Facile hydrothermal synthesis and electrochemical properties of (NH4)2V10O25·8H2O nanobelts for high-performance aqueous zinc ion batteries

Journal Pre-proof Facile hydrothermal synthesis and electrochemical properties of (NH4)2V10O25·8H2O nanobelts for high-performance aqueous zinc ion batteries Hanmei Jiang, Yifu Zhang, Zhenghui Pan, Lei Xu, Jiqi Zheng, Zhanming Gao, Tao Hu, Changgong Meng PII:

S0013-4686(19)32378-3

DOI:

https://doi.org/10.1016/j.electacta.2019.135506

Reference:

EA 135506

To appear in:

Electrochimica Acta

Received Date: 15 September 2019 Revised Date:

30 November 2019

Accepted Date: 12 December 2019

Please cite this article as: H. Jiang, Y. Zhang, Z. Pan, L. Xu, J. Zheng, Z. Gao, T. Hu, C. Meng, Facile hydrothermal synthesis and electrochemical properties of (NH4)2V10O25·8H2O nanobelts for highperformance aqueous zinc ion batteries, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2019.135506. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Facile Hydrothermal Synthesis and Electrochemical Properties of (NH4)2V10O25·8H2O Nanobelts for High-Performance Aqueous Zinc Ion Batteries Hanmei Jiang1, Yifu Zhang*,1,2, Zhenghui Pan*,2, Lei Xu1, Jiqi Zheng1, Zhanming Gao1, Tao Hu*,1, Changgong Meng1 1

School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China

2

Department of Materials Science and Engineering, National University of Singapore, 117574

Singapore, Singapore *E-mail address: [email protected] (Y. Zhang), [email protected] (Z. Pan); [email protected] (T. Hu)

Abstract Aqueous rechargeable Zn-ion batteries (ARZIBs) are highly desirable for grid-scale applications because of their high safety, low cost, sustainability, and environmental friendliness. However, the lack of suitable cathode materials possessing satisfactory cycle performance and energy density limits their wide applications. In this work, (NH4)2V10O25·8H2O nanobelts, the ammonium ion, and water expanded VO skeletons, are synthesized by a facile hydrothermal synthesis and reported as a cathode material in ARZIBs. The Zn//(NH4)2V10O25·8H2O nanobelts battery achieves capacities as high as 417, 366, 322, 268 and 209 mA h g−1 at 0.1, 0.2, 0.5, 1.0 and 2.0 A g−1 respectively, showing high rate capacity. In addition, the battery not only exhibits an excellent cycle lifespan of 310 mA h g−1 after 100 cycles (0.1 A·g−1), but delivers a high energy density of 320 Wh kg−1 at the power density of 77 W kg−1. These results demonstrate that the Zn//(NH4)2V10O25·8H2O nanobelts battery possesses significantly enhanced electrochemical performances, which is superior to most state-of-the-art cathode materials for ARZIBs. Furthermore, the reaction mechanism of the reversible Zn2+ intercalation/deintercalation into (NH4)2V10O25·8H2O is studied by multiple analytical methods. This work not only demonstrates that (NH4)2V10O25·8H2O nanobelts can act as a promising cathode candidate, but also provides an attractive solution for the synthesis of cathode materials for ARZIBs and other multivalent metal-ion batteries. Keywords: (NH4)2V10O25·8H2O nanobelts; Zn2+ storage; Cathode materials; Aqueous zinc-ion battery; High capacity; Electrochemical mechanism 1

1.

Introduction Aqueous rechargeable Zn-ion batteries (ARZIBs) with high safety, eco-friendly, low-cost and

