1.The function and selection requirements of adhesives
1.1.The function of adhesives
Cathode and anode slurries:
Provide viscosity to ensure that particles in the slurry do not easily settle and maintain slurry stability
Provide viscosity for good fluidity
Provide viscosity to facilitate effective dispersion of materials
Cathode and anode electrodes：
Provide adhesive force to ensure good contact between active materials, conductive carbon, and current collectors.
Provide adhesive force to ensure good contact between particles.
The rolling process is resistant to deformation and breakage.
Pouch Cell :
Good adhesion to metal foils and will not peel off the metal foils due to the use of electrolyte, charging, and discharging.
Good electrochemical stability within a wide voltage range.
High melting point and a low swelling rate. Even under high temperatures, the combined structure of the adhesive and the active material needs to remain stable. Adhesives usually undergo swelling, and if the swelling exceeds a certain degree, it will affect the conductivity between the active material and the current collector, leading to capacity fade of the battery. Therefore, it is necessary to control its swelling rate.
Excellent ion transport properties and electronic conductivity.
2.Common classifications and introductions
2.1.Common classifications of adhesives
2.1.1.Commonly Used adhesives:
PVDF— Commercially Used Cathode adhesive
CMC,SBR—Commercially Used Anode adhesive
PTFE—Adhesive for Supercapacitors
2.1.2.New adhesives
PAA—High-Strength Silicon Anode adhesive
PAN—Improved Rate Capability and Low-Temperature Performance
PMMA—Experimental Type
PAI—Experimental Type
PI—Experimental Type
PVA(Polyvinyl Alcohol) —Experimental Type
Sodium Alginate - Experimental Type
2.2.PVDF
PVDF is a polymer material with a high dielectric constant. It exhibits excellent chemical stability, temperature characteristics, mechanical properties, and processability, which positively contribute to enhancing bonding performance. PVDF is widely used in lithium-ion batteries as an adhesive for both positive and negative electrodes. The PVDF monomer consists of two hydrogen atoms and two fluorine atoms (—CH2—CF2). The electrons of the hydrogen atoms tend to be biased towards other atoms, making them donors. The covalent bond electron cloud is biased towards the fluorine atoms, thus fluorine atoms are referred to as acceptors. Among the crystalline forms of PVDF, the α-form is the most stable, while the remaining part is amorphous. The amorphous region serves as a good matrix for polar molecules, allowing lithium ions to penetrate through the swollen thin layer of PVDF.
2.3.CMC
CMC can effectively increase the viscosity of the slurry,improve its fluidity, and enhance the coating appearance.
2.4.SBR
SBR plays a binding role in the electrod
SBR is insoluble in water, it exists in the form of an emulsion
3.Physical parameters
4.test method
The following four methods re the most commonly used adhesive testing methods in the industry
The swelling of the adhesive can be tested by making the adhesive into a sheet and then soaking it in electrolyte
The ionic conductivity of the adhesive can be analyzed by making it into a battery and conducting EIS testing
Viscosity test
Solid content test
Slurry stability
5.Development direction of adhesives
Functionalization
Improvement in rate performance and low-temperature performance
Enhanced adhesive strength
High energy density (low usage)
Suppression of expansion (surface coating - such as silicon anode, lithium-sulfur)
Low cost
Low cost or reduced usage
Easier processing
Better solubility
Flexible electrode for easy rolling
6.Common problems with coating and previews for next course
6.1.Common issues in slurry coating
6.2. Preview of next course content
How to choose among the numerous brands of PVDF, CMC, and SBR?
If PVDF solution discolors, can it still be used?
Gelation often occurs in high-nickel slurries. How can we solve this problem?
Why are there few commercialized new adhesives? What are the reasons?
Other topics of interest to everyone
7.Canrd Brief Introduce
Canrd use high battery R&D technology(core members are from CATL) and strong Chinese supply chain to help many foreign companies with fast R&D. We provide lab materials, electrodes, custom dry cells, material evaluation, perfomance and test, coin/pouch/cylindrical cell equipment line, and other R&D services.
Email： contact@canrd.com Phone/Wechat/WhatsApp： +86 19867737979
Canrd Official Web Canrd Company Vedio Canrd Company profile
Website : www.canrud.com
8.
Q & A
In this Q&A session, Dr. Ke also answered all the questions raised by everyone one by one
亦又: "For PVDF, is a higher or lower molecular weight better?"
Dr. Ke:
"A higher molecular weight generally provides better binding strength, and you can use less of it. However, it also makes dispersion more difficult."
呱呱....: "What type of binder is better for self-supporting electrodes without current collectors? What should the selection criteria be?"
Dr. Ke:
"I understand that it should be a binder with good adhesion properties and flexible long-chain polymers, since there's no current collector, the binder also needs to play part of the role of the current collector."
985: "Which is better, PVDF or LA233?"
Dr. Ke:
"PVDF and LA133 have different applications. One is NMP-based, and the other is water-based. PVDF is mostly used in the positive electrode, while LA133 is mainly used in the negative electrode to improve low temperature and rate performance, though it can also be used in the positive electrode for LiFePO4."
亦: "In lithium-sulfur batteries, there are many binder types. What standard should be followed? Which type is better for handling expansion?"
