Nitrogen-doped carbon nanofibers derived from polyacrylonitrile for use as anode material in sodium-ion batteries

CARBON 94 (2015) 189–195

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CARBON journal homepage: www.elsevier.com/locate/carbon

Nitrogen-doped carbon nanofibers derived from polyacrylonitrile for use as anode material in sodium-ion batteries Jiadeng Zhu, Chen Chen, Yao Lu, Yeqian Ge, Han Jiang, Kun Fu, Xiangwu Zhang ⇑ Fiber and Polymer Science Program, Department of Textiles Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USA

a r t i c l e

i n f o

Article history: Received 14 April 2015 Received in revised form 21 June 2015 Accepted 29 June 2015 Available online 29 June 2015

a b s t r a c t Nitrogen-doped carbon nanofibers (N-CNFs) derived from polyacrylonitrile were successfully synthesized by a combination of electrospinning and thermal treatment processes. The as-prepared N-CNFs were used as anode material for sodium-ion batteries due to their unique fabric and weakly-ordered turbostratic structure as well as large spacing between graphene layers. Results show that N-CNFs carbonized at 800 °C delivered a high reversible capacity of 293 mAh g 1 at a current density of 50 mA g 1 in the first cycle. Even though the first-cycle Coulombic efficiency was 64%, it increased to nearly 100% only after a few initial cycles. Additionally, these N-CNFs showed excellent cycling and high-rate performance, and maintained a capacity of up to 150 mAh g 1 even at an extremely high current density of 1000 mA g 1 for over 200 cycles. It is, therefore, demonstrated that N-CNFs prepared under appropriate conditions are promising anode material candidate for sodium-ion batteries. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Lithium-ion batteries (LIBs) with high energy density and long cycle life have been widely used in our daily life such as electrical vehicles, mobile phones, laptops, etc [1–3]. However, the use of LIBs in larger-scale applications, such as smart power grids and stationary energy storage, is hindered by the limited global Li supply. Sodium-ion batteries (SIBs) have recently obtained significant attention not only because Na and Li have similar electrochemical properties but Na is one of the most abundant elements in the world. As sodium ion has a larger ionic radius than lithium ion (102 nm for Na+ and 76 nm for Li+), typical graphite material, which has been extensively used as the anode material in LIBs, shows poor electrochemical performance in SIBs because only a limited number of sodium can be inserted into graphite layers [4,5]. Many attempts have been made to find suitable carbonaceous anode materials for SIBs, including expanded graphite, biomass derived hard carbon, reduced graphene oxide, carbon microspheres, and porous carbon fibers [6–10]. The carbonaceous materials have been tested with capacities between 100 and 300 mAh g 1 under various conditions. However, these capacity values could be only obtained at low current densities or at high temperatures with limited cycle numbers. Hence, alternative carbon materials are still needed in order to achieve satisfactory performance at room temperature and high current densities for SIBs. ⇑ Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.carbon.2015.06.076 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved.

It is worth noting that doping nitrogen into the carbon structure is an efficient method to improve the electrochemical performance of carbon-based Li-storage anodes because N-doping can generate extrinsic defects and hence enhance the electrode reaction [11–13]. This strategy could be employed to develop carbonaceous Na-storage anode materials for achieving high electrochemical performance. Herein, in this study, we demonstrate a new Na storage material with excellent electrochemical performance by using N-doped carbon nanofibers (N-CNFs) derived from polyacrylonitrile. The as-prepared N-CNFs exhibit high capacity, good rate capability as well as excellent cycle stability due to their weakly-ordered turbostratic structure and large interlayer spacing between graphene sheets. In addition, the fabric structure formed by continuous N-CNFs provides a good conducing connectivity which is beneficial for Na ion de/insertion. As a result, N-CNFs prepared under appropriate conditions show a high reversible capacity of up to 293 mAh g 1 at a current density of 50 mA g 1 in the first cycle and 150 mAh g 1 (a retention of 93.7%) even at an extremely high current density of 1000 mA g 1 over 200 cycles, as well as an excellent rate capability.

