New Strategy for the Morphology-Controlled Synthesis of V2O5

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New strategy for the morphology-controlled synthesis of V2O5 microcrystals with enhanced capacitance as battery-type supercapacitor electrodes Jiqi Zheng, Yifu Zhang, Tao Hu, Tianming Lv, and Changgong Meng Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00776 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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Crystal Growth & Design

New strategy for the morphology-controlled synthesis of V2O5 microcrystals with enhanced capacitance as battery-type supercapacitor electrodes Jiqi Zheng, Yifu Zhang*, Tao Hu, Tianming Lv, Changgong Meng School of Chemistry, Dalian University of Technology, Dalian 116024, PR China *Corresponding author. E-mail address: [email protected]

Abstract. Porous vanadium pentoxide (V2O5) microcrystals with different morphologies were synthesized through the decomposition of butterfly-like, rhombohedral and flower-like ammonium metavanadate (NH4VO3) microcrystals, which were synthesized by the drowning-out crystallization of hydrothermal NH4VO3 aqueous solution using ethanol as both the antisolvent and the template for the self-assemble of vanadate ions. The effects of reaction conditions on the morphologies of products were characterized by scanning electron microscope (SEM) and the possible growth mechanism was proposed. The electrochemical properties of the produced porous V2O5 with different morphologies were studied as battery-type electrodes for supercapacitors using 1M LiClO4/ propylene carbonate (PC) as the electrolyte. Rhombohedral V2O5 exhibited the highest initial specific capacitance of 641 F·g−1 at 0.5 A·g−1 among the three obtained morphologies, as well as excellent rate capability and cycling stability, with a retention of over 119% after 2000 cycles, making it a promising electrode material for supercapacitors. The influences of morphologies on the capacitance and cycle performance are analyzed. The results indicate that the increasing complexity of the structure leads to lower specific capacitance because of higher degree of electrode polarization and higher resistance. While the structure stability of the microcrystals is related to the rate capability as well as the cycling performance. Keywords: Morphology-controlled synthesis; V2O5 microcrystal; Porous structure; Battery-type electrode

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1.

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Introduction Vanadium oxides (VOx) have attracted much interest and been widely used in various fields such as

catalysis and energy storage because of their unique layered structure, multiple oxidation states (III – V), as well as earth abundance and low cost

1-6

. Among numerous VOx, vanadium pentoxide (V2O5) is the

most promising material for energy storage because it has the highest oxidation state of vanadium, enabling it to store more electron per formula unit with the highest stability

7-10

. Nevertheless, the

electrochemical properties of V2O5 as electrodes for supercapacitors are limited because of its poor conductivity and cycling stability, especially the dissolution and volume expansion during the circulation process 11-13. Based on previous studies, the morphologies and structures of the materials have great influence on their electrochemical properties. Therefore, it is an effective and convenient route to improve the electrochemical performance of electrode materials through the development of novel architectures, which can shorten the access for ion diffusion and enhance the structural stability

7, 14-16

have been made to obtain V2O5 with different morphologies, such as nanobelts nanofibers

19

, layer-by-layer V2O5 quadrate structures

and hollow microspheres

23

20

, hierarchical spheres

21

17

. Many efforts

, nanowires

, porous octahedrons

18

,

22

, etc. While little literatures focused on the comprehension among the

properties of V2O5 with different morphologies and discuss the effects of morphologies on the electrochemical properties. Qian et al. 24 reported the synthesis of V2O5 nanowires, flower-like flakes, and curly bundled nanowires in the presence of polyethylene glycol 6000 (PEG), sodium dodecylbenzene sulfonate (SDBS), and Pluronic P-123 (P123). V2O5 nanowires had the highest capacitance among these three morphologies (349 F·g−1 at 5 mV·s−1) but showed poor cycling stability with a retention of only 34% after 200 cycles. While the curly bundled nanowires showed an upward trend during the circulation because of the existence of nanopores. Zhang et al.

25

compared the electrochemical properties of V2O5

nanobelts, nanoparticles and microspheres, and V2O5 microspheres exhibited the highest capacitance of 308 F·g−1 at 1 A·g−1, which is attributed to the large surface area. The result indicates that though most of the reported researches focused on the development of nanostructures, microstructures V2O5 also have enormous potential in the application of energy storage due to the possible advanced properties of individual particles as well as the novel collective properties and enhanced tunable functions caused by the particle ensembles 26. While neither of the researches gives regularity of how the morphology of the product affects its electrochemical performance, though it has great significance for the design of

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electrode materials with new structures. Thus, it is an urgent need to find the relations between the morphologies of electrode materials and their electrochemical performance. To realize the morphology-controlled synthesis of V2O5, various methods were used. For instance, thermal evaporation method was used to synthesize layered-structure nanoribbons

27

; electro-deposition

was used to obtain 3D porous V2O5 hollow sphere 28; spray pyrolysis method was employed to produce V2O5 with various porous structures

29

, etc. Among them, hydrothermal reaction is the most commonly

used method due to its facile conditions, and the morphology of the products can be well controlled. While templates were usually needed to fabricate special morphologies and the produces were normally nanomaterials, 30, 31. Drowning-out crystallization is an effective method to get pure crystals through the recrystallization by adding antisolvent in the generated supersaturated solution to reduce the solubility of the solute in the component solvent, and it is possible to control the morphology and size of the crystals by changing kinetic factors like the amount of antisolvent, concentration of the solute and the crystallization temperature

