【作者】 张扬；

【导师】 左伟伟； 朱美芳；

【作者基本信息】 东华大学 ， 材料学， 2022， 博士

【摘要】 近年来,可穿戴电子器件和人工智能设备市场迎来快速发展,市场对配套的柔性储能设备的需求逐年增长。目前主流的储能器件锂离子电池（LIBs）由于具备高能量密度、低自放电等优点,已经在多个领域内获得了大量的关注与广泛的应用,因此,为了满足柔性市场对储能器件巨大的使用需求,亟需开发纤维状的电极用于柔性LIBs的组装。常规方法来讲,电极通过将活性材料、粘结剂和导电剂涂覆在金属薄膜上制备得到,由于电极本身刚性较大、弯折后难以恢复以及形状不匹配等问题,导致其难以直接作为柔性电极应用于纤维状电池。石墨烯纤维是由石墨烯片层通过层层堆叠的方式组装而成的新型碳基材料,一方面,其继承了石墨烯单元优异的性能,以湿法纺丝、干法纺丝、干喷湿纺、限域水热等加工方法为基础,结合多种后处理方式,能够实现纳米尺度的部分优异性能遗传到宏观纤维上,因此纤维在力学、电学和电化学等方面均有优异的表现;另一方面,石墨烯片层尺度及片层之间结构可调,微观孔结构能够精确控制,通过纺丝原液杂化以及纤维表面修饰等手段能够实现功能粒子的可控负载,因此,为纤维的柔性化以及功能化带来了可能性,有望在下一代纤维状储能器件领域发挥巨大的作用。对于多样化的应用场景,储能器件对容量的衡量指标往往会发生变化,不同于常规电池注重于质量比容量和体积比容量,由于纤维特殊的一维线状结构,线容量在应用过程中成为了更为重要的应用指标,更高的线容量能够在轻量化和微型化两方面改善纤维状电池的性能。但是,石墨烯纤维直接作为纤维状电池仍旧存在以下问题:（1）相较于金属氧化物、硅单质等活性材料,石墨烯的理论比容量较低,本身储锂能力有限;（2）引入的高容量功能组分在循环时往往出现巨大的体积膨胀的现象,导致电池表现出较差的电化学稳定性;（3）由于纤维制备过程中片层之间存在强大的相互作用,导致纤维内部片层排列过于紧密,大大降低了电解液在纤维内部的扩散速率。因此,如何通过纤维结构的可控设计以及多组分间的杂化来实现纤维状电极线容量的有效提升成为了目前面临的主要问题。针对以上问题,本论文采用湿法纺丝的方式连续化制备多孔石墨烯杂化纤维,系统研究了石墨烯杂化纤维的物理特性及电化学性能,并探讨了其在高线容量纤维状锂离子电池（F-LIBs）中应用的可行性。具体研究内容如下:（1）碳纳米管杂化石墨烯纤维的构筑及其用于F-LIBs的研究。以氧化石墨烯（GO）和碳纳米管（CNT）为原料,通过超声的方式将两者均匀分散于去离子水中,利用GO和CNT之间强烈的π-π相互作用,获得不同CNT含量的纺丝液前驱体。以GO-CNT的水溶液为纺丝液,高分子量壳聚糖水溶液为凝固浴,通过湿法纺丝的方式连续化制备GO-CNT杂化纤维,进一步通过惰性气体高温热还原获得具有微观导电网络结构的r GO-CNT纤维。分析杂化纤维的物理性能发现,由于湿法纺丝过程中喷丝孔处强烈的应力剪切作用,纯r GO纤维和r GO-CNT杂化纤维内部石墨烯片层都表现出明显的取向结构,但由于CNT的存在,r GO-CNT杂化纤维内部片层间排列由紧密变得更为疏松,层间距明显增大,形成了贯穿的多孔网络结构,该结果同样在比表面积上得到体现,由于堆叠程度的降低,r GO-CNT杂化纤维相较于r GO纤维比表面积从127.5 m² g^-1提升至204.5 m² g^-1。最终以r GO-CNT杂化纤维为负极材料组装成纤维状LIBs,并对其电化学性能进行表征,探究了不同CNT含量下柔性电极的容量,在CNT含量为5%,电流密度0.05 A g^-1时,质量容量为522 m Ah g^-1(0.287 m Ah m^-1)。另外,500 m A g^-1电流密度下循环600圈,容量依旧保持374 m Ah g^-1(0.206 m Ah m^-1)。为了进一步解释一维材料在石墨烯纤维制备过程中杂化作用的原理,纺制石墨烯/银纳米线（r GO-Ag）纤维作为对照,揭示了一维碳基材料在提高石墨烯基纤维线容量方面的巨大作用。（2）氧化锡原位沉积石墨烯杂化纤维的构筑及其用于F-LIBs的研究。通过一维CNT碳材料作为石墨烯纤维的杂化材料来抑制石墨烯片层的堆叠,暴露更多的活性位点,可显著提升石墨烯纤维的线容量。为了进一步提升纤维的理论容量,通过添加高质量容量的活性材料（如金属氧化物）则可进一步提升纤维的线容量。将GO均匀分散于去离子水中作为纺丝液,以壳聚糖水溶液作为凝固浴连续纺制GO纤维并将其浸泡于Sn Cl₂的盐酸/氨水溶液中进行原位还原沉积,通过后续的洗涤、干燥和热处理,获得层层堆叠的r GO-Sn O₂杂化纤维,对纤维的物理性能进行表征,氨水添加量提升,石墨烯杂化纤维片层间距、Sn O₂含量逐渐提高,而比表面积和纤维强度则逐渐降低,这是由于氨水的加入提高了还原溶液的p H,使得GO纤维在溶胀过程中内部片层间静电相斥作用力增强,造成相邻片层拥有更大的间距。对r GO-Sn O₂纤维的电化学性能进行测试,在半电池测试中,随着氨水含量的增加,纤维状电极的容量先增大后减小,循环稳定性则逐渐降低,当氨水添加量为0.6 m L时,r GO-Sn O₂杂化纤维具有最为优异的循环和倍率性能,0.1 A g^-1电流密度下,线容量达到1.35 m Ah m^-1,循环100次后线容量保持1.16 m Ah m^-1,电流密度增至5 A g^-1时,其线容量达到0.312 m Ah m^-1,表明活性材料氧化物的添加将有效提高纤维状电极的线容量。以氨水添加量为0.6 m L时制备的r GO-Sn O₂杂化纤维作为负极材料,Li Fe PO₄/碳纤维作为正极材料,探究该杂化纤维在全电池中应用的可行性,在50 m A g^-1的电流密度下循环50次,其线容量为0.51 m Ah m^-1,且该电池在各种弯折条件下仍旧保持良好的电流输出。本章发展了活性材料金属氧化物在石墨烯片层之间精准负载的方法,揭示了石墨烯与功能组分杂化的原理,制备了具有高线容量且循环稳定的石墨烯/金属氧化物杂化纤维。（3）多孔石墨烯-碳纳米管-氧化锡纤维的构筑及其用于F-LIBs和钠离子电池（F-SIBs）的研究。Sn O₂作为活性材料将有效提升纯石墨烯纤维电极的线容量,为了实现更高的线容量,需进一步提高石墨烯纤维的线密度,即增大纤维直径,但随着纤维状电极越来越粗,电解液在纤维内部的扩散将受到更大的阻力,从而使得内部活性材料难以与电解质接触而导致容量的损耗,因此,如何构建合理的纤维微观结构,在直径不断增加时,仍旧保持较大的线容量将是本章需要解决的主要问题。将GO、CNT和Sn O₂纳米粒子在水溶液中充分分散后获得纺丝前驱体,以壳聚糖水溶液为凝固浴,湿法纺丝得到GO-CNT-Sn O₂凝胶纤维,通过后续的冻干、水合肼还原以及冷压处理获得多孔的扁形r GO-CNT-Sn O₂（PP-GCS）纤维,冻干赋予了纤维贯穿多孔的微观结构使得线容量增大时电解液在内部的快速扩散得以保证,冷压过程则实现了纤维内部片层的取向重排而使多孔纤维具备优异的柔性,相较于常规r GO-CNT-Sn O₂（C-GCS）纤维和多孔r GO-CNT（PP-GC）,r GO-Sn O₂（PP-GS）纤维,该纤维作为负极材料组装成F-LIBs的电化学性能更为优异,在0.1 A g^-1的电流密度下循环100次,其容量保持17.43 m Ah m^-1,1 A g^-1条件下循环500次,容量保持9.12 m Ah m^-1。并且,随着纤维线密度不断增大,该柔性电极的质量容量并未发生明显衰减,其线容量却不断提升,当纺丝速率为900μL min^-1时,所制备的PP-GCS纤维在0.1 A g^-1的电流密度下最高线容量达到47.78 m Ah m^-1。进一步将PP-GCS纤维作为负极材料组装成F-SIBs,在0.05 A g^-1电流密度下,其线容量达到15.74 m Ah m^-1,在且在多次弯折下仍旧保持良好的柔性特征,有望在下一代高线容量柔性可穿戴储能器件中获得广泛应用。本章发展了冷压驱动石墨烯多孔纤维的方法,实现了线密度和线容量之间的线形增长关系,制备了具有更高线容量、循环稳定且柔性的多孔石墨烯杂化纤维。更多还原

