Introduction to the manufacturing process of square lithium-ion batteries

The composition of lithium-ion batteries is relatively complex, mainly including cathod, anode, separators, electrolytes, current collectors, binders, conductive agents, etc. The reactions involved include electrochemical reactions of positive and negative electrodes, ion conduction and electronic conduction, and heat diffusion.

The general process of lithium-ion battery manufacturing is: active materials, binders and conductive agents are mixed to prepare slurry, and then coated on both sides of copper or aluminum current collectors. After drying, the solvent is removed to form dry pole pieces. The pole piece particle coating is compacted and densified, and then cut or stripped. Then the cathod/anode and separators are assembled into battery cells, and the electrolyte is injected after packaging. After charging and discharging activation, the battery products are finally formed.

According to the core package forming form, it can be divided into square batteries, cylindrical batteries and soft-pack batteries. Conventionally, the battery manufacturing process can be divided into three sections: front-end process segment, middle-end process assembly section and back-end process test section.

The production goal of the front-end process segment is to complete the production of cathod and anode. Its process routes include pulping, coating, rolling, slitting, sheeting, and die-cutting, and the related equipment includes mixers, coaters, roller presses, slitting machines, sheeting machines, and die-cutting machines.

The production goal of the assembly section of the middle process is to complete the manufacture of the battery cell. There are differences in the technical routes and production line equipment of the middle process of different types of lithium batteries. The essence of the middle process is the assembly process. Specifically, it is to assemble the (positive and negative) electrode sheets made in the previous process with the diaphragm and electrolyte in an orderly manner.

Due to the different structures of square (rolled), cylindrical (rolled) and soft-pack (layered) batteries, there are obvious differences in the technical routes and production line equipment of different types of lithium batteries in the middle process. Specifically, the main processes of the middle process of square and cylindrical batteries include winding, liquid injection, and packaging. The equipment involved mainly includes winding machines, liquid injection machines, packaging equipment (shelling machines, groove rolling machines, sealing machines, welding machines), etc.; the main processes of the middle process of soft-pack batteries include lamination, liquid injection, and packaging. The equipment involved mainly includes lamination machines, liquid injection machines, packaging equipment, etc.

The production goal of the back-end process is to complete the formation and packaging. As of the middle process, the functional structure of the battery cell has been formed. The significance of the back-end process is to activate it, and after testing, sorting, and assembly, a lithium battery product with safe use and stable performance is formed.

The main processes of the back-end process include formation, capacity division, testing, sorting, etc., and the equipment involved mainly includes charging and discharging machines, testing equipment, etc.

The front-end process film making process

① Pulping: Pulping is the process of mixing active material powder, binder, conductive agent, etc. and solvent in a certain order and conditions to form a stable suspension.

The slurry of lithium batteries is divided into positive electrode slurry and negative electrode slurry. The formula of the slurry, the uniformity of dispersion, the viscosity, adhesion, stability and consistency of the slurry have a significant impact on the performance of lithium batteries. The active materials, conductive carbon, thickeners, binders, additives, solvents, etc. that make up the electrode are put into the mixer in a certain proportion and order through the automatic feeding control system. With the help of the revolution stirring and dispersion crushing of the mixer, a uniformly dispersed solid-liquid suspension slurry is obtained to facilitate coating.

Three elements of stirring: wetting, dispersion and stability.

The stirring paddle has an impact on the dispersion speed. The stirring paddle generally includes snake-shaped, butterfly-shaped, spherical, paddle-shaped, gear-shaped, etc. Generally, snake-shaped, butterfly-shaped, and paddle-shaped stirring paddles are used to handle materials or ingredients that are difficult to disperse in the initial stage; spherical and gear-shaped paddles are used for states with less difficulty in dispersion, and the effect is better.

The influence of stirring speed on the degree of dispersion. Generally speaking, the higher the stirring speed, the faster the dispersion speed, but the greater the damage to the material structure itself and the equipment.

The effect of viscosity on the degree of dispersion. Generally, the lower the viscosity of the slurry, the faster the dispersion speed. However, too thin a slurry will lead to material waste and increased slurry precipitation.

The effect of viscosity on bonding strength. The higher the viscosity, the higher the flexibility and bonding strength. The lower the viscosity, the lower the bonding strength.

The effect of vacuum on the degree of dispersion. High vacuum is conducive to the discharge of gas from the gaps and surfaces of the material, reducing the difficulty of liquid adsorption. The difficulty of evenly dispersing the material will be greatly reduced when the material is completely weightless or the gravity is reduced.

The effect of temperature on the degree of dispersion. At a suitable temperature, the slurry has good fluidity and is easy to disperse. Too hot slurry is prone to crusting, and too cold slurry will greatly reduce its fluidity.

The mixing of ingredients is the basis of the subsequent process of lithium batteries. High-quality mixing is the basis for the high-quality completion of subsequent coating and roller pressing processes, which will directly or indirectly affect the safety and electrochemical properties of the battery.

