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Hollow Porous VOx/C Nanoscrolls as High-Performance Anodes for Lithium-Ion Batteries Bao-Rui Jia,* Ming-Li Qin,* Zi-Li Zhang, Shu-Mei Li, De-Yin Zhang, Hao-Yang Wu, Lin Zhang, Xin Lu, and Xuan-Hui Qu Institute for Advanced Materials and Technology, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, 100083, Beijing, P. R. China S Supporting Information *
ABSTRACT: Novel hollow porous VOx/C nanoscrolls are synthesized by an annealing process with the VOx/octadecylamine (ODA) nanoscrolls as both vanadium and carbon sources. In the preparation, the VOx/ODA nanoscrolls are first achieved by a two-phase solvothermal method using ammonium metavanadat as the precursor. Upon subsequent heating, the intercalated amines between the vanadate layers in the VOx/ODA nanoscrolls decompose into gases, which escape from inside the nanoscrolls and leave sufficient pores in the walls. As the anodes of lithium-ion batteries (LIBs), such hollow porous VOx/C nanoscrolls possess exceedingly high capacity and rate capability (904 mAh g−1 at 1 A g−1) and long cyclic stability (872 mAh g−1 after 210 cycles at 1 A g−1). The good performance is derived from the unique structural features of the hollow hierarchical porous nanoscrolls with low crystallinity, which could significantly suppress irreversible Li+ trapping as well as improve Li+ diffusion kinetics. This universal method of annealing amine-intercalated oxide could be widely applied to the fabrication of a variety of porous electrode materials for high-performance LIBs and supercapacitors. KEYWORDS: VOx/C nanoscrolls, hollow, low-crystalline, lithium-ion batteries, anodes cycles.17 Shi et al. reported that porous V2O3/C composites delivered more than 2-fold higher capacity in comparison with V2O3, approaching a capacity ∼750 mAh g−1 at 250 mA g−1.18 Sun et. al coated uniform amorphous V2O5 films on graphene via atomic layer deposition, and the nanocomposite displayed an exceptional capacity of 900 mAh g−1 at 200 mA g−1.21 However, the dramatic volume variation and structure change during practical discharge/charge processes usually could result in the undesirable pulverization in the electrodes and poor cycling stability. Moreover, the power performance of vanadium oxide anode materials still needs to be improved by applying a highly porous nanostructure. Vanadium oxide nanoscrolls have usually been prepared via hydrothermally treating the vanadium oxide intercalated with organic amines, and they exhibit a multiwalled and open-ended tubular structure.24−34 Because vanadium oxide nanoscrolls are supposed to possess a lot of transport channels for Li+ ions and many active sites, they have been studied as cathode materials for LIBs.25,28,29 However, the performance of nanoscrollstructured vanadium oxides in anode of LIBs was not investigated in previous studies in detail, and this may be due to the absence of porous structure and existence of organic amines in vanadium oxide nanoscrolls. So it is necessary and
1. INTRODUCTION Lithium-ion batteries (LIBs) are remarkably practical and effective devices for electrochemical energy storage1−4 and have attracted significant interest in a broad range of applications including electric vehicles and mobile communication technology.5 With rapidly increasing demands in LIB market, the development of energy storage devices with high power density, energy density, and excellent cycling stability has become a critical requirement. To meet these demands, a variety of metal oxides have been exploited as the novel anode materials for LIBs. Generally, the lithiation reactions in the metal oxide anodes involve two major routes, namely, conversion and insertion reactions. For example, the oxides of late-transitionmetal elements (like CoO 6 and Fe2O3 7) and main-group elements (like SnO2 8) belong to the conversion reaction mechanism, and the oxides of early transition-metal elements (like TiO2 9 and MoO2 10) share the insertion reaction mechanism. Vanadium oxides have attracted considerable interest because of their low cost, abundance, and high theoretical reversible capacity.11−13 Among various vanadium oxides, V2O3 and V2O5 are considered as the most promising anode materials for LIBs and hold conversion and insertion reactions in the lithiation, respectively.14−23 For example, Dong et al. prepared V2O3/ carbon nanocomposites, which delivered a highly retained capacity of 780 mAh g−1 at 200 mA g−1 after 100 cycles.15 Jiang et al. synthesized carbon-coated yolk−shell V2O3 microspheres, which had a capacity of 437.5 mAh g−1 at 0.1 A g−1 after 100 © 2016 American Chemical Society
Received: June 20, 2016 Accepted: September 9, 2016 Published: September 9, 2016 25954
DOI: 10.1021/acsami.6b07439 ACS Appl. Mater. Interfaces 2016, 8, 25954−25961
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a−c) Illustration of the fabrication for the hollow porous VOx/C nanoscrolls; (d) structure of VOx/ODA nanoscrolls; (e) structure of VOx/C nanoscrolls; (f) electrolyte diffusion and charge transfer in VOx/C nanoscrolls.
