A Multinuclear NMR Study on the Speciation in the Liquid-Phase Synthesis of Sulfide-Based Electrolytes for All-Solid-State Lithium Batteries

The preparation of Li2S–P4S10 (LPS) solid electrolytes for all-solid-state lithium batteries through liquid-phase methods has attracted much attention over the last years. One of the most critical aspects to be clarified is how speciation affects the ionic conductivity of the synthesized materials, which is often lower than that obtained by other synthetic approaches. This paper shed some light on the species formed upon the reaction of Li2S and P4S10 in acetonitrile under mild conditions, exploring a wide range of Li2S/P4S10 molar ratios, by means of multinuclear/multidimensional NMR spectroscopy and single-crystal X-ray diffraction. Specifically, 31P NMR spectroscopy clearly evidenced the initial formation of P4S112– (two triplets at 91.2 and 74.3 ppm, 2JPP = 23.16 Hz) from the reaction of P4S10, solubilized likely via electrostatic interactions (singlet at 81.9 ppm), with Li2S. On the way to the LPS solid electrolyte, the twisted (118.3 ppm) and chair (54.3 ppm) isomers of P2S82–, as well as (PS3–)n (86.5 ppm) and the solvent adduct [PS3(CH3CN)]− (299.0 ppm), were intercepted. As for (PS3–)n, the observation of a sharp singlet in the 31P NMR spectrum and the evaluation by diffusion 31P NMR spectroscopy of a hydrodynamic volume of ca. 800 Å3, substantially the same than that of P4S112–, suggest the formation of a cyclic P4S124– species. Consistently, also P2S62–, potentially derived by the dissociation in two equal parts of P4S124–, was observed in the 31P NMR spectrum as a singlet at 31.2 ppm. For all the investigated molar ratios, rather broad resonances in the −3.0/–1.5 ppm range were observed in the 7Li NMR spectra, reflecting the dynamic nature of these systems. Metathetic exchange of Li+ with the bis(triphenylphosphine)iminium cation was found to be a successful strategy to obtain good-quality single crystals of P4S112–, chair and twisted P2S82–, and P2S62–, whose solid-state structures were determined by X-ray diffractometric studies.