Semi‐Chelation Solvation Enables High Interface Compatibilities with both Anode and Cathode in a Bulky Mg[B(HFIP) ₄ ] ₂ Electrolyte

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Magnesium fluorinated alkoxyl organoborates (Mg[B‐(OR F ) 4 ] 2 ) electrolytes have received large attention due to the adjusted branched chain, high oxidation stability, and high ion conductivity in rechargeable magnesium batteries (RMBs). However, the most promising Mg[B‐(OCH(CF 3 ) 2 ) 4 ] 2 /1,2‐dimethoxyethane (Mg[B(HFIP) 4 ] 2 /DME) electrolyte still faces the significant challenges of unstable, non‐uniform electrode/electrolyte interphases (SEI/CEI), resulting from the decomposition of strong‐chelation [Mg(DME) 3 ] 2+ ion. Herein, a semi‐chelation solvation structure is designed in Mg[B(HFIP) 4 ] 2 electrolytes by achieving a balance between weak Mg 2+ ‐solvent interactions, ring‐like molecular configurations, and sufficient salt dissociation. The semi‐chelation Mg[B(HFIP) 4 ] 2 electrolyte exhibits lower desolvation energy and processes higher proportions of contact ion pair. The decompositions of semi‐chelation Mg[B(HFIP) 4 ] 2 electrolytes induce the formation of uniform organic–inorganic composite SEI/CEI layers with rich MgB x O y and MgF 2 components. The stable and high‐conductivity SEI/CEI layers enable high interface compatibilities with Mg anode and high‐voltage cathodes. Therefore, Mg anode exhibits a high average coulombic efficiency (99.4%, 500 cycles) and long‐term cycling stability (2000 h). Notably, the high capacity retention (83% after 500 cycles) of Mg||Prussian blue full cell with a discharge platform of 2.1 V can be achieved. This strategy provides a new sight for designing high‐voltage and high‐compatibility boron‐based electrolytes for RMBs.