FeVO4⋅nH2[email protected] nanocomposite as high performance cathode materials for aqueous Zn-ion batteries

Journal of Alloys and Compounds 818 (2020) 153372

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FeVO4,nH2O@rGO nanocomposite as high performance cathode materials for aqueous Zn-ion batteries Binxu Lan a, 1, Chen Tang a, 1, Lineng Chen c, Wenwei Zhang a, Wen Tang a, Chunli Zuo a, Xudong Fu a, Shijie Dong b, **, Qinyou An c, Ping Luo a, * a

Hubei Provincial Key Laboratory of Green Materials for Light Industry, Collaborative Innovation Center of Green Light-Weight Materials and Processing, School of Materials and Chemical Engineering, Hubei University of Technology, Hubei, Wuhan, 430068, PR China Hubei University of Economics, Hubei, Wuhan, 430205, PR China c State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, Hubei, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 October 2019 Received in revised form 9 December 2019 Accepted 10 December 2019 Available online 11 December 2019

Aqueous zinc ion batteries (ZIBs) have recently attracted an increasing attention as an environmental friendliness, low cost and highly potential novel energy storage system. Although vanadium-based materials serve as a capable of arousing and holding the attention cathode materials for ZIBs, whereas low conductivity and confusing zinc ion storage mechanisms remain an obstacle. Herein, FeVO4,nH2O@rGO composite is employed to evaluate zinc ion storage capability as a cathode for the first time in 2 M Zn(TFSI)2 electrolyte. Benefiting from the large lattice spacing, dual electrochemical activity and fast electron transfer, the composite delivers excellent rate performance and long life cycle (the capacity retained ~100 mAh g
1 at 1.0 A g
1 after 1000 cycle). In addition, the electrochemical performance of graphene-modified FeVO4,nH2O is superior to other precursors for comparison. Furthermore, the intercalated/de-intercalated mechanism of zinc ion is distinct from the traditional conversion reaction, which is confirmed by in-situ X-ray diffraction and various ex-situ techniques. © 2019 Elsevier B.V. All rights reserved.

Keywords: FeVO4,nH2O@rGO Aqueous zinc ion batteries Cathode Energy storage mechanism

1. Introduction Nowadays, lithium-ion batteries (LIBs) play an important role in the market of portable electronics and electric vehicles because of their high energy density, long lifespan and high operating voltage [1,2]. However, the deficiency including high cost, flammable organic electrolytes, insufficient lithium resources and environmental impact motivate will hinder its further development and extensively applied [3]. Therefore, it is urgent to develop new energy storage devices with high safety, lower cost, high energy density and green sustainable. In this trend, aqueous rechargeable batteries have been considered as the next generation new energy storage device candidate benefiting from better safety, lower cost, easier processing, and higher ionic conductivity compared with the case of organic electrolytes [4,5]. In the study of aqueous

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Dong), [email protected] (P. Luo). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.153372 0925-8388/© 2019 Elsevier B.V. All rights reserved.

rechargeable batteries using naturally abundant metal ions (Naþ, Kþ, Zn2þ, Mg2þ, Ca2þ, Al3þ) as charge carriers [6e11], aqueous ZIBs have gradually captured researchers’ attention because of the high theoretical capacity of 820 mAh g
1, low redox potential (
0.76 V vs. SHE), excellent stability in water, and nontoxicity [5,12]. However, finding a appreciable host cathode material to facilitate the reversible charge and discharge of zinc ions is still a challenge for the development of aqueous ZIBs. Till date, the exploration of cathode materials for aqueous ZIBs have focused on vanadium-based materials [13e19], manganesebased oxides [20,21] and prussian blue analogues [22,23]. However, in the above cathode materials, the manganese-based oxides are abruptly attenuated due to structural instability during the cycle [24], and the limited capacity of prussian blue analogues restrict their extremely application [25,26]. In contrast, vanadiumbased materials exhibit excellent electrochemical zinc storage properties for the cathode of aqueous zinc ion batteries, which may be attributed to vanadium element often possess rich valence states, it can facilitate to achieve local electron-eutrality and relieve the polarization problem caused by multivalent ions [27]. Vanadium-based materials, including vanadium oxides (VO2, etc.),

