polymer lithium battery

The impact of fast charging on polymer lithium battery life

With the gradual increase in the popularity of electric vehicles, electric vehicles have increasingly entered the daily lives of ordinary consumers. The service life of power batteries is often one of the focuses of people's concerns, which also leads to the second-hand new energy This is an important reason for the low value retention rate of automobiles, so cycle life is also a very important assessment indicator for power batteries. Generally speaking, the cycle life of lithium-ion batteries and the selection of positive and negative electrode materials and electrolytes have the greatest impact on the cycle life. Secondly, the use strategy of lithium-ion batteries, such as charge and discharge system, operating temperature, etc., will all have an impact on the cycle life of lithium-ion batteries. have a significant impact.

Peter Keil (first author, corresponding author) and Andreas Jossen of the Technical University of Munich in Germany analyzed the impact of charging strategies on different types of power polymer lithium battery cycles.

First, let’s analyze some common charging strategies.

1.Constant current and constant voltage (CCCV)

Constant current and constant voltage charging is the most common and common charging method. At the beginning of charging, the constant current charging mode is used to charge the lithium-ion battery. When the set voltage is reached, the voltage is controlled to remain unchanged and the charging current is continuously reduced until the current is reached. The set value is reached or the time reaches the set value (as shown in figure a below). The charging time of this charging strategy is mainly affected by the size of the constant current charging current Icc, and the charging capacity is mainly affected by the charging cut-off voltage Vch and the constant voltage charging cut-off current Iend. Related research also shows that high charging cut-off voltage Vch and large The charging current Icc will cause significant degradation in the life of the lithium-ion battery.

2. Multi-step constant current charging method (MSCC)

In this charging strategy, constant voltage charging is no longer used, but a multi-step constant current charging strategy with decreasing charging current is adopted. For example, after using I1 constant current to charge to the cut-off voltage, continue to use a smaller current I2 to charge to the cut-off voltage. voltage, and so on until the current decreases to the final cut-off current (as shown in Figure b below).

3. Pulse charging method (PC)

This charging method can be found in some relevant literature reports. In this charging strategy, the charging process may use a series of short-time pulses to adjust the charging current or even the charging direction (discharge). Common pulse charging There are two strategies, one is to replace only the constant voltage charging part of CCCV charging with pulse charging, and the other is to replace the entire process with pulse charging (as shown in Figure d below).

4. Accelerated charging (BC)

The so-called accelerated charging mode adds a high-current CC or constant power charging process to the CCCV charging mode to achieve the purpose of reducing the charging time (as shown in Figure e below). Studies have shown that the accelerated charging mode can be effective. Improve the charging efficiency of lithium-ion batteries without significantly affecting the cycle life of lithium-ion batteries.

In the experiment, the author chose three power-type 18650 lithium-ion batteries from Sanyo, Sony and A123. The rated capacity of the battery is about 1A. The information of the three batteries is shown in the table below. Figure a below shows the three types of A, B and C. The discharge curve of the battery under low current. Four charging systems were used in the experiment, CCCV (constant current and constant voltage), CCPC (constant current pulse), PC (pulse) and BC (accelerated charging).

Since the CCCV charging strategy is the most common charging method, the CCCV charging method is also used as the baseline for reference in this experiment. Table 2 shows the strategies used in CCCV charging, mainly examining different charging currents (1A, 3A and 5A). and the impact of charging cut-off voltage. Table 3 shows the CCPC (constant current pulse) charging strategy, using 3A as the charging current. Table 4 shows the PC (pulse) charging strategy, in which the high current is 5A and the low current is 1A. The high and low currents have the same duration. The impact of different frequencies on the battery is mainly tested. Table 5 shows the BC (accelerated charging) charging strategy, which mainly tests the impact of accelerated charging on battery performance in different SoC ranges.

1.1 Influence of charging current

The figure below shows the time required for battery charging when different charging currents are used in the CCCV charging strategy. It can be seen that most of the capacity of batteries A and C is completed during the constant current charging process, while the constant voltage charging capacity only accounts for a very small amount. part of the battery, while the constant voltage charging time of battery B accounts for a larger proportion.

The biggest impact on the battery charging time is the charging current. For example, if we increase the charging current from 1A to 3A, the charging time can be shortened by 50%. If we continue to increase the charging current to 5A, only the charging time of batteries A and C will be shortened by 1/ 3. Since Battery B has a longer constant voltage charging time, increasing the charging current to 5A does not significantly shorten the charging time compared to 3A.

To illustrate the impact of different charging currents on the battery cycle life when using the CCCV charging strategy, it can be seen that for battery A, a large charging current will cause a significant decrease in battery life. For example, the battery life can reach 1,000 times when charged at 1A, and up to 1,000 times when charged at 3A. When the charging current reaches 5A, the battery life will drop to 800 times, and when the charging current reaches 5A, the battery life will only be about 600 times. However, different charging currents for battery B have no significant impact on the cycle life of the battery. For battery C, the capacity retention rate can still reach 97% after 1,200 cycles when charged at 1A. The capacity retention rate after 1,200 cycles when charged at 3A is 95%. However, the battery life will decline rapidly when charged at 5A. Less than 700 times.

