nanostructures for enhanced lithium storage performance

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Strongly coupled N-doped carbon/Fe3 O4 /N-doped carbon hierarchical micro/nanostructures for enhanced lithium storage performance Tiantian Ma a, Xianghong Liu a,b,∗, Li Sun a, Yongshan Xu a, Lingli Zheng a, Jun Zhang a,b,∗

Q1

a b

College of Physics, Qingdao University, Qingdao 266071, Shandong, China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 14 August 2018 Revised 12 September 2018 Accepted 22 September 2018 Available online xxx Keywords: Iron oxide Micro/nanostructures Carbon tubes Anode Coupling

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a b s t r a c t A strong interface coupling is of vital importance to develop metal oxide/carbon nanocomposite anodes for next-generation lithium ion batteries. Herein, a rational N-doped carbon riveting strategy is designed to boost the lithium storage performance of Fe3 O4 /N-doped carbon tubular structures. Polypyrrole (PPy) has been used as the precursor for N-doped carbon. N-doped carbon-riveted Fe3 O4 /N-doped carbon (N–C@Fe3 O4 @N–C) nanocomposites were obtained by pyrolysis of PPy-coated FeOOH@PPy nanotubes in Ar atmosphere. When tested as an anode for LIBs, the N–C@Fe3 O4 @N–C displays a high reversible discharge capacity of 675.8 mA h g−1 after 100 cycles at a current density of 100 mA g−1 and very good rate capability (470 mA h g−1 at 2 A g−1 ), which significantly surpasses the performance of Fe3 O4 @N–C. TEM analysis reveals that after battery cycling the FeOx particles detached from the carbon fibers for Fe3 O4 @N–C, while for N–C@Fe3 O4 @N–C the FeOx particles were still trapped in the carbon matrix, thus preserving good electrical contact. Consequently, the superior performance of N–C@Fe3 O4 @N–C is attributed to the synergistic effect between Fe3 O4 and N-doped carbon combined with the unique structure properties of the nanocomposites. The strategy reported in this work is expected to be applicable for designing other electrode materials for LIBs. © 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

1. Introduction In recent years, electrochemical energy storage systems are engaging more and more attention because of the high risk of pollution on the use of fossil energy. High requirements thus have been proposed for future lithium ion batteries (LIBs). The graphite-based materials are currently used as anode materials, but the low theoretical capacity (ca. 372 mA h g−1 ) limits its use in future LIBs. Therefore, it is of great importance to study novel electrode materials to peruse higher capacity, higher energy density and better rate stability [1–4]. Metal oxides such as SnO2 [5–7], Fe2 O3 [8–10], and Fe3 O4 [11–14], are widely investigated as promising electrode materials for future LIBs. Among these anode materials, Fe3 O4 has attracted much interest because of its high theoretical capacity (926 mA h g−1 ), low cost and environmental compatibility [14,15]. However, the low conductivity and the large volume change caused by delithiation/lithiation during charge/discharge processes often

∗

Corresponding authors at: College of Physics, Qingdao University, Qingdao 266071, Shandong, China. E-mail addresses: [email protected] (X. Liu), [email protected] (J. Zhang).

lead to poor cycling performance and decrease of capacity [16,17]. To enhance the conductivity of Fe3 O4 anodes, various carbon materials have been used as a conductive support due to their high conductivity and stability. For example, Wu and co-workers reported that Fe3 O4 /carbon nanotube nanocomposites prepared by magnetron sputtering exhibited a high reversible capacity over 800 mA h g−1 [18]. Wan et al. prepared Fe3 O4 -anchored carbon nanofiber composites and obtained a high reversible capacity of 755 mA h g−1 [19]. The pulverization of metal oxide particles during charge/discharge processes, which will lead to detachment of metal oxide particles from the carbon support, is another major concern for achieving extended cycling life. To solve this problem, researchers have tried to fill Fe3 O4 particles into the carbon nanotubes. For example, Wang et al. encapsulated Fe3 O4 nanoparticles in carbon nanotubes (CNTs) and claimed that the formation of solid electrolyte interphase (SEI) film occurred on the surface of CNTs, thus improving the cycling stability of Fe3 O4 materials [20]. Very recently, Liu et al. [21] introduced Fe3 O4 nanoparticles into multi-walled carbon nanotubes by a wet chemical injecting method and obtained a specific capacity of 703.7 mA h g−1 after 350 cycles at 100 mA g−1 . Although the above reports delivered improvements in the electrochemical performances of Fe3 O4 , an alternative and feasible strategy to obtain a strong interface

