Green Synthesis of Vanadate Nanobelts at Room Temperature for

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Green Synthesis of Vanadate Nanobelts at Room Temperature for Superior Aqueous Rechargeable Zinc-Ion Batteries Zhiqiang Xie, Jianwei Lai, Xiuping Zhu, and Ying Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01378 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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ACS Applied Energy Materials

Green Synthesis of Vanadate Nanobelts at Room Temperature for Superior Aqueous Rechargeable Zinc-Ion Batteries

Zhiqiang Xiea,1, Jianwei Laia,1, Xiuping Zhub,2, Ying Wanga,* a

Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, USA.

b

Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803, USA. * Corresponding author: Prof. Ying Wang, E-mail: [email protected]. 1

The authors contributed equally to this work.

Abstract Rechargeable aqueous zinc-ion batteries are emerging as new promising energy storage devices for potential grid-scale applications, owing to their high safety and low cost. However, the limited choice of cathode materials and lack of green and scalable synthesis strategies have largely hindered their practical applications. Herein, a universal synthesis approach is developed to produce a variety of nanostructured layered vanadates, i.e., nanobelts of NaV3O8·1.35H2O (NVO), Zn3V2O8·1.85H2O (ZnVO) and KV3O8·0.51H2O (KVO), at room temperature. When examined as new cathodes for zinc-ion battery system with aqueous ZnSO4 as electrolyte, all three nanobelts exhibit excellent electrochemical performances, particularly the NVO and ZnVO electrodes, delivering high specific capacities of 366 and 328 mAh g-1 at 0.1 A g-1 respectively. In addition, at an ultra-high current density of 10 A g-1, the NVO shows an initial capacity of 186 1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mAh g-1 with retained capacity of 200 mAh g-1 after 200 cycles, while ZnVO provides an initial capacity of 205 mAh g-1 with remained capacity of 191 mAh g-1. Such remarkable electrochemical performances make layered vanadates especially the NVO and ZnVO very promising cathode candidates for new-generation aqueous zinc-ion batteries. Keywords: sodium vanadate, zinc vanadate, potassium vanadate, aqueous zinc ion battery, electrochemical characterizations.

2 ACS Paragon Plus Environment

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1. Introduction Li-ion batteries have been widely applied as energy storage devices for several decades.13

Nevertheless, the safety issues and limited lithium resources of the organic electrolytes cannot

meet the increasing demands for grid storage and electric vehicles.4-6 Recently, there have been tremendous efforts in developing new aqueous rechargeable batteries. Some abundant alkaline ions such as K+ and Na+ and divalent cations such as Mg2+ and Zn2+ can work as charge carriers in aqueous batteries.7-10 Among them, the aqueous zinc ion batteries (ZIBs) are becoming more and more attractive because of the abundance and low cost of zinc anode as well as high stability in aqueous electrolytes.11-14 A typical aqueous Zn-ion battery consists of the oxide nanostructure (e.g., V2O5) as cathode and Zn metal as anode, with the aqueous electrolyte in between.9,15 It should be noted that the cathode material is more crucial to the capacity and specific energy of batteries, because cathode material has lower capacities than anode material and thus affect the battery capacity more. Although vanadium-based materials (e.g., Zn0.25V2O5·nH2O),9 manganese-based oxides (e.g., MnO2),16 Prussian blue analogs,17 have been studied as cathodes for ZIBs, the capacity, rate capability and cycle stability of ZIBs are still unsatisfactory for future practical applications. For example, MnO2 suffers from fast capacity decay and poor rate capability owing to its intrinsic poor conductivity. Prussian blue provides ~30 mAh g−1 limited capacity and suffers from oxygen evolution occurs when operated at ~1.7 V.16,17 Recent studies reveal that layered vanadates (Na2V6O16·1.63H2O,18 Zn2V2O7,19 Zn3V2O7(OH)2·2H2O,20) stand out as potential cathode materials for ZIBs, due to the facile electrochemical Zn2+ intercalation and high reversibility. Mai et al. reported that the existence of interlayer metal ions and structural water can contribute significant enhancement to the cycling stability of ZIBs, since they probably serve



