What is the lithium ion battery assembly process and technology?
According to the structural design and design parameters of the battery, how to prepare the selected battery materials and effectively combine them together, and assemble a battery that meets the design requirements, is a problem to be solved by the battery production process. It can be seen that whether the battery production process is reasonable is the key to whether the assembled battery meets the design requirements, and it is the most important step that affects the battery performance.
Refer to the production process of AA-type Cd/Ni and MH/Ni batteries, combined with the structural design of AA-type lithium-ion batteries and the performance characteristics of lithium-ion battery materials and repeated tests to determine the production process of AA-type lithium-ion batteries .
The production process of AA lithium-ion battery involves four processes:
① Preparation of positive and negative plates;
② Winding of the battery cell;
③Assemble;
④ Sealing.
This is not much different from the traditional AA-type Cd/Ni battery production process, but in terms of technology, lithium-ion batteries are much more complicated, and the requirements for environmental conditions are much more stringent. The manufacturing process technology of lithium-ion batteries is very strict and complex.
Among them, the preparation of the positive and negative electrode slurry, the coating, drying, and rolling of the positive and negative electrodes, and the winding of the battery have the greatest impact on the battery performance, and are the most critical steps in the lithium-ion battery manufacturing technology. A brief introduction to these processes is given below.
In order to prevent the precipitation of metal lithium on the copper part of the negative electrode and cause safety problems, it is necessary to improve the process of the pole piece, and both sides of the copper foil need to be coated with carbon paste.
The main steps of the process flow of the lithium ion battery are as follows.
①Pulping Use special solvents and binders to mix with powdered positive and negative active materials, and mix them evenly at high speed to make slurry-like positive and negative materials.
②Coating: Coat the prepared slurry evenly on the surface of the metal shrinkage, and dry it to make positive and negative pole pieces respectively.
③Assemble according to the order of positive electrode sheet, diaphragm, negative electrode sheet and diaphragm from top to bottom. The battery core is made by winding, and then the process of injecting electrolyte and sealing to complete the assembly process of the battery and make the finished product. Battery.
④Formation Use special battery charging and discharging equipment to conduct charging and discharging tests on finished batteries. Each battery is tested, and qualified finished batteries are screened out before leaving the factory.
1.1 Pulping
Special solvents and binders are mixed with the powdery positive and negative active materials in a certain proportion, and after high-speed stirring, the positive and negative materials are made into slurry. The binders commonly used in lithium-ion batteries are PVDF and PTFE.
In the whole pulping process, the preparation of electrode active material, conductive agent and binder is the most important link. Taking lithium cobalt oxide as the positive electrode active material and graphite as the negative electrode as an example, the basic knowledge of some ingredients is introduced below.
Under normal circumstances, the electrode is composed of active material, conductive agent, binder and lead. The difference is that the binder type of the positive and negative materials is different, and the additives need to be added to the negative electrode material. The addition of additives is mainly to improve the binding agent. Adhesion ability, etc.
The batching process is actually to mix the various components of the slurry in a standard proportion and prepare the slurry to facilitate uniform coating and ensure the uniformity of the pole pieces. The ingredients roughly include five processes, namely the pretreatment, blending, soaking, dispersion and flocculation of raw materials.
(1) Positive electrode ingredients
①Physical and chemical properties of raw materials
a. Lithium cobaltate: non-polar substance, irregular shape, particle size D50 is generally 6~8μm, water content ≤0.2%, usually alkaline, pH value is 10~11.
Lithium manganate: non-polar material, irregular shape, particle size D50 is generally 5~7μm, water content ≤
0.2%, usually weakly alkaline, with a pH of about 8.
b. Conductive agent: non-polar substance, grape chain, water content 3%~6%, oil absorption value is about 300,
The particle size is generally 2~5μm; there are mainly ordinary carbon black, superconducting carbon black, graphite milk, etc. In mass applications, superconducting carbon black and graphite milk are generally selected as a compound; usually neutral.
c. PVDF binder: non-polar substance, chain-like substance, its molecular weight ranges from 300,000 to 3,000,000; the molecular weight decreases after absorbing water, and the viscosity becomes worse.
d. NMP (N-Methylpyrrolidone): weakly polar liquid, used to dissolve/swell PVDF, and at the same time as a solvent to dilute the slurry.
