Hybridizing δ-type NaxV2O5·nH2O with graphene towards high-performance aqueous zinc-ion batteries

Electrochimica Acta 321 (2019) 134689

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Hybridizing d-type NaxV2O5$nH2O with graphene towards highperformance aqueous zinc-ion batteries Weijun Zhou a, Jizhang Chen a, *, Cuilan He a, Minfeng Chen a, Xinwu Xu a, Qinghua Tian b, Junling Xu c, Ching-Ping Wong c, d, ** a

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, 210037, China Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou, 310018, China Department of Electronic Engineering, The Chinese University of Hong Kong, NT, Hong Kong, China d School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, USA b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 June 2019 Received in revised form 4 August 2019 Accepted 12 August 2019 Available online 13 August 2019

Thanks to low cost and high safety, aqueous zinc-ion batteries (AZIBs) are attractive candidates for largescale energy storage applications, while suitable cathode materials are needed urgently. In this work, dNaxV2O5$nH2O with a large interlayer spacing of 10.6 Å is reported as the cathode material for aqueous zinc-ion batteries for the first time. It is found that d-NaxV2O5$nH2O performs better than Na2V6O16$nH2O under the same condition, and its electrochemical performances can be significantly improved after hybridizing it with reduced graphene oxide. The obtained nanocomposite can deliver high reversible capacity of 433.5 mAh g
1 at 0.1 A g
1, superior rate capability of 244.1 mAh g
1 at 5 A g
1, and good cyclability of 70.5% over 1000 cycles. Such great performances origin from the synergetic effect of d-NaxV2O5$nH2O and graphene, which enables rapid Zn2þ diffusion and large capacitive contribution. Besides, the ex-situ measurement results demonstrate good structural stability and reversible Zn2þ ion storage behavior of reduced graphene oxide/d-NaxV2O5$nH2O nanocomposite. This work extends the knowledge into the material technology of AZIBs. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Sodium vanadate Layered structure Graphene Aqueous batteries Zinc ion storage

1. Introduction With the advantage of high energy density, lithium-ion batteries (LIBs) are dominating the commercial rechargeable battery market. However, their large-scale applications are restricted by high price and poor safety, as well as the finite lithium-based resources [1e3]. As an alternative battery chemistry, aqueous zinc-ion batteries (AZIBs) have proven highly promising [4], primarily owing to the following three factors: (1) metallic Zn is stable in water, has a relatively low redox potential (
0.76 V vs. standard hydrogen electrode), and possesses a high theoretical specific capacity (820 mAh g
1, 5855 mAh cm
3); (2) zinc-based resources are abundant on the earth; and (3) the usage of aqueous electrolytes not only reduces the battery manufacturing cost but also enhances

* Corresponding author. ** Corresponding author. Department of Electronic Engineering, The Chinese University of Hong Kong, NT, Hong Kong, China. E-mail addresses: [email protected], [email protected] (J. Chen), [email protected] (C.-P. Wong). https://doi.org/10.1016/j.electacta.2019.134689 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

safety and favors high rate capability [5e7]. Nevertheless, the Zn anode in AZIBs suffers from the problems of dendrite growth, H2 evolution, and surface passivation, to address which the strategies including constructing Zn anode with three-dimensional hierarchical architecture, coating thin inert layer onto the Zn anode, and introducing additives into the electrolyte have been reported [3e5,8]. What's more, the development of AZIBs is hindered by the lack of suitable cathode materials [2,3]. The reason behind that is the large atomic mass and high valence state of Zn2þ, resulting in sluggish transport of Zn2þ within cathode materials. Manganesebased oxides are the most studied cathode materials for AZIBs [9e13], whereas they usually suffer from unsatisfactory rate performances due to slow reaction kinetics of conversion reactions [14]. Other materials such as Prussian blue analogues [15e17], polyanionic compounds [18,19], Mo-based compounds [20], and organic and polymer compounds [21e24] have also been investigated as the cathode materials, however, are confronted with the problem of low specific capacitance. Ever since Nazar et al. reported Zn0.25V2O5$nH2O with the layered structure as a high-capacity and long-life cathode material

