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Understanding PVDF Binder for Lithium Ion Battery

Polyvinylidene fluoride (PVDF) combines the benefits of fluoro resins and general-purpose resins, offering excellent resistance to chemicals, oxidation, high temperatures, weather, and radiation, along with unique properties like piezoelectricity, dielectricity, and thermoelectricity.

It is the second most produced fluoro resin after PTFE and is widely used in weather-resistant coatings, injection molding, lithium batteries, and photovoltaic backsheet films. As new energy industries like lithium batteries and photovoltaics grow rapidly, the demand for PVDF is increasing, especially for lithium battery applications.

Lithium Batteries Becoming a Trend

Lithium-ion batteries are gradually replacing traditional lead-acid and nickel-metal hydride batteries due to their advantages such as high energy density, strong actual load capacity, long cycle life, low self-discharge rate, and small volume. In lithium batteries, the electrodes are composed of binders, active materials, current collectors, and conductive agents.

Although binders account for a small portion, approximately 1% to 10% of the total mass of the electrode sheet, their role is irreplaceable. The quality of the binder directly affects the various electrochemical performances of the battery.

Role and Performance of Binders

  1. Ensure uniformity and safety during the slurry preparation of active materials
  2. Act as an adhesive between active material particles
  3. Bond active materials to the current collector
  4. Maintain adhesion between active materials and between active materials and the current collector
  5. Facilitate the formation of a solid electrolyte interphase (SEI) film on the surface of carbon materials (graphite)

Performance Requirements for Lithium Battery Binders

  1. Adhesive Properties: Binders must have good adhesive properties, high tensile strength, good flexibility, and low Young’s modulus. They should provide sufficient bonding strength to ensure that during the production and use (storage, cycling) of the battery, the active material does not detach from the electrode sheet due to repeated expansion and contraction, and the bonding between electrode particles remains intact.
  2. Chemical and Electrochemical Stability: Binders must remain stable under high potential (cathode binders) or low potential (anode binders) conditions. Cathode binders should not oxidize under high voltage, and anode binders should not reduce under low voltage. During storage and cycling (battery charging and discharging), binders should not undergo side reactions with active materials, lithium, or other substances.
  3. Electrolyte Compatibility: Binders should be insoluble in the electrolyte solution or have a low swelling coefficient, not react chemically with the electrolyte, and maintain their shape, structure, and properties in the electrolyte.
  4. Processing Performance: Binders should have good dispersibility in the slurry medium, facilitating the uniform bonding of active materials to the current collector, and should provide good slurry, electrode sheet, and battery processing performance.
  5. Kinetic Performance: Binders should have minimal impact on the conduction of electrons and ions within the electrode.

PVDF: The Most Widely Used Binder in Lithium Batteries

Polyvinylidene fluoride (PVDF) is one of the most widely used binders for both the cathode and anode in lithium batteries, drawing significant attention from researchers.

Advantages of PVDF Binders

  • PVDF has a broad electrochemical stability window, maintaining stable electrochemical performance from 0 to 5V (Li/Li+).
  • It possesses good oxidation resistance and chemical inertness, making it less prone to degradation.
  • PVDF also exhibits excellent swelling properties, which enhances the wettability of electrode sheets with the electrolyte.

Disadvantages of PVDF Binders

  • PVDF is a semi-crystalline polymer, and despite its excellent electrochemical and chemical stability, it has inherently weak electronic and ionic conductivity.
  • The adhesive effect of PVDF generally arises from van der Waals forces between molecules and hydrogen bonds formed between the C-F bonds on its main chain and other substances in the electrode. Particularly when used with silicon anodes, which undergo significant volume expansion, it can lead to capacity loss and disruption of the conductive network.
  • Additionally, PVDF’s molecular weight decreases upon absorbing water, reducing its adhesive properties, thus requiring stringent humidity control in the environment.
  • PVDF requires N-methyl-2-pyrrolidone (NMP) as a solvent, which has a high evaporation temperature, poses environmental pollution risks, and is costly.

Therefore, researchers often modify PVDF binders to enhance the overall performance of batteries.

PVDF Binding Mechanism

The primary binding mechanism of conventional PVDF involves van der Waals forces. These forces are facilitated by hydrogen bonds formed between the fluorine atoms on the long chains of PVDF and other component particles in the electrode sheet. The hydrogen bonds help link the component particles together, with intermolecular forces providing the adhesive effect.

In modified PVDF, the binding mechanism consists of two parts: one is the van der Waals forces resulting from the high molecular weight, and the other is the chemical bonds formed between the modified PVDF and the foil material.

Factors affecting the adhesive properties of PVDF include molecular weight, crystallinity, PVDF modification, types of cathode materials, and conductive agents. The larger the molecular weight of PVDF, the stronger the adhesion, leading to greater peel strength of the electrode sheet. Higher crystallinity results in tighter packing between molecular chains, increasing intermolecular forces and improving adhesion.

Modified PVDF Binders

Copolymer Modified PVDF Binders

Copolymer modification is the most commonly used method for preparing PVDF binders in batteries. VDF is often copolymerized with fluorinated or non-fluorinated monomers to obtain functional binders. For instance, copolymerizing with fluorinated monomers like hexafluoropropylene and tetrafluoroethylene can enhance the flexibility of the resin. The main issue with using PVDF as a binder is insufficient adhesion.

A common solution is copolymerizing with monomers containing polar groups such as carboxyl or hydroxyl groups to improve adhesion with polar substances and metal electrodes.

Graft Modified PVDF Binders

Graft modification of PVDF binders typically involves using PVDF as the main body with small molecules or inorganic particles dispersed within it, often resulting in chemical bonds between them. For example, reports indicate that simply physically mixing graphene with PVDF can lead to graphene aggregation.

However, by simply heating and stirring to incorporate graphene into the insulating PVDF binder, hydrogen bonds and other chemical bonds can form between graphene and PVDF. This can create a good electrical network in lithium titanate composite electrodes, reducing charge transfer resistance and increasing lithium-ion diffusion rates.

Compound Modified PVDF Binders

In the modification of PVDF binders, compound modification is a simple and effective method. It generally involves compounding another polymer with PVDF to obtain a binder with improved properties. Common polymers compounded with PVDF include polyethylene glycol (PEG), polymethyl methacrylate (PMMA), and polyvinyl acetate (PVAc).

The modified binder typically shows a moderate reduction in PVDF crystallinity, better compatibility with the electrolyte in composite electrodes, faster lithium-ion transport, and improved battery performance.

Conclusion

To overcome these issues like not being strong enough and not conducting electricity well, researchers are exploring new methods mentioned above. These approaches aim to make PVDF binders more flexible, improve their chemical bonding, and enhance their compatibility with electrode materials.

As research continues, further innovations in PVDF modification are expected to contribute to the development of more efficient, durable, and high-performance batteries, supporting the growing needs of modern energy storage applications.

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