Nitrogen-Doped Carbon-Encapsulated Antimony Sulfide Nanowires

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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Nitrogen-Doped Carbon-Encapsulated Antimony Sulfide Nanowires Enable High Rate Capability and Cyclic Stability for Sodium-Ion Batteries Yucheng Dong,†,§,⊥ Mingjun Hu,‡ Zhenyu Zhang,§ Juan Antonio Zapien,§ Xin Wang,*,† Jong-Min Lee,*,⊥ and Wenjun Zhang*,§

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International Academy of Optoelectronics at Zhaoqing, South China Normal University, Guangzhou, Guangdong Province 526060, China ‡ School of Materials Science and Engineering, Beihang University, Beijing 100191, China § Center of Super Diamond and Advanced Films, Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong 999077, Hong Kong, China ⊥ School of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore S Supporting Information *

ABSTRACT: Antimony sulfide (Sb2S3) has been employed for materials of the potential anode in sodium-ion batteries (SIBs) because it possesses a high theoretical capacity. However, volume variations coupled with sluggish diffusion kinetics cause rapid capacity degradation and cyclic instability during the sodiation/desodiation process. Here, we introduce a simple strategy to develop nitrogen-doped carbon-encapsulated antimony sulfide nanowire (Sb2S3@N-C) composites for the anode in SIBs. The resulting composites display excellent electrochemical characteristics with remarkable rate capability, ultrahigh capacity, and excellent stability derived from the synergistic effect between a one-dimensional Sb2S3 nanowire and a nitrogen-doped carbon, thus demonstrating the Sb2S3@ N-C composites as a material with potential characteristics for the anode in next-generation storage devices. Electrochemical analysis reveals that pseudocapacitive behavior dominates the overall electrochemical process of the Sb2S3@N-C composites, which is responsible for the fast capacitive charge storage. KEYWORDS: nitrogen-doped carbon, antimony sulfide nanowire, pseudocapacitive behavior, superior cyclic stability, electrochemistry, sodium-ion batteries

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INTRODUCTION Lithium-ion batteries (LIBs) have revolutionized and dominated commercial markets for electrical vehicles, portable electronics, and large-scale storages.1,2 However, insufficient resources of lithium on Earth have hindered practical applications of LIBs. Sodium-ion batteries (SIBs) possess the potential to replace LIBs in the next energy storage system because of sodium’s natural abundance and environmental benignity.3,4 Sodium and lithium possess similar physiochemical properties (e.g., intercalation behavior), but the current electrode materials suitable for LIBs cannot be simply transplanted to SIBs because the radius of the sodium ion (r = 1.02 Å) is much bigger compared to the lithium ion (r = 0.69 Å), which can cause slower sodium-ion diffusion efficiency, larger volume variation, and more severe pulverization of the sodium host active materials.5,6 For instance, the graphite anode successfully used in commercial LIBs is unsuitable as an © XXXX American Chemical Society

anode material for SIBs because its interlayer distance is too narrow to host sodium ions. Therefore, the design of appropriate materials with high rate capability, specific capacity, and highly durable cyclic stability is required to enhance the performance of SIBs.6,7 Various materials have been studied for the anode in SIBs, including carbonaceous materials, alloy materials, and metal oxides/sulfides.8−12 Despite these efforts, suitable anode materials are still in demand of practical applications. Anode materials with alloying reaction processes (e.g., tin and antimony) during cycling processes are promising because of their intrinsic high specific energy capacity per mass compared with those for conventional carbon-based anode materiReceived: December 24, 2018 Accepted: February 26, 2019

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DOI: 10.1021/acsanm.8b02335 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

