- Email: [email protected]
C nanosheets for effective lithium-ion storage
Journal of Alloys and Compounds 691 (2017) 592e599
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Hierarchically structured Fe3O4/C nanosheets for effective lithium-ion storage Qiang Xin, Ligang Gai*, Yang Wang, Wanyong Ma, Haihui Jiang, Yan Tian Institute of Advanced Energy Materials and Chemistry, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2016 Received in revised form 20 May 2016 Accepted 31 August 2016 Available online 1 September 2016
Carbon-coated magnetite (Fe3O4/C) nanocomposites with hierarchical structure have been subjected to extensive research due to their relatively large specific surface area, extra free voids, and thus enhanced electrochemical performance for lithium-ion batteries (LIBs). In this paper, Fe3O4/C nanosheets with hierarchical structure in the form of carbon matrix supporting Fe3O4 nanocrystals have been prepared by annealing the nanosheet-like iron alkoxide precursor in N2 atmosphere. The Fe3O4/C samples are characterized with X-ray powder diffraction, Raman, scanning electron microscopy, transmission electron microscopy, N2 sorption isotherms, and thermogravimetricedifferential scanning calorimetric techniques, and further evaluated in the role of anode materials for LIBs. The specific discharge capacity of Fe3O4/C nanosheets obtained by annealing the precursor in N2 at 600 C for 3 h retains 647 and 546 mAh g1 after 100 cycles at 100 and 200 mA g1, respectively, exhibiting superior electrochemical performance to many other Fe3O4/C nanocomposites in previous reports. The Fe3O4/C nanosheets presented here not only enrich the nanomaterial community but also provide a promising candidate functioning as an anode material for effective lithium-ion storage. © 2016 Elsevier B.V. All rights reserved.
Keywords: Fe3O4/C nanosheets Hierarchical structure Electrochemical performance Anode material Lithium-ion batteries
1. Introduction Nanomaterials of transition metal oxides (TMOs) functioning as anode materials in lithium-ion batteries (LIBs) have been subjected to extensive research due to their improved safety and higher reversible capacities compared with commercial graphite [1e3]. Among the TMOs, magnetite (Fe3O4)-based nanomaterials have attracted particular interest because of their low cost, environmental friendliness, and high theoretical capacity (ca. 924 mAh g1) [4e17]. However, pristine Fe3O4 suffers from large irreversible capacity loss, low initial Coulombic efficiency, poor rate capability and cycling stability, severe volume variation (200%), and rapid dissolution/aggregation during the repeated lithiation/delithiation process [5,12]. This hinders its practical application in LIBs. So far, many approaches have been developed to prepare Fe3O4based nanomaterials with enhanced electrochemical performance [4e18]. Although individual Fe3O4 nanocrystals with unique crystal structure have been demonstrated to exhibit improved capacity and rate capability [10,11], the rate performance still remains
* Corresponding author. E-mail address: [email protected] (L. Gai). http://dx.doi.org/10.1016/j.jallcom.2016.08.331 0925-8388/© 2016 Elsevier B.V. All rights reserved.
insufficient to fully realize their potential in LIBs due to the lack of favorable electrical/ionic conductivity and the continuous growth of unstable solid electrolyte interface (SEI) films at the Fe3O4/ electrolyte interface during cycles [14,17]. A well-accepted strategy is combining Fe3O4 nanocrystals with coating layers of metal [18], metal oxide (e.g. Al2O3 [12]), iron carbide [5], carbon [4e8,13e17], and/or graphene [9,11,15] to offer magnetic nanocomposites with superior electrochemical performance, because the coating layers not only improve the electrochemical kinetics at the electrode/ electrolyte interface but also suppress particle aggregation and serve as buffer layers to improve the structural stability of the active materials [12,14,16]. Among the carbon-coated magnetic nanocomposites, hierarchical structures have received much attention because of their relatively large specific surface area which can provide more lithium storage sites and a large electrode/electrolyte contact area for high Li-ion flux across the interface [8,14]. Also, hierarchical structures provide extra free voids for alleviating the structural strain and accommodating the large volume variation during Li-ion insertion/extraction [19]. However, carbon coating in previous reports usually requires an additional step to deposit a layer of amorphous carbon on the surface of the pre-synthesized electroactive material [6,14]. This complicates the synthetic
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
procedure [16]. Therefore, it is more desirable to develop a simple approach for preparing hierarchically structured Fe3O4/C nanocomposites without adding any additional carbon source. In this paper, we report on a simple synthesis of hierarchically structured Fe3O4/C nanosheets. The synthetic procedure involves solvothermal synthesis of the iron alkoxide precursor, followed by annealing the precursor in N2 atmosphere. More importantly, the as-obtained hierarchical Fe3O4/C nanosheets exhibit superior electrochemical performance to many other Fe3O4/C nanocomposites in previous reports [4,6,7,20e22]. 2. Experimental 2.1. Synthesis of hierarchically structured Fe3O4/C nanosheets Fe3O4/C nanosheets with hierarchical structure were obtained by annealing an iron alkoxide precursor in N2 atmosphere. For the synthesis of the iron alkoxide precursor, 1 mmol iron (III) acetylacetonate and 5 mmol sodium acetate were added into a flask containing 30 mL of glycerol/ethanol mixed solution (glycerol/ ethanol ¼ 4:1, v/v). After vigorous agitation at room temperature for 0.5 h, the mixture was transferred into a Teflon-line autocalve with capacity of 35 mL. The autoclave was sealed and heated at 190 C for 12 h, and then allowed to cool to room temperature. The pale green precipitate was collected by centrifugation, washed with water and ethanol several times, and then dried in a vacuum oven. To obtain the hierarchical Fe3O4/C nanosheets, the dried precipitate was transferred into a tubular furnace and annealed in N2 atmosphere at 350 C for 2 h and then at 500e650 C for 3 h, with a heating rate of 2 C min1. The samples were named Fe3O4/C-T, where T denotes the annealing temperature. For comparison, carbon-coated Fe3O4 nanoparticle aggregates (hereafter referred to as Fe3O4/C-NP) were acquired by annealing the precursor at 600 C for 3 h, where the precursor was obtained by a solvothermal synthesis at 190 C for 1 h.
593
the adsorption data. Before recording the nitrogen sorption isotherms, the samples were degassed at 200 C for 6 h. 2.3. Electrochemical tests The electrochemical performance of the samples was evaluated by assembling CR2032 coin cells in configuration of Li metal()j electrolytejsample powders(þ) with organic electrolyte, using microporous polypropylene film (Celgard 2400) as the separator. The electrolyte was 1 mol L1 LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, v/v) solution (MTI Kejing Group, Hefei, China). The positive electrode was prepared by blade-coating homogeneously blended slurry onto a copper foil, and then dried in a vacuum oven at 80 C for 12 h. The slurry consists of 70 wt% active material, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) in N-methylpyrrolidone. The aluminium foil with dried coating was subjected to double rolling to make a smooth surface, and then cut into circular sheets with diameter of 12 mm. The mass of active material in a single electrode was in the range of 2e4 mg. The assembly of cells was completed in an Ar-filled glove box, in which oxygen and moisture were kept below 5 ppm. Cyclic voltammograms (CV) were measured on a CHI 660E electrochemical workstation (Shanghai CH Instruments Co., China). The applied potential range was 0.01e3 V for Fe3O4/C vs. Liþ/Li. Galvanostatic charge/discharge (GCD) curves of the coin cells were collected on a LANHE CT2001A battery tester (Wuhan Landian Co., China), at the current rates ranging from 50 to 1000 mA g1. The specific capacity of Fe3O4/C was calculated by inclusion of the mass of the carbon component. Alternating current (AC) electrochemical impedance spectroscopy (EIS) measurements were performed on the CHI 660E electrochemical workstation with AC amplitude of 5 mV, at a frequency range of 102e105 Hz. 3. Results and discussion
2.2. Characterization
3.1. Structure, composition, and morphology
X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with Cu Ka radiation (l ¼ 1.5406 Å), operating at 40 kV and 40 mA. Raman spectra were collected at room temperature on a Horiba Jobin Yvon laser confocal micro-Raman spectrometer with a 532 nm yttrium aluminum garnet (YAG) laser. Carbon (C), hydrogen (H), and nitrogen (N) elemental (CHN) analysis was performed on an Elementar Analysensysteme Vario EL III element analyzer. Scanning electron microscopy (SEM) images were taken on a FEI Quanta 200 and a Nova™ NanoSEM field-emission scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G20 high-resolution transmission electron microscope, operating at an accelerating voltage of 200 kV. Thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis were carried out on a Perkin-Elmer DSC2C thermogravimetric analyzer, operating at a heating rate of 10 C min1 in air and nitrogen, respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific ESCALAB 250 X-ray photoelectron spectrometer, using monochromatic Al Ka radiation (1486.6 eV). Binding energies for the high-resolution spectra were calibrated by setting C1s at 284.8 eV. Nitrogen sorption isotherms were collected at 77.3 K using a Micromeritics TriStar II 3020 sorption analyzer. The BrunauereEmmetteTeller (BET) method was utilized to calculate the specific surface areas (SBET). Pore size distributions (PSD) were determined by the BarretteJoynereHalenda (BJH) method, using
The composition and morphology of the iron alkoxide precursor were examined by the CHN, TG/DSC, and SEM techniques (Supplementary material, S1). The results show: (i) the formula of the precursor is Fe(C3H5O3); (ii) the iron alkoxide precursor is composed of microparticles constituted by crumpled nanosheets with thickness of ca. 60 nm, presenting a hierarchical structure (Fig. S2d); (iii) the tridentate chelation of glycerol towards Fe3þ plays an important role in the formation of sheet-like precursor. Subsequent anneal treatment in N2 atmosphere causes carbonization of the organic component in the precursor to form partially graphitic carbon layer, concomitant with partial reduction of Fe3þ to produce Fe3O4 nanocrystals [16], leading to the formation of hierarchically structured Fe3O4/C nanosheets. The crystallographic structure and phase purity of the Fe3O4/C samples were examined by the XRD technique (Fig. 1). The diffraction peaks marked in Fig. 1a can be indexed to (220), (311), (400), (511), and (440) planes of face-centered cubic Fe3O4 (JCPDS 88-0866). It can be discerned that the diffraction peaks from Fig. 1aed, corresponding separately to Fe3O4/C-650, Fe3O4/C-600, Fe3O4/C-550, and Fe3O4/C-500, gradually decreases in intensity. The result indicates that the degree of crystallinity of the samples is enhanced as the anneal temperature increases [23]. The gradual increase in intensity at the 2q range from 15 to 30 is related to partially graphitic carbon upon Fe3O4. This can be confirmed by the Raman spectra (Fig. 2), XPS spectra (Fig. 3), and the TEM images (Fig. 5) of the samples. It is worth noting that a peak centered at ca. 44.7 occurs in Fig. 1a. This peak corresponds to the (110) plane of
594
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
Fig. 1. XRD spectra of the samples: (a) Fe3O4/C-650; (b) Fe3O4/C-600; (c) Fe3O4/C-550; (d) Fe3O4/C-500; (e) Fe3O4/C-NP; the dot in (a) indicates the (110) plane of cubic Fe (JCPDS 87-0721).
