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Hydrothermal synthesis and electrochemical properties of 3D Zn2V2O7 microsphere for alkaline rechargeable battery
Journal of Power Sources 439 (2019) 227087
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Hydrothermal synthesis and electrochemical properties of 3D Zn2V2O7 microsphere for alkaline rechargeable battery Qi Zhang, Qiqi Shi, Hongdong Li, Zhenyu Xiao, Kun-Peng Wang, Lingbo Zong, Lei Wang * State Key Laboratory Base of Eco-chemical Engineering, Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Three-dimensional Zn2V2O7 micro sphere covered by nanoparticles. � Excellent capacity and good cycle sta bility in alkaline rechargeable battery. � High energy density for Zn2V2O7//AC battery-supercapacitors hybrid device. � High performance results from unique structure and synergistic effect.
A R T I C L E I N F O
A B S T R A C T
Keywords: Zinc vanadate Alkaline rechargeable battery Energy storage
In this work, 3D microsphere Zn2V2O7 is obtained by calcined the precursor Zn3(OH)2V2O7⋅2H2O. The 3D structure of Zn2V2O7 inherits from the precursor which is composed by layered structures with nanoparticles fully covered. The unique structure endows it higher specific surface area and porous structure. Benefitting from the structure stability and the synergistic effect of the mixed metal oxides, the Zn2V2O7 electrode shows high specific capacity of 84.8 mAh g 1 at 1 A g 1, good rate capability and long cycle life. In addition, the batterysupercapacitors hybrid device is assembled with Zn2V2O7 as positive electrode and AC as negative electrode. The device shows high cycling stability with 89.8% retention after 6000 cycles and maximum energy density of 34.99 Wh kg 1 at 850 W kg 1. The high specific capacity and energy density suggest that 3D Zn2V2O7 micro sphere electrode material is suitable for application in the field of alkaline rechargeable battery.
1. Introduction Recently, growing global energy demands and the environmental pollution resulted from the extensive use of the fossil fuels have pro moted the development of clean, efficient, sustainable green energy source [1–4]. The wind, solar, tidal and hydroelectric power are alter native energy sources with endless supplies, how to store the energy is
still a challenge [5]. As the most promising energy storage devices, supercapacitors (SCs) and lithium-ion batteries (LIBs) have drew worldwide attention and become the dominant power sources in portable electronic device [6–14]. However, the sluggish kinetic of LIBs and low energy density of SCs limit their wide application in large scale grid energy storage [15,16]. Alkaline rechargeable batteries (ARBs) can be bridged the gap between LIBs and SCs due to the high energy density
* Corresponding author. E-mail address: [email protected] (L. Wang). https://doi.org/10.1016/j.jpowsour.2019.227087 Received 1 February 2019; Received in revised form 28 June 2019; Accepted 29 August 2019 Available online 5 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 439 (2019) 227087
and power density, superior rate capacity, fast surface reaction [17–20]. Therefore, much attention have been paid on the development of ARBs to obtain the electrode with high electrochemical performance. The ARBs can store charge by the reversible faradic reaction which occurs in the surface and near-surface electrode materials in alkaline electrolyte solution [21–23]. Based on the storage mechanism, transi tion metal oxides (TMOs) such as MnO2, Co, Ni-based oxides/hydroxides have emerged as electrode materials for ARBs in the past decades due to their high theoretical capacity, abundant redox centers and environ mental benignity [18–20,24,25]. Vanadium oxides have been widely investigated as electrode materials in alkaline electrolyte because of their unique layered structure, rich oxidation states and low cost [26–29]. Various V2O5 nanostructures have been synthesized and served as electrode materials in alkaline electrolyte, the electrochemical per formance shows that the specific capacitance of ca. 300–600 F g 1 and capacitance retention of 6%–132% after 100–2000 cycles [30–32]. However, many drawbacks such as the toxicity of V2O5, the poor elec trical conductivity of single metal oxides and the expand volume during in charge-discharge process make them unable to meet the needs in modern society [33]. In order to solve these problems, mixed transition metal oxides (MTMOs) which contain different metal cations are intro duced. The complex chemical composition and synergistic effect of the mixed metal endow them outstanding electrochemical performance, which make them promising candidates for ARBs [34]. Metallic vanadates exhibit outstanding electrochemical properties due to the various valence states of vanadium element, wide operating potential window and the unique crystal structure [35–38]. Therefore, many vanadate compounds have been synthesized and used in optical device, lithium-ion batteries (LIBs) and photocatalytic fields [39–41]. Among them, zinc vanadium oxides are promising candidates for energy storage due to the low cost and high conductivity of zinc and the mul tiple valence states of vanadium [42]. Butt et al. synthesized ZnV2O4 glomerulus microspheres and investigated the potential application in hydrogen storage [43]. Liu et al. reported hollow flower-like Zn2V2O7 electrode material for LIBs with high cycle stability and capacity of 882.5 mAh g 1 after 100 cycles [44]. Sun et al. reported ZnV2O6 nanobelts as Na-ion battery material with high capacity of 480.5 mAh g 1 and cycle stability of 51.4% capacity retention after 100 cycles [45]. The Zn3V2O8 nanoplates electrode material reported by Ryu et al. showed a maximum capacitance of 302 F g 1 and high cycle life in KOH electrolyte [46]. Liu et al. reported Zn2V2O7 nanowires which synthe sized by microwave-assisted method and showed high capacitance reached up to 427.7 F g 1 at 1 mA cm 2 [47]. Kim et al. synthesized Zn2V2O7 nanopowders and mixed with multi-walled carbon nanotubes as the electrode of SCs, which showed excellent cycle stability [48]. To the best of our knowledge, 3D microsphere Zn2V2O7 served as the electrode material for ARBs has not been reported and much effort should be paid on the zinc vanadates. In this work, we report the synthesis of 3D microsphere Zn2V2O7 by a hydrothermal method followed by annealing treatment. SEM and TEM images show the Zn2V2O7 inherites the 3D skeleton from the precursor Zn3(OH)2V2O7⋅2H2O, and the 3D microsphere Zn2V2O7 is constructed by layered structure with nanoparticles fully covered. The unique structure of Zn2V2O7 shows the following advantages for ARBs. Firstly, the 3D skeleton is assembled by 2D nanosheets and the nanosheets are covered with 0D nanoparticles, so the material owns the advantages of 0D, 2D, 3D structure: high specific surface area, excellent electron and ionic conductivity and stability. Secondly, the synergistic effect of the mixed oxide improved the comprehensive electrochemical performance. As a consequence, the prepared Zn2V2O7 microsphere electrode pos sesses high specific capacity of 84.8 mAh g 1 and capacity retention of 91.1% after 6000 cycles. The battery-supercapacitor hybrid (BSH) de vice also exhibits remarkable energy density of 34.99 Wh kg 1 at the power density of 850 W kg 1. Moreover, the BSH device shows excellent cycling stability with capacity retention of 89.8% after 6000 cycles, which is higher than the most reported vanadates materials.
2. Experimental 2.1. Synthesis of Zn2V2O7 All of the reagents used in the experiments were of analytical grade and used directly without further purification. Typically, 1.0 mmol NH4VO3 was dissolved in 15 mL deionized water and 20 mL ethylene glycol (EG) in 100 mL beaker to form a homogeneous solution, named A solution. Meanwhile, 1.0 mmol Zn(CH3COO)2 and 1 μmol polyethylene glycol 2000 (PEG 2000) were dissolved in 5 mL deionized water with stirring for 20 min to dissolve completely, called B solution. Under vigorous stirring, the solution A was slowly added to the solution B dropwise. The obtained mixture was transferred into a Teflon-lined stainless-steel autoclave and heated at 160 οC for 24 h. After the solu tion cooled down to room temperature naturally, the sample was washed and centrifuged with deionized water for several times, and then dried in vacuum oven at 60 οC overnight. At last, the obtained sample was calcined at 450 οC for 5 h at a heating rate of 1 οC min 1 and the final products of Zn2V2O7 was obtained. In comparison, V2O5 nanosheets were also synthesized by a hydrothermal method according to literature [49]. 2.2. Characterization The X-ray diffraction (XRD) was used to examine the crystallinity and phase purity of the products on a Bruker-D8 Advance diffractometer using Cu Kɑ radiation (λ ¼ 1.5406 Å) with operation voltage of 40 kV. The surface morphology and elemental composition of the products were characterized using a field-emission scanning electron microscope (FESEM, JEOL JSM-7500F) and energy dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) images were performed using JEOL-2010 with an accelerating voltage of 200 kV under 91 μA. The BET surface area was investigated by micrometrics ASAP 2020 nitrogen adsorption/desorption experiments. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with Mg-Kα excitation source were used to analyze the surface element composition. 2.3. Electrochemical measurement in a three-electrode system All the electrochemical measurements were carried out using a CHI760E electrochemical working station (Chenhua, Shanghai). In the traditional three-electrode system, the platinum foil and Ag/AgCl elec trode were served as the counter electrode and reference electrode, respectively, while the modified nickel foam (NF) electrode as the working electrode. Electrochemical performance was carried out in 3 M KOH electrolyte to study the electrochemical behavior of the Zn2V2O7 electrode. Cyclic voltammetry (CV) was measured with different scan rates at a potential window of 0–0.6 V. Galvanostatic charge–discharge (GCD) curves were obtained with the potential window of 0–0.45 V at various current densities, and electrochemical impedance spectroscopy (EIS) experiments were conducted at a frequency range from 100 kHz to 0.01 Hz at the open circuit potential of 0.179 V. 2.4. Fabrication of BSH device The BSH device was assembled by using an activated carbon (AC) on nickel foam (NF) (1 cm � 2 cm) as the negative electrode and Zn2V2O7 on NF as the positive electrode in 1 M KOH as the electrolyte. The working electrodes Zn2V2O7//NF were prepared by grinding Zn2V2O7 (70 wt%), acetylene black (20 wt%), polytetrafluoroethylene (PTFE, 10 wt%), and a few drops of acetone to form a homogeneous slurry. Then the slurry was coated onto NF. The NF was dried at 80 � C for 12 h under a vacuum circumstance. The AC//NF electrode was prepared by the similar process, but the homogeneous slurry contains AC and PTFE with a weight ratio of 9:1 in acetone. The loading weight of the electrode was depended on the mass difference of the NF before and after coating 2
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active materials. The mass loading of Zn2V2O7 is about 2 mg, and the AC is about 3.5 mg.
