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Journal of Power Sources 344 (2017) 74e84

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

In-depth nanocrystallization enhanced Li-ions batteries performancewith nitrogen-doped carbon coated Fe3O4 yolk�shell nanocapsulesQianhui Wu a, c, Rongfang Zhao a, c, Wenjie Liu b, c, Xiue Zhang a, c, Xiao Shen a, c,Wenlong Li a, c, Guowang Diao a, c, **, Ming Chen a, c, *

a School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR Chinab School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, PR Chinac Key Laboratory of Environmental Materials & Environmental Engineering of Jiangsu Province, Yangzhou, Jiangsu, 225002, PR China

h i g h l i g h t s

* Corresponding author. School of Chemistry and Chou University, Yangzhou, 225002, PR China.** Corresponding author. School of Chemistry and Chou University, Yangzhou, 225002, PR China.

E-mail addresses: gwdiao@yzu.edu.cn (G. D(M. Chen).

http://dx.doi.org/10.1016/j.jpowsour.2017.01.1010378-7753/© 2017 Elsevier B.V. All rights reserved.

g r a p h i c a l a b s t r a c t

� Fe3O4@C-N yolk�shell nanocapsuleswere synthesized without sacrificialtemplate.

� Yolk�shell structure allows Fe3O4 toexpand freely without breaking car-bon shell.

� The volume expansion of Fe3O4 re-sults in the in-depthnanocrystallization.

� In-depth nanocrystallization of Fe3O4enhances the capability.

� Fe3O4@C-N-700 delivers a high ca-pacity and excellent cycling stability.

a r t i c l e i n f o

Article history:Received 16 December 2016Received in revised form18 January 2017Accepted 23 January 2017Available online 1 February 2017

Keywords:Yolk�shell structureIn-depth nanocrystallizationNitrogen-doped carbonFe3O4 nanocapsulesLithium ion battery

a b s t r a c t

In this paper nitrogen-doped carbon-encapsulation Fe3O4 yolk�shell magnetic nanocapsules (Fe3O4@C-N nanocapsules) have been successfully constructed though a facile hydrothermal method and subse-quent annealing process. Fe3O4 nanoparticles are completely enclosed in nitrogen-doped carbon shellswith void space between the nanoparticle and the shell. The yolk�shell structure allows Fe3O4 nano-particles to expand freely without breaking the outer carbon shell during the lithiation/delithiationprocesses. The volume expansion of Fe3O4 results in the in-depth nanocrystallization. Fortunately, thenew generated small nanoparticles can increase the capability with the cycle increase due to the uniqueconfinement effect and excellent electronic conductivity of the nitrogen-doped carbon shells. Hence,after 150 cycles, the discharge capacity of Fe3O4@C-N-700 nanocapsules still remained 832 mA h g�1 at500 mA g�1, which corresponds to 116.7% of the lowest capacity (713 mA h g�1) at the 16th cycle. Webelieve that the yolk�shell structure is conducive to enhance the capacity of easy pulverization metaloxidation during the charge/discharge processes.

© 2017 Elsevier B.V. All rights reserved.

hemical Engineering, Yangz-

hemical Engineering, Yangz-

iao), chenming@yzu.edu.cn

1. Introduction

Recently, lithium-ion batteries are considered as the mostpromising energy storage devices for electric vehicles and hybridelectronic vehicles, because of their high energy density, without

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memory effect completely, little self-discharge and environmen-tally friendly [1e5]. However, due to the enormous demands ofenergy consumption, the traditional graphite material as the anodematerial can not meet the requirements for energy storage [6e10].Therefore, transition metal oxides having a high theoretical specificcapacity (typically greater than 600 mA h g�1), much higher thanthe graphite material, they have become a new hot spot for lithium-ion battery anode materials.

Among transition metal oxides, Fe3O4 has received the mostattention, due to the high theoretical capacity (926 mA h g�1), lowcost, earth abundance and ecological friendliness [4,9]. However,Fe3O4 as the anode material for lithium batteries still exist someproblems (the large irreversible capacity loss, poor cycle stabilityand low rate capacity, etc) owing to the low electric conductivityand severe volume expansion during Liþ insertion and extractionprocess [11]. In order to solve the above problems, many studieshave been carried out [12e26]. For example, Zhu et al. have suc-cessfully fabricated hierarchical Fe3O4@polypyrrole nanocages andthe composite exhibited excellent electrochemical performances[14]. Among these solutions, the carbon-coated has been regardedas the most effective ways to improve the conductive characteris-tics and alleviating volume expansion characteristics [16e26]. Theadvantage of carbon-coated is due to the following reasons. First,the carbon layer is beneficial to remain intact of Fe3O4 bodymorphology and prevent material to chalking agglomeration [14].Second, the carbon coating layer improves the electron conduc-tivity of the material [9]. Third, the carbon coating layer is condu-cive to form excellent and stable SEI film on the surface of thematerial. The nitrogen-doped carbon material can better improvethe material specific capacity and cycling stability [27e30]. It isattributed to the following reasons: (1) Because the radius of ni-trogen atom is smaller than carbon atom and the electronegativityis larger than carbon atom, the doping of nitrogen atoms canchange the crystal structure of carbon materials, increase the de-gree of disorder of carbon materials, introduce more defects andvacancies and provide more channels for the diffusion of lithiumions [31e33]. (2) Nitrogen doping improves the electrical conduc-tivity of the carbon material, and the N atoms incorporated intographitic networks can facilitate the formation of stronger in-teractions between the N-doped carbon structure and the lithiumions, which are favorable for Li insertion [31e33]. In addition, yolk-shell nanostructure as a new architecture of lithium ion batterymaterials has been widespread attention. The yolk-shell structurein combination with carbon shell has provided a new strategy tosolve the shortcomings existing in electrode materials, such as thedrastic volume change, low electric conductivity and low rate ca-pacity. Mai et al. have synthesized a manganese oxide/carbon yolk-shell nanorods by a solgel method, using dopamine as carbonsource, and the composite showed a high reversible capacity of634 mA h g�1 at 500 mA g�1 [16]. Zhao et al. have fabricatedyolkeshell structured SnO2@C nanospheres through a two-stepsolegel coating process by using tetraethyl orthosilicate andresorcinoleformaldehyde as precursors, the materials manifestsuperior electrochemical performance with a high lithium storagecapability, a good cycling performance and an excellent rate capa-bility [34].

In this paper, we demonstrate a one-step in situ nanospace-confined synthesis strategy to fabricate nitrogen-doped carbon-encapsulation Fe3O4 yolk�shell magnetic nanocapsules (Fe3O4@C-N nanocapsules) with very sufficient internal void space. In elec-trochemistry experiment, we found that carbon shells in theyolk�shell structure cannot restrain the volume expansion of Fe3O4nanoparticles, which result in the in-depth nanocrystallization toform smaller nanoparticles. It is fortunate that the new generatedsmall nanoparticles can enhance the capability due to the in-depth

nanocrystallization of electrode materials, and the uniqueconfinement effect and excellent electronic conductivity of thenitrogen-doped carbon shells. As a result, Fe3O4@C-N nanocapsulesas anodematerial for lithium batteries exhibit high specific capacityand excellent cycling stability, which indicated that Fe3O4@C-Nnanocapsules could be employed as an excellent anode material forhigh-performance lithium batteries.

2. Experimental section

2.1. Materials

Iron(III) chloride hexahydrate (FeCl3$6H2O, AR), sodium hy-droxide (NaOH, AR), dopamine (AR) and polyvinylidene difluoride(PVDF, AR) are supplied by Shanghai Chemical Corp. The electrolytesolution with 1 M LiPF6/ethylene carbonate (EC)/diethyl carbonate(DMC)/ethyl methyl carbonate (EMC) (1: 1: 1 by volume) werepurchased from Guangzhou Tinci Materials Technology Co. Ltd.Other chemicals and solvents are reagent grade and commerciallyavailable. Deionized water is used for all experiments.

