Lithium ion battery applications of molybdenum disulfide (MoS₂) nanocomposites

Abstract

This is the first targeted review of the synthesis – microstructure – electrochemical performance relations of MoS₂ – based anodes and cathodes for secondary lithium ion batteries (LIBs). Molybdenum disulfide is a highly promising material for LIBs that compensates for its intermediate insertion voltage (∼2 V vs. Li/Li⁺) with a high reversible capacity (up to 1290 mA h g⁻¹) and an excellent rate capability (e.g. 554 mA h g⁻¹ after 20 cycles at 50 C). Several themes emerge when surveying the scientific literature on the subject: first, we argue that there is excellent data to show that truly nanoscale structures, which often contain a nanodispersed carbon phase, consistently possess superior charge storage capacity and cycling performance. We provide several hypotheses regarding why the measured capacities in such architectures are well above the theoretical predictions of the known MoS₂ intercalation and conversion reactions. Second, we highlight the growing microstructural and electrochemical evidence that the layered MoS₂ structure does not survive past the initial lithiation cycle, and that subsequently the electrochemically active material is actually elemental sulfur. Third, we show that certain synthesis techniques are consistently demonstrated to be the most promising for battery applications, and describe these in detail. Fourth, we present our selection of synthesis methods that we believe to have a high potential for creating improved MoS₂ LIB electrodes, but are yet to be tried.

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Tyler Stephenson

Tyler Stephenson is currently finishing his PhD in Materials Engineering at the University of Alberta. His strong interest in electrochemistry and energy storage is an asset for his lithium ion battery research. His other projects focus around corrosion and fouling for the oil and gas industry, nuclear industry, and on the remediation of tailings from the Alberta oil sands. In 2010, Tyler completed his BSc in Materials Engineering, with a specialization in nano and functional materials, at the University of Alberta.

Zhi Li

Zhi Li received his bachelor and master degree at the Shanghai Jiaotong University, China. Thereafter he joined the department of Chemistry and Geochemistry, Colorado School of Mines, USA, and received his PhD in 2009. In 2010 he joined the University of Alberta and National Institute for Nanotechnology in Canada as a post-doctoral research fellow. His research interests are in the field of materials for electrochemical energy storage and catalysis.

Brian Olsen received his BSc in Engineering Physics and MSc in Materials Engineering from the University of Alberta. He is currently working at the University of Alberta as a research engineer specializing in energy storage, microscopy, and data analysis.

David Mitlin

David Mitlin is a Professor in the Department of Chemical and Materials Engineering at the University of Alberta and a Principal Research Officer at the National Institute for Nanotechnology NRC. Dr Mitlin has published over 100 peer-reviewed journal articles, holds two U.S. patents, is an Editor for Journal of Materials Science, and serves on the Board of Review for Metallurgical and Materials Transactions. Prior to joining the University of Alberta in 2004, Dr Mitlin was at Los Alamos National Laboratory (USA), where he was awarded a Directors Funded Post Doctoral Fellowship. From 2000 to 2002, Dr Mitlin worked as an Integration Engineer at the IBM Semiconductor Research and Development Center, in Hopewell Junction NY. He received a Doctorate in Materials Science from U.C. Berkeley in 2000.

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Broader context

The unique properties of molybdenum disulfide engender a versatility that has enabled its use in a wide range of scientific fields. The global prevalence of lithium ion battery (LIB) technology creates a strong driving force for the development of advanced electrode materials. This is the first targeted review on the synthesis – microstructure – electrochemical performance relationships of MoS₂ electrodes for secondary LIBs. The use of MoS₂ is discussed both in terms of its application for LIB anodes, and for cathodes – an emerging and highly promising area. A comparative assessment of MoS₂ as an electrode material places it in a highly competitive position. While MoS₂–lithium metal batteries were unsuccessfully commercialized in the 1980's, we will show that advancements in nanostructuring of the material, ionic liquid electrolytes, and nanocomposites have revitalized the research effort. We present a detailed and critical analysis regarding what is known about the lithiation/delithiation mechanism in MoS₂, and highlight key areas requiring further study.

1 Introduction

In this review we focus on synthesis – microstructure – electrochemical performance relationships for MoS₂ – based electrodes for secondary lithium ion batteries (LIBs). The use of MoS₂ is discussed both in terms of its application for LIB anodes (where most work has been done) and for cathodes (an emerging and highly promising area). We treat these aspects in detail, while avoiding discussions regarding other dichalcogenides. We present a detailed and critical review regarding what is known about the lithiation/delithiation mechanism in MoS₂; a topic that is currently under significant debate in the energy storage community. The outline of this review is as follows: Section 2.0 presents other applications of MoS₂ beyond rechargeable batteries. Section 3.1 outlines the long and rich history of MoS₂ in the battery field, including its commercial applications in the 1980's by Moli Energy. Section 3.2 contrasts the scope and the aims of this article with those of several other recent reviews that adopt a broader context for the various energy and non-energy applications of dichalcogenides. Section 4.0 provides an overview of MoS₂ in the LIB material landscape, contrasting its electrochemical performance to that of other promising electrode materials. Section 5.0 details the structure and bonding of MoS₂. In Section 6.0 we discuss the lithiation mechanisms of MoS₂, which are in fact highly dependent on the cut-off voltage at discharge. Section 7.0 presents a detailed discussion on MoS₂ nanocomposites for LIBs. Here we consider the interrelation between the various synthesis approaches that have been employed for creating nanostructured MoS₂. In addition we examine the resultant microstructures that often contain a secondary nanodispersed carbon phase, and the several criteria used to judge electrochemical cycling performance (e.g. maximum reversible capacity, cycle 1 and steady-state cycling coulombic efficiency, cycling capacity retention, and rate capacity retention). Finally in Section 8.0, we present an overview of what we believe are the additional promising approaches for achieving MoS₂ – based nanomaterials for lithium storage, but which have not been investigated to date. Section 9.0 contains the concluding remarks.

2 Applications of MoS₂

Molybdenum disulfide exhibits a remarkably diverse range of unique properties, many of which are effectively summarized by the reviews of Strano et al.¹ and Zhang et al.² Due to these properties MoS₂ is currently the focus of research groups across a broad range of disciplines. In the field of tribology, MoS₂ is often referred to as a super lubricant. The material has long been recognized for its exceptional lubricity, owing to the weak van der Waals bonds between S–Mo–S layers, and to the relative inertness of the sulfur basal planes.³ It has been a favored dry lubricant for aerospace applications due to its excellent lubricity in vacuo and under high load.^4,5 MoS₂ is also a key additive in many types of commercial lubricating fluids and greases. More recently, a fascinating negative compressibility effect for MoS₂ under conditions of dynamic shear has revealed a new property of the interlayer glide mechanism.⁶ MoS₂ is also making an impact in the field of chemical sensing. Owing to the variance in reactivity between basal and edge sites, functionalized MoS₂ nanosheets have been used to immobilize DNA strands and immunoglobins for biosensing applications.^7,8 Additionally, flexible MoS₂ based thin film transistors have successfully been used to detect minute changes in nitrogen oxide (NO),⁹ and nitrogen dioxide (NO₂)¹⁰ concentrations under ambient conditions. This type of sensor is highly sought after in the field of toxic gas detection.

The tunable direct band gap and highly deformable nature of a monolayer of MoS₂ has led many researchers to consider it as a viable material for photovoltaic applications.^11–15 In addition to this, the use of MoS₂ in catalysis is widespread due again to its band gap (which couples well with the solar spectrum), as well as the catalytic activity of its edge sites.^16,17 As a photocatalyst for the oxidation of environmental contaminants, MoS₂ is frequently coupled with an additional semiconducting phase (such as TiO₂) as a nanocomposite with great positive effect.^18,19 Its catalytic effects for hydrodesulfurization in oil refining are well known.²⁰ MoS₂ has also been shown as a synergistic catalytic support for gold nanoparticles²¹ as well as for a nickel–iron alloy used in the electrooxidation of hydrazine, an important fuel.²² Furthermore, many groups are exploring the catalytic effect of MoS₂ for hydrogen evolution.^23–28 Finally, methods are being elucidated to employ MoS₂ nanocomposites as flexible and rewritable memory diodes.²⁹ While this article will focus on the application of MoS₂ to the field of energy storage in lithium ion batteries, the material remains exquisitely versatile.

3 History

3.1 Background and Motivation

Efficient energy storage is a long-standing technological and scientific problem that has global implications for all of humanity. The requirements of progressively smaller scale and larger capacity for a wide range of portable, automotive and stationary systems continue to be strong driving forces for the development of advanced lithium ion batteries. Though lithium has been incorporated into battery systems since the late 1950's,^30–32 commercial secondary lithium ion batteries remain a challenge for many applications requiring capacity retention over thousands of cycles and progressively higher energy storage densities. Early work revealed the propensity of mixed valence state transition metal dichalcogenides to intercalate alkali metal ions.^33–35 These studies led to various commercial primary lithium battery systems in the 1970's. Of these, lithium sulfur dioxide (Li/SO₂), lithium thionylchloride (Li/SOCl₂), and lithium sulfurylchloride (Li/SO₂Cl₂) were influential. In addition, the viability of molybdenum and other disulfides was also explored.^36–43

Given the emergence of nanostructured materials, MoS₂ is once again becoming the subject of significant attention as a battery anode material.^44,45 The material is quite promising as a negative electrode, since its capacity can be three and a half times that of commercial graphite anodes (372 mA h g⁻¹). For example, reversible values as high as 1290 mA hg⁻¹ have been reported for nanostructured MoS₂–graphene composite electrodes.⁴⁶ Moreover, compared to other emerging negative electrode materials, such as silicon or germanium, MoS₂ generally displays much better rate capability and lower rates of cycling induced degradation. While silicon anodes possess initial capacities around 3500 mA h g⁻¹ when tested at low rates such as 0.1 or 0.2 C, they retain minimal capacity at rates of 10 C and higher, e.g.⁴⁷ Conversely, as will be discussed later, properly designed MoS₂ electrodes are capable of being cycled at high current densities, retaining a capacity of 554 mA h g⁻¹ after 20 cycles at a rate of 50 C.⁴⁸ From a practical battery design perspective, MoS₂ electrodes are also quite attractive since they possess significantly less volumetric expansion upon lithiation compared to some other conversion materials. For example, while silicon expands 280% upon full lithiation (Li₁₅Si₄), the conversion reaction of MoS₂ to Li₂S and molybdenum yields “only” 103% expansion.

A key disadvantage of employing MoS₂ based anodes is their intermediate lithiation voltages (1.1–2.0 V vs. Li/Li⁺, depending on the degree of lithiation), which substantially narrows the voltage window and hence the net energy density of a full cell LIB. While one can argue that a higher potential versus lithium makes MoS₂ safer than graphite due to a reduction in the driving force for lithium dendrite formation, there is always a trade-off.⁴⁹ Moreover, recent studies by Cui et al.⁵⁰ and Xie et al.⁵¹ have demonstrated that by employing heteronanostructures that can accommodate the volume expansion, the cyclability of silicon may be substantially improved. As is going to be documented in this review, nanostructured MoS₂-based anodes are also highly stable during cycling. However a polysulfide shuttling problem, well known for Li–S batteries, may cause premature electrode failure via electrochemical degradation of the active material.