Homogenization equipment is divided into intermittent homogenization and continuous homogenization according to the operation mode. The representative equipment of intermittent homogenization is double planetary stirring homogenization and disperser circulating homogenization; continuous homogenization is mainly double helix continuous grinding and dispersing homogenization.

②Coating: Coating is the process of evenly coating the positive (negative) suspension slurry on the aluminum foil (copper foil) surface and then drying it to form a film.

According to the slurry parameters, the pump speed is adjusted, and the thickness and uniformity of the extrusion head gasket are controlled by adjusting the pressure of the extrusion head cavity to control the coating thickness, so that the slurry is evenly coated on the current collector substrate, and the slurry solvent spread on the substrate is removed by oven drying and heating, so that the solid material is well bonded to the substrate to form the positive and negative electrode rolls respectively.

The execution quality of the coating process profoundly affects the consistency, safety and life cycle of the finished battery, so the coater is the most valuable equipment in the front process.

③ Rolling: Rolling makes the active material and the current collector in close contact, reduces the moving distance of electrons, reduces the thickness of the pole piece, increases the filling amount, reduces the internal resistance of the battery, increases the conductivity, and increases the volume utilization rate of the battery, thereby increasing the battery capacity.

④ Slitting: According to the process and the size of the incoming material, use a slitting machine to cut the film roll into multiple rolls of the same size. Slit the pole piece into the designed width to meet the battery cell size requirements. (The order of die cutting and slitting in the process is not necessarily, and die cutting and slitting are also carried out at the same time.)

⑤ Die cutting: The anode and cathode membranes are cut by a forming die or laser to form the pole ears and pole ear spacing of specific shapes and specifications. Figure 5 is a state diagram of the die-cutting product.

Mid-stage assembly process

Assembly process flow: Winding → Hot pressing → X-ray detection (according to product requirements) → Cell pairing → Soft connection welding → Ultrasonic welding → Insulation bottom shell → Cell shell → Top cover welding → Air tightness detection → Vacuum baking → Liquid injection → Standing.

① Winding

Winding is the process of winding the positive electrode, negative electrode and diaphragm in a certain order to make a jelly roll. It is mainly used in the production of square and round lithium batteries.

Compared with cylindrical winding, the square winding process has higher requirements for tension control, so the square winding machine is more technically difficult. The items that need to be monitored in the winding process include damage to the electrode or diaphragm, metal foreign matter on the surface of the material, overhang of the double-sided coating of the electrode, defective incoming materials, folding and folding of the pole ear, etc.; the process has a correction mechanism, tension control component, electrode length measurement component and other controls to ensure that the parameters of the wound battery meet the specification requirements.

②Hot pressing: The purpose of hot pressing of core pack is mainly to reshape the battery cell, reduce the deviation of the pole piece diaphragm during the core pack transportation process, resulting in short circuit or overhang (i.e. the distance controlled by the anode pole piece and the cathode pole piece), eliminate the diaphragm wrinkles, drive out the air inside the battery cell, make the diaphragm and the positive and negative pole pieces fit together tightly, shorten the lithium ion diffusion distance, reduce the internal resistance of the battery, improve the flatness of the lithium ion battery, make the battery cell thickness meet the requirements and have high consistency, and control the thickness of the core pack within a relatively consistent specification range, laying the foundation for the battery cell into the shell and battery consistency.

The main process parameters of hot pressing shaping of battery cells are pressurization pressure, pressurization time and template temperature. Under appropriate process parameters, there is almost no air inside the thick battery cell, the diaphragm and the pole piece fit together tightly, and the loose battery cell can become a hard block state. However, for the ceramic diaphragm used in recent years, due to the existence of the ceramic layer, it is difficult for the diaphragm to fit together with the pole piece to form this state.

In the process determination test, the test items include the permeability and thickness change of the diaphragm, whether the thickness of the battery cell meets the requirements for shell entry, whether the pole piece is broken, etc. As the core component of the battery, the battery diaphragm plays a key role in isolating the positive and negative pole pieces, or the current collector contact short circuit while allowing lithium ions to pass back and forth between the two poles. The microporous structure on the diaphragm is an important channel for these ions to travel back and forth between the positive and negative poles. Its permeability will directly affect the performance of the battery. The permeability of the diaphragm refers to the amount of gas that passes through the diaphragm under a certain time and pressure. If the permeability of the diaphragm is not good, it will affect the transfer of lithium ions between the positive and negative poles, and then affect the charging and discharging of the lithium battery.

The process of the diaphragm permeability test is: fix the battery diaphragm, apply air pressure on one side of the diaphragm, measure the air pressure drop and the time used, and test the permeability of the diaphragm. The shorter the time used, the better the permeability. During the hot pressing process, the diaphragm may be severely compressed, and the thickness of the diaphragm changes greatly, resulting in the pores being blocked. The diaphragm will become transparent when observed with the naked eye. This situation indicates that the hot pressing shaping has exceeded the limit on the battery cell and will affect the lithium ion transmission.

