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Metallic silver doped vanadium pentoxide cathode for aqueous rechargeable zinc ion batteries
Journal of Alloys and Compounds 787 (2019) 9e16
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Metallic silver doped vanadium pentoxide cathode for aqueous rechargeable zinc ion batteries Binxu Lan a, b, Zhuo Peng c, Lineng Chen c, Chen Tang a, b, Shijie Dong a, b, **, Chen Chen a, b, Min Zhou a, b, Cheng Chen a, b, Qinyou An c, ***, Ping Luo a, b, * a b c
Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, PR China Collaborative Innovation Center of Green Light-weight Materials and Processing, Hubei University of Technology Wuhan 430068, PR China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2018 Received in revised form 31 January 2019 Accepted 6 February 2019 Available online 8 February 2019
Recently, rechargeable aqueous zinc-ion batteries have received considerable attention in relation to large-scale energy storage, mainly because of their advantages of environmental friendliness, nontoxicity, and favorable safety. However, little researches have been conducted on the cathode materials used in zinc-ion batteries. In this work, an aqueous zinc-ion battery is constituted based on metallic silver doped vanadium pentoxide cathode, Zn(CF3SO3)2 aqueous electrolyte, and zinc anode, which exhibits excellent electrochemical performance. At a current density of 0.1 A g1, the specific capacity of about 215 mAh g1 was retained after 50 cycles. It also delivers long-cycle stability at 3 A g1, which remains at 80 mAh g1 after 700 cycles. Furthermore, the reaction mechanism is confirmed by X-ray diffraction, ex-situ Raman, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy analysis. This outstanding performance is attributed to the high conductivity of the cathode material and the vacancy exchange mechanism of Zn2þ/Agþ. This research shows metallic silver doped vanadium pentoxide cathode which could be considered a promising cathode material for aqueous zinc-ion batteries. © 2019 Published by Elsevier B.V.
Keywords: Aqueous zinc-ion battery Silver vanadium oxide Cathode material
1. Introduction With the deterioration of the global climate and the rapid consumption of fossil fuels, it is imperative that new types of energy conversion and storage devices are developed [1]. Secondary Batteries are one of the most important energy storage systems for renewable energy, including wind energy, solar energy, tidal energy, biomass energy, and geothermal energy [2]. As one of the most important secondary batteries, Lithium-ion batteries have been incorporated widely in electric vehicles, large-scale smart grid storage systems, mobile phones, and portable computers in recent years [3e5]. However, their use is associated with high cost, safety
* Corresponding author. Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, PR China. ** Corresponding author. Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, PR China. *** Corresponding author. E-mail addresses: [email protected] (S. Dong), [email protected] (Q. An), [email protected] (P. Luo). https://doi.org/10.1016/j.jallcom.2019.02.078 0925-8388/© 2019 Published by Elsevier B.V.
concerns, scarcity of lithium resources, and environmental pollution [3,4]. In view of the above shortcomings, many novel alternative secondary batteries have been developed recently, lithiumeair batteries [6], lithiumesulfur batteries [7],magnesiumion batteries [8], aqueous rechargeable lithium batteries [9,10], aqueous zinc-ion batteries (ZIBs) [11] and others [12]. Among these new secondary batteries, aqueous ZIBs have attracted considerable attention because of their favorable safety aspects, and environmental friendliness [13]. In addition, not only the ionic conductivity of the aqueous electrolyte is twoethree orders of magnitude higher than that of an organic electrolyte [14], but also the negative electrode use the metal-rich zinc which can provide a high capacity of 820 mAh g1 [15,16]. Although the advantages of ZIBs are outstanding, the selection of the cathode material is limited because the requirement of the electrode material for the insertion and extraction of the divalent zinc ion is different from that of the monovalent alkaline metal ion [17]. Previously, the following types of cathode material have been reported: Manganese-based oxide [18e21], NASICON [13,22], transition metal oxide [23], vanadium-based compounds [24e32],
