Iodine Boosted Fluoro‐Organic Borate Electrolytes Enabling Fluent Ion‐Conductive Solid Electrolyte Interphase for High‐Performance Magnesium Metal Batteries

Rechargeable magnesium batteries are regarded as a promising multi-valent battery system for low-cost and sustainable energy storage applications. Boron-based organic magnesium salts with terminal substituent fluorinated anions (Mg[B(OR^F)₄]₂, R^F=fluorinated alkyl) have exhibited impressive electrochemical stability and oxidative stability. Nevertheless, their deployment is hindered by the complicated synthesis routes and the surface passivation of Mg metal anode. Herein, we report the design of an advanced electrolyte formulation comprised of tri(hexafluoroisopropyl) borate (B(HFIP)₃) and iodine (I₂) in 1,2-dimethoxyethane (DME) solvent, which eventually convert into a Mg[B(HFIP)₄]₂/DME-MgI₂ electrolyte system upon interacting with Mg anode. The Mg anode reacts with I₂ and the electron-accepting B(HFIP)₃, leading to the in situ formation of a solid-electrolyte interphase layer composed of MgF₂ and MgI₂ species that can facilitate fast and stable Mg plating/stripping. Compared with the pristine Mg[B(HFIP)₄]₂/DME electrolyte, the Mg[B(HFIP)₄]₂/DME-MgI₂ electrolyte exhibited superior electrochemical performance including an ultra-low overpotential (~80 mV), high Coulombic efficiency and a long-cycling period over 1500 h. In result, the rechargeable magnesium batteries with Mg[B(HFIP)₄]₂/DME-MgI₂ electrolyte and Chevrel-phase Mo₆S₈ cathode show outstanding compatibility, rapid kinetics, and stable cyclability for over 1200 cycles, surpassing all previously reported boron-based electrolytes. This work introduces a promising halogen-enhancement strategy for boron-based Mg-ion electrolytes, with the overarching goal of establishing favorable solid-electrolyte interphases that are pivotal for the advancement and optimization of multi-valent secondary batteries.