What are the influencing factors of lithium battery capacity diving

Simon F. Schuster (first author and corresponding author) of the Technical University of Munich, Germany, decomposed the influence of battery voltage window interval, charging current and temperature on the nonlinear decay of power lithium-ion battery. The research stated that a wider voltage window and larger The higher charging current and lower temperature will accelerate the growth of the negative electrode SEI film, resulting in poor negative electrode kinetic conditions, thereby accelerating the appearance of lithium precipitation on the negative electrode surface, and leading to the earlier appearance of non-linear battery degradation. It is a curve of a typical lithium battery transition from linear decay to non-linear decay (NMC/graphite system). From the figure, it can be seen that the decay rate of the battery in the non-linear decay stage is more than 7 times that of the linear decay stage. . Generally, we believe that the most important factor in the capacity decline of lithium batteries in the early linear decay stage is the loss of active Li caused by the growth of the SEI film. In the non-linear decay stage, the growth of the SEI film leads to the deterioration of the kinetic conditions of the negative electrode and metal lithium. The precipitation of metal lithium on the surface of the negative electrode further promotes the analysis of the electrolyte and the growth of the SEI film, which intensifies the precipitation of metal lithium, and causes the degradation rate of the lithium battery to be greatly accelerated. The battery used by Simon F. Schuster in the test is the IHR18650A battery from E-oneMoliEnergy, the cathode material is NMC material, the anode material is graphite, and the nominal capacity is 1.95Ah. In the experiment, the influence of voltage window, charge rate, discharge rate and temperature on the non-linear decay of the battery is mainly decomposed. 1. The influence of the operating voltage window The cycle performance curve of the battery in different voltage window ranges. From the figure a below, we can It can be seen that as the battery working voltage window expands, the node at which the battery has a non-linear decay is clearly advanced. For example, compared to a battery with a voltage window of 1.2V (3.0-4.2V), the voltage window is 0.94V (3.17-4.11V) The length of the linear decay section of the battery is increased by about 42%. The author believes that this is mainly due to the increase in the dissolution of transition metal elements in the positive electrode under a wide electrochemical window, and the migration of the dissolved transition metal elements to the surface of the negative electrode leads to the acceleration of the growth of the negative electrode SEI film, which leads to the accelerated degradation of the negative electrode kinetic conditions. Therefore, lithium metal is precipitated earlier in the negative electrode, leading to an earlier appearance of non-linear decay. 2. The influence of charge-discharge rate Since the non-linear degradation of lithium batteries is mainly caused by the precipitation of metallic lithium on the surface of the negative electrode, the charge and discharge current is also closely related to the occurrence of non-linear degradation of lithium batteries. The following figure a is different The cycle performance curve of the battery under the charging and discharging current. From the figure, it can be noticed that the charging current of the battery has the greatest impact on the non-linear degradation of the battery. The battery charged at the 1C rate almost exhibits non-linear degradation from the beginning. However, if we reduce the charging current to 0.5C, the time node of the non-linear decay of the battery will be greatly delayed, and the impact of the discharge current on the non-linear decay of the battery is almost negligible. This is important because as the charging current increases, the polarization of the negative electrode will also increase significantly, which leads to a significant increase in the risk of lithium precipitation in the negative electrode. The precipitated porous structure of metallic lithium will prompt the analysis of the electrolyte. Thereby accelerating the decline of the negative electrode dynamics, leading to the early appearance of nonlinear decay. 3. The influence of temperature The temperature has a very significant influence on the kinetic characteristics of the negative electrode, so the temperature has a distinct influence on the time when the non-linear decline of the battery occurs. The following figure a is a curve of the cycle performance of the battery at 25, 35 and 50℃. From the figure, we can see that within the voltage window range of 3.0-4.2V, the battery cycled at 25℃ has the earliest non-linear degradation. The second is the battery cycled at 50°C, and the battery cycled at 35°C exhibits nonlinear decay at the latest. If we reduce the voltage window of the battery to 3.17-4.11V, the decay rate of the battery cycled at 35°C and 50°C in the early stage is relatively consistent, but the battery cycled at 35°C at the end of life begins to show a non-linear decay. This is mainly due to the deterioration of battery dynamics conditions at low temperatures, which makes the negative electrode easier to release lithium, thereby accelerating the growth of the SEI film, which leads to further deterioration of the negative electrode dynamics conditions, leading to the earlier appearance of the non-linear decay of the lithium battery. From the previous decomposition, it is not difficult to see that the growth of the negative electrode SEI film during the cycle leads to the deterioration of the kinetic performance of the negative electrode. Lithium precipitation on the surface of the negative electrode is an important factor leading to the non-linear degradation of lithium batteries. The surface morphology of the positive and negative electrodes before and after the non-linear decay. From the figure, we can see that the morphology of the positive electrode hardly changes in this process, and the morphology of the negative electrode is before the non-linear decay. A thin SEI film can be observed on the surface of the active material particles, and a thicker SEI film can be observed in some areas. After nonlinear decay, a distinct SEI film has appeared on the surface of the negative electrode particles, and many areas are also There are very thick SEI films. This also verifies our previous guess that the deterioration of the kinetic conditions caused by the continued growth of the negative electrode SEI film causes the lithium precipitation on the negative electrode surface to be an important factor leading to the non-linear degradation of lithium batteries. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete it. Previous: Why didn't the car replace the lead-acid battery with the lithium battery?