【作者】 袁艳；

【导师】 陈白珍；

【作者基本信息】 中南大学 ， 冶金工程， 2014， 博士

【摘要】 摘要：锂硫电池由于具有能量密度高、原料来源丰富、成本低廉、绿色环保等优势,成为极具发展潜力的新型二次电池体系之一,有望在便携式电子通信、储能装备和电动汽车等高能量密度需求领域实现广泛应用。然而,受限于当前技术水平、自身储能方式和材料性能缺陷,锂硫电池仍然存在着活性材料利用率低、循环寿命差、倍率性能差、白放电高等不足,这严重制约了其下一步的发展。针对锂硫电池存在的上述问题,本论文以提升电池综合性能、揭示内部失效机制为主要目的,从电极材料和电解液两个方面开展了研究工作。论文研究了有机溶剂的加入对离子液体基电解液各项性能指标的影响,优化了三元电解液体系组分,考察了其在锂硫电池上应用的可行性；合理选择了液相沉积法的制备工艺,研究了所制备的纳米硫与硫／纳米炭黑复合材料在三元电解液中的性能,分析了硫与纳米炭黑的不同结合方式引起的不同性能表现；研究了不同制备条件下硫／中空碳纤维复合材料的组成、结构形貌与电化学性能之间的关系,分析了中空碳纤维对电池性能的改善作用；研究了不同种类电解液添加剂对电池性能的影响,探讨了添加剂对穿梭效应的抑制行为与相关机理。得到的主要研究结论如下：(1) PEGDME的加入在一定程度上降低了PYR14TFSI基电解液的热稳定性,但有利于提高电解液的导电性、降低体系粘度、保障电极反应活性与可逆性,且PEGDME含量越高,作用效果越明显。优化含量下的LiTFSI+PYR14TFSI+PEGDME三元电解液在常温下的离子电导率可达4.4×10-3S/cm,稳定电位窗口为0～4.1V(vs.Li+Li),且与金属锂电极的相容性良好,能够保证电池在长期循环过程中的电化学稳定性,尤其是在适度高温的环境下,因此适合在锂硫电池中应用。(2)液相沉积法在合适工艺条件下可制备出粒径在100nm以内、尺寸较为均一且分散性良好的类球形纳米硫材料,其在三元电解液中0.02C首次放电比容量达到1050mAh/g,循环期间库仑效率接近100%,但容量衰减较快。通过液相沉积法制备的硫／纳米炭黑复合材料,实现了硫碳的均匀分布和紧密接触,其0.02C首次放电比容量高达1200mAh/g,经50次充放电循环后,容量仍保持有700mAh/g,相比单质纳米硫,复合材料增强了电极的导电性和结构稳定性,提高了活性材料的利用率,改善了电池的循环性能。(3)高温处理法制备硫／中空碳纤维复合材料容易产生较为严重的团聚结块,硫碳分布不均匀。液相沉积法可以保证硫碳的良好分散和较小尺寸的硫颗粒,适中载硫量下硫碳接触紧密、结合力强,部分硫颗粒可嵌入至碳纤维的中空通道内,材料0.1C首次放电比容量达到1165mAh/g,经充放电循环40次后,容量仍维持有520mAh/g,基于第二次放电的容量保持率达到75.7%,且高倍率循环稳定性良好。中空碳纤维构建的富孔、高导、稳定的三维导电网络,促进了电荷/离子的快速传递,实现了更多的活性物质装载,增强了电极与电解液的界面接触,减少了不溶产物沉积导致的活性物质损失,抑制了硫体积的反复变化对电极的破坏,因而综合提高了复合材料的容量发挥、循环性能和倍率性能。(4)穿梭效应在充电过程表现尤为显著,充电曲线的高电位平台容量与穿梭效应密切相关,此时多硫化物存在着氧化和还原反应的竞争过程。LiNO3通过自身消耗参与负极成膜反应,改变了金属锂界面的组分状态,增强了负极表面SEI膜的结构稳定性,因而有效阻断了多硫化物与负极锂的接触和反应,抑制了穿梭效应,合适添加量下电池0.1C首次放电比容量达到1135mAh/g,经40次循环后保持有580mAh/g,循环期间库仑效率始终接近100%,因此对电池过充的限制和对性能的改善作用比较明显。而PYR14TFSI由于具有多硫化物溶解度低、粘度高等特性,对多硫化物在电解液中的溶解和扩散同时起到了限制作用,进而对穿梭效应实现了抑制,但整体作用效果相比LiNO3有一定的差距。更多还原