sustainability have garnered recent attention towards grid-level energy storage systems (ESSs), compared to Li-ion batteries (LIBs) and Sodium-ion batteries (SIBs) [1-9]. Zinc has some advantages over Li and Na as the anode materials, and it provides high gravimetric and volumetric capacities (820 mAh g−1, 5855 mAh cm−3) as well as a low negative potential (−0.762 V vs. SHE) [10]. Furthermore, apart from environmental benignity and high safety, it is also abundant in the earth crust (about 300 times higher than Li), which contributes to its low cost. However, the practical applications of ARZIBs are still hindered by the inadequacy of suitable cathode materials with satisfactory cycle stability and energy density [10-12]. Currently, cathode materials including Prussian blue analogues (PBA)-, Mn-, V-, Mo-, Co-based materials have been developed for ARZIBs [10, 12-17]. Since Linda F. Nazar’s group reported an intercalated cathode Zn0.25V2O5·nH2O with high capacity (ca. 300 mAh g−1 at 50 mA g−1) and excellent rate performance [13], V-based cathode materials have attracted tremendous interest due to some of V-based materials’ outstanding advantages: abundant, multiple redox reactions, low-cost and appropriate layered-structures with adjustable tunnels favoring the efficient ingress/egress of Zn2+ [4, 13, 18-28]. So far, V-based materials, such as, MxV2O5·nH2O (M = Zn, Ca, Mg) [13, 18, 23], V2O5·nH2O [24], V2O5 [5, 29, 30], VO2 [21], H2V3O8·[20], sodium vanadate (e.g.: Na2V6O16·3H2O [25], Na2V6O16·1.63H2O [26], Na0.33V2O5·[27], et al.), potassium vanadate (e.g.: K2V6O16·2.7H2O [31]), VS2 [19], Zn3V2O7(OH)2·2H2O [22] and so on, have been explored as promising cathode materials for ARZIBs. For example, Liang’s group reported several sodium/potassium vanadates with various compositions for ARZIBs and the improvements in the performances of the reported cathodes are mainly ascribed to their layered open-framework nanostructures and the multiple oxidations states in vanadium [32, 33]. The recent advances of V-based layered materials for ARZIBs exhibiting satisfactory electrochemical performance motivate the development of layered ammonium vanadates as electrode materials for ARZIBs. On the basis of the previous research studies, the application of ammonium vanadates to ARZIBs’ cathode materials is deemed as a potential choice to realize good electrochemical performance because of their intriguing characteristics [34-36]: ( ) they have a smaller molecular weight and density, endowing higher specific gravimetric and volumetric capacities compared to other vanadates (e.g.: sodium, potassium, etc.); ( ) the intercalated NH4+ act as “pillars” between the VO layers to prevents destructive structural changes during the reversible intercalation/deintercalation of Zn2+; ( )

2

hydrogen bonds (N–H/O) between the NH4+ and VO layer enhance the structural stability, which plays a key role in extraordinary cycle performance. Typically, Yang et al. [37] and Tang et al. [38] manufactured the NH4V4O10 for ARZIBs with high energy density and long cycle life, respectively. As the family of ammonium vanadates, (NH4)2V10O25·8H2O has received rare attention other that Wei et al. who reported (NH4)2V10O25·8H2O as a cathode material for ARZIBs with an energy density of 225.4 W·h·kg−1 and good cycle ability (90.1% after 5000 cycles) [39], and Jiang et al. who synthesized (NH4)2V10O25·8H2O urchin-like hierarchical arrays as electrodes for supercapacitors [40]. However, the specific capacity of the resultant (NH4)2V10O25·8H2O, that is synthesized by the hydrothermal method of solution A (NH4VO3 and LiOH) mixed with solution B (MnSO4·H2O and HCOOH), is ~229 mAh g−1 at 0.1 A g−1 [39], is comparatively low compared to the reported V-based materials (Tables S1 and S2). On the other hand, (NH4)2V10O25·8H2O depicts a characteristic layered structure made up of VO layers and interstitial NH4+ and H2O (insert in Figure 1a). NH4+ and H2O in (NH4)2V10O25·8H2O are located at the tetrahedral sites between the layers of V and O atoms and among the VO layers, which are available for occupation by metal ions [37, 39, 41, 42]. As the ionic radius of Zn2+ (0.76 Å) is smaller than that of NH4+ (1.43 Å) and H2O (4 Å), the reversible intercalation/deintercalation of Zn2+ into the (NH4)2V10O25·8H2O cathode will be allowed and the volume expansion is overcome. Therefore, in this work, we focus on the facile hydrothermal synthesis of (NH4)2V10O25·8H2O nanobelts and improve their storage properties of Zn2+. As expected, we demonstrate that (NH4)2V10O25·8H2O nanobelts are successfully prepared by a facile hydrothermal synthesis and the as-obtained (NH4)2V10O25·8H2O nanobelts show excellent electrochemical storage of Zn2+ as a potential positive electrode material for high-performance ARZIBs. The Zn//(NH4)2V10O25·8H2O nanobelts battery exhibits excellent electrochemical performance, which enables the practical applicability of the as-synthesized (NH4)2V10O25·8H2O nanobelts for ARZIBs.

2.

Experimental section

2.1. Synthesis of (NH4)2V10O25·8H2O nanobelts All reagents of analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd and used without any further purification. In detail, 0.63 g of H2C2O4·2H2O and 0.585 g of NH4VO3 were added to 35 mL distilled water. After the mixture was magnetic stirred for 30 min, they were transferred

3

into a 50 mL Teflon-lined stainless-steel autoclave and heated at 120 or 130 °C for the given times. The products were filtered off, rinsed repeatedly with deionized H2O and absolute ethanol, and then dried in vacuum at 75 °C for 12 h to obtain (NH4)2V10O25·8H2O nanobelts. The representative XRD patterns of the samples obtained at 120 °C for various reaction times are shown in Figure S1a. When the reaction time is 12 h or less, a mixture of (NH4)2V10O25·8H2O and NH4V3O8·0.5H2O is synthesized (Figure S1a). With increasing reaction time, the phase of NH4V3O8·0.5H2O gradually reduces and disappear. When the reaction time reaches 24 h or more, the pure phase of (NH4)2V10O25·8H2O is synthesized (Figure S1a). Figure S1b shows the XRD patterns of the products obtained at 130 °C for various reaction times, and (NH4)2V10O25·8H2O

is

obtained.