【Abstract】 With the rapid development of wearable electronic devices and artificial intelligence devices,the market demand for flexible energy storage devices is also increasing year by year.LIBs have been applied in lots of fields and achieved enough attention as mainstream energy storage devices due to high energy density and low self-discharge.Accordingly,it is necessary that develop fiber shaped electrode for assembling flexible lithium ion batteries in order to meet the application scenarios and application requirement of flexible electronics market.The electrode for traditional LIBs could be prepared by mixing active materials,conductive material and binder following coated on metal foil.Owning to the high rigidity,easy deformation after bending and unmatched macroscopic shape,traditional electrode is difficult to apply in fiber shaped LIBs directly.Graphene fiber,as a novel carbon based materials,could be constructed via layer-by-layer stacking between graphene sheets byπ-πinteraction.On the one hand,graphene fiber inherits exccellent properties coming from graphene sheets.Based on the various process methods such as wet spinning,dry spinning,dry-jet wet spinning and limited hydrothermal and combined with post processing methods,graphene fiber could express a part of properties belonging to the graphene nanosheets,including mechanical,electrical and electrochemical performance.In the other hands,the layer size and diatance,stacking types and pore structure could be accurately controlled by in situ hybridization in wet spinning process and fiber surface modification,which contributed to the the active materials loading precisely.Therefore,it is possible that graphene fiber applied in fiber shaped energy storage devices for high flexibility and functionalization.The targets for lithium ion battery in different application scenarios show an obvious difference.Different from the traditional lithium ion batteries which pay attention to mass capacity and volume capacity,the line capacity become a more important indicator for evaluating electrochemical performance for fiber shaped battery for one-dimensional structure.However,there are several problems for applying graphene fiber in flexible LIBs.（1）Compared with metal oxide and silican active materials,graphene show low capacity and poor adaptation in flexible LIBs as electrode;（2）The fiber LIBs usually presented poor cycling performance owning to introducing high capacity active material which express huge volume expansion in cycling;（3）Because of strong interaction between two sheets leading to tight array of fiber structure,electrolyte usually presented low diffusion rate in graphene fiber.Therefore,how to achieve remarkable increase in line capacity for fiber shaped batteries by the control design of fiber structure and the hybridization of multi-component has been the main problem.This dissertation focuses on the preparation