Reaction mechanisms of MnMoO4 for high capacity anode material of Li secondary battery
Introduction
The Li-ion rechargeable batteries are considered as the most suitable power sources for portable electronic devices due to their high capacity and energy density. Generally, Li-ion rechargeable batteries consist of intercalation compounds for both cathode and anode electrode materials. One is a lithiated transition metal oxide as the cathode and the other is graphite as the anode. However, the graphite anode material commonly used in Li-ion rechargeable batteries suffers from small capacity per unit weight (about 350 mA h/g) and/or per unit volume due to its low density in spite of its low redox potential and good cycle life. Furthermore, due to the diffusivity, the rate capability of graphite material also needs improvement. To overcome these disadvantages, considerable amounts of attempts have been made to find out alternative anode materials, including metal oxide (MO, M=Co, Ni, Fe) [1], tin-based material [2], vanadium-based oxide materials [3], [4], in place of graphite anodes. Notably, vanadium-based oxides as active materials for Li secondary battery have been studied, since the vanadium oxides have interesting characteristics from a standpoint of variety of oxidation states. Recently, several researchers including our group have described the low potential Li-insertion behavior in vanadium-based oxides such as RVO4 (R=In, Cr, Fe, Al, Y) [3] and MnV2O6 [4]. Molybdenum oxides should be attractive as anode material because they also have various oxidation states like vanadium. Previously, the molybdenum oxide MoO2 as anode material of the lithium rocking chair battery was proposed by Auborn and Baberio [5] more than 10 years ago, but their study became limited by experimental conditions such as poor stability of the electrolyte at low potential.
In this study, we synthesized molybdenum-based oxide MnMoO4 as a new anode material and described the lithium insertion/removal behavior at low potential. The XRD measurement and X-ray absorption study of O K-edge, Mn and Mo L-edge have provided insight on the structural transformation and electrode reaction mechanism.
Section snippets
Experimental
MnMoO4 powder sample was prepared by conventional solid reaction methods. Starting material used was MnCO3 (99.9% Soekawa chemicals) and MoO3 (99.9% Soekawa). These reagents were mixed in stoichiometric ratio in agate mortar and the mixture was heat-treated at 600 °C for 24 h in air atmosphere. From here, we could get crystalline MnMoO4 powdered sample. The phase identification was carried out by powder X-ray diffractometry using Rigaku RINT2500V with CuKα radiation. The samples for the
Results and discussion
The crystal structure of the synthesized powder was examined by X-ray diffractometry analysis. The well-defined peak obtained confirms that the synthesized compound is MnMoO4 without any impurity phases and the JCPDS data (card number 27-1280) also provides the exact match. Fig. 1 shows the measured powder diffraction profile for MnMoO4. The schematic diagram of MnMoO4 structure is shown in Fig. 2. This structure has been known as alpha-MnMoO4, which is composed of octahedrally coordinated Mn
Conclusions
We reported the electrochemical properties of MnMoO4 as a new type of anode material for lithium secondary battery. This high capacity anode, realized during the first lithiation (∼1800 A h/kg) and in the subsequent lithiation (∼1000 A h/kg), could be attributed to the oxygen contribution to lithium insertion by the accommodation of electrons in the hybridization orbital. During the first lithiation, amorphization, which has an intermediate state of NaCl structure, was observed in XRD
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