O2 Adsorption Associated with Sulfur Vacancies on MoS2

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

O2 Adsorption Associated with Sulfur Vacancies on MoS2 Microspheres Guiru Sun,† Fei Li,‡ Tong Wu,† Lina Cong,† Liqun Sun,† Guochun Yang,‡ Haiming Xie,*,† Alain Mauger,§ Christian M. Julien,§ and Jia Liu*,†

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/15/19. For personal use only.

†

Nation & Local United Engineering Laboratory for Power Batteries, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China ‡ Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Department of Physics, Northeast Normal University, Changchun 130024, P. R. China § Sorbonne Université, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), CNRS UMR 7590, 4 Place Jussieu, 75005 Paris, France S Supporting Information *

ABSTRACT: MoS2 is well-known for its catalytic properties, mainly to adsorb hydrogenous or carbonaceous materials. However, the effect of MoS2 on the oxygen adsorption has been investigated only a few times thus far. In this work, we first studied the adsorbability of O2 by MoS2 through the analysis of Li2O2 growth on the surface of flower-like MoS2 microspheres with different concentrations of sulfur vacancies, which can be applied as the highly active electrocatalysts for Li−O2 batteries. The enhancement of battery performance for the Def-MoS2@CTs (CTs = carbon textile substrates) with a larger concentration of sulfur vacancies (S/Mo = 1.61) can be achieved. The experimental and theoretical results confirm that the sulfur vacancies play a crucial role in the adsorption process and thus affect the morphology and nucleation of Li2O2. In addition, a fundamental catalytic mechanism for this adsorption process is also proposed. These results provide a new insight into the development of a highly active electrocatalyst by introducing a large concentration of defects for Li−O2 batteries.

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blocks the porous channels,15 leading to the sluggish kinetics. Therefore, being an optimized microstructure, a hierarchical 3D structure is expected to enhance the catalyst performance due to the high surface area, the sufficient porous channels, and the short ion and/or electron diffusion path.16,17 Besides the morphology, there are many other factors that can affect the catalytic activity of MoS2, such as the synthesis method and sulfur vacancy generation. Introduction of sulfur vacancies into the planes is a promising approach to enhance the catalytic kinetics of MoS2.4 At the S vacancy site, the adjacent Mo atoms exposed to the surface are uncoordinated, allowing their d-states to form bonds with adsorbing species. These Mo atoms are thus the active sites. Usually, we simply speak of a sulfur vacancy as an active site to make it short, but actually, the active sites should be the Mo atoms next to the vacancies. Therefore, by introducing a controlled concentration of sulfur vacancies, it is possible to create a much larger number of active sites than that found at the edges. Equally, the sulfur vacancies can act as donors, which results in the

INTRODUCTION Molybdenum disulfide (MoS2) is considered to be a promising and potential candidate for the different application fields including electronics, optoelectronics, energy storage, catalysis, etc.1−7 Being a typical layered structure similar to graphite, MoS2 is extensively studied as an anode material for Li-ion batteries, which can facilitate the insertion and extraction of Li+.8 In addition, MoS2 is also known due to its catalytic property, which has been used for photoelectrocatalysis,9 hydrogen evolution reaction (HER),10 conversion of synthesis gases (syngas),11 Li−O2 batteries,12,13 etc. Typically, MoS2 exhibits the sheet-like structure due to its two-dimensional (2D) nature.13,14 However, the sheet morphology is not favorable for catalytic properties of MoS2, because the sheet has no dangling bond at the basal planes terminated by sulfur atoms, which leads to the catalytic insertion of basal planes. As a consequence, if the plane morphology is to be kept, the only possibility is to reduce the plane size from the macroscale to the nanoscale. This strategy has been employed to construct a cathode based on MoS2 nanoflakes for a Li−O2 battery.12 Equally, the aggregation and/or restacking of 2D nanosheet impedes the full utilization of surface area; in particular, it © XXXX American Chemical Society

