García Daza Fabián A, Bonilla Mauricio R, Llordés Anna, Carrasco Javier, Akhmatskaya Elena
BCAM-Basque Center for Applied Mathematics , Alameda de Mazarredo 14 , E-48009 Bilbao , Spain.
CIC EnergiGUNE , Albert Einstein 48 , E-01510 Miñano , Spain.
ACS Appl Mater Interfaces. 2019 Jan 9;11(1):753-765. doi: 10.1021/acsami.8b17217. Epub 2018 Dec 24.
Garnet-structured LiLaZrO is a promising solid electrolyte for next-generation solid-state Li batteries. However, sufficiently fast Li-ion mobility required for battery applications only emerges at high temperatures, upon a phase transition to cubic structure. A well-known strategy to stabilize the cubic phase at room temperature relies on aliovalent substitution; in particular, the substitution of Li by Al and Ga ions. Yet, despite having the same formal charge, Ga substitution yields higher conductivities (10 S/cm) than Al (10 S/cm). The reason of such difference in ionic conductivity remains a mystery. Here, we use molecular dynamic simulations and advanced sampling techniques to precisely unveil the atomistic origin of this phenomenon. Our results show that Li vacancies generated by Al and Ga substitution remain adjacent to Ga and Al ions, without contributing to the promotion of Li mobility. However, while Ga ions tend to allow limited Li diffusion within their immediate surroundings, the less repulsive interactions associated with Al ions lead to a complete blockage of neighboring Li diffusion paths. This effect is magnified at lower temperatures and explains the higher conductivities observed for Ga-substituted systems. Overall, this study provides a valuable insight into the fundamental ion transport mechanism in the bulk of Ga/Al-substituted LiLaZrO and paves the way for rationalizing aliovalent substitution design strategies for enhancing ionic transport in these materials.
石榴石结构的LiLaZrO是下一代固态锂电池中一种很有前景的固体电解质。然而，电池应用所需的足够快的锂离子迁移率仅在高温下向立方结构的相变时才会出现。一种在室温下稳定立方相的著名策略依赖于异价取代；特别是用铝离子和镓离子取代锂离子。然而，尽管铝离子和镓离子具有相同的形式电荷，但用镓取代产生的电导率（10 S/cm）高于用铝取代（10 S/cm）。这种离子电导率差异的原因仍然是个谜。在这里，我们使用分子动力学模拟和先进的采样技术来精确揭示这一现象的原子起源。我们的结果表明，由铝和镓取代产生的锂空位与镓和铝离子相邻，无助于促进锂的迁移率。然而，虽然镓离子倾向于允许锂在其紧邻环境中有限地扩散，但与铝离子相关的排斥力较小导致相邻锂扩散路径完全受阻。这种效应在较低温度下会放大，并解释了在镓取代体系中观察到的较高电导率。总体而言，这项研究为深入了解镓/铝取代的LiLaZrO本体中的基本离子传输机制提供了有价值的见解，并为合理化这些材料中增强离子传输的异价取代设计策略铺平了道路。
