Layered Transition Metal Sulfides Research Articles

C3N4-digested intercalation MoS2 nano-flowers with expanded interlayer spacing towards high-performance rechargeable zinc-ion batteries

Molybdenum disulfide (MoS2) materials are very competitive cathode materials for aqueous zinc-ion batteries (AZIBs) due to their sandwich structure, large surface area and low cost. Also, the inherent low capacity and high energy barrier limit its further development. Here, the C3N4-digested intercalation MoS2 (MSCN) has been achieved via a hydrothermal strategy by using g-C3N4 nanosheets as a self-sacrificial template. The introduction of g-C3N4 greatly improves the MoS2 conductivity, which further enhances the Zn2+ diffusion dynamics, and succeeds in increasing the spacing between the layers. With a discharge capacity of 168 mAh g−1 at 5 A g−1, Zn-ion batteries based on the MSCN cathode assembly exhibit good cycle stability and excellent rate performance, under the same current density circulation capacity remain a rate of 92.9 % after 1000 cycles. And beyond that, flexible quasi-solid ZIBs with MSCN cathodes which have good Zn2+ storage properties and promising for future applications. The research in this article is of great significance for the design and development of AZIBs layered transition metal sulfide cathodes.

Layered Transition Metal Sulfides for Supercapacitor Applications

AbstractSupercapacitor (SC) devices holds an important position between traditional capacitors and ion batteries in terms of energy density and power density values. In particular, SC′s greater power density values than Li‐ion batteries make them useful for some specific applications, such as storing energy in hybrid cars while the car slows down. Increasing energy density values is one of the key challenges for the SC community. Transition metal dichalcogenides (TMDCs), are one of the new developing important material systems to have this potential, compared to their counterparts’ transition metal oxides and conductive polymers. This review is about giving insight into the electrochemical performances of two‐dimensional (2D) layered transition metal sulfides (TMS) such as MoS2, WS2, TaS2, NbS2, VS2, TiS2 and ZrS2 materials. On the other hand, the methods mostly used in synthesizing these materials are presented.

Insight into the intrinsic activity of various transition metal sulfides for efficient hydrogen evolution reaction.

The electrocatalytic hydrogen evolution reaction (HER) is an efficient approach to convert sustainable energy sources into clean energy carriers, H2. Although various transition metal sulfides (TMSs) have been reported as promising alternatives to precious metal-based catalysts, the top catalyst among TMSs remains unclear as there is a dearth of high-quality studies that provide a 'fair' comparison of the performance of these TMSs synthesized and tested under the same conditions. In this work, layered transition metal sulfides (MS2: MoS2, WS2, VS2) and non-layered transition metal sulfides (MxSy: FeS2, CoSx, NiS) were obtained by a straightforward hydrothermal method, and thus a comprehensive platform was established for the comparison of the intrinsic activity of these materials in the HER. Experimental results demonstrate that layered MS2 exhibits better performance than non-layered MxSy in acidic electrolytes, while CoSx and NiS can catalyze hydrogen evolution more effectively under alkaline conditions due to structural reconfiguration. MoS2 shows the best HER performance in both acidic and alkaline electrolytes, particularly in 1 M KOH solution. This work provides guidance for the optimal design of transition metal electrocatalysts, and structural engineering strategies can be used to further enhance their catalytic activity.

Cobalt phosphide integrated manganese-doped metallic 1T-vanadium disulfide: Unveiling a 2D-2D tangled 3D heterostructure for robust water splitting

The rational integration of a 2D layered transition metal sulfide and phosphide in a single heterostructure can bring a breakthrough to produce hydrogen through controlled manipulation of electronic structure. Here, we report a novel design of a hierarchical 2D−2D tangled 3D heterostructure of manganese-doped 1T vanadium disulfide (1T−Mn−VS2), assembled with cobalt phosphide (Co2P), at the flexible carbon cloth as 1T−Mn−VS2@Co2P@CC. The HER active 1T−Mn−VS2 bearing catalytically active edge sites, with numbers of basal planes and OER active Co2P, create synergism and hastening rection kinetics towards bifunctional water splitting. The heterostructure display significantly low over potential of 98 and 250 mV for HER and OER at 10 mA cm−2. The 1T−Mn−VS2@Co2P@CC (+,-) electrochemical cell needs 1.50 V to reach a current density of 10 mAcm−2. The implication of defect engineering along with interface modeling validated by DFT study, induces the regulation of electronic structure for fast redistribution of local charges/electron transfer.

