【作者】 乔彦强；

【导师】 涂江平；

【作者基本信息】 浙江大学 ， 材料学， 2012， 博士

【摘要】 单斜Li3V2(PO4)3具有比容量高(197 mAh g-1)、平台电压高和循环结构稳定性好等优点,被认为是动力电池热点正极材料之一。层状LiV3O8具有高比容量、价格较低等特点,也是极具发展潜力的锂离子电池正极材料之一。本文主要针对Li3V2(PO4)3和LiV3O8的一些不足,通过各种途径对其进行改性研究,以提高它们的电化学性能。采用碳热还原法,分别以聚苯乙烯、硬脂酸和抗坏血酸作为碳源制备了Li3V2(PO4)3/C材料。采用聚苯乙烯为碳源时,研究了碳含量对Li3V2(PO4)3/C电化学性能的影响。对比发现,聚苯乙烯为碳源包覆Li3V2(PO4)3效果要明显优于乙炔黑为碳源包覆的材料。在3.0-4.3 V电压范围内,低倍率下较少碳含量的Li3V2(PO4)3/C具有最高放电比容量。但是在高倍率下,较高碳含量的Li3V2(PO4)3/C (碳层厚度约23-27 nm)具有最好的电化学性能。采用硬脂酸为碳源时,分析发现700℃下烧结的Li3V2(PO4)3/C具有最好的电化学性能。该材料在3.0-4.3 V和3.0-4.8 V电压范围内0.1 C下的首次放电比容量分别为130.6和185.9mAhg-1。在高倍率15 C下,3.0-4.3 V和3.0-4.8 V电压范围内放电比容量仍能分别达到103.3和112.1 mAh g-1。采用抗坏血酸为碳源制备Li3V2(PO4)3/C不适合采用碳热还原法,需要采用水热法处理前驱体,这样制备出的Li3V2(PO4)3/C颗粒均匀细小,且碳包覆性好,具有最优的电化学性能。采用聚乙烯醇(PVA-124)为碳源,利用碳热还原法制备了Li3V2(PO4)3/C。在-20℃至65℃的电化学性能测试发现,在较低倍率0.1 C下,3.0-4.3 V和3.0-4.8 V电压范围内-20℃下的首次放电比容量分别为84.3和118.9 mAh g-1。但是在高倍率-20℃下,Li3V2(PO4)3/C几乎没有容量。在高温下,3.0-4.3 V电压范围内,Li3V2(PO4)3/C的电化学性能会随温度升高而提高,但是在3.0-4.8 V电压范围内,温度升高反而会恶化Li3V2(PO4)3/C的电化学性能。电化学阻抗谱分析发现电荷转移阻抗可能是影响低温性能的主要因素。而高温电化学性能较差的原因则可能是电解液和Li3V2(PO4)3/C的不稳定性造成的。采用水合肼为球化剂,制备了多孔球形Li3V2(PO4)3/C。该材料的多孔性提供了较高的电化学反应界面,表现出优异的电化学性能,在0.2 C下,3.0-4.3 V和3.0-4.8 V电压范围内的放电比容量分别可达129.1和183.8mAhg-1。在高倍率15 C下,该多孔Li3V2(PO4)3/C在3.0-4.3 V和3.0-4.8 V电压范围内的放电比容量仍分别可达100.5和121.5 mAh g-1。采用甘氨酸为形貌控制剂制备了层片状Li3V2(PO4)3/C。该层片状材料在3.0-4.3 V和3.0-4.8 V电压范围内的首次放电比容量分别达125.2和133.1 mAh g-1(3 C),500次循环后放电比容量仍能分别保持到111.8和97.8 mAh g-1,表现出了较好的循环稳定性。采用尿素辅助的流变相法合成一维棒状LiV3O8.由于一维材料具有一维电子传输通道和缓冲充放电过程中材料的体积变化的优点,因此具有更为理想的电化学性能。对棒状LiV3O8电化学性能测试发现500℃合成的棒状LiV3O8电化学性能最为优异。在2.0-4.0 V电压范围内,50和120 mA g-1电流密度下的首次放电比容量分别为273.6和250.4 mAh g-1,而且具有较好的循环性能。采用甘氨酸辅助的溶液法合成多孔块状L-V-O (LiV3O8和Uio3V2O5两相组成)。对比不同温度烧结的材料发现,温度太低,材料结晶性差；温度升高会使材料的结晶性提高,但同时也缩小了(100)面面间距,影响了锂离子的扩散传输,不利于电化学性能的提高。通过对L-V-O材料的电化学性能测试发现,400℃样品具有最好的电化学性能。在50 mA g-1和120 mA g-1电流密度下的放电比容量分别为265.7 mAh g-1和237.0 mAh g-1。在480 mA g-1电流密度下.该多孔材料还能保持144.7 mAh g-1的放电比容量。多孔块状L-V-O良好的电化学性能因其具有大的比表面积、良好的电解液浸透性,因此减小了锂离子在SEI膜的迁移阻抗和电荷在电化学活性界面上的转移阻抗。恒流间歇滴定法(GITT)计算得到充电态和放电态的锂离子扩散系数在10-14-10-9 cm2 s-1之间。更多还原

【Abstract】 Monoclinic Li3V2(PO4)3 is an attractive cathode material for the application in electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to its high theoretical capacity (197mAh -1), high operate voltage, and structure stability during the cycling. Layered lithium vanadate oxide LiV3O8,is also regarded as a promising cathode material in rechargeable lithium batteries because of its potentially high specific capacity and low cost. The main objective in this research is to overcome the drawbacks of Li3V2(PO4)3 and LiV3O8 by several methods, in order to improve their electrochemical performance.Li3Vi(PO4)3/C cathode materials were synthesized by carbon-thermal reduction method using polystyrene, stearic acid and ascorbic acid as the carbon sources, respectively. When employed polystyrene as the carbon source, we mainly focused on the effects of carbon