Transition metal dichalcogenide based nanomaterials

Chemical Engineering Journal 307 (2017) 189–207

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Review

Transition metal dichalcogenide based nanomaterials for rechargeable batteries Songping Wu ⇑, Yao Du, Shuijing Sun School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China

h i g h l i g h t s This is an important and timely review to advanced energy materials. A brief commentary on state-of-the-art progress of TMDs has been presented. Potential research directions and important scientific problems are also proposed.

a r t i c l e

i n f o

Article history: Received 21 June 2016 Received in revised form 4 August 2016 Accepted 8 August 2016 Available online 8 August 2016 Keywords: Transition metal dichalcogenides MoS2 FeS2 Nanomaterials Rechargeable batteries

a b s t r a c t Transition metal dichalcogenide based (TMD-based) nanomaterials have emerged as important candidates of electrode materials for rechargeable batteries due to their unique physical properties. TMDs are abundant in environmentally friendly natural ore and have excellent large-current charge/discharge capability, ultra-long life and wide operation temperature region. In this review, a brief introduction of recent developments about TMD-based nanocomposite electrodes was provided. Subsequently, synthetic routes to TMD-based nanocomposites and their electrochemical performances in rechargeable batteries were stated in detail. The state-of-the-art advances in the relation between rationally designed structures and electrochemical performances of TMD-based nanocomposite electrode materials were summarized. Potential research directions and important scientific problems of TMD-based materials were also proposed in the hope of solving the emerging problems of TMD-based batteries. It is predicted that TMD materials, particularly MoS2 and FeS2, will slowly re-establish themselves as promising candidates for crucial components of heavy-duty energy-storage devices such as electric cars, energy-storage stations, smart grids and so on. Ó 2016 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brief introduction of TMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li-ion batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Layered materials for LIBs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. MoS2–based materials for LIBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Other layered TMDs for LIBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Non-layered TMDs for LIBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. FeS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. CoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium-ion batteries and others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Layered materials for SIBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Direct-exfoliation in solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Hydrothermal route . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Non-Layered MX2 materials (FeS2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (S. Wu). http://dx.doi.org/10.1016/j.cej.2016.08.044 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

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S. Wu et al. / Chemical Engineering Journal 307 (2017) 189–207

Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

1. Introduction In the ongoing fight against the increasingly serious environmental degradation, rechargeable batteries have been emerging as robust roles to change the game rule [1–3]. Among the current candidates, rechargeable rocking-chair batteries such as Li-ion batteries (LIBs) and sodium-ion batteries (SIBs) have established their dominant positions from universal power sources for portable energy storages (such as smart phones, tablets and laptops) and medical sectors (prosthetics, pacemakers, biointerfacial devices) to high-power supplies, including electrical vehicles, stationary energy storage stations, smart power grids, etc. [4]. Recently, rapid rise of commercially available high power applications has imposed a critical challenge on the rechargeable ultrahigh energy storage devices, which are generally operated in hostile and changeable operation environments. For example, the energy-storage stations are usually built in the mountainous regions for wind energy and geothermal energy, and deserts for solar energy, where the huge temperature differences might be a problem to be concerned. Power battery for electric vehicles must be enoughly robust to endure both the internal self-generated heat energy and the sharp change of environmental temperature as they are operated in different geographical areas. Smart grids have harsh requirements for reliability against the current shock and seasonal change of temperature. The recent commercial graphite anode materials have some obvious shortcomings such as a low theoretical capacity of 372 mA h g1 [5], severe capacity degradation, poor reliability [6,7] and temperature effect [8] under extreme environmental conditions. As a result, it is of extreme difficulty for traditional battery in meeting the requirement of increasingly heavy-duty applications, which were greatly different with the portable consumer electronics. Accordingly, novel high-performance electrode materials are highly desirable to replace conventional graphite anodes for LIBs [9,10]. As of late, significant efforts have focused on developing alternatives (i.e., oxides [11–13] sulfides [14,15] and alloys) to replace current anode materials (i.e. graphite) [16]. Among them, transition metal dichalcogenides (TMDs), whose generalized formula is MX2, have aroused strong interest due to their crystal structures, excellent mechanical properties and robust probabilities in catalysis [17], sensor [18], energy storage and conversion [19], and electronic devices [20] such as field-effect transistors and logic circuits. From the viewpoint of technology, TMDs have good performances in large current discharging and ultralong life, wide operation temperature (40 °C to 60 °C), inexpensive and abundant natural resources, no polluted composition included, over 10 years of shelf life (2 years under 60 °C) and lighter [21]. Therefore, the investigations into TMD-based materials mostly manufactured from natural mineral have arisen as one of the hottest topics in the advanced energy field. It is noted that there are several recent reviews on transition metal dichalcogenide materials for energy storage and conversion [22–24]. Herein, we concisely analyze those published references to differentiate them from current article. In the review by David et al. [22], a considerable length was spent to introduce the background knowledge of MoS2. Subsequently, lithiation potential of 1.1 V was noted. However, the more important lithiation potential of 1.9 V, which was the trigger

point for an electrochemical reaction, was unexpectedly ignored. Finally, the review concentrated its interest on the synthesis routes. In-depth survey into the relation between structure and performance was not presented in the reference. In another mini review by Zhang [23], FeS2 was considered to experience a redox mechanism for Li or Na cycling, instead of the traditional Li-insertion followed by redox mechanism. However, the representative reduction peaks appeared at 1.9 V and 2.25 V for Li-S battery, structural stability of cubic-phase materials such as Li2MnO4 during Li-cycling, and frequently-appeared amorphous intermediate species would be presented against this viewpoint [25]. In a latest published review by Zhang et al. [26], the recent progress in the development of MoS2 nanostructures was reported in a broad range of application, such as energy conversion, energy storage and sensors. However, the review did not consider rechargeable batteries in detail. The systematical discussion on the structure and performance for LIBs or SIBs was not fully explored. As stated above, a timely and critical review that concentrates exclusively on TMDs for applications in rechargeable batteries is fairly lacking until now. This review aims to provide an up-todate and comprehensive summary of recent advances in artificially structural design, construction of 3D MoS2-based composites and the correlation between structure and electrochemical performance, which are matters of grave concern for energy application of MoS2–based materials. Li-cycling mechanisms have been specifically highlighted for each specific instance. Notably, we herein focus our attention on the TMDs materials based on the lithium-intercalation followed by a redox mechanism, including layered 2D TMDs (MoS2, VS2, TiS2 and WS2) and nonlayered TMDs (mostly FeS2 and CoS2). As a consequence, the special layered MX2 such as SnS2, which adopted a redox and alloying Li-cycling mechanism, will not be explored in this article.

2. Brief introduction of TMDs According to the crystal structure and application characteristics in rechargeable batteries, TMDs could be classified by layered and non-layered structures. Comparatively speaking, layered TMDs have recently received more attention because of the interesting graphene-like structure. As mentioned in Fig. 1, group 4–7 are predominantly layered, whereas some of group 8–10 TMDs are commonly found in nonlayered structures such as pyrite [27]. In general, single-layered TMDs with lamellar structure have been believed to possess an opportunity for energy storage and conversion; however, nonlayered structural FeS2 also has outstanding manifestation in the related fields. Molybdenum disulfide (MoS2) is considered as a typical layered dichalcogenide, where a trigonal prismatic coordination sphere, bound to six sulfide ligands, is occupied by each Mo(IV) center, and each sulfur center is connected to three molybdenum centers. A layered structure, where molybdenum atoms are located between layers of sulfur atoms, is the consequence of interconnected trigonal prisms. Natural MoS2 is commonly found in the ‘2H phase’; however, synthetic MoS2 often contains the 3R phase [27]. It is well known that intercalation with alkali metals induces phase changes in some TMDs. For example, lithium intercalation in

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Fig. 1. Structure of monolayered TMDs. About 40 different layered TMD compounds exist. The transition metals and three chalcogen elements that predominantly crystallize in those layered structure are highlighted in the periodic table. Partial highlights for Co, Rh, Ir and Ni indicate that only some of the dichalcogenides form layered structures. For example, NiS2 is found to have a pyrite structure but NiTe2 is a layered compound. Reproduced with permission from Ref. [27]. Copyright 2013, Macmillan Publisher Lts.

