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[ Instrument R&D of instrument network ] The complex structural chemistry brought by the different composition of layered metal oxides (NaxTMO2, TM=transition metal) plays a decisive role in the layered stacking structure, sodium ion conductivity and redox activity. The study of sexual materials has opened up new ways. NaxTMO2 mainly includes two structures, O3 and P2, and the P2 structure has a faster Na+ diffusion rate due to the open triangular prism diffusion channel. However, the initial charging capacity of the P2-type structure is low (<80 mAh g-1), and the average voltage is less than 3.2 V. When charging to a higher voltage, it will undergo a phase change from P2 to O2 or OP4/"Z" resulting in cycle stability. decline.
Sodium content is a key factor affecting the structural stability of P2. During the charging process, the P to O phase transition will occur as the TMO2 layer slips due to the extraction of sodium. If the sodium content in the P2 structure is sufficient during charging, the P2 structure can also remain stable during charging and discharging. At the same time, the higher sodium content in the P2 structure makes the average valence of TM lower, enabling higher sodium storage capacity at lower voltages. Therefore, it is important to systematically investigate the effect of sodium ion content in the P2 phase layered oxide on the structure and properties of the material.
Under the guidance of Hu Yongsheng, a researcher of the Institute of Physics, and Marnix Wagemaker, a researcher at the Institute of Physics, Dr. Chenglong Zhao from the E01 group of the Clean Energy Key Laboratory of the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, studied the P2 layered oxidation Potential sodium ion content in the substance, and observed that high sodium content can improve the stability of the structure, and at the same time can promote the oxidation of low-valent cations to a high-valent state. For the sodium ion P2 phase material, it is very important to realize the transition from Ni2+ to Ni4+ at a voltage of 4.0 V. Under normal circumstances, the transition from Ni2+ to Ni4+ will occur above about 4.2 V. Such high voltage is usually accompanied by structural evolution, electrolyte decomposition, and oxygen loss due to oxygen ion oxidation, etc., causing the material to suffer performance degradation during cycling . Introducing a high sodium content in the P2 phase structure will reduce the average valence state of the transition metal ions in the structure, which will promote the transition of the transition metal with the lowest oxidation state to its high valence state and achieve higher capacity. During the sodium removal process, a large amount of sodium ions in the P2 phase structure can greatly improve the stability of the structure.
For P2 phase cathode materials with low sodium content, when charged and when the sodium ion content in the structure is less than about 1/3, the original P2 phase structure will be transformed into O2 phase or OP4/"Z" phase. When the same sodium ion is extracted, the high sodium content can leave more sodium in the structure, weaken the repulsion between adjacent transition metal layers, and inhibit the transition metal layer slip. For P2-Na45/54Li4/54Ni16/54Mn34/54O2 materials with high sodium content, there is no transition from P to O phase during charging; the newly formed P2 phase has the same structure as the original P2 phase, and has more The large sodium ion layer spacing will promote the diffusion of sodium ions to a certain extent and achieve higher reversibility. Studies have shown that within a 2.0-4.0 V electrochemical window, the material can achieve a multi-electron transfer reaction from Ni2+ to Ni4+, showing a high reversible capacity of ~100 mAh g-1, and stable cycling. Compared with the traditional low sodium content P2 phase materials, the high sodium content P2 phase materials provide new insights into the electronic structure and chemical structure for the development of new electrode materials to a certain extent.
The research results are published in the Journal of the American Chemical Society under the title of Revealing High Na-Content P2-Type Layered Oxides for Advanced Sodium-Ion Cathodes. Relevant work was supported by the National Natural Science Foundation of China, the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences, the Beijing Municipal Science and Technology Commission and the Beijing Natural Science Foundation-Haidian Original Innovation Joint Fund, and the National Public Education Fund Committee.