【作者】 李国春；

【导师】 高学平；

【作者基本信息】 南开大学 ， 无机化学， 2013， 博士

【摘要】 单质硫作为一种轻质、多电子反应正极活性材料,其理论比容量为1675mAh/g;以金属锂为负极、单质硫为正极所组成的锂硫电池的理论能量密度高达2600Wh/kg。此外,单质硫还具有资源丰富、成本低廉、安全性好和无环境污染等优点。因此,锂硫电池被视为下一代极具应用潜力的高比能二次电池体系。由于硫活性物质本身的电子和离子绝缘特性,以及硫电极在放电过程中形成的中间产物多硫化锂易溶解于有机电解液中,且放电最终产物硫化锂也是电子的不良导体等众多因素,导致锂硫电池循环寿命较差,且硫的活性物质利用率和倍率性能偏低,因而严重制约了锂硫电池的实用化进程。在本论文中,为了克服上述问题,采用将单质硫与导电炭黑进行复合,制备了一系列硫基复合材料。特别是,利用导电炭黑基体较强的导电性能、较高的比表面积和吸附特性,来提高硫基复合材料的导电性,以期抑制锂硫电池放电中间产物的溶解穿梭,从而提高硫活性物质的利用率,改善硫电极的循环性能以及倍率性能。首先,采用商业化导电炭黑Ketjenblack EC600JD作为基体,将单质硫和导电炭黑按一定的比例通过机械球磨和热处理的的方法,制备了硫／碳复合材料(S/C),然后采用原位化学氧化聚合的方法制备出具有核壳结构的聚苯胺包覆硫/碳多重复合材料(PANI@S/C)。研究结果表明,硫含量为43.7wt%的PANI@S/C多重复合材料表现出最佳的电化学性能。其中,结构分析显示,厚度为5-10nm的导电聚苯胺均匀包覆在S/C纳米颗粒的表面,活性物质硫则高度分散于导电炭黑基体的纳米孔中,从而形成具有核壳结构的复合材料。在1C充放电倍率下,该复合材料首周放电容量达1405.5mAh/g,循环100周以后容量保持在596mAh/g。即使在10C倍率下,该复合材料最高放电容量仍可达635mAh/g,并且经过180周循环后,容量保持率为60%。由于导电炭黑和聚苯胺的协同作用,显着提高了复合电极的电化学性能。同时,电极体系的电子导电性得到增强,且电极反应的穿梭效应得到有效抑制,从而使电极表现出优异的高倍率充放电性能、良好的循环稳定性和较高的活性物质利用率。为进一步提高复合材料中硫的负载量,本文又以商业化导电炭黑BP2000(CCB)为原料,采用KOH高温活化扩孔的方法,制备了具有高比表面积和高孔容的活化导电炭黑材料(A-CCB)。然后,通过两步热处理的方法制备了硫含量分别为64wt%和73wt%的硫/活化导电炭黑复合材料(S/A-CCB),以及硫含量为65wt%的硫/导电炭黑复合材料(S/CCB),并研究了它们的电化学性能。结构分析显示,导电炭黑经活化后其比表面积和孔体积显著增大。电化学测试表明,硫含量为64wt%的S/A-CCB复合材料表现出最佳的电化学性能。在160mA/g电流密度下,该复合材料首周放电容量为956.7mAh/g,循环100周后,容量保持在531.9mAh/g;当电流密度为800mA/g时,硫电极经过数周活化后其最高放电容量可达771.8mAh/g,表现出较好的倍率性能。在同等条件下,采用CCB作为导电基体或提高S/A-CCB复合材料中硫的负载量均导致硫电极较差的电化学性能。最后,考虑到硫化聚丙烯腈和硫/碳复合材料均可作为锂硫电池正极材料,以此可构筑多元复合材料。本论文仍以导电炭黑BP2000作为基体,首先采用溶解-沉淀法获得了硫/导电炭黑/聚丙烯腈的三元混合物,随后将混合物高温预煅烧,再将预煅烧产物经过不同时间的热处理制备得到三种硫/碳/硫化聚丙烯腈复合材料(S/C/sPAN)。同时,采用1M LiPF6(PC+EC+DEC, v/v/v=1:4:5)的碳酸酯类电解液和1M LiTFSI+0.2M LiNO3(DOL+TEGDME, v/v=1:1)的醚类电解液对这三种复合材料进行循环伏安和恒流充放电性能表征。研究结果表明,三种样品在这两种电解液中具有不同的电化学反应特性。经过对比,热处理7个小时得到的S/C/sPAN-7复合材料在碳酸酯类电解液中表现出最佳的电化学性能。以复合材料整体作为活性物质计算,在40mA/g电流密度下,该复合材料首周放电容量高达1103.4mAh/g,循环40周后,容量保持在350mAh/g。当电流密度增大到100mA/g时,复合材料经过200周循环后容量还保持在243.5mAh/g,容量保持率从第二周开始计算时高达77%。在300mA/g电流密度下,该复合材料也表现出优异的循环稳定性,并且充放电库仑效率接近100%。总之,本论文以商业化的导电炭黑为基体,设计合成了两类新颖的硫/碳/聚合物多重复合材料,并研究了它们的结构和电化学性能的关系。此外,以导电炭黑为原料,制备了具有高比表面积和高孔体积的活化导电炭黑材料,获得了电化学性能优异的硫基复合材料。本论文从稳定的商业化碳材料着手,为今后设计新型硫基复合材料和开发廉价实用的高比能锂硫电池提供了有力的实验依据和理论基础。更多还原

