C composite as anode material for lithium-ion batteries

C composite as anode material for lithium-ion batteries

Accepted Manuscript Title: Micro-tube biotemplate synthesis of Fe3 O4 /C composite as anode material for lithium-ion batteries Authors: Jun Du, Yu Din...

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Accepted Manuscript Title: Micro-tube biotemplate synthesis of Fe3 O4 /C composite as anode material for lithium-ion batteries Authors: Jun Du, Yu Ding, Liangui Guo, Li Wang, Zhengbing Fu, Caiqin Qin, Feng Wang, Xinyong Tao PII: DOI: Reference:

S0169-4332(17)31868-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.213 APSUSC 36413

To appear in:

APSUSC

Received date: Revised date: Accepted date:

15-3-2017 7-6-2017 20-6-2017

Please cite this article as: Jun Du, Yu Ding, Liangui Guo, Li Wang, Zhengbing Fu, Caiqin Qin, Feng Wang, Xinyong Tao, Micro-tube biotemplate synthesis of Fe3O4/C composite as anode material for lithium-ion batteries, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.213 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.

Micro-tube biotemplate synthesis of Fe3O4/C composite as anode material for lithium-ion batteries Jun Dua, Yu Dinga, Liangui Guoa, Li Wanga, Zhengbing Fua, Caiqin Qina, Feng Wanga * and Xinyong Tao b * a

College of Chemistry and Materials Science, Hubei engineering university, Xiaogan, 432000, PR China,

E-mail: [email protected] b College

of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, 310014,

PR China, E-mail: [email protected]

Graphical abstract

Figure for Table of Content Fe3O4/C nanoparticles were successfully synthesized via a facile micro-tube method using kapok fibers as both the carbon source and the template.

Highlights 

Fe3O4/C nanoparticles were successfully synthesized via a facile micro-tube method.

 

Based on the structural, morphological, and elemental analyses, a micropipette mechanism was proposed. As-prepared Fe3O4/C nanocomposite electrode exhibits high reversible capacity, excellent cycling stability, and good rate capability.

Abstract: Kapok fibres were used as micro-tube biotemplate and bio-carbon source to synthesise Fe3O4/C composites, which were then utilized as anode materials. Fe3O4 nanoparticles were grown uniformly onto the external surface and internal channel of kapok carbon fibres. The flexibility, high specific surface area and electronic conduction of kapok fibres can buffer the volume expansion as well as inhibit the aggregation of Fe3O4 nanoparticles. Thus, the electrical integrity and structural of the Fe3O4/C composites electrode during lithiation/delithiation processes. The Fe3O4/C composites electrode delivers a high reversible capacity of 596 mAh·g−1 after 100 cycles and an ultra-high coulombic efficiency approaching 100%. The high electrochemical performance of the Fe3O4/C composites can be caused by the synergistic effect of the Fe3O4 nanoparticles and the structure of kapok carbon fibres.

Keywords: Fe3O4/C composite, micro-tube biotemplate, anode material, lithium-ion batteries 1. Introduction The demand for materials with superior properties has increased daily in recent years. Fe3O4 has attracted considerable attention because of its low cost, high theoretical capacity, good environmental friendliness and electrochemical stability [1-15]

. Nevertheless, its application in the practical application of lithium-ion batteries

(LIBs) is still largely hampered by its poor cycling performance arising from the huge volume change and severe aggregation of Fe3O4 particles during Li insertion/extraction

[3, 16-20]

. To date, various techniques, including hydrothermal

synthesis[18], templating[2] and other methods[5-7, 21] have been developed to fabricate the C-based Fe3O4/C composite. New ideas are required with the advancement of science and technology; thus, numerous researchers began focus on systems found in nature[22-25]. The kapok fibres are the fruit fibres of kapok tree, which originated in tropical India and Southeast Asia. These fibres, with diameters ranging from 20 µm to 45 µm, are currently utilised as packing materials for pillows and soft toys, buoyant materials, and oil absorbents because of their thin cell wall, low density, large lumen and hydrophobic properties[26, 27]. Kapok fibres are recognised as the world’s finest and lightest fibres[26,

