We perform static and dynamic ab initio simulations to investigate the structural and the thermodynamic properties of Li₆PS₅Cl, a solid electrolyte actively considered for solid-state batteries. Our simulations account for the disorder in the structure where the Li atoms can rotate either around sulfur or chlorine atoms. Li₆PS₅Cl presents a non-uniform distribution of Li ions around S and Cl atoms, which tends to become more homogeneous at higher temperature. This specific short-range order of Li has a significant impact on the stability of Li₆PS₅Cl. Comparing with recent X-ray and neutron diffraction studies, we confirm one Li crystallographic site position (Li1) and amend the coordinates of a second one (Li2). We then address the calculation of the heat capacity C_p with a combination of ab initio trajectories and a so-called temperature remapping approximation. Indeed, the standard quasi-harmonic approximation is not able to capture the complex energy landscape experienced by the mobile lithium atoms. To the best of our knowledge, there exists no experimental or theoretical C_p value for Li₆PS₅Cl in the literature, despite the importance of this thermodynamic quantity. Finally we use this more reliable C_p to investigate the thermodynamic stability of Li₆PS₅Cl against the decomposition reaction leading to Li₂S, Li₃PS₄ and LiCl. We show that Li₆PS₅Cl is stable above 700 K, which is consistent with the high synthesis temperatures.

中文翻译：

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使用第一性原理计算的 Li6PS5Cl 固体电解质的结构和热力学性质


我们执行静态和动态 ab initio 模拟，以研究 Li 6 PS 5 Cl 的结构和热力学特性，Li₆PS₅Cl 是一种积极考虑用于固态电池的固体电解质。我们的模拟解释了结构中的无序，其中 Li 原子可以围绕硫原子或氯原子旋转。Li₆PS₅Cl 在 S 和 Cl 原子周围呈现出不均匀的锂离子分布，在较高温度下趋于更加均匀。Li的这种特定的短程有序对Li₆PS₅Cl的稳定性有重大影响。与最近的X射线和中子衍射研究相比，我们确认了一个Li晶体位点位置（Li1）并修正了第二个位置（Li2）的坐标。然后，我们结合从头计算轨迹和所谓的温度重映射近似值来计算热容 C_。事实上，标准的准谐波近似无法捕捉移动锂原子所经历的复杂能量景观。据我们所知，尽管这个热力学量很重要，但文献中没有 Li₆PS₅Cl 的实验或理论 C_p 值。 最后，我们利用这种更可靠的 C_p 来研究 Li₆PS₅Cl 对导致 Li₂S、Li₃PS₄ 和 LiCl 的分解反应的热力学稳定性。结果表明，Li₆PS₅Cl 在 700 K 以上是稳定的，这与高合成温度一致。