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Energy storage performance and mechanism of the novel copper pyrovanadate Cu3V2O7(OH)2·2H2O cathode for aqueous zinc ion batteries
Journal Pre-proof Energy storage performance and mechanism of the novel copper pyrovanadate Cu3V2O7(OH)2·2H2O cathode for aqueous zinc ion batteries Linlin Chen, Zhanhong Yang, Jian Wu, Hongzhe Chen, Jinlei Meng PII:
S0013-4686(19)32219-4
DOI:
https://doi.org/10.1016/j.electacta.2019.135347
Reference:
EA 135347
To appear in:
Electrochimica Acta
Received Date: 18 September 2019 Revised Date:
12 November 2019
Accepted Date: 20 November 2019
Please cite this article as: L. Chen, Z. Yang, J. Wu, H. Chen, J. Meng, Energy storage performance and mechanism of the novel copper pyrovanadate Cu3V2O7(OH)2·2H2O cathode for aqueous zinc ion batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135347. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Energy storage performance and mechanism of the novel copper pyrovanadate Cu3V2O7(OH)2.2H2O cathode for aqueous zinc ion batteries
Linlin Chena,b, Zhanhong Yanga,*, Jian Wua,b, Hongzhe Chena,b, Jinlei Menga,b a Hunan Province Key Laboratory of Chemical Power Source, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China b Innovation Base of Energy and Chemical Materials for Graduate Students Training, Central South University, Changsha, 410083, China * Corresponding author. E-mail address: [email protected] (Z. Yang).
Abstract: Aqueous zinc ion batteries (ZIBs) are expected to be used in large-scale energy storage because of their major merits of high-safety, low cost as well as excellent energy density. However, the discovery of high-performance cathode materials and the exploration of zinc storage mechanism are still facing great challenges.
Herein,
a
new
cathode
material
of
copper
pyrovanadate
Cu3V2O7(OH)2.2H2O (CuVO) is demonstrated to be potential in zinc ion storage. A series of ex-situ characterization results reveal a hybrid mechanism involving phase transitions and classical insertion/extraction reaction. First, zinc ions are embedded into CuVO and replace Cu2+ to form a new phase of Zn3(OH)2V2O7.2H2O (ZnVO), and then repeative intercalation of Zn2+ in the ZnVO lattice dominates the subsequent electrochemical reactions. Meantime, a highly conductive Cu0 matrix is generated 1
upon cycling, which effectively enhance the electronic transport. In addition, electrochemical reaction kinetics demonstrate that the CuVO electrode has a significant capacitance contribution and a fast zinc ion solid-state diffusion rate. As a consequence, the CuVO electrode delivers a high specific discharge capacity of 216 mAh g−1 at 0.1 A g−1, and a high capacity retention of 89.3% after 500 cycles at 0.5 A g−1. Keywords: Copper pyrovanadate; Zinc ion batteries; Zinc storage mechanism; Electrochemical kinetics.
1. Introduction Rechargeable aqueous metal-ion batteries have become one of the emerging alternatives for grid energy storage owing to their inherent safety related to the use of non-toxic and non-flammable electrolyte [1]. Compared with an organic conventional system, the high ionic conductivity of aqueous solution (100 times higher than that of organic) endow this ion battery family high rate performance. Moreover, rigorous assembly and sealing operations under anhydrous and anaerobic conditions are avoided, which greatly improve production efficiency and reduce technical costs. Among various aqueous rechargeable batteries (such as Li+, Na+, K+, Ca2+, Mg2+, Zn2+, Al3+, etc.), aqueous zinc ion batteries (ZIBs) stand out on account of the advantages of metallic Zn, including high volumetric energy density (5851 mAh cm−3), low redox potential (-0.76 V versus normal hydrogen electrode), high stability in water, low cost, and high abundance [2,3]. The exploration of high performance cathode materials is the key to improve the 2
electrochemical capabilities of zinc ion batteries. Recently, various vanadium-based electrodes considering on the rich chemical valences (V2+, V3+, V4+, V5+) and facile distortion of V–O octahedral have attracted extensive attention [4]. Nazar et al. [5] assembled a high-efficiency Zn metal/Zn0.25V2O5 cell, which delivers a superhigh discharge capacity of 300 mAh g−1, offers an energy density of ∼450 Wh l−1, and displays a superior cycle stability with 80% capacitance retention after 1000 cycles. Since then, many other vanadium oxides VxOy have also been proven to be effective as the positive electrodes for ZIBs, such as V2O5 [6,7], VO2 (B) [8], V6O13 [9], V3O7·H2O [10], etc. In addition to VxOy, it is particularly important to note that transition metal vanadates MxVnOm (M= Ca2+, Cu2+, Li+, K+, Na+, Al3+, Ag+, et al.) exhibit outstanding electrochemical properties due to the regulation of interlayer spacing