| Lithium ion batteries have been considered as the best energy storage and conversion devices due to their high specific capacity, high energy density, high working potential and environmental advantageous. The cathode material is a key part of lithium ion batteries and has been intensively studied for over two decades, Among numorous cathode materials under consideration, layered Li2MnO3-like compounds have been considered as promising ones. In this study, a typical Li2MnO3-like cathode material, Li[Li0.23Co0.3Mn0.47]O2 was prepared by a simple citric acid assised sol-gel method. The structure and elctrochemical properties of the material were detailedly studied by various techniques. The major conclusions of this work are summaried as below.(1) Synthesis of the material. The Li[Li0.23Co0.3Mn0.47]O2 material was studied by a simple citric acid method using LiCH3COO·2H2O, Co(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O as the raw material. The entire synthetic route was composed of the preparation of sol solution, formation of the gelation, followed by obtaining of the precursor material. Then the precursor material was heat treated at 900 oC for 15 h, followed by quickly quenching in N2 liquild to get the target product.(2) Structure of the material. X-ray diffraction showed that Li[Li0.23Co0.3Mn0.47]O2 had a similarα-NaFeO2 structure as that of LiCoO2. However, the material had a lower symmetry degree due to the superlattice cation ordering in the transition metal layer. Rietveld refinement showed that the Li+ ions occupy the 3b (0, 0, 0.5) and 3a (0, 0, 0) sites. The Co3+ and Mn4+ ions locate at the 3a (0, 0, 0) site, and the O2- ions at the 6c (0, 0, 0.2588) site. The lattice parameters of the material were calculated to be a = 2.832(9) ? and c = 14.220(4) ?. The c/a ratio of the material was smaller than those of LiCoO2 and LiNiO2, indicating a more layered structure of the material. Li[Li0.23Co0.3Mn0.47]O2 showed more Raman bands than LiCoO2, indicating significant differences between the local structures of the materials. This is because of the co-existance of Li,Mn,Co atoms in the transition metal layer, which lowered the symmetrical degree of the local structure. The broadening of the Raman bands of Li[Li0.23Co0.3Mn0.47]O2 indicates the cation disordering in the transition metal layer. In addition, a detailed investigation of the A1g band showed that Li[Li0.23Co0.3Mn0.47]O2 had a stronger M-O bond than that of the Co-O bond in LiCoO2, which resulted in a shorter c parameter of the material lattice.(3) Morphology of the material. X-ray photoelectron spectroscopy confirmed the +3 and +4 valence state of Co and Mn in Li[Li0.23Co0.3Mn0.47]O2. Based on the analysis of the C 1s and O 1s XPS curves, a Li2CO3 impurity phase was supposed in the material, which was further confirmed by FTIR spectroscopy. TEM analysis showed that the material had a small particle size about 100 nm. In addition, a Li2CO3 layer with thickness of 30? was clearly observed by TEM.(4) Cyclic voltammetry. The electrochemical properties of the materail were studied using a two-electrode battery cell. The initial charge process of the battery showed two oxidation curves, between which the one at 4.1 V was attributed to the oxidation of Co3+, whereas that at 4.6 V was assigned to the release of oxygen and the extraction of Li+. The intensity of the 4.6 V oxidation peak was weakened significantly in the following cycles. This indicates that the oxygen release process was almost completed in the first cycle.(5) Charge-discharge cycling performance. The charge-discharge cycling performance of Li[Li0.23Co0.3Mn0.47]O2 was investigaed in the potential region of 2.0～4.8 V. About 0.23 mol of Co3+ ions were oxidized in the first charge process, and about 0.38 mol of oxygen was released from the material lattice. The material showed an initial discharge capacity of 222,183 and 110 mAhg-1 at the C/4, 1C and 3C rates, respectively, which was 156,145 and 120 mAhg-1 after 50 cycles. The cycling performance of the mateiral obtained by this work was very promising comparing with the previous reports on Li[Li0.23Co0.3Mn0.47]O2 prepared by PVA or combustion methods. This was attributed to the nano size of the material, which provided short pathways and large specific surface aera for Li+ diffusion.(6) Li+ diffusion kinetic properties. The lithium diffusion coefficients (DLi+) of the material were studied by PITT technique. The DLi values between 4.0 and 4.3V were～10-13cm2s-1 during the first charge. Afterwards, the DLi values immediately decreased to10-14 cm2s-1 and reached to 10-15 cm2s-1 at 4.5V. The small DLi values indicate that the electrochemical kinetics after 4.3V was very slow. This could be due to the reason that the electrochemical reaction was retarded by high kinetic barrier associated with the simultaneous oxygen release, lithium extraction and structural modification. Any oxygen loss and structural modifications were completed after the first charge. The following ectrochemical process was dominated by reversible Li+ intercalation. We obtained larger DLi values of 10-14～10-13cm2s-1in the second charge.The electrochemical kinetic behaviors of Li[Li0.23Co0.3Mn0.47]O2 were also studied by electrochemical impedance spectroscopy. The total impedance of the electrode at lower potentials, which included the SEI resistance and charge transfer resistance, was smaller that those at higher potentials. This was attributed to the decomposition of the electrolyte, as well as the release of oxygen which facilitated the decomposition of electrolyte. This suggestes that better cycling performance could be obtained if the material was cycled in a lower upper potential. Acturally, the material indeed showed much better cycling performance in the 2.0- 4.55 V potential region than in the 2.0– 4.8 V region.
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