【作者】 彭春丽；

【导师】 陈白珍； 张宝；

【作者基本信息】 中南大学 ， 冶金物理化学， 2011， 博士

【摘要】 该论文系统地对锂电池正极材料LiFePO4和Li2FeSiO4的合成、改性及其电化学性能进行了研究。采用液相共沉淀-碳热还原法合成了LiFePO4,首次研究了反应物溶液浓度对前躯体FePO4·xH2O和LiFePO4的影响。结果表明：反应物溶液浓度为0.1M、0.3M、0.5M、1.0M和1.5M时制备的FePO4·xH2O均为纯相,1.0M条件下合成的FePO4·xH2O含两个结晶水,即x=2。以反应物浓度为1.0M时合成的FePO4·xH2O为原料制备出的LiFePO4电化学性能最好,以O.1C倍率首次放电容量达159mAh·g-1,循环30次后容量保持率为99.7%。采用线性极化法和恒电位阶跃法,研究了LiFePO4在不同嵌锂状态下的动力学性能。结果表明LiFePO4材料的交换电流密度随嵌锂量的增加而增大,锂离子在LiFePO4材料中的扩散系数数量级为10-15-10-14cm2·s-1,体相掺杂能改变LiFePO4材料的交换电流密度和锂离子在LiFePO4材料中的扩散系数。通过Mg、Ni掺杂对LiFePO4进行了改性,对掺杂后的LiFePO4的晶形结构、表面形貌及电化学性能进行了系统地研究。结果表明,通过Mg、Ni掺杂后LiFePO4的橄榄石型结构没有发生变化,但颗粒的粒度均有所减小且分布较为均匀。虽然掺杂后的样品的首次充放电容量均有所减小,但其库仑效率和循环性能明显得到了提高。LiFePO4、LiMg0.02Fe0.98PO4、LiNi0.03Fe0.97PO4样品的首次放电容量分别为159、158和156mAh·g-1,在0.5C倍率条件下循环100次后放电容量分别为114、139和134 mAh·g-1;在1C倍率条件下循环50次后的放电容量分别为106、128和120 mAh·g-1。对LiMg0.02Fe0.98PO4和LiNi0.03Fe0.97PO4电极进行交流阻抗研究,发现LiMgo.02Feo.98PO4和LiNio.03Feo.97PO4电极的电化学阻抗减小。对LiMg0.02Fe0.98PO4和LiNi0.03Fe0.97PO4样品进行了循环伏安研究。结果表明氧化峰和还原峰的电位差分别减小到0.24V和0.29V,电极反应的可逆性得到了明显的提高。采用新型机械活化-高温固相法制备了铁位掺镍的Li2Fe1-xNixSiO4和铁位掺锰的Li2Fe1-xMnxSiO4/C复合材料。通过镍掺杂对Li2FeSiO4的结构、形貌和电化学性能的影响进行了系统地研究。研究表明：适量的镍掺杂对Li2FeSiO4材料基本结构不会有任何影响,产物的表面形貌得到了改善,颗粒形貌趋于规则,一定程度上提高了材料的充放电容量和循环性能,其中Li2Fe0.7Ni0.3SiO4样品放电容量和容量保持率最好。首次放电容量为118.7 mAh·g-1,循环30次后放电容量为103.5 mAh·g-1,容量保持率为87.2%。在C/8倍率下的首次放电容量和循环性能都有大幅度的下降,其首次放电容量为102.6 mAh·g-1,循环30次放电容量仅为74.9 mAh·g-1,容量衰减率为27%。同时,系统研究了掺锰量、焙烧温度、焙烧时间、Li/Si配比对Li2Fe1-xMnxSiO4/C复合材料物理性能及电化学性能影响,对优化合成条件下制备的Li2Fe1-xMnxSiO4/C材料的倍率性能及不同含量的蔗糖和葡萄糖为碳源的Li2Fe1-xMnxSiO4/C复合材料的电化学性能进行了系统地考察。结果表明：Li2Fe0.9Mn0.1SiO4/C材料最佳的合成条件为：Li/Si=2.04、合成温度为600℃、反应时间为16h。在最佳的合成条件下产品Li2Fe0.9Mn0.1SiO4/C具有最佳的电化学性能：以C/16倍率条件充放电,在1.5-4.8V电压区间首次放电比容量为149.8mAh·g-1,循环30次后的容量保持率仍有90.1%。随着碳含量的增加,Li2Fe0.9Mn0.1SiO4/C颗粒逐渐减小。与蔗糖相比,葡萄糖作为碳源合成的材料的粒径分布更均匀,颗粒的表面形貌更规则,葡萄糖掺入量为15%的样品具有较好的电化学性能,在0.1C倍率下的首次放电比容量为154.7mAh/g,循环30次后的容量保持率为92.2%。首次采用循环伏安(CV)和交流阻抗图谱法(EIS)对改性前后的Li2FeSiO4的嵌锂动力学过程进行了研究,并提出与之匹配的等效电路图。从动力学方面进一步阐明了材料改性前后的性能差异。更多还原

