【作者】 田冰冰；

【导师】 王海辉；

【作者基本信息】 华南理工大学 ， 化学工程， 2011， 硕士

【摘要】 目前锂离子电池广泛采用的石墨类碳负极材料不可逆容量损失较大,而且存在安全性问题。尖晶石Li₄Ti₅O₁₂作为锂离子电池负极材料时具有优异的安全性能、简单的制备工艺、低廉的成本和良好的环境特性,并且在充放电过程中结构几乎不发生变化被称为“零应变”材料,具有非常优越的循环性能,因而引起人们的广泛关注。然而,Li₄Ti₅O₁₂材料较低的电子电导率和离子传导率(固有电导率仅为10^-9 Scm^-1)影响其电化学性能特别是高倍率下的电化学性能,而成为它实现工业化应用的最大障碍。本文旨在通过Nb掺杂、表面石墨烯包覆等技术途径,提高其电导率进而改善其电化学性能。为了进一步挖掘尖晶石型Li₄Ti₅O₁₂的可逆容量和倍率性能,本文还研究了其在低电位下的电化学行为并阐述了导电剂对材料电化学行为的影响。第一章综述了锂离子电池及其负极材料的研究进展和尖晶石Li₄Ti₅O₁₂负极材料的研究现状,提出了本论文的选题意义以及需要解决的相关科学问题。第二章简要的介绍了制备尖晶石型Li₄Ti₅O₁₂及其改性样品过程中所采用的化学药品、实验仪器及测试电池过程中所采用的仪器及表征手段。第三章采用醋酸溶胶凝胶法制备了尖晶石型Li₄Ti₅O₁₂及一系列铌掺杂的Li₄Ti₅O₁₂,研究不同掺杂量对Li₄Ti₅O₁₂电化学性能的影响,深入研究了最佳掺杂量时材料的电化学性能。研究结果表明,通过用Nb⁵⁺部分取代Ti⁴⁺后,所得材料导电性提高,充放电容量有所增大,循环性能得到了很大提高。该法合成的Li₄Ti_4.95Nb_0.05O₁₂材料的循环伏安曲线具有较高的氧化还原峰值,并且对称性也较Li₄Ti₅O₁₂的好,具有良好的循环性能。组装成测试电池的电化学阻抗结果和导电率测试结果都显示,Li4Ti4.95Nb0.05O12材料具有较Li₄Ti₅O₁₂材料更高的锂离子传导速率和电子传导速率。第四章运用柠檬酸溶胶-凝胶法合成了Li₄Ti₅O₁₂和Li₄Ti_4.95Nb_0.05O₁₂材料,重点对其深度放电行为进行研究,将放电截止电压由1.0 V扩至0 V后,尖晶石型钛酸锂的可逆容量得到了提高。本章还就铌掺杂对Li₄Ti₅O₁₂材料深度放电行为的影响作了深入探讨。为钛酸锂材料容量挖潜提供了有益参考。第五章通过一种简单的溶胶凝胶法合成石墨烯包覆的Li₄Ti₅O₁₂材料。利用石墨烯片在溶液中的特殊作用,合成了Li₄Ti₅O₁₂颗粒细小且粘附在石墨烯片上的Li₄Ti₅O₁₂/石墨烯复合电极材料。元素分析结果显示该复合电极材料碳含量约为7.8%wt,XRD分析表明石墨烯包覆的钛酸锂材料仍表现为单一的尖晶石结构。扫描电镜结果显示,钛酸锂和石墨烯具有协同作用。石墨烯的加入可以阻止Li₄Ti₅O₁₂的团聚,Li₄Ti₅O₁₂分布于石墨烯片层之间和孔洞结构中也可以阻止石墨烯片的堆积。因此Li₄Ti₅O₁₂/石墨烯复合电极材料中Li₄Ti₅O₁₂颗粒更细小。石墨烯包覆的Li₄Ti₅O₁₂材料表现出了良好的电化学性能,尤其是大倍率下的电化学性能。在10 C倍率下,Li₄Ti₅O₁₂/石墨烯复合材料第100次的放电比容量是110.3 mAhg^-1,高于Li₄Ti₅O₁₂的84.6 mAhg^-1。循环伏安测试结果显示, Li₄Ti₅O₁₂/石墨烯复合电极材料有更好的导电性和循环性能。更多还原

