Enhanced Reversible Zinc Ion Intercalation in Deficient Ammonium Vanadate for High?Performance Aqueous Zinc?Ion Battery

Quan Zong, Wei Du, Chaofeng Liu, Hui Yang, Qilong Zhang?, Zheng Zhou, Muhammad Atif, Mohamad Alsalhi, Guozhong Cao?

ABSTRACT Ammonium vanadate with bronze structure (NH4V4O10) is a promising cathode material for zinc?ion batteries due to its high specific capacity and low cost. However, the extraction of at a high voltage during charge/discharge processes leads to irreversible reac?tion and structure degradation. In this work, partial ions were pre?removed from NH4V4O10 through heat treatment; NH4V4O10 nanosheets were directly grown on carbon cloth through hydrothermal method. Defi?cient NH4V4O10 (denoted as NVO), with enlarged interlayer spacing, facilitated fast zinc ions transport and high storage capacity and ensured the highly reversible electrochemical reaction and the good stability of layered structure. The NVO nanosheets delivered a high specific capac?ity of 457 mAh g?1 at a current density of 100 mA g?1 and a capacity retention of 81% over 1000 cycles at 2 A g?1. The initial Coulombic efficiency of NVO could reach up to 97% compared to 85% of NH4V4O10 and maintain almost 100% during cycling, indicating the high reaction reversibility in NVO electrode.

KEYWORDS Deficient ammonium vanadate; Large interlayer spacing; Reversible redox reaction; Electrochemical mechanism

1 Introduction

The global energy crisis and environmental issues continu?ously drive the development of renewable energy sources such as solar, wind and tidal energy [1, 2]. Electrical energy storage (EES) systems are vital to enable and guarantee the reliability and scalability of these renewable energies [3, 4]. Among various EES systems, lithium?ion batteries (LIBs) have been widely employed in the smart electronics and electric vehicles owing to their high energy density and the long?term lifespan [5—7]. Nevertheless, the safety issues, high cost, environmental concerns, as well as limited lithium resources have strongly restricted their large?scale and sus?tainable applications [8, 9]. Recently, a variety of aqueous batteries based on alkali metal cations (Li+, Na+, and K+) and multivalent charge carriers (Mg2+, Ca2+, Zn2+, and Al3+) have attracted great attention [10—14]. Aqueous zinc?ion bat?teries (ZIBs) are increasingly developed owing to their low cost, excellent safety, relatively high theoretical capacity (820 mAh g?1) and low redox potential (? 0.76 Vvs.the standard hydrogen electrode) [15—17]. To date, a lot of atten?tion has been paid on the cathode materials for ZIBs includ?ing manganese oxides [18—20], vanadium?based compounds [21—25], Prussian blue and its analogs [26], transition metal dichalcogenides [27, 28], and polymers [29]. However, some disadvantages seriously hinder the practical application of these materials, such as irreversible phase transition of man?ganese oxides, low specific capacity of Prussian blue and its analogs (＜ 100 mAh g?1), and low electrical conductivity of transition metal dichalcogenides [30—32].

In this work, we prepared 2D NVO nanosheets through heat treatment of NH4V4O10nanosheets grown on carbon cloth in air at a low temperature (300 ℃). After the heat treatment, the interplanar spacing is enlarged and the ions transfer pathways are increased due to the loss of most NH+4ions, which is beneficial for rapid Zn?ion intercalation/dein?tercalation. The layered structure of NVO remains to be sta?ble and the NVO electrode exhibits high reaction reversibil?ity because the deammoniation from the interplanar spacing is effectively prevented, contributing to a long?life span. The electrochemical mechanism in NVO, involving the process of ions intercalation/deintercalation, has been elaborated.

