Enhancing the Anode Performance of Antimony through Nitrogen

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Enhancing the Anode Performance of Antimony through Nitrogen-Doped Carbon and Carbon Nanotubes Xia Liu, Yichen Du, Xin Xu, Xiaosi Zhou, Zhihui Dai, and Jianchun Bao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11926 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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Enhancing the Anode Performance of Antimony through Nitrogen-Doped Carbon and Carbon Nanotubes Xia Liu, Yichen Du, Xin Xu, Xiaosi Zhou,* Zhihui Dai,* and Jianchun Bao Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China

Abstract: Antimony is a promising high-capacity anode material in sodium-ion batteries, but it generally shows poor cycling stability because of its large volume changes during sodium ion insertion and extraction processes. To alleviate or even overcome this problem, we develop a hybrid carbon encapsulation strategy to improve the anode performance of antimony through the combination of antimony/nitrogen-doped carbon (Sb/N-carbon) hybrid nanostructures and carbon nanotube (CNT) network. When evaluated as an anode material for sodium ion batteries, the as-synthesized Sb/N-carbon+CNTs composite exhibits superior cycling stability and rate performance in comparison with Sb/N-carbon or Sb/CNTs composite. A high charge capacity of 543 mA h g−1 with initial charge capacity retention of 87.7% are achieved after 200 cycles at a current density of 0.1 A g−1. Even under 10 A g−1, a reversible capacity of 258 mA h g−1 can be retained. The excellent sodium storage properties can be attributed to the formation of Sb−N bonding between antimony nanoparticle and the nitrogen-doped carbon shell in addition to the electronically conductive and flexible CNT network. The hybrid carbon encapsulation strategy is simple yet very effective, and it also provides new avenues for designing advanced anode materials for sodium-ion batteries.

1. Introduction

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Sodium-ion batteries (SIBs) have drawn increasing attention for large-scale energy storage applications as a promising alternative to lithium-ion batteries because sodium resources are ubiquitous around the world and are essentially inexhaustible.1−3 Recent studies on sodium-ion battery cathode materials have demonstrated properties comparable to their lithium-ion battery counterparts.4−13 The major challenge for high-energy-density SIBs therefore relies on developing a new anode material with a high specific capacity and a suitable redox potentials.14−16 Compared with various carbonaceous anodes,17−34 which show specific capacities typically less than 300 mA h g−1, group V elements have recently attracted great interest as potential high-capacity anode materials.35,36 In specific, phosphorus (P) and antimony (Sb) are two attractive candidates because of their high theoretical capacities of 2596 and 660 mA h g−1, respectively.37−43 Despite Sb has received less attention than phosphorus due to its higher cost, the increased interest in Sb could cause a decrease in its cost as a result of large-scale production. In addition, compared with P, Sb is superior for high-power SIBs because of its high electrical conductivity.44 However, the practical application of Sb in SIBs is still hampered by the poor cycle stability, similar to that of P.45−49 The main reason is that Sb experiences large volume changes during sodiation/desodiation that results in pulverization of Sb particles, exfoliation from the current collector, and the continuous production of a thick solid electrolyte interphase (SEI) on Sb surfaces, leading to a poor cycle life and rate performance.50−52 To circumvent this problem, great effort has been devoted to designing materials that can accommodate the volume change, including the use of nanosized Sb particles,44,53−55 the fabrication of Sb hollow microspheres,56 embedding in carbon nanofibers,45,46 and encapsulating with graphene sheets.57−60 Among the various

