Production Processes And Electrochemical Properties Of Soft-Packed Lithium-Ion Batteries

Lithium secondary batteries are the new types of cleanliness and recyclable energy source. They have high voltage and high energy density. Recently they are widely used in digital products, e.g., mobile phone, notebook, bluetooth headset, MP3, digital camera, et al. Soft-packed lithium-ion battery is a new type of the lithium secondary batteries, which has excellent characteristics including safety, light weight, flexible, but its low rate of dischargeability makes it can not compete with Ni-MH and Ni-Cd batteries at the little-scaled high power battery markets.At the aims of the improvement of the high rate discharge ability of the soft-packed lithium-ion battery, the production processes of the soft-packed lithium-ion battery, including the selection of the raw materials of the electrode and the fabrication techniques, were optimized in this thesis. The electrochemical properties of the lithium-ion battery were tested by the electrochemical impedance spectra (EIS), etc. The microstructure and the composite of the solid-electrolyte interphase (SEI) were analyzed by scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX) and Fourier transform infrared spectroscopy. The decay mechanism of the negatives electrode was also discussed.The results show that the bonding of active materials of the cathode and the copper foil was enhanced when the raw materials used for fabricating soft-packed battery including LiCoO₂, carbon, binder (polyvinylidene, PVDF), conductive carbon, solvent, aluminum foil and copper foil were dehydrated, thus, decreased the resistance of the battery and increased the high rate of discharge ability of the battery. By the comparison of two kinds of performance methods, constant current (CC) charge-discharge mode and ladder charge mode, it was found that the internal resistance of batteries was lower, and the capacity was higher when the CC mode was used. However, with the increasing of the discharge rate, the discharge flat decreased faster in the former mode.The study of the relationship between the capacity of negative and positive (N/P) showed that, when the value of N/P increased, the capacity and cycling stability was improved, but the high rate dischargeability was decreased. The EIS studies indicated that, When the value of N/P increased, the surface film impedances (R_film) and the charge transfer impedance (Rc.t.) increased first (N/P=0.8¹.6) and then decreased (N/P=1.8). It was inferred that, when the value of N/P ≦ 1.0, the complex SEI films with large impedance were formed on the negative surface.SEM micrographs of negative electrode surface showed that, the cracks on the electrode surface were increased with electrochemical cycling. Also some white reduction products were formed on the surface of electrode with cycling. The EXAF studies showed that the contents of carbon and oxygen were 56.97%, 43.03% respectively in the white regions. It was inferred that the electrolyte was reduced on the surface of negative electrodes. The formation of reduction products which were covered on the electrode surface increased the internal resistance of batteries. The capacity and discharge voltage plateau were also decreased. The FTIR studies showed that the SEI film on the negative electrode surface were mainly composed and(CH2OCO2Li)2.