Lithium-ion battery cells are accompanied by continuous reversible capacity degradation during the cycle, which eventually leads to the failure of lithium-ion batteries, which leads to the reversible capacity decline of lithium-ion batteries. Generally, we believe that the continuous growth of SEI membranes leads to lithium ions. The main factors of battery decay, in addition to the reduction of the reversible capacity caused by the structural decay of the positive electrode material, and the lithium deposition of the negative electrode are all important reasons for the decline of the capacity of the lithium ion battery, but specific to the specific system and specific use methods need to be targeted Sexual analysis.
Recently, YangGao (first author) and Jiuchun Jiang (corresponding author) of Beijing Jiaotong University have 0–20%, 20%–40%, 40%–60%, 60%–80%, 80 for NCM/graphite batteries. In the range of %–100% and 0–100% SoC, the decay mechanism of the cycle at 6C rate was analyzed. It was found that the cycle between 0-20% caused the lithium ion battery to generate more internal resistance and less. The capacity loss, while in the 80-100% cycle will lead to more capacity loss of the battery. The mechanism study of the decay shows that the ratio of positive active material loss and active Li loss at 100% DOD is comparable, but at 20 Under %DOD, the main cause of the decline of lithium-ion battery is the loss of active Li.
The battery parameters used in the test are shown in the following table. The battery capacity is 8Ah, the positive NCM material, and the negative electrode are graphite materials. The test is carried out using Arbin test equipment. The battery is placed in an incubator at 25 degrees Celsius during the whole cycle to reduce the temperature. The impact on battery decay.
The data of lithium-ion batteries circulating in different SoC ranges are shown in the figure below. In order to compare the cycle performance of batteries with 20% DOD and 100% DOD discharge depth, the authors used 5 times of 20% DOD cycle as an equivalent cycle (integral discharge). The capacity is equivalent to 100%DOD). It can be seen from the following figure that the decay rate in the cycle is fast and slow, respectively, '80%, 100%'>'20%, 40%'>'40%, 60%' ≈ '60%, 80%' > '0, 20%', the three SoC ranges in the middle are very close in the decay rate. The battery capacity decay rate of 100% DOD and 80% SoC-100% SoC cycle Be significantly faster than other 20% DOD cycle batteries.
Yang Gao uses the pulse current method to measure the change trend of the internal resistance of the battery in the cycle. Because the response speed of different impedances inside the lithium ion battery is different, the ohmic impedance is generally the fastest, so the author thinks that the impedance measured on the order of 10mS is the fastest. Mainly for ohmic impedance, and the battery polarization resistance is slightly slower, so the impedance after 10mS includes ohmic impedance and battery polarization, so the ohmic impedance and polarization resistance inside the lithium-ion battery can be separated according to these characteristics.
From the test results in the figure below, the ohmic impedance change of the battery in the cycle is small, and the ohmic impedance of the battery of 100% DOD and 0-20% SoC cycle is higher than that of other ranges, but in comparison, the battery pole The increase in the impedance is more significant. From the figure below, it can be seen that the maximum increase in polarization resistance is 100% DOD, and the polarization impedance of the 0-20% SoC cycle in a 20% DOD cycle battery. Increase the maximum.
After completing the cycle test, Yang Gao conducted a capacity test at a small magnification of 0.05 C to eliminate the influence of polarization factors, obtain the maximum reversible capacity Cmax, and then discharge at different magnifications, using the maximum reversible capacity. Subtracting the capacity at different magnifications results in a loss of capacity due to reduced kinetic properties. From the test data in the figure below, the maximum reversible capacity loss of the battery in the 80% SoC-100% SoC cycle is the highest, 0%-20 The %SoC cycled battery has the lowest maximum reversible capacity loss, but it can be seen from the following figure b that the 0%-20% SoC cycle of the battery has the largest capacity loss due to the deterioration of the dynamics. This indicates that the cycle is high in the SoC range. Lithium-ion battery can cause large loss of reversible capacity and circulate in the low SoC range, which will cause the deterioration of the dynamic characteristics of lithium-ion battery.
In order to analyze the capacity decay mechanism that causes the lithium-ion battery to circulate in different SoC ranges, YangGao used the incremental capacity method ICA and the voltage difference method DVA to analyze the lithium-ion battery. First, the author measured the three-electrode battery separately. The positive voltage of the whole battery, the voltage change of the negative electrode during charging and the dV/dQ and dQ/dV curves of the positive electrode, the negative electrode and the whole battery respectively (as shown in the figure below, interested friends can view our previous article "Anli One" A powerful attenuation mechanism analysis tool - dV/dQ curve), from the following figure b, we can see that there are two main peaks in the whole battery, dividing the whole battery into three main reaction zones, and this The main peaks of both are from the negative electrode. The author divides the dV/dQ curve into several parts in the following figure b according to the position of the characteristic peak.
Figures a and b below show the variation of the dV/dQ curve of the battery under different active Li losses. It can be seen from the figure that the positive voltage curve of the lithium ion battery does not change significantly when the active Li is lost, but the curve of the negative electrode It will shift to the right. From the following figure b, it can be seen that the two characteristic peaks generated by the negative electrode will shift to the right as the Li loss increases, and the shape changes. The following figures c and d reflect the positive electrode. The effect of active material loss on the voltage curve, it can be seen from the figure that the loss of positive active material has no effect on the full battery voltage curve and the negative electrode curve, and the characteristic peak in the dV/dQ curve also has no significant change, mainly because The active Li in a lithium ion battery is actually insufficient, so that a part of the loss of the positive electrode active material does not have a large influence on the capacity of the lithium ion battery. Similarly, in view of a significant excess of the negative electrode, a certain amount of negative active material is lost in the cycle. It does not produce a significant change in the capacity of the full battery, but it will cause the characteristic peaks in the dV/dQ curve to shift and the area to decrease.
According to the above data, YangGao believes that the capacity of PEarea I and NEpeak III, and the height of NEpeak II represent the loss of active Li in the lithium ion battery. The capacity of PEarea II mainly represents the loss of active material of the positive electrode, and the main reaction of the height and capacity of NEpeak I The loss of the negative active material.
The figure below shows the change of the characteristic peak of the lithium ion battery during the cycle. The following figure a shows the change of PEareaII in the cycle, which mainly shows the loss of the active material of the positive electrode. The figure b below mainly reflects the loss of the active Li. The positive electrode active material loss of the battery with 100% DOD cycle is the largest, while the loss of active Li is the largest for the 20% DOD cycle. At the same time, the loss of active Li in the cycle is accelerating, while the loss of positive active material is decelerating. This indicates that the loss of active Li is the main factor leading to the decline of 20% DOD battery capacity. The following figures e and f show the change in the height and capacity of NEpeak I, respectively, reflecting the loss of the active material of the negative electrode, which can be seen in the figure. More negative electrode active material loss occurs in the 0-20% cycle of the battery in the cycle, but the negative electrode active material loss is still much smaller than the active Li loss and the positive active material loss, which indicates that it is in the NCM/graphite battery. The loss of negative active material 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 reversible capacity decline, while for the 100% DOD cycle battery, the loss of positive active material and the loss of active Li are both causing its reversible capacity. An important factor in decline.
Yang Gao's work shows that different usage systems lead to different decay mechanisms. The battery capacity of cycling at 20% DOD is slower, but the internal resistance increases faster, but both capacity decline and internal resistance increase. A battery slower than 100% DOD. For a 20% DOD cycle battery, the loss of active Li is the main cause of reversible capacity loss, while the battery positive active material loss and active Li loss of 100% DOD cycle are battery reversible capacity. The main factor of decline.
