A battery cell is the smallest unit of a battery system. Multiple battery cells form a module, and multiple modules form a battery pack. This is the basic structure of a car power battery. A battery is like a container for storing electrical energy. The amount of capacity it can store is determined by the amount of active material covered by the positive and negative electrodes. The design of the positive and negative electrode plates needs to be tailored to different models. The gram capacity of the positive and negative electrode materials , the ratio of active materials, the thickness of the electrode plates, the compaction density, etc. are also crucial to the capacity.
During the lithium battery production process, the battery cells and shells used in the liquid injection process have been dried and are therefore extremely sensitive to moisture in the air in the workshop. Once the humidity in the air in the room is too high and is absorbed by the battery, it will cause battery swelling, leakage and many other problems. Therefore, limited environmental control of the lithium battery injection process is a very important part of the lithium battery production process.
First, the requirements for humidity in the production process environment of the lithium battery workshop:
Lithium battery drying room New energy drying room To prevent moisture from affecting lithium batteries, the lithium battery production line must be placed in a drying room. A drying room is a room that uses a dehumidifier to reduce the air humidity in the room to a certain required range. A lithium battery drying room refers to a room with relatively low humidity for the production of lithium batteries.
The moisture content in lithium battery materials is one of the main sources of moisture in the battery cell, and the higher the ambient humidity, the easier it is for the battery materials to absorb moisture from the air. Conversely, the better the ambient humidity control, the more limited the battery materials' ability to absorb moisture from the air. Currently, there is a recommended humidity control ladder for lithium battery production workshops (for reference only, humidity control is carried out according to the actual situation of each battery company):
Relative humidity ≤ 30% workshop (such as mixing, coating machine head, tail, etc.)
Relative humidity ≤ 20% workshop (such as rolling, sheet making, baking, etc.)
Relative humidity ≤ 10% workshop (such as lamination, winding, assembly, etc.)
Dew point temperature ≦ -45℃ workshop (such as battery cell baking, liquid injection, sealing, etc.)
Even if the humidity gradient control is followed as above, the residence time of the process needs to be controlled.
The lithium battery drying room is to control the humidity of the environment in the lithium battery production line environment.
Slurrying of active materials - mixing process
Stirring is to stir the active materials into a slurry through a vacuum mixer. This is the first process in battery production. The quality control of this process will directly affect the quality of the battery and the qualified rate of the finished product. Moreover, this process is complex and has high requirements for the raw material ratio, mixing steps, stirring time, etc.
What is being stirred here is the active material of the battery.
CATL's mixing workshop has strict dust control. In addition, dust needs to be strictly controlled during the mixing process to prevent dust from affecting battery consistency. The level of dust control in CATL's production workshop is equivalent to medical level.
Apply the stirred slurry on the copper foil - coating process
This process is to evenly apply the slurry that has been stirred in the previous process to the upper and lower surfaces of the 4,000-meter-long copper foil at a speed of 80 meters per minute. The copper foil before coating is only 6 microns thick, which can be described as "as thin as a silkworm's wing."
The most important thing in the coating process is the consistency of thickness and weight
Coating is very important. It is necessary to ensure that the thickness and weight of the pole piece are consistent, otherwise it will affect the consistency of the battery. Coating must also ensure that no particles, debris, dust, etc. are mixed into the pole piece. Otherwise, it will cause the battery to discharge too quickly and even cause safety hazards.
Compress the negative electrode material on the copper foil and then cut it - cold pressing and pre-cutting
In the rolling workshop, the pole sheets with positive and negative electrode materials attached are rolled by rollers. On the one hand, this makes the coated materials tighter, improves the energy density, and ensures the consistency of thickness. On the other hand, it will further control dust and humidity.
Cold pressing is to compact the positive and negative electrode materials on the aluminum foil, which is also important for improving energy density.
The cold-pressed electrodes are cut into pieces according to the size of the batteries to be produced, and the generation of burrs (the burrs here can only be seen clearly under a microscope) is fully controlled. This is to prevent the burrs from piercing the diaphragm and causing serious safety hazards.