Received: November 27, 2018

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DOI: 10.1021/acs.inorgchem.8b03300 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Schematic Illustration of the Fabrication Process of MoS2@CTs and Def-MoS2@CTs

lithium perchlorate (LiClO4, Aladdin Reagent, AR) were dried at 120 °C overnight in a vacuum oven. Preparation of MoS2@CTs and Def-MoS2@CTs. The CTs (2 cm × 5 cm) were cleaned by sonication in deionized water and ethanol for 30 min to remove the surface impurities and then dried in an oven at 50 °C for 10 h. NaMoO4·2H2O (0.38 mmol) and H2NCSNH2 (1.40 mmol) were dispersed in deionized water (30 mL) and stirred at room temperature. After stirring for 30 min, the mixture and the CTs were transferred to a hydrothermal autoclave with a capacity of 50 mL, and maintained at 200 °C for 24 h. After the hydrothermal crystallization, the product was filtered and washed using deionized water and ethanol three times and then dried at 50 °C for 10 h in an oven. MoS2@CTs were thus obtained. According to the sulfur vacancies that form under H2 reduction,24 Def-MoS2@CTs was obtained after annealing MoS2@CTs at 600 °C for 6 h in the flowing gas composed of H2 and Ar in a volume ratio of 8:92 with a heating rate of 5 °C min−1. The synthesis procedure for Def-MoS2@CTs is illustrated in Scheme 1. The obtained MoS2@CTs and Def-MoS2@ CTs were then formed into a disc with a diameter of 1.2 cm that was directly used as a cathode for Li−O2 cells. The loading mass density for Def-MoS2 or MoS2 is about 0.45 mg cm−2. Preparation of MoS2 and Def-MoS2 Powders and a Reference Electrode. MoS2 and Def-MoS2 powders were fabricated by a synthetic method similar to that mentioned above with the only difference being the lack of CT participation. The corresponding morphologies and XRD patterns for MoS2 and Def-MoS2 powders are shown in Figures S1 and S2 in Supporting Information (SI). The reference electrode was prepared on the basis of Def-MoS2 and PVDF binder as follows: Def-MoS2 powder and PVDF in a weight ratio of 90:10 were mixed in NMP to make a slurry, which was evenly coated onto the CTs with a diameter of 1.2 cm. Subsequently, the electrode was dried to remove residual NMP solvent at 120 °C for 12 h under vacuum. The total loading of Def-MoS2 for the reference electrode was the same as that of MoS2 in the Def-MoS2@CTs electrode. Characterization. X-ray diffraction (XRD) diffractometer (Rigaku D/max 2500) with Cu Kα radiation (λ = 1.5406 Å) was used to analyze the crystal phases of CTs, MoS2@CTs, Def-MoS2@CTs, and the cathodes at the different electrochemical states at 40 kV and 30 mA. The Raman spectra were measured using a Raman spectrometer (LabRAM HR Evolution-HORIBA) with an excitation laser wavelength (λ = 488 nm). The Raman shift range was 100−3500 cm−1, and the width of the laser beam was 2−3 μm. The chemical state of the surface elements for MoS2@CTs and Def-MoS2@CTs were characterized by X-ray photoelectron spectroscopy (XPS, ESZALB 250XL) with Al Kα radiation (hν = 1486.7 eV) and an emission angle of 90°. All XPS spectra were calibrated using the C 1s peak assigned to the C−H bond at 284.8 eV. The morphology and particle size of CTs, MoS2@CTs, Def-MoS2@CTs, and cathodes at the different electrochemical states were observed by scanning electron microscopy (SEM, Hitachi S-5500). The element mapping information on DefMoS2@CTs was obtained via energy dispersive spectroscopy (EDS, AMETEK) coupled with SEM. The microstructure and crystal information on MoS2 and Def-MoS2 powders were obtained via high resolution transmission electron microscopy (HRTEM, JEOL 2100F) operating at 200 kV. Preparations of Li−O2 Cells and Electrochemical Performance Measurements. Coin-type Li−O2 cells were assembled in an Ar-filled glovebox (Mbraun, PRS380/S11-0736, H2O < 1.0 ppm, O2 < 1.0 ppm) with a Li anode (China Energy Lithium Co., Ltd.), a glass fiber separator (GF/D, Whatman) soaked in an electrolyte (140 μL, 1

