【作者】 吴宪；

【导师】 张乃庆；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2020， 博士

【摘要】 锂硫电池正极理论质量比容量高达1675 mAh g^-1,具有很好的开发和应用前景,得到了世界范围内的广泛关注。然而,锂硫电池的发展仍然面临着诸多问题和挑战,其中放电中间产物多硫化锂的溶解引起的穿梭效应和充放电过程中反应动力学迟缓的问题尤为严重。针对以上问题,本论文通过对碳材料的掺杂以及复合改性,设计四种高性能硫正极载体材料。以低成本的海苔为生物质原料制备氮、氧共掺杂的生物质碳材料（ANOC）。氮、氧掺杂可以在碳材料中提供极性吸附位点,抑制多硫化锂的溶解。在此基础上,选用KOH活化的方式在碳材料表面引入微孔结构,提升比表面积和孔体积,有利于硫的均匀分布,提高活性物质的利用率和倍率性能。制备的电池在硫含量为81.2%时,在0.2 C的电流密度下经过100次循环后放电比容量可以保持为657mAh g^-1。杂原子掺杂的方式引入的吸附位点有限,对穿梭效应的抑制效果有待进一步提升。金属氧化物具有更多的极性位点,可以更加有效地抑制多硫化锂的扩散。通过水热法制备二氧化钼/氮掺杂石墨烯（MoO₂/G）复合材料。极性的MoO₂有效吸附多硫化锂,抑制其溶解;氮掺杂的石墨烯可以提供导电网络并保证活性物质的均匀分布。MoO₂与石墨烯之间存在C-O-Mo化学键,可以在材料间构建电子转移通道,提升电化学反应动力学。得益于MoO₂/G结构的特点,制备的电池在1 C的电流密度下经过500次循环后容量保留为82%。尽管MoO₂可以抑制多硫化锂的溶解,但是锂硫电池氧化还原反应困难的问题依旧存在。采用一步水解法合成BiOCl和石墨烯的复合材料（BiOCl/G）。BiOCl表面暴露大量的金属Bi原子,可以与多硫化锂之间形成Bi-S键,BiOCl层间的Cl离子可以降低表面金属的电子云密度,增强对多硫化锂的吸附能力。并且BiOCl可以加快锂硫电池充放电过程中的氧化还原反应速率,降低电化学极化并提升循环稳定性。得益于BiOCl的吸附和催化能力,用其作为正极载体材料在0.1C的电流密度下首次放电比容量高达1414 mAh g^-1,并且在2 C时经过2000次循环,平均每次循环的容量衰减率仅为0.007%。多硫化锂的氧化还原反应能垒高,导致锂硫电池反应动力学迟缓。采用反溶剂法制备3,4,9,10-苝四甲酰二亚胺/石墨烯（PTCDI/G）复合材料。PTCDI具有2对可逆的氧化还原反应过程,可以作为氧化还原媒介体动态地促进多硫化锂的反应,调节锂硫电池反应动力学。PTCDI中的酰胺基团可以与多硫化锂之间形成Li-N键,抑制多硫化锂的溶解,有效地提升电池的循环稳定性和倍率性能。制备的电池在0.05 C时放电比容量高达1496 mAh g^-1,并且在5 C的电流密度下经过1500次循环,平均每次循环的容量衰减率仅为0.009%。更多还原

【Abstract】 Sulfur cathodes combined with lithium anodes could provide a capacity of 1675mAh g^-1,which affords times higher energy density than the commercial lithium ion batteries.Nevertheless,their attainability for further application of Li-S batteries is seriously questioned,especially for the difficulty of sluggish kinetics and shuttle effects.Based on the issues mentioned above,this thesis designs four sulfur host materials with chemical adsorption capacity to lithium polysulfides through doping and compositing with polar materials.The design intentions focus on understanding and improving the factors that affect the reaction kinetics of lithium sulfur batteries,and put forward new ways to improve the reaction kinetics,providing new design concept for the preparation of lithium sulfur batteries.A nori-derived carbon（ANOC）via carbonization and subsequent activation is prepared as sulfur host.The obtained ANOC has abundant nitrogen and oxygen,which provide multiple polar sites to anchor polysulfides.Furthermore,the 3D layered hierarchical structure of the ANOC provides an electron transfer network to perform a high discharge capacity even at a high discharge rate.The result demonstrates that even with a high sulfur content of 81.2 wt%,the cathode still reveals a capacity of 657 mAh g^-1 after 100 cycles at 0.2 C.A hollow MoO₂ sphere anchored on nitrogen-doped graphene is synthesized as sulfur host.The nitrogen-doped graphene could provide a conductive network and ensure a high sulfur loading.Meanwhile,the C-O-Mo bond between MoO₂ and N-doped graphene explores a pathway for electron transfer.The aforesaid synergistic effect would enhance the kinetics of lithium sulfur batteries profitably.The discharge capacity could reach 615 mAh g^-1 at the current of 2 C,and retain 82%after 500 cycles at 1 C.BiOCl is employed as sulfur host material in Li-S batteries,which could integrate suppression of shuttle effect and promotion kinetics redox reactions of lithium polysulfides.The polar BiOCl performs robust chemical adsorption ability to polysulfides,and the electrocatalytic activity of BiOCl would accelerate the fragmentation of polysulfides into shorter chains.The battery with BiOCl/G-S cathode reveals a high capacity of 1414 mAh g^-1 at the current density of 0.1 C and the low capacity decay rate of 0.007%during 2000 cycles at 2 C.PTCDI/G is employed as redox mediator to accelerate the conversion process between polysulfides and sulfur.PTCDI has redox reaction ability,and the product serves as reducing and oxidizing agent in facilitating the process of both charging and discharging.Moreover,PTCDI has strong adsorption ability with polysulfides to restrain shuttle effect,resulting in promotional kinetics and cycle stability.Therefore,a high initial capacity of 1496 mAh g^-1 at 0.05 C and slight capacity decay of 0.009%per cycle at 5 C over 1500 cycles is achieved with PTCDI/G host.更多还原