Unveiling the High‐Voltage Reactivity and Gas Evolution With Aluminum‐Based Chloride and Oxychloride Catholytes in Solid‐State Sodium Batteries

ABSTRACT All‐solid‐state sodium batteries (ASSBs) employing halide solid electrolytes (SEs) offer a cost‐effective and energy‐dense alternative to conventional liquid electrolyte systems. However, their high voltage (>4 V vs. Na/Na + ) performance remains limited by interfacial instability between the cathode active material (CAM) and the SE. We present here the electrochemical and interfacial behaviors of crystalline NaAlCl 4 and amorphous sodium–aluminum–oxychloride (NACO) SEs when combined with NaNi 0.5 Mn 0.5 O 2 cathode. While oxygen incorporation in NACO enhances ionic conductivity by nearly three orders of magnitude relative to NaAlCl 4 , it does not improve high‐voltage cycling stability. Cells employing NACO exhibit accelerated capacity fade, increased cell impedance growth, and intrinsic oxygen evolution above 4.5 V vs. Na 3 Sn, as revealed by operando electrochemical mass spectrometry. In contrast, the NaAlCl 4 ‐based cells show no detectable gas release, underscoring their superior high‐voltage stability and safety. Time‐of‐flight secondary‐ion mass spectrometry confirms the formation of Al─O and Ni/Mn─Cl species, respectively, in the SE and CAM, indicating redox‐driven anion exchange that contributes to kinetic hindrance of high‐voltage phase transitions. The findings establish that while oxygen incorporation enhances ionic transport, it can compromise interfacial stability, suggesting pure chloride SEs may offer a more robust and intrinsically safer pathway for developing high‐energy ASSBs.