Fig. 2. Raman spectra of: (a) Fe3O4/C-600; (b) Fe3O4/C-500; (c) Fe3O4/C-NP; inset is the magnification profile corresponding to spectrum (a) with range from 2000 to 3500 cm1.
cubic Fe (JCPDS 87-0721). The occurrence of Fe and/or FeO has also been observed in the XRD spectrum of Fe3O4/C, which was derived from annealing a Fe-containing alkoxide precursor in nitrogen atmosphere at 600 C for 3 h [24]. This is attributed to the deep reduction of Fe3þ to Fe0 in a carbon-thermal environment. However, there are no diffraction peaks corresponding to FeO or Fe in Fig. 1bed, indicating a high phase purity of Fe3O4 in the other four Fe3O4/C composites annealed at relatively lower temperatures. The coexistence of Fe3O4 and partially graphitic carbon in the magnetic composites can be revealed by the Raman spectra (Fig. 2). The weak peaks around 385, 507, and 680 cm1 arise from Eg, T2g, and A1g vibration modes of Fe3O4, respectively [11,25,26]. The peaks centered at 1336 and 1594 cm1 correspond separately to the fundamental D and G bands of partially graphitic carbon [9,25,26]. The weak and broad peak around 2500e3000 cm1 in Fig. 2a can be fitted into two peaks at 2676 and 2931 cm1 (Fig. 2, inset). The
former peak corresponds to the overtone of the D band (2D), and the latter corresponds to the (D þ G) band [9]. The peak intensity ratio between D and G bands (ID/IG) is considered a measure of the degree of graphitization of the carbon component, i.e. the lower the ratio of ID/IG, the higher the degree of graphitization [17]. In the present case, the ID/IG for Fe3O4/C-600 is 0.90, lower than that of 1.58 for Fe3O4/C-500 and 1.06 for Fe3O4/C-NP, indicating that an elevated temperature favors enhancement in degree of graphitization of the carbon component. Compared with Fe3O4/C-NP obtained at the same annealing temperature, Fe3O4/C-600 derived from the precursor constituted by nanosheets (Fig. S2d) benefits from an enhanced carbonization. This is attributed to the nanosheet precursor composed of crumpled sheets with thickness of ca. 60 nm (Fig. S2d), smaller than the size of nanoparticles (200 nm) constituting the nanoparticle precursor (Fig. S2a). The enhanced carbonization is beneficial to improve the electrical conductivity of Fe3O4/C-600 functioning as an electrode material (discussion below). To confirm the coexistence of Fe3O4 with partially graphitic carbon in Fe3O4/C composites, Fe3O4/C-600 was selected for further characterization with the XPS technique, as shown in Fig. 3. Elemental C, O, and Fe can be observed in the survey XPS spectrum (Fig. 3a). According to the XPS analysis, the concentration of carbon is estimated to be 47.8 at.% (Table S2), approximately amounting to 22.8 wt%. This value is higher than the weight percentage of carbon (11 wt%) for Fe3O4/C-600, as determined by the TG analysis (Fig. S4). This is due to the adventitious carbon contamination inevitably occurred in XPS measurements [27]. The high-resolution Fe 2p spectrum (Fig. 3b) exhibits two peaks centered at 724.3 and 710.6 eV, corresponding separately to Fe 2p1/2 and Fe 2p3/2 of Fe3O4 [28]. Also, there is no satellite peak at ca. 719 eV in Fig. 3b, confirming the existence of Fe3O4 rather than Fe2O3 in the composite sample [28]. Through Gaussian functions, the high-resolution C 1s spectrum (Fig. 3c) can be fitted into three bands centered at 284.8 (50.2% in total area), 285.8 (14.4%), and 288.5 (35.4%) eV, corresponding separately to C]C, CeC/CeO, and C]O components [29]. Likewise, three components centered at 530.1 (23%), 530.6 (40.4%), and 532.1 (36.6%) eV can be resolved in the O 1s spectrum (Fig. 3d). They are attributed to OeFe [30,31], O]C [29], and surface hydroxyls [29,31], respectively. The relatively high concentration of C]O component in the spectra of C 1s (Fig. 3c) and O 1s (Fig. 3d) is responsible for the relatively high atomic ratio of O/Fe (2.19) estimated by the XPS data (Table S2). Fig. 4 shows the SEM images of the samples. The Fe3O4/C composites retain the sheet-like morphology after anneal treatment (Fig. 4aec). The nanosheets are enchased by spherical nanoparticles with size ranging from 5 to 40 nm (Fig. 4d,e). The thickness of the sheets is in the range of 15e40 nm (Fig. 4d). Also, a large amount of pores especially mesopores are created due to discontinuity of the nanoparticles (Fig. 4d). However, Fe3O4/C-NP exhibits disordered nanoparticle aggregates with wide particle size distribution ranging from 10 to 300 nm (Fig. 4f and inset). The nanosheets constituted by spherical nanoparticles with narrow size distribution are also confirmed by the TEM image (Fig. 5). The lattice fringes marked in the high-resolution TEM image (Fig. 5b) with lattice spacing of ca. 0.253 and 0.242 nm correspond separately to d311 and d222 of cubic Fe3O4. The amorphous regions in Fig. 5b are attributed to carbon. The in situ generated carbon serves not only as a reducing agent to ensure the formation of Fe3O4 phase, but also as a physical barrier to suppress the growth of nanoparticles [24], producing Fe3O4/C nanosheets in the form of carbon layer matrix supporting Fe3O4 nanocrystals. Through the TG/DSC technique (Fig. S4), the weight percentage of the carbon
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
595
Fig. 3. XPS spectra of Fe3O4/C-600: (a) survey spectrum; (b) Fe 2p; (c) C 1s; (d) O 1s.
component in Fe3O4/C-600, Fe3O4/C-550, Fe3O4/C-500, and Fe3O4/ C-NP is estimated to be ca. 11%, 12.4%, 14%, and 15.3%, respectively.
features make the Fe3O4/C nanosheets a promising candidate in the role of anode materials for LIBs.
3.2. BET analysis
3.3. Electrochemical performance
Fig. 6 shows the N2 sorption isotherms and the PSD plots of the samples. The sorption isotherms (Fig. 6a) exhibit type-IV adsorption/desorption with type-H3 hysteresis loop according to BDDT classification [32]. The hysteresis loops occur in a wide relativepressure (P/P ) range of 0.44e0.98, suggesting the coexistence of mesopores (2e50 nm) and macropores (>50 nm) [17,26]. The bimodal characteristic is also reflected by the PSD plots (Fig. 6b), where mesopores with pore width smaller than 15 nm are dominant. The textural parameters derived from the isotherms are summarized in Table S3. It is found that the SBET of Fe3O4/C-600 (77.9 m2 g1) is close to that of Fe3O4/C-NP (86.4 m2 g1), but smaller than that of Fe3O4/C-550 (138.4 m2 g1) and Fe3O4/C-500 (143.1 m2 g1). However, the ratio of Spore/SBET for Fe3O4/C-600 (65%) is higher than that for Fe3O4/C-550 (59%) and Fe3O4/C-500 (58.6%), where Spore is the total area in pores. The coexistence of mesopores and macropores is beneficial to improve the electrochemical performance of electrode materials, because the pores not only facilitate electrolyte-ion diffusion to active sites with less resistance, but also tolerate the volume change of Fe3O4 during charge/discharge cycles [26]. On the basis of the above analysis, Fe3O4/C nanosheets have been achieved by annealing the iron alkoxide counterparts in N2 atmosphere. The sheet-like iron alkoxide counterparts in the form of Fe(C3H5O3) are obtained through a solvothermal synthesis at 190 C for 12 h, using glycerol/ethanol as the reaction medium without any surfactant. The Fe3O4/C nanosheets are featured with hierarchical structure in the form of carbon matrix supporting Fe3O4 nanocrystals, a moderate SBET, and a bimodal porosity. These
The electrochemical performance of the samples was tested with CV, GCD, and EIS techniques. Fig. 7a shows the CV curves of Fe3O4/C-600. In the first cathodic scan, the weak peak around 1.56 V is assigned to the irreversible reaction with electrolyte [25]. The weak peak around 0.78 V and the strong peak at 0.5 V correspond separately to the two steps of lithiation in Fe3O4 as represented by eqs (1) and (2), accompanying with the formation of SEI film on the electrode surface due to irreversible reactions [13,25,33]. Fe3O4 þ xLiþ þ xe / LixFe3O4
(1)
LixFe3O4 þ (8 x)Liþ þ (8 x)e / 3Fe0 þ 4Li2O
(2)
The conversion reaction potential of Fe3O4/C-600 (~0.5 V) is lower than that of bare Fe3O4 and Fe3O4/C nanocomposites (~0.65 V) [7,13], probably due to the double protection of Fe3O4 by carbon on both sides, a situation that slows down the conversion of Fe3O4 upon Liþ insertion [7]. In the first anodic scan, the broad peak centered at 1.62 V with a shoulder peak around 1.87 V can be attributed to reversible oxidation of Fe0 to Fe2þ/Fe3þ [7,9,14,25]. The imperceptible stepwise process of the anodic peak is due to kinetics effects [34]. In the second scans, both the cathodic and anodic peaks shift to higher voltage and the corresponding current peaks decrease, revealing the presence of certain degree of irreversibility of the redox reaction [9,25]. From the third cycle onwards, the cathodic peaks are merged into one reduction peak at 0.72 V, and the peak position, intensity, and integral area of the CV scans are nearly overlapped. This is attributed to the decrease in
596
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
Fig. 4. SEM images of: (a) Fe3O4/C-500; (b) Fe3O4/C-550; (c,d) Fe3O4/C-600; (e) size distribution of Fe3O4 in Fe3O4/C-600; (f) Fe3O4/C-NP.
