【作者】 葛昊；

【导师】 李宁；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2009， 博士

【摘要】 尖晶石型Li4Ti5O12由于具有良好的循环性能、突出的安全性能、十分小的体积变化及低廉的成本而成为目前的研究热点,它被美国能源部列为第二代锂离子动力电池的负极材料。本文采用TG(Thermogravimetry analysis,热重分析法)、XRD(X-ray radial diffraction,X射线衍射分析)、XPS(X-ray photoelectron spectroscopy,X射线光电子能谱)、SEM(Scanning electron microscope,扫描电子显微镜观察)、循环伏安(Cyclic Voltammetry,CV)、充放电试验、电化学阻抗谱(Electrochemical impedance spectroscopy,EIS)等测试手段,从制备、掺杂改性、结构特征、电化学性能、理论容量、嵌锂机制等方面对尖晶石型Li4Ti5O12进行了深入的研究。通过对高温固相和液相法合成工艺的优化,制备出了性能优良的尖晶石型钛酸锂。研究表明,Li/Ti配比、焙烧温度和焙烧时间均对材料的结构和电化学性能有较大影响。高温固相法优化后的工艺条件为:以Li/Ti配比为0.86的比例投料后于800℃焙烧12h。在此工艺条件下所合成的尖晶石型Li4Ti5O12结晶好、粒度分布均匀,在0.1C下,其可逆容量为158mAh·g-1左右,50次循环后的容量保持率高达97.5%。由于液相法前驱体粒度分布十分均匀而且水解所产生的TiO2为非晶态,其在较低Li/Ti配比、较低的合成温度和较短的反应时间的条件下就能合成出性能优良的尖晶石型Li4Ti5O12。最佳工艺条件为:以Li/Ti配比为0.83的比例投料后于750℃焙烧8h。在此工艺条件下所合成的尖晶石型Li4Ti5O12结晶良好且粒度分布十分均匀,在0.1C下,其可逆容量为169mAh·g-1左右,50次循环后的容量保持率高达99.5%。降低反应物料的粒度和反应激活能能够有效地降低焙烧温度和缩短反应时间,进而降低了目标产物的平均粒度,为电极材料获得良好的电化学性能奠定基础。为了改善尖晶石型Li4Ti5O12的倍率性能,研究了16d位Li掺杂和体相Ag掺杂对材料结构和电化学行为的影响。Li掺杂能够有效地改善Li4+xTi5-xO12-δ(0≤x≤0.2)样品的电导率和锂离子扩散系数,进而显著地提高材料的倍率性能。随Li含量的升高,Li4+xTi5-xO12-δ(0≤x≤0.2)样品的电导率和锂离子扩散系数逐渐增大。但当Li掺杂量较大时,Li4+xTi5-xO12-δ(0≤x≤0.2)样品中较多的O空位会降低材料自身结构的稳定性,进而影响材料的循环性能。当Li掺杂量x=0.1时,材料Li4+xTi5-xO12-δ(0≤x≤0.2)具有良好的循环性能和高倍率性能;由于Ag+的半径较大,无论是采用简单的液相法还是超声波分散辅助法均不能使Ag进入尖晶石型Li4Ti5O12的晶格。Ag的存在能够有效地提高Li4Ti5O12/Ag样品的电导率,随Ag含量的增加,材料的电导率逐渐增大。但由于Ag不能嵌锂,较高的Ag含量会使Li4Ti5O12/Ag样品的可逆容量受到影响。与简单的液相法相比,超声波分散辅助法能够提高Ag颗粒的分散性、降低Ag颗粒的粒度,在较低Ag含量时,就能显著提高Li4Ti5O12/Ag样品的电导率,从而使Li4Ti5O12/Ag样品具有良好的倍率性能和较高的可逆容量。本文还深入地研究了尖晶石型钛酸锂在2.0~1.0V电压范围内的电化学行为。因为Li7Ti5O12具有较高的电导率和锂离子扩散系数,所以,尖晶石型钛酸锂展现出了十分优良的高倍率充电性能。根据材料的这一特点,我们考察了不同工作模式对其倍率性能的影响。在慢放-快充模式下,尖晶石型钛酸锂的高倍率性得到了极大的改善,即使在30C下,其可逆容量仅比0.5C时减少9.6%。为了进一步挖掘尖晶石型钛酸锂的可逆容量和倍率性能,本文首次研究了其在低电位下的电化学行为并阐述了乙炔黑对材料电化学行为的影响。将放电截止电压由1.0V扩至0.01V后,尖晶石型钛酸锂的可逆容量得到了提高,但其循环稳定性没有受到影响。在0.6V以下,由于导电剂乙炔黑开始嵌锂,其既是电子传导剂又是锂离子传导剂,所以电极的反应面积显著地得到了提高,进而使得尖晶石型钛酸锂在0.6V以下具有十分优良的高倍率性能。因此,将放电截止电压由1.0V扩至0.01V,尖晶石型钛酸锂的可逆容量和高倍率性能均得到了明显的提高。最后,根据对尖晶石型钛酸锂结构的分析,结合相应的XRD和充放电测试结果,我们阐述了尖晶石型钛酸锂在低电位下的嵌锂机制,并修正了尖晶石型钛酸锂的理论容量值。在0.6V以下,Li离子能够嵌入了Li7Ti5O12的四面体空位。尖晶石型Li4Ti5O12的理论容量受其得电子能力限制而不是由空位数所决定的,对应的理论容量为291.8mAh·g-1,而不是175.1mAh·g-1或350.2mAh·g-1。由于锂离子在2.0~0.6V和0.6~0.01V之间的嵌入方式不同,因此尖晶石型钛酸锂在这两个电压范围内的放电曲线形态截然不同。更多还原