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approaches, Sb nanoparticles encapsulated with carbon coating are appealing in improving the electrochemical performance. On one hand, reducing the size of Sb particles to nanoscale can effectively alleviate the internal stress during sodium ion insertion and extraction processes, thus bringing about less particle pulverization and fracture. On the other hand, the carbon coating with appropriate kinetic properties for effective transport of sodium ions and electrons can significantly minimize the mechanical strain induced by the volume change of Sb. This strategy often results in improved capacity in initial several cycles but does not suggest a long cycle lifespan and high power density because of the weak interactions between Sb and carbon shells and sluggish electrolyte infiltration. It remains a big challenge to improve both the cycling stability and rate performance of Sb anode materials. Carbon nanotubes (CNTs) are a good candidate to load active materials for SIBs owing to their superior electrical conductivity, high flexibility, large surface area, and remarkable chemical stability. Recently, various nanocomposites connecting by carbon nanotubes have been reported as desirable electrode materials.61−63 Compared with famous graphene sheets, carbon nanotubes show the advantages of a smart electronic wire as well as serving as a more open and reliable support network for active materials during the charge−discharge processes, which is beneficial to the goal of high-rate capability and long-term stability. However, active materials wired by CNTs may undergo continual growth of SEI layer if they are directly exposed to the electrolyte during cycling. Thus, if a strategy can strongly bind active materials in carbon coating and then interweave the carbon-encapsulated active materials by CNTs, then the aforementioned three issues including weak interactions between active materials and carbon shells, slow sodium ion diffusion, and unstable SEI layer would be

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addressed and therefore excellent cyclability and rate capability can be achieved. Herein, we propose and design a hybrid carbon coating for the Sb anode through a ball-milling process of polyacrylonitrile (PAN), CNTs, and Sb powder and subsequent pyrolysis treatment. Sb nanoparticles are first coated by PAN-derived nitrogen-doped carbon (N-carbon) shell, which can not only strongly couple Sb nanoparticles through Sb−N bonding but also effectively avoid the direct contact between Sb nanoparticles and the electrolyte. Flexible CNT network is then employed for interweaving and electrically wiring the Sb/N-carbon core−shell composite. When measured as an anode material for SIBs, the hybrid carbon-coated Sb nanoparticles show outstanding sodium storage properties in terms of high capacity, long cycle life, and high rate performance. Moreover, with a simple synthetic procedure and excellent battery performance, this hybrid carbon coating strategy offers a highly promising candidate for commercialized anode materials of SIBs.

2. Experimental Section 2.1. Synthesis of the Sb/N-carbon+CNTs Composite In a typical synthesis, 800 mg of Sb powder and 200 mg of multiwalled CNTs were dispersed into 10 mL of 4.0 wt % PAN/N, N-dimethylformamide (DMF) solution using a ball-milling method (~800 rpm, 24 h), producing a homogeneous dispersion. Afterward, thin Sb/PAN+CNTs films were made by blade-casting on glass substrates pretreated using a piranha solution (sulfuric acid/hydrogen peroxide, 7:3). The moving speed of the manual blade coater was operated in the range of 40−60 cm min−1 to ensure surface smoothness. After drying under infrared light, the as-exfoliated films were collected using a pair of tweezers. Then, the films were placed in a crucible in a tube furnace, heated to 600 oC, and

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maintained at that temperature for 5 h under argon atmosphere with a heating rate of 5 oC min−1, forming the Sb/N-carbon+CNTs composite. For comparison, the Sb/N-carbon composite was prepared using the same process without addition of multiwalled CNTs. The Sb/CNTs composite was fabricated by the same procedure except for the use of PAN. 2.2. Materials Characterization Scanning electron microscopy (SEM) was conducted on a JEOL JSM-7600F scanning electron microscope operated at 10 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were performed on a JEOL JEM-2100F transmission electron microscope operated at 200 kV. Scanning transmission electron microscopy (STEM) meaurement as well as elemental mapping analysis were carried out on the JEOL JEM-2100F transmission electron microscope equipped with a Thermo Fisher Scientific energy-dispersive X-ray spectrometer. Nitrogen adsorption and desorption isotherms at 77.3 K were determined on an ASAP 2050 surface area-pore size analyzer. Thermogravimetric analysis (TGA) was measured on a NETZSCH STA 449 F3 in air atmosphere with a heating rate of 10 oC min−1 from room temperature to 900 oC. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALab250Xi electron spectrometer from VG Scientific using 300 W Al Kα radiation. 2.3. Electrochemical Measurements Electrochemical experiments were carried out using CR2032 coin cells. The working electrodes were fabricated by mixing Sb/N-carbon+CNTs, Sb/N-carbon, or Sb/CNTs with Super-P carbon black and carboxymethyl cellulose sodium with a weight ratio of 70:20:10 in water using a mortar and pestle. The resulting slurry was coated onto pure Cu foil (99.9 %,

