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镍基电极活性材料的制备、改性及其电化学储能性能研究
中文摘要

过去数十年来,随着能源危机日趋严峻以及传统化石能源所带来的环境问题,可再生的清洁能源的开发迫在眉睫。其中,设计高性能的储能设备已经成为当前环境和能源可持续发展最为迫切的挑战之一。超级电容器作为新型的能源存储和转换设备,因其具有快速充放电、良好的循环稳定性和较高的功率密度等优点而受到广泛的关注,并且在移动电子设备、混合电动汽车和高功率设备等领域都具有广阔的应用前景。金属化合物在目前所报导的各类电极活性材料中具有较高的比电容,是超级电容器中最有希望实现高能量密度的一类电极活性材料。本论文研究了镍基硫化物和氢氧化物的水热和溶剂热制备、物相调控规律及电化学储能性能。通过改变溶剂热合成条件以及运用热处理工艺构筑出一系列不同形貌、结构和物相的镍基化合物,并且系统地研究镍基化合物及其复合材料的超级电容性能与形貌及结构、成分的关系。主要内容包括以下三个部分: 1.采用一种简单的溶剂热合成方法在相对较低的温度条件下,通过改变反应条件制备不同物相硫化镍纳米材料,包括单相α-NiS、Ni₃S₄和NiS₂全面的研究了反应条件和表面活性剂(CTAB)产物形貌和物相的影响,分析了溶剂热合成的硫化镍相控制机理。在CTAB存在的条件下,当水热反应时间为9 h时成功合成中空结构的NiS₂。随着反应时问的延长,溶剂热合成的硫化镍更倾向于硫化学计量比低的物相,有效实现表面活性剂对硫化镍合成物相的调控。并且进一步探究了硫化镍电极材料微观形貌、物相和其电化学超级电容性能的关系。结果表明α-NiS电极材料电化学性能明显优于Ni₃S₄和NiS₂电极材料,在0.5 Ag⁻¹电流密度下比电容达800 F g⁻¹,而Ni₃S₄和NiS₂电极材料的比电容为515和394 F g⁻¹,这主要归因于其更小的颗粒尺寸、高电导率和独特的晶体结构。α-NiS电极材料在10A g⁻¹电流密度下恒流充放电循环2000圈以后仍然保持初始比电容的81.2%,并且保持较高的库伦效率(>98%),这表明αNiS电极具有高的比电容和优异的循环稳定性。本研究工作实现了可控物相硫化镍的低温制备,阐明了溶剂热合成过程中硫化镍相演变机制,为不同应用领域合成制备可控物相硫化镍提供理论指导。 2.以Ni₃S₂@MoS₂为例,选用纳米镍粉为原料和模板,采用一种简单的水热合成方法及后续退火处理工艺,成功设计并制备了一种高导电率分级核壳结构的硫化物复合材料,该方法可实现大规模应用。Ni₃S₂@MoS₂复合材料作为超级电容器的电极材料,其在0.5A g⁻¹电流密度条件下的比电容为1418.5F g⁻¹,并且在5A g⁻¹大电流密度充放电条件下循环1250圈仍然保持75.8%的容量保持率。这种后续退火后得到的高度结晶的Ni₃S₂@MoS₂复合材料的比电容是其对应的为退火处理前的Ni〓S〓@MoS₂复合材料的比电容的1.9倍。由MoS₂纳米片相互交织构成的壳层提供了与电解质离子更加充分有效接触面积,有利于电解质离子的扩散传输。利用MoS₂纳米片层作为核层的柔性保护,包裹高导电率的金属性核材料(Ni₃S₂),这种分级复合材料提供多孔分级结构的均质原子异质结界面,实现电子传输过程的协同作用,有效增强电极的电化学性能。独特的结构使得Ni₃S₂@M0S₂复合材料作为电极材料具有更高的比电容和倍率性能。而且,以Ni₃S₂@MoS₂为正极材料和还原氧化石墨烯(rGO)为负极材料组装的非对称超级电容器(Ni₃S₂@MoS₂//rGO)在0.5A g⁻¹电流密度下的比电容为61 F g⁻¹。在400 W ㎏⁻¹功率密度下的能量密度为21.7 Wh ㎏⁻¹,在2400 W ㎏⁻¹功率密度下的能量密度为12 Wh ㎏⁻¹。并且在10 A g⁻¹电流密度下循环4000圈容量保持率为72%,表现出优异的循环性能。本工作为设计高结晶性核壳分级结构复合材料提供了一种有效的策略,在能量存储应用领域为制备高性能电极活性材料提供了新的思路。 3.采用简单的溶剂热法合成制备镍锰层状双金属氢氧化物(NiMn LDH)和镍锰层状双金属氢氧化物/石墨烯(NiMn LDH/rGO)复合材料。对所制备的NiMn LDH/rGO复合材料进行结构和形貌表征,发现石墨烯可以有效改善NiMn LDH团聚,进而增加其比表面积。在三电极体系下,对所制备的NiMn LDH和NiMn LDH/rG0复合材料进行了电化学性能测试,结果表明石墨烯可以明显改善复合电极材料的电化学性能。 LDH/rGO-4电极材料在1 Ag⁻¹电流密度下比电容高达1500F g⁻¹,并且在20A g⁻¹大电流密度下循环5000圈容量保持率为初始容量的90.5%,显示出优异的循环稳定性。在此基础上,将LDH/rGO-4和rGO分别作为正极和负极,组装成两电极体系,并对其电化学性能进行表征。所组装的非对称超级电容器在0.5A g⁻¹电流密度下比电容为82.5F g⁻¹,对应的能量密度为29.3 Wh ㎏⁻¹。电化学测试结果表明镍锰层状双金属氢氧化物/石墨烯复合材料是一种具有实际应用前景的超级电容器电极材料。 关键词:金属硫化物;层状双金属氢氧化物;电化学;水热合成;退火;超级电容器

英文摘要

Over the past decades, there has been a huge demand on renewable energy to meet the ever-growing energy crisis and to address the devastating environmental issues. Therefore, designing of high-performance energy storage devices has become one of the most urgent challenges heading towards finding sustainable solutions to the energy crisis and environmental problem. Many efforts have been dedicated to the advancement of energy storage and conversion devices, such as supercapacitors and secondary batteries. Among them, supercapacitors have attracted much attention due to their unique