Rodlike Sb2Se3 Wrapped with Carbon: The Exploring of

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Rod-like Sb2Se3 Wrapped with Carbon: The Exploring of Electrochemical Properties in Sodium-Ion Batteries Peng Ge, Xiao-Yu Cao, Hongshuai Hou, Sijie Li, and Xiaobo Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10886 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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ACS Applied Materials & Interfaces

Rod-like Sb2Se3 Wrapped

with

Carbon:

The

Exploring of Electrochemical Properties in SodiumIon Batteries Peng Ge,1 Xiaoyu Cao,2 Hongshuai Hou1, Sijie Li1 and Xiaobo Ji 1* 1

College of Chemistry and Chemical Engineering, Central South University,

Changsha, 410083, China. 2

College of Chemistry, Chemical and Environmental Engineering, Henan

University of Technology, Zhengzhou, 450000, China. ABSTRACT: One-dimensional Sb2Se3/C rods are prepared through self-assembly from the inducing of anisotropy, and the corresponding sodium storage behaviors are evaluated, presenting excellent electrochemical performances with superior cycle stability and rate capability. The Sb2Se3 delivers a high initial charge capacity of 657.6 mAh g-1 at current density of 0.2 A g-1 between 2.5 and 0.01 V. After 100 cycles, the reversible capacity of Sb2Se3/C remain 485.2 mAh g-1. Even at high rate current density of 2.0 A g-1, the charge capacity still remains 311.5 mAh g-1. Through the analysis of CV and in-situ EIS, the in-depth understanding for high rate performances are explored effectively. Briefly, the sodium storage performance of Sb2Se3/C is observably enhanced, benefiting from the 1D structure and the introduction of carbon layer with robust structure stability and conductivity. KEYWORDS: Sb2Se3, carbon coating, conductivity, sodium-ion batteries, electrochemistry

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1. INTRODUCTION The exploitation of electrode materials with superior electrochemical sodium storage performances is pivotal in the development of advanced sodium-ion battery, which has been regarded as the promising alternative for conventional lithium-ion batteries.1-12 In considering multi-electron reaction from the conversion and alloying reaction during the charge-discharge process, generally, metal-chalcogenides can exhibit high sodium storage capacity as anode materials for SIBs.13-14 In spite of increased toxicity, the electrical conductivity of selenium (1×10-3 S m-1) is much higher than that of sulfur (5×10-28 S m-1) as reported,15 the generated selenium in the conversion reaction process can largely enhance the electrode conductivity of the sample, resulting in the rate capability.16 So far, the sodium storage activity of several metal selenides have been studied, e. g., NiSe2, CoSe2, SnSe, SnSe2 and Sb2Se3.17-21Among them, Sb2Se3 with layered structure is of great interest because of the high theoretical specific capacity

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and enhanced electrochemical

utilization.19 Regretably, the large volume change in the sodiation-desodiation process of the Sb2Se3 would lead to the serious destruction of electrode structure, giving rise to the rapid deterioration of capacity. To accommodate the volume change is quite necessary to obtain applicable Sb2Se3 anode material. It has been demonstrated that constructing carbon composite is an effective strategy to improve the electrochemical properties, due to the good flexibility and electronic conductivity of carbon.23-29 Moreover, the novel nanostructure also plays an important role in promoting the electrochemical performances.30-32 As the unique architecture, 1D nanostructures display special electrical conductivity, mechanical and thermal stability, which can be attributed to the special geometric and morphology characteristics.33 Profiting from the uniform size distribution and long-range orientation of the crystalline lattice, the sodium ions can


