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Co3V2O8 Sponge Network Morphology Derived from MetalOrganic Framework as an Excellent Lithium Storage Anode Material Vaiyapuri Soundharrajan, Balaji Sambandam, Jinju Song, Sungjin Kim, Jeonggeun Jo, Seokhun Kim, Seulgi Lee, Vinod Mathew, and Jaekook Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01047 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

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ACS Applied Materials & Interfaces

Co3V2O8 Sponge Network Morphology Derived from Metal-Organic Framework as an Excellent Lithium Storage Anode Material Vaiyapuri Soundharrajan1, Balaji Sambandam1, Jinju Song, Sungjin Kim, Jeonggeun Jo, Seokhun Kim, Seulgi Lee, Vinod Mathew and Jaekook Kim* Department of Materials Science and Engineering, Chonnam National University, 300Yongbong-dong, Bukgu, Gwangju 500-757, South Korea; Fax: +82-62-530-1699; Tel: +8262-530-1703 *E-mail: [email protected] 1

These authors contributed equally to this work.

ABSTRACT Metal-organic framework (MOF) based synthesis of battery electrodes has presntly become a topic of significant research interest. Considering the complications to prepare Co3V2O8 due to the criticality of its stoichiometric composition, we report on a simple MOF based solvothermal synthesis of Co3V2O8 for use as potential anodes for lithium battery applications. Characterizations by X-ray diffraction, X-ray Photoelectron Spectroscopy, high resolution electron microscopy, and porous studies revealed that the phase pure Co3V2O8 nanoparticles are inter connected to form a sponge-like morphology with porous properties. Electrochemical measurements exposed the excellent lithium storage (∼ 1000 mAhg-1 at 200 mAg-1) and retention properties (500 mAhg-1 at 1000 mAg-1 after 700 cycles) of the prepared Co3V2O8 electrode. A notable rate performance of 430 mAhg-1 at 3200 mAg-1 was also observed and ex-situ investigations confirmed the morphological and structural stability of this material. These results validate that the unique nanostructured morphology arising from the use of the ordered array of MOF networks is favorable for improving the cyclability and rate capability in battery electrodes. The synthetic strategy presented herein may provide solutions to develop phase pure mixed metal oxides for high performance electrodes for useful energy storage applications. Keywords: Metal-organic frameworks, solovothermal synthesize, Co3V2O8 sponge morphology, lithium ion batteries; long life stability 1 ACS Paragon Plus Environment


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1. INTRODUCTION Transition metal oxides based anode materials have attracted much attention due to their improved capacities, cycling performances, and initial Columbic efficiencies.1 However, their practical applications are limited because of various parameters including capacity fading, volume expansion, crystallinity, and particle size, etc. Hence, by careful designing/controlling of the morphology and particle sizes of these materials, the above limitations can be addressed. In fact, researchers have paid attention to improving their performance by developing various alternative anodes with high capacity, long cycle life, and high safety. As a result, considerable efforts are now being made to develop new highperformance electrode materials for the next-generation lithium-ion batteries (LIBs).1-4 Mixed

metal

oxides

exhibit

more

improved

electrochemical

properties

synergistically than the single phase materials, including electrical and ionic conductivity, reversible capacity, and mechanical stability.5 Although multi-valent metal oxides with different metal cations have shown high electrochemical performance, possibly due to their interfacial effects and to the synergistic effects of mixed cation species, the mechanism that completely explains the improved characteristics remain complicated to understand.6 Among the various metal oxides, metal vanadates have wide-spread applications in many fields, including photocatalytic water oxidation7 and oxygen evolution reactions.8 Specific to energy storage applications, few attempts including synthesis tuning and morphology tailoring have been made to improve the performance of metal vanadates as anode materials for LIBs.9-11 For example, Wu et al., recently demonstrated that Co2V2O7 hexagonal microplates show high specific capacity and long-term cycling stability since capacities of 866 mAhg−1 with nearly 100% Coulombic efficiency could be retained after 150 cycles.10 Similarly, α-CuV2O6 nanowires through a hydrothermal route,12 hollow ZnV2O4 microspheres,13 and amorphous FeVO4 nanosheet arrays14 are a few examples of vanadium based mixed metal oxides showing good performance in lithium storage properties. Metal vanadates have also been utilized as cathodes for Li-ion batteries. A recent LiV3O8 nanorod cathode with high-power and long-life rechargeable properties was reported for LIBs.15 According to previous reports, cobalt vanadate (Co3V2O8; CVO) materials have a crystalline structure in a Kagome’ staircase geometry, and show interesting magnetic behavior.16,17 Recent reports have demonstrated that Co3V2O8 material can be used as an 2 ACS Paragon Plus Environment

