LiNi x Co1−x O2 solid solutions are synthesised and their electrochemical performance as cathodes is examined in lithium cells. Through DTA/TG analysis and self-discharge tests, it is found that both the thermal stability at high temperature and the electrochemical stability in the fully-charged state for LiNi x Co1−x O2 increases with increasing content of cobalt in the compound. The average operating potentials for the solid solutions are lower than those for pure LiCoO2. This indicates that both cobalt and nickel take part in the redox reactions during lithium insertion and extraction. The substitution of Ni3+ for Co3+ in LiNi x Co1−x O2 can improve the specific capacity, but with a small sacrifice in cycleability.