Carbon-Encapsulated Hollow Porous Vanadium-Oxide Nanofibers for

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Carbon-Encapsulated Hollow Porous Vanadium-Oxide Nanofibers for Improved Lithium Storage Properties Geon-Hyoung An,† Do-Young Lee,‡ and Hyo-Jin Ahn*,†,‡ †

Program of Materials Science & Engineering, Convergence Institute of Biomedical Engineering and Biomaterials, Seoul National University of Science and Technology, Seoul 139-743, Korea ‡ Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 139-743, Korea S Supporting Information *

ABSTRACT: Carbon-encapsulated hollow porous vanadium-oxide (C/HPV2O5) nanofibers have been fabricated using electrospinning and postcalcination. By optimized postcalcination of vanadium-nitride and carbon-nanofiber composites at 400 °C for 30 min, we synthesized a unique architecture electrode with interior void spaces and well-defined pores as well as a uniform carbon layer on the V2O5 nanofiber surface. The optimized C/HPV2O5 electrode postcalcined at 400 °C for 30 min showed improved lithium storage properties with high specific discharge capacities, excellent cycling durability (241 mA h g−1 at 100 cycles), and improved high-rate performance (155 mA h g−1 at 1000 mA g−1), which is the highest performance in comparison with previously reported V2O5-based cathode materials. The improved electrochemical feature is due to the attractive properties of the carbon-encapsulated hollow porous structure: (I) excellent cycling durability with high specific capacity relative to the adoption of carbon encapsulation as a physical buffer layer and the effective accommodation of volume changes due to the hollow porous structure, (II) improved high-rate performance because of a shorter Li-ion diffusion pathway resulting from interior void spaces and well-defined pores at the surface. This unique electrode structure can potentially provide new cathode materials for high-performance lithium-ion batteries. KEYWORDS: Li-ion battery, cathode, vanadium oxide, hollow porous structure, carbon encapsulation

1. INTRODUCTION Lithium-ion batteries (LIBs) are high-energy storage devices. LIBs have been widely applied in portable electronics such as smartphones, laptops, and tablet PCs, because of their high capacity, outstanding cycling durability, absence of memory effect, and environmentally friendly features.1−3 Currently, applications of LIBs in electric vehicles (EVs) and plug-in hybrid (PHEVs) are limited owing to the low capacity, low cycling durability, and deficient high-rate capability in electrode materials.4 LIBs are composed of four main parts: cathode, anode, electrolyte, and membrane. In particular, the relatively poor electrochemical performance and high cost of the cathode materials compared to the anode materials are main challenges for the expanded development of LIBs for EVs and PHEVs.5,6 Thus, much exertion has been focused for the improvement of several electrode materials for cathode with high capacity, outstanding cycling durability, and improved high-rate perform© XXXX American Chemical Society

ance for high-performance LIBs. Among the potential electrode materials for the cathode, vanadium pentoxide (V2O5) is a one of the most encouraging cathode candidates because it can capture large quantities of electrons owing to the layered structure, which can provide an excellent performance during the lithiation/delithiation process7,8 V2O5 can deliver a high theoretical capacity (294 mA h g−1, versus Li/Li+), which is a higher value compared to commercialized electrode materials for a cathode such as LiCoO2 (140 mA h g−1), LiMn2O4 (148 mA h g−1), and LiFePO4 (170 mA h g−1, ) as well as the latest cathode materials such as LiNi0.33Co0.33Mn0.33O2 (278 mA h g−1) and LiNi0.8Co0.15Al0.05O2 (279 mA h g−1).9−12 Nevertheless, the realistic use of V2O5 in LIB cathodes has been Received: May 4, 2016 Accepted: July 12, 2016

A

DOI: 10.1021/acsami.6b05307 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the ideal C/HPV2O5-30 synthesis process. (a) PAN and VO(acac)2 nanofiber obtained by electrospinning. (b) VN nanoparticles/CNF composite obtained after a carbonization process. (c) C/HPV2O5-30 with interior void spaces and well-defined pores as well as a uniform carbon layer on the V2O5 surface prepared by postcalcination.