2.2. Characterization

The morphology of the samples was characterized using ascanning electron microscope (SEM, Hitachi S-4800) and a Trans-mission Electron Microscopy (TEM, Philips Tecnai-12), High-reso-lution TEM (HRTEM) and high-angle annular dark-field scanningtransmission electron microscopy (HAADF-STEM) were conductedusing a FEI Tecnai G2 F30 STWIN (USA) operating at 200 kV. X-raydiffraction (XRD) was carried out on a graphite monochromatorand Cu Ka radiation (l ¼ 0.1541 nm) on a D8 advance superspeedpowder diffractometer (Bruker). Raman spectra were recorded by aRenishaw in Via Raman microscope. X-ray photoelectron spectro-scopic (XPS) measurements were made on a Thermo Escalab 250system. The energy-dispersive X-ray (EDX) analysis was performedon a KEVEX X-ray energy detector. The surface area and pore sizedistribution were measured by N2 adsorption-desorption tech-nique in an automated surface area and porosity analyzer (ASAP2020, HD88). The magnetic measurement used a vibrating samplemagnetometer (VSM) (EV7, ADE, USA).

2.3. Electrochemical tests

Electrochemical performance were characterized using 2032type coin cells which consist of the test anode, lithium foil ascathode, 1 M LiPF6 dissolved in a mixture of EC/DMC/EMC (1:1:1 byvolume) as electrolyte, the Celgard 2400 as separator and assem-bled in an high-purity argon-filled glovebox (Vacuum AtmospheresCo., Ltd). The test anode was prepared by mixed Fe3O4@C-Nnanocapsules, carbon blacks, and PVDF binders at a weight ratio of8:1:1 in N-methyl-2-pyrrolidinone (NMP) and cast onto a copperfoil current collector. The load of activematerial is about 2mg cm�2.After coating, the electrodes were dried at 80 �C for 10 h to removethe solvent before pressing. The electrodes were punched in theform of disks and then vacuum-dried at 120 �C for 12 h.

Cyclic voltammetry (CV) measurements were performed usingan electrochemical workstation (CHI660 E, Chenghua, CHN) at ascan rate of 0.1 mV s�1 between 0.01 and 3.0 V. Electrochemicalimpedance spectroscopic (EIS) were performed using an AutolabElectrochemical Analyzer (Ecochemie, Netherlands). The chargeand discharge performances were carried out using a battery testsystem (CT-3008W, Xinwei, CHN) within a range of 0.01e3 V atdifferent current densities.

Scheme 1. Schematic illustration of the synthesis process of the Fe3O4@C-N magnetic nanocapsules.

Q. Wu et al. / Journal of Power Sources 344 (2017) 74e8476

2.4. Preparation of nitrogen-doped carbon-encapsulation Fe3O4yolk�shell nanocapsules

The fusiform b-FeOOH was prepared by a simple hydrothermalmethod according to reference with some modifications [35].50 mg b-FeOOHwas dispersed in 50 ml Tris-buffer (10 mM, pH 8.5)by ultrasonication then added 15 mg dopamine (DA) stirred 24 h at30 �C. The product was washed with deionized water and ethanol,followed by drying under vacuum at 80 �C for 12 h the as-preparedb-FeOOH@PDA were annealed at 400 �C for 2 h with a heating rateof 1 �C min�1 and further heated at 600 or 700 �C for 2 h with aheating rate of 5 �C min�1 under argon atmosphere to obtain Fe3O4@C-N yolk�shell nanocapsules. The samples obtained at differentcalcination temperature were named Fe3O4@C-N-600 andFe3O4@C-N-700, respectively.

3. Results and discussion

3.1. Characterization of Fe3O4@C-N nanocapsules

The Fe3O4@C-N nanocapsules were synthesized through a one-step in situ nanospace-confined pyrolysis strategy for robust yolk-shell magnetic nanocapsules with very sufficient internal voidspace, in which Fe3O4 nanoparticle is well confined in thecompartment of a hollow carbon nanospindle. Scheme 1 illustratesthe synthetic process of the Fe3O4@C-N nanocapsules. The

Fig. 1. TEM and SEM images of (aec)

morphology andmicrostructure of the as-prepared b-FeOOH and b-FeOOH@PDA were characterized by SEM and TEM. In Fig. 1a-c, theuniform b-FeOOH nanoparticles are nanospindles with 40e50 nmin diameter and 200e400 nm in length. Dopamine (DA) was usedas carbon and nitrogen sources to be uniformly coated on thesurface of b-FeOOH to form b-FeOOH@PDA, in which the thicknessof the shell is about 5 nm as shown in Fig. 1d-f.

When b-FeOOH@PDA composites were annealed at differenttemperature under argon atmosphere, DA layer is carbonized andb-FeOOH is reduced to Fe3O4 and the morphologies are shown inFig. 2. Fig. 2 (a, b) show the typical TEM image of Fe3O4@C-N-600which was obtained by b-FeOOH@PDA annealed at 400 �C for 2 hwith a heating rate of 1 �C min�1 further heated at 600 �C with aheating rate of 5 �C min�1 under argon atmosphere under argonatmosphere. Fig. 2c, d shows the typical TEM image of Fe3O4@C-N-700 nanocapsules which were obtained by b-FeOOH@PDAannealed at 400 �C for 2 h with a heating rate of 1 �C min�1 furtherheated at 700 �C with a heating rate of 5 �C min�1 under argonatmosphere. It can be clearly seen that the cores are shrunk, whichresults in Fe3O4 nanoparticles are confined in a carbon shell with alarge internal void space. Compared with Fe3O4@C-N-600,Fe3O4@C-N-700 nanocapsules have larger internal void and smallerFe3O4 cores, so they are standard yolk�shell structure. From thehigh magnification TEM image (Fig. 2e), it is obvious that Fe3O4 iscompletely wrapped by uniformly graphitization carbon layer(5e8 nm) and the lattice fringe pitch of 0.2 nm corresponds well

b-FeOOH, (def) b-FeOOH@PAD.

Fig. 2. TEM images of (a, b) Fe3O4@C-N-600 with different magnifications, (c, d) Fe3O4@C-N-700 with different magnifications. HRTEM images of (e, f) Fe3O4@C-N-700 withdifferent magnifications.

Q. Wu et al. / Journal of Power Sources 344 (2017) 74e84 77

with the d-spacing of the [400] reflection of Fe3O4. The yolk�shellstructure of Fe3O4@C-N-700 nanocapsules is further confirmed byenergy-dispersive X-ray (EDX) spectroscopy. The distributions ofFe, O, C and N are shown in Fig. 3. Within the yolk-shell nano-capsules, the Fe and O elements are distributed in the internalstructure with large internal void and the C and N elements uni-formly are distributed on the external layer, indicating that Nelement is successfully doped into carbon shell.

The formation mechanism of yolk-shell Fe3O4@C-N nano-capsules is discussed. On the one hand, the transformation of thecomposition from b-FeOOH@PDA to Fe3O4@C-N has two steps. Thefirst step is the dehydration reaction of b-FeOOH and the reactionequation is as follow: 2b-FeOOH /Fe2O3 þ H2O. Meanwhile, thecarbonization of PDA forms the carbon shell. Subsequently, in thesecond step, Fe2O3 is reduced by carbon to generate Fe3O4 under

the protection of N2. On the other hand, the change of themorphology from core-shell to yolk-shell structure is attributed tothe speciality of b-FeOOH. In the dehydration process, when watermolecular is released from the spindle-like b-FeOOH, some holesare left on the materials surface. This phenomenon has been alsoobserved by Zhou et al., who demonstrated the formation of mes-oporous Fe3O4 nanorods owing to the dehydration reaction of a-FeOOH nanorods [13]. With the increase of calcination tempera-ture, the porous surface causes the looseness and collapse ofspindle structure, leading to the generation of some small frag-ments. At high calcination temperature (600 or 700 �C), smallfragments are fused inside the carbon shell, which results in thevolume shrinkage of core and the formation of internal void spaceand the yolk-shell structure. When b-FeOOH without PDA coatinglayer is annealed at the same reaction condition, the final product is

Fig. 3. (a) HAADF-STEM image of Fe3O4@C-N-700 nanocapsules and element mapping of (b) Fe Ka1, (c) O Ka1, (d) C Ka1, (e) N Ka1.

