1. Composition of electrodes:
1. Positive electrode composition:
a. Lithium cobalt oxide: positive electrode active material, lithium ion source, providing lithium source for the battery.
b. Conductive agent: improve the conductivity of the positive electrode sheet and compensate the electronic conductivity of the positive electrode active material. Increase the liquid absorption of the electrolyte of the positive electrode sheet, increase the reaction interface, and reduce polarization.
c. PVDF adhesive: bond lithium cobaltate, conductive agent and aluminum foil or aluminum mesh together.
d. Positive electrode lead: made of aluminum foil or aluminum strip.
2. Composition of negative electrode:
a. Graphite: Negative electrode active material, the main material that constitutes the negative electrode reaction; it is mainly divided into two categories: natural graphite and artificial graphite.
b. Conductive agent: improve the conductivity of the negative electrode sheet, and compensate the electronic conductivity of the negative electrode active material. Improve the reaction depth and utilization. prevent the formation of dendrites. Utilize the liquid absorption ability of conductive materials to improve the reaction interface and reduce polarization. (Can choose to add or not add according to graphite particle size distribution).
c. Additives: reduce irreversible reactions, improve adhesion, increase slurry viscosity, and prevent slurry precipitation.
d. Water-based adhesive: bond graphite, conductive agent, additives and copper foil or copper mesh together.
e. Negative electrode lead: made of copper foil or nickel strip.
Second, the purpose of ingredients:
The batching process is actually to mix the various components in the slurry according to the standard ratio to prepare the slurry to facilitate uniform coating and ensure the consistency of the pole piece. Batching generally includes five processes, namely: raw material pretreatment, blending, soaking, dispersion and flocculation.
Matching (1), the principle of positive electrode ingredients
1. Physical and chemical properties of raw materials.
(1) Lithium cobaltate: non-polar substance, irregular shape, particle size D50 is generally 6-8μm, water content ≤0.2%, usually alkaline, pH value is about 10-11.
Lithium manganate: non-polar substance, irregular shape, particle size D50 is generally 5-7μm, water content ≤0.2%, usually weakly alkaline, pH value is about 8.
(2) Conductive agent: non-polar substance, grape chain, water content 3-6%, oil absorption value ~300, particle size generally 2-5μm; mainly ordinary carbon black, superconducting carbon black, graphite milk, etc. , in large-scale applications, the compound of superconducting carbon black and graphite milk is generally selected; it is usually neutral.
(3) PVDF (polyvinylidene fluoride) adhesive: non-polar substances, chains, molecular weight ranging from 300,000 to 3,000,000; after absorbing water, the molecular weight decreases and the viscosity becomes worse.
(4) NMP (N-methylpyrrolidone): a weakly polar liquid, used to dissolve/swell PVDF and dilute the slurry.
2. Pretreatment of raw materials
(1) Lithium cobaltate: dehydration. Generally, it is baked at 120°C under normal pressure for about 2 hours.
(2) Conductive agent: dehydration. Generally, it is baked at 200°C under normal pressure for about 2 hours.
(3) Adhesive: Dehydration. Generally, it is baked at 120-140°C under normal pressure for about 2 hours, and the baking temperature depends on the size of the molecular weight.
(4) NMP: Dehydration. Use dry molecular sieves to dehydrate or use special reclaiming facilities for direct use.
3. Blending of raw materials:
(1) Dissolution of adhesive (according to standard concentration) and heat treatment.
(2) Ball milling of lithium cobaltate and conductive agent: the powder is initially mixed, lithium cobaltate and conductive agent are bonded together, and the agglomeration and conductivity are improved. After being made into slurry, it will not be distributed in the binder alone, and the ball milling time is generally about 2 hours; in order to avoid mixing impurities, agate balls are usually used as ball milling mesons.
4. Dispersion and soaking of dry powder:
(1) Principle: The solid powder is placed in the air, and as time goes by, it will absorb part of the air on the surface of the solid. After the liquid binder is added, the liquid and the gas begin to compete for the solid surface; if the solid and gas adsorption The adsorption force is stronger than that of the liquid, and the liquid cannot wet the solid; if the adsorption force of the solid to the liquid is stronger than that of the gas, the liquid can wet the solid and squeeze out the gas.
When the wetting angle is less than or equal to 90 degrees, the solid is wetted.
When the wetting angle > 90 degrees, the solid is not wetted.
All members in the positive electrode material can be wetted by the binder solution, so the positive electrode powder is relatively easy to disperse.
(2) The impact of the dispersion method on dispersion:
A. Standing method (long time, poor effect, but does not damage the original structure of the material);
B. Stirring method; rotation or rotation plus revolution (short time, good effect, but may damage the structure of individual materials).
1. The influence of the stirring paddle on the dispersion speed. Stirring paddles generally include snake-shaped, butterfly-shaped, spherical, paddle-shaped, gear-shaped, etc. Generally, snake-shaped, butterfly-shaped, and paddle-shaped stirring paddles are used to deal with the initial stage of materials or ingredients that are difficult to disperse; spherical and gear-shaped are used for states that are less difficult to disperse, and the effect is good.
