Property, electronic and crystal structures, thermodynamic stability, and cathode performance of Lix (Mn, Co, Ni, M) O2 (M=Al, Ti, Fe) as a cathode active material for Li secondary battery

In order to investigate the property, electronic and crystal structures, thermodynamic stability, and cathode performance of Li-x (Mn, Co, Ni, M) O-2 (M=Al, Ti, Fe) as a cathode active material for Li secondary batteries, LiMn0.3Co0.3Ni0.3M0.1O2 (M=Al, Ti, Fe) and LiMn1/3Co1/3-0.1Ni1/3Al0.1O2 were prepared by the solution method and solid-state method. These materials have the same layered structure (R (3) over barm) as the LiMn1/3Co1/3Ni1/3O2. Li-x (Mn, Co, Ni, Al) O-2, which was obtained by the solution method, had a higher discharge capacity and better cycle performance than that obtained by the solid-state method. Based on the results of the electronic and crystal structures, the change in the covalent bond and cation mixing were small by substitution. The enthalpy change per mol of atoms for the reaction, Delta H-R, was calculated from the heat of dissolution. Delta H-R increased with the decreasing Li content and LixMn1/3Co1/3-0.1Ni1/3Al0.1O2, which was obtained by the solution method, was more thermodynamically stable than that obtained by the solid-state method irrespective of the Li content. LixMn1/3Co1/3Ni1/3O2 is more thermodynamically stable than Lix (Mn, Co, Ni, Al) 02, which was obtained by the same synthesis method. It was suggested that the structural and thermodynamic stability should provide an effective cycle performance.