A self-healing lamellar structure boosts highly-stable zinc-storage

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A self-healing lamellar structure boosts highly-stable zinc-storage property of bilayered vanadium oxides Gongzheng Yang, Tongye Wei, and Chengxin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10849 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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ACS Applied Materials & Interfaces

A Self-Healing Lamellar Structure Boosts Highly-Stable Zinc-Storage Property of Bilayered Vanadium Oxides

Gongzheng Yang1, Tongye Wei1, Chengxin Wang1,2*

1

State Key Laboratory of Optoelectronic Materials and Technologies, School of

Physics Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China 2

The Key Laboratory of Low-carbon Chemistry & Energy Conservation of

Guangdong Province, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China

Keywords: Energy storage, Aqueous battery, Zinc-ion battery, Cathode, Bilayer, Self heal, Long life

*Correspondence and requests for materials should be addressed to C. X. Wang. E-mail: [email protected]

1

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Abstract Rechargeable aqueous zinc-ion batteries have been considered one of the promising alternative energy storage systems to lithium ion batteries owing to their low cost and high safety. However, there is lack of long-life positive materials severely, which severely restricts the development of zinc-ion batteries. The strong interactions present between the intercalated multivalent cations and host materials inevitably cause structural distortions and create large migration barriers to the diffusion of cations, resulting in poor cycling stability and limited rate performance. Here, we report the application of bilayered ammonium vanadium oxide (NH4V4O10) as the cathode material for zinc-ion batteries. A self-healing lamellar structure, which combines a macroscopically reversible morphological transformation and a microscopically adjustable interlayer spacing to accommodate the strong interactions, is observed upon insertion and release of cations. This stable architecture enables a specific capacity of 147 mAh g–1 at a current density of 200 mA g–1 (voltage window: 1.7 – 0.8 V vs. Zn2+/Zn) and a capacity retention of more than 70.3% over 5000 cycles (5000 mA g–1). Our finding provides a new alternative for zinc-ion batteries and inspiration for how to further develop advanced positive electrodes by employing materials with flexible microarchitectures.

2


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Introduction With the increasing demands for sustainable and clean energy, the state-of-the-art lithium-ion batteries have clearly been unable to satisfy the needs of large-scale energy storage systems 1–3. The high cost and large safety risk of lithium-ion batteries are the top two remaining issues and motivate researchers worldwide to explore alternative battery solutions

4–8

. Making full use of the existing research on Li-ion

technologies for reference (e.g., Li-S

9,10

and Li-metal batteries

11

), rechargeable

batteries based on multivalent ions, such as Mg2+, Ca2+, Zn2+, Al3+, by coupled with the corresponding metallic anodes have attracted intensive attention because of their low cost and, in particular, their abilities to achieve higher energy densities beyond lithium ion batteries 8,12–18. On the other hand, aqueous batteries that are fabricated in a simple and environmentally friendly process have recently aroused substantial interest and are believed to be capable of solving the safety problems associated with lithium-ion batteries 19–23. Therefore, innovative researchers have quickly learned that the aqueous batteries using multivalent cations as charge carriers may be an effective solution to realizing large-scale energy storage

24,25

. Among the available multivalent

ion-based cells, aqueous Zn2+ ions-based batteries hold promise owing to the merits of metallic Zn, including its nontoxicity, abundant reserves, low redox potential (−0.78 V vs. the standard hydrogen electrode), and facile reversible plating/stripping in a Zn2+-containing aqueous solution, which delivers a high theoretical capacity of 820 mAh g−1 26,27. Notable energy densities within the range of 250 to 290 Wh kg−1 (based on the active materials) have recently been reported for aqueous zinc-ion batteries 28,29. Remarkably, a total energy density of 75.2 Wh kg−1 was successfully achieved in a zinc-ion pouch cell; this value is almost twice that of commercial lead-acid batteries, and therefore, this system is regarded to be promising for large-scale energy storage applications 30. A variety of cathode materials, such as layered metal oxides

29,31–33

, tunnel-type

MnO2 27,34–43, Prussian blue analogs 44–48, and polyanionic compounds 49–51, have been reported as intercalation hosts for divalent Zn2+-based cations. Two redox events, 3



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which are denoted the conversion and intercalation mechanisms, dominate the electrochemistry

during

the

operation

of

the

cells.

The

representative

conversion-based cathode material is α-MnO2, where a highly reversible phase transformation between α-MnO2 and MnOOH provides a high capacity and an excellent cycling performance (an impressive capacity retention of 92% over 5000 cycles) 37. Among the materials for the intercalation mechanism, Nazar and coworkers developed a high-capacity and long-life aqueous rechargeable zinc-ion battery using a Zn0.25V2O5·nH2O

nanobelt

cathode

that

underwent

reversible

Zn2+

ion

(de)intercalation and, exhibited a capacity retention of more than 80% over 1000 cycles

29

. In view of these long-life positive electrodes, we note that a robust or

reversible framework structure is vital for counteracting the effects of the intercalated divalent Zn2+ ions in the manufacture of advanced aqueous Zn2+ ion-based batteries. Vanadium oxide and its derivates, which have rich redox chemistry due to their variety of accessible oxidation states and common open layer architecture