32, 33

. According to the above discussion, it’s necessary to develop a new

synthetic method which can produce a series of materials with different morphologies, so the systematic analysis of the relations between the morphologies of materials and their electrochemical performance can be performed. In this contribution, an innovative, effective and environmental friendly strategy that combines the hydrothermal method and the drowning-out crystallization, along with the subsequent thermal treatment was developed to synthesize porous V2O5 microcrystals with three different morphologies. More importantly, the regularity of how the morphologies of the crystals affect their electrochemical performance is summarized. Herein, hydrothermal reaction was used to get supersaturated solution of NH4VO3, in which ethanol was added as both antisolvent and template to obtain NH4VO3 microcrystals. Various morphologies including butterfly-like, rhombohedral and flower-like NH4VO3 were obtained through self-assembling of vanadate ions by control the initial concentration of NH4 VO3, hydrothermal temperature, amount of ethanol and crystallization temperature. The possible mechanism during this process was discussed. Then the obtained NH4VO3 microcrystals were decomposed into V2O5 through the thermal treatment in ambient-air and their electrochemical properties were studied as battery-type electrode materials for supercapacitors. The correlations among the structures of the crystals and their corresponding electrochemical performances including specific capacitances, rate capability and cycling stability were analyzed. All the V2O5 with three morphologies exhibit remarkable initial capacitances of 641, 556 and

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609 F·g−1 at 0.5 A·g−1 of rhombohedral, butterfly-like and flower-like V2O5, respectively, which is higher than most of the reported V2O5 electrode materials, as well as excellent cycling stabilities. Development of these new structures not only provides promising candidate materials for high performance supercapacitors, but also gives a significant opportunity for the study of how the morphologies of the crystals affect their electrochemical properties.

2. 2.1.

Experimental section Synthesis Materials Both ammonium metavanadate (NH4VO3) (analytical grade with purity over 99.0%) and ethanol

(analytical grade with purity over 99.7%) in this work were used without purification. In a typical synthesis, as shown in Scheme 1, 5 mmol (0.585 g) commercial NH4VO3 with a morphology of irregular bulks (Fig. S1, Supporting information) was dispersed into 30 mL distilled water with continuous magnetic stirring for 4 h. The suspension was then transferred into a Teflon-lined stainless-steel autoclave (50 mL volume) and heated at a certain temperature for 24 h, after which yellowish solution was obtained. A certain amount of ethanol was added into the clear solution and yellowish precipitate formed immediately. The precipitate was centrifuged and through further crystallization. Finally, the crystalline product was calcined in a muffle furnace at 400 °C for 4 h in ambient-air to get pure vanadium pentoxide (V2O5). Three different morphologies were synthesized and their corresponding conditions are summarized in Table 1.

Scheme 1. A schematic illustration of the synthetic process of V2O5 with different morphologies. 4


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Table 1. The experimental conditions of the synthesis of NH4VO3 with different morphologies. Hydrothermal NH4VO3/mmol

H2O/mL

Crystallization Ethanol/mL

temperature/°C

Morphology temperature/°C

5

30

100

70

40

Butterfly-like microcrystals

5

30

100

40

8

Rhombohedral microcrystals

5

30

180

40

40

Flower-like microcrystals

2.2.

Material characterizations The morphologies of the samples were observed by both the QUANTA450 scanning electron

microscope with tungsten filament (working distance is about 10 mm) and the NOVA NanoSEM 450 field-emission scanning electron microscopy with working distances of about 10 mm at low magnification and 5 mm at high magnification. X-ray diffraction (XRD, PANalytical X’Pert powder) of the samples was identified using Ni-filtered Cu Kα radiation with an operation voltage of 40 kV and a current of 40 mA. Fourier transform infrared spectroscopy (FTIR, Nicolet 6700) was characterized using KBr pellet technique from 4000 to 400 cm−1. Raman spectra were obtained using a 532 nm excitation line on a Thermo Scientific spectrometer. Brunauer-Emmet-Teller (BET) method was performed on Micromeritics ASAP-2020 with a degassed temperature of 100 °C. Energy-dispersive X-ray spectrometer (EDS) was also used to reveal the chemical composition and the element mapping, and the instrument was attached to the SEM. 2.3.

Electrochemical characterizations The electrochemical properties were carried out by a CHI-660D electrochemical workstation. The

working electrodes were prepared by mixing the samples, carbon black and polyvinylidene difluoride in a mass ratio of 8: 1: 1, and moderate amount of N-methyl-2-pyrrolidone was added to form slurry, which was coated on nickel foam (1 cm* 1 cm) and then dried at 100 °C overnight. The working electrodes had similar mass loadings of about 3 mg/cm2. In the three-electrode experimental cell, a graphite rod was served as the counter electrode and a saturated calomel electrode (SCE) was used as the reference

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electrode. 1 mol·L−1 LiClO4/propylene carbonate (PC) solution was used as the electrolyte. To perform the electrochemical properties of the samples, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were taken. Moreover, the specific capacitances (C, F·g−1) of the active materials can be calculated from GCD curves based on the following equation:

=

∙∆

(1)

∙∆

Where I (A) is the constant charge-discharge current; ∆t (s) is the discharge time in the potential window ∆V (V) during the discharge process; m (g) is the mass loading of the active material on the electrode.

3.

