【作者】 肖政伟；

【导师】 胡国荣； 彭忠东；

【作者基本信息】 中南大学 ， 电化学工程， 2008， 博士

【摘要】 橄榄石结构LiFePO₄具有原料丰富、无毒、热稳定性强、循环性能好、理论容量高(170mAh·g^-1)等优点,能很好的符合信息社会对新型能源的需求和环保的需要,是新一代商品化锂离子电池正极材料中的有力竞争者。但其固有的晶体结构和元素组成,造成材料振实密度低、导电性能差、电化学活性低。这些性能亟待改善。本文系统研究了不同铁源和碳源对合成LiFePO₄/C材料性能的影响;探索新原料新工艺合成LiFePO₄/C,以突破国外制备工艺的专利垄断;选取具有代表性的离子,研究了Li位和Fe位掺杂对LiFePO₄材料的晶体结构和电化学性能的影响;对LiFePO₄/C电极进行表面碳包覆改性进一步提高了电极的倍率性能;采用CV和EIS研究了LiFePO₄/C电极在较高温度下的电化学行为,为进一步开辟提高材料电化学性能的途径提供理论参考。以Fe₂O₃为铁源,详细考察了合成温度对LiFePO₄/C材料晶体性能、碳含量、振实密度和电化学性能的影响。较FeC₂O₄·2H₂O,采用Fe₂O₃合成了振实密度更高电化学性能优良的LiFePO₄/C材料。研究了不同球磨分散介质对LiFePO₄/C的充放电容量和振实密度的影响。较乙醇和水,采用丙酮作为球磨分散介质制备了充放电容量和平台以及循环和倍率性能更优、振实密度更高的LiFePO₄/C材料。首次提出以Fe₂O₃和NH₄H₂PO₄为原料一步固相法制备Fe₂P₂O₇的工艺。以Fe₂P₂O₇和Li₂CO₃为原料制备的LiFePO₄保持了较好的颗粒形貌和均匀的粒度分布。在不额外加入还原剂的情况下实现了Fe³⁺→Fe²⁺:Fe₂O₃→Fe₂P₂O₇→LiFePO₄。以丙酮代替乙醇作球磨分散介质合成了性能优良的LiFePO₄/C材料。以还原铁粉、LiFePO₄和葡萄糖为原料,经球磨机械活化后高温下制备了LiFePO₄/C正极材料。700℃是合成LiFePO₄的最佳温度。具有纳米级一次粒径的软团聚LiFePO₄/C粉末材料表现出了较好的电化学性能。大部分有机碳前驱体的无氧热解碳呈蓬松状态,在LiFePO₄/C材料颗粒间形成有效的导电连接。采用均苯四甲酸酐、柠檬酸和蔗糖可制备电化学性能优良的LiFePO₄/C材料。单独以石墨或乙炔黑对LiFePO₄进行包覆/掺杂对提高材料的电化学性能有限。研究了LiFePO₄的Li位和Fe位掺杂对材料电化学性能的影响。一定浓度的Mg²⁺和Ca²⁺的Li位掺杂均可以不同程度改善LiFePO₄/C的倍率性能。Co²⁺、Ni²⁺和Mn²⁺的Fe位掺杂对LiFePO₄/C电化学性能的影响取决于掺杂浓度。首次发现了正磷酸盐中Co³⁺/Co²⁺和Ni³⁺/Ni²⁺电对在较低电位下的电化学活性。LiFe_0.94Co_0.06PO₄/C和LiFe_0.94Ni_0.06PO₄/C电极的CV曲线在4.0V左右出现了Co³⁺/Co²⁺和Ni³⁺/Ni²⁺电对的氧化还原峰。提出了碳包覆修饰LiFePO₄/C电极以提高其电化学性能:大幅度提高了电极充放电容量和倍率性能;分别降低和提高了Fe³⁺ /Fe²⁺电对氧化和还原电位;减小了电极过程电荷转移阻抗。分别采用CV和EIS研究了LiFePO₄/C在较高温度下的电化学性能。结果均表明:提高LiFePO₄/C电极的工作温度,可以增加电极的Li⁺扩散系数,增大电极的可逆性,有利于活性材料充放电容量的发挥,提高电极的深度充放能力。更多还原

