Ionic transport in the Ge-substituted argyrodite-type materials Li_6+xGe_xP_1–xS₅I (x = 0–0.6) as investigated by ⁷Li and ³¹P solid-state NMR spectroscopy and neutron powder diffraction

Argyrodite-type materials show very promising properties as solid electrolytes. Li6PS5X (X = Cl, Br, I) has attracted considerable interest and many groups tried to improve the already high ionic conductivity. Recent attempts explored the substitution of P by Ge, which allowed a higher Li-content in Li6PS5I.1-2 Substitution resulted in increased ionic conductivity, however, the origin of the faster ionic transport is not yet fully elucidated. Here, we investigated ion dynamics in the substitution series Li6+xGexP1–xS5I (x = 0–0.6) by means of neutron powder diffraction and solid-state NMR. Rietveld analysis of the diffraction data revealed the occupation of new Li-sites. Additional to the sites forming cage-like Li-structures in Li6PS5I, samples with a Ge-content of 30at% and 60at% accommodate also type 2 Li sites close to the Li cages and type 4 sites, placed in between the Li cages. The introduced site disorder is also directly sensed by high resolution 31P MAS NMR; already for 10at% Ge five different chemical environments are revealed with increasing disorder for higher levels of substitution. 7Li NMR spin-lattice relaxation measurements point to correlated motion and allow the distinction of two different Li-jump processes. The low-temperature flank of the 1/T1(1/T) rate peak is characterized by an activation energy of approximately 0.1 eV and indicates short-ranged motion. It likely represents the intercage jumps of the Li+ ions. Complementary, spin-lock NMR experiments, being sensitive to slower dynamic processes, reveal a second jump process characterized by a higher barrier, which we assign to exchange processes connecting the Li-rich cages. The corresponding rate peak is also observed for the unsubstituted Li6PS5I, however, shifted toward significantly higher temperatures. As revealed by NMR, Ge substitution clearly facilitates intercage ion dynamics that finally enable the ions to move over long distances in argyrodite-type Li6+xGexP1–xS5I.
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