Fig. 5. TEM images of Fe3O4/C-600: (a) low magnification; (b) high-resolution image corresponding to the squared area in (a).
electrochemical polarization of the electrode and the formation of a stable SEI film upon the active material [17,33]. Fig. 7b shows the GCD curves of Fe3O4/C-600 at 100 mA g1. The discharge capacity (Cd) of the first cycle is 1292 mAh g1, much
higher than the theoretical capacity of Fe3O4/C-600 (863 mAh g1) with 11 wt% carbon, based on the theoretical capacity of 924 mAh g1 for Fe3O4 [17] and 372 mAh g1 for graphitic carbon [35]. The first discharge curve exhibits a slope from open circuit
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
597
Fig. 6. (a) Sorption isotherms; (b) plots of BJH adsorption dV/dD pore volume vs. pore width.
Fig. 7. (a) CV curves of Fe3O4/C-600 at 0.1 mV s1; (b) GCD curves of Fe3O4/C-600 at 100 mA g1; (c) rate capability of Fe3O4/C-600; (d) cycle performance of Fe3O4/C-600 and Fe3O4/ C-NP; (e) Coulombic efficiency plots of Fe3O4/C-600; (f) Nyquist plots.
potential to 0.7 V, a long voltage plateau at ca. 0.7 V, and a subsequent slope down to 0.01 V. The former two parts correspond separately to the two steps of lithiation in Fe3O4 [13,33], as mentioned before. The latter is ascribed to lithium intercalation into carbon [13,20], as represented by eq (3) [13]:
C þ xLiþ þ xe / LixC
(3)
In the following discharge curves, there are sloping plateaus in the potential range of 1e0.75 V, higher than the voltage plateau of 0.7 V for the first cycle. Also, the charge curves move to relatively
598
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
higher potentials after the first cycle. The visible shifts to relatively higher potentials of the discharge and charge curves are attributed separately to the decreased resistance due to reduction of Fe3O4 and the polarization of the electrode [20]. The specific charge capacity of the first cycle is 801 mAh g1, rendering an initial Coulombic efficiency of 62%. The capacity loss is attributed to the formation of SEI film, decomposition of electrolyte, and formation of lithium organic compounds, which are common to most anode materials [5,10,17,31]. In the second and third cycles, the Coulombic efficiency dramatically increases to 97.1% and 99.0%, indicating a rapid stabilization of the SEI film [5]. Like Fe3O4/C-600, the Fe3O4/C-550, Fe3O4/C-500, and Fe3O4/CNP samples possess a relatively high initial Cd of 1067, 1229, and 1202 mAh g1 at 100 mA g1 and a relatively low Coulombic efficiency of 61%, 62%, and 61% in sequence (Fig. S5aec). However, the Cd of the three samples after 100 cycles retains separately 387, 357, and 386 mAh g1, much lower than that of 647 mAh g1 for Fe3O4/ C-600. This result indicates superior cycle performance of Fe3O4/C600 to that of Fe3O4/C-550, Fe3O4/C-500, and Fe3O4/C-NP. As mentioned before, the SBET of Fe3O4/C nanosheets decreases (Table S3) while the degree of graphitization of the carbon component increases (Fig. 2), as the anneal temperature increases. As a result, the superior electrochemical performance of Fe3O4/C600 is attributed to the enhanced degree of graphitization. This improves the electronic conductivity and thus reduces the charge transfer resistance (Fig. 7f, discussion below). For Fe3O4/C-650, the initial Cd is 861 mAh g1 at 100 mA g1, and the initial Coulombic efficiency is 45% (Fig. S5d). From the second to the 10th cycle, the Cd decreases to and stabilizes at 656 mAh g1. Considering that Fe3O4/ C-650 is doped with Fe, Fe3O4/C-600 was selected for further electrochemical tests. It is found that Fe3O4/C-600 delivers Cd of 742, 651, 572, 410, and 309 mAh g1 after every ten cycles at 50, 100, 200, 500, and 1000 mA g1, and resumes 729 mAh g1 when recycled at 50 mA g1 (Fig. 7c). This indicates a good rate capability. Fig. 7d shows the cycle performance of Fe3O4/C-600 and Fe3O4/CNP. When cycled at 200 mA g1, the Cd dramatically decreases to 617 from 977 mAh g1 in the first two cycles, and then slowly decreases and retains 546 mAh g1 after 100 cycles. Even cycled at 500 mA g1 for 100 cycles, the Cd remains 317 mAh g1. The Coulombic efficiency separately approximates 100% during the cycles at 200 and 500 mA g1, aside from the initial cycle (Fig. 7e). However, the Cd of Fe3O4/C-NP remains only 47 mAh g1 after 100 cycles at 200 mA g1. This result indicates superior cycle performance of Fe3O4/C-600 to that of Fe3O4/C-NP. Also, the electrochemical performance of Fe3O4/C-600 nanosheets is superior to that of Fe3O4/C nanocomposites [4,20,21], erythrocyte-like Fe3O4/C [6], C/Fe3O4 nanotubes [7], and Fe3O4/Fe/MWCNT [22] (Table 1). The superior electrochemical performance of Fe3O4/C-600 is due to that (i) the hierarchically structured nanosheets with relatively large SBET can provide sufficient electrode/electrolyte contact area for high Li-ion flux across the interface [8,14,16]; (ii) the porous two-dimensional (2D) nanosheet structure is able to shorten the Liion diffusion pathway and to tolerate the volume change of active materials [8,14,26]; (iii) the partially graphitic carbon layer not only improves the electronic conductivity and thus reduces the charge transfer resistance, but also positively contributes to the structural integrity of the electrode materials during the cycling process [16,26]. The structural integrity is reflected by the SEM image of Fe3O4/C-600 collected after 100 cycles at 100 mA g1, where the sheet-like structure can be discerned (Fig. S6). To clarify the difference in electrochemical performance of the samples, EIS spectra were collected as shown in Fig. 7f. The intercept at the real axis of impedance (Z0 ) reflects the total ohmic resistance of electrolyte and electrical contact (Re). The semicircle in the middle frequency region represents the charge transfer
Table 1 Comparison of the electrochemical performance of Fe3O4/C nanocomposites. Sample
Cycle number/ remnant Cd (mAh g1)
Current density (mA g1)
Ref.
