Conductive Polymer-Coated VS4 Submicrospheres As Advanced

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Conductive polymer coated VS4 sub-microspheres as advanced electrode materials in lithium-ion batteries Yanli Zhou, Yanlu Li, Jing Yang, Jian Tian, Huayun Xu, Jian Yang, and Weiliu Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04444 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016

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ACS Applied Materials & Interfaces

Conductive Polymer Coated VS4 Sub-Microspheres as Advanced Electrode Materials in Lithium-Ion Batteries Yanli Zhou,† Yanlu Li,‡ Jing Yang,‡ Jian Tian,§ Huayun Xu,† Jian Yang,*,† and Weiliu Fan*,† †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education School of Chemistry

and Chemical Engineering, Shandong University, Jinan 250100, PR China ‡

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China School of Materials Science and Engineering, Shandong University of Science and Technology,

§

Qingdao 266590, PR China1

KEYWORDS: solvothermal, sulfides, nanomaterials, conductive polymer, lithium ion batteries (LIBs)

*

Corresponding Authors

Email: [email protected] (J. Yang), and [email protected] (W. Fan)

ACS Paragon Plus Environment

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ABSTRACT: VS4 as an electrode material in lithium-ion batteries, holds intriguing features like high content of sulfur and one-dimensional structure, inspiring the exploration in this field. Herein, VS4 sub-microspheres have been synthesized via a simple solvothermal reaction. But they quickly degrade upon cycling as an anode material in lithium-ion batteries. So, three conductive polymers, polythiophene (PEDOT), polypyrrole (PPY) and polyaniline (PANI), are coated on the surface to improve the electron conductivity, suppress the diffusion of polysulfides and modify the interface between electrode/electrolyte. PANI is the best in the polymers. It improves the coulombic efficiency to 86 % for the first cycle and keeps the specific capacity at 755 mAh g-1 after 50 cycles, higher than the cases of naked VS4 (100 mAh g-1), VS4@PEDOT (318 mAh g-1) and VS4@PPY (448 mAh g-1). The good performances could be attributed to the improved charge-transfer kinetics and the strong interaction between PANI and VS4 supported by theoretical simulation. The discharge voltage about ~2.0 V makes them promising cathode materials.


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1. INTRODUCTION Lithium ion batteries (LIBs) as one of high-efficient energy storage devices, are applied in a variety of applications, from portable electronics to electric transportation.1,2 The applications stimulate the development of LIBs, particularly for advanced electrode materials.3 The commercial electrode materials like graphite and LiCoO2, are limited by their low capacities. So, many efforts have been devoted to seek after the alternatives.4-11 Until recently, transitional metal sulfides come into sight.12 The moderate bonding between M/S could reduce the electrode polarization,13 improve the energy efficiency. The relatively good electron conductivity of transitional metal sulfides to the oxide counterparts,14,15 could benefit the charge transportation, promote the rate capability. Finally, transitional metal sulfides have a myriad of distinctive crystal structures, which might offer unexpected opportunities for LIBs. More interestingly, some of sulfides also exhibit the electrochemical characteristics of Li-S batteries.16-19 For example, MoS2 in a typical two-dimensional structure, were converted to metallic Mo and Li2S after the first discharge. Afterwards, the following electrochemical reactions happened between Li2S and S.20 Metallic Mo seldom participates these electrochemical reactions, but it could absorb the polysulfides via the strong Mo-S bonding. So, it inhibited the diffusion of the polysulfides into the electrolytes. Meanwhile, metallic Mo would increase the electron conductivity of the electrode.16 These results indicate that the electrochemical behaviors of transitional metal sulfides maybe complicate, which has been recently exemplified by VS4. At first, Shin and Cho proposed that VS4 as an anode material was cycled via Li-S mechanism, based on ex-situ XRD patterns, TEM images and EDS mapping on the fully discharged/charged


