C composites as excellent anode materials for lithium-ion batteries

C composites as excellent anode materials for lithium-ion batteries

Materials Today Energy 5 (2017) 187e195 Contents lists available at ScienceDirect Materials Today Energy journal homepage: www.journals.elsevier.com...

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Materials Today Energy 5 (2017) 187e195

Contents lists available at ScienceDirect

Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/

Cotton-assisted combustion synthesis of Fe3O4/C composites as excellent anode materials for lithium-ion batteries Cheng-Gong Han, Nan Sheng, Chunyu Zhu*, Tomohiro Akiyama Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 February 2017 Received in revised form 23 June 2017 Accepted 1 July 2017

A facile one-pot cotton-assisted combustion synthesis in several minutes was employed to prepare Fe3O4/C composites. Phase structure, morphologies, specific surface areas and compositions were characterized by X-ray diffraction, scanning/transmission electron microscopy, N2 adsorption-desorption isotherms and thermogravimetry, respectively. Well-dispersed Fe3O4 nanoparticles in cotton-derived carbon matrix resulted in excellent electrochemical performance. The highest gravimetric and volumetric discharge capacity of 803 mAh g1 and 948 mAh cm3, respectively, were delivered in Fe3O4/C-8 composite (36.0% carbon) after 300 cycles at a current of 0.4 A g1. The presence of carbon played a key role on buffering the volume changes and suppressing the agglomeration of Fe3O4 nanoparticles during Liþ ion insertion/extraction process, thereby resulting in the high electrochemical performance. The facile and scalable combustion-synthesized Fe3O4/C composites with excellent electrochemical performance provide a possibility as anode materials used for lithium-ion batteries. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Oxide Fe3O4 Carbon Combustion reaction Li ion battery

1. Introduction Rechargeable Li-ion batteries (LIBs) are becoming the most attractive and promising storage devices used in portable electronics (e.g. laptops, cameras, mobile phones) and pure/hybrid electric vehicles due to their high power/energy densities [1,2]. A rechargeable LIB consists of an anode and a cathode, which are normally separated by a membrane soaked with an electrolyte solution [3]. Exploiting new anode materials with high storage capacity is crucial for the urgent demands of high power/energy Liion batteries because of the relatively low theoretical capacity (only 372 mAh g1) of commercialized graphite anodes [4,5]. High capacity of 700 mAh g1 as well as 100% capacity retention up to 100 cycles have been achieved in transition metal oxides [6]. And capacities with 2e3 times higher than graphite are also observed in NiO [7,8], MnO [9,10], CoO [11,12], and Fe3O4 [13e15], evidencing the availability of transition metal oxides as anode materials. Amongst these transition metal oxides, Fe3O4 possesses the highest theoretical capacity of 924 mAh g1, low cost, abundance and easy preparation [16], hence Fe3O4 is highlighted as the potential anode material. Such electrochemical reaction occurs during the Liþ ion

* Corresponding author. E-mail address: [email protected] (C. Zhu). http://dx.doi.org/10.1016/j.mtener.2017.07.001 2468-6069/© 2017 Elsevier Ltd. All rights reserved.

insertion process for Fe3O4, resulting in the formation of metallic Fe and amorphous Li2O [17].

Fe3 O4 þ 8 Liþ þ 8 e /3 Fe0 þ 4 Li2 O

(1)

The higher potential of 0.9 V vs. Li/Liþ for Fe3O4 than graphite can keep the safety of battery by evading the risks of high-surfacearea Li plating at the end of fast recharge process [18]. In addition, a higher volumetric capacity can be expected in Fe3O4 due to its larger density of 5.17 g cm3 than 2.2 g cm3 for graphite. However, Fe3O4 suffers from the poor cycling stability arising from the huge volume changes and serious agglomerations of active materials during the Liþ ion insertion/extraction process. Many efforts of controlling morphologies and preparing methods have been devoted to address the undesirable issues. Nanostructure is considered as one effective strategy based on the characteristic of fast diffusion rate, short diffusion path and quick insertion/extraction for Liþ ions without any deterioration of electrodes [19]. Hence, morphologies with nanotubes [20,21], nanowires [22,23], nanoflowers [24], and nanoparticles [25] have been studied, demonstrating the high electrochemical performance of Fe3O4. On the other hand, introducing the carbon into Fe3O4 anode materials exhibits the excellent electrochemical performance due to the enhanced electrical conductivity, alleviated volume changes and agglomeration of active particles during the cyling [26e29].

