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MWCNTs modified α-Fe2O3 nanoparticles as anode active materials and carbon nanofiber paper as a flexible current collector for lithium-ion batteries application
Accepted Manuscript MWCNTs modified α-Fe2O3 nanoparticles as anode active materials and carbon nanofiber paper as a flexible current collector for lithium-ion batteries application Xu Chen, Zhen Zhao, Yan Zhou, Yun Shu, Muhammad Sajjad, Qinsong Bi, Yang Ren, Xu Wang, Xiaowei Zhou, Zhu Liu PII:
S0925-8388(18)34066-0
DOI:
https://doi.org/10.1016/j.jallcom.2018.10.367
Reference:
JALCOM 48185
To appear in:
Journal of Alloys and Compounds
Received Date: 1 August 2018 Revised Date:
10 October 2018
Accepted Date: 27 October 2018
Please cite this article as: X. Chen, Z. Zhao, Y. Zhou, Y. Shu, M. Sajjad, Q. Bi, Y. Ren, X. Wang, X. Zhou, Z. Liu, MWCNTs modified α-Fe2O3 nanoparticles as anode active materials and carbon nanofiber paper as a flexible current collector for lithium-ion batteries application, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.367. 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.
ACCEPTED MANUSCRIPT MWCNTs modified α-Fe2O3 nanoparticles as anode active materials and carbon nanofiber paper as a flexible current collector for lithium-ion batteries application
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Xu Chena, Zhen Zhaoa, Yan Zhoua, Yun Shua, Muhammad Sajjada, Qinsong Bia, Yang Rena, Xu Wangc, Xiaowei Zhoua,* and Zhu Liua,b,* a
Department of Physics and Astronomy, Yunnan University, Kunming, Yunnan
Province, China 650091
Micro and Nano-materials and Technology Key Laboratory of Yunnan Province,
Kunming City, Yunnan Province, China 650091
Materials Science and Engineering Program, University of Houston, Houston, TX
77204, USA * *
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c
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b
Corresponding Author E-mail: [email protected]
Corresponding Author E-mail: [email protected]
Abstract
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α-Fe2O3 nanoparticles/multi-wall carbon nanotubes (MWCNTs) hybrids were successfully prepared via a facile one-step hydrothermal method. The MWCNTs enhances the conductivity and provides space for stress and strain during volume
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expansion of α-Fe2O3 semiconductor as the anode for LIBs. The composite shows impressive electrochemical performance when it was used as the electrodes of coin
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cells. The α-Fe2O3 electrode with 50 wt.% MWCNTs retains its capacity of 816.8 mAh g-1 after 50 cycles at the current density of 200 mA g-1, which is superior to those of contrast samples (bare α-Fe2O3, pure MWCNTs, 10 wt.%, 30 wt.%, 70 wt.% MWCNT/α-Fe2O3 hybrids). Furthermore, the flexible carbon nanofiber paper (CNP) has also been investigated as a novel current collector with lithium storage for application
in
LIBs,
and
the
capacity
of
free-standing
α-Fe2O3/50
wt.%MWCNTs/CNP hybrids electrode can retain 467.2 mAh g-1 after 50 cycles under the current density of changes from 2500 mA g-1 to 200 mA g-1. The MWCNTs improved cycle performance, enhanced reversible capacities and rate capability of
ACCEPTED MANUSCRIPT MWCNTs/α-Fe2O3 anodes can attribute to the inherent conducting network, shorten electron pathway and faster reaction kinetics. Keywords: Multi-wall carbon nanotubes, Carbon nanofiber paper, Kinetics, α-Fe2O3 1.Introduction
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Li-ion batteries (LIBs) are considered to be energy storage sources with high-energy density, safe and high cycle life in various high technology applications.[1-2] Nanostructured metal oxide in carbon modification is a promising anode material for safer LIB designs.[3-6] Hematite (α-Fe2O3), the most stable iron
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oxide anode material for LIBs, has attracted great attention for its eco-friendliness, abundance, low cost, as well as the high theoretical capacity of 1005 mAh g-1.[7-11]
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However, the low electronic conductivity, easy agglomeration and poor capacity retention have hindered its practical implementation in LIBs.[8, 12-15] Therefore, improving the conductivity and alleviating the collapse of α-Fe2O3 during the intercalation/deintercalation of Li+ are the research hotspots when it was used as an anode material for LIBs.[16-21] Thus, great efforts have been devoted to the
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exploitation of α-Fe2O3 materials with higher conductivity and good capacity retention. The effective methods are using nanosize structures or high conductivity carbon-based hybrids which can improve the electron/ion transfer rates and mitigate
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the structural strain during Li+ intercalation/deintercalation.[22-27] Up to now, improving conductivity and avoiding structure collapse of Fe2O3 nanostructure anode materials mainly include introducing graphite, carbon nanotubes
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or graphene into the matrix of the Fe2O3 nanostructure or performing carbon coating.[28-30] We reported on the previous work that Fe3O4 nanoparticles were electrochemically grown on free-standing buckypaper for light-weighted and flexible lithium-ion battery.[31] Cuo et al. reported CNT/graphene nanosheet hybrid anode for LIBs using a chemical vapor deposition method which exhibited a high specific capacity of 984 mAh g-1.[32] Liu′s group prepared Fe2O3-filled CNTs anode by the AAO template method with a reversible capacity of 768 mAh g-1 after 40 cycles.[33] Carbon coated α-Fe2O3 hollow nanohorns on carbon nanotube (CNT) anode with very stable capacity retention of 800 mAh g-1 over 100 cycles at a high current density of
ACCEPTED MANUSCRIPT 500 mA g-1 have been constructed by Lou′s team. Yu et al. successfully fabricated graphene sheets/α-Fe2O3 sandwiched nanostructure hybrids for anode with a high capacity of 658.5 mAh g-1 at 1000 mA g-1 up to 200 cycles.[34] Chen et al. designed a novel hybrid nanostructure by coating Fe2O3 nanoparticles which used as the anode in
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a Li-ion battery, this hybrid material (70.32 wt.% carbon nanotubes, 29.68 wt.% Fe2O3) showed a reversible discharge capacity of 515 mAh g-1 after 50 cycles at a density of 100 mA/g.[35] Shen′s team prepared Fe2O3/COOH-MWCNT composites and these composites were used as electrode material in lithium-ion batteries, a
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reversible capacity of 711.2 mAh·g-1 at a current density of 500 mA·g-1 after 400 cycles was obtained.[36] TiO2 films coated α-Fe2O3 nanoparticles anchored on
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MWCNTs exhibiting a steady discharge capacity of 670 mAh g-1 even cycled at a large current density of 1000 mA g-1.[37] However, there is a limited study on the impact of the proportion of carbon nanotubes in α-Fe2O3 nanocomposite electrodes on the electrochemical performance and application of suitable composite materials in anode materials for flexible LIBs. Furthermore, most active materials embedded in
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the inner nanotube cannot be effectively accessed by the electrolyte, resulting in ineffective Li+ storage capacity. Therefore, more efforts should be devoted to the design of α-Fe2O3 on the surface of high conductivity carbons composite which has
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excellent structural stability, easy accessibility for the electrolyte and fast electron/ion transport pathways as anodes for LIBs. Herein, α-Fe2O3 was anchored on the surface of MWCNTs outer wall with
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different weight ratios of MWCNTs to α-Fe2O3 by a simple hydrothermal method. The cycle ability and stability of the as-prepared hybrids have been studied by applying it as the anodes of LIBs. It is noteworthy that the α-Fe2O3/50 wt.%MWCNTs anodes can maintain their reversible capacity of 816.8 mAh g-1 with the initial coulombic efficiency of 96.6 % after 50 cycles. In addition, α-Fe2O3/MWCNTs/CNP electrodes with carbon nanofiber paper (CNP) as the current collector to synthesize by suction filtration method instead of the metallic foil in traditional LIBs. Therefore, a light-weight, flexible, stable, and high-capacity α-Fe2O3/MWCNTs hybrids anode is realized. The novel free-standing α-Fe2O3/50
ACCEPTED MANUSCRIPT wt.%MWCNTs/CNP hybrid showed a discharge capacity of 574.8 mAh g-1 at a current density of 200 mA g-1 after 50 cycles. Our work provides a simple way for the fabrication of 3D porous, light-weight, flexible and high conductivity anode materials for LIBs.
