Applied Chemistry for Engineering (공업화학)

The Korean Society of Industrial and Engineering Chemistry (한국공업화학회)
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Properties, Preparation, and Energy Storage Applications of Two-dimensional Molybdenum Disulfide

2차원 이황화몰리브덴의 성질, 제조 및 에너지 저장 소자 응용

  • Choi, Bong Gill (Department of Chemical Engineering, Kangwon National University)
  • 최봉길 (강원대학교 화학공학과)
  • Received : 2019.02.28
  • Accepted : 2019.03.11
  • Published : 2019.04.10
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Abstract

Two-dimensional (2D) ultrathin molybdenum dichalcogenides $MoS_2$ has gained a great deal of attention in energy conversion and storage applications because of its unique morphology and property. The 2D $MoS_2$ nanosheets provide a high specific surface area, 2D charge channel, sub-nanometer thickness, and high conductivity, which lead to high electrochemical performances for energy storage devices. In this paper, an overview of properties and synthetic methods of $MoS_2$ nanosheets for applications of supercapacitors and rechargeable batteries is introduced. Different phases triangle prismatic 2H and metallic octahedral 1T structured $MoS_2$ were characterized using various analytical techniques. Preparation methods were focused on top-down and bottom-up approaches, including mechanical exfoliation, chemical intercalation and exfoliation, liquid phase exfoliation by the direct sonication, electrochemical intercalation exfoliation, microwave-assisted exfoliation, mechanical ball-milling, and hydrothermal synthesis. In addition, recent applications of supercapacitors and rechargeable batteries using $MoS_2$ electrode materials are discussed.

Keywords

1. Introduction

Ever-increasing demand for energy to address depletion of fossil fuels significantly require the development of renewable and sustainable energy storage and conversion systems[1-4]. Rechargeable and storage batteries or secondary cells, can be charged, and discharged into a load, and recharged many times for use, which are designed to deliver high energy and power densities in many energy applications[5-8]. The rechargeable batteries are used in portable electronics and power supplies, electric vehicles, and energy storage power stations, because of their high energy density, low cost, lightweight, size, and long lifetime. A lithiumion battery is one of the most developed batteries, which consists of anode (e.g., graphite) and cathode (e.g., lithium metal oxide) separated by the electrolyte. Recently, in order to increase energy performance of Li-ion batteries and address limitation of lithium resources, sodium and magnesium-ion batteries have attracted a great deal of attention because of their low cost, natural abundance, and similar insertion mechanism to lithium ions[9-12]. Supercapacitors or electrochemical capacitors store electrical charges by typically two mechanisms: (1) formation of an electrical double layer between electrode and electrolyte and (2) reversible Faradaic reaction at the electrode/electrolyte interface[13,14]. Compared to rechargeable batteries, supercapacitors are capable of delivering higher power density, higher rate capability, and more stable cycle life. However, most supercapacitors suffered from lower energy densities than batteries[15-19].

Ultrathin and layered two-dimensional (2D) nanosheets, especially, graphene materials, have drawn tremendous attention and shown great promise for a wide range of applications[20,21]. Since graphene materials have shown a great potential for the application of energy storage systems, and other 2D materials, such as, metal oxides, hydroxides, dichalcogenides, and carbides have been extensively investigated[22-25]. Among them, molybdenum disulfide (MoS2) is widely interested in many applications because of its unique 2D morphology and physicochemical properties[26-29]. The 2D morphology enables MoS2 to provide high specific surface area compared to its bulk counterpart. The ultrathin layers of MoS2 allow to high flexibility and strong mechanical strength in contrast to MoS2 bulk materials. The attractive merits of MoS2 have been demonstrated in many applications of electronics, sensors, and biomedicine. In particular, the high conductivity, large surface area, and active edge sites of MoS2 made it a promising electrode material for development of energy conversion and storage applications, such as, rechargeable batteries, supercapacitors, electrocatalytic reaction, and solar cells.

