Three-Dimensional Porous Iron Vanadate Nanowire Arrays as a High

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A New Three-dimensional Porous Iron Vanadate Nanowire Arrays as a High-Performance Lithium-ion Battery Yunhe Cao, Dong Fang, Ruina Liu, Ming Jiang, Hang Zhang, Guangzhong Li, Zhiping Luo, Xiaoqing Liu, Jie Xu, Chuanxi Xiong, and Weilin Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08282 • Publication Date (Web): 27 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015

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ACS Applied Materials & Interfaces

A New Three-dimensional Porous Iron Vanadate Nanowire Arrays as a High-Performance Lithium-ion Battery Yunhe Caoa, Dong Fang*a, Ruina Liua, Ming Jianga, Hang Zhanga, Guangzhong Lib, Zhiping Luoc, Xiaoqing Liud, Jie Xua, Chuanxi Xiongad, Weilin Xu*a a

Key Lab of Green Processing and Functional Textiles of New Textile Materials, Ministry of Education, College of

Material Science and Engineering, Wuhan Textile University, Wuhan, P. R. China E-mail: [email protected];

[email protected].

b

State Key Laboratory of Porous Metal Material, Northwest Institute for Non-ferrous Metal Research, Xi’an, P. R. China.

c

Department of Chemistry and Physics, Fayetteville State University, Fayetteville, NC 28301, USA.

d

School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People's Republic of

China.

Abstract: Development of three-dimensional nano-architectures on current collectors has emerged as an effective strategy for enhancing rate capability and cycling stability of the electrodes. Herein, a new type of three-dimensional porous iron vanadate (Fe0.12V2O5) nanowire arrays on a Ti foil has been synthesized by a hydrothermal method. The as-prepared Fe0.12V2O5 nanowires are about 30 nm in diameter and several micrometers in length. The effect of reaction time on the resulting morphology is investigated and the mechanism for the nanowire formation is proposed. As an electrode material used in lithium ion batteries, the unique configuration of the Fe0.12V2O5 nanowire arrays presents enhanced capacitance, satisfying rate capability and good cycling stability, as evaluated by cyclic voltammetry and galvanostatic discharge-charge cycling. It delivers a high discharge capacity of 293 mAh·g-1 at 2.0-3.6 V or 382.2 mAh·g-1 at 1.0-4.0 V after

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50 cycles at 30 mA·g-1. Keyword: iron vanadate, electrode, nanowire array, one-dimensional, lithium-ion battery

1. Introduction The versatility of vanadium in terms of accessible valence states (from 3 to 5), coordination number (4-6), and coordination polyhedral shapes (tetrahedron to octahedron with intermediate square pyramid) allows a large variety of structure types to be obtained.1 Considerable research efforts have been devoted to constructing vanadium oxides or vanadate nanostructures, due to their applications in the areas of catalysis and lithium ion batteries (LIBs).2-5 For instance, vanadium oxides (V2O5) have attracted extensive interest as potential battery materials, which can deliver much higher theoretical capacities than those of commercialized lithium cobalt oxides (LiCoO2) 6. However, their viability in practical batteries is hampered by their tendency to become amorphous upon cycling, which can be changed and improved by introduction of secondary metal cations (M= Li, Na, K, Fe, Cu, or Ag).7-11 The embedded metal cations can raise the electronic conductivity of the material through replacedment of V by metal cation.12 Further, the [MO6] octahedral chains formed in the framework of MxV2O5 prevent the deformation of the material structure during electrochemical cycling.2,

13-15

For example, Fe0.11V2O5.16 has a better

electrochemical performance than the undoped V2O5, because Fe3+ stabilizes the V2O5 host lattice and delays the ε→δ→γ phase transitions at a potential close to 2.0 V vs. Li+/Li.16 Li et al. also demonstrated a special 3D-porous Fe0.1V2O5.15 thin film with a high reversible capacity (255 mAh g-1) and stable capacity retention.17 As alternatives to the presently used positive electrode materials for LIBs (layered LiCoO2-type or spinel LiMn2O4-type), numerous polyanionic compounds, such as, -PO4 18, -P2O7

