The Influence of Electrolyte Formulation on Gas Evolution in Sodium‐Ion Batteries with NaMnO₂ Cathode

Sodium-ion batteries (SIBs) are considered a promising alternative to lithium-ion batteries due to the high availability of sodium resources. Among the various candidates for the positive electrode, layered (O3-type) NaMnO2 has attracted considerable attention. However, understanding of its interfacial stability remains limited. Differential electrochemical mass spectrometry (DEMS) is a powerful tool for monitoring gas evolution and therefore provides valuable insights into side reactions occurring at the interface between anode/cathode and electrolyte. In this work, the gassing behavior of SIB half-cells with NaMnO2 cathode and six representative electrolyte formulations is investigated using DEMS. The results show that electrolytes with fluoroethylene carbonate effectively suppress parasitic reactions and promote the formation of passivating interphases, resulting in improved performance and limited gas release. PC-based electrolytes appear to be more stable than EC-based electrolytes, especially in combination with NaClO4. The use of NaPF6 is associated with increased H2 evolution and possible manganese dissolution, thereby impairing interfacial stability and releasing more lattice oxygen. An increase in the upper cutoff potential enhances gas release, indicating more severe (electro)chemical oxidation of the electrolyte. Overall, this study paves the way for new strategies for tailoring electrolytes to improve the cyclability and safety of SIBs.