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

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.