Ion Association Dynamics Governs Low-Temperature Ion Transport of Sodium-Ion Battery Electrolytes

Abstract Sodium-ion batteries (SIBs) are being explored for low-temperature energy storage, yet the molecular origin of severe ion-transport degradation upon cooling remains unclear. Here, we combine machine-learned molecular dynamics with in situ variable-temperature Raman spectroscopy to establish a quantitative static-dynamic framework that directly correlates Na+-solvent coordination, ion association, and ion transport in the organic electrolytes of sodium-ion batteries. Quantitative structural analysis reveals only modest changes in Na+-solvent coordination upon cooling, while solvent-separated ion pairs remain the most mobile transport species throughout the investigated temperature range. Instead, cooling only slightly increases the populations of ion association but markedly prolongs their lifetimes and suppresses their dynamic reorganization, progressively reducing the fraction of rapidly migrating Na+ ions and thereby accounting for the pronounced decline in electrolyte conductivity at low temperatures. These findings identify ion-association dynamics, rather than static solvation structures, as the dominant molecular factor governing low-temperature ion transport.