Study On The Conductive System And The Rate Discharge Performance Of LiFePO₄ Cathode

The high power lithium ion batteries are a new style power supply, which is based on the fast transmission of Li⁺ and electron. The electronic conductivity pathways among the active materials and the pathways between the active materials and the current collector can be provided by adding conductive agents. The resultant pathways can reduce the electrode contact resistance, speed up the electronic conductance and improve the lithium-ion migration rate within the active material effectively. It can improve the high-rate performances of active material.The micro-sized LiFePO₄ was used as the cathode active material in this paper. Three different conductive system of the cathode for Li-ion batteries was formed using acetylene black, carbon nanotubes and the multiplex conductive agents, respectly. The effect of the species and content of conductive agents on the the electrical conductivity and the high-rate discharge performances of the active material was investgated systemly. The relative influence mechanisms were also explored and studied.The results showed that the electrical conductivity and the discharge capacity retention of the cathode improved with the increase of the acetylene black. The short-range electrical conductivity between the acetylene black and the active material, the long-range electrical conductivity among the acetylene black were formed when the content of the acetylene black was about 12 wt%. The system had the highest discharge capacity retention and median voltage. The discharge capacity retention at 20C and 40C were 67.0% and 52.6%, respectly. It can discharge at 100C. The electrical conductivity can also improve when the content of the acetylene black was more 12 wt%. But the discharge capacity retention and the median voltage decreased. The cathode polarization can be reduced by decreasing the thickness of the electrode, which can enhance the high-rate discharge capacity retention and the voltages of the positive electrode.The carbon nanotubes can effectively improve the electrical conductivity of the system because it has good long-range electrical conductivity. The electrical conductivity of the system that the content of the carbon nanotubes is 6 wt% was the same to the system that the content of the acetylene black is 9 wt%. But the interface between the carbon nanotubes and the active material, the short-range electrical conductivity was less than the acetylene black. Then the high rate discharge properties of the acetylene black system was better than the carbon nanotubes system.The acetylene black and the carbon nanotubes can improve the short-range electrical conductivity and the long-range electrical conductivity of the LiFePO₄, respectively. The multiplex conductive agents were formed based on the each advantage. The results showed that the electrical conductivity and the high rate discharge properties of the multiplex conductive agents that the content was 9 wt% (the ratio of acetylene black/carbon nanotubes was 1:0.005) was better than the acetylene black system that the content was 12 wt%. The KD-T19 type lithium ion battery was prepared using the multiplex conductive agents. The inside resistance of the battery was about 8⁹ m?. The discharge capacity retention of the battery at 25C was about 75.5%.