% VO(acac)2 were dissolved in DMF. For the electrospinning process, the DC voltage and feed rate were fixed at 13 kV and 0.03 mL/h.3 The as-spun PAN/VO(acac)2 nanofibers were stabilized in air and carbonized in nitrogen atmosphere.3 After carbonization, the vanadium nitride/carbon-nanofiber (VN/ CNF) composites were successfully formed. To obtain the carbon-encapsulated hollow porous V2O5 nanofibers, three sets of samples were sintered in air for 10, 30, and 60 min using a box furnace at 400 °C. The three types of nanofibers calcined for 10, 30, and 60 min are hereafter referred to as C/HPV2O510, C/HPV2O5-30, and C/HPV2O5-60, respectively. 2.3. Characterization. The structures were examined by field emission−scanning electron microscopy and transmission electron microscopy (MULTI/TEM; Tecnai G2 , KBSI Gwangju Center). The energy-dispersive X-ray spectrometer (EDS) was used to perform TEM−EDS mapping to examine the dispersion of elements in the samples. The contents were determined by thermogravimetric analysis (TGA) in the temperature range from 200 to 600 °C at a heating rate of 10 °C min−1 in air. The crystal structures were examined by Xray diffractometry (XRD) in the range from 10° to 90° with a step size of 0.02°. The chemical bonding were characterized by X-ray photoelectron spectroscopy (XPS) with an Al Kα X-ray source. The electrical conductivity of the electrodes was measured using a Hall effect measurement system. The electrodes to measure the electrical conductivity were coated on a glass substrate and dried in air at 100 °C for 12 h. 2.4. Electrochemical Characterization. Electrochemical measurements were performed using coin cells (CR2032), which are composed of C/HPV2O5 as the cathode, Li metal foil as the anode, a porous polypropylene membrane as the separator, and a 1.0 M LiPF6 solution in a mixture of ethylene carbonate−dimethyl carbonate (1:1) as the electrolyte. For electrodes, slurries consisting of the active materials (80 wt %), poly(vinylidene difluoride) (10 wt %) as a binder, and Ketjen black (10 wt %) as a conducting material in an N-methyl-2pyrrolidinone solvent were coated on Al foil, and the resultant electrode was dried in an oven at 100 °C for 12 h. The mass loading of C/HPV2O5 was optimized and fixed with 10.5 ± 0.5 mg cm−2. All of the LIB coin cells were constructed in a highpurity argon-filled glovebox with H2O and O2 contents less than 5 ppm. The charge−discharge tests in the potential range of 2.0−4.0 V (versus Li/Li+) were carried out using a battery cycler system (WonATech Corp., WMPG 3000) at 25 °C in an incubator. The cycling durability was measured up to 100 cycles at a current density of 100 mA g−1. The high-rate performance was investigated at current densities of 50, 100, 300, 500, 700,

limited due to the poor cycling durability and poor high-rate capability arising from its large volume changes causing deterioration and pulverization of V2O5, low electrical conductivity (10−3 to 10−2 S cm−1), and low diffusion coefficient of Li ions (∼10−12 cm2 s−1) during the lithiation/ delithiation process.8,13−15 In order to overcome these issues for performance improvement, much effort has been focused on the fabrication of nanostructured V2O5 to obtain improved electrochemical performance. In this connection, advanced nanostructured V2O5 structures have been reported, such as core−shell structures, hollow structures, porous structures, and composite structures, which are especially important for improved electrochemical performance of the electrodes.16−18 In particular, these nanostructures can serve a large active surface area and a shorter Li-ion diffusion pathway and can relax the mechanical stress during cycling.16−18 Nonetheless, for the electrochemical performance of nanostructured V2O5 electrodes, cycling durability and high-rate capability are still not sufficient. Therefore, we have designed unique electrode architectures, including hollow structures, porous structures, and carbon encapsulation, to solve the above-mentioned problems. As is well-known, hollow structures show attractive advantages, such as good mechanical stability and excellent cycling durability due to interior void spaces that are active during cycling.19,20 Furthermore, porous structures provide a reduced ion diffusion pathway because the pores at the surface provide high contact areas between the electrode materials and electrolyte, resulting in outstanding high-rate capability.21,22 In addition, carbon materials on V2O5 surface play an important role as a physical buffer and also enhance the electrical conductivity, resulting in excellent cycling durability, especially at high rates.23,24 Thus, we synthesized a cathode material for LIBs with a unique architecture of a carbon-encapsulated hollow porous structure based on V2O5 using electrospinning and postcalcination. These strategies allow synthesis of cathode materials with excellent structural stability and high electrical conductivity.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Polyacrylonitrile (PAN, Mw = 150 000), N,N-dimethylformamide (DMF, 99.8%), and vanadyl acetylacetonate (VO(acac)2, 98%) were purchased from SigmaAldrich. 2.2. Synthesis of Carbon-Encapsulated Hollow Porous V2O5 Nanofibers. Carbon-encapsulated hollow porous V2O5 nanofibers (C/HPV2O5) were successfully synthesized using electrospinning and calcination. First, 10 wt % PAN and 15 wt B


