Long-Lasting Nb2O5

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Long-lasting Nb2O5-based nanocomposite materials for Li-ion storage Min Yeong Song, Na Rae Kim, Hyeon Ji Yoon, Se Youn Cho, Hyoung-Joon Jin, and Young Soo Yun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11444 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Applied Materials & Interfaces

Long-Lasting Nb2O5-Based Nanocomposite Materials for Li-Ion Storage Min Yeong Song,1,† Na Rae Kim,1,† Hyeon Ji Yoon,1 Se Youn Cho,1 Hyoung-Joon Jin,1,* and Young Soo Yun2,*

1

Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea

2

Department of Chemical Engineering, Kangwon National University, Samcheok 245-711, Korea

†,*

These authors contributed equally to this work.

KEYWORDS: niobium pentoxide · nanocomposite · porous carbon · electrode · Li-ion · hybrid capacitor

ACS Paragon Plus Environment

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ABSTRACT

Advanced nanostructured hybrid materials can help us overcome the electrochemical performance limitations of current energy storage devices. In this study, three-dimensional porous carbon nanowebs (3D-CNWs) with numerous included orthorhombic Nb2O5 (T-Nb2O5) nanoparticles were fabricated using a microbe-derived nanostructure. The 3D-CNW/T-Nb2O5 nanocomposites showed an exceptionally stable long-term cycling performance over 70,000 cycles, a high reversible capacity of ~125 mA h g–1, and fast Li-ion storage kinetics in a cointype two-electrode-system using Li metal. In addition, energy storage devices based on the above nanocomposites achieved a high specific energy of ~80 W h kg–1 together with a high specific power of ~5,300 W kg–1 and outstanding cycling performance with ~80% capacitance retention after 35,000 cycles.


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INTRODUCTION

The increasing demand for more compact mobile electronics and competitive electric vehicles requires advanced power sources with a high energy density, high power characteristics, and a long cycling lifetime.1,2 Supercapacitors exhibit fast and stable charge delivery by virtue of physical adsorption and desorption of charges on the surface of electrode materials.3,4 However, the physically accumulated charges forming electrochemical double layers (EDLs) are limited to the electrode surface and/or areas close to it, leading to a low energy density.3,4 In contrast, conventional Li-ion batteries (LIBs) use intercalation-based bulk host structures as active electrode materials.5,6 LIBs show high energy densities, since Li ions can be stored inside the bulk structure, and the intercalation compounds have a high operating voltage.2–4 However, the sluggish solid-state diffusion of Li ions in the bulk host structure leads to low power characteristics and large volume changes during repetitive intercalation/extraction processes, resulting in insufficient cycling performance and limited ability to meet the recent demands.7,8 Although the rate capabilities and cycling performance of intercalation-based active materials can be boosted by downsizing them to a nanometer-scale, this approach leads to a dramatic enhancement of unwanted side reactions in the anodic potential region, deteriorating the energy characteristics.9 Therefore, advanced electrode materials with sophisticated design are required for better energy storage devices with high energy and power densities as well as long cycling lifetimes. Niobium pentoxide (Nb2O5) is a promising electrode material with a fast Li-ion diffusion pathway, exhibiting pseudocapacitive charge storage behavior in the stable potential range of conventional carbonate-based electrolytes.10,11 In addition, the insertion of lithium into Nb2O5


