Process And Mechanism Research On Microwave Synthesis Of LiFePO₄ Materials

The ever-growing demand for portable batteries with high energy density is exerting pressure for the development of advanced lithium-ion batteries. The olivine phase of LiFePO₄ has shown considerable promise as a cathode material in Li-ion batteries due to its high cyclability, stability, low toxicity, and low cost. Since the insulating LiFePO₄ phase was first proposed as a cathode material, research has been mostly devoted to designing LiFePO₄ materials with better conductivities. Another problem of the LiFePO₄ is that synthesis is not easy because of the iron oxidation state. The oxidation state control has been usually done by the furnace heating with reductive or inert gas flow for several hours. In addition, alternative synthetic processes have developed continually such as hydrothermal synthesis, co-precipitation, sol-gel and microwave processing.The microwave irradiation has been shown to provide a novel, rapid and economical method of preparing many important materials. Microwave processing is a self-heating process that occurs through the absorption of electromagnetic energy. Since the microwave energy is directly absorbed by the sample, uniform and rapid heating can be achieved within a short period of time. We have used the microwave heating as an alternative synthetic method because it is hard to synthesize pure iron (II) compound using the furnace heating. The particle morphology was observed by SEM and X-ray diffraction was used to characterize the phase homogeneity.The process technology of LiFePO₄ has been studied on microwave single-mode and multi-mode microwave cavity respectively. X-ray diffraction revealed that a single phase lithium iron phosphate powder can be synthesized in both microwave cavities. The optimum processing with single-mode cavity is 500W in microwave power for 2 minutes. Two-step synthesis method has been used in multi-mode cavity. After 500W in microwave power for 20 minutes the sample was reground and repressed. Then, the sample was synthesized using multi-mode within 20 minutes and 750W in microwave power. The particle morphology shows that the average size of LiFePO₄ particles is approximately 3μm. This size is smaller than that synthesized in single-mode.We also discussed the feasibility of using activated carbon as microwave absorber and reductive atmosphere maker. In addition, the impacts of processing time and temperature to the particle size and morphology have been studied. According to the characteristics of microwave electromagnetic heating, the mechanism of microwave synthesis has been investigated primarily. Distribution on relative peak intensity of X-ray diffraction has been found different between microwave single-mode processing and conventional method.