Study On The Preparation Of Nickel-based Cathode Materials For Lithium-ion Batteries And The Surface Coating Technology By Solid State Reaction At Room Temperature

Nickel-based cathode material is one of the main candidate materials for lithium ion batteries cathodes due to its high specific capacity.In the meantime,LiNi_0.8Co_0.2O₂,as the optimum lithium ion batteries cathode materials,has been widely used in electric vehicle, storage power plant,military weapon and etc,which need industrial batteries with high capacity and high power.However,the loss of specific capacity during cycling and poor storage performance have seriously hindered its application and marketlization in large scale.In order to improve the electrochemical performance,the research work was focused on improving the traditional preparation method,finding out new preparation technique and carrying out surface coating.In this dissertation,the LiNi_0.8Co_0.2O₂ cathode material was prepared by "controlled crystallization" method and spray pyrolysis method respectively.The nickel-based cathode with surface coating for lithium ion batteries by solid state reaction at room temperature was also studied systematically and thoroughly for the first time.The spherical Ni_0.8Co_0.2(OH)₂ precursor was prepared by "controlled crystallization" method in 12 liter agitated reactor.The effects of stirring speed,flowing speed of nickel cobalt vitriol solution, NH₃/(Ni+Co),pH value,reaction temperature,reaction time and etc,on Ni_0.8Co_0.2(OH)₂ precursor morphology as well as tap-density were studied systematically.The spherical LiNi_0.8Co_0.2O₂ cathode material with tap-density of 3.0g/cm³ was prepared in the optimum condition. LiNi_0.8Co_0.2O₂ cathode material showed an initial discharge capacity of 195.3mAh/g at a 0.2 C rate between 3.0 and 4.3 V,and still delivered 177.0mAh/g after 50 cycles.The LiNi_0.8Co_0.2O₂ cathode material was first prepared by the self-made spray pyrolysis equipment in China.The effects of the main controlled condition in the process of spray pyrolysis,such as solution type,solution concentration,flow rate of carrier gas,pyrolysis temperature and additive on the morphology of LiNi_0.8Co_0.2O₂ precursor were studied.The spherical LiNi_0.8Co_0.2O₂ cathode material prepared in the optimum condition showed fully developed crystallinity and uniform particle size distribution with size of about 1μm.The initial discharge capacity of LiNi_0.8Co_0.2O₂ material was 187.3 mAh/g, and its capacity retention was 96.8%after 30 cycles.And the formation mechanism of spherical and dense particles with additive was studied during the spray pyrolysis.In this dissertation,the solid state reaction at room temperature was introduced to the surface coating the of cathode materials for lithium ion battery for the first time.The LiNi_0.8Co_0.2O₂ powders were coated by Al₂O₃,AlPO₄ and Co₃(PO₄)₂ respectively.The cycleability of LiNi_0.8Co_0.2O₂ cathode material was greatly improved by coating with Al₂O₃ and AlPO₄,but the effects were different in different coating materials.Although the electrochemical performance of the Al₂O₃-coated LiNi_0.8Co_0.2O₂ material was almost the same as the AlPO₄-coated one,the thermal stability of the AlPO₄-coated LiNi_0.8Co_0.2O₂ material was better.The Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ material was systematically studied in this dissertation.The results showed the Co₃(PO₄)₂ coating was mainly in the form of a Li_xCoPO₄ phase on the surface of LiNi_0.8Co_0.2O₂ powders.The Li_xCoPO₄ coating layer could suppress and eliminate the lithium impurity(LiOH and Li₂CO₃)in the LiNi_0.8Co_0.2O₂ powders surface,reduce the loss of discharge capacity during cycling and improve the storage stability.The initial discharge capacity of 1wt.% Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ material was 181.1 mAh/g at 1C,and the discharge capacity was 160.9mAh/g after 60 cycles.The discharge capacity of Al₂O₃-coated,AlPO₄-coated and the uncoated material after 60 cycles was 155.8mAh/g,156.1mAh/g and 148.5mAh/g respectively, which showed the Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ material had good cycleability.And the cycleability improvement mechanism of LiNi_0.8Co_0.2O₂ material with surface modification was investigated by electrochemical impedance spectroscopy(EIS)technique.The spherical Ni(OH)₂ precursor was coated with Co/Mn(OH)₂ on the surface by the solid state reaction at room temperature and then the LiNiO₂ coated with Co/Mn was prepared.The effects of the controlled conditions in the process,such as different Co/Mn ratio,calcing atmosphere,calcing temperature,calcing time on the crystal structure and electrochemical performance were studied.The initial capacity at 1C of the Co/Mn-coated LiNiO₂ prepared in the optimum condition was 181.3 mAh/g,and the capacity loss was only 8.9%after 50 cycles,while the uncoated LiNiO₂ lost about 26.9%.The function of Co/Mn coating layer on LiNiO₂ in the process of charge-discharge was interpreted by the cyclic voltammogram and electrochemical impedance spectroscopy technique.And the reaction and surface coating mechanism of solid state reaction at room temperature was investigated.