Dodecahedron-Shaped Porous Vanadium Oxide and Carbon

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Dodecahedron-Shaped Porous Vanadium Oxide and Carbon Composite for High-Rate Lithium Ion Batteries Yifang Zhang, Anqiang Pan, Yaping Wang, Weifeng Wei, Yanhui Su, Jimei Hu, Shuquan Liang, and Guozhong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04866 • Publication Date (Web): 10 Jun 2016 Downloaded from http://pubs.acs.org on June 13, 2016

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ACS Applied Materials & Interfaces

Dodecahedron-Shaped Porous Vanadium Oxide and Carbon Composite for High-Rate Lithium Ion Batteries

Yifang Zhang,a Anqiang Pan,a, b,* Yaping Wang,a Weifeng Wei,b Yanhui Su,a Jimei Hu,a Guozhong Cao,c and Shuquan Liang,a,*

a

School of Materials Science & Engineering, Central South University, Changsha,

410083, Hunan, China b

State Key Laboratory of Powder Metallurgy, Central South University, Changsha,

410083, China c

Department of Materials Science & Engineering, University of Washington, Seattle,

WA, 98195, USA

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A. Q. Pan); [email protected] (S. Q. Liang). 1

ACS Paragon Plus Environment


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ABSTRACT: Carbon-based nanocomposites have been extensively studied in energy storage and conversion systems because of their superior electrochemical performance. However, the majority of metal oxides are grown on the surface of carbonaceous material. Herein, we report a different strategy of constructing V2O5 within the metal organic framework derived carbonaceous dodecahedrons. Vanadium precursor is absorbed into the porous dodecahedron-shaped carbon framework firstly, and then in situ converted into V2O5 within the carbonaceous framework in the annealing process in air. As cathode materials for lithium ion batteries, the porous V2O5@C composites exhibit enhanced electrochemical performance, due to the synergistic effect of V2O5 and carbon composite. KEYWORDS: lithium ion batteries, cathode, vanadium oxides, metal organic framework, carbon

1. INTRODUCTION Vanadium pentoxide (V2O5) have been extensively studied as cathode materials in lithium ion batteries because of their high capacity, easy fabrication and abundant resources in storage.1 However, they are suffering from the low Li+ ion diffusion coefficient2 and poor electron conductivity.3,

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Recently, great efforts have been

endeavored to the fabrication of nanomaterials, which exhibit enhanced electrochemical performance.5, 6 To date, V2O5 with various morphologies, such as nanoparticles, nanowires/nanorods, nanosheets and three-dimensional hierarchical

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microspheres have demonstrated better rate capability than their bulk counterpart.7-10 The improvement of rate performance is attributed to the reduced particle size, which shortens the required Li+ ion diffusion and electron transportation distances. However, the intrinsic low electronic conductivity of V2O5 is still not addressed. More recently, nanocomposites from V2O5 and conductive materials have attracted tremendous attention because of their structural advantages. Among them, carbon and V2O5 nanocomposites are mostly reported, such as V2O5/carbon nanotubes and V2O5/graphene.11, 12 The rate capabilities are greatly improved because of the kinetic enhancement for electron transportation and faster Li+ ion diffusion in nanoscaled active materials. In these works, vanadium oxides are mainly grown on the pre-treated carbonaceous materials.11,

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However, their capacity retention

capability still needs further improvement. The inferior cycling stability may be raised from the direct exposure of V2O5 with the electrolyte. As we know, the nanostructured V2O5 experiences repeated volume expansion and contraction upon cycling, which may cause structural break into electrolyte and capacity fading. Based on this consideration, constructing a protective layer on the surface of V2O5, such as carbon coating or inert metal oxide coating, has been demonstrated an effective way to improve their cyclic stability.14,

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The chemical and physical features of the

coating layer are very important for the obtaining of good electrochemical properties. The coating layer should be porous and of reasonable thickness so that the electrolyte can penetrate inside easily. Moreover, the coating layer should also be robust enough

