【作者】 龚正良；

【导师】 杨勇；

【作者基本信息】 厦门大学 ， 物理化学， 2007， 博士

【摘要】 正交结构正硅酸盐聚阴离子型化合物由于其高的理论容量和突出的安全性能等优点成为很有发展潜力的新一代锂离子电池正极材料。聚阴离子型正极材料通常具有价格低廉、电化学循环稳定性及热稳定性好等优点,但其低的电导率制约了其在高功率型电极材料方面的应用。本文采用改进的固相反应、溶胶凝胶法、及水热辅助溶胶凝胶法分别制备了Li₂FeSiO₄/C、Li₂Mn_xFe_1-xSiO₄/C及Li₂CoSiO₄系列电极材料,研究了合成方法对材料的结构、形貌、物理化学性质及电化学性能的影响。建立了一种新型的合成方法—水热辅助溶胶凝胶法,通过水热辅助溶胶凝胶法合成了具有良好性能的上述材料。采用多种结构分析、表面分析、磁性测量以及电化学研究方法等实验手段,深入分析研究了所合成材料的结构特征、物理化学性质及电化学性能,考察了影响材料电化学性能的主要因素。对不同方法合成Li₂FeSiO₄/C复合材料的研究表明由于Fe-O-Li-O-Fe的长程相互作用Li₂FeSiO₄在奈尔温度（T_N=20K）以下为反铁磁有序。对顺磁区域进行分析,得到居里常数C_p=3.06emu·K·mol^-1和奈尔温度θ_p=-38.1K。不同方法所合成的Li₂FeSiO₄/C复合材料的电化学研究表明材料的形貌、微结构及相纯度是影响Li₂FeSiO₄材料性能的主要因素。水热辅助溶胶凝胶法所合成材料显示出良好的电化学性能,具有高的放电容量、良好的高倍率性能和循环稳定性。以C/16电流充放电时,首次放电容量高达160mAhg^-1。在2C倍率下充放电,其首次放电容量达125mAhg^-1。即使在5C和10C倍率下充放电,其首次放电容量仍维持在91和78mAhg^-1,放电容量开始随循环次数的增加略有上升然后维持稳定,循环50次后其放电容量没有衰减。水热辅助溶胶凝胶法所合成材料显示出良好的电化学性能,主要是由于其多孔纳米结构和高的相纯度以及通过表面碳包覆提高材料的电导率。高的放电容量及良好的高倍率充放电性能和循环稳定性显示所合成Li₂FeSiO₄/C复合正极材料是理想的动力电池（电动车/混合电动车）用锂离子电池正极材料。对不同方法合成Fe掺杂Li₂Mn_xFe^1-xSiO₄的研究表明Li₂Mn_xFe_1-xSiO₄可以在整个组成范围内形成很好的固熔体,掺杂后材料的结构没有明显变化。磁化率测量结果显示,Li₂Mn_xFe_1-xSiO₄的有效磁矩随Mn含量的增大而增大,与材料计量比相符。Li₂Mn_xFe_1-xSiO₄可以实现超过一个Li的可逆交换,水热辅助溶胶凝胶法所合成Li₂Mn_0.5Fe_0.5SiO₄/C复合正极材料的首次放电容量高达235mAhg^-1。,相当于可逆的嵌脱1.42个Li。Li₂Mn_xFe_1-xSiO₄（0<x≤1）材料循环容量衰减是材料自身的性质。Mn³⁺的Jahn-Teller效应引起的体积变化效应导致材料结构破坏可能是Li₂Mn_xFe_1-xSiO₄（0<x≤1）材料循环容量衰减的主要原因。单纯的Fe掺杂对Li₂MnSiO₄的循环性能改善不明显。对不同方法合成Li₂CoSiO₄材料的研究表明Li₂CoSiO₄具有多种结构异形体,改变合成方法和合成条件可以得到不同相结构的Li₂CoSiO₄材料。磁化率材料结果显示,由于Co-O-Li-O-Co的长程相互作用固相法所合成Li₂CoSiO₄材料在奈尔温度（T_N=18K）以下为反铁磁有序。对顺磁区域进行分析,得到居里常数C_p=2.36emu·K·mol^-1和外斯常数θ_p=-31.95K。其有效磁矩μ_eff与高自旋四面体和假四面体CO²⁺的磁矩值一致。在Li₂CoSiO₄材料中电子的轨道角动量没有被完全冻结而存在着某些“残余”部份。低的电导率以及无法采用原位碳包覆和有效的球磨碳包覆的方法来提高其电导率是导致Li₂CoSiO₄材料低的电化学活性的主要原因。通过改进合成方法、优化合成条件以及采用机械球磨的方法对材料进行碳包覆可以显著提高Li₂CoSiO₄材料的电化学活性。电化学测试结果显示Li₂CoSiO₄具有较高的充放电电位,第一个Li的嵌脱电位大约在4.1V。水热辅助溶胶凝胶法所合成材料具有最优的电化学性能,良好的电化学性能主要归因与其较小的粒径、均一粒径分布及高的相纯度。包覆碳纳米管后的Li₂CoSiO₄-C材料首次放电容量达到101mAhg^-1。更多还原

