过渡金属硫化物改性锂硫电池正极材料

樊潮江, 燕映霖, 陈利萍, 陈世煜, 蔺佳明, 杨蓉

化学进展 ›› 2019, Vol. 31 ›› Issue (8) : 1166-1176.

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化学进展 ›› 2019, Vol. 31 ›› Issue (8) : 1166-1176. DOI: 10.7536/PC190140 CSTR: 32298.14.PC190140

过渡金属硫化物改性锂硫电池正极材料

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Transition-Metal Sulfides Modified Cathode of Li-S Batteries

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摘要

锂硫电池(LSBs)由于单质硫正极具有超高能量密度(2600 Wh/kg)和超高理论比容量(1675 mAh/g),且环境友好、成本低廉,被认为是最有前景的储能体系之一。然而,硫正极的绝缘性和严重体积膨胀以及多硫化物(LiPSs)的“穿梭效应”等问题导致活性物质利用率低、循环稳定性差及电化学反应动力不足,严重阻碍了LSBs的商业化发展。最新研究表明,过渡金属硫化物作为载体或添加剂能够显著改善LSBs正极材料的电化学性能。本文从等效/共正极作用、导电性增强作用、LiPSs吸附作用和电化学反应催化作用四个方面梳理了过渡金属硫化物在LSBs正极材料中的改性机理,并指出多元过渡金属硫化物复合﹑纳米结晶和量子化作为增加比表面积和活性位点的方法是过渡金属硫化物用于锂硫电池正极材料的重要发展方向,可大幅提升LSBs的电化学性能。

Abstract

Lithium-sulfur batteries(LSBs) are considered as one of the most promising energy storage systems due to the ultra-high theoretical energy density(2600 Wh/kg) and ultra-high theoretical specific capacity(1675 mAh/g), environmental friendliness and low-cost of sulfur cathode. However, the insulation of sulfur cathode, the volumetric strain and the “shuttle effect” of polysulfides lead to problems such as low utilization rate of active materials, poor cycle stability and low redox kinetics, which seriously hinder the commercial development of LSBs. Recent studies have shown that transition metal sulfides as host or additives can significantly improve the electrochemical performance of LSBs cathode materials. In this paper, the modification mechanism of transition metal sulfides in LSBs cathode materials is reviewed from four aspects: equivalent/common positive electrode effect, conductivity enhancement, LiPSs adsorption and electrochemical reaction catalysis. It is pointed out that multi-transition metal sulfides composite, nano-crystallization and quantization as important areas for increasing the specific surface area and active sites should be used as transition metal sulfides for lithium-sulfur battery cathode materials, which can greatly improving the electrochemical performance of LSBs.

关键词

锂硫电池 / 过渡金属硫化物 / 电化学性能 / 穿梭效应 / 吸附作用 / 催化作用

Key words

lithium sulfur batteries / transition-metal sulfides / electrochemical performance / shuttle effect / adsorption / catalysis

引用本文

导出引用
樊潮江, 燕映霖, 陈利萍, . 过渡金属硫化物改性锂硫电池正极材料[J]. 化学进展, 2019, 31(8): 1166-1176 https://doi.org/10.7536/PC190140
Chaojiang Fan, Yinglin Yan, Liping Chen, et al. Transition-Metal Sulfides Modified Cathode of Li-S Batteries[J]. Progress in Chemistry, 2019, 31(8): 1166-1176 https://doi.org/10.7536/PC190140
中图分类号: TM911.3;O646   

Contents

1 Introduction
2 Transition-metal sulfides equivalent/co-cathode
2.1 Equivalent cathode
2.2 Common-cathode
3 Conductivity enhancement
3.1 Transition-metal sulfides addition
3.2 Structural modification
4 Adsorption of polysulfides
4.1 Computational study of binding energy
4.2 Adsorption mechanism
4.3 Influencing factors of adsorption
5 Catalysis of electrochemical reaction kinetics
6 Conclusion

1 引言

太阳能、风能和潮汐能等新型能源是应对全球能源危机的理想方案。然而受限于时间与空间的间歇性,新型能源的应用严重依赖于高效储能系统[1]。另外,随着电子器件、电动汽车的迅速发展,便携式高功率储能系统的需求急剧上升。现有的锂离子电池难以满足日益增长的需求[2]。锂硫电池(LSBs)单质正极超高理论能量密度(2600 Wh/kg)和理论比容量(1675 mAh/g),且成本低、资源丰富和环境友好等优点,受到了广泛关注与研究,有望成为下一代高效高功率储能体系[3]
然而,LSBs的发展和应用仍面临着诸多问题,如单质硫和Li2S的绝缘性、中间体硫化物(LiPSs)易溶于电解液导致的“穿梭效应”以及循环过程正极材料的体积膨胀[4]。特别是“穿梭效应”,极大地限制了LSBs实际比容量、循环寿命和倍率性能[5]。针对以上问题,研究人员已经从新型电解液的开发[6,7]、锂金属负极表面修饰与新型负极的开发[8]正极的复合改性[9,10,11]等多方面进行了大量研究工作。其中,正极材料的改性主要集中在将单质硫与导电材料相复合,从而提高导电性和稳定性,并利用物理和化学吸附缓解LiPSs在电解液中的穿梭,从而减少活性物质的损失,提高电化学性能。目前主流的复合正极材料有硫/碳复合材料[12]、硫/金属化合物复合材料[13]、硫/导电聚合物复合材料等[14]
碳导电材料,包括多孔碳[10]碳纳米管[15]、碳空心球[16]石墨[17]等,作为复合正极最重要的载体,研究最为广泛。虽然这类材料能够有效提高正极导电性,并对LiPSs起到了一定的束缚作用。但由于碳材料非极性表面与极性LiPSs之间相互作用力较弱,对“穿梭效应”的缓解作用非常有限[18]
过渡金属氧化物较强的极性,能够有效吸附LiPSs,有望显著缓解“穿梭效应”,改善LSBs性能。因此,众多金属氧化物(MgO、Al2O3、CaO、La2O3、CeO2等)被用于正极单质载体[19]。然而,由于金属氧化物导电性较差,需外加导电剂导致活性物质比例过低[20]
过渡金属硫化物(TMSs),如TiS2,锂化状态表现出半金属金属的行为,并且具有极性和高导电性,最早用于可充电锂电池的层状插层正极材料并且已经在第一代锂离子电池中商业化[21,22]。近年来,过渡金属硫化物(TMSs,M=Co[23]、Ti[22]、Fe[24]、Ni[25]、Cu[26]、Zn[27]、W[28]、V[29]、Mo[30]、和Mn[31])由于导电性较好,逐步替代金属氧化物,作为载体添加剂引入LSBs正极材料中,不仅提升了正极材料的有效容量,而且显著改善了其循环性能和倍率性能[32]。此外,TMSs作为LSBs隔膜的添加剂、涂层或主体材料等,可提高LSBs的活性材料利用率和循环寿命[33]
本文梳理了过渡金属硫化物在LSBs正极材料中的改性机制,包括等效/共正极材料作用、导电性增强作用、LiPSs的吸附作用和电化学反应催化作用四个方面,并对未来研究方向进行了展望。
图1 过渡金属硫化物用于LSBs促进作用示意图

