Heteroatom doping and activation of carbon nanofibers enabling ultrafast and stable sodium storage

Electrochimica Acta 276 (2018) 304e310

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Heteroatom doping and activation of carbon nanofibers enabling ultrafast and stable sodium storage Yue Bao, Yuping Huang, Xiong Song, Jin Long, Suqing Wang**, Liang-Xin Ding, Haihui Wang* School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou, 510640, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 March 2018 Received in revised form 17 April 2018 Accepted 27 April 2018 Available online 30 April 2018

Activation is a common strategy to tailor microstructure of carbon materials. A new activation combined with heteroatom doping of carbon nanofibers is proposed to synthesize the N, S-doped carbon nanofibers (NSCNFs). After electrospinning and followed thermal treatment with thiourea, the diameter of the as-obtained NSCNFs decreases with much more abundant pore structure and functionalized with Scontaining oxygen groups compared with that obtained without activation. The increased specific surface area and pore volume are beneficial to facilitate the Naþ diffusion and provide more active sites to increase adsorption capacitance and facilitate pseudo-capacitive reactions. Also, the S-containing oxygen groups contribute to pseudo-capacitance by faradaic reactions. As a result, capacitive behavior is boosted hence leading to a excellent rate capability and cycling stability. The flexible and freestanding NSCNFs film is directly used as anode for sodium-ion batteries (NIBs) and shows excellent rate capability with reversible capacity of 147 and 133 mA h g-1 at 10 and 30 A g1, respectively. Remarkably, ultra-long cyclic life is also achieved with 90.8% capacity retention after 6000 cycles at 10 A g1. Such electrochemical performance makes NSCNFs promising to be used in high power and durable NIBs. This activation strategy can also be expanded to functionalize other carbon materials. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Sodium-ion batteries Activated carbon Heteroatom doping Sodium storage mechanism

1. Introduction Lithium-ion batteries (LIBs) develop fastest among all types of commercial batteries due to their outstanding energy and power density. They have been widely used in small-size battery market as 3C products, electric bicycles and power tools [1]. For application in large-scale energy storage, the concern about high cost and limited lithium reserves is increasing. Large research efforts have been devoted to explore reliable alternative large-scale energy storage technology. The NIBs have attracted extensive attention due to the abundance of sodium resources and similar physical and chemical properties between sodium and lithium [2]. However, the ionic radius of Naþ is much larger than that of Liþ which leads to severe volume change and sluggish kinetics during electrochemical reactions, consequently leading to poor rate capability and inferior cyclic stability [3].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (H. Wang). https://doi.org/10.1016/j.electacta.2018.04.207 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

To date, a large number of anode materials for NIBs such as carbon materials [4], metal alloys [5], oxides [6] and sulfides [7] have been intensive investigated. Among them, carbon materials are the most promising anodes for NIBs due to their low cost, abundance, environment-friendliness and adjustable porosity [8e12]. Our group developed polyimide based N-doped carbon nanofibers to be anode material for NIBs. Interconnected and stable structure with uniform micropores and abundant N made it show excellent electrochemical performance [8]. Wu et al. prepared disordered hard carbon/graphene which delivered a reversible capacity of 181 mA h g-1 at 200 mA g1 [9]. Zhou et al. assembled a NIB using hard carbon as the anode with an ether-based electrolyte and a relative high capacity of 217 mA h g-1 at 900 mA g1 was achieved [10]. Yang et al. reported rGO as anode material for NIBs in an ether-based electrolyte, the initial Coulombic efficiency achieved 74.6% due to the formation of thin and ionic conducting SEI film benefit from the ether-based electrolyte [11]. However the rate capability still need to be further improved. It is well known that increasing the proportion of capacitance is an achievable way to retain more capacity at high current densities. Capacitive contribution is composed of electric double-layer capacitance and