https://doi.org/10.1016/j.jechem.2018.09.017 2095-4956/© 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Please cite this article as: T. Ma et al., Strongly coupled N-doped carbon/Fe3 O4 /N-doped carbon hierarchical micro/nanostructures for enhanced lithium storage performance, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.017

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Scheme 1. Schematic synthesis of N–C@Fe3 O4 @N–C hybrid materials.

2.3. Preparation of N–C@Fe3 O4 @N–C Scheme 1 illustrates the schematic synthesis of N–C@Fe3 O4 @ N–C. The as-prepared PPy nanotubes (0.05 g) were dispersed in 35 mL water under sonication. Then 0.2433 g FeCl3 , 0.8499 g NaNO3 and 20 μL HCl were added and the mixture was stirred for 30 min at room temperature. The mixture was transferred into a 50 mL Teflon-sealed autoclave and maintained at 100 °C for 10 h. The product was centrifuged and washed with water, and re-dispersed in 40 mL ethanol under sonication. 0.25 mL Py was injected into the mixture, followed by addition of 10 mL 1 M FeCl3 solution under stirring. After 20 h, the product was collected by centrifugation and washed with water and ethanol, and dried at 60 °C overnight. N–C@Fe3 O4 @N–C nanocomposites were obtained by annealing the product in Ar at 450 °C for 2 h with a temperature rate of 5 °C/min. For comparison, Fe3 O4 @N–C was also prepared by annealing the product derived from the hydrothermal synthesis. Fig. 1. The XRD patterns of (1) the precursor, (2) Fe3 O4 @N–C and (3) N–C@Fe3 O4 @N–C.

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coupling between metal oxide and carbon support is still of great importance. Herein, we report the rational design of strong interface coupling between hierarchical N-doped carbon-riveted Fe3 O4 /N-doped carbon (N–C@Fe3 O4 @N–C) nanofibers and their enhanced lithium storage performance as an anode for LIBs. Electrochemistry experiments revealed that the N–C@Fe3 O4 @N–C nanocomposites displayed much better properties than Fe3 O4 @N–C in terms of reversible capacity, cycling and rate performance. Based on various electrochemical tests and post-TEM analysis, we confirm that the enhanced lithium storage properties are ascribed to the functional N-doped carbon riveted structure, and the synergistic effect between Fe3 O4 and N-doped carbon support. 2. Experimental

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2.1. Materials

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Pyrrole (Py), anhydrous iron (III) chloride (FeCl3 ), Methyl orange (MO, (CH3 )2 NC6 H4 N = NC6 H4 SO3 Na), ethanol, sodium nitrate (NaNO3 ) and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co. Ltd.

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2.2. Preparation of PPy nanotubes

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Typically, 0.4866 g FeCl3 and 3 mmol MO were dissolved in 60 mL water under stirring for 30 min. Then 0.21 mL Py was added into the mixture, and the solution was stirred at room temperature for 12 h. The product was washed with water/ethanol and then dried at 80 °C for 12 h.