Fig. 1 Schematic diagram of the effect of transition metal sulfides on the promotion of LSBs

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2 过渡金属硫化物等效/共正极作用

2.1 等效正极作用

过渡金属硫化物用于锂硫电池正极材料的改性机制研究中,Li等[34]提出了硫等效正极材料(sulfur-equivalent cathode material)的概念,即过渡金属硫化物替代传统单质硫,单独作为LSBs正极材料。主要在于过渡金属硫化物本身含有S,可以作为活性物质参与反应并提供容量,而且Mn+离子可以参与到离子反应中,促进反应的发生。研究人员首先对比了晶体MoS2和非晶MoS3作为硫等效正极材料时的电化学性能,结果表明非晶MoS3等效正极,表现出类似硫的行为,在1.9 V左右呈现出放电平台,初始比容量达到667 mAh/g,1000次循环后保持在383 mAh/g(图2d),循环性能优异。而晶体MoS2需要经过一个“激活”过程(首次放电到0.01 V)才能在1.9 V左右出现放电平台(图2b),且比容量仅为277 mAh/g。
图2 MoSx作为LSBs硫等效材料的电化学性能:(a)MoS2等效正极充放电曲线,(b)“激活”后MoS2等效正极充放电曲线,(c,d) MoS3等效正极电化学性能曲线[34]

Fig. 2 Electrochemical performance of MoSx as a sulfur equivalent material for LSBs:(a)MoS2 equivalent positive charge and discharge curve,(b)MoS2 equivalent positive charge and discharge curve after “activation”,(c,d) MoS3 equivalent positive electrode electrochemical performance curves[34]

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NiS2作为LSBs等效正极在1.2~1.3 V和1.9~2.1 V分别观察到氧化峰和还原峰,并且表现出较好的电化学性能(在C/10、C/20和C/50下的初始放电比容量分别为300、450和550 mAh/g,在100次循环后仅衰减23%)[35]
多元金属硫化物的混合材料作为LSBs等效正极,也表现出优异的电化学性能。如CVD法制备的NiS2/FeS泡沫等效正极具有良好的导电性和高度多孔结构,表现出了优异的电化学性能(10 mA/cm3电流下首次放电比容量达到了560 mAh/g)(图3)[36]
图3 NiS2/FeS用作LSBs等效正极电化学性能[36]

Fig. 3 Electrochemical performance of NiS2/FeS as equivalent positive electrode of LSBs[36]

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以上研究结果表明,过渡金属硫化物是一种高能量且稳定性较强的LSBs等效正极材料。然而,并不是所有过渡金属硫化物都具有电化学活性。例如,Co9S8[37]在LSBs典型的测试电压范围内仅表现出13~36 mAh/g的初始放电比容量,性能极差;MoS2在测试电压范围内也几乎没有容量[37]。更为重要的是,目前研究的过渡金属硫化物作为等效正极时,其能量密度和比容量较低,并不能满足实际应用的要求。因此,仍需要针对过渡金属硫化物材料进行更多的研究,以实现综合使用性能的提升。

2.2 共正极作用

为了保持锂硫电池高能量密度和高理论比容量的特征,将过渡金属硫化物与单质硫进行复合后作为正极材料,是过渡金属硫化物作为锂硫电池正极材料的热点。
最早作为锂离子电池插层正极材料之一的TiS2,亦最早用于LSBs共正极。Garsuch等[2, 11]研究了S/C/TiS2球磨复合LSBs电极材料,发现添加TiS2电池循环寿命显著改善,但导致单位活性物质的比容量降低;Su等[38]在硫复合电极中也用TiS2代替碳添加剂获得了与Garsuch等类似的实验结果,即循环稳定性显著增加,但活性物质的容量有所降低。
Liu等[39]将Ni泡沫作为集流体与S复合原位生成NiS2,NiS2/S作为锂硫电池正极材料表现出优异的电化学性能,0.1 C首次放电比容量可以达到1000 mAh/g,相比于NiS2等效正极比容量大幅提高。此外,在超过10个循环后仍然保持880 mAh/g的高容量而没有任何衰减,在70次循环后放电比容量仍保持在850 mAh/g,循环性能也有大幅提升(图4)。
图4 NiS2/S共正极材料的表征[39]

Fig. 4 Characterization and performance of NiS2/S cathode[39]

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CuS/CNTs/S[40],CoS2/rGO/S[41]、WS2/S[42]、CoS2/C/S[43]、NiS/S[44]等均被用作LSBs共正极材料,都表现出了较好的电化学性能。等效/共正极材料为高性能LSBs正极材料的研究与应用开辟了新的思路。因此,在选择LSBs正极材料时,应合理利用过渡金属硫化物和导电基体材料的协同优势,构建综合性能优异的等效正极或共正极材料体系,从而推动LSBs实用化发展。

3 导电性的增强作用

3.1 过渡金属硫化物添加增强正极导电性

载体材料的导电性在LSBs性能中起关键性作用,其不仅影响电子和离子传输速率,还与Li-S转化体系氧化还原动力学密切相关
与过渡金属氧化相比,过渡金属硫化物通常表现出更高的导电性,如表1中所示,过渡金属硫化物的电导率明显高于金属氧化物,特别是CoS2(6.7×103 S/cm),远高于其他过渡金属硫化物。因此,在单质正极中引入导电性优异的过渡金属硫化物作为等效正极或共正极,不仅可以稳定可逆充放电比容量,还可以大大地提高复合正极材料的电导率,增强电化学反应动力,缓解由于单质硫绝缘性对LSBs电化学性能产生的负面影响。
表1 不同过渡金属硫/氧化电导率对比表

Table 1 Different transition-metal sulfide/oxide conductivity

TMSs Conductivity(S/cm) ref
FeS 80 36
Ni3S2 1.8×10-5 49
Co9S8 2.9×102 46
CuS 8.7×102 49
CoS2 6.7×103 33
Graphene 106 50
Graphene oxide 1.7×102 50
MoS2 3.1 47
Fe3O4 4.0×10-3 51
NiO 10-13 55
V2O5 3.7×10-2 45
TiO2 10-10 17
CoO 2.0×10-4 33
Ti4O7 3.2 48
MnO2 10-5 48
WS2 6.7 52
TiS2 30~50 53
VS2 0.1 17
FeS2 0.6 54
Graphite 1~1000 33
例如,将具有高导电性的CoS2引入碳/硫正极中,提高了正极材料的整体导电性,对电子的传输速率有较大的提高,在工作条件下促进了LiPSs与CoS2之间的强相互作用,并且高导电性的CoS2加速了离子和电子的传导,加快了多硫化物的转变,循环效率提高了10%,而且在2000次循环期间具有高放电容量和稳定的循环性能,在2 C下的放电容量衰减率仅为0.034%/循环,并且在0.5 C下实现了1368 mAh/g的高初始放电比容量[33]。优异导电性的CuS在循环过程电解质也会腐蚀铜以释放Cu2+/Cu+氧化反应相结合,促进离子电子转移,加速氧化还原反应,提高电池性能(首次放电比容量高达1300 mAh/g,硫的利用率达到77.6%,而库仑效率接近100%)[49]
然而,过渡金属硫化物的电导率差异很大,既有半金属( msup [33]和TiS2/VS2[45]),也有半导体( msup [45] msup [48])。因此依靠寻找导电性优异的过渡金属硫化物作为锂硫电池正极材料,选择范围过窄,电化学提升的效果较为有限。