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Composites, consisting of nanostructured Fe3O4 and carbon, are expected to possess the attractive electrochemical performance as anode materials. A facile and economic method for preparing nanoFe3O4/C composites with high capacity and cycling stability is becoming urgent for the further applications, against those complex and long-time procedures [25,30,31]. Combustion synthesis as a simple and time/energy saving method, has drawn more attentions to prepare various oxides with nanosizes based on a selfsustaining and highly exothermic redox chemical reaction [32e37], which demonstrates its feasibility for the high performance Fe3O4/C composites in one step. In the present work, cotton-assisted combustion synthesis is employed to fabricate a series of Fe3O4/C composites. Cotton, as a cheap, widely available, chemically and mechanically robust biotemplate, is used to synthesize carbon-containing nanomaterials with the porous structure, which cellulose of cotton includes many glucose monosaccharide units, and eOH, CH2eOeCH2 groups as CO sources [38e41]. In addition, following the multi ions as-sucked into the mini pores of cotton, the cotton fibers can effectively hinder the agglomeration of primary particles during the solvent evaporation, producing the small sizes and porous structures [42]. The combination of combustion synthesis and cotton as both the template and carbon source, opens a new facile and scalable route for fabricating Fe3O4/C composites in a one-pot reaction. Phase structure, morphologies, and the electrochemical performance are investigated in detail by tailoring the carbon amounts of composites. High electrochemical performance is delivered in obtained Fe3O4/C composites via cotton-assisted combustion synthesis. 2. Experimental Iron nitrate (Fe(NO3)3$9H2O, 99%, Kishida Chemical Co. Ltd. Japan) and commercial absorbent cotton were employed as the raw materials without further purification. 1 g commercial cotton was immerged in the 6 ml iron nitrate aqueous solutions, consisting of 6, 8, 10 and 12 mmol Fe(NO3)3$9H2O, respectively. After the ultrasonic treatment and adequate absorption, the wet cotton was treated at 70  C. The dried cotton was placed in an Ar-flow filled vertical tube furnace, which was heated from room temperature at a 10  C/min. When a violent combustion reaction accompanying with the emission of large amounts of gases were observed, the power of furnace was tuned off immediately. The final powders were obtained by pulverizing the nature-cooled residues in an agate mortar. For simplicity, these obtained samples were marked by Fe3O4/C-6, Fe3O4/C-8, Fe3O4/C-10, and Fe3O4/C-12, respectively. Phase structure was determined by powder X-ray diffraction (XRD) with a graphite monochromator and 1D silicon strip detector (Rigaku Miniflex600, D/teX Ultra 2, Cu Ka). Morphologies as well as element distribution and sizes were characterized by scanning electron microscopy (SEM, JEOL, JSM-7001FA) together with an Xray energy dispersive spectrometry (EDS) and transmission electron microscopy (TEM, JEOL, JEM-2010F). Carbon content in the products and thermal behavior were studied by a combined thermogravimetric and differential scanning calorimetry (TG-DSC, Mettler Toledo) analyzer. Raman spectra of the samples were detected by a RENISHAW Raman spectrometer with an excitation wavelength of 532 nm. Brunauer-Emmett-Teller (BET) specific surface area of samples was carried out by N2 sorption using a BELSORP-mini surface area analyzer. A modified-Swagelok cell, two-electrode union-joint cell, was assembled in an Ar-filled glove-box. The working electrode was prepared by mixing the active material, polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) (1:1 in weight) binders, and acetylene black conductive carbon in a weight ratio of 75:10:15. Celgard polypropylene membrane was served as a separator, while lithium

disk was employed as the reference and counter electrode. 1 M solution of LiPF6 dissolved in a 50:50 (v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte. Discharge/charge measurement was galvanostaticly performed in an Arbin Instrument (MSTAT4, USA) in a potential range of 3.0e0.01 V at 25  C. Cyclic voltammetry (CV) was conducted by using a potentiostat/galvanostat (Autolab, PGSTAT128N) in a range of 3.0e0.01 V at a scan rate of 0.2 mV s1. Electrochemical impedance spectroscopy (EIS) of Fe3O4/C electrodes of the Li-Fe3O4/ C cells was carried out at room temperature by employing an Autolab instrument with a frequency response analyzer (FRA), in the frequency range from 1000 kHz to 0.1 Hz under a sine wave with amplitude of 10 mV.