2. Experimental 2.1. N-CNF Preparation A viscous and homogeneous solution was first prepared by adding 2.4 g polyacrylonitrile (PAN, Mw = 150,000, Sigma–Aldrich) to 30 g N,N-dimethylformamide (DMF, >99.5%, Sigma–Aldrich),

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followed by mechanical stirring at 50 °C for 24 h. The solution was transferred to a plastic syringe and was electrospun into nanofibers under a high voltage of 15 kV. During electrospinning, the needle-collector distance and the feed rate were fixed at 15 cm and 0.75 ml min 1, respectively [14]. The as-spun nanofibers were first stabilized at 250 °C for 2 h in air with a heating rate of 5 °C min 1, and then carbonized under a nitrogen atmosphere at 700, 800, and 900 °C, respectively, for 2 h with a heating rate of 2 °C min 1. The nitrogen atmosphere (Airgas National Welders, USA) was used for the purpose of N-doping. The N-CNFs prepared at the three different carbonization temperatures were denoted as N-CNF-700, N-CNF-800, and N-CNF-900, respectively. 2.2. Characterization X-ray diffraction (XRD) patterns were recorded on Rigaku D/max 2400, Japan, with Cu Ka (k = 1.5418 Å) radiation in a 2-Theta range from 10° to 60°. Raman spectroscopy (Renishaw Raman 2000, USA) was used to detect the graphitic carbon structure and the presence of defects for N-CNFs using a 514 nm laser beam. The morphology of N-CNFs was investigated by using the field-emission scanning electron microscopy (FESEM, FEI Quanta 3D, USA) at 10 kV and the high-resolution transmission electron microscopy (HR-TEM, JEOL-2010F, Japan). To prepare samples for HR-TEM observation, N-CNFs were dispersed in ethanol, which was deposited on the copper grid and dried at 60 °C for three hours. Surface area was measured by using N2 adsorption–desorption isotherms at 77 K on a Gemini VII 2390 Series analyzer (Micrometrics, USA). X-ray photoelectron spectroscopy (XPS, SPECS FlexMod, Germany) was used to execute elemental analysis and identify surface functional groups for N-CNFs at room temperature with a Kratos Analytical spectrometer and monochromatic Mg Ka X-ray source. The element analysis was performed by using an elemental analyzer (Perkin Elmer 2400 Series II CHNS/O). 2.3. Electrochemical measurement The working electrodes were prepared by mixing 80 wt% active material (N-CNF-700, N-CNF-800, and N-CNF-900, respectively), 10 wt% Super P conductor (TIMCAL, Graphite & Carbon Ltd, C-65) and 10 wt% alginic acid sodium salt binder (MP Biomedicals, LLC., USA) in deionized water. The electrodes were dried at 80 °C in vacuum oven overnight to remove the solvent after coating the slurry on the copper foil. To test the electrochemical properties, 2032 type coin cells were assembled using glass fiber mat (GE healthcare) as the separator and sodium metal (Na, Sigma–Aldrich) as the counter electrode in an argon-filled glove box. The electrolyte used was 1 M sodium perchlorate (NaClO4, P98.0%, Sigma– Aldrich) dissolved in a mixture of ethylene carbonate (EC, 99%, Sigma–Aldrich) and dimethyl carbonate (DMC, P99%, Sigma– Aldrich) (EC/DMC = 1:1 by volume). The assembled cells were tested by a LADN-CT 2001A battery system with a voltage range between 2.5 V and 0.001 V vs Na+/Na at ambient temperature. Cyclic voltammetry (CV) measurements were carried out by using a Gamay reference 600 Potentiostat/Galvanostat/ZRA device with a scan rate of 0.1 mV s 1 in a potential range of 2.5–0.001 V. Electrochemical impedance spectroscopy (EIS) data of N-CNFs before and after cycling were obtained with the same device from 1 MHz to 0.01 Hz with an AC voltage amplitude of 5 mV. 3. Results and discussion 3.1. Morphology and structure characterization The morphology and microstructure of N-CNFs were observed by FE-SEM, as shown in Fig. 1. With the carbonization

