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Niobium oxide nanoparticle core–amorphous carbon shell structure for fast reversible lithium storage
Electrochimica Acta 240 (2017) 316–322
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Niobium oxide nanoparticle core–amorphous carbon shell structure for fast reversible lithium storage Kyungbae Kima , Sang-Gil Woob , Yong Nam Job , Jaegab Leea , Jae-Hun Kima,* a b
School of Advanced Materials Engineering, Kookmin University, Seoul 02707, Republic of Korea Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi 13509, Republic of Korea
A R T I C L E I N F O
Article history: Received 12 December 2016 Received in revised form 23 February 2017 Accepted 10 April 2017 Available online 18 April 2017 Keywords: niobium oxide nanoparticle core-shell structural evolution lithium storage hybrid supercapacitor
A B S T R A C T
The hybrid supercapacitor concept involving a battery electrode and a supercapacitor electrode was recently introduced to meet the demand for high energy and power electrochemical energy storage devices. To successfully apply this device, high-capacity and rate electrode materials for Li storage should be developed. Niobium pentoxide (Nb2O5) has recently attracted considerable attention owing to its reasonable capacity, excellent rate capability, and cycling stability. However, the low electronic conductivity of the material is a major limitation. To address this issue, carbon incorporation is usually performed. Herein, we report Nb2O5 nanoparticle core-amorphous carbon shell materials prepared by hydrothermal reaction and one-step carbon formation with annealing. During the one-step process, it was found that pyrolysis of a carbon precursor could significantly influence the structural evolution of Nb2O5 with increasing temperature. In addition, Nb2O5 is reduced to NbO2 in Ar atmosphere with further increase in temperature. The niobium oxide-carbon core-shell structure was thoroughly examined by using transmission electron microscopy. It was demonstrated that the proposed carbon-coated materials exhibited excellent electrochemical properties in terms of rate and cycle performances. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the times of ever-growing energy demands, electrochemical energy storage has become a core technology in various applications [1–3]. Among energy storage devices, rechargeable batteries and electrochemical capacitors (supercapacitors) are the major ones that store electrical energy via electrochemical reactions at the electrodes. The two systems are contrary to each other in terms of their energy storage mechanisms and electrochemical performance. Batteries have the advantage of relatively high energy density because ions can be stored in the bulk of electrode materials by diffusion-limited insertion. Conversely, supercapacitors exhibit the superior characteristics of high power (rate) and cycle life, resulting from the surface charge storage mechanisms involving Faradaic electrochemical reactions or formation of electrical double layers. Owing to the difference in the charge storage methods of two devices, better energy density and power/cycling stability characteristics are difficult to achieve simultaneously.
* Corresponding author. E-mail address: [email protected] (J.-H. Kim). http://dx.doi.org/10.1016/j.electacta.2017.04.051 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
To overcome the gap between batteries and supercapacitors, the hybrid supercapacitor (HSC) concept has been introduced by adopting a battery electrode and a supercapacitor electrode [4–8]. In this device, the battery-type electrode materials such as Li4Ti5O12 and TiO2 are used for anodes by using Li insertion chemistry and the capacitor-type electrode materials such as activated carbon are employed as cathodes by adopting surface charge storage mechanism. To narrow the kinetic gap between two different type electrodes, the anode should have high-rate capability with moderate capacities. As a result, hybrid supercapacitors can exhibit high power with considerable energy densities by combining the advantages of Li-ion batteries and nonaqueous supercapacitors. In this context, niobium pentoxide (Nb2O5) materials have recently attracted considerable interest as anode materials for HSCs because they can provide (1) relatively high theoretical capacity (200 mAh g 1 = 720 C g 1) compared to conventional pseudocapacitor materials such as RuO2 and MnO2, (2) excellent rate capability induced by pseudocapacitive intercalation reactions, and (3) relative abundance in nature. Actually, Nb2O5 has great potential for use in various applications because of its unique performance characteristics resulting from its energy band structure and the existence of several crystal structures and
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morphologies [9,10]. This material has mainly been used in specific applications, such as gas sensors, electrochromics, and catalysts [11,12]. In recent years, however, its applications have expanded to rechargeable batteries and supercapacitors based on its ability to store alkali ions [12–17]. Recently, Nb2O5 materials with a specific crystal structure were reported to have the advantage of fast Li intercalation reactions [18–30]. However, to use this material in HSCs, two prerequisites must be satisfied. One is crystal structure and morphology, as mentioned above. Dunn et al. reported interesting results on this issue [20,21]. According to the papers, nanostructured orthorhombic Nb2O5 is beneficial for relatively high-capacity and high-rate lithium storage owing to its ability to rapidly transport Li+ ions through its open and layered crystallographic structure [31]. The other issue is the low electronic conductivity of the materials [32,33]. Stoichiometric Nb2O5 is known to be an insulator with an electrical conductivity (s) of 3 10 6 S cm 1 [34]. To address this issue, carbon and conducting polymer incorporation have been performed [22–27,35]. In this study, we introduce a one-step annealing and carbon coating process after simple hydrothermal synthesis of Nb2O5 nanoparticles. The as-synthesized niobium oxide particles showed a less-crystalline phase, and the particles were annealed to obtain an orthorhombic structure, which is favorable for fast Li+ storage. During this annealing process, carbon coating by chemical vapor deposition (CVD) through sublimation of a solid precursor can be performed simultaneously. Here, we report the effect of the carbon coating process on the crystal structure transition of niobium oxide for the first time. A simple annealing process without carbon sources only leads to structural evolution from less-crystalline to orthorhombic and tetragonal Nb2O5 sequentially with increasing temperature, as reported in the literature [9–12]. By contrast, onestep annealing and carbon coating with a carbon precursor causes the crystal structure change of the as-prepared niobium oxide into pseudohexagonal and orthorhombic Nb2O5, and tetragonal NbO2 at different temperatures. The material characterization of the prepared carbon-coated Nb2O5 was carried out by several analytical tools. The electrochemical characterization demonstrated that the Nb2O5 nanoparticles with amorphous carbon shell layer had excellent rate and cycle performances.