2 Experimental

2.1 Synthesis of NVO Nanosheets

All chemical reagents were used without any further purifi?cation. Prior to the synthesis, a piece of carbon cloth (1 × 2 cm2) was treated by sonication in 3 M HCl solution for 30 min, followed by sonication in acetone, deionized (DI) water, and absolute ethanol sequentially for 30 min each, and dried at 60 °C under vacuum for 12 h. The NH4V4O10nanosheets were fabricated through a facile hydrothermal reaction. 4 mmol of NH4VO3(98%, Fisher Scientific) and 4.8 mmol of H2C2O4·2H2O (98%, Fisher Scientific) were added into 80 mL DI water and kept stirring for 30 min. The mixture was transferred to a 100 mL?Teflon?lined stainless?steel autoclave, and then a piece of carbon cloth protected by Polytetrafluoroethylene tape was immersed into the reaction solution. The autoclave was sealed and maintained at 180 °C for 6 h. After cooling to ambient temperature naturally, the carbon cloth was carefully taken out and rinsed by ethanol and deionized water several times and then dried at 60 °C under vacuum for 12 h. The NVO nanosheets were obtained by annealing NH4V4O10nanosheets at 300 ℃ in air for 2 h with the heating speed of 5 ℃ min?1. For comparison, the NH4V4O10nanosheets were annealed at 400 ℃ in air for 2 h with the same heat?ing speed to prepare V2O5nanosheets. The mass load of NH4V4O10, NVO and V2O5nanosheets was about 0.7, 0.65 and 0.5 mg cm?2, respectively.

2.2 Material Characterizations

The phase of the sample was identified by a Bruker X?ray diffractometer (XRD, D8 Discover with IμS 2?D detection system) at an accelerating voltage of 50 kV and a working current of 1000 μA. Thermogravimetric analysis (TGA) was conducted using a TGA5500 to analyze the thermal stabil?ity of the sample within 30—600 °C. Raman spectra were recorded on a Horiba Scientific LabRAM HR Evolution with a laser excitation sources of 514 nm. Fourier transform infra?red spectroscopy (FTIR) pattern was collected on Thermo Fisher Nicolet iS5 from 4000 to 400 cm?1using the ATR technique. The morphologies and structures of the materials were observed using scanning electron microscopy (SEM, SU?8010) and transmission electron microscopy (TEM, Tecnai G2 F20) equipped with an energy?dispersive X?ray spectrometer (EDS) operated at 200 kV. X?ray photoelectron spectroscopy (XPS) technique was carried out on Thermo Scientific K?Alpha with an Al Kα radiation (1486.6 eV) to determine the valent state of elements.

2.3 Electrochemical Characterizations

Electrochemical performance was tested using CR2032 coin?type cells, which were assembled by binder?free nanosheets as the cathode, Zn metal as the anode (0.15—0.25 mm thick), 80 μL of 3 M zinc trifluoromethanesulfonate (98%, Zn(CF3SO3)2) aqueous solution as electrolyte and a glass fiber filter (Whatman, Grade GF/A) as the separator in an air atmosphere. Cyclic voltammetry (CV) in a voltage window of 0.2—1.6 V and electrochemical impedance spectroscopy (EIS) in the frequency range from 0.01 Hz to 100 kHz were tested on a Solartron electrochemical station (SI 1287) cou?pled with an electrochemical impedance spectroscopy sys?tem (EIS, SI 1260). The galvanostatic charge—discharge was obtained on a Neware tester (CT?4008). The galvanostatic intermittent titration technique (GITT) was applied to ana?lyze the reaction and diffusion kinetics at a current density of 50 mA g?1and a charge/discharge time and interval of 10 min for each step.

3 Results and Discussion

3.1 Phase and Structural Characterizations

Fig. 1 Phase and structural characterization of NH4V4O10 and NVO. a XRD patterns. The (001) peak in NVO shifts toward a low angle, indicat?ing a large interlayer spacing. b TGA analysis of NH4V4O10. The inset is the crystal structures of bi?layered NH4V4O10 and single layered V2O5. c Raman and d FTIR spectra, the similar spectra indicate similar VO framework in two samples while the peak changes originate from the loss of NH

SEM images in Fig. 2a, b show that the ultrathin NVO nanosheets are grown on the activated carbon cloth uni?formly. The hydrophilic surface of activated carbon cloth is suitable for the uniform growth of nanosheets (Fig. S2). The nanosheets almost maintain the original morphology after annealing in 300 °C compared to the pristine NH4V4O10(Fig. S3). TEM images (Fig. 2c, d) display the ultrathin nanosheets with a lattice spacing of 0.194 nm, which cor?responds to the spacing of (403) plane of NH4V4O10. EDS mapping in Fig. 2e reveals the homogeneous distribution of N, V, and O elements in NVO nanosheets. The trace amount of N element suggests the loss of many ammonium ions between the layers. The high resolution XPS spectrum is used to analyze the valence state of V in two electrodes shown in Figs. 2f and S3. For NVO, the peaks at 517.4 ad 516.0 eV are assigned to the V 2p3/2electrons of V5+and V4+, respectively [61, 62]. The ratio of V5+/V4+can be cal?culated by the integration areas of the fitted peak areas. The V5+ratio of 60.4% in NVO is higher than that of NH4V4O10(51.6%) because of the oxidation of V4+after the heat treat?ment in air.

