【作者】 张文华；

【导师】 艾新平；

【作者基本信息】 武汉大学 ， 物理化学， 2012， 博士

【摘要】 探索和发展具有高能量密度、低成本和环境友好的二次电池新体系是便携式电子产品、电动汽车、储能电站等应用领域的迫切需求。锂硫二次电池具有高达2600Wh/kg的理论比能量,而且单质硫具有资源丰富、环境友好等特点,极具应用开发前景,是当前化学电源领域的研究热点。然而,单质硫是电子和离子绝缘体,且硫电极的放电中间产物易在电解液中溶解流失,导致硫电极的电化学活性低、循环稳定性差,严重制约了锂硫二次电池的开发进程。本论文以发展高循环稳定性的硫／碳复合正极材料为目的,在多孔碳基体的制备及结构优化方面开展了系统的研究。主要研究内容和结果如下：1.分别采用化学活化法和模板法制备了三种不同结构的介孔碳,包括以石墨化中间相碳微球(MCMB)和未石墨化中间相碳微球(CMS)为原料的化学活化介孔碳AMCMB和ACMS,以及以蔗糖为碳源、SBA-15为模板制备的规则孔道结构介孔碳CMK-3,并分别以它们为导电载体制备了硫/碳复合材料。考察了介孔碳/硫复合材料在有机醚类电解液中的电化学性能,探讨了碳基体孔结构对单质硫电化学行为的影响。研究结果表明,三种介孔碳基体均可以有效抑制多硫化锂中间产物的溶解流失,从而使复合硫电极在循环过程中表现出高达99%的充放电库伦效率。其中,有序介孔碳CMK-3因具有孔容高、孔径分布窄、孔道结构长等孔结构特征,以之为基体的复合硫电极不仅硫的负载量高,而且比容量大、循环性能好,表明具有半开放体相孔道结构的介孔碳,在抑制多硫化物的溶解流失方面具有结构上的优势。硫含量为70%的CMK-3/S复合材料,首周放电比容量为1395mAhg-1；循环50周以后,仍保持有848mAhg-1的比容量。2.为了探究抑制多硫化锂中间产物溶解流失的更有效方法,提出了以具有丰富体相孔结构的微球碳为基体分散负载硫,同时配合使用对多硫化物难溶或不溶的电解液体系,发展高循环稳定性硫电极体系的新思路。为了证实这一设想,实验选择蔗糖为碳源,分别采用水热法和加热回流法制备出多种微孔碳球(MCS),并通过气相转移法制备了硫/微孔碳复合材料(S/MCS).通过比较不同微孔碳球与硫组成的硫碳复合材料在可抑制多硫化锂溶解的有机碳酸酯电解液体系中的电化学性能,考察了碳球粒径、导电添加剂对硫电极反应活性和循环稳定性的影响。研究结果表明,以气相沉积碳纤维(VGCF)为导电添加剂、0.15M低浓度蔗糖溶液为反应液,通过加热回流方法制备的MCS-VGCF复合碳微球,因具有小的碳球粒径和良好的三维导电网络结构,与硫复合形成的硫/碳复合材料S/MCS-VGCF在1M LiPF6/PC-EC-DEC电解液中,不仅具有较高的比容量、良好的循环稳定性,而且在充放电过程中,表现出100%的超高库伦效率,说明因多硫化物中间产物溶解而引发的“穿梭效应”得到了完全抑制。其初始可循环容量为1200mAhg-1；100周后,比容量仍保持在720mAh g-1以上,展示出良好的应用前景。3.通过比较S/MCS-VGCF复合材料在有机醚类电解液和有机碳酸酯电解液中的充放电行为,探讨了微球碳/硫复合材料在有机碳酸酯电解液中的反应机理。研究结果表明,采用具有锂离子传导性的微孔碳材料为基体分散负载单质硫,同时使用对多硫化物中间产物不溶或难溶的有机碳酸酯电解液体系,可以实现活性硫在硫/碳固-固界面上的直接原位还原与氧化,而不需要电解液相的直接参与,为发展高循环稳定性硫电极提供了新的思路和技术途径。4.为探讨聚合物表面修饰对硫/碳复合材料电化学性能的影响,分别选用高比表面的BP2000和高导电性的KS6石墨作为碳基体,制备了硫碳复合材料；并通过进一步的聚丙烯腈(PAN)包覆,制备出两类碳-硫-聚丙烯腈三元复合材料,考察了复合硫电极在碳酸酯电解液中的电化学性能。结果表明,PAN的表面修饰有效改善了硫/碳复合材料的电性能。PAN含量为40%的PAN-BP2000/S复合材料,初始可循环容量为568mAh g-1,循环100周后,可逆容量仍保持为557mAhg-1；单质硫、PAN.KS6的质量比为4：5：1的KS6-PAN/S复合材料,初始可逆容量为1162mAh g-1,循环50周后,容量仍保持在1000mAh g-1以上更多还原

