新型磷酸铁锂复合正极材料的制备与性能研究
发布时间：2018-09-04 09:33
【摘要】：橄榄石结构的磷酸铁锂(LiFePO4)由于具有原料来源丰富、环境友好、优良的热稳定性和化学稳定性、比容量高(理论比容量为170mAh·g-1)、高而平坦的工作电压(对Li/Li+电位为3.43V)等优点被认为是目前最具前景的锂离子电池正极材料之一,但是纯相LiFePO4在应用时存在本征电子电导率和锂离子扩散系数较低的缺陷,这导致其大电流充放电性能难以满足需求,给LiFePO4的大规模商业化应用尤其是在动力电池方面的应用带来了极大的障碍。本文针对LiFePO4的两大缺陷,着力通过各种改性途径制备了改性的LiFePO4复合材料,以加快其商业化进程。 首先,通过酯化反应制备了PEG接枝的多壁碳纳米管(MWCNTs-g-PEG, MP),并将其与锂盐掺杂后作为一种新型的导电剂用于LiFePO4电极中,制备了LiFePO4/MP复合正极材料,分别研究了不同PEG分子量和不同MP添加量对复合正极材料的结构、电化学性能、导电和导热性能等的影响。结果表明,PEG在MWCNTs表面的均匀包覆能有效促进碳管在活性物质中的分散,有利于在电极中形成良好的导电和导热网络,在MP添加量仅为5wt.%的条件下,也能表现出优于传统导电剂乙炔黑添加量为20wt.%的电池性能,而且LiFePO4/MP的锂离子扩散系数较LiFePO4/乙炔黑的增加了近两个数量级,且PEG分子量越低,性能越好；而当MP-350(接枝PEG的分子量为350)含量为10wt.%时,其倍率性能、循环性能和导热性能均达到最优,且其低温充放电性能也得到了明显改善。 其次,通过原位聚合包覆法将既具有导电性又具有电化学活性的聚苯胺(PANI)成功包覆在LiFePO4颗粒表面得到了LiFePO4/PANI复合材料,探讨了制备过程中不同盐酸浓度和复合物中不同PANI含量对产物结构和性能的影响,研究表明,浓度为1M的盐酸由于酸性过强易使LiFePO4发生溶解,难以得到可用的复合物,而盐酸浓度太小则由于酸掺杂浓度过低使得PANI的导电性能受到影响,选择0.1M的盐酸能使所得LiFePO4/PANI复合物具有较理想的电化学性能；而PANI含量为10.2wt.%的LiFePO4/PANI (400μL)复合物由于具有合适的包覆结构,能大大增加LiFePO4颗粒的表面电子电导率,减少电池的极化,表现出最优异的电化学性能,经过100次O.1C循环之后,其放电比容量仅衰减3.2%,仍可达153mAh·g-1,倍率为2C时的放电比容量仍维持在,～122mAh·g-1,但更高倍率下的性能仍有待改善。 再次,为了改善LiFePO4/PANI在大倍率下的充放电性能,采用两种方法制备了PANI-PEG共聚物,通过掺杂使其获得电子和离子混合传导性,并将其用于改性LiFePO4正极材料。结果显示,通过在PEG末端引入苯胺基团,然后引发苯胺聚合,并在此过程中对LiFePO4进行原位包覆,能实现对颗粒的均匀完整包覆,所得LiFePO4/PANI-PEG复合物表现出极其优异的电化学性能,0.1C的放电比容量高达165mAh·g-1,5C时的比容量也可达到125mAh·g-1,比容量保持率为76%,并且其锂离子扩散系数DLi+值(3.4×10-13cm2·s-1)也较LiFePO4的DLi+值(3.2×10-14cm2·s-1)增加了一个数量级。 最后,利用多巴胺在聚合过程中可以附着在任何物质表面形成一层致密聚多巴胺(PDA)纳米薄膜的特性,选择PDA作为一种新型碳源,制备了完整碳包覆的LiFePO4/C复合物,碳含量和碳包覆层厚度可以通过多巴胺与LiFePO4前驱体之间的比例来进行调控,而颗粒之间由于PDA强粘附性所形成的“碳桥”结构以及颗粒表面包覆的碳层在整个活性物质中形成了三维纳米导电网络,极大地增加了LiFePO4颗粒之间的电子传导能力,使得LiFePO4在0.1C的放电比容量由无碳的84mAh·g-1增至135mAhg·-1(含碳2.02wt.%),即使在10C的大倍率情况下,其放电比容量仍可维持在～70mAh·g-1,有望作为一种新的碳包覆方法而应用于LiFePO4/C复合物的实际生产中。
[Abstract]:Olivine-structured lithium ferric phosphate (LiFePO4) is considered as one of the most promising cathode materials for lithium-ion batteries because of its abundant raw materials, environment-friendly, excellent thermal and chemical stability, high specific capacity (theoretical specific capacity 170mAh g-1), high and flat working voltage (for Li/Li + potential 3.43V). It is the defect of low intrinsic electronic conductivity and lithium ion diffusion coefficient in pure phase LiFePO4 that makes its high current charge-discharge performance difficult to meet the demand and brings great obstacles to the large-scale commercial application of LiFePO4, especially in power battery. Modified LiFePO4 composites were prepared by various modification methods to speed up the commercialization process.