②Pretreatment of raw materials
a. Lithium cobaltate: dehydration. Generally use 120℃ normal pressure baking for about 2h;
b. Conductive agent: dehydration. Generally use 200℃ atmospheric pressure baking for about 2h;
c. Binder: dehydration. Generally use 120~140℃ atmospheric pressure baking for about 2 hours, and the baking temperature depends on the molecular weight;
d. NMP: dehydration. Use dry molecular sieve for dehydration or use special reclaiming facilities for direct use.
③ Blending of raw materials
a. Dissolution of binder (according to standard concentration) and heat treatment;
b. Lithium cobalt oxide and conductive agent ball milling: the powder is initially mixed to bond the lithium cobalt oxide and the conductive agent together to improve their agglomeration and conductivity. After mixing into the slurry, it will not be distributed separately in the binder. The milling time is generally about 2h; in order to avoid mixing impurities, agate balls are usually used as the milling medium.
④Dispersion and wetting of dry powder
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 will compete to escape from the solid surface; 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 less than or equal to 90, the solid is wetted.
When the wetting angle>90, the solid does not wet.
All components in 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: static method (featured by long dispersion time and poor effect, but does not damage the original structure of the material); stirring method rotation or rotation plus revolution (short time, good effect, but may damage individual The structure of the material itself).
The effect of stirring paddle on the dispersion speed. Stirring paddles generally include snake-shaped, butterfly-shaped, spherical, paddle-shaped, gear-shaped, and so on. Generally, serpentine, butterfly, and paddle-shaped stirring blades are used to deal with the initial stages of materials or ingredients that are difficult to disperse; spherical and gear-shaped are used for 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 speed of dispersion. Under normal circumstances, the smaller the slurry concentration, the faster the dispersion speed, but too thin slurry 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 bonding strength; the lower the concentration, the lower the bonding strength.
The effect of vacuum on the 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 speed of dispersion. At a suitable temperature, the slurry has good fluidity and is easy to disperse. Too hot slurry is easy to skin, too cold slurry will greatly reduce the fluidity.
⑤Dilution
Adjust the slurry to an appropriate concentration for easy coating.
(2) Negative ingredients
The principle is roughly the same as that of positive electrode ingredients.
①Physical and chemical properties of raw materials
a. Graphite: non-polar substance, easy to be polluted by non-polar substance, easy to disperse in non-polar substance; not easy to absorb water, and 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.
b. Water-based binder (SBR): Small molecule linear chain emulsion, easily soluble in water and polar solvents.
c. Anti-precipitation agent (CMC): polymer compound, easily soluble in water and polar solvents.
d. Isopropanol: a weakly polar substance that 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 bonding strength.
e. Ethanol: 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 linear cross-linking of the binder and improve the bonding strength (The role of isopropanol and ethanol is essentially the same. When mass production, you can consider cost factors and then choose which additive).
f. Deionized water (or distilled water): diluent, added appropriately to change the fluidity of the slurry.
②Pretreatment of raw materials
a. Graphite: After mixing to homogenize the raw materials, and then baking at 300~400℃ under normal pressure to remove surface oily substances, improve the compatibility with water-based binders, and round the edges and corners of the graphite surface (some materials are to maintain surface characteristics, Bake is not allowed, otherwise the efficiency will be reduced).
b. Water-based binder: Properly dilute to improve dispersion ability.