In addition, the intermediate voltage profile versus Li/Li⁺ has led researchers to consider MoS₂ as a positive electrode, with the system often being pre-lithiated prior to device assembly. The application of MoS₂ as a cathode was patented in 1980 (ref. 52) and has since been explored by others.^53–57 Similar to the case of using MoS₂ as an anode, the intermediate voltage of MoS₂ is generally viewed as a disadvantage for its use as a cathode. The cathodic voltage of LiMoS₂ is lower than other commercial cathode materials such as lithium cobalt oxide (LiCoO₂) and related four and five component oxides (2.5–4.5 V vs. Li/Li⁺).^58–61 However in some applications this may be compensated by the higher charge storage capacity of LiMoS₂ to yield energy densities on par or even higher than LiCoO₂ and related materials (for example 1.5 V × 1000 mA h g⁻¹vs. 3.5 V × 150 mA h g⁻¹).^62,63 Secondary lithium metal batteries using MoS₂ as a cathode and lithium as an anode were commercialized in the early 1980's by Moli Energy. However these batteries were prone to the growth of lithium dendrites from the anode which resulted in poor cycle life and safety concerns due to short circuiting.

Archer et al.⁶⁴ have recently shown that this effect can be mitigated by the use of carefully selected ionic liquid electrolytes. These researchers constructed a half cell of MoS₂ and lithium, using an electrolyte consisting of a blend of silica nanoparticles with 1-methyl-3-propylimidazolium bis(trifluoromethanesulfone) and propylene carbonate (SiO₂-IL-TFSI/PC). The cell retained a reversible capacity of 750 mA h g⁻¹ after 15 cycles. Moreover, the use of their hybrid electrolyte prolonged short circuit times by an order of magnitude compared to a “standard” electrolyte of ethylene carbonate and dimethyl carbonate. This achievement should provide a research path forward to optimize pre-lithiated MoS₂-based microstructures for positive electrode applications. Not only would the revival of the lithium–metal battery solve electrode compatibility issues, it would enable the use of a wider range of high capacity cathodic materials. The emergence of nanostructured materials has led to a performance enhancement of a number of traditional lithium ion battery materials. As a result, molybdenum disulfide is presently being re-explored as an advanced lithium ion battery material and will hence be the focus of this article.

3.2 Related reviews

Recently there have been several high quality review articles on the synthesis and structure of a variety of sulfides which include their general application in a multitude of functional and energy storage fields. The relatively broad and excellent review by Chen et al.⁶⁵ discussed a wide range of sulfides and chalcogenides, including MoS₂. This review gave a comprehensive description of chalcogenide properties including optical, magnetic, electrical, field-emission, photoelectric, thermoelectric, and photocatalytic activity. In addition to the energy storage applications of these materials (lithium ion batteries), the authors discussed a wide range of others including fuel cells, solar cells, and electrical nanogenerators.

The comprehensive article by Yu et al.⁶⁶ provides another exceptional discussion on a wide variety of nanostructured metal chalcogenides, including sulfur, selenium and tellurium compounds, for energy conversion and storage. The thrust of their work is to summarize and critically compare winning synthesis and modification strategies across a range of energy related applications, making the review highly pertinent across those fields. Emphasis is placed on methods for creating a wide array of nanomaterials, including discussions of an array of liquid-phase synthesis methodologies and strategies for modification of metal chalcogenide nanomaterials. A diverse range of synthesis methods are covered including liquid exfoliation, hot-injection, mixed solvent, microwave, Kirkendall-effect-induced and photochemical. Modification of metal chalcogenide nanomaterials with carbon, noble metals, metal oxides and with other metal chalcogenides is also discussed in detail. The applications covered include fuel cells, water splitting, supercapacitors and solar cells, in addition to lithium ion batteries.

The highly relevant review by Zhang et al.⁶⁷ focuses on metal dichalcogenide (mostly MoS₂) nanosheets, and covers a broad array of synthesis methods, properties and applications. The section on preparation methods emphasizes the optimum approaches for yielding such morphologies as 2D graphene-like single and multi-layers. Moreover this particular article offers a uniquely in-depth discussion of device and sensor applications of MoS₂. The authors begin with the synthesis approach that really began the “graphene revolution” i.e. mechanical cleavage, and demonstrate how it has been applied to a variety of materials such as sulfides, nitrides, selenides, and oxides. The review then covers synthesis by electrochemical lithium intercalation, exfoliation, and sonication in various solvents as well as CVD growth. The authors provide a critical review of MoS₂ crystal structure (structures 2H and 1T are emphasized), mechanical properties, electronic structure and optical properties. In the Applications section the review provides a detailed discussion regarding the use of MoS₂ nanosheets for electronic devices, optoelectronic devices, sensing platforms, and energy storage devices that includes both electrochemical supercapacitors and lithium ion batteries.

Researchers focused a state-of-the-art review on several highly technologically promising two-dimensional layered nanomaterials: molybdenum trioxide (MoO₃), disulphide (MoS₂), diselenide (MoSe₂) and ditelluride (MoTe₂).⁶⁸ Their manuscript provides an accurate overview of the crystal structure and bonding of the oxide and of each dichalcogenide, and explains and contrasts their electronic band structure, electrical, optical, mechanical, thermal and magnetic properties. Synthesis methods for layered crystals including vapor phase deposition (PVD and CVD methods), liquid phase deposition and solid state reactions are discussed. Moreover this review offers a detailed section discussing methods for layer exfoliation and identification. Approaches such as mechanical exfoliation, liquid exfoliation, laser thinning, as well as AFM and optical methods for thickness and layer number identification are examined. The authors span numerous application fields by discussing uses of these materials as lubricants, in electrochromic systems, in electronic devices, in battery electrodes, as catalysts, in optical devices, in sensors and in superconductors.

The key aspect differentiating this review from others is that while being quite comprehensive, we focus almost entirely on synthesis – microstructure – electrochemical performance relationships of MoS₂ – based electrodes. These are discussed in terms of their application to anodes and cathodes in LIBs. We treat these aspects in substantial detail, and keep our focus relegated to MoS₂. We limit our discussion of the synthesis methods to those approaches that have either been demonstrated to be optimum for LIBs or to those that in our opinion have much promise. Moreover, a substantial portion of this manuscript contains a critical discussion regarding the ambiguity in the battery literature concerning the actual lithiation sequence of MoS₂, with and without the addition of nanostructured carbons.

4 MoS₂ in the LIB material landscape

While bulk MoS₂ offers little in the way of exciting electrochemical properties for lithium storage, its nanostructured counterparts are the focus of much attention. The nanostructuring of materials for lithium ion batteries embodies a number of well-known advantages and disadvantages.^44,69–71 The trend that nanostructured analogues routinely outperform their bulk equivalents in terms of capacity and cycle life has been demonstrated in many other emerging negative electrode materials. As such, there exists a wide range of nanostructured materials that provide a highly competitive landscape in terms of electrochemical performance in LIBs. Compared to these, MoS₂ stacks up well in terms of experimental reversible specific capacity, and values as high as 1290 mA h g⁻¹ have been reported for nanostructured MoS₂–graphene composite electrodes.⁴⁶ While there are a few materials with higher theoretical capacity, such as certain nanostructured carbons, silicon,⁷² tin,^73,74 and tin dioxide (SnO₂),^75,76 MoS₂ is advantageous in terms of rate capability and capacity retention, as well as cost (e.g. sub-micron MoS₂ powder retails for dollars per kg).

Nanostructured carbons show significantly higher lithium storage capacity than bulk graphite, especially at high current densities.^77,78 Single-walled carbon nanotubes exhibit a range of capacities between 400 and 460 mA h g⁻¹, while multi-walled carbon nanotubes have a capacity of 340 mA h g⁻¹, similar to graphite (372 mA h g⁻¹).^79–81 Reversible capacities from 540–780 mA h g⁻¹ have achieved for graphene, which can be further enhanced by forming mixtures with other carbon allotropes such as carbon nanotubes and fullerenes.^82–84 Furthermore, an impressive 800 mA h g⁻¹ specific capacity was reported for oxidized graphene nanoribbons.⁸⁵ More recently, researchers often employ graphene as a nanocomposite additive with great positive effect.⁸⁶ However, the low packing density of carbons, especially for the nanostructured variety limits their volumetric energy density, one of the most important parameters for portable applications.^77,87 Also, the high surface area often leads to the excessive formation of solid electrolyte interphase (SEI) which results in large irreversible capacities and capacity fading.^77,88 In addition, most varieties of nanostructured carbons, such as graphene and carbon nanotubes, remain far too expensive for commercial electrode applications.

Among the other emerging anode materials, nanostructured metal oxides remain attractive in terms of capacity, though they generally fall short in their rate capability, significant overpotential, and capacity retention. For example, SnO₂ can exhibit a large specific capacity (∼800 mA h g⁻¹) when coupled with carbon.⁸⁹ However, similar to what is found for most conversion oxides, poor cycling performance has impeded tin dioxide's usefulness. Cobalt oxide (Co₃O₄) is also promising owing to its large theoretical specific capacity (890 mA h g⁻¹).⁹⁰ However it too demonstrates poor cycling stability. There are some general exceptions to the rule that oxides cycle poorly, and have poor rate performance. Some notable cases are the insertion electrodes based on TiO₂ nanostructures.^91–94 However, the capacity of TiO₂ (250 mA h g⁻¹) is less than that of graphite. Molybdenum dioxide (MoO₂) graphene nanocomposites (there is some debate concerning whether these are conversion or insertion electrodes) have been reported to retain 675 mA h g⁻¹ after 100 cycles.⁹⁵ Recently, an MoO₂ nanocomposite with multiwalled carbon nanotubes was reported to have a reversible capacity of 1144 mA h g⁻¹ after 200 cycles.⁹⁶ MoO₂ has also been successfully employed as a single component anode demonstrating a simple, low cost fabrication process.⁹⁷ Other materials, such as nickel oxide (NiO) also exhibit enhanced performance (1031 mA h g⁻¹ after 40 cycles) when formed as a nanocomposite with graphene.^98–100

Other materials which exhibit high capacities for lithium storage include silicon and sulfur compounds. Silicon nanoparticles in a composite with graphene, as well as aluminum coated silicon nanowires have been shown to exhibit large reversible capacities of 1866 mA h g⁻¹ (ref. 101) and 3300 mA h g⁻¹ (ref. 102) respectively. However the majority of silicon nanostructures are prone to rapid capacity degradation due to volumetric expansion upon lithium intercalation which pulverizes the electrode.^103–106 The samples in ref. 102 suffered a 25% capacity degradation after 50 cycles. Tin disulfide (SnS₂) is also being considered as a replacement for the commercial graphite anode. SnS₂ nanoplates were shown to retain a capacity of 583 mA h g⁻¹ after 30 cycles but did not survive for longer durations.^107,108 Elemental sulfur has long been recognized for its large theoretical specific capacity of 1675 mA h g⁻¹.¹⁰⁹ Unfortunately, lithium–sulfur batteries suffer from poor rate capability (due to poor electrical conductivity of sulfur) and dissolution of lithium–sulfur compounds. Promising efforts are underway to stabilize these compounds (usually with a carbon phase) and capacities as high as 455 mA h g⁻¹ after 50 cycles at higher current densities have been reported.¹¹⁰ These materials, as well as others are effectively summarized in recent reviews.^111–121

Table 1 provides a summary of the electrochemical performance of various LIB electrode materials. From the comparison, we can see that MoS₂ is a highly competitive LIB material in terms of charge capacity, rate capability and cycle life. The main disadvantage of MoS₂ is the intermediate voltage of 2.0 V that prevents it from coupling well with other materials in a full cell. Reported voltages vs. Li/Li⁺ are experimentally measured values.