In this article, we reviewed unique structures and properties and preparation methods of MoS2 nanosheets. Various experimental techniques were introduced to investigate MoS2 nanosheets. We summarized the recent synthetic methods for preparation of MoS2 materials to achieve nanostructure, extraordinary properties, and enhanced electrochemical performances, including mechanical exfoliation, chemical intercalation and exfoliation, liquid phase exfoliation by direct sonication, electrochemical intercalation exfoliation, microwave-assisted exfoliation, mechanical ball-milling, and hydrothermal synthesis. Following that, the main applications of MoS2 materials are reviewed, involving energy storage systems of supercapacitors and rechargeable batteries.

2. Properties of MoS2

Natural MoS2 materials have a crystalline structure of trigonal prismatic or an octahedral Mo coordination[30]. Particularly, MoS2 has different polymorphs depending on the electron filling in the valence d-orbitals of Mo atoms, involving 3R of rhombohedral polymorph with three-layers and Mo atoms in trigonal prismatic coordination, 2H of hexagonal with two-layers and Mo atoms in trigonal prismatic coordination, and 1T of trigonal with one-layer and Mo atoms in octahedral coordination[31]. MoS2 shows diverse electrical and optical properties according to the different atomic arrangement of MoS2. The 2H MoS2is naturally abundant and stable, and exhibits indirect bandgap of 1.29 eV for bulk state and direct bandgap of 1.90 eV for monolayer state[32]. The 1T MoS2 shows hydrophilic and metallic nature; the electrical conductivity of 1T is 107 times higher than 2H phase. The 1T MoS2 has a similar electrical conductivity with metallic materials of copper and gold. The attractive features of 1T MoS2 have made it as a promising electrode material in applications of energy storage and conversion.

The transformation of 2H into 1T phase in MoS2 can be characterized by various analytical techniques, such as, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman, photoluminescence (PL), extended X-ray absorption fine structure (EXAFS), and ultraviolet-visible (UV-vis)[30]. The measurement of XRD using bulk 2H MoS2 and 1T MoS2 samples provides exfoliation states and crystalline structure of MoS2. The exfoliated 1T MoS2 compared to 2H MoS2 exhibits reduced (002) peak with blue shift, indicating expansion of the interlayer spacing and low crystallinity. In addition, most XRD peaks of 1T MoS2 are reduced when MoS2 sheets are exfoliated. Raman spectroscopy provides a clear evidence for the formation of 1T phase MoS2. The 2H MoS2 shows characteristic peaks of 380 cm-1 (E12g mode associated with opposite vibration of two S atoms with respect to the Mo atom) and 400 cm-1(A1g mode associated with the vibration of only S atoms in opposite directions)[30]. Compared to 2H MoS2, the 1T MoS2 shows prominent peaks at around 150, 220, 280, 330, and 400 cm-1, corresponding J1, J2, E1g, J3, and A1g modes, respectively. As the 1T phase increases, E1g, and A1g decrease, while J1, J2, and J3 modes increase[30]. This observation in Raman spectroscopy indicates the presence of the as-syn-thesized 1T phase in MoS2 sheets. The atomic arrangement of Mo and S between the triangle prismatic and octahedral coordination structures differ from each other. Hence, Mo 3d and S 2p XPS investigation is one of the most promising way to detect 1T phase of MoS2. In addition, peak fitting of Mo 3d and S 2p XPS provides accurate 1T contents of MoS2. The different atomic structures of MoS2 result in different optical properties, and thus UV-vis absorption spectrum of 2H MoS2 and 1T MoS2 are a good indicator to distinguish phases of MoS2. Typically, 2H MoS2 shows three peaks at 440 (attributed to the quantum effect of small lateral-sized MoS2 sheets), 610, and 650 nm (attributed to the energy split large lateral dimensions)[33]. The formation of 1T phase removes the characteristic peaks of 2H MoS2, indicating metallic properties of 1T MoS2.

3. Preparation of MoS2

Tremendous efforts have been attempted to synthesize MoS2 nano-sheets in terms of production cost and scalability. Various methods have been employed to prepare MoS2 in order to achieve nanostructure, special properties, and superior performance, including mechanical exfoliation, chemical intercalation and exfoliation, liquid phase exfoliaftion by direct sonication, electrochemical intercalation exfoliation, mi-crowave-assisted exfoliation, mechanical ball-milling, and hydrothermal synthesis[34-46].

