【作者】 简泽浪；

【导师】 陈文； 胡勇胜；

【作者基本信息】 武汉理工大学 ， 材料物理与化学， 2012， 博士

【摘要】 能源短缺与环境污染已经成为威胁人类生存和发展的严峻问题。解决能源与环境问题最有效的方法是大力发展可再生能源,然而这些可再生能源由于不连续性、不稳定性,需要采用储能装置进行转换和存储。在众多储能器件中,储能电池是其中重要的一种。锂离子电池具有工作电压高、比能量大、质量轻、体积小、循环寿命长、无记忆效应、可快速充放电和无环境污染等一系列显著的优点,是非常有前途的储能电池；同时钠离子电池由于资源丰富、成本低,也适合做储能电池。本文首先是对现有储能电池电极材料进行改性,开发了低温数层石墨烯包覆技术,得到数层石墨烯包覆的Li4Ti5O12复合材料；其次开发更高容量的电池电极材料,首次研究了LiNb3O8作为锂离子电池负极材料的电化学性能；最后研究了Na3V2(PO4)3/C作为钠离子电池材料的结构、电化学性能以及储钠机理。主要研究结论及成果如下：碳包覆技术对于提高材料的表面电子电导率和颗粒间的电子接触有着重要的作用。目前的碳包覆温度一般都在700℃以上,然而这么高的温度既不利于节约能源,也可能导致电极材料在包覆过程中被还原。我们开发了一种数层石墨烯低温包覆技术,这种包覆技术操作温度在400℃,利用该技术成功地在Li4Ti5O12表面包覆上均匀的数层石墨烯,形成离子、电子的混合导电网络。包覆后样品在5C和10C倍率下的容量分别达到131和104mAh/g,并且具有非常好的循环性能,2C倍率下2400周循环后,容量从最初的144.6降到124.4mAh/g,容量保持率为86%。同时采用该技术在Li2MnO3表面成功包覆上数层石墨烯,其循环性能也大大提高,结构仍然保持,Mn的价态也没有发生变化。这种技术方法简单,非常容易普及到其他电极材料。虽然包覆后的Li4Ti5O12电化学性能得到显著提高,但是由于其理论容量只有175mAh/g,这就要求我们开发新的替代产品以满足高能量密度锂离子电池的需要。由于Nb5+/Nb4+和Nb4+/Nb3+的氧化还原电对都在1-3V之间,能够避免SEI膜的生成,可能实现两电子转移,因此铌基化合物具有非常高的理论容量。我们首次将LiNb3O8作为锂离子电池负极材料,利用高能球磨将固相法制备的LiNb3O8与乙炔黑共混,降低颗粒尺寸,构建离子、电子的混合导电网络,可以更好地提高LiNb3O8材料的电化学性能,LiNb3O8纳微复合材料首周充放电容量分别为212和351mAh/g,对应于3.3个Li可逆地从Li6.4Nb3O8中脱出。放电态Nb5+不能完全转变成Nb3+,只能是由Nb3+和Nb4+共存；充电态也不能完全回到Nb5+,仍有部分Nb4+存在,说明LiNb3O8纳微复合材料可以部分实现两电子转移。同时采用溶胶-凝胶法制备了纳米LiNb3O8,利用非原位XRD和XANES技术,结合充放电曲线研究了LiNb3O8在充放电过程中结构变化。随着锂离子电池逐渐在电动汽车领域的大规模使用,锂的需求量不断增加,而锂的资源有限,为了缓解这个矛盾,钠离子电池目前受到广泛关注和研究,以替代锂离子电池材料在储能方面的应用。对比锂离子电池,钠离子电池具有潜在的优点,成本低,安全性能高。我们首次报道了采用一步固相法,成功制备了碳包覆Na3V2(PO4)3复合材料,形成离子、电子的混合导电网络。碳包覆Na3V2(PO4)3复合材料具有3.4和1.6V vs. Na+/Na两个平台。作为正极材料,3.4V平台高于目前报道的绝大部分钠离子电池正极材料的平均电压。通过优化电解液和包覆碳含量,首次采用NaFSI/PC新型电解液体系,碳包覆Na3V2(PO4)3复合材料首周库仑效率高达98.7%,随后每周库仑效率都高于99.8%,首周可逆容量高达107mAh/g,80周循环后容量仍然保持在99.5mAh/g,容量保持率为92.9%。循环稳定后,极化只有50mV,低于在其它电解液体系中的极化。采用XRD, in-situ XRD, NMR和STEM研究了Na3V2(PO4)3作为钠离子电池正极的储钠机理。Na3V2(PO4)3充放电过程是一个典型的两相反应,两相分别为Na3V2(PO4)3和NaV2(P04)3。充放电过程中结构变化可逆,由Na3V2(PO4)3转变为NaV2(PO4)3的体积变化仅为8.26%。Na3V2(PO4)3中Na有两种占位,分别为6b(Nal)和18e(Na2)位,其中Na2位置的数量约为Nal位置数量的2倍。Nal在结构变化中基本不发生变化,而18e位置的Na2可以进行可逆的脱嵌。更多还原

