Toward high stability of O3-type NaNi1/3Fe1/3Mn1/3O2 cathode material with zirconium substitution for advanced sodium-ion batteries

We successfully synthesized a series of O3-type NaNi_1/3Fe_1/3Mn_1/3−xZr_xO₂ (x = 0, 0.01, 0.02, 0.04) cathode materials by the solid-state reaction method. Energy dispersion spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy results confirmed the successful incorporation of Zr elements into the lattice to substitute Mn. Due to the introduction of Zr⁴⁺, the crystal structure modulation of O3-NaNi_1/3Fe_1/3Mn_1/3O₂ has been realized. By increasing the Zr⁴⁺ content, the width of the sodium diffusion layer expands, thereby facilitating the diffusion of sodium ions. Consequently, the material exhibits a remarkable enhancement in high-rate capability. At the same time, increasing the Zr⁴⁺ content results in a notable decrease in both the average bond length of TM−O and the thickness of the TMO₆ octahedron in the transition metal layer, resulting in a significant improvement in the cycling performance and structural stability of the cathode material. Additionally, the in-situ XRD results demonstrate that the optimized cathode composition of O3-NaNi_1/3Fe_1/3Mn_1/3–0.02Zr_0.02O₂ (NFMZ2) undergoes a reversible phase transition of O3 → O3 + P3 → P3 → O3 + P3 → O3 during the charge–discharge process.