Letter pubs.acs.org/NanoLett
3D V6O13 Nanotextiles Assembled from Interconnected Nanogrooves as Cathode Materials for High-Energy Lithium Ion Batteries Yuan-Li Ding,‡ Yuren Wen,§ Chao Wu,‡ Peter A. van Aken,§ Joachim Maier,‡ and Yan Yu*,†,‡ †
Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, 230026 Anhui, P. R. China ‡ Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany § Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, 70569 Stuttgart, Germany S Supporting Information *
ABSTRACT: Three-dimensional (3D) hierarchical nanostructures have been demonstrated as one of the most ideal electrode materials in energy storage systems owing to the synergistic combination of the advantages of both nanostructures and microstructures. In this work, 3D V6O13 nanotextiles built from interconnected 1D nanogrooves with diameter of 20−50 nm were fabricated via a facile solution-redox-based self-assembly route at room temperature, and the mesh size in the textile structure can be controllably tuned by adjusting the precursor concentration. It is suggested that the formation of 3D fabric structure built from nanogrooves is attributed to the rolling and self-assembly processes of produced V 6O13 nanosheet intermediates. When evaluated as cathodes for lithium ion batteries (LIBs), the products delivered reversible capacities of 326 mAh g−1 at 20 mA g−1 and 134 mAh g−1 at 500 mA g−1, and a capacity retention of above 80% after 100 cycles at 500 mA g−1. Importantly, the resulting textiles exhibit a specific energy as high as 780 Wh kg−1, 44−56% higher than those of conventional cathodes, that is, LiMn2O4, LiCoO2, and LiFePO4. Furthermore, the 3D architectures retain good structural integrity upon cycling. Such findings reveal a great potential of V6O13 nanotextiles as high-energy cathode materials for LIBs. KEYWORDS: Nanotextiles, nanogrooves, self-assembly, lithium ion batteries, vanadium oxides
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low cost, and wide availability.19−22 Compared to well-known V2O5, mixed-valence V6O13 is a relatively less-studied vanadium oxide though having shown better electrochemical performance.22,23 As shown in Figure 1, V6O13 is composed of alternating single and double vanadium oxide layers. Valence bond sum calculations suggest that V(1) single layer sites as well as V(3) sites of the double layer are occupied by V4+ (green octahedra), while V(2) sites (gray octahedra) of the double layer possess a V5+ character.22 The single and double layers alternate in the V6O13 structure and share corners and provide much more sites for lithium intercalation. Theoretically, V6O13 can electrochemically incorporate up to 8 Li per formula unit with all the V ions being reduced to 3+ oxidation state, which gives a high theoretical specific capacity and energy of 417 mAh g−1 and 900 Wh kg−1,22 respectively, much higher than those of conventional LiMn2O4 (148 mAh g−1, 500 Wh kg−1),24 LiCoO2 (140 mAh g−1, 540 Wh kg−1),25 or LiFePO4 (170 mAh g−1, 500 Wh kg−1).26 Moreover, V6O13 shows a metallic character at room temperature,27 which is beneficial for
anostructuring has become one of the most powerful means to improve electrochemical performance of electrode materials for energy storage devices, including solar cells,1,2 fuel cells,3,4 supercapacitors,5,6 and rechargeable lithium ion batteries (LIBs),7−13 due to their distinct advantages of large specific surface area and reducing the transport length for electrons and ions. Although low-dimensional nanomaterials, such as 0D nanoparticles, 1D nanostructures (nanowires, nanorods, and nanotubes), and 2D nanomaterials (nanosheets, nanoflakes) have shown superior electrochemical performance in LIBs, they still suffer from self-aggregation and pulverization,14−16 which lead to poor cycling stability. One effective way out is to assemble nanobuilding blocks into a robust 3D architecture (nano-0D/1D ⊂ micro-3D), which synergistically combines the advantages of both nanostructures and microstructures.17,18 In such structures, Li+ insertion/extraction is much faster than that in the bulk counterpart because of the nanoscaled dimension, while the 3D architecture at the submicrometer or micrometer scale can effectively avoid the self-aggregation of active nanomaterials.17 Among various potential cathode candidates for LIBs, vanadium oxides (e.g., V2O5, VO2, V3O7, V4O9, V6O13, etc.) have been extensively studied due to multiple vanadium oxidation states (V2+, V3+, V4+, V5+), high specific capacities, © XXXX American Chemical Society
Received: December 8, 2014 Revised: January 18, 2015
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cathodes for LIBs, the 3D textiles exhibit superior Li+ storage properties in terms of high capacity and energy, superior rate capability, and stable cyclability. Furthermore, 3D cross-linked architecture can be retained upon cycling, which is beneficial for effectively impeding the self-aggregation of nanoscaled building blocks. The fabrication process for 3D V6O13 textiles is illustrated in Figure 2. Their controllable growth with different size mesh is
Figure 1. Crystal structure of V6O13 along [010]; the V(1)O6 and V(3)O6 octahedra are shown in green, while the V(2)O6 octahedra are displayed in gray. The V(4+), V(5+), and O atoms are represented in yellow, blue, and red, respectively.