Table 1 Summary of electrochemical performance data for various LIB electrode materials^a

Anode materials	Theoretical specific capacity (mA h g^−1)	Voltage vs. Li/Li^+	First discharge capacity (mA h g^−1)	First charge capacity (mA h g^−1)	Reversible capacity after (W) cycles (mA h g^−1)	Coulombic efficiency after (X) cycles (%)	Current density	Reversible capacity (mA h g^−1) after (Y) cycles at (Z) current density	Reference
a * – indicates a value estimated from a published graph.
MoS 2	669–1675	2.0	1062	917	907 (50)	98* (50)	1 C	554 (20) (50 C)	48
MoS2–GNS	669–1675	2.0	1300	2200	1290 (50)	99.2 (50)	100 mA g^−1	1050 (5) (1000 mA g^−1)	46
MoO2–MWCNT	840	1.6	2270	1243	1144 (200)	99 (200)	100 mA g^−1	408 (5) (1 A g^−1)	96
TiO2	335	1.5	334	245	243 (30)	98.7 (30)	66 mA g^−1	6 (10) (6.67 A g^−1)	91
Co3O4	890	1.2	1285	1108	1004 (50)	98 (50)	50 mA g^−1	790 (5) (1 A g^−1)	90
Sn–C	994	0.6	490*	350*	510* (200)	99* (200)	0.8 C	200 (10) (5 C)	74
SnO2–GNS	790	0.6	1875*	1120*	872 (200)	99.5 (200)	100 mA g^−1	519 (10) (2 A g^−1)	75 and 76
SiNW–Al	4200	0.5	3347	3105	1300 (100)	98.8 (100)	0.1 C	1300 (100) (0.1 C)	102
NiO–GNS	718	0.5	1600*	1056	1031 (40)	98 (40)	0.1 C	460* (5) (5 C)	98 and 99
Graphene	372–1116	0.5	945	650	460 (100)	99* (100)	1 C	460 (100) (1 C)	82–84
Graphite	372	0.3	320*	320*	240 (20)	99* (1)	50 mA g^−1	240 (20) (50 mA g^−1)	84
Li	3600	0.0	—	—	—	—	—	—	111
Cathode materials
Li Ni0.5Mn1.5O4	331	4.6	311	367	294 (80)	99* (80)	0.3 C	245* (30) (7 C)	58
LiCoO2	272	4.5	190	153	110* (14)	—	47 mA g^−1	110* (14) (47 mA g^−1)	62 and 63
Sulfur	1675	2.0	960	830*	650 (40)	95 (30)	0.2 C	350 (45) (1 C)	109

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5 Structure and bonding of MoS₂

A considerable amount of work has been done to characterize the electronic, optical and physical properties of molybdenum disulfide.^122–145 It is known to be a polytypic material,^125,127,130 and along with a myriad of higher order sulfides,¹²⁵ three main structural polytypes of the material have been identified: 2H–MoS₂, 3R–MoS₂, and 1T–MoS₂. Fig. 1 shows idealized molecular models of these structures, as well as their lithiated counterparts. Fig. 1a and b show the neat and the lithiated 2H–MoS₂ structures, with a 5% lattice expansion in the c-direction and a-direction due to lithium intercalation. Fig. 1c shows the lithiated 1T–MoS₂, with the lithium ions occupying octahedral interstices. Fig. 1d shows 3R–MoS₂ while Fig. 1e shows Li₂S, which will be pertinent when discussing the lithiation-cycled structures. Of these, the 2H and 3R polytypes have been found to be naturally occurring (the 2H is found in much higher quantities) and thus their structures are very well characterized.¹²⁴ The 1T polytype is a synthetic material, and as a result, there is some disagreement in open literature regarding its structure. The 2H and 3R polytypes exhibit stacking sequences of ABA, and ABC respectively, with the molybdenum cations having trigonal prismatic coordination.^123,142Fig. 1a shows the stable 2H structure (space group P6₃/mmc), with lattice parameters of a = 3.16 Å, c = 12.29 Å and Wyckoff positions of molybdenum and sulfur atoms being: 2 Mo at ±(1/3,2/3,1/4) and 4 S at ±(1/3,2/3,z and 1/3,2/3,1/2 − z) with z = 0.621.^122,130Fig. 1d shows the 3R structure which is of the R3m space group, with lattice parameters of a = 3.16 Å and c = 18.37 Å.¹³¹ Here, Wyckoff positions of molybdenum and sulfur were reported as Mo at (2/3,1/3,0), and S at (0,0, ±z) with z = (2/3 × 0.127).¹³¹

Fig. 1 Molecular models of (a) 2H–MoS₂. (b) Lithiated 2H–MoS₂ showing a 5% lattice expansion in the c-direction and a-direction due to intercalation. (c) Lithiated 1T–MoS₂ showing lithium ions occupying octahedral interstices. (d) 3R–MoS₂ (e) Li₂S (domains of this phase would be interspersed with molybdenum nanoparticles). Dimensions are shown in Angstroms.

While theory predicts the trigonal (1T) polytype of MoS₂, its unstable nature has made it difficult to characterize.^126,127,130 In literature pertaining to lithium ion batteries, the 1T polytype is only observed after lithiation. We employed the crystallographic data from the study by Py and Haering to construct Fig. 1c.¹⁴⁶ The lattice parameters for the 1T polytype are reported as a = 3.36 Å and c = 6.29 Å.¹⁴⁶ The model in Fig. 1c was constructed using the space group P1, a = b = 3.36 Å, c = 6.29 Å, α = β = 90°, γ = 120°, with Wyckoff positions of Li at (0,0,1/2), Mo at (0,0,0) and S at (1/3,2/3,3/4) and (2/3,1/3,1/4).

As a layered transition metal dichalcogenide the electrical, optical, and physical properties of MoS₂ are extremely anisotropic.¹²³ Electrical and thermal conductivities are orders of magnitude smaller in the direction perpendicular to the basal plane, and thermal expansion is an order of magnitude greater.¹²³ The layered hexagonal crystal structure is formed by strong Mo–S covalent bonds in the layers, and weak van der Waals forces between S–Mo–S layers.^122,129 The van der Waals gap has been measured at approximately 3.49 Å via XRD.^123,130,143 Within the S–Mo–S layers, the intermediate difference in electronegativities between sulfur and molybdenum lead to covalent bonds that are partially polarized. The molybdenum cations give up (primarily d-band) valence electrons to the sulfur anions and are left in an oxidation state of (4+) while the oxidation state of the sulfur anions becomes (2−).¹²³ However, within the layer, each molybdenum atom is coordinated with six sulfur atoms while each sulfur atom becomes coordinated with three molybdenum atoms to give the hexagonal unit cell of 2H–MoS₂, as shown in Fig. 1a. Looking in the direction perpendicular to the basal plane, the molybdenum and sulfur atoms are arranged in hexagonal sheets.¹²³

The trigonal prismatic coordination of the molybdenum atoms gives rise to six equivalent cylindrical bond functions. These are a combination of the 4d, 5s, and 5p orbitals.^{129,133,134,143} This type of orbital combination has been described as d⁴sp hybridization by the work of Pauling¹⁴⁷ and Hultgren.¹⁴⁸ On the molybdenum atom, four valence electrons primarily from the 4d orbital are responsible for the bonding to the sulfur atoms, and the remaining two valence electrons of molybdenum reside in non-bonding orbitals. Each sulfur atom achieves coordination to three molybdenum atoms via hybridization of 3p and 3d orbitals. The van der Waals bonding between S–Mo–S sandwiches is a result of the interaction between saturated sulfur 3s subshells, which extend perpendicular to the basal plane.^123,129 The weak inter-layer van der Waals bonding allows for expansion of the bulk structure in the c-direction upon intercalation.

6 Lithiation mechanism of MoS₂

6.1 Lithiation of MoS₂ from 3.0 to 1.1 V

Intercalation of lithium into MoS₂ is known to occur within the voltage range of 3–0 V with a significant change in lithiation mechanism occurring below approximately 1.1 V versus Li/Li⁺. In the range of 3 through 1.1 V, lithium ion insertion is fully reversible which is shown as reaction (1) and idealized in Fig. 1b. This sequence, which is well agreed upon in literature, is commonly observed up to the voltage plateau occurring at approximately 1.1 V during initial discharge. Here, x is in the range of 0 ≤ x ≤ 1.

MoS₂ + xLi⁺ + xe⁻ → Li_xMoS₂ (∼1.1 V vs. Li/Li⁺) (1)

The theoretical specific charge capacity of this reaction is 167 mA h g⁻¹, corresponding to the intercalation of one lithium ion per molybdenum atom.¹⁴⁶ At voltages below this plateau there appears to be one or several disproportionation reactions as well as the presence of intermediate metastable sulfide species. These reactions have been suspected since early work on Li–MoS₂ was conducted,³⁶ and were partially clarified in the late 1980's when Selwyn et al.¹⁴⁹ published seminal work on the decomposition of molybdenum and tungsten dichalcogenides during lithiation. The exact potential onset as well as the nature of these reactions have only recently been elucidated, and are still not fully understood. These will be discussed in the next section of the review.

Since then, lithium intercalation into molybdenum disulfide has been studied in detail by various groups.^{53,142,150–154} More recent work has also been instrumental in the understanding of the complex mechanisms involved in lithium intercalation into molybdenum disulfide host lattices.^155–159 Various methods were employed to study the insertion of lithium into molybdenum disulfide. These include physical vapor deposition (PVD) of lithium onto cleaved MoS₂ in high vacuum,¹⁵⁰ liquid phase intercalation by immersion of MoS₂ in n-butyl lithium (C₄H₉Li),^{40,42,156,157} and intercalation via electrochemical methods.^{36–43,55,151,158} Intercalated samples were characterized via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).