Fe3O4/C-600 nanosheets Fe3O4/C-600 nanosheets Fe3O4/C-600 nanosheets Fe3O4/C nanocomposite erythrocyte-like Fe3O4/C C/Fe3O4/nanotubes Fe3O4/C nanocomposite Fe3O4/C nanocomposite Fe3O4/Fe/MWCNT
100/647 100/546 100/317 50/610 160/470 150/561 100/430 100/471 50/460
100 200 500 100 200 150 100 100 168
This work This work This work [4] [6] [7] [20] [21] [22]
resistance (Rct). The inclined line in the low frequency region indicates the Warburg impedance (Zw) associated with Li-ion diffusion in the electrode materials. The kinetics data of the samples are listed in Table 2. It is apparent that there is no distinct difference in Re value of the samples, indicating the cells work under consistent conditions [13]. However, the Rct of Fe3O4/C-600 is much smaller than that of the other samples, indicating rapid electron transfer during the electrochemical reaction. This is due to the enhanced degree of graphitization of the carbon component in Fe3O4/C-600, as reflected by the Raman spectra (Fig. 2). Also, the inclined line of Fe3O4/C-600 approaches nearer to the imaginary axis (Z00 ), suggesting an improvement of the Li-ion diffusion [25]. This result is associated with the porous 2D nanosheet structure of Fe3O4/C-600 (Fig. 4d). In addition, the electrode reaction kinetics can be reflected by the exchange current density (i0), which is calculated according to eq (4) [13]: i0 ¼ RT/nFRct
(4)
where R is the gas constant (8.314 J mol1 K1), T is the absolute temperature (298.15 K), n is the number of transferred electrons per Fe3O4 during lithiation, and F the Faraday constant (96,485 C mol1). In the case of Fe3O4/C-600, the value of i0 is much higher than that of the other Fe3O4/C samples (Table 2). This result indicates good electrode kinetics in Fe3O4/C-600, supporting its superior electrochemical performance. 4. Conclusion In summary, hierarchically structured Fe3O4/C nanosheets have been prepared by annealing the iron alkoixde counterparts in N2 atmosphere. The Fe3O4/C nanocomposites have been evaluated in the role of anode materials for LIBs. The scientific significance of this research lies in: (1) iron alkoxide nanosheets can be obtained through a solvothermal synthesis in a glycerol/ethanol reaction medium without adding any surfactant; (2) the Fe3O4/C nanosheets derived from the iron alkoixde counterparts are featured with hierarchical structure in the form of carbon matrix supporting Fe3O4 nanocrystals, an enhanced degree of graphitization of the carbon component, and a bimodal porosity; (3) the Cd of Fe3O4/C-600 can retain 647 and 546 mAh g1 after 100 cycles at 100 and 200 mA g1,
Table 2 Kinetics data derived from the EIS spectra. Sample
Re (U)
Rct (U)
i0 (A cm2)
Fe3O4/C-600 Fe3O4/C-550 Fe3O4/C-500 Fe3O4/C-NP
16.1 5.8 8.7 9.4
74.0 210.4 506.5 497.1
4.34 1.53 6.34 6.46
105 105 106 106
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
respectively, making it superior to Fe3O4/C-NP and many other Fe3O4/C nanocomposites in previous reports; (4) the Fe3O4/C-600 nanosheets presented here provide a promising candidate functioning as an anode material for effective Li-ion storage. Acknowledgments This research is financially supported by National Natural Science Foundation of China under Grant No. 51472278 and 51272143. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.08.331. References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496e499. [2] X. Xia, Y. Zhang, D. Chao, C. Guan, Y. Zhang, L. Li, X. Ge, I.M. Bacho, J. Tu, H.J. Fan, Solution synthesis of metal oxides for electrochemical energy storage applications, Nanoscale 6 (2014) 5008e5048. [3] W.W. Lee, J.M. Lee, Novel synthesis of high performance anode materials for lithium-ion batteries (LIBs), J. Mater. Chem. A 2 (2014) 1589e1626. [4] X.Y. Chen, B.H. Liu, Z.P. Li, Fe3O4/C composites synthesized from Fe-based xerogels for anode materials of Li-ion batteries, Solid State Ionics 261 (2014) 45e52. [5] J. Zhang, K. Wang, Q. Xu, Y. Zhou, F. Cheng, S. Guo, Beyond yolkeshell nanoparticles: Fe3O4@Fe3C core@shell nanoparticles as yolks and carbon nanospindles as shells for efficient lithium ion storage, ACS Nano 9 (2015) 3369e3376. [6] L. Wang, J. Liang, Y. Zhu, T. Mei, X. Zhang, Q. Yang, Y. Qian, Synthesis of Fe3O4@ C coreeshell nanorings and their enhanced electrochemical performance for lithium-ion batteries, Nanoscale 5 (2013) 3627e3631. [7] Q. Qu, J. Chem, X. Li, T. Gao, J. Shao, H. Zheng, Strongly coupled 1D sandwichlike C@Fe3O4@C coaxial nanotubes with ultrastable and high capacity for lithium-ion batteries, J. Mater. Chem. A 3 (2015) 18289e18295. [8] Y. Wan, Z. Yang, G. Xiong, R. Guo, Z. Liu, H. Luo, Anchoring Fe3O4 nanoparticles on three-dimensional carbon nanofibers toward flexible high-performance anodes for lithium-ion batteries, J. Power Sources 294 (2015) 414e419. [9] Y. Dong, K.C. Yung, R. Ma, X. Yang, Y.S. Chui, J.M. Lee, J.A. Zapien, Graphene/ acid assisted facile synthesis of structure-tuned Fe3O4 and graphene composites as anode materials for lithium ion batteries, Carbon 86 (2015) 310e317. [10] T. Xia, X. Xu, J. Wang, C. Xu, F. Meng, Z. Shi, J. Lian, J.M. Bassat, Facile complexcoprecipitation synthesis of mesoporous Fe3O4 nanocages and their high lithium storage capacity as anode material for lithium-ion batteries, Electrochimica Acta 160 (2015) 114e122. [11] J.S. Xu, Y.J. Zhu, Monodisperse Fe3O4 and g-Fe2O3 magnetic mesoporous microspheres as anode materials for lithium-ion batteries, ACS Appl. Mater. Interfaces 4 (2012) 4752e4757. [12] Q.H. Wu, B. Qu, J. Tang, C. Wang, D. Wang, Y. Li, J.G. Ren, An alumina-coated Fe3O4-reduced graphene oxide composite electrode as a stable anode for lithium-ion battery, Electrochimica Acta 156 (2015) 147e153. [13] F. Wu, R. Huang, D. Mu, B. Wu, S. Chen, New synthesis of a foamlike Fe3O4/C composite via a self-expanding process and its electrochemical performance as anode material for lithium-ion batteries, ACS Appl. Mater. Interfaces 6 (2014) 19254e19264. [14] X. Lin, J. Shao, X. Xiao, L. Chen, X. Wang, S. Li, H. Ge, Carbon encapsulated 3D hierarchical Fe3O4 spheres as advanced anode materials with long cycle lifetimes for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 14641e14648.
599
[15] Z. Zhang, F. Wang, Q. An, W. Li, P. Wu, Synthesis of graphene@ Fe3O4@C coreeshell nanosheets for high-performance lithium ion batteries, J. Mater. Chem. A 3 (2015) 7036e7043. [16] G. Gao, S. Lu, B. Dong, Z. Zhang, Y. Zheng, S. Ding, One-pot synthesis of carbon coated Fe3O4 nanosheets with superior lithium storage capability, J. Mater. Chem. A 3 (2015) 4716e4721. [17] C. He, S. Wu, N. Zhao, C. Shi, E. Liu, J. Li, Carbon-encapsulated Fe3O4 nanoparticles as a high-rate lithium ion battery anode material, ACS Nano 7 (2013) 4459e4469. [18] P.L. Taberna, S. Mitra, P. Poizot, P. Simon, J.M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nat. Mater 5 (2006) 567e573. [19] S. Xu, C.M. Hessel, H. Ren, R. Yu, Q. Jin, M. Yang, H. Zhao, D. Wang, a-Fe2O3 multi-shelled hollow microspheres for lithium ion battery anodes with superior capacity and charge retention, Energy Environ. Sci. 7 (2014) 632e737. [20] P. Wang, M. Gao, H. Pan, J. Zhang, C. Liang, J. Wang, P. Zhou, Y. Liu, A facile synthesis of Fe3O4/C composite with high cycle stability as anode material for lithium-ion batteries, J. Power Sources 239 (2013) 466e474. [21] M. Hu, Y. Jiang, M. Yan, Scalable synthesis of Fe3O4/C composites with enhanced electrochemical performance as anode materials for lithium-ion batteries, J. Alloy. Compd. 582 (2014) 563e568. [22] D.H. Lee, S.D. Seo, G.H. Lee, H.S. Hong, D.W. Kim, One-pot synthesis of Fe3O4/ Fe/MWCNT nanocomposites via electrical wire pulse for Li ion battery electrodes, J. Alloy. Compd. 606 (2014) 204e207. [23] F. Li, E. Shangguan, J. Li, L. Li, J. Yang, Z. Chang, Q. Li, X.Z. Yuan, H. Wang, Influence of annealing temperature on the structure and electrochemical performance of the Fe3O4 anode material for alkaline secondary batteries, Electrochimica Acta 178 (2015) 34e44. [24] S. Jin, H. Deng, D. Long, X. Liu, L. Zhan, X. Liang, W. Qiao, L. Ling, Facile synthesis of hierarchically structured Fe3O4/carbon micro-flowers and their application to lithium-ion battery anodes, J. Power Sources 196 (2011) 3887e3893. [25] Y. Dong, K. Md, Y.S. Chui, Y. Xia, C. Cao, J.M. Lee, J.A. Zapien, Synthesis of CNT@ Fe3O4-C hybrid nanocables as anode materials with enhanced electrochemical performance for lithium ion batteries, Electrochimica. Acta 176 (2015) 1332e1337. [26] G. Zhou, D.W. Wang, F. Li, L. Zhang, N. Li, Z.S. Wu, L. Wen, G.Q. Lu, H.M. Cheng, Graphene-wrapped Fe3O4 anode material with improved reversible capacity and cyclic stability for lithium ion batteries, Chem. Mater 22 (2010) 5306e5313. , I. Liascukiene, C. Me thivier, [27] J. Landoulsi, M.J. Genet, S. Fleith, Y. Toure P.G. Rouxhet, Organic adlayer on inorganic materials: XPS analysis selectivity to cope with adventitious contamination, Appl. Surf. Sci. 383 (2016) 71e83. [28] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2þ and Fe3þ ions in oxide materials, Appl. Surf. Sci. 253 (2008) 2441e2449. [29] Q. Niu, K. Gao, Z. Lin, W. Wu, Amine-capped carbon dots as a nanosensor for sensitive and selective detection of picric acid in aqueous solution via electrostatic interaction, Anal. Methods 5 (2013) 6228e6233. [30] K. Idczak, R. Idczak, R. Konieczny, An investigation of the corrosion of polycrystalline iron by XPS, TMS CEMS, Phys. B 491 (2016) 37e45. [31] G. Gao, Q. Zhang, K. Wang, H. Song, P. Qiu, D. Cui, Axial compressive a-Fe2O3 microdisks prepared from CSS template for potential anode materials of lithium ion batteries, Nano Energy 2 (2013) 1010e1018. rol, [32] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouque T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 87 (1985) 603e619. [33] X. Fan, S. Li, H. Zhou, L. Lu, One-pot high temperature hydrothermal synthesis of Fe3O4@C/graphene nanocomposite as anode for high rate lithium ion battery, Electrochimica Acta 180 (2015) 1041e1049. [34] J. Liu, J. Ni, Y. Zhao, H. Wang, L. Gao, Grapecluster-like Fe3O4@C/CNT nanostructures with stable Li-storage capability, J. Mater. Chem. A 1 (2013) 12879e12884. [35] M. Latorre-Sanchez, A. Primo, H. Garcia, Green synthesis of Fe3O4 nanoparticles embedded in a porous carbon matrix and its use as anode material in Li-ion batteries, J. Mater. Chem. 22 (2012) 21373e21375.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Hierarchically structured Fe3O4/C nanosheets for effective lithium-ion storage Qiang Xin, Ligang Gai*, Yang Wang, Wanyong Ma, Haihui Jiang, Yan Tian Institute of Advanced Energy Materials and Chemistry, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2016 Received in revised form 20 May 2016 Accepted 31 August 2016 Available online 1 September 2016
Carbon-coated magnetite (Fe3O4/C) nanocomposites with hierarchical structure have been subjected to extensive research due to their relatively large specific surface area, extra free voids, and thus enhanced electrochemical performance for lithium-ion batteries (LIBs). In this paper, Fe3O4/C nanosheets with hierarchical structure in the form of carbon matrix supporting Fe3O4 nanocrystals have been prepared by annealing the nanosheet-like iron alkoxide precursor in N2 atmosphere. The Fe3O4/C samples are characterized with X-ray powder diffraction, Raman, scanning electron microscopy, transmission electron microscopy, N2 sorption isotherms, and thermogravimetricedifferential scanning calorimetric techniques, and further evaluated in the role of anode materials for LIBs. The specific discharge capacity of Fe3O4/C nanosheets obtained by annealing the precursor in N2 at 600 C for 3 h retains 647 and 546 mAh g1 after 100 cycles at 100 and 200 mA g1, respectively, exhibiting superior electrochemical performance to many other Fe3O4/C nanocomposites in previous reports. The Fe3O4/C nanosheets presented here not only enrich the nanomaterial community but also provide a promising candidate functioning as an anode material for effective lithium-ion storage. © 2016 Elsevier B.V. All rights reserved.