3.2 结构改性导电性增强作用

鉴于过渡金属硫化电导率的差异性,探寻其他提升导电性的方法,也是这一领域的研究热点。近年来,研究人员发现通过过渡金属硫化物的微观结构设计能够进一步增强正极材料的导电性。
过渡金属硫化物的纳米化是微观结构设计提高导电性的有效方式之一。纳米尺寸的过渡金属硫化物弥散分布在正极材料之中,能够大大缩短扩散路径,有效降低离子和电子通过电极的传输距离。另外,由于纳米结构的高比表面效应,能够增加活性物质和电解液的接触及浸润性,界面离子和电子传输速率显著提升,从而有效提高活性物质利用率并推动内部电化学反应的进行[56]。例如,纳米化后的Ni3S2内层的Ni金属提供电子传输路径并有效地增加正极电导率及其在Ni金属表面位形成的Ni3S2具有相当低的电阻率而具有更高的电子导电性[36],其较高的电子电导率有效地缓解了“穿梭效应”,提供了有利于电化学环境的低界面电阻。
另外,设计多种微观结构过渡金属硫化载体,也是有效增强导电性的研究思路。Archer等[57]采用CVD法一步合成具有三维多孔结构的TiS2,通过三维结构的Ti泡沫前驱体和S8的热反应形成互连的混合3D正极结构,利用TiS2本身的高导电性改性三维结构设计增强了电子和离子传导性;Yuan等[33]将CoS2石墨烯混合构建成CoS2@石墨烯层状材料,形成层状导电框架到CoS2表面的电子路径,通过层状结构的设计进一步增强正极材料的导电性。
选择导电性优异的过渡金属硫化物,并设计其微观结构在改进复合正极材料的导电性方面非常有效,并取得了显著的成果。然而,锂硫电池的“穿梭效应”仍是阻碍其商品化的核心问题。因此,研究提高LSBs正极单质载体材料对LiPSs的吸附作用,进而有效缓解“穿梭效应”的研究意义非凡。

4 对LiPSs的吸附作用

锂硫电池正极充放电中间产物LiPSs易溶于电解液导致的“穿梭效应”仍是阻碍其商品化的核心问题。相比非极性多孔碳材料,大多过渡金属硫化物具有较强的极性,通过对LiPSs较强的化学相互作用和吸附作用从而抑制“穿梭效应”[8]。研究人员从结合能的计算研究﹑吸附机理和吸附强度的影响因素三方面对过渡金属硫化物进行了许多卓有成效的研究工作。

4.1 结合能的计算研究

多种过渡金属硫化物(MoS2、TiS2、SnS2、Cu2S/CuS、Ni3S2、WS2、VS2、ZrS2)与LiPSs的强极性化学相互作用被认为是LiPSs的Li+金属硫化物的硫离子的键合,结合能越大,相互作用越强,锚定效果越好。因此,研究人员计算了多种过渡金属硫化物和LiPSs/Li2S之间的强的结合能,其计算结果如表2。通过对比研究结果,可以明显地看出,过渡金属硫化物与LiPSs/Li2S之间的结合能远高于石墨石墨烯。如Li2S2石墨结合能仅仅为0.28 eV,而TiS2和Li2S的结合能达到2.99 eV,Co9S8结合能甚至高达6.06 eV,是目前报道的最高结合能。鉴于过渡金属硫化物和LiPSs/Li2S之间的强的结合能,过渡金属硫化物通常被用作载体材料或LSBs中多硫化吸附的中间层[58]。在选择过渡金属硫化物作为LSBs硫载或添加剂时,应注重通过结合能的强度来判断和选择。此外,过渡金属硫化物还可以提供与非极性硫的化学相互作用,从而抑制活性物质硫的溶解。
表2 过渡金属硫化物对LiPSs结合能

Table 2 Binding energy of transition-metal sulfides to LiPSs

TMSs Crystal face LiPSs Binding energy(eV) ref
Co9S8 (002) Li2S2 2.22 35
(202) 3.29
(008) 6.06
Co3S4 (111) Li2S4 2.26 33
Li2S6 1.61
Li2S8 1.68
(220) Li2S4 2.76
Li2S6 2.18
Li2S8 2.18
CoS2 (111) Li2S4 1.97 64
Li2S6 1.01
TiS2 Li2S 2.99
Li2S6 1.02
MoS2 Li2S2 0.82 43
NbS2 Li2S2 0.76
FeS Li2S6 0.87 60, 57
SnS2 Li2S6 0.80 62
VS2 Li2S6 1.04 62
NiS2 (111) Li2S4 2.06 35
Ni3S2 Li2S6 0.72 60
WS2 S8 0.30 27
Li2S8 0.52
Li2S6 0.85
Li2S4 0.80
Li2S2 1.05
Li2S 1.45
ZrS2 Li2S2 2.70 59
TiO2 Li2S 1.66
Graphene Li2S4 0.34 35
Graphite Li2S 0.60

4.2 吸附机理研究

除了Li+与硫离子的结合吸附,还有一些研究人员试图发现其他的过渡金属硫化物与LiPSs/Li2S结合方式。
Zhang等[61]研究发现过渡金属硫化物主要通过硫键与LiPSs相互作用,并且过渡金属硫化物对LiPSs吸附能按元素周期律顺序呈折线型趋势(如图5所示)。该研究很好地在元素周期律的层面上解释了不同金属硫化吸附LiPSs的本质差异,提出了用于筛选硫正极载体材料的标准,为LSBs材料的理性设计与高通量筛选提供了依据。
图5 元素对结合能的影响[61]

Fig. 5 Effect of elements on binding energy[61]

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还有研究认为过渡金属离子与硫离子和Li+离子的偶联性相互作用是过渡金属硫化吸附LiPSs的内在机理。即依赖 Sn2- -Mδ+和Li+-Sδ-结合之间的高亲合力,减少LiPSs的溶解并改善硫正极的循环稳定性[35,47]。例如,Co9S8[48]中的Coδ+ S22- 可以分别和LiPSs中的 S22- 和Li+成键,形成偶联性相互作用,高效地吸附电解液中的LiPSs,抑制LiPSs在电解液中的穿梭,提高电极材料稳定性
另外,FeS2可以与Li2Sn通过化学反应形成活性Li2FeS2+n配合物,可显著减少硫正极中溶解的Li2Sn向外扩散,从而改善锂硫电池的循环性能[62]