MoS2 with large concentration sulfur vacancies being an n-type semiconductor.18 This ensures that the MoS2 exhibits better electric conductivity, facilitating the kinetics of electrochemical reactions. Currently, MoS2 with sulfur vacancies as catalyst mainly adsorbs hydrogenous or carbonaceous material.11,19 However, the effect of sulfur vacancies in MoS2 on the oxygen adsorption has not been investigated thus far. Herein, an O2-participating battery system (Li−O2 battery) was thus employed to evaluate the influence of sulfur vacancies on the oxygen adsorption. We successfully prepared a selfstanding cathode of carbon textile substrates (CTs) decorated by flower-like MoS 2 microspheres with two different concentrations of sulfur vacancies (sulfur vacancies determined by S/Mo = 1.91 (MoS2@CTs) and 1.61 (Def-MoS2@CTs)). Typically, an electrocatalyst-based cathode for Li−O2 batteries consists of the carbonaceous materials increasing the electric conductivity, the chemical binder to make it possible to case on the substrate, and the electrocatalyst. However, carbon particles in a cathode mixing with polymer binder sometimes exhibit tight aggregation, which inevitably decreases the O2 diffusion and limits the Li2O2 deposition space.20 Additionally, it has been widely recognized that carbonaceous powder and binder have a negative influence on the long-term stability for cells.20,21 Therefore, considering the low conductivity of MoS2 and the negative effects of commonly used carbon powder and binder, the CTs are chosen as the conductive carbon substrates here.22 Note that the capacity contributed from pure CTs could be ignored.23 This suggested, however, that intrinsic adsorption properties of MoS2 catalysts can be obtained, implying that the capacities obtained in this work are 1 order of magnitude smaller than those obtained with a full cathode that includes the carbonaceous component in “real” Li−O2 batteries. Compared with the cell performance of MoS2@CTs, the cell with Def-MoS2@CTs exhibits a lower overpotential, a longer cycle life, and better reversibility. Additionally, the experimental results show that the concentration of sulfur vacancies has major effects on the Li2O2 morphology and the catalytic activity. Furthermore, the density functional theory (DFT) computations give enlightenment on this effect, and the catalytic mechanism is thus proposed. These results contribute to the development of a highly active electrocatalyst to the O2-participating reactions by introducing the high concentration of sulfur vacancies, which can be employed for broad fields (e.g., fuel cells, metal−O2 batteries, and water electrolyzers).

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EXPERIMENTAL DETAILS

Chemicals and Materials. Sodium molybdate (NaMoO4·2H2O, Tianjin Chemical Reagent Factory, AR), thiourea (H2NCSNH2, Tianjin Tiantai Fine Chemical Co., Ltd., AR), ethanol (CH3CH2OH, Beijing Chemical Works, AR), N-methyl-2-pyrrolidone (NMP, Aladdin Reagent, AR), dimethyl sulfoxide (DMSO, Aladdin Reagent, AR, ≥99.7%), and CTs (CeTech Co., Ltd.) were used without any purification. Polyvinylidene fluoride (PVDF, Solef5130, Solvay) and B

