Study of Microstructure Change of Carbon Nanofibers as Binder-Free

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Study of Microstructure Change of Carbon Nanofibers as Binder-Free Anode for High-Performance Lithium-Ion Batteries Ting Wang,† Shaojun Shi,† Yuhong Li,†,‡ Mengxi Zhao,† Xiaofeng Chang,‡ Di Wu,‡ Haiying Wang,† Luming Peng,‡ Peng Wang,§ and Gang Yang*,† †

Jiangsu Laboratory of Advanced Functional Material, School of Chemistry and Materials Engineering, Changshu Institute of Technology, Changshu 215500, P. R. China ‡ Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P.R. China § National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: Flexible and binder-free film of N, O-doped carbon nanofibers (CNFs) is the ideal anode for high-energy-density batteries. Here, CNFs flexible films which the N, O dopant give defect in graphite structure results in high specific surface area more than 500 m2 g−1. A flexible film of CNF800 carbonized at 800 °C delivers initial capacities of 2000 and 755 mAh g−1 at the current densities of 5 and 10 A g−1, respectively. After 500 cycles, CNF800 remains the capacities of 1251, 865, 702, and 305 mAh g−1 at 0.5, 1, 5, and 10 A g−1, respectively. The microstructures of CNFs under various state of charge are studied by HRTEM, XPS, 13C NMR, and so forth. The lithiation/delithiation mainly happens to the interlayer of graphite domain of CNFs. The dopants of nitrogen and oxygen involve in lithiation, but much of LiN is irreversible. The excellent performances of CNFs film can be attributed to the N, O doped structure of graphite domain that has increased the conductivity and lithium storage ability. Further development of N, O doped CNFs may enable practical applications as flexible anode in high-performance lithium-ion batteries. KEYWORDS: lithium-ion batteries, binder-free anode, carbon nanofibers, flexible film, electrochemical performance

1. INTRODUCTION Lithium ion batteries (LIBs) play a key role in the expanding growth of electronic vehicles (EVs), due to their high energy density and power density, long cycle performance and environmental friendliness, and so forth. The performance of LIB mainly relies on the capacities of electrode materials.1−3 Graphite is the most widely used anode material for current LIBs. However, it cannot meet the increasing demands and requirements for EVs and electricity storage stations used in large-scale, because of its low specific capacity of 372 mAhg−1 and poor rate performance.4,5 Therefore, other forms of carbon materials are widely studied as candidates to increase the energy density and rate performance of the LIBs. A number of disordered carbons prepared below 1000 °C termed “turbostratic” carbon and exhibited random stacking, have attracted much attention in their specific capacities up to 850 mAhg−1 in the past decade.6−8 The disordered carbons are usually classified into graphitizable (“soft”) and nongraphitizable (“hard”) carbons. Both types have small graphene sheets stacked in a roughly graphite domain with short-range order.9,10 The disordered nature of these carbon materials may lead to improving the specific capacities, because of the enlarged © XXXX American Chemical Society

interlayer spacing (compared to pure graphite), and they usually provide the prospect of improved lifetimes because the distortion of the graphene lattice can be minimized during lithium insertion/extraction.11−13 Recently, nitrogen has been used as dopants in carbon materials further improving the electrochemical performance of LIBs. Kleinsorge et al. concluded that the hybridized N-doped carbon materials could be tuned by the %N by reaction conditions. Bellowing 10% N usually induces C sp2 clustering effects, thereby reducing the band gap and increasing conductivity.14 Such an effect allows for the possibility of favorable LiN interaction appearing in carbon materials as the anode during lithiation/delithiation.15−18 Several experimental and theoretical studies showed that the nitrogen played the positive role improving the specific capacities. Nitrogendoped carbon materials as anode reach to 1000 mAh g−1 as anode, much higher than graphite of 372 mAh g−1. Xiao et al. concluded the stronger interaction between the nitrogen-doped Received: September 21, 2016 Accepted: November 11, 2016 Published: November 11, 2016 A

DOI: 10.1021/acsami.6b11996 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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can be inferred that both N-dopant and porous structures of CNF should be important factors for high-performance LIBs. Until now, the interaction between lithium and N-doped CNF during lithiation and delithiation still need to be pursued. The versatile N state of CNFs synthesized by PAN is a good example to study the origin of the high-performance of Ndoped CNFs as anode for LIBs. In this work, we synthesized N-doped CNF flexible films with controllable morphologies and N content. The flexible film of CNF800 carbonized at 800 °C delivers the initial capacities of 3150, 2000, and 755 mAh g−1 at the current densities of 0.5, 5, and 10 A g−1, respectively. After 500 cycles, CNF800 remains 305 mAh g−1 at the current density of 10 A g−1. Importantly, the fiber structure and flexibility of the film still remain. The main lithiation/delithiation still happen to the interlayer of the graphite domain of CNFs. The dopant of nitrogen is seriously involved in lithiation, but much of the Li N is irreversible. The excellent performances of CNFs film can be attributed to the N, O doped structure of graphite domain that has increased the conductivity and lithium storage ability. Further development of N, O doped CNFs may enable practical applications as flexible anodes in high-rate lithium-ion batteries.