Keywords: Fe3O4/C nanosheets Hierarchical structure Electrochemical performance Anode material Lithium-ion batteries
1. Introduction Nanomaterials of transition metal oxides (TMOs) functioning as anode materials in lithium-ion batteries (LIBs) have been subjected to extensive research due to their improved safety and higher reversible capacities compared with commercial graphite [1e3]. Among the TMOs, magnetite (Fe3O4)-based nanomaterials have attracted particular interest because of their low cost, environmental friendliness, and high theoretical capacity (ca. 924 mAh g1) [4e17]. However, pristine Fe3O4 suffers from large irreversible capacity loss, low initial Coulombic efficiency, poor rate capability and cycling stability, severe volume variation (200%), and rapid dissolution/aggregation during the repeated lithiation/delithiation process [5,12]. This hinders its practical application in LIBs. So far, many approaches have been developed to prepare Fe3O4based nanomaterials with enhanced electrochemical performance [4e18]. Although individual Fe3O4 nanocrystals with unique crystal structure have been demonstrated to exhibit improved capacity and rate capability [10,11], the rate performance still remains
* Corresponding author. E-mail address: [email protected] (L. Gai). http://dx.doi.org/10.1016/j.jallcom.2016.08.331 0925-8388/© 2016 Elsevier B.V. All rights reserved.
insufficient to fully realize their potential in LIBs due to the lack of favorable electrical/ionic conductivity and the continuous growth of unstable solid electrolyte interface (SEI) films at the Fe3O4/ electrolyte interface during cycles [14,17]. A well-accepted strategy is combining Fe3O4 nanocrystals with coating layers of metal [18], metal oxide (e.g. Al2O3 [12]), iron carbide [5], carbon [4e8,13e17], and/or graphene [9,11,15] to offer magnetic nanocomposites with superior electrochemical performance, because the coating layers not only improve the electrochemical kinetics at the electrode/ electrolyte interface but also suppress particle aggregation and serve as buffer layers to improve the structural stability of the active materials [12,14,16]. Among the carbon-coated magnetic nanocomposites, hierarchical structures have received much attention because of their relatively large specific surface area which can provide more lithium storage sites and a large electrode/electrolyte contact area for high Li-ion flux across the interface [8,14]. Also, hierarchical structures provide extra free voids for alleviating the structural strain and accommodating the large volume variation during Li-ion insertion/extraction [19]. However, carbon coating in previous reports usually requires an additional step to deposit a layer of amorphous carbon on the surface of the pre-synthesized electroactive material [6,14]. This complicates the synthetic
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
procedure [16]. Therefore, it is more desirable to develop a simple approach for preparing hierarchically structured Fe3O4/C nanocomposites without adding any additional carbon source. In this paper, we report on a simple synthesis of hierarchically structured Fe3O4/C nanosheets. The synthetic procedure involves solvothermal synthesis of the iron alkoxide precursor, followed by annealing the precursor in N2 atmosphere. More importantly, the as-obtained hierarchical Fe3O4/C nanosheets exhibit superior electrochemical performance to many other Fe3O4/C nanocomposites in previous reports [4,6,7,20e22]. 2. Experimental 2.1. Synthesis of hierarchically structured Fe3O4/C nanosheets Fe3O4/C nanosheets with hierarchical structure were obtained by annealing an iron alkoxide precursor in N2 atmosphere. For the synthesis of the iron alkoxide precursor, 1 mmol iron (III) acetylacetonate and 5 mmol sodium acetate were added into a flask containing 30 mL of glycerol/ethanol mixed solution (glycerol/ ethanol ¼ 4:1, v/v). After vigorous agitation at room temperature for 0.5 h, the mixture was transferred into a Teflon-line autocalve with capacity of 35 mL. The autoclave was sealed and heated at 190 C for 12 h, and then allowed to cool to room temperature. The pale green precipitate was collected by centrifugation, washed with water and ethanol several times, and then dried in a vacuum oven. To obtain the hierarchical Fe3O4/C nanosheets, the dried precipitate was transferred into a tubular furnace and annealed in N2 atmosphere at 350 C for 2 h and then at 500e650 C for 3 h, with a heating rate of 2 C min1. The samples were named Fe3O4/C-T, where T denotes the annealing temperature. For comparison, carbon-coated Fe3O4 nanoparticle aggregates (hereafter referred to as Fe3O4/C-NP) were acquired by annealing the precursor at 600 C for 3 h, where the precursor was obtained by a solvothermal synthesis at 190 C for 1 h.
593
the adsorption data. Before recording the nitrogen sorption isotherms, the samples were degassed at 200 C for 6 h. 2.3. Electrochemical tests The electrochemical performance of the samples was evaluated by assembling CR2032 coin cells in configuration of Li metal()j electrolytejsample powders(þ) with organic electrolyte, using microporous polypropylene film (Celgard 2400) as the separator. The electrolyte was 1 mol L1 LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, v/v) solution (MTI Kejing Group, Hefei, China). The positive electrode was prepared by blade-coating homogeneously blended slurry onto a copper foil, and then dried in a vacuum oven at 80 C for 12 h. The slurry consists of 70 wt% active material, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) in N-methylpyrrolidone. The aluminium foil with dried coating was subjected to double rolling to make a smooth surface, and then cut into circular sheets with diameter of 12 mm. The mass of active material in a single electrode was in the range of 2e4 mg. The assembly of cells was completed in an Ar-filled glove box, in which oxygen and moisture were kept below 5 ppm. Cyclic voltammograms (CV) were measured on a CHI 660E electrochemical workstation (Shanghai CH Instruments Co., China). The applied potential range was 0.01e3 V for Fe3O4/C vs. Liþ/Li. Galvanostatic charge/discharge (GCD) curves of the coin cells were collected on a LANHE CT2001A battery tester (Wuhan Landian Co., China), at the current rates ranging from 50 to 1000 mA g1. The specific capacity of Fe3O4/C was calculated by inclusion of the mass of the carbon component. Alternating current (AC) electrochemical impedance spectroscopy (EIS) measurements were performed on the CHI 660E electrochemical workstation with AC amplitude of 5 mV, at a frequency range of 102e105 Hz. 3. Results and discussion
2.2. Characterization
3.1. Structure, composition, and morphology
X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with Cu Ka radiation (l ¼ 1.5406 Å), operating at 40 kV and 40 mA. Raman spectra were collected at room temperature on a Horiba Jobin Yvon laser confocal micro-Raman spectrometer with a 532 nm yttrium aluminum garnet (YAG) laser. Carbon (C), hydrogen (H), and nitrogen (N) elemental (CHN) analysis was performed on an Elementar Analysensysteme Vario EL III element analyzer. Scanning electron microscopy (SEM) images were taken on a FEI Quanta 200 and a Nova™ NanoSEM field-emission scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G20 high-resolution transmission electron microscope, operating at an accelerating voltage of 200 kV. Thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis were carried out on a Perkin-Elmer DSC2C thermogravimetric analyzer, operating at a heating rate of 10 C min1 in air and nitrogen, respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific ESCALAB 250 X-ray photoelectron spectrometer, using monochromatic Al Ka radiation (1486.6 eV). Binding energies for the high-resolution spectra were calibrated by setting C1s at 284.8 eV. Nitrogen sorption isotherms were collected at 77.3 K using a Micromeritics TriStar II 3020 sorption analyzer. The BrunauereEmmetteTeller (BET) method was utilized to calculate the specific surface areas (SBET). Pore size distributions (PSD) were determined by the BarretteJoynereHalenda (BJH) method, using
The composition and morphology of the iron alkoxide precursor were examined by the CHN, TG/DSC, and SEM techniques (Supplementary material, S1). The results show: (i) the formula of the precursor is Fe(C3H5O3); (ii) the iron alkoxide precursor is composed of microparticles constituted by crumpled nanosheets with thickness of ca. 60 nm, presenting a hierarchical structure (Fig. S2d); (iii) the tridentate chelation of glycerol towards Fe3þ plays an important role in the formation of sheet-like precursor. Subsequent anneal treatment in N2 atmosphere causes carbonization of the organic component in the precursor to form partially graphitic carbon layer, concomitant with partial reduction of Fe3þ to produce Fe3O4 nanocrystals [16], leading to the formation of hierarchically structured Fe3O4/C nanosheets. The crystallographic structure and phase purity of the Fe3O4/C samples were examined by the XRD technique (Fig. 1). The diffraction peaks marked in Fig. 1a can be indexed to (220), (311), (400), (511), and (440) planes of face-centered cubic Fe3O4 (JCPDS 88-0866). It can be discerned that the diffraction peaks from Fig. 1aed, corresponding separately to Fe3O4/C-650, Fe3O4/C-600, Fe3O4/C-550, and Fe3O4/C-500, gradually decreases in intensity. The result indicates that the degree of crystallinity of the samples is enhanced as the anneal temperature increases [23]. The gradual increase in intensity at the 2q range from 15 to 30 is related to partially graphitic carbon upon Fe3O4. This can be confirmed by the Raman spectra (Fig. 2), XPS spectra (Fig. 3), and the TEM images (Fig. 5) of the samples. It is worth noting that a peak centered at ca. 44.7 occurs in Fig. 1a. This peak corresponds to the (110) plane of
594
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
Fig. 1. XRD spectra of the samples: (a) Fe3O4/C-650; (b) Fe3O4/C-600; (c) Fe3O4/C-550; (d) Fe3O4/C-500; (e) Fe3O4/C-NP; the dot in (a) indicates the (110) plane of cubic Fe (JCPDS 87-0721).
