Toward Positive Electrode Materials with High-Energy Density: Electrochemical and Structural Studies on LiCo Mn_2- O₄...

To obtain positive electrode materials with higher energy densities (Ws), we performed systematic structural and electrochemical analyses for LiCo Mn_2- O₄ (LCMO) with 0 ≤ x ≤ 1. X-ray diffraction measurements and Raman spectroscopy clarified that the samples with x ≤ 0.5 are in the single-phase of a spinel structure with the Fd3̅m space group, whereas the samples with x ≥ 0.75 are in a mixture of the spinel-phase and Li₂MnO₃ phase with the C2/m space group. The x-dependence of the discharge capacity (Q _dis) indicated a broad maximum at x = 0.5, although the average operating voltage (E _ave) monotonically increased with x. Thus, the W value obtained by Q _dis × E _ave reached the maximum (=627 mW h·g^-1) at x = 0.5, which is greater than that for Li[Ni_1/2Mn_3/2]O₄. Furthermore, the change in the lattice volume (ΔV) during charge and discharge reactions approached 0%, that is, zero-strain, at x = 1. Because ΔV for x = 0.5 was smaller than that for Li[Ni_1/2Mn_3/2]O₄, the x = 0.5 sample is found to be an alternative positive electrode material for Li[Ni_1/2Mn_3/2]O₄ with a high W.