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2.4. Characterization

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2.4.1. Sample characterization Powder X-ray diffraction (XRD) was used to characterize the structures of hybrid materials on a Rigaku Smartlab with Cu Kα radiation (λ = 0.15418 nm). The morphology and structure of samples were observed by a scanning electron microscope (SEM, Carl Zeiss SIGMA 300) and transmission electron microscope (TEM, FEI JEM2010) respectively. High-resolution TEM (HRTEM) was obtained on a FEI Tecnai G20. The elemental mapping was tested on FEI JEM2010. Raman spectroscopy and thermogravimetric analysis in air atmosphere were conducted on Renishaw RM20 0 0 and SDT Q600, respectively. The N2 adsorption-desorption isotherms of samples were performed on a Quantachrome Autosorb-iQ3. Analysis of surface species and elemental chemistry was characterized by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250).

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2.4.2. Electrochemical measurements To fabricate the electrodes, the prepared samples were mixed with carbon black and polyvinylidene fluoride (80:10:10) in Nmethyl-2-pyrrolidone. The slurry was coated onto a Cu current collector and dried at 120 °C overnight in a vacuum oven. The electrochemical test cells were assembled in glove box under a dry argon atmosphere. Electrolyte was 1 M LiPF6 dissolved in ethylene carbonate/diethyl carbonate/ethyl methyl carbonate in a 1/1/1 volume ratio. The mass loading of the electrodes is in the range of 1.2–1.5 mg/cm2 . CR2025 coin-type cell was used for cyclic voltammetry (CV) test, galvanostatic discharge–charge cycling, as well as electrochemical impedance spectra (EIS). CV profiles were obtained in the range of 0.01–3 V at a scanning rate of 0.1 mV s−1 and EIS were obtained in a frequency range from 0.01 Hz to 105 Hz with a signal amplitude of 5 mV from an electrochemical workstation (Bio-Logic VSP). The charge-discharge curves were recorded in a potential window from 0.01–3.0 V on a Land CT2001A battery test system.

Please cite this article as: T. Ma et al., Strongly coupled N-doped carbon/Fe3 O4 /N-doped carbon hierarchical micro/nanostructures for enhanced lithium storage performance, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.017

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Fig. 2. The SEM images of (a, b) Fe3 O4 @N–C, and (c, d) N–C@Fe3 O4 @N–C.

Fig. 3. The TEM images of (a, b) Fe3 O4 @N–C and (d, e) N–C@Fe3 O4 @N–C; HRTEM images of (c) Fe3 O4 @N–C and (f) N–C@Fe3 O4 @N–C, and the elemental mapping of (g) N–C@Fe3 O4 @N–C and (h) Fe3 O4 @N–C.

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3. Results and discussion N doping has been approved to contribute to improved electron conductivity, more lithium storage sites and better ion permeability of carbon, thereby resulting in enhanced electrochem-

ical performance [22–24]. Thus PPy was selected as the precursor to make N-doped carbon (N–C). PPy nanotubes were first synthesized and used as a support for the hydrothermal growth of FeOOH nanoparticles. A functional layer of PPy was further polymerized on FeOOH@PPy nanofibers. After annealing in Ar

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Fig. 4. (a) Raman spectra, (b) TG curves of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C and N2 adsorption-desorption isotherms of (c) Fe3 O4 @N–C and (d) N–C@Fe3 O4 @N–C (Insets in (c) and (d) are the pore distribution curve).