and the structural stabilization effects from ion pillar and coordination [11-16]. Liang’s group [13] used the Ag0.4V2O5 nanobelts as a novel cathode for ZIBs, which revealed a new type of combination displacement / intercalation (CDI) reaction mechanism. Meanwhile, the highly conductive Ag0 matrix generated in situ can facilitate electron conduction, thus enabling outstanding rate and ultra-long cycling performance. Liu et al. [17] proposed a high-performance CuV2O6 cathode material for ZIBs; in their study, a complex mechanism that involves the divalent cations replacement between Cu2+ and Zn2+, reduction/oxidation of metallic Cu nanoparticles, and the insertion/extraction of zinc ion, together endow the system excellent electrochemical behaviour. Alshareef et al. [18] reported an advanced aqueous rechargeable ZIBs using ultralong zinc pyrovanadate (Zn3V2O7(OH)2·2H2O, ZVO) 3
nanowires as cathode material. The assembled Zn//ZVO cells deliver a high energy density of 214 Wh kg−1, showing a very prominent advantage over commercial lead– acid batteries. Cu3V2O7(OH)2.2H2O (CuVO) is a rare copper vanadate mineral with layered structure. It possess a crystal structure similar to Zn3V2O7(OH)2·2H2O, and its structure is built up of Cu3O6(OH)2 layers separated by V2O7 pillars and unligated water molecules (Fig. 1a). Actually, CuVO electrodes have been successfully employed as a cathode material for LIBs because of their unique layered structure and rapid kinetics, delivering a high capacity of 620 mAh g−1 at a current density of 20 mA g−1 [19]. However, few attempts have been made so far to investigate the reversible zinc storage properties and mechanism. To capitalize on these features, herein we demonstrate a new and high-performance ZIB system by using CuVO nanoribbon cathode. The Zn//CuVO battery exhibits a high specific discharge capacity of 216 mAh g−1 at 0.1 A g−1, and a high capacity retention of 89.3% after 500 cycles at 0.5 A g−1. 2. Experimental 2.1. Preparation of CuVO nanoribbon The CuVO nanoribbon was synthesized by a simple hydrothermal process as reported in literature [20]. Typically, 0.3064 g of CuCl2·2H2O was dissolved into 24 mL deionized water to obtain a blue transparent solution. Next, 0.1404 g of NH4VO3 was dissolved into another 24 mL of deionized water by stirring at 80 °C, and the resulting solution was added dropwise into the above CuCl2 solution under ambient 4
temperature, a yellow-green product was generated immediately. After vigorous stirring for about 10 minutes, the obtained suspension was transferred to 100 mL Teflon-lined stainless steel autoclave and hydrothermal treated at 180 °C for 20 h. Finally, the as-synthesized precipitate was washed several times with distilled water and anhydrous ethanol, and then dried at 60 °C for further use. 2.2. Materials characterization PANayltical
Empyrean
powder
X-ray
diffractometer
(XRD)
using
monochromatic Cu Kα radiation was employed to characterize the crystal structures of CuVO at pristine and different cycle states. The morphologies were observed through the utilization of FEI Helios NanoLab G3 UC scanning electron microscope (SEM) and JEM-2100F transmission electron microscope (TEM). The sample surface elemental composition and changes in the valence were examined on Phi Quantum 2000 spectrophotometer utilizing Al KαX-ray radiation. The Raman spectra were recorded on a Renishaw inVia spectrometer. 2.3. Electrochemical tests To evaluate the electrochemical capability, CR2025 coin-type batteries were constructed by using the as-obtained CuVO as working electrode, 0.1 mm Zn metal foil as counter electrode, and 2.5 M Zn(CF3SO3)2 aqueous solution as electrolyte. The cathode was prepared by pressing a slurry consisting of 70 wt.% CuVO active material, 20 wt.% conductive carbon (Super P) and 10 wt.% PTFE binder onto titanium mesh, and then drying overnight under vacuum at 65 °C. The mass loading of the active materials is around 1.3–1.6 mg cm–2. The galvanostatic charge/discharge 5
(GCD) measurements were performed over the potential range of 0.2 - 1.6 V utilizing Neware CT-4008 instrument (Shenzhen, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were all conducted on an electrochemical workstation (CHI 660E, Chenhua). The EIS plots were recorded at a frequency range of 10-2 to 105 Hz with an amplitude of 5 mV. 3. Results and discussion The illustration of the synthesis process of the CuVO powder is presented in Fig. S1. The CuVO was synthesized by a simple hydrothermal method. The crystal phase of the obtained sample was confirmed by XRD measurements. It can be seen from Fig. 1b that all characteristic diffraction peaks could be well assigned to the monoclinic Cu3V2O7(OH)2.2H2O phase with space group of C2/m (JCPDs No. 80-1170, a =10.61 Å, b = 5.86 Å, c = 7.21 