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Benjamin Morgan1

1, Univ of Bath, Bath, , United Kingdom

Halide-substituted lithium-argyrodites, Li6PS5X (X=Cl, Br, I) are a promising family of lithium-ion solid electrolytes, with potential applications in all-solid-state lithium-ion batteries. Changing X from I to Cl produces a strong increase in lithium-ion conductivity, which has been attributed to increased crystallographic disorder for X and S ions across 4a and 4c sites. A previous molecular dynamics study [1] has predicted that efficient long-ranged Li-ion transport only occurs for such X/S disordered systems, with this attributed to changes in the rates of lithium-ion jumps between lattice sites. A microscopic explanation for this change in lithium-ion dynamics, however, is lacking.

To study this behaviour, we have performed a series of ab initio molecular dynamics simulations of ordered and disordered Li6PS5X. In contrast to previous computational studies, we have analysed the lithium-ion dynamics in terms of transitions between local potential minima (inherent structures) in lithium-ion configuration space, which allows us to resolve non-trivial lithium motion [3].

We find that in fully ordered Li6PS5X, the lithium ions arrange in six-coordinate octahedra around the (4a/4c) S anions. Interestingly, this is the case for X ordered over the 4a sites and ordered over the 4c sites, indicating a strong preference for separated (4a/4c) S ions at both sites to be octahedrally coordinated by Li. Lithium motion consists of concerted processes within individual octahedra; specifically, octahedral rotations and internal reorganisations via trigonal prismatic configurations; neither of which give long ranged lithium diffusion.

For anion-disordered Li6PS5X, this preferred octahedral coordination is disrupted. We observe a number of LixS coordination environments with x ≠ 6, and 6-fold coordination environments increasingly deviate from ideal octahedral symmetry. This increased Li-coordination disorder facilitates long-ranged Li-ion diffusion between LixS polyhedra. We note that for pairs of S ions in adjacent 4a / 4c sites, it is not possible for both S ions to simultaneously achieve ideal octahedral coordination. We therefore propose that the capacity for long-ranged lithium transport in anion-disordered Li6PS5X arises from geometric frustration of preferred octahedral lithium configurations, which produces a highly disordered network of lithium-ion polyhedra, and enables fast lithium-ion diffusion.

[1] Rao et al. Sol. Stat. Ionics 2013, 230, 72.
[2] de Klerk et al. Chem. Mater. 2016, 28, 7955.
[3] Stillinger and Weber, Science 1984, 225, 983.

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