Below approximately x = 1, intercalation of lithium into molybdenum disulfide is commonly described as an ion/electron transfer topotactic reaction producing a metallic paramagnetic product.^36,38,155 For lithium atoms, the occupied valence electron states have a higher energy than the unoccupied conduction band states of the molybdenum cations in MoS₂ (primarily the 4d bands), and so electron transfer results upon intercalation.¹⁵⁵ The weak van der Waals forces that hold the S–Mo–S layers together in 2H–MoS₂ allow for the insertion of lithium between them. As intercalation proceeds, lithium diffuses between the MoS₂ planes and occupies vacant octahedral interstices in the hexagonal crystal lattice as shown in Fig. 1b. It has been suggested that the diffusion of lithium ions involves tetragonal interstices in the van der Waals gap, alluding to the fact that these ions are fast diffusers.^{36,39,43,153,160} The occupation of tetragonal interstices is however, less likely, since the energy to occupy a tetrahedral site is larger. Alkali ion conductivity in transition metal sulfides is due to a number of factors.⁴³ First, there is a high concentration of potential charge carriers. Second, there is a high concentration of vacancies and interstitial sites, and third, the activation energy for ionic hopping to adjacent sites is relatively low (on the order of the formation enthalpy for the ternary compound Li_xMX₂).⁴³ Diffusion of lithium ions takes place primarily in two-dimensional intercalation planes in the van der Waals gap.⁴¹ For Li⁺ ions in MoS₂ this occurs primarily between octahedral interstices parallel to the basal plane.¹⁶¹

Below approximately x = 0.1, intercalation proceeds into the 2H–MoS₂ lattice with little disruption. The host lattice supplies intercalation sites and redox centers, but otherwise assumes a passive role in the reaction. As the lithium concentration increases in the 2H–MoS₂ lattice, the addition of 1 or more electrons to the host phase, creating [MoS₂]⁻, leads to the formation of a distorted octahedral environment for the metal cations, which experience substantial alterations in electronic band structure and electrochemical potentials.^38–40,142 Researchers have commonly observed a lithium superlattice forming in the van der Waals gap as the lithium concentration rises in the range of approximately 0.1 ≤ x ≤ 1.^{157,160–164} This superlattice, described as 2a_o × 2a_o, is an ordering of lithium ions in the van der Waals gap, which occupy octahedral and then tetrahedral interstices above the saturation limit. This effects a phase change in which the coordination of the molybdenum atoms shifts from trigonal prismatic in the 2H phase, to octahedral in the 1T phase. This phase change is associated with the voltage plateau that is commonly observed around 1.1 V vs. Li/Li⁺ in galvanostatic cycling. The mechanism of this phase change, as explained by the work of Py and Haering,¹⁴⁶ is described as a glide process between the molybdenum and sulfur planes. A similar process has been observed in the lithiation of graphitic carbons.¹⁶⁵ Mulhern¹⁶⁶ has gone on to demonstrate that the resulting intercalated phase of MoS₂ is highly distorted, and undergoes appreciable lattice expansion. Intercalation-induced lattice strain in the basal plane causes the formation of dislocations, which fragment the lattice and may create diffusion pathways for lithium ions, allowing them to penetrate further into the host material.¹⁶⁰ Furthermore, this fragmentation may lead in part to the disproportionation reaction associated with the formation of lithium sulfide (Li₂S) and molybdenum metal particles that will be outlined in the next section.

An examination of 2H–MoS₂ octahedral site radius and lithium ionic radius indicates that the two are approximately 0.7 Å.¹⁶⁷ Here we consider that the van der Waals gap in 2H–MoS₂ is approximately 3.49 Å and the sulfur atom covalent radius is 1.04 Å.¹⁶⁷ Therefore, it can be inferred that lithium intercalation should proceed into molybdenum disulfide without any appreciable change in host lattice parameters. However experimentally we know this is not the case. XRD studies of intercalated MoS₂ indicate that there is an approximate 4–6% lengthening of the c-axis and a-axis in the hexagonal unit cell upon intercalation with lithium ions as shown in Fig. 1b and c.^{36,38,146,160} While it is true that a portion of this expansion in the c-direction is attributed to the co-intercalation of solvent molecules (most of the intercalation studies are done using n-butyl lithium solutions in hexane), there is still appreciable expansion that results from lithium ion insertion.

Nuclear magnetic resonance studies have shown that lithium atoms can vary in size based on their ionic character (having a fully ionized radius of approximately 0.7 Å and a neutral radius of approximately 1.4 Å).⁴⁰ It is thought that the ionicity of the lithium decreases, as its concentration in the host lattice rises.^38,40 Therefore, the ionicity will have an effect on the lattice strain being exerted by the lithium ions. The first lithium atoms to intercalate each donate an electron to the empty 4d band of the molybdenum cations and are thus stored in a completely ionized state. However, it has been shown that as the lithium ion concentration increases, their ionicity decreases, and the ions themselves are thought to undergo a slight increase in ionic radius. This brings about an expansion of the lattice in the c-direction.^36–39,146 In MoS₂, lattice expansion in the a-direction is partially attributed to alterations in the electronic band structure of the host as intercalation continues.¹⁵⁰ It is known that the transition metal dichalcogenides have strong overlap–covalency interactions between the metal d-bands, and the chalcogen s–p bands.¹⁴³ Therefore, as intercalation continues, the sulfur atoms in MoS₂ experience a slight increase in atomic radius as the electron density in the host lattice rises.^40,143 These affects, along with solvent co-intercalation, are the likely source of the observed increase in the lattice parameters of MoS₂ during intercalation (Fig. 1b).

6.2 Lithiation of MoS₂ from 1.1 to 0 V

More recently, an increasing number of authors are validating the existence of one, or a series of decomposition reactions when MoS₂ cells are discharged below 1.1 V vs. Li/Li⁺.^46,168–182 Furthermore, since these decomposition reactions all yield a lithium sulfide product, an examination of some of the more prevalent lithium sulfur (Li–S) battery literature provides us with some clarification of the reaction mechanisms. For this system the formation of polysulfides and the sulfide shuttling effect are well characterized.^183–193 For the more “bulk-like” structures of MoS₂, significant voltage plateaus are observed upon the first discharge at approximately 1.1 and 0.6 V vs. Li/Li⁺, although the exact values vary by study.^46,168–182

For the case of truly nanoscale structures the five issues that arise when interpreting or even comparing the charging profiles are: first, the morphology of the nanostructured MoS₂ electrodes deviates significantly from bulk, for example possessing much larger interlayer spacing and surface area to volume ratio, and/or a much higher defect density.^{168,180,181,194–198} For the case of interlayer spacing, it has been shown that MoS₂ nanostructures with a wider spacing resulted in enhancements in electrochemical performance in terms of the initial lithiation kinetics and the charge storage capacity.^{168,177,180–182} Here, the effect is attributed to the increased volume associated with the layer expansion leading to faster ionic diffusion and better material utilization during the initial lithiation. The effect of enhanced lithium storage in a wider van der Waals gap has also been conclusively demonstrated with graphene.^{82,84,101,199} The increased surface area effect is expected to remain influential during the life of the material. However, effects due to the non-equilibrium spacing of the basal planes in MoS₂ or high defect content will become unimportant after the structure irreversibly decomposes to molybdenum and Li₂S.

Interestingly, an examination of literature pertaining to MoS₂ nanostructures as hydrodesulfurization (HDS) catalysts leads to some useful information regarding point defects in the material.^200–204 The structure of HDS catalysts must be well characterized, as it is usually defect sites on the crystal that lead to the bonding of the sulfur functional group (or sulfur-containing groups) in organic compounds. Using techniques such as scanning tunneling microscopy (STM), the morphology of these MoS₂ nanostructures (such as nanosheets and fullerenes) have been meticulously characterized by these researchers. In the nanostructures, they see a much higher defect density in the form of vacancies (mainly sulfur edge vacancies) that lead to sub-coordinated molybdenum centers, and have concluded that they exhibit increased reactivity due to the presence of dangling bonds. It seems plausible that these same sites could act as adsorption sites for lithium ions, and may help to explain the elevated lithium storage capacities that are so often observed in the MoS₂ nanostructures during the first cycle.¹⁵⁷ Also, the presence or creation of point defects along the sulfur basal plane may serve as nucleation points for the formation of the lithium superlattice that has been observed to develop in the van der Waals gap.^{157,160,162,164} Researchers are therefore encouraged to attempt atomic force microscopy and STM studies on lithiated MoS₂ samples to observe lithiation patterns, similar to what was done by Kalinin et al.²⁰⁵

Second, the structures often contain significant amounts of carbon-based phases, which are electrochemically active towards lithium.^{170,172,174,181,182} For example Archer et al.¹⁷² describe a system with 22 wt% carbon that had the best electrochemical performance. In many studies, the authors employ carbon nanostructures that possess charge storage capacities well in excess of graphite's 372 mA h g⁻¹.^{77,78,82–85} Materials like multilayer “graphene”, highly graphitic nanoparticles, or carbon nanotubes bind lithium via adsorption, pore filling, and intercalation, routinely yielding capacities as high as 650 mA h g⁻¹. Were the carbons to also contain substantial amounts of nitrogen heteroatoms, reversible capacities as high as 1780 mA h g⁻¹ are possible.^88,206 In the authors' opinion the coaptation of an active material with carbons goes a long way towards accounting for the tremendously enhanced capacity of the nanocomposites, since in general this contribution is neglected in the calculations.

Third, in nanoscale materials, the formation of an SEI layer (commonly described as a gel-like polymeric layer) can also have a significant contribution to the overall voltage profile.¹⁷⁰ Such capacity contribution is of course detrimental, resulting in poor coulombic efficiency of the electrodes. Fourth, it is likely that similar to the lithium-sulfur battery, full charging–discharging for the Li–MoS₂ system involves intermediate “molecular” polysulfides. For the Li–S system these reactions are well characterized,^183–193 however their exact nature remains to be elucidated in the Li–MoS₂ system.^177–182 Fifth, it is therefore plausible that the surfaces of molybdenum nanoparticles present after full discharge have dangling bonds that will attract and immobilize polysulfides, and could serve as adsorption sites for lithium ions in subsequent cycles. As a corollary, sub-coordinated molybdenum centers have been shown to have a high affinity for sulfur-containing molecules.²⁰⁰ However molybdenum metal nanoparticles are generally X-ray amorphous and are thus difficult to track during the lithiation studies.¹⁸¹

The lower voltage plateau evident upon first discharge, occurring at approximately 0.6 V vs. Li/Li⁺, has been attributed to either the reversible conversion reaction of MoS₂ to Li₂S and metallic molybdenum through reactions (1) and then (2),^46,168–176 or the irreversible decomposition with the same redox chemistry, followed by cycling between Li₂S (Fig. 1e) and elemental sulfur (reaction (3)).^177–182 The theoretical specific charge capacity of reaction (1) is 167 mA h g⁻¹, while the theoretical capacity of MoS₂ lithiating by reactions (1) and then by (2) (i.e. full discharge) is 669 mA h g⁻¹.

