碳纳米材料构建高性能锂离子和锂硫电池研究进展

吴怡芳, 崇少坤, 柳永宁, 郭生武, 白利锋, 张翠萍, 李成山

材料工程 ›› 2020, Vol. 48 ›› Issue (4) : 25-35.

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材料工程 ›› 2020, Vol. 48 ›› Issue (4) : 25-35. DOI: 10.11868/j.issn.1001-4381.2019.000590
纳米材料专栏

碳纳米材料构建高性能锂离子和锂硫电池研究进展

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Research progress on carbon nano-materials to construct Li-ion and Li-S batteries of high performance

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摘要

碳作为单一元素可形成像零维碳纳米球、一维碳纳米管、二维石墨烯等多种碳纳米结构,它们在锂离子和锂硫电池中的表现也有所不同。需要阐明的是,碳纳米管和石墨烯由于具有以下缺点不适合直接作为锂离子或锂硫电池电极材料:(1)第一次不可逆容量大,首次充放电效率低;(2)在充放电曲线中电压滞后现象严重;(3)缺少稳定的电压平台;(4)容量衰减快。科学家们一直在为获得具有更高能量密度和更广阔应用前景的锂离子电池和锂硫电池而努力,由于可充电电池的性能主要取决于阴极和阳极的性能,因此,设计先进的电极材料以及制备具有特定成分和结构的电极成为近年来的研究热点。本文综述了碳纳米材料在构建高性能锂离子、锂硫电池电极材料和特定电极方面的作用。首先,从促进电子和离子传输、固定多硫化物位置以及缓冲体积膨胀三个方面讨论了碳纳米材料在修饰电活性材料的作用;其次,从作为导电添加剂、电流集流体和导电中间层三个方面讨论了碳纳米材料在最优化非活性组分的作用;然后,从作为非导电基体上的导电相、柔性电流集流体和自支撑复合电极三个方面讨论了碳纳米材料在柔性电池设计的作用。最后,本文对碳纳米材料的未来发展趋势作了概述,兼具多种功能的碳纳米材料被认为是今后的研究重点。

Abstract

Carbon solely can form a lot of nanostructures, such as zero-dimensional nanosphere, one-dimensional nanotube and two-dimensional graphene. They perform differently in Li-ion and Li-S batteries. It is worth noting that CNTs and graphene are not appropriate to be used as electroactive materials for Li-ion or Li-S batteries for four reasons. First, when CNTs and graphene are used as an anode, they often exhibit high specific capacities during the first lithiation step, but a large fraction of lithium ions is irreversibly consumed instead of reversibly stored, leading to a low Coulombic efficiency of the cell. Second, a graphene-based anode has a large voltage hysteresis in the charge/discharge curves. Third, it has been reported a CNT-based anode lacks a steady voltage plateau with large change in voltage during discharge. Fourth, despite their high initial capacities, graphene and CNT-based anodes often suffer from fast capacity decay after a few tens of cycles. Continuous efforts have been made to build better lithium batteries with a higher energy density and wider applicability, including both current state-of-the-art Li-ion batteries and near-term Li-S batteries. Because the behavior of a rechargeable battery is mainly based on the performance of its anode and cathode, designing advanced electrode materials as well as electrode with tailored compositions and structures has been the hot topic in recent years. The role of carbon nano-materials to construct electrode materials and tailored electrodes in Li-ion and Li-S batteries in high performance was reviewed in the paper from three aspects. Firstly, the role of carbon nano-materials in modifying the electroactive materials was discussed from three aspects:electron- and ion-transport facilitators, immobilization sites and volume expansion buffering. Secondly, the role of carbon nano-materials in optimizing the inactive components was considered as follows:conducting additives, current collectors and conductive interlayers. Thirdly, the role of carbon nano-materials in designing the bendable and stretchable devices are discussed from three aspects:conductive phases in nonconductive substrates, flexible current collectors and freestanding composite electrode. Finally, perspectives on future development of Li-ion and Li-S batteries were presented. It is considered that multi-functional carbon nano-materials will be main research focus in the future.

