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What does the discharge capacity of lithium batteries have to do with?

by:dcfpower     2021-03-20

The influence of tabs on the performance of high-rate batteries. Under high-rate discharge conditions, the discharge voltage curve of high-rate batteries will have a voltage peak, and the discharge capacity of the battery will also increase. A comparatively detailed study of the thermal behavior of lithium-ion batteries under high-rate discharge conditions by infrared thermal imaging shows that the electrode reactions in various regions during the discharge process of potassium-ion batteries are very unbalanced. Under the condition of high-rate discharge, the resistance in the area near the battery tabs is relatively small at the beginning, and the current density is relatively large. Therefore, the heat generated in this part is relatively large and the temperature rises quickly. In the later stage of the discharge process, the capacity near the tab area is exhausted. If the temperature of the part far away from the tab area rises slowly, the discharge process will be terminated, otherwise the voltage will rise. When designing a high-current discharge lithium battery pack, you can weld a few more ears on the pole piece of the electric floor, so that under the condition of high-rate discharge, there will be multiple areas inside the battery at the early stage of high-rate discharge. The internal resistance is relatively small and the current density is relatively high. Larger, faster response speed, thereby alleviating the violent response in the case of monopolar ears. In fact, when there are multiple tabs welded on the electrode, it is equivalent to connecting several small batteries in parallel to form a large battery, and the battery voltage U remains unchanged.

Uu003dUu003dUu003dU'· …

The large battery resistance R is composed of several small battery resistances R1, R3, R... etc. in parallel, which is approximately expressed as:

Ru003d1/(1/R, +1/R2+l/R·…+l/R)

R must be less than any one of R1, R2, R3, Rn..., if the battery has only one tab, then

Ru003dR, +R+R+…+R.

The result is that the total internal resistance of high-rate batteries is greatly reduced after paralleling. In the case of high-current discharge, the temperature of the high-rate battery rises quickly. The battery with multiple tabs spot welded has lower internal resistance, the temperature rise is moderated, and the surface temperature of the battery decreases.

When the high-rate battery is discharged at 5C, the discharge capacity of the single-pole battery is about 1060mAh, and the discharge capacity of the bipolar battery is about 1030mAh, which is slightly lower than that of the single-pole battery. However, the initial discharge voltage of the bipolar battery is higher than that of the single-ear battery, and the discharge platform is not as flat as the single-ear battery. Taking a certain type of soft-packaged lithium battery pack as the research object, increasing the number of tabs on the pole piece will inevitably increase the thickness of the battery (because the spot welding of the tabs on the electrode piece will occupy a part of the thickness of the battery), so for a fixed size . The capacity of this bipolar battery will be slightly reduced. Therefore, the discharge capacity of the bipolar battery is lower than that of the unipolar battery. The rapid drop in voltage platform is not caused by the poor electrochemical performance of the bipolar battery, but due to the increase of the battery’s tabs, which will inevitably increase the thickness of the battery, reduce the length of the pole piece, and thus reduce the active material. Caused by usage.

When the high-rate battery model becomes larger and the size becomes larger, the thickness increase caused by the number of tabs will be relatively reduced. At this time, the quality of the active material used in the single and double tab batteries is equivalent, and the battery capacity There will be little difference, and the high-rate battery discharge curve will be smoother. The 381530 and 063465 batteries adopt the capacity changes of single and bipolar ear technology respectively, and the capacity changes have a greater impact on small batteries. Comprehensive analysis believes that increasing the number of battery tabs can indeed improve the high-rate discharge performance of the battery, and it is especially suitable for large-size and large-size flexible packaging batteries. From the 8C discharge curve of the battery, we can clearly see. In terms of high-rate discharge performance, bipolar batteries do have incomparable advantages over monopolar batteries. It can be seen from the discharge curve of the battery that the initial discharge voltage of the bipolar ear battery is higher and the discharge curve is smoother, but the discharge capacity is lower. In addition, it can be found from the discharge curve of a single-ear battery that the voltage of a single-ear battery does decrease relatively quickly when discharging, and then a voltage peak appears when it rises, but there is no voltage peak on the discharge curve of a bipolar battery. Therefore, this article believes that increasing the number of battery tabs is a research direction to improve the high-rate discharge performance of lithium-ion batteries. We can also weld 3 tabs on the positive and negative electrode sheets, but this will inevitably increase the difficulty of heat sealing of the aluminum-plastic film, which is easy to overheat and cause problems such as battery short circuit leakage. Of course, if the heating conditions are properly controlled, it is possible to achieve beautiful sealing and good heat sealing (we have successfully done experiments in this area, the yield is low, and the heat sealing conditions are demanding), but the number and width of the lugs used are specific The size of the battery is related, and the specific type of battery should also be tested to prove how many and what size tabs should be used.

Increasing the number of tabs for high-rate batteries is indeed effective in reducing the internal resistance of the battery (obviously for large batteries, such as 063465), but increasing the number of tabs results in a battery that can be used The active substance is reduced.

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