Mechanistic Origin of Superionic Lithium Diffusion in Anion-Disordered Li₆PS₅X Argyrodites

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 Li 6 PS 5 X (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 3 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 Li 6 PS 5 I and Li 6 PS 5 Cl 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 6 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.

A Versatile Li6.5In0.25P0.75S5I Sulfide Electrolyte Triggered by Ultimate‐Energy Mechanical Alloying for All‐Solid‐State Lithium Metal Batteries

“…However, many previous reports have demonstrated the existence of Li 6 PS 5 X (X = Cl, Br and I) electrolytes at room temperature. [ 41–43 ] This indicates that the decomposition energy is relatively small and can be stabilized by the kinetic barrier at room temperature, thus forming metastable Li 6 PS 5 X (X = Cl, Br and I) electrolytes. Furthermore, according to the DFT calculations, the incorporation of indium does not lead to a significant increase in the decomposition energy of Li 6 PS 5 I, illustrating that the In‐doped Li 6 PS 5 I electrolyte can be prepared potentially.…”

Understanding fast-ion conduction in solid electrolytes

“…In many fast-ion conductors, however, iontransport proceeds via complex transport mechanisms. Ions may diffuse via highly concerted mechanisms in which groups of mobile ions undergo cooperative near-synchronous motion [16][17][18][19][20][21], or there may be strong coupling between the dynamics of the host-framework atoms and that of the mobile ions, with ion diffusion promoted by specific 'breathing' motions or by polyhedral rotations within the host framework [22][23][24]. The occurrence of complex conduction mechanisms in fast-ion conductors makes the analysis of experimental data and the development of explanatory or predictive theoretical models particularly challenging, especially in those materials where the microscopic mechanisms of ion transport have not yet been well characterized.…”

“…Within this context, there have been a number of recent studies that have presented new perspectives on the mechanistic origin of fast-ion conduction, or that have highlighted various concepts that can help to explain fast-ion conduction in specific materials. These factors include the role of various forms of inherent crystallographic disorder within the host-framework substructure [18,[26][27][28] and the presence of occupational disorder of the mobile-ion species across available sites [29][30][31]. Other studies have analysed fast-ion conduction by considering ways in which a given system may be 'frustrated', where competing energetic considerations prevent the mobile ions from adopting a single ordered low-energy configuration, with these mobile ions instead sampling a large number of disordered configurations with similar energies [18,26,[32][33][34][35].…”

“…These factors include the role of various forms of inherent crystallographic disorder within the host-framework substructure [18,[26][27][28] and the presence of occupational disorder of the mobile-ion species across available sites [29][30][31]. Other studies have analysed fast-ion conduction by considering ways in which a given system may be 'frustrated', where competing energetic considerations prevent the mobile ions from adopting a single ordered low-energy configuration, with these mobile ions instead sampling a large number of disordered configurations with similar energies [18,26,[32][33][34][35]. In other materials, fast-ion conduction has been associated with specific hostframework dynamics, such as dynamical 'breathing' of diffusion-limiting bottlenecks [22], or polyanion rotations that may directly couple to the motion of the charge-carrying ions [24,[36][37][38].…”

Aliovalent doping response and impact on ionic conductivity in the antiperovskite solid electrolyte Li3OCl

“…A practical solid electrolyte should have a high ionic conductivity to allow fast charge and discharge rates and high power output. To achieve this goal, considerable research effort has been directed towards both the discovery of new materials with intrinsically high ionic mobilities [3][4][5][6] and to deriving general principles that can explain how chemical or structural variations within solid electrolytes modulate lithium transport [7][8][9][10].…”