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electrodes.21 Furthermore, they thought that VS4 could not be prepared without a graphitic layer.22 But Grey et. al. have revisited this material again very recently, using a series of powerful tools to characterize the short-range structures of discharged/charged electrodes.23 They thought that the reaction from VS4 to Li2S + V was partially reversible, most likely was stopped at a metastable phase Li3+xVS4. VS4 is attracting attention in this family, due to its high content of sulfur, one-dimensional crystal structure and unique electrochemical behavior.24,25 The high content of sulfur might increase the theoretical capacity, as sulfur is involved in the electrochemical reaction. The chainlike crystal structure likely facilitates the charge-transfer kinetics, because the interaction between neighbouring chains is quite weak. The high discharge voltage at ~2.0 V makes it promising cathode materials.21,22 But the reports about electrochemical performances of VS4 are quite limited.21,22,26 Herein, VS4 sub-microspheres are synthesized by a simple solvothermal reaction at a low temperature in the absence of graphitic layers. When these sub-microspheres are examined as the electrode material, they present the poor cycling stability and rate capability. Thus, three conductive polymers, PEDOT, PPY and PANI, are deposited on the surface of the VS 4 submicrospheres to improve the electron conductivity, suppress the dissolution of polysulfides and modify the interface between electrode/electrolyte. To our knowledge, such a polymer coating on VS4 has not been reported before. All the composites exhibit enhanced electrochemical performances, compared with the naked VS4 sub-microspheres. The coulombic efficiency for the first cycle could be as high as ~ 86 % for PANI-coated VS4 sub-microspheres. They also exhibit the best cycling stability and rate capability in the polymer-coated VS4. The insights about why


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PANI is the best polymer for the surface coating, are disclosed on the basis of the interaction between the conductive polymers and VS4 via theoretical simulations. 2. EXPERIMENTAL SECTION 2.1. Reagents Ammonium metavanadate (NH4VO3, 99 %), thioacetamide (TAA, 99 %), pyrrole (99 %), aniline (99.5 %), thiophene (EDOT, 99 %), ammonium persulfate (98 %), sodium dodecyl benzene sulfonate (SDBS, ≥98%), polyvinylpyrrolidone (PVPK30, 99.8%), camphorsulfonic acid (99 %), ethylene glycol (EG, 99 %), hydrochloric acid (HCl, 36.5%), and absolute alcohol (99

%)

were

purchased

from

Sinopharm

Chemical

Reagent

Co.,

Ltd.,

China.

Carboxymethylcellulose sodium (CMC, 99%) was produced by Aladdin Reagent Co., Ltd., China. Deionized (DI) water was produced from a water purification system (ULUPURE, China). 2.2. Chemical Synthesis 2.2.1. Synthesis of Hierarchical VS4 Sub-Microspheres All the chemicals were analytical grade and used without further purification. In a typical protocol, 3 mmol of ammonium metavanadate (NH4VO3) was added into 25 mL of deionized water. Then the suspension was heated to 60 oC to form a clear pale-yellow solution. After 25 mL of 0.6 M thioacetamide (TAA) in ethylene glycol (EG) was introduced, this solution was transferred into a Teflon-lined stainless steel autoclave with a capacity of 65 mL. The autoclave was sealed and maintained at 160 oC for 24 h. The product was rinsed by deionized water and absolute alcohol for several times, and dried in a vacuum oven at 60 oC for 10 h.


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2.2.2. Synthesis of Polymer-Coated VS4 Sub-Microspheres 1 g of SDBS and 0.1 mg of VS4 sub-microspheres were mixed in 50 mL of deionized water, which was sonicated for 30 min. Then, different amounts of pyrrole (50, 125 or 250 μL) were added to the suspension. After the suspension was stirred for 1 h in an ice-water bath, 5 mL of 0.1 M ammonium persulfate was slowly dripped into the above suspension. This suspension was then stirred for another 12 h in an ice-water bath to finish the polymerization reaction, generating PPY-coated VS4 sub-microspheres. The product was rinsed with deionized water and absolute alcohol for several times, and dried in a vacuum oven at 60 oC for 10 h for the later characterization. As to PANI-coated VS4 sub-microspheres, the similar procedure was carried out, except aniline was used as the monomer and the pH of the solution was controlled to ~ 2 by HCl. For PEDOT-coated VS4 sub-microspheres, EDOT was used as the monomer and the pH was adjusted to 3 to 4 by camphorsulfonic acid, in which PVP were the surfactant. 2.3. Sample Characterization Powder X-ray diffraction (XRD) patterns were collected by the use of graphitemonochromatized Cu Kα radiation (λ = 1.5418 Å) on a Bruker D8 advanced X-ray diffractometer. Transmission electron microscope (TEM) images, high-resolution transmission electron microscope (HRTEM) images, and selected area electron diffraction (SAED) patterns were recorded on a high-resolution transmission electron microscope (JEOL-2100, Japan) at an acceleration voltage of 200 kV. Scanning electron microscope (SEM) images, energy-dispersive