temperature increased from 700, 800, to 900 °C, the diameter of N-CNFs decreases from 250, 200, to 150 nm since larger amount of small molecules were released at higher temperatures during carbonization. Fig. 2 shows the HRTEM images of N-CNFs. Turbostratic structure and rough fiber surface are observable, indicating the presence of amorphous structure and structural defects which are favorable for Na-ion diffusion from various orientations and can provide sufficient contact area between active materials and the electrolyte [13]. The nitrogen-sorption data of the N-CNFs are shown in Table 1. The specific Brunauer– Emmett–Teller (BET) surface area and pore volume of N-CNFs increase with increase in carbonization temperature. The BET surface area is 16.23, 84.27, and 513.89 m2 g 1 for N-CNF-700, N-CNF-800 and N-CNF-900, respectively, with pore volume of 0.011, 0.058, and 0.354 cm3 g 1. The XRD patterns of N-CNFs in Fig. 3(a) present a weak and broad peak at around 24.25°, corresponding to the (0 0 2) plane, a characteristic of the disordered carbon material. The interlayer spacing (d002) of N-CNFs was calculated by the Bragg’s law and the results are shown in Table 1. With increase in carbonization temperature from 700, 800, to 900 °C, the interlayer spacing decreases slightly from 0.367, 0.363, to 0.359 nm, but is always greater than that (0.336 nm) of graphite. For carboneous anodes, the turbostratic structure and large interlayer spacing between graphene sheets are beneficial to the reversible storage of large sodium ions [7]. Raman spectra were recorded to study the degree of graphitization of the as-prepared N-CNFs, as shown in Fig. 3(b). The broad D (disordered portion) and G bands (ordered graphitic structure) suggest that N-CNFs contain partially graphitized carbon along with amorphous carbon. The graphitized carbon generally consists of assemblies of graphitic layers, which are expected to act as a good host for the storage of sodium ions [7]. The relative intensity ratio (ID/IG) between the D and G bands indicates the degree of the disorder nature of carbon structure. As shown in Table 1, the ID/IG ratio decreases with increase in carbonization temperature, representing the transformation of disordered carbon into graphitic carbon during the carbonization process. However, it is noteworthy that the co-existence of a prominent D band and a wide G band show the loss of long-range ordering between the graphene sheets, which is consistent with the XRD and TEM results. Comparing Fig. 3(a) and (b), it is obvious that the absence of (1 0 0) and (0 0 4) planes (in XRD patterns) and the presence of the D peak (in Raman spectra) indicate that the PAN-derived N-CNFs are typical non-graphitized carbon [15]. Table 2 presents the elemental analysis results for N-CNFs. The hydrogen (H) and nitrogen (N) contents decrease with increasing carbonization temperature, while the content of carbon (C) increases. The weight percentages of H and N decrease from 1.27 and 16.28 wt% to 0.39 and 10.68 wt%, respectively; while a 5.26 wt% increment can be observed for C when the carbonization temperature increases from 700 °C to 900 °C. XPS was performed to further identify the surface functionalities of N-CNFs, and the obtained high-resolution N1s spectra are shown in Fig. 4. All three N-CNFs show two N1s peaks, which can be fitted by three component peaks at 398.2, 400.2 and 401.8 eV, which are assigned to pyridinic (N-6), pyrrolic/pyridine (N-5), and quaternary nitrogen (N-Q), respectively. From Fig. 4, it is seen that with increase in carbonization temperature, the relative amount of N-5 decreases, which indicates that N-5 has been chemically transformed into other nitrogen species, such as N-6, N-Q, etc., at higher carbonization temperatures [16]. Notably, the electrochemical performance, especially for rate performance and conductivity, could be improved by the existence of pyridinic and quaternary structures [10].

