Synthesis, structure, and ionic transport properties of lithium monothiophosphate pentahydrate Li3PO3S·5H2O and its anhydrous form Li3PO3S

Abstract

Mixed anion materials are promising for application as solid electrolytes in all-solid-state batteries, offering enhanced chemical and electrochemical stability alongside balanced ionic conductivity. This work reports a novel lithium oxythiophosphate Li₃PO₃S·5H₂O, synthesized at a low temperature in an aqueous medium. The crystal structure and properties of Li₃PO₃S·5H₂O were investigated through 3-dimensional electron diffraction, combined with powder X-ray and neutron diffraction, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, and electrochemical impedance spectroscopy. Li₃PO₃S·5H₂O crystallizes in the hexagonal space group P6₃cm with a = 16.48462(4), c = 5.36224(2) Å, V = 1261.928(6) ų, and Z = 6. ³¹P solid-state MAS NMR spectra revealed the presence of pure and ordered PO₃S tetrahedra. The dehydration of Li₃PO₃S·5H₂O was studied via TGA and temperature-dependent XRD, demonstrating the formation of the anhydrous phase Li₃PO₃S at 200 °C. The conductivity of anhydrous Li₃PO₃S (2.6 × 10⁻⁶ S cm⁻¹ at room temperature and E_a = 0.51 eV) is three orders of magnitude higher than that of the parent material Li₃PO₄ prepared by further heating the sample at 400 °C, with a significantly lower activation energy. Our findings highlight the potential of mixed anion materials in improving solid electrolyte performance. This work brings new insight into the crystal chemistry of oxythiophosphate anions, which is key in developing fast ionic conductors for solid-state batteries.