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iron oxide hybrid architectures for lithium ion battery anode
Accepted Manuscript Carbon nanotubes branched on three-dimensional, nitrogen-incorporated reduced graphene oxide/iron oxide hybrid architectures for lithium ion battery anode Yingbo Kang, Xu Yu, Manikantan Kota, Ho Seok Park PII:
S0925-8388(17)32633-6
DOI:
10.1016/j.jallcom.2017.07.264
Reference:
JALCOM 42680
To appear in:
Journal of Alloys and Compounds
Received Date: 20 May 2017 Revised Date:
22 July 2017
Accepted Date: 25 July 2017
Please cite this article as: Y. Kang, X. Yu, M. Kota, H.S. Park, Carbon nanotubes branched on threedimensional, nitrogen-incorporated reduced graphene oxide/iron oxide hybrid architectures for lithium ion battery anode, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.264. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
Carbon Nanotubes Branched on Three-Dimensional,
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Nitrogen-Incorporated Reduced Graphene Oxide/Iron Oxide Hybrid Architectures for Lithium Ion Battery
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Anode
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Yingbo Kang, Xu Yu, Manikantan Kota, Ho Seok Park*
*Corresponding author. Tel: +82-31-299-4715, Email: [email protected], (Ho Seok Park)
ACCEPTED MANUSCRIPT Carbon Nanotubes Branched on Three-Dimensional, Nitrogen-Incorporated Reduced Graphene Oxide/Iron Oxide Hybrid Architectures for Lithium Ion Battery Anode
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Yingbo Kang, Xu Yu, Manikantan Kota, Ho Seok Park*
School of Chemical Engineering, Sungkyunkwan University, Suwon 440 746, Republic
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of Korea.
*Corresponding author. Tel: +82-31-299-4715, Email: [email protected], (Ho Seok Park)
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ACCEPTED MANUSCRIPT Abstract The carbon nanotubes (CNTs) branched on three-dimensional (3D) macroporous, nitrogen-incorporated reduced graphene oxide (NG)/iron oxide
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(CNT/NG-Fe) hybrid architectures have been prepared via an ice templating and microwave synthesis. Compared with the pristine RGO, the CNTs can be more readily and uniformly grown on the 3D NG surfaces due to the good electronic
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conductivity by N-type configurations. As demonstrated by the electrochemical
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performances, the discharge capacity of the 3D CNT/NG-Fe is 1208 mAh g-1 at 50 mA g-1 which is greater than 890 and 820 mAh g-1 of the CNT/G-Fe and NG. When the rate increases from 100 to 1000 mAh g-1, the capacity retention reaches 52 % of initial capacity corresponding to the discharge capacity of 947 mAh g-1. After 130
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cycles at 100 mA g-1, the capacity gradually increases to 1020 mAh g-1 with the Coulombic efficiency of > 98.5 %. The enhanced capacity, rate capability and cyclic stability of the CNT/NG-Fe are associated with the doping effect of N-configuration
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and unique hierarchical structure consisting of the dense CNT branches on 3D
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macroporous continuity.
Keywords: Hierarchical architecture; Nitrogen doping; Porous Graphene; Carbon Nanotube; Hybrid; Lithium Ion Battery
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ACCEPTED MANUSCRIPT 1. Introduction Graphene is two-dimensional (2D) arrays of carbon atoms arranged in a hexagonal
pattern
[1],
whereas
carbon
nanotube
(CNT)
forms
tubular
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one-dimensional (1D) structure [2]. Both of CNT and graphene have received significant attention due to their outstanding properties such as high electrical conductivity, large surface area, good mechanical property, and electrochemical and
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thermal stabilities for energy-related applications. For instance, they were applied for
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a broad range of applicative fields, such as supercapacitors [3], battery [4], solar cells [5], electrocatalyst [6], sensors [7], etc. Moreover, hybrid materials which take advantages of integrating CNT and graphene into multi-dimensional hierarchical architecture are considered as a promising candidate for lithium ion battery (LIB) [8].
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Recently, the three-dimensional (3D) hierarchical architecture of carbon nanomaterials has been investigated to achieve prominent structural features such as large available area, fast ion and mass transport, percolated electron transfer and
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structural integrity [9]. Accordingly, the poor rate and cyclic capabilities of transition
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metal oxides, which are a critical challenge of conversion-based material [10], could be improved by depositing on such 3D macroporous internetworked reduced graphene oxide (RGO) [11]. For instance, 3D RGO/TiO2, RGO/SnO2 [12], RGO/MnO2 [13], RGO/Fe2O3 [14], RGO/NiO [15], RGO/WO3 [16], RGO/CuO [17], and RGO/CoO [18] hybrid architectures were developed for LIB applications. Another important strategy to improve electrochemical properties of carbon nanomaterials is the incorporation of heteroatoms such as nitrogen (N) [19], oxygen 3
ACCEPTED MANUSCRIPT (O) [20], phosphorus (P) [3], sulfur (S) [11] and fluorine (F) [21] into the graphitic lattice. Such heteroatom chemistry leads to modify electronic structure and electrochemical reactivity depending on the chemical identity, bonding configuration
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and composition [22]. Motivated by these findings, N-RGO, O-RGO, P-RGO, S-RGO and F-RGO were synthesized to achieve enhanced performances of LIB [23].
Taking full advantages of the afore-mentioned chemistries, for the first time,
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we demonstrate the unique hierarchical architecture, where a bunch of CNTs are
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brached on the 3D macroporous, N-incorporated RGO/iron oxide (CNT/NG-Fe) hybrids, constructed via an ice templating and microwave synthesis. The features of this complex hybrid architecture can be described along the following lines. (1) The 3D N-incorporated RGOs act as conductive networking substrate to provide large
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surface area for the deposition of iron oxide nanoparticles, to facilitate Li ion transport and to delocalize stress created by volume expansion during charge/discharging process. (2) The CNT branches offer 1D conducting pathway between intra- or
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interparticles and inhibiting restacking of RGO nanosheets. (3) The iron oxide
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nanoparticles are high capacity materials for enhancing charge storage capacity of carbon nanomaterials.
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ACCEPTED MANUSCRIPT 2. Experimental section 2.1 Synthesis of 3D NG The graphene oxide (GO) was synthesized by modified Hummers method [24]. Firstly,
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100 mg of as-obtained GOs was dispersed into 10 mL of deionized (DI) water, and ultrasonicated for 1 hour to make the homogenous dispersion. Secondly, 500 mg of melamine was added into 20 mL of DI water, and then the mixture was stirred and for 1 hour. Finally, two dispersions were mixed together and stirred to
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heated at 80
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make the homogenous solution. The final solution was frozen by liquid nitrogen and freeze-dried for 3 days to remove the solvent. The as-obtained sample was placed in the tube furnace and heat treated at two steps: (1) at 300
for 1 hour; (2) 900
for 1
min-1 under an argon atmosphere. The
hour with the increasing rate of 5
was noted as NG.
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temperature was naturally cooled down to room temperature, and the obtained sample
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2.2 Growth of CNT branches
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50 mg of ferrocene was mixed with 3 mL of acetylacetonate (ACN) solution, the mixture was stirred for 30 minutes and bath-sonicated for 1 hour. Then, 10 mg of NG was added and stirred for another 1 hour, and the solution was named A. 50 mg of azodicarbonamide (ADC) was mixed with 2 mL of ACN and stirred for 30 minutes, which was named as the B. Finally, the mixture of A and B was stirred for another 1 h to make the final homogeneous dispersion. The final solution was placed in the home-made microwave and irradiated for 40 seconds, then the sample was 5
ACCEPTED MANUSCRIPT continuously irradiated for another 150 seconds for the growth of CNT on N-doped RGO. The final product was obtained and noted as CNT/NG-Fe. The growth of CNT
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on RGO without N doping was noted as CNT/G-Fe.
