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材料研究学报, 2023, 37(7): 481-494 DOI: 10.11901/1005.3093.2022.360

综述

MXene在锂硫电池中应用的研究进展

季雨辰, 刘树和,, 张天宇, 查成

昆明理工大学冶金与能源工程学院 昆明 650093

Research Progress of MXene Used in Lithium Sulfur Battery

JI Yuchen, LIU Shuhe,, ZHANG Tianyu, ZHA Cheng

Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China

通讯作者: 刘树和，副研究员，2538234121@qq.com，研究方向为锂离子电池和高比能锂硫电池的电极材料设计、制备、储能性能和机理

责任编辑: 黄青

收稿日期: 2022-07-01   修回日期: 2022-12-06  

基金资助: 国家自然科学基金(51264016)

Corresponding authors: LIU Shuhe, Tel:15912595890, E-mail:2538234121@qq.com

Received: 2022-07-01   Revised: 2022-12-06  

Fund supported: National Natural Science Foundation of China(51264016)

作者简介 About authors

季雨辰，男，1997年生，硕士

摘要

MXene是一种新兴的二维过渡金属碳化物或碳氮化物，优异的金属导电性、丰富的表面官能团和超薄二维结构使其在电化学储能方面的应用有巨大的潜力。锂硫电池的理论比容量较高，在新一代储能器件中极具竞争力。二维MXene及其组装的三维材料作为一种先进的硫载体可通过多种途径克服锂硫电池固有的导电性差和放电产物溶解严重的问题。本文综述了目前二维和三维结构的MXene材料在锂硫电池中的应用，分析了性能与结构之间的关系，总结了目前存在的挑战和困难并对未来的设计方向提出一些看法。

关键词： 评述; MXene; 锂硫电池; 复合材料

Abstract

As an emerging two-dimensional transition metal carbide or carbonitride, MXene exhibits excellent metallic conductivity, abundant surface functional groups and ultrathin two-dimensional structure, which endows it great potential in electrochemical energy storage. Lithium-sulfur batteries have a high theoretical specific capacity and become a very competitive choice in the new generation of energy storage devices. Two-dimensional MXenes and the assembled three-dimensional materials, as advanced sulfur carriers, can effectively improve the inherent poor conductivity and serious dissolution of discharge products of lithium-sulfur batteries. This paper reviews the current applications of MXene materials with two-dimensional and three-dimensional structures in lithium-sulfur batteries, analyzes the relationship between different structures and performance, summarizes the current challenges and difficulties, and gives a view on the direction to proceed for future designs.

Keywords： review; MXene; lithium sulfur battery; composites

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本文引用格式

季雨辰, 刘树和, 张天宇, 查成. MXene在锂硫电池中应用的研究进展[J]. 材料研究学报, 2023, 37(7): 481-494 DOI:10.11901/1005.3093.2022.360

JI Yuchen, LIU Shuhe, ZHANG Tianyu, ZHA Cheng. Research Progress of MXene Used in Lithium Sulfur Battery[J]. Chinese Journal of Materials Research, 2023, 37(7): 481-494 DOI:10.11901/1005.3093.2022.360

新能源汽车的应用和便携式电子设备的发展，极大地促进了储能器件的研究并对电池的性能提出了更高的要求[1~3]。与锂离子电池相比，锂硫电池的理论比容量(1675 mAh·g-1)和能量密度(2600 Wh·kg-1)有很大的优势[4,5]。锂硫电池由硫正极、锂负极、隔膜和电解液组成[6~8]，其反应机理基于硫与锂的氧化还原反应[7,9,10]。锂硫电池的放电过程为单质硫(S8)参与16电子还原反应，先与锂生成可溶性多硫化物中间体(Li2S x，2<x≤8)和最终还原成硫化锂(Li2S)。锂硫电池的充电过程，是施加一定电位差的外电场使Li2S分解成Li和S(图1a)[11~15]。锂硫电池的放电过程有两个电压平台(图1b)，上电压平台对应硫向长链多硫化物转化和溶解，低电压平台对应溶解于电解液中的多硫化物进一步锂化成短链多硫化物并沉淀到电极上。这种固相—液相—固相的转变使锂硫电池与其他电池系统不同，在实际应用中也存在着更大的挑战[16, 17]。

图1

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图1   锂硫电池的电化学反应原理和充放电曲线示意图[7,10]

Fig.1   Electrochemical reaction schematic diagram (a) and charge-discharge profile (b) of lithium-sulfur battery[7,10]


