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 P₄S₁₁^2- (two triplets at 91.2 and 74.3 ppm, ²<i>J</i>_PP = 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 P₂S₈^2-, as well as (PS₃^-)_<i>n</i> (86.5 ppm) and the solvent adduct [PS₃(CH₃CN)]^- (299.0 ppm), were intercepted. As for (PS₃^-)_<i>n</i>, 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 ų, substantially the same than that of P₄S₁₁^2-, suggest the formation of a cyclic P₄S₁₂^4- species. Consistently, also P₂S₆^2-, potentially derived by the dissociation in two equal parts of P₄S₁₂^4-, 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 P₄S₁₁^2-, chair and twisted P₂S₈^2-, and P₂S₆^2-, whose solid-state structures were determined by X-ray diffractometric studies.