Deciphering the Halide Chemistry of Cl – and Br – in Enhancing Kinetics of Mg Plating/Stripping

In promising rechargeable magnesium batteries (RMBs), while empirical Cl – addition to boron-based electrolytes optimizes Mg 2+ solvation structure and desolvation process, the scarcity of comparative halide anion studies has hindered the development of a systematic halide chemistry theory for electrolyte design. Br – offers moderate solubility and mass efficiency comparable to Cl –, superior to F – and I –, positioning it as an ideal platform to decouple the effects of halide identity on reaction kinetics, bridging a critical gap in fundamental mechanistic insights. Herein, leveraging the magnesium phenyl fluoroborate complex (MPFBC) electrolyte, we systematically synthesize MPFBC-Cl, MPFBC-Br, and the control electrolytes to elucidate how Cl – and Br – influence Mg plating/stripping kinetics from the bulk electrolyte to the interface. Specifically, the lower charge density and larger ionic radius of Br – compared with Cl – weaken Coulombic interactions with Mg 2+, thereby facilitating ionic cluster dissociation and boosting bulk conductivity. The higher polarizability and chemical softness of Br – relative to Cl – drive stronger specific adsorption at the Mg interface, thereby reconstructing the inner Helmholtz plane and reducing the nucleation overpotential. Furthermore, Br – promotes the formation of Mg–Br species in the SEI, which accelerates Mg 2+ transport across the interface. Benefiting from multiscale modification, the Mg|MPFBC-Br|Mg cell demonstrates improved Mg plating/stripping kinetics, evidenced by a low overpotential (<220 mV) and stable cycling for 1000 h at 1.0 mA cm –2, outperforming MPFBC-Cl and most conventional electrolyte systems. This study unveils the halide chemistry of Br – in accelerating Mg plating/stripping kinetics, offering critical insights toward the rational electrolyte design for next-generation RMBs.