【作者】 曹雁冰；

【导师】 胡国荣； 彭忠东；

【作者基本信息】 中南大学 ， 材料冶金， 2010， 博士

【摘要】 聚阴离子型铁系材料LiFePO4和Li2FeSiO4具有原料丰富、环境友好、循环稳定性好、安全性高等突出优点,成为很有发展潜力的新一代锂离子电池正极材料。但聚阴离子型铁系材料的本身结构决定了其电导率低,电化学活性差,限制了其大规模应用。为此,本文对铁系材料的合成方法和改性工作开展了系列的研究,以改善其电化学性能。以LiH2PO4和FeC2O4-2H2O为原料,聚乙烯醇为碳源,通过机械化学活化辅助固相法合成了原位碳包覆的LiFePO4材料。详细考察了合成条件对LiFePO4/C材料晶体结构、物理和电化学性能的影响,并通过拉曼光谱对复合材料中碳的结构进行了分析,焙烧温度对碳的石墨化结构含量影响最大。优化工艺条件下,热解碳在LiFePO4颗粒表面形成了良好的纳米导电层,放电比容量为155.4 mAh/g (0.1C)和137.6 mAh/g (1C),振实密度为0.87 g/cm3。在碳包覆基础上,通过锰离子掺杂制备了较低碳含量的LiFe0.99Mn0.01PO4/C材料,O.1C放电容量达到158.9 mAh/g,在1C和2C倍率下的放电容量分别为140.1和130 mAh/g,循环性能优良,同时具有较高的振实密度(1.02g/cm3)。以LiH2PO4和还原铁粉为原料,聚乙烯醇为碳源,通过机械液相活化沉淀法首先合成了分散均匀、反应活性较高的[Fe3(PO4)2-8H2O +Li3PO4]前驱体,分析了前驱体制备的机械活化反应过程。TGA-DSC分析显示合成LiFePO4的晶化过程与有机碳源的碳化过程同步,制备的前驱体经过一步焙烧可生成原位碳包覆LiFePO4材料。优化结果表明,LiFePO4晶粒表面原位形成了5nm左右分布均匀的碳导电层,获得了颗粒细小均匀、结晶良好的LiFePO4/C材料,放电比容量为156.8 mAh/g (0.1C)、140.7 mAh/g (1C)和130.2 mAh/g (2C),振实密度为1.12g/cm3。并通过碳包覆和铁位掺杂复合改性制备了LiFe0.99M0.01P04/C材料(M= Mn2+,V3+,Si4+),显示了良好的倍率充放电能力和循环稳定性。样品LiFe0.99Si0.01P04/C在高倍率3C下放电比容量为137.1 mAh/g,循环40次后保持在136 mAh/g。从缺陷化学角度分析了高价离子铁位掺杂改性的作用,离子掺杂使LiFePO4晶格内产生了锂空位,扩展了锂离子扩散的通道,降低了O对Li的束缚,从而有利于Li+的快速传输。交流阻抗分析和循环伏安测试结果也表明,离子掺杂提高了LiFePO4的导电性能,有利于减少电极过程中的电荷传递电阻和锂离子扩散阻力,降低了充放电过程中的动力学限制,提高了电极的可逆性。选择不同原料体系通过机械活化结合多元醇合成法制备了不同形貌的纯相LiFePO4材料,探索了一种常压低温下高效合成LiFePO4的工艺。以棒状形貌的[Fe3(PO4)2·8H2O+Li3PO4]为前驱体,在三甘醇介质中沸腾回流制备了结晶良好的棒状LiFePO4材料。以聚乙烯醇为碳源,对纯相LiFePO4进行了碳包覆改性,以改善纯相材料的电导率。获得的LiFePO4/C复合材料1C、2C下放电容量分别为139.8 mAh/g、129.5 mAh/g。以三甘醇为溶剂和还原剂,Li2CO3和FePO4为原料,采用多元醇还原法直接制备了颗粒细小均匀、结晶良好的纯相LiFePO4,无须后续热处理。通过碳包覆改性处理合成了LiFePO4/C材料,在0.1C下放电容量为157.3 mAh/g,2C倍率下放电比容量保持在136.2 mAh/g,具有良好的倍率性能和循环稳定性。将微波加热技术应用于Li2FeSiO4/C正极材料的制备。以Li2CO3、FeC2O4·2H2O和纳米Si02为原料,聚乙烯醇和超导电炭黑为复合碳源,采用微波固相法制备了Li2FeSiO4/C材料。在选用的微波合成体系下,超导碳和聚乙烯醇热分解的无定形碳不仅利于合成反应的顺利进行,而且在Li2FeSiO4/C材料中形成有效的导电通路,提高了Li2FeSiO4的整体导电性能,60℃下0.05C倍率首次放电容量为124.2mAh/g,0.5C倍率首次放电比容量102.3 mAh/g。进一步改善前驱体制备方式,采用低热固相反应获得了分散均匀的β-FeOOH/SiO2前驱体,再通过微波碳热还原法合成了结晶好、晶粒细小均匀、高纯度的Li2FeSi04/C材料。经过优化后的Li2FeSiO4/C材料在60℃下0.05C倍率首次放电容量为129.6 mAh/g,0.5C倍率下有107.5 mAh/g。更多还原

