新型磷酸鐵鋰復合正極材料的制備與性能研究
發(fā)布時間：2018-09-04 09:33
【摘要】：橄欖石結構的磷酸鐵鋰(LiFePO4)由于具有原料來源豐富、環(huán)境友好、優(yōu)良的熱穩(wěn)定性和化學穩(wěn)定性、比容量高(理論比容量為170mAh·g-1)、高而平坦的工作電壓(對Li/Li+電位為3.43V)等優(yōu)點被認為是目前最具前景的鋰離子電池正極材料之一,但是純相LiFePO4在應用時存在本征電子電導率和鋰離子擴散系數(shù)較低的缺陷,這導致其大電流充放電性能難以滿足需求,給LiFePO4的大規(guī)模商業(yè)化應用尤其是在動力電池方面的應用帶來了極大的障礙。本文針對LiFePO4的兩大缺陷,著力通過各種改性途徑制備了改性的LiFePO4復合材料,以加快其商業(yè)化進程。 首先,通過酯化反應制備了PEG接枝的多壁碳納米管(MWCNTs-g-PEG, MP),并將其與鋰鹽摻雜后作為一種新型的導電劑用于LiFePO4電極中,制備了LiFePO4/MP復合正極材料,分別研究了不同PEG分子量和不同MP添加量對復合正極材料的結構、電化學性能、導電和導熱性能等的影響。結果表明,PEG在MWCNTs表面的均勻包覆能有效促進碳管在活性物質中的分散,有利于在電極中形成良好的導電和導熱網(wǎng)絡,在MP添加量僅為5wt.%的條件下,也能表現(xiàn)出優(yōu)于傳統(tǒng)導電劑乙炔黑添加量為20wt.%的電池性能,而且LiFePO4/MP的鋰離子擴散系數(shù)較LiFePO4/乙炔黑的增加了近兩個數(shù)量級,且PEG分子量越低,性能越好；而當MP-350(接枝PEG的分子量為350)含量為10wt.%時,其倍率性能、循環(huán)性能和導熱性能均達到最優(yōu),且其低溫充放電性能也得到了明顯改善。 其次,通過原位聚合包覆法將既具有導電性又具有電化學活性的聚苯胺(PANI)成功包覆在LiFePO4顆粒表面得到了LiFePO4/PANI復合材料,探討了制備過程中不同鹽酸濃度和復合物中不同PANI含量對產物結構和性能的影響,研究表明,濃度為1M的鹽酸由于酸性過強易使LiFePO4發(fā)生溶解,難以得到可用的復合物,而鹽酸濃度太小則由于酸摻雜濃度過低使得PANI的導電性能受到影響,選擇0.1M的鹽酸能使所得LiFePO4/PANI復合物具有較理想的電化學性能；而PANI含量為10.2wt.%的LiFePO4/PANI (400μL)復合物由于具有合適的包覆結構,能大大增加LiFePO4顆粒的表面電子電導率,減少電池的極化,表現(xiàn)出最優(yōu)異的電化學性能,經(jīng)過100次O.1C循環(huán)之后,其放電比容量僅衰減3.2%,仍可達153mAh·g-1,倍率為2C時的放電比容量仍維持在,～122mAh·g-1,但更高倍率下的性能仍有待改善。 再次,為了改善LiFePO4/PANI在大倍率下的充放電性能,采用兩種方法制備了PANI-PEG共聚物,通過摻雜使其獲得電子和離子混合傳導性,并將其用于改性LiFePO4正極材料。結果顯示,通過在PEG末端引入苯胺基團,然后引發(fā)苯胺聚合,并在此過程中對LiFePO4進行原位包覆,能實現(xiàn)對顆粒的均勻完整包覆,所得LiFePO4/PANI-PEG復合物表現(xiàn)出極其優(yōu)異的電化學性能,0.1C的放電比容量高達165mAh·g-1,5C時的比容量也可達到125mAh·g-1,比容量保持率為76%,并且其鋰離子擴散系數(shù)DLi+值(3.4×10-13cm2·s-1)也較LiFePO4的DLi+值(3.2×10-14cm2·s-1)增加了一個數(shù)量級。 最后,利用多巴胺在聚合過程中可以附著在任何物質表面形成一層致密聚多巴胺(PDA)納米薄膜的特性,選擇PDA作為一種新型碳源,制備了完整碳包覆的LiFePO4/C復合物,碳含量和碳包覆層厚度可以通過多巴胺與LiFePO4前驅體之間的比例來進行調控,而顆粒之間由于PDA強粘附性所形成的“碳橋”結構以及顆粒表面包覆的碳層在整個活性物質中形成了三維納米導電網(wǎng)絡,極大地增加了LiFePO4顆粒之間的電子傳導能力,使得LiFePO4在0.1C的放電比容量由無碳的84mAh·g-1增至135mAhg·-1(含碳2.02wt.%),即使在10C的大倍率情況下,其放電比容量仍可維持在～70mAh·g-1,有望作為一種新的碳包覆方法而應用于LiFePO4/C復合物的實際生產中。
[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.
【學位授予單位】：華中科技大學
【學位級別】：博士
【學位授予年份】：2014
【分類號】：O646;TM912
本文編號：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.
【學位授予單位】：華中科技大學
【學位級別】：博士
【學位授予年份】：2014
【分類號】：O646;TM912
【參考文獻】
相關期刊論文 前10條
1 吳海燕;王劍華;李斌;郭玉忠;;鋰離子電池正極材料LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2的研究進展[J];材料導報;2007年S3期
2 王延敏;;新型導電聚苯胺衍生物制備方法研究進展[J];材料導報;2009年01期
3 俞琛捷;莫祥銀;康彩榮;倪聰;丁毅;;鋰離子電池磷酸鐵鋰正極材料的制備及改性研究進展[J];材料科學與工程學報;2011年03期
4 張愛勤;王力臻;張勇;;鋰離子電池正極材料聚吡咯的性能[J];電池;2008年02期
5 梁英;饒睦敏;蔡宗平;趙靈智;李偉善;;鋰離子電池正極材料LiMn_2O_4改性研究進展[J];電池工業(yè);2009年01期
6 林克芝;王曉琳;徐艷輝;;化學處理對多壁碳納米管電化學儲鋰性能的影響[J];電源技術;2006年02期
7 付亞娟;郭春雨;郝明明;韓宇;高英;劉興江;;磷酸鐵鋰材料的改性研究進展[J];電源技術;2010年09期
8 劉全兵;羅傳喜;宋慧宇;廖世軍;;磷酸鐵鋰包覆與摻雜改性研究進展[J];電源技術;2011年03期
9 唐致遠,阮艷莉,宋全生,盧星河;橄欖石LiFePO_4復合正極材料的合成及其電化學性能研究[J];高等學�；瘜W學報;2005年10期
10 景遐斌,王利祥,王獻紅,耿延候,王佛松;導電聚苯胺的合成、結構、性能和應用[J];高分子學報;2005年05期
相關博士學位論文 前2條
1 楊志方;苯胺齊聚物或聚苯胺嵌段共聚物的合成與組裝[D];華中科技大學;2010年
2 范長嶺;導電聚合物PPy和PAn在鋰離子電池正極中的應用[D];湖南大學;2012年
，本文編號：2221692
本文鏈接：http://www.sikaile.net/kejilunwen/dianlilw/2221692.html