carbon material and lithium is favorable for lithiation/ delithiation, because nitrogen with intrinsic electronegativity of 3.5 is higher than that of carbon of 3.0.19 However, the intrinsic extra-lithiation of nitrogen-doped carbon is still being pursued, because there are several states of N in carbon materials.20,21 Usually, there are two major CN binding states; one is a direct substitutional graphite-like structure and the other is a pyridine-like defect structure with lone-pair electrons that require the rearrangement of the neighboring C atoms.18,22,23 Y.C. Lin et al. demonstrated an efficient method to selectively produce graphitic-N and pyridinic-N defects in graphene by using the mixture plasma of ozone and nitrogen. They found that the pyridinic-N exhibited higher chemical activity and tends to trap metal atoms.24 Cho et al. suggests that both pyridine- and graphite-like structures can be effective for lithium intercalation. Their first-principles calculations of the graphene sheets confirmed that the large storage capacity of both N-doping structures comes from the formation of dangling bonds around the pyridine-like local motives upon lithium intercalation.22Y. F. Li and co-workers investigated pyridine like structures in a series of single-walled CNx nanotubes. They predicted that pyridine-like structures enhanced the capacity due to the large adsorption energy and low energy barrier for lithiation, but graphite-like N atoms formed an electron-rich structure and hinder lithiation.25 However, the different N-doping structures in carbon materials improved the capacities still need to be extended. Polyacrylonitrile (PAN) with abundant nitrogen is widely used on a large scale in the textile industry, and one of the most suitable candidates for making high performance N-doped carbon nanofibers (CNFs).26−28 CNF produced by PAN similar to other N-doped carbon materials has four kinds of nitrogen groups, sp 2 hybridized pyridinic nitrogen, sp 3 hybridized pyrrolic/pyridone nitrogen, sp2 hybridized quaternary nitrogen, and sp2 hybridized pyridonic-nitrogen oxide.29 Peng et al. synthesized a series of CNFs by electrospinning and carbonization. The as-prepared CNF films were directly assembled in LIBs, which delivered 434 mAh g−1 under a current density of 150 mA g−1.28 Considering other kinds of carbon materials, both N-doping and defects in the graphite structure play important roles in high performance of LIBs. D. Golberg’s group reported that the N-doped graphene (NG) as anode delivered an initial capacity of 1284 mAh g−1 at 0.2 C rate. They found evidence that the defects-discrepancy metrics of NG for energy storage, and enlarged edge spacing and surface hole defects have resulted in the improved surface capacitive effects. They concluded that the high rate capability and high specific capacity were due to short-distance orderings formed on edges and profound surface defects during discharging.15 B. Xu’s group reported high-performance lithium storage in nitrogen-rich mesoporous carbon materials. They concluded that the CN bonding could be partially broken and resulted in a dangling bond reacted with lithium to improve the capacities.17 H.M. Jeong’s group reported nitrogen-doped multiwall CNT for lithium storage with extremely high capacity. Their nitrogen-doped CNT contained wall defects through which lithium ions could diffuse so as to occupy a large portion of the interwall space as storage regions.18 Koltonow et al. found that restacked exfoliated sheets created interconnected nanofluidic channels for ion transport.30 To obtain high capacity, good rate performance as well as excellent cycle stability, the intrinsic turbostratic structure and enlarged interlayer spacing of the graphene-like sheets should exist. It

2. EXPERIMENTAL SECTION 2.1. Synthesis. Polyacrylonitrile (PAN, Aldrich, Mw = 150 000) and dimethylformamide (DMF) are analytical reagents. One gram of PAN was stirred and dissolved in DMF for 12 h to produce a 10 wt % solution. 0.05 g urea was dissolved in the solution of PAN/DMF. The mixture was held in a spinning nozzle with a tip diameter of 1 mm. The working high voltage is 15 kV, and the distance between the needle and collector is 16 cm in the electrospinning system. After continuous electrospinning for 4 h, a white-colored film was removed from the aluminum collector. First, the white film was preoxidized in air at 250 °C for 2 h to increase the stability of PAN. Subsequently, the film was heated in N2 atmosphere at 800 °C for 3 h, and the products are simply named as CNF800. 2.2. Characterization. XRD patterns were collected at 0.02° step widths from 10° to 80° (Rigaku diffractometer, Cu Kα radiation). The products were measured using a field emission scanning electron microscope and energy dispersive X-ray detector (FE-SEM and EDX, SIGMA, ZEISS), FT-IR spectra (BRUKER VECTOR 22 spectrometer). Raman spectra were run on a Renishaw microscope (INVIA REFLEX 12-80000). X-ray photoelectron spectroscopy measurements (XPS Thermo Scientific Escalab250 Xi) were carried out with Al−Kα as the radiation source. Transmission electron microscope (TEM) analysis was conducted on a FEI Tecnai F20 TEM with a field emission gun. All imaging results were obtained with an accelerating voltage of 200 kV unless otherwise specified. High angle annular dark field imaging of scanning transmission electron microscopy (STEMHAADF) was achieved using a HAADF detector (Fischione, Model 3000) with an inner semiangle of 36 mrad and an outer semiangle of 180 mrad. The binding energies obtained in the XPS analysis were calibrated against the C 1s peak that was locked at 284.6 eV. 13C Hahn-echo MAS NMR spectra were measured on a 9.4T Bruker Avance III spectrometer using 4.0 mm MAS probes, with the rotor spinning at 14 kHz. All samples were packed into rotors in a N2 glovebox.13C chemical shift is referenced to adamantane, which have two narrow resonances and the more intense one appearing at 38.4 ppm. A rotor synchronized Hahn-echo sequence (π/2−τ−π−τ− acquisition) and a recycle delay of 0.5 s were used to obtain the 13C NMR spectra. 2.3. Electrochemical Test. The flexible CNF film was directly used as the anode without binder or conductive additive. The electrochemical tests were evaluated in coin cell (CR2016). The loading mass of CNFs electrode was about 1.1 mg cm−2. The coin cells were assembled in an argon-filled glovebox using lithium metal as the B


