| 热解炭和纳米炭纤维包覆改性锂离子电池活性物质的研究 | |
| Alternative Title | Modification of Li-ion battery active materials by coating pyrolytic carbon and carbon nanofiber |
| 刘树和 | |
| Subtype | 博士 |
| Thesis Advisor | 成会明 |
| 2008-05-31 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料学 |
| Keyword | 热解炭 纳米炭纤维 化学气相沉积 核壳结构 锂离子电池 电化学性能 |
| Abstract | 作为一种高效储能器件，锂离子电池已在人类的生产和生活中扮演着重要角色。价格低廉、比容量较高的天然石墨（Natural Graphite, NG）和稳定性高、循环性能优良的磷酸铁锂(LiFePO4)是极有应用价值的锂离子电池活性物质。但是由于天然石墨不可逆容量高、循环性能差，LiFePO4的离子和电子导电能力差，均无法直接作为锂离子电池活性物质加以应用。热解炭(Pyrolytic Carbon,PyC)和纳米炭纤维(Carbon Nanofiber,CNF)具有优异的力学性能、导电性能和与电解液的良好相容性，用其对天然石墨和LiFePO4进行包覆改性能极大改善锂离子电池活性物质的电化学性能，可望实现工业化应用。 本论文采用化学气相沉积（chemicalvapor deposition,CVD）工艺，首先制备了一维PyC材料--炭丝，研究了制备工艺与其形貌和微观结构的关系，提出了其生长机理。然后采用不同的流化床工艺（振动床和搅动床）制备了PyC/CNF包覆天然石墨和LiFePO4的核壳结构复合材料。系统研究了PyC或CNF壳层的形貌、结构、分布及含量等对复合材料物理性质和电化学性能的影响，得到如下结果： 1. 利用CVD工艺在不添加任何催化剂的条件下制备出多种形貌的炭丝。炭丝一般为锥状形貌，茎部由沿径向排列的年轮状石墨层组成，而头部由同心的球形石墨片层套构而成。选择合适的工艺参数可以制备出长径比大、生长密度高的炭丝材料。炭丝的拉伸强度从石墨化前的187.9 MPa提高到石墨化后的221.7 MPa，其拉伸断裂行为可以用剑鞘断裂机理来解释。 2. 利用流化床工艺制备了PyC/NG和CNF/NG核壳结构复合材料。适当厚度、结构和含量的PyC包覆壳层能够有效地降低天然石墨的比表面积、防止了石墨的粉化剥落，从而显著改善其首次库仑效率和循环性能。由于颗粒状结构的PyC壳层有利于降低活性物质颗粒间的接触电阻并增加有机粘结剂对活性物质颗粒之间的粘结强度，因此可以得到较高循环性能。 由于CNF/NG的比表面积随CNF含量的增加而增大，引起首次循环的不可逆容量增加和首次库仑效率下降,但随后的充放电过程中循环稳定性显著增加。CNF能够在天然石墨颗粒之间形成良好的导电网络，并抑制天然石墨破坏，因此能够有效地提高天然石墨的循环性能。 在天然石墨上复合包覆CNF和PyC，能够在改善循环性能的基础上，提高首次库仑效率。 3. 利用流化床工艺制备了PyC/LiFePO4和CNF/LiFePO4两种核壳结构复合材料。两种复合材料的电阻率均随包覆PyC或CNF含量的增加而显著降低。适当含量的PyC或CNF包覆层能够有效地提高LiFePO4的容量和循环性能。PyC和CNF包覆层均能在LiFePO4颗粒之间构筑导电网络，但与PyC相比，由于CNF具有一维结构和优异的力学性能，因此更适于作为LiFePO4电极材料的高效导电剂。 4. 炭丝是由反应空间中的粘滞性小液滴定向聚集、融并、碳化生成的，符合PyC沉积的液滴理论。按照不同的流化工艺，流化床中天然石墨球上PyC的沉积遵循不同的机理：振动床工艺中沉积的PyC主是颗粒状形貌，以气相均相成核（液滴理论）为主要沉积机理；搅动床工艺中沉积的PyC是光滑形貌，符合表面生长沉积机理。天然石墨球和LiFePO4颗粒上CNF的生长遵循“吸附-扩散-沉积”模型，以顶端生长方式从附着在活性物质表面的Ni催化剂上生长而成。 |
| Other Abstract | As a high efficiency energy storage device, lithium ion secondary battery begins to act as an important role in our life. Natural graphite (NG) with low price and high capacity and LiFePO4 with high stablity and excellent cyclability are the active materials with high potential for lithium ion secondary battery (LIB). However, they can not be used directly as the active materials of LIB because of the high irreversible capacity and poor cyclability of NG, and the low diffusion of lithium ion in LiFePO4 and poor electron conductivity of LiFePO4. Due to their excellent mechanical property, electron conductance and compatible with electrolyte, pyrolytic carbon (PyC) and carbon nanofibers (CNFs) can greatly improve the electrochemical performances of NG and LiFePO4 by coating them on the active materials to form shell-core structured composites, and these composites are hopeful for industrial application. In this dissertation, one dimensional PyC, carbon filament, was fabricated by chemical vapor deposition (CVD) method and the effect of fabricating parameters on the morphology and microstructure of carbon filaments was explored and their growth mechanism was proposed. Then, shell-core structured PyC/CNF coated NG and LiFePO4 were synthesized by different types of fluidized bed CVD (vibrated bed and agitated bed). The effect of the morphology, structure, distribution, and content of the PyC or CNF shell on the physical and electrochemical properties of the composites was systematically studied and the following results were obtained: 1. Carbon filaments