关键词

碳纳米材料 / 锂离子 / 锂硫电池 / 研究进展

Key words

carbon nano-material / Li-ion / Li-S battery / research progress

引用本文

导出引用
吴怡芳, 崇少坤, 柳永宁, 郭生武, 白利锋, 张翠萍, 李成山. 碳纳米材料构建高性能锂离子和锂硫电池研究进展[J]. 材料工程, 2020, 48(4): 25-35 https://doi.org/10.11868/j.issn.1001-4381.2019.000590
Yi-fang WU, Shao-kun CHONG, Yong-ning LIU, Sheng-wu GUO, Li-feng BAI, Cui-ping ZHANG, Cheng-shan LI. Research progress on carbon nano-materials to construct Li-ion and Li-S batteries of high performance[J]. Journal of Materials Engineering, 2020, 48(4): 25-35 https://doi.org/10.11868/j.issn.1001-4381.2019.000590
中图分类号: O646.21   

随着日益增长的能源需求,锂离子电池作为一种新兴的能源,已日益渗入到生活的方方面面。小到日常所用的电子设备、小型电动和交通工具电源,大到电动汽车、风光伏储能系统、移动通信基站电源,锂离子电池的应用无处不在[1-8]。通常,为了增加锂离子电池中电极材料的导电性,会在电极材料表面包覆碳层,或添加导电炭黑、导电石墨等碳材料[9-11]。为了获得更高性能的锂离子电池,近年来,采用碳纳米技术提高锂离子材料性能的研究也相当活跃[12-18]
锂硫电池由于高理论比容量(1672 mAh·g-1)和低成本等优势成为目前高能量密度电池研发的重点[19-24]。然而锂硫电池在实际应用中存在一些问题,一个是S和放电产物Li2S是绝缘的,而Li2S的低密度(相对于S)意味着正极在放电过程中发生显著的体积膨胀,导致硫的利用率降低;另一个是“穿梭效应”,电解质溶解的锂聚硫化物(LiPS)中间体在正负极之间的迁移,导致正负极逐渐降解。这些问题导致Li-S电池的容量快速衰减和较低的库仑效率,而采用碳纳米材料是提高锂硫电池性能的一个重要方向。
碳作为单一元素可形成像零维碳纳米球、一维碳纳米管、二维石墨烯等多种碳纳米结构,它们在锂离子和锂硫电池中的表现也有所不同。需要阐明的是,碳纳米材料由于具有以下缺点[25-26]不适合直接作为电极材料:(1)第一次不可逆容量大,首次充放电效率低;(2)在充放电曲线中电压滞后现象严重;(3)缺少稳定的电压平台;(4)容量衰减快。
能量密度决定续航里程,功率密度决定加速度。对能量储存装置而言,制备高能量密度或高功率密度的锂离子或锂硫电池是目前的研究重点。本文从碳纳米材料在构建高性能锂离子和锂硫电池方面的作用入手,从以下三个方面阐述碳纳米材料在锂离子和锂硫电池中的作用:(1)修饰电活性材料;(2)最优化非活性组分;(3)柔性电池设计。

1 碳纳米材料在修饰电活性材料方面的作用

1.1 促进电子和离子传输

电子和锂离子传输特性在电化学反应中起到至关重要的作用。研究表明[27],电活性材料无论是被碳纳米材料固定、包裹、胶囊化、形成三明治结构、层状结构或混合结构,化学性能都会有显著的提升。碳纳米/电活性复合材料的结构模型示意图如图 1所示。
图 1 碳纳米/电活性复合材料的结构模型示意图(其中红色代表电活性材料,蓝色代表碳纳米材料)[27]

(a)固定模型; (b)包裹模型; (c)胶囊化模型; (d)三明治模型; (e)层状模型; (f)混合模型

Fig.1 Schematic of structural models of carbon nano-materials/electroactive composites (red:electroactive materials; blue:carbon nano-materials)[27]

(a)anchored model; (b)wrapped model; (c)encapsulated model; (d)sandwich-like model; (e)layered model; (f)mixed model

