高性能锂硫电池正极及隔膜修饰研究

发布时间：2018-10-18 07:12

【摘要】：锂硫电池体系因其高理论能量密度受到广泛关注,最有望成为下一代动力新能源体系,而我国硫矿资源丰富,分布广泛,也为锂硫电池商业化提供了物质条件。利用导电性较好的碳材料与硫复合,为硫提供导电网络骨架,解决硫及其还原产物L2S或L2S2导电性较差的问题,已经被广泛研究和应用。对于锂硫电池正极集流体而言,通常选用铝箔,但自然状态下易被氧化,铝箔表面生成4-5 nm厚度的Al2O3氧化层,该氧化层可以保护深层铝箔,但其导电性较差,影响正极电子的传输,并且高负载硫正极片还存在正极材料干裂脱粉问题。另外,锂硫电池充放电过程中产生高阶多硫化物,易溶于电解液,并且随电解液通过隔膜穿梭到负极,在负极金属锂表面沉积,造成活性物质硫的不可逆损失,从而降低电池容量,增大电池容量衰减率,缩短电池寿命。针对上面提到的问题,本文主要做了以下三部分工作:首先是利用化学气相沉积(CVD)技术合成的超长阵列碳管交织成三维(3D)集流体来代替铝箔集流体。将掺氮石墨烯与硫的复合材料与超长碳管砂芯抽滤成3D碳管骨架基底,制成graphene@S-CNT柔性正极。柔性正极可以弯曲,具有良好的机械性能,适用于一些柔性器件。但是柔性极片丰富的大孔结构不能限域多硫化物。本文采用简单的碾压过程改善正极机械性能、导电性、孔结构及孔分布来限域多硫化物,提高电池循环稳定性。但是柔性电极主要用于部分柔性器件,而且在扩大的电池(如软包电池)中,正负极片难连接极耳,所以在保持原有铝箔集流体基础上,在其表面增加石墨烯/碳纳米管杂化物修饰层,在铝箔表面形成3D导电网络,在铝箔与正极活性材料硫界面,提高界面导电性2.7倍。在面载硫量为1.1 mg cm-2,电流密度0.5 C(1 C=1675 mAh gs-1)条件下,首圈放电容量达到1113 mAh gs-1,硫利用率达到67%,极化现象明显减小,硫利用率提高,容量提高。并且修饰层能够减小表面张力,增加集流体与正极复合材料的粘结性,解决极片脱粉问题。除修饰集流体的方法外,对于与正极面直接相对的隔膜进行修饰也是抑制多硫化物穿梭效应的有效方法。隔膜修饰层通过物理和化学吸附溶解在电解液中的多硫化物,抑制多硫化物穿梭,促进多硫化物相转变,回收穿梭的多硫化物。对于纽扣式锂硫电池,以多孔石墨烯修饰的聚丙烯膜作为隔膜,在电极面载硫量1.8-2.0 mg cm-2、电流密度0.05 C条件下,硫的利用率达到86.5%,自放电后容量保持率90%,并且提高了倍率性能。此外,将多孔石墨烯修饰隔膜扩大用于软包电池(30×50 cm2),面载硫量7.8 mg cm-2,电流密度0.05 C,电池初始放电比容量达到1135 mA h gs-1。与聚丙烯膜相比,多孔石墨烯功能隔膜极大的提高了电池容量以及循环稳定性。
[Abstract]:Because of its high theoretical energy density, lithium-sulfur battery system is expected to become the next generation of new energy system. However, China is rich in sulfur resources and widely distributed, which provides material conditions for the commercialization of lithium-sulfur batteries. It has been widely studied and applied to solve the problem of poor conductivity of sulfur and its reducing product L _ 2S or L2S2 by using carbon materials with good conductivity and sulfur composite to provide conductive network skeleton for sulfur. For lithium-sulfur batteries, aluminum foil is usually used as cathode collector, but it is easy to be oxidized in natural state. The surface of aluminum foil forms a Al2O3 oxide layer of 4-5 nm thickness, which can protect deep layer aluminum foil, but its electrical conductivity is poor. The electron transport of positive electrode is affected, and there is the problem of dry cracking and demineralization of positive electrode material in high load sulfur positive plate. In addition, high order polysulfide is produced during charging and discharging of lithium-sulfur batteries, which is easily dissolved in electrolyte, and the electrolyte shuttles through the diaphragm to the negative electrode, and deposits on the cathode metal lithium surface, resulting in the irreversible loss of sulfur, the active substance. Thus, the battery capacity is reduced, the battery capacity attenuation rate is increased, and the battery life is shortened. In order to solve the problems mentioned above, the following three parts have been done in this paper: firstly, the ultra-long array carbon tubes synthesized by chemical vapor deposition (CVD) technology are interwoven into three-dimensional (3D) collecting fluid instead of aluminum foil collecting fluid. The graphene@S-CNT flexible positive electrode was prepared by filtering the nitrogen-graphene / sulfur composite material and the ultra-long carbon tube sand core into a 3D carbon tube skeleton substrate. Flexible positive electrode can be bent, with good mechanical properties, suitable for some flexible devices. However, the flexible pole-rich macroporous structure can not limit the region of polysulfide. In this paper, a simple rolling process is used to improve the positive pole mechanical properties, conductivity, pore structure and pore distribution to limit the range of polysulfides and improve the cycle stability of the battery. But flexible electrodes are mainly used in some flexible devices, and in expanded batteries (such as soft-clad batteries), positive and negative electrodes are difficult to connect to the polar ears, so on the basis of retaining the original aluminum foil to collect fluid, Graphene / carbon nanotube (CNT) hybrid modification layer was added on the surface, and 3D conductive network was formed on the surface of aluminum foil. The conductivity of the interface was improved by 2.7 times at the interface between aluminum foil and active cathode material sulfur. At 1. 1 mg cm-2, current density of 0. 5 C (1 C ~ (1) C), the discharge capacity of the first loop reaches 1113 mAh gs-1, sulfur utilization ratio, the polarization phenomenon is obviously reduced, the sulfur utilization efficiency is increased, and the capacity is increased. The modified layer can reduce the surface tension, increase the adhesion between the collector and the positive composite material, and solve the problem of electrode demineralization. In addition to the method of modifying the collector the modification of the diaphragm directly opposite to the positive pole is also an effective method to suppress the shuttle effect of polysulfide. The membrane modified layer absorbs the polysulfide dissolved in electrolyte by physical and chemical adsorption, which inhibits the polysulfide shuttle, promotes the polysulfide phase transition, and recovers the shuttle polysulfide. For the button type lithium-sulfur battery, the porous graphene modified polypropylene membrane was used as the diaphragm. At the current density of 1.8-2.0 mg cm-2, on the electrode surface, the sulfur utilization rate reached 86.5%, the capacity retention rate after self-discharge was 90%, and the performance of the ratio was improved. In addition, the porous graphene modified separator was expanded to be used in a soft-coated battery (30 脳 50 cm2), the surface sulfur content was 7.8 mg cm-2, current density 0.05C, and the initial discharge specific capacity of the cell reached 1135 mA h gs-1.. Compared with polypropylene membrane, the porous graphene functional membrane greatly improved the battery capacity and cycle stability.
【学位授予单位】：曲阜师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM912;O646.5

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