lithium 3400mah 3.7v 18650 battery

　　18650 battery cycle life decline reasons

　　The 18650 battery was first introduced by Sony Corporation of Japan. After decades of development, it has become the most mature and stable general-purpose model. It has reached a very high level in terms of consistency and safety. And after years of development, the industrial chain of 18650 batteries is very mature and has become a

　　The 18650 battery was first introduced by Sony Corporation of Japan. After decades of development, it has become the most mature and stable general-purpose lithium-ion battery model. It has reached a very high level in terms of consistency and safety. And after years of development, the industrial chain of 18650 battery is very mature and the cost is low. This series of advantages made Tesla finally choose Panasonic's 18650 battery as the power lithium battery of its electric vehicles (it has been gradually replaced by 21700 Battery).

　　During the cycle of lithium-ion batteries, the reversible capacity will continue to decline due to electrochemical, mechanical and other factors, which will affect the service life of lithium-ion batteries. Due to the characteristics of the sealed structure of lithium-ion batteries, our research on the reasons for the decline of lithium-ion batteries usually uses destructive means to dissect the battery. However, the emergence of some advanced detection methods in recent years, such as neutron diffraction, CT, etc., allows us to study the reasons for the decline of lithium-ion batteries without destroying the structure of lithium-ion batteries. Recently, A.Pfrang (first author) and E.Figgemeier (corresponding author) of the European Commission's Joint Research Center used CT to study the decay mechanism of lithium-ion batteries during cycling. CT analysis shows that even when there is a central needle in the center of the battery, the battery cell still undergoes significant deformation after long-term cycling, and the deformation will cause the positive electrode active material to peel off from the surface of the Al current collector, causing an accelerated decline in capacity. The inhomogeneity of the cell structure caused by the positive tab is an important factor causing the deformation of the cell, which should be optimized in the subsequent design.

　　The battery used in the experiment is the 18650 battery produced by Sanyo Company. The capacity of the battery is 2.05Ah, and the energy density is 165Wh/kg. The structure is scanned (CT equipment is from GE company, the maximum resolution is 5um).

　　First, A.Pfrang dissected the internal structure of Sanyo’s battery. The structure of Sanyo’s battery is shown in the figure below. The width of the positive electrode is 56mm, the width of the negative electrode is 58mm, and the width of the diaphragm is 59mm. The ear is left at the tail, and both the positive and negative electrodes have a part of the light foil at the tail, which is mainly to prevent the direct contact between the positive active material and the negative active material during the acupuncture test, thereby improving the safety of acupuncture. A hollow center needle is placed in the center of the battery cell. Different from the center needle of other manufacturers, the center needle of Sanyo battery has a slit, which may be because the designer hopes that the gas that appears in the battery during thermal runaway has enough diffusion channels, or It is hoped that the electrolyte can infiltrate through the middle of the cell to improve the infiltration effect of the electrolyte.

　　Although the center pin is added to the middle part of the battery cell of Sanyo battery, theoretically it can provide a certain support for the battery cell and prevent the battery cell from collapsing, but after cycling, the battery cell part of the 18650 battery still undergoes significant deformation (as shown in the figure below ). Through the analysis of the cycled battery, it was found that most of the cell deformation of the battery occurred between the positive tab and the center pin, which indicated that the presence of the positive tab may have caused the inhomogeneity of the cell structure , leading to the appearance of cell deformation during cycling.

　　In order to analyze the factors of battery internal deformation and battery cycle life decline, A.Pfrang analyzed four batteries of No. 93, 98, 133 and 138, and all four batteries were cycled between 70-90% SoC (1C , 33 degrees Celsius), in which the capacity loss rate of No. 133 battery is slightly higher than that of No. 98 and No. 135 batteries during the cycle, and a sudden capacity dive occurred at the end of the service life, while the capacity of No. 93 battery declined during the entire cycle. The drop rate is very fast. By comparing the CT images of several batteries, it can be seen that the cells of the 93 and 133 batteries have undergone very obvious deformation after cycling, while the cells of the 98 and 135 batteries with better cycle performance No significant deformation occurred.

　　The figure below compares the cell deformation of three batteries with relatively fast decay speeds. Among them, the depth of discharge of batteries No. 1 and No. 59 is 100% DOD, the median value of SoC is 50%, and the depth of discharge of No. 133 batteries is 20% DOD , SoC median value is 80%. We have found very significant deformation in all batteries with severe reversible capacity decline, and we can see from the figure that almost all the deformation occurs in the range from the positive tab to the center of the battery, which is mainly because the positive The presence of tabs causes unevenness in the internal structure of the cell, so deformation is more likely to occur when stress occurs inside the cell.

　　The deformation of the battery cell during the cycle will cause a very large curvature on the positive electrode, resulting in the peeling of the positive electrode active material and the Al foil. Through the dissection of the battery after cycling, it is found that the positive electrode active material has peeled off at these very large curvature positions. The phenomenon (as shown on the right of the figure below), which is also an important factor causing the reversible capacity loss of lithium-ion batteries during cycling.

　　A. Pfrang found through CT scanning that even if there is a center needle in the center of the battery (generally we believe that the existence of the center needle can effectively prevent the deformation of the battery), after a long-term cycle we can still find obvious in the battery. The deformation, especially in the battery with faster cycle decay, this deformation is more obvious. Comparing the positions where these deformations occur, it can be found that most of the deformations occur between the positive tab and the middle of the cell. This may be because the existence of the positive tab leads to the inhomogeneity of the cell structure, which introduces additional stress. Causes deformation to occur more easily here. Through anatomy, it was found that at the position with large deformation, the positive electrode active material and the Al foil current collector have peeled off, causing the loss of active material, which is also an important factor causing the reversible capacity loss of lithium-ion batteries. The application of CT technology allows us to have a more accurate understanding of the reasons for the capacity loss of lithium-ion batteries during cycling, which is of great significance to continuously improve battery performance.