【Abstract】 Polyanion iron-base cathode materials, LiFePO4 and Li2FeSiO4 are seen as highly promising candidates for large-scale Li-ion battery applications due to the merits of abundant raw materials, environmental friendliness, excellent cyclic stability, and high safety. However, the intrinsic crystal structure of polyanion iron-base cathode materials results in poor conductivity and electrochemical inertness, which seriously hinder their application in large scale. In order to improve the electrochemical performance, the serial research work is focused on synthesis methods and modification technique of iron-base cathode materials.In-situ carbon-coated LiFePO4 cathode material was prepared by ball-milling activation and subsequent solid state reaction, with LiH2PO4 and FeC2O4·2H2O as starting materials, and polyvinyl alcohol (PVA) as carbon source. The influences of synthetic conditions on the crystal structure, physical property and electrochemical performance of LiFePO4/C composites were investigated in detail. And the structure of carbon in the composites was studied by Raman spectrum, which indicated the sintering temperature played the significant role on the graphitization degree of carbon pyorlyzed from PVA. Under the optimum conditions, the residual pyorlyzed carbon in situ formed a well conductive nano-film on the surface of LiFePO4 grain, and the obtained LiFePO4/C cathode delivered reversible discharge specific capacity of 155.4 mAh/g at 0.1C rate and 137.6 mAh/g at 1C rate, with tap density of 0.87 g/cm3. On the basis of carbon coating modification, Mn-ion-doping LiFe0.99Mn0.01P04/C composites were prepared with lower carbon content, displaying good cyclic capability and higher reversible discharge specific capacity of 158.9 mAh/g at 0.1C rate and 140.1 mAh/g at 1C rate and 130 mAh/g at 2C rate, as well as higher tap density of 1.02 g/cm3.Homogeneous distribution of [Fe3(PO4)2·8H2O+Li3PO4] precursor with the high reacting activity was prepared by mechano-chemical liquid phase activation technique, with LiH2PO4 and reduction iron powder as starting materials, and PVA as carbon source. The reaction course between LiH2PO4 and reduction iron powder under mechanical activation is analyzed. PVA pyrolyzed during the formation of LiFePO4 synchronously, as a result, carbon-coated LiFePO4 composites were synthesized in situ by one-step solid state reaction of as-prepared precursor. The optimum results showed that, uniform in-situ carbon coating (5nm or so) was formed on the surface of LiFePO4 crystalline. Consequently, well-crystallized LiFePO4/C composites with homogeneous fine particle size were obtained, which had the discharge specific capacity of 156.8 mAh/g at 0.1C rate and 140.7 mAh/g at 1C rate and 130.2 mAh/g at 2C rate, and tap density of 1.12 g/cm3. In order to further enhance the electrochemical performance of LiFePO4/C, LiFe0.99M0.01P04/C (M=Mn2+,V3+,Si4+) composites cathode were prepared by carbon coating combination with Fe-site doping, then the rate performance and cyclic stability of LiFePO4 was improved significantly. It was found that the sample LiFe0.99Si0.01P04/C displayed 137.1 mAh/g at 3C discharge rate and maintained 136 mAh/g after 40th cycles. The modification effect of aliovalent doping on Fe site was discussed from the viewpoint of defect chemistry. The dopant substituting on the Fe sites can create lithium vacancies in olivine LiFePO4 lattice by charge-compensation mechanism. The existences of lithium vacancy may expand Li+diffusion channels in the structure, weaken the bound of oxygen to lithium, and increase lithium mobility. And the analyses of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) also indicated that aliovalent doping on Fe site improved conduction property of LiFePO4, reduced charge transfer impedance and Li ion diffusion resistance in the electrode process, increased phase transformation kinetics during cycling, and enhanced reversibility of LiFePO4 electrode.Pure LiFePO4 materials with different morphology were prepared by mechanochemical-activation-assisted polyol synthesis processing, using different raw materials, and a low temperature approach for efficient preparation of LiFePO4 cathode was explored. LiFePO4 rods with good crystallization were synthesized in boiling tetraethyleneglycol (TEG) medium by re fluxing processing, using rod-shaped [Fe3(PO4)2·8H2O +Li3PO4] as precursor. In order to improve the electrical conductivity, carbon coating modification on the pure LiFePO4 was carried out with PVA as carbon source. The prepared LiFePO4/C composite delivered discharge capacity of 139.8 mAh/g at 0.1 C rate and 129.5 mAh/g at 2C rate. Highly crystalline LiFePO4 ultrafine particles were synthesized by a polyol reduction processing without post annealing, with TEG acting as solvent and reductant, FePO4 and Li2CO3 as the starting material. Carbon-coating LiFePO4 composites were prepared by calcining the mixture of as-prepared pure LiFePO4 and PVA. And the obtained composites delivered discharge capacity of 157.3 mAh/g at 0.1C rate and 136.2 mAh/g at 2C rate, displaying good rate capability and cyclic stability.Microwave heating technique was introduced into the synthesis of Li2FeSi04/C cathode materials. Li2FeSiO4/C cathode materials were synthesized by microwave solid-state processing, with Li2CO3 FeC2O4·2H2O、and nano-SiO2 as the starting materials, PVA and super-P carbon as the carbon sources. Under the selective microwave synthesis system, super-P carbon powder and pyrolyzed amorphous carbon not only effectively provided the high temperature to induce the complete reaction but also formed the conductive network to enhance the electronic conductivity. The prepared Li2FeSiO4/C composite had discharge capacity of 124.2 mAh/g at 0.05C rate and 102.3 mAh/g at 0.5C rate at 60℃. To improve preparation route of precursor, the homogeneous distribution of FeOOH/SiO2 was prepared from FeCl24H2O and Na2SiO3"9H2O by low-heating solid-state reaction. Then Li2FeSiO4/C composites were prepared by microwave carbothermal reduction method, with PVA and super-P carbon as reductants. Under optimum conditions, highly pure Li2FeSiO4/C material with uniform and fine particle size is obtained, showing discharge capacity of 129.6 mAh/g at 0.05 C rate and 107.5 mAh/g at 0.5Crate at 60℃.更多还原