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Letter

Na-Storage Capability Investigation of Carbon Nanotubes-Encapsulated Fe S Composite 1-x

Ying Xiao, Jang-Yeon Hwang, Ilias Belharouak, and Yang-Kook Sun ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00660 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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Na-Storage Capability Investigation of Carbon Nanotubes-Encapsulated Fe1−xS Composite ║

Ying Xiao†, Jang-Yeon Hwang†, Illias Belharouak ‡, and Yang-Kook Sun†,* †

Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea

‡

Qatar Environment and Energy Research Institute (QEERI), Hamad bin Khalifa University

(HBKU), Qatar Foundation, Doha, Qatar. ║

College of Science and Engineering, Hamad bin Khalifa University (HBKU), Doha, Qatar

E-mail: [email protected] ABSTRACT: A promising anode material consisting of Fe1−xS nanoparticles and bamboo-like carbon nanotubes (CNTs) has been designed and prepared by an effective in situ chemical transformation. The resultant Fe1−xS@CNTs with three-dimensional network can not only provide high conductivity paths and channels for electrons and ions, but also offer the combined merits of iron sulfide and CNTs in electrochemical energy storage applications, leading to an outstanding performance as an anode material for sodium-ion batteries. When tested in half-cell, a high capacity of 449.2 mAh g−1 can be retained after 200 cycles at 500 mA g−1, corresponding to a high retention of 97.4%. Even at 8000 mA g−1, the satisfactory capacity of 326.3 mAh g−1 can be delieved. While tested in the full-cell, a capacity of 438.5 mAh g-1 with capacity retention of 85.0% is manifested after 80 cycles based on the mass of anode. The appealing structure and electrochemical performance of this material demonstrate its great promise applications in practical rechargeable batteries. 750

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Owing to the long life span and high energy density, lithium-ion batteries (LIBs) have received a worldwide application in our daily life for powering the portable devices and burdening electric vehicles (EVs) and hybrid EVs.1-3 The rapid development of society and ever-increasing market demands drive the exploring of low-cost batteries with high energy capacity, long life and high power density. Therefore, on the one hand, much effort has been made to search appropriate anodes to substitute commercial-used graphite. On the other hand, as a potential alternative to the prevailing LIBs, sodium-ion batteries (SIBs) have received significant attention in recent years owing to their greater safety as well as the greater natural abundance of sodium.4-6 However, SIBs still face challenges that prevent their widespread practical application. The major obstacle for SIBs, and especially sodium-ion full-cells, is the deficiency of suitable electrode materials for reversible and rapid Na+ insertion and extraction. Until now, various anode materials have been shown to be reversible uptake and removal of sodium, and thus have exhibited promise for sodium-storage applications; these materials include carbon materials such as graphene1 and Ndoped carbon;7 materials based upon alloys of Sn,8,9 Sb,10 and P;5,11 metal oxides such as SnO2,12 TiO2,13 NaTiO2,14 CuO,15 and Fe3O4;16 and some organic anodes etc.17-18 Nevertheless, the low specific capacity of carbon materials and organic materials, coupled with the huge volume changes and slow sodium-ion diffusion kinetics within alloy-based materials and metal oxides, has led to unsatisfactory specific capacity and rate capability, hindering the full utilization of these materials. Thus, designing low-cost electrode materials with the desired high performance is still a challenge and represents a key factor in the development and practical application of SIBs. Metal sulfides, identified as greatly promising anodes for SIBs, have attracted tremendous interest recently owing to their rich earth reserve, intrinsic safety, and large theoretical capacity resulting from the conversion reaction mechanism.19,20 Compared to their reduced metal and 2