【Abstract】 Spinel Li4Ti5O12 is a promising candidate of negative electrode materials for lithium-ion batteries due to its excellent cycle performance and safety, low volume change and cost. It has been ranked as the secondary negative electrode material for lithium-ions power battery by DOE(Department of Energy, USA). A study on the synthesis, improvements, structural characteristics, electrochemical performance, theoretical capacity and intercalation mechanism of spinel Li4Ti5O12 were carried out systemically and in detail through test measures such as TG, XRD, XPS, SEM, CV, EIS, etc.Spinel Li4Ti5O12 with excellent performances was prepared via optimized solid state method and hydrolyzation method, respectively. Experiment results indicated that the ratio of raw materials Li/Ti, calcination temperature and reaction time played an important role in the electrochemical performances. The optimized condition for solid state method was: The raw materials Li/Ti with the ratio of 0.86 were calcinated at 800℃for 12h. The prepared sample at this developed condition possessed well-distributed morphology, high phase purity and degree of crystallinity, its reversible capacity and capacity retention after 50 cycles were 158mAh·g-1 and 97.5%, respectively. The hydrolyzation method could synthesize spinel Li4Ti5O12 at lower ratio of raw materials Li/Ti and calcination temperature and shorter reaction time for its lower-sized and amorphous TiO2 precursor. The optimized condition for hydrolyzation method was: The raw materials Li/Ti with the ratio of 0.83 were calcinated at 750℃for 8h. The prepared sample at this developed condition possessed well-distributed morphology, high phase purity and degree of crystallinity, its reversible capacity and capacity retentionafter 50 cycles were 169mAh·g-1 and 99.5%, respectively. Reducing the reactants’granularity and the actived energy of reaction could effectively lower the calcination temperature and shorten the reaction time, thus reducing the average size of the obtained samples, make excellent groundwork for electrode materials to possess better electrochemical performance.In order to improve the rate performance, the effect of 16d sites Li doping and bulk Ag doping on the structure and electrochemical performances of spinel Li4Ti5O12 were studied, respectively. Li doping could effectively increase the lithium-ion and electronic conductivity of Li4+xTi5-xO12-δ(0≤x≤0.2), evidently improve its rate performance. With the increasing of Li doping amount, lithium-ion and electronic conductivity of Li4+xTi5-xO12-δ(0≤x≤0.2) increased, however its cycling stability was depressed when the Li doping was of x=0.2 for the higher amount of oxygen vacancy. The Li doping of x=0.1, the appropriate Li doping amount, showed improved rate capability and better high rate performance comparing to undoped sample; Because the ionic radius of Ag+ was much bigger than that of the Ti4+, the Ag+ could be doped into the lattice of spinel Li4Ti5O12. Ag could increase the electronic conductivity of Li4Ti5O12/Ag remarkably, the electronic conductivity of Li4Ti5O12/Ag increased with the content of Ag increasing. Because metal Ag could not accommodate Li, the high Ag content would decrease the reversible capacity of Li4Ti5O12/Ag. Compared to simple hydrolyzation method, ultrasonic-assisted method could enhance the dispersity of Ag particles and reduce the particle size of Ag particles. It could evidently increase the electronic conductivity of Li4Ti5O12/Ag at lower Ag content, contributing to the better rate performance and higher reversible capacity of Li4Ti5O12/Ag.The electrochemical performances of spinel lithium titanate in the voltage of 2.0~1.0V were also deeply explored. The higher electronic conductivity and lithium-ions diffusion coefficient of the reduction product Li7Ti5O12 contributed to the excellent high power charge performance of spinel lithium titanate. According to this situation, the effect of working modes on the rate performances of spinel lithium titanate were stuied. Under low discharge and fast charge mode, the high rate performances of spinel Li4Ti5O12 were greatly improved. It only lost 9.6% of the reversible capacity of 0.5C even at 30C.In order to further empolder the reversible capacity and rate performances of spinel lithium titanates, we had stuied its electrochemical performances under low potential and the influence of acetylene back on its electrochemical performances. When discharge voltage of Li4Ti5O12 extended from 1.0V to 0.01V, its cycling stability was not affected and its reversible capacity was increased. Under 0.6V, acetylene back was both electronic conducting additive and lithium-ion conducting additive for Li4Ti5O12, which made the reaction area of electrode be inhanced markedly and contributd to the excellent high rate performances of spinel lithium titanate under 0.6V. Accordingly, both the reversible capacity and high rate performance of spinel lithium titanate were improved when the discharge voltage extended from 1.0V to 0.01V. At last, combining the electrochemical and XRD results with the crystal structure of spinel lithium tinitades, we demonstrated the corresponding reaction mechanism of the low-potential intercalation behavior of Li4Ti5O12 and modified the classical viewpoint on the theoretical capacity of Li4Ti5O12. Under 0.6V, lithium-ions could intercalate into the tetrahedral sites of Li7Ti5O12. The theoretical capacity of Li4Ti5O12 was limited by the number of tetravalent titanium, but not the octahedral or tetrahedral sites to accommodate lithium-ions in the voltage range of 2.0V to 0.01V, corresponding to 291.8mAh·g-1, but not 291.8mAh·g-1 or 291.8mAh·g-1. The shape of the discharge curves of spinel lithium titanate were totally different due to the different way of lithium-ions being accommodated in the interstitial positions in the corresponding voltage ranges of 2.0~0.6V and 0.6~0.01V.更多还原