【作者】 张坤；

【导师】 魏秉庆；

【作者基本信息】 西北工业大学 ， 材料学， 2018， 硕士

【摘要】 锂硫电池因硫正极高的理论比容量和高能量密度,有望突破目前电动汽车中动力电池能量密度瓶颈。然而,由于硫的绝缘性、电极体积膨胀、多硫化物在电解液中造成的穿梭效应、锂枝晶生长和有效正极硫负载量不足等问题,导致锂硫电池电化学性能较差,致使应用受限。提高锂硫电池的活性物质负载量进而全面提高电池的已经成为研究者关注的重点问题之一。本文聚焦于锂硫电池面负载量和能量密度的提升,对锂硫电池的正极、负极和制备工艺分别提出了改进方法。首先制备了质轻、载硫能力强的吡咯调控的三维石墨烯气凝胶;其次,通过在负极引入石墨烯纸集流体,制备了GP@Li/Py-NGF@S的锂硫全电池;最后,通过喷涂的方法在不明显增加极片质量的情况下,包覆氮化碳保护层,并研究了自放电性能,主要研究结果如下:(1)通过在石墨烯水热还原过程中添加吡咯,制备出轻质、载硫能力强的三维石墨烯气凝胶,从正极角度提高器件的能量密度。通过三维石墨烯提供的物理限制作用和吡咯小分子对多硫化物原位化学吸附作用,提升锂硫电池的电化学性能,在面负载量高达6.2mg/cm~2的情况下,首次放电比容量为985.8mA h/g,在100次循环后,容量保持率为81%。(2)在第一个实验的基础上,在正极中加入抗坏血酸进一步提高石墨烯还原性,以提高比表面积和孔隙度进而改善正极的含硫量;在负极采用石墨烯自组装技术制备的石墨烯纸集流体,达到抑制锂枝晶的效果。二者配合组装成GP@Li/Py-NGF@S的锂硫全电池。在单位面积硫负载量达到5.4 mg/cm~2的情况下,初始放电比容量为990mA h/g,考虑到正负极和电解液的质量,全电池能量密度为540W h/kg,400次循环保持率为87.6%。(3)制备了石墨相氮化碳纳米片,并利用了喷涂技术在正极极片表面制备了氮化碳薄膜。通过可视化实验和电化学性能测试,证明氮化碳薄膜可以有效抑制锂硫电池充放电过程中的穿梭效应和静置过程中存在的自放电现象。这主要是由于氮化碳对多硫化物有强吸附作用。相对于传统的刮涂等技术,喷涂技术有助于简易大规模进行隔膜、极片等的改性处理,成本低廉,有广泛的发展前景。更多还原

【Abstract】 Traditional Li-ion batteries are facing problems due to the intrinsic limitation of their low energy density.As one type of promising Li battery,lithium–sulfur(Li–S)batteries have an advantage of high theoretical energy density(2600 W h/kg).Nevertheless,the highly insulating nature of elemental sulfur,the huge volume expansion of sulfur upon lithiation/delithiation,the notorious polysulfide shuttle and low sulfur loading are the main obstacles to widespread practical utilization of Li–S batteries.Thus,improving the sulfur loading of the cathode in Li-S batteries to enhance the energy density of the whole devices has been drawing researchers’sight during recent years.In this paper,we focus on surfur loading and energy density enhancement of the lithium-sulfur batteries through the improvement of both positive and negative electrodes of lithium-sulfur batteries and the preparation process.Firstly,we design and construct a pyrrole modified graphene aerogel foam(Py-GF)by a simple hydrothermal and freeze drying method as the sulfur host;secondly,we replaced Li metal anode with GP@Li and fabricated GP@Li/Py-NGF@S full cell;thirdly,we warp sulfur cathode with g-C3N4 film through spray method.Compared with traditional process,this method is highly scalable and flexible.The main contents and research results are as follows:(1)We design and construct a pyrrole modified graphene aerogel foam(Py-GF)by a simple hydrothermal and freeze drying method as the sulfur host,where pyrrole provides strong chemical bonding for polysulfide anchoring and graphene aerogel foam serves as a matrix to enhance the conductivity as well as increase the sulfur loading of the cathode simultaneously.The Py-GF@S cathode,with a high sulfur loading of about6.2 mg/cm~2,displays an improved initial specific capacity(1220 mA h/g at 0.2C and985.8 mA h/g at 0.5C)and cycle stability(capacity retention of 81%after 100 cycles at0.5C).We anticipate that the work described here will be helpful to develop Li–S batteries that meet the requirements of practical applications.(2)On the basis of the first experiment,ascorbic acid was added to the positive electrode to further increase the reducibility of graphene to improve the specific surface area and porosity and to improve the sulfur content of the positive electrode.In the negative electrode,we use graphene self-assembly technology to prepare graphene paper.We assemble Li-S full cell based on cathode and anode.The full cell,with a high sulfur loading of about 5.4 mg/cm~2,displays an improved initial specific capacity(990mA h/g at 0.5C),high energy density(540W h/kg based on the mass of full cell)and cycle stability(capacity retention of 87.6%after 400 cycles at 0.5C).(3)We prepared graphite carbon nitride sheet,and the use of spray technology in the positive pole piece surface prepared carbon nitride film.Through the visualization experiment and the electrochemical performance test,it is proved that the carbon nitride thin film can effectively inhibit the shuttle effect during the charge-discharge process and the self-discharge phenomenon during standing still.This is mainly due to the strong adsorption of polysulfides by carbon nitride.Compared with the traditional scraping and other technologies,spray technology can facilitate the simple and large-scale membrane modification,low cost and broad prospects for development.更多还原