advantages, such as rapid charging/discharging time, good cycle stability and high power density, and broad potentials applications in portable electronic equipment, hybrid electric vehicles and high power equipments. The metal compounds are considered to be one of the most promising electrode active materials in the high energy density supercapacitors due to the high specific capacity. The main research work of this thesis is to study the preparation of nickel based compounds and their supercapacitor properties by changing the synthesis method systematically, including nickel-based sulfide, nickel-molybdenum sulfide and nickel-based hydroxide. The main results include the following three parts: Firstly, phase-controlled solvothermal synthesis has been proposed for the synthesis of nickel sulfide of single phase including α-NiS, Ni₃S₄ and NiS₂ by tuning the reaction time and the addition of surfactant. The phase evolution of nickel sulfide proceeded with the increase of sulfur stoichiometry with longer reaction time in the presence of surfactant. With the addition of hexadecyl-trimethyl-ammonium bromide (CTAB), a higher sulfur stoichiometry NiS₂ phase with hollow sphere geometry was synthesized at 9 h, time was much shorter due to the enrichment of S² on CTAB micelle surface, followed by the transformation to single phase Ni₃S₄ finally due to dissipation of enriched sulfur to the bulk solution. The application of these three single phase materials in supercapacitors was investigated. The α-NiS electrode material outperformed the Ni₃S₄ and NiS₂ electrodes, exhibiting a higher specific capacitance of 800 F g⁻¹ at 0.5 A g⁻¹ due to the small particle size, high electrical conductivity and the unique hexagonal crystal structure. The cycle performance and coulombic efficiency for the α-NiS were evaluated by charging/discharging between 0 and 0.5 V at a current density of 10 A g⁻¹ over 2000 cycles. The α-NiS electrode exhibited excellent electrochemical stability with 81.2% of the initial available specific capacitance remaining after 2000 cycles, while retaining high coulombic efficiency (> 98%). The results demonstrated that the α-NiS based electrode displayed a high specific capacitance and excellent cycling stability. This work sheds light on the phase evolution mechanism of nickel sulfide during solvothermal synthesis in the presence of surfactant and provide useful guidance on the identification of different nickel sulfide phases for various applications. Secondly, a hierarchical Ni₃S₂@MoS₂ hybrid structure was synthesized by an effective strategy by combining hydrothermal route and subsequent annealing treatment. When tested as supercapacitor electrodes, the Ni₃S₂@MoS₂ composites exhibited high specific capacitance of 1418.5 F g⁻¹ at 0.5 A g⁻¹, which also showed