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Nanostructured metal sulfides for energy storage Cite this: Nanoscale, 2014, 6, 9889

Xianhong Rui,†abc Huiteng Tan†bd and Qingyu Yan*bcd Advanced electrodes with a high energy density at high power are urgently needed for high-performance energy storage devices, including lithium-ion batteries (LIBs) and supercapacitors (SCs), to fulfil the requirements of future electrochemical power sources for applications such as in hybrid electric/plugin-hybrid (HEV/PHEV) vehicles. Metal sulfides with unique physical and chemical properties, as well as high specific capacity/capacitance, which are typically multiple times higher than that of the carbon/ graphite-based materials, are currently studied as promising electrode materials. However, the implementation of these sulfide electrodes in practical applications is hindered by their inferior rate performance and cycling stability. Nanostructures offering the advantages of high surface-to-volume ratios, favourable transport properties, and high freedom for the volume change upon ion insertion/ extraction and other reactions, present an opportunity to build next-generation LIBs and SCs. Thus, the development of novel concepts in material research to achieve new nanostructures paves the way for improved electrochemical performance. Herein, we summarize recent advances in nanostructured metal sulfides, such as iron sulfides, copper sulfides, cobalt sulfides, nickel sulfides, manganese sulfides, molybdenum sulfides, tin sulfides, with zero-, one-, two-, and three-dimensional morphologies for LIB

Received 4th June 2014 Accepted 22nd June 2014

and SC applications. In addition, the recently emerged concept of incorporating conductive matrices, especially graphene, with metal sulfide nanomaterials will also be highlighted. Finally, some remarks are

DOI: 10.1039/c4nr03057e

made on the challenges and perspectives for the future development of metal sulfide-based LIB and SC


1. Introduction The continuous burning of fossil fuels (e.g., petroleum, coal and natural gas) has intensied the global warming issue. In an effort to combat the soaring greenhouse gases emission caused by the combustion of these nite resources, society is racing towards the use of renewable resources, such as solar, wind, hydropower and geothermal energy, which offer sustainable energy without sacricing environmental quality. However, these intermittent energy resources rely heavily on natural conditions (e.g., sunshine, rain, wind and location); thus, technology is required to convert the off-peak electricity into different forms for storage to ll the energy shortfalls during the on-peak period. To strike a balance between the needs of widespread energy demands and the ecological issues, investment in clean electrochemical energy, in particular lithium-ion batteries (LIBs) and supercapacitors (SCs), is needed to usher in


School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui, 243002, China


School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore. E-mail: [email protected]


Energy Research Institute, Nanyang Technological University, 637459, Singapore


TUM CREATE Centre for Electromobility, Nanyang Technological University, 637459, Singapore † These authors contributed equally to this work.

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a green energy future. Both these energy storage devices are expected to play essential roles in our daily lives as the dominant power sources for portable consumer electronics (e.g., smartphones, tablets, notebook PCs and camcorders) and in hybrid electric/plug-in-hybrid (HEV/PHEV) vehicles.1–8 A typical LIB or SC system mainly consists of a negative electrode (anode), a positive electrode (cathode), an electrolyte and a separator sandwiched between the parallel facing electrodes (Fig. 1).9–15 The working principle of both devices involves the voltage-driven migration of cations (Li+, Na+, K+, and H+) or anions (OH) through the electrolyte towards the electrodes for reversible electrochemical reactions. Moreover, the electrons ow through the external circuit to maintain the charge balance. Despite having similarities in the device architecture, manufacturing and technology (rocking-chair system), their charge storage mechanisms are markedly different. For LIBs, the electrochemical reaction of lithium ions in the bulk is realized along different chemical pathways, e.g. via an intercalation/de-intercalation reaction, conversion reaction and alloying/de-alloying reaction.16–21 Therefore, as shown in Fig. 2, LIBs generally contribute high energy density (120–200 W h kg1) at the expense of power density (0.4–3 kW kg1) due to the sluggish kinetics of lithium ion diffusion in the bulk. Unlike LIBs, SCs, including electric double-layer capacitors (EDLCs) and pseudo-capacitors, have the advantage of delivering energy in a shorter time scale by exploiting their fast surface or near

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Fig. 2 A Ragone plot showing the specific energy density versus power density for batteries and capacitors. Reproduced with permission from ref. 4. Copyright 2008, Nature Publishing Group.

