【作者】 庄大高；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料学， 2006， 博士

【摘要】 橄榄石型LiFePO₄因理论比容量高、价格低廉、对环境友好、循环性能优良、安全性能突出等优点而被认为最具开发和应用潜力的新一代锂离子电池正极材料。但是纯LiFePO₄因电子电导率极低而造成可逆容量低,大电流充放电性能差。针对这种情况,人们尝试采取减小材料颗粒尺寸,添加导电剂,掺杂金属离子等措施来改善LiFePO₄的性能,目前取得了一定的进展。 本文以寻求性能达到商业化应用需求的LiFePO₄作为研究的目的。采用水热法合成了LiFePO₄、Li（Mn,Fe）PO₄、Li（Co,Fe）PO₄、Li（Ni,Fe）PO₄及相应的经高温碳包覆的复合材料。用一步高温固相法合成了LiFePO₄/C及（Li,Nb）FePO₄/C复合材料。利用XRD、SEM、TEM、Rietveld Refinement拟合等技术对产物的微观结构和形貌进行了分析。采用恒流充放电、循环伏安（CV）和电化学阻抗谱（EIS）技术测试其电化学性能。对水热合成和固相合成的LiFePO₄/C存在的性能差别作了研究。系统研究了合成工艺条件对产物电化学性能的影响。 以FeSO₄、H₃PO₄和LiOH为原料,用水热合成法制备纯度高、结晶好、颗粒分布均匀且细小的LiFePO₄粉体。分析了合成温度和时间对产物LiFePO₄的形貌和电化学性能的影响。研究表明,晶体生长完整、颗粒小的LiFePO₄具有较好的电化学性能。在本文的实验条件下,150℃、5h合成的LiFePO₄具有最好的电化学性能。对水热法合成LiFePO₄反应机理的研究表明,要得到单相的LiFePO₄,必须先将LiOH和H₃PO₄混合,形成中间产物Li₃PO₄,再与Fe²⁺反应才能得到单相的LiFePO₄。利用聚丙烯高温裂解对水热合成产物进行碳包覆制备了LiFePO₄/C复合材料。最佳的包覆温度为550℃,在此条件下得到几乎球形化的LiFePO₄/C复合材料。在以0.05C和0.1C充放电时,LiFePO₄/C正极材料的初始放电容量达到162mAhg^-1和159mAhg^-1,接近理论容量。但大电流放电性能离实际应用仍有较大的距离。 用水热法对合成了Mn、Co、Ni分别掺杂的LiMn_xFe_1-xPO₄、LiCo_xFe_1-xPO₄、LiNi_xFe_1-xPO₄及其高温碳包覆的复合材料。对掺杂的最佳合成工艺和最佳掺杂量都进行了探索。结果发现,Mn的掺杂在一定程度上改善了LiFePO₄正极材料的性能,170℃、3h合成的LiMn_0.15Fe_0.85PO₄的比容量比纯LiFePO₄高出约17%。但经碳包覆之后,大电流充放电性能提高幅度不大。镍的最佳掺杂量为5%,160℃、3h水热合成LiNi_0.05Fe_0.95PO₄的比容量比纯LiFePO₄高33%。经碳包覆之后,1C放电放容量达到130mAhg^-1。镍掺杂能明显改善LiFePO₄/C的循环寿命,经1C充放电100次后,容量不仅没有减少反而提高。钴掺杂之后,无论包覆与否LiFePO₄/C的电化学性能没有提高。更多还原

【Abstract】 Olivine-type LiFePO₄ is considered as the most promising candidate for the next-generation cathode materials of Li-ion batteries because it has high theoretical specific capacity, is cheap and environmentally friendly, and also has good cycling characteristics and excellent safety. However, pure LiFePO₄ shows low reversible capacity and poor charge-discharge characteristics at high current density due to its poor electronic conductivity. Some measures such as reducing particle size, adding conductive additive, and cation ion doping were taken to improve the electrochemical performances of LiFePO₄, and some progresses have been achieved so far.The research aims at developing LiFePO4 that can reach a level of practical application. LiFePO₄, Li （Mn, Fe） PO₄, Li （Co, Fe） PO₄, Li （Ni, Fe） PO₄ have been synthesized by the solvothermal method and the corresponding carbon-coated composites were also prepared using high-temperature heat treatment route. LiFePCVC and （Li, Nb） FePO₄ /C composites have been synthesized by one-step solid phase reactions. The microstructures and morphologies of these composites were investigated by XRD, SEM, TEM, and Rietveld refinement. The electrochemical performances have been evaluated by galvanostatic charge-discharge cycling, cyclic voltammetry （CV） and electrochemical impedance spectra （EIS）. The differences in performances between solvothermally and solid phase synthesized products have been studied. The effects of the synthesis methods and synthesis parameters on the electrochemical performances of LiFePO₄ have been systemically investigated.LiFePO₄ has been synthesized by the solvothermal method using FeSO₄, H₃PO₄ and LiOH as raw materials. The product shows a high purity, good crystallinity, small particle size and homogenous size distribution. The effects of synthesis temperature and time on the morphologies and electrochemical performances of LiFePO₄ have been analyzed. It was found that well-crystallized LiFePO₄ with small particle size shows good electrochemical performances. In the present case, LiFePO₄ synthesized at 150℃ for 15 h shows best electrochemical performances. The study on the formation mechanism of LiFePO₄ by the solvothermal method shows that in order to obtain single-phase LiFePO₄, LiOH and H₃PO₄ must be mixed first to form intermediate