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X-ray spectra (EDS) and elemental mapping were acquired from a field-emission scanning electron microscope (Nova 450, USA). Raman spectra were acquired on a MicroRaman spectrometer using a laser of 515 nm as an excitation (OLYM-PUS FV500, Japan). Nitrogen sorption isotherm was achieved at 77.3 K on a Micromeritics ASAP2020HD88 gas sorptometer (Micromeritics, USA). Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA851 thermal analyzer in air at a heating rate of 10 oC min-1. Infrared spectra were performed on Fourier-transformed infrared Spectrometer (FT-IR, Tensor 27, Bruker). 2.4. Electrochemical Measurements Electrochemical performances of these sub-microspheres as anode materials of LIBs, were evaluated by CR2032 coin cells. The working electrode was made of active material, acetylene black, and carboxyl methyl cellulose in a weight ratio of 7:2:1. These materials were milled in the presence of droplets of water and the obtained slurry was pasted on a clean copper foil. After dried in vacuum at 60 oC for 10 h, the coated foil was roll-pressed and punched into discs. The mass loading of the active material was about 1.0-1.5 mg cm-2. Then, the disc as the working electrode was assembled with a lithium foil as the counter electrode, a Celgard 2400 microporous polypropylene membrane as the separator, and 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v) as the electrolyte in an argon-filled glove box. The galvanostatic discharge and charge tests of the cells were performed in a range of 0.01-3 V on battery cyclers (Land CT2001A, China). Electrochemical impedance spectra (EIS) were measured on an electrochemical workstation (AUTOLAB PGSTAT302N, Switzerland) over a frequency range of 100 kHz to 0.01 Hz with an amplitude of 10 mV. Cyclic voltammograms (CV) were obtained from an electrochemical workstation (LK 2005A, China) over 0.01-3 V at 0.1 mV s-1. All the electrochemical tests were carried out at 25 oC.


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2.5. Computation Details First-principle total energy calculations were performed within the framework of density functional theory (DFT), as implemented in the program CASTEP.27 The electron-ion interaction was described by Vanderbilt-type ultrasoft pseudopotential,28 and the exchange-correlation terms were treated with generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) form.29 The one-electron valence state was expanded on a basis of plane wave with a cutoff energy of 300 eV, which makes the tolerance for the Hellmann-Feynman forces about 0.05 eV/Å. Brillouin- zone integration has been performed over Monkhorst-Park meshes of dense 1 x 1 x 1 k-point for all the atomic force relaxation, the total energy and electronic properties calculations. The vacuum between a polymer and its image as well as the distance between VS 4 cluster and its image all exceeds 20 Å along the periodic directions to avoid any artificial interaction caused by the periodic boundary condition. The binding energy Ebind here is defined as the energy difference between VS4 cluster + polymer and polymer adsorbed VS4, shown as following: Ebind=Epolymer+EVS4-Epolymer/VS4 A larger binding energy indicates strong binding ability of VS4 and polymer. 3. RESULT AND DISCUSSION Polymer-coated VS4 sub-microspheres were prepared via a wet-chemical process, as shown in Figure 1. First, NH4VO3 was mixed with TAA at room temperature in a mixed solvent of water


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and ethylene glycol. Then, the solution went through a typical solvothermal reaction, producing hierarchical VS4 sub-microspheres. After a simple purification, these particles were redispersed in an aqueous solution by SDBS or PVP. The polymerization reaction was conducted in an icewater

Figure 1. Synthesis of polymer-coated VS4 sub-microspheres. bath to guarantee the uniform coating of the newly-formed polymer on the surface of VS4 submicrospheres. Figure 2a shows the XRD pattern of VS4 sub-microspheres. All the diffraction peaks could be indexed as monoclinic-phase VS4 in a space group of I2/a (JCPDS Card No. 211434), where V atoms are sandwiched by S rectangles in a chain structure along the monoclinic c axis (Figure 2b). No peaks from impurities are observed, implying the high purity of VS4. Raman spectrum, as an effective and sensitive tool to the structure, is shown in Figure S1, which is the identifical to the previous reports on VS4.24,25 The peaks at 190 and 280 cm-1 are ascribed to the stretching (A1) and bending (B1) modes of V-S, respectively.24 The origins of the other peaks are still to be explored.


