Carbon Fibers Derived from Fe-Carrageenan

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Energy, Environmental, and Catalysis Applications

Highly Porous FeS/Carbon Fibers Derived from Fe-Carrageenan Biomass: High-capacity and Durable Anodes for Sodium-ion Batteries Daohao Li, Yuanyuan Sun, Shuai Chen, Jiuyong Yao, Yuhui Zhang, Yanzhi Xia, and Dongjiang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03059 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Applied Materials & Interfaces

Highly Porous FeS/Carbon Fibers Derived from FeCarrageenan Biomass: High-capacity and Durable Anodes for Sodium-ion Batteries Daohao Li,† Yuanyuan Sun,† Shuai Chen,‡ Jiuyong Yao,† Yuhui Zhang,† Yanzhi Xia,*, † Dongjiang Yang*,†,§ †

Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of

Shandong Province, Institute of Marine Biobased Materials, College of Environmental Science and Engineering, Qingdao University, Qingdao 266071, P R China. ‡

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

Sciences, Taiyuan 030001, P R China. §

Queensland Micro- and Nanotechnology Centre (QMNC), Griffith University, Nathan,

Brisbane, Queensland 4111, Australia. *Corresponding Authors: [email protected] (D. Yang); [email protected] (Y. Xia).

KEYWORDS: double helix structures; Fe-carrageenan biomass; FeS/carbon fibers; anode materials; sodium-ion batteries

ABSTRACT: The nanostructured metal sulfides have been reported as promising anode materials for sodium ion batteries (SIBs) due to their high theoretical capacities, but suffered from the unsatisfactory electronic conductivity and poor structural stability during charge/discharge process, thus limiting their applications. Herein, the one-dimensional (1D) porous FeS/carbon fibers (FeS/CFs) micro/nanostructures are fabricated through facile pyrolysis of double-helix structured Fe-carrageenan fibers. The FeS nanoparticles are in-situ formed by interacting with sulfur-containing group of natural material ι-carrageenan and uniformly embedded in the unique 1D porous carbon fabrous matrix, significantly enhancing the sodium-ion storage performance. The obtained FeS/CFs with optimized sodium storage performance benefits from the appropriate carbon content (20.9 wt%). The composite 1 ACS Paragon Plus Environment

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exhibits high-capacity and excellent cycling stability (283 mAh g−1 at current density of 1 A g−1 after 400 cycles) and rate performance (247 mAh g−1 at 5 A g−1). This work provides a simple strategy to construct 1D porous FeS/CFs micro/nanostructures as high performance anode materials for SIBs via a unique sustainable and environmentally friendly way.

1. INTRODUCTION

Sodium ion batteries (SIBs) with the advantages of geographical distribution of sodium and cost effectiveness have been attracted extensive attentions.1, 2 Ideal SIBs should be featured with long-term cycling life and high energy density. Developing high performance electrode materials with high specific capacities, excellent rate performances and cycling stability are of great significance for SIBs.3-5 For SIBs anode materials, the high theoretical specific capacity materials, such as Sn, Sb, P, and metal sulfides (MSs), etc, were reported as promising candidates.6-8 Among them, the MSs, such as FeS2, CoS2, SnS2, and MoS2, have been successfully employed as anode materials for SIBs.9-14 Unfortunately, MSs usually suffer from the unsatisfactory electronic conductivity and poor structural stability during Na+ de/insertion reaction process, leading to poor rate performance and cycling stability. Construction of core/shell structured carbon encapsulated MSs nanoparticles (NPs) composites is effective way to enhance the welldesigned structure, which can not only enhance the electrons transport, but also maintain the structural stability of the MSs-based active materials during charge/discharge process, consequently, resulting in enhanced cycling stability.15-19 However, some studies have reported that an excess of carbon content in the MSs/C composites results in the decreased active materials content and the reduced sodium ion transport capacity, leading to the inferior low specific capacities and rate performance.20-22 Apparently, designing an optimal content 2 ACS Paragon Plus Environment

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carbon encapsulated MSs NPs composites would extra promote the electrolyte ions diffusion to enhance capacities and rate performance. The carbon-based composites with one-dimensional (1D) porous micro/nanostructures can demonstrate significant impacts for energy storage, achieving high-capacity, outstanding rate performance and cycling properties.23, 24 The 1D continuous structure can offer interconnected electron transport channels through active materials to the current collector. Besides, the 1D porous composites can provide large surface area and appropriate pore sizes distribution, which allow for high contact area of electrode to electrolyte to shorten the electrolyte ions diffusion

distance.