3.1. Top-down approaches for preparation of MoS2

Similar to graphene materials, MoS2 nanosheets can be obtained from mechanical exfoliation of bulk layered MoS2 powers. Novoselov and co-workers successfully exfoliated MoS2 into single-layer MoS2 nanosheet with a high quality of crystalline structure and micro-sized lateral size, but the very small quantities are limited to many practical applications[36]. In order to enhance production rate of MoS2 nano-sheets, lithium ion-intercalation and exfoliation methods have extensively been developed[38,39]. Typically, this process is carried out in an atmosphere protected by inert gas, such as, an argon-filled glove box, using n-butyl-lithium at a certain temperature for a certain time of 5~70 h (Figure 1). The lithium-ion intercalation can be accelerated by the assistance of sonication, microwaves, and ball-milling. And also, a lithium-ion battery system was attempted to intercalate Li-ion into layered MoS2 sheets, in which bulk MoS2 and lithium foil were used as the cathode and anode, respectively[37]. By discharging, Li-ion effectively intercalates into bulk MoS2 sheets, thereby achieving efficient exfoliation of MoS2. Other alkali metals have been employed to exfoliate MoS2. The use of alkali metals as the intercalants are basically electron donors, and thus inducing strain force by the electron transfer from the intercalant. The strain force leads to facilitate the phase transition of MoS2 from 2H to 1T[34]. Compared to 2H MoS2, 1T MoS2 exhibits higher electrical conductivity, larger spacing distance, and more abundant active site surfaces. These features of 1T MoS2 are especially favorable to the electrochemical applications that require high electron and ion conductivity. The 1T phase of MoS2 was confirmed by various analytical instruments, such as, TEM, XRD, Raman, STEM, and XPS.

GOOOB2_2019_v30n2_133_f0001.png 이미지

Figure 1. Schematic illustration of exfoliation of MoS2 using alkali metals as intercalants and post-treatments of sonication and ball milling methods.

On the other hand, other top-down approaches have been developed. The mechanical ball-milling process can be used to prepare MoS2nanosheets without any additives or any other additional treatment for several hours. In particular, the milling time influenced the size of MoS2 and phase transition (2H to 1T)[40]. The use of microwaves was also reported by Reshmi et al. for exfoliating MoS2 and phase conversion of 2H into 1T[41]. Recently, supercritical CO2-assisted exfoliation method has been also developed by Xu and coworkers (Figure 2)[42]. The use of supercritical CO2 gas effectively exfoliated bulk MoS2 into mono- or few-layer MoS2 nanosheets. The pre-dispersion of MoS2 flakes in a mixture of ethanol and water was treated in supercritical CO2 gas condition at 80 °C and 16 MPa under stirring for 6 h. The introduction of CO2 gas induced stain forces into MoS2 sheets, and thus leading to phase transform of the 2H into the 1T phase of MoS2. Because the 1T phase of MoS2 is important in applications of electrochemical devices, numerous methods have been developed to control the phase of MoS2. The phase transition of 2H and 1T can be occurred by mechanical tensile and compressive strains onto the MoS2surfaces. Chi et al. reported that high pressure up to 81 GPa induced sufficient strain forces into MoS2 sheets, and thus leading to 1T phase by mechanical layer sliding[43]. However, this compressive pressure is extremely high and the process is very low throughput, which is hard to realize. In order to induce phase transition of 2H into 1T, electron injection, argon-plasma treatment, and chemical vapor treatment have successfully developed. More recently, our group reported scalable and straightforward process for exfoliating MoS2 materials using fluid dynamins into metallic MoS2 sheets, resulting in an impressive yield performance of 76.9% and a high concentration of 20 mg/mL[35]. In this process, ionic liquid was used as additives for stabilizing MoS2 dispersion and 1T phase.

GOOOB2_2019_v30n2_133_f0002.png 이미지

Figure 2. Schematic illustration of adsorption of CO2 on the singlelayer MoS2 and TEM image of 1T and 2H phases of MoS2 sheet (Reprinted with permission form ref. 42. Copyright (2016) American Chemical Society).