2.1 Preparation of α-Fe2O3/MWCNT nanohybrids
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2.Experimental and methods
All chemicals were reagent grade and used as received from the supplier. The α-Fe2O3 nanoparticles and fabrication of α-Fe2O3/MWCNT nanohybrids were
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prepared by hydrothermal method. In a typical procedure, 1) Fe(NO3)3⋅9H2O (Corresponding to the iron source quality of samples was bare α-Fe2O3 4.8 g, 10 wt.%
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4.8 g, 30 wt.% 4.8 g, 50 wt.% 2.4 g, 70 wt.% 1.6 g, respectively) and 15 mL (0.025 g) Cetyltrimethyl Ammonium Bromide (CTAB, Tianjin science and technology co., LTD) was dissolved in 15 mL deionized water and stirred for 15 min to obtained homogeneous solution; 2) Carbon nanotubes (MWCNTs, Shenzhen port of nano-science and technology co ., LTD) (Corresponding to the MWCNTs, quality of
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samples is bare MWCNTs 1.0 g, 10 wt.% 0.1 g, 30 wt.% 0.3 g, 50 wt.% 0.5 g, 70 wt.% 0.7 g, respectively) and 1.3 g NaOH ( Sigma Aldrich Shanghai trading co., LTD), were dissolved in 15 mL deionized water and stirred for 15 min which results in a
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homogeneous solution; 3) Mixed the solution of foregoing two steps and stirred for 15 min to obtain a homogeneous solution with outgrowth precipitate Fe(OH)3; 4) Then
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sealed the solution in a 50 mL stainless steel autoclave and put it in the 160 °C environment for 12 h; 5) Then the solid precipitate particles were filtered from the solution, rinsed by DI water and alcohol for 15 min and dried at 60 °C for 24 h. We selected appropriate reagents to prepare a series of α-Fe2O3/MWCNTs hybrids with a different mass ratio of α-Fe2O3 to MWCNTs. Carbon nanofiber paper (CNP, 0.27 mm thickness, Kunming Natai Energy Co., Ltd, China) was used as the novel free-standing flexible electrode to replace traditional copper current collector. 2.2 Material Characterization
ACCEPTED MANUSCRIPT The structure of the as-synthesized α-Fe2O3, α-Fe2O3/MWCNTs composite and modified CNP samples were characterized by an X-ray diffraction (XRD, Cu Kα radiation 1.5406 Å Rigaku TTRШ) and transmission electron microscopy (TEM, JEM-2100 Japan). The morphologies of the samples were investigated by scanning
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electron microscopy (SEM, FEI QUANTA200). The Raman spectra were obtained by an inVia (Renishaw USA) at room temperature. Chemical bonding analysis of α-Fe2O3 and α-Fe2O3/MWCNTs composite were carried out by X-ray photoelectron spectroscopy (XPS, Thermo scientific K-ALPHA+). The Fourier transform-infrared
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(FT-IR, NicoletS10) spectra of the α-Fe2O3/MWCNTs hybrids were acquired by a Thermo Nicolet spectrometer using the KBr pellet technique (Avatar 360, USA).
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Particle size distribution of different α-Fe2O3/MWCNTs hybrids were demonstrated by light scattering analysis using a Mastersizer 3000E analyzer (Malvern Instruments, Malvern-UK) using the Fraunhofer approximation model. UV-vis measurements were performed on a UV-Vis spectrophotometer (AVATAR, FT-IT, 360). The reaction kinetics of α-Fe2O3/50 wt.%MWCNTs were studied by thermogravimetric analysis
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(TGA, Instrument SDT 2960 USA).
2.3 Electrochemical characterization
The working anodes were prepared by mixing the active material (α-Fe2O3,
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α-Fe2O3/MWCNTs or α-Fe2O3/MWCNTs/CNP), a conductive agent (carbon black), and binder (PVDF) in a weight ratio of 8:1:1. The typical loading density of active materials was ∼ 2.0 mg cm-2. The specific discharge capacities values were calculated
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based on grams of α-Fe2O3/MWCNTs or grams of α-Fe2O3/MWCNTs/CNP hybrid. Coin-type cells (CR2025) were assembled using the lithium metal as the counter electrode, a polypropylene (PP) microporous film as the separator, and LiPF6 (1 M) in ethylene carbonate (EC)-dimethyl carbonate (DMC) and diethyl carbonate (DEC) (volume ratio of EC:DMC:DEC=1:1:1) as the electrolyte in an argon-filled glove box (MIKROUNA super, O2 ≤ 0.1 ppm, H2O ≤ 0.1 ppm). The galvanostatic measurements were carried out on a LAND-CT2001A battery tester with a voltage window between 0.01 V and 3.0 V at various current rates. The cyclic voltammetry (CV) was performed between 0.01 V and 3.0 V using an electrochemical workstation
ACCEPTED MANUSCRIPT (CHI604E) at a scan rate of 0.1 mV s-1. The electrochemical impedance spectra (EIS) of the cells were measured on the electrochemical workstation (CHI604E) with the frequency range from 0.1 Hz to 100 kHz. Nyquist plots derived from EIS were simulated using Z-view software. 3.Results and discussion
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SEM images of α-Fe2O3/MWCNTs hybrids with different mass ratios are shown in Fig. 1. The average diameter and length of as-prepared MWCNTs is ∼ 60 nm and ∼ 15 µm, respectively, as shown in Fig. 1a. The spherical bare α-Fe2O3 nanocrystals
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with an approximate diameter of 150 nm are shown in Fig. 1b. As shown in Fig. 1c-f the α-Fe2O3 particles are stably stuck in the compact network structure formed by the
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stacked different mass ratio of MWCNTs (10 wt.%, 30 wt.%, 50 wt.%, 70 wt.%). As the MWCNTs amount increased, the particle size of α-Fe2O3 gradually becomes larger, which is consistent with the results of the granularity analysis as shown in Fig. S1. The network spaces become larger thickening with the increase of MWCNTs mass ratio, which may control dimensional growth and determine the diameter of
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α-Fe2O3 particles. In addition, with the increase of MWCNTs weight ratio in the composite, the absorption spectrum of α-Fe2O3/MWCNTs is gradually enhanced (Fig. S2). The band gap of α-Fe2O3/MWCNTs hybrids are around ~ 2.0 eV, as presented in
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Fig. S3, and the hybrids can absorb a wide range of the solar spectrum.
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Fig. 1. SEM images of α-Fe2O3/MWCNTs hybrids with different mass ratios of
MWCNTs (a) pure MWCNTs, (b) bare α-Fe2O3,(c) 10 wt.%, (d) 30 wt.%, (e) 50 wt.%, (f) 70 wt.%. The powder of MWCNTs, α-Fe2O3, α-Fe2O3/MWCNTs hybrids were further characterized by X-ray diffraction patterns (Fig. 2). The reflections result of bare α-Fe2O3 were in excellent accordance with a hexagonal hematite structure (JCPDS No. 99-0060) which belongs to the space group R-3c (Number 167). From
ACCEPTED MANUSCRIPT α-Fe2O3/MWCNTs hybrids, several strong diffraction peaks can be observed corresponding to the (012), (104), (110) and (113) peaks, which precisely match with those of pure α-Fe2O3 crystals. The synthesized α-Fe2O3/MWCNT hybrids display a broad peak at 2θ = 25.6° which can be assigned to the characteristic (002) plane of MWCNTs. As the MWCNTs mass ratio increases, the carbon peak appeared which
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suggests α-Fe2O3 crystals formed on MWCNTs, which is consistent with the results in Fig. 1c-f. Furthermore, thermogravimetric (TGA) analysis in Fig. S4 shows that the percentage content of α-Fe2O3/50 wt.%MWCNT is in accordance with the theoretical
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calculation (ratio of mass 1:1).
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Fig. 2. XRD patterns of MWCNTs, pure α-Fe2O3 and α-Fe2O3/MWCNTs hybrids. The structure and morphology of α-Fe2O3/MWCNTs hybrids were investigated by TEM. As demonstrated in Fig. 3a, the α-Fe2O3 is nanosphere morphology with
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diameter range from 20 nm to 200 nm, which the most widely distributed diameter around 150 nm. The α-Fe2O3 particles have monocrystal properties (Fig. 3a),
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displaying clear lattice fringes corresponding to the (012) plane of α-Fe2O3 with an interplanar crystal spacing of 3.68 Å, as presented in Fig. 3b. Fig. 3c-d manifests the morphology network architectures of the as-prepared α-Fe2O3/50 wt.% MWCNTs hybrids. It can be seen that the α-Fe2O3 are loaded on the surface of one-dimensional MWCNTs. Obviously, different from nanosphere bare α-Fe2O3, the morphology of α-Fe2O3 nanoparticles in the hybrids network has a hexagonal structure with the size range from 50 nm to 200 nm. A similar phenomenon for the α-Fe2O3/70 wt.%MWCNTs appears in Fig. 3e-f. The α-Fe2O3 is basically distributed outward along the surface of MWCNTs, this is because that the surface of oxidized MWCNTs
ACCEPTED MANUSCRIPT has numerous oxygen-containing groups, which may have a dispersive effect for the nuclei of Fe3+ precipitates in the hydrothermal reaction. The formation mechanism of α-Fe2O3 on the outer wall of MWCNTs prepared by hydrothermal has been analyzed in Fig. S5. The surface of the MWCNTs applied contains functional groups such as
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-COOH and -OH. During the reaction, the Fe3+ in precursor solution is captured by the oxygen-containing functional groups on the surface of the MWCNTs and α-Fe2O3 formed on the outer wall of MWCNT at high temperature and pressure. The α-Fe2O3 nanoparticles grown on the outer wall surface of MWCNT, as shown in the TEM
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embedded in the MWCNTs matrix (Fig. S7).
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image of Fig. S6. The α-Fe2O3 nanoparticles with a smooth surface are well
Fig. 3. TEM images of α-Fe2O3 hybrids with different mass ratio of MWCNTs (wt.%): (a-b) pure α-Fe2O3, (c-d) 50 wt.%, (e-f) 70 wt.%.
ACCEPTED MANUSCRIPT Raman spectra of the pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids were shown in Fig. 4. As we know, hematite is an antiferromagnetic material 6 which belongs to the ܦ3d crystal space group. There were seven phonon lines in the
Raman spectrum. The five phonon lines were named two A1g modes (217 cm-1 and
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404 cm-1) and three Eg modes (283 cm-1, 494 cm-1 and 603 cm-1), respectively.[26] Two obvious shark peaks located at 1346 cm-1 and 1580 cm-1 correspond to typical D-band and G-band which can be observed in pure MWCNTs and all α-Fe2O3/MWCNTs hybrids samples. The A1g and Eg peaks arise from α-Fe2O3 can be
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observed in Raman spectra at the range from 150 cm-1 to 600 cm-1. The peaks located at 217 cm-1 and 404 cm-1 were assigned to the A1g vibration modes and peaks at 283
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cm-1, 495 cm-1 and 603 cm-1 were caused by Eg. The intense feature named 2LO at 1311 cm-1 is assigned to a two-magnon scattering which arises from the interaction of two magnons created on antiparallel close spin sites. Infrared spectroscopy shows that oxygen-containing functional groups (-COOH, -OH) exist in all α-Fe2O3/MWCNTs
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hybrids (Fig. S8).