Synthesis and characterization of mesoporous vanadium sulfides as environmental catalysts for the cycloaddition of CO2 with 2-(phenoxymethyl)oxirane) and oxidation reactions

Metal sulfide is a vital species of compound semiconductor materials, whose metallic ions are in mono-, bi-, or maybe in multi-form. Considering such materials, layered transition metal sulfide (LTMS) including layered transition metal dichalcogenides (LTMDs) like VS2 and VS4 (also called patronite mineral) has attracted growing interest. Moreover, it can be regarded as graphene analogs due to its large interlayer spacing and high theoretical specific capacity, enabling such materials to be applied in different fields i.e. photoelectric staffs, electro-catalysis phenomena, Li-containing batteries, and energy storage usages. Herein, a practical synthetic protocol is designated to construct mesoporous vanadium sulfides (VS2 and VS4) with controlled morphologies from VS2 (Flower-like), VS4 (Urchin-like), VS4 (Microsphere), and VS2 (Nanosheet assemblies) via adjusting the molar ratio of vanadium acetylacetonate and L-cysteine using a one-step solvothermal method. The catalytic activity of vanadium sulfides as an environmental catalyst has been considered for cycloaddition reaction of CO2 and 2-(phenoxymethyl)oxirane), oxidation of dibenzyl sulfide, and 1-phenylethan-1-ol. The prepared mesoporous vanadium sulfides have a higher surface area than the previously reported ones. The results and analysis revealed that the catalytic behavior of vanadium sulfides significantly depends on physical features such as morphology and specific surface area. The structure of the prepared materials was studied using a variety of characterization techniques: FTIR, XRD, TGA, BET, FE-SEM, EDX, and X-ray mapping

Enhanced Photoelectrochemical Performance in Engineered Interface 2D Metal Sulfide Heterostructures for Water Splitting

Photoelectrochemical water splitting (PEC) is a viable solution for addressing future environmental and energy concerns. However, due to the high cost of photoelectrodes and the limitations of sluggish kinetics, practical applications are limited. To achieve that goal, effective solutions are still needed to create low-cost photoelectrodes suited for PEC. Transistion metal sulfides (TMS) materials have a high potential for use as photoelectrodes due to their favorable and impressive properties1. The photoactivity of TMS is typically unlimited due to low reflection at the grain boundary. The individual atomic thin sheets with higher surface area compared to bulk materials allow for faster diffusion of photoinduced charge carriers primarily due to the less interface between their grains. Their lower interfacial contact resistance enables enhanced charge dynamics and, as a result, improved electrochemical reaction kinetics. When compared to other TMS, CdS has a direct bandgap and excellent photoactivity. However, its toxicity and photocorrosion make it unsuitable for long-term practical use. WS2 is well-known layered transition metal sulfide that has a 2D molybdenite structure. It is non-toxic in acidic/neutral solutions that are chemically stable. It has been demonstrated to be more stable against oxidation and photo corrosion than CdS, which is extensively employed in photocatalysis2. However, because of the recombination of photoinduced charge carriers, pure WS2 exhibits relatively poor photocatalytic efficiency and stability. As a result, combining WS2 based photoactive nanomaterials with other suitable materials has helped overcome this challenge. Transistion metal sulfides with acceptable band structures for heterostructure construction with WS2 may enable breakthroughs in photocatalytic and photo-electrochemical processes.The primary focus of this work is to construct heterostructures by combining CdS and WS2 and to investigate their role as photoanodes of semiconductor materials in PEC water splitting. In this work, we have synthesised heterostructures using a facile solution-reaction method and Successive Ionic Layer Adsorption Reaction deposition (SILAR) technique which is intrinsically cost effective and scalable3. The heterostructure is characterised by XRD, SEM and TEM. We also studied the performance enhancement of this heterostructure using Linear sweep voltammetry and Electrochemical impedance spectroscopy. Pristine WS2 and CdS showed photocurrent densities of 3 μA cm-2 and 20 μA cm-2. However, heterostructures formation led to much improved photocurrents. Photocurrent densities of 151 μA cm-2 were observed for WS2/CdS heterostructures respectively at 1.23 V vs RHE. The enhanced improvement in photoelectrochemical performance in the case of the heterostructures could be due to the following factors: (a) the intimate interaction of this unique heterostructure improves adhesion and lowers the photoinduced charge transfer barrier; (b) increased light harvesting capability; and (c) improved charge dynamics of photoinduced charge carriers due to proper band alignment. The current study paves the way for further research into the creation of metal sulfide heterostructures that generate analogous photoelectrode materials.Keywords: Photoelectrochemical water splitting, metal sulfides, photoelectrodes.

