The positive and negative electrodes and separators of lithium-ion batteries have a porous structure, and the parameters such as the porosity and the curvature of the electrodes and separators will have an effect on the diffusion of Li+ in them. The mechanical pressure will affect the porosity and curl of porous materials. It has a certain influence, so the mechanical pressure will affect the diffusion rate of Li+ in the positive and negative electrodes and the separator, and then affect the electrochemical performance of lithium-ion batteries. This point we have in the article "Strive for a miracle? Mechanical-electrochemical model reduction" pressure 'The effect on the electrical performance of lithium-ion batteries' is discussed in detail. Although the external high mechanical pressure will have a negative impact on the electrochemical performance of lithium-ion batteries, but in actual production, lithium-ion batteries use hard-shell structure Design, so the mechanical pressure is difficult to avoid in the use of lithium-ion batteries, so we need to more detailed understanding of the impact of pressure on the electrical properties of lithium-ion batteries, and guide the design of lithium-ion batteries.
Recently, bdilbari Shifa Mussa et al. at the Royal Institute of Technology in Sweden conducted a detailed study of the capacity decline and increase in internal resistance of lithium-ion batteries under different pressures. It was found that as the pressure increases, the diffusion resistance of Li+ in the battery increases. Obviously increased, of course, mechanical pressure is not without any benefit, bdilbari Shifa Mussa found that 1.3MPa pressure can help reduce the loss of active Li, slow down the rate of lithium-ion battery capacity decline.
In the experiment, bdilbari Shifa Mussa et al. used a single-layer NCM111/graphite battery as the research object. The information of the positive and negative electrode materials is shown in the following table. The separator of the experimental cell was Celgard's 2320 separator, with a thickness of 20 μm and a porosity of 39%. After fixing, fix it with the device shown in the figure below, and apply the corresponding pressure (0.66, 0.99, 1.32 and 1.98 MPa).
The following figure shows the impedance EIS data of the new electrode under different pressures. From the figure below, we can first notice that the intercept of the curve and real part (X-axis) is affected only by the highest pressure. The increase in the impedance of the surface is mainly due to the diaphragm. The mid-frequency semicircular and low-frequency diffusion curves increase with increasing pressure, indicating that the interface dynamics and electrochemical diffusion of the electrode are at higher pressures. After being restrained, the increase order of the internal resistance of the battery under different pressures was 1.32 MPa.<0.99MPa<0.66MPa<1.98MPa, 说明在1.32MPa是最佳压力, 能够有效的减少锂离子电池容量衰降. 对正负极在相同压力条件下的研究表明, 在高压下正负极的界面阻抗都会随着增加, 共同影响锂离子电池的界面动力学条件. 在扩散阻抗方面只有负极的扩散阻抗会随着压力的增大而增加, 因此锂离子电池在高压力下的扩散阻抗增加主要来自负极.
The EIS analysis of the positive (Fig. a) and negative (b) below the cycle shows that the increase in the internal resistance of the lithium-ion battery due to the discovery cycle is mainly due to the increase in the ohmic and charge exchange impedances of the positive and negative electrodes, and the negative electrode. The Li+ diffusion resistance increases.
Although pressure can have a significant effect on the battery's impedance, pressure seems to have a negligible effect on the capacity of lithium-ion batteries. With a 50% increase in pressure at 3C rate, the capacity will only decrease by about 1.2%. Cycling tests at different pressures show that , The pressure will have a significant impact on the capacity decline of lithium-ion batteries. Figure a below shows the effect of different pressures on the capacity drop of lithium-ion batteries. You can see the capacity of pressure vs. lithium-ion batteries after 600 cycles of 3C discharge. The decline has a significant effect (capacity decline from small to large is 1.32MPa<0.99MPa<1.98MPa<0.66MPa) . 为了将电池内阻的变化对电池放电容量的影响降到最小, bdilbariShifa Mussa降上述电池在C/25倍率下进行了测试 (如下图b所示) , 同样的出了上述结论, 这表明1.3MPa是最为合适的压力, 压力过高或者过低都会加速锂离子电池的容量衰降.
