What is the reason for the capacity decline of 18650 lithium battery?

With the increase in the number of charge and discharge cycles of the 18650 lithium battery, the capacity obtained in the constant current charging process shows a downward trend with the increase in the number of cycles. On the contrary, the supplementary charging capacity obtained in the constant voltage charging process is keep improving. This is because the internal resistance of the battery continues to rise during the cycle, and the polarization of the battery continues to increase, resulting in a decrease in the charging capacity of the battery in the constant current section and an increase in the charging capacity in the constant voltage section. The internal resistance of the 18650 lithium battery decreases with the increase of its open circuit voltage, and increases significantly with the increase of the number of cycles. In the state of full charge (100% SOC), the internal resistance of the battery rises from formation to 200 cycles, which is one of the reasons for the decline of battery capacity. With the increase of the number of cycles of 18650 lithium batteries, the charge transfer resistance (R) of the charge transfer process at the interface between the electrode and the electrolyte increases significantly. This may be due to the high-impedance passivation film deposited on the positive and negative active materials, and lithium It is caused by the decrease of the effective position of ion extraction/intercalation, and the increase of charge transfer impedance will lead to the decline of battery dynamic performance, which leads to the capacity decline of the battery during long-term cycling. The capacity decay rate of the positive and negative electrodes is not much different, and with the increase of the number of cycles, the contribution of the capacity loss of the positive and negative electrodes to the total battery capacity loss decreases, while the straight loss of active lithium ions and the lithium ion between the positive and negative electrodes The decrease in inter-migration capacity contributes to the increase in the attenuation of the full battery capacity. The lattice constant and the diffraction peak intensity of the crystal plane and the crystal plane of the positive pole powder of the 18650 lithium battery before and after the cycle. The phase and structure of the positive electrode material of the battery did not change before and after the cycle, and the pure layered LiCoO2 crystal phase was always maintained. After 200 cycles, no impurity phase was detected. It is stated that the positive electrode material did not undergo phase change during the cycle. . With the increase of the number of cycles, the value of the lattice constant a remains unchanged, while the value of c gradually increases, which indicates that LiCoO, the Li/Co ratio in the material decreases, that is, the amount of lithium ions decreases, which indicates that the lithium battery is Active lithium ions are reduced. The value of kmy/lom gradually decreases, which means that as the number of cycles increases, the regularity of the layered structure of the positive electrode LiCoO2 material decreases, and the mixing degree of Li and Co* ions increases. This may cause Li-insertion and release to be hindered, resulting in a loss of capacity. The XRD spectra of the negative electrode of the battery before and after the cycle of the 18650 lithium battery indicate that the phase and structure of the negative electrode material of the battery have not changed before and after the cycle, and the graphite crystal phase is always maintained. After 200 cycles, there is no phase change, and the diffraction peak intensity is The drop. With the increase of the number of cycles, the lattice constant does not change much, and the om value gradually increases. Use the Mering-Maire formula (also known as the Franklin formula) to calculate the degree of graphitization of the anode material: Gu003d(0.3440-dom)/(0.3440-0.3354)? 00%. Where G is the degree of graphitization, %; 0.3440 is the interlayer spacing of completely non-graphitized carbon, nm, and 0.3354 is the interlayer spacing of ideal graphite crystals, which is 1/2 of the c-axis lattice constant of hexagonal graphite, nm, oa is the interplanar spacing of the (002) crystal plane of the carbon material, in nm. The calculation shows that the degree of graphitization of the negative electrode material drops from 87.2% to 75.6% from the formation of the battery to the battery after 200 cycles. The decrease in the degree of graphitization of the negative electrode material will increase the resistance of Li+ insertion and de-intercalation, resulting in a loss of capacity. After 200 cycles of the 18650 lithium battery, the battery capacity decay rate was 15.6%; while the positive and negative electrode capacities lost 6.6% and 4.3%, respectively. It is therefore speculated that the capacity decay of the battery during the first 200 cycles is mainly due to the loss of active lithium ions. As well as the loss of electrode active materials; the loss of active lithium ions is mainly caused by the continuous consumption of active lithium ions by the reaction between the electrolyte and the positive and negative electrode active materials during the cycle; the regularity of the layered structure of the positive electrode active material decreases and the degree of ion mixing Increased surface charge transfer resistance increases, resulting in a decrease in its ability to extract lithium, resulting in a loss of capacity; the capacity loss of the negative electrode is mainly due to the deposition of a passivation film on the negative electrode active material, which leads to a substantial increase in charge transfer resistance. In addition, the negative electrode material Decreased graphitization degree and increased crystal defects will lead to a decrease in the ability of the negative electrode to deintercalate lithium and a loss of capacity; in addition, the porosity of the separator decreases, which prevents lithium ions from moving between the positive and negative electrodes, resulting in a loss of capacity; the choice is made in battery design Proper N/P ratio, improving the structural stability of the positive and negative materials, optimizing the film-forming additive formula in the electrolyte to stabilize the SEI film, and solving the surface of the diaphragm to prevent the blocking of the diaphragm pores. Improve the battery, which is expected to further improve the battery Cycle life. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: Measures to extend the life of lithium batteries