09: The Role of Binders in Lithium-Ion Batteries

Hi everyone! In this post, I will explain the important tasks of binders in lithium-ion batteries.

Binders

Binders in lithium-ion batteries serve several vital functions that are crucial for both the performance and longevity of these batteries. One of their primary roles is to maintain the structural integrity of the electrode materials. They help hold together the active material particles (like lithium cobalt oxide or graphite) and conductive additives in a cohesive composite. Additionally, they also ensure good electrical contact within the electrode through maintaining a connection between the active material particles and the current collector, which is vital for efficient electron transfer and overall battery performance. While binders themselves are generally electrical insulators, they must not impede the movement of lithium ions. This ensures that the battery maintains good ionic conductivity, crucial for its charging and operational efficiency.

Binders must also accommodate the volume changes that occur in the electrode materials during charging and discharging. A good binder is flexible enough to absorb these changes without cracking or losing contact with the active material or current collector. This flexibility contributes significantly to the battery’s durability and cycle life. Binders also need to be chemically stable and inert in the battery’s electrolyte to prevent chemical degradation, which can lead to the breakdown of the electrode structure and battery failure.

In the manufacturing process, binders aid in creating a uniform slurry of the active material, which can be coated smoothly onto the current collector. This uniformity is essential for consistent battery quality and performance. Moreover, binders contribute to the environmental stability of the battery, protecting the electrodes from humidity, temperature variations, and other environmental factors.

Polyvinylidene fluoride (PVDF) is a common binder used in lithium-ion battery electrodes due to its chemical stability, mechanical strength, and good adhesion properties. It helps in forming a strong bond between the active material particles and the current collector, enhancing the electrode’s structural stability during repeated charge-discharge cycles.

How They Work

Most binders realize the cohesion by physical interlocking or chemical bonding. Adhesion is a complicated process that is influenced by the bulk composition, surface chemistry, and the morphology (size, aspect ratio, and surface roughness) of both binders and the materials to be cohered. Moreover, the environmental temperature and humidity also affect the adhesion forces and even the mechanisms, which makes the process more intricate. One most typical example is dopamine and its derivatives, which are the most widely studied binder family. The catechol group is believed to be capable of interacting with almost all surfaces including metals, oxides and polymers via different mechanisms.

A recent study found that dopamine is capable of forming high-strength yet reversible bonds with both organic and inorganic surfaces via the adjustment of its own oxidation states. It developed a facile surface modification approach, in which a self-polymerization of dopamine produced an adherent coating on a wide variety of surfaces, where the wettability, processibility and many other characteristics of the materials could be highly changed. The study also discovered a highly reversible nanostructured adhesive, which could be even applied under water.

Another example commonly used in LIBs is carboxymethyl cellulose (CMC). CMC is believed to act as a surfactant to enhance the dispersity of the hydrophobic graphite. SBR contributes most of the adhesion forces in electrodes by physical interlocking. In Si-based electrodes, it is the carboxyl groups in CMC that form dynamic H-bonds and covalent bonds with the hydroxyl groups on the Si surfaces. That is to say, the same binder may work in different ways when used in different systems. Therefore, it is necessary to understand the properties of AMs in batteries to develop binders with better performance.

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