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How much do you know about square 6s 22.2v lipo battery?

Classified by shape Square 6s 22.2v lipo battery has three types: square, cylindrical and thin. There are many types of square 6s 22.2v lipo batteries, which are mainly used in mobile phones, digital cameras and other fields; cylindrical square 6s 22.2v lipo batteries currently include 18650 (18mm×65mm), 26650 (26mm×65mm), 21700 (21mm×70mm) Three models are mainly used in the field of notebook computers and power tools; the sheet-shaped square 6s 22.2v lipo battery can meet the thinning requirements of computers and cameras. Today CNHL will introduce the relevant knowledge of square 6s 22.2v lipo battery in detail.

1. Components of square 6s 22.2v lipo battery

Square A typical square 6s 22.2v lipo battery's main components include a top cover, a casing, a positive plate, a negative plate, a lamination or winding composed of a diaphragm, insulating parts and safety components. Among them, two safety structures are included: Nail Safety Device (NSD) and Overcharge Safety Device (OSD). NSD adds a metal layer, such as copper foil, to the outermost core of the roll. When acupuncture occurs, the local high current generated at the acupuncture position quickly reduces the current per unit area through a large area of ​​copper foil, which can prevent local overheating at the acupuncture position and slow down the occurrence of thermal runaway.

The OSD safety design can be seen on many square 6s 22.2v lipo batteries. Generally, a metal sheet is used with the melt (Fuse). The melt can be designed on the positive current collector. When overcharging, the square 6s 22.2v lipo battery is used. The internal pressure causes the OSD to trigger an internal short circuit, which generates an instantaneous high current and fuses the melt, thereby cutting off the internal current loop of the square 6s 22.2v lipo battery. The shell is generally a steel shell or an aluminum shell. With the drive of the market's pursuit of energy density and the progress of the production process, the aluminum shell has gradually become the mainstream.

2. Features of square 6s 22.2v lipo battery

Advantages of the square 6s 22.2v lipo battery: The square 6s 22.2v lipo battery has high packaging reliability; high system energy efficiency; relatively light weight and high energy density; An important option for energy density; if the monomer capacity is large, the system configuration is relatively simple, making it possible to monitor the monomers one by one; another benefit of the simple system is relatively good stability.
Disadvantages of square 6s 22.2v lipo battery: Since square 6s 22.2v lipo battery can be customized according to the size of the product, there are thousands of models on the market, and because there are too many models, it is difficult to unify the process; production automation The level is not high, and the monomers are quite different. In large-scale applications, there is a problem that the system life is much lower than the life of the monomer. The national recommended standard GB/T 34013-2017 "Power storage square 6s 22.2v lipo battery product specifications and dimensions for electric vehicles" gives 8 series of dimensions for the square 6s 22.2v lipo battery.

Guiding the size of the battery cell may not have a particularly obvious effect in the short term. Some people even think that giving guidance will restrict the development of the industry, and changing the product size is not only a problem of tooling and molds for battery cell production, but also its impact very big. However, as a recommended standard, as long as it can give manufacturers a tendency to prepare for new production capacity and adjust production lines, in the long run, it will inevitably promote the gradual development of specifications and sizes in the direction of serialization.

The consistency of cells and modules is the premise for the real realization of cascade utilization. The square 6s 22.2v lipo battery is easier to increase the capacity than the cylindrical battery, and there are fewer restrictions in the process of increasing the capacity. The number of cells in the system is small, which should be one of the important competitiveness of the square 6s 22.2v lipo battery. However, as the volume of the monomer increases, some problems also arise, such as serious side bulge, difficulty in heat dissipation, and increased non-uniformity, which hinders its development. The following introduces the typical problems and solutions of the square 6s 22.2v lipo battery.

3. Typical problems and solutions of square 6s 22.2v lipo battery

(1) Side bulging problem The square 6s 22.2v lipo battery has a certain pressure inside the square 6s 22.2v lipo battery during the charging and discharging process (experience data is 0.3-0.6MPa), under the same pressure, the larger the stress area , the more serious the deformation of the shell wall of the square 6s 22.2v lipo battery. During the first charging and discharging process of the liquid square 6s 22.2v lipo battery, the electrode material and the electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material. The formed passivation film can effectively prevent the passage of solvent molecules, but lithium ions can be freely embedded and de-embedded through the passivation layer, which has the characteristics of solid electrolyte, so this passivation film is called the solid electrolyte interface. (Solid Electrolyte Interface, SEI) membrane.

