DOI: 10.1002/cssc.201500139
Communications
Hierarchical Vanadium Pentoxide Spheres as HighPerformance Anode Materials for Sodium-Ion Batteries Dawei Su,*[a, b] Shixue Dou,*[b] and Guoxiu Wang*[a] such as the phosphate polyanion,[2] fluoride-based NaMF3 (M = Fe, Mn, V, and Ni) materials,[3] layered transition metal oxides,[4] fluorophosphate,[5] and fluorosulfate;[6] however, they can only achieve a reversible capacity of around 150 mA h g 1. Recently, it was revealed that transition-metal materials such as a-MnO2, and b-MnO2[7] can yield much larger capacities when applied as cathode material for sodium-ion batteries. Various materials have been studied for use as anode, from carbon-based anode materials, such as hard carbonaceous materials, with good cyclability but low capacity[8] to alloy anode materials such as tin, antimony, phosphorus, and lead, showing high storage capacities.[9] There is also great potential in exploring transition-metal oxides as anode materials for sodium-ion batteries, including Na2Ti3O7, SnO2, and SnO,[10] and TiO2,[11] as well as others. If the electrode materials have exposed crystal planes that present tunnels, the performance can be improved,[12] because armchair rechargeable batteries rely on an oxide host from/into which alkali ions can be extracted/inserted reversibly. The high ionization potential and large ionic diameter of the sodium ion (1.02 ) compared to the lithium ion (0.76 ) limit the structural variability and choice of sodium insertion materials in crystalline materials.[1b] There remains a lack of appropriate active materials with sufficiently large interstitial spaces within their crystal structure to host sodium ions and offer a satisfactory electrochemical performance. Therefore, finding and optimizing suitable electrode materials is crucial for the development of sodium-ion batteries. We have recently developed a layer-structured vanadium pentoxide (V2O5) as electrode material for sodium-ion batteries, showing that its suitability is due to its large interlayer distance along the c-axis in its crystal structure.[13] We also noted that the spherical architecture can buffer volume changes (i.e., tolerate expansion and shrinkage during cycling) and increase the contact area between the electrode and electrolyte.[14] Herein, we synthesize hierarchical V2O5 spheres consisting of nanocrystals. Via field emission scanning microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), it is revealed that the as-prepared V2O5 shows predominantly exposed (110) facets. This is important for accommodating more sodium ions. Electrochemical testing shows that the as-prepared hierarchical V2O5 spheres can achieve a high capacity of 271 mA h g 1, which is much higher than that achieved by previously synthesized V2O5.[13] This capacity enhancement can be attributed to the presence of more open interlayers in the as-prepared V2O5 crystal structure owing to the predominantly exposed (110) crystal facets. The as-prepared V2O5 also demonstrates a superior high rate capacity and cyclability, so that it can exhibit 179 and 140 mA h g 1 discharge capacity
We report the synthesis of hierarchical vanadium pentoxide (V2O5) spheres as anode materials for sodium-ion batteries (Naion batteries). Through field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy characterizations, it was found that the as-prepared V2O5 spheres are composed of primary nanoparticles with pores between them. The as-prepared hierarchical V2O5 spheres achieved a discharge capacity of 271 mA h g 1 at a current density of 40 mA g 1, and 177 mA h g 1 discharge capacity after 100 cycles. Even at high current densities, V2O5 spheres still delivered high capacity and superior cyclability (179 and 140 mA h g 1 discharge capacities at 640 and 1280 mA g 1 current densities, respectively). The promising electrochemical performances of V2O5 spheres should be ascribed to the unique architecture of hierarchical spheres and the predominant exposed (110) facets, which provides open interlayers for facile sodium ion intercalation. Each nanoparticle contains predominantly exposed (110) crystal planes. The ex situ FESEM analysis revealed that the pores formed by the primary nanocrystals effectively buffer volume changes in the electrode during cycling, contributing to the excellent cycling performance.
Sodium-ion batteries have been comprehensively investigated recently, because they are being considered as a suitable alternative to lithium-ion batteries.[1] Because of their low cost and the abundant reserves of sodium resources (the fourth-mostabundant element in the Earth’s crust). Research on electrode materials and electrolytes for sodium-ion batteries has received much attention, and this attention has resulted in significant progress. There are many different framework compounds that can be used as cathode materials for sodium-ion batteries,
[a] Dr. D. Su, Prof. Dr. G. Wang Centre for Clean Energy Technology School of Chemistry and Forensic Science University of Technology Sydney Broadway, Sydney, NSW 2007 (Australia) E-mail: [email protected] [email protected] [b] Dr. D. Su, Prof. Dr. S. Dou Institute for Superconducting and Electronic Materials University of Wollongong Wollongong, NSW 2522 (Australia) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500139. This publication is part of a Special Issue on the “Future Energy” conference in Sydney, Australia. A link to the issue’s Table of Contents will appear here once it is complete.