DOI: 10.1021/acs.inorgchem.8b03300 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) XRD patterns of pristine CTs, MoS2@CTs, and Def-MoS2@CTs; (b) the magnified pattern enclosed by the pink dashed line in part a; (c) Raman spectra of pristine CTs, MoS2@CTs, and Def-MoS2@CTs; and XPS spectra of (d) C 1s, (e) Mo 3d−S 2s, and (f) S 2p for MoS2@ CTs and Def-MoS2@CTs. M LiClO4/DMSO), and a cathode which is described above. The galvanostatic measurements of all cells were carried out on a LAND system (CTA2001A, Wuhan Land Electronic Co., Ltd.) in an electrochemical window 2.2−4.5 V vs Li/Li+ in an O2-filled glovebox (Mikrouna, Super (1220/750/900) + Universal (1800/750/900), H2O < 1 ppm). The delivered capacities for all cells were calculated on the basis of the entire area of as-prepared cathodes. Cyclic voltammetry (CV) measurements were performed on a potentiostat (PARSTAT 4000, Ametek Inc.) at a scan rate of 0.1 mV s−1 in an electrochemical window 2.2−4.5 V vs Li/Li+. Electrochemical impedance spectroscopy (EIS) was conducted on a workstation within a frequency range 106−10−1 Hz. Computational Details. Structural relaxations and total energy calculations were performed in a framework of DFT within the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP).25 The parametrization of Perdew−Wang (PW91) electron−ion interactions was described using all-electron projector augmented-wave (PAW) potentials,26 with valence configurations of 4p64d55s1 for Mo, 3s23p4 for S, and 2s22p4 for O. van der Waals interactions were included in the calculations.27 The cutoff energy of 600 eV and appropriate Monkhorst−Pack kmeshes with spacing of 2π × 0.05 Å−1 were used. Isolated monolayers for MoS2 were modeled by a 3 × 3 supercell with a vacuum space of 15 Å to avoid the interaction between adjacent layers.28 All configurations were optimized until the Hellmann−Feynman forces acting on each atom minimize to a value below 0.01 eV Å−1.

concentration of sulfur vacancy could exist in the DefMoS2@CTs sample, which can also be suggested by the shifts of the XRD peaks (Figure 1b). The Raman spectra of pristine CTs, MoS2@CTs, and DefMoS2@CTs show the signals of D and G bands at 1357 and 1595 cm−1, respectively, characteristics of the carbon (Figure 1c).30,31 The D band is associated with the disorders and defect modes, and the G band arises from the well-graphitized carbon. The intensity ratio of the D and G bands is the same for both composites as expected. For MoS2@CTs, the peaks at 382 and 407 cm−1 are identified as the in-plane (E12g) and outof-plane (A1g) vibration of MoS2, respectively.32 For DefMoS2@CTs, these peaks shift to smaller wavenumbers at 380 and 405 cm−1, respectively. These shifts arise from the phonon confinement by point defects,33 which is consistent with the XRD results. The surface composition and chemical states of MoS2@CTs and Def-MoS2@CTs were studied by XPS. The survey spectra consist of C, Mo, and S contributions without any other elements (Figure S3 in SI). The deconvolution of the C 1s spectra shows two carbon contributions (Figure 1d). C1 at 284.8 eV is the characteristic of the standard carbon;34 C2 at 285.4 eV is attributed to CO.35 The Mo 3d5/2 and 3d3/2 doublets are deconvoluted into two sets of peaks, as shown in Figure 1e. One set of peaks (in green) at 230.5 and 233.6 eV corresponds to the stoichiometric MoS2 (s-MoS2 peaks).36 The other one (in blue) at 229.6 and 232.8 eV is the signature of sulfur defects in MoS2 (d-MoS2 peaks).35,36 The peak areas of s-MoS2 and d-MoS2 for MoS2@CTs and Def-MoS2@CTs are listed in Table S1 in SI. Note that Def-MoS2@CTs shows a higher peak area of d-MoS2 and a lower peak area of s-MoS2 than those of MoS2@CTs, additional evidence of the sulfur vacancies in [email protected] The S 2p spectra of MoS2@ CTs and Def-MoS2@CTs are displayed in Figure 1f. For MoS2@CTs, the S 2p spectra at 162.5 eV (S 2p3/2) and 163.7 eV (S 2p1/2) are ascribed to S2−.29 The peaks of S 2p3/2 and S 2p1/2 are shifted to the lower binding energy side for DefMoS2@CTs, which has been discussed in previous reports,36

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RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of pristine CTs, MoS2@ CTs, and Def-MoS2@CTs. The diffraction peaks for all samples at 2θ = 25.3° and 42.7° correspond to reflections of graphitic carbon,21 while the other peaks located at 2θ = 14.3°, 33.5°, 58.3°, and 60.1° correspond to the (002), (101), (110), and (008) plane reflections of the MoS2 standard pattern (JCPDS PDF 37-1492),24 which are characteristics of MoS2 and Def-MoS2, providing evidence of successful preparations of MoS2@CTs and Def-MoS2@CTs. The lattice parameters for MoS2@CTs are a = b = 3.1596(7) Å, c = 12.2968(7) Å, in good agreement with the previous results,29 while the lattice parameters for Def-MoS2@CTs are a = b = 3.1513(7) Å, c = 12.2859(7) Å. This result indicates that the a larger C

