Development of a novel large-scale manufacturing technology for sulfide solid electrolytes

A research group in the doctoral program of Toyohashi University of Technology’s Department of Electrical and Electronic Information Engineering that includes a doctoral student Hirotada Gamo and specially appointed assistant professor Jin Nishida, specially appointed associate professor Atsushi Nagai, assistant professor Kazuhiro Hikima, professor Atsunori Matsuda and others, developed a large-scale manufacturing technology of Li₇P₃S₁₁ solid electrolytes for all-solid-state lithium-ion secondary batteries.

This method involves the addition of an excessive amount of sulfur (S) along with Li₂S and P₂S₅, the starting materials of Li₇P₃S₁₁, to a solvent containing a mixture of acetonitrile (ACN), tetrahydrofuran (THF) and a slight amount of ethanol (EtOH). This helped to shorten the reaction time from 24 hours or longer to only two minutes. The final product obtained using this method is highly pure Li₇P₃S₁₁ without an impurity phase that showed high ionic conductivity of 1.2 mS cm^-1 at 25 °C. These results enable us to produce a large quantity of sulfide solid electrolytes for all-solid-state batteries at low cost. The results of the research were published online by Advanced Energy and Sustainability Research on April 28, 2022.

Details

All-solid-state batteries are expected to be the next generation of batteries for electric vehicles (EVs) because they are very safe and enable a transition to high energy density and high output power. Sulfide solid electrolytes, which show good ionic conductivity and plasticity, have been actively developed with a view toward the applications for all-solid-state batteries in EVs. However, no large-scale manufacturing technology for sulfide solid electrolytes has been established at the level of commercialization, as sulfide solid electrolytes are unstable in the atmosphere and the process for synthesizing and processing them requires atmospheric control. For this reason, there is an urgent need to develop the liquid-phase manufacturing technology of sulfide solid electrolytes that offers low-cost and high scalability.

Li₇P₃S₁₁ solid electrolytes exhibit high ionic conductivity and thus are one candidate solid electrolyte for all-solid-state batteries. The liquid-phase synthesis of Li₇P₃S₁₁ generally occurs in an acetonitrile (ACN) reaction solvent via precursors including insoluble compounds. Conventional reaction processes like this take a long time as they go through a kinetically disadvantageous reaction from an insoluble starting material to an insoluble intermediate. Worse, it is possible that the insoluble intermediate creates non-uniformity through a complicated phase formation, leading to an increase in large-scale manufacturing costs.

Against this background, the research group worked on the development of a technology for liquid-phase production of highly ion conductive Li₇P₃S₁₁ solid electrolytes via uniform precursor solutions. It has been shown that the recently developed method can obtain a uniform precursor solution containing soluble lithium polysulfide (Li₂S_x) in just two minutes, by adding Li₂S and P₂S₅, the starting materials of Li₇P₃S₁₁, and an excessive amount of S to a solvent containing a mixture of ACN, THF and a small amount of EtOH. The key to the 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 elucidate the mechanism of the reaction in this method, ultraviolet-visible (UV-Vis) spectroscopy was used to investigate the chemical stability of Li₂S_x with and without the added EtOH. The study showed that the presence of EtOH made Li₂S_x more chemically stable. Thus, the reaction in this method would take the following steps. First, lithium ions are strongly coordinated with EtOH, a highly polar solvent. Next, shielding polysulfide ions against lithium ions stabilizes highly reactive S₃^･– radical anions which are a kind of polysulfide. The generated S₃^･– attacks the P₂S_5, breaking the cage structure of P₂S₅ and causing the reaction to progress. The reaction forms lithium thiophosphate which dissolves into a highly soluble mixed solvent containing ACN and THF solvents. This may have helped to obtain uniform precursor solutions very rapidly. The final product, Li₇P₃S₁₁, could be prepared in two hours without the necessity of ball milling or high energy treatment in the process of reaction.

The ion conductivity of the Li₇P₃S₁₁ obtained using this method was 1.2 mS cm^-1 at 25 °C, higher than the Li₇P₃S₁₁ synthesized using the conventional liquid-phase synthesis method (0.8 mS cm^-1) or ball milling (1.0 mS cm^-1). The method proposes a new path for the synthesis of a sulfide solid electrolyte and achieves a large-scale manufacturing technology with low cost.

Future Outlook

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