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metal oxide counterparts, metal sulfides are expected to exhibit improved mechanical stability benefiting from their smaller volume change and higher first-cycle efficiency endowed by their greater reversibility,21,22 and thus show more promise for practical applications. Among all reported metal sulfides, iron sulfides manifest appealing properties including cost-effectiveness, natural abundance, low toxicity and high theoretical capacity.23,24 However, the iron sulfides have also shown inferior electrochemical properties owing to their low conductivity, sluggish kinetics, possible dissolution of sulfur and severe volume changes during repeated cycling.23 Thus, the design of effective strategies to achieve high capacity and rate capability is imperative for practical application. Because of their high conductivity, carbon nanotubes have been frequently considered as a matrix material for improving the electrochemical performance of various materials,25-27 and previous works have also demonstrated the effect of CNTs in stabilizing active components by partially preventing dissolution into the electrolyte.28,29 Until now, however, there are seldom reports related to CNT/iron sulfide composites for battery applications. Inspired by the abovementioned works and considering the drawbacks of iron sulfides applied in rechargeable batteries, in the present work, Fe1−xS nanoparticles encapsulated within carbon nanotubes were designed and prepared for the first time through a facile in-situ chemical transformation. The as-prepared composite comprised small Fe1−xS nanoparticles encapsulated in bent CNTs that formed a three-dimensional (3D) network. The electrochemical behaviors of the designed material were investigated in detail when used as SIB anode materials. The CNT encapsulants offer a strong buffer effect to mitigating the large volume change upon cycling, accommodate the resultant sulfur-based species, and also improve the electroconductivity of the whole electrode. The small size of the encapsulated Fe1−xS particles increases the electrode/electrolyte contact area and shortens the diffusion paths for electrons and ions. The generated 3D network provides effective conductive paths for rapid transportation of electrons. 3


ACS Energy Letters

These structural properties not only lead to the remarkable electrochemical performance of the Fe1−xS@CNTs composite in terms of high reversible capacity, long cycling stability, and good rate capability in SIBs, but also confer the resulting electrode with outstanding Li-storage capability. Thus, the proposed material exhibits significant potential for use as a highperformance anode material in rechargeable batteries. RESULT AND DISCUSSION

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(f)

Fe 2p Fe

Fe

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2-

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Fe1-xS@CNTs

Intensity (a.u.)

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(d)

Precursor

1375.8 1345.0

Fe 3p

(c)

Precursor Fe1-xS@CNTs

Intensity (a.u.)

(b)

C 1s

(a)

Intensity (a.u.)

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C 1s

C-N-H N-H

C-N

404

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Bind energy (eV)

Figure 1. (a) Schematic illustration of the formation process of Fe1−xS@CNTs. (b) XRD patterns and (c) Raman spectra of precursor and Fe1−xS@CNTs composite. (d) Survey XPS spectra of Fe1-xS@CNTs composite, (e) High-resolution Fe 2p XPS spectrum, (f) S 2p XPS spectrum and (g) C 1s XPS spectrum combined with N 1s XPS spectrum (inset).

Figure 1a schematically illustrates the formation process of Fe1-xS@CNTs composite. Our strategy was to generate iron sulfides within CNTs by means of an in-situ solid-state approach. In 4


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the first step, a nitrogen-containing organic material (melamine) was homogeneously mixed with FeCl3 through stirring at low temperature, followed by mechanical grinding. A Fe-based precursor was then generated by pyrolyzing the abovementioned mixture under an inert atmosphere. Some C/N species liberated from the decomposition of melamine cracked on the surface of Fe-catalysis derived from FeCl3 decomposition and diffused through Fe particles to form a Fe3C/Fe@CNTs composite, subjecting to a vapor–liquid–solid (VSL) mechanism.30-33 During the pyrolysis process, numerous pores were generated owing to the decomposition of organic materials; this was demonstrated by the high surface area (269.5 m2 g−1) and high pore volume (0.674 cm−3 g−1) of the resulting material (Figure S1). Then, a facile chemical conversion was carried out to in-situ transform the Fe3C/Fe@CNTs precursor to a Fe1−xS@CNTs composite; the ease of this process benefited from the porous structure of the Fe3C/Fe@CNTs composite. Figure 1b shows X-ray diffraction (XRD) patterns of the precursor and annealed products. Almost all the XRD peaks of the annealed sample can be well assigned to pyrrhotite Fe1−xS (JCPDS Card No. 29-726) with hexagonal phase, indicating the successful transformation of Fe3C/Fe to iron sulfide through the sulfidation treatment. A small peak at ca. 44.6° was assigned to residual Fe. However, no distinct peaks belonging to CNTs could be detected in these XRD patterns, which may be caused by the relatively strong intensity of peaks related to iron sulfide. Raman analysis was carried out to characterize the presented CNTs in the composite. As seen in Figure 1c, Raman spectra of both the precursor and the Fe1−xS@CNTs composite displayed similar peaks in the range of 500–3000 cm−1. The peaks at 1345.0 and 1375.8 cm−1 correspond to characteristic D band associated with defects and G band related to graphitic, respectively. Relatively small peaks at ca. 2681.4 cm−1 were assigned to the 2D band of graphite.30,31,33,34 The Fe1−xS@CNTs composite exhibit a higher IG/ID ratio (1.24) than that of its precursor (1.01), indicating high and enhanced graphitic degree of the sulfide composite, which will be beneficial 5