3. Results and discussion Fig. 1 displays the schematic illustration for preparing Fe3O4/C composites. The required absorbent cotton is immersed into a homogenous Fe(NO3)3 aqueous solution. After the ultrasonic treatment and drying the water, the cotton that is well-absorbed by Fe(NO3)3 is subjected to the combustion reaction in an Ar atmosphere within several minutes. The carbon composite with welldispersed and embedded Fe3O4 nanoparticles is finally obtained. In order to monitor the process of combustion reaction, TG-DSC for the decomposition of precursor Fe(NO3)3$9H2O, absorbent cotton, and the mixture of both is carried out, as shown in Fig. 2. All the samples were heated from room temperature at a rate 10  C/min in an Ar flow of 200 ml/min. In Fig. 2 (a), a weight loss of around 22% in TG curve occurs bellow 150  C, ascribing to the evaporation of partial crystal water in Fe(NO3)3$9H2O. In the range of 150e200  C, a remarkably decreased weight loss of 48% in TG curve is observed, accompanying with a sharp endothermic peak at 165  C in DSC curve, which can be attributed to the decomposition of Fe(NO3)3 to form the iron oxide and release NOx gases [43]. The stable TG curve as well as no extra peaks in DSC curve above 200  C indicate the full decomposition of precursor. In Fig. 2 (b), the inset gives the chemical structure of cellulose of cotton, possessing lots of C-O-Hcontaining groups, e.g. ‒OH, and CH2‒O‒CH2 [39]. A large weight loss of 87% is displayed in TG curve in the range of 300e400  C, together with an endothermic peak at 367  C in DSC curve, which is arising from the thermal degradation of cotton with the release of volatile due to the dehydration, decarboxylation and decarbonylation, thereby resulting in the formation of carbon [44,45]. In Fig. 2 (c), an obvious weight loss of 25% in TG curve in the range of 100e200  C is noticed, corresponding to an exothermic peak at 156  C in DSC curve. It can be ascribed to the redox combustion reaction between oxidant Fe(NO3)3 and reductant cotton, not to the evaporation of water and/or decomposition of Fe(NO3)3 due to the absence of endothermic peak. The possible combustion reaction can be shown as follows:

3 FeðNO3 Þ3 þ ðC6 H10 O5 Þn /Fe3 O4 þ x CO2 ðgÞ þ ð6n  xÞCOðgÞ þ 5n H2 OðgÞ þ ð14  6n  xÞNO2 ðgÞ þ ð6n þ x  5Þ NOðgÞ (2) Compared with the entropy of Fe2O3 (825.503 KJ/mol, 298 K) and FeO (272.044 KJ/mol, 298 K), entropy of Fe3O4 displays the most negative value of 1118.383 KJ/mol (298 K), indicating the easiest formation of Fe3O4 after combustion reaction according to the most negative DG (DG¼DHTDS) at the same conditions. A further decreased weight is noticed in the range of 300e400  C, corresponding to the downward TG curve in Fig. 2 (b), which is due to the thermal degradation of extra cotton of the precursor to form the carbon. The complete formation of Fe3O4/C composites is

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Fig. 1. Schematic illustration for preparing Fe3O4/C composites.

Fig. 2. TG-DSC curves of Fe(NO3)3$9H2O (a), absorbent cotton (b), mixture for preparing Fe3O4/C-8 sample (c).