a good capacitance retention of 75.8% at 5 A g⁻¹ after 1250 cycles. The Ni₃S₂@MoS₂ composites demonstrated 1.9 fold higher specific capacitance compared to the amorphous shell counterpart (Ni〓S〓@MoS₂). The improved electrochemical performance is ascribed to the synergetic effect of the large accessible surfaces and optimal contacts between the MOS₂ and the electrolyte, and high capacitance of the metallic Ni₃S₂ core. Furthermore, the assembled asymmetric supercapacitor (Ni₃S₂@MoS₂//rGO) also demonstrated capacitance of 61 F g⁻¹ at 0.5 A g⁻¹, with energy and power densities of 21.7 Wh ㎏⁻¹ at 400 W ㎏⁻¹ and 12 Wh ㎏⁻¹ at 2400 W ㎏⁻¹ under a voltage operating window of 1.6 V. The asymmetric supercapacitor also showed favourable cycle stability with 72% capacity retention over 4000 cycles at 10 A g This work proposes a universal technique for the synthesis of a phase-controlled nickel sulfide which can also be extended to other nanostructured systems, such as cobalt sulfide, which providing new alternatives for the preparation of advanced functional materials. Thirdly, pure NiMn layered double hydroxide (NiMn LDH) and flower-like 3D-architectured NiMn LDH/reduced graphene oxide (rGO) composite are both fabricated by a simple solvothermal synthesis method. The structural characterization and morphological observation of the composites suggest the NiMn LDH was restacking on the surface of the rGO flakes by means of electrostatic interactions, leading to the composites with high specific surface area. The electrochemical performances of pure NiMn LDHs and NiMn LDH/GO composites with different content of GO were investigated and compared. Electrochemical measurements prove that rGO can improve its capacitance and cyclic stability of the hybrid materials due to the incorporation of NiMn LDH and rGO flakes, compared with the pristine counterpart. The LDH/rGO-4 exhibits a higher specific capacitance of 1500 F g⁻¹ at 1 A g⁻¹ in 6 M KOH solution and more stable rate capability than others. Furthermore, the capacitance of the composite maintained 90.5% of the initial capacitance even at a high current density of 20 A g⁻¹ after 5000 cycles. A hybrid capacitor with LDH/rGO-4 as positive electrode and rGO as negative electrode is assembled. It possesses a specific capacitance of 82.5 F g⁻¹ at 0.5 A g⁻¹ and an energy density of 29.3 Wh ㎏⁻¹ within a potential window of 1.6 V. These results indicate that the LDH/rGO-4 composite is a promising material for supercapacitors. Our presented work not only provides a facile preparation approach to LDH/rGO hybrid materials, but also sheds light on electrode design for supercapacitors. Key words: Mental sulfide; NiMn LDH; Electrochemisty; Hydrothermal synthesis; Annealing; Supercapacitors

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