On electrochemical impedance measurements of Li_xCo_0.2Ni_0.8O₂ and Li_xNiO₂ intercalation electrodes

Nyquist plots measured from thin Li_xCo_0.2Ni_0.8O₂ electrodes at different potentials in Li-salt solutions include two semicircles, one relating to the high-frequency domain and the other referring to the medium-to-low frequency range. Based on fine features of the dependence of the diameter of these two semicircles on the potential (R_sl and R_ct versus E), and the estimated values of related capacities, we assigned these semicircles to (i) Li ion migration through a surface layer covering the active mass particles and (ii) interfacial charge-transfer, respectively. We have applied an approach based on a simple Frumkin-type sorption isotherm in order to explain the experimental R_ct versus E relationship. The absolute values of the double-layer capacity (several mF per 1 cm² of visible surface area) can be rationalized in terms of the porous structure of the composite electrodes and assuming an increase in surface area and appearance of disorder on the surface layer|particle interface after the electrodes cycling. Nyquist plots of these electrodes can be successfully modeled by a combination of two (RC) semicircles with the so-called Frumkin and Melik-Gaykazyan impedance that provides a good fit to the experimental plots measured down to a mHz range. Only minor differences in the shape of the impedance spectra measured from Li_xCo_0.2Ni_0.8O₂ and Li_xNiO₂ electrodes could be observed. The former electrode seems to undergo a somewhat more rapid degradation at high anodic polarization than the latter, as follows from both cyclic voltammetry and electrochemical impedance measurements. However, this more rapid degradation in capacity is not related to interfacial changes, but rather to changes in the bulk material.