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Nitrogen doped porous carbon fibres as anode materials for sodium ion batteries with excellent rate performance† Lijun Fu,b Kun Tang,bd Kepeng Song,c Peter A. van Aken,c Yan Yu*ab and Joachim Maierb

Received 9th October 2013 Accepted 5th November 2013

Nitrogen-doped activated porous carbon fibres (ACFs) were prepared as anode materials for Na-ion batteries. They exhibit excellent electrochemical performance, especially rate performance. The

DOI: 10.1039/c3nr05374a

excellent rate performance is ascribed to the fibre-like morphology and the facilitated charge transfer. The influence of nitrogen functionalities on charge transfer and electrochemical performance of

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N-doped carbon anodes for Na ion batteries is discussed.

Introduction In the past decades, research on lithium ion batteries led to a successful development in portable electronic devices. Superior properties such as high energy density and environmental benignity make lithium ion batteries promising as power sources for electric vehicles. In view of the growing demands for lithium ion batteries, limited reserves of lithium and the expected drastic cost rise of lithium,1 the exploration of new low-cost energy storage devices is highly required. Recently, sodium ion batteries gained increasing attention, because of their abundant reserves and relatively even geological distribution.2 However, the practical implementation of sodium ion batteries is hampered by lower energy density and a

School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, China. E-mail: [email protected]; Tel: +86-551-63607179

b

Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart, 70569, Germany. E-mail: l.fu@f.mpg.de; Fax: +49-7116891722; Tel: +49-7116891725

c Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, Stuttgart, 70569, Germany d

National Institute of Clean-and-Low-Carbon Energy, P. O. Box 001, Shenhua NICE, Future Science & Technology City, Changping District, Beijing 102209, P. R. China

† Electronic supplementary information (ESI) available: SEM and HR-TEM images of ACFs-3 and ACFs-5; Raman spectra of ACFs and ACFs-C, N2 adsorption and desorption isotherms and pore size distribution of ACFs-3 and ACFs-5; N1S XPS spectra of the as-prepared ACFs and ACFs-C aer sputtering for 10 min; cyclic voltammograms of ACFs-3 and ACFs-5; galvanostatic discharge–charge proles of ACFs-3 and ACFs-5 at a current density of 50 mA g1; cycle performance of ACFs-C at a current density of 5 A g1 in the potential range of 0.005–3 V vs. Na+/Na; rate performance of ACFs-C in the potential range of 0.005–2 V vs. Na+/Na; galvanostatic discharge–charge proles of the Ppy precursor at different current densities; cyclic voltammograms of ACFs at different scan rates; and the plots of i/v1/2 vs. v1/2 used for calculating a1 (slope) and a2 (intercept) at different potentials for ACFs; separation of charge storage contributions of ACFs-3 and ACFs-5 from (pseudo) capacitance and a diffusion-controlled process at a scan rate of 5 mV s1; compositional analysis of nitrogen doped carbon bers aer sputtering with Ar ion for 10 min. See DOI: 10.1039/c3nr05374a

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poor cyclability. The reason for the poor cycle life is attributed to the larger ionic radius of the sodium ion (55% larger in radius than Li+ ions), which makes it more difficult to be reversibly inserted into and extracted from host materials. A typical example is given by graphite, the favourite anode material in present commercial lithium ion batteries, which can barely accommodate sodium ions under moderate conditions.3 Besides kinetic issues, the larger Na+ radius can also be relevant for possible structural change4 during insertion/extraction. Thus exploitation of electrode materials with high capacities and high rate performances is crucial for further development of sodium ion batteries. Many electrode materials, such as carbon, Li4Ti5O12,5 Na2Ti3O7,6 tin7 and sodium terephthalate (Na2C8H4O4),8,9 and NaMnO2,10 have been considered as electrode candidates for Na ion batteries. As anodes, disordered carbons did not only attract great interest due to the existing versatile preparation methods, but also in view of structural advantages such as the larger interlayer space, facilitating Na+ insertion and extraction.11 Aer the rst report of electrochemical sodium ion insertion into carbon by Doeff et al.,12 which describes a reversible capacity of 85 mA h g1, various kinds of carbon materials have been investigated, such as carbon black,13 carbon microspheres,14 and mesocarbon microbeads (MCMB).15 Although a high reversible capacity (300 mA h g1) can be achieved for the ballmilled hard carbon derived from pyrolyzed glucose, there have been only a few reports demonstrating good cycling stability during extended operation, in particular at high charge–discharge rates. Recently, the critical impact of particle size and morphology on mass transfer and thus on electrochemical performance for lithium ion batteries has been emphasized.16 Along these lines, hollow carbon nanospheres have been considered as anode materials for sodium ion batteries, delivering high capacity and excellent rate capability.17