Fig. 2. (a) Typical layered structure: Schematic of functionalization scheme. Upper section, side view of the 2H and 1T phases. Bottom section, the 2H phase of TMDs is converted to the 1T phase via lithiation using butyllithium (BuLi), and the 1T phase is negatively charged. n – indicates the excess charges carried by the exfoliated 1T-phase nanosheets. Reproduced with permission from Ref. [29]. Copyright 2015 Macmillan Publisher Lts. (b) Typical non-layered structure: Crystal structure of FeS2. Reproduced with permission from Ref. [23]. Copyright 2015, The Royal Chemical Society.

2H-MoS2 results in partial transformation to the 1T polymorph (Fig. 2a) [28–30]. A non-layered TMD pyrite (FeS2), known as fool’s gold, adopts a cubic structure, where unit cell is composed of a Fe face-centered cubic sublattice into which the S ions are embedded. In the first bonding sphere, the Fe atoms are surrounded by the nearest six sulfur neighbours in a distorted octahedral arrangement (Fig. 2b). Single-layered 2 dimensional (2D) TMDs (M = Mo, Ti, V, and W, X = S or Se) enable them to possess great potential for alternative electrode materials. In general, layered MX2 has strong covalent bonds within layers and weak Van der Waals forces between layers, which provide ideal space for intercalation of ions with small radius such as Li or Na ions. Non-layered TMDs, typical FeS2, experience a similar Li-cycling reaction as compared with layered MoS2. 3. Li-ion batteries 3.1. Layered materials for LIBs Up to now, as a typical layered dichalcogenide, MoS2 has presented some crucial applications in transistors [31], biosensor [32], photodetectors [33], terahertz modulation [34] and following advanced energy storage devices [27]. The interlayer spacing of MoS2 can provide a suitable space for the accommodation of a variety of guest species [35–37], and a hole mobility of 741 cm2 V1 S1 [38], enabling MoS2 to be promising new-concept energy materials for LIBs [27], Mg batteries [39,40], all-solid-state lithium batteries [41], Li-O2 batteries [42], vanadium redox flow batteries (VRFB), etc. [43]. Among them, a little earlier, MoS2 was utilized as the cathode materials for LIBs [44,45]. However, the anode applications of MoS2 have aroused

much interest over the years because of its unique electrochemical characteristics. As an anode material, a typical Li insertion and extraction into an idealized MoS2 can be summarized as follows [46–48]: þ

Intercalation : MoS2 þ xLi þ xe $ Lix MoS2 þ

Conversion : Lix MoS2 þ ð4 xÞLi þ ð4 xÞe $ Mo þ 2Li2 S

ð1Þ ð2Þ

Bulk TMD materials such as MoS2 and WS2 could be utilized as electrode materials for rechargeable batteries because Li+ or Na+ can be easily inserted or extracted from these materials based on their layered structure. However, lithiation of these compounds inevitably gives rise to a severe damage for the integrity of the whole electrode structure. Very recently, systematic DFT calculations and ab initio molecular dynamic simulations were applied to gain insights into the structural evolution and capacity fading mechanism of MoS2-based nanomaterials as LIB anodes [49]. The results demonstrated that continuous intercalation of Li ions induces structural destruction of 2H phase MoS2 nanosheets in the discharge process. To suppress the dissociation of MoS2 nanosheets in the lithiation process, construction of a sandwichlike graphene/MoS2/graphene structure was proposed by the authors. Compared with the three-dimensional (3D) bulk TMDs, twodimensional (2D) single-layered TMDs displayed dramatic differences in the electronic structure interlayer coupling, quantum confinement and symmetry elements. Exfoliated 2D nanosheets are attractive as anodes for rechargeable batteries because the loosely stacked nanosheets could accommodate so-called ‘‘tidal volume” derived from periodic reversible volume changes during Licycling. Hence, synthesis of the 2D TMD nanosheets and further

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Fig. 3. (I) Direct sonication exfoliation: A) Photographs of dispersions of MoS2 (in NMP), WS2 (in NMP), B) An SEM image of the surface of a MoS2 film. C) He ion microscope image of the edge of a WS2 film. Reproduced with permission from Ref. [57]. Copyright 2011, Macmillan Publisher Lts. (II) Chemical co-exfoliation: A) Schematic of fabrication processes. Pre-exfoliated MoS2 reacts with A+C10H 8 to form an intercalation sample, and then exfoliates to single-layer sheets in water. B) Photograph of Na-exfoliated singlelayer MoS2 dispersion in water. Reproduced with permission from Ref. [59]. Copyright 2014, Macmillan Publisher Lts. (III) A) The cut-off voltage used to optimize the electrochemical lithiation conditions. B) The atomic force microscope images of MoS2, WS2 and TiS2 on SiO2 substrates. The insets of the AFM images represent height profiles from the substrate onto the nanosheets. The height of the step at the edge indicates the thickness of the nanosheets. Reproduced with permission from Ref. [27]. Copyright 2013, Macmillan Publisher Lts.

3.1.1. MoS2–based materials for LIBs 3.1.1.1. MoS2 and its non-carbon hybrids. 3.1.1.1.1. Pristine MoX2 (X = S, Se). Several synthetic approaches to MoS2 have been explored, including thermal decomposition [50], solution lithiation [36,51], atomic layer deposition (ALD) [52], gas phase synthesis [53], solution phase followed by heat treatment [54], and chemical vapor deposition (CVD) [55]. Recently, melting-infiltration assisted nano-replication method was also adopted to MoS2, MoSe2, WS2 and WSe2 [56], delivering reversible Li-storage capacities of 710 mA h g1 for MoS2, 744 mA h g1 for MoSe2, 501 mA h g1 for WS2 and 427 mA h g1 for WSe2 at 2C, respectively, without a remarkable fading of capacity.

The emergence of superlattice by chain clusterization of metal atoms has been observed in Li-intercalated MoS2 and WS2. Interestingly, the distorted phase can be metastable after the intercalant is removed [61]. It is easy to understand that the properties of such distorted TMDs are significantly different when compared with their undistorted counterparts. Therefore, liquid phase co-exfoliation of TMDs experiences serious issues such as unstable structures, long durations (1 week) and unmanageable process. In-depth investigations into the structural evolution of exfoliated graphene-like MoS2 via intercalation-exfoliation and its influence on the electrochemical performance are of great interest. Now, we turned our attention to another synthesis strategy for pristine MoS2, i.e. solution-phase high-pressure reaction, including hydrothermal and solvothermal methods.

3.1.1.1.1.1. Liquid exfoliation method. Liquid exfoliation method is likely suitable for fundamental and proof-of-concept applications where large quantities of materials are required [27]. Actually, co-exfoliation could be classified as direct sonication exfoliation in commonly used solvents such as dimethylformamide and Nmethyl-2-pyrrolidone [57,58], chemical co-exfoliation [59] and electrochemical co-exfoliation (Fig. 3) [27,60].

3.1.1.1.1.2. Solution-phase high-pressure reaction. Solution-phase high-pressure reaction is a frequently-utilized route to MoS2 nanomaterials. In the pressure vessel, MoS2 particles were generally self-assembled with the nanosheets in order to achieve thermodynamic stability by lowering the free energy. Therefore, versatile morphologies have emerged in resultant MoS2 nanomaterials. Typical hydrothermal morphologies include nanosheets [62], nanowall

construction of rationally designed 3D structure are of great importance.


Fig. 4. Typical morphologies of MoS2 via synthesis process in a pressure vessel. (a) Hydrothermal MoS2 nanowalls. Reproduced with permission from Ref. [63]. Copyright 2013, The American Chemical Society. (b) Hydrothermal nanosheets. Reproduced with permission from Ref. [62]. Copyright 2015, The Royal Chemical Society. (c) Solvothermal worm-like MoS2. Reproduced with permission from Ref. [72]. Copyright 2015, The Royal Chemical Society. (d) Solvothermal nanotube. Reproduced with permission from Ref. [71]. Copyright 2014, The Royal Chemical Society.