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(b)

(a)

1 um

1 um

5 um

5 um

(c)

1 um

5 um Fig. 1. SEM images of (a) N-CNF-700, (b) N-CNF-800, and (c) N-CNF-900. The insets are high-resolution images.

3.2. Electrochemical performance The electrochemical behavior of the N-CNFs was evaluated by cyclic voltammetry (CV) analysis and Fig. 5 shows the discharge (Na insertion)/charge (Na extraction) curves for the first five CV cycles. It is seen that N-CNF-700 exhibits three cathodic peaks located at around 1.3, 0.6, and 0 V, respectively, in the first cycle. The peak at around 1.3 V can be ascribed to the reaction between sodium ions and surface functional groups. This peak is still present in the subsequent cycles even though it becomes less distinctive, indicating that the reaction is partially reversible. The peak at 0.6 V originates from the decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI) layer on the surface of N-CNFs, and it disappears in the subsequent cycles. The cathodic peak at 0 V is attributed to sodium ion insertion into porous carbon which is similar for lithium insertion in carbonaceous materials [17,18]. Besides the abovementioned sharp peaks, the broad reduction range (1.75–0.2 V) is ascribed to the mechanism of sodium adsorption/insertion on/between graphene layers in the N-CNFs. For the anodic process, the peak at around 0.1 V in the first and subsequent cycles indicates the extraction of sodium ions from carbonaceous materials [19]. Sodium removal also takes place in a wide potential window (0.2–1.75 V), resulting in the broad oxidation range in the CV curves. After the first scan cycle, the CV curves exhibit good repeatability, indicating the high reversibility of the subsequent reactions. Comparing Fig. 5 (a)–(c), it is seen that the CV behaviors of N-CNF-800 and N-CNF-900 are similar to that of N-CNF-700. It is interesting to note that all the above mentioned peaks left shift with increase in carbonization temperature, probably due to their different structures such as surface area, pore size, functional group content, etc.

Fig. 6 shows the galvanostatic discharge/charge profiles of N-CNFs, tested at a current density of 50 mAh g 1. The reversible capacities of N-CNF-700, N-CNF-800, and N-CNF-900 are 255, 293, and 299 mAh g 1, with irreversible capacities of 51, 162, and 471 mAh g 1, respectively. The initial irreversible capacity could be originated from the decomposition of electrolyte components to form a SEI layer on the electrode surface and/or the irreversible sodium insertion into special positions, such as in the vicinity of residual H atoms in the carbon structure [18,20,21]. Among all three N-CNFs, N-CNF-900 has the highest irreversible capacity, resulting in the lowest initial Coulombic efficiency of 38.8%, mainly owing to its large surface area (513.89 m2 g 1), which is more than 6 and 32 times greater than those of N-CNF-800 (84.27 m2 g 1) and N-CNF-700 (16.23 m2 g 1), respectively. Larger surface area leads to the formation of greater solid electrolyte interphase (SEI), which is irreversible. Similar result has been found by Bommier et al. in their study on hard carbon anodes [22]. In addition to large surface area, N-CNF-900 also has higher micro-pore volume, which can also lead to a higher irreversible Na storage and thus result in poor cycling performance [18]. The cycling performance of the N-CNFs at a current density of 50 mA g 1 is shown in Fig. 7. N-CNF-700, N-CNF-800 and N-CNF-900 exhibit reversible capacities of 185, 254 and 218 mAh g 1 at the 200th cycle, corresponding to high capacity retentions of 77.5%, 86.7%, and 72.9%, respectively. The Coulombic efficiencies of all three N-CNFs reach nearly 100% after the initial several cycles, which indicate the high cyclability of N-CNFs. It should be noted that N-CNF-900 exhibits poorer cycling stability than N-CNF-800. It is probably because the relatively smaller intergraphitic spacing of N-CNF-900 (Table 1) makes sodium ions transport and storage more difficult in this composite.