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Figure 2. (a) XRD pattern and (b) crystal structure of VS4, red atoms: V; yellow atoms: S.

Figure 3. (a) and (b) SEM images, (c) and (d) TEM images, (e) HRTEM image and (f) SAED pattern of VS4 sub-microspheres.


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Figure 3 displays SEM and TEM images of VS4 sub-microspheres. As presented in Figure 3a, all the particles exhibit a spherical shape and a relatively narrow size distribution from 500 to 800 nm. The magnified image on the surface of these sub-microspheres (Figure 3b), discloses a trunk-like feature. This preferential growth could be associated with the highly anisotropic structure of monoclinic VS4, the unique chain-like structure. EDX spectrum and ICP-AES data of the sub-microspheres (Figure S2 and Table S1) gives a molar ratio of S: V at 4: 1, close to the stoichiometric ratio of VS4. Besides S and V, carbon and oxygen are also observed, which might come from the adhesive tape or the surface oxidation. Elemental mapping of S and V (Figure S3) indicates that they are uniformly distributed in the sub-microspheres, also consistent with VS4. Figure 3c and 3d show the TEM images of these sub-microspheres, where they are well dispersed on the copper grids. This result suggests their good dispersity. HRTEM image at the edge of a particle (Figure 3e), shows clear lattice fringes, corresponding to (011) planes of monoclinic-phase VS4. The SAED pattern consists of diffused diffraction rings, suggesting that the trunk-like units are randomly oriented inside the sub-microspheres. The specific surface area is achieved from nitrogen sorption isotherms (Figure S4), ~ 5.8 m2 g-1 on the basis of BrunauerEmmett and Teller (BET) theory. VS4 sub-microspheres were explored as an electrode material for LIBs. Figure 4a shows the typical CV curves of the VS4 sub-microspheres in the voltage range of 0.01-3.0 V at a scan rate of 0.1 mV s-1 for the first five cycles. In the first cathodic sweep, the two peaks at 1.83 and 1.47 V could be attributed to the lithium insertion into VS4 then the formation of Li3+xVS4.23 The small peak at 0.67 V is likely to come from the formation of a solid electrolyte interphase (SEI) film and the complete reduction to metallic V and Li2S.23,30 The first anodic sweep exhibits the two peaks at 1.78 and 2.4 V, corresponding to the deliathion of Li2S and the formation of


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amorphous VS4 and Li3+x’VS4.23 Here, Li3+x’VS4 is slightly different from Li3+xVS4 in terms of Li contents and oxidation state of V. From the second cycle, the cathodic peak at 1.83 V shifts to the positive, while that at 1.47 V moves to the negative. Moreover, the peak at 1.47 V gradually decays upon cycling and disappears after the 4th sweep. This degradation indicates the poorly electrochemical stability of intermediate, consistent with the continuous decreasing of the reversible capacity. The anodic peaks basically keep their positions, but the intensities decrease as compared to those at the first sweep, also indicating the degradation of the electrochemical performance. This phenomenon becomes more apparent in the following cycles, which is also visible in the galvanostatic discharge-charge profiles. As shown in Figure 4b, the first discharge/charge capacities are 1652 and 1306 mAh g-1, corresponding to a coulombic efficiency of 79%. The irreversible capacity probably originates from the inevitable decomposition of the electrolyte, the formation of a SEI film, the trapping of lithium inside the active material, and so on.31,32 Figure 4c shows the cycling performance of the VS4 sub-microspheres at a current density of 100 mA g-1. Its reversible capacity quickly drops to 100 mAh g-1 after 50 cycles, reflecting its poor electrochemical performances. The same conclusion could be drawn from its rate performances (Figure 4d). At a low current density like 200 mA g-1, there is only a capacity of ~ 200 mAh g-1.


