Sodium manganese oxide thin films as cathodes for Na-ion batteries

This paper presents the fabrication and characterization of sodium manganese oxide cathode thin films for rechargeable Na-ion batteries. Layered oxide compounds of nominal compositions Na_0.6MnO₂ and Na_1.0MnO₂ have been prepared by radio frequency magnetron sputtering and post-annealing at high temperatures under various conditions. The Na_0.6MnO₂ thin films possess either a hexagonal or orthorhombic structure while the Na_1.0MnO₂ films crystallize in a monoclinic structure, as shown by X-ray diffraction and X-ray absorption spectroscopy results. The potential profiles of the film cathodes are characterized by features similar to those measured for the powders and exhibit reversible storage capacities in the range of 50-60 μAh cm^-2 μm^-1, which correspond to about 120-140 mAh g^-1, and are maintained over 80 cycles.