2.3 Fabrication of coin cell
The electrochemical performance of CNT/NG-Fe was performed by coin cell
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(CR2032), which was fabricated in the glove box. The anode electrode was prepared
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as follows: a slurry consists of 80% of active material, 10% of polyvinylidence fluoride as binders and 10% of carbon black as the conducting agents. In additional, the NMP was used as the solvent. All components were mixed and grinded in the agate mortar with few drops of NMP for 30 minutes to make homogeneous slurry. for overnight in
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Then, the slurry was pasted on the copper foil, and dried at 80
vacuum oven. The test cell was fabricated in an argon-filled glove box by pressing anode electrode with a lithium metal foil and 1 M LiPF6 dissolved in a solution of
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ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by volume ratio) mixture as
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electrolyte. The cell was galvanostatically cycled between 0 and 3 V vs Li/Li+ at various current densities.
2.4 Characterization
Transmission electron microscopy (TEM) images were collected on a JEM-3010 HR TEM (300 kV). Scanning electron microscopy (SEM) image were obtained using a field emission scanning electron microscope (Philips SEM 535M), equipped with a 6
ACCEPTED MANUSCRIPT Schottky-based field emission gun. A scanning TEM (STEM) was operated with a probe focused to 0.2 nm and camera length of 20 cm. The scan raster was 512 × 512 points with a dwell time of 8.5 seconds per scan. Chemical analysis was performed
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using a VG Electron Spectroscope (ESCA 2000) at high vacuum (10-10 torr). It was equipped with monochrometer (quartz), twin X-ray source (Mg/Altarget) and hemispherical analyzer. The Brunauer-Emmett-Teller (BET) surface areas and
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nitrogen adsorption-desorption isotherms were measured at 78 K using BELSORP
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analysis program. Horvath-Kawazoe (HK) and Barrett-Joyner-Halenda (BJH) analyses are used to calculate the average micro- and meso-pore sizes. In-situ Raman spectra were measured using confocal micro-raman spectrometer NRS-3100, Jasco-Japan system with microscope having 100X lens and an excitation laser beam
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source of 532 nm wavelength. X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo MultiLab 2000system with an Al-Mg α X-ray source. There
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are no electron transfer processes during the XPS measurement of all samples.
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ACCEPTED MANUSCRIPT 3. Results and discussion As shown in Fig. 1, the 3D CNT/NG-Fe was synthesized through an ice templating and microwave irradiation synthesis. First, the 3D macroporous NG was
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synthesized via a simple and facile ice-templating method using GO and melamine precursor. The resulting material was used as the internetworked substrate for the growth of CNT branches. The homogeneous mixture of NG and ferrocene catalysts
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was treated under microwave irradiation to deposit CNT branches onto the surface of
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NG. During the microwave irradiation process, ADC was decomposed to produce urea that can react with oxygen molecules under air atmosphere and to make GO reduced by decreasing the oxygen content. At the meantime, the ferrocene is decomposed into metallic iron and hydrocarbons, as demonstrated in the equation
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below [25]:
Fe(C H ) → Fe + H + CH + C H + ⋯ The metallic iron particles were deposited on the surface of NG. and then, NG/iron
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particle produced strong magnetic-heating effect [26] to heat up microwave reactor
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[27]. At a high temperature, hydrocarbons were decomposed into molecular carbons, which will preferentially cover the surface of iron nanoparticles. Furthermore, this dissolved carbons start to be nucleated and grown into CNTs on iron nanoparticle via a tip growth mechanism after the saturation concentration is reached [28]. In addition to the decomposition of ADC, air reacts with iron to form iron oxide as the temperature continues to increase during the period of CNT growth.
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ACCEPTED MANUSCRIPT The morphology and microstructure of the CNT/G-Fe and CNT/NG-Fe were characterized by SEM and TEM images in Fig. 2. Low and high magnification SEM images of CNT/G-Fe as shown in Fig. 2a and 2b. In Fig. 2c and 2d, the CNT/NG-Fe
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preserved the 3D macroporous structure assembled by ice-templating method, indicating no significant damage of N configurations on the 3D architecture (Fig. S1a and S1b). The CNT branches were much denser for the 3D N-incorporated RGO than
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only 3D RGO substrate presumably due to the role of N configuration (Fig. S1c and
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S1d) as the activated site of depositing ferrocene catalyst and the large surface area. The N-containing groups of 3D NG acted as anchoring iron nanoparticles [29], which was deposited without critical loss of metal precursors and oxidized into iron oxide at low temperatures. The length of CNT branches was in the range of several
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micrometers, and the average diameters ranged up to 60 nm as shown in Fig. 2e and 2f. The CNT branches were randomly distributed and deposited on the surface of 3D macrorpous NG-Fe hybrids, and some iron oxide particles were positioned on both
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the CNT and NG surface. As shown in TEM images of the CNT/NG-Fe in Fig. 2g,
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the CNTs were not straight but curved. The large size of iron oxide nanoparticles obviously remained and was deposited at the tip part of the CNTs. As shown in STEM images, the spots of N, O and Fe elements were localized as marked by red contour, indicating iron oxide nanoparticles were deposited onto the 3D NG surface. By contrast, the C signals were distributed over the whole region of NGs and CNTs which were not distinguished. Since N configuration was solely existent onto the NG, red contour corresponds to the domain of NG. This result means that the iron oxide 9
ACCEPTED MANUSCRIPT particles were localized onto the surface of NG due to a favorable interaction with N-containing groups, where CNT branches were nucleated and grown. Meanwhile, the diameters of the CNT branches were mostly in the range from 30 to 60 nm
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strongly influenced by the particle size of Fe catalyst, which is consistent with the SEM result. As marked by blue contour, the CNT branches showed much weaker signals of O, N, and Fe compared to those in red contour of NG. These findings
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indicate that N-configuration strongly influence the growth of CNT branches.
