【作者】 罗巍；

【导师】 黄云辉； 胡先罗；

【作者基本信息】 华中科技大学 ， 材料学， 2012， 博士

【摘要】 随着人口与经济的迅猛增长,全球能源消耗日益增加。高效率、低成本及环境友好型的可再生能源转换及存储系统的研究已成为科研热点。同时,随着对便携式电子设备和下一代电动汽车需求的日益增长,锂离子电池作为能量转换及存储技术的核心,变得越来越不可或缺。然而,石墨,传统的锂离子电池负极材料,理论容量仅为372mAh/g,远远不能满足高能量密度与高功率密度锂离子电池的要求。自从Tarascon等人首次报道了过渡金属氧化物电极材料具有优异的比容量、循环寿命及倍率性能后,过渡金属氧化物在锂离子电池领域引起了广泛的关注。但在电化学转换反应中,过渡金属氧化物会被还原为金属团簇,同时锂离子会与氧反应形成Li2O。这将导致非常大的体积膨胀及结构的严重破坏,进而引发容量的迅速衰退。为了解决这些问题,包括电极材料的纳米化、材料的形貌控制及各种碳基添加剂的引入在内的很多方法被采用。这些方法显著地提高了过渡金属氧化物基负极材料的储锂性能。过去几十年,纳米纤维、纳米棒、纳米管及纳米线等一维纳米材料因其较高的比表面积、优异的轴向电导性而在高性能锂离子电池上有了长足的发展。静电纺丝是一种构筑一维纳米材料的简单、普适的方法。受此启发,我们利用静电纺丝结合不同的后处理工艺制备了一系列一维纳米过渡金属氧化物,并研究了其储锂性能。作为一种储锂材料,二氧化钼(MoO2)因其具有低电子阻抗、高稳定性及高密度(6.5g/cm3)等性质而引起广泛关注。我们采用单针头静电纺丝、空气稳定和还原碳化这一简单制备过程,制备了具有核壳结构的碳包覆MoO2纳米纤维。这种纳米纤维的核是约20nm大小的MoO2晶粒,壳为约3nm厚的碳层。这种单针头静电纺丝的方法省略了常用到的复杂的同轴针头设备和特殊的不互溶前驱体溶液,并且得到的碳包覆M002纳米纤维表现出了优异的储锂性能,其在50mA/g的电流密度下,充放电循环100次后,比容量仍高达762.7mAh/g.这不仅说明纳米尺寸的MoO2具有很高的电化学活性,而且也证明了碳包覆可以有效地维持MoO2在充放电过程中结构的稳定性,从而改善其循环性能。二氧化钛(TiO2)因其优越的安全性、低廉的成本、高化学稳定性及无毒性等优点而有望取代石墨成为一类很有前景的负极材料。然而,Ti02固有的低离子扩散速率及低电导率致使其作为锂离子电池负极材料时容量衰减较快及倍率性能低下。为了解决这些问题,本文采用基于静电纺丝的层层(Layer-by-Layer,简称LBL)自组装的方法合成了MoO2表面修饰的TiO2纳米纤维。这些纳米纤维由处于纤维芯层的TiO2纳米晶粒和处于表面的具有类金属导电性的MoO2纳米层构成,并且表面MoO2的量可通过改变LBL过程中溶液的浓度或LBL自组装的次数来调节。当被用作锂离子电池负极材料时,MoO2表面修饰的TiO2纳米纤维在0.2C的电流密度下,充放电循环50次后,容量仍高达到514.5mAh/g,表现出了较高的比容量。同时,这种材料的倍率性能和循环性能也得到了改善。这表明,表面修饰MoO2可以实现TiO2纳米纤维储锂性能的提高。最后,我们还通过静电纺丝及简单的一步烧结法成功制备出多孔钴酸锌(ZnCo2O4)纳米管。这种独特的多孔管状结构的形成主要取决于热处理过程中不同物质的不同迁移率和有机物的分解与燃烧。在最初的热处理过程中Zn(NO3)2/Co(NO3)2/PVP纳米纤维表面的Zn(NO3)2和Co(NO3)2分解并氧化生成金属氧化物,处于纤维内部的金属硝酸盐由于未接触氧气而没有及时反应,从而造成了纤维内部与表面之间金属硝酸盐的浓度差。受此浓度差的驱动,纤维内部的金属硝酸盐处于不稳定状态并趋于向表面迁移。同时,纤维表面与内部之间也存在着金属氧化物的浓度梯度,其趋向于向纤维内部迁移。当温度达到PVP的玻璃化转变温度时,PVP分子链段将变得高度柔韧,这有利于金属硝酸盐的扩散,从而促使纤维表面富集金属氧化物,而其内部富集PVP。当加热温度进一步升高时,PVP完全分解,纳米纤维结构将转变为中空的纳米管结构,并且由于烧结过程中气体的放出及ZnCo2O4晶粒的长大,纳米管的管壁必将产生大量的孔隙。这种独特的多孔纳米管结构的形成机理可归结于Kirkendall effect效应(熔融金属硝酸盐相对于新形成的金属氧化物具有更高的扩散速率)。当被用作锂离子电池负极材料时,多孔ZnCo2O4纳米管不仅表现出了较高的比容量(100mA/g电流密度下充放电循环30次后容量高达1454mAh/g),而且显示出优越的倍率性能(2000mA/g电流密度下充放电循环30次后容量仍有794mAh/g).可见,多孔的纳米管结构即有助于电解液的浸润以及锂离子的扩散,还对结构的稳定性起了积极的作用,从而保证了电极材料在充放电过程中实现优异的倍率性能和循环性能。更多还原