Results and discussion

3.1. Morphology and composition of NH4VO3 with different morphologies The SEM images of the crystals synthesized with a hydrothermal temperature of 100 °C and an ethanol volume of 70 mL, then crystallized at 40 °C are shown in Fig. 1. It can be observed that the product consists of microcrystals, which present a homogeneous butterfly-like morphology. The crystal is composed of a hollow cylinder in the middle and a pair of symmetrical wing-like structures at both sides, and sulci can be clearly observed on the whole crystal. The crystals are uniform in size, with lengths in the range of 25-30 µm, widths in the range of 15-20 µm and thicknesses among 1.5-3 µm depending on the different parts of the crystal. As crystals were formed immediately when the ethanol was added, part of the sediment was separated and dried in 2 min to observe the morphology. As shown in Fig. 2, the sample crystalized for 2 min still possesses basic features of the butterfly-like morphology, while the edges are rough. This result illustrates that the formation of the special structure is rapid, and the further crystallization mainly aimed at the improvement of the crystallinity and forming intact crystals with smooth edges. The speculative reaction process is as follows. NH4VO3 was dissolved thoroughly and formed homogeneous aqueous solution through hydrothermal reaction. With the addition of abundant ethanol, the NH4VO3 recrystallized immediately because it is almost insoluble in ethanol. During this process, the vanadate ions self-assembled to form the butterfly-like crystals, and the ethanol might be used as the template for the hollow-structure

34

, as shown in Fig. S2, Supporting information. Further

crystallization was necessary for the formation of smooth crystals. Thus, the uniform butterfly-like NH4VO3 microcrystals were synthesized.

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In order to explore the influence of synthesis conditions on the morphologies of the products, three mainly parameters were discussed: the concentration of NH4VO3, the hydrothermal temperature and the volume of ethanol in the drowning-out crystallization process. The different synthetic conditions are summarized in Table S1 (Supporting information). Crystals with install dosages of 4 mmol and 6 mmol NH4VO3 were synthesized under the same conditions, and their SEM images are shown in Fig. S3, Supporting information. When the install dosage was 4 mmol, the whole crystal became lager with a length up to 70 µm. The hollow cylinder part became smaller and turned into a rhombus, while the flanking wing-like structure became wider with obvious angles. This reason for this phenomenon might be that lower concentration of vanadate ions was not enough to form enough crystal nuclei, therefore the remanent vanadate ions in the solution could only keep growing along the existing limited nuclei, resulting in bigger scale of the crystals. The crystals synthesized with 6mmol NH4VO3 retained the butterfly-like morphology. Nevertheless, the hollow cylinder part of some crystals, which is the most unstable structure of the whole crystal, was broken where new crystalline blocks grown up, which might be caused by the continued crystallization of higher concentration of vanadate ions. Hydrothermal temperatures also had great impact on the morphologies of the crystals. Increase or decrease of the temperature couldn’t lead to unique or regular morphologies (Fig. S4, Supporting information), which might because different hydrothermal temperature result in different species of polyoxovanadate

34

. The

moderate amount of ethanol is another necessary condition for the formation of the special morphology. When the volume of ethanol decreased to 40 mL, the product was basically a blend of rhombohedral and flower-like crystals (Fig. S5, Supporting information). With further increase of ethanol to 50 and 60 mL, the basic outline of the butterfly-like shape emerged gradually. Irregular structures appeared again when the volume of ethanol exceeded the moderate amount, reaching 80 mL. The above results suggest that optimal conditions were used in the synthesis of butterfly-like NH4VO3 microcrystals.

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Fig. 1. SEM images of the as-obtained butterfly-like NH4VO3 microcrystals.

Fig. 2. SEM images of the butterfly-like NH4VO3 microcrystals crystalized for 2 min.

It is worth noting that when the ethanol volume was 40 mL, crystals with a morphology similar to rhombus could be obtained, but there are still some flower-like crystals in the product. In order to synthesize uniform rhombohedral crystals, the crystallization temperature was reduced from 40 °C to 8 °C to lower the crystallization rate and prevent the residual vanadate ions in the solution from forming new crystals. The morphology of the obtained product is shown in Fig. 3. In this condition, homogeneous 8


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rhombus crystals were produced, with a length of 20 µm, a width of 7.5 µm and a thickness of 5 µm. According to the abovementioned suppositional mechanism, less amount of ethanol (40 mL) was only enough to precipitate part of the NH4VO3, while there wasn't sufficient ethanol to be the template for the hollow structure. Besides, high concentration of vanadate species remained in the solution would keep growing and cover the sulci on the surface during the aging processes at low crystallization temperature, and thus eventually formed rhombohedral crystals. The effect of the synthesis conditions to the rhombohedral morphology was also studied, and the related synthetic conditions are listed in Table S2 (Supporting information). Low concentration of NH4VO3 (4 mmol) resulted in irregular and larger scales of the rhombohedral crystals with lengths in the range of 40-75 µm (Fig. S6a, Supporting information), which is consistent with the phenomenon observed in the research of butterfly-like crystals. Increasing the dosage of NH4VO3 to 6mmol also couldn’t get uniform crystals (Fig. S6b, Supporting information). SEM images shown in Fig. S6(c, d) indicate that the morphologies of rhombohedral crystals had huge changes when the hydrothermal temperature changed. The product turned into unevenly sized plate-like crystals with a hydrothermal temperature of 80 °C, while many flower-like crystals appeared with a hydrothermal temperature of 180 °C, which might be related to the difference in the species of polyoxovanadates formed at different temperatures. Fig. S7 Supporting information shows the influence of the ethanol volumes on the products’ morphologies. 30 mL ethanol can also result in rhombohedral crystals, while their shapes and scales were not quite uniform. Besides, the reduced dosage of ethanol caused lower yield of the precipitated crystals. Excess amount of ethanol (50 mL) caused an outline of the butterfly-like morphology, only without the hollow cylinder part. When the amount of ethanol increased to 60 mL, almost all the crystals exhibited a butterfly-like morphology. This confirms the previous speculation that ethanol is the template for the formation of the butterfly-like morphology, and lack of ethanol led to a variation to rhombohedral morphology. Besides, Fig. S7d (crystallized at 8 °C) exhibits less flower-like crystals than that in Fig. S5c (crystallized at 40 °C), indicates that low crystallization temperature is advantageous for the inhibition of flower-like morphology, which is consistent with the previous speculation.