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gallop orderly in the superhighway and the volume exist regular expansion along the direction, which can stabilize cycling properties and enhance rate performance.34 Herein, 1D rod-like Sb2Se3 was obtained from hydrothermal reaction and further coated by carbon layer to form Sb2Se3/C. The as-prepared samples were used as anode for SIBs, showing the excellent electrochemical properties. The Sb2Se3/C can retain the reversible capacity of 485.2 mAh g-1 after 100 cycles at current density of 0.2 A g-1 between 0.01 and 2.5 V (vs. Na+/Na). 2. EXPERIMENTAL SECTION 2.1 Preparation of Sb2Se3 and Sb2Se3/C In a typical synthesis, 0.625 g SbCl3 was added to 75 mL of ethylene glycol in 200 mL Teflon lining with 1 h magnetic stirring to form transparent A solution at room temperature. At ice water temperature, 0.5 g Se uniformly dispersed in the 75 mL distilled water and 0.75 g NaBH4 was added subsequently. After 30 min string, the transparent solution (NaHSe) can be obtained (termed: B solution). B solution was added dropwise to the A solution with stirring to form orange-red solution. The Teflon lining was transferred to the electric oven at 180 oC for 24 h and cooled to room temperature naturally. The black precipitates were collected after ethanol and water washing several times and dried under vacuum. Through previous reported carbonization methods,35 the product (Sb2Se3/C) was thermal-treated under an Ar flow at 400 oC for 2 h with a heating rate of 3 oC/min. Without the carbonize process, the Sb2Se3 was obtained in the same condition. 2.3 Materials characterization X-Ray diffraction (XRD) were performed by Rigaku D/max 2550 VB+ 18 kW (Cu, Ka radiation, λ = 0.1546 nm, V = 40 kV, I = 30 mA) at a scan rate of 2o min-1. The content of the carbon was carried out on the instrument TGA (NETZSCH STA449F3) at the air atmosphere in the interval


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from room temperature to 800 oC at a heating rate of 10 oC min-1. The scanning electron microscopy (SEM, a JEM-2100F instrument at 200 kV) was used to character the morphology and size of the obtained samples. The samples dispersed uniformly in the ethanol were dropwise added on the copper grid. The as-prepared grids were used on the Transmission electron microscopy (TEM, JEM-2100F) at current density of 10 um with an accelerating voltage of 200 kV. 2.4 Electrochemical characterization The electrochemical performances of as-prepared products were determined by assembling a 2016-type coin cell with Na metal (purity, 99.5%) as counter electrode in a MBraun glove box with argon concentrations of moisture and oxygen below 5 ppm. The slurry came from the active materials (Sb2Se3 or Sb2Se3/C), conductive agent (carbon black) and binder agent (carboxymethyl cellulose) with the weight ratio (70 : 15 : 15) in deionized water. The uniformly mixing slurry was used to coat on the copper foil and dried in the vacuum oven at 80 oC for 12 h, which is used as the working electrodes. The electrolyte solution consisted of 1M NaClO4 in propylene carbonate (PC) and the annexing agent of 5% FEC (fluoroethylene carbonate) in argon glove and polypropylene separator (Celgard 2400). Galvanostatic cycling and rate performances were tested by Land CT 2001 battery and Arbin battery cycler (BT2000) at a serious of current densities between 0.01 and 2.5 V (vs Na+/Na). Cyclic voltammetric (CV) was undertaken on the AUTOLABEL instruments at the scan rate of 0.1 mV s-1. Electrochemical impedance measurements (EIS) were conducted on the (CHI 660D, Chenhua Instrument Company, Shanghai, China) in a frequency range from 0.01 Hz to 100 kHz. The temperature of all electrochemical tests were maintained at 298 K.


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3. RESULTS AND DISCUSSION

Figure 1. XRD patterns (a), TGA curve (b), Raman spectroscopy (c) and XPS spectroscopy (d) of Sb2Se3 and Sb2Se3/C, XPS high resolution spectra of Sb 3d (e), Se 3d (f), O 1s (g) for Sb2Se3 and Sb2Se3/C.


