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Reduced graphene oxide enwrapped vanadium pentoxide nanorods as cathode materials for lithium-ion batteries
Physica E 56 (2014) 231–237
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Reduced graphene oxide enwrapped vanadium pentoxide nanorods as cathode materials for lithium-ion batteries Dezhi Chen a,n, Hongying Quan b, Shenglian Luo a,n, Xubiao Luo a, Fang Deng a, Hualin Jiang a a Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, School of Environmental and Chemical Engineering, Nanchang Hangkong University, No. 696, Fenghe Nan Avenue, Nanchang 330063, China b School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China
H I G H L I G H T S
rGO enwrapped V2O5 nanorods composites were prepared for the first time. The electrochemical properties were studied as cathode materials. rGO/V2O5 composites show excellent electrochemical lithium storage activity. The structure-component-property relationships were discussed.
art ic l e i nf o
a b s t r a c t
Article history: Received 10 August 2013 Received in revised form 19 September 2013 Accepted 23 September 2013 Available online 1 October 2013
Novel reduced graphene oxide/vanadium pentoxide (rGO/V2O5) nanocomposites were fabricated by coassembly between negatively charged graphene oxide and positively charged oxide nanorods. A series of characterization including X-ray diffraction, Raman spectrum, scanning electron microscopy and transmission electron microscopy indicated that the V2O5 nanorods with the width of about 50 nm and the length from a few hundred nanometers to several micrometers were enwrapped by rGO layers to form core–shell nanostructures. Compared with the pristine V2O5 nanorods, the as-prepared rGO/V2O5 nanocomposites with 13 wt% rGO showed a significantly enhanced electrochemical performance with high reversible capacities, good cycling stabilities and excellent rate capabilities as a cathode material for lithium batteries. The rGO/V2O5 nanocomposites electrodes delivered a stable discharge capacity around 140 mA h g 1 at a current density of 150 mA g 1 for 100 cycles in the voltage range of 2.5–4.0 V. Furthermore, the nanocomposites electrodes delivered discharge capacities of 287 mA h g 1 and 207 mA h g 1 during the first and 50th cycles in the voltage range of 2.0–4.0 V at a current density of 100 mA g 1, respectively. The as-synthesized nanocomposites are promising candidates for electrical energy storage applications. & 2013 Elsevier B.V. All rights reserved.
Keywords: Reduced graphene oxide Vanadium pentoxide Cathode Lithium-ion battery Nanocomposite
1. Introduction Lithium-ion batteries (LIBs) have been considered as the most effective technologies for electrochemical energy storage over the last two decades [1]. Electrode materials play dominant roles in the performance of LIBs. To meet the ever-growing demand for portable power sources, especially for high-energy and highpower applications such as electrical vehicles (EVs) and hybrid electrical vehicles (HEVs), numerous efforts have been devoted to develop cost-effective and high-performance electrode materials [2–6]. As a kind of promising cathode material, vanadium pentoxide (V2O5) with a layered structure has attracted much
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Corresponding authors. Tel.: þ 86 791 83953373. E-mail addresses: [email protected] (D. Chen), [email protected] (S. Luo).
1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.09.009
attention due to its high capacity, high output voltage, and low cost [7]. However, the practical application of V2O5 is limited by its intrinsic low-diffusion coefficient of lithium ions ( 10 12 cm2 s 1) and moderate electronic conductivity (10 2–10 3 S cm 1) [8,9]. One generally accepted strategy to overcome these obstacles is to reduce the size of oxides to nanoscale and load onto conductive carbonaceous matrices [1,4,10]. For a given electrode material, the decrease in size can effectively shorten the diffusion time of lithium ions within the material. Meanwhile, the introduction of conductive carbonaceous materials can obviously improve the electronic conductivity and electrochemical stability of the whole electrode. Recently, a new carbon nanomaterial, graphene has received considerable attention for its unique properties such as superior electrical conductivity, excellent mechanical flexibility, large surface area and high thermal stability [11]. Furthermore, low cost
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graphene can be produced in bulk, efficiently through chemical oxidation of graphite to hydrophilic graphite oxide, which can then be exfoliated as individual graphene oxide (GO) sheets by ultrasonication [12]. The GO is then converted to conducting graphene by the chemical reduction process. Generally, these graphene sheets obtained by the chemical reduction method are also known as reduced graphene oxide (rGO). Consequently, rGO/V2O5 nanocomposites have been fabricated as cathode materials for LIBs by different synthesis routes [13–16]. Compared with pristine V2O5 nanostructures, these as-prepared nanocomposites exhibited significantly enhanced reversible capacities and cycling stabilities as well as excellent rate performance. However, the high quality percentage content (420 wt%) of rGO in these nanocomposites decreased the specific reversible capacity of the whole cathode. Although lower rGO (4.5 wt%) can also enhance the electrochemical performance of the as-prepared nanocomposites for lithium ion storage [17], these V2O5 nanomaterials are still prone to strong aggregation during the cycle processes because of nonintimate contact between rGO layers and V2O5. This leads to a decrease in capacity of nanocomposites by 20–50% of their first reversible capacity after 30 cycles. One of the most promising approaches to resolve these problems is to prepare monodisperse V2O5 nanocrystals wrapped by ultrathin and high electrical performance rGO layers. Recently, some groups have synthesized rGO-wrapped metal oxides nanocomposites through electrostatic interactions between negatively charged GO sheets and positively charged metal oxide nanocrystals modified by surface grafting [18–20]. The GO is then converted to conducting rGO by the chemical reduction process. The novel strategy is not only simple, but also able to pre-select nanostructures with desired functionalities and morphology. Herein, we prepared rGO-encapsulated V2O5 nanocomposites by coassembly between negatively charged GO and positively charged oxide nanorods. The process is driven by the mutual electrostatic interactions of the two species, and is followed by thermal reduction. The as-synthesized nanocomposites possess flexible and ultrathin rGO shells that effectively enwrap the oxide nanorods. As a cathode material for LIBs, the nanocomposites with 13 wt% rGO show high reversible capacities, good cycling stabilities and excellent rate capabilities.