Å). Meanwhile, these diffraction peaks are shape and intense, indicating a high degree of crystallinity. Raman spectrum displayed in Fig. 1c further proves the phase composition of CuVO. Evidently, all bands present the same characteristics as the standard data of volborthite achieved from the RRUFF Raman database (R070710, 532 nm). Fig. 1d shows SEM micrographs of as-synthesized CuVO, from which we can observe that the sample adopt a typical nanoribbon-like appearance with length of several microns and diameters of approximate 150-200 nm. These nanoribbons adhere to each other, forming a substantially continuous network. In addition, TEM investigations were also conducted for evaluating the appearance and lattice structure of CuVO. The low-magnification TEM observation in Fig. 1e reveals the ribbon-like architecture again, and the high-resolution TEM image in Fig. 6
1f presents clearly resolved lattice fringe with a distance of 0.256 nm, which agrees well with the (220) plane of CuVO.
Systematic electrochemical measurements are executed to demonstrate the potential of zinc ion storage by using CuVO electrodes. Fig. 2a displays the first five CV profiles of CuVO at a scan speed of 0.5 mV s-1 within the potential window of 0.2 - 1.6 V. It can be noticed that the first CV profiles display a different shape compared to the following cycle scans, especially the cathodic peak at low potential, which is usually thought to be connected with some irreversible reactions occurred in the initial process. During the subsequent CV scans, there are three couples of well-defined redox peaks situated at 1.39/1.33, 1.10/0.94, and 0.59/0.44 V, implying the stepwise electrochemical insertion/extraction of Zn2+ into/from the CuVO structure. The first several GCD curves of CuVO cathode at 0.2 A g-1 are shown in Fig. 2b. Interestingly, a discharge platform appears at ca. 0.25V in the second discharge process and gradually disappears in the following cycles, which may be related to the pH evolution of electrolyte and the disappearance of some side reactions [21,22]. A fairly high initial discharge capacity of 299.8 mAh g-1 with a columbic efficiency of 78.2% is achieved, while it tardily decreases to 157.7 mAh g-1 after three cycles. Such rapid capacity attenuation is due to an electrochemically irreversible reaction, which will be discussed later. The specific discharge capacity tends to stabilize after the 3rd cycle, and about 100.1 mAh g-1 (63.5% of the value in the 3rd cycle) could be retained undergo 130 cycles (Fig. 2c). Meanwhile, the EIS Nyquist plots during the cycling 7
were also studied, as displayed in Fig. S2. The charge transfer resistance (semicircle in the high frequency area) and the Warburg impedance (oblique line in the low frequency range) decrease significantly undergoing three initial cycles, implying improved electron transfer at the electrode/electrolyte interface and accelerated zinc ion diffusion rate. After 5 cycles, the impedance of these two parts remains basically unchanged, and both remain at a very impressive low value, which could be attributed to the electrochemical performance. The rate capability of the CuVO cathode is displayed in Fig. 2d. The average specific capacities of 216, 159, 148, 133, 127, and 105 mAh g-1 are attained under varying current densities of 0.1, 0.3, 0.5, 0.8, 1.0, and 2.0 A g-1, respectively. As the current rate gradually reduces to 0.1 A g-1, an impressive capacity of 198 mAh g-1 could be achieved, demonstrating superior structural stability of the CuVO electrode under a high rate charge and discharge. Meanwhile, the electrode delivers a very small changes in capacity over a wide current range studied. The corresponding GCD profiles under different applied current densities are presented in Fig. 2e. It must be mentioned that the charge/discharge plateaus can be well discerned even at a high rate of 2.0 A g-1. Notably, a high discharge capability of 92 mAh g-1 with about 89.3% capacitance retention can be achieved after 500 cycles at 0.5 A g-1 (Fig. 2f). The increasing specific capacities in the initial 100 cycles was considered to be due to the electrode activation. At first, it needs time for cathode materials to be fully infiltrated by electrolyte; secondly, with the repeated Zn2+ insertion/extraction and phase transition, more active sites would be generated, resulting in the enhancement of the utilization of active materials. Similar activation 8
phenomena are often prevalent in previously reported vanadium oxide electrodes [13,17,23]. Moreover, the coulomb efficiency stabilizes at around 100% throughout the repetitive cyclic testing, and these cycles reflect a good potential-time response (inset of Fig. 2f), suggesting the excellent reversibility and superior cyclic durability of the CuVO electrode.