52,53

, have

long been studied as hosts for accommodating guest ions. Very recently, Ceder et al. 54 predicted the feasibility of Zn2+ ion insertion into orthorhombic V2O5 by means of first-principles calculations, and the results of these calculations were soon validated by the demonstration of reversible Zn intercalation in xerogel-V2O5 17,29. Nonetheless, to date, very few vanadium oxide-based host materials for aqueous Zn2+ ion-based batteries have been explored, and the reported materials commonly suffer from a poor cycling performance

30,32,33

. The strong interactions present between a multivalent

intercalant and the surrounding anionic environment not only create the large migration barriers for the cations but may also destroy the host structure, leading to low ion mobility and possible structural collapse

55

. Vanadium oxides with a low

degree of crystallinity are proposed to suffer from less mechanical stress during the intercalation and release of large cations and may offer more insertion sites than their crystalline counterparts, resulting in a higher capacity and enhanced ion mobility 56. Many impressive battery performances were successfully achieved by using bilayered vanadium oxides, which were assumed to be amorphous due to the lack of long-range 4


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structural order, as host materials for rechargeable batteries

29,57–70

. The

abovementioned Zn0.25V2O5·nH2O species is one kind of this bilayered material

29

.

However, the exact structural and morphological evolution of the Zn0.25V2O5·nH2O nanobelts has not been investigated, and the superiority of the bilayer structure compared with a monolayer material has yet to be clearly revealed. Herein, we examine these questions by designing both bilayered and monolayer vanadium oxides with similar dimensions, evaluating their zinc storage properties, studying the changes in their structural, morphological, and chemical states upon cycling, and ultimately clarifying their electrochemical mechanisms and especially the differences in their structural evolutions with cation intake/removal. As expected, the bilayered NH4V4O10 exhibits a specific capacity of 147 mAh g–1 in the voltage window of 0.8 − 1.7 V with an average working voltage of ~1.1 V and a capacity retention of 70.3 % over 5000 cycles at a high rate of 2000 mA g–1, corresponding to a much better performance than that of monolayered V2O5. Detailed characterization reveals that the bilayered NH4V4O10 material possesses an electrochemically responsive layered structure with an adjustable intralayer spacing and a highly reversible morphological transition between nanosheets and nanobelts that allow the material to adapt to volumetric changes, whereas the monolayered V2O5 shows poor cycling stability because of its continuously decomposing skeleton. Thus, the enhanced zinc storage properties of bilayered NH4V4O10 can be attributed to its self-healing lamellar structure. This study offers a reference for exploiting suitable cathode materials for aqueous Zn2+ ion-based batteries.

Experimental section Materials synthesis. In a typical synthesis of NH4V4O10 nanobelts, 585 mg NH4VO3 (0.5 mmol) powders were firstly added into 99 mL deionized water and heated to 80 o

C, forming a light yellow solution. Then, 1 mL 1mol L–1 nitric acid was added drop

by drop into the solution. Upon severe stirring, 265 mg C10H14O5V was dissolved in the above solution. Subsequently, the obtained dark green dispersion was poured into 5



a Teflon-lined autoclave and heated to 180 oC for 6 hours. Finally, the precipitate in the bottom of vessel was collected, followed with washing by deionized water and absolute alcohol three times, and dried in air at 70 oC for 12 hours. The α-V2O5 nanobelts are obtained by annealing the NH4V4O10 in air at 450 oC for 2 hours. Characterizations. The crystal structure of the products was confirmed by X-ray diffraction (XRD) measurements (Rigaku Co., Japan) using Cu Kα radiation at a generator voltage of 40 kV, while the XRD data was refined by the Maud Rietveld refinement program. A JSM-7001F type scanning electron microscope (SEM, JEOL, 15 kV) and a transmission electron microscopy (TEM, FEI Tecnai G2 F30, 300 kV) equipped with energy dispersive spectroscopy were introduced to examine the morphologies and elemental distributions. X-ray photoelectron spectroscopy (XPS) analyses were performed on PerkinElmer PHI 1600 ESCA. Raman spectra were recorded from a Renisha INVIA micro-Raman spectroscopy system. XAS tests were performed on the BL14W1 beamline of Shanghai Synchrotron Radiation Facility and the data were analyzed with software of Ifeffit Athena 30. Electrochemical tests. Electrochemical properties of the materials were evaluated by CR2032 coin-type cells. An aqueous 3 mol L–1 Zn(CF3SO3)2 solution (pH = 3.6), a zinc foil with a diameter of 14 mm, a glass fiber (GF/A, whatman) were used as the electrolyte, counter electrode, and separator, respectively. The cathode was prepared by mixing 70 wt% NH4V4O10, 20 wt% carbon black, and 10 wt% polyvinylidene fluoride in N-methyl-2-pyrrolidone solvent, which was then pasted on a graphite paper with a thickness of 150 µm. After drying in air at 90 oC for 12 hours, the slurries were cut into disks with a diameter of 12 mm. The mass loading of active materials (NH4V4O10) was approximately 2 mg cm–2. The manufacturing processes of the cells were performed in air at ambient temperature. Cyclic voltammetry (CV) studies were conducted on electrochemistry workstation (CHI660, CH Instruments, China) between 0.8 − 1.7 V at various scanning rates. Galvanostatic charge-discharge tests were performed using a battery test system (Newware, Shenzhen). For the ex situ XRD, TEM, and Raman investigations, the electrodes were prepared by dispersing the 6


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slurries on titanium foil that further were assembled in pouch cells. The cycled electrodes could easily peel off from the titanium foil after washing with deionized water and dried naturally in air.