Fig. 4. Raman spectra of bare MWCNTs and α-Fe2O3 with 10/30/50/70 wt.% MWCNTs.
To further characterize the chemical components and valence states of the MWCNTs and α-Fe2O3 with 10/30/50/70 wt.% MWCNTs, XPS analyses are performed as depicted in Fig. 5. The XPS survey spectra of pure MWCNTs, bare α-Fe2O3 and MWCNT/α-Fe2O3 hybrids are presented in Fig. 5a. The survey spectra are calibrated with the C1s line of adventitious carbon at 284.4 eV. Fig. 5b shows the
ACCEPTED MANUSCRIPT zoom in XPS spectra of the C1s of α-Fe2O3 with 50 wt.% MWCNTs at 284.4 eV which arise from carbon. Fig. 5c shows that the O1s spectrum of the α-Fe2O3 nanoparticles corresponding to Fe−O−Fe bonds (532.3 eV). The measured binding energies of Fe2p3/2 and Fe2p1/2 are given in Fig. 5d at 709.4 eV and 722.8 eV,
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respectively. The peak top distance is 13.4 eV, which corresponds to the standard spectral phase of Fe2p in α-Fe2O3. It can demonstrate the presence of Fe3+. Similar analysis indicates the oxygen content of the oxide in Fig. 5c. These demonstrate that the oxide within all MWCNTs/α-Fe2O3 hybrids can be attributed to ferric oxides,
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which coincides with XRD results.
Fig. 5. XPS spectrum of as-prepared simples:(a) bare MWCNTs, α-Fe2O3 and α-Fe2O3 with 10/30/50/70 wt.% MWCNTs, (b) C−C in α-Fe2O3 with 50 wt.%
MWCNTs, (c) O1s in α-Fe2O3 with 50 wt.% MWCNTs,(d) Fe 2p in α-Fe2O3 with 50 wt.% MWCNTs. The electrochemical performances of pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids were investigated by a cyclic charge-discharge. CVs of the pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids electrodes at a scan
ACCEPTED MANUSCRIPT rate of 0.1 mV s-1 for 3 cycles are shown in Fig. 6. Fig. 6a is a CV of pure MWCNTs electrode with the first discharge platform at 0.7 V and has good stability over the next 2 cycles. Fig. 6b shows the CV of bare α-Fe2O3 with two reduction peaks (0.39 V/1.22 V, a bulky peak and a tiny broad peak) for in the 1st discharge process. In the first cathodic scan, the small peak located at 1.22 V is caused by the initial lithium
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insertion into α-Fe2O3 (α-Fe2O3+xLi++xe-→LixFe2O3, LixFe2O3+(2-x)Li++(2-x)e-→ Li2Fe2O3), and the large peak observed at 0.39 V is ascribed to the reduction of Fe2O3 to Fe and the formation of Li2O when the Fe2O3 phase reacts with Li+
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(Li2Fe2O3+4Li++4e-→2Fe+3Li2O) and formation of solid electrolyte interphase (SEI) film.[38-40] During the 1st charge cycle, the peak at 1.75 V is related to the oxidation
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of Fe0 to Fe2+ and then Fe3+, finally reforming Fe2O3. In the subsequent 2 cycles, the peak intensity at 0.39 V decreases with the discharge cycles, indicating that an irreversible phase transformation occurred during Li+ insertion and extraction in the 1st cycle. In the charge-discharge process of subsequent 2 cycles, the broad cathodic peak and the oxidation peak shift to high potential indicates a good reversibility of the
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Fe3+ to Fe0 and Fe0 to Fe3+ reaction, respectively. The 1st broad discharge peaks at ∼ 0.60 V shown in Fig. 6c-f, which is similar to the bare α-Fe2O3 indicating the irreversible
phase
transformation
occurred
during
the
1st
cycle
of
the
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α-Fe2O3/MWCNTs hybrids electrode. All α-Fe2O3/MWCNTs hybrids electrodes with a broad cathodic peak and oxidation peak in the charge-discharge process of 2nd and 3rd cycles correspond to the Fe3+ to Fe0 and Fe0 to Fe3+ reaction. It is worth noting
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that two almost overlap cathodic peaks located at near 0.84 V and 0.88 V for α-Fe2O3/50 wt.% MWCNTs emerge in the following two cycles, respectively. Compare with bare α-Fe2O3, the cathodic peak of α-Fe2O3/50 wt.% MWCNTs electrode shift to high potential while the oxidation peak potential almost unchanged, indicating a good reversibility of the Fe0 to Fe3+ reaction and the inhibiting of formation for amorphous Li2O particles which are deemed as the main reason for the irreversible capacity during the discharge process. It indicates that the α-Fe2O3/50 wt.% MWCNTs electrode is sufficient to form Li+ insertion reduction than that of bare α-Fe2O3 and the electrochemical cycling process is more stable.
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Fig. 6. CVs of anodes membrane at a scan rate of 0.1 mV s-1 for three cycles: (a) pure
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MWCNTs, (b) bare α-Fe2O3, (c) α-Fe2O3/10 wt.% MWCNTs, (d) α-Fe2O3/30 wt.% MWCNTs, (e) α-Fe2O3/50 wt.% MWCNTs and (f) α-Fe2O3/70 wt.% MWCNTs. Representative charge-discharge profiles of the electrode based on pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids electrodes with 50 cycles at current rate of 200 mA g-1 in the potential range of 0.01 V - 3.0 V are shown in Fig. 7. The
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charge-discharge characteristics of pure MWCNT electrode for the 1st, 2nd and 50th cycles are given in Fig. 7a. The 1st discharge-charge capacities are 409.2 mAh g-1 and 388.4 mAh g-1 with the coulombic efficiency of 94.8 %. The rapid decay of the
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discharge capacity of the 2nd cycle to 225.3 mAh g-1 is caused by the formation of an irreversible SEI film. The 3rd cycle discharge capacity also slightly attenuates
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compared to the 2nd cycle. Fig. 7b shows charge-discharge characteristics of the bare α-Fe2O3 electrode at the 1st, 2nd and 50th cycles. Its 1st discharge and charge capacities are 1004.5 mAh g-1 and 580.8 mAh g-1 with the coulombic efficiency of 57.8 %. This low coulombic efficiency is due to the irreversible lithium ions embedded in electrode material and SEI film formation. The discharge capacity for 3rd cycle also hugely attenuates to 164.2 mAh g-1 caused by volume expansion cracking of the active material then it′s peeling off from the current collector, and while the irreversible lithium ions remain inside the pulverizing α-Fe2O3. Fig. 7c-f show that α-Fe2O3 with different amount of MWCNTs hybrids electrodes have higher
ACCEPTED MANUSCRIPT specific capacity retention compared to pure MWCNTs and bare α-Fe2O3 after 50 cycles. It is observed that the 1st discharge and charge capacities are 1139.8 mAh g-1 and 976.5 mAh g-1 for α-Fe2O3/50 wt.% MWCNTs anodes, indicating an initial coulombic efficiency of 85.7 %. The α-Fe2O3/50 wt.% MWCNTs anodes 1st cycle
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reversible capacity is 976.5 mAh g-1 that is much higher compared to pure MWNTs (388.4 mAh g-1). The discharge capacity is 845.4 mAh g-1 with the initial coulombic efficiency of 96.6 % which is much higher than that of bare α-Fe2O3 anodes with a capacity of 164.5 mAh g-1 after 50 cycles. The irreversible capacity of α-Fe2O3/
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MWCNTs hybrids anodes can be assigned to the formation of a SEI film on the
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surface of active materials and the irreversible insertion of lithium ions into Fe2O3.
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Fig. 7. Galvanostatic charge-discharge profiles of the membrane electrode: (a) pure MWCNTs, (b) bare α-Fe2O3, (c) α-Fe2O3/10 wt.%MWCNTs, (d) α-Fe2O3/30
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wt.%MWCNTs, (e) α-Fe2O3/50 wt.%MWCNTs and (f) α-Fe2O3/70 wt.%MWCNTs. Furthermore, the cycling performance of the pure MWCNTs, bare α-Fe2O3
nanoparticles and α-Fe2O3/MWCNTs hybrid electrodes are shown in Fig. 8a. It is observed that after 50 cycles the α-Fe2O3/MWCNTs hybrid electrode with 10 %, 20 %, 50 % and 70 % MWCNTs mass ratios still exhibited superior cycling performance with the discharge capacities of 426.2 mAh g-1, 580.6 mAh g-1, 778.2 mAh g-1 and 338.1 mAh g-1 at the current rate of 200 mA g-1. In contrast, the bare α-Fe2O3 nanoparticles electrode showed rapid fading of discharge capacity, display a capacity of less than 114.6 mAh g-1 after 50 cycles. It can be observed that the pure
ACCEPTED MANUSCRIPT MWCNTs shows higher capacity retention compared with bare α-Fe2O3 electrode. The pure MWCNTs retained a capacity value of 227.3 mAh g-1 at the current density of 200 mA g-1 even after 50 cycles. Rate performances of α-Fe2O3/MWCNTs hybrid and counterpart have also been investigated at various current densities (200, 500,
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1000, 1500, 2000 and 2500 mA g-1), and the results are demonstrated in Fig. 8b. It is worth noting that, when current density varies from 200 mA g-1 to 2500 mA g-1, the reversible capacity of the hybrid electrode does not rise with the increase of the weight ratio of MWCNTs within the samples during the 50 cycles. The capacities of
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α-Fe2O3 with 50 wt.% MWCNTs electrode retains at 817.3 mAh g-1 after the current density increases from 200 mA g-1 to 2500 mA g-1 and then back to 200 mA g-1,
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whose value is higher than those of the counterparts and pure MWCNTs.