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ACS Applied Materials & Interfaces counter electrode, Celgard 2500 as the separator, and 1 M LiPF6 (dissolved in ethylene carbonate, dimethyl carbonate, and ethyl-methyl carbonate in a 1:1:1 volume ratio) as the electrolyte. The galvanostatic charge and discharge cycling was studied at room temperature in the voltage range of 0.001−3.0 V (vs Li+/Li) at different current densities by LAND CT2001A battery testing system (Wuhan, China). Cyclic voltammetry (CV) was collected on the electrochemical workstation (PARSTAT2273, Princeton Applied Research, U.S.A.) between voltages 0.001 and 3.0 V (vs Li+/Li) at a scan rate of 0.1 mV s−1. To measure the state of CNFs electrode after lithiation/delithiation, the CNF electrode films were removed from the disassembled coin cell, and rinsed with solvent to remove the residual electrolyte for several time. The rinsed CNFs films were dried in argon-filled glovebox overnight to ensure that the entire solvent removed.

(Figure 2). It can be seen that the distribution of C, O, and N elements are homogeneous in the whole fiber.

3. RESULTS AND DISCUSSION 3.1. Preparation and Morphologies of CNFs Films. Figure 1 shows the schematic structure of PAN nanofiber,

Figure 2. SEM image of the cross section of CNF800, and the linear distribution of C, O, and N elements.

3.2. Electrochemical Properties and Characterization of CNFs after Lithiation/Delithiation. The conductivity of freestanding film CNF800 is collected at different bending angles (Figure S1 in the Supporting Information, SI). CNF films present the same conductivity values (0.62 S cm−1) at the dihedral angle range from 0 to 180°. The flexible films of CNF as anode electrode are assembled coin-cell without traditional mixing with conductive carbon and the binder of PVdF. They present good electrochemical properties as shown in Figure 3. The CV and EIS profiles are shown in Figure 3a,b. There are two cathodic peaks at about 0.2 V (not obvious in the first cycle) and 1.25 V, which are indexed to lithium insertion into graphite-like carbon and lithiation of the defective sites on the carbon nanofibers, respectively.10,29,35,36 Another cathodic peak in the first cycle of CNF800 at about 0.65 V could be attributed to the irreversible formation of a solid electrolyte interface (SEI) film.29,35 In the subsequent well-overlapped CV curves, only a very weak peak close to 1.25 V appears in Figure 3a. EIS of CNF800 as binder free anodes carried out at a fresh state of CNF800, the first discharging state (CNF800 firstDC), and the first charging state (CNF800 firstC) are shown in Figure 3b that each curve is composed of depressed semicircles in high and middle frequency and a straight line in low frequency. An equivalent circuit fits the EIS parameters, and the fitted parameters are listed in Table S1 (in the SI). The weight factor x2 is close to 10−4 indicating that the fitted parameters are reliable. The semicircles are always related with two interface impedances including the charge transfer resistance and the polarization resistance between the electrode and the electrolyte interface, and the straight line is associated with ion diffusion toward the electrode (Warburg resistance). Rs, the solution resistance of the cell, presents similar values of the CNF800 anode at various states of charge. Due to CNF800 including porous structure, charge transfer resistances might happen at graphite structure and micropores in CNF they are assigned to Rct and Rct′, respectively. Rct (the charge transfer resistance related with graphite structure) and Rp (polarization resistance) of CNF800 as anode at various states of charge present similar values, indicating a good conductivity of CNF800 at lithiation and delithiation state. At pure state, porous structure of CNF800 produces a very large value of Rct′

Figure 1. Schematic structure of PAN nanofiber (a), stabilized nanofibers (b), and carbon nanofibers (c).

stabilized nanofibers, and carbon nanofibers. The Mw of polyacrylonitrile (CH2CH2CN)n used in this work is 150 000 g mol−1, that is, n is about 2830. The length of a single PAN chain is estimated about 700 nm, and thus a single PAN fiber with micrometers length includes a tremendous amount of PAN chains (as shown in Figure 1a). The thermal stability of the stabilized fiber is attributed to the formation of the ladder structure due to cyclization of the nitrile groups in acrylic molecule (as shown in Figure 1b).31−34 In the following carbonization, the cyclized and oxidized PAN chains are reacted accompanying the removal of small molecules, such as H2O, CO2, N2, and so forth. Finally, PAN converts to carbon nanofiber which a single carbon nanofibers (CNFs) includes a large amount of graphite domain (as shown in Figure 1c). The turbostratic structure of graphite domain in N, O-doped CNFs will be beneficial for the lithiation and delithiation of highperformance LIBs. The elemental distribution in CNF800 is recorded by the linear scanning the cross section of fiber C


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Figure 3. CV profiles (a) carried out at scanning rate of 0.1 mV s−1 and EIS profiles (b) of CNF800 as anode carried out at pure state (CNF800), the 1st discharging (CNF800 1stDC) and 1st charging state (1stC). The initial charge/discharge curves (c) and the cyclic performance (d) of CNF800 at various current densities (the inset is the digital photo of CNF800 film). The SEM images of CNF800 as anode film at the 500th discharge state (e and f).

Figure 4. Characterization of XRD (a), Raman (b), and BET (c) profiles. TEM of CNF800 (d), CNF800_1DC (e), and CNF800_1C (f), and the insets are the SAED images.