as “pillars” to improve the structural stability during repeated charge/discharge process and provide a fast zinc ion diffusion path.18,21 However, the synthesis methods of most layered vanadates require complicated processes and precise control of synthesis parameters such as pH, temperature and concentrations of the solutions, and thus pose challenges for industrial production of ZIBs.9,18-25 More importantly, the newly developed cathode materials for ZIBs cannot achieve a decent capacity of above 150 mAh g−1 at high rates of above 5 A g−1. For instance, Kim et al. synthesized Zn2V2O7 nanowires at 210oC via a hydrothermal method, which delivered a low capacity of 138 mAh g−1 at 4 A g−1.19 Alshareef et al. reported that Zn3V2O7(OH)2·2H2O nanowires prepared at 180oC by a hydrothermal method provided a low capacity of only 76 mAh g−1 at 3 A g−1.20 Therefore, there is a timely need for developing “green” and scalable synthesis of vanadate-based cathode materials with high durability, high capacity and low cost. In this regard, Chen et al. proposed a scalable “liquid-solid stirring” approach to produce NaV3O8·1.5H2O nanobelts, and discovered that the NaV3O8·1.5H2O nanowires as cathode material can provide a low capacity of 84 mAh g−1 at 1A g−1 after 100 cycles in ZnSO4 electrolyte, with a capacity retention of only 34%, but the addition of NaSO4 into the ZnSO4 electrolyte could improve its capacity retention up to 90% by inhibiting the dissolution of NaV3O8·1.5H2O in the electrolyte and suppressing the Zn dendrite deposition.26 Moreover, Mai et al. examined Zn/V2O5 battery system with aqueous hybrid-ion (Zn2+ and Li+) electrolyte, displaying a low capacity of 238 mAh g-1 at 50 mA g-1 but a higher discharge platform can be achieved.27 Nevertheless, there is still plenty of room to improve its cycling stability and rate performance for practical applications in ZIBs. In addition, it would be very promising to extend such “liquid-solid stirring” approach into large-scale production of other vanadate nanobelts such as potassium or zinc vanadate nanobelts for new aqueous rechargeable batteries.


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Herein, we prepared NaV3O8·1.35H2O (NVO) nanobelts at room temperature through a modified “liquid-solid stirring” approach by a simple sonication pretreatment of vanadium precursors during the synthesis process. When examined as new cathode for zinc-ion battery system with aqueous ZnSO4 electrolyte, the NVO nanobelts exhibit a high specific capacity of 366 mAh g−1 at 0.1 A g−1 and retain the capacity of 204 mAh g−1 at an extremely high current density of 10 A g−1 after 200 cycles, with capacity retention of as high as ~97%. Meanwhile, it displays a high Coulombic efficiency within the range of 98-100%. Noteworthily, for the first time, we discovered that such a room-temperature synthesis can be generalized to obtain Zn3V2O8·1.85H2O (ZnVO) and KV3O8·0.51H2O (KVO) nanobelts by simply replacing the sodium precursors (NaCl) with other low-cost zinc and potassium precursors (e.g., ZnSO4, KCl). When evaluated as new cathodes for ZIBs, ZnVO and KVO nanobelts exhibit excellent cycle stabilities over 200 cycles with highly reversible capacities of 205 and 110 mAh g-1 at 10 A g-1, respectively. In comparison with the recent reports on vanadium-based cathode materials, the NVO and ZnVO nanobelts demonstrate the superior battery performance based on rate performances and cycle stabilities. This work offers a new strategy for large-scale production of layered vanadates for promising applications as aqueous ZIB cathode materials.