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XRD patterns of the as-obtained samples are displayed in Figure 1a. It is found that all the diffraction peak positions of Sb2Se3 and Sb2Se3/C well correspond to those of the standard orthorhombic Pbnm(62) Sb2Se3 (JCPDS: 15-0861).36-38 Moreover, the diffraction peaks located at 15.029, 16.874, 21.446, 24.151, 27.394, 28.199, 31.159, 34.075 and 41.304o can be assigned to (200), (120), (220), (101), (230), (211), (221), (240) and (250), confirming the high purity of Sb2Se3. Through refining the parameters of the cell lattice, the a = 11.637 Å, b = 11.72 Å and c = 3.968 Å (β = 90o) can be calculated, which is in agreement with those of the single crystal Sb2Se3. The strongest peak of the as-sample (Sb2Se3) is indexed to (230), while that of standard pattern of Sb2Se3 is (221) peak. Meanwhile, the intensities of all the (hk0) peaks were improved abnormally, revealing that the rods possess a preferential orientation along the tendency of (001).39 The broad and week peak at 25o is associated with the existing amorphous carbon.40-41 The high crystallinity of the samples could be substantiated by the narrow and sharp peaks, the lowered peak intensity of Sb2Se3/C than that of Sb2Se3 should be ascribed to the amorphous carbon.10 In addition, the Raman was conducted to further investigate the crystalline structure of carbon as displayed in Fig. 1(c). It is clear that the carbon has been decomposed on the Sb2Se3 rods, and the ID/IG is about 0.9, verifying the high electronic conductivity. In order to determine the content of carbon for Sb2Se3/C, TGA was carried out, which is displayed in Figure 1b. The increased part of the curves between 300 and 400 oC is observed for Sb2Se3 and Sb2Se3/C, which is due to that the Sb2Se3 decomposed into the Sb2O4 and SeO2 with the sublimation slowly. The steeply decreasing part is ascribed to the sublimation of SeO2 for Sb2Se3 and the leaving of SeO2 as well as CO2 for Sb2Se3/C. From the DSC, the combustion of carbon was differentiated. Interestingly, when the temperature was raised to 390 oC, the combustion of carbon is obvious from the peaks of DSC. The 35.19 % weight loss can testify that the Sb2Se3 transforms into


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Sb2O4 completely, according to the following equation in Figure 1b. From the results of TGA, the carbon content of Sb2Se3/C can be calculated to ∼8%. XPS was performed to investigate the compositions and valence state of the as-obtained samples in detail, and the full survey spectra is shown in Fig. 1(d). Clearly, the structure of the samples was not altered obviously. In comparison with the high spectra of Sb 3d, Se 3d for Sb2Se3 and Sb2Se3/C, it is found that the peaks of Sb2Se3/C move toward high waves, agreeing well with the previous report.42 Meanwhile, the peaks located at 529.19 eV, 530.49 eV for Sb2Se3/C and the peaks sited at 529.04 eV, 529.64 eV for Sb2Se3, which are assigning to Sb 3d5/2 (Sb III, and Sb III, V).43 The peaks appeared at 538.5 eV is relation with Sb 3d3/2. The peaks indexed to O 1s also are found, and that of Sb2Se3/C is stronger than that of Sb2Se3, which is ascribed to the oxygen on the surface of Sb2Se3 and carbon layer. Moreover, the peak and the satellite peak situated at 54.0 eV is deriving from the Sb 3d3/2 and Sb 3d5/2. The peak of carbon also is found in the Sb2Se3, indicating the existing of carbon.


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Figure 2. SEM micrographs (a-c), TEM (d), HRTEM (e) and EDX (h) of Sb2Se3, TEM (f) and HRTEM (g) of Sb2Se3/C.


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The surface morphology and internal structure of the as-prepared samples are displayed in Figure 2, in the different magnifications. Figure 2(a-c) exhibits SEM images of Sb2Se3 and the well-defined rods can be found to stack loosely, which are beneficial for improving the contacting area with electrolyte and increasing more active sites for electrochemical reaction during cycling as compared to solid compaction structure. Rod-like structure would shorten the sodium ions shuttling paths and make Na+ transfer in order, resulting in the excellent electrochemical properties.30 As displayed in Figure 2d, the typical rod-like TEM images of Sb2Se3 matches well with SEM images (the diameter of 100 nm). From the HRTEM images of Sb2Se3, the lattice strip is clearly observed. The interplanar distances of 0.369 and 0.576 nm are relative to (101) and (200), corresponding to the phase of standard Sb2Se3. The marked angel (19.7o) is deemed as the angel of (101), (001). Taking consideration into the above results, the rods grown preferentially towards [001] direction can be deduced.44 These results are consistence with the analysis of XRD. Expectedly, the TEM images of Sb2Se3/C can verity that the amorphous carbon has been decorated on the surface and the rod-like structure keeps unchanged in Figure 2f. In addition, the high solution TEM image of Sb2Se3/C is exhibited in Figure 2g and it is noted that the uniform rods are wrapped by the carbon coming from the pyrolysis of polyvinylidene fluoride (PVDF). Meanwhile, the mapping of Sb2Se3/C was conducted to better understand the carbon coating structure. Obviously, the carbon is wrapped on the surface of the samples. The carbon matrix has been regarded as the best choice to buffer the volume expansion with the extra advantages of increasing electronic conductivity, which is conductive to long-time cycling stability and rate performances.45-47 In Figure 2h, the Energy Dispersive X-ray analysis (EDX) of Sb2Se3 is displayed, the clear atomic ratio of antimony and selenium (∼ 2 : 3 ) further demonstrates that the target products (Sb2Se3) have been fabricated successfully.


