You must understand the principle and process of lithium-ion batteries

　　Lithium-ion battery is a kind of secondary battery (rechargeable battery), which mainly relies on Li+ intercalation and deintercalation between two electrodes to work. With the continuous development of downstream industries such as energy vehicles, the production scale of lithium-ion batteries is constantly expanding.

　　China Energy Storage Network News: Lithium-ion battery is a kind of secondary battery (rechargeable battery), which mainly relies on Li+ to intercalate and deintercalate between two electrodes to work. With the continuous development of downstream industries such as energy vehicles, the production scale of lithium-ion batteries is constantly expanding.

　　This topic is divided into two parts, the upper part and the lower part. The first part focuses on the principle, formula and process flow of lithium-ion batteries. The next part explains the production and performance of lithium batteries. This article is the first part of this topic.

　　One, working principle

　　1, positive electrode structure

　　LiCoO2 + conductive agent + adhesive (PVDF) + current collector (aluminum foil)

　　2, negative structure

　　Graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)

　　3, working principle

　　3.1 Charging process

　　A power supply charges the battery. At this time, the electron e on the positive electrode runs from the external circuit to the negative electrode, and the positive lithium ion Li+ "jumps" from the positive electrode into the electrolyte, and "climbs" over the small curved hole in the diaphragm. "Swimming" reaches the negative electrode and combines with the electrons that have ran over. at this time:

　　The reaction that occurs on the positive electrode is:

　　The reaction on the negative electrode is:

　　3.2 Battery discharge process

　　discharge has constant current discharge and constant resistance discharge. Constant current discharge is actually adding a variable resistance to the external circuit that can change with voltage. The essence of constant resistance discharge is to add a resistance to the positive and negative electrodes of the battery to allow electrons to pass. It can be seen that as long as the electrons on the negative electrode cannot run from the negative electrode to the positive electrode, the battery will not discharge. Both electrons and Li+ act at the same time, with the same direction but different paths. When discharging, the electrons run from the negative electrode to the positive electrode through the electronic conductor, and the lithium ion Li+ "jumps" from the negative electrode into the electrolyte, and "climbs" over the diaphragm. "Swimming" to the positive electrode in the small hole, combined with the electrons that ran over long ago.

　　3.3 charge and discharge characteristics

　　The positive electrode of the battery cell adopts LiCoO2, LiNiO2, LiMn2O2, of which LiCoO2 is a crystal type with a very stable layer structure, but when x Li ions are removed from LiCoO2, its structure may change, but whether it changes depends on the value of x size.

　　Through research, it is found that when x>0.5, the structure of Li1-xCoO2 is extremely unstable, crystal collapse will occur, and its external appearance is the overwhelming end of the battery. Therefore, during use of the battery cell, the value of x in Li1-xCoO2 should be controlled by limiting the charging voltage. Generally, the charging voltage is not greater than 4.2V and x is less than 0.5. At this time, the crystal form of Li1-xCoO2 is still stable.

　　The negative electrode C6 has its own characteristics. After the first formation, the Li in the positive electrode LiCoO2 is charged into the negative electrode C6. When discharged, Li returns to the positive electrode LiCoO2, but a part of Li must remain in the center of the negative electrode C6 after the formation. In order to ensure the normal insertion of Li in the next charge and discharge, otherwise the overwhelming of the cell is very short. In order to ensure that a part of Li remains in the negative electrode C6, it is generally achieved by limiting the lower discharge voltage: safe charging upper voltage ≤ 4.2V, lower discharge limit Voltage ≥2.5V.

　　The principle of the memory effect is crystallization, and this kind of reaction hardly occurs in a lithium battery. However, the capacity of a lithium-ion battery will still decrease after many times of charging and discharging, and the reasons are complex and diverse. It is mainly the change of the positive and negative materials themselves. From the molecular level, the hole structure of the positive and negative electrodes will gradually collapse and block; from a chemical point of view, it is the active passivation of the positive and negative materials, and side reactions appear to be stable. Other compounds. Physically, the positive electrode material will gradually peel off. In short, the number of lithium ions that can move freely during charging and discharging in the battery is ultimately reduced.

　　Overcharge and overdischarge will cause permanent damage to the positive and negative electrodes of lithium-ion batteries. From the molecular level, it can be intuitively understood that overdischarge will cause the negative electrode carbon to excessively release lithium ions and cause its sheet structure to collapse. Overcharging will force too much lithium ions into the carbon structure of the negative electrode, and some of them will no longer be able to be released.

　　Unsuitable temperature will trigger other chemical reactions inside the lithium-ion battery to produce undesirable compounds, so there are protective temperature-controlled diaphragms or electrolyte additives between the positive and negative electrodes of many lithium-ion batteries. When the battery heats up to a certain level, the pores of the composite membrane are closed or the electrolyte is denatured, the internal resistance of the battery increases until it is disconnected, and the battery no longer heats up to ensure that the battery charging temperature is normal.