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for AZIBs [25], considerable research efforts have been devoted to developing vanadium-based cathode materials, which feature large interlayer spacing, high theoretical specific capacity, and low price [26e29]. For instance, Zhou et al. constructed cotton-like LixV2O5$nH2O with a rather large interlayer spacing of 13.77 Å and achieved great performances [30]. Particularly, several research works have focused on sodium vanadates that manifested favorable electrochemical performances [31e34]. For example, Kim et al. reported Na2V6O16$3H2O nanorods with layered structure, which exhibit high capacity of 361 mAh g
1 at 0.1 A g
1, great rate performance (230 mAh g
1 at 6 A g
1), and good cycling stability (80% capacity retention over 1000 cycles) [31]. Meantime, Mai et al. and Liang et al. disclosed that b-NaxV2O5 with three-dimensional (3D) tunneled structure can also deliver great performances [32,33]. As yet, d-NaxV2O5$nH2O (denoted d-NVO) with the layered structure has not been studied for AZIBs, although this material has presented great performances for LIBs [35,36], sodium-ion batteries [37,38], and supercapacitors [39]. In d-NVO, the interlayer H2O molecules and Naþ ions could act as pillars to stabilize the V2O5 layers and shielding the electrostatic interactions between inserted cations during the charge/discharge process. Nevertheless, like other vanadium-based cathode materials, d-NVO is intrinsically low in electrical conductivity, which is definitely negative for electrochemical energy storage. To mitigate this issue, graphene has been used by several groups as the scaffold to support vanadium-based cathode materials [34,40e42]. Firstly, graphene functions as a super highway for electrons, therefore improving the overall electrical conductivity of the composites. Secondly, the large surface to volume ratio of graphene can restrain vanadium-based materials from aggregation, thus endowing the composites with high access to electrolyte ions. Last but not least, the graphene scaffold can effectively buffer the strain/stress associated with electrochemical redox reactions of vanadium-based materials. Herein, we demonstrate for the first time d-NVO with a large interlayer spacing of 10.6 Å as the cathode materials for AZIBs, which outperforms previously reported Na2V6O16$nH2O under the same test environment. A facile one-step hydrothermal method is used to combine d-NVO with highly conductive reduced graphene oxide (rGO) scaffold. The obtained rGO/d-NVO nanocomposite can give significantly improved performances for AZIBs in comparison with the contrast samples. In order to elucidate the great performances of rGO/d-NVO, follow-up examinations were conducted. It is found that pseudocapacitive Zn2þ intercalation accounts for a large proportion of the total capacity, e.g., 78.4% at 1 mV s
1. In addition, rGO/d-NVO possesses much higher Zn2þ ion diffusion coefficient than that of Na2V6O16$nH2O and much lower charge transfer impedance than that of Na2V6O16$nH2O and d-NVO. We also paid attention to the fundamental Zn2þ storage mechanism of rGO/d-NVO, and noticed that the layered structure of d-NVO remains stable upon electrochemical process with no new phase detected, and the Zn2þ intercalation is accompanied by reversible redox transitions of vanadium element and reversible H2O uptake/ release. Besides, it is found that the (de)intercalation-induced morphological and structural transformations of rGO/d-NVO are slight over cycles. 2. Experimental section 2.1. Material synthesis Graphene oxide (GO) was synthesized according to our previous report [43]. For the synthesis of rGO/d-NVO, 240 mg of GO was dispersed in 60 mL aqueous solution containing 0.05 M V2O5 and 0.05 M NaOH. Subsequently, the suspension was transferred into an autoclave and kept at 180  C for 24 h. The precipitate (i.e., rGO/d-

NVO) from hydrothermal reaction was washed with deionized (DI) water for three times, and then freeze dried. Besides, neat rGO was synthesized via a same procedure except for that V2O5 was not added. For comparison, hydrothermal reaction was also carried out when GO was not added while other conditions remain the same as that of rGO/d-NVO. The obtained product is Na2V6O16$nH2O rather than d-NVO. Instead, d-NVO can be produced when the concentration of NaOH was reduced from 0.05 to 0.04 M in the hydrothermal reaction. 2.2. Characterization The morphology and microstructure were analyzed by field emission scanning electron microscope (FE-SEM, JEOL JSM-7600F) and transmission electron microscope (TEM, JEOL JEM-2100F) equipped with an energy dispersive X-ray spectroscopy (EDX) detector. X-ray diffraction (XRD) measurements were performed on a Rigaku Ultima IV powder X-ray diffractometer with Cu Ka radiation source (l ¼ 1.5406 Å). Raman spectra were collected by a Themor DXR532 Raman spectrometer (l ¼ 532 nm). The elemental oxidation states, Na/V ratios, and porous information were examined by X-ray photoelectron spectrometer (XPS, Kratos AXIS UltraDLD), inductively coupled plasma optical emission spectrometer (ICPOES, Varian VISTA-MPX), and N2 adsorption/desorption analyzer (Quantachrome Autosorb-iQ2-MP), respectively. The chemical compositions were further measured using thermogravimetric analyzer (TGA, TA Instruments Q5000 IR) and elemental analyzer (Elementar Vario EL Cube). The solubility of rGO/d-NVO in the electrolyte after 1000 cycles was evaluated by inductively coupled plasma mass spectrometer (ICP-MS). 2.3. Electrochemical measurements A slurry of 70% active material, 20% carbon black (Super-P), and 10% polyvinylidene fluoride (PVDF) dispersed in N-methylpyrrolidone (NMP) was pasted onto titanium foils and then dried at 80  C for 6 h. After pressed at 10 MPa, the titanium foils were cut into round pieces (12 mm in diameter) as working electrodes. CR2016-type coin cells were assembled in the air atmosphere, using zinc metal foil, Whatman filter paper (GF/A), and 3 M Zn(CF3SO3)2 aqueous solution as the counter/reference electrode, separator, and electrolyte, respectively. The cells were aged for 12 h before conducing galvanostatic charging/discharging (GCD) tests on a LAND battery testing system (CT2001A) at different current densities between 1.6 and 0.2 V vs. Zn2þ/Zn. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI 660E electrochemical workstation. 3. Results and discussion The rGO/d-NVO nanocomposite was synthesized via a one-step hydrothermal method using GO, V2O5, and NaOH as the reactants, as projected in Fig. 1a. The crystallographic structures of rGO/dNVO was studied by XRD, as shown in Fig. 1b. All the diffraction peaks can be indexed to monoclinic d-NaxV2O5$nH2O that consists of double layers of VO5 square pyramids with a considerably large interlayer distance of 10.6 Å, between which Naþ ions and water molecules reside [35e39]. The peaks from rGO are not seen in the XRD pattern due to the low crystallinity of rGO. To obtain more information, the XRD pattern of rGO/d-NVO was refined with the Rietveld method by the FULLPROF software using ICSD 84879 (space group: C2/m) as the original model. The lattice parameters of d-NaxV2O5$nH2O are identified as a ¼ 11.88 Å, b ¼ 3.66 Å, c ¼ 11.26 Å, and b ¼ 109.2 . For comparison, neat rGO,