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atmosphere, the PPy-coated FeOOH@PPy materials were converted into N–C@Fe3 O4 @N–C, where Fe3 O4 particles were fully enveloped by the N–C matrix. Fig. 1 presents the XRD patterns of the products obtained at different stages of the synthesis. The diffraction peaks of the asprepared precursor, as shown in Fig. 1(1), can be perfectly indexed to the tetragonal phase FeOOH (JCPDS 34-1266). After calcination, the FeOOH@PPy was transformed into Fe3 O4 @N–C (JCPDS 750033), with a minor content of hematite Fe2 O3 (JCPDS 33-0664). In Fig. 1(3) it is noted that all the diffraction peaks can be assigned to Fe3 O4 (JCPDS 75-0033), while no obvious diffraction for impurities can be seen. The formation of Fe2 O3 is attributed to dehydration of FeOOH [25], while formation of Fe3 O4 is from reduction of Fe2 O3 by carbon during the pyrolysis of PPy at elevated temperature [26]. A series of characterizations on the morphology and composition of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C are presented in Figs. 2 and 3. The typical SEM images of the samples are shown in Fig. 2. As can be seen, the Fe3 O4 @N–C (Fig. 2a and b) and N–C@Fe3 O4 @N–C (Fig. 2c and d) display rather different surface architectures. For N–C@Fe3 O4 @N–C, as shown in Fig. 2(d), many bud-like particles are observed to grow on the fibers, exhibiting a hierarchical architecture. Further details are provided by the TEM observation. In Fig. 3(a and b) of Fe3 O4 @N–C, many Fe3 O4 particles are randomly dis-

persed on carbon support, and the HRTEM in Fig. 3(c) exhibits a lattice spacing of 0.25 nm, corresponding to the (311) plane of Fe3 O4 . In Fig. 3(d and e), the N–C@Fe3 O4 @N–C materials is observed to possess a different morphology from that of Fe3 O4 @N–C. The Fe3 O4 particles in N–C@Fe3 O4 @N–C show an elongated shape, differing from the round particle morphology of Fe3 O4 in Fe3 O4 @N–C. The elongated shape of Fe3 O4 is also in accordance with the SEM observation (Fig. 2d, a large amount of bud-like particles). This difference indicates that the carbon coating layers help to preserve the elongated shape of bud-like particles. Without carbon coating the Fe3 O4 in Fe3 O4 @N–C transforms into round particles due to Ostwald ripening. In Fig. 3(e), the carbon coating layers can be seen clearly, as marked by the arrows. The Fe3 O4 particles in N–C@Fe3 O4 @N–C are seen to be wrapped by carbon. The lattice spacing in Fig. 3(f) is measured to be 0.3 nm, which can be ascribed to the (220) plane of Fe3 O4 . Fig. 3(g and h) shows the scanning TEM (STEM) and the corresponding elemental mapping images of N–C@Fe3 O4 @N–C and Fe3 O4 @N–C, showing strong signal of Fe, O, C and N elements. Raman analysis was used to further investigate the carbon structure of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C, as shown in Fig. 4(a). Two defined bands at about 1355 and 1590 cm−1 are assigned to D and G bands for carbon, respectively. For both samples, it is seen that the intensity of G bands is higher than that of

Please cite this article as: T. Ma et al., Strongly coupled N-doped carbon/Fe3 O4 /N-doped carbon hierarchical micro/nanostructures for enhanced lithium storage performance, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.017

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Fig. 5. The XPS spectra of (a) C, (b) N and (c) Fe of Fe3 O4 @N–C and (d) C, (e) N and (f) Fe of N–C@Fe3 O4 @N–C.