V5S8 nanoparticles anchored on carbon nanofibers for fast and durable sodium and potassium ion storage

Sodium- and potassium-ion batteries (SIBs and PIBs), as the next-generation batteries, attract a lot of attention due to the abundance and even distribution of sodium and potassium sources. However, both SIBs and PIBs need high-performing active anode materials to widen their industrial applications. Transition metal sulfides, possessing high theoretical capacity, show promise for SIB and PIB anodes. However, the structural instability of these sulfides occurring due to the volume changes during the active ion insertion and release limits their applications. In this work, V5S8 nanoparticles anchored on carbon nanofibers (V5S8@CNF) were fabricated for SIB and PIB anodes. The SIB cells exhibited outstanding performance with 351.0 mA h g−1 specific capacity after 400 cycles at 0.2 A g−1. A similar PIB cell demonstrated 289.2 mA h g−1 specific capacity after 200 cycles at 0.1 A g−1. Long-term stability of SIBs and PIBs for 10,000 and 2000 cycles at 2 and 1 A g−1, respectively, showed very stable specific capacities equal to 251.0 and 190.1 mA h g−1. The outstanding energy-storage-related properties of the sulfide-based anodes provide insights into techniques for improving Na+ and K+ electrochemical reactions assisted by two-dimensional layered transition metal sulfides.

Engaging tailored capacity of layered WS2 via sulphur bonding coupled with polyetherimide (WS2@NC) nanocomposite for high power and improved lithium-ion storage

Layered transition metal sulfides have drawn great research attention due to their excellent properties for energy storage and conversion devices. However, there is a need to innovate effective approaches to enhance the performance of metal sulfides for modern energy storage devices. Herein, a rational surface engineering strategy was adapted to design tungsten disulfide nanosheets coated polyetherimide (WS2@PEI) nanocomposite by facile sol-gel method combined with a subsequent pyrolysis. The WS2@NC nanocomposite exhibits excellent lithium-ion storage properties as compared to pristine WS2 nanosheets. The origin of enhanced performance of WS2@NC nanocomposite is carbon-sulphur (C–S) bonding with WS2, offered by thermally stable polyetherimide (N–C). The increased strength of charging is subjected to C–S and N–C bindings, which significantly enhanced the active sites and defects in WS2@NC nanocomposite framework. The thin porous carbon layer provided elasticity to control the volume expansion during the lithium ions (Li+) insertion/deinsertion, while nitrogen and sulphur components offered more active sites and defects in the nanocomposite for reversible adsorption of Li+ ions. Thus, the WS2@NC nanocomposite could deliver high reversible specific capacity of 712 mAh g−1 at 0.1 A g−1 after 100 cycles and 417.6 mAh g−1 even at high current density of 0.8 A g−1 with 99% average coulombic efficiency.

Atom removal on the basal plane of layered MoS2 leading to extraordinarily enhanced electrocatalytic performance

Efficient utilization of sulfur atom in MoS2-based electrocatalysts on the basal plane is highly desirable and expectable to optimize the electrocatalytic performance for hydrogen evolution reaction (HER). Herein, removing atoms on the basal plane of layered MoS2 by Li-ion intercalation leading to extraordinarily enhanced electrocatalytic performance is newly reported, which provides a novel way to active the basal planes of layered transition metal sulfides towards hydrogen evolution. It is found that the desulfurization of layered MoS2 can bring atomic vacancies on the basal plane, which is confirmed to be active sites towards hydrogen evolution experimentally and theoretically. The first-principle calculations reveal that the desulfurization induced atomic vacancies not only reduce the free energy of hydrogen adsorption (ΔGH*), but also accelerate the electron transfer on the basal plane. Besides, the phase transformation from 2H to 1T during the electrochemical lithium-ion intercalation could reach higher electronic conductivity, thus further optimize the HER kinetics through phase engineering. For these reasons, the desulfurized MoS2 displays improved HER activity with lowered overpotential of 200 mV to obtain 10 mA cm−2, high stability, decreased Tafel slope of 65 mV dec−1 and enhanced TOF of 0.093H2 s−1 at overpotential of 250 mV, which are much better than that of the pristine MoS2 and other optimized MoS2-based catalysts.

