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

Abstract Mg(SO₃CF₃)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 [Mg²⁺‐OTf⁻] ion pair on Mg. Herein, the OTf⁻ anion is successfully grafted on low‐cost fluoride boronic esters (B(OC_xH_yF_2x‐y+1)₃) to obtain the asymmetric and weak‐coordination boron‐center [B(OC_xH_yF_2x‐y+1)₃OTf]⁻ anion in ether electrolytes. The ‐OCH₂CF₃ (TFE) groups in B(TFE)₃ effectively realize the charge delocalization of the OTf⁻ and B‐O plane, restraining the independent decomposition of the [Mg²⁺‐OTf⁻] ion pair. The co‐decomposition of the asymmetric [B(TFE)₃OTf]⁻ 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/Mo₆S₈ 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.