Rechargeable Mg–Br<sub>2</sub> Battery with\nUltrafast Bromine Chemistry

A sustainable society necessitates the support of diversified\nenergy\nstorage systems. Magnesium metal batteries, known for the environmental\nfriendliness, safety of dendrite-less, cost-effective, and high volumetric\ncapacity of magnesium metal, exhibit promising prospects. However,\nthe high charge density of the magnesium ion leads to sluggish ion\ndiffusion in cathodes, posing challenges for developing magnesium\nmetal battery systems with high power and high energy density. Here,\ninspired by the Hard–Soft-Acid–Base theory, we propose\na soft-anion-induced bond weakening strategy to address the diffusion\ndifficulty. The bulky and broadly electron-distributed succinimide\nion (SN<sup>–</sup>) in SN–Mg–Br significantly\nweakens the Mg–Br bond, promoting rapid magnesium ion transport\nand enabling ultrafast bromine chemistry, thus realizing a highly\nrechargeable Mg–Br<sub>2</sub> battery prototype. Benefiting\nfrom the solubilization of SN<sup>–</sup>, the Mg–Br<sub>2</sub> batteries achieve a high discharge plateau of 2.7 V, a remarkable\nspecific capacity of 326 mAh g<sub>Br</sub><sup>–1</sup>, and\nan impressive lifespan of 400 cycles. Attributed to the half–half\ndiffusion/adsorption–desorption control process mechanism,\nthe batteries can be well cycled under high-rate charging at 10 C\nand ultralow temperatures down to −55 °C. This bond weakening\nstrategy may stimulate the development of battery systems with similar\nhigh charge density to magnesium ion, toward high power and high energy\ndensity, paving the way for sustainable energy storage systems.