VOx Hollow Microboxes Derived from Metal

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Carbon-Coated Fe3O4/VOx Hollow Microboxes Derived from Metal-OrganicFrameworks as A High-Performance Anode Material for Lithium-Ion Batteries Zhi-Wei Zhao, Tao Wen, Kuang Liang, Yi-Fan Jiang, Xiao Zhou, Cong-Cong Shen, and An-Wu Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15110 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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ACS Applied Materials & Interfaces

Carbon-Coated

Fe3O4/VOx

Hollow

Microboxes

Derived

from

Metal-Organic-Frameworks as A High-Performance Anode Material for Lithium-Ion Batteries Zhi-Wei Zhao, Tao Wen, Kuang Liang, Yi-Fan Jiang, Xiao Zhou, Cong-Cong Shen, An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, 230026, China

1

ACS Paragon Plus Environment


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ABSTRACT: As the ever-growing demand for high-performance power sources, lithium-ion batteries with high storage capacities and outstanding rate performance have been widely considered as a promising storage device. In this work, starting with metal-organic frameworks (MOFs), we have developed a facile approach to the synthesis of hybrid Fe3O4/VOx hollow microboxes via the process of hydrolysis and ion exchange and subsequent calcination. In the constructed architecture, the hollow structure provides efficient lithium ions diffusion pathway and extra space to accommodate the volume expansion during the insertion and extration of Li+. With the assistance of carbon coating, obtained Fe3O4/VOx@C microboxes exhibit excellent cyclability and enhanced rate performance when employed as an anode material for lithium-ion batteries. As a result, the obtained Fe3O4/VOx@C delivers a high Coulombic efficiency (near 100%) and outstanding reversible specific capacity of 742 mAh g–1 after 400 cycles at a current density of 0.5 A g–1. Moreover, a remarkable reversible capacity of 556 mAh g–1 could be retained even at a current of 2 A g–1. This study provides fundamental understanding for the rational design of other composite oxides as high-performance electrode materials for lithium-ion batteries. KEYWORDS: Fe3O4/VOx, hollow microboxes, nanohybrid, carbon layers, lithium-ion batteries

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INTRODUCTION The recent rapidly growing use of mobile electronics such as mobile phones, notebook PCs, and camcorders necessitates the development of low-cost, reliable and efficient electric energy conversion/storage devices.1-3 Among those various energy conversion/storage systems, advanced lithium-ion batteries (LIBs) have attracted much attention and have been considered to be the ideal choice in this field because of their high energy density and long cycle life.4-6 Presently, graphite is a well-commercialized material for anodes due to its low cost and efficient cyclic performance. However, graphite suffers from a low theoretical discharge capacity of 372 mAh g–1, resulting in low power output.7,8 Therefore, developing electrode materials with both high energy and stability is urgent for satisfying demand of energy storage. Recently, metal alloys, silicon, transition metal oxides and chalcogenides with high capacity have been widely explored as anodes to replace graphite in lithium ion batteries.9-12 Among them, iron oxides have shown desirable properties, such as high theoretical capacity, low toxicity, low cost, and natural abundance, making them a possible candidate as the dominant anode material for commercial scale LIBs.13,14 Unfortunately, these materials usually suffer from poor electronic conduction and fail to maintain integrity and large volume change during discharge/charge cycles, thus hindering the industrialization as substitutions for graphite.15,16 Morphology-specific nano/micro-materials have aroused much interest because of their unique structure, outstanding physical-chemical properties and potential applications in energy areas.17,18 As one type of promising architectures, 3



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hollow/porous structure with a large specific surface area has recently stimulated tremendous research interest. For LIBs, the hollow and porous structure enables better access to Li+ ions since porous structure provides large amounts of cavities and high surface areas, which can increase the contact area between electrode and electrolyte and active sites for lithium redox reactions.19 In particular, the hollow interior could provide enough space to buffer the mechanical stresses that accompany the volume changes during repeated Li+ insertion/extraction processes, thereby preventing anode degredation and improving cycle performance. Inspired by the unique properties, many researches have been dedicated to the rational design and synthesis of hollow/porous structure including Kirkendall effect,20 chemical etching,21 ionic exchange,22 solid-state decomposition23 and self-organization.24 For example, template-engaged methods are generally used to construct hollow structure via the growth of desirable shell materials with the assistance of sacrificial templates.25 Such approach is effective to synthesize available hollow materials, yet the obtained hollow particles only present relatively single configurations. Most recently, hollow structure materials containing multiple components exhibit highly desirable features and enhanced performance compared to single component hollow structure. As such, considerable attention should be devoted to extending the organization of hierarchical hollow hybrid materials.