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results in a solid solution with no apparent phase changes and negligible changes to lattice constants and unit cell volume,11 achieving stable cycling. The theoretical Li-ion storage capacity of Nb2O5 is ~200 mA h g–1, corresponding to 728 C g–1, which is much higher than the values of typical supercapacitor electrodes.12 However, its poor electrical conductivity (~3.4 × 10–6 S cm–1) obstructs the fast charge storage kinetics, which can be resolved by reducing the size of Nb2O5 particles and constructing a hybrid structure with a conducting material.13,14 Kong et al. reported that pseudocapacitors based on orthorhombic Nb2O5 (T-Nb2O5)/graphene paper composites showed a high specific energy of 47 W h kg–1 and a specific power of 18 kW kg–1.15 Additionally, Nb2O5@carbon core-shell nanocrystal-based hybrid supercapacitors showed a specific energy of ~63 W h kg–1 and a specific power of ~16,530 W kg–1.16 The emphasis of these hybrid devices is their stable long-term cycling performance over several thousands of cycles, which is much better than that of rechargeable batteries.15–18 These results demonstrate the synergistic effects between carbon-based materials and Nb2O5 and the feasibility of Nb2O5-based energy storage devices. Nevertheless, the cycling performance of Nb2O5-based energy storage devices is still inferior to that of supercapacitors; moreover, their energy and power characteristics do not exceed those of LIBs. Their tepid electrochemical performance lacks any distinguishing point and makes these devices resemble a generic version of the popular LIBs. Herein, we demonstrate that super-stable long-term cycling performance with high specific energy and power characteristics can be achieved by an exquisite design of Nb2O5-based nanocomposite materials supported by three-dimensional porous carbon nanowebs (3D-CNWs). The nanocomposite materials were constructed by simple heating of pyrolized bacterial cellulose (BC) pellicles composed of carbon nanofibers with adsorbed niobium ions. The resulting 3DCNWs with numerous included nanometer-scale T-Nb2O5 particles showed an outstanding


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cycling performance over 70,000 cycles, a high reversible capacity of ~125 mA h g–1, and rapid Li-ion storage kinetics. In addition, energy storage devices based on 3D-CNW/T-Nb2O5 nanocomposites and the previously reported activated carbon nanosheets (ACNs)19 as the anode and cathode, respectively, showed a high specific energy of ~80 W h kg–1 and a specific power of ~5,300 W kg–1 with an exceptional capacitance retention of ~80% after 35,000 cycles.


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EXPERIMENTAL Preparation of 3D-CNW/T-Nb2O5 nanocomposites. BC pellicles were prepared from Acetobacter xylinum BRC 5 in the Hestrin and Schramm (HS) medium using a previously reported method.20 The as-obtained BC hydrogels were immersed in tert-butanol (99.0%, SigmaAldrich, USA). After freezing at –30 °C for 6 h, the frozen BCs were freeze-dried at −45 °C and 4.5 Pa for 72 h. The prepared BC cryogels were heated to 800 °C under N2 atmosphere at a rate of 5 °C min−1 and held at the final temperature for 2 h. The resulting 3D-CNWs (150 mg) were immersed in 30 mL of an N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich, USA) solution of NbCl5 (0.93 mM, 99.9%, Sigma-Aldrich, USA). The 3D-CNWs with included Nb precursor were dried in a convection oven at 120 °C. Subsequently, the resulting material was heated to 800 °C for 2 h under N2 atmosphere at a rate of 10 °C min−1. The final product (3D-CNW/TNb2O5 nanocomposites) was stored in a vacuum oven at 30 °C.

Characterization. Sample morphology was examined by field emission scanning electron microscopy (FE-SEM, S-4300, Hitachi, Japan) and field emission transmission electron microscopy (FE-TEM, JEM2100F, JEOL, Japan). Raman spectra were recorded using a continuous-wave linearly polarized laser (514.5 nm, 2.41 eV, 16 mW). The laser beam was focused by a 100× objective lens, resulting in a spot with ~1 µm diameter. An acquisition time of 10 s and three scans were used to collect each spectrum. X-ray diffraction (XRD, Rigaku DMAX 2500) analysis was performed using Cu Kα radiation with λ = 0.154 nm at 40 kV and 100 mA. The chemical composition of the samples was examined by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA, USA) with monochromatic Al Kα radiation (hν = 1,486.6 eV). The pore


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structures were analyzed using nitrogen adsorption and desorption isotherms recorded by a surface area and porosimetry analyzer (Tristar, Micromeritics, USA) at –196 °C.

