Impedance spectra of porous, composite intercalation electrodes: The origin of the low-frequency semicircles

Impedance spectra of composite, graphite electrodes, which are highly porous with a poor orientation of their particles show well-developed low-frequency semicircles (LFSs) with formally very high characteristic capacitances (about 160 mF cm^-2), which cannot be rationalized on the basis of the porous electrodes models known from the literature. We use herein a new model (recently reported in full detail) considering two (or more) porous sublayers of different thicknesses, which readily reproduces the well-developed LFSs with very high characteristic capacitances (C_LFS in the range from 0.05 to 5 F cm^-2 using typical values of electrodes parameters, calculated from ω_max). A detailed analysis shows that the LFSs characterizing the impedance spectra of highly porous graphite electrodes, have a simple "geometric" meaning. They result from a parallel combination of the intercalation capacitance of the thinnest sublayer and the high, active resistance of the thicker layer.