Research On Preparation And Electrochemical Performance Of Layered Li-rich Cathode Material For Lithium Ion Battery

Along with the development of global economy, the problems in environment degradation and the shortage of resources have aroused the global attention. Many countries are trying their best to develop efficient and environmentally friendlynew energy, in the hope of alleviating effectively or solving the problems in environment and resources. With the advantages, such as large power density, low toxicity and rare pollution, lithium ion batteries can alleviate effectively the problems in environment and resources to a great extent for its wide use in portable electronic devices, aerospace, military areas and its tentative use in light electric vehicle(LEV), new energy automobile and so on. However, lithium ion batteries must be improved in the density and power density to be applied widely. In addition, cathode material plays a vital role in comprehensive performance of the lithium ion battery, especially in capacity, so it is very necessary to do more research on cathode for lithium ion battery.The layered Li-rich Li1+2M1-zO2, which be of new generation cathode (M is one or more than one kind of transition metal elements, z≥0), has attracted a wide attention of many researchers for its high discharge capacity, excellent cycleability and new charge and discharge mechanism and is regarded as a promising cathode material for future Li-ion battery. Based on such materials, the main contents of this thesis are as follows:(1) Layered Li-rich Li[Li0.2Ni0.26Mn0.54]O2cathode material was firstly synthesized via conventional co-precipitation and then solid state reaction method. Physical characterization revealed that the prepared sample behaves the typical layered a-NaFeO2structure and an irregular morphology, consisted of small grains. The electrochemical tests indicated that the material provides a discharge specific capacity of212mAh/g in first cycle. After30cycles, the discharge capacity remains81%of that of the first cycle. However, the prepared material shows a poor rate capability.(2) Under ordinary pressure and water bath condition, the as-prepared coprecipitation precursor Mn0.67Ni0.33(OH)2was treated using Co(NOs)2solution via cation exchange method, and then series of Co-cotaining layered Li-rich material Li[Li0.2Ni0.26Mn0.54Cox]O2were obtained. The research results proved that the initial discharge capacity becomes bigger and cycling stability becomes better with the increasing of temperature. This is closely accordant to the increasing Co content entering into Li[Li0.2Ni0.26Mn0.54]02as temperature increases. On one hand, this law can be attributed to that the Co can act as active material to make a bigger specific capacity, and on the other hand, Co can reduce the Ni content to weaken the disordering between Li and Ni in the metal layers and lithium layers, which may cause the phenomenon of Ni ion occupying in lithium layer to block the pathway of Li+diffusion, and improve the cycle stability of the material.(3) Under hydrothermal condition, as well as high pressure and higher temperature, the precursor Mno.67Nio.33(OH)2was treated using Co(NO3)2solution via cation exchange, and then series of Co-cotaining layered Li-rich materials Li[Li0.2Ni0.26Mn0.54Cox]O2were prepared. It came out that all the Li-rich Co-containing materials prepared under hydrothermal condition offere a bigger initial discharge capacities, better cycle stability and rate performance than the material Li[Li0.2Ni0.26Mn0.54]O2without Co. As hydrothermal temperature increases to150℃from110℃, the Co content in corresponding materials increases and the comprehensive properties of the obtained materials get better. When temperature increases to160℃and continues to increase to180℃from150℃, the Co content increases first and then decreases. However, the electrochemical properties of materials become worse gradually. It can thus be concluded that, in spite of without the most Co content, the material obtained at150℃shows the biggest initial discharge capacity, as well as the best cycle stability and rate performance. All the properties of these materials are superior to the optimum Co-containing Li-rich material obtained at water bath temperature. On the one hand, the above results can be attributable to that participation of Co can play the role of active elements and weaken the disordering between Li and Ni in the metal layers and lithium layers to prevent the Ni ion occupying in lithium layer to block the pathway of Li+diffusion. On the other hand, this is because, under high pressure hydrothermal condition, Co-cotaining material with a higher crystallinity can be obtained. However, when Mn content is insufficient to support MnO6octahedral structure framework, owing to the Co content increasing to to a certain value, the electrochemical performance will get worse instead.(4) Quasi spherical MnzNi1-z(OH)2with uniform size was prepared via similar cation exchange method mentioned above, and then series of layered materials containing different content of Ni and Mn were prepared. And the effect of different amount of lithium on the performance was investigated. It turned out that the initial discharge capacity increases as the amount of Li increases, and also a better cycling stability appeares. When the Li content increases to a certain extent, the obtained material exhibits the inherent characteristic of layered Li-rich λLi2MnO3-(1-λ,)LiMO2(M=Ni、Co、Mn、Cr or any combination of them) in the initial charging curve. It proved that layered Li-rich material with a fine morphology is likely to be prepared by the method mentioned above. However, the materials obtained showes a poor comprehensive performance, especially a lower specific capacity. It seemed that, to obtain the Li-rich layered material with a fine morphology and an excellent electrochemical property, it is very necessary to make more attempts and explorations, such as grinding way and time, sintering system and so on.