Electronic and electrochemical properties of LixNi1-yCoyO2 cathodes studied by impedance spectroscopy

An electrochemical impedance spectroscopy (EIS) study of three members of the family of compounds LixNi1-yCoyO2 (y = 0, 0.2, and 1) has been performed by taking spectra at closely spaced bias potentials over the potential windows of utilization as cathodes in Lithium-ion batteries. The spectra have been interpreted in terms of electronic and ionic transport properties. It has been found. that, for x values greater than about 0.9, all the compounds show semiconductive properties with a specific resistance that increases with decreasing Ni content. For x close to about 0.9-0.8, the properties change into those of a metal-like material in which the ionic conductivity becomes the limiting factor. The transition between these two limiting conditions clearly appears in the impedance spectra.