Results and Discussion The hydrothermally synthesized products display a belt-like morphology, as depicted in Fig. 1a and Figure 1. A low-magnification scanning electron microscopy (SEM) image reveals that the products consist of a mixture of bundled, split, and exfoliated nanobelts, which are usually associated with the well-known “Ostwald ripening mechanism” in the growth of layered vanadium oxides

71

. Generally, the exfoliated

nanobelts present a rectangular shape several micrometers in length and ca. 100 − 400 nm in width (Figure 1a). The crystalline phase of the as-synthesized products was determined by X-ray diffraction (XRD) (Figure 1b) experiments. All peaks can be indexed to monoclinic ammonium vanadium oxide (NH4V4O10) (JCPDS no. 31−0075). To investigate the microstructure, transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) analyses were employed. A typical TEM image of an individual NH4V4O10 nanobelt is illustrated in Figure 1c. The corresponding SAED pattern (Figure 1c, inset) verifies its single crystalline nature and can be readily indexed along the [001] zone axis, matching well with the strongest peak at (001) in the XRD pattern (Figure 1a). From a high-resolution TEM (HRTEM) image (Figure 1d and Figure S1), we can directly observe a d-spacing of 0.576 nm, which belongs to the (200) planes and runs parallel to the length of the nanobelt, indicating that the nanobelt grows along the [010] direction. Obviously, the {001} facets with large interlayers along the c-axis are predominantly exposed, which facilitates the chemical intercalation of cations. Moreover, this growth feature creates a unique self-healing lamellar structure during the reversible intake/removal of cations in NH4V4O10, which will be discussed in the following sections. No impurities are detected in the sample by energy-dispersive X-ray analysis (EDX, Figure 1d) and thermogravimetric analysis is further conducted to examine the chemical composition 7



of the products (Figure S2). Coin cells (2032) with a metallic zinc (Zn foil with a diameter of 14 mm and a thickness of 0.03 mm) as both reference and counter electrode, 3 M Zn(CF3SO3)2 electrolyte, and glass fiber separator were used to evaluate the electrochemical performance of the materials. As shown in Figure S3, we first performed cyclic voltammetry (CV) studies of the first two cycles between 0.8 and 1.7 V to observe the electrochemical behavior of the cell. An open-circuit voltage of ca. 1.3 V versus Zn2+/Zn is detected, and the observation of large difference between the two CV curves suggests that an irreversible phase change happens in the first cycle. Figure 2a shows the typical voltage-capacity profiles of a NH4V4O10//Zn cell cycled at 200 mA g–1. During the discharge process, the potential initially rapidly drops to a short plateau located at 1.3 V and then decreases smoothly to the cut-off voltage. During the charge process, the potential gradually increases to 1.3 V and then suddenly rises to an obvious plateau at 1.5 V. The voltage of 1.3 V noticeably acts a critical point that divides the electrochemical events occurring before and after it. The subsequent overlapped charge-discharge profiles are consistent with the cycling performance illustrated in Figure 2b, which implies a reversible zinc storage property. In addition, the large coulombic efficiency (96.9% and ≥99.8% in the second cycle and subsequent cycles, respectively) further suggests the full utilization of the transferred cations and electrons. The variations in the pronounced redox peaks with increasing sweep rates, v, during voltammetry experiments are often examined to distinguish the charge storage mechanism of a nanomaterial electrode 72,73. Figure 2c depicts the CV plots obtained from 0.1 to 0.5 mV s–1, which possess well-preserved shapes. The qualitative relationship between each peak current i and v is expressed as i=avb, where both a and b are constants

72,73

. Specially, the value of b provides kinetic information: b=0.5 is

regarded as diffusion-controlled behavior, whereas b=1.0 presents a dominant pseudocapacitive contribution

72–74

. In this system, the b values calculated from the

two redox peaks (Figure 2c) are 0.84 and 0.77 (Figure 2d), which suggest the 8


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existence of extrinsic pseudocapacitive effects. More intuitive contributions of the two kinetics contributions are given in Figure 2e and f, where the capacitive contribution increases from 70.1% to 85.1% as the sweep rate increases. The gradually increasing ratio of the pseudocapacitance, namely, the occurrence of more redox reactions on the surface of the electrode materials, indicates an excellent rate performance. As displayed in Figure 3a, the insertion/extraction of cations at various currents only slightly affects the electrochemical zinc storage property of the electrode. A specific discharge capacity of 77 mAh g–1 is initially obtained at a current of 50 mA g–1 (Figure S3) and increases to a maximum value of 147 mAh g–1 after several cycles (Figure 3b). The specific capacities obtained at fast discharge rates of 200 and 600 mA g–1 are 126 and 104 mAh g–1, respectively, which are ~86 and 71% of that obtained at 50 mA g–1. Even at 1000 and 2000 mA g–1, which correspond to the completion of a full charge/discharge process in ca. 10 and 5 minutes, respectively, impressive capacities of 91 and 72 mAh g–1 are achieved, confirming the high-power capability. The Ragone plot in Figure 3c shows the energy densities of recently reported vanadium-based cathode materials

