Estimation and the Material Design for Functional Oxides by Thermodynamic Measurement

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LiMn_2O_4 has an advantage of low cost compared with LiCoO_2 and it pays attention as a positive electrode active material for the Lithium secondary battery, which has high charge-discharge potential. The relation between the structure and the electrode characteristics from the viewpoint of thermodynamics was investigated. First, the samples was synthesized of the Li rich spinel, Li_<1.33>Mn_<1.67>O_4, the defect spinel, Li_<0.89>Mn_<1.78>O_4, and substitution spinel to partially replace Mn by Al, LiMn_<2-x>Al_xO_4. It was investigated the relation between property, structure, and ode performance of the samples. Furthermore, the standard enthalpy of formation, Δ_1H^0, and the enthalpy change of reaction, ΔH, were calculated form the heat of dissolution. Thermodynamic stability was examined by these parameters. The Mn valence and lattice parameter a changed with change of structure and metal contents. The crystal structure analysis by neutron diffraction of LiMn_<1.8>Al_<0.2>O_4 and Li_ … More <1.1>Mn_<1.7>Al_<0.2>O_4 was examined. As a result, the bond distance of (Mn, Li, Al)-O decreased and absolute value of the lattice energy (Fourier method) increased with increasing Li and Al. Moreover, the first principle calculation (DV-Xα) method was investigated. From the results, covalent bond of the Mn-O bond was stronger than the Li-O bond. Furthermore, the heats of dissolution of the samples were measured by the solution calorimetry method. When ΔA_fH^0 and ΔH decreased, it brought to a good cycle performance. On the other hand, a change in lattice energy before and after the cell reaction were calculated. From the results, when thermodynamic stability of a positive electrode active material increased and the change of lattice energy decreased, it brought good cycle performance.Accordingly, it is suggested that the stability of the host structure and the existence of Li, which is ionic state in the positive electrode active material, brought to improvement for the cycle performance. Less