Realization of High Performance Lithium Iron Phosphorus Oxynitride Thin Film Cathodes for Lithium Ion Batteries

Experimental realization of nitrogen doping into LiFePO4 films has been carried out by reactive sputtering. LiFePOxNy films were deposited under various N-2/(Ar/H-2) flow ratios followed by thermal annealing. The amounts of nitrogen doping increase with the N-2/(Ar/H-2) flow ratios. Enhanced Li-N signals were recorded, where Nitrogen substitutes for Oxygen at LiO6 octahedral structure in LiFePO4. The binding energy bands of the double and triple coordinated N-P bonds were found and the triple bond increases with increased nitrogen doping. With nitrogen doping, Fe2+ remained unchanged, whereas Fe-O defects in FeO8 octahedron increased. Nitrogen doping has elevated the conductivity of the films to the order of 10(4) S, and also reduced the electrochemical impedance. The charge-discharge tests show that capacity of 140 mAh/g at 0.2 C can be obtained with appropriate nitrogen doping. Even at a current rate of 10 C, the optimized LiFePOxNy film can delivers a capacity of 100 mAh/g.