Gryaznov Denis, Stauffer Shannon K, Kotomin Eugene A, Vilčiauskas Linas
Center for Physical Sciences and Technology (FTMC), Saulėtekio al. 3, LT-10257 Vilnius, Lithuania.
Phys Chem Chem Phys. 2020 Jun 7;22(21):11861-11870. doi: 10.1039/d0cp00772b. Epub 2020 May 20.
Sodium Super Ionic Conductor (NASICON) structured phosphate framework compounds represent a very attractive class of materials for their use as Na-ion battery electrodes. A series of NASICON-structured NaTi(PO) compounds corresponding to varying degrees of sodiation (x = 1-4) have been investigated using high-level hybrid density functional theory calculations using the Linear Combination of Atomic Orbitals and Gaussian-type basis set formalism together with hybrid B1WC and HSE06 exchange-correlation functionals. Using primitive cells of NaTi(PO) compounds with different stoichiometry, sodium sublattice structure and titanium oxidation states are constructed and analyzed using group theoretical symmetry considerations. The existence of mixed titanium oxidation states for x = 4 (Ti/Ti) and x = 2 (Ti/Ti) and a single oxidation state for x = 1 (Ti) and x = 3 (Ti) has been demonstrated. The results show a necessary set of symmetry reductions taking place due to the highest possible sodium/vacancy and titanium charge ordering with changing x. For each composition, an electroneutrality condition for the oxidation states of all atoms was applied which led to the discovery of several energy minima corresponding to different electronic configurations as identified by different Ti magnetic moments. An interesting relation between the bulk electronic properties of NaTi(PO) compounds and the variation of sodium content was also found. In addition to sodium and titanium oxidation state charge ordering, the existence of large differences between the origin and the size of the band gap is shown. The band gap changes from the 4.05 eV 2p-3d gap in NaTi(PO) to the 0.59 eV 3d-3d gap in NaTi(PO) with extra states due to mixed titanium valence. These results serve as an important electronic structure benchmark for further studies of such polyanion materials and help to explain some important properties of these systems relevant to battery applications.
钠超离子导体（NASICON）结构的磷酸盐骨架化合物因其可用作钠离子电池电极而成为一类极具吸引力的材料。使用原子轨道线性组合和高斯型基组形式主义以及混合B1WC和HSE06交换相关泛函，通过高水平混合密度泛函理论计算研究了一系列对应于不同程度钠化（x = 1 - 4）的NASICON结构的NaTi(PO)化合物。使用具有不同化学计量比的NaTi(PO)化合物的原胞，利用群论对称性考虑构建并分析了钠亚晶格结构和钛氧化态。已证明对于x = 4（Ti/Ti）和x = 2（Ti/Ti）存在混合钛氧化态，而对于x = 1（Ti）和x = 3（Ti）存在单一氧化态。结果表明，随着x的变化，由于钠/空位和钛电荷的最高可能有序排列，会发生一组必要的对称性降低。对于每种组成，应用了所有原子氧化态的电中性条件，这导致发现了几个对应于不同电子构型的能量最小值，这些电子构型由不同的Ti磁矩确定。还发现了NaTi(PO)化合物的体电子性质与钠含量变化之间的有趣关系。除了钠和钛氧化态电荷有序排列外，还显示了带隙起源和大小之间存在很大差异。带隙从NaTi(PO)中的4.05 eV 2p - 3d带隙变化到NaTi(PO)中的0.59 eV 3d - 3d带隙，由于混合钛价态存在额外的态。这些结果为进一步研究此类聚阴离子材料提供了重要的电子结构基准，并有助于解释这些系统与电池应用相关的一些重要性质。