27]

, offering environmental advantages over long-cycle renewable

resources[26, 27]. Although kapok fibres have been used as raw material for textiles and handicrafts in Asia for thousands of years, their potential contribution to sustainable natural biomass resource management have only been recognised recently. Furthermore, the study found that kapok fibres possess a micro-tubular structure[26, 27]. When kapok fibres form contact with the surface of a liquid, the liquid flows into the microtubules; this phenomenon is known as capillarity. This phenomenon presents important implications for the synthesis of new materials. Herein, we report a new method named “micro-tube method,” which is inspired by the capillary phenomenon and the concept of biological template[23-28]. Fe3O4/C

nanoparticles can be synthesised via a simple, cost-effective and convenient micro-tube method. The obtained Fe3O4/C exhibited considerable energy storage performance as new electrode materials for LIBs with unique composite structures and the synergetic effects of Fe3O4 nanoparticles. Kapok carbon fibres, a natural biomass

renewable

material,

should

present

more

applications

in

green

material-enabled energy storage systems. 2. Experimental section Material synthesis. Kapok fibres, which were cut into small pieces of 2–3 mm after cleaning and drying, were dispersed in anhydrous ethanol. Then, dissolving 5 g of Fe(NO3)3·9H2O in 5 mL of deionised water. Kapok fibres were removed from anhydrous ethanol, drained and dipped in Fe(NO3)3 solution. After the colour of all kapok fibres changed to yellow, the remaining liquid on the surface was removed and drained. Then, all kapok fibres were prepared on nickel foam substrates at the top of concentrated ammonia. Fe(NO3)3,which remains in the kapok fibres, reacts with volatilised ammonia to generate Fe(OH)3, and the colour of kapok fibres tend to change from yellow to brown. The reaction was stopped when the colour of all kapok fibres turned brown. Then the kapok fibres were placed into a sealed graphite crucible. They were heated to 500 °C at a heating rate of 5 °C/min and maintained at that temperature for 4 h under a constant Ar gas flow (350 sccm). After that, the furnace was naturally cooled to room temperature. Characterisation. The phase compositions of the as-prepared products were determined by X-ray diffraction (XRD) using an XPert Pro diffractometer with a step

size of 0.02 for Cu Kα radiation (λ=1.5418Å). The microstructure and morphology of products were investigated by scanning electron microscopy (SEM, Hitachi s-4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F30) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. Electrochemical measurements. Electrochemical performance of the Fe3O4/C composite was carried out in the CR2025 coin-type cell. And the metallic lithium was used as the counter and reference electrodes. The testing electrode was fabricated by mixing active materials, a conductivity agent (Super P) and a poly (vinylidene fluoride) binder at a weight ratio of 80:10:10 in an N-methyl-2-pyrrolidene solvent. After the testing electrodes were dried at 80 °C under vacuum overnight, the coin cells were assembled inside an argon-filled glove box. The electrolyte was 1 M LiPF6 in ethylene carbonate–dimethyl carbonate (1:1 by volume), and the separator was a polypropylene membrane. The galvanostatical charge (Li+ extraction) and discharge (Li+ insertion) were conducted in the voltage range of 0.01–3.0 V versus Li/Li+ at different current densities by using a battery testing system at ambient temperature (Shenzhen Neware Battery, China). Cyclic voltammetry (CV) curves were measured at a scanning rate of 0.1 mV·s−1 within the potential range of 0.002–3.0 V versus Li/Li+ by using a CHI660D electrochemistry working station (Shanghai Chenhua, China). 3. Results and discussion Typically, in the fabrication process of the Fe3O4/C composite, the particle size of Fe3O4 influences coating efficiency and uniformity and indirectly affects

electrochemical performance, such as the specific capacitance[2,

14, 16, 17, 29-32]

