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The phase relationship between LiNiO₂ and its related compounds was examined by X-ray diffraction, neutron diffraction, DTA-TG, and electrical, magnetic, and electrochemical measurements. Decomposition of LiNiO₂ proceeded at higher temperatures to Li_1-xNi_1+xO₂ with the partially disordered α-NaFeO₂ structure and Li_xNi_1-xO with the fully disordered rock-salt structure. Higher oxygen partial pressure in the atmosphere suppressed the decomposition reaction, which was considered to be the reduction from Ni³⁺ to Ni²⁺ state. The nearly stoichiometric "LiNiO₂" was synthesized at reaction temperatures below 700°C in an oxygen gas flow with the starting materials Li₂O₂ and NiO. Neutron diffraction measurement confirmed the composition Li_0.996Ni_1.006O₂, which is very close to the stoichiometry. The deintercalation mechanism of LiNiO₂ and the spinel transformation process from Li_1-xNiO₂ to Li_yNi₂O₄ …