Research On Capacity Fading Mechanism During Long-term Cycling Of Over-charged/Over-discharged/External Short-circuited LiCoO₂/MCMB Battery

The lithium ion battery has been widely applied in the fields of electronic products owing to its advantages such as high voltage, high energy density, l ow pollution and so on. However, the differences between monomer batteries in the battery pack and faulty operation may lead to fault accident（short-circuiting, overcharging and over-discharging）. If the fault degree is slight and the security problems do not happen, it has great significance to maximum use of the battery, amending and perfecting lithium ion battery safety standards, and the designation of battery management system（BMS）. In this paper, the changes of electrochemical properties of the battery during the long-term charge/discharge cycling were examined by electrochemical impedance spectroscopy（EIS） and potentiostatic intermittent potentiostatic intermittent titration technique（PITT） tests. Then the morphology and structure changes were studied using scanning electron microscopy（SEM）, X-ray absorption spectroscopy（XAS）, X-ray diffraction（XRD） and so on. The capacity fading mechanisms during long-term cycling of failure LiCoO₂/mesocarbon microbeads（MCMB） were obtained（short circuiting, overcharging, and over-discharging）.The effect of short-time external short circuiting on the capacity fading mechanism during long-term cycling of LiCoO₂/MCMB battery is studied in our work. The cycling life of battery can be shortened due to the serious short circuiting. The short circuiting can lead to the high discharge current that the instant contact current can instantly reach to be about 34.8C, and the voltage can instantly reduce to almost 0.00 V. The high discharge current can lead to high temperature. The temperature is 33°C, 63°C, and 76°C for battery short circuited by 0.6 mΩ 1s, 5.0 mΩ 30 s, and 0.6 mΩ 30 s, respectively, and the temperature of battery short-circuited by 0.6 mΩ 180 s can reach to be 115°C. The high temperature can accelerate the decomposition of electrolyte, and accelerate the side reactions between electrode/electrolyte interfaces. High current can damage the electrode leaving vacancies in the structure and releasing small pieces of electrode into the electrolyte. According to the results of PITT, the reduction of the chemical diffusion coefficient of lithium（D） and the increase of polarization relate with the ion disorder. The capacity loss of LiCoO₂（11.31%） is larger than MCMB（6.38%）. The cycling can intensify the damage of LiCoO₂, the increase of polarization, and the thickness of SEI film on surface of anode. The polarization and the capacity loss of electrodes are main reason of capacity fading of shorted battery during long-term cycling and the polarization is mainly caused by LiCoO₂ electrode.Degradation mechanism during long-term cycling of over-charged LiCoO₂/MCMB is studied. The battery was over-charged to different voltages（4.4 V, 4.5 V, 4.6 V, and 4.7 V）, and then the over-charged battery was cycled for 1000 times with 30% DOD. The slight over-charging（4.4 V and 4.5 V） for one time has little effect on cyling performance of battery, while serious over-charging can intensify the capacity fading of the battery. The capacity loss of MCMB is higher than that of LiCoO₂ for the battery over-charged to 4.4 V and 4.5 V. The capacity fading is attitude to the MCMB electrode which is related with the increase of SEI film according to SEM、XRD、XPS and water washing test. The surface of MCMB particles form battery over-charged to 4.6 V and 4.7 V has many roughening facets and small amounts of needle-like feature can be observed on the surface of MCMB particle according to the results of SEM and XPS. The over-charge reaction causes some of the Co3+ ions at the electrode surface to be reduced from XAS. The cobalt can be detected on the surface of MCMB after 1000 cycles, indicating that the cycling can lead to the dissolution of cobalt. Thus, the capacity fading of the battery serious over-charged for one time is related to the generation of dendrite, the increase of SEI film, and the dissolution of cobalt. Continuous over-charging for 10 times is also studied in order to study the relation of over-charging voltage and overcharging number. Continuous serious over-charging can intensify the capacity fading of battery obviously. The performance degradation occurs by different mechanisms for battery over-charged to different voltage. The capacity fading of full battery over-charged to 4.4 V and 4.5 V can be mainly attributed to the MCMB electrode, whereas capacity fading over-charged to 4.6 V and 4.7 V is attributed primarily to the LiCoO₂ electrode. The capacity loss of MCMB may be attributed to the SEI film on the surface of MCMB electrode and the capacity loss of LiCoO₂ electrode may be related to the reduction and the dissolution of cobalt. The effect of over-charging voltage on the battery performance is higher than over-charging number and the over-charging for ten times can intensify the capacity fading of the battery.The capacity fading mechanism of during long-term cycling over-discharged LiCoO₂/MCMB is studied. The LiCoO₂/MCMB batteries are over-discharged to 102% DOD, 105% DOD, and 115% DOD for one time, respectively, then are fully charged and cycled 1000 times at 0.6C with 30% DOD. The slight over-discharging（102% DOD and 105% DOD） has little effect on capacity fading of the battery while the serious over-discharging（115% DOD） can accelerate the capacity fading. The structure of LiCoO₂ remain unchanged after different over-discharging according to the results of SEM 、 EDS and XRD. While the copper metal from the current collector is oxidized and dissolved during the serious over-discharging process, and then is reduced and deposited on the surface of MCMB during the following charging process. Cu-sputtering experiment is used to simulate and analyze the effect of deposited Cu on the performance of MCMB electrode. The copper on the surface of MCMB electrode may hinder the lithium intercalation and deintercalation. The cycling can lead to the increase of SEI film. The main reason for the capacity loss of MCMB electrode is the increase of SEI film by the water washing test. The structure of LiCoO₂ and MCMB still remained unchanged after cycles while the discharging performance of MCMB material can be degraded. Continuous slight over-discharging for 10 times has a little effect on the battery performance and capacity fading of the battery is intensified. While continuous serious over-discharging can lead to the failure of battery. During charge/overdischarge cycling, Cu dendrites continued growing from the cathode side, penetrating through the separator and forming a copper bridge between the anode and cathode. The copper bridge caused micro-shorting and eventually led to the failure of the battery. The effect of over-discharging depth on the battery performance is higher than over-discharging number and the over-discharging for ten times can intensify the capacity fading of the battery.