Hydrothermal Synthesis Of Lithium-rich Cathode Materials And Modification Study

With the rapid development of science and technology, the requirement for lithiumi-ion battery is increasing day by day. So it is urgent to develop a high-energy and high-power cathode materials to replace the present commercial LiCoO2material. In recent years, lithium-rich layered oxide material because of its high specific capacity, good thermal stability, low cost and environmentally friendly, it has attracted extensive research around the world. However, lithium-rich layered oxide material still exists some problems, such as low initial coulombic efficiency, poor rate performance. Therefore, to improve the preparation and performance for the lithium-rich layered oxide possesses important theoretical and practical significance.In this dissertation, the uniform nanoscale lithium-rich cathode materials are successfully prepared via a one-step hydrothermal synthesis by using MnSO4, NiSO4, LiOH and LiNO3as the raw materials,(NH4)2S2O8as the oxidant. The effects of the lithium ion concentration and reaction time on the hydrothermal products are investigated systematically. The materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the lithium ion concentration is confirmed as the critical factor in preparation lithium-rich materials. Under the optimum lithium ion concentration of2.5M, the obtained product has a Lio.94[Li0.14Ni0.26Mn0.60]O2stoichiometry with plate-like morphology and a uniform particle size distribution. The size of particle is ca.80nm and the thickness is about20nm. The Brunauer-Emmett-Teller surface area measurement indicates that it possesses a high surface area of35.60m2g-1. The electrochemical tests show that its discharge capacity can reach to278mAh g-1and185mAh g-1at0.1C and5C, respectively.The Co-doped uniform nanoscale Li0.96[Li0.14Co0.09Ni0.21Mn0.55]O2lithium-rich material is obtained by a similar method and it is successfully coated by AlF3. The results show that Co-doped is helpful to stabilize layered structure and enhance conductivity. And AIF3coating can raise the specific capacity and the initial coulombic efficiency of the electrode (from85.3%to90.8%). Electrochemical impedance spectroscopy analysis show that AIF3coating can pre-activate the surface of lithium-rich material and reduce the initial diffusion impedance.We attempt to prepare the controllable materials by using different additives and find that the surfactants have little effect on the morphology of products. However, the polymer—PVP has a significant effect on the morphology of product. The product varies from plate to grain and gets more uniform. The usage amount of ionic liquid has a major impact on the product. When the usage amount is large (more than1ml), the undesired product is obtained. While if the usage amount is0.2ml, the plate-like object product can be obtained. Electrochemical tests show that the use of surfactants and PVP has little influence on the electrochemical performance of materials. While the usage of ionic liquid make the electrochemical performance of material deteriorates that the discharge capacity is only254mAh g-1at0.1C. Due to the more uniform particle distribution, the rate performance of the material prepared with PVP is improved to a certain extent that it can reached to214mAh g-1at5C...