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vanadates (Zn0.25V2O5$xH2O, etc.), vanadyl phosphates (Na3V2(PO4)2F3, etc.) and oxygen-free vanadium compounds (VS2, etc.), has been extensively studied for ZIB cathodes [12,28e31]. For example, Liang et al. reported a series of sodium vanadate and provided a new perspective of Zn2þ storage mechanism in aqueous ZIB systems [32]. Mai et al. prepared a water-lubricated intercalation cathode material (V2O5$nH2O), which manifested a capacity of 372 mAh g
1 [10]. It is worth noting that most of the reported vanadium-based cathode materials containing crystal water can provide higher specific capacity and better cycle performance compared to the cathode without crystals water [16,33]. The reason is that water molecules can work as a charge shield for the zinc ions during the insertion/extraction process, reducing their effective charges and thus enhance the capacity and rate performance [10,13]. Although the synthesis of vanadium-based cathode material containing crystal water maybe a viable strategy for aqueous ZIBs, its inherent low conductivity and intricate zinc storage mechanism are still a significant challenge. Graphene possesses huge attractions for electrochemical community due to its high specific surface area and fast electron transport capabilities [34,35]. Additionally, benefiting from the mechanical strength of graphene, it can act as a buffer to accommodate volume changes of the electrode material during ion insertion/extraction, which is advantageous for stabilizing the original crystal structure [36,37]. Besides, FeVO4 has been considered as an excellent energy storage material for Li-, Mg- and K-ion batteries, mainly due to the both electrochemical activity (Fe and V) during charge and discharge [38e40]. Therefore, the composite materials, which consisted of graphene and FeVO4 containing crystal water, maybe a great potential cathode material for aqueous ZIBs. In this work, we report a kind of ZIBs consisting of FeVO4,nH2O@rGO (FVO@rGO) nanocomposite and metallic zinc as the cathode and anode, respectively. The distinguishing features of fast electron transport result in FVO@rGO cathode materials with superior zinc ion storage performance compared to other graphenefree modified precursors. Furthermore, neither metal V nor Fe is produced during charge storage by in-situ X-ray diffraction (XRD) and various ex-situ techniques characterization, indicating that the zinc ion de/intercalation mechanism of FVO@rGO isn’t belong to traditional conversion reactions. The excellent electrochemical performance proved that FVO@RGO cathode is a potential candidate cathode material for aqueous zinc ion battery, and it also provides a reference for understanding the zinc storage mechanism of vanadium-based materials. 2. Experimental section 2.1. Material synthesis All chemicals were of analytical grade and were used as received without further purification. The FVO@rGO nanocomposite was synthesized by replacing the deionized water with GO solution dispersion according to the hydrothermal synthesis process reported in the prior literature [36]. In a typical procedure, 2 mmol FeCl3,6H2O was placed in 10 ml of GO solution (2 mg ml
1, Hengqiu Graphene Technology (Suzhou) Co, Ltd.) magnetically stirred until completely dissolved, while 2 mmol of NH4VO3 was added to 10 ml of deionized water magnetic stirring in water bath at 70  C until form a transparent yellow solution. Next, the NH4VO3 solution was added dropwise to the FeCl3,6H2O suspension and magnetic stirring was continued for 30 min. After that, it was transferred into 50 ml autoclave heated at 180  C for 3 h. The precursor was collected and washed three times with deionized water and absolute ethanol. Finally, it was dried at 70  C overnight. For

comparison, pure FeVO4,nH2O (FVO) nanorods were synthesized via similar method without graphene. The synthesis of FeVO4 (FVO500) was performed by placing the precursor FeVO4,nH2O in a tube furnace sintering 500  C, and the heating rate was 5  C min
1, for 2 h.