Synthesis of MoS₂/TiO₂ Nanophotocatalyst and Its Enhanced Visible Light Driven Photocatalytic Performance.

Molybdenum disulfide (MoS₂), as a typical layered transition metal sulfide, has been widely used in photocatalysis. Here, we report layered MoS₂ nanosheet-coated TiO₂ heterostructures that were prepared using a simple photo-assisted deposition method. The as-prepared samples were investigated in detail by using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Results demonstrated that the MoS₂ nanosheets uniformly covered the outer surface of TiO₂. The visible light-sensitive photocatalytic activity was evaluated by the removal of methylene blue (MB) and 2-chlorophenol (2-CP) in aqueous solution. Thus, the MoS₂/TiO₂ heterostructures exhibited improved photocatalytic degradation activity under visible light compared with the pure TiO₂. Under visible light irradiation for 90 min, the degradation efficiencies of MB and 2-CP over the MoS₂/TiO₂ sample (sunlight irradiation time: 30 min) are as high as 93.6% and 70.6%, respectively. Furthermore, the corresponding mechanism of enhanced photocatalytic activity is proposed on the basis of the comprehensively investigated results from the radical trapping experiments, photoluminescence spectroscopy, and electron spin resonance analysis. The hole oxidation, hydroxyl radicals, and superoxide anion radicals act as the active species simultaneously in the photodegradation of the dye molecules. However, of these species, hole oxidation played the most important roles in the photocatalytic reaction.

Intercalation of Solvated Magnesium-Ions into Layered Transition Metal Sulfide for Fast Mg Storage

Rechargeable magnesium (Mg) batteries (RMBs) are potential candidates for new generation battery technologies due to the attractive properties of Mg as a metal anode, especially the intrinsic nature of homogeneous Mg deposition and abundant Mg reserves. Compared to the monovalent counterpart (e.g. Li+), the divalent Mg ions have a greater charge density and thus need to overcome much stronger electrostatic forces for insertion and diffusion inside the host structures. One of the strategies to enhance Mg2+ ion intercalation kinetics is to reduce its charge density. Herein, we present the intercalation chemistry of the solvated ions [Mg(DME)3]2+, in which the improved ion mobility has been proven by various electrochemical and spectroscopic characterizations.1 The novel concept was realized with Mg[B(hfip)4]2/DME electrolyte2, 3 and a layered MoS2 cathode coupled with a Mg anode. The cell configuration exhibited a long-term cycling stability (>200 cycles) and high rate capability (47 mA h g-1 at 0.5 A g-1). In addition, the mechanism of Mg storage has been investigated in detail, revealing the reversible 2H to 1T MoS2 phase transition during Mg ions de-/intercalation. Compared to other reported intercalation with [Mg(H2O)x]2+ or MgCl-,4, 5 the DME molecules will neither passivate the anode nor corrode the cell components, which makes this approach more practical. Moreover, the concept of solvated Mg2+ ions can be potentially extended to other intercalation cathode materials and applied to other multivalent ions intercalation. References Z. Li, X. Mu, Z. Zhao-Karger, T. Diemant, R. J. Behm, C. Kübel and M. Fichtner, Nature Communications, 2018, 9, 5115.Z. Zhao-Karger, M. E. Gil Bardaji, O. Fuhr and M. Fichtner, J. Mater. Chem. A, 2017, 5, 10815-10820.Z. Zhao-Karger, R. Liu, W. Dai, Z. Li, T. Diemant, B. P. Vinayan, C. Bonatto Minella, X. Yu, A. Manthiram, R. J. Behm, M. Ruben and M. Fichtner, ACS Energy Letters, 2018, 3, 2005-2013.K. W. Nam, S. Kim, S. Lee, M. Salama, I. Shterenberg, Y. Gofer, J.-S. Kim, E. Yang, C. S. Park, J.-S. Kim, S.-S. Lee, W.-S. Chang, S.-G. Doo, Y. N. Jo, Y. Jung, D. Aurbach and J. W. Choi, Nano Letters, 2015, 15, 4071-4079.H. D. Yoo, Y. Liang, H. Dong, J. Lin, H. Wang, Y. Liu, L. Ma, T. Wu, Y. Li, Q. Ru, Y. Jing, Q. An, W. Zhou, J. Guo, J. Lu, S. T. Pantelides, X. Qian and Y. Yao, Nature Communications, 2017, 8, 339.