of porous graphene fiber by wet spinning.The physical feature and electrochemical performance was measured systematacially and the possibility of application in fiber shaped lithium ion batteries was discussed.The main results are as follows:（1）The preparation of graphene hybrid carbon nanotubes fiber for the application in flexible lithium ion battery.The uniform spinning solution could be obtained by the ultrasound process for GO and CNT in deionized water depending on the strongπ-πinteraction between two carbon based materials.Then,the continuous GO/CNT fiber was prepared by wet spinning that chitosan solution regard as coagulating bath.Finally,the r GO-CNT fiber was received after reduced at 900^oC under the N₂ atmosphere A network with high alignment,optimal porosity and low junction contact resistance is formed.Because of the strong shear force at wet spinning,the r GO and r GO-CNT fiber presented high orientation and lay-by-layer stacking structure.However,r GO-CNT fiber showed loosen situation compared with r GO fiber owning to existence of CNT as insert,leading to the larger distance for adjacent graphene sheets and more number for continuous pores.As a result,the specific surface area of r GO-CNT increased from127.5 m² g^-1 to 204.5 m²g^-1 after adding CNT.The half-cell fiber shaped lithium ion battery was assembly that r GO-CNT fiber regarded as anode materials for measuring the electrochemical performance.At the 5%content of CNT,the r GO-CNT fiber presented maximum line capacity reached to 522 m Ah g^-1(0.287 m Ah m^-1)at current density of 0.05 A g^-1.What’s more,after 600 cycles the retaining capacity was 374 m A h g^-1(0.206 m Ah m^-1)at the current density of 500 m A g^-1.Finally,to demonstrate the principle of one dimensional material as insert in graphene fiber system,the graphene/silver nanowire（r GO-Ag）fiber was fabricated for anode as control group,revealing that carbon based material played a more significant role in capacity growth.（2）The preparation of tin oxide in situ deposition on graphene hybrid fiber for the application in flexible lithium ion battery.Inserting CNT between adjacent graphene sheets for r GO fiber capacity growth was achieved mainly by preventing the stacking of graphene sheets and exposing more active site for lithium reaction.To improve the theoretical capacity of graphene hybrid fiber,a feasible method is adding enough active material such as metal oxide.The uniform spinning solution could be obtained by the ultrasound process for GO in deionized water.Then,the continuous GO fiber was prepared by wet spinning that chitosan solution regard as coagulating bath.Subsequently,GO fibers were immersed into Sn Cl₂ concentrated hydrochloric acid/ammonium hydroxide solution for in situ GO reduction and Sn O₂ deposition.After that,GO fibers with precipitates were washed by distilled water,dried,and annealing successively,resulting in the final r GO-Sn O₂ fibers.With the increase amount of ammonium hydroxide,the distance of neighbor graphene sheets and the content of Sn O₂ were enhanced simultaneously,while the specific surface area and fiber strength were decreased.These results attributed to the