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D bands, indicating a high degree of graphitization of the carbon [27,28]. The relatively weak band at around 660 cm−1 is attributed to A1g in Fe3 O4 [28,29]. To confirm the Fe3 O4 content in the hybrids, TG analysis was carried out and the results are present in Fig. 4(b). The weight loss below 150 °C is caused by the release of adsorbed water. The major weight loss due to carbon combustion takes place at 250–500 °C, giving rise to an observed weight loss of 30.2 wt% and 47.4 wt% for Fe3 O4 @N–C and N–C@Fe3 O4 @N–C, respectively. Based on the following reaction,4Fe3 O4 + O2 = 6Fe2 O3 , the content of Fe3 O4 in Fe3 O4 @N–C and N–C@Fe3 O4 @N–C is calculated to be 63.8 wt % and 43.8 wt%, respectively. The N2 adsorption–desorption analysis of the two samples has been performed, and the results are shown in Fig. 4(c and d). The specific surface area of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C is tested to be 21.68 m2 g−1 and 28.41 m2 g−1 , respectively. X-ray photoelectron spectroscopy (XPS) analysis is employed to further characterize the products. Fig. 5(a–c) shows the high resolution XPS spectra of C 1s, N 1s and Fe 2p. In Fig. 5(a), the three peaks at 284.6, 285.2 and 287.4 eV of C 1s are assigned to C=C, C–C and C–N [30,31]. The XPS spectra of N 1s (Fig. 5b) can be fitted into three peaks at 398.4, 400.2 and 402.4 eV, which can be attributed to pyridinic-N, pyrrolic-N and graphitic-N [24,32]. In Fig. 5(c), the Fe 2p3/2 and Fe 2p1/2 signals have a binding energy of 710.9 and 724.6 eV, respectively, assigning to Fe2+ and Fe3+ of Fe3 O4 [33,34]. For N–C@Fe3 O4 @N–C, the high resolution XPS spectra of C 1s, N 1s and Fe 2p in Fig. 5(d–f) show similar fitting results, except for the varied ratio of different components. And the amount of N doping in N–C@Fe3 O4 @N–C and Fe3 O4 @N–C is 8.87 at.% and 3.37 at.%, according to XPS analysis. To highlight the unique structure of N–C@Fe3 O4 @N–C, we tested the electrochemical performance of N–C@Fe3 O4 @N–C and Fe3 O4 @N–C as the anode in LIBs. CV measurements are performed at a low scan rate of 0.1 mV s−1 between 0.01 and 3.0 V vs. Li+ /Li. As shown in Fig. 6(a), the cathodic peaks in the first cycle of

Fe3 O4 @N–C at 0.5 V are ascribed to the reduction of Fe3+ or Fe2+ to Fe (Lix Fe3 O4 + (8 − x)Li + + (8 − x)e − ↔3Fe0 + 4Li2 O) and the formation of SEI film. The broad anodic peak at 1.53 V is attributed to reversible oxidation reaction of Fe in the first cycle. While for N–C@Fe3 O4 @N–C (Fig. 6b), the main peak at 0.55 V corresponds to the reduction of Fe3+ or Fe2+ to Fe and the formation of SEI film, and other peaks at 1.04 and 1.5 V are attributed to the lithium interaction reaction and formation of Lix Fe3 O4 and the structure change. After the second cycle, the CV curves of both N–C@Fe3 O4 @N–C and Fe3 O4 @N–C nearly overlap, demonstrating good reversibility of the products [35,36]. Fig. 6(c) displays the cycling performance of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C at a current density of 100 mA g−1 in the voltage range of 0.01–3.0 V. The Fe3 O4 @N–C delivers an initial discharge/charge capacity of 1171.4 and 582.3 mA h g−1 , corresponding to a Coulombic Efficiency (CE) of 49.7%, while N–C@Fe3 O4 @N–C exhibits a discharge/charge capacity of 1119.1 and 605.1 mA h g−1 with an initial CE of 54.1%. After 100 cycles, the N–C@Fe3 O4 @N–C possesses a discharge capacity of 675.8 mA h g−1 , much higher than that (418.1 mA h g−1 ) of Fe3 O4 @N–C. Furthermore, it is seen that the specific capacities of both Fe3 O4 @N–C and N–C@Fe3 O4 @N–C show a decrease and then a gradual increase along with the cycling. This phenomenon has been widely observed for metal oxide anode materials [37–39]. The increasing capacity has been attributed to activation of the anode materials during cycling [40,41] or contribution from capacitive storage [39]. Herein, based on the discharge/charge profiles and CV tests, we are able to correlate the increasing capacities to the reversible reaction of (Fe3 O4 + xLi + + xe − ↔Lix Fe3 O4 ), which will be discussed later. The rate performances were tested at different current densities from 0.1 to 2 A g−1 . As shown in Fig. 6(d), the average reversible capacity of N–C@Fe3 O4 @N–C is about 700, 695, 620, 560 and 470 mA h g−1 at 0.1, 0.2, 0.5, 1 and 2 A g−1 , respectively. When the current density is switched back to 0.1 A g−1 , the capacity increases to 780 mA h g−1 . Further cycling at the current density of