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李进等[28]将介孔碳纳米微球粉末与磷酸铁锂(LiFePO4)、石墨混合(介孔碳含量为8%)作为电池的正负极材料,组装锂离子电池。添加碳纳米微球后,电池的可逆性和比容量均优于未添加的,尤其在大电流9.0 A充放电时,充电电压平台约低0.25 V,充放电效率高出约88.5%;输出容量与放电电流为0.8 A时相比高约15%。在低温0,-10 ℃和-20 ℃放电时,添加碳纳米微球的电池输出容量与额定容量的比值比未添加的分别高12.4%,14.9%和16.7%。
Wang等[29]制备了分级结构的多孔碳包覆的LiFePO4/碳纳米管(CNT)微球复合物(C@LFP/CNTs),如图 2所示。其中,碳纳米管作为电子导体原位均匀地嵌入到开孔微球中,形成导电性的碳纳米管网络,交错的碳纳米管孔网络还有利于锂离子的传输,使得每一个C@LiFePO4/CNT微球成为一个有效的电化学反应微反应器。同时,无定形碳与碳纳米管桥接,进一步提高了整个复合物的电导性。除此之外,复合物的微球结构确保了复合物具有高的振实密度,提高了电池的体积比能量密度。碳纳米管作为良好的电容材料容量响应迅速,它的引入还有利于提高电池的倍率性能和循环稳定性。电化学测试表明,分级结构的多孔C@ LiFePO4/CNT微球具有极高的倍率性能,达73 mAh·g-1@60C,其循环稳定性优异,在10 C的放电倍率下,循环1000次容量保持率达98%,体积比能量密度也高达443 Wh·L-1
图 2 多孔碳包覆的LiFePO4/碳纳米管(CNT)微球复合物制备示意图[29]

Fig.2 Schematic illustrating of the preparation process of C@LFP/CNTs[29]

Full size|PPT slide

Wang等[30]还制备了氮掺杂石墨烯气凝胶改性的LiFePO4正极材料,如图 3所示。其中,(010)面取向的LiFePO4纳米片被氮掺杂的石墨烯气凝胶包裹,它具有三维多孔结构,比表面积高达199.3 m2·g-1。在这一复合物中,氮掺杂的石墨烯气凝胶交织的多孔网络为快速的电子和锂离子传输提供了通道,而具有大的(010)比表面积的LiFePO4纳米片提高了锂活性位点,缩短了锂扩散距离。电化学测试表明,氮掺杂石墨烯气凝胶改性的LiFePO4正极材料具有极高的倍率性能,达78 mAh·g-1 @100C。循环稳定性优异,在10 C的放电倍率下,循环1000次容量保持率达89%。
图 3 氮掺杂石墨烯气凝胶改性的LiFePO4正极材料示意图[30]

(a)氮掺杂石墨烯气凝胶改性的LiFePO4正极材料示意图(LiFePO4纳米片为绿色,多孔的氮掺杂石墨烯气凝胶网络为灰色,充满电解液的孔隙为淡蓝色); (b)氮掺杂石墨烯气凝胶改性的LiFePO4正极材料的四种主要阻抗; (c)LiFePO4晶体中锂离子扩散示意图; (d)氮掺杂石墨烯气凝胶改性的LiFePO4正极材料制备示意图

Fig.3 Schematic of nitrogen doped graphene modified aerogel LiFePO4(LFP@N-GA) cathode material[30]

(a)schematic of a battery based on LFP@N-GA cathode(LFP NPs active phase is green, the porous N-GA network is grey, and the electrolyte filled in the pores is pale blue in color); (b)illustration of the four primary resistances in LFP@N-GA cathode; (c)schematic of the Li+ diffusion process in LFP crystal; (d)preparation process of LFP@N-GA