Firstly, PEG-grafted multi-walled carbon nanotubes (MWCNTs-g-PEG, MP) were synthesized by esterification and doped with lithium salts as a new type of conductive agent in LiFePO4 electrode. LiFePO4/MP composite cathode materials were prepared. The structure and electrochemical properties of the composite cathode materials with different molecular weight of PEG and different amount of MP were studied. The results show that the uniform coating of PEG on the surface of MWCNTs can effectively promote the dispersion of carbon nanotubes in the active materials, and is conducive to the formation of a good conductive and thermal conductive network in the electrode. Moreover, the lithium ion diffusivity of LiFePO4/MP is increased by nearly two orders of magnitude compared with LiFePO4/acetylene black, and the lower the molecular weight of PEG, the better the performance; and when the content of MP-350 (350 molecular weight grafted PEG) is 10wt.%, the ratio performance, cycling performance and thermal conductivity of LiFePO4/MP are the best, and its charge-discharge performance at low temperature is also proved. Significant improvement.
Secondly, LiFePO4/PANI composites were successfully coated on the surface of LiFePO4 particles by in-situ polymerization. The effects of different hydrochloric acid concentration and different PANI content on the structure and properties of the composites were discussed. The results showed that the concentration of 1 M was the best. Hydrochloric acid is easy to dissolve LiFePO4 because of its strong acidity, so it is difficult to get the available compound. However, the conductivity of PANI is affected by the low concentration of hydrochloric acid. Choosing 0.1M hydrochloric acid can make the LiFePO4/PANI composite have ideal electrochemical performance, while the content of PANI is 10.2wt.% LiFePO4/PANI (PANI). Because of the proper coating structure, the composite can greatly increase the surface electronic conductivity of LiFePO4 particles, reduce the polarization of the battery and exhibit the best electrochemical performance. After 100 O.1C cycles, the discharge specific capacity of the composite decreases only by 3.2%, and it can still reach 153mAh g 1. The discharge specific capacity of the composite at the rate of 2C is still maintained at ~122 mAh. G-1, but the performance at higher magnification is still to be improved.
Thirdly, in order to improve the charge-discharge performance of LiFePO4/PANI at high rate, PANI-PEG copolymers were prepared by two methods, which were doped to obtain mixed conductivity of electrons and ions and used to modify LiFePO4 cathode materials. The LiFePO4/PANI-PEG composites prepared by in-situ coating of LiFePO4 can achieve uniform and complete coating of particles. The electrochemical properties of the composites are excellent. The specific capacity of 0.1C is up to 165mAh.g-1,5C, and the specific capacity can reach 125 mAh.g-1, and the specific capacity retention rate is 76%, and the lithium ion diffusion coefficient DLi+ is 3.4 *10-13cm2. S-1) also increased by an order of magnitude over the LiFePO4 DLi+ value (3.2 * 10-14cm2. S-1).