Li_xMoS₂ + (4 − x)Li⁺ + (4 − x)e⁻ → Mo + 2Li₂S (∼0.6 V vs. Li/Li⁺) (2)

The reaction of elemental sulfur and lithium may be described by equation (3), which yields a theoretical capacity of 1675 mA h g⁻¹ if the weight of the molybdenum is not taken into account.

S + 2Li⁺ + 2e⁻ → Li₂S (∼2.2 V vs. Li/Li⁺) (3)

Though the majority of open literature favors the reversible conversion reaction sequence, recent detailed XRD studies indicated that upon delithiation, MoS₂ was no longer detectable.^177–181 This may be due to an amorphization of the electrode materials. However since both the 1.1 and the 0.6 V plateaus are observed only during the first lithiation, we believe that the irreversible decomposition of MoS₂ is more likely. At room temperature solid-state sulfidation reactions are quite sluggish, so it is unlikely that MoS₂ would re-form especially at higher charging rates. This conclusion is also based on a recent TEM study of post-cycled MoS₂ electrodes (Fig. S8 in their ESI†).¹⁸² The authors conclusively detected metallic molybdenum in the delithiated state. One difficulty in characterizing the cycle 1 discharge plateaus for the MoS₂ half-cell (vs. Li/Li⁺) is that the plateau at 0.6 V is difficult to distinguish from the one attributed to SEI formation. Unfortunately, the formation of an SEI layer occurs in most battery anodes during the first discharge cycle around 0.6 V. In fact it may be very difficult to separate the two processes for high surface area electrodes where substantial capacity is lost due to SEI formation, while the initial lithiation reaction irreversibly alters the lithium active phases. However extensive microstructural evidence, discussed in this manuscript, supports the formation of metallic molybdenum during discharge cycle 1. Moreover low surface area “bulk” MoS₂, where the total irreversible capacity due to SEI should be relatively low, clearly demonstrates the 0.6 V plateau (as seen by the CV peak at ∼0.6 V in Fig. 2a).¹⁷⁰

Fig. 2 Cyclic voltammograms for (a) MoS₂ powder.¹⁷⁰ (b) MoS₂–graphene nanocomposite (scan rate 0.5 mV s⁻¹).¹⁷⁰ (Copyright 2011 American Chemical Society) (c) MoS₂–amorphous carbon nanocomposite (scan rate 0.2 mV s⁻¹).¹⁷² (d) Commercial MoS₂ powder in a smaller voltage window (scan rate 0.05 mV s⁻¹).¹⁷⁹ (Copyright 2012 Elsevier Ltd.). (e) Sulfur (scan rate 0.05 V).²⁰⁷ Adapted from ref. 170 (DOI: 10.1021/nn200659w), ref. 172 (DOI: 10.1039/c2jm32468g), ref. 179 (DOI: 10.1016/j.electacta.2012.07.020), and ref. 207 (DOI: 10.1039/c2jm15041g) with permission.

The irreversibility of the MoS₂ decomposition reaction upon initial discharge is further supported by the trends observed in cyclic voltammetry (CV). Fig. 2a (ref. 170) and 2d (ref. 179) highlight CV curves for commercial MoS₂ powders in different voltage ranges (scan rates 0.5 and 0.05 mV s⁻¹ respectively). Fig. 2b (ref. 170) shows a typical CV curve for an MoS₂/graphene nanocomposite (scan rate 0.5 mV s⁻¹). Fig. 2c (ref. 172) is a CV curve for an amorphous carbon–MoS₂ nanocomposite (scan rate 0.2 mV s⁻¹). Fig. 2e (ref. 207) is a CV curve for a typical lithium–sulfur redox couple, shown for comparison (scan rate 0.05 mV s⁻¹). In Fig. 2e, the two cathodic peaks at ∼2.3 and ∼2.1 V are attributed to the stepwise reduction of sulfur to Li₂S. The first step (∼2.3 V) involves the reduction of sulfur to intermediate lithium polysulfides (Li₂S_n, 2 < n < 8) and the second step (∼2.1 V) is attributed to the reduction of higher order polysulfides to Li₂S.^109,118 The dominant oxidation peak at 2.4 V is recognized as the conversion of all lithium polysulfides to S₈²⁻ accomplished by facile charge transfer kinetics.¹⁰⁹ This CV plot also demonstrates the excellent reversibility of the lithium–sulfur redox couple.

Fig. 2a–c show similar trends in all cathodic and anodic sweeps. In the first cathodic sweep, peaks at approximately 1.0 V are observed on all plots and attributed to the formation of Li_xMoS₂ and the resulting 2H to 1T phase transition.^170,177 This peak is also observed in 2d, although here, the authors only discharge to 0.8 V in an attempt to investigate the reversibility of reaction (1) in this voltage range. The large cathodic peak at approximately 0.4 V (Fig. 2a–c) is attributed to the conversion reaction of MoS₂ to Li₂S and molybdenum (reaction (2)). The irreversibility of this reaction is supported by the disappearance of these peaks in subsequent reduction cycles. Instead, a dominant cathodic peak at approximately 2.0 V (consistent with 2e) is observed in Fig. 2a–c, while the peaks at 1.0 and 0.4 V previously discussed are greatly diminished in subsequent cycles. The dominant cathodic peak forming at ∼2.0 V is well known in lithium–sulfur battery systems and is attributed to the formation of Li₂S.^{109,186,188,208} Upon recharging, Fig. 2a–c show two anodic peaks (one shallow peak at ∼1.7 V and a large peak at ∼2.4 V). The first shallow anodic peak is likely due to the delithiation of residual Li_xMoS₂ which has not undergone conversion. The dominant anodic peak at ∼2.4 V is due to the conversion of Li₂S to S₈²⁻ consistent with Fig. 2d and e.^{110,187,208–210}

The material in Fig. 2d (ref. 179) was discharged too deeply to demonstrate full reversibility of reaction (1). However by initiating it in a stepwise manner the MoS₂ conversion reaction was successfully observed. This is similar to the results obtained by Py and Haering,¹⁴⁶ where the formation of Li_xMoS₂ was conclusively identified with the long voltage plateau at 1.1 V. In Fig. 2d, the first discharge to 0.8 V (ref. 179) allowed the conversion of MoS₂ to Li₂S and molybdenum metal, but not completely. The first anodic sweep reveals the delithiation of Li_xMoS₂ (the doublet centered at 2.0 V) as well as a large broad peak at ∼2.5 V, which is attributed to the formation of sulfur (similar to Fig. 2e). With subsequent cycling the anodic doublet centered around 2.0 V and the dominant cathodic peak at around 1.0 V both get weaker. This indicates the consumption of the active materials initially present, i.e. Li_xMoS₂ and MoS₂. Such cycling induced degradation has also been observed in other studies^172,178 and attests to the instability of the Li_xMoS₂ compound, and its tendency to decompose at higher values of x. By the third cycle, two small cathodic peaks at 1.75 and 2.34 V appear (shown inset in Fig. 2d), which are likely due to the reduction of higher order lithium polysulfides.¹⁰⁹ Based upon the trends observed in literature, the pertinent redox reaction after first discharge involves lithium and sulfur as the electro-active species (reaction (3)).

Fig. 3 presents recent XRD data that further supports the argument regarding a lithiation sequence that involves the irreversible formation of molybdenum.^172,177Fig. 3a shows an XRD scan of an MoS₂ electrode in the as-received state (bottom) and after it was discharged to 0.01 V (top).¹⁷² The as-received material is clearly 2H–MoS₂, while the discharged material is Li₂S and molybdenum metal. Fig. 3b shows an XRD pattern of a commercial MoS₂ powder after the first discharge to 0.01 V.¹⁷⁷ Again, there is strong evidence that Li₂S and molybdenum are the dominant phases. Fig. 3c shows the Li₂S + molybdenum composite (material analyzed in Fig. 3b) after it was recharged to 3.0 V. While there is substantial peak broadening due to partial amorphization and/or nanocrystallization, elemental sulfur and molybdenum metal are definitively present in the charged state. This also indicates that the material was not fully lithiated, consistent with it being a micro-scale powder rather than a nanocomposite. In situ XRD studies of MoS₂ have been performed, though no study has done this across the entire voltage range of 0.01 to 3 V.^146,179 The elucidation of the microstructural evolution in MoS₂ based electrodes during cycling would benefit greatly from an in situ XRD study throughout the entire voltage range. XRD and FTIR have been used by others to track the lithiation of MoS₂, with the results being in agreement with the trends discussed here.^179,181

Fig. 3 XRD scans of MoS₂ electrode at various states of charge. (a) (Bottom) as received, and (top) after discharge to 0.01 V. Peaks marked by * are from the copper current collector.¹⁷² (b) After discharge to 0.01 V.¹⁷⁷ (c) After recharge to 3.0 V.¹⁷⁷ (Copyright 2012 Wiley-VHC Verlag GmbH &Co, KGaA, Weinheim) Adapted from ref. 172 (DOI: 10.1039/c2jm32468g), and ref. 177 (DOI: 10.1002/asia.200100796) with permission.

It appears that after the first discharge, the molybdenum nanoparticles may have a multifunctional beneficial role: first, the particles serve to enhance the electrical conductivity of the Li₂S matrix, which partially alleviates the poor electrical conductivity concern associated with both the sulfur and Li₂S phases. Second, the nanoparticles may serve as pinning sites for soluble polysulfides, preventing their dissolution, and thus mitigating the shuttling effect that causes electrochemical degradation of lithium–sulfur batteries. While the first effect is quite reasonable and should be expected were the MoS₂ to irreversibly decompose, the second effect is hypothetical and requires substantial experimental evidence before being considered a real benefit. Authors have argued that it may be the carbon that in fact pins the soluble polysulfide anions.^186,187,191

The theoretical specific capacity of reaction (2) is 669 mA h g⁻¹ while that of reaction (3) is 1675 mA h g⁻¹. Therefore reaction (2) does not fully explain the enhanced specific capacities well in excess of 700 mA h g⁻¹ that are commonly observed in the Li–MoS₂ system.^46,169–182 While reaction (3) is quite likely, it can only follow reaction (2), which means the mass of molybdenum must also be taken into account. Therefore in the authors' opinion, reactions (2) and (3) do not fully capture the complexity of the charging/discharging process in the MoS₂-based system. We believe that there are three additional, and by no means mutually exclusive, contributions to the net charge storage: first, nanostructured molybdenum particles may also participate to some extent in the lithiation reaction, serving as physical adsorption sites for the Li⁺ ions. Bulk molybdenum is inactive towards lithiation, so any binding would have to be at or near the surface.²¹¹ Reports in literature are consistent in showing a much higher capacity for materials that are nanoscale rather than their bulk counterparts.^{170,174,180,181,185,212–214} Researchers have characterized these effects well for a variety of transition metal nanoparticles formed from oxide conversion electrodes.^215–218 These authors comment on the various charge storage mechanisms in conversion electrodes and allude to enhancements from capacitive effects brought about by the high surface area metallic nanoparticles. Authors also point out that there may be a contribution to the reversible capacity from the polymeric SEI layer that forms around the metallic nanoparticles.²¹⁵ However in our opinion such reactions would be either fully irreversible or very poorly reversible, and would only adversely affect the coulombic efficiency without boosting the reversible capacity. Another possibility is that more than two lithium ions react per sulfur atom in reaction (3). We believe that this is unlikely since no analogue has been reported for the well-characterized Li–S system.