2.2. Material characterization X-ray diffraction (XRD) measurements were performed using a Bruker D8 Discover X-ray diffractometer with Cu Ka radiation (l ¼ 1.5418 Å). Raman spectra were obtained using a Renishaw INVIA micro-Raman spectroscopy system. X-ray photo-electron spectrometry (XPS) of the product was collected on a VG K-Alpha Probe spectrometer (Thermofisher Scientific) with Al Ka radiation as the excitation source. The TG analysis was conducted on a NETZSCH-STA449c/3/G thermoanalyzer under an air atmosphere from 30 to 800  C with a heating rate of 10  C min
1. The field emission scanning electron microscopy (FESEM) images and energy dispersive spectrometry (EDS) elemental mappings were collected with a JEOL-7100F microscope. Transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) element mapping, and high-resolution transmission electron microscopy (HRTEM) images were recorded using a Tecnai G2 F20 S-TWIN TMP transmission electron microscope. In-situ XRD mechanism characterization was performed using a Bruker D8 Advance X-ray diffractometer.

2.3. Electrochemical measurement Zn(CF3SO3)2 (zinc trifluoromethanesulfonate) and Zn(TFSI)2 (zinc(II) bis(trifluoromethanesulfonyl)imide) electrolyte salts were purchased from Shanghai Macklin Biochemical Co., Ltd and Huizhou DADO New Material Technology Co., Ltd. The FVO@rGO electrode was prepared by mixing FVO@rGO, acetylene black and polytetrafluoroethylene (PTFE) in a weight ratio of 7:2:1, then grinding the slurry, tableting and punching into a shape and finally placing it in an oven at 70  C overnight. Zinc foil and glass fiber membranes (GF/D What-man) were used as the anode and the separator, respectively, and two types of electrolytes (2 M Zn(TFSI)2, and 2 M Zn(CF3SO3)2). The electrochemical performances of the prepared samples were measured on CR2016 coin cells. Galvanostatic intermittent titration technique (GITT)/galvanostatic chargeedischarge tests were performed using a LAND battery testing system (CT2001A, LANHE) in the potential range of 0.2e1.4 V at room temperature. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted using an electrochemical workstation (CHI600E, shanghai and VMP3 multichannel electrochemical workstation (Bio-Logic France).

2.4. Electrolyte conductivity test Two stainless steel discs (1.6 cm in diameter, 0.2 cm in thickness) and glass fiber (GF/D What-man) were used as the electrode and separator, respectively. Then assembled the CR2016 buttontype symmetrical battery separately with two different electrolytes (2 M Zn(TFSI)2, 2 M Zn(CF3SO3)2). The resistance test of the button battery was performed using the CHI660 electrochemical workstation. Under the other same conditions, the resistance value is inversely proportional to the conductivity value. In other words, the resistance of the button battery can directly reflect the conductivity of different electrolytes.

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3. Results and discussion XRD patterns of the FVO@rGO, FVO and FVO-500 are presented in Fig. S1a, in which all diffraction peaks of the precursors FVO and FVO@rGO correspond to FeVO4$1.1H2O. After annealing in air, all characteristic peaks were indexed to FeVO4 (PDF #38-1372), indicating that the precursor FVO was completely converted to the FeVO4 phase. In addition, the EDS spectral results of FVO@rGO composite show that the atomic ratio of Fe, V, and O is close to 1:1:4 (Fig. S2). As can be seen from TG-DSC curves (Figs. S1bec), the FVO@rGO show a weight loss of 2.26% up to 100  C, which is mainly attributed to the physically absorbed water. Besides, a weight loss of 14.86% occurred from 150 to 500  C, which mainly included approximately 5.94% structural water loss and about 8.92% burned rGO. SEM and TEM images (Fig. 1aeb, d) show that FVO@rGO are composed of flake graphene and nanorods, wherein the nanorods with a typical size of ~2 mm in length and ~350 nm in width. Simultaneously, the precursor FVO nanorods are observed in Fig. S3a, which are similar in shape and size to FVO@rGO. However, the FVO-500 nanorods gradually distorted become porous structure (Fig. S3b), which was attributed to the evaporation of structured water at a high temperature of 500  C [41]. The considerable lattice fringe of 0.89 nm and a thin amorphous layer on the surface of nanorods can be clearly observed in the HRTEM image (Fig. 1c), and the lattice fringe is in agreement with the (100) plane of FeVO4$1.1H2O [42]. Furthermore, TEM-EDS (Fig. 1e) and SEM-EDS mappings (Figs. S3ced) can show that the elements of Fe, V, O are distributed homogeneously in the as-prepared sample, and further illustrate that the graphene film is well coated. As expected, carbon Raman signal is discernible for FVO@rGO, the Raman spectrum (Fig. 1f and Fig. S4) possesses two characteristic bands at approximately 1334 cm
1 and 1605 cm
1, which correspond to Dband and G-band, respectively. Considering that the electrolyte contains the bulky CF3SO
3/ TFSI
salt anion (versus SO2
with double charge), which can 4