D


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The TEM image of CNF800 reveals a homogeneous fiber structure, and the diameter of CNF800 is 180 nm (Figure 4d). After the first lithiation process, there is a SEI film about 50 nm in thickness grown on CNFs. The HAADF images (Figure S2), CNFs at the fifth lithiation state also present rough surfaces and show an observed interface on the fibers, instead of a relatively smooth surface in pure CNFs. The fiber of CNF800_1DC can be observed to have a somewhat black and white area, and the diameter of fiber increases to 338 nm, because lithium cations insert into the graphitic layers and stay with the N and O dopant of CNFs. The inhomogeneous Li distribution is also observed from spatially resolved Li EELS maps in X. Wang’s work that lithium ions were stored in N-doped graphene.15 After the first delithiation process, the fiber of CNF800_1C appears to have a rough surface morphology of SEI, and the diameter of CNFs is 240 nm. The shrinkage of the fiber diameter indicates the removal of lithium from the graphite layer and delithiation with N and O dopant of CNFs. The selected area electron diffraction (SAED) pattern (insets in Figure 4d−f) presents a feature corresponding to amorphous materials with turbostratic microstructure also reported in previous publications.36 In the low magnified TEM images (Figure S3), CNF800, CNF800_1DC, and CNF800_1C remain in the long fiber morphology, and only the fiber diameter of the CNFs show expansion in CNF800_1DC and almost shrinks back in CNF800_1C. From the HRTEM image of CNF800_1C, the turbostratic structure of the graphite domain can be distinguished in CNFs (Figure S3b′). Using the measurement of HRTEM, the elemental distribution of lithiated/delithiated CNFs are recorded by line scanning, after the SEI film was removed by electron beam. Figure 5 shows the high-angle annular dark-field (HAADF) image and elemental distribution of a single nanofiber of CNF800_5DC. It can be seen that the elements of C, N, O, F

(the charge transfer resistance related with micropores of CNF800), but after the first lithiation/delithiation, the SEI film filled in micropores of CNF800 plays an important role in decreasing the resistance of Rct′. Figure 3c shows the initial charge/discharge curves of CNF800. CNF800 as anode delivers the initial capacities of 3150, 2250, 2000, and 755 mAh g−1 at the current densities of 0.5, 1, 5, and 10 A g−1, respectively. The first discharge curves show a plateau at about 0.7 V, which is attributed to the formation of solid electrolyte interface (SEI) film on the surface of CNFs.35 After 500 cycles, CNF800 remains the capacities of 1251, 865, 702, and 305 mAh g−1 at the current densities of 0.5, 1, 5, and 10 A g−1, respectively (Figure 3d). Table S2 gives a comparison of electrochemical performances between the current study and the binder-free anodes made from CNFs films. Comparison with the previous publication, CNF800 is a potential binder-free anode for LIBs. The SEM images of CNFs as anodes cycled at 1 A g−1 for 500 times are made to show the mechanical stability of the binder free electrode of CNFs. As shown in Figure 3e,f, the flexible fiber morphologies remain excellent with the exception of the roughness on the fiber surface that is the solid electrolyte interface (SEI) film on the surface of CNFs. Figure 4a shows the XRD patterns of pure CNF800, the first lithiation (CNF800_1DC) and first delithiation (CNF800_1C) state. Pure CNF800 presents the typical wide diffraction peaks (2θ) observed at 23.8° and 43.5° corresponding to the (002) and (001) planes of the graphite-like structure, respectively. Compared with the narrow and strong diffraction peak at 26.5° in graphitic carbon, the shifted and widened peaks are similar to the disordered carbons reported in the previous publications due to the lower graphitic degree and smaller graphite domain in CNFs, the ordered carbon layers damaged and the interlayer distance enlarged in the existence of nitrogen and oxygen groups, and so forth.17 The increased interplanar spacing of the (002) plane is positive for the reversible lithium intercalation/ extraction. After the initial discharge (lithiation) and charge (delithiation), the typical pattern (002) remains widened and weak, but shift to a lower degree of 21.3°. As shown in Figure 4a, the peak position is almost the same value in the electrode of CNF800_1DC and CNF800_1C, because the interlayer of graphite is inserted in lithium and a little irreversibly stays in. It suggests that introducing N, O dopants in CNF significantly enhances the insertion capacity by providing more activity sites and enlarging the interlayer distance of graphite domain in CNFs. Raman is sensitive to the subtle structure change in a carbonbased material. As shown in Figure 4b, the Raman spectra of pure CNF800, CNF800_1DC, and CNF800_1C display two strong peaks at 1350.9 cm−1 (D-band) and 1586.0 cm−1 (Gband). The ID/IG value of CNF800 is calculated to be 1.4, which are larger values than those of graphite and graphene attributed to the low crystallinity and disordered orientations of graphite domain in CNFs. After lithiation and delithiation of CNF800, the D-band becomes stronger in CNF800_1DC and CNF800_1C than those of pure CNF800. The Raman shift should be attributed to lithium ions inserted into the graphite layers and lithiation with nitrogen and oxygen of CNF800 that results in the graphite structure of CNFs to more disorder. FTIR of CNF800_1DC and CNF800_1C appears a new peak at 1507.4 and 865.8 cm−1 in comparison with pure CNF800 (Figure 4c), which further confirms the N, O dopant involving in the lithiation/delithiation.