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Goodfellow) and then dried in a vacuum oven at 40 oC for 12 h. The mass loading of active material is 1.0−1.2 mg cm−2. The electrolyte solution for all tests was 1 M NaClO4 in ethylene carbonate/diethyl carbonate/fluoroethylene carbonate (FEC) (1:1:0.1 v/v/v). Glass fiber (GF/D) from Whatman and sodium metal were used as separators and counter electrodes, respectively. The coin cells were assembled in an argon-filled glovebox (H2O, O2 < 0.1 ppm, MBraun, Germany). Galvanostatic charge−discharge tests of the batteries were performed on a Land CT2001A multichannel battery testing system at different current densities in the fixed voltage window between 0.01 and 2 V vs Na+/Na at room temperature. Electrochemical impedance spectroscopy (EIS) was acquired on a PARSTAT 4000 electrochemical workstation by applying a sine wave with an amplitude of 10.0 mV over the frequency range from 100 kHz to 100 mHz.

3. Results and Discussion Figure 1 illustrates the synthesis process for the Sb/N-carbon+CNTs composite. First, Sb particles and multiwalled CNTs, together with PAN, were dispersed into DMF assisted by a ball-milling method. The resulting viscous dispersion was readily spread out on the glass substrates to generate thin Sb/PAN+CNTs films (Supporting Information, Figure S1) by a facile blade-casting approach. Solvent evaporation from these thin films condensed the nonvolatile components upon drying, generating black composite sheets, which were collected using a pair of tweezers. Subsequent pyrolysis process resulted in the formation of Sb/N-carbon+CNT composite with robust and effective carbon matrix for sodium ion and electron transportation. Besides, the Sb/CNTs and Sb/N-carbon composites were prepared through the same procedure as those of Sb/N-carbon+CNTs except for the use of PAN and

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CNTs, respectively. More experimental details can be found in the Experimental Section.

Figure 1. Schematic illustration of the synthesis procedure for the Sb/N-carbon+CNTs composite. To study the morphology of these materials, we conducted SEM and TEM characterizations. SEM and TEM images show that most of Sb nanoparticles are agglomerated and uneven distributed within N-carbon and CNT network in the samples of Sb/N-carbon and Sb/CNTs (Figure 2a,b,d,e), respectively. By contrast, the sample of Sb/N-carbon+CNTs displays that the Sb nanoparticles are well covered by N-carbon with threading CNTs on the surface (Figure 2c). Additionally, the TEM image (Figure 2f) reveals that the Sb nanoparticles are homogeneously dispersed in the hybrid carbon matrix in the sample of Sb/N-carbon+CNTs. The interpenetration of CNTs in the hybrid of Sb nanoparticles and N-carbon should lead to a higher surface area if they form an interconnecting network. The HRTEM image (Supporting Information, Figure S2) show Sb nanocrystal is encapsulated by N-carbon and CNTs. As can be seen, the measured interlayer spacing of 0.34 and 0.23 nm in Figure S2 corresponds to the (002) plane of CNTs and (012) plane of Sb, respectively. The XRD patterns of Sb/CNTs and Sb/N-carbon+CNTs display highly crystalline Sb, with characteristic graphitic peaks of CNTs at 26.4o (Supporting

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Information, Figure S3). By contrast, no CNTs peak in the XRD pattern of Sb/N-carbon is found while the Sb peaks are still pronounced, implying the amorphous nature of N-carbon. The Brunauer−Emmett−Teller (BET) surface area decreases from 20.9 m2 g−1 in Sb/CNTs to 16.5 m2 g−1 in Sb/N-carbon+CNTs and 4.2 m2 g−1 in Sb/N-carbon (Supporting Information, Figure S4), while the Sb contents in these composites are close. The respective amount of Sb in Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs is ~73, 66, and 64 wt % through TGA (Supporting Information, Figure S5), based on the weight loss of carbon combustion and the weight increase of Sb2O4 formation.57 This indicates that the introduction of CNT network into the Sb/N-carbon composite does increase the surface area (Supporting Information, Figure S4), which are favorable for fast sodium ion transport. To confirm whether the Sb nanoparticles are homogeneously embedded within the hybrid carbon coating, the composition of the Sb/N-carbon+CNTs composite was analyzed by elemental mapping. The energy dispersive X-ray spectrometry (EDX) elemental mapping images (Figure 2g−j) indicate that Sb and N-carbon are uniformly combined with each other in the composite, guaranteeing a high electronic conductivity for the whole composite.

