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

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Among the four major materials of secondary batteries, the electrode has the greatest impact on performance. Cathodes and anodes are made by mixing active materials, conductive agents, and binders with solvents to create a slurry, which is then coated onto a current collector (aluminum foil or copper foil). (For current collectors and conductive agents, refer to Complete Battery Mastery episodes 24, 26, and 27.)

The binder accounts for less than 5% by mass in the electrode, so it has not received much attention until now. However, as research on next-generation battery technologies has become active recently, interest in binders is increasing. As the materials and manufacturing methods of cathodes and anodes change, binders that can deliver optimal performance have become necessary. Binders are expected to be a key factor in opening the door to next-generation batteries.

Binders are 'adhesives'... Directly linked to battery lifespan and capacity

In lithium-ion batteries, binders play the role of physically stabilizing the electrode. Binders are materials used to secure adhesion force or cohesion force between the active material and the current collector. In other words, they are additives that help the active material mix well with the conductive agent and be uniformly coated onto the current collector.

Binders are closely related to battery performance, including lifespan and energy density. When lithium-ion batteries undergo repeated charge and discharge cycles, insertion and extraction of lithium ions can cause cracks or swelling in the active material, which binders can mitigate or prevent. Improving binder performance can reduce its content and allow more active material to be added, thereby increasing energy density.

To be used as binder materials, several requirements must be met. First, they must maintain stable adhesion over long-term use. They must also not cause side reactions chemically or electrochemically with the electrolyte. Chemical stability means they should not undergo oxidation or reduction reactions with the electrolyte. Electrochemical stability means they should not decompose within the operating voltage range of lithium-ion batteries, which is 3 to 5 volts.

Binders must also have heat resistance to withstand temperatures up to 200 degrees Celsius during electrode manufacturing and must not break under high pressure.

In wet electrode processes, solvents are used to make the slurry. Binders are classified into organic (non-aqueous) and aqueous binders depending on the type of solvent. This means binders used with organic solvents and binders used with aqueous solvents, respectively. Generally, PVDF, an organic binder, is widely used for cathodes, while SBR/CMC, aqueous binders, are commonly used for anodes.

PVDF is classified as a line-contact binder because it maintains adhesion by connecting particles with lines. SBR/CMC belong to the point-contact binder category. Point-contact binders have better fixation strength than line-contact binders.

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 organic processes are applied instead of aqueous processes. PVDF (Polyvinylidene Fluoride) is mainly used for cathodes along with NMP (N-Methyl-2-Pyrrolidone), an organic solvent.

These two materials have excellent dispersibility and adhesion to electrode active material particles and conductive agents. They are stable and do not easily oxidize or reduce in organic electrolytes. However, PVDF has the drawback that as its molecular weight increases, the slurry viscosity also increases, reducing dispersibility. It is also known to cause delamination issues between the current collector and electrode coating layer at high temperatures.

PVDF has excellent weather resistance (resistance to various climates) and anti-fouling properties, making it widely used in solar cell films and water intake plant membranes.

For anodes, aqueous binders such as SBR (Styrene-Butadiene Rubber) and CMC (Carboxymethyl Cellulose) are mixed and used.

SBR/CMC binders bind the active material and conductive agent in a point-contact manner, providing excellent cohesion. However, stronger binders are needed to suppress the volume expansion of next-generation anode materials like silicon.

The PVDF binder market is oligopolized by several overseas companies such as Kureha in Japan, Solvay in Belgium, and Arkema in France. Domestically, ChemTros acquired PVDF manufacturing process technology from the Korea Research Institute of Chemical Technology in March 2019 and has been conducting research and development for commercialization through a pilot line since 2021.

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

Zeon in Japan is a major producer of SBR binders. Domestically, Hansol Chemical has succeeded in localizing anode binders and supplies them to Samsung SDI and SK On. LG Chem and Kumho Petrochemical also produce anode binders.

SNE Research forecasts that the global lithium-ion battery binder market will grow from 89,000 tons in 2025 to 232,000 tons in 2030. In terms of value, it is expected to reach approximately 4.4 trillion KRW by 2030.

What is PTFE used in dry electrodes?

As research on next-generation batteries progresses, new binder materials are also being developed. A representative example is dry electrodes. Tesla applied dry electrodes in manufacturing its 4680 cylindrical batteries. Since dry electrodes are made without solvents, new binder materials are required.

In the conventional wet process, slurry is coated onto the current collector, and NMP is absorbed using hot air. NMP is expensive and an environmental pollutant, so domestic companies recover and purify it for reuse. South Korea relies entirely on imports for NMP, which is monopolized by BASF in Germany and Ashland (formerly ISP) in the United States.

