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Lithium ion full battery production process–Adhesive Section(two)

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  1. Provide viscosity to ensure that particles in the slurry do not easily settle and maintain slurry stability
  2. Provide viscosity for good fluidity
  3. Provide viscosity to facilitate effective dispersion of materials
  1. Provide adhesive force to ensure good contact between active materials, conductive carbon, and current collectors.
  2. Provide adhesive force to ensure good contact between particles.
  3. The rolling process is resistant to deformation and breakage.

Common classifications of adhesives

(1)Commonly Used adhesives:
PVDF— Commercially Used Cathode adhesive
CMC,SBR—Commercially Used Anode adhesive
PTFE—Adhesive for Supercapacitors
(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

Major Manufacturers of Commercial Adhesives

Usage Suggestions

With numerous manufacturers and a wide range of adhesive models available,how do we make the right choice?
(1)Choose mainstream manufacturers’ products to ensure quality
(2)Opt for products that have been used in commercial batches for reliable performance
(3)Clarify the application requirements of the product and make targeted selections of adhesives based on the characteristics of the research materials

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Problem of PVDF Glue Solution Discoloration

In 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?

Mechanism of Color Change

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.

Mechanism of Color Change – Conjugation Effect

Main Manifestations of Conjugation Effect:
(1) Averaging of Electron Density and Resulting Average Bond Length
(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.

Effects of discoloration

Note: The content of PVDF is 5% .

  1. Different types of PVDF can lead to different coloration results.
  2. Low-purity NMP, which contains more free ammonia, is more susceptible to discoloration.
  3. 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).

There is no significant difference in adhesion strength when comparing the electrodes prepared with conventional and color-changing glue solutions.

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.

PVDF Glue Solution Color Change Summary

(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)
(2) The 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.
(3)During high-speed stirring and dispersion, maintain a relatively low temperature of the solution
(4) The high molecular weight, copolymerization, and modification of PVDF can help reduce the discoloration phenomenon.
(5) Generally, the color change reaction does not significantly impact the performance of PVDF, and it can be used based on specific circumstances.

Issues Regarding Gelation in High-Nickel Slurries

Three Essential Factors for Gelation:
(1)PH value (primarily derived from cathode materials, with a PH of approximately 11.75 for high-nickel materials)
(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)
(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.
1. Conduct copolymerization and modification of PVDF to prevent the formation of large π-π conjugated bonds.
2. 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%

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.

Difficulties in the application of new adhesives

PI (polyimide) adhesive Advantages:
(1) The PI molecular structure contains an imide group, which has higher tensile strength 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:
(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 the 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

Adhesion Principles

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.
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
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

Company introduction

Welcome to Canrd Company (Canrd stands for “Creating Avenues for New Research Development”,website: www.canrd.com ). We specialized in:

  1. Electrochemical experiment consumables, including coin cell cases, cathodes, anodes, electrodes, electrolytes, and various other materials.
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