Electron microscopy studies to explore the effects of doping elements on thermal stability of O3-type layered cathode materials for sodium-ion batteries

초록

In this study, we synthesized O3-type sodium layered cathode materials (NaNi0.3Fe0.2TM0.1Mn0.4O2, TM: Co, Cu) with different dopants to investigate their effects on thermal stability. The Cu-doped material exhibits pronounced structural stability as the temperature increases, as shown through X-ray diffraction (XRD) and in-situ transmission electron microscopy (TEM) structural investigations. Using high-resolution electron energy loss spectroscopy (EELS), we closely monitored the thermal degradation mechanisms of the cathodes under elevated temperature conditions. Detailed particle-scale depth profiling reveals that Cu doping effectively delays the reduction of Mn and suppresses oxygen release inside the particles, with severe thermal degradation confined only at the particle surface. Conversely, Co-doped cathodes were more susceptible to Mn reduction and underwent earlier phase transition to degraded forms. The superior thermal stability of Cu-doped material is attributed to enhanced structural stability associated with the strong covalency of the Cu-O bond and Cu-related charge compensation. These characteristics provide a distinct advantage in mitigating thermal risks under severe thermal stress. This work provides essential insights for designing next-generation cathode materials with enhanced thermal resilience, promoting the advancement of safer, high-performance sodium-ion batteries. © 2026 Elsevier B.V.

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