Molecular-Level Design of Localized High-Concentration Electrolytes for Sodium-Ion Batteries: A Combined DFT and MD Study

The development of stable high-voltage electrolytes is crucial for advancing sodium-ion batteries (SIBs). While high-concentration electrolytes (HCEs) improve oxidative stability, their high viscosity severely limits ion transport. Localized high-concentration electrolytes (LHCEs) offer a promising solution by incorporating inert diluents to preserve the solvation structure of HCEs while mitigating their kinetic drawbacks. Herein, we elucidate the molecular-level mechanism of diluents in LHCEs using a combination of density functional theory (DFT) calculations and molecular dynamics (MD) simulations. By introducing 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) and m-fluorotoluene (mFT) as diluents into a carbonate ester-based NaPF6 electrolyte, we successfully constructed LHCEs dominated by anion-aggregated (AGG-type) solvation shells. Although the diluents do not directly coordinate with Na+, they significantly increase the coordination number of PF6– anions and the proportion of AGG-type solvation shells, replicating the desirable solvation structure of HCEs. Notably, mFT exhibits a slightly stronger regulatory effect than TTE and leads to a higher coordination number of PF6– (NF = 4.11) in the solvation shell. The high participation of PF6– in the solvation shell promotes the formation of PF6– monodentate coordination. This unique structure reduces free solvent molecules, enhances oxidation stability, simultaneously liberates Na+ from restricted motion within solvation shell clusters, and increases the ionic diffusion coefficients in the LHCEs by nearly an order of magnitude when compared with the HCEs at room temperature. Even more pronounced improvements of ionic diffusion are found in mFT-based LHCEs at low temperatures. This study provides fundamental theoretical insights for designing high-performance LHCEs and their significant potential for durable and wide-temperature-range SIBs.