source and carbon content on the electrochemical performance of Li3V2(PO4)3/CO After comparison, the residual carbon produced by the pyrolysis of PS produced fine particle sizes and uniform carbon distribution on the Li3V2(PO4)3 particle surface; better than in composite with acetylene black. In the potential range of 3.0-4.3 V, the lower polystyrene added Li3V2(PO4)3/C with a thin carbon coating possesses the highest initial discharge capacity at lower current densities. However, at high current densities, the higher polystyrene added Li3V2(PO4)3/C with a thicker carbon coating (23-27 nm) shows best electrochemical performance. By using stearic acid as a carbon source, it is found that the Li3V2(PO4)3/C composite synthesized at 700℃shows the best electrochemical performance. The Li3V2(PO4)3/C shows a high initial discharge capacity of 130.6 mAh g-1between 3.0 and 4.3 V, and 185.9 mAh g-1 between 3.0 and 4.8 V at 0.1 C, respectively. Even at a charge-discharge rate of 15 C, the Li3V2(PO4)3/CO still can deliver a discharge capacity of 103.3 and 112.1 mAh g-1in the potential region of 3.0-4.3 V and 3.0-4.8 V, respectively. It is improper for to adopt carbon-thermal reduction method to prepare Li3V2(PO4)3/CO composite by using ascorbic acid as a carbon source. When the precursor was treated by hydrothermal method, the Li3V2(PO4)3/CO with fine particles and well carbon coating can be obtained, exhibiting good electrochemical performance.cathode material was synthesized by carbon-thermal reduction method using polyvinyl alcohol (PVA-124) as a carbon source. The electrochemical properties of the Li.V2(PO4)3/C material at various temperatures (-20.0.25,40 and 65℃) were tested. At-20℃the Li3V2(PO4)3/C electrode presents an high initial discharge capacity of 84.3 mAh g-1 between 3.0 and 4.3 V, and 118.9 mAh g-1 between 3.0 and 4.8 V at 0.1 C, respectively. However, the electrode can only deliver small discharge capacities at -20℃at 10 C rate. At higher temperatures, the capacity increases with the temperature between 3.0 and 4.3 V. but decreases between 3.0 and 4.8 V. EIS analysis reveals that the Ra is considered to be a predominant factor to influence the capacity of the electrode at low temperatures. In the potential range of 3.0-4.8 V. the lower discharge capacity would be mainly resulted from the larger crystal structural distortion and non-uniformity of SEI layer at high temperatures.Spherical porous Li3V2(PO4)3/CO composites were synthesized by a soft chemistry route using hydrazine hydrate as the spheroidizing medium. This porous structure can provide sufficient contact between active materials and electrolyte, thus the electrochemical performance of Li3V2(PO4)3/C composites are enhanced. The spherical porous Li3V2(PO4)3/C electrode shows a high discharge capacity of 129.1 mAh g"1 between 3.0 and 4.3 V, and 183.8 mAh g-1 between 3.0 and 