【Abstract】 ABSTRACT:Lithium-sulfur battery, considered as one of the most potential secondary battery system, is expected to achieve a wide range of application in the field of high energy density demands, such as portable electronic communication, energy storage equipments and electric vehicles, due to the advantages of high energy density, abundant resources, low cost and environmental friendliness. However, the low utilization of the active material, poor cycle life, low rate performance and high self-discharge etc., which are derived from the limitation of current techniques, storage mechanism and defects of the electrode materials, severely restricts the development of lithium-sulfur battery.In view of above problems existed in lithium-sulfur batteries, the researches on both electrode materials and electrolytes were carried out in order to improve the battery performance and reveal the internal fading mechanism. In this paper, the effects of the performance by adding an organic solvent to ionic liquid based electrolyte were studied; the components of ternary electrolyte system were optimized; the feasibility of its application in lithium/sulfur batteries was evaluated. Meanwhile, optimized liquid phase precipitation process was choosed to make nanomaterials; the performances of sulfur and sulfur-carbon nanomaterials in the ternary electrolyte were characterized, the different performances caused by the different combination type between sulfur and nano-carbon black were analyzed as well. Also, the relationship among components, morphology, structure and electrochemical properties of sulfur/hollow carbon fiber composite materials prepared under different conditions was studied; the role of hollow carbon fiber on improving battery performance was analyzed. At last, the effects of different types of electrolyte additives on battery performance were explored; the inhibition mechanisms of different additives on the shuttle behavior were discussed. Here come the main conclusions:(1) Although the thermal stability of the PYR14TFSI based electrolyte was reduced by the addition of PEGDME, it helps improving the conductivity of electrolyte, reducing the viscosity of the system, and maintaining the reactivity and reversibility of electrode. Besides, the more content of PEGDME were added, the greater improvement it got. The ionic conductivity of optimized LiTFSI+PYRi4TFSI+PEGDME mixture can reach up to4.4×10-3S/cm at room temperature and the stable electrochemical window is0-4.1V(vs.Li+/Li). The ternary electrolyte mixture had good compatibility with lithium electrode, and ensured good electrochemical stability of Li/Li symmetric cell during long-term cycles, especially under proper elevated temperature, which is suitable for being used in the lithium-sulfur battery.(2) Spherical sulfur nanomaterial with size in range of100nm was successfully synthesized by proper liquid phase precipitation process. The initial specific capacity of the nanosulfur electrode can reach to1050mAh/g at a rate of0.02C, the coulombic efficiency is almost100%, but the capacity fades rapidly. The sulfur/carbon black composite prepared via liquid phase precipitation method, realized uniform distribution and close contact of the sulfur and carbon, which is beneficial to the electrode conductivity. The sulfur/carbon composite electrode exhibited an initial specific capacity of1200mAh/g at a rate of0.02C, and the capacity retained a700mAh/g after50cycles. Compared to the element sulfur nanomaterial, sulfur/carbon black composite enhanced the conductivity and structral stability of the electrode, increased the utilization of active material, and improved the cycle performance of the cell.(3) Sulfur/hollow carbon fiber composite material prepared by high temperature processing method prone to agglomerate severely, and the distribution of sulfur/carbon was not uniform. Good distribution of sulfur/carbon particles and small sulfur particles could be ensured via liquid phase precipitation method. Sulfur and carbon particles could be contacted closely when load of sulfur content is suitable, some sulfur particles could insert into the hollow path of the carbon fiber. The initial specific capacity of the composite got to1165mAh/g at a rate of0.1C, the capacity remained520mAh/g after40cycles. The capacity retention rate reached to75.7%compared to the2nd cycle, and cycle stability at high rate was pretty good. The stable three-dimensional conductive network with abundant holes constructed by the hollow carbon fiber, promoted the fast delivery of the charge and ion, realized more load of active material, enhanced the contact of electrode and electrolyte, reduced the loss of active material which was attributed to the precipitation of insoluble products, and restrained the destruction of the electrode from the repeated volume changes. Thus, it improved the specific capacity, cycle performance and rate performance of the composite material.(4) Shuttle effect during the charge process was particularly significant; the capacity of the high potential platform of the charge curve was closely related to the shuttle effect, meanwhile, the electrochemical oxidation process of the polysulfide competed with the reduction process of shuttle effect. The self-consumption of LiNO3for film-forming, changed the components status of lithium metal surface and improved the structural stability of SEI film on the anode, which effectively blocked the reaction between polysulfide and the anode and restricted the shuttle behavior. The initial specific capacity reached up to1135mAh/g at a rate of0.1C under a suitable content of LiNO3, the capacity remained580mAh/g after40cycles; the coulombic efficiency was closely to100%. Hence, it obviously restricts the overcharge and improves the performance of the cell. Moreover, Low solubility of polysulfide in PYR14TFSI and high viscosity of PYR14TFSI, which inhibited dissolving and diffusing of polysulfide in the electrolyte, actually suppressed the shuttle effect. However, the general effects of adding PYR14TFSI were still not very good compared with adding LiNO3.更多还原