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(a)

(b)

(c)

Fig. 2. HRTEM images of (a) N-CNF-700, (b) N-CNF-800, and (c) N-CNF-900.

Table 1 Physical parameters of N-CNFs.

c d

Surface areaa (m2 g 1)

Pore volumeb (cm3 g 1)

d002c (nm)

ID / IGd

N-CNF-700 N-CNF-800 N-CNF-900

16.23 ± 1.01 84.27 ± 6.38 513.89 ± 45.84

0.011 0.058 0.354

0.367 0.363 0.359

1.11 1.01 0.94

Surface area was obtained by Brunauer–Emmett–Teller (BET) method. Total pore volume was determined at a relative pressure of 0.98. d002 was calculated by Bragg’s law. ID and IG were the integrated intensities of D-band and G-band.

(a) Intensity (a.u.)

a b

Samples

N-CNF-900

N-CNF-800

N-CNF-700

10

20

(b)

30

40

500

50

2 theta (o)

D

60

G

Intensity (a.u.)

Therefore, good electrochemical properties of N-CNFs can only be obtained by optimizing the thermal treatment conditions, such as treatment temperature and time, and balancing the structural parameters including graphitic interlayer spacing, pore structure, etc [23,24]. Fig. 8 shows the cycling performance of N-CNF-800 at a significantly higher current density of 1000 mA g 1. It is seen that at such a high capacity density, N-CNF-800 still has an initial capacity of 159 mAh g 1, which reduces slowly to 150 mAh g 1 at the 200th cycle, corresponding to a high capacity retention of 93.7% (only a 0.0293% capacity loss per cycle). In addition, N-CNF-800 possesses a Coulombic efficiency of nearly 100%, starting from the second cycle. Consequently, these results indicate N-CNFs have excellent electrochemical performance even at high current densities. It should be noted that this is an exceptionally high capacity (159 mAh g 1 at 1000 mA g 1) compared with other results reported on carbonaceous materials [7,8,25,26].

N-CNF-900 N-CNF-800 N-CNF-700

1000

1500

2000

2500

3000

3500

Raman Shift (cm-1) Fig. 3. (a) XRD patterns and (b) Raman spectra of N-CNFs.

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J. Zhu et al. / CARBON 94 (2015) 189–195 Table 2 Elemental analysis of N-CNFs.

(a)

Element (wt%)

N-CNF-700 N-CNF-800 N-CNF-900

C

H

N

71.31 74.88 76.57

1.27 1.12 0.39

16.28 11.21 10.68

0

Current (µA)

Samples

50

-50

1st 2nd 3rd 4th 5th

-100

(a) Intensity (a.u.)

Raw Fitted N-5 N-6 N-Q

-150

0.0

0.5

1.0

1.5

2.0

2.5

Voltage (V) 50

(b)

406

404

402

400

398

396

394

Binding Energy (eV)

(b)

Current (µA)

0 -50 -100

Raw Fitted N-5 N-6 N-Q

Intensity (a.u.)

1st 2nd 3rd 4th 5th

-150 0.0

0.5

1.0

1.5

2.0

2.5

Voltage (V) 50

(c)

406

404

402

400

398

396

394

Binding Energy (eV)

(c)

Current (µA)

0

1st 2nd 3rd 4th 5th

-100 -150

Raw Fitted N-5 N-6 N-Q

Intensity (a.u.)

-50

-200

0.0

0.5

1.0

1.5

2.0

2.5

Voltage (V) Fig. 5. Cyclic voltammetry of (a) N-CNF-700, (b) N-CNF-800, and (c) N-CNF-900 at a scan rate 0.1 mV s 1.