2. Material and methods 2.1. Preparation of V2O5 nanorods In a typical synthesis, 0.3 g of ammonium metavanadate (Tianjing Jinghua) and 0.50 g of triblock copolymer P123 (EO20PO70EO20, EO ¼ethylene oxide, PO ¼propylene oxide; Sigma) were dissolved in 30 mL of DI water containing 1.5 mL of 2 M HCl. This mixture was stirred at room temperature for 7 h and then transferred to an autoclave and heated to 120 1C for 24 h. The resulting precipitate was vacuum-filtered and rinsed with DI water. This as-prepared precipitate was calcined at 500 1C for 2 h to obtain V2O5 nanorods (Fig. S1). 2.2. Preparation of rGO/V2O5 nanocomposites V2O5 nanorods (0.4 g) were first dispersed in 200 mL ethanol by sonication for 30 min. Then, aminopropyltrimethoxysilane (APTS, 1 mL) was added, and refluxed for 4 h to obtain APTS-modified V2O5 nanorods. A certain amount of negatively charged GO suspension (0.2 mg/mL, obtained by the modified Hummers method [21]), as shown in Fig. S2, was added into positively charged aminefunctionalized V2O5 nanorods dispersion under vigorous stirring at pH 6 to fabricate GO-enwrapped V2O5 nanorods (GO/V2O5). After
mixing for 1 h, the mixture was centrifuged and washed with deionized water. Finally, the as-prepared product was calcined at 300 1C for 30 min to obtain rGO-enwrapped V2O5 nanorods (rGO/ V2O5) nanocomposites. 2.3. Characterization X-ray diffraction (XRD) analyses were carried out on an X-ray diffractometer (XRD-6000, Shimadzu Scientific Instruments). The XRD patterns with Cu Kα radiation (λ ¼1.5406 Å) at 40 kV and 40 mA were recorded in the range of 2θ ¼ 10–601. The XRD specimens were prepared by means of flattening the powder on the small slides. Scanning electron microscope (SEM) images were achieved by an FEI Quanta 250 field-emission gun environmental scanning electron microscope at 10 kV with the samples obtained from the thick suspension dropping on the silicon slice. Transmission electron microscopy (TEM) images were obtained using a JEM-2100F transmission electron microscope (JEOL Ltd., Japan) operated at 200 kV. Energy dispersive spectrometry (EDS) was obtained from an Apollo300 field-emission gun scanning electron microscope. Raman spectra were recorded from 1100 to 1800 cm 1 on a LabRAM HR800 Laser Raman spectroscope (HORIBA Jobin Yvon Co. Ltd., France) using a 632.5 nm argon ion laser. All samples were deposited on silicon wafers in powder form without using any solvent. Thermogravimetric (TG) analysis was performed in air using a Pyris Diamond TG analyzer (PerkinElemer Inc., USA). The samples were heated from 50 1C to 550 1C at 5 1C/min. 2.4. Electrochemical measurements The electrochemical properties of the rGO/V2O5 nanocomposites and pristine V2O5 nanorods as cathode materials in lithium ion cells were evaluated by a galvanostatic charge/discharge technique. The test electrodes were prepared by mixing active materials (nanocomposites 80% or pristine V2O5 70%) with carbon black (nanocomposites 10% or pristine V2O5 20%) and 10% polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone to form a slurry; CR2032 type coin cells were finally assembled in a highlypure argon-filled glovebox using the test electrodes and the metallic lithium counter/reference electrode, a polypropylene separator (Celgard 2400) and an electrolyte of 1 mol/L LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DMC) (EC/DMC, 1/1 vol). Charge–discharge measurements were carried out galvanostatically using a battery test system (LAND CT2001A model, Wuhan Jinnuo Electronics. Ltd., China). The electrochemical impedance measurements were performed on a CHI660D electrochemical workstation (Shanghai Chenhua Co. Ltd., China) at an AC voltage of 5 mV amplitude in the range of 100 kHz–0.01 Hz.
3. Results and discussion The overall synthesis procedure for the rGO-enwrapped V2O5 nanorods composites (rGO/V2O5) is illustrated in Fig. 1. First, V2O5 nanorods were modified by surface grafting of APTS to render the V2O5 surface positively charged. The modified V2O5 nanorods were then assembled with negatively charged GO by electrostatic interactions. Finally, the aggregates were thermally reduced to prepare rGO/V2O5 nanocomposites. Fig. 2 shows digital photographs of pristine V2O5 nanorods, GO/V2O5 and rGO/V2O5. As presented in Fig. 2a, pure V2O5 is orange, and then turns to brown (Fig. 2b) for GO layers. Finally, after the thermal treatment, the product becomes black (Fig. 2c). Fig. 3a shows the XRD patterns of pristine V2O5 nanorods, GO/ V2O5 and rGO/V2O5. All diffraction peaks can be attributed to
D. Chen et al. / Physica E 56 (2014) 231–237
orthorhombic V2O5 (JCPDS No. 89-0612). It indicates that the process of wrapping GO and the thermal reduction does not affect the crystallization of V2O5. Additionally, there is no evidence in the XRD patterns for the presence of GO or rGO, suggesting that these nanosheets are homogenously dispersed in the hybrids [18]. Fig. 3b shows the Raman spectra of GO/V2O5 and rGO/V2O5 between 1100 cm 1 and 1800 cm 1. The characteristic peaks at 1348 and 1585 cm 1 in Raman spectra of the nanocomposites are attributed to the D band (breathing mode of k-point phonons of A1g symmetry) and the G band (the first-order scattering of the E2g phonons), respectively [22]. Thermal reduction treatment resulted in a clear increase of the D/G intensity ratio from 0.9 for GO in GO/V2O5 to 1.1 for rGO in rGO/V2O5 nanocomposites, which is
Fig. 1. Schematic illustration of the fabrication of rGO enwrapped V2O5 nanorods.