ACS Applied Nano Materials als.13−15 In particular, chalcogenide materials (e.g., Bi2S3, SnS, SnS2, and Sb2S3) have alloying processes and sequential conversions during the steps of sodiation/desodiation, leading to high specific capacity.12,16−19 Among them, Sb2S3 is a potential candidate for SIBs, providing a high theoretical capacity of 946 mAh g−1 with 12 equiv of sodium per unit in the process.20−24 In addition, it has a small volume change, good gravimetric energy density, and good electrochemical behavior because of the formation of Na2S during its sodiation process, which possesses higher reversibility than its equivalent oxide.25,26 However, the electrochemical performance of Sb2S3 needs to be further improved because of the intrinsic drawbacks of poor electrical conductivity and severe volume variation, which happen during the process of sodiation/ desodiation. Several strategies have been suggested to solve the issues and enhance the electrochemical characteristics of the material by controlling its shape and structure. This is because the nanostructured materials possibly increase the interfacial area and reduce the diffusion paths for ions and electrons.27 Another effective approach is to disperse Sb2S3 into a carbonaceous matrix, which serves as a structural buffer with the dual purpose of reducing the problems of volume change and also improving the conductivity.28 For example, Pan et al. designed Sb2S3-nanoparticle-anchored carbon-nanotube-backbone composites that afforded a capacity of 412.3 mAh g−1 (0.05 A g−1) even after 50 cycles.16 Xu et al. found that Sb2S3 nanoparticles dispersed on reduced graphene provided a capacity of 581.2 mAh g−1 (0.05 A g−1) after 50 cycles.28 It has been proven that carbon materials doped with nitrogen can effectively improve their electronic conductivity and surface wettability and create active sites and defects for enhancement of the electrochemical performance.11,29−32 Therefore, the development of nitrogen-doped carbon-encapsulated nanostructured active materials is an attractive approach in the design of advanced sodium-ion anodes with greatly enhanced electrochemical performance. Dopamine contains abundant catechol and amine functional groups, and its ability to selfpolymerize in basic aqueous solutions makes it an ideal contributor to the construction of a uniform polydopamine (PDA) layer on the electrode surface. This, in turn, can be converted into nitrogen-doped carbon under thermal treatment.33,34 Even though many scientists have tried to improve the performance, their long-term stability and high rate capability are still unsatisfactory because of the loss of metal sulfides from the carbonaceous matrix by volume variation during the process. Herein, nitrogen-doped carbon-encapsulated Sb2S3 nanowire (Sb2S3@N-C) composites are favorably developed through a simple coating method, which is followed by heat treatment. The PDA layer is turned into amorphous nitrogen-doped carbon, which improves the electronic conductivity and further facilitates the diffusion of ions and electrons. Meanwhile, the stable C−S bonds between the Sb2S3 nanowire and carbon coating endow good structural stability to the prepared composites. Accordingly, the Sb2S3@N−C composites present excellent electrochemical characteristics with rate capability, excellent capacity, and cyclic stability for SIBs through the synergistic effect between a one-dimensional Sb2S3 nanowire and a nitrogen-doped carbon coating. This simple synthesis and superior electrochemical performance of Sb2S3@N-C composites provide an alternative way to cultivate highperformance electrode materials for future SIBs.