3.2. Bottom-up approaches for preparation of MoS2

Hydrothermal and solvothermal reactions are common for preparing MoS2 nanosheets[44-46]. A mixture of ammonium molybdate tetrahydrate ((NH4)6Mo7O24⋅4H2O) and thiourea was used as the precursors for the Mo and S elements. The precursors were thermally treated at 200 °C for 20 h. The typically obtained MoS2 sheets have a lateral size of 200 nm with 2~5 layers. In addition, 1T phase of MoS2 was also obtained with 1T contents of < 70%. In order to increase 1T contents, the temperature-assisted phase transform of MoS2 has been developed. The hydrothermal treatment at 180 °C for 24 h induced metallic phase up to 92.4%. The high temperature above 200 °C caused to the formation of dominant 2H phase (> 90%). The composition ratio of precursors is also critical factor for control the phase of MoS2. Liu et al.reported that changing the precursor ratio significantly influenced metallic contents of MoS2[45].

4. Applications of MoS2

Since the 2H MoS2 was synthesized for many applications, the 1T MoS2 has also synthesized and now been widely used in various application fields, such as, electrocatalysts (Figure 3), electronic devices, photovoltaic, sensors, and energy storage and conversion systems[27,34].

GOOOB2_2019_v30n2_133_f0003.png 이미지

Figure 3. (a) Cyclic voltammograms of various solutions. (b)~(e) Schematic illustrations for the evolution process of the 1T phase based on Mo (Reprinted with permission form ref. 46. Copyright (2017) American Chemical Society).

4.1. Supercapacitor

Supercapacitors, also called ultracapacitors or electrochemical capacitors, are power supply devices to bridge the gap between batteries and capacitors[1-10]. By separating electrolyte ions, supercapacitors can store electrical charges and provides high power density. The desirable supercapacitors can operate at high charge and discharge rates within a few seconds during long-term. Recent issue of supercapacitors is how to increase energy densities, which is generally low than lithium-ion batteries. Since the discovery of graphene materials, ultra thin-layered 2D materials are promising electrode materials to achieve a high capacitance, resulting in high energy density of supercapacitors [5]. Various 2D materials, including MXenes, metal oxides, metal hydroxides, and disulfide family, have been extensively attempted to enhance electrochemical performances of supercapacitors[5]. Among them, MoS2 has been attracted a great deal of attention for development of supercapacitors because of its high theoretical capacitance value and unique structural properties of MoS2. The porous MoS2 thin films were prepared by Choudhary and coworkers based on a direct magnetron sputtering method[47]. The MoS2 film electrode showed a high gravimetric capacitance of 330 F/g at 10 V/s (high rate capability) and a good cycling stability of 97% retention over 5,000 cycles of charging/discharging measurement. Karade et al. synthesized ultrathin and layer MoS2 nanosheets by a chemical deposition method, and demonstrated as energy storage electrodes by a high specific capacitance of 576 F/g at 5 mV/s and an excellent cycle life (82% retention over 3000 cycles)[48]. However, 2H MoS2 electrodes limited to practical supercapacitor electrodes because of their poor electrical conductivity. In contrast, the metallic MoS2 electrodes provide more advantages of high electrical conductivity, a high hydrophilicity, and a large interlayer space. Acerce and coworkers successfully fabricated 1T MoS2 film electrodes by a chemical intercalation-exfoliation and post vacuum