Furthermore,

the

small

size

active

materials

NPs

in

1D

micro/nanostructure could be accommodated the volume expansion to restrict degradation of NPs during cycling. Nevertheless, the traditional methods for fabricating such 1D porous MSs/C micro/nanostructures are usually either complicated or used nonrenewable petrochemical resources and environmentally unfriendly sulfides (such as H2S) as precursors. Thus, the newly green and sustainable method must be developed to manufacture optimal carbon content 1D porous MSs/C micro/nanostructures as high-capacity and durable anode for SIBs. Recently, the renewable and abundant biomass resources, especially for the more available and potential seaweed biomass, have been used as precursors or template for fabricating the energy storage materials.25-27 Herein, we used the Fe-carrageenan biomass as precursor to prepare 1D porous and core/shell structured FeS NPs (25-45 nm)/carbon fiber (a diameter of about 10-15 µm) composites (FeS/CFs). The Fecarrageenan fibers were prepared through wet spinning carrageenan using the coagulating bath containing Fe3+ ions. Carrageenan macromolecules undergo crosslinking with Fe3+ through forming double helix structures.28-30 After pyrolysis of the Fe-carrageenan fibers, the structures of sulfate groups binding Fe3+ ions are converted to FeS NPs without any additional sulfur source, and the carrageenan 3 ACS Paragon Plus Environment


macromolecules are decomposed into H2O, CO, CO2, and a spot of SO2 to generate porous structure and finally transformed into porous carbon, forming 1D porous FeS/CFs micro/nanostructures. The confinement effect of double helix structure for Fe3+ can lead to uniform distribution of the FeS NPs in the 1D porous carbonaceous microfibers with core/shell structure. A carbon removal process was carried out using CO2 at 600 °C with different removal time (t hours) to obtain FeS/CFs-t (t = 1 and 2). We found the FeS/CFs-1 with 20.9 wt% carbon content exhibited high reversible capacities, excellent rate performance and cycling stability (283 mAh g−1 at current density of 1 A g−1 after 400 cycles, 247 mAh g−1 at 5 A g−1).

2. EXPERIMENTAL SECTION 2.1. Materials The ι-carrageenan was provided by the Qingdao Haizhilin Biotechnology Development Limited Company. Iron (III) chloride hexahydrate (FeCl3·6H2O) was purchased from Sinopharm Chemical Reagent (Shanghai, China). 2.2. Preparation of FeS/CFs-t The 7.0 wt% ι-carrageenan solution was prepared at 80 oC and made homogeneous with deionized water under high speed stirring, and then, this solution was extruded from a spinneret into a coagulating bath containing 3.0 wt % FeCl3 ethanol solution to form Fecarrageenan fibers. The obtained Fe-carrageenan fibers were washed with distilled water and dried. The Fe-carrageenan fibers were pyrolyzed at 800 °C at the heating rate of 5 °C min−1 for 1 h in Ar atmosphere to obtain FeS/CFs. Finally, the FeS/CFs were exposed to calcination at 600 °C at a heating rate of 7 °C min−1 for t h in CO2 atmosphere to obtain FeS/CFs-t (t = 1 and 2). 2.3. Characterizations 4 ACS Paragon Plus Environment

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Coupled thermogravimetric and mass spectrum (TG-MS) were obtained using Evolution 16/18 and OMNI star instruments. The phase structures were characterized with X-ray diffraction (XRD, DX2700, China) operating with Cu Kα radiation (l=1.5418 Å) at a scan rate (2θ) of 1° min−1 with the accelerating voltage of 40 kV. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method from the data in a relative pressure (P/P0) range between 0.05 and 0.25 and pore structures were characterized. The chemical composition was investigated by X-ray photoelectron spectroscopy (XPS) using an ESCALab250 electron spectrometer (Thermo Scientific Corporation) with mono-chromatic 150 W Al Kα radiation. The morphologies and structures of the samples were characterized by scanning electron microscopy (SEM; SU8020) and transmission electron microscopy (TEM) and high-resolution TEM (FEI Tecnai G20). 2.4. Electrochemical measurements The samples were mixed with carbon black and PVDF in a weight ratio of 80:10:10 in NMP. The slurry was coated on the Cu foil and dried for 10 h at 120 °C under vacuum. The 2016 type coin cells were assembled in an Ar-filled glove box. Sodium metal was used as the reference electrode and counter electrode, and the separator was Whatman GF/D. The electrolyte was 1.0 M solution of NaClO4 in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 vol.%) containing 5 wt% fluoroethylene carbonate. The electrochemical performances were tested in a voltage window of 0.01-3.0 V vs Na+/Na using a cell testing instrument (LAND CT2001A and CHI 760E). The specific capacities and current densities are based on the mass of FeS/CFs and FeS/CFs-t.

