Regulate the Na content for structurally stable and high-performance sodium ion batteries

The P2-type Ni/Mn layered oxide is a promising cathode material due to its high specific capacity and unique two-dimensional Na⁺ diffusion path. However, some drawbacks such as P2-O2 irreversible phase transition during charging/discharging, Na⁺/vacancy ordering transition, and poor air stability have seriously affected its commercialization. Based on above problems of layered oxides, a series of cathode materials Na_xLi_0.13Ni_0.20Mn_0.67O₂ (x = 0.6, 0.7, 0.8, 0.9, 1.0) with different sodium contents are prepared, correspondingly investigating the effect of sodium content on the crystal structure and electrochemical performance of cathode materials. The results show that the crystal structure is unstable and prone to P2-O2 irreversible phase transition when the sodium content is low. But the higher sodium content easily to bring Na⁺/vacancy ordering transition. Besides, the higher sodium content leads to poor air stability of cathode materials, which is not easy for subsequent wafer preparation and storage. Compared with other sodium content samples, the Na0.8 possesses the best cycling stability, which does not undergo P2-O2 irreversible phase transition and Na⁺/vacancy ordered transition, and the capacity retention rate is as high as 93.32 % after 100 cycles. The air stability of Na0.8 is also excellent, with the unbroken crystal structure, without impurity increasing on the surface, and still maintaining satisfactory cycling stability after exposed for 72 h in the air. This paper develops a new idea for the design of layered oxide cathode materials for sodium-ion batteries, and also provides a research basis for the effect of sodium content on the structure and performance of the materials.