the ODA in VOx/ODA nanoscrolls was decomposed into gases and small amounts of carbon. The released gases were responsible for creating a porous structure in the nanoscroll wall. The residual carbon remained between the porous vanadium oxide layers, forming a novel hollow porous nanoscroll structure. The scroll-like morphology of the VOx/ODA nanoscrolls has been confirmed by SEM and TEM images. As shown in Figure 2a and Figure 2b, the internal and external diameters are about 30 and 150 nm, respectively, with a length reaching several
interesting to investigate the relationship between this special scroll nanostructure and anode performance of LIBs. Here, first, we synthesized a new kind of low-crystallized hollow hierarchical porous VOx/C nanoscrolls material. The VOx/octadecylamine (ODA) nanoscrolls were first achieved using ammonium metavanadat as the precursor. Upon the subsequent heating, the intercalated amines between the vanadate layers in the VOx/ODA nanoscrolls decomposed into gases, which escaped from inside the nanoscrolls and left sufficient pores in the walls, forming the hollow hierarchical porous nanoscroll structure. Second, we studied their performance as anode of LIBs, and the material exhibited a good cycle stability, showing a highly reversible capacity of 907 mA h g−1 after 150 cycles at a current density of 1 A g−1. As illustrated in Figure 1f, the hollow hierarchical porous nanoscrolls could facilitate the contact between electrolyte and electrode and accelerate the diffusion of Li+ ions. And the low-crystallized mixed-valence VOx phase with a large amount of disordered sites would largely suppress the irreversible Li+ trapping. Furthermore, the effect of annealing on the structure, composition, phase as well as anode performance of the materials was also discussed.
2. RESULTS AND DISCUSSION Figure 1 illustrates the fabrication procedure of the hollow porous VOx/C nanoscrolls. First, VOx/ODA nanoscrolls were synthesized by a ODA−water two-phase solvothermal method using ammonium metavanadate (NH4VO3) as the precursor. At room temperature, white solid ODA and NH4VO3 were added into water. Upon heating, ODA melted into a liquid state and floated on NH4VO3 aqueous solution, forming a two-phase system. Then the vanadium solute species transferred from the water into the melted ODA and were hydrolyzed to vanadium oxide by water molecules. Similar to the formation mechanism of the many reported vanadium oxide nanoscrolls, three steps were involved: (i) formation of a vanadium oxide/ODA composite, (ii) reduction of V5+, (iii) triggering of scrolling process due to the interplay between amines and vanadium oxide sheets.24,32−34 Finally, a waxy black solid product floating on the clear water surface can be obtained. The excess ODA was removed by washing, and the residues were the VOx/ODA nanoscrolls. The detailed synthesis processes and the photoimages are provided in Supporting Information. Dissoluble vanadium source NH4VO3 is chosen as the precursor in our synthesis, not commonly used V2O5 or vanadium alkoxide.35−38 Moreover, ODA layer of two-phase system is reductive. The VOx/ODA nanoscrolls were then annealed at 400 °C for 3 h in argon gas. Upon heat treatment,
Figure 2. (a) SEM image, (b) TEM image, (c) XRD pattern, (d) XPS spectrum, and (e) V 2p3/2 XPS spectra of the VOx/ODA nanoscrolls; (f) V XPS spectrum of ammonium metavanadate raw material. The set in (b) shows an enlarged TEM image. 25955
DOI: 10.1021/acsami.6b07439 ACS Appl. Mater. Interfaces 2016, 8, 25954−25961
Research Article