2. Experimental Synthesis of vanadate nanobelts Synthesis of NaV3O8·1.35H2O: In a typical synthesis, 0.5g commercial V2O5 powder was mixed with 50 mL DI H2O under sonication for 30 min, and then 0.1 M NaCl was added into the above solution at room temperature (~25 °C) under vigorous stirring for 3 days. The final



product was washed using DI water and ethanol repeatedly. Then, the NaV3O8·1.35H2O nanobelt was collected by centrifugation, and dried at 80°C for 12 h.

Synthesis of Zn3V2O8·1.85H2O: In a typical synthesis, 0.5g commercial V2O5 powder was mixed with 50 mL DI H2O under sonication for 30 min, and then 0.1 M ZnSO₄·7H₂O was added into the above solution at room temperature under vigorous stirring for 6 days. After repeated wash with DI water and ethanol, the sample was collected by centrifugation.

Synthesis of KV3O8·0.51H2O: followed the same synthesis procedure as NVO by simply replacing the NaCl precursor with KCl for potassium sources.

Materials characterizations The X-ray diffraction (XRD) measurements were conducted using a Rigaku MiniFlex Xray diffractometer (Cu Ka radiation) with a scan rate of 2° min-1. The scanning electron microscopy (SEM) images were captured on a FEI Quanta 3D FEG FIB/SEM. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations were carried out using a JEOL JEM-2010 microscope at 200 kV. The thermogravimetry (TG) measurements were performed using an SII STA7300 analyzer under the nitrogen atmosphere.

Electrochemical measurements The working electrode was fabricated by rolling Vanadate nanobelts (60 wt%) as the active material, Super P (30 wt%) as the conductive material, and polytetrafluoroethylene (10 wt%) as the binder into thin film. To evaluate the electrochemical performances, the battery


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testing was conducted in 2032 coin-type cells between 0.4-1.4 V, with 1 M ZnSO4 solution as the electrolyte, glass microfiber filter as separator, and Zn metal as anode. Before electrochemical measurements, 12 h aging treatment of all assembled coin cells was performed. Galvanostatic charge/discharge experiments were conducted on an battery analyzer (MTI Corporation) between 0.4 and 1.4 V. Cyclic voltammetry (CV) measurements were done employing an electrochemical work station (CHI 6504C) between 0.4 and 1.4 V at 0.1 mV s-1.

3. Results and discussion The typical synthesis procedure of NVO, ZnVO, and KVO nanobelts are schematically presented in Figure 1. As the vanadium precursor, commercial V2O5 powders are first pretreated with sonication in water to break them into smaller particles. Then the metal salts are added into the dispersion under vigorous stirring for a certain time at room temperature. During this process, the dissolution of V2O5 generates free vanadium species (e.g., VO2+ and [V10O28]6-), then such species react with Na+/Zn2+/K+ to form nuclei of metal vanadates on the surface of V2O5 particles and gradually grow into nanobelt-like structure, as the color of the solution can be observed to turn dark yellow.26-27 After washing, three kinds of nanobelts are obtained. The sonication pretreatment of vanadium precursor can not only shorten the reaction time to generate NVO from previously reported 96 h to 72 h, but also may affect the morphology and structure of final products, which can likely benefit their electrochemical performances.26 The NVO nanobelts are synthesized via a cost-effective, additive/solvent-free method without any additional annealing step, thus this facile green method possesses a huge potential for large-scale industrial production. More importantly, we demonstrate that this room-temperature synthesis strategy can