Finally, by using the characteristics that dopamine can adhere to any material surface to form a dense layer of poly (dopamine) (PDA) nano-film during the polymerization process, a complete carbon-coated LiFePO4/C composite was prepared using PDA as a new carbon source. Carbon content and carbon-coated thickness can be determined by the ratio of dopamine to LiFePO4 precursor. The "carbon bridge" structure formed by the strong adhesion of PDA between particles and the carbon layer coated on the surface of particles formed a three-dimensional nanoconductive network in the whole active material, which greatly increased the electronic conductivity between LiFePO4 particles, and increased the discharge specific capacity of LiFePO4 at 0.1C from 84mAh g-1 to 135m. Ahg (-1) (containing 2.02wt.%) of carbon, the discharge specific capacity of Ahg (-1) can be maintained in the range of ~70mAh (-1) even at a high rate of 10C. It is expected to be used in the production of LiFePO4/C composites as a new carbon coating method.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：O646;TM912
本文编号：2221692
[Abstract]:Olivine-structured lithium ferric phosphate (LiFePO4) is considered as one of the most promising cathode materials for lithium-ion batteries because of its abundant raw materials, environment-friendly, excellent thermal and chemical stability, high specific capacity (theoretical specific capacity 170mAh g-1), high and flat working voltage (for Li/Li + potential 3.43V). It is the defect of low intrinsic electronic conductivity and lithium ion diffusion coefficient in pure phase LiFePO4 that makes its high current charge-discharge performance difficult to meet the demand and brings great obstacles to the large-scale commercial application of LiFePO4, especially in power battery. Modified LiFePO4 composites were prepared by various modification methods to speed up the commercialization process.
Firstly, PEG-grafted multi-walled carbon nanotubes (MWCNTs-g-PEG, MP) were synthesized by esterification and doped with lithium salts as a new type of conductive agent in LiFePO4 electrode. LiFePO4/MP composite cathode materials were prepared. The structure and electrochemical properties of the composite cathode materials with different molecular weight of PEG and different amount of MP were studied. The results show that the uniform coating of PEG on the surface of MWCNTs can effectively promote the dispersion of carbon nanotubes in the active materials, and is conducive to the formation of a good conductive and thermal conductive network in the electrode. Moreover, the lithium ion diffusivity of LiFePO4/MP is increased by nearly two orders of magnitude compared with LiFePO4/acetylene black, and the lower the molecular weight of PEG, the better the performance; and when the content of MP-350 (350 molecular weight grafted PEG) is 10wt.%, the ratio performance, cycling performance and thermal conductivity of LiFePO4/MP are the best, and its charge-discharge performance at low temperature is also proved. Significant improvement.
Secondly, LiFePO4/PANI composites were successfully coated on the surface of LiFePO4 particles by in-situ polymerization. The effects of different hydrochloric acid concentration and different PANI content on the structure and properties of the composites were discussed. The results showed that the concentration of 1 M was the best. Hydrochloric acid is easy to dissolve LiFePO4 because of its strong acidity, so it is difficult to get the available compound. However, the conductivity of PANI is affected by the low concentration of hydrochloric acid. Choosing 0.1M hydrochloric acid can make the LiFePO4/PANI composite have ideal electrochemical performance, while the content of PANI is 10.2wt.% LiFePO4/PANI (PANI). Because of the proper coating structure, the composite can greatly increase the surface electronic conductivity of LiFePO4 particles, reduce the polarization of the battery and exhibit the best electrochemical performance. After 100 O.1C cycles, the discharge specific capacity of the composite decreases only by 3.2%, and it can still reach 153mAh g 1. The discharge specific capacity of the composite at the rate of 2C is still maintained at ~122 mAh. G-1, but the performance at higher magnification is still to be improved.