1.2 Influence of charging cut-off voltage

The charging cut-off voltage will affect the electrode potential of the positive and negative electrodes, so it will have a significant impact on the cycle life of lithium-ion batteries. The figure below shows the changes in the capacity of the three batteries A, B and C with the charging cut-off voltage. It can be seen that the charging cut-off voltage of batteries A and B has a significant impact on the battery capacity. Every time the charging cut-off voltage decreases by 100mV, the battery capacity It will decay by 10-20%. Since battery C uses the LFP positive electrode, the battery capacity above 3.4V has almost nothing to do with the battery's charging cut-off voltage.

The following is the cycle performance of the battery under different cut-off voltages. Overall, the cycle performance of the battery is constantly decreasing as the charge cut-off voltage increases. For battery A, the impact of the charge cut-off voltage is relatively small. If the charge is The cut-off voltage is reduced from 4.2V to 4.1V, and the lifespan is only increased by about 100 times. To significantly increase the lifespan of battery A, it needs to be reduced to 4.0V, but this will reduce the battery capacity by 30%, and increase the charging voltage of the battery to 4.25 V will only slightly reduce the cycle life of the battery.

Battery B is much more sensitive to charging voltage. After reducing the charging voltage from 4.1V to 4.0V, the capacity retention rate of the battery increases by 5% after 800 cycles. However, further reducing the charging cut-off voltage to 3.9V will affect the cycle life of the battery. There is no significant impact, but if the charging cut-off voltage is increased to 4.15V, the degradation rate of battery B will be significantly accelerated.

Battery C is the most sensitive to the charge cut-off voltage. If the charge cut-off voltage is increased from 3.6V to 3.65V, the cycle life of the battery will be significantly reduced, but reducing the charge cut-off voltage has no significant impact on the cycle life of the battery.

2.1 Constant current pulse (CCPC)

The constant current pulse charging system is close to the constant current and constant voltage charging system, except that the constant voltage charging stage is replaced by pulse charging. The charging time of CCPC is actually longer than the common constant current and constant voltage charging, so the charging time CCPC has no advantage in terms of time.

For the cycle data of the three batteries using CCPC and PC charging strategies, it can be seen that when the CCPC charging strategy is used for the three batteries, the cycle life of the battery is actually the same as that of the CCCV charging strategy using the same charging current.

2.2 Pulse charging (PC)

Different from the previous CCPC charging strategy, the PC charging strategy uses pulses throughout the charging process. Since the polarization is relatively large during the pulse charging process, pulse charging will reduce the charging capacity of the battery, such as Under the pulse charging strategy, battery A can only charge to 80% of its capacity, while battery C can charge to 95% of its capacity. In terms of charging time, the PC charging strategy is almost the same as the CCCV charging system with the same current. From the cycle data in the figure above, we can see that the cycle life of the battery with the PC charging strategy is almost the same as the CCCV charging strategy with the same current.

The accelerated charging strategy is to use a larger charging current (5A) for charging in a lower SoC range. After reaching the cut-off voltage, it switches to a small current (1A) CCCV charging method for charging. Therefore, the charging time is relatively shortened. After testing the battery The charging time of A using BC charging strategy is 57min, and that of battery C is 48min, which is between the CCCV charging time of 1A and 3A current.

The cycle performance of the BC charging strategy for batteries A and C. It can be seen from the figure that for battery A, if accelerated charging (5A) is used from 0%, the battery's degradation rate will increase significantly, which is the same as the 5A current. CCCV charging strategy is close. If the charging is accelerated from 10% SoC and 20%, the cycle life will be greatly improved, which is close to the CCCV charging strategy of 3A current. For battery C, if the SoC that starts accelerating charging is 0% and 10%, the decline rate of the battery will be significantly accelerated, while if accelerating charging starts at 20% SoC, it will have less impact on the cycle life of the battery. This also shows that accelerated charging at lower SoC will cause the cycle life of the battery to be significantly affected.

Peter Keil's work shows that the biggest impact on battery cycle life is charging current. Charging batteries A and C at a high current of 5A will cause the cycle life of lithium-ion batteries to be severely shortened. However, the cycle life of battery B seems to be affected by the charging current. The impact is not big. The second is the charge cut-off voltage of the battery. A high charge cut-off will seriously shorten the service life of the lithium-ion battery. Especially for batteries B and C, increasing the charge cut-off voltage by 50mV will seriously affect the cycle life of the battery.

From the perspective of charging strategy, the pulse charging strategy has no significant impact on the cycle life of the battery (compared to CCCV), but pulse charging does not reduce the charging time. In particular, the CCPC charging strategy will also extend the charging time of the battery. .

The accelerated charging (BC) strategy can reduce a certain amount of charging time, but the accelerated charging strategy requires careful selection of the SoC range for accelerated charging. Try not to use accelerated charging in an SoC range that is too low to avoid affecting the cycle life of the battery.

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