Study On Synthesis And Modification Of Layered LiMnO₂

The increasing concerns on energy,environmental and the rapid development of modern science and technology have more demands on batteries.Lithium ion batteries are attractive for use in the field of mobile power source with the favorable properties of high voltage,long cycling life,high energy density,non-memory effect and pollution-free.Intensive research is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode materials.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 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 two mainly 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 layered LiMnO₂ compound prepared as the cathode materials for Li-ion batteries.The main results are as follows:i synthesis and comparison of two different layered LiMnO₂Monoclinic and orthorhombic LiMnO₂ are prepared by melting and sol-gel 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 tendncy to transform to a spinel-like cation ordering during electrochemical cycling between 2.2-4.5V.ii synthesis and electrochemistry of LiMnO₂ with Ni dopingLayered o-LiMnO₂ was prepared and modified with Ni²⁺ by sol-gel process. The results...