1.The function of adhesives
Cathode and anode slurries：
Provide viscosity to ensure that particles in the slurry do not easily settle and maintain sslurry 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.
2.1.Commonly Used adhesives:
PVDF— Commercially Used Cathode adhesive
CMC,SBR—Commercially Used Anode adhesive
PTFE—Adhesive for Supercapacitors.
2.2.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
With numerous manufacturers and a wide range of adhesive models available,how do we make the right choice?
4.1.Choose mainstream manufacturers' products to ensure quality
4.2.Opt for products that have been used in commercial batches for reliable performance
4.3.Clarify the application requirements of the product and make targeted selections of
adhesives based on the characteristics of the research materials
Recommended that everyone clearly consider the product requirements and material characteristics before use.
If you have any questions or need advice, please feel free to consult with Canrd Technology.
5.Problem of PVDF Glue Solution
DiscolorationIn practical use, PVDF glue solutions often encounter the issue of darkening in color, sometimes even turning a dark red hue. What is the reason for this? Is it still usable?
Formation of Conjugated Double Bonds: When PVDF is dissolved in an NMP solution with high alkalinity (free ammonia) or the glue solution is stored at a high temperature for a long time, the alkaline groups attack adjacent C-F and C-H bonds. PVDF is prone to bimolecular elimination reactions, resulting in the formation of some double bonds on the molecular chain.
Mechanism of Color Change: Conjugated double bonds have a basic unit of C=C-C=C. As the degree of conjugation increases, there are certain ultraviolet characteristics observed: a red shift in the maximum absorption wavelength; if there is fluorescence, a red shift in the maximum excitation wavelength and a red shift in the maximum emission wavelength; if there is color, it gradually deepens. Compounds with conjugated double bonds exhibit an interaction between alternating π bonds (π-π conjugation effect), resulting in the formation of a large π bond. Since the energy levels of the large π bond are closely spaced, electrons are easily excited, leading to an increase in the wavelength of the absorption peak and a significant enhancement of chromogenic effects.
6.1.Conjugation EffectMain Manifestations of Conjugation Effect:
6.1.1.Averaging of Electron Density and Resulting Average Bond Length
6.1.2.Lowering of Energy in the Conjugated System.The energy differences between various energy levels decrease, resulting in lower excitation energies for electrons in the molecule. This causes the absorption spectrum of the conjugated system to shift towards longer wavelengths. As the conjugated chain grows longer, the absorption spectrum shifts towards even longer wavelength regions, entering the visible light spectrum.
The longer the conjugated chain is, the red shift of the absorption spectrum
wavelength occurs, and the coloration becomes more obvious.
7.Effects of discolorationDifferent types of PVDF can lead to different coloration results.
Low-purity NMP, which contains more free ammonia, is more susceptible to discoloration.
When PVDF in the colloidal solution is precipitated into a solid, it is observed that the color is completely different from that of the solution, and the discoloration phenomenon is not prominent (which validates the previously mentioned discoloration principle).
Note: The content of PVDF is 5%
There is no significant difference in adhesion strength when comparing the electrodes prepared with conventional and color-changing glue solutions.
Using UV to analyzed the absorption of light
Varying the storage temperature of the glue solution, observed changes in light absorption while also monitoring the molecular weight
As the temperature increases, the color change of the glue solution becomes more apparent, and the absorbance also increases (shown by the red line). However, there is no significant change in the molecular weight, indicating that there is no essential change in PVDF.
8.PVDF Glue Solution Color Change Summary
8.1.The primary reasons for the color change in PVDF glue solution are high temperature and alkaline groups (such as the concentration of free ammonia in NMP)
8.2.he presence of alkaline groups triggers a de-HF reaction in PVDF, leading to the formation of conjugated double bonds. These conjugated double bonds absorb light, resulting in the discoloration of the solution.
8.3.During high-speed stirring and dispersion, maintain a relatively low temperature of the solution
8.4.The high molecular weight, copolymerization, and modification of PVDF can help reduce the discoloration phenomenon.
8.5.Generally, the color change reaction does not significantly impact the performance of PVDF, and it can be used based on specific circumstances.
