Solvation structure-property relationships in high-concentration sodium electrolytes: Insights from molecular simulations

Sodium-ion batteries (SIBs) are promising cost-effective and sustainable alternatives to lithium-ion batteries (LIBs) for large-scale energy storage. However, their development is limited by slow ion transport due to the larger ionic radius of Na⁺ and challenges in forming stable solid electrolyte interphases (SEIs). Moreover, fundamental differences in ionic size, chemical reactivity, and electrochemical behavior between Na⁺ and Li⁺ hinder the direct transfer of design principles from LIBs to SIBs. To address these issues, electrolyte engineering, particularly with high-concentration electrolytes, has shown potential to enhance interfacial stability and suppress dendrite formation. In this study, we applied a combination of density functional theory (DFT), ab initio molecular dynamics (AIMD), and classical molecular dynamics (CMD) simulations to investigate solvation structure-property relationships in sodium bis(fluorosulfonyl)imide dimethoxyethane (NaFSI/DME) electrolytes. Our results reveal that, at low concentrations (1–2 M), Na⁺ primarily forms solvent-separated and contact ion pairs, and ion transport is dominated by vehicular motion involving co-diffusion with FSI⁻. Free DME molecules are readily reduced, forming reactive intermediates such as sodium methoxide (NaOCH₃), which destabilize the SEI. At high concentrations (4–5 M), Na⁺ is preferentially coordinated by FSI⁻, forming extended ionic aggregates. The transport mechanism shifts to structural diffusion, where Na⁺ migrates by hopping between FSI⁻ sites, resulting in increased correlated transference numbers. Moreover, the reduction of Na-FSI-DME clusters favors the formation of stable SEI components such as NaF and Na₂O. These findings elucidate how solvation environments and ion transport mechanisms evolve with concentration and highlight the critical role of high-concentration electrolytes in stabilizing interfacial chemistry. This work offers molecular-level insights to guide the design of Na electrolytes with enhanced ion dynamics and robust SEI formation.