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EXPERIMENTAL SECTION

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RESULTS AND DISCUSSION

Preparation of Sb2S3 Nanowires. The Sb2S3 nanowires were obtained with a hydrothermal method.35 First, SbCl3 (0.228 g) and Na2S·9H2O (0.48 g) were separately dissolved in 20 mL of ethylene glycol; the former solution was dropped into the latter solution slowly under stirring. A homogeneous solution was obtained after several hours of mixing of the resultant solution. The final solution was moved to an autoclave, where it was maintained at 200 °C for 10 h. Later after the cooling process, precipitates were prepared by centrifugation, then cleaned with deionized (DI) water and ethanol, and afterward dried at 60 °C for 12 h under vacuum. Preparation of Sb2S3@N-C Composites. The as-synthesized Sb2S3 nanowires (100 mg) were dispersed thoroughly into a 10 mM Tris buffer solution (150 mL, pH = 8.5) with sonication for 10 min, followed by the addition of dopamine (100 mg) under vigorous stirring for 24 h in order to synthesize polymerized dopamineencapsulated Sb2S3 (Sb2S3@PDA) composites. The resulting solution was obtained via centrifugation, further cleaned with DI water and then ethanol, consecutively, and freeze-dried. For Sb2S3@N-C composites, the as-prepared Sb2S3@PDA nanowires were transferred to an argon-flowing tubular furnace for pyrolysis at 400 °C for 3 h at 3 °C min−1 (heating rate). For comparison, the Sb2S3 nanowires (100 mg) were cleaned in a 10 mM Tris buffer solution (100 mL), and then dopamine (100 mg) was added to obtain the Sb2S3@PDA composites. The carbon-encapsulated Sb2S3 nanowire composites were obtained under the same experimental conditions. The composite prepared with a higher concentration of dopamine in the Tris buffer solution is referred to as Sb2S3@N-HC. Characterization. X-ray diffraction (XRD) was utilized to determine crystallographic structures through Bruker D8 X-ray with a Renishaw inVia Raman microscope. X-ray photoelectron spectroscopy (XPS) was employed with a VG Escalab 220i-XL. Thermogravimetric analysis (TGA) was utilized with a TGA Q50 analyzer under an air atmosphere until 700 °C at 10 °C min−1 (heating rate) to determine the carbon mass of Sb2S3@N-C composites. Scanning electron microscopy (SEM; Philips XL-30FEG), transmission electron microscopy (TEM; Philips CM20) with energy-dispersive spectroscopy (EDS), and high-resolution TEM (HRTEM; CM200 FEG) were used to reveal the sample morphology. Electrochemical Measurements. The electrochemical characteristics of Sb2S3@N-C were examined with assembly coin cells in a glovebox. In sodium half-cells, a metal foil of sodium was utilized for the counter and reference electrodes. For working electrodes, Sb2S3@ N-C composites (70 wt %), Super-p (20 wt %), and sodium alginate (10 wt %) binder dissolved in DI water were mixed to produce a homogeneous slurry. The slurry was deposited onto a copper foil, dried under vacuum, and then prepared as electrode films. A total of 1.1 mg cm−2 was used for the average loading of mass of the material. A glass fiber membrane (GF/D) served as the separator. NaPF6 (1.25 M) in ethyl methyl carbonate was utilized as the electrolyte. Cyclic voltammetry (CV) was conducted on a CHI-660C analyzer at different sweep rates. Galvanostatic discharge/charge measurements were carried out at different currents in 0.01−3.0 V versus Na/Na+ with an analyzer (MACCOR 4000). Electrochemical impedance spectroscopy (EIS) with a ZAHNER-elektrik IM 6 was utilized. The capacity of Sb2S3@N-C was calculated with the total mass of active materials. All of the electrochemical tests at room temperature were done.

The specific strategy used for the fabrication of Sb2S3@N-C composites is illustrated in Figure 1. First, the Sb2S3 nanowires were prepared via a simple hydrothermal method. Afterword, the obtained Sb2S3 nanowires were coated with PDA to form Sb2S3@PDA composites. Finally, the as-prepared Sb2S3@PDA composites were heated at 400 °C for 3 h under an agron atmosphere. In this step, the PDA layer was carbonized to form Sb2S3@N-C composites. B

