正极材料LiFePO_4的合成与性能研究
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摘要
自从1997年Padhi等首次报道具有橄榄石型结构的LiFePO4能作为锂离子电池正极材料以来,许多科研小组对其做了深入研究,认为LiFePO4原材料丰富、对环境友好、安全稳定,可作为锂离子电池理想的正极材料。LiFePO4的主要问题是电子导电率低和离子扩散性能差,目前主要采用三种方法改进其导电性能,(1)采用“软化学”合成方法和手段来控制产物颗粒的大小和形貌;(2)在颗粒表面包覆导电物质提高复合材料的导电能力;(3)离子掺杂以改善LiFePO4的电子电导率。因此在本论文中我们采用sol-gel法合成LiFePO4来控制产物粒径和形貌,采用原位聚合的方法在LiFePO4粒子表面包覆导电聚合物聚苯胺来改进LiFePO4的电化学性能。
首先我们对锂离子电池正极材料的研究历史和发展过程进行了回顾,重点讨论了LiFePO4的合成方法和改性手段,分析了LiFePO4走向实际应用面临的问题和未来的研究方向。在第三章中我们采用高温固相法和sol-gel法合成了LiFePO4,对sol-gel法合成LiFePO4的条件进行了优化,并对在此优化条件下合成的LiFePO4样品利用X射线衍射(XRD)、扫描电镜(SEM)等方法对所得样品的晶体结构、表观形貌、粒径大小等进行了分析研究,并组装成模拟电池进行电池性能测试。充放电实验结果表明工艺优化样品的首次放电容量为131.3 mAh/g,充放电效率为91.5%,10次循环后容量保持率为95.5%。
在第四章中我们采用化学氧化聚合的方法合成了导电聚合物聚苯胺,研究了氧化剂浓度,反应温度,反应时间,酸浓度对聚苯胺电导率,聚合产率的影响。得到优化的合成参数为过硫酸铵与苯胺单体的摩尔比为1:1,反应温度为5℃,反应时间为6 h,酸浓度为1 mol/L。在第五章中我们采用原位聚合的方法合成了不同聚苯胺含量的聚苯胺包覆的LiFePO4复合正极材料。研究结果表明,在LiFePO4粒子表面包覆导电聚合物聚苯胺可以提高其放电容量,改善其大电流放电时的循环性能。其中原位包覆25%聚苯胺的PAn-LiFePO4复合正极材料性能最好,以0.1 C进行充放电时,首次放电容量为140.3 mAh/g,以1 C进行充放电时,放电容量为116.5 mAh/g,循环50次后容量为118.7 mAh/g。
Since Padhi et al found lithium iron phosphate (LiFePO4) could be used as cathode material for the lithium ion batteries, many research groups have been devoted to study the performance of this material. They think that LiFePO4 is one of the most promising cathode materials for the lithium ion batteries because it is abundant, environmentally benign, stable and safe. The disadvantage of LiFePO4 is the low electrical conductivity and Li-ion diffusivity. Numerous approaches directed at overcoming these problems have been described in the literature, including adopting low temperature liquid-phase process to control particle size and morphology, coating conductive material by carbon or fine metal particles to enhance the electrical conductivity, and attempting at doping with supervalent cations to improve the stability of material structure. So we decide to adopt sol-gel method to control particle size and morphology and adopt in-situ polymerization method to coat polyaniline on the surface of LiFePO4 particle to improve the electrochemical performance of LiFePO4. The main contents of this paper are given as following.
Firstly, we retrospect the development of cathode material for lithium ion batteries, focusing on the synthesis and improvement of properties of LiFePO4. The problems in application and researching direction in the future are analyzed and pointed out. Then the LiFePO4 has been synthesized by solid-state method and sol-gel method, and the synthetic conditions of sol-gel method were optimized. The crystalline structure, morphology of particles and crystalline size of the LiFePO4 samples prepared at this optimized condition were investigated by X-ray diffraction and scanning electron microscopy. The charge-discharge test results of the optimal LiFePO4 sample show that the first discharge capacity is 131.3 mAh/g, the charge-discharge efficiency is 91.5% and the capacity retain ratio is 95.5% after 10 cycles.
Polyaniline (PAn), one of conductive polymers, is synthesized from aniline (An) by chemical oxidation polymerization method in chapter four. The effects of several factors (including oxidant concentration, reaction temperature, reaction time, acid concentration) were studied. The optimized parameter of synthesis is that the mole ratio of ammonium persulfate (APS) and An is 1:1, reaction temperature is 5℃, reaction time is 6 h, and acid concentration is 1mol/L, respectively. A series of polyaniline-LiFePO4 (PAn-LiFePO4) composites were synthesized by in-situ polymerization in chapter five. Research results showed that PAn-LiFePO4 composites had higher discharge capacity and better cycling performance. The PAn-LiFePO4 composite with 25 wt% polyaniline showed the best electrochemical performance.Its first specific discharge capacity was 140.3 mAh/g at 0.1 C and its specific discharge capacity was 118.7 mAh/g after 50 cycles at 1 C.