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Figure 2. Low-resolution (a−d) and high-resolution (e−h) SEM images of VN/CNF, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60. The insets in (e−h) show magnified SEM images.

Figure 3. Low-resolution (a−d) and high-resolution (e−h) TEM images of VN/CNF, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60.

and 1000 mA g−1. To investigate the electrochemical kinetics, electrochemical impedance spectroscopy (EIS) measurements were performed using fresh cells in a frequency range of 105 to 10−2 Hz at an AC signal of 5 mV. Cyclic voltammetry (CV) measurements were prepared with a potentiostat/galvanostat in the potential range of 2.0−4.0 V (versus Li/Li+) at a scan rate of 0.1 mV s−1.

morphology without blobs of agglomerated VN, which means that VN nanoparticles were imbedded in the CNF matrix. After postcalcination, C/HPV2O5-10 (Figure 2b,f with diameters ranging from 435−441 nm presented rough surfaces. The C/ HPV2O5-30 (Figure 2c,g) with diameters ranging from 440− 453 nm showed the hollow porous structure due to the diffusion of VN, a phase transition from VN to V2O5, a decomposition of carbon, and grain growth of V2O5. The V2O5 grains connect to each other and form the porous structure, leading to the formation of pores along the body. Thus, we discovered a noticeable morphological change in the nanofiber surface, leading to the generation of hollow porous structures during postcalcination. In addition, the electrospinning solution was optimized using 15 wt % VO(acac)2 to obtain the successful formation of hollow porous structures, as shown in Figure S1. Similarly, C/HPV2O5-60 (Figure 2d,h) with diameters ranging from 437−449 nm displayed the hollow porous structure without any morphological changes. However, at low temperatures of 300 and 350 °C (Figure S2a and b) during postcalcination, nanofibers did not form the hollow porous structure, whereas the structure of the nanofibers almost collapsed at high temperature of 450 °C (Figure S 2d) because of excessive grain growth of V2O5. Therefore, the optimized hollow porous structure was obtained by postcalcination at 400 °C. This unique architecture of hollow porous structure in LIBs

3. RESULTS AND DISCUSSION Figure 1 is an illustration of an ideal preparation process of C/ HPV2O5-30. The as-spun nanofiber composed of VO(acac)2 and PAN was fabricated by electrospinning, as shown in Figure 1a. The as-spun nanofiber was carbonized under a nitrogen atmosphere to obtain VN/CNF, as shown in Figure 1b. Finally, C/HPV2O5-30 (Figure 1c), which formed the carbonencapsulated hollow porous structure because of the diffusion of VN, the decomposition of carbon, the grain growth of V2O5, and the presence of residual carbon, was prepared by postcalcination at 400 °C for 30 min. The morphologies of C/HPV2O5 were investigated by SEM measurements. Figure 2 presents low-resolution (Figure 2a−d) and high-resolution (Figure 2e−h) SEM images of the VN/ CNF, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60. As shown in Figure 2a,e, VN/CNF with diameters ranging from 531−558 nm showed smooth surfaces and consistent C


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Figure 4. Schematic illustration of the formation mechanism of C/HPV2O5-30.