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Scheme 1. Schematic Illustration of the Sb2Se3/C rods.

Based on the above analysis, the formation of Sb2Se3 and carbonizing process of Sb2Se3/C are illuminated in Scheme. 1. The reducing agent was dropwise added to the Sb3+ solution to form Sb-Se composite. At high temperature and pressure, the mixing solvent (water and glycol) provides more mobility of the Sb-Se composites, further nucleating and growing into the Sb2Se3 seed. In addition, the chain structures play a crucial role in the growth of the rod-like morphology towards (001) direction. Utilizing the unique property of PVDF, it can be dissolved into the N methyl pyrrolidone (NMP) but separated with water. When the Sb2Se3 rods were distributed uniformly in the solution, the dissolved PVDF were existed on the surface of Sb2Se3. With the introduction of water, the PVDF was separated out and coated the samples. With the protection of Ar，the PVDF was carbonized, further forming the carbon layer successfully.


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Figure 3. Cycle performances of Sb2Se3 (a) and Sb2Se3/C (b), charge-discharge platforms of Sb2Se3 (c) and Sb2Se3/C (d), SEM of the cycled electrode material Sb2Se3 (e) and Sb2Se3/C (f). For evaluating the electrochemical performances of Sb2Se3 and Sb2Se3/C as SIBs anodes, the galvanostatic charge–discharge cycling was undertaken at current density of 0.2 A g-1 in the voltage interval from 0.01 to 2.5 V. The cycling curve of Sb2Se3 is exhibited in Figure 3a. The initial discharge and charge capacity deliver 1049.3 and 657.6 mAh g-1, respectively, yielding a


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low columbic efficiency (CE) of 62.67 %. However, the charge capacity is close to the theoretical Na+-storage value (670 mAh g-1), indicating that the as-obtained sample could provide extensive sites for electrochemical reaction. The sodiation reaction can be divided to two parts, conversion reaction and alloying reaction, which is marked as follows (Eqs. 1 and 2). The reaction of 1 mol Sb2Se3 need 12 mol Na+ for transforming into 3 mol Na2Se and 2 mol Na3Sb.19 Conversion : Sb2Se3 + 6 Na+ + 6 e- → 2Sb + 3Na2Se

Eqs. 1

Alloying : 2Sb + 6 Na+ + 6 e- → 2Na3Sb

Eqs. 2

In the first 10 cycles, the increased charge capacity is noticed, which can be ascribed to the activation process. The acute fluctuating of CE and the serious fading of charge/discharge capacity can be found in the subsequent 20 cycles. The side reaction, pulverization and falling of the samples should be responsible for the capacity decreasing. After 100 cycles, the charge capacity of Sb2Se3 only remain 308.2 mAh g-1 with a per fading of 3.49 mAh g-1. Figure 3b exhibits the cycling curve of Sb2Se3/C. Restricted by the low capacity of carbon, the Sb2Se3/C exhibits the initial charge and discharge capacity (577.5, 831.2 mAh g-1) with the CE of 69.48%. Both Sb2Se3 and Sb2Se3/C show the low CE, which is associated with the side reaction and the formation of SEI film. Not surprisingly, Sb2Se3/C also has an activation process in the first 10 cycles. During the repeated sodaition/desodiation processes, the rod structures are destroyed by the large volume expansion/shrink, giving rise to the gradually decreasing capacity. Fortunately, the existing carbon matrix can effectively lower electrode polarization, alleviate the volume change and inhibit the serious side reaction on the surface of Sb2Se3.48 After 100 cycles, Sb2Se3/C still delivers a reversible capacity of 485.2 mAh g-1 with a per decreasing of 0.92 mAh g-1 and CE of 99.1%, which is due to the loss Na+ in the surface of carbon layer. Figure 3(c, d) exhibit the platform curves of the samples, it is found that the platform above 0.8 V is related to


