Site Disorder Drives Cyanide Dynamics and Fast Ion Transport in Li₆PS₅CN

Halide argyrodite solid-state electrolytes of the general formula Li6PS5X exhibit complex static and dynamic disorder that plays a crucial role in ion transport processes. Here, we unravel the rich interplay between site disorder and dynamics in the plastic crystal argyrodite Li6PS5CN and the impact on ion diffusion processes through a suite of experimental and computational methodologies, including temperature-dependent synchrotron powder X-ray diffraction, AC electrochemical impedance spectroscopy, 7Li solid-state NMR, and machine learning-assisted molecular dynamics simulations. Sulfide and (pseudo)halide site disorder between the two anion sublattices unilaterally improves long-range lithium diffusion irrespective of the (pseudo)halide identity, which demonstrates the importance of site disorder in dictating bulk ionic conductivity in the argyrodite family. Furthermore, we find that anion site disorder modulates the presence and time scales of cyanide rotational dynamics. Ordered configurations of anions enable fast, quasi-free rotations of cyanides that occur on time scales of 1011 Hz at T = 300 K. In contrast, we find that cyanide dynamics are slow or frozen in Li6PS5CN when site disorder between the cyanide and sulfide sublattices is present at T = 300 K. We rationalize the observed differences in cyanide dynamics in the context of elastic dipole interactions between neighboring cyanide anions and local strain induced by the configurations of site disorder that may impact the energetic landscape for cyanide rotational dynamics. Through this study, we find that anion disorder plays a decisive role in dictating the extent and time scales of both lithium ion and cyanide dynamics in Li6PS5CN.卤化物银锗矿（argyrodite）固态电解质的通式为Li6PS5X，这类材料展现出复杂的静态与动态无序，其在离子输运过程中发挥关键作用。本研究针对塑晶态银锗矿Li6PS5CN，通过一系列实验与计算表征手段厘清了位点无序与动力学过程间的复杂相互作用，及其对离子扩散过程的影响；所采用的手段包括变温同步辐射粉末X射线衍射（synchrotron powder X-ray diffraction）、交流电化学阻抗谱（AC electrochemical impedance spectroscopy）、固态7Li核磁共振（7Li solid-state NMR），以及机器学习辅助分子动力学模拟（machine learning-assisted molecular dynamics simulations）。研究发现，两个阴离子亚晶格间的硫离子与（伪）卤离子位点无序，无论（伪）卤离子的种类如何，均能单向提升长程锂离子扩散性能，这证明了位点无序在决定银锗矿型电解质体系体相离子电导率中的重要性。此外，我们发现阴离子位点无序可调控氰根旋转动力学的存在性与时间尺度：阴离子有序排布可使氰根实现快速准自由旋转，该过程在300 K下的时间尺度可达10^11 Hz；与之相反，当300 K下氰根与硫离子亚晶格间存在位点无序时，Li6PS5CN中的氰根动力学过程会变得缓慢甚至被冻结。我们通过相邻氰根阴离子间的弹性偶极相互作用，以及位点无序构型所诱导的局域应变（该应变可影响氰根旋转动力学的势能环境），对观测到的氰根动力学差异进行了合理化阐释。本研究表明，阴离子无序在决定Li6PS5CN中锂离子与氰根动力学的程度与时间尺度方面发挥决定性作用。