3

decrease the number of water molecules around the Zn2þ cation and reduces the solvation effect, leading to a better electrochemical properties [43,44]. Fig. 2a displays the cyclic voltammetry (CV) profiles of FVO@rGO at a scan rate of 0.1 mV s
1 with 2 M Zn(TFSI)2 electrolyte. With the increase of the cycle, the oxidation peak migrates from 0.57 V to 0.60 V and becomes smaller, while the reduction peak at 0.70 V gradually disappears, and another one moves from 1.05 V to 1.14 V. The apparent irreversibility may be attributed to the first insertion of zinc ions, which destroy the disordered change in the stacking of a crystal plane in the host material. However, the subsequent reversible cycles indicate the excellent zinc storage capacity of FVO@rGO electrode. It is worth noting that the FVO@rGO and FVO electrodes also exhibit similar CV behavior in 2 M Zn(CF3SO3)2 and 2 M Zn(TFSI)2, respectively (Figs. S5aeb). Therefore, the rGO sheet does not significantly affect the mechanism of the redox reaction in the FVO@rGO cathode. As for FVO-500, its CV curves are different from the former two, which is attributed to the transformation of the crystal system from monoclinic to a triclinic single phase after calcination (Fig. S5c). Simultaneously, the corresponding cycle performance (Fig. 2b) and rate data (Fig. 2d) reveal that FVO@rGO possesses the higher specific capacity and batter rate performance compared to FVO and FVO-500. Unfortunately, the graphene-modified method still fails to improve the capacity retention rate of FVO@rGO (Table S1). The reason for rapid capacity decay is attributed to strong electrostatic interaction between FVO@rGO and divalent Zn2þ, which results in slow reaction kinetics and inhibits Zn2þ reversible storage. In addition, irreversible decay may be caused by two possible parasitic reactions involving the reduction of dissolved O2 and the salt anion [45]. The Ragone plots in Fig. S6 show that a high energy density of 142.1 Wh kg
1 and a high power density of 72.8 W kg
1 (based on the mass of cathode material) were obtained for FVO@rGO electrode, which were superior to the previously reported other aqueous storage batteries [39,46e50]. However, it has a slight disadvantage compared to other vanadium-based materials for zinc ion batteries [29,30,51]. The irreversible polarization phenomenon

Fig. 1. Characterization of FVO@rGO cathodes: (a) SEM, (b, d) TEM and (c) HRTEM images. (e) TEM-EDS elemental mappings. (f) Raman spectrum.

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Fig. 2. Electrochemical performance of FVO@rGO/FVO/FVO-500 electrode at voltage window of 0.2e1.4 V in 2 M Zn(TFSI)2 electrolyte (a) CV curves of FVO@rGO electrode at 0.1 mV s
1, (b) Cycling performance at 0.2 A g
1, (c) Galvanostatic charge-discharge curve of FVO@rGO, (d) Rate performance, (e) Long term cycling performance at 1 A g
1.