obtained above 400  C based on the unchanged TG-DSC curves. Note that, when using 1 g cotton as raw material in the experiment, a vigorous exothermic reaction is observed around 200  C, asfeatured by the emission of large amount of gases in several seconds and a temperature jump-up monitored by a thermocouple. The released reaction heat can further push the temperature to a higher value, so as to pyrolyze the raw materials into a stable carbon-oxide composite, even such a sintering at this high temperature is not performed. The crystal structures of Fe3O4/C composites are identified by XRD patterns, as shown in Fig. 3. All the diffraction peaks of the obtained composites are well matched with that of the cubic spinel-like structure of magnetite (PDF#19-0629), indicating the

formation of single Fe3O4 phase. An increased trend of intensity for diffraction peak (311) is observed as increasing the contents of Fe3O4. The broad diffraction peaks demonstrate the fine particle sizes, benefiting the electrochemical performance. Based on DebyeScherrer’s formula [46]: D¼0.89l/Bcosq, where l is the X-ray wavelength, B is the full width at half maximum, and q is Bragg angle in degree, an average crystallite size of 7.9, 10.5, 15.6, and 22.0 nm is calculated from the diffraction peak (311) for Fe3O4/C-6, 8, 10, and 12, respectively. The results indicate that the more carbon contents can restrain the growth of Fe3O4 crystallite sizes. No diffraction peaks belonged to carbon are observed, due to its disordered structure as-confirmed by Raman analysis. Fig. 4 displays the Raman spectra of Fe3O4/C composites. Two peaks located

Fig. 3. XRD patterns of Fe3O4/C composites.

Fig. 4. Raman spectra of Fe3O4/C composites.

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at 217.2 and 278.9 cm1 are shown in all samples, which are assigned to the characteristic bands of Fe3O4 not Fe2O3 [47,48]. Two obvious peaks at around 1350 and 1580 cm1 correspond to graphitic peaks of D-band and G-band, respectively. D-band is related to the carbon vibration in disordered graphite, while Gband represents the vibration of sp2-bonded carbon atoms in 2Dgraphite [49,50]. Intensity ratio of D-band to G-band, ID/IG, manifests a value of 0.74, 0.76, 0.78, and 0.82 for Fe3O4/C-6, 8, 10, and 12 composite, respectively. The slightly increased ID/IG value as increasing the amounts of Fe3O4 demonstrates a higher amount of defects in the carbon structure, which can facilitate the diffusion of Liþ ion through the disordered region [51]. In order to confirm the carbon contents of Fe3O4/C composites, TG-DSC analysis was operated in the air flows from room temperature up to 700  C at a rate of 10  C/min. Fig. 5 displays the TG-DSC of Fe3O4/C composites. An obvious weight loss in the temperature range of 200e350  C, as well as two exothermic peaks in DSC curve, is exhibited for all the samples. The weight loss is due to the combustion of carbon and oxidation of Fe3O4 to Fe2O3 in air. Although ~3.4% mass increase occurs during the oxidation process from Fe3O4 to Fe2O3 (the mass of Fe3O4 and Fe2O3 is 77.2 and 79.8 g for 1 mol Fe element, respectively) [52], no obvious weight increase can be observed in the TG curve, which can be attributed to a dominant role of carbon combustion [53]. The first exothermic peak of 200e300  C in DSC curve is mainly arising from the carbon combustion, while the second peak from 300 to 350  C is due to the further decomposition of carbon radiuses. No weight loss as well as exo/endo-thermic peaks appear in TG-DSC curves for all the samples when temperature is more than 400  C, indicating the complete combustion of carbon and stable formation of Fe2O3. The reaction occurred in the temperature range of 200e350  C can be depicted as follows:

4 Fe3 O4 þ C þ 2 O2 ðgÞ/6 Fe2 O3 þ CO2 ðgÞ

(3)

The corresponding weight loss of Fe3O4/C-6, 8, 10, and 12 samples is 40.9%, 33.8%, 26.5%, and 23.4%, respectively. The mass ratio of carbon in Fe3O4/C-6, 8, 10, and 12 samples is calculated to be 42.9%, 36.0%, 29.0%, and 26.0%, respectively. N2 physisorption measurement was carried out to confirm the porous nature of Fe3O4/C composites. Fig. 6(a) and (b) show the N2 adsorption-desorption isotherms and pore size distribution of Fe3O4/C composites, respectively. In Fig. 6 (a), the typical N2-sorption isotherms with hysteresis loops at relative pressure of 0.6e0.9 are displayed, demonstrating the mesoporous characteristics of composites [39]. BET specific surface area of composites is increased in the order of Fe3O4/C-6, 8, 10, 12, showing the values of

Fig. 5. TG-DSC curves of Fe3O4/C composites.