29,32,75−77

(calculated based on the active

materials). The most inspiring merit of the cell is its outstanding cycling stability. During long-term operation at 500, 1000 (Figure S4), and 2000 mA g–1 (Figure 3d), the cell exhibits considerable capacity retentions of 90.6, 87.5, and 70.3%, respectively, of the highest capacities delivered at high rates. Based on comparison with other reported electrode materials (Table S1), the NH4V4O10 nanobelts demonstrate a superior cycling performance, which may result from the structural stability and facile diffusion paths of cations; this high performance motivates us to further study the zinc storage mechanism of the NH4V4O10 nanobelts. To elucidate the chemical states of the electrode materials, X-ray photoelectron spectroscopy (XPS) analysis was carried out. Figure 4a gives the survey spectra of the pristine material and the fully discharged and charged electrodes in the first cycle, showing all the elements present in the electrode. Interestingly, neither discharging 9



nor charging produce the signals corresponding to nitrogen (Figure 4b). The simplest explanation for this observation is that the NH4+ groups are expelled from the open spaces between the large layers of V2O5 chains during the insertion of divalent cations as a result of the electrostatic repulsion. Evidence obtained the Fourier transform infrared (FT-IR) spectroscopy further suggests that this phenomenon happens at the very beginning of cation insertion (Figure S5). The extraction of NH4+ groups inevitably leads to rearrangement of the lamellar structure, and the vacated sites provide unoccupied space for hosting intercalated cations. The mixed valence states of the V component can be determined from the V 2p signals of the three materials. Therefore, to gain insight into the variation in the V oxidation level and electronic structure, synchrotron X-ray adsorption spectroscopy (XAS) characterization was employed. Figure 4c exhibits the normalized V K-edge X-ray adsorption near edge structure (XANES) profiles of pristine NH4V4O10 and the cycled electrodes at selected states in the first cycle. The pre-edge peak, known as the 1s−3d electronic transition, is located ca. 5470 eV and usually provides an indication of the symmetry around the vanadium site

78

. Obviously, a significant and irreversible change is

observed in the intensity of the pre-edge peak after cation intercalation, indicating the formation of a distorted square pyramidal geometry at V2O5 sites and the occurrence of an irreversible structural phase transition. All three electrodes have an edge step between the V4+ and V5+ standard spectra. Surprisingly, the oxidation state of V does not decrease but rather increases to a higher value in the fully discharged state. Upon recharging, the maximum oxidation state, which is extremely close to V5+, is observed. Figure S6 displays the extended X-ray absorption fine structure (EXAFS) spectra of the cycled NH4V4O10 electrodes at different states. The first two strong peaks at 0.15 and 0.25 nm are dominated by single scattering contributions from the first and second coordination spheres of V-O and V-V correlations. No obvious variation in the two peaks is observed after the electrochemical reactions. This lack of change is abnormal and contrary to the results obtained for other vanadium oxide-based nonaqueous batteries 17, as discussed in the supporting information. The small change 10


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in the valence state induces a change in the color of the electrodes, as exhibited in Figure S7. In addition, the O 1s spectra (Figure 4d) deserve special attention. The peaks at binding energies of 530.4, 531.3, and 532.5 eV are attributed to the O 1s orbitals of VOx, OH, and crystalline H2O molecules, respectively 79. The intensity of the last peak is clearly dramatically increased in the full discharged state and disappears upon recharge, consistent with the insertion and extraction of Zn2+ ions. Early on, Oka et al. proposed the existence of a 6-fold coordination shell around the Zn2+ ions in hydrated Zn0.25V2O5·H2O

80

. Combined with the XPS results, it is

reasonable to conclude that Zn2+ ions coordinated by H2O molecules (Zn2+ hydrate: Zn(H2O)m2+) are the most probable interstitial cations for intercalation into NH4V4O10. The Zn 2p component centered at 1022 eV has a much higher atomic percentage obtained upon discharging to 0.8 V than that in the extracted state, which verifies the reversible (de)intercalation of Zn2+-based cations (Figure 4e). Figure 4f displays the XANES spectra recorded at the Zn K-edge of a fully discharged NH4V4O10 electrode (first cycle) and the aqueous electrolyte (3 M Zn(CF3SO3)2). A six-coordinate conformer of zinc is dectected in the electrolyte (Zn(H2O)62+, octahedral coordination), while there is only a slight decrease in the density collected from the discharged electrode