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Figure 4. (a) CV curves, (b) galvanostatic discharge/charge profiles, (c) cycling performance and coulombic efficiency, and (d) rate performance of VS4 sub-microspheres. In order to improve the electrochemical properties, carbon coating on the surface is always the first choice. However, either the hydrothermal carbonization by organic molecules, or the decomposition of C2H2 at a high temperature for carbon coating led to the decomposition of VS 4 or the appearance of impurities (Figure S5 and S6). So, three conductive polymers, PPY, PANI, and PEDOT, are coated on the surface of VS4 sub-microspheres by the polymerization at ambient conditions. The successful deposition of PPY, PANI, and PEDOT on VS4 submicrospheres are clearly observed in the SEM, TEM images and FTIR spectra. As shown in Figure 5a-5c, all the products keep their spherical shape, no matter what kind of polymers are involved. Meanwhile, their surface becomes smooth after coating, confirming the polymer coating. This result is also supported by TEM images (Figure 5d-5f), because a low-contrast but uniform layer caused by the formation of polymers could be easily visualized on the surface.


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More importantly, the average thickness of different polymers is quite close, if their monomer amounts are controlled as the same. This result is also independently concluded by TG analysis (Figure 6a). The different polymer coating in a similar thickness, offers a solid basis for us to evaluate the effect of different polymers on electrochemical properties of VS4 sub-microspheres.

Figure 5. SEM images (a-c) and TEM images (d-f) of polymer-coated VS4 sub-microspheres. (a,d) PEDOT, (b,e) PPY, (c,f) PANI. Compared to SEM and TEM images, FTIR spectra give direct evidences on the polymer formation. As targeted in Figure 6b, the bands at ~549 and 994 cm-1 arise from the doubly bonded/bridged S2- (V-S-V) and terminal S stretching of VS4.23 These bands exist in all the FTIR spectra of the polymer-coated sub-microspheres, indicating the stability of VS4 through the polymerization. Besides the signals of VS4, the characteristic bands of different polymers are also observed after the polymerization. VS4@PEDOT presents the C–O–R and C–S vibrations of PEDOT at ~1194 and 932 cm-1.33-35 VS4@PPY shows the C=C and C-N stretching vibrations of PPY at 1550 and 1194 cm-1.36-39 VS4@PANI exhibits benzenoid- and quinoid-ring vibrations of PANI at 1471 and 1565 cm-1. The bands between 1200 and 1400 cm-1 belong to the C–N


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stretching modes of an aromatic amine.40 These results confirm the successful formation of PPY, PEDOT, or PANI on the surface of VS4.

Figure 6. (a) TG curves and (b) FTIR spectra of VS4, VS4@PEDOT, VS4@PPY and VS4@PANI. The electrochemical properties of polymer-coated VS4 are summarized in Figure 7. The discharge/charge profiles of all the polymer-coated VS4 at the first cycle, show a flat discharge plateau at 1.9 V and a reversible charge plateau at ~ 2.3 V, similar to the case of VS 4 only. But these capacities represented by this plateau are much higher than those in VS 4, indicating more lithium ions are inserted via this reaction. The corresponding CV curves of polymer-coated VS4 are shown in Figure S7, it can be found that these curves are similar to those of VS4, but the stabilities are improved, particularly for the cases of VS4@PANI and VS4@PPY. This result is also in good agreement with that from discharge-charge profiles. The potential difference between discharge/charge plateaus is ~ 0.4 V, much smaller than many of conversion-type transitional metal oxides and sulphides.12,41-44 The coulombic efficiencies of the polymer-coated VS4 at the first cycle are 82 % for PPY, 86 % for PEDOT, and 86% for PANI. All these data are higher than the case of VS4, suggesting less irreversible reactions during the first cycle. Such a