Cut out the small ears of the positive and negative poles on the battery - ear die cutting and striping
The tab die cutting process is to use a die cutting machine to form the conductive tabs for the battery. We know that the battery is divided into positive and negative poles, and the tabs are the metal conductors that lead the positive and negative poles out of the battery. In layman's terms, the tabs at the positive and negative poles of the battery are the contact points during charging and discharging.
The next slitting process is to cut the battery electrodes using a cutting knife.
The ear die cutting is to make small ears with positive and negative poles.
Completing the prototype of the battery cell - winding process
Here, the positive electrode, negative electrode and separator of the battery are wound together to form a bare cell. Advanced CCD visual inspection equipment can realize automatic inspection and automatic deviation correction to ensure that the cell electrodes are not misaligned.
After the winding process, the prototype of the battery cell is basically formed
With the assistance of CCD visual inspection equipment, CATL's battery production workshop is one of the most automated battery production workshops in the world.
Removing moisture and injecting electrolyte – baking and filling
Moisture is the enemy of the battery system. The battery baking process is to make the internal moisture of the battery meet the standard and ensure that the battery has good performance throughout its life cycle.
In order to remove moisture, the battery cell needs to be baked
Injection is to inject electrolyte into the battery cell. The electrolyte is like the blood flowing in the body of the battery cell. The exchange of energy is the exchange of charged ions. These charged ions are transported from the electrolyte to the other electrode to complete the charge and discharge process. The injection amount of electrolyte is the key among the keys. If the injection amount of electrolyte is too large, it will cause the battery to heat up or even fail directly. If the injection amount is too small, it will affect the battery's cyclability.
The process of battery activation - formation
Formation is the process of activating the battery cell after injection. Through charging and discharging, a chemical reaction occurs inside the battery cell to form a SEI film (SEI film: a passivation film is formed when the electrolyte and the negative electrode material react at the solid-liquid phase level during the first cycle of the lithium battery, just like coating the battery cell with a mask.), ensuring the safety, reliability and long cycle life of the subsequent battery cells during the charge and discharge cycle. To activate the performance of the battery cell, it must also go through a series of "physical examination processes" such as X-ray monitoring, insulation monitoring, welding monitoring, and capacity testing.
The formation process also includes the second filling of electrolyte, weighing, welding of the filling port, and air tightness testing after the battery cell is "activated"; self-discharge testing, high-temperature aging and static standing ensure product performance.
Each manufactured battery cell has a separate QR code that records the date of birth, manufacturing environment, performance parameters, etc. The powerful traceability system can record any information. If an abnormality occurs, production information can be retrieved at any time; at the same time, these big data can provide targeted data support for subsequent improved designs.
A single battery cell cannot be used. It can only be used directly by combining multiple battery cells together, adding a protective circuit and a protective shell. This is the so-called battery module.
A battery module is composed of many cells. Cells with good consistency need to be strictly screened and assembled into modular battery modules according to precise design, and single cell monitoring and management devices are installed. CATL's module production line is fully automated, and the entire process is completed by more than a dozen precision manipulators. In addition, each module has its own fixed identification code, so that the entire process can be traced if there is a problem.
Loading
The battery cells are transported to the designated location, and the robot automatically grabs them and sends them to the module assembly line.
In CATL's workshop, everything from automatically transporting materials to feeding equipment is 100% automated.
Give the battery a bath - plasma cleaning process
The surface of each battery cell is cleaned (CATL uses plasma treatment technology to ensure cleanliness). Ion cleaning is used here to ensure that pollutants in the process do not adhere to the bottom of the battery cell.
Why should we use plasma cleaning technology? The reason is that plasma cleaning technology is the most thorough stripping cleaning method. Its biggest advantage is that there is no waste liquid after cleaning. Its biggest feature is that it can handle metals, semiconductors, oxides and most polymer materials well, and can achieve overall and local cleaning as well as complex structure cleaning.
Assembling the cells - gluing the cells
Before assembling the battery cells, the surface needs to be coated with glue. In addition to fixing, the coating also serves the purpose of insulation and heat dissipation. CATL uses the most advanced high-precision coating equipment and robot collaboration in the world. It can coat the glue with a set trajectory and monitor the coating quality in real time to ensure the coating quality, further improving the consistency of each group of different battery modules.