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2. Experimental Materials synthesis: First, niobium oxide nanoparticles were synthesized according to the following hydrothermal synthesis procedure. 4.56 g of ammonium niobate oxalate hydrate (C4H4NNbO9xH2O, Sigma Aldrich) and 0.8 g of Pluronic F-127 (Sigma Aldrich) were dissolved in individual de-ionized (DI) water solutions (80 ml) under stirring for 20 min. Then, the ammonium niobate solutions and HCl (2 M, 1 ml) were added to the F-127 solution under stirring. The solutions were transferred into a Teflon-lined steel autoclave. The sealed autoclave was heated to 200 C and maintained at the temperature for 24 h to facilitate hydrothermal reaction. The resulting suspension was centrifuged to separate the white precipitates. After thorough washing with DI water and ethyl alcohol several times, the precipitates were dried at 80 C. For annealing and carbon-coating, the precipitates were placed in a quartz tube within a vertical furnace, as reported elsewhere [36]. The furnace was heated to each temperature (800, 850, 900, and 1000 C) with/without the carbon precursor of naphthalene in Ar atmosphere. After maintaining at each target temperature for 3 h, the furnace was cooled to room temperature. Materials characterization: The crystal structures of the synthesized materials were identified by X-ray diffraction (XRD, Rigaku D/MAX-2500 V with Cu Ka radiation). X-ray photoelectron spectroscopy (XPS, Thermo Scientific Ka) was used to analyze the chemical state of the materials after carbon coating. The morphology and microstructures of the samples were observed using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7401F) and a high-resolution transmission electron microscope (HR-TEM, JEOL ARM-200F) with a probe Cs aberration corrector (CEOS GmbH) and an energy dispersive spectroscope (EDS). Raman spectroscopy (Raman Microscope, Renishaw) was employed to characterize the phase transition of Nb2O5 and the carbon shell layer. Thermogravimetric analysis (TGA, TA Instruments Q600 V20.9 Build 20) was performed to determine the carbon content of the carbon-coated Nb2O5 nanoparticles. Electrochemical characterization: For the half-cell test, carbon-coated Nb2O5 material (80 wt%), a conducting agent (Super P, 10 wt%), and polyvinylidene fluoride binder (PVDF, 10
Fig. 1. XRD patterns of Nb2O5 samples annealed in Ar atmosphere for 3 h at various temperatures: (a) without and (b) with carbon precursor.
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wt%) were dissolved in an N-methyl-2-pyrrolidone solution, and the slurry was coated onto Cu foil substrates. After coating, the electrode was pressed and dried under vacuum at 120 C for 12 h and then cut into 12 mm-diameter discs, each containing approximately 2 mg cm 2 of active material. Using the abovedescribed working electrode, CR2032 coin-type half-cells were fabricated using a porous polyethylene separator (Asahi Kasei Chemicals) and a Li metal as both counter and reference electrodes. The electrolyte was 1 M LiClO4 in propylene carbonate solution (Panax Etec). Cyclic voltammetry (CV) measurements were carried out by a potentiostat (BioLogic VSP) within a voltage window of 1.0–3.0 V vs. Li+/Li. Galvanostatic discharge-charge (i.e., Li+ insertion-extraction) tests were performed at several constant currents. For cycle performance comparison of the differently treated samples, the electrodes were pre-cycled 3 times at 0.1C (1C = 200 mA g 1) and then cycled 500 times at 1C. For long-term cycling of the sample prepared at 850 C, the electrodes were also pre-cycled 3 times at 0.1C and then cycled 4000 times at 5C. For rate performance tests, the electrodes were pre-cycled 3 times at 0.05C and 30 times at 0.1C to precisely measure the capacity at each rate after full activation of the materials for Li+ storage. 3. Results and discussion Fig. 1 shows the XRD patterns of the annealed Nb2O5 powders and the carbon-coated Nb2O5 powders annealed with the carbon precursor, both in Ar atmosphere. In both cases, various crystalline phases can be observed after annealing at temperatures higher than 800 C, which were transformed from the less-crystalline asprepared powders (Fig. S1) after hydrothermal synthesis. When the as-prepared sample was annealed without the carbon precursor (Fig. 1a), the phases were converted from less-crystalline to orthorhombic (T-Nb2O5, ICDD-JCPDS No. 30-0873) at 800 and 850 C. As the temperature increased to 900 C, the phase transformed into a mix of orthorhombic and tetragonal (MNb2O5, ICDD-JCPDS No. 72-1484). At the annealing temperature of
1000 C, the tetragonal phase was dominant. This phase evolution of Nb2O5 with increasing annealing temperature is in good agreement with the results reported in the literature [9–12]. By contrast, when the as-prepared sample was annealed with the carbon precursor, a different result was observed (Fig. 1b). When the samples were treated at 800 and 850 C, pseudohexagonal (TT-Nb2O5, ICDD-JCPDS No. 28-0317) and orthorhombic phases were found, respectively. In the case of one-step carbon coating, the orthorhombic phase was found upon annealing at 850 C. Because the pyrolysis of naphthalene is an endothermic reaction, a higher temperature is required to obtain the same phase [37,38]. When the temperature was increased to 900 C, the orthorhombic Nb2O5 phase coexisted with a small amount of the tetragonal NbO2 phase (ICDD-JCPDS No. 43-1043). When the temperature was increased to 1000 C, the orthorhombic phase was completely converted to the tetragonal NbO2 phase. During the one-step carbon coating process with annealing, the naphthalene precursor is pyrolyzed, leaving behind residual carbon and a few vapors such as CO, CO2, and H2O [37]. Here, the carbon itself can act as a reducing agent and the vaporization may have an influence on the reduction of Nb2O5 (Nb5+) phase to NbO2 (Nb4+). Upon annealing without the carbon precursor, orthorhombic Nb2O5 changed to tetragonal Nb2O5, which is the thermodynamically favorable phase transformation at the annealing temperature of 1000 C. In the case of the one-step annealing and carbon coating, the pyrolysis reaction inducing carbon coating can lead to different structural evolution of the mother phase to a reduced state at the same time. The reduction phenomenon is possibly induced from surface to bulk and thus, the smaller particle size may be further easily reduced. This is the first report of the above finding and it is expected to be useful in various applications. To further investigate the chemical states of the Nb2O5 materials, XPS was used. Fig. 2a shows the C 1s core-level spectra of the carbon-coated Nb2O5 samples prepared at different temperatures. The profiles are similar, and their shapes agree with the results of amorphous or pyrolyzed carbon in the literature
Fig. 2. XPS spectra of carbon-coated Nb2O5 powders prepared at different temperatures: (a) C 1s and (b) Nb 3d.