In order to reveal the zinc ion storage mechanism, ex-situ XRD data of CuVO under varied states during the first cycle were analysed, as displayed in Fig. 3a. During the discharge process, some new phases located at 43.3° and 50.4°were detected, which is indicative of the formation of the reduced products Cu. Meantime, the characteristic peaks attributed to CuVO were obviously weakened, especially the strong peak at 12.3°, suggesting the decrease of active component CuVO content owing to phase transition. In addition, some new peaks appear at 9.8°, 10.9°and 16.4°, and their peak intensity gradually become stronger with increasing depth of discharge. The new peaks present a structure analogous to zinc chlorate hydroxide (Zn4ClO4(OH)7·nH2O, PDF#41-0715) [24], so we infer that its appearance may be related to the electrolyte byproduct of zinc hydroxide trifluoromethanesulfonate produced on the electrode surface. Ex-situ Raman spectra of the CuVO electrode at different states (Fig. S3) further reveal the formation of the electrolyte byproduct. Meanwhile, SEM images of the CuVO electrode after fully discharging were also investigated, as shown in Fig. S4. It can be seen that many large flakes appear on the electrode surface, showing a similar morphology to zinc hydroxide sulfate hydrate 9
reported in the literature [25]. Energy-dispersive spectroscopy (EDS) mapping (Fig. S4b) presents that these flake-like product contains abundant Zn, C, F, and S elements, which confirm that the formed flakes are zinc hydroxide trifluoromethanesulfonate. Surprisingly, some bright particles that covered by some thin flakes are observed from the high-magnification SEM images (Fig. S4c,d), presumably to be in-situ reduced highly conductive Cu0 nanoparticles. Conversely, this electrolyte byproduct and the new phase of Cu disappear during the subsequent charging process, accompanied by the enhancement of the diffraction peak of the main active component. Surprisingly, the charging product can match well with the Zn3(OH)2V2O7.2H2O ( defined as ZnVO, PDF#87-0417), not CuVO, implying that CuVO has been transformed into ZnVO through a complete charge-discharge cycle. This phenomenon can be explained by the following: during the discharge process, the Cu-O bond breaks to form a reduced metallic Cu0, and the embedded zinc ions can be bonded with O atoms to readjust the interlayer structure, eventually forming a frame structure constructed through alternating Zn–O with tetrahedral layer and V–O with octahedral layer, namely ZnVO. Since the electronegativity of Zn is stronger than that of Cu, the formed Zn-O bond is much more stable than the Cu-O bond [26], so the resultant ZnVO can be stably present. The irreversible phase transition in the first cycle can be recognized as an important factor for the unsatisfactory columbic efficiency and subsequent capacity decay. As shown in Fig. 3b, this ZnVO phase dominates the subsequent insertion and extraction process, and only the expansion and contraction of the crystal lattice is noticed. Compared with the discharge state, the peak in the charged state shift towards 10
lower 2θ angles (partial enlargement of Fig. 3b), corresponding to the increase of the lattice distance in view of Bragg's equation, which can be attributed to the extraction of Zn2+. Furthermore, the generation of Cu0 can still be detected in the discharge state, which can effectively enhance the electrical conductivity of electrode materials. In addition, a discharge byproduct of Cu(OH)2·H2O occurs in the 2nd and 3rd cycles, which is ubiquitous and difficult to avoid in aqueous electrolyte [27]. The illustration of the reaction mechanism of the CuVO electrode is depicted in Fig. S5.