3. RESULTS AND DISCUSSION 3.1. Synthesis of the FeS/Carbon Fibers

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Figure 1. Schematic illustration on the synthesis process of FeS/CFs-t. As illustrated in Figure 1, a wet-spinning process was used to fabricate biomass microfibers. First, the ι-carrageenan solution (prepared at temperature of 80 °C) was extruded from a spinneret into a coagulating bath containing Fe3+ ethanol solution to form Fe-carrageenan fibers (Figure S1). In this step, random coil carrageenan macromolecules would gelate with Fe3+. The Fe3+ ions not only assist the formation of double helix structure, but also promote the aggregation of different double helices to form junction zones, which enable long-range cross-linking. The weight percentage of Fe3+ species in Fe-carrageenan fibers was measured to be about 16.4 wt% using thermogravimetric (TG) analysis (Figure S2). After facile pyrolysis process of Fecarrageenan fibers in inert atmosphere, the organic and inorganic Fe-carrageenan fibers were converted to FeS/CFs. Finally, the FeS/CFs-t (t = 1 and 2) were obtained through calcination process at 600 °C in CO2 atmosphere for t hours. 3.2. Characterizations of the FeS/Carbon Fibers and the Precursors


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Figure 2. (a) TG (pink) and DTG (olive) curves of Fe-carrageenan fibers in Ar atmosphere (heating rate of 5 °C min−1). (b) The analysis of the outlet gases from the decomposition of Fe-carrageenan fibers TG-MS: H2O (pink), CO (olive), CO2 (purple) and SO2 (orange). To research the decomposition mechanism of Fe-carrageenan fibers during pyrolysis process, the coupled thermogravimetric and mass spectrum (TG-MS) was executed. The TG curve presents that the whole process of thermal decomposition for the Fecarrageenan fibers between 50-700 °C with 70.9 wt% weight loss is the result of decomposition of carrageenan macromolecules and the structures of sulfate groups binding Fe3+ ions are converted to FeS NPs (Figure 2a). The derivative thermograms (DTG) curve shows two distinct weight losses at the temperatures of around 100240 °C and 420-450 °C, corresponding the fast thermal decompositions of carrageenan in these stages. The corresponding mass spectra (Figure 2b) illustrates the evolved gases are steam (H2O, m/z = 18), carbon monoxide (CO, m/z = 28), carbon dioxide (CO2, m/z = 44), and sulfur dioxide (SO2, m/z = 64). A mass of H2O and CO and a spot of SO2 are evolved during the whole process of thermal decomposition, which are derived from the oxygen-containing groups and sulfate groups. The majority of sulfate groups interacting with Fe3+ through ionic forces were gradually converted to FeS. The purple curve assigned to CO2 has two peaks at around 170 and 460 °C, which is related 7 ACS Paragon Plus Environment


to the two fast weight loss (Figure 2a), indicating the fast consumption of carbon by oxygen in precursor at these temperatures. To sum up, the whole process of thermal decompositions for the Fe-carrageenan fibers under Ar atmosphere is the result of decomposition of carrageenan macromolecular, where the oxygen-containing groups (C-O-C-, –OH, and –SO4) were converted to H2O, CO, CO2, and SO2, and the residue is FeS/CFs.