Full size|PPT slide

Fang等[31]报道了高电压的LiNi0.5Mn1.5O4/多壁碳纳米管(MWCNT)复合材料。由于不需要使用黏结剂、导电添加剂和金属电流集流体,LiNi0.5Mn1.5O4/ MWCNT复合物具有柔韧性高和质量轻的优点。由于使用了高电压的LiNi0.5Mn1.5O4粒子,电池的能量密度显著提高了。此外,由于多壁碳纳米管的高导电性,LiNi0.5Mn1.5O4/MWCNT复合电极在20 C放电电流下仍能承载140 mA·g-1放电电流下80%的容量。同时,在10 C的大电流放电条件下,电池的循环性能良好,100次循环后没有明显的容量衰减。由于具有高电压、高倍率性能和质量轻的优点,LiNi0.5Mn1.5O4/MWCNT复合电极将成为一种有前景的高功率电池替代品。此外,由于它具有质量轻、柔韧性好的优点,有望被用于下一代超轻柔性电子装置。
Zhang等[32]采用简易的机械研磨方法制备了LiNi0.8Co0.15Al0.05O2/碳纳米管(NCA/CNT)复合材料。高电导性的碳纳米管不但提高了电池的电导性,而且有效地避免了电极活性材料与电解液的副反应。与纯NCA电极相比,它的循环性能和倍率性能都有所提高,在0.25 C的放电倍率下循环60次,NCA/CNT复合电极的可逆容量达181 mAh·g-1,容量保持率达96%,明显高出纯NCA电极(可逆容量153 mAh·g-1,容量保持率达90%)。在5 C的高倍率放电条件下,NCA/CNT复合电极的可逆容量达160 mAh·g-1,而纯NCA电极仅有140 mAh·g-1
金属氧化物阳极,理论容量高但电荷转移动力学缓慢[33-41]。Zhou等[39]报道了NiO纳米片在石墨烯上生长,提高了锂储存容量;在电荷转移动力学方面,原位TEM显示[40], 石墨烯使锂离子扩散速率提高了2个数量级,表明石墨烯为锂离子扩散提供了快速通道,这归功于NiO与石墨烯之间强烈的界面反应,确保了足够多的界面锂离子扩散通道。在反应动力学方面,NiO/石墨烯复合物中锂离子与NiO的反应动力学获得极大的提高。在NiO/石墨烯复合物中,NiO锂化的平均反应时间为5 s,是纯NiO材料的15.4倍。在纯NiO材料中,第二片NiO纳米片的锂化是在第一片NiO纳米片完全锂化后开始的,随着反应的进行,NiO纳米片完全锂化的时间越来越长,这是由于锂离子的传输距离更长,使得锂化越来越困难。而对NiO/石墨烯复合物来说,石墨烯的存在提供了大量的界面锂离子扩散通道,从而使这种材料具有快速的反应动力学。Lou等[41]设计了MoS2管状结构,内部嵌入了碳纳米管CNTs作为导电支架,碳纳米管作为锂离子快速传输的导电通道,多孔管状结构为快速锂离子扩散提供短的扩散距离。MoS2/CNT复合物可逆容量达1100 mAh·g-1,200循环后容量几乎没有衰减。

1.2 固定多硫化物位置

电解液中多硫中间化合物的严重溶解和迁移是Li-S电池保持长期稳定性所面对的最具挑战性的问题。因此,在硫阴极中包含多硫化物锁定位置非常重要。考虑到多硫化物(极化)和碳(非极化)的不同化学键性质,多硫化物和多孔结构碳的物理结合由于表面结合不紧密不足以固定迁移的多硫化物,而通过两极交互反应的多硫化物的化学吸附被认为可以有效提高多硫化物和电极之间的结合程度。氧化石墨烯(GO)表面含有丰富的含氧官能团,具有强化学吸附能力,可以固定多硫化物,有效地防止它在循环过程中在电解液中溶解。Ji等[42]采用从头计算方法表明氧化石墨烯中的环氧基和羟基都能提高多硫化物中硫原子和石墨烯中碳原子之间的结合力,从而提高多硫化物和石墨烯之间的交互作用,较好地固定多硫化物。
另一种固定多硫化物的表面化学策略是在碳纳米管或石墨烯中引入具有不同电负性的杂环原子掺杂。氮掺杂是最常使用的杂环原子掺杂手段[43-46],因为氮比碳的电负性高,可以提高碳表面的电负性,有利于多硫化物的化学吸附。

