Co‐Solvation Tunes Ion Pairing and Interphases to Stabilize High‐Voltage Sodium Metal Batteries

ABSTRACT Electrolytes that enable sodium‐metal batteries to operate reliably at sub‐zero temperatures must simultaneously satisfy fast bulk transport, low Na + desolvation barriers, high‐voltage stability, and mechanically robust interphases requirements that are intrinsically coupled in conventional single‐solvent systems. Here we introduce a solvation‐engineering strategy that decouples these constraints by enforcing carbonate/ether co‐solvation in a NaPF 6 ‐based electrolyte and demonstrate its practical impact in full cells using a high‐voltage Na‐rich layered oxide cathode, Na 2 NiFeMnO 6 , a cathode chemistry that remains largely unexplored despite its high reversible capacity and structural stability. Molecular dynamics and FTIR spectroscopy show that tuning the PC/THF ratio systematically regulates ion‐association equilibria, while density functional theory reveals that co‐solvation simultaneously suppresses oxidative reactivity (lower HOMO) and decreases Na + desolvation penalties. These molecular‐level effects translate directly into interfacial kinetics and durability: the tuned 50:50 PC/THF electrolyte forms a mixed organic–inorganic SEI/CEI dominated by NaF with controlled carbonaceous passivation, verified by XPS and XRD, enabling low impedance growth and stable charge transfer. As a result, Na∥Na 2 NiFeMnO 6 cells deliver near‐theoretical reversible capacity (≈140 mAh g − 1 ) at room temperature with negligible capacity decay (∼3% over 280 cycles at 0.5C) and sustain reversible sodiation/desodiation at −30°C (≈84 mAh g − 1 ).