Synthesis And Characterization Of LiMn₂O₄ As Cathode Materials For Lithium Ion Batteries

Spinel LiMn₂O₄ is regarded as one of the most promising cathode materials for lithium ion batteries due to its stability, high voltage, low cost and environmental advantageous. In this thesis, we gave a study on the spinel LiMn₂O₄ and LiFexMn_2-xO₄ materials. The synthesis technique, structural characterization, electronic structure and the conductive properties of the materials are reported. The spinel LiMn₂O₄ was synthesized by a simple wet chemical method, drying emulsion, using lithium nitrate [LiNO₃] and manganese acetate [Mn(CH₃COO)₂.4H₂O] as raw materials. Well-crystallized spinel LiMn₂O₄ were obtained in the temperature range from 500℃to 800℃which was confirmed by X-ray diffraction (XRD) study. Moreover, the lattice constants of the materials increased with the sinter temperature. XRD study showed that the crystal structure of LiMn₂O₄ was almost independent of the sintering time, so sintering temperature is the main factor for material preparation. Jahn-Teller effect of LiMn₂O₄ near 280K causes its poor electrochemical performance. An insight study of the electronic structure of LiMn₂O₄ can reveal the nature mechanism of Jahn-Tell effect, which is more helpful for the improving of the cycling properties of the material. According to the crystal-field theory, the orbits of the 3d electrons from Mn will split into four levels in the elongated octahedron under Jahn-Teller distortion. The energy gaps, ?E(b1g –a1g) and ?E(b2g –eg), characterize strength of the Jahn-Teller effect in the crystal and reflect the degree of Jahn-Teller distortion of [MnO6] octahedron. Mossbauer spectroscopy is an efficient tool for studying the hyperfine structure of materials. We can obtain more information about the crystal field of [MnO6] octahedron using this technique, and get a deepper understanding about the Jahn-Teller effect in this material. Since the ionic radius of Fe3+ in six-fold octahedron coordination is 0.645?, almost as same as that of Mn3+ in the same site, rare substitutions of Fe3+ for Mn3+ in the spinel LiMn2O4 do not change the local structure much, i.e., the information obtained from the light doped [(Fe, Mn)O6] octahedron reflects the nature of the local structure of [MnO6]. We got the Mossbauer spectroscopy of LiFe0.1Mn1.9O4 using few Fe3+ ions as the probe ion. Then we established the relationship between the Mossbauer quadrupole splitting and the Mn-3d electronic structure. Using this relationship, we calculated the energy gaps ?E(b1g –a1g) and ?E(b2g –eg) in different temperature. And we obtained 0.44eV and 0.33eV when T=80K and 0.42 eV and 0.31 eV when T=295K respectively. The Mossbauer spectroscopy may be useful in the study of the electronic structure for other manganese-based oxides. To summarize, both LiMn2O4 and LiFexMn2-xO4 were synthesized using drying emulsion method. A systematical study of the electronic properties and crystal structure of these materials were carried out by...