The present authors believe that much of the capacity enhancement beyond 669 mA h g⁻¹ is largely due to the presence of a nanostructured carbon phase whose contribution to the total electrode capacity is either underappreciated or perhaps not accounted for at all. As was discussed previously, many of the carbon allotropes interspersed with MoS₂ possess quite a high lithium storage capability that well-exceeds that of carbon black and even that of graphite. There is also a likely synergy between the two nanodispersed phases that cannot be captured by a standard rule of mixtures calculation even when the capacity of each phase is obtained separately. The enhancement of lithium capacity by carbon phases is routinely demonstrated in MoS₂ and Li–S literature, though the exact mechanism remains unclear.^{46,169,170,172,180–182,185,187,193}

7 MoS₂ nanocomposites in LIBs

With the development of nanostructured materials, there is a resurgence of synthesis research directed at creating MoS₂-based nanocomposite structures for lithium ion battery applications. The effort is primarily directed at utilizing MoS₂ as an anode material to be used against a pre-lithiated cathode. A particle size refinement of MoS₂ down to the nanoscale greatly shortens the lithium ion diffusion distances, providing a substantial boost in the rate-dependent capacity retention as compared to the more “micro” MoS₂ counterparts.^{44,46,159,172,174,180,182} In addition, it is consistently demonstrated that it is the hybrid MoS₂–carbon systems, in particular the ones with nanoscale structure, that offer the optimum combination of energy density, cycling stability, and high rate capability.^{190,219–229}

There are several microstructural scenarios during the conversion reaction, where nanostructured carbon would enhance electrode performance. These include one or a combination of (a) carbon acting as a binder between the S/Li₂S and the molybdenum nanoparticles; (b) carbon encapsulating both phases providing an electrically conductive path down the current collector; (c) carbon acting as a “skeleton” which provides both an electrically conductive pathway down to the current collector and prevents material agglomeration during cycling. The possibility of (a) vs. (b) vs. (c) would also depend on the type of carbon added. It is intuitive that crystalline/particulate phases like carbon nanotubes or graphene nanoflakes would be more effective in providing a skeleton, while materials like amorphous carbon would be more effective as coatings and/or binder. At this point in the case of MoS₂ there is not enough microstructural evidence to conclusively identify such enhancement mechanisms. There exists a wide range of techniques by which to synthesize electrode-grade molybdenum disulfide of various morphologies. Among these, hydrothermal, assisted hydrothermal, solvothermal, and template assisted techniques are the most successfully employed and will be presently discussed.

7.1 MoS₂ nanostructures by hydrothermal techniques

A significant portion of the recent published work uses carbon-templated hydrothermal synthesis to create nanocomposite structures of MoS₂ dispersed in a conductive matrix such as graphene, carbon nanotubes (CNTs) or amorphous carbon. The key advantage of hydrothermal methods is that they may be employed to create commercial or near commercial mass loadings (∼10 mg cm⁻²) on the electrodes.²³⁰ Electrically conducting multiwalled carbon nanotubes, possessing large open specific surface areas and excellent chemical and thermal stabilities, are perhaps the most widely employed nanometer-sized templates.^{190,219–224} Graphene is also becoming a popular choice of support, progressively gaining greater scientific attention as compared to CNTs.^225–229 The rationale for this stems from a real cost and scalability advantage of wet methods used to synthesize graphene/graphene oxide versus chemical vapor deposition generally employed to fabricate CNTs.^231–233

It is well known that, similar to graphene, exfoliated MoS₂ often exists in the form of nanosheets due to its layered structure. The presence of 2D graphene sheets in the hydrothermal process could further guide the formation of MoS₂ sheets and generate a sheet-on-sheet structure. Despite common literature claims of such structures possessing a long-term benefit (i.e. over numerous charge/discharge cycles), there is little microstructural or electrochemical evidence that suggests they survive past the initial lithiation step. Nevertheless the increased interfacial contact between carbon and Mo/S would promote cycling stability by reducing the rates of material aggregation. Graphene, CNTs and related materials are known to template the growth of various sulfides and oxides from solution, resulting in orders of magnitude reduction in particle sizes as compared to the non-templated baselines.^228,234 Since such nanocarbons are very effective in refining the as-synthesized microstructure and hence shortening the lithium diffusion distances, they substantially improve the rate dependent capacity retention. It has also been suggested that during cycling, the electrochemically active surface area of these electrodes can increase due to a gradual breakdown of the graphene and resulting introduction of defect sites.²²⁸ These sites serve to trap more lithium ions during intercalation and could explain the gradual increase in specific capacity that is often observed. A highly interspersed carbon phase would also substantially improve the electrical conductivity of the electrode down to the current collector, regardless of the lithium-active phases present.

By introducing graphene nanosheets into the hydrothermal synthesis process for MoS₂, authors were able to create a true nanocomposite.⁴⁶Fig. 4a and b highlight the resulting as-synthesized microstructure, which exhibits significantly improved electrochemical performance over the graphene-free baseline.⁴⁶Fig. 4c and d show the cycling results, demonstrating a stable reversible capacity of approximately 1290 mA h g⁻¹ and an excellent rate capability. The capacity retention of this material was 99.2% after 50 cycles (current density 100 mA g⁻¹). This is among the best performance, in terms of capacity and cycling stability, reported in literature for an MoS₂-based anode. The graphene additive has significantly decreased the size of the MoS₂ nanosheets, which likely led to better material utilization during the conversion to Li₂S and molybdenum. Authors elaborated upon the synergistic behavior of MoS₂ and graphene, and provide a detailed investigation of the electronic and atomic structure of the nanocomposite.²³⁵ The work provides evidence that the creation of a graphene–MoS₂ nanocomposite improves the overall electrical conductivity of the electrode. Furthermore, the authors discuss the weak van der Waals and electrostatic interactions of the two materials, which would allow for facile expansion at the graphene–MoS₂ interface during initial lithiation.²³⁵ This may actually influence the subsequent cycling behavior in terms of allowing all the MoS₂ to be converted. In contrast, studies repeatedly show that for micro-scale MoS₂ much of the material does not react with lithium during the first cycle or afterwards (capacities well below 669 mA h g⁻¹).

Fig. 4 As-synthesized MoS₂–graphene nanocomposite. (a and b) SEM and TEM micrographs respectively. (c) Cycling behavior of the nanocomposite (with graphene-free MoS₂ as the baseline). (d) Cycling behaviour of MoS₂–graphene nanocomposite at various current densities. Adapted from ref. 46 (DOI: 10.1039/c1cc10631g) with permission.

Amorphous carbons formed during the hydrothermal synthesis of MoS₂ can also increase the electrode performance. Authors prepared amorphous carbon–MoS₂ nanostructures via a hydrothermal/carbonization technique.¹⁷²Fig. 5a and b display SEM and TEM micrographs of the as-prepared MS-22 nanostructures (MoS₂ + 22 wt% carbon). Fig. 5c shows that the capacity retention of this composite had a very strong carbon loading dependence. At a carbon loading of 22 wt%, the stable capacity was approximately 875 mA h g⁻¹ for over 100 cycles. The authors attributed this stability to the MoS₂ being fully coated with carbon, which allowed for full material utilization during electrochemical cycling. Moreover, the carbon coating may mitigate the SEI layer formation, though more evidence is needed for this hypothesis. Fig. 5d shows capacity–voltage profiles for charging and discharging, which are typical of MoS₂–carbon composites. Here, the first discharge exhibits plateaus at approximately 1.1 and 0.6 V, indicative of the 2H to 1T MoS₂ (∼1.1 V) phase transformation, and subsequent conversion to Li₂S and molybdenum metal (∼0.6 V). These disappear in subsequent cycles, indicating that this reaction is irreversible. XRD scans from this material (shown in Fig. 3a) suggested the presence of Li₂S and molybdenum metal after first discharge. Moreover, the discharge (∼2.0 V) and charging (∼2.3 V) voltage plateaus for the 100^th cycle are indicative of the lithium–sulfur redox couple.^186,188,210

Fig. 5 Hydrothermally synthesized MoS₂–amorphous carbon nanocomposite. (a and b) SEM and TEM micrographs of MS-22 (MoS₂ + 22 wt% C). (c) Cycling stability of pure MoS₂ and various MoS₂–carbon composites, MS-X stands for MoS₂ with X wt% C. (d) Voltage capacity profiles for MS-22 (current density 100 mA g⁻¹). Adapted from ref. 172 (DOI: 10.1039/c2jm32468g) with permission.

Researchers have employed polystyrene microspheres to tailor the dispersion and the microscopic assembly of MoS₂ nanosheets (MoS₂–NS).¹⁷⁴ Through a post hydrothermal synthesis annealing treatment in an inert atmosphere, the template polystyrene microspheres were decomposed. The resultant ultrathin MoS₂ nanosheet assemblies assumed microsphere morphologies with a wide spacing between the layers. This material had a BET surface area of 36 m² g⁻¹, with a primarily mesoporous structure due to the stacking of the individual nanosheets. The hierarchical structure of the MoS₂–NS microspheres was quite advantageous for battery applications: the high surface area of the nanosheets increased the overall charge storage capacity, while the void space buffered the volumetric changes allowing for facile lithiation/delithiation. Fig. 6a and b show TEM micrographs of these resultant hierarchical structures.¹⁷⁴Fig. 6c and d show the cycling performance of the baseline MoS₂ flakes (I) and the MoS₂–NS microspheres (II) at a current density of 100 mA g⁻¹ (Fig. 6c) and at various current densities (Fig. 6d). The MoS₂–NS microspheres consistently outperformed the baseline in terms of overall capacity, cycling capacity retention and rate capability, supporting the authors' argument regarding the essential role of polystyrene microsphere assisted synthesis.

Fig. 6 MoS₂–nanosheet microspheres. (a) TEM micrograph of as-synthesized MoS₂ nanosheet microspheres. (b) HRTEM image of several MoS₂ nanosheets; the inset shows an HRTEM image of a single MoS₂ nanosheet. (c) Cycling performance of MoS₂ flakes (I) and MoS₂–NS microspheres (II) at a current density of 100 mA g⁻¹. (d) Cycling performance of MoS₂ flakes (I) and MoS₂–NS microspheres (II) at different current densities (mA g⁻¹). Adapted from ref. 174 (DOI: 10.1039/c1nr11552a) with permission.