Andreev reflection spectroscopy of ferromagnetic Fe<sub>0.26</sub>TaS<sub>2</sub> with layered structure

<sec>An elementary mission of spintronics research is to prevent the interface reacting in spin device and extract spin polarization of ferromagnetic material reliably. Layered transition metal sulfide has very strong anisotropic magnetism, magnetoresistance, and unique Hall effect. It provides a good platform for studying the magnetic order related physical phenomena and may lay a foundation for spintronic applications. In this work, the magnetism, electronic transport and Andreev reflection spectrum of a novel ferromagnetic material Fe<sub>0.26</sub>TaS<sub>2</sub> with a layers-stacked structure are measured. Strong magnetic anisotropy, double-peak magnetoresistance and anomalous Hall effect are found. In the magnetic measurement, the strong magnetic anisotropy behavior in Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal is observed. Curie temperature <i>T</i><sub>C</sub> of the Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal is confirmed by zero field cooling, field cooling and Arrot plot. The electronic transport in the Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal also reveals strong anisotropic behaviors, such as butterfly-like magnetoresistance and obvious anomalous hall effect below <i>T</i><sub>C</sub>.</sec><sec>To obtain the spin polarization of Fe<sub><i>x</i></sub>TaS<sub>2</sub>, we fabricate an Fe<sub><i>x</i></sub>TaS<sub>2</sub>/superconductor Andreev junction to measure the spin polarization that is fitted by the modified Blonder-Tinkham-Klapwijk (BTK) theory. Perhaps the diffusion of Pb can form an alloy structure, creating another superconductor behavior. The two-gap BTK theory confirms our hypothesis, and the result spin polarization can reach 26%. To avoid the interference from Pb alloy superconductor, we also fabricate an Fe<sub>0.26</sub>TaS<sub>2</sub>/Al/Pb superconductor junction by evaporating Al and then Pb film on the surface of Fe<sub>0.26</sub>TaS<sub>2</sub> in sequence. The results of BTK fit show that the spin polarization from the first technical route cannot be reliable due to the tunneling layer on the Al interface. In order to obtain a clean interface, Fe<sub>0.26</sub>TaS<sub>2</sub>/NbSe<sub>2</sub> junction is fabricated through mechanical-exfoliation and dry-transfer method. Through the Andreev reflection spectrum of this junction, the spin polarization of Fe<sub>0.26</sub>TaS<sub>2</sub> is extracted to be 47% ± 7%. For various two-dimensional ferromagnetic materials, our work suggests that the dry-transfer method is well applicable in spin polarization extraction. The results of spin polarization indicate that the Fe<sub>0.26</sub>TaS<sub>2</sub> is a promising candidate of next-generation material of spintronics.</sec>

A kinetics study on intercalation pseudocapacitance of layered TiS2 in K-ion batteries.

The boundary between a capacitor and a battery is vague for layered transition metal sulfides, especially when it comes to TiS2 with a multi-phase transition process. In this work we analyzed both the quasi-static process and dynamic process of K+ storage in TiS2 with carbonate based electrolytes, which proved to be an intercalation pseudocapacitive behavior.