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Prussian blue analogues [33e36], and others [12,37]. Among the cathode materials mentioned above, vanadium-based compounds have demonstrated high specific capacity, which could be attributed to their rich chemical valence. However, their development is limited by the poor conductivity. In view of this, highly conductive metallic silver doped vanadium pentoxide (SVO) is used as the cathode material to improve the conductivity compared to pure V2O5. Furthermore, the zinc ion intercalation/de-intercalation mechanism in the silver doped vanadium pentoxide is confirmed by X-ray diffraction (XRD), ex-situ Raman, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analysis. This excellent performance is attributed to two aspects: on the one hand, the conductivity of the positive electrode material doped with metallic silver is improved to facilitate the transmission of electrons and zinc ions. On the other hand, the vacancy exchange of Agþ and Zn2þ in the crystal structure of the cathode material forms a good stable structure. 2. Results and discussion The crystallographic structure of the prepared SVO samples was characterized by XRD measurement. It can be seen clearly from Fig. 1a that all the diffraction peaks could be indexed perfectly as monoclinic Ag0.33V2O5 (a ¼ 15.386 Å, b ¼ 3.615 Å, c ¼ 10.069 Å, Space group: C 2/m), consistent with the standard card (JCPDS card No. 81-1740). The XRD model of pure V2O5 is provided in Fig. S2 (Supplementary Information), which also fits perfectly with the standard card (JCPDS card No. 76-1803). In addition, the monoclinic structure of the SVO, drawn using VESTA software, is shown in Fig. 1b. Its crystallographic structure is similar to that reported previously for Na0.33V2O5 [38,39], which could provide feasible access for the insertion/extraction of zinc-ion guests. The Fourier transform infra-red (FTIR) spectrum of the SVO, illustrated in Fig. 1c, shows two peaks at 3453 and 1631 cm1, which correspond to the stretching of OeH and the bending vibrations of OeH in free water, respectively. The bands at 1007, 973, 939, 920, and 547 cm1
can be attributed to the asymmetric and symmetric stretching vibrations of VO3 groups. The bands at 875 and 712 cm1 are associated with the VO3 anti-symmetric stretching vibrations. Furthermore, the observable peak at 1427 cm1 could reflect the nitrate residue from the reactants [40e44]. The XPS survey spectrum, shown in Fig. 1d, demonstrates that the as-prepared SVO sample contains only vanadium, silver, and oxygen. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s to 284.8 eV. The two strong peaks at the Ag region of 366.9 and 372.9 eV (Fig. 1e) can be assigned to Ag 3d5/2 and Ag 3d3/2, respectively, whereas the three peaks at 516.6, 524.5, and 529.4 eV correspond to V 2p3/2, V 2p1/2, and O 1s, respectively (Fig. 1f) [38,45]. The SEM and TEM images were used to investigate the morphology and microstructure of the as-prepared SVO samples. Fig. 2a shows the typical SEM image of nanorods with length size of about 100e300 nm. Similarly, the SEM image Fig. S2 (Supplementary Information) of pure V2O5 can also exhibit nanorods with a length of approximately 300e500 nm. The TEM image of individual nanorods, shown in Fig. S3 (Supplementary Information), further confirms the nanorods of SVO. In Fig. 2d and Fig. S3 (Supplementary Information), the EDX mappings of the synthesized SVO also demonstrate that the elements of Ag, V, and O were distributed homogeneously within the sample. The High-resolution transmission electron microscopy (HRTEM) images (Fig. 2c) of individual SVO nanorods show a clear lattice fringe with interplanar spacing is about 2.17 Å, which corresponds to the (601) crystal plane, indicating high crystallinity of the SVO sample. The selected area electron diffraction (SAED) pattern (Fig. 2d) also corresponds to the d-spacing values of the (601) crystal plane of the SVO. The SVO/Zn ZIBs were built using SVO as the positive electrode, aqueous Zn(CF3SO3)2 solution as electrolyte and metallic zinc as the negative electrode. The electrochemical performance was evaluated by cyclic voltammetry (CV), galvanostatic chargeedischarge, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) measurements. Fig. 3a shows the initial five cycles versus Zn/Zn2þ of the CV pattern of the
Fig. 1. Characterization of as-synthesized SVO nanorods: (a) XRD pattern, (b) SVO 3D tunnel structure, (c) FTIR spectrum, (d) XPS spectra of the SVO samples, (e) Ag 3d spectrum, and (f) V 2p spectrum.
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Fig. 2. (a) SEM image, (b) SEM-EDx image, (c) TEM image, (d) SAED pattern.
SVO electrode within a voltage window of 0.2e1.6 V at a scan rate of 0.1 mV s1. From the CV curve (Fig. 3a), it can be seen clearly that the first cycle is somewhat different from the other cycles. In the forward scan, a sharp peak can be seen at 0.604 V, followed by three small shoulder peaks at 0.40, 0.915, and 1.082 V, indicating the electrochemical insertion of Zn2þ into the 3D channel-structured. In the reverse scan, three oxidation peaks can be seen at 0.62, 1.017, and 1.002 V, demonstrating that Zn2þ was de-intercalated from the host structure. The curve of this initial cycle is different from the curve of the subsequent cycle, which may be attributed to the irreversible phase transition of the material caused by zinc ion intercalation. The clear redox peak on the CV curve of the subsequent cycle is almost at the same point on the abscissa, indicating that the SVO electrode has good Zn2þ insertion/extraction ability in the late cycle. In addition, the SVO