p H influence of ammonium hydroxide leading to the different swelling degree.In half-cell battery,the specific capacity of r GO-Sn O₂ fiber reached a peak at 0.6 m L ammonium hydroxide added.The proposed device acquire a remarkable line-capacity reaching to 1.35 m Ah m^-1 and still maintaining 1.16 m A h m^-1 after 100 cycles at 0.1 A g^-1.When the current density enhanced to 5 A g^-1,the line capacity still retain to 0.312 m A h m^-1,which indicated that active materials could promote the line capacity obviously for fiber shaped electrode.In addition,matching with Li Fe PO₄/carbon fiber（Li Fe PO₄/CF）,the full cell shows volume capacity at 0.51 m A h m^-1 after 50cycles at the current density of 50 m A g^-1 and holds a stable charge discharge cycle performance in different bending angle.This chapter developed a method for controlled loading metal oxide nanoparticles between graphene sheets,a principle about the hybridization of graphene and fuctional components and a fiber with high line capacity and stable cycle performance.（3）The preparation of porous graphene/carbon nanotube/tin oxide hybrid fiber for the application in flexible lithium and sodium ion battery.Sn O₂ as active materials could improve the line capacity proved in part two.To further improve the line capacity,enlarging fiber diameter is another method.However,the diffusion rate of electrolyte in fiber will become slower along with the increase of diameter leading to a poor line capacity.Therefore,how to construct rational structure could provide enough channels for electrolyte as diameter large enough is the main point that we should solved.GO powder,CNT and Sn O₂ nanoparticles were suspended into deionized water by sonicating.The profiled r GO-CNT-Sn O₂ aerogel hybrid（PP-GCS）fiber could be assembled via wet-spinning following freeze-dried and mechanically compressed.The graphene hybrid aerogel fiber was fabricated by wet spinning following freeze dried for building porous structure assuring the faster and thicker diffusion of electrolyte,and the fiber flexibility was promoted by the mechanical power derived rearrangement of graphene sheets.Compared with traditional r GO-CNT-Sn O₂（C-GCS）fiber and porous r GO-CNT（PP-GC）,r GO-Sn O₂（PP-GS）,the assembled fiber shaped battery demonstrated a stable cycle performance in that the discharge capacity maintained 17.43 m A h m^-1 after 100 cycles in 0.1 A g^-1 and 9.12 m Ah m^-1 after 500cycles in 1 A g^-1.Besides,with the increase of PP-GCS fiber diameter,the mass-capacity still keeps a high level.The highest line capacity of porous fiber,which prepared at inject rate at 900μL min^-1,reached to 47.78 m A h m^-1 at current density of 0.1 A g^-1.PP-GCS hybrid fiber,assembled to fiber shaped SIBs as anode,possessed high line capacity,reached to 15.74 m Ah m^-1at 0.05 A g^-1.Therefore,this work provided a booming future in the preparation of high energy device in next-generation wearable electronics.This chapter developed a method for porous graphene fiber by cold compress drived,achieved relationship between line density and line capacity and prepared a fiber with higher line capacity,stable cycle performance and flexibility.更多还原