81

. This result suggests that the inserted zinc ions possess a similar

coordination geometry as those in the electrolyte, namely, Zn(H2O)62+, agreeing well with the above XPS analysis. To investigate the morphological evolution of the NH4V4O10 electrodes, we used TEM to observe the microstructures at various stages. As shown in Figure 5a, no evident changes in the morphology are observed when the electrode is discharged to 0.8 V in the first cycle. However, the corresponding HRTEM image (Figure 5b) exhibits a narrowed interlayer distance of 0.45 nm, corresponding to the lattice fringe of the (200) planes; this distance is far less than that of pristine NH4V4O10 (Figure 1d). The well-defined but alternative diffraction plot of the collected SAED patterns (Figure 5b, inset) unambiguously reveals that a phase change takes place during the operation of the NH4V4O10//Zn cells. EDX and element mapping analyses together 11



illustrate that the nanobelts have a homogeneous distribution of V, O, and Zn (Figure 5c and d). For the charged state, the morphologies of the electrodes are well maintained until the cell is charged over 1.3 V. As mentioned above, the charge profile of the cell experiences a quick rise beyond this voltage. To our surprise, upon charging to 1.5 V, a substantial portion of the nanobelts vanishes and is replaced by a stacked sheet-like morphology constituting a mixed nanobelts and nanosheets (Figure S8), both of which consist of uniform distributions of V, O, and Zn. When the cell is fully charged to 1.7 V, nanosheets become the dominate morphology of the electrode (Figure 5 e to g), and polycrystallinity is observed (Figure 5g, inset). Only a very small amount of Zn is observed by EDX spectroscopy, implying the thorough extraction of Zn(H2O)62+ (Figure 5i and j). The remaining lattice fringes shown in Fig. 5h confirm the crystalline nature of these nanosheets. Moreover, the interlayer d-spacing of the (001) planes is dramatically enlarged to 1.3 nm based on careful measurement of the edges of the curled nanosheets (Figure 5k and l). Subsequently, we performed TEM and XPS analyses on the electrodes after the second cycle. Astonishingly, these nanosheets revert back to the original nanobelts with good crystallinity (Figure S9) upon discharging to 0.8 V and once again convert to nanosheets after the cell is fully charged to 1.7 V (Figure S10). The exact same evolution in the chemical states is also observed (Figure S11), and the reversible morphological variation can be observed even after 1000 cycles (Figure S12), demonstrating an interesting self-healing behavior upon the intake/removal of Zn(H2O)62+. To gain a better understanding of the zinc-storage mechanism, it is important to clarify the structural response of the electrodes to the (de)intercalation of Zn(H2O)62+. Following the initial discharge process (Figure 6a), most diffraction peaks vanish, leaving only a set of 00l reflections characteristic of bilayered vanadium oxides with a c-axis preferred orientation. The layer spacing of the (001) reflections is stretched from the initial 0.97 to 1.03 nm as a result of the intercalation of Zn2+ hydrate cations (radius of Zn(H2O)62+: 0.43 nm) that replace the NH4+ groups (radius: 0.148 nm) 82. 12


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By virtue of the continuous intercalation of large cations, the electrostatic attraction present between layers consistently impacts the electrochemical behavior. A slight shrinkage in the c values is clearly observed with increasing voltage, analogous to the results obtained for 2D Ti3C2

24

. The charged material exhibits the opposite trend in

the lamellar structure. Of particular concern is the diffraction pattern collected of the electrode charged to 1.5 V, where another set of distinct 00l reflections possessing a much larger interplanar distance (1.3 nm) appears. The 1.3 nm spacing matches well with the results of our HRTEM experiments (Figure 5i), and the intensity of the corresponding diffraction pattern gradually increases to the maximum intensity observed upon charging to 1.7 V. In the 2nd discharge process, an abrupt change in the XRD pattern occurs when the cell is discharged beyond to the critical voltage of 1.3 V, in which a noticeable reduction in the d-spacing of the (001) reflections to nearly the same value as that of the initial discharging state is observed. The subsequent overlapped XRD patterns of both the discharged and charged states in the second cycle (Figure 6a) demonstrate the structural stability and reversible zinc-storage property of the material. Furthermore, a table summarizing the changes in the (200) and (001) planes upon charging and discharging based on the TEM and XRD results is provided to clearly illustrate the structural stability (Table S2). Elucidation of the molecular vibration modes of the material can provide more structural information on the intercalation of Zn(H2O)62+. Figure 6b shows the Raman scattering spectra obtained various points in the first two cycles. Generally, the low-frequency peaks (100 and 170 cm–1) below 200 cm–1 are attributed to the bending vibration of the skeleton and are strongly associated with the layered structure 58. The high-wavenumber peaks at 269, 428, 509, and 689 cm–1 correspond to the bending vibrations of the O-V-O, V-O, and V-O-V bonds and the stretching vibration of the V-O bonds, respectively, in the bilayered V2O5 chains 58. Intriguingly, all the Raman bands exhibit regular shifts in conjunction with the intake and removal of cations. Specifically, an opposite trend in the Raman shifts is observed for the peaks below and above 200 cm–1 under both discharging and charging conditions. For instance, in 13