electrochemical performance is better than pure V2O5 at a current density of 0.1 A g1 (Figs. S4 and SI); however, its initial discharge reaches approximately 418 mAh g1 higher than the initial cycle charge capacity (335 mAh g1). The considerable capacity for the first discharge may not only be the intercalation of zinc ions, but the silver ions are reduced to metallic silver. The reason for the smaller charging capacity may be the inserted Zn ions, located at “dead Zn2þ sites,” cannot be extracted from the SVO lattice during the subsequent charge process [26]. At a current density of 0.5 A g1, the SVO cycle stability (Fig. 3b) is superior to pure V2O5 (Figs. S5 and SI), which may be the role of highly conductive metallic silver in subsequent cycles. Rate performance of SVO and pure V2O5 electrode are measured as shown in Fig. 3c and Fig. S5 (Supplementary Information). The average discharge capacities of SVO at the current densities of 0.2, 0.5, 1.0,
and 2.0 A g1 are 200, 150, 118, and 96 mAh g1, respectively. However, the specific capacity of pure V2O5 decreases sharply with increasing current density (Figs. S5 and SI). Accordingly, the energy and power densities of the Zn-SVO battery are calculated and shown in the Ragone plot (Figs. S6 and SI). An energy density of 118 Wh kg1 at a power density of 0.136 kW kg1 and a power density of 1.3 kW kg1 at an energy density of 58 Wh kg1 can be achieved, which is superior to the previously reported materials [18,33,36]. We also noticed that the SVO cathode material can exhibit long-term cycle stability, as shown in Fig. 3e, even at a current density of 3 A g1, the capacity is still stable after 700 cycles. To investigate the zinc ion diffusion rate of the prepared SVO cathode material, GITT measurements were performed on the SVO/ Zn cell, which was discharged/charged at a constant current flux (0.1 A g1) at 1 h intervals. Surprisingly, the prepared SVO delivers a specific capacity of 432.3 mAh g1 in the GITT test (Fig. 4a and b). The remarkable specific capacity might be attributable to the 3D tunnel framework of the SVO cathode material, which facilitates the insertion/extraction of metallic zinc ions. The Zn2þ ion diffusion coefficient can be obtained via following equation [29]:
D¼
4
mB VM pt MB S
DEs DEt
(1)
Where t is the pulse duration, MB is the molar mass of SVO, VM is the molar volume, mB and S are the active mass and electrode/ electrolyte contact area. DES and DEt can be obtained from the GITT curves (Fig. 4c). The zinc ion diffusion rate of SVO (about 1010 109 cm2 s1, Fig. 4d) is higher than that of pure V2O5 (about 1012 -
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Fig. 3. Electrochemical performance of SVO in the potential range of 0.2e1.6 V, running in 2 M Zn (CF3SO3)2 electrolyte: (a) CV curves of SVO at a scan rate of 0.1 mV s1, (b) Cyclic performance of SVO nanorods at 0.5 A g1, and (c) Rate performance, (d) Charge and discharge curves at the current density from 0.2 to 2.0 A g1, (e) Cycling performance of SVO at 3.0 A g1.
1011 cm2 s1, Figs. S5 and SI). In Table S1 (Supplementary Information), compared the zinc ion diffusion rate of the reported cathode. SVO has a higher zinc ion diffusion coefficient may be attributed to the high conductivity of metallic silver in the SVO cathode material during charge and discharge. To further understand the electrochemical performance of SVO and pure V2O5, electrochemical impedance spectroscopy (EIS) measurements were performed during different charge and discharge processes, as illustrated in Fig. S7 (Supplementary Information). In a typical equivalent circuit (inset of Fig. S7), Rs stands for all equivalent series resistance, while Rct represents the charge transfer resistance between the electrode/electrolyte interface and the contacts with the electrode material and a sloped line (ZW) associated with the transfer of Zn2þ. The low equivalent series
resistance further confirms that the silver-doped cathode material has higher conductivity than pure V2O5. A high Rct is a feature of the freshly assembled cell. However, Rct drops significantly after the 20th cycle, not only because the electrolyte penetrates into the SVO channel-structure, but also the metal silver plays a role in the charge and discharge process. In order to explore the mechanism of zinc ion intercalation/ extraction of host materials, ex-situ XRD, ex-situ Raman, ex-situ XPS, ex-situ SEM and ex-situ TEM were applied under different charge and discharge conditions. Fig. 5a shows the ex-situ XRD pattern with a 2q of 15e50 , a major peak 2q at 29.20 is repeatedly moved during constant current discharge and charging, which corresponds to a change in the (401) crystal plane. In addition, a new diffraction peak located at 27.33 corresponds to the new
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Fig. 4. (aec) GITT curve of the as-prepared SVO at a current density of 100 mA g1 (d) Diffusion coefficients of Zinc ion.
Fig. 5. (a) The ex-situ XRD and (b) ex-suit Raman patterns of SVO electrode at different charge/discharge states.