Schematic illustration of (a) a lithium ion battery and (b) an electrochemical supercapacitor. (a) is reproduced with permission from ref. 10. Copyright 2014, Elsevier. (b) is reproduced with permission from ref. 11. Copyright 2011, Royal Society of Chemistry.

Fig. 1

surface (several tens of nanometers from the surface) reactions (physical absorption/desorption or Faradaic processes).22–26 This characteristic renders SCs with a high power density (5– 30 kW kg1) that far exceeds the level offered by LIBs despite having limited energy density (5–8 W h kg1) (Fig. 2). For the past few decades, metal suldes (MSs) have emerged as the most prominent candidates for LIBs and SCs. Their unique physical and chemical properties (e.g., higher electrical conductivity, mechanical and thermal stability than those of their corresponding metal oxides), as well as the rich redox chemistry that contributes to their high specic capacity/ capacitance (several times higher than those of carbon/ graphite-based materials), make them stand out from other electrode materials.8,27–35 Unfortunately, the sluggish diffusivity of lithium ion in the MSs limits the rate of lithium insertion/ extraction (low power), and the low surface area of the bulk electrodes restrains their capacitive contribution (low energy). To realize high-efficiency energy storage of MSs, tremendous research effort has been made in the nanoengineering of electrode materials to improve their power density for LIBs and energy density for SCs. Moving from conventional bulk to nanostructures offers several advantages for LIBs and SCs, including: (i) increase in electrode/electrolyte contact area per unit mass, which permits a higher lithium-ion ux across the interface for LIBs (i.e., higher charge/discharge rates) and more ion adsorption sites for double-layer formation and charge-

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transfer reactions for SCs (i.e., higher specic capacitances); (ii) shorter path lengths for ionic and electronic transport, resulting in a faster diffusion rate (i.e., higher power); (iii) better accommodation of the mechanical strain and structural distortion generated from ion insertion/extraction and other reactions (i.e., longer cycle life); and (iv) the occurrence of new reactions that are not possible for bulk materials.3,11,35–47 The strong correlation between the nanostructures and their electrochemical properties has prompted a surge in research to perform the controlled synthesis of innovative electrode materials with tailored nanostructures. Furthermore, for LIB applications, the polysulde Li2Sx (2 < x < 8) intermediates generated during the lithiation process easily react with or dissolve into the organic electrolyte, leading to an irreversible capacity decay of the MS anodes.48–53 The dissolution of polysuldes also results in the so-called shuttle effect, in which the soluble long-chain polysuldes diffuse to the surface of the cathode and are reduced to shortchain polysuldes. However, the re-oxidation of these species aer being transported back to the anode does not contribute to the overall capacity. Thus, this process progressively decreases the active mass utilization and markedly reduces the coulombic efficiency. Moreover, the insulating polysuldes layer deposited on the surface of the electrode may deteriorate its conductivity and prevent further electrochemical reactions from taking place. To solve these challenges, nanostructured MSs that can hybridize with conductive matrices, such as carbon components, were further developed as composite electrodes to prevent the dissolution of polysuldes and reduce the interface resistance, leading to

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higher specic capacities at high charge/discharge rates, and an improved cyclability of the lithium storage. In this review, an overview of recent progress made towards MS electrode engineering strategies for LIBs and SCs applications is provided. Moreover, MS crystal structures, charge storage mechanisms and electrochemical performances in terms of specic capacity/capacitance, cycling stability and rate capability are summarized. Thereaer, discussion on fundamental reasons that may make engineered MS a viable candidate will be covered in hope of paving a new research direction to bring this family one step forward towards the commercial implementation of this material. Moreover, insights into the future research and development of MS compounds for nextgeneration LIB and SC devices are also provided.