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high coulombic efficiency for the first cycle is much higher than the previous reports on VS4, even if it was coupled with reduced graphene oxide (rGO).22 The cycling stability of the polymer-coated VS4 is tested at a current density of 100 mA g-1. Although all the discharge capacities of the polymer-coated VS4 degrade upon cycling (Figure 7b), that of VS4@PANI could be still kept at ~ 755 mAh g-1 after 50 cycles, higher than the cases of VS4 (100 mAh g-1), VS4@PEDOT (318 mAh g-1) and VS4@PPY (448 mAh g-1). The capacity degradation might be related to the dissolution of sulfur-containing species into the electrolyte, which is supported by the increase of sulfur in the electrolyte during the repeated lithiation/deliathion processes (Figure S8). The similar results are also achieved from the rate capability of the polymer-coated VS4 (Figure 7c), where VS4@PANI exhibits a reversible capacity of 400 mAh g-1 at 2 A g-1. When the current density returns to 0.1 A g-1, the reversible capacity could recover to 680 mAh g-1. All of them are better than those of VS4 sub-microspheres without surface coating. But PANI is the best polymer for the surface coating of VS4. This result is totally different from what reported for conductive polymer-coated hollow sulfur particles,45 where the electrochemical performances of different polymers was in the order of PEDOT > PPY > PANI. This result was correlated with the chemical bonding between the polymer heteroatoms and LixS (0< x ≤ 2), using Ab inito simulations in the framework of density functional theory. In our case, these conductive polymers are coated on VS4 or Li3+x’VS4 formed after cycles, where V


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Figure 7. (a) Galvanostatic discharge/charge profiles VS4@PEDOT, VS4@PPY and VS4@PANI, (b) cycling performances, (c) rate performances of above three kinds of polymer coated composites and VS4 sub-microspheres, and (d) Nyquist plots of the electrodes built on VS4@PEDOT, VS4@PPY and VS4@PANI. The discharge/charge profiles and cycling performances were obtained at 0.1 A g-1. atoms are entrapped by multiple sulfur atoms. The interaction between them and conductive polymers is also disclosed by density functional theory. To simplify the calculation, the repeating unit of each polymer is chosen as the model molecule to estimate the interaction between the polymers and VS4. Meanwhile, VS4 clusters, rather than crystalline VS4, are used, because they approach the status of VS4 after cycles.23 Figure 8 shows most stable configurations, electron density differences and binding energies of VS4 clusters to PEDOT, PANI and PPY. For PANI


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and PPY, VS4 clusters interact with nitrogen atoms in the polymers via the hydrogen bond of S∙∙∙H−N, resulting in the high binding energies of 4.11 eV for PANI/VS 4 and 3.59 eV for PPY/VS4. On the contrary, the interaction between VS4 clusters and PEDOT is quite weak (~ 0.67 eV), due to the absence of the hydrogen bond. This order is in good agreement with the cycling stability of polymer-coated VS4. The strong binding affinity of PANI with VS4 can effectively reduce the dissolution of sulfur into the electrolyte, thereby improving cycling stability compared to PPY and PEDOT.

Figure 8. (a) Chemical structures of PANI, PPY and PEDOT, m and n show the doped and undoped parts in these polymers, and X indicates the counterion incorporated during polymerization. (b) The most stable configurations and (c) corresponding electron density difference of VS4 interacted with PANI, PPY and PEDOT calculated by first-principles. Gray, blue, yellow, red and white balls represent C, N, S, O, and H atoms. Yellow and blue regions


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represent electron depletion and accumulation, respectively. The distance between S and H as well as the angle of S∙∙∙H−N/C are indicated in the configurations. The binding energies of VS4 with polymers are labeled in the brackets. The Nyquist plots of the composites are measured to gain more insights about the understanding on their performances. Before the measurements, all the composites are charged to 3 V after 50 cycles at 100 mA g-1. As shown in Figure 7d, our Nyquist plots are composed of an incomplete semicircle and a diagonal line, which is well fitted by the proposed equivalent circuit. Rs and Rf are the electrolyte resistance, and the resistance related to the surface passivation layers. Rct reflects the charge-transfer resistance from electrolyte to active materials. The Li-ion diffusion inside the active materials is delegated by Warburg resistance (Zw). On the basis of this fitting, VS4 @PANI shows the smallest Rct and Rf (Table S2), indicating its best charge-transfer kinetics. This result is consistent with its electrochemical performances. In order to optimize the electrochemical performance of VS4@PANI, the different shell thicknesses of PANI are examined. Because the shell thickness of PANI could be tuned by controlling the monomer volume in the synthesis (Figure 9a-9f), the composite in different shell thicknesses is denoted as VS4@PANI-X, where X indicates the monomer volume. It is found that VS4@PANI-125 shows the best results during the cycling test (Figure 9g). A thin shell of PANI would weaken its suppression on the dissolution of sulfur. So, VS4@PANI-50 quickly falls to 190 mAh g-1 after 50 cycles. The thick shell of PANI is much better in this aspect. But its capacity is very limited (VS4@PANI-250), because the thick shell of PANI increases the “dead weight” in the electrode.