【Abstract】 The ever-increasing demand for electric energy storage, ranging from portable electronics to electric vehicles and to renewable power stations, stimulates the development of improved rechargeable lithium batteries with substantially enhanced energy density and greatly reduced cost. In this technology development, lithium-sulfur batteries (Li-S) are revisited as a promising candidate for large scale electric storage, because of its high theoretical energy density of2600Wh Kg-1and particularly the natural abundance and low toxicity of sulfur. Despite a great progress has been achieved, commercial development of Li-S batteries is still hindered by the insufficient cyclability and low utilization of the electrode-active materials, which are given rise by the insulating nature of sulfur and the dissolution loss of the intermediates generated during discharge of sulfur. This PhD. work focuses on developing S/C composite cathode material with high capacity and good cycling performance. The main results are summarized as follows:1. Three kinds of mesoporous carbons, AMCMB, ACMS and CMK-3. were prepared and then used as conductive substrate for encapsulating sulfur to form S/C composite. The AMCMB and ACMS mesoporous carbon substrates were prepared by chemically activating graphitized mesophase carbon microbeads (MCMB) and non-graphitized mesophase carbon microbeads (CMS) using KOH as activation reagent, respectively. Highly ordered CMK-3mesoporous carbon was synthesized by a nanocasting method using silaceous SBA-15as hard template and sucrose as carbon source. The electrochemical performance of the S/C composites using these mesoporous carbon substrates were characterized by gavanostatically cycling them in organic ether-based electrolyte. The results showed that all three S/C composite electrods exhibit a superhigh coulombic efficiency of～99%, indicating that the mesoporous carbons as prepared in this work could effectively suppress the dissolution loss of the polysulfide intermediates. Benefiting from the highly-ordered and half-opened channel structure of CMK-3, the CMK-3/S compostie not only posseses high sulfur loading content, but also exhibits high capacity and good cycling performance. The initial discharge capacity of CMK-3/S composite with70wt.%sulfur content is1395mAh g-1. After50cycles, the composite still keeps a capacity of～848mAh g-1, corresponding to a capacity retention of61%.2. An alternative way to suppress the dissolution loss of the ploysulfide is proposed by dispersing or encapsulating sulfur into the microporous cabon spheres, which has rich inner micropores and small particle size, and simultaneously using the electrolytes with only poorer polysulfide solubility. To demonstrate the feasibility of this idea, several kinds of microporous carbon microspheres(MCS) were synthesized by hydrothermal method and heating reflux method with sucrose as carbon source, respectively, and then the S/MCS composites were prepared by vaporizing sulfur into the micropores of carbon spheres at elevated temperature. The effects of carbon sphere size and conductive additive on the electrochemical behaviors of the S/MCS composites were investigated by gavanostatic charge and discharge test. The experiment results showed that, in the carbonate electrolyte of1M LiPF6/PC-EC-DEC, the S/C composite obtained from a conductive matrix of nanofiber-wired carbon spheres, exhibits not only stable cycling performance with a reversible capacity of720mAh g-1after100cycles, but also superior high coulombic efficiency of～100%upon extended cycling (except the first three cycles), showing a good application prospect in Li-S batteries.3. The structural and electrochemical analysis indicates that the improved electrochemical behaviors of the S/MCS composite arise from a new reaction mechanism, in which Li+ions and electrons transport through the carbon matrix into the interior of the cathode and then react with the embedded sulfur in the S/C solid-solid interfaces, avoiding the dissolution of the intermediates into the bulk electrolyte. This working mechanism suggested a new idea for developing high perfrmance sulfur electrode in the future.4. To investigate the influence of conductive polymer coating on the electrochemical behaviors of S/C composite, we chose a high specific area carbon BP2000and a high electrical conductive carbon KS6as conductive supports to prerpare two kinds of PAN-coated S/C composites by ball milling the mixtures of conductive carbon with sulfur, and then spraying a suspension of S/C composite and PAN/DMF solution into a water bath, and their electrochemical performances were also evaluated by galvanostatic discharge—charge cycling in carbonate-based eletrolyte. The results showed that the PAN-BP2000/S composite can deliver a reversible capacity of568mAh g-1at the first cycle. After100cycles, the capacity still remains at557mAh g-1. The initial reversible capacity of the PAN-KS6/S composite with a mass ratio of5:1:4is1162mAh g-1.After50cycles, the reversible capacity still keeps at1000mAh g-1, indicating a highly stable cycling performance.更多还原