DOI: 10.1021/acsanm.8b02335 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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region for O 1s and Sb 3d is displayed in Figure 3a. The two main peaks of O 1s in the range of ∼530 and 536 eV may come from the absorbed species (i.e., O2, CO2, and H2O) on the composite. The Sb 3d peaks at ∼539.7 and ∼530.6 eV suggest the presence of Sb3+ in the prepared composites,39 which may be indexed to Sb 3d3/2 and Sb 3d5/2, respectively. Detailed spectra of the deconvoluted S 2p region shown in Figure 3b display the peaks at ∼160.8, ∼161.9, and ∼163.6 eV. The first two peaks correspond to S 2p1/2 and S 2p3/2, respectively, and may be indexed to the single doublet from the bonds between sulfur and antimony,40 while the last peak at ∼163.6 eV can be assigned to C−SOx−C groups, suggesting that sulfur was clearly placed in the carbon structure. The spectra of C 1s in Figure 3c consist of five peaks centered at ∼285.1, ∼285.6, ∼286.3, ∼287.0, and ∼298.2 eV, which can be assigned to C−C, C−N/C−S, C−O, CO, and OC O, respectively. The high intensity of the C−N/C−S bonds in the C 1s spectra further confirms the favorable doping of sulfur and nitrogen in the carbon structure.41 The strong binding between Sb2S3 nanowires and carbon mainly comes from the stable C−S bonds, possibly enhancing the electronic conductivity due to the synergistic bridging effect. Importantly, it also improves the performance by suppressing the dissolution of sulfur during the cycling process, resulting in long-term stability for SIBs.26,42 In Figure 3d, the two peaks at ∼399.1 and ∼400.3 eV in the N 1s spectrum demonstrate the presence of nitrogen in the amorphous carbon structure, which may be associated with pyridinic and pyrrolic nitrogen, respectively.15,43 The nitrogen dopants are known to create active sites and defects for fast diffusion processes, thus improving the electrochemical properties of electrode materials.44 The morphology and structure of Sb2S3@N-C were delved into and are displayed in Figure 4. The as-prepared Sb2S3 nanowires are ∼20−100 nm in diameter and up to ∼50 μm in length in Figure S3. They have a smooth surface and present no indication of a layered structure. In contrast, the SEM images of the Sb2S3@N-C composites, displayed in Figure 4a,b, reveal the carbon-encapsulated nanowires with lengths of several micrometers, which are shorter than those of pristine Sb2S3 nanowires because of the damage of long nanowires during the fabrication process. The carbon-encapsulated nanowires also present a fine surface due to the uniform polymerization process. Figure 4c further confirms the uniform distribution of the nanowire’s coating layer in Sb2S3@N-C composites.

Figure 1. Schematic diagram of the construction of Sb2S3@N-C composites.

The crystallographic structure of Sb2S3@N-C was analyzed by XRD. For Sb2S3@N-C, all peaks are indexed to the crystallized phase for Sb2S3 (PDF 00-042-1393) and have strong intensities as shown in Figure 2a, implying its good crystallinity and its carbon amorphousness. The orthorhombic Pbnm (No. 62) is a = 11.239 Å, b = 11.313 Å, and c = 3.8411 Å. Later structural information on Sb2S3@N-C in Figure 2b was revealed using Raman spectroscopy. Peaks before 400 cm−1 correspond to the characteristic features of crystalline Sb2S3.26,36 Meanwhile, the two bands at ∼1350 and ∼1548 cm−1 are associated with the D and G bands of the carbon due to the disordered and graphitic structures, respectively, indicating the graphitization of polymerized dopamine after high-temperature pyrolysis.37 The peak intensity ratio of the D and G bands (ID/IG) is estimated to be ∼0.91, suggesting the presence of amorphous carbon and the agreement with XRD analysis. TGA was done at 10 °C min−1 (heating rate) to estimate the content of carbon in the final product. When Sb2S3 was converted to Sb2O4 at 700 °C in air,38 the decrease of the theoretical weight was ∼9.4 wt %. The carbon contents of the Sb2S3@N-C and Sb2S3@N-HC composites were determined to be ∼9.8 and ∼12.6 wt %, respectively, based on the TGA curves in Figure S1. The content of carbon was further confirmed by dissolving Sb2S3@N-C composites in concentrated HCl and weighing the remains, which was estimated to be ∼10.02 wt %. XPS measurement for the Sb2S3@N-C composites in 0− 1000 eV provides further information on the elemental composition and oxidation state. The span spectrum indicates the existence of five elements (antimony, oxygen, sulfur, carbon, and nitrogen) in Figure S2. No additional element was identified in the wide-range XPS survey scan (Figure S2), confirming the expected elemental composition of the prepared Sb2S3@N-C composites. The high-resolution spectral

Figure 2. (a) XRD pattern of Sb2S3@N-C. (b) Raman spectra of Sb2S3 nanowires and Sb2S3@N-C composites. C

DOI: 10.1021/acsanm.8b02335 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 3. XPS spectra of Sb2S3@N-C composites: (a) Sb 3d and O 1s; (b) S 2p; (c) C 1s; (d) N 1s.

