Mass Manufacturing Technology for Solid-State Lithium-Ion Secondary Batteries

A research group at Toyohashi University of Technology’s Department of Electrical and Electronic Information Engineering established a major manufacturing technology of Li₇P₃S₁₁ solid electrolytes for all-solid-state batteries.

An excessive amount of sulfur (S) is added to a solvent comprising acetonitrile (ACN), tetrahydrofuran (THF), and a small amount of ethanol, together with Li₂S and P₂S₅, which are the starting ingredients for Li₇P₃S₁₁ (EtOH), decreasing the reaction time from up to 24 hours to only two minutes.

At 25 °C, the final product produced using this process is very pure Li₇P₃S₁₁ with no impurity phase and high ionic conductivity of 1.2 mS cm^-1. These findings allow mass-producing sulfide solid electrolytes for all-solid-state batteries at a reasonable cost. On April 28^th, 2022, Advanced Energy and Sustainability Research released the study’s findings online.

Details

Since they are exceedingly safe and enable a shift to high energy density and high output power, all-solid-state batteries are predicted to be the next generation of batteries for electric vehicles (EVs). Sulfide solid electrolytes with high ionic conductivity and flexibility have been actively explored for use in all-solid-state batteries in electric vehicles.

However, since solid sulfide electrolytes are unstable in the environment and the process of synthesizing and processing them needs atmospheric control, no large-scale production technique for them was devised at the commercialization level.

As a result, there is a pressing need to create a low-cost, high-scalability liquid-phase manufacturing process for solid sulfide electrolytes.

Considering Li₇P₃S₁₁ solid electrolytes have high ionic conductivity, they are a potential contender for all-solid-state batteries. Li₇P₃S₁₁ is typically synthesized in a liquid-phase reaction solvent such as acetonitrile (ACN) using insoluble precursors.

This type of reaction takes a long time because it involves a kinetically inefficient reaction from an insoluble starting material to an insoluble intermediate. Furthermore, the insoluble intermediate might cause non-uniformity by causing a difficult phase development, resulting in higher large-scale production costs.

In light of this, the research team sets out to create a method for producing highly ion-conductive Li₇P₃S₁₁ solid electrolytes using uniform precursor solutions in the liquid phase.

It has been demonstrated that 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, a uniform precursor solution containing soluble lithium polysulfide (Li₂S_x) can be obtained in just two minutes.

The creation of lithium polysulfide by adding a modest quantity of EtOH or an excessive amount of S is the key to this method’s fast synthesis.

UV-Vis spectroscopy was utilized to analyze the chemical stability of Li₂S_x with and without the addition of EtOH to understand the mechanism of the reaction in this approach. The addition of EtOH increased the chemical stability of Li₂S_x, according to the research.

Lithium ions have strong coordination with EtOH, a polar solvent, and by shielding polysulfide ions from lithium ions, extremely reactive S₃^.- radical anions—a kind of polysulfide—are stabilized.

The produced S₃^.- assaults P₂S₅, causing the reaction to develop by destroying the cage structure of P₂S₅. Lithium thiophosphate is formed, which dissolves in a highly soluble mixed solution combining the solvents ACN and THF.

This might have aided in the speedy production of consistent precursor solutions. Li₇P₃S₁₁, the end product, could be made in two hours without using ball milling or high-energy treatment during the reaction.

At 25 °C, the Li₇P₃S₁₁ synthesized using this approach had an ion conductivity of 1.2 mS cm^-1, which was greater than the Li₇P₃S₁₁ produced using the traditional liquid-phase synthesis method (0.8 mS cm^-1) or ball milling (1.0 mS cm^-1). The approach presents a new route for synthesizing a solid sulfide electrolyte and results in low-cost, large-scale production technology.

Future Outlook

The research team believes that the low-cost approach suggested in this study for the large-scale manufacture of solid sulfide electrolytes for all-solid-state batteries would be critical in commercializing EVs using all-solid-state batteries.

The study concentrated on Li₇P₃S₁₁ as a solid sulfide electrolyte. This approach can also be used to make different sulfide solid electrolytes besides Li₇P₃S₁₁.

Journal Reference:

Gamo, H., et al. (2022) Solution Processing via Dynamic Sulfide Radical Anions for Sulfide Solid Electrolytes. AESR doi/10.1002/aesr.202200019.