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for enhancing electric conductivity and promoting charge transfer during electrochemical reactions. The carbon content in the Fe1−xS@CNTs composite was calculated to be 29.6 wt% according to TG analysis (Figure S2), which is similar to the element analysis results (27.5 wt%). Besides, X-ray photoelectron spectra (XPS) technique was performed to further investigate the surface composition of the Fe1−xS@CNTs composite. As displayed in Figure 1d, the survey spectrum clearly indicated the presence of Fe, S, C, and O elements in the composite. Peaks observed at ca. 707.2 eV in the high-resolution Fe 2p XPS spectrum were attributed to metallic Fe (3.5 at%), whereas peaks centered at ca. 713.3 (23.8 at%) and 726.5 eV (10.8 at%) combined with satellite peaks at ca. 719.5 (10.9 at%) and 731.5 eV (3.6 at%) indicated the presence of Fe2+ in the composite (Figure 1e). Other peaks at ca. 710.9 (30.8 at%) and 724.3 eV (16.6 at%) were related to Fe3+.35-37 In the high-resolution S 2p XPS spectrum, five peaks at ca. 161.4, 162.6, 163.6, 164.8, 168.6 and 169.7 eV were detected (Figure 1f). The peaks at ca. 161.4, 162.6, 163.6, 164.8 eV corresponded to S 2p, whereas the other two peaks at 168.6 and 169.7 eV corresponded to oxidized groups (SOx).38,39 Based on the literature, these results verify the successful formation of Fe1−xS species along with a negligent amount of Fe in the designed iron sulfide@CNTs composite. Additionally, high-resolution C 1s and N 1s spectra (Figures 1g) confirmed the presence of N-doped carbon,40 which will provide more active sites for electrochemical reactions.30,31,40,41 Scanning electron microscopy (SEM) images shown in Figure 2a indicated that the Fe1−xS@CNTs composite had a hollow bamboo-like structure with a diameter of 20–50 nm, similar to the precursor material (Figure S3). Fe1−xS nanoparticles with good dispersion can be obviously detected within CNTs not limited to the end of CNTs (Figure S4), providing evidence that the composite was formed by means of a VSL mechanism. To gain further insight into the microstructure of the designed product, transmission electron microscopy (TEM) analysis was 6


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ACS Energy Letters

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fe

S

C

Figure 2. (a) SEM image of Fe1−xS@CNTs composite. (b-e) TEM images of Fe1−xS@CNTs composite at various magnifications. (f,g) HRTEM images of Fe1−xS@CNTs composite, selected from different areas. (h) STEM image and corresponding element mapping images of Fe1−xS@CNTs composite.

performed. As shown in Figures 2b-e, after chemcial transformation, the Fe1−xS@CNTs retained the same morphology as their precursor, displaying a 3D connected network. The magnified images clearly demonstrated that the Fe1−xS nanoparticles were highly dispersed throughout and well encapsulated in the bamboo-like CNTs. The lattice fringes of 0.173 nm and 0.208 nm detected in HRTEM images (Figures 2f,g), respectively corresponding to the (220) and (2022) crystal planes of Fe1−xS and revealing the crystalline property of the Fe1−xS nanoparticles. STEM images and elements mapping shown in Figure 2h confirmed the distribution of Fe1−xS in the carbon nanotubes.

