Research On Capacity Loss Mechanisms Of Graphitic Carbon Anodes In Lithium Ion Batteries

The lithium ion battery has some advantages such as high voltage, high energy density, few pollution and so on. It has been applied in the fields of electronic products and electric vehicle. The capacity of lithium ion battery decreased gradually during use. If the reasons for the capacity loss were explored, the preparation of the battery would be improved targetedly and lithium ion batteries with high capacity and long life could be obtained, which is of great importance. In this paper, capacity loss mechanisms of mesocarbon microbeads (MCMB) and graphite anodes applied in lithium ion battery widely were studied. The changes of electrochemical properties of the anodes during the long-term charge/discharge cycling were examined by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tests. Then the growth of solid electrolyte interface (SEI) film on the surface of the anode, structure changes and the growth of the lithium deposits were studied using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and so on.The capacity fade behaviors of MCMB were studied by the long-term charging and discharging of MCMB coin cells. The results indicated that d002and Lc of MCMB bulk increased and the degree of graphitization decreased slightly with the cycling, while d110and La did not change obviously, and the disorder extent of MCMB surface increased. Li2O was formed firstly, and then LiF was formed in SEI film on the surface of MCMB. Due to the growth of SEI film, it was more difficult for lithium ions to transfer through SEI film, which contributed to the increase of film resistance. Moreover, the increase of LiF content indicated that LiPF6was continously consumed during the growth of SEI film, leading to the increase of ohmic resistance. The increase of these resistances led to the polarization increase of the lithium insertion and deinsertion reaction, and the decline of reaction current, which contributed to the capacity loss of the anode.The capacity loss mechanism of MCMB anode in LiCoO2/MCMB battery during the long-term cycling was studied. The capacity of LiCoO2/MCMB battery declined quickly during the first200charge/discharge cycles. The MCMB materials in the single-side coated area near the plate lug of anode detached from current collector. Adhesion between anode material and current collector in the single-side coated area was small from test result of adhesion. Detachment of anode material from current collector was considered as one of the reasons for quick capacity loss at the early stage of cycling. Li2CO3and ROCO2Li appeared in the SEI film generated in the later period of cycling. d002of MCMB increased slightly, and d110and La did not change during the long-term cycling. The effect of SEI film on the capacity loss increased with the cycling and the effect of the structure change of MCMB material on the capacity loss was smaller than that of SEI film. The effects of SEI film generated in the both earlier and later periods of cycling on the electrochemical resistances from big to small were Rf, Rct and Rb. Lithium deposition on the MCMB anode grew gradually with the cycling. The deposition appeared on the surface next to the current collector firstly, and then on the surface next to the separator. The outer region of both lower and upper deposition layers consisted of Li2CO3, LiOH, ROCO2Li and ROLi. The inner region of etched lower deposition layer mainly consisted of Li2O, LiF and Li2CO3, and that of etched upper deposition layer mainly consisted of Li2CO3, ROCO2Li, ROLi and LiF. SEI film hindered the intercalation of lithium ions into carbon layers obviously and ovcharged LiCoO2cathode provided more lithium ions, which brought up the formation of the lithium deposition during long-term cycles. The thickness of the deposition ranged from tens to several hundreds of micrometres. The deposition hindered the intercalation of lithium ions, and it made MCMB layer peeled off from current collector locally. Rb of the cell increased slightly, and Rf and Rct increased significantly, leading to the capacity decline of the anode. The capacity loss of anode was larger than that of cathode after2400cycles. The growth of SEI film on the anode was the main reason for the capacity decline, and it accounted for68%of the capacity loss of the anode.d002of the anode in the LiCoO2/graphite battery increased and Lc decreased slightly, and La did not change obviously during the long-term charge/discharge cycling. When the battery was charged and discharged for600cycles, lithium deposition began to appeare unevenly on the surface of graphite anode and the deposition grew continuously with the cycling. The composition of SEI film on the surface of lithium deposition was similar with that on the surface of graphite. The appearance of lithium deposition could consume the electrolyte of the battery and decrease the ionic conductivity of the battery. Moreover, the formation of lithium deposition itself would also consume the active lithium of the battery and led to the capacity loss. The exfoliation of graphite material led to the decline of anode active material with the charge/discharge cycling, making metal lithium deposition appear on the surface of graphite anode. The effect of cathode and anode on the LiCoO2/graphite battery capacity loss was the first largest and that of electrolyte was the second largest. Moreover, the effect of anode on the property decline of the battery was greater than that of cathode.