首先，单质S和放电产物Li2S的导电性能较差(25℃下分别为5×10-30 S·cm-1和10-13 S·cm-1)降低了电子迁移速率和活性物质利用率以及反应动力学[16]。其次，在放电过程中在正极侧生成的可溶性多硫化物(尤其是高阶多硫化物)通过隔膜扩散到负极，与Li金属发生寄生反应还原成低阶多硫化物和不溶性Li2S2/Li2S。生成的还原性硫化物集中在负极侧与其他溶解的多硫化物重新结合而扩散回正极，并在充电过程中重新氧化为原始的Sm2-(2≤m≤8)。中间体在正负极间的迁移和还原氧化，形成严重的“穿梭效应”(图2)[17,18]。这不仅使活性硫物质的损失，还因为放电产物在负极的聚集使其表面状态恶化，使电池的容量衰减、库伦效率和倍率性能降低[19~21]。此外，单质S和Li2S的密度不同使充放电过程中发生约80%的体积收缩与膨胀，材料结构的破坏使导电基体坍塌和电化学性能降低[22,23]。其中穿梭效应能否受到抑制是影响锂硫电池性能的重要因素。

图2

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图2   “穿梭效应”的示意图[19]

Fig.2   Diagram of "shuttle effect"[19]


针对锂硫电池的不足之处，科研人员对硫正极进行优化，制备出一系列硫基复合材料，包括硫/碳复合材料、硫/金属化合物复合材料和硫/导电聚合物复合材料等。碳材料的多种维度使其孔容、孔径和比表面积各异，在负载硫和提供反应活性位点方面有很高的研究价值[24]。对碳材料进行杂原子掺杂，利用杂原子对多硫化物的吸附和提高导电性，可进一步提高界面反应动力学[25]。与常规碳材料相比，金属化合物，包括金属氧化物、金属硫化物和金属氮化物等，其表面有更多的极性位点，亲硫性更强。金属化合物良好的本征导电性能和较高的固有密度，可提高锂硫电池的体积能量密度[26]。导电聚合物是一种具有一定电学导电性的高分子聚合物，其疏松的结构可以吸附并容纳多硫化物，良好的弹性和柔韧性则可维持正极材料的结构稳定，因此也在锂硫电池中得到了应用[27]。作为新型过渡金属碳/氮化物二维纳米层状材料的MXene材料，近年来也在锂硫电池中得到了应用，本文将对此详细综述。

1 MXene材料

MXene是过渡金属碳化物和碳氮化物等新型二维材料的简称，具有优异的导电性、丰富的表面极性官能团和优秀的机械柔性[28]。MXene(记作M n+1X n Tx)材料是用选择性蚀刻分子式为M n+1AX n 的MAX相中结合力较弱的A位材料制备的(图3)[29]，其中M是过渡金属，如Ti或V；A主要是IIIA或IVA族元素，如Al、Si和Ga等，X则是C或/和N[30,31]。常用的蚀刻剂为氢氟酸(HF)或盐酸(HCl)与氟化物的混合溶液(图4)[32，35]。反应除去A元素后在A元素原本的位置和M层表面产生官能团Tx(Tx：-OH、-F和-O)。

图3

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图3   MXene蚀刻前后的SEM图片[31]

Fig.3   SEM images of MXene before (a) and after (b) eatching[31]


图4

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图4   MXene的结构形成示意图[35]

Fig.4   Structural formation diagram of MXene[35]


锂硫电池体系中理想的载硫体需具有高导电性、对多硫化物的强亲和力以及加速硫物种氧化还原的能力，以提高锂硫电池反应动力学和抑制穿梭效应。作为载硫体时，MXene材料的二维层状结构对硫物种能进行有效的物理限制，高电导率提供了优异的导电骨架特性，能促进电子的快速转移和硫物种间的转化。末端的金属位点通过强路易斯酸碱和极性作用锚定多硫化物[33]，表面的极性官能团则利用极性作用阻止多硫化物的溶解。此外，经过改性优化的MXene纳米片还可用作锂硫电池体系的隔膜。与常规的聚丙烯(PP)隔膜相比，MXene的高导电性和强多硫化物吸附能力可提供更加彻底的氧化反应和更快的反应动力学，降低活性物质的损失和容量衰减。但是，范德华力使MXene材料容易出现重堆积的现象，会导致离子的传输通道变少，捕获多硫化物和电催化的功能表面大量损失，最终降低锂硫电池的综合性能和使用寿命。为了解决这一问题，可从二维结构出发，在保留结构特征的基础上进行MXene材料的性能改进，包括在表面进行杂原子掺杂[34]、附着新型材料[35]和制造缺陷[36]等；也可以构造三维MXene材料，利用其互联结构和良好的离子传输通道[37]提高反应动力学和活性物质利用率，从而提高锂硫电池的性能的。