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, -MoO4,20 -SO4,20 and -BO6,21 have been studied. Among these, LiFePO4 exhibits the most

promising electrochemical performance, with safety, stability and cost advantages over the layered cobalt nickel oxides.22 Vanadate groups behave structurally like -POx and -SOx polyanions, but vanadium may be electrochemically active, and therefore may contribute to theoretical capacity.2 Therefore, various iron vanadates possess high lithium ion storage capacities owing to multistep reductions and more electron transfers upon lithium ion intercalations, which can be prepared by metal organic pyrolysis,3 low temperature preparation23 or hydrothermal synthesis.24,

25

The

hydrothermal synthesis is a promising method to prepare samples with controlled shapes, nanostructures, or even single crystals,26 which have been recognized for possible use in energy storage and conversion. 27, 28 In present work, a new type of three-dimensional porous iron vanadate (Fe0.12V2O5) nanowire arrays on Ti foil is synthesized, for the first time, by a facile one-step hydrothermal method. Furthermore, the effects of the reaction time and annealing temperature on the product morphology and structure are also investigated. The novel nanostructure will shorten diffusion pathways for lithium ions during intercalation/de-intercalation benefiting lithium ion (Li+ ion) flux crossing the solid-liquid interfaces.24 Due to the unique properties of these nanowire arrays, such as high surface areas, crystallinity, good conductivity and direct growth on conductive substrates, they have potential applications in LIBs, chemical sensing, electrochemical or photocatalysis, field emission, and electrochromic devices.29,

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As an electrode material for LIBs, the Fe0.12V2O5

nanowire arrays on Ti foil presents outstanding lithium ion storage properties.

2. Experimental Section 2.1 Materials Synthesis

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The Fe0.12V2O5 nanowires were synthesized through a hydrothermal method using a Ti foil as the substrate. All the chemicals were of analytical grade and used as received without any further purification. The mixed solution of FeCl3·6H2O (1.5 mmol), NH4VO3 (1.5 mmol) and H2C2O4·2H2O (4.2 mmol) was added into a 50 mL Teflon container, followed by addition of hexamethylenetetramine (HMTA, 3 mmol) with stirring. For comparison, different amount of HMTA (0, 1.5, 3, 6, 12 mmol) was added into the reaction solution while NH4VO3, FeCl3·6H2O were used in the same amount in each hydrothermal reaction. A piece of Ti foil (99.5% purity) was placed into the container. Then the container was sealed in an autoclave and transferred to an electrical oven to keep at 150 °C for different time (20 min, 30 min, 40 min, 1 h and 2 h). The samples synthesized at 150 °C for 1 h were further annealed at different temperatures (200, 250 and 400 °C) for 2 h, other samples were annealed at 250 °C. 2.2 Materials Characterization X-ray diffraction (XRD) patterns of the as-prepared products and that annealed at different temperatures were collected on a Bruker D8 Advanced X-Ray Diffractometer. Surface morphology of the samples was examined with field-emission scanning electron microscopy (FE-SEM) (Hitachi S-4800). Transmission electron microscopy (TEM) images were taken on JEOL-2100F at an accelerating voltage of 200 kV. The chemical compositions of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, KAlpha 1063, Thermo Fisher Scientific, UK). The Fourier Transform infrared spectroscopy (FTIR) spectra were recorded using a Nicolet 6700 FTIR spectrometer. 2.3 Electrochemical Measurements The coin cells (CR 2016) were assembled to investigate the electrochemical properties; and

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sample of Fe0.12V2O5 nanowires on Ti foil (about 1.6 mg) was directly used as the working electrode. The cells were assembled in an argon-filled glove box with water and oxygen contents less than 1 ppm. The cells were assembled based on the configuration of Li/electrolyte/Fe0.12V2O5 nanowires configuration with a liquid electrolyte (1.0 M LiPF6 in ethylene carbonate/diethyl carbonate). A metallic Li foil was used as the counter electrode in 2-electrode cell, and Celgard 2400 was used as a separator. The cells were then aged for 8 h before measurement. Cyclic voltammetry (CV) studies were carried out on an electrochemical workstation (Corrtest Instruments, model CS120). The cells were galvanostatically charged and discharged using a Neware battery tester CT 3008. The electrochemical impedance measurements (EIS) were carried out by applying 100 kHz to 0.001 Hz frequency ranges with acoscillation amplitude of 5 mV on an AUTOLAB electrochemical workstation (PG302N).

