| With the development of modern society, more and more electronic devices, for instance, mobile phones, laptop computers, increasingly come into our daily life. Along with it, long-life, environmental friendly and low-cost batteries are urgently requested to meet these crucial demands. Lithium-ion batteries have won much attention because of their high voltage, high specific energy and low self-discharge.Up to now, the vast majority of lithium-ion batteries use lithium cobalt oxides, LiCoO2, as their positive electrode materials. But the scarcity, high price and toxicity of cobalt urge researchers to exploit other cathode materials for Li-ion batteries. Among all the candidates, manganese oxides attract great interest because of its low price and environmental consideration. Spinel phase LiMn2O4 has been extensively studied. However, its own crystal structure leads to the limited specific capacity. Stability problems also occur at elevated temperature. So a lot of effort has been focused on another kind of manganese oxide, LiMnO2 as a promising active material for Li-ion batteries because it has high theoretical capacity of 285mAhg"1 and low cost relative to LiCoO2.Orthorhombic LiMnO2 has an ordered rock salt structure of space group Pmnm. It transforms irreversibly to a material with spinel-like ordering upon electrochemical cycling. Another phase of interest in this system is monoclinic LiMnO2 which is analogous to LiCoO2. Unfortunately, pure LiMnO2, obtained by traditional solid-state reaction crystallizes in an orthorhombic structure instead of monoclinic one. According to the references, there are only two methods, ion-exchange and hydrothermal reactions, have been applied successfully to the synthesis of pure monoclinic LiMnO2. But both of the two methods requires a long preparation process and excessive consumption of lithium. These disadvantages greatly limit the mass production and its practical application.In this thesis, the work is focused on the investigation of the layerd LiMnO2 compound prepared by a new method as the cathode materials for Li-ion batteries. The main results are as follows:i synthesis and comparison of two different layered LiMnO2 Monoclinic and orthorhombic LiMnO2 are prepared by hydrothermal and rheological phase methods, respectively. Comparison of them has been made in terms of physical characteristics and electrochemical performance. Under the current density of 50mAg-1, both the monoclinic and orthorhombic LiMnO2 compounds have a tendency to transform to a spinel-like cation ordering during electrochmical cycling between 2.0-4.4V.ii synthesis and electrochemistry of LiMnO2 with Cr doping LiMn1-xCrxO2 compound with a-NaFeO2 layered-type crystal structure has been prepared by the rheological phase method. Compared with the samples synthesized from high temperature solid-state reaction, the compound prepared by rheological phase method exhibits much higher initial discharge capacity (180mAhg-1) and better cycling ability. Stable cycling could be achieved between 2.0-4.4V because Cr substitution prohibits a further evolution from layered phase to spinel one after the initial stages of electrochemical cycling. With the increasing of Cr substitution content, the initial discharge capacity and its fading rate of samples decreases correspondingly. After an all-rounded consideration, it is suggested that a proper amount of Cr is 15%.iii Stabilization of monoclinic LiMnO2 by Mg and Cr additionAnother kind of element, Mg, is also introduced into the monoclinic LiMn0.85Cr0.15O2. The results show that the layered structure could be stabilized through the "pillar" effect of Mg2+ between the MnO2 layers. Even when a large amount of lithium ions de-insert from the MnO2 layers, the layered structure would not collapse like the mono-doping phase LiMn1-xCrxO2. Therefore, the cycling stability is improved substantially.
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