Fig. 8. (a) Cycling performance of α-Fe2O3/MWCNTs hybrids anodes at a current
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density of 200 mA g-1, (b) Rate cyclability at various current densities (200/500/1000/2000/2500 mA g-1) at the voltage range of 0.01 V - 3.0 V.
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Fig. 9 shows the Nyquist plots of AC impedance (Frequency range from 100 kHz
to 0.1 Hz) for the bare α-Fe2O3, pure MWCNTs and α-Fe2O3 with MWCNTs hybrids electrodes, which were measured at an open circuit voltage (~3.0 V) state using fresh cells at room temperature. Six profiles show a semicircle at the high-to-medium frequencies and a sloping line at low frequency which is related to the charge transfer at the electrode/electrolyte interface and lithium ions diffusion process in electrodes, respectively. The fitting equivalent circuit for the cell system is depicted in the inset of the Figure. The Nyquist plots for the six samples were similar in shape except for the diameters of semicircles. From the Nyquist plots, it can be found that bulk resistance
ACCEPTED MANUSCRIPT (Rs) and charge transfer resistance (Rct) of the pure MWCNTs, α-Fe2O3/70 wt.%MWCNTs, α-Fe2O3/50 wt.%MWCNTs, α-Fe2O3/30 wt.%MWCNTs, α-Fe2O3/10 wt.%MWCNTs and bare α-Fe2O3 electrode are Rs = 5.5, 6.7, 5.3, 4.1, 3.3, 8.0 Ω and Rct = 195.2, 224.3, 244.3, 421.8, 614.0, 1009.0 Ω, respectively. Compared with EIS
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spectrum of bare α-Fe2O3 nanoparticle sample, the pure MWCNTs, and corresponding hybrids display relatively smaller Rct, indicating a lower impedance value with better conductivity. While for the electrode of pure MWCNTs, the electrons are first transferred from carbon tube bulk to surface with a shorter path. It
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can be deduced that the transfer of Li+ in hybrids electrode become faster and easier
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when the mass ratio of MWCNTs increased from 10 wt.% to around 50 wt.%.
Fig. 9. Typical electrochemical impedance spectra measured with bare α-Fe2O3, pure
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MWCNTs and α-Fe2O3/MWCNTs hybrid electrodes (inset is the equivalent circuit used to fit the EIS).
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Furthermore, an ultra-light, excellent conductive and flexible carbon nanofiber
paper (CNP), which only has one-sixth of the mass of copper-based current collector, was adopted as a novel current collector. The total specific capacity of the new flexible α-Fe2O3/50 wt.%MWCNT/CNP electrode (467.2 mAh g-1) is four times large than that of the α-Fe2O3/50 wt.%MWCNT electrode with a conventional copper collector (106.8 mAh g-1) after 50 cycles at a current density of 200 mA g-1 (Table S1). Fig. 10 shows the XRD patterns of CNP and the novel α-Fe2O3/50 wt.%MWCNT/CNP hybrid electrode. The characteristic diffraction peaks result of α-Fe2O3/50 wt.%MWCNT/CNP were in excellent accordance with a hexagonal
ACCEPTED MANUSCRIPT hematite structure, which can be indexed to the (012), (104), (110), (113), (024) and (116) plane from bare α-Fe2O3 crystals. The α-Fe2O3/50 wt.%MWCNT/CNP hybrids exhibited two peaks at 2θ = 25.6° and 44.2° which can be assigned to the (002) and
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(100) characteristic plane of carbon. This is the same with results of CNP.
Fig. 10. XRD patterns of pure CNP and α-Fe2O3/50 wt.%MWCNTs hybrids fabricated on CNP.
As shown in Fig. 11a and b, the morphology of the 3D porous CNP and
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α-Fe2O3/50 wt.%MWCNTs/CNP hybrids were examined. Fig. 11a shows SEM images of the as-obtained 3D porous structure CNP which consists of nanofibers with a diameter of ∼300 nm, length of several µm and thickness of ∼130 µm. An
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interconnected network with uniformly shaped fibers ranging from 200 nm to 300 nm provides a conductive 3D electronic transportation pathway and facilitates the access
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of the electrolyte. The picture of the real flexible CNP products network is given in Fig. S5. SEM image (Fig. 11b) shows a rough surface caused by the loading of α-Fe2O3/50 wt.%MWCNTs hybrids filtered on CNP. The α-Fe2O3 nanoparticles are uniformly distributed inside the 3D porous network structure.
Fig. 11. SEM image of pure CNP and α-Fe2O3/50 wt.% MWCNTs hybrids fabricated CNP.
ACCEPTED MANUSCRIPT Representative charge-discharge profiles of the electrode based on pure CNP and α-Fe2O3/50 wt.%MWCNTs/CNP at the current rate of 200 mA g-1 at the potential range of 0.01 V - 3.0 V are shown in Fig. 12. For pure CNP, its initial discharge capacity is 100.0 mAh g-1 and maintains at 29.7 mAh g-1 after 50 cycles (Fig. 12a).
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The reversible discharge-charge capacity of the 1st cycle for α-Fe2O3/50 wt.%MWCNTs/CNP anodes is 1322.2 mAh g-1 and 641.5 mAh g-1, respectively (Fig. 12b). The discharge capacity still remains at 574.8 mAh g-1 that is higher compared with graphene after 50 cycles. The discharge capacity still remains at 574.8 mAh g-1
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after 50 cycles, which is obviously higher than those of bare CNP and commercial
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graphite.
Fig. 12. Galvanostatic charge-discharge profiles of electrode: (a) pure CNP, (b) the α-Fe2O3/50 wt.%MWCNTs/CNP membrane.
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The cycling performance of the flexible α-Fe2O3/50 wt.%MWCNTs/CNP hybrid, pure CNP electrode are shown in Fig. 13. In contrast, the flexible bare CNP electrode and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid showed discharge capacity
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of 29.7 mAh g-1 and 574.8 mAh g-1 at a current density of 200 mA g-1 after 50 cycles (Fig. 13a), respectively. It is noteworthy noting that the capacity of free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode is higher than that of commercial graphite.[41] The rate capability of the porous CNP anodes and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode in LIBs was also evaluated by cycling at different current densities from 200 mA g-1 to 2500 mA g-1 (Fig. 13b). The free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode delivers a higher capacity of 201.2 mAh g-1 at a current rate of 2500 mA g-1. When its current density
ACCEPTED MANUSCRIPT converts from 2500 mA g-1 back to 200 mA g-1, the discharge capacity rises to 467.2
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mAh g-1.
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Fig. 13. (a) Cycling performance of CNP and α-Fe2O3/50 wt.%MWCNTs/CNP anodes at a current density of 200 mA g-1, (b) Rate cyclability at various current
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densities (200/500/1000/2000/2500 mA g-1) at the voltage range of 0.01 V - 3.0 V. The Nyquist plots of the fresh cell which fabricated by pure CNP, α-Fe2O3/50 wt.%MWCNTs and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP were shown in Fig. 14 with a frequency range from 100 kHz to 0.1 Hz at open circuit potential(~3.0 V). From the Nyquist plots, it can be found that bulk resistance Rs are 1.8, 1.3, 1.3 Ω,
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and charge transfer resistance Rct are 84.8, 140.6, 198.8 Ω corresponding to pure CNP, α-Fe2O3/50 wt.%MWCNTs and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP electrodes,
respectively.
The
conductivity
of
free-standing
α-Fe2O3/50
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wt.%MWCNTs/CNP electrode is superior to α-Fe2O3/50 wt.%MWCNTs with the
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copper current collector.
ACCEPTED MANUSCRIPT Fig. 14. Typical electrochemical impedance spectra measured with pure CNP, α-Fe2O3/50 wt.% MWCNTs and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP electrode. 4.Conclusion Hematite α-Fe2O3 nanoparticles interconnected by the carbon nanotubes
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(MWCNTs) have been successfully prepared as LIBs anodes. The α-Fe2O3 hybrid electrode with 50 wt.%MWCNTs retains relatively higher capacity of 778.2 mAh g-1 after 50 cycles under the current density of 200 mA g-1. Moreover, the carbon
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nanofiber paper (CNP) was used as a novelty current collector to replace the traditional copper foil for application in LIBs. The discharge capacity of free-standing
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α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode still remains at 467.2 mAh g-1 after 50 cycles when the current density varies gradually from 200 mA g-1 to 2500 mA g-1, and then back to 200 mA g-1. The inherently conducting network of MWCNTs with shorten electron pathway and faster reaction kinetics in α-Fe2O3 improve the cycling performance, resulting in higher reversible capacities and rate capability.
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Acknowledgment
We gratefully acknowledge the financial support from the Natural Science Foundation of China (grant No. 61664009, 51771169) and the Youth Project of
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Applied basic research of Yunnan Science and Technology Department (grant No. 2015FD001). This work is also funded in part by the High-end Scientific and Technological Talents Introduction Project of Yunnan Province (grant No.
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2013HA019). References
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ACCEPTED MANUSCRIPT Highlights: α-Fe2O3/MWNT nanocomposite electrodes for flexible LIBs.
2.
α-Fe2O3 embedded in the outer carbon nanotube.