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Communications at 640 and 1280 mA g 1 current densities, and maintain 96 and 85 mA h g 1, respectively, after 100 cycles. The improved cyclability should be ascribed to the porous spherical architecture, which can accommodate large volume changes arising from sodium ion insertion and extraction. As revealed by ex situ FESEM analysis, the overall spherical shape of the as-prepared V2O5 persists after long term cycling. The morphology of the V2O5 spheres, characterized by FESEM, is shown in Figure 1. The low-magnification FESEM image (Figure 1 a) shows that the as-prepared V2O5 particles are spherical in shape, with a homogeneous size distribution.
sodium ions through ex- and in-situ synchrotron characterization.[2] Along the c-axis, the V2O5 exhibits a noticeable distance between the nearest bilayer stacks (ca. 4.37 ), which is an ideal layer structure for intercalation of sodium ions. The crystal structure of the as-prepared V2O5 was further identified by XRD measurements (Figure 2). All the diffraction
Figure 2. X-ray diffraction pattern of hierarchical V2O5 spheres.
peaks can be indexed to the Pmmn space group, with lattice parameters a = 11.512 , b = 3.564 , and c = 4.368 , which match the JCPDF 65-0131 standard. The (110) peak is the most intense one. As discussed previously, for bulk Pmmn V2O5 the (001) peak should be the one with the strongest intensity because of the preferred crystal structure of sheets in the abplane.[13] The relative intensities of the (001) and (110) diffraction peaks [(110):(001) = 1.5] suggest that the as-prepared V2O5 spheres show more (110) crystal planes. As compared in Figure S1 (Supporting Information), the (110) crystal planes presents interlayers, while there are no interlayers in the (001) crystal planes. Therefore, the (110) crystal planes can provide more space for the storage of sodium ions. Furthermore, we have demonstrated that the V2O5 with preferred (110) orientation (with the ratio between the (110) and (001) crystal planes around 1.3) can achieve promising electrochemical performance in sodium-ion batteries.[13] Therefore, we obtained V2O5 with predominantly exposed (110) crystal planes, which is expected to result in improved electrochemical performances. To confirm the architecture and structure of the as-prepared V2O5 spheres, we characterized them by using TEM and high resolution TEM (HRTEM). A low-magnification TEM image (Figure 3 a) clearly shows the spherical shape of the as-prepared V2O5. The gradual contrast from the edge to the centre of a typical free-standing V2O5 sphere (Figure 3 b) indicates that the entire sphere has a porous architecture and is composed of self-organized nanocrystals. The corresponding selected area electron diffraction (SAED) pattern (Figure 3 b, inset) can be indexed to the orthorhombic symmetry of V2O5, and the diffraction ring in the SAED pattern indicates its polymorphous crystalline nature. From the high-magnification TEM image (Figure 3 c), it can be clearly seen that nanorods self-assemble
Figure 1. a) Low-, and b–e) high-magnification FESEM images of as-prepared V2O5 spheres. f) Model of the as-prepared V2O5 and the crystal structure of V2O5.
The high-magnification FESEM image (Figure 1 b) shows that the V2O5 spheres are ca. 2 mm in diameter. Figure 1 c–e shows that a typical free-standing V2O5 sphere is composed of nanorods with a particle size of 200 nm (Figure 1 d), and these nanorods further consist of primary nanocrystals with a particle size of less than 20 nm (Figure 1 e). This hierarchical spherical architecture is schematically illustrated in Figure 1 f. Furthermore, the crystal structure of V2O5 consists of two-dimensional (2D) bilayer stacks, which are built up from distorted trigonal bipyramidal coordination polyhedra of oxygen around vanadium. Tepavcevic et al. revealed the relationship between the electrochemically response of a V2O5 structure and its adjustable interlayer spacing, which accommodates intercalation of
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Figure 3. a) Low-magnification TEM image of as-prepared V2O5 spheres. b) Typical TEM image of a single V2O5 sphere. c) High-magnification TEM image of as-prepared V2O5 sphere. d) Lattice-resolved HRTEM image, from which the interlayer structure of V2O5 can be directly observed. The Right top inset in (d) is a simulated V2O5 crystal plane (with V and O atoms in green and red color, respectively).