Hot pressing temperature exceeding the specification will cause the diaphragm to close the pores, increase the DC resistance (DCR) inside the battery cell, block the lithium ion channel, and lead to insufficient capacity. If the pole piece is relatively brittle, the battery cell bend is prone to powder loss or even breakage during hot pressing shaping, which will lead to limited electron transmission and increase the internal resistance of the battery. Therefore, the hot pressing shaping of the battery cell must also avoid this situation. These aspects require that the hot pressing shaping pressure is as small as possible and the time is as short as possible.

Conventionally, preheating can be performed before hot pressing of the battery to shorten the heating time of the hot pressed battery cell, thereby shortening the hot pressing time. Most test lines use manual hot presses, and mass production lines use automatic hot presses.

③X-ray inspection: The dimensions of the cells that have been wound and hot-pressed are reviewed. The process department decides whether to conduct a full inspection or a random inspection based on the actual situation to prevent unqualified cells from entering the subsequent process. Figure 9 shows an X-ray tester.

④Cell pairing: For square batteries or stacked batteries, in order to meet customer needs for capacity, multiple JR cells (i.e., multiple parallel cells in one aluminum shell, JR stands for jelly roll, representing the core package) are derived, and cells above 2JR form a new battery process. The main reason for the emergence of multiple JRs is that on the one hand, the scrapping cost of a single JR is reduced; on the other hand, the length of the cell pole piece of the winding equipment is too long, and the equipment control capability is difficult to meet, which will lead to misalignment of the pole ear and waste of incoming materials. Its process action is to sort the A/B side cells on the conveyor line and achieve stacking and pairing.

⑤Soft connection welding: Soft connection welding, also known as current collector welding, is used to complete the welding of the battery top cover and the soft connection sheet.

The main inspections are welding mark size and welding tension. If the welding mark size is too small, the residual area may be too small, the current capacity is poor, and the abnormal welding tension may also cause the welding machine to be cold-welded, affecting the current capacity.

⑥ Ultrasonic welding: Weld the positive and negative pole ears of the core pack to the soft connecting piece respectively, so that the pole on the top cover is connected with the pole ear of the battery cell.

⑦ Insulation bottom into shell: Before the wound battery cell is inserted into the aluminum shell, an insulation bottom is placed into the bottom of the aluminum shell to prevent internal short circuit of the battery, which is the same for general batteries.

⑧ Battery cell into shell: Battery cell into shell is to put the finished core pack into the aluminum shell, which is convenient for adding electrolyte and protecting the battery cell structure.

The equipment used for shelling is mostly to transfer the aluminum shell to the fixed fixture through the manipulator, and push the battery cell into the aluminum shell smoothly through the high-precision track. After the battery cell enters the shell, it should be short-circuited to prevent bad batteries from flowing into the subsequent process.

⑨ Top cover welding: The battery after shelling is welded.

⑩ Air tightness test: Helium test is mostly used. By injecting helium into the battery after the top cover is welded and detecting whether it leaks helium, it is determined whether there are pinholes or gaps in the aluminum shell and the top cover of the battery cell.

? Vacuum baking: High-temperature baking in a vacuum environment is used to reduce the water content in the battery cell to a safe limit value. Therefore, the battery should be tested for water content afterwards.

? Injection: The electrolyte is injected into the battery according to a certain capacity. The electrolyte and the electrode react chemically and serve as a medium for ion transport.

? Standing: After the injection and sealing are completed, the battery cell needs to be stood still first. According to different processes, it can be divided into high-temperature standing and normal-temperature standing. The purpose of standing still is to allow the injected electrolyte to fully moisturize the electrode and fully diffuse between the electrode.

Back-end process testing process

As of the assembly process, the functional structure of the lithium battery cell has been formed. The significance of the test process is to activate it, and after testing, sorting, and assembly, a lithium battery product with safe use and stable performance is formed. Its process route is roughly divided into: chemical formation and capacity system → laser cleaning → sealing pin welding → cleaning → size measurement.

① Chemical formation and capacity system: that is, when the battery is charged for the first time, a protective film is formed on the anode, called the solid electrolyte interphase layer (SEI), to achieve the "initialization" of the lithium battery and discharge the gas in the battery cell by vacuuming. It can prevent the anode from reacting with the electrolyte, which is a key factor for safe operation, high capacity, and long life of the battery. After several charge and discharge cycles, the battery is aged for 2 to 3 weeks, and the micro-short-circuited battery is removed. After capacity sorting and packaging, it becomes a commodity. After the formation, the battery cell with severe electrolyte loss can be refilled with electrolyte.

② Laser cleaning: Laser cleaning the injection port to ensure the quality of sealing pin welding.

③ Sealing pin welding: After the battery is charged, a certain amount of inert gas will be charged into the negative pressure of the battery, and then the sealing pin will be inserted for sealing welding.

④ Cleaning: Clean the surface of the battery shell.

⑤ Dimension measurement: Ensure the consistency of the battery cell size.