Figure 5. HAADF image and elemental distribution of single nanofiber of CNF800_5DC. E


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Figure 6. High-resolution XPS spectra of the deconvoluted Li 1s peak of CNF800_1DC (a) and CNF800_1C (b) without Ar+ etching and CNF800_1DC (c) and CNF800_1C (d) with Ar+ etching.

present homogeneous distribution in the fiber, and the intensities of each of the elements are almost linear with their atomic composition in the CNF. Similar results can be collected in the HAADF image and elemental distribution of CNF800_5C (Figure S4). 3.3. Elemental State of CNF Electrodes after Lithiation/Delithiation. On the basis of the above results, lithiation of CNFs includes the lithium insertion into the graphite layer, with lithium staying with N, O dopants of CNF, accompanying the formation of an SEI film. X-ray photoemission spectroscopy (XPS) is a powerful method to study the elemental state of CNFs before and after lithiation/delithiation. The survey scan spectrum of pure CNFs showed the principal C 1s, N 1s, and O 1s core levels from XPS analysis, without evidence of impurities (Figure S5). After the initial lithiation and delithiation, the XPS of CNF800_1DC and CNF800_1C presents a much larger change. The F 1s and Li 1s core levels appear, but the N 1s core level disappears in the CNF800_1DC electrode, indicating that no N element is present in the SEI film. To better show the elemental state of the CNF, the SEI film was removed by Ar+ etching. The Ar+ etching time is determined by the atomic change (Figure S6). After Ar+ etching, the N 1s core level appears in both CNF800_1DC and CNF800_1C electrodes (Figure S6b). Figure 6 shows the high-resolution XPS profiles of the deconvoluted Li 1s peak of CNF800_1DC and CNF800_1C with and without Ar+ etching. After the spectrum was corrected for any background signals using the Shirley algorithm prior to curve resolution, the Li 1s core level peak of electrodes can be resolved and clearly provide the information on Li-contained component and the state of lithium. Without Ar+ etching, the Li 1s core level peak of CNF800_1DC is resolved into two components centered at 55.2 and 56.2 eV represented Li2CO3 and LiF which is known the main components of SEI film (Figure 6a).37 The main component is Li2CO3 of 93.5% (atomic percentage), in comparison with LiF of 6.5%. For the electrode of CNF800_1C, the Li 1s core level peak can be

resolved into three components centered at 54.4, 55.8, and 56.1 eV represented LiCx, Li2CO3, and LiF (Figure 6b). The appearance of LiCx might be due to the reason that the rough surface of CNF800_1C lets minor LiCx can be detected. The component of the irreversible LiCx detected on the surface is the lowest value of 5.9%. Moreover, the component of Li2CO3 decreases to 32.9% but LiF increases to 61.2%, which indicates minor decomposition of Li2CO3 and continuous decomposition of electrolyte to form LiF during charging process. After Ar+ etching removes the SEI film, the components of lithium are observed in Figure 6c,d. The Li 1s core level peak of CNF800_1DC is resolved into LiCx at the center of 54.5 eV, LiN/Li2CO3 at 55.2 eV, LiO at 55.8 eV, and LiF at 56.4 eV. The major component of LiCx (45.7%) confirms major lithiation in graphite. The components of LiN and LiO show the lithiation existed with the N, O dopant of CNFs, however, Li 1s of LiN/Li2CO3 mixed at 55.2 eV needs further N 1s core level to distinguish them. For the sample of CNF800_1C, the Li 1s core level is resolved into a minor Li Cx (54.2 eV) remaining at only 2.1%, LiN/Li2CO3 (55.2 eV) about 23.6%, and the major component of LiF (56.1 eV) about 74.1%. Because LiF and Li2CO3 nanoparticles are dispersed in the SEI film, the Ar+ etching cannot totally remove them. And thus, Li 1s of LiF and Li2CO3 appeared as a major component when the component of LiCx is very low. It can be seen that the initial delithiation process removes most of lithium from CNFs. According to the Li 1s core level, it is still hard to distinguish the contribution of N and O dopant of CNFs for the capacity of LIBs. The high-resolution O 1s XPS spectra of CNF800 at various states are shown in Figure S7. A weak O 1s core level appears in pure CNF800 that is attributed to the CO of CNF. After the initial lithiation, a very strong O 1s core level appears at the center of 531.9 eV assigned to the component Li2CO3 of SEI film. During delithiation, some of Li 2 CO 3 might be decomposed and thus the O 1s core level of CNF800_1C F


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Figure 7. High-resolution XPS spectra of the deconvoluted N 1s peak of CNF800_pure (a), CNF800_1C without Ar+ etching (b), and CNF800_1DC (c) and CNF800_1C (d) with Ar+ etching.

Figure 8. High-resolution XPS spectra of the deconvoluted C 1s peak of CNF800_pure (a), CNF800_1DC (b), and CNF800_1C (c) without Ar+ etching, and CNF800_1DC (d) and CNF800_1C (e) with Ar+ etching.

electrode moves to 532.4 eV, and its intensity decreases a lot. If the SEI film is removed, then the O 1s core level is decreased 70% in intensity. The weak and widened O 1s core level at 531.6 and 532.1 eV can be assigned to CO of CNF. A new O 1s core level at 528.5 eV is LiO of CNF800_1DC, but disappears in CNF800_1C. This confirms that the lithiation of CNF involves the O of the CNF. In the recent publications, incorporation of N atoms into carbon materials (e.g., graphene, carbon nanotubes, etc.) has been regarded as one of the best ways to improve the

electrochemical performance. We all know that there are four kinds of nitrogen state, N-6: sp2 hybridized pyridinic nitrogen, N-5: sp3 hybridized pyridone nitrogen, N-Q: sp2 hybridized quarternary nitrogen, and N-X: sp2 hybridized pyridonicnitrogen oxide.20,21 Which nitrogen is preferred involving in the lithiation/delithiation is still unknown. Here we are the first to study the N 1s core level of CNFs at the state of lithiation/ delithiation. Figure 7 shows the high-resolution XPS spectra of the deconvoluted N 1s peak of CNF800 at various states. The N 1s G


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Figure 9. 13C NMR spectra of pure CNF800 (a), CNF800_1DC (b), and CNF800_1C (c).