Figure 4. (a and b) SEM images of Sb2S3@N-C at different magnifications. (c) TEM image of Sb2S3@N-C composites. (d) HRTEM image of Sb2S3@N-C with a pattern of the Fourier transform (inset). (e) TEM image and its EDS mapping of (f) antimony, (g) sulfur, (h) carbon, and (i) nitrogen in the single carbon-encapsulated Sb2S3 nanowire. Scale bars in parts f−i are all 100 nm.

Figure 4d demonstrates the HRTEM image of the individual carbon-encapsulated Sb2S3 nanowire, and the inset illustrates the corresponding Fourier transition. A total of 0.5 nm of the lattice spacing is consistent with the d spacing of (120) of the antimonite Sb2S3, further confirming the well-textured Sb2S3 in the prepared composites. The TEM image of a single carbonencapsulated Sb2S3 nanowire in Figure 4e clearly displays its well-carbon-encapsulated architecture features. The corresponding EDS mapping analysis in Figure 4f−i further proves the uniformity in the distribution of carbon, nitrogen, sulfur, and antimony elements throughout the whole nanowire. The

carbon coating layer tightly combined with the Sb2S3 nanowire through the stable C−S bonds whose thickness of ∼14 nm is clearly seen on the Sb2S3 nanowire surface, which is a nitrogendoped amorphous carbon structure due to the pyrolysis process, possibly creating the stable solid/electrolyte interface (SEI) and thus enhancing the conductivity of the electrode.45 Electrochemical characteristics of Sb2S3@N-C composites were analyzed with CV and charge/discharge measurements. The CV curves of Sb2S3@N-C composites for the initial 3 cycles at 0.1 mV s−1 in the window (vs Na/Na+) are displayed in Figure 5a. The peak at ∼1.15 V may be associated with D

DOI: 10.1021/acsanm.8b02335 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 5. (a) CV curves of Sb2S3@N-C composites at 0.1 mV s−1. (b) Charge/discharge curves of Sb2S3@N-C composites at 100 mA g−1.

discharge and charge capacities decrease to ∼820 and ∼790 mAh g−1, respectively, with ∼96% of CE, indicating good capacity retention and reversibility. The rate capability corresponding to Sb2S3@N-C composites was evaluated for 10 cycles at different current densities in Figure 6a. The Sb2S3@N-C composites retain a capacity of