③ Blending, wetting and dispersing
a. Graphite and binder solutions have different polarities and are not easy to disperse.
b. The graphite can be preliminarily wetted with an alcohol aqueous solution, and then mixed with the binder solution.
c. The stirring concentration should be appropriately reduced to improve the dispersibility.
d. The dispersion process is to reduce the distance between polar and non-polar materials and increase their potential energy or surface energy. Therefore, it is an endothermic reaction, and the overall temperature drops during stirring. If conditions permit, the stirring temperature should be appropriately increased to make it easier to absorb heat, while improving fluidity and reducing the difficulty of dispersion.
e. The stirring process, such as adding a vacuum degassing process to remove gas and promote solid-liquid adsorption, has a better effect.
f. The dispersion principle and dispersion method are the same as the related content in the positive electrode ingredients, which are discussed in detail above and will not be explained in detail here.
④Dilution
Adjust the slurry to a suitable concentration for easy coating.
(3) Matters needing attention in ingredients
①Prevent other impurities from being mixed;
②Prevent the slurry from splashing;
③The concentration (solid content) of the slurry should be gradually adjusted from high to low;
④ Pay attention to scraping the edges and the bottom during the intermittent mixing process to ensure uniform dispersion;
⑤The slurry should not be left for a long time to avoid its precipitation or decrease in uniformity;
⑥The materials to be baked must be sealed and cooled before being added in order to avoid changes in the properties of the component materials;
⑦The length of the mixing time is based on the design considerations of the equipment performance and the amount of materials added;
⑧The use of the stirring paddle is difficult to replace when the slurry is dispersed. If it cannot be replaced, the speed can be adjusted from slow to fast to avoid damage to the equipment;
⑨Sifter the slurry before discharging to remove large particles to prevent breakage during coating;
⑩ Strengthen training of ingredients personnel to ensure that they master professional technology and safety knowledge;
⑪The key to ingredients is to be evenly dispersed, master the center, and other methods can be adjusted by themselves.
1.2 Coating
The prepared slurry is evenly coated on the surface of the metal foil, and dried to prepare positive and negative pole pieces respectively. There are about 20 kinds of coating methods that may be used to coat the liquid material on the support, and each technology has many special configurations, so there are many coating methods to choose from. Commonly used coating methods include extruder, reverse roll coating and knife coating.
In the research phase of the lithium ion battery laboratory, a simple coating experiment device such as a scraper, a scraper or an extrusion can be used to coat the pole piece, which can only coat a small amount of experimental research samples.
Compared with knife coating, slot die and reverse roll coating processes are generally preferred in large-scale production lines, because they are easy to handle positive and negative slurries with different viscosities and change the coating rate, and it is easy to control the coating on the web thickness of. This is very useful for the production of lithium-ion batteries where the thickness of the electrode sheet coating is relatively high, so that the thickness deviation of the coating can be controlled within ±3μm. Among them, roller coating has many forms. According to the rotation direction of roller coating, it can be divided into two types: forward roller coating and reverse roller coating. In addition, there are more than 10 kinds of roller coating methods such as 3 rollers and 4 rollers.
The slurry involves the positive and negative electrodes of the battery, that is, the problem of coating the active material on aluminum foil or copper foil. The uniformity of the active material coating directly affects the quality of the battery. Therefore, the pole piece slurry coating technology and equipment are lithium ion One of the keys to battery development and production.
Generally, the choice of coating method needs to consider the following aspects, including the number of layers to be coated, the thickness of the wet coating, the rheology of the coating liquid, the required coating accuracy, the coating support or substrate, and the coating Speed etc.
How to choose a suitable coating method for pole piece slurry?
In addition to considering the above factors, a comprehensive analysis must be combined with the specific conditions and characteristics of pole piece coating.
The characteristics of battery pole piece coating are: double-sided single-layer coating; thicker slurry wet coating (100 ~ 300μm); slurry is a non-Newtonian high-viscosity fluid; equivalent to general coating products, pole piece coating The fabric precision requirements are high, which is similar to the film coating precision; the aluminum foil and copper foil with a thickness of 10-20μm are coated on the support; compared with the film coating speed, the pole piece coating speed is not high.