Thirdly, in order to improve the charge-discharge performance of LiFePO4/PANI at high rate, PANI-PEG copolymers were prepared by two methods, which were doped to obtain mixed conductivity of electrons and ions and used to modify LiFePO4 cathode materials. The LiFePO4/PANI-PEG composites prepared by in-situ coating of LiFePO4 can achieve uniform and complete coating of particles. The electrochemical properties of the composites are excellent. The specific capacity of 0.1C is up to 165mAh.g-1,5C, and the specific capacity can reach 125 mAh.g-1, and the specific capacity retention rate is 76%, and the lithium ion diffusion coefficient DLi+ is 3.4 *10-13cm2. S-1) also increased by an order of magnitude over the LiFePO4 DLi+ value (3.2 * 10-14cm2. S-1).
Finally, by using the characteristics that dopamine can adhere to any material surface to form a dense layer of poly (dopamine) (PDA) nano-film during the polymerization process, a complete carbon-coated LiFePO4/C composite was prepared using PDA as a new carbon source. Carbon content and carbon-coated thickness can be determined by the ratio of dopamine to LiFePO4 precursor. The "carbon bridge" structure formed by the strong adhesion of PDA between particles and the carbon layer coated on the surface of particles formed a three-dimensional nanoconductive network in the whole active material, which greatly increased the electronic conductivity between LiFePO4 particles, and increased the discharge specific capacity of LiFePO4 at 0.1C from 84mAh g-1 to 135m. Ahg (-1) (containing 2.02wt.%) of carbon, the discharge specific capacity of Ahg (-1) can be maintained in the range of ~70mAh (-1) even at a high rate of 10C. It is expected to be used in the production of LiFePO4/C composites as a new carbon coating method.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：O646;TM912
【参考文献】
相关期刊论文 前10条
1 吴海燕;王剑华;李斌;郭玉忠;;锂离子电池正极材料LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2的研究进展[J];材料导报;2007年S3期
2 王延敏;;新型导电聚苯胺衍生物制备方法研究进展[J];材料导报;2009年01期
3 俞琛捷;莫祥银;康彩荣;倪聪;丁毅;;锂离子电池磷酸铁锂正极材料的制备及改性研究进展[J];材料科学与工程学报;2011年03期
4 张爱勤;王力臻;张勇;;锂离子电池正极材料聚吡咯的性能[J];电池;2008年02期
5 梁英;饶睦敏;蔡宗平;赵灵智;李伟善;;锂离子电池正极材料LiMn_2O_4改性研究进展[J];电池工业;2009年01期
6 林克芝;王晓琳;徐艳辉;;化学处理对多壁碳纳米管电化学储锂性能的影响[J];电源技术;2006年02期
7 付亚娟;郭春雨;郝明明;韩宇;高英;刘兴江;;磷酸铁锂材料的改性研究进展[J];电源技术;2010年09期
8 刘全兵;罗传喜;宋慧宇;廖世军;;磷酸铁锂包覆与掺杂改性研究进展[J];电源技术;2011年03期
9 唐致远,阮艳莉,宋全生,卢星河;橄榄石LiFePO_4复合正极材料的合成及其电化学性能研究[J];高等学校化学学报;2005年10期
10 景遐斌,王利祥,王献红,耿延候,王佛松;导电聚苯胺的合成、结构、性能和应用[J];高分子学报;2005年05期
相关博士学位论文 前2条
1 杨志方;苯胺齐聚物或聚苯胺嵌段共聚物的合成与组装[D];华中科技大学;2010年
2 范长岭;导电聚合物PPy和PAn在锂离子电池正极中的应用[D];湖南大学;2012年
，本文编号：2221692
本文链接：https://www.wllwen.com/kejilunwen/dianlilw/2221692.html