be generalized to obtain nanobelt-structured potassium vanadate (KVO) and zinc vanadate (ZnVO) by simply replacing sodium precursor with potassium and zinc precursors. The XRD pattern of NVO nanobelts is well indexed with a standard PDF card (Space group: P21/m, PDF#16-0601), confirming the crystalline structure and phase purity of NVO (Figure 2a). The strong peaks from the XRD pattern located at 11.70, 25.93, 28.35, 29.77, 40.06, 46.26 and 50.83° are consistent with previous reports.26,28 In the NVO system, hydrated Na+ ions exist between the V3O8 layers to form a layered structure, where the V3O8 layers are composed of VO6 octahedrons and edge-sharing VO5 tetragonal pyramid units. The weight percentage of crystalline water is determined by thermogravimetric analysis (TGA) between 100 and 350 °C, as revealed in Figure 2 d, e, and f. The weight loss is attributed to the evaporation of physically absorbed water (30-100 °C) and crystalline water (100-350 °C). The weight loss of lattice water is 6.4 wt%, corresponding to 1.35 molecules of water per NaV3O8 unit (Figure 2d). In addition, the crystalline phase of the as-synthesized ZnVO and KVO are also tested by X-ray diffraction, as shown in Figure 2b-c. The diffraction peaks of ZnVO nanobelts sample are in good agreement with Zn3V2O8, well indexed with a standard PDF card (PDF#19-1468), where ZnVO is comprised of distorted octahedral zinc ions bonded with vanadium tetrahedrons. The strong peaks from the XRD pattern of KVO at 11.07, 22.15, 25.83, 28.35, 50.67° are well matched with the strong peaks from a standard PDF card (PDF#51-0379), confirming the high purity of KVO, where the KVO consists of V3O8 layers and interstitial hydrated K+. It should be noted that the crystal structure of NVO is very similar to KVO by employing different hydrated metal ions (Na+ or K+) acting as pillars in the V3O8 layers, while ZnVO shows a porous framework structure. According to TGA analysis shown in Figure 2e-f, the weight percentages of crystalline water are


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6.6 wt% and 2.7 wt% for ZnVO and KVO samples, and the molecular formulas are Zn3V2O8·1.85H2O and KV3O8·0.51H2O, respectively. Scanning electron microscopy (SEM) imaging is conducted to compare the morphologies of the as-synthesized vanadate nanobelts. The belt-shape nanostructure is confirmed from the SEM images in Figure 3a-c. Such unique nanobelt architecture is quite different from the commercial V2O5 power with uneven sizes ranging from 24 to 667 µm. It is observed that individual nanobelts with average widths of ~25nm are entangled into small bundles for NVO. The ZnVO is composed of nanobelts with average widths of ~40 nm entangled into bundles, and the bundles are much larger than NVO. The KVO shows that the average widths of individual nanobelts is ~25nm, which is very similar to the morphology of NVO. Furthermore, transmission electron microscopy (TEM) images in Figure 4a and b present the formation of small bundles through stacking of individual nanobelts. From the HRTEM image in Figure 4b, a single nanobelt shows distinct lattice fringe areas apart from areas without fringe, suggesting that the NVO nanobelts contain crystalline and amorphous areas. The interlayer spacing of 0.19 nm matches well with the (204) lattice fringe. The electrochemical performances of the NVO, ZnVO, and KVO nanobelts are examined for the assembled Zn/Vanadate batteries using a 1M mild aqueous ZnSO4 electrolyte. The initial three CV curves of NVO, ZnVO and KVO at a scan rate of 0.1 mV s-1 with a voltage range of 0.4-1.4 V vs Zn2+/Zn are shown in Figure 5a-c, respectively. As for the NVO electrode, it can be seen that a small shoulder peak at 0.9 V and two reduction peaks at 0.81 V and 0.52 V in the first CV curve, resulted from the multistep Zn2+ intercalation into the layered structure. Two individual oxidation peaks at 0.75 V and 0.97 V followed by a small peak at 1.07 V are attributed to the corresponding deintercalation of zinc ions. The reduction scan of the first cycle