3.
Hybrids electrode exhibits superior electrochemical properties.
4.
Free-standing carbon nanofiber paper applied to light-weighted LIBs.
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1.
S0925-8388(18)34066-0
DOI:
https://doi.org/10.1016/j.jallcom.2018.10.367
Reference:
JALCOM 48185
To appear in:
Journal of Alloys and Compounds
Received Date: 1 August 2018 Revised Date:
10 October 2018
Accepted Date: 27 October 2018
Please cite this article as: X. Chen, Z. Zhao, Y. Zhou, Y. Shu, M. Sajjad, Q. Bi, Y. Ren, X. Wang, X. Zhou, Z. Liu, MWCNTs modified α-Fe2O3 nanoparticles as anode active materials and carbon nanofiber paper as a flexible current collector for lithium-ion batteries application, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.367. 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.
ACCEPTED MANUSCRIPT MWCNTs modified α-Fe2O3 nanoparticles as anode active materials and carbon nanofiber paper as a flexible current collector for lithium-ion batteries application
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Xu Chena, Zhen Zhaoa, Yan Zhoua, Yun Shua, Muhammad Sajjada, Qinsong Bia, Yang Rena, Xu Wangc, Xiaowei Zhoua,* and Zhu Liua,b,* a
Department of Physics and Astronomy, Yunnan University, Kunming, Yunnan
Province, China 650091
Micro and Nano-materials and Technology Key Laboratory of Yunnan Province,
Kunming City, Yunnan Province, China 650091
Materials Science and Engineering Program, University of Houston, Houston, TX
77204, USA * *
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c
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b
Corresponding Author E-mail: [email protected]
Corresponding Author E-mail: [email protected]
Abstract
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α-Fe2O3 nanoparticles/multi-wall carbon nanotubes (MWCNTs) hybrids were successfully prepared via a facile one-step hydrothermal method. The MWCNTs enhances the conductivity and provides space for stress and strain during volume
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expansion of α-Fe2O3 semiconductor as the anode for LIBs. The composite shows impressive electrochemical performance when it was used as the electrodes of coin
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cells. The α-Fe2O3 electrode with 50 wt.% MWCNTs retains its capacity of 816.8 mAh g-1 after 50 cycles at the current density of 200 mA g-1, which is superior to those of contrast samples (bare α-Fe2O3, pure MWCNTs, 10 wt.%, 30 wt.%, 70 wt.% MWCNT/α-Fe2O3 hybrids). Furthermore, the flexible carbon nanofiber paper (CNP) has also been investigated as a novel current collector with lithium storage for application
in
LIBs,
and
the
capacity
of
free-standing
α-Fe2O3/50
wt.%MWCNTs/CNP hybrids electrode can retain 467.2 mAh g-1 after 50 cycles under the current density of changes from 2500 mA g-1 to 200 mA g-1. The MWCNTs improved cycle performance, enhanced reversible capacities and rate capability of
ACCEPTED MANUSCRIPT MWCNTs/α-Fe2O3 anodes can attribute to the inherent conducting network, shorten electron pathway and faster reaction kinetics. Keywords: Multi-wall carbon nanotubes, Carbon nanofiber paper, Kinetics, α-Fe2O3 1.Introduction
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Li-ion batteries (LIBs) are considered to be energy storage sources with high-energy density, safe and high cycle life in various high technology applications.[1-2] Nanostructured metal oxide in carbon modification is a promising anode material for safer LIB designs.[3-6] Hematite (α-Fe2O3), the most stable iron
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oxide anode material for LIBs, has attracted great attention for its eco-friendliness, abundance, low cost, as well as the high theoretical capacity of 1005 mAh g-1.[7-11]
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However, the low electronic conductivity, easy agglomeration and poor capacity retention have hindered its practical implementation in LIBs.[8, 12-15] Therefore, improving the conductivity and alleviating the collapse of α-Fe2O3 during the intercalation/deintercalation of Li+ are the research hotspots when it was used as an anode material for LIBs.[16-21] Thus, great efforts have been devoted to the
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exploitation of α-Fe2O3 materials with higher conductivity and good capacity retention. The effective methods are using nanosize structures or high conductivity carbon-based hybrids which can improve the electron/ion transfer rates and mitigate
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the structural strain during Li+ intercalation/deintercalation.[22-27] Up to now, improving conductivity and avoiding structure collapse of Fe2O3 nanostructure anode materials mainly include introducing graphite, carbon nanotubes
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or graphene into the matrix of the Fe2O3 nanostructure or performing carbon coating.[28-30] We reported on the previous work that Fe3O4 nanoparticles were electrochemically grown on free-standing buckypaper for light-weighted and flexible lithium-ion battery.[31] Cuo et al. reported CNT/graphene nanosheet hybrid anode for LIBs using a chemical vapor deposition method which exhibited a high specific capacity of 984 mAh g-1.[32] Liu′s group prepared Fe2O3-filled CNTs anode by the AAO template method with a reversible capacity of 768 mAh g-1 after 40 cycles.[33] Carbon coated α-Fe2O3 hollow nanohorns on carbon nanotube (CNT) anode with very stable capacity retention of 800 mAh g-1 over 100 cycles at a high current density of
ACCEPTED MANUSCRIPT 500 mA g-1 have been constructed by Lou′s team. Yu et al. successfully fabricated graphene sheets/α-Fe2O3 sandwiched nanostructure hybrids for anode with a high capacity of 658.5 mAh g-1 at 1000 mA g-1 up to 200 cycles.[34] Chen et al. designed a novel hybrid nanostructure by coating Fe2O3 nanoparticles which used as the anode in
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a Li-ion battery, this hybrid material (70.32 wt.% carbon nanotubes, 29.68 wt.% Fe2O3) showed a reversible discharge capacity of 515 mAh g-1 after 50 cycles at a density of 100 mA/g.[35] Shen′s team prepared Fe2O3/COOH-MWCNT composites and these composites were used as electrode material in lithium-ion batteries, a
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reversible capacity of 711.2 mAh·g-1 at a current density of 500 mA·g-1 after 400 cycles was obtained.[36] TiO2 films coated α-Fe2O3 nanoparticles anchored on
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MWCNTs exhibiting a steady discharge capacity of 670 mAh g-1 even cycled at a large current density of 1000 mA g-1.[37] However, there is a limited study on the impact of the proportion of carbon nanotubes in α-Fe2O3 nanocomposite electrodes on the electrochemical performance and application of suitable composite materials in anode materials for flexible LIBs. Furthermore, most active materials embedded in
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the inner nanotube cannot be effectively accessed by the electrolyte, resulting in ineffective Li+ storage capacity. Therefore, more efforts should be devoted to the design of α-Fe2O3 on the surface of high conductivity carbons composite which has
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excellent structural stability, easy accessibility for the electrolyte and fast electron/ion transport pathways as anodes for LIBs. Herein, α-Fe2O3 was anchored on the surface of MWCNTs outer wall with
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different weight ratios of MWCNTs to α-Fe2O3 by a simple hydrothermal method. The cycle ability and stability of the as-prepared hybrids have been studied by applying it as the anodes of LIBs. It is noteworthy that the α-Fe2O3/50 wt.%MWCNTs anodes can maintain their reversible capacity of 816.8 mAh g-1 with the initial coulombic efficiency of 96.6 % after 50 cycles. In addition, α-Fe2O3/MWCNTs/CNP electrodes with carbon nanofiber paper (CNP) as the current collector to synthesize by suction filtration method instead of the metallic foil in traditional LIBs. Therefore, a light-weight, flexible, stable, and high-capacity α-Fe2O3/MWCNTs hybrids anode is realized. The novel free-standing α-Fe2O3/50
ACCEPTED MANUSCRIPT wt.%MWCNTs/CNP hybrid showed a discharge capacity of 574.8 mAh g-1 at a current density of 200 mA g-1 after 50 cycles. Our work provides a simple way for the fabrication of 3D porous, light-weight, flexible and high conductivity anode materials for LIBs.
2.1 Preparation of α-Fe2O3/MWCNT nanohybrids
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2.Experimental and methods
All chemicals were reagent grade and used as received from the supplier. The α-Fe2O3 nanoparticles and fabrication of α-Fe2O3/MWCNT nanohybrids were
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prepared by hydrothermal method. In a typical procedure, 1) Fe(NO3)3⋅9H2O (Corresponding to the iron source quality of samples was bare α-Fe2O3 4.8 g, 10 wt.%
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4.8 g, 30 wt.% 4.8 g, 50 wt.% 2.4 g, 70 wt.% 1.6 g, respectively) and 15 mL (0.025 g) Cetyltrimethyl Ammonium Bromide (CTAB, Tianjin science and technology co., LTD) was dissolved in 15 mL deionized water and stirred for 15 min to obtained homogeneous solution; 2) Carbon nanotubes (MWCNTs, Shenzhen port of nano-science and technology co ., LTD) (Corresponding to the MWCNTs, quality of
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samples is bare MWCNTs 1.0 g, 10 wt.% 0.1 g, 30 wt.% 0.3 g, 50 wt.% 0.5 g, 70 wt.% 0.7 g, respectively) and 1.3 g NaOH ( Sigma Aldrich Shanghai trading co., LTD), were dissolved in 15 mL deionized water and stirred for 15 min which results in a
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homogeneous solution; 3) Mixed the solution of foregoing two steps and stirred for 15 min to obtain a homogeneous solution with outgrowth precipitate Fe(OH)3; 4) Then
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sealed the solution in a 50 mL stainless steel autoclave and put it in the 160 °C environment for 12 h; 5) Then the solid precipitate particles were filtered from the solution, rinsed by DI water and alcohol for 15 min and dried at 60 °C for 24 h. We selected appropriate reagents to prepare a series of α-Fe2O3/MWCNTs hybrids with a different mass ratio of α-Fe2O3 to MWCNTs. Carbon nanofiber paper (CNP, 0.27 mm thickness, Kunming Natai Energy Co., Ltd, China) was used as the novel free-standing flexible electrode to replace traditional copper current collector. 2.2 Material Characterization
ACCEPTED MANUSCRIPT The structure of the as-synthesized α-Fe2O3, α-Fe2O3/MWCNTs composite and modified CNP samples were characterized by an X-ray diffraction (XRD, Cu Kα radiation 1.5406 Å Rigaku TTRШ) and transmission electron microscopy (TEM, JEM-2100 Japan). The morphologies of the samples were investigated by scanning
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electron microscopy (SEM, FEI QUANTA200). The Raman spectra were obtained by an inVia (Renishaw USA) at room temperature. Chemical bonding analysis of α-Fe2O3 and α-Fe2O3/MWCNTs composite were carried out by X-ray photoelectron spectroscopy (XPS, Thermo scientific K-ALPHA+). The Fourier transform-infrared
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(FT-IR, NicoletS10) spectra of the α-Fe2O3/MWCNTs hybrids were acquired by a Thermo Nicolet spectrometer using the KBr pellet technique (Avatar 360, USA).