observed in the first few cycles (Fig. 2c) was caused by the incomplete release of the intercalated Zn2þ from the electrode lattice [18]. Furthermore, a long-lasting and considerable discharge capacity after 1000 cycle can be obtained at 1.0 A g
1, further demonstrating the remarkable cycling performance of FVO@rGO electrode in 2 M Zn(TFSI)2 electrolyte (Fig. 2e). In comparison, the pure FVO and FVO-500 cathode (Fig. S5d) performed worse under the same conditions. The slow decay in the initial cycle of the FVO@rGO electrode and high stability in the later stage indicate that graphene can act as a buffer to accommodate the volume change of the electrode material during ion insertion/extraction. This method of successfully modified the cathode using rGO has been similarly reported in aqueous ZIBs [52e54]. We also noticed that the ZIBs system with Zn(TFSI)2 electrolyte exhibits more stable electrochemical performance compared to Zn(CF3SO3)2 electrolyte, which was attributed to the higher ionic conductivity (Fig. S7) of bulky TFSI
anions (versus CF3SO
3 ). In order to gain insight into the electrochemical behavior of different electrodes, the EIS measurement was performed after the first cycle (Fig. S8). In terms of equivalent circuits, Rs represents ohmic resistance, while Rct stands charge transfer resistance, the sloped line (ZW) is attributed to the diffusion of Zn2þ in the electrode [18]. It is apparent that the Rct values of 49.6, 79.8 and 168.4 U correspond to the FVO@rGO, FVO and FVO-500 electrodes, respectively. The result further demonstrates that rGO can enhance the kinetics of charge transfer [55] and act as a bridge between Zn2þ and electron transport during cycling. To clarify the zinc ion storage mechanism of the FVO@rGO//Zn batteries system, a series of characterization measurements including XRD, TEM and SAED were performed under different charge and discharge conditions. There are two main peaks, located at 27.8 and 30.5 , with minor changes observed in different states

(Fig. S9). When discharged to 0.2 V (from state “a” to “c”), the two diffraction peaks slightly shifted to the left and the intensity of peak gradually become weaker. In contrast, the diffraction peak shifted to the right from state “c” to “f”, but the intensity of the peak becomes weaker than the original state, which is attributed to the fact that the inserted zinc ions are located at the “dead Zn2þ sites,” causing irreversible damage to the host material [18,51]. In addition, it can be observed that the subsequent cycles (from state “g” to “r”) exhibit a good reversibility, which corresponding to the CV test results. The HRTEM and SAED patterns (Fig. 3aec) show a slight difference in lattice spacing and electron diffraction for the different state electrodes, respectively. In the insertion state, the lattice fringes increased by 7.8% and the diffraction pattern were slightly confused (Fig. 3a and Fig. S10), while it can recover by 6.6% and become relatively ordered when the zinc ions are released. Meanwhile, TEM-EDS element mapping indicates that the Zn2þ ions already intercalated into the FVO@rGO electrode (Fig. 3dee). Not unexpected, some of the zinc ions cannot be removed from the host material in the extraction state (Fig. S11). Not only does this result coincide with the analysis of ex-situ XRD, but it further explains the irreversible phenomenon of the initial cycle. In order to further insight the intercalation reaction mechanism of the FVO@rGO electrode in different depths of charge and discharge process, ex-situ XPS and in-situ XRD were performed. Fig. 4a-c and Fig. S12 display the XPS spectra of Zn 2p, V 2p and Fe 2p in different states (original, fully discharged and fully charged stage). Among them, a small amount of the V5þ signal (517.4 eV) was shifted to the left (518.2 eV) and most of V5þ were reduced to V4þ(517.4 eV) and V3þ(516.3 eV) during the discharge process, which may be attributed to the insertion of zinc ions and the concomitant bonding rearrangements of (V4þ/V5þ) [43]. When charged to 1.4 V, few of V4þ signal can still be observed at full

B. Lan et al. / Journal of Alloys and Compounds 818 (2020) 153372

5

Fig. 3. The characterization of zinc storage mechanism of FVO@rGO in 2 M Zn(TFSI)2 electrolyte: Ex-situ HRTEM image and SAED patterns of (a) pristine state, (b) fully discharged state and (c) fully charged state; (d-e) TEM-EDS element mapping images of the insertion state electrode.