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The crystalline structure of the CNT/NG-Fe was investigated by XRD spectroscopy in Fig. 3a. A strong sharp reflection in the diffraction profile of the CNT/G-Fe at 2θ=25.6° is attributed to the (002) reflection of the 3D RGO. By contrast, the (002) reflection of the CNT/NG-Fe was observed at 2θ=26.4°, where N
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incorporation did not significantly change the structure of 3D RGO. Both CNT/G-Fe and CNT/NG-Fe showed the characteristic XRD patterns of α-Fe2O3 phase, where the peaks appeared at 2θ of 24.1°, 33.2°, 35.6°, 40.8°, 49.5°, 54.1°, 62.5°, and 64.1°,
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corresponding to (012), (104), (110), (113), (024), (116), (214), and (300) planes, as
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marked by the letter “A” (JCPDS no. 89-0596/33-0664) [30]. On the other hand, the XRD pattern of the CNT/NG-Fe was dominated by α-Fe2O3 phase, indicating higher phase purity than the CNT/G-Fe. The different modes of the CNT/NG-Fe for the growth of Fe2O3 phase were attributed to the distinct chemical circumstances of the NG given by the N configuration. The presence and purity of hematite phase of α-Fe2O3 for the CNT/NG-Fe was expected to improve the anode capacity as demonstrated by Tarascon group [31]. The low cycle stability and conductivity of this 10
ACCEPTED MANUSCRIPT phase can be improved depositing onto the porous conductive material of 3D NG and connecting with CNT branches. The porous structure of the CNT/NG-Fe was characterized using nitrogen
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adsorption and desorption isotherm as shown in Fig. 3b. Both of the CNT/NG-Fe and CNT/G-Fe hybrids revealed type IV isotherm, confirming the existence of mesoporosity. As analyzed by BET and BJH, the specific surface area and the average
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mesopore diameter of the CNT/NG-Fe and the CNT/G-Fe were 87.4 and 53.7 m2 g-1
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and 28.1 and 25.2 nm, respectively. It notes that the enlarged surface area of the CNT/NG-Fe is presumably due to the heteroatom nitrogen doping and the growth of dense CNT branches. The pore volume of the CNT/NG-Fe and CNT/G-Fe was 0.61 cm3 g-1and 0.34 cm3 g-1, respectively. Therefore, the hierarchical porous structure of
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the CNT/NG-Fe can provide shorter ion transfer pathway and easy ion accessibility for the enhanced electrochemical performance. The chemical structure and bonding nature of the CNT/NG-Fe are
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demonstrated by XPS data, as shown in Fig. 4. As determined by wide XPS spectra,
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CNT/NG-Fe is consist of C, O, N and Fe, which are centered at 285.0 eV, 399.0 eV, 533.0 eV and 711.0 eV, respectively (Fig. S 2c). The binding energy region plotted of C 1s showed four deconvoluted peaks, which are centered at 284.8 eV, 285.8 eV, 286.9 eV and 289.1 eV, corresponding to C=C, C-N C=N and C=O bondings (Fig. 4a). From the high resolution N1s spectra (Fig. 4b), two major peaks were identified as pyridinic and graphitic-N centered around 398.2 eV and 401.3 eV [32], indicating the incorporation of two major N configurations. Moreover, two minor peaks were 11
ACCEPTED MANUSCRIPT observed at around 399.1 eV and 404.5 eV corresponding to pyrollic and oxidized pyridinic-N bonds [32]. The presence of majority of pyridinic and graphitic peaks have advantage in terms of improving the electronic conductivity by altering the local
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electron density on the graphene surface [33]. CNT/NG-Fe showed different ratios of C and N configurations from NG (Fig. S2), predicting that CNT/NG-Fe has a different surface chemistry by branching of CNT. The formation of hematite phase
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with the oxidation state of Fe3+ was confirmed by the high resolution of Fe2p
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spectrum, where two peaks of Fe2p3/2 and Fe2p1/2 were observed at 711.4 eV and 725 eV (Fig. 4c). The atomic % of N and Fe for the CNT/G-Fe and the CNT/NG-Fe were estimated as 2.39 and 4.09, respectively. The C/O ratio of the CNT/NG-Fe and CNT/G-Fe was 8.2 and 9.8, respectively. The decrease in C/O ratio in CNT/NG-Fe
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could be attributed to the incorporation higher percentage of nitrogen atoms. Raman spectroscopy was used to evaluate defects and structural change of carbonaceous materials as shown in Fig. 4d. For the case of NG, a D band (disordered
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structure) is observed at 1335 cm-1 and G band (stretching of the C-C bond) at 1584
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cm-1. The CNT/G-Fe hybrids exhibited D and G bands at 1343 cm-1 and 1572 cm-1. In contrast to the CNT/G-Fe, the D band of the CNT/NG-Fe was blue-shifted to 1319 cm-1, and G band was red-shifted to 1579 cm-1, which are associated with the outcome of N-doping effect and graphitic nature of grown CNT [34]. The intensity ratio of the D and G band (ID/IG) is a crucial factor to verify the defects. The ID/IG value of the CNT/NG-Fe was measured as 1.03, which is lower than 1.16 of NG, indicating that decoration of iron oxide nanoparticles and subsequent growth of CNT can restore the 12
ACCEPTED MANUSCRIPT defects. However, the ID/IG value of the CNT/NG-Fe is higher compared to that of CNT/G-Fe, which means that the incorporation of nitrogen configuration increases the defect sites. The existence and phase purity of α-Fe2O3 was further confirmed by
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the Raman peaks of the CNT/NG-Fe at 224.7, 296.1, 412.5, 506.4, and 611.5 cm-1, as consistent with the XPS results.
To investigate the electrochemical reactions of the CNT/NG-Fe as an anode
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material for lithium ion battery, CV curves of the CNT/NG-Fe for the initial three
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cycles was measured with a potential window of 0.05 to 3V at a scan rate of 0.1 mV/s, as shown in Fig. 5a. In the cathodic polarization process of the 1st cycle, the cathodic peak appeared at 0.6 V, which is attributed to the reduction of Fe3+ to Fe0 and insertion by lithium ion into the CNT/NG-Fe to form Li2O [23]. The anode peak
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presents at 1.75 V during the discharging process, corresponding to the reversible reaction of Fe0 to Fe3+ [8]. Meanwhile, the discharge peak shifts to 0.8 V from the 2nd cycle due to the irreversible phase transformation at the 1st cycle, resulting from the
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decomposition of electrolyte and formation of solid electrolyte interface (SEI) layers.
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The mechanism of lithium insertion can be verified by CV as follow reactions [14]: Fe O + Li + e → Li (Fe O ) (1) Li (Fe O ) + (2 − )Li + (2 − )e → Li (Fe O ) (2)
Li (Fe O ) + 4Li + 4e → 2Fe + 3Li O (3)
For the 2nd and 3rd cycles of the CNT/NG-Fe, the shape of CV curves was almost identical, that means the insertion and desertion of lithium ions are reversible.
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ACCEPTED MANUSCRIPT As shown in Fig. 5b, the voltage versus specific capacity plots conducted at 50 mA g-1 between 0 and 3 V versus Li/Li+. For the 1st cycle, the discharge plateau exhibits at ~1.6 V and ~1 V, which is attributed to the transformation of Fe3+ to Fe2+
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and Li ion insertion into the CNT/NG-Fe to form Li2O. The discharge plateau appeared near 0.85 V corresponds to formation of irreversible LixFe2O3 and further reduce to metallic iron in Eq.(1). The discharge capacity of the CNT/NG-Fe is 1208
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mAh g-1, which is also higher than 897 and 827 mAh g-1 of the CNT/G-Fe and NG
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due to the combination of the structure effect and chemical composition effect (Fig 5b). The capacity fading after the 1st cycle was78 % due to the unavoidable formation of SEI film and electrolyte decomposition, which is commonly observed for most anode materials [35]. Nonetheless, this value of 98.5 % Coulombic efficiency for the
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CNT/NG-Fe was higher than 98 % and 97% of the CNT/G-Fe and NG, indicating the restricted irreversible reaction by the hierarchical architecture and N-configuration. To further confirm the electrochemical behavior of the CNT/NG-Fe, the GCD
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curves were verified at different current densities from 100 to 1000 mA g-1 (Fig. 5c
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and Fig. S3a). At the current density of 1000 mA g-1, the discharge capacity is reach to 492 mAh g-1 with the capacity retention of 52 % of initial capacity corresponding to the discharge capacity of 947 mAh g-1. By contrast, the CNT/G-Fe and NG showed 46 % and 47 % of rate capabilities at the same conditions. The improved rate capability of CNT/NG-Fe was attributed to the hierarchical structure for fast lithium ion transport and N configuration for facilitated charge transfer as shown in Nyquist
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at the current density of 100 mA g-1, and after 90 cycles, the discharge capacity is greatly increased to 1020 mAh g-1 with the coulombic efficiency in excess of 98%. At the following 40 cycles, the discharge capacity becomes almost constant. These
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results are attributed to the further activation of the CNT/NG-Fe after many
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charge-discharge cycles. Cyclic stability at maximum cycles (130 cycles) at 100 mA g-1, the capacity gradually increases to 1020 mAh g-1 with the Coulombic efficiency of > 98.5%. Even though the initial capacity of CNT/G-Fe is 439 mAh g-1, the maximum capacity after 130 cycles was observed to be 573 mAh g-1 which is half of
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the capacity compared to CNT/NG-Fe. Consequently, the CNT/NG-Fe revealed enhanced rate capability and cyclic stability due to the heteroatom doping effect and
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hierarchical structure with the growth of dense CNT branches.
