橄榄石型磷酸盐LiMPO4(M=Fe,Mn)的水热合成机理及电化学性能研究

IMR OpenIR

	橄榄石型磷酸盐LiMPO4(M=Fe,Mn)的水热合成机理及电化学性能研究
	秦学
学位类型	博士
导师	周延春 ; 王晓辉
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	锂离子电池 Lifepo4 水热合成电化学高能量密度 Lithium-ion Battery Lifepo4 Hydrothermal Synthesis Electrochemistry High Volumetric Energy
其他摘要	" 当今，清洁能源包括风能、太阳能、核能和潮汐能，方兴未艾。锂离子电池在储存这些清洁能源中扮演着重要角色。它们正被用于大到电动汽车，小到微型集成电路片等多种装置中。锂离子电池主要由四部分组成，即正极、负极、电解液和膈膜。与负极材料的比容量相比，正极材料的比容量要小得多。因此，锂离子电池的能量密度主要取决于正极。与众多的正极材料如LiCoO2、LiMn2O4和LiNiO2相比，具有橄榄石结构的LiFePO4被认为是一种较为理想的正极材料。它具有诸多优点，如较高的理论比容量(170 mAh/g)、较好的循环稳定性、低成本、环境友好和优良的本征热稳定性。与传统的固相合成方法相比，水热法具有低成本、产品粒径分布均匀、反应速度快和产物尺寸可控等优点。然而，水热合成LiFePO4的机理尚不清楚。因此，本文系统地研究了微米LiFePO4和纳米LiFePO4的水热合成机理和晶体生长机制。在此基础上，提出一种新颖的LiFePO4块体电极概念。主要结论如下： (1) 以LiOH、H3PO4、MSO4 (M = Fe, Mn)和抗坏血酸为合成原料，通过向H3PO4、MSO4与抗坏血酸的混合溶液中缓慢添加LiOH溶液制备出粘稠状前躯体。利用诸如XRD、SEM和TEM表征手段，我们系统地研究了微米LiFePO4菱形板片在水热合成过程中系统内部的原位温度、pH值变化、物相演变和晶体生长方向。结合水热合成LiMnPO4的物相演变和FTIR分析结果，表明在前躯体中存在一种无定形Fe2P4O12化合物。该大分子化合物使得前躯体变得粘稠。并且，它在水热过程中的水解引起水热系统中pH值的下降。观察到前躯体明显的溶解过程，较快的产物形成，增加前躯体的过饱度以及引入大比表面积形核质点可降低产物粒径等实验事实，综合推断出水热合成LiFePO4菱形板片为溶解－沉淀机制。 (2) 系统地研究了纳米LiFePO4水热合成机理。通过向LiOH溶液中缓慢加入H3PO4，随后引入抗坏血酸和FeSO4成功获得新型悬浊液状前躯体。研究表明，两种不同添加顺序所获得的前驱体溶解速率的差异导致了产物粒度的不同，并且首次提出了两步生长机制合理地解释了所获得的不同尺度的LiFePO4形貌的迥然差异。研究了乙二醇对水热合成LiFePO4的影响。结果表明，乙二醇优先在LiFePO4{010}面上吸附，从而抑制LiFePO4晶粒沿[010]方向生长。同时，结合第一原理计算和实验证据，证明了反位缺陷浓度的降低可以通过PO4四面体中P-O对称伸缩振动模式峰(983-1003 cm-1)的红移来指示。根据此规律，发现乙二醇可以降低LiFePO4晶体内的Li/Fe反位缺陷浓度。 (3) 利用水热合成出的LiFePO4具有优良的烧结性这一优点，通过传统的陶瓷烧结方法结合化学气相沉积工艺，制备出多孔导电LiFePO4块体电极。与制备传统的薄膜电极相比，该块体电极不使用任何粘合剂、溶解剂和集流体，并且制备工艺简单、造成本低、大幅度缩短了工艺流程。同时，所制备出的块体电极具有超高的体积比能量和稳定的循环性能。 (4) 利用不同尺度和形貌的尿素甲醛树脂，通过牺牲模板法对块体电极进行进一步的优化。最终，在真空热处理条件下制备具有高孔隙率的互相贯穿的层次块体电极。因此，所获得的块电极同时具有较高的能量密度和良好的倍率性能。"; " Clean energies including wind, solar, nuclear and tide power now are in the ascendant. Lithium-ion batteries are currently playing a key role in storing these clean energies, and powering the diverse devices from electric vehicles to microchips. Lithium-ion batteries are generally made up of four parts like cathode, anode, electrolyte and separator. Compared with the specific capacity of anode materials, the specific capacity of cathode materials are innately low. Therefore, the energy density of lithium-ion battery is largely determined by the cathode materials. As compared with other cathode materials like LiCoO2, LiMn2O4 and LiNiO2, the olivine-type LiFePO4 is considered as highly praised cathode materials. This battery material has many advantages such as high theoretical specific capacity (170 mAh/g), superior cycling stability, low cost, environmental friendliness, and excellent innate thermal safety. In comparison with conventional solid-phase synthesis methods, hydrothermal method presents low cost, homogeneous particle size distribution, fast reaction rate, and facile product size control, etc. However, mechanism for hydrothermal synthesizing LiFePO4 is still unclear. Consequently, the crystal growth mechanism of microscaled and nanoscaled LiFePO4 is systematically studied in this dissertation. Based on this, we propose a novel concept of LiFePO4-based bulk electrode. The main conclusions are as follows: (1) We used LiOH, H3PO4, MSO4 (M = Fe, Mn) and ascorbic acid as raw materials. The sticky precursor was prepared by the addition of LiOH solution into the mixed solution of H3PO4, MSO4 and ascorbic acid. With the aid of characterizationtechnique