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

Abstract 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 Mg 2+ 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 I 0 /I 3 − /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.