in a characteristic way. The hierarchical nanoparticles align along the radial direction, forming the channels. Those channels can facilitate the diffusion of electrolyte and sodium ions. The atom-resolved HRTEM image in Figure 3 d further reveals that the primary nanoparticles are less than 20 nm in size and have pores between them. The (001) crystal planes, with 4.368 d-spacing, can be readily observed. As demonstrated by the corresponding crystal structure (in Figure 3 d, inset) the as-prepared V2O5 shows many spaces for sodium ion storage and intercalation. Moreover, the pores between the primary nanoparticles can be clearly seen (Figure 3 d), which can buffer volume changes generated by sodium ion intercalation, thus leading to better electrochemical performance in sodium-ion batteries. The electrochemical performance was evaluated by galvanostatic charge–discharge cycling. As shown in Figure 4, when the hierarchical V2O5 spheres are cycled at 40 mA h g 1, the galvanostatic charge–discharge profile demonstrates the high initial discharge capacity of 271 mA h g 1 (Figure 4 a). A 200 mA h g 1 discharge capacity can be obtained in the second cycle. The decreased discharge capacity between the 1st and 2nd cycles could be due to the generation of a solid electrolyte interphase (SEI), which is formed by electrolyte decomposition.[13] After 100 cycles, the as-prepared V2O5 sphere electrode still achieved a high discharge capacity of 177 mA h g 1, representing 88.5 % of the capacity of the second cycle. This suggests the good cyclability of the as-prepared V2O5 spheres for sodium-ion batteries. The charge and discharge reactions between V2O5 and the sodium ions can be observed from the differential curves of the charge and discharge profiles, as shown in Figure 4 b. There are two peaks in the first cathodic curves, which are located at 1.97 V and 1.36 V, respectively. The first peak should correspond to the decomposition of electrolyte, forming the SEI.[13] The second peak could be assigned to inChemSusChem 0000, 00, 0 – 0
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sertion of sodium ions into the lattice framework of V2O5. During the anodic process, there is an obvious peak at around 1.84 V, corresponding to the extraction of sodium ions from V2O5 crystals. The following cycles also show the two anodic peaks at 1.90 V and 1.45 V, and one broad anodic peak at around 1.84 V. Up to 100 cycles, the insertion process can be readily observed at around 1.43 V, as shown in Figure 4 b. Because of the exposure of large interlayers in the as-prepared V2O5 spheres, the material demonstrates a very good high-rate performance, as verified by the measurements at high current densities. It delivers discharge capacities of 229, 225, 212, 179, and 140 mA h g 1, at current densities of 80, 160, 320, 640, and 1280 mA g 1, respectively (Supporting Information, Figure S2). The 2nd-cycle charge and discharge profiles of the V2O5 sphere electrodes are shown in Figure 5 a, with high capacity values of 183, 176, 159, 123, and 116 mA h g 1, respectively. Figure 5 b displays the high reversibility of the V2O5 spheres over 100 cycles at high current densities. Re-
Figure 4. a) The 1st, 2nd, and 100th cycle discharge and charge profiles of hierarchical V2O5 spheres at 40 mA g 1 current density. b) The corresponding differential curves of the discharge and charge profiles at 40 mA g 1 current density.
versible capacities of 185, 178, 157, and 96 mA h g 1 were retained after 100 cycles at current densities of 80, 160, 320, and 640 mA g 1. Even when the current density was as high as 1280 mA g 1, a discharge capacity of 85 mA h g 1 was retained 3
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Communications performance of commercial V2O5 nanocrystals (Supporting Information, Figure S3 and S4) was also tested. The commercial V2O5 showed a discharge capacity of about 60 mA h g 1. This confirms the superior electrochemical performance of the asprepared V2O5 spheres. We also investigated the hierarchical V2O5 sphere electrodes at varied current rates, as shown in Figure 5 c. At low current density, a high discharge capacity was obtained. Although the capacity decreased with increasing current density, the electrodes still maintained high discharge capacities. Furthermore, a capacity of 197 mA h g 1 was recovered when the current density was reversed back to 40 mA g 1. This indicates that the hierarchical V2O5 spheres can tolerate varied current densities and have excellent rate capability and cycling stability. The hierarchical spherical architecture also has benefits for enhanced cyclability. This is because the pores between the assembled nanoparticles can tolerate the deformation of the electrode during the sodium ion insertion and extraction processes. The primary nanoparticles can squeeze out in the radial direction via inelastic flow, releasing the stress and thereby avoid large volume expansions. This has been verified by ex situ FESEM analysis on the cycled electrode materials. As shown in Figure 6, an overall spherical shape is maintained
Figure 6. a) Low-magnification, and b) high-magnification FESEM images of hierarchical V2O5 sphere electrodes after 100 cycles tested at 40 mA g 1 current density. Figure 5. a) Discharge and charge profiles of hierarchical V2O5 spheres for the 2nd cycle at 80, 160, 320, 640, and 1280 mA g 1 current densities. (b) Capacity versus cycle number at current densities of 80, 160, 320, 640, and 1280 mA g 1. (c) Rate performance of hierarchical V2O5 spheres at varied current densities.