[63], nanoflower [64–67], cockscomb-like [68], hierarchical microspheres [69], tremella-like [70], etc. (Fig. 4). A representative reversible discharge capacity of 589 mA h g1 at 100 mA g1 after 80 cycles could be yielded for MoS2 nanosheets [62]. Subsequently, a high reversible capacity of 883 mA h g1 at 400 mA g1 after 30 cycles has been displayed for hydrothermally nanoflowerstructured MoS2 electrodes [65]. Of particular note is that the solvothermal MoS2 also constructed various and interesting morphologies such as hierarchical hollow MoS2 nanotubes [71], worm-like ultra-long MoS2 nanostructures [72], 3D architectures composed of MoS2-graphene hybrid nanosheets [73], and 3D flower-like MoS2 spheres [74]. In brief summary, solution-phase high-pressure reaction has a probability of yielding diversified MoS2 nanostructures, and affording a fairly stable reversible capacity of 850 mA h g1 after 50 cycles at 100 mA g1. However, good rate performance for MoS2 is highly desirable to meet the requirement of rapid charge/discharge. As a consequence, oxide hybrids and surface decoration were introduced into the MoS2, respectively. 3.1.1.1.2. MoS2-oxides composites. The restacking or re-aggregation is a potential issue for the MoS2 nanosheets during Li-ion cycling. Therefore, oxides with high reversible capacity were employed as activators of MoS2 nanosheets for electrolyte penetration and spacers to prevent MoS2 nanosheets from restacking. TiO2 is a promising candidate as an anode material for LIBs owing to its low cost, easy availability, and eco-friendly characteristics. Like MoS2, TiO2 experiences a similar intercalation/deintercalation reaction mechanism (Eq. (3)) [75], and retains a robust structural stability during cycling. þ

TiO2 þ xLi þ xe $ Lix TiO2

ð3Þ

In general, strong synergistic effect was believed to exist between MoS2 and TiO2, leading to significant facilitation in transportation of ions and electrons across the interfaces. Hydrothermal

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MoS2 nanosheet-coated TiO2 nanobelts [76], TiO2 microspheres@MoS2 [77] and few-layered MoS2@TiO2 [78] delivered superior cycling capabilities of 714–740 mA h g1 after 150–200 cycles at 100 mA g1. Very interestingly, a high capacity of 120 mA h g1 at 40 C and a long-term capacity retention of 75.2% (at 6 C) after 1000 cycles for mesoporous MoS2-TiO2 fibrous nanocomposites have been reported [79]. TiO2 could be also utilized as a bonding interface to create a strong adhesion between MoS2 and Ti substrate, allowing electrons to easily transport throughout the whole electrode. Therefore, additive /binder-free MoS2 electrodes could deliver a discharge capacity of 1189 mA h g1 at a current density of 1.0 A g1 after 600 cycles [80]. Low theoretical capacity (335 mA h g1) of TiO2 was an obvious disadvantage for MoS2@TiO2 composites. Hence, other compounds with high theoretical capacity such as Fe3O4 [81], SnO2 [82,83], MoS2/C encapsulated in Sn@SnOx nanoparticles [84], molybdenum oxide nanoparticles [85] and SiCN [86] were also employed to form the MoS2-based composites, affording representative reversible capacities of 1200 mA h g1 at 500 mA g1 at 560th cycle for Fe3O4/MoS2 composites [81] and 602 mA h g1 under 1 A g1 after 230 cycles for SnO2/MoS2 nanocomposites [82]. However, how to suppress the so-called ‘‘volume tidal” of oxides during Li-cycling still remains ambiguous. In brief summary, the typical morphologies of TiO2-MoS2 composites via hydrothermal route and corresponding electrochemical performance were exhibited in Fig. 5. TiO2-MoS2 composites maintain a remarkable structural integrity during cycling, particularly under a large current density [79]. However, low reversible capacity of TiO2 might have limited the improvement of overall performance for TiO2-MoS2 composites. In addition, the low electrical conductivity and large capacity loss at the first cycle of MoS2 electrodes remain large challenge to be addressed before MoS2 nanosheets become useful for energy storage applications. Next, conductive polymer was introduced into MoS2-based nanocomposites to improve the conductivity of MoS2. 3.1.1.1.3. Introduction of conductive polymer to MoS2. The introduction of polymer was a practicable approach to decorate and modify the MoS2. Polymer could improve conductivity, i.e. 1.0 101 S cm1 for poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) [87], rebuild hierarchical porous structure and enhance Li+ transportation during Li+ insertion/extraction [88]. MoS2/PEDOT: PSS composites exhibited a reversible capacity of 712 mA h g1 at a current density of 50 mA g1, with 81% capacity retention after 100 cycles [87]. An excellent rapid charge characteristic has been afterwards achieved in porous MoS2/polyaniline (MoS2/PANI) composites with a tremella-like hierarchical structure, e.g. reversible capacities of 369 mA h g1 at a current density of 4 A g1 and 915 mA h g1 after 200 cycles at 1 A g1, respectively [88]. The addition of polymers inevitably reduced the content of active MoS2, leading to a low reversible capacity as compared with carbon-based MoS2 hybrids. More troublesomely, conductive polymers generally possess conjugated structural main-chain. The rigidity of the chain makes them difficult to accommodate the volume change of electrochemically active materials. In addition, their characteristics, including exorbitant price, insolubility and infusibility, produce difficulties in freeform fabrication for conductive polymers. The salient features are discussed here: (1) In general, Li-cycling properties of pristine MoS2 are fairly stable at a low current density, implying huge potential for rechargeable application. The introduction of oxide or polymer exhibits a probability in improving rate performance of MoS2-based composites, for example, a reversible capacity of 915 mA h g1 at a current density of 1 A g1 after 200 cycles for porous MoS2/polyaniline composite.

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Fig. 5. Typical morphologies of TiO2-MoS2 composites via hydrothermal route. (a) TiO2 nanobelt@few-layered MoS2. Reproduced with permission from Ref. [76]. Copyright 2014, The Royal Chemical Society. (b) Firework-shaped TiO2 microspheres embedded with few-layer MoS2. Reproduced with permission from Ref. [77]. Copyright 2015, The Royal Chemical Society. (c) 3D few-layered MoS2 coated TiO2 nanosheet core–shell nanostructures. Reproduced with permission from Ref. [78]. Copyright 2015, The Royal Chemical Society. (d) Comparative cycling performance of pure MoS2, F-TiO2@MoS2 and pure TiO2, corresponding to materials in Fig. 4b, at a current density of 100 mA g1. Reproduced with permission from Ref. [77]. Copyright 2015, The Royal Chemical Society.

However, the addition of polymer or oxide inevitably leads to a low reversible capacity. Therefore, rationally design of 3D MoS2 structure via a facile and inexpensive route maybe the next goal to be solved. Next, other approaches are investigated, whereby the MoS2 is combined by carbon materials, including graphene, amorphous carbon and carbon nanotube or fiber. 3.1.1.2. MoS2-graphene hybrids. As a new carbon material, graphene has attracted great attention due to its large specific surface area, excellent electron mobility, high thermal conductivity and planar sp2-hybridized carbon framework, and accordingly, frequently selected as a matrix to support anode materials [89–92] or cathode materials [93] for LIBs. MoS2-graphene hybrids were allowed to provide a possibility in constructing 3D conductive networks, which could effectively suppress aggregation, contribute more active sites, lower the diffusion energy barrier of Li ions [94], and then garner improved electrochemical performance during the lithiation/delithiation process. 3.1.1.2.1. Liquid phase co-exfoliation. In 2013, co-exfoliated MoS2/graphene composites afforded reversible capacity of 750 mA h g1 at a current density of 100 mA g1 after 50 cycles, with 350 mA h g1 at a current density of 2 A g1 [95]. Subsequently, a flexible and robust MoS2-graphene hybrid paper was fabricated (Fig. 6) [96]. Thereafter, intercalation-exfoliation of few-layered MoS2 was introduced to reduced graphene oxide (rGO). The resultant layer-by-layer MoS2/rGO hybrids delivered a capacity of 940 mA h g1 at a current density of 100 mA g1 after 75 cycles [51]. Interestingly, high capacities have been recently achieved in graphene-like MoS2 nanosheets/graphene nanocomposites via hydrolysis of LiMoS2 by Liu et al. [97], i.e. 1351 mA h g1 at a

