XRD analysis, IR spectroscopy, magnetic susceptibility measurements and thermal analysis are used to investigate the effect of Co substitution on the Ni3+ content and long-range order in LixNi2-xO2 (0.7 < x < 1). Kinetic factors influence the lithium amount in Lix(Co1-yNiy)2-xO2 (0.8 less-than-or-equal-to y less-than-or-equal-to 1) and are associated with the competition between the solid state reaction and the volatility of the unreacted lithium salt: Co-dopants in NiO enhance the reactivity of the oxide towards lithium salts (LiNO3), as a result of which the lithium amount (the M3+ content, correspondingly) in the reaction product Lix(Co1-yNiy)2-xO2 increases from x almost-equal-to 0.65 to x almost-equal-to 0.92. It has been proved that the nonstoichiometry of Lix(Co1-yNiy)2-xO2 with respect to lithium is associated with the stabilization of Ni2+ ions but not of Co2+ ions. Further enhancement of the cobalt amount (0 less-than-or-equal-to y < 0.8) results in the formation of ''ideal'' Lix(Co1-yNiy)2-xO2 solid solutions with nearly stoichiometric composition (0.96 < x less-than-or-equal-to 1). Moreover, Co-substitution in lithium-nickel oxide over the whole concentration range (0 less-than-or-equal-to y less-than-or-equal-to 1) stabilizes the layered crystal structure, which is manifested by the increase of the trigonal distortion of the cubic lattice. The shift of the IR bands with the contraction of the unit cell during cobalt substitution confirms the formation of mixed nickel-cobalt layers.