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anode for lithium batteries and as a positive electrode for supercapacitors with superior performance.18 Moreover, Yang et al.,19 reported the self-assembly of multilayered Co3V2O8 exhibiting outstanding reversible capacity (1114 mAhg-1 retained after 100 cycles) and excellent rate performance (361 mAhg-1 at a high current density of 10 Ag-1). This excellent performance was attributed to its unique morphologies and particularly to the surface-to-surface constructions and formation of quantum dots during the discharge process. On the other hand, investigations for electrochemical supercapacitor applications revealed that Co3V2O8 maintained specific capacitance of 739 Fg-1 and cycling stability of 704 Fg-1 was retained after 2000 cycles.20 Further, Shaoo et al.,21 demonstrated ultrathin 2D- Co3V2O8 sheet showing high specific capacitance of 4194 Fg-1 at 1 Ag-1 current density for longer cycles. Wu et al.,22 reported hollow hexagonal prismatic pencil shaped CVO showing impressive lithium storage properties with good cycling stability and superior rate capability. The CVO displayed a discharge capacity of 847 mAhg-1 which can be retained with a corresponding Columbic Efficiency (CE) >98% even after 200 cycles. In general, although the few available reports show promising results, Co3V2O8 appears to still remain underexplored possibly due to the complication during synthesis to achieve the desired phase selection.8,22 In a broad view, metal oxides obtained using MOF intermediates show excellent cyclability especially at high current densities and exhibit superior rate performance for LIB applications.23,24 However, to the best of our knowledge, all the preparative techniques involving MOF-based strategies have been utilized for synthesis of simple metal oxides and simple stoichiometric mixed metal oxides spinel such as ZnCo2O425, NiCo2O426, and other mixed metal oxides.27-29 On the other hand, our method exploits the synthesis of a non-stoichiometric (with respect to the corresponding metal) mixed metal oxide, namely, Co3V2O8 using the MOF-based intermediate. The present study therefore offer opportunities for synthesizing mixed metal oxides, especially non-stoichiometric mixed metal oxides for useful energy storage applications.