Figure 3. (a) XRD patterns of the FeS/CFs, FeS/CFS-1, and FeS/CFs-2. (b) Nitrogen adsorption-desorption isotherm and (c) the corresponding pore size distribution curves for FeS/CFs, FeS/CFs-1, and FeS/CFs-2. X-ray diffraction (XRD) was used to measure the structures of the samples. As shown in Figure 3a, the strong and sharp diffraction peaks at 2 θ values of 30.0, 33.8, 43.1, 53.2 and 71.3 are observed from the XRD patterns of all the FeS/CFs, FeS/CFs-1, and FeS/CFs-2 samples, corresponding to the diffractions of (110), (112), (114), (310), and (224) planes, respectively, indicating the pure phase of FeS (JCPDF no. 00-0370477) in the composites.31,

32

Compared the XRD pattern of Fe-carrageenan fibers

(Figure S3), the Fe3+ ions are completely converted to FeS species through pyrolysis in Ar atmosphere. In addition, the peak intensities of FeS and the amorphous carbon located at about 20-25° are increased and reduced with the increase of carbon removal 8 ACS Paragon Plus Environment

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time, respectively. The average diameters of FeS NPs in the samples roughly estimated through the Scherrer's formula, which are 62, 59, and 102 nm for FeS/CFs, FeS/CFs-1, and FeS/CFs-2, respectively. The carbon contents of the obtained FeS/CFs, FeS/CFs-1, and FeS/CFs-2 were determined to be about 35.7, 20.1, and 9.4 wt%, respectively (Figure S4). The porous characteristics of the samples were also investigated. As shown in Figure 3b, the FeS/CFs and FeS/CFs-1 exhibit the typical type II adsorption isotherm, and the absorption presents at low relative pressure (P/P0 < 0.02), which is corresponding to the adsorption in micropores. The micropores of the FeS/CFs should be attributed to the thermal decomposition of carrageenan macromolecule. However, there is no microporous adsorption for FeS/CFs-2. In addition, the hysteresis loops of all the isotherms arise from the mesoporous structure.27 The Brunauer-Emmett-Teller (BET) specific surface area (SBET) of FeS/CFs, FeS/CFs-1, and FeS/CFs-2 are 374, 265, and 78 m2 g−1, respectively. The density functional theory (DFT) pore size distributions calculated from the adsorption branch of the isotherms exhibit the FeS/CFs and FeS/CFs-1 contain micro- and mesopores, whereas FeS/CFs-2 has small amount of micro- and mesoporous structures (Figure 3c).



Figure 4. (a) XPS spectra of FeS/CFs, FeS/CFs-1, and FeS/CFs-2. The high resolution XPS spectra of FeS/CFs-1, (b) Fe 2p spectrum, (c) S 2p spectrum, and (d) O 1s spectrum. The X-ray photoelectron spectroscopy (XPS) measurement was carried out to research the detailed elemental composition of FeS/CFs and FeS/CFs-t (Figure 4 and S5). The survey spectra show the presence of S, C, O, and Fe elements in all the samples (Figure 4a). The peak intensity of Fe species is obviously increased through carbon remove process, and the peak intensities of C and O are reduced. As shown in Figure 4b and S5a-b, all the three high-resolution of Fe 2p spectra can be decomposed into five peaks in the binding energy range of 706-730 eV. The two unfitted peaks at about 712 and 725 eV are assigned to the Fe 2p3/2 and Fe 2p1/2, respectively. The four fitted peaks at around 711.1, 713, 724.6 and 726.5 eV confirm the existence of Fe-S bond.33,

34

The peak at 719.0 eV is corresponding to the Fe-O bond, which may

originate from the surface oxidation of samples in air. For S 2p spectra, the peaks located at 161.7 and 163.0 eV are ascribed to S 2p3/2 and S 2p1/2, respectively, corresponding well with FeS (Figure 4c and S5c-d).34, 35 The bonding of sulfur with oxygen (S-O bond) is supported by the peak centered at 166.6 and 167.8 eV. In addition, the spectra of C 1s of all the samples are fitted to four peaks, which can be assigned to C-C (284.4 eV), C=C (285.0 eV), C-O (286.0 eV), and C=O (288.5 eV) (Figure 4d and S5e-f). The residual oxygen-containing groups (C-O, C=O, and S-O) in the samples are originated from the thermal decomposition of carrageenan macromolecule (Figure S5g-i).