. In

addition, a stable carbon template affects the electrochemical stability of the electrodes[2, 14, 16, 17, 30-32] In this work, the micro-tubular structure of kapok fibres, which were used as the carbon source and the biotemplate, is conducive to the infiltration of the precursor solution. The morphologies of the as-prepared products were characterised by SEM. Figure 2a shows the low magnification SEM image of kapok fibres. The image clearly reveals that the kapok fibres possess a hollow structure with large lumen and diameters ranging from 10 µm to 25 μm. This structure is conducive to the infiltration of the precursor solution. The clear image (Figure 2b) proves that the hollow structure is maintained after heat treatment at 500 °C. Figure 2c, who is the magnified image of Figure 1b, reveals that nanoparticles are deposited in the surface of kapok fibres. In the enlarged view in Figure 2d, the particle size ranges from 1 nm to 10 nm. A typical XRD pattern of the product is shown in Figure 2e.The peaks can be assigned to the diffraction of (220), (311), (400), (511) and (440) planes of the Fe3O4/C (JCPDS 82-1533) [4, 6, 21]. Figure 2f is the synthetic principle of the Fe3O4/C composites. The formation of Fe3O4/C composites can be divided into two steps. The first step reflects the reaction of Fe(NO3)3 with NH3·H2O to generate Fe(OH)3 (Eq. 1) [33]:

Fe(NO3)3(aq) + 3OH−→Fe(OH)3↓+ 3NO3−

(1)

The colour of kapok fibre impregnated with the aqueous ferric nitrate solution changes from yellow to brown. The reaction must be immediately stopped once the

transition is completed in order to avoid the formation of complexes. The precursor is pyrolysed and calcined in a tube furnace with flowing argon. The second step is as follows (Eq. 2, Eq. 3)[16, 33]:

3Fe(OH)3→ Fe2O3 + 3H2O(g)

(2)

3Fe2O3 +C → 2Fe3O4 + CO(g)

(3)

Fe(OH)3 was decomposed to Fe2O3 and then reacted into Fe3O4[33]. In this work, kapok fibres acted as the renewable carbon source and provided a suitable reducing atmosphere for the reduction reactions to proceed. As shown in Figures 3a and 3b, Fe3O4 nanoparticles were well-dispersed in the carbon matrix with an average particle size of 5 nm. The TEM images also confirmed the SEM results. In Figure 3c, the HRTEM image shows the clear lattice fringes of Fe3O4 nanoparticles embedded in the carbon matrix. The lattice fringes spacing of 0.25 nm corresponds to the (311) plane of the cubic Fe3O4, which is in agreement with the XRD results. The high-quality encapsulation structure of the synthesised Fe3O4/C composites possess was confirmed by evaluating the chemistry of the surface using surface-sensitive high-resolution X-ray photoelectron spectroscopy (XPS). Figure 3d shows a typical full XPS spectrum of the Fe3O4/C composites, in which the photoelectron lines at binding energies of 285 and 532 eV are attributed to C 1s and O 1s, respectively. The XPS result (Figure 3e) shows the typical characteristics of Fe3O4 of the final electrode, with two peaks located at 710.9 and 724.3 eV corresponding to the Fe 2p3/2 and Fe 2p1/2 states[3, 5, 34], respectively. These findings indicate that the