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Figure 2. (a, d) SEM and TEM images of Sb/CNTs. (b, e) SEM and TEM images of Sb/N-carbon. (c, f) SEM and TEM images of Sb/N-carbon+CNTs. (g) STEM image and corresponding (h) C, (i) N, and (j) Sb elemental mapping images of the Sb/N-carbon+CNTs composite. XPS measurements were carried out on N-carbon, Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs after their surface contaminants were removed (Supporting Information, Figure S6). The results show that apparent N peak can be observed in the N-carbon, Sb/N-carbon or Sb/N-carbon+CNTs but does not exist in the Sb/CNTs sample, suggesting that N-doping is originated from PAN-derived N-carbon. In order to investigate possible chemical bonding occurring between Sb and N-carbon, N-carbon, Sb/CNTs, Sb/N-carbon,

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and Sb/N-carbon+CNTs have been studied by high-resolution XPS spectra (Figures 3 and S7, Supporting Information).64 As displayed in Figure 3a, the high-resolution N 1s XPS spectra show pyridinic group (N2, C−N=C, 398.1 eV) and substitutional graphite group (N3, N bonded with three carbon atoms, 399.8 eV) in N-carbon.65 Compared to N-carbon, the pyridinic group in the high-resolution N 1s XPS spectrum of Sb/N-carbon+CNTs (Figure 3b) or Sb/N-carbon (Supporting Information, Figure S7) not only shifts to lower binding energy from 398.1 to 397.7 eV but also has a larger peak area than does the substitutional graphite group, suggesting that there may be a strong interaction between Sb and the pyridinic group of N-carbon. As shown in Figure 3c, the Sb/CNTs composite represents a Sb5+ 3d3/2 main line at 540.2 eV with a Sb 3d3/2 satellite peak at 537.8 eV.66 The presence of a main line together with a satellite peak results from air oxidation of Sb during the preparation process. When N-carbon is combined with Sb/CNTs through ball-milling and subsequent pyrolysis, the overall shape of the Sb 3d core peak is retained, as demonstrated in Figure 3d. However, two important modifications can be noticed: (i) a significant weakening of the main peak toward the lower binding energy and (ii) a complete disappear of the satellite peak. Upon N-carbon introduction the full width at half-maximum (fwhm) of the Sb5+ 3d3/2 main peak decreases from 1.31 to 1.27 eV and the relative satellite peak area decreases from 11.8% to 0 as well as the binding energy of the main peak shifts from 540.2 to 539.3 eV. These changes indicate that the Sb nanoparticles are tightly wrapped by the hybrid carbon coating and the polar Sb−N bonds are probably formed.67,68 To our knowledge, the precise binding energy positions and the relative intensity of the main peaks for Sb nanoparticles in N-doping carbon environment have not been reported. However, it is obvious that the significant increase of

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the relative pyridinic group area (from 42.4% to 50.9%) observed in Figure 3a,b, together with the remarkable weakening of the Sb 3d main peak and its peak shifts to a lower binding energy in Figures 3c,d, can be attributed to the formation of Sb−N bonding between Sb nanoparticles and the hybrid carbon coating. This chemical bonding enables N-carbon to strongly bond the ball-milling-formed Sb nanoparticles, and thus helps stabilize Sb and NaxSb intermediates during cycling.

Figure 3. (a, b) High-resolution N 1s XPS spectra of N-carbon and Sb/N-carbon+CNTs. (c, d) High-resolution Sb 3d XPS spectra of Sb/CNTs and Sb/N-carbon+CNTs. Figure 4a displays the first charge/discharge profiles of the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes. The onset slopes at around 0.98 V correspond to the production of SEI films. The discharge plateaus located at about 0.55 V are ascribed to the