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Fig. 3. SEM images of the as-obtained rhombohedral NH4VO3 microcrystals.

It is worth noting that in the abovementioned experiments, both of the two products synthesized with a hydrothermal temperature of 180 °C (Fig. S4d and S6d, Supporting information) have flower-like crystals. According to the previous experiences, flower-like crystals tend to formed at high crystallization temperature, and excess amount of ethanol might result in butterfly-like morphology, so a new sample was synthesized with a hydrothermal temperature of 180 °C, 40 mL ethanol and a crystallization temperature of 40 °C, and the SEM images are shown in Fig. 4. The obtained crystals are basically flower-like, which consist of many little crystals, with overall diameters among 20-25 µm. Table S3 (Supporting information) summarizes different synthetic conditions and their corresponding results. The growth mechanism can be explained by the SEM of products synthesized with different amount of ethanol (Fig. S8, Supporting information). When the volume of ethanol increased to 50 and 60 mL, both flower-like morphology and butterfly-like morphology with broken middle part can be observed. Compared with the synthesis of butterfly-like morphology, insufficient amount of ethanol caused the unstable structure of the hollow cylinder, where is easy to broken and grow new crystals, especially under high hydrothermal temperature and high crystallization temperature. Fig S8b clearly shows the process of

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the growth of little crystals from the broken hollow and finally formed the flower-like morphology. While as little crystals grown randomly in all directions, each flower-like polycrystals is different. Higher concentration of vanadate ions is beneficial to the formation of new crystals, therefore increasing amount of NH4VO3 had little effects on the morphology of the product, as shown in Fig. S8d. To summarize, a new drowning-out crystallization method was developed to synthesize butterfly-like, rhombohedral and flower-like NH4VO3. Each of the morphologies was synthesized more than three times and this method showed good reproducibility. Factors that can affect the morphology of the crystals include the concentration of commercial NH4VO3, the hydrothermal temperature, the volume of ethanol and the crystallization temperature. According to this research, lower concentration of vanadium leads to less crystal nuclei and lager scale of the crystals; Polyoxovanadate produced at lower hydrothermal temperature tends to grown along xy plane; Lower crystallization temperature can slow down the crystallization rate and prevent the growth of new flower-like crystals; Ethanol not only serves as the antisolvent for the drowning-out crystallization process, but also plays a main role in the formation of the hollow cylinder of the butterfly-like structure during the self-assembling process. Insufficient ethanol caused incomplete hollow structure. As low hydrothermal temperature (100 °C) and crystallization temperature (8 °C) are beneficial to the crystallization of polyoxovanadate on xy plane, the rhombohedral morphology can be obtained at this condition. While high hydrothermal temperature (180 °C) and crystallization temperature (40 °C) are beneficial to the crystallization of polyoxovanadate on z axis, so the flower-like morphology was obtained. But further research is still needed to study the specific mechanism during this crystallization process.

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Fig. 4. SEM images of the as-obtained flower-like NH4VO3 microcrystals.

The XRD patterns of all the three products with different morphologies are shown in Fig. 5a. It can be observed that though synthesized under different conditions, all the products are pure NH4VO3 and their diffraction peaks can all be indexed to JCPDS No.25-0047, orthorhombic NH4VO3. Moreover, all the products are well-crystallized with high crystallinity. The FTIR spectrum was used to get further information about the synthesized crystals, and products with different morphologies showed similar FTIR curves. Fig. S9, Supporting information shows the FTIR spectrum of butterfly-like NH4VO3, which is almost the same with that of commercial NH4VO3. Peaks at 3495 and 1655 cm-1 are assigned to the O-H stretching vibration and H-O-H bending vibration of the water in the samples. The wavenumber of the H-O-H bending vibration is higher than usual because of the hydrogen bonds between water and amino

26

. Peaks at 3201 and 1417 cm-1 confirm the existence of NH4+ groups, corresponding to the

stretching vibration and symmetric bending vibration respectively 35. Peaks at 2929 and 2787 cm-1 are the characteristics of NH4+ groups, which are caused by the doubly and triply degenerate bending of N-H-N bonds. The peaks of symmetric and asymmetric vibration of VO2 groups and V–O–V bonds are located at 892 and 491 cm-1, 677 and 496 cm-1, respectively 36. All the peaks in the FTIR spectrum can be indexed

12


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to NH4VO3. EDS spectra of the butterfly-like NH4VO3 (Fig. 5b), rhombohedral NH4VO3 and flower-like NH4VO3 (Fig. S10, Supporting information) demonstrate that all the products are composed of V, O, N and C elements, among which C comes from the conducting resin used for the fixation of the powder products. Through the elemental mapping images shown in Fig. 5(c-f) and Fig. S11, Supporting information, it can be observed that V, O and N elements have homogeneous distribution in the crystals. All the above-mentioned measurements including XRD, IR and EDS confirm that all the crystallographic products with different morphologies are pure NH4VO3.