Thus,

the

optimized

parameter

for

the

preparation

of

(NH4)2V10O25·8H2O is at 120 °C for 24 h or more, and at 130 °C for various reaction times. The sample synthesized at 130 °C for 8 h is chosen to characterize the electrochemical properties of (NH4)2V10O25·8H2O nanobelts for the cathode materials of ARZIBs owing to its short synthesis time. 2.2. Materials characterization The composition of (NH4)2V10O25·8H2O nanobelts were confirmed by X-ray powder diffraction (XRD, Panalytical X’Pert powder diffractometer at 40 kV and 40 mA with Ni-filtered Cu Kα radiation), energy-dispersive X-ray spectrometer (EDS) and EDS mapping attached to SEM (QUANTA450), X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo Fisher Scientific), Raman spectra (Thermo Scientific spectrometer). Fourier transform infrared spectroscopy, FTIR, were recorded by Nicolet 6700 spectrometer (from 4000 to 400 cm−1 with a resolution of 4 cm−1). The morphologies of (NH4)2V10O25·8H2O nanobelts were observed by FE-SEM (NOVA NanoSEM 450, FEI) and TEM (FEITecnai F30, FEI). HRTEM and selected area electron diffraction (SAED) were also collected on FEITecnai F30. Before TEM characterization, (NH4)2V10O25·8H2O nanobelts sample were prepared by ultrasonic dispersion in absolute ethanol. For the ex-situ XRD, FE-SEM and XPS characterizations, the cycled electrodes were washed with distilled water and dried naturally in air. Electrochemical tests of (NH4)2V10O25·8H2O nanobelts were carried out in CR2032 coin-type cells. (NH4)2V10O25·8H2O nanobelts, a conductive agent (Super-P, Sigma-Aldrich), and PVDF (Sigma-Aldrich) were mixed in a weight ratio of 7 : 2 : 1 to make the cathode electrode, of which the mixture was coated on Ti sheet. After drying in oven at 80 °C for 12 h, the total mass of the electrode materials was around 2 mg. Then, with zinc foil (thickness = 0.25mm) and glass fiber (Whatman GF/A) employed as the anode and separator, the electrodes were assembled in coin-type cells (CR2032). A 3 M Zn(CF3SO3)2 aqueous 4

solution was used as the electrolyte. Cyclic voltammetry (0.3–1.3 V) and EIS were carried out at a CHI 660D electrochemical workstation. The charge/discharge curves at different current rates were performed using a Wuhan LAND battery tester.

3.

Results and discussion

3.1. Characterization of (NH4)2V10O25·8H2O nanobelts In this work, a convenient and large-scale hydrothermal route was applied to prepare (NH4)2V10O25·8H2O nanobelts using NH4VO3, H2C2O4·2H2O and H2O as the starting materials. Figure 1a shows the XRD pattern of the sample obtained at 130 °C for 8 h and it confirms the successful synthesis of (NH4)2V10O25·8H2O (JCPDS, No. 26-0097). The energy-dispersive X-ray spectrometer (EDS) spectrum (Figure 1b) and elemental mapping images (Figure S2) demonstrate that N, O, and V elements are highly dispersed in the as-obtained (NH4)2V10O25·8H2O. The structure information of the as-obtained (NH4)2V10O25·8H2O is provided by the Raman spectrum (Figure 1c) and Fourier-transform infrared spectroscopy (FTIR) spectrum (Figure S3). The prominent Raman frequencies are ranged from 100 to 1200 cm−1. The low-frequency Raman bands are strongly related to the layered structure and revealed the chain translation [43]. The Raman peaks are approximately the signature of VO framework with layered structures [39, 43, 44], suggesting excellent storage ability for Zn-ions [24]. As shown in Figure S3, all peaks are the characteristic V−O, V−O−O, V=O, O−H and N−H vibrations [45]. In detail, the wide absorption band at about 3500 cm−1 is closely associated with O−H bond stretching, and the band at about 1630 cm−1 is assigned to the stretching of H2O [46-49], further proving the existence of H2O in (NH4)2V10O25·8H2O. Two peaks at about 3150 cm−1 and 1400 cm−1 are fitted well with N−H stretching mode, attributing to the ammonium ion in (NH4)2V10O25·8H2O. In the region ranging from 1100 cm−1 to 400 cm−1, three common bands at about 1000 cm−1, 770 cm−1 and 530 cm−1 are indexed to the stretching modes of V=O, V−O and V−O−V [43], respectively. Thus, the Raman and FTIR spectra further confirm the successful preparation of (NH4)2V10O25·8H2O.