final product is Fe3O4 instead of Fe2O3. This characteristic is important in distinguishing between Fe3O4 (magnetite) and γ-Fe2O3 (magnetite) since the two have the same crystalline structure but differ only in the valence state of iron ions. The O 1s spectrum of the composite contains four components: (O1) O2− bonded with iron at 530.9 eV, (O2) oxygen in O-H at 531.2 eV, (O3) oxygen in O-C=O at 532.3 eV and (O4) oxygen in C-O at 533.8 eV[35]. As shown in Figure 4, the successful fabrication Fe3O4/C nanocomposites for LIB anodes are evident because of the unusual excellent electrochemical properties. Figure 4a presents the discharge/charge profiles of Fe3O4/C hybrid electrodes between 0.01 and 3.0 V at a current density of 100 mA·g−1. The first discharge and charge capacities are 921 and 632 mAh·g−1, corresponding to an initial columbic efficiency of 68.63%. Despite the first irreversible capacity loss in the initial several cycles, which is generally observed in many carbon-metal oxide hybrids[1-9]. The main causes of the first irreversible capacity loss are the inevitable formation of solid electrolyte interphase (SEI) and the decomposition of electrolyte. The discharge voltage plateau at ~0.8 V in the first cycle is different from those of other cycles at ~1.0 V, further indicating that an irreversible reaction occurred in the first cycle. Notably, no evident change was observed in both charge/discharge profiles even after 100 cycles, further indicating that Fe3O4/C is extraordinarily stable during circulation. Figure 4b demonstrates the cycling performance and coulombic efficiency of composite material electrode at a current density of 100 mA·g−1 and below 20 °C. It is apparent that the Fe3O4/C composite material electrode exhibits an upward trend in

capacity and a high reversible specific capacity of over 710 mAh·g−1 after 100 cycles, accounting for 110.6% of the second cycle (642 mAh·g−1). This value is higher than Fe3O4 nanoparticles on graphene[19] and close to Fe3O4-NPs/EG composite[31]. The stable cyclic performance is considerably higher than that of current carbon-based electrode materials. Evidently, the enhanced electrochemical performance is closely correlated to the novel composition and nanostructure of the electrode material. On one hand, the Fe3O4/C composite material electrode shows superior capacity retention because the peculiar composite structure can efficiently accommodate the volumetric change during Li+ insertion/extraction processes. On the other hand, Fe3O4 nanoparticles, which possess high specific surface area, can increase the contact area between the active materials and shorten Li+ diffusion distance. As shown in Figure 4c, it presents the first three CV curves of the Fe3O4/C composite electrode between 0.01 and 3.0 V at a scan rate of 0.1 mV·s−1 and at room temperature. In agreement with large literature reports, the CV curve of the first cycle is completely different from those of subsequent cycles, especially for the discharge process. Two clearly-defined peaks are observed at 0.97 and 0.60 V (versus Li+/Li) in the first discharge cycle and are usually attributed to the occurrence of side effects on the electrode interfaces and surfaces owing to the formation of SEI and the two steps of the lithiation reactions of Fe3O4 (step 1, Fe3O4+ 2Li+ + 2e− → Li2(Fe3O4); and step 2, Li2(Fe3O4) + 6 Li+ + 6e− → 3Fe0+ 4Li2O)[2, 5, 21]. In comparison, the distinct peaks appear at 0.81 V during discharge and at 1.65 and 1.90 V during charge from the second cycle onward; these peaks exclusively correspond to the electrochemical

reduction/oxidation (Fe3O4 ↔ Fe) reactions accompanying lithium ion insertion (lithiation) and extraction (delithiation), in accordance with those previously reported in the literature for Fe3O4-based electrodes. Apparently, the peak intensity drops significantly in the second cycle, indicating the occurrence of several irreversible reactions with the formation of an SEI film. Notably, after the first cycle, the voltage–current curves almost overlapped, indicating that a stable SEI film formed on the surfaces and interfaces of carbon shells in the first cycle can prevent the direct contact of encapsulated Fe3O4 nanoparticles with the electrolyte and safeguard the structural integrity of interior Fe3O4 during subsequent charge/discharge cycles, thus leading to high coulombic efficiency, superior and stable reversibility of the sample. In order to verify the good performance of the Fe3O4/C electrode, ac impedance measurements were carried out to identify the relationship between the electrode kinetics and electrochemical performance at frequencies from 100 KHz to 0.01 Hz. From Figure 3d, we can see that the fresh Fe3O4/C cell possesses a large interface resistance of 1280 Ω, which decreases to 673 Ω after three cycles and continues to decrease to 36.2 Ω after 50 cycles. This value is much lower than Fe3O4/RGO or M-Fe3O4/RGO(84 Ω or 200 Ω)[19]. As the number of cycle increases, the electrode material is constantly being activated, consequently improving reaction kinetics. The ac impedance measurements are in accordance with the cycling performance as shown in Figure 3b. Figure 4f shows the rate capability of the Fe3O4/C composite material electrodes. The electrodes deliver a reversible capacity of 596.3 mAh·g−1 when first cycled at 100

