| Due to high energy density, high voltage platform, good power performance and long cycle life, Li ion battery became a star battery since its commercialization and played an importance roll in consumer electronics, power battery and energy storage. However, along with the widely application of Li ion battery, more and more safety accidents were reported. Therfore, study the failure mechanism for Li ion battery and the lithium intercalation /deintercalation mechanism is of great significance in improving the safety and the overall performance.In this paper, the nano-LiFePO4 18650 cells, chosen for research, were provided by A123 systems. The performance of these batteries under normal, overcharge and overdischarge condition, as well as the lithium dynamic intercalation/deintercalation behavior, have been systematically studied. The dissertation focuses on the following parts:In Chapter 2, the long term cycle test was performed on 18650 cells under current rates of 1C,2C and 3C. The cell voltage, current, cell surface temperature and the AC impedance during the 2000 cycles were monitored. It was found that the failure mechanism of Li ion battery under normal condition was mainly due to the capacity loss which is more serious under high current rates. The three-electrode system test and the AC impedance test showed that the performance degradation of the cell was caused by the rise of anode potential as well as the battery internal resistance. The high energy synchrotron XRD demonstrated that the crystal structure of the active materials in cathode and anode was not degraded. However, the LiFePO4/FePO4 ratio at 0%SOC was reduced after long time cycle test, indicating the loss of the active lithium. Therefore, transformation of active lithium to SEI on the surface of anode, increasing the internal resistance, was considered to be the main reason for the capacity decay.In chapter 3, the cell voltage, current, cell surface temperature and the AC impedance under different depth of overdischarge were monitored. It was found that the overdischarge tolerance decreased rapidly as the increase of depth of overdischarge and the battery failed directly at 120% DOD. By the three-electrode system, the anode potential was found to increase quickly and reaches above 4.50V during overdischarge process. The test of Cu electrode in Li ion battery electrolyte showed that the copper corrosion reaction would occur since the oxidation potential is only 3.92V and 4.17V. The AC impedance results confirmed the dissolution of the anode current collector. The in situ high energy synchrotron XRD data showed that the active materials were not destroyed under overdischarge process. The SEM images showed new layers formed in the overdischarge cell and the EDS results confirmed the presence of Cu in cathode, anode and the separator. Therefore, the micro-shorting led by corrosion and re-deposit of Cu in anode and cathode was the main failure mechanism of Li ion battery under overdischarge condition.In chapter 4, the cell voltage, current, cell surface temperature and the AC impedance under different depth of overcharge were monitored. The cell voltage and the surface temperature reached 5.22V and 45℃. The overcharge tolerance of Li ion battery decreased fast as the increase of depth of overcharge. The three-eletrode system test showed that the potential for anode and cathode achieved-0.34V and 4.93V respectively at 110%DOC, largely deviating from the lithium intercalation/deintercalation potential. Also, Fe corrosion would occur during this potential in Li ion battery electrolyte. The in situ high energy synchrotron XRD data confirmed that the active materials were not destroyed under overcharge process. Lots of spots were found in the SEM images of the failed anode, and the EDS observed the presence of Fe, Cr and Ni in the spots. Therefore, the failure mechanism of Li ion battery during overcharge condition was considered to be related to the stainless steel.In chapter 5, the in situ high energy synchrotron XRD was employed to study 18650 Li ion battery. At current rate of 4C, the crystal structure changes in both cathode and anode were observed simultaneously. In anode, the Li-rich phase closed to LiC6 and a series of intermediate phase were observed, and indicated that the dynamic lithium intercalation into graphite might not follow the classical stage mechanism. According to the variation of cell parameters and content of LiFePO4/FePO4, the charge/discharge process was divided into three periods with different components of phases:LiFePO4+lithium-deficient solid solution phase (period Ⅰ), LiFePO4+ FePO4 (period Ⅱ), FePO4+ lithium-rich solid solution phase (period Ⅲ). It also found that the division of the three periods was related to the current density. It is believed that this dynamic mechanism better describes the lithium intercalation/deintercalation behavior under real working conditions.In chapter 6, to explain the new phenomenon in anode found in chapter 5, the in situ high energy synchrotron XRD was applied on a Li/graphite half cell. It was found that the voltage platform corresponding to stage compounds disappears at higher current rate. The absence of superlattic peaks in XRD patterns confirmed that the dynamic lithium intercalation did not follow the classical stage mechanism which based on thermodynamic equilibrium condition. This proposed dynamic mechanism was considered to reflect the lithium ion intercalation behavior under real operating conitions. |
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