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attributed to the formation of new and smaller sp2 domains during the reduction [23]. From the typical SEM image of rGO/V2O5 nanocomposites shown in Fig. 4, we can observe that the width of these V2O5 nanorods is about 50 nm, and the length ranges from a few hundred nanometers to several micrometers. Furthermore, we can also see that the surface of rod-like V2O5 nanocrystals is covered with a wrinkled layer of rGO. The above results were further confirmed by TEM observation. Fig. 5 shows the typical TEM and HRTEM images of rGO/V2O5 nanocomposites. The TEM image in Fig. 5a shows that the rod-like V2O5 nanocrystals are embedded in rGO nanosheets. From the TEM image (Fig. 5b), we can clearly see that the V2O5 nanorod is completely enwrapped by rGO layers, which helps to prevent V2O5 from agglomeration and enables a good dispersion of these oxide nanorods in rGO shells. The thickness of rGO shells is about 5–10 nm. It means that the number of rGO nanosheets is about 5–10. From the HRTEM image in Fig. 5c, we can obviously observe the interfacial structure between the V2O5 nanorod and the rGO layer. Meanwhile, the corrugated parallel fringes with a basal spacing of about 0.37 nm can be assigned to the (002) plane of several-layer rGO. The HRTEM image in Fig. 5d shows a welldefined crystallinity of V2O5 with a lattice spacing of 0.41 nm, which is assigned to the (101) plane of orthorhombic V2O5 (JCPDS No. 89-0612). EDS mapping for the elements vanadium, oxygen and carbon was used to understand the distribution of V2O5 nanorods in the nanocomposites. From the elemental distribution of vanadium, oxygen, and carbon shown in Fig. 6, we can recognize the uniform dispersion of the V2O5 nanorods in the stacked structure consisting of rGO-enwrapped V2O5 nanorods. The TEM and EDS elemental mapping characterization confirms that thin rGO layer is uniformly coated on the surface of V2O5 nanorods.
Fig. 2. Digital photographs of (a) pristine V2O5 nanorods, (b) GO/V2O5 and (c) rGO/V2O5.
Fig. 3. (a) XRD patterns and (b) Raman spectra of V2O5 nanorods, GO/V2O5 and rGO/V2O5.
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For quantifying the amount of rGO in the nanocomposites, thermogravimetric analysis (TGA) was carried out in air. The samples were heated from 50 to 550 1C at a rate of 5 1C/min. Fig. 7 shows the TGA curve of rGO/V2O5 hybrids. It can be seen that the rGO/V2O5 nanocomposites are almost stable between 50 and 300 1C, which indicates that oxygen-containing functional groups of GO are almost removed by the previous thermal reduction. The rapid mass loss between 300 and 450 1C is attributed to the combustion of rGO. According to the TGA curve in Fig. 7, we can calculate that the content of rGO is 13 wt%.
Fig. 4. Typical SEM image of rGO/V2O5.
The electrochemical performance of rGO/V2O5 nanocomposites and pristine V2O5 nanorods as cathode materials was evaluated by galvanostatic discharge/charge testing. Fig. 8a shows the initial, second and 20th discharge/charge curves of these materials at a current density of 150 mA g 1 (rate of approximately 1C) in a voltage range of 2.5–4.0 V vs. Li/Li þ . During the initial discharge curves, the nanocomposites and pristine V2O5 electrodes exhibited the typical voltage profile of V2O5, which showed two flat plateaus at 3.4 and 3.2 V corresponding to two different two-phase regions of V2O5–Li0.5V2O5 coexistence and Li0.5V2O5–LiV2O5 coexistence, respectively. As shown in Fig. 8a, when the cells were discharged to 2.5 V, the rGO/V2O5 nanocomposites and pristine V2O5 electrodes delivered a specific initial discharge capacity of 146 and 147 mA h g 1 based on the mass of V2O5, respectively. Fig. 8b shows the specific discharge capacities of the rGO/V2O5 nanocomposites and pristine V2O5 electrodes for the 200 cycles at different current densities. After the first 100 cycles at the current density of 150 mA g 1, the nanocomposites electrodes exhibited a specific discharge capacity of 139.3 mA h g 1, which corresponds to 95.4% of the initial capacity. The performance of the cells cycled at 2.5– 4.0 V vs. Li/Li þ shows excellent cycling stability for 1Li/V2O5, with only modest decay after 100 cycles ( 0.046% decay per cycle). However, for the V2O5 electrodes, only a specific discharge capacity of 96.1 mA h g 1 was delivered after 100 cycles at a current of 150 mA g 1. It indicates that the rGO can effectively improve the reversible capacity of lithium ion storage for V2O5. Furthermore, good high rate performances are desirable in developing high performance lithium ion batteries. The cycling responses of the rGO/V2O5 nanocomposites and pristine V2O5 electrodes at different C rates were also evaluated and are shown in Fig. 8b. After 100 cycles at 1C, the current density was increased
Fig. 5. (a, b) Typical TEM images rGO/V2O5. (c, d) HRTEM images of selection in (b).
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Fig. 6. (a) SEM image and (b)–(d) EDS mappings of rGO/V2O5; (b)–(d) corresponds to elements of vanadium, oxygen, and carbon, respectively.