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Figure 5. SEM images of (a, c) Fe-carrageenan fibers, (d) FeS/CFs, (e) FeS/CFs-1, and (f) FeS/CFs-2. (b) The diameter distribution of the Fe-carrageenan fibers. (g) The EDS mappings of FeS/CFs-1 for C, Fe, S elements. (h) The high-magnification and (i) the cross section SEM images of FeS/CFs-1. The morphology of the samples was measured by scanning electron microscopy (SEM). As shown in Figure 5a, the Fe-carrageenan fibers are 1D longish and straight with a diameter of about 20-27 µm (Figure 5b, by statistical using Nano Measurer software). A mass of wrinkles formed in the wet spinning process are existed on the surface of Fe-carrageenan fibers (Figure 5c),. After pyrolysis of Fe-carrageenan fibers, the obtain FeS/CFs can retain fibrous structure and show rugged surface with a large shrinkage of diameter (Figure 5d), and the FeS/CFs-1 and FeS/CFs-2 with reduced carbon content exhibits the similar morphology compared with FeS/CFs (Figure 5e, f). The SEM energy dispersive X-ray spectroscopy (EDS) was used to analyze the elemental compositions of the samples. The C, Fe, and S elements are homogeneously distributed in the FeS/CFs-1 (Figure 5g). In addition, the C element in FeS/CFs-2 (Figure S6) is obvious fewer than the other two samples through the EDS mapping for C (Figure 5g and S7), indicating more carbon removed with the increase of carbon 11 ACS Paragon Plus Environment


removal time. A large amount of macropores can be observed on the surface of the sample with diameter of about 100 nm (Figure 5h), which can act as reservoirs for electrolyte to promote the diffusion of Na+. In addition, the obtain 1D porous micro/nanostructure composites display typical lotus-root-like porous structure (Figure 5i), which may due to the decomposition of most organic moieties of the Fe-carrageenan fibers through pyrolysis process.

Figure 6. TEM images of (a) FeS/CFs, (b) FeS/CFs-1, and (c) FeS/CFs-2. (d) HRTEM image of FeS/CFs-1. The transmission electron microscopy (TEM) and high solution TEM (HRTEM) further characterize the structure and component of FeS/CFs and FeS/CFs-t. Large amounts of FeS NPs are uniformly distributed in the FeS/CFs (Figure 6a). For FeS/CFs-1, the diameter of FeS NPs is similar with FeS/CFs (Figure 6b), and the NPs are still well covered by carbon layer. With the increase of carbon removal time, the size of FeS NPs increased, attributing to the aggregation of NPs with less carbon coated during the process of carbon removed (Figure 6c). The HRTEM image of FeS/CFs-1 exhibits a set of lattice fringes with a spacing of 0.208 nm 12 ACS Paragon Plus Environment

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corresponding to the (114) plane of FeS (Figure 6d). Furthermore, abundant micropores are observed in the amorphous carbon surrounding the FeS NPs, which benefits the promotion of Na+ diffusion to enhance the energy storage performance. 3.3. Electrochemical Performance of the FeS/Carbon Fibers

Figure 7. (a) CV curves of FeS/CFs-1 in the first three cycles. (b) The charge and discharge curves of FeS/CFs-1 at the 1st, 2nd, and 3rd between 0.01 and 3.0 V (vs. Na+/Na) at a current density of 1 A g−1. (c) The specific capacities and cycling performance of FeS/CFs, FeS/CFs1, and FeS/CFs-2 at a current density of 1 A g−1. (d) The specific capacity of the FeS/CF-1 at different current densities. The samples were used as anode materials for SIBs to evaluate the sodium storage performances by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. FeS undergoes the sodium storage perfformance through the redox reaction (FeS + 2Na+ + 2e− → Na2S + Fe) with a theoretical capacity of about 609 mAh g−1.19 Figure 7a and S8 show the CV curves of the FeS/CFs-1, FeS/CFs, and FeS/CFs-2 in the first three cycles with a scan rate of 0.1 mV s−1 in the potential window of 0.01-3.0 V (vs. Na+/Na). For the FeS/CFs-1, the cathodic reduction peaks 13 ACS Paragon Plus Environment


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locates at 1.43, 0.87 and 0.37 V in the first cathodic scan are attributed to the electrochemical reduction of FeS with Na+ to form the Na2S and Fe,41,