core level peak of pure CNF800 is resolved into N-6 at the center of 398.3 eV, N-5 at 400.6 eV, N-Q at 401.0 eV, and N-X at 403.7 eV, respectively (Figure 7a). The compositions of N-6, N-5, N-Q, and N-X are 35.9 at%, 13.7 at%, 42.8 at%, and 7.6 at %, respectively, which are similar to those reported in the previous report.38N-6 and N-Q are the major component of CNF produced by PAN.29,31 At the initial lithiation and delithiation processes, none of the N 1s core level peaks are collected in CNF800_1DC and CNF800_1C (Figure 7b) due to the absence of nitrogen in the SEI film. When the Ar+ etching removes the SEI film, CNF800_1DC and CNF800_1C present a single strong N 1s core level peak (Figure 7c and 7d). The N 1s core level peak of CNF800_1DC is resolved into N-6 at the center of 398.3 eV, N-5 at 400.6 eV, N-Q at 399.2 eV,

and N-X at 403.2 eV, respectively. The composition of N-Q decreases a lot from 42.8% of CNF800 to 9.0% of CNF800_1DC, and N-6 decreases from 35.9% to 22.9% of CNF800_1DC, meanwhile N-5 greatly increases from 13.7% to 60.6%. We can conclude that the nitrogen of CNFs are seriously involved in the lithiation process accompanyied with the appearance of some CN bonds broken and LiN bonds. FTIR of CNF800 and CNF800_1DC appears a new peak at 1507.4 and 865.8 cm−1 assigned to CN and LiN, respectively (Figure 4c). The N 1s core level peak of CNF800_1C can be resolved into N-6 at 398.4 eV, N-5 at 399.3 eV, N-Q at 400.7 eV, and N-X at 402.7 eV, respectively. The composition of N-6 shows a minor increase from 22.9% to 30.2%, N-Q increases from 9.0% to 10.9%; meanwhile N-5 and H


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component of 73.1%. The composition CN is greatly decreased from 19.4% of CNF800_1DC to 9.5% of CNF800_1C, and CLi relatively decreases from 4.9% of CNF800_1DC to 2.0% of CNF800_1C. Moreover, the CO of CNF800 at the lithiation/delithiation state appears in decreased composition, from 9.8% in pure CNF800 to 4.2% in CNF800_1DC and 3.2% in CNF800_1C. We further conclude that N and O are involved in the lithiation/delithiation apart from the major lithiation/delithiation happening to the graphite domain in CNFs. 3.4. 13C NMR Spectra of CNF Electrodes after Lithiation/Delithiation. Solid-state 13C MAS NMR is an important method to characterize carbon materials at the molecular level.40−42 Figure 9 shows 13C MAS spectra of pure CNF800, CNF800_1DC, and CNF800_1C, respectively. The spectra show a considerably stronger sp2 peak close to 126 ppm and some weaker peaks related with the complex lithiation/delithiation state of CNFs. The least-squares fit shows that the 13C NMR spectrum of pure CNF800 can be assigned to the Csp2 of graphite (the major peak at 125 ppm), and small amounts of other 13C species corresponding to weak peaks, containing CN of aryl (151.6 ppm), CO of aryl (169.1 ppm), CO of pyridinic (195.0 ppm), COC of aryl (65.0 ppm) and CH of aryl (30.7 ppm).40,43−45 The compositions of the above components detected by 13C NMR are in accord with those recorded by elemental analysis and XPS. At the state of the initial lithiation, the 13C NMR of CNF800_1DC presents great change. Because the lithiation is involved in the graphite, N, and O elements, the peak positions experience a relatively minor shift. The least-squares peak fits of 13C NMR of CNF800_1DC are assigned the major peak at 127 ppm (Csp2 of graphite), and other weak peaks at 154.5 ppm (CN of aryl), 169.2 ppm (CO of aryl), 200.1 ppm (CO of pyridinic), 65.0 ppm (COC of aryl), and 29.0 ppm (CH of aryl). Due to the formation of SEI and lithiation in CNFs, the ratio of Csp2 decreases from 69.3% in pure CNF800 to 53.6% in CNF800_1DC. The yield of CN decreases from 13.4% in CNF800 to 8.3% in CNF800_1DC which further confirms N involving the lithiation. Because the CO includes the contribution of SEI and CNFs, it is hard to distinguish which involves in the lithiation. There are two new peaks appearing at 85 ppm (COLi of aryl), 53 ppm (C NLi of aryl). The respective COLi and CNLi yields are 3.1% and 8.0%, which give evidence of the N and O dopants of CNF involving the lithiation. Because some of the COLi yields from SEI, the peak area intensity of CO Li is much higher than that of CNLi. At the state of delithiation, the least-squares peak fits of 13C NMR of CNF800_1C are assigned the major peak at 126.7 ppm (Csp2 of graphite), and other weak peaks at 156 ppm (CN of aryl), 170 ppm (CO of aryl), 210 ppm (CO of pyridinic), 65.4 ppm (COC of aryl), 29.8 ppm (CH of aryl), 85.2 ppm (COLi of aryl), and 51.6 ppm (CNLi of aryl). After the delithiation, the yield of COLi decreases from 8.0% in CNF800_1DC to 6.2% in CNF800_1C, and that of CNLi also decreases from 3.1% in CNF800_1DC to 1.6% in CNF800_1C, which are evidence of the N and O dopant of CNF involved in the lithiation/delithiation processes.