current density of 100 mA g1 after 200 cycles and 591 mA h g1 at 1 A g1, respectively. Very recently, in-situ conversion of dopamine to highly conductive nitrogen-doped graphene (NDG) in the interlayer space of MoS2 led to the formation of NDG/MoS2 nanocomposites [98], delivering a high reversible capacity of above 820 mA h g1 at a current density of 1 A g1 until the 100th cycle. 3.1.1.2.2. Hydrothermal method followed by annealing. As early as 2011, single-layered MoS2/graphene@amorphous carbon [99] and layered MoS2/graphene [100] delivered reversible capacities of 1100 mA h g1 under a current density of 100 mA g1 after 100–300 cycles. Subsequently, few-layered MoS2/graphene composites [101], layered MoS2/N-doped graphene/porous G-C3N4 nanosheets hybrids [102], MoS2/graphene hybrid nanoflowers [94], MoS2/graphene nanosheets (GNs) composites [103], ultrathin MoS2/graphene heterostructures [104], and MoO2-MoS2/graphene composites [105] delivered similar Li-cycling performance. Very recently, a core-shell structural MoS2 nanosheets grown on hollow carbon microspheres afforded a stable capacity of 970 mA h g1 for over 100 cycles at a current density of 250 mA g1 [106]. Repeatedly confirmed data on the hydrothermal MoS2/graphene composites may be indicative of the reliability of hydrothermal MoS2/graphene composites under a low current density. However, an excellent performance under a largecurrent density for the special materials is what we highly hope for. 3.1.1.2.3. CVD method. Today, chemical vapor deposition (CVD) has arisen as a powerful technique for single or few-layered graphenelike materials [55]. Flower-like MoS2 particles anchored on graphene [107], MoS2-coated 3D graphene network (3DGN) [108], and ultrathin MoS2 nanosheets vertically aligned on the


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Fig. 6. Upper panel: (a) Photograph, (b) Cross-sectional SEM image and (c) Structural scheme of a piece of flexible and robust MoS2–rGO hybrid paper chemically cross-linked by poly(ethylene oxide) (PEO). Bottom panel: (a) Charge–discharge (current density: 100 mA g1) and (b) cyclic voltammetry curves (scan rate = 0.1 mV S1) of the MoS2– rGO–PEO film anode. Reproduced with permission from Ref. [96]. Copyright 2013, The Royal Chemical Society.

single-layered graphene sheets (MoS2-NS/G) [109] have been fabricated, giving rise to a 10th-cycle capacity of 466 mA h g1 at a high current density of 4 A g1 [108] and 620 mA h g1 at 4 A g1 for 1000 cycles due to fast electron transportation and ion diffusion in the unique vertical design [109]. As showed in Fig. 7, hydrothermal and CVD routes have provided significant strategies to MoS2 with various morphologies such as nanoflowers, nanosheets and heterostructures. Generally speaking, hydrothermal MoS2/graphene composites appeared to afford a higher reversible capacity under a low current density as compared with those via CVD method. The reasons may be the emergence of complex chemical groups and robust chemical bonds between MoS2 and graphene in the extreme reaction environment. From another perspective, large-current performance for MoS2 graphene hybrids via CVD is also admirable. 3.1.1.2.4. Solution-phase method followed by annealing. The combination of facile solution process and subsequent heat treatment could garner layered MoS2/graphene (MoS2/G) composites [110], delivering a reversible capacity of 808 mA h g1 at a current density of 100 mA g1 after 100 cycles. Nitrogen and sulfur co-doped graphene supported MoS2 afforded a similar reversible capacity of 1010 mA h g1 at 150 mA g1 after 50 cycles [111]. Intriguingly, superior electrochemical performances under a large-current density have aroused great attention. Recently, rGO-anchored MoS2 composites were believed to display a recommendable reversible charge capacity of 670 mA h g1 after 250 cycles at a large current density of 1 A g1 [112]. Freestanding and interwoven MoS2@graphene nanocables delivered a reversible capacity of 900 mA h g1 at a current density of 5 A g1 after 700 cycles (Fig. 8) [113]. MoS2/32% graphene sheets (GS) films [114] showed a high-rate capability of 552 mA h g1 at a current density of 10 A g1 after 7500 cycles. The synergistic effect of few-layered MoS2, intimate contact and morphological compatibility between

MoS2 and graphene sheets, and the absence of non-conducting PVDF binder could greatly promote the transportation of interfacial lithiumion and electron mobility. These unique characteristics led to the outstanding rate capability of the free-standing and binderless MoS2/graphene sheets electrode [114]. In brief summary, several strategies to MoS2@graphene hybrids have been established. CVD and solution-phase process, including ultrasonic co-exfoliation, solution process followed by heat treatment and hydrothermal method, have been testified as the frequently-used routes to synthesize MoS2/graphene composites with diversified morphologies. Among them, Freestanding MoS2@graphene nanostructural electrodes are believed as the promising candidate for large power LIB applications because of a facile route and a high reversible capacity after ultralong cycles, i.e. 552 mA h g1 at a current density of 10 A g1 after 7500 cycles. Most importantly, solution-phase heat-induced approach has an obvious advantage in forming the homogeneous distribution of porous MoS2 on the graphene and robust chemical bonds between them. However, how to control the evolution of morphology is of very importance for investigation into the relation between morphology and performance. Further, the universal rule to the formation of functional groups and natural characteristic of chemical bonds between MoS2 and graphene still retains unobvious and need to be revealed. 3.1.1.3. MoS2-amorphous carbon hybrids. Amorphous carbon is inexpensive carbon material for LIBs application. Commercial CMK (carbon material from Korea) matrix has been testified to possess a capability of improving cycling performance and rate capability for MoS2 due to the enlargement of interlayer distance and favourable conductivity [37]. Hydrothermal (solvothermal) method has received much great interest on forming the MoS2/amorphous carbon (a-C) composites.

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Fig. 7. Upper panel (MoS2 via hydrothermal route): (a) TEM image of the hydrothermal MoS2/graphene heterostructures, and (b) cycle performance at a current density of 0.1 A g1 for pristine MoS2 and MoS2/graphene. (a) and (b) reproduced with permission from Ref. [104]. Copyright 2015, Elsevier; (c) The SEM images of hydrothermal MoS2 nanoflowers/graphene composite (Inset is the pristine MoS2), and (d) cycling behaviors of the three structures: C-MoS2, pristine MoS2, and MoS2/rGO composites, respectively. (c) and (d) reproduced with permission from Ref. [94]. Copyright 2015, The American Chemical Society. Bottom panel (MoS2 via CVD): (e) Microstructure of the assynthesized MoS2 nanoflowers on freestanding graphene film (FSG). A photograph of a flexible freestanding graphene film (FSG) film is displayed in the inset. (f) First discharge profile shows different first and second charge. Reproduced with permission from Ref. [107]. Copyright 2014, The American Chemical Society. (g) Illustration of transport paths of Li+ ions and electrons in the ultrathin MoS2 nanosheets (MoS2-NS)/graphene electrode, and (h) Discharge capacity as a function of cycle number of the electrodes at current density of 400 lA cm2 for corresponding electrode. Reproduced with permission from Ref. [109]. Copyright 2015, Elsevier.

As early as 2011, Chang et al. reported that hydrothermal 3D graphene-like MoS2/amorphous carbon (a-C) composites afforded a reversible capacity of 962 mA h g1, and retained 912 mA h g1 at a current density of 100 mA g1 after 100 cycles (Fig. 9) [115].

Until now, similar performances were also repeatedly verified by several research groups for different MoS2-amorphous carbon hybrids, such as hydrothermal 3D MoS2–carbon nanostructures [116], MoS2 nanoflowers with carbon-coating [117], MoS2


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Fig. 8. (a) The schematic diagram of MoS2@G, where graphene rolls up into a hollow nanotube and thin MoS2 nanosheets are uniformly standing on the inner surface of graphitic nanotubes. (b) TEM image of MoS2@graphene. (c) Electrochemical cycling performance of MoS2@G and MoS2–F@G electrodes under charge/discharge cycles from 3 to 0.1 V. (d) Cycling performance and Coulombic efficiency of MoS2@G at 5 A g1. All the capacities are based on the whole electrode. Reproduced with permission from Ref. [113]. Copyright 2014, The Royal Chemical Society.