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phase Zn2 (V3O8)2 (PDF#24-1482) can also be observed when discharging from 1.0 to 0.2 V. Conversely, this diffraction peak slowly disappears when charging from 0.4 to 1.6 V, which indicates that zinc ions insertion/extraction from the host material. Fig. 5b shows the ex-situ Raman spectra of the SVO electrode in different states. Typical Raman bands around 252, 428.2, 684.3, 853.2, 900.4, and 988.9 cm1 were observed. Among them, the most intense peak at 252 cm1 is assigned to be the channel-structured silver vanadate reported in the literature [46,47,50e52]. The bands observed at 684.3 cm1 can be attributed to the VOV asymmetric vibration [48,49]. In addition, the band at 853.2 cm1 can be associated with the stretching vibrations of VO3 groups in the (V2O7)4ion [50e52]. The Raman frequencies at 900.4 cm1 correspond to the stretching vibration of VeO bonds and terminal V-O-Ag or V-O-V vibrations [46e52]. However, the Raman band at 428.2 and 988.9 cm1 may be correlated with the signal from binder and conductive agent. During the discharge from 1.0 to 0.2 V, a distinct Raman band at 252.2 cm1 became weaker and wider, while the Raman band at 900.4 cm1 become stronger and narrower, which may be inserted Zn2þ accompanied by silver oxidation from Agþ to Ag0. On the contrary, when charging from 0.4 to 1.6 V, it is obvious that the Raman band at 252.2 cm1 become stronger and narrower, and gradually returns to the original state at 900.4 cm1, which is caused by the extraction of Zn2þ from the lattice. The ex-situ XPS and ex-situ TEM analyses of the SVO electrodes in the discharged and charged states were further applied to verify the results of ex-situ XRD and ex-situ Raman. Fig. 6a is the XPS of Zn 2p electrodes in three different states (original, discharged, charged). It is very obvious that the Zn 2p intensity of charged state electrode is much weaker than the discharge one, which indicates that the Zn ions do have intercalated into the SVO cathode [29]. The corresponding energy dispersive spectroscopy (Figs. S8-9, SI) shows that Zn ions are evenly distributed throughout the SVO cathode material. In the fully charged state, the weaker Zn 2p can be
observed; indicating that the Zn2þ intercalation is irreversible in the electrode, which further confirms the analysis results of the above ex-situ XRD and ex-situ Raman, and similar phenomenon have been reported [53]. It can be seen from the Ag 3d XPS spectrum (Fig. 6b) that in the fully discharged state, the metallic silver is reduced, and once charged, the metallic silver is oxidized to silver ions, indicating that the replacement of metallic silver by the insertion of zinc ions into the host material, while the zinc ions occupy the vacancies of the silver atoms to form a new phase. In addition, in the curve fitting V 2p (Fig. 6c), from the original state to the discharge state, V5þ (517.1 eV) peak becomes weaker, V4þ (516.4 eV) peak becomes stronger indicating that V5þ is reduced to V4þ. After recharging, V4þ is oxidized to V5þ, accompanied by the extraction of Zinc ion. Furthermore, TEM and TEM-EDX element mapping of different states are collected (Fig. 6def). The (002) crystal plane nanorods of the original SVO exhibit lattice fringes with d-spacing of 0.473 nm, which is consistent with the XRD data (Figs. S10 and SI). When discharge to 0.2 V, A set of interplanar distances are clearly observed (Fig. 6d), which indicates that the intercalation of Zn ions will occupy a part of the Ag vacancies of SVO to form a new phase Zn2(V3O8)2, while Agþ is reduced to Ag0. In addition, the fully discharged EDX element mapping can also see the metal silver element precipitated, and the distribution of some Zn elements can be seen in the vicinity of the silver, which may be the formation of the new phase Zn2(V3O8)2. On the contrary, during the charging process, zinc ions are de-intercalated from the cathode material, and the corresponding vacancies are reoccupied by the oxidized Agþ, resulting in a transition to the original state. The change in lattice fringe spacing exactly matches this result. These findings match with those deduced from the ex-situ XRD, Raman and XPS studies. In short, from the above analysis, the reaction mechanism of the SVO/Zn battery changes as shown in Fig. 7. The outstanding electrochemical properties of SVO/Zn system are mainly attributed to
Fig. 6. (a) Zn 2p, (b) Ag 3d, (c) V 2p XPS spectra, (d) ex-situ HRTEM images at fully discharged state and (e) EDX element mapping (f) ex-situ HRTEM images at fully charge state.
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Fig. 7. Schematic diagram of the vacancy exchange mechanism of SVO/Zn batteries.