Fig. 7. TGA curve of rGO/V2O5.
to 300 mA g 1 (2C), 750 mA g 1 (5C), 1.5 A g 1 (10C) and 4.5 A g 1 (30C) successively. At each current density, the cells were tested for 20 cycles to ensure the reliability of the reported readings. When the current density was increased to 2C, the nanocomposites electrodes can deliver a specific capacity of about 125 mA h g 1. By further increasing the current density to 5C, a specific capacity of 97 mA h g 1 can be obtained, and it still retained 75 mA h g 1 and 40 mA h g 1 even at a high current density of 10C and 30C. The retention of the rate capacity is calculated to be 86% (2C), 66% (5C), 51% (10C), and 27% (30C) on the basis of the capacity value of 146 mA h g 1 at a current density of 150 mA g 1 (1C). The corresponding discharge/charge curves of the nanocomposites electrode at different rates are shown in Fig. 8c. For comparison, the specific capacity of pristine V2O5 electrodes was 88 mA h g 1 at 2C and 65 mA h g 1 at 5C, and it delivered only 40 mA h g 1 and 14 mA h g 1 at 10C and 30C,
respectively. Furthermore, when the rate was returned to 1C, the specific discharge capacities can be recovered to 140 and 90 mA h g 1 for the nanocomposites and pristine V2O5 electrodes, respectively. Compared with the pristine V2O5 nanorods, the rGO/V2O5 nanocomposites exhibit a significantly enhanced electrochemical performance for lithium ion storage. It may be attributed to the unique nature of the nanocomposites structures and the outstanding electrical conductivity and mechanical property of rGO. First, the highly conductive rGO shells and rGO networks can effectively increase the electrical conductivity of the electrode materials and contact areas with electrolyte [18]. Herein, AC impedance measurements were performed after 50 cycles at 1C. Nyquist plots (Fig. 8d) show that the diameter of the semicircle for rGO/V2O5 electrodes in the highmedium frequency region is much smaller than that of bare V2O5 electrodes, suggesting that rGO/V2O5 electrodes possess lower contact and charge-transfer impedances [24]. Second, in the rGO/V2O5 electrodes, V2O5 nanorods are enwrapped in the rGO shells homogeneously. Such a dimensional confinement of the V2O5 nanorods by the surrounding rGO sheets provides good buffering effects to maintain the electrodes structures during the cycling process. This result confirms that the rGO shells could not only preserve the high conductivity of the overall electrode, but also largely improve the electrochemical activity of V2O5 nanorods during the cycle processes. Furthermore, the electrochemical performance of electrodes in the voltage range of 2.0–4.0 V vs. Li/Li þ was also tested and is shown in Fig. 9. Fig. 9a and b shows the 1st, 2nd, 5th, 10th and 20th discharge/charge curves of pristine V2O5 and rGO/V2O5 electrodes. As shown in Fig. 9a, the first two plateaus at approximately 3.3 and 3.1 V are attributed to the phase transitions from V2O5 to Li0.5V2O5, and from Li0.5V2O5 to LiV2O5, respectively. The plateau at approximately 2.2 V is ascribed to the formation of Li2V2O5 for the intercalation of the second lithium ion into LiV2O5 [25]. A specific initial discharge capacity of 254 mA h g 1 for the bare V2O5 electrodes was obtained at a current density of
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Fig. 8. Electrochemical performance of rGO/V2O5 and pristine V2O5 as cathode materials in the voltage range of 2.5–4 V vs. Li/Li þ . (a) Galvanostatic discharge/charge profiles of rGO/V2O5 electrode at 150 mA g 1. (b) Cycling performances of rGO/V2O5 and pristine V2O5 electrodes. (c) Galvanostatic discharge/charge profiles of rGO/V2O5 electrodes at different rates. (d) Nyquist plots of rGO/V2O5 and pristine V2O5 electrodes.
Fig. 9. Discharge–charge curves of the 1st, 2nd, 5th, 10th and 20th cycles of (a) pristine V2O5 and (b) rGO/V2O5 electrodes in the voltage range of 2–4 V vs. Li/Li þ .
100 mA g 1 within a voltage range of 2–4 V. The value increased slightly to 287 mA h g 1 in the second cycle, which is close to the theoretic capacity (294 mA h g 1 for 2Li þ intercalation). At the 20th cycle, a specific discharge capacity of 205 mA h g 1 can be obtained. For the rGO/V2O5 electrodes (Fig. 9b), a specific initial discharge capacity of 284 mA h g 1 was obtained at a current density of 100 mA g 1. A specific discharge capacity of 253 mA h g 1 can be retained in the 20th cycle, which corresponds to 89% of the initial discharge capacity. Although the Li2V2O5 phase transition related to the second lithium ion is not as reversible as the Li0.5V2O5 and LiV2O5 phase transition for the first lithium ion intercalation/deintercalation [26], the introduction of rGO can effectively enhance the Li storage performance of V2O5. Fig. 10 shows the cycling performance of the pristine V2O5 and rGO/V2O5 electrodes. After 50 cycles, the discharge capacity of the pristine V2O5 electrodes was reduced to 156 mA h g 1. The rGO/V2O5 electrodes still delivered a specific discharge capacity of
207 mA h g 1, which is consistent with previous rGO/V2O5 (46 wt%) nanocomposites [14].
4. Conclusion In summary, rGO/V2O5 nanocomposites with core–shell structures were successfully fabricated by electrostatic interactions for the first time. The unique structures of the composites and the outstanding electrical conductivity and mechanical property of rGO can effectively improve lithium-storage performance of V2O5, including highly reversible capacities, good cycling stabilities and excellent rate capabilities as a cathode material for LIBs. The ease of fabrication and excellent electrochemical properties of rGO-based nanocomposites make the rGO-enwrapped V2O5 electrochemically active material an ideal candidate for energy storage.
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Fig. 10. Specific discharge capacity of pristine V2O5 and rGO/V2O5 electrodes vs. cycle number in the voltage range of 2–4 V vs. Li/Li þ .
Acknowledgment This work was supported by the School Foundation of Nanchang Hangkong University (EA201302072). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.physe.2013.09. 009. References [1] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy & Environmental Science 4 (2011) 3243. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496.