W. Zhou et al. / Electrochimica Acta 321 (2019) 134689

3

Fig. 1. (a) Illustration of the synthesis procedure and crystallographic structure, (b) XRD pattern and the corresponding Rietveld refinement result, (c, d) SEM images, (e, f) TEM images, and (g) STEM image and corresponding EDX mapping images of rGO/d-NVO.

Na2V6O16$nH2O (JCPDS 16-0601), and d-NVO were also synthesized (see details in Experimental section), and their crystalline phases are confirmed by the XRD patterns (Fig. S1) and Raman spectra (Fig. S2), which coincide with previous reports [31,39,44e46]. For rGO/d-NVO, the Raman peaks located at 163, 267, 423, 502, 688, and 1005 cm
1 (Fig. S2) are attributed to various stretching and bending modes of VeO and VeOeV bonds [32,36,39]. It should be noted that although d-NVO and Na2V6O16$nH2O both have monoclinic phase and layered structure, they have different space group, and the interlayer distance of d-NVO is much larger than that of Na2V6O16$nH2O (7.7 Å). Liang et al. and Cao et al. have confirmed that the large interlayer spacing of vanadates can facilitate fast Zn2þ diffusion and improve electrochemical stability [30,47]. In Fig. 1cee, SEM and TEM images of rGO/d-NVO reveal that dNVO nanobelts are grown on crumpled rGO nanosheets. The nanobelts are a few micrometers long and several hundred nanometers wide, and are randomly oriented. The SEM images of the counterparts are provided in Fig. S3. The rGO shows typical crumpled nanosheet morphology. As for neat vanadate samples, Na2V6O16$nH2O possesses a wire-like morphology, while d-NVO is comprised by nanobelts. In the high-resolution TEM image of rGO/ d-NVO (Fig. 1f), a lattice fringe of 0.196 nm can be observed, belonging to the (
601) plane of d-NaxV2O5$nH2O. Furthermore, scanning transmission electron microscopy (STEM) technology is employed to investigate rGO/d-NVO, since rGO is not easy to be seen from TEM. It is further evidenced by the STEM image and the corresponding EDX mapping images (Fig. 1g) that d-NVO nanobelts are distributed on the surface of rGO. According to TGA (Fig. S4), ICP-OES, and elemental analysis, the mass fraction of rGO in the rGO/d-NVO can be estimated to be 13.52% and the exact molecular formula of d-NVO in the nanocomposite can be determined to be

Na0.48V2O5$0.36H2O. These measurements also reveal that the stoichiometric formulas for neat Na2V6O16$nH2O and d-NVO are Na2V6O16$2.1H2O and Na0.38V2O5$0.48H2O, respectively. Although its mass ratio in the rGO/d-NVO is not high, the rGO plays a great role in affecting the porosity of d-NVO. On the basis of nitrogen adsorption/desorption measurements (the corresponding isotherms are shown in Fig. S5), the Brunauer-Emmett-Teller (BET) surface area (192 m2 g
1) and total pore volume (0.59 cm3 g
1) of rGO/d-NVO are considerably larger than that of neat d-NVO (49 m2 g
1, 0.15 cm3 g
1), thus endowing rGO/d-NVO with much higher access to the electrolyte. As for rGO and Na2V6O16$nH2O, their BET surface area values are 278 and 132 m2 g
1, respectively. The electrochemical Zn2þ storage properties were evaluated by assembling coin cells using 3 M Zn(CF3SO3)2 aqueous solution as the electrolyte and zinc metal foil as the anode. Fig. 2a shows the initial three CV cycles of rGO/d-NVO at a scan rate of 0.2 mV s
1 within a potential window of 0.2e1.6 V vs. Zn2þ/Zn. In the cathodic scan, multiple peaks can be observed, unveiling a multistep Zn2þ intercalation mechanism due to continuous reduction from V5þ to V4þ and V3þ. Subsequently in the anodic scan, multiple peaks are attributed to the converse process. Interestingly, the cathodic branch of the 1st cycle is slightly different from the rest of cycles in respects of peak positions, while the anodic branches in different cycles hold almost the same profile. The shift of cathodic peaks can be attributed to the gradual activation of d-NVO, as observed in previously reported Na2V6O16$3H2O barnesite nanorods [31]. In addition, after 1st cycle, the succeeding CV curves maintain a good reproducibility, demonstrating good electrochemical reversibility. The GCD curves of rGO/d-NVO at 0.1 A g
1 in initial three cycles are presented in Fig. 2b. The nanocomposite delivers an initial discharge capacity of 362.4 mAh g
1, which increases to 416 and

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W. Zhou et al. / Electrochimica Acta 321 (2019) 134689

Fig. 2. (a) CV curves of rGO/d-NVO in initial three cycles at 0.2 mV s
1. (b) GCD curves of rGO/d-NVO in initial three cycles at 0.1 A g
1. (c) Rate performances of four samples at current densities from 0.1 to 5 A g
1. (d) GCD curves of rGO/d-NVO at different current densities.