the special frame structure of the material and the electrical conductivity. 3. Conclusions In summary, a one-step hydrothermal method was employed to synthesize silver-doped vanadium pentoxide cathode material, metallic zinc was used as a negative electrode material, and 2 M Zn(CF3SO3)2 was used as an electrolyte to produce an aqueous ZIB that demonstrated excellent electrochemical performance compared to pure V2O5. Furthermore, the zinc ion intercalation/deintercalation mechanism is verified by XRD, Raman, and XPS, TEM analysis. This excellent performance is attributed to two aspects: on the one hand, the conductivity of the positive electrode material doped with metallic silver is improved to facilitate the transmission of electrons and zinc ions. On the other hand, the vacancy exchange of Agþ and Zn2þ in the crystal structure of the cathode material forms a good stable structure. This work demonstrate not only that vanadium-based compounds could be suitable candidate cathode materials for ZIBs, but also that they could be used for further development of rechargeable aqueous ZIBs, which have the characteristics of being environmentally friendly, and sufficiently secure for large-scale energy storage systems. 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), and 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). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.02.078. References [1] N.S. Lewis, Research opportunities to advance solar energy utilization, Science 351 (6271) (2016) 1920, https://doi.org/10.1126/science.aad1920. [2] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (1) (2014) 19e29, https://doi.org/
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Metallic silver doped vanadium pentoxide cathode for aqueous rechargeable zinc ion batteries Binxu Lan a, b, Zhuo Peng c, Lineng Chen c, Chen Tang a, b, Shijie Dong a, b, **, Chen Chen a, b, Min Zhou a, b, Cheng Chen a, b, Qinyou An c, ***, Ping Luo a, b, * a b c
Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, PR China Collaborative Innovation Center of Green Light-weight Materials and Processing, Hubei University of Technology Wuhan 430068, PR China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2018 Received in revised form 31 January 2019 Accepted 6 February 2019 Available online 8 February 2019
Recently, rechargeable aqueous zinc-ion batteries have received considerable attention in relation to large-scale energy storage, mainly because of their advantages of environmental friendliness, nontoxicity, and favorable safety. However, little researches have been conducted on the cathode materials used in zinc-ion batteries. In this work, an aqueous zinc-ion battery is constituted based on metallic silver doped vanadium pentoxide cathode, Zn(CF3SO3)2 aqueous electrolyte, and zinc anode, which exhibits excellent electrochemical performance. At a current density of 0.1 A g1, the specific capacity of about 215 mAh g1 was retained after 50 cycles. It also delivers long-cycle stability at 3 A g1, which remains at 80 mAh g1 after 700 cycles. Furthermore, the reaction mechanism is confirmed by X-ray diffraction, ex-situ Raman, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy analysis. This outstanding performance is attributed to the high conductivity of the cathode material and the vacancy exchange mechanism of Zn2þ/Agþ. This research shows metallic silver doped vanadium pentoxide cathode which could be considered a promising cathode material for aqueous zinc-ion batteries. © 2019 Published by Elsevier B.V.
Keywords: Aqueous zinc-ion battery Silver vanadium oxide Cathode material
1. Introduction With the deterioration of the global climate and the rapid consumption of fossil fuels, it is imperative that new types of energy conversion and storage devices are developed [1]. Secondary Batteries are one of the most important energy storage systems for renewable energy, including wind energy, solar energy, tidal energy, biomass energy, and geothermal energy [2]. As one of the most important secondary batteries, Lithium-ion batteries have been incorporated widely in electric vehicles, large-scale smart grid storage systems, mobile phones, and portable computers in recent years [3e5]. However, their use is associated with high cost, safety
* Corresponding author. Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, PR China. ** Corresponding author. Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, PR China. *** Corresponding author. E-mail addresses: [email protected] (S. Dong), [email protected] (Q. An), [email protected] (P. Luo). https://doi.org/10.1016/j.jallcom.2019.02.078 0925-8388/© 2019 Published by Elsevier B.V.
concerns, scarcity of lithium resources, and environmental pollution [3,4]. In view of the above shortcomings, many novel alternative secondary batteries have been developed recently, lithiumeair batteries [6], lithiumesulfur batteries [7],magnesiumion batteries [8], aqueous rechargeable lithium batteries [9,10], aqueous zinc-ion batteries (ZIBs) [11] and others [12]. Among these new secondary batteries, aqueous ZIBs have attracted considerable attention because of their favorable safety aspects, and environmental friendliness [13]. In addition, not only the ionic conductivity of the aqueous electrolyte is twoethree orders of magnitude higher than that of an organic electrolyte [14], but also the negative electrode use the metal-rich zinc which can provide a high capacity of 820 mAh g1 [15,16]. Although the advantages of ZIBs are outstanding, the selection of the cathode material is limited because the requirement of the electrode material for the insertion and extraction of the divalent zinc ion is different from that of the monovalent alkaline metal ion [17]. Previously, the following types of cathode material have been reported: Manganese-based oxide [18e21], NASICON [13,22], transition metal oxide [23], vanadium-based compounds [24e32],