4.3 吸附作用的影响因素

过渡金属硫化物与LiPSs之间的相互作用强弱受到多种因素的影响,如元素种类及价态、晶面结构和结合位点等,研究人员基于此也展开了大量工作。
4.3.1 元素及其价态
通过吸附试验,可直观目测过渡金属硫化物添加后,LiPSs溶液的颜色变化,从而检测金属硫化物对LiPSs的吸附作用强弱。图6所示添加不同过渡金属元素的硫化物后,溶液颜色逐渐变浅或变成无色,说明对LiPSs具有不同的吸附能力[33,62,63]
图6 过渡金属硫化物对LiPSs的吸附实验图[33,62,63]

Fig. 6 Adsorption test of transition metal sulfides on LiPSs[33,62,63]

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过渡金属离子的价态对其硫化物与LiPSs的相互作用能力也有较大的影响。比如,具有不同价态的Co9S8﹑Co3S4﹑CoS2对同种多硫化物的结合能不同[35,33]。可以利用金属硫化物的价态来判断其与LiPSs的强相互作用,为研究过渡金属硫化物与LiPSs的吸附作用提供了一种新的研究思路。
4.3.2 晶面结构
晶体结构作为过渡金属硫化物一项重要的参数,也对LiPSs吸附能力和锚定能力具有较大的影响(图7)。根据表2中的详细数据对比可以看出LiPSs与过渡金属硫化物不同晶面结构的结合能不同,且不同的晶面结构对不同LiPSs分子结合能也不同。
图7 金属硫化物对LiPSs的吸附能:(a)Ni3S2对不同LiPSs吸附能的吸附能,(b)不同金属硫化物对不同类型LiPSs吸附能,(c)不同晶面结构硫化物对LiPSs吸附[35,63]

Fig. 7 Adsorption energy of transition metal sulfides to LiPSs:(a)Adsorption energy of Ni3S2 for adsorption energy of different LiPSs,(b)Adsorption energy of different metal sulfides for different types of LiPSs,(c)Adsorption energy of LiPSs by sulfides of different crystal plane structures[35,63]

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因此,应注重设计具有可控晶体结构的过渡金属硫化物或者利用不同过渡金属硫化物对不同LiPSs分子的具有协同效应的两种或多种金属硫化物进行复合,已获得最佳的吸附能,最大化TMSs与LiPSs的相互作用,优化LSBs性能。
4.3.3 结合位置
除了不同元素对LiPSs结合能力有差别之外,过渡金属硫化物的晶面晶棱、顶点等不同位置对LiPSs结合能力的强弱差异巨大。如MoS2的边缘Mo原子和S原子与Li2S的结合能分别高达4.48和2.70 eV,远高于MoS2的中心位置原子(0.87 eV)[43]。因此二维层状过渡金属硫化物封装Li2S复合材料利用结构的优势,实现了对Li2S的强吸附,吸附能可以达到2.99 eV,大幅提高了LSBs材料的高硫量负载和面积容量[43]。此外,CuS量子点[36]、纳米球形NiS[65]、层状纳米 msup [26]泡沫3D结构 msup [57]等边缘结构丰富的过渡金属硫化物拥有大量的活性吸附位点,能够从物理及化学方面锚定活性物质硫并吸附LiPSs。因此,设计优化过渡金属硫化物的微观结构,能够有效吸附LiPSs更高效的结构缓解“穿梭效应”(图8)。
图8 TMSs对LiPSs的吸附模型示意图:(a)物理模型[66],(b)化学模型[67]

Fig. 8 Schematic diagram of the adsorption model of TMSs on LiPSs:(a) physical model[66],(b) chemical model[67]

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通过以上诸多实验研究与理论研究的相互印证,进一步证实了过渡金属硫化物对LiPSs的强吸附作用,且可以通过筛选强吸附元素、调节最优价态、调控晶面结构和结合位置等多种方法提升其对LiPSs吸附作用,进而更加有效地抑制了“穿梭效应”,推动LSBs的产业化与商品化。

5 电化学反应动力催化作用

过渡金属硫化物作为LSBs硫载体添加剂对LiPSs电化学反应的催化作用亦是近年来的研究热点。LSBs体系中是一个多电子氧化还原反应过程:一个S8分子需要得到16个电子才能被完全还原为Li2S。氧化还原过程中涉及到多种多硫离子自由基,而且,多硫离子自由基之间十分复杂的相互转化反应关系,使得锂硫电池体系内部电化学反应更加复杂。已有研究表明加入过渡金属硫化添加剂,能够平衡添加剂对LiPSs的吸附并减少LiPSs向电解质的扩散,从而催化锂硫电池内部电化学氧化还原反应,有效地提高反应动力学,改善电池性能,包括更高的初始放电比容量、更低的极化,更高的库仑效率,优异的循环稳定性和倍率性能[41]
电化学反应动力学主要由两个因素决定:首先,足够的结合亲合力允许吸附具有足够的活性物质表面覆盖;其次,在液-固边界上的有效电荷转移促进吸附物在氧化还原过程中的电子传输(如图9所示)[68]。在电化学反应过程中,固体产物的表面反应和随后的沉积可以在相对丰富的表面位点上进行,并且加速了固-液转化过程中的电子和离子传输,这是极性过渡金属硫化物作为LSBs正极具有电化学动力学催化作用的的根本原因。
图9 极性导体材料在增强电化学反应动力学中的作用[68,69]

Fig. 9 The role of polar conductor materials in enhancing electrochemical reaction kinetics[68,69]

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CoS2@石墨[48]正极材料中既包含了CoS2与LiPSs的静态吸附作用,又包含了动态加速LiPSs转化的电化学反应。对比图10a和图10b,CoS2的添加从两方面加快了LiPSs在其表面的可逆转化,对电化学反应的催化作用主要表现在极性CoS2对LiPSs的结合亲合力,极大程度上使LiPSs的吸附扩散达到协调,并提供了更多的活性位点,使得保证足够的活性物质参与电化学化学反应。此外,Co2+的引入进一步增强了电荷的有效转移和电子的传输。共同作用促进了高阶多硫化物向低阶多硫化物(Li2S8↔Li2S6↔Li2S4)的转化。因此,CoS2添加的石墨烯硫正极相对于无CoS2的硫正极电池能量效率提高了10%。在2 C下容量衰减率仅为0.034%/循环,在0.5 C下的初始放电比容量高达1368 mAh·g-1
图10 CoS2添加对氧化还原反应催化作用示意图[33]

Fig. 10 Schematic diagram of the catalytic action of CoS2 on redox reaction[33]