attributed to the cross-link of the thin nanosheets to form 3D architec ture. The HRTEM image of the precursor in Fig. 3b confirms the for mation of the layered structure, and a representative lattice fringe of 0.262 nm matches well with the (020) plane of the Zn3(OH)2V2O7⋅2H2O (PDF no# 50–0570). Fig. 3c and d show the low and high magnificent TEM images of Zn2V2O7. The different shades of color area indicates the formation of 3D microstructure, which composed by numerous nano particles. The shallow area in Fig. 3c represents the stack of nano particles while the transparent area suggests the space between nanoparticles, which provides path for electrolyte ions and electronics. Fig. 3c also shows the nanoparticles with uniform size about 30 nm. In the nanoparticles area of Fig. 3d, the lattice fringe of 0.241 nm with high crystallinity results from the ( 114) plane of Zn2V2O7, which further demonstrates the formation of Zn2V2O7. In addition, the EDS spectra and EDS mapping analysis are performed to verify the elements distribution of the Zn2V2O7. As shown in Fig. 3e, the sample is composed of Zn, V, O with molar ratio of 23 : 19: 57. Fig. 3f shows three colors represented Zn, V, O respectively distribute throughout the region, which indicates the homogeneous distribution of the three elements on the whole microsphere. Considering the surface charge storage mechanism of the ARBs, the surface oxidation state and elemental composition are characterized by XPS. As shown in Fig. 4a, the complete spectrum of Zn2V2O7 is obtained, which contains typical Zn 2p, V 2p, O 1s and C 1s regions. The Zn 2p region in Fig. 4b includes two peaks at 1021.39 and 1044.39 eV, the former is assigned to the Zn 2p3/2 and the latter is Zn 2p1/2. As shown in Fig. 4c, two peaks located at binding energies of 530.1 and 531 eV are observed in the high resolution spectrum of O 1s, which implies two kinds of O bind environments in the Zn2V2O7. The peak at 530.1 eV named O1 can be attributed to the typical metal-oxygen (V-O) bond, while the 531 eV named O2 may come from the surface adsorbed oxygen species such as H2O. For the high resolution V 2p spectrum, in Fig. 4d, the peak at 517.42 and 524.84 eV can be ascribed to the V5þ 2p3/2 and V5þ 2p1/2, respectively, while the small peak at 516.7 eV is assigned to the V4þ 2p3/2, indicating the mixed oxidation states of V element on the surface of Zn2V2O7. The integrate area of the V element suggests the V5þ are predominant over V4þ. The XPS analysis demonstrates the compo sition and oxidation state of the element in Zn2V2O7, which is consistent with XRD and EDS.
3. Result and discussion 3.1. Characterization of the products The synthesis procedure for Zn2V2O7 contains two steps: a hydro thermal process to obtain the precursor and then calcine the precursor in 450 � C for Zn2V2O7. XRD analyses are performed to verify the crystalline of the obtained samples. The XRD pattern of the precursor is shown in Fig. 1a, all the diffraction peaks are in good agreement with the monoclinic phase of Zn3(OH)2V2O7⋅2H2O (JCPDS card file no. 50–0570), and no other impurity peaks exist. Fig. 1b shows the XRD pattern of Zn2V2O7, after calcined the precursor, the three ultra-strong diffraction peaks locate at 16.7� , 28.6� , 28.7� can be definitely indexed to the ( 111) (022) ( 113) lattice planes of the Zn2V2O7 (JCPDS card file no. 29–1396), and other remaining peaks in Fig. 1b are also indexed to the standard card with no other reflection peaks are detected. In addition, Fig. 1b shows all the peaks with sharp and narrow Full Width at Half Maximum (FWHM), indicating the high purity and crystallinity of Zn2V2O7. The morphologies of the obtained samples before and after calcined are characterized by SEM. As shown in Fig. 2a–c, the images of the precursor with different magnifications indicate that the diameter of the formed microsphere is distributed 2–4 μM. The microsphere conducted by layered structures shows smooth surface and uniform thickness of 40 nm. The images of Zn2V2O7 are shown in Fig. 2d–f, after calcined in 450 � C, the 3D microsphere structure is still remained, suggesting the high stability of the structure in high temperature. Fig. 2e shows the 3D skeleton is composed by 2D nanosheets with rough surface and porous structures. The cross-link between 2D nanosheets provides path for ions and electrons. The high magnification SEM image in Fig. 2f displays that the nanosheets are covered by numerous nanoparticles. The porous and rough structure is formed by the release of H2O and the transformation of crystalline under high temperature, during this process, Zn3(OH)2 V2O7⋅2H2O microspheres are impacted and formed porous structures. Compared with the smooth surface of the precursor, this rough surface owned higher specific surface area to load more active sites. N2 ab sorption/desorption measurements are conducted to investigate the structure change before and after calcined (Fig. S1). The specific surface area of the as-prepared products is analyzed by BET surface area mea surement, and the value increases from 11.9 m2 g 1 to 19.4 m2 g 1 after calcined, which is in consistent with the SEM results. The TEM figures are utilized for further investigation of the detailed structure of the precursor and sample. The TEM image of the precursor can be seen in Fig. 3a, a shallow color in the layered areas reveals the sheet-like structure, while the dark area among them is mainly
3.2. Electrochemical capacitive properties of the Zn2V2O7 electrode To evaluate the advantage of the prepared material for potential application in energy storage, the Zn2V2O7 as the electrode material is carried out in three-electrode system. We make a comparison of CV curves between naked NF and Zn2V2O7 electrodes at a scan rate of 30 mV s 1 in the potential window range of 0–0.6 V. As shown in Fig. 5a, as the substrate material, the CV curve of the naked NF is nearly a
Fig. 1. XRD patterns of (a) the precursor Zn3(OH)2V2O7⋅2H2O and (b) Zn2V2O7 material. 3
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Journal of Power Sources 439 (2019) 227087
Fig. 2. SEM patterns of (a–c) the precursor Zn3(OH)2V2O7⋅2H2O and (d–f) Zn2V2O7 material.
