Lithium batteries - Materials - Performance comparison of common binders

The electrode structure of lithium-ion batteries is delicate and critical, mainly composed of core elements such as active materials, binders, conductive agents, and current collectors. 

Although the proportion of binders and conductive agents in the composition is relatively low, their impact on battery performance cannot be underestimated. 

The core function of the binder is to tightly bond solid particles such as active substances and conductive agents into a whole, and firmly attach the electrode coating to the current collector.

The specific function of adhesive is reflected in:

Ensure mechanical stability, stabilize the structure and volume changes of positive and negative electrode active materials during charging and discharging processes, 

prevent active material detachment, and enhance the cycling stability of electrode sheets;

Reduce the internal resistance of the battery, collaborate with conductive agents to construct a conductive network, and effectively promote electronic conduction within the electrode;

Optimize the wetting performance of the electrolyte, enhance the transport efficiency of lithium ions between the electrode and electrolyte interface by adsorbing the electrolyte.

The performance requirements for lithium battery binders are mainly reflected in the following aspects:

Adhesive performance: The adhesive is required to have excellent adhesive performance, high tensile strength, good flexibility, 

and low Young's modulus to ensure that the active material will not detach from the electrode during repeated expansion and contraction during battery production, use (storage, cycling),

 and the bonding between electrode particles will not be damaged;

Chemical stability and electrochemical stability: Whether in high potential (positive electrode binder) or low potential (negative electrode binder) environments, 

the binder should maintain its stability and not undergo side reactions with active materials, Li, and other substances;

Electrolyte compatibility: It is required that the binder maintains stable shape, structure, and properties in the electrolyte, 

is insoluble in the electrolyte solution or has a low swelling coefficient, and does not undergo chemical reactions with the electrolyte;

Processing performance: It should have good dispersibility in the slurry medium, 

which is conducive to the uniform bonding of active substances on the current collector and provides convenience for the processing of slurry, electrode sheets, and batteries;

Dynamics performance: The negative impact on electron and ion conduction in the electrode should be minimized as much as possible.

There are various types of adhesives commonly used in lithium-ion battery electrodes, including PVDF, PTFE, poly (acrylic acid) (PAA), styrene butadiene rubber (SBR), polyethylene oxide (PEO), 

carboxymethyl cellulose sodium (CMC), and alginate. 

Yu Xiqian, a research team from the Institute of Physics of the Chinese Academy of Sciences/National Research Center for Condensed Matter Physics in Beijing, 

conducted in-depth research and comparison on the performance parameters of these commonly used adhesives.

The basic properties of these adhesives, such as tensile and compressive mechanical properties, adhesive strength, and thermal properties, have been detailed in Tables 1, 2, 3, and 4.

 Among them, the tensile performance reflects the electrode's resistance to volume expansion, while the compressive performance demonstrates the electrode's ability to maintain structural integrity under pressure.

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When conducting in-depth comparative evaluations of the performance of these seven common adhesives, we considered their adhesion, tensile strength, elasticity, swelling, ionic conductivity, 

thermal stability, and oxidation stability, as shown in Figure 1.

PVDF (polyvinylidene fluoride) exhibits excellent comprehensive performance among numerous adhesives. 

As a key polymer, PVDF is produced on a large scale mainly through lotion or suspension polymerization technology, using vinylidene fluoride monomer, surfactant and initiator.

 Its unique molecular structure, in which H atoms are partially replaced by F atoms, endows PVDF with extremely high chemical, electrochemical, and thermal stability.

 Its electrochemical window width is about 5V, and its thermal decomposition temperature is as high as 400 ℃. 

In addition, the molecular structure of PVDF facilitates the formation of hydrogen bonds, 

ensuring excellent mechanical and adhesive strength through van der Waals forces and interactions between hydrogen bonds and polymer chains or adhesive surfaces between molecules.

 Compared with fully fluorinated polytetrafluoroethylene (PTFE), PVDF not only has superior tensile strength and adhesion, 

but also exhibits good swelling behavior and crystallinity, thus providing excellent ionic conductivity after absorbing electrolytes.

 However, for ultra-high quality load electrodes (>20 mg/cm ²) or significant changes in electrode volume, the adhesion of PVDF may be slightly insufficient, 

due to its lower electron cloud density and polarizability resulting in weaker van der Waals force interactions.

PTFE (polytetrafluoroethylene) exhibits extremely unbalanced properties in its perfluorinated form.

 Its unique CF2-CF2 unit endows it with unparalleled chemical stability, mechanical properties, and oxidative stability, ranking among the top in many adhesives. 

However, its adhesion and conductivity are relatively weak. The hexagonal crystal structure of PTFE results in lower inter chain cohesion and easier sliding along the chain axis. 

Therefore, under shear load, PTFE crystals are prone to crystal slip along the c-axis, forming a high aspect ratio nanofiber structure, and thus constructing a three-dimensional network structure, 

effectively polymerizing active materials and carbon black. This characteristic has enabled PTFE to be widely used in dry electrode processes.

From a mechanical performance perspective, although PAA (polyacrylic acid), CMC (carboxymethyl cellulose sodium), and alginate are not as good as PVDF, 

their water solubility and abundant carboxyl or hydroxyl groups contribute to stronger adhesion. 

SBR (styrene butadiene rubber) is known for its high elasticity and is often mixed with CMC to complement each other's shortcomings.

CMC, as a multi-component weak acid, can dissociate to form carboxylate anion functional groups, 

which interact with hydroxyl groups on the surface of materials such as silicon/carbon to form an ideal carbon gel phase network.

Although CMC has low cost, good thermal stability, and environmental friendliness, it exhibits strong rigidity and brittleness as a water-based adhesive, 

which may cause cracks on the electrode surface and even lead to gaps between the electrode material and the current collector. 

To solve this problem, SBR is often used as an elastic additive for CMC, reducing the brittleness of the electrode and improving the adhesion strength of the electrode to the current collector.

 PAA, as a water-soluble polymer, has abundant carboxylic acid groups that enable it to form strong interactions with active substances and aluminum foil, making it a potential high-performance binder for electrodes.

 During the battery cycling process, PAA also helps to form a stable CEI (solid electrolyte interface), improving the cycling stability of the battery.

 PEO (polyethylene oxide) is known for its excellent ion conductivity, but its oxidation resistance at high voltages is slightly insufficient.