3.2 Electrochemical Reaction Kinetics

Fig. 2 Structural and morphological characterization of the NVO nanosheets. a, b SEM images, the ultrathin NVO sheets are grown on the car?bon cloth uniformly. c TEM and d HRTEM images, a lattice spacing of 0.194 nm corresponds to the spacing of (403) plane. e Elemental map?pings of N, O, and V. Few N elements are detected due to the loss of ammonium ions. f XPS spectrum of V 2p3/2

Fig. 3 Electrochemical reaction kinetics. Comparison of a 1st cycle and b 3rd cycle CV curves of NH4V4O10 and NVO at a scan rate of 0.1 mV s?1. c CV curves of NVO electrode in the first three cycles, the overlapped curves mean a highly reversible reaction. d CV curves of NVO at different scan rates. e The relationship between peak currents and scan rates. f Capacitive contribution at 0.5 mV s?1 in NVO (54.6%). g The percentages of capacitive and diffusion contributions at different scan rates, increasing from 45.9% to 67.4%. h Nyquist plots of NH4V4O10 and NVO before and after CV test. i Relationship between the real part of impedance and low frequencies, smaller slopes of the lines mean fast zinc ion diffusion

Table 1 Comparison of the peak positions and voltage gaps of three samples in the first and third cycle of CV

The CV curves at different scan rates from 0.2 to 1.2 mV s?1were measured to analyze the electrochemical reaction kinetics. All CV curves show similar shapes with the increase of scan rate as shown in Fig. 3d. The cathodic peaks shift toward higher potential and the anodic peaks move to lower voltages because of the polarization effect. The relationship of peak currents (i) and sweep rate (v) was investigated by using the following equation [65, 66]:

whereaandbare adjustable parameters. The value ofbcan be determined by the slope of the straight line of logivs logv. Ab?value of 1 indicates that the charge storage is surface?capacitive dominated, while theb?value of 0.5 rep?resents a mass diffusion?controlled process. Theb?values of peaks a?f in NVO were calculated to be 0.50, 0.94, 1.0, 0.86, 0.73, 0.59, respectively (Fig. 3e), whereas the correspond?ing values in NH4V4O10were 0.50, 0.9, 1.0, 0.76, 0.65, 0.50 (Fig. S6). Theb?values suggest that the kinetics of the NVO and NH4V4O10are controlled by a combination of diffusion and capacitive behaviors, and the higherb?values in NVO indicate faster ion diffusion.

In addition, the contributions of capacitive (k1v) and diffu?sion?controlled (k2v1/2) processes could be quantitatively cal?culated by the current density (i) at a particular potential (V) and scan rate (v), based on the following equation [67, 68]:

The fitted CV curve at a scan rate of 0.5 mV s?1is shown in Fig. 3f, in which the shadow area represents the capacitive contribution with a high value of 54.6%. With increasing scan rates from 0.2 to 1.2 mV s?1, the contribution ratio of capacitive increases from 45.9% to 67.4% (Fig. 3g), indicat?ing that the electrochemical behavior of the NVO nanosheets is mainly dominated by the capacitive process.

Nyquist plots consist of a semi?circle in the high?fre?quency section and a straight line in the low?frequency region. The diameter of the semicircle represents the charge transfer resistance (Rct) and the slope of the line represents Warburg resistance (ZW) associating with ion diffusion. In Fig. 3h, theRctof NVO electrode before CV test is about 81 Ω, which is higher than the 42 Ω of NH4V4O10. However, after 15 cycles of CV test, electrodes are activated, leading to reducedRctof two samples, theRctof NVO is decreased to 11 Ω, which is lower than the 30 Ω of NH4V4O10, due to fewer ammonium ions and larger interlayer spacing than NH4V4O10. The relationship between low frequencies and the real part of impedance can be used to calculate the Zn?ion diffusion coefficients (DZn2+) (details shown in ESI) [69, 70]. The NVO electrode exhibits a higherDZn2+of 2.4 × 10?13cm2s?1than NH4V4O10of 6.3 × 10?14cm2s?1, and retains the superiority after 15 cycles of CV test, with 8.7 × 10?13cm2s?1higher than NH4V4O10of 3.4 × 10?13cm2s?1. These results show that the NVO electrode presents small charge transfer resistance and high Zn?ion diffusion coefficient due to the weak interaction force between zinc ions and ammonium ions and large interlayer spacing, which are favorable for fast redox reaction.

