【作者】 王超；

【导师】 黄云辉；

【作者基本信息】 华中科技大学 ， 材料学， 2015， 博士

【摘要】 锂硫电池是最有前途的下一代电化学储能器件。硫的理论容量高达1675mAh g-1,能量密度远超现有的锂离子电池正极材料。但是锂硫电池的循环性能差,电化学循环过程中的效率较低。本论文中,作者将石墨烯材料引入到锂硫电池中,合成硫-石墨烯复合正极材料,希望通过石墨烯的特性来改善锂硫电池多硫化物的穿梭现象,增加导电性,进而提高锂硫电池的电化学性能。本论文的工作主要可以分为下面三点：(1)作者用三维结构的多孔氮掺杂石墨烯作为基底材料,通过溶液法合成了石墨烯和硫的复合材料(3D-NGS)。这种材料具有很高的硫含量,为87.6wt.%,并且硫颗粒被石墨烯包裹的很好。电化学测试显3D-NGS复合材料具有良好的循环和倍率性能：在600mAg-1的电流密度下循环145次后,放电比容量可以维持在792mAhg-1,平均每次循环的容量衰减率为0.05%；在1500mA g-1的高电流密度下也表现出了很好的循环性能,循环200次后放电比容量保持在671mAh g-1。作者认为3D-NGS具有比较好的电化学性能和石墨烯的结构密不可分：第一,石墨烯在氮掺杂后有比较好的导电性,并且自组装形成多孔的三维结构,同时均匀的包覆硫颗粒,使得复合材料具有良好的电子和锂离子传输能力；第二多孔的结构可以缓解充放电过程中活性物质的体积变化,保持电极片的稳定性；第三,氮掺杂后硫和石墨烯的相互作用变强了,因此能够更好地抑制多硫化物的迁移,从而提高复合材料的电化学性能。(2)作者制备了一种具有大孔结构的自支撑纳米硫-石墨烯纸复合材料。该复合材料可以直接用来作为锂硫电池的电极片。硫-石墨烯是通过冻干和低温热处理的方法得到的。在硫-石墨烯材料中,硫全部铺展在石墨烯的表面,混合均匀。柔性的硫-石墨烯在电化学反应的过程中不仅起到了导电网络的作用,其大孔结构还可以缓解体积效应。硫-石墨烯复合材料展现了优异的倍率和循环性能：在300mAg-1的电流密度下循环200次后放电比容量还保持在800mAh g-1,平均每次循环容量衰减率为0.035%；在高电流密度1500mA g-1下循环也表现出了良好的电化学性能。其优异的电化学性能可以归功于硫-石墨烯具有稳定的多孔结构以及石墨烯与硫的强相互作用。(3)作者研究了硫化锂正极材料,硫化锂是一种很有前景的锂硫电池正极材料,其理论比容量为1166mAh g-1,它可以和非锂金属负极相匹配,从而避免了金属锂负极的安全性问题。研究表明,大颗粒硫化锂的电化学性能很差,因此它的导电性很差并且存在多硫化物的穿梭问题,另外,多硫化锂在空气中也不稳定。为此作者研究制备了一种柔性的不需要调浆电极材料,纳米硫化锂-还原性石墨烯片。其制备方法很简单,可通过自然滴制法得到。硫化锂-还原性石墨烯片具有自支撑结构,不需要粘合剂和金属基底,可以直接作为电极片使用,显著的降低了电极片的质量。这种硫化锂-还原性石墨烯片复合材料展现出很好的循环和倍率性能,在7C的倍率下还能保持597mAh g-1的放电比容量。它优异的电化学性能主要源于硫化锂-还原性石墨烯片具有不错的电导率,柔性电极的优点以及对电解液的吸附能力。在硫化锂-还原性石墨烯片中硫化锂的颗粒尺寸为25-50nm,若不用石墨烯,同样方法得到的硫化锂颗粒为3-5μm,石墨烯的存在显著降低了硫化锂颗粒的大小。更多还原

【Abstract】 Lithium-sulfur battery is a promising electrochemical device for future energy conversion and storage. Its theoretical capacity is1675mAh g-1,much higher than that of conventional lithium-ion battery. However, it suffers from rapid capacity decay and low energy efficiency. In this thesis, we are mainly use graphene into the cathode materials to inhibit the polysulfides shuttle and increase the conductivity, hence improve the electrochemical preformance of lithium sulfur battery. Our work in this thesis can be divided into three parts:(1) A porous three-dimensional nitrogen-doped graphene (3D-NG) was introduced as an interconnected framework for sulfur in lithium-sulfur battery using a solution react method. The3D-NG-sulfur composite (3D-NGS) has a high sulfur content of87.6wt.%and sulfur was wrapped very well by graphene. The as-designed3D-NGS composite exhibits excellent rate capability and cyclability. The discharge specific capacity is792mAh g-1after145cycles at a current density of600mA g-1and the capacity fading rate is0.05%per cycle. Even at a high rate of1500mA g-1, the composite still shows a good cycle performance with a capacity of671mAh g-1after200cycles. The outstanding electrochemical performance can be attributed to the flexible porous3D structure and N-doping in graphene. The flexible3D-NG can provide a conductive framework for electron transport and alleviate the volume effect during cycling; N-doping can facilitate the penetration of Li ions across the graphene and restrain sulfur due to the strong chemical bonding between S and nearby N atoms.(2) A macroporous free-standing nano-sulfur/graphene (S-rGO) paper is introduced directly as an electrode for lithium-sulfur battery. The S-rGO paper is synthesized via a facile freeze drying route followed by low-temperature heat treatment. The flexible S-rGO paper not only provides a conductive framework for electron transport but also alleviates volume effect during cycling. The as-designed S-rGO paper exhibits excellent rate capability and cyclability. The specific discharge capacity is800mAh g-1after200cycles at a current density of300mA g-1and the capacity fading rate is only0.035%per cycle. Even at a high current density of1500mA g-1, it still shows a good performance. We ascribe the high performance of the S-rGO paper to stable macroporous structure and strong interaction between sulfur nanoparticles and graphene.(3)In order to forbid the use of lithium metal as the anode in lithium sulfur battery, we developed lithium sulfide as a cathode. Lithium sulfide is a promising cathode material for Li-S batteries with high capacity (theoretically1166mAh g-1), and can be paired with non-lithium-metal anodes to avoid potential safety issues. However, the cycle life of coarse Li2S particles suffers from poor electronic conductivity and polysulfide shuttling. Li2S is not stable in air. Here we develop a flexible slurryless Li2S/reduced graphene oxide cathode paper (Li2S/rGO paper) by simple drop-coating. The Li2S/rGO paper can be directly used as a free-standing and binder-free cathode without metal substrate, which leads to significant weight savings. It shows excellent rate capability (up to7C with discharge capacity of597mAh g"1) and cycle life in coin cell tests, due to the high electron conductivity, flexibility and strong solvent absorbency of rGO paper. The Li2S particles that precipitate out of the solvent on rGO have diameters25-50nm, in contrast to the3-5μm coarse Li2S particles without rGO.更多还原