Regulating solvation sheath by introducing multifunctional fluoride boronic esters for highly efficient magnesium stripping/plating

Rechargeable magnesium batteries (RMBs) are considered a highly promising energy storage system. However, the lack of low-cost and highly effective electrolytes severely hinders the development of RMBs. Herein, a tris(2,2,2-trifluoroethyl) borate (B(Otfe)3) co-solvent is introduced into the magnesium-aluminum-chloride complex electrolyte. The electrophilic B(Otfe)3 as an electron pair acceptor can react with the passivation layer to avoid the electrolytic conditioning process. The B(Otfe)3-modified solvation structure with low desolvation energy increases the concentration of active cation, effectively improving the ionic conductivity and area current density. Meanwhile, the B(Otfe)3-modified solvation ions with lower LUMO energy induce the formation of the B-F/B-O-containing and MgF2-containing solid electrolyte interphase (SEI), reducing the interfacial transfer impedance. The prepared B(Otfe)3/MgCl2/AlCl3 (BMA) electrolyte exhibits excellent electrochemical performance with low overpotential (∼122 mV), high average coulombic efficiency (CE) (∼98.2%) and long-term cycling stability (1000 h). Moreover, the BMA electrolyte shows good compatibility with Mo6S8 and CuS cathodes. The Mg|CuS full cell keeps a high specific capacity of 231 mAh g−1 after 80 cycles at 56 mA g−1, and the Mg|Mo6S8 cell operates 1000 cycles with an average CE of nearly 100%. This work provides an effective way to simultaneously improve the electrolyte solvation sheath and SEI structure for stable magnesium batteries.