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Prussian blue analogues [33e36], and others [12,37]. Among the cathode materials mentioned above, vanadium-based compounds have demonstrated high specific capacity, which could be attributed to their rich chemical valence. However, their development is limited by the poor conductivity. In view of this, highly conductive metallic silver doped vanadium pentoxide (SVO) is used as the cathode material to improve the conductivity compared to pure V2O5. Furthermore, the zinc ion intercalation/de-intercalation mechanism in the silver doped vanadium pentoxide is confirmed by X-ray diffraction (XRD), ex-situ Raman, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analysis. This excellent performance is attributed to two aspects: on the one hand, the conductivity of the positive electrode material doped with metallic silver is improved to facilitate the transmission of electrons and zinc ions. On the other hand, the vacancy exchange of Agþ and Zn2þ in the crystal structure of the cathode material forms a good stable structure. 2. Results and discussion The crystallographic structure of the prepared SVO samples was characterized by XRD measurement. It can be seen clearly from Fig. 1a that all the diffraction peaks could be indexed perfectly as monoclinic Ag0.33V2O5 (a ¼ 15.386 Å, b ¼ 3.615 Å, c ¼ 10.069 Å, Space group: C 2/m), consistent with the standard card (JCPDS card No. 81-1740). The XRD model of pure V2O5 is provided in Fig. S2 (Supplementary Information), which also fits perfectly with the standard card (JCPDS card No. 76-1803). In addition, the monoclinic structure of the SVO, drawn using VESTA software, is shown in Fig. 1b. Its crystallographic structure is similar to that reported previously for Na0.33V2O5 [38,39], which could provide feasible access for the insertion/extraction of zinc-ion guests. The Fourier transform infra-red (FTIR) spectrum of the SVO, illustrated in Fig. 1c, shows two peaks at 3453 and 1631 cm1, which correspond to the stretching of OeH and the bending vibrations of OeH in free water, respectively. The bands at 1007, 973, 939, 920, and 547 cm1
can be attributed to the asymmetric and symmetric stretching vibrations of VO3 groups. The bands at 875 and 712 cm1 are associated with the VO3 anti-symmetric stretching vibrations. Furthermore, the observable peak at 1427 cm1 could reflect the nitrate residue from the reactants [40e44]. The XPS survey spectrum, shown in Fig. 1d, demonstrates that the as-prepared SVO sample contains only vanadium, silver, and oxygen. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s to 284.8 eV. The two strong peaks at the Ag region of 366.9 and 372.9 eV (Fig. 1e) can be assigned to Ag 3d5/2 and Ag 3d3/2, respectively, whereas the three peaks at 516.6, 524.5, and 529.4 eV correspond to V 2p3/2, V 2p1/2, and O 1s, respectively (Fig. 1f) [38,45]. The SEM and TEM images were used to investigate the morphology and microstructure of the as-prepared SVO samples. Fig. 2a shows the typical SEM image of nanorods with length size of about 100e300 nm. Similarly, the SEM image Fig. S2 (Supplementary Information) of pure V2O5 can also exhibit nanorods with a length of approximately 300e500 nm. The TEM image of individual nanorods, shown in Fig. S3 (Supplementary Information), further confirms the nanorods of SVO. In Fig. 2d and Fig. S3 (Supplementary Information), the EDX mappings of the synthesized SVO also demonstrate that the elements of Ag, V, and O were distributed homogeneously within the sample. The High-resolution transmission electron microscopy (HRTEM) images (Fig. 2c) of individual SVO nanorods show a clear lattice fringe with interplanar spacing is about 2.17 Å, which corresponds to the (601) crystal plane, indicating high crystallinity of the SVO sample. The selected area electron diffraction (SAED) pattern (Fig. 2d) also corresponds to the d-spacing values of the (601) crystal plane of the SVO. The SVO/Zn ZIBs were built using SVO as the positive electrode, aqueous Zn(CF3SO3)2 solution as electrolyte and metallic zinc as the negative electrode. The electrochemical performance was evaluated by cyclic voltammetry (CV), galvanostatic chargeedischarge, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) measurements. Fig. 3a shows the initial five cycles versus Zn/Zn2þ of the CV pattern of the
Fig. 1. Characterization of as-synthesized SVO nanorods: (a) XRD pattern, (b) SVO 3D tunnel structure, (c) FTIR spectrum, (d) XPS spectra of the SVO samples, (e) Ag 3d spectrum, and (f) V 2p spectrum.
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Fig. 2. (a) SEM image, (b) SEM-EDx image, (c) TEM image, (d) SAED pattern.
