Microstructure modification strategies on P2-NaxNi1/3Fe1/3Mn1/3O2 cathode materials by high temperature solid-state method for enhanced sodium ion storage

Abstract

Na_xNi_1/3Fe_1/3Mn_1/3O₂ cathode material with high stability and high capacity is favored by researchers. However, during the charge-discharge process, Na_xNi_1/3Fe_1/3Mn_1/3O₂ undergoes a series of complex phase transitions, which lead to the attenuation of its electrochemical performance and severely limit its application. In this paper, a series of Na_xNi_1/3Fe_1/3Mn_1/3O₂ (0.6 ≤ x ≤ 1.0) cathode materials are synthesized by the high-temperature solid-state method, and the influence of varying sodium contents on the structure, Na occupancy, and various performance of the materials is analyzed. The results indicate that all four samples are P2 phase. However, they have different sodium sites (directly under the transition metal element (Na_f) and directly under the gap of the three transition metal elements (Na_e)) occupancy. It is found that the Na_f to Na_e ratio of Na_0.9Ni_1/3Fe_1/3Mn_1/3O₂ material is 0.55, at which time Na_f/Na_e is lower and the electrochemical performance is the best. Its initial discharge specific capacity is 112mAg⁻¹, and after the multiplicative performance test, the discharge specific capacity is 95.7mAg⁻¹. The study shows that by adjusting the initial sodium content, the occupancy rate of different sodium sites is changed, which significantly affects the morphology, capacity, and cycling stability of the material.