9.Issues Regarding Gelation in High-NickelSlurries
Three Essential Factors for Gelation:
9.1.PH value (primarily derived from cathode materials, with a PH of approximately 11.75 for high-nickel materials)
9.2.PVDF (Conventional PVDF is susceptible to alkaline groups, which can lead to the loss of HF and the formation of conjugated double bonds. The presence of these conjugated double bonds can easily cause molecules to form a viscous state due to intermolecular interactions）
9.3.Humidity (Moisture content in the slurry and the environment)
Improvement Measures
Reduce the PH value of high-nickel materials, for instance, some material manufacturers currently employ water washing processes.
Conduct copolymerization and modification of PVDF to prevent the formation of large π-π conjugated bonds.
Control the moisture content of the slurry and the environment. Generally, the relative humidity in the mixing workshop for high-nickel materials should be less than 10%
10.Application Challenges of Novel Adhesives
Lithiated PAA
Advantages: Possesses high tensile strength and is easy to form a film on the surface of materials, making it suitable for materials with large volume expansion. Additionally, the - COOLi moiety exhibits excellent ionic conductivity, improving the rate capability.
Due to the material characteristics of lithiated PAA, various issues arise during the battery manufacturing process, which is why it has not yet been commercially applied in large-scale production.
PI (polyimide) adhesive
Advantages：
1）The PI molecular structure contains an imide group, which has higher tensile trength and elastic resilience, and can effectively adapt to the expansion and contraction of the electrode.
2)The thermal decomposition temperature of the material is high and the safety is good.
Application Questions:4
1）Need to carry out high-temperature polycondensation reaction at a temperature of ~250 °C, that is, after the electrode is ready, it need to carry out high-temperature polycondensation in a high-temperature oven, which is difficult to process.
2） The rigidity of the material is more than enough, and the flexibility is insufficient, which makes it difficult to smooth the electrode after deformation, which affects the interface
3）The material contains an imide group with lone pairs, which will consume lithium ions in he first lithium intercalation, reducing the efficiency of the first time
4）High material cost
Here are two examples of new adhesive applications, to illustrate that although many materials have some good properties, but to make a product, you need to consider the overall performance, just like LiPF6, a lithium salt, each performance is not optimal, but the comprehensive performance is the most suitable for the application
11.Adhesion Principles
11.1.Wettability Theory
When viewed under magnification, any solid surface appears to be uneven. To enable the adhesive to completely adapt to the "terrain" of the solid surface, it must turn into a
liquid during the bonding process and fully wet the surface of the solid.Complete wettability is a necessary condition for achieving high-strength joints. If wettability is incomplete, numerous bubbles will appear at the interface. Under stress, stress concentration will occur around these bubbles, leading to a significant reduction in strength. Since the surface tensions of organic liquids do not vary greatly, the time required for wettability mainly depends on the viscosity of the liquid and the contact angle. Lower viscosity results in shorter wetting time, allowing for thorough infiltration into surface gaps. A smaller contact angle leads to faster wetting speeds.
11.2.Mechanical theory
The mechanistic theory holds that the adhesive must penetrate into the voids on the surface of the adherent and exclude the air adsorbed at its interface in order to produce an adhesive effect. Mechanical embedding is an important factor when bonding porous substrates such as Styrofoam. Adhesives bond dense materials with a sanded surface better than dense materials with smooth surfaces because:
(1) mechanical mounting
(2) formation of a clean surface
(3) generation of reactive surface
(4) surface area increase. Since the surface is rougher due to sanding, it can be assumed that the physical and chemical properties of the surface layer have changed, thus increasing the bond strength
11.3.Adsorption theory
Adsorption theory states that bonding is caused by the contact between the molecules of two materials and the generation of interfacial forces. The main sources of adhesion are intermolecular forces, including hydrogen bonding force and van der Waals force. The process of continuous contact between the adhesive and the adherent is called wetting, to make the adhesive wet the solid surface, the surface tension of the adhesive should be less than the critical surface tension of the solid, and the adhesive is immersed in the depressions and voids of the solid surface to form good wetting. If the adhesive is hollowed out in the recess of the surface, the actual contact area between the adhesive and the adherent object is reduced, thus reducing the bond strength of the joint.