431 mAh g
1 at 2nd and 3rd cycles, respectively. Such increase is ascribed to the gradual activation of d-NVO. Similar tendency has been reported in previous work about vanadates [32,34]. Accordingly, the curve profile at 1st cycle is slightly different from that at 2nd and 3rd cycles, while the latter two nearly overlap, in good agreement with the CV results. It is also seen that all the charge/ discharge plateaus connect together to form sloped curves, which is a common phenomenon for sodium vanadate cathode materials [31e33]. In the case of rGO, its CV and GCD curves (Figs. S6a and S6d) are typical of that of electrical double layer capacitors (EDLCs). In fact, a Zn-ion hybrid capacitor would be constructed if employing rGO as the cathode material. As for Na2V6O16$nH2O and d-NVO, they also show obvious redox characteristics in CV and GCD curves, whereas their capacities are lower than that of rGO/d-NVO, as shown in Fig. S6. High rate capability is critical for practical applications. As can be seen from Fig. 2c, rGO/d-NVO gives much higher capacities than the three contrast samples throughout all the current densities. When the current density is increased from 0.1 A g
1 progressively, the discharge capacity of rGO/d-NVO at the end of each current density drops from 433.5 to 421.9, 407.6, 387.6, 352.8, and 244.1 mAh g
1 at 0.2, 0.5, 1, 2, and 5 A g
1, respectively. In contrast, the capacity of Na2V6O16$nH2O is 318 mAh g
1 at 0.1 A g
1, which drops rapidly to 86.7 and 20.8 mAh g
1 at 2 and 5 A g
1, respectively. Interestingly, the d-NVO first evaluated in this study can give much higher rate capabilities than that of Na2V6O16$nH2O at current densities from 0.1 to 2 A g
1. It should be mentioned that

although Na2V6O16$nH2O cathode materials with similar morphology and structure to ours were reported to possess good rate performances [31,45,48], our measurement results show dNVO acts better than Na2V6O16$nH2O when they are tested under the same condition. As for rGO, it is not surprising that it exhibits a low specific capacity of 69.8 mAh g
1 at 0.1 A g
1, and when the current density is increased to 5.0 A g
1, 71.6% of that capacity can be retained, since Zn//rGO system functions as a Zn-ion hybrid capacitor. Remarkably, when rGO is combined with d-NVO, thanks to the synergistic effect, the rGO/d-NVO nanocomposite is superior to most of the cathode materials reported to date for AZIBs in terms of rate capability. In Table S1, the capacities of rGO/d-NVO at certain current densities are compared with that of state-of-the-art vanadium-based cathode materials. It can be clearly seen that our rGO/ d-NVO outperforms nearly all these materials, such as rGO/ Na1.1V3O7.9 (72 mA h g
1 at 2 A g
1) [34], b-NaxV2O5 (96.4 mA h g
1 at 2 A g
1) [32], Zn0.25V2O5$0.85H2O (223 mA h g
1 at 4.5 A g
1) [25], Mg0.34V2O5$0.84H2O (81 mA h g
1 at 5 A g
1) [27], (NH4)2V10O25$8H2O (123.6 mA h g
1 at 5 A g
1) [28], and K2V6O16$2.7H2O (207.5 mA h g
1 at 4 A g
1) [49]. The great rate performance of rGO/d-NVO can also be reflected by Fig. S7. In addition, as the current density returns back to 0.1 A g
1, rGO/dNVO is able to recover a reversible capacity of ~420 mAh g
1, manifesting good structural robustness and great electrochemical stability. The corresponding GCD curves of rGO/d-NVO at various current densities are depicted in Fig. 2d. The curve profiles at different rates, especially at high rates such as 2 A g
1, display the

W. Zhou et al. / Electrochimica Acta 321 (2019) 134689

same shape and small polarization, implying fast charge transfer kinetics. In order to further investigate the electrochemical kinetics of three sodium vanadate samples, CV measurements were carried out at various scan rates. The CV curves of rGO/d-NVO from 0.2 to 1.0 mV s
1 are shown in Fig. 3a. With the increase of scan rate, the anodic peaks slightly shift to the positive direction, while the cathodic peaks move to the opposite direction at the meantime, and these peaks become broader gradually, due to the increased polarization at higher scan rates, which is a common phenomenon. Besides, these CV curves maintain similar shapes at all the scan rates, according well with the great rate capability demonstrated by Fig. 2c. The general relationship between the peak current (i) and scan rate (n) can be described using the following equation if assuming that i obeys a power-law relationship with n [50]:

i ¼ anb

(1)

