Lithium ion conduction in lithium magnesium thio-phosphate

Synthesis and Characterization of Three New Lithium‐Scandium Hexathiohypodiphosphates: Li4–3xScxP2S6 (x = 0.358), m‐LiScP2S6, and t‐LiScP2S6

“…The P atoms reside mostly in the layer containing Sc, while the layer containing lithium mostly contains vacancies on the respective P position. This distribution of the P 2 dumbells is reflected by the site occupation factors of SOF(P1) = 0.925 and SOF(P2) = 1 – SOF(P1), which agrees well with the site distribution found in Li 4–2 x Mg x P 2 S 6 ( x = 2/3) …”

“…These bond lengths are in good agreement with the isostructural Li 2 FeP 2 S 6 and other related A ScP 2 S 6 compounds ( A = Ag, Na) , . The Li‐ion conductivity of Li 4 P 2 S 6 , was previously measured to be very low (see Table ) but it increases with increasing magnesium content x in the range of x ≤ 2/3 (see Table ); beyond that value it decreases again . Therefore, we suspected that Li 4–3 x Sc x P 2 S 6 could show enhanced Li ion conductivity, compared to the parent compound Li 4 P 2 S 6 .…”

“…The lithium ion motion in the parent crystalline compound Li 4 P 2 S 6 was reported to be four orders of magnitude lower (1.6 × 10 –10 S · cm –1 ). Partial substitution of Li + ions by Mg 2+ ions in Li 4–2 x Mg x P 2 S 6 , increases the ionic conductivity by several orders of magnitude (1 × 10 –6 S · cm –1 ), similarly to the conductivity of Li 4–3 x Sc x P 2 S 6 . Probing electronic conductivity by dc polarization experiments was not possible due to high signal‐to‐noise ratios at low excitation voltages.…”

“…The refined structure is very similar to the structure of Li 4 P 2 S 6 , but Sc prefers to occupy a single site (Li1), thus leading to an ordered variant of Li 4 P 2 S 6 with lower symmetry. The compound is isostructural to Li 2 FeP 2 S 6 and to the Li ion conductor Li 4–2 x Mg x P 2 S 6 . It is composed of sulfur atoms that are arranged in a hcp packing, where Li, Sc, and P 2 dumbbells occupy the octahedral holes.…”

“…Other examples (see Table ) include the ternary compounds Li 3 PS 4 , Li 2 P 2 S 6 , Li 4 P 2 S 6 ,, argyrodite Li 7 PS 6 and Li 7 P 3 S 11 ,, or the quaternary compound Li 4 PS 4 I . Further interesting thiophosphate based ion conductors are Li 4–2 x Mg x P 2 S 6 ( x = 1/3, 2/3), the argyrodite‐type phases Li 6 PS 5 X ( X = Cl, Br, I),, and tetragonal Li 10 GeP 2 S 12 (LGPS), along with its derivatives, including Li 11 Si 2 PS 12 , Li 10 SnP 2 S 12 , and Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 , which are closely related to the thio‐LISICON (LIthium SuperIonic CONductor) family of compounds. LGPS‐type materials are currently the record holders in terms of Li + ion conductivity in the solid state with up to 1.6 × 10 –2 S · cm –1 , …”