【Abstract】 With the rapid growth of population and economy, global energy consumption increases strongly, which has stimulated intense research on renewable energy conversion and storage systems with high efficiency, low cost and environmental friendliness. In particular, rechargeable Lithium-ion batteries (LIBs), as the heart of key renewable energy conversion and storage technologies, become increasingly imperative with the increasing need of portable electronic devices and the upcoming electric vehicles. However, graphite, the conventional anode material in LIBs, only has a theoretical capacity of372mAh/g, which cannot meet the growing need for LIBs with higher capacity and power.Recently, transition metal oxides (TMOs) had been widely investigated since Tarascon et al first report that electrodes made of TMOs exhibit perfect reversible capacity, cyclic life and rate performance as alternative anode materials for LIBs. However, the TMOs are reduced in a conversion reaction to small metal clusters with the oxygen reacting with Li ion to form Li2O, leading to large volume expansion and destruction of the structure upon electrochemical cycling, and thus resulting in a rapid capacity fading and limiting their wide applications. Considerable approaches have been carried out to tackle this issue, including the use of nanostructure, porous structure, and the introduction of various carbon additives. These studies have brought significantly improvements of the lithium-storage performance of TMOs-based anodes.The past decade has witnessed significant growth in one dimensional (1D) nanomaterials, such as nanofibers, nanorods, nanotubes and nanowires, for high-performance LIBs by their high surface-to-volume ratios and excellent electrical conductivity along the lateral direction. Electrospinning is a simple and versatile method that provides direct and controllable fabrication strategies to construct well-defined1D nanostructure. Inspired by this, a series of1D nanostructured TMOs have been fabricated by electro spinning combining with post-heating. Furthermore, the lithium-storage properties of these1D nanostructured TMOs have been attempted to be integrated in this study.Molybdenum dioxide (MoO2) has recently received much attention as host substances for lithium-storage, owing to the low electrical resistivity, high stability, and high density (6.5g/cm). By employing a facile route based on single-nozzle electrospinning, air stabilization, and reduction/carbonization processes,1D carbon-coated MoO2nanofibers were prepared. The as-obtained1D carbon-coated MoO2nanofibers comprise hierarchically assembled MoO2nanocrystals of～20nm that are encapsulated within a thin carbon shell. This method does not require a complex coaxial-nozzle electrospinning device or a specialized immiscible solution-based precursor that is usually indispensable in an electrospinning process to form1D core-shell composites. The electrode made of the as-formed MoO2@C nanofibers exhibits excellent cyclability and a reversible capacity as high as762.7mAh/g at50mA/g after100cycles.Moreover, Titanium dioxide (TiO2) has also been fabricated as promising alternative anodes to graphite in LIBs because of their superior safety, low cost, chemical stability, and non-toxicity. Nevertheless, the main weakness of TiO2lies in the intrinsically slow kinetics of lithium ions diffusion and low electronic conductivity, resulting in the deterioration of reversible capacity and rate capability. To overcome this issue, an economical route based on electrospinning and layer-by-layer (LBL) self-assembly processes has been developed to synthesize unique MoO2-modified TiO2nanofibers, comprising a core of TiO2nanofibers and a thin metal-like MoO2nanolayer. The thickness of the MoO2nanolayer can be tuned by altering the precursor concentration or the LBL cycles. When evaluated for their lithium-storage properties, the MoO2modified TiO2nanofibers exhibit a high discharge capacity of514.5mAh/g at0.2C over50cycles and excellent rate capability, demonstrating that enhanced physical and/or chemical properties can be achieved through proper surface modification.Lastly, we have successfully fabricated porous ZnCo2O4nanotubes by an electrospinning method followed by annealing procedures. The formation mechanism of the unique porous nanotubular structure mainly relies on the different rate of mass transfer and the removal of polymer during the heating process. Zn(NO3)2and Co(NO3)2near the surface of the as-spun Zn(NO3)2/Co(NO3)2/PVP nanofibers decompose oxidatively to generate metal oxide nanocrystals during the thermal treatment initially. Meanwhile, the included metal nitrates in the core region do not react promptly due to lack of oxygen, but are unstable and tend to diffuse to the surface, driven by the concentration gradient. In addition, there exists another concentration gradient of metal-oxide nanocrystals from the surface to the interior of the nanofibers. Furthermore, PVP chains are highly flexible when the temperature is above Tg of PVP, facilitating the diffuse of metal nitrates. When the heating temperature is further enhanced, PVP decomposes completely. Therefore, the as-spun fibers have a great tendency to form hollow tubes eventually under annealing in air, owing to the Kirkendall effect related to the higher diffusion speed of melting metal nitrates in comparison to the newly-formed metal oxides. When evaluated for their lithium-storage properties, the porous ZnCo2O4nanotubes exhibit not only a high specific capacity of1454mAh/at100mA/g, but also a perfect rate capability of794mAh/g at a current density as high as2000mA/g, indicating a promising anode candidate for lithium-ion batteries.更多还原