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Typical problems and solutions of prismatic batteries side bulge problem

2022-03-14

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Lithium-ion batteries have a certain pressure inside the battery during the charging and discharging process (experience data 0.3-0.6 MPa). Under the same pressure, the larger the force area, the more serious the deformation of the battery shell wall.

An important reason for the expansion of the battery: gas is generated during the formation of SEI during formation, and the air pressure in the battery increases. Due to the poor pressure resistance of the square battery planar structure, the shell is deformed; the lattice parameters of the electrode material change during charging, resulting in The electrode expands, and the electrode expansion force acts on the casing, causing deformation of the battery casing;

When 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 battery case. 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 square batteries is a common problem, especially for large-capacity square lithium-ion batteries. cycle life.

Strengthen the shell strength, 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 separately.

The effect of reinforcing structures can be clearly observed. Taking the case of fixed width as an example, under the pressure of 0.3Mpa, the deformation amount of the reinforced structure is 3.2mm, while the deformation amount of the shell without the reinforcement structure reaches 4.1mm, and the deformation amount is reduced by more than 20%.

Compression under the condition of fixed width:

Pressing under the condition of fixed length:

To optimize the arrangement of cells in the module, the researchers compared the two arrangements, as shown in the figure below, and the amount of deformation is shown in the table below. By comparison, it is found that the deformation in the thickness direction of arrangement II is significantly smaller than that of arrangement I.

The heat dissipation performance of the large prismatic battery deteriorates

As the volume of the cell increases, the distance between the heating part inside the battery and the casing becomes longer and longer, and there are more and more conductive media and interfaces, making it difficult to dissipate heat, and on the cell, the heat distribution is uneven. The problem became more and more obvious.

Wu Weixiong et al. conducted a study in which a 3.2 V/12 Ah square lithium-ion battery was used in the experiment. The bases are shown in Table 1. The battery charging and discharging equipment is Xinwei CT-3001W-50V120ANTF. During the test, the ambient temperature is 31 ℃, the heat dissipation method is air cooling, and the temperature change of the battery is recorded with a temperature inspection instrument.

Experimental steps:

1) Voltage charging, charge the battery with 12A current until the charging cut-off voltage is 3.65V and the current is 1.8A;

2) Set aside for 1 hour after charging to make the battery stable;

3) Constant current discharge, discharge at different rates to the discharge cut-off voltage of 2 V. Among them, the discharge rate is set as 1C, 2C, 3C, 4C, 5C, and 6C, respectively.

The phase change material is used as a heat conduction medium and is attached to the surface of the single cell, and the heat dissipation effect is greatly improved.

For lithium battery systems, to prevent thermal runaway, the ideal is to directly detect the parameters of each cell (basic temperature, voltage, current, etc.) With the advent of sensors, early warning and disposal of thermal runaway will also be possible. The number of cells in the system is small, which should be one of the important competitiveness of square batteries. 

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