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to tolerate the volume changes. However, the fabrication process is always quite tedious and requires careful treatment. Encapsulating V2O5 nanoparticles within the porous carbon framework may be an alternative way to obtain good comprehensive electrochemical properties. For example, Zhang et al. reported the fabrication of V2O5/carbon composite with high capacity and good rate capability.16 In this way, the vanadium oxides will not break into electrolyte directly and the structural integrity can be improved. Moreover, the porous carbon framework can allow the easy electrolyte penetration and improve the electronic conductivity of the electrode materials. Carbonaceous frameworks derived from metal organic frameworks (MOFs) have attracted particular attention in recent years because of their diverse structures and applications.17, 18 MOFs derived carbon inherits MOFs’ high surface area,19 large pore volume20 and tunable intrinsic characteristics,21-23 which are considered as new promising carbon materials24 in energy storage,25-27 adsorption,28,

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sensors,30 and

catalysts.31 Moreover, the structural diversity of the carbonaceous framework allows the fabrication of carbonaceous composites with different morphologies. Herein, MOFs derived mesoporous carbon framework was employed to synthesize dodecahedron-shaped V2O5@C composite by a liquid penetration of vanadium precursor into the framework and a subsequent annealing process in air. The weight percentage of carbon in the composite can be engineered by the annealing durations in air. As cathode materials for lithium ion batteries, the V2O5@C

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composites exhibit good rate capability and cycling stability.

2. EXPERIMENTAL SECTON Synthesis of dodecahedron-shaped carbon framework. Zeolitic imidazolate frameworks (ZIFs), as metal organic frameworks, were prepared via a precipitation method according to the earlier report.32 One mmol of cobalt nitrate hexahydrate and 5 mmol of 2-methylimidazole were added into 50 mL methanol, respectively. The above solutions were mixed under vigorous stirring for 30 s and incubated for another 24 h to obtain the precipitates, which were collected by centrifugation and washed with ethanol for several times. After dried in vacuum, the prepared zeolitic imidazolate frameworks were carbonized in a tube furnace under argon gas flow at 900 oC for 4 h. The obtained products were further etched with 1M HCl to remove the anchored cobalt metal, and were dried for later use. Synthesis of V2O5@carbon composite. VOC2O4 was prepared as the liquid vanadium precursor solution. In a typical synthesis, V2O5 (2.728 g) and H2C2O4·2H2O (5.673 g) in a molar ratio of 1:3 were stirred in 10 mL deionized water at 80 oC for several hours until the formation of a clear blue VOC2O4 solution (3M). Then the above prepared carbon frameworks were dispersed in VOC2O4 solution by sonication, allowing the penetration of VOC2O4 solution into its porous framework. After that, they were collected by centrifugation and annealed in air at 350 oC for 4h, 3h, and 2h with a temperature ramping rate of 1 oC min-1. And the obtained samples

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were designated as V@C-1, V@C-2, and V@C-3, respectively. For comparison, bare V2O5 was also prepared in a similar process with 2 hours of calcination, without using carbon framework. Materials characterization. The powder X-ray diffraction patterns were recorded on a Rigaku D/max 2500 XRD (Cu Kα radiation, λ=1.54178Å). The thermogravimetric analysis (TGA, NETZSCH STA 449C) was conducted under ambient atmosphere with a heating rate of 5 oC min-1. Raman spectroscopy measurements were performed at room temperature in a spectrometer (LabRAM Hr800) with a back illuminated charge coupled detector (CCD) attachment. The morphologies were characterized by scanning electron microscopy (SEM, Quanta FEG 250) and transmission electron microscopy (TEM, JEOL JEM-2100F). Nitrogen adsorption-desorption measurements were conducted at 77K (NOVA 4200e, Quantachrome Instruments). Electrochemical measurement. The electrode materials were mixed with acetylene black and polyvinylidene fluoride (PVDF) at a weight ratio of 70:20:10 in N-methyl-2-pyrrolidone (NMP) solution to form slurries, which were uniformly coated on aluminum foils before drying in vacuum at 100 oC. The mass loading was about 1 mg cm-2. 2025 type coin cells were assembled in a argon-gas filled glove box (Mbraun, Germany), using Li metal foil as anode, 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC)-diethyl carbonate (DEC) (1:1:1 in volume) as the electrolyte, and polypropylene membrane as the separator. Cyclic voltammetry (CV)