Nature of extra capacity in MoS2 electrodes: Molybdenum atoms accommodate with lithium

Two-dimensional (2D) layered transition metal sulfide based materials are promising candidates for electrodes of lithium ion batteries (LIBs). As one of the representatives, molybdenum disulfide (MoS2), exhibits additional reversible capacity beyond their theoretical value although the lithium storage mechanism is still poorly understood. In this report, we developed a highly conducting metallic phase 1T(octahedral)-MoS2 based electrode with metal Mo atoms confined in “graphene nanoreactor” of abnormal lithium-storage sites, which delivered a high specific capacity of ~1800 mAh g−1 at 1 A g-1, about 2.6 times of its intercalation reactions value. Contrary to previous reports, a new lithium-storage mechanism for the high and ever-increasing capacity was proposed. The theoretical calculations and experiments show that a major contribution to the extra capacity in MoS2 anodes is due to Mo atoms accommodate with Li+. The Mo precipitates and their full contact with Li2S matrix enabled the reversible Mo→LixMo→Mo→1T-MoS2 reaction during charging/discharging processes. Over prolonged cycling, per Mo atomic could accommodate six Li+ ions. This new proposal could help to establish an accurate electrochemical reaction mechanism of MoS2 LIBs, which may lead to inspiration on new methods to greatly improve the performance of the MoS2 anode.

Fabrication of 3D Porous Hierarchical NiMoS Flowerlike Architectures for Hydrodesulfurization Applications

Layered transition-metal sulfides such as MoS2 often show a range of intriguing electronic, catalytic, and optical properties. Because of their high surface energy, layered materials generally tend to stack and prevent the exposure of additional edge sites. Here, we demonstrate a facile approach for the preparation of hierarchical NiMoS nanoflowers via SiO2-assisted hydrothermal synthesis. The structure and morphology of the nanomaterials are characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman, and X-ray photoelectron spectroscopy analyses, revealing that different sizes of NiMoS nanoflowers assembled from various nanosheet thicknesses can be tuned by modifying the Si/Mo molar ratio. The key aspect of this strategy is to construct the three-dimensional nanostructures around the SiO2 nanospheres while maintaining a flowerlike feature. The correlation between the nanosheet thickness, surface area, and total dispersion of the Mo atoms indicated that the l...

Activating layered LiNi0.5Co0.2Mn0.3O2 as a host for Mg intercalation in rechargeable Mg batteries

Layered crystal structures are some of the most intensively studied intercalation hosts for guest ion insertion. For Mg insertion, layered transition metal sulfides or selenides have been used for reversible Mg intercalation; however, far less intercalation hosts have been found for layered oxides most likely because of the strong interaction between Mg2+ and the oxide host. Here, we demonstrate that layered LixNi0.5Co0.2Mn0.3O2 (NCM523), which is an important commercial electrode for Li-ion batteries but has been regarded as electrochemically inactive in rechargeable Mg batteries, can function as a reversible host for Mg2+ if water opens up the layers and screens the electrostatic interaction between Mg2+ and the host. Upon the formation of water-intercalated NCM523, the discharge capacity dramatically increases utilizing the multi-redox reaction of Ni2+/Ni3+/Ni4+, which exhibits an average voltage of ∼3.1V (vs. Mg/Mg2+) in rechargeable Mg batteries with an energy density of 589Whkg−1 in the first discharge.

High-energy asymmetric supercapacitors based on free-standing hierarchical Co-Mo-S nanosheets with enhanced cycling stability.

Layered transition metal sulfides (TMS) are emerging as advanced materials for energy storage and conversion applications. In this work, we report a facile and cost-effective anion exchange technique to fabricate a layered, multifaceted, free standing, ultra-thin ternary cobalt molybdenum sulfide nanosheet (Co-Mo-S NS) architecture grown on a 3D porous Ni foam substrate. The unique Co-Mo layered double hydroxides are first synthesized as precursors and consequently transformed into ultra-thin Co-Mo-S NS. When employed as an electrode for supercapacitors, the Co-Mo-S NS delivered an ultra-high specific capacitance of 2343 F g-1 at a current density of 1 mA cm-2 with tremendous rate capability and extraordinary cycling performance (96.6% capacitance retention after 20 000 cycles). Furthermore, assembled Co-Mo-S/nitrogen doped graphene nanosheets (NGNS) in an asymmetric supercapacitor (ASC) device delivered an excellent energy density of 89.6 Wh kg-1, an amazing power density of 20.07 kW kg-1, and superior cycling performance (86.8% capacitance retention after 50 000 cycles). Such exceptional electrochemical performance of Co-Mo-S NS is ascribed to the good electrical contact with the 3D Ni foam, ultra-high contact area with the electrolyte, and enhanced architectural softening during the charging/discharging process. It is expected that the fabricated, unique, ultra-thin Co-Mo-S NS have great potential for future energy storage devices.