2 二维MXene复合材料

为了解决锂硫电池存在的库伦效率低、循环寿命差等不足，合适的载硫基体极为重要。二维纳米材料的高导电性、大表面积和优异的机械柔性，使其适用于锂硫电池[38]。其中二维MXene材料独特的表面官能团保证了快速的电子转移和对多硫化物的有效吸附[39]，超薄结构使大量的缺陷和丰富的边缘暴露在电解液中，能捕获多硫化物其边缘还能改变电子转移路径，具有一定催化作用[40]。因此，多种类型的二维MXene材料已经应用于锂硫电池。用原子掺杂修饰碳材料表面，能优化材料的极性表面性质[41]。

基于此类研究，Bao等[34]利用静电将带正电荷的三聚氰胺插入带负电荷的MXene薄片中，热退火后制备出褶皱的氮掺杂MXene纳米片(Crumpled N-Ti3C2T x )(图5)。三聚氰胺作为氮源的同时还起隔膜作用，缓解了MXene材料的重堆积。

图5

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图5   褶皱N-Ti3C2T x /S纳米片的合成示意图[37]

Fig.5   Synthesis schematic of the crumpled N-Ti3C2T x /S nanosheets[37]


退火产生的层次化孔隙使氮掺杂进材料的活性中心，极大地提高了纳米片的表面性能。载硫后，复合正极在0.2C下的首次循环放电容量高达1144 mAh·g-1，200次循环后的容量保持率为83%。倍率为2C时初始容量和1000次循环后的容量分别为825 mAh·g-1和610 mAh·g-1，全面优于未掺氮的对照组。Yang等[42]用熔盐法将钴纳米颗粒嵌入MXene材料，用硫脲原位硫化使钴纳米粒子转化为CoS2并掺入氮元素(图6)。由此制备的氮掺杂MXene-CoS2复合材料(N-MX-CoS2)具有强耦合杂化结构，氮的成功掺杂增强了材料对多硫化物的吸附能力而产生了更多催化位点。双功能异质结的协同效应抑制了多硫化物的穿梭，提高了氧化还原动力学。以硫/碳纳米管(CNT)混合物作为正极，该复合材料用作隔膜组装的电池，具有在1C下1031 mAh·g-1和4C下775 mAh·g-1的优秀初始性能，1C的长循环中每圈的容量衰减率只有0.052%。同样是构造异质结，Meng等[43]将共价三嗪骨架(CTF)纳米片固定在Ti3C2纳米片(TNS)上构建了CTF/TNS异质结构(图7)。异质结先通过物理约束实现了高质量负载，又用Ti-S和Li-N键实现了对多硫化物的双位化学锚定。硫的质量含量为76%时，正极在0.2C下获得了1441 mAh·g-1的高可逆容量，面积硫负载量提高至5.6 mg·cm-2时100次循环后的容量保持率达到94%。金属氧化物的强极性使其具有优异的捕获多硫化物的能力，其中TiO2的应用最为广泛。Qiu等[44]在MXene纳米片上制造了有序的TiO2纳米阵列，比原位氧化保留了更优异的导电性和柔韧性。如图8所示，丰富的中孔有利于电解质的运输和硫物种的容纳，路易斯酸碱和极性相互作用为复合材料提供了强大的物理屏障和化学吸附，极大地抑制了穿梭效应。协同作用下的正极在2C下循环500次后保持了612.7 mAh·g-1的容量，每个循环的容量衰减率只有0.058%。

图6

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图6   硫脲原位硫化示意图[45]

Fig.6   Schematic diagram of the in-situ sulfidation by thiourea[45]


图7

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图7   CTF/TNS异质结构示意图[46]

Fig.7   Schematic diagram of CTF/TNS heterostructure[46]


图8

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图8   MXene@TiO2纳米阵列的SEM图[47]

Fig.8   SEM image of MXene@TiO2 nanoarrays[47]