in the CV curves is slightly different from the reduction scan of the second and third cycles based on the peak positions, but the oxidation curves have very similar peak positions for these three cycles. The reduction peak shift can be ascribed to the activation of NVO nanobelts during the discharge process.24 The subsequent cycles exhibit two oxidation peaks at 0.96 and 0.76 V, and two reduction peaks at 0.84 and 0.59 V, which is consistent with previous reports.18,24 Similar to the NVO electrode, the ZnVO and KVO electrodes are in the activation process during the first CV curves. The next two cycles of ZnVO have oxidation peaks at 0.96 and 0.80 V, and reduction peaks at 0.82 and 0.61 V, while subsequent CV curves of KVO display oxidation peaks at 1.0 and 0.81 V, and reduction peaks at 0.85 and 0.61 V. Furthermore, subsequent CV curves of NVO, ZnVO, and KVO show similarity and repeatability except for the initial cycle, indicating the superior electrochemical reversibility of these three electrodes. In addition, according to their CV curves in Figure 5a-c, they show very similar CV curves after three cycles, suggesting that the capacities mainly originate from the valence state change of vanadium in these layered vanadates during Zn2+ intercalation/deintercalation process. The cations (Na+, Zn2+, or K+) only act as pillars to stabilize the V3O8 layers and may not be extracted during cycling. Therefore, in our case, these cations (Na+, Zn2+, and K+) contribute no capacity for our zinc-ion battery system, which is consistent with previous publicatons.18, 26 The galvanostatic charge and discharge curves of different electrodes between 0.4 V and 1.4 V under different current densities are shown in Figure 6a-c and Figure S1-S3. The rate capability is conducted from 0.1 to 10 A g-1, and then goes back to 0.5 A g-1. The chargedischarge curves of NVO at 0.1 A g-1 show stable plateaus located at 0.79/ 1.02 V for the charge process, and 0.58/ 0.89 V for the discharge process, which are in agreement with the oxidation and reduction peaks from the CV curves (Figure 5a, Figure S2). The ZnVO electrode displays


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stable plateaus at 0.75/ 1.03 V for the charge process and 0.59/ 0.8 V for the discharge process, while the KVO electrode presents stable plateaus at 0.78/ 1.01 V for the charge process and 0.61/ 0.81 V for the discharge process at 0.1 A g-1 (Figure S3-S4). The oxidation and reduction peaks of CV curves from both ZnVO and KVO are well consistent with their plateaus as displayed in the charge-discharge curves (Figure 5b-c). In addition, all the electrodes demonstrate excellent rate capabilities. At 0.1 A g-1, the corresponding discharge capacities of NVO, ZnVO, and KVO are 366, 328, and 316 mAh g-1 respectively, and the electrodes retain capacities of 149.4, 122.8, and 113.1 mAh g-1 even at ultra-high current density of 10 A g-1. These values are higher than the previously reported Na2V6O16·0.51H2O (353 and 162 mAh g-1 at 0.05 and 2 A g-1),

18

Zn3V2O7(OH)2·2H2O (200 and 54 mAh g-1 at 0.05 and 3 A g-1),20 KV3O8 (87 mAh g-1 at 0.5 A g1

) respectively,25 and comparable to other studies reported in literature.15,19,21,24,26 After the

cycling test at extremely high current densities, all these three electrodes exhibit average reversible capacities of 278, 242.9, and 246.5 mAh g-1 at 0.5 A g-1 respectively, well demonstrating

their

superior

rate

capabilities.

This

implies

that

the

zinc

ion

intercalation/deintercalation process is not hampered at high current densities. In summary, the NVO electrode exhibits the best rate capability with the slightest capacity fading under various current densities, and ZnVO displays comparable rate performance to KVO. To further evaluate cycling performances of these three electrodes at high current densities, the electrodes are tested at 1 A g-1 for over 100 cycles. As observed from Figure 6d, the NVO electrode delivers a high discharge capacity of 321 mAh g-1 from the second discharge process, maintaining as high as 251 mAh g-1 (78% capacity retention) after 100 cycles with a Coulombic efficiency around 98% after the first cycle. The ZnVO and KVO electrodes exhibit high initial discharge capacities of 281.67 and 228.1 mAh g-1 with 77.7% and 90% capacity