where a and b are adjustable parameters. b ¼ 0.5 indicates the current is controlled by semi-infinite diffusion of Zn2þ, while a b value of 1.0 illustrates surface-induced capacitive behavior. That is, a higher b value would reflect more favored electrochemical kinetics. Based on the slope of log (i) vs. log (n) plots (see Fig. S8c), the b values associated with O1, O2, R1, and R2 peaks (the positions of these peaks are marked in Fig. 3a) of rGO/d-NVO are fitted to be 0.788, 0.910, 0.884, and 0.881, respectively, meaning that the corresponding redox reactions are dominated by capacitive kinetics. Such phenomenon is similar to pseudocapacitive intercalation proposed by Dunn et al. [50]. By analyzing the CV curves of the other two samples (Fig. S8), the b values of Na2V6O16$nH2O and dNVO can also be collected and are compared with that of rGO/dNVO in Fig. 3b. It is found that the b values of rGO/d-NVO are much higher, confirming that the addition of rGO scaffold improves electrochemical kinetics enormously. Furthermore, the current

5

response, i(V) can be quantitatively divided into capacitive (k1n) and diffusion-controlled (k2n1/2) parts using the following equation [27,29,51,52]:

iðVÞ ¼ k1 n þ k2 n1=2

(2)

By determining k1, it is possible to calculate the fraction of current that arises from pseudocapacitive intercalation as a function of the potential. For instance, in Fig. 3c, at a scan rate of 1.0 mV s
1, the shaded area stands for the capacitive part, illustrating the capacitor-like charge storage behavior in rGO/d-NVO occurs to a great extent. Fig. 3d displays the diffusion-controlled and capacitive contributions to the i(V) of rGO/d-NVO at various scan rates. As the scan rate rises, the capacitive contribution ratio increases from 58.5% to 78.4% gradually. This trend is in good agreement with that in previous reports [27,29,51]. The above results indicate that the pseudocapacitive intercalation occupies a higher proportion than the diffusion-controlled one, especially at high rates, which is why rGO/d-NVO can give great rate capability in Fig. 2c. The Zn2þ ion diffusion coefficients (D2þ Zn ) in rGO/d-NVO were determined utilizing the Galvanostatic Intermittent Titration Technique (GITT) proposed by Weppner and Huggins [53]. Before GITT measurements, the cells were first run for 50 cycles to obtain a stable state. Subsequently, a galvanostatic pulse of 20 min at 50 mA g
1 followed by 3 h relaxation was repeatedly applied in order to allow relaxation back to equilibrium until the cutoff potentials are reached, and the corresponding GITT curves are shown in Fig. 3e. The D2þ Zn can be calculated according to the following equation since DEt has a linear relationship with t1/2 (see Fig. S9b):

DZn2þ ¼

    nM VM 2 DEs 2 pt S DEt 4

(3)

where t is the constant current pulse duration (1200 s); nM and VM

Fig. 3. (a) CV curves of rGO/d-NVO recorded at various scan rates from 0.2 to 1.0 mV s
1. (b) b values of different redox peaks of three samples determined from the log (i) vs. log (n) plots. (c) Capacity contribution analysis of rGO/d-NVO at 1.0 mV s
1. (d) Contribution ratios of diffusion controlled and capacitive capacities of rGO/d-NVO at different scan rates. (e) Discharge/charge GITT curves of rGO/d-NVO and (f) the corresponding D2þ Zn values compared with that of Na2V6O16$nH2O and d-NVO.

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W. Zhou et al. / Electrochimica Acta 321 (2019) 134689

are the moles and molar volume of rGO/d-NVO, respectively; S is the geometric area of the electrode; and DEs and DEt are the change in the steady state voltage and the overall cell voltage regardless of the IR-drop during a single GITT pulse, respectively (see Fig. S9a). Despite the divalent nature of Zn2þ ion, the D2þ Zn of rGO/d-NVO is still as high as 10
8.5 to 10
9.5 cm2 s
1, which is comparative to that of neat d-NVO, and is higher than that of Zn0.25V2O5$nH2O [25], H2V3O8 [26], and Fe5V15O39(OH)9$9H2O [54]. Besides, a general decrease trend is observed as more Zn2þ is inserted, due to the growing repulsive force between Zn2þ ions. Moreover, the D2þ Zn of dNVO is more than one order of magnitude higher than that of Na2V6O16$nH2O (10
10.5 to 10
12 cm2 s
1), owing to larger interlayer spacing of d-NVO than that of Na2V6O16$nH2O. The above result also discloses why d-NVO performs better than Na2V6O16$nH2O in terms of rate capability, thus highlighting the importance of this work. Cycling stability is an important parameter for electrode materials. As shown in Fig. 4a, no capacity decay is observed for rGO upon prolonged cycling up to 1000 cycles, since it adopts an EDLCtype charge storage mechanism. Such excellent cycling stability of rGO also reveals that there is no problem with our test environment. For rGO/d-NVO, at 2 A g
1, it provides an initial discharge capacity of 303.7 mAh g
1, which gradually drops to 214.2 mAh g
1 after 1000 cycles, corresponding to a capacity retention of 70.5%. The GCD curves of rGO/d-NVO at different cycles during the longterm cycling are shown in Fig. S10, implying good reversibility. Instead, when rGO is not employed, neat d-NVO shows much inferior cyclability (27.1% capacity retention after 1000 cycles), indicating that the rGO scaffold plays a significant role in improving the electrochemical stability of d-NVO. After 1000 cycles, the electrolyte of rGO/d-NVO coin cells was collected by washing the electrodes, separators, and coin cell shells with DI water. Then the collected electrolyte was analyzed by ICP-MS, which reveals merely 5.93 ppm of vanadium in the rGO/d-NVO can be dissolved.