straight line, and no obvious peak current is observed, while the Zn2V2O7 electrode shows a pair of redox peaks with large current, which indicates the capacity of the Zn2V2O7/NF almost derives from the contribution of Zn2V2O7 material. The CV profiles of Zn2V2O7 electrode have acquired at different scan rates from 5 to 100 mV s 1 are shown in Fig. 5b. Increasing the scan rate led to peak current increase, and the redox peaks in the window range are clearly retained and the shape shows no obvious change in the high scan rate, which suggests the good charge/discharge property and rate capability, the obvious redox peaks and the slightly shift of peak potential with the increasing sweep rate suggest the typical battery-type material. The redox peaks between 0.1 and 0.4 V are assigned to the faradaic reaction of V5þ/V4þ, which may resulting from the intercalation and deintercalation of Kþ from elec trolyte solution. XPS analysis were used to better understand the insertion mechanism (Fig. S2), in the discharge state, the peak at 524.8 eV which corresponds to of V 2p1/2 disappears, while the remaining V 2p3/2 peak is deconvoluted into two peaks at 517.6 eV and 516.8 eV, which correspond to V5þ 2p3/2 and V4þ 2p3/2, respectively. Compared with the Zn2V2O7 before charge/discharge process, the comparable integrate area of these two peaks indicate the nearly same amount of V5þ and V4þ. That’s reasonable, in the discharge state, reduction reaction occurs in the positive electrode Zn2V2O7, the V5þ accepting electrons and reduce to V4þ. The possible reaction mechanism is show in Eq. (1) [47]: Zn2V2O7 þx Kþ þxe ↔KxZn2V2O7
capacity of the electrode material for ARBs. Fig. 5c presents the typical GCD curves at different current densities, the nonlinear shape from 0 to 0.45 V with a voltage plateau indicates the typical battery behavior of the electrode. Based on the data from GCD curves, the specific capacity as a function of the current density is shown in Fig. 5d. The highest specific capacity reaches up to 84.8 mAh g 1 at 1 A g 1, with the current density increased to 10 A g 1, the discharge time decreases gradually, and the corresponding specific capacity values are 69.0, 56.7, 50.0, 45.3, 41.6 mAh g 1, about 50% of the initial specific capacity is retained, implying the good rate capability of the electrode. Fig. S3 shows the contribution fraction of the capacitive and diffusion-control process, we find that most of the current contribution comes from the diffusion-limit storage. The result also confirms the battery-type behavior due to the insertion/deinsertion of Kþ. The high specific ca pacity of 3D Zn2V2O7 microsphere may originate from the 2D nano sheets with thin thickness, the fully covered nanoparticles and high percentage of the exposed active sites. The unique structure may provide high specific surface area to store more charge, while the thin thickness and the cross-link of the layer structure offer short path for ions and electrons, which can benefit for ions transport rate. Long-term cycling stability of the electrode is a crucial parameter for the application in ARBs. The cycle stability of the Zn2V2O7 electrode is carried out by repeating the GCD test at 8 A g 1 for 6000 cycles (Fig. 5e). It can be observed that the specific capacity of first cycle was 48.7 mAh g 1, the value retention gradually decreases over the initial 360 cycles, after which, the capacity retention remains a constant value upon the repeated cycling to 6000 cycles. The initial decrease of the specific
(1)
The GCD tests can be used to accurately determine the specific 4
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Fig. 3. TEM images of the precursor Zn3(OH)2V2O7⋅2H2O at (a) low and (b)high magnification; TEM images of sample Zn2V2O7 at (c) low and (d) high magnifi cation; (e) The EDS spectra with molar ratio of elements; (f) EDS mapping of Zn2V2O7.
capacity may be caused by the loss of the material during the repeat cycling. The specific capacity of the electrode is 44.35 mAh g 1 in the last cycle, corresponding to 91.1% of its initial capacity, which dem onstrates the excellent cycling stability during the repeat chargedischarge cycling process. The long cycle life may be caused by the stability of the 3D skeleton, which suffers from the high temperature without decomposition. In addition, volume expansion of the metal oxide material is inevitable during the repeat GCD process, but the interspace between nanoparticles may provide enough space for them. In order to further understand the conductivity of the electrode material, the EIS measurement is performed. The Nyquist plots of Zn2V2O7 electrode before and after 6000 cycles are shown in Fig. 5f, in the high frequency, the intercept of the X-axis before the semicircle represents the equivalent series resistance (ESR), which contains intrinsic resistance of the electro-active material, electrolyte resistance and interface contact resistance [50]. The after cycling material exhibits lower ESR (0.41 Ω) than the before material (0.6 Ω). Considering in the same system, the electrolyte resistance, intrinsic resistance and current collector are same, the lower ESR donates a lower contact resistance between the material and electrolyte, which may be caused by the increased wettability during the charging/discharging process. In the same frequency region, we can figure out that the material after cycling exhibits a semicircle with small diameter (5.1 Ω), in contrast, the same sample before GCD cycle shows no obvious semicircle. The low charge
transfer resistance (Rct) at the interface of the electrolyte and electrode indicates the large electro-active surface area of the electrode. The synergistic effect between Zn and V elements may explain the low resistance. The electrochemical performance of V2O5 nanosheets (Fig. S4) syn thesized by a facile hydrothermal method are also tested to investigate the synergistic effect of the mixed metal oxide. In the control experi ments, the 3D Zn2V2O7 microsphere is insteaded by the V2O5 nanosheets as the electrode material with the same experimental condition. The CV curves with different scan rates in the potential range of 0–0.6 V are shown in Fig. S5a, obvious redox peaks are obtained in the low scan rates, and when the scan rate exceeded to 60 mV s 1, the redox peaks display noticeable shift and then disappear quickly, which implies the poor reversibility and low rate capability. The obvious plateau in the GCD curves (Fig. S5b) indicates the typical battery-type behavior, which is consistent with the CV curves. The specific capacities calculated from the GCD curves are shown in Fig. S5c, high specific capacity of 67.5 mAh g 1 is achieved at a current density of 1 A g 1, and the value decreases to 23.9 mAh g 1 at 10 A g 1, the 35.4% of the capacity retention implies the low rate capability. Then the cycle stability of V2O5 is tested at 10 A g 1 in Fig. S5d, 36.4% deterioration of its initial specific capacity is obtained after 1000 cycles, suggesting the poor stability. The notable weaker electrochemical performance than Zn2V2O7 may be due to the poor conductivity of the single metal oxides (Fig. S6). 5
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Fig. 4. (a) XPS survey of Zn2V2O7.(b–d) High resolution of Zn 2p, O1s and V 2p.
curves of the Zn2V2O7 and AC electrode at a scan rate of 30 mV s 1 in three-electrode system show the potential widow range of Zn2V2O7 and AC are 0–0.6 V and 1.0–0 V, respectively. The data indicates the continuous and complementary potential window of the two materials. Secondly, the specific capacity of the two materials should be equiva lent. Considering the Zn2V2O7 electrode and AC electrode own different specific capacity and potential window, balance the mass ratio of the electrode materials is important. In a detailed process, the GCD curves of the two materials is conducted at the same current, and the discharge time ratio of the two materials is mass ratio, which is calculated by the charge balance theory, according to the Eq. (2) as follows [55]:
The above results demonstrate that Zn2V2O7 electrode material owns high capacity, excellent cycling stability and good rate capability, and the electrochemical performance is better or comparable with other previous reported vanadium oxide-based electrodes (Table 1). The good performance can be attributed to the following two aspects: (1) Struc ture advantage. The 3D microsphere structure constructed by 2D layered structure, and the layered structure covered by numerous nanoparticles. The structure inherits the properties of 2D layered structure and 0D nanoparticles, which endow it high specific surface area and low diffusion path. In addition, the structure of Zn2V2O7 microsphere formed by the calcination process and constructed by nanoparticles, the spaces between nanoparticles not only improve the specific surface area to store more charge, but also provide space for the volume expansion during the charge/discharge process; (2) Component advantage. The synergistic effect of the mixed metal oxides is the key to high electro chemical performance. V element provides various oxidation states while Zn improves the high electrical conductivity, therefore, the 3D Zn2V2O7 microsphere shows enhanced capacity and durability.
mþ = m ¼ C V =Cþ V þ
(2)
here mþ and m- (g) represent the mass of positive and negative material, Cþ and C- (F g 1) are the specific capacity of the materials at the same current in the three-electrode system, and Vþ and V- (V) is the discharge voltage range of the individual electrode. The optimum mass ratio of Zn2V2O7 and AC is calculate 2:3.5. Fig. S8a and Fig. S8b show the GCD curves of the two electrodes with optimum mass ratio at the same cur rent of 4 mA. The discharge time is 224 s and 226 s, respectively. The nearly same discharge time at same current suggests the two electrodes can be assembled to BSH device. The electrochemical performance of the BSH device based on Zn2V2O7 and AC is conducted. Firstly, the stable voltage window of the BSH device is studied by CV curves with increased potential window form 0–1.4 V to 0–1.8V. As shown in Fig. 6a, the representative CV curves of the BSH are investigated at 30 mV s 1 with different voltage windows, increased potential window brings bigger closed area, indi cating the higher capacity. However, obvious polarization peak is observed when the potential window up to 0–1.8 V, which is attributed to the decomposition of the electrolyte. Therefore, the stable potential
3.3. The electrochemical properties of the Zn2V2O7//activated carbon BSH device To further evaluate the practical application of the prepared 3D microsphere Zn2V2O7 electrode, BSH device is assembled with Zn2V2O7 as the positive and the AC as the negative electrode. As we all know, two conditions should be met for the two materials assembled into BSH device. Firstly, the potential range of the two materials should be complementary, that is, the potential window of them have one inter section at least. In order to understand the feasibility of the positive and negative material in BSH device, the CV curves and the capacity matching experiments are carried out. As shown in Fig. S7, the CV 6
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Fig. 5. The electrochemical performance of Zn2V2O7 electrode. (a) CV curves of Zn2V2O7 electrode and NF electrode at 30 mV s 1; (b) CV curves at different scan rates; (c) GCD curves at different current densities; (d) Specific capacity calculated from GCD curves; (e) Cycling performance at 8 A g 1; (f) Nyquist plots in the frequency ranging from 0.01 to 100 kHz.