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Introduction of heteroatoms,18 particularly nitrogen atoms, is a popular approach to improve the electrochemical performance of carbonaceous materials as anode materials for lithium ion batteries, since the electric conductivity19 and capacity20,21 can both be enhanced. It is also reported that aer nitrogen doping, a pseudo-capacitance can be generated due to the interaction between the electrolyte and N species on the surface.22 Given the similarity between Li and Na ion batteries, it is logical to apply the same strategy to disordered carbon as an anode material for sodium ion batteries. Cao et al. investigated hollow carbon nanowires derived from polyaniline,11 which can deliver a reversible capacity of 251 mA h g1. Wang et al. synthesized porous nitrogen-doped nanosheets from graphene oxide (GO)–polypyrrole composites, and found that a reversible capacity of 200 mA h g1 is achieved aer 250 cycles at a current density of 50 mA g1, and still 50 mA h g1 at 10 A g1.23 Huang et al. reported N-doped carbon nanobers which can deliver a reversible capacity of 73 mA h g1 at the current density of 20 A g1.24 These studies clearly demonstrated nitrogen-doped porous carbon bres to be attractive anode constituents for Na-ion batteries. However, there are few reports investigating the inuence of the heteroatoms on the charge transfer, hence the electrochemical performance of nitrogendoped carbon anodes for sodium ion batteries. Herein, we report on nitrogen doped porous carbon bres derived from polypyrrole (Ppy). As anode materials for sodium ion batteries, they exhibit high reversible capacity and excellent rate performance. Furthermore, we emphasize the inuence of heteroatoms, particularly nitrogen functionalities on charge transfer and electrochemical performance of N-doped carbon anodes for Na ion batteries.

Characterization of nitrogen-doped porous carbon fibers Nitrogen doped porous carbon materials were synthesized by pyrolysis of a Ppy nanober precursor, which directly provides the N doping source. The Ppy nanobers were prepared via an oxidative template assembly route,25 then calcined in an inert atmosphere, followed by an activation process by using KOH as an activating agent to yield the activated porous carbon bres (ACFs-C), and the activation process was performed at 650  C. KOH is an effective chemical activation agent to yield carbon materials with high surface area and narrow pore distribution.26 For comparison, a simultaneous carbonization–activation process was applied to prepare activated porous carbon bres by directly mixing KOH with a Ppy precursor followed by a calcination step in an Ar atmosphere at 650  C. ACFs-3 and ACFs-5 denote the carbon bres activated with 3 and 5 times the amount of KOH (by weight), respectively. The morphologies of the carbon bres were characterized by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The as-prepared ACFs-C have a uniform bre shape with a diameter of 50–70 nm (Fig. 1a), cross-linked with each other, which is benecial for both the electron and ion transfers.27 Observation from high resolution

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Fig. 1

(a) SEM and (b) HR-TEM images of ACFs-C.