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Particle size distribution of different α-Fe2O3/MWCNTs hybrids were demonstrated by light scattering analysis using a Mastersizer 3000E analyzer (Malvern Instruments, Malvern-UK) using the Fraunhofer approximation model. UV-vis measurements were performed on a UV-Vis spectrophotometer (AVATAR, FT-IT, 360). The reaction kinetics of α-Fe2O3/50 wt.%MWCNTs were studied by thermogravimetric analysis
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(TGA, Instrument SDT 2960 USA).
2.3 Electrochemical characterization
The working anodes were prepared by mixing the active material (α-Fe2O3,
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α-Fe2O3/MWCNTs or α-Fe2O3/MWCNTs/CNP), a conductive agent (carbon black), and binder (PVDF) in a weight ratio of 8:1:1. The typical loading density of active materials was ∼ 2.0 mg cm-2. The specific discharge capacities values were calculated
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based on grams of α-Fe2O3/MWCNTs or grams of α-Fe2O3/MWCNTs/CNP hybrid. Coin-type cells (CR2025) were assembled using the lithium metal as the counter electrode, a polypropylene (PP) microporous film as the separator, and LiPF6 (1 M) in ethylene carbonate (EC)-dimethyl carbonate (DMC) and diethyl carbonate (DEC) (volume ratio of EC:DMC:DEC=1:1:1) as the electrolyte in an argon-filled glove box (MIKROUNA super, O2 ≤ 0.1 ppm, H2O ≤ 0.1 ppm). The galvanostatic measurements were carried out on a LAND-CT2001A battery tester with a voltage window between 0.01 V and 3.0 V at various current rates. The cyclic voltammetry (CV) was performed between 0.01 V and 3.0 V using an electrochemical workstation
ACCEPTED MANUSCRIPT (CHI604E) at a scan rate of 0.1 mV s-1. The electrochemical impedance spectra (EIS) of the cells were measured on the electrochemical workstation (CHI604E) with the frequency range from 0.1 Hz to 100 kHz. Nyquist plots derived from EIS were simulated using Z-view software. 3.Results and discussion
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SEM images of α-Fe2O3/MWCNTs hybrids with different mass ratios are shown in Fig. 1. The average diameter and length of as-prepared MWCNTs is ∼ 60 nm and ∼ 15 µm, respectively, as shown in Fig. 1a. The spherical bare α-Fe2O3 nanocrystals
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with an approximate diameter of 150 nm are shown in Fig. 1b. As shown in Fig. 1c-f the α-Fe2O3 particles are stably stuck in the compact network structure formed by the
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stacked different mass ratio of MWCNTs (10 wt.%, 30 wt.%, 50 wt.%, 70 wt.%). As the MWCNTs amount increased, the particle size of α-Fe2O3 gradually becomes larger, which is consistent with the results of the granularity analysis as shown in Fig. S1. The network spaces become larger thickening with the increase of MWCNTs mass ratio, which may control dimensional growth and determine the diameter of
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α-Fe2O3 particles. In addition, with the increase of MWCNTs weight ratio in the composite, the absorption spectrum of α-Fe2O3/MWCNTs is gradually enhanced (Fig. S2). The band gap of α-Fe2O3/MWCNTs hybrids are around ~ 2.0 eV, as presented in
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Fig. S3, and the hybrids can absorb a wide range of the solar spectrum.
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Fig. 1. SEM images of α-Fe2O3/MWCNTs hybrids with different mass ratios of
MWCNTs (a) pure MWCNTs, (b) bare α-Fe2O3,(c) 10 wt.%, (d) 30 wt.%, (e) 50 wt.%, (f) 70 wt.%. The powder of MWCNTs, α-Fe2O3, α-Fe2O3/MWCNTs hybrids were further characterized by X-ray diffraction patterns (Fig. 2). The reflections result of bare α-Fe2O3 were in excellent accordance with a hexagonal hematite structure (JCPDS No. 99-0060) which belongs to the space group R-3c (Number 167). From
ACCEPTED MANUSCRIPT α-Fe2O3/MWCNTs hybrids, several strong diffraction peaks can be observed corresponding to the (012), (104), (110) and (113) peaks, which precisely match with those of pure α-Fe2O3 crystals. The synthesized α-Fe2O3/MWCNT hybrids display a broad peak at 2θ = 25.6° which can be assigned to the characteristic (002) plane of MWCNTs. As the MWCNTs mass ratio increases, the carbon peak appeared which
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suggests α-Fe2O3 crystals formed on MWCNTs, which is consistent with the results in Fig. 1c-f. Furthermore, thermogravimetric (TGA) analysis in Fig. S4 shows that the percentage content of α-Fe2O3/50 wt.%MWCNT is in accordance with the theoretical
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calculation (ratio of mass 1:1).
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Fig. 2. XRD patterns of MWCNTs, pure α-Fe2O3 and α-Fe2O3/MWCNTs hybrids. The structure and morphology of α-Fe2O3/MWCNTs hybrids were investigated by TEM. As demonstrated in Fig. 3a, the α-Fe2O3 is nanosphere morphology with
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diameter range from 20 nm to 200 nm, which the most widely distributed diameter around 150 nm. The α-Fe2O3 particles have monocrystal properties (Fig. 3a),
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displaying clear lattice fringes corresponding to the (012) plane of α-Fe2O3 with an interplanar crystal spacing of 3.68 Å, as presented in Fig. 3b. Fig. 3c-d manifests the morphology network architectures of the as-prepared α-Fe2O3/50 wt.% MWCNTs hybrids. It can be seen that the α-Fe2O3 are loaded on the surface of one-dimensional MWCNTs. Obviously, different from nanosphere bare α-Fe2O3, the morphology of α-Fe2O3 nanoparticles in the hybrids network has a hexagonal structure with the size range from 50 nm to 200 nm. A similar phenomenon for the α-Fe2O3/70 wt.%MWCNTs appears in Fig. 3e-f. The α-Fe2O3 is basically distributed outward along the surface of MWCNTs, this is because that the surface of oxidized MWCNTs
ACCEPTED MANUSCRIPT has numerous oxygen-containing groups, which may have a dispersive effect for the nuclei of Fe3+ precipitates in the hydrothermal reaction. The formation mechanism of α-Fe2O3 on the outer wall of MWCNTs prepared by hydrothermal has been analyzed in Fig. S5. The surface of the MWCNTs applied contains functional groups such as
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-COOH and -OH. During the reaction, the Fe3+ in precursor solution is captured by the oxygen-containing functional groups on the surface of the MWCNTs and α-Fe2O3 formed on the outer wall of MWCNT at high temperature and pressure. The α-Fe2O3 nanoparticles grown on the outer wall surface of MWCNT, as shown in the TEM
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embedded in the MWCNTs matrix (Fig. S7).
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image of Fig. S6. The α-Fe2O3 nanoparticles with a smooth surface are well
Fig. 3. TEM images of α-Fe2O3 hybrids with different mass ratio of MWCNTs (wt.%): (a-b) pure α-Fe2O3, (c-d) 50 wt.%, (e-f) 70 wt.%.
ACCEPTED MANUSCRIPT Raman spectra of the pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids were shown in Fig. 4. As we know, hematite is an antiferromagnetic material 6 which belongs to the ܦ3d crystal space group. There were seven phonon lines in the
Raman spectrum. The five phonon lines were named two A1g modes (217 cm-1 and
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404 cm-1) and three Eg modes (283 cm-1, 494 cm-1 and 603 cm-1), respectively.[26] Two obvious shark peaks located at 1346 cm-1 and 1580 cm-1 correspond to typical D-band and G-band which can be observed in pure MWCNTs and all α-Fe2O3/MWCNTs hybrids samples. The A1g and Eg peaks arise from α-Fe2O3 can be
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observed in Raman spectra at the range from 150 cm-1 to 600 cm-1. The peaks located at 217 cm-1 and 404 cm-1 were assigned to the A1g vibration modes and peaks at 283
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cm-1, 495 cm-1 and 603 cm-1 were caused by Eg. The intense feature named 2LO at 1311 cm-1 is assigned to a two-magnon scattering which arises from the interaction of two magnons created on antiparallel close spin sites. Infrared spectroscopy shows that oxygen-containing functional groups (-COOH, -OH) exist in all α-Fe2O3/MWCNTs
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hybrids (Fig. S8).