(Figure 6 a). The diameter of the sphere is still around 2 mm (Figure 6 b), without obvious deformation and expansion after cycling, indicating that the V2O5 spheres can withstand extended cycling. In summary, hierarchical V2O5 spheres are synthesized by a hydrothermal method. SEM, XRD, and TEM analyses reveal that the V2O5 spheres demonstrate a preferred (110) orientation, showing open interlayers in the crystal structure. When tested in sodium-ion batteries, the as-prepared V2O5 spheres achieve a high discharge capacity of ~ 200 mA h g 1 at 40 mA g 1 current density. Furthermore, the electrode also exhibited superior high rate capability, with discharge capacities of 179 and 140 mA h g 1 at 640 and 1280 mA g 1 current densi-
after 100 cycles. These values are much higher than that of our previously reported V2O5 material.[13] The achieved capacities from the as-prepared V2O5 spheres are also higher than those achieved by V2O5 aerogels[15] and amorphous V2O5[16] electrodes for sodium-ion batteries. This performance should be ascribed to the predominance of exposed (110) crystal planes in the asprepared V2O5 spheres. For comparison, the electrochemical
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ties, respectively, and maintained these high values for 100 cycles. This promising electrochemical performance in Naion batteries could be ascribed to the unique hierarchical sphere architecture and preferred exposure of (110) crystal planes. The pores formed by primary nanocrystals can readily tolerate volume expansion and contraction during cycling, resulting in very good cyclability. Such mechanism has been verified by ex-situ FESEM analysis. Furthermore, the predominantly exposed (110) crystal planes present more open interlayers, promoting Na ion accessibility and insertion/extraction into/ from the crystal structure of the as-prepared V2O5, which has benefits for enhanced capability and high rate performances.
Acknowledgements This original research was proudly supported by the Commonwealth of Australia through the Automotive Australia 2020 Cooperative Research Centre (AutoCRC). The authors acknowledge the use of facilities within the UOW Electron Microscopy Centre. The authors also would like to thank Dr. Tania Silver for critical reading.
Experimental Section Keywords: anodes · batteries · metal oxides · porous materials · sodium
Synthesis and method: In a typical synthesis process, 6.5 mmol commercial V2O5 (Sigma–Aldrich, ~ 500 nm) and 19.5 mmol oxalic acid (H2C2O4, Sigma–Aldrich, 99.0 %) were dissolved in 40 mL distilled water. The mixture was vigorously stirred at 80 8C for several hours until V2O5 nanoparticles were completely dissolved and a clear blue VOC2O4 solution was formed. Then, 5 mL of the as-prepared VOC2O4 solution was added into 30 mL isopropanol, and after stirring for a few minutes, the solution was sealed in a Teflon container (50 mL in capacity) and heated up to 200 8C. The container was kept at this temperature for 12 h. After being cooled down to room temperature naturally, the products were collected and washed with distilled water and ethanol several times. After further drying at 60 8C in a vacuum oven overnight, a powder was obtained. The final V2O5 products were synthesized by sintering the powder precursor at 350 8C for 2 h with a 2 8C min 1 heating rate. Structural and physical characterization: The phase and crystal structure of the material were characterized by X-ray diffraction (XRD, Siemens D5000) using CuKa radiation with a scanning step of 0.018 s 1. The morphology of the material was observed by field emission scanning electron microscopy (FESEM, Zeiss Supra 55VP). The details of the material and crystal structure were analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL JEM-2011). Selected area electron diffraction (SAED) patterns were recorded by a Gatan charge-coupled device (CCD) camera in a digital format. Electrochemical testing: The electrodes were first prepared: 80 wt % as-prepared materials, 10 wt % acetylene carbon black, and 10 wt % poly(vinylidene fluoride) binder (PVDF, (CH2CF2)n, Sigma–Aldrich) were dispersed in N-methyl-2-pyrrolidone (NMP, C5H9NO, Sigma–Aldrich, 99.5 %) to form a slurry, and the slurry was then pasted onto aluminum foil and dried in a vacuum oven for 12 h, which was followed by pressing at 200 kg cm 2. The mass loading of each electrode was approximately 1.1 mg. Electrochemical measurements were carried out using CR2032 coin cells with Na metal (Sigma–Aldrich, 99.95 %) as reference and counter electrode and glass nanofiber (Whatman) as the separator. The CR2032 coin cells were assembled in an argon-filled glove box (UniLab, Mbraun, Germany) with low levels of H2O and O2 (H2O < 0.01 ppm, O2 < 0.01 ppm). The electrolyte solution was 1 m sodium perchlorate (NaClO4, Sigma–Aldrich, 99 %) dissolved in a mixture of ethylene carbonate (EC, C3H4O3, Sigma–Aldrich, 99 %) and propylene carbonate (PC, C4H6O3, Sigma–Aldrich, 99.7 %) with a volume ratio of 1:1. The galvanostatic charge-discharge measurements were performed at room temperature at different current densities in the voltage range from 1.0 to 4.0 V. During the test, the cell was first discharged, followed by charging process. In order to investigate the
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COMMUNICATIONS In higher spheres: Hierarchical V2O5 spheres are synthesized and demonstrate a preferred (110) orientation, exposing more open interlayers of its crystal structure. When tested in sodium-ion batteries, the as-prepared V2O5 spheres achieve high discharge capacity, superior high rate capability, and excellent cyclability. The promising electrochemical performance is ascribed to the unique hierarchical spherical architecture and preferred exposure of the (110) crystal planes.