5

Figure 1. Characterizations of (NH4)2V10O25·8H2O nanobelts: (a) XRD pattern, insert the structure of (NH4)2V10O25·8H2O; (b) EDS spectrum; (c) Raman spectrum; (d) FE-SEM image; (e) TEM image; (f) HRTEM image.

Figure 1d displays the Field Emission scanning electron microscopes (FE-SEM) image of (NH4)2V10O25·8H2O and it exhibits that the belt-like morphology dominated. The transmission electron microscope (TEM) image in Figure 1e further exhibits the 1D structures of (NH4)2V10O25·8H2O. The crystal plane distance of 0.266 nm can be observed in the High-resolution TEM (HRTEM) image (Figure 1f), corresponding to (400) plane of the crystal (NH4)2V10O25·8H2O. Thus, the combination of FE-SEM and TEM images suggest the (NH4)2V10O25·8H2O nanobelts are synthesized here. Further compositions of the (NH4)2V10O25·8H2O nanobelts are explored by the X-ray photoelectron spectroscopy (XPS) test, as shown in Figure 2. The survey XPS spectrum (Figure 2a) reveals (NH4)2V10O25·8H2O nanobelts consist of N, O and V three elements. The peak of N1s locates at 401.69 eV (Figure 2b), which is the typical characteristic of ammonium ion [50]. From the O1s core-level spectrum (Figure 2c), two deconvoluted peaks at 530.23 eV and 531.07 eV are respectively fitted well with the existence of O2− and H2O [46, 51]. The V2p XPS region in Figure 1d mainly contains two peaks, which are assigned to V2p3/2 and V2p1/2. Both V2p3/2 peak and V2p1/2 peak are split into two signals, including V2p3/25+ (517.72 eV) and V2p3/24+ (516.25 eV); and V2p1/25+ (525.46 eV) and V2p1/24+ (524.03 eV) [46, 52]. Based on all the above results, (NH4)2V10O25·8H2O nanobelts are successfully synthesized by this facile hydrothermal method.

6

Figure 2. XPS spectra of (NH4)2V10O25·8H2O nanobelts: (a) full spectrum; (b-d) core-level spectra of N1s, O1s and V2p, respectively.

3.2. Electrochemical properties and mechanism study of (NH4)2V10O25·8H2O nanobelts applied to ARZIBs To assess the merits of (NH4)2V10O25·8H2O nanobelts, the CR-2032 coin-type cells are assembled with a Zn foil as the anode and (NH4)2V10O25·8H2O nanobelts mixed with the additive attached to a Ti foil as the cathode (Figure 3a). Figure S4 and Figure 3b represent the cyclic voltammetry (CV) profiles of Zn//(NH4)2V10O25·8H2O nanobelts battery within the initial 4 cycles at 0.1 mV·s−1. After the 1st cycle, the CV curves are highly reversible with three main pairs of redox peaks centered at ca. 0.532/0.385 V, 0.741/0.602 V and 1.023/0.836 V (vs. Zn2+/Zn), which are attributed to the continuous Zn2+ intercalation/deintercalation in the (NH4)2V10O25·8H2O layered structure. The phenomenon proves a multistep reaction mechanism of (NH4)2V10O25·8H2O [23, 39]. The CV curves are mostly overlapped, suggesting

excellent

reversibility

of

ingress/egress

of

Zn2+

into/from

the

layer-structured

(NH4)2V10O25·8H2O. Figures 3c-e show the charge/discharge profiles with different cycles of Zn//(NH4)2V10O25·8H2O nanobelts battery at 0.1, 0.2 and 0.5 A·g−1, respectively. These curves show the 7

charge/discharge platforms concur well with the CV curves in Figure S4 and Figure 3b. Figures 3f-h exhibit the corresponding cycling performance and coulombic efficiency of Zn//(NH4)2V10O25·8H2O nanobelts battery at 0.1, 0.2 and 0.5 A·g−1 respectively. A high initial specific capacity of 423 mA h g−1 was observed and it was maintained at 310 mA h g−1 after 100 cycles (Figure 3c and f) at 0.1 A·g−1. As for 0.2 and 0.5 A·g−1, the specific capacities could retain about 252 mA h g−1 after 100 cycles (Figure 3d and g), and about 191 mA h g−1 after 500 cycles (Figure 3e and h) respectively. As shown in Figure 3f-h, the coulombic efficiencies are almost 100% at each cycle. The results demonstrate that the Zn//(NH4)2V10O25·8H2O nanobelts battery delivers a good reversible capacity at various current densities, and the achieved performance is superior to or comparable to other state-of-the-art cathode materials for ARZIBs (Table S1), such as V2O5·nH2O [24], Zn3V2O7(OH)2·2H2O [22], VS2 [19], et al. The FE-SEM test is carried out to study the structural stability of the cathode material in Zn//(NH4)2V10O25·8H2O battery after cycling. After 500 cycles, the morphology and structure of (NH4)2V10O25·8H2O are retained (Figure S5a-d), demonstrating a rather stable structure and its long lifespan. The capacity fade during the long cycling is suspected to be caused by the following points: (1) dissolution of a small amount of active material into the electrolyte [53]; (2) the formation of Zn dendrites and passivation layer on Zn, which impedes Zn-ion diffusion as shown in Figure S5e-f [54, 55].

