An investigation of cell-impedance-controlled lithium transport through LiCoO2/Li1−δMn2O4 bilayer film electrode prepared by rf magnetron sputtering

Abstract

Lithium transport through LiCoO2/Li1−δMn2O4 bilayer film electrode prepared by radio-frequency (rf) magnetron sputtering was investigated in a 1M solution of LiClO4 in propylene carbonate. From the analyses of the AC-impedance spectra experimentally measured from the Li1−δMn2O4 single-layer and LiCoO2/Li1−δMn2O4 bilayer film specimens, the internal cell resistance of the LiCoO2/Li1−δMn2O4 bilayer film electrode was determined to be smaller in value than that of the Li1−δMn2O4 single-layer film electrode over the whole potential range, which can be accounted for by the kinetic facility for the interfacial charge-transfer reaction in the presence of the more conductive LiCoO2 surface film. Moreover, from the analyses of the anodic current transients measured from both the film specimens, it was suggested that the cell-impedance-controlled constraint at the electrode surface is changed to the diffusion-controlled constraint simultaneously characterised by the large potential step and the small amount of lithium transferred during lithium transport. In addition, in the case of the LiCoO2/Li1−δMn2O4 bilayer film electrode, it was found that the critical value of the applied potential step needed for the mechanism transition is reduced, which strongly indicates that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport. Furthermore, from the comparison of the cathodic current transients measured on the Li1−δMn2O4 single-layer film specimens with various thicknesses, it was experimentally verified that the diffusion resistance is explicitly distinguished from the internal cell resistance.