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D. Su,* S. Dou,* G. Wang* && – && Hierarchical Vanadium Pentoxide Spheres as High-Performance Anode Materials for Sodium-Ion Batteries
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Hierarchical Vanadium Pentoxide Spheres as HighPerformance Anode Materials for Sodium-Ion Batteries Dawei Su,*[a, b] Shixue Dou,*[b] and Guoxiu Wang*[a] such as the phosphate polyanion,[2] fluoride-based NaMF3 (M = Fe, Mn, V, and Ni) materials,[3] layered transition metal oxides,[4] fluorophosphate,[5] and fluorosulfate;[6] however, they can only achieve a reversible capacity of around 150 mA h g 1. Recently, it was revealed that transition-metal materials such as a-MnO2, and b-MnO2[7] can yield much larger capacities when applied as cathode material for sodium-ion batteries. Various materials have been studied for use as anode, from carbon-based anode materials, such as hard carbonaceous materials, with good cyclability but low capacity[8] to alloy anode materials such as tin, antimony, phosphorus, and lead, showing high storage capacities.[9] There is also great potential in exploring transition-metal oxides as anode materials for sodium-ion batteries, including Na2Ti3O7, SnO2, and SnO,[10] and TiO2,[11] as well as others. If the electrode materials have exposed crystal planes that present tunnels, the performance can be improved,[12] because armchair rechargeable batteries rely on an oxide host from/into which alkali ions can be extracted/inserted reversibly. The high ionization potential and large ionic diameter of the sodium ion (1.02 ) compared to the lithium ion (0.76 ) limit the structural variability and choice of sodium insertion materials in crystalline materials.[1b] There remains a lack of appropriate active materials with sufficiently large interstitial spaces within their crystal structure to host sodium ions and offer a satisfactory electrochemical performance. Therefore, finding and optimizing suitable electrode materials is crucial for the development of sodium-ion batteries. We have recently developed a layer-structured vanadium pentoxide (V2O5) as electrode material for sodium-ion batteries, showing that its suitability is due to its large interlayer distance along the c-axis in its crystal structure.[13] We also noted that the spherical architecture can buffer volume changes (i.e., tolerate expansion and shrinkage during cycling) and increase the contact area between the electrode and electrolyte.[14] Herein, we synthesize hierarchical V2O5 spheres consisting of nanocrystals. Via field emission scanning microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), it is revealed that the as-prepared V2O5 shows predominantly exposed (110) facets. This is important for accommodating more sodium ions. Electrochemical testing shows that the as-prepared hierarchical V2O5 spheres can achieve a high capacity of 271 mA h g 1, which is much higher than that achieved by previously synthesized V2O5.[13] This capacity enhancement can be attributed to the presence of more open interlayers in the as-prepared V2O5 crystal structure owing to the predominantly exposed (110) crystal facets. The as-prepared V2O5 also demonstrates a superior high rate capacity and cyclability, so that it can exhibit 179 and 140 mA h g 1 discharge capacity
We report the synthesis of hierarchical vanadium pentoxide (V2O5) spheres as anode materials for sodium-ion batteries (Naion batteries). Through field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy characterizations, it was found that the as-prepared V2O5 spheres are composed of primary nanoparticles with pores between them. The as-prepared hierarchical V2O5 spheres achieved a discharge capacity of 271 mA h g 1 at a current density of 40 mA g 1, and 177 mA h g 1 discharge capacity after 100 cycles. Even at high current densities, V2O5 spheres still delivered high capacity and superior cyclability (179 and 140 mA h g 1 discharge capacities at 640 and 1280 mA g 1 current densities, respectively). The promising electrochemical performances of V2O5 spheres should be ascribed to the unique architecture of hierarchical spheres and the predominant exposed (110) facets, which provides open interlayers for facile sodium ion intercalation. Each nanoparticle contains predominantly exposed (110) crystal planes. The ex situ FESEM analysis revealed that the pores formed by the primary nanocrystals effectively buffer volume changes in the electrode during cycling, contributing to the excellent cycling performance.
Sodium-ion batteries have been comprehensively investigated recently, because they are being considered as a suitable alternative to lithium-ion batteries.[1] Because of their low cost and the abundant reserves of sodium resources (the fourth-mostabundant element in the Earth’s crust). Research on electrode materials and electrolytes for sodium-ion batteries has received much attention, and this attention has resulted in significant progress. There are many different framework compounds that can be used as cathode materials for sodium-ion batteries,
[a] Dr. D. Su, Prof. Dr. G. Wang Centre for Clean Energy Technology School of Chemistry and Forensic Science University of Technology Sydney Broadway, Sydney, NSW 2007 (Australia) E-mail: [email protected] [email protected] [b] Dr. D. Su, Prof. Dr. S. Dou Institute for Superconducting and Electronic Materials University of Wollongong Wollongong, NSW 2522 (Australia) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500139. This publication is part of a Special Issue on the “Future Energy” conference in Sydney, Australia. A link to the issue’s Table of Contents will appear here once it is complete.