ACS Applied Materials & Interfaces micrometers. The TEM images indicate that the typical nanoscolls approximately contain 20 layers, with the average interlayer spacing of around 2.5 nm. The ODA molecules are intercalated between the vanadium oxide layers. The wellordered layered structure in the VOx/ODA nanoscrolls was also confirmed by XRD. The highly sharp and intense {001} diffraction peaks at low scattering angles in Figure 2c indicate a well-ordered layered structure. According to (002) peak, the average interlayer spacing is determined to be 2.4 nm, which is in agreement with the TEM results. At higher scattering angles, broader {hk0} reflections are observed, which confirms the crystalline nature of the vanadate walls.28−30 XPS was used to examine the valence of vanadium in the VOx/ODA nanoscrolls and the obtained binding energy was corrected to be 284.6 eV, according to the C 1s spectrum. Figure 2d shows the XPS spectrum, where the peaks are indexed to C, N, O, and V elements. The V 2p3/2 peaks of the nanoscrolls in Figure 2e present the mixed vanadium valence of V4+ (516.4 eV) and V3+ (515.4 eV).39−41 Additionally, the vanadium valence can also be determined by the difference (D) between V 2p3/2 and O 1s level binding energies.42 In the present work, the D values for the deconvoluted V4+ and V3+ are 13.6 and 14.6 eV, respectively, which are very consistent with the reported ones in the literature.43,44 The XPS result of NH4VO3 is also given in Figure 2f, demonstrating that the V5+ in the raw material was reduced to V3+ and V4+ after the reaction. It is suggested that low-valence VOx/ODA nanoscrolls containing V3+ may be achieved by the two-phase solvothermal method, different from previously reported nanoscrolls composed of V 5+ and V4+.25−30,32−34 The VOx/ODA nanoscrolls were transformed into the hollow porous VOx/C nanoscrolls after annealing in Ar. The SEM image (Figure 3a) shows that the product after annealing still maintains a tube-like morphology. From the TEM images in Figure 3b and Figure 3c, the VOx/C nanoscrolls exhibit a porous structure and become somewhat distorted. The TEM image of an unfolded nanoscroll is given in Figure 3d, where a porous thin layer proves the nanoscroll structure. XRD was used to study the structure and phase of the VOx/C nanoscrolls. As shown in Figure 3e, the strong diffraction peaks of the VOx/ODA nanoscrolls have disappeared after annealing and only several broad peaks are found in the XRD pattern of VOx/C nanoscrolls, which suggests that the VOx/C nanoscrolls of small nanocrystals are not well crystallized. Although the diffraction peak at 25° can be indexed into VO2, the precise crystalline structure may not be recognized due to the low crystallinity. XPS was used for surface analysis of the valence of vanadium in the VOx/C nanoscrolls. According to Figure 3f, the V 2p3/2 peaks at 516.4 and 517.4 eV correspond to V4+ and V5+, respectively.43,44 The V3+ existing in the VOx/ ODA nanoscrolls is not observed in the product after annealing, which is mainly due to the surface oxidation on the VOx/C nanoscrolls. According to a previous report, vandium(III) oxide is very easy to be oxidized in air at room temperature.45 Since the annealing process was performed under the inert gas atmosphere, the surface oxidation might occur during storage of the VOx/C nanoscrolls. The VOx/C nanoscrolls have a high surface area and low crystallinity, and thus the V(III) oxide at the surface is active for the oxidation. The carbon amount of the sample was determined by element combustion analysis. As presented in Table S1, the content of carbon is decreased from 42.9 to 8.6 wt % during annealing, which proves that the organic ODA in VOx/ODA nanoscrolls was decomposed.46
Figure 3. (a) SEM image, (b−d) TEM images, (e) XRD pattern, and (f) V 2p3/2 XPS spectra of the hollow porous VOx/C nanoscrolls.
