The capacity degradation of 18650 lithium batteries is approximately linearly related to the number of cycles at room temperature. After 200 cycles, the capacity degradation rate of the battery is 15.6%. As the number of cycles increases, the charging and discharging capacitance of the battery gradually decreases, the charging voltage platform increases, the discharging voltage platform significantly decreases, and the discharging voltage platform becomes shorter. According to Ohm's law, there is a relationship between the input and output voltage, resistance, and current of a battery during constant current charging and discharging: V=E-IR. In the formula, E is the electromotive force of the battery at equilibrium potential, I is the constant current charging and discharging current (negative current during charging and positive current during discharging), and R is the total internal resistance of the battery, including solution resistance, polarization resistance of positive and negative electrodes, and contact resistance between active substances and solutions, as well as between active substances and current collectors. The battery is cycled under a constant current charging and discharging system, and the charging and discharging current remains constant. As the number of cycles increases, the total internal resistance of the battery continues to increase. Therefore, during the battery cycle, the charging voltage platform gradually increases while the discharge voltage platform gradually decreases.
As the number of battery charging and discharging cycles increases, the capacity obtained during constant current charging shows a decreasing trend with the increase of cycle times. On the contrary, the supplementary charging capacity obtained during constant voltage charging continues to increase. This is because during the cycling process, the internal resistance of the battery continuously increases, and the polarization of the battery continuously increases, resulting in a decrease in the constant current charging capacity of the battery and an increase in the constant voltage charging capacity.
The internal resistance of 18650 lithium batteries decreases with the increase of their open circuit voltage and significantly increases with the increase of the number of cycles. At full charge (100% SOC), after 200 cycles, the internal resistance of the battery increases, which is one of the reasons for the battery capacity degradation.
As the number of cycles increases, the charge transfer impedance (R) of the charge migration process at the electrode electrolyte interface in the 18650 lithium battery significantly increases. This may be due to the deposition of high impedance passivation films on the positive and negative active materials, as well as the reduction of effective positions for lithium ion extraction/insertion. The increase in charge transfer impedance can lead to a decrease in the dynamic performance of the battery, resulting in capacity degradation during long-term cycling.
The decay rates of positive and negative electrode capacity are not significantly different, but as the number of cycles increases, the contribution of positive and negative electrode capacity loss to the total battery capacity loss decreases, while the direct loss of active lithium ions and the decrease in lithium ion migration ability between positive and negative electrodes increase the contribution to the total battery capacity decay.
The lattice constant and diffraction peak intensity of the positive electrode powder of 18650 lithium battery before and after cycling. Before and after cycling, the phase and structure of the positive electrode material of the battery did not change, and remained pure layered LiCoO2 crystal phase. After 200 cycles, no impurities were detected, indicating that there was no phase change in the positive electrode material during the cycling process. As the number of cycles increases, the lattice constant a value remains unchanged, while the c value gradually increases, indicating that the Li/Co ratio in LiCoO materials decreases, that is, the amount of lithium ions decreases, indicating a decrease in active lithium ions in lithium-ion batteries. The value of kmy/lom gradually decreases, indicating that as the number of cycles increases, the regularity of the layered structure of the positive electrode LiCoO2 material decreases, and the miscibility of Li and Co * ions increases. This may hinder the insertion and extraction of Li, resulting in a loss of capacity.
The XRD spectra of the negative electrode of the 18650 lithium battery before and after cycling indicate that the phase and structure of the negative electrode material did not change before and after cycling, and the graphite crystal phase remained unchanged. After 200 cycles, no phase transition occurred, and the diffraction peak intensity decreased. As the number of cycles increases, the lattice constant does not change much, and the om value gradually increases. Calculate the degree of graphitization of negative electrode materials using the Mering Maire formula (also known as the Franklin formula):
G=(0.3440 dom)/(0.3440-0.3354)? 00%. In the formula, G represents the degree of graphitization,%; 0.3440 is the interlayer spacing of completely non graphitized carbon, nm, 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 carbon material (002) crystal plane, nm. According to calculations, the degree of graphitization of the negative electrode material in the battery decreased from 87.2% to 75.6% after 200 cycles. The decrease in the degree of graphitization of negative electrode materials will increase the resistance of Li+insertion and extraction, leading to a loss of capacity.
After 200 cycles of 18650 lithium batteries, the battery capacity degradation rate is 15.6%; And the capacity losses of the positive and negative electrodes are 6.6% and 4.3%, respectively; It can be inferred that the capacity degradation of the battery during the first 200 cycles mainly comes from the loss of active lithium ions and 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 cycling process; The regularity of the layered structure of the positive electrode active material decreases, the ion mixing degree increases, and the surface charge transfer impedance increases, resulting in a decrease in its lithium removal ability and a loss of capacity; The capacity loss of the negative electrode is mainly due to the deposition of a passivation film on the active material of the negative electrode, which leads to a significant increase in charge transfer impedance. In addition, the degree of graphitization of the negative electrode material is reduced, and crystal defects are increased, which will lead to a decrease in the lithium removal ability of the negative electrode and a loss of capacity; In addition, the decrease in membrane porosity hinders the movement of lithium ions between positive and negative electrodes, leading to capacity loss; Improvements in battery design, such as selecting an appropriate N/P ratio, improving the structural stability of positive and negative electrode materials, optimizing the formulation of film-forming additives in the electrolyte to stabilize the SEI film, and surface treatment of the separator to prevent clogging of the separator holes, are expected to further enhance the battery's cycle life.