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Additionally, N2 adsorption/desorption isotherms of the Fe1−xS@CNTs products indicated that the mesoporous characteristics of the resultant materials (Figure S5). Compared with those of the precursor, the Brunauer–Emmett–Teller (BET) surface area and pore volume of the composite product were greatly decreased from 269.5 m2 g−1 and 0.674 cm−3 g−1 to 126.2 m2 g−1 and 0.365 cm3 g−1, respectively. These reductions were ascribed to the successful introduction of sulfur into the CNT structure, suggesting the effectiveness of chemical transformation. The high surface area endowed the Fe1−xS@CNTs composite with a large electrode/electrolyte contact area, whereas the composite’s porosity was expected to facilitate the transport of ions and the penetration of electrolyte molecules as well as to buffer volume changes during repeated cycling.21,42 To evaluate the electrochemical performance of the Fe1−xS@CNTs as an anode material for SIBs, a coin-type half-cell was first prepared and tested. Figure 3a shows charge/discharge curves of the Fe1−xS@CNTs composite electrode at 500 mA g−1 (the first cycle was performed at 200 mA g−1) collected over the voltage range of 0.01–2.3 V. These profiles are similar to those of FeS-based materials.21 The initial discharge and charge capacities were 637.7 and 478.7 mAh g−1, respectively, corresponding to a Coulombic efficiency of 75.1%. The first-cycle capacity loss was mainly attributed to the formation of a solid–electrolyte interface (SEI) film caused by the reversible reactions and electrolyte decomposition.19,42 Figure 3b shows the corresponding cycle performance of the Fe1−xS@CNTs composite electrode. The Fe1−xS@CNTs composite electrode manifested excellent stability and high reversibility. A capacity of up to 449.2 mAh g−1 was delivered after 200 cycles, representing the high capacity retention of 97.4% relative to the capacity value of the third cycles. Such value is much higher than those of most previous iron sulfide–based anodes in SIBs.21,43-45 By contrast, pure CNTs without iron sulfides tested under the same conditions exhibited the low capacity of 75.4 mAh g−1 after 100 cycles (Figure S6), demonstrating the prominent contribution of Fe1−xS in the composite electrode. Additionally, the 8


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ACS Energy Letters

Voltage (V vs. Na /Na)

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Figure 3. Electrochemical performance of as-obtained Fe1−xS@CNTs samples in SIBs. (a) Charge/discharge profiles acquired at 500 mA g−1. (b) Cycle performance and Coulombic efficiency of Fe1−xS@CNTs composite at 500 mA g−1. (c,d) Cycling performance and corresponding charge/discharge profiles of Fe1−xS@CNTs composite at various current densities. (e) Model and (f) cycling performance of the designed Fe1−xS@CNTs/Na[Ni0.61Co0.12Mn0.67]O2 full cell.

Fe1−xS@CNTs composite electrode exhibited remarkable rate capability when tested at various current densities ranging from 200 to 8000 mA g−1. The Fe1−xS@CNTs composite electrode exhibited good capacity retention with increasing the current density (Figures 3c and d); specifically, the average capacities of 492.7, 442.7, 438.2, 409.6, 365.3, and 326.3 mAh g−1 were 9


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retained at 200, 800, 1000, 2000, 5000, and 8000 mA g−1, respectively. Even after testing at 8000 mA g−1, high reversible capacity similar to the initial value could be achieved after returning the current density to 200 mA g−1. Long-term testing at 1000 mA g−1 (Figure S7) verified the Fe1−xS@CNTs composite’s excellent cyclic stability upon prolonged cycling, confirming its significant rate capability. Besides, TEM examination for the cycled electrode (Figure S8) reveal that the sample could retain encapusulated morphology after cycling, and no nanoparticles fall off the composite, indicating the superior structural stability. In our previous work,46 Na[Ni0.61Co0.12Mn0.27]O2 having a spoke-like nano-assembled (SNA) structure was prepared that displayed high capacity, and good physical and thermal stability, demonstrating promise as a cathode material for SIBs. Considering the outstanding performance of the as-prepared Fe1−xS@CNTs composite, a full cell constructed with Fe1−xS@CNTs anode and a Na[Ni0.61Co0.12Mn0.27]O2

cathode

was

fabricated

and

tested

at

0.5

C-rate.