首先我们对锂离子电池正极材料的研究历史和发展过程进行了回顾,重点讨论了LiFePO4的合成方法和改性手段,分析了LiFePO4走向实际应用面临的问题和未来的研究方向。在第三章中我们采用高温固相法和sol-gel法合成了LiFePO4,对sol-gel法合成LiFePO4的条件进行了优化,并对在此优化条件下合成的LiFePO4样品利用X射线衍射(XRD)、扫描电镜(SEM)等方法对所得样品的晶体结构、表观形貌、粒径大小等进行了分析研究,并组装成模拟电池进行电池性能测试。充放电实验结果表明工艺优化样品的首次放电容量为131.3 mAh/g,充放电效率为91.5%,10次循环后容量保持率为95.5%。
在第四章中我们采用化学氧化聚合的方法合成了导电聚合物聚苯胺,研究了氧化剂浓度,反应温度,反应时间,酸浓度对聚苯胺电导率,聚合产率的影响。得到优化的合成参数为过硫酸铵与苯胺单体的摩尔比为1:1,反应温度为5℃,反应时间为6 h,酸浓度为1 mol/L。在第五章中我们采用原位聚合的方法合成了不同聚苯胺含量的聚苯胺包覆的LiFePO4复合正极材料。研究结果表明,在LiFePO4粒子表面包覆导电聚合物聚苯胺可以提高其放电容量,改善其大电流放电时的循环性能。其中原位包覆25%聚苯胺的PAn-LiFePO4复合正极材料性能最好,以0.1 C进行充放电时,首次放电容量为140.3 mAh/g,以1 C进行充放电时,放电容量为116.5 mAh/g,循环50次后容量为118.7 mAh/g。
Since Padhi et al found lithium iron phosphate (LiFePO4) could be used as cathode material for the lithium ion batteries, many research groups have been devoted to study the performance of this material. They think that LiFePO4 is one of the most promising cathode materials for the lithium ion batteries because it is abundant, environmentally benign, stable and safe. The disadvantage of LiFePO4 is the low electrical conductivity and Li-ion diffusivity. Numerous approaches directed at overcoming these problems have been described in the literature, including adopting low temperature liquid-phase process to control particle size and morphology, coating conductive material by carbon or fine metal particles to enhance the electrical conductivity, and attempting at doping with supervalent cations to improve the stability of material structure. So we decide to adopt sol-gel method to control particle size and morphology and adopt in-situ polymerization method to coat polyaniline on the surface of LiFePO4 particle to improve the electrochemical performance of LiFePO4. The main contents of this paper are given as following.
Firstly, we retrospect the development of cathode material for lithium ion batteries, focusing on the synthesis and improvement of properties of LiFePO4. The problems in application and researching direction in the future are analyzed and pointed out. Then the LiFePO4 has been synthesized by solid-state method and sol-gel method, and the synthetic conditions of sol-gel method were optimized. The crystalline structure, morphology of particles and crystalline size of the LiFePO4 samples prepared at this optimized condition were investigated by X-ray diffraction and scanning electron microscopy. The charge-discharge test results of the optimal LiFePO4 sample show that the first discharge capacity is 131.3 mAh/g, the charge-discharge efficiency is 91.5% and the capacity retain ratio is 95.5% after 10 cycles.
Polyaniline (PAn), one of conductive polymers, is synthesized from aniline (An) by chemical oxidation polymerization method in chapter four. The effects of several factors (including oxidant concentration, reaction temperature, reaction time, acid concentration) were studied. The optimized parameter of synthesis is that the mole ratio of ammonium persulfate (APS) and An is 1:1, reaction temperature is 5℃, reaction time is 6 h, and acid concentration is 1mol/L, respectively. A series of polyaniline-LiFePO4 (PAn-LiFePO4) composites were synthesized by in-situ polymerization in chapter five. Research results showed that PAn-LiFePO4 composites had higher discharge capacity and better cycling performance. The PAn-LiFePO4 composite with 25 wt% polyaniline showed the best electrochemical performance.Its first specific discharge capacity was 140.3 mAh/g at 0.1 C and its specific discharge capacity was 118.7 mAh/g after 50 cycles at 1 C.
引文
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