42

and the

formation of solid electrolyte interphase (SEI) film. These peaks shift to 1.54, 0.91, and 0.27 V in the subsequent two cathodic scans, which are corresponding to sodiation processes. For the anodic scans, the peaks located at 1.34, 1.78, and 2.11 V demonstrate transition of Na2S and Fe to NaxFeS and Na2FeS2 species during the desodiation process (Figure 7a).19 In addition, the pair of oxidation/reductive peaks appear in 0.10 and 0.03, corresponding to the extraction/insertion of Na+ to carbon. The redox peaks in CV curves of FeS/CFs and FeS/CFs-2 are similar to those of FeS/CFs-1, and the initial cathodic scans of the three samples are distinct attribute to the different of carbon content and porous structure. The second and third CV curves of FeS/CFs and FeS/CFs-1 show no obvious change, indicating good stability. Figure 7b exhibits the galvanostatic charge/discharge curves of FeS/CFs-1 anode at the current density of 1 A g−1. For the first discharge curve, the plateaus at around about 0.9 and 0.6 V are corresponding to the conversion reaction of FeS to Fe and Na2S and these plateaus in subsequent discharge curves shift to about 0.9-1.3 and 0.35 V. Two charge platforms at 1.3 and 1.8 V are found in all charge curves. In addition, the first discharge and charge specific capacities of FeS/CFs-1 are 460 and 317 mAh g-1 at 1 A g−1 with a Coulombic efficiency (CE) of 68.9 %. The irreversible capacity during the first cycle is mainly attributed to formation of SEI on the surface of FeS/CFs-1, and the irreversible insertion of Na+. The specific capacities and long cycling stability of carbon fibers without FeS NPs (CFs), FeS/CFs, FeS/CFs-1, and FeS/CFs-2 were evaluated. As shown in Figure 7c, the reversible charge capacity of CFs is only about 75 mA h g−1 at 1 A g−1, which suggested that such pure carbon material is not a high-performance anode material for SIBs. After compositing with FeS NPs, the reversible charge capacity of FeS/CFs-1 14 ACS Paragon Plus Environment

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can attain 283 mAh g−1 after 400 cycles at 1 A g−1, exhibiting obvious advantages over CFs and FeS/CFs (207 mA h g−1). For FeS/CFs-2, the specific capacity fades seriously after 100 cycles, although it exhibits high specific capacity at the initial cycles. The high specific capacity and excellent cycling stability of the FeS/CFs-1 anode result from the optimal content of carbon in the composite (Figure S4). The sodium storage performance of the FeS/CFs-1 electrode is compared with the reported iron sulfidebased electrodes, indicating the high specific capacity and cycling stability of it (Figure S9). The rate performance of the FeS/CFs-1 is shown in Figure 7d.

The charge

specific capacities are 438, 376, 332, 303, 280, and 247 mAhg−1 at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g−1, respectively. When the current density returns back to 5 A g−1 after 60 cycles, the charge specific capacity is recovered to 431 mAh g−1, indicating that the excellent rate capability of FeS/CFs-1.

Figure 8. (a) Nyquist plots of CFs, FeS/CFs. FeS/CFs-1, and FeS/CFs-2 electrodes. (b) The relationship between -Zim and the reciprocal square root of the angular frequency in the low-



frequency region. Schematic illustration of Na+ de/insertion reaction process of (c) FeS/CFs-1, and (d) FeS/CFs-2. The electrochemical impedance spectra (EIS) is further measured to evaluate the electrochemical performance of CFs, FeS/CFs, FeS/CFs-1, and FeS/CFs-2 (the fitted same equivalent circuit shown in Figure S10). As shown in Figure 8a, the Nyquist plots of the CFs, FeS/CFs, FeS/CFs-1, and FeS/CFs-2 electrodes before initial charge/discharge process show the typical semicircle at high frequency and the linear slope at low frequency region. The semicircle, high-frequency intercept at the Zre axis, and the inclined line illustrate the charge-transfer resistance of the interface between electrodes and electrolyte (Rct), resistance of the electrolyte and other cell components (Re), and the Warburg impedance (W), respectively. The Rct values of CFs, FeS/CFs, FeS/CFs-1, and FeS/CFs-2 electrodes are determined to be 202.2, 272.9, 253.4, and 627.3 Ω, respectively. The Rct of FeS/CFs-2 with unsatisfactory porous carbon structure is much larger than those of CFs, FeS/CFs and FeS/CFs-1, suggesting the porous carbon coating FeS NPs can decrease the resistance of the electrodes and promote Na+ diffusion and charge transfer. The EIS can be devoted to calculate the values of Na+ ion diffusion coefficient (DNa+), using the following equation (1): DNa+ = R2T2/2A2n4F4C2σ2