3. Results and Discussion (b)

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Fig. 1 (a), (b) FE-SEM; (c), (d) TEM images and the inset is a SAED pattern; (e) HR-TEM image of as-prepared sample (1h). The schematics in Fig. S1 illustrates the formation steps of Fe0.12V2O5 nanowires. Firstly, the precursor nanowires were grown on the Ti foil by a facile low-temperature hydrothermal route. The Ti foil was uniformly covered by nanowires. Annealing treatment in air at different temperatures was performed to convert the precursor into high crystalline Fe0.12V2O5. The digital picture is presented under their corresponding schematic diagram. The morphology of the as-prepared samples (for 1 h) was investigated in detail by FE-SEM and TEM. Fig. 1(a) and (b) are the typical FE-SEM images of the as-prepared nanowires, with a diameter about 30-70 nm. Furthermore, the SEM images of the Fe0.12V2O5 spheres collect from the solution were shown in Fig. S2. Fig. 1(c) and (d) present the TEM images of the as-prepared nanowires. The morphology of the nanowires is consistent with the observation in the FE-SEM images. Moreover, the HR-TEM image in Fig. 1(e) shows the as-prepared samples with a low crystallinity, which is also indicated by the diffused rings in the associated selected area electron diffraction (SAED) patterns (inset in Fig. 1(d)). To determine the chemical composition of the as-prepared samples, XPS measurement was carried out in the region of 0-1300 eV (Fig. S3(a)). A survey image shows the sample contains Cl, C, N, V, O and Fe elements. The C element may be from surface contamination of CO2. 16 After annealing at 250 °C, a full range XPS spectral analysis in Fig. S3(b) reveals four main peaks located at about 284.66, 522.06, 529.13 and 709.91 eV, in the curve corresponding to C 1s, V 2p, O 1s and Fe 2p, respectively. The heat treatment can eliminate the nitrogen and chlorine species from the as-prepared sample. All the V

2p1/2

and V

2p3/2

peaks in Fig. S3(c) are centered at

approximately 524.77 and 517.08 eV, respectively, but these peaks have an asymmetric shape with

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high and/or low shoulders that points to a multicompotent nature of the V 2p peaks. The binding energies of V2p3/2 at 517.42 eV and V2p1/2 at 524.81 eV can be assigned to V5+. The peaks at 517.04 eV and 523.52 eV are attributed to the V 2p3/2 and V2p1/2 respectively, which are centered at the V4+ position. Similarly, we can assign the binding energies of Fe2p3/2 at 710.74 eV and 712.62 eV to Fe2+ and Fe3+, respectively, as shown in Fig. S3(d). The peak at 720 eV is the satellite peak of Fe3+.31, 32 In order to study the effect of HMTA in the formation of iron vanadate nanowires, different amount of HMTA was added into the reaction solution, while NH4VO3, FeCl3·6H2O were used in the same amount in each hydrothermal reaction. The morphologies of the obtained samples are shown in Fig. S4. Without HMTA, particles were obtained which were similar to that after adding 12 mmol HMTA into the reaction solution. After adding 1.5 mmol HMTA, the as-prepared nanowires have a larger size in diameter than by adding 3 mmol HMTA. However, further increasing the quantity of HMTA to 6 mmol, the nanowires were cross-linked to form thin film-like structure. Further, the FTIR spectrum of the as-prepared nanowires is tested as shown in Fig. S5. The peaks at 1009.3 cm-1 and 774.6 cm-1 are due to V-O stretching,33, 34 and the peak at 529.4 cm-1 is assigned to Fe-O stretching.25 Moreover, the bands at 3122.9 and 1394.4 cm-1 are attributed to the asymmetric stretching vibration and the symmetric bending vibration of NH4+, 35 and the peak at 1602 cm-1 is the bending mode of O-H vibration.36 On the basis of the experimental results, the reactions for the synthesis of the precusor nanowires can be formulated as follows: 2NH4VO3+4H2C2O4=(NH4)2(VO)2(C2O4)3+2CO2 +4H2O (1)