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pure a-Fe2O3, which is confirmed by XRD pattern (Fig. S1). Addi-tionally, the evolution morphology of b-FeOOH was characterizedby TEM and the images were shown in Fig. S2, which also confirmthe crush, fusion and volume shrinkage of a-Fe2O3 at the hightemperature.

The crystal structures of these samples are analyzed by X-raydiffraction (XRD) in Fig. 4A. In pattern a, all the diffraction peaks ofthe precursor sample can be well assigned to the pure b-FeOOH(JCPDS Card No. 34-1266). In pattern b, the XRD pattern of b-FeOOH@PDA still displays the diffraction peaks of b-FeOOH. Afterthe annealing process at different temperature, the character peaksof final products appear at 18.3�, 30.0�, 35.4�, 43.0�, 57.0�, and 62.5�,corresponding well with the (111), (200), (311), (400), (511) and(440) diffraction peaks of Fe3O4 (JCPDS card no. 19-0629), whichindicates the final product is determined to be Fe3O4 without im-purity (pattern c and d). In order to evaluate the graphitic quality ofthe carbon shell, the Raman spectra are given in Fig. 4B. The twopeaks located at 1350 and 1590 cm�1 can be assigned to the D bandand G band of carbon, respectively. Typically, the D band of carbonindicates the defects and disordered portions, while the G bandstands for the ordered graphitic crystallites of carbon [36,37]. Thepeak intensity ratio of D band and G band (ID/IG) of Fe3O4@C-N-600and Fe3O4@C-N-700 is measured to be 0.89 and 0.87, implying ahigher graphitization degree of carbon shell in Fe3O4@C-N-700nanocapsules, which is beneficial to improve the electric conduc-tivity of nanocapsules. However, when the calcination temperatureis increased to 800 �C, the XRD pattern (Fig. S3a) displays that theidentified diffraction peaks are clearly ascribed to cubic Fe (JCPDS,card 06-0696) [38], the product is named Fe@C-N-800. And theTEM image of Fe@C-N-800 was shown in Fig. S3b. Therefore, thecalcination temperature should not exceed 700 �C.

Fig. 4C shows the X-ray photoelectron spectroscopy (XPS)spectrum of Fe3O4@C-N-700 nanocapsules, proving the presence ofC, N, O and Fe elements. In curve b, the high-resolution XPS

spectrum of the Fe 2p shows typical characteristics of Fe3O4 withbroad peaks around 711.0 and 724.5 eV, corresponding to Fe 2p3/2and Fe 2p1/2 states, respectively [39]. The absence of the satellitepeaks also corroborated the assignment of the product to Fe3O4(magnetite) rather than g-Fe2O3 (maghemite) [40]. For the C 1sspectrum (curve c), four peaks are assigned to graphitized carbon(284.80 eV), C]N double bond (286.20 eV), carbonyl (287.41 eV),and carboxyl/ester groups (288.66 eV), respectively [41]. Oxygen-containing functional groups of carbon shell are conducive to therapid formation of stable SEI film on the electrode surface, whichcontributes to the lithium-ion transport and the protection of thecore material. In curve d, the high-resolution N1s XPS spectrum ofthe Fe3O4@C-N-700 nanocapsules is identified to the pyridinic ni-trogen structure at 398.42 eV, the pyrrolic nitrogen structure at400.19 eV and the quaternary nitrogen structure at 401.18 eV [42],proving the presence of nitrogen in the structure of the carbonlayer. The doping amount of nitrogen in carbon shell is about 10.8%based on the XPS measurement. In order to confirm the content ofFe3O4 in the Fe3O4@C-N-600 and Fe3O4@C-N-700 nanocapsules,the EDX analysis were employed and the results are shown inFig. S4. The EDX spectra reveal that the sample is composed of C, Fe,O, and N elements. The mass percentages of Fe3O4 in Fe3O4@C-N-600 and Fe3O4@C-N-700 nanocapsules are about 76.2% and 73.8%,respectively.

N2 adsorption/desorption isotherm and the pore size distribu-tion are shown in Fig. 5a. The isotherm is the type IV, in which thehysteresis loop confirms the existence of mesoporous in carbonlayer [43]. The Brunauer-Emmett-Teller (BET) surface area ofFe3O4@C-N-700 nanocapsules is measured to be about 94.2 m2 g�1,which is higher than those of previously reported Fe3O4-basedmaterials [44] (Table S1, Supporting Information). Such a largespecific surface area is beneficial to increasing the contact area ofelectrolyte with active materials and provides more active sites forthe diffusion of electrolyte. The pore size distribution (Fig. 5b)

Fig. 4. (A) XRD patterns of (a) b-FeOOH, (b) b-FeOOH@PDA, (c) Fe3O4@C-N-600, (d) Fe3O4@C-N-700. (B) Raman spectra of (a) Fe3O4@C-N-600, (b) Fe3O4@C-N-700. (C) XPS spectra ofthe Fe3O4@C-N-700. (a) survey, high-resolution XPS spectra of (b) Fe 2p, (c) C1s and (d) N1s.

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obtained though the Barrett�Joyner�Halenda (BJH) method showsthat themain pore size of Fe3O4@C-N-700 nanocapsules range from10 to 12 nm. This porous structure greatly provides an effectivewayfor the transfer of lithium-ions, accelerating the transmission effi-ciency. Fig. 5c shows the magnetization curve of Fe3O4@C-N-700nanocapsules which were measured at room temperature withmaximum applied field of 10 kOe in either direction. The Fe3O4@C-N-700 nanocapsules exhibit ferromagnetic behavior at room tem-perature and its saturation magnetization is 38 eum g�1. As shownin Fig. 5d, the Fe3O4@C-N-700 nanocapsules can be easily separatedfrom the solution by a magnet, indicating that they have the po-tential to become a magnetic carrier.

3.2. Electrochemical performance of yolk-shell Fe3O4@C-Nnanocapsules

The electrochemical properties of the as-prepared yolk-shell

Fe3O4@C-N nanocapsules were systematically investigated byassembling them into coin-type 2032 cell with metal lithium as thecounter electrode. Fig. 6 shows the charge-discharge profiles forfirst, second, 50th and 150th cycles of Fe3O4@C-N-600 (a), andFe3O4@C-N-700 (b), Fe/C-N-800 (c) and pure Fe3O4 (d) at a currentdensity of 500 mA g�1 between 0.01 and 3 V. As shown in Fig. 6a, b,the charge/discharge specific capacities of Fe3O4@C-N-600 nano-capsules are 703 and 1108 mA h g�1, respectively, in the firstcharge-discharge curve, while Fe3O4@C-N-700 nanocapsulesdisplay higher charge/discharge specific capacities (817 and1141 mA h g�1). The initial discharge capacities of Fe3O4@C-N-600and Fe3O4@C-N-700 nanocapsules are higher than theoretical ca-pacity of bulk Fe3O4 (926 mA h g�1), but the first coulombic effi-ciency is only 63.4% and 71.6%, which is ascribed to the formation ofsolid SEI layer on the surface of electrodematerial. The formation ofsolid SEI film is an irreversible process, resulting in the lowcoulombic efficiency. In subsequent cycles, the charge-discharge

Fig. 5. (a) N2 adsorptionedesorption isotherm of Fe3O4@C-N-700 nanocapsules and (b) The corresponding pore size distribution. (c) The magnetization curves of Fe3O4@C-N-700nanocapsules measured at room temperature. (d) Photo images of Fe3O4@C-N-700 nanocapsules dispersed and separated from the solution.