Many synthetic adhesives are susceptible to wetting metal substrates, and most solid adhesives have less surface tension than the adhesive. In fact, good wetting is achieved when the adhesive has a lower surface tension than the adherent, which is why epoxy adhesives are excellent for metal bonding, while untreated polymers such as polyethylene, polypropylene, and fluoroplastics are difficult to bond. The adhesive is in close contact with the adherent object through wetting, and the permanent bond is mainly produced by intermolecular forces.
There are four types of chemical bonds including adhesion and cohesion:
(1)ionic bonds, (2) covalent bonds, (3) metallic bonds, and (4) van der Waals forces
12.Factors influencing adhesionPhysical influencing factors：
1.Surface roughness
2.Surface treatment
3.Infiltration
4.Migration
5.Stress
6.Adhesive layer thickness
7. Load stress includes shear stress, peeling stress and alternating stress
Chemical influencing factors：
1.Polarity
2.Molecular weight
3.Sidechain
4.PH value
5.Cross-linking
6.Solvents and plasticizers
7. Packing
13.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
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Website : www.canrud.com
14.Q & A
Finally, let’s briefly discuss the relevant influencing factors. Here, I will focus solely on the physical factor—pressure. This explains why the adhesion strength of the electrode is often significantly better after calendering compared to before. As mentioned earlier, calendering creates more interfacial contact between the material and the current collector. In some cases, particles are even pressed into the current collector, forming strong physical bonds.
That concludes today’s lecture. Thank you all for your attention, and I welcome further discussions and exchanges!
During this Q&A session, Dr. Ke also addressed each of the questions raised by the participants:
Chongqing-Lithium Battery: "Which type of PVDF is used for NCA modification?"
Dr. Ke:
"NCA can use general types of PVDF, such as 5130 or 900."
Moon Bay: "How long should LA132 adhesive be mixed?"
Dr. Ke:
"LA132/LA133 adhesives are pre-dissolved and can be used directly."
Jiang iaobai: "Why isn’t PVDF used for the anode?"
Dr. Ke:
"This is primarily due to considerations of cost, environmental impact, and safety."
★Xiu & Si★: "Does adding too much conductive agent lead to a low first-cycle efficiency?"
Dr. Ke:
"Currently, both conductive agents and binders are used in very small amounts in the industry, as conductive agents typically have inherently low first-cycle efficiency."
nnn: "Why does the binder reduce rate performance?"
Dr. Ke:
"Binders themselves have very poor ionic conductivity. In electrode materials, they evenly coat the surface of the active material particles, essentially creating a 'cement layer' on the active material, which hinders ion transport. Therefore, the amount of binder is minimized as much as possible while ensuring sufficient adhesion. For example, some positive electrode formulations use as little as 1% binder."
/Weiqu & Cangsang/: "Isn’t 1% binder for the cathode a bit low?"
Dr. Ke:
"Generally, materials with a large specific surface area adsorb more binder on their surface, leaving less binder available to bond between active material particles. However, in current mass-production formulas, binders as low as 1% are already being used."
980101741: "What does modified PVDF refer to?"
Dr. Ke:
"It's a general term. Modifications can include adding side chains, using non-homopolymers, or selecting copolymers."
Angel Offline: "Once, during slurry mixing, there was no circulating water, and the temperature rose significantly. How can we determine if the slurry is still usable?"
Dr. Ke:
"To assess the quality of the slurry, you can refer to our previous live class notes. Key metrics to check include particle size, viscosity, and slurry rheology."
Material Development: "What are the typical goals for modifying PVDF?"
Dr. Ke:
"The main goal is to enhance the stability of PVDF. For high-nickel materials, it can also prevent gelation."
Material Development: "If the binder amount is reduced, can we just add more water? Adding water can lower viscosity by adjusting solid content, but does that risk sedimentation?"
Dr. Ke:
"Adding too much water and lowering the solid content will definitely affect the stability of the slurry."
tzhowever: "Regarding PI application, do we still need to polycondense the PI monomer? Isn't PI already polycondensed?"
Dr. Ke:
"If the PI is fully polycondensed, it cannot dissolve in a solution, so it can't be made into a slurry. Here, I'm referring to using partially polymerized PI in solution to prepare electrode films. These PIs undergo further polycondensation at high temperatures to form a polymer network with sufficient strength."
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