Solid-state ionic conductors are of primary importance for the design of tomorrow’s batteries. In lithium- or sodium-ion-based materials, the alkali cations diffuse through three-dimensional channels consisting of interconnected tetrahedral or octahedral sites with low free energy barriers between them. Fluoride ion conductors stand out in this landscape since the materials with the highest conductivities belong to the MSnF₄ family (in which M²⁺ is a divalent cation), whose structure is layered and characterized by double-layers of Sn²⁺ and M²⁺ cations along a given direction. Importantly, these materials display stereoactive electron lone pairs (LPs) that seemingly play an important role not only in stabilizing the Sn–Sn layer but also in modulating the fluoride ion diffusive behavior. However, despite previous experimental and simulation studies, the involvement of the LPs in the fluoride ion conduction mechanism remains to be quantitatively understood. In this work, we simulate the BaSnF₄ tetragonal structure using machine learning-based molecular dynamics, in which the interaction potential is trained on density functional theory data. We investigated the role of the Sn–LP–Sn layer in lowering the diffusion energy landscape. In particular, we show how the F^– ions jump across this layer and occur much more frequently than in the Ba–F–Ba one, resulting in the formation of vacancies in the Ba–Sn layers. Concurrently, the LP stereochemical activity fluctuates to accommodate the F ions jumping. In addition, the presence of the LP layer enhances the flexibility of the Sn ions, which leads to an increase in two-dimensional diffusion by several orders of magnitude. These results contribute to our understanding of the interplay between LPs and ionic diffusion, helping to explain the good performance of the material in fluoride-ion batteries.

中文翻译：

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立体电子孤对电子促进氟离子在四方 BaSnF4 中的扩散


固态离子导体对于未来电池的设计至关重要。在锂离子或钠离子基材料中，碱阳离子通过三维通道扩散，该通道由相互连接的四面体或八面体位点组成，它们之间具有低自由能势垒。氟离子导体在这种景观中脱颖而出，因为具有最高电导率的材料属于 MSnF₄ 家族（其中 M²⁺ 是二价阳离子），其结构是分层的，其特征是沿给定方向的双层 Sn²⁺ 和 M²⁺ 阳离子。重要的是，这些材料显示出立体活性电子孤对电子 （LP），它们似乎不仅在稳定 Sn-Sn 层方面发挥着重要作用，而且在调节氟离子扩散行为方面也发挥着重要作用。然而，尽管之前进行了实验和模拟研究，但 LP 在氟离子传导机制中的参与仍有待定量理解。在这项工作中，我们使用基于机器学习的分子动力学模拟了 BaSnF₄ 四方结构，其中相互作用势是在密度泛函理论数据上训练的。我们研究了 Sn-LP-Sn 层在降低扩散能景观中的作用。特别是，我们展示了 F^– 离子如何跳过该层，并且比 Ba-F-Ba 层更频繁地发生，从而导致在 Ba-Sn 层中形成空位。同时，LP 立体化学活性波动以适应 F 离子跳跃。此外，LP 层的存在增强了 Sn 离子的柔韧性，这导致二维扩散增加了几个数量级。 这些结果有助于我们理解 LP 和离子扩散之间的相互作用，有助于解释该材料在氟离子电池中的良好性能。