3.3 Electrochemical Properties of NVO

Fig. 4 Electrochemical performance of NVO and NH4V4O10. a Comparison of GCD plots at 100 mA g?1. b GCD curves of NVO in first three cycles. c Rate performance of NVO and NH4V4O10. d Cycling stability with the corresponding coulombic efficiencies at 2 A g?1. e Ragone plots in comparison with other cathodes, such as (NH4)2V6O16·1.5H2O nanobelts [48], (NH4)2V6O16·1.5H2O nanowires [32], NH4V4O10 [50], (NH4)2V4O9 [51], V2O5 [73], Ca0.25V2O5·nH2O [74], PANI?VOH [75]

3.4 Charge Storage Mechanism of NVO

Someex-situcharacterizations were employed to study the charge storage mechanism of NVO electrode in Fig. 5. The phases changes of NVO nanosheets during the first charg?ing?discharging process were analyzed using ex situ XRD characteristics in Fig. 5a. Compared to pristine NVO, new peaks located at 12.3°, 32.1°, 60.6°, and 62.6° are indexed to Zn3V2O7(OH)2·2H2O (JCPDS No. 87—0417), which has been reported in previous studies [47, 52, 76]. Generally, Zn2+could be coordinated with water molecules in the aque?ous electrolyte to form large [Zn(H2O)6]2+[77]. Because of the weakened O—H bond affected by Zn2+in H2O, the OH?might originate from the broken O—H bond and react with VO in the layers to form Zn3V2O7(OH)2·2H2O. At the same time, we suspect that the generating H+would not exist in the electrolyte but possibly insert into the cathode mate?rials [78]. In addition, during the first discharge, the (001) peak of NVO shifted slightly to high angle area, indicating shrinkage of the interlayer spacing upon Zn?intercalation. When discharged to 0.2 V, the (001) peak moves to the high angle of 9.06° with the interlayer distance decreased to 9.8 ?, which is possibly attributed to the strong electrostatic interaction between Zn2+and negative single?connected oxy?gen. Upon the first charge, the (001) peak of NVO returns and the peaks of Zn3V2O7(OH)2·2H2O disappear, demon?strating the reversibility of the phase transition.

Ex situ SEM was conducted to study the changes of morphologies during the first charge/discharge process in Fig. 5b?e. When discharged to 0.6 V, some nanoparticles appear on the surface of the NVO nanosheets (Fig. 5b), which could be the growth of Zn3V2O7(OH)2·2H2O. After fully discharged, the surface of the electrode is fully covered by this by?product (Fig. 5c). When charged to 0.8 V, the Zn3V2O7(OH)2·2H2O layer begins to be decomposed and form the nanoparticles (Fig. 5d). As shown in Fig. 5e, after the electrode is charged to 1.6 V, the Zn3V2O7(OH)2·2H2O by?product is decomposed completely and the surface becomes smooth. The results of ex situ SEM suggest that the formation and decomposition of Zn3V2O7(OH)2·2H2O are highly reversible. This process was further confirmed by the ex situ TEM in Fig. S12. A lattice spacing of 0.246 nm can be observed at the fully discharged state, corresponding to the (201) plane of the new phase of Zn3V2O7(OH)2·2H2O (Fig. S12a). After charging to 1.6 V, the electrode exhibits the characteristic of NVO, suggesting the good reversibility (Fig. S12b). EDS mapping in Fig. S12c, d shows the distri?bution of N, V, O, and Zn elements at the fully discharged and charged states. The Zn signal at the fully charged could originate from the absorbed Zn2+or Zn2+existing in the crystal lattice.

Fig. 5 Electrochemical reaction process studies. a Ex situ XRD patterns of NVO in different discharge/charge states, the shifts and recovery of (001) peaks suggest a reversible reaction in the electrochemical processes. Ex situ SEM of NVO in the first discharge/charge process, b discharge to 0.6 V, c discharge to 0.2 V, d charge to 0.8 V, e charge to 1.6 V. Ex situ XPS spectra of f V 2p, g O 1 s, h Zn 2p in the initial, discharge to 0.2 V and charge to 1.6 V states in the first cycle