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过渡金属硫化物的添加对于LiPSs转化反应中最重要的过程之一的Li2S氧化过程(Li2S沉积的逆过程)也起到了较大的催化作用[43]。比如,VS2与rGO混合正极材料中具有较丰富活性位点,允许更多的活性物质硫参与到反应中来,促进Li2S的氧化。过渡金属硫化物还加速了反应过程中物质的转换,与具有光滑rGO表面和不规则Li2S颗粒的循环rGO/S正极相比,循环rGO-VS2/S表现出均匀沉积在rGO-VS2上的Li2S而没有散落的Li2S颗粒,对液-固转化过程具有较大促进作用[17]
除了单一过渡金属硫化物对电化学反应动力学的催化作用研究外,Cui等[62]对一系列过渡金属硫化物进行了对比实验研究,从图11a可以看出,添加VS2、CoS2和TiS2的S@G/CNT正极表现出比Ni3S2、SnS2、FeS更快的扩散速率,更好的反应动力学及更高的反应转化催化活性。实验后发现其主要原因在于Ni3S2、SnS2和FeS的扩散势垒比CoS2,VS2和TiS2扩散势垒大0.1 eV(图11b),这很好地解释了为什么含CoS2、VS2和TiS2正极显示出更好的反应动力学。较高的反应动力学因素赋予载体材料表面较快的扩散速率,使锂和硫之间的氧化还原反应变得更容易进行。因此,含有VS2,TiS2和CoS2正极具有更高的结合能和更低的扩散活化能垒,而具有更好的电化学性能(图11c)。
图11 各种金属硫化表面的锂离子扩散特性及机理分析:(a)氧化过程CV峰值电流与扫描速率的平方根,(b)各种金属硫化扩散过程的能量分布,(c)不同复合电极在0.5 C下300次循环的循环性能和库仑效率[62]

Fig. 11 Lithium ion diffusion properties on the surface of various metal sulfides with mechanism analysis:(a)oxidation process versus the square root of the scan rates,(b)energy profiles for diffusion processes of Li ion on Ni3S2, SnS2, FeS, CoS2, VS2, TiS2, and graphene and(c)cycling performance and coulombic efficiency of the different composite electrodes at 0.5 C for 300 cycles[62]

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用于LiPSs吸附和催化的过渡金属硫化物的选择与它们所能提供的活性位点及与LiPSs的极性相互作用对电子和离子传导的促进作用密切相关。在电化学反应过程中,LiPSs与具有强相互作用的载体材料保持紧密的电接触时,可以真正实现低电子转移电阻﹑高电子传输速率和LiPSs转换过程的快速氧化还原反应动力学。
另外,TMSs改性LSBs综合性能的方法,不仅限于改性LSBs正极,还可以用于改性其他组件,以增强LiPSs的吸附增强和提供更快的氧化还原反应,大幅地提升和改善锂硫电池电化学性能。表3罗列了近年来TMSs作为隔膜添加剂、间隔层或隔膜主体时,LSBs性能改进的相关报道的数据。
表3 TMSs作为LSBs不同组件的数据报告

Table 3 Data report of TMSs as different components in LSBs

TMSs Initial capacity
(mAh·g-1)
Sulfur loading
(mg·cm-2)
ref
CoS2 interlayer 1240 at 0.2 C 1.55 70
CoS2 additive 1326 at 0.1 C 2.3 33
Co9S8 host 1130 at 0.05 C 1.5 37
Co9S8-Celgard 1385 at 0.1 C 2.0 71
TiS2 additive 1000 at 0.1 C N/A 22
TiS2 encapsulation 1156 at 0.2 C 2 17
MoS2 additive 1270 at 0.2 C 2 72
MoS2 coating 950 at 0.2 C N/A 73
NiS2 additive 1203 at 0.1 C 2.0~3.3 25
WS2 host 1581 at 0.1 C 2 28
WS2 interlayer 1454 at 0.02 C 4 74

6 结论

本文重点综述了过渡金属硫化物用于锂硫电池正极载体添加剂的基础和改性的研究,着重结合过渡金属硫化物等效/共正极作用、导电性增强作用、对LiPSs吸附作用和电化学反应催化作用四个方面展开讨论:(1)部分过渡金属硫化物可单独作为LSBs正极材料具有一定的容量,并且TMSs与S复合并发生反应以实现共正极的高比容量;(2)TMSs作为硫载体或者添加剂以及通过结构改性增强其电子和离子导电性,进而增加正极材料及锂硫电池整体的导电性,提高电子传输速率,取得了显著的成效;(3)TMSs与LiPSs强极性吸附作用,较高的结合能使得多硫化物的吸附扩散达到平衡,为解决“穿梭效应”难题提供了可行的方案;(4)极性TMSs的强结合亲合力,吸附充足的活性物质并提供丰富的活性位点以及其在氧化还原过程中对的电子传输促进作用,极大程度上催化了电化学反应。各方面协同作用,缓解“穿梭效应”,提升活性物质利用率,增强正极材料的导电性,从而提高充放电比容量﹑循环稳定性和倍率性能,实现LSBs性能的大幅提升。
尽管过渡金属硫化物具有如上所述的诸多优点,但它们颗粒大且比表面积和孔体积的低等固有缺陷使其具有一定的局限性。多元过渡金属硫化物复合,实现多孔化结构设计和高硫量负载,多组分协同作用应进一步被开发和研究。设计构建内/外异质结构并具有高度吸附成分和催化作用组分,也是实现LiPSs捕获-扩散-转换的有效方法。过渡金属硫化物的纳米结晶量子化来提升比表面积并增加活性位点亦是过渡金属硫化物作为锂硫电池正极材料的重要发展方向之一,实现正极高硫量负荷(> 6 mg·cm-2)、低电解质/硫比(<2 mL·g-1)是实现高能量密度的最关键参数之一。

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Fully lithiated lithium sulphide (Li2S) is currently being explored as a promising cathode material for emerging energy storage applications. Like their sulphur counterparts, Li2S cathodes require effective encapsulation to reduce the dissolution of intermediate lithium polysulphide (Li2Sn, n=4-8) species into the electrolyte. Here we report, the encapsulation of Li2S cathodes using two-dimensional layered transition metal disulphides that possess a combination of high conductivity and strong binding with Li2S/Li2Sn species. In particular, using titanium disulphide as an encapsulation material, we demonstrate a high specific capacity of 503 mAh g(-1)(Li2S) under high C-rate conditions (4C) as well as high areal capacity of 3.0 mAh cm(-2) under high mass-loading conditions (5.3 mg(Li2S) cm(-2)). This work opens up the new prospect of using transition metal disulphides instead of conventional carbon-based materials for effective encapsulation of high-capacity electrode materials.

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Zhang Y, Wang L Z, Zhang A Q, Song Y H, Li X F, Feng H, Wu X B, Du P P . Solid State Ionics, 2010,181(17):835.
[19]
Zheng S Y, Chen Y, Xu Y H, Feng Y, Liu Y H, Yang J H, Wang C S . ACS Nano, 2013,7(12):10995.