8

Figure 3. Characterizations of (NH4)2V10O25·8H2O nanobelts as the cathode material in ARZIBs: (a) Schematic illustration of Zn//(NH4)2V10O25·8H2O nanobelts battery. (b) CV curves during the initial 2-4 cycles at 0.1 mV·s−1. (c, d and e) Discharge/charge profiles with different cycles at 0.1, 0.2 and 0.5 A·g−1, respectively. (f, g and h) Cycling performances and coulombic efficiencies at 0.1, 0.2 and 0.5 A·g−1, respectively.

Figure 4a-c depicts the rate capability of Zn//(NH4)2V10O25·8H2O nanobelts battery at a range of current densities from 0.1 to 2.0 A g−1, and it provides a specific capacity of 417, 366, 322, 268 and 209 mA h g−1 at 0.1, 0.2, 0.5, 1.0 and 2.0 A g−1 respectively. As the current density decreases back to 0.1 A g−1 after 80 cycles, the specific capacity recovers to about 350 mA h g−1 with a recovery ratio of 84%. The as-obtained (NH4)2V10O25·8H2O nanobelts exhibit greatly enhanced capacity superior to the reported ammonium vanadates for ARZIBs, including (NH4)2V3O8 (~160 mAh g−1 at 0.1 A g−1) [38], NH4V4O10 (380.3 mAh g−1 at 0.1 A g−1) [38], NH4V3O8 (~160 mAh g−1 at 0.1 A g−1) [38], NH4V4O10 nanobelt (147 mAh g−1 at 0.05 A g−1) [37], (NH4)2V10O25·8H2O (~229 mAh g−1 at 0.1 A g−1) [36]. The achieved electrochemical performance Zn//(NH4)2V10O25·8H2O nanobelts battery is much higher than or 9

comparable to the state-of-the-art V-based cathode materials for ARZIBs summarized in Table S2. Thus, these findings demonstrate that the Zn//(NH4)2V10O25·8H2O nanobelts battery exhibits a good rate performance and can satisfy the requirement of high capacity and rapid charge/discharge process. It is noted that the specific capacity of the synthesized (NH4)2V10O25·8H2O in this work by a facile hydrothermal route is much higher than that of (NH4)2V10O25·8H2O reported by Wang’s group (~229 mAh g−1 at 0.1 A g−1) [39], which is synthesized by the hydrothermal method of solution A (NH4VO3 and LiOH) mixed with solution B (MnSO4·H2O and HCOOH). The high specific capacity of the synthesized (NH4)2V10O25·8H2O is attributed to the water and the NH4+ between the larger layers of VO chains, which provide the good kinetics and the excellent ion mobility during the insertion/extraction of an abundance of divalent cations discussed below. Figure 4d describes the Ragone plots of this work and the reported V-based cathode materials, which verifies the inspiring merits of the as-obtained (NH4)2V10O25·8H2O. It delivers energy densities as high as 320 Wh kg−1 and 234 Wh kg−1 at the power densities of 77 W kg−1 and 873 W kg−1 respectively, which are much higher than or comparable to the values of some state-of-the-art V-based materials for ARZIBs such as, VO2(B) nanobelts (271.8 Wh kg−1) [56], VS2 (123 Wh kg−1) [19], Ca0.25V2O5·nH2O (267 Wh kg−1 at 53.4 W kg−1) [23], et al. Consequently, to the best of our knowledge, this Zn//(NH4)2V10O25·8H2O nanobelts battery shows one of the most promising performances from the perspective of rate capability, high capacity, and long cycle stability (Tables S1-S2 and Figures 3-4), supporting the essential potential for multivalent ion storage and enlightening its advantages for an eco-friendly atmosphere.