金属有机框架材料(MOF)及其衍生物具有明确的多孔结构，自带的杂原子掺杂和活性金属中心有限制多硫化物和催化双重效应[45]，但其不连续颗粒状结构在充放电过程中的聚集和分散在一定程度上影响了电池的性能。Ye等[35]用MOF材料在掺氮MXene纳米片上自组装，用原位硒化制备出双金属硒化物(CoZn-Se@N-MX)(图9)。MXene作为催化底物可实现快速的电子和离子传输，防止硒化物在多硫化物转化中的聚集而硒化物反过来阻止了纳米片的重堆积，保留了更多的活性表面，加速了氧化还原反应。以S/CoZn-Se@N-MX为正极的锂硫电池在0.1、0.2、0.5、1、2和3C下的可逆放电容量分别为1500、1412、1159、1050、910和844 mAh·g-1。硫是面积负载量为2.9 mg·cm-2时电极在0.2C下的首次循环依然有1147 mAh·g-1的容量。Jiang等[46]用ZIF-67在MXene纳米片上的原位成核和热解合成了多孔碳修饰含氮MXene纳米片(N-Ti3C2/C)，将其涂覆到PP膜上制备出新型改性隔膜。由ZIF-67热解均匀修饰的MXene纳米片避免了重堆积并有稳定的二维几何结构，大量的活性路易斯酸性表面和氮原子的掺入使纳米片具有优异的化学吸附能力。隔膜在抑制多硫化物穿梭的同时，还控制了锂负极中的枝晶生长。面积硫负载量为3.4 mg·cm-2时，电极在0.5C下循环500次后的容量保持在716 mAh·g-1。高硫负载下的优异性能，使其具有极大的实际应用价值。

图9

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图9   CoZn-Se@N-MX的FESEM图[38]

Fig.9   FESEM images of CoZn-MOFs@MX[38]


孔隙率，与活性中心的引入和电子离子传输通道的构建有密切的关系[47]。在二维形态的基础上创造多孔结构，不仅可打开堵塞的通道还能引入更多的边界和缺陷，为捕获多硫化物提供更多可利用的表面和活性中心，使氧化还原转化效率提高。Xiong等[36]用部分氧化再蚀刻去除的方法制备出带孔的MXene纳米片，与CNT发生静电吸附后沉积在PP隔膜上可制备出MXene改性隔膜。以硫/导电炭黑混合物作为正极组装成的锂硫电池，具有显著的循环稳定性。在0.5C下电池的初始容量为1105 mAh·g-1，150次循环后保持在801 mAh·g-1。将不同的孔径与孔密度进行实验对照，证实了适当的孔隙率能保证正极稳定的电化学反应，也能限制多硫化物的溶解。

总之，以二维MXene材料为主体构建的锂硫电池具有增强的电化学性能，其比容量和循环稳定性比常规锂硫电池体系有一定的提高，但是还有不可忽视的缺陷。首先，片状结构的载硫量有限，不利于电池性能的进一步提高；其次，二维材料的重堆积现象依然存在；最后，二维的层状形态产生了较大的离子横向传输距离，使反应动力学降低[48]。因此，构建一种多孔、高比表面积、短离子传输距离的三维结构极为重要。

3 三维MXene复合材料

与二维MXene材料相比，三维结构能负载并容纳更多的硫，也有更多的位置锚定多硫化物。三维材料各部连结与支撑的特性能防止MXene纳米片的重新堆叠，互联导电网络可使锂离子的传输速度较高[49]。作为载硫体，三维材料有足够多的接触位点和良好的电解液渗透性，有助于提高硫的利用率。这些优势特性，可使三维MXene材料用作锂硫电池正极材料。根据结构进行分类，三维MXene复合材料大体上可份为三维层状结构和三维球状结构。

3.1 三维层状结构

3.1.1 表面修饰构建层状结构

三维层状结构继承了二维材料的优点，与叠层二维纳米片相比，三维多层结构可避免MXene纳米片的重新堆叠，实现锂离子的高速传输。单一材料锚定多硫化物的能力有限，不能很好地缓解穿梭效应和加速硫物种的转化，须在MXene纳米片表面引入复合层。复合层的加入增大了层间距，使材料具有更大的暴露面积和储硫空间，在提高硫负载的同时还将多硫化物限制在正极中抑制了穿梭效应。