Li₃PO₄, and then Li₃PO₄ reacts with Fe²⁺ to form pure LiFePO₄. LiFePO₄/C composites are synthesized by carbon-coating the hydrothermally synthesized LiFePO₄ through pyrolyzing polypropylene at high temperature. The study suggested that the best coating temperature is 550℃. Under this condition, ball-likeLiFePCVC was obtained, which shows a first discharge capacity of 162 and 159 mA h g"1 at 0.05 C and 0.1 C, respectively, close to the theoretical capacity of LiFePC>4. However, the electrochemical performances of the materials at large current density are far from practical application.LiMxFei.xPO4 and their corresponding carbon-coated composites have been synthesized by the solvothermal method using Mn, Co and Ni as doping atoms and the corresponding heat treatment route. This research has explored the best synthesis technology and best doping content for solid solution doping. It was found that the electrochemical performances of LiFePO4 could be improved to some extent by Mn doping. LiMno.isFeo.ssPC^ prepared by hydrothermal route at 170°C for 3 h shows a 17% higher specific capacity than pure LiFePO4. After carbon coating, however, the charge-discharge performances at large current is not obviously improved. In this research, it is found that the optimized Ni doping content is 5%. LiNio.o5Feo.95P04 prepared by hydrothermal route at 160°C for 3 h shows a 33% higher specific capacity than pure LiFePO4. Under this condition, the capacity of LiNio.osFeo.gsPC^ is 33% higher than LiFePO4. After carbon coating, this material yields a discharge capacity of 130 mA h g"1 at 1 C. The cycling stability of LiFePCVC can be obviously improved after Ni doping. When cycled at 1 C for 100 times, this material exhibits capacity increase instead of decrease. However, Co doping cannot help improve the electrochemical performances of LiFePCVC whether it was undergone carbon coating or not.Carbon-coated LiFePC>4 has been developed by solid-state reaction using inexpensive Fe2C>3, NH4H2PO4 and LiOH as the raw materials and polypropylene as the reductive agent and the carbon source and the synthesis technology was explored. It was found that LiFePCVC composite prepared at 600°C for 10 h exhibits good electrochemical performances, yielding a discharge capacity of 136 mA h g"’at 1 C. Based on the above research, Lii-sjtNbxFePCVC has been synthesized by a one-step solid-state reaction through Li+ sites doping using Nb5+, whose radius is close to that of Li+. It was found that when the molar ratio of doping content is 0.008, the discharge capacity of this material reaches 148 mA h g"1 at 1 C and 130 mA h g"1 at 2 C, and the capacity fade is only 2.6% after 100 cycles at 1 C. Cyclic voltammetry tests of Lii.5lNb,FePO4/C indicate that polarization decreases obviously after doping. EIS tests show that the sum of/?ctand Rf shows a continuous decrease with increasing Nb doping content in Lii.sjcNbxFePCVC. When the Nb doping content increases from 0 to 0.01, the sum of Rct and R{ decreases from 300 to 65 Q. It indicates that Nb doping greatly improves the conductivity of LiFePCVThis research has analyzed the reason why the electrochemical performances of LiFePCVC prepared by the solvothermal method are not so good as that prepared by solid-state reactions. Rietveld refinement fitting shows that the length of Li-0 bond is larger of LiFePO4 by solid-state reactions compared with that prepared by hydrothermal route. It is suggested that binding force is weakened because of the large bond length, which facilitates diffusion of Li+ in solid phase. As a result, the LiFePCVC prepared by solid phase reactions exhibits better electrochemical performances.更多还原