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Physica E journal homepage: www.elsevier.com/locate/physe
Reduced graphene oxide enwrapped vanadium pentoxide nanorods as cathode materials for lithium-ion batteries Dezhi Chen a,n, Hongying Quan b, Shenglian Luo a,n, Xubiao Luo a, Fang Deng a, Hualin Jiang a a Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, School of Environmental and Chemical Engineering, Nanchang Hangkong University, No. 696, Fenghe Nan Avenue, Nanchang 330063, China b School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China
H I G H L I G H T S
rGO enwrapped V2O5 nanorods composites were prepared for the first time. The electrochemical properties were studied as cathode materials. rGO/V2O5 composites show excellent electrochemical lithium storage activity. The structure-component-property relationships were discussed.
art ic l e i nf o
a b s t r a c t
Article history: Received 10 August 2013 Received in revised form 19 September 2013 Accepted 23 September 2013 Available online 1 October 2013
Novel reduced graphene oxide/vanadium pentoxide (rGO/V2O5) nanocomposites were fabricated by coassembly between negatively charged graphene oxide and positively charged oxide nanorods. A series of characterization including X-ray diffraction, Raman spectrum, scanning electron microscopy and transmission electron microscopy indicated that the V2O5 nanorods with the width of about 50 nm and the length from a few hundred nanometers to several micrometers were enwrapped by rGO layers to form core–shell nanostructures. Compared with the pristine V2O5 nanorods, the as-prepared rGO/V2O5 nanocomposites with 13 wt% rGO showed a significantly enhanced electrochemical performance with high reversible capacities, good cycling stabilities and excellent rate capabilities as a cathode material for lithium batteries. The rGO/V2O5 nanocomposites electrodes delivered a stable discharge capacity around 140 mA h g 1 at a current density of 150 mA g 1 for 100 cycles in the voltage range of 2.5–4.0 V. Furthermore, the nanocomposites electrodes delivered discharge capacities of 287 mA h g 1 and 207 mA h g 1 during the first and 50th cycles in the voltage range of 2.0–4.0 V at a current density of 100 mA g 1, respectively. The as-synthesized nanocomposites are promising candidates for electrical energy storage applications. & 2013 Elsevier B.V. All rights reserved.
Keywords: Reduced graphene oxide Vanadium pentoxide Cathode Lithium-ion battery Nanocomposite
1. Introduction Lithium-ion batteries (LIBs) have been considered as the most effective technologies for electrochemical energy storage over the last two decades [1]. Electrode materials play dominant roles in the performance of LIBs. To meet the ever-growing demand for portable power sources, especially for high-energy and highpower applications such as electrical vehicles (EVs) and hybrid electrical vehicles (HEVs), numerous efforts have been devoted to develop cost-effective and high-performance electrode materials [2–6]. As a kind of promising cathode material, vanadium pentoxide (V2O5) with a layered structure has attracted much
n
Corresponding authors. Tel.: þ 86 791 83953373. E-mail addresses: [email protected] (D. Chen), [email protected] (S. Luo).
1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.09.009
attention due to its high capacity, high output voltage, and low cost [7]. However, the practical application of V2O5 is limited by its intrinsic low-diffusion coefficient of lithium ions ( 10 12 cm2 s 1) and moderate electronic conductivity (10 2–10 3 S cm 1) [8,9]. One generally accepted strategy to overcome these obstacles is to reduce the size of oxides to nanoscale and load onto conductive carbonaceous matrices [1,4,10]. For a given electrode material, the decrease in size can effectively shorten the diffusion time of lithium ions within the material. Meanwhile, the introduction of conductive carbonaceous materials can obviously improve the electronic conductivity and electrochemical stability of the whole electrode. Recently, a new carbon nanomaterial, graphene has received considerable attention for its unique properties such as superior electrical conductivity, excellent mechanical flexibility, large surface area and high thermal stability [11]. Furthermore, low cost
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graphene can be produced in bulk, efficiently through chemical oxidation of graphite to hydrophilic graphite oxide, which can then be exfoliated as individual graphene oxide (GO) sheets by ultrasonication [12]. The GO is then converted to conducting graphene by the chemical reduction process. Generally, these graphene sheets obtained by the chemical reduction method are also known as reduced graphene oxide (rGO). Consequently, rGO/V2O5 nanocomposites have been fabricated as cathode materials for LIBs by different synthesis routes [13–16]. Compared with pristine V2O5 nanostructures, these as-prepared nanocomposites exhibited significantly enhanced reversible capacities and cycling stabilities as well as excellent rate performance. However, the high quality percentage content (420 wt%) of rGO in these nanocomposites decreased the specific reversible capacity of the whole cathode. Although lower rGO (4.5 wt%) can also enhance the electrochemical performance of the as-prepared nanocomposites for lithium ion storage [17], these V2O5 nanomaterials are still prone to strong aggregation during the cycle processes because of nonintimate contact between rGO layers and V2O5. This leads to a decrease in capacity of nanocomposites by 20–50% of their first reversible capacity after 30 cycles. One of the most promising approaches to resolve these problems is to prepare monodisperse V2O5 nanocrystals wrapped by ultrathin and high electrical performance rGO layers. Recently, some groups have synthesized rGO-wrapped metal oxides nanocomposites through electrostatic interactions between negatively charged GO sheets and positively charged metal oxide nanocrystals modified by surface grafting [18–20]. The GO is then converted to conducting rGO by the chemical reduction process. The novel strategy is not only simple, but also able to pre-select nanostructures with desired functionalities and morphology. Herein, we prepared rGO-encapsulated V2O5 nanocomposites by coassembly between negatively charged GO and positively charged oxide nanorods. The process is driven by the mutual electrostatic interactions of the two species, and is followed by thermal reduction. The as-synthesized nanocomposites possess flexible and ultrathin rGO shells that effectively enwrap the oxide nanorods. As a cathode material for LIBs, the nanocomposites with 13 wt% rGO show high reversible capacities, good cycling stabilities and excellent rate capabilities.
