| Research Abstract |
We investigated the relation between thermodynamic stability, crystal structures, electronic states and electrode performance of LiMn_<2-x>M_xO_4(M=Al, Ni, Co, Cr, Zn, Mg)as cathode materials for lithium secondary batteries.The cycle performance is improved by substitution of Mn with M.The heat of dissolution of these samples was measured by the solution calorimetry method.The enthalpy change per mole of atoms for the formation reaction, ΔH_R, were calculated from the heat of dissolution.The larger |ΔH_R| means a thermodynamically more stable sample.A more thermodynamically stable sample makes a stable crystal structure and leads to a good cycle performance.The crystal structure was determined by Rietveld analysis using powder neutron diffraction.The electron density distribution was obtained by XRD using MEM/Rietveld method and DV-Xα method.The electronic density maps of LiMn_<2-x>M_xO_4 shows the covalent bonding of Mn-O of LiMn_<2-x>M_xO_4 is stronger than that of LiMn2O4.On the other hands, we calculated that the net charge of each atom, the bond overlap population of Li-O, Mn-O, M-O, the density of states, and the electron density of LiMn_<1.75>M_<0.25>O_4 using first principles calculation by DV-Xα method.Li ****y always keeps high and the covalent bonding of Mn-O in the octahedron of each LiMn_<1.75>M_<0.25>O_4 is stronger than that of LiMn_2O_4.****ver, we investigated the electronic states of Li_yMn_<2-x>M_xO_4 dependence of Li content.We calculated the electronic states of Li_yMn_<1.75>M_<0.25>O_4 by DV-Xα method.The net charge of each atom, and the bond overlap populations of Mn-O and M-O were calculated.The strong covalency between Mn and 0 existed in the Li defect model and Li_yMn_<2-x>M_xO_4.The bonding change in Mn-O is small and maintained the spinel structure during the charge-discharge process by substitution of Mn with M.From these results, the stability of the host structure is important for cycle performance.
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