【作者】 刘强；

【导师】 胡国荣；

【作者基本信息】 中南大学 ， 冶金工程， 2013， 硕士

【摘要】 摘要：镍系锂离子电池正极材料是一种极具希望的下一代锂离子正极材料。相对于传统LiCoO2正极材料,其具有比容量更高、价格更低、资源更广等优势,但其也面临着循环稳定性不好、热稳定性欠佳和不耐过充等问题。本论文选取了LiNi0.9Co0.1-x[Mn1/2Mg1/2]xO2体系作为研究对象。考察了材料的合成方法和性能,以及掺杂元素的影响规律和基本机理。以LiNi0.9Co0.04[Mn1/2Mg1/2]0.06O2为例,研究了正极材料的合成路线和工艺条件。首先通过控制结晶法制备氢氧化物前驱体；然后利用高温煅烧法合成正极活性材料。在控制结晶条件实验中,控制氨水浓度为0.6mol·L-1,反应温度为50℃、搅拌速度为500rpm、加料速度为15mL·min-1等一系列工艺参数,研究了pH值变化对前驱体成分、颗粒形貌和振实密度的影响。研究表明,在pH值为11.5±0.02的条件下制备得到的前驱体,成分符合设计比例、属类球形,振实密度为2.2g·cm-3；将前驱体粉末与锂盐充分混合,通过高温煅烧制备得到正极活性材料,研究了煅烧温度、时间和锂过量系数对材料结构和电化学性能的影响。结果表明,优化的煅烧工艺条件参数为：煅烧温度750℃、保温时间16h、锂过量5%。讨论了Co-Mn-Mg掺杂体系不同的掺杂配比,即LiNi0.9Co0.1-x[Mn1/2Mg1/2]xO2中x值变化对材料结构和性能的影响。得到的系列材料都具有相互接近的类球形的形貌,成分符合实际目标值。XPS(X射线光电子能谱分析)的分析结果表明,Ni、Co、Mn、Mg四种主要金属元素的存在价态分别为+3,+3,+4和+2。等量的Mn-Mg掺杂,能够保持LiMO2层状化合物中的M综合价态为+3价。XRD(X射线衍射)精修结果表明,Mg可以占据3b和3a两种位置,Mg在3b位置的出现能够减少Li/Ni混排的程度,同时增大晶胞参数c(a大致保持不变),有利于锂离子的脱嵌。同样从不同循环次数后,处于充电状态电池的EIS(交流阻抗)曲线分析结果可以看出：Mn-Mg对Co的替代能够有效抑制循环过程中正极材料电子导电电阻的升高。电化学充放电测试则表明,Mn-Mg对Co的替代小幅地降低了材料的首次放电比容量,却能有效提高材料对比容量的保持能力。在x=0.06时,材料具有最佳的容量保持能力和较高的比容量：0.5C倍率下,首次放电比容量为181.3mAhg-1,循环50次比容量保持率为93.2%；0.2C倍率下,首次放电比容量为194.4mAhg-1,循环15次比容量保持率为97.7%。在优化x=0.06的掺杂材料再进行F阴离子掺杂,发现F掺杂能够有效提高材料的容量保持能力。当F掺杂量为2%时,材料在低倍率时(0.2C)兼具优良的循环性能(50次循环容量保持率为97.4%)和较高的首次放电比容量(185mAhg-1)。在较高的倍率下(0.5C),虽然材料的循环性能优良(50次循环容量保持率为98.3%),但是其首次放电比容量较低(165mAhg-1)。更多还原

【Abstract】 Abstract:Lithium nickel-based oxide is a kind of promising cathode material to power next generation of lithium-ion batteries. Compared to commerical LiCoO2, it has advantages on specific capacity, price and abundant resources. However, it is faced with problems like structural unstability, thermal unstability and meanwhile it cannot undergo overcharging. Two of the most common and useful ways to resolve these problems are doping with other foreign ions and surface modification repectively. This thesis is focused on muti-elements ions doping nickel-rich system LiNio.9Coo.1-x[Mn1/2Mg1/2]x02cathode materials. The synthesis method and properties of the materials have been invetigated. Then the effect of ions doping on the cathode materials and the basic improvement mechanism have also been discussed.The synthesis method and process of the LiNio.9Coo.1-x[Mn1/2Mg1/2]x02cathode materials were examplified by the synthesis of LiNio.9Coo.o4[Mn1/2Mg1/2]o.o602. Firstly, nickel hydroxide precusor was prepared by controlled crystallization process; then lithium nickel-based oxides active material was obtained by traditional calcination process. In the conditional experiments of the pH values, the crystallization system is fixed:ammonia concentration at0.6mol·L-1, reaction temperature at50℃, stirring rate by500rpm, feed rate at15mL·min-1. The effect of pH on the chemical compositions, morphologies and tape density was studied. It is found that precursors with proper chemical composition, near spherical morphology and a tap density of2.2g·cm-3can be obtained when the pH value is adjusted around11.5±0.02. For the calcination process, the calcination temperature, reaction time and excess amount of lithium were studies. The optimized calcination parameters are as following:reaction temperature at750℃for16h with an excess lithium amount of5%.The effect of variation of x in the LiNi0.9Coo.1-x[Mn1/2Mg1/2]x02compounds on the structure and electrochemical properties were discussed. The LiNio.9Coo.1-x[Mn1/2Mg1/2]x02particles are of almost the same spherical morphologies confirmed by SEM. The chemical compositions of this series of LiNi0.9Coo.1-x[Mn1/2Mg1/2]xO2are quite close to the designed compositions. The XPS results revealed that Ni, Co, Mn and Mg elements are at+3,+3,+4and+2chemical valence states respectively. This means that equal amount of Mn and Mg has the nominative valence state of+3, which justify the correctness of our prior assumption that equal amount of Mn and Mg should be added. In the XRD refinements, Mg ions are detected to be located at both3b and3a sites. Meanwhile, the introduction of Mg ions into the nickel-based layered structure can lower the cation mixing of Li/Ni and increase the c parameter, which has a positive effect on fast lithium intercalation/deintercalation. The substitution of Mn-Mg to Co in the compounds can also suppress the charge-transfer growth during the electrochemical cycling, which confirmed by the EIS analysis. Mn-Mg substitution to Co improves the capacity retentation capability of the nickel-based compounds, although at a cost of little capacity decrease. When x is0.06, the cathode material has the best capacity retentation capability,93.2%of capacity is maintained after50cycles at0.5C-rate with an intial discharge capacity of194.4mAhg-1. F doping into the LiNi0.9Coo.o4[Mn1/2Mg1/2]o.o602compound can further improve the cycling capability efficiently and meanwhile keep the relative high capacity at low rate (0.2C) when the amount is chosen at2%. The initial discharge capacity at0.2C rate is185mAhg-1,97.4%of the initial discharge capacity is maintained after50cycles. However, at higher rate (0.5C), F doping seems to be detrimental to the initial discharge capacity (only165mAhg-1) although with an improved cycling performance (capacity retention rate98.3%for50cycles).更多还原