Two-Dimensional Substitution Series Na3P1–xSbxS4–ySey: Beyond Static Description of Structural Bottlenecks for Na+ Transport

Highly conductive solid electrolytes are fundamental for all solid-state batteries with low inner cell resistance. Such fast solid electrolytes are often found by systematic substitution experiments in which one atom is exchanged for another, and corresponding changes in ionic transport are monitored. With this strategy, compositions with the most promising transport properties can be identified fast and reliably. However, the substitution of one element does not only influence the crystal structure and diffusion channel size (static) but also the underlying bonding interactions and with it the vibrational properties of the lattice (dynamic). Since both static and dynamic properties influence the diffusion process, simple one-dimensional substitution series only provide limited insights to the importance of changes in the structure and lattice dynamics for the transport properties. To overcome these limitations, we make use of a two-dimensional substitution approach, investigating and comparing the four single-substitution series Na3P1–xSbxS4, Na3P1–xSbxSe4, Na3PS4–ySey, and Na3SbS4–ySey. Specifically, we find that the diffusion channel size represented by the distance between S/Se ions cannot explain the observed changes of activation barriers throughout the whole substitution system. Melting temperatures and the herein newly defined anharmonic bulk modulusas descriptors for bonding interactions and corresponding lattice dynamicscorrelate well with the activation barriers, highlighting the relevance of lattice softness for the ion transport in this class of fast ion conductors.高导电固体电解质是构建低内阻全固态电池的核心基础材料。此类快离子固体电解质通常通过系统性取代实验筛选获得：实验中以一种原子替换另一种原子，并实时监测离子输运特性的相应变化。借助该策略，可快速且可靠地筛选出具备最优输运性能的材料组分。然而，单一元素的取代不仅会改变晶体结构与离子扩散通道尺寸（静态特性），还会影响体系内的键合相互作用，进而改变晶格的振动特性（动态特性）。由于静态与动态特性均会影响离子扩散过程，仅依靠一维取代序列的研究，难以全面揭示结构与晶格动力学变化对输运性能的影响机制。为克服上述局限，本研究采用二维取代策略，对四种单取代体系Na₃P₁₋ₓSbₓS₄、Na₃P₁₋ₓSbₓSe₄、Na₃PS₄₋ᵧSeᵧ与Na₃SbS₄₋ᵧSeᵧ展开研究并进行对比分析。具体而言，本研究发现：以S/Se离子间距表征的扩散通道尺寸，无法解释整个取代体系中活化能垒的观测变化。熔融温度与本文首次定义的非谐体积模量（anharmonic bulk modulus）——作为键合相互作用与对应晶格动力学的描述符——与活化能垒呈现良好的相关性，凸显了晶格柔软性在该类快离子导体离子输运过程中的重要作用。