Fig. 4. Raman spectra of bare MWCNTs and α-Fe2O3 with 10/30/50/70 wt.% MWCNTs.
To further characterize the chemical components and valence states of the MWCNTs and α-Fe2O3 with 10/30/50/70 wt.% MWCNTs, XPS analyses are performed as depicted in Fig. 5. The XPS survey spectra of pure MWCNTs, bare α-Fe2O3 and MWCNT/α-Fe2O3 hybrids are presented in Fig. 5a. The survey spectra are calibrated with the C1s line of adventitious carbon at 284.4 eV. Fig. 5b shows the
ACCEPTED MANUSCRIPT zoom in XPS spectra of the C1s of α-Fe2O3 with 50 wt.% MWCNTs at 284.4 eV which arise from carbon. Fig. 5c shows that the O1s spectrum of the α-Fe2O3 nanoparticles corresponding to Fe−O−Fe bonds (532.3 eV). The measured binding energies of Fe2p3/2 and Fe2p1/2 are given in Fig. 5d at 709.4 eV and 722.8 eV,
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respectively. The peak top distance is 13.4 eV, which corresponds to the standard spectral phase of Fe2p in α-Fe2O3. It can demonstrate the presence of Fe3+. Similar analysis indicates the oxygen content of the oxide in Fig. 5c. These demonstrate that the oxide within all MWCNTs/α-Fe2O3 hybrids can be attributed to ferric oxides,
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which coincides with XRD results.
Fig. 5. XPS spectrum of as-prepared simples:(a) bare MWCNTs, α-Fe2O3 and α-Fe2O3 with 10/30/50/70 wt.% MWCNTs, (b) C−C in α-Fe2O3 with 50 wt.%
MWCNTs, (c) O1s in α-Fe2O3 with 50 wt.% MWCNTs,(d) Fe 2p in α-Fe2O3 with 50 wt.% MWCNTs. The electrochemical performances of pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids were investigated by a cyclic charge-discharge. CVs of the pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids electrodes at a scan
ACCEPTED MANUSCRIPT rate of 0.1 mV s-1 for 3 cycles are shown in Fig. 6. Fig. 6a is a CV of pure MWCNTs electrode with the first discharge platform at 0.7 V and has good stability over the next 2 cycles. Fig. 6b shows the CV of bare α-Fe2O3 with two reduction peaks (0.39 V/1.22 V, a bulky peak and a tiny broad peak) for in the 1st discharge process. In the first cathodic scan, the small peak located at 1.22 V is caused by the initial lithium
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insertion into α-Fe2O3 (α-Fe2O3+xLi++xe-→LixFe2O3, LixFe2O3+(2-x)Li++(2-x)e-→ Li2Fe2O3), and the large peak observed at 0.39 V is ascribed to the reduction of Fe2O3 to Fe and the formation of Li2O when the Fe2O3 phase reacts with Li+
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(Li2Fe2O3+4Li++4e-→2Fe+3Li2O) and formation of solid electrolyte interphase (SEI) film.[38-40] During the 1st charge cycle, the peak at 1.75 V is related to the oxidation
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of Fe0 to Fe2+ and then Fe3+, finally reforming Fe2O3. In the subsequent 2 cycles, the peak intensity at 0.39 V decreases with the discharge cycles, indicating that an irreversible phase transformation occurred during Li+ insertion and extraction in the 1st cycle. In the charge-discharge process of subsequent 2 cycles, the broad cathodic peak and the oxidation peak shift to high potential indicates a good reversibility of the
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Fe3+ to Fe0 and Fe0 to Fe3+ reaction, respectively. The 1st broad discharge peaks at ∼ 0.60 V shown in Fig. 6c-f, which is similar to the bare α-Fe2O3 indicating the irreversible
phase
transformation
occurred
during
the
1st
cycle
of
the
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α-Fe2O3/MWCNTs hybrids electrode. All α-Fe2O3/MWCNTs hybrids electrodes with a broad cathodic peak and oxidation peak in the charge-discharge process of 2nd and 3rd cycles correspond to the Fe3+ to Fe0 and Fe0 to Fe3+ reaction. It is worth noting
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that two almost overlap cathodic peaks located at near 0.84 V and 0.88 V for α-Fe2O3/50 wt.% MWCNTs emerge in the following two cycles, respectively. Compare with bare α-Fe2O3, the cathodic peak of α-Fe2O3/50 wt.% MWCNTs electrode shift to high potential while the oxidation peak potential almost unchanged, indicating a good reversibility of the Fe0 to Fe3+ reaction and the inhibiting of formation for amorphous Li2O particles which are deemed as the main reason for the irreversible capacity during the discharge process. It indicates that the α-Fe2O3/50 wt.% MWCNTs electrode is sufficient to form Li+ insertion reduction than that of bare α-Fe2O3 and the electrochemical cycling process is more stable.
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Fig. 6. CVs of anodes membrane at a scan rate of 0.1 mV s-1 for three cycles: (a) pure
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MWCNTs, (b) bare α-Fe2O3, (c) α-Fe2O3/10 wt.% MWCNTs, (d) α-Fe2O3/30 wt.% MWCNTs, (e) α-Fe2O3/50 wt.% MWCNTs and (f) α-Fe2O3/70 wt.% MWCNTs. Representative charge-discharge profiles of the electrode based on pure MWCNTs, bare α-Fe2O3 and α-Fe2O3/MWCNTs hybrids electrodes with 50 cycles at current rate of 200 mA g-1 in the potential range of 0.01 V - 3.0 V are shown in Fig. 7. The
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charge-discharge characteristics of pure MWCNT electrode for the 1st, 2nd and 50th cycles are given in Fig. 7a. The 1st discharge-charge capacities are 409.2 mAh g-1 and 388.4 mAh g-1 with the coulombic efficiency of 94.8 %. The rapid decay of the
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discharge capacity of the 2nd cycle to 225.3 mAh g-1 is caused by the formation of an irreversible SEI film. The 3rd cycle discharge capacity also slightly attenuates
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compared to the 2nd cycle. Fig. 7b shows charge-discharge characteristics of the bare α-Fe2O3 electrode at the 1st, 2nd and 50th cycles. Its 1st discharge and charge capacities are 1004.5 mAh g-1 and 580.8 mAh g-1 with the coulombic efficiency of 57.8 %. This low coulombic efficiency is due to the irreversible lithium ions embedded in electrode material and SEI film formation. The discharge capacity for 3rd cycle also hugely attenuates to 164.2 mAh g-1 caused by volume expansion cracking of the active material then it′s peeling off from the current collector, and while the irreversible lithium ions remain inside the pulverizing α-Fe2O3. Fig. 7c-f show that α-Fe2O3 with different amount of MWCNTs hybrids electrodes have higher
ACCEPTED MANUSCRIPT specific capacity retention compared to pure MWCNTs and bare α-Fe2O3 after 50 cycles. It is observed that the 1st discharge and charge capacities are 1139.8 mAh g-1 and 976.5 mAh g-1 for α-Fe2O3/50 wt.% MWCNTs anodes, indicating an initial coulombic efficiency of 85.7 %. The α-Fe2O3/50 wt.% MWCNTs anodes 1st cycle
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reversible capacity is 976.5 mAh g-1 that is much higher compared to pure MWNTs (388.4 mAh g-1). The discharge capacity is 845.4 mAh g-1 with the initial coulombic efficiency of 96.6 % which is much higher than that of bare α-Fe2O3 anodes with a capacity of 164.5 mAh g-1 after 50 cycles. The irreversible capacity of α-Fe2O3/
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MWCNTs hybrids anodes can be assigned to the formation of a SEI film on the
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surface of active materials and the irreversible insertion of lithium ions into Fe2O3.
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Fig. 7. Galvanostatic charge-discharge profiles of the membrane electrode: (a) pure MWCNTs, (b) bare α-Fe2O3, (c) α-Fe2O3/10 wt.%MWCNTs, (d) α-Fe2O3/30
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wt.%MWCNTs, (e) α-Fe2O3/50 wt.%MWCNTs and (f) α-Fe2O3/70 wt.%MWCNTs. Furthermore, the cycling performance of the pure MWCNTs, bare α-Fe2O3
nanoparticles and α-Fe2O3/MWCNTs hybrid electrodes are shown in Fig. 8a. It is observed that after 50 cycles the α-Fe2O3/MWCNTs hybrid electrode with 10 %, 20 %, 50 % and 70 % MWCNTs mass ratios still exhibited superior cycling performance with the discharge capacities of 426.2 mAh g-1, 580.6 mAh g-1, 778.2 mAh g-1 and 338.1 mAh g-1 at the current rate of 200 mA g-1. In contrast, the bare α-Fe2O3 nanoparticles electrode showed rapid fading of discharge capacity, display a capacity of less than 114.6 mAh g-1 after 50 cycles. It can be observed that the pure
ACCEPTED MANUSCRIPT MWCNTs shows higher capacity retention compared with bare α-Fe2O3 electrode. The pure MWCNTs retained a capacity value of 227.3 mAh g-1 at the current density of 200 mA g-1 even after 50 cycles. Rate performances of α-Fe2O3/MWCNTs hybrid and counterpart have also been investigated at various current densities (200, 500,
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1000, 1500, 2000 and 2500 mA g-1), and the results are demonstrated in Fig. 8b. It is worth noting that, when current density varies from 200 mA g-1 to 2500 mA g-1, the reversible capacity of the hybrid electrode does not rise with the increase of the weight ratio of MWCNTs within the samples during the 50 cycles. The capacities of
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α-Fe2O3 with 50 wt.% MWCNTs electrode retains at 817.3 mAh g-1 after the current density increases from 200 mA g-1 to 2500 mA g-1 and then back to 200 mA g-1,
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whose value is higher than those of the counterparts and pure MWCNTs.