electrolyte decomposition, leading to the formation of a SEI, intercalation of sodium ions into Sb2S3, and formation of antimony nanoparticles in Na2S.20,21 The other two peaks at ∼0.88 and ∼0.46 V may come from the conversion of sulfur in Sb2S3 (Sb2S3 + 6Na+ + 6e− → 2Sb + 3Na2S) and the process of alloying between antimony and sodium to form Na3Sb (2Sb + 6Na+ + 6e−1 → 2Na3Sb).38 The peak at ∼0.09 V may be assigned to the intercalation of sodium ions in the nitrogendoped carbon structure.46 In the initial anodic scan, the peak at ∼0.72 V is associated with the process of dealloying (2Na3Sb → 2Sb + 6Na+ + 6e−). The peak at ∼1.29 V is associated with the formation of Sb2S3 due to the reaction of Na2S with antimony (2Sb + 3Na2S → 2Sb2S3 + 6Na+ + 6e−), while the rise of the peak at ∼1.57 V due to the deintercation of sodium ions from the layered structure.38,47 The following broad peak at ∼2.11 V may come from the extraction of sodium ions and formation of Sb2S3 during the deintercalation process.48 Additionally, the small peak located at ∼0.05 V indicates sodium-ion extraction from the carbon layer. As seen in the following scans, the CV curve of Sb2S3@N-C composites for the 1st cycle is very different from the later ones due to the activation steps for sodiation.17,49 The cutoff voltage is important to improving the electrochemical reversibility on the metal sulfide materials. The CV curves of Sb2S3@N-C composites at 0.1 mV s−1 for the first 3 cycles are demonstrated in Figure S4. The positions of the peaks in Figure S4 are almost similar to those for Sb2S3@N-C composites in Figure 5a. The discharge/charge characteristics of Sb2S3@N-C composites at 0.1 A g−1 are demonstrated in Figure 5b. The capacities of Sb2S3@N-C composites for the first discharge and charge are ∼1050 and ∼840 mAh g−1, respectively, with a corresponding Coulombic efficiency (CE) of ∼80%. The initial capacity of Sb2S3@N-C composites is higher than that for Sb2S3 possibly because of the undesired side reactions at the interface, which houses extra sodium ions through adsorption/desorption during SEI layer formation.47 The high CE corresponding to the 1st cycle may come from the development of a uniform and stable SEI layer outside the carbon coating layer. The uniform nitrogen-doped amorphous carbon structure may contribute to the formation of the stable SEI layer, generate more sodium storage sites, and enhance the conductivity of the electrode. The capacity decrease may result from electrolyte decomposition, and the characteristics of the polarization hysteresis during the reaction process can cause the reduction of Sb2S3 by sodium ions to antimony before the reaction of alloying and undesired byproducts during the reactions.50 For the 3rd cycle, it is very evident that the

Figure 6. (a) Rate capability of Sb2S3@N-C at different current densities. (b) Stability of Sb2S3@N-C composites tested at 1.0 A g−1.

∼765 mAh g−1 at 0.1 A g−1 after 10 cycles. As the current rate gradually increased (0.2−6.0 A g−1), the capacities are ∼655 mAh g−1 at the 20th cycle, ∼600 at the 30th, ∼530 at the 40th, ∼500 at the 50th, ∼460 at the 60th, and ∼440 at the 70th, respectively. At 10 A g−1, it increases up to ∼410 mAh g−1 after 80 cycles and higher than that for graphite. The capacity decreases when the current increases because of the process of slow diffusion for the electrons and ions, indicating the importance of the diffusion rate at a high current density.51 In comparison, the Sb2S3@N-HC composites tested at the same electrochemical conditions present a capacity of ∼410 mAh g−1 at 10 A g−1, which is lower than that for Sb2S3@N-C composites, as shown in Figure S5a. The Sb2S3 nanowire electrode yields a relatively lower reversible capacity. In Figure S6a, the specific capacity gradually dropped to ∼405 mAh g−1 at 4.0 A g−1. The enhanced rate performance of Sb2S3@N-C composites indicates, on the one hand, that the nitrogen dopants provide sufficient defects and electrochemical active E

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Figure 7. (a) CV curves at different scan rates. (b) Plots for log(i) versus log(v) (peak current, i; scanning rate, v). (c) Bar charts at different scanning speeds. (d) Capacitive contribution at 1.0 mV s−1.