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curves of Fe3O4@C-N-700 are nearly coincident after the secondlap. The charge-discharge capacity of Fe3O4@C-N-700 nanocapsulesis about 832 mA h g�1 which are 89.8% of the theoretical specificcapacity and the coulomb efficiency up to 99.1%. In Fig. 6c, d, Fe@C-N-800 show the lowest specific capacity (257 mA h g�1) in firstcharge-discharge curve with an obvious decrease in subsequentcycles and pure Fe3O4 show a significant decrease in both chargeand discharge profiles in subsequent cycles, which indicating thatthey are not the ideal electrode material. Therefore, it can be seenthat Fe3O4@C-N-700 nanocapsules showed the highest specificcapacity and coulombic efficiency.

Electrochemical impedance spectroscopy (EIS) measurementswere carried out in the frequency range from 0.1 Hz to 100 kHz forthe pure Fe3O4, Fe3O4@C-N-600 and Fe3O4@C-N-700 nanocapsuleelectrodes after the 20th cycle as shown in Fig. 6e. The curvesexhibit depressed semicircle in the high/medium-frequency regionfollowed by an inclined line in the low-frequency region. Thesemicircle is associated with charge-transfer resistances (Rct). Theinclined line represents Warburg impedance (Zw) related to thediffusion of lithium ions within the bulk of the electrode material[45]. The smaller the radius of the semicircle on behalf of thesmaller the value of the charge-transfer resistance is, the steeperthe line on behalf of the faster of the lithium ion diffusion is. As canbe seen from Fig. 6e, Fe3O4@C-N-600 and Fe3O4@C-N-700 nano-capsules have smaller charge-transfer resistance and faster lithiumion migration than pure Fe3O4, indicating that nitrogen-dopedcarbon shell improves electrical conductivity.

The cyclic voltammograms (CVs) of Fe3O4@C-N-700 nano-capsules were tested for 4 cycles in the 0.01e3 V rang at a sweeprate of 0.1 mV s�1. As shown in Fig. 6f, it can be seen that there aretwo obviously reduction peaks in the initial discharge process. Aweak reduction peak appearing at ~0.88 V, corresponding to the

structure transition caused by the insertion of Liþ into Fe3O4(Fe3O4 þ 2 Liþ þ 2 e / Li2Fe3O4). Another major reduction peak atabout 0.62 V can be attributed to the transformation of Li2Fe3O4 toFe0 (Li2Fe3O4 þ 6 Liþ þ 6 e�/ 3 Fe0 þ 4 Li2O) along with theirreversible reactions of electrolyte to form a solid electrolyteinterface (SEI) layer [46e48]. Two broad oxidation peaks at ~1.65and ~1.88 V can be explained as the reversible oxidation of Fe0 toFe2þ/Fe3þ [45,49]. In the subsequent cycles, the cathodic peak shiftsto 0.84 and 1.5 Vwith the current drop and oxidation peaks remainsat ~1.7 and ~2.0 V and the CV curves almost overlap, which impliesthat a stable and complete SEI film is well formed on the surface ofcarbon shell, and the redox reactions of lithium insertion/extrac-tion are highly reversible. From the CV measurement, it can beinferred that Fe3O4@C-N-700 nanocapsules display high cycle sta-bility during the repeated lithiation/delithiation process.

Fig. 6g is a charge-discharge cycle performance comparison ofFe3O4@C-N-600, Fe3O4@C-N-700 nanocapsules, Fe/C-N-800 andpure Fe3O4 at 500 mA g�1. The discharge capacity of pure Fe3O4decreases to 73 mA h g�1 by the 150th cycle. And the dischargecapacity of Fe/C-N-800 decreases to 143 mA h g�1 after the 150thcycle far below the discharge performance of iron oxide compos-ites. It is believed that when the calcination temperaturewas raisedto 800 �C, the as-prepared material has no longer high lithiumstorage performance. The discharge capacity of Fe3O4@C-N-600and Fe3O4@C-N-700 nanocapsules shows a slowly rise duringcycling. After 150 cycles, the discharge capacity of Fe3O4@C-N-700nanocapsules still remained 832 mA h g�1, which corresponds to102% of the second cycle discharge capacity. Compared to thelowest capacity (713 mA h g�1) at the 16th cycle, the dischargecapacity of Fe3O4@C-N-700 nanocapsules at 150th cycle increased16.7%. When the metal oxides are used as electrode materials inLIBs, the slow increases of discharge capacities in the long cycle

Fig. 6. Voltage profiles of (a) Fe3O4@C-N-600, (b) Fe3O4@C-N-700, (c) Fe/C-N-800 and (d) pure Fe3O4 at 500 mA g�1. (e) Nyquist plots after 20 cycles at 500 mA g�1 in the frequencyrange from 100 kHz to 0.1 Hz. (f) Cyclic voltammograms for the first four cycles of Fe3O4@C-N-700. (g) Cycling performance of Fe3O4@C-N-600, Fe3O4@C-N-700, Fe/C-N-800 andpure Fe3O4 at 500 mA g�1. (h) Cycling performance of Fe3O4@C-N-600 and Fe3O4@C-N-700 nanocapsules at different current density.

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Q. Wu et al. / Journal of Power Sources 344 (2017) 74e8482

have been reported in many articles [50e54]. In this experiment, itmight be attributed to the following factors (1) The Fe3O4 cores ofFe3O4@C-N-700 broke up into smaller Fe3O4 nanoparticles so thatFe3O4 has a larger surface area and a larger contact area with thecarbon shell and electrolyte, which contribute to improve theability of electron transfer and shorten the diffusion path of lithiumions and electrons. (2) The doping of nitrogen atoms can change thecrystal structure of carbon materials, increase the degree of disor-der of carbon materials, and introduce more defects to providemore active sites for lithium ions.With the increase of the cycle, thedefects continue to increase and active sites are activated [31e33].Therefore, Fe3O4@C-N-700 nanocapsules exhibit a higher revers-ible specific capacity and better cycling performance than the otherthree materials.

The rate performances of Fe3O4@C-N-700 and Fe3O4@C-N-600nanocapsules are tested and the result is shown in Fig. 6h. Thereversible capabilities of Fe3O4@C-N-600 and Fe3O4@C-N-700nanocapsules are kept at 937 and 997 mA h g�1 after the 10th cycleat 100 mA g�1, respectively. With increasing current density, thedischarge capacities of the two materials drop significantly. Whenthe current density is increased to 1000 mA g�1, the specific ca-pacity of Fe3O4@C-N-600 nanocapsules stabilizes at 490 mA h g�1

while the Fe3O4@C-N-700 even stabilizes at 633 mA h g�1. More-over, when the current rate returns to 100 mA g�1 after 80 cycles,both electrode materials still restore the specific capacity withoutevident capacity loss. From Fig. 6h, Fe3O4@C-N-700 nanocapsulesexhibit much more excellent rate performance than Fe3O4@C-N-

Fig. 7. (A) TEM images of Fe3O4@C-N-700 nanocapsules after the (a, b) 50th cycle, (c, d)volumetric changes process of Fe3O4@C-N nanocapsules during electrochemical cycling.

600, which is due to the unique yolk-shell structure of Fe3O4@C-N-700 nanocapsules resulting into the rapid transfer of lithium ions aswell as powerful protect the integrity of activity material.