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formation of NaxSb (0 ≤ x ≤ 3) alloys.42 The charge plateaus appeared at around 0.78 V relate to the phase transition from NaxSb to amorphous Sb. Figure 4b shows the cycling performances of the Sb/CNT, Sb/N-carbon, and Sb/N-carbon+CNTs composites tested at a current density of 0.1 A g−1 for 200 cycles. It is worth noting that the specific capacity is calculated on the basis of the total mass of Sb/CNTs, Sb/N-carbon, or Sb/N-carbon+CNTs. Under 0.1 A g−1, the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes show initial charge capacities of 574, 556, and 543 mA h g−1 and discharge capacities of 808, 780, and 837 mA h g−1, respectively, corresponding to respective Coulombic efficiency of 71.0%, 71.3%, and 64.9%. The initial capacity loss is mainly attributed to the reductive decomposition of the electrolyte and the generation of SEI layer as well as irreversible insertion of sodium into Sb nanoparticles and carbon matrix. The relatively poor initial Coulombic efficiency of the Sb/N-carbon+CNTs electrode is due to the relatively low content of Sb (Supporting Information, Figure S5). After 100 cycles, reversible capacities of 206, 463, and 501 mA h g−1 are maintained for the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes, respectively. In comparison with the initial cycle, the capacity retention in the 100th cycle for these electrodes are ordered as follows: Sb/N-carbon+CNTs (92.3%) > Sb/N-carbon (83.2%) > Sb/CNTs (35.9%). After 200 cycles, reversible capacities of 108, 408, and 476 mA h g−1 are still retained for Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs, respectively, corresponding to capacity retentions of 18.8%, 73.4%, and 87.7% of their initial capacities. It is noted that the Sb/N-carbon+CNTs composite manifests excellent cycling stability in comparison with many other Sb-based anode materials (Supporting Information, Table S1). Although the relatively low Coulombic efficiencies for Sb/CNT, Sb/N-carbon, and

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Sb/N-carbon+CNTs in the first cycle, all electrodes exhibit much higher average Coulombic efficiencies

of

96.2%

for

Sb/CNTs,

96.4%

for

Sb/N-carbon,

and

97.2%

for

Sb/N-carbon+CNTs over 200 cycles (Supporting Information, Figure S8). Importantly, the synthetic procedure for fabricating Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs are identical, indicating that binding force is effective between Sb nanoparticles and CNTs or N-carbon, which leads to different electronic transport stability during sodium ion uptake and release processes, and must play an essential role in the final cycling performance of the Sb/CNTs, Sb/N-carbon, or Sb/N-carbon+CNTs electrode.

Figure 4. (a) Galvanostatic charge−discharge profiles for the first cycles of the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes at a current density of 0.1 A g−1. (b) Cycling performances of the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes under 0.1 A

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g−1. (c) Rate capabilities of the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes. To further evaluate the electrode kinetics and stability, we tested the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes at various current densities. Figure 4c shows the rate capability performances of the electrodes. When first discharged these electrodes at a current density of 0.1 A g−1 for 10 cycles, the electrodes made from Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs exhibit reversible capacities of 555, 550, and 533 mA h g−1, respectively. Subsequent cell testing at 0.2, 0.5, 1, 2, 5, and 10 A g−1 (each current density for 10 cycles) clearly shows that the rate capabilities of the electrodes decrease in the following

order:

Sb/N-carbon+CNTs

>

Sb/N-carbon

>

Sb/CNTs,

implying

that

Sb/N-carbon+CNTs possesses the fastest reaction kinetics. Specifically, even under 10 A g−1, the Sb/N-carbon+CNTs electrode could still deliver a charge capacity of 258 mA h g−1, which greatly outperforms the corresponding values of 87 mA h g−1 for the Sb/CNTs counterpart and 91 mA h g−1 for the Sb/CNTs counterpart. This is also supported by the cycling performance of these electrodes at 0.1 A g−1 (Figure 4b). It is worth mentioning that 53%, 84%, and 95% of the former capacities were, respectively, recovered for the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs electrodes when the current density was suddenly switched from 10 to 0.1 A g−1 (Figure 4c), indicating the superior structural stability of the Sb/N-carbon+CNTs composite. To reveal the reason for the improved electrochemical performance of the Sb/N-carbon+CNT electrode, the tested cell was disassembled after 200 cycles in fully desodiation condition, and the structure of the composite was investigated by elemental mapping (Figure 5a−e). Integrated with the STEM image, the elemental mappings display