window for this BSH device was 0–1.7 V. To evaluate the value of the rate capability and specific capacity the of the BSH device, Fig. 6b shows the CV curves at different scan rates with potential window of 0–1.7 V. With the increasing of the scan rate, the closed area of the curves increase while the shape of the curves re mains unchanged, suggesting the high rate capability. Furthermore, the GCD curves of the BSH device at different current densities from 1 A g 1 to 10 A g 1 are shown in Fig. 6c, all the GCD curves are symmetric in shape during in the charge and discharge process, and the discharge curves are nonlinear, indicating the existence of pseudocapacitance. The specific capacity from the GCD curves is calculated and shown in Fig. 6d. The values reach to 41.2, 35.6, 30.7, 28.2, 25.3, 23.0 mAh g 1 at 1, 2, 4, 6, 8, 10 A g 1, respectively. The device shows 55.9% retention of its initial capacity when the current density reaches up to 10 A g 1, indi cating the excellent specific capacity and rate capability of the device. Fig. 6e presents the specific capacity of the BSH device as a function of
Table 1 Comparison of the electrochemical performances of the as-prepared Zn2V2O7 with previously reported vanadium-based electrode in alkaline electrolyte. Materials
Specific capacity (mAh g 1)
Cycling performance (cycles)
Morphology
Ref
GR/ BiVO4 Zn3V2O8 Zn2V2O7 ZnV2O4 GR/ BiVO4 Zn3V2O8 Zn2V2O7
87 (5 mV s 1)
80.3%(2000)
Fern-like
[41]
29 (2 A g 1) 59 (1 mA cm 2) 50 (1 A g 1) 294 (5 A g 1)
98%(2000) 84%(1000) 89% (1000) 91%(2500)
nanoplatelets nanoparticle nanosphere monolith
[46] [47] [51] [52]
125 (0.8A g 1) 85 (1 A g 1)
76% (5000) 91%(6000)
sheet microsphere
[54] This work
7
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Fig. 6. The performance of Zn2V2O7//AC BSH device. (a) CV curves at different potential windows; (b) CV curves at different scan rates ranging from 5 to 100 mV s 1; (c) GCD curves at various current densities; (d) Specific capacity variation with different current densities; (e) Cycling performance for the BSH at 8 A g 1; (f) Comparison of energy densities with other vadanates from literature.
charge/discharge cycling numbers. We find that the BSH device shows 82.3% retention of the specific capacity after 1000 cycles at 8 A g 1, then slight recovery can be found in the later 5000 cycles. It is noting that the specific capacity retention can be reached up to 89.8% after 6000 cycles, suggesting a long cycle life of the BSH device. The specific energy density and specific power density of the BSH device are the most important parameters to verify the feasibility in practical application. The Ragone plot of Zn2V2O7//AC BSH device is prepared from the GCD curves. The relation between energy density and power density of the device can be calculated according to the following equations [47]: � E ¼ ðC ⋅ ΔV 2 Þ 2 � 3:6 (3) P ¼ 3600E=Δt
where E is the energy density, P is the power density, Δt is the discharge time, C is the specific capacity and ΔV is the voltage range. As shown in Fig. 6f, the maximum energy density is 34.99 Wh kg 1 at a power den sity of 850 W kg 1, and the energy density retains 19.57 Wh kg 1 at a high power density of 8500 W kg 1. The high energy density of the BSH device is comparable with other reports based on vanadate materials, such as Ni3(VO4)2&NiO//Ni3(VO4)2&NiO (46 Wh kg 1 at 101W kg 1) [39], WV2O7NSG electrode (27.8 Wh kg 1 at 950Wkg 1) [40]; rGO/ BiVO4//rGO/BiVO4 (33.7 Wh kg 1 at 8000W kg 1) [41], Zn2V2O7/ Ni-foam//AC (18.7 Wh kg 1 at 272.7Wkg 1) [47], 1 GR/BiVO4//GR/BiVO4 (45.69 Wh kg at 800 W kg 1) [52], AlV3O9//AC (37.2 Wh kg 1 at 1124.4W kg 1) [53], GR/V2O5//GR cell (26.22 Wh kg 1 at 425 W kg 1) [55].
(4)
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4. Conclusion
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Yang, Y. Li, X.S. Sun, Y. Fang, J. Li, Mesoporous hybrids of reduced graphene oxide and vanadium pentoxide for enhanced performance in lithium-ion batteries and electrochemical capacitors, ACS Appl. Mater. Interfaces 8 (2016) 9200–9210. [31] Y. Zhang, J. Zheng, Q. Wang, S. Zhang, T. Hu, C. Meng, One-step hydrothermal preparation of (NH4)2V3O8/carbon composites and conversion to porous V2O5 nanoparticles as supercapacitor electrode with excellent pseudocapacitive capability, Appl. Surf. Sci. 423 (2017) 728–742. [32] T. Qian, N. Xu, J. Zhou, T. Yang, X. Liu, X. Shen, J. Liang, C. Yan, Interconnected three-dimensional V2O5/polypyrrole network nanostructures for high performance solid-state supercapacitors, J. Mater. Chem. A 3 (2015) 488–493. [33] H.F. Jiang, X.Y. Cai, Y. Qian, C.Y. Zhang, L.J. Zhou, W.L. Liu, B.S. Li, L.F. Lai, W. Huang, V2O5 embedded in vertically aligned carbon nanotube arrays as freestanding electrodes for flexible supercapacitors, J. Mater. Chem. 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Kim, Zn3V2O8 porous morphology derived through a facile and green
In summary, 3D microsphere Zn2V2O7 is prepared through hydro thermal method. The unique 3D microsphere conducted by 2D layered structure and the 2D nanosheets contain numerous nanoparticles, incorporating the synergistic effect of Zn and V, endow the Zn2V2O7 to be a promising candidate for electrode of ARBs. As expected, the ob tained Zn2V2O7 material shows high specific capacity and outstanding cycle stability. The BSH device Zn2V2O7//AC is assembled, and the device can be reversibly charged for 6000 times with capacity loss less than 11%. The BSH device can deliver an energy density of 34.99 Wh kg 1 at power density of 850 W kg 1, which is better than the reported vanadate materials, demonstrating the great potential in practical application on ARBs. Acknowledgment The authors would like to thank the National Natural Science Foundation of China (Nos: 51572136, 21571112, 21702116, 51372125, 21805155); Taishan Scholars Program; the Natural Science Foundation of Qingdao, China (16-5-1-83-jch). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227087. References [1] J. Wang, H. Tang, H. Wang, R. Yu, D. Wang, Multi-shelled hollow micro-/ nanostructures: promising platforms for lithium-ion batteries, Mater. Chem. Front. 1 (2017) 414–430. [2] P. Chen, Y. Wu, Y. Zhang, T.-H. Wu, Y. Ma, C. Pelkowski, H. Yang, Y. Zhang, X. Hu, N. Liu, A deeply rechargeable zinc anode with pomegranate-inspired nanostructure for high-energy aqueous batteries, J. Mater. Chem. A 6 (2018) 21933–21940. [3] X. Li, K. Ding, B. Gao, Q. Li, Y. Li, J. Fu, X. Zhang, P.K. Chu, K. Huo, Freestanding carbon encapsulated mesoporous vanadium nitride nanowires enable highly stable sulfur cathodes for lithium-sulfur batteries, Nano Energy 40 (2017) 655–662. [4] Z. Li, T.R. Klein, D.H. Kim, M. Yang, J.J. Berry, M.F.A.M. van Hest, K. Zhu, Scalable fabrication of perovskite solar cells, Nat. Rev. Mater. 3 (2018) 18017. [5] N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Asymmetric supercapacitor electrodes and devices, Adv. Mater. 29 (2017) 1605336. [6] M. Salman, X. Chu, T. Huang, S. Cai, Q. Yang, X. Dong, K. Gopalsamy, C. Gao, Functionalization of wet-spun graphene films using aminophenol molecules for high performance supercapacitors, Mater. Chem. Front. 2 (2018) 2313–2319. [7] G. Zhang, Y. Han, C. Shao, N. Chen, G. Sun, X. Jin, J. Gao, B. Ji, H. Yang, L. Qu, Processing and manufacturing of graphene-based microsupercapacitors, Mater. Chem. Front. 2 (2018) 1750–1764. [8] J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang, D. Wang, Accurate control of multishelled Co3O4 hollow microspheres as highperformance anode materials in lithium-ion batteries, Angew. Chem. Int. Ed. 52 (2013) 6417–6420. [9] H. Li, H. Ma, M. Yang, B. Wang, H. Shao, L. Wang, R. Yu, D. Wang, Highly controlled synthesis of multi-shelled NiO hollow microspheres for enhanced lithium storage properties, Mater. Res. Bull. 87 (2017) 224–229. [10] P. Bandyopadhyay, T.T. Nguyen, N.H. Kim, J.H. Lee, Facile synthesis of 4,4’diaminostilbene-2,2’-disulfonic-acid-grafted reduced graphene oxide and its application as a high-performance asymmetric supercapacitor, Chem. Eng. J. 333 (2018) 170–184. [11] Y. Wang, H. Dou, J. Wang, B. Ding, Y. Xu, Z. Chang, X. Hao, Three-dimensional porous MXene/layered double hydroxide composite for high performance supercapacitors, J. Power Sources 327 (2016) 221–228. [12] J. Zhang, W. Zhang, M. Han, J. Pang, Y. Xiang, G. Cao, Y. Yang, Synthesis of nitrogen-doped polymeric resin-derived porous carbon for high performance supercapacitors, Micropor. Mesopor. Mat. 270 (2018) 204–210. [13] H. Jiang, L. Yang, C. Li, C. Yan, P.S. Lee, J. Ma, High–rate electrochemical capacitors from highly graphitic carbon–tipped manganese oxide/mesoporous carbon/manganese oxide hybrid nanowires, Energy Environ. Sci. 4 (2011) 1813–1819. [14] P.J. Hall, M. Mirzaeian, S.I. Fletcher, F.B. Sillars, A.J.R. Rennie, G.O. Shitta-Bey, G. Wilson, A. Cruden, R. Carter, Energy storage in electrochemical capacitors: designing functional materials to improve performance, Energy Environ. Sci. 3 (2010) 1238–1251. [15] X. Lai, J.E. Halperta, D. Wang, Recent advances in micro-/nano-structured hollow spheres for energy applications: from simple to complex systems, Energy Environ. Sci. 5 (2012) 5604–5618.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Hydrothermal synthesis and electrochemical properties of 3D Zn2V2O7 microsphere for alkaline rechargeable battery Qi Zhang, Qiqi Shi, Hongdong Li, Zhenyu Xiao, Kun-Peng Wang, Lingbo Zong, Lei Wang * State Key Laboratory Base of Eco-chemical Engineering, Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Three-dimensional Zn2V2O7 micro sphere covered by nanoparticles. � Excellent capacity and good cycle sta bility in alkaline rechargeable battery. � High energy density for Zn2V2O7//AC battery-supercapacitors hybrid device. � High performance results from unique structure and synergistic effect.
