Study on internal resistance of Li-ion power battery

The internal resistance of a single power lithium-ion battery is an important representation of its power characteristics (Hong et al., 2023). When the current flows through a single battery with high internal resistance, the amount of heat generated is relatively large, resulting in accelerated deterioration of the single power battery, further increasing the internal resistance, accelerating the deterioration of the single power battery with high internal resistance, and seriously affecting the endurance and safety performance of new energy vehicles (Chen et al., 2020). Therefore, this paper studies the mechanism and improvement plan of low internal resistance consistency of single battery.

The internal resistance of a battery is the expression of the electrical characteristics displayed by the chemical reaction inside the battery (Du et al., 2019). The internal resistance of the battery is related to the conductivity and concentration distribution of the electrolyte and collector inside the battery, the permeability of the diaphragm, and the resistance of the battery shell (Dai et al., 2014). Extract different batches of batteries in production and test the internal resistance distribution as shown in Figure 1.

From the C3 batch, the internal resistance of the battery suddenly increased significantly, with an average increase of 0.07 mΩ, and the spread became larger. The abnormal fluctuation of battery internal resistance is analyzed as follows.

According to fault tree analysis (FTA), the internal resistance of the battery is analyzed as shown in Figure 2, which is mainly divided into the internal resistance of the battery cover and the internal resistance of the pole group, which mainly includes ionic resistance, electronic resistance and contact internal resistance (Li et al., 2020). The internal resistance tests of different battery batches are shown in Table 1.

As can be seen from Table 1, from batch C4, the measured AC internal resistance of the battery increases by 0.07 mΩ, in which the internal resistance of the pole group increases by 0.09 mΩ and the internal resistance of the battery cover increases by 0.09 mΩ. The internal resistance of battery cover, electrolyte, diaphragm and pole plate was analyzed for the failed batch battery.

The internal resistance of the battery cover is analyzed, and its structure is shown in Figure 3. The internal resistance of the battery cover is mainly composed of positive terminal resistance and negative terminal resistance (Tang et al., 2007). The design requires an internal resistance of less than 0.15 mΩ, and the internal resistance of the tested battery cover is shown in Figure 4.

From the above data, it can be seen that the internal resistance of the positive terminal of the battery cover fluctuates greatly, and the difference between batches reaches 20%. By improving the manufacturing engineering capability of the battery cover, the process capability of the positive internal resistance of the battery cover is shown in Figure 5, and the consistency process capability of the internal resistance of the battery cover is improved from 0.90 to 1.35.

In order to analyze the influence of the conductivity and concentration distribution of the electrolyte inside the electrode group on the internal resistance of the battery, the internal resistance data of the battery under the same batch of electrolyte were tested, as shown in Figure 6.

As can be seen from Figure 6, the internal resistance of the battery mutates during the use of the same batch of electrolyte, so the electrolyte batch is not the cause of the internal resistance fluctuation of the battery. Test the conductivity of the electrolyte and conduct process capability analysis, as shown in Figure 7. The CPK value of the electrolyte conductivity is 1.75, that is the consistency of the electrolyte conductivity is good.

The air permeability of the diaphragm reflects the ability of ions to pass through the diaphragm, and has a certain influence on the ionic resistance of the pole group (Reynolds et al., 2021). The diaphragm air permeability test values of different batches were analyzed, as shown in Figure 8, and no major fluctuations occurred. Therefore, it can be concluded that the internal resistance of the battery is not related to the diaphragm batch.

In order to detect the influence of the battery electrode plate on the internal resistance, three groups of batteries were selected to test the corresponding battery internal resistance and the positive electrode plate resistance. The test results are shown in Table 2.

The battery resistance corresponding to the batch of batteries with large internal resistance is significantly higher than the previous electrode resistance.

The electrode sheet is divided into composite layer (active material layer), collector and its interface (Zhao et al., 2022). The test is shown in Figure 9. The constant current power supply is used to measure the multipoint voltage through the probe, and then the resistance is calculated. The resistivity of the electrode composite layer and the interface resistance between the electrode composite layer and the collector layer of two different slurry batches were respectively tested, as shown in Table 3 and Table 4. The corresponding test results of the battery internal resistance were shown in Table 5.

