[Complete Battery Mastery](29) Are Binders Harmful Substances? ... Battery Industry on Alert for EU Environmental Regulations

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Among the four key secondary battery materials, electrodes have the greatest impact on performance. Both cathodes and anodes are manufactured by mixing active materials, conductive agents, and binders with a solvent to create a slurry, which is then coated onto a current collector (aluminum foil or copper foil). (For more information on current collectors and conductive agents, refer to episodes 24, 26, and 27 of Battery Mastery.)

The proportion of binders in electrodes is less than 5% by mass, which is not significant, so they have traditionally garnered little attention. However, recent advances in next-generation battery technology have fueled interest in binders. As the materials and manufacturing processes for cathodes and anodes evolve, there is a growing need for optimized binders that can maximize performance. As such, binders are increasingly seen as key to unlocking the doors to next-generation batteries.

Binders Are 'Adhesives'...Directly Linked to Battery Lifespan and Capacity

In lithium-ion batteries, binders play a role in physically stabilizing the electrode. A binder is a material used to secure the adhesion or cohesion forces between the active material and the current collector. Simply put, it is an additive that ensures the active material mixes well with the conductive agent and can be uniformly coated onto the current collector.

The binder is closely related to the performance of a battery, including lifespan and energy density. As lithium-ion batteries undergo repeated charge and discharge cycles, the insertion and extraction of lithium ions can create cracks or cause expansion in the active material, which binders help to alleviate or prevent. Improved binder performance allows for lower binder content and a higher proportion of active material, which in turn increases energy density.

Several requirements must be met for a substance to be used as a binder. First, it should maintain stable adhesion over long-term use. The binder must also exhibit chemical and electrochemical stability in relation to the electrolyte, meaning it should not react chemically with the electrolyte or undergo unwanted side reactions. Chemical stability means the binder must not induce oxidation or reduction reactions with the electrolyte. Electrochemical stability means it should not degrade within the operating voltage range of 3–5V typical of lithium-ion batteries.

The binder must possess heat resistance, withstanding temperatures up to 200 degrees Celsius during electrode manufacturing, as well as high pressure without fracturing.

In the wet electrode process, a solvent is used to make the slurry. Depending on the type of solvent, binders are classified as organic (non-aqueous) or aqueous. Simply put, an organic binder is used with organic solvents, while an aqueous binder is used with aqueous solvents. Generally, cathodes utilize the organic binder PVDF, while anodes use the aqueous binder SBR/CMC.

PVDF is classified as a linear contact binder because it maintains adhesion by connecting particles in a linear fashion. SBR/CMC falls into the point contact binder category. Point contact types generally exhibit stronger fixing capability than linear contact types.

Schematic diagram of PVDF, a direct contact binder, and SBR/CMC binder, an indirect contact type. Image source: Journal of Polymer Science and Technology, Vol. 27, No. 3

Cathode active materials do not dissolve well in water, so the organic process is used instead of the aqueous one. The cathode typically utilizes PVDF (Poly vinylidene Fluoride) alongside the organic solvent NMP (N-Methyl-2-Pyrrolidone).

These two substances offer excellent dispersibility with the electrode’s active material particles and conductive agents, and have outstanding cohesion. They are also chemically stable, resisting oxidation or reduction even in organic electrolytes. However, a drawback of PVDF is that increasing its molecular weight results in increased slurry viscosity, which lowers dispersibility. Additionally, PVDF is known to be prone to delamination between the current collector and the electrode coating layer at high temperatures.

PVDF also features excellent weather resistance and anti-fouling characteristics under varied environmental conditions, leading to its wide use in solar cell films and water treatment membranes.

The anode typically uses the aqueous binder SBR (Styrene-Butadiene Rubber) mixed with CMC (Carboxy Methyl Cellulose).

The SBR/CMC binder binds the active material and conductive agents together through point contacts, resulting in superior cohesion. However, for next-generation anodes using silicon, a stronger binder is needed to suppress the significant volume expansion of silicon.

The PVDF binder market is dominated by a few overseas companies, including Kureha from Japan, Solvay from Belgium, and Arkema from France. In South Korea, Chemtrus began research and development for commercialization via a pilot line in 2021 after acquiring PVDF manufacturing process technology from the Korea Research Institute of Chemical Technology in March 2019.

Schematic diagram of binder for anode material. Image source=LG Chem

Among the major producers of SBR binder is Zeon from Japan. In South Korea, Hansol Chemical has successfully localized the production of anode binders and supplies them to Samsung SDI and SK On. Both LG Chem and Kumho Petrochemical also manufacture anode binders.

According to SNE Research, the global lithium-ion battery binder market is expected to increase from 89,000 tons in 2025 to 232,000 tons in 2030. In monetary terms, it is projected to reach approximately 4.4 trillion won by 2030.

What is PTFE, Used in Dry Electrodes?

As research into next-generation batteries progresses, the development of new binder materials is advancing in tandem. One prime example is dry electrodes. The dry electrode process, which Tesla applied to its 4680 cylindrical batteries, requires new binder materials because it does not use solvents in manufacturing.

In the conventional wet process, the slurry is coated onto the current collector and NMP is removed using hot air. Since NMP is expensive and an environmental pollutant, Korean companies are recovering and refining it for reuse. NMP is fully imported into Korea, with German BASF and U.S.-based Ashland (formerly ISP) holding a virtual monopoly over its supply.

Dry processing does not use NMP, instead coating the current collector directly with a powder mixture of active materials or forming the powder into a film, which is then attached to the current collector.