A R T I C L E I N F O
A B S T R A C T
Keywords: Zinc vanadate Alkaline rechargeable battery Energy storage
In this work, 3D microsphere Zn2V2O7 is obtained by calcined the precursor Zn3(OH)2V2O7⋅2H2O. The 3D structure of Zn2V2O7 inherits from the precursor which is composed by layered structures with nanoparticles fully covered. The unique structure endows it higher specific surface area and porous structure. Benefitting from the structure stability and the synergistic effect of the mixed metal oxides, the Zn2V2O7 electrode shows high specific capacity of 84.8 mAh g 1 at 1 A g 1, good rate capability and long cycle life. In addition, the batterysupercapacitors hybrid device is assembled with Zn2V2O7 as positive electrode and AC as negative electrode. The device shows high cycling stability with 89.8% retention after 6000 cycles and maximum energy density of 34.99 Wh kg 1 at 850 W kg 1. The high specific capacity and energy density suggest that 3D Zn2V2O7 micro sphere electrode material is suitable for application in the field of alkaline rechargeable battery.
1. Introduction Recently, growing global energy demands and the environmental pollution resulted from the extensive use of the fossil fuels have pro moted the development of clean, efficient, sustainable green energy source [1–4]. The wind, solar, tidal and hydroelectric power are alter native energy sources with endless supplies, how to store the energy is
still a challenge [5]. As the most promising energy storage devices, supercapacitors (SCs) and lithium-ion batteries (LIBs) have drew worldwide attention and become the dominant power sources in portable electronic device [6–14]. However, the sluggish kinetic of LIBs and low energy density of SCs limit their wide application in large scale grid energy storage [15,16]. Alkaline rechargeable batteries (ARBs) can be bridged the gap between LIBs and SCs due to the high energy density
* Corresponding author. E-mail address: [email protected] (L. Wang). https://doi.org/10.1016/j.jpowsour.2019.227087 Received 1 February 2019; Received in revised form 28 June 2019; Accepted 29 August 2019 Available online 5 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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and power density, superior rate capacity, fast surface reaction [17–20]. Therefore, much attention have been paid on the development of ARBs to obtain the electrode with high electrochemical performance. The ARBs can store charge by the reversible faradic reaction which occurs in the surface and near-surface electrode materials in alkaline electrolyte solution [21–23]. Based on the storage mechanism, transi tion metal oxides (TMOs) such as MnO2, Co, Ni-based oxides/hydroxides have emerged as electrode materials for ARBs in the past decades due to their high theoretical capacity, abundant redox centers and environ mental benignity [18–20,24,25]. Vanadium oxides have been widely investigated as electrode materials in alkaline electrolyte because of their unique layered structure, rich oxidation states and low cost [26–29]. Various V2O5 nanostructures have been synthesized and served as electrode materials in alkaline electrolyte, the electrochemical per formance shows that the specific capacitance of ca. 300–600 F g 1 and capacitance retention of 6%–132% after 100–2000 cycles [30–32]. However, many drawbacks such as the toxicity of V2O5, the poor elec trical conductivity of single metal oxides and the expand volume during in charge-discharge process make them unable to meet the needs in modern society [33]. In order to solve these problems, mixed transition metal oxides (MTMOs) which contain different metal cations are intro duced. The complex chemical composition and synergistic effect of the mixed metal endow them outstanding electrochemical performance, which make them promising candidates for ARBs [34]. Metallic vanadates exhibit outstanding electrochemical properties due to the various valence states of vanadium element, wide operating potential window and the unique crystal structure [35–38]. Therefore, many vanadate compounds have been synthesized and used in optical device, lithium-ion batteries (LIBs) and photocatalytic fields [39–41]. Among them, zinc vanadium oxides are promising candidates for energy storage due to the low cost and high conductivity of zinc and the mul tiple valence states of vanadium [42]. Butt et al. synthesized ZnV2O4 glomerulus microspheres and investigated the potential application in hydrogen storage [43]. Liu et al. reported hollow flower-like Zn2V2O7 electrode material for LIBs with high cycle stability and capacity of 882.5 mAh g 1 after 100 cycles [44]. Sun et al. reported ZnV2O6 nanobelts as Na-ion battery material with high capacity of 480.5 mAh g 1 and cycle stability of 51.4% capacity retention after 100 cycles [45]. The Zn3V2O8 nanoplates electrode material reported by Ryu et al. showed a maximum capacitance of 302 F g 1 and high cycle life in KOH electrolyte [46]. Liu et al. reported Zn2V2O7 nanowires which synthe sized by microwave-assisted method and showed high capacitance reached up to 427.7 F g 1 at 1 mA cm 2 [47]. Kim et al. synthesized Zn2V2O7 nanopowders and mixed with multi-walled carbon nanotubes as the electrode of SCs, which showed excellent cycle stability [48]. To the best of our knowledge, 3D microsphere Zn2V2O7 served as the electrode material for ARBs has not been reported and much effort should be paid on the zinc vanadates. In this work, we report the synthesis of 3D microsphere Zn2V2O7 by a hydrothermal method followed by annealing treatment. SEM and TEM images show the Zn2V2O7 inherites the 3D skeleton from the precursor Zn3(OH)2V2O7⋅2H2O, and the 3D microsphere Zn2V2O7 is constructed by layered structure with nanoparticles fully covered. The unique structure of Zn2V2O7 shows the following advantages for ARBs. Firstly, the 3D skeleton is assembled by 2D nanosheets and the nanosheets are covered with 0D nanoparticles, so the material owns the advantages of 0D, 2D, 3D structure: high specific surface area, excellent electron and ionic conductivity and stability. Secondly, the synergistic effect of the mixed oxide improved the comprehensive electrochemical performance. As a consequence, the prepared Zn2V2O7 microsphere electrode pos sesses high specific capacity of 84.8 mAh g 1 and capacity retention of 91.1% after 6000 cycles. The battery-supercapacitor hybrid (BSH) de vice also exhibits remarkable energy density of 34.99 Wh kg 1 at the power density of 850 W kg 1. Moreover, the BSH device shows excellent cycling stability with capacity retention of 89.8% after 6000 cycles, which is higher than the most reported vanadates materials.