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[39–41]. The fitted peaks can be attributed to C–C bonds and oxygen-containing functional groups [40,41]. The Nb 3d core-level XPS spectra of the samples are shown in Fig. 2b (See Fig. S2 for direct comparison). In the profile of the 800 C sample, the peaks at 206.7 and 209.5 eV can be assigned to pseudohexagonal Nb2O5. As the temperature increased to 850 C, the Nb 3d peaks shifted to higher binding energies because the crystal structure changed to orthorhombic [33]. For the 900 C sample, the Nb 3d peaks of Nb2O5 (Nb5+) still remained at the same position, while a slight shoulder with the Nb 3d5/2 peak was observed at a lower binding energy of 205.8 eV. The sample prepared at the pyrolysis and annealing temperature of 1000 C displays a prominent shoulder at the same binding energy. The two peaks at 205.8 and 209.0 eV can be attributed to NbO2 (Nb4+) [42–45]. It was confirmed that annealing at 1000 C could lead to simultaneous reduction of Nb2O5 and pyrolysis of the carbon precursor. From the results, a disagreement is found. In XRD, the Nb2O5 phase was completely converted to NbO2 at 1000 C, but in XPS, the Nb2O5 peaks remained dominant. This is probably because the surface of NbO2 is easily oxidized to Nb2O5 in ambient atmosphere, as well demonstrated in the literature [43–45]. Raman spectroscopy was performed as well to further examine the structural evolution of the carbon-coated Nb2O5 materials and the results are shown in Fig. 3. The spectra of the 800 and 850 C samples are very similar because the crystal structure of TT-Nb2O5 is analogous to that of T-Nb2O5 and TT-Nb2O5 is only a less crystalline form of T-Nb2O5, stabilized by impurities [10,12]. As the annealing temperature further increased to 900 and 1000 C, significant changes were observed. The Raman bands in the lowwavenumber region of 100–300 cm 1 are characteristic of the bending modes of the Nb–O–Nb linkages [46]. When the crystal structure of Nb2O5 is transformed into tetragonal NbO2, the Nb–O– Nb bending becomes stronger. As a result, the peaks in the region turn to sharper [47]. The signal in the region of 550–750 cm 1 can be attributed to the symmetric stretching mode of Nb–O bond [46]. For the 900 and 1000 C sample, new sharp or broad peaks appeared at 460, 840, 900, and 990 cm 1, which can be assigned to the bands in tetragonal NbO2 structure [46–48] and the Raman bands in the high-wavenumber region of 900–1200 cm 1 are attributed to the terminal Nb=O symmetric stretching mode on the surface [46]. These all changes are related to the structural change from T-Nb2O5 (lattice parameter: a = 6.175 Å, b = 29.175 Å, and
Fig. 3. Raman spectra of carbon-coated Nb2O5 powders prepared at different temperatures.
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c = 3.93 from ICDD-JCPDS No. 30-0873) to tetragonal NbO2 (lattice parameter: a = b = 9.693 Å and c = 5.985 Å). The bonding structure such as distance and type of bonds between Nb and O atoms was changed with increasing annealing temperature. These Raman results are consistent with the structural changes confirmed in the XRD analysis results. For further material and electrochemical evaluations, we selected the carbon-coated Nb2O5 material prepared at 850 C because T-Nb2O5 is known to exhibit the best electrochemical performance, as confirmed in our pre-experimental stage of the present study. Fig. 4a and b show the FE-SEM and the lowmagnification TEM images of the sample, respectively, from which it can be observed that micron-sized particles consist of fine primary particles measuring a few tens of nanometers. Fig. 4c displays the high-magnification TEM image, in which the overall uniform coating layer and Nb2O5 nanoparticle–thin amorphous carbon core–shell structure can be seen clearly. Additional TEM bright- and dark-field images are available in the Supporting Information (Fig. S3). The HR-TEM images with the fast Fourier transform (FFT) pattern (Fig. 4d and e) confirm that the core Nb2O5 particles were well crystallized, and the measured d-spacing of 3.9 Å corresponds to the (001) reflection of the orthorhombic structure, as verified by the FFT pattern (Fig. 4d inset). An amorphous carbon layer was found on the surface of the nanoparticles, and the thickness of this layer was measured to be approximately 5 nm. The amount of coated carbon layer was evaluated to be about 5.3 wt% by TGA analysis (See Fig. S4). As shown in Fig. 4f, the elemental mapping results obtained using EDS indicate that a uniform, well-distributed, thin carbon coating was formed on the Nb2O5 nanoparticles by using the CVD process along with the naphthalene precursor. The D and G bands related to the carbon shell layer, too, were observed in the Raman results (Fig. 3). Fig. 5a presents the CVs of the carbon-coated Nb2O5 electrode (850 C) at scan rates of 0.1–10 mV s 1. The CV curves corresponding to 0.1 mV s 1 exhibit symmetric cathodic and anodic broad peaks at about 1.7 and 1.8 V, respectively, which can be ascribed to pseudocapacitive reactions by Li+ insertion and extraction into/ from T-Nb2O5. As the scan rate was increased, the similar broad peak shapes and peak shifts were observed. The peak current, used for analyzing the charge storage mechanism and rate performance, is displayed in Fig. 5b. Voltammetric current obeys the power law (I = avb, a: prefactor). From the b values, surface charge transfercontrolled (b = 1) and cation diffusion-controlled (b = 0.5) can be distinguished [49–53]. For the carbon-coated T-Nb2O5, b was 0.91, which indicates that charge storage is largely controlled by surface reactions [49–53]. We confirmed that fast Li+ transport in T-Nb2O5 can be achieved by uniform carbon coating, which facilitates fast electron conduction. Fig. 5c shows that the voltage profiles obtained at various rates and sloping curves agree well with the CVs, which is typical for TNb2O5. The profiles of the other samples are compared in Fig. S5. The reversible capacity of carbon-coated T-Nb2O5 at 0.1C (1C = 200 mA g 1) is approximately 190 mAh g 1. The cycle performance of the carbon-coated samples prepared at different temperatures is compared in Fig. 5d (for as-annealed samples without carbon-coating, see Fig. S6a). For all four samples, capacity retention is very stable up to 500 cycles. For long-term cycling (4000 cycles), the performance of the 850 C sample at 5C is shown in Fig. 5e. When compared to that of the as-annealed samples, the cycling stability is enhanced greatly. The excellent cycling stability can be directly attributed to the vital role of carbon coating in suppressing the morphological changes in particles. To highlight the role of carbon coating, rate capability of the electrodes was measured under galvanostatic conditions, and the results are shown in Fig. 5f (for as-annealed Nb2O5, see Fig. S6b). For carbon-coated niobium oxides, a relatively good rate
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Fig. 4. Electron microscopy analyses of carbon-coated Nb2O5 particles prepared at 850 C: (a) FE-SEM image, (b) low-magnification TEM image, (c) high-magnification TEM image (d) HR-TEM image with FFT pattern (inset), (e) HR-TEM image, and (f) EDS elemental mapping results.
