【作者】 刘艳；

【导师】 胡国荣； 杜柯；

【作者基本信息】 中南大学 ， 材料冶金， 2010， 硕士

【摘要】 橄榄石结构的磷酸铁锂(LiFePO4)由于原料来源广泛、价格便宜、环境友好,作正极材料时具有热稳定性好、循环性能优良等突出特点,成为最有前途的锂离子正极材料之一。但由于其固有的晶体结构,LiFePO4具有极低的电子电导率,大电流放电性能差,成为限制其应用的最大障碍。目前改善LiFePO4导电性的研究集中在碳包覆与金属离子掺杂两个方面。本文以合成高性能的LiFePO4/C复合材料作为研究目标。首先以不同的磷酸铁盐(FePO4·xH2O、FePO4·2H2O和FePO4)为铁源,合成了LiFePO4/C复合材料。通过正交实验优化了以FePO4·2H2O为原料制备LiFePO4/C的工艺条件。在最优条件下制备的LiFePO4/C在0.1C(1C=170mA.g-1)倍率下首次放电比容量有151.1mAh·g-1,在2C倍率放电时有126.8mAh·g-1。研究了不同温度预处理FePO4-2H2O对原料的结构,以及对合成的LiFePO4/C的形貌、电化学性能等的影响；考察了煅烧时间、煅烧温度对合成的LiFePO4/C性能的影响。确定了最佳的原料预处理温度500℃、合成LiFePO4/C的温度650℃以及合成时间6h,在此条件下制备的LiFePO4/C材料在0.1C下首次放电比容量155.5 mAh.g-1,40次循环后容量保持率为98.2%。研究了LiFePO4/C的铁位掺杂对材料电化学性能的影响。研究表明,V5+的掺杂改善了LiFePO4材料的电化学性能,特别是高倍率放电性能。以FePO4为铁源合成的LiFe0.99V0.01PO4/C材料在0.1C、1C和2C时的放电比容量为160.6、145.7和135 mAh·g-1；以FeC2O4·2H2O为铁源合成的LiFe1-δ-xVxPO4/C材料,在x=0.005时有最佳的电化学性能,在0.1C、1C和2C时放电比容量分别为160.7、147.2和139.9 mAh·g-1；采用两步固相反应合成了锂、铁双位掺杂的锂离子电池正极材料Li0.99Nb0.01Fe1-xMgxPO4/C(x=0,0.01,0.02,0.03,0.04)。Nb5+、Mg2+双位掺杂使晶体内部产生特殊的晶体结构缺陷,有效地提高了LiFePO4/C复合材料的电子电导率,有利于锂离子的快速传输；减小了材料充放电平台的起始电压差,降低了正极材料的电极极化,大幅度提高了材料的大电流放电能力。Li0.99Nb0.01Fe0.97+Mg0.0.PO4/C复合材料具有最好的电化学性能。0.2C、1C、2C、4C倍率充放电其首次放电比容量分别为153.7mAh·g-1、149.7mAh·g-1、144.6mAh·g-1、126.4mAh·g-1,即使在8C倍率下放电其放电比容量也有92.2mAh·g-1,且具有较好的循环稳定性。更多还原

【Abstract】 Olivine-structured LiFePO4 has become one of the most promising cathode material for its wide material source, low row materials cost, and environmental friendliness. Being used as cathode material, it has the characteristic of good thermal stability, excellent circle performance and so on. Its intrinsic crystal structure, however, result in properties of low electronic conductivity and poor discharge capability at high rate, which has been the greatest obstacle for application of LiFePO4 Nowadays, the main methods for improving electronic conductivity of LiFePO4 focus on carbon coating and metal ion doping. In this paper, the main work is to synthesize LiFePO4/C composite with good electrochemical properties.Firstly, LiFePO4/C composite was sintered with different iron phosphate (FePO4·xH2O, FePO4·2H2O and FePO4) as the iron source. The synthetic conditions of LiFePO4/C were optimized by orthonormal experiment using FePO4·2H2O as raw material. The sample prepared in optimized technical conditions has the highest reversible discharge specific capacity of 151.1 mAh·g-1 at 0.1C(1C=170mA·g-1) rate and 128.6 mAh·g-1 at 2C rate. The effects of pretreatment temperature on the crystal structure of FePO4·2H2O, the morphology and electrochemical properties of LiFePO4/C were investigated. The influences of synthesis temperature, synthesis time on the performance of LiFePO4/C were studied. The results showed the optimal conditions for synthesizing LiFePO4/C were as follows:pretreatment temperature of FePO4·2H2O was 500℃, synthesis temperature was 650℃, synthesis time was six hours. The sample prepared in these conditions exhibited excellent cyclability,delivering a discharge specific capacity of 153.0 mAh·g-1 at 0.1C rate after 40 cycles.The doping effects of Fe site on the electrochemical properties of LiFePO4 were investigated. The results showed V5+doping can improve electrochemical performance of LiFePO4, especially the discharge capability at high rate. LiFe0.99V0.01PO4/C has the discharge capacity of 160.6,145.7 and 135 mAh·g-1 at 0.1 C,1C and 2C rate using FePO4 as the iron source; LiFe1-δ-xVxPO4/C(x=0.005) has the discharge capacity of 160.7,147.2 and 139.9 mAh·g-1 at 0.1C,1C and 2C rate using FeC2O4·2H2O as the iron source.The multiple doping Li-ion battery cathode materials Li0.99Nb0.01Fe1-xMgxPO4/C(x=0,0.01,0.02,0.03,0.04) were synthesized by two-step solid state reaction. Special crystal defect existence of the Nb5+, Mg2+ multiple doping samples, can improve electrochemical performance of LiFePO4, which is propitious to the transfer of Li ions. Li0.99Nb0.01Fe0.97Mg0.03PO4/C has the best electrochemical performance. The initial discharge capacity of the sample delivers 153.7,149.7,144.6 and 126.4 mAh·g-1 at 0.2,1,2 and 4C rates, even at 8C rate, the initial discharge capacity of the sample remains 92.2 mAh·g-1. In addition, it shows excellent cycle stability at different rates.更多还原