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pseudo-capacitance. For electric double-layer capacitance, specific surface area and pore structure are key factors for increasing the adsorption capacity because more active cites are provided. In order to tune the pore structure and increase the specific surface area of carbon materials, the physical or chemical activation is often required [13]. Physical activation is usually carried out at high temperatures (600e1200  C) in the oxidizing atmosphere of CO2 [14], steam [15], air or mixture of them after carbonization of carbon precursors. Chemical activation generally involves mixing carbon materials with activating agents (KOH [16], ZnCl2 [17], etc.) and carbonization, and gets higher yields and specific surface area at lower activation temperature compared with physical activation [18]. However a washing process is necessary to remove residual reactant for chemical activation. Also, exorbitant surface area will induce electrolyte decomposes easily to form more SEI film which results in low initial Coulombic efficiency [19]. Therefore, activation procedure is needed to be simplified and optimized for industrial battery applications. Heteroatoms doping in the carbonaceous materials is also a popular approach to improve the electrochemical performance, in which the heteroatoms can react with Naþ and contribute to pseudo-capacitance [20]. Huang et al. presented functionalized PPy nanofibers with high N content as anode materials for NIBs. The materials exhibited high rate capability and delivered 73 mA h g-1 at 20 A g1 [21]. Introduction of nitrogen (N) in the carbon structure can generate extrinsic defects, tune pore structure and demonstrated to improve the electron donor capability and generate pseudo-capacitance [22]. Recently, another heteroatom of sulfur (S) was also introduced to the carbon structure to improve the electrochemical performance of carbon-based anodes [23]. Chen et al. prepared sulfur covalently bonded graphene, the chemically doped S improved the capacity and boosted the rate capability. As a result, it delivered 262 and 83 mA h g-1 at 0.1 and 5 A g1 [24]. The S doping can enlarge the interspace of carbon, hence improve the kinetics of Naþ transport and facilitate de-intercalation/ intercalation reactions [25,26]. S-containing oxygen groups can contribute to pseudo-capacitance by faradaic reactions and CeS bonds are effective active sites to improve capacity [27]. These studies clearly demonstrate that heteroatom doping is a viable way to endow carbon materials with good rate capability and cycling stability by boosting capacitive behavior, facilitating ion diffusion and modifying electronic property. In this work, we propose a new simple activation to synthesize flexible N, S-doped carbon nanofibers (NSCNFs). During the activation process, the gases (H2S and SOx) from thermal decomposition of thiourea corrode the carbon and tune pore structure with highly increased specific surface area, and also as S sources to dope or functionalize the carbon nanofibers. The improved microstructure facilitates the Naþ diffusion and provides more active sites to increase electric double-layer capacitance. Moreover, the introduction of S-containing oxygen groups contributes to increase the pseudo-capacitance. Hence, the obtained NSCNFs used as anode materials for NIBs exhibits excellent cycling performance and high rate capability. 2. Experimental 2.1. Materials synthesis 2.1.1. Preparation of flexible PAN nanofibers In a typical process, 0.435 g polyacrylonitrile (PAN, Mw ¼ 150,000, Sigma Aldrich) was dissolved in 5 g N,N-dimethylformamide (DMF, Aladdin Co. Ltd., China) and stirring at 40  C for 2 h to form viscous solution. The solution was then transferred into a 5 mL disposable syringe equipped with a 21-guage stainless