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was tested on an electrochemical workstation (CHI660C, China) at different scan rates in the voltage range of 2.5-4 V (vs. Li/Li+). The galvanostatic charge/discharge performances of the electrodes were conducted at room temperature on a Land Battery Tester (Land CT 2001A, Wuhan, China). The electrochemical impedance spectroscopy (EIS) was obtained using a ZAHNER-IM6ex Electrochemical Station (ZAHNER Co., Germany) in the frequency range of 100 kHz to 10 mHz.

3. RESULTS AND DISCUSSION Figure 1 schematically illustrates the formation process of V2O5@carbon dodecahedrons. ZIF-67 dodecahedrons were firstly fabricated by a precipitation method and were later carbonized at 900 oC. The obtained carbon framework was further acid leached to remove the cobalt species and get the porous carbon dodecahedrons. After carbonization, the organic linkers of MOFs become graphitized carbon after high temperature calcinations, obtaining the Co/C composite. Cobalt is removed by chloride acid leaching. Then, the obtained carbon dodecahedrons were dispersed into VOC2O4 solution to allow their sufficient penetration. After annealing in air at 350 oC for several hours, the VOC2O4 solution located in porous carbonaceous framework was in situ converted into V2O5@carbon dodecahedrons.

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Fig. 1 Schematic illustration of the preparation process of V2O5@C composites.

The powder X-ray diffraction (XRD) pattern of metal organic framework of ZIF67 is in good consistent with the simulated XRD pattern (see Figure S1, supporting information) and previous reported results,33 demonstrating its successful fabrication. As shown in Figure S2, the metal organic framework is composed of uniform dodecahedrons in a narrow size distribution ranging from 1.5 to 2 µm. The dodecahedron-shaped carbonaceous frameworks exhibit well-preserved morphology from the metal organic frameworks (Figure 2a, b). According to the TEM images (Figure 2c, d), the carbon dodecahedrons are of high porosity, which is very important for the subsequent absorption of VOC2O4 solution into its framework. The carbonaceous framework can keep its structures after VOC2O4 penetration (Figure S3). In order to get the desired V2O5@carbon composite, an appropriate annealing temperature in air should be selected that the carbon framework can be partially reserved and vanadium species can be completely converted into V2O5. According to

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the TG and DTA analysis result (see Figure S4), the decomposition temperature of VOC2O4 is around 262 oC and its oxidation temperature into V2O5 is around 350 oC.34 And the fully carbon combustion temperature is around 440 oC. Based on the TG and DSC analysis results, 350 oC was selected as the calcination temperature.

Fig. 2 SEM images (a, b), TEM images (c, d) and HRTEM images (e, f) of ZIF-67 derived carbonaceous frameworks. The carbonaceous frameworks exhibit jagged edges as can be seen from (e), and have high density nanosized voids enclosed by graphite layers (f) which are produced during the pyrolyzing process of ZIF-67 with the catalytic effect of cobalt.

Figure 3 shows the XRD patterns and Raman spectrums of the products annealed in air at 350 oC for 2h. The XRD patterns of V2O5@carbon and V2O5 are in good agreement with the orthorhombic V2O5 phase (JCPDS card No. 75-0457: Pmn21(31),

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a=11.48 Å, b=4.36 Å, c=3.555 Å), demonstrating the fully oxidation of vanadium species into V2O5. However, the higher peak intensity and narrower peak breadth of V2O5 than V2O5@carbon composite suggest its larger particle size. A strong XRD peak at around 26o has been detected for the carbonaceous framework, indicating the existence of highly graphitized carbon in the framework. The broader peak at 26o for the V2O5@carbon composite than V2O5 suggests the existence of residual carbon in the final products. The Raman spectrums of V2O5@carbon dodecahedrons (Figure 3b) show the typical D and G bands at around 1350 cm-1 and 1570 cm-1,35,