In order to reveal the essence of the capacity growth phenom-enon, the morphologies of the Fe3O4@C-N-700 nanocapsules afterthe 50th, 100th, and 150th cycles were observed and TEM imagesare shown in Fig. 7A. Compared with Fe3O4@C-N-700 nanocapsulesbefore cycle (Fig. 2e), Fe3O4 cores displayed distinct volumeexpansion and broke up into small nanoparticles after the 50thcycle (Fig. 7A(a-b)). The transformation of Fe3O4 cores is conduciveto increase the active site of electrode materials. This process thatthe Fe3O4 cores broke up into Fe3O4 smaller nanoparticles might benamed in-depth nanocrystallization, which contributes to theenhancement of capacity. With the increase of cycle (Fig. 7A(c-d)),the yolk-shell structure almost disappeared owing to the volumeexpansion of Fe3O4 cores, but new generated Fe3O4 nanoparticleswere still confined in the carbon shells completely, so the capacityincreased continuously (Fig. 6g). After the 150th cycle (Fig. 7A(e-f)),Fe3O4 core was sustained severe volume expansion and broken upinto smaller nanoparticles, however, since the buffer of internalspace and the protection of carbon shells, the overall structuremaintains intact and the volume of Fe3O4@C-N-700 nanocapsuleshas no change. It is believed that the yolk-shell structure providesthe large internal void space to effectively buffer the volumeexpansion of Fe3O4 cores. Though, it is well known that the volumeexpansion of electrode materials is a bad side. However, coin hastwo sides. This time we got the fortunate side that the new

100th cycle and (e, f) 150th cycle. (B) Schematic illustration of the morphology and

Q. Wu et al. / Journal of Power Sources 344 (2017) 74e84 83

generated Fe3O4 nanoparticles can increase the capacity due to thein-depth nanocrystallization during the charge/discharge processesand the confined effect of the N-doped carbon shell. Schematicillustration in Fig. 7B shows the morphology and volume changesprocess of Fe3O4@C-N nanocapsules during electrochemicalcycling.

4. Conclusions

In summary, yolk-shell Fe3O4@C-N nanocapsules have beensuccessfully prepared by a simple synthesis method. Fe3O4@C-N-700 nanocapsules exhibit excellent cycle performance and rateperformance. The cycle capacity profiles at 500 mA g�1 are prettygood, remaining at 832 mA h g�1 after 150 cycle. The enhancementof electrochemical performance of Fe3O4@C-N nanocapsules ismainly attributed to the following reason: (1) The unique yolk-shellstructure provides sufficient buffer space for freely volumeexpansion of Fe3O4 in intercalation/deintercalation lithium ionprocess. (2) The volume expansion of Fe3O4 results in in-depthnanocrystallization and generates smaller Fe3O4 nanoparticles,which efficiently promote the capacity with the increase of cycle.(3) N-doped carbon shell improves the conductivity of Fe3O4nanoparticles enhancing the ability to gain and loss of electrons,and greatly displays the confined effect for new generated Fe3O4nanoparticles, thus maintains the integrity structure of nano-capsules and ensures the cycling stability.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (Grant No. 21511140282), NaturalScience Foundation of Jiangsu Province (BK20161329), and a ProjectFunded by the Priority Academic Program Development of JiangsuHigher Education Institutions. The work was also sponsored byAdvanced Catalysis and Green Manufacturing Collaborative Inno-vation Center, Changzhou University, and Jiangsu Key Laboratory ofAdvanced Catalytic Materials and Technology (Grant number:BM2012110). The work was also sponsored by Qing Lan Project ofHigher Learning Institutions in Jiangsu Province, the support ofhigh-end talent plan of Yangzhou university. The authors alsoacknowledge the Testing Center of Yangzhou University for TEM,HRTEM, XRD, EDX, Raman and XPS experiments.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2017.01.101.

References

[1] J.-M. Kim, H.-S. Park, J.-H. Park, T.-H. Kim, H.-K. Song, S.-Y. Le, Conductingpolymer-skinned electroactive materials of lithium-ion batteries: ready formonocomponent electrodes without additional binders and conductiveagents, ACS Appl. Mater. Interfaces 6 (2014) 12789e12797.

[2] J.B. Goodenough, Evolution of strategies for modern rechargeable batteries,Acc. Chem. Res. 46 (2013) 1053e1061.

[3] S. Sallard, E. Castel, C. Villevieille, P. Novak, A low-temperature benzyl alcohol/benzyl mercaptan synthesis of iron oxysulfide/iron oxide composite materialsfor electrodes in Li-ion batteries, J. Mater. Chem. A 3 (2015) 16112e16119.

[4] Y. Dong, R. Ma, M. Hu, H. Cheng, Q. Yang, Y.Y. Li, J.A. Zapien, Thermalevaporation-induced anhydrous synthesis of Fe3O4-graphene composite withenhanced rate performance and cyclic stability for lithium ion batteries, Phys.Chem. Chem. Phys. 15 (2013) 7174e7181.

[5] Y. Wang, L. Yang, R. Hu, W. Sun, J. Liu, L. Ouyang, B. Yuan, H. Wang, M. Zhu,A stable and high-capacity anode for lithium-ion battery: Fe2O3 wrapped byfew layered graphene, J. Power Sources 288 (2015) 314e319.

[6] S. Guo, C. Li, Y. Chi, Z. Ma, H. Xue, Novel 3-D network SeSx/NCPAN compositesprepared by one-pot in-situ solid-state method and its electrochemical per-formance as cathode material for lithium-ion battery, J. Alloys Compd. 664

(2015) 92e98.[7] H. Zhang, C. Ma, Y. Hu, A. Maclennan, H. Wang, C. Wang, Effect of sulfate in

mineral precursor on capacitance behavior of prepared activated carbon, Ind.Eng. Chem. Res. 53 (2014) 10125e10132.

[8] L. Fan, L. Tang, H. Gong, Z. Yang, R. Guo, Carbon-nanoparticles encapsulated inhollow nickel oxides for supercapacitor application, J. Mater. Chem. 32 (2012)16376e16381.

[9] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature407 (2000) 496e499.

[10] Y. Yan, B. Li, W. Guo, H. Pang, H. Xue, Vanadium based materials as electrodematerials for high performance supercapacitors, J. Power Sources 329 (2016)148e169.

[11] X. Wang, H. Guan, S. Chen, H. Li, T. Zhai, D. Tang, Y. Bando, D. Golberg, Self-stacked Co3O4 nanosheets for high-performance lithium ion batteries, Chem.Commun. 47 (2011) 12280e12282.

[12] H. Xia, Y. Wan, G. Yuan, Y. Fu, X. Wang, Fe3O4/carbon coreeshell nanotubes aspromising anode materials for lithium-ion batteries, J. Power Sources 241(2013) 486e493.

[13] S.M. Yuan, J.X. Li, L.T. Yang, L.W. Su, L. Liu, Z. Zhou, Preparation and lithiumstorage performances of mesoporous Fe3O4@C microcapsules, ACS Appl.Mater. Interfaces 3 (2013) 705e709.

[14] J. Liu, X. Xu, R. Hu, L. Yang, M. Zhu, Uniform hierarchical Fe3O4@Polypyrrolenanocages for superior lithium ion battery anodes, Adv. Energy Mater. (2016),http://dx.doi.org/10.1002/aenm.201600256.

[15] H. Zhang, R. Hu, H. Liu, W. Sun, Z. Lu, J. Liu, L. Yang, Y. Zhang, M. Zhu,A spherical SneFe3O4 @graphite composite as a long-life and high-rate-capability anode for lithium ion batteries, J. Mater. Chem. A 4 (2016)10321e10328.