2. Experimental 2.1. Synthesis of Zn2V2O7 All of the reagents used in the experiments were of analytical grade and used directly without further purification. Typically, 1.0 mmol NH4VO3 was dissolved in 15 mL deionized water and 20 mL ethylene glycol (EG) in 100 mL beaker to form a homogeneous solution, named A solution. Meanwhile, 1.0 mmol Zn(CH3COO)2 and 1 μmol polyethylene glycol 2000 (PEG 2000) were dissolved in 5 mL deionized water with stirring for 20 min to dissolve completely, called B solution. Under vigorous stirring, the solution A was slowly added to the solution B dropwise. The obtained mixture was transferred into a Teflon-lined stainless-steel autoclave and heated at 160 οC for 24 h. After the solu tion cooled down to room temperature naturally, the sample was washed and centrifuged with deionized water for several times, and then dried in vacuum oven at 60 οC overnight. At last, the obtained sample was calcined at 450 οC for 5 h at a heating rate of 1 οC min 1 and the final products of Zn2V2O7 was obtained. In comparison, V2O5 nanosheets were also synthesized by a hydrothermal method according to literature [49]. 2.2. Characterization The X-ray diffraction (XRD) was used to examine the crystallinity and phase purity of the products on a Bruker-D8 Advance diffractometer using Cu Kɑ radiation (λ ¼ 1.5406 Å) with operation voltage of 40 kV. The surface morphology and elemental composition of the products were characterized using a field-emission scanning electron microscope (FESEM, JEOL JSM-7500F) and energy dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) images were performed using JEOL-2010 with an accelerating voltage of 200 kV under 91 μA. The BET surface area was investigated by micrometrics ASAP 2020 nitrogen adsorption/desorption experiments. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with Mg-Kα excitation source were used to analyze the surface element composition. 2.3. Electrochemical measurement in a three-electrode system All the electrochemical measurements were carried out using a CHI760E electrochemical working station (Chenhua, Shanghai). In the traditional three-electrode system, the platinum foil and Ag/AgCl elec trode were served as the counter electrode and reference electrode, respectively, while the modified nickel foam (NF) electrode as the working electrode. Electrochemical performance was carried out in 3 M KOH electrolyte to study the electrochemical behavior of the Zn2V2O7 electrode. Cyclic voltammetry (CV) was measured with different scan rates at a potential window of 0–0.6 V. Galvanostatic charge–discharge (GCD) curves were obtained with the potential window of 0–0.45 V at various current densities, and electrochemical impedance spectroscopy (EIS) experiments were conducted at a frequency range from 100 kHz to 0.01 Hz at the open circuit potential of 0.179 V. 2.4. Fabrication of BSH device The BSH device was assembled by using an activated carbon (AC) on nickel foam (NF) (1 cm � 2 cm) as the negative electrode and Zn2V2O7 on NF as the positive electrode in 1 M KOH as the electrolyte. The working electrodes Zn2V2O7//NF were prepared by grinding Zn2V2O7 (70 wt%), acetylene black (20 wt%), polytetrafluoroethylene (PTFE, 10 wt%), and a few drops of acetone to form a homogeneous slurry. Then the slurry was coated onto NF. The NF was dried at 80 � C for 12 h under a vacuum circumstance. The AC//NF electrode was prepared by the similar process, but the homogeneous slurry contains AC and PTFE with a weight ratio of 9:1 in acetone. The loading weight of the electrode was depended on the mass difference of the NF before and after coating 2
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active materials. The mass loading of Zn2V2O7 is about 2 mg, and the AC is about 3.5 mg.
attributed to the cross-link of the thin nanosheets to form 3D architec ture. The HRTEM image of the precursor in Fig. 3b confirms the for mation of the layered structure, and a representative lattice fringe of 0.262 nm matches well with the (020) plane of the Zn3(OH)2V2O7⋅2H2O (PDF no# 50–0570). Fig. 3c and d show the low and high magnificent TEM images of Zn2V2O7. The different shades of color area indicates the formation of 3D microstructure, which composed by numerous nano particles. The shallow area in Fig. 3c represents the stack of nano particles while the transparent area suggests the space between nanoparticles, which provides path for electrolyte ions and electronics. Fig. 3c also shows the nanoparticles with uniform size about 30 nm. In the nanoparticles area of Fig. 3d, the lattice fringe of 0.241 nm with high crystallinity results from the ( 114) plane of Zn2V2O7, which further demonstrates the formation of Zn2V2O7. In addition, the EDS spectra and EDS mapping analysis are performed to verify the elements distribution of the Zn2V2O7. As shown in Fig. 3e, the sample is composed of Zn, V, O with molar ratio of 23 : 19: 57. Fig. 3f shows three colors represented Zn, V, O respectively distribute throughout the region, which indicates the homogeneous distribution of the three elements on the whole microsphere. Considering the surface charge storage mechanism of the ARBs, the surface oxidation state and elemental composition are characterized by XPS. As shown in Fig. 4a, the complete spectrum of Zn2V2O7 is obtained, which contains typical Zn 2p, V 2p, O 1s and C 1s regions. The Zn 2p region in Fig. 4b includes two peaks at 1021.39 and 1044.39 eV, the former is assigned to the Zn 2p3/2 and the latter is Zn 2p1/2. As shown in Fig. 4c, two peaks located at binding energies of 530.1 and 531 eV are observed in the high resolution spectrum of O 1s, which implies two kinds of O bind environments in the Zn2V2O7. The peak at 530.1 eV named O1 can be attributed to the typical metal-oxygen (V-O) bond, while the 531 eV named O2 may come from the surface adsorbed oxygen species such as H2O. For the high resolution V 2p spectrum, in Fig. 4d, the peak at 517.42 and 524.84 eV can be ascribed to the V5þ 2p3/2 and V5þ 2p1/2, respectively, while the small peak at 516.7 eV is assigned to the V4þ 2p3/2, indicating the mixed oxidation states of V element on the surface of Zn2V2O7. The integrate area of the V element suggests the V5þ are predominant over V4þ. The XPS analysis demonstrates the compo sition and oxidation state of the element in Zn2V2O7, which is consistent with XRD and EDS.
3. Result and discussion 3.1. Characterization of the products The synthesis procedure for Zn2V2O7 contains two steps: a hydro thermal process to obtain the precursor and then calcine the precursor in 450 � C for Zn2V2O7. XRD analyses are performed to verify the crystalline of the obtained samples. The XRD pattern of the precursor is shown in Fig. 1a, all the diffraction peaks are in good agreement with the monoclinic phase of Zn3(OH)2V2O7⋅2H2O (JCPDS card file no. 50–0570), and no other impurity peaks exist. Fig. 1b shows the XRD pattern of Zn2V2O7, after calcined the precursor, the three ultra-strong diffraction peaks locate at 16.7� , 28.6� , 28.7� can be definitely indexed to the ( 111) (022) ( 113) lattice planes of the Zn2V2O7 (JCPDS card file no. 29–1396), and other remaining peaks in Fig. 1b are also indexed to the standard card with no other reflection peaks are detected. In addition, Fig. 1b shows all the peaks with sharp and narrow Full Width at Half Maximum (FWHM), indicating the high purity and crystallinity of Zn2V2O7. The morphologies of the obtained samples before and after calcined are characterized by SEM. As shown in Fig. 2a–c, the images of the precursor with different magnifications indicate that the diameter of the formed microsphere is distributed 2–4 μM. The microsphere conducted by layered structures shows smooth surface and uniform thickness of 40 nm. The images of Zn2V2O7 are shown in Fig. 2d–f, after calcined in 450 � C, the 3D microsphere structure is still remained, suggesting the high stability of the structure in high temperature. Fig. 2e shows the 3D skeleton is composed by 2D nanosheets with rough surface and porous structures. The cross-link between 2D nanosheets provides path for ions and electrons. The high magnification SEM image in Fig. 2f displays that the nanosheets are covered by numerous nanoparticles. The porous and rough structure is formed by the release of H2O and the transformation of crystalline under high temperature, during this process, Zn3(OH)2 V2O7⋅2H2O microspheres are impacted and formed porous structures. Compared with the smooth surface of the precursor, this rough surface owned higher specific surface area to load more active sites. N2 ab sorption/desorption measurements are conducted to investigate the structure change before and after calcined (Fig. S1). The specific surface area of the as-prepared products is analyzed by BET surface area mea surement, and the value increases from 11.9 m2 g 1 to 19.4 m2 g 1 after calcined, which is in consistent with the SEM results. The TEM figures are utilized for further investigation of the detailed structure of the precursor and sample. The TEM image of the precursor can be seen in Fig. 3a, a shallow color in the layered areas reveals the sheet-like structure, while the dark area among them is mainly
3.2. Electrochemical capacitive properties of the Zn2V2O7 electrode To evaluate the advantage of the prepared material for potential application in energy storage, the Zn2V2O7 as the electrode material is carried out in three-electrode system. We make a comparison of CV curves between naked NF and Zn2V2O7 electrodes at a scan rate of 30 mV s 1 in the potential window range of 0–0.6 V. As shown in Fig. 5a, as the substrate material, the CV curve of the naked NF is nearly a
Fig. 1. XRD patterns of (a) the precursor Zn3(OH)2V2O7⋅2H2O and (b) Zn2V2O7 material. 3
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Fig. 2. SEM patterns of (a–c) the precursor Zn3(OH)2V2O7⋅2H2O and (d–f) Zn2V2O7 material.