1.3 对体积膨胀起缓冲作用

电活性材料在脱嵌锂过程中存在严重的体积膨胀,极大地影响了它们的循环性能。例如,硫的体积膨胀达80%[47],硅的体积膨胀高达300%[48],金属氧化物如NiO[39-40],Co3O4[49]和V3O7[50]也在循环过程中有明显的体积膨胀。反复的体积膨胀和收缩直接导致了电极的粉化和电活性材料之间的分离,是容量衰减的最主要原因。当活性材料膨胀和收缩时,固体电解质界面(solid electrolyte interface, SEI)膜生成又破裂,当新的SEI膜在新鲜暴露的表面形成时,锂离子的不可逆消耗将导致较低的库仑效率。此外,SEI膜的累积导致了内阻的增加,阻碍了锂离子迁移通道,对活性材料的电化学反应起到负面作用。
Mai等[50]设计了V3O7/石墨烯复合物,其中V3O7纳米线被半空心的石墨烯纳米卷包裹。中空的石墨烯卷为V3O7纳米线在锂化过程中的体积膨胀提供了自由空间,有效地抑制V3O7纳米线的团聚,确保了循环过程中V3O7/石墨烯复合物的结构稳定性以及锂离子和电子传输通道的通畅性。V3O7/石墨烯复合物可逆容量达321 mAh·g-1(100 mA·g-1),400次循环下容量保持率达87.3%(2000 mA·g-1)。Yu等[51]制备Si纳米粒子填充的碳纳米管材料,原位TEM观察表明,碳纳米管壁制约了Si锂化过程中的体积膨胀而自身的管状结构没有被破坏。

2 碳纳米材料在最优化非活性组分方面的作用

2.1 作为导电添加剂

高功率电池由于电荷转移速率快,导电添加剂的作用就显得额外重要。碳纳米材料作为导电添加剂有两大优点:(1)碳纳米材料可以构建高效的导电网络而几乎不增加额外的质量,相当于可以相对地增加活性材料的含量,从而增加电极的容量。(2)碳纳米材料良好的热导性使电极具有良好的散热性,减少了电池的热失控,提高了电池的安全性。
Landi等[52]报道了1%(质量分数,下同)的单壁碳纳米管添加的LiNi0.8Co0.2O2正极材料,相比4%炭黑添加的LiNi0.8Co0.2O2正极材料,它具有更好的渗流网络。在较高的放电电流(16.4 mA·cm-1)下,单壁碳纳米管添加的LiNi0.8Co0.2O2正极材料的比容量是炭黑添加的正极材料的3倍。并且,单壁碳纳米管添加剂提高了电极的热稳定性,在脱锂过程中正极的放热能减少了40%。
Yang等[53]比较了石墨烯导电添加剂和商品导电添加剂对LiFePO4软包电池的影响,如图 4所示。商品导电添加剂一般添加7%的炭黑和3%的导电石墨,Yang的研究表明,2%石墨烯导电添加剂的LiFePO4软包电池比商品导电添加剂的软包电池具有更高的容量。尽管添加石墨烯的LiFePO4由于构建了有效的导电网络具有高容量,但从其充放电平台曲线上可以看出它的极化较大,这是由于锂离子传输通道在一定程度上被石墨烯的二维平面结构阻塞了空间位阻效应,导致电化学反应所需的锂离子流量不足。也就是说,尽管石墨烯导电网络确保了快速的电子传输,但受阻的锂离子传输导致反应动力学方面的负面效应,从而导致较高的极化。然而,采用1%的炭黑替代部分石墨烯,电池的极化大大减弱,从而改善了锂离子传输性能。
图 4 2%石墨烯导电添加剂的2 Ah(1)和1%石墨烯和1%炭黑导电添加剂的2.6 Ah(2)磷酸铁锂软包电池与商品导电添加剂的软包电池比较[53]

(a)循环性能; (b)充放电平台曲线

Fig.4 Comparasion of cycling performance(a) and charge/discharge profiles(b) of two 2.0 Ah Li-ion batteries using 2% graphene and commercial conducting additives(1) and two 2.6 Ah Li-ion batteries using 1% graphene plus 1% carbon black and commercial conducting additives(2)[53]

