A high-rate and air-stable cathode material for sodium-ion batteries: yttrium-substituted O3-type Ni/Fe/Mn-based layered oxides

Abstract

Cobalt-free NaNi_1/3Fe_1/3Mn_1/3O₂ is considered as one of the most promising cathode materials for sodium-ion batteries due to its high specific capacity, low cost and facile synthesis method. However, its electrochemical performance deteriorates rapidly due to serious structural degradation in the charge–discharge process, and it is difficult to store O3-type layered oxides in air, which seriously affects its commercialization process. Therefore, in this work, the yttrium element was used to partially substitute the NaNi_1/3Fe_1/3Mn_1/3O₂ material to achieve microscopic modulation of the crystal structure. The strong Y–O bond can stabilize the crystal structure, inhibit the slipping of the transition metal layer, and prevent the occurrence of an irreversible phase transition, thus improving the cycling performance of the material. At the same time, the sodium-ion diffusion layer expands with the introduction of Y, and the sodium-ion diffusion coefficient and rate capability of the material are significantly enhanced. In addition, Y³⁺ substitution reduces the ratio of Mn³⁺/Mn⁴⁺ in the material, mitigating the Jahn–Teller effect, which is another important factor for increasing the stability of the layered structure. Moreover, even after storing NaNi_1/3Fe_1/3−0.01Mn_1/3Y_0.01O₂ in air for 7 days, its crystal structure did not undergo significant changes, indicating a substantial improvement of air stability compared to the pristine material of NaNi_1/3Fe_1/3Mn_1/3O₂.