　　2. Lithium battery formula and process flow

　　1. Positive and negative electrode formula

　　1.1 Positive electrode formula: LiCoO2 + conductive agent + binder + current collector (aluminum foil)

　　LiCoO2(10μm): 96.0%

　　Conductive agent (CarbonECP) 2.0%

　　adhesive (PVDF761) 2.0%

　　NMP (increased cohesiveness): The weight ratio of solid matter is about 810:1496

　　A) Positive electrode viscosity control 6000cps (temperature 25 rotor 3);

　　B) The weight of NMP must be adjusted appropriately to meet the viscosity requirements;

　　C) Pay special attention to the influence of temperature and humidity on viscosity

　　Positive active material:

　　Lithium cobalt oxide: positive electrode active material, lithium ion source, to improve the lithium source for the battery. 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 material, irregular shape, particle size D50 is generally 5-7μm, water content ≤0.2%, usually weakly alkaline, pH value is about 8.

　　Conductive agent: chain-like substance, water content <1%, particle size is generally 1-5μm. Superconducting carbon blacks with excellent conductivity are usually used, such as Ketjen Carbon Black CarbonECP and ECP600JD. Its role is to improve the conductivity of the positive electrode material, compensate for the electronic conductivity of the positive electrode active material, and increase the amount of electrolyte absorption of the positive electrode sheet. Increase the reaction interface and reduce polarization.

　　PVDF adhesive: non-polar substance, chain-like substance, molecular weight ranging from 300,000 to 3,000,000; after absorbing water, the molecular weight decreases and the viscosity becomes worse. Used to bond lithium cobalt oxide, conductive agent and aluminum foil or aluminum mesh together. Commonly used brands are Kynar761.

　　NMP: weakly polar liquid, used to dissolve/swell PVDF, and at the same time to dilute the slurry.

　　Current collector (positive lead): made of aluminum foil or aluminum tape.

　　1.2 negative electrode formula: graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)

　　Anode material (graphite): 94.5%

　　Conductive agent (CarbonECP): 1.0% (Ketjen superconducting carbon black)

　　Binder (SBR): 2.25% (SBR=styrene butadiene rubber latex)

　　Thickener (CMC): 2.25% (CMC = sodium carboxymethyl cellulose)

　　Water: The weight ratio of solid matter is 1600:1417.5

　　A) Negative electrode viscosity control 5000-6000cps (temperature 25 rotor 3)

　　B) The weight of the water needs to be adjusted appropriately to meet the viscosity requirements;

　　C) Pay special attention to the influence of temperature and humidity on viscosity

　　2, positive and negative mixture

　　Graphite: negative electrode active material, the main material constituting the negative electrode reaction; mainly divided into two categories: natural graphite and artificial graphite. Non-polar substances are easily contaminated by non-polar substances, and are easy to disperse in non-polar substances; they are not easy to absorb water and are not easy to disperse in water. Contaminated graphite is easily reunited after being dispersed in water. Generally, the particle size D50 is about 20 μm. The particle shapes are diverse and irregular, mainly spherical, flake, fibrous, etc.

　　Conductive agent: Its function is:

　　A) Improve the conductivity of the negative electrode sheet to compensate for the electronic conductivity of the negative electrode active material.

　　B) Improve the depth of reaction and utilization.

　　C) Prevent the generation of dendrites.

　　D) Utilize the liquid absorption capacity of conductive materials to improve the reaction interface and reduce polarization. (It can be added or not added according to the graphite particle size distribution).

　　Additives: reduce irreversible reactions, improve adhesion, increase slurry viscosity, and prevent slurry precipitation.

　　Thickener/Precipitation Prevention Agent (CMC): High molecular compound, easily soluble in water and polar solvents.

　　Isopropanol: a weakly polar substance, after adding it can reduce the polarity of the binder solution and improve the compatibility of graphite and the binder solution; it has a strong defoaming effect; it is easy to catalyze the network cross-linking of the binder , Improve the bond strength.

　　Ethanol: a weakly polar substance, after adding it can reduce the polarity of the adhesive solution and improve the compatibility of graphite and the adhesive solution; it has a strong defoaming effect; it is easy to catalyze the linear cross-linking of the adhesive and increase the viscosity. Bond strength (Isopropanol and ethanol have the same effect in essence, and cost factors can be considered in mass production and then choose which one to add).

　　Water-based adhesive (SBR): Bond graphite, conductive agent, additives and copper foil or copper mesh together. Small molecule linear chain emulsion, easily soluble in water and polar solvents.

　　Deionized water (or distilled water): diluent, added appropriately to change the fluidity of the slurry.

　　negative lead: made of copper foil or nickel tape.

　　2.1 Positive electrode mixture:

　　2.1.1 Pretreatment of raw materials

　　1) Lithium cobaltate: dehydration. Generally, it is baked for about 2 hours at 120°C under normal pressure.

　　2) Conductive agent: dehydration. Generally use 200°C normal pressure baking for about 2 hours.