10.8, 13.6, 15.8, 21.9 m2 g1, respectively. The increased BET is arising from the more pore volumes (Fig. 6 (b)) due to the release of more NO2 and NO gases during the combustion reaction (Equation. (2)). A narrow distribution of pore size based on Barrett-JoynerHalenda (BJH) method is observed in Fig. 6 (b), which manifests a pore diameter peak located at 5e10 nm. In addition, the pore volume is increased as increasing the amounts of Fe3O4, due to the enlarged reactive point between Fe(NO3)3 and cotton in the combustion process. The high BET and pore volume as well as nanopore size, provide the buffering spaces against the volume changes of Fe3O4 nanoparticles during the insertion/extraction of Li ions, and increase the contact areas between composites and additives, benefiting the cycling stability and Liþ ion diffusion for Fe3O4/C composites [27,54]. However, the increased specific surface area and pore volume can decrease the tap density of obtained electrode material, so as to decrease its volumetric capacity; furthermore, the increased specific surface area also raise the risk of side reaction during cycling such as the decomposition of electrolyte. Therefore, it is important to obtain a composite with optimized specific surface area, carbon content and tap density, so as to obtain both high gravimetric and volumetric capacity. Fig. 6 (c) displays the tap density of Fe3O4/C composites. All the powders were vacuum-dried for 24 h and oscillated for 30 min in the equipment for measuring tap density. A tap density is increased in the order of Fe3O4/C-6, 8, 10, and 12, displaying the value of 1.06, 1.18, 1.26, and 1.45 g cm3. The higher tap density is arising from the less carbon content and BET specific surface area. The increased tap density of Fe3O4/C composites is attributed to the predominant of carbon content over BET specific surface area. Fig. 7 shows the SEM images of Fe3O4/C composites together with the cotton-derived carbon before and after pulverization. The cotton-like morphology can be observed in the cotton-derived carbon before pulverization, displaying the diameter of <10 mm. After pulverization, the cotton-like fibers fragment the small parts. Such cotton-like morphologies are still maintained for all the Fe3O4/C composites after the combustion reaction. Fe3O4 particles and carbon are combined together to form the composites. The high counts of C in EDS shown in the inset of Fe3O4/C-8 sample confirm the presence of carbon. In order to clearly distinguish the Fe3O4 particles and carbon, TEM of Fe3O4/C-8 sample is given in Fig. 8. In Fig. 8 (a), nanosized Fe3O4 particles with the size of <10 nm are well-dispersed and embedded in carbon matrix, which is consistent with the Debye-Scherrer’s results in XRD. Electron diffraction pattern confirms the Fe3O4 structure in the selected area of Fig. 8 (b). In addition, a lattice space of 0.255, 0.256, 0.258 nm is observed in Fig. 8 (c), which is well assigned to the theoretical value of 0.254 nm for (311) plane of Fe3O4, evidencing the existence of Fe3O4. Electrochemical performance of Fe3O4/C composites was carried for evaluating possibility as anode materials. Fig. 9 (a) shows the cycling performance of Fe3O4/C composites. The calculation of specific capacity is based on the total amount of the Fe3O4/C composites. The Fe3O4/C-8 sample exhibits the best electrochemical performance, displaying the highest discharge capacity of 803 mAh g1 after 300 cycles at 0.4 A g1, which is attributed to the minimized agglomeration of Fe3O4 particles and buffered volume changes during the cycling process. This high discharge capacity of Fe3O4/C composite prepared by a facile and simple method can be comparable with the reported values shown in Table 1 [55e61]. However, Fe3O4/C-6 sample delivers the lowest discharge capacity, less than 372 mAh g1 of commercial graphite, probably due to its largest amounts of carbon. Fe3O4/C-8, 10 and 12 samples display the decreased discharge capacity below 50 cycles, owing to the lithiation-induced mechanical degradation and unstable formation of solid electrolyte interphase (SEI) [4,62]. As increasing the cycle

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Fig. 6. (a) Nitrogen adsorption-desorption isotherms, (b) pore size distribution, and (c) tap density of Fe3O4/C composites.

Fig. 7. SEM images of Fe3O4/C composites together with the cotton-derived carbon before and after pulverization.