2. Nanostructured metal sulfide electrodes 2.1

Iron suldes

On the basis of Fe–S phase diagram,54 iron suldes have a multitude of possible stoichiometries and crystal structures dependent on the temperature, pressure, and sulfur concentration. Among them, troilite FeS, greigite Fe3S4, and pyrite FeS2 have long been studied as interesting high-capacity anode materials for LIBs.55–57 There are, however, only few reports on iron suldes for SCs. Troilite FeS shows a hexagonal crystal structure based on the ˚ c ¼ 5.04 A) ˚ with alternating c-planes of NiAs structure (a ¼ 3.67 A, Fe2+ and S2 ions (Fig. 3a).58–60 The lithiation and delithiation of FeS are described as FeS + 2Li+ + 2e 4 Li2S + Fe, and occur at the potential plateau of ca. 1.6 V vs. Li/Li+ with a theoretical capacity of 609 mA h g1.34 The cubic close-packed crystal structure of greigite Fe3S4 is similar to that of magnetite Fe3O4, in which S2 ions substitute O2 ions, and the tetrahedral A-sites are occupied by Fe3+ ions and the octahedral B-sites are occupied

Fig. 3 Crystalline structures of stoichiometric (a) troilite FeS along different directions, (b) greigite Fe3S4, and (c) pyrite FeS2.

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half by Fe3+ ions and half by Fe2+ ions (Fig. 3b).59,61 The theoretical specic capacity of Fe3S4 is determined to be 725 mA h g1, based on the electrochemical reaction of Fe3S4 + 8Li+ + 8e 4 4Li2S + 3Fe at ca. 1.5 V vs. Li/Li+.62 The cubic pyrite FeS2 phase (space ) is among the most abundant of the iron disuldes, group Pa3 and its crystalline structure is relatively complicated. The Fe2+ ions sit inside the sulfur octahedra and are at the corners and face centers of the cubic unit cell, while the S ions are tetrahedrally coordinated by three Fe2+ ions and one S ion (Fig. 3c).63–65 The electrochemical reaction of FeS2 with lithium involves two steps: FeS2 + 2Li+ + 2e / Li2FeS2 at 2 V vs. Li/Li+, and Li2FeS2 + 2Li+ + 2e / 2Li2S + Fe0 at 1.5 V vs. Li/Li+, yielding a theoretical capacity as high as 894 mA h g1.55,66 Many groups have attempted to synthesize iron sulde nanostructures using a variety of chemical or physical approaches for various applications.60,64,66–73 For example, needle-like FeS was prepared by the solvothermal decomposition of a precursor complex [Fe3(m3-O)(m2-O2CCH2Cl)6(H2O)3]NO3$H2O in the presence of thiourea at 150  C for 12 h.67 Oriented-FeS nanotubes were synthesized via the suldation of hematite (a-Fe2O3) nanowire arrays with H2S gas at relatively low temperatures (200–300  C).60 Vertically oriented single-crystalline FeS2 nanowires were fabricated via the thermal suldation of low-carbon steel foil at 350  C for 2 h.68 Single-crystalline FeS2 nanorods, nanobelts and nanoplates were synthesized by the suldation reaction of iron dichloride (FeCl2) and iron dibromide (FeBr2).69 For LIB anode applications, Xia et al.74 developed a facile hydrothermal process for the preparation of FeS2 nanocrystals with diameters ranging from 10 to 35 nm, which show an initial discharge capacity of 885 mA h g1 at a current density of 89 mA g1 between 0.8 and 2.4 V vs. Li/Li+. Aer 40 cycles, the capacity deceases to 408 mA h g1 and with a capacity retention of 46%. Li et al.75 demonstrated the large-scale

Fig. 4 (a) TEM, (b and c) HRTEM images of greigite Fe3S4 nanoplatelets. (d) Corresponding CV curves vs. Li metal at a scan rate of 0.5 mV s1. Reproduced with permission from ref. 76. Copyright 2011, American Chemical Society.