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Communications at 640 and 1280 mA g 1 current densities, and maintain 96 and 85 mA h g 1, respectively, after 100 cycles. The improved cyclability should be ascribed to the porous spherical architecture, which can accommodate large volume changes arising from sodium ion insertion and extraction. As revealed by ex situ FESEM analysis, the overall spherical shape of the as-prepared V2O5 persists after long term cycling. The morphology of the V2O5 spheres, characterized by FESEM, is shown in Figure 1. The low-magnification FESEM image (Figure 1 a) shows that the as-prepared V2O5 particles are spherical in shape, with a homogeneous size distribution.
sodium ions through ex- and in-situ synchrotron characterization.[2] Along the c-axis, the V2O5 exhibits a noticeable distance between the nearest bilayer stacks (ca. 4.37 ), which is an ideal layer structure for intercalation of sodium ions. The crystal structure of the as-prepared V2O5 was further identified by XRD measurements (Figure 2). All the diffraction
Figure 2. X-ray diffraction pattern of hierarchical V2O5 spheres.
peaks can be indexed to the Pmmn space group, with lattice parameters a = 11.512 , b = 3.564 , and c = 4.368 , which match the JCPDF 65-0131 standard. The (110) peak is the most intense one. As discussed previously, for bulk Pmmn V2O5 the (001) peak should be the one with the strongest intensity because of the preferred crystal structure of sheets in the abplane.[13] The relative intensities of the (001) and (110) diffraction peaks [(110):(001) = 1.5] suggest that the as-prepared V2O5 spheres show more (110) crystal planes. As compared in Figure S1 (Supporting Information), the (110) crystal planes presents interlayers, while there are no interlayers in the (001) crystal planes. Therefore, the (110) crystal planes can provide more space for the storage of sodium ions. Furthermore, we have demonstrated that the V2O5 with preferred (110) orientation (with the ratio between the (110) and (001) crystal planes around 1.3) can achieve promising electrochemical performance in sodium-ion batteries.[13] Therefore, we obtained V2O5 with predominantly exposed (110) crystal planes, which is expected to result in improved electrochemical performances. To confirm the architecture and structure of the as-prepared V2O5 spheres, we characterized them by using TEM and high resolution TEM (HRTEM). A low-magnification TEM image (Figure 3 a) clearly shows the spherical shape of the as-prepared V2O5. The gradual contrast from the edge to the centre of a typical free-standing V2O5 sphere (Figure 3 b) indicates that the entire sphere has a porous architecture and is composed of self-organized nanocrystals. The corresponding selected area electron diffraction (SAED) pattern (Figure 3 b, inset) can be indexed to the orthorhombic symmetry of V2O5, and the diffraction ring in the SAED pattern indicates its polymorphous crystalline nature. From the high-magnification TEM image (Figure 3 c), it can be clearly seen that nanorods self-assemble
Figure 1. a) Low-, and b–e) high-magnification FESEM images of as-prepared V2O5 spheres. f) Model of the as-prepared V2O5 and the crystal structure of V2O5.
The high-magnification FESEM image (Figure 1 b) shows that the V2O5 spheres are ca. 2 mm in diameter. Figure 1 c–e shows that a typical free-standing V2O5 sphere is composed of nanorods with a particle size of 200 nm (Figure 1 d), and these nanorods further consist of primary nanocrystals with a particle size of less than 20 nm (Figure 1 e). This hierarchical spherical architecture is schematically illustrated in Figure 1 f. Furthermore, the crystal structure of V2O5 consists of two-dimensional (2D) bilayer stacks, which are built up from distorted trigonal bipyramidal coordination polyhedra of oxygen around vanadium. Tepavcevic et al. revealed the relationship between the electrochemically response of a V2O5 structure and its adjustable interlayer spacing, which accommodates intercalation of
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Figure 3. a) Low-magnification TEM image of as-prepared V2O5 spheres. b) Typical TEM image of a single V2O5 sphere. c) High-magnification TEM image of as-prepared V2O5 sphere. d) Lattice-resolved HRTEM image, from which the interlayer structure of V2O5 can be directly observed. The Right top inset in (d) is a simulated V2O5 crystal plane (with V and O atoms in green and red color, respectively).