Fig. 8. TEM of Fe3O4/C-8 sample.

numbers, reactivation of electrodes occurs and the increased storage capacity is observed, which is ascribed to the gradual formation of stable SEI [4,62]. Interestingly, this phenomenon is invisible for Fe3O4/C-6 sample due to the suppressed volume changes and mechanical degradation from excess carbon amounts. High volumetric capacity of anode material is critical for the practical applications. Fig. 9 (b) gives the volumetric capacity with the

dependence of cycle number for Fe3O4/C composites. A high volumetric capacity of 948 mAh cm3 after 300 cycles is delivered for Fe3O4/C-8 sample at 0.4 A g1, which is larger than the value of 558 mAh cm3 for commercial graphite (tap density of 1.5 g m3) [63]. Appropriate amounts of carbon in the Fe3O4/C composites can result in the best electrochemical performance.

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Fig. 9. Cycling performance of Fe3O4/C composites with gravimetric capacity (a) and volumetric capacity (b) at a current density of 0.4 A g1. The dish line corresponds to the graphite as comparison.

Table 1 Comparison of the reported and current values about the preparing method and discharge capacity. Materials

Preparing method

Discharge capacity (mAh g1)

Current density (A g1)

Ref.

Fe3O4/C composite N-doped urchin-like [email protected] Graphene [email protected] C-encapsulated nano- Fe3O4 Fe3O4 nanoparticle [email protected] nanosheets [email protected] nanocapsules Fe3O4 nanospheres

Cotton-assisted combustion synthesis Hydrothermal method Atomic layer deposition Hydrothermal method Base-catalyzed method De-alloying method Hydrothermal method Hydrothermal method

803 800 870 806 857 724 832 800

0.4 0.5 0.93 0.1 0.46 0.3 0.5 0.2

This work 55 56 57 58 59 60 61

Fig. 10 manifests the discharge-charge curves of Fe3O4/C composites. In the first cycle, a droped potential from 1.6 to 0.8 V is observed for all the samples, arising from the formation of intermediate LixFe3O4 [64]. The following plateau at 0.8 V appears, indicating the reduction reaction of Fe3þ and Fe2þ to Fe0 [65]. The plateau shifts to the higher potential in the second cycle, demonstrating the irreversible redox reaction due to the formation of SEI film as well as the decomposition of electrolyte [66]. As increasing the cycle number, the increased voltage difference between charge and discharge curves appears for all the samples, especially for Fe3O4/C-6 sample, indicating the increased polarization during the cycling. Fig. 11 displays the CV curves of Fe3O4/C composites in the voltage range of 0e3.0 V at a scan rate of 0.2 mV s1. The similar CV curves are observed for all the samples. During the first cathodic scan, a weak peak of 1.5 V is noticed, which is related to the irreversible reaction with electrolyte [52,67]. The tiny peak in the range of 0.75e1 V is shown in all the samples, indicating the formation of LixFe3O4 intermediate [4]. A sharp cathodic peak located at 0.49, 0.64, 0.59, 0.55 V is observed for Fe3O4/C-6, 8, 10, 12 sample, respectively, which is assigned to the reduction of LixFe3O4 to Fe

(300 (100 (500 (100 (200 (200 (150 (160

cycle) cycle) cycles) cycle) cycle) cycle) cycles) cycles)

and amorphous Li2O [39]. The lower value (0.49 V) of Fe3O4/C-6 sample than others is likely owing to the double protection of Fe3O4 by more carbon on both sides, which slows down the conversion of Fe3O4 upon Liþ insertion [68]. A reductive peak around 0.01 V is accredited to the intercalation of Li into carbon to form LiyC6 [39]. A wide anodic peak around 1.75 V corresponds to the transformation of Fe0 and Li2O to Fe3O4 [69]. In the subsequent cycles, both the cathodic and anodic peaks positively shift to a higher potential and the corresponding current peaks decrease, demonstrating the presence of irreversible redox reaction and formation of an SEI film on the surface during the first cycle [70,71]. The overlapped CV curves after the second cycle indicate a good reversibility and capacity stability due to the alleviated volume changes and agglomeration of Fe3O4 particles at the assist of carbon [72]. EIS was carried out to evaluate the excellent electrochemical performance and conductivity of Fe3O4/C composites. Fig. 12 shows the Nyquist plots of Fe3O4/C composites after 10 cycles at a constant current of 0.4 A g1. The corresponding equivalent circuit in the inset is used to fit the spectra. Rs, Rf, and Rct represent the resistance of ohm, Liþ ion diffusion in SEI film, and charge transfer,

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Fig. 10. Discharge-charge curves of Fe3O4/C composites.

