Enhancing the Interphasic Stability of Sodium-Ion Batteries via Structural Distortion-Driven Defluorination of Aromatic Additives

Abstract The commercial viability of sodium-ion batteries is hindered by the interphasic instability of hard carbon (HC) anodes. Here, we reveal a fundamentally new interphasic engineering mechanism driven by the reduction-induced structural distortion of fluorinated aromatic additives (FAAs). We demonstrate that 1,2-difluorobenzene (2FB) preferentially accumulates at the HC interface via strong π–π interactions. Crucially, upon reduction, 2FB undergoes profound structural distortion driven by π* and σ* orbital mixing. This deformation significantly weakens the C–F bond, triggering facile defluorination to construct a robust, sodium fluoride (NaF)-rich inorganic solid electrolyte interphase (SEI). This highly protective, thin layer suppresses continuous SEI dissolution and hazardous Na plating while accelerating Na+ transport kinetics. Consequently, HC || Na(Ni1/3Fe1/3Mn1/3)O2 full cells delivered enhanced cycling stability under 3C fast-charging conditions. This work establishes a molecular design principle for engineering advanced electrochemical interfaces through orbital-level structural manipulation.