Property, Electronic and Crystal Structures, Thermodynamic Stability, and Cathode Performance of Li_x(Mn, Co, Ni, M)O₂ (M=Al, Ti, Fe) as a Cathode Active Material for Li Secondary Battery

Abstract

In order to investigate the property, electronic and crystal structures, thermodynamic stability, and cathode performance of Li_x(Mn, Co, Ni, M)O₂ (M=Al, Ti, Fe) as a cathode active material for Li secondary batteries, LiMn_0.3Co_0.3Ni_0.3M_0.1O₂ (M=Al, Ti, Fe) and LiMn_1/3Co_1/3-0.1Ni_1/3Al_0.1O₂ were prepared by the solution method and solid-state method. These materials have the same layered structure (R¯3m) as the LiMn_1/3Co_1/3Ni_1/3O₂. Li_x(Mn, Co, Ni, Al)O₂, 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, ΔH_R, was calculated from the heat of dissolution. ΔH_R increased with the decreasing Li content and Li_xMn_1/3Co_1/3-0.1Ni_1/3Al_0.1O₂, which was obtained by the solution method, was more thermodynamically stable than that obtained by the solid-state method irrespective of the Li content. Li_xMn_1/3Co_1/3Ni_1/3O₂ is more thermodynamically stable than Li_x(Mn, Co, Ni, Al)O₂, which was obtained by the same synthesis method. It was suggested that the structural and thermodynamic stability should provide an effective cycle performance.