【Abstract】 As a light-weight, multi-electrons reaction cathode active material, the elemental sulfur has a theoretical capacity of1675mAh/g. The theoretical energy density of lithium-sulfur battery, based on the metallic lithium as anode and elemental sulfur as cathode, can reach up to2600Wh/kg. Besides, the elemental sulfur has the advantages of natural abundance, low cost, safety and environmentally friendliness. Therefore, lithium-sulfur battery is considered as a promising high specific energy secondary battery system for the next generation. However, the pratical application of lithium-sulfur battery is seriously restricted by the following fundamental problems:(1) electrically and ironically insulated nature of elemental sulfur;(2) solubility of polysulfides produced during the discharge process of sulfur electrode into organic electrolytes;(3) poor electrical conductivity of final reduction product Li2S, et.al. These problems lead to serious decay of cycle life, low utilization of active material sulfur and poor rate performance for lithium-sulfur battery. In this work, sulfur was incorporated into conductive carbon black to prepare a series of sulfur-based composites in order to overcome the problems mentioned above. The introduction of conductive carbon black with excellent electrical conductivity, high specific surface area and good absorption capability is expected to improve the conductivity of sulfur-based composites, effectively suppress the dissolution and shuttle of intermediate products, thereby improving the utilization of sulfur active material and also improving cycle performance and rate performance of sulfur electrode.Firstly, commercial conductive carbon black Ketjenblack EC600JD was used as matrix to prepare sulfur/carbon composites by ball-milling sulfur and conductive carbon black in a certain proportion and a subsequent heat treatment. Then, polyaniline coated sulfur/carbon multi-composites(PANI@S/C) with unique core/shell structure were successfully prepared by an in-situ chemical oxidative polymerization method. It’s demonstrated that the PANI@S/C composite with43.7wt%sulfur presents the optimum electrochemical performance. The conductive polyaniline with a thickness of ca.5-10nm, is uniformly coated onto the surface of the S/C composite to form a core/shell structure, while sulfur is highly dispersed in the nanopores of conductive carbon black matrix. The initial discharge capacity can reach up to1405.5mAh/g and a discharge capacity of596mAh/g can still be retained after100cycles at1C. Even at10C rate, a maximum discharge capacity of635mAh/g could be obtained for the composite, and the capacity retention is over60%after180cycles. The synergistic effect of the conductive carbon black and polyaniline can significantly improve the electrochemical performance of the composites electrode. Meanwhile, the electronic conductivity of the electrode system could be improved and the shuttle effect of the electrode actions suppressed, thus leading to excellent high rate charge/discharge performance, good cycle stability and relatively high utilization of sulfur.In order to further improve the sulfur loading in the composites, commercial conductive carbon black BP2000(CCB) was used as raw material and KOH as pore-expanding agent to prepare activated conductive carbon black(A-CCB) material with high specific surface area and large pore volume by activating at high temperature. Then, sulfur/activated conductive carbon black(S/A-CCB) composites with64wt%and73wt%sulfur, and sulfur/conductive carbon black(S/CCB) composite with65wt%sulfur were further prepared by two-step heat treatment, and the electrochemical performance for the composites were comparatively investigated. It’s demonstrated that the specific surface area and pore volume of commercial CCB were significantly improved after activation. The S/A-CCB composite with64wt%sulfur shows the optimum electrochemical performance.. The initial discharge capacity of S/A-CCB composite with64wt%sulfur is956.7mAh/g and can be maintained at531.9mAh/g after100cycles at the current density of160mA/g. The maximum discharge capacity of771.8mAh/g can still be obtained after several cycles’activation at the current density of800mA/g, indicating a satisfactory rate performance. However, the results show that the employment of CCB as conductive matrix or the increase of sulfur loading in S/A-CCB composite, under the same condition, would lead to a poorer electrochemical performance for sulfur electrode.Finally, considering that both sulfurized polyacrylonitrile and sulfur/carbon composites can be served as cathode materials for lithium-sulfur battery, to construct multi-composites, sulfur/carbon/polyacrylonitrile ternary mixtures were firstly obtained through dissolution-precipitation process by still using conductive carbon black BP2000as matrix. Then, the mixture were pre-calcinated and heat treated at different calcinating time to prepared three sulfur/carbon/sulfurized polyacrylonitrile composites(S/C/sPAN). The CVs and charge/discharge performance of the as-prapared composites were further characterized in1M LiPF6(PC+EC+DEC, v/v/v=1:4:5) carbonate-based electrolyte and1M LiTFSI+0.2M LiNO3(DOL+TEGDME, v/v=1:1)ether-based electrolyte, respectively. The results show that the three samples demonstrate different electrochemical response characterizations in the two electrolytes. By comparison, the S/C/sPAN-7composite with7h heat-treatment shows the optimum electrochemical performance in carbonate-based electrolyte. The initial discharge capacity of1103.4mAh/g can be obtained at the current density of40mA/g when calculated by the whole composite as active material, and the discharge capacity can be maintained at350mAh/g after40cycles. Moreover, the discharge capacity of243.5mAh/g can be maintained after200cycles when the current increased to100mA/g, and the capacity retention can reach up to77%from the second cycle. The composite can also show good cycle performance at300mA/g, together with ca.100%charge/discharge coulombic efficiency.In summary, in this paper, two novle sulfur/carbon/polymer multi-composites were designed and synthesized by employing commercial conductive carbon black as matrix. The relationship between the structures and electrochemical performance of the composites were then discussed. In addition, the activated conductive carbon material with high specific surface and pore volume was also prepared using conductive carbon black as raw material, and the sulfur-based composites with good electrochemical performance were obtained. Starting with stable and commercial carbon materials, we are confident that the work would provide forceful experimental and theoretical basis to design new sulfur-based materials and develop low cost and practical high specific energy lithium-sulfur battery in future.更多还原