reaction kinetics in aqueous electrolyte, as revealed by the combined spectroscopic analysis and theoretical simulations. The co-storage strategy enables an attractive specific capacity of 110 mAh g⁻¹ at 0.2 A g⁻¹ and capacity retention of 80% at 2 A g⁻¹, compared to 60 mAh g⁻¹ and 56% for single Mg²⁺ insertion in the Mg-ion battery. A similar phenomenon is observed in the Ca-ion system, thus allowing the stable cycling of 1.1 V-class aqueous Mg-/Ca-ion full cells for over 2000 cycles. This work suggests the great potential of building high-power and long-lifespan aqueous divalent ion batteries enabled by the proton-assisted storage behavior.