Halogenated
argyrodites Li6PS5Br, Li6PS5Cl, and Li6PS5I exhibit
large differences in the measured Li ionic conductivities. Crystallographic
analysis has shown that these differences may be related to occupations
of specific Wyckoff sites in different argyrodite types, but detailed
understanding of the relationship between the atomic structure and
operating diffusion mechanisms is still lacking. In this work, we
employed ab initio molecular dynamics simulations to calculate the
Li diffusivity for different argyrodite structure types. Our calculations
show that the Li diffusivity does not depend implicitly on the type
of halogen but is rather governed by the degree of structural disorder.
Assuming disordered structures to arise naturally from the ordered
structure type by thermally activated antisite defects, we are able
to explain the degree of disorder found for the different types of
halogens from the calculated defect formation energies. Comparing
the calculated formation energies to the ionic radii of the halogen
atoms, we find a strong correlation between the radii and energies
required for introducing the antisite defects.
A new diffusion path comprising the concerted migration of lithium ions is proposed for Li4P2S6. Detailed analysis of the underlying migration process reveals a significantly reduced energy barrier compared to purely interstitial migration.
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