论文标题
NAMNO $ _2 $的Jahn-Teller失真和相位稳定性的声子研究
Phonon study of Jahn-Teller distortion and phase stability in NaMnO$_2$ for sodium-ion batteries
论文作者
论文摘要
阴极材料在充电/放电过程中经历了各种相变，并且结构过渡会严重影响电池性能。尽管声子特性可以为结构稳定性和过渡提供直接的线索，但在钠阴极材料中探索了它。在这里，使用第一原理计算，我们研究了各种分层namno $ _2 $材料的声子和电子属性，尤其是专注于Mn $^{3+} $的Jahn-Teller扭曲的依赖性。声子分散曲线表明，带Jahn-Teller失真的O $'$ 3和p $'$ 2结构与未呈现的O3和P2结构相比，动态稳定。 O3和P2结构的结构不稳定性是从假想的声子频率中直接观察到的，作为所谓的声子软模式，其相应的位移来自沿Mno $ _6 $ _6 $ octoctra的局部Mn-O键沿局部Mn-O键方向扭曲的O原子。这与高NA浓度下的Jahn-Teller失真的实验稳定性和结构过渡一致。此外，状态的轨道构成密度呈现出Jahn-Teller失真的轨道再分配，例如$ e_g $ band拆分，o $ $ 3和p $'$ 2的稳定性对电子 - 电子相关性不敏感。我们的结果表明，声子分析对于进一步了解阴极材料中的结构稳定性和相变的重要性。
Cathode materials undergo various phase transitions during the charge/discharge process, and the structural transitions significantly affect the battery performance. Although phonon properties can provide a direct clue for structural stability and transitions, it has been less explored in sodium cathode materials. Here, using the first-principles calculations, we investigate phonon and electronic properties of various layered NaMnO$_2$ materials, especially focusing on the dependency of the Jahn-Teller distortion of Mn$^{3+}$. The phonon dispersion curves show that the O$'$3 and P$'$2 structures with the Jahn-Teller distortion are dynamically stable in contrast to undistorted O3 and P2 structures. The structural instability of O3 and P2 structures is directly observed from the imaginary phonon frequencies, as so-called phonon soft modes, whose corresponding displacements are from O atoms distorting along the local Mn-O bond direction in the MnO$_6$ octahedra. This is consistent with the experimental stability and a structural transition with the Jahn-Teller distortion at the high Na concentration. Furthermore, the orbital-decomposed density of states presents the orbital redistribution by the Jahn-Teller distortion such as $e_g$-band splitting, and the stability of O$'$3 and P$'$2 is not sensitive to the electron-electron correlation. Our results demonstrate the importance of phonon analysis to further understand the structural stability and phase transitions in cathode materials.
