Thermal evolution of cathode materials used in commercial sodium-ion batteries

The structural evolution of Na_xNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathodes extracted from commercial 18650 sodium-ion batteries at various states of charge, fully charged (FC), partially charged (PC), and fully discharged (FD) is explored. X-ray absorption spectroscopy (XAS) performed at room-temperature on extracted samples indicate changes in the structure and oxidation states of all three transition metals across the different states of charge. Variable-temperature X-ray diffraction (XRD) revealed a progression of thermally induced phase transformations from the initial layered O3 (R3̄m) and P3 (R3m) structures to a secondary O3 phase, followed by the formation of cubic metal oxide and metallic phases (Fm3̄m). Thermogravimetric and differential scanning calorimetric analyses show varying degrees of mass loss and significant exothermic activity, particularly in FC samples, associated with structural changes and cathode decomposition as corroborated by variable-temperature XRD under similar atmospheric conditions. Post-thermal treatment scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis identifies morphological and compositional differentiation, with irregular Mn/Ni-rich particles and Ni/Fe-enriched spherical domains exhibiting oxygen deficiency. These findings provide an overall view of the thermal response and phase evolution of layered NFM cathodes at different electrochemical conditions.