【作者】 邓新荣；

【导师】 胡国荣； 彭忠东；

【作者基本信息】 中南大学 ， 电化学工程， 2008， 博士

【摘要】 镍系正极材料是几种新一代锂离子电池正极材料中的主要候选材料之一,具有很高的可逆比容量,是将锂离子电池应用到电动汽车、蓄能电站、军事武器等高容量、大功率的工业大电池领域的首选材料。然而,由于镍系正极材料在循环过程中存在容量衰减和贮存性能差等问题,严重阻碍了其大规模的运用和市场化。针对这种情况,目前研究工作主要通过改进传统制备方法、摸索新的制备途径、并对正极材料进行表面改性、以提高锂离子电池镍系正极材料的电化学性能。为此,论文分别采用控制结晶法和新兴的喷雾热分解法制备了LiNi_0.8Co_0.2O₂正极材料,并首次对室温固相反应法表面改性锂离子电池镍系正极材料进行了系统深入的研究。采用控制结晶法在12.0L的反应釜中进行制备球形Ni_0.8Co_0.2（OH）₂前驱体实验。分析了前驱体制备过程中,搅拌速度、加料速度、NH₃/（Ni+Co）、体系pH值、温度、反应时间等控制因素对Ni_0.8Co_0.2（OH）₂前驱体形貌和振实密度的影响。在最佳实验条件下得到球形度好,振实密度高达3.0 g/cm³的LiNi_0.8Co_0.2O₂正极材料。在3-4.3V、0.2C倍率下,LiNi_0.8Co_0.2O₂正极材料首次放电容量达到195.3 mAh/g,50次循环后仍保持177.0 mAh/g。率先在国内运用自行设计制造的喷雾热分解设备制备LiNi_0.8Co_0.2O₂正极材料。实验研究了喷雾热分解过程的溶液体系、溶液浓度、载气流量、热分解温度、助剂等对LiNi_0.8Co_0.2O₂材料前驱体形貌的影响。最佳实验条件下可制备出结晶度完善,粒径约为1μm左右,粒度分布均匀的球形实心LiNi_0.8Co_0.2O₂正极材料。材料首次放电容量为187.3mAh/g,30次循环后的容量保持率为96.8%。还研究了喷雾热分解过程通过加入助剂合成球形实心粉体颗粒的机理。论文首次将室温固相反应法引入锂离子电池正极材料的表面包覆,制备了Al₂O₃、AlPO₄和Co₃（PO₄）₂分别包覆LiNi_0.8Co_0.2O₂正极材料。研究结果表明:表面包覆能够有效改善LiNi_0.8Co_0.2O₂材料的循环性能,不同包覆物的改性效果存在差异。虽然表面包覆Al₂O₃与表面包覆AlPO₄对改善LiNi_0.8Co_0.2O₂材料的电化学性能差别不大,但表面包覆AlPO₄的LiNi_0.8Co_0.2O₂材料热稳定性能优于表面包覆Al₂O₃的材料。对表面包覆Co₃（PO₄）₂的LiNi_0.8Co_0.2O₂正极材料进行了系统研究,研究结果表明:表面包覆Co₃（PO₄）₂的LiNi_0.8Co_0.2O₂材料主要是以Li_xCoPO₄的形式包覆出现在LiNi_0.8Co_0.2O₂粉末表面,其可以有效抑制并消除LiNi_0.8Co_0.2O₂表面的锂杂质（LiOH和Li₂CO₃）,改善其贮存性能,并减少容量损失。1wt.%Co₃（PO₄）₂包覆LiNi_0.8Co_0.2O₂材料的首次放电比容量为181.1mAh/g,60次循环的容量为160.9 mAh/g（1C）。而经AlPO₄包覆、Al₂O₃包覆和未包覆材料60次循环的容量分别为156.1 mAh/g,155.8 mAh/g,148.5 mAh/g。Co₃（PO₄）₂包覆LiNi_0.8Co_0.2O₂材料显示了更好的循环性能。同时,通过交流阻抗技术研究了表面包覆改善LiNi_0.8Co_0.2O₂材料循环性能的机理。采用室温固相法在球形Ni（OH）₂的表面包覆Co/Mn（OH）₂,制备LiNiO₂正极材料。研究了不同Co/Mn配比、焙烧气氛、焙烧温度和焙烧时间对产物结构与电化学性能的影响。最佳实验条件下制备的Co/Mn包覆LiNiO₂正极材料在1C倍率下的首次放电比容量为181.3mAh/g,50次循环后材料的容量损失率仅为8.9%,而未经包覆材料的容量损失率为26.9%。从循环伏安和交流阻抗的角度,解释了表面包覆Co/Mn改善LiNiO₂正极材料循环性能的作用。并探讨了室温固相反应及其表面包覆的机理更多还原

【Abstract】 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.更多还原