Formation Mechanism of Thiophosphate Anions in the Liquid-Phase Synthesis of Sulfide Solid Electrolytes Using Polar Aprotic Solvents

Abstract

Preparation of sulfide-based Li-ion conductive solid electrolytes by a liquid-phase process has been proposed as an effective route for industrial scaling-up. However, reaction mechanisms in the liquid-phase synthesis or the role of solvents in the reactions are not yet well understood. Here, the reaction between Li2S and P2S5 in the composition of 50:50 mol% mediated by a polar aprotic solvent (acetonitrile) was investigated. The study of the crystal and local structure of the 50Li2S:50P2S5 sample revealed large chemical changes upon crystallization, highlighting the initial formation of a polymer-like (PS3 − )n intermediate. In the base of the (PS3 − )n intermediate, the reaction pathways for the formation of the P2S6 2− , PS4 3− , and P2S7 4− anions are elucidated. These findings give a deeper insight into the reaction mechanisms governing the liquid-phase synthesis of sulfide solid electrolytes and provide more specific criteria by which to design novel materials with superior characteristics through this liquid-phase approach.

Chemical Stability and Ionic Conductivity of LGPS-Type Solid Electrolyte Tetra-Li7SiPS8 after Solvent Treatment

“…For the pristine sample, 31 P solid-state MAS NMR clearly shows the two signals of LiSiPS at 94 and 73 ppm, which correspond to isolated [PS 4 ] 3− units on the 4d site (occupied by Si and P) and 2b (occupied exclusively by P) phosphorus site, respectively (Figure 3a). 1 Based on previous reports, 1,24,53 we assign the additional peak at 90 ppm to ortho-LiSiPS and the peaks at 86 and 84 ppm to an amorphous orthothiophosphate-related "Li 3 PS 4 "-type side phase and a polymeric PS 3…”

“…The suspensions show a variety of different colors (from colorless, yellow, over green to blue) depending on the solvent, as shown in the photographs in Figure 4a, similar to previous observations on thiophosphate suspensions. 8,10,13,19,24,25,59 UV/vis measurements suggest the formation of various polysulfide species in the solvent phase of the LiSiPS suspensions. These species are tentatively assigned according to previous literature in Figure 4b.…”

The effect of solvent on reactivity of the Li2S–P2S5 system in liquid-phase synthesis of Li7P3S11 solid electrolyte

“…At the same time, the Li 7 P 3 S 11 precursor involves two phases of the Li 2 S P 2 S 5 and Li 3 PS 4 complexes 18 . According to a recent study, following a drying process at 100 °C, Li 2 S P 2 S 5 with ACN exhibits a halo-shaped XRD pattern originating from its amorphous structure 28 . Therefore, the Li 7 P 3 S 11 precursor with ACN solvent contains crystalline Li 3 PS 4 ACN and amorphous Li 2 S P 2 S 5 .…”

On the underestimated influence of synthetic conditions in solid ionic conductors

“…131 The reactants typically do not fully dissolve and a heterogeneous reaction system is the norm. , S 0 /S-S. 65,87,123,132,133 This can also apply locally; 75 : 25 Li 2 S : P 2 S 5 (typical stoichiometry for Li 3 PS 4 ) under certain conditions in acetonitrile (ACN) resulted in Li 7 P 3 S 11 with Li 2 S, likely the result of local stoichiometry closer to 70 : 30 due to unreacted Li 2 S. 134 Thus incomplete reaction and uncontrolled heterogeneous distribution of reactants can be a potential pitfall in synthesis. There seems to be no ideal solvent that fully dissolves starting materials and/or products without inducing unwanted reactions (thus making isolation of products difficult)alcohols such as ethanol may dissolve Li 2 S and LiX, but may tend to attack and decompose P 4 S 10 .…”