SVO electrode within a voltage window of 0.2e1.6 V at a scan rate of 0.1 mV s1. From the CV curve (Fig. 3a), it can be seen clearly that the first cycle is somewhat different from the other cycles. In the forward scan, a sharp peak can be seen at 0.604 V, followed by three small shoulder peaks at 0.40, 0.915, and 1.082 V, indicating the electrochemical insertion of Zn2þ into the 3D channel-structured. In the reverse scan, three oxidation peaks can be seen at 0.62, 1.017, and 1.002 V, demonstrating that Zn2þ was de-intercalated from the host structure. The curve of this initial cycle is different from the curve of the subsequent cycle, which may be attributed to the irreversible phase transition of the material caused by zinc ion intercalation. The clear redox peak on the CV curve of the subsequent cycle is almost at the same point on the abscissa, indicating that the SVO electrode has good Zn2þ insertion/extraction ability in the late cycle. In addition, the SVO electrochemical performance is better than pure V2O5 at a current density of 0.1 A g1 (Figs. S4 and SI); however, its initial discharge reaches approximately 418 mAh g1 higher than the initial cycle charge capacity (335 mAh g1). The considerable capacity for the first discharge may not only be the intercalation of zinc ions, but the silver ions are reduced to metallic silver. The reason for the smaller charging capacity may be the inserted Zn ions, located at “dead Zn2þ sites,” cannot be extracted from the SVO lattice during the subsequent charge process [26]. At a current density of 0.5 A g1, the SVO cycle stability (Fig. 3b) is superior to pure V2O5 (Figs. S5 and SI), which may be the role of highly conductive metallic silver in subsequent cycles. Rate performance of SVO and pure V2O5 electrode are measured as shown in Fig. 3c and Fig. S5 (Supplementary Information). The average discharge capacities of SVO at the current densities of 0.2, 0.5, 1.0,
and 2.0 A g1 are 200, 150, 118, and 96 mAh g1, respectively. However, the specific capacity of pure V2O5 decreases sharply with increasing current density (Figs. S5 and SI). Accordingly, the energy and power densities of the Zn-SVO battery are calculated and shown in the Ragone plot (Figs. S6 and SI). An energy density of 118 Wh kg1 at a power density of 0.136 kW kg1 and a power density of 1.3 kW kg1 at an energy density of 58 Wh kg1 can be achieved, which is superior to the previously reported materials [18,33,36]. We also noticed that the SVO cathode material can exhibit long-term cycle stability, as shown in Fig. 3e, even at a current density of 3 A g1, the capacity is still stable after 700 cycles. To investigate the zinc ion diffusion rate of the prepared SVO cathode material, GITT measurements were performed on the SVO/ Zn cell, which was discharged/charged at a constant current flux (0.1 A g1) at 1 h intervals. Surprisingly, the prepared SVO delivers a specific capacity of 432.3 mAh g1 in the GITT test (Fig. 4a and b). The remarkable specific capacity might be attributable to the 3D tunnel framework of the SVO cathode material, which facilitates the insertion/extraction of metallic zinc ions. The Zn2þ ion diffusion coefficient can be obtained via following equation [29]:
D¼
4
mB VM pt MB S
DEs DEt
(1)
Where t is the pulse duration, MB is the molar mass of SVO, VM is the molar volume, mB and S are the active mass and electrode/ electrolyte contact area. DES and DEt can be obtained from the GITT curves (Fig. 4c). The zinc ion diffusion rate of SVO (about 1010 109 cm2 s1, Fig. 4d) is higher than that of pure V2O5 (about 1012 -
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Fig. 3. Electrochemical performance of SVO in the potential range of 0.2e1.6 V, running in 2 M Zn (CF3SO3)2 electrolyte: (a) CV curves of SVO at a scan rate of 0.1 mV s1, (b) Cyclic performance of SVO nanorods at 0.5 A g1, and (c) Rate performance, (d) Charge and discharge curves at the current density from 0.2 to 2.0 A g1, (e) Cycling performance of SVO at 3.0 A g1.
1011 cm2 s1, Figs. S5 and SI). In Table S1 (Supplementary Information), compared the zinc ion diffusion rate of the reported cathode. SVO has a higher zinc ion diffusion coefficient may be attributed to the high conductivity of metallic silver in the SVO cathode material during charge and discharge. To further understand the electrochemical performance of SVO and pure V2O5, electrochemical impedance spectroscopy (EIS) measurements were performed during different charge and discharge processes, as illustrated in Fig. S7 (Supplementary Information). In a typical equivalent circuit (inset of Fig. S7), Rs stands for all equivalent series resistance, while Rct represents the charge transfer resistance between the electrode/electrolyte interface and the contacts with the electrode material and a sloped line (ZW) associated with the transfer of Zn2þ. The low equivalent series
resistance further confirms that the silver-doped cathode material has higher conductivity than pure V2O5. A high Rct is a feature of the freshly assembled cell. However, Rct drops significantly after the 20th cycle, not only because the electrolyte penetrates into the SVO channel-structure, but also the metal silver plays a role in the charge and discharge process. In order to explore the mechanism of zinc ion intercalation/ extraction of host materials, ex-situ XRD, ex-situ Raman, ex-situ XPS, ex-situ SEM and ex-situ TEM were applied under different charge and discharge conditions. Fig. 5a shows the ex-situ XRD pattern with a 2q of 15e50 , a major peak 2q at 29.20 is repeatedly moved during constant current discharge and charging, which corresponds to a change in the (401) crystal plane. In addition, a new diffraction peak located at 27.33 corresponds to the new
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Fig. 4. (aec) GITT curve of the as-prepared SVO at a current density of 100 mA g1 (d) Diffusion coefficients of Zinc ion.
Fig. 5. (a) The ex-situ XRD and (b) ex-suit Raman patterns of SVO electrode at different charge/discharge states.