the charging process of the first cycle, the Raman fingerprint of the layer-type structure shows a significant redshift (–12.3 cm–1), whereas the vibrations of the V2O5 chains exhibit obvious universal blueshifts (typically +10 cm–1). Strain is known to induce shifts in the vibrational frequencies, and consequently, the Raman shifts can be applied to evaluate the increase or release of stresses in the material. Therefore, it can be concluded that the interlayer stress in the material increases during cation intercalation and is then released upon cation extraction, while the in-plane stress of the V2O5 chains the results are opposite. We combined the results of all the above electrochemical, XPS, FT-IR, XAS, HRTEM, XRD, and Raman experiments to develop a plausible mechanism for the reversible Zn(H2O)62+ insertion/extraction process occurring in NH4V4O10, as illustrated in Figure 6c. Starting from the initial breakthrough of Zn(H2O)62+ cations, the NH4+ groups between the bilayers are continuously expelled into the electrolyte as a result of electrostatic interactions. This process is irreversible and leads to transformation of the architecture to bilayered ZnyV4O10-x·mH2O nanobelts and the disappearance of NH4+ groups from the electrodes. Upon recharge, the extraction of cations produces electrostatic repulsion between the bilayers, which favors the release of stress from the interlayer structure. This process increases the d spacing, accelerates the bending vibrations of the skeleton, and ultimately engenders the slip of the expanded layers, leading to the formation of few-layer nanosheet. The occurrence of this dislocation and gliding process is evidenced by the steep climb in the charge curve from 1.3 to 1.7 V and the elevated strain values of the skeleton measured using Raman spectroscopy. With the reinjection of cations, the Zn(H2O)62+ cations serve as a binder repair the mismatched nanograins with the aid of electrostatic attraction between the large cations and bilayers, which quickly (discharge to 1.3 V) recover their original positions in situ, thereby reestablishing the belt-like morphology. Note that the morphological evolution from nanobelts to nanosheets is strictly conducted in a limited space. From combination of this fact with the highly reversible electrochemistry, it is reasonable to propose that the intercalation process, which is a 14


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solid-solution reaction, dominates the electrochemical behavior of the NH4V4O10//Zn battery. The sloping plateaus in both the charge and discharge profiles are assigned to the structural changes resulting from variations in the stress. Thus, the framework structure of the NH4V4O10 electrode is flexible and capable of electrochemical self-healing. This enhanced flexibility during the gliding and buckling of the layers provides a suitable environment for accommodating guest species, and the subsequent self-healing process further provides the structural stability; both of these properties contribute to an excellent cycling performance. The electrochemical reactions of the NH4V4O10//Zn battery can be described using the following equations (excluding the irreversible reaction in the first discharge process: NH4V4O10 + 0.55 Zn(H2O)62+ + 1.1 e– → Zn0.55V4O10·mH2O + NH4+; the number of electrons gained or lost is calculated from the specific capacities in the electrochemical tests): V4O10–x·nH2O + 1.05 Zn(H2O)62+ + 2.1 e– ↔ Zn1.05V4O10–x·mH2O Inspired by these interesting findings, the following crucial question will inevitably be brought forward: is the observed self-healing a universal behavior of layered vanadium oxides when used as cathode materials for rechargeable zinc-ion batteries? For this purpose we have examined the zinc-storage properties of layered V2O5 nanomaterials which are fabricated by calcinating NH4V4O10 at 450 oC in air. On the basis of the XRD patterns, the as-prepared V2O5 belongs to the orthorhombic system (α-V2O5) and is made up of single layer of VO5 square pyramids with a narrow d-spacing of 0.44 nm (Figure S13 and 14). The incorporation of Zn2+-based cations into the orthorhombic electrode is accompanied by a single phase transition, as indicate by the observation of only one plateau in the discharge curve. Some broadening of the background and changes in the peak intensities are observed in the XRD patterns (Figure S16). However, no notable changes in the morphology and molecular vibration modes are observed in the HRTEM and Raman studies (Figure S 15 and 16). This opposite result may be attributable to the incomplete reaction of V2O5 (the theoretical capacity of V2O5 (300 mAh g−1) is much higher than that the 15



value we obtained (120 mAh g−1)), which suggests that the phase transition must be irreversible and cause disorder in V2O5, leading to the formation of amorphous electrodes. As expected, the charge/discharge plateaus become increasingly blurred and finally disappear with prolonged cycling (Figure S17). Consequently, the V2O5 electrode exhibits a very poor cycling performance (Figure S18). This result demonstrates that the change in the crystal structure of V2O5 is associated with the fading of the electrode performance, as its lamellar structure is unable to withstand the ongoing intercalation of large cations (Zn2+ hydrate, see Figure S19) and gradually collapses into numerous tiny domains. By comparing the distinct zinc-storage properties of V2O5 and NH4V2O5, we deem that the bilayer structure of the latter is the key to the electrochemical performance. First, V2O5 slab, which is stacked from double chains propagating down the b-axis by co-sharing the edges of VO6 octahedra, is linked via interchain V-O bonds and arranged in parallel along the c-axis to form bilayered material. In contrast, the α-V2O5 is an ordered assembly of square pyramidal VO5 units. With the cations intercalation, the based composition unit of bilayered material is better to keep the stability of structure, which may because the stronger interactions between V-O bonds than the van der Waals forces in the monolayer material. Second, the open 2D channels with large distances, derived from the double chains arranged in parallel via van der Waals forces, provide facile diffusion pathways for cation transport. A schematic illustration of the superiority of bilayered structure is shown in Figure S20.