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ACCEPTED MANUSCRIPT 4. Conclusion In summary, we have successful synthesized a novel CNT growth on hierarchical NG by microwave-assisted method and applied as high-capacity anode materials in to
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LIBs. 3D macroporous architecture of NG was very crucial for depositing the iron oxide nanoparticles during the microwave irradiation, which assisted the uniform growth of CNT onto the NG skeleton. This hierarchical structure consisting of 1D
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CNT branches, iron oxide nanoparticles and 3D NG was beneficial for facilitating
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lithium ion transport, and for reducing the charge transfer resistance. The specific discharge capacity of the CNT/NG-Fe was 1020 mAh g-1 at 100 mA g-1, which was superior to the CNT/G-Fe and NG. Moreover, the CNT/NG-Fe showed better rate capability and cyclic stability compared to those of CNT/G-Fe and NG. These results
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indicate that the CNT/NG-Fe are regarded as a promising electrode material for LIBs.
Acknowledgment.
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ACCEPTED MANUSCRIPT This work was supported by both the financial support from the R&D Convergence Program (CAP-15-02-KBSI) of NST (National Research Council of Science & Technology) and the Energy Efficiency & Resources program of the Korea of Energy
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Technology Evaluation and Planning (KETEP), and was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No.
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20152020105770).
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properties, Appl. Surf. Sci. 314 (2014) 228–232. [16] S. Thangavel, M. Elayaperumal, G. Venugopal, Synthesis and properties of tungsten oxide and reduced graphene oxide nanocomposites, Mater. Express. 2 (2012) 327–334. [17] Y. Liu, Y. Ying, Y. Mao, L. Gu, Y. Wang, X. Peng, CuO nanosheets /rGO hybrid lamellar films with enhanced capacitance, Nanoscale. 5 (2013) 9134–9140. [18] S. Mao, Z. Wen, T. Huang, Y. Hou, J. Chen, High-performance bi-functional electrocatalysts of 3D crumpled graphene–cobalt oxide nanohybrids for oxygen reduction and evolution reactions, Energy Environ. Sci. 7 (2014) 609–616. [19] M. Kota, X. Yu, S. H. Yeon, H. W. Cheong, H. S. Park, Ice-templated three dimensional nitrogen doped graphene for enhanced supercapacitor performance, J. Power Sources. 303 (2016) 372–378. [20] M. Wu, C. Cao, J. Z. Jiang, Light non-metallic atom (B, N, O and F)-doped graphene: a first-principles study, Nanotechnology. 21 (2010) 505202. [21] B. Shen, J. Chen, X. Yan, Q. Xue, Synthesis of fluorine-doped multi-layered graphene sheets by arc-discharge, RSC Adv. 2 (2012) 6761–6764. [22]H. Terrones, M. Terrones, E. Hernández, N. Grobert, J. C. Charlier, P. M. Ajayan, New metallic allotropes of planar and tubular carbon, Phys. Rev. Lett. 84 (2000) 1716. [23]B. Guo, L. Fang, B. Zhang, J. R. Gong, Graphene doping: A Review, Insciences J. 1 (2011) 80–89. [24] W. S. Hummers, R. E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339–1339. [25] A. Leonhardt, S. Hampel, C. Müller, I. Mönch, R. Koseva, M. Ritschel, D. Elefant, K. Biedermann, B. Büchner, Synthesis, properties, and applications of ferromagnetic-filled carbon nanotubes, Chem. Vap. Depos. 12 (2006) 380–387. [26] A. M. Schwenke, S. Hoeppener, U. S. Schubert, Synthesis and modification of carbon nanomaterials utilizing microwave heating, Adv. Mater. 27 (2015) 4113– 4141. [27] E. O. Pentsak, E. G. Gordeev, V. P. Ananikov, Noninnocent nature of carbon support in metal/carbon catalysts: Etching/pitting vs nanotube growth under microwave irradiation, ACS Catal. 4 (2014) 3806–3814. [28] S. Yellampalli, ed., Carbon nanotubes - synthesis, characterization, applications, InTech, 2011. [29] V. M. Dhavale, S. K. Singh, A. Nadeema, S. S. Gaikwad, S. Kurungot, Nanocrystalline Fe–Fe2O3 particle-deposited N-doped graphene as an activity-modulated Pt-free electrocatalyst for oxygen reduction reaction, Nanoscale. 7 (2015) 20117–20125. [30] X. Zhang, Y. Niu, X. Meng, Y. Li, J. Zhao, Structural evolution and characteristics of the phase transformations between α-Fe2O3, Fe3O4 and γ-Fe2O3 nanoparticles under reducing and oxidizing atmospheres, CrystEngComm. 15 (2013) 8166–8172. [31] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, 19
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Nature. 407 (2000) 496–499. [32] Z. Lin, M. Song, Y. Ding, Y. Liu, M. Liu, C. Wong, Facile preparation of nitrogen-doped graphene as a metal-free catalyst for oxygen reduction reaction, Phys. Chem. Chem. Phys. 14 (2012) 3381–3387. [33] N. Daems, X. Sheng, I. F. J. Vankelecom, P. P. Pescarmona, Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A. 2 (2014) 4085–4110. [34] T. Sharifi, E. Gracia-Espino, H. R. Barzegar, X. Jia, F. Nitze, G. Hu, P. Nordblad, C. W. Tai, T. Wågberg, Formation of nitrogen-doped graphene nanoscrolls by adsorption of magnetic γ-Fe2O3 nanoparticles, Nat. Commun. 4 (2013). [35] S. K. Park, K. Choi, S. H. Lee, I. K. Oh, S. Park, H. S. Park, CNT branching of three-dimensional steam-activated graphene hybrid frameworks for excellent rate and cyclic capabilities to store lithium ions, Carbon. 116 (2017) 500–509. [36] J. X. Chen, D. L. Zhao, R. R. Yao, C. Li, X. J. Wang, F. F. Sun, Hedgehog-like CuO/nitrogen-doped graphene nanocomposite for high-performance lithium-ion battery anodes, J. Alloys Compd. 714 (2017) 419–424.
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and microwave irradiation.
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Fig. 1. Synthetic procedure of the CNT/NG-Fe through an ice-templating assembly
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Fig. 2. (a) and (b) Low and high magnification SEM images of CNT/G-Fe. (c), (d), and (e) Low and high magnification SEM images of CNT/NG-Fe. (f) TEM and (g)
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dark-field TEM and overlaid and elemental mapping images of CNT/NG-Fe at the tip
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region (carbon, iron, oxygen, and nitrogen of elemental mapping images).
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b 400 Α: α-Fe2O3 (Hematite)
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Α Α Α
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0.8
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o 2θ ( )
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Fig. 3. (a) XRD curves of the CNT/NG-Fe, CNT/G-Fe, and NG. (b)
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BJH pore size distribution).
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Nitrogen-adsorption and desorption isotherms of CNT/NG-Fe and CNT/G-Fe (inset is
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a
b
C-N C=N HO-C=O
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Pyrrolic Graphitic
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Fe 2p 3/2
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c
N1s
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C=C
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Fig. 4. (a) C1s, (b) N1s, (c) Fe2p XPS scans of the CNT/NG-Fe. (d) Raman spectra
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of the CNT/NG-Fe, CNT/G-Fe, and NG.