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2.2 作为电流集流体

铝和铜箔等传统的电流集流体需要面对的问题是:(1)它们质量轻,增加了非活性组分的比例,导致了能量密度的降低。(2)活性材料和光滑金属表面的弱连接使界面性能变差,使活性材料有从电流集流体上剥离的危险。(3)金属在有机电解液中不稳定,在长期的电化学循环下会被腐蚀。
碳纳米管和石墨烯可以被装配成自支撑膜用作电流集流体,它具有质量轻、力学性能好和化学性能稳定的优点[54-57]。Wang等[54]报道了使用超轻的碳纳米管作为导电集流体制备自支撑电极,与传统的金属电流集流体相比,它具有更好的润湿性、更强的附着力、更好的力学性能和电极/碳纳米管界面较低的接触电阻,因而,循环稳定性、倍率性能和质量比能量密度均有所提高。Shi等[55]报道了采用高导性的石墨烯膜作为LiFePO4阴极和Li4Ti5O12阳极的导电集流体,与采用传统的金属集流体相比,它们具有更高的比容量和更稳定的循环性能,尤其是在高电流密度下。此外,碳纳米管和石墨烯作为导电集流体用于Li-S电池可以在循环过程中存储多硫化物,有利于提高电池的循环稳定性。Hsieh等[56]报道了一种来自生物灵感的电极结构,它成功地将sp2型碳和sp3型碳整合到同一整体结构(图 5),其中sp2型碳负责电荷转移,sp3型碳使活性物质具有高的载量。具体来讲,它在三维石墨烯网络(3DG)上原位生长了氮掺杂的碳纳米管(NCNT)作为电流集流体,使电子可以在氮掺杂碳纳米管和三维石墨烯网络之间快速传输,同时氮掺杂碳纳米管的亲水特性又保证了活性物质Li4Ti5O12 (LTO)的高载量。这种特别设计的LTO-NCNT-3DG电极的活性物质载量达到整个电极的74%, 而传统金属箔的活性物质载量仅能达到20%。这种LTO-NCNT-3DG电极在5 C倍率下具有158 mAh·g-1的比容量,在10 C的倍率下2000次循环的容量保持率达94%。
图 5 来自生物灵感的同时具有疏水和亲水特性,能够促进电子的快速传输和活性物质的均匀负载,不需要任何金属导电集流体或黏结剂的NCNT-3DG电极结构[56]

Fig.5 Bio-inspired NCNT-3DG with hydrophobic and hydrophilic features within the same structure, promoting a fast electron transfer and a uniform loading of active materials in a monolithic structure free of any metal current collector or binder[56]

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2.3 作为导电中间层

多硫化物在电解液中的溶解和迁移会导致活性材料在循环过程中严重的不可逆损失,是Li-S电池所面临的最具挑战性的问题,而在硫阴极和聚合物隔膜之间引入导电中间层可以有效地解决这一问题。碳纳米管和石墨烯由于具有高电导性、管状多孔结构和合适的表面特性可以用来设计导电中间层。在Li-S电池中,导电中间层作为嵌入性的导电网络,可以拦截迁移的多硫化物,使它们可以再利用,提高了硫利用率和电池的循环稳定性。Su等[58]在Li-S电池中引入导电性的多壁碳纳米管中间层,它在电极中形成额外的电流通路,使电子可以快速传输,降低了电池内阻,减缓了溶解的多硫化物扩散到阳极,提高了电池的容量和循环稳定性。Zhou等[59]设计了一种复合隔膜,它将导电性的石墨烯中间层涂覆在传统的聚合物隔膜上,被证实是一种提高Li-S电池性能的简单而有效的方法。Fang等[60]采用部分氧化的石墨烯拦截多硫化物,不但捕获了迁移的多硫化物,而且通过化学交互反应固定了它们,提高了电池的电化学性能。

3 碳纳米材料在柔性电池设计方面的作用

碳纳米材料由于具有优异的力学性能和独特的一维、二维结构特征,在制作柔性电池方面有极大的优势[61-72]。首先,碳纳米管和石墨烯的一维和二维纳米结构使它们可以有极小的弯曲半径,在弯曲时有高耐受度。其次,碳纳米管和石墨烯很容易装配成自支撑膜或3D结构,为制备不同的柔性电极提供了多种可能。再次,碳纳米管束和石墨烯片之间的强交互作用,使它们在反复弯曲和拉伸时可保持完整性。

3.1 作为非导电基体上的导电相

聚合物基体,例如聚二甲基硅氧烷(PDMS)、聚对苯二甲酸乙二酯(PET)、聚偏氟乙烯(PVDF)、纤维素纸(cellulose paper)和聚酯纤维(polyester fabrics)由于弹性高成为最常使用的柔性支撑物。然而,聚合物基体通常是绝缘的,必须要加入导电添加剂以降低其内阻。碳纳米管和石墨烯具有高电导性和良好的本征柔韧性,通常采用喷涂、溅射、真空抽滤等多种方法组装到聚合物基体中,以改善柔性基体的电导性,提高它们的电化学性能。Weng等[73]采用真空抽滤方法制备了一种复合石墨烯/纤维素纸,这种以石墨烯作为导电框架的复合材料具有优异的力学柔韧性,较高的比容量和优异的循环性能。

