Synthesis and Characterization of Lithium Cobalt Manganese Oxides (LiCoO2-Li2MnO3) As Positive Electrode Material for Lithium-Ion Batteries

Abstract

The lithium-rich materials of LiMO2-Li2MnO3 (M : Co, Mn, etc.) are one of the most attractive positive electrode materials for high-capacity lithium-ion batteries, because the materials show the largest rechargeable capacity of more than 300 mAhg-1 among lithium insertion materials reported so far. The lithium-rich materials show unique behavior in charge-discharge curves. For example, voltage plateau at 4.5 V at the initial charge, large voltage hysteresis at the subsequent cycles, and change in voltage profiles during the cycling. Such anomalous behavior cannot be seen for conventional lithium insertion materials of LiMO2 with layered structure and LiM2O4 with spinel structure. In this study we prepared and characterized lithium cobalt manganese oxides of LiCoO2-Li2MnO3 to understand a origin of anomalous behavior observed in the lithium-rich materials. A series of LiCoO2-Li2MnO3 materials were prepared by a solid-state method in a molar ratio of LiCoO2 : Li2MnO3 = 3 : 1, 5 : 3, 1 : 1, and 1 : 3. X-ray diffraction (XRD) patterns indicate all samples have layered structure although crystal systems are different, a monoclinic lattice for the Mn-rich sample (LiCoO2 : Li2MnO3 = 1 : 3) and a hexagonal lattice for Co-rich samples (LiCoO2 : Li2MnO3 = 3 : 1 and 5 : 3). Superlattice lines caused by cation ordering between lithium ions and transition metal ions were observed in the XRD pattern of the Mn-rich sample. In order to compare crystal structures of a series of LiCoO2-Li2MnO3 materials, a hexagonal lattice for the Co-rich samples was reduced to a monoclinic lattice and hexagonal lattice parameters were converted to monoclinic ones. Unit cell volume of the LiCoO2-Li2MnO3 materials was calculated by using monoclinic lattice parameters decreased monotonously with increasing cobalt contents, indicating that the LiCoO2-Li2MnO3 materials show a characteristic of solid solution. In order to investigate electrochemical behavior of the LiCoO2-Li2MnO3 materials, charge and discharge tests were carried out by employing Li[Li1/3Ti5/3]O4 (LTO) as a negative electrode.at a constant current. The cells were cycled in the voltage range in 0.5 – 3.3 V vs. LTO, corresponding to ca. 2.0 – 4.8 V vs. Li, at a current density of 0.25 mAcm-2. During the cycling, capacity of a Co-rich material faded gradually while that of the Mn-rich material was increased. The capacity of the LiCoO2-Li2MnO3 material with 1:1 ratio of LiCoO2 : Li2MnO3 maintained capacity to be 132 mAhg-1 after 250 cycles. Significant difference among LiCoO2-Li2MnO3 materials were observed in a change in voltage profiles during the cycling. In order to show solid-state redox potentials in the LiCoO2-Li2MnO3 materials, differential chronopotentiograms (dQ/dE plot) for each sample taken at every 10 cycles were calculated (Figure). In the LiCoO2-Li2MnO3 material with 3:1 ratio of LiCoO2 : Li2MnO3, intensity of the solid-state redox peaks became weak cycle by cycle, indicating the material deteriorated without any change in the solid-state redox reactions. In contrast, solid-state redox potentials were dramatically changed for the Mn-rich material. New oxidation peaks were appeared at about 1.4 and 2.3 V vs. LTO and a reduction peak at about 1.8 V vs. LTO was shifted to lower potential. From these results combined with ex-situ XRD examinations after cycle tests, we will discuss lithium insertion mechanism of a series of LiCoO2-Li2MnO3 materials with emphasis on change in solid-state redox potentials. Figure 1