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b3.0
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Fig. 5. (a) CV curves of the CNT/NG-Fe at the 1st, 2nd and 3rd cycles. (b) GCD curves of the CNT/NG-Fe, CNT/G-Fe, and NG at the first cycle. (c) Rate capabilities of the
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CNT/NG-Fe, CNT/G-Fe, and NG at various rates. (d) Cycle stabilities of the CNT/NG-Fe, CNT/G-Fe, and NG at 100 mA g-1, and the columbic efficiency of the
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ACCEPTED MANUSCRIPT Highlights
Carbon Nanotubes Branched on Three-Dimensional, Nitrogen-Incorporated Reduced Graphene
Ion Battery Anode
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Oxide/Iron Oxide Hybrid Architectures for Lithium
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Yingbo Kang, Xu Yu, Manikantan Kota, Ho Seok Park*
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Unique hybrid hierarchical architectures are created.
Dense CNTs are branched on 3D macroporous NG-Fe hybrid. A high purity of α-Fe2O3 phase is formed for the 3D CNT@NG-Fe. Anode performances are enhanced by N configuration and hierarchical
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structure.
*Corresponding author. Tel: +82-31-299-4715, Email: [email protected], (Ho Seok Park)
S0925-8388(17)32633-6
DOI:
10.1016/j.jallcom.2017.07.264
Reference:
JALCOM 42680
To appear in:
Journal of Alloys and Compounds
Received Date: 20 May 2017 Revised Date:
22 July 2017
Accepted Date: 25 July 2017
Please cite this article as: Y. Kang, X. Yu, M. Kota, H.S. Park, Carbon nanotubes branched on threedimensional, nitrogen-incorporated reduced graphene oxide/iron oxide hybrid architectures for lithium ion battery anode, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.264. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
Carbon Nanotubes Branched on Three-Dimensional,
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Nitrogen-Incorporated Reduced Graphene Oxide/Iron Oxide Hybrid Architectures for Lithium Ion Battery
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Anode
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Yingbo Kang, Xu Yu, Manikantan Kota, Ho Seok Park*
*Corresponding author. Tel: +82-31-299-4715, Email: [email protected], (Ho Seok Park)
ACCEPTED MANUSCRIPT Carbon Nanotubes Branched on Three-Dimensional, Nitrogen-Incorporated Reduced Graphene Oxide/Iron Oxide Hybrid Architectures for Lithium Ion Battery Anode
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Yingbo Kang, Xu Yu, Manikantan Kota, Ho Seok Park*
School of Chemical Engineering, Sungkyunkwan University, Suwon 440 746, Republic
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of Korea.
*Corresponding author. Tel: +82-31-299-4715, Email: [email protected], (Ho Seok Park)
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ACCEPTED MANUSCRIPT Abstract The carbon nanotubes (CNTs) branched on three-dimensional (3D) macroporous, nitrogen-incorporated reduced graphene oxide (NG)/iron oxide
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(CNT/NG-Fe) hybrid architectures have been prepared via an ice templating and microwave synthesis. Compared with the pristine RGO, the CNTs can be more readily and uniformly grown on the 3D NG surfaces due to the good electronic
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conductivity by N-type configurations. As demonstrated by the electrochemical
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performances, the discharge capacity of the 3D CNT/NG-Fe is 1208 mAh g-1 at 50 mA g-1 which is greater than 890 and 820 mAh g-1 of the CNT/G-Fe and NG. When the rate increases from 100 to 1000 mAh g-1, the capacity retention reaches 52 % of initial capacity corresponding to the discharge capacity of 947 mAh g-1. After 130
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cycles at 100 mA g-1, the capacity gradually increases to 1020 mAh g-1 with the Coulombic efficiency of > 98.5 %. The enhanced capacity, rate capability and cyclic stability of the CNT/NG-Fe are associated with the doping effect of N-configuration
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and unique hierarchical structure consisting of the dense CNT branches on 3D
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macroporous continuity.
Keywords: Hierarchical architecture; Nitrogen doping; Porous Graphene; Carbon Nanotube; Hybrid; Lithium Ion Battery
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ACCEPTED MANUSCRIPT 1. Introduction Graphene is two-dimensional (2D) arrays of carbon atoms arranged in a hexagonal
pattern
[1],
whereas
carbon
nanotube
(CNT)
forms
tubular
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one-dimensional (1D) structure [2]. Both of CNT and graphene have received significant attention due to their outstanding properties such as high electrical conductivity, large surface area, good mechanical property, and electrochemical and
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thermal stabilities for energy-related applications. For instance, they were applied for
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a broad range of applicative fields, such as supercapacitors [3], battery [4], solar cells [5], electrocatalyst [6], sensors [7], etc. Moreover, hybrid materials which take advantages of integrating CNT and graphene into multi-dimensional hierarchical architecture are considered as a promising candidate for lithium ion battery (LIB) [8].
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Recently, the three-dimensional (3D) hierarchical architecture of carbon nanomaterials has been investigated to achieve prominent structural features such as large available area, fast ion and mass transport, percolated electron transfer and
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structural integrity [9]. Accordingly, the poor rate and cyclic capabilities of transition
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metal oxides, which are a critical challenge of conversion-based material [10], could be improved by depositing on such 3D macroporous internetworked reduced graphene oxide (RGO) [11]. For instance, 3D RGO/TiO2, RGO/SnO2 [12], RGO/MnO2 [13], RGO/Fe2O3 [14], RGO/NiO [15], RGO/WO3 [16], RGO/CuO [17], and RGO/CoO [18] hybrid architectures were developed for LIB applications. Another important strategy to improve electrochemical properties of carbon nanomaterials is the incorporation of heteroatoms such as nitrogen (N) [19], oxygen 3
ACCEPTED MANUSCRIPT (O) [20], phosphorus (P) [3], sulfur (S) [11] and fluorine (F) [21] into the graphitic lattice. Such heteroatom chemistry leads to modify electronic structure and electrochemical reactivity depending on the chemical identity, bonding configuration
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and composition [22]. Motivated by these findings, N-RGO, O-RGO, P-RGO, S-RGO and F-RGO were synthesized to achieve enhanced performances of LIB [23].
Taking full advantages of the afore-mentioned chemistries, for the first time,
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we demonstrate the unique hierarchical architecture, where a bunch of CNTs are
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brached on the 3D macroporous, N-incorporated RGO/iron oxide (CNT/NG-Fe) hybrids, constructed via an ice templating and microwave synthesis. The features of this complex hybrid architecture can be described along the following lines. (1) The 3D N-incorporated RGOs act as conductive networking substrate to provide large
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surface area for the deposition of iron oxide nanoparticles, to facilitate Li ion transport and to delocalize stress created by volume expansion during charge/discharging process. (2) The CNT branches offer 1D conducting pathway between intra- or
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interparticles and inhibiting restacking of RGO nanosheets. (3) The iron oxide
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nanoparticles are high capacity materials for enhancing charge storage capacity of carbon nanomaterials.
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ACCEPTED MANUSCRIPT 2. Experimental section 2.1 Synthesis of 3D NG The graphene oxide (GO) was synthesized by modified Hummers method [24]. Firstly,
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100 mg of as-obtained GOs was dispersed into 10 mL of deionized (DI) water, and ultrasonicated for 1 hour to make the homogenous dispersion. Secondly, 500 mg of melamine was added into 20 mL of DI water, and then the mixture was stirred and for 1 hour. Finally, two dispersions were mixed together and stirred to
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heated at 80
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make the homogenous solution. The final solution was frozen by liquid nitrogen and freeze-dried for 3 days to remove the solvent. The as-obtained sample was placed in the tube furnace and heat treated at two steps: (1) at 300
for 1 hour; (2) 900
for 1
min-1 under an argon atmosphere. The
hour with the increasing rate of 5
was noted as NG.
