【作者】 刘媛媛；

【导师】 谷俊杰；

【作者基本信息】 天津大学 ， 化学工程， 2016， 博士

【摘要】 橄榄石型磷酸铁锂（LiFePO₄）作为目前最有发展前景的锂离子动力电池正极材料之一,具有高比容量、好的循环可逆性能、低的原料成本、高的安全性能和环境友好等优点,目前已经成为电池界竞相开发和研究的热点。但其低的电子导电率和锂离子扩散系数,导致了LiFePO₄材料在高倍率充放电条件下比容量衰减迅速,从而严重地阻碍了LiFePO₄的商业化应用。针对以上存在的问题,本文通过调控LiFePO₄材料的颗粒大小和晶体生长方向,以及通过碳改性等手段来改性LiFePO₄材料,最终提高其充放电性能。本文取得了如下主要结果:首先,采用实验室规格的高剪切混合器来制备前驱体,经水热结晶,成功地实现了对LiFePO₄/C材料粒度大小的调控。通过研究获得了高剪切混合器转子转速对前驱体沉淀和LiFePO₄/C材料的晶体结构、颗粒形貌、大小及其分布的影响规律,揭示了高剪切混合器辅助的水热法实现粒度可控的关键。在高剪切混合器转速为1.3×104 rpm下制备得到的LiFePO₄/C样品,其粒度减小至²20 nm,表现出了优异的电化学性能,在0.1 C和20 C倍率下,其放电比容量分别达到160.1mAh·g^-1和90.8 mAh·g^-1。其次,在高剪切混合器辅助的水热工艺基础上,继续在前驱体的混合过程中加入了非离子型表面活性剂Tween-80。研究表明,表面活性剂Tween-80分子在水热合成过程中可以降低LiFePO₄产物粒度大小,并且可以调控LiFePO₄产物沿（010）晶面生长。制备得到的LiFePO₄/C材料其颗粒粒度减小至¹00 nm,I（020）/I（111）比值高达1.19,表现出了优异的电化学性能,在0.1 C和20 C倍率下,放电容量高达166.5 mAh·g^-1和119.6 mAh·g^-1。最后,采用金属有机框架MIL^-100（Fe）同时作为模板和原料,制备得到了由LiFePO₄纳米颗粒嵌入多孔连续碳骨架内部的三维多孔LFP/CNWs材料。为了进一步提高该材料的导电性能,将LFP/CNWs样品同三聚氰胺（C3N3（NH2）3）混合后,经碳热还原反应制备得到了氮改性的LFP/N-CNWs材料。该氮改性的多孔碳骨架（N-CNWs）环绕在LiFePO₄纳米颗粒表面,增大了LiFePO₄颗粒同电解液之间的接触面积,加快了锂离子和电子在整个电极材料内的传导速率,提高了LiFePO₄材料的有效利用率和倍率性能。LFP/N-CNWs样品在0.1 C和20 C倍率下的放电容量分别达到161.1 mAh·g^-1和93.6 mAh·g^-1。更多还原

【Abstract】 Olivine-structured LiFePO₄,as the most promising cathode material for lithium ion batteries,has been received intensive research and development owing to its merits of high theoretical capacity,long cycle life,low cost and toxicity,superior safety,as well as environmental benignity.However,the inherent shortcomings of LiFePO₄ comparing with other cathode materials,such as the material’s low electrical conductivity and slow Li-ion diffusion,result in noteworthy capacity loss during high-rate discharge and poor electrochemical performance,making it unsuitable for commercial applications.Tremendous efforts have been devoted in our study to overcome these issues and improve the overall electrochemical performance of LiFePO₄,including decreasing Li FePO₄ particles to nanometer size,adjusting the crystal orientation of LiFePO₄ or coating with electronic conductive carbon.The research results and conclusions are listed as follow:Firstly,a high shear mixer has been adopted during the precursor preparation process,and size controlling of Li FePO₄ particles has been successfully achieved via this high shear mixer assisted hydrothermal method.Effect of rotor speed of the high shear mixer on the phase structure,morphology and size distribution of the precursor precipitate and Li FePO₄ particles were studied,in order to understand the reason that high shear mixer can successfully control the particle size of LiFePO₄.It is indicated that the Li FePO₄/C particles synthesized by the high shear mixer at the stirring rate of 1.3×104 rpm exhibit a reduced particle size of 220 nm,performing high discharge capacity of 160.1 mAh·g^-1 at 0.1 C and even high rate capacity of 90.8 m Ah·g^-1 at 20 C.Secondly,non-ionic surfactant Tween-80 is added during the precursor preparation process on the basis of high shear mixer assisted hydrothermal method.It is indicated that the use of Tween-80 can successfully reduce the grain size and modulate the crystal growth along the（010）facet of LiFePO₄ crystal.The LiFePO₄ particles synthesized via the Tween-80 modified hydrothermal method exhibit small mean diameter of ¹00 nm and a large I（020）/I（111）ratio of 1.19,which performing excellent discharge capabilities of 166.5 mAh·g^-1 and 119.6 mAh·g^-1 at the current rates of 0.1 C and 20 C,respectively.Lastly,a three-dimensional porous LiFePO₄/C composite with nano-sized LiFePO₄ particles embedded in an interconnected carbon network is successfully synthesized using unique Fe-based metal organic framework MIL^-100（Fe）as a porous template and the starting material of Fe and C by a carbothermal reduction reaction under reducing atmosphere.Then,the prepared LFP/CNWs composites are mixed with melamine to synthesize nitrogen-modified LFP/N-CNWs composites,expecting to further increase the electrical conductivity and consequently enhance the electrochemical performance of Li FePO₄ cathodes materials.This N-CNWs acted as a continuous conductive framework for the embedded Li FePO₄ nanosparticles,which increases the contact area between electrode materials and electrolyte,and then fasts the diffusion coefficient for electron and Li-ion,resulting in efficient utilization of lithium iron phosphate and high rate electrochemical performance.This synthesized LFP/N-CNWs composites deliver excellent discharge capacities of 161.5 mAh·g^-1 and 93.6 mAh·g^-1 at the current rate of 0.1 C and 20 C,respectively.更多还原