2. Material and methods 2.1. Preparation of V2O5 nanorods In a typical synthesis, 0.3 g of ammonium metavanadate (Tianjing Jinghua) and 0.50 g of triblock copolymer P123 (EO20PO70EO20, EO ¼ethylene oxide, PO ¼propylene oxide; Sigma) were dissolved in 30 mL of DI water containing 1.5 mL of 2 M HCl. This mixture was stirred at room temperature for 7 h and then transferred to an autoclave and heated to 120 1C for 24 h. The resulting precipitate was vacuum-filtered and rinsed with DI water. This as-prepared precipitate was calcined at 500 1C for 2 h to obtain V2O5 nanorods (Fig. S1). 2.2. Preparation of rGO/V2O5 nanocomposites V2O5 nanorods (0.4 g) were first dispersed in 200 mL ethanol by sonication for 30 min. Then, aminopropyltrimethoxysilane (APTS, 1 mL) was added, and refluxed for 4 h to obtain APTS-modified V2O5 nanorods. A certain amount of negatively charged GO suspension (0.2 mg/mL, obtained by the modified Hummers method [21]), as shown in Fig. S2, was added into positively charged aminefunctionalized V2O5 nanorods dispersion under vigorous stirring at pH 6 to fabricate GO-enwrapped V2O5 nanorods (GO/V2O5). After
mixing for 1 h, the mixture was centrifuged and washed with deionized water. Finally, the as-prepared product was calcined at 300 1C for 30 min to obtain rGO-enwrapped V2O5 nanorods (rGO/ V2O5) nanocomposites. 2.3. Characterization X-ray diffraction (XRD) analyses were carried out on an X-ray diffractometer (XRD-6000, Shimadzu Scientific Instruments). The XRD patterns with Cu Kα radiation (λ ¼1.5406 Å) at 40 kV and 40 mA were recorded in the range of 2θ ¼ 10–601. The XRD specimens were prepared by means of flattening the powder on the small slides. Scanning electron microscope (SEM) images were achieved by an FEI Quanta 250 field-emission gun environmental scanning electron microscope at 10 kV with the samples obtained from the thick suspension dropping on the silicon slice. Transmission electron microscopy (TEM) images were obtained using a JEM-2100F transmission electron microscope (JEOL Ltd., Japan) operated at 200 kV. Energy dispersive spectrometry (EDS) was obtained from an Apollo300 field-emission gun scanning electron microscope. Raman spectra were recorded from 1100 to 1800 cm 1 on a LabRAM HR800 Laser Raman spectroscope (HORIBA Jobin Yvon Co. Ltd., France) using a 632.5 nm argon ion laser. All samples were deposited on silicon wafers in powder form without using any solvent. Thermogravimetric (TG) analysis was performed in air using a Pyris Diamond TG analyzer (PerkinElemer Inc., USA). The samples were heated from 50 1C to 550 1C at 5 1C/min. 2.4. Electrochemical measurements The electrochemical properties of the rGO/V2O5 nanocomposites and pristine V2O5 nanorods as cathode materials in lithium ion cells were evaluated by a galvanostatic charge/discharge technique. The test electrodes were prepared by mixing active materials (nanocomposites 80% or pristine V2O5 70%) with carbon black (nanocomposites 10% or pristine V2O5 20%) and 10% polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone to form a slurry; CR2032 type coin cells were finally assembled in a highlypure argon-filled glovebox using the test electrodes and the metallic lithium counter/reference electrode, a polypropylene separator (Celgard 2400) and an electrolyte of 1 mol/L LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DMC) (EC/DMC, 1/1 vol). Charge–discharge measurements were carried out galvanostatically using a battery test system (LAND CT2001A model, Wuhan Jinnuo Electronics. Ltd., China). The electrochemical impedance measurements were performed on a CHI660D electrochemical workstation (Shanghai Chenhua Co. Ltd., China) at an AC voltage of 5 mV amplitude in the range of 100 kHz–0.01 Hz.
3. Results and discussion The overall synthesis procedure for the rGO-enwrapped V2O5 nanorods composites (rGO/V2O5) is illustrated in Fig. 1. First, V2O5 nanorods were modified by surface grafting of APTS to render the V2O5 surface positively charged. The modified V2O5 nanorods were then assembled with negatively charged GO by electrostatic interactions. Finally, the aggregates were thermally reduced to prepare rGO/V2O5 nanocomposites. Fig. 2 shows digital photographs of pristine V2O5 nanorods, GO/V2O5 and rGO/V2O5. As presented in Fig. 2a, pure V2O5 is orange, and then turns to brown (Fig. 2b) for GO layers. Finally, after the thermal treatment, the product becomes black (Fig. 2c). Fig. 3a shows the XRD patterns of pristine V2O5 nanorods, GO/ V2O5 and rGO/V2O5. All diffraction peaks can be attributed to
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orthorhombic V2O5 (JCPDS No. 89-0612). It indicates that the process of wrapping GO and the thermal reduction does not affect the crystallization of V2O5. Additionally, there is no evidence in the XRD patterns for the presence of GO or rGO, suggesting that these nanosheets are homogenously dispersed in the hybrids [18]. Fig. 3b shows the Raman spectra of GO/V2O5 and rGO/V2O5 between 1100 cm 1 and 1800 cm 1. The characteristic peaks at 1348 and 1585 cm 1 in Raman spectra of the nanocomposites are attributed to the D band (breathing mode of k-point phonons of A1g symmetry) and the G band (the first-order scattering of the E2g phonons), respectively [22]. Thermal reduction treatment resulted in a clear increase of the D/G intensity ratio from 0.9 for GO in GO/V2O5 to 1.1 for rGO in rGO/V2O5 nanocomposites, which is
Fig. 1. Schematic illustration of the fabrication of rGO enwrapped V2O5 nanorods.
