Basic knowledge of ingredients
First, the composition of the electrode:
1, the positive electrode composition:
The composition of the electrode: a, lithium cobaltate: positive electrode active material, lithium ion source, for the battery to improve the lithium source.
b。 Conductive agent: Improve the conductivity of the positive electrode sheet and compensate for the electronic conductivity of the positive electrode active material。 The liquid absorption of the electrolyte of the positive electrode sheet is increased, the reaction interface is increased, and the polarization is reduced。
c. PVDF binder: Lithium cobaltate, a conductive agent and an aluminum foil or an aluminum mesh are bonded together.
d。 Positive lead: Made of aluminum foil or aluminum strip。
2, the composition of the negative electrode:
a, graphite: negative electrode active material, which constitutes the main substance of the negative electrode reaction; mainly divided into two major categories of natural graphite and artificial graphite.
b. Conductive agent: improves the conductivity of the negative electrode sheet and compensates for the electronic conductivity of the negative electrode active material. Improve reaction depth and utilization. Prevent the generation of dendrites. The liquid absorption capacity of the conductive material is utilized to increase the reaction interface and reduce polarization. (Can be selected according to the graphite particle size distribution plus or not).
c. Additives: reduce irreversible reactions, improve adhesion, increase slurry viscosity, and prevent slurry precipitation.
d。 Aqueous binder: Bonding graphite, conductive agent, additive and copper foil or copper mesh together。
e, negative lead: made of copper foil or nickel strip。
Second, the purpose of the ingredients: the batching process is actually the various components in the slurry are mixed together according to the standard ratio, prepared into a slurry, in order to facilitate uniform coating, to ensure the consistency of the pole pieces. The ingredients generally comprise five processes, namely: pretreatment, blending, wetting, dispersion and flocculation of the raw materials.
Third, the principle of ingredients:
(A), the principle of positive dosing
1。 Physical and chemical properties of raw materials。
(1) Lithium cobaltate: non-polar substance, irregular shape, particle size D50 is generally 6-8 μm, water content ≤ 0。2%, usually alkaline, PH value is about 10-11。
Lithium manganate: non-polar substance, irregular shape, particle size D50 is generally 5-7 μm, 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 is generally 2-5 μm; mainly common carbon black, superconducting carbon black, graphite milk, etc。 In high-volume applications, superconducting carbon black and graphite emulsion are generally selected; usually neutral。
(3) PVDF binder: non-polar substance, chain, molecular weight ranging from 300,000 to 3,000,000; molecular weight decreases after water absorption, viscosity is deteriorated。
(4) NMP: a weakly polar liquid used to dissolve/swell PVDF and to dilute the slurry。
3. Mixing of raw materials:
(1) Dissolution of the binder (according to the standard concentration) and heat treatment。
(2) Lithium cobaltate and conductive agent ball milling: The powder is initially mixed, and lithium cobaltate and conductive agent are bonded together to improve agglomeration and conductivity. After being formulated into a slurry, it is not separately distributed in the binder, and the ball milling time is generally about 2 hours; in order to avoid mixing impurities, an agate ball is usually used as a ball mill meson.
4. Dispersion and wetting of dry powder:
(1) Principle: The solid powder is placed in the air。 As time passes, part of the air will be adsorbed 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 adsorbed The liquid is stronger than the liquid, and the liquid cannot wet the solid; if the solid-liquid adsorption force is stronger than the gas's adsorption force, the liquid can wet the solid and extrude the gas。 When the wetting angle is ≤ 90 degrees, the solid is wetted。 When the wetting angle is >90 degrees, the solids are not wetted。 All members of the positive electrode material can be wetted by the binder solution, so the positive powder dispersion is relatively easy。
(2) The effect 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 it may damage the structure of individual materials).
1. The influence of the stirring paddle on the dispersion speed. The agitating paddles generally include a serpentine shape, a butterfly shape, a spherical shape, a paddle shape, a gear shape, and the like. Generally, the serpentine, butterfly, and paddle type agitating paddles are used to deal with the initial stage of materials or ingredients that are difficult to disperse; the spherical and gear shapes are used to disperse the less difficult state, and the effect is good.
2. The effect 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 material itself and the equipment.
3。 The effect of concentration on the dispersion speed。 Generally, the smaller the slurry concentration, the faster the dispersion speed, but too thin will result in waste of material and aggravation of slurry precipitation。
4。 The effect of concentration on bond strength。 The higher the concentration, the greater the softening strength and the higher the bonding strength; the lower the concentration, the smaller the bonding strength。
5。 The effect of vacuum on the dispersion speed。 The high vacuum is beneficial to the gas discharge of the material gap and the surface, and the difficulty of liquid adsorption is reduced; the difficulty of uniformly dispersing the material under the condition of complete weight loss or reduced gravity will be greatly reduced。
6。 The effect of temperature on the dispersion speed。 At a suitable temperature, the slurry has good fluidity and is easy to disperse。 Too hot slurry is easy to crust, and the fluidity of too cold slurry will be greatly reduced。
7。 Dilute。 The slurry is adjusted to a suitable concentration to facilitate coating。