Guo Yuhong, Xu Li-Chun, Zhao Wenyang, Guo Chunli, Yang Zhi, Liu Ruiping, Shao Jian-Li, Xue Lin, Li Xiuyan
College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China.
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, P. R. China.
Phys Chem Chem Phys. 2022 Aug 17;24(32):19362-19370. doi: 10.1039/d2cp02227c.
Sodium ion technology is increasingly investigated as a low-cost solution for grid storage applications. Among the reported cathode materials for sodium-ion batteries, NaV(PO)FO is considered as one of the most promising materials due to its high operation voltage and good cyclability. Here, the de-sodiumization process of NaV(PO)FO has been systematically examined using first-principles calculations to uncover the fundamental questions at the atomic level. Four stable intermediate products during the de-sodiumization process are firstly determined based on the convex hull, and three voltage platforms are then predicted. Except for two voltage platforms (3.37 V and 3.75 V) close to the experimental values, the platform up to 5.28 V exceeds the stability window (4.8 V) of a typical electrolyte, which was not observed experimentally. Excitingly, the change of volume is only 2% during the sodiumization process, which should be the reason for the good cycling stability of this material. Electronic structure analysis also reveals that the valence states of V ions will be changed from V to V during the sodiumization process, resulting in a weak Jahn-Teller distortion in VOF octahedra, and then making the lattice-constants asymmetrically change. More seriously, combined with a bandgap of 2.0 eV, the conduction band minimum mainly composed of V-t non-bonding orbitals has strong localized characteristics, which should be the intrinsic origin of poor electron transport properties for NaV(PO)FO. Nonetheless, benefiting from the layer-like structure features with F-segmentation, this material has an ultrafast sodium ionic conductivity comparable to that of NASICON, with an activation energy of only 82 meV. Therefore, our results indicate that maintaining layer-like features and regulating V atoms will be important directions to improve the performance of NaV(PO)FO.
钠离子技术作为一种用于电网储能应用的低成本解决方案，正受到越来越多的研究。在已报道的钠离子电池阴极材料中，NaV(PO)FO因其高工作电压和良好的循环稳定性而被认为是最有前景的材料之一。在此，通过第一性原理计算系统地研究了NaV(PO)FO的脱钠过程，以揭示原子层面的基本问题。基于凸包首先确定了脱钠过程中的四种稳定中间产物，然后预测了三个电压平台。除了两个接近实验值的电压平台（3.37V和3.75V）外，高达5.28V的平台超出了典型电解质的稳定窗口（4.8V），这在实验中未被观察到。令人兴奋的是，在钠化过程中体积变化仅为2%，这应该是该材料具有良好循环稳定性的原因。电子结构分析还表明，在钠化过程中V离子的价态将从V变为V，导致VOF八面体中出现弱的 Jahn-Teller 畸变，进而使晶格常数不对称变化。更严重的是，结合2.0eV的带隙，主要由V-t非键轨道组成的导带最小值具有很强的局域化特征，这应该是NaV(PO)FO电子传输性能差的内在原因。尽管如此，受益于具有F分段的层状结构特征，该材料具有与NASICON相当的超快钠离子电导率，激活能仅为82meV。因此，我们的结果表明，保持层状特征和调节V原子将是提高NaV(PO)FO性能的重要方向。