Metal-organic frameworks (MOFs), emerging as a new class of porous crystalline materials with a coordination network of inorganic components (metal ions or metal clusters) and organic components (organic ligands) linked together, have become a 4


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hot research topic in the last decades.26-28 Previous studies have demonstrated that MOFs can act as both templates and precursors for the synthesis of new nanostructured materials.29,30 Compared with traditional templates/precursors, MOFs offer distinct advantages to the formation of hollow structures because of their unique morphology and porosity, providing a unique opportunity to develop a new class of tailored anode materials. For example, Prussian blue (PB) and its analogues were utilized to prepare Co3O4 nanocages through a thermolysis procedure.31 By using Cu-based MOFs as the sacrificial template, Hu et al. fabricated CuO/Cu2O hollow polyhedra for LIBs with a long cycling life.32 Despite some progresses have been achieved, hollow structure materials with high complexity derived from MOFs and their applications in energy storage are still lack of systematic study, because it is difficult to integrate different materials with various physical/chemical properties into the synthetic process simultaneously.

In this work, we report a large-scale and facile procedure for the synthesis of Fe3O4/VOx hollow microboxes based on template-engaged reactions between PB template and sodium orthovanadate, followed by thermal treatment of poly(dopamine) (PDA) coating. By taking advantage of the unique structure and chemical behavior of MOFs, the obtained carbon coated Fe3O4/VOx (Fe3O4/VOx@C) product is porous and uniform, which would efficiently restrict the volume expansion and reduce transport distance of electrons and ions during the charging/discharging process. When evaluated as anode for LIBs, the MOF-derived Fe3O4/VOx@C composite exhibits

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superior Li+ storage properties in terms of capacity, rate capability, and stability of cycling performance.

EXPERIMENTAL SECTION Chemicals. Polyvinyl pyrrolidone (PVP, K30, MW~40000), HCl, Na3VO4, K4Fe(CN)6·3H2O and tris(hydroxymethyl)aminomethane (Tris) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dopamine was purchased from Sigma Aldrich. All the chemicals were of analytical grade without further purification. Synthesis of Prussian Blue Microcubes. In a typical experiment, 0.5 g of K4Fe(CN)6·3H2O was added to a 200 mL of HCl solution (0.1 M) containing 12.0 g PVP under magnetic stirring. After vigorous stirring for 30 min, a bright solution was obtained, and the beaker was then sealed and heated at 80 oC for 24 h. After cooled to room temperature naturally, the blue precipitates were separated by centrifugation, washed several times with distilled water and ethanol respectively and then dispersed into 80 mL of alcohol for further use. Synthesis of Fe3O4/VOx@C Microboxes. To obtain Fe(OH)3/FeVO4·xH2O hybrid microboxes, 10 mL of as-prepared PB suspension in ethanol was mixed with 17 mL deionized water containing 0.12 g of sodium orthovanadate. After the mixture was shaken for about 15 min at room temperature, the precipitates were filtered and washed several times with deionized water and ethanol and then dried under vacuum at 80 oC for 12 h. To coat PDA layer on the surface of microboxes, 100 mg of Fe(OH)3/FeVO4·xH2O was dispersed in 50 mL Tris-HCl buffer (pH ~ 8.5) solution with 45 mg of dopamine through ultrasonication and then polymerized for 24 hours. 6


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After centrifugal washing with ethanol and drying at 60 oC in electrical oven, the PDA-coated Fe(OH)3/FeVO4·xH2O was collected. Finally, Fe3O4/VOx@C hollow hybrid microboxes were obtained by annealing the above black powder at 500 oC for 2 h in N2 with a slow heating rate of 2 oC min-1. Characterization X-ray diffraction (XRD) patterns were taken with Philips X’Pert Pro Super X-ray diffractometer using Cu-Kα radiation (λ = 1.54178 Å). The sample morphologies were observed by scanning electron microscopy (SEM, JEOL JSM-6330F, 15.0 kV). High-resolution transmission electron microscopic (HRTEM) images, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) elemental mapping analyses were performed on a JEOL JEMARF200F atomic resolution analytical microscope with a spherical aberration corrector. The Barrett-Emmett-Teller (BET) specific surface areas of samples were measured using N2 adsorption/desorption isotherms Micromeritics ASAP-2010 system at 77 K. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. Thermogravimetric analysis (TGA) was carried out using a Shimadzu-50 thermoanalyser under air gas flow at 10 oC min-1 in the temperature range of 30-800 oC. The mass ratio of Fe/V in the multi-compositional microboxes was analyzed by inductively coupled plasma (ICP) optical emission spectroscopy. Electrochemical Measurements The electrochemical tests were carried out by assembly of 2032 coin cells in a 7