The V 2p XPS spectra of NVO at different electrochemi?cal stages are shown in Fig. 5f. When discharged to 0.2 V, the V5+cations are reduced to V4+and V3+due to the insertion of zinc ions. The existence of V5+state could be attributed to the Zn3V2O7(OH)2·2H2O by?product and the incomplete redox reaction results that the delivered specific capacity is lower than theoretical capacity. In addition, the peaks shift towards higher binding energy in the fully dis?charged state because the inserted zinc ions could affect the distribution of electron [79]. After charged to 1.6 V, the V cations are oxidized, and the V 2p spectrum is similar to that of initial state, suggesting good reversible redox reactions. Figure 5g shows the XPS spectra of O 1 s. After the first discharging process, the peaks located at around 532.5 and 533.0 eV are attributed to water molecules and OH?, respec?tively, indicating to the insertion of hydrated zinc ions and the formation of Zn3V2O7(OH)2·2H2O [80]. After charging to 1.6 V, the H2O signal is retained while the OH?signal disappears, which further suggests the decomposition of Zn3V2O7(OH)2·2H2O. There is no peaks of Zn 2p in the pristine NVO (Fig. 5h), while two peaks corresponding to Zn 2p1/2(1045.6 eV) and Zn 2p3/2(1022.5 eV) appear in the fully discharged state, indicating the intercalation of zinc ions [81]. The Zn 2p signals in the fully charged state are also detected, which is consistent with the results of ex situ TEM.

GITT plots were measured at a current density of 50 mA g?1to calculate the diffusion coefficients of the zinc ions as shown in Fig. 6a, b (details supplied in Supporting information) [82, 83]. TheDZnvalues of NVO during both the insertion and extraction processes are within the orders of 10?10to 10?12cm2s?1, superior to those of NH4V4O10, indicating the fast Zn?ion diffusion ability, which is agree?ment with the EIS results. From ex situ characterizations and GITT plots, the zinc ions insertion/extraction process in NVO could be deduced. In the discharging process, theDZnvalue goes through four stages: stabilization, gradual decline, fluctuating rise, and sharp decline. TheDZnvalues remain unchanged at the beginning of discharging process (Region I) due to large interlayer space for the insertion of hydrated zinc ions. When discharged to 0.9 V (Region II), theDZnvalues decrease gradually which is possibly attributed to the difficult insertion of large sized [Zn(H2O)6]2+. Then, theDZnvalue exhibits a tendency of fluctuating increasement (Region III) which is possibly attributed to the desolvation of [Zn(H2O)6]2+and easier intercalation process of small Zn2+. After discharged to 0.5 V (Region IV), theDZnvalue decreases dramatically because no channels are available for ions intercalation. When charged to 0.8 V (Region I), theDZnvalue decreases gradually owing to the deintercalation of Zn2+. After 0.8 V (Region II), the increase inDZnvalue is attributed to decomposition of Zn3V2O7(OH)2·2H2O and then producing more Zn2+.

Fig. 6 Analysis of ions insertion/extraction process. a GITT tests at a current density of 50 mA h g?1. b Corresponding ion diffusion coefficients of NVO and NH4V4O10

Supplementary InformationThe online version contains supplementary material available at https:// doi. org/ 10. 1007/ s40820? 021? 00641?3.

4 Conclusions

Pre?removing some ammonium ions from NH4V4O10not only expanded the interlayer spacing but also produced many active sites, which offered more diffusion pathways, facili?tated the zinc ions insertion/extraction and alleviate cycling decay caused by the irreversible dissolution of ammonium ions during cycles. The NVO electrode exhibits a high initial Coulombic efficiency of 97% compared to 85% of NH4V4O10electrode and delivers a high specific capacity of 457 mAh g?1at a current density of 100 mA g?1com?pared to 363 mAh g?1of NH4V4O10electrode as well as a long?term cycling stability (81% of initial capacity after 1000 cycles) compared to 40% of NH4V4O10electrode. Theex-situcharacterizations (XRD, SEM, TEM, and XPS) demonstrated reversible Zn3V2O7(OH)2·2H2O formation/decomposition in NVO during charge/discharge processes. This work provides a novel strategy of deionized method for designing high?performance cathode materials for ZIBs and other multivalent ion batteries.

AcknowledgementsThis work was supported by the National Science Foundation (CBET?1803256), National Natural Science Foundation of China (Grant No. 51772267), the National Key R&D Program of China (Grant No. 2016YFB0401501), and the Key R&D Program of Zhejiang Province (Grant No. 2020C01004). The author acknowledges the financial support from China Schol?arship Council (No. 201906320198) and 2019 Zhejiang Univer?sity Academic Award for Outstanding Doctoral Candidates. The authors are grateful to the Deanship of Scientific Research, King Saud University for funding through Vice Deanship of Scientific Research Chairs.

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