Electrochemically active LiCoO₂ and LiNiO₂ made by cationic exchange under hydrothermal conditions

The layered LiMO₂ (M = Co, Ni) compounds, which are of potential interest for Li-ion batteries, were synthesized at low temperatures by treatment under hydrothermal conditions of LiOH · H₂O aqueous solutions containing powdered H_xMO₂ phases. We studied the reaction mechanism and the influence of temperature, pressure, water dilution, and precursor ratio on the degree of progress of the ion exchange process. Single-phase LiMO₂ can be obtained in 48 h at 160°C under an air pressure of 60 bars from an MOOH/LiOH · H₂O/H₂O mixture. The degree of advancement of the exchange reaction for M = Co was monitored in situ using an autoclave which allows the withdrawal of samples in the course of the reaction. From transmission electron microscopy coupled with x-ray diffraction studies we conclude that the reaction occurs by surface H⁺/Li⁺ exchange and is accompanied by a progressive breaking of the particles due to an interfacial collapse phenomenon. Infrared studies indicate that the LiCoO₂ and LiNiO₂ phases obtained are contaminated by carbonates that can more easily be eliminated in the case of LiCoO₂ by water washing and post-heating treatments under primary vacuum at 200°C for 2 days. Once the ion-exchange parameters are controlled, the LiMO₂ products exhibit electrochemical performances comparable to those of high-temperature made phases.