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attributed to the formation of new and smaller sp2 domains during the reduction [23]. From the typical SEM image of rGO/V2O5 nanocomposites shown in Fig. 4, we can observe that the width of these V2O5 nanorods is about 50 nm, and the length ranges from a few hundred nanometers to several micrometers. Furthermore, we can also see that the surface of rod-like V2O5 nanocrystals is covered with a wrinkled layer of rGO. The above results were further confirmed by TEM observation. Fig. 5 shows the typical TEM and HRTEM images of rGO/V2O5 nanocomposites. The TEM image in Fig. 5a shows that the rod-like V2O5 nanocrystals are embedded in rGO nanosheets. From the TEM image (Fig. 5b), we can clearly see that the V2O5 nanorod is completely enwrapped by rGO layers, which helps to prevent V2O5 from agglomeration and enables a good dispersion of these oxide nanorods in rGO shells. The thickness of rGO shells is about 5–10 nm. It means that the number of rGO nanosheets is about 5–10. From the HRTEM image in Fig. 5c, we can obviously observe the interfacial structure between the V2O5 nanorod and the rGO layer. Meanwhile, the corrugated parallel fringes with a basal spacing of about 0.37 nm can be assigned to the (002) plane of several-layer rGO. The HRTEM image in Fig. 5d shows a welldefined crystallinity of V2O5 with a lattice spacing of 0.41 nm, which is assigned to the (101) plane of orthorhombic V2O5 (JCPDS No. 89-0612). EDS mapping for the elements vanadium, oxygen and carbon was used to understand the distribution of V2O5 nanorods in the nanocomposites. From the elemental distribution of vanadium, oxygen, and carbon shown in Fig. 6, we can recognize the uniform dispersion of the V2O5 nanorods in the stacked structure consisting of rGO-enwrapped V2O5 nanorods. The TEM and EDS elemental mapping characterization confirms that thin rGO layer is uniformly coated on the surface of V2O5 nanorods.
Fig. 2. Digital photographs of (a) pristine V2O5 nanorods, (b) GO/V2O5 and (c) rGO/V2O5.
Fig. 3. (a) XRD patterns and (b) Raman spectra of V2O5 nanorods, GO/V2O5 and rGO/V2O5.
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For quantifying the amount of rGO in the nanocomposites, thermogravimetric analysis (TGA) was carried out in air. The samples were heated from 50 to 550 1C at a rate of 5 1C/min. Fig. 7 shows the TGA curve of rGO/V2O5 hybrids. It can be seen that the rGO/V2O5 nanocomposites are almost stable between 50 and 300 1C, which indicates that oxygen-containing functional groups of GO are almost removed by the previous thermal reduction. The rapid mass loss between 300 and 450 1C is attributed to the combustion of rGO. According to the TGA curve in Fig. 7, we can calculate that the content of rGO is 13 wt%.
Fig. 4. Typical SEM image of rGO/V2O5.
The electrochemical performance of rGO/V2O5 nanocomposites and pristine V2O5 nanorods as cathode materials was evaluated by galvanostatic discharge/charge testing. Fig. 8a shows the initial, second and 20th discharge/charge curves of these materials at a current density of 150 mA g 1 (rate of approximately 1C) in a voltage range of 2.5–4.0 V vs. Li/Li þ . During the initial discharge curves, the nanocomposites and pristine V2O5 electrodes exhibited the typical voltage profile of V2O5, which showed two flat plateaus at 3.4 and 3.2 V corresponding to two different two-phase regions of V2O5–Li0.5V2O5 coexistence and Li0.5V2O5–LiV2O5 coexistence, respectively. As shown in Fig. 8a, when the cells were discharged to 2.5 V, the rGO/V2O5 nanocomposites and pristine V2O5 electrodes delivered a specific initial discharge capacity of 146 and 147 mA h g 1 based on the mass of V2O5, respectively. Fig. 8b shows the specific discharge capacities of the rGO/V2O5 nanocomposites and pristine V2O5 electrodes for the 200 cycles at different current densities. After the first 100 cycles at the current density of 150 mA g 1, the nanocomposites electrodes exhibited a specific discharge capacity of 139.3 mA h g 1, which corresponds to 95.4% of the initial capacity. The performance of the cells cycled at 2.5– 4.0 V vs. Li/Li þ shows excellent cycling stability for 1Li/V2O5, with only modest decay after 100 cycles ( 0.046% decay per cycle). However, for the V2O5 electrodes, only a specific discharge capacity of 96.1 mA h g 1 was delivered after 100 cycles at a current of 150 mA g 1. It indicates that the rGO can effectively improve the reversible capacity of lithium ion storage for V2O5. Furthermore, good high rate performances are desirable in developing high performance lithium ion batteries. The cycling responses of the rGO/V2O5 nanocomposites and pristine V2O5 electrodes at different C rates were also evaluated and are shown in Fig. 8b. After 100 cycles at 1C, the current density was increased
Fig. 5. (a, b) Typical TEM images rGO/V2O5. (c, d) HRTEM images of selection in (b).
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Fig. 6. (a) SEM image and (b)–(d) EDS mappings of rGO/V2O5; (b)–(d) corresponds to elements of vanadium, oxygen, and carbon, respectively.
