Lithium batteries are the most used batteries in the digital field. Its most prominent advantage is its high energy density, which is suitable for digital products that are very compact and portable. At the same time, compared to the previous dry batteries, lithium-ion batteries can be recycled and have advantages in environmental protection. Lithium-ion battery positive and negative materials can absorb and release lithium ions. However, the potential energy of lithium ions in the positive and negative electrodes is different. The lithium ion chemical potential in the negative electrode is high, and the lithium ion chemical potential in the positive electrode is low. When lithium ions are discharged, lithium ions stored in the negative electrode are released and absorbed by the positive electrode. Since the potential energy of lithium ions in the negative electrode is higher than that of the positive electrode, this potential energy difference is released as electric energy. The charging process is a reversal of the above process, releasing lithium ions in the positive electrode into the negative electrode. Due to the migration of lithium ions in the positive and negative electrodes, the lithium ion battery is also called a rocking chair battery.
The 18650 is the most common lithium-ion encapsulation method at present, whether it is the most popular ternary material at present, or the lithium iron phosphate that the country pushes, and lithium titanate that has not been popular yet, all have the specification of 18650. 18650 type batteries, Cylindrical cylindrical package, this battery core diameter 18mm, length 65mm, widely used in charging treasure, electric cars, notebooks, light flashlights and other fields, the benefits of such packages are unified specifications, easy automation , Large-scale production, with high mechanical strength, high impact resistance, high yield characteristics; In addition there are Prismatic square soft package, common in mobile phones and tablet computers, the most direct benefit of such a package is thin, small, portable .
In the era of notebook computers, 18650 batteries are just behind the scenes of digital products. With the popularity of smart devices such as smart phones and tablets, the 18650 has also begun to move from behind the scenes to the front desk and is known to the public. So how does the seemingly simple 18650 battery come into being and what is its secret? Next, let us explore the birth process. I will illustrate the birth of the 18650 battery from the three aspects of coating, assembly, and testing.
The first production process of the battery core: coating
Before entering the production floor, you need
to wear a mask and shoe cover to avoid inhalation of dust and static
electricity. First of all, from the coating process, large copper foil (yellow)
and aluminum foil (silver) can be seen in this process. Aluminum foil is used
to coat nickel-cobalt-manganese NCM ternary materials; conversely, copper foil
is used to coat negative active material graphite; white is the separator.
Global lithium battery separator is mainly occupied by Asahi, Celgard, SK,
toray, W-SCOP and other manufacturers, these foreign companies control nearly
70% of the market share. The market share of Chinese diaphragm companies
accounts for about 30%, and lithium battery separators are constantly making
localization breakthroughs. The capacity of the cell is based on the area of
the formulation of these formulations.
A complete roll of coated positive and negative electrode material is about 126mm wide. Next, it needs to be cut into 7 small rolls with a width of about 18mm. Each roll will be evenly divided into several sections. Each section represents the required materials for a single cell. According to XCMG, an engineer of the cell core factory, the current price of ternary cathode materials is 120,000 yuan per ton, which can be used to produce 50,000 cells per ton of material. Currently, the average daily production capacity of solar cells is 500,000, and 10 tons need to be used. Ternary cathode material, this expenditure of light daily needs 600,000 yuan.
The automated machine will mark each section
with a nickel tape and an insulating high temperature resistant Mylar sheet.
The next step is to carry out five levels of weighing, namely, weight bias, A1,
A2, A3, and lighter. This is the same as the selection of CPU wafers. Different
physiques will appear in the same batch of cutting process. According to different
physiques, the corresponding capacity will be divided and shipped.
The second production process of the battery
The well-defined positive and negative electrodes will be fully automated here. The white material is a separator. The battery core is still not working properly. It is necessary to add an electrolyte as a medium to allow the positive and negative electrodes to undergo a chemical reaction. Lithium ions can migrate between the positive and negative electrodes, and a charged ion output energy lithium battery can be charged and discharged. Automatic roll-packing directly on the assembly line, into the next assembly step into the steel shell.