straight line, and no obvious peak current is observed, while the Zn2V2O7 electrode shows a pair of redox peaks with large current, which indicates the capacity of the Zn2V2O7/NF almost derives from the contribution of Zn2V2O7 material. The CV profiles of Zn2V2O7 electrode have acquired at different scan rates from 5 to 100 mV s 1 are shown in Fig. 5b. Increasing the scan rate led to peak current increase, and the redox peaks in the window range are clearly retained and the shape shows no obvious change in the high scan rate, which suggests the good charge/discharge property and rate capability, the obvious redox peaks and the slightly shift of peak potential with the increasing sweep rate suggest the typical battery-type material. The redox peaks between 0.1 and 0.4 V are assigned to the faradaic reaction of V5þ/V4þ, which may resulting from the intercalation and deintercalation of Kþ from elec trolyte solution. XPS analysis were used to better understand the insertion mechanism (Fig. S2), in the discharge state, the peak at 524.8 eV which corresponds to of V 2p1/2 disappears, while the remaining V 2p3/2 peak is deconvoluted into two peaks at 517.6 eV and 516.8 eV, which correspond to V5þ 2p3/2 and V4þ 2p3/2, respectively. Compared with the Zn2V2O7 before charge/discharge process, the comparable integrate area of these two peaks indicate the nearly same amount of V5þ and V4þ. That’s reasonable, in the discharge state, reduction reaction occurs in the positive electrode Zn2V2O7, the V5þ accepting electrons and reduce to V4þ. The possible reaction mechanism is show in Eq. (1) [47]: Zn2V2O7 þx Kþ þxe ↔KxZn2V2O7
capacity of the electrode material for ARBs. Fig. 5c presents the typical GCD curves at different current densities, the nonlinear shape from 0 to 0.45 V with a voltage plateau indicates the typical battery behavior of the electrode. Based on the data from GCD curves, the specific capacity as a function of the current density is shown in Fig. 5d. The highest specific capacity reaches up to 84.8 mAh g 1 at 1 A g 1, with the current density increased to 10 A g 1, the discharge time decreases gradually, and the corresponding specific capacity values are 69.0, 56.7, 50.0, 45.3, 41.6 mAh g 1, about 50% of the initial specific capacity is retained, implying the good rate capability of the electrode. Fig. S3 shows the contribution fraction of the capacitive and diffusion-control process, we find that most of the current contribution comes from the diffusion-limit storage. The result also confirms the battery-type behavior due to the insertion/deinsertion of Kþ. The high specific ca pacity of 3D Zn2V2O7 microsphere may originate from the 2D nano sheets with thin thickness, the fully covered nanoparticles and high percentage of the exposed active sites. The unique structure may provide high specific surface area to store more charge, while the thin thickness and the cross-link of the layer structure offer short path for ions and electrons, which can benefit for ions transport rate. Long-term cycling stability of the electrode is a crucial parameter for the application in ARBs. The cycle stability of the Zn2V2O7 electrode is carried out by repeating the GCD test at 8 A g 1 for 6000 cycles (Fig. 5e). It can be observed that the specific capacity of first cycle was 48.7 mAh g 1, the value retention gradually decreases over the initial 360 cycles, after which, the capacity retention remains a constant value upon the repeated cycling to 6000 cycles. The initial decrease of the specific
(1)
The GCD tests can be used to accurately determine the specific 4
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Fig. 3. TEM images of the precursor Zn3(OH)2V2O7⋅2H2O at (a) low and (b)high magnification; TEM images of sample Zn2V2O7 at (c) low and (d) high magnifi cation; (e) The EDS spectra with molar ratio of elements; (f) EDS mapping of Zn2V2O7.
capacity may be caused by the loss of the material during the repeat cycling. The specific capacity of the electrode is 44.35 mAh g 1 in the last cycle, corresponding to 91.1% of its initial capacity, which dem onstrates the excellent cycling stability during the repeat chargedischarge cycling process. The long cycle life may be caused by the stability of the 3D skeleton, which suffers from the high temperature without decomposition. In addition, volume expansion of the metal oxide material is inevitable during the repeat GCD process, but the interspace between nanoparticles may provide enough space for them. In order to further understand the conductivity of the electrode material, the EIS measurement is performed. The Nyquist plots of Zn2V2O7 electrode before and after 6000 cycles are shown in Fig. 5f, in the high frequency, the intercept of the X-axis before the semicircle represents the equivalent series resistance (ESR), which contains intrinsic resistance of the electro-active material, electrolyte resistance and interface contact resistance [50]. The after cycling material exhibits lower ESR (0.41 Ω) than the before material (0.6 Ω). Considering in the same system, the electrolyte resistance, intrinsic resistance and current collector are same, the lower ESR donates a lower contact resistance between the material and electrolyte, which may be caused by the increased wettability during the charging/discharging process. In the same frequency region, we can figure out that the material after cycling exhibits a semicircle with small diameter (5.1 Ω), in contrast, the same sample before GCD cycle shows no obvious semicircle. The low charge
transfer resistance (Rct) at the interface of the electrolyte and electrode indicates the large electro-active surface area of the electrode. The synergistic effect between Zn and V elements may explain the low resistance. The electrochemical performance of V2O5 nanosheets (Fig. S4) syn thesized by a facile hydrothermal method are also tested to investigate the synergistic effect of the mixed metal oxide. In the control experi ments, the 3D Zn2V2O7 microsphere is insteaded by the V2O5 nanosheets as the electrode material with the same experimental condition. The CV curves with different scan rates in the potential range of 0–0.6 V are shown in Fig. S5a, obvious redox peaks are obtained in the low scan rates, and when the scan rate exceeded to 60 mV s 1, the redox peaks display noticeable shift and then disappear quickly, which implies the poor reversibility and low rate capability. The obvious plateau in the GCD curves (Fig. S5b) indicates the typical battery-type behavior, which is consistent with the CV curves. The specific capacities calculated from the GCD curves are shown in Fig. S5c, high specific capacity of 67.5 mAh g 1 is achieved at a current density of 1 A g 1, and the value decreases to 23.9 mAh g 1 at 10 A g 1, the 35.4% of the capacity retention implies the low rate capability. Then the cycle stability of V2O5 is tested at 10 A g 1 in Fig. S5d, 36.4% deterioration of its initial specific capacity is obtained after 1000 cycles, suggesting the poor stability. The notable weaker electrochemical performance than Zn2V2O7 may be due to the poor conductivity of the single metal oxides (Fig. S6). 5
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Fig. 4. (a) XPS survey of Zn2V2O7.(b–d) High resolution of Zn 2p, O1s and V 2p.
curves of the Zn2V2O7 and AC electrode at a scan rate of 30 mV s 1 in three-electrode system show the potential widow range of Zn2V2O7 and AC are 0–0.6 V and 1.0–0 V, respectively. The data indicates the continuous and complementary potential window of the two materials. Secondly, the specific capacity of the two materials should be equiva lent. Considering the Zn2V2O7 electrode and AC electrode own different specific capacity and potential window, balance the mass ratio of the electrode materials is important. In a detailed process, the GCD curves of the two materials is conducted at the same current, and the discharge time ratio of the two materials is mass ratio, which is calculated by the charge balance theory, according to the Eq. (2) as follows [55]:
The above results demonstrate that Zn2V2O7 electrode material owns high capacity, excellent cycling stability and good rate capability, and the electrochemical performance is better or comparable with other previous reported vanadium oxide-based electrodes (Table 1). The good performance can be attributed to the following two aspects: (1) Struc ture advantage. The 3D microsphere structure constructed by 2D layered structure, and the layered structure covered by numerous nanoparticles. The structure inherits the properties of 2D layered structure and 0D nanoparticles, which endow it high specific surface area and low diffusion path. In addition, the structure of Zn2V2O7 microsphere formed by the calcination process and constructed by nanoparticles, the spaces between nanoparticles not only improve the specific surface area to store more charge, but also provide space for the volume expansion during the charge/discharge process; (2) Component advantage. The synergistic effect of the mixed metal oxides is the key to high electro chemical performance. V element provides various oxidation states while Zn improves the high electrical conductivity, therefore, the 3D Zn2V2O7 microsphere shows enhanced capacity and durability.
mþ = m ¼ C V =Cþ V þ
(2)
here mþ and m- (g) represent the mass of positive and negative material, Cþ and C- (F g 1) are the specific capacity of the materials at the same current in the three-electrode system, and Vþ and V- (V) is the discharge voltage range of the individual electrode. The optimum mass ratio of Zn2V2O7 and AC is calculate 2:3.5. Fig. S8a and Fig. S8b show the GCD curves of the two electrodes with optimum mass ratio at the same cur rent of 4 mA. The discharge time is 224 s and 226 s, respectively. The nearly same discharge time at same current suggests the two electrodes can be assembled to BSH device. The electrochemical performance of the BSH device based on Zn2V2O7 and AC is conducted. Firstly, the stable voltage window of the BSH device is studied by CV curves with increased potential window form 0–1.4 V to 0–1.8V. As shown in Fig. 6a, the representative CV curves of the BSH are investigated at 30 mV s 1 with different voltage windows, increased potential window brings bigger closed area, indi cating the higher capacity. However, obvious polarization peak is observed when the potential window up to 0–1.8 V, which is attributed to the decomposition of the electrolyte. Therefore, the stable potential
3.3. The electrochemical properties of the Zn2V2O7//activated carbon BSH device To further evaluate the practical application of the prepared 3D microsphere Zn2V2O7 electrode, BSH device is assembled with Zn2V2O7 as the positive and the AC as the negative electrode. As we all know, two conditions should be met for the two materials assembled into BSH device. Firstly, the potential range of the two materials should be complementary, that is, the potential window of them have one inter section at least. In order to understand the feasibility of the positive and negative material in BSH device, the CV curves and the capacity matching experiments are carried out. As shown in Fig. S7, the CV 6
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Fig. 5. The electrochemical performance of Zn2V2O7 electrode. (a) CV curves of Zn2V2O7 electrode and NF electrode at 30 mV s 1; (b) CV curves at different scan rates; (c) GCD curves at different current densities; (d) Specific capacity calculated from GCD curves; (e) Cycling performance at 8 A g 1; (f) Nyquist plots in the frequency ranging from 0.01 to 100 kHz.