Bao等[50]用插层法将还原氧化石墨烯(RGO)插入MXene纳米片中，制备出三明治结构的3D MXene/RGO杂化纳米片(图10)。MXene的亲水表面能结合多硫化物，而RGO能阻止MXene纳米片的重堆积，因此载硫量70%的正极在0.5C下的初始容量达到了1144.2 mAh·g-1，300次循环后保持在878.4 mAh·g-1。相对而言，RGO的非极性与多硫化物间的作用力较弱，因此Zhang等[51]使用MoS2带正电的S空位与带负电的多硫化物之间的强结合作用合成了3D分层的MXene/MoS2-C纳米化合物(图11)。MoS2覆盖在MXene材料上，丰富的正电缺陷能捕获多硫化物，分层多孔结构则能在缓冲体积膨胀的同时提供极短的离子扩散路径，使其电化学性能显著提高。作为正极时，0.5C下经历300次循环后每个循环的容量衰减率只有0.07%。Wang等[52]用一步水热法将阳离子钴(Co)掺杂到MoSe2中，与MXene材料原位杂化后制备出Co-MoSe2/MXene纳米片，加入低含量的氧化石墨烯(GO)制备出以Co-MoSe2/MXene为载硫主体的高密度正极(图12)。Co的掺入优化了MoSe2的电子结构和反应位点，提高了材料的催化活性和极性作用。GO在反应中还原成石墨烯，避免了导电添加剂的加入并间接提高了正极的硫含量。正极具有1.88 g·cm-3的体积密度和8.7 mg·cm-2的面积硫负载量，并在0.1C下获得了1454 mAh·g-1的高初始容量和3659 Wh·L-1的高体积能量密度。Wen等[53]以四氟硼酸铵(NH4BF4)和盐酸(HCl)的混合物对MXene材料进行蚀刻处理，使其表面产生大量AlF3纳米粒子，并用一层Ni(OH)2纳米片包覆其整体(图13)。AlF3粒子不仅阻止了材料的重新堆叠还通过极性作用吸附多硫化物，使Ni(OH)2纳米片发挥了渗透电解液和抑制多硫化物扩散的双重作用。复合正极具有优异的电化学性能，包括0.2C下1184 mAh·g-1的高可逆容量和4C下635 mAh g-1的倍率性能。在1C下循环1000次，每次循环的容量衰减率仅为0.048%。Zhang等[54]用聚二烯丙基二甲基氯化铵(PDDA)对活性碳进行功能化处理，制备出C/PDDA纳米粒子。这种粒子与蚀刻后的MXene材料发生静电吸附，真空过滤后C/PDDA均匀分布在MXene纳米片之间的三维材料(图14)。这一结构，除了防止MXene纳米片的重堆积和提高电子导电性，还避免了物理层面的多硫化物穿梭和相邻官能团之间的化学吸附。以0.2C进行循环，正极的初始放电容量达到了1016.8 mAh·g-1，在0.2C到3C的区间内放电容量分别为1001.5、859.4、772.7、720.4和672.5 mAh·g-1。

图10

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图10   MXene/RGO纳米片的合成示意图和SEM图[52]

Fig.10   Synthesis schematic and SEM image of MXene/RGO nanosheets[52]


图11

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图11   MXene/MoS2-C纳米复合材料的SEM和HRTEM图像[53]

Fig.11   SEM and HRTEM images of MXene/MoS2-C nanocomposite[53]


图12

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图12   (浸硫)Co-MoSe2/MXene合成过程示意图[54]

Fig.12   Schematic diagram of the synthesis process of (sulfur-impregnated) Co-MoSe2/MXene[54]


图13

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图13   Ti3C2T x @AlF3和分层Ti3C2T x @AlF3/NH复合材料示意图[56]

Fig.13   Schematic diagrams of Ti3C2T x @AlF3 and layered Ti3C2T x @AlF3/NH composites[56]


图14

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图14   dTCP材料的结构示意图和横截面SEM图片[57]

Fig.14   Schematic diagram of the structure of dTCP and cross-sectional SEM image