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steel needle. A piece of Al foil was grounded on the roller as a collector and the revolving speed of roller was 20 rpm min1. The distance between the needle tip and roller was 16 cm. The high voltage and temperature applied for electrospinning was 13 kV and 40  C. The flow rate of solution was 1.2 mL h1. 2.1.2. Preparation of doped carbon nanofibers A porcelain boat loaded with appropriate amount of thiourea (Aladdin Co. Ltd., China) was put on the left side of the as-spun white membrane. And then, stabilized at 350  C for 2 h with a heating rate of 2  C min1 followed by carbonization at 800  C for 1 h with a heating rate of 5  C min1 under Ar atmosphere. The resulting N, S-doped carbon nanofibers is denoted as NSCNFs. For comparison, the N-doped carbon nanofibers (NCNFs) without activation and functionalization was prepared similar to that of NSCNFs except for no thiourea was added during the thermal treatment. 2.2. Structural characterization The crystal structures of the samples were characterized by a Bruker D8 Advance X-ray diffractometer using filtered Cu-Ka radiation. The morphology was investigated by scanning electron microscopy (SEM, NOVA NANOSEM 430) and transmission electron microscopy (TEM, JEM-2010HR). The X-ray photoelectron spectroscopy (XPS) spectra were collected on an ESCALAB 250 X-ray photoelectron spectrometer using an Al Ka X-ray source. The specific surface was measured by the Brunauer-Emmett-Teller (BET) method (Micromeritics analyzer ASAP 2460 (USA)), pore size distribution (PSD) was investigated using the density functional theory (DFT) method (Micromeritics). Raman spectra were collected on LabRAM Aramis Raman spectrometer with a laser wavelength of 532 nm. 2.3. Electrochemical measurements The flexible NSCNFs was cut into pieces and directly used as the working electrodes. The electrochemical performance was carried out using 2032-type coin cells assembled in an argon-filled glove box with both content of oxygen and water less than 1 ppm. 1 M sodium hexafluorophosphate (NaPF6) in dimethoxyethane (DME) was used as electrolyte, glass fiber (GF/B) from Whatman was used as separator, and Na metal foil as the counter and reference electrode. The galvanostatic charge/discharge were conducted on Battery Testing System (Neware Electronic Co., China) in a voltage range of 0.01e3 V. Cyclic voltammetry (CV) measurements were conducted on an electrochemical workstation (CHI604D, Chenhua Instrument Company, Shanghai, China) in 0.01e3 V at a scanning rate from 0.2 to 1.0 mV s1. Electrochemical impedance spectra (EIS) were collected by the electrochemical workstation (Zahner IM6ex) in the frequency range from 100 kHz to 10 mHz. 3. Results and discussion From the XRD patterns of NCNFs and NSCNFs films, two broad peaks at 24.9 and 43.3 corresponding to (002) and (100) diffraction are observed which illustrates an amorphous nature of the carbon structure (Fig. S1). The interlayer distance is calculated to be 0.356 nm (larger than 0.336 nm for graphite). From the analysis of Raman spectra (Fig. S2), the higher intensity ratio of D-band to Gband of NSCNFs (1.09) than that of NCNFs (1.00) also indicates that the carbon activation and functionalization help to create more defects and make the carbon structure more distorted [28]. The SEM images are shown in Fig. 1. Long nanofibers connect with each other and form an integrated 3D carbon network which

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Fig. 1. (a) Low-magnification SEM image and photograph and (b) High-magnification SEM images of NSCNFs; (c) Histogram of fiber diameters of NSCNFs. (d) Low-magnification SEM image and (e) High-magnification SEM images of NCNFs; (f) Histogram of fiber diameters of NCNFs.

can favor electron transfer and ion transportation [29]. From the photograph of the NSCNFs (inset of Fig. 1a), ultra-long nanofibers makes NSCNFs freestanding and flexible which can be directly used as anode for NIBs without using additives and substrates, thereby achieves higher energy density. Compared with NCNFs without activation and functionalization, the average diameter of carbon nanofibers in NSCNFs decreases to 141 nm which is smaller than 242 nm of NCNFs (Fig. 1c and f), hence results in larger specific surface area and shorter diffusion path of Naþ for NSCNFs [30]. From the TEM images (Fig. 2), the NSCNFs shows much fluffier surface compared with NCNFs. The smaller diameter and fluffier

surface of NSCNFs are caused by consumption of carbon of CNFs due to the corrosion of carbon by gases (e.g. H2S, SOx) generated from thiourea decomposition. The BET result shows that the specific surface area and the pore volume of the NSCNFs (698.4 m2 g1 and 0.295 cm3 g1) are highly increased compared with that of NCNFs (212.0 m2 g1 and 0.094 cm3 g1) (Fig. 2c and f). The increased specific surface area and pore volume are beneficial for electrochemical performance due to the increased contact area between electrode and electrolyte and more active sites can be provided for pseudo-capacitive reactions and adsorption. X-ray photoelectron spectroscopy (XPS) is used to analyze the

Fig. 2. (a) Low-magnification TEM image and (b) High-magnification TEM image of CNF in NCNFs; (c) Nitrogen adsorption-desorption isotherms and pore size distribution curve (the inset) of NCNFs. (d) Low-magnification TEM image and (e) High-magnification TEM image of CNF in NSCNFs; (f) Nitrogen adsorption-desorption isotherms and pore size distribution curve (the inset) of NSCNFs.

