【作者】 倪江锋；

【导师】 苏光耀； 周恒辉；

【作者基本信息】 湘潭大学 ， 物理化学， 2004， 硕士

【摘要】 磷酸铁锂（LiFePO4）作为一种理想的锂离子电池正极材料，具有原材料便宜、能量密度高、性能稳定、对环境友好等优点。然而由于晶体结构的固有限制，LiFePO4具有极低的电子导电率，这已成为限制其应用的最大障碍。目前改善LiFePO4导电性的研究主要集中在碳包覆及金属或金属离子掺杂两方面。2002年，麻省理工学院的Chiang及合作者的研究表明，高价离子掺杂能提高LiFePO4电子导电系数达8个数量级。高价金属离子掺杂，造成了LiFePO4晶格中Li和Fe的缺陷，从而在FeO6次层形成Fe2+/Fe3+共存的混合价态结构，有效的提高了LiFePO4的导电性能，提高了LiFePO4的实际比容量。同时，由于数量很少，所以掺杂离子基本不影响LiFePO4的晶体结构和其它物理特征。因此，金属离子掺杂是一种很有前景的提高LiFePO4电化学性能的改性方法。然而，目前这方面的研究尚未深入，离子掺杂的类型和方式也没有更多的报道。因此在本论文中，我们依据Chiang的离子掺杂机理，努力拓展掺杂离子的种类、掺杂前驱体的类型及掺杂的方式，并对掺杂LiFePO4的合成工艺进行优化，制备出电化学性能优良的Li0.99M0.01FePO4。我们还对合成的Li0.99M0.01FePO4样品进行各种表征和分析，从材料结构与电化学性能关系、微观电子转移和宏观能量转换的角度来阐明离子掺杂对提高LiFePO4电化学性能的作用及影响。本论文的主要内容如下:首先，我们综述了锂离子电池正极材料的研究历史和现状，重点回顾了LiFePO4最近几年的研究成果，分析指出了LiFePO4尚需解决的问题和未来的研究方向，并给出了本论文的主要研究内容和研究方法。随后，为了与离子掺杂的LiFePO4相对比，我们对未掺杂的LiFePO4进行了合成和初步研究，重点在于考察热处理温度对材料结构和性能的影响。结果表明675℃合成的LiFePO4不仅结晶完整，而且形貌均匀，粒度适中，具有最佳的<WP=4>电化学性能。在第四章中，我们以Cu2+掺杂为基础，系统研究了各种合成条件对Li1-xCuxFePO4电化学性能的影响，其中包括热处理制度、掺杂比例、锂铁比例、预烧温度和时间、烧结温度和时间，确定了最佳的合成工艺。优化合成的Li0.99Cu0.01FePO4在常温下以0.1C放电，能够得到157mAh/g的可逆容量，即使电流密度提高到1C，放电容量也达132mAh/g。我们还进一步研究了不同掺杂方式对Li0.99Cu0.01FePO4电化学性能的影响，结果表明共沉淀掺杂是一条低含量离子掺杂的有效途径；喷雾法虽然也是一种很有应用前景的掺杂方式，但是尚有许多问题需要解决。在第五章中，我们详细讨论了Li0.99M0.01FePO4的电化学性能与掺杂离子本质的关系。数据表明掺杂的效果与前驱体的类型关系不大，有机酸盐和氧化物掺杂都能有效的提高材料的电化学性能。但是掺杂的效果与离子半径和价态密切相关，半径适中、价态较高的掺杂离子，其提高Li0.99M0.01FePO4电化学性能的效果比较明显。更多还原

【Abstract】 Lithium iron phosphate (LiFePO4) has been a promising cathode material for Lithium ion batteries among the simplest, most widely studied and potentially most useful cathode materials. Much attention had been paid to it for its merits: safety, environmental friendliness, low material cost, high energy density, and ease of synthesis and stability in common organic electrolyte systems. However, differing significantly from that in the layered and spinel structures, there is no continuous network of FeO6 edge-shared octahedral that might contribute to electronic conductivity, resulting in a extremely low electronic conductivity, which has been the greatest obstacle for application of lithium iron phosphate. When current density increases, its reversible capacity drops dramatically. The main methods for improving electronic conductivity of LiFePO4 focus on carbon coating and metal particle and ion doping. In 2002, Chiang and co-workers in Massachusetts institute of technology firstly reported that by supervalent ion doping, the electronic conductivity of LiFePO4 could be improved by eight orders of magnitude to 10-2 S/cm. The little dopant doesn’t influence the crystal structure and other physico-chemical characters, but enhance the capacity delivery and cyclability. So supervalent ion doping is an effective method to improve LiFePO4 electrochemical property. However, few deeper research results were reported, and there are still lots of problems unsolved. In the paper, LiFePO4 doped with ions was thoroughly studied based on Cu2+ ion doping. The issues systematically studied include the doping ion type, the doping manner and the dopant type of precursor. The optimal technological parameters have been attained. The physico-chemical properties of LiFePO4 doped with Cu2+ ion, Li0.99Cu0.01FePO4, following the optimal route were investigated thoroughly. Furthermore, deep <WP=6>discussion was carried out from the aspects of micro electronic structure and conductive essence, crystalline structure and material electrochemical property. The main contents are given as following.Firstly, the paper retrospects the history and development of cathode materials for lithium ion batteries, focusing on reviewing the research progress on LiFePO4 during the last several years, and pointing out the problems unsolved and the future researching direction. The main working plans and research approaches are given. Then undoping LiFePO4 has been synthesized, and the temperature effects are investigated. The results indicate that LiFePO4 synthesized at 675℃ exhibits the optimal electrochemical property due to its ideal olivine structure and medium particle size.Doping LiFePO4 is thoroughly studied in chapter four. All kinds of synthesis factors including heat-treated manner, dopant content, Li/Fe ratio, decomposing temperature and time, sintering temperature and time are systemically investigated based on Cu2+ ion doping, and the optimal technological parameters have been attained. LiFePO4 doped with 1% Cu2+ ion (Li0.99Cu0.01FePO4) synthesized following the optimal route exhibited excellent electrochemical properties. It can deliver a reversible capacity as high as 157 mAh/g at a discharging rate 0.1C at room temperature, even though the discharging current rate rise to 2C, it is able to deliver a capacity of 124 mAh/g. Temperature effects and overcharge behaviors are studied to versify its super thermodynamic stability. Another two doping manners, co-precipitation doping and spray doping, are also studied in this paper. The former is an effective route used to improve doping effect and near 130 mAh/g reversible capacity of Li0.99Cu0.01FePO4 was achieved when discharge current rate rise to 2C. Spray is a widely used manner to synthesize electrode materials, but it has not expressed the results as expected in our experiments. Much work <WP=7>should be loaded. In chapter five, versatile supervalent ions are investigated for ion doping. The results seemingly indicate that the effects of doping have little concern with dopant precu更多还原