The generated gases escaped from the nanoscrolls and created plenty of pores in the nanoscroll walls. The uniform distribution of carbon element in the VOx/C nanoscrolls was further revealed by EDS mapping analysis in Figure S2 in the Supporting Information. Figure 4 presents the N2 adsorption/desorption isotherms and Barrett−Joyner−Halenda (BJH) pore-size distribution curves of the VOx/ODA nanoscrolls and hollow porous VOx/C nanoscrolls. According to the Brunauer, Deming, Deming, and Teller (BDDT) classification, the VOx/C nanoscrolls display a type IV isotherm with a hysteresis loop, indicating mesoporous structure (2−50 nm in size).47 The Brunauer−Emmett−Teller (BET) specific surface area of the VOx/C nanoscrolls is 55.9 m2 g−1, much larger than the VOx/ ODA nanoscrolls (7.4 m2 g−1). The pore sizes of hollow porous VOx/C nanoscrolls exhibit a bimodal distribution for mesopore and macropore. In view of the observed nanoscroll morphology, the mesopores of ∼10 nm may come from the porous walls, whereas the large macropores of ∼80 nm may be attributed to the cylinder-shaped cavities in the nanoscrolls. In order to understand the chemical reactions in the annealing process of VOx/C nanoscrolls, TG and MS analyses were used. As shown in the TG curve (Figure 5a), the observed weight loss from room temperature to 250 °C is assigned to the thermal evaporation of absorbed water. An abrupt weight loss is observed at ∼350 °C, which is caused by the decomposition and evaporation of ODA and water molecules intercalated in 25956
DOI: 10.1021/acsami.6b07439 ACS Appl. Mater. Interfaces 2016, 8, 25954−25961
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ACS Applied Materials & Interfaces
Figure 4. (a, b) Nitrogen adsorption/desorption isotherm and corresponding BJH pore-size distribution curve of the VOx/ODA nanoscrolls. (c, d) Nitrogen adsorption/desorption isotherm and corresponding BJH pore-size distribution curve of the hollow porous VOx/C nanoscrolls.
Figure 5. (a) TG curve and (b) MS analysis of the VOx/ODA nanoscrolls under Ar gas atmosphere.