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conversion reaction and the platform below 0.7 V is associated with alloying reaction, which is consistence with the CV curves in Figure 5. Furthermore, the platform curves of Sb2Se3/C are smoother than those of Sb2Se3, which is attributed to the polarization lowered by the introduction of carbon.49 Through the detailed comparison of curves, these of Sb2Se3/C overlap to larger extent than that of Sb2Se3, indicating the better cycling stability and smaller polarization, which benefits from the coated carbon layer. The larger polarization of Sb2Se3 is clearly observed, which is ascribed to the reduction of Sb2Se3 by sodium ions to Sb metal before alloying in sodiation/desodiation process. As well-known, the carbon layer also plays an important role in enhancing the electronic conductivity, which is also in favour of improving the rate performances.45, 47 After the discharge-charge cycling, the corresponding SEM were conducted and shown in Figure. 5(e, f). Clearly, the Sb2Se3/C rods still keep the primitive structure, and the Sb2Se3 rods can be found because of the pulverisation. It is concluded that the introduction of carbon would stabilize the rod-like structure.


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Figure 4. Rate performances of Sb2Se3, Sb2Se3/C (a) and comparison of the rate performances of Sb2Se3/C against other similar materials (b). Figure 4 exhibits the rate performances of Sb2Se3 and Sb2Se3/C in various current densities between 0.01 and 2.5V. From average capacity of every 5 cycles in Figure 4a, that of Sb2Se3 can deliver 621.4, 519.8, 427.5, 310.1 and 237.9 mAh g-1 at 0.2, 0.4, 0.8, 1.0 and 2.0 A g-1. When the current density returned to 0.2 A g-1, the charge capacity can recover up to 553.87 mAh g-1 but only remain 446.8 mAh g-1 after the following cycles. For Sb2Se3/C, the Na+-storage capacities


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of 548.5, 489.3, 443.5, 382.9 and 311.7 mAh g-1 can be kept at the stepwise current densities. Even though the current density got back to 0.2 A g-1, the reversible capacity still can restore to 528.3 mAh g-1 and maintain 519.9 mAh g-1 after some charge/discharge cycling expectedly. It is striking to note that Sb2Se3/C displays the excellent rate performances at large current densities, which may be due to the improved electronic conductivity.11,

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Moreover, the excellent

reversible property of Sb2Se3/C derives from the stabilization of carbon layer. The existing carbon matrix protects the rod-like structure from the damage coming from the fast transferring of Na ions. As displayed in Figure 4b, when the current density increased to 0.8 A g-1, the asprepared Sb2Se3/C delivers a charge capacity of 443.5 mAh g-1, higher than that of the previous reports about anodes for SIBs (396 and 234 mAh g-1).50-51 In addition, in comparison with Sb2Se3 anode materials, the Sb2Se3/C shows superior rate capacity, which may be ascribed to the rodlike structure wrapped completely with the carbon.


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Figure 5. CV of Sb2Se3/C: the three curves at 0.1 mV s-1 (a), four curves at different scans (c), log i ~ log v plots at different sodiation/desodiation states (e); CV of Sb2Se3: the three curves at 0.1 mV s-1 (b), four curves at different scans (d), log i ~ log v plots at different sodiation/desodiation states (f).