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temperature was naturally cooled down to room temperature, and the obtained sample
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2.2 Growth of CNT branches
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50 mg of ferrocene was mixed with 3 mL of acetylacetonate (ACN) solution, the mixture was stirred for 30 minutes and bath-sonicated for 1 hour. Then, 10 mg of NG was added and stirred for another 1 hour, and the solution was named A. 50 mg of azodicarbonamide (ADC) was mixed with 2 mL of ACN and stirred for 30 minutes, which was named as the B. Finally, the mixture of A and B was stirred for another 1 h to make the final homogeneous dispersion. The final solution was placed in the home-made microwave and irradiated for 40 seconds, then the sample was 5
ACCEPTED MANUSCRIPT continuously irradiated for another 150 seconds for the growth of CNT on N-doped RGO. The final product was obtained and noted as CNT/NG-Fe. The growth of CNT
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on RGO without N doping was noted as CNT/G-Fe.
2.3 Fabrication of coin cell
The electrochemical performance of CNT/NG-Fe was performed by coin cell
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(CR2032), which was fabricated in the glove box. The anode electrode was prepared
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as follows: a slurry consists of 80% of active material, 10% of polyvinylidence fluoride as binders and 10% of carbon black as the conducting agents. In additional, the NMP was used as the solvent. All components were mixed and grinded in the agate mortar with few drops of NMP for 30 minutes to make homogeneous slurry. for overnight in
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Then, the slurry was pasted on the copper foil, and dried at 80
vacuum oven. The test cell was fabricated in an argon-filled glove box by pressing anode electrode with a lithium metal foil and 1 M LiPF6 dissolved in a solution of
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ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by volume ratio) mixture as
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electrolyte. The cell was galvanostatically cycled between 0 and 3 V vs Li/Li+ at various current densities.
2.4 Characterization
Transmission electron microscopy (TEM) images were collected on a JEM-3010 HR TEM (300 kV). Scanning electron microscopy (SEM) image were obtained using a field emission scanning electron microscope (Philips SEM 535M), equipped with a 6
ACCEPTED MANUSCRIPT Schottky-based field emission gun. A scanning TEM (STEM) was operated with a probe focused to 0.2 nm and camera length of 20 cm. The scan raster was 512 × 512 points with a dwell time of 8.5 seconds per scan. Chemical analysis was performed
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using a VG Electron Spectroscope (ESCA 2000) at high vacuum (10-10 torr). It was equipped with monochrometer (quartz), twin X-ray source (Mg/Altarget) and hemispherical analyzer. The Brunauer-Emmett-Teller (BET) surface areas and
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nitrogen adsorption-desorption isotherms were measured at 78 K using BELSORP
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analysis program. Horvath-Kawazoe (HK) and Barrett-Joyner-Halenda (BJH) analyses are used to calculate the average micro- and meso-pore sizes. In-situ Raman spectra were measured using confocal micro-raman spectrometer NRS-3100, Jasco-Japan system with microscope having 100X lens and an excitation laser beam
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source of 532 nm wavelength. X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo MultiLab 2000system with an Al-Mg α X-ray source. There
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are no electron transfer processes during the XPS measurement of all samples.
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ACCEPTED MANUSCRIPT 3. Results and discussion As shown in Fig. 1, the 3D CNT/NG-Fe was synthesized through an ice templating and microwave irradiation synthesis. First, the 3D macroporous NG was
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synthesized via a simple and facile ice-templating method using GO and melamine precursor. The resulting material was used as the internetworked substrate for the growth of CNT branches. The homogeneous mixture of NG and ferrocene catalysts
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was treated under microwave irradiation to deposit CNT branches onto the surface of
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NG. During the microwave irradiation process, ADC was decomposed to produce urea that can react with oxygen molecules under air atmosphere and to make GO reduced by decreasing the oxygen content. At the meantime, the ferrocene is decomposed into metallic iron and hydrocarbons, as demonstrated in the equation
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below [25]:
Fe(C H ) → Fe + H + CH + C H + ⋯ The metallic iron particles were deposited on the surface of NG. and then, NG/iron
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particle produced strong magnetic-heating effect [26] to heat up microwave reactor
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[27]. At a high temperature, hydrocarbons were decomposed into molecular carbons, which will preferentially cover the surface of iron nanoparticles. Furthermore, this dissolved carbons start to be nucleated and grown into CNTs on iron nanoparticle via a tip growth mechanism after the saturation concentration is reached [28]. In addition to the decomposition of ADC, air reacts with iron to form iron oxide as the temperature continues to increase during the period of CNT growth.
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ACCEPTED MANUSCRIPT The morphology and microstructure of the CNT/G-Fe and CNT/NG-Fe were characterized by SEM and TEM images in Fig. 2. Low and high magnification SEM images of CNT/G-Fe as shown in Fig. 2a and 2b. In Fig. 2c and 2d, the CNT/NG-Fe
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preserved the 3D macroporous structure assembled by ice-templating method, indicating no significant damage of N configurations on the 3D architecture (Fig. S1a and S1b). The CNT branches were much denser for the 3D N-incorporated RGO than
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only 3D RGO substrate presumably due to the role of N configuration (Fig. S1c and
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S1d) as the activated site of depositing ferrocene catalyst and the large surface area. The N-containing groups of 3D NG acted as anchoring iron nanoparticles [29], which was deposited without critical loss of metal precursors and oxidized into iron oxide at low temperatures. The length of CNT branches was in the range of several
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micrometers, and the average diameters ranged up to 60 nm as shown in Fig. 2e and 2f. The CNT branches were randomly distributed and deposited on the surface of 3D macrorpous NG-Fe hybrids, and some iron oxide particles were positioned on both
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the CNT and NG surface. As shown in TEM images of the CNT/NG-Fe in Fig. 2g,
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the CNTs were not straight but curved. The large size of iron oxide nanoparticles obviously remained and was deposited at the tip part of the CNTs. As shown in STEM images, the spots of N, O and Fe elements were localized as marked by red contour, indicating iron oxide nanoparticles were deposited onto the 3D NG surface. By contrast, the C signals were distributed over the whole region of NGs and CNTs which were not distinguished. Since N configuration was solely existent onto the NG, red contour corresponds to the domain of NG. This result means that the iron oxide 9
ACCEPTED MANUSCRIPT particles were localized onto the surface of NG due to a favorable interaction with N-containing groups, where CNT branches were nucleated and grown. Meanwhile, the diameters of the CNT branches were mostly in the range from 30 to 60 nm
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strongly influenced by the particle size of Fe catalyst, which is consistent with the SEM result. As marked by blue contour, the CNT branches showed much weaker signals of O, N, and Fe compared to those in red contour of NG. These findings
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indicate that N-configuration strongly influence the growth of CNT branches.