406

404

402

400

398

396

2.5

394

Binding Energy (eV)

The electrochemical impedance spectroscopy (EIS) measurements were carried out to characterize N-CNFs before and after 200 cycles at a current density of 1000 mA g 1 and the corresponding Nyquist plots are shown in Fig. 9(a) and (b), respectively. The semicircle in the high frequency region is corresponded to the charge transfer resistance. It is seen that the charge transfer resistances of N-CNF-700, N-CNF-800 and N-CNF-900 before cycling are 78, 48, and 43 ohm cm2, respectively, which increase to 140, 98, and 90 ohm cm2 after cycling. The high charge transfer resistance of N-CNF-700 is probably due to its lower electrical conductivity

Voltage (V)

Fig. 4. N1s XPS spectra of (a) N-CNF-700, (b) N-CNF-800, and (c) N-CNF-900.

2.0 1.5 1.0 0.5 0.0

(a) 0

(b)

(c)

100 200 300 400 500 600 700 800

Specific Capacity (mAh g -1)

Fig. 6. Galvanostatic charge/discharge profiles of (a) N-CNF-700, (b) N-CNF-800, and (c) N-CNF-900 at a current density of 50 mA g 1 for the first cycle.

J. Zhu et al. / CARBON 94 (2015) 189–195

100 300 80 60

200

40 100 0

N-CNF-700 N-CNF-800 N-CNF-900

0

25

50

75

100 125 Cycle Number

150

175 1

.

300

100 80

200

1000 mA g-1

60 40

100 charge discharge

0

0

25

50

75

100 125 Cycle Number

150

175

Fig. 8. Cycle performance of N-CNF-800 at a current density of 1000 mA g

100

(a)

N-CNF-700 N-CNF-800 N-CNF-900

-Z'' (ohm cm2)

80 60 40 20 0 0

25

50

75 100 125 150 175 200

Z' (ohm cm2) 200 N-CNF-700 N-CNF-800 N-CNF-900

(b) -Z'' (ohm cm2)

0

Coulombic Efficiency (%)

Specific Capacity (mAh g-1)

Fig. 7. Cycle performance of N-CNFs at a current density of 50 mA g

200

20

Coulombic Efficiency (%)

Specific Capacity (mAh g-1)

194

150

200

20 0

1

.

and weakly-ordered turbostratic carbon structure of N-doped CNFs [26–28]. To assess the feasibility of using N-CNFs in SIBs for high-power applications, their rate performance was evaluated by increasing the current density stepwise from 50 to 1000 mA g 1 every 10 cycles and then returning to 50 mA g 1 after cycling at 1000 mA g 1 (Fig. 10). It is seen that N-CNF-800 possesses the best rate performance among all three N-CNFs. The reversible capacities of N-CNF-800 are retained at 250, 228, 209, 184, and 159 mAh g 1, respectively, when the current density increases from 50, 100, 200, 500, to 1000 mA g 1. The capacity of N-CNF-800 returns to 243 mAh g 1 when the current density goes back to 50 mA g 1 after the rate performance test, indicating its excellent reversibility. The superior electrochemical performance of N-CNF-800 is attributed to the unique nanofibrous structure with appropriate surface area. As illustrated in Fig. 11, first of all, the unique nanofibrous carbon structure formed by continuous N-CNFs ensures an efficient and uninterrupted electron transport. Secondly, the large interlayer spacing between graphene layers facilitates sodium ion transport and storage, which are extremely important for sodium ions since they are much larger than lithium ions [10].

100

4. Conclusions

50

0

0

100

200

300

Z' (ohm cm2)

400

500

Fig. 9. Electrochemical impedance spectroscopy analyses of the N-CNFs (a) before and (b) after 200 cycles at 25 °C and under a current density of 1000 mA g 1.

compared to N-CNF-800 and N-CNF-900 [15]. It is important to note that the steeper tail in low frequency region of each N-CNF demonstrates high sodium ion conductivity in such electrode material, which may be attributed to the unique fabric assembling