The evolution of chemical composition and element valence states of CuVO upon cycling was investigated by utilizing ex-situ XPS measurements. As can be seen from the core level Zn 2p XPS spectrum displayed in Fig. 3c, the pristine CuVO does not contain Zn element because no signal for zinc is captured. While discharging to 0.2 V, a pair of distinct peaks at binding energies of 1044.8 (Zn 2p1/2) and 1021.6 eV (Zn 2p3/2) are observed, confirming the successful embedding of Zn2+ into CuVO. It is noteworthy that the peak of Zn2p still exists in the subsequent full-charge state but reduce in intensity, which is related to the phase transition from CuVO to ZnVO and the incomplete extraction of Zn2+. Fig. 3d displays the core level XPS spectra of Cu element, in which two main peaks situated at 955.2 and 935.5 eV can be ascribed to the typical Cu2+ line shape of Cu 2p1/2 and Cu 2p3/2, respectively, and the peak in the range of 940-946 eV corresponds to the Cu2+ satellite response [28,29]. Note that a small amount of Cu0 might be generated during the hydrothermal process [11]. Relative to pristine state, most of Cu2+ is reduced to Cu+ and Cu0, and the 11
characteristic satellite peak representing Cu2+ disappears in the discharge state. After recharging to 1.6 V, a Cu2+ feature appears, implying the partial oxidation of Cu0 and Cu+ accompanied by de-intercalation of Zn2+. The V2p spectrum in Fig. 3e also reveals a similar phenomenon that the insertion and extraction of Zn2+ can drive the redox reaction between V5+ and V4+.
For electrode materials involving the (de)-intercalation mechanism, it is necessary and meaningful to have a comprehensive insight into the electrochemical kinetics. Fig. 4a exhibits the CV curves of CuVO at various sweep speeds ranging from 0.1 to 1.0 mV s-1. It should be pointed out that the peak current intensity gradually enhances as the sweep speed increases. Generally speaking, the peak current (i) versus the sweep speed (v) satisfy a power exponential relationship: i=avb, where a and b are alterable parameters [30]. Based on the formula, the value of b can be determined by the fitted slopes of log (i) vs. log (v), which can be used to estimate the electrochemical rate-limiting step and thus reveal the charge storage mechanism. In particular, the b-value closer to 1.0 suggests a surface-dominated capacitive behavior, whereas b=0.5 represents a total diffusion driven process. As can be seen in Fig. 4b, the calculated b values of peak 1, 2, 5, 6 are 0.81, 0.90, 0.95, and 0.85, respectively, implying that the capacitive effect contributes most to the capacity. To be more exact, the specific capacitance contribution percentages can be quantified through the following formula: i=k1v+k2v1/2, in which the first half (k1v) represents the fraction of current derived from the surface capacitive behavior, and the latter 12
(k2v1/2) stands for the contribution determined by the diffusion-induced insertion process [31]. At a fixed sweep speed of 0.8 mV s-1, about 77.2% of the current is provided by the capacitive properties (Fig. 4c). In addition, the capacitance contribution at other scan speeds is also calculated, as presented in Fig. 4d. The results show that within the investigated range, the capacitive effect holds dominant position throughout the entire electrochemical process, and its contribution proportion gradually increases with improvement of the scan rates. It is generally believed that a high capacitance contribution is helpful for executing high rate capability. To further inquire into the Zn2+ diffusion and transport ability in the CuVO electrode, galvanostatic intermittent titration technique (GITT) measurement was carried out based on the previous method. During a single GITT process, the voltage variation under constant current display a good linear relationship with the root of pulse time τ1/2 (Fig. S6). Fig. 4e exhibits the GITT profiles and corresponding calculated Zn2+ diffusion coefficients (DZn). As expected, the CuVO cathode presents a high DZn values of 10-9-10-12 cm2 s−1, which is more competitive than those of other cathodes including V10O24·12H2O [32], KV3O8 [33], VO2 (B) nanofibers [8], Z0.5N2VPF [34], and K2V6O16·1.57H2O [33]. Excellent diffusion kinetics is the guarantee of high specific capacity and outstanding rate performance. It's worth noting that the Zn2+ ion diffusivity is apparently higher on charge than on discharge at early stage, which can be explained by the following reasons: firstly, some in-situ formed metallic Cu0 has not been oxidized at the initial stage of charging, which endow the electrode high electrical conductivity and fast electron transfer, leading to rapid extraction of Zn2+; 13
secondly, at the initial stage of discharge, Zn2+ will be embedded in the bulk of the electrode active particles through a long diffusion distance and a large resistance, while the charging process is reversed, and Zn2+ at the surface and interface will be preferentially and rapidly extracted. In addition, a large fluctuation of Zn2+ diffusion coefficient is observed during the charging process. The D value decreases on the charge platform, while it rises suddenly when reaching the next platform, and then decreases. The diffusion of zinc ions between two platforms is relatively easier, but it is more difficult on the platforms due to the redox reactions [35].