[16] Z. Cai, L. Xu, M. Yan, C. Han, L. He, K.M. Herculeet, C. Niu, Z. Yuan, W. Xu, L. Qu,K. Zhao, L. Mai, Manganese oxide/carbon yolk-shell nanorod anodes for highcapacity lithium batteries, Nano Lett. 15 (2014) 738e744.

[17] Y. Luo, X. Zhou, Y. Zhong, M. Yang, J. Wei, Z. Zhou, Preparation of coreeshellporous magnetite@carbon nanospheres through chemical vapor deposition asanode materials for lithium-ion batteries, Electrochim. Acta 154 (2015)136e141.

[18] N. Liu, W. Hui, T.M. McDowell, Y. Yao, C. Wang, Y. Cui, A yolk-shell design forstabilized and scalable Li-Ion battery alloy anodes, Nano Lett. 12 (2012)3315e3321.

[19] X. Fang, S. Liu, J. Zang, C. Xu, M. Zheng, Q. Dong, D. Sun, N. Zheng, Preciselycontrolled resorcinol-formaldehyde resin coating for fabricating core-shell,hollow, and yolk-shell carbon nanostructures, Nanoscale 5 (2013)6908e6916.

[20] N. Ding, Y. Lum, S. Chen, S. Chien, T. Hor, Z. Liu, Y. Zong, Sulfur-carbon yolk-shell particle based 3D interconnected nanostructures as cathodes forrechargeable lithium-sulfur batteries, J. Mater. Chem. A 3 (2015) 1853e1857.

[21] L. Jiang, Y. Qu, Z. Ren, P. Yu, D. Zhao, W. Zhou, L. Wang, H. Fu, Situ carbon-coated yolkeshell V2O3 microspheres for lithium-ion batteries, ACS Appl.Mater. Interfaces 7 (2015) 1595e1601.

[22] R. Liu, Y. Guo, G. Odusote, F. Qu, R.D. Priestley, Core-shell Fe3O4 polydopaminenanoparticles serve multipurpose as drug carrier, catalyst support and carbonadsorbent, ACS Appl. Mater. Interfaces 5 (2013) 9167e9171.

[23] Z. Sun, K. Xie, Z.A. Li, L. Sinev, P. Ebbinghauset, A. Erbe, M. Farle,M. Schuhmann, M. Muhler, E. Ventosa, Hollow and yolk-shell iron oxidenanostructures on few-layer graphene in Li-Ion batteries, Chem. Eur. J. 20(2014) 2022e2030.

[24] W. Ni, Y. Wang, R. Xu, formation of Sn@C yolkeshell nanospheres and cor-eesheath nanowires for highly reversible lithium storage, Part. Part. Syst.Charact. 30 (2013) 873e880.

[25] T. Yao, T. Cui, J. Wu, Q. Chen, X. Yin, F. Cui, K. Sun, Preparation of acid-resistantcore/shell Fe3O4@C materials and their use as catalyst supports, Carbon 50(2012) 2287e2295.

[26] H. Liu, Z. Li, Y. Liang, R. Fu, D. Wu, Facile synthesis of MnO multi-core@nitrogen-doped carbon shell nanoparticles for high performance lithium-ionbattery anodes, Carbon 84 (2015) 419e425.

[27] Y. Tang, B.L. Allen, D.R. Kauffman, A. Star, Electrocatalytic activity of nitrogen-doped carbon nanotube cups, J. Am. Chem. Soc. 131 (2009) 13200e13201.

[28] L. Qie, W.-M. Chen, Z.-H. Wang, Q.-G. Shao, X. Li, L.-X. Yuan, X.-L. Hu, W.-X. Zhang, Y.-H. Huang, Nitrogen-doped porous carbon nanofiber webs asanodes for lithium ion batteries with a superhigh capacity and rate capability,Adv. Mater. 24 (2012) 2047e2050.

[29] Y. Li, J. Wang, X. Li, J. Liu, D. Geng, J. Yang, R. Li, X. Sun, Nitrogen-doped carbonnanotubes as cathode for lithiumeair batteries, Electrochem. Commun. 13(2011) 668e672.

[30] L. Chen, Z. Huang, H. Liang, Q. Guan, S. Yu, Bacterial-cellulose-derived carbonNanofiber@MnO2 and nitrogen-doped carbon nanofiber electrode materials:an asymmetric supercapacitor with high energy and power density, Adv.Mater. 25 (2013) 4746e4752.

[31] F. Zheng, Y. Yang, Q. Chen, High lithium anodic performance of highlynitrogen-doped porous carbon prepared from a metal-organic framework,Nat. Commun. 5 (2014) 5261e5271.

[32] X. Li, D. Geng, Y. Zhang, X. Meng, R. Li, X. Sun, Superior cycle stability ofnitrogen-doped graphene nanosheets as anodes for lithium ion batteries,Electrochem. Commun. 13 (2011) 822e825.

[33] H. Wang, C. Zhang, Z. Liu, L. Wang, P. Han, H. Xu, K. Zhang, S. Dong, J. Yao,