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glove box filled with pure argon gas. The electrode was prepared by mixing as-prepared active materials, carbon black (super P) and polymer binder (polyvinylidene fluoride, PVDF) (8:1:1, weight ratio) on a piece of Cu foil, and then dried in a vacuum oven at 120 oC overnight. The mass loading of the active materials on copper foil was about 1.0 mg cm−2. The electrolyte solution was composed of 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1. A lithium disk was used as both the counter and reference electrode. Galvanostatic charge/discharge cycling tests were performed between 0.01 and 3.0 V on a NEWARE battery tester at room temperature. Cyclic voltammetry (0.1 mV s–1) and AC-impedance spectra (100 kHz-0.01 Hz) were carried out on a CHI-660E (CH Instruments, Inc.) workstation. RESULTS AND DISCUSSION The overall formation processes of the multi-compositional Fe3O4/VOx@C hollow microboxes are illustrated in Figure 1. Highly uniform Prussian blue (PB, Fe4[Fe(CN)6]3) microcubes were firstly synthesized by a simple hydrothermal method. A subsequent conversion process was performed on the template-engaged reaction between unique PB templates and Na3VO4. Sodium orthovanadate could produce hydroxide ions due to the partial hydrolysis in solution. PB has a very small solubility product constant (Ksp = 3.3×10−41), however, PB is unstable in alkaline solution and undergoes an ion exchange with vanadate anions and hydroxide ions to form insoluble precipitate, giving rise to Fe(OH)3 and FeVO4·xH2O at room temperature. Under appropriate conditions, the process of hydrolysis and ion exchange take place 8


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simultaneously. Therefore, the reaction occurs from the outside to inside to form yolk shell/hollow Fe(OH)3/FeVO4·xH2O hybrid microboxes. Finally, poly(dopamine) (PDA) was coated on the surface of the as-synthesized products and then Fe3O4/VOx@C microboxes were obtained through annealing treatment in N2 atmosphere. Along with the release of volatile matter such as H2O, CO, and CO2, a large amount of voids were generated under high temperature (500 oC). VO43-

VO43-

PDA Annealing

Fe4[Fe(CN)6]3

Fe(OH)3/FeVO4·xH2O

Yolk shelled Microbox

Fe3O4/VOx@C

Figure 1. Schematic illustration of fabrication of Fe3O4/VOx@C microboxes starting from Prussian blue cubic. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to investigate the morphology and detailed structure of the as-obtained products. As shown in Figure 2a and 2d, it can be observed that PB has a three-dimensional microcubes shape with a uniform size. At the beginning, the addition of Na3VO4 caused the transformation of PB into multi-compositional composites in the nearsurface region through hydrolysis and ion exchange.18 As illustrated in Figure 1, a visible interlayer gap appears between the shell and the remaining PB core after 10 minutes of reaction (Figure 2b and 2f). Compared with PB solid cubes, the appearance of yolk-shell structure reveals that the reaction occurs from the outside to inside. As the reaction proceeds to 20 minutes, it is worth noting that the core disappeared and the hollow microboxes were observed (Figure 2c and 9



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2g). A continuous PDA layer was then coated on the surface of sample. After annealing at 500 °C in N2, as-prepared material was transformed into multi-compositional composites (Figure 2d). After carbonization, TEM image confirms the formation of carbon layer with a thickness of ~20 nm on the surface of microboxes (Figure 2h). For comparison, the collapse of sample was found in the absence of PDA layer (Figure S1), indicating that the carbon layer is beneficial for maintaining integrity of the structure during pyrolysis treatment. To estimate the carbon content of these products, thermogravimetric analysis (TGA) was carried out in air (Figure S2). The major weight loss between 300 and 450 °C can be ascribed to the combustion of carbon in air. The carbon content was estimated to be 18.24 wt% for Fe3O4/VOx@C sample. For Fe3O4/VOx microboxes, a small weight loss (2.56 wt%) is believed to come from PVP, which was adsorbed on the surface of sample acting as carbon source during the pyrolysis process. Moreover, there was a slight weight gain from 200 to 300 °C, attributing to the oxidation of Fe3O4 to Fe2O3.

