ETRI develops ultra-thin solid electrolyte membrane for all-solid-state secondary batteries

Korean researchers have succeeded in developing a key technology for all-solid-state secondary batteries, known as next-generation lithium-ion batteries due to their high safety. The work was published online as a cover study in SMALL, the world’s leading journal for energy materials, at the end of last year.

Electronics and Telecommunications Research Institute (ETRI) developed a separation membrane based on a binder material that easily becomes fibrillized when subjected to mechanical shearing (force applied) through a mixing process with solid electrolyte powder without using a solvent. This solid electrolyte membrane is simple and fast to manufacture and is extremely thin and robust.

In general, in research on all-solid-state secondary batteries, the thickness is set to several hundred micrometers (㎛) to 1 millimeter (㎜) to increase the durability of the membrane when using a hard solid electrolyte in the manufacturing process. However, this has the disadvantage of being too thick compared to conventional polymer separation membranes, resulting in a very large loss of energy density.

The research team applied a binder material that exhibits fibrillation behavior when mechanical shear (force) is applied, and manufactured an ultra-thin solid electrolyte membrane with a thickness of 18㎛, which is close to the thickness of existing commercialized lithium-ion battery separation membranes, through a dry process.

Through this, the research team significantly reduced the cell volume and created a high-energy density and high-performance all-solid-state secondary battery. It can be said that the energy density has been increased by up to 10 times compared to a 1 mm thick solid electrolyte membrane.

This research will enable the development of all-solid-state secondary batteries with high energy density by improving the ion transfer rate between charge and discharge while significantly reducing cell volume and weight through a solid electrolyte membrane with a thickness close to that of existing commercialized polymer separation membranes.

The study also revealed a correlation between the molecular weight of the binder material and the degree of robust entanglement, providing a process standard for developing optimized ultra-thin solid electrolyte membranes. This makes it possible to produce cost-effective membrane shapes with the correct amount of binder in the manufacturing process.

All-solid-state secondary battery, which are gaining attention as next-generation secondary batteries, is a battery system that significantly improves safety by changing the medium for ion transfer from a liquid to a solid-state material, eliminating risks such as ignition, explosion, and leakage.

The key material in these all-solid-state secondary batteries is a solid-state electrolyte membrane that transfers ions while preventing direct contact between the anode and cathode. In a conventional lithium-ion cell, the membrane acts as both a liquid electrolyte and a separator.

In the cell production process, liquid electrolytes are manufactured through direct injection, while solid electrolytes are manufactured separately in the form of membranes and applied to cell production.

On the other hand, the dry process, which mechanically mixes powdered solid electrolyte with a fibrous binder to produce a membrane, minimizes the binder content and eliminates the use of solvents, resulting in a highly ionic conductive solid electrolyte membrane that is stronger and easier to control regarding thickness than conventional slurry-based tape casting processes.

ETRI researchers have succeeded in producing ultra-thin solid electrolyte membranes by optimizing a mechanical shear process that maximizes the degree of entanglement of the fibrous binder, which is essential for the dry process.

The researchers quantified the correlation between the molecular weight of the polymeric binder and the degree of entanglement during fibrillation through structural analysis. By optimizing the process temperature and time during shearing, it was possible to induce up to 98% polymer binder fibrillation, forming a binder network with a strong entanglement structure.

Park Young Sam, the principal researcher at ETRI’s Smart Materials Research Section, said, “The success of creating large-scale solid electrolyte membranes with separator-level thickness is expected to significantly improve energy density, which will increase the commercialization potential of all-solid-state secondary batteries with high price competitiveness.”

Shin Dong Ok, the principal researcher at ETRI’s Smart Materials Research Section, also said, “Through an in-depth understanding of the polymer binder fibrillation, we have solved the problem of ultra-thin solid electrolyte membranes, which has been a challenge, with a simple and fast process.”

In particular, the researchers said that the results of the study are significant because they provide an optimal shearing process standard that has not been addressed in the existing dry process, which can be expanded to composite anodes and cathodes of all-solid-state secondary batteries and can eliminate the use of solvents that cause environmental pollution.

While ETRI researchers focused on solid-state electrolyte thinning in this study, they plan to conduct research to further improve ion conductivity performance and achieve stable interface control with the electrode. The researchers also manufactured a pouch-type cell with an ultra-thin solid electrolyte membrane applied and reported stable charge/discharge results, suggesting the possibility of commercialization.

This study was participated in by Shin Dong Ok and Park Young Sam, the principal researchers at ETRI, as corresponding authors, and Yoon Seok Yoon, a combined master’s and doctoral student at UST, as the first author.

1) SMALL (IF: 13.0, JCR by field: 93.258), URL (https://doi.org/10.1002/smll.202407882) Published online (November 12, 2024), selected as a cover study

2) Fibrillatable binder material: A grass-like material that separates into strands like a rope when a large mass is subjected to force

3) Shear force: When forces of equal magnitude and opposite directions act simultaneously on an object, the force that acts parallel to the surface within the object

4) Solid electrolyte membrane: Thin and flexible solid electrolyte membrane

5) Separation membrane: Separation membrane made of polymers with a thickness of 10 to 20 um, used in liquid electrolyte-based Li-ion batteries

6) Fibrillation: The process by which materials such as polymers are transformed into long, thin fibrils

7) Dry process: A process of forming electrodes using solid powders without the use of solvents, which saves energy and cost, enables the manufacture of high energy density electrodes, and has environmental benefits by minimizing the use of hazardous materials

8) Entanglement: A phenomenon in which polymer binders are entangled with each other while being separated into fibrils, which has structural characteristics that improve mechanical properties

9) All-solid-state secondary battery: A battery that replaces liquid-phase electrolytes with solid-phase electrolytes

10) Apply: Typically, solid electrolyte membranes are manufactured through a tape casting process by preparing a slurry. However, the solid electrolyte easily reacts with the solvent used, resulting in poor performance, and a solvent removal process is required after casting, making it difficult to produce a solid electrolyte membrane with uniform thickness and high density.

11) Slurry: Mixed suspensions of solid electrolyte powders, binders, and solvents

12) Tape casting process: A process in which a slurry of solids, binders, and additives mixed with a solvent is applied in a thin layer over a substrate and dried to produce a film.

13) Correlation between the molecular weight of the polymeric binder and the degree of entanglement during fibrillation: This strongly entangled structure of the binder enabled the production of a solid electrolyte membrane with a thickness of 18㎛ over an area of 12x12㎠, despite the application of a very low 0.5wt% content. Ion conduction performance was improved by 30 times compared to conventional pellet-type solid electrolyte layers. In addition, it was shown that the energy density of 191 Wh/kg and 481 Wh/L was achieved even in a single-cell structure during cell manufacturing, indicating that the energy density can be significantly improved simply by reducing the thickness of the solid electrolyte membrane.

photo caption) 0.5wt%: Binder weight to total solid electrolyte membrane ratio

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In addition, this research was conducted with the support of ETRI’s Future Source Creative Research Section project, the Ministry of Trade, Industry and Energy’s ‘Lithium-based Next-Generation Secondary Battery Performance Improvement and Manufacturing Technology Development’ project, and the National Research Council of Science and Technology’s Global TOP Strategic Research Group project.