Synthesis And Characterization Of Manganese Based Cathode Materials For Lithium Ion Batteries

Spinel LiMn2O4 is regarded as one of the most promising cathodematerials for lithium ion batteries due to its stability, high voltage, lowcost and environmental advantageous. In this thesis, we gave asystematical study on the spinel LiMn2O4 and LiNixMn2-xO4 materials.The synthesis technique, structural characterization, electronicstructure and the conductive properties of the materials are reported. The spinel LiMn2O4 was synthesized by a simple wet chemicalmethod, drying emulsion, using lithium nitrate [LiNO3] and manganeseacetate [Mn(CH3COO)2.4H2O] as raw materials. Differential thermalanalysis (DTA) and thermogravimetric analysis (TGA) analysis of theprecursor showed the spinel LiMn2O4 can be obtained at a temperaturerange from 300℃~900℃. Well-crystallized spinel LiMnO4 were obtainedin the temperature range from 500℃ to 800℃ which was confirmed byX-ray diffraction (XRD) study. Moreover, the lattice constants of thematerials increased with the sinter temperature. The morphology of thematerials was obtained by SEM, which showed the samples undergo acrystallizing process from loose to aggregate, and then finally towell-ordered crystalline grains with the increasing of sinteringtemperature. The sample sintered at 800℃ has a uniform particle sizeabout 0.5μm, and the grains show an fine polyhedral shape and almostconnected each other. XRD study showed that the crystal structure of LiMn2O4 was almostindependent of the sintering time, so sintering temperature is the mainfactor for material preparation. Furthermore, it was found out by XRDanalysis that the lattice constant of nonstoichiometric spinelLi1+ Mn2- O decreased with lithium content, due to the larger Coulomb δ δ 4interaction of Mn4+-O than that of Mn3+-O. Jahn-Teller effect of LiMn2O4 near 280K causes its poorelectrochemical performance. An insight study of the electronicstructure of LiMn2O4 can reveal the nature mechanism of Jahn-Telleffect, which is more helpful for the improving of the cycling properties ofthe material. According to the crystal-field theory, the orbits of the 3delectrons from Mn will split into four levels in the elongated octahedronunder Jahn-Teller distortion. The energy gaps, ?E(b1g – a1g) and ?E(b2g– eg), characterize strength of the Jahn-Teller effect in the crystal andreflect the degree of Jahn-Teller distortion of [MnO6] octahedron.Mossbauer spectroscopy is an efficient tool for studying the hyperfinestructure of materials. We can obtain more information about the crystalfield of [MnO6] octahedron using this technique, and get a deepperunderstanding about the Jahn-Teller effect in this material. Since the ionic radius of Fe3+ in six-fold octahedron coordination is0.645?, almost as same as that of Mn3+ in the same site, raresubstitutions of Fe3+ for Mn3+ in the spinel LiMn2O4 do not change thelocal 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 usingfew Fe3+ ions as the probe ion. Then we established the relationshipbetween the Mossbauer quadrupole splitting and the Mn-3d electronicstructure. Using this relationship, we calculated the energy gaps, ?E(b1g– a1g) and ?E(b2g – eg), being 0.41ev and 0.30ev respectively, which isclose to those calculated by the first principle calculations. The firstprinciple calculations give us a theoretical base for the design ofmaterials, and the Mossbauer spectroscopy may be also useful in thestudy of the electronic structure for other manganese-based oxides. Substitutes of Mn by some other metal ions such as Al, Cr, Ti, Co,Ni et al. can improve the electrochemical performance of LiMn2O4. Weprepared nickel-doped LiMn2O4 (x=0, 0.1…0.5) using drying emulsionmethod and studied their structural and physical properties using variesof techniques. XRD study showed pure spinel LiNix...