High Performance Lithium-ion Battery Design for Fast Charging

I have a dream: "One day I can design a lithium-ion battery that has both fast charge, high specific energy, and long life characteristics!" These features are difficult to achieve at the same time. Our lithium-ion battery designers are aware that fast charging can seriously affect the life of lithium-ion batteries, often because Li+ is rapidly embedded in the negative graphite grid, which can cause severe mechanical stress in the graphite material, resulting in graphite anode The problem of delamination and particle breakage of the material, in addition to too fast charging speed or low battery temperature during charging, may cause metal Li to precipitate on the surface of the negative electrode, which will lead to loss of reversible capacity of the lithium-ion battery, and decay of cycle life. The power cell is higher than the energy, so reducing the charging time of the power cell is a more challenging task. In order to solve this problem, Franz B. Spingler of the University of Technology in Munich, Germany, analyzed the relationship between negative and irreversible lithium, irreversible cell volume expansion, and battery capacity loss, and designed a fast charging system for high-capacity batteries based on this., This system can reduce the charging time by 11 % and the capacity decline by 16 %(200 cycles) compared to the 1C double constant current-constant pressure charging.

The NCM/graphite flexible pack battery was used in the experiment with a capacity of 3.3 Ah. The basic characteristics of the battery are shown in the table below. The battery is placed in a thermostat. Throughout the process of charging and discharging, the laser thickener will continuously measure its thickness along the length of the battery and use an infrared temperature sensor to track the temperature changes on the surface of the lithium ion battery(as shown in the figure below).

Franz B. Spingler first analyzed the effect of temperature on the expansion characteristics of lithium-ion batteries. When the battery temperature returns from 0 °C to 45 °C, the average expansion rate of the entire battery is 1.2 um / °C. From figure B below, we can also note that the expansion of the entire battery is not uniform, the edge of the battery is larger, and the local expansion speed of the battery is in the range of 0.6 um / °C to 3.4 um / °C. The coefficient of expansion is equivalent to 1.2 X 10-4 / °C to 7.0 X 10-4 / °C, with an average of 2.5 x 10-4 / °C. The main reason for measuring the temperature of the lithium-ion battery expansion is that the temperature of the lithium-ion battery will increase during the charging process. This will also cause the expansion of the lithium-ion battery and the need to separate the temperature expansion from the total expansion of the lithium-ion battery.

The following figure shows the volumetric expansion during the charging of CC-CV using 0.5 C, 1.0 C, 1.5 C, and 2C, respectively, where the line segment curve is the directly measured battery expansion curve. The solid line is the expansion curve of the battery after deducting the expansion factor caused by the temperature. We can note that when the battery is charged with a large current(1.5 C and 2.0 C), the battery expansion begins to show an expansion peak(overshoot) in the early stages of the battery charging from constant current charging to constant pressure charging, and then falls down and disappears before the constant pressure charging ends. First, we look at the 2.0 C charge, which has a volume-expanding peak of about 40um, accounting for 25 % of the total volume expansion from 0-100 % SoC batteries. The size of this volumetric expansion peak is closely related to the battery's recharging ratio, which is 25um at 1.5 C, while 0.5 C and 1C have no such peak expansion. Franz B. Spingler believes that the main reason for this peak expansion may be that the metal Li precipitated on the negative surface during rapid charging and re-embedded into the graphite negative electrode at the end of constant pressure charging.

If the peak of the battery expansion is due to negative surface lithium, then a platform will be generated on the voltage curve during the re-embedding of the metal Li into the negative electrode, so Franz B. Spingler verifies whether the above assumption is correct. When the battery is charged at 90 %(the top of the peak volume expansion) at different magnification rates, the battery voltage is interrupted and the changes in the battery voltage are recorded(as shown in the figure below). As we can see from the static voltage curve, The voltage of batteries recharged at 0.5 C and 1.0 C is rapidly declining after the charge is interrupted, while batteries with recharging ratio above 1.5 C have a significant voltage platform during the voltage drop after the charge is interrupted. In particular, the battery voltage platform that is charged at 2.0 C and 2.5 C is very obvious. This shows that with the increase of the charging ratio, the phenomenon of negative surface metal Li has become more obvious. It also shows that the peak of volume expansion that occurs in lithium-ion batteries during large current charging has a close relationship with lithium negative surface analyzer.

The volume expansion generated by a lithium-ion battery during charging is not completely reversible. The figure below shows the capacity loss, average irreversible volume expansion, and maximum irreversible volume expansion of the battery for each cycle at different charge ratios. From the figure, we noticed that the irreversible volume expansion of the battery has a strong correlation with battery capacity loss. The calculation shows that the correlation between the average irreversible volume expansion and the battery capacity loss is 0.945, and the maximum irreversible volume expansion has a correlation with the battery capacity loss. Up to 0.996.

Franz B. Spingler's study found that the irreversible volumetric expansion of the battery at the edge of the battery is often more serious. To explain this phenomenon, Franz B. Spingler will perform an autopsy on the recharged battery at 0.5-2 .0 C magnification. The following figure shows the anatomy. Two negative poles, From the following figure we can see that the position of the battery edge is often more irreversible and volumetric expansion is more serious. At the dissected negative surface of the battery, we find that there is a significant metal Li precipitated at these positions. This shows that the irreversible volume expansion and capacity loss of the battery are closely related to the precipitation of the metal Li on the negative surface.

From the above analysis, it is not difficult to see that the irreversible metal Li precipitation on the negative surface, the irreversible volume expansion of the battery, and the capacity loss of the battery are closely related. Therefore, we should avoid the precipitation of negative and irreversible metal Li when we design the fast charging system of lithium-ion batteries. 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 multiplier of 0.5-3 .0 C, and then 0.5 C constant current-constant pressure discharge to 0 % SoC, the maximum irreversible volumetric expansion of the battery is then recorded to guide the design of the fast charging system. The test results are shown in the figure below. From the figure, we can notice a trend that the greater the charge ratio, the higher the end of SoC, the greater the maximum irreversible volume expansion of the battery, which means that the battery's capacity is lost. The greater.

In order to minimize the maximum irreversible volume expansion, Franz B. Spingler uses a segmented charge, in which 2.4 C charges are used in the 0-10 % SoC range and then reduced one by one(as shown in Figure C below), through this optimized charging system., The charging time of lithium-ion batteries can be reduced by up to 21 %(compared to the 1C magnification CC-CV system), effectively reducing the charging time.

The optimized charging system effectively improves the cycle life of lithium-ion batteries by reducing irreversible volume expansion. The following figure shows the battery cycle curve using the optimized charging system, the 1C ratio CC-CV, and the 1.4 C ratio CC-CV charging system., It can be seen that compared to the ordinary CC-CV curve, the battery cycle performance after the optimized charging system has been significantly improved(200 weeks of cycle, 16 % of capacity loss). From the anatomical results of the battery, After optimizing the charging system, the negative extremely irreversible lithium of the battery has also been significantly reduced.

Franz B. Spingler revealed the relationship between negative polar irreversible lithium analysis caused by lithium-ion batteries charging at different magnification rates and the irreversible volumetric expansion of the battery and the loss of battery capacity. The reason for the rapid charging of lithium-ion batteries 'capacity decline and acceleration, According to the irreversible volumetric expansion of batteries caused by different charging ratios, an optimized charging system was developed, which reduced the charging time by 21 % and the capacity loss by 16 % (200 cycles) compared to the 1C doubling CC-CV charging system.

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