In order to analyze the cause of the lithium-ion battery capacity decline, bdilbariShifa Mussa disassembled the recycled lithium-ion battery and tested the positive and negative electrodes. The following figure shows the voltage difference curve of the negative electrode. It can be seen that the peak of the new electrode at around 60 mAh/g shifts to 20-40 mAh/g, indicating the presence of active Li loss, while the cell drift of 1.32 MPa cycle is minimal, indicating that the loss of active Li is minimal. Under different pressures The distance between the 20-40 mAh/g peak and the 90-100 mAh/g peak of the negative electrode after the cycle did not change, indicating that different pressure cycles will not have a significant effect on the active material loss of the negative electrode.
The following figure shows the capacity test curves for the positive and negative electrodes. From the figure, it can be seen that almost no active material loss occurs in the NCM111 electrode after cycling, and the negative electrode is circulating around the negative electrode for about 4% of the activity after cycling. Material loss, which is consistent with the previous analysis. In summary, 1.32MPa pressure to reduce the lithium ion battery capacity decline is the main mechanism of action to reduce the loss of active Li.
The following figure shows an SEM image of the separator after cycling under different pressures (a1.98 MPa, b 1.32 MPa, c 0.99 MPa, d 0.66 MPa, e new diaphragm). From the figure, it can be seen that the circulatory diaphragm has occurred. Different degrees of local closed-cell phenomenon, which will cause local current density increase, accelerate the decline of the life of lithium-ion batteries. The closed-cell condition of the diaphragm can be judged from the ohmic resistance of the diaphragm (f), and can be seen under 1.32MPa The ohmic resistance of the circulatory diaphragm is minimal, but bdilbari Shifa Mussa believes that the septum obturator is mainly caused by the decomposition product of the electrolyte during the circulatory decay.
From the above experimental results, we found that the external mechanical pressure has a significant impact on the impedance and capacity degradation of Li-ion batteries, and there is uneven pressure between the Li-ion battery cells and the battery cells. Phenomenon, resulting in uneven current distribution and aging speed, which will accelerate the battery life decline rate, in order to study the impact of different pressure on the current distribution between the cells, bdilbariShifa Mussa will be two single cells Connected in parallel, and applied different pressures (0.66MPa and 1.32MPa) to detect the current distribution between the two batteries (as shown in the figure below). From the figure we can see the battery due to the pressure of 0.66MPa. The impedance is small, so the current is significantly higher than the battery under 1.32MPa, which will cause the battery under 0.66MPa to charge to a higher SoC state than the battery under 1.32MPa, and then cause the battery life decline under 0.66MPa to accelerate. This point can be confirmed from the test results in the following figure f, 1.32MPa under the cycle of the battery than the 0.66MPa cycle of battery capacity retention is much higher, but with 1.32MPa battery The capacity decay rate of Lian's 0.66 MPa battery is very close to that of two parallel 0.66 MPa batteries. This shows that uneven current distribution caused by different pressures does not significantly affect the rate of battery decay, and the battery's attenuation decreases. The speed is mainly affected by the pressure on the battery.
In the past, we often focused on the effects of temperature, charge/discharge rate, and charge/discharge depth on the rate of cell decay. However, bdilbari Shifa Mussa's research shows that the mechanical stress on the battery also has an important impact on the decline of lithium-ion batteries. It shows that 1.32MPa is a good pressure for the NCM111/graphite battery. If the pressure is too large or too small, the lithium-ion battery will experience a faster capacity decline during the cycle. This suggests that we need to pay attention to the design of the lithium-ion battery. The effect of the mechanical pressure of the battery case and the battery pack structure on the cell and the cell for the lithium-ion battery cycle life, targeted optimization.
