High capacity battery cycle expansion force

High capacity battery cycle expansion force

Introduction

The change of expansion force in the cycle process of lithium-ion battery has an important influence on the module and system design. The accelerated attenuation of the capacity in the process of the battery cycle has a certain correlation with the excessive squeeze pressure of the high capacity battery. Therefore, it is important to study the change of expansion force in the cycle process for high capacity battery, which is of great significance to the optimal design of the battery and the system.

Experiment for high capacity battery

With different models of square lithium iron phosphate power high capacity battery as samples (Model1, Model2, Model3), placed in the limit pressure tooling, respectively, 1 C constant current constant voltage charging and 1C discharge cycle at room temperature (25 ± 5) ℃ and high temperature (45 ± 5) ℃, charge and discharge interval of 30 min.

Different models of square lithium iron phosphate power high capacity battery

And the pressure sensor to record the battery expansion force during the charge and discharge cycle, analyze expansion force. And if you want to know more about lithium iron phosphate power battery, please read the Top 10 lithium iron battery manufacturers in China.

Results and discussion for cycle expansion force

Change of the expansion force during the cycle process

The square lithium iron phosphate batteries of high capacity battery of different capacity and sizes were studied, and expansion force in the cycle process was recorded. The expansion force of the 500 cycles of Model1 battery showed nonlinearly along with the charging / discharge.

The change of expansion force during charging, with 30%SOC being the peak of the first expansion force and 100%SOC being the second peak, and it increases overall as the cycle number increases, and shows the same regularity.

Expansion force during charging and discharge in cycle progress

The expansion force changes during the discharge process, the peak of the expansion force is 0% discharge depth (DOD) and 70%DOD, namely the corresponding 100%SOC and 30%SOC. Similarly, with the number of cycles, the expansion force shows an obvious increasing law.

Analysis of the change law of the expansion force

In high capacity battery, when expansion force in the cycle is summarized and analyzed, the change law in different SOC is slightly different: 30%SOC is the first peak of the expansion force. With the increase of cycles, the increase is greater than 100%SOC, the expansion force of the 0%SOC as F0, 30%SOC as F30, and so on, 100%SOC expansion force as F100, to observe the change law of F30 / F100 during the cycle.

We find that expansion force of 30%SOC will progressively approach expansion force of 100%SOC. As shown in the data, from 600 cycles, F30 starts to be greater than F100, and the ratio of F30 to F100 gradually increases to about 103%, and it remains basically constant at this value.

Effect of the temperature on the change of the expansion force

The change pattern of F30 / F100 was observed at different temperatures. For Model1 cells, the peak ratio change pattern of the expansion force at 45℃ and 25℃ was different. When the initial pretension force (0%SOC) is about 0 kN, the peak ratio of expansion force in normal temperature cycle increases slowly. During 25℃ cycle, after 800 cycles, F30 / F100 from 100 cycles, F30 / F100 reaches 90% to 100%.

Mutual correspondence of expansion force and capacity

Normal temperature cycle and 45℃ high temperature cycle are the two commonly used cycle systems to evaluate the service life of batteries. Model1 batteries have the change rules of capacity attenuation and peak expansion force ratio under the condition of 25 and 45℃:

Effect of the temperature on the change of the expansion force

● During the normal temperature cycle, the ratio growth of cyclic expansion force is relatively slow. With the increase of F30 / F100 to more than 100%, the capacity decay curve of high capacity battery changes from linear to slightly curved curve. From the data, the decay rate of the battery capacity before 1 000 cycles is about 0.35% per 100 cycles, and about 7% per 1 000 cycles.

● The 45℃ cycle curve also presents a similar rule, regardless of the first 100 rapid attenuation, the first 600 capacity attenuation rule is 0.85% per 100 capacity attenuation. When F30 / F100 increases above 100%, the 600 to 1 500 decay rule is 0.95% per 100 capacity attenuation.

It can be seen that the change of high capacity battery cycle expansion force is related to the attenuation of battery capacity. The results show that there is a close correlation between the continuous growing pressure of lithium-ion batteries and the reversible capacity loss, and the capacity decay rate of lithium-ion batteries can be predicted by measuring the internal pressure change of lithium-ion batteries, for example, you can measure the internal pressure changes of lithium-ion golf cart batteries to predict the capacity decay rate of lithium-ion batteries.

Analysis of the expansion force of different batteries

The test results found that the changing trend of the cyclic expansion force of different models of high capacity battery is very close. Compared with the change curve of F30 / F100 in different models of batteries in the 45℃ cycle, this change rule is also obvious.

Expansion force of different batteriesThe ratio of F30 / F100 of different models of batteries reaches 100% for about 600 times, and about 103% for about 800 times and remains constant. From these data, we can see that the cycle expansion force of different batteries has the same change law, and the value of F30 / F100 is closely related to the cycle attenuation.

High capacity battery disassembly analysis

The battery in the initial state and the circulating battery are disassembled, and the battery thickness of the battery and the pole sheet thickness of the battery in different charge states are measured. The pole sheet thickness is greatly increased compared with the initial state. The pole sheet rebound rate is defined as the percentage of the increment of the pole sheet thickness, that is:
Rebound rate = (cyclic posterior pole thickness subtract initial electrode thickness) / initial electrode thickness 100%

It can be seen from the data that the change of anode electrode sheet thickness is the same as the trend of stress change in the battery charge and discharge process, and the inferred change of force is related to the anode electrode.

Disassemble the cycle battery, record the initial thickness of 100% and 101.55%; add the battery housing thickness to 101.52%; infer the increase of battery thickness and the increase of the battery expansion during the cycle process mainly comes from the increase of the battery thickness.

Disassembly of high capacity battery

Conclusion

In the process of lithium-ion batteries cycle of high capacity battery, expansion force and capacity attenuation have a certain relationship:

● The expansion force in the charging and discharge process of high capacity battery of lithium iron phosphate shows a nonlinear change trend, similar to the sine wave distribution, with two peaks, the first peak is about 30%SOC, and the second peak is 100%SOC.

● As the cycle proceeds, the battery expansion force will gradually increase. The growth rate of the 30%SOC and the 100%SOC expansion force is different, with the initial F30 less than the F100; as the cycle proceeds, the F30 will gradually be greater than the F100.

● The test results show that the capacity decay accelerates after F30 more than F100.
Based on this rule, the cycle life of the battery can be predicted by the pressure change during the cycle.

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