1.Mixing
1.1.Slurry stirring is the process of adding active materials, conductive carbon black, dispersants,
binders, additives, and other components to a mixing equipment in a certain proportion and
order. Under the mechanical actions such as agitation, kneading, and shearing generated by the
equipment, these components are mixed together to form a uniform, stable solid-liquid
suspension system suitable for coating.
Goal: Achieve uniformity and consistency at both macro and micro levels
1.2.Goal: Wettability, dispersion, and stabilization are the three key indicators of good slurry
1.3.Good wettability can reduce the difficulty of dispersion and improve mixing efficiency.
1.4.Mechanical stirring dispersion refers to the process of causing strong turbulent motion of the liquid flow through intense mechanical stirring, which leads to the breakup and suspension of particle aggregations.
1.5.The spatial hindrance effect of the dispersant adsorbed on the surface of the particles generates a strong repulsive force between the particles.
The cathode and anode slurries utilize polymeric materials (PVDF/CMC) to provide steric repulsion, maintaining the stable suspension of particles.
2.Mixing equipment
universities mainly use magnetic mixing and ultrasonic dispersion. Advantages: Less material usage. Problem: Poor dispersion effect.
Commercial mixing primarily uses planetary mixing. Advantages: excellent dispersion effects. Problem: requires a larger amount of material.
magnetic mixing simple dispersion disk planetary mixing
3.Mixing device
The mixing tank has both revolution and rotation (dispersion disk)
The tank is equipped with a circulating cooling water system to control the temperature of the entire slurry.
The tank has a vacuum pump to remove air bubbles.
4.Mixing Process
The mixing process is similar to the positive electrode, except for the final step add SBR (binder).
The left figure shows the general mixing process (revolution speed~20rpm)
5.Preparation of PVDF glue solution
5.1.Mainstream CMC gel solution
5.2.The process of formula improvement based on actual needs, incorporating new binders such as PAN, PAA, etc.
6.Slurry dispersion
Control the temperature of the slurry during the entire mixing process, and a vacuum pumping step is performed before shipment to ensure that the slurry coating is free of bubbles (similar to the preparation of cathode slurry)
add SP,disperse for 10min add SP,disperse for 2h
add active material, disperse for 2h adjust viscosity, then sieve
7.CMC and Surface Wetting
1)Due to the hydrophobic nature of the graphite surface, when graphite powder is added to water, it floats on the surface and does not wet properly. Therefore, it is necessary to add CMC (carboxymethylcellulose sodium) to improve the surface condition, allowing the graphite to wet and disperse in the water.
2)Some surfaces of secondary agglomerated graphite are difficult to wet completely. In such cases, kneading and mixing are needed to improve the wetting condition of the surfaces. (Detailed discussion will be share next time.)
3)CMC is an excellent thickener that increases the viscosity and fluidity of the slurry, resulting in a good coating appearance and improved weight uniformity
8.SBR (Styrene-Butadiene Rubber)
1)SBR exists as a binder in tab, bonding the active material with the current collector and particles together.
2)With the development and advancement of materials, many new binders have emerged to improve bonding, enhance low-temperature discharge, and improve rate charge-discharge performance, etc.
9.Precautions for the Use of SBR
1)SBR should be added last when stirring the slurry, and the stirring speed should be reduced to less than half of the high speed to prevent high-speed stirring from destroying the SBR's glue bundles, which can cause demulsification and slurry agglomeration.
2)After coating the slurry, it is necessary to control the baking speed during the drying process. Too fast can easily cause uneven distribution of SBR inside the electrode, known as SBR floating, affect bonding and demolding.
Analysis of Electrode Section
normal conditions SBR tends to float
10.Demulsification Phenomenon
1)Demulsification: refers to the complete disruption of an emulsion into two immiscible phases.
2)SBR (Styrene-Butadiene Rubber) used in lithium batteries is a type of aqueous binder, specifically a latex of styrene-butadiene copolymer. It is produced through emulsion polymerization of styrene and butadiene monomers in an aqueous medium with the addition of emulsifiers and initiators. The resulting product is a water-based emulsion with a solid content of approximately 50%. This emulsion remains stable due to the formation of colloidal micelles that are oil-philic inside and water-philic outside. Under normal conditions, these micelles remain stable. However, when the stirring speed is too fast and the mechanical force is strong, the micelles can be destroyed, exposing the oil-philic and hydrophobic interior, which then becomes a new phase that is immiscible with water. This is known as solid precipitation.