Conclusion In summary, we have reported a new intercalation cathode material that can reversibly react with Zn2+ hydrate ions by extruding NH4+ groups from the bilayer structure. Through investigation of the material upon discharge/charge, a self-healing lamellar structure that depends on the electrostatic interactions between injected cations and bilayered chains is observed. The adjustable intralayer spacing ensures barrier-free access to accommodate Zn(H2O)62+ ions, and the flexible architecture effectively 16


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eliminates the stresses created by cation insertion/extraction; both of these properties contribute to highly stable and high-rate electrochemical properties. The greatly increased capacity relative to the initial value indicates that the interlayer spaces are fully available for insertion of guest cations, leading to the maximum utilization of the material. Moreover, the cathode electrode reported here supplies a specific energy of 155 Wh kg–1 (cathode only) and a capacity retention of 70.3% after 5000 cycles, demonstrating the promising applications of this material in rechargeable zinc-ion batteries.

Supporting information Supplementary TEM, FT-IR, XPS, Raman characterizations, and electrochemical testing data are included. This material is available free of charge on the ACS Publications website or requests from the authors.

Author contributions C.X.W proposed the research. G.Z.Y and T.Y.W performed the synthesis, characterizations, and electrochemical measurements of materials. G.Z.Y conducted the HRTEM. T.Y.W carried out the XRD and Raman studies. All authors contributed to the discussions of scientific issues.

Competing interests The authors declare no competing financial interests.

Acknowledgements This work is supported by the National Natural Science Foundation of China (U1401241)

and

Guangdong

Natural

Science

(2015A030310462).

ORCID Gongzheng Yang: 0000-0001-9193-7437 17


Foundation

of

China


Tongye Wei: 0000-0001-7232-8644 Chengxin Wang: 0000-0001-8355-6431

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Figure 1. Morphological and structural characterization of NH4V4O10. a, Panoramic SEM image. b, XRD patterns. The blue line in b belonging to the standard characteristic peaks of NH4V4O10 (JCPDS no. 31−0075). c, TEM image. An arrow parallel to the long axis of nanobelt highlighting its growth orientation. The inset in c showing the corresponding SAED pattern. d. HRTEM image. The inset in d illustrates the EDX result.

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Discharge Charge

40

70

0 0

40

80

120

60 0

50

100

-1

-1

s -1

0.1 mV s -1 0.2 mV s -1 0.3 mV s -1 0.4 mV s -1 0.5 mV s

0.4

0.1 -

Current (mA)

0.5

mV

0.8

0.0

d log (i, current)

1.2

Peak 1

200

Cycle number


c

150

0.0

-0.2

b = 0.84

b = 0.77 -0.4

-0.4

Peak 1 Peak 2

-0.6

-0.8

Peak 2 0.8

1.0

1.2

1.4

-1.0

1.6

-0.8

2+

e

1.2

Capacitive -1 0.5 mV s

f

-0.4

0.4

0.0

-0.4

Diffusion-controlled Capacitive 120

Contribution ratio (%)

Current (mA)

0.8

-0.6

log (V, scan rate)


70.1%

76.5%

0.1

0.2

77.0%

81.6%

85.1%

90

60

30

-0.8 0 0.8

1.0

1.2

1.4

1.6

2+


0.3 0.4 -1 Sweep rate (mV s )

0.5

Figure 2. Electrochemical performance and quantitative capacitive analysis of zinc storage behavior. a, Galvanostatic discharge-charge profiles of the NH4V4O10 electrode that are obtained at a current density of 200 mA g–1 in different cycles. b, Cycling performance at a current density of 200 mA g–1. c, CV curves at different scan rates. d, Log(i) versus log(v) plots at specific peak currents. e, Capacitive (sky-blue) 28

–1

contribution to charge storage at 0.5 mV s . f, Contribution ratio of diffusion-controlled and capacitive capacities at different scan rates.


Coulombic efficiency (%)

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

Page 28 of 33

Page 29 of 33

2.1

c 1500

-1


3

10

100

300

800

1.8

Specific energy (Wh kg )

a 2+

1.5

1.2

0.9

-1

1000

2000

Unit: mA g

0.6

600

50

200

2

10

this work ZnV2O7(OH)2⋅2H2O H2V3O8 LiV3O8 VS2 Zn0.25V2O5

1

10 0

30

60

90

120

0

150

10

-1

1

100 -1

80

-1

300 600 800

60

1000 1500

80

2000 40

40

20

Discharge Charge 0

0 40

60

80

100

Capacity retention (%)

50 200

20

4

10

10

120 100 100

100

0

-1

80 80 60

-1

60

2000 mA g

70.3% 40

40

20

20

0

120

0

Cycle number

1000

2000

3000

4000

Cycle number

Figure 3. Rate and long-term cycling performances of the NH4V4O10//Zn cell. a, Galvanostatic discharge-charge profiles that are obtained at different current densities. b, Rate performance. c, Comparison of the Ragone plot of NH4V4O10//Zn cell with other vanadium oxides-based cathode materials (the specific energy and power densities are calculated based on active materials). d, Long-term cycling properties at a current density of 2000 mA g–1.

29


0 5000



d

50 unit: mA g

120

3

10

Specific power (W kg )

b 160

2

10



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



a

Pristine Full Discharge Full Charge

d

O 1s

Pristine

F 1s

V 2p -

OH

H2O

Zn 2p

O 2s C 1s O (KLL)

Zn (LMM)

V 3s

Intensity (a.u.)