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confirming the existence of carbon in the composite. The high intensity of G band indicates the graphitic feature of the carbon, which is consistent with the XRD results. The D band demonstrates the existence of large amounts of defects. All other Raman peaks for V2O5@carbon composites can be assigned to characteristic features of vanadium oxide (Table S1, supporting information).37-39 The result demonstrates the successfully preparation of V2O5 and carbon composite.

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Fig. 3 XRD patterns (a) and Raman spectrums (b) of the V2O5@C composite, V2O5 and carbon framework.

The morphologies of the V2O5@carbon composites annealed for different durations were studied by FESEM. All three samples are composed of uniform dodecahedrons, indicating the good structural reservation from the carbonaceous framework. However, with the increase of calcining time, the surfaces of the V2O5@carbon dodecahedrons are getting rough. The sample calcined for 2h (V@C3) exhibits smooth surfaces, as shown in Figure 4a, b. For sample V@C-2, part of

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the carbon skeleton is burned off, resulting in less smooth surface and more clearly observed V2O5 particles (Figure S5c, d). After long period of calcination, the V@C-1 sample shows rough surfaces constructed by V2O5 particles with size of around 20-50 nm (Figure S5a, b). The differences of detailed morphologies for three samples are owing to the different extents of carbon combustion. According to the TG analysis result (Figure S6), the weight percentages of carbon in the composite are 7.3%, 16.6% and 37.8% for V@C-1, V@C-2, and V@C-3, respectively. For the bare V2O5 electrode, it is composed of irregular and densely packed particles (see Figure S7). The results demonstrate the good structural design of the composite by the confinement of the carbonaceous framework. The interior structures of the V@C-3 composite are further studied by TEM. As can be seen from Figure 4c, d, most of the pores within the carbonaceous framework are filled with V2O5 nanoparticles, which is quite different from the pristine carbon framework (Figure 2). The selected area diffraction pattern indicates the polycrystalline feature of V2O5 in the carbon framework (Figure 4d). The result is reasonable because V2O5 nanoparticles are encapsulated by the porous carbon framework. Figure 4e shows the lattice fringes of 3.47 Å, which is in good agreement with the planar distance of (210) for V2O5. The elemental mapping results (Figure 4f) demonstrate the homogeneous distribution of C, V and O elements for the composite dodecahedrons, suggesting the uniform formation of V2O5 within the carbon framework.

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Fig. 4 SEM images (a, b), TEM images (c, d), SAED image (inset of d), HRTEM image (e) and elemental mapping results (f) of V2O5@C composite.

Nitrogen adsorption-desorption analysis was also carried out to study the structural changes after making V2O5@carbon composite. Figure 5 shows the nitrogen adsorption-desorption isothermal curves and pore size distribution of carbonaceous framework and V2O5@carbon composite. According to Brunauer-Emmett-Teller method, the surface area changes significantly from 420.8 m2 g-1 to 64.2 m2 g-1 after making V2O5@carbon composite. Moreover, the pore volume decreases dramatically for the pores less than 10 nm. These results demonstrate the pores within the carbon framework are largely occupied by V2O5 nanoparticles. However, the V2O5 and carbon composite is still of high porosity, which is vital for the electrolyte penetration in lithium ion batteries. 13



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Fig. 5 Nitrogen adsorption-desorption isotherm (a) and pore size distribution (b) of the carbon framework and V2O5@C composite.