2. EXPERIMENTAL SECTION 2.1. Sample preparation

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In a typical procedure, 3 mmol of cobaltacetylactonate and 2 mmol of vanadiumacetylactonate were mixed with 2.5 mmol of terephtalic acid (benzene dicarboxylic acid, BDC) in 40 ml of N,N-Dimethylmethanamide (DMF). After stirring for 20 min, the homogeneous dark red clear solution was transferred into a 50 ml Teflon-lined autoclave, filled to 80% of its capacity. The autoclave was sealed and placed in an oven and heated at 180 °C for 24 h. The final products were washed by distilled water and ethanol several times and dried in an oven at 70 °C for 8 hrs. The as synthesized material was then annealed in air atmosphere at 450 °C for 4 h in order to obtain phase pure CVO. 2.2. Structure characterization and electrochemical measurements Powder X-ray powder diffraction measurements (XRD, Cu Kα radiation, with λ= 1.5406 Å), were carried out using Shimadzu X-ray diffractometer. The surface morphology was analyzed by Field-emission scanning electron microscopy (FE-SEM) using S-4700 Hitachi with an EDS detector and lattice fringes were analyzed using Field emission transmission electron microscopy (FE-TEM, Philips Tecnai F20 at 200 kV in KBSI Chonnam National University) equipped with selected area electron diffraction (SAED). The elemental oxidation states were examined by X ray Photoelectron Spectroscopy (XPS, Thermo VG Scientific instrument, Multilab 2000 in Chonnam Center for Research Facilities) using Al Kα as X-ray source. The spectrometer was calibrated with respect to the C1s peak binding energy of 284.6 eV. The surface area and pore size distribution of the sample were determined based on the nitrogen adsorption and desorption isotherms using Brunauer–Emmett–Teller (BET, Micromeritics ASAP2010 Instrument Co., Norcross, GA, USA). Electrochemical experiments were performed with 2032 coin type cells using a Li foil as the counter electrode. A working electrode was prepared by mixing CVO nanoparticles, Super P as conducting carbon, and PAA binder in DI water to form a slurry at a weight ratio of 70:15:15, respectively. The obtained mixture was then uniformly pasted on a pure Cu-foil current collector and dried overnight under vacuum. The dried material was then pressed between stainless steel twin rollers at room temperature. The foil was then punched into circular discs, and the coin cells were assembled with lithium metal as the counter electrode and a membrane (Celgard 2400) together with glass fiber as a separator. The active material loading on the cathode was approximately, 0.84 mg. The electrolyte was 1 M LiPF6 in EC:DMC (1:1 in volume). Charge/discharge cycling was performed on a BTS-2004H (Nagano, Japan) battery test instrument


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at 0.01–3.0 V. A cyclic voltammetric (CV) measurement of the electrode was performed using a Bio Logic Science Instrument (VSP 1075) with a scan rate of 0.2 mV s-1. 3. RESULTS AND DISCUSSION Figure 1a shows the XRD pattern for cobalt vanadium based MOF (Co-V-MOF) as an intermediate product and exhibits peaks in the lower angle region around 5-15°, revealing the formation of an array of cobalt and vanadium in the BDC linker’s network. The 2θ positions of Co-V-MOF

(a)

Intensity (arb.unit)

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Co V O 3 2 8

(b) JCPDS:16-0675 (311)

(440)

(400)

10

20

30

40 2θ (degree)

50

60

70

Figure 1. XRD pattern for (a) MOF intermediate and (b) Co3V2O8 after annealing at 450 °C. The standard pattern of Co3V2O8 is also provided for comparison purposes. the different planes of the MOF network normally depend on the way in which the linker coordinates with metal ions and solvents being trapped on the cavity.30 In our case, the peak is observed at 2θ value of 8.7°, thus revealing the formation of the MOF network. However, the determination of the exact crystal structure of the Co-V-MOF intermediate is complicated and requires further study. Figure 1b highlights the XRD pattern of the product obtained from the annealing of Co-V-MOF at 450 °C in air. The diffraction planes are well indexed to the cubic crystal form of Co3V2O8 (JCPDS No.16-0675) and is also in congruence with the reported results.19,20 The thermogravimetric (TG) and derivative thermogravimetric analysis (DTG) analyses (supporting information Figure S1) were performed to confirm the decomposition point of the material under oxygen atmosphere. The results reveal two different decomposition points at 5 ACS Paragon Plus Environment


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330 and 405°C, the former being attributed to the decomposition of the trapped solvent from the cavity network of the Co-V-MOF while the latter point at 405 °C is most likely related to the complete removal of the MOF network from the corresponding oxide of Co3V2O8. Hence, based on these observations, the Co-V-MOF intermediary product was annealed at 450 °C in air to obtain the MOF free/phase pure CVO in the present study. The SEM images, in Figures. 2a and 2b, of the present CVO reveals closely connected

Figure 2. SEM images of Co3V2O8 at (a) low and (b) high magnification. TEM images at (c) low and (d) high magnification. The lattice fringes are marked with arrows. nanoparticles that tend to form nanoscale voids. This well-defined sponge like network 6 ACS Paragon Plus Environment