Highly stable sulfur/microporous carbon (S/MC) composites are prepared by vacuum infusion of sulfur vapor into microporous carbon at 600 °C, and lithium sulfide/microporous carbon (Li2S/MC) cathodes are fabricated via a novel and facile in situ lithiation strategy, i.e., spraying commercial stabilized lithium metal powder (SLMP) onto a prepared S/MC film cathode prior to the routine compressing process in cell assembly. The in situ formed Li2S/MC film cathode shows high Coulombic efficiency and long cycling stability in a conventional commercial Li-ion battery electrolyte (1.0 M LiPF6 + EC/DEC (1:1 v/v)). The reversible capacities of Li2S/MC cathodes remain about 650 mAh/g even after 900 charge/discharge cycles, and the Coulombic efficiency is close to 100% at a current density of 0.1C, which demonstrates the best electrochemical performance of Li2S/MC cathodes reported to date. Furthermore, this Li2S/MC film cathode fabricated via our in situ lithiation strategy can be coupled with a Li-free anode, such as graphite, carbon/tin alloys, or Si nanowires to form a rechargeable Li-ion cell. As the Li2S/MC cathode is paired with a commercial graphite anode, the full cell of Li2S/MC-graphite (Li2S-G) shows a stable capacity of around 600 mAh/g in 150 cycles. The Li2S/MC cathodes prepared by high-temperate sulfur infusion and SLMP prelithiation before cell assembly are ready to fit into current Li-ion batteries manufacturing processes and will pave the way to commercialize low-cost Li2S-G Li-ion batteries.

[20]
Xu Z, You H H, Zhang L, Yang Q H . Carbon, 2017,124:722.
[21]
Wu S P, Ge R Y, Lu M J, Xu R, Zhang Z . Nano Energy, 2015,15:379
[22]
Sun K, Zhang Q, Bock D C, Tong X, Su D, Marschilok A C, Takeuchi K J, Takeuchi E S, Gan H . Journal of the Electrochemical Society, 2017,164(6):A1291.
[23]
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[24]
Zhang Y J, Qu J, Hao S M, Chang W, Ji Q Y, Yu Z Z . ACS Applied Materials & Interfaces, 2017,9(48):41878.

<![CDATA[Increasing demands for lithium-ion batteries (LIBs) with high energy density and high power density require highly reversible electrochemical reactions to enhance the cyclability and capacities of electrodes. As the reversible formation/decomposition of the solid electrolyte interface (SEI) film during the lithiation/delithiation process of Fe3S4 could bring about a higher capacity than its theoretical value, in the present work, synthesized Fe3S4 nanoparticles are sandwich-wrapped with reduced graphene oxide (RGO) to fabricate highly reversible and long cycling life anode materials for high-performance LIBs. The micron-sized long slit between sandwiched RGO sheets effectively prevents the aggregation of intermediate phases during the discharge/charge process and thus increases cycling capacity because of the reversible formation/decomposition of the SEI film driven by Fe nanoparticles. Furthermore, the RGO sheets interconnect with each other by a face-to-face mode to construct a more efficiently conductive network, and the maximum interfacial oxygen bridge bonds benefit the fast electron hopping from RGO to Fe3S4, improving the depth of the electrochemical reactions and facilitating the highly reversible lithiation/delithiation of Fe3S4. Thus, the resultant Fe3S4/RGO hybrid shows a highly reversible charge capacity of 1324 mA h g-1 over 275 cycles at a current density of 100 mA g-1, even retains 480 mA h g-1 over 500 cycles at 1000 mA g-1, which are much higher than reported values.]]>

[25]
Lu Y, Li X N, Liang J W, Hu L, Zhu Y C, Qian Y T . Nanoscale, 2016,8(40):17616.

<![CDATA[S/NiS2-C composites were fabricated using a sample melting-diffusing-reacting strategy. When the elemental sulfur content in the composite is 54.9 wt%, it shows good cyclic performance and delivers a specific capacity of 730 mA h g-1 after 200 cycles at 0.5 C and 544 mA h g-1 after 500 cycles at 2 C. The elemental sulfur content can be adjusted, and when the elemental sulfur content is increased to 64.8 wt% and 76.0 wt%, the cathodes can maintain a stable cyclic capacity of 386 and 312 mA h g-1 after being discharged/charged at 2 C for 500 cycles respectively. The carbon structure and the uniformly distributed nickel disulfide in the S/NiS2-C composites efficiently stabilizes the polysulfides and also facilitates the conversion of electrochemical reactions.]]>

[26]
Sun K, Zhao C, Lin C H, Stavitski E, Williams G J, Bai J, Dooryhee E, Attenkofer K, Thieme J, Gan H . Scienpngic Reports, 2017,7(1):12976.

<![CDATA[Conductive metal sulfides are promising multi-functional additives for future lithium-sulfur (Li-S) batteries. These can increase the sulfur cathode's electrical conductivity to improve the battery's power capability, as well as contribute to the overall cell-discharge capacity. This multi-functional electrode design showed initial promise; however, complicated interactions at the system level are accompanied by some detrimental side effects. The metal sulfide additives with a chemical conversion as the reaction mechanism, e.g., CuS and FeS2, can increase the theoretical capacity of the Li-S system. However, these additives may cause undesired parasitic reactions, such as the dissolution of the additive in the electrolyte. Studying such complex reactions presents a challenge because it requires experimental methods that can track the chemical and structural evolution of the system during an electrochemical process. To address the fundamental mechanisms in these systems, we employed an operando multimodal x-ray characterization approach to study the structural and chemical evolution of the metal sulfide-utilizing powder diffraction and fluorescence imaging to resolve the former and absorption spectroscopy the latter-during lithiation and de-lithiation of a Li-S battery with CuS as the multi-functional cathode additive. The resulting elucidation of the structural and chemical evolution of the system leads to a new description of the reaction mechanism.]]>

[27]
Xu J, Zhang W X, Fan H B, Cheng F L, Su D W, Wang G X . Nano Energy, 2018,51:73.
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Lei T Y, Chen W, Huang J W, Yan C Y, Sun H X, Wang C, Zhang W L, Li Y R, Xiong J . Advanced Energy Materials, 2017,7(4):1601843.
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Cheng Z B, Xiao Z B, Pan H, Wang S Q, Wang R H . Advanced Energy Materials, 2017,8(10):1702337.
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Sreedharan R, Vijayakumari A M, Satpati B, Roy A, Basu P K, Bhattacharjee K . Materials Research Express, 2017,4(11):115012.
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Hong X, Liang J, Tang X, Yang H, Li F . Chemical Engineering Science, 2019,194:148.
[33]
Yuan Z, Peng H J, Hou T Z, Huang J Q, Chen C M, Wang D W, Cheng X B, Wei W F, Zhang Q . Nano Letters, 2016,16(1):519.

Lithium-sulfur (Li-S) battery system is endowed with tremendous energy density, resulting from the complex sulfur electrochemistry involving multielectron redox reactions and phase transformations. Originated from the slow redox kinetics of polysulfide intermediates, the flood of polysulfides in the batteries during cycling induced low sulfur utilization, severe polarization, low energy efficiency, deteriorated polysulfide shuttle, and short cycling life. Herein, sulfiphilic cobalt disulfide (CoS2) was incorporated into carbon/sulfur cathodes, introducing strong interaction between lithium polysulfides and CoS2 under working conditions. The interfaces between CoS2 and electrolyte served as strong adsorption and activation sites for polar polysulfides and therefore accelerated redox reactions of polysulfides. The high polysulfide reactivity not only guaranteed effective polarization mitigation and promoted energy efficiency by 10% but also promised high discharge capacity and stable cycling performance during 2000 cycles. A slow capacity decay rate of 0.034%/cycle at 2.0 C and a high initial capacity of 1368 mAh g(-1) at 0.5 C were achieved. Since the propelling redox reaction is not limited to Li-S system, we foresee the reported strategy herein can be applied in other high-power devices through the systems with controllable redox reactions.