Figure 5. (a) TEM-EDS mapping data of C/HPV2O5-30. (b) TGA curves of the VN/CNF, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60 from 200 to 600 °C at a heating rate of 10 °C min−1 in air.

buffer and provide increased electrical conductivity, resulting in excellent cycling durability and superb high-rate performance. Similarly, C/HPV2O5-60 (Figure 3d,h) showed the hollow porous structure and exhibited lattice fringes with a spacing of 0.27 and 0.58 nm, corresponding to the (011) and (200) planes of V2O5, respectively.28,29 However, C/HPV2O5-60 did not exhibit carbon layers on the V2O5 surface because of total exhaustion of the carbon source during the long postcalcination time of 60 min. Therefore, the successful formation of carbonencapsulated hollow porous structure was obtained using the optimized condition of postcalcination of VN/CNF at 400 °C for 30 min. Depending on the SEM and TEM results, a probable formation mechanism of C/HPV2O5-30 is illustrated in Figure 4. Remarkably, we synthesized novel carbon-encapsulated hollow porous structure with interior void spaces and welldefined pores. First, the formation of a hollow structure with interior void spaces is attributed to the diffusion of VN, a phase transition from VN to V2O5, and a decomposition of carbon. That is, during postcalcination in air, the VN nanoparticles in the core region should diffuse to the surface to encounter oxygen and reach thermodynamic stability via the reaction 4VN +9O2 → 2 V2O5+4NO2. Then the carbon materials decompose

can effectively accommodate the volume changes during the lithiation/delithiation process and provide a shorter Li-ion diffusion pathway, leading to the excellent cycling durability and improved high-rate performance. To further examine the nanostructure, TEM measurements were carried out. Figure 3 presents low-resolution (Figure 3a− d) and high-resolution (Figure 3e−h) TEM images of VN/ CNF, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60. The dark spots in the VN/CNF (Figure 3a,e) are well-distributed VN nanoparticles, 11−15 nm in size, inside the CNF matrix. After postcalcination, C/HPV2O5-10 (Figure 3b,f) showed spherical V2O5 grains without pores because of the short time allowed for grain growth. Interestingly, C/HPV2O5-30 (Figure 3c) displayed a hollow porous structure with pores along the body and flat V2O5 grains owing to sufficient time for grain growth. In general, V2O5 grows to a flat structure via oligomerization of vanadium.25−27 HRTEM images with lattice fringes of C/HPV2O5-30 (Figure 3g) show an interplanar spacing of 0.34 nm, which corresponds to the (110) plane of V2O5.28,29 Moreover, a uniform carbon layer on the V2O5 nanofiber surface with a thickness of about 6.7 nm was absolutely observed by HRTEM, as shown in Figure 3g. These uniform carbon layers on the V2O5 surface serve as a physical D


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Figure 6. (a) XRD patterns of VN/CNF, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60. XPS spectra of (b) V 2p and (c) C 1s of C/HPV2O530. (d) Electrical conductivity of commercial V2O5, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60.

via the reaction C+O2 → CO2. As a result, the nanofibers form hollow structures with interior voids. Second, the generation of a hollow structure with pores is attributed to the grain growth of V2O5 by the Ostwald ripening process during postcalcination, leading to the generation of interconnected nanostructures.30 Finally, the carbon encapsulation occurs because of residual carbon in the CNF matrix after postcalcination. Thus, the CNF matrix serves as a carbon source for formation of carbon encapsulation. To confirm the uniform distribution of vanadium, oxygen, and carbon atoms, TEM−EDS mapping of C/HPV2O5-30 data was performed as shown in Figure 5a. The EDS result confirms that V and O atoms are consistently distributed along the nanofibers. Therefore, V2O5 was successfully synthesized using electrospinning and postcalcination. Furthermore, the EDS results indicate that carbon atoms are consistently dispersed on the V2O5 surface, which means the existence of a carbon layer. To further examine the content of VN/CNF, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60, TGA analysis was papered from 200 to 600 °C in air, as shown in Figure 5b. VN/CNF presented a weight loss of 56.1% due to the subsistence of VN nanoparticles in the CNFs. In addition, C/HPV2O5-10 and C/ HPV2O5-30 exhibited weight losses of 98.3% and 99.2%, which indicates that the two samples holed a small amount of carbon materials after postcalcination. However, C/HPV2O5-60 reached a weight loss of 99.9%, implying the absence of carbon materials. Theses TGA results are in accordance with the HRTEM results. Figure 6a shows XRD patterns used to investigate the crystal structures of the samples. All samples indicated broad peaks at around 25° which correspond to the (002) layers of graphite.31,32 The foremost characteristic diffraction peak of VN/CNF was observed at 43.7 corresponding to the (200) plane of the VN phase with a face-centered structure (JCPDS card no. 78-1315). The main characteristic diffraction peaks of C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60 are ob-