(1)

where A, C, F, T, R, n, and σ represent the surface area of the electrode, concentration of Na+, Faraday constant, absolute temperature, the gas constant, the number of electrons per molecule during redox process, and the Warburg factor, respectively. The values of σ can be estimated from low-frequency inclined lines using the equation of – Zim = σω-1/2, where ω is the angular frequency. Figure 8b exhibits the relation between –Zim and ω-1/2 through linear fitting of the spots. The FeS/CFs-1 exhibits the smaller σ value than FeS/CFs and FeS/CFs-2, resulting in the higher DNa+ calculated from 16 ACS Paragon Plus Environment

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equation (1). The order of

DNa+ is the same as Rct, further indicating that the

appropriate carbon coated FeS NPs is favorable for promoting the Na+ diffusion. The FeS/CFs-1 anode for SIBs exhibits excellent sodium storage performance due to their distinctive 1 D porous micro/nanostructure. As shown in Figure 8c, the carbonaceous microfiber matrix can retain the structure stability of FeS NPs during charge/discharge process, which is crucial for promoting cycling stability.36, 37 For the unsatisfactory carbon content and large sizes of FeS NPs in FeS/CFs-2, the serious volume change of FeS after the Na+ insertion could cause serious agglomeration, pulverization, and the loss of active materials from the electrodes, resulting in the poor cycling performance (Figure 8d). In addition, the porous structure of FeS/CFs-1 can store the electrolyte as the “reservoirs”, increasing interface between active materials and electrolyte, shortening the diffusion distance of Na+,38 and restraining the intermediate polysulfide dissolution into electrolyte during the charge/discharge process to prevent the loss of active substance. Furthermore, the 1D structure can serve as the channels for fast electron transport to promote the electrochemical conductivity of the anode, enhancing the rate performance. 4. CONCLUSIONS In summary, the double helix structured Fe-carrageenan fibers were used to fabricate 1D porous FeS NPs/carbonaceous microfiber micro/nanostructures. The composite with core/shell structure was in situ formed in the pyrolysis process of Fe-carrageenan fibers without any addition of toxic sulfur source. The nanoconfined iron ions in the unique double helix structure can ensure that the FeS NPs are uniformly embedded in the 1D porous carbon fabrous matrix. When evaluated as anode material for SIBs, the carbon coated FeS NPs can effectively improve the structural stability of FeS NPs during Na+ de/insertion reaction process, guaranteeing the excellent cycling stability. The 1 D porous carbon fibrous matirx can enhance ion and electron transport kinetics to 17 ACS Paragon Plus Environment


improve the rate performance. The FeS/CFs-1 with 20.9 wt% carbon content as anode for SIBs exhibits high reversible capacities and excellent cycling stability (283 mAh g−1 at current density of 1 A g−1 after 400 cycles) and rate performance (247 mAh g−1 at 5 A g−1). Such sustainable biomass conversion strategy provides a facile, green and effective route to fabricate high-performance metal sulfide-based anode materials for SIBs.

ASSOCIATED CONTENT Supporting Information Supporting Information contains wet spinning process for the preparation of Fe-carrageenan fibers (Figure S1), TG curve of Fe-carrageenan fibers in air (Figure S2), XRD pattern of Fecarrageenan-fibers (Figure S3), TG curves of the FeS/CFs, FeS/CFs-1, and FeS/CFs-2 in air (Figure S4), XPS spectra of FeS/CFs and FeS/CFs-2 (Figure S5), FESEM images of of FeS/CFs-2 (Figure S6) and FeS/CFs (Figure S7) and the corresponding EDS mappings for C, Fe, S elements, CV curves of FeS/CFs and FeS/CFs-2 (Figure S8), Comparision of electrochemical performance between FeS/CFs-1 and recently reported high-performance iron sulfide-based electrodes (Figure 9), the fitted same equivalent circuit of the EIS measurement (Figure S10). This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (D. Yang); [email protected] (Y. Xia). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work is financially supported by the National Natural Science Foundation of China (No. 51473081 and 51672143), Taishan Scholars Program, and Outstanding Youth of Natural Science in Shandong Province (JQ201713).

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TABLE OF CONTENTS GRAPHIC




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Carbon Fibers Derived from Fe-Carrageenan

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