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(440) (531) (442)

(422) (511)

(400) (331)

(111)

(222)

(220)

Fe3O4/VOx@C

Intensity (a.u.)

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(311)

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Fe3O4/VOx Fe3O4 (JCPDS: No. 88-0315) 10

20

30

40

50

60

70

2θ (degree)

Figure 2. SEM (a-d) and TEM (e-h) images of samples: PB microcubes (a, e), PB@Fe(OH)3/FeVO4·xH2O yolk-shelled microboxes (b, f), Fe(OH)3/FeVO4·xH2O (c, g), and Fe3O4/VOx@C hollow boxes (d, h); (i) XRD patterns

of Fe3O4/VOx and

Fe3O4/VOx@C microboxes; HRTEM images (j, k), STEM (l) and EDX element mappings (m) of Fe3O4/VOx@C sample.

The chemical compositions and the crystallographic structures, the obtained samples were determined by powder X-ray diffraction (XRD) (Figure 2i and Figure S3). The precursor can be identified as Fe4[Fe(CN)6]3 without impurities (Figure S3a). However, two broad peaks are presented in the XRD pattern of Fe(OH)3/FeVO4·xH2O after dried at 80 oC for 12 h as shown in Figure S3b, indicating that a part of amorphous Fe(OH)3 could be converted to Fe2O3 during a low-temperature drying 11



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process.18

After

annealing

process,

the

diffraction

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peaks

of

as-prepared

_

Fe3O4/VOx@C can be assigned to Fe3O4 in Fd3m space group (JCPDS: No. 88-0315). Fe3O4 component was generated from the thermal decomposition and carbothermal reduction of Fe(OH)3/FeVO4·xH2O@PDA microboxes.33 No peaks of any vanadium oxides can be observed. However, the Fe/V atomic ratio (2.1:1) determined by the inductively coupled plasma (ICP) analysis proves the existence of amorphous vanadium oxide. Moreover, the material without PDA layer were determined to be Fe3O4/VOx after annealing treatment in N2 atmosphere, which was reduced by carbon from PVP existing in Fe3O4/VOx during annealing process, as confirmed by the TGA analysis of Fe3O4/VOx sample (Figure S2). The HRTEM images further demonstrate this point (Figure 2j and 2k). From Figure 2k, a lattice fringe with d-spacing of approximately 0.298 nm is apparently observed, corresponding to the (220) plane of Fe3O4. In addition, a typical view of carbonaceous layer and amorphous VOx can be clearly seen in the HRTEM images of Fe3O4/VOx@C sample. In order to further confirm the inner architecture and compositions, the spatial distribution of different elements in the Fe3O4/VOx@C was measured by the elemental mapping (Figure 2m). One can see that Fe, V, O, and C elements are distributed homogeneously all over whole sample, indicating the successful formation of Fe3O4/VOx@C with different composites. These evidences suggest the successful coupling of the hydrolysis process and the ion exchange reaction to form uniform hollow composites. Energy-dispersive X-ray spectroscopy (EDX, Figure S4) result further confirms the presence of V element in the sample, resulting from the ion exchange reaction as 12