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morphology is most likely related to the initial evolution of the peripheral of the extended network of the metal organic framework before heat treatment and the subsequent cleaving during annealing. However, the present morphology, shown more closely in Figure 2b, appears to be unique for MOF based syntheses. The SEM-EDS pattern obtained for the present CVO also confirms the phase purity of the sample (Figure S2). The low resolution transmission electron microscopy (TEM) image in Figure 2c, clearly reveals the sponge-like network with nanometer sized holes within them. The high resolution image, in Figure 2d, displays the measured interplanar distance, the d value of 0.251 nm revealing the (311) plane corresponding to the mother peak of the cubic Co3V2O8 in Figure 1. However, although the particles appear very dense and affect the resolution of the TEM images, additional images along with the standard area electron diffraction (SAED) pattern are also provided in the supporting information (Figure S3). More than one SAED pattern demonstrated the dense or multilayers of particles and polycrystalline behavior of Co3V2O8.

Scheme 1. Scheme representation of Co3V2O8 sponge like network formation obtained through Co-V-MOF intermediate and Ostwald ripening processes. Based on these observations, the possible mechanism for the formation of the sponge-like morphology with zig-zag network morphology of the present Co3V2O8 is illustrated in Scheme 1. In the initial stage, the metal salts of cobalt and vanadium in the precursor solution coordinate feasibly with the linker groups under moderate temperatures and pressure generated in the thermal bomb. At higher temperatures, an MOF network is generated via Co-BDC-Co-, Co-BDC-V- or VBDC-V- linkages, depending on the availability and coordination ability between the two metal precursors, whereas the solvent, DMF targets the cavities in the MOF network. As the solvothermal reaction progresses, the nucleation of Co-V-MOF nuclei leads to form spherical or oval-like particles. In the next stage, these nuclei tend to attract each other to form a macroscopic network due to Ostwald ripening in a pressurized environment. Finally, the Co-V-MOF 7 ACS Paragon Plus Environment


intermediate annealed at 450 °C in air produces a phase pure Co3V2O8 with sponge-like morphology consisting of pores formed most likely due to the evaporation of organic reactants. The surface electronic state and composition of the Co3V2O8 sponge network morphology were analyzed by XPS using Al Kα as X-ray source as depicted in Figure 3. The C 1s peak profile in Figure 3a shows four distinct peaks at 282.8, 284.9, 286.8, and 289.3 eV which are due to carbon C 1s

O 1s

(b)



(a)

280

(c)

285 290 Binding Energy (eV)

295

Co 2p

2p3/2 2p1/2

+2

+3 Shake-up lines

770

780

525

790 800 Binding Energy (eV)

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810

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545

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(d)

2p3/2


275


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+5

2p1/2

+4

516

519 522 525 Binding Energy (eV)

528

Figure 3. XPS pattern of (a) C 1s, (b) O 1s, (c) Co 2p, and (d) V 2p for Co3V2O8 sponge network morphology. in the lattice as well as on the particle surface. The highly intense peak at 282.8 eV may be associated with the metal-carbon feature namely, C-M-O (where M = Co or V) due to carbon substitution in the oxygen sites of the cubic lattice.31 This might have originated from the MOF network since the process of annealing influences the rearrangement and hence lead to the carbon 8 ACS Paragon Plus Environment