Oriented Stacking along Vertical (002) Planes of MoS2: A Novel Assembling Style to Enhance Activity for Hydrogen Evolution

The layered structures of MoS2 tend to stack and aggregate and result in decreasing of rims and edges of MoS2 nanosheets, which greatly limited the improvement of activity of MoS2 as electrocatalysts for hydrogen evolution reaction (HER). Herein, owing to the inert nature of (002) basal plane of MoS2 for HER, we propose that oriented stacking along vertical (002) plane can form nanocolumn structure, which may expose an increased number of the active sites per mass and accelerate charge transfer rate for HER. The electrochemical measurements confirm that the nanocolumns of oriented stacking of MoS2 (O-MoS2) have much better HER activity than bulk MoS2 and randomly stacking of MoS2 with about 25 times enhancement of electrochemically active surface area (ECSA). Therefore, assembling style along vertical inert plane may be a novel way to enhance HER performances of layered transition metal sulfides.

The High-Temperature Transformation from 1T- to 3R-Li x TiS2 (x = 0.7, 0.9) as Observed in situ with Neutron Powder Diffraction

Abstract Layered titanium disulfide is used as lithium-ion intercalating electrode material in batteries. The room-temperature stable trigonal 1T polymorphs of the intercalates Li x TiS2(x ≤ 1) are widely-investigated. However, the rombohedral 3R polymorphs, being stable at higher temperatures for large x, are less well known. In this study, we report on the synthesis of phase-pure 1T-Li x TiS2(x = 0.7, 0.9) and its transformation to the 3R phase between 673 and 873 K as monitored using high-temperature neutron powder diffractometry. For the 3R polymorph, full Rietveld refinements show lithium ions to be statistically distributed over octahedral voids at the fractional coordinates 0, 0, 1/2 , exclusively. The comparison of Madelung energies with results of periodic quantum-chemical calculations reveals that the evolution of lattice parameters and the room-temperature stability of the 1T phase are not governed by electrostatics, but by correlation and polarization. The insights gained do not only elucidate the structure of 3R-Li x TiS2, but also help to understand and control polymorphism in layered transition-metal sulfides.

Ultrathin MoS2/Nitrogen‐Doped Graphene Nanosheets with Highly Reversible Lithium Storage

The extensive range of possibilities for using a rechargeable lithium ion battery (LIB) for stationary power storage applications has stimulated signifi cant research to improve its energy, power density and cycling life. The current use of graphite as a commercially available anode material cannot fully meet the energy density requirements for LIB applications in electric vehicles due to its relatively small capacity (372 mAh/g). In the next generation LIB, graphite should be replaced by alternative higher capacity materials and graphene based composite is one of the promising choices. The discovery of graphene has attracted wide-ranging interest due to its unique physical and chemical properties and potential for applications in electronic devices and sensors. [ 1–3 ] This has led to the development of a signifi cant number of graphene-based materials for use as LIB anodes and results have been generally promising due to high specifi c capacity and improvement of its cycling ability. In recent years, chemical substitutional doping has been used to enhance the properties of graphene—further improving its properties beyond just morphology and size control. This includes the use of sulfur, boron, and nitrogen doping, the latter of which is particularly effective in modulating the electronic properties of graphene. [ 4–6 ] Based on these results, we expect that nitrogen-doped (N-doped) graphene will be used as a template for synthesis of active electrode materials. Compared to graphene-based metal oxide materials, some graphene-based transition metal sulfi des possess consummate 2D-layered structures. The matched structure between transition metal sulfi des and graphene avoids the exfoliation of electrode materials and capacity fading during the charge/discharge process. [ 7–9 ] As a typical layered transition metal sulfi de, MoS 2 has a structure similar to graphite, consisting of three atom layers of S-Mo-S stacked together through van der Waals interactions. [ 10 , 11 ] This layered structure enables the effi cient intercalation and deintercalation of lithium ions. In recent years, many published reports have articulated various processes for preparing MoS 2 or MoS 2 composites as LIB anodes, which