Therefore, the active material dissolution is not the reason of capacity fading for rGO/d-NVO. However, 18.5% of Naþ in the rGO/dNVO was dissolved in the electrolyte. Compared to d-NVO, Na2V6O16$nH2O offers much lower specific capacity (188.5 mAh g
1) at the 1st cycle, whereas it also suffers from poor cycling stability (34.5% capacity retention at 1,000th cycle). Indeed, we have noticed that Na2V6O16$nH2O exhibits excellent cycling performances in other reports [31,45,48]. Although the Na2V6O16$nH2O synthesized by us performs not well, our contrast test results highlight the high electrochemical stability of rGO/d-NVO. The EIS measurements were carried out with frequency ranging from 0.01 to 100,000 Hz and the amplitude being set as 5 mV, to grasp more insights about the electrochemical kinetics. The Nyquist plots of four electrode materials under two-electrode configuration (coin cells) are shown in Fig. 4b. Generally, the depressed semicircle in the high frequency region represents charge transfer resistance (Rct) at the electrode/electrolyte interface, while the sloped line in the low frequency region is ascribed to ion diffusion in the bulk electrode [55,56]. It is obvious that before cycles, the Rct of rGO/dNVO (85 U) is close to that of rGO, and is significantly lower than that of Na2V6O16$nH2O (340 U) and d-NVO (1139 U), indicative of improved charge transfer kinetics and enhanced activity when dNVO is modified by rGO conductive network. At the end of 1000 cycles, the change of rGO in Nyquist plots is rather small, which is in good line with its great cyclability. As for rGO/d-NVO, the Rct increases slightly to 125 U after 1000 cycles, which is much lower than that of Na2V6O16$nH2O (1050 U) and d-NVO (809 U), supporting the results of Fig. 4a. The EIS measurements were also conducted under three-electrode configuration, in which zinc metal foil and Hg/HgSO4 electrode were used as the counter electrode and reference electrode, respectively. As is shown in Fig. S11, the impedance values of rGO/d-NVO before cycles are much lower than that of Na2V6O16$nH2O and d-NVO, demonstrating a same trend as that under two-electrode configuration. After 1000 cycles,

Fig. 4. (a) Cycling performances at 2 A g
1 and (b) Nyquist plots before and after cycles under two-electrode configuration of rGO/d-NVO, rGO, Na2V6O16$nH2O, and d-NVO. Surface SEM images of (c) fresh rGO/d-NVO electrode and (d, e) rGO/d-NVO electrode after 1000 cycles.

W. Zhou et al. / Electrochimica Acta 321 (2019) 134689

the coin cells were dissembled, and the rGO/d-NVO electrodes were examined by SEM. As can be seen from Fig. 4cee, the surface morphology of the electrode remains nearly unchanged during 1000 cycles. In Fig. 4d, the long dent is caused by the glass fiber from the separator. And it can be clearly observed from Fig. 4e that the nanobelt morphology of d-NVO is well preserved after 1000 cycles, which explains high reversibility of rGO/d-NVO. The charge storage mechanism of rGO/d-NVO was investigated by ex-situ XRD measurements during the second cycle (the corresponding GCD curve is shown in Fig. 5a), and the results are shown in Fig. 5b. Before ex-situ studies, the electrodes were separated from the coin cells after realizing a certain state, rinsed with DI water for three times, and then dried at room temperature under vacuum. The two diffraction peaks marked with dotted box come from the Ti foil substrate. Except for these two peaks, all the other peaks shift to higher degree during discharging, suggesting that the d-NaxV2O5$nH2O lattice shrinks upon Zn2þ intercalation. In particular, the interlayer spacing decreases by ~5% at 0.2 V vs. Zn2þ/ Zn in comparison with that at 1.6 V (Fig. 5c). Subsequently during the charge process, the lattice is found to expand reversibly upon Zn2þ extraction. Such phenomenon is consistent with previously reported Na2V6O16$1.63H2O [45], and may be ascribed to that the

7

intercalated Zn2þ would screen the interlayer electrostatic repulsion [57,58]. Furthermore, no new peaks are observed during the whole process. As depicted in Fig. S12, the ex-situ Raman measurements were also applied to study rGO/d-NVO. All the spectra are roughly identical when the electrode is at different states, indicating the Zn2þ (de)intercalation-induced structural transformation is slight. On the basis of the above results, it can be inferred that d-NaxV2O5$nH2O incorporates Zn2þ via a solidsolution process. What's more, the structural evolutions of rGO/dNVO after 50 cycles were investigated by XRD and Raman measurements, as shown in Figs. S13 and S14. The pattern/spectrum remains nearly the same as that of the fresh electrode, illustrating great structural robustness of d-NaxV2O5$nH2O, which benefits from the pillaring effect of interlayer Naþ and structural water and the buffering effect of rGO. Furthermore, the ex-situ XPS measurements were also performed on rGO/d-NVO, as shown in Fig. 5d and e, in which the binding energy values were calibrated using the C 1s peak at 284.8 eV. At the initial state, the V 2p signal of the electrode can be deconvoluted into V5þ (2p3/2: 516.3 eV) and V4þ (2p3/2: 514.0 eV) contributions, the latter of which originates from partial reduction of the V2O5 framework owing to the existence of indigenous Naþ. It