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doping into the lattice. It is worth noting that carbon substituting oxygen in the lattice (C@O) tends to influence energy and photocatalytic parameters.31 The remaining carbon peaks can be attributed to the adventitious carbon on the surface of the material. The peak at 284.9 eV is most likely due to the graphitic-like carbon (C—C, from the MOF network) condensed on the surface and the remaining peaks at 286.8 and 289.3 eV can be due to the formation of the C—OH (and C—O— C) and C═O (and COO) bonds, respectively, of the carbonate-like species32 originating from the oxidized carbon species present in the material. It is highly possible that these residue carbon species is formed due to the cleaving and subsequent evaporation of the MOF network during the annealing process at moderate temperatures of 450 °C. The oxygen deconvoluted profile clearly shows the (Figure 3b) different environmental states of oxygen in this material. The peak positions at 530.4, 531.9, and 533.1 eV can be ascribed to the M—O (O2– lattice oxygen), C═O (and COO), and C—OH (and C—O—C) species, respectively. However, the peak at 531.9 eV is also usually associated with oxygen defects,33 and a number of other species such as hydroxyls, chemisorbed oxygen, and under-coordinated lattice oxygen.34 The XPS profile of Co 2p consists of two major peaks at 780.1 and 795.8 eV, respectively, due to the spin orbit coupling energy states of 2p3/2 and 2p1/2 with a splitting energy of 15.7 eV. Two observations are easily noticeable after deconvolution of the Co 2p peaks as shown in Figure 3c: (a) two different oxidation states of Co and (b) shake-up satellite lines. The deconvoluted 2p3/2 peak is comprised of +2 and +3 oxidation states at 780.1 and 782.6 eV, respectively, with the shake-up satellite peak at 786.5 eV.35 The two prominent shake-up satellite peaks (at 786.5 and 803.0 eV) provide evidence of the Co in +2 oxidation state.36 Further, it is worth mentioning that the shake-up satellite line at 786.5 eV provides evidence for Co2+ ions in octahedral coordination. The electronic structure of these kagome staircase type compounds (Co3V2O8 and Ni3V2O8), in general, reveal the transition metal (Co) to be in octahedral coordination with the surrounding oxygen with specified bond lengths.37 Hence the Co 2p lines demonstrate the oxidation state of +2 and +3, with +2 being predominant over +3. Similarly, the vanadium 2p lines in the Co3V2O8 sponge network shaped morphology show that the spin orbit coupling value of 7.4 eV between 2p3/2 and 2p1/2 is 517.3 and 524.7 eV, respectively. As expected, the resultant deconvolution (Figure 3d) reveals that the V 2p encompassing of the +4 and +5 oxidation states at 516.8 and 517.5 eV, respectively wherein the peak with high binding energy (517.5 eV) is assigned to V5+ and that with the lower binding energy (516.8 eV) is ascribed to the V4+ species on the 9 ACS Paragon Plus Environment


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surface.38 Both elements confirm the quality of the material and these values are well matched with reported work.22 Thus, on the surface of Co3V2O8, both cobalt (+2 and +3) and vanadium (+5 and +4) show variable oxidation states without affecting the original phase of cubic Co3V2O8. The overall survey spectrum provides the purity of this material (Figure S4). The textural properties of CVO (Figure S5a) include a type III isotherm revealing mesoporous solids with weak adsorption. The nitrogen adsorption/desorption isotherm can be attributed to type III with a distinct hysteresis loop observed in the range of ∼0.3–1.0 P/P0. In addition, the sponge network morphology validates the surface area of 8.0 m2g-1 from BET analysis with a corresponding pore volume of 0.0476 cm3g-1 from Barrett–Joyner–Halenda (BJH) pore size distribution curve for this material (Figure S5b). As anticipated, the pore size distribution shows major distributions centered in the mesoporous range in addition to the presence of small amount of macropores. Further, the distribution derived from the BJH equation shows the rapid aggregation influencing the absence of larger voids, which is possible due to the fusion of smaller voids. The sharp and gradual desorption pattern reveals the ordered crystallinity of the Co3V2O8. Figure 4a depicts the cyclic voltammogram (CV) profile obtained at a scan rate of 0.2 mVs1

in the potential window of 3 V to 10 mV. In the first cathodic sweep or during the lithation step,

three reduction peaks centered at 1.47, 0.55 V and 0.05 V are assigned to the reduction of Co3V2O8 to CoO accompanied with the formation of LixV2O5, further reduction into metallic Co, and alloying formation of Li-Co respectively. On the other hand, the anodic sweep of the first cycle shows two distinguished oxidation peaks at 1.25 and 2.39 V. The former peak is assigned to the dealloying reaction of Li-Co; extraction of Li ions from the LixV2O5 matrices; decomposition of Li2O and oxidation of Co (0) to CoO, whereas the latter is due to further extraction of Li ions from LixV2O5 matrixes, during the delithation process, which exactly concurs with reported works.19,22 These two anodic peaks are observed to be retained for the remaining cycles. In the second cathodic step, in addition to the existing peaks from the first cycle with notable shifting (1.47 to 1.44 V; 0.55 to 0.51 V and 0.05 to 0.24 V), yet another new peak is observed at 0.95 V and all these peaks are retained on subsequent cycling. This new peak can be attributed to the further insertion of Li ions into the LixV2O5 matrix (LixV2O5 + y Li