high-rate charge and discharge. Owing to the combination of these properties, V6O13 has been considered as a promising cathode candidate for high-performance LIBs since the original work of Murphy in 1979.28 Nevertheless, as a mixed-valence vanadium oxide, it faces the challenges of controllable structural synthesis.29 On the other hand, the four phase transitions, decrease of electronic conductivity upon lithium intercalation, and the loss of electrode integrity hinder its practical applications.22 One effective solution to enhance the electrochemical performance of V6O13 electrode is the design and construction of nanostructures with large specific surface and shortened electron/ion transport pathways.3,30 To date, some V6O13 nanomaterials, such as microflower,31 nanowire,32 and nanosheets,33 have shown the potential as electrode materials for supercapacitors or LIBs since they exhibit enhanced Li+ diffusion kinetics and remarkably increased specific capacities compared to the bulk counterpart. However, as mentioned previously, the large volume expansion/contraction of simplex nanostructure during Li+ intercalation and deintercalation results in self-aggregation or pulverization that may interrupt the electronic and ionic contact pathways in the electrodes, leading to a rapid capacity fading. Hence, it is highly desirable to build an electrode that consists of a robust 3D cross-linked architecture assembled by continuous interconnected 1D nanoscaled building blocks, which provide not only direct and rapid electron/ion transport pathways in the radial direction, but also long-range electron conduct channels along the axis direction.34,35 More importantly, 3D network structures can counteract self-aggregation of building blocks and ensure mechanical stability of the whole network by better withstanding volume changes. To our knowledge, most reported V6O13 materials with simplex structure are prepared either by a high-temperature calcination method in inert atmosphere or by a hydrothermal or solvothermal route.22,23,33 The fabrication of 3D V6O13 architectures with interconnected nanoscaled building blocks through a simple synthesis procedure remains a great challenge. Herein, we report the fabrication of 3D V6O13 nanotextiles assembled from continuous interconnected 1D nanogrooves via a facile solution-redox-based self-assembly method at room temperature. The mesh size in the textile structure can be controllably tuned by varying the concentration of VOSO4 precursor solution. The obtained products possess well-suited mesh structures resulting from the formation and simultaneous self-assembly of V6O13 nanogrooves. When evaluated as
Figure 2. Schematic diagram of 3D V6O13 nanotextiles with interconnected nanogrooves prepared by a solution-redox-based selfassembly route between δ-MnO2 nanosheets and VO2+ at room temperature.
achieved by a facile solution-redox-based self-assembly method at room temperature. δ-MnO2 nanosheets (Figure S1, Supporting Information) were used as both oxidant and template, and VO2+ is employed as both reducer and the vanadium source. The driving force for this reaction comes from the difference in the reduction potentials of the two redox pairs MnO2/Mn2+ (1.23 V vs SHE) and VO2+/VO2+ (0.991 V vs SHE). In the reaction, the V6O13 nanogrooves form and spontaneously self-assemble into a cross-linked 3D fabric structure. After the redox transformation, a dark green suspension was obtained, which revealed complete conversion from δ-MnO2 to V6O13. The morphology and composition of the material were systematically examined by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). Figure 3, panels a−c display typical morphology of the products at different magnifications. As can be seen, a highly interconnected and porous 3D fabric-like architecture assembled from numerous 1D building structures is obtained. Further investigations show that these 1D building blocks exhibit the morphology and structure of well-defined nanogrooves with diameters of 20−50 nm; they are randomly connected with the adjacent grooves to self-assemble into continuous and interconnected 3D fabric architecture. This internal cross-linked structure effectively inhibits the selfaggregation of building blocks and prevents mechanical failure upon repeated Li+ intercalation and deintercalation, which thus ensures durability and good electron/ion contact. The mesh size of 30−200 nm can be easily tuned by adjusting the concentration of VOSO4 precursor solution. The formation of the mesh is probably due to the formation and self-assembly process of nanogrooves. B
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Figure 3. SEM images of 3D V6O13 nanotextiles synthesized by using VOSO4 precursor with different concentrations: (a−c) 0.08 M, the white arrows in panel c indicate some grooves; (d−f) 0.05 M; and (g−i) 0.02 M. (j) Low-magnification TEM, (k) high-magnification TEM, and (l) HRTEM images of 3D V6O13 nanotextiles prepared by using 0.05 M VOSO4.