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In order to better investigate the differences of charge/discharge behaviours between Sb2Se3 and Sb2Se3/C, the CV curves were carried out. Figure 5 exhibits the CV curves of 1st, 3rd and 5th at a scan rate of 0.1 mV s-1. In the first cathodic scan of Sb2Se3, three reduction peaks appear at about 1.26, 0.80 and 0.26 V, which is associated with the SEI formation, conversion reaction and alloying reaction, respectively. For the anode scan, the oxidation peaks at 0.79 and 1.5 V correspond to the dealloying and conversion reaction.21 Due to the activation process, the obvious changes take place between 3rd, 5th curves and 1st curves. Clearly, with the introduction of carbon layer, the characteristic peaks of Sb2Se3/C were lowered, which is in accordance with the analysis of platforms. By comparing with the 3rd, 5th curves of the asobtained materials, the more overlapping curves of Sb2Se3/C were observed, indicating that the sodiation/desodiation behaviours of sodium ions tend to be more stable and higher reversibility in the subsequence cycles, which is accordance with the analysis of charge/discharge platforms. According to the previous reports, the high rate performances are associated with the capacitive effect, which were investigated through the CV curves at 0.5, 0.7, 0.9 and 1.5 mV s-1 as displayed in Figure 5(c, d). Note that, no distinct changes in the shapes were displayed, suggesting that the Na-storage behaviours of samples are kept similar. Two components of capacity are concluded: (a) the diffusion-controlled contribution which comes from the intercalation/alloying/conversion reaction; (b) the capacitive contribution from the electrical double-layer effect and the charge-transfer with surface/subsurface atoms.52-55 The pattern of electrochemical charge carrier transfer is often analysed using the equation, ݅ = ܽ‫ ܾݒ‬, where i is the peak current, v is the scan rate, a and b are fitting parameters. b = 0.5 corresponds to a totally diffusion-controlled process, while b = 1 corresponds to a pseudocapacitive process. In the case of sodium ion batteries, it means the transfer of sodium ions only occurs on the surface of active


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materials which are more dynamically advantageous than diffusion inside active materials.56-58 Figure 5(e-f) is the plot of log(i) versus log(v). Both samples show a good linear relationship, but b values of Sb2Se3 is 0.50 and 0.54, while for the Sb2Se3/C rods, they are 0.61 and 0.68, which mean that the latter possesses a higher percentage of pseudocapacitive charge carrier transfer, and reasonably shows a better rate performance, as demonstrated in Figure 4. Therefore, it is inferred that the improved surface-controlled behaviours should be ascribed to the introduction of carbon layer, which reduce the internal resistances, enhance the activity of electrode as well as the electric charge collection effectivity.

Table 1. The values of fitting liners in low-frequency regions for various cycles. 5th

8th

11th

14th

17th

30th

40th

50th

60th

70th

Sb2Se3

2500

1824

1017

1559

1509

1458

1480

1491

1500

1580

Sb2Se3/C

2430

2280

2100

2115

2098

1500

1200

998

950

970

Sb2Se3

208

209

1258

677

217

487

450

468

431

453

Sb2Se3/C

276

345

358

322

310

245

261

255

248

262

Slope

Intercept


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Figure 6. The impedance plots of Sb2Se3/C (a, c), Sb2Se3 (b, d), the Z’ versus ω-1/2 in the lowfrequency regions Sb2Se3/C (e), Sb2Se3 (f), impedance plots of samples and equivalent circuit (inset) (g), the fitting liner (h) after 100 cycles.