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surface chemistry of obtained carbon materials (Fig. 3). In the N 1s spectra of NCNFs and NSCNFs, four apparent peaks at 398.2, 399.9, 401.0 and 403.7 eV are assigned to pyridinic (N-6), pyrrolic-N (N-5), quaternary-N (N-Q) and oxidized-N (NeO), respectively [31]. The main existing forms of N are N-6 and N-Q (Table S1). It has been reported that N-6 can introduce pseudo-capacitance effect into the carbon and increase the electrical double layer capacitance, and NQ can facilitate electron transfer [22,32e34]. Compared with the NCNFs, apparently decrease of the N-Q content for NSCNFs indicates that N-Q is partially substituted by doping S (Fig. 3aeb). Moreover, N-5 is also substituted to some extent as the ratio of N-5 decreases fractionally. In the S 2p spectrum of NSCNFs (Fig. 3c), two apparent peaks at binding energy of 163.8 and 165.1 eV are assigned to S 2p2/3 and S 2p1/2 peaks for the aromatic sulfides (CeSeC covalent bond); the broad peak at 167.9 eV is assigned to the existing of oxidized sulfur [23]. These results validate that surface of carbon successively functionalized by S-containing oxygen groups. The sulfone and sulfoxides can contribute to pseudocapacitance through redox faradaic reactions [35,36]. The ratio of C in NSCNFs (76.5%) is relative lower compared with that of NCNFs (84.1%) and the loss part may relate to the gas corrosion or substitution of S. The detail element compositions of NCNFs and NSCNFs from XPS results are shown in Table S1. To obtain the detail element compositions, the elemental analysis is also carried out and the results are shown in Table S2, the N and S contents of NSCNFs are 10.9% and 2.8%, respectively. The elemental mapping of NSCNFs shown in Fig. S3 reveals that N and S are uniformly distributed within carbon nanofibers. The galvanostatic charge/discharge behaviors are investigated between 0.01 and 3.0 V. Fig. 4a and b shows charge/discharge curves of NCNFs and NSCNFs, respectively. The first discharge and charge capacities of NCNFs are 327 and 217 mA h g-1 with low initial Coulombic efficiency (ICE) of 66.3% (Fig. 4a), while NSCNFs can deliver higher discharge and charge capacities of 352 and 325 mA h g-1 corresponding to ICE of 92.3% (Fig. 4b). It is worthwhile to notice that the ICE of NSCNFs is much higher than previous studies on N and/or S doped disordered carbon (mostly reported results are below 75%). The high ICE of NSCNFs can be ascribed to thin SEI film benefit from ether-based electrolyte, high reversible reactions of S groups with Naþ and capacitive controlled sodium storage mechanism [10,23,26,37e39]. Fig. 4c compares the rate capability between the NCNFs and NSCNFs. It is clear that the NSCNFs exhibits much better rate capability than NCNFs. The reversible capacities of NSCNFs are 201, 163 and 152 mA h g-1 at 0.05, 1, and 5 A g1, respectively. More impressively, even at 30 A g1, a high capacity of 133 mA h g-1 can still be retained for NSCNFs. As we know, it is among the best performances of all carbon materials for NIBs (Table S3).

