Mechanistic Origin of Superionic Lithium Diffusion in Anion-Disordered Li6PS5 X Argyrodites.

Benjamin J Morgan

doi:10.1021/acs.chemmater.0c03738

Abstract

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 Li6PS5X (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 ×103 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 Li6PS5I and Li6PS5Cl 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 SLi6 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.

Highlights

Lithium-ion-conducting solid electrolytes are considered candidate materials for use in future all-solid-state lithium-ion batteries.[1−3] Present-day commercial lithium-ion batteries use liquid-organic electrolytes; these are flammable, raising safety issues, and have narrow electrochemical stability windows, preventing their use with energy-dense high-voltage electrodes
The rate at which individual lithium ions diffuse through a solid electrolyte is described by the lithium self-diffusion coefficient, which can be calculated from molecular dynamics simulations as the slope of the lithium mean-squared displacement (MSD) versus time, in the long time limit.[72]
The MSDs of the anion-disordered systems (50% site inversion) show qualitatively different behavior: these MSDs continually increase at long times, corresponding to nonzero diffusion coefficients and long-ranged lithium diffusion

Summary

Introduction

Lithium-ion-conducting solid electrolytes are considered candidate materials for use in future all-solid-state lithium-ion batteries.[1−3] Present-day commercial lithium-ion batteries use liquid-organic electrolytes; these are flammable, raising safety issues, and have narrow electrochemical stability windows, preventing their use with energy-dense high-voltage electrodes. One family of promising lithium-ion solid-electrolytes are the lithium argyrodites Li6PS5X (X = Cl, Br, or I).[11,21−25] While cLcooi6nnPddSuu5ccCttiilvveiatnie(dsσ(RLσTiR6PT≈S≈51B10r0−−6e3xShSibccmimt −−h11)i)g,.h2L6i,6r2Po7SoT5mIh-itesecmloapnrgeseirdaetduriarffebelryieolnnecsiecs between X = {Cl,Br} and X = I is notable because these three materials have topologically identical crystal structures, suggesting the same lithium-ion diffusion pathways should exist in each system. This inverse correlation between anion size and ionic conductivity runs counter to the trend seen in other families of solid electrolytes, for example, thio-LISICON and NASICON, in which larger, more polarizable, lesselectronegative anions are associated with increased ionic conductivities[2] with this relationship often attributed to a combination of larger anions giving an increased accessible volume for the diffusing lithium ions and weaker lithium−anion electrostatic interactions

Methods

Results

Conclusion