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C6H20N4 + 6H2O→ 6HCHO +4NH3 (2) NH3+H2O=NH4++OH- (3) Fe3++ Cl- + (NH4)2(VO)2(C2O4)3+ OH-→ (NH4)xFe0.24V4O10Cly+ NH3+ C2O42- + CO + H2O (4) HMTA is a highly water soluble, non-ionic tetradentate cyclic tertiary amine with the chemical formula (CH2)6N4. HMTA decomposes upon heating to form formaldehyde and ammonia above 90 °C (eq 2).37 Ammonia reacts with water to produce hydroxide (OH-) (eq 3), which drives the precipitate of (NH4)xFe0.24V4O10Cly (eq 4). The added HMTA may play multiple roles in the synthesis process. During the reaction, HMTA buffers the pH and provides a convenient and continuous source of OH-.38 The localized [OH-] will be absorbed on the nanocrystal surface and change the surface free energies of the various crystallographic planes.39, 40 The facets with higher free energy would grow relatively faster, breaking the natural growth habit of the crystal. The added HMTA can creat additional growth anisotropy, that is, it has the effect to balance the growth rate of different facets. 0 min

20 min

1h

40 min

2h

4h

Fig. 2 Schematic illustration of the growth mechanism of the as-prepared nanowires..

To understand the growth mechanism of the as-prepared nanowires, time-dependent experiments were carried out. The representative morphologies of products collected at five different reactions stages are shown in Fig. 2. At the preliminary stage (20 min), a layer of irregular particles on Ti foil is formed (Fig. S6(a)). In the sample intermediated for 40 min, the irregular particles act as the seed crystals, resulting thin and short nanowires on Ti foil (Fig. S6(b)). The product is consisted of longer and larger nanowires after 1 h of reaction, and all irregular 8


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particles disappear (Fig. S6(c)). After 2 h of reaction, the nanowires are cross-linked to form thin film-like structure (Fig. S6(d)), and this phenomenon is further demonstrated after 4 h (Fig. S6(e)). Fig. S7 displays the XRD patterns of the powders obtained by peeling off the samples from the Ti substrate using a scalpel blade. The as-prepared samples with different reaction time have a similar structure, and the degree of crystallization become higher along with the reaction time extension. After annealing at 250 °C for 2 h, the phenomenon of crystallization is reversed, and the sample obtained at 20 min has a higher crystallinity than that of other samples. The nanowires obtained for 60 min or 120 min have a lower crystal with amorphous or in a low crystalline region. One reason is attributed to the different Fe doping amount in V2O5 layers, which may inhibition or slow down crystallization rate.41 The FTIR spectra of the samples after annealing at 250 °C are shown in Fig. S8, and the peaks at 1009 cm-1 and 529 cm-1 are attributed to V-O and Fe-O stretching, respectively.33, 34 The ratio of the FTIR peak intensity of V-O and Fe-O is shown in Table S1.The ratio of the sample obtained with a longer reaction time becomes lower, which means more Fe ions doping into the V2O5 layers. The spaces in each plane from XRD results are shown in Table S2 and the (200) space is increasing with more Fe ions doping. Compared to the standard crystal card, the sample fabricated at 1 h after annealing at 250 °C is Fe0.12V2O5 (PDF #49-0805, orthorhombic) with lattice parameters of a=11.54 Å, b=3.56 Å and c=4.36 Å.

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(a)

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(b)

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d=20.8 nm

(200)

Fig. 3 (a) (b) FE-SEM images; (c) TEM images; (d) HR-TEM image of Fe0.12V2O5 nanowires annealed at 250 ºC. Inset in (c) is a SAED pattern of Fe0.12V2O5 nanowires. After calcination at 250 ºC in air, the as-prepared nanowire arrays with taupe color were changed to brown-yellow. From Fig. 3, the morphology and size of the nanowires are nearly unchanged. Interestingly, there are numerous pores on the wires, indicating that porous structures are formed after the annealing process. The structure can provide a large amount of void spaces, which can accommodate the structure strain during lithium ions insertion/extraction process, thus leading to improved lithium-storage properties. The HR-TEM image was taken from the edge of a nanowire (in Fig. 3(c), the blue frame) and the lattice fringes are clearly visible with spacing of 5.8 Å, corresponding to (200) plane of Fe0.12V2O5. A SAED pattern is shown in the inset of Fig. 3(c).

