【作者】 高旭光；

【导师】 胡国荣； 彭忠东；

【作者基本信息】 中南大学 ， 材料冶金， 2009， 博士

【摘要】 提高锂离子电池性能和降低电极材料的成本一直是锂离子电池领域的主要研究方向。本文基于这种趋势在详细考查了锂离子电池正极材料研究发展的基础上,选取了橄榄石型LiFePO₄材料作为研究对象,并运用水淬冷法对其合成和改性工作进行了较为深入和系统的研究。研究对比了随炉冷却方式和水淬冷方式对合成纯相LiFePO₄微观结构和电化学性能的影响。研究发现水淬冷方式较随炉冷却方式制备的LiFePO₄具有更精细的颗粒尺寸,通过Rietveld法结构精修分析发现采用淬冷法制备的样品晶格内存在一定的缺陷,并造成晶胞体积发生收缩;从化学热力学、动力学和晶体学角度对淬冷法制备的LiFePO₄的作用机理进行分析,提出了淬冷法制备的样品中可能存在离子扩散的空位和通道,这些空位和通道可能为锂离子的扩散提供路径,有利于固相法制备LiFePO₄电化学性能的改进;通过正交实验优化了淬冷固相法制备LiFePO₄的工艺条件。在最优条件下制备的纯相LiFePO₄在0.1C倍率下可逆放电容量达151.1 mAh·g^-1,0.5C倍率下达132.0 mAh·g^-1,明显优于常规固相法制备的LiFePO₄。采用水淬冷法合成了碳包覆的LiFePO₄/C化合物,并借助拉曼（Raman）光谱手段研究了碳在制备LiFePO₄/C中的作用机理,研究表明碳有效改进LiFePO₄的电化学性能需要以下三个条件:（ⅰ）.磷酸铁锂颗粒多为sp²型的碳所包覆;（ⅱ）.包覆碳能形成良好的导电网络;（ⅲ）.碳包覆均匀。采用淬冷固相法通过正交实验优化了低碳高活性LiFePO₄/C的合成条件,经过优化后合成的LiFePO₄/C复合材料0.1C放电容量达163.8 mAh·g^-1,在1C和2C倍率下的放电容量分别达143.4和124.7 mAh·g^-1,循环性能优良。而复合材料中的原位碳包覆量仅为0.98%,比表面积为8.9 m²·g^-1,极大地改善了材料的物理性能。采用水淬冷法合成了铁位掺杂和锂位掺杂的LiFePO₄化合物,并借助四探针法测电导率、粉末微电极循环伏安法和交流阻抗法探讨了铁位掺杂和锂位掺杂对LiFePO₄电导率的改善机理。研究表明:（ⅰ）.通过铁位掺杂Mg²⁺和Ni²⁺不但将样品的电导率提高了2-3个数量级,而且掺杂还降低了O对Li的束缚,从而有利于Li⁺的快速传输。（ⅱ）.锂位掺杂Ti⁴⁺和Nb⁵⁺不但可以将LiFePO₄的电导率提高近100倍,而且锂位掺杂在一定程度上还可以抑制脱锂.嵌锂过程中LiFePO₄的相变,从而有利于Li⁺扩散。但是,锂位引入过多的杂原子会导致材料电化学性能的下降。为了进一步提高磷酸铁锂的电导率并优化LiFePO₄/C的性能,本文研究了水淬冷法制备低碳掺杂型LiFePO₄的合成与性质。研究发现经过低碳量包覆和金属离子掺杂方法可以将LiFePO₄的电导率提高到10^-2S·cm^-1数量级,材料的电化学性能尤其是倍率性能得到明显的改善。样品LiFe_0.99Mg_0.01PO₄/C在2C倍率下首次放电容量达到133.9mAh·g^-1,经过50次循环几乎无衰减,而样品中的原位碳含量仅为1.05%。Li_1-xFePO₄和Li_1-xFe_0.99Mg_0.01PO₄/C（x=0～0.8）的交换电流密度数值分别介于0.04-0.10和0.1-0.42 mA·cm^-2之间,并且交换电流密度随着锂含量的变化而变化,并在x=0.4～0.5附近出现最大值。研究了喷雾干燥法制备球形高密度LiFePO₄的工艺参数,结果表明,在较低固体含量（20%）和高离心转速(18000r·min^-1)下可制得实心球形粒子前驱体。以实心粒子前驱体所合成的产物仍为实心,振实密度达到1.51g·cm^-3,所制备的实心多孔样品在0.2C、1C和2C倍率下的可逆比容量分别为150.2、144.0和129.8 mAh·g^-1,而且材料循环性能良好。更多还原