The important reasons for the expansion of the square 6s 22.2v lipo battery are:
① During the formation, gas is generated during the formation of the SEI film, and the air pressure in the square 6s 22.2v lipo battery increases. Due to the poor pressure resistance of the flat structure of the square 6s 22.2v lipo battery, the casing is deformed;
②The lattice parameters of the electrode material change during charging, causing the electrode to expand, and the electrode expansion force acts on the shell, resulting in the deformation of the square 6s 22.2v lipo battery shell;
③When stored at high temperature, a small amount of electro-hydraulic decomposition and the increase of gas pressure due to temperature effect will cause the deformation of the square 6s 22.2v lipo battery shell. Among the above three reasons, the expansion of the casing caused by the expansion of the electrode is the most important.

The bulging problem of the square 6s 22.2v lipo battery is a common problem, especially the large-capacity square 6s 22.2v lipo battery is more serious. The bulging of the square 6s 22.2v lipo battery will increase the internal resistance and local electrolysis Liquid depletion or even shell rupture seriously affects the safety performance and cycle life of the square 6s 22.2v lipo battery.
Solution:
① Adopt small structure to strengthen the shell strength;
②Optimize the arrangement. Through the above two methods, the bulging problem of the square 6s 22.2v lipo battery can be effectively solved. Strengthening the shell strength is to design the original plane shell into a reinforced structure, and test the effect of the shell strengthening structure design by pressing the inside of the shell. According to the different fixing methods (fixed length direction and fixed width direction) test, the effect of the strengthening structure can be clearly observed. Taking the case of fixed width as an example, under the pressure of 0.3MPa, the deformation of the shell with the reinforcement structure is 3.2mm, while the deformation of the shell without the reinforcement structure reaches 4.1mm, and the deformation amount is reduced by more than 20%. The arrangement of the cells in the module is different, and the deformation in the thickness direction is also different. The solution to optimize the arrangement is to compare and select the arrangement with the smallest deformation as possible.

(2) The heat dissipation performance of the large square 6s 22.2v lipo battery deteriorates. With the increase of the volume of the monomer, the distance from the heating part inside the square 6s 22.2v lipo battery to the shell is getting farther and farther, and the conductive medium and interface are getting more and more It is difficult to dissipate heat, and the problem of uneven heat distribution on the monomer is becoming more and more obvious. Wu Weixiong and others from the Department of Physics, Tsinghua University conducted a study. The experiment used a square 6s 22.2v lipo battery of 3.2V/12A h, and the square 6s 22.2v lipo battery charging and discharging equipment was Xinwei CT-3001W-50V120-ANTF. During the process, the ambient temperature was 31°C, the heat dissipation method was air cooling, and the temperature change of the square 6s 22.2v lipo battery was recorded with a temperature inspection instrument.

The experimental steps are as follows:
1) Constant current charging, charge the square 6s 22.2v lipo battery with a current of 12A until the charging cut-off voltage is 3.65V.
2) Shelving, 1h after charging to make the square 6s 22.2v lipo battery stable.
3) Constant current discharge, discharge at different rates to the discharge cut-off voltage of 2V. Among them, the discharge rate is set according to 1C, 2C, 3C, 4C, 5C and 6C respectively.

Under different discharge rates, the temperature of the surface of the square 6s 22.2v lipo battery will change. As the rate increases, the temperature is also getting higher and higher. The maximum surface temperature of the square 6s 22.2v lipo battery corresponding to each discharge rate is 38.1°C, 48.3°C, 56.7°C, 64.4°C, 72.2°C, and 76.9°C, respectively. At 3C rate discharge, the maximum temperature has exceeded 50℃, and at 6C, the temperature reached 76.9℃ and the time exceeding 50℃ was 470s, accounting for two-thirds of the entire discharge process, which is safe and continuous for the square 6s 22.2v lipo battery to work. Very unfavorable.

The phase change material has the ability to change its physical state within a certain temperature range. Therefore, the phase change material is used as a heat transfer medium and attached to the surface of the single cell, and the heat dissipation effect can be greatly improved. In addition, there are also plans to combine the thermally conductive material with water cooling, so that the water cooling system can transfer the heat absorbed by the thermally conductive material to the outside of the system.
For the square 6s 22.2v lipo battery system to prevent thermal runaway, the ideal is to be able to directly detect the parameters of each cell (basic temperature, voltage and current, etc.) The emergence of new sensors with good functions will make it possible to warn and deal with thermal runaway.

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