Fig. 5. Electrochemical properties of carbon-coated Nb2O5 electrodes: (a) cyclic voltammograms, (b) normalized peak current as a function of scan rate, (c) voltage profiles for galvanostatic cycling, (d) cycle performance at 1C, (e) long-term cycle performance at 5C, and (f) rate performance.
performance was observed. As the pyrolysis temperature increases, the electrical conductivity of the obtained carbon is known to increase [54,55]. This would enhance the rate capability of the carbon-coated materials studied herein. However, the crystal structure also varies with temperature, and thus, the optimum point between crystal structure and carbon pyrolysis temperature should be addressed. Among the carbon-coated
niobium oxide materials employed herein, the sample prepared at 850 C shows balanced electrochemical properties in terms of capacity and rate capability. The electrode does not show the best capacity retention ratio with increasing constant current (rate), but it does retain a capacity of approximately 130 mAh g 1 at a high rate of 20C (4 A g 1). The temperature of the one-step annealing and carbon coating process can affect both the properties of the
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carbon coat and the crystal structure of the mother phase. Moreover, the pyrolysis reaction can significantly influence the mother phase. To achieve optimum electrochemical performance of carbon-coated materials for lithium storage, these facts should be considered carefully. 4. Conclusions Nb2O5 nanoparticle core-amorphous carbon shell materials were prepared by a hydrothermal method and a one-step carbon coating process with annealing. XRD analysis demonstrated that the amorphous carbon formation via pyrolysis of a carbon precursor can significantly affect structural changes in the mother phase. During the process, the phase of Nb2O5 was changed with increasing temperature and finally reduced to NbO2 in Ar atmosphere. TEM results showed that Nb2O5 primary particles measuring a few tens of nanometers were uniformly coated with thin amorphous carbon layers. The carbon-coated Nb2O5 exhibited significantly enhanced rate and cycle performance compared to the non-coated Nb2O5 nanoparticles, which can be attributed to the carbon layer. The carbon layer had vital roles such as providing electronic network and suppressing the morphological changes in particles. In addition, for achieving balanced electrochemical performance, the pyrolysis and annealing temperatures should be selected carefully by considering both the mother phase and the carbon shell layer. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2009-0093814) and the Ministry of Science, ICT & Future Planning (MSIP, 2015R1A5A7037615 and 2016M3C1B5906958). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2017. 04.051. References [1] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (2004) 4245–4270. [2] R.R. Salunkhe, Y.V. Kaneti, J. Kim, J.H. Kim, Y. Yamauchi, Nanoarchitectures for metal–organic framework-derived nanoporous carbons toward supercapacitor applications, Acc. Chem. Res. 49 (2016) 2796–2806. [3] M.-S. Park, J. Kim, K.J. Kim, J.-W. Lee, J.H. Kim, Y. Yamauchi, Porous nanoarchitectures of spinel-type transition metal oxides for electrochemical energy storage systems, Phys. Chem. Chem. Phys. 17 (2015) 30963–30977. [4] G.G. Amatucci, F. Badway, A. Du Pasquier, T. Zheng, An asymmetric hybrid nonaqueous energy storage cell, J. Electrochem. Soc. 148 (2001) A930–A939. [5] L. Cheng, H.-J. Liu, J.-J. Zhang, H.-M. Xiong, Y.-Y. Xia, Nanosized Li4Ti5O12 prepared by molten salt method as an electrode material for hybrid electrochemical supercapacitors, J. Electrochem. Soc. 153 (2006) A1472– A1477. [6] K. Naoi, W. Naoi, S. Aoyagi, J.-i. Miyamoto, T. Kamino, New generation “nanohybrid supercapacitor”, Acc. Chem. Res. 46 (2013) 1075–1083. [7] J.-H. Kim, J.-S. Kim, Y.-G. Lim, J.-G. Lee, Y.-J. Kim, Effect of carbon types on the electrochemical properties of negative electrodes for Li-ion capacitors, J. Power Sources 196 (2011) 10490–10495. [8] X. Wang, G. Shen, Intercalation pseudo-capacitive TiNb2O7@carbon electrode for high-performance lithium ion hybrid electrochemical supercapacitors with ultrahigh energy density, Nano Energy 15 (2015) 104–115. [9] H. Schäfer, R. Gruehn, F. Schulte, The modifications of niobium pentoxide, Angew. Chem. Int. Ed. 5 (1966) 40–52. [10] I. Nowak, M. Ziolek, Niobium compounds: Preparation, characterization, and application in heterogeneous catalysis, Chem. Rev. 99 (1999) 3603–3624. [11] A.V. Rosario, E.C. Pereira, Influence of the crystallinity on the Li+ intercalation process in Nb2O5 films, J. Solid State Electrochem. 9 (2005) 665–673.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Niobium oxide nanoparticle core–amorphous carbon shell structure for fast reversible lithium storage Kyungbae Kima , Sang-Gil Woob , Yong Nam Job , Jaegab Leea , Jae-Hun Kima,* a b
School of Advanced Materials Engineering, Kookmin University, Seoul 02707, Republic of Korea Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi 13509, Republic of Korea
A R T I C L E I N F O
Article history: Received 12 December 2016 Received in revised form 23 February 2017 Accepted 10 April 2017 Available online 18 April 2017 Keywords: niobium oxide nanoparticle core-shell structural evolution lithium storage hybrid supercapacitor
A B S T R A C T