Fig. 9. Typical morphologies of MoS2 via hydrothermal approach. (a) 3D graphene-like architecture MoS2/amorphous carbon (a-C) composites (carbon source: glucose). Reproduced with permission from Ref. [115]. Copyright 2011, The Royal Chemical Society. (b) MoS2 nanoflowers with carbon-coating (Carbon source: glucose). Reproduced with permission from Ref. [117]. Copyright 2014, The Royal Chemical Society. (c) MoS2 quasi-hollow microspheres-encapsulated porous carbon (carbon source: sulfonated polystyrene). Reproduced with permission from Ref. [118]. Copyright 2014, Wiley. (d) Hierarchical carbon-sandwiched monolayered MoS2 building blocks (carbon source: polyvinylpyrrolidone). Reproduced with permission from Ref. [119]. Copyright 2015, The American Chemical Society. (e) 3D hierarchical hydrothermal MoS2/C nanoflowers (carbon source: polyaniline). Reproduced with permission from Ref. [120]. Copyright 2014, The American Chemical Society.

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quasi-hollow microsphere-encapsulated porous carbon [118], carbon-sandwiched monolayered MoS2 building blocks [119], 3D hydrothermal MoS2/C nanoflowers [120], few-layered MoS2 nanosheets/porous carbon frameworks [121], and sphere-like Sn/ MoS2/C [122]. Intriguingly, in a full-battery application, hydrothermal 3D MoS2 nanoflake array/carbon cloth has demonstrated a practical application as a power source to light a commercial red LED [123]. In another instance, a sheet-like MoSe2/C composite via hydrothermal method and subsequent annealing treatment (carbon source: glucose) afforded a reversible specific capacity of about 576.7 mA h g1 at a current density of 100 mA g1 after 50 cycles [124]. It is worthwhile noting that carbon-coated nanobowl MoS2 [125] and MoS2 nanoleaves with a spherical onion-like carbon core [126] using solvothermal method followed by annealing could afford comparable reversible capacity, i.e. reversible capacity of 853 mA h g1 at a current density of 50 mA g1 after 60 cycles [126]. Besides the above mentioned routes, pyrolysis of polymer [127,128], microwave heating [129] and NaCl-templated method [130,131] were also developed as feasible strategies to MoS2/carbon composites. Very recently, the high rate performance of coexfoliated MoS2@N-doped amorphous carbon has been reported, e.g. a reversible capacity of 1147 mA h g1 at a current density of 100 mA g1 after 50 cycles and a high cycling capability of 820 mA h g1 at 2 A g1 after 200 cycles [127]. In brief summary, the crucial influential factor may be the active MoS2 in MoS2-amorphous carbon hybrids, affording reversible capacity of 900 mA h g1 at a current density of 100 mA g1 after 100 cycles. Most importantly, the construction of compact amorphous carbon layer for resultant MoS2-based structure should robustly accommodate volume change (so-called tidal volume) of

active substances on electrodes of LIBs. The co-exfoliation route followed by heating is promising owing to the formation of uniform and compact carbon layer via a facile, low-cost and flexible technology. 3.1.1.4. MoS2-CNT hybrids. Carbon nanotubes (CNTs) are cylindrical allotropes of carbon, and found in unique position as additives instead of carbon black in LIB electrode materials owing to their extraordinary thermal conductivity, mechanical and electrical properties. CNTs are generally categorized as single-walled (SWCNTs) and multi-walled nanotubes (MWCNTs). MoS2/CNT hybrids could be gathered by dry grinding [132], microwave irradiation technique [133], hydrothermal route [134], etc. Hydrothermal route has produced coaxial CNT [134], CNT backbone [135], and carbon nanofiber (CNF) [136] to support MoS2 nanosheets. The MoS2/CNT hybrids afforded similar electrochemical properties as compared with MoS2/graphene or amorphous carbon hybrids. Very recently, excellent performances have been achieved in a 3D interconnected carbon nanotube/layered MoS2 nanohybrid network via hydrothermal route, delivering discharge capacities of 512 mA h g1 at a current density of 100 A g1 and 1679 mA h g1 over 425 cycles at 1 A g1, accompanying with 96% discharge capacity retention (Fig. 10) [137]. Flexible and free-standing MoS2 hybrids, based on porous carbon nanofiber (PCNF) [138], CNT [58,139], active carbon fiber (ACF) [140], and graphene/CNT frameworks [141], have attracted much attention to date. Notably, dually protected flower-like MoS2 exhibited a superior rate capability of 300 mA h g1 at a current density of 20 A g1 and an ultralong cyclability of 728 mA h g1 at 5 A g1 after 1000 cycles [142]. In brief summary, CNT provides an excellent chance to improve the rate performance of MoS2/CNT hybrids, particularly under an

Fig. 10. TEM images of (a, b) MoS2/CNT. (c) The batteries using MoS2/CNT nanohybrid as active materials were tested under various current densities ranging from 1 A g1 to 100 A g1. Reproduced with permission from Ref. [137]. Copyright 2015, Elsevier.


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Fig. 11. (a) Three-dimensional representation of the structure of MoS2. Single layers, 6.5 Å thick, can be extracted using scotch tape-based micromechanical cleavage. Reproduced with permission from Ref. [31]. Copyright 2011 Macmillan Publisher Lts. (b) Schematic images of MoS2 nanoflowers, nanoplates and micorspheres via different approaches. (c) Voltage profile of 60MoS2 free-standing electrode along with its corresponding structure. Inset marked by arrows: schematic representation showing the predicted mechanism for Na insertion and extraction into an idealized MoS2/rGO free-standing composite paper electrode. Reproduced with permission from Ref. [48]. Copyright 2015, Elsevier.

ultra-large current density. However, there exist several great challenges for CNTs (CNFs) to be tackled. On one hand, as to the counterpart as conventional conductive agent (carbon black) or collector (copper foil), a high cost-performance of CNTs (CNFs) may be of considerable importance; on the other hand, the accommodation mechanism of volume expansion of MoS2 during cycling for CNTs (CNFs) is also a problem to be dealt with due to the unique tubular or fibrous structure. We depicted the typical MoS2 morphologies and their structural evolution during Li/Na discharge/charge in the Fig. 11. Molybdenum disulfide has established itself as the most promising intercalation host candidate in LIBs, and displayed the leading example of layered materials for energy storage and energy conversion. However, others are also of great interest in similar applications. 3.1.2. Other layered TMDs for LIBs Other layered TMDs mainly include WS2, TiS2 and VS2. WS2 acts as semiconductor with a layered structure. As a sharp contrast, TiS2 and VS2 could be classified into semimetal and metal, respectively. All of them would be stated as follows. 3.1.2.1. WS2. WS2/carbon hybrids possess a strong Li-storage capability [143,144]. Shiva et al. [143] produced uniform graphene-like few-layered WS2 supported on rGO which delivered specific capacities of 400–450 mA h g1 at a current density of 100 mA g1 at 50 cycles, and steady capacities of 180–240 mA h g1 at 4 A g1. Longterm Li cycling stability is also worth noting. WS2 anchored on rGO displayed a reversible charge capacity of 369 mA h g1 after 250 cycles at a large current density of 1 A g1 [112]. In a recent interesting work, free-standing sandwich-type WS2–nanotubes/GNs hybrids were observed to maintain a capacity of 318.6 mA h g1 over 500 cycles at a current density of 1 A g1 [145]. WS2/carbon nanofibers (WS2/CNFs) obtained by an electrospinning method afforded a first-cycle discharge/charge capacity of 941/756 mA h g1 at 100 mA g1 and maintained a capacity of 458 mA h g1 after 100 cycles at 1 A g1 [146]. Nonmetal-doped WS2 also recently aroused some interest. The sulfur-doped WS2 delivered a reversible capacity of 566.8 mA h g1 at a current density of 0.8 A g1 after 50 cycles. The sulfuration process can be readily extended to other dichalcogenides, and may provide a class of versatile electrode materials for lithium-ion