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expected. For further investigation of the chemical states and compositions of samples, X-ray photoelectron spectroscopy (XPS) measurements were carried out on the Fe(OH)3/FeVO4·xH2O and Fe3O4/VOx@C to determine the oxidation states of the iron and vanadium. The XPS survey reveals the presence of Fe 2p, V 2p, O 1s and C 1s in the as-prepared samples (Figure S5), in accordance with EDX analysis. For Fe(OH)3/FeVO4·xH2O sample, the corresponding high resolution XPS of Fe 2p spectrum (Figure 3a) indicates that two strong peaks located at the binding energies of 711.0 eV for Fe3+ 2p3/2 and 724.6 eV for Fe3+ 2p1/2, accompanied by a strong satellite peak that appeared at around 719 eV.34 In the V 2p spectrum as shown in Figure 3c, a core peak at 516.9 eV is indicative of the typical character of V5+ 2p3/2.8,35 Therefore, the oxidation states of Fe and V in Fe(OH)3/FeVO4·xH2O exist as Fe3+ and V5+, respectively. However, for the XPS Fe 2p spectrum of Fe3O4/VOx@C hollow boxes, a pair of characteristic peaks appear at the binding energies of 709.1 eV and 722.3 eV, corresponding to the Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively (Figure 3b).33 It is noted that the V 2p3/2 core peak spectrum for Fe3O4/VOx@C (Figure 3d) is composed of two components located at 516.9 eV and 515.6 eV, which can be attributed to two formal oxidation degrees of V5+ and V4+, respectively.8 For C 1s spectrum of Fe3O4/VOx@C (Figure S6a), the peak positions at 284.7 and 285.6 eV can be ascribed to the carbon atoms in the form of C–C and C–O.36 Raman spectrum of carbon-coated Fe3O4/VOx is given in Figure S6b, indicating the existence of carbon in the composite.34 N2 adsorption/desorption was measured to characterize the porous structure of 13



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as-prepared samples and acquire information about the specific surface area and pore size. As shown in Figure 3e and 3f, the Brurauer-Emmerr-Teller (BET) surface area of Fe3O4/VOx@C micro-boxes is 94 m2 g-1, which is larger than that of Fe3O4/VOx (66 m2 g-1). Without carbon coating, the collapse of Fe3O4/VOx cubic shape leads to the decrease of surface area during annealing process, as confirmed by SEM and TEM observations (Figure S1). The corresponding pore size distribution indicates that an average pore diameter of Fe3O4/VOx@C is about 3.89 nm (inset in Figure 3e and 3f). The results imply that the as-prepared samples have a loose mesoporous structure, which is beneficial for providing more active sites and is able to buffer the volume changes of microboxes during electrochemical reactions.

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(c)

(a)

3+

Satellite

705

710

5+

V

3+

Fe 2p1/2 Intensity (a.u.)

Intensity (a.u.)

Fe 2p3/2

715

720

725

730

514

515

516

(b)

3+

2+

Fe 2p1/2

2+

705

Fe 2p1/2

710

715

720

725

4+

V

730

514

515

80

3

-1

100

-1

0.006 0.004

3

100

0.002 0.000 1

10

100

Pore Diameter (nm)

60 40

2

20

517

518

519

120

Pore Volume (cm g nm)

2.84 nm

Volume Adsorbed (cm g STP)

3

-1 3

(f)

0.008

-1

Pore Volume (cm g nm )

120

516

Binding Energy (eV)

Binding Energy (eV)

(e) 140

519

V

Intensity (a.u.)

Intensity (a.u.)

Fe 2p3/2

518

5+

(d)

3+

Fe 2p3/2

517

Binding Energy (eV)

Binding Energy (eV)

Volume Adsorbed (cm g STP)

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66 m g

-1

80

60

0.05

3.89 nm

0.04 0.03 0.02 0.01 0.00 1

10

Pore Diameter (nm)

100

40

20

2

-1

94 m g 0

0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Relative Pressure (P/P0)

Figure 3. Fe 2p XPS spectra of Fe(OH)3/FeVO4xH2O (a) and Fe3O4/VOx@C (b); high-resolution XPS spectrum of V 2p of Fe(OH)3/FeVO4·xH2O (c) and Fe3O4/VOx@C (d); nitrogen adsorption-desorption isotherms and corresponding pore size distribution (inset) of Fe3O4/VOx (e) and Fe3O4/VOx@C sample (f). To evaluate the performance of Fe3O4/VOx@C hollow microboxes in lithium-ion batteries (LIBs), the electrochemical properties versus Li/Li+ were systematically investigated using half cell. The representative cyclic voltammetry (CV) curves of Fe3O4/VOx@C nanocomposite electrode were measured at a scan rate of 0.1 mV s−1 in 15



