Electrolyte Engineering Assisted by Pseudo‐Diluent for Stable and High‐Voltage Sodium Metal Batteries

Abstract

Practical sodium metal batteries (SMBs) remain hindered by Na dendritic growth and limited cathode areal loading (<1 mAh cm−2). Typical localized high concentration electrolytes (LHCEs) can offer wide electrochemical windows and improved physical properties, while critical challenges persist: (a) sluggish Na+ interfacial exchange kinetics arising from a constrained Na+ hopping mechanism, and (b) protracted solid electrolyte interphase (SEI) formation. Herein, a quasi‐localized high concentration electrolyte (q‐LHCE1) is designed by introducing a pseudo‐diluent, 2,2,2‐trifluoroethyl trifluoromethanesulfonate (TFE‐OTf), into sodium bis(fluorosulfonyl)imide (NaFSI)/diglyme (G2) system. TFE‐OTf not only strengthens cation‐anion interactions to promote a NaF‐dominated poly‐inorganic SEI, but also participates in the inner solvation sheath to enhance connectivity among solvation clusters. The resultant q‐LHCE1 enables superior cycling stability for 4.0 V Na|| NaNi0.33Fe0.33Mn0.33O2 (NFM) cells with a high cathode loading of 14 mg cm−2, and retains 80% of its initial capacity over 430 cycles at 0.3 C. The constructed NFM||Na cell with limited Na reservoir (N/P = 10.6) also achieves 96.8% capacity retention after 40 cycles at 0.1 C. This work sheds light on the Na+ interfacial exchange kinetics and poly‐inorganic SEI formation, and offers a promising strategy for practical SMBs applications.