Tuning Na⁺ Solvation and Association in Fluorine-Free NaClO₄ Electrolytes via Carbonate Cosolvents

Fluorine-free electrolytes are increasingly pursued for sodium-ion batteries (SIBs) to mitigate environmental and safety concerns associated with per- and polyfluoroalkyl substance (PFAS)-containing salts. However, the molecular mechanisms governing ion transport in sodium perchlorate (NaClO₄)-based electrolytes remain insufficiently understood. Here, classical molecular dynamics (MD) simulations were employed to investigate how carbonate cosolvents influence solvation structure and ion association in 1 M NaClO₄ electrolytes. Three solvent systems were examined: pure diglyme (G2), G2/propylene carbonate (PC), and G2/vinylene carbonate (VC). The simulations show that G2 strongly coordinates Na⁺ and promotes extensive ion association, dominated by multi-ion aggregates, resulting in limited ionic mobility. Incorporation of VC increases ionic conductivity (5.89 → 20.48 mS/cm, Nernst-Einstein estimate), whereas PC produces moderate enhancement. Structural analyses reveal that VC does not enter the primary Na⁺ solvation shell. Instead, it increases the dielectric screening of the electrolyte and weakens Na⁺-ClO₄ ^- electrostatic interactions. This reduction in ion association promotes ion dissociation and facilitates faster solvent exchange dynamics, thereby improving Na⁺ transport. These results indicate that dielectric modulation of ion-anion interactions, rather than changes in the primary solvation structure, plays a key role in governing ion transport in NaClO₄-based fluorine-free electrolytes. The findings provide molecular-level insights into the role of cosolvent dielectric properties in regulating ion association and highlight the utility of molecular simulations for understanding electrolyte behavior.