retention respectively for over 100 cycles (Figure 6e-f). Moreover, the NVO, ZnVO, and KVO electrodes deliver highly-reversible discharge capacities of 204, 212, and 111 mAh g-1 in average for over 200 cycles respectively under an extremely high current density of 10 A g-1(Figure 6g-i). It is remarkable that the NVO and KVO show no capacity loss after long-term cycles, maintaining as high as 200 and 111 mAh g-1 respectively, and ZnVO shows over 97% retention of the highest discharge capacity, meanwhile they display high Coulombic efficiencies in the range of 97-100%. The different electrochemical performances of three vanadates are likely attributed to the following reasons: First, as shown in Figure 3 a-c, the NVO, ZnVO, and KVO display similar nanobelt-like shapes, but the degree of nanobelts entanglement and average widths are different. The NVO and KVO show relative sparse distribution and smaller nanobelt widths when compared to ZnVO. The sparse distribution and slender morphologies of NVO and KVO nanobelts may effectively inhibit the aggregation during long-term cycling, leading to a more stable cyclability with better capacity retention than ZnVO. Second, according to previous publications,15,18,24 the water molecules in the crystal structure of cathode materials play a positive role in enhancing their electrochemical performances in Zn-ion batteries. In this regard, we speculate that different crystal water content in the as-prepared NVO, ZnVO and KVO may affect their electrochemical performances as well. The specific compositions of these three vanadates are NaV3O8·1.35H2O, Zn3V2O8·1.85H2O and KV3O8·0.51H2O, in which the NVO and ZnVO possess more lattice water compared to KVO. Benefiting from sufficient structural water, the NVO and ZnVO electrodes show higher capacities of 366 and 328 mAh g-1 respectively than KVO with a capacity of 316 mAh g-1 at the same specific current of 0.1 A g-1. The synthesis method and electrochemical performance of the recently reported vanadium-based cathode materials for zinc-ion battery are summarized in Table S1. The NVO,


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ZnVO, and KVO materials in this work are synthesized through a cost-effective and roomtemperature approach with a great potential for commercial production, while the recently reported vanadium-based cathode materials require complex and multistep synthesis procedure. Furthermore, the NVO, ZnVO, and KVO materials demonstrate superior electrochemical performance with respect to specific capacity among recently reported cathode materials. It should be noted that the sonication treatment of commercial vanadium oxide precursor plays an important role in the morphology of the vanadates, and may affect the electrochemical performance of the NVO nanobelts. The NVO-2 nanobelts are synthesized following the same procedure as NVO without the pre-sonication of vanadium oxide precursor (denoted as NVO-2). As shown in Figure S5a, the characteristic diffraction peaks of NVO-2 are consistent with NVO, well indexed with the standard PDF card (#16-0601), confirming they are the same compound. The SEM image of the NVO-2 displays small bundles entangled by nanobelts without uniformity of the NVO sample, with the widths ranging from 280 nm to 610 nm (Figure S5b). Moreover, the cycling performances of NVO and NVO-2 under the same current density of 1 A g-1 are evaluated in Figure S6. It shows that the NVO electrode provides an initial discharge capacity of 247 mAh g-1, higher than the initial discharge capacity of 159 mAh g-1 from the NVO-2 electrode. The NVO electrode could maintain as high capacity as 251 mAh g-1 after 100 cycles, while the capacity of NVO-2 electrode decrease drastically to 145 mAh g-1. It is worthwhile to note that the difference of average discharge capacity between NVO electrode and NVO-2 electrode is 103 mAh g-1, indicating pre-sonication of vanadium oxide precursor can effectively reduce aggregation of the finial product, and further boost the electrochemical performance of vanadates.