Electrochemical characterization. The electrochemical properties of 3D-CNW/T-Nb2O5 nanocomposites and their full-cell devices using ACNs were characterized with the help of a Wonatech automatic battery cycler and CR2032-type coin cells. For the half-cell experiments, coin cells were assembled in a glove box filled with argon, using 3D-CNW/T-Nb2O5 nanocomposites as the working electrode and metallic Li foil as the reference and counter electrode. LiClO4 (1 M; Aldrich, 99.99%) was dissolved in a solution of propylene carbonate (PC) and used as an electrolyte for Li-ion storage. A glass microfiber filter (GF/F, Whatman) was used as a separator. The working electrodes were prepared by mixing the active material (80 wt.%) with conductive carbon (10 wt.%) and polyvinylidene fluoride (10 wt.%) in N-methyl-2pyrrolidone. The resulting slurries were uniformly applied on Al foil. The electrodes were dried at 120 °C for 2 h and roll-pressed. The active material mass loading was ca. 1 mg cm–2, and the total electrode weight was ca. 2~3 mg. For the full-cell experiments, coin cells were assembled in a glove box filled with argon using 3D-CNW/T-Nb2O5 nanocomposites and ACNs as the anode and cathode, respectively. The same electrolyte and separator as for the half-cells were used, and the total electrode weight of the full cells was 4~5 mg.


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RESULTS AND DISCUSSION

3D-CNW/T-Nb2O5 nanocomposites exhibit a three-dimensional macroporous structure composed of randomly entangled carbon nanofibers [Fig. 1(a)]. This morphology is similar to that of 3D-CNWs fabricated from BC pellicles by the same method without the Nb precursor [Fig. S1]. However, the presence of T-Nb2O5 nanoparticles is not detected even in the magnified FE-SEM image [Fig. 1(b)]. This could be due to the homogeneous dispersion of very small TNb2O5 nanoparticles, which cannot be detected under the low resolution of FE-SEM. In contrast, FE-TEM images show numerous T-Nb2O5 nanoparticles well dispersed on the surface of carbon nanofibers [Figs. 1(c)–1(e)], with additional TEM images found in Figs. S2–S4. The nanoparticles have a size of 5~10 nm and a well-ordered crystal structure [Figs. 1(e) and S2–S4]. The observed lattice spacing of ~0.39 nm corresponds to the (001) plane of orthorhombic Nb2O5 [Fig. 1(e)]. The XRD pattern of 3D-CNW/T-Nb2O5 nanocomposites also shows several sharp peaks of the orthorhombic crystal structure, in contrast to the XRD pattern of 3D-CNWs, which shows a broad graphite (002) plane only [Fig. 2(a)]. The Raman spectra of 3D-CNWs and 3DCNW/T-Nb2O5 nanocomposites exhibit distinct D and G bands at ~1,340 and ~1,593 cm–1, respectively. These bands are attributed to aromatic polyhexagonal carbon structures, where the D band originates from the disordered carbon structure of 3D-CNWs in the A1g breathing mode of the aromatic ring near the basal edge, and the G band stems from the polyhexagonal carbon structure related to the E2g vibration mode of sp2-hybridized C atoms.21 The intensity ratio of the above bands (IG/ID) allows one to estimate the size of the carbon basic structural units (BSUs). These ratios are equal to 1.04 and 1.00 for 3D-CNWs and 3D-CNW/T-Nb2O5 nanocomposites, respectively, indicating the presence of a few nanometer-scale hexagonal carbon domains. The similar IG/ID ratios of 3D-CNWs and 3D-CNW/T-Nb2O5 nanocomposites suggest that the


