Optimization of synthesis process for carbon-mixed LiFePO4 composite thin-film cathodes deposited by bias sputtering

Thin films of carbon-mixed LiFePO4 were deposited on Si and stainless steel substrates by radio-frequency magnetron sputtering. The goal of this study was to optimize the conditions of sputter deposition for LiFePO4/C thin films to obtain a film with pure LiFePO4 phase. Negative electric bias ( 0 - 80 V ) was applied on the substrates during deposition to modify the film properties. After deposition, the films were annealed at different temperatures ranging from 150 - 650 S C without leaving the vacuum deposition chamber. Suitable bias was found to purify the LiFePO4 olivine phase and reduce the grain size. Too high a substrate bias and anneal temperature resulted in undesired second phases. Carbon mixing effectively lowered the resistivity of the films to the order of 102 Omega cm. Charge-discharge and cyclic voltammetry curves revealed the different electrochemical characteristics of these films, which were attributed to the modified crystallography and morphology. The films deposited under substrate bias of - 20 V and anneal temperature of 500 S C exhibited a capacity of similar to 170 mAh g +/- 7.5% with a 3.4 V plateau. (c) 2006 The Electrochemical Society.