Fig. 7. TGA curve of rGO/V2O5.
to 300 mA g 1 (2C), 750 mA g 1 (5C), 1.5 A g 1 (10C) and 4.5 A g 1 (30C) successively. At each current density, the cells were tested for 20 cycles to ensure the reliability of the reported readings. When the current density was increased to 2C, the nanocomposites electrodes can deliver a specific capacity of about 125 mA h g 1. By further increasing the current density to 5C, a specific capacity of 97 mA h g 1 can be obtained, and it still retained 75 mA h g 1 and 40 mA h g 1 even at a high current density of 10C and 30C. The retention of the rate capacity is calculated to be 86% (2C), 66% (5C), 51% (10C), and 27% (30C) on the basis of the capacity value of 146 mA h g 1 at a current density of 150 mA g 1 (1C). The corresponding discharge/charge curves of the nanocomposites electrode at different rates are shown in Fig. 8c. For comparison, the specific capacity of pristine V2O5 electrodes was 88 mA h g 1 at 2C and 65 mA h g 1 at 5C, and it delivered only 40 mA h g 1 and 14 mA h g 1 at 10C and 30C,
respectively. Furthermore, when the rate was returned to 1C, the specific discharge capacities can be recovered to 140 and 90 mA h g 1 for the nanocomposites and pristine V2O5 electrodes, respectively. Compared with the pristine V2O5 nanorods, the rGO/V2O5 nanocomposites exhibit a significantly enhanced electrochemical performance for lithium ion storage. It may be attributed to the unique nature of the nanocomposites structures and the outstanding electrical conductivity and mechanical property of rGO. First, the highly conductive rGO shells and rGO networks can effectively increase the electrical conductivity of the electrode materials and contact areas with electrolyte [18]. Herein, AC impedance measurements were performed after 50 cycles at 1C. Nyquist plots (Fig. 8d) show that the diameter of the semicircle for rGO/V2O5 electrodes in the highmedium frequency region is much smaller than that of bare V2O5 electrodes, suggesting that rGO/V2O5 electrodes possess lower contact and charge-transfer impedances [24]. Second, in the rGO/V2O5 electrodes, V2O5 nanorods are enwrapped in the rGO shells homogeneously. Such a dimensional confinement of the V2O5 nanorods by the surrounding rGO sheets provides good buffering effects to maintain the electrodes structures during the cycling process. This result confirms that the rGO shells could not only preserve the high conductivity of the overall electrode, but also largely improve the electrochemical activity of V2O5 nanorods during the cycle processes. Furthermore, the electrochemical performance of electrodes in the voltage range of 2.0–4.0 V vs. Li/Li þ was also tested and is shown in Fig. 9. Fig. 9a and b shows the 1st, 2nd, 5th, 10th and 20th discharge/charge curves of pristine V2O5 and rGO/V2O5 electrodes. As shown in Fig. 9a, the first two plateaus at approximately 3.3 and 3.1 V are attributed to the phase transitions from V2O5 to Li0.5V2O5, and from Li0.5V2O5 to LiV2O5, respectively. The plateau at approximately 2.2 V is ascribed to the formation of Li2V2O5 for the intercalation of the second lithium ion into LiV2O5 [25]. A specific initial discharge capacity of 254 mA h g 1 for the bare V2O5 electrodes was obtained at a current density of
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Fig. 8. Electrochemical performance of rGO/V2O5 and pristine V2O5 as cathode materials in the voltage range of 2.5–4 V vs. Li/Li þ . (a) Galvanostatic discharge/charge profiles of rGO/V2O5 electrode at 150 mA g 1. (b) Cycling performances of rGO/V2O5 and pristine V2O5 electrodes. (c) Galvanostatic discharge/charge profiles of rGO/V2O5 electrodes at different rates. (d) Nyquist plots of rGO/V2O5 and pristine V2O5 electrodes.
Fig. 9. Discharge–charge curves of the 1st, 2nd, 5th, 10th and 20th cycles of (a) pristine V2O5 and (b) rGO/V2O5 electrodes in the voltage range of 2–4 V vs. Li/Li þ .
100 mA g 1 within a voltage range of 2–4 V. The value increased slightly to 287 mA h g 1 in the second cycle, which is close to the theoretic capacity (294 mA h g 1 for 2Li þ intercalation). At the 20th cycle, a specific discharge capacity of 205 mA h g 1 can be obtained. For the rGO/V2O5 electrodes (Fig. 9b), a specific initial discharge capacity of 284 mA h g 1 was obtained at a current density of 100 mA g 1. A specific discharge capacity of 253 mA h g 1 can be retained in the 20th cycle, which corresponds to 89% of the initial discharge capacity. Although the Li2V2O5 phase transition related to the second lithium ion is not as reversible as the Li0.5V2O5 and LiV2O5 phase transition for the first lithium ion intercalation/deintercalation [26], the introduction of rGO can effectively enhance the Li storage performance of V2O5. Fig. 10 shows the cycling performance of the pristine V2O5 and rGO/V2O5 electrodes. After 50 cycles, the discharge capacity of the pristine V2O5 electrodes was reduced to 156 mA h g 1. The rGO/V2O5 electrodes still delivered a specific discharge capacity of
207 mA h g 1, which is consistent with previous rGO/V2O5 (46 wt%) nanocomposites [14].
4. Conclusion In summary, rGO/V2O5 nanocomposites with core–shell structures were successfully fabricated by electrostatic interactions for the first time. The unique structures of the composites and the outstanding electrical conductivity and mechanical property of rGO can effectively improve lithium-storage performance of V2O5, including highly reversible capacities, good cycling stabilities and excellent rate capabilities as a cathode material for LIBs. The ease of fabrication and excellent electrochemical properties of rGO-based nanocomposites make the rGO-enwrapped V2O5 electrochemically active material an ideal candidate for energy storage.
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Fig. 10. Specific discharge capacity of pristine V2O5 and rGO/V2O5 electrodes vs. cycle number in the voltage range of 2–4 V vs. Li/Li þ .
Acknowledgment This work was supported by the School Foundation of Nanchang Hangkong University (EA201302072). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.physe.2013.09. 009. References [1] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy & Environmental Science 4 (2011) 3243. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496.
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