【摘要】：橄欖石結(jié)構(gòu)的磷酸鐵鋰(LiFePO4)由于具有原料來(lái)源豐富、環(huán)境友好、優(yōu)良的熱穩(wěn)定性和化學(xué)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(duì)Li/Li+電位為3.43V)等優(yōu)點(diǎn)被認(rèn)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(yīng)用時(shí)存在本征電子電導(dǎo)率和鋰離子擴(kuò)散系數(shù)較低的缺陷,這導(dǎo)致其大電流充放電性能難以滿(mǎn)足需求,給LiFePO4的大規(guī)模商業(yè)化應(yīng)用尤其是在動(dòng)力電池方面的應(yīng)用帶來(lái)了極大的障礙。本文針對(duì)LiFePO4的兩大缺陷,著力通過(guò)各種改性途徑制備了改性的LiFePO4復(fù)合材料,以加快其商業(yè)化進(jìn)程。 首先,通過(guò)酯化反應(yīng)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(dǎo)電劑用于LiFePO4電極中,制備了LiFePO4/MP復(fù)合正極材料,分別研究了不同PEG分子量和不同MP添加量對(duì)復(fù)合正極材料的結(jié)構(gòu)、電化學(xué)性能、導(dǎo)電和導(dǎo)熱性能等的影響。結(jié)果表明,PEG在MWCNTs表面的均勻包覆能有效促進(jìn)碳管在活性物質(zhì)中的分散,有利于在電極中形成良好的導(dǎo)電和導(dǎo)熱網(wǎng)絡(luò),在MP添加量?jī)H為5wt.%的條件下,也能表現(xiàn)出優(yōu)于傳統(tǒng)導(dǎo)電劑乙炔黑添加量為20wt.%的電池性能,而且LiFePO4/MP的鋰離子擴(kuò)散系數(shù)較LiFePO4/乙炔黑的增加了近兩個(gè)數(shù)量級(jí),且PEG分子量越低,性能越好；而當(dāng)MP-350(接枝PEG的分子量為350)含量為10wt.%時(shí),其倍率性能、循環(huán)性能和導(dǎo)熱性能均達(dá)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(guò)原位聚合包覆法將既具有導(dǎo)電性又具有電化學(xué)活性的聚苯胺(PANI)成功包覆在LiFePO4顆粒表面得到了LiFePO4/PANI復(fù)合材料,探討了制備過(guò)程中不同鹽酸濃度和復(fù)合物中不同PANI含量對(duì)產(chǎn)物結(jié)構(gòu)和性能的影響,研究表明,濃度為1M的鹽酸由于酸性過(guò)強(qiáng)易使LiFePO4發(fā)生溶解,難以得到可用的復(fù)合物,而鹽酸濃度太小則由于酸摻雜濃度過(guò)低使得PANI的導(dǎo)電性能受到影響,選擇0.1M的鹽酸能使所得LiFePO4/PANI復(fù)合物具有較理想的電化學(xué)性能；而PANI含量為10.2wt.%的LiFePO4/PANI (400μL)復(fù)合物由于具有合適的包覆結(jié)構(gòu),能大大增加LiFePO4顆粒的表面電子電導(dǎo)率,減少電池的極化,表現(xiàn)出最優(yōu)異的電化學(xué)性能,經(jīng)過(guò)100次O.1C循環(huán)之后,其放電比容量?jī)H衰減3.2%,仍可達(dá)153mAh·g-1,倍率為2C時(shí)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(guò)摻雜使其獲得電子和離子混合傳導(dǎo)性,并將其用于改性LiFePO4正極材料。結(jié)果顯示,通過(guò)在PEG末端引入苯胺基團(tuán),然后引發(fā)苯胺聚合,并在此過(guò)程中對(duì)LiFePO4進(jìn)行原位包覆,能實(shí)現(xiàn)對(duì)顆粒的均勻完整包覆,所得LiFePO4/PANI-PEG復(fù)合物表現(xiàn)出極其優(yōu)異的電化學(xué)性能,0.1C的放電比容量高達(dá)165mAh·g-1,5C時(shí)的比容量也可達(dá)到125mAh·g-1,比容量保持率為76%,并且其鋰離子擴(kuò)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(gè)數(shù)量級(jí)。 最后,利用多巴胺在聚合過(guò)程中可以附著在任何物質(zhì)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(fù)合物,碳含量和碳包覆層厚度可以通過(guò)多巴胺與LiFePO4前驅(qū)體之間的比例來(lái)進(jìn)行調(diào)控,而顆粒之間由于PDA強(qiáng)粘附性所形成的“碳橋”結(jié)構(gòu)以及顆粒表面包覆的碳層在整個(gè)活性物質(zhì)中形成了三維納米導(dǎo)電網(wǎng)絡(luò),極大地增加了LiFePO4顆粒之間的電子傳導(dǎo)能力,使得LiFePO4在0.1C的放電比容量由無(wú)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(yīng)用于LiFePO4/C復(fù)合物的實(shí)際生產(chǎn)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(xué)位授予單位】：華中科技大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2014
【分類(lèi)號(hào)】：O646;TM912

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