Li content dependence of thermodynamic stability and the crystal structure of LixMn1-yMyO2(M = Mn,Al,Cu) as a cathode active material for Li secondary battery

We investigated the relationship between the thermodynamic stability and Li content in the crystal structure of the orthorhombic LiMn1-xMxO2 (M = Mn, Al, Cu). The enthalpy change for mol of atoms for the reaction, Delta H-R, were calculated from the heat of dissolution. Delta H-R increased with the decreasing 1:1 content except for the two-phase region and significantly increased the two-phase region for each sample. For the same Li content, the LixMnO2 sample obtained by reflux heating was more thermodynamically stable than the other samples. Furthermore, the LixMn0.95Al0.05O2 sample was unstable, and the LixMn0.95Cu0.05O2 sample was more stable than that of LixMnO2,which was obtained by the same method. The crystal structure of LixMn1-yMyO2 (M = Mn, Al) was analyzed by the neutron powder diffraction method. The Madelung energy and bond lengths of each sample were calculated using the Rietveld analysis. The Madelung energy of the spinel phase was low, and the spinel phase was more stable than the orthorhombic phase. The distributions of the nuclear density and electron density were calculated by MEM in order to determine the bonding state. The nuclear density decreased with the decreasing Li content. In o-LiMn0.95M0.05O2 (M = Al, Cu), the Li localization and strong covalence of (Mn,M)-O were estimated from the distribution of the electron densities.