Fig. 8. (a) Cycling performance of α-Fe2O3/MWCNTs hybrids anodes at a current
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density of 200 mA g-1, (b) Rate cyclability at various current densities (200/500/1000/2000/2500 mA g-1) at the voltage range of 0.01 V - 3.0 V.
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Fig. 9 shows the Nyquist plots of AC impedance (Frequency range from 100 kHz
to 0.1 Hz) for the bare α-Fe2O3, pure MWCNTs and α-Fe2O3 with MWCNTs hybrids electrodes, which were measured at an open circuit voltage (~3.0 V) state using fresh cells at room temperature. Six profiles show a semicircle at the high-to-medium frequencies and a sloping line at low frequency which is related to the charge transfer at the electrode/electrolyte interface and lithium ions diffusion process in electrodes, respectively. The fitting equivalent circuit for the cell system is depicted in the inset of the Figure. The Nyquist plots for the six samples were similar in shape except for the diameters of semicircles. From the Nyquist plots, it can be found that bulk resistance
ACCEPTED MANUSCRIPT (Rs) and charge transfer resistance (Rct) of the pure MWCNTs, α-Fe2O3/70 wt.%MWCNTs, α-Fe2O3/50 wt.%MWCNTs, α-Fe2O3/30 wt.%MWCNTs, α-Fe2O3/10 wt.%MWCNTs and bare α-Fe2O3 electrode are Rs = 5.5, 6.7, 5.3, 4.1, 3.3, 8.0 Ω and Rct = 195.2, 224.3, 244.3, 421.8, 614.0, 1009.0 Ω, respectively. Compared with EIS
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spectrum of bare α-Fe2O3 nanoparticle sample, the pure MWCNTs, and corresponding hybrids display relatively smaller Rct, indicating a lower impedance value with better conductivity. While for the electrode of pure MWCNTs, the electrons are first transferred from carbon tube bulk to surface with a shorter path. It
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can be deduced that the transfer of Li+ in hybrids electrode become faster and easier
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when the mass ratio of MWCNTs increased from 10 wt.% to around 50 wt.%.
Fig. 9. Typical electrochemical impedance spectra measured with bare α-Fe2O3, pure
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MWCNTs and α-Fe2O3/MWCNTs hybrid electrodes (inset is the equivalent circuit used to fit the EIS).
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Furthermore, an ultra-light, excellent conductive and flexible carbon nanofiber
paper (CNP), which only has one-sixth of the mass of copper-based current collector, was adopted as a novel current collector. The total specific capacity of the new flexible α-Fe2O3/50 wt.%MWCNT/CNP electrode (467.2 mAh g-1) is four times large than that of the α-Fe2O3/50 wt.%MWCNT electrode with a conventional copper collector (106.8 mAh g-1) after 50 cycles at a current density of 200 mA g-1 (Table S1). Fig. 10 shows the XRD patterns of CNP and the novel α-Fe2O3/50 wt.%MWCNT/CNP hybrid electrode. The characteristic diffraction peaks result of α-Fe2O3/50 wt.%MWCNT/CNP were in excellent accordance with a hexagonal
ACCEPTED MANUSCRIPT hematite structure, which can be indexed to the (012), (104), (110), (113), (024) and (116) plane from bare α-Fe2O3 crystals. The α-Fe2O3/50 wt.%MWCNT/CNP hybrids exhibited two peaks at 2θ = 25.6° and 44.2° which can be assigned to the (002) and
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(100) characteristic plane of carbon. This is the same with results of CNP.
Fig. 10. XRD patterns of pure CNP and α-Fe2O3/50 wt.%MWCNTs hybrids fabricated on CNP.
As shown in Fig. 11a and b, the morphology of the 3D porous CNP and
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α-Fe2O3/50 wt.%MWCNTs/CNP hybrids were examined. Fig. 11a shows SEM images of the as-obtained 3D porous structure CNP which consists of nanofibers with a diameter of ∼300 nm, length of several µm and thickness of ∼130 µm. An
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interconnected network with uniformly shaped fibers ranging from 200 nm to 300 nm provides a conductive 3D electronic transportation pathway and facilitates the access
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of the electrolyte. The picture of the real flexible CNP products network is given in Fig. S5. SEM image (Fig. 11b) shows a rough surface caused by the loading of α-Fe2O3/50 wt.%MWCNTs hybrids filtered on CNP. The α-Fe2O3 nanoparticles are uniformly distributed inside the 3D porous network structure.
Fig. 11. SEM image of pure CNP and α-Fe2O3/50 wt.% MWCNTs hybrids fabricated CNP.
ACCEPTED MANUSCRIPT Representative charge-discharge profiles of the electrode based on pure CNP and α-Fe2O3/50 wt.%MWCNTs/CNP at the current rate of 200 mA g-1 at the potential range of 0.01 V - 3.0 V are shown in Fig. 12. For pure CNP, its initial discharge capacity is 100.0 mAh g-1 and maintains at 29.7 mAh g-1 after 50 cycles (Fig. 12a).
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The reversible discharge-charge capacity of the 1st cycle for α-Fe2O3/50 wt.%MWCNTs/CNP anodes is 1322.2 mAh g-1 and 641.5 mAh g-1, respectively (Fig. 12b). The discharge capacity still remains at 574.8 mAh g-1 that is higher compared with graphene after 50 cycles. The discharge capacity still remains at 574.8 mAh g-1
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after 50 cycles, which is obviously higher than those of bare CNP and commercial
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graphite.
Fig. 12. Galvanostatic charge-discharge profiles of electrode: (a) pure CNP, (b) the α-Fe2O3/50 wt.%MWCNTs/CNP membrane.
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The cycling performance of the flexible α-Fe2O3/50 wt.%MWCNTs/CNP hybrid, pure CNP electrode are shown in Fig. 13. In contrast, the flexible bare CNP electrode and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid showed discharge capacity
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of 29.7 mAh g-1 and 574.8 mAh g-1 at a current density of 200 mA g-1 after 50 cycles (Fig. 13a), respectively. It is noteworthy noting that the capacity of free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode is higher than that of commercial graphite.[41] The rate capability of the porous CNP anodes and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode in LIBs was also evaluated by cycling at different current densities from 200 mA g-1 to 2500 mA g-1 (Fig. 13b). The free-standing α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode delivers a higher capacity of 201.2 mAh g-1 at a current rate of 2500 mA g-1. When its current density
ACCEPTED MANUSCRIPT converts from 2500 mA g-1 back to 200 mA g-1, the discharge capacity rises to 467.2
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mAh g-1.
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Fig. 13. (a) Cycling performance of CNP and α-Fe2O3/50 wt.%MWCNTs/CNP anodes at a current density of 200 mA g-1, (b) Rate cyclability at various current
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densities (200/500/1000/2000/2500 mA g-1) at the voltage range of 0.01 V - 3.0 V. The Nyquist plots of the fresh cell which fabricated by pure CNP, α-Fe2O3/50 wt.%MWCNTs and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP were shown in Fig. 14 with a frequency range from 100 kHz to 0.1 Hz at open circuit potential(~3.0 V). From the Nyquist plots, it can be found that bulk resistance Rs are 1.8, 1.3, 1.3 Ω,
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and charge transfer resistance Rct are 84.8, 140.6, 198.8 Ω corresponding to pure CNP, α-Fe2O3/50 wt.%MWCNTs and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP electrodes,
respectively.
The
conductivity
of
free-standing
α-Fe2O3/50
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wt.%MWCNTs/CNP electrode is superior to α-Fe2O3/50 wt.%MWCNTs with the
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copper current collector.
ACCEPTED MANUSCRIPT Fig. 14. Typical electrochemical impedance spectra measured with pure CNP, α-Fe2O3/50 wt.% MWCNTs and free-standing α-Fe2O3/50 wt.%MWCNTs/CNP electrode. 4.Conclusion Hematite α-Fe2O3 nanoparticles interconnected by the carbon nanotubes
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(MWCNTs) have been successfully prepared as LIBs anodes. The α-Fe2O3 hybrid electrode with 50 wt.%MWCNTs retains relatively higher capacity of 778.2 mAh g-1 after 50 cycles under the current density of 200 mA g-1. Moreover, the carbon
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nanofiber paper (CNP) was used as a novelty current collector to replace the traditional copper foil for application in LIBs. The discharge capacity of free-standing
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α-Fe2O3/50 wt.%MWCNTs/CNP hybrid electrode still remains at 467.2 mAh g-1 after 50 cycles when the current density varies gradually from 200 mA g-1 to 2500 mA g-1, and then back to 200 mA g-1. The inherently conducting network of MWCNTs with shorten electron pathway and faster reaction kinetics in α-Fe2O3 improve the cycling performance, resulting in higher reversible capacities and rate capability.
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Acknowledgment
We gratefully acknowledge the financial support from the Natural Science Foundation of China (grant No. 61664009, 51771169) and the Youth Project of
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Applied basic research of Yunnan Science and Technology Department (grant No. 2015FD001). This work is also funded in part by the High-end Scientific and Technological Talents Introduction Project of Yunnan Province (grant No.
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2013HA019). References
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ACCEPTED MANUSCRIPT Highlights: α-Fe2O3/MWNT nanocomposite electrodes for flexible LIBs.
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α-Fe2O3 embedded in the outer carbon nanotube.
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Hybrids electrode exhibits superior electrochemical properties.
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Free-standing carbon nanofiber paper applied to light-weighted LIBs.
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