The hybrid supercapacitor concept involving a battery electrode and a supercapacitor electrode was recently introduced to meet the demand for high energy and power electrochemical energy storage devices. To successfully apply this device, high-capacity and rate electrode materials for Li storage should be developed. Niobium pentoxide (Nb2O5) has recently attracted considerable attention owing to its reasonable capacity, excellent rate capability, and cycling stability. However, the low electronic conductivity of the material is a major limitation. To address this issue, carbon incorporation is usually performed. Herein, we report Nb2O5 nanoparticle core-amorphous carbon shell materials prepared by hydrothermal reaction and one-step carbon formation with annealing. During the one-step process, it was found that pyrolysis of a carbon precursor could significantly influence the structural evolution of Nb2O5 with increasing temperature. In addition, Nb2O5 is reduced to NbO2 in Ar atmosphere with further increase in temperature. The niobium oxide-carbon core-shell structure was thoroughly examined by using transmission electron microscopy. It was demonstrated that the proposed carbon-coated materials exhibited excellent electrochemical properties in terms of rate and cycle performances. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the times of ever-growing energy demands, electrochemical energy storage has become a core technology in various applications [1–3]. Among energy storage devices, rechargeable batteries and electrochemical capacitors (supercapacitors) are the major ones that store electrical energy via electrochemical reactions at the electrodes. The two systems are contrary to each other in terms of their energy storage mechanisms and electrochemical performance. Batteries have the advantage of relatively high energy density because ions can be stored in the bulk of electrode materials by diffusion-limited insertion. Conversely, supercapacitors exhibit the superior characteristics of high power (rate) and cycle life, resulting from the surface charge storage mechanisms involving Faradaic electrochemical reactions or formation of electrical double layers. Owing to the difference in the charge storage methods of two devices, better energy density and power/cycling stability characteristics are difficult to achieve simultaneously.
* Corresponding author. E-mail address: [email protected] (J.-H. Kim). http://dx.doi.org/10.1016/j.electacta.2017.04.051 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
To overcome the gap between batteries and supercapacitors, the hybrid supercapacitor (HSC) concept has been introduced by adopting a battery electrode and a supercapacitor electrode [4–8]. In this device, the battery-type electrode materials such as Li4Ti5O12 and TiO2 are used for anodes by using Li insertion chemistry and the capacitor-type electrode materials such as activated carbon are employed as cathodes by adopting surface charge storage mechanism. To narrow the kinetic gap between two different type electrodes, the anode should have high-rate capability with moderate capacities. As a result, hybrid supercapacitors can exhibit high power with considerable energy densities by combining the advantages of Li-ion batteries and nonaqueous supercapacitors. In this context, niobium pentoxide (Nb2O5) materials have recently attracted considerable interest as anode materials for HSCs because they can provide (1) relatively high theoretical capacity (200 mAh g 1 = 720 C g 1) compared to conventional pseudocapacitor materials such as RuO2 and MnO2, (2) excellent rate capability induced by pseudocapacitive intercalation reactions, and (3) relative abundance in nature. Actually, Nb2O5 has great potential for use in various applications because of its unique performance characteristics resulting from its energy band structure and the existence of several crystal structures and
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morphologies [9,10]. This material has mainly been used in specific applications, such as gas sensors, electrochromics, and catalysts [11,12]. In recent years, however, its applications have expanded to rechargeable batteries and supercapacitors based on its ability to store alkali ions [12–17]. Recently, Nb2O5 materials with a specific crystal structure were reported to have the advantage of fast Li intercalation reactions [18–30]. However, to use this material in HSCs, two prerequisites must be satisfied. One is crystal structure and morphology, as mentioned above. Dunn et al. reported interesting results on this issue [20,21]. According to the papers, nanostructured orthorhombic Nb2O5 is beneficial for relatively high-capacity and high-rate lithium storage owing to its ability to rapidly transport Li+ ions through its open and layered crystallographic structure [31]. The other issue is the low electronic conductivity of the materials [32,33]. Stoichiometric Nb2O5 is known to be an insulator with an electrical conductivity (s) of 3 10 6 S cm 1 [34]. To address this issue, carbon and conducting polymer incorporation have been performed [22–27,35]. In this study, we introduce a one-step annealing and carbon coating process after simple hydrothermal synthesis of Nb2O5 nanoparticles. The as-synthesized niobium oxide particles showed a less-crystalline phase, and the particles were annealed to obtain an orthorhombic structure, which is favorable for fast Li+ storage. During this annealing process, carbon coating by chemical vapor deposition (CVD) through sublimation of a solid precursor can be performed simultaneously. Here, we report the effect of the carbon coating process on the crystal structure transition of niobium oxide for the first time. A simple annealing process without carbon sources only leads to structural evolution from less-crystalline to orthorhombic and tetragonal Nb2O5 sequentially with increasing temperature, as reported in the literature [9–12]. By contrast, onestep annealing and carbon coating with a carbon precursor causes the crystal structure change of the as-prepared niobium oxide into pseudohexagonal and orthorhombic Nb2O5, and tetragonal NbO2 at different temperatures. The material characterization of the prepared carbon-coated Nb2O5 was carried out by several analytical tools. The electrochemical characterization demonstrated that the Nb2O5 nanoparticles with amorphous carbon shell layer had excellent rate and cycle performances.