11.Monitoring methods
Usually, the following methods are used to monitor the slurry.
viscosity testing solid content testing scraper fineness meter
slurry stability Membrane internal resistance test
12.Canrd Brief Introduce
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13.Q & A
During the Q&A session after the lecture, everyone actively asked questions, and Dr. Ke Lu De patiently provided answers. He also graciously accepted all the course suggestions offered by the students.
1234567890: "Is the planetary mixer used to improve mixing efficiency? Can't we just filter out the undissolved PVDF? Is this process similar to ink production?"
Dr. Ke Lu De:
"The planetary mixer is used both for high efficiency and achieving good dispersion. Filtering out undissolved PVDF would reduce the amount of PVDF added, which in turn affects the viscosity and reduces the stability of the slurry. Additionally, less PVDF will impact the bonding performance."
Fan: "If conductive agents need SP and CNT, how can we determine whether they are well dispersed?"
Dr. Ke Lu De:
"Generally, you can use a particle size analyzer, a scraper fineness gauge, or a microscope to observe whether the conductive agents are dispersed or agglomerated. Battery manufacturers typically optimize the dispersion process during the experimental stage, and during production, you just need to control the stirring process parameters."
Star Fragments: "Do CNTs need pre-dispersion? Aren't MW-CNTs severely entangled?"
Dr. Ke Lu De:
"CNTs do need pre-dispersion. Currently, commercial CNTs are already dispersed into a slurry. For single-walled CNTs, they are even harder to disperse, requiring dispersants and better dispersion techniques like bead milling."
Tan Gui: "Why do some 2% PVDF solutions appear completely transparent, while others are milky? And why does PVDF and NMP mix turn yellow?"
Dr. Ke Lu De:
"From our understanding and practice, two main factors influence this phenomenon. One is the humidity: high humidity can cause this issue. The second factor is the amount of water and ammonium ions in the NMP. The ammonium ions can attack the F-bonds of PVDF, forming carbon-carbon double bonds, which are reactive and can form color-changing functional groups. The water content and temperature of the slurry should be controlled, especially for high-nickel cathode materials."
Arbin: "How is the solid content of the slurry tested? Also, how do you test the internal resistance of the membrane and the electronic conductivity of the membrane?"
Dr. Ke Lu De:
"Testing the solid content of the slurry is simple. You weigh the slurry, then dry it to remove the solvent and calculate the proportion of solids. To test the internal resistance of the membrane, specialized equipment is needed. The principle involves a four-point probe test. The slurry is coated onto a non-conductive substrate like PET, and then the test is conducted."
Fan: "If the mixing time is too long, will it absorb water?"
Dr. Ke Lu De:
"This is related to the differences in commercial mixers. The mixing tanks can be sealed and vacuumed, so the time wouldn't affect the material. Typically, we control the ambient humidity to be below 45%. If the material is highly sensitive to moisture, we maintain humidity below 10%. Water content control in the slurry mainly involves controlling the NMP content, and the incoming NMP should have less than 200ppm water."
NNN: "What are the differences in the preparation process for 811 and 622, 532?"
Dr. Ke Lu
De: "Generally, 622 and below can use standard mixing conditions with an ambient humidity of around 45%. For 811 or NCA, due to the higher alkalinity of the material's surface and its sensitivity to water, many manufacturers control the humidity to below 10%. Of course, materials are improving, and some manufacturers have managed to control the alkalinity, making the materials less sensitive to water."
Fan: "If there is a gritty texture in the slurry during coating, does that mean it wasn't well dispersed?"
Dr. Ke Lu De:
"Yes, a gritty texture in the coating process typically indicates poor dispersion. Of course, it's important to determine if the grittiness is due to isolated particles or a lot of them."
Fan: "If the slurry is well-mixed but requires a long time and oil-based systems are sensitive to water, do you have any suggestions for laboratory operations?"
Dr. Ke Lu De:
"You can optimize the formulation and mixing method based on your actual ratios and processes. You don’t need to use a very high BET conductivity carbon; sealing during the stirring process can help improve it. If the water content in NMP is controlled at 200ppm, that’s sufficient for most materials, so students don't need to focus on dewatering."
Star Fragments: "How do manufacturers handle NMP with excessive water content?"
Dr. Ke Lu De:
"In industry, battery companies purchase NMP with battery-grade purity. The NMP producers use large-scale chemical techniques, such as distillation towers. Using molecular sieves is costly and less efficient."
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