to those of the reported Sb2S3-based materials. In contrast, Sb2S3@N-HC composites possessed a capacity of ∼645 mAh g−1 at 1.0 A g−1 for the 6th cycle and then gradually decreased to ∼475 mAh g−1 after 1000th cycles with a capacity retention of ∼74% in Figure S5b. Furthermore, the cyclic stability of Sb2S3 nanowires is shown in Figure S5b, with a reversible capacity of ∼455 mAh g−1 after 300 cycles. The cycle performance of Sb2S3 nanowire decay could be attributed to continuous sulfur dissolution into the electrolyte.52 To clarify the relationship between the performance and structure, we examined the morphology of Sb2S3@N-C composites upon cycling. In Figure S9, the structure of carbon-encapsulated Sb2S3 nanowires was well preserved after the long cycling process. The results proved that the carbon coating on the Sb2S3 nanowire surface served as a buffer, which could accommodate volume changes and preserve the electrode structural integrity upon cycling, leading to the enhanced electrochemical characteristics. To further prove the outstanding cyclability of Sb2S3@N-C composites, a new cell was prepared for testing. The Sb2S3@NC composite provided a high capacity of ∼450 mAh g−1 at 2 A g−1 after 2000 cycles in Figure S7. Accordingly, the Sb2S3@NC composites exhibited remarkable long-term cyclic stability and rate capability, possibly because of the synergistic effect between the Sb2S3 nanowires and nitrogen-doped carbon coating. EIS was later utilized to characterize the kinetic information on the Sb 2 S 3 @N-C composites and Sb 2 S 3 nanowires in Figure S8. The typical impedance spectrum contains a semicircle and slope. The semicircle is related to the SEI layer impedance (RSEI) and charge-transfer impedance (Rct) between the electrode/electrolyte interface at high frequency, and the slope at low frequency is recognized as

sites for the fast diffusion process and, on the other hand, that the strong synergistic bridging effect through the C−S bonds provides a robust composite structure to maintain the original structure during the cycling process at the high current densities. Moreover, the reversible capacity of Sb2S3@N-C composites was swiftly recovered as the current decreased to 0.2 A g−1, suggesting good sodium-ion diffusion kinetics and excellent structure stability, which is very important for SIBs with high specific energy and excellent cycle life. As the current was increased to 2.0 A g−1, the capacity initially decreased to ∼540 mAh g−1 and then gradually steadied at ∼500 mAh g−1. The long-term stability should be evaluated for practical applications. The cyclic stability of Sb2S3@N-C composites was first cycled at 0.1 A g−1 for 5 cycles and then at 1.0 A g−1 for 1000 cycles, as shown in Figure 6b. After the 6th cycle, the capacity gradually dropped to ∼590 mAh g−1 after 50 cycles and dropped to ∼650 mAh g−1 after ∼100 cycles. The capacity fade during the initial cycles may come from (i) the continuous formation and stabilization of the SEI layer on the electrode surface and (ii) the influence of undesired side reactions with sodium ions and the functional groups from the carbon.14,31,48,49 A high capacity of ∼625 mAh g−1 at 1.0 A g−1 was maintained after 1000 cycles, and the reversible capacity retention estimated from the 6th to 1000th cycles is up to 81%. The CE was consistently over 99% from the 15th cycle, suggesting the stabilization of SEI. The long-term stability for SIBs might have originated from the stable C−S bonds between the Sb2S3 nanowires and carbon coating layer, which reduces the dissolution of sulfur during the cycling process. Table S1 compares Sb2S3@N-C composites and previously reported Sb2S3-based electrodes for SIBs. The electrochemical characteristics of Sb2S3@N-C composites are excellent or close F

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abundant extrinsic defects that act as active sites to improve the overall conductivity and facilitate efficient diffusion for Na+ ions and electrons. A combination between the Sb2S3 nanowire and nitrogen-doped carbon through stable C−S bonds forms a robust composite architecture, resulting in superior rate capability and stability. These composites also exhibited superior electrochemical characteristics with excellent rate capability, high capacity, and remarkable long-term stability. The kinetics study reveals that pseudocapacitive behavior is important in the overall electrochemical process for the Sb2S3@N-C composites, which lead to fast charge-transfer and ion-diffusion kinetics. The great electrochemical characteristics mainly originated from the synergistic effect of several merits, including nanoscaled active material, uniform nitrogendoped carbon coating, and robust structural stability. The simple synthetic can be used as a useful tool for developing advanced materials for the anode in high-performance SIBs.