assembled from 1D building blocks was clearly observed. A representative high-resolution TEM (HRTEM) image in Figure 3, panel l indicates a relatively good crystallinity, which is in good agreement with the XRD pattern, as displayed in Figure 4, panel a. The XRD pattern is well indexed to monoclinic V6O13 phase (JCPDS no.: 01−075−1140), which confirms the above conversion from δ-MnO2 to V6O13. To further identify the obtained V6O13 phase, we carried out Raman scattering. As shown in Figure 4, panel b, the Raman spectrum displays a very strong peak located at 140 cm−1, which corresponds to Bg symmetry. Some peaks with medium intensity situated at 95, 277, 875, 923, and 990 cm−1; a shoulder at 300 cm−1; and broad peaks with weak intensity at 190, 478, 520, 688, and 836 cm−1 are also identified, which is in good agreement with previously reported results.36 The high-frequency Raman peak situated at 990 cm−1, which is assigned to the stretching mode of vanadyl oxygen, indicates the attainment of V6O13 and absence of V2O5 and VO2 characteristic modes.36 The peaks located at 277, 300, 403, and 478 cm−1 can be assigned to the bending vibrations of the V−O bonds, the bridging V−O−V, and the three-coordinated O−V(3) bonds, respectively. To
The V6O13 nanotextiles synthesized using the VOSO4 solutions of 0.02, 0.05, and 0.08 M are represented by VO-T02M, VO-T-05 M, and VO-T-08M, respectively. As shown in Figure 3, panel b, VO-T-08 M shows a relatively dense fabric structure. Upon decreasing the concentration of VOSO4 solution from 0.08 to 0.05 M, mesh of 50−100 nm was obtained (Figure 3e), which is further increased to 100−200 nm when using a lower concentration of VOSO4 solution (0.02 M, Figure 3h). Such findings reveal that a higher precursor concentration facilitates the reaction between δ-MnO2 nanosheets and VO2+ and thus the production of nanogrooves, which leads to a denser textile. Importantly, such 3D textile networks can be realized in high quality no matter which concentration of VOSO4 was used, which reveals the controllability of the process. The formed mesh structure allows for easy penetration of electrolyte, which ensures excellent ionic contact together with the facilitated fast electron/ion transport, which thus leads to superior electrochemical performance. The textile structure was further investigated by transmission electron microscopy (TEM). As shown in Figure 3, panels j and k, a cross-linked textile structure C
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copy (XPS) measurements. The binding energies obtained in the XPS analysis were corrected by referencing the C 1s line to 284.6 eV. The XPS spectra of V 2p together with O 1s are presented in Figure 4, panel c. The peaks of both V 2p3/2 and V 2p1/2 were identified at 516.0, 517.3 eV and 524.1, 525.2 eV, corresponding to V4+ (516.0 and 524.1 eV) and V5+ (517.3 and 525.2 eV), respectively,37,38 which confirm the presence of mixed-valence vanadium oxides. The intensity of the V5+ peak at 517.3 eV is a little higher than that of the V4+ peak at 516.0 eV, which is due to the easy oxidation of V4+ to V5+ on the surface considering that the detection depth of the XPS is only a few nanometers.38 The growth mechanism of the products can be divided into two stages: (i) the formation of V6O13 nanosheets based on the redox reaction between VO2+ and δ-MnO2, and (ii) the rolling and self-assembly processes of V6O13 nanosheets. First, VO2+ is oxidized to V6O13 by δ-MnO2, whereby δ-MnO2 is reduced to soluble Mn2+. During the subsequent process, the newly produced V6O13 nanosheets tend to crimp on the edges owing to high surface energy, forming the groove structures, and simultaneously self-assemble into the 3D architectures, as illustrated in Figure 5. Considering the small standard reduction potential difference value between MnO2/Mn2+ (1.23 V) and V2O5/VO2+ (0.957 V), δ-MnO2 is in view of overvoltages and concentration effect, not expected to completely oxidize VO2+ to V2O5, but forms the end-product mixed-valence V6O13. The actual reactions between VO2+ and δ-MnO2 possibly involve two steps. First, VO2+ can be assumed to be oxidized to VO2+ intermediates (eq 4), which further react with VO2+ in the solution and form the more stable V6O13 phase in the subsequent step (eq 5). The reactions are as follows: Half reaction: MnO2 + 4H+ + 2e− → Mn 2 + + 2H 2O E ⊖ = 1.23V
(1)
Half reaction: VO+2 + 2H+ + e− → VO2 + + H 2O E ⊖ = 0.991V
(2)
Half reaction: V2O5 + 6H+ + 2e− → 2VO2 + + 3H 2O E ⊖ = 0.957V
Figure 4. (a) The XRD pattern, (b) Raman spectra, and (c) XPS spectra of the as-prepared 3D V6O13 nanotextiles.
