Impedance spectroscopy was used to investigate the stability and the catalytic activity of the lithium–nickel mixed oxides with high lithium content (Li x Ni1−x O, x=0.30–0.40) in a eutectic melt (Li:K) at 650°C under a corrosive atmosphere (CO2:O2 4:1). The results were compared with a NiO reference cathode material. A modified transmission line model allowed to give a physical-meaning to the impedance spectra. All Li–Ni oxides showed similar catalytic activity and their impedance values were one order of magnitude lower than NiO. During the first 100h of immersion, the samples showed structural changes due to the loss of lithium. Later on, the structure kept stable. The loss of lithium was confirmed by chemical analysis and X-ray diffraction (XRD). All Li–Ni oxide samples reduced the nickel dissolution in the eutectic in one order of magnitude in relation to NiO. In general, similar morphology was observed by scanning electron microscopy (SEM) for the fresh samples. After their immersion, the Li–Ni oxides did not show morphological change except for the sample with x=0.30, for which a reduction of the particle size was observed. NiO presented an important morphological change due to its lithiation in situ.