N-X components are decreased. It can be concluded that N elements of CNF happen to the lithiation, which is in accord with the change of LiN in Li 1s core level (Figure 7c,d). Nevertheless, the nitrogen of the N-Q component involves in an irreversible lithiation/delithiation because of the change from 42.8% in pure CNF800 to 9.0% in CNF800_1DC and 10.9% in CNF800_1C, and some of nitrogen of the N-6 is involved in a reversible lithiation/delithiation for the change from 35.9% in CNF800 to 22.9% in CNF800_1DC and 30.2% in CNF800_1C. According to N state change after lithiation/ delithiation, we can conclude that (1) N-6 is the major state in the contribution of lithium storage, and most of the N-6 can be reversed back after delithiation; (2) most of the N-Q state irreversibly changes to the N-5 state after the initial lithiation and stays in the N-5 structure during delithiation; and (3) the N dopant mainly plays the role of making the CNF structure disordered and the greater defect position in the graphite domain improves its ability for lithium storage. The main lithiation/delithiation has been found in the interlayer of graphite domain of CNFs. Figure 8 shows the high-resolution XPS spectra of the deconvoluted C 1s peak of CNF800 at various states. The C 1s core level peak of CNF800 is resolved into C sp2 at the center of 284.8 eV, CN at 286.0 eV, CN at 286.6 eV, and CO at 288.8 eV, respectively. The component sp2C-sp2C is the major 69.6% in CNF800, and the components of CN, CN, and CO are 12.6%, 8.0%, and 9.8%, respectively. After the initial lithiation/deliathiation, the XPS spectra of C 1s present much change (Figure 8b−d) because of the lithiation in the interlayer of graphite domain and with nitrogen, oxygen, and the formation of SEI, and so forth. Without Ar + etchcing, the C 1s core level peak of CNF800_1DC (Figure 8b) is resolved into C sp2 at 284.8 eV, CN at 286.2 eV, CN at 286.6 eV, CO at 288.8 eV,9,39 and two new peak of Li2CO3 at 290.1 eV and CLi at 282.8 eV. The major components of CNF800_1DC are Csp2 40.1% and Li2CO3 51.6%. The strong peak of Li2CO3 comes from the SEI film. The C 1s core level peak of CNF800_1C (Figure 8c) can be resolved into C sp2 at 284.8 eV, CN at 285.7 eV, CN at 286.8 eV, CO at 288.8 eV, Li2CO3 at 290.3 eV, and CLi at 282.8 eV. The component of C sp2 returns back to the major 85.9% and Li2CO3 component dramatically decreases to 5.8%, which the result is in accordance with that of Li 1s core level peaks. It can be concluded that the SEI film is still unstable at the initial lithiation/delithiation processes. After Ar+ removes the SEI films, the high-resolution C 1s XPS spectra of CNF800_1DC and CNF800_1C are demonstrated (Figure 8d,e). Both electrodes present weak peak of Li2CO3, which further confirms the Ar+ etching method is an efficient way to remove SEI film and better show the lithiation/delithiation state of CNFs. The C 1s core level peak of CNF800_1DC (Figure 8d) is resolved into C sp2 at 284.6 eV, CN at 285.5 eV, CN at 286.6 eV, CO at 288.8 eV, Li2CO3 at 290.2 eV, and CLi at 282.7 eV. In comparison with the components of pure CNF800, the composition of C sp 2 decreases from 69.6% of CNF800 to 55.7% of CNF800_1DC, CN increases from 12.6% of CNF800 to 19.4% of CNF800_1DC due to lithiation. The C 1s core level peak of CNF800_1C (Figure 8e) is resolved into C sp2 at 284.6 eV, CN at 285.7 eV, CN at 286.6 eV, CO at 288.8 eV, Li2CO3 at 290.5 eV, and CLi at 282.8 eV. The component of Li2CO3 is the lowest only 1.2%, but C sp2 is the majority

4. CONCLUSIONS In summary, we developed a scalable, practical, and low-cost preparation of flexible and binder-free N, O-doped CNFs using I