N-CNFs derived from PAN were fabricated by a combination of electrospinning and thermal treatment. These N-CNFs exhibit stable cyclability and excellent rate capability because of their unique fabric and weakly-ordered turbostratic structure as well as large spacing between graphene layers. Especially for N-CNF-800, it delivered a high reversible capacity of 293 mAh g 1 at a current density of 50 mAh g 1 in the first cycle with a Coulombic efficiency of 64% which increased to nearly 100% only after a few initial cycles. Additionally, it maintained a reversible capacity up to 150 mAh g 1 even at a high current density of 1000 mA g 1 with a retention of 93.7% after 200 cycles. Therefore, it is demonstrated that N-CNFs prepared under appropriate conditions is promising anode material candidate for SIBs.

195

Specific Capacity (mAh g-1)

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50 mA g-1

300

100 mA g-1

200 mA g-1

200 100 0

50 mA g-1 500 mA g

-1

1000 mA g-1

N-CNF-700 N-CNF-800 N-CNF-900

0

10

20

30 Cycle Number

40

50

60

Fig. 10. Capacity retention of N-CNFs with varying current densities.

Intercalated Na+ Absorbed Na+

Na+ in nanopores

e-

Na+

Carbon

Fig. 11. Schematic illustration of sodium storage mechanism in N-CNFs.

Acknowledgment The authors appreciate the use of Analytical Instrumentation Facility (AIF) at North Carolina State University.

References [1] P.G. Bruce, B. Scrosati, J.M. Tarascon, Nanomaterials for rechargeable lithium batteries, Angew. Chem. Int. Ed. 47 (2008) 2930–2946. [2] J. Ji, H. Ji, L.L. Zhang, X. Zhao, X. Bai, X. Fan, et al., Graphene-encapsulated Si on ultrathin-graphite foam as anode for high capacity lithium-ion batteries, Adv. Mater. 25 (2013) 4673–4677. [3] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Recent development in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682–2699. [4] Y.W. Cheng, C.K. Lin, Y.C. Chu, A. Abouimrane, Z. Chen, Y. Ren, et al., Electrically conductive ultrananocrystalline diamond-coated natural graphite-copper anode for new long life lithium-ion battery, Adv. Mater. 26 (2014) 3724–3729. [5] K. Zaghib, X. Song, A. Guerfi, R. Rioux, K. Kinoshita, Purification process of natural graphite as anode for Li-ion batteries: chemical versus thermal, J. Power Sources 119–121 (2003) 8–15. [6] Y. Wen, K. He, Y. Zhu, F. Han, Y. Xu, I. Matsuda, et al., Expanded graphite as superior anode for sodium-ion battery, Nat. Commun. (2014) 5. [7] K. Hong, L. Qie, R. Zeng, Z. Yi, W. Zhang, D. Wang, et al., Biomass derived hard carbon used as a high performance anode material for sodium ion batteries, J. Mater. Chem. A 2 (2014) 12733–12738. [8] Y.X. Wang, S.L. Chou, H.K. Liu, S.X. Dou, Reduced graphene oxide with superior cycling stability and rate capability for sodium storage, Carbon 57 (2013) 202– 208. [9] R. Alcántara, P. Lavela, G.F. Ortiz, J.L. Tirado, Carbon microspheres obtained from resorcinol-formaldehyde as high-capacity electrodes for sodium-ion batteries, Electrochem. Solid-State Lett. 8 (4) (2005) A222–A225. [10] L. Fu, K. Tang, K. Song, P.A. Aken, Y. Yu, J. Maier, Nitrogen doped porous carbon fibers as anode materials for sodium ion batteries with excellent rate performance, Nanoscale 6 (2014) 1384–1389. [11] Z. Zhou, X. Gao, J. Yan, D. Song, M. Morinaga, A first-principles study of lithium absorption in boron or nitrogen-doped single-walled carbon nanotubes, Carbon 42 (2004) 2677–2682. [12] L. Qie, W.M. Chen, Z.H. Wang, Q.G. Shao, X. Li, L.X. Yuan, et al., Nitrogen-doped porous carbon nanofiber webs as anodes for lithium ion batteries with a superhigh capacity and rate capability, Adv. Mater. 24 (2012) 2047–2050.