transmission electron microscopy (HRTEM) of ACFs-C reveals a turbostatic structure (Fig. 1b), indicating its amorphous structure due to the relatively low pyrolysis temperatures (650  C). ACFs-3 and ACFs-5 show similar morphologies (Fig. S1†), except for ACFs-5, the surface of which is rougher compared with the other two. The X-ray diffraction (XRD) patterns of the as-prepared ACFsC, ACFs and polypyrrole (Ppy) precursor are shown in Fig. 2a. The Ppy shows a broad peak around 25 , indicating an amorphous structure. For the ACFs-C and ACFs, two peaks can be observed at 22.7 and 43.6 , corresponding to the (002) and (100) diffraction modes of the disordered carbon structure. The interlayer spacing (d002) is calculated to be 0.4 nm, larger than that of graphite (0.336 nm), which is essential for the reversible storage of sodium. However, one can also observe that the (002) peak is overlapped with a broader peak, ascribed to amorphous carbon, and the (100) peak at 43.6 is relatively weak, indicating the low crystalline structure of the as-prepared ACFs-C and ACFs, consistent with the Raman (Fig. S2†) and HRTEM results. The porous structures of carbon bres were investigated by N2 absorption–desorption

(a) XRD spectra of ACFs-C, ACFs-3 and ACFs-5; (b) N2 adsorption and desorption isotherms and pore size distribution of ACFs-C; (c) N1S XPS spectra of the as-prepared samples; (d) illustration of nitrogen doping carbon with different doping sites. Fig. 2

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Table 1 Structural and compositional analyses of nitrogen doped carbon fibers

Element concentration (atomic %) Sample

BET surface area (m2 g1)

Average pore size (nm)

C

N

O

ACFs-3 ACFs-5 ACFs-C

941.2 1508.0 372.4

2.18 1.93 3.34

77.4 80.1 79.1

6.6 5.9 8.8

16.0 14.0 12.1

measurements (Fig. 2b and S3†). The specic Brunauer–Emmett– Teller (BET) surface area for ACFs-C, ACFs-3, and ACFs-5 is 372.4, 941.2, 1508.0 m2 g1, and the average pore radius is 3.34, 2.18, 1.93 nm, respectively (Table 1). X-ray photoelectron spectroscopy (XPS) was used to analyse the different functional groups of the porous carbon bres aer carbonization. The surface atomic content of oxygen for ACFs-C, ACFs-3, and ACFs-5 is 12.1%, 16.0% and 14.0%, and the surface nitrogen atomic content is 8.8%, 6.6%, and 5.5%, respectively (Table 1). The N1s XPS spectra of the as-prepared porous carbon and its precursor polypyrrole (Fig. 2c) indicate that the pyrrolic N (peaked at 399.6 eV, Fig. 2d), coming from the pyrrolic ring in the precursor, converts into pyridinic and quaternary N, located at 398.0 and 400.9 eV,22 respectively. Increasing the amount of activation agent, or degree of pre-calcination, benets the transformation process. According to the XPS spectra, ACFs-C obsess the highest amount of pyridinic and quaternary N, among all the as-prepared porous carbons, followed by ACFs-5 and ACFs-3 (in the descending order). Aer 10 min Ar ion sputtering, the XPS spectra show similar results for the asprepared samples (Fig. S4 and Table S1†), indicating analogous doping properties between the inner part and surface of each sample.

Electrochemical performance as anodes in Na ion batteries The electrochemical performance of the as-prepared porous carbon bres was investigated by galvanostatic and cyclic voltammetric (CV) methods. It is reported that the sodium ion insertion into the graphene layer and pores occurs in the potential regime below 1.2 V vs. Na+/Na. In the case of ACFs-C, the cathodic reaction starts at 2 V with three cathodic peaks located at around 1.1, 0.4 and 0 V during the rst scan (Fig. 3a). The peak around 1.1 V is ascribed to the reaction between the sodium ion and the surface functional group(s).28 The peak is still present in the following cycles, although it becomes less distinctive, indicating the reaction to be partially reversible, which is consistent with our previous results.17 The peak at 0.4 V is due to the decomposition of the propylene carbonate (PC) electrolyte and formation of a solid electrolyte interface (SEI) lm,29 which disappears in the subsequent cycles. The cathodic peak located around 0 V is attributed to sodium ion insertion into porous carbon.11,23 For the anodic process, the peak located