Fig. 5. Composition characterizations of the as-obtained NH4VO3: (a) XRD patterns of three products with different morphologies; (b) EDS spectrum and (c-f) A SEM image and its corresponding elemental mapping images of the butterfly-like NH4VO3.

3.2. Composition and Morphology of V2O5 with different morphologies The synthesized NH4VO3 with different morphologies were calcined at 400 °C for 4 h in ambient-air atmosphere to obtain pure V2O5. During this process, NH4VO3 decomposed into V2O5, ammonia, and water, as shown in Equation (2). Based on pervious experiences, 400 °C is the optimal temperature for the synthesis of V2O5 through the calcination of NH4VO3 because of the appropriate crystalline and porous structure of the product, which would lead to a higher capacity 37. 2NH4VO3 → 2NH3 + V2O5 + H2O 13


(2)


XRD patterns of the calcined products with different morphologies (Fig. 6a) indicate that all the products are well-crystallized, and their diffraction peaks can be indexed to JCPDS No.41-1426, orthorhombic V2O5. These three morphologies exhibited similar Raman, FTIR, EDS and XPS spectra, which were used to get further information about the structure and the composition of the calcined V2O5. The vibrational modes in Raman spectrum are characteristics of orthorhombic V2O5, as shown in Fig. 6b. The peaks located at 141 and 284 cm-1 can be assigned to B2g and Ag modes of bending vibrations of O– V=O. Peaks at 196 and 406 cm-1 can be indexed to the bending vibrations of V–O–V (Ag mode) of the O atoms at different locations. Peaks at 303 and 479 cm−1 are corresponding to the bending vibrations of V– O–V in B1g and B2u mode respectively. Peaks at 525 and 698 cm−1 are the characteristics of the Ag and B2g mode of V–O stretching vibration. The peak at 996 cm−1 is due to the symmetric stretching vibration of the V=O (Ag mode) 10, 38. FTIR spectrum is inserted in Fig. 6b. The peak at 1026 cm−1 confirms that the product is orthorhombic V2O5, which is due to the symmetric stretching vibration of the V5+=O bonds. The peak at about 820 cm−1 is related to the vibration of O–(V)3. Peaks among 610 and 475 cm−1 originate from the asymmetric and symmetric stretching vibration of V–O–V, respectively 39, 40. The EDS spectrum of the calcined product shown in Fig. S12, Supporting information reveals that the product consists of two elements V and O. As mentioned before, C comes from conducting resin in the test. The elemental mapping images of the three different morphologies are shown in Fig. 6(c-e) and Fig. S13 (Supporting information), indicating that both the V and O elements are homogeneously distributed in the sample. XPS measurement was used to further confirm the oxidation state of vanadium. Samples with three morphologies showed similar XPS spectra, as shown in Fig. S14, Supporting information. All the peaks can be assigned to V, O and C elements, among which C comes from the adsorbed carbon dioxide in the pores. The core-level XPS spectrum of V 2p3/2 shows binding energies at 517.2 eV, corresponding to V (+5) 41. All the above results of XRD, Raman, IR, EDS and XPS confirm that the products are pure V2O5.

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Fig. 6. (a) XRD patterns of the calcined products with different morphologies; (b) Raman spectrum of the butterfly-like V2O5, insert the IR spectrum; (c-e) A SEM image and its corresponding elemental mapping images.

SEM images of all V2O5 products with three different morphologies were taken to study the influence of the calcination to the morphologies of the products, as shown in Fig. 7. The three products still kept their original morphologies, though a fraction of the crystals had little breakage. While compared to the NH4VO3 crystals, which surfaces were quite smooth (Fig. S15, Supporting information), there were many pores and cracks on the surfaces of the calcined V2O5. The porous structure was formed because under high temperature in ambient-air, the NH4VO3 decomposed and released plenty ammonia. 20

During this process, NH4VO3 blocks were impacted and formed irregular pores

. N2

adsorption-desorption confirms the porous structure of the calcined V2O5. All the three morphologies have similar BET surface areas of about 10 m2/g. While compared with other two morphologies, rhombohedral V2O5 has the highest pore volumes in the tested range of pore diameters, as shown in Fig. S16, Supporting information. Higher pore volume is favorable for the accessibility of the electrolyte, rapid ion diffusion and storage, which might be beneficial for the electrochemical properties

15


42

. All the


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above characterizations prove that porous butterfly-like, rhombohedral and flower-like V2O5 crystals were successfully prepared.

Fig. 7. SEM images of the butterfly-like (a-c), rhombohedral (d-f) and flower-like (g-i) V2O5.