batteries with improved electrochemical characteristics [147]. Compared with the pristine WS2, the WS2-super P nanocomposites prepared by hydrothermal and sulfide reduction reactions, exhibited initial discharge capacity of 421 mA h g1, initial Coulombic efficiency (81%), and good retentive capacity of 389 mA h g1 after 200 cycles [148]. Up to now, the investigations into 2D TMDs have mostly been focused on MoS2, WS2 and SnS2. However, these 2D TMDs are all semiconductors with poor conductivity, to some extent, limiting their electrochemical performances. Logically, semimetal and metallic TMDs have attracted extensive scientific attention. 3.1.2.2. TiS2. TiS2 adopts a hexagonal close packed (hcp) CdI2 – type structure, different from MoS2. In this motif, half of the octahedral holes are filled with a Ti4+, whose center is surrounded by six sulfide ligands in an octahedral structure. Each sulfide is connected to three Ti centers with a pyramidal sulfur geometry. Notably, titanium disulfide is a semimetal, meaning small overlap of the conduction band and valence band. As early as 1997, microtubular TiS2 by CVD delivered a capacity of 256 mA h gl [149], with an average intercalation voltage of 2.16 eV (LiTi2S4) [150]. However, its discharge voltage was relatively low as compared with other cathode materials for lithium batteries. Therefore, further investigation showed an excellent cycling performance of hexagonal TiS2 as an anode (1.40–3.0 V) because of dispelling the irreversible dissolution of Li2S into the electrolyte below 0.50 V [151]. In 2013, the introduction of multi-walled carbon nanotubes (MWCNTs) to TiS2 as an anode gathered an initial specific capacity of 450 mA h g1 with 80% capacity retention at a current density of 100 mA g1 after 50 discharge-charge cycles [152]. Very recently, another type of rechargeable battery, i.e. TiS2-S battery, was also reported by Ma et al. [153]. TiS2 also demonstrated important applications in all-solid-state Li batteries for hybrid electric vehicles and plug-in electric vehicles [154–156]. As a typical characteristic, an all-solid-state TiS2 battery retained a high capacity of 180 mA h g1 at 0.2 C over 300 discharge-charge cycles when operated at 393 K [156]. Influence of heterovalent substitutions Ti–V and Ti–Cr on the electrochemical intercalation of lithium into TiSe2 has been investigated, suggesting that the substitution of vanadium for titanium

200


provides a significant increase in the intercalation capacity of the material [157]. 3.1.2.3. VS2. Different from MoS2, VS2 monolayer is metallic with a spin-polarized ground state [158]. VS2 has a layered structure with a high potential for Li intercalation, generally considered as a cathode for LIBs due to an open circuit voltage of 2.43 V (vs Li/Li+) [159]. However, computed average open circuit voltage (OCV) of 0.93 V (vs Li/Li+) for Li intercalation on metallic VS2 monolayer suggested that VS2 monolayer can be utilized as a promising anode material for LIBs, with a higher theoretical capacity of 466 mA h g1 and higher Li diffusion rate than those of MoS2 or graphite [158]. In a recent research, Li-cycling reaction was exhibited in Eq. (4). The result explicitly verified the Murugan’s conclusion about VS2 as the cathode material of Li-ion battery.

VS2 þ xLi þ xe $ Lix VS2 ð2:2—2:48 V v s Li=Li Þ: þ

þ

ð4Þ

The VS2, just like NASICON-type structural LiTi2(PO4)3 [47], possesses an average voltage at which Li insertion-extraction occurred from 2.2 V to 2.5 V. This voltage, unfortunately, was too high when used as an anode material and too low for a cathode material in LIBs. In short summary, layered TMDs, i.e. WS2, TiS2 and VS2, as electrode materials remain intriguing because of high theoretical capacity and power density. As a consequence, they are promising electrode materials for some special occasions such as solid-state lithium batteries. 3.2. Non-layered TMDs for LIBs Generally speaking, materials with a high voltage (>2 V vs. Na (Li)) are defined as cathode materials for LIBs or SIBs. Accordingly, FeS2 (1.6 V vs. Li) could be an anode material for LIBs. 3.2.1. FeS2 Pyrite, the most common sulfide minerals, belongs to semiconductor material with a band gap of 0.95 eV. FeS2 was ever utilized to make thermal battery [160,161], polymer electrolyte battery [162], and non-rechargeable lithium batteries by Energizer brand as a newer commercial application [21]. In the late ‘90 s, the charge-discharge mechanism of FeS2 in room-temperature was explored [163], and ever utilized as the cathode materials (theoretical energy density of 860 Wh kg1 on the average voltage of 1.6 V) [21,164], accompanying with the presence of Li2FeS2 [165]. In 2015, Walter et al. [166] also assigned the FeS2 as a cathode for LIBs, delivering a capacity of 715 mA h g1 and an average energy density of 1237 Wh kg1 after 100 cycles. The reaction mechanism was exhibited as follows: þ

Intercalation : FeS2 þ xLi þ xe $ Lix FeS2 ð0 < x < 2; 1:5—1:7 VÞ ð5Þ þ

Conversion : Lix FeS2 þ ð4 xÞLi þ ð4 xÞe $ Fe þ 2Li2 S ð1:5 VÞ ð6Þ In a recent report [167], author started with the fundamental chemistry of FeS2 for capacity fading mechanism of Li/FeS2 batteries and proposed three facile strategies for stabilizing capacity, i.e. use of vinylene carbonate (VC) as the electrolyte additive to form robust solid electrolyte interphase (SEI), promotion of dense and uniform deposition of Li, and addition of carbon interlayer between the FeS2 electrode and separator. In present age, further progress has been made to investigate the rechargeable application of FeS2. Several synthesis approaches have been explored to yield FeS2 materials with different morphologies and structures, including solid-state method [168], wet solution synthesis [169], high-temperature solvent method [166]

and hydrothermal approach, etc. [170,171]. Typical morphologies include nanocrystal [166], hierarchical nanostructured hollow microspheres [172], porous FeS2 particles [168], rGO-wrapped FeS2 microspheres [173], micro-spherical FeS2/CNT powders, etc. In 2012 [170], CTAB-assisted hydrothermal FeS2 powders afforded a reversible capacity of 413 mA h g1 at a current density of 445 mA g1 after 35 cycles. Subsequently, similar electrochemical characteristics were observed in hydrothermally microspherical FeS2/CNT powders, i.e. 491 and 370 mA h g1 after 50 cycles at 0.1 and 1 C (1 C = 890 mA g1), respectively [174]. Application of graphene was believed as a viable approach to improve the cycling performance because of the strong accommodation to volume expansion of active materials during Li-cycling. For example, hydrothermal FeS2/rGO microparticles, as an anode material for LIBs, were allowed to afford a reversible capacity of 1001 mA h g1 at a current density of 100 mA g1 over 60 cycles [175]. In other substances, remarkable cycling stabilities have been corroborated in the solvothermal FeS2/N-doped graphene (FeS2/ N-G) composites [176], pyrite FeS2 microspheres wrapped by rGO [173] and core-shell nano-FeS2@N-doped graphene [177], i.e. 970 mA h g1 at a current density of 890 mA g1 after 300 cycles, 380 mA h g1 even at a current density of 8900 mA g1 (10 C) after 2000 cycles [173], and a remarkable specific energy of 950 W h kg1 at 0.15 kW g1 and a stable cycling performance of 600 Wh kg1 at 0.75 kW g1 after 400 cycles [177]. Very recently, FeS2@carbon fiber electrode, through electrospinning followed by vacuum sulfidation [178], demonstrated a lithiation capacity of 600 mA h g1 with an initial discharge energy density of 1000 Wh kg1, and 840 Wh kg1 after 100 cycles in the voltage range of 1.0–3.0 V (vs Li/Li+). It is expected that the rechargeable Li–FeS2 system will become competitive in the future. 3.2.2. CoS2 CoS2 also belongs to the pyrite group, which was ever used in thermal battery cathode instead of pyrite. As a rechargeable battery electrode material, the electrochemical reaction mechanism of CoS2 is still unobvious because of two models emerged in the references. We list the CoS2 here for providing more insights into the TMDs. (1) Calculated Gibbs free energy change (DG) and electromotive force (E) values suggested possible reaction mechanism of CoS2 anode during intercalation of Li ions [179], i.e. intercalation followed by redox (Eqs. (7), (8)). Jin et al. [180] adopted the theory. þ

