锂氧气电池多孔正极的构筑与反应机理研究

发布时间：2018-06-18 22:30

  本文选题：锂氧气电池 + 过氧化锂　； 参考：《南京大学》2016年博士论文

【摘要】：随着电动汽车和智能电网市场规模的不断扩大,电化学储能技术越来越受到人们的关注。锂氧气电池理论比能量极高,是锂离子电池的数倍,并且成本低廉、环境友好,因而被普遍认为是极具希望的下一代储能系统。和锂离子电池的研究方式一样,目前普遍采用基于正极材料质量的比容量来评估锂氧气电池的电化学性能。因此,有必要探究锂氧气电池是否合适采用这种方式来评估电化学性能,即是否满足电池容量和正极材料质量呈线性关系的前提条件。此外,目前实际比容量低、充电过电位大、循环性能差等问题严重限制了锂氧气电池的实用化进程。就我们所知,锂氧气电池的电化学性能主要取决于氧气电极的结构和固有化学性质。因此,本论文中我们在优化氧气电极的多孔结构和提高其稳定性方面进行了一系列研究。此外,我们还综合利用X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)、差分电化学质谱(DEMS)等手段重点研究了锂氧气电池的充电反应机理。主要研究内容如下：首先,我们探究了影响锂氧气电池放电容量的因素。通过验证锂离子电池的容量随LiCoO2的质量线性增加,说明采用质量比容量这一参数来评估锂离子电池的电化学性能是合理的。同时,我们发现锂氧气电池的放电容量也随正极SP炭黑的质量增加而增加,但是两者不再呈线性关系。XRD的分析结果表明Li202是主要的放电产物,从而保证锂氧气电池的反应机理。考虑到氧气是放电过程的反应物,我们进一步研究了氧气窗口面积对电池放电容量的影响。实验结果表明,锂氧气电池的放电容量随氧气窗口面积的增大线性增加。通过SEM观察到锂氧气电池放电过程中Li202的沉积几乎只发生在电极暴露在氧气氛围下的区域,因此,我们将电极暴露在氧气氛围的区域称为Li202沉积的有效区域。此外,我们还合理地提出了锂氧气电池氧气电极有效区域的形成途径。基于上述实验结果,我们通过一步法成功合成了具有自支撑和多阶孔道结构的石墨烯气凝胶,并直接用作锂氧气电池不含粘结剂的正极材料。石墨烯气凝胶的三维多阶孔道结构方便电解液的渗透、氧气的扩散和电子的转移,大比表面积能够提供足够多的活性反应位置,高孔容可以存储更多的放电产物。因此,基于石墨烯气凝胶的放电比容量高达10000mAhg-1. Ru纳米颗粒进一步修饰在石墨烯片层的表面(Ru-GA),表现出对氧气析出反应(OER)很好的催化活性。Ru-GA正极可以有效增加锂氧气电池的放电比容量(12000 mAh g-1),降低充电过电位,提升循环稳定性(在500 mAh g-1的容量截止条件下可以循环50圈)。更重要的是,根据原位DEMS的分析结果,我们提出了锂氧气电池的充电过程可分为三个氧化阶段的反应机理。再者,为了解决循环过程中碳腐蚀的问题,我们通过硬模板法制备了孔径不同的有序多孔Ru02材料,并用作锂氧气电池的无碳正极。同时,我们还系统研究了有序多孔Ru02材料的孔隙结构参数对锂氧气电池性能的影响。电化学测试和分析结果表明,基于孔径为16nmRuO2(RuO2-16)正极的锂氧气电池放电比容量最大,能量转换效率最高,并且在100 mAg-1电流密度和2.5-4.0 V电压区间内可以稳定循环70圈。锂氧气电池如此优异的电化学性能得益于Ru02-16正极良好的电子导电性、较大的BET比表面积、合适的孔径大小和对OER良好的催化活性。原位DEMS的分析结果表明,和碳材料正极相比,RuO2-16无碳正极能够有效减少副反应的产生。最后,我们深入研究了具有高比表面积的活化石墨烯吸附的水汽对锂氧气电池电化学性能和反应机理的影响。活化石墨烯用作正极的锂氧气电池在充电过程中出现了两个电压平台。通过XRD和SEM对不同充放电阶段的产物进行表征,我们发现锂氧气电池的放电产物中既有Li2O2,也有LiOH,并且生成的LiOH会在电池充电至3.5 V时完全分解。这与原位DEMS的分析结果吻合。但是,当SP炭黑用作正极时,并没有发现LiOH。这表明LiOH的形成与活化石墨烯有关。卡尔-费休滴定的结果表明,活化石墨烯由于极高的比表面积,吸附了空气中大量的水汽。因此,H20与放电产物Li202反应生成LiOH, LiOH的分解造成了较低的充电电压平台。
[Abstract]:With the expansion of the market size of electric vehicles and smart grid, the electrochemical energy storage technology has attracted more and more attention. The theory of lithium oxygen battery is more than energy, many times the lithium ion battery, and the cost is low, and the environment is friendly. Therefore, it is generally considered as the very promising next generation energy storage system. And the lithium ion battery research In the same way, it is widely used to evaluate the electrochemical performance of lithium Oxygen Batteries Based on the mass specific capacity of positive materials. Therefore, it is necessary to explore whether lithium oxygen batteries are suitable for evaluating the electrochemical performance, that is, whether the battery capacity and the material quantity of positive electrode have a linear relationship. The problems of low specific capacity, large overpotential and poor cycling performance seriously restrict the practical process of lithium oxygen batteries. As we know, the electrochemical performance of the lithium oxygen cell depends mainly on the structure and natural chemical properties of the oxygen electrode. Therefore, in this paper, we are optimizing the porous structure of the oxygen electrode and improving its stability. In addition, X ray diffraction (XRD), scanning electron microscope (SEM), X ray photoelectron spectroscopy (XPS) and differential electrochemical mass spectrometry (DEMS) are used to study the reaction mechanism of lithium oxygen batteries. The main contents are as follows: first, we explored the influence of lithium oxygen battery discharge. By verifying that the capacity of the lithium ion battery increases linearly with the mass of LiCoO2, it is reasonable to use the mass specific capacity to evaluate the electrochemical performance of the lithium ion battery. At the same time, we find that the discharge capacity of the lithium oxygen battery increases with the increase of the quality of the positive SP carbon black, but both of them are no longer linear. The analysis of the relationship.XRD shows that Li202 is the main discharge product, thus ensuring the reaction mechanism of the lithium oxygen