PTFE (PolyTetraFluoroEthylene) is widely used as a binder in the dry electrode process. PTFE was developed in 1938 by the American chemical company DuPont and is better known under the trade name Teflon.

PTFE, composed of extremely strong carbon-fluorine bonds, offers excellent heat and chemical resistance. Its smooth surface makes it a popular non-stick coating for frying pans, as well as gaskets, bearings, container linings, valves and pump parts, and saw blades.

PTFE appears as a white powder and has two transition temperatures. It exhibits a fibrillation phenomenon when stress is applied above 19 degrees Celsius. This effect becomes even more pronounced above 30 degrees Celsius.

PTFE fiberization phenomenon. Image source=International Polymer Processing

Leveraging this property, after mixing active material, conductive agent, and binder, the blend can be extruded into thin films even without a solvent. Maxwell, a company acquired by Tesla in 2019 for the development of the 4680 cylindrical cell, uses this process for manufacturing dry electrodes.

Specialized Binders Needed for Silicon Anodes

To increase battery capacity, more manufacturers are now attempting to apply silicon to anodes. Silicon has a theoretical capacity of 4,200 milliampere-hours (mAh)/g, about ten times that of graphite (372 mAh/g).

However, silicon is prone to severe swelling during charge/discharge cycles, and the SEI (Solid Electrolyte Interphase) layer forms irregularly, which shortens battery lifespan. To address this, carbon nanotube (CNT) conductive agents are used. (For more on silicon anodes, see episode 11 of Battery Mastery.)

Moreover, recent research has shown that using a robust polymer binder can prevent silicon anode cracking during charge/discharge cycles and enhance the electrochemical performance of batteries. Binders that improve the cohesion between electrode materials help mitigate issues such as swelling.

PAA (Poly Acrylic Acid) and PI (Poly Imide) are gaining attention as binders for silicon anodes. Both are aqueous binders offering superior tensile strength and adhesion compared to conventional binders, effectively suppressing the volume expansion of silicon anodes. They also encapsulate the active material to form a stable SEI layer.

Binder for silicon anode materials acquired by Aegyeol Chemical in collaboration with Samsung SDI. Image source=Eugene Investment & Securities

In South Korea, Aekyung Chemical has developed a binder for silicon anodes and is preparing it for commercialization. In May of last year, Aekyung Chemical completed both domestic and international patent registrations for its high-capacity silicon anode binder and is currently conducting tests with customers at home and abroad. According to the Korean Intellectual Property Office, Aekyung Chemical and Samsung SDI jointly obtained the final patent for a polymer binder for silicon anodes in June 2021.

PFAS Environmental Regulations in Europe: A Variable

The most closely watched issue on binders in the battery industry is now the environmental regulations of the European Union (EU). The European Chemicals Agency (ECHA) is pushing to restrict the use of nearly all per- and polyfluoroalkyl substances (PFAS). On February 2023, ECHA disclosed over 10,000 targeted fluorinated compounds for regulation, including PVDF and PTFE.

ECHA solicited opinions on the PFAS regulations until September last year and is currently conducting its review. It plans to submit its final opinion to the European Commission this year. If approved by the European Parliament and Council, PFAS regulations are expected to be implemented after a transition period, starting as early as 2026–2027.

Last September, the Korean government and domestic companies officially communicated to ECHA and the World Trade Organization (WTO) that a "cautious review is needed" regarding a comprehensive ban on PFAS. PFAS are used not only in secondary batteries but also in semiconductors and display panels. Arkema, Solvay and others are lobbying for the exemption of fluoropolymers from PFAS restrictions.

PFAS Regulation Roadmap. Image source=European Chemicals Agency

The battery industry hopes that PVDF and PTFE will ultimately be excluded from the final regulation. Son Eun-ho, Director of the Surface Materials & Chemistry Process Research Center at the Korea Research Institute of Chemical Technology, explained, "Whether PVDF and PTFE are included in the final statement will likely depend on the availability of alternatives and the degree of hazard. Contrary to popular belief, these two substances are not particularly hazardous."

PFAS are a category of persistent organic pollutants that do not decompose easily in nature and can accumulate in the environment and in living organisms. The best-known PFAS include PFOA (Perfluorooctanoic acid) and PFOS (Perfluorooctane sulfonic acid). PTFE, which is widely used as a pan or pot coating, is a fluoropolymer with a chemical structure and physical properties entirely different from PFOA and PFOS. Even if you inadvertently ingest a chip of fluororesin from a pan coating, it is not absorbed by the body and is excreted as is, so the risk to human health is considered negligible.

International polymer processing, Polytetrafluoroethylene Paste Extrusion: A Fibrillation Model and its Relation to Mechanical Properties, 2013.6
Journal of Polymer Science and Technology, Functional Polymers for Secondary Batteries, 2016.6.3
Ministry of Food and Drug Safety, Q&A on Perfluorochemicals, 2017.3.23
Charged, Maxwell’s Teflon-fibrilizing electrode process could save Tesla big bucks on battery manufacture, 2019.3.21
European Chemicals Agency, ECHA publishes PFAS restriction proposal, 2023.2.7
Eugene Investment & Securities, Binders for Aekyung Chemical’s Anode Materials, 2023.4.12
Law News, EU and U.S. PFAS usage regulations, 2023.8.9
C&EN, The battle over PFAS in Europe, 2023.9.18
Korea IR Association, Looking Forward to Chemtrus in 2024, 2023.11.2

This content was produced with the assistance of AI translation services.