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(4) Flandrois, S.; Simon, B. Carbon Materials for Lithium-Ion Rechargeable Batteries. Carbon 1999, 37, 165−180. (5) Wang, F.; Graetz, J.; Moreno, M. S.; Ma, C.; Wu, L. J.; Volkov, V.; Zhu, Y. M. Chemical Distribution and Bonding of Lithium in Intercalated Graphite: Identification with Optimized Electron Energy Loss Spectroscopy. ACS Nano 2011, 5, 1190−1197. (6) Cao, S. M.; Feng, X.; Song, Y. Y.; Liu, H. J.; Miao, M.; Fang, J. H.; Shi, L. Y. In Situ Carbonized Cellulose-Based Hybrid Film as Flexible Paper Anode for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 1073−1079. (7) Zhong, C.; Wang, J. Z.; Wexler, D.; Liu, H. K. Microwave Autoclave Synthesized Multi-Layer Graphene/Single-Walled Carbon Nanotube Composites for Free-Standing Lithium-Ion Battery Anodes. Carbon 2014, 66, 637−645. (8) Yang, Z. B.; Ren, J.; Zhang, Z. T.; Chen, X. L.; Guan, G. Z.; Qiu, L. B.; Zhang, Y.; Peng, H. S. Recent Advancement of Nanostructured Carbon for Energy Applications. Chem. Rev. 2015, 115, 5159−5223. (9) Wang, S. X.; Yang, L. P.; Stubbs, L. P.; Li, X.; He, C. B. LigninDerived Fused Electrospun Carbon Fibrous Mats as High Performance Anode for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2013, 5, 12275−12282. (10) Montoro, L. A.; Matsubara, E. Y.; Rosolen, J. M. Lithium Intercalation into Single-Walled Carbon Nanotubes Network Electrode: Storage Mechanisms and Impurity Effects. J. Power Sources 2014, 257, 205−212. (11) Yildirim, H.; Kinaci, A.; Zhao, Z. J.; Chan, M. K. Y.; Greeley, J. P. First-Principles Analysis of Defect-Mediated Li Adsorption on Graphene. ACS Appl. Mater. Interfaces 2014, 6, 21141−21150. (12) Thinius, S.; Islam, M. M.; Heitjans, P.; Bredow, T. Theoretical Study of Li Migration in Lithium−Graphite Intercalation Compounds with Dispersion-Corrected DFT Methods. J. Phys. Chem. C 2014, 118, 2273−2280. (13) Yao, F.; Günes, F.; Ta, H. O.; Lee, S. M.; Chae, S. J.; Sheem, K. Y.; Cojocaru, C. S.; Xie, S. S.; Lee, Y. H. Diffusion Mechanism of Lithium Ion through Basal Plane of Layered Graphene. J. Am. Chem. Soc. 2012, 134, 8646−8654. (14) Kleinsorge, B.; Ferrari, A. C.; Robertson, J.; Milne, W. I.; Waidmann, S.; Hearne, S. Bonding Regimes of Nitrogen in Amorphous Carbon. Diamond Relat. Mater. 2000, 9, 643−648. (15) Wang, X.; Weng, Q. H.; Liu, X. Z.; Wang, X. B.; Tang, D.-M.; Tian, W.; Zhang, C.; Yi, W.; Liu, D. Q.; Bando, Y.; Golberg, D. Atomistic Origins of High Rate Capability and Capacity of N-Doped Graphene for Lithium Storage. Nano Lett. 2014, 14, 1164−1171. (16) Qie, L.; Chen, W. M.; Wang, Z. H.; Shao, Q. G.; Li, X.; Yuan, L. X.; Hu, X. L.; Zhang, W. X.; Huang, Y. H. Nitrogen-Doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047− 2050. (17) Mao, Y.; Duan, H.; Xu, B.; Zhang, L.; Hu, Y. S.; Zhao, C. C.; Wang, Z. X.; Chen, L. Q.; Yang, Y. S. Lithium Storage in NitrogenRich Mesoporous Carbon Materials. Energy Environ. Sci. 2012, 5, 7950−7955. (18) Shin, W. H.; Jeong, H. M.; Kim, B. G.; Kang, J. K.; Choi, J. W. Nitrogen-Doped Multiwall Carbon Nanotubes for Lithium Storage with Extremely High Capacity. Nano Lett. 2012, 12, 2283−2288. (19) Xiao, K.; Liu, Y.; Hu, P.; Yu, G.; Sun, Y.; Zhu, D. n-Type FieldEffect Transistors Made of an Individual Nitrogen-Doped Multiwalled Carbon Nanotube. J. Am. Chem. Soc. 2005, 127, 8614−8617. (20) Wang, H.; Maiyalagan, T.; Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and its Potential Applications. ACS Catal. 2012, 2 (2), 781−794. (21) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Evolution of Nitrogen Functionalities in Carbonaceous Materials during Pyrolysis. Carbon 1995, 33, 1641−1653. (22) Cho, Y. J.; Kim, H. S.; Im, H.; Myung, Y.; Jung, G. B.; Lee, C. W.; Park, J.; Park, M. H.; Cho, J.; Kang, H. S. Nitrogen-Doped Graphitic Layers Deposited on Silicon Nanowires for Efficient Lithium-Ion Battery Anodes. J. Phys. Chem. C 2011, 115, 9451−9457.

electrospinning and carbonization. Both the N content and porous structure play important roles in the electrochemical properties of CNFs. The flexible film of CNF800 delivers the initial capacities of 3150, 2000, and 755 mAh g−1 at the current densities of 0.5, 5, and 10 A g−1, respectively. After 500 cycles, CNF800 remains 305 mAh g−1 at current density of 10 A g−1. The lithiation/delithiation mainly happens to the interlayer of the graphite domain of CNFs, however, the dopants of nitrogen and oxygen seriously involve in lithiation. The excellent performances of the CNF film can be attributed to the N, O doped structure of graphite domain that has increased the conductivity and lithium storage ability, but much of the LiN is irreversible. Here, N-6 is the major state in the contribution of lithium storage, and most of the N-6 can be reversed back after delithiation, most of N-Q state irreversibly changed to the N-5 state after the initial lithiation and stays in the N-5 structure during delithiation. The N dopant mainly plays the roles of making the structure of CNFs disordered and providing the defect position in the graphite domain that have improved ability in lithium storage. After all, the flexible and binder-free film of N, O-doped CNFs would be an ideal anode for highenergy-density batteries if the dopant compositions and porous structure exist in CNFs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11996. The conductivity of CNF film vs the bending dihedral angles, an equivalent circuit, and EIS fitted parameters, comparison table of electrochemical performances between the current study and binder-free anodes made from CNFs films, HAADF images of a single nanofiber of CNF800 at the 5th discharge state, TEM, HRTEM images, HAADF image, and elemental distribution of electrodes, XPS survey spectra, elemental change with the Ar+ etching time, and high-resolution O 1s XPS spectra of CNF-800C at various states (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.Y.). ORCID

Gang Yang: 0000-0001-7706-7236 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of the National Natural Science Foundation of China (Grant Nos. 51172032 and 11474147), and Natural Science Foundation of Jiangsu Province of China (Grant No. BK20141229). We would also like to acknowledge the technical support received at the Testing Center of Yangzhou University.

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REFERENCES

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Study of Microstructure Change of Carbon Nanofibers as Binder-Free

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