Analysis on Decline and Decline Mechanism of Life of NCM Battery

Lithium-ion batteries will be accompanied by a continuous reversible capacity decline during the cycle process, which will eventually lead to the failure of lithium-ion batteries, resulting in more factors in the decline of the reversible capacity of lithium-ion batteries. Generally, we believe that the continuous growth of the SEI membrane is the main cause of the decline of lithium-ion batteries. factor, in addition, the reduction of reversible capacity caused by the structural decay of the cathode material and the negative polar lithium chromatography are important reasons for the decline in the capacity of lithium-ion batteries, but specific systems and specific ways of use require targeted analysis.

Recently, Yang Gao(first author) and JiuchunJiang(communication author) of Beijing Jiaotong University and others targeted NCM/graphite batteries at 0 -- 20 %, 20 % -- 40 %, 40 % -- 60 %, 60 % -- 80 %, 80 % -- 100 % and 0 -- 100 % SoC range, The decline and recline mechanism of the 6C cycle was analyzed. The study found that the cycle between 0-20 % will lead to more internal resistance and less capacity loss for lithium-ion batteries, while the 80-100 % cycle will result in more batteries. Capacity loss. The study of the mechanism of decline shows that the ratio of the loss of positive polar active substances and the loss of active Li under 100 % DOD is equivalent, but under 20 % DOD, the main reason for the decline of lithium-ion batteries is the loss of active Li.

The battery parameters used in the test are shown in the following table. The battery capacity is 8Ah, positive pole NCM material, and negative pole graphite material. The test was conducted using Arbin test equipment. During the entire cycle, the battery was placed in a thermostat at 25 degrees Celsius to reduce the impact of temperature on the battery decline.

The data of lithium-ion batteries circulating in different SoC ranges is shown in the figure below. In order to compare the cycling performance of batteries with 20 % DOD and 100 % DOD discharge depth, the author uses 5 20 % DOD cycles as an equivalent cycle(the overall discharge capacity is equivalent to 100 % DOD).), From the figure below, it can be seen that the decay rate in the cycle is fast and slow. 80 %, 100 %] & GT; [ 20 %, 40 %] & GT; [ 40 %, 60 %] <UNK>[ 60 %, 80 %] & GT; [ 0, 20 %] The three SoC ranges in the middle are very close to the decay rate. The 100 % DOD and 80 % SoC-100 % SoC cycle battery capacity decays significantly faster than the other 20 % DOD cycle batteries.

YangGao uses the pulse current method to measure the trend of resistance in the cycle in the battery. Since the response speed of different impedance inside the lithium-ion battery is different, the ohmic impedance is generally the fastest. Therefore, the author believes that the impedance measured on the order of 10mS is mainly ohmic impedance. The battery polarization impedance reacts slightly slower, so the impedance after 10mS contains ohmic impedance and battery polarization impedance, so the ohmic impedance and polarization impedance inside the lithium-ion battery can be separated based on these characteristics.

From the test results in the following figure, the battery has a small change in the cyclic Zhongoumu impedance, and the battery ohmic impedance of 100 % DOD and 0-20 % SoC cycles is higher than that of other ranges of batteries, but in contrast, the battery polarization impedance the increase is more significant. From the figure below, it can be seen that the largest increase in polarization impedance is the 100 % DOD battery, while the polarization impedance of the 0-20 % SoC battery in the 20 % DOD cycle increases the most.

After completing the cycle test, YangGao conducted a capacity test at low magnification of 0.05 C to eliminate the influence of polarization factors, obtain the maximum reversible capacity Cmax, and then discharge at different magnification rates. Using the maximum reversible capacity minus the capacity at different magnification, the volume loss caused by the reduction of the kinetic characteristics is obtained. According to the test data in the figure below, the maximum reversible capacity loss is the highest for batteries in the SoC-100 % SoC cycle and the lowest for batteries in the 0 % -20 % SoC cycle but can be found in figure B below. The 0 % -20 % SoC cycle battery has the largest capacity loss due to poor kinetic properties. This shows that the circulation in the high SoC range will lead to a large loss of reversible capacity of lithium-ion batteries, and the circulation in the low SoC range will lead to a serious decline in the kinetic properties of lithium-ion batteries.