The

Fe1−xS@CNTs/Na[Ni0.61Co0.12Mn0.27]O2 full cell was determined to deliver the high initial capacity of 123 mAh g−1 at 0.5 C-rate based on the mass of the cathode, and exhibited stable cycle retention of 85% after 80 cycles (Figures 3e, 3f and S9). According to the mass of the anode, a stable capacity of 438.5 mAh g−1 could be retained (Figure S10), indicating the promise of our materials for practical application. Compared with previously reported iron sulfides-based anodes, the present material manifested great competitive in SIBs application (Table 1). Table 1. Comparison of the Na-storage performance with reported FeS-based materials. Types of materials

Steps in

a

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density

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b

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voltage

efficiency

retention

−1

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method

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FeS@C

6

100

300

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67.6%

21

FeS-RGO

2

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50

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71.6%

103.2%

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FeS@C/C cloth

2

91 (730)

50 (200)

0.05–3.0

~64.1%

86.0% (46.9%)

43

Fe1−xS@CNTs

2

500 (1000)

200 (300)

0.01–2.3

75.1%

97.4% (67.4%)

This work

500

100

0.01–3.0

79.0%

124.1%

This work

10


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Note: The current density values in brackets in column a correspond to the cycle number in brackets in column b and the capacity retention in brackets in column d. The cycling performance of Fe1−xS@CNTs composite tested at 0.0.1– 3.0 V is shown in Figure S11.

To investigate the electrochemical Na-storage mechanism of the as-prepared novel electrode, ex-situ XRD was performed. Based on the charge/discharge profiles and dQ dV−1 curves, various potentials for the first and 30th cycles were selected as the final cutoff voltages. Figure 4a shows representative dQ dV−1 curves of the Fe1−xS@CNTs. In the first cycle, an obvious cathodic peak appeared at ca. 0.85 V, which may be related to the formation of SEI film; when the cell was discharged to 0.01 V, a sodiation reaction occurred to form Fe, Na2S, and Na-rich phases.21,48 As for the initial charge process, peaks centered at ca. 1.35 V and ca.1.80 V corresponded to the

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⊗

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formation of NaaFe1−xSb and Nay-aFe1−xSb, respectively. The Na storage mechanism was

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& Na2CO3

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ACS Energy Letters

C1.1 C1.4

1st

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Figure 4. Electrochemical mechanism investigation of as-obtained Fe1−xS@CNTs composite in SIBs. (a) dQ dV−1 curves of Fe1−xS@CNTs composite. Ex-situ XRD patterns of Fe1−xS@CNTs electrodes in various (b) discharged states and (c) charged states for the first cycle. (d) Ex-situ XRD patterns of Fe1−xS@CNTs electrodes in various charged states after the 30th cycle.

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investigated in further detail by means of ex-situ XRD. All diffraction peaks related to Fe1−xS disappeared between D0.68 V and D0.01 V (Figure 4b). Several peaks between 31.5° and 40.0° could not be clearly assigned to a single phase that include Na2S and NaaFe1−xSb (Na3FeS3 and Na2FeS2). These results were similar to those reported previously for metal sulfide materials, suggesting a conversion reaction mechanism. After charging at ca. 1.8 V (Figure 4c), no sharp peaks were observed; two wide ones at ca. 25.0° and 39.1° corresponded to Na2CO3 and Na2S, respectively. In the fully charged state (2.3 V), three peaks were observed at ca. 27.7°, 29.0°, and 29.6°, ascribed to Na2CO3. Throughout the whole process, the Fe peaks remained closely similar to the original one (Figure S12), which may have been due to the ultrasmall crystal size or the amorphous nature of the Fe particles generated from the conversion reaction.21 Besides, the XRD patterns for the charged and discharged states after the 30th cycle were similar to those of 1st. Based on the above results and the related literature, the Na-storage mechanism of the composite can be expressed as follows: Fe S + 2Na + 2e → Na S + Fe

(1)

Na S + Fe ↔ Na Fe S + 2Na + 2e (a,b=2 or 3)

(2)

Na Fe S ↔ Na Fe S + a − yNa + a − ye

(3)