To

understand

the

electrochemical

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insertion/extraction

mechanism

during

discharge/charge processes of the Zn//NVO batteries, the ex situ XRD analysis is performed as displayed in Figure 7a. The XRD patterns of the NVO electrode material before and after (charge at 1.4 V) electrochemical reactions show a similar pattern, demonstrating the reversible intercalation/deintercalation mechanism in NVO. When discharging at 0.4 V, the (001) peak shifts to a lower angle (from 11.52° to 10.52°), which corresponds to an increase of cell parameter c from 0.767 nm to 8.40 nm, possibly ascribed to the Zn2+ (0.074 nm) intercalation. It should be noted that we perform the ex situ XRD characterization of three different electrochemical states (initial state, discharge at 0.4V and charge at 1.4V). The three states are from three coin cells, as a coin cell cannot be taken apart and reassembled. The loading masses of the electrodes in three coin cells may be slightly different. Therefore, the intensity of XRD is stronger after discharging at 0.4V probably due to its higher loading mass than other electrodes. As presented in Fig. 7a, the ex situ XRD results reveal that there is no noticeable formation of new phases, but only a peak shift of (001) occurs due to the Zn2+ insertion into the V3O8 layers of these vanadates at fully discharged state. Subsequently, the sample at fully charged state displays a very similar XRD pattern to the initial state and no new peaks appear, confirming that these Na+, Zn2+, and K+ cations remain in the crystal structure. To

further

investigate

the

multistep

redox

reactions

during

Zn2+

intercalation/deintercalation, ex situ XPS spectra of NVO electrode at initial, discharge and charge states are presented in Figure 7b-d. As seen from Figure 7b, the NVO electrode at initial state shows the V 2p3/2 peak at 517.55 eV, and the V 2p1/2 peak at 525.15 eV,26,29 corresponding to the V 2p3/2 -V 2p1/2 spin-orbit doublet for V (V), and the V2p3/2/V2p1/2 peaks located at 516.21/523.70 eV are attributed to V (IV), which may be caused by the impurity of V2O5


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precursor. When discharged to 0.4 V, The V2p3/2/V2p1/2 binding energy values of 517.85/526.06, 516.69/524.46, and 515.34/522.98 eV are assigned to V (V), V (IV) and V (III), respectively. This result indicates that the V (V) is reduced to V (IV) and V (III) during the discharge process. When recharged to 1.4 V, it is found that most of the V (IV) and V (III) can be reoxidized to V (V), confirming the highly reversible intercalation/deintercalation of zinc ions in NVO. The excellent electrochemical performances of these electrodes are attributed to the open layered structure of the vanadates as well as high surface area and short diffusion distance provided by the nanobelt nanostructure. The facile, green, room-temperature synthesis method in this work can be widely applicable to other high-valence-state metal oxides, opening new research revenue in large-scale production of electrode materials for next-generation rechargeable batteries.

4. Conclusions In summary, nanobelts of various vanadate compounds have been successfully synthesized via a green, cost-effective, easily large-scale, and universal method without any additional calcination step. When evaluated in aqueous zinc ion rechargeable batteries, all vanadates nanobelts, including NVO, ZnVO, and KVO show great electrochemical performance in terms of the reversible specific capacity, long-term cycle stability, and rate performance. Among them, the NVO nanobelts exhibit the highest specific capacity of 366 mAh g-1 at 0.1 A g1

, reversible capacity of 322 mAh g-1 at 1 A g-1, and retained capacity of 200 mAh g-1 up to 200

cycles at an ultra-high current density of 10 A g-1. In addition, ZnVO nanobelts show a high capacity of 328 mAh g-1 at 0.1 A g-1, 294 mAh g-1 at 1 A g-1, and remained capacity of 206 mAh g-1 at 10 A g-1 after 200 cycles, while KVO nanobelts display comparable capacity of 316 mAh g-1 at 0.1 A g-1, 253 mAh g-1 at 1 A g-1, and 111 mAh g-1 at 10 A g-1 after 200 cycles respectively.



