Structure, Morphology, and Electrochemical Investigation of LiMn₂O₄ Thin Film Cathodes Deposited by Radio Frequency Sputtering for Lithium Microbatteries

Spinel lithium manganese oxide (LiMn 2 O 4 ) cathode thin films were successfully deposited on different substrates such as Si, Pt/Al, and indium tin oxide (ITO)/Pt/Al by radio frequency (RF) sputtering. These thin films were investigated as positive electrodes for lithium microbatteries. The thickness of the deposited films was estimated by surface-profilometer at different deposition time intervals from 30 min to 15 h, and the thickness was found to be 20−600 nm. The structure and surface morphology deposited thin films of LiMn 2 O 4 were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The phase transformation from amorphous to crystalline phase was found at around 700 °C for the electrode films deposited on Si substrate. The LiMn 2 O 4 deposited with ITO interlayer on Pt/Al substrate was found to be well crystallized relatively at lower temperature (≤300 °C). No significant changes were observed on the surface morphology between the as-deposited and the annealed film. Electrochemical performance was measured for LiMn 2 O 4 films deposited with or without ITO interlayer on Pt/Al substrate. The discharge capacity around 60.9 and 67.5 mAh g −1 are obtained, respectively, for the 500 °C-annealed LiMn 2 O 4 /Pt/Al film and as-deposited LiMn 2 O 4 /ITO/Pt/Al film. Because the crystallization temperature of LiMn 2 O 4 /ITO/Pt/Al film was found to be relatively lower (≤300 °C), this work could possibly be extended to design and fabricate flexible lithium microbatteries using polymer substrates.