Science and technology
How to efficiently dissipate heat for lithium-ion batteries?
Lithium-ion batteries generate a certain amount of heat during charging and discharging, especially in the process of charging and discharging large currents. A large amount of heat is generated inside the lithium-ion battery, but the thermal conductivity of the lithium-ion battery is different in different directions. There is a significant difference in the rate of thermal conductivity in the direction parallel to the pole piece, which is significantly higher than the thermal conductivity in the direction perpendicular to the pole piece. Therefore, different heat dissipation methods not only have significant differences in efficiency, but also improper heat dissipation methods will generate severe temperature gradients inside the lithium ion battery, affecting the internal current distribution of the battery, resulting in inconsistent internal attenuation of the battery. Affect the life of lithium-ion batteries.
Recently, YanZhao (first author) and Teng Zhang (communication author) and Gregory J. Offer (communication author) of Imperial College of the United Kingdom have “surface heat dissipation” and “ear-cooling” for lithium-ion batteries by establishing a two-dimensional model. "The effects of the two heat dissipation methods and the impact on the life of the lithium ion battery were studied, and suggestions were made on how to optimize the "ear heat dissipation".
First, the author created a two-dimensional model (shown below) in MATLAB R2017 that simulates the temperature distribution of the cell in a cross-section (length x thickness).
In order to verify the validity of the model, the author used three kinds of soft-packed batteries to verify the experimental results. The positive materials of the three batteries are NCM622 materials, the negative electrodes are all graphite materials, the battery capacity is 16Ah, and the size is 117mm×101mm. ×11.5mm, the structure and size of the positive electrode, the negative electrode and the separator are completely the same, and only the structure and position of the tab are different (as shown in the figure below). The details of the structure of the tabs of the three batteries are shown in the following table.
The temperature profile of the battery intermediate position surface, the battery tab and the cooling bath measured in the experiment, can be seen from the following figure a (negative pole cooling), the C70 battery compared to the narrow-ear ear, the wide-ear design C70 The surface temperature of the battery is 2.3 °C lower at the center position and 1.5 °C lower at the negative electrode tab. From the following figure b (cooling of the positive electrode tab), it can be seen that the wide-ear design C70 battery still has an advantage in cooling effect, at the center position. The temperature of C70 battery is 2 °C lower and the position of the ear is 0.7 °C lower, which indicates that the wide-ear design can effectively reduce the thermal resistance of the battery. At the same time, compared with the heat dissipation of the positive electrode, the thermal resistance of the negative electrode is smaller and can be improved. heat radiation.
In order to verify the effect of different heat dissipation methods on the surface temperature of lithium-ion batteries, the authors used the surface cooling method to dissipate the heat of the S30 battery, and then used the S30, C30 and C70 batteries for heat dissipation. All batteries were first charged at 1C. Then, discharge was performed at a rate of 5 C, followed by charging at 2 C, and the surface temperature change of the battery during this process was as shown in the following figure.
As can be seen from the figure below, the S30 battery with surface heat dissipation has the lowest temperature on the surface of the battery during the whole process. The maximum temperature at the end of discharge is 33.3 °C. The temperature of the S30 and C30 batteries with the ear heat is the highest, at the end of the discharge. The battery temperatures reached 45.5 ° C and 44.5 ° C, respectively, while the temperature of the C70 battery with a wide-aperture design showed a significant decrease. The maximum temperature at the end of the discharge was 6 ° C lower than the narrow-ear design S30 and C30 batteries, indicating a wide The ear design can effectively improve the heat dissipation effect.
From the above experimental data, we can see that although the wide-aur design can effectively improve the heat dissipation of the ear, the maximum temperature is still 7 °C higher than the surface heat dissipation. In order to further optimize the heat dissipation effect of the ear, the author adopts two-dimensional. The model simulates the battery. The following figure shows the simulation results for the C70 battery. From the following figure c, we can see that the simulated battery temperature curve is highly consistent with the actual temperature curve of the battery, which indicates that the model can simulate the lithium ion battery very well. Temperature changes in actual work.
