A New Sythetic Route for Preparing Carbon-Coated LiFePO4

Introduction Lithium iron phosphate (ordered olivine-type structure) is emerging as the most promising material to replace lithiated transition metal oxide as cathode material for secondary lithium-ion batteries[1,2]. LiFePO4 cathode with a structure of olive-type has been offering the optimal combination of low cost ,favorable electrochemical activity and low environmental impact. However there is a basic problem hindering the practical application of this material.The poor electronic and ionic conduction in the crystalline lattice of LiFePO4.To solve the problem, a great deal of effort has been done by coating LiFePO4 particles with electrically conductive materials like carbon[3,4]and, metal and metal oxides[5].In the solid-state synthesis of LiFePO4, the reaction condtion,such as atmosphere and temperature, need to be strictly controlled to prevent the oxidation of iron ions. In this research Olive LiFePO4 synthesis by modified solid state reaction.The powders were calcined for 8h at 690°C.The sample synthesized by this technique has the high reversible discharge specific capacity and good cyclic performance. The LiFePO4/C composite can deliver a high discharge capacity of 149mAh/g at 0.2C rate. Experimental LiFePO4 was prepared by the solid-state reaction of stoichiometric Li2CO3(99%),H3PO4(85%), Fe2O3(95%),glucose(99%) ,citric acid(99%)and distilled water as raw materials, The precursors were weighed, according to the stoichiometric ratio, mixed,parched at 120°C and grounded by a planetary ball mixer.The obtained powders were calcined for 8h at 690°C in inert atmosphere (high purity Ar). The prepared powders were examined by XRD,SEM,etc. For evalution of electrochemical performances, charge/discharge cycle tests. Cells were composed of a cathode and a lithium metal anode separated by a porous polypropylene film (Celgard 2400).The cathode mixture consisted of 85Wt% LiFePO4 powder 10Wt% acetylene black and 5Wt% PVDF binder on aluminum coupon. All cells were assembled in a dry box. Cells were cycled in the voltage range of 2.5-4.2V at a constant current of 0.3mA/cm(about 0.2C). Results and discussion Figure 1 shows X-ray diffraction (XRD) pattern of LiFePO4/C Which was calcined at 690°C.Compared with the published XRD data of Li-Fe-P-O compounds.All the samples can be assigned to olivine LiFePO4 with orthorhombic lattice(Pnma). Figure 2 shows the SEM micrograph of LiFePO4/C powder.It was found that the particle size has slightly micro-aggregate phenomenon. The sample with the characteristics of fine partice and good crystalline LiFePO4/C phase may have good electrochemical performance. Figure 3 shows the discharge curve of LiFePO4/C cells which deliver a high discharge capacity of 149mAh/g. And the cycling capacity is quite stable.