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formation of T-Nb2O5 does not affect the development of the 3D-CNW carbon microstructure. The content of T-Nb2O5 in the nanocomposites was estimated by thermogravimetric analysis (TGA) in O2 atmosphere, as shown in Fig. 3. While the TGA curve of nanocomposites shows a residue of approximately 64.7 wt.%, that of 3D-CNWs exhibits a residue of ~4.0 wt.% after heating at 600 °C. This result indicates that the content of T-Nb2O5 incorporated in the nanocomposites is about 60 wt.%. The pore structures of 3D-CNWs and 3D-CNW/T-Nb2O5 nanocomposites were characterized by nitrogen adsorption and desorption isotherms, as shown in Fig. 4(a). Both isotherm curves exhibited an IUPAC type-II shape, indicating a macroporous structure. The specific nanocomposite surface area is ~103.0 m2 g–1, while that of 3D-CNWs is higher, being equal to 184.5 m2 g–1. The reduced specific surface area could be attributed to the introduction of T-Nb2O5 nanoparticles, since they have a relatively high density compared to carbon-based materials. Pore size distribution data of 3D-CNW and 3D-CNW/T-Nb2O5 also show that they have the similar pore size distribution, however, 3D-CNW has larger pore volume than 3DCNW/T-Nb2O5 in overall pore diameters. The surface properties of 3D-CNWs and 3D-CNW/T-Nb2O5 nanocomposites were investigated by XPS, as shown in Fig. 5. The C 1s XPS spectra of both materials [Fig. 5(a)] show the presence of distinct C–C and C–O bonds together with a minor amount of C=O bonds, indicating their similar carbon structure.22 However, in the case of 3D-CNW/T-Nb2O5 nanocomposites, the C–O and C=O binding energies are shifted to lower energies compared to those of 3D-CNWs. This could originate from the interaction between T-Nb2O5 nanoparticles and the oxygenated functional groups of 3D-CNWs.23 The O 1s XPS spectrum of 3D-CNWs shows two overlapping peaks corresponding to O=C and O–C bonds, while that of 3D-CNW/T-


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Nb2O5 nanocomposites shows a large new peak centered at 530.6 eV, corresponding to O–Nb bonds and including the O=C and O–C bonds [Fig. 5(b)]. In addition, the Nb 3d XPS spectrum of 3D-CNW/T-Nb2O5 nanocomposites clearly shows two Nb 3d5/2 peaks centered at 207.6 and 210.3 eV, supporting the presence of T-Nb2O5 [Fig. 5(c)]. The C/O ratios of 3D-CNWs and 3DCNW/T-Nb2O5 nanocomposites are 12.7 and 2.5, respectively. The dramatic oxygen content increase results from the introduction of numerous T-Nb2O5 nanoparticles. Moreover, the C/Nb ratio is equal to 8.0, in agreement with the TGA results. The electrochemical performance of 3D-CNW/T-Nb2O5 nanocomposites was tested in a voltage range of 1.2–3.0 V vs. Li+/Li in 1 M LiClO4 dissolved in PC as an electrolyte [Fig. 6]. Cyclic voltammetry (CV) curves of T-Nb2O5 nanocomposites show a mount up of the anodic and cathodic currents from 2.3 V, with a dramatic increase around 2.0 V [Fig. 6(a)]. The broad anodic and cathodic peaks in the potential range of 1.2~2.3 V result from the following redox reaction: T-Nb2O5 + xLi+ + Xe– ↔ LixNb2O5 where x is molar ratio of the inserted Li ion (0 to 1).24 The peak current (i) increase with increasing sweep rate (v) can be represented as i = avb, where a and b are adjustable values.11 For a diffusion-controlled reaction, the b-value is close to ~0.5, while it is ~1 for a surface-controlled pseudocapacitive reaction. 3D-CNW/T-Nb2O5 nanocomposites characterized using sweep rates from 0.1 to 10 mV s–1 exhibit a b-value of ~0.94, indicating that their Li-ion storage is mainly due to the surface-driven pseudocapacitive processes [Fig. 6(b)]. The galvanostatic discharge (Li-ion insertion) profile of the nanocomposites at a current density of 0.5 A g–1 shows a drastic increase of specific capacity around 2.0 V [Fig. 6(c)], which agrees with the CV curve. Moreover, the voltage continuously decreases without exhibiting any plateau section between 2.0 and 1.2 V


