The electrochemical characteristics of Li1−x Ni1+x O2 samples have been extensively investigated as a function of the lithium deficiency x in the narrow composition range 0.008–0.065. In particular, the electrochemical and structural behaviours of the quasi-stoichiometric compound (x=0.008) have been proved to be strongly different from those exhibited by a sample with x=0.025. The faster kinetics for Li transport combined with structural changes which are minimized in the quasi-ideal compound explain the lack of notable polarization voltage and the good cycle life of the latter cathode material with a stable capacity of 140 A h/kg over 40 cycles at C/10 rate. In addition, interesting and useful correlations (i) between the cathode impedance and the lithium deficiency and (ii) between the evolution of the cathode impedance and the cycling performance have been established.