【Abstract】 Lithium metal orthosilicates （Li₂MSiO₄, M = Fe, Mn, Co） are a new class of ’polyanion’ compounds containing compact tetrahedral ’anion’ structural units （SiO₄）^4- with strong covalent bonding. Orthosilicates are promising candidates for next generation of lithium ion batteries, due to their high theoretical capacity and excellent safety performance. Polyanion cathode materials are attractive for low-cost, good cyclic stability, and excellent thermal stability. However, their low electronic conductivity impedes their use as electrode materials in high-power batteries. In this work, Li₂FeSiO₄/C, Li₂Mn_xFe_1-xSiO₄/C and Li₂CoSiO₄ electrode materials were prepared by modified solid state reaction, sol-gel method and hydrothermal reaction. An in-situ carbon coating method by adding sucrose to the synthetic precursor with ball-milling techniques was introduced in order to improve the low electronic conductivity of these materials. More important, a new synthesis route （hydrothermal assisted sol-gel method） has been developed for the preparation of these materials. The structure character, physico-chemical properties and electrochemical performance of the prepared materials are studied in detail by various methods and techniques, including structural analysis, surface analysis, magnetization measurements and electrochemical techniques. The factors affecting the electrochemical performance of the materials were investigated.The studies of Li₂FeSiO₄/C composite materials prepared by different methods show that Li₂FeSiO₄ powders possess an antiferromagnetic ordering below T_N = 20 K due to long range Fe-O-Li-O-Fe interactions. Analysis of the paramagnetic region giving the Curie-Weiss parametersθ_p = - 38.1 K and C_p = 3.06 emuK·mol^-1, show the divalent state of Fe cations. The morphology, micro-structure and phase purity are the main factors affecting the electrochemical performance of the material. The Li₂FeSiO₄/C composite material prepared through hydrothermal assisted sol-gel process displays a large discharge capacity of ca. 160 mAhg^-1 at C/16 rate and shows superior charge and discharge capabilities under high rate conditions, which could be, at least in part, attributed to the high phase purity, porous aggregate nano-structure, and improved electronic conductivity through carbon connection. The high rate capability and excellent capacity retention of the Li₂FeSiO₄ material shows high potential for cathode materials for high-power lithium-ion batteries.The studies of Li₂Mn_xFe_1-xSiO₄/C composite materials prepared by different methods show that Li₂Mn_xFe_1-xSiO₄ solid solutions can be achieved in a wide compositional range. Magnetic susceptibility experiments give evidence that the effective momentμ_eff increases with the increase in Mn content x, consistent with the stoichiometric. Electrochemical tests show that more than one Li reversible exchange can be achieved for Li₂Mn_xFe_1-xSiO₄. An optimized capacity and energy density for Li₂Mn_xFe_1-xSiO₄ was achieved at x = 0.5. The Li₂Mn_0.5Fe_0.5SiO₄/C composite material prepared through hydrothermal assisted sol-gel process shows a capacity as high as 235 mAhg^-1. The studies of the possible fading mechanism of Li₂Mn_xFe_1-xSiO₄ （x > 0） materials show that the poor cyclic performance of Li₂Mn_xFe_1-xSiO₄ （x > 0） is its intrinsic property. The poor cycling performance of the materials is associated with the Jahn-Teller effect of Mn³⁺, which cause the volumetric effect and destroys the structure of the materials.The studies of Li₂CoSiO₄ materials prepared by different methods show that two modifications of Li₂CoSiO₄ were prepared by different methods at various synthesis conditions. Magnetic susceptibility experiments give evidence that Li₂CoSiO₄ powers posses an antiferromagnetic ordering below T_N = 18 K due to long range Co-O-Li-O-Co interactions. Analysis of the paramagnetic region giving the Curie-Weiss parametersθ_p = - 31.49 K and C_p= 2.31 emu·K·mol^-1. The low conductivity of this compound and the difficulty of performing in-situ carbon coating to improve its low electronic conductivity result in its poor electrochemical performance. Through improving the synthesis methods, optimizing the synthesis conditions and coating with carbon by ball milling process, we successfully synthesized Li₂CoSiO₄ material with uniform nanoparticles and high phase purity, which shows high electrochemical activity for the first time. Reversible extraction and insertion of the first lithium from and into Li₂CoSiO₄ at -4.1 V vs. lithium have shown that this material is a potential candidate for new high-voltage cathodes in lithium-ion batteries. After coated with carbon nanotubes （CNTs） the Li₂CoSiO₄ prepared through hydrothermal assisted sol-gel process shows a discharge capacity of 101 rnAhg^-1 in the first cycle. The low electronic conductivity of Li₂CoSiO₄ and the parasitic reaction with the electrolytes contribute to the irreversible capacity loss of the material in the first charge-discharge cycle.更多还原