Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li₁₀Ge_1–xSn_xP₂S₁₂

Abstract

Strategies to enhance ionic conductivities in solid electrolytes typically focus on the effects of modifying their crystal structures or of tuning mobile-ion stoichiometries. A less-explored approach is to modulate the chemical bonding interactions within a material to promote fast lithium-ion diffusion. Recently, the idea of a solid-electrolyte inductive effect has been proposed, whereby changes in bonding within the solid-electrolyte host framework modify the potential energy landscape for the mobile ions, resulting in an enhanced ionic conductivity. Direct evidence for a solid-electrolyte inductive effect, however, is lacking—in part because of the challenge of quantifying changes in local bonding interactions within a solid-electrolyte host framework. Here, we consider the evidence for a solid-electrolyte inductive effect in the archetypal superionic lithium-ion conductor Li 10 Ge 1– x Sn x P 2 S 12 . Substituting Ge for Sn weakens the {Ge,Sn}–S bonding interactions and increases the charge density associated with the S 2– ions. This charge redistribution modifies the Li + substructure causing Li + ions to bind more strongly to the host framework S 2– anions, which in turn modulates the Li + ion potential energy surface, increasing local barriers for Li + ion diffusion. Each of these effects is consistent with the predictions of the solid-electrolyte inductive effect model. Density functional theory calculations predict that this inductive effect occurs even in the absence of changes to the host framework geometry due to Ge → Sn substitution. These results provide direct evidence in support of a measurable solid–electrolyte inductive effect and demonstrate its application as a practical strategy for tuning ionic conductivities in superionic lithium-ion conductors.

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

“…The set of lithium ions that define a specific coordination polyhedron (all those within r coord of the central atom) can be described by a vertex list of these ion indices, e.g. (1,3,7,20,52,100). The edge topology connecting these ions is described by an undirected edge graph, where we consider an edge formed between any two vertices of a polyhedron with a separation smaller than a threshold distance r edge .…”

“…Identifying new solid lithium-ion electrolytes is an active area of research [3], with strategies ranging from targeted chemical modification of known solid electrolytes, to improve their conductivities [7][8][9][10][11], to high-throughput screening of new materials [12][13][14][15]. In both cases, it is useful to understand why some materials are highly-conducting, yet others are not * b.j.morgan@bath.ac.uk [3,[16][17][18][19][20]. Such understanding can help inform chemical strategies for optimising the ionic conductivities of known materials, or can provide selection criteria for identifying new promising electrolytes.…”

Paradigms of frustration in superionic solid electrolytes

“…Diffusion may also be limited to one-dimensional channels, which can be easily blocked. The importance of having a percolating network of low-barrier transitions has [84,128,129] structural: destabilized energy minima in amorphous phase due to lack of crystalline order [95]; structural arrangement and local coordination symmetry of anions impacts diffusivity [130,131]; vacancy-induced site disorder and partial site occupancy induce mobility [91]; atomic substitution can tune lattice volume and relative cation site preference to maximize frustration [132,133]; ion conduction correlates with lack of cation site preference [134] dynamical: enhanced [PS 4 ] chemical: inductive effect through S interaction affects barriers [38][39][40]; substitution with O changes bonding [41] structural: site occupancies disorder above transition temperature [93]; Ge/P site disorder can increase conductivity [135]; O substitution for S changes site occupancy [41]; distorted intrinsic site symmetry, fluctuations in the coordination environment, and lack of clear site preference enhance frustration in Ti-based variant [58]; conduction pathway changes with local site volume [40] dynamical: enhanced [PS 4 ] 3− reorientations from structural or chemical modification lead to faster conductivity [89,93,94]; highly correlated migration in sulfide and oxygen-substituted variants [10,41] . [68] and by Morgan in the context of argyrodites [61].…”

Understanding fast-ion conduction in solid electrolytes

“…It is therefore useful to understand why certain materials exhibit particularly high ionic conductivities, while others, including materials that appear structurally or chemically similar, do not. Understanding the factors that promote fast-ion conduction in specific families of solid electrolytes can help direct the development of generalized 'design principles' that may then be used to design and synthesize new materials with improved ionic conductivities [3][4][5][6][7][8] or to identify completely new families of potential fast-ion conductors, through, for example, high-throughput computational screening [9][10][11][12]. The development of quantitative models of ion transport that can describe, and ideally also explain, the exceptional ionic conductivities of fastion conducting materials presents an additional intriguing challenge.…”