Fig. 2. Raman spectra of: (a) Fe3O4/C-600; (b) Fe3O4/C-500; (c) Fe3O4/C-NP; inset is the magnification profile corresponding to spectrum (a) with range from 2000 to 3500 cm1.
cubic Fe (JCPDS 87-0721). The occurrence of Fe and/or FeO has also been observed in the XRD spectrum of Fe3O4/C, which was derived from annealing a Fe-containing alkoxide precursor in nitrogen atmosphere at 600 C for 3 h [24]. This is attributed to the deep reduction of Fe3þ to Fe0 in a carbon-thermal environment. However, there are no diffraction peaks corresponding to FeO or Fe in Fig. 1bed, indicating a high phase purity of Fe3O4 in the other four Fe3O4/C composites annealed at relatively lower temperatures. The coexistence of Fe3O4 and partially graphitic carbon in the magnetic composites can be revealed by the Raman spectra (Fig. 2). The weak peaks around 385, 507, and 680 cm1 arise from Eg, T2g, and A1g vibration modes of Fe3O4, respectively [11,25,26]. The peaks centered at 1336 and 1594 cm1 correspond separately to the fundamental D and G bands of partially graphitic carbon [9,25,26]. The weak and broad peak around 2500e3000 cm1 in Fig. 2a can be fitted into two peaks at 2676 and 2931 cm1 (Fig. 2, inset). The
former peak corresponds to the overtone of the D band (2D), and the latter corresponds to the (D þ G) band [9]. The peak intensity ratio between D and G bands (ID/IG) is considered a measure of the degree of graphitization of the carbon component, i.e. the lower the ratio of ID/IG, the higher the degree of graphitization [17]. In the present case, the ID/IG for Fe3O4/C-600 is 0.90, lower than that of 1.58 for Fe3O4/C-500 and 1.06 for Fe3O4/C-NP, indicating that an elevated temperature favors enhancement in degree of graphitization of the carbon component. Compared with Fe3O4/C-NP obtained at the same annealing temperature, Fe3O4/C-600 derived from the precursor constituted by nanosheets (Fig. S2d) benefits from an enhanced carbonization. This is attributed to the nanosheet precursor composed of crumpled sheets with thickness of ca. 60 nm (Fig. S2d), smaller than the size of nanoparticles (200 nm) constituting the nanoparticle precursor (Fig. S2a). The enhanced carbonization is beneficial to improve the electrical conductivity of Fe3O4/C-600 functioning as an electrode material (discussion below). To confirm the coexistence of Fe3O4 with partially graphitic carbon in Fe3O4/C composites, Fe3O4/C-600 was selected for further characterization with the XPS technique, as shown in Fig. 3. Elemental C, O, and Fe can be observed in the survey XPS spectrum (Fig. 3a). According to the XPS analysis, the concentration of carbon is estimated to be 47.8 at.% (Table S2), approximately amounting to 22.8 wt%. This value is higher than the weight percentage of carbon (11 wt%) for Fe3O4/C-600, as determined by the TG analysis (Fig. S4). This is due to the adventitious carbon contamination inevitably occurred in XPS measurements [27]. The high-resolution Fe 2p spectrum (Fig. 3b) exhibits two peaks centered at 724.3 and 710.6 eV, corresponding separately to Fe 2p1/2 and Fe 2p3/2 of Fe3O4 [28]. Also, there is no satellite peak at ca. 719 eV in Fig. 3b, confirming the existence of Fe3O4 rather than Fe2O3 in the composite sample [28]. Through Gaussian functions, the high-resolution C 1s spectrum (Fig. 3c) can be fitted into three bands centered at 284.8 (50.2% in total area), 285.8 (14.4%), and 288.5 (35.4%) eV, corresponding separately to C]C, CeC/CeO, and C]O components [29]. Likewise, three components centered at 530.1 (23%), 530.6 (40.4%), and 532.1 (36.6%) eV can be resolved in the O 1s spectrum (Fig. 3d). They are attributed to OeFe [30,31], O]C [29], and surface hydroxyls [29,31], respectively. The relatively high concentration of C]O component in the spectra of C 1s (Fig. 3c) and O 1s (Fig. 3d) is responsible for the relatively high atomic ratio of O/Fe (2.19) estimated by the XPS data (Table S2). Fig. 4 shows the SEM images of the samples. The Fe3O4/C composites retain the sheet-like morphology after anneal treatment (Fig. 4aec). The nanosheets are enchased by spherical nanoparticles with size ranging from 5 to 40 nm (Fig. 4d,e). The thickness of the sheets is in the range of 15e40 nm (Fig. 4d). Also, a large amount of pores especially mesopores are created due to discontinuity of the nanoparticles (Fig. 4d). However, Fe3O4/C-NP exhibits disordered nanoparticle aggregates with wide particle size distribution ranging from 10 to 300 nm (Fig. 4f and inset). The nanosheets constituted by spherical nanoparticles with narrow size distribution are also confirmed by the TEM image (Fig. 5). The lattice fringes marked in the high-resolution TEM image (Fig. 5b) with lattice spacing of ca. 0.253 and 0.242 nm correspond separately to d311 and d222 of cubic Fe3O4. The amorphous regions in Fig. 5b are attributed to carbon. The in situ generated carbon serves not only as a reducing agent to ensure the formation of Fe3O4 phase, but also as a physical barrier to suppress the growth of nanoparticles [24], producing Fe3O4/C nanosheets in the form of carbon layer matrix supporting Fe3O4 nanocrystals. Through the TG/DSC technique (Fig. S4), the weight percentage of the carbon
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
595
Fig. 3. XPS spectra of Fe3O4/C-600: (a) survey spectrum; (b) Fe 2p; (c) C 1s; (d) O 1s.
component in Fe3O4/C-600, Fe3O4/C-550, Fe3O4/C-500, and Fe3O4/ C-NP is estimated to be ca. 11%, 12.4%, 14%, and 15.3%, respectively.
features make the Fe3O4/C nanosheets a promising candidate in the role of anode materials for LIBs.
3.2. BET analysis
3.3. Electrochemical performance
Fig. 6 shows the N2 sorption isotherms and the PSD plots of the samples. The sorption isotherms (Fig. 6a) exhibit type-IV adsorption/desorption with type-H3 hysteresis loop according to BDDT classification [32]. The hysteresis loops occur in a wide relativepressure (P/P ) range of 0.44e0.98, suggesting the coexistence of mesopores (2e50 nm) and macropores (>50 nm) [17,26]. The bimodal characteristic is also reflected by the PSD plots (Fig. 6b), where mesopores with pore width smaller than 15 nm are dominant. The textural parameters derived from the isotherms are summarized in Table S3. It is found that the SBET of Fe3O4/C-600 (77.9 m2 g1) is close to that of Fe3O4/C-NP (86.4 m2 g1), but smaller than that of Fe3O4/C-550 (138.4 m2 g1) and Fe3O4/C-500 (143.1 m2 g1). However, the ratio of Spore/SBET for Fe3O4/C-600 (65%) is higher than that for Fe3O4/C-550 (59%) and Fe3O4/C-500 (58.6%), where Spore is the total area in pores. The coexistence of mesopores and macropores is beneficial to improve the electrochemical performance of electrode materials, because the pores not only facilitate electrolyte-ion diffusion to active sites with less resistance, but also tolerate the volume change of Fe3O4 during charge/discharge cycles [26]. On the basis of the above analysis, Fe3O4/C nanosheets have been achieved by annealing the iron alkoxide counterparts in N2 atmosphere. The sheet-like iron alkoxide counterparts in the form of Fe(C3H5O3) are obtained through a solvothermal synthesis at 190 C for 12 h, using glycerol/ethanol as the reaction medium without any surfactant. The Fe3O4/C nanosheets are featured with hierarchical structure in the form of carbon matrix supporting Fe3O4 nanocrystals, a moderate SBET, and a bimodal porosity. These
The electrochemical performance of the samples was tested with CV, GCD, and EIS techniques. Fig. 7a shows the CV curves of Fe3O4/C-600. In the first cathodic scan, the weak peak around 1.56 V is assigned to the irreversible reaction with electrolyte [25]. The weak peak around 0.78 V and the strong peak at 0.5 V correspond separately to the two steps of lithiation in Fe3O4 as represented by eqs (1) and (2), accompanying with the formation of SEI film on the electrode surface due to irreversible reactions [13,25,33]. Fe3O4 þ xLiþ þ xe / LixFe3O4
(1)
LixFe3O4 þ (8 x)Liþ þ (8 x)e / 3Fe0 þ 4Li2O
(2)
The conversion reaction potential of Fe3O4/C-600 (~0.5 V) is lower than that of bare Fe3O4 and Fe3O4/C nanocomposites (~0.65 V) [7,13], probably due to the double protection of Fe3O4 by carbon on both sides, a situation that slows down the conversion of Fe3O4 upon Liþ insertion [7]. In the first anodic scan, the broad peak centered at 1.62 V with a shoulder peak around 1.87 V can be attributed to reversible oxidation of Fe0 to Fe2þ/Fe3þ [7,9,14,25]. The imperceptible stepwise process of the anodic peak is due to kinetics effects [34]. In the second scans, both the cathodic and anodic peaks shift to higher voltage and the corresponding current peaks decrease, revealing the presence of certain degree of irreversibility of the redox reaction [9,25]. From the third cycle onwards, the cathodic peaks are merged into one reduction peak at 0.72 V, and the peak position, intensity, and integral area of the CV scans are nearly overlapped. This is attributed to the decrease in
596
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
Fig. 4. SEM images of: (a) Fe3O4/C-500; (b) Fe3O4/C-550; (c,d) Fe3O4/C-600; (e) size distribution of Fe3O4 in Fe3O4/C-600; (f) Fe3O4/C-NP.