with various morphologies were fabricated without the use of any catalyst by CVD. Most of the filaments are cone-shaped, the stems of which are formed of annular layers arranged in a tree ring structure and the heads are formed of concentrical graphite layers. Carbon filaments with high aspect ratio and density can be synthesized by properly selected parameters. The tensile strength of the filaments averages 187.9 MPa, which reaches 221.7 MPa after graphitization. The tensile break behavior of the carbon filaments can be explained by the sword-in-sheath fracture mechanism. 2. Shell-core structured PyC/NG and CNF/NG composites were synthesized by fluidized bed CVD. Coating layer with proper thickness, structure and content of PyC can effectively reduce the specific surface area (SSA) of NG and refrain the exfoliation of graphite, and therefore improves the first coulombic efficiency (CE) and cyclability of NG. Due to the increase of the surface roughness on sub-micron scale by coating with granular PyC film to decrease the contacting resistance and increase the adhesion among the graphite particles by the organic binders termed anchor effect, the cyclability of NG is greatly improved. SSA of CNF/NG increases with the increase of CNF content and therefore the irreversible capacity in the first cycle increases and the first CE decreases. However, the cycling stability greatly increases in the following discharge and charge process. CNFs can form a good conductive network among NG particles and refrain the exfoliation of graphite, so they can improve the cyclability of NG effectively. The first CE of CNF/NG composite can be increased by coating PyC on it while the cyclability of NG is improved by CNF coating. 3. Shell-core structured PyC/LiFePO4 and CNF/LiFePO4 composites were synthesized by fluidized bed CVD. The resistance of the composites decreases markedly with the increase of the PyC or CNF content.The capacity and cyclability of LiFePO4 were effectively improved by proper amount of PyC or CNF coating. A good conductive network can be formed among LiFePO4 particles through PyC or CNF coating.Compared to PyC, CNF is more suitable to be used as the conductive additive for the active material of LIB because of the one dimensional structure and excellent mechanical properties. 4. Carbon filament is formed through the aggregation, coalescence and carbonization of the viscid droplets formed in the reaction zone, and therefore its formation can be explained by the droplet theory of PyC deposition. According to different fluidized CVD, PyC deposition on NG and LiFePO4 follows different mechanisms: in the vibrated bed, PyC deposited is mainly granular and is formed through homogenous nucleation of the intermediate species formed in the gas phase (droplet theory); while in the agitated bed, PyC deposited is smooth and is formed through surface growth mechanism. The growth of CNF on the surface of NG and LiFePO4 follows the “adsorption-diffusion-precipiation” mechanism, and grows in tip growth model from the nickel catalyst adhered on the surface of active materials. |
| Pages | 146 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/16894 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 刘树和. 热解炭和纳米炭纤维包覆改性锂离子电池活性物质的研究[D]. 金属研究所. 中国科学院金属研究所,2008. |
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