【作者】 周克勤；

【导师】 郑军伟；

【作者基本信息】 苏州大学 ， 物理化学， 2010， 硕士

【摘要】 本文采用高温固相法合成了LiV_xFe_1-xPO₄/C正极材料。通过X射线衍射（XRD）研究表明制得的LiV_xFe_1-xPO₄/C晶体结构与标准LiFePO₄的晶体结构相一致;循环伏安谱图未观察到V的氧化还原峰,表明V取代了Fe2+的位置进入了LiFePO₄骨架而未改变其骨架结构。扫描电子显微镜（SEM）和透射电子显微镜（TEM）分析表明加入的碳源在LiFePO₄颗粒外形成了碳包覆层。研究表明,LiV_xFe_1-xPO₄/C比纯的LiFePO₄/C具有更高的充放电容量和更好循环性能。通过考察不同焙烧条件及V掺杂量对LiV_xFe_1-xPO₄/C性能的影响发现,随着焙烧温度的提高,LiV_xFe_1-xPO₄/C的结晶度增加,但其放电容量和循环性能降低;研究在同一焙烧温度下改变焙烧时间时发现,焙烧时间越长其样品的结晶度越好,但其放电容量和循环性能却随之降低。随着V掺杂量的提高材料的放电容量也随之增加。V的掺杂有利于提高LiFePO₄/C的循环性能,但当V的参杂量进一步增加时LiV_xFe_1-xPO₄/C的循环性能却随之降低,研究表明V的掺杂量为x=0.05时材料循环性能最好。采用微波高温固相法和溶胶凝胶法分别合成了Li₃V₂（PO₄）₃/C复合材料和纯Li₃V₂（PO₄）₃材料。考察微波高温焙烧温度和反应时间对Li₃V₂（PO₄）₃/C样品晶体结构以及电化学性能的影响。充放电研究表明:以过量5%抗坏血酸为碳源在微波700℃下烧结10 min合成样品电化学性能较好。该样品在充放电截止电压为3-4.2V,0.1 C倍率进行充放电,首次放电比容量为112 mAh/g,且循环40次后,放电比容量仍为106.5 mAh/g。考察了溶胶凝胶法制备纯Li₃V₂（PO₄）₃相的晶体结构以及电化学性能,充放电研究表明:溶胶凝胶法制备的Li₃V₂（PO₄）₃前驱物在微波750℃下烧结10 min合成样品的在充放电截止电压为3-4.2 V时,以0.05 C倍率进行充放电,首次放电比容量为106.4 mAh/g。更多还原

【Abstract】 LiV_xFe_1-xPO₄/C cathode materials were synthesized by high-temperature solid-phase reaction method. The X-ray diffraction （XRD） studies show that the crystal structure of LiV_xFe_1-xPO₄/C is similar to that of LiFePO₄; The fact that the redox peaks of vanadium can not be observed in cyclic voltammogram indicates that the vanadium atoms partially replace the Fe2+ in the LiFePO₄ skeleton, such a replacement have no effect on the skeleton structure. Both scanning electron microscopy （SEM） and transmission electron microscopy （TEM） analysis show that the surface of LiFePO₄ particles is coated with carbon generated from the organics during the high temperature treatment in the inner atmosphere.The charge-discharge studies have shown that the LiV_xFe_1-xPO₄/C possesses a higher charge-discharge capacity and better cycling performance than LiFePO₄/C. Further examination of the different calcination conditions and the different amount of V doped in the LiV_xFe_1-xPO₄/C, it was found that the crystallinity of LiV_xFe_1-xPO₄/C increased as the calcination temperature increased, however, the discharge capacity and cycle performance was degraded. Under the same calcination temperature, it was found that prolonging calcination time results in a better crystallinity of the sample, but worse discharge capacity and cycle performance.The discharge capacity increases with increasing of the amount of doped vanadium. Vanadium-doping seems to improve the cycle performance of LiFePO₄/C. The best cycling performance of material was obtained when the amount of the doped V was x=0.05. A further increase of the amount of doped vanadium lead to the degrade of the cycle performance of LiV_xFe_1-xPO₄/C.In this paper, Li3V（2PO₄）3/C composite materials and pure Li3V（2PO₄）3 were also synthesized by microwave-assistant high-temperature solid reaction method and sol-gel method, respectively. ?In the process of the microwave-assistant high-temperature synthesis, the effects of experimental conditions, such as the reaction temperature and reaction time, on crystal structure and electrochemical performance were investigated. Charge-discharge test results show that a better electrochemical properties of the samples could be obtained by using excess of 5% ascorbic acid as carbon source and under the microwave sintering at 700℃for 10min. In the voltage range of 3.0-4.3 V, the first reversible capacity of pure Li₃V₂（PO₄）₃/C is 112 mAh/g at 0.1 C rate. After 40 cycles, the discharge capacity still remained 106.5 mAh/g. For comparison, we also studied the crystal structure and electrochemical properties of pure Li3V2 （ PO₄ ） 3 prepared by sol-gel method. Charge-discharge test results showed that the samples synthesized with sol-gel precursor under the same experimental conditions reached 106.4 mAh/g in the first discharge capacity of the sample in the voltage range of 3-4.2 V at 0.05C rate.更多还原