【Abstract】 Intensive research and development work is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode materials.Based on this trend and reviewing the development of cathode materials for lithium ion batteries,this dissertation focusing on olivine LiFePO₄ cathode materials.The synthesis and modification of LiFePO₄ cathode materials were studied by water quenching treatment in details.The effect of common cooling mode and water quenching（WQ） mode on the microstructure and electrochemical performance of LiFePO₄ were studied.In contrast to the common cooling mode,LiFePO₄ synthesized by water quenching treatment had more fine grain size. Crystal defect existence of the sample prepared by water quenching treatment was found by Rietveld refinement,which produced the crystal volume contraction.The working mechanism of water quenching treatment was discussed from chemical thermodynamics,chemical dynamics and crystallography.And the existences of vacancy and diffusion channel were brought on in the sample by water quenching method.These vacancies were helpful to improve the electrochemical performance of LiFePO₄ for offering the diffusion path of lithium ion in the electrode process.The synthetic conditions of pure LiFePO₄ were optimized by orthonormal experiment.The pure LiFePO₄ prepared under optimum condition had the highest reversible discharge specific capacity of 151.1 mAh·g^-1 at 0.1C rate and 132.0 mAh·g^-1 at 0.5C rate.LiFePO₄/C composite was prepared by water quenching treatment and the working mechanism of carbon in the LiFePO₄/C composite was studied by Raman spectrum method.The results showed that three necessary conditions determine the improvement of LiFePO₄/C.（ⅰ）.The particles of LiFePO₄ need be coated by sp² type carbon;（ⅱ）.The good electronic conductive network among particles need be formed;（ⅲ）.The uniformity of carbon on the surface of LiFePO₄ particle is also necessary. The synthesis of LiFePO₄/C with low carbon content and good activity by water quenching treatment was optimized by orthonormal experiment. The optimum sample with good cyclic capability displayed the highest reversible discharge specific capacity of 163.8 mAh·g^-1 at 0.1C rate and 143.4 mAh·g^-1 at 1C rate and 124.7 mAh·g^-1 at 2C rate.And the in situ carbon content of the sample was only 0.98%.The specific surface area was 8.9 m²·g^-1.Hence the physical property of the sample was improved greatly.The Fe sites（M2） doping and Li sites（M1） doping LiFePO₄ compound were synthesized by WQ method and the mechanism of the improvement of electronic conductivity was also discussed by four-probe method,powder microelectrode cyclic voltammetry and electrochemical impedance spectroscopy measurements.The study indicated:（ⅰ） the Fe sites doping Mg²⁺ and Ni²⁺ did not only improve the electronic conducivity by 2～3 order of magnitude but also weakened the bound of oxygen to lithium,which is propitious to the transport of Li ions;（ⅱ） the electronic conducivity of doping samples in Li sites by Ti⁴⁺ and Nb⁵⁺ was improved by 100 times.Meanwhile,structure change due to phase transformation was inhibited to some extent.But electrochemical performance was poor with doping excess atoms in Li sites.The synthesis and capability of doping type sample with low carbon content were investigated in order to further improve the electronic conductivity of LiFePO₄ and optimize the performance of LiFePO₄/C. The electronic conductivity was enhanced to 10^-2 S·cm^-1 by this method. The electrochemical property especially the rate capability was improved greatly.The sample LiFe_0.99Mg_0.01PO₄/C showed 133.9 mAh·g^-1 at 2C discharge rate and almost no loss after 50^th cycles.And the in situ carbon in the sample was only 1.05%.The ranges of exchange current density（i₀） in Li_1-xFePO₄ and Li_1-xFe_0.99Mg_0.01PO₄/C（x=0～0.8） were 0.04～0.10 and 0.1～0.42 mA·cm^-2,respectively.The exchange current density were changed with lithium content in the electrode,and the max value was obtained near at x=0.4～0.5.The synthetic conditions of spherical LiFePO₄ with high tap density were studied by spray drying（SD） method.The results showed dense spherical precursor was prepared at the low solid content（20%） and high speed of centrifugal atomizer for 18000 r·min^-1.The product with dense particle was synthesized by using dense precursor.And the tap density was 1.51 g·cm^-3.The sample with good cycle capability could reach 150.2, 144.0 and 129.8 mAh·g^-1 at the discharge rate of 0.2C、1C and 2C, respectively.更多还原