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Figure 9. SEM and TEM images of (a) and (d) VS4@PANI-50, (b) and (e) VS4@PANI-125, and (c) and (f) VS4@PANI-250 composites, (g) cycling performances of VS4@PANI-50, VS4@PANI-125 and VS4@PANI-250 composites.

4. CONCLUSIONS In summary, VS4 sub-microspheres have been successfully synthesized by a solvothermal reaction at a low temperature. As an electrode material in rechargeable lithium batteries, these naked particles display inferior electrochemical performances, both in cycling stability and in rate capability. So, three conductive polymers, PEDOT, PPY and PANI, are coated on their


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surface to improve their performances. It is found that all of them effectively increase the cycling stability, rate capability and coulombic efficiency particularly at the first cycle. PANI is the best of them. The PANI-coated particles show a coulombic efficiency of 86% for the first cycle, and a reversible capacity of ~ 755 mAh g-1 after 50 cycles at 100 mA g-1. The superior performances of the PANI-coated particles might be related to its strong interaction with VS 4, which is supported by the theoretical simulation.

ASSOCIATED CONTENT Supporting Information Raman spectrum, EDX spectrum, SEM image, EDS elemental mapping, Nitrogen adsorptiondesorption isotherms and pore size distribution plot for VS4, XRD patterns and TEM images for carbon coated VS4, CV curves for three polymer coated VS4, S contents in electrolytes for polymer coated VS4 and naked VS4, and fitting data from EIS spectra for polymer coated VS4. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the 973 Project of China (No. 2011CB935901), National Nature Science Foundations of China (No. 51172076 and 21471090), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (JQ 201205), and Taishan Scholarship in Shandong Province (No. ts201511004). REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. (2) Winter, M.; Besenhard, Spahr, M. E.; Novak, P. Insertion Electrode Materials for Rechargeable Lithium Batteries. Adv. Mater. 1998, 10, 725-763. (3) Deng, D. Li-Ion Batteries: Basics, Progress, and Challenges, Energy Sci. Eng. 2015, 3, 385418. (4) Liang, M. H.; Zhi, L. J. Graphene-Based Electrode Materials for Rechargeable Lithium Batteries. J. Mater. Chem. 2009, 19, 5871-5878. (5) Yao, Y.; McDowell, M. T.; Ryu, I.; Wu, H.; Liu, N.; Hu, L.; Nix, W. D.; Cui, Y. Interconnected Silicon Hollow Nanospheres for Lithium-Ion Battery Anodes with Long Cycle Life. Nano Lett. 2011, 11, 2949-2954. (6) Zhou, Y.; Jiang, X.; Chen, L.; Yue, J.; Xu, H.; Yang, J.; Qian, Y. Novel Mesoporous Silicon Nanorod as An Anode Material for Lithium Ion Batteries. Electrochim. Acta 2014, 127, 252-258. (7) Derrien, G.; Hassoun, J.; Panero, S.; Scrosati, B. Nanostructured Sn-C Composite as an Advanced Anode Material in High-Performance Lithium-Ion Batteries. Adv. Mater. 2007, 19, 2336-2340.


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Conductive Polymer Coated VS4 Sub-Microspheres as Advanced Electrode Materials in Lithium-Ion Batteries Yanli Zhou,† Yanlu Li,‡ Jing Yang,‡ Jian Tian,§ Huayun Xu,† Jian Yang,*,†and Weiliu Fan*,† †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education School of

Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China ‡

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR

China

§

School of Materials Science and Engineering, Shandong University of Science and

Technology, Qingdao 266590, PR China

Graphical abstract


Conductive Polymer-Coated VS4 Submicrospheres As Advanced

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