filtration methods[49]. The 1T MoS2 film electrode had a thickness of 5 μm and film packing density of 2.5 mg/cm2, in which 1T content is approximately 70%. The 1T MoS2 film electrode showed an extremely high volumetric capacitance of 400~650 F/cm3 at a scan rate of 20 mV/s. In addition, this MoS2 film electrode had high specific capacitance at high scan rate of 200 mV/s. Hence, high energy and power densities were achieved to 0.11 Wh/cm3 and 1.1 W/cm3, respectively. Other researchers have focused on synthesis of 1T MoS2 materials and applied them into supercapacitor electrodes. Thi et al. reported that synthesized 1T MoS2 nanoflower-based electrodes show a high specific capacitance of 259 F/g at 5 mV/s with a good long-term stability during 1,000 cycles[50]. On the other hand, MoS2/carbon composite electrodes have been developed in order to address intrinsically low electrical conductivity of MoS2. Various nanocarbon materials have been employed, including carbon nanofibers, graphene, carbon nanotubes, carbon spheres, and conducting polymers. Huang and coworkers synthesized composite electrodes consisted of carbon aerogel/MoS2, showing 260 F/g at 1 A/g with a good cycle life of 924% capacitance retention over 1,500 cycles[51]. The MoS2/graphene-incorporated into multiwalled carbon nanotube electrodes have been also prepared, showing a high specific capacitance, a high rate capability, and a good long-term stability. To increase ion and electron transport properties in electrode materials, three-dimensional (3D) MoS2/graphene aerogel composites were synthesized by a self-assembly of graphene oxide under hydrothermal treatment. As-synthesized 3D MoS2/graphene electrodes showed a high capacitance of 231 F/g with high energy (26 Wh/kg) and power (6,443 W/kg) densities[52]. Zhang and coworkers fabricated a rambutan-like composite electrode of MoS2-incorporated into carbon spheres[53]. This composite electrode exhibited a high specific capacitance of 411 F/g at an applied current density of 1 A/g and a good long-term stability of 93.2% capacitance retention during 1,000 cycles. The conducting polymers have been also attempted to enhance the electrical conductivity of MoS2. For instance, poly(3,4-ethylenedioxy- thiophene) was in situ polymerized onto the MoS2 surface. The coated conducting polymer enhanced the electrochemical performance of composite electrodes, showing a high specific capacitance of 405 F/g and an excellent capacitance retention of 90% over 1,000 cycles[54]. Yang et al. synthesized core-shell structured electrode consisted of polyaniline shell and 1T MoS2 core. This unique structured electrode exhibited an impressive specific capacitance of 678 F/g at 1 mV/s with a good capacitance retention of 80% during 10,000 cycles[55]. The polypyrrole was polymerized onto the MoS2 surface, showing a high specific capacitance of 700 F/g at a scan rate of 10 mV/s and a good cycling stability of 85% capacitance retention during 4,000 cycles[56]. The controlled morphology of conducting polymer/MoS2 composite electrodes showed enhanced electrochemical performances for supercapacitor applications. The Mn3O4 was incorporated into MoS2 using a hydrothermal and chemical precipitation method. This hybrid electrode showed a high specific capacitance of 119.3 F/g after 2,000 cycles at an applied current density of 1 A/g with a capacitance retention of 69.3%[57]. The ultrathin layered 2D hybrid films of graphene/MoS2 was fabricated by vacuum filtration method, showing an extremely high volumetric capacitance of 1,292.0 F/cm3 at 1 A/g and an excellent cycle life[58]. 