This group also reported similar results using glucose as an additive in the presence of CNTs during hydrothermal synthesis.¹⁷⁵ This was shown to significantly decrease the thickness of the MoS₂ nanosheets. The glucose adsorbed on the surface of the CNT@MoS₂ was further converted into a thin amorphous carbon layer during the calcination process, and hence acted as an additional conductive and perhaps protective coating. The BET surface area was reported at 30 m² g⁻¹. The glucose-assisted material consistently outperformed the glucose-free baseline (in terms of capacity retention and rate capability). The capacity of the glucose-assisted CNT@MoS₂ was nearly 1000 mA h g⁻¹, decreasing to approximately 800 mA h g⁻¹ after 60 cycles.¹⁷⁵

Further to their previous work,⁴⁶ authors present a study on L-cysteine assisted hydrothermal synthesis of MoS₂ graphene nanocomposites.¹⁷⁰ While biomolecular-assisted synthesis methods have been employed to create other types of sulfide nanostructures, this was the first study of its kind for MoS₂. The electrochemical results were very promising. The synthesized materials were true graphene–MoS₂ nanocomposites and possessed a synergistic charge capacity of nearly 1200 mA h g⁻¹ which was shown to be dependent on the graphene–MoS₂ ratio (optimal was 2 : 1). The strong dependence of the electrode capacity on the amount of graphene added substantiates the argument that it is a major contributor to the net charge storage. The fact that there is an optimum ratio of graphene to MoS₂ does not need to be rationalized in terms of any profound electronic effects or a fundamental modification of the MoS₂ structure (it does not exist past cycle 1). Rather it can be explained by correlating this ratio to the best microstructural dispersion of the two phases, i.e. a mixture that is the most “nano”. Unfortunately there are no existing literature reports where authors have demonstrated the variations in the key microstructural parameters (i.e. MoS₂ crystallite and particle size, total porosity and pore size distribution, electrical conductivity of the composite, degree of encapsulation by the carbon of the MoS₂ particles, etc.) with the loading of a given carbon phase. However it is quite reasonable to expect a “volcano” type of electrochemical performance curve versus carbon mass loading, with the peak corresponding to the optimum overall capacity retention and rate capability (e.g. in Fig. 7f). At lower carbon loadings the dispersion would not be optimized due to an insufficient amount of the carbon phase, while at higher mass loading agglomeration would reduce the amount of electrochemically accessible material and drive up the electrode resistivity.

Fig. 7 L-cysteine assisted hydrothermal synthesis of graphene–MoS₂ nanocomposites. (a) SEM micrograph of the MoS₂ baseline. (b and c) SEM and TEM micrographs of the 2 : 1 by weight graphene–MoS₂ nanocomposite. (d and e) Charge–discharge curves for the baseline MoS₂ and for the 2 : 1 nanocomposite, respectively. (f) Cycling stability of the nanocomposites: (1) MoS₂ (2) G/MoS₂ (1 : 1) (3) G/MoS₂ (2 : 1) (4) G/MoS₂ (4 : 1). Adapted from ref. 170 (DOI: 10.1021/nn200659w) with permission from the American Chemical Society, Copyright 2011.

Fig. 7 shows SEM and TEM micrographs of their baseline material (a) which was nearly monolithic, and their 2 : 1 graphene–MoS₂ nanocomposite (b and c).¹⁷⁰ Though the two materials appear to be well interspersed, further evidence in terms of analytical mapping, Z-contrast imaging, HRTEM, etc. would have been useful. Electrochemical tests (Fig. 7d–f) highlight the significant differences between the baseline MoS₂ and the various nanocomposites that were analyzed. Fig. 7f shows that the total capacity and the cycling stability are much better for the 2 : 1 nanocomposite (marked 3) as compared to the graphene-free MoS₂ baseline (marked 1). The 1 : 1 graphene/MoS₂ (marked 2) and 4 : 1 graphene/MoS₂ (marked 4) nanocomposites are both inferior to the 2 : 1, likely for the reasons previously discussed. There are also significant differences in the voltage–capacity profiles for the baseline (Fig. 7d) and the 2 : 1 nanocomposite (Fig. 7e). The previously described plateaus at ∼1.1 and 0.6 V vs. Li/Li⁺ during the first discharge of MoS₂ were much less conspicuous for the nanocomposite (Fig. 7e). It is difficult to quantitatively compare the voltage–capacity profiles for the two materials during subsequent cycling, since neither possesses well-defined plateaus. However one can qualitatively state that the voltage profiles did vary with the graphene content, supporting the argument that it had a substantial contribution to the net capacity. Furthermore, the lack of discernible voltage plateaus in subsequent cycles indicates that both materials went through similar phase changes as a result of their initial lithiation. The stable cycling capacity of nearly 1200 mA h g⁻¹ after 100 cycles is among the highest reported in literature for any MoS₂-based electrodes. The 2 : 1 graphene–MoS₂ nanocomposite not only displayed over twice the capacity of the MoS₂ baseline but was also much more stable. This is shown in Fig. 7f.

It is also important to note that additives such as ionic liquids, glucose and biomolecular compounds show a strong impact on the morphologies of hydrothermally synthesized MoS₂. Similar effects have been demonstrated in the hydrothermal synthesis of metal oxides,^236–239 where the solution-phase interactions are generally better understood. Hence any comparisons with the carbon-free baselines are further obscured since not only is the resultant carbon content and dispersion different, but also the morphology of the MoS₂ phase. Varying the relative amount of precursor would also have an effect on the microstructure of MoS₂. For instance, in the previous study¹⁷⁰ the microstructure of MoS₂ would not only differ between the graphene-free MoS₂ baseline and the graphene-MoS₂ samples, but also between the 1 : 1, 2 : 1 and 4 : 1 specimens. These types of questions would be better resolved through the application of more robust microscopy analysis on the as-synthesized and post cycled specimens.

7.2 MoS₂ prepared by solvothermal synthesis

Despite the success of the MoS₂ electrode in a half-cell configuration, published work on a full cell is limited. Cho et al.⁴⁸ are one of the only groups to successfully test and publish results on such a cell, using a lithium cobalt oxide cathode and an anode consisting of graphene-like MoS₂ nanoplates. The nanoplates were synthesized via a liquid phase solvothermal reaction of molybdenum hexacarbonyl (Mo(CO)₆) and sulfur in xylene. Fig. 8a and b display SEM and TEM images of the synthesized nanostructures, respectively. The SEM images indicate that the particulate size of the MoS₂ is sub-100 nm scale, while the TEM micrographs indicate that the structure is highly disordered. Examining the published micrograph one observes a structure that is quite heterogeneous even on the sub-5 nm scale (dimension of the scale marker), with both crystalline and amorphous regions being present throughout. For the crystalline sections, a variety of contrast fringe spacings are observed. These may not actually be conclusively ascribed to any given set of interplanar spacings due to an unknown orientation of the polycrystalline specimen relative to the electron beam. Moreover some regions of the micrograph show contrast synonymous with Moiré patterns, which are caused by overlapping crystallites. It is not clear as to what kind of feature the marker showing 0.69 nm spacing is referring too, though we believe it to be a Moiré fringe.

Fig. 8 Disordered graphene-like MoS₂ achieved via liquid phase solvothermal technique. (a and b) SEM and TEM (with FFT insert) images of the synthesized nanostructures. (c and d) Charging–discharging curves and cycling stability results for the half-cell. Adapted from ref. 48 (DOI: 10.1021/nl202675f) with permission from the American Chemical Society, Copyright 2011.

Fig. 8c and d show the potential–capacity curves and cycling stability results for the half-cell, which are very impressive. The dominant discharge (∼2.0 V) and charge (∼2.3 V) plateaus are consistent with other work¹⁷² and are indicative of the lithium–sulfur redox couple. The authors correctly point out that larger interlayer spacing in their nanostructured MoS₂ would alter the intercalation thermodynamics and kinetics. However, this effect will only be realized during the first lithiation and cannot contribute to the electrochemical performance in the subsequent cycles. The charge storage capacities are in excess of the theoretical value. Though the authors argued that the structure is only porous MoS₂, the residual presence of a substantial amount of carbon from the molybdenum hexacarbonyl precursor cannot be ruled out. Neither TEM nor XRD analysis presented by the authors was sufficiently detailed to negate that possibility. Given the reported surface area of 80 m² g⁻¹, there may also be a contribution to the net capacity due to the surface adsorption of lithium on metallic molybdenum after the conversion reaction (in addition to the almost certain adsorption of lithium on any residual carbon). The authors' rate dependence results in Fig. 8d (nearly 800 mA h g⁻¹ at 30 C and 700 mA h g⁻¹ at 50 C) may only be realized with a high charge transfer surface area and extremely short diffusion distances, implying that the very fine, high surface area microstructure remains stable throughout the cycling.

7.3 Ordered mesoporous MoS₂ through templating

The use of hard templates has been recently and successfully employed to synthesize highly porous MoS₂ nanostructures with high surface areas. The surface areas of these materials are large enough to justify lithium ion surface adsorption and lithium metal pore filling (nanoplating) as an important secondary contributor to the net charge storage.^177,182,240 In such cases a charge storage capacity beyond 669 mA h g⁻¹ is expected analogous to the high surface area/high porosity carbons that exceed 372 mA h g⁻¹. Fig. 9 highlights one of the more interesting and better performing examples of such an approach.²⁴⁰ The specific surface area and pore volume of mesoporous MoS₂ were calculated to be 130 m² g⁻¹ and 0.24 cm³ g⁻¹, and are expected to further increase after the first lithiation. Fig. 9a shows a low magnification SEM image revealing macroscopic clusters of rod-like interconnected MoS₂ nanowires. The mesoporous channels are partially revealed by the TEM micrograph in Fig. 9b, with SAED inset indexed to 2H–MoS₂. Fig. 9c shows the excellent cycling stability of the material, retaining a reversible capacity of 876 mA h g⁻¹ after 100 cycles at a current density of 0.1 A g⁻¹. Fig. 9d shows the exceptional rate capability, and capacity recovery of the material, even after cycling at a current density of 10 A g⁻¹. This capability was attributed to the large electrode/electrolyte interface allowed by the mesoporous channels (which had a narrow size distribution). This led to ultrafast lithium intercalation over a large surface area. Similar results have also been obtained for other mesoporous materials.²⁴¹ The authors comment that the high rate capability is also due to the enhanced layer spacing of their MoS₂, however this is doubtful after the first cycle. The high rate capability could only be realized in a highly conductive matrix which suggests the presence of a finely dispersed metallic phase. The authors employed an excessive amount of carbon black in their electrode recipe (70 : 20 : 10, active material : carbon black : PVDF binder) which may partially account for the capacity enhancement, though in this case we believe that it is a secondary issue relative to the porosity effect.

Fig. 9 Templated mesoporous MoS₂. (a) Low magnification SEM image revealing MoS₂ microstructures. (b) TEM micrograph (with SAED inset) revealing the mesoporosity of MoS₂ crystallites and wire-like arrays. (c) Cycling performance at a current density of 0.1 A g⁻¹. (d) Cycling performance at different current densities. Adapted from ref. 240 (DOI: 10.1002/aenm.201200087) with permission from Wiley-VHC Verlag GmbH &Co, KGaA, Weinheim, Copyright 2012.