DOI: 10.1021/acs.inorgchem.8b03300 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

to the expected pattern of MoS2, which is in agreement with XRD results. Additionally, compared with the well-defined lattice fringes of MoS2, Def-MoS2 displays some disorder and discontinuous lattice fringes (Figure 2h), further confirming the presence of a large concentration of sulfur vacancies in DefMoS2.37 The CV curves of Li−O2 cells with MoS2@CTs and DefMoS2@CTs cathodes at a scan rate of 0.1 mV s−1 in an electrochemical window 2.2−4.5 V vs Li/Li+ are shown in Figure 3a. The cell with Def-MoS2@CTs exhibits lower oxygen

demonstrating the existence of sulfur vacancies. The results also indicate that the atomic ratios of S and Mo for MoS2@ CTs and Def-MoS2@CTs are 1.91 and 1.61, respectively, confirming a larger concentration of sulfur vacancies for DefMoS2@CTs than that for MoS2@CTs. The morphology and particle size information for MoS2@ CTs and Def-MoS2@CTs were studied by SEM and TEM. The pristine CTs are woven by the carbon fibers with a diameter of 8 μm (Figure S4 in SI). The MoS2 and Def-MoS2 are well-dispersed on the CTs (Figure 2a,b). In Figure 2c,d,

Figure 3. (a) CV curves of Li−O2 cells using pristine CTs, MoS2@ CTs, and Def-MoS2@CTs cathodes in an O2 atmosphere at a scan rate of 0.1 mV s−1, and the discharge−charge profiles of Li−O2 cells with (b) pristine CTs, (c) MoS2@CTs, and (d) Def-MoS2@CTs at a constant current density of 0.1 mA cm−2 with a capacity limit of 0.5 mAh cm−2.

evolution reaction (OER) and higher oxygen reduction reaction (ORR) onset potentials, as well as higher intensities of cathodic and anodic peaks than the cells with pristine CTs and MoS2@CTs, indicating a higher catalytic activity of DefMoS2@CTs toward ORR and OER than that of MoS2@CTs. The delivered discharge capacity of a cell using Def-MoS2@ CTs reaches 6.7 mAh cm−2, against 1.0 mAh cm−2 for the cell using pristine CTs and 4.8 mAh cm−2 with MoS2@CTs (Figure S7 in SI). The cycle abilities of the cells with the different electrodes are measured at a constant current density of 0.1 mA cm−2 with a capacity limit of 0.5 mAh cm−2. In comparison to the cycling stabilities for cells using pristine CTs of 2 cycles (Figure 3b and Figure S8a in SI) and MoS2@CTs of 35 cycles (Figure 3c and Figure S8b in SI), the cell using Def-MoS2@CTs shows a much better cyclability of 100 cycles (Figure 3d and Figure S8c in SI). The cell with Def-MoS2@ CTs displays an overpotential of 0.68 V in the first cycle (Figure 3d), much lower than that of a cell with pristine CTs (1.60 V, Figure 3b) and MoS2@CTs (0.89 V, Figure 3c). Moreover, the cells with Def-MoS2@CTs exhibit good rate performances, which can sustain 78 and 39 cycles at current densities of 0.2 and 0.4 mA cm−2 with a capacity limit of 0.5 mAh cm−2, respectively (Figure S9 in SI). For the cell with a reference electrode made of Def-MoS2 powder and binder (Figure S10 in SI), it only sustains 52 cycles with an overpotential of 0.87 V on the first cycle, which shows a relatively poor battery performance than a cell using Def-

Figure 2. (a, c) SEM images of MoS2@CTs, (b, d) SEM images of Def-MoS2@CTs, TEM images of (e) MoS2 and (f) Def-MoS2, and HRTEM images of (g) MoS2 and (h) Def-MoS2.