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Fig. 4 Cycling performance and coulombic efficiency of Fe0.12V2O5 grown on Ti foil between 2.0 and 3.6 V at 30 mA·g-1 (a) and 500 mA·g-1 (b). (c) The second specific capacity at different current densities in the range of 2.0 to 3.6 V. (d) Galvanostatic charge-discharge profiles of Fe0.12V2O5 nanowire arrays grown on Ti foil at different rates. Due to the appealing structural features, the aligned Fe0.12V2O5 nanostructures on Ti foil may posses an excellent lithium storage property for LIBs. Fig. 4(a) displays the cycling performance and coulombic efficiency of Fe0.12V2O5 nanowires on Ti foil at 30 mA·g-1 for 100 cycles in the voltage range of 2.0 to 3.6 V. A reversible discharge capacity of 278 mAh·g-1 is retained after 100 cycles, which is much higher than the reported results in the Table S3.42-44 When the current density is increased to 500 mA·g-1, a capacity of 190 mAh·g-1 can still be delivered after 100 cycles (Fig. 4(b)). The enhanced electrochemical properties may due to the Fe0.12V2O5 nanowire arrays attached on Ti foil directly, which acts as current collector to improve the electron transmission.45 The rate capability of the Fe0.12V2O5 was tested at different current densities, which was shown in Fig. 4(c). This material delivered a reversible discharge capacity of 124.86 11


Coulombic efficiency (%)

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mAh·g-1 at a high current density of 1920 mA·g-1. The rate capability of Fe0.12V2O5 nanowire arrays in Fig. 4(d) presents the high steady discharge capacity of 312.70, 283.81, 260.26, 227.06, 199.41, 159.25, 122.70 mAh·g-1 at 30, 60, 120, 240, 480, 960, 1920 mA·g-1, respectively. When the current was turned back to 30 mA·g-1, the remained capacity is 295.91 mAh·g-1 about 94.63% of the initial capacity. Even at a high current density of 1920 mA·g-1, the electrode delivered a reversible capacity of 124.86 mAh·g-1.

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Fig. 5 Comparison of charge-discharge curves and cycling performance of Fe0.12V2O5 nanowires grown on Ti foil with different reaction time ((a), (b)) and at various annealing temperatures ((c), (d)). Fig. 5(a) compares the second cycle capacity of samples with different reaction time after annealing at 250 °C (resulting different morphologies). It is clear that the samples show drastically different capacity. The capacity of Fe0.12V2O5 with reaction time of 1 h is higher than others, which is 291.58 mAh·g-1. The porous net frame is built by these disorder nanowires (Fig. 3(c)),

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which makes the electrolyte soaking well into the active material, facilitates the kinetics of lithium ion diffusion and can effectively endure volume expansion during the repetitious insertion and extraction of Li+. The capacity of the Fe0.12V2O5 nanowires with reaction 20 min and 40 min was smaller, which is due to the shorter reaction time resulting the mixture of the short nanowires and nanoparticles (Fig. S6 (a) and (b)). This structure may not be able to store the lithium ion effectively and fail to endure volume expansion, which was further confirmed by the cycling performance in Fig. 5(b). In the sample produced in 2 h of reaction time, in the space the nanowires were cross-linked to form thin film-like structures, which lowered the specific surface area. Further, according to the FTIR and XRD results FexV2O5 prepared at 20 min has a lower Fe doping amount and a higher crystalline structure than other samples, while that obtained at 2 h has a higher Fe doping amount and a lower crystalline structure. The electrochemical performance presents that the capacity for FexV2O5 prepared at 20 min and 2 h is extremely low. The Fe doping amount and crystalline of FexV2O5 may also play important roles on their lithium storage performances beside their morphology influence. The second cycle charge-discharge profiles for the as-prepared samples, which annealed at different temperatures were as electrode tested in the potential range of 2.0-3.6 V (vs Li+/Li) at a constant current of 50 mA·g-1 (Fig. 5(c)). The as-prepared sample and that calcined at 200, 250, and 400 ºC, respectively, delivered a reversible discharge capacity of 174.77, 281.65, 292.43 and 173.62 mAh·g-1. With increasing the annealed temperature, the charge-discharge capacity of the Fe0.12V2O5 electrode increase drastically during room temperature to 250 ºC. Meanwhile, the cycling performances of Fe0.12V2O5 samples treated at different temperature are also exhibited in Fig. 5(d). The as-prepared samples and that treated at 200 ºC only retained 80.3% and 79.1% of

