Preparation and Characteristics of LiFePO4 Thin Film by Radio Frequency Magnetron Sputtering for Lithium Microbatteries

LiFePO4 thin films were deposited by radio frequency (rf) magnetron sputtering. The effect of substrate temperature during the rf magnetron sputtering on the morphology and characteristics of the LiFePO4 thin films has been investigated. Different substrate temperatures, from 25 to 500 degrees C, were applied during deposition. When the substrate temperature increased, the film structure changed from amorphous to crystalline, as characterized by X-ray diffraction. At the high substrate temperature of 500 degrees C, an impurity phase of Li3Fe2(PO4)(3) was developed, and carbon particles in the film tended to aggregate in clusters on the substrate surface. The surface and cross-sectional morphology of the thin film was observed by using scanning electron microscopy. Electrochemical tests showed that the different characteristics of the as-deposited films could be attributed to the crystallography and morphology, The film deposited at the 400 degrees C substrate temperature exhibited an initial discharge capacity of 56 mu Ah/(cm(2).mu m) at a current density of 10 mu A/cm(2) between 3.0 and 4.3 V, with good cyclability, suggesting that LiFePO4 thin film can be used as the cathode film for lithium microbatteries.