To visually demonstrate the potential of our assembled Zn//CuVO batteries, the Ragone plots showed in Fig. 5a were proposed to compare with other ZIBs systems reported in the literature (see Fig. S7 for detailed calculation process). As can be observed, our Zn//CuVO cell delivers a high energy density of 228 Wh kg−1 at a power density of 96 W kg−1, and still achieves an energy density of 109 Wh kg−1 at an extra high power density of 2075 W kg−1 based on the mass of the cathode material, which is obviously superior to the previously recorded cell system based on VS2 [36], Zn3[Fe(CN)6]2
[37],
Mn2O3
[38],
ZnHCF@MnO2
[39],
Todorokite
[40],
Zn3V2O7(OH)2.2H2O [18], and ZnMn2O4 cathodes [41]. The actual application value of our proposed Zn//CuVO cells was further confirmed by driving a 1.5 V temperature sensor through a prototype device (two coin batteries in series, Fig. 5b). Impressively, it can supply power to maintain the temperature sensor for working about 6 h. 14
4. Conclusion In this work, CuVO nanoribbon was prepared via a simple hydrothermal procedure, and evaluated as a cathode material for ZIBs. After the first GCD cycling, the CuVO electrode undergoes an irreversible phase transition to form a stable ZnVO phase, and then zinc ions are repeatedly inserted / extracted into / from the ZnVO lattice. Meanwhile, the inserted Zn2+ could drive the valence change of Cu element to form a highly conductive metallic Cu0 matrix, which is beneficial to electron transport. In addition, the assembled Zn//CuVO cell exhibits impressive EIS results, which could be attributed to the electrochemical performance. Benefiting from the above findings, the CuVO cathode displays a superior specific capacity of 216 mAh g−1 at 0.1A g−1, outstanding cycle stability (89.3% capacity retention after 500 cycles at 0.5 A g−1), and excellent rate capability. This research not only expands the choice of cathode materials for ZIBs, but also stimulates the utilization of the CuVO electrode in other aqueous metal ion batteries.
Declarations of interest None.
Acknowledgements This work was supported by the Natural Science Foundation of China (no. 21371180) and Hunan Provincial Science and Technology Plan Project (no. 15
2017TP1001)
Appendix A. Supplementary data Supplementary data related to this article can be found at…
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23
Fig. 1. (a) Crystal structure, (b) XRD patterns, (c) Raman spectra, (d) SEM, (e) TEM, and (f) HRTEM images of CuVO.
24
Fig. 2. Electrochemical performance of the CuVO electrode. (a) CV profiles at 0.5 mV s-1, (b) GCD curves for the initial ten cycles at 0.2 A g-1, (c) cycling stability at 0.2 A g-1, (d) rate performance, (e) GCD curves obtained at different current densities, (f) cycle capability at 0.5 A g-1, the inset is the GCD curves between 200th to 220th cycles.
25
Fig. 3. Ex-situ XRD patterns of CuVO obtained during the (a) 1st, and (b) 2nd and 3rd cycles at different states. Ex-situ XPS spectra of (c) Zn 2p, (d) Cu 2p, and (e) V 2p in pristine, fully discharged, and charged states.
26
Fig. 4. (a) CV profiles of CuVO at varied scan rates, (b) Logarithmic plots of current and scan rates, (c) capacitive contribution at 0.8 mV s-1, (d) contribution ratios arising from capacitance or diffusion-limited process at various sweep speeds, (e) GITT curves and calculated DZn values in the charge/discharge process.
27
Fig. 5. (a) Comparison of the Ragone plots of Zn//CuVO battery with other previously recorded Zn-based cell system, (b) Digital photos of our assembled Zn//CuVO batteries powering a 1.5 V temperature sensor.
28
A new cathode material of CuVO is demonstrated to be potential in ZIBs. A hybrid mechanism consisting of conversion and insertion/extraction is proposed. Cu0 matrix generated upon cycling can provide high electronic conductivity.
Author Contribution Statement
CRediT author statement: Linlin Chen: Data curation, Roles/Writing - original draft, Writing - review & editing. Zhanhong Yang: Conceptualization, Methodology, Funding acquisition, Supervision. Jian Wu: Investigation. Hongzhe Chen: Formal analysis, Visualization. Jinlei Meng: Validation.
