The cell-impedance-controlled lithium transport through LiMn 2O 4 film electrode with fractal surface by analyses of ac-impedance spectra, potentiostatic current transient and linear sweep voltammogram

Abstract

Lithium transport through the fractal LiMn 2O 4 film electrode under the cell-impedance-controlled constraint was investigated by employing ac-impedance spectroscopy, potentiostatic current transient technique and linear sweep voltammetry. For this purpose, the flat and fractal LiMn 2O 4 film electrodes were prepared on the as-deposited Pt/polished Al 2O 3 substrate and the surface modified Pt/unpolished Al 2O 3 substrate, respectively. From the analysis of the ac-impedance spectra obtained from the flat and fractal electrodes, it is found that the apparent self-similar fractal dimension reduces the charge-transfer resistance, and the phase angle of the diffusion impedance under the semi-infinite diffusion condition positively deviates in absolute value from 45° with increasing fractal dimension. All the potentiostatic current transients experimentally measured from the flat and fractal LiMn 2O 4 electrodes showed non-generalised Cottrell behaviour which resulted from the cell-impedance-controlled constraint during lithium transport. In the case of linear sweep voltammogram theoretically calculated and experimentally measured from the flat and fractal LiMn 2O 4 electrodes, the power dependence of the peak current on the scan rate hardly exhibited the generalised Randles–Sevčik behaviour. From the analyses of the potentiostatic current transients and the linear sweep voltammograms, furthermore, it is experimentally confirmed that the internal cell resistance mainly determining the cell-impedance-controlled lithium transport strongly depends upon the fractal dimension as well as such external parameters as the applied potential step and the amount of lithium transferred during lithium transport.