Intensity (a.u.)

V 2s

V (LMM) C (KLL)

1200

O 1s

2-

O

F (KLL)

1000

800

Zn (LMM)

Zn 3p Zn 3s

N 1s

V 3p

600

400

200

Full Discharge

Full Charge

0

Binding energy (eV)

b

N1s

Pristine Full discharge Full Charge

535

534

Intensity (a.u.)

e Intensity (a.u.) 404

402

400

0.9

531

530

529

1045

1040

f

Ref. VO2 Pristine

0.6

0.3 Reduction Oxidation 0.0

1035

1030

1025

1020

1015

Binding energy (eV)

Pristine Full Discharge Full Charge Ref. V2O3 Ref. V2O5

Zn2p3/2

Zn2p1/2

398

Normalized adsorption (a.u.)

1.2

532

Full Discharge Full Charge

Binding energy (eV)

c

533

Binding energy (eV)

406

Normalized adsorption (a.u.)

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

Page 30 of 33

5480

5482

2.1

Zn K-edge XANES

1.4

0.7

Full Discharge Ref. Zn(CF3SO3)2-Solution

5484

0.0

5470

5480

5490

5500

9660

9680

9700

9720

Energy (eV)

Energy (eV)

Figure 4. XPS and XAS characterization of the electrode materials. a, Survey spectra of the electrodes obtained at different stages. The fluorine coming from the binder of PVDF in manufacturing electrodes. b, N 1s region. c, Normalized V K-edge XANES spectra for V2O3 (V3+ standard), VO2 (V4+ standard), NH4V4O10, fully discharged NH4V4O10, fully charged NH4V4O10, and V2O5 (V5+ standard) electrodes. The inset showing a magnified image in the XANES profiles marked with yellow dotted circle. d. O 1s region. e. Zn 2p region. f, XANES30spectra recorded at Zn K-edge of fully discharged NH4V4O10 electrode and 3 M aqueous Zn(CF3SO3)2 solution.


Page 31 of 33 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

a


c

O

b

V

1st cycle Full Discharge

0.45 nm

(200)

Zn Cu Zn

V

0

100 nm

2

4

6

10

d

10 nm

O

e

8

g

V

Zn

i

C 1st cycle Full Charge

V O F Zn 0

Cu Zn Cu

V 2

4

6

8

10

j 500 nm

50 nm V

f

h

k

O

l d=1.3 nm

500 nm

10 nm

50 nm

10 nm

Figure 5. Morphological characterization of the electrodes. a-d, NH4V4O10 electrodes that are discharged to 0.8 V in the first cycle. a, TEM image of the well-preserved nanobelt. b, HRTEM image and SAED pattern suggesting the high crystallinity of the nanobelt. c, EDX spectrum and quantitative analysis. d, STEM and elemental mappings collected from the blue rectangular region. e-l, NH4V4O10 electrodes that are charged to 1.7 V in the first cycle. e, f) TEM, STEM images and g, k) magnified TEM images revealing that the electrodes entirely changed to sheet-like morphologies. The crystallinity of the nanosheets is identified by the FFT patterns in inset of g. h, l, HRTEM images show the dramatically increased d-spacing from 0.97 to 1.3 31nm in the (001) plane. i, j, EDX analyses and STEM-EDS mappings verify the full extraction of Zn2+-based ions.



a

b

Pristine Discharge 500

Charge

nd

nd

2 Cha. 1.6 V nd

400

2 Cha. 1.5 V

2 Cha. 1.7 V

Intensity (a.u.)

nd

2 Cha. 1.7 V

nd

2 Cha. 1.3 V nd

2 Dis. 0.8 V

nd

2 Dis. 1.3 V st

1 Cha. 1.7 V

nd

2 Cha. 1.3 V st

1 Cha. 1.3 V nd

st

2 Cha. 1.1 V

1 Dis. 0.8 V

nd

2 Cha. 0.9 V

Pristine

nd

2 Dis. 0.8 V

200 300

Intensity (a.u.)

nd

2 Dis. 0.9 V nd

2 Dis. 1.0 V nd

2 Dis. 1.1 V nd

2 Dis. 1.3 V nd

2 Dis. 1.5 V 200 st

1 Cha. 1.7 V

c

400

600

800

1000

-1

Wavenumber (cm )

-1

st


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

Page 32 of 33

1 Cha. 1.6 V st

1 Cha. 1.5 V st

1 Cha. 1.3 V st

1 Cha. 1.1 V

100

st

1 Cha. 0.9 V st

1 Dis. 0.8 V st

1 Dis. 0.9 V st

1 Dis. 1.0 V st

1 Dis. 1.1 V

0

pristine 10

20

30

40

2θ (degree)

50

60

70

80

1.5

1.2

0.9 2+


Figure 6. Ex situ XRD and Raman analyses of the electrodes. a) XRD patterns and b) Raman spectra of the NH4V4O10 electrodes that are obtained in the initial two cycles at selected states. The black, blue, and pink lines correspond to the pristine, discharged, and charged states, respectively. c, Scheme illustration of the possible zinc-storage mechanism of the NH4V4O10 electrodes.

32


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TOC graphic

33


A self-healing lamellar structure boosts highly-stable zinc-storage

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