10

Figure 4. (a, b and c) The rate performance of Zn//(NH4)2V10O25·8H2O nanobelts battery. (d) Comparison of Ragone plots of Zn//(NH4)2V10O25·8H2O nanobelts battery with those reported vanadium-based electrodes applied to ARZIBs [Ca0.25V2O5·nH2O (Ref. [23]), V2O5 (Ref. [5]), V2O5 hollow spheres (Ref. [57]), VS4@rGO (Ref. [58]), V10O24·12H2O (Ref. [59]), K2V6O16·2.7H2O nanorods (Ref. [31]), a-Zn2V2O7 (Ref. [60]), H2V3O8 nanowire/Graphene (Ref. [20]), V2O5·nH2O/graphene (Ref. [24]), RGO/VO2 composite (Ref. [61]),]. (All values are based on the mass of the cathode material)

To investigate the electrochemical kinetics of Zn//(NH4)2V10O25·8H2O nanobelts battery, several tests were carried out. As depicted in Figure 3f-h, the capacity of Zn//(NH4)2V10O25·8H2O nanobelts battery decreases in the first several cycles, and then it slightly increases, and finally, it gradually reduces with the consecutive cycles. EIS test is a useful tool to explain this phenomenon. Therefore, EIS test is performed

to

reveal

the

transport

efficiency

(impedance

changes)

for

high-performance

Zn//(NH4)2V10O25·8H2O nanobelts battery during the cycling process. As shown in Figure 5a, the slopes of the Nyquist plots of Zn//(NH4)2V10O25·8H2O nanobelts battery at low frequencies indicate the fast diffusion of ions [62], and the Nyquist plots at an open-circuit potential show a semicircle at high frequency associated with charge transfer resistance (Rct) and a sloped line at low frequency denoted as Warburg diffusion impedance related to the transfer of zinc ions [63-65]. The innate resistance (Rc) is 11

determined by the intercept at high frequency and these values are approximately similar (about 1 Ω) after the 1st, 20th and 50th cycle at 0.1 A·g−1. The Rct values (the diameter of the semicircle) measure about 77, 5 and 12 Ω after the 1st, 20th and 50th cycle at 0.1 A·g−1 respectively, whose trend may result in the above phenomenon. Figure 5b gives the fitting of Z’−ω−1/2, and the slope of Zn//(NH4)2V10O25·8H2O nanobelts battery at

20th

cycle

is

the

smallest,

revealing

that

the

Zn2+

diffusion

coefficient

of

Zn//(NH4)2V10O25·8H2O nanobelts battery at 20th cycle is smallest among the 1st, 20th and 50th cycle based on the following equation (1) (detail analysis is represented in Supporting Information) [62]. D୉୍ୗ ୞୬మశ =

ୖమ ୘ మ

(1)

ଶ୅మ ୬ర ୊ర େమ ஢మ ౭

The above results are corresponding with the EIS observations (Figure 5a). Hence, the EIS results from Figure 5a-b support the low inter-resistance and high-rate performance of Zn//(NH4)2V10O25·8H2O nanobelts battery. To further disclose the merits of (NH4)2V10O25·8H2O, the galvanostatic intermittent titration technique (GITT) is performed to evaluate the Zn2+ diffusion coefficient, as shown in Figures 5c-d and Figure S6. Based on the following equation (2) (detail analysis is represented in Supporting Information), the Zn2+ diffusion coefficient in Zn//(NH4)2V10O25·8H2O nanobelts battery is calculated to be around 10−10 cm−2 s−1 (Figure 5d), which is comparable to the value of V-based cathode materials in ARZIBs [18, 39]. ‫ܦ‬௓௡మశ =

ସ

గఛ

ቀ

௠ಳ ௏ಾ ଶ ெಳ ௌ

ቁ ቀ

∆ாೄ ଶ ∆ாഓ

ቁ

(2)

The above results suggest that (NH4)2V10O25·8H2O nanobelts can provide good access to the fast migration of zinc ions, resulting in good rate performance.

12

Figure 5. Nyquist plots (a) and fitting of Z’-ω-1/2 (b) of the Zn//(NH4)2V10O25·8H2O nanobelts battery after different discharge/charge processes. GITT curves of Zn//(NH4)2V10O25·8H2O nanobelts battery (c) and the corresponding diffusivity coefficient for Zn2+ in Zn//(NH4)2V10O25·8H2O nanobelts battery vs. specific capacity (d).

The CV tests are carried out at 0.2~1.0 mV·s−1 to further study the electrochemical kinetics of Zn//(NH4)2V10O25·8H2O nanobelts battery. Figure 6a shows that they retain similar shapes with increasing scan rate. It could be observed that the characteristic redox peaks become broader and shift a bit to lower (reduction peaks) and higher (oxidation peaks) potential. Some previous researches have manifest that the current (i) is originated from two parts: diffusion-controlled process and surface-controlled capacitive process [13, 19]. The degree of capacitive process is qualitatively calculated according to the equation (3): i = aυb

(3)

log i = log a + blog υ

(4)

where υ is the scan rate (mV·s−1), a and b represent adjustable parameters, and b-value is between 0.5 and 1. The value b = 0.5 suggests that the capacity is ascribed to diffusion-controlled process, while the value