http://dx.doi.org/10.1016/j.jpowsour.2017.01.101http://dx.doi.org/10.1016/j.jpowsour.2017.01.101http://refhub.elsevier.com/S0378-7753(17)30111-8/sref1http://refhub.elsevier.com/S0378-7753(17)30111-8/sref1http://refhub.elsevier.com/S0378-7753(17)30111-8/sref1http://refhub.elsevier.com/S0378-7753(17)30111-8/sref1http://refhub.elsevier.com/S0378-7753(17)30111-8/sref1http://refhub.elsevier.com/S0378-7753(17)30111-8/sref2http://refhub.elsevier.com/S0378-7753(17)30111-8/sref2http://refhub.elsevier.com/S0378-7753(17)30111-8/sref2http://refhub.elsevier.com/S0378-7753(17)30111-8/sref3http://refhub.elsevier.com/S0378-7753(17)30111-8/sref3http://refhub.elsevier.com/S0378-7753(17)30111-8/sref3http://refhub.elsevier.com/S0378-7753(17)30111-8/sref3http://refhub.elsevier.com/S0378-7753(17)30111-8/sref4http://refhub.elsevier.com/S0378-7753(17)30111-8/sref4http://refhub.elsevier.com/S0378-7753(17)30111-8/sref4http://refhub.elsevier.com/S0378-7753(17)30111-8/sref4http://refhub.elsevier.com/S0378-7753(17)30111-8/sref4http://refhub.elsevier.com/S0378-7753(17)30111-8/sref4http://refhub.elsevier.com/S0378-7753(17)30111-8/sref4http://refhub.elsevier.com/S0378-7753(17)30111-8/sref5http://refhub.elsevier.com/S0378-7753(17)30111-8/sref5http://refhub.elsevier.com/S0378-7753(17)30111-8/sref5http://refhub.elsevier.com/S0378-7753(17)30111-8/sref5http://refhub.elsevier.com/S0378-7753(17)30111-8/sref5http://refhub.elsevier.com/S0378-7753(17)30111-8/sref5http://refhub.elsevier.com/S0378-7753(17)30111-8/sref6http://refhub.elsevier.com/S0378-7753(17)30111-8/sref6http://refhub.elsevier.com/S0378-7753(17)30111-8/sref6http://refhub.elsevier.com/S0378-7753(17)30111-8/sref6http://refhub.elsevier.com/S0378-7753(17)30111-8/sref6http://refhub.elsevier.com/S0378-7753(17)30111-8/sref7http://refhub.elsevier.com/S0378-7753(17)30111-8/sref7http://refhub.elsevier.com/S0378-7753(17)30111-8/sref7http://refhub.elsevier.com/S0378-7753(17)30111-8/sref7http://refhub.elsevier.com/S0378-7753(17)30111-8/sref8http://refhub.elsevier.com/S0378-7753(17)30111-8/sref8http://refhub.elsevier.com/S0378-7753(17)30111-8/sref8http://refhub.elsevier.com/S0378-7753(17)30111-8/sref8http://refhub.elsevier.com/S0378-7753(17)30111-8/sref9http://refhub.elsevier.com/S0378-7753(17)30111-8/sref9http://refhub.elsevier.com/S0378-7753(17)30111-8/sref9http://refhub.elsevier.com/S0378-7753(17)30111-8/sref9http://refhub.elsevier.com/S0378-7753(17)30111-8/sref10http://refhub.elsevier.com/S0378-7753(17)30111-8/sref10http://refhub.elsevier.com/S0378-7753(17)30111-8/sref10http://refhub.elsevier.com/S0378-7753(17)30111-8/sref10http://refhub.elsevier.com/S0378-7753(17)30111-8/sref11http://refhub.elsevier.com/S0378-7753(17)30111-8/sref11http://refhub.elsevier.com/S0378-7753(17)30111-8/sref11http://refhub.elsevier.com/S0378-7753(17)30111-8/sref11http://refhub.elsevier.com/S0378-7753(17)30111-8/sref11http://refhub.elsevier.com/S0378-7753(17)30111-8/sref11http://refhub.elsevier.com/S0378-7753(17)30111-8/sref12http://refhub.elsevier.com/S0378-7753(17)30111-8/sref12http://refhub.elsevier.com/S0378-7753(17)30111-8/sref12http://refhub.elsevier.com/S0378-7753(17)30111-8/sref12http://refhub.elsevier.com/S0378-7753(17)30111-8/sref12http://refhub.elsevier.com/S0378-7753(17)30111-8/sref12http://refhub.elsevier.com/S0378-7753(17)30111-8/sref12http://refhub.elsevier.com/S0378-7753(17)30111-8/sref13http://refhub.elsevier.com/S0378-7753(17)30111-8/sref13http://refhub.elsevier.com/S0378-7753(17)30111-8/sref13http://refhub.elsevier.com/S0378-7753(17)30111-8/sref13http://refhub.elsevier.com/S0378-7753(17)30111-8/sref13http://refhub.elsevier.com/S0378-7753(17)30111-8/sref13http://dx.doi.org/10.1002/aenm.201600256http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref15http://refhub.elsevier.com/S0378-7753(17)30111-8/sref16http://refhub.elsevier.com/S0378-7753(17)30111-8/sref16http://refhub.elsevier.com/S0378-7753(17)30111-8/sref16http://refhub.elsevier.com/S0378-7753(17)30111-8/sref16http://refhub.elsevier.com/S0378-7753(17)30111-8/sref17http://refhub.elsevier.com/S0378-7753(17)30111-8/sref17http://refhub.elsevier.com/S0378-7753(17)30111-8/sref17http://refhub.elsevier.com/S0378-7753(17)30111-8/sref17http://refhub.elsevier.com/S0378-7753(17)30111-8/sref17http://refhub.elsevier.com/S0378-7753(17)30111-8/sref17http://refhub.elsevier.com/S0378-7753(17)30111-8/sref18http://refhub.elsevier.com/S0378-7753(17)30111-8/sref18http://refhub.elsevier.com/S0378-7753(17)30111-8/sref18http://refhub.elsevier.com/S0378-7753(17)30111-8/sref18http://refhub.elsevier.com/S0378-7753(17)30111-8/sref19http://refhub.elsevier.com/S0378-7753(17)30111-8/sref19http://refhub.elsevier.com/S0378-7753(17)30111-8/sref19http://refhub.elsevier.com/S0378-7753(17)30111-8/sref19http://refhub.elsevier.com/S0378-7753(17)30111-8/sref19http://refhub.elsevier.com/S0378-7753(17)30111-8/sref20http://refhub.elsevier.com/S0378-7753(17)30111-8/sref20http://refhub.elsevier.com/S0378-7753(17)30111-8/sref20http://refhub.elsevier.com/S0378-7753(17)30111-8/sref20http://refhub.elsevier.com/S0378-7753(17)30111-8/sref21http://refhub.elsevier.com/S0378-7753(17)30111-8/sref21http://refhub.elsevier.com/S0378-7753(17)30111-8/sref21http://refhub.elsevier.com/S0378-7753(17)30111-8/sref21http://refhub.elsevier.com/S0378-7753(17)30111-8/sref21http://refhub.elsevier.com/S0378-7753(17)30111-8/sref21http://refhub.elsevier.com/S0378-7753(17)30111-8/sref21http://refhub.elsevier.com/S0378-7753(17)30111-8/sref22http://refhub.elsevier.com/S0378-7753(17)30111-8/sref22http://refhub.elsevier.com/S0378-7753(17)30111-8/sref22http://refhub.elsevier.com/S0378-7753(17)30111-8/sref22http://refhub.elsevier.com/S0378-7753(17)30111-8/sref22http://refhub.elsevier.com/S0378-7753(17)30111-8/sref22http://refhub.elsevier.com/S0378-7753(17)30111-8/sref23http://refhub.elsevier.com/S0378-7753(17)30111-8/sref23http://refhub.elsevier.com/S0378-7753(17)30111-8/sref23http://refhub.elsevier.com/S0378-7753(17)30111-8/sref23http://refhub.elsevier.com/S0378-7753(17)30111-8/sref23http://refhub.elsevier.com/S0378-7753(17)30111-8/sref24http://refhub.elsevier.com/S0378-7753(17)30111-8/sref24http://refhub.elsevier.com/S0378-7753(17)30111-8/sref24http://refhub.elsevier.com/S0378-7753(17)30111-8/sref24http://refhub.elsevier.com/S0378-7753(17)30111-8/sref24http://refhub.elsevier.com/S0378-7753(17)30111-8/sref24http://refhub.elsevier.com/S0378-7753(17)30111-8/sref25http://refhub.elsevier.com/S0378-7753(17)30111-8/sref25http://refhub.elsevier.com/S0378-7753(17)30111-8/sref25http://refhub.elsevier.com/S0378-7753(17)30111-8/sref25http://refhub.elsevier.com/S0378-7753(17)30111-8/sref25http://refhub.elsevier.com/S0378-7753(17)30111-8/sref25http://refhub.elsevier.com/S0378-7753(17)30111-8/sref26http://refhub.elsevier.com/S0378-7753(17)30111-8/sref26http://refhub.elsevier.com/S0378-7753(17)30111-8/sref26http://refhub.elsevier.com/S0378-7753(17)30111-8/sref26http://refhub.elsevier.com/S0378-7753(17)30111-8/sref27http://refhub.elsevier.com/S0378-7753(17)30111-8/sref27http://refhub.elsevier.com/S0378-7753(17)30111-8/sref27http://refhub.elsevier.com/S0378-7753(17)30111-8/sref28http://refhub.elsevier.com/S0378-7753(17)30111-8/sref28http://refhub.elsevier.com/S0378-7753(17)30111-8/sref28http://refhub.elsevier.com/S0378-7753(17)30111-8/sref28http://refhub.elsevier.com/S0378-7753(17)30111-8/sref28http://refhub.elsevier.com/S0378-7753(17)30111-8/sref29http://refhub.elsevier.com/S0378-7753(17)30111-8/sref29http://refhub.elsevier.com/S0378-7753(17)30111-8/sref29http://refhub.elsevier.com/S0378-7753(17)30111-8/sref29http://refhub.elsevier.com/S0378-7753(17)30111-8/sref29http://refhub.elsevier.com/S0378-7753(17)30111-8/sref30http://refhub.elsevier.com/S0378-7753(17)30111-8/sref30http://refhub.elsevier.com/S0378-7753(17)30111-8/sref30http://refhub.elsevier.com/S0378-7753(17)30111-8/sref30http://refhub.elsevier.com/S0378-7753(17)30111-8/sref30http://refhub.elsevier.com/S0378-7753(17)30111-8/sref30http://refhub.elsevier.com/S0378-7753(17)30111-8/sref31http://refhub.elsevier.com/S0378-7753(17)30111-8/sref31http://refhub.elsevier.com/S0378-7753(17)30111-8/sref31http://refhub.elsevier.com/S0378-7753(17)30111-8/sref31http://refhub.elsevier.com/S0378-7753(17)30111-8/sref32http://refhub.elsevier.com/S0378-7753(17)30111-8/sref32http://refhub.elsevier.com/S0378-7753(17)30111-8/sref32http://refhub.elsevier.com/S0378-7753(17)30111-8/sref32http://refhub.elsevier.com/S0378-7753(17)30111-8/sref33

Q. Wu et al. / Journal of Power Sources 344 (2017) 74e8484

G. Cui, Nitrogen-doped graphene nanosheets with excellent lithium storageproperties, J. Mater. Chem. 21 (2011) 5430e5434.