4.2. Rechargeable batteries

Increasing energy demands in modern society has required the development of the high energy and power density of energy devices. Lithium-ion batteries have offered high energy densities for commercially available electronic products, such as, personal mobile phones, notebooks, portable electronics, and electric or hybrid vehicles[59]. They can store electrical charges by the electrochemical reactions between anode and cathode materials. Graphite is one of the most promising anode materials in commercial Li-ion batteries. However, the intrinsically low theoretical capacity of graphite (372 mAh/g) limited to application of Li-ion batteries. In this regard, MoS2 has been attracted much attention as anode materials for development of lithium ion batteries because of its high theoretical capacity of 670 mAh/g, low cost, and natural abundance. The 2D layer structure of MoS2 allows accumulating Li+, Na+, and Mg2+ ions. Despite of these attractive features of MoS2 in battery applications, MoS2-based electrodes suffered from poor stability and low rate capability originated from formation of Li2S after the first cycle[60]. Thus, recent researches focused on high rate capability and long-term stability of MoS2-based electrodes for lithium ion batteries.

Various morphologies and structures of MoS2 materials have been fabricated. For instance, Xu et al. reported that 3D microspheres of MoS2 nanoflakes through a solid phase reaction method show a high discharge capacity of 850.9 mAh/g at 100 mA/g over 50 cycles[61]. The 2D MoS2 nanosheet electrodes synthesized by Veeramalai and coworkers showed 1,097 mAh/g at 50 mA/g after 25 cycles[62]. In addition, 3D hierarchical structured MoS2 with porous structure prepared by Wang et al. exhibited a discharge capacity of 845 mAh/g at 100 mA/g after 50 cycles with a good stability even at a high rate of 500 mA/g over 100 cycles[63]. The MoS2-based electrodes suffered from low electrical conductivity. Hence, 1T MoS2 has been intensively investigated because of its high electrical conductivity and expanded interlayer space. The pristine 2H MoS2 typically exhibited 600 mAh/g at the first cycle and decreased dramatically to ~200 mAh/g in the second cycle. In contrast, 1T MoS2 delivered as high as specific capacity of 1,000 mA/g and maintained 50% retention after 50 cycles. In order to address the issues of MoS2-based electrodes, carbon materials have incorporated into electrode materials[34]. The graphene/MoS2 composites with 2H and 1T hybrid phase through a hydrothermal synthesis showed a high discharge capacity of 1,086 mAh/h at 500 mA/g and a high capacity retention of 82.6% (897 mAh/g) after 50 cycles[64]. The sandwich-structured MoS2/graphene composite electrodes exhibited an extremely high specific capacity of 1,800 mA even at a high rate of 1 A/g. The controlled solvothermal method enabled to deposition of MoS2 nanosheets onto the carbon cloth surface, showing 1,789 mAh/g at 0.1 A/g and 853 mAh/g after 140 cycles[65]. In addition, nitro-gen-doped MoS2 deposited onto carbon cloth delivered a high discharge capacity of 1,308 mAh/g at 0.1 A/g and 1,125 mAh/g after 100 cycles[66]. The MoS2 was incorporated into metal oxides, such as, TiO2. Chen et al. reported that 3D porous MoS2/TiO2 nanocomposite electrodes showed an excellent cycling performance of 95.9% retention after 100 cycles[67].

Apart from lithium ion batteries, MoS2 was used as an electrode material for application of lithium-sulfur batteries. Compared to lithium ion batteries, Li-S batteries provide higher energy density (2,800 Wh/kg) and higher theoretical capacity of 1,675 mAh/g. However, Li-S batteries suffered from the intrinsically low electrical conductivity of sulfur, the formation of polysulfides, and significant volume expansion. To address these issues, Wang et al. described that hierarchical and porous structured SnO2/1T MoS2 nanosheets deposited onto the carbon cloth surface showed 448 mAh/g after 4,000 cycles[68]. Jeong and coworkers fabricated 1T MoS2 and carbon nanotube composite electrode, showing 670 mAh/g after 500 cycles at a high rate of 1C[69]. The availability of MoS2 sheets with the intercalation of alkali metal ions allowed them to accumulate sodium ions among the MoS2 sheets, resulting in fabrication of Na-ion batteries. The graphene/MoS2 composite electrodes exhibited a high rate capability of 284 mAh/g at a high rate of 20 A/g with an excellent cycle life of 95% retention after 250 cycles[70]. In addition, expanding MoS2 sheets with polyethylene oxide led to a high specific capacity of 225 mAh/g and maintained 148 mAh/g after 70 cycles[71]. Because the magnesium can accumulate charges two times higher than those of sodium and lithium metal, magnesium ion batteries have been attracted a great deal of attention as potential rechargeable energy storage devices. Liang and coworkers reported that MoS2/polyethylene oxide composite electrodes exhibited a specific capacity of 75 mAh/g at 5 mA/g[72]. In addition, Liu et al.fabricated graphene/MoS2 composite electrode, showing a high discharge capacity of 115.9 mAh/g and a good cycle life of 82.5 mAh/g after 50 cycles[73].

5. Conclusions

We summarized the properties, preparation methods, and energy storage applications of MoS2 materials. Particularly, the properties of MoS2 depend on morphologies and phase. Thus, various synthetic methods of MoS2 were introduced involving top-down and bottom-up approaches. Supercapactors and rechargeable batteries (i.e., lithium-ion, magnesium-ion, and sodium-ion batteries) using MoS2 electrode mate-rials were described. The metallic MoS2 nanosheets have showed a promising potential in energy storage applications, because of their high electrical conductivity, hydrophilicity, high active surface area, and expanded interlayer distance. Although MoS2 materials has been employed as advanced electrode materials in energy storage field, there still are many challenges in terms of mass production, fine control of phase and morphology, and unit cost production.

Acknowledgment

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT (grant no. 2018R1A2A3075668)

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