With the purpose of understanding the influence of structure on the anode’s performance, different samples have also been prepared, with results summarized in Table S1. The product after annealing at 600 °C in Ar and the product after annealing at 400 °C in Air were abbreviated as NS-600 and NS-400Air, respectively. The C content in NS-600 is 10.5 wt %, and Figure 6a demonstrates that NS-600 has a hollow porous structure similar to the VOx/C nanoscrolls. The C content of NS-400Air is 0.3 wt %, showing that the ODA in the VOx/ODA nanoscrolls has been completely decomposed into gases. As demonstrated in Figure 6b, most of the nanoscroll structures can be damaged in NS-400Air. This confirms that the residual carbon between porous vanadate lamellars can suppress the agglomeration of nanoparticles and maintain the porous nanoscrolls structure after annealing. As presented in Figure S3, the specific surface area of NS-600 reaches ∼139 m2 g−1, larger than that of the VOx/C nanoscrolls prepared at 400 °C. The specific surface area NS-400Air is only 19.1 m2 g−1 (Figure S4). Besides the VOx phase, the XRD pattern in Figure 6c shows that NS-600 contains some well-crystallized V2O3.50 The high annealing temperature promotes crystallization process
the nanoscrolls. No meaningful weight loss is found when the temperature is over 450 °C, leading to a total of 35% weight loss during the whole annealing process. The MS spectrum in Figure 5b presents the evolution curves of H2 (m/z = 2), NH3 (m/z = 17), H2O (m/z = 18), and CO2 (m/z = 44) gases from VOx/ODA nanoscrolls during the annealing process.48,49 The H2O peak can be found at ∼200 °C, indicating the thermal evaporation of H2O. At ∼450 °C, the H2, NH3, CO2, and H2O peaks are observed, where the H2, NH3, and CO2 result from the decomposition of ODA and the H2O is the intercalated water between vanadium oxide layers in the nanoscrolls. The MS result is consistent with the TG result. The escape of these gases is responsible for the formation of pores in the walls of nanoscrolls. Specifically, the significant H2 signal may result from the condensation reaction, which implies that some carbon has been generated during the pyrolysis, consistent with the results aformentioned. The ODA in the VOx/ODA nanoscrolls is decomposed to H2, NH3, CO2, and residual carbon at ∼400 °C, resulting in the formation of hollow porous VOx/C nanoscrolls. 25957
DOI: 10.1021/acsami.6b07439 ACS Appl. Mater. Interfaces 2016, 8, 25954−25961
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ACS Applied Materials & Interfaces
disappeared in the second cycle. The peak at ∼1.1 V indicates the formation of LiyVOx solid solution due to the intercalation of Li+. The anodic peak at ∼2.5 V indicates the oxidation of vanadium cations.23 In the second cycle, the lithiation scan curve is distinctively different from the first one, and a new cathodic peak at ∼1.6 V appeared. This is due to the formation of amorphous phase in the charge/discharge process, consistent with a previous report.20,21,23 Figure 7b presents the charge/ discharge voltage profiles of the VOx/C nanoscrolls at 1 A g−1 for the first, second, and 150th cycles. It can be found that the electrode shows a sloping voltage profile without obvious plateaus. This characteristic is because of the low crystalline nature observed in the VOx/C nanoscrolls. Such behavior has also been found in less crystalline carbon,52 nanosized Li4TiO12,53 nanosized TiO2,54 and amorphous vanadium oxide.20 The Li+ storage sites are cluster gaps, void spaces, vacancies, and so on, which are electrochemically nonequivalent to each other.52 The initial charge and discharge capacities are 1283 and 698 mAh g−1, respectively, with the initial Coulombic efficiency of ∼55%. The low initial Coulombic efficiency is caused by two possibilities: (1) Owing to the high BET surface area of VOx/C nanoscrolls, lots of irreversible reactions take place, forming solid electrolyte interface (SEI) layer.55 (2) In contrast to high crystal vanadium oxides, the low crystalline nature of VOx/C nanoscrolls can suppress the Li trappings; however, some Li+ ions may still be trapped in the lattice.20,23 For the second cycle, the discharge capacity decreases to 755 mAh g−1 and the Coulombic efficiency reaches 90%. When the cycle number is up to 150, the discharge capacity and Coulombic efficiency have been promoted to 907 mAh g−1 and 99.4%, respectively. Moreover, the lithiation reaction mainly takes place below 1.0 V (approximately three-fourths of the whole capacity), which is because the low crystalline nature and the incorporation of V3+ in VOx/C nanoscrolls extend the potential window.51,56 Figure 7c shows the charge and discharge capacity upon cycling ranging from 0.01 to 3 V vs Li+/Li at 1 A g−1. In the first 10 cycles the capacity decreases to 663 mhA g−1 and subsequently recovers to a level above the initial capacity. After 125 cycles, a capacity of 901 mhA g−1 is achieved. From the tenth to the 125th cycle a capacity of ∼240 mhA g−1 is raised, which means the presence of a reactivation process. Reactivation is attributed to the refinement of anode structure and the optimization of stable SEI layer57 and has also been found in Co3O4,57,58 copper vanadium oxide,59 Fe2O3,60 Co9S8,61 FeMoO4, and other metal oxide/sulfide anode materials.62,63 The capacity almost does not fade up to 210 cycles and retains 872 mhA g−1. Furthermore, the rate capability of the material was also evaluated by stepwise increasing and decreasing the current density between 0.1 and 10 A g−1 in the potential range of 0.01−3 V vs Li+/Li. As shown in Figure 8, the VOx/C nanoscrolls can achieve a high capacity of 404 mAh g−1 at 5 A g−1, higher than the greatest value (370 mAh g−1) of the commercial graphite anode material. A capacity of 285 mAh g−1 can still be obtained even at a high current density of 10 A g−1. Especially, when the current density is decreased to 0.1 A g−1 once again, the capacity recovers to ∼1200 mAh g−1, higher than the capacity at the fifth cycle, 918 mAh g−1. The raised capacity implies the existence of a reactivation process, consistent with the result in Figure 7. The four samples including VOx/ODA nanoscrolls, VOx/C nanoscrolls, NS-600, and NS-400Air were tested to compare the performance. Figure 9a gives their capacities upon cycling at
Figure 6. (a, b) TEM images of NS-600 and NS-400Air. (c, d) XRD patterns of NS-600 and NS-400Air.
and crystal growth, and well-crystallized V2O3 appears. Moreover, the carbothermal reduction reaction may occur between the high-valence vanadium oxide and residual carbon at 600 °C, forming low-valence V2O3. The XRD result in Figure 6d shows that NS-400Air is well-crystallined V2O5,51 which means that the vanadium oxide in the VOx/ODA nanoscrolls has been totally oxidized by Air. The obtained VOx/C nanoscrolls have some unique features, including hierarchical porous hollow structure, low degree of crystallinity, and mixed vanadium valence, by which they are expected to be good anode materials for LIBs. Figure 7a shows the representative CV curves of the VOx/C nanoscrolls in the range of 0.01−3.0 V (vs Li+/Li) at 0.5 mV s −1. In the first lithiation process, the cathodic peak at ∼0.4 V seems to correspond to the formation solid electrolyte interface (SEI) layer and irreversible lithiation of VOx, since it almost
Figure 7. (a) CV curves in the voltage range of 0.01−3.0 V versus Li+/ Li at a scan rate of 0.5 mV s−1, (b) typical charge/discharge curves at a constant current density of 1 A g−1, and (c) cycling performance ranging from 0.01 to 3.0 V versus Li/Li+ at a current density of 1 A g−1 of the hollow porous VOx/C nanoscrolls. 25958
DOI: 10.1021/acsami.6b07439 ACS Appl. Mater. Interfaces 2016, 8, 25954−25961
Research Article
ACS Applied Materials & Interfaces
Figure 8. Rate capability and cycling performance of the hollow porous VOx/C nanoscrolls at various rates.