in a characteristic way. The hierarchical nanoparticles align along the radial direction, forming the channels. Those channels can facilitate the diffusion of electrolyte and sodium ions. The atom-resolved HRTEM image in Figure 3 d further reveals that the primary nanoparticles are less than 20 nm in size and have pores between them. The (001) crystal planes, with 4.368 d-spacing, can be readily observed. As demonstrated by the corresponding crystal structure (in Figure 3 d, inset) the as-prepared V2O5 shows many spaces for sodium ion storage and intercalation. Moreover, the pores between the primary nanoparticles can be clearly seen (Figure 3 d), which can buffer volume changes generated by sodium ion intercalation, thus leading to better electrochemical performance in sodium-ion batteries. The electrochemical performance was evaluated by galvanostatic charge–discharge cycling. As shown in Figure 4, when the hierarchical V2O5 spheres are cycled at 40 mA h g 1, the galvanostatic charge–discharge profile demonstrates the high initial discharge capacity of 271 mA h g 1 (Figure 4 a). A 200 mA h g 1 discharge capacity can be obtained in the second cycle. The decreased discharge capacity between the 1st and 2nd cycles could be due to the generation of a solid electrolyte interphase (SEI), which is formed by electrolyte decomposition.[13] After 100 cycles, the as-prepared V2O5 sphere electrode still achieved a high discharge capacity of 177 mA h g 1, representing 88.5 % of the capacity of the second cycle. This suggests the good cyclability of the as-prepared V2O5 spheres for sodium-ion batteries. The charge and discharge reactions between V2O5 and the sodium ions can be observed from the differential curves of the charge and discharge profiles, as shown in Figure 4 b. There are two peaks in the first cathodic curves, which are located at 1.97 V and 1.36 V, respectively. The first peak should correspond to the decomposition of electrolyte, forming the SEI.[13] The second peak could be assigned to inChemSusChem 0000, 00, 0 – 0
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sertion of sodium ions into the lattice framework of V2O5. During the anodic process, there is an obvious peak at around 1.84 V, corresponding to the extraction of sodium ions from V2O5 crystals. The following cycles also show the two anodic peaks at 1.90 V and 1.45 V, and one broad anodic peak at around 1.84 V. Up to 100 cycles, the insertion process can be readily observed at around 1.43 V, as shown in Figure 4 b. Because of the exposure of large interlayers in the as-prepared V2O5 spheres, the material demonstrates a very good high-rate performance, as verified by the measurements at high current densities. It delivers discharge capacities of 229, 225, 212, 179, and 140 mA h g 1, at current densities of 80, 160, 320, 640, and 1280 mA g 1, respectively (Supporting Information, Figure S2). The 2nd-cycle charge and discharge profiles of the V2O5 sphere electrodes are shown in Figure 5 a, with high capacity values of 183, 176, 159, 123, and 116 mA h g 1, respectively. Figure 5 b displays the high reversibility of the V2O5 spheres over 100 cycles at high current densities. Re-
Figure 4. a) The 1st, 2nd, and 100th cycle discharge and charge profiles of hierarchical V2O5 spheres at 40 mA g 1 current density. b) The corresponding differential curves of the discharge and charge profiles at 40 mA g 1 current density.
versible capacities of 185, 178, 157, and 96 mA h g 1 were retained after 100 cycles at current densities of 80, 160, 320, and 640 mA g 1. Even when the current density was as high as 1280 mA g 1, a discharge capacity of 85 mA h g 1 was retained 3
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Communications performance of commercial V2O5 nanocrystals (Supporting Information, Figure S3 and S4) was also tested. The commercial V2O5 showed a discharge capacity of about 60 mA h g 1. This confirms the superior electrochemical performance of the asprepared V2O5 spheres. We also investigated the hierarchical V2O5 sphere electrodes at varied current rates, as shown in Figure 5 c. At low current density, a high discharge capacity was obtained. Although the capacity decreased with increasing current density, the electrodes still maintained high discharge capacities. Furthermore, a capacity of 197 mA h g 1 was recovered when the current density was reversed back to 40 mA g 1. This indicates that the hierarchical V2O5 spheres can tolerate varied current densities and have excellent rate capability and cycling stability. The hierarchical spherical architecture also has benefits for enhanced cyclability. This is because the pores between the assembled nanoparticles can tolerate the deformation of the electrode during the sodium ion insertion and extraction processes. The primary nanoparticles can squeeze out in the radial direction via inelastic flow, releasing the stress and thereby avoid large volume expansions. This has been verified by ex situ FESEM analysis on the cycled electrode materials. As shown in Figure 6, an overall spherical shape is maintained
Figure 6. a) Low-magnification, and b) high-magnification FESEM images of hierarchical V2O5 sphere electrodes after 100 cycles tested at 40 mA g 1 current density. Figure 5. a) Discharge and charge profiles of hierarchical V2O5 spheres for the 2nd cycle at 80, 160, 320, 640, and 1280 mA g 1 current densities. (b) Capacity versus cycle number at current densities of 80, 160, 320, 640, and 1280 mA g 1. (c) Rate performance of hierarchical V2O5 spheres at varied current densities.