In order to analyze the mechanism of capacity decline that causes lithium-ion batteries to circulate in different SoC ranges, YangGao used the incremental capacity method ICA and the voltage difference method DVA to analyze lithium-ion batteries. First of all, the authors used the three-electrode battery method to measure the voltage changes and dV/dQ and dQ/dV curves of the positive and negative poles in the whole battery during the charging process(as shown in the figure below). Interested friends can look at our previous article "Amway's powerful decline mechanism analysis tool-dV/dQ curve"). From Figure B below, you can see that there are two main peaks in the whole battery. The whole battery is divided into three main reaction regions, and the main peaks of both are from the negative poles. The author divides the dV/dQ curve into several parts in figure B below according to the position of the characteristic peak.

The following figures a and B show the changes in the dV/dQ curve of the battery under the loss of different active Li. From the figure, it can be seen that the positive electrode voltage curve of the lithium-ion battery does not change significantly when the active Li loss occurs, but the negative pole the curve will shift to the right. The two features generated by the negative pole can be seen in figure B below. The summit shifts to the right as Li's loss increases overall and changes in shape. The following figures C and D reflect the effect of the loss of positive polar active substances on the voltage curve. It can be seen from the figure that the loss of positive polar active substances has no effect on the voltage curve and negative polar curve of the whole battery. At the same time, there is no significant change in the characteristic peaks in the dV/dQ curve. This is mainly because the active Li in the lithium-ion battery is actually insufficient, so the loss of a portion of the positive active material will not have a large impact on the capacity of the lithium-ion battery. Similarly, in view of the significant excess of the negative electrode, in the cycle, a certain amount of negative active material loss does not significantly change the capacity of the entire battery, but it will cause the characteristic peaks in the dV/dQ curve to shift and the area. Decrease.

According to the above data, YangGao believes that the capacity of PareaI and NEpeakIII, as well as the height of NEpeakII, represent the loss of active Li in lithium-ion batteries, and the PareaII capacity mainly represents the loss of positive active substances. The height and capacity of NEpekI mainly reflect the loss of negatively active substances.

The following figure shows the change of characteristic peaks in the cycle of lithium-ion batteries. The following figure shows the change of PareaII in the cycle, mainly indicating the loss of positive polar active substances. Figure B in the figure below mainly reflects the loss of active Li. From the figure, it can be seen that the loss of positive polar substances in 100 % of the DOD cycle is the greatest, while the loss of active Li in 20 % of the DOD cycle batteries is the greatest, and the loss of active Li in the cycle is constantly accelerating. The loss of positive polar active substances is constantly decelerating, which indicates that the loss of active Li is the main factor that causes the decline of 20 % of DOD battery capacity. The following figures E and F indicate changes in the height and capacity of NEpekI, respectively, reflecting the loss of negative polar active substances. From the figure, it can be seen that more negative active material losses occur in batteries circulating in 0-20 % of the cycle. However, compared to the loss of active Li and the loss of positive polar active substances, the loss of negative polar active substances is still much smaller, which indicates that the loss of negative active substances in NCM/graphite batteries is not the main factor leading to the decline of reversible capacity.

In general, for a 20 % DOD cycle battery, the loss of active Li is the main factor leading to the decline of reversible capacity, while for a 100 % DOD cycle battery, the loss of active matter and the loss of active Li are the causes of its reversible capacity. The important factor of decline.

The work of YangGao shows that different use systems can lead to different decay mechanisms. The battery capacity of the 20 % DOD cycle has a slower decline, but the internal resistance has increased faster. However, both the capacity decline and the internal resistance increase are slower than 100 % DOD batteries. For a 20 % DOD cycle battery, the loss of active Li is the main cause of reversible capacity loss, while the loss of 100 % DOD cycle battery positive active material and the loss of active Li are the main factors for the decline of reversible battery capacity.

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