Additionally, up to now, there has been just one prior literature regarding the use of Fe1−xS as an anode material in LIBs. Thus, in order to further verify the promising application of our designed material in enenrgy storage areas, the electrochemical performance of Fe1−xS@CNTs for Li-storage was also investigated. Cyclic voltammetry (CV) was carried out firstly to evaluate the related electrochemical performance. Figure S13a shows typical CV curves for the Fe1−xS@CNTs. During the first cathodic process, one peak at ca.1.97 V corresponded to the reaction between Fe1−xS and Li to form Li2Fe1−xS2; a sharp peak at ca. 1.22 V was related to the transformation between Fe1−xS and Fe; and a peak at 0.72 V was assigned to the formation of SEI 12


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film, which gradually disappeared with cycling.49 During the anodic sweep process, two peaks were detected at ca. 1.95 and 2.03 V, which were ascribed to the oxidation of Fe into Li2−yFe1−xS2. In the following cycles, a peak observed at ca. 1.38 V during the discharging process represented the formation of Li2Fe1−xS2, whereas a peak observed at 1.93 V during the charging process was related to the transformation from Li2Fe1−xS2 to Li2−yFe1−xS2.49,50 The peaks observed during the fifth and tenth cycles overlapped well, implying its good electrochemical reversibility. On the basis of the CV analysis and the previously reported storage mechanism of Fe1−xS and FeS,43,48,49 the electrochemical reactions occurring during the charge/discharge process for the current material can be expressed as follows: Fe S + 2Li + 2e → Li Fe S + Fe

(4)

Fe S + 2Li + 2e → Li S + Fe

(5)

Li S + Fe − yLi − ye ↔ Li Fe S

(6)

Figure S13b displays typical charge/discharge profiles of Fe1−xS@CNTs measured in the voltage window of 0.01-3.0 V. In the first discharge curve, a long voltage plateau was observed between 1.35 and 1.20 V, in addition to a small slope at ca. 0.72 V, corresponding to the formation of Fe, Li2S, and Li-rich phases as well as SEI, in a manner consistent with the CV results. The first discharge and charge capacities were respectively 1080.8 and 788.3 mAh g−1 based on the total mass of the composite electrode, corresponding to a high initial Coulombic efficiency of 73.0%. The irreversible capacity loss was probably caused by the formation of SEI films and the irreversible decomposition of the electrolyte. Figure S13c depicts the cycle performance of Fe1−xS@CNTs at 500 mA g−1. The capacity of ca. 1335.0 mAh g−1 was retained after 200 cycles, batter than the pure CNTs (Figure S14) and most of the previously reported iron sulfides (Table S1), indicating the material’s excellent cycle stability and promise for application to LIBs. Upon the cycling, reversible capacity increasing can be clearly detected, common for 13


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transition metal-based materials. According to the literature, the reason maybe ascribes to the progressive formation of electrochemistry active polymeric gel-like films, the interfacial Listorage, or the generation of LiOH and its subsequent transformation to Li2O and LiH.51-53 Additionally, related cycling performance tests conducted at various current densities confirmed the outstanding rate capability of the Fe1−xS@CNTs as an anode material for LIBs (Figures S15). The TEM characterization for cycled electrode indicated its good structural stability (Figure S16). The related electrochemical testing and analysis results reveal the superior performance of the present material in rechargeable batteries. The reasons for the material’s remarkable electrochemical performance including high capacity, excellent rate capability and significant capacity retention may be ascribed to its novel structure and the synergistic effect between the Fe1−xS particles and the high-conductivity elastic carbon coating. The material’s hollow structure provides void space that can alleviate the large volume variations of Fe1−xS during the sodiation– desodiation/intercalation–deintercalation processes;21,54,55 the small Fe1−xS particles can effectively shorten the diffusion paths for electrons and ions, coupled with providing large electrode/electrolyte contact areas;42,56 and the porous carbon shells improve the conductivity of the electrode and facilitate the diffusion of the electrolyte.57,58 All these aspects contributed to the remarkable Na/Li storage performance of the Fe1−xS@CNTs. CONCLUSIONS In summary, Fe1−xS@CNTs composite was prepared by a facile chemical transformation from Fe3C/Fe to sulfides. The resulting product featured a three-dimensional network with porous and hollow structure, endowing it with excellent cycling stability and rate capability as an anode material. When included in a full cell, the Fe1−xS@CNTs composite enabled high capacity of up to 123.0 mAh g−1 based on cathode mass and capacity retention of 85.0%, demonstrating its great promise for practical application. The combined effects of the highly conductive connected 14


