【作者】 张明；

【导师】 王太宏；

【作者基本信息】 湖南大学 ， 材料科学与工程， 2014， 博士

【摘要】 进入21世纪以来,能源安全问题引起了全球各国的极大关注。以化石燃料为代表的能源面临着资源枯竭的问题,以太阳能、风能为代表的可再生能源存在稳定性不足的问题,因此,研究稳定的、高效的能源设备迫在眉睫。此外,便携式设备的爆发式增长对便携式储能设备提出了更高的要求。理论综合性能优异的锂离子电池是解决以上问题的可能途径之一。现今使用的锂离子电池负极——碳材料的理论容量仅为372mA h/g,难以满足人们对高容量、高功率锂离子电池的需求。以过渡金属氧化物(氧化铁、氧化钴)、锡基化合物(硫化锡、氧化锡)为代表的高理论容量负极材料是提高锂离子电池性能的备选方案,但是他们的稳定性和功率性能较差。据已发表的论文可知制备以上材料与碳材料(例如石墨烯、碳纤维)的复合材料是提高锂离子电池负极性能的非常有前途的策略。然而,传统的石墨烯基氧化铁、锡基化合物复合材料存在合成方法繁琐、环境危害大、合成效率低等不足。此外,碳纤维基氧化钴复合材料的合成机理、功率性能、机械稳定性有待进一步提高。本论文以材料物理与化学、无机化学和电化学相互交叉的锂离子电池为研究对象,从合成机理、结构调控、电化学性能等方面研究石墨烯、碳纤维基过渡金属氧化物、锡基化合物复合纤维。围绕这个目标,本文开展了以下几个方面的研究工作。(1)结合氧化石墨烯强烈吸收微波的特点,本文采用了微波加热的方法,以抗坏血酸为还原剂,在30min内将氧化石墨烯还原为石墨烯。通过热重分析发现该方法得到的石墨烯的热稳定温度较普通方法得到的石墨烯提高了50℃。仪器表征结果表明,微波加热得到的石墨烯具有较低的缺陷密度和较高的石墨化度。(2)首次采用微波加热法,以绿色环保的抗坏血酸为还原剂,将Fe3+原位沉积在氧化石墨烯表面,通过后续的热处理将Fe3+部分转化为Fe2+而合成了具有高储锂性能的石墨烯/Fe304复合负极材料,其中Fe304纳米颗粒被石墨烯分开。石墨烯/Fe304展现了良好的储锂循环性能(50个循环后的可逆容量为690mA h/g)和优秀的倍率性能(5C倍率下的容量为350mA h/g)。(3)考虑到Sn2+在碱性条件下具有较强的还原性,采用微波加热法,以Sn2+还原氧化石墨烯而制备高性能石墨烯/Sn02复合负极材料,提出“Sn2+还原氧化石墨烯”机制。石墨烯/Sn02复合材料作为锂离子电池负极表现出良好的循环性能(100个循环后的可逆容量为550mA h/g)和高倍率性能(5C倍率下的容量为460mA h/g)。(4)根据“Sn2+还原氧化石墨烯”机制,考虑到Fe2+在碱性条件下也具有较强的还原性,采用微波加热法,Fe2+还原氧化石墨烯,大规模、高效率制备了高性能的石墨烯/Fe203复合负极材料,提出了“可变价金属离子还原氧化石墨烯”机制。作为锂离子电池负极材料,该方法合成的石墨烯/Fe203复合材料要优于普通机械混合法所制得的复合材料和单纯的Fe203,其在100个循环后的可逆容量高达800mA h/g o(5)根据“可变价金属离子还原氧化石墨烯”机制,利用氧化石墨烯氧化亚锡离子而制备具有优异储锂性能的石墨烯/硫化锡复合材料。在氧化石墨烯作用下,石墨烯/硫化锡复合材料中硫化锡的平均粒径为5nm,且均匀分散在石墨烯片层上。由于石墨烯/硫化锡具有以上独特的结构,由硫化锡分解产生的Li2S可能在一个相对较低的电位下可逆性分解而储锂。基于该储锂机理,石墨烯/硫化锡以0.2C的倍率在150次循环以后的放电容量为860mA h/g,且其倍率容量同样高于单纯的SnS。(6)考虑到高倍率负极材料的迫切需求和静电纺丝法制备的聚丙烯腈基碳/钴纤维中钴的价态存在争议的问题,本文通过静电纺丝和热处理的方法得到了CoO均匀分布的碳/CoO纳米纤维网络结构。作为无粘结剂锂离子电池负极材料,在650℃得到的碳/CoO纳米纤维在0.1A/g的电流密度下第52个循环的放电容量高达633mA h/g。该比容量高于在550℃和600℃所获得碳/CoO纳米纤维网络,同样高于在600℃和650℃获得的纯碳纤维网络。此外,碳/CoO纳米纤维网络结构(650℃)的倍率容量(2A/g的电流密度下为420mA h/g)也高于在600℃合成的碳/CoO纳米纤维网络结构和在650℃制备的纯碳纤维网络。碳/CoO纳米纤维网络结构的优良性能与碳纤维提高电子传输效率,离子扩散速率和维护CoO纳米颗粒的稳定性有关。(7)考虑到碳/CoO纤维作为无粘结剂负极的脆性,结合石墨烯优异的力学、电学性能,本文采用静电纺丝法和随后的热处理工艺制得了碳-石墨烯-氧化钴无粘结剂柔性毡。结构表征表明氧化石墨烯可以在合成过程中控制CoO的颗粒尺寸。作为锂离子电池无粘结剂负极材料,碳-石墨烯-氧化钴无粘结剂柔性毡展现出了比CoO-C,石墨烯-碳,和纯碳纳米毡更好的循环稳定性和很高的比容量(0.5A/g的电流密度下352次循环后690mA h/g)以及更强的倍率容量(2A/g的电流密度下400mA/g)。这些性能提高可以归因于柔性毡保证Li+快速扩散,具有良好的机械性能和良好的导电能力的石墨烯不但控制了CoO的颗粒尺寸,还改善了毡的机械强度和导电性,且引入CoO后使有助于碳纤维储锂性能的缺陷增多。更多还原

【Abstract】 Since the2000, many countries paid much attention to the energy safety. Fossil fuels may be exhausted in the nearly future. Wind and solar energy as renewable energy systems are unstable based on current technology. So it is urgent to develop stable and high efficiency energy devices. Besides, the demand of increasing portable devices on the portable energy devices became higher. Lithium ion batteries with high theoretical properties may be one of the possible strategies to solve above issues. Now, the common anodes for lithium ion batteries are carbon, whose theoretical capacity only is372mA h/g. Those lithium ion batteries cannot meet the demands of the humans for lithium ion batteries with high capacities and high