Synthesis and Structural Characterization of Three Lithium Dithiophosphates: Li3PO2S2·11H2O, Li3PO2S2·H2O, and Li3PO2S2

Abstract

Solid-state batteries require electrolytes with high Li-ion conductivity, together with good interfacial and chemical stability. Oxythiophosphate-type solid electrolytes appear to achieve such a balance, but their chemistry remains little investigated. Here, we report the low-temperature aqueous synthesis of lithium dithiophosphate undecahydrate, Li3PO2S2·11H2O, and its dehydration pathway to the monohydrate Li3PO2S2·H2O and to the anhydrous phase Li3PO2S2. Li3PO2S2·11H2O crystallizes in the P212121 space group, isostructural to the parent compound Na3PO2S2·11H2O. Upon annealing at 100 °C, a monohydrate forms with a C2cm structure solved ab initio from XRPD and PND data. Further dehydration at 200 °C yields anhydrous Li3PO2S2, which adopts a Pbca structure isostructural to the Na analog. Solid-state NMR spectroscopy confirms dominant (PO2S2) units in the hydrate and reveals partial transformation to (PO3S)/(PO4) units upon annealing. Li3PO2S2·11H2O exhibits a room-temperature ionic conductivity of ∼10–5 S cm–1. The ionic conductivities of Li3PO2S2·H2O and Li3PO2S2 are lower: σ = 8.7 × 10–8 S·cm–1 and 2.6 × 10–8 S·cm–1 (at 110 °C), with activation energies of ∼0.70 and ∼0.69 eV, respectively. These findings expand the oxythiophosphate structural family with new hydrated and anhydrous phases, providing a basis for tuning O/S composition and Li-ion transport through targeted cation substitution.