Fig. 5. TEM images of Fe3O4/C-600: (a) low magnification; (b) high-resolution image corresponding to the squared area in (a).
electrochemical polarization of the electrode and the formation of a stable SEI film upon the active material [17,33]. Fig. 7b shows the GCD curves of Fe3O4/C-600 at 100 mA g1. The discharge capacity (Cd) of the first cycle is 1292 mAh g1, much
higher than the theoretical capacity of Fe3O4/C-600 (863 mAh g1) with 11 wt% carbon, based on the theoretical capacity of 924 mAh g1 for Fe3O4 [17] and 372 mAh g1 for graphitic carbon [35]. The first discharge curve exhibits a slope from open circuit
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
597
Fig. 6. (a) Sorption isotherms; (b) plots of BJH adsorption dV/dD pore volume vs. pore width.
Fig. 7. (a) CV curves of Fe3O4/C-600 at 0.1 mV s1; (b) GCD curves of Fe3O4/C-600 at 100 mA g1; (c) rate capability of Fe3O4/C-600; (d) cycle performance of Fe3O4/C-600 and Fe3O4/ C-NP; (e) Coulombic efficiency plots of Fe3O4/C-600; (f) Nyquist plots.
potential to 0.7 V, a long voltage plateau at ca. 0.7 V, and a subsequent slope down to 0.01 V. The former two parts correspond separately to the two steps of lithiation in Fe3O4 [13,33], as mentioned before. The latter is ascribed to lithium intercalation into carbon [13,20], as represented by eq (3) [13]:
C þ xLiþ þ xe / LixC
(3)
In the following discharge curves, there are sloping plateaus in the potential range of 1e0.75 V, higher than the voltage plateau of 0.7 V for the first cycle. Also, the charge curves move to relatively
598
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
higher potentials after the first cycle. The visible shifts to relatively higher potentials of the discharge and charge curves are attributed separately to the decreased resistance due to reduction of Fe3O4 and the polarization of the electrode [20]. The specific charge capacity of the first cycle is 801 mAh g1, rendering an initial Coulombic efficiency of 62%. The capacity loss is attributed to the formation of SEI film, decomposition of electrolyte, and formation of lithium organic compounds, which are common to most anode materials [5,10,17,31]. In the second and third cycles, the Coulombic efficiency dramatically increases to 97.1% and 99.0%, indicating a rapid stabilization of the SEI film [5]. Like Fe3O4/C-600, the Fe3O4/C-550, Fe3O4/C-500, and Fe3O4/CNP samples possess a relatively high initial Cd of 1067, 1229, and 1202 mAh g1 at 100 mA g1 and a relatively low Coulombic efficiency of 61%, 62%, and 61% in sequence (Fig. S5aec). However, the Cd of the three samples after 100 cycles retains separately 387, 357, and 386 mAh g1, much lower than that of 647 mAh g1 for Fe3O4/ C-600. This result indicates superior cycle performance of Fe3O4/C600 to that of Fe3O4/C-550, Fe3O4/C-500, and Fe3O4/C-NP. As mentioned before, the SBET of Fe3O4/C nanosheets decreases (Table S3) while the degree of graphitization of the carbon component increases (Fig. 2), as the anneal temperature increases. As a result, the superior electrochemical performance of Fe3O4/C600 is attributed to the enhanced degree of graphitization. This improves the electronic conductivity and thus reduces the charge transfer resistance (Fig. 7f, discussion below). For Fe3O4/C-650, the initial Cd is 861 mAh g1 at 100 mA g1, and the initial Coulombic efficiency is 45% (Fig. S5d). From the second to the 10th cycle, the Cd decreases to and stabilizes at 656 mAh g1. Considering that Fe3O4/ C-650 is doped with Fe, Fe3O4/C-600 was selected for further electrochemical tests. It is found that Fe3O4/C-600 delivers Cd of 742, 651, 572, 410, and 309 mAh g1 after every ten cycles at 50, 100, 200, 500, and 1000 mA g1, and resumes 729 mAh g1 when recycled at 50 mA g1 (Fig. 7c). This indicates a good rate capability. Fig. 7d shows the cycle performance of Fe3O4/C-600 and Fe3O4/CNP. When cycled at 200 mA g1, the Cd dramatically decreases to 617 from 977 mAh g1 in the first two cycles, and then slowly decreases and retains 546 mAh g1 after 100 cycles. Even cycled at 500 mA g1 for 100 cycles, the Cd remains 317 mAh g1. The Coulombic efficiency separately approximates 100% during the cycles at 200 and 500 mA g1, aside from the initial cycle (Fig. 7e). However, the Cd of Fe3O4/C-NP remains only 47 mAh g1 after 100 cycles at 200 mA g1. This result indicates superior cycle performance of Fe3O4/C-600 to that of Fe3O4/C-NP. Also, the electrochemical performance of Fe3O4/C-600 nanosheets is superior to that of Fe3O4/C nanocomposites [4,20,21], erythrocyte-like Fe3O4/C [6], C/Fe3O4 nanotubes [7], and Fe3O4/Fe/MWCNT [22] (Table 1). The superior electrochemical performance of Fe3O4/C-600 is due to that (i) the hierarchically structured nanosheets with relatively large SBET can provide sufficient electrode/electrolyte contact area for high Li-ion flux across the interface [8,14,16]; (ii) the porous two-dimensional (2D) nanosheet structure is able to shorten the Liion diffusion pathway and to tolerate the volume change of active materials [8,14,26]; (iii) the partially graphitic carbon layer not only improves the electronic conductivity and thus reduces the charge transfer resistance, but also positively contributes to the structural integrity of the electrode materials during the cycling process [16,26]. The structural integrity is reflected by the SEM image of Fe3O4/C-600 collected after 100 cycles at 100 mA g1, where the sheet-like structure can be discerned (Fig. S6). To clarify the difference in electrochemical performance of the samples, EIS spectra were collected as shown in Fig. 7f. The intercept at the real axis of impedance (Z0 ) reflects the total ohmic resistance of electrolyte and electrical contact (Re). The semicircle in the middle frequency region represents the charge transfer
Table 1 Comparison of the electrochemical performance of Fe3O4/C nanocomposites. Sample
Cycle number/ remnant Cd (mAh g1)
Current density (mA g1)
Ref.