Lix+yV2O5). The fact that no further


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shifts occur in the anodic and cathodic peak positions during the remaining cycles indicates the structural integrity of the material. A feasible mechanism as described by Yang et al.,19 has dominated here; initially, the 1.0

3.0

(a)

-0.5 st 1 cycle nd 2 cycle rd 3 cycle 4thcycle

-1.0 -1.5 -2.0

0.0

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th 5 cycle

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Cycle

Figure 4. (a) Cyclic voltammetry profile for Co3V2O8, (b) charge and discharge profile for Co3V2O8 at 200 mA g-1 current density, (c) cyclability curves at 200 and 500 mA g-1, and (d) rate performance at various current densities. Inset in (a) shows magnified region at 1.47 V. Co3V2O8 crystal structures are gradually destroyed, resulting in the formation of CoO particles hanging on to the remaining disordered lithiated-CVO (LixCo3V2O8). Upon further insertion of lithium ions on the network, the reduced CoO is further reduced to metallic Co particles and attaches to the remaining network of LixV2O5 matrix as evidenced from the peak at 1.47 V in the cathodic profile. In addition, in our case, further insertion of Li ions and an alloy formation of LiCo occurred at 0.05 V. Whereas, in the anodic region, as expected, electrochemical lithium extraction reactions occur to produce CoO and LixV2O5 at 1.25 and 2.39 V, respectively. 11 ACS Paragon Plus Environment


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The galvanostatic dis-charge and charge profile for the CVO electrode at a current density of 200 mAg-1 in the voltage range of 0.005 V vs Li/Li+ to 2.5 V vs Li/Li+ is shown in Figure 4b. This profile consists of 1st, 2nd, 50th, 100th, and 140th cycles of charge and discharge curves. In the first discharge curve, the potential drops quickly to 0.95 V, then shows a plateau, and drops gradually to reach the specific capacity of 1536 mAhg-1. This process is usually ascribed to irreversible reactions, i.e., decomposition of the electrolyte (formation of a solid electrolyte interphase (SEI) layer) and defined as interfacial lithium storage.39 Thus, the initial charge and discharge capacities are 934 and 1536 mAh g-1, respectively, with a Coulombic efficiency (CE) of 61%. This higher initial discharge capacity is due to the formation of the unavoidable SEI layer. According to the Faraday equation, the amount of lithiation and de-lithiation was estimated to be 23.3 and 14.2 Li per formula unit for the 1st discharge and charge curves, respectively. It is important to note that the theoretical capacity and reactive kinetics are not clearly investigated for this material. Thus, the specific capacity initially decreased and reached 820 mAh g-1 (at 10th cycle) after which the capacity is increased gradually. At a given current density of 200 mAg-1, the specific capacity increases as the number of cycles are increased until the 63rd cycle when a maximum capacity of 1030 mAh g-1 is attained. Further cycling leads to saturation in the specific capacity values and at the end of the 140th cycle, this value remained at around 980 mAhg-1, which is still higher than the initial capacities (2nd cycle, ~ 934 mAhg-1). Figure 4c depicts the cycling performance of the sponge network-like morphology of CVO at two different current densities of 200 and 500 mAg-1. Overall, it can be observed that the reversible capacities of the electrode gradually increased with increasing number of cycles except for a small drop in the capacities during the initial few cycles. Precisely, the performance initially decreased (after the 1st cycle) to 934 and 726 mAhg-1 for 200 and 500 mAhg-1, respectively; however, after the 10th and 30th cycle of the former and latter, respectively, their specific capacities gradually increased. This sudden specific capacity decrease and increasing phenomenon is commonly observed for transition metal oxides based anode electrodes, and is normally attributed to the presence of a possible activation process in the electrode though further studies are required in the present case.9,40 As mentioned earlier, the capacity for 200 mAg-1 exhibits 1030 mAh g-1 at the 65th cycle and it finally sustained the value of ~980 mAh g-1 at the 140th cycle. On the other hand, for the current density of 500 mAg-1, the maximum value of 780 mAh g-1 was observed at the 100th cycle, after which the value stabilized around 730 mAhg-1 at the 140th cycle. The 12 ACS Paragon Plus Environment