the MoS2 and Def-MoS2 particles are flower-like microspheres of 1.3 μm in diameter. The microspheres consist of nanosheets interconnected with each other, forming a hierarchical porous microstructure. The N2 adsorption/desorption test shows the pore sizes of the microstructure in the range 8−240 nm (Figure S5 in SI). The EDS elemental mapping images of DefMoS2@CTs show that the Mo and S elements are uniformly distributed on the particles (Figure S6 in SI). Considering the difficulty to acquire TEM information for self-standing electrodes, MoS2 and Def-MoS2 powders were thus prepared. Figure 2e,f shows that MoS2 and Def-MoS2 are composed of nanosheets, which is very consistent with the SEM results. Note that MoS2 and Def-MoS2 particles display similar morphology and size, indicating that the introduction of sulfur vacancies does not affect the morphology and size. In Figure 2g,h, the HRTEM images for MoS2 and Def-MoS2 exhibit the lattice fringes with a distance of 6.16 Å, conforming D

DOI: 10.1021/acs.inorgchem.8b03300 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. SEM images of (a) MoS2@CTs and (b) Def-MoS2@CTs after discharge. SEM images of (c) MoS2@CTs and (d) Def-MoS2@CTs after recharge. The broken circles in part c show regions where residues are observed. (e) Raman spectra, (f) XRD patterns, and (g, h) FTIR spectra of MoS2@CTs and Def-MoS2@CTs at different electrochemical states at a constant current density of 0.1 mA cm−2 with a limit capacity of 1 mAh cm−2.

4g,h, respectively. In Figure 4g, it is clear that the Li2CO3 formed on recharge of MoS2@CTs, further confirming that the residual product is Li2CO3. Additionally, the XRD and FTIR results indicate that the Li2O2 forms during discharge and then decomposes during recharge for Def-MoS2@CTs (Figure 4f,h), further indicating a superior reversibility of DefMoS2@CTs, which is consistent with the Raman results. Figure 5 shows the Nyquist plots of the cells with MoS2@ CTs and Def-MoS2@CTs before and after cycling under the

MoS2@CTs, confirming the advantages of the binder-free DefMoS2@CTs electrode. To explore the reversibilities of as-prepared MoS2@CTs and Def-MoS2@CTs cathodes, the morphologies for the cathode at the different electrochemical states at a constant current densities of 0.1 mA cm−2 within the limit capacity of 1.0 mA h cm−2 were studied by SEM. For MoS2@CTs, toroidal-like products with a particle size of 200 nm are deposited on the MoS2@CTs surface (Figure 4a). In contrast, the discharge products are uniformly deposited with a film-like morphology on the whole surface of Def-MoS2@CTs (Figure 4b). After recharge, most toroidal-like particles disappear for MoS2@ CTs, but still some residues remain (Figure 4c). However, for Def-MoS2@CTs, the film-like products are totally decomposed after recharge (Figure 4d), indicating a good reversibility of Def-MoS2@CTs. To identify the deposit natures, Raman spectra, XRD, and FTIR were recorded for the different cathodes after discharge and recharge. In Figure 4e, after discharge, the peaks of Li2O2 at 788 cm−1 are clearly observed for both MoS2@CTs and DefMoS2@CTs, proving that the toroidal-like and film-like discharge products are Li2O2.37 After the subsequent recharge, the Li2O2 peak vanishes and no new peak generates for DefMoS2@CTs, confirming a high reversibility of Def-MoS2@CTs accompanied by the formation and decomposition of Li2O2. For MoS2@CTs, the Li2CO3 peak at 1085 cm−1 is observed after recharge, indicating that the residual product on the MoS2@CTs surface is Li2CO3 (Figure 4c),38,39 which might be caused by the electrolyte decomposition under a high charge voltage.38 However, no obvious peak corresponding to Li2CO3 is observed in Figure 4f, which could be attributed to an unavailable amount detected by XRD. The FTIR spectra data were collected for MoS2@CTs and Def-MoS2@CTs in Figure