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that pulverization of larger Sb nanoparticles occurred during cycling and the resultant Sb nanoparticles are uniformly dispersed in the hybrid carbon matrix consisting of PAN-derived N-doped carbon and CNTs, indicating that the strong interaction between Sb and N-doped carbon coating can effectively avoid the agglomeration of Sb formed during sodiation/desodiation, which is consistent with the superior cycling stability (Figure 4b). Moreover, we conducted electrochemical impedance spectroscopy (EIS) measurements of the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs cells after 5 cycles (Figure 5f). Apparently, the diameter of the semicircle for the Sb/N-carbon+CNTs cell in the high-medium frequency region is smaller than that for the Sb/CNTs or Sb/N-carbon cell, suggesting that the combined resistance Rf+ct composed of the SEI resistance (Rf) and the charge transfer resistance (Rct) of Sb/N-carbon+CNTs is lower than that of Sb/CNTs or Sb/N-carbon. Furthermore, the slope of the line for the Sb/N-carbon+CNTs cell in the low frequency region is higher than that for the Sb/CNTs or Sb/N-carbon cell, implying that Sb/N-carbon+CNTs possesses a smaller solid-state diffusion resistance. The lower SEI and charge transfer as well as solid-state diffusion resistances of Sb/N-carbon+CNTs indicate its more stable SEI film and better electronic and ionic transportation, which are responsible for the outstanding sodium storage properties.

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Figure 5. (a) STEM image and (b−e) corresponding C, N, Sb, and Na elemental mapping images of the cycled Sb/N-carbon+CNTs electrode material. (f) Nyquist plots of the Sb/CNTs, Sb/N-carbon, and Sb/N-carbon+CNTs cells after 5 cycles. The enhanced cycling stability and rate performance of the Sb/N-carbon+CNTs electrode can be attributed to the following aspects. First, the as-prepared Sb/N-carbon+CNTs has Sb−N bonding between Sb and N-doped carbon coating that effectively binds the encapsulated Sb nanoparticles. This unique composite does not only help provide fast and efficient electronic and ionic transport, but also well buffer the large volume variation of Sb during sodium ion insertion/extraction processes. Second, based on the rate capability result, the uniform distribution of Sb nanoparticles in the hybrid carbon matrix can strongly increase the physical connection and electrical contact of Sb with the hybrid conductive network, thus boosting the effective electrochemical utilization of Sb and guaranteeing the reversible sodium uptake/release processes even under high current densities. Third, the tight hybrid

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carbon encapsulation is beneficial to generate a stable SEI film and prevents the electrolyte from further decomposition while still allows for sodium ion transport into the embedded Sb.

4. Conclusions In summary, we have reported a Sb/N-carbon+CNTs composite via a facile ball-milling method, leading to a hybrid carbon encapsulation for Sb nanoparticles through the PAN-derived carbon shell and the interspersed electronically conductive and flexible CNT network. When evaluated as an anode material for SIBs, the Sb/N-carbon+CNTs composite shows high specific capacity, rate capability and cycle life as compared to the Sb/CNTs or Sb/N-carbon composite, which arises from the hybrid carbon coating with strong interaction between Sb and N-doped carbon and the electronically conductive and flexible CNT network. Such a hybrid carbon coating strategy could render an effective and versatile route to enhance the electrochemical properties of high-capacity electrode materials with huge volume changes and poor electrical conductivities in SIBs.

Supporting Information Photograph of the Sb/PAN+CNTs film; HRTEM image, XRD patterns, nitrogen adsorption/desorption isotherms, TGA curves, XPS survey scans, high-resolution N 1s XPS spectum,

and

Coulombic efficiencies

of

N-carbon,

Sb/CNTs,

Sb/N-carbon,

and

Sb/N-carbon+CNTs. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Authors *(X.Z.) E-mail: [email protected]. Telephone/Fax: +86-25-85891027.

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*(Z.D.) E-mail: [email protected]. Telephone/Fax: +86-25-85891051.

Notes The authors declare no competing financial interest.

Acknowledgements. This work was supported by the National Natural Science Foundation of China (Grant Nos. 21503112, 51577094, and 21475062), the Natural Science Foundation of Jiangsu Province of China (BK20140915), the Scientific Research Foundation for Advanced Talents of Nanjing Normal University (2014103XGQ0073), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Program of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials.

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