Solvent's Covert Role: Concerted Anion‐Solvent Co‐Intercalation Rewrites Voltage Rules for Dual‐Ion Batteries

Abstract Anion intercalation into graphite cathodes governs dual‐ion battery (DIB) performance but suffers from unexplained solvent‐driven voltage shifts (>500 mV) and capacity variations (12‐fold) across electrolyte solvents. Prevailing thermodynamic models overlook solvent involvement due to unresolved anion‐solvent coupling dynamics. Using a spatially resolved operando Raman platform, we detect solvent‐specific vibrational fingerprints and graphite G‐band splitting at identical intercalation thresholds, confirming simultaneous anion‐solvent insertion. This redefines solvents as active thermodynamic directors, revealing two hidden energetic contributions: 1) cation desolvation penalties governed by solvent donor number (DN) and 2) dielectric constant ( ε )‐modulated screening between anions and graphene layers. Integrating these into a revised Nernst model yields a DN‐ ε descriptor that quantitatively predicts intercalation voltages across solvents. We further demonstrate that salt‐concentrated electrolytes disrupt this mechanism by depleting solvent activity, shifting pathways from co‐intercalation to anion‐dominant insertion. This work resolves long‐standing DIB anomalies in anion‐inserted graphite cathode reactions and establishes solvent properties as central levers for energy‐dense DIBs.