Co‐Regulating Planar Mg Deposition and Bromine‐Rich Mg Anode‐Electrolyte Interface by Multifunctional Organic Bromine Additive

Abstract Rechargeable magnesium batteries (RMBs) are highly promising candidates for next‐generation energy storage systems due to their superior energy density and intrinsic safety. However, severe magnesium (Mg) anode passivation in conventional electrolytes and non‐uniform Mg deposition significantly compromise their reversibility and cycling stability. Here, for the first time, a multifunctional 1‐bromooctane (OctylBr) additive is introduced into the strongly passivating magnesium bis(hexamethyldisilazide) (Mg(HMDS)₂) electrolyte to engineer a Br‐enriched solid electrolyte interphase (SEI) and promote uniform planar Mg deposition. This strategic electrolyte modification extends the Mg cycling lifespan of conventional electrolytes from 0 to 3600 h with an exceptionally low overpotential of 45 mV in a symmetric cell and a high coulombic efficiency of 99.34% in an asymmetric cell. The planar deposition enabled by the electrolyte allows for reversible Mg plating/stripping across a broad range of areal capacities (0.5–25 mAh cm⁻²). Notably, at high areal capacity of 10 mAh cm⁻², the electrolyte sustains a lifespan of 640 h in a symmetric cell, underscoring its effectiveness for high‐energy applications. Full‐cell evaluations with Mo₆S₈ cathodes further validate the enhanced cycling performance, and the approach proves effective across various conventional Mg salts. These findings position OctylBr as a generally compatible electrolyte additive, unlocking new pathways for high‐performance, long‐life RMBs.