Activation Energies for Structural Evolution of Na(Ni1-2xFexMnx)O2 (x = 0.3, 1/3, and 0.4) as Cathode Materials for Secondary Sodium Batteries

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Abstract
In this study, O3-type layered Na(Ni1-2xFexMnx)O2 (x = 0.3, 1/3, and 0.4) cathode materials for sodium-ion batteries are investigated to clarify the effects of Ni content and temperature-dependent phase-transition behavior. Charge–discharge tests are performed at 20–50 °C, and ex situ X-ray diffraction (XRD) analysis with Rietveld refinement is carried out at selected potentials to quantify temperature-dependent phase transitions and structural parameters. All samples are obtained as single-phase O3-type layered materials with the R-3m space group. The c-axis parameter, slab thickness, and inter-slab distance decrease with increasing Ni content. Analysis of the temperature-dependent phase transitions confirms that the O3/P3/P′3/P3″ transitions are accelerated at higher temperatures and higher Ni contents. Formation of the P′3 and P3″ phases is accompanied by stepwise changes in slab thickness and inter-slab distance, indicating substantial volume expansion. The O3/P3 transition in the Na-rich region exhibits a higher apparent activation-energy barrier, whereas the P3/P′3/P3″ transitions at lower Na contents show lower apparent activation energies and proceed more rapidly. These results demonstrate that Ni content and operating temperature strongly affect the phase-transition behavior and structural stability of O3-type Na(Ni, Fe, Mn)O2 cathode materials.

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