more promising reversible capacity than the other reported vanadium oxide materials. Usually structure and lattice stress can be generated in the electrode material during the insertion/desertion of Li+, especially for a large amount of Li+ ions. The large volume variation of material can lead to the pulverization of electrode and the decrease of the capacity upon cycling.20−23 Here, as shown in Figure S6, for the VOx/C nanoscrolls, the hollow nanotube structure can be remained after 200 cycles. According to previous research, hollow and porous structures can effectively tolerate the structure stress and volume variation caused by the insertion/desertion of Li+ and are often adopted in the design of the metal oxide anodes of LIBs. The same purpose is realized for VOx/C nanoscrolls. At the same time, the hollow hierarchical porous nanoscroll structure can increase the contact area with electrolyte, by the large holes in the center of nanoscrolls and the small pores in the shell of nanoscrolls, which facilitate the lithiation and delithiation reactions.23,67 Moreover, low crystallinity VOx phase produces a large amount of structural disorders, which can suppress the irreversible Li+ trapping in vanadium oxide and increase the storage sites of Li+.23 The voids and vacancies in the low crystalline phase can be a benefit for the faster diffusion of Li+ and avoid lattice stress, greatly improving the rate capability and the cycling life.23
Figure 9. (a, b) Cycling performance at a current density of 1 A g−1 in the voltage range of 0.01−3 V versus Li+/Li and rate capability of VOx/C nanoscrolls, VOx/ODA nanoscrolls, NS-600, and NS-400Air. (c) Electrochemical performance of hollow porous VOx/C nanoscrolls anode material compared with other vanadium oxide materials in previous works (A, V2O3/graphene;64 B, C-coated yolk−shell V2O3 microspheres;17 C, V2O5 nanorods;65 D, V2O3/ordered mesoporous carbon;19 E, amorphous V2O5;20 F, V2O5−SnO2 nanocapsules;51 G, one-dimensional V2O3@C nanocomposite;50 H, V2O5 aerogel;22 I, porous V2O3/C composite;18 J, V2O3/C;15 K, carbon encapsulated V2O3 nanowires;66 L, amorphous vanadium oxide/graphene;21 M, hydrangea-like amorphous mixed-valence VOx microspheres23).
3. SUMMARY In conclusion, we demonstrated the fabrication of hollow porous VOx/C nanoscrolls by a two-step procedure, which involved a water−ODA two-phase solvothermal method and an annealing process. VOx/ODA nanoscrolls were first synthesized and then annealed into VOx/C nanoscrolls. As the anodes material of LIBs, the material had a high capacity, good stability, and rate capability. After 210 cycles, a reversible capability of 872 mAh g−1 could still be maintained at a current density of 1 A g−1. The excellent electrochemical behavior may be attributed to its hollow hierarchical porous structure and low crystallinity VOx phase. Furthermore, the universal method of annealing amine-intercalated oxide would be very helpful in the synthesis of the other porous nanomaterials.
1 A g−1 in the potential range of 0.01−3 V vs Li+/Li. At the 150th cycle, the capacities of NS-400Air, NS-600, and VOx/ ODA nanoscrolls are 688, 601, and 282 mhA g−1, respectively, lower than that of the VOx/C nanoscrolls. The rate capabilities in Figure 9b show that the VOx/C nanoscrolls have a better performance than the other three samples. Although NS-600 has a similar porous nanoscroll structure and 2-fold higher specific surface area in comparison with the VOx/C nanoscrolls, the observed lower capacity in NS-600 is because of the different phase consistent between NS-600 and VOx/C nanoscrolls. There is some well-crystallined V2O3 in NS-600. Likewise, NS-400Air has a more suppressed performance than the VOx/C nanoscrolls, which is due to the well-crystallined V2O5 phase and low specific surface area of NS-400Air. Figure 9c shows a performance comparison of the hollow porous VOx/C nanoscrolls in this study and vanadium oxide materials reported in the open literature. The VOx/C nanoscrolls exhibit
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07439. 25959
DOI: 10.1021/acsami.6b07439 ACS Appl. Mater. Interfaces 2016, 8, 25954−25961
Research Article
ACS Applied Materials & Interfaces
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Experimental details, a table showing comparison of samples, and figures showing reaction process, EDS mapping, isotherms, and SEM images (PDF)
AUTHOR INFORMATION
Corresponding Authors
*B.-R.J.: e-mail,
[email protected]. *M.-L.Q.: e-mail,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation Program of China (Grants 51574031, 51574030, and 51574029), the Natural Science Foundation Program of Beijing (Grant 2162027), the National 863 Program (Grant 2013AA031101), the China Postdoctoral Science Foundation (Grant 2016M591073), and the Fundamental Research Funds for the Central Universities (Grant FRF-TP-15-062A1).
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