charge state, which should be caused by the zinc ions at the “dead zinc” site cannot be removed from the electrode [14,18,51]. The corresponding Zn 2p signal spectra (Fig. 4a) further illustrates the above situation. Furthermore, from the original state to the discharged state, Fe3þ (711.4 eV) is reduced to Fe2þ (710.1 eV). After charging, the Fe 2p peak signal shifted back to the site corresponding to Fe3þ. Fig. 4d and Fig. S13 display the in-situ XRD patterns, which reveal two more intuitive change peaks at 27.8 and 30.5 , respectively, during the insertion/extraction of Zn2þ processes. Can be clearly seen from Table S2, both peaks shifted to the left and the peak intensity decreases, indicating that the intercalation of zinc ions leads to a larger interplanar spacing according to the Bragg equation; In contrast, the diffraction peak returns to identical position and the intensity was lower than original, which was provided powerful evidence to partial irreversibility. Meanwhile, the reversible peaks shift in the subsequent cycle reveals the excellent Zn2þ de/intercalation ability of the FVO@rGO electrode. Comprehensive analysis of ex-situ XPS and in-situ XRD characterization, we noticed that neither metallic V nor Fe is produced, which indicating that the FeVO4 cathode is distinct from the traditional conversion reaction during the de/intercalation of zinc ions [40,56]. The electrochemical reaction formula of Zn//FVO@rGO cell can be summarized as follows: Anode: x Zn2þ þ 2x e
4 x Zn

(1)

Cathode: FeVO4,nH2O@rGO þ x Zn2þ þ 2x e
4 ZnxFeVO4,nH2O@rGO

(2)

4. Conclusions In summary, a promising FVO@rGO nanocomposite was constructed by a facile one-step hydrothermal method and served as a cathode material of aqueous ZIBs for the first time. Owing to the large lattice spacing, abundant variable valence elements (Fe and V) and high electrical conductivity, the FVO@rGO cathode material delivered outstanding zinc ion storage capability during charge and discharge process. In addition, benefit from the higher ionic conductivity, the electrolyte containing the TFSI
salt anion displayed more intuitive stability in long-term cycling than CF3SO
3 anion. Furthermore, a series of in situ and ex-situ characterizations have also demonstrated that the mechanism of zinc insertion/extraction reactions was fundamentally different from conventional conversion reactions. This work indicated that graphene-modified FeVO4,nH2O was a potential cathode material for aqueous ZIBs. We also believed that vanadium-based materials with various electrochemical activity elements will cause great interest among researchers of energy storage electrode materials.

Author contributions section Author Contributions: conceptualization: Binxu Lan, Ping Luo, Qinyou An Author Contributions: methodology: Binxu Lan, Lineng Chen, Ping Luo Author Contributions: software: Wenwei Zhang, Xudong Fu Author Contributions: validation: Wen Tang Author Contributions: formal analysis: Chunli Zuo Author Contributions: data curation: Binxu Lan, Chen Tang

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B. Lan et al. / Journal of Alloys and Compounds 818 (2020) 153372

Fig. 4. Ex-situ XPS scheme of (a) Zn 2p, (b) V 2p and (c) Fe 2p; (d) 2D in-situ XRD patterns of FVO @rGO electrode for initial three cycles.

Author Contributions: writing (original draft preparation): Binxu Lan Author Contributions: writing (review and editing): Chen Tang, Ping Luo, Qinyou An Author Contributions: visualization: Xudong Fu, Lineng Chen Author Contributions: supervision: Shijie Dong, Ping Luo Author Contributions: funding acquisition: Shijie Dong, Ping Luo

Declaration of competing interest There are no conflicts to declare.

Acknowledgments This work was supported by the National Natural Science Foundation of China (51771071, 51602239), the International Science & Technology Cooperation Program of China (Grant No.2016YFE0124300), the open fund of Collaborative Innovation Center of Green Light-weight Materials and Processing and Hubei Provincial Key Laboratory of Green Materials for Light Industry (Grant No. 201710A05 and 201611A07) and the guidance project of scientific research program from Hubei Provincial Department of Education (Grant No.B2019046).

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