served at 15.4°, 20.3°, 21.7°, 26.2°, and 32.4°, which correspond to the (200), (010), (110), (101), and (011) planes, respectively. These patterns are in accordance with those of the V2O5 phase with a primitive structure (JCPDS card no. 86-2248), which are consistent with the HRTEM results. Interestingly, the intensities of diffraction peaks of C/HPV2O530 and C/HPV2O5-60 increased compared to those of C/ HPV2O5-10 due to a long phase transition time from VN to V2O5 during postcalcination. Also, the V2O5 phase formed at 400 °C during postcalcination, as shown in Figure S3. To further examine the chemical bonding states, XPS measurements were carried out, as shown in Figure 6b,c as well as Figure S4. The V 2p XPS spectral peaks of C/HPV2O5-30 (Figure 6b) exhibited two signals at ∼517.0 eV and ∼524.0 eV, which correspond to the V 2p3/2 and V 2p1/2 photoelectrons of the V2O5 phase (V5+), respectively.33,34 However, C/HPV2O510 (Figure S4b) showed a small amount of V2O3 phase (V3+) because of a short postcalcination time. In particular, it is wellknown that the V2O3 phase does not participate in the lithiation/delithiation process at the cathode in the voltage range of 2.0−4.0 V (versus Li/Li+) due to charge localization, leading to poor LIB performance.31 Furthermore, the disassembly of the C 1s spectra of C/HPV2O5-30 (Figure 6c) was observed at 284.5, 286.0, 287.4, and 288.9 eV corresponding to C−C groups, C−O groups, CO groups, and O−CO groups, respectively.35,36 These results indicate the existence of the carbon layer on the V2O5 surface, which is in accordance with the HRTEM results. With increasing postcalcination time, the amount of oxygen-containing functional groups such as C−O groups, CO groups, and O−C O groups decreased because of an exhaustion of carbon materials during the long postcalcination time from 10 to 60 min, as shown in Figure S4e−h. These XPS results are consistent with the HRTEM and TGA results. In addition, the electrical conductivity of the samples was investigated by a Hall effect measurement system, as shown in Figure 6d. For E


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Figure 7. Charge−discharge curves of (a) commercial V2O5, (b) C/HPV2O5-10, (c) C/HPV2O5-30, and (d) C/HPV2O5-60 electrodes at a current density of 100 mA g−1 in the voltage range 2.0−4.0 V (versus Li/Li+) for 1st, 2nd, 10th, 50th, and 100th cycles.

Figure 8. Electrochemical properties of C/HPV2O5 electrodes. (a) Cycling durability of the charge−discharge capacities of commercial V2O5, C/ HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60 up to 100 cycles. (b) The high-rate performance at current densities of 50, 100, 300, 500, 700, 1000, and 50 mA g−1. (c) Nyquist plots in the frequency range of 105 to 10−2 Hz before the charge−discharge tests. (d) CV curves of C/HPV2O5-30 electrode in the potential range of 2.0−4.0 V (versus Li/Li+) at a scan rate of 0.1 mV s−1.

comparison, the conductivity of commercial V2O5 (99.2%, Alfa Aesar, 11093) was measured. The electrical conductivities of V2O5, C/HPV2O5-10, C/HPV2O5-30, and C/HPV2O5-60 were 1.1 × 10−2, 3.9 × 10−1, 2.7 × 10−1, and 9.3 × 10−2 S cm−1, respectively. The C/HPV2O5-30 exhibited relatively higher electrical conductivity than C/HPV2O5-60, which means that

the existence of the carbon layer on the V2O5 nanofiber surface contributes to the increase of electrical conductivity. Figure 7 exhibits the galvanostatic charge−discharge profiles of commercial V2O5, C/HPV2O5-10, C/HPV2O5-30, and C/ HPV2O5-60 electrodes measured at a current density of 100 mA g−1. All electrodes exhibit three plateaus at approximately F