13

b = 1 represents that the current is a complete surface-controlled capacitive process. Figure 6b describes the plots of log i vs. log υ for two redox peaks in Figure 6a. The slopes (b-values) of two redox peaks measure 0.74 and 0.89, showing that the capacity of (NH4)2V10O25·8H2O nanobelts is mainly influenced by the surface-controlled capacitive process. Furthermore, the current (i = aυb) can be divided into two parts, as displayed in equation (5): i = k1υ + k2υ1/2

(5)

i/υ1/2 = k1υ1/2 + k2

(6)

where, k1υ means the capacitive process, and k2υ1/2 represents the diffusion-controlled process. According to the current response from the capacitive and diffusion-controlled contributions at a particular voltage, the capacitive contribution could be quantitatively calculated. Figure 6c represents a typical result at 0.4 mV s−1. The shaded area (the capacitive contribution) accounts for 71% of the total capacity. Figure 6d exhibits the results of the surface-controlled capacitive contribution at 0.2~1.0 mV·s−1. The contribution ratio increases from 61% to 83%, which indicates that the capacitive process is the dominant contributor to the total capacity of (NH4)2V10O25·8H2O nanobelts cathode electrode.

14

Figure 6. (a) CV curves at different scan rates and (b) the corresponding log (i) vs. log (v) plots of the redox peaks in (a), (c) capacitive separation curves and (d) capacitive contribution ratios at different scan rates of Zn//(NH4)2V10O25·8H2O nanobelts battery.

The relationship between the electrochemical reaction and the structural evolution of (NH4)2V10O25·8H2O is further studied by the ex-situ X-ray diffraction (XRD) and XPS tests. Figure 7b-c represents the ex-situ XRD result of the cathode at different discharge/charge states (Figure 7a) at the current density of 0.1 A g−1 in Zn//(NH4)2V10O25·8H2O nanobelts battery, which disclose the evolution of the crystal structure during the charge/discharge process. The diffraction peaks of (NH4)2V10O25·8H2O reduce to a certain degree and a new phase Zn3(OH)2V2O7·2H2O (JCPDS, No. 50-0570) appears as shown in Figure 7b. This suggests that a small amount of (NH4)2V10O25·8H2O suffers an irreversible phase transformation and converts to Zn3(OH)2V2O7·2H2O, which are consistent with some V-based cathode materials [38, 39]. Then, the mixtures of (NH4)2V10O25·8H2O and Zn3(OH)2V2O7·2H2O co-existed and they display highly reversible and long cycle performance (Figure 3c-h), which is supported by the structure evolution in Figure 7b-c and the cycle performances in Figure 3c-h. The shapes of the charge/discharge curves of Zn//(NH4)2V10O25·8H2O nanobelts battery in Figure 3c-e remain

15

unchanged, which indicates that the layered structure of VO framework in (NH4)2V10O25·8H2O (insert in Figure 1a) is not damaged during the Zn2+ ingress/egress. To be specific, Figure 7c shows the variation of the characteristic peak (001) of (NH4)2V10O25·8H2O representing the intercalation/deintercalation of Zn2+ into its VO layered structures. It shifts to low 2θ degree during the Zn2+ intercalation (the discharge process), and it recovers to large 2θ degree during the Zn2+ deintercalation (the charge process). These results mean that the interlayer spacing of (100) peak of (NH4)2V10O25·8H2O increases with the Zn2+ intercalation, and then reduces with the Zn2+ deintercalation, which is attributed to the mobility of zinc ions during the intercalation/deintercalation into the interlayer of VO skeletons. With the continuous cycles, the XRD patterns of the cathode show the reversible peaks, which indicates the reversible structural transition of the cathode materials during the insertion/extraction of Zn2+. Thus, the consequent charge/discharge processes display the almost similar evolution after the intercalation/deintercalation of Zn2+ and demonstrate the long structural stability, ensuring the excellent cycle stability and coulombic efficiency of the cathode material of (NH4)2V10O25·8H2O. Ex-situ XPS tests are also used to study the surface chemical states of the cathode material at the states of the pristine, 20th full discharge and 20th full charge (Figure 7d-g and Figure S7). As shown in Figure S7, the signal of Zn is not detected at the pristine state. At the state of 20th full discharge, two peaks at 1045.61 eV and 1022.49 eV are observed, which belong to Zn2p1/2 and Zn2p3/2, respectively. Both the full XPS spectra and the core-level spectra of the 20th full charge reveal that Zn element remains in the cathode material. The peaks of Zn2p1/2 and Zn2p3/2 shift to 1045.14 eV and 1022.07 eV, owing to the formation of Zn3(OH)2V2O7·2H2O that acts as a pillar to ensure the cathode material’s structural stability during the discharge/charge processes [39]. The above findings precisely demonstrate the Zn2+ intercalation/deintercalation into the cathode material and the formation of Zn3(OH)2V2O7·2H2O to stabilize the structure, which is in agreement with the ex situ XRD observation (Figure 7b). Figure 7d-g depicts the core-level XPS spectra of O1s and V2p to reveal their evolution during the reversible Zn2+ insertion/extraction at the states of 20th full discharge and 20th full charge. The spectra of O1s is divided into three peaks including O2−, OH− and H2O (Figure 7d-e). Compared with the pristine state (two peaks O2− and H2O), the new peak of OH− supports the formation of Zn3(OH)2V2O7·2H2O again. There are much more H2O in the full discharge state. The peaks of H2O greatly strengthen/weaken as the discharge/charge process, demonstrating the hydrated Zn2+ ion intercalation/deintercalation. The H2O during the discharge/charge process can offer electrostatic shielding for Zn2+, leading to good kinetics, ion

