Anomalous behaviour in diffusion impedance of intercalation electrodes

Abstract

The non-ideal behaviours of diffusion impedance observed in such intercalation electrodes as hydride-forming alloys and transition metal oxides were theoretically treated in terms of (i) the diffusion length distribution, (ii) diffusion towards and from the fractal surface and (iii) diffusion coupled with the trapping reaction or the homogeneous reaction. Firstly, when the diffusion lengths were widely distributed across the electrode, the absolute value of the phase angle deviated positively from 45 degrees in the high frequency range, and it deviated negatively from 90 degrees in the low frequency range. Secondly, the ac-impedance spectrum for diffusion through the fractal electrode exhibited a power-law behaviour of frequency dispersion with the absolute phase angle between 45 degrees and 90 degrees in the high frequency range, depending on the fractal dimension of the electrode surface. Finally, an arc appeared in the Nyquist plot of the ac-impedance spectrum, in case diffusion was coupled with either the trapping reaction or the homogeneous reaction.