Low-temperature synthesis of LiNiO₂: Reaction mechanism, stability, and electrochemical properties

A thorough study was made concerning the formation of LiNiO₂ by a low-temperature ion-exchange reaction under hydrothermal conditions. LiNiO₂ with a low degree of cationic mixing and good electrochemical performance was prepared either from pure β-NiOOH, γ-NiOOH, or a mixture of both where the γ phase was the majority. The synthesized product (LT-LiNiO₂) presents electrochemical properties similar to LiNiO₂ prepared by conventional powder synthesis (HT-LiNiO₂) in spite of the difference in Brunauer, Emmett, and Teller (BET) surfaces, around 20 m²/g for LT-LiNiO₂ and 0.7 m²/g for HT-LiNiO₂. The thermal and moisture stabilities of LT-LiNiO₂ were studied and turned out to be much lower than those of HT-LiNiO₂. This moisture instability was found (i) to be intrinsic to LiNiO₂, since ground HT-LiNiO₂ samples presented a similar behavior, (ii) to be enhanced by decreasing the LiNiO₂ particle size, and (iii) to be much larger than that of LiCoO₂. In view of electrochemical applications, cobalt-substituted products should be used in order to ensure the stability against both reduction-delithiation and hydrolysis.