In the dry process, NMP is not used; instead, powder-form active material mixtures are directly coated onto the current collector or made into films and then attached to the current collector.

The binder material widely used in dry electrode processes is PTFE (Polytetrafluoroethylene). PTFE was developed in 1938 by the American chemical company DuPont and is well known to us by its trade name, Teflon.

PTFE consists of very strong carbon (C) and fluorine (F) bonds, making it heat-resistant and chemically resistant. Its smooth surface makes it widely used as a coating for frying pans and is also used in gaskets, bearings, container interiors, valve and pump parts, and saw blades.

PTFE is a white powder and has two transition temperatures. When stress is applied above 19°C, fibrillation occurs. Above 30°C, fibrillation becomes more active.

PTFE fiberization phenomenon. Image source=International Polymer Processing

Using these properties of PTFE, when active material, conductive agent, and binder are mixed and extruded, thin films can be made without solvents. Maxwell, acquired by Tesla in 2019 to develop the 4680 cylindrical battery, manufactures dry electrodes using this method.

Binders suitable for silicon anode materials

Recently, attempts to apply silicon to anodes to increase battery capacity have increased. The theoretical capacity of silicon is 420 milliampere-hours (mAh)/g, about 10 times that of graphite (372 mAh/g).

However, silicon suffers from severe swelling during charge and discharge, and the SEI (Solid Electrolyte Interphase) layer forms irregularly, shortening battery life. Carbon nanotube (CNT) conductive agents are used to compensate for this. (For silicon anode materials, see Complete Battery Mastery episode 11.)

Additionally, recent studies show that using strong polymer binders can prevent cracking of silicon anode materials during charge and discharge and improve the battery's electrochemical performance. Binders that enhance adhesion among electrode materials can help mitigate swelling and other issues.

PAA (Polyacrylic Acid) and PI (Polyimide) are attracting attention as binders for silicon anodes. Both PAA and PI are aqueous binders with higher tensile strength and adhesion than conventional binders, suppressing volume expansion of silicon anode materials. They also encapsulate active materials 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

Domestically, Aekyung Chemical is developing binders for silicon anodes and preparing for commercialization. In May last year, Aekyung Chemical completed domestic and international patent registrations for high-capacity silicon-based anode binders and is conducting tests with domestic and international clients. According to the Korean Intellectual Property Office, Aekyung Chemical obtained the final patent for polymer binders for silicon anodes jointly filed with Samsung SDI in June 2021.

European PFAS environmental regulations as a variable

The most notable issue related to binders in the battery industry currently is the environmental regulations of the European Union (EU). The European Chemicals Agency (ECHA) is pushing for usage restrictions on almost all per- and polyfluoroalkyl substances (PFAS). In February 2023, ECHA disclosed over 10,000 regulated fluorine compounds, including PVDF and PTFE.

ECHA conducted public consultations on PFAS regulations until September last year and is currently reviewing them. The final opinion is expected to be submitted to the EU Commission this year. Once confirmed by the EU Parliament and Council, PFAS regulations are expected to be implemented after a transition period starting around 2026?2027.

The Korean government and domestic related companies conveyed their opinion to ECHA and the World Trade Organization (WTO) last September, stating that "careful review is necessary" regarding the full PFAS ban. Perfluoroalkyl substances are used not only in secondary batteries but also in semiconductors and displays. Companies like Arkema and Solvay are requesting exemptions for fluoropolymers from PFAS regulations.

PFAS Regulation Roadmap. Image source=European Chemicals Agency

The battery industry expects that PVDF and PTFE will be excluded from the final opinion. Eunho Son, head of the Interface Materials Chemistry and Process Research Center at the Korea Research Institute of Chemical Technology, explained, "Whether PVDF and PTFE are included in the final opinion will depend on whether alternatives exist and the degree of their harmfulness. Contrary to common belief, these two substances are not highly toxic."

Perfluoroalkyl substances are persistent organic pollutants that do not naturally degrade easily and can accumulate in nature or the body. Representative PFAS include perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). PTFE, used as a coating for frying pans and pots, is a polymer material with a chemical structure and physical properties completely different from PFOA and PFOS. Even if small pieces of fluororesin coated on frying pans are accidentally ingested, they are not absorbed by the body and are excreted as is, so there is almost no risk of harm to humans.

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Polymer Science and Technology, Functional Polymers for Secondary Batteries, 2016.6.3
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Charged, Maxwell’s Teflon-fibrilizing electrode process could save Tesla big bucks on battery manufacture, 2019.3.21
ECHA, ECHA publishes PFAS restriction proposal, 2023.2.7
Eugene Investment & Securities, Aekyung Chemical’s Anode Binder, 2023.4.12
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