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ACS Applied Materials & Interfaces 3.3, 3.1, and 2.2 V in the first discharge processes, which can be ascribed to the phase changes during lithiation.37,38 These phase changes are reversible in the next charge processes and following cycles. The specific discharge capacities were 288 mA h g−1 for a commercial V2O5 electrode, 246 mA h g−1 for the C/HPV2O5-10 electrode, 293 mA h g−1 for the C/HPV2O5-30 electrode, and 290 mA h g−1 for the C/HPV2O5-60 electrode at the first cycle. Among these, the C/HPV2O5-10 electrode showed the lowest initial specific discharge capacity due to the low crystallinity of V2O5 and the presence of V2O3 phases. The commercial V2O3 electrode showed a lower specific discharge capacity of 20 mA h g−1 after 100 cycles (Figure S5). Impressively, the C/HPV2 O5 -30 electrode (Figure 7c) exhibited a high specific capacity, near to the theoretical capacity of 294 mA h g−1. These results are due to the carbonencapsulated hollow porous structure with improved electrical conductivity and increased electroactive sites, leading to high specific capacity. In addition, the Coulombic efficiency of all electrodes was consistently maintained at 100% during the first cycles, indicating no irreversible capacity loss. As shown in Figure 8a, the cycling durability was evaluated by galvanostatic charge−discharge tests up to 100 cycles. The specific discharge capacities decreased continuously up to 100 cycles. The commercial V2O5 electrode showed a rapid drop of the specific discharge capacity to 153 mA h g−1 after 100 cycles with a capacity retention of 53%. Noticeably, the C/HPV2O530 electrode presented impressive cycling durability with a high specific discharge capacity of 241 mA h g−1 after 100 cycles with a capacity retention of 82%, which is the highest performance in comparison with previously reported V2O5 cathode materials with various morphologies and structure, as summarized in Table S1.18,28,39−51 Thus, we believe that the observed excellent cycling durability of C/HPV2 O5 -30 electrode is mainly ascribed to the novel architecture with carbon-encapsulated hollow porous structure. There are two reasons for the excellent cycling durability: (I) efficient accommodation of volume changes during the lithiation/ delithiation process using the interior void spaces and welldefined pores, (II) introduction of carbon encapsulation as a physical buffer layer that prevents structural breakdown during the lithiation/delithiation process. However, because of the absence of carbon encapsulation, the C/HPV2O5-60 electrode indicated a relatively low cycling durability with a specific discharge capacity of 195 mA h g−1 after 100 cycles and with a poor capacity retention of 67%. Figure 8b shows the high-rate performance of all electrodes obtained at current densities of 50, 100, 300, 500, 700, 1000, and 50 mA g−1. Remarkably, the C/HPV2O5-30 electrode exhibited excellent high-rate performance from 300 to 155 mA h g−1 with current densities of 50 and 1000 mA g−1 and were then regained to 281 mA h g−1 with a capacity retention of 94% when the current density reverted to 50 mA g−1. These results can be attributed to the more positive conditions for Li-ion diffusion at high current densities due to the shorter Li-ion diffusion pathway depending on the hollow porous structure, thus leading to improved high-rate performance. To investigate the influence of carbon-encapsulated hollow porous structure on the electrochemical kinetics of the lithiation/delithiation process, EIS measurement was performed using fresh cells. Figure 8c shows Nyquist plots of the electrodes. The semicircle in the high-frequency region is ascribed to the charge transfer resistance (Rct) at the cathode− electrolyte interface, and the straight line in the low-frequency

