Asymmetric SO3CF3−-Grafted Boron-Center Anion Enables Boron-Containing Interphase for High-Performance Rechargeable Mg Batteries

Mg(SO 3 CF 3 ) 2 (Mg(OTf) 2 ) is a simple and cost-effective magnesium salt, which can promote the future applications of rechargeable magnesium batteries (RMBs). However, the simple Mg(OTf) 2 /ether electrolytes suffer from poor electrochemical properties due to the low solubility of Mg(OTf) 2 and the serious decomposition passivation of the [Mg 2+ -OTf − ] ion pair on Mg. Herein, the OTf − anion is successfully grafted on low-cost fluoride boronic esters (B(OC x H y F 2x-y+1 ) 3 ) to obtain the asymmetric and weak-coordination boron-center [B(OC x H y F 2x-y+1 ) 3 OTf] − anion in ether electrolytes. The -OCH 2 CF 3 (TFE) groups in B(TFE) 3 effectively realize the charge delocalization of the OTf − and B-O plane, restraining the independent decomposition of the [Mg 2+ -OTf − ] ion pair. The co-decomposition of the asymmetric [B(TFE) 3 OTf] − induces the formation of the B-containing organic/inorganic interphase, thus achieving a reversible Mg plating/stripping. After the further solubilization reaction, the obtained electrolyte exhibits a high average coulombic efficiency of 98.13% and long-term cycling stability (1000 h). Notably, the long cycling life (capacity retention of 90.2% after 600 cycles at 1 C) and high-rate capacity (43.0 mAh g −1 at 5 C) of the Mg/Mo 6 S 8 full cell demonstrate a favorable electrolyte/cathode compatibility. This work brings new insights to design the new-type and low-cost Mg-salts and high-performance electrolytes for commercial RMBs.