3.3. Electrochemical properties of the V2O5 with different morphologies To investigate the effects of the morphologies on the electrochemical properties of the as-obtained V2O5 crystals, CV, GCD and EIS measurements of the three as-obtained morphologies were taken on the three-electrode system. Fig. 8a shows the CV curves of the three products at a scan rate of 1 mV·s−1 in the potential range from -0.1 to 0.7 V. All the CV curves exhibit two pairs of prominent redox peaks, which are caused by the reversible faradaic redox reaction 43: V2O5 + xLi + xe- ⇌ LixV2O5 (0≤x≤1)

16


(2)

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The charge-storage mechanism in V2O5 is the intercalation/extraction of Li ions from the electrolyte 44

, which can be ascribed to the faradaic battery-type mechanism

45, 46

. The oxidation peaks of

rhombohedral, flower-like and butterfly-like V2O5 shift to higher potential respectively and their reduction peaks shift to lower potential, which is the characteristic of electrode polarization 47. Electrode polarization, also known as electrode overpotential, is the offset of a redox reaction's experimentally observed potential from its theoretically potential. There are mainly two kinds of electrode polarization, electrochemical polarizations and concentration polarization. Electrochemical polarization is related to the rate of the redox reaction. Slower redox reaction lead to higher degree of electrochemical polarization. Concentration polarization refers to the overpotential caused by differences in the concentration of ions between electrolyte and the electrode surface. When redox reaction is rapid than ion diffusion, the concentration of ions would be lower than that in electrolyte, so the reaction potential must be higher than the theoretically potential to facilitate the reaction. Meanwhile, the reaction current would also be limited by the ion diffusion and leads to lower specific capacitance. In this system, more complex structures lead to higher degrees of electrode polarization and lower peak currents, which means lower specific capacitances. As the active materials of all the electrode are pure V2O5, their electrochemical polarizations are similar. The differences of the electrode polarization mainly result from the concentration polarization arising from the different morphologies. It is easier for the electrolyte to reach the active surfaces when the structure is simple (rhombus), results in the lowest concentration polarization; while butterfly-like V2O5 with the complex hollow structure has the highest concentration polarization degree. GCD curves of all the three products exhibit non-line shape with two plats on each segment, which potentials correspond to the positions of the redox peaks in CV curves, as shown in Fig. 8b. The specific capacitances of rhombohedral, flower-like and butterfly-like V2O5 calculated from discharge curves based on equation (1) are 608, 544 and 483 F·g−1 respectively, which are consistent with the results of CV curves. This result can be explained by Nyquist plots of the three morphologies V2O5 shown in Fig. 8c. In low frequency region, rhombohedral V2O5 exhibits highest slope, indicating the fastest ion diffusion and enhanced capacitive behavior

3, 48

. Rhombohedral V2O5 also exhibits lower

equivalent series resistance (ESR, about 0.76 Ω) than that of flower-like V2O5 (about 1.17 Ω) and butterfly-like V2O5 (about 1.22 Ω), which is the intersection of the curve at x-axis

49

. As in the same

system, the electrolyte resistance and the contact resistance between the active material and the current collector are almost the same, lower ESR denotes lower intrinsic resistance of the active material.

17



Besides, the negative shift of the rhombohedral V2O5 indicates lower interfacial charge transfer resistance 48

. In the same frequency region, the semicircle of butterfly-like V2O5 is more obvious than that in

flower-like V2O5, denotes more complex pore structure, which is in agreement with the hollow cylinder structure of butterfly-like morphology observed in SEM images. All the products exhibit similar Warburg diffusion stage, which is related to efficient access for ion diffusion and can be observed by the length of 45° sloped portion between high and low frequency region 50. Porous structure of the obtained V2O5 leads to longer path for ion diffusion. The above analyses suggest that rhombohedral V2O5 exhibit the lowest electrode polarization and highest specific capacitance among the three morphologies due to its low ESR, low interfacial charge transfer resistance and high rate of ion diffusion. While the flower-like and butterfly-like V2O5 have more serious polarization and lower capacitance because of higher resistance caused by their complex structure.

Fig. 8. The comparison of the V2O5 with different morphologies: (a) CV curves collected at a scan rate of 1 mV·s−1; (b) GCD curves collected at a current density of 1 A·g−1; (c) Nyquist plots in the frequency ranging from 100 kHz to 0.01 Hz.

To further discuss the effect of morphology on the chemical performance of V2O5 crystals and evaluate the rate capability of the electrode, CV curves at different scan rates and GCD curves at various current densities were tested. CV curves of rhombohedral V2O5 (Fig. 9a) present good symmetry at all scan rates from 1 to 20 mV·s−1. The peak currents of both oxidation and reduction are almost the same and increase with the scan rate, demonstrating the rapid ionic transportation and good reversibility of the redox reactions at high scan rate

51

. The redox peaks still exist but shift only slightly because of higher

electrode polarization degree at high scan rates, indicating the good rate of ionic and electron conduction 52

. CV curves of butterfly-like V2O5 (Fig. S17a, Supporting information) also exhibit good symmetry at

18


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high scan rates, though the shape changes a little because of the electrode polarization. While flower-like V2O5 shows asymmetric CV curves at high scan rates, as shown in Fig. S17b, Supporting information. One oxidation peak and two reduction peaks can be observed and their peak currents are different, indicating its unsatisfactory rate capability, which can be further proved by the GCD measurement.

Fig. 9. (a) CV curves of rhombohedral V2O5 collected at different scan rates; (b) GCD curves of rhombohedral V2O5 collected at different current densities; (c) Specific capacitances of V2O5 with different morphologies calculated from GCD curves at different current densities; (d) Cycling performance of V2O5 with different morphologies collected at 100 mV·s−1.