【Abstract】 With the merits of abundant raw materials, non-toxicity, good thermal stability, excellent charge/discharge cycle ability and high theoretical capacity (170mAh·g^-1), olivine-structured LiFePO₄ can meet the needs of both demand of new energy-consuming information society and environmental protection. LiFePO₄ has become one of the most promising contenders among the new generation of commercial cathode materials. Its intrinsic crystal structure and elemental composition, however, result in properties of low tap density, poor electronic conductivity and electrochemical inertness, which urgently needed to be improved.In the paper, the impacts of different iron sources and carbon precursors on the properties of LiFePO₄/C material were studied systematically: the tap density of LiFePO₄/C was enhanced with improved technological conditions; new raw materials and new techniques were explored for synthesis of LiFePO₄/C in order to bypass the monopoly of foreign patents; doping effects of representative ions on both Li and Fe sites on the crystal structure and electrochemical properties of LiFePO₄ were investigated; the rate performance of LiFePO₄/C electrode was further improved by coating the electrode surface with carbon; CV and EIS were used to investigate the electrochemical behavior of LiFePO₄/C electrode at higher working temperatures, which provided a theoretical reference for opening up a method to further improve material’s electrochemical performance.Using Fe₂O₃ as the iron source, the influences of synthesis temperature on the crystal property, carbon content, tap density and electrochemical performance of LiFePO₄/C were investigated in detail. LiFePO₄/C material of high tap density and excellent electrochemical performance was prepared when using Fe₂O₃, compared to FeC₂O₄·2H₂O as an iron source. The effects of different ball-milling dispersive media on charge/discharge capacity and tap density of LiFePO₄/C were studied. Compared to ethanol and water, using acetone as dispersive medium, LiFePO₄/C cathode material of better charge/discharge capacity and plateau, better cycling and rate performance and higher tap density was synthesized.In the paper, one-step solid state reaction was for the first time put forward to prepare Fe₂P₂O₇ using Fe₂O₃ and NH₄H₂PO₄ as raw materials. Fe₂P₂O₇ and Li₂CO₃ were employed to synthesize LiFePO₄ which exhibited good particle morphology and even size distribution. Fe³⁺→Fe²⁺ was realized with no extra addition of reducing agent: Fe₂O₃→Fe₂P₂O₇→LiFePO₄. LiFePO₄/C of good property was synthesized by using acetone instead of ethanol as dispersive medium.Reduced iron, LiH₂PO₄ and glucose were mechanically activated by ball-milling and LiFePO₄/C cathode material was prepared at high temperature. 700℃was the optimum synthesis temperature. With nano-sized primary articles, LiFePO₄/C powder material showed a good electrochemical performance.Most carbon from oxygen free pyrolysis of organic precursors exhibited a fluey morphology and formed an effective conducting connection between LiFePO₄/C particles. LiFePO₄/C of good electrochemical properties could be prepared using pyromellitic anhydride, citric acid and sucrose as carbon precursor respectively. Little improvement in electrochemical performance could be obtained by modifying LiFePO₄ with using only graphite or acetylene black.The doping effects of Li and Fe site on the electrochemical properties of LiFePO₄ were investigated. Electrochemical performance of LiFePO₄ could be enhanced to different extent by Mg²⁺ and Ca²⁺ doping on Li site with a certain concentration. The influences of Fe site doping by Co²⁺, Ni²⁺ and Mn²⁺ depended on doping concentration. The electrochemical activity at lower potential of Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ redox couples in orthophosphate was for the first time discovered in the paper. 4V redox peaks for Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ were revealed on the CV curves for LiFe_0.94Co_0.06PO₄/C and LiFe_0.94Ni_0.06PO₄/C electrode.Electrochemical properties of LiFePO₄/C electrode were improved by carbon coating modification: discharge capacity and rate ability weregreatly enhanced; oxidation and reduction voltages for Fe³⁺ /Fe²⁺ couple were decreased and increased respectively; charge transfer impedance was greatly reduced.The electrochemical properties of LiFePO₄/C at higher temperatures were examined with CV and EIS. Results indicated diffusion coefficient for Li⁺ and irreversibility for LiFePO₄ electrode were raised by increasing working temperature. Higher temperature could benefit exertion of charge/discharge capacity and raise the depth of charge and discharge.更多还原