The pole piece needs to be coated with slurry on both sides of the metal foil. The coating technology route decides to choose single-layer coating, on the other hand, it is coated again after drying. Considering that the pole piece coating belongs to the thick coating coating. Scraper, doctor blade and air knife coating are only suitable for coating thinner coatings, not for pole piece slurry coating. The coating thickness is affected by the viscosity of the coating slurry and the coating speed, and it is difficult to perform high-precision coating.
Comprehensive consideration of the various requirements of pole piece slurry coating, extrusion coating or roll pressing are available.
Extrusion coating can be used for coating of higher viscosity fluids, and higher precision coatings can be obtained. To obtain a uniform coating, using slit extrusion coating, the design and operating parameters of the extrusion nozzle must be within an appropriate range, that is, the critical condition that must be called the "coating window" in the coating technology Within the range, normal coating can be carried out.
Detailed data on the rheological properties of the coating slurry are required for design. Once the extrusion nozzle is designed and processed according to the provided rheological data, the coating accuracy may be affected when the rheological properties of the coating slurry are greatly changed. The extrusion coating equipment is more complicated, and the operation needs to be specialized. Technology.
Roll coating can be applied to the coating of pole piece slurry. There are many forms of roller coating, which can be divided into two types: forward roller and reverse roller coating according to the direction of rotation of roller coating. In addition, there are more than 10 types of roller coating, such as 3 rollers and 4 rollers. Which roll coating method to use depends on the rheological properties of various slurries. That is to say, the designed roll coating form, structure size, operating conditions, physical properties of the coating liquid and other conditions must be within a reasonable range, that is, the operating conditions enter the coating window to be coated with excellent performance. .
The viscosity of the pole piece slurry is extremely high, exceeding the viscosity of the general coating solution, and the required coating amount is large, which cannot be uniformly coated by the current conventional coating method. Therefore, it is necessary to select an appropriate coating method for the pole piece slurry based on its flow mechanism, combined with the rheological properties of the pole piece slurry and coating requirements.
The length of each pole piece required by different types of lithium-ion batteries is also different. If continuous coating is used, and then cut to length to produce pole pieces, when assembling the battery, it is necessary to scrape off the slurry coating at one end of each pole piece to expose the metal foil. The process route of continuous coating and fixed-length slitting has low efficiency and cannot meet the needs of final large-scale production.
Therefore, if you consider the use of a fixed-length segmented coating method, perform segmental coating according to the coating and blank length required by the battery specifications during coating. It is difficult to realize the segmental coating of the length required by different battery specifications with a simple mechanical device. Computer technology is used in the design of the coating head, and the pole piece coating head is designed as a coating device with integrated intelligent control of light, machine and electricity. The operating parameters are input into the computer with the keyboard before coating, and are controlled by the computer during the coating process to automatically carry out fixed-length segmentation and double-sided overlapping coating.
The pole piece slurry coating is relatively thick, the coating amount is large, and the drying load is large. Drying methods such as ordinary hot air convection drying method or dryer heat conduction drying method are inefficient, and the optimized hot air impingement drying technology can be used, which can improve the drying efficiency and allow uniform and rapid drying. The dried coating has no external dry and internal wet Or surface wrinkles and other maladies.
In the pole piece coating production line, from unwinding to rewinding, there are many links such as coating and drying. The pole piece (substrate) is driven by multiple transmission points. In view of the extremely thin aluminum foil, copper foil, poor rigidity, easy tearing and wrinkles, etc., special technical devices are adopted in the design to keep the pole pieces flat in the coating area and strictly control the tension gradient of the film path to make The tension of the entire circuit is within the safety limit. In the transmission design of the coating line, it is advisable to adopt the intelligent speed control technology of DC motor to keep the speed of the coating spot and the film path stable, so as to ensure the longitudinal uniformity of the coating.