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due to pseudocapacitive charge storage.25 The charge (Li-ion extraction) profile also shows a similar specific capacity change around 2.0 V and a Coulombic efficiency above 99%. At increased current density, the working voltage ranges gradually decreased due to increased polarization, and specific capacities were reduced. The reversible capacity at a current density of 0.5 A g–1 was ~125 mA h g–1, corresponding to a specific capacitance of ~250 F g–1, which is calculated by whole weights of the nanocomposites including both 3D-CNW and T-Nb2O5. Contributions of 3D-CNWs for the specific capacities can be confirmed in Fig. S5. Considering only the pseudocapacitive Li-ion storage voltage range of 1.2–2.2 V, the capacitance value corresponds to ~377 F g–1 [Fig. 6(d)]. The reversible capacities at current densities of 1, 2, 5, and 10 A g–1 are ~118, 107, 95, and 82 mA h g–1, respectively, corresponding to specific capacitances of 356, 322, 283, and 240 F g–1, respectively [Figs. 6(c) and 6(d)], in a voltage range of 1.2–2.2 V. In addition, the IR drops for 0.5 to 10 A g–1 are just within 8.8~240.7 mV [Fig. 6(d)]. These results demonstrate the high reversible capacity and rapid charge storage kinetics of 3D-CNW/T-Nb2O5 nanocomposites. Even at a further increased current density of 30 A g–1, a high specific capacitance of 140 F g–1 was maintained, and a small IR drop of ~430 mV was observed [Fig. 6(d)]. In addition, the 3D-CNW/T-Nb2O5 nanocomposites show highly stable cycling over 70,000 charge/discharge cycles [Fig. 7(a)]. Overall, a Coulombic efficiency of nearly 100% was maintained, indicating the potential use of the above nanocomposites as electrodes

for

energy

storage

devices

[Fig.

7(a)].

Additionally,

the

galvanostatic

discharge/charge profiles at the 100th and every 10,000th cycle are similar to each other, demonstrating the highly stable cycling of 3D-CNW/T-Nb2O5 nanocomposites [Fig. 7(b)]. Further specific analysis of structural changes after the first cycle and during following cycles was conducted by performing ex-situ XRD and obtaining FE-TEM images, of which


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results are shown in Figs. S6 and S7. While the crystal structure of T-Nb2O5 did not change with respect to the state of charges (SoCs) during Li-ion insertion, the (001) plane of orthorhombic Nb2O5 gradually reduced with Li-ion extraction for the first cycle [Fig. S6]. This result indicates that the arrangement of niobium ions lying in a sheet parallel to (001) was disordered after the first cycle, whereas other crystal structures were not affected. Ex-situ FE-TEM images after the first cycle, which show that the crystal structure of T-Nb2O5 is particularly damaged for the (001) plane, support these XRD results [Fig. S7]. In addition, for the following cycles, the (001) plane continuously reduced and eventually disappeared after 70,000 cycles, as shown in Fig. 7(c), and other peaks also considerably reduced after 70,000 cycles. These results suggest that the crystal structure of T-Nb2O5 is damaged after long-term cycling. FE-TEM images, however, confirm the presence of numerous nanoparticles on the surface of carbon nanofibers even after 70,000 cycles, with the former exhibiting similar morphologies to the as-prepared T-Nb2O5 [Figs. 7(d) and 7(e)]. Based on these results, we assume that the somewhat amorphized T-Nb2O5 could reversibly store Li ions. Full-cell devices (Li-ion hybrid capacitors) were assembled using 3D-CNW/T-Nb2O5 nanocomposites and ACNs as the anode and cathode, respectively. The electrochemical performance of ACNs was pre-tested at voltages between 2.0 and 4.5 V vs. Li+/Li. ACNs showed linear profiles and a specific capacitance of ~135 F g–1 at a current density of 0.5 A g–1 [Fig. S8]. Details of the cathode materials are provided in Figs. S8–S10. The voltage of the anode/cathode pair was controlled at 2.2 V vs. Li+/Li by lithiation via pre-cycling [Fig. S11].26 The Li-ion hybrid capacitors were operated in a range of 0.5–3.3 V, with their cyclic voltammograms showing a large charge storage area that gradually decreases with increasing sweep rates [Fig. 8(a)]. Nevertheless, the nearly tetragonal shape was well-maintained even at a