further understand the oxidation state of vanadium in the asprepared product, we performed X-ray photoelectron spectros-
(3)
step 1: MnO2 + 2VO2 + → Mn 2 + + 2VO2+
(4)
step 2: 2VO2+ + 4VO2 + + 5H 2O → V6O13 + 10H+
(5)
Figure 5. Schematic illustration of the formation of 3D V6O13 nanotextiles constructed by interconnected nanogrooves based on the rolling mechanism. D
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Figure 6. (a) CV curves of 3D V6O13 nanotextile electrode (VO-T-05M) between 1.0 and 4.0 V at a scan rate of 0.1 mV s−1; (b) the first three-cycle discharge and charge profiles of VO-T-05 M at 20 mA g−1; (c) the energy density comparison of our 3D nanotextile electrode and conventional LiMn2O4, LiCoO2, LiFePO4, and LiNi0.5Mn1.5O4 electrodes; (d) the rate capability of VO-T-05 M electrode from 50−1000 mA g−1; (e) the cycling performance of VO-T-08M, VO-T-05M, and VO-T-02 M at 200 and 500 mA g−1; (f) the schematic diagram of Li+ intercalation process for 3D nanotextile electrodes.
Although CV plots show two pairs of main redox peaks, these peaks are not very sharp, which suggests that the crystallinity of the product fabricated at room temperature is not pronounced, as confirmed by XRD pattern (Figure 4a). In addition, a multiple-step phase transition process is involved during Li intercalation/deintercalation for V6O13 electrode.39 Thus, the corresponding discharge and charge profiles display relatively sloped plateaus. As shown in Figure 6, panel b, two sloped voltage plateaus at around 2.5 and 2.8 V are identified in the charge curves. The first discharge and charge capacities of the nanotextile electrode are 359.1 and 326.8 mAh g−1, respectively, which correspond to an initial Coulombic efficiency (CE) as high as 91%. In the subsequent two cycles, CEs are 99.5% and 102%, which further confirm excellent Li+ storage reversibility in the 3D fabric electrode. Importantly, our V6O13 material shows an energy density as high as 780 Wh kg−1 (Figure 6c), 44−56% higher than those of conventional LiMn2O4 (500 Wh kg−1), LiCoO2 (540 Wh kg−1), or LiFePO4 (500 Wh kg−1). Even compared to high-voltage LiNi0.5Mn1.5O4 (650 Wh kg−1),40 our 3D nanotextiles still deliver a 20% higher specific energy. Besides the obtained high specific energy, the 3D textile electrodes demonstrate satisfactory rate capability as well. As
During this redox process, the dissolution of Mn will induce a large number of surface and structural defects on the edges of newly formed nanosheets. Thus, the produced V6O13 nanosheets would possess high surface energies and tend to scroll, which would lead to the formation and simultaneous selfassembly into cross-linked architecture of nanogrooves. The increase of the diameter of some grooves is mainly due to the merging of parallel oriented multiple grooves, as evidenced by SEM images in Figure S2 (Supporting Information). The interesting 3D fabric architectures assembled from interconnected nanogrooves offer shorter Li+ diffusion channels and more effective long-range electron transports compared to conventional materials and thus are promising as cathode materials for LIBs. Figure 6, panel a displays typical cyclic voltammetry (CV) curves of the products between 1.0 and 4.0 V at a scan rate of 0.1 mV s−1. In the first cycle, two pairs of main redox peaks at 3.01/2.79 V and 2.79/2.46 V can be clearly identified, similar to the previously reported characteristics of V6O13 electrode.33 In addition, broad cathodic and anodic peaks at 1.73 and 2.5 V were also observed, which indicate a multiplestep phase transition process.39 During the subsequent two cycles, CVs display almost overlapping profiles, which reveal structural preservation and excellent Li+ storage reversibility. E
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electrolyte contact and facilitate rapid electron/ion transport.41 Furthermore, the cross-linked networks provide continuous electronic migration pathways in all directions. (ii) A large number of nanomesh in the 3D nanotextile architecture is favorable for the penetration of electrolyte into the whole 3D structure and also for accommodating volume changes upon cycling, as illustrated in Figure 6, panel f. Thus, self-aggregation of nanogrooves can be effectively impeded or mitigated due to the mechanical stability of cross-linked networks. (iii) Such compact textured architectures would exhibit a relatively high packing density and deliver high volumetric energy and power density. In summary, we have successfully prepared 3D V6O13 nanotextiles assembled from continuous interconnected nanogrooves via a facile solution-redox-based self-assembly route at room temperature. The size of the mesh in the textiles can be controllably tuned by adjusting the precursor concentration. The synthesis approach is simple, environmentally benign, lowcost, and easily up-scalable. The as-prepared textiles possess not only a nano-1D ⊂ micro-3D hierarchical architecture, but also a robust cross-linked conductive network and hence favor fast electron/ion transport. When employed as cathodes in LIBs, the 3D nanotextiles exhibit excellent lithium storage properties and also maintain the structural integrity upon cycling. Such findings show the usefulness of combining different dimensionalities on different length scales, a future that offers a great potential for further optimization.