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The crystalline structure of the CNT/NG-Fe was investigated by XRD spectroscopy in Fig. 3a. A strong sharp reflection in the diffraction profile of the CNT/G-Fe at 2θ=25.6° is attributed to the (002) reflection of the 3D RGO. By contrast, the (002) reflection of the CNT/NG-Fe was observed at 2θ=26.4°, where N
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incorporation did not significantly change the structure of 3D RGO. Both CNT/G-Fe and CNT/NG-Fe showed the characteristic XRD patterns of α-Fe2O3 phase, where the peaks appeared at 2θ of 24.1°, 33.2°, 35.6°, 40.8°, 49.5°, 54.1°, 62.5°, and 64.1°,
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corresponding to (012), (104), (110), (113), (024), (116), (214), and (300) planes, as
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marked by the letter “A” (JCPDS no. 89-0596/33-0664) [30]. On the other hand, the XRD pattern of the CNT/NG-Fe was dominated by α-Fe2O3 phase, indicating higher phase purity than the CNT/G-Fe. The different modes of the CNT/NG-Fe for the growth of Fe2O3 phase were attributed to the distinct chemical circumstances of the NG given by the N configuration. The presence and purity of hematite phase of α-Fe2O3 for the CNT/NG-Fe was expected to improve the anode capacity as demonstrated by Tarascon group [31]. The low cycle stability and conductivity of this 10
ACCEPTED MANUSCRIPT phase can be improved depositing onto the porous conductive material of 3D NG and connecting with CNT branches. The porous structure of the CNT/NG-Fe was characterized using nitrogen
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adsorption and desorption isotherm as shown in Fig. 3b. Both of the CNT/NG-Fe and CNT/G-Fe hybrids revealed type IV isotherm, confirming the existence of mesoporosity. As analyzed by BET and BJH, the specific surface area and the average
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mesopore diameter of the CNT/NG-Fe and the CNT/G-Fe were 87.4 and 53.7 m2 g-1
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and 28.1 and 25.2 nm, respectively. It notes that the enlarged surface area of the CNT/NG-Fe is presumably due to the heteroatom nitrogen doping and the growth of dense CNT branches. The pore volume of the CNT/NG-Fe and CNT/G-Fe was 0.61 cm3 g-1and 0.34 cm3 g-1, respectively. Therefore, the hierarchical porous structure of
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the CNT/NG-Fe can provide shorter ion transfer pathway and easy ion accessibility for the enhanced electrochemical performance. The chemical structure and bonding nature of the CNT/NG-Fe are
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demonstrated by XPS data, as shown in Fig. 4. As determined by wide XPS spectra,
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CNT/NG-Fe is consist of C, O, N and Fe, which are centered at 285.0 eV, 399.0 eV, 533.0 eV and 711.0 eV, respectively (Fig. S 2c). The binding energy region plotted of C 1s showed four deconvoluted peaks, which are centered at 284.8 eV, 285.8 eV, 286.9 eV and 289.1 eV, corresponding to C=C, C-N C=N and C=O bondings (Fig. 4a). From the high resolution N1s spectra (Fig. 4b), two major peaks were identified as pyridinic and graphitic-N centered around 398.2 eV and 401.3 eV [32], indicating the incorporation of two major N configurations. Moreover, two minor peaks were 11
ACCEPTED MANUSCRIPT observed at around 399.1 eV and 404.5 eV corresponding to pyrollic and oxidized pyridinic-N bonds [32]. The presence of majority of pyridinic and graphitic peaks have advantage in terms of improving the electronic conductivity by altering the local
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electron density on the graphene surface [33]. CNT/NG-Fe showed different ratios of C and N configurations from NG (Fig. S2), predicting that CNT/NG-Fe has a different surface chemistry by branching of CNT. The formation of hematite phase
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with the oxidation state of Fe3+ was confirmed by the high resolution of Fe2p
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spectrum, where two peaks of Fe2p3/2 and Fe2p1/2 were observed at 711.4 eV and 725 eV (Fig. 4c). The atomic % of N and Fe for the CNT/G-Fe and the CNT/NG-Fe were estimated as 2.39 and 4.09, respectively. The C/O ratio of the CNT/NG-Fe and CNT/G-Fe was 8.2 and 9.8, respectively. The decrease in C/O ratio in CNT/NG-Fe
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could be attributed to the incorporation higher percentage of nitrogen atoms. Raman spectroscopy was used to evaluate defects and structural change of carbonaceous materials as shown in Fig. 4d. For the case of NG, a D band (disordered
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structure) is observed at 1335 cm-1 and G band (stretching of the C-C bond) at 1584
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cm-1. The CNT/G-Fe hybrids exhibited D and G bands at 1343 cm-1 and 1572 cm-1. In contrast to the CNT/G-Fe, the D band of the CNT/NG-Fe was blue-shifted to 1319 cm-1, and G band was red-shifted to 1579 cm-1, which are associated with the outcome of N-doping effect and graphitic nature of grown CNT [34]. The intensity ratio of the D and G band (ID/IG) is a crucial factor to verify the defects. The ID/IG value of the CNT/NG-Fe was measured as 1.03, which is lower than 1.16 of NG, indicating that decoration of iron oxide nanoparticles and subsequent growth of CNT can restore the 12
ACCEPTED MANUSCRIPT defects. However, the ID/IG value of the CNT/NG-Fe is higher compared to that of CNT/G-Fe, which means that the incorporation of nitrogen configuration increases the defect sites. The existence and phase purity of α-Fe2O3 was further confirmed by
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the Raman peaks of the CNT/NG-Fe at 224.7, 296.1, 412.5, 506.4, and 611.5 cm-1, as consistent with the XPS results.
To investigate the electrochemical reactions of the CNT/NG-Fe as an anode
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material for lithium ion battery, CV curves of the CNT/NG-Fe for the initial three
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cycles was measured with a potential window of 0.05 to 3V at a scan rate of 0.1 mV/s, as shown in Fig. 5a. In the cathodic polarization process of the 1st cycle, the cathodic peak appeared at 0.6 V, which is attributed to the reduction of Fe3+ to Fe0 and insertion by lithium ion into the CNT/NG-Fe to form Li2O [23]. The anode peak
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presents at 1.75 V during the discharging process, corresponding to the reversible reaction of Fe0 to Fe3+ [8]. Meanwhile, the discharge peak shifts to 0.8 V from the 2nd cycle due to the irreversible phase transformation at the 1st cycle, resulting from the
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decomposition of electrolyte and formation of solid electrolyte interface (SEI) layers.
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The mechanism of lithium insertion can be verified by CV as follow reactions [14]: Fe O + Li + e → Li (Fe O ) (1) Li (Fe O ) + (2 − )Li + (2 − )e → Li (Fe O ) (2)
Li (Fe O ) + 4Li + 4e → 2Fe + 3Li O (3)
For the 2nd and 3rd cycles of the CNT/NG-Fe, the shape of CV curves was almost identical, that means the insertion and desertion of lithium ions are reversible.