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high sweep rate of 30 mV s–1, indicating rapid charge storage kinetics. The galvanostatic charge/discharge profiles of Li-ion hybrid capacitors show a triangular shape, indicating capacitive charge storage [Fig. 8(b)]. At a current density of 0.1 A g–1, the specific capacitance of Li-ion hybrid capacitors equaled 58.1 F g–1, corresponding to 44.2 mA h g–1 [Fig. 8(c)], and the average voltage was calculated as 1.81 V [Table S1]. When the current density increased to 4 A g-1, the specific capacitance and average voltage decreased to 23 F g–1 and 1.32 V, respectively. Therefore, the specific energy decreased with increasing current density, while the specific power increased. A high specific energy of ~80.0 W h kg–1 was achieved at a specific power of 180 W kg–1, while the latter reached ~5,300 W kg–1 at a specific energy of ~24.0 W h kg–1. Further details on the electrochemical performance of Li-ion hybrid capacitors are listed in Table S1. As shown in Fig. 8(d), the Ragone plots for 3D-CNWs/T-Nb2O5//ACN (this work) [squares], Nb2O5-CNT//AC [circles],27 Nb2O5@C//AC [triangles],16 TiO2B//CNT [inverse triangles],28 and C-Li4Ti5O12//AC [diamonds],29 display the relationship between specific energy and specific power. The Li-ion hybrid capacitor based on 3D-CNWs/T-Nb2O5//ACN shows the highest specific energy and power values in the depicted Ragone plots. In addition, the above capacitor shows an outstanding cycling performance with nearly 100% Coulombic efficiency after 35,000 cycles [Fig. 8(e)]. The stable cycles maintained in overall cycles, and a capacitance retention of ~80% was achieved after 35,000 cycles [Fig. 8(e)].


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CONCLUSION

In summary, 3D-CNWs were fabricated from BC pellicles by simple heating, with T-Nb2O5 nanoparticles homogeneously introduced onto their surface up to a content of ~60 wt.%. The 3DCNW/T-Nb2O5 nanocomposites showed remarkable cycling performance over 70,000 cycles as well as a high specific capacity of ~125 mA h g–1 and high rate capabilities for current densities of 0.1 to 30 A g–1. After long-term cycling over 70,000 cycles, the initial morphologies of nanometer-scale T-Nb2O5 nanoparticles were well-maintained on the surface of the supporting carbon nanofibers as a highly amorphous microstructure. The Li-ion hybrid capacitors based on a 3D-CNW/T-Nb2O5 nanocomposite anode and a previously reported ACN cathode exhibited an outstanding capacitance retention of ~80% after 35,000 cycles. In addition, a high specific energy of ~80 W h kg–1 and a high specific power of ~5,300 W kg–1 were achieved. The simple fabrication method and practicable electrochemical performance of 3D-CNW/T-Nb2O5 nanocomposites and related energy storage devices can open up a new pathway towards a betterperforming power source.


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AUTHOR INFORMATION Corresponding Author *

H. –J. Jin, E-mail address: [email protected]

*

Y. S. Yun, E-mail address: [email protected]

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea grant funded by the Korean Government (MEST) (NRF-2012M1A2A2671806) and the Basic Science Research Program funded by the Ministry of Education (NRF-2013R1A1A2A10008534). Additional support was also provided by the Industrial Strategic Technology Development Program (Project No. 10050477, Development of a separator with low thermal shrinkage and electrolyte with high ionic conductivity for Na-ion batteries), funded By the Ministry of Trade, Industry & Energy (MI, Korea). S. J. H. and Y. W. P. acknowledge the support from the Swedish-Korean Basic Research Cooperative Program (2014R1A2A1A12067266) of the NRF Korea.

