Research Of Preparation And Performance For LiFePO₄ Cathode

LiFePO4 is an stable olivine-type structure, low cost, rich resource and environment friendly material, so it has attracted significant interest as a cathode material for Li-ion batteries. In this paper, our investigations were focused on the technics of preparation and the high-temperature storage and cycling characteristics of the LiFePO4 Li-ion batteries. And also the effect of the charge current on the discharge rate was studied with the LiFePO4 Li-ion batteries. Then our investigation results of the tolerance to the various abuse disturbances for tested single SCBs (steel can batteries) with the system of LiFePO4/MCMB Li-ion batteries at different conditions (charge state, cycling and operating etc) were followed.The techniques of preparation of the LiFePO4 cahode was investigated and optimized. Comparison of performance between different LiFePO4 positive electrode materials from different manufacturers showed that the particle sizes, uniformity grade and area ratio of LiFePO4 had great effect on solidification quality of slurry and electrochemical performance of the LiFePO4 cathode. In conjunction with the results from cycling and IR tests, which showed that LiFePO4 positive electrode material with the particle size of 4μm, uniformity grade and surface area ratio of 20m2/g has good electrochemical performance.Concentration of PVDF/NMP solution was 30% which could soaked LiFePO4 positive electrode materials and conduct additive completely. Increase of active materials and decrease of conduct additive could improve capacity of the cells, while the IR of cells also increased. In this paper, we showed a optimized direction 83:10:7 (LiFePO4: conduct additive: PVDF mass %).The cycling performance at room temperature and the mechanism of capacity fades during high temperature storage and cycling for over two hundred LiFePO4/MCMB SCBs were studied. The battery retained 91% of its initial capacity and IR (impedances resistance) rised from 41.97m? to 44.54 m? after 460 cycles. The cells showed capacity loss when the cells stored and cycling at high temperature, especially 70 and 80℃, IR and capacity loss perform worse. In conjunction with the results from ICP, SEM and IR tests, which showed that a small amount LiFePO4 was dissolved in the electrolyte and maybe the dissolved Fe2+ was reduced at the surface of anode and that the iron metal played a catalytic role in the formation and growth of an interfacial film at the anode surface, leading to the huge rise of the IR and capacity loss of the cell during storage and cycling.The tolerant abilities to abuse for LiFePO4/MCMB Li-ion batteries after cycling were investigated. In the cycling range of this experiment, there were obvious effects of cycling on electrical and thermal safety, however, hardly on mechanical safety. For overcharge, short-circuit and oven tests, in the cycling range of this experiment, all of cells were safe, but unsafe possibility of the cells increased when cycling numbers increased. In this paper, the triggering factors of thermal runway were also discussed in detail.