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From the cycling performances, some unfavourable side reactions are conjectured in the cycling processes, which destroy the stable sodiation /desodiation reaction. In order to investigate the roles of carbon layer in protecting the electrode, EIS after different cycling (5th, 8th, 11th, 14th, 17th, 30th, 40th, 50th, 60th and 70th) were carried out, and the Nyquist plots of samples are displayed in Figure 6(a-d). The semicircle at high frequencies corresponds to the charge-transfer resistance and the straight sloping line at low frequency is related to the diffusion of sodium ions in the electrode. The inset-illustration of Figure 6g exhibits the equivalent circuit. Re is associated with the resistance of the materials in cell, Rf and Rct represent the impedances of SEI and charge-transfer, Zw is the diffusion of sodium ions into the bulk electrode. Note that a large variation of resistances for Sb2Se3 were displayed, indicating that the drastic side reactions have been taken place in the charge/discharge process of Sb2Se3. It is found that the inner resistances in the Sb2Se3/C is stable, perhaps resulting from that the carbon layer prevent the electrode from the direct intruding of electrolyte, which inhibits the side reaction taken place. The fitting lines about Z’ versus ω-1/2 (ω = 2πf) in the low-frequency regions are shown in Figure 6(e, f) and the relative values are exhibited in Table 1. Based on the previous reports (Z = Re + Rf +Rct + aω-1/2),59-60 the major inner resistances of electrodes are according to the values of intercept. Clearly, the dramatic variations were observed for Sb2Se3 at 8th (209Ω), 11th (1258Ω), 14th (677Ω), 17th (217Ω), deriving from serious side reaction with electrode polarization, which is consistent with the results of cycling curves. Meanwhile, the resistance of Sb2Se3/C is about 276Ω, after the following 3 cycling, it was increased to 345Ω, perhaps resulting from the active process. Through 70 discharge/charge cycling, the final resistance was stable at 262Ω for Sb2Se3/C (970Ω for Sb2Se3), indicating the faster electrons transfer. Moreover, comparing the slopes of the as-prepared samples, the smaller slopes of Sb2Se3/C demonstrated the better Na-


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kinetics in the internal electrodes. Importantly, the more stable resistances for Sb2Se3/C were observed, facilitating the steady internal reaction, which is beneficial for the electrochemical properties. Owing to incompletion of the conversion and alloying reactions, the residues of Na2Se with Sb particles are helpful to the conductivity, giving rise to the decreasing resistance in sequence. After 100 sodiation/desodiation cycles, the Rct of Sb2Se3/C is 186.5Ω, smaller than that of Sb2Se3 (398.2Ω), indicating that it is easier for sodium ions to diffuse in the matrix. Figure 6h is the liner relation between real impedance and the inverse square root of frequency, which can be used to calculate the Na+ diffusion coefficient based on the equation (3). The σ is the slope of liner in Figure 6h and the slope of Sb2Se3 line is larger than that of Sb2Se3/C line. In addition, the R, T, S, n, F and C keep consistence in this work. The larger σ, the smaller DNa+. The DNa+ of Sb2Se3 is calculated to 3.37×10-17 cm2s-1, meanwhile, that of Sb2Se3/C can reach 1.58×10-16 cm2s1

. The larger DNa+ can confirm that the carbon matrix facilitates the fast transport of sodium ions,

which is in favor of improving the electrochemical performances.61 D = 0.5 R2T2/S2n4F4C2σ2

Eqs. 3

Table 2. The values of fitting liners in low-frequency regions for various voltages. 2.3 V 2.0V 1.7V 1.4V 1.1V

0.8V

0.5V 0.2V 0.01V

Sb2Se3

2507

2130

1777

1846

1706

169

230

350

801

Sb2Se3/C

3370

4510

4435

4310

4012

809

885

832

2450

Sb2Se3

1232

1197

1287

1313

1369

458

522

655

1036

Sb2Se3/C

1393

1408

1389

1400

1426

576

642

685

1112

Slope

Intercept


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Figure 7. The EIS at various voltages for the first cycling, Sb2Se3/C (a), Sb2Se3 (c), the fitting lines in the low-frequency for Sb2Se3/C (b), Sb2Se3 (d), the discharge platform versus the relative phase transform (e).


