Lithium Battery Manufacturing Technology and Process

Manufacturing process of square lithium-ion batteries

The composition of lithium-ion batteries is relatively complex, mainly including positive/negative electrode plates, separators, electrolytes, current collectors and binders, conductive agents, etc.

 The reactions involved include electrochemical reactions of the positive and negative electrodes, ion and electron conduction, and diffusion of heat.

The general manufacturing process of lithium-ion batteries involves mixing active materials, binders, and conductive agents to prepare a slurry, 

which is then coated on both sides of a copper or aluminum current collector. After drying, the solvent is removed to form a dry electrode sheet. 

The particle coating of the electrode sheet is compacted and densified, and then cut or divided into strips.

 Then the positive/negative electrode plates and separator are assembled into the battery cell, packaged and injected with electrolyte, activated by charging and discharging,

 and finally formed into the battery product.

According to the shape of the core pack, it can be divided into square batteries, cylindrical batteries, and soft pack batteries. 

Typically, the battery manufacturing process can be divided into three stages: the front-end manufacturing section, the back-end assembly section, and the back-end testing section.

The production goal of the front-end process is to complete the production of positive and negative electrode plates. 

The process route includes pulping, coating, rolling, slitting, film making, and die-cutting, 

and related equipment such as mixers, coating machines, roller presses, slitting machines, film making machines, die-cutting machines, etc.

The production goal of the mid process assembly section is to complete the manufacturing of battery cells,

 and there are differences in the mid process technology routes and production line equipment for different types of lithium batteries. 

The essence of the intermediate process is the assembly process, specifically the orderly assembly of the (positive, negative) pole pieces made in the intermediate process with the diaphragm and electrolyte.

Due to the different structures of square (rolled), cylindrical (rolled), and soft pack (layered) batteries,

 there are significant differences in the technical routes and production line equipment of different types of lithium batteries in the intermediate process.

 Specifically, the main processes of the middle section of square and cylindrical batteries include winding, liquid injection, and packaging.

 The equipment involved mainly includes winding machines, liquid injection machines, packaging equipment (such as shell entry machines,

 groove rolling machines, sealing machines, welding machines), etc;

 The main processes of the intermediate stage 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 subsequent process is to complete the formation and packaging. 

As of the middle stage process, the functional structure of the battery cells has been formed. 

The significance of the later stage process is to activate them, and after testing, sorting, and assembly, form lithium battery products that are safe to use and have stable performance.

The main processes of the later stage include chemical conversion, volume separation, testing, sorting, etc. 

The equipment involved mainly includes charging and discharging motors, testing equipment, etc.

Pre production process: Film making technology

① Pulping: Pulping is the process of uniformly mixing active substance powder, binders, conductive agents,

 and solvents in a certain order and under certain conditions to produce a stable suspension.

The slurry of lithium batteries is divided into positive electrode slurry and negative electrode slurry. 

The formula, dispersion uniformity, viscosity, adhesion, stability, and consistency of the slurry have a significant impact on the performance of lithium batteries. 

The active material, conductive carbon, thickener, binder, additive, solvent, etc. 

that make up the electrode are fed into the mixer through an automatic feeding control system in a certain proportion and order. 

With the help of the mixer's revolving stirring and dispersing crushing effect, a uniformly dispersed solid-liquid suspension slurry is obtained for coating.

The three elements of stirring are wetting, dispersion, and stability.

The stirring blade has an impact on the dispersion speed, and the stirring blade generally includes snake shaped, butterfly shaped, spherical, blade shaped, gear shaped, etc. 

Generally, snake shaped, butterfly shaped, and paddle shaped impellers are used in the initial stage of handling materials or ingredients that are difficult to disperse;

 Spherical and gear shapes are used for dispersing difficult states, with excellent results.

The influence of stirring speed on dispersion degree generally means that the higher the stirring speed, the faster the dispersion speed,

 but the greater the damage to the material's own structure and equipment.

The effect of viscosity on dispersion degree is usually that the lower the viscosity of the slurry, the faster the dispersion speed. 

However, being too thin will lead to material waste and increased precipitation of the slurry.

The influence of viscosity on bond strength, the higher the viscosity, the higher the flexural strength, and the greater the bond strength; 

The lower the viscosity, the lower the bonding strength.

The influence of vacuum degree on dispersion degree, high vacuum degree is beneficial for gas discharge from material gaps and surfaces, reducing the difficulty of liquid adsorption; 

The difficulty of dispersing materials uniformly under complete weightlessness or reduced gravity will be greatly reduced.

The influence of temperature on dispersion degree: at an appropriate temperature, the slurry has good fluidity and is easy to disperse. 

Too hot slurry is prone to skin formation, while too cold slurry will greatly reduce its fluidity.

The mixing of ingredients is the foundation of the subsequent process of lithium batteries, 

and high-quality mixing is the basis for the high-quality completion of subsequent coating and rolling processes, 

which will directly or indirectly affect the safety and electrochemical performance of the battery.