ASSOCIATED CONTENT Supporting Information. Additional information about FE-SEM and FE-TEM images of 3DCNWs and 3D-CNWs/T-Nb2O5 nanocomposites, electrochemical performances of 3D-CNWs and ACNs, ex situ XRD and FE-TEM characterizations of 3D-CNW/T-Nb2O5 nanocomposites for the first cycle, materials characteristics of ACNs and schematic image and table for a Li-ion hybrid capacitor is included in the Supporting Information.


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Figure 1. Morphological characteristics of 3D-CNW/T-Nb2O5 nanocomposites. (a), (b) FE-SEM images at different magnifications. (c), (d) FE-TEM images at different magnifications. (e) Highresolution FE-TEM image and (inset) diffraction pattern of the selected area.


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Figure 2. (a) XRD patterns and (b) Raman spectra of 3D-CNW/T-Nb2O5 nanocomposites and 3D-CNWs.


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Figure 3. TGA curves of 3D-CNW/T-Nb2O5 nanocomposites and 3D-CNWs.


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Figure 4. (a) Nitrogen adsorption and desorption isotherms of 3D-CNW/T-Nb2O5 nanocomposites and 3D-CNWs, and (b) their pore size distribution data.


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Figure 5. XPS (a) C 1s and (b) O 1s spectra of 3D-CNWs and 3D-CNW/T-Nb2O5 nanocomposites, and (c) XPS Nb 3d spectrum of 3D-CNW/T-Nb2O5 nanocomposites.


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Figure 6. Electrochemical performance of 3D-CNW/T-Nb2O5 nanocomposites tested using a two-electrode configuration with Li metal in an electrolyte composed of 1 M LiClO4 dissolved in PC, in a voltage window between 1.2 and 3.0 V. (a) Cyclic voltammograms at different sweep rates and (b) specific peak currents characterized at different sweep rates. (c) Galvanostatic discharge/charge profiles at different current densities and (d) rate capabilities characterized at current densities from 0.5 to 30 A g–1.


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Figure 7. (a) Cycling performance of 3D-CNW/T-Nb2O5 nanocomposites tested over 70,000 cycles and (b) galvanostatic discharge/charge profiles at the 100th and every 10,000th cycle. (c) Ex situ XRD patterns of 3D-CNW/T-Nb2O5 nanocomposites after the 100th, 10000th and 70,000th cycle. Ex situ (d) FE-TEM and (e) high-resolution FE-TEM image of 3D-CNW/T-Nb2O5 nanocomposites after 70,000 cycles. Inset of (e) shows the selected area diffraction pattern of 3D-CNW/T-Nb2O5 nanocomposites.


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Figure 8. Electrochemical performance of Li-ion hybrid capacitors based on 3D-CNWs/TNb2O5//ACN tested in an electrolyte composed of 1 M LiClO4 dissolved in PC, in a voltage window between 0.5 and 3.3 V. (a) Cyclic voltammograms at different sweep rates. (b) Galvanostatic charge/discharge profiles at different current densities. (c) Rate capabilities characterized at current densities from 0.1 to 4 A g–1. (d) Ragone plots of several energy storage devices based on 3D-CNWs/T-Nb2O5//ACN (squares), Nb2O5-CNT//AC (circles),27 Nb2O5@C//AC (triangles),16 TiO2B//CNT (inverse triangles),28 and C-Li4Ti5O12//AC (diamonds).29 (e) Cycling performance and Coulombic efficiency tested over 35,000 cycles.


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Table of Contents

Long-lasting Nb2O5-based nanocomposite materials for Liion storage Min Yeong Song,† Na Rae Kim,† Hyeon Ji Yoon, Se Youn Cho, HyoungJoon Jin,* and Young Soo Yun*

Text for Table of Contents

Three-dimensional porous carbon nanowebs (3D-CNWs) with numerous included orthorhombic Nb2O5 (T-Nb2O5) nanoparticles were fabricated using a microbe-derived nanostructure, showing a remarkably stable and long-term cycling performance with high energy and power characteristics.


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Long-Lasting Nb2O5

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