[13] W.H. Shin, H.M. Jeong, B.G. Kim, J.K. Kang, J.W. Choi, Nitrogen-doped multiwall carbon nanotubes for lithium storage with extremely high capacity, Nano Lett. 12 (2012) 2283–2288. [14] B.-Y. Hsieh, J. Kim, J. Zhu, S. Li, X. Zhang, X. Jiang, A laser ultrasound transducer using carbon nanofibers-polydimethylsiloxane composite thin film, Appl. Phys. Lett. 106 (2015) 021902. [15] C. Kim, K.S. Yang, M. Kojima, K. Yoshida, Y.J. Kim, Y.A. Kim, et al., Fabrication of electrospinning-derived carbon nanofiber webs for the anode material of lithium-ion secondary batteries, Adv. Funct. Mater. 16 (18) (2006) 2393–2397. [16] Z. Wang, Y. Li, X.-J. Lv, N-doped mesoporous carbon as high performance anode material in sodium ion batteries at room temperature, RSC Adv. 4 (2014) 62673–62677. [17] Y. Cao, L. Xiao, M.L. Sushko, W. Wang, B. Schwenzer, J. Xiao, et al., Sodium ion insertion in hollow carbon nanowires for battery applications, Nano Lett. 12 (2012) 3783–3787. [18] J.R. Dahn, T. Zheng, Y. Liu, J.S. Xue, Mechanisms for lithium insertion in carbonaceous materials, Science 270 (1995) 590–593. [19] N.A. Kaskhedikar, J. Maier, Lithium storage in carbon nanostructures, Adv. Mater. 21 (2009) 2664–2680. [20] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novlk, Insertion electrode materials for rechargeable lithium batteries, Adv. Mater. 10 (10) (1998) 725–763. [21] S. Wang, S. Yata, J. Nagano, Y. Okano, H. Kinoshita, H. Kikuta, et al., A New carbonaceous material with large capacity and high efficiency for rechargeable Li-ion batteries, J. Electrochem. Soc. 147 (7) (2000) 2498–2502. [22] C. Bommier, W. Luo, W.-Y. Gao, A. Greaney, S. Ma, X. Ji, Predicting capacity of hard carbon anodes in sodium-ion batteries using porosity measurements, Carbon 76 (2014) 165–174. [23] J. Jin, Z. Shi, C. Wang, Electrochemical performance of electrospun carbon nanofibers as free-standing and binder-free anodes for sodium-ion and lithium-ion batteries, Electrochim. Acta 141 (2014) 302–310. [24] J. Ding, H. Wang, Z. Li, A. Kohandehghan, K. Cui, Z. Xu, et al., Carbon nanosheet frameworks derived from peat moss as high performance sodium ion battery anodes, ACS Nano 7 (12) (2013) 11004–11015. [25] Z. Wang, L. Qie, L. Yuan, W. Zhang, X. Hu, Y. Huang, Functionalized N-doped interconnected carbon nanofibers as an anode material for sodium-ion storage with excellent performance, Carbon 55 (2013) 328–334. [26] T. Chen, Y. Liu, L. Pan, T. Lu, Y. Yao, Z. Sun, et al., Electrospun carbon nanofibers as anode materials for sodium ion batteries with excellent cycle performance, J. Mater. Chem. A 2 (2014) 4114–4121. [27] L. Ji, X. Zhang, Generation of activated carbon nanofibers from electrospun polyacrylonitrile-zinc chloride composites for use as anodes in lithium-ion batteries, Electrochem. Commun. 11 (2009) 684–687. [28] L. Ji, X. Zhang, Fabrication of porous carbon nanofibers and their application as anode materials for rechargeable lithium-ion batteries, Nanotechnology 20 (15) (2009) 155705.