【Abstract】 Energy shortage and environment pollution threat the survival and development of humanbeing. The most effective solution is to develop renewable resources. However, those resources, because of instability and discontinuity, need large-energy energy storage equipment which could achieve energy storage and transfer between different energy resources. As energy storage equipment, lithium ion batteries have been considered as promising stationary batteries because of their high output voltage, high energy density, high power density, long cycle life, no memory and environmental friendly features; sodium ion batteries could also be used as stationary batteries because of their abundant resources and low cost. In this Ph. D. dissertation, we coated the Li4Ti5O12with a few-layer graphene via a new low-temperature coating technique; secondly, we introduce a novel material of LiNb3O8with high capacity as an anode electrode for lithium ion batteries; we report the use of carbon coated Na3V2(PO4)3as a promising cathode material for sodium ion batteries and investigate the sodium storage mechanism. The main findings and conclusions are as follows:As far as we know, carbon coating is a powerful method to enhance the surface electrical conductivity of electrode materials. It is an effective way of improving electrochemical performance. Typically, carbon coating of electrode materials is conducted over700℃. Under such a high temperature, some electrode materials, including lithium transition metal oxides, may be reduced or may become unstable during the carbon coating process. We successfully coated few-layer graphene on the surface of Li4Ti5O12by a low-temperature carbon coating technique at400℃. Capacities of131and104mAh/g can be reached at current rates of5C and10C, respectively. Moreover, cyclic performance is significantly improved after coating. The capacity decreases from144.6to124.4mAh/g after2400cycles at a current rate of2C in a half cell versus Li+/Li, with high capacity retention of86%. Furthermore, few-layer graphene coated Li2MnO3was also prepared using the same approach. The crystalline structure and chemical state of Mn remain unchanged after few-layer graphene coating. The cyclic performance is also significantly improved. This new graphene-coating technique could be easily extended to other active electrode materials for electrochemical devices.Nevertheless, the theoretical capacity of Li4Ti5O12is only175mAh/g. In order to further improve the energy density of a battery, it is desirable to explore new anode materials with high reversible capacity. Here we exploited a novel anode material of LiNb3O8. For enhancing electrical contact and deducing the particle size of electrode material, the pure LiNb3O8was ball-milled with carbon black. The initial discharge/charge capacities of the resulting sample are351and212mAh/g, respectively. The partial two-electron transfer reaction from Nb5+to Nb3+is realized in this material The result is confirmed by XANES and ex-situ XRD.Due to the low abundance of lithium in the earth’s crust, large-scale applications of lithium ion batteries become questionable. We first report the use of carbon to coat Na3V2(PO4)3materials by one-step solid state reaction approach. The sample shows flat voltage plateaus at3.4V and1.63V vs. Na+/Na. The storage voltage of3.4V is relatively higher than the average voltage of other cathode materials for sodium ion batteries. The capacity is improved by reducing carbon content of Na3V2(PO4)3/C and coulombic efficiency is enhanced by optimized electrolytes. The initial coulombic efficiency for sample after reducing carbon content inNaFSI/PC is up to98.7%and it can maintain at99.8%in the subsequent cycles. The initial reversible capacity is107.1mAh/g, which is close to its theoretical capacity and its capacity retention is92.9%after80cycles. The behavior of structural changes between charge/discharge of Na3V2(PO4)3/C is a typical two-phase reaction between Na3V2(PO4)3to NaV2(PO4)3with volume change of ca.8.26%.更多还原