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Cyclic voltammetry (CV) tests at different scan rates are carried out to evaluate the electrochemical kinetics of NCNFs and NSCNFs toward Naþ (Fig. 5a and b). The CV curves of NSCNFs at various scan rates from 0.2 to 1 mV s1 display similar shapes. The weak redox peaks and rectangular shapes imply that some capacitive behavior during the sodium storage process. The relationship between log i and log v can be described as log(i)¼blog(v)þlog(a) and the b values are listed in Table S4. It can be seen almost all the b values of NSCNFs are higher and close to 1.00 which demonstrates the electrochemical reactions of NSCNFs are mainly dominated by capacitive behavior [40]. The exact ratio of capacitive contribution can be further quantified by separating the current (i) at a given potential into contribution of ionic diffusion (k2n1/2) and capacitance effect (k1n) according to the equation: i(V) ¼ k1n þ k2n1/2 [41e43]. The exact ratios of capacitive contribution are quantified which show improved results with increasing the scan rate (Fig. 5c). The higher ratios of capacitive contribution of NSCNFs as a result of larger specific area, improved pore structure and pseudocapacitance effect of S-containing oxygen groups is consistent with the better rate capability of NSCNFs. The capacitive contribution reaches 87% of the charge storage at a scan rate of 1 mV s1 for NSCNFs and the typical profile for capacitive current (red region) in comparison with the total current is shown in Fig. 5d. The Nyquist plots of NCNFs and NSCNFs are carried out and shown in Fig. S5. It can be seen that the charge transfer resistance of NCNFs and NSCNFs are 6.6 and 25.6 U, respectively. The lower electrical conductivity of NSCNFs probably caused by introduction of S groups [44e47]. The long-term cycling performances are also investigated. As shown in Fig. S4, both of the NCNFs and NSCNFs show excellent cycling stability with capacity retention rates beyond 100% at 1 A g1 after 2500 cycles. And the NSCNFs can deliver a high capacity of 204 mA h g-1 while it is only ~100 mA h g-1 for NCNFs. Even up to 10 A g1, NSCNFs can retain 164.3 mA h g-1 after 6000 cycles (Fig. 6a). Moreover, it should be pointed that all the capacities are calculated based on the overall mass of the entire electrode due to the freestanding characteristic of NSCNFs. The morphology of tested NSCNFs after 500 cycles is investigated by SEM and TEM (Fig. 6bec), no obvious changes can be observed. Accordingly, the good structural stability is beneficial for long cyclic life of NSCNFs. 4. Conclusion In summary, a novel heteroatom doping combined with activation strategy is proposed to synthesize NSCNFs. The obtained freestanding and flexible NSCNFs can be directly used as anode for sodium-ion batteries. The electric double-layer capacitance is improved by increased specific surface area and pore volume with facilitated ion diffusion. Additionally, S-containing oxygen groups

Fig. 3. High-resolution XPS spectra of (a) N 1s of NCNFs and (b) NSCNFs, (c) S 2p of NSCNFs.

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Fig. 4. Typical charge/discharge curves of (a) NCNFs and (b) NSCNFs at 50 mA g1 for the first two cycles; (c) Rate capability of NCNFs and NSCNFs.

Fig. 5. CV curves at varies scan rates of the (a) NCNFs and (b) NSCNFs; (c) Contribution ratios of capacitance at different scan rates; (d) Capacitive contribution (red) and diffusion contribution (blue) of NSCNFs at 1.0 mV s1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 6. (a) Long cycling performance of NSCNFs at 10 A g1. (b) SEM and (c) TEM images of NSCNFs after 500 cycles.

in the carbon structure also contribute to pseudo-capacitance to boost capacitive behavior of NSCNFs. As a result, the specific capacity and rate capability are significantly improved through activation and functionalization. Moreover, high electrochemical reversibility and structural stability allow NSCNFs to deliver superior cyclic stability. In view of the excellent performance, NSCNFs is very promising to be applied in high power density NIBs.

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Acknowledgements The authors greatly acknowledge the National Natural Science Foundation of China (Nos. 21576100, 21536005 and 21776098), National Key Research Program of China (No. 2016YFA0202600), Guangdong Natural Science Funds for Distinguished Young Scholar (2017A030306022) and Pearl River S&T Nova Program of Guangzhou (201610010062).

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2018.04.207.

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