Structural characterization and electrochemical properties of RF-sputtered nanocrystalline Co₃O₄ thin-film anode

Nanocrystalline Co₃O₄ thin-film anodes were deposited on Pt-coated silicon and 304 stainless steel by radio frequency (RF) magnetron sputtering. The as-deposited and annealed cobalt oxide thin films showed smooth and crack-free morphologies. Both the as-deposited and annealed films exhibited spinel Co₃O₄ phase with nanocrystalline structure. High-temperature annealing enhanced the crystallinity of RF-sputtered cobalt oxide films due to rearrangement of cobalt and oxygen atoms. Electrochemical characterization of RF-sputtered films was carried out by cyclic voltammetry and charge/discharge tests in the voltage range of 0.3-3.0 V. Cyclic voltammetry plots showed that the RF-sputtered Co₃O₄ thin films were electrochemically active. X-ray photoelectron spectrometer (XPS) showed that the fresh cobalt oxide films had two peaks of Co₃O₄. In addition to the binding energy of cobalt oxide, the XPS spectrum of discharged film presented two additional binding energies correspond to Co metal. The first discharge capacities of as-deposited, 300, 500, and 700 °C-annealed films were 722.8, 772.5, 868.4, and 1059.9 μAh cm^-2 μm^-1, respectively. High-temperature annealing could enhance the capacity and cycle retention obviously. After 25 cycles discharging, the annealed films showed better cycle retention than as-deposited film. The 700 °C-annealed film exhibited excellent discharge capacity approximated to the theoretical capacity.