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Fig. 3 (a) Cyclic voltammograms of ACFs-C for the first 5 cycles in the potential range of 0–3 V vs. Na+/Na at a scan rate of 0.1 mV s1; (b) galvanostatic discharge–charge profiles of ACFs-C at a current density of 50 mA g1; (c) cycle performance of ACFs-3, ACFs-5 and ACFs-C at a current density of 50 mA g1; (d) rate performance of ACFs-3, ACFs5 and ACFs-C in the potential range of 0–3 V vs. Na+/Na.

around 0.1 V observed in the rst and subsequent cycles indicates sodium ion extraction from carbonaceous materials. However, a hysteresis occurs, which may result from the defect structure of the carbon materials, as reported for lithium ion batteries.30 Aer the second scan, the CV curves almost overlap with each other, indicating good cycle performance of ACFs-C during sodium ion insertion and extraction. Similar CV behaviours were observed for ACFs-3 and ACFs-5 as shown in Fig. S5.† Fig. 3b shows the galvanostatic charge–discharge proles of ACFs-C. The rst discharge and charge steps deliver a specic capacity of 646 and 296 mA h g1 (corresponding to a coulombic efficiency of 46%, comparable to the reported carbon anodes for Na-ion batteries11,17) at a current density of 50 mA g1, respectively. The large irreversible capacity results from the irreversible reaction between sodium and surface functional group(s) as well as with the electrolyte forming a SEI layer. The initial discharge and charge capacities for ACFs-3 and ACFs-5 are 601, 310 and 702, 288 mA h g1, respectively (Fig. S6†). All ACFs-3, ACFs-5 and ACFs-C show excellent capacity retention with high reversible capacities of 215, 222 and 243 mA h g1 maintained even aer 100 cycles. Aer several cycles, the coulombic efficiency approaches 100% (Fig. 3c). The rate performance of the nitrogen doped carbon bres is extraordinary (Fig. 3d). Especially for the ACFs-C, the reversible capacity at a current of 1, 2, 5, and 10 A g1 can still be as high as 153, 134, 101 and 72 mA h g1, respectively. The bre-like morphology of this nitrogen doped carbon bres plays an important role in optimizing and even enhancing the electrochemical performance. First, the bres interconnect with each other, which is benecial concerning the transfer of sodium

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ions and electrons. Second, the nitrogen doping enhances22 the (pseudo) capacitive function in the rate performance. The capacity retention ability of ACFs-C at 5 A g1 is excellent, aer 100 cycles, still a capacity of 100 mA h g1 is maintained (Fig. S7†). In contrast, the Ppy precursor shows very poor electrochemical performance when used as an anode for Na-ion batteries, which deliver capacities of 136 and 33 mA h g1 at 0.1 and 1 A g1 (Fig. S8†). These results indicate that transformation from Ppy to carbon with carbonization and activation processes favours the sodium ion storage. The average discharge–charge voltage for ACFs-C is 1.1 V, which might be ascribed to the low crystallinity, as shown from XRD, Raman and TEM results. In the potential range of 0.005–2 V vs. Na+/Na, ACFs-C also show superior rate performance; the capacities of 95, 66, and 33 mA h g1 are realized for current densities of 1, 2, and 5 A g1 (Fig. S9†), indicating the practical applicability of ACFs-C. Further insight comes from the CV curves and galvanostatic discharge–charge proles. In detail, for all the nitrogen doped porous carbon materials, a rectangular shaped CV curve was observed in the potential range of 0–3 V, with the galvanostatic discharge–charge proles being quite linear. This reveals the (pseudo) capacitive properties of such an anode material for Na ion batteries. To better understand the (pseudo) capacitance contribution to the total charge storage, cyclic voltammetry with multi-scan rates was applied (Fig. S10†). The total current resulting from diffusion controlled Faradic processes and double layer charging can be represented as: i(V) ¼ a1v + a2v1/2