Fig. 5. (b) The ex-situ XRD patterns and (c) magnified region with the 2q ranging from 7e11 of the rGO/d-NVO electrode at five different charge/discharge states, and (a) the corresponding GCD curve. The ex-situ high-resolution XPS spectra of (d) V 2p and (e) Zn 2p regions at the initial, fully discharged (0.2 V), and fully charged (1.6 V) states of the rGO/ d-NVO electrode.

8

W. Zhou et al. / Electrochimica Acta 321 (2019) 134689

is worthwhile mentioning that the mixed vanadium valences are beneficial to electrochemical performances [59]. When the electrode is discharged to 0.2 V (2nd cycle), the peak area ratio of V4þ to V5þ increases, while a new peak corresponding to V3þ appears. Besides, a blue shift of the binding energy of V 2p is observed at the discharged state, which is consistent with previous reports and is probably related to the bonding rearrangements at the V sites after Zn2þ intercalation [25,27]. Moreover, when the electrode is charged back to 1.6 V (2nd cycle), the spectrum considerably resembles that of the initial state, confirming that the intercalation of Zn2þ into dNaxV2O5$nH2O is accompanied by the reversible redox transitions of vanadium element for charge balance. It can also be observed from Fig. 5d that the O 1s peak at 532.1 eV that is assigned to H2O molecules enhances significantly when the electrode is discharged to 0.2 V, indicating that H2O is incorporated into rGO/d-NVO along with Zn2þ insertion [28,60]. The H2O molecules inside d-NVO can effectively shield the electrostatic field of the inserted Zn2þ ions, thus contributing to good electrochemical performances. As can be seen from Fig. 5e, when the electrode is discharged to 0.2 V, it exhibits obvious Zn 2p signals that correspond to the intercalated Zn2þ. It is also seen that most of the intercalated Zn2þ can be extracted when the electrode is charged back to 1.6 V, which confirms good reversibility of rGO/d-NVO. Here at 1.6 V, the residual Zn2þ in the electrode is the “dead Zn2þ” that cannot be extruded during the charge process, and is probably the partial origin of capacity decay that is observed in the above electrochemical measurements. In Fig. S15, we can see that the Naþ in rGO/d-NVO is generally preserved during the 2nd cycle. In summary, the charge/ discharge process of rGO/d-NVO can be described as follows:

d
Na0:48 V2 O5 , nH2 O þ xZn2þ þ 2xe
þ mH2 O4d
Na0:48 Znx V2 O5 ,ðn þ mÞH2 O

(4)

On the basis of the above results, we speculate that the capacity decay of rGO/d-NVO is due to the following three factors. Firstly, the (de)intercalation process of d-NVO is partially irreversible. That is, some inserted Zn2þ ions, located at the “dead sites”, are unable to be extruded from the lattice of active material during the following charge process [26,61]. Secondly, as is shown in Fig. S16, the formation of Zn dendrite and surface passivation could definitely cause capacity degradation. Thirdly, Naþ would be partially dissolved in the electrolyte after long-term cycles, weakening the pillaring effect. 4. Conclusion Aqueous zinc-ion batteries (AZIBs) have been attracting growing interest, whereas their development is hindered by suitable cathode materials. Thus, it is an urgent challenge to explore new material system. In this study, d-NVO is firstly employed as the cathode material for AZIBs. Notably, the electrochemical performances of dNVO are better than that of Na2V6O16$nH2O under the same test environment. Moreover, the performances of d-NVO can be greatly improved after modification with rGO, owing to the synergistic merits of large interlayer spacing of d-NVO (10.6 Å) and high electrical conductivity of rGO. At 0.1 A g
1, the rGO/d-NVO nanocomposite can deliver high reversible capacity of 433.5 mAh g
1, 81.4% and 56.3% of which can be maintained when the current density is increased to 2 and 5A g
1, respectively. And the capacity retention reaches 70.5% over 1000 cycles. The reason behind the great performances of rGO/d-NVO is large specific surface area (192 m2 g
1), low charge transfer impedance (85 U), high Zn2þ ion diffusion coefficient (10
8.5 to 10
9.5 cm2 s
1), and more than half contribution of pseudocapacitive Zn2þ intercalation to the total capacity. The ex-situ measurements indicate that the intercalation

