Ion-Docking Effect Enabling Rechargeable High-Voltage Magnesium–Iodine/Chlorine Battery

Rechargeable magnesium (Mg) batteries represent a promising energy storage system by offering low cost and dendrite-less propensity. However, the limited selection of cathode materials, and often with low voltage and capacity, constrain Mg batteries. Herein, by exploiting the ion-docking effect between two halogen species—iodine cations (I⁺) and chlorine anions (Cl⁻)—we activate the cathodic activity of halogens and develop a magnesium–iodine/chlorine (Mg-I/Cl) battery prototype with high energy and power density. The ion-docking effect enables I⁺ and Cl⁻ to mutually balance and disperse their charges, and weakens the coordination strength between Cl⁻ and Mg²⁺ while enhancing the stability of I⁺, thus facilitating the multi-electron (2 + 1/3) redox reactions of halogens. We also find the solvation state of iodine species determines the reaction process of the I⁰/I₃⁻/I⁻ redox couples. The here-developed magnesium–iodine/chlorine battery features an impressively high discharge plateau of up to 3.0 V with a high capacity exceeding 400 mAh g⁻¹, and demonstrates a stable lifespan for 500 cycles, with the ability of ultra-fast charging at 20C and low-temperature cycling under −30 °C. These findings may provide new insights for developing high-energy-density Mg battery systems.