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corresponding discharge and charge curves at 500 mAg-1 are provided in the supporting information (Figure S6). The reason for increased performance could be the generation of increasingly more metallic cobalt during the lithiation (discharge) process. The gradual increasing performance at different current densities is well supported by other reported works.19,22 Similarly, the cyclability curve for another intermediate current density of 300 mAg-1 was also examined and the results are shown in Figure S7. At this current density, the specific capacity remained at 1020 mAhg-1 for about 100 cycles with CE of ~ 100%. As seen from the profile, the specific capacity initially decreased and gradually increased from the 11th cycle onwards. The value is sustained after the 80th cycle, and at the 100th cycle, the measured capacity was found to be 1020 mAhg-1. The voltage-specific capacity (C-rate) was tested against cycle times for this sample at various current densities from 50 mAg-1 to 3200 mAg-1; the corresponding graph is shown in Figure 4d. Initially, at the charge-discharge current of 50 mAg-1, the average discharge capacity of 1017 mAhg-1 was obtained. Further increasing the current density to 100 mAg-1, the value further reduced and reached 900 mAhg-1. The average specific capacities were found to be 840.8, 741, 655.6, 543.5, and 434.6 mAhg-1 for various current densities of 200, 400, 800, 1600, and 3200 mAg-1, respectively. Further, at the current density of 50 mAg-1 for the cycle completion, the measured average specific capacity was found to be 945 mAhg-1 and it reaches close to the initial specific capacity value for the same current density. It is worth noting that at the maximum current density of 3200 mA g-1, the material retains an average capacity of 430 mAhg-1 with CE reaching close to 100%, revealing the material stability. In addition, the performance of the present CVO electrode with the sponge network morphology was investigated in detail at a high current density of 1000 mAg-1 and the results are shown in Figure 5. The discharge/charge profile of the 1st, 2nd, 100th, 200th, 300th, 400th, 500th, 600th and 700th cycle with CE close to 100% are found in Figure 5a. The specific capacity initially decreases and reaches around 400 mAhg-1 at the 50th cycle. The performance, however, increases and retained its value of 501 mAhg-1 even at 700th cycle. This behavior could be attributed to the unused active materials being pulverized into active Co nanoparticles and as a result the latter gets



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accumulated. This synergetic behavior of Co particles can tend to accommodate increasingly more lithium ions, leading to increased capacities during extended cycling.19,22 Although the rate performance of 430 mAhg-1 at the current density of 3200 mAg-1 is lower than the reported values19,22 of Co3V2O8, the cyclability curves for 500 and 1000 mAg-1 show the impressive cycle life of the present material and in the latter case appears

Figure 5. (a) Charge and discharge profile for Co3V2O8 at high current density of 1000 mAg-1 and (b) corresponding cyclability curve for the same. Ex-situ SEM images of electrode after 700 cycles at the current density of 1000 mAg-1 (c) low and (d) high magnification. to retain the structural stability even after 700 cycles. In order to analyze the structural stability of the present electrode, SEM and TEM measurements were performed (ex-situ) for the electrode recovered after the cycling test at 1000 mAg-1 and the obtained SEM images are projected in Figure 5c and 5d. The low and high magnified images imply that the original sponge network is almost retained even after prolonged cycling at high discharge/charge rates. The corresponding EDS pattern (SI, Figure S8) of this electrode 14 ACS Paragon Plus Environment

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agrees well with that of the original material and thereby proves the stability of the present electrode. A high resolution TEM image is also provided in the SI (Figure S9). Nevertheless, a few key points including morphology tailoring, particles size tuning and monitoring reaction/synthetic protocols are currently being addressed to improve the performance of this material.