The following figure shows the temperature variation curve of the battery when the three types of batteries simulated by the model are used for surface heat dissipation and the heat dissipation of the ear. For the S30 battery with surface heat dissipation, the maximum temperature of the model and the actual temperature are at the S3 position. For 1.5 ° C, the maximum error of the C30 battery with the ear cooling at the S1 and S3 positions is -1.5 ° C and 1.9 ° C, respectively. For the S30 and C70 battery models with the ear cooling, the simulation results are not much different from the actual results. 1 ° C, indicating that the model can provide temperature prediction with acceptable accuracy.
The following figure shows the internal temperature change, current distribution change and local SoC variation of the S70 battery with surface heat dissipation and the C70 battery with the ear cooling method. From the following figure a we can see that the S30 battery passes through the top of the battery. The surface heat dissipation method dissipates the battery, so it can be seen that there is a significant temperature gradient inside the battery. In contrast, the temperature of the C70 battery using the ear cooling method is much more uniform.
Since the kinetics of the lithium-ion battery is closely related to the temperature of the battery, due to the existence of the temperature gradient, we can observe that there is a significant uneven current distribution inside the S30 battery, compared to the temperature of the C70 battery. The distribution should be even more uniform. At the same time, due to the uneven current distribution, the local SoC of the S30 battery is uneven. The current near the cooling surface is smaller, so the final SoC is obviously higher than other positions.
From the above analysis, it can be seen that the polar heat dissipation can effectively improve the internal temperature, current and SoC uniformity of the battery during the heat dissipation process, but it is limited by the area where the heat dissipation of the ear is small, so the absolute heat dissipation is still higher than the surface heat dissipation. Small, so the author further studied and analyzed the effect of the shape of the ear on the heat dissipation of the ear.
Based on the C70 battery with better heat dissipation, the author optimized the structure of the tab. Figure b below shows the effect of the width of the tab on the average temperature, maximum temperature and minimum temperature of the battery during discharge. It can be seen that the width of the battery ear is increased from 10mm to 90mm. At the end of discharge, the average temperature of the battery drops from 44.5°C to 39.5°C. The maximum temperature difference of the battery in the length direction increases from 1°C to 2.4°C, due to the weight of the ear. Increasingly, the weight energy density of the battery was reduced from 210 Wh/kg to 207 Wh/kg.
It can be seen from the following figure c that when the thickness of the tab is increased from 0.2 mm to 1 mm, the average temperature of the battery after the end of discharge is reduced from 40.4 ° C to 34.4 ° C, and the temperature gradient inside the battery is also increased from 2 ° C. At 3.7 ° C, the weight energy density of the battery dropped from 210 Wh / kg to 197 Wh / kg due to the increase in the weight of the tab.
Figure d below shows the effect of the thickness variation of the collector on the heat dissipation effect of the battery. It can be seen from the figure that increasing the thickness of the current collector has less influence on improving the heat dissipation effect of the battery, and the thickness of the current collector is increased by 90%, and the average of the battery at the end of discharge. The temperature only drops by 0.25 ° C, the temperature gradient in the length direction drops from 2 ° C to 1.5 ° C, but the weight energy density of the battery drops from 210 Wh / kg to 177 Wh / kg due to the increase in the weight of the collector, it can be said by increasing the thickness of the current collector The method of improving the heat dissipation of the battery is inefficient.
Yan Zhao's work shows that the bottleneck of the ear cooling is mainly in the cross-sectional area of the ear. Whether it is to increase the width and thickness of the ear, it can effectively improve the heat dissipation effect of the battery, especially after the thickness is increased to 1mm, the heat of the ear is cooled. The effect can be close to the surface heat dissipation, and the ear cooling can effectively reduce the temperature gradient inside the battery, thereby improving the internal current of the battery and the uniformity of the SoC, thereby achieving the purpose of improving the cycle life of the battery.