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the potential range from 0.01 to 3.0 V at room temperature (Figure 4a). In general, the cathodic and anodic peaks are ascribed to the Li+ insertion/extraction processes for the active material, respectively. According to previous reports,37-39 it is noted that the first CV curve is obviously different from that of subsequent cycles, especially for the discharge branch. Therefore, two well-defined peaks can be identified at 0.84 V and 0.61 V in the first discharge cycle, which is usually attribute to the two steps of the lithiation reactions of Fe3O4.39 Fe3O4 + 2Li+ + 2e → Li2(Fe3O4) Li2(Fe3O4) + 6Li+ + 6e → 3Fe0 + 4Li2O And, a weak peak can be observed accompanied by the formation of LixVOy, when discharged between 1.5 and 2.0 V as shown in Figure 4a. During the anodic sweep, the result shows two distinguished oxidation peaks at 1.26 and 2.5 V. The former peak is assigned to decomposition of Li2O and oxidation of Fe(0), whereas the latter is due to extraction of Li ions from LixVOy matrix.8,40 Especially, the amorphous matrix of VOx, which not only functions as the reaction sites but also serves as the separator,

preventing

the

agglomeration

of

the

nanograins

during

lithiation/delithiation process.41 Apparently, the peak intensity reduced noticeably in the second cycle, accompanied a slight shift of its voltage position, indicating the occurrence of some irreversible processes with the formation of a solid electrolyte interface (SEI) film in the electrode material. Additionaly, it is noteworthy that the CV curves of Fe3O4/VOx@C microboxes in the subsequent two cycles almost overlap, revealing that the electrochemical reaction proceeded to a similar extent. This 16


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phenomenon can be attributed to the formation of a stable SEI film on the surface and interface in the first cycle, which can prevent the direct contact of Fe3O4/VOx@C microboxes with electrolyte during subsequent cycles, thus leading to the stable and superior reversibility. Figure 4b shows representative charge-discharge curves of Fe3O4/VOx@C hollow boxes for the 1st, 2nd, 5th, and 10th cycles at a current density of 0.1 A g–1 with a cutoff window of 0.01-3.0 V. It can be seen that the initial discharge capacity of the lithium battery storage is up to 1138 mAh g–1, but a relative low reversible capacity of 793 mAh g–1 is achieved. The irreversible capacity loss in the first cycle might be primarily ascribed to the decomposition of electrolyte and the formation of SEI film. Noticeably, the discharge potential plateau at about 0.84 V versus Li/Li+ in the first cycle is observed for Fe3O4/VOx@C sample, suggesting that the lithium insertion reactions as well as irreversible reactions occurred, this is in good agreement with the CV result.42,43 0.2

(b) 1.26 V

Anodic

2.5 V

3.0

-1

Current (A g )

0.0

-0.1

Cathodic

0.84 V 0.74 V

-0.2

0.61 V

1st 2nd 3rd

-0.3

Charge 2.0 1.5 1.0

Discharge

0.5 0.0

-0.4 0.0

0.5

1.0

1.5

2.0

2.5

0

3.0

900

1200

1500

120

1600

1500

(d)

Fe3O4/VOx@C (Discharge) Fe3O4/VOx@C (Charge)

1200

-1

Fe3O4/VOx (Discharge) Fe3O4/VOx (Charge)

0.1 A g

-1

900

Specific Capacity (mAh g )

-1

600

Spectific Capacity (mAh g )

Voltage (V vs. Li/Li )


300

-1

+

(c)

1st 2nd 5th 10th

2.5 +

0.1

Voltage (V vs. Li/Li )

(a)