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2. Experimental Materials synthesis: First, niobium oxide nanoparticles were synthesized according to the following hydrothermal synthesis procedure. 4.56 g of ammonium niobate oxalate hydrate (C4H4NNbO9xH2O, Sigma Aldrich) and 0.8 g of Pluronic F-127 (Sigma Aldrich) were dissolved in individual de-ionized (DI) water solutions (80 ml) under stirring for 20 min. Then, the ammonium niobate solutions and HCl (2 M, 1 ml) were added to the F-127 solution under stirring. The solutions were transferred into a Teflon-lined steel autoclave. The sealed autoclave was heated to 200 C and maintained at the temperature for 24 h to facilitate hydrothermal reaction. The resulting suspension was centrifuged to separate the white precipitates. After thorough washing with DI water and ethyl alcohol several times, the precipitates were dried at 80 C. For annealing and carbon-coating, the precipitates were placed in a quartz tube within a vertical furnace, as reported elsewhere [36]. The furnace was heated to each temperature (800, 850, 900, and 1000 C) with/without the carbon precursor of naphthalene in Ar atmosphere. After maintaining at each target temperature for 3 h, the furnace was cooled to room temperature. Materials characterization: The crystal structures of the synthesized materials were identified by X-ray diffraction (XRD, Rigaku D/MAX-2500 V with Cu Ka radiation). X-ray photoelectron spectroscopy (XPS, Thermo Scientific Ka) was used to analyze the chemical state of the materials after carbon coating. The morphology and microstructures of the samples were observed using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7401F) and a high-resolution transmission electron microscope (HR-TEM, JEOL ARM-200F) with a probe Cs aberration corrector (CEOS GmbH) and an energy dispersive spectroscope (EDS). Raman spectroscopy (Raman Microscope, Renishaw) was employed to characterize the phase transition of Nb2O5 and the carbon shell layer. Thermogravimetric analysis (TGA, TA Instruments Q600 V20.9 Build 20) was performed to determine the carbon content of the carbon-coated Nb2O5 nanoparticles. Electrochemical characterization: For the half-cell test, carbon-coated Nb2O5 material (80 wt%), a conducting agent (Super P, 10 wt%), and polyvinylidene fluoride binder (PVDF, 10
Fig. 1. XRD patterns of Nb2O5 samples annealed in Ar atmosphere for 3 h at various temperatures: (a) without and (b) with carbon precursor.
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wt%) were dissolved in an N-methyl-2-pyrrolidone solution, and the slurry was coated onto Cu foil substrates. After coating, the electrode was pressed and dried under vacuum at 120 C for 12 h and then cut into 12 mm-diameter discs, each containing approximately 2 mg cm 2 of active material. Using the abovedescribed working electrode, CR2032 coin-type half-cells were fabricated using a porous polyethylene separator (Asahi Kasei Chemicals) and a Li metal as both counter and reference electrodes. The electrolyte was 1 M LiClO4 in propylene carbonate solution (Panax Etec). Cyclic voltammetry (CV) measurements were carried out by a potentiostat (BioLogic VSP) within a voltage window of 1.0–3.0 V vs. Li+/Li. Galvanostatic discharge-charge (i.e., Li+ insertion-extraction) tests were performed at several constant currents. For cycle performance comparison of the differently treated samples, the electrodes were pre-cycled 3 times at 0.1C (1C = 200 mA g 1) and then cycled 500 times at 1C. For long-term cycling of the sample prepared at 850 C, the electrodes were also pre-cycled 3 times at 0.1C and then cycled 4000 times at 5C. For rate performance tests, the electrodes were pre-cycled 3 times at 0.05C and 30 times at 0.1C to precisely measure the capacity at each rate after full activation of the materials for Li+ storage. 3. Results and discussion Fig. 1 shows the XRD patterns of the annealed Nb2O5 powders and the carbon-coated Nb2O5 powders annealed with the carbon precursor, both in Ar atmosphere. In both cases, various crystalline phases can be observed after annealing at temperatures higher than 800 C, which were transformed from the less-crystalline asprepared powders (Fig. S1) after hydrothermal synthesis. When the as-prepared sample was annealed without the carbon precursor (Fig. 1a), the phases were converted from less-crystalline to orthorhombic (T-Nb2O5, ICDD-JCPDS No. 30-0873) at 800 and 850 C. As the temperature increased to 900 C, the phase transformed into a mix of orthorhombic and tetragonal (MNb2O5, ICDD-JCPDS No. 72-1484). At the annealing temperature of
1000 C, the tetragonal phase was dominant. This phase evolution of Nb2O5 with increasing annealing temperature is in good agreement with the results reported in the literature [9–12]. By contrast, when the as-prepared sample was annealed with the carbon precursor, a different result was observed (Fig. 1b). When the samples were treated at 800 and 850 C, pseudohexagonal (TT-Nb2O5, ICDD-JCPDS No. 28-0317) and orthorhombic phases were found, respectively. In the case of one-step carbon coating, the orthorhombic phase was found upon annealing at 850 C. Because the pyrolysis of naphthalene is an endothermic reaction, a higher temperature is required to obtain the same phase [37,38]. When the temperature was increased to 900 C, the orthorhombic Nb2O5 phase coexisted with a small amount of the tetragonal NbO2 phase (ICDD-JCPDS No. 43-1043). When the temperature was increased to 1000 C, the orthorhombic phase was completely converted to the tetragonal NbO2 phase. During the one-step carbon coating process with annealing, the naphthalene precursor is pyrolyzed, leaving behind residual carbon and a few vapors such as CO, CO2, and H2O [37]. Here, the carbon itself can act as a reducing agent and the vaporization may have an influence on the reduction of Nb2O5 (Nb5+) phase to NbO2 (Nb4+). Upon annealing without the carbon precursor, orthorhombic Nb2O5 changed to tetragonal Nb2O5, which is the thermodynamically favorable phase transformation at the annealing temperature of 1000 C. In the case of the one-step annealing and carbon coating, the pyrolysis reaction inducing carbon coating can lead to different structural evolution of the mother phase to a reduced state at the same time. The reduction phenomenon is possibly induced from surface to bulk and thus, the smaller particle size may be further easily reduced. This is the first report of the above finding and it is expected to be useful in various applications. To further investigate the chemical states of the Nb2O5 materials, XPS was used. Fig. 2a shows the C 1s core-level spectra of the carbon-coated Nb2O5 samples prepared at different temperatures. The profiles are similar, and their shapes agree with the results of amorphous or pyrolyzed carbon in the literature
Fig. 2. XPS spectra of carbon-coated Nb2O5 powders prepared at different temperatures: (a) C 1s and (b) Nb 3d.