As revealed by the ex situ XRD and XPS characterizations, the vanadate electrodes with such high capacities are attributed to the highly reversible Zn ions intercalation/deintercalation process. Our work demonstrates that the vanadate nanobelts are not only highly promising cathode materials for ZIBs, but also boost further development of vanadium-based layered structures for grid-scale energy storage applications.

ACKNOWLEDGMENTS We acknowledge the Economic Development Assistantship from LSU graduate school, and the Research Enhancement Award (REA) sponsored by LaSPACE for financial support. We would like to thank the shared Instrumentation Facilities (SIF) at Louisiana State University for using XPS, XRD, SEM, and TEM. We thank Prof. Q. L. Wu, and Dr. X. X. Sun at School of Renewable Natural Resources of LSU for using Thermal Gravimetric Analyzer.

Supporting Information The Supporting Information is available: The SEM image of commercial V2O5 powder; chargedischarge curves of the NVO, ZnVO, KVO at different current densities ranging from 0.1 to 10 A g-1; X-ray diffraction pattern of the NVO-2 nanobelts and corresponding SEM image; cycling performances of NVO and NVO-2 at 1 A g-1; comparison of our work with recently reported vanadium-based cathode materials.


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Figures Captions: Figure 1. Schematic illustration of green synthesis of NVO, ZnVO, KVO nanobelts at room temperature. Figure 2. (a-c) X-ray diffraction patterns of NVO, ZnVO, and KVO nanobelts respectively, (d-f) Corresponding thermogravimetric analysis (TGA) of NVO, ZnVO, and KVO. Figure 3. (a-c) SEM images of NVO, ZnVO, and KVO nanobelts respectively. Figure 4. (a) TEM image of NVO, (b) high-resolution TEM image with lattice fringes of 0.19 nm, corresponding to the (204) plane. Figure 5. (a-c) Cyclic voltammograms of NVO, ZnVO, and KVO between 0.4 and 1.4 V during the first three cycles. Figure 6. (a-c) Rate capability of NVO, ZnVO, and KVO respectively with current densities ranging from 0.1 to 10 A g-1, and corresponding cycling performances at (d-f) 1 A g-1 and (g-i) 10 A g-1. Figure 7. Ex situ XRD and XPS of the NVO electrodes at different electrochemical states under the current density of 0.2 A g-1: (a) Ex situ XRD patterns, (b) Ex situ XPS of V 2p at initial state, (c) the 1st discharge (0.4 V), and (d) the 1st charge (1.4 V). Table of Contents Graphic


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Figure 1. Schematic illustration of green synthesis of NVO, ZnVO, KVO nanobelts at room temperature.



Figure 2. (a-c) X-ray diffraction patterns of NVO, ZnVO, and KVO nanobelts respectively, (d-f) Corresponding thermogravimetric analysis (TGA) of NVO, ZnVO, and KVO.


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Figure 3. (a-c) SEM images of NVO, ZnVO, and KVO nanobelts respectively.



Figure 4. (a) TEM image of NVO, (b) high-resolution TEM image with lattice fringes of 0.19 nm, corresponding to the (204) plane.


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Figure 5. (a-c) Cyclic voltammograms of NVO, ZnVO, and KVO between 0.4 and 1.4 V during the first three cycles.



Figure 6. (a-c) Rate capability of NVO, ZnVO, and KVO respectively with current densities ranging from 0.1 to 10 A g-1, and corresponding cycling performances at (d-f) 1 A g-1 and (g-i) 10 A g-1.


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Figure 7. Ex situ XRD and XPS of the NVO electrodes at different electrochemical states under the current density of 0.2 A g-1: (a) Ex situ XRD patterns, (b) Ex situ XPS of V 2p at initial state, (c) the 1st discharge (0.4 V), and (d) the 1st charge (1.4 V).



Table of Contents Graphic:


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Green Synthesis of Vanadate Nanobelts at Room Temperature for

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