4. CONCLUSION In summary, the Co3V2O8 synthesized successfully from the MOF network showed good cyclability and specific capacity of 700 cycles and 501 mAhg-1, respectively, at a current density as high as 1000 mAg-1. The impressive rate performance (430 mAhg-1 at 3200 mAg-1) is ascribed to the unique morphology arising from the use of the ordered array of MOF networks for the preparation of the present CVO. Although, MOFs do not have specified shapes, the cleaving and the subsequent removal of the network during the annealing process of the cobalt-vanadium based MOF intermediary product appears to influence the formation of the sponge like morphology in the Co3V2O8 prepared in the present study. Although further investigations are required, the present study may open up new opportunities in not only synthesizing MOF networks but also utilizing them to derive phase pure mixed metal oxides with unique particle morphologies for promising high power active materials.

ACKNOWLEDGEMENTS This work was supported by National research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2014R1A2A1A10050821). This research was also supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078875) or (2013-073298).

Supporting Information



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Supporting information contains TGA profile, SEM-EDS, HRTEM images of CVO, XPS survey and surface area for CVO, charge/discharge (300 and 500 mAg-1) and cyclability (300 mAg-1) patterns for CVO, TEM image for CVO sponge network morphology after 700 cycles at 1000 mAg-1 current density. REFERENCES 1) Lee, W. W.; Lee, J. M. Novel Synthesis of High Performance Anode Materials for Lithium-Ion Batteries (LIBs). J. Mater. Chem. A 2014, 2, 1589-1626. 2) Wang, H.; Dai, H. Strongly Coupled Inorganic–Nano-Carbon Hybrid Materials for Energy Storage. Chem. Soc. Rev. 2013, 42, 3088-3113. 3) Liu, D. H.; Lü, H. Y.; Wu, X. L.; Hou, B. H.; Wan, F.; Bao, S. D.; Yan, Q.; Xie, H. M.; Wang, R. S. Constructing the Optimal Conductive Network in MnO-Based Nanohybrids as High-Rate and Long-Life Anode Materials for Lithium-Ion Batteries. J. Mater. Chem. A 2015, 3, 19738-19746. 4) Hou, B.-H.; Wu, X.-L.; Wang, Y.-Y.; Lü, H.-Y.; Liu, D.-H.; Sun, H.-Z.; Zhang, J.-P.; Guan, H.-Y. Full Protection for Graphene-Incorporated Micro-/Nanocomposites Containing Ultra-small Active Nanoparticles: the Best Li-Storage Properties. Part. Part. Syst. Charact. 2015, 32, 1020–1027. 5) Yuan, C. Z.; Wu, H. B.; Xie, Y.; Lou, X. W. Mixed Transition-Metal Oxides: Design, Synthesis, and Energy-Related Applications. Angew. Chem. Int. Ed. 2014, 53, 14881504. 6) Li, H. Q.; Liu, X. Z.; Zhai, T. Y.; Li, D.; Zhou, H. S. Li3VO4: A Promising Insertion Anode Material for Lithium-Ion Batteries. Adv. Energy Mater. 2013, 3, 428-432. 7) Zhao, Y.; Liu, Y.; Du, X.; Han, R.; Ding, Y. Hexagonal Assembly of Co3V2O8 Nanoparticles Acting as an Efficient Catalyst for Visible Light-Driven Water Oxidation. J. Mater. Chem. A 2014, 2, 19308-19314. 8) Li, P.; Umezawa, N.; Abe, H.; Ye, J. Novel Visible-Light Sensitive Vanadate Photocatalysts for Water Oxidation: Implications from Density Functional Theory Calculations. J. Mater. Chem. A 2015, 3, 10720-10723.


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