window for this BSH device was 0–1.7 V. To evaluate the value of the rate capability and specific capacity the of the BSH device, Fig. 6b shows the CV curves at different scan rates with potential window of 0–1.7 V. With the increasing of the scan rate, the closed area of the curves increase while the shape of the curves re mains unchanged, suggesting the high rate capability. Furthermore, the GCD curves of the BSH device at different current densities from 1 A g 1 to 10 A g 1 are shown in Fig. 6c, all the GCD curves are symmetric in shape during in the charge and discharge process, and the discharge curves are nonlinear, indicating the existence of pseudocapacitance. The specific capacity from the GCD curves is calculated and shown in Fig. 6d. The values reach to 41.2, 35.6, 30.7, 28.2, 25.3, 23.0 mAh g 1 at 1, 2, 4, 6, 8, 10 A g 1, respectively. The device shows 55.9% retention of its initial capacity when the current density reaches up to 10 A g 1, indi cating the excellent specific capacity and rate capability of the device. Fig. 6e presents the specific capacity of the BSH device as a function of
Table 1 Comparison of the electrochemical performances of the as-prepared Zn2V2O7 with previously reported vanadium-based electrode in alkaline electrolyte. Materials
Specific capacity (mAh g 1)
Cycling performance (cycles)
Morphology
Ref
GR/ BiVO4 Zn3V2O8 Zn2V2O7 ZnV2O4 GR/ BiVO4 Zn3V2O8 Zn2V2O7
87 (5 mV s 1)
80.3%(2000)
Fern-like
[41]
29 (2 A g 1) 59 (1 mA cm 2) 50 (1 A g 1) 294 (5 A g 1)
98%(2000) 84%(1000) 89% (1000) 91%(2500)
nanoplatelets nanoparticle nanosphere monolith
[46] [47] [51] [52]
125 (0.8A g 1) 85 (1 A g 1)
76% (5000) 91%(6000)
sheet microsphere
[54] This work
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Fig. 6. The performance of Zn2V2O7//AC BSH device. (a) CV curves at different potential windows; (b) CV curves at different scan rates ranging from 5 to 100 mV s 1; (c) GCD curves at various current densities; (d) Specific capacity variation with different current densities; (e) Cycling performance for the BSH at 8 A g 1; (f) Comparison of energy densities with other vadanates from literature.
charge/discharge cycling numbers. We find that the BSH device shows 82.3% retention of the specific capacity after 1000 cycles at 8 A g 1, then slight recovery can be found in the later 5000 cycles. It is noting that the specific capacity retention can be reached up to 89.8% after 6000 cycles, suggesting a long cycle life of the BSH device. The specific energy density and specific power density of the BSH device are the most important parameters to verify the feasibility in practical application. The Ragone plot of Zn2V2O7//AC BSH device is prepared from the GCD curves. The relation between energy density and power density of the device can be calculated according to the following equations [47]: � E ¼ ðC ⋅ ΔV 2 Þ 2 � 3:6 (3) P ¼ 3600E=Δt
where E is the energy density, P is the power density, Δt is the discharge time, C is the specific capacity and ΔV is the voltage range. As shown in Fig. 6f, the maximum energy density is 34.99 Wh kg 1 at a power den sity of 850 W kg 1, and the energy density retains 19.57 Wh kg 1 at a high power density of 8500 W kg 1. The high energy density of the BSH device is comparable with other reports based on vanadate materials, such as Ni3(VO4)2&NiO//Ni3(VO4)2&NiO (46 Wh kg 1 at 101W kg 1) [39], WV2O7NSG electrode (27.8 Wh kg 1 at 950Wkg 1) [40]; rGO/ BiVO4//rGO/BiVO4 (33.7 Wh kg 1 at 8000W kg 1) [41], Zn2V2O7/ Ni-foam//AC (18.7 Wh kg 1 at 272.7Wkg 1) [47], 1 GR/BiVO4//GR/BiVO4 (45.69 Wh kg at 800 W kg 1) [52], AlV3O9//AC (37.2 Wh kg 1 at 1124.4W kg 1) [53], GR/V2O5//GR cell (26.22 Wh kg 1 at 425 W kg 1) [55].
(4)
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4. Conclusion
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In summary, 3D microsphere Zn2V2O7 is prepared through hydro thermal method. The unique 3D microsphere conducted by 2D layered structure and the 2D nanosheets contain numerous nanoparticles, incorporating the synergistic effect of Zn and V, endow the Zn2V2O7 to be a promising candidate for electrode of ARBs. As expected, the ob tained Zn2V2O7 material shows high specific capacity and outstanding cycle stability. The BSH device Zn2V2O7//AC is assembled, and the device can be reversibly charged for 6000 times with capacity loss less than 11%. The BSH device can deliver an energy density of 34.99 Wh kg 1 at power density of 850 W kg 1, which is better than the reported vanadate materials, demonstrating the great potential in practical application on ARBs. Acknowledgment The authors would like to thank the National Natural Science Foundation of China (Nos: 51572136, 21571112, 21702116, 51372125, 21805155); Taishan Scholars Program; the Natural Science Foundation of Qingdao, China (16-5-1-83-jch). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227087. References [1] J. Wang, H. Tang, H. Wang, R. Yu, D. Wang, Multi-shelled hollow micro-/ nanostructures: promising platforms for lithium-ion batteries, Mater. Chem. Front. 1 (2017) 414–430. [2] P. Chen, Y. Wu, Y. Zhang, T.-H. Wu, Y. Ma, C. Pelkowski, H. Yang, Y. Zhang, X. Hu, N. Liu, A deeply rechargeable zinc anode with pomegranate-inspired nanostructure for high-energy aqueous batteries, J. Mater. Chem. A 6 (2018) 21933–21940. [3] X. Li, K. Ding, B. Gao, Q. Li, Y. Li, J. Fu, X. Zhang, P.K. Chu, K. Huo, Freestanding carbon encapsulated mesoporous vanadium nitride nanowires enable highly stable sulfur cathodes for lithium-sulfur batteries, Nano Energy 40 (2017) 655–662. [4] Z. Li, T.R. Klein, D.H. Kim, M. Yang, J.J. Berry, M.F.A.M. van Hest, K. Zhu, Scalable fabrication of perovskite solar cells, Nat. Rev. Mater. 3 (2018) 18017. [5] N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Asymmetric supercapacitor electrodes and devices, Adv. Mater. 29 (2017) 1605336. [6] M. Salman, X. Chu, T. Huang, S. Cai, Q. Yang, X. Dong, K. Gopalsamy, C. Gao, Functionalization of wet-spun graphene films using aminophenol molecules for high performance supercapacitors, Mater. Chem. Front. 2 (2018) 2313–2319. [7] G. Zhang, Y. Han, C. Shao, N. Chen, G. Sun, X. Jin, J. Gao, B. Ji, H. Yang, L. Qu, Processing and manufacturing of graphene-based microsupercapacitors, Mater. Chem. Front. 2 (2018) 1750–1764. [8] J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang, D. Wang, Accurate control of multishelled Co3O4 hollow microspheres as highperformance anode materials in lithium-ion batteries, Angew. Chem. Int. Ed. 52 (2013) 6417–6420. [9] H. Li, H. Ma, M. Yang, B. Wang, H. Shao, L. Wang, R. Yu, D. Wang, Highly controlled synthesis of multi-shelled NiO hollow microspheres for enhanced lithium storage properties, Mater. Res. Bull. 87 (2017) 224–229. [10] P. Bandyopadhyay, T.T. Nguyen, N.H. Kim, J.H. Lee, Facile synthesis of 4,4’diaminostilbene-2,2’-disulfonic-acid-grafted reduced graphene oxide and its application as a high-performance asymmetric supercapacitor, Chem. Eng. J. 333 (2018) 170–184. [11] Y. Wang, H. Dou, J. Wang, B. Ding, Y. Xu, Z. Chang, X. Hao, Three-dimensional porous MXene/layered double hydroxide composite for high performance supercapacitors, J. Power Sources 327 (2016) 221–228. [12] J. Zhang, W. Zhang, M. Han, J. Pang, Y. Xiang, G. Cao, Y. Yang, Synthesis of nitrogen-doped polymeric resin-derived porous carbon for high performance supercapacitors, Micropor. Mesopor. Mat. 270 (2018) 204–210. [13] H. Jiang, L. Yang, C. Li, C. Yan, P.S. Lee, J. Ma, High–rate electrochemical capacitors from highly graphitic carbon–tipped manganese oxide/mesoporous carbon/manganese oxide hybrid nanowires, Energy Environ. Sci. 4 (2011) 1813–1819. [14] P.J. Hall, M. Mirzaeian, S.I. Fletcher, F.B. Sillars, A.J.R. Rennie, G.O. Shitta-Bey, G. Wilson, A. Cruden, R. Carter, Energy storage in electrochemical capacitors: designing functional materials to improve performance, Energy Environ. Sci. 3 (2010) 1238–1251. [15] X. Lai, J.E. Halperta, D. Wang, Recent advances in micro-/nano-structured hollow spheres for energy applications: from simple to complex systems, Energy Environ. Sci. 5 (2012) 5604–5618.
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