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships □
that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(19)32219-4
DOI:
https://doi.org/10.1016/j.electacta.2019.135347
Reference:
EA 135347
To appear in:
Electrochimica Acta
Received Date: 18 September 2019 Revised Date:
12 November 2019
Accepted Date: 20 November 2019
Please cite this article as: L. Chen, Z. Yang, J. Wu, H. Chen, J. Meng, Energy storage performance and mechanism of the novel copper pyrovanadate Cu3V2O7(OH)2·2H2O cathode for aqueous zinc ion batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135347. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Energy storage performance and mechanism of the novel copper pyrovanadate Cu3V2O7(OH)2.2H2O cathode for aqueous zinc ion batteries
Linlin Chena,b, Zhanhong Yanga,*, Jian Wua,b, Hongzhe Chena,b, Jinlei Menga,b a Hunan Province Key Laboratory of Chemical Power Source, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China b Innovation Base of Energy and Chemical Materials for Graduate Students Training, Central South University, Changsha, 410083, China * Corresponding author. E-mail address: [email protected] (Z. Yang).
Abstract: Aqueous zinc ion batteries (ZIBs) are expected to be used in large-scale energy storage because of their major merits of high-safety, low cost as well as excellent energy density. However, the discovery of high-performance cathode materials and the exploration of zinc storage mechanism are still facing great challenges.
Herein,
a
new
cathode
material
of
copper
pyrovanadate
Cu3V2O7(OH)2.2H2O (CuVO) is demonstrated to be potential in zinc ion storage. A series of ex-situ characterization results reveal a hybrid mechanism involving phase transitions and classical insertion/extraction reaction. First, zinc ions are embedded into CuVO and replace Cu2+ to form a new phase of Zn3(OH)2V2O7.2H2O (ZnVO), and then repeative intercalation of Zn2+ in the ZnVO lattice dominates the subsequent electrochemical reactions. Meantime, a highly conductive Cu0 matrix is generated 1
upon cycling, which effectively enhance the electronic transport. In addition, electrochemical reaction kinetics demonstrate that the CuVO electrode has a significant capacitance contribution and a fast zinc ion solid-state diffusion rate. As a consequence, the CuVO electrode delivers a high specific discharge capacity of 216 mAh g−1 at 0.1 A g−1, and a high capacity retention of 89.3% after 500 cycles at 0.5 A g−1. Keywords: Copper pyrovanadate; Zinc ion batteries; Zinc storage mechanism; Electrochemical kinetics.
1. Introduction Rechargeable aqueous metal-ion batteries have become one of the emerging alternatives for grid energy storage owing to their inherent safety related to the use of non-toxic and non-flammable electrolyte [1]. Compared with an organic conventional system, the high ionic conductivity of aqueous solution (100 times higher than that of organic) endow this ion battery family high rate performance. Moreover, rigorous assembly and sealing operations under anhydrous and anaerobic conditions are avoided, which greatly improve production efficiency and reduce technical costs. Among various aqueous rechargeable batteries (such as Li+, Na+, K+, Ca2+, Mg2+, Zn2+, Al3+, etc.), aqueous zinc ion batteries (ZIBs) stand out on account of the advantages of metallic Zn, including high volumetric energy density (5851 mAh cm−3), low redox potential (-0.76 V versus normal hydrogen electrode), high stability in water, low cost, and high abundance [2,3]. The exploration of high performance cathode materials is the key to improve the 2
electrochemical capabilities of zinc ion batteries. Recently, various vanadium-based electrodes considering on the rich chemical valences (V2+, V3+, V4+, V5+) and facile distortion of V–O octahedral have attracted extensive attention [4]. Nazar et al. [5] assembled a high-efficiency Zn metal/Zn0.25V2O5 cell, which delivers a superhigh discharge capacity of 300 mAh g−1, offers an energy density of ∼450 Wh l−1, and displays a superior cycle stability with 80% capacitance retention after 1000 cycles. Since then, many other vanadium oxides VxOy have also been proven to be effective as the positive electrodes for ZIBs, such as V2O5 [6,7], VO2 (B) [8], V6O13 [9], V3O7·H2O [10], etc. In addition to VxOy, it is particularly important to note that transition metal vanadates MxVnOm (M= Ca2+, Cu2+, Li+, K+, Na+, Al3+, Ag+, et al.) exhibit outstanding electrochemical properties due to the regulation of interlayer spacing and the structural stabilization effects from ion pillar and coordination [11-16]. Liang’s group [13] used the