16

mobility and good rate performance [24]. Thus, the H2O acts as a pillar to maintain the structure of VO skeletons of the cathode material during the discharge/charge processes. As for the XPS spectra of V2p, the intensity of V2p (4+) peak increases and the intensity of V2p (5+) peak decreases at 20th full discharge state (Figure 7f), compared to the values at 20th full charge state (Figure 7g). This reveals the reduction of V

to

V

with

the

Zn2+

intercalation.

The

above

results

demonstrate

the

Zn2+

intercalation/de-intercalation reaction mechanism, which is in line with the reported vanadium-based cathode materials in ARZIBs [13, 18, 23, 66]. Based on the above analysis, Figure 7h describes a schematic diagram about the crystal structure change of (NH4)2V10O25·8H2O during the charge/discharge process. First, the interlayer spacing of (NH4)2V10O25·8H2O is very large, equaling about 10.3 Å, which benefits from the crystal water in the layers.

The

reversible

intercalation/deintercalation

of

Zn2+

into

the

cathode

material

of

(NH4)2V10O25·8H2O is allowed and the volume expansion is overcome. The electrostatic attraction between Zn2+ and the host material can be effectively weakened, which ensures the stability in the following discharge/charge process. During the first irreversible intercalation of Zn2+, the diffraction peaks of (NH4)2V10O25·8H2O become weak to form Znx(NH4)yV10O25·zH2O and Zn3(OH)2V2O7·2H2O. After that, two phases of Znx(NH4)yV10O25·zH2O and Zn3(OH)2V2O7·2H2O co-existed and they exhibit high reversibility during the following discharge/charge process, which is in agreement with the reported process [13, 67].

17

Figure 7. Ex situ studies of the cathode materials at different discharge/charge states in Zn//(NH4)2V10O25·8H2O nanobelts battery: (a) Ex situ points at discharge/charge curves; (b, c) Ex situ XRD patterns; (d, e) Ex situ O1s core-level XPS spectra; (f, g) Ex situ V2p core-level XPS spectra. (h) Schematic diagram about the crystal structure change of (NH4)2V10O25·8H2O nanobelts in the discharge/charge process. 4.

Conclusion In conclusion, (NH4)2V10O25·8H2O nanobelts are synthesized by a facile hydrothermal synthesis and

developed as the cathode material for ARZIBs. Owing to the structure of the ammonium ion and water expanded

VO

skeletons,

it

shows

excellent

electrochemical

storage

of

Zn-ions.

The

Zn//(NH4)2V10O25·8H2O nanobelts battery exhibits high rate capacity, excellent cycle performance (about 252 mA h g−1 after 100 cycles at 0.2 A·g−1), and high energy density (234 Wh kg−1 at 873 W kg−1), which enables the practical applicability of the as-synthesized (NH4)2V10O25·8H2O nanobelts for ARZIBs. The

18

ex-situ XRD and XPS tests at various charge/discharge states reveal that a new phase Zn3(OH)2V2O7·2H2O

is

formed

and

the

co-existed

structures

of

(NH4)2V10O25·8H2O

and

Zn3(OH)2V2O7·2H2O are highly stable and reversible for the intercalation/deintercalation of Zn2+. This work not only highlights (NH4)2V10O25·8H2O nanobelts as a promising cathode material for high safety and highly durable ARZIBs, but also provides an attractive solution for the low-cost synthesis of cathode materials for other multivalent metal-ion batteries.

Acknowledgements This work was partially supported by the National Natural Science Foundation of China (Grant No. 21771030, 21601026), Fundamental Research Funds for the Central Universities (DUT18RC(6)008) and the China Sponsorship Council (201806065025).

Supporting Information Supporting Information associated with this article can be found, in the online version. References

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Author Contributions Section Hanmei Jiang and Yifu Zhang conceived and designed the study. Hanmei Jiang, Lei Xu, Jiqi Zheng, and Zhanming Gao performed the experiments. Hanmei Jiang and Yifu Zhang wrote the paper. Hanmei Jiang, Zhenghui Pan, Yifu Zhang, Tao Hu, and Changgong Meng reviewed and edited the manuscript. All authors read and approved the manuscript.

Declaration of Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.