Morgan Benjamin J
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
The Faraday Institution, Quad One, Harwell Science and Innovation Campus, Didcot OX11 0RA, U.K.
Chem Mater. 2021 Mar 23;33(6):2004-2018. doi: 10.1021/acs.chemmater.0c03738. Epub 2021 Mar 3.
The rational development of fast-ion-conducting solid electrolytes for all-solid-state lithium-ion batteries requires understanding the key structural and chemical principles that give some materials their exceptional ionic conductivities. For the lithium argyrodites LiPSX (X = Cl, Br, or I), the choice of the halide, X, strongly affects the ionic conductivity, giving room-temperature ionic conductivities for X = {Cl,Br} that are ×10 higher than for X = I. This variation has been attributed to differing degrees of S/X anion disorder. For X = {Cl,Br}, the S/X anions are substitutionally disordered, while for X = I, the anion substructure is fully ordered. To better understand the role of substitutional anion disorder in enabling fast lithium-ion transport, we have performed a first-principles molecular dynamics study of LiPSI and LiPSCl with varying amounts of S/X anion-site disorder. By considering the S/X anions as a tetrahedrally close-packed substructure, we identify three partially occupied lithium sites that define a contiguous three-dimensional network of face-sharing tetrahedra. The active lithium-ion diffusion pathways within this network are found to depend on the S/X anion configuration. For anion-disordered systems, the active site-site pathways give a percolating three-dimensional diffusion network; whereas for anion-ordered systems, critical site-site pathways are inactive, giving a disconnected diffusion network with lithium motion restricted to local orbits around S positions. Analysis of the lithium substructure and dynamics in terms of the lithium coordination around each sulfur site highlights a mechanistic link between substitutional anion disorder and lithium disorder. In anion-ordered systems, the lithium ions are pseudo-ordered, with preferential 6-fold coordination of sulfur sites. Long-ranged lithium diffusion would disrupt this SLi pseudo-ordering, and is, therefore, disfavored. In anion-disordered systems, the pseudo-ordered 6-fold S-Li coordination is frustrated because of Li-Li Coulombic repulsion. Lithium positions become disordered, giving a range of S-Li coordination environments. Long-ranged lithium diffusion is now possible with no net change in S-Li coordination numbers. This gives rise to superionic lithium transport in the anion-disordered systems, effected by a concerted string-like diffusion mechanism.
全固态锂离子电池快速离子传导固体电解质的合理开发需要理解一些材料具有优异离子电导率的关键结构和化学原理。对于锂辉石LiPSX（X = Cl、Br或I），卤化物X的选择强烈影响离子电导率，使得X = {Cl,Br}时的室温离子电导率比X = I时高×10倍。这种变化归因于S/X阴离子无序程度的不同。对于X = {Cl,Br}，S/X阴离子发生替代无序，而对于X = I，阴离子亚结构是完全有序的。为了更好地理解替代阴离子无序在实现快速锂离子传输中的作用，我们对具有不同S/X阴离子位点无序量的LiPSI和LiPSCl进行了第一性原理分子动力学研究。通过将S/X阴离子视为四面体密堆积亚结构，我们确定了三个部分占据的锂位点，它们定义了一个由共面四面体组成的连续三维网络。发现该网络内的活性锂离子扩散途径取决于S/X阴离子构型。对于阴离子无序系统，活性位点 - 位点途径给出一个渗流三维扩散网络；而对于阴离子有序系统，关键的位点 - 位点途径是无活性的，给出一个断开的扩散网络，其中锂的运动限于S位置周围的局部轨道。根据每个硫位点周围的锂配位情况对锂亚结构和动力学进行分析，突出了替代阴离子无序与锂无序之间的机制联系。在阴离子有序系统中，锂离子是准有序的，硫位点优先形成6配位。长程锂扩散会破坏这种SLi准有序，因此是不利的。在阴离子无序系统中，由于Li - Li库仑排斥，准有序的6配位S - Li受到阻碍。锂位置变得无序，产生一系列S - Li配位环境。现在长程锂扩散成为可能，而S - Li配位数没有净变化。这导致阴离子无序系统中的超离子锂传输，由协同的线状扩散机制实现。