Dr. Ke:
"Lithium-sulfur is similar to silicon-based materials, where the binder needs to control expansion. So, binders used for silicon anodes can be considered. Also, binders that can conduct ions are worth exploring."
冰蝶: "Why do 5% Nafion solutions behave differently with different catalysts, some even detaching severely in solution? How to solve this?"
Dr. Ke:
"Is there a difference in the particle size or surface area of the catalysts? If that's the case, you need to adjust the binder content accordingly."
锂盐: "Since we can use self-supporting electrodes, why still use current collectors, and what role do they play?"
Dr. Ke:
"Self-supporting electrodes serve many purposes. For example, they prevent active material layers from settling and help the active material layer adhere well to the current collector. Additionally, due to coating process limitations, using current collectors with larger porosity can lead to thicker and heavier electrodes with lower energy density. Self-supporting electrodes can have higher energy density."
月亮湾湾: "How does PVDF crystallinity affect its performance?"
Dr. Ke:
"Crystallinity affects the binder’s tensile properties, mechanical strength, and adhesion stability. However, higher crystallinity makes the binder harder to dissolve and requires higher dissolution temperatures."
黑黑: "Does a high-strength binder have a larger elastic modulus? Is high viscosity the reason for moisture absorption?"
Dr. Ke:
"High strength refers to rigidity. Some binders, like PAA (polyacrylic acid), have high moisture absorption due to their carboxyl groups, which is why they are used in diapers."
小花匠: "When referring to different types of PVDF, does this mean different molecular weights?"
Dr. Ke:
"Not only molecular weight but also copolymerization degree, synthesis method, and functional groups. The type differences include different synthesis methods, molecular weights, and functional groups. As polymer preparation is complex, there are large variations between different PVDF manufacturers."
黑黑: "Binders need to have both rigidity and flexibility, right?"
Dr. Ke:
"Exactly, a balance of both rigidity and flexibility."
小白菜: "Does the type of binder mainly affect battery cycle performance?"
Dr. Ke:
"Binders can affect almost all electrochemical properties of the battery."
亦正亦奇: "Which binder is suitable for high-nickel ternary cathode materials?"
Dr. Ke:
"For high-nickel, it’s best to use binders with optimized functional groups that are resistant to alkaline attack."
♟: "Could a binder that is too strong block the ion channels of the active material?"
Dr. Ke:
"It depends on the type of binder. If it wraps around the active material completely, it could indeed hinder ion transport."
猛将兄: "Is there a typical range for binder usage, for example, in the full battery manufacturing process? Is there a minimum amount?"
Dr. Ke:
"From what I’ve seen in industrial formulations, it’s usually between 1% to 6%."
奋斗: "If PVDF is used for the negative electrode, does it affect the performance much? How does it compare to CMC and SBR as binders?"
Dr. Ke:
"We’ve tested PVDF for the negative electrode, and it provides weaker bonding compared to water-based binders, which also leads to reduced first-cycle efficiency. While it helps with low temperature and rate performance, the long-term cycling performance is worse than with CMC+SBR."
小花匠: "Have you analyzed the reasons for this? Also, are we assuming graphite as the negative electrode material?"
Dr. Ke:
"Yes, we've analyzed and compared different binders. We suspect PVDF might break bonds and participate in reactions. PVDF has better ionic conductivity compared to SBR, but the poor bonding leads to worse cycling performance."
转身、未来: "If the negative electrode material is hydrophilic, but the literature uses PVDF for mixing and slurry preparation, I face sedimentation with PVDF but CMC works better. How should I choose?"
Dr. Ke:
"Just set up a comparison group based on your situation. We suggest using water-based CMC+SBR."
阿斗: "What is the typical shelf life of PVDF?"
Dr. Ke:
"PVDF shouldn’t be stored too long. Its shelf life is usually about two years from the production date."
北极海: "If the slurry coating easily detaches from the copper foil to form a self-supporting film, how much does this affect the final performance?"
Dr. Ke:
"It indicates that the adhesion between the electrode coating and current collector is insufficient, which increases the resistance between the coating and the collector and negatively impacts performance. You can try: 1. Add more binder to increase adhesion; 2. Try using carbon-coated copper foil."
mecate: "In literature on silicon anode binders, PVDF is often used for comparison, but PVDF isn’t commercially used for negative electrodes. Doesn’t CMC perform better for silicon anodes?"
Dr. Ke:
"This is mostly for comparison purposes, and there’s no issue with that. But I agree, water-based binders are better for silicon anodes in practice."
NEU-硅基: "Which binder provides the best cycling performance for silicon-carbon anodes?"
Dr. Ke:
"For silicon anodes, PAA-based binders currently provide the best performance."
不倒翁+先进院+锂电: "When using PVDF as a binder for the negative electrode, for a button half-cell where the active material acts as the positive electrode, what binder should be used?"
Dr. Ke:
"For lithium metal, both positive and negative electrodes with binders and conductive agents are considered the positive electrode. In research, PVDF is used for the negative electrode, but for commercial use, CMC+SBR is more common, and data supports better results."
亦正亦奇: "Which is better, acetylene black or SP as a conductive agent?"
Dr. Ke:
"I haven’t compared them directly, but acetylene black is not commonly used in industry."
┌.呓噫: "How can SBR be re-dispersed in water after drying? Or is there another solvent that can dissolve it?"
Dr. Ke:
"Once dried, SBR cannot be dissolved."
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