Studies On Microwave Synthesis Of Carbon Coating Lithium Iron Phosphate

As an alternative cathode material for lithium-ion battery, LiFePO4has high theoretical capacity (170mAh/g), stable charge-discharge voltage platform, high security, good thermal stability, and is environment-friendly. R&D of LiFePO4is then a hot area in rechargeable lithium-ion battery. The influences of carbon source on the morphology, structure and electrochemical capacity of LiFePO4encapsulized by carbon (LiFePO4/C) are envestigated through materials characterization such as Differential Scanning Calorimetry (DSC), Thermo Gravimetric-Differential Thermal Analysis (TG-DTA), X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and electrochemical analyses such as galvanostatic charge-discharge and cyclic voltammetry (CV). FePO4·2H2O is used as the iron source, LiOH·H2O as the lithium source, a2450MHz,675W electromagnetic waves as the heating source. The precusors are prepared by drying corresponding rheologic phases consisting of Li-source, FePO4and carbon source through heating or spray.It is found that the carbon source with low permittivity, such as polyethylene glycol (PEG10000), is not good for synthesis of LiFePO4/C through microwave heating due to its incomplete carbonization, resulting to decay of electrochemical capacity because of the residue of carbonization. When using0.05mol LiOH·H2O, FePO4·2H2O as Li source and Fe source to prepare the precursor by heating rheological phase, sucrose and starch are beneficial of builting better connection between FePO4particles with carbon material, compared with spray drying the precursor. After optimizing carbon mass and microwave heating time (polyethylene glycol addition:6g, microwave heating30min) for preparing0.05mol of LiFePO4, a maximum discharge capacity of107mAh/g at0.1C rate is achieved. Similarly,2.5g of sucrose and starch is suitable for synthesis of0.05mol of LiFePO4by microwave heating25min. The discharge capacity of the synthesized LiFePO4/C with sucrose and starch reaches111and122mAh/g at0.1C rate, respectively. SEM and electrochemical analysis shows that microwave synthesis of LiFePO4/C particle agglomeration, electrode reaction kinetics in poor performance adopting rheological phase precursor.The precursors are prepared by the method of rheological phase and spray drying. The effects of precursor on LiFePO4’s morphology and electrochemical performance are investigated. Compared with spray drying precursor, synthesized LiFePO4/C from rheological phase agglomerates seriously when using sucrose, polyethylene glycol (PEG10000), starch, glucose as a carbon source. However, the electrochemical properties of the synthesized LiFePO4/C from rheological phase precursor are better than that from spray drying precursor. When using citric acid as the carbon source, the feed bed volume increases3-5times after reaction if rheological phase as the precursor, whereas it shrinks if using spray drying precursor. The LiFePO4’s electrochemical properties from spray drying precuesor are better than that from rheological phase precursor. The precursor’s density and water content is vey critical to morphology and electrochemical properties of microwave synthesized LiFePO4.