mA·g−1 for 10 cycles. Upon increasing the discharge/charge current density to 200, 500, 1000, 2000 and 3000 mA·g−1, the reversible capacity of the Fe3O4/C composite material electrode still remains at 521.2, 479.4, 378.9, 349.7 and 320 mAh·g−1, respectively. In addition, when the current rate finally returned to its initial value of 100 mA·g−1, the retention rate of the discharge capacity at the 60th cycle (538.4 mAh·g−1) relative to the discharge capacity at the 10th cycle is 90.3% (596.3 mAh·g−1). After 110 cycles, the specific capacity increased to a value of 606.6 mAh·g−1 and is flushed with the initial charge-discharge capacity, as is consistent with the cycling performance shown in Figure 3b. The results show that the structure of Fe3O4/C remains exceedingly stable even under high rate charge/discharge cycling condition. Good cyclability, high specific capacity and superior rate capability make Fe3O4/C composites a promising potential anode materials for high power commercial LIBs. During the synthesis process, Fe3O4 nanoparticles are uniformly dispersed on the surface of Fe3O4/C composites and inset of kapok fibres against agglomeration. It has been demonstrated that this structure can shorten the pathway for Li ion transfer and increase the Li ion diffusion coefficient. In addition, Fe3O4 nanoparticles may be fixed in the internal structure of carbon layers, which will present the huge volume change during the lithiation and delithiation processes. For kapok carbon, it is that maintaining the capacity without completely destroying the fibre structure. Meanwhile, kapok fibres can provide unique properties such as electrical conductivity for Fe3O4. Therefore, the synergetic effects of Fe3O4 and kapok carbon can enhance

the excellent rate capability and high capacity.

4. Conclusions We demonstrated a simple approach to prepare Fe3O4/C composite via micro-tube method. Easily available, carbon-rich kapok fibres play a dual role as the carbon source and the biotemplate. Fe3O4 particles with diameter ranging from 1 nm to 10 nm are completely embedded in the porous carbon matrix of the Fe3O4/C composite. The photo electrochemical measurements prove that the Fe3O4/C electrode exhibits notable cycle potential for anode material for LIBs. We believe that this facile, green and economical method will extend the range of biotemplate synthesis to other materials for various applications, such as gas sensing, catalysis and photonics.

5. Acknowledgment The work presented in this paper is financially supported by the NSFC (21301054 and 51402096), Hubei Provincial NSF of China (2012FFB00503 and 2016CFB311), Hubei Provincial SF of China (Q20152706) and the Program for Scientific and Technological Innovation Team of Hubei Province (T201517).

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Figure 1. Schematic of the synthesis routes of Fe3O4/C composites.

Figure 2. (a) SEM image of kapok fibres; (b) A low-magnification SEM image of the Fe3O4/C composite materials; (c) SEM image of an individual sample; (d) Partial enlargement SEM image in (c); (e) XRD pattern of Fe3O4/C nanoparticles; (f) Synthetic principle of the Fe3O4/C composites.

Figure 3. (a, b) Low-magnification TEM images of Fe3O4/C composites; (c) High-resolution TEM images Fe3O4/C composites; (d) XPS spectra of Fe3O4/C composites; (e, f) The high-resolution spectra of Fe 2p and O 1s.

Figure 4. (a) Charge/discharge profiles of Fe3O4/C sample; (b) cycling performance and coulombic efficiency of Fe3O4/C electrodes;(c) The Nyquist plots of Fe3O4/C nanocomposites in different cycles; (d) Nyquist plots of Fe3O4/C nanocomposites in different cycles; (e) charge/discharge profiles of Fe3O4/C electrodes at various current densities; (f) rate and cycling performances of Fe3O4/C electrodes.