Synthesis and Characterization of a New Lithium Oxythiophosphate Ionic Conductor

Solid electrolytes (SEs) have attracted significant attention for developing all-solid-state batteries (ASSBs), which could enable the use of Li metal as the anode, thus improving both energy density (>1500 Wh/L) and safety¹. Extensive research has been focused on developing new SE materials, leading to the discovery of a new lithium superionic conductor, Li₁₀GeP₂S₁₂, (LGPS) with higher ionic conductivity (>10^-2 S cm^-1) compared to the liquid ones². However, LGPS faces issues in terms of chemical and electrochemical stabilities³. To overcome this issue, one approach is to partially substitute sulfur with oxygen, forming oxythiophosphate compositions Li₃PS_4-xO_x with LGPS like-structure. Such materials exhibit both improved electrochemical stability against Li metal and chemical stability in contact with air³.
Most reports in the literature use high-temperature synthetic pathways, which limits the range of possible compositions in the Li-P-S-O phase diagram as only phases with x < 1 were found pure. Additionally, this approach results in a complex mixture of different anions (PO₄^3-, PO₃S^3-, PO₂S₂^3-, POS₃^3-, PS₄^3-) at the local scale, without precise control over their proportions and distribution.
The goal of this work is to explore the Li-P-O-S system by using low-temperature approaches, in order to achieve better control of the stoichiometry and repartition of oxythiophosphates anions. In that context, a novel lithium oxythiophosphate phase Li₃PO₃S was obtained and MAS ³¹P NMR confirmed the presence of a pure (PO₃S) environment⁴. The powder XRD pattern of the product indicates the formation of a new phase with a different crystal structure from those of the Na₃PO₃S analog and the parent Li₃PO₄ and Li₃PS₄ compounds. The ratio of S/P was checked by EDX and found to be 0.98, which is consistent with the starting composition. Electron diffraction revealed that the new phase crystallizes in the hexagonal system with the space group P6₃cm, and its crystal structure was resolved through microcrystal electron diffraction. The ionic conductivity of Li₃PO₃S was studied after different thermal treatments. These findings present an effective method for stabilizing lithium oxythiophosphate phases that could not be obtained through high-temperature approaches, opening a new window for further exploration in solid-state ionic conductors.

(1) Albertus, P.; Anandan, V.; Ban, C.; Balsara, N.; Belharouak, I.; Buettner-Garrett, J.; Chen, Z.; Daniel, C.; Doeff, M.; Dudney, N. J.; Dunn, B.; Harris, S. J.; Herle, S.; Herbert, E.; Kalnaus, S.; Libera, J. A.; Lu, D.; Martin, S.; McCloskey, B. D.; McDowell, M. T.; Meng, Y. S.; Nanda, J.; Sakamoto, J.; Self, E. C.; Tepavcevic, S.; Wachsman, E.; Wang, C.; Westover, A. S.; Xiao, J.; Yersak, T. Challenges for and Pathways toward Li-Metal-Based All-Solid-State Batteries. ACS Energy Lett. 2021, 6 (4), 1399–1404. https://doi.org/10.1021/acsenergylett.1c00445.
(2) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A Lithium Superionic Conductor. Nat. Mater. 2011, 10 (9), 682–686. https://doi.org/10.1038/nmat3066.
(3) Xu, M.; Song, S.; Daikuhara, S.; Matsui, N.; Hori, S.; Suzuki, K.; Hirayama, M.; Shiotani, S.; Nakanishi, S.; Yonemura, M.; Saito, T.; Kamiyama, T.; Kanno, R. Li10GeP2S12-Type Structured Solid Solution Phases in the Li9+δP3+δ′S12–kOk System: Controlling Crystallinity by Synthesis to Improve the Air Stability. Inorg. Chem. 2022, 61 (1), 52–61. https://doi.org/10.1021/acs.inorgchem.1c01748.
(4) Pompetzki, M.; Jansen, M. Natriummonothiophosphat(V): Kristallstruktur und Natriumionenleitfähigkeit. Z. Für Anorg. Allg. Chem. 2002, 628 (3), 641–646. https://doi.org/10.1002/1521-3749(200203)628:3<641::AID-ZAAC641>3.0.CO;2-8.