3.1.2 通过分区构建层状结构

三维MXene材料本体依然缺少对多硫化物的物理限制，两者接触的不足影响了后续反应中多硫化物的高效转换。构建不同材料组成的分区结构，一方面可最大程度的发挥每个部位的特殊功能，还有在协同效应下发挥很好的物理限制和化学吸附双重作用。Zhang等[55]用二维纳米片对多硫化物的阻隔作用，提出了一种平行互联的MXene/碳纳米管复合材料(PA-MXene/CNT)并用单向冷冻干燥法成功制备出来。定向生长的冰晶可使MXene纳米片相互连接在碳纳米管堆积中，形成微观上的层状结构(图15)。此结构对多硫化物具有极佳的物理封闭能力，CNT在保证良好导电网络的同时有效阻止了材料的重堆积，很好的抑制了高硫负载下的穿梭效应。面积硫负载量为7 mg·cm-2时，电池在0.5C下循环800次后容量保持在570 mAh·g-1，面积硫负载量升至10 mg·cm-2时0.5C下历经300次循环其比表面积容量依然大于6 mAh·cm-2，每次循环的容量衰减率仅为0.01%。Tian等[56]用静电纺丝法将超声处理的MXene纳米片限制在多通道纳米纤维(MCNF)中，碳化处理后制备出MXene@MCNF复合材料。多通道的空心结构保证了实现高硫负载和缓冲体积变化所需空间，碳纳米纤维作为多硫化物的物理屏障也让分层的MXene纳米片维持三维导电框架，有效缓解了重堆积现象。因此，载硫后的正极在0.2C时具有1177 mAh·g-1的高容量，在高倍率下也有优异的循环稳定性。Liu等[57]以TEMPO氧化的碳纳米纤维(T-CNF)和还原氧化石墨烯(RGO)为框架，用低温水热法将MXene纳米片均匀涂覆其上构建了类似钢筋混凝土的结构。T-CNF和RGO确保了优异的机械强度和高速离子扩散路径，包覆的MXene层则提供了强导电性和对多硫化物的有效吸附。电极在0.1C和5C下的初次循环其容量分别为1470 mAh·g-1和744 mAh·g-1，倍率性能突出。Hou等[58]设计了一种分区层状结构，将正极分为硫源、电催化和多功能吸附区(图16a)。三区分别由嵌入石墨烯中的升华硫、负载TiN的碳纳米纤维(CNFs)和MXene构成，协同作用之下实现了高硫负载下优秀的长循环性能。硫负载量为6.2 mg·cm-2时，0.1C下循环100次容量保持在886.9 mAh·g-1，比表面积容量稳定在5.5 mAh cm-2。如图16b所示，硫含量高达10 mg·cm-2时电池在0.05C下其初始容量达到1141.4 mAh·g-1，对应于7.3 mAh·cm-2的高比表面积容量。在整个循环过程中库伦效率(CE)维持在96.1%的高位，体现了超高面积负荷下优秀的活性硫利用率。Xu等[59]用水热法在氢氟酸(HF)蚀刻的MXene纳米片上自催化生长碳纳米管(CNT)。由于不添加额外的催化剂，合成的Ti3C2T x -CNT复合材料呈现高度集成的形貌(图17)。水热反应催化了碳纳米管的原位生长，致密的网状结构保证了足量的电解液渗透、畅通的离子传输和丰富的反应活性位点。电极在0.2C下的初始放电容量为1368 mAh·g-1，电流密度升至 10C仍有251 mAh·g-1的容量，切换回0.2C后能恢复至1052 mAh·g-1。高硫含量和充放电电流是锂硫电池日后商用的一个重要条件，在材料制备过程中实现材料形状的可调和根据作用分区设计也开拓了MXene材料在锂硫电池系统中的应用前景。

图15

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图15   PA-MXene/CNT的单向冷冻干燥制备示意图[58]

Fig.15   Schematic of synthesizing PA-MXene/CNT with unidirectional freeze-drying process[58]


图16

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图16   (a)“三区”示意图和(b)硫负载量10 mg·cm-2时0.05C下的循环性能图[61]

Fig.16   (a) Schematic diagram of the “three regions” and (b) cycling performance of the cathode at 0.05C for sulfur loading of 10 mg·cm-2[59]


图17

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图17   Ti3C2T x -CNT复合材料的结构示意图和SEM图片[62]

Fig.17   Structural schematic and SEM images of Ti3C2T x -CNT composites[62]


三维层状复合结构扩展了层间距防止了层间堆叠，在实现高硫负载的同时还限制多硫化物。但是，大部分复合材料在合成过程有方向不可控和层间距不均匀的不足，且大多呈现开放体系，在长期循环中阻止多硫化物溶解的能力有限，难以得到实际应用。

