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

LiFePO 4 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 LiFePO 4 thin films has been investigated. Different substrate temperatures, from 25 to 500 °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 °C, an impurity phase of Li 3 Fe 2 (PO 4 ) 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 °C substrate temperature exhibited an initial discharge capacity of 56 μAh/(cm 2 ·μm) at a current density of 10 μA/cm 2 between 3.0 and 4.3 V, with good cyclability, suggesting that LiFePO 4 thin film can be used as the cathode film for lithium microbatteries.