[34]
Ye H L, Ma L, Zhou Y, Wang L, Han N, Zhao F P, Deng J, Wu T P, Li Y G, Lu J . Proceedings of the National Academy of Sciences, 2017,114(50):13091.
[35]
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Babu G, Masurkar N, Al Salem H, Arava L M R . Journal of the American Chemical Society, 2016,139(1):171.

<![CDATA[Lithium-sulfur (Li-S) chemistry is projected to be one of the most promising for next-generation battery technology, and controlling the inherent &quot;polysulfide shuttle&quot; process has become a key research topic in the field. Regulating intermediary polysulfide dissolution by understanding the metamorphosis is essential for realizing stable and high-energy-density Li-S batteries. As of yet, a clear consensus on the basic surface/interfacial properties of the sulfur electrode has not been achieved, although the catalytic phenomenon has been shown to result in enhanced cell stability. Herein, we present evidence that the polysulfide shuttle in a Li-S battery can be stabilized by using electrocatalytic transition metal dichalcogenides (TMDs). Physicochemical transformations at the electrode/electrolyte interface of atomically thin monolayer/few-layer TMDs were elucidated using a combination of spectroscopic and microscopic analysis techniques. Preferential adsorption of higher order liquid polysulfides and subsequent conversion to lower order solid species in the form of dendrite-like structures on the edge sites of TMDs have been demonstrated. Further, detailed electrochemical properties such as activation energy, exchange current density, rate capabilities, cycle life, etc. have been investigated by synthesizing catalytically active nanostructured TMDs in bulk quantity using a liquid-based shear-exfoliation method. Unveiling a specific capacity of 590 mAh g-1 at 0.5 C rate and stability over 350 cycles clearly indicates yet another promising application of two-dimensional TMDs.]]>

[43]
Hong X D, Li S L, Tang X N, Sun Z H, Li F . Journal of Alloys and Compounds, 2018,749:586.
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Liu Z, Zheng X, Luo S L, Xu S Q, Yuan N Y, Ding J N . Journal of Materials Chemistry A, 2016,4(35):13395.
[45]
Wang H T, Zhang Q F, Yao H B, Liang Z, Lee H K, Hsu P C, Zheng G Y, Cui Y . Nano Letters, 2014,14(12):7138.

Exploring the chemical reactivity of different atomic sites on crystal surface and controlling their exposures are important for catalysis and renewable energy storage. Here, we use two-dimensional layered molybdenum disulfide (MoS2) to demonstrate the electrochemical selectivity of edge versus terrace sites for Li-S batteries and hydrogen evolution reaction (HER). Lithium sulfide (Li2S) nanoparticles decorates along the edges of the MoS2 nanosheet versus terrace, confirming the strong binding energies between Li2S and the edge sites and guiding the improved electrode design for Li-S batteries. We also provided clear comparison of HER activity between edge and terrace sites of MoS2 beyond the previous theoretical prediction and experimental proof.

[46]
Tran D T, Dong H, Walck S D, Zhang S S . RSC Advances, 2015,5(107):87847.
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Ye C, Zhang L, Guo C X, Li D D, Vasileff A, Wang H H, Qiao S Z . Advanced Functional Materials, 2017,27(33):1702524.
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Pang Q, Kundu D, Cuisinier M, Nazar L F . Nature Communications, 2014,5:4759.

The lithium-sulphur battery relies on the reversible conversion between sulphur and Li2S and is highly appealing for energy storage owing to its low cost and high energy density. Porous carbons are typically used as sulfur hosts, but they do not adsorb the hydrophilic polysulphide intermediates or adhere well to Li2S, resulting in pronounced capacity fading. Here we report a different strategy based on an inherently polar, high surface area metallic oxide cathode host and show that it mitigates polysulphide dissolution by forming an excellent interface with Li2S. Complementary physical and electrochemical probes demonstrate strong polysulphide/Li2S binding with this 'sulphiphilic' host and provide experimental evidence for surface-mediated redox chemistry. In a lithium-sulphur cell, Ti4O7/S cathodes provide a discharge capacity of 1,070 mAh g(-1) at intermediate rates and a doubling in capacity retention with respect to a typical conductive carbon electrode, at practical sulphur mass fractions up to 70 wt%. Stable cycling performance is demonstrated at high rates over 500 cycles.

[49]
Wei T X, Liu Y F, Dong W J, Zhang Y, Huang C Y, Sun Y, Chen X, Dai N . Applied Materials & Interfaces, 2013,5(21):10473.

We demonstrated revertible shifts of surface-dependent localized surface plasmon resonances (LSPRs) in CuS nanodisks. Oleylamine (OYA) served as a solvent and surface ligand covering on CuS nanodisks during the thermolysis of single-source precursor copper ethylxanthate (Cu(ex)2). When OYA ligand was unloaded and reloaded on the surface of CuS nanodisks, the wavelength of LSPRs blue-shifted due to more oxygen exposure and then reverted through surface repassivation. The surface-dependent shift of LSPRs was dominated by the concentration of free holes in CuS nanodisks, which was modulated by the coverage and exchange of surface ligands, and the oxygen exposure dose and time. The semiconductor nanocrystals with tunable LSPRs have great potential in advanced plasmonics.

[50]
Wang Z Y, Dong Y F, Li H J, Zhao Z B, Wu H B, Hao C, Liu S H, Qiu J S, Lou X W . Nature Communications, 2014,5(1):5002.
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杨华(Yang H) . 天津大学硕士论文( Master Dissertation of Tianjin University), 2013.
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陈翔(Chen X), 侯廷政(Hou T Z), 彭翃杰(Peng H J), 程新兵(Cheng X B), 黄佳琦(Huang J Q), 张强(Zhang Q) . 储能科学与技术( Energy Storage Science and Technology), 2017,6(3):500.
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Rubha P, Kannan A G, Jun H A, Kim D W . ACS Applied Materials & Interfaces., 2016,8(6):4000.
摘要

Trapping lithium polysulfides formed in the sulfur positive electrode of lithium-sulfur batteries is one of the promising approaches to overcome the issues related to polysulfide dissolution. In this work, we demonstrate that intrinsically hydrophilic magnesium oxide (MgO) nanoparticles having surface hydroxyl groups can be used as effective additives to trap lithium polysulfides in the positive electrode. MgO nanoparticles were uniformly distributed on the surface of the active sulfur, and the addition of MgO into the sulfur electrode resulted in an increase in capacity retention of the lithium-sulfur cell compared to a cell with pristine sulfur electrode. The improvement in cycling stability was attributed to the strong chemical interactions between MgO and lithium polysulfide species, which suppressed the shuttling effect of lithium polysulfides and enhanced the utilization of the sulfur active material. To the best of our knowledge, this report is the first demonstration of MgO as an effective functional additive to trap lithium polysulfides in lithium-sulfur cells.