shown in Figure 6, panel d, on increasing current densities from 50−100, 200, 500, and 1000 mA g−1, reversible capacities of 295, 255, 215, 134, and 88 mAh g−1 are obtained, respectively. It should be noted that, after the continuous cycling with increasing current densities, a specific capacity as high as 280 mAh g−1 could be well recovered at a current density of 50 mA g−1, that is, about 95% retention of the specific capacity in the second cycle, which confirms the excellent Li+ storage reversibility. The 3D textile electrodes undergo a relatively fast capacity drop in the initial cycles, but on further cycles, the capacity stabilized, which is probably attributed to an activation process (Figure 6d). As for cycling tests in Figure 6, panel e, we generally activated them at a relatively low current density for 3−5 cycles after cell assembly, before we carried out further cycling measurements at higher current densities. As expected, the obtained textile electrodes also exhibit superior cycling stability. As shown in Figure 6, panel e, the reversible capacity of 178 mAh g−1 can be maintained for VO-T-05 M after 100 cycles at 200 mA g−1. Even at a relatively high current density of 500 mA g−1, three electrodes (VO-T-02M, VO-T-05 M, and VO-T-08M) deliver capacity retentions of above 80%. Such cycling performance and rate capability are remarkably better than some literature reports of simplex V6O13 nanostructures.29,33 To compare electrochemical kinetics for the 3D textile electrodes prepared by using different concentration VO2+, electrochemical impedance spectroscopy (EIS) measurements were carried out. As shown in Figure S3 (Supporting Information), the Nyquist plots of 3D textile electrodes display similar shapes, with a semicircle appearing in the highfrequency domain and a straight line in the low-frequency region, corresponding to the charge transfer process and diffusion process of lithium ions,13 respectively. VO-T-02 M and VO-T-05 M deliver almost the same charge transfer resistance (∼140 Ω), a bit lower than that of VO-T-08 M (∼180 Ω), which suggests a relatively faster electrochemical kinetics and higher electronic conductivity of VO-T-02 M and VO-T-05M. To investigate the structural stability upon cycling, we carried out SEM measurements for the as-prepared electrode after 100 cycles. As shown in Figure 7, the 3D network assembled from
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Experimental section; SEM images of δ-MnO2 nanosheets and 3D V6O13 nanotextiles; and EIS plots of the products after three charge/discharge cycles from 100 kHz to 0.01 Hz. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Alexander von Humboldt Foundation (Sofja Kovalevskaja award), the National Natural Science Foundation of China (No. 21171015 and No. 21373195), the “Recruitment Program of Global Experts”, the program for New Century Excellent Talents in University (NCET-12-0515), the Fundamental Research Funds for the Central Universities (WK2060140014, WK2060140016), the Collaborative Innovation Center of Suzhou Nano Science and Technology, the Max Planck Society, and the European Union Seventh Framework Programme (FP7/2007-2013) under Grant agreement No. 312483 (ESTEEM2).
Figure 7. (a) SEM image of 3D V6O13 nanotextile electrode after 100 cycles at 500 mA g−1; (b) the magnified view taken from the boxed area in panel a.
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interconnected1D nanostructures is maintained without obvious structural damage. The structure is obviously robust enough to endure repeated Li+ insertion/extraction, leading not only to superior rate capability, but also to good cycling stability. The excellent electrochemical performance of such V6O13 nanotextiles might be related to the structure in several aspects. (i) Nanogrooves offer adequate electrode and
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