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phase Zn2 (V3O8)2 (PDF#24-1482) can also be observed when discharging from 1.0 to 0.2 V. Conversely, this diffraction peak slowly disappears when charging from 0.4 to 1.6 V, which indicates that zinc ions insertion/extraction from the host material. Fig. 5b shows the ex-situ Raman spectra of the SVO electrode in different states. Typical Raman bands around 252, 428.2, 684.3, 853.2, 900.4, and 988.9 cm1 were observed. Among them, the most intense peak at 252 cm1 is assigned to be the channel-structured silver vanadate reported in the literature [46,47,50e52]. The bands observed at 684.3 cm1 can be attributed to the VOV asymmetric vibration [48,49]. In addition, the band at 853.2 cm1 can be associated with the stretching vibrations of VO3 groups in the (V2O7)4ion [50e52]. The Raman frequencies at 900.4 cm1 correspond to the stretching vibration of VeO bonds and terminal V-O-Ag or V-O-V vibrations [46e52]. However, the Raman band at 428.2 and 988.9 cm1 may be correlated with the signal from binder and conductive agent. During the discharge from 1.0 to 0.2 V, a distinct Raman band at 252.2 cm1 became weaker and wider, while the Raman band at 900.4 cm1 become stronger and narrower, which may be inserted Zn2þ accompanied by silver oxidation from Agþ to Ag0. On the contrary, when charging from 0.4 to 1.6 V, it is obvious that the Raman band at 252.2 cm1 become stronger and narrower, and gradually returns to the original state at 900.4 cm1, which is caused by the extraction of Zn2þ from the lattice. The ex-situ XPS and ex-situ TEM analyses of the SVO electrodes in the discharged and charged states were further applied to verify the results of ex-situ XRD and ex-situ Raman. Fig. 6a is the XPS of Zn 2p electrodes in three different states (original, discharged, charged). It is very obvious that the Zn 2p intensity of charged state electrode is much weaker than the discharge one, which indicates that the Zn ions do have intercalated into the SVO cathode [29]. The corresponding energy dispersive spectroscopy (Figs. S8-9, SI) shows that Zn ions are evenly distributed throughout the SVO cathode material. In the fully charged state, the weaker Zn 2p can be
observed; indicating that the Zn2þ intercalation is irreversible in the electrode, which further confirms the analysis results of the above ex-situ XRD and ex-situ Raman, and similar phenomenon have been reported [53]. It can be seen from the Ag 3d XPS spectrum (Fig. 6b) that in the fully discharged state, the metallic silver is reduced, and once charged, the metallic silver is oxidized to silver ions, indicating that the replacement of metallic silver by the insertion of zinc ions into the host material, while the zinc ions occupy the vacancies of the silver atoms to form a new phase. In addition, in the curve fitting V 2p (Fig. 6c), from the original state to the discharge state, V5þ (517.1 eV) peak becomes weaker, V4þ (516.4 eV) peak becomes stronger indicating that V5þ is reduced to V4þ. After recharging, V4þ is oxidized to V5þ, accompanied by the extraction of Zinc ion. Furthermore, TEM and TEM-EDX element mapping of different states are collected (Fig. 6def). The (002) crystal plane nanorods of the original SVO exhibit lattice fringes with d-spacing of 0.473 nm, which is consistent with the XRD data (Figs. S10 and SI). When discharge to 0.2 V, A set of interplanar distances are clearly observed (Fig. 6d), which indicates that the intercalation of Zn ions will occupy a part of the Ag vacancies of SVO to form a new phase Zn2(V3O8)2, while Agþ is reduced to Ag0. In addition, the fully discharged EDX element mapping can also see the metal silver element precipitated, and the distribution of some Zn elements can be seen in the vicinity of the silver, which may be the formation of the new phase Zn2(V3O8)2. On the contrary, during the charging process, zinc ions are de-intercalated from the cathode material, and the corresponding vacancies are reoccupied by the oxidized Agþ, resulting in a transition to the original state. The change in lattice fringe spacing exactly matches this result. These findings match with those deduced from the ex-situ XRD, Raman and XPS studies. In short, from the above analysis, the reaction mechanism of the SVO/Zn battery changes as shown in Fig. 7. The outstanding electrochemical properties of SVO/Zn system are mainly attributed to
Fig. 6. (a) Zn 2p, (b) Ag 3d, (c) V 2p XPS spectra, (d) ex-situ HRTEM images at fully discharged state and (e) EDX element mapping (f) ex-situ HRTEM images at fully charge state.
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Fig. 7. Schematic diagram of the vacancy exchange mechanism of SVO/Zn batteries.
the special frame structure of the material and the electrical conductivity. 3. Conclusions In summary, a one-step hydrothermal method was employed to synthesize silver-doped vanadium pentoxide cathode material, metallic zinc was used as a negative electrode material, and 2 M Zn(CF3SO3)2 was used as an electrolyte to produce an aqueous ZIB that demonstrated excellent electrochemical performance compared to pure V2O5. Furthermore, the zinc ion intercalation/deintercalation mechanism is verified by XRD, Raman, and XPS, TEM analysis. This excellent performance is attributed to two aspects: on the one hand, the conductivity of the positive electrode material doped with metallic silver is improved to facilitate the transmission of electrons and zinc ions. On the other hand, the vacancy exchange of Agþ and Zn2þ in the crystal structure of the cathode material forms a good stable structure. This work demonstrate not only that vanadium-based compounds could be suitable candidate cathode materials for ZIBs, but also that they could be used for further development of rechargeable aqueous ZIBs, which have the characteristics of being environmentally friendly, and sufficiently secure for large-scale energy storage systems. 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), and 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). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.02.078. References [1] N.S. Lewis, Research opportunities to advance solar energy utilization, Science 351 (6271) (2016) 1920, https://doi.org/10.1126/science.aad1920. [2] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (1) (2014) 19e29, https://doi.org/
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