range is referred to as the Warburg impedance corresponding to Li-ion diffusion in the cathode.52−54 In general, a smaller diameter of the semicircle in the high-frequency region signifies low charge-transfer resistance. The C/HPV2O5-30 electrode shows the lowest Rct and lower Warburg impedance compared to the other electrodes owing to the unique architecture of the carbon-encapsulated hollow porous structure, indicating that carbon encapsulation could contribute not only to reduction of the Rct value but also to prevention of structural breakdown. However, the C/HPV2O5-60 electrode exhibited a relatively high Rct because of its low electrical conductivity (Figure 6d) resulting from the absence of carbon encapsulation. To observe the electrochemical properties of C/HPV2O5-30 electrode, CV measurements were performed. Figure 8d shows the CV curve of C/HPV2O5-30 electrode with the three anodic and cathodic process. The first anodic and cathodic process A1/C1 (3.56 V/ 3.25 V) corresponds to the conversion reaction between V2O5 and ε-Li0.5V2O5, which can convey a theoretical capacity of 73.5 mA h g−1. The second anodic and cathodic process A2/C2 (3.40 V/3.03 V) is attributed to a conversion reaction between ε-Li0.5V2O5 and δ-LiV2O5, which can exhibit a theoretical capacity of 73.5 mA h g−1. The third anodic and cathodic process A3/C3 (2.67 V/2.11 V) is attributed to a conversion reaction between δ-LiV2O5 and γ-Li2V2O5. The reaction at this stage can also release a theoretical capacity of 147 mA h g−1. Therefore, the sum of the theoretical capacity of V2O5 is 294 mA h g−1.55−57 The CV curves of the C/HPV2O5-30 electrode suggest good reversibility of the lithiation/delithiation process, which is in accordance with the charge−discharge profiles. Thus, the electrochemical conversion reaction of the C/ HPV2O5-30 electrode can be summarized using the following steps:55−57 V2O5 + 0.5Li+ + 0.5e− ↔ ε − Li 0.5V2O5

(1)

ε − Li 0.5V2O5 + 0.5Li+ + 0.5e− ↔ δ − LiV2O5

(2)

δ − LiV2O5 + Li+ + e− ↔ γ − Li 2V2O5

(3)

The exceptional lithium storage properties of the C/HPV2O530 electrodes can be explained by their unique architecture. The existence of carbon encapsulation as a physical buffer layer can noticeably provide improved electrochemical performance, such as excellent cycling durability due to superb prevention of the structural pulverization during cycling and an enhanced high-rate performance due to improved electrical conductivity. In addition, the carbon-encapsulated hollow porous structure with interior void spaces and well-defined pores can accommodate the structural volume changes of cathode materials during the lithiation/delithiation process and allow an increase in electroactive sites, leading to excellent cycling durability with high specific capacity. Finally, the interior void spaces and well-defined pores possibly provide a favorable environment for Li-ion diffusion at high current densities and a shorter Li-ion diffusion pathway, leading to the high-rate performance of the electrode.

4. CONCLUSIONS C/HPV 2O 5 was synthesized using electrospinning and postcalcination. By optimizing the postcalcination conditions, we synthesized a unique architecture consisting of a carbonencapsulated hollow porous structure; the structure includes interior void spaces and well-defined pores as well as a uniform carbon layer on the V2O5 nanofiber surface. The optimized C/ G


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HPV2O5-30 electrode exhibited improved lithium-storage properties, including the highest specific discharge capacities among the samples, excellent cycling durability (241 mA h g−1 at 100 cycles), and improved high-rate performance (155 mA h g−1 at 1000 mA g−1) compared to the C/HPV2O5-10 and C/ HPV2O5-60 electrodes as well as commercial V2O5. The improved electrochemical performance can be clarified by two main influences. First, the excellent cycling durability with high specific capacity is attributed to the introduction of carbon encapsulation as a physical buffer layer and the efficient accommodation of volume changes due to the hollow porous structure. Second, the improved high-rate performance is ascribed to shorter Li-ion diffusion pathway resulting from interior void spaces and well-defined pores at the surface. We believe that this unique structure of the electrode has great potential for use in LIBs as well as various energy storage devices, such as electrochemical capacitors and full cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05307. Supplementary figures and the table of cycling durability comparison of previously reported V2O5-based cathode materials with various morphologies and structure in LIBs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for H.-J.A.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2015R1A1A1A05001252).

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REFERENCES

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Carbon-Encapsulated Hollow Porous Vanadium-Oxide Nanofibers for

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