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their initial capacity after 50 cycles, respectively. While, there was only a slightly decrease in capacity of the nanowires calcined at 250 ºC after 50 cycles (less than 9%). Even after 100 cycles, the nanowires synthesized at 250 ºC remained a discharge capacity of 248.98 mAh·g-1, which corresponding to 84.9% capacity retention. According to XRD results and the observed morphologies, the enhanced electrochemical performance of the Fe0.12V2O5 nanowires synthesized at 250 ºC is possibly a result of the formation of appropriate crystalline structures, porous structure and a suitable nanowire size, which lead to reversible phase transition and effective accommodation of the structure strain during cycling. At lower temperatures, a low capacity is obtained due to the existence of less accessible sites in the defective crystalline structure, which is also less stable during the charge-discharge process. While, at 400 ºC, significant crystal growth takes place along with the nanowires aggregation and collapse (Fig. S9), which affects its electrochemical performances.

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10

20

30

40

50

0

100

Cycle number

450 -1

Capacity (mAh g )

200

300

400

0 500

Cycle number

500 30 mA g-1

(e)

60 mA g

30 mA g

-1

-1

120 mA g

-1

400 240 mA g

-1

Charge Discharge

350 480 mA g

-1

300 960 mA g

-1

250 1920 mA g

-1

200 0

10

20

30


1.0

Capacity (mAh g )

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40

Cycle number

Fig. 6 (a) Cyclic voltammograms of Fe0.12V2O5 nanowire at a scan rate of 0.5 mV s-1 between 1.0 and 4.0 V. (b) Charge-discharge voltage profiles of the Fe0.12V2O5 nanowire for the first five cycles. (c,d) Capacity and Coulombic efficiency of Fe0.12V2O5 nanowire at current rates of 30 mA g-1 and 500 mA g-1, respectively. (e) Galvanostatic charge-discharge profiles of Fe0.12V2O5 nanowire arrays grown on Ti foil at different rates between 1.0 and 4.0 V. To our knowledge, the energy density is dependent on the specific capacity and the working potential of electrode materials. The high cut-off potentials tested result in high specific capacity of cathode materials.46-48 As a result, increasing the cut-off potential is an important approach to enhance lithium ion battery energy density. However, according to reports in literature,31 the charge-discharge cut-off potential will affect the stability of cell cycle. Usually, the cycle stability of the battery is worse using a larger potential window. To demonstrate the effect of higher cut-off 15



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potential on the electrochemical performance of the Fe0.12V2O5 nanowire arrays cathode, testing of assembled cells was conducted in the voltage range of 1.0-4.0 V. Fig. 6(a) shows the first three cycles of the Fe0.12V2O5 nanowires grown on Ti foil, during the initial sweeping to 1.0 V, two well-defined reduction peaks are observed at 2.83 V, 2.52V, respectively, which correspond to the complicated multi-step electrocemical lithium ion intercalation processes involving the reduction of both iron and vanadium.7 When sweeping towards back to 4.0 V, two characteristic peaks indicative of lithium ion extraction are located at 2.97, 2.70 and 2.01 V, respectively. For comparison, the CV curves of the sample between 2.0 and 3.6 V have been provided in Fig. S10 where there are two pairs of peaks in the curves. The differences between these two kinds of CV curves are the extra pair peaks between 1.0-2.0 V. From the XRD result shown in Fig. S7(b), the Fe0.12V2O5 nanowires obtained for 1 h and annealed at 250 °C have a lower crystal structure with amorphous or in a low crystalline region. The peaks are likely attributed to Li+ ions insertion/extraction in the amorphous or low crystalline region.49 Whereas, Chen et al.50 and Whittingham et al.51 also have reported that the voltage ranges at 4.0-2.6 V, 4.0-2.1 V and 4.0-1.5 V corresponding to one, two and three lithium intercalations per formula unit V2O5, respectively. The peak at 1.61 V for lithium insertion in the CV curves may be related to another lithium intercalation into Fe0.12V2O5. Fig. 6(b) shows the charge-discharge voltage profiles of the Fe0.12V2O5 nanowire for the first five cycles. The initial discharge capacity of Fe0.12V2O5 nanowires cathode is 478.47 mAh·g-1, which is higher than the voltage range of 2.0-3.6 V (337 mAh·g-1). The good reversibility of Fe0.12V2O5 nanowires electrode indicates not occur irreversible structural degradation even the lower cut-off voltage is set at 1.0 V instead of 2.0 V, which is different from the reported Fe2V4O13