battery. Considering the oxygen is the reactant of the discharge process, we further study the effect of the oxygen window area on the discharge capacity of the battery. The experimental results show that the discharge capacity of the lithium oxygen battery increases with the increase of the oxygen window area. By SEM, it is observed that the deposition of Li202 in the discharge process of the lithium oxygen cell is almost only in the area exposed to the oxygen atmosphere by the electrode, so we call the area of the oxygen atmosphere of the electrode known as the effective region of the Li202 deposition. In addition, we also reasonably put forward the shape of the oxygen electrode in the lithium oxygen battery. Based on the above experimental results, we successfully synthesized graphene aerogels with self support and multistage channel structure by one step method and directly used as cathode materials for lithium oxygen batteries without adhesives. The three-dimensional multi-channel structure of graphene aerogels is convenient for the permeation of the electrolyte, the diffusion of oxygen and the transfer of electrons. The specific surface area can provide enough active reaction position, and the high pore volume can store more discharge products. Therefore, the discharge specific capacity of the graphene aerogel is up to 10000mAhg-1. Ru nanoparticles to be further modified on the surface of the graphene lamellar (Ru-GA), showing a good catalytic active.Ru-GA positive electrode for the oxygen exhalation reaction (OER). It can effectively increase the discharge specific capacity of the lithium oxygen battery (12000 mAh g-1), reduce the overpotential and improve the cycle stability (50 cycles can be circulate under the capacity cut-off condition of 500 mAh g-1). Furthermore, in order to solve the problem of carbon corrosion in the cycle process, we have prepared porous Ru02 materials with different pore sizes by hard template method and used as carbon free cathode of lithium oxygen batteries. At the same time, we also systematically studied the effect of pore structure parameters of ordered porous Ru02 materials on the performance of lithium oxygen batteries. The results show that the discharge capacity of the lithium oxygen battery based on the aperture of 16nmRuO2 (RuO2-16) is the highest, the energy conversion efficiency is the highest, and the 70 cycles can be stabilized in the 100 mAg-1 current density and the 2.5-4.0 V voltage range. The excellent electrochemical properties of the lithium oxygen battery can benefit from the good electronic conductivity of the Ru02-16 positive electrode. The large BET specific surface area, suitable aperture size and good catalytic activity for OER. In situ DEMS analysis shows that RuO2-16 carbon free positive electrode can effectively reduce the production of the side reaction. Finally, we have studied the electrochemical properties of the lithium oxygen battery adsorbed by activated graphene with high specific surface area. There are two voltage platforms in the charging process of the living fossil ink as the positive electrode. Through XRD and SEM, the products of different charge discharge stages are characterized. We find that the discharge products of the lithium oxygen battery have both Li2O2 and LiOH, and the generated LiOH will finish when the battery is charged to 3.5 V. Full decomposition. This coincides with the analysis of in situ DEMS. However, when SP carbon black is used as a positive pole, no LiOH. is found to indicate that the formation of LiOH is associated with the living fossil graphene. The results of Carle - Fischer titration showed that the living fossil graphene adsorbed a large amount of water vapor in the air because of its high specific surface area. Therefore, H20 was reacted with the discharge product Li202. The decomposition of LiOH and LiOH results in a lower charging voltage platform.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TM911.41
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本文编号：2037084