power density. Transition metal oxides (including FexOy, CoOx) and tin-based compounds are anodes with high theoretical capacity, and may be the candidates for carbon. However, their stabilities are relative poor and their power densities are relative low. According to the literature, preparing the composites based on above materials and carbon materials (such as graphene and carbon fibers) are promising route to improve the properties of anodes. Whereas, the traditional methods for the synthesis of graphene-FexOy and graphene-tin-based compounds are tedious, environmental pollution and low efficiency. Besides, the synthesis mechanism, power density and mechanical stability of carbon fibers-CoO composites need further investigation.The present thesis will investigate the lithium ion batteries, which include the science of material physics and chemistry, inorganic chemistry and electrochemistry. The graphene-based and carbon fibers based transition metal oxides and tin compounds will be investigated in detail from the angles of synthesis mechanism, microstructure modification and their electrochemical properties. This study will focus on the following aspects to achieve the goals.(1) Taking into account of microwave absorbability of graphene oxide, graphene sheets were prepared in30min by microwave irradiation using ascorbic acid as reductants. Thermo gravimetric analysis results discovered that the temperature corresponding to the thermal stability of the graphene sheets prepared by microwave irradiation was50℃higher than that of graphene sheets prepared by other heat methods. The enhancement could be attributed to the lower densities of defect sites and higher graphitization degree of graphene sheets synthesized by microwave irradiation. (2) Graphene/Fe3O4composite anodes were synthesized by employing microwave irradiation as heat sources, using nontoxic ascorbic acid as reductants to in situ deposit Fe3+on the surface of graphene oxide, and following heat treatment to partial reduce Fe3+to Fe2+. The Fe3O4nanoparticles were separated by graphene sheets. Graphene/Fe3O4composites displayed enhanced cyclic stability (showing a reversible capacity of690mA h/g after50cycles) and excellent rete capacity (350mA h/g at a rate of5C).(3) Taking into account of the relative high reducibility of Sn2+in basic condition, graphene/SnO2composites with high properties were prepared by microwave irradiation and the theory of "Sn2+reducing graphene oxide to graphene" was proposed. As anodes for lithium ion batteries, graphene/SnO2composites showed good cyclic properties (a reversible capacity of550mA h/g after100cycles) and excellent rate capacities (460mA h/g at a rate of5C).