like XRD, SEM and TEM, in-situ variation of temperature and pH value in reaction-system, phase evolution and crystal growth direction are systematically studied during hydrothermal synthesizing LiFePO4 rhombus platelets. Combined with phase evolution during hydrothermal synthesizing LiMnPO4 and FTIR analysis results, it is indicated that there exist amorphous Fe2P4O12 compound in precursor. This macromolecular compound leads the obtianed precursor to be sticky, and hydrolysis of tetraphosphate cause the pH value drop during hydrothermal treatment. As evidenced by apparent precursor dissolution, fast hydrothermal formation, and significant decrease in product size with increasing precursor supersaturation and introducing nucleation sites with large specific surface area into the reaction medium in the reaction system, a dissolution-precipitation mechanism is accounted for the hydrothermal synthesis of LiFePO4 rhombus platelets. (2) The mechanism of hydrothermal synthesizing nanoscaled LiFePO4 is systematically studied. The novel suspensible precursor was successfully obtained through slow adding H3PO4 into LiOH solution and subsequently introducing FeSO4 and ascorbic acid. The results show that different dissolution rate of precursors prepared by different feeding sequence of starting materials lead to different particle sizes. Furthermore, we propose a two-step growth mechanism to explain the difference of LiFePO4 morphology with different particle sizes. The effects of ethylene glycol in hydrothermal synthesizing LiFePO4 are investigated. It is indicated that prior adsorption of ethylene glycol on the {010} faces of LiFePO4 inhibits the crystal growth of LiFePO4 along [010] direction. At same time, combined with first-principles calculation and experimental evidence, we find that the reduction of Fe• Li antisite defect concentration could be denoted by a red shift of the vibration band corresponding to the symmetric stretching P-O vibration of the PO4 tetrahedron (983-1003 cm-1). Based on this principle, we show that ethylene glycol plays an essential role in reducing the Fe• Li antisite defect concentration. (3) With the aid of nice sintering activity of hydrothermally synthesized LiFePO4, porous and electrically conductive LiFePO4-based bulk electrodes are successfully fabricated by traditional ceramic sinter and chemical vapor deposition methods. In comparison with conventional thin film electrode, the as-prepared bulk electrodes are binder-, solvent- and current-collector-free. Furthermore, the preparation process of the low-cost bulk electrodes is much simplified, which would greatly shorten process. Moreover, the as-obtained bulk electrodes have the characteristics of high volumetric energy and stable cycle life. (4) The bulk electrodes are subsequently optimized bysacrificial template-directed procedure using different-sized and -morphologic urea formaldehyde resin UFR as templates. The hierarchical and interpenetrating bulk electrodes with high porosity are successfully prepared under vacuum heat treatment. Thus, the obtained bulk electrodes simultaneously have high volumetric energy and improved rate capability. "
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64463
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	秦学. 橄榄石型磷酸盐LiMPO4(M=Fe,Mn)的水热合成机理及电化学性能研究[D]. 北京. 中国科学院金属研究所,2012.