[34] J. Wang, W. Li, F. Wang, Y. Xia, A.M. Asiri, D. Zhao, Controllable synthesis ofSnO2 @C yolkeshell nanospheres as a high-performance anode material forlithium ion batteries, Nanoscale 6 (2014) 3217e3222.

[35] C. Lei, F. Han, Q. Sun, W. Li, A. Lu, Confined nanospace pyrolysis for thefabrication of coaxial Fe3O4@C hollow particles with a penetrated meso-channel as a superior anode for Li-Ion batteries, Chem. Eur. J. 20 (2014)139e145.

[36] K. Liang, T. Cheang, T. Wen, X. Xie, X. Zhou, Z. Zhao, C. Shen, N. Jiang, A. Xu,Facile preparation of porous Mn2SnO4/Sn/C composite cubes as high perfor-mance anode material for lithium-ion batteries, J. Phys. Chem. C 120 (2016),3369e3376.36.

[37] G. Chen, M. Zhou, J. Catanach, T. Liaw, L. Fei, Deng. S, Luo. H, Solvothermalroute based in situ carbonization to Fe3O4@C as anode material for lithium ionbattery, Nano Energy 8 (2014), 126e132.6.

[38] Z. Wen, C. Suqin, F. Zhang, X. Feng, S. Cui, S. Mao, S. Luo, Z. He, J. Chen, Ni-trogen-enriched core-shell structured Fe/FeC3-C nanorods as advanced elec-trocatalysts for oxygen reduction reaction, Adv. Mater. 24 (2012),1399e1404.9e367.

[39] W. Wei, S. Yang, H. Zhou, I. Lieberwirth, X. Feng, K. Müllen, 3D graphenefoams cross-linked with pre-encapsulated Fe3O4 nanospheres for enhancedlithium storage, Adv. Mater. 25 (2013) 2909e2914.

[40] J. Luo, J. Liu, Z. Zeng, C.F. Ng, L. Ma, H. Zhang, J. Lin, Z. Shen, H. Fan, Three-dimensional graphene foam supported Fe3O4 lithium battery anodes withlong cycle life and high rate capability, Nano Lett. 13 (2013) 6136e6143.

[41] M. Chen, J. Jiang, X. Zhou, G. Diao, Preparation of akaganeite nanorods andtheir transformation to sphere shape hematite, J. Nanosci. Nanotechnol. 8(2008) 3942e3948.

[42] Y. Liu, K. Huang, H. Luo, H. Li, X. Qi, J. Zhong, Nitrogen-doped graphene-Fe3O4architecture as anode material for improved Li-ion storage, RSC Adv. 4 (2014)17653e17659.

[43] J. Zhang, S. Guo, J. Wei, W. Yan, J. Fu, S. Wang, M. Cao, Z. Chen, High-efficiencyencapsulation of Pt nanoparticles into the channel of carbon nanotubes as anenhanced electrocatalyst for methanol oxidation, Chem. Eur. J. 19 (2013)16087e16092.

[44] H. Jiang, Y. Hu, S. Guo, C. Yan, P.S. Lee, C. Li, Rational design of MnO/carbon

nanopeapods with internal void space for high-rate and long-life Li-Ion bat-teries, ACS Nano 8 (2014) 6038e6046.

[45] Y. Zuo, G. Wang, J. Peng, G. Li, Y. Ma, F. Yu, B. Dai, X. Guo, C. Wong, Hybrid-ization of graphene nanosheets and carbon-coated hollow Fe3O4 nano-particles as a high-performance anode material for lithium-ion batteries,J. Mater. Chem. A 4 (2016) 2453e2460.

[46] T. Xia, X. Xu, J. Wang, C. Xu, F. Meng, Z. Shi, J. Lian, J.M. Bassat, Facile complex-coprecipitation synthesis of mesoporous Fe3O4, nanocages and their highlithium storage capacity as anode material for lithium-ion batteries, Electro-chim. Acta 160 (2015) 114e122.

[47] F. Han, L. Ma, Q. Sun, C. Lei, A. Lu, Rationally designed carbon-coated Fe3O4coaxial nanotubes with hierarchical porosity as high-rate anodes for lithiumion batteries, Nano Res. 7 (2014) 1706e1717.

[48] C. Ding, Y. Zeng, L. Cao, L. Zhao, Q. Meng, Porous Fe3O4-NCs-in-Carbonnanofoils as high-rate and high-capacity anode materials for lithium-ionbatteries from Na-Citrate mediated growth of super-thin Fe-Ethylene glyco-late nanosheets, ACS Appl. Mater. Interfaces 8 (2016) 7977e7990.

[49] A.S. Hameed, M.V.V. Reddy, B.V.R. Chowdari, J.J. Vittal, Preparation of RGOwrapped magnetite nanocomposites and its energy storage properties, RSCAdv. 4 (2014) 64142e64150.

[50] Y. Chen, B. Song, M. Li, L. Lu, J. Xue, Fe3O4 nanoparticles embedded in uniformmesoporous carbon spheres for superior high-rate battery applications, Adv.Funct. Mater. 24 (2014) 319e326.

[51] S.K. Behera, Enhanced rate performance and cyclic stability of Fe3O4-graphenenanocomposites for Li ion battery anodes, Chem. Commun. 47 (2011)10371e10373.

[52] J. Zhang, K. Wang, Q. Xu, Y. Zhou, F. Cheng, S. Guo, Beyond yolk-shell nano-particles Fe3O4@Fe3C core@shell nanoparticles as yolks carbon nanospindlesas shells for efficient lithium ion storage, ACS Nano 9 (2015) 3369e3376.

[53] Y. Sun, C. Wang, Y. Xue, Q. Zhang, R.G. Mends, L. Chen, T. Zhang, T. Gemming,M.H. Rümmeli, X. Ai, F. Lei, Coral-inspired nanoengineering design for long-cycle and flexible lithium-ion battery anode, ACS Appl. Mater. Interfaces 8(2016) 9185e9193.

[54] M. Chen, X. Shen, K. Chen, Q. Wu, P. Zhang, X. Zhang, G. Diao, Nitrogen-dopedmesoporous carbon-encapsulation urchin-like Fe3O4 as anode materials forhigh performance Li-ions batteries, Electrochim. Acta 195 (2016) 94e105.

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In-depth nanocrystallization enhanced Li-ions batteries performance with nitrogen-doped carbon coated Fe3O4 yolk−shell nano ...1. Introduction2. Experimental section2.1. Materials2.2. Characterization2.3. Electrochemical tests2.4. Preparation of nitrogen-doped carbon-encapsulation Fe3O4 yolk−shell nanocapsules

3. Results and discussion3.1. Characterization of Fe3O4@C-N nanocapsules3.2. Electrochemical performance of yolk-shell Fe3O4@C-N nanocapsules

4. ConclusionsAcknowledgementsAppendix A. Supplementary dataReferences