Dual Oxygen Precursors Boosting the Ionic Conductivity of Glassy Electrolytes for All‐Solid‐State Sodium Batteries

Amorphous halide‐based solid electrolytes (SEs) are promising candidates for all‐solid‐state Na batteries (ASSNaBs) due to their structural flexibility and favorable mechanical properties. Among them, aluminum‐based halide electrolytes are particularly attractive owing to their low cost and oxidative stability; however, previously reported systems typically exhibit limited room‐temperature ionic conductivity (<1 mS cm −1 ). In this work, we report the synthesis of a transparent, viscoelastic Na–Al SE with the specific composition 0.6NaClO–AlCl 3 –0.175SeO 2 , achieved through the strategic introduction of dual oxygen sources (NaClO and SeO 2 ). This approach enables the modulation of charge carrier concentrations while simultaneously supplying sufficient oxygen. Furthermore, we introduce the concept of deoxygenation enthalpy to rationalize the selection of these dual oxygen sources among various oxide candidates. The resulting electrolyte achieves a high Na + conductivity of 2.03 mS cm −1 at ambient temperatures, among the highest reported for Na–Al halide electrolytes. Molecular dynamics simulations confirm that segmental motion within the disordered framework actively facilitates Na + transport, underpinning the observed viscoelastic behavior. When integrated into ASSNaB with uncoated NaNi 0.4 Fe 0.2 Mn 0.4 O 2 cathode, the electrolyte enables stable long‐term cycling and superior thermal compatibility, demonstrating practical applicability. This work establishes a new paradigm in rational precursor design for high‐performance viscoelastic SEs.