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ACCEPTED MANUSCRIPT As shown in Fig. 5b, the voltage versus specific capacity plots conducted at 50 mA g-1 between 0 and 3 V versus Li/Li+. For the 1st cycle, the discharge plateau exhibits at ~1.6 V and ~1 V, which is attributed to the transformation of Fe3+ to Fe2+
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and Li ion insertion into the CNT/NG-Fe to form Li2O. The discharge plateau appeared near 0.85 V corresponds to formation of irreversible LixFe2O3 and further reduce to metallic iron in Eq.(1). The discharge capacity of the CNT/NG-Fe is 1208
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mAh g-1, which is also higher than 897 and 827 mAh g-1 of the CNT/G-Fe and NG
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due to the combination of the structure effect and chemical composition effect (Fig 5b). The capacity fading after the 1st cycle was78 % due to the unavoidable formation of SEI film and electrolyte decomposition, which is commonly observed for most anode materials [35]. Nonetheless, this value of 98.5 % Coulombic efficiency for the
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CNT/NG-Fe was higher than 98 % and 97% of the CNT/G-Fe and NG, indicating the restricted irreversible reaction by the hierarchical architecture and N-configuration. To further confirm the electrochemical behavior of the CNT/NG-Fe, the GCD
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curves were verified at different current densities from 100 to 1000 mA g-1 (Fig. 5c
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and Fig. S3a). At the current density of 1000 mA g-1, the discharge capacity is reach to 492 mAh g-1 with the capacity retention of 52 % of initial capacity corresponding to the discharge capacity of 947 mAh g-1. By contrast, the CNT/G-Fe and NG showed 46 % and 47 % of rate capabilities at the same conditions. The improved rate capability of CNT/NG-Fe was attributed to the hierarchical structure for fast lithium ion transport and N configuration for facilitated charge transfer as shown in Nyquist
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ACCEPTED MANUSCRIPT plots (Fig. S3b). These values were much greater compared to capacity and rate capability of other N-doped RGO as previously reported [36]. As shown in Fig. 5d, the discharge capacity of the CNT/NG-Fe is 817 mAh g-1
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at the current density of 100 mA g-1, and after 90 cycles, the discharge capacity is greatly increased to 1020 mAh g-1 with the coulombic efficiency in excess of 98%. At the following 40 cycles, the discharge capacity becomes almost constant. These
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results are attributed to the further activation of the CNT/NG-Fe after many
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charge-discharge cycles. Cyclic stability at maximum cycles (130 cycles) at 100 mA g-1, the capacity gradually increases to 1020 mAh g-1 with the Coulombic efficiency of > 98.5%. Even though the initial capacity of CNT/G-Fe is 439 mAh g-1, the maximum capacity after 130 cycles was observed to be 573 mAh g-1 which is half of
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the capacity compared to CNT/NG-Fe. Consequently, the CNT/NG-Fe revealed enhanced rate capability and cyclic stability due to the heteroatom doping effect and
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hierarchical structure with the growth of dense CNT branches.
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ACCEPTED MANUSCRIPT 4. Conclusion In summary, we have successful synthesized a novel CNT growth on hierarchical NG by microwave-assisted method and applied as high-capacity anode materials in to
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LIBs. 3D macroporous architecture of NG was very crucial for depositing the iron oxide nanoparticles during the microwave irradiation, which assisted the uniform growth of CNT onto the NG skeleton. This hierarchical structure consisting of 1D
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CNT branches, iron oxide nanoparticles and 3D NG was beneficial for facilitating
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lithium ion transport, and for reducing the charge transfer resistance. The specific discharge capacity of the CNT/NG-Fe was 1020 mAh g-1 at 100 mA g-1, which was superior to the CNT/G-Fe and NG. Moreover, the CNT/NG-Fe showed better rate capability and cyclic stability compared to those of CNT/G-Fe and NG. These results
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indicate that the CNT/NG-Fe are regarded as a promising electrode material for LIBs.
Acknowledgment.
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ACCEPTED MANUSCRIPT This work was supported by both the financial support from the R&D Convergence Program (CAP-15-02-KBSI) of NST (National Research Council of Science & Technology) and the Energy Efficiency & Resources program of the Korea of Energy
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Technology Evaluation and Planning (KETEP), and was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No.
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20152020105770).
Reference: 17
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Nature. 407 (2000) 496–499. [32] Z. Lin, M. Song, Y. Ding, Y. Liu, M. Liu, C. Wong, Facile preparation of nitrogen-doped graphene as a metal-free catalyst for oxygen reduction reaction, Phys. Chem. Chem. Phys. 14 (2012) 3381–3387. [33] N. Daems, X. Sheng, I. F. J. Vankelecom, P. P. Pescarmona, Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A. 2 (2014) 4085–4110. [34] T. Sharifi, E. Gracia-Espino, H. R. Barzegar, X. Jia, F. Nitze, G. Hu, P. Nordblad, C. W. Tai, T. Wågberg, Formation of nitrogen-doped graphene nanoscrolls by adsorption of magnetic γ-Fe2O3 nanoparticles, Nat. Commun. 4 (2013). [35] S. K. Park, K. Choi, S. H. Lee, I. K. Oh, S. Park, H. S. Park, CNT branching of three-dimensional steam-activated graphene hybrid frameworks for excellent rate and cyclic capabilities to store lithium ions, Carbon. 116 (2017) 500–509. [36] J. X. Chen, D. L. Zhao, R. R. Yao, C. Li, X. J. Wang, F. F. Sun, Hedgehog-like CuO/nitrogen-doped graphene nanocomposite for high-performance lithium-ion battery anodes, J. Alloys Compd. 714 (2017) 419–424.
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and microwave irradiation.
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Fig. 1. Synthetic procedure of the CNT/NG-Fe through an ice-templating assembly
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Fig. 2. (a) and (b) Low and high magnification SEM images of CNT/G-Fe. (c), (d), and (e) Low and high magnification SEM images of CNT/NG-Fe. (f) TEM and (g)
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dark-field TEM and overlaid and elemental mapping images of CNT/NG-Fe at the tip
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region (carbon, iron, oxygen, and nitrogen of elemental mapping images).
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b 400 Α: α-Fe2O3 (Hematite)
1.0
10
300
Α
Α Α Α
ΑΑ
Α
200
3
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CNT/NG-Fe
CNT/G-Fe
100
CNT/NG-Fe CNT/G-Fe
0.8
dVp/dlogdp
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B
Va/cm (STP)g
Intensity(arb.u.)
B: C (Carbon) 0.6 0.4 0.2
0.0 0 10
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Relative Pressure (p/p )
o 2θ ( )
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Fig. 3. (a) XRD curves of the CNT/NG-Fe, CNT/G-Fe, and NG. (b)
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BJH pore size distribution).
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Nitrogen-adsorption and desorption isotherms of CNT/NG-Fe and CNT/G-Fe (inset is
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a
b
C-N C=N HO-C=O
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Binding energy (eV)
Oxide pyridinic
390
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Intensity (arb.u.)
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Binding energy (eV)
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Fe 2p 1/2
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Fe 2p 3/2
700
Pyridinic
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c
N1s
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280
C1s
Intensity (arb.u.)
Intensity (arb.u.)
C=C
CNT/G-Fe
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600
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Raman shift (cm )
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Fig. 4. (a) C1s, (b) N1s, (c) Fe2p XPS scans of the CNT/NG-Fe. (d) Raman spectra
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of the CNT/NG-Fe, CNT/G-Fe, and NG.
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b3.0
1 cycle 2 cycle 3 cycle
0.2
2.5
Voltage (V)
CNT/NG-Fe CNT/G-Fe NG
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Fig. 5. (a) CV curves of the CNT/NG-Fe at the 1st, 2nd and 3rd cycles. (b) GCD curves of the CNT/NG-Fe, CNT/G-Fe, and NG at the first cycle. (c) Rate capabilities of the
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CNT/NG-Fe, CNT/G-Fe, and NG at various rates. (d) Cycle stabilities of the CNT/NG-Fe, CNT/G-Fe, and NG at 100 mA g-1, and the columbic efficiency of the
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CNT/NG-Fe (blue color).
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ACCEPTED MANUSCRIPT Highlights
Carbon Nanotubes Branched on Three-Dimensional, Nitrogen-Incorporated Reduced Graphene
Ion Battery Anode
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Oxide/Iron Oxide Hybrid Architectures for Lithium
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Yingbo Kang, Xu Yu, Manikantan Kota, Ho Seok Park*
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Unique hybrid hierarchical architectures are created.
Dense CNTs are branched on 3D macroporous NG-Fe hybrid. A high purity of α-Fe2O3 phase is formed for the 3D CNT@NG-Fe. Anode performances are enhanced by N configuration and hierarchical
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structure.
*Corresponding author. Tel: +82-31-299-4715, Email: [email protected], (Ho Seok Park)