4.8 V at 0.2 C, respectively. Even at a charge-discharge rate of 15 C, this material can still deliver a discharge capacity of 100.5 and 121.5 mAh g-’in the potential region of 3.0-.3 V and 3.0-4.8 V, respectively. Plate-like Li3V2(PO4)3/C composite was synthesized via a solution route followed by CTR by using glycine as the morphology control agent. At a charge-discharge rate of 3 C, the plate-like Li3V2(PO4)3/C exhibits an initial discharge capacity of 125.2 and 133.1 mAh g-1in the voltage ranges of 3.0-4.3 V and 3.0-4.8 V, respectively. After 500 cycles, the electrodes still can deliver a discharge capacity of 111.8 and 97.8 mAh g’correspondingly, showing a good cycling stability.Rod-like LiVsOs composites were fabricated by using a carbamide-assisted rheological phase reaction method. These one-dimensional (ID) materials have been considered as an effective way for achieving high-rate capability and enhancing power performance because they can provide efficient one-dimensional electron transport pathways and accommodate the volume changes during charge/discharge processes. The rod-like LiV3O8 calcined at 500℃has the optimal performance, delivering an initial discharge capacity of 273.6 and 250.4 mAh g-1between 2.0 V and 4.0 V at a current density of 50 and 120 mA g-1, respectively. After 60 cycles by applying 50 mA g-1a discharge capacity of 213.0 mAh g-1is obtained, showing a good cycling performance.Wafer-liked porous xLiV3O8-VyLi0.3V2O5 (Li-V-O) composites are synthesized by a facile self-assembled synthesis using a glycine-assisted solution route followed by a low temperature reaction. The compound synthesized at lower temperature shows low cry stall inity. The higher calcining temperature will result in good crystallinity which leads to the compound have a slightly lower a value, indicating the preferred orientation along the (100) plane and a slightly smaller interlayer spacing in the structure which would lead to a longer diffusion path for the lithium ions and thus depress the electrochemical performance. Among these Li-V-0 composites, the one synthesized at 400℃, which has 27.06 wt.%Li0.3V2O5, exhibits the highest initial discharge capacities of 265.7 and 237.0 mAh g-1at current densities of 50 and 120 mA g-1between 2.0 and 4.0 V, respectively. Even at a high current density of 480 mA g-1 it still can deliver a discharge capacity of 144.7 mAh g"1. The good electrochemical performance of the as-synthesized composite can be attributed to the porous structure, thus highly improves the specific surface area, enhances the contact with electrolyte, and decreases the impedance of Li+migration through surface-passivating layer. In addiction, the diffusion coefficients of Li ions in this composite determined by galvanostatic intermittent titration technique are in the region of 10-’4 to 10-9 cm2 s-1 in the charge/discharge processes.更多还原