ð7Þ

þ

ð8Þ

CoS2 þ xLi þ xe $ Lix CoS2 ð1:6 VÞ CoS2 þ 4Li þ 4e $ Co þ 2Li2 S ð0:02 VÞ

(2) In-situ TEM investigations showed that electrochemical reaction of CoS2 in LIBs can be expressed as CoS2 þ þ4Li þ 4e $ Co þ 2Li2 S ð8Þ [181]. The researches by Shadike et al. [182], Wang et al. [183], and He et al. [184] adopted the viewpoint. In our opinions, it was very likely that CoS2 adopted the redox mechanism to cycle Li ions because the cyclic voltammetry pattern explicitly exhibited the reduction potential of 1.5 V, responding to the reaction given in Eq. (8). Not any peak appeared in the high potential for Li-insertion like as MoS2. In brief summary, traditional sulfide minerals are widely distributed in nature, and possess some obvious advantages such as low cost and mild toxicity. However, the traditional pyrorefining process of sulfides can cause serious environmental pollution, exerting great pressure on fragile natural ecological environment. Hence, natural mineral FeS2 directly utilized as advanced electrode materials will increase their economic value and decrease the environmental harm.


4. Sodium-ion batteries and others Today, sodium ion batteries (SIBs) have been considered as cheaper ways to store energy for so-called ‘‘large format” applications [185,186], including stationary energy storage, grid storage, renewable energy storage as wind and solar power, backup systems as uninterrupted power supply, and automotive. More detailed facts on energy density of SIBs can refer to the review by Yabuuchi et al. [187]. 4.1. Layered materials for SIBs Recently, MoS2 and TiS2 are verified to possess a wide application in new-concept rechargeable batteries such as magnesiumion battery [188], and calcium-ion batteries for TiS2 [189]. In a recent report [190], thin TiS2 nanoplates are capable of fast and reversible Na+ intercalation and deintercalation, delivering a large capacity close to full Na+ intercalation (186 mA h g1), high rate capability (similar to 100 mA h g1 at 10 C) and satisfactory cycling stability. Application of MoS2 for SIBs will be afterward described in detail as follows. It is generally accepted that, similar to the reaction mechanism of Li with MoS2 [46,191], Na insertion and extraction into an idealized MoS2-based composite electrode could be exhibited in Eqs. (9), (10). As a graphene-like structure, monolayer MoS2 could achieve a reversible capacity of 146 mA h g1 (average potential: 0.75–1.25 V) for SIBs [192].

Intercalation : MoS2 þ xNaþ þ xe $ Nax MoS2

ð9Þ

Conversion : Nax MoS2 þ ð4 xÞNaþ þ ð4 xÞe $ Mo þ 2Na2 S

201

MoS2 nanoflowers/rGO composites were hopeful for affording high reversible capacity of sodium ion batteries (SIBs) [198]. Subsequently, the role of hydrothermal MoS2/rGO for storage of Na+ was also surveyed, showing Na-cycling capability of 227 mA h g1 at 320 mA g1 after 300 cycles [199]. Finally, further attempts have made certain progress. 3D hydrothermal MoS2 nanoflowers/rGO composites demonstrated a reversible specific capacity of 575 mA h g1 at 100 mA g1 [200], the binder-free electrode made of MoS2 and CNF could still deliver a charge capacity of 283.9 mA h g1 after 600 cycles at a current density of 100 mA g1, indicating a very promising anode for long-life SIBs [201]. Similar result was also observed in the hydrothermal MoS2/C nanospheres, i.e. 400 mA h g1 for 300 cycles at 1 C [202]. In addition, solvothermal MoS2 nanostructures were comparable with those via hydrothermal route. Typical ultralong wormlike MoS2 nanostructures delivered a sodium insertion capacity of 675.3 mA h g1 at 61.7 mA g1 in the first cycle while possessing a capacity of 410.5 mA h g1 after 80 cycles [72]. Recently, several reports on MoS2-based sodium ion batteries (SIBs) have been published. MoS2@C-CMC electrodes, in which MoS2 nanosheets were vertically aligned on carbon paper with hydrothermal route followed by heat treatment, exhibited a stable cycling performance of a charge capacity of 286 mA h g1 when cycled at a current density of 80 mA g1 after 100 cycles [203]. In addition, the microwave-assisted synthesis [204], electrospinning method [201], electrospinning combined with atomic layer deposition (ALD) [205], and spray pyrolysis [128] were also allowed to garner the MoS2-carbon composites for anode materials of sodium ion batteries (SIBs). Notably, Electrochemically derived 2H-to-1T MoS2 for rechargeable magnesium-anode batteries was also reported [206].

ð10Þ 4.1.1. Direct-exfoliation in solution Directly-exfoliated few-layered MoS2 nanosheets exhibited a high reversible sodium storage capacity of 386 mA h g1 at a current density of 40 mA g1 after 100 cycles [193]. In another reference, single-layered MoS2 sheets directly exfoliated in 1-methyl2-pyrrolidinone (NMP) afforded a sodiation capacity of 165 mA h g1 after 50 cycles when measured at a current density of 20 mA g1 [194]. Polymer-decorated MoS2, e.g. poly(ethylene oxide)-intercalated MoS2 composites via chemical exfoliationrestacking method, exhibited a specific capacity of 148 mA h g1 at the 70th cycle under a current density of 50 mA g1 [195]. From 2014, several researches into MoS2-rGO composites for sodium ion battery application have been carried out [191]. The group of David [191] fabricated a flexible paper anode composed of acid-exfoliated few-layered MoS2 and rGO by means of vacuum filtration. The crumpled hybrid afforded a Na cycling property, e.g. about 230 mA h g1 after 20 cycles at 25 mA g1. Obviously, the exfoliated MoS2 nanosheets and their rGO composites possessed a relatively low reversible capacity; more importantly, the exfoliated MoS2 could not sustain the large-current shock. Two possible reasons were given as follows: (1) the absence of 3D structure in MoS2 nanosheets, and (2) considerable difficulty in the formation of robust chemical bond between MoS2 nanosheets and matrix. Fortunately, the hydrothermal route is of great potential in addressing the above-mentioned two questions. 4.1.2. Hydrothermal route Large-current density charge/discharge performances have been achieved in hydrothermal MoS2 microflowers [196,197], delivering Na–storage capabilities of 236 mA h g1 at 10 C [196], 350 mA h g1 at 50 mA g1 and 195 mA h g1 at 10 A g1 after 1500 cycles [197].

4.2. Non-Layered MX2 materials (FeS2) In 2008, Kim et al. [207] suggested that the conversion reaction mechanism could be adopted for Na-cycling in FeS2, affording the first discharge capacity of 447 mA h g1 at a current density of 50 mA g1 with severe capacity fading in the afterward chargedischarge cycling. In subsequent researches, the suggestion has been seriously challenged. In 2014, Kitajou pointed out that the Na-cycling mechanism of FeS2 was similar with that of MoS2 [208]. As sodium-ion anode materials, FeS2 nanocrystals via hightemperature solvent method delivered a reversible capacity above 500 mA h g1 for 400 cycles at a current density of 1 A g1 [166]. However, the important problem encountered in rechargeable SIB batteries with carbonate-based electrolytes was the limited cycle life caused by the conversion-type reaction (FeS2 + 4Na ? Fe + 2Na2S, 0.8 V). Therefore, a compatible NaSO3CF3/diglyme electrolyte [209] and the cut-off voltage of 0.8 V have been utilized in FeS2 microspheres to yield the high-rate capability (170 mA h g1 at 20 A g1) and unprecedented long-term cyclability (90% capacity retention for 20,000 cycles) because of stable electrically conductive layer-structured NaxFeS2 [210]. Such a method has been previously proposed for the electrochemical exfoliation of MoS2 in Fig. 3.

