Dipole Reorientation Induced Temperature-Dependent Solvation Structure in Low-Temperature Sodium Metal Batteries

Sodium metal batteries (SMBs) are promising candidates for next-generation energy storage systems due to their high energy density and abundant sodium resources. However, their application is hindered by sluggish interfacial kinetics at low temperatures. We designed a temperature-dependent electrolyte by balancing ion–dipole and dipole–dipole interactions. The electron-withdrawing effect of fluorine (F) atoms increases the instability of the fluoroethylene carbonate (FEC) dipole orientation within the solvation shell. As temperature decreases, FEC’s dipole reorientation triggers a shift from an ethyl methyl carbonate (EMC)-dominated solvation structure to an FEC-dominated one. This significantly reduces the desolvation energy of solvent-separated ion pairs (SSIPs), contact ion pairs (CIPs), and aggregates (AGGs), accelerating interfacial kinetics. It also reorganizes the solvation structure and alters the decomposition pathway of the electrolyte, forming a thin, organic-rich CEI layer at low temperatures. As a result, P2-Na2/3Ni1/3Mn2/3O2 (P2-NNMO) and O3-NaNi1/3Fe1/3Mn1/3O2 (O3-NFM) cells demonstrate reliable performance, achieving a high-capacity retention of 92.4% after 1000 (1 C) and 84.7% after 900 (0.5 C) cycles at −20 °C. Notably, the P2-NNMO and O3-NFM cells deliver reversible discharge capacities of 80.6 and 102.3 mAh g–1 at −40 °C, respectively. This work offers valuable insight for advancing energy storage technologies.