the Warburg impedance from the diffusion of sodium ions in electrode materials.53 From the Nyquist plots, the Sb2S3@N-C composites display a small semicircle, suggesting a small interfacial resistance. The calculated interface impedances (Rint) of Sb2S3 nanowires and Sb2S3@N-C composites are ∼130 and ∼48 Ω, respectively. The improved conductivity of Sb2S3@N-C composites originated from the addition of nitrogen-doped carbon-structure-encapsulated Sb2S3 nanowires, showing the enhanced electrochemical performance of Sb2S3@N-C composites. To further characterize the kinetic origin of Sb2S3@N-C composites, a series of CV tests were conducted in Figure 7. The CV curves of Sb2S3@N-C composites at different scan rates are displayed in Figure 7a. The peak shape of the obtained CV profiles is well preserved, and a slight shift occurs during the processes. A clear positive shift for the anodic peaks and a negative shift for the cathodic peaks were obtained because of polarization during the process.54 The CV profiles indicate that sodium-ion storage in Sb2S3@N-C composites obeys a combination of the diffusion-controlled reaction and capacitive process.55 The capacitive contribution is estimated by the relationship between the peak current (i) and scan rate (v) (eq 1): i = avb

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b02335. Additional characterizations of materials including TGA, XPS, SEM, CV, rate and cyclic capabilities, Nyquist plots, and electrochemical performance (PDF)

(1)

where both a and b are constants. The surface capacitive process dominates when b is close to 1.0, while for a solid-state diffusion-controlled process, b is close to 0.5.56 Figure 7b exhibits the plots of log(i) versus log(v), from which the value of b for the cathodic and anodic steps are estimated to be ∼0.77 and ∼0.75, respectively, suggesting a more favored capacitive kinetics of Sb2S3@N-C composites. The analysis approach proposed by Dunn et al. was applied to analyze the pseudocapacitive contributions and diffusioncontrolled process contributions to the total capacity.57 The current (I) at a given potential (V) is composed of the surface capacitive-controlled effects (k1v) and diffusion-controlled process (k2v1/2) according to eq 258 I(V ) = k1v + k 2v1/2

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AUTHOR INFORMATION

Corresponding Authors

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

Mingjun Hu: 0000-0002-5474-6022 Xin Wang: 0000-0002-4771-8453 Jong-Min Lee: 0000-0001-6300-0866 Wenjun Zhang: 0000-0002-4497-0688 Notes

(2)

The authors declare no competing financial interest.

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where k1 and k2 are adjustable parameters. The bar charts of calculated percentages of pseudocapacitive contributions in the Sb2S3@N-C composites are displayed at different scanning speeds in Figure 7c. At the different scanning speeds, the pseudocapacitive contributions can be calculated as ∼62.9, ∼66.7, ∼71.7, ∼76.3, and ∼82.6%, respectively, revealing the excellent rate capability and stability of sodium-ion storage of the designed Sb2S3@N-C composites. Figure 7d illustrates the typical voltage response for the pseudocapacitive contribution (blue-shaded region) compared with the whole area (black line). The large part of the pseudocapacitive contribution of ∼76.3% was developed at 1 mV s−1, implying a favorable charge-transfer kinetics of Sb2S3@N-C composites and good agreement with the obtained b value.

ACKNOWLEDGMENTS The authors acknowledge support from the Cultivation Project of National Engineering Technology Center (Grant 2017B090903008), National Natural Science Foundation of China Program (Grant 51602111), Special Fund Project of Science and Technology Application in Guangdong (Grant 2017B020240002), Basic Research Project of Knowledge Innovation Program of Shenzhen City (Grant JCYJ20160229165250876), and Xijiang R&D Team.

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CONCLUSION We have prepared Sb2S3@N-C composites via a simple carbon-coating technique with the thermal treatment as advanced anode materials for SIBs. The PDA coating layer, acting as a nitrogen-doped carbon source, was converted into the amorphous nitrogen-doped carbon with the help of the thermal treatment process. Nitrogen dopants can create G

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