The test results show that the internal resistance of the electrode sheet, the internal resistance of the composite layer and the interface resistance of the composite layer and the fluid collector are significantly different between different slurry batches, and the internal resistance of the battery corresponding to the electrode sheet with large internal resistance is also significantly increased (Amanor-Boadu and Guiseppi-Elie, 2020).

The positive and negative slurry of lithium-ion battery is a typical multi-component system (active particles, binders, conductors, etc.), and the preparation of positive and negative slurry includes a series of technological processes such as mixing, dissolution and dispersion between liquid and liquid, liquid and solid materials (Nie et al., 2019). In the production of high solid content slurry, the active particles, conductive agents and other solid particles are easy to settle and agglomerate during pulping mixing, the particle material is easy to destroy, and it is difficult to form a stable connected bond and conductive network structure to ensure the safety and performance of the battery. The existing pulping processes, such as double-planet stirring, tend to form a dispersed “dead zone” far from the impeller area, resulting in uneven slurry mixing. Meanwhile, solid components need to be added in batches in different batches, and the pulping speed is low, which is difficult to meet the application requirements of new energy vehicle power batteries (Yang et al., 2021). Therefore, the twin-screw pulping process of power lithium-ion battery was independently developed and designed, as shown in Figure 10.

The mathematical expression of the shear rate (γ) of the twin screw system at the edge of the cylinder wall is:

Where D₀ represents the outer diameter of the screw (mm), N represents the speed of the main screw (rpm), and h represents the distance between the wall of the screw cylinder and the point where it is located (mm).

The viscosity of twin-screw extruder is defined as: (Zhuang et al., 2020) (see Figure. 11)

Where K is the fluid consistency coefficient, and the greater K is, the greater the flow resistance. n is a non-Newtonian exponent, and in general, for most polymer fluids, K and n can be regarded as constants over a small range of variations. Then the viscosity dependence on shear can be expressed as:

Where, when n = 1, $dη/dγ=0$⁠,the fluid is Newton fluid; When n < 1, $dη/dγ<0$⁠, The fluid is pseudoplastic. When n > 1, $dη/dγ>0$⁠, the fluid is dilatant plastic (Forouzan et al., 2018). It can be seen from the relationship between shear viscosity and shear rate that pseudoplastic fluid has shear thinning behavior (lithium battery slurry belongs to this property), and dilatant plastic fluid has shear thickening behavior (Song et al., 2020).

Use the double -screw pulp process, the planet’s mixing pulp process to make the pulp, and make the polar sheet according to different coating speeds to perform the resistance test of the compound layer. The measurement results are shown in Figure 12.

It can be seen that the internal resistance of the polar section of the different pulp production is large. Under the same slurry process, the internal block resistance of the polar section of the coating speed is small. Combined with the above, the slurry process is solidified, the internal blockage of the polar sheet is monitored, and the coating speed is changed to ensure the stable process.

According to the document, it may also affect the internal resistance of the battery (Zhang et al., 2020) to the internal resistance test of the internal resistance test of the internal resistance test of the internal resistance test of the internal resistance test. (See Figure 13)

It can be seen that as the temperature rises, the internal resistance value is declining, the temperature rises by 10 °C, and the internal resistance is reduced by 0.078 MΩ. The relationship between the internal resistance of the battery and the real -time temperature of the production is shown in Figure 14. There is no obvious relationship with the temperature. The temperature has a certain effect on the internal resistance of the battery, but the actual production process is not the dominant factor (Li et al., 2014). Therefore, the control workshop temperature is 25–28 °C, which can ensure the consistency of the battery internal resistance testing test.

By the improvement of the production control, uniform slurry and coating process of the battery cover, and the improvement of the temperature control of the workshop, the internal resistance of the battery produced has significantly improved. (see Figure 15).

According to the actual production results, the battery internal resistance data produced by different production lines does not fluctuate obviously, and the difference in the standard difference between the internal battery resistance from 0.017 is reduced from 0.017 to 0.0097.

Through the analysis and improvement of the internal resistance of power lithium-ion batteries, a comprehensive understanding of the internal resistance of the power lithium-ion battery has a comprehensive understanding of the influencing factors of power lithium-ion batteries. The use of power lithium-ion batteries on new energy vehicles has played a supporting role (Li, 2018).