Fe3O4/C-600 nanosheets Fe3O4/C-600 nanosheets Fe3O4/C-600 nanosheets Fe3O4/C nanocomposite erythrocyte-like Fe3O4/C C/Fe3O4/nanotubes Fe3O4/C nanocomposite Fe3O4/C nanocomposite Fe3O4/Fe/MWCNT
100/647 100/546 100/317 50/610 160/470 150/561 100/430 100/471 50/460
100 200 500 100 200 150 100 100 168
This work This work This work [4] [6] [7] [20] [21] [22]
resistance (Rct). The inclined line in the low frequency region indicates the Warburg impedance (Zw) associated with Li-ion diffusion in the electrode materials. The kinetics data of the samples are listed in Table 2. It is apparent that there is no distinct difference in Re value of the samples, indicating the cells work under consistent conditions [13]. However, the Rct of Fe3O4/C-600 is much smaller than that of the other samples, indicating rapid electron transfer during the electrochemical reaction. This is due to the enhanced degree of graphitization of the carbon component in Fe3O4/C-600, as reflected by the Raman spectra (Fig. 2). Also, the inclined line of Fe3O4/C-600 approaches nearer to the imaginary axis (Z00 ), suggesting an improvement of the Li-ion diffusion [25]. This result is associated with the porous 2D nanosheet structure of Fe3O4/C-600 (Fig. 4d). In addition, the electrode reaction kinetics can be reflected by the exchange current density (i0), which is calculated according to eq (4) [13]: i0 ¼ RT/nFRct
(4)
where R is the gas constant (8.314 J mol1 K1), T is the absolute temperature (298.15 K), n is the number of transferred electrons per Fe3O4 during lithiation, and F the Faraday constant (96,485 C mol1). In the case of Fe3O4/C-600, the value of i0 is much higher than that of the other Fe3O4/C samples (Table 2). This result indicates good electrode kinetics in Fe3O4/C-600, supporting its superior electrochemical performance. 4. Conclusion In summary, hierarchically structured Fe3O4/C nanosheets have been prepared by annealing the iron alkoixde counterparts in N2 atmosphere. The Fe3O4/C nanocomposites have been evaluated in the role of anode materials for LIBs. The scientific significance of this research lies in: (1) iron alkoxide nanosheets can be obtained through a solvothermal synthesis in a glycerol/ethanol reaction medium without adding any surfactant; (2) the Fe3O4/C nanosheets derived from the iron alkoixde counterparts are featured with hierarchical structure in the form of carbon matrix supporting Fe3O4 nanocrystals, an enhanced degree of graphitization of the carbon component, and a bimodal porosity; (3) the Cd of Fe3O4/C-600 can retain 647 and 546 mAh g1 after 100 cycles at 100 and 200 mA g1,
Table 2 Kinetics data derived from the EIS spectra. Sample
Re (U)
Rct (U)
i0 (A cm2)
Fe3O4/C-600 Fe3O4/C-550 Fe3O4/C-500 Fe3O4/C-NP
16.1 5.8 8.7 9.4
74.0 210.4 506.5 497.1
4.34 1.53 6.34 6.46
105 105 106 106
Q. Xin et al. / Journal of Alloys and Compounds 691 (2017) 592e599
respectively, making it superior to Fe3O4/C-NP and many other Fe3O4/C nanocomposites in previous reports; (4) the Fe3O4/C-600 nanosheets presented here provide a promising candidate functioning as an anode material for effective Li-ion storage. Acknowledgments This research is financially supported by National Natural Science Foundation of China under Grant No. 51472278 and 51272143. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.08.331. References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496e499. [2] X. Xia, Y. Zhang, D. Chao, C. Guan, Y. Zhang, L. Li, X. Ge, I.M. Bacho, J. Tu, H.J. Fan, Solution synthesis of metal oxides for electrochemical energy storage applications, Nanoscale 6 (2014) 5008e5048. [3] W.W. Lee, J.M. Lee, Novel synthesis of high performance anode materials for lithium-ion batteries (LIBs), J. Mater. Chem. A 2 (2014) 1589e1626. [4] X.Y. Chen, B.H. Liu, Z.P. Li, Fe3O4/C composites synthesized from Fe-based xerogels for anode materials of Li-ion batteries, Solid State Ionics 261 (2014) 45e52. [5] J. Zhang, K. Wang, Q. Xu, Y. Zhou, F. Cheng, S. Guo, Beyond yolkeshell nanoparticles: Fe3O4@Fe3C core@shell nanoparticles as yolks and carbon nanospindles as shells for efficient lithium ion storage, ACS Nano 9 (2015) 3369e3376. [6] L. Wang, J. Liang, Y. Zhu, T. Mei, X. Zhang, Q. Yang, Y. Qian, Synthesis of Fe3O4@ C coreeshell nanorings and their enhanced electrochemical performance for lithium-ion batteries, Nanoscale 5 (2013) 3627e3631. [7] Q. Qu, J. Chem, X. Li, T. Gao, J. Shao, H. Zheng, Strongly coupled 1D sandwichlike C@Fe3O4@C coaxial nanotubes with ultrastable and high capacity for lithium-ion batteries, J. Mater. Chem. A 3 (2015) 18289e18295. [8] Y. Wan, Z. Yang, G. Xiong, R. Guo, Z. Liu, H. Luo, Anchoring Fe3O4 nanoparticles on three-dimensional carbon nanofibers toward flexible high-performance anodes for lithium-ion batteries, J. Power Sources 294 (2015) 414e419. [9] Y. Dong, K.C. Yung, R. Ma, X. Yang, Y.S. Chui, J.M. Lee, J.A. Zapien, Graphene/ acid assisted facile synthesis of structure-tuned Fe3O4 and graphene composites as anode materials for lithium ion batteries, Carbon 86 (2015) 310e317. [10] T. Xia, X. Xu, J. Wang, C. Xu, F. Meng, Z. Shi, J. Lian, J.M. Bassat, Facile complexcoprecipitation synthesis of mesoporous Fe3O4 nanocages and their high lithium storage capacity as anode material for lithium-ion batteries, Electrochimica Acta 160 (2015) 114e122. [11] J.S. Xu, Y.J. Zhu, Monodisperse Fe3O4 and g-Fe2O3 magnetic mesoporous microspheres as anode materials for lithium-ion batteries, ACS Appl. Mater. Interfaces 4 (2012) 4752e4757. [12] Q.H. Wu, B. Qu, J. Tang, C. Wang, D. Wang, Y. Li, J.G. Ren, An alumina-coated Fe3O4-reduced graphene oxide composite electrode as a stable anode for lithium-ion battery, Electrochimica Acta 156 (2015) 147e153. [13] F. Wu, R. Huang, D. Mu, B. Wu, S. Chen, New synthesis of a foamlike Fe3O4/C composite via a self-expanding process and its electrochemical performance as anode material for lithium-ion batteries, ACS Appl. Mater. Interfaces 6 (2014) 19254e19264. [14] X. Lin, J. Shao, X. Xiao, L. Chen, X. Wang, S. Li, H. Ge, Carbon encapsulated 3D hierarchical Fe3O4 spheres as advanced anode materials with long cycle lifetimes for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 14641e14648.
599
[15] Z. Zhang, F. Wang, Q. An, W. Li, P. Wu, Synthesis of graphene@ Fe3O4@C coreeshell nanosheets for high-performance lithium ion batteries, J. Mater. Chem. A 3 (2015) 7036e7043. [16] G. Gao, S. Lu, B. Dong, Z. Zhang, Y. Zheng, S. Ding, One-pot synthesis of carbon coated Fe3O4 nanosheets with superior lithium storage capability, J. Mater. Chem. A 3 (2015) 4716e4721. [17] C. He, S. Wu, N. Zhao, C. Shi, E. Liu, J. Li, Carbon-encapsulated Fe3O4 nanoparticles as a high-rate lithium ion battery anode material, ACS Nano 7 (2013) 4459e4469. [18] P.L. Taberna, S. Mitra, P. Poizot, P. Simon, J.M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nat. Mater 5 (2006) 567e573. [19] S. Xu, C.M. Hessel, H. Ren, R. Yu, Q. Jin, M. Yang, H. Zhao, D. Wang, a-Fe2O3 multi-shelled hollow microspheres for lithium ion battery anodes with superior capacity and charge retention, Energy Environ. Sci. 7 (2014) 632e737. [20] P. Wang, M. Gao, H. Pan, J. Zhang, C. Liang, J. Wang, P. Zhou, Y. Liu, A facile synthesis of Fe3O4/C composite with high cycle stability as anode material for lithium-ion batteries, J. Power Sources 239 (2013) 466e474. [21] M. Hu, Y. Jiang, M. Yan, Scalable synthesis of Fe3O4/C composites with enhanced electrochemical performance as anode materials for lithium-ion batteries, J. Alloy. Compd. 582 (2014) 563e568. [22] D.H. Lee, S.D. Seo, G.H. Lee, H.S. Hong, D.W. Kim, One-pot synthesis of Fe3O4/ Fe/MWCNT nanocomposites via electrical wire pulse for Li ion battery electrodes, J. Alloy. Compd. 606 (2014) 204e207. [23] F. Li, E. Shangguan, J. Li, L. Li, J. Yang, Z. Chang, Q. Li, X.Z. Yuan, H. Wang, Influence of annealing temperature on the structure and electrochemical performance of the Fe3O4 anode material for alkaline secondary batteries, Electrochimica Acta 178 (2015) 34e44. [24] S. Jin, H. Deng, D. Long, X. Liu, L. Zhan, X. Liang, W. Qiao, L. Ling, Facile synthesis of hierarchically structured Fe3O4/carbon micro-flowers and their application to lithium-ion battery anodes, J. Power Sources 196 (2011) 3887e3893. [25] Y. Dong, K. Md, Y.S. Chui, Y. Xia, C. Cao, J.M. Lee, J.A. Zapien, Synthesis of CNT@ Fe3O4-C hybrid nanocables as anode materials with enhanced electrochemical performance for lithium ion batteries, Electrochimica. Acta 176 (2015) 1332e1337. [26] G. Zhou, D.W. Wang, F. Li, L. Zhang, N. Li, Z.S. Wu, L. Wen, G.Q. Lu, H.M. Cheng, Graphene-wrapped Fe3O4 anode material with improved reversible capacity and cyclic stability for lithium ion batteries, Chem. Mater 22 (2010) 5306e5313. , I. Liascukiene, C. Me thivier, [27] J. Landoulsi, M.J. Genet, S. Fleith, Y. Toure P.G. Rouxhet, Organic adlayer on inorganic materials: XPS analysis selectivity to cope with adventitious contamination, Appl. Surf. Sci. 383 (2016) 71e83. [28] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2þ and Fe3þ ions in oxide materials, Appl. Surf. Sci. 253 (2008) 2441e2449. [29] Q. Niu, K. Gao, Z. Lin, W. Wu, Amine-capped carbon dots as a nanosensor for sensitive and selective detection of picric acid in aqueous solution via electrostatic interaction, Anal. Methods 5 (2013) 6228e6233. [30] K. Idczak, R. Idczak, R. Konieczny, An investigation of the corrosion of polycrystalline iron by XPS, TMS CEMS, Phys. B 491 (2016) 37e45. [31] G. Gao, Q. Zhang, K. Wang, H. Song, P. Qiu, D. Cui, Axial compressive a-Fe2O3 microdisks prepared from CSS template for potential anode materials of lithium ion batteries, Nano Energy 2 (2013) 1010e1018. rol, [32] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouque T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 87 (1985) 603e619. [33] X. Fan, S. Li, H. Zhou, L. Lu, One-pot high temperature hydrothermal synthesis of Fe3O4@C/graphene nanocomposite as anode for high rate lithium ion battery, Electrochimica Acta 180 (2015) 1041e1049. [34] J. Liu, J. Ni, Y. Zhao, H. Wang, L. Gao, Grapecluster-like Fe3O4@C/CNT nanostructures with stable Li-storage capability, J. Mater. Chem. A 1 (2013) 12879e12884. [35] M. Latorre-Sanchez, A. Primo, H. Garcia, Green synthesis of Fe3O4 nanoparticles embedded in a porous carbon matrix and its use as anode material in Li-ion batteries, J. Mater. Chem. 22 (2012) 21373e21375.