Please cite this article as: T. Ma et al., Strongly coupled N-doped carbon/Fe3 O4 /N-doped carbon hierarchical micro/nanostructures for enhanced lithium storage performance, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.017

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Fig. 6. CV curve of (a) Fe3 O4 @N–C and (b) N–C@Fe3 O4 @N–C at a scan rate of 0.1 mV s−1 between 0.01 and 3.0 V; (c) Cycling performances a current density of 100 mA g−1 and (d) rate properties of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C. The rate test was conducted after the cycling at 100 mA g−1 .

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2 A g−1 for 40 cycles, a large reversible capacity of 510 mA h g−1 is retained, indicating a superior rate capability in the followed 40 cycle. The average capacity of Fe3 O4 @N–C is about 450, 420, 350, 285 and 220 mA h g−1 at 0.1, 0.2, 0.5, 1 and 2 A g−1 , respectively. And the retained capacity is 320 mA h g−1 after the following 40 cycle at 2 A g−1 . By comparison, the N–C@Fe3 O4 @N–C shows much better rate performances than Fe3 O4 @N–C, which can be attributed to the peculiar structure. This excellent capacity is much better than that of Fe3 O4 @N–C. Fig. 7(a–d) shows the charge/discharge profiles of both samples at a current density of 100 mA g−1 . For Fe3 O4 @N–C at a current density of 100 mA g−1 , it shows discharge capacities of 1171.4, 553 and 430 mA h g−1 and charge capacities of 582.3, 431.5 and 391.7 mA h g−1 at the 1st, 2nd and 3rd cycle, respectively. As for N–C@Fe3 O4 @N–C (Fig. 7c), it exhibits discharge capacities of 1119.1, 574.7 and 455.9 mA h g−1 and charge capacities of 605.1, 472.2 and 415.8 mA h g−1 , respectively. It can be seen that the 1st discharge curves of both samples can be divided into three regions. The first region from 0.85 to 3.0 V is due to insertion of Li+ into Fe3 O4 (Fe3 O4 + xLi + + xe − ↔Lix Fe3 O4 ). The potential plateau in the second region from 0.75 to 0.85 V is attributed to the conversion of Fe3 O4 to Fe. The third region from 0.01 to 0.75 V is corresponding to the insertion of Li+ into carbon (C + xLi + + xe − ↔Lix C), respectively. It is noted that the po-

tential plateau in the second region gradually disappears from the 2nd discharge profile and the discharge capacity declines to the lowest value until 40th cycle for Fe3 O4 @N–C and 20th for N–C@Fe3 O4 @N–C. In the following discharge profiles, it is interesting to see that the potential plateau in 0.75–0.85 V reappears. The recurrence of this plateau is more obvious in the 70th–100th discharge profiles. This observation indicates that the conversion reaction of Fe3 O4 to Fe is reversible, but might be subject to a long activation process during cycling [42–45]. To further clarify the electrochemical process, CV tests were performed after the 40 cycles at 2 A g−1 . As shown in Fig. 7(e and f), the Fe3 O4 @N–C and N–C@Fe3 O4 @N–C display similar CV curves with high similarity and stability, respectively. The cathodic peak at 0.83 eV corresponds to the formation of Li2 O and the reduction of Fe3 O4 and the anodic peak at 1.61 eV are attributed to the oxidation of Fe [11]. This information clearly confirms that conversion reaction of Fe3 O4 to Fe is reversible. To interpret the better performance of N–C@Fe3 O4 @N–C, EIS tests were conducted after cycling at 100 mA g−1 for 100 cycles. In Fig. 8, the Nyquist plots of both samples consist of both the arc in the high frequency region and the inclined line in the low frequency region. The data were analyzed by fitting to the equivalent circuit model (inset in Fig. 8). Re , Rf , Rct , CPE and Zw denote the electrolyte resistance, the resistance of the SEI film formed on the

