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

Abstract 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.