One would expect that the high surface area would lead to excessive SEI layer formation, however the templated electrode is shown to remain quite stable with good coulombic efficiency (97–98%) throughout cycling. This agrees with what is commonly reported in literature for templated carbons with highly ordered porosity, which are also stable upon cycling and demonstrate good coulombic efficiency.²⁰⁶ Apparently after the first cycle, SEI formation is at least partially inhibited. While no post-cycled TEM was completed in this work, others have shown SEI formation on mesoporous MoS₂ with partial retention of the mesoporous structure after initial charge and discharge.¹⁷⁸ For this material, the pore volume is also expected to accommodate the volume expansions and distortions associated with lithiation and the conversion reaction. The stability of the material suggests that the mesoporous structure may have been partially retained after many cycles, however this was not proven.

Table 2 provides a summary of the electrochemical data from the various literature sources. As can be seen there is a substantial variation not only in the charge storage capacities but also in the coulombic efficiencies between the studies. Interestingly, nominally similar techniques, e.g. hydrothermal synthesis, can result is radically different electrochemical performance outcomes. The implication of this is that both subtle changes in the experimental synthesis parameters, the electrode/cell fabrication techniques, and the morphology of the as-synthesized material can all play a major role in determining how well the battery performs.

Table 2 Summary of recent electrochemical data collected for MoS₂. In all cases electrodes were tested in a half-cell configuration versus Li/Li⁺ and fully cycled across a voltage window of^a ∼0.01 to 3 V

Material	Synthesis method	First discharge capacity (mA h g^−1)	First charge capacity (mA h g^−1)	Reversible capacity after (X) cycles (mA h g^−1)	Coulombic efficiency after (Y) cycles (%)	Current density	Highest current density tested	Reference
a * – indicates a value estimated from a published graph.
MoS2–PEO (plate-like particles)	Exfoliation/hydrolysis	1131	822*	890 (50)	95* (50)	50 mA g^−1	50 mA g^−1	180
MoS2–GNS–PEO (nanoparticles)	Exfoliation/hydrolysis	1130	830*	1000 (180)	93* (180)	50 mA g^−1	10 A g^−1	181
MoS2–GNS (nanoparticles)	Hydrothermal	2200*	1300	1290 (50)	99.2 (50)	100 mA g^−1	1000 mA g^−1	46
MoS2–GNS (nanoparticles)	Hydrothermal	1571	1031	1187 (100)	99* (100)	100 mA g^−1	1000 mA g^−1	170
MoS2–CNTs (nanosheets)	Hydrothermal	1434	862	698 (60)	94 (3)	100 mA g^−1	1000 mA g^−1	174
MoS2–CNTs (nanosheets)	Hydrothermal	710*	390*	390 (50)	98 (50)	0.6 mA cm^−2	0.6 mA cm^−2	171
MoS2–amC (nanosheets)	Hydrothermal	1175	870*	852 (40)	94* (40)	60 mA g^−1	60 mA g^−1	159
MoS2–amC (nanoparticles)	Hydrothermal	1340*	869	633 (50)	65 (1)	100 mA g^−1	400 mA g^−1	173
MoS2–amC (nanoparticles)	Hydrothermal	1160	791	585 (70)	95 (3)	100 mA g^−1	1000 mA g^−1	174
MoS2–amC (nanoparticles)	Hydrothermal	2100*	930*	912 (100)	99* (100)	100 mA g^−1	100 mA g^−1	169
MoS2-amC (nanowires)	Template-assisted	880	625	630 (20)	98.5 (20)	33 mA g^−1	669 mA g^−1	177
MoS2@CMK-3 (nanorods)	Template-assisted	1056	824	602 (100)	97* (100)	250 mA g^−1	2000 mA g^−1	182
MoS2–AB (nanorods)	Template-assisted	1060*	1052	876 (100)	98* (100)	100 mA g^−1	10 A g^−1	240
MoS2–amC (nanoparticles)	Solvothermal	1062	917	907 (50)	87 (1)	1062 mA g^−1	53.1 A g^−1	48

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8 Promising MoS₂ nanomaterials not investigated for lithium storage

As a nanostructured material, MoS₂ can exist in a diverse range of morphologies and microstructures. These include fullerene-like MoS₂ (layered onion-like nanospheres),^{200,201,242–247} MoS₂ nanotubes,^{194–196,248–252} MoS₂ nanowires with various stoichiometries,^253–255 nanoribbons and nanosheets.^256–262 As summarized in previous review papers,^197,263,264 these nanostructures can be synthesized via a wide range of methods. At this stage the vast majority of these structures have not been investigated as electrode materials for lithium storage. We believe that these MoS₂ nanostructures could hold great promise for electrode applications as many of the synthesis techniques offer opportunities for nanoscale carbon incorporation. In this section, we will give a brief overview of the nanostructures achieved using two of the most scalable methods: template-assisted and gas-phase synthesis techniques. These techniques are already widely utilized in sectors such as microelectronics and thin films coatings for various industrial applications, making the technology mature and transferrable to the energy storage sector. Moreover the described approaches yield arrays of interconnected nanostructures that offer direct electron conductions paths down to the current collectors; a key advantage over techniques that result in isolated crystallites or particles.

8.1 Various MoS₂ nanostructures synthesized through templating

Many MoS₂ nanostructures can be synthesized through the templating strategy. Authors performed seminal work on template-assisted synthesis of monodispersed microscale MoS₂ nanofibers and nanotubes.²⁶⁵ They utilized thermal decomposition of two different ammonium thiomolybdate precursors within the confines of a porous aluminum oxide template. Their technique resulted in a dense fibrous network of MoS₂ nanotubes that extended parallel to the substrate. This is appealing for lithium storage due to the high degree of interconnectivity and potential for flexibility.²⁶⁶ Additional work describe routes to MoS₂ nanotube and fullerene synthesis.^267,268 Authors describe a procedure to synthesize MoS₂ nanotubes of different chiralities, similar to carbon nanotubes.²⁶⁷ While drawing comparisons the authors mentioned that the unique sandwich-like structure of MoS₂ may offer greater resistance to nanotube buckling and kinking than for CNTs. This fact may prove useful for the creation of flexible lithium ion batteries that can withstand large amounts of deformation.

Researchers have also developed an alternate template-assisted technique to create a coaxial–binary system of graphene and MoS₂ nanotubes.²⁶⁹ Since it is known that a capacity enhancement is accompanied by the coordination of graphene with MoS₂ nanosheets, the creation of a coaxial–binary system of MoS₂ and carbon nanotubes may exhibit excellent electrochemical performance. Additionally, a template-assisted method using silica for producing mesoporous MoS₂ has also been successfully completed.²⁷⁰ Here, the authors have developed a method of synthesizing tubular mesoporous domains of MoS₂ which are highly layered and nanoscale. They go on to describe a dimensional tunability, which is difficult to achieve with other synthesis techniques and could therefore be useful for creating MoS₂ nanostructures with controllable size for lithium storage. This technique may be useful for quantifying the dimensional effect of MoS₂ on charge capacity.

8.2 MoS₂ nanostructures synthesized though gas-phase techniques

Gas-phase synthesis techniques are amongst the most intriguing since they yield a wide range of unique microstructures and offer a high degree of versatility. Authors describe methods which involve aerosol assisted CVD processes that form MoS₂via the decomposition of a single source precursor gas.^271,272 These methods have led to some very distinctive MoS₂ microstructures, which exhibit a plate-like morphology that was found to vary with annealing temperature. Although complex, the result was the deposition of nanoscale MoS₂ structures over centimeter square areas which may be useful for conformal coatings on complex geometries.²⁷¹ Additionally, researchers also report on a novel CVD method of synthesizing MoS₂ monolayers over large areas.²⁷³ These procedures are advantageous since many of the methods previously discussed yield interconnected particulates of MoS₂ that are difficult to deposit as large-area coatings. While these coatings are usually evaluated for anti-friction applications, their adaptation for lithium storage on complex geometries could be beneficial, since electrode templates often have intricate, high surface area morphologies.

Authors have shown that it is also possible to synthesize high surface area molybdenum disulfide nanotubes directly from a reaction of molybdenum metal and sulfur powder together with iodine flakes reacted in a glass ampoule at 850 °C.²⁷⁴ In their method, C₆₀ was added at 5 wt% and used as a growth catalyst in their reactions but was removed in subsequent processing steps. Others have reported an electrochemical enhancement by adding C₆₀.⁸⁴ These tubes were observed to have a high defect density along their length and be of relatively uniform diameter. Furthermore, they demonstrate that it is possible to grow vertically aligned MoS₂ nanotube forests across a substrate surface, similar to CNTs.²⁶³

Researchers have achieved physical vapor deposition of MoS₂ thin films using reactive magnetron sputtering of a solid molybdenum target and magnetron sputtering a solid MoS₂ target.^198,275 In this work the authors noticed that a significant portion of crystallites would form with their c-axis parallel to the substrate, which would present dangling bonds in the form of edge vacancies to the outer surface as well as provide potential inter-planar diffusion pathways to incident lithium ions. Authors describe a technique for the synthesis of MoS₂ nanoparticles using pulsed laser ablation.²⁷⁶ Here pure, fullerene-like nanoparticles with a very uniform size distribution were synthesized by the ablation of a target in water. This technique could be adapted as a simple way to fabricated nanocomposites with carbon, via carbon incorporation into the pressed molybdenum disulfide target pellet. The propensity for unique facile nanostructures involving carbon encapsulation and incorporation seems plausible with this technique.

9 Concluding thoughts

While LiMoS₂ cathodes may offer little in the way of capacity enhancement compared to LiCoO₂, the material is still worth considering due to its exceptional rate performance and cycling stability. As an anode, the capacity enhancement of MoS₂ over graphite has been well demonstrated. There is still significant debate regarding the actual lithiation reaction sequence during charging/discharging. However, there is progressively more evidence to support what is currently the minority view, that the primary lithium active phase is elemental sulfur beyond the first cycle. Metallic molybdenum appears to have a secondary albeit very important role of both enhancing the electrical conductivity of the electrode and perhaps stabilizing the shuttling of polysulfides that are known to be the source of premature failure in Li–S batteries. However for the case of Li–MoS₂, the role of polysulfides and their interaction both with the molybdenum and the various nanocarbon phases interspersed within the electrodes remains quite poorly understood. This brings up the second outstanding scientific issue: the experimentally reported reversible charge storage capacities for MoS₂-based architectures are consistently above the theoretical capacity of the literature-proposed lithiation conversion reactions. These involve either MoS₂ or sulfur (keeping in mind the weight contribution of the “inactive” molybdenum). While scenarios such as nanoscale molybdenum actually being electro-active towards lithium are possible and should be further explored, we believe that the charge capacity of the nanodispersed carbons present in the composites is often underestimated.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ee42591f

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