　　3) Binder: dehydration. Generally, it is baked at 120-140°C under normal pressure for about 2 hours, and the baking temperature depends on the molecular weight.

　　4) NMP: dehydration. Use dry molecular sieve for dehydration or use special reclaiming facilities for direct use.

　　2.1.2 Material ball milling:

　　1) At the end of 4 hours, the ball mill is separated through a sieve;

　　2) Pour LiCoO2 and CarbonECP into the bucket, and add the grinding balls at the same time (dry material: grinding balls=1:1), and perform ball milling on the roller bottle and the rotating speed control above 60rpm

　　2.1.3 Blending of raw materials:

　　1) Dissolution of adhesive (according to standard concentration) and heat treatment.

　　2) Ball milling of lithium cobalt oxide and conductive agent: the powder is initially mixed, and the lithium cobalt oxide and conductive agent are bonded together to improve agglomeration and conductivity. The slurry will not be separately distributed in the binder after the slurry is prepared. The milling time is generally about 2 hours; in order to avoid mixing impurities, agate balls are usually used as the ball milling meson.

　　2.1.4 Dry powder dispersion and soaking:

　　Principle: When the solid powder is placed in the air, 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 adsorption force ratio of the solid to the gas is compared with that of the liquid The adsorption force of the liquid is strong, and the liquid cannot wet the solid; if the adsorption force of the solid and the liquid is stronger than the adsorption force of the gas, the liquid can wet the solid and squeeze the gas out.

　　When the wetting angle is ≤90°, the solid is wetted. When the wetting angle>90°, the solid will not wet.

　　All members of the positive electrode material can be wetted by the binder solution, so the positive electrode powder is relatively easy to disperse.

　　The influence of dispersion method on dispersion:

　　1) Static method (long time, poor effect, but does not damage the original structure of the material);

　　2) Stirring method: rotation or rotation plus revolution (short time, good effect, but it may damage the structure of individual materials).

　　The effect of stirring paddle on the dispersion speed: The stirring paddle roughly includes serpentine shape, butterfly shape, spherical shape, paddle shape, gear shape and so on. Generally, serpentine, butterfly, and paddle-shaped stirring blades are used to deal with the initial stage of materials or ingredients that are difficult to disperse; spherical and gear-shaped are used in the state of low dispersing difficulty, and the effect is good.

　　The effect of stirring speed on 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 and the equipment.

　　The effect of concentration on the dispersion rate. Under normal circumstances, the smaller the slurry concentration, the faster the dispersion speed, but too thin will lead to waste of materials and aggravation of slurry precipitation.

　　The effect of concentration on bond strength. The greater the concentration, the greater the flexibility strength, and the greater the bonding strength; the lower the concentration, the lower the bonding strength.

　　The effect of vacuum degree on dispersion speed. The high vacuum is beneficial to the discharge of gas from the material gap and surface, reducing the difficulty of liquid adsorption; the difficulty of uniform dispersion of the material under the condition of complete weightlessness or reduced gravity will be greatly reduced.

　　The effect of temperature on the dispersion rate. At a suitable temperature, the slurry has good fluidity and is easy to disperse. Too hot slurry is easy to crust, and too cold slurry's fluidity will be greatly reduced.

　　Dilution: Adjust the slurry to an appropriate concentration for easy coating.

　　2.1.5 Operation steps

　　A) Pour NMP into the power mixer (100L) to 80°C, weigh PVDF into it, and turn it on; parameter setting: rotation speed 25±2 rpm, stirring for 115-125 minutes;

　　b) Turn on the cooling system, add the ground positive electrode dry material in four times, with an interval of 28-32 minutes. The third addition depends on the material needs to add NMP. After the fourth addition, add NMP; power mixer Parameter setting: Rotation speed is 20±2 rpm

　　C) 30±2 minutes after the fourth feeding, high-speed stirring is carried out, the time is 480±10 minutes; the power mixer parameter setting: revolution is 30±2 revolutions/min, rotation is 25±2 revolutions/min;

　　D) Vacuum mixing: connect the power mixer to vacuum, keep the vacuum at -0.09Mpa, stir for 30±2 minutes; power mixer parameter settings: revolution is 10±2 minutes, rotation is 8±2 rpm

　　E) Take 250-300 ml of slurry and use a viscometer to measure the viscosity; test conditions: rotor number 5, rotation speed 12 or 30 rpm, temperature range 25 °C;

　　F) Take the positive electrode material out of the power mixer for colloidal grinding and sieving. At the same time, mark the stainless steel basin, and then it can flow into the drawing operation process after being handed over with the drawing equipment operator.

　　2.1.6 Notes

　　A) Complete, clean up machinery and equipment and working environment;

　　B) When operating the machine, pay attention to safety and avoid smashing the head.

　　2.2 negative electrode mixture

　　2.2.1 Pretreatment of raw materials:

　　1) Graphite:

　　A, mixing, homogenize the raw materials and improve consistency.

　　B, baking 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 bake for maintaining surface characteristics)