3.2 三维球状结构

3.2.1 中空球体复合结构

与层状结构相比，球形结构具有较大的内腔、坚固的壳体和互联的网格支架。较大的内部空间可以储存大量的硫，还能缓解充放电过程中体积膨胀和收缩的应力。外壳作为物理屏障，可抑制多硫化物的扩散。Song等[60]用三聚氰胺甲醛(MF)为模板设计了多孔氮掺杂的MXene空心球(P-NTC)(图18)。三维框架为多硫化物的化学捕获和催化转化提供了充足的位点，氮掺杂的协同作用促进了多硫化物的分解与转化。在强化学吸附和对氧化还原的高效催化下，S/P-NTC正极具有较高的初始容量、倍率性能和长期循环稳定性。在高硫负载下制作的软包电池，依然保持着优异的电化学性能。Wang等[61]以聚甲基丙烯酸甲酯(PMMA)为模板进行喷雾干燥使MXene表面产生氧化位点，退火后选择性蚀刻Ti-O键制备出三维e-Ti3C2微球。高孔隙率的微球可提供高效的电荷传输和电解质渗透路径，表面的不饱和配位键和丰富边缘能显著增强多硫化物的化学吸附和催化转化。融硫后的正极可提供0.2C下1205.9 mAh·g-1的高放电容量和5C下772.4 mAh·g-1的高倍率性能。

图18

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图18   P-NTC复合材料的MF模板合成示意图[63]

Fig.18   Schematic illustration of the MF-templating synthesis of P-NTC composites[63]


在构建并利用球形结构进一步引入具有大比表面积和大孔体积的轻质碳材料，可更好避免材料的重堆积，保持优异的导电性和循环稳定性。Xiong等[62]用MXene中带负电荷的官能团均匀生长ZIF材料，热解后制备出钴氮共掺杂的含碳纳米管空心MXene球(hollow Co-CNT@MXene)(图19)。杂原子的引入增强了材料的化学吸附能力，导电碳纳米管支撑的钴进一步加速了硫物种的转化。载硫后的电极具有出色的倍率性能，在2C和3C下分别具有893.3和733 mAh·g-1的初始容量。Wang等[63]先在碳纳米管表面原位合成丝状硫形成多孔微球，然后将超薄MXene纳米片包裹其上，形成了三维S-CNT@MXene笼状结构(图20)。碳纳米管和MXene构建的导电网络，降低了固有电阻和提高反应动力学。MXene涂层有效抑制了穿梭效应并提供硫化锂成核位置，笼内充足的空间可适应体积变化和使硫化锂均匀沉积。固定在导电碳纳米管上的硫纳米颗粒比熔融状态下的块状硫实现了更优的硫利用率和锂离子扩散速率。材料直接用作正极，在0.1C下获得了1375.1 mAh·g-1的容量，在4C下循环150次后保留了656.3 mAh·g-1的容量。

图19

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图19   中空Co-CNT@MXene的SEM和EDS图像[65]

Fig.19   SEM and EDS images of hollow Co-CNT@MXene[65]


图20

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图20   S-CNT和S-CNT@MXene球的SEM图片[66]

Fig.20   SEM images of S-CNT and S-CNT@MXene spheres[66]


3.2.2 实心球体复合结构

作为另一种球形结构，实心球体比中空球体有更高的机械强度与密度，有助于提高电极工作中的结构稳定性和体积能量密度。Zhao等[64]利用静电作用使粉碎的MXene纳米片包裹涂有聚多巴胺(PDA)的硫球再分散到大片的MXene材料中，创造了三维层状包覆球体的先进结构(图21)。PDA涂层防止了破碎MXene纳米片的重堆积，保留了足够的活性位点，大片的MXene材料则充当导电支架并将球体材料紧密容纳其中。这种结构使电流密度从0.12C提高到3C，电极保持着典型的充放电平台，25倍的电流密度下保有了起初倍率下55%的容量(从1560 mAh·g-1降至854 mAh·g-1)，表现出极佳的倍率性能。Wang等[65]合成了掺氮碳纳米管(CNT)/MXene复合材料(N‑Ti3C2@CNT)并通过喷雾干燥和一步热解成功制备出微球(图22)。含氮的两种材料在微球内相互作用不仅提高了材料的导电性，也增强了对多硫化物的锚定能力，而由CNT支撑的互连多孔网络则保证了电子和离子的高速传输。N-Ti3C2@CNT/S正极在0.1C下有1339.2 mAh·g-1的高可逆容量和1C下每次循环0.016%的低衰减率。

图21

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图21   S@PCL复合材料的合成示意图和横截面的SEM图片[67]

Fig.21   Synthesis schematic and cross-section SEM images of S@PCL


图22

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图22   N‑Ti3C2@CNT复合材料的合成示意图和SEM图片[68]

Fig.22   Synthesis schematic and SEM images of N‑Ti3C2@CNT composites[68]