GCD curves of V2O5 crystals with three different morphologies at various current densities are shown in Fig. 9b and Fig. S17c, d (Supporting information). The initial specific capacitance of rhombohedral V2O5 at 0.5 A·g−1 is 641 F·g−1, which is the highest among these three morphologies, also higher than most of the reported pure V2O5 like 3D V2O5 architectures 7; hollow V2O5 spheres

53

, and

comparable with many composites of V2O5 and rGO/CNT, as shown in Table 2. Furthermore, when the current density increases to 10 A·g−1, the capacitance can still remain 84.2% and reach 540 F·g−1 (Fig. 9c), 19



Page 20 of 30

confirming the excellent rate capability of rhombohedral V2O5. Though butterfly-like V2O5 achieves the lowest initial capacitance of 556 F·g−1 at 0.5 A·g−1 among the three products, it also exhibits excellent rate capability with a retention of 73.2% (407 F·g−1) when the current density increased to 10 A·g−1. While flower-like V2O5 can only remain 45.6% of the initial capacitance (609 F·g−1 at 0.5 A·g−1) at 10 A·g−1, which is consistent with the result of CV measurement. Though VOx normally suffer a sharp decline at high current density, these porous V2O5 microcrystals have excellent rate capabilities. Such good rate capability can be attributed to the low resistances and high rates of ion diffusion caused by the porous structures, as confirmed by EIS. On one hand, the pores formed from the released gas is interconnected, which can not only improve the area of electrode-electrolyte interface, but also shorten the pathway of ion diffusion. Thus, this porous architecture realized high efficient transportation and make up for the poor conductivity of V2O5 42. On the other hand, the destruction of the structure caused by severer volume expansion at higher current density is a main reason of the capacitance decline. While the porous structure of the obtained V2O5 formed during calcination process is quite stable and provide enough space for the volume expansion, leads to higher rate capability.

Table 2. Comparison of the electrochemical performance of V2O5 microcrystals with important literature reports. Types of V2O5 material

Electrolyte

Specific capacitance/F·g-1

Rhombohedral V2O5

1 M LiClO4 in PC

641, 0.5 A·g-1

Cyclic performance 119.8%

after

Reference 2000 this work

cycles

132.6% Butterfly-like V2O5

1 M LiClO4 in PC

after

2000

556, 0.5 A·g-1

this work cycles

Flower-like V2O5

1 M LiClO4 in PC

609, 0.5 A·g-1

70.4% after 2000 cycles

this work

3D V2O5 architectures

1 M Na2SO4

521, 5 mV·s-1

90% after 4000 cycles

7

V2O5 nanobelt arrays

1 M LiNO3

498, 1 A·g-1


17

Layer-by-layer V2O5 quadrate

1 M LiClO4 in PC

430, 0.1 A·g-1


20

Interconnected V2O5 network

0.5 M K2SO4

304, 0.1 A·g-1


54

Hollow V2O5 spheres

5 M LiNO3

479, 5 mV·s-1


53

V2O5/MWCNT aerogels

1M Na2SO4

450, 1 A·g-1

120 after 20000 cycles

55

2D V2O5 nanosheets and rGO

1M KCl

635, 1 A·g-1


56

mesoporous rGO-V2O5 hybrid

1M Na2SO4

466, 2 mV·s-1


57

20


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Surface-uneven V2O5

1 M LiClO4 in PC

380, 1 A·g-1


58

Porous V2O5 nanoparticles

1 M LiClO4 in PC

545, 1 A·g-1


10

V2O5⋅H2O nanowires

2 M KCl

349, 5 mV·s-1


24

Hollow spherical V2O5

5 M LiNO3

479, 5 mV·s-1


43

V2O5/ppy core/shell network

5 M LiNO3

448, 0.5 A·g-1


59

Notably, the cycling stabilities of these three products have similar trends with their rate capabilities, as shown in Fig. 9d. Both rhombohedral V2O5 and butterfly-like V2O5 have excellent cycling stabilities. The specific capacitances of rhombohedral V2O5 and butterfly-like V2O5 increase gradually and reach up to 134% and 139% of their initial capacitances respectively after 200 cycles due to the active process. This phenomenon is common in porous materials and can be attributed to the increasing wettability of the active material during the cycling, which leads to larger electrode-electrolyte interface and higher utilization of active materials

60-63

. As there is more complex pore structure in butterfly-like V2O5, it has

higher upward potential. Then the capacitances of rhombohedral V2O5 and butterfly-like V2O5 represent slightly fluctuating decline, but still have extremely high retentions of 119.8% and 132.6% after 2000 cycles and 109.7% and 127.4% after 3000 cycles (Fig. S18, Supporting information). The poor cycling stability of VOx normally caused by the dissolution of active material in aqueous solutions and the structure degradation during the intercalation/deintercalation of Li+ in the charging/discharging process. Previous research confirmed that organic electrolyte used in this research can significantly reduce the dissolution without sacrifice the capacitance 58. The pores formed during long-time calcination are stable and can provide space for the volume expansion. Besides, the crystals of these two morphologies are whole and well crystallized, so the microstructures also have high stability, thus the structure degradation during cycles can be avoided. Whereas the capacitance of flower-like V2O5 only increases slightly to 102% of the initial capacitance, then suffers a decline to only 70.4% in 2000 cycles and has a retention of 60.9% after 3000 cycles. As the dissolution of V2O5 in organic electrolyte is slight, the lower cycling stability of flower-like V2O5 is due to the structure degradation 3. The flower-like microcrystal consists of minor crystals grown irregularly, so the whole structure might be less stable then the other two morphologies, which can be further proved by the SEM images of the electrode after cycles, as shown in Fig. S19, Supporting information. It can be observed that the morphologies of both rhombohedral and butterfly-like microcrystals were well maintained after cycles. While some of flower-like microcrystals disintegrated 21



into disconnected little blocks and lost their original structures, confirming the instability of flower-like structures is the main reason of its worse cycling stability compared to the other two morphologies. The unstable structures of flower-like V2O5 might also be the reason of its unsatisfactory rate capability. Based on the above discussion, in this system, the complex structure of the microcrystals leads to higher degree of electrode polarization, higher resistance and lower specific capacitance. The structure stability of the crystals is related to their rate capability as well as the cycling performance.