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The in-situ EIS were employed to investigate the internal reaction for the first cycling as displayed in Figure 7(a, c). It is clear that the electrodes have similar variation tendency for the resistance at 2.3, 2.0, 1.7, 1.4, 1.1, 0.8, 0.5, 0.2, 0.01V, retrospectively. From the 1.1 V to 0.8 V, the slopes were lowered sharply, suggesting the enhanced electron transfer. From the analysis of platform in Figure 3, when the cells were discharged to 0.8 V, the electrodes have major conversion to form Na2Se. As well-known, sulfur is able to provide a matrix with high Li+ ion conductivity, thereby reducing internal resistance.62-64 Hence, the selenium plays a similar role in quickening the Na+ with electron transfer. Furthermore, the inner resistances have a dramatic changing (from 1369 to 458Ω for Sb2Se3, from 1426 to 579Ω for Sb2Se3/C) in Table 2. However, the relative large resistances for Sb2Se3/C were observed, perhaps coming from the inadequately active process. With the following sodiation process, the above-obtained Sb nanoparticles were reacted with sodium ions to form the Na3Sb, resulting in the gradual volume expansion, which would enlarge the resistances. The 1st discharge platform of Sb2Se3 is shown in Figure 7e, where three platforms corresponding to various sodiation processes are observed. In the discharge process, it is concluded as following, (a) insertion reaction (Sb2Se3 + xNa+ → NaxSb2Se3 for 2.5 1.1 V), (b) conversion reaction (NaxSb2Se3 + Na → 2Sb + 3Na2Se for 1.1 – 0.7 V), (c) alloying reaction (Sb + 3Na+ → Na3Sb for 0.7 – 0.01V)，which is similar to the previous report.65 It is expected that the analysis of in-situ EIS are consistence with those of CV curves in Figure 5. Taking consideration into the above discussion, it is clear that the carbon layer effectively alleviates the volume expansion and reducing the internal resistances, which is in favor of Nastorage behaviors.


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4. CONCLUSIONS In summary, the Sb2Se3/C rods have been obtained through Sb2Se3 coming from selfassembly and ex-situ carbonize process. The uniform Sb2Se3 rods are found to grow toward [001] direction. The electrochemical properties of Sb2Se3 and Sb2Se3/C are investigated as anode materials for SIBs. Benefitting from special structures, the initial Na+-storage capacity of Sb2Se3 can reach 657.6 mAh g-1. The Sb2Se3/C exhibits the excellent cycling stability and rate performance. The Sb2Se3/C can keep the reversible capacity of 485.2 mAh g-1 after 100 cycles and remain 311.5 mAh g-1 at large current density of 2.0 A g-1. This research shows that Sb2Se3/C may be a promising anode materials for SIBs. Significantly, it provides an effective method to investigate the internal chemical reaction and gives an in-depth understanding of the improved rate performance and the present work can open opportunities for other fields.

AUTHOR INFORMATION Corresponding Author * Email address: [email protected]; Tel: +86 731-88879616; Fax: +86 731- 88879616 REFERENCES (1) Jiang, Y.; Wei, M.; Feng, J.; Ma, Y.; Xiong, S. Enhancing the Cycling Stability of Na-ion Batteries by Bonding SnS2 Ultrafine Nanocrystals on Amino-functionalized Graphene Hybrid Nanosheets. Energy Environ. Sci. 2016, 9, 1430-1438. (2) Ma, C.; Xu, J.; Alvarado, J.; Qu, B.; Somerville, J.; Lee, J. Y.; Meng, Y. S. Investigating the Energy Storage Mechanism of SnS2-rGO Composite Anode for Advanced Na-Ion Batteries. Chem. Mater. 2015, 27, 5633-5640. (3) Qu, B.; Ma, C.; Ji, G.; Xu, C.; Xu, J.; Meng, Y. S.; Wang, T.; Lee, J. Y. Layered SnS2Reduced Graphene Oxide Composite - A High-Capacity, High-Rate, and Long-Cycle Life Sodium-Ion Battery Anode Material. Adv. Mater. 2014, 26, 3854-3859. (4) Zhou, T.; Pang, W. K.; Zhang, C.; Yang, J.; Chen, Z.; Liu, H. K.; Guo, Z. Enhanced Sodium-


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Ion Battery Performance by Structural Phase Transition from Two-Dimensional Hexagonal-SnS2 to Orthorhombic-SnS. Acs Nano 2014, 8, 8323-8333. (5) Broux, T.; Bamine, T.; Fauth, F.; Simone, L.; Olszewski, W.; Marini, C.; Menetrier, M.; Carlier, D.; Masquelier, C.; Croguennec, L. Strong Impact of the Oxygen Content in Na3V2(PO4)2F3-yOy (0

Rodlike Sb2Se3 Wrapped with Carbon: The Exploring of

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