三维球状结构的优良性能大体上源于以下几点：(1) 微球的高比表面积和多种类型的孔隙产生了充足的活性硫容纳空间，并为多硫化物提供了更多的可吸附位点；(2) MXene和碳材料的高效结合促进了电子在微球中的传输，提高了电化学反应动力学；(3) MXene的高极性和杂原子的掺入有效地将多硫化物锚定在微球中，从而使其具有高度稳定的循环性能；(4) 丰富的边缘活性位点与MXene材料的协同结合加速了多硫化物的催化转化。但是三维球状结构的孔隙率各异，在硫负荷与传输动力学间有较大的差距。大孔结构在储硫能力和缓冲体积变化方面表现优秀，但是因硫的分散性差而产生聚集，降低了硫利用率和反应动力学。松散的小孔避免了硫的大规模聚集，但是造成过量的电解液吸收，加速了多硫化物的溶解，降低了库伦效率低和循环性能差。实心球体可提高整体电极密度但是使其硫负载量降低和倍率能力难以提高。因此，合适的孔隙结构对于三维球状MXene复合材料极其重要。

4 结语和展望

锂硫电池的理论能量密度较高，是一种很有前途的电化学储能器件。MXene材料有导电性高、表面官能团丰富和结构可调等特征，捕获多硫化物及后续处理方面表现突出。本文综述了二维和三维结构的MXene及其复合材料在锂硫电池中的应用。二维材料表面的极性官能团、大量的缺陷和丰富的边缘可实现与多硫化物间的强相互作用，但是范德华力易造成其重堆积，导致低硫负荷、缓慢的离子传输动力学和较差的多硫化物吸附能力。三维结构比二维材料具有扩展的表面积、互联传输通道和优异的结构稳定性。三维层状结构中插层物种的加入使材料有更大的层间距，不仅增加了硫负载量也对多硫化物有更好的限制作用。同时，开放结构的层状纳米片无法限制多硫化物在长期循环过程中的溶解。进一步构建的三维球状结构因孔隙结构差异巨大，很难在储硫能力与反应动力学间取得很好的平衡。在未来的研究中，需要综合上述结构的优点，实现高速反应动力学、优秀的载硫能力、良好的多硫化物限制和高体积能量密度的多重性能要求。

目前，MXene材料在锂硫电池的应用处于初级阶段。(1) 在已经发现的三十几种MXene材料中，常用的类型仅限于Ti3C2和Ti2C，其他种类的MXene在锂硫电池中的应用还十分欠缺。(2) MXene材料在锂硫电池体系中起化学吸附并催化转化多硫化物作用但是其机理还不十分清楚。虽然有研究阐明了MXene表面极性官能团的作用，但是锂硫电池的充放电过程伴随着多种副产物的产生，其间的相互作用以及影响因素尚需深入探究。吸附和催化机理的深入研究是一项重要并且极具难度的任务，需要更加先进的表征技术和理论计算。(3) MXene材料的电化学性能受表面端基和掺杂原子的影响，不同的端基在多硫化物的吸附和转化中的差异很大。通过合适的刻蚀方法使材料的表面性质可控调节，则可提高材料整体的固硫能力和催化活性。(4) 与实验室中常用的纽扣电池相比，商用锂硫电池真正取代成熟的锂离子电池对硫含量和电解液用量的要求更高，还要以更加环保安全的方法大规模制备MXene材料，以满足实际应用所需。MXene材料还有很多待发现的特性，巨大的潜力使其在未来得到高度关注和快速发展，并进一步带动锂硫电池向商用转化。

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[J]. Nano-Micro Letters, 2020, 12(1): 40

DOI      [本文引用: 3]

Scalable fabrication of high-rate micro-supercapacitors (MSCs) is highly desired for on-chip integration of energy storage components. By virtue of the special self-assembly behavior of 2D materials during drying thin films of their liquid dispersion, a new inkjet printing technique of passivated graphene micro-flakes is developed to directly print MSCs with 3D networked porous microstructure. The presence of macroscale through-thickness pores provides fast ion transport pathways and improves the rate capability of the devices even with solid-state electrolytes. During multiple-pass printing, the porous microstructure effectively absorbs the successively printed inks, allowing full printing of 3D structured MSCs comprising multiple vertically stacked cycles of current collectors, electrodes, and sold-state electrolytes. The all-solid-state heterogeneous 3D MSCs exhibit excellent vertical scalability and high areal energy density and power density, evidently outperforming the MSCs fabricated through general printing techniques.

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