Lithium-ion batteries with organic liquid electrolytes are known to be effective. However, they are unsafe and have a limited lifetime. Because of this, solid-state electrolytes are considered to be possible solutions to these problems. Using sulfur-based solid-state electrolytes can be a useful, sustainable, and green alternative. With their high lithium-ion conductivity and transference number, electrochemical stability; sulfur-based solid-state electrolytes are ideal in lithium-ion batteries. The ionic conductivity of sulfur-based electrolytes highly depends on the crystalline phases of the products. Li₂S and P₂S₅ systems are used to form glass sulfide electrolytes which have the benefits of isotropic conduction, absence of grains, a low melting point that helps control morphology, and numerous compositions. On the other hand, glass-ceramic sulfide electrolytes have the same positive properties, but are metastable and cannot be synthesized by a regular solid-state reaction. Depending on the synthesis conditions, these solid-state electrolytes could have over a few orders of magnitude difference in their ionic conductivity. The goal of this study is to understand how these phase transformations affect electrolyte properties and the effects of these transformations on the ionic and electronic conductivity within the cathode nanostructures.

Synthesized from traditional Li₂S and P₂S₅ compounds, sulfur-based electrolytes form Li₃PS₄ which is a glass sulfide electrolyte. However under different conditions PS₄^3- has been found to be a precursor to the formation of Li₇P₃S₁₁, a sulfide glass-ceramic electrolyte. The formation of P₂S₇^4- and P₂S₆^4- in the local structure also aids in the production of Li₇P₃S₁₁, achieving the ionic conductivity of 10^-2 S cm^-1 at room temperature, which is close to the ionic conductivity of organic liquid electrolytes. Though promising, sulfide electrolytes possess a few challenges, such as instability and decreased ionic conductivity at room temperature due to formation of sulfide compounds with very low ionic conductivity, such as P₂S₆^4- (hypo-thiodiphosphate).

This research aims to investigate the molar concentrations of Li₂S and P₂S₅, using xLi₂S·(100-x)P₂S₅ in compositions of 65≤x≤70 mol%, to achieve the highest conductivity and evaluate phase transitions at different temperatures and in presence of cathode components. Dry ball milling will be compared with the synthesis in liquid phase using acetonitrile. In this synthesis, acetonitrile and the selected xLi₂S·(100-x)P₂S₅ at 65≤x≤70 mol% of Li₂S-P₂S₅ will be mixed and ultrasonicated and dried under vacuum. The powders are heat-treated at 220℃ and 250℃ to promote crystallization. The second approach involves ultrasonication under the same conditions without a liquid median. Morphology of the sulfide solid electrolyte particles will be obtained by using a scanning electron microscope (SEM). The crystal phase and chemical composition of the sulfide electrolytes will be obtained by in-situ X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical impedance spectroscopy will be conducted to evaluate the ionic conductivity, and the charge rate will be completed by C-rate testing. The authors acknowledge financial support from the NSF IUCRC program for supporting the “Center for solid-state electric power storage” (#2052631), and the South Dakota “Governor’s Research Center for electrochemical energy storage”.