Conductive and electrochemical properties as well as thermal stability of LiaNil-xCoxO2 where x = 0-0.3 and a = 1.0-0.4 have been studied using impedance spectroscopy, galvanostatic intermittent titration technique, cyclic voltammetry, electrochemical cycling, and differential scanning calorimetry (DSC). The electronic conductivity of partially charged material (a less than or equal to 0.8) increases substantially when the cobalt content is increased to x = 0.3 from 0.2. The mobility, chemical diffusion coefficient, and Li+ ion diffusion coefficient in the partially charged material are in the neighborhood of 10(-6) cm(-2)/V s, 10(-8) cm(2)/s, and 5 x 10(-10) cm(2)/s, respectively, and are rather insensitive to the values of a and x. Results of electrochemical and DSC studies show that LiNi0.7Co0.3O2 is the best cathode material for a Li cell of all materials studied here in view of the best cycle stability, the smallest irreversible capacity, and the greatest thermal stability with minimal reduction of specific capacity from that of LiNiO2.