High‐Energy Aqueous Magnesium Ion Batteries with Capacity‐Compensation Evolved from Dynamic Copper Ion Redox

Abstract The low specific capacity and low voltage plateau are significant challenges in the advancement of practical magnesium ion batteries (MIBs). Here, a superior aqueous electrolyte combining with a copper foam interlayer between anode and separator is proposed to address these drawbacks. Notably, with the dynamic redox of copper ions, the weakened solvation of Mg²⁺ cations in the electrolyte and the enhanced electronic conductivity of anode, which may offer effective capacity‐compensation to the 3,4,9,10‐perylenetetracarboxylic diimide (PTCDI)‐Mg conversion reactions during the long‐term cycles. As a result, the unique MIBs using expanded graphite cathode coupled with PTCDI anode demonstrate exceptional performance with an ultra‐high capacity (205 mAh g⁻¹, 243 Wh kg⁻¹ at 5 A g⁻¹) as well as excellent cycling stability after 600 cycles and rate capability (138 mAh g⁻¹, 81 Wh kg⁻¹ at 10 A g⁻¹).