[60]
Zhang S S, Tran D T . Electrochimica Acta, 2015,176:784.
[61]
Chen X, Peng H J, Zhang R, Hou T Z, Huang J Q, Li B, Zhang Q . ACS Energy Letters, 2017,2(4):795.
[62]
Zhou G M, Tian H Z, Jin Y, Tao X Y, Liu B F, Zhang R F, Seh Z W, Zhuo D, Liu Y Y, Sun J, Zhao J, Zu C X, Wu D S, Zhang Q F, Cui Y . Proceedings of the National Academy of Sciences of the United States of America, 2017,114(5):840.
[63]
Li Z, Zhang S G, Xu M, Tatara R, Dokko K, Watanabe M, Zhang J H . ACS Applied Materials & Interfaces, 2017,9(44):38477.

<![CDATA[Lithium-sulfur (Li-S) batteries have attracted interest as a promising energy-storage technology due to their overwhelming advantages such as high energy density and low cost. However, their commercial success is impeded by deterioration of sulfur utilization, significant capacity fade, and poor cycle life, which are principally originated from the severe shuttle effect in relation to the dissolution and migration of lithium polysulfides. Herein, we proposed an effective and facile strategy to anchor the polysulfides and improve sulfur loading by constructing a three-dimensionally hierarchical Ni/Ni3S2/S cathode. This self-supported hybrid architecture is sequentially fabricated by the partial sulfurization of Ni foam by a mild hydrothermal process, followed by physical loading of elemental sulfur. The incorporation of Ni3S2, with high electronic conductivity and strong polysulfide adsorption capability, can not only empower the cathode to alleviate the shuttle effect, but also afford a favorable electrochemical environment with lower interfacial resistance, which could facilitate the redox kinetics of the anchored polysulfides. Consequently, the obtained Ni/Ni3S2/S cathode with a sulfur loading of ∼4.0 mg/cm2 demonstrated excellent electrochemical characteristics. For example, at high current density of 4 mA/cm2, this thick cathode demonstrated a discharge capacity of 441 mAh/g at the 150th cycle.]]>

[64]
Liu D H, Zhang C, Zhou G M, Lv W, Ling G W, Zhi L J, Yang Q H . Advanced Science, 2017,5(1):1700270.
摘要

<![CDATA[Lithium-sulfur (Li-S) battery has emerged as one of the most promising next-generation energy-storage systems. However, the shuttle effect greatly reduces the battery cycle life and sulfur utilization, which is great deterrent to its practical use. This paper reviews the tremendous efforts that are made to find a remedy for this problem, mostly through physical or chemical confinement of the lithium polysulfides (LiPSs). Intrinsically, this &quot;confinement&quot; has a relatively limited effect on improving the battery performance because in most cases, the LiPSs are &quot;passively&quot; blocked and cannot be reused. Thus, this strategy becomes less effective with a high sulfur loading and ultralong cycling. A more &quot;positive&quot; method that not only traps but also increases the subsequent conversion of LiPSs back to lithium sulfides is urgently needed to fundamentally solve the shuttle effect. Here, recent advances on catalytic effects in increasing the rate of conversion of soluble long-chain LiPSs to insoluble short-chain Li2S2/Li2S, and vice versa, are reviewed, and the roles of noble metals, metal oxides, metal sulfides, metal nitrides, and some metal-free materials in this process are highlighted. Challenges and potential solutions for the design of catalytic cathodes and interlayers in Li-S battery are discussed in detail.]]>

[65]
韩东梅(Han D M), 王拴紧(Wang S J), 沈培康(Shen P K), 孟跃中(Meng Y Z), . 电池( Battery Bimonthly), 2013,43(1):6.
[66]
Kaiser M R, Liang X, Liu H K, Dou S X, Wang J Z . Carbon, 2016,103:163.
[67]
Fei L F, Li X G, Bi W T, Zhuo Z W, Wei W F, Sun L, Lu W, Wu X J, Xie K Y, Wu C Z, Chan H, Wang Y . Advanced Materials, 2015,27(39):5936.

A space-confined "sauna" reaction system is introduced for the simultaneous reduction and functionalization of graphene oxide to unique graphene-sulfur hybrid nanosheets, in which thin layers of amorphous sulfur are tightly anchored on the graphene sheet via strong chemical bonding. Upon being used as the cathode material in lithium-sulfur batteries, the as-synthesized composite shows an excellent electrochemical performance.

[68]
Peng H J, Zhang G, Chen X, Zhang Z X, Xu W T, Huang J Q, Zhang Q . Angewandte Chemie, 2016,128(42):13184.
[69]
Xu J, Zhao W X, Fan H B, Cheng F L, Su D W, Wang G X . Nano Energy, 2018,51:73.
[70]
Ma Z L, Li Z, Hu K, Liu D D, Huo J, Wang S Y . Journal of Power Sources, 2016,325:71.
[71]
He J R, Chen Y F, Manthiram A . Energy & Environmental Science, 2018,11:2560.
[72]
Dirlam P T, Park J, Simmonds A G, Domanik K, Arrington C B, Schaefer J L, Pyun J . ACS Applied Materials & Interfaces, 2016,8(21):13437.
摘要

The practical implementation of Li-S technology has been hindered by short cycle life and poor rate capability owing to deleterious effects resulting from the varied solubilities of different Li polysulfide redox products. Here, we report the preparation and utilization of composites with a sulfur-rich matrix and molybdenum disulfide (MoS2) particulate inclusions as Li-S cathode materials with the capability to mitigate the dissolution of the Li polysulfide redox products via the MoS2 inclusions acting as "polysulfide anchors". In situ composite formation was completed via a facile, one-pot method with commercially available starting materials. The composites were afforded by first dispersing MoS2 directly in liquid elemental sulfur (S8) with sequential polymerization of the sulfur phase via thermal ring opening polymerization or copolymerization via inverse vulcanization. For the practical utility of this system to be highlighted, it was demonstrated that the composite formation methodology was amenable to larger scale processes with composites easily prepared in 100 g batches. Cathodes fabricated with the high sulfur content composites as the active material afforded Li-S cells that exhibited extended cycle lifetimes of up to 1000 cycles with low capacity decay (0.07% per cycle) and demonstrated exceptional rate capability with the delivery of reversible capacity up to 500 mAh/g at 5 C.

[73]
Ghazi Z A, He X, Khattak A M, Khan N A, Liang B, Iqbal A, Wang J X, Sin H, Li L S, Tang Z Y . Advanced Materials, 2017,29(21):1606817.
[74]
Park J, Yu B C, Park J S, Choi J W, Kim C, Sung Y E, Goodenough J B . Advanced Energy Materials, 2017,7(11):1602567.

基金

国家国际科技合作专项(2015DFR50350)
和国家自然科学基金项目(51702256)

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