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network, the volume buffering effect of its hollow structure and carbon encapsulation, and the shortened transport paths provided by its numerous pores conferred the resultant cell display excellent performance. The proposed simple and effective synthesis strategy in the present work could also be used to fabricate other electrode materials for highly efficient batteries. METHODS Synthesis of Fe3C/Fe@CNTs composite In a typical procedure, 2 g of iron trichloride and 10 g of melamine were dissolved in deionized water and stirred at 80 °C overnight. The resulting precursor was collected and ground to form a homogeneous powder. The Fe-based presursor was obtained after calcining the resulted homogeneous powder at 900 °C for 4 h at Ar atmorspheres. Synthesis of Fe1−xS@CNTs composite Fe1−xS@CNTs composite was prepared by mixing the Fe3C/Fe@CNTs precursor with sulfur in the weight ratio of 1:1, followed by annealing at 650 °C for 2 h under Ar atmosphere. As a comparison material, pure CNTs were obtained by immersing Fe1−xS@CNTs in aqua regia for two weeks. Characterizations Samples’ phase and composition were characterized by means of X-ray diffraction (Rint-2000, Rigaku) using a Cu Kα radiation source. The morphology and microstructure of the samples were revealed by means of SEM (JEOL JSM 6400) and TEM (JEOL 2010) analyses. Scanning transmission electron microscope (STEM) mapping images were acquired using an FEI Technai G2 F20 instrument. To further characterize the chemical composition of the hybrid, X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 spectrometer (Perkin-Elmer). Raman spectra were recorded on an Invia Raman spectrometer. The Brunauer–Emmett–Teller

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surface areas of the as-synthesized samples were measured using a Quantachrom Autosorb-1 instrument. Electrochemical measurements A mixture of 80 wt% of the active material, 10 wt% of carbon black (Super P), and 10 wt% of sodium carboxymethyl cellulose (CMC) binder (added as a 1 wt% CMC solution) were fully mixed by using a mortar. The resulting homogeneous slurry was cast onto Cu foil by using an applicator and was then dried overnight at 80 °C in a vacuum glass oven. A coin cell (CR2032) with Na metal as the cathode was used to study the electrochemical performance of the samples. During the assembly process, a 1 M solution of NaClO4 in ethylene carbonate (EC) and polycarbonate (PC) (1:1, vol%) with 5 wt% of FEC served as an electrolyte for sodium-ion batteries; a 1 M solution of LiPF6 in ethylene carbonate (EC) and polycarbonate (DMC) (1:1, vol%) with 5 wt% of FEC served as an electrolyte for lithium-ion batteries. The mass loading of the active materials was around 2.0 mg. The capacity of the electrode was calculated based on the total weight of the samples. ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website. the N2 adsorption-desorption isotherms of the precursor; TG curve of Fe1-xS@CNTs and the corresponding description; SEM and TEM images of the precursor; the N2 adsorption-desorption isotherms of the Fe1-xS@CNTs; Characterizations of the CNTs: including XRD pattern (along with some statement), SEM and TEM images, cycling performance for SIBs at 500 mA g-1; Cycling performance of Fe1-xS@CNTs for SIBs at 1000 mA g-1; TEM images of Fe1-xS@CNTs electrodes after 100 cycles at 500 mA g-1 for SIBs; Charge/discharge profiles of Fe1−xS@CNTs/Na[Ni0.61Co0.12Mn0.27]O2 during cycling at 0.5 C-rate; Cycling performance 16


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Na[Ni0.61Co0.12Mn0.27]/Fe1-xS@CNTs Na full cell at 0.5 C (capacities are calculated based on anode mass); Cycle performance of Fe1-xS@CNTs tested in the voltage window of 0.01~3.0 V; Ex-situ XRD of Fe1-xS@CNTs between 40~50 °C for the first cycle; Cycle performance of Fe1xS@CNTs

as anode in LIBs; TEM images of Fe1-xS@CNTs electrodes after 100 cycles at 500

mA g-1 for LIBs. AUTHOR INFORMATION *Corresponding author Yang-Kook Sun, e-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was mainly supported by the Global Frontier R&D Program (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, Information & Communication Technology (ICT) and the Human Resources Development program (No. 20154010200840) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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