(1)

with v representing the scan rate, a1v corresponding to the current of (pseudo) capacitance contribution, while a2v1/2 corresponds to the diffusion controlled process, according to Cottrell's equation.31,32 Aer interpreting the results from CV curves with scan rates of 0.5, 1, 2, 5, and 10 mV s1, one can determine the a1 and a2 values (see Fig. S10†), as well as the (pseudo) capacitive current contribution at a given potential, by plotting i(V)/v 1/2 versus v1/2. Note that here only the sodium ion insertion (cathodic) process was considered, since the baseline of the anodic process is difficult to determine in this case. The total charge storage of ACFs-C at 5 mV s1 is 659 C g1, out of which 477 C g1 corresponds to (pseudo) capacitance contribution (Fig. 4). For ACFs-3 and ACFs-5, the total charge storage is 540 and 587 C g1, while the 360 and 370 C g1 originate from the (pseudo) capacitance effect, respectively (Fig. 4 and S11†). The analysis above indicates the (pseudo) capacitance contribution to be signicantly associated with storage for the asprepared porous carbon bres at relatively high scan rates, i.e. high current load. The prime contribution of the (pseudo) capacitance is attributed to the surface reaction. It is surprising that ACFs-C with the smallest surface area (Table 1) deliver the largest (pseudo) capacitance charge storage among all the asprepared porous carbon bres. A similar phenomenon is observed for the rate performance: when a high current load was applied, ACFs-C present superior capacity retention ability compared with the other two (Fig. 3d). The explanation then

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(a) Separation of charge storage contributions of ACFs-C from (pseudo) capacitance and a diffusion-controlled process at a scan rate of 5 mV s1, marked with sparse and concentrated stripes, respectively and (b) charge contributions from (pseudo) capacitance and a diffusion-controlled process for ACFs-C, ACFs-3 and ACFs-5 at a scan rate of 5 mV s1. Fig. 4

should root in the different surface chemistry due to the different roles of the N atoms in N doped carbonaceous materials. This gives rise to different electrochemical properties in the application of aqueous supercapacitors,22 e.g. pyridinic-N and graphitic-N can generate pseudocapacitance by interacting with protons in an acidic electrolyte. In our case, among all the samples, the ACFs-C with the highest amount of pyridinic-N and quaternary-N display the best electrochemical performances, especially rate performance and highest (pseudo) capacitance. Another reason which can be ascribed to the excellent electrochemical performance is observed from the impedance spectra (Fig. 5). ACFs-C electrodes measured before cycling

Electrochemical impedance spectra of ACFs-C, ACFs-3 and ACFs-5 before cycling.

Fig. 5

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exhibit the smallest charge transfer resistance, which guarantees excellent rate performance, superior to those of the other two samples. The superior charge transfer property may also stem from a higher amount of pyridinic and quaternary N, not only on the surface, but also in the inner part of ACFs-C (Fig. S4†). The relatively smaller content of oxygen of ACFs-C may also be ascribed to their higher conductivity, compared with ACFs-3 and ACFs-5.33

Conclusion Nitrogen doped porous carbon bres as anode materials for sodium ion batteries were investigated, among which the ACFsC, with a relatively small surface area, show excellent capacity retention ability and extraordinary rate performance. The reversible capacity at 0.05 A g1 is 296 mA h g1 and could still maintain 72 mA h g1 at a current of 10 A g1. The good electrochemical performance is attributed to the bre-like morphology with large interlayer distance, and easier charge transfer properties, according to the higher amount of pyridinic and quaternary N on the surface and the bulk, compared with ACFs-3 and ACFs-5, facilitating both sodium ion and electron transfers. Though the average voltage is 1 V vs. Na+/Na, the ACFs-C can still be considered as promising anode candidates for low-cost sodium ion batteries, due to their excellent rate performance. The increase of the crystallinity may further improve their electrochemical performance.