Steel shell is a one-piece stamping molding, the thickness of less than 1mm, so the high strength and quality requirements of steel, this appliance core factory is selected to import Hansteel materials.
This step connects the negative electrode tab to the bottom of the steel shell by spot welding. The factory uses a Japanese Miyachi laser spot welding machine to ensure spot welding accuracy and product quality. The spot welder can't weld the bottom of the steel shell, insert the bottom from the reserved hole first, and then put it into the spot welder. Squeeze it, and the negative pole will make a good nickel strip and connect it to the bottom of the steel shell.
The electrolyte is injected into the glove box filled with inert gas argon in a sealed box. The oxygen concentration in the sealed box gloves must be less than 10 ppm (close to an oxygen-free vacuum environment) to prevent electrode oxidation. The electrolyte argon gas is sent to the sealed box through the circulating purification device. The catalyst contains hydrogen and hydrogen, which can be removed by the reaction of hydrogen and oxygen. At the same time, the desiccant in the device absorbs moisture and ensures the drying of the atmosphere inside the box.
After the electrolyte is injected into the cell, the cap and the steel shell have been joined together by laser welding. Rows of neat and orderly piled up, ready to go.
A new battery was born. Each cell needs to wear "new clothes" - PVC sleeves sorted by capacity.
Different capacities correspond to different PVC jackets. Here we see green, blue, pink and other colors. The rich appearance color adds a sense of fashion to the batteries.
The Final production process of the battery core: First charge and test
After the electrolyte is injected into the electrolyte, no electricity is actually stored. At this time, the states of the positive and negative electrodes have not reached a stable state, and they must be charged through the first charge before they can be used normally. This first charge is called “chemical conversion”. When charging for the first time, extra charge is required to create a protective film on the electrode surface. This protective film is the secret of the low self-discharge of the lithium-ion battery. At the same time, the properties of the protective film also affect the performance and life of the battery. Therefore, the formation process is very important. Chemical conversion is accomplished using a volumetric cabinet.
The batteries are chemicals. In order to ensure the safety and longevity of the products, they must be packed in boxes before packing, 200 boxes per box. Ensure three consistency: consistent capacity, consistent internal resistance, and consistent voltage. Only in this way can it be delivered normally, otherwise it will be thrown into the cold room and become B-level and C-level batteries.
The first test, capacity. In order to ensure the accuracy of the capacity test, the factory purchased the industry-leading Bell automatic weighing cabinet to meet the needs of tens of thousands of daily sub-capacity. The newly born batteries need to do five cycles of burn-in tests and pick out those that cannot work or have a deviating capacity. According to the national standard GB/T18287-2013 "General Specification for Lithium-ion Battery and Battery Packs for Mobile Phones", 0.2C charge and discharge tests are to be conducted. Charge with appropriate current to 4.20V cut-off (high voltage version needs to be charged to 4.30V or 4.35V), let stand for a period of time (more than 15min) so that the battery temperature is close to room temperature and the polarization within the battery is basically dissipated. The test uses a 0.2C discharge to a 2.75V termination voltage to reach the marked capacity before it is considered qualified. Here, C is a unit of magnification, taking a 2600 mAh cell discharge as an example, and 0.2 C as a 520 mA discharge.
The second test, internal resistance. Now the batteries factory is equipped with internal resistance automatic screening machines that can be screened. 18650 three yuan materials, internal resistance within 70 milliohms, are counted as qualified products; less than 30 milliohms is a special pick. If it is lithium iron phosphate or lithium titanate, the internal resistance can be within 20 milliohms.
The third test, voltage. The battery voltage in the same box is 3.7V±0.05, which is convenient for multi-section parallel connection or series use. Each box of batteries has been made with three consistent pairs, so it is not recommended to use across the box. This is a common method in the world.
In addition to the conformance test, each batch
of batteries also needs to perform dozens of destructive tests, such as impact,
vibration, and puncture, to eliminate hidden dangers and ensure the best
quality of each batch.