Homogenization equipment can be divided into intermittent homogenization and continuous homogenization according to the operation mode. 

Representative equipment for intermittent homogenization includes double planetary stirring homogenization and disperser circulation homogenization; 

Continuous homogenization is mainly achieved through double helix continuous grinding and dispersion of homogenate.

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

Adjust the pump speed according to the slurry parameters, 

and control the coating thickness by adjusting the pressure of the extrusion head chamber through the thickness and uniformity of the extrusion head gasket, 

so that the slurry is evenly coated on the current collector substrate. 

Dry and heat the slurry solvent spread on the substrate through an oven, and make the solid material adhere well to the substrate to form positive and negative electrode rolls, respectively.

The quality of the coating process has a profound impact on the consistency, safety, and lifespan of the finished battery,

 so the coating machine is the most valuable equipment in the front-end process.

③ Roller pressing: By roller pressing, the active substance is in close contact with the current collector

, reducing the distance of electron movement, lowering the thickness of the electrode, increasing the loading capacity, 

and reducing the internal resistance of the battery to improve conductivity, thereby increasing the battery capacity utilization rate.

④ Slitting: According to the process and incoming material size, use a slitting machine to cut the film roll into multiple rolls of the same size. 

Cut the polarizer into the designed width to meet the size requirements of the battery cell.

 (The order before and after die-cutting and slitting in the process is not necessarily the same, and die-cutting and slitting can also be performed simultaneously.)

⑤ Die cutting: The anode cathode film is cut by a forming die or laser to form specific shapes and specifications of pole ears and pole ear spacing.

Mid process assembly technology

Assembly process flow:

 winding → hot pressing → X-ray inspection (according to product requirements) → cell pairing → soft connection welding → ultrasonic welding → insulation bottom into the shell →

 cell into the shell → top cover welding → air tightness testing → vacuum baking → liquid injection → standing.

① Winding up

Winding is the process of making a jelly roll by winding the positive electrode sheet, negative electrode sheet, and separator in a certain order. 

Mainly used for the production of square and circular lithium batteries.

Compared to cylindrical winding, the square winding process has higher requirements for tension control, 

so the technical difficulty of the square winding machine is greater. 

The items that need to be monitored in the winding process include pole piece or diaphragm damage, metal foreign objects on the material surface,

 misalignment of the double-sided coating of the pole piece, incoming defective products, pole ear folding and folding, etc; 

The process is equipped with correction mechanisms, tension control components, pole piece length measurement components, 

and other controls to ensure that the parameters of the wound battery cells meet the specifications.

② Hot pressing: The main purpose of hot pressing the core pack is to shape the battery cell,

 reduce the deviation of the electrode separator during the core pack transportation process, 

which may cause short circuits or overhangs (i.e., changes in the distance controlled between the anode and cathode electrode sheets), 

eliminate membrane wrinkles, drive out air inside the battery cell, 

make the membrane and positive and negative electrode sheets tightly adhere to each other, shorten the diffusion distance of lithium ions, reduce the internal resistance of the battery, 

improve the flatness of the lithium-ion battery, make the thickness of the battery cell 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 battery shell insertion and battery consistency.

The main process parameters for hot pressing and shaping of battery cells include pressure, pressure time, and template temperature. 

Under appropriate process parameters, there is almost no air inside the thick battery cell, 

and the diaphragm and pole piece are tightly adhered together, allowing the loose battery cell to become a hard block state.

 However, for the ceramic membranes used in recent years, due to the presence of ceramic layers, it is difficult for the membranes to adhere to the electrodes and form this state.

In the process determination test, the testing items include the permeability and thickness change of the diaphragm, 

whether the thickness of the battery cell meets the requirements for entering the shell, and whether the pole piece has broken. 

The battery separator, as the core component of the battery, plays a crucial role in isolating the positive and negative electrodes, 

or allowing lithium ions to pass back and forth between the two poles while short circuiting the current collector. 

The microporous structure on the separator is an important channel for these ions to travel back and forth between the positive

 and negative electrodes, and its permeability directly affects the performance of the battery. 

The permeability of the separator refers to the amount of gas that can pass through it under certain time and pressure.

 If the permeability of the separator is poor, it will affect the transfer of lithium ions between the positive and negative electrodes, thereby affecting the charging and discharging of lithium batteries.

The process of testing the permeability of the separator is as follows: fix the battery separator, apply air pressure on one side of the separator, 

measure the pressure drop and time used, and test the permeability of the separator. 

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 may vary greatly, causing the micropores to be blocked. 

The diaphragm will turn transparent when observed with the naked eye.

 This situation indicates that the effect of hot pressing on the battery cell exceeds the limit, which will affect the transmission of lithium ions.

Exceeding the standard hot pressing temperature can cause the diaphragm to close, resulting in an increase in the direct current resistance (DCR) inside the battery cell, 

obstruction of the lithium-ion channel, 

and insufficient capacity utilization. If the electrode is relatively brittle, the bending of the battery cell during hot pressing is prone to powder loss or even breakage,

 which can limit electronic transmission and increase the internal resistance of the battery. 