3.2 作为柔性电流集流体

石墨烯和碳纳米管电流集流体具有高稳定性、良好的浸润性、低接触电阻、强黏附力,以及良好的柔韧性,被用作柔性电极和柔性全电池电流集流体[74-76]。例如,Aliahmad等[74]通过一种逐层自组装技术在木质微纤维上沉积单壁碳纳米管制备了柔性电流集流体,其中碳纳米管的载量达到10.1 μg/cm2,并与V2O5/石墨烯活性材料相结合制备了具有高的容量和能量密度的柔性电极。Gwon等[75]采用石墨烯膜作为电流集流体制备了锂离子柔性电极,V2O5电极生长于石墨烯的表面,不仅提高了电极的力学柔韧性,与传统电极相比,还具有优异的电化学性能。Li等[76]报道了一种超轻薄、柔性的LiFePO4/Li4Ti5O12全电池,它使用3D石墨烯泡沫作为电流集流体,具有优异的柔韧性,在反复弯曲并且弯曲半径小于5 mm时也没有结构失效,它在弯曲状态下能够点亮红色发光二极管。并且采用高耐受度的3D石墨烯泡沫作为导电集流体的电池,在伸直和弯曲状态下充放电曲线几乎没有不同,具有良好的循环稳定性。

3.3 作为自支撑复合电极

碳纳米管和石墨烯组装到活性材料中,被广泛用作柔性构件自支撑复合电极[77-85]。碳纳米管和石墨烯具有高柔韧性,能够构造具有高耐受度、高力学性能的骨架。此外,这种方法不使用导电集流体、导电添加剂和黏结剂,极大地降低了非活性组分的含量,提高了柔性电池的能量密度。例如,Yousaf等[81]采用溶剂热合成方法制备了一种碳纳米管海绵上生长的MoSe2纳米结构自支撑复合电极(图 6),它具有高的活性物质载量。MoSe2以层状结构生长并且层数可控。其中,核壳结构的CNT@MoSe2(MoSe2为10层)电极在电流密度为100 mA·g-1下100个循环后仍具有820 mAh·g-1的可逆容量。先位研究表明,随循环次数的增加MoSe2从晶态向部分无定型态转变,这将使容量随循环次数的增加而持续增长。Ding等[82]采用溶剂蒸发法制备了3D石墨烯/LiFePO4自支撑复合电极,在100次反复弯曲后,柔性电极仍没有明显的容量衰减。Yuan等[83]报道了碳纳米管基的自支撑硫电极,它采用短的多壁碳纳米管作为短程电导框架来存储硫,采用超长的碳纳米管作为长程导电网络和具有良好柔韧性的内部机械骨架。采用54%的中等硫含量和6.3 mg·cm-2载硫率,自支撑的碳纳米管电极在电流密度为0.38 mA·cm-2下150个循环后仍具有700 mAh·g-1的可逆容量。表明碳纳米管在其中起了双层作用,一是作为有效的导电网络,二是作为柔性构件单元。
图 6 不同层数MoSe2在碳纳米管上生长的示意图(a)及其循环容量(b)[81]

Fig.6 Schematic representation(a) and their capacity over cycle number(b) of different layers of MoSe2 over CNT[81]

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4 结束语

近年来对碳纳米材料的研究表明,它们在实际应用中并不十分适合作为锂离子或锂硫电池电极材料。但是碳纳米材料在修饰电活性材料、最优化非活性组分以及柔性电池设计方面已经彰显了其优势,具有广阔的应用前景。
值得一提的是,碳纳米材料有时可以同时兼具多重作用。例如,采用多孔氮掺杂的碳纳米微球锂硫电池,它丰富的孔隙结构保证硫的有效利用率及电池的稳定循环性能。氮掺杂表面能够实现对多硫化物的有效吸附,独立的纳米结构之间的中孔孔隙可以有效缓解硫在充放电过程中的体积膨胀问题。紧密堆叠的纳米球结构也能更好地实现离子和电子的有效传输。独特的碳纳米结构也为制备平滑紧密的高负载电极材料提供了可能。可以预见,这种兼具多种功能的碳纳米材料是今后的研究重点。

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