2. The influence of stirring speed on the dispersion speed. Generally speaking, the higher the stirring speed, the faster the dispersion speed, but the greater the damage to the structure of the material itself and the equipment.
3. The effect of concentration on dispersion speed. Generally, the smaller the slurry concentration, the faster the dispersion speed, but too thin will lead to waste of materials and aggravation of slurry precipitation.
4. Effect of concentration on bond strength. The greater the concentration, the greater the flex strength and the greater the bonding strength; the lower the concentration, the smaller the bonding strength.
5. The effect of vacuum degree on dispersion speed. High vacuum is conducive to the discharge of gas from the gaps and surfaces of materials, and reduces the difficulty of liquid adsorption; the difficulty of uniform dispersion of materials in the case of complete weightlessness or reduced gravity will be greatly reduced.
6. The influence of temperature on the dispersion speed. At a suitable temperature, the slurry has good fluidity and is easy to disperse. If the slurry is too hot, it will easily form a skin, and if it is too cold, the fluidity of the slurry will be greatly reduced.
5. Dilution. Adjust the slurry to a suitable concentration for easy coating.
(2) The principle of negative electrode ingredients (roughly the same as the principle of positive electrode ingredients)
1. Physical and chemical properties of raw materials.
(1) Graphite: non-polar substance, easy to be polluted by non-polar substances, easy to disperse in non-polar substances; not easy to absorb water, and not easy to disperse in water. Contaminated graphite is easy to re-agglomerate after being dispersed in water. The general particle size D50 is about 20 μm. The particle shapes are various and irregular, mainly spherical, flake, fibrous and so on.
(2) Water-based adhesive (SBR: styrene-butadiene rubber): small molecular linear chain emulsion, easily soluble in water and polar solvents.
(3) Anti-precipitation agent (CMC: carboxymethyl cellulose): polymer compound, easily soluble in water and polar solvents.
(4) Isopropanol: a weak polar substance, which can reduce the polarity of the binder solution and improve the compatibility of graphite and binder solution; it has a strong defoaming effect; it is easy to catalyze the binder network cross-linked to increase bond strength.
Ethanol: a weak polar substance, which can reduce the polarity of the binder solution and improve the compatibility between graphite and binder solution; has a strong defoaming effect; it is easy to catalyze the linear cross-linking of the binder and improve the viscosity of the binder solution. Knot strength (Isopropanol and ethanol are essentially the same, and you can choose which one to add after considering the cost factor in mass production).
(5) Deionized water (or distilled water): Thinner, add as appropriate to change the fluidity of the slurry.
2. Pretreatment of raw materials:
(1) Graphite: A. Mixing to homogenize raw materials and improve consistency. B. Bake at 300~400°C under normal pressure to remove oily substances on the surface, improve compatibility with water-based adhesives, and round the edges and corners of the graphite surface (some materials are not allowed to be baked in order to maintain surface characteristics, otherwise the performance will be reduced).
(2) Water-based adhesive: properly diluted to improve dispersibility.
3. Blending, soaking and dispersing:
(1) The polarity of graphite and binder solution is different, and it is not easy to disperse.
(2) The graphite can be preliminarily wetted with an aqueous alcohol solution, and then mixed with the binder solution.
(3) The stirring concentration should be appropriately reduced to improve the dispersibility.
(4) The dispersion process is to reduce the distance between polar and non-polar substances and increase the potential energy or surface energy, so it is an endothermic reaction, and the overall temperature drops when stirring. If conditions permit, the stirring temperature should be appropriately increased to facilitate heat absorption, improve fluidity and reduce the difficulty of dispersion.
(5) If a vacuum degassing process is added to the stirring process to remove gas and promote solid-liquid adsorption, the effect is better, and the vacuum high-speed dispersing machine of Bocheng Machinery can be used.
(6) The principle and method of dispersion are the same as those in the positive electrode ingredients, which are discussed in detail in III. (1) and 4, and will not be explained in detail here.
4. Dilution. Adjust the slurry to a suitable concentration for easy coating.
4. Precautions for ingredients:
1. Prevent mixing with other impurities;
2. Prevent the slurry from splashing;
3. The concentration (solid content) of the slurry should be adjusted gradually from high to low, so as not to increase trouble;
4. Pay attention to edge scraping and bottom scraping during the intermittent stirring process to ensure uniform dispersion;
5. The slurry should not be left for a long time to avoid precipitation or uniformity reduction;
6. The materials to be baked need to be sealed and cooled before they can be added to avoid changes in the properties of the component materials;
7. The length of the stirring time is mainly based on the performance of the equipment and the amount of materials added; the use of the stirring paddle is replaced by the difficulty of slurry dispersion. If it cannot be replaced, the speed can be adjusted from slow to fast to avoid damage to the equipment;
8. Sieve the slurry before discharging to remove large particles to prevent belt breakage during coating;
9. Strengthen training for ingredients personnel to ensure that they have professional knowledge to avoid catastrophe;
10. The key to the ingredients is to disperse evenly, master the center, and adjust other methods by yourself.