Experimental Synthesis of ACFs-C, ACFs-3 and ACFs-5 The nitrogen doped porous carbon bers were synthesized by pyrolysis of Ppy nanober precursors. The Ppy nanobers were prepared via an oxidative template assembly route.25 Typically, 0.45 g cetrimonium bromide (CTAB) was dissolved in 120 mL 1 M HCl at 0  C and forms a transparent solution. Then 0.82 g ammonium peroxydisulfate (APS) was added, aer which a white precipitate was formed. Then 960 mL pyrrole was added and the solution was mildly stirred for 24 h at 0–5  C. The black precipitate was ltered and washed with 1 M HCl, deionized water and acetone until the ltrate became neutral and colorless. The as-prepared Ppy nanobers were dried at 80  C under vacuum. The ACFs-C were prepared by rst carbonizing the Ppy nanobers at 650  C for 0.5 h in an Ar atmosphere, with a slow temperature increase (3  C min1), followed by mixing with KOH (KOH : Ppy ¼ 3 : 1, weight ratio) and activation at 650  C in an Ar atmosphere for 0.5 h. Finally the ACFs-C were collected by washing with 0.1 M HCl, deionized water and drying at 80  C under vacuum. The ACFs-3 and ACFs-5 were prepared according to a similar treatment but without the pre-carbonization procedure. The weight ratio of KOH and the Ppy precursor for ACFs-3 and ACFs-5 was 3 and 5, respectively. Characterization The morphology of the prepared samples was investigated with scanning electron microscopy (SEM, JEOL, Tokyo) and

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transmission electron microscopy (TEM, JEOL 4000FX). Raman spectra were recorded on a Jobin Yvon LabRam spectrometer (excitation wavelength: 632.8 nm). Nitrogen adsorption and desorption isotherms were measured with a Quanta Chrome Adsorption Instrument. X-ray photoelectron spectroscopy (XPS) spectra were recorded with an Axis Ultra Instrument (Kratos Analytical Ltd., UK) to investigate the surface components on the surface. The structures of the as-prepared samples were recorded by X-ray diffraction (XRD) (Philips) using Cu Ka radiation.

Electrochemical measurements Two-electrode Swagelok cells were used for electrochemical testing. The working electrode was prepared by spreading 80% active materials (here ACFs-C, ACFs-3 and ACFs-5), 10% Super P and 10% poly(vinylidene diuoride) (PVDF) onto a copper foil, followed by drying at 80  C under vacuum overnight and cutting into pellets with a diameter of 8 mm (with 1 mg active material on each pellet, 10 mm). The cell was assembled in a glove box lled with argon. Sodium metal was used as the counter electrode, glass ber (GF/D, Whatman) as a separator and 1 M NaClO4 in propylene carbonate (PC) as the electrolyte. Cyclic voltammetry measurement was carried out with a Voltalab 80 electrochemical workstation with multi-scan rates of 0.1, 0.2, 0.5, 1, 2, 5, and 10 mV s1. The galvanostatic charge–discharge proles were recorded on an Arbin MSTAT battery test station in the potential range of 0.005–3 V vs. Na+/Na. Electrochemical impedance measurements were carried out (for electrodes prepared without adding super P) on a VoltaLab 80 electrochemical workstation as well, in the frequency range from 100 kHz to 0.01 Hz, and the voltage perturbation was 5 mV.

Acknowledgements We thank Prof. Chunlei Wang for useful discussion, Ms A. Fuchs for SEM and BET analysis, Ms G. G¨ otz for XRD measurements, Dr M. Konuma and Mr A. Schulz for XPS and Raman measurements. This work was nancially supported by the Soa Kovalevskaja award from Alexander von Humboldt Foundation, the National Natural Science Foundation of China (no. 21171015 and 21373195), the “1000 plan” from the Chinese Government and the program for New Century Excellent Talents in University (NCET). K. Song acknowledges nancial support from the Doctor Training Program between the Max Planck Society and the Chinese Academy of Sciences. The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement no. 312483 (ESTEEM2).

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Nanoscale, 2014, 6, 1384–1389 | 1389

Nitrogen doped porous carbon fibres as anode materials for sodium ion batteries with excellent rate performance.

Nitrogen-doped activated porous carbon fibres (ACFs) were prepared as anode materials for Na-ion batteries. They exhibit excellent electrochemical per...
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