Figure 5. Nyquist impedance plots of (a) MoS2@CTs and (b) DefMoS2@CTs at the pristine, discharged, and recharged states at a constant current density of 0.1 mA cm−2 with a limit capacity of 1.0 mA h cm−2.

same cycling conditions. The corresponding impedances are fitted by the equivalent circuit (Figure S11 in SI), as shown in Table S2 in the SI. The EIS spectra before cycling are the same for the two cells and have the classical shape of a semicircle followed by a diffusive part. The resistance of the cells is almost the same (∼30 Ω) for pristine MoS2@CTs and Def-MoS2@ CTs. After discharge, the impedance increases significantly for both of the cells, resulting from the deposition of the insulating Li2O2 on the cathode surface.40 Note that the resistance in the E

DOI: 10.1021/acs.inorgchem.8b03300 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry MoS2@CTs cell (∼116 Ω) is larger than in the Def-MoS2@ CTs cell (∼100 Ω), despite the fact that the discharge capacity for both cells is the same, indicating that nearly equal Li2O2 has been generated. This result thus shows that the resistance of the deposit depends not on the amount of Li2O2 but also on its morphology studied in the previous section. After recharge, the Li2O2 decomposition reduces the impedance again for both cathodes. However, for the MoS2@CTs cell, the impedance after cycling remains at ∼44 Ω which is significantly larger than that of the pristine cathode. In addition, the semicircle has been replaced by a flat arc, indicating that products remain irreversibly on the cathode surface (Figure 5a). This could be in agreement with the SEM and Raman analyses showing the existence of Li2CO3 residue. On the contrary, the impedance for Def-MoS2@CTs after recharge almost reduces to that of the initial state. Moreover, the initial semicircle is almost recovered (Figure 5b), confirming that Li2O2 is completely decomposed on the Def-MoS2@CTs. These results confirm that Def-MoS2@CTs displays a better reversibility than MoS2@CTs during cell cycling, giving evidence of the higher catalytic properties of Def-MoS2@CTs induced by sulfur vacancies. DFT Analysis. As shown in this work, the morphology and distribution of Li2O2 are greatly influenced by the introduction of the sulfur vacancies in MoS2. Some reports have disclosed that the O2 adsorption ability of the electrocatalyst plays a crucial role in Li 2 O 2 formation. 41−43 We used DFT computation to investigate the adsorption mechanism between the slabs of MoS2 or Def-MoS2 and O2. We constructed the perfect models of MoS2 and Def-MoS2 on the top and side views, as seen in Figure 6a,b. The most stable configurations of

indicating that the Def-MoS2 surface has the stronger intrinsic O2 adsorption ability as compared to that of the MoS2 surface. In addition, there have been previous reports that sulfur vacancies on MoS2 prefer to form rows.44 Therefore, we constructed the models of Def-MoS2 which contained two sulfur vacancies that are next to each other (Figure S12a in SI) and further analyzed the O2 adsorption energy of Def-MoS2 with two adjacent sulfur vacancies. The results show that the adsorption energy of two O2 molecules on the two sulfur vacancies was simply twice the adsorption energy on one sulfur vacancy, indicating that the adsorption energies increase linearly with the number of O2 molecules absorbed. Therefore, the computations on a single vacancy are suffcient to model the strong O2 adsorption ability of Def-MoS2. Mechanism Proposal. Typically, for a cathode with electrocatalyst, the formation pathways of Li2O2 can be described as the following steps:41,45 O2 + S → O2 *

(1)

Li+ + O2 * + e− → LiO2 *

(2)

Li+ + e− + LiO2 * → Li 2O2 *

(3)

2LiO2 → Li 2O2 + O2

(4)

2LiO2 * → Li 2O2 * + O2 *

(5)

