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

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.