【摘要】：橄欖石型磷酸鐵鋰(LiFePO_4)作為目前最有發(fā)展前景的鋰離子動(dòng)力電池正極材料之一,具有高比容量、好的循環(huán)可逆性能、低的原料成本、高的安全性能和環(huán)境友好等優(yōu)點(diǎn),目前已經(jīng)成為電池界競(jìng)相開發(fā)和研究的熱點(diǎn)。但其低的電子導(dǎo)電率和鋰離子擴(kuò)散系數(shù),導(dǎo)致了LiFePO_4材料在高倍率充放電條件下比容量衰減迅速,從而嚴(yán)重地阻礙了LiFePO_4的商業(yè)化應(yīng)用。針對(duì)以上存在的問題,本文通過調(diào)控LiFePO_4材料的顆粒大小和晶體生長(zhǎng)方向,以及通過碳改性等手段來(lái)改性LiFePO_4材料,最終提高其充放電性能。本文取得了如下主要結(jié)果:首先,采用實(shí)驗(yàn)室規(guī)格的高剪切混合器來(lái)制備前驅(qū)體,經(jīng)水熱結(jié)晶,成功地實(shí)現(xiàn)了對(duì)LiFePO_4/C材料粒度大小的調(diào)控。通過研究獲得了高剪切混合器轉(zhuǎn)子轉(zhuǎn)速對(duì)前驅(qū)體沉淀和LiFePO_4/C材料的晶體結(jié)構(gòu)、顆粒形貌、大小及其分布的影響規(guī)律,揭示了高剪切混合器輔助的水熱法實(shí)現(xiàn)粒度可控的關(guān)鍵。在高剪切混合器轉(zhuǎn)速為1.3×104 rpm下制備得到的LiFePO_4/C樣品,其粒度減小至~220 nm,表現(xiàn)出了優(yōu)異的電化學(xué)性能,在0.1 C和20 C倍率下,其放電比容量分別達(dá)到160.1mAh·g~(-1)和90.8 mAh·g~(-1)。其次,在高剪切混合器輔助的水熱工藝基礎(chǔ)上,繼續(xù)在前驅(qū)體的混合過程中加入了非離子型表面活性劑Tween-80。研究表明,表面活性劑Tween-80分子在水熱合成過程中可以降低LiFePO_4產(chǎn)物粒度大小,并且可以調(diào)控LiFePO_4產(chǎn)物沿(010)晶面生長(zhǎng)。制備得到的LiFePO_4/C材料其顆粒粒度減小至~100 nm,I(020)/I(111)比值高達(dá)1.19,表現(xiàn)出了優(yōu)異的電化學(xué)性能,在0.1 C和20 C倍率下,放電容量高達(dá)166.5 mAh·g~(-1)和119.6 mAh·g~(-1)。最后,采用金屬有機(jī)框架MIL~(-1)00(Fe)同時(shí)作為模板和原料,制備得到了由LiFePO_4納米顆粒嵌入多孔連續(xù)碳骨架內(nèi)部的三維多孔LFP/CNWs材料。為了進(jìn)一步提高該材料的導(dǎo)電性能,將LFP/CNWs樣品同三聚氰胺(C3N3(NH2)3)混合后,經(jīng)碳熱還原反應(yīng)制備得到了氮改性的LFP/N-CNWs材料。該氮改性的多孔碳骨架(N-CNWs)環(huán)繞在LiFePO_4納米顆粒表面,增大了LiFePO_4顆粒同電解液之間的接觸面積,加快了鋰離子和電子在整個(gè)電極材料內(nèi)的傳導(dǎo)速率,提高了LiFePO_4材料的有效利用率和倍率性能。LFP/N-CNWs樣品在0.1 C和20 C倍率下的放電容量分別達(dá)到161.1 mAh·g~(-1)和93.6 mAh·g~(-1)。
[Abstract]:Olivine type lithium ferric phosphate (LiFePO_4) is one of the most promising cathode materials for lithium-ion power battery. It has the advantages of high specific capacity, good recycling reversibility, low raw material cost, high safety performance and environmental friendliness. At present, battery industry has become a hot spot in the field of development and research. However, the low electron conductivity and lithium ion diffusion coefficient lead to the rapid decline of specific capacity of LiFePO_4 materials under high rate charge and discharge conditions, which seriously hinder the commercial application of LiFePO_4. In order to improve the charge-discharge performance of LiFePO_4 materials, the particle size, crystal growth direction and carbon modification were adjusted to improve the charge-discharge properties of LiFePO_4 materials. The main results obtained in this paper are as follows: firstly, the precursor is prepared by using a high shear mixer in laboratory specification, and the particle size of LiFePO_4/C material is successfully controlled by hydrothermal crystallization. The effects of rotor speed of high shear mixer on the crystal structure, morphology, size and distribution of precursor precipitates and LiFePO_4/C materials were obtained, and the key to realize particle size control by hydrothermal method assisted by high shear mixer was revealed. The LiFePO_4/C samples prepared at high shear mixer speed of 1.3 脳 10 ~ 4 rpm showed excellent electrochemical properties when the particle size was reduced to ~ 220 nm,. The specific discharge capacity of the samples reached 160.1mAh g ~ (-1) and 90.8 mAh g ~ (-1) at the rate of 0.1 C and 20 C, respectively. Secondly, on the basis of hydrothermal process assisted by high shear mixer, Nonionic surfactant Tween-80. was added to the mixing process of the precursor. The results show that the surfactant Tween-80 molecule can reduce the particle size of LiFePO_4 products during hydrothermal synthesis and can regulate the growth of LiFePO_4 products along the (010) crystal plane. The particle size of the prepared LiFePO_4/C material was reduced to ~ 100 nm,I (0 20) / I (111) as high as 1.19, showing excellent electrochemical performance. The discharge capacity reached 166.5 mAh g ~ (-1) and 119.6 mAh g ~ (-1) at the ratio of 0.1C and 20C. Finally, MIL~ (-1) 00 (Fe), a metal-organic framework, was used as a template and a raw material to prepare a three-dimensional porous LFP/CNWs material embedded into the porous continuous carbon skeleton by LiFePO_4 nanoparticles. In order to further improve the electrical conductivity of the material, the nitrogen-modified LFP/N-CNWs material was prepared by carbothermal reduction after mixing the LFP/CNWs sample with melamine (C3N3 (NH2) 3). The nitrogen modified porous carbon skeleton (N-CNWs) encircles the surface of LiFePO_4 nanoparticles, increases the contact area between LiFePO_4 particles and electrolyte, and accelerates the lithium ion and electron conduction rate in the whole electrode material. The effective utilization ratio and the performance of LiFePO_4 were improved. The discharge capacity of LFP/N-CNWs samples at the rate of 0.1 C and 20 C was 161.1 mAh g-1 and 93.6 mAh g-1, respectively.
【學(xué)位授予單位】：天津大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2015
【分類號(hào)】：TQ131.11;TM912

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