On electrochemical impedance measurements of LixCo0.2Ni0.8O2 and LixNiO2 intercalation electrodes

Nyquist plots measured from thin LixCo0.2Ni0.8O2 electrodes at different po tentials in Li-salt solutions include two semicircles, one relating to the high-frequency domain and the other referring to the medium-to-low frequenc y 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 estimate d 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 ex perimental R-ct versus E relationship. The absolute values of the double-la yer capacity (several mF per 1 cm(2) of visible surface area) can be ration alized in terms of the porous structure of the composite electrodes and ass uming an increase in surface area and appearance of disorder on the surface layer I particle interface after the electrodes cycling. Nyquist plots of these electrodes can be successfully modeled by a combination of two (RC) s emicircles with the so-called Frumkin and Melik-Gaykazyan impedance that pr ovides a good fit to the experimental plots measured down to a mHz range. O nly minor differences in the shape of the impedance spectra measured from L ixCo0.2Ni0.8O2 and LixNiO2 electrodes could be observed. The former electro de seems to undergo a somewhat more rapid degradation at high anodic polari zation than the latter, as follows from both cyclic voltammetry and electro chemical impedance measurements. However, this more rapid degradation in ca pacity is not related to interfacial changes, but rather to changes in the bulk material. (C) 2000 Elsevier Science Ltd. All rights reserved.