【Abstract】 The synthesis, modification and electrochemical performance of rechargeable lithium batteries and cathode materials were reviewed in detail.The cathode material LiFePO4 for lithium ion battery was prepared by co-precipitation-carbothermal reduction reaction method. The effect of solution concentration on the FePO4·xH2O precursor and LiFePO4 was studied. The results show that no impurities exist in the FePO4·xH2O synthesized with solution concentration from 0.1M to 1.5M. LiFePO4/C made from FePO4·2H2O precursor under the condition that the solution concentration of 1M show excellent electrochemical performance. The initial discharge capacity of LiFePO4 prepared under the optimized condition was 159 mAh·g-1 at 0.1 C rate and capacity retention remains 99.7 % after 30 cycles.The kinetics behaviors of LiFePO4 material for lithium ion battery were investigated by means of linear sweep voltammetry and chronoamperometry. It is shown that with lithium interaction into LiFePO4 material, the exchange current density was increased gradually. Meanwhile, the level of diffusion coefficient (DLi+) for LiFePO4 material is between 10-15 cm2·s-1 and 10-14 cm2·s-1.And doping has something with the exchange current density and diffusion coefficient (DLi+).The modification of LiFePO4 by Mg and Ni doping was investigated. The crystal structure, surface morphology and electrochemical properties of LiFePO4 were systematically researched. The results indicated that the structure of the material wasn’t changed after Mg and Ni doping. Appropriate amount of Mg and Ni doping can get the particle size of LiFePO4 smaller and uniform distribution. Apparently, the initial charge and discharge capacity of LiFePO4 samples went down but the corresponding coulombic efficiency and cycling performance were enhanced. The discharge capacity of LiFePO4, LiMgo.o2Fe0.98P04, and LiNi0.03Fe0.97PO4 were 159,158 and 156 mAh·g-1, respectively. And, the capacity was respectively kept 114,139 and 134 mAh·g-1 at 0.5 C after 100 cycles and the capacity retained 106,128 and 120 mAh·g-1 cycling at 1 C after 50 cycles. The Mg-doped and Ni-doped LiFePO4 samples were discussed by AC impedance and the Rct value of LiMg0.02Fe0.98PO4 and LiNi0.03Fe0.97PO4 was decreased. The potential difference between the oxidation potential and the reduction potential of LiMgo.o2Fe0.98PO4 and LiNi0.03Fe0.97PO4 were decreased to 0.24V and 0.29V respectively, resulting in the obvious enhancement of the reversibility of electrode reaction.Li2FeSi04 materials doped with Ni and Mn were synthesized via a novel mechanical activation-high temperature solid state method. The effect of Ni doping on physical structure and electrochemical performance of Li2FeSiO4 was investigated. The results were shown that appropriate Ni doping was unable to influence the structure of product. And then, the morphology of product tended to be regular, increasing the charge-discharge capacity and improving the cycle performance. Li2Fe0.7Ni0.3SiO4 shows the highest discharge capacity and the best cyclying stability. The initial discharge capacity was 118.7 mAh·g-1 and capacity retention remains 87.2% after 30 cycles. The effect of Mn doping, sintering temperature, sintering time and Li/Si ratio on Li2Fe1-xMnxSiO4/C’s performance were systematically researched. The cycle performance of Li2Fe1-xMnxSiO4/C prepared at 600℃for 16h and a Li/Si moral ratio of 2.04 as the optimum condition and the electrochemical properties of Li2Fe0.9Mn0.1SiO4/C synthetized using sucrose and glucose as carbon source and reductant. It delivered an initial capacity of 149.8mAh/g between 1.5V and 4.8V at C/16 rate and a capacity retention ratio of 90.1% after 30 cycles. The cycle performance of Li2Fe0.9Mn0.1SiO4/C became worse with increasing the discharge rate and capacity remains 74.9 mAh·g-1 after 30 cycles. Simultaneously, Li2Fe0.9Mn0.1SiO4/C with different content of sucrose and glucose were prepared. The grain size of Li2Fe0.9Mn0.1SiO4/C decreased with the increase of carbon content. Compared with Li2Fe0.9Mn0.1SiO4/C using sucrose as carbon source, Li2Fe0.9Mn0.1SiO4/C using glucose displayed smaller particles and more homogeneous distribution. the sample with 15%(wt,%) glucose exhibited excellent performance, with an initial discharge capacity of 154.7mAh/g and a capacity retention rate of 92.2% after 30 cycles.The lithium deintercalation-intercalation kinetics of modified Li2FeSiO4 materials were investigated by cyclic voltammetry, electrochemical impedance spectroscopy methods, and a fitting equivalent circuit diagram was raised. The results further proved the modified Li2FeSi04 had better electrochemical performance than the pristine one.更多还原