ESD fabricated thin films of spinel LiMn₂O₄ for lithium microbatteries: I. Effects of thickness

Electrochemical properties of thin films of LiMn₂O₄ spinels of several thicknesses prepared by electrostatic spray deposition (ESD) were studied in 1 M LiClO₄/propylene carbonate solution using cyclic voltammetry and electrochemical impedance spectroscopy. The shapes of the cyclic voltammograms (CVs) is extremely affected by the scan rate. In general, at a constant film thickness, the slower the scan rates the sharper and narrower are the peaks, A progressive decrease in the charge-transfer resistance was always found with raising the film thickness and the applied potential. Both the film thickness and the charge injection process effectively limit the region over which diffusion control is observed. The diffusion control domain shrinks with decreasing film thickness in agreement with the theoretical predictions. The diffusion coefficient of Li ions in the solid phase varied in a similar manner with the potential, irrespective of the film thickness. The diffusion coefficient exhibited a peak-shaped function of the electrode's potential with minima at potentials close to the peak potentials of the CV. Importantly, operating at a constant potential, the chemical diffusion coefficient decreases within the 1-0.5 μm range, and is almost constant for smaller thicknesses.