(Figure 6 a). The diameter of the sphere is still around 2 mm (Figure 6 b), without obvious deformation and expansion after cycling, indicating that the V2O5 spheres can withstand extended cycling. In summary, hierarchical V2O5 spheres are synthesized by a hydrothermal method. SEM, XRD, and TEM analyses reveal that the V2O5 spheres demonstrate a preferred (110) orientation, showing open interlayers in the crystal structure. When tested in sodium-ion batteries, the as-prepared V2O5 spheres achieve a high discharge capacity of ~ 200 mA h g 1 at 40 mA g 1 current density. Furthermore, the electrode also exhibited superior high rate capability, with discharge capacities of 179 and 140 mA h g 1 at 640 and 1280 mA g 1 current densi-
after 100 cycles. These values are much higher than that of our previously reported V2O5 material.[13] The achieved capacities from the as-prepared V2O5 spheres are also higher than those achieved by V2O5 aerogels[15] and amorphous V2O5[16] electrodes for sodium-ion batteries. This performance should be ascribed to the predominance of exposed (110) crystal planes in the asprepared V2O5 spheres. For comparison, the electrochemical
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ties, respectively, and maintained these high values for 100 cycles. This promising electrochemical performance in Naion batteries could be ascribed to the unique hierarchical sphere architecture and preferred exposure of (110) crystal planes. The pores formed by primary nanocrystals can readily tolerate volume expansion and contraction during cycling, resulting in very good cyclability. Such mechanism has been verified by ex-situ FESEM analysis. Furthermore, the predominantly exposed (110) crystal planes present more open interlayers, promoting Na ion accessibility and insertion/extraction into/ from the crystal structure of the as-prepared V2O5, which has benefits for enhanced capability and high rate performances.
Acknowledgements This original research was proudly supported by the Commonwealth of Australia through the Automotive Australia 2020 Cooperative Research Centre (AutoCRC). The authors acknowledge the use of facilities within the UOW Electron Microscopy Centre. The authors also would like to thank Dr. Tania Silver for critical reading.
Experimental Section Keywords: anodes · batteries · metal oxides · porous materials · sodium
Synthesis and method: In a typical synthesis process, 6.5 mmol commercial V2O5 (Sigma–Aldrich, ~ 500 nm) and 19.5 mmol oxalic acid (H2C2O4, Sigma–Aldrich, 99.0 %) were dissolved in 40 mL distilled water. The mixture was vigorously stirred at 80 8C for several hours until V2O5 nanoparticles were completely dissolved and a clear blue VOC2O4 solution was formed. Then, 5 mL of the as-prepared VOC2O4 solution was added into 30 mL isopropanol, and after stirring for a few minutes, the solution was sealed in a Teflon container (50 mL in capacity) and heated up to 200 8C. The container was kept at this temperature for 12 h. After being cooled down to room temperature naturally, the products were collected and washed with distilled water and ethanol several times. After further drying at 60 8C in a vacuum oven overnight, a powder was obtained. The final V2O5 products were synthesized by sintering the powder precursor at 350 8C for 2 h with a 2 8C min 1 heating rate. Structural and physical characterization: The phase and crystal structure of the material were characterized by X-ray diffraction (XRD, Siemens D5000) using CuKa radiation with a scanning step of 0.018 s 1. The morphology of the material was observed by field emission scanning electron microscopy (FESEM, Zeiss Supra 55VP). The details of the material and crystal structure were analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL JEM-2011). Selected area electron diffraction (SAED) patterns were recorded by a Gatan charge-coupled device (CCD) camera in a digital format. Electrochemical testing: The electrodes were first prepared: 80 wt % as-prepared materials, 10 wt % acetylene carbon black, and 10 wt % poly(vinylidene fluoride) binder (PVDF, (CH2CF2)n, Sigma–Aldrich) were dispersed in N-methyl-2-pyrrolidone (NMP, C5H9NO, Sigma–Aldrich, 99.5 %) to form a slurry, and the slurry was then pasted onto aluminum foil and dried in a vacuum oven for 12 h, which was followed by pressing at 200 kg cm 2. The mass loading of each electrode was approximately 1.1 mg. Electrochemical measurements were carried out using CR2032 coin cells with Na metal (Sigma–Aldrich, 99.95 %) as reference and counter electrode and glass nanofiber (Whatman) as the separator. The CR2032 coin cells were assembled in an argon-filled glove box (UniLab, Mbraun, Germany) with low levels of H2O and O2 (H2O < 0.01 ppm, O2 < 0.01 ppm). The electrolyte solution was 1 m sodium perchlorate (NaClO4, Sigma–Aldrich, 99 %) dissolved in a mixture of ethylene carbonate (EC, C3H4O3, Sigma–Aldrich, 99 %) and propylene carbonate (PC, C4H6O3, Sigma–Aldrich, 99.7 %) with a volume ratio of 1:1. The galvanostatic charge-discharge measurements were performed at room temperature at different current densities in the voltage range from 1.0 to 4.0 V. During the test, the cell was first discharged, followed by charging process. In order to investigate the
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Received: January 27, 2015 Published online on && &&, 0000
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COMMUNICATIONS In higher spheres: Hierarchical V2O5 spheres are synthesized and demonstrate a preferred (110) orientation, exposing more open interlayers of its crystal structure. When tested in sodium-ion batteries, the as-prepared V2O5 spheres achieve high discharge capacity, superior high rate capability, and excellent cyclability. The promising electrochemical performance is ascribed to the unique hierarchical spherical architecture and preferred exposure of the (110) crystal planes.
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D. Su,* S. Dou,* G. Wang* && – && Hierarchical Vanadium Pentoxide Spheres as High-Performance Anode Materials for Sodium-Ion Batteries
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