-1

0.1 A g

-1

0.2 A g

-1

0.5 A g

-1

1Ag

-1

2Ag

600

300

100 1200

80 -1

0.5 A g

60

800

40 400

20 0

0

0 0

10

20

30

40

50

60

0

100

Cycle Number

200

Cycle Number

17


300

400

Coulombic Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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Figure 4. (a) CV curves of Fe3O4/VOx@C microboxes at a sweep rate of 0.1 mV s–1 in the potential range from 0.01 to 3.0 V versus Li/Li+; (b) charge-discharge curves of Fe3O4/VOx@C at the current density of 0.1 A g–1; (c) rate capabilities of Fe3O4/VOx@C and Fe3O4/VOx samples at various rates; (d) cycling stability of Fe3O4/VOx@C hollow boxes at the high current density of 0.5 A g–1. To highlight the unique superiority of our hollow microboxes for anode materials of LIBs, the rate capabilities of Fe3O4/VOx and Fe3O4/VOx@C are compared from Figure 4c, it can be clearly seen that the reversible capacity of Fe3O4/VOx@C material was maintained at 796 mAh g–1 after 10th cycle at 0.1 A g–1. Upon increasing the discharge-charge rates to 0.2 A g–1, 0.5 A g–1, 1 A g–1 and 2 A g–1, the measured discharge capacities were about 746 mAh g–1, 669 mAh g–1, 622 mAh g–1 and 556 mAh g–1, respectively. When the current rate was finally returned to its initial value of 0.1 A g–1 after a total of 50 cycles, a capacity of 833 mAh g–1 was recoverable and an interesting phenomenon of gradually increasing capacity was observed up to the 60th cycle, indicating the structure refinement and formation of a thin and stable SEI without fracture.44 In contrast, Fe3O4/VOx sample displays significantly lower capacity, which confirms the advantages of Fe3O4/VOx@C microboxes used for electrode materials. The initial coulombic efficiency of carbon-coated Fe3O4/VOx is 70%, which is higher than that of Fe3O4/VOx hollow microboxes (65%), showing a decrease of the irreversible capacity loss. The higher coulombic efficiency of carbon-coated Fe3O4/VOx electrode indicates that a stable SEI film formed on the surfaces and interfaces of carbon layers in the first cycle can avoid the direct contact 18


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of Fe3O4/VOx with electrolyte and defend the interior Fe3O4/VOx from structural damage during long-term charge-discharge cycles.34 To compare the cycling stability of Fe3O4/VOx and Fe3O4/VOx@C, the electrodes were further investigated at a current density of 0.5 A g–1 for 400 cycles under the same conditions. As anticipated, Fe3O4/VOx@C materials electrode exhibits a good cycling stability, as displayed in Figure 4d. Obviously, the discharge capacity fades in the first several dozens of cycles. After that, the discharge capacity starts to increase and reaches a high capacity of 742 mAh g–1 after 400 cycles at a current density of 0.5 A g–1. Apparently, such performance of carbon-coated Fe3O4/VOx in terms of cycling capability is superior to that of Fe3O4 and Fe3O4@C based anodes reported in previous works (Table S1, Supporting Information), demonstrating the advantages of this new structure with improved the lithium storage properties. It is well-known that the electrode surface would be covered by a SEI layer during the charge-discharge process, which forms due to the electrolyte decomposition.42 Under a high-rate charge/discharge and long cycles, the electrode material will undergo high mechanical degradation owing to the drastic volume changes associated with the repeated Li+ insertion/extraction processes. The aforementioned phenomenon may be attributed to the fracture and reform of the outside SEI, leading to an unstable and thick SEI film during cycling test and capacity fading. After an extremely long cycle, the reactivated electrode materials reveal an outstanding cycling stability by the formation of a stable SEI film.44 Furthermore, their Coulombic efficiency rapidly increases from 62.7% for the first cycle to nearly 100% after fifteen cycles and remains thereafter. For Fe3O4/VOx material without 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

carbon layers, it shows a much lower capacity than Fe3O4/VOx@C microboxes (Figure S7). The discharge capacity decreases to 308 mAh g–1 after 400 cycles at a current density of 0.5 A g–1. As for Fe3O4/VOx@C material, the porous and uniform structure would efficiently restrict the volume expansion and reduce transport distance of electrons and ions during the charging/discharging process, thus leading to a much higher capacity as compared to Fe3O4/VOx. Moreover, the existence of carbon layer can allow Fe3O4/VOx@C to expand upon lithium ions insertion without breaking the carbon shell. To optimize the ratio between Fe3O4 and VOx, Fe3O4/VOx@C-1, Fe3O4/VOx@C-2 and Fe3O4/VOx@C-3 with different Fe/V atomic ratio were prepared by adding different amount of sodium orthovanadate and the LIB performance was measured (see Figure S8). As a result, the sample at a Fe/V atomic ratio of 2.1:1 displays an outstanding capacity, which confirms the advantages of Fe3O4/VOx@C-2 used for electrode materials. The morphology of the cycled carbon-coated Fe3O4/VOx and Fe3O4/VOx electrode was investigated to reveal its structure evolution after 100 cycles at a current density of 0.5 A g−1 (Figure S9). Obviously, the integrity of carbon-coated Fe3O4/VOx hollow microboxes can be well maintained even after 100 cycles, while for Fe3O4/VOx sample without carbon coating, the structure was severely broken. This characteristic agrees well with the result that Fe3O4/VOx@C materials electrode exhibits a higher cycling stability.

