Development of a new large-scale technology for the production of sulfide solid electrolytes

Research group in the doctoral program of the Department of Electrical and Electronic Information Engineering of the University of Toyohashi, which includes doctoral student Hirotada Gama and specially appointed associate professor Jin Nishida, specially appointed associate professor Atsushi Nagai, associate professor Kazuhira Hishimano, professor of technology₇P₃S₁₁ solid electrolytes for solid lithium-ion secondary batteries.

This method involves adding an excessive amount of sulfur (S) along with Li₂C and P₂S₅Lee’s source materials₇P₃S₁₁, to a solvent containing a mixture of acetonitrile (ACN), tetrahydrofuran (THF) and a small amount of ethanol (EtOH). It helped cut back reaction time from 24 hours and more to just two minutes. The final product obtained by this method is high purity lithium₇P₃S₁₁ without the impurity phase, which showed a high ionic conductivity of 1.2 mSm see^-1 at 25 ° C. These results allow us to produce large quantities sulfide solid electrolytes for all-solid state batteries at a low price. The results of the study were published online Advanced research in energy and sustainable development April 28, 2022

Details

It is expected that solid state batteries will be the next generation of batteries for electric vehicles (EV), because they are very safe and allow you to move to high energy density and high power output. Sulfide solid electrolytes, which exhibit good ionic conductivity and ductility, are being actively developed for use in fully solid-state EV batteries. However, at the level of commercialization, no large-scale technology for the production of sulfide solid electrolytes has been created, as sulfide solid electrolytes are unstable in the atmosphere and the process of their synthesis and processing requires atmospheric control. For this reason, there is an urgent need to develop liquid-phase technology for the production of sulfide solid electrolytes, which provides low cost and high scalability.

Lee₇P₃S₁₁ solid electrolytes exhibit high ionic conductivity and are therefore one of the candidates for solid electrolytes for fully solid state batteries. Liquid-phase synthesis of Li₇P₃S₁₁ usually occurs in solvent acetonitrile reaction (ACN) through precursors, including insoluble compounds. Conventional reaction processes, such as this one, are time consuming as they go through a kinetically unfavorable reaction from an insoluble starting material to an insoluble intermediate. What is worse, it is possible that the insoluble intermediate creates unevenness through the complex phase, leading to increased costs for large-scale production.

Against this background, the research team worked on the development of technology for liquid-phase production of high-ionic Li₇P₃S₁₁ solid electrolytes through homogeneous solutions of precursors. It has been shown that a newly developed method allows to obtain a homogeneous solution of the precursor containing soluble lithium polysulfide (Li₂S_x) in just two minutes, adding Li₂C and P₂S₅Lee’s source materials₇P₃S₁₁, and an excessive amount of S to a solvent containing a mixture of ACN, THF and a small amount of EtOH. The key to rapid synthesis in this method is the formation of lithium polysulfide through the addition of a small amount of EtOH or an excessive amount of S.

To determine the mechanism of the reaction in this method, ultraviolet (UV-Vis) spectroscopy was used to study the chemical stability of Li₂S_x with and without EtOH. The study found that the presence of EtOH made Li₂S_x more chemically stable. Therefore, the reaction in this method will take the following steps. First, lithium ions are strongly coordinated with EtOH, a highly polar solvent. Further, shielding polysulfide ions from lithium ions stabilizes highly reactive S₃^･– radical anions, which are a type of polysulfide. Generated C₃^･– attacks P₂S_5, violation of cell design P₂S₅ and elicits a reaction to progress. The reaction produces lithium thiophosphate, which is soluble in a mixed solvent containing ACN and THF solvents. This may have helped to obtain homogeneous solutions of the precursors very quickly. The final product, Li₇P₃S₁₁could be cooked in two hours without the need for ball crushing or high energy processing during the reaction.

Ionic conductivity Li₇P₃S₁₁ obtained by this method was 1.2 ms see^-1 at 25 ° C, higher than Li₇P₃S₁₁ synthesized using the usual method of liquid-phase synthesis (0.8 mSm see^-1) or ball milling (1.0 ms cm)^-1). The method offers a new pathway for the synthesis of sulfide solid electrolyte and achieves large-scale production technology with low cost.

Prospects for the future

The research team believes that low-cost technologies for large-scale production of sulfide solid electrolytes for fully solid-state batteries proposed in this study may be important in the commercialization of EVs equipped with fully solid-state batteries. The study focused on Lee₇P₃S₁₁ for use as a solid sulfide electrolyte. We also want to apply this technology to the synthesis of sulfide solid electrolytes other than Li₇P₃S₁₁.