I have a dream: 'One day I can design a lithium-ion battery with fast charge, high specific energy and long life characteristics!' and at the current state of the art these features are difficult to achieve at the same time. Lithium ion battery designers are well aware that fast charging can severely affect the life of lithium-ion batteries. This is often the result of Li+ rapidly embedding into the graphite grid of the negative electrode and causing severe mechanical stress in the graphite material, resulting in graphite negative electrode materials. The problem of delamination and particle crushing occurs. In addition, excessively fast charging speed or too low battery temperature during charging may cause metal Li to precipitate on the negative electrode surface, which will lead to the loss of the reversible capacity of the lithium ion battery and the decline of the cycle life.
The specific energy of the power battery is higher, so reducing the charging time of the power battery is a more challenging matter. In order to solve this problem, Franz B. Spingler et al. of German Technical University in Munich analyzed the irreversible volume of the negative electrode irreversible lithium. The relationship between expansion and battery capacity loss is based on the design of a high-energy battery with a fast charging system. Compared to 1C rate constant current-constant voltage charging, this system can reduce the charging time by 11% and 16% decline in capacity (200 cycles).
The NCM/graphite soft-pack battery used in the experiment has a capacity of 3.3 Ah. The basic characteristics of the battery are shown in the following table. The battery is placed in a constant temperature chamber. The laser thickness gauge will follow the length of the battery during the entire charge and discharge process. The direction of the thickness of its continuous measurement, and the use of infrared temperature sensor to track the surface temperature of lithium-ion battery changes (as shown below).
Franz B.Spingler first analyzed the effect of temperature on the expansion characteristics of lithium-ion batteries. When the battery temperature was restored from 0°C to 45°C, the average expansion rate of the entire battery was 1.2um/°C. From the figure b below, we can also notice the entire The expansion of the battery is not uniform, and the edge expansion of the battery is larger. The local expansion speed of the battery is in the range of 0.6 um/°C to 3.4 um/°C, and the expansion coefficient is 1.2 x10-4/°C to 7.0 x 10-4/ °C, the average is 2.5x10-4/°C. The main reason for measuring the temperature-induced lithium-ion battery expansion is because the lithium-ion battery will increase in temperature during charging, which will also cause the expansion of the lithium-ion battery. The temperature expansion is separated from the overall expansion of the lithium ion battery.
The figure below shows the volumetric expansion during the charging process using 0.5C, 1.0C, 1.5C and 2C CC-CV respectively. The curve of the line is the battery expansion curve obtained by direct measurement. The solid line is after deducting the temperature and causing the expansion factor. The battery's expansion curve. We can notice that in the early stage of charging the battery from constant current charging to constant voltage charging at high current (1.5C and 2.0C) charging, the battery expansion begins to show an overshoot of expansion, and then Falling and disappearing before constant voltage charging. First we look at 2.0C charging. This volume expansion overshoot reaches about 40um, which accounts for 25% of the total volume expansion of 0-100% SoC battery. This volume expansion The magnitude of the peak is closely related to the charge rate of the battery. At 1.5C, the height of this peak is 25um, and this peak expansion does not occur at 0.5C and 1C rates. Franz B. Spingler believes that the main reason for this swelling may be It is during the rapid charging process that metal Li precipitates on the surface of the negative electrode and is re-embedded into the graphite negative electrode at the end of constant voltage charging.
If the peak of battery expansion is due to lithium deposition on the negative electrode surface, then a platform will be created on the voltage curve during the re-embedding of metal Li into the negative electrode. Therefore, in order to verify whether the above assumption is correct, Franz B. Spingler uses different batteries. When the CC-CV charge reaches 90% (the top of the volume expansion peak), it is interrupted. Then the change of the battery voltage is recorded (as shown in the figure below). From the static voltage curve, we can see that the charge is 0.5C and 1.0C. The battery quickly drops after the charge is interrupted, and the battery with the charge rate above 1.5C has an obvious voltage platform during the voltage drop after charging is interrupted, especially the battery charged at 2.0C and 2.5C rate. The voltage platform is very obvious. This shows that as the charging rate increases, the phenomenon of Li metal precipitation on the surface of the negative electrode becomes more apparent. It also shows that the volume expansion peak of the Li-ion battery during high-current charging is closely related to lithium deposition on the negative electrode surface. relationship.
The volume expansion of lithium-ion batteries during charging is not all reversible. The following figure shows the capacity loss of each cycle of the battery at different charging rates, the average irreversible volume expansion and the maximum irreversible volume expansion. From the figure we It is noted that the irreversible volumetric expansion of the battery has a strong correlation with the capacity loss of the battery. The calculation shows that the correlation between the average irreversible volumetric expansion and the battery capacity loss is 0.945, and the correlation between the maximum irreversible volumetric expansion and the battery capacity loss is as high as 0.996.
Franz B.Spingler's study found that the irreversible volume expansion of the battery at the edge of the battery tends to be more severe. To explain this phenomenon, Franz B. Spingler dissected the battery after charging at a 0.5-2.0C rate. After dissecting the two negative electrodes, we can see from Figure a below that the position of the battery edge is often more serious than the irreversible volume expansion. At the disassembled negative surface of the battery we find that it is precisely at these positions that there is a clear precipitation of metallic Li. This shows that The irreversible volumetric expansion and capacity loss of the battery are closely related to the precipitation of metallic Li on the negative electrode surface.
From the above analysis, it is easy to see that the irreversible metal Li on the surface of the negative electrode precipitates, and the irreversible volumetric expansion of the battery has a close relationship with the capacity loss of the battery. Therefore, we must avoid causing the negative electrode to be irreversible when designing the lithium-ion battery rapid charging system. Precipitation of metal Li. In order to design a charging system that can quickly charge and avoid rapid decay of battery life, Franz B. Spingler charges the battery to 10-100% SoC using a 0.5-3.0C rate, and then 0.5C constant. Stream - constant voltage discharge to 0% SoC, and then record the maximum irreversible volume expansion of the battery, and to guide the design of the rapid charging system. Test results are shown in the figure below, from the figure we can notice a trend, that is, charge rate The larger the SoC is, the higher the maximum irreversible volumetric expansion of the battery is, which means the greater the capacity loss of the battery.
In order to minimize the maximum irreversible volume expansion, Franz B. Spingler adopts the method of segment charging, which uses 2.4C charging in the range of 0-10% SoC, and then successively decreases (as shown in Figure C below) through this optimization. After the charging system, the charging time of lithium-ion battery can be reduced by up to 21% (compared with 1C rate CC-CV system), which effectively reduces the charging time.
The optimized charging system can effectively improve the cycle life of lithium-ion batteries by reducing the irreversible volume expansion. The following figure shows the battery cycle curves of the 1C-rate CC-CV and 1.4C-rate CC-CV charging systems. It can be seen that compared with the ordinary CC-CV curve, the cycle performance of the battery after optimization of the charging system has been significantly improved (cycle 200 weeks, capacity loss is reduced by 16%), from the perspective of the battery's anatomy, optimized charging After the system, the irreversible lithium in the negative electrode of the battery is also significantly reduced.
Franz B. Spingler investigates the relationship between the irreversible volumetric expansion of the negative electrode and the irreversible volume expansion of the battery and the capacity loss of the battery caused by the charging of the lithium ion battery at different magnifications, and reveals the reason that the rapid charge causes the lithium ion battery capacity to decline in acceleration. , And based on the irreversible volume expansion of the battery caused by different charging rates, an optimized charging system was developed. Compared with the 1C rate CC-CV charging system, the charging time was reduced by 21% and the capacity loss was reduced by 16% (200 cycles).