Figure 6 shows the electrochemical performance of the electrode materials in coin cells. The electrodes were firstly investigated by cyclic voltammograms (CVs) at a scan rate of 0.1 mV s-1. As shown in Figure 6a, for sample V@C-3, the detection of two cathodic peaks at 3.38 and 3.18 V reveals the multi-step Li+ ions intercalation process, corresponding to the phase changes from α-V2O5 to ε-Li0.5V2O5, and then to δ-LiV2O5, respectively.40, 41 The anodic peaks at 3.25 and 3.45 V correspond to the multi-step Li+ ions de-intercalation process and the phase changes backward from δ-

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LiV2O5 to ε-Li0.5V2O5, and to α-V2O5, respectively.42 The voltage gaps between the cathodic and anodic peaks are only 75 mV (a vs. a’) and 65 mV (b vs. b’), respectively for V@C-3 composite. The gaps get bigger with the decrease of carbon content. Voltage gaps of 192 mV (a vs. a’) and 140 mV (b vs. b’) are detected for the bare V2O5 electrode. The result demonstrates the polarization of the V2O5@carbon composite is greatly reduced. The enlarged contact area between electrode and electrolyte, and the good contact between V2O5 and carbon framework may be the two main reasons for the lower polarization.

Fig. 6 (a) CV curves of V2O5@C composite with different carbon contents and the comparison CV curve for bare V2O5 at scan rate of 0.1 mV s-1; (b) CV curves of

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V@C-3 cathode under different scan rates; (c) CV curves of bare V2O5 cathode under different scan rates; (d) linear fitting result of Ip vs ν1/2 for the peaks indicated by b and a’.

Figure 6b shows the CV curves of V@C-3 composite at various scan rates. Two couples of redox peaks are well presented at high scan rate of 1 mV s-1. The CV curves at various scan rates for the other two V2O5@carbon samples are also tested and shown in Figure S8. With the decrease of carbon content, the redox peaks at high scan rates become less distinguishable. Only one broad peak is detected for bare V2O5 electrode (Figure 6c). As shown in Figure 6b, the small peak shifts with the increase of scan rates indicate low polarization and fast kinetics of the redox reaction for V2O5@carbon composites.43 The Li+ ions diffusion coefficient is calculated based on the Randles-Sevchik equation (eq. 1) for semi-infinite diffusion of Li+ ions into the cathode materials,44

= 2.69 × 10 ⁄ ⁄ ⁄

(1)

where Ip is the peak current, n is the number of electrons transferred per molecule, A is the active surface area of the electrode, D is the apparent ion diffusion coefficient, ν is the scanning rate, and C0 is the concentration of lithium ions in the cathode. As presented in Figure 6d, the corresponding peak current has a linear relationship with the square root of scan rate. The calculated Li+ ion diffusion coefficients of V@C-3 are 3.86×10-11cm2 s-1 and 3.18×10-11cm2 s-1 at the indicated peak positions of b and 16


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a’, respectively. While the values for V2O5 electrodes are 1.59 × 10-11 cm2 s-1 and 7.8 × 10-12 cm2 s-1, respectively. The result demonstrates the Li+ ions diffusion coefficient of V@C-3 is 2 and 4 times higher than V2O5 at the peak positions of b and a’, respectively. Detailed Li+ ions diffusion coefficient values for all samples are listed in Table S2. Figure 7a shows the discharge/charge profiles of the V@C-3 composite at various rates. Multiple plateaus are clearly observed on both discharge/charge profiles, which are in good accordance with the CV curves. Even at 32 C, the discharge/charge plateaus can be easily distinguished. The composite electrode delivered the specific discharge capacities of 140.2, 135.4, 132.8, 129.2, 125.2, 121.6 and 117.7 mA h g-1 at the rate of 1C, 2C, 4C, 8C, 16C, 32C and 64C, respectively. When the current is reset to 1 C, a capacity of 138.2 mA h g-1 can be recovered (Figure 7b). The slight capacity decay at higher rates demonstrates good rate capability of the composite electrode. For comparison, the discharge/charge profiles at various rates for the other three samples with different carbon contents are also given in Figure S9. A comprehensive comparison in Figure 7b shows the rate performance has gradually gone worse with the decrease of carbon content. Although similar specific discharge capacities can be released for the electrodes at low rates (

Dodecahedron-Shaped Porous Vanadium Oxide and Carbon

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