Fig. 11. CV curves of Fe3O4/C composites in the voltage range of 0e3.0 V at a scan rate of 0.2 mV s1.

respectively. CPE1 and CPE2 are the SEI film and double-layer capacitance, respectively. CPE3, substituting finite Warburg element, is employed to properly fit the inclined line in the low frequency region [73e75], which is related to the Liþ ions diffusion in anode materials. The simulated Rf and Rct values by Zsimpwin Software are listed in Table 2. The lowest values of Rf ¼1.15 and Rct ¼1.47 U are obtained for Fe3O4/C-8 sample, indicating the lowest resistances thereby exhibiting the excellent electrochemical performance. The superior electrochemical performance can be attributed to the homogenously dispersed Fe3O4 nanoparticles by

appropriate amounts of carbon that can minimize the diffusion path of Liþ ions, buffer the volume changes, and alleviate the agglomeration of Fe3O4 nanoparticles during the cycling process [76]. 4. Conclusions A series of Fe3O4/C composites were prepared by a facile cottonassisted combustion reaction. The occurrence of combustion reaction was confirmed by TG-DSC, resulting in the formation of Fe3O4/

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Fig. 12. Nyquist plots of Fe3O4/C composites after 10 cycles at a current density of 0.4 A g1.

Table 2 Rf and Rct of Fe3O4/C composites after 10 cycles at a constant current of 0.4 A g1. Sample

Rf (U)

Rct (U)

Fe3O4/C-6 Fe3O4/C-8 Fe3O4/C-10 Fe3O4/C-12

5.30 1.15 2.15 2.33

7.20 1.47 3.11 5.51

C composites with the nanopores of 5e10 nm as well as the tap density of 1.06e1.45 g cm3. Fe3O4 nanoparticles with the size of <10 nm were well-embedded in the carbon matrix, giving rise to the excellent electrochemical performance. Fe3O4/C-8 sample with 36.0% carbon, exhibited a highest gravimetric capacity of 803 mAh g1 and volumetric capacity of 948 mAh cm3 after 300 cycles at the current of 0.4 A g1. The presence of carbon in composites was in favor of the well-dispersion and minimized agglomeration of Fe3O4 nanoparticles, high electronic conductivity, and the buffered volume changes during the Liþ ion insertion/extraction, thereby improving the electrochemical performance. The combustion reaction given in this study is promising for preparing other nanosized metal oxide/carbon composites as anode materials with high electrochemical performance. Acknowledgements This work is financed and supported partially by JSPS KAKENHI (JP15K18325). References [1] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Nanomaterials for rechargeable lithium batteries, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [2] C.M. Hayner, X. Zhao, H.H. Kung, Materials for rechargeable lithium-ion batteries, Annu. Rev. Chem. Biomol. Eng. 3 (2012) 445e471. [3] M. Lübke, N.M. Makwana, R. Gruar, C. Tighe, D. Brett, P. Shearing, Z. Liu, J.A. Darr, High capacity nanocomposite Fe3O4/Fe anodes for Li-ion batteries, J. Power Sources 291 (2015) 102e107. [4] K. Zhu, Y. Zhang, H. Qiu, Y. Meng, Y. Gao, X. Meng, Z. Gao, G. Chen, Y. Wei, Hierarchical Fe3O4 microsphere/reduced graphene oxide composites as a capable anode for lithium-ion batteries with remarkable cycling performance, J. Alloys Comp 675 (2016) 399e406. [5] H. Gu, D. Cao, J. Wang, X. Lu, Z. Li, C. Niu, H. Wang, Micro-CaCO3 conformal template synthesis of hierarchical porous carbon bricks: as a host for SnO2 nanoparticles with superior lithium storage performance, Mater. Today Energy 4 (2017) 75e80. [6] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition-metaloxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496e499.

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