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with irreversible structural and a poor reversibility at a cut-off voltage lower than 1.7 V. The cycle performances of both the electrodes at different current densities are compared in Fig. 6(c) and (d). Actually, at a lower rate of 30 mA·g-1, the sample has an initial discharge capacity of 470.89 mAh·g-1 and 382.15 mAh·g-1 after 50 cycles. Long-life cycles of Fe0.12V2O5 nanowires cathode at high current density of 500 mA·g-1 is subsequently shown. The initial discharge capacity of 337.84 mAh·g-1 is achieved. The capacity retention 92% after 250 cycles and 86% after 500 cycles of the initial discharge capacity are remained. To evaluate the tolerance toward varied current densities of Fe0.12V2O5 nanowires, a stepwise density test was conducted. As shown in Fig. 6(e) , results from rate capability experiments for the Fe0.12V2O5 cathodes upon cycling at different current densities of 30, 60, 120, 240, 480, 960, 1920 mA·g-1. The specific capacity was stable at a constant current rate, while changes in current density resulted in stepwise dependence of the capacity. The cathodes were able to provide 91%, 85%, 77%, 66%, and 58% of the initial capacity at 30 mA·g-1 when the cycling current was increased by 2, 4, 16, 32, and 64 times. And the electrode recovered to the 95% of the initial capacity at 30 mA·g-1 as long as the current density reversed back to low current densities. The enhanced rate performance indicates that it has stable structure during the insertion and extraction of lithium ion when the voltage range is raised to 1.0-4.0 V. Since the Fe0.12V2O5 is composed of Fe2+, Fe3+and V4+, V5+, multistep reductions of Fe0.12V2O5 during lithium intercalation are complex and the theoretical capacity of the Fe0.12V2O5 has not been reported. In order to solve the above problems, X-ray photoelectron spectra (XPS) were used to test the valence of the key elements (such as Fe and V). Before the XPS test, the surface of the samples was thinned (~5 nm) using plasma cleaner. The XPS survey of Fe0.12V2O5

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discharged to 2.0 V and 1.0 V are shown in Fig. S11 (a) and Fig. S11 (b), respectively. After discharging to 2.0 V, the binding energies of V are at 516.50 and 515.10 eV in Fig. S11 (c), which is attributed to the V4+ 2p3/2 and V3+ 2p3/2, respectively.52 The binding energies in Fig. S11 (d) at 709.04 and 711.26 eV are corresponding to Fe2+ 2p3/2 and Fe3+ 2p3/2, respectively.53 With increasing the discharge depth to 1 V, the binding energy in Fig. S11 (e) is at 515.90 eV corresponding to the V3+2p3/2.54 The oxidation states of iron also have been carried out and the binding energies at 709.05 and 711.15 eV are attributed to Fe2+2p3/2 and Fe3+2p3/2, respectively, in Fig. S11 (f).58 On the basis of the above results, an interpretation of lithium intercalation in Fe0.12V2O5 follows. Firstly, the reduction of Fe0.12V2O5 nanowires begins with the reduction of V5+ to V4+, as well as the partial reductions of V4+ to V3+ and Fe3+ to Fe2+, corresponding to the higher voltage peaks in the CV curves between 2-3.6 V. Secondly, with increasing the discharge depth, the lower voltage peak at about 1.61 V can be attributed to the dominant reduction of V4+ to V3+ as well as the partial reduction of Fe3+ to Fe2+.55 Therefore, both the reductions of Fe ion and V ion in Fe0.12V2O5 with lithium intercalation are demonstrated. According to the peaks areas of V and Fe elements in Fig. S11, the capacity results are in the follows. Fe0.12V2O5+xLi++xe-→LixFe0.12V2O5 (0

Three-Dimensional Porous Iron Vanadate Nanowire Arrays as a High

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