Therefore, the hot pressing shaping of battery cells must also avoid this situation. 

These aspects require that the pressure for hot pressing and shaping should be as low as possible, and the time should be as short as possible.

Normally, preheating treatment can be carried out before hot pressing the battery to shorten the heating time of the hot pressed cell, thereby reducing the hot pressing time. 

Most experimental lines use manual hot press machines, while mass production lines use automatic hot press machines.

③ X-ray inspection: Conduct a dimensional review of the battery cells that have been wound and hot pressed.

 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.

④ Cell pairing: For square or laminated batteries, in order to meet customer demand for capacity, 

multiple JR cells (i.e. multiple parallel cells inside an aluminum shell, JR stands for jelly roll,

 representing the core pack) have been derived, and a new battery process is composed of 2 JR or more cells. 

The main reason for the emergence of multiple JRs is to reduce the scrap cost of a single JR on one hand;

 On the other hand, if the length of the battery cells in the winding equipment is too long and the control capability of the equipment is difficult to meet,

 it may lead to misalignment of the electrode ears and waste of incoming materials. 

The process action is to sort the A/B surface cells on the conveyor line and achieve stacking pairing.

⑤ Soft connection welding: Soft connection welding, also known as current collector welding, 

is used to complete the welding work between the battery top cover and the soft connection piece.

The main focus is on detecting the size of the weld print and welding tension.

 If the size of the weld print is too small, the residual area may be too small, the overcurrent capacity may be poor,

 and abnormal welding tension may also lead to virtual welding of the welding machine, affecting the overcurrent capacity.

⑥ Ultrasonic welding: Weld the positive and negative pole ears of the core package separately to the soft connection piece, 

so that the pole on the top cover is connected to the pole ears of the battery cell.

⑦ Insulation bottom in shell: Before inserting the wound battery cell into the aluminum shell, 

placing an insulation bottom in the bottom of the aluminum shell is to prevent internal short circuits in the battery, 

which is the same for general batteries.

⑧ Cell in shell: Cell in shell refers to packaging the finished core into an aluminum shell, which facilitates the addition of electrolyte and protects the structure of the cell.

The equipment used for shell insertion is mostly to transfer the aluminum shell to a fixed fixture through a robotic arm, 

and push the battery cell smoothly into the aluminum shell through a high-precision track. 

After the battery cells are put into the shell, they should be subjected to short-circuit detection to prevent defective cells from entering the subsequent process.

⑨ Top cover welding: Weld the top cover of the battery after it is put into the shell.

⑩ Airtightness testing: Helium gas testing is commonly used to determine

 whether there are pinholes or gaps in the aluminum shell and top cover of the battery cell by injecting helium gas into the battery after welding the top cover and testing for helium gas leakage.

⑪ Vacuum baking: High temperature baking in a vacuum environment to reduce the water content in the battery cell and achieve a safe threshold. 

Therefore, it is necessary to conduct water content testing on the battery afterwards.

⑫ Liquid injection: Inject electrolyte into the battery according to a certain capacity, 

and the electrolyte and electrode will undergo a chemical reaction while serving as a medium for ion transport.

⑬ Rest: After the injection and sealing are completed,

 the battery cell needs to be first left to rest. Depending on the process, it can be divided into high-temperature rest and room temperature rest. 

The purpose of rest is to allow the injected electrolyte to fully moisturize the electrodes and diffuse between them.

Testing process for the later stage of the process

As of the assembly process, the functional structure of the lithium battery cells has been formed.

 The significance of the testing process is to activate them, and after testing, sorting, and assembly, form lithium battery products that are safe to use and have stable performance. 

The process route is roughly divided into: chemical capacity separation system → laser cleaning → sealing nail welding → cleaning → size measurement.

① Chemical capacity separation system: refers to the formation of a protective film on the anode during the first charge of the battery, 

called the solid electrolyte interphase layer (SEI), to achieve the "initialization" of the lithium battery and exhaust the gas inside the cell through vacuum pumping. 

It can prevent the anode from reacting with the electrolyte, which is a key element for safe operation, high capacity, and long lifespan of batteries. 

After several cycles of charging and discharging, the battery is aged for 2-3 weeks, and the micro short circuit battery is removed. 

After capacity sorting and packaging, it becomes a commodity. Cells with severe electrolyte loss after formation can be refilled with electrolyte through secondary injection.

② Laser cleaning: Perform laser cleaning on the injection port to ensure the quality of sealing nail welding.

③ Sealing nail welding: After being divided into two parts, a certain amount of inert gas will be injected into the negative pressure of the battery, 

and then sealing nails will be inserted for sealing and welding.

④ Cleaning: Clean the surface of the battery casing.

⑤ Size measurement: Ensure the consistency of cell size.

The above is an introduction to the conventional production process of square batteries.