of Zn2þ is realized by a reversible and stable solid solution process accompanied by reversible redox transitions of vanadium element and reversible H2O uptake/release. In summary, this work not only opens a new door to the practical application of d-NVO in AZIBs but also broadens the horizon in understanding the Zn2þ storage mechanism. Acknowledgements This work was supported by the Natural Science Foundation of Jiangsu Province (BK20170917, China), the Scientific Research Foundation for High-Level Talents of Nanjing Forestry University (GXL2016023, China), and the Jiangsu Specially-Appointed Professor Program (China). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134689. References [1] M. Du, D. Song, A. Huang, R. Chen, D. Jin, K. Rui, C. Zhang, J. Zhu, W. Huang, Stereoselectively assembled metal-organic framework (MOF) host for catalytic synthesis of carbon hybrids for alkaline-metal-ion batteries, Angew. Chem. 58 (2019) 5307e5311. [2] J. Ming, J. Guo, C. Xia, W. Wang, H.N. Alshareef, Zinc-ion batteries: materials, mechanisms, and applications, Mater. Sci. Eng. R. 135 (2019) 58e84. [3] A. Konarov, N. Voronina, J.H. Jo, Z. Bakenov, Y.K. Sun, S.T. Myung, Present and future perspective on electrode materials for rechargeable zinc-ion batteries, ACS Energy Lett 3 (2018) 2620e2640. [4] P. Yu, Y. Zeng, H. Zhang, M. Yu, Y. Tong, X. Lu, Flexible Zn-ion batteries: recent progresses and challenges, Small 15 (2019) 1804760. [5] G.Z. Fang, J. Zhou, A.Q. Pan, S.Q. Liang, Recent advances in aqueous zinc-ion batteries, ACS Energy Lett 3 (2018) 2480e2501. [6] M. Song, H. Tan, D.L. Chao, H.J. Fan, Recent advances in Zn-ion batteries, Adv. Funct. Mater. 28 (2018) 1802564. [7] Y.B. Li, J. Fu, C. Zhong, T.P. Wu, Z.W. Chen, W.B. Hu, K. Amine, J. Lu, Recent advances in flexible zinc-based rechargeable batteries, Adv. Energy Mater. 9 (2019) 1802605. coli, I. Pavlovic, C. Barriga, F. La Mantia, Layered double [8] M.A. Gonz alez, R. Tro hydroxides as a suitable substrate to improve the efficiency of Zn anode in neutral pH Zn-ion batteries, Electrochem. Commun. 68 (2016) 1e4. [9] N. Zhang, F.Y. Cheng, J.X. Liu, L.B. Wang, X.H. Long, X.S. Liu, F.J. Li, J. Chen, Rechargeable aqueous zinc-manganese dioxide batteries with high energy and power densities, Nat. Commun. 8 (2017) 405. [10] S. Islam, M.H. Alfaruqi, V. Mathew, J. Song, S. Kim, S. Kim, J. Jo, J.P. Baboo, D.T. Pham, D.Y. Putro, Y.K. Sun, J. Kim, Facile synthesis and the exploration of the zinc storage mechanism of beta-MnO2 nanorods with exposed (101) planes as a novel cathode material for high performance eco-friendly zinc-ion batteries, J. Mater. Chem. A 5 (2017) 23299e23309. [11] J.H. Huang, Z. Wang, M.Y. Hou, X.L. Dong, Y. Liu, Y.G. Wang, Y.Y. Xia, Polyaniline-intercalated manganese dioxide nanolayers as a high-performance cathode material for an aqueous zinc-ion battery, Nat. Commun. 9 (2018) 2906. [12] J.W. Hao, J. Mou, J.W. Zhang, L.B. Dong, W.B. Liu, C.J. Xu, F.Y. Kang, Electrochemically induced spinel-layered phase transition of Mn3O4 in high performance neutral aqueous rechargeable zinc battery, Electrochim. Acta 259 (2018) 170e178. [13] G.Z. Fang, C.Y. Zhu, M.H. Chen, J. Zhou, B.Y. Tang, X.X. Cao, X.S. Zheng, A.Q. Pan, S.Q. Liang, Suppressing manganese dissolution in potassium manganate with rich oxygen defects engaged high-energy-density and durable aqueous zincion battery, Adv. Funct. Mater. 29 (2019) 1808375. [14] H.L. Pan, Y.Y. Shao, P.F. Yan, Y.W. Cheng, K.S. Han, Z.M. Nie, C.M. Wang, J.H. Yang, X.L. Li, P. Bhattacharya, K.T. Mueller, J. Liu, Reversible aqueous zinc/ manganese oxide energy storage from conversion reactions, Nat. Energy 1 (2016) 16039. coli, G. Kasiri, F. La Mantia, Phase transformation of copper hex[15] R. Tro acyanoferrate (KCuFe(CN)6) during zinc insertion: effect of co-ion intercalation, J. Power Sources 400 (2018) 167e171. [16] Q. Zhang, C. Li, Q. Li, Z. Pan, J. Sun, Z. Zhou, B. He, P. Man, L. Xie, L. Kang, X. Wang, J. Yang, T. Zhang, P.P. Shum, Q. Li, Y. Yao, L. Wei, Flexible and highvoltage coaxial-fiber aqueous rechargeable zinc-ion battery, Nano Lett. 19 (2019) 4035e4042.  coli, F. La Mantia, An aqueous zinc-ion battery based on copper hex[17] R. Tro acyanoferrate, ChemSusChem 8 (2015) 481e485. [18] W. Li, K. Wang, S. Cheng, K. Jiang, A long-life aqueous Zn-ion battery based on Na3V2(PO4)2F3 cathode, Energy Storage Mater 15 (2018) 14e21.

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