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[39–41]. The fitted peaks can be attributed to C–C bonds and oxygen-containing functional groups [40,41]. The Nb 3d core-level XPS spectra of the samples are shown in Fig. 2b (See Fig. S2 for direct comparison). In the profile of the 800 C sample, the peaks at 206.7 and 209.5 eV can be assigned to pseudohexagonal Nb2O5. As the temperature increased to 850 C, the Nb 3d peaks shifted to higher binding energies because the crystal structure changed to orthorhombic [33]. For the 900 C sample, the Nb 3d peaks of Nb2O5 (Nb5+) still remained at the same position, while a slight shoulder with the Nb 3d5/2 peak was observed at a lower binding energy of 205.8 eV. The sample prepared at the pyrolysis and annealing temperature of 1000 C displays a prominent shoulder at the same binding energy. The two peaks at 205.8 and 209.0 eV can be attributed to NbO2 (Nb4+) [42–45]. It was confirmed that annealing at 1000 C could lead to simultaneous reduction of Nb2O5 and pyrolysis of the carbon precursor. From the results, a disagreement is found. In XRD, the Nb2O5 phase was completely converted to NbO2 at 1000 C, but in XPS, the Nb2O5 peaks remained dominant. This is probably because the surface of NbO2 is easily oxidized to Nb2O5 in ambient atmosphere, as well demonstrated in the literature [43–45]. Raman spectroscopy was performed as well to further examine the structural evolution of the carbon-coated Nb2O5 materials and the results are shown in Fig. 3. The spectra of the 800 and 850 C samples are very similar because the crystal structure of TT-Nb2O5 is analogous to that of T-Nb2O5 and TT-Nb2O5 is only a less crystalline form of T-Nb2O5, stabilized by impurities [10,12]. As the annealing temperature further increased to 900 and 1000 C, significant changes were observed. The Raman bands in the lowwavenumber region of 100–300 cm 1 are characteristic of the bending modes of the Nb–O–Nb linkages [46]. When the crystal structure of Nb2O5 is transformed into tetragonal NbO2, the Nb–O– Nb bending becomes stronger. As a result, the peaks in the region turn to sharper [47]. The signal in the region of 550–750 cm 1 can be attributed to the symmetric stretching mode of Nb–O bond [46]. For the 900 and 1000 C sample, new sharp or broad peaks appeared at 460, 840, 900, and 990 cm 1, which can be assigned to the bands in tetragonal NbO2 structure [46–48] and the Raman bands in the high-wavenumber region of 900–1200 cm 1 are attributed to the terminal Nb=O symmetric stretching mode on the surface [46]. These all changes are related to the structural change from T-Nb2O5 (lattice parameter: a = 6.175 Å, b = 29.175 Å, and
Fig. 3. Raman spectra of carbon-coated Nb2O5 powders prepared at different temperatures.
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c = 3.93 from ICDD-JCPDS No. 30-0873) to tetragonal NbO2 (lattice parameter: a = b = 9.693 Å and c = 5.985 Å). The bonding structure such as distance and type of bonds between Nb and O atoms was changed with increasing annealing temperature. These Raman results are consistent with the structural changes confirmed in the XRD analysis results. For further material and electrochemical evaluations, we selected the carbon-coated Nb2O5 material prepared at 850 C because T-Nb2O5 is known to exhibit the best electrochemical performance, as confirmed in our pre-experimental stage of the present study. Fig. 4a and b show the FE-SEM and the lowmagnification TEM images of the sample, respectively, from which it can be observed that micron-sized particles consist of fine primary particles measuring a few tens of nanometers. Fig. 4c displays the high-magnification TEM image, in which the overall uniform coating layer and Nb2O5 nanoparticle–thin amorphous carbon core–shell structure can be seen clearly. Additional TEM bright- and dark-field images are available in the Supporting Information (Fig. S3). The HR-TEM images with the fast Fourier transform (FFT) pattern (Fig. 4d and e) confirm that the core Nb2O5 particles were well crystallized, and the measured d-spacing of 3.9 Å corresponds to the (001) reflection of the orthorhombic structure, as verified by the FFT pattern (Fig. 4d inset). An amorphous carbon layer was found on the surface of the nanoparticles, and the thickness of this layer was measured to be approximately 5 nm. The amount of coated carbon layer was evaluated to be about 5.3 wt% by TGA analysis (See Fig. S4). As shown in Fig. 4f, the elemental mapping results obtained using EDS indicate that a uniform, well-distributed, thin carbon coating was formed on the Nb2O5 nanoparticles by using the CVD process along with the naphthalene precursor. The D and G bands related to the carbon shell layer, too, were observed in the Raman results (Fig. 3). Fig. 5a presents the CVs of the carbon-coated Nb2O5 electrode (850 C) at scan rates of 0.1–10 mV s 1. The CV curves corresponding to 0.1 mV s 1 exhibit symmetric cathodic and anodic broad peaks at about 1.7 and 1.8 V, respectively, which can be ascribed to pseudocapacitive reactions by Li+ insertion and extraction into/ from T-Nb2O5. As the scan rate was increased, the similar broad peak shapes and peak shifts were observed. The peak current, used for analyzing the charge storage mechanism and rate performance, is displayed in Fig. 5b. Voltammetric current obeys the power law (I = avb, a: prefactor). From the b values, surface charge transfercontrolled (b = 1) and cation diffusion-controlled (b = 0.5) can be distinguished [49–53]. For the carbon-coated T-Nb2O5, b was 0.91, which indicates that charge storage is largely controlled by surface reactions [49–53]. We confirmed that fast Li+ transport in T-Nb2O5 can be achieved by uniform carbon coating, which facilitates fast electron conduction. Fig. 5c shows that the voltage profiles obtained at various rates and sloping curves agree well with the CVs, which is typical for TNb2O5. The profiles of the other samples are compared in Fig. S5. The reversible capacity of carbon-coated T-Nb2O5 at 0.1C (1C = 200 mA g 1) is approximately 190 mAh g 1. The cycle performance of the carbon-coated samples prepared at different temperatures is compared in Fig. 5d (for as-annealed samples without carbon-coating, see Fig. S6a). For all four samples, capacity retention is very stable up to 500 cycles. For long-term cycling (4000 cycles), the performance of the 850 C sample at 5C is shown in Fig. 5e. When compared to that of the as-annealed samples, the cycling stability is enhanced greatly. The excellent cycling stability can be directly attributed to the vital role of carbon coating in suppressing the morphological changes in particles. To highlight the role of carbon coating, rate capability of the electrodes was measured under galvanostatic conditions, and the results are shown in Fig. 5f (for as-annealed Nb2O5, see Fig. S6b). For carbon-coated niobium oxides, a relatively good rate
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Fig. 4. Electron microscopy analyses of carbon-coated Nb2O5 particles prepared at 850 C: (a) FE-SEM image, (b) low-magnification TEM image, (c) high-magnification TEM image (d) HR-TEM image with FFT pattern (inset), (e) HR-TEM image, and (f) EDS elemental mapping results.