Intercalation : FeS2 þ xNaþ þ xe $ Nax FeS2

ð11Þ

Conversion : Nax FeS2 þ ð4 xÞNaþ þ ð4 xÞe $ Fe þ 2Na2 S ð12Þ However, the up-to-date research showed that the quantumconfined length scales of pyrite nanoparticles sustained reversible conversion reactions in sodium ion and lithium ion batteries attrib-

202


Fig. 12. Scheme illustrating the benefit of using ultrafine nanoparticles in sodium-sulfur conversion systems and the kinetic and thermodynamic limitations that make cycling of bulk electrode materials irreversible and ultrafine nanoparticles reversible. LD corresponds to the diffusion length of Fe ions to perform cation exchange, and DFeS2 is the diameter of the FeS2 bulk or nanoparticle. Reproduced with permission from Ref. [211]. Copyright 2015, The American Chemical Society.

Table 1 Physical properties and electrochemical Li/Na cycling data of typical TMD-based materials. Morphology

Synthesis route

Current density (mA g1)

Reversible capacity (mA h g1)

Capacity retention after n cycles

Refs.

MoS2/TiO2 Ultrathin MoS2/GNs MoS2/GNs films MoS2/GNs/CNT FeS2/rGO MoS2 microflowers MoS2/C nanospheres FeS2 microspheres

Wetness impregnation CVD Solution-phase/Annealing Hydrothermal Solvothermal Hydrothermal Hydrothermal Solvothermal

6C 4 A g1 10 A g1 5 A g1 8.9 A g1 (10 C) 10 A g1 1C 20 A g1

160 620 553 728 380 195 400 170

75.2% (n = 1000) 100% (n = 1000) 90% (n = 7500) 100% (n = 1000) 100% (n = 150 1000) 100% (n = 1500) 100% (n = 300) 90% (n = 20,000)

[79] (LIBs) [109] (LIBs) [114] (LIBs) [142] (LIBs) [173] (LIBs) [197] (SIBs) [202] (SIBs) [210] (SIBs)

uted to a nanoparticle size [211]. Reversible capacities over 500 and 600 mA h g1 for sodium and lithium storage were observed in ultrafine nanoparticles (Fig. 12). Mg-ion battery and Al-ion battery with FeS2 cathode were also surveyed. The investigation into the discharge/charge reaction mechanism of FeS2 cathode material for aluminum rechargeable battery at 55 °C was carried out [212]. As a result of the redox reaction, FeS2 was transformed into low crystalline FeS and amorphous Al2S3. In addition, pyrite FeS2 chemically combined with lithium polysulphide (Li2Sn) to form active Li2FeS2+n complexes, which can be used in lithium-sulfur batteries [213]. In brief summary, transition metal dichalcogenides have demonstrated huge potential as anode materials in SIBs due to their high capacity, low cost and abundance. Despite those, the research into transition metal dichalcogenide materials is obviously inadequate as compared with other materials including metal oxides and pure metal anode materials.

5. Conclusions and outlook As demonstrated in the Table 1, TMDs possessed remarkable cycling and high-rate performances for Li-ion or Na-ion, enabling

them to be promising candidates for large-powered and highreliable devices. In this review, a brief commentary on the current state-of-theart TMD synthesis and challenges of performance has been presented. To meet the demand of the emerging power battery, and overcome the shortcomings of traditional electrode materials such as low capacity, poor environmental adaptability and insufficient large-current charge/discharge capability, artificially designed TMD-based hybrids have been synthesized. With specific attention on rechargeable electrodes, a number of reports have provided frameworks of TMD-based nanohybrid categories, ranging from graphene-containing, CNT-containing, amorphous carbon-containing materials to their artificially designed combinations. To garner remarkable electrochemical performances for TMDbased nanomaterials, several strategies concerning structure may be helpful. (1) Existence of complex chemical groups and robust chemical bonds between MX2 and matrix to accommodate volume expansion of electrochemically active materials during discharge/ charge. Hydrothermal method is a viable approach to form robust chemical bonds. (2) Artificially designed structures to improve electron transportation and ion diffusion, such as MoS2 vertically aligned on the single-layered graphene sheets (MoS2-NS/G) [109],


MoS2 vertically aligned on carbon paper [203], freestanding MoS2@graphene nanostructure [114], compact amorphous carbon layer for MoS2-based structure [127], 3D interconnected CNT/ MoS2 network [137], FeS2 microspheres wrapped by rGO [173] and core-shell nano-FeS2@N-doped graphene [177]. (3) Quantum-confined length scales of nanoparticles to change electrochemical mechanism [211]. However, several challenges remain to be addressed in the future: (a) Low-cost, scalable and environmentally-friendly exfoliation of single-layered TMDs is also of great significance for the practical application of TMD-based composites. Solutionbased intercalations including direct ultrasonic and chemical-assisted exfoliation are proved to be advantageous due to their high yields and easy implementations. However, the subtle combination of various synthetic routes is vital to yield more morphologies, and further give TMDs more features. (b) Almost all TMDs are semiconductor with poor conductivity and unsatisfactory flexibility. Therefore, to improve highrate and long-term cycling performance of corresponding electrodes, the decoration of TMD nanosheets and construction of 3-D microstructure by means of carbon materials may be of huge importance in increasing the conductivity and maintaining structural integrity. How to construct 3Dstructured architectures needs to be deliberately considered, and the ingenious synthetic route needs to be rationally designed. Hydrothermal method may be a practical and scalable approach to 3D-structured nanoparticles. Hightemperature solution (using Molybdenyl acetylacetonate as Mo source) is also an attemptable route to form nanosized 3D structure. (c) How to improve the dispersity of these nanocomposites and enhance the interface interaction between nanoparticles and TMDs nanomaterials during charge/discharge should be addressed. In our opinions, there exist several routes to solve above questions. (1) Introduction of graphite-based carbon materials such as reduced graphene oxide or graphene may be helpful because the few-layered graphite nanosheets could effectively block the re-aggregation of active materials, and simultaneously, provide necessary conductivity. (2) Synthesis and application of monodispersed nanoparticles could increase the contact areas between active materials and electrolyte and enhance the Li (Na) ion diffusion path. (3) Carefully selection of electrolyte with suitable additives such as fluoroethylene carbonate could be useful to improve electrochemical performance of corresponding electrodes. And (4) 3D-structured nanoparticles mentioned above should be constructed. (d) To some extent, although the rechargeable performance of TMD-based materials has been studied, the correspondence between structure and performance is not clear yet. The actual mechanism underlying these electrochemical characters is somewhat mystifying to researchers. Further investigations into the trigger origin of electrochemical reaction and the principle of capacity retention need to be carried out. In-situ TEM observation [214], in-situ Li-NMR observation [215] and Operando X-ray techniques [216] combined with cyclic voltammetry technology are strong tools for researches of electrochemical mechanism. First principles density functional theory (DFT) is an important method for theoretical evaluation [217]. (e) TMD-based composites have displayed many unique synergistic attributes. However, in-depth understanding of the small size effect and coupling effect between

203

simple-layered TMDs and nano-sized carbon materials may be necessary to provide critical insight into reaction mechanism and relation between structure and performance. (f) The high potential of TMDs is a double-edged sword, making them into a dilemma. As a cathode material, the Li dendrite should be a hidden danger for safety; as an anode, low opencircuit voltage inevitably results in a low reversible capacity. Thus, applicable occasions should be carefully thought over. Generally speaking, TMDs could be safely employed as anode materials for power battery because the ultra-large current charge/discharge capability and the superior cycling stability could be sufficient to make up for the shortcomings derived from high potential. (g) The research into the temperature effect fairly lacks data. Thermodynamics and kinetics of TMD electrode should be highlighted to clearly understand the change origin of electrochemical performance with the environmental change. Because of the superior long-term cycling stability, largecurrent discharge probability and low-cost, TMD materials, particularly MoS2 and FeS2, have slowly re-established themselves as promising candidates for vital components of high-energy storage devices. There is much evidence that TMD-based nanomaterials will soon appear in heavy-duty energy-storage devices such as electric cars, energy-storage stations and smart grids in the predictable future.

Acknowledgements This work is supported by the Department of Science and Technology of Guangdong (No. 2014B010123001, No. 2015B090901030, No. 2016B050502004), Guangzhou Science Technology and Innovation Commission (No. 2016201604030013), and Foshan shunde CG Electronic Industry Co., Ltd.

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