【Abstract】 Lithium-ion batteries have been widely utilized as a power source for portable electronic devices. Recently, much effort has been made to promote their application in hybrid electric vehicles and dispersed energy storage systems, which demand light weight, small volume, high energy density and safety. As a promising anode material for the lithium-ion batteries, spinel Li₄Ti₅O₁₂ exhibits many advantages compared to the currently used graphite. For example, there is negligible change in the unit cell volume of Li₄Ti₅O₁₂ during lithium insertion and extraction, so it was named as zero strain material. It also has very flat voltage plateau at around 1.55 V vs. Li/Li+, which is higher than the reduction potential of most organic electrolytes. Therefore, Li₄Ti₅O₁₂ is much safer and more stable than carbon-based materials. Despite the advantages mentioned above, however, there are some obstacles for the development and commercialization of Li₄Ti₅O₁₂. One of the main obstacles is its low electronic conductivity, which leads to its low rate capacity. In order to improve the conductivity, three methods were proposed: （1） improving the synthesis route to get nano-sized particles, because small particle size will shorten lithium ion diffusion path and broaden the electrode/electrolyte contact surface obviously; （2） adding a second conductive phase into the Li₄Ti₅O₁₂, such as metal powder and carbon; （3） substituting Li or Ti by other metal cations, such as Cr3+, V5+, Mn4+, Fe3+, Al3+, Ga3+, Co3+, Ta5+, Cu2+. In this dissertation, sol-gel synthesis methods were carried out to prepare electrode materials. Nb-doping and grephene coating are mainly used to enhance the electrical conductivity of Li₄Ti₅O₁₂ materials and then improve the electrochemical performance.?To further extend the reversible capacity, we also investigated the deep discharge electrochemical behavior of Li₄Ti₅O₁₂ and modified Li₄Ti₅O₁₂ materials.In the first chapter, the current survey on lithium ion batteries anode?materials, especially on spinel Li₄Ti₅O₁₂ material was introduced. And correlative scientific problems needed to solve were brought forward.In the second chapter,?a brief introduction to the chemicals used in the synthetic process, experimental equipments and analysis technics were declared in this part.In the third chapter, Nb doped lithium titanate with the composition of Li₄Ti_4.95Nb_0.05O₁₂ has been prepared by a sol-gel method. X-ray diffraction （XRD） and scanning electron microscope （SEM） are employed to characterize the structure and morphology of Li4Ti4.95Nb0.05O12. The Li₄Ti_4.95Nb_0.05O₁₂ electrode presents a higher specific capacity and better cycling performance than the Li₄Ti₅O₁₂ electrode prepared by the similar process. The Li₄Ti_4.95Nb_0.05O₁₂ exhibits an excellent rate capability with a reversible capacity of 135 mAh g-1 at 10 C, 127 mAh g-1 at 20 C and even 80 mAh g^-1 at 40 C. Electrical resistance measurement and electrochemical impedance spectra （EIS） reveal that the Li₄Ti_4.95Nb_0.05O₁₂ exhibits a higher electronic conductivity and faster lithium ion diffusivity than the Li₄Ti₅O₁₂, which indicates that niobium doped lithium titanate （Li4Ti4.95Nb0.05O12） is promising as a high rate anode for the lithium-ion batteries.In the fourth chapter, Li₄Ti₅O₁₂ and Li₄Ti_4.95Nb_0.05O₁₂ has been synthesized by a citric acid-assistant sol-gel method. The structures of the samples were confirmed by X-ray diffraction （XRD）. The electrochemical performance of the Li₄Ti₅O₁₂ down to 1.0 V, 0.5 V and 0 V has been investigated. Cyclic voltammograms （CV） and XRD show that the Li₄Ti₅O₁₂ electrode possesses a good stability during the process of the Li+ deep intercalation and extraction. The electrochemical performances of the Li₄Ti_4.95Nb_0.05O₁₂ and the Li₄Ti₅O₁₂ in the range from 0 to 2.5 V are also investigated. The Li₄Ti_4.95Nb_0.05O₁₂ presents a higher specific capacity and better cycling stability than the Li₄Ti₅O₁₂ due to the improved conductivity. The Li₄Ti_4.95Nb_0.05O₁₂ exhibits a capacity as high as 231.2 mAh g-1 after 100 cycles, which is much higher than the Li₄Ti₅O₁₂ （111.1 mAh g-1）. The effect of Nb-doping on electrochemical performance of Li₄Ti₅O₁₂ discharged to 0 V has also been discussed.In the fifth chapter, Li₄Ti₅O₁₂/graphene composite was prepared by a facile sol-gel method. The lattice structure and morphology of the composite were investigated by X-ray diffraction （XRD） and scanning electronic microscopy （SEM）. The electrochemical performances of the electrodes have been investigated compared with the pristine Li₄Ti₅O₁₂ synthesized by a similar route. The Li₄Ti₅O₁₂/graphene composite presents a higher capacity and better cycling performance than Li₄Ti₅O₁₂ at the cutoff of 2.5 - 1.0 V, especially at high current rate. The excellent electrochemical performance of Li₄Ti₅O₁₂/graphene electrode could be attributed to the improvement of electronic conductivity from the graphene sheets. When discharged to 0 V, the Li₄Ti₅O₁₂/graphene composite exhibited a quite high capacity over 274 mAhg^-1 below 1.0 V, which was quite beneficial for not only the high energy density but also the safety characteristic of lithium-ion batteries.更多还原