20


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800 700 600

-Z'' (ohms)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500 400 300 200

Fe3O4/VOx@C

100

Fe3O4/VOx

0 0

100

200

300

400

500

600

700

800

Z' (ohms)

Figure 5. Nyquist plots of Fe3O4/VOx@C and Fe3O4/VOx material anodes at fresh coin cells over the frequency range from 100 kHz to 0.01 Hz. To

further

understand

the

mechanism

for

the

improved

performance,

electrochemical impedance spectroscopy (EIS) measurements, a promising tool for investigating diffusion issues, were carried out for Fe3O4/VOx and Fe3O4/VOx@C electrodes at frequencies from 100 kHz to 0.01 Hz. As presented in Figure 5, the Nyquist plots share the common feature with a high-frequency depressed semicircle and an inclined line in the low-frequency region, corresponding to the electrode reaction and lithium ion diffusion in the solid.45,46 Besides, the diameter of the semicircle for Fe3O4/VOx@C material in the high-frequency region is obvious smaller than that of Fe3O4/VOx microboxes, indicating that the electrode of Fe3O4/VOx@C facilitate rapid charge transport during the electrochemical Li+ insertion/extraction reaction. It is thus concluded that the unique hierarchical porous microboxes and the carbon layer provides a conductive pathway to charge transfer across the 21



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electrode/electrolyte interface, resulting in excellent cycle performance and enhanced lithium storage. Conclusions In summary, novel Fe3O4/VOx hollow microboxes are rationally designed and fabricated through template-engaged reaction between Prussian blue cubes and Na3VO4 and subsequent calcinations. The obtained carbon-coated Fe3O4/VOx@C hollow microboxes possess an uniform size and compositional complexity. In this architecture, the hollow structure and nanopores provide efficient lithium ions diffusion pathway and free space to accommodate the volume expansion during the charging/discharging procedures. Furthermore, the carbon layers are favorable for stabilizing the original structure as well as enhance the electric conductivity of the electrode materials. When evaluated as anode materials for LIBs, the Fe3O4/VOx@C sample exhibits a better cycling performance with a high reversible capacity of 742 mAh g–1 after 400 charge/discharge cycles compared to the carbon-free counterpart. As a result, Fe3O4/VOx@C microboxes are promising candidate for the next generation of LIBs with high energy and power density. The present work provides a facile and general approach to design other advanced hollow porous hybrid materials derived from MOFs for potential applications in high performance LIBs, supercapacitors, sensors, catalysts.

ASSOCIATED CONTENT

Supporting Information 22


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SEM, TEM, TGA curve, XPS survey of Fe3O4/VOx microbox; TGA, EDX and XPS characterization of Fe3O4/VOx@C material; XRD pattern of Prussian blue; XRD pattern and XPS spectrum of Fe(OH)3/FeVO4·xH2O after dired at 80

o

C;

Charge/discharge capacities of the Fe3O4/VOx@C as anode of LIBs. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel: +86 0551 63602346. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The special funding support from the Scientific Research Grant of Hefei Science Center of CAS (2015SRG-HSC048), the National Basic Research Program of China (2011CB933700), the National Natural Science Foundation of China (51572253, 21271165, 81372821) and Scientific Research Grant of Hefei Science Center of CAS (2015SRG-HSC048) is acknowledged. REFERENCES (1) Liu, B.; Zhang, J.; Wang, X. F.; Chen, G.; Chen, D.; Zhou, C. W.; Shen, G. Z. Hierarchical Three-Dimensional ZnCo2O4 Nanowire Arrays/Carbon Cloth Anodes for a Novel Class of High-Performance Flexible Lithium-Ion Batteries. Nano Lett. 2012, 12, 3005-3011. (2) Pasta, M.; Wessells, C. D.; Huggins, R. A.; Cui, Y. A High-Rate and Long Cycle Life Aqueous Electrolyte Battery for Grid-Scale Energy Storage. Nat. Commun. 2012, 3, 1149. 23



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29



Table of Content

120

-1


1600

100 1200

80

-1

0.5 A g

60

800

40 400

20

Coulombic Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0

0 0

100

200

300

400

Cycle number

30


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VOx Hollow Microboxes Derived from Metal

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