Electrical conductivity and Li-6,Li-7 NMR studies of Li1+yCoO2

The battery cathode material LiCoO2 was synthesized with a deliberate excess of Li, according to Li1+yCoO2, where y = 0.08 and 0.35 (nominally). The effect of divalent doping with Mg2+ was also explored for some samples, with y values of 0.0 (stoichiometric) and 0.08. Electrical conductivity measurements of the stoichiometric material, without Mg, as functions of oxygen partial pressure and temperature exhibit p-type semiconducting behaviour and suggest that the defects primarily responsible for the generation of holes are cobalt ion vacancies. Excess Li increases the electrical conductivity, while the incorporation of Mg leads to a more dramatic enhancement in conductivity, the latter interpreted as a transition to metallic behaviour. NMR spectroscopic measurements of both Li-6,Li-7 isotopes suggest that only a small fraction (< 20%) of the excess Li in the y = 0.35 material enters the structure ionically while reducing the formal Co valence. Most of the excess consists of Li2CO3 and possibly other impurity phases, the former also having been identified by X-ray diffraction. Another small portion of the excess Li (about 10%) appears to enter interstitial sites in close proximity to paramagnetic Co2+ ions.