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Nanoscale Table 1

Performance comparison of metal sulfides with or without carbonaceous material coating for LIBs

Catalogue Bare Carbonaceous coating

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Electrical conductivity

Volume expansion

Structure stability

Polysulde dissolution

Cycle stability

Rate capability

Low High

Large Small

Bad Good

Signicant Light

Bad Good

Inferior Superior

synthesis of FeS2 nanowires via the chemical transformation of iron uoride nanowires. Such nanoelectrodes exhibit a capacity of 668 mA h g1 during the rst discharge process over the

voltage range of 2.4–1.1 V vs. Li/Li+ at 89 mA g1, and retain 350 mA h g1 aer 50 cycles (capacity retention: 52%). Furthermore, Fe3S4 nanoplatelets were synthesized using 3-methyl catechol as

(a) SEM, (b) TEM, (c) high-magnification TEM, and (d and e) HRTEM images of C@FeS nanosheets. (f) High-rate capability and (g) cycling performance of C@FeS nanosheet, nanoplate, nanoparticle, and bare FeS nanocrystal electrodes. Reproduced with permission from ref. 80. Copyright 2012, American Chemical Society.

Fig. 5

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the growth moderator and phase-control agent in the presence of sulfur, thiosulfate, octadecylamine and divalent iron ions (Fe2+).76 As seen from the transmission electron microscopy (TEM) image (Fig. 4a), the as-synthesized nanocrystals have platelet-like morphologies with lateral dimensions of 10–20 nm. Both multicrystalline and monocrystalline Fe3S4 nanoplatelets can be identied separately in the high-resolution (HR) TEM images (Fig. 4b and c) by their distinctive lattice sets of greigite. These Fe3S4 nanoplatelets display attractive electrochemical activities towards lithiation/delithiation, as indicated by the cyclic voltammetry (CV) curves (Fig. 4d) with repeated broad cathodic peaks at 1.5 and 0.5 V vs. Li/Li+ and anodic peaks at 1.5 and 2.0 V vs. Li/Li+ (labeled as III, IV, I and II). Despite these successes, the practical use of these iron sulde nanomaterials is still hindered by their high irreversible capacity loss during the rst few charge–discharge cycles (i.e., low coulombic efficiency) and by the gradual breakdown of the electrode aer multiple charge–discharge cycles (i.e., capacity fading). The underlying reasons for this are due to the volume expansion accompanied by the formation of the aforementioned polysuldes, which leads to capacity degradation.34,77–79 Surface modication of iron suldes with carbonaceous materials such as amorphous carbon,80–82 and reduced graphene oxide (rGO)50 has been proven to be an effective strategy to alleviate these problems (Table 1). For example, Xu et al.80 reported a facile approach to prepare carbon-coated troilite FeS (C@FeS) nanosheets via a surfactant-assisted solution-based technique, in which 1-dodecanethiol (DDT) was used as the sulfur source and surfactant. The detailed synthetic procedure involved reacting Fe(acac)3 and DDT with a molar ratio of Fe(acac)3 : DDT of 1 : 20 in oleylamine at 220  C, followed by annealing at 400  C for 2 h under an Ar atmosphere. The scanning electron microscopy (SEM) image in Fig. 5a and the TEM image in Fig. 5b reveal the formation of graphene-like FeS nanosheets with lateral dimensions of about 100–200 nm and thicknesses of less than 10 nm. The top-view high-magnication TEM (Fig. 5c) indicates the polycrystalline nature of an individual nanosheet comprising small nanocrystals (

Nanostructured metal sulfides for energy storage.

Advanced electrodes with a high energy density at high power are urgently needed for high-performance energy storage devices, including lithium-ion ba...
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