The synthesis, characterization and electrochemical behavior of the layered LiNi_0.4Mn_0.4Co_0.2O₂ compound

The mixed transition metal layered compound, LiNi_0.4Mn_0.4Co_0.2O₂, with the α-NaFeO₂ layer structure has been synthesized and characterized. The optimum temperature of synthesis was found to be 800–900 °C. Rietveld refinement showed that cobalt suppresses transition metal ion migration into the Li sites whereas nickel promotes the migration. XPS analysis shows that the Co and about 20% of the Ni and Mn are in the 3+ oxidation state, while 80% of the Ni and Mn are in the 2+ and 4+ oxidation states, respectively. LiNi_0.4Mn_0.4Co_0.2O₂ shows Curie–Weiss paramagnetic behavior above 150 K, and the value of the Curie constant is consistent with the above oxidation states. In lithium electrochemical cells the composition LiNi_0.4Mn_0.4Co_0.2O₂ gave the highest reversible capacity among the studied compositions. It shows excellent rate capability, giving reversible capacities ranging from 180 to 155 mA h g⁻¹ at current densities from 0.1 to 2.0 mA cm⁻².