The O2 is adsorbed on the active sites (S) at the beginning of ORR (step 1). Then, the O2* traps an electron and combines with Li+ to form LiO2* (step 2). The LiO2 can convert to Li2O2 by an electrochemical reaction (step 3) and/ or a disproportionation reaction in a solution phase (step 4) or a surface phase (step 5). Therefore, the concentration of active sites and the O2 adsorption ability play the key roles in the Li2O2 formation.41,42,46 For MoS2@CTs, the limited active sites only can provide a part of the nucleation sites of Li2O2 by the electrochemical reaction (step 3). Most Li2O2 growth occurs in the electrolyte by step 4 due to the weak O2 adsorption ability of MoS2. Therefore, the toroid-like Li2O2 is deposited according to a homogeneous nucleation and then the dynamic growth, which has been investigated elsewhere.47 In contrast, Def-MoS2@ CTs with a strong O2 adsorption ability possesses a large concentration of active sites, which leads to the Li2O2 being uniformly deposited on the active sites by step 3 and step 5. Therefore, the film-like Li2O2 forms by inhomogeneous nucleation. In comparison with the isolated toroid-like Li2O2, the film-like Li2O2 with good contact on the Def-MoS2 nanosheets can maximize the utilization of active sites in nanosheets and improve the electron transfer, resulting in the enhancement of reaction kinetics. Therefore, the strong O2 adsorption ability and the large concentration of sulfur vacancies of Def-MoS2 are believed to be the factors for the improved cell performance.

Figure 6. Top and side views of the ground state structure of (a) MoS2 and (b) Def-MoS2, and the most stable configurations of O2 adsorbed on (c) MoS2 and (d) Def-MoS2.

O2 adsorbed on MoS2 and Def-MoS2 are depicted in Figure 6c,d. For the MoS2 surface, the O2 locates on the top of the S sites. The distance between S and O is 3.38 Å. The O−O bond length of 1.23 Å is similar to that for isolated O2 of 1.21 Å. For the Def-MoS2 surface, one O atom of the O2 coordinates with three Mo atoms with the Mo−O bond length of 2.17 Å, while the length of the O−O bond is elongated to 1.38 Å. To analyze the affinity between O2 and the as-prepared sample, the adsorption energy Ea is defined as

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CONCLUSION The geometrical morphology and distributed growth of Li2O2 can be largely influenced by the electrocatalyst for Li−O2 batteries. In this work, we successfully synthesized the CTs decorated by the flower-like MoS2 microspheres with two different concentrations of sulfur vacancies, MoS2@CTs (S/ Mo = 1.91) and Def-MoS2@CTs (S/Mo = 1.61), which were directly used as self-standing cathodes for Li−O2 cells. The experimental and theoretical results indicate that a larger

Ea = E(O2 /slab) − E(slab) − E(O2 )

where E(O2/slab) is the total energy of the O2 adsorbed configuration, E(slab) is the energy of the isolated slab (MoS2 or Def-MoS2), and E(O2) is the energy of the isolated O2. The adsorption energies for the most stable configurations of O2 on the MoS2 and Def-MoS2 are −0.14 and −2.25 eV, respectively, F

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concentration of sulfur vacancies exhibits a stronger adsorbability to O2, leading to the changes of the morphology and nucleation mechanism of Li2O2. Furthermore, a fundamental catalytic mechanism for this adsorption process is also proposed. These results represent a new strategy for the development of a highly active electrocatalyst by introducing a large concentration of defects for some O2-participating reactions (i.e., ORR and OER), which can be applied for broad fields (e.g., fuel cells, metal−O2 batteries, and water electrolyzers).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03300.

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Additional characterization details including SEM images, isotherms, and electrical performance information (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mial: [email protected]. *E-mial: [email protected]. ORCID

Guochun Yang: 0000-0003-3083-472X Haiming Xie: 0000-0002-7653-4071 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the following entities: Special Fund of Key Technology Research and Development Projects (20180201097GX, 20180201099GX, 20180201096GX), Jilin Province Science and Technology Department; Key Subject Construction of Physical Chemistry of Northeast Normal University; The R&D Program of Power Batteries with Low Temperature and High Energy, Science and Technology Bureau of Changchun (19SS013); National Key R&D Program of China (2016YFB0100500); General Financial Grant from the China Postdoctoral Science Foundation (Grant 2016M601363); Fundamental Research Funds for the Central Universities (Grant 2412017QD011); Jilin Scientific and Technological Development Program (Grant 20180520143JH); and National Natural Science Foundation of China (Grant 201805030).

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