Fig. 5. Electrochemical properties of carbon-coated Nb2O5 electrodes: (a) cyclic voltammograms, (b) normalized peak current as a function of scan rate, (c) voltage profiles for galvanostatic cycling, (d) cycle performance at 1C, (e) long-term cycle performance at 5C, and (f) rate performance.
performance was observed. As the pyrolysis temperature increases, the electrical conductivity of the obtained carbon is known to increase [54,55]. This would enhance the rate capability of the carbon-coated materials studied herein. However, the crystal structure also varies with temperature, and thus, the optimum point between crystal structure and carbon pyrolysis temperature should be addressed. Among the carbon-coated
niobium oxide materials employed herein, the sample prepared at 850 C shows balanced electrochemical properties in terms of capacity and rate capability. The electrode does not show the best capacity retention ratio with increasing constant current (rate), but it does retain a capacity of approximately 130 mAh g 1 at a high rate of 20C (4 A g 1). The temperature of the one-step annealing and carbon coating process can affect both the properties of the
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carbon coat and the crystal structure of the mother phase. Moreover, the pyrolysis reaction can significantly influence the mother phase. To achieve optimum electrochemical performance of carbon-coated materials for lithium storage, these facts should be considered carefully. 4. Conclusions Nb2O5 nanoparticle core-amorphous carbon shell materials were prepared by a hydrothermal method and a one-step carbon coating process with annealing. XRD analysis demonstrated that the amorphous carbon formation via pyrolysis of a carbon precursor can significantly affect structural changes in the mother phase. During the process, the phase of Nb2O5 was changed with increasing temperature and finally reduced to NbO2 in Ar atmosphere. TEM results showed that Nb2O5 primary particles measuring a few tens of nanometers were uniformly coated with thin amorphous carbon layers. The carbon-coated Nb2O5 exhibited significantly enhanced rate and cycle performance compared to the non-coated Nb2O5 nanoparticles, which can be attributed to the carbon layer. The carbon layer had vital roles such as providing electronic network and suppressing the morphological changes in particles. In addition, for achieving balanced electrochemical performance, the pyrolysis and annealing temperatures should be selected carefully by considering both the mother phase and the carbon shell layer. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2009-0093814) and the Ministry of Science, ICT & Future Planning (MSIP, 2015R1A5A7037615 and 2016M3C1B5906958). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2017. 04.051. References [1] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (2004) 4245–4270. [2] R.R. Salunkhe, Y.V. Kaneti, J. Kim, J.H. Kim, Y. Yamauchi, Nanoarchitectures for metal–organic framework-derived nanoporous carbons toward supercapacitor applications, Acc. Chem. Res. 49 (2016) 2796–2806. [3] M.-S. Park, J. Kim, K.J. Kim, J.-W. Lee, J.H. Kim, Y. Yamauchi, Porous nanoarchitectures of spinel-type transition metal oxides for electrochemical energy storage systems, Phys. Chem. Chem. Phys. 17 (2015) 30963–30977. [4] G.G. Amatucci, F. Badway, A. Du Pasquier, T. Zheng, An asymmetric hybrid nonaqueous energy storage cell, J. Electrochem. Soc. 148 (2001) A930–A939. [5] L. Cheng, H.-J. Liu, J.-J. Zhang, H.-M. Xiong, Y.-Y. Xia, Nanosized Li4Ti5O12 prepared by molten salt method as an electrode material for hybrid electrochemical supercapacitors, J. Electrochem. Soc. 153 (2006) A1472– A1477. [6] K. Naoi, W. Naoi, S. Aoyagi, J.-i. Miyamoto, T. Kamino, New generation “nanohybrid supercapacitor”, Acc. Chem. Res. 46 (2013) 1075–1083. [7] J.-H. Kim, J.-S. Kim, Y.-G. Lim, J.-G. Lee, Y.-J. Kim, Effect of carbon types on the electrochemical properties of negative electrodes for Li-ion capacitors, J. Power Sources 196 (2011) 10490–10495. [8] X. Wang, G. Shen, Intercalation pseudo-capacitive TiNb2O7@carbon electrode for high-performance lithium ion hybrid electrochemical supercapacitors with ultrahigh energy density, Nano Energy 15 (2015) 104–115. [9] H. Schäfer, R. Gruehn, F. Schulte, The modifications of niobium pentoxide, Angew. Chem. Int. Ed. 5 (1966) 40–52. [10] I. Nowak, M. Ziolek, Niobium compounds: Preparation, characterization, and application in heterogeneous catalysis, Chem. Rev. 99 (1999) 3603–3624. [11] A.V. Rosario, E.C. Pereira, Influence of the crystallinity on the Li+ intercalation process in Nb2O5 films, J. Solid State Electrochem. 9 (2005) 665–673.
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