(4) Based on the theory of "Sn+reducing graphene oxide to graphene" and the reducibility of Fe2+in basic condition, graphene/Fe2O3composites with excellent properties were synthesis by microwave irradiation. This method was of high efficiency to prepare the composites at large scale. A theory of "the metal ions with variable valence reducing graphene oxide to graphene" was proposed based the experimental results. As anodes for lithium ion batteries, graphene/Fe2O3composites showed enhanced properties compared with mechanical mixture of graphene-Fe2O3, bare Fe2O3, whose reversible capacity was about800mA h/g after100cycles.(5) Based on the theory of "the metal ions with variable valence reducing graphene oxide to graphene", graphene/tin sulfide composites with excellent properties were prepared using graphene oxide to oxidize Sn2+to Sn4+. With the help of graphene oxide, the tin sulfide nanoparticles in graphene/tin sulfide with diameters of5nm were dispersed homogenously on graphene sheets. Since the special structure, the Li2S arising from the decomposition of tin sulfide may be reversibly decomposed at a relative low potential. Based on this mechanism, graphene/tin sulfide composites could deliver a discharge capacity of860mA h/g at a rate of0.2C, higher than that of bare SnS. Their rate capacities also were higher than those of bare SnS.(6) Because the valence of cobalt in carbon fiber/cobalt composites was controversial and the demands for anodes with high power density were increasing, this study investigated the synthesis of carbon/CoO nanofiber networks by electrospinning and following heat treatment. As binder-free anodes for lithium ion batteries, the carbon/CoO nanofiber networks synthesized at650℃could deliver a discharge capacity of633mA h/g after52cycles at a current density of0.1A/g. This value was higher than those networks obtained at550and600℃, and pure carbon fiber networks synthesized at600and650℃. In addition, the rate capacities of the carbon/CoO nanofiber networks (650℃) also were higher than those of carbon/CoO networks (600℃) and pure carbon fiber networks (650℃). The excellent electrochemical properties of carbon/CoO nanofiber networks could be ascribed to the enhanced conductivity, fast diffusion rate, and the stability of CoO by carbon fibers.(7) Since the brittleness of carbon/CoO as binder-free anodes and the excellent mechanical and electronic properties of graphene, the binder-free flexible mats of carbon-graphene-CoO were prepared by electrospinning and following heat treatment. The structure characterization results showed that the graphene oxide could control the particle sizes of CoO. As binder-free anodes for lithium ion batteries, the carbon-graphene-CoO binder-free flexible mats displayed enhanced cyclic stability (690mA h/g after352cycles at a current density of0.5A/g) and excellent rate capacity (400mA h/g at a current density of2A/g) compared with CoO-carbon, graphene-carbon, and pure carbon fiber mats. The improvement could be attributed to the flexible mats to make sure the fast diffusion of Li+and to improve the mechanical strength and conductance. Besides, the defect sites will be improved by the introduction of CoO.更多还原