4.

Conclusion In summary, a new strategy was developed to synthesize porous butterfly-like, rhombohedral and

flower-like V2O5 microcrystals through the calcination of NH4VO3 with different morphologies in ambient-air. The morphology of NH4VO3 crystals can be controlled through the drowning-out crystallization process of the hydrothermal aqueous solution using ethanol as both the antisolvent and template. The influences of concentration of vanadium, hydrothermal temperature, amount of ethanol and the crystallization temperature were studied and the possible crystallization mechanism was proposed. The relations among morphologies of microcrystals and their electrochemistry properties as battery-type electrodes for supercapacitors were studied. The porous rhombohedral, butterfly-like and flower-like V2O5 microcrystals exhibited high capacitances of 641, 556, 609 F·g−1 at 0.5 A·g−1 a respectively, which is related to their structure complexities. In this system, the variation of the rate capabilities (84.2%, 73.2% and 45.6% retention at 10 A·g−1of rhombohedral, butterfly-like and flower-like V2O5) is almost consistent with the variation of their cycling stabilities (remained 119.8%, 132.6% and 70.4% after 2000 cycles), which are both determined by stabilities of crystals. The synthetic process in this work is facile, environment-friendly and low cost, and the products especially rhombohedral V2O5 exhibited excellent electrochemical performance, making them promising battery-type electrode materials for supercapacitors. This work not only studied the relations among morphologies and electrochemical properties, which can be the guidance for the design of new morphologies, but also provides an extensible strategy through recrystallization for the morphology-controlled synthesis of metal oxides. Acknowledgement

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This work was partially supported by the National Natural Science Foundation of China (Grant No. 21601026, 21771030), Fundamental Research Funds for the Central Universities (DUT16RC(4)10), and Doctoral Research Foundation of Liaoning Province (201601035). Supporting Information SEM image of the commercial NH4VO3 (Fig. S1); Suggested formation mechanism of the hollow-structure under the templating effect of ethanol (Fig. S2); SEM images of the microcrystals synthesized with different conditions (Fig.S3 - Fig.S8); FTIR spectra of commercial NH4VO3 and the synthesized butterfly-like NH4VO3 (Fig. S9); EDS spectra, SEM images and their corresponding elemental mapping images of rhombohedral and flower-like NH4VO3 microcrystals (Fig.S10 - Fig.S11); EDS spectrum of the butterfly-like V2O5 (Fig. S12); SEM images and their corresponding elemental mapping images of rhombohedral and flower-like V2O5 microcrystals (Fig.S13); XPS spectra of the typical V2O5 sample (Fig.S14); FE-SEM images of NH4VO3 microcrystals (Fig.S15); Pore size distribution data of V2O5 (Fig.S16); Electrochemical performance and cycling performance of V2O5 (Fig.S17 - Fig. S18); SEM images of electrodes before and after cycles (Fig.S19); The different synthetic conditions of NH4VO3 microcrystals and their corresponding morphologies (Table 1-3). Conflicts of interest There are no conflicts of interest to declare. References (1) Yan, Y.; Li, B.; Guo, W.; Pang, H.; Xue, H., Vanadium based materials as electrode materials for high performance supercapacitors. J. Power Sources 2016, 329, 148-169. (2) Zhang, Y.; Zheng, J.; Jing, X.; Meng, C., A strategy for the synthesis of VN@C and VC@C core-shell composites with hierarchically porous structures and large specific surface area for high performance symmetric supercapacitors. Dalton Trans. 2018, 47, 8052-8062 (3) Yu, M.; Zeng, Y.; Han, Y.; Cheng, X.; Zhao, W.; Liang, C.; Tong, Y.; Tang, H.; Lu, X., Valence-optimized vanadium oxide supercapacitor electrodes exhibit ultrahigh capacitance and super-long cyclic durability of 100 000 cycles. Adv. Funct. Mater. 2015, 25, 3534-3540.

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V2O5/polypyrrole

network

nanostructures

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supercapacitors. J. Mater. Chem. A 2015, 3, 488-493.

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New strategy for the morphology-controlled synthesis of V2O5 microcrystals with enhanced capacitance as battery-type supercapacitor electrodes Jiqi Zheng, Yifu Zhang*, Tao Hu, Tianming Lv, Changgong Meng

A facile route was developed to prepare porous butterfly-like, rhombohedral and flower-like V2O5 microcrystals by the drowning-out crystallization of hydrothermal NH4VO3 aqueous solution and subsequent calcination. The porous V2O5 with different morphologies displayed high capacitances and excellent cycling stabilities. The effects of morphologies on the electrochemical performances were analyzed.

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New Strategy for the Morphology-Controlled Synthesis of V2O5

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