Rechargeable Mg-Br₂ Battery with Ultrafast Bromine Chemistry

A sustainable society necessitates the support of diversified energy storage systems. Magnesium metal batteries, known for the environmental friendliness, safety of dendrite-less, cost-effective, and high volumetric capacity of magnesium metal, exhibit promising prospects. However, the high charge density of the magnesium ion leads to sluggish ion diffusion in cathodes, posing challenges for developing magnesium metal battery systems with high power and high energy density. Here, inspired by the Hard-Soft-Acid-Base theory, we propose a soft-anion-induced bond weakening strategy to address the diffusion difficulty. The bulky and broadly electron-distributed succinimide ion (SN^-) in SN-Mg-Br significantly weakens the Mg-Br bond, promoting rapid magnesium ion transport and enabling ultrafast bromine chemistry, thus realizing a highly rechargeable Mg-Br₂ battery prototype. Benefiting from the solubilization of SN^-, the Mg-Br₂ batteries achieve a high discharge plateau of 2.7 V, a remarkable specific capacity of 326 mAh g_Br^-1, and an impressive lifespan of 400 cycles. Attributed to the half-half diffusion/adsorption-desorption control process mechanism, the batteries can be well cycled under high-rate charging at 10 C and ultralow temperatures down to -55 °C. This bond weakening strategy may stimulate the development of battery systems with similar high charge density to magnesium ion, toward high power and high energy density, paving the way for sustainable energy storage systems.