Fluorinated Halide Solid Electrolytes for High-Voltage All-Solid-State Sodium-Ion Batteries Enabling Reversible Oxygen Redox

Fluorinated halide solid electrolytes (FHSEs) enable integration with high-voltage layered oxides in all-solid-state sodium-ion batteries (ASSSIBs). Here, we demonstrate high-voltage ASSSIBs by pairing a fluorine-substituted UCl 3 -type chloride solid electrolyte, Na 0.6 Ta 0.2 La 0.8 Cl 3.7 F 0.3, with a P2-type Na 0.8 Li 0.1 Ni 0.2 Mn 0.7 O 2 cathode. The ASSSIB achieves reversible oxygen redox up to 4.6 V and delivers 132 mAh g –1 with long-term cycling stability, surpassing the performance of liquid electrolytes and nonfluorinated halide counterparts. Electrochemical and structural analyses reveal that, in liquid-electrolyte cells, oxygen redox above 4.2 V becomes irreversible due to solvent oxidation and surface-layer formation, while phase transitions further degrade structural reversibility. In contrast, the FHSE preserves the P2 framework and stabilizes oxygen-redox activity, enabling higher capacity and long-term cycling stability. Supported by integrated theoretical calculations, electrochemical analyses, and advanced characterizations, this work presents a viable strategy for advancing high-voltage ASSSIBs. Overall, FHSEs enable stabilized oxygen redox above 4.2 V and realize durable, high-energy ASSSIBs.