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Fig. 7. Discharge/charge voltage profiles of (a, b) Fe3 O4 @N–C and (c, d) N–C@Fe3 O4 @N–C for cycling at 100 mA g−1 ; CV curve of (e) Fe3 O4 @N–C and (f) N–C@Fe3 O4 @N–C at a scan rate of 0.1 mV s−1 between 0.01 and 3.0 V after 100 cycles at 100 mA g−1 .

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electrode, the charge-transfer resistance, the double layer capacitance and the Warburg impedance [46, 47]. The result shows that the SEI film resistance (Rf ) and charge transfer resistance (Rct ) for N–C@Fe3 O4 @N–C are lower than those for Fe3 O4 @N–C. The influence of charging/discharging on the structural stability and phase composition of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C was studied by XRD and TEM. The XRD patterns in Fig. 9 indicate that the materials are composed of a poor crystalline phase

(FeOx ), which is very different from the original structure of Fe3 O4 (Fig. 1). However, this phase cannot be assigned to any of known iron oxide. The morphology evolution of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C is reflected by the scheme in Fig. 10. For Fe3 O4 @N–C (Fig. 10a), after extended cycling, the FeOx particles are observed to detach from the carbon fibers, which might result in a poor electrical contact. For N–C@Fe3 O4 @N–C (Fig. 10b), the FeOx particles are seen to be well wrapped by the carbon matrix. This

Please cite this article as: T. Ma et al., Strongly coupled N-doped carbon/Fe3 O4 /N-doped carbon hierarchical micro/nanostructures for enhanced lithium storage performance, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.017

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Fig. 10. Schematic illustration of morphology evolution of (a) Fe3 O4 @N–C and (b) N–C@Fe3 O4 @N–C and the corresponding TEM images of the materials after rate test.

Fig. 8. Nyquist plots of Fe3 O4 @N–C and N–C@Fe3 O4 @N–C tested after 100 cycles at 100 mA g−1 .

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Nos. 21601098 and 51602167), Shandong Provincial Science Foundation (ZR2016EMB07 and ZR2017JL021) and Key Research and Development Program (2018GGX102033), and Qingdao Applied Fundamental Research Project (16-5-1-92-jch and 17-1-1-81-jch).

Fig. 9. XRD patterns of electrodes after battery cycling. The diffraction of Cu is due to the Cu foil.

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result indicates that the outer carbon layer of N–C@Fe3 O4 @N–C could well accommodate the volume change of FeOx during charging/discharging, thus retaining the material structure and giving rise to better electrochemical performance.

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4. Conclusions

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In summary, we proposed a rational carbon riveting strategy to enhance the structure stability of Fe3 O4 anodes during cycling. As discussed, the tubular N–C@Fe3 O4 @N–C nanocomposites demonstrated significantly enhanced electrochemical performances against Fe3 O4 /N–C in terms of reversible capacity, rate capability and cycling stability due to the enhanced materials structure and electrical conductivity. Post TEM analysis approved the important role of the carbon riveting layers in retaining the materials structure during cycling. The superior performance of N–C@Fe3 O4 @N–C is due to the synergistic effect between Fe3 O4 and N-doped carbon combined with the strong interface coupling of the unique structure. Furthermore, the increasing capacity widely observed in anode materials is correlated to the reversible conversion reaction of Fe3 O4 . This work provides a potential strategy for designing high performance anode materials for LIBs.

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2018.09.017.

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Please cite this article as: T. Ma et al., Strongly coupled N-doped carbon/Fe3 O4 /N-doped carbon hierarchical micro/nanostructures for enhanced lithium storage performance, Journal of Energy Chemistry (2018), https://doi.org/10.1016/j.jechem.2018.09.017

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