Gluing process of battery cells
Building a home for the battery cell - welding of the end plate and the side plate
Battery modules are mostly made of welded aluminum end plates and side plates, and are laminated and welded by robots.
Wiring harness isolation plate assembly
After the welding monitoring system accurately locates the welding position, it binds the wire harness isolation board material barcode to the MES production scheduling management system and generates a separate code for traceability. After coding, the wire harness isolation board is automatically loaded into the module by a robot.
Complete the series and parallel connection of batteries——laser welding
Through automatic laser welding, the connection between the pole and the connecting piece is completed to realize the series and parallel connection of the batteries.
An important step before going offline - offline testing
Before going offline, the module is fully inspected for performance, including module voltage/resistance, battery cell voltage, withstand voltage test , and insulation resistance test. The standardized module design principle can be customized to match different models, and each module can also be installed in the best suitable space and predetermined position in the vehicle.
Each battery pack contains several battery cells, integrated with connectors, controllers and cooling systems, and is wrapped in an aluminum shell. They are automatically fastened by bolts and connected by electrical connectors. Even if a failure occurs, only a single module needs to be replaced, not the entire battery pack. The maintenance workload and danger are greatly reduced. Replacing a module only requires disassembling the cooling system, and does not involve other components.
In fact, electric vehicles must use various methods to ensure safety to the greatest extent from the initial design stage. However, even the most perfect design still needs to be tested in practice. At CATL, only battery products that have successfully passed these tests can be released for use.
590 degrees Celsius fire test
What does it mean to burn batteries at 590 degrees Celsius? We know that the surface temperature of Venus is 464 degrees Celsius. At such a high temperature, metal materials such as lead and zinc have long melted. However, the battery pack has to "survive" at such a high temperature.
In terms of safety performance, the national standard is that the battery will not catch fire or explode after external combustion for 130 seconds. However, as an industry leader, CATL has higher requirements. Not only has it achieved the national standard that the battery can still work normally after external combustion for 130 seconds, but it has also achieved the national standard that the battery will not explode after continuous combustion for 1 hour at 590 degrees Celsius.
Continuous 21 hours vibration test
In daily car use, it is inevitable to pass through some bumpy roads. The vibration generated by the battery may cause poor fixation of substandard battery products, loose components, or even cracked casings, ultimately leading to safety failures.
Therefore, we need to simulate the impact of vehicle vibration on the battery pack. The vibration table is used to simulate the bumpy road conditions that the battery pack will encounter in actual use, the environmental chamber is used to provide different temperature environments, and the charger and discharger are used to provide the actual working conditions of charging and discharging. These three parts constitute a vibration test system with temperature and load, which truly simulates the scene when the actual vehicle is used.
CATL has a 20-ton vibration table that simulates the bumpy road conditions that battery packs will encounter in actual use, but the vibration intensity is even greater than the actual road conditions. In the test, the battery pack is vibrated 200 times per second, and the battery cell module is vibrated 2,000 times. Even more stringent is that the battery pack needs to vibrate randomly for 21 hours in an environmental condition of -30°C to 60°C, which can simulate the fatigue of driving hundreds of thousands of kilometers
Impact test with acceleration up to 100G
Similar to the vibration test, the impact test is used to test the mechanical stability of the battery pack. It simulates the impact of the instantaneous bump on the battery pack structure when the vehicle passes through a roadblock. In addition, there is a one in ten thousand chance of the battery falling during the battery replacement process. As the saying goes, it is better to be safe than sorry. CATL conducts a free fall test on the battery from a height of 1 meter and ensures that all functions are still normal.
In CATL's impact test, the highest acceleration can reach 100G. It should be noted that the maximum acceleration that an average person's heart can withstand is 50G. The current record shows that the human body can withstand an acceleration limit of about 40G. Under such a strong acceleration impact, the battery pack still operates normally.
The squeeze test that is closest to a real accident
The extrusion test is used to simulate the situation where the battery is squeezed in a traffic accident. As the degree of deformation of the battery increases, the positive and negative current collectors will be torn first. A very large current is generated at the short-circuit point, and the heat is released in a concentrated manner, causing the temperature of the short-circuit point to rise sharply, which can easily cause thermal runaway, and then cause fire or explosion.
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