The general process flow of pole piece coating is as follows:
Unwinding → Splicing → Pulling → Tension Control → Automatic Deviation Correction → Coating → Drying → Automatic Deviation Correction → Tension Control → Automatic Deviation Correction → Rewinding
The coated substrate (metal foil) is discharged from the unwinding device into the coating machine. The head and tail of the substrate are connected into a continuous belt at the splicing table, and then sent to the tension adjusting device and the automatic correction device by the pulling device, and then into the coating device. The pole piece slurry is coated in the coating device according to the predetermined coating amount and blank length. During double-sided coating, it automatically tracks the length of the front side and the blank length for coating. The coated wet pole piece is sent to the drying tunnel for drying, and the drying temperature is set according to the coating speed and coating thickness.
1.3 Slitting
Slitting is to cut the rolled electrode tape into the length and width required to assemble the battery according to different battery models, and prepare for assembly.
1.4 Winding
After placing the positive electrode sheet, the negative electrode sheet, and the separator in order, they are wound into batteries on a winding machine. In order to make the thickness of the battery core evenly and tightly wound, in addition to the coating error of the positive and negative plates as small as possible, the shearing error of the positive and negative plates is also required to be as small as possible, so that the positive and negative plates are as consistent as possible. The required rectangle.
In addition, during the winding process, the operator should adjust the positions of the positive and negative plates and the diaphragm in time to prevent uneven thickness of the battery cell, uneven tightness between the front and back, and the negative plate cannot be aligned with the positive plate on both sides, especially the battery cell. The occurrence of short-circuit conditions. Winding requires the surface of the diaphragm and pole piece to be flat and free of wrinkles, otherwise the internal resistance of the battery will be increased. The up and down deviations of the positive and negative sheets or separators after rolling are all δ<-0.5mm. The winding tightness must meet the tightness design requirements, and the battery core is easy to pack but not too loose. Only in this way can the battery assembled with this battery cell be uniform and consistent, and the test structure can be guaranteed to have good accuracy, reliability and reproducibility.
Finally, it should be noted that: Except for the coating process of the film, other processes are carried out in the drying chamber, especially after the cell battery is wound and packaged, it must be dried in a vacuum drying oven at 80°C for about 12 hours. , Inject liquid into a glove box with a relative humidity of 5% or less; after liquid injection, the battery must be placed for at least 6 hours, and the electrode diaphragm can be formed and circulated after the electrode diaphragm is fully wetted.
1.5 Assembly
Put the positive electrode sheet-the diaphragm-the negative electrode sheet-the diaphragm from top to bottom in the order of winding to form a battery cell, and then through the process of injecting electrolyte and sealing to complete the assembly process of the battery and make the finished battery.
1.6 Formation
Use special battery charging and discharging equipment to perform charging and discharging tests on finished batteries, and screen out qualified finished batteries before leaving the factory.
The formation of lithium-ion batteries has two main functions: one is to convert the active material in the battery into a substance with normal electrochemical effects by the first charge; the other is to make the electrode mainly the negative electrode form an effective passivation film or SEI film, in order to form a uniform SEI film on the surface of the negative carbon material, a stepwise charging and discharging method is usually used. At different stages, the charging and discharging current is different, and the time of storage is different. It should be mastered according to the material used and the process route. , Usually the time of formation is controlled at about 24h.
The passivation film on the surface of the negative electrode plays an important role in the electrochemical reaction of the lithium ion battery for the stability of the battery. Therefore, in addition to classifying the materials and manufacturing process as confidential by battery manufacturers, the formation conditions have also been classified as important secrets for companies to manufacture batteries. During the formation of the battery, the first few charge and discharge will reduce the discharge capacity of the battery in the initial stage due to the irreversible reaction of the battery. After the electrochemical state of the battery is stable, the battery capacity becomes stable.
Therefore, some formation procedures include multiple charge and discharge cycles to achieve the purpose of stabilizing the battery. This requires battery testing equipment to provide multiple step settings and cycle settings. Taking the BS9088 device as an example, 64 step parameters can be set, and a maximum of 256 cycles can be set, and the cycle method is not limited; you can perform a small current charging and discharging cycle first, and then a high current charging and discharging cycle, and vice versa.