Ag0.4V2O5 nanobelts as a novel cathode for ZIBs, which revealed a new type of combination displacement / intercalation (CDI) reaction mechanism. Meanwhile, the highly conductive Ag0 matrix generated in situ can facilitate electron conduction, thus enabling outstanding rate and ultra-long cycling performance. Liu et al. [17] proposed a high-performance CuV2O6 cathode material for ZIBs; in their study, a complex mechanism that involves the divalent cations replacement between Cu2+ and Zn2+, reduction/oxidation of metallic Cu nanoparticles, and the insertion/extraction of zinc ion, together endow the system excellent electrochemical behaviour. Alshareef et al. [18] reported an advanced aqueous rechargeable ZIBs using ultralong zinc pyrovanadate (Zn3V2O7(OH)2·2H2O, ZVO) 3
nanowires as cathode material. The assembled Zn//ZVO cells deliver a high energy density of 214 Wh kg−1, showing a very prominent advantage over commercial lead– acid batteries. Cu3V2O7(OH)2.2H2O (CuVO) is a rare copper vanadate mineral with layered structure. It possess a crystal structure similar to Zn3V2O7(OH)2·2H2O, and its structure is built up of Cu3O6(OH)2 layers separated by V2O7 pillars and unligated water molecules (Fig. 1a). Actually, CuVO electrodes have been successfully employed as a cathode material for LIBs because of their unique layered structure and rapid kinetics, delivering a high capacity of 620 mAh g−1 at a current density of 20 mA g−1 [19]. However, few attempts have been made so far to investigate the reversible zinc storage properties and mechanism. To capitalize on these features, herein we demonstrate a new and high-performance ZIB system by using CuVO nanoribbon cathode. The Zn//CuVO battery exhibits a high specific discharge capacity of 216 mAh g−1 at 0.1 A g−1, and a high capacity retention of 89.3% after 500 cycles at 0.5 A g−1. 2. Experimental 2.1. Preparation of CuVO nanoribbon The CuVO nanoribbon was synthesized by a simple hydrothermal process as reported in literature [20]. Typically, 0.3064 g of CuCl2·2H2O was dissolved into 24 mL deionized water to obtain a blue transparent solution. Next, 0.1404 g of NH4VO3 was dissolved into another 24 mL of deionized water by stirring at 80 °C, and the resulting solution was added dropwise into the above CuCl2 solution under ambient 4
temperature, a yellow-green product was generated immediately. After vigorous stirring for about 10 minutes, the obtained suspension was transferred to 100 mL Teflon-lined stainless steel autoclave and hydrothermal treated at 180 °C for 20 h. Finally, the as-synthesized precipitate was washed several times with distilled water and anhydrous ethanol, and then dried at 60 °C for further use. 2.2. Materials characterization PANayltical
Empyrean
powder
X-ray
diffractometer
(XRD)
using
monochromatic Cu Kα radiation was employed to characterize the crystal structures of CuVO at pristine and different cycle states. The morphologies were observed through the utilization of FEI Helios NanoLab G3 UC scanning electron microscope (SEM) and JEM-2100F transmission electron microscope (TEM). The sample surface elemental composition and changes in the valence were examined on Phi Quantum 2000 spectrophotometer utilizing Al KαX-ray radiation. The Raman spectra were recorded on a Renishaw inVia spectrometer. 2.3. Electrochemical tests To evaluate the electrochemical capability, CR2025 coin-type batteries were constructed by using the as-obtained CuVO as working electrode, 0.1 mm Zn metal foil as counter electrode, and 2.5 M Zn(CF3SO3)2 aqueous solution as electrolyte. The cathode was prepared by pressing a slurry consisting of 70 wt.% CuVO active material, 20 wt.% conductive carbon (Super P) and 10 wt.% PTFE binder onto titanium mesh, and then drying overnight under vacuum at 65 °C. The mass loading of the active materials is around 1.3–1.6 mg cm–2. The galvanostatic charge/discharge 5
(GCD) measurements were performed over the potential range of 0.2 - 1.6 V utilizing Neware CT-4008 instrument (Shenzhen, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were all conducted on an electrochemical workstation (CHI 660E, Chenhua). The EIS plots were recorded at a frequency range of 10-2 to 105 Hz with an amplitude of 5 mV. 3. Results and discussion The illustration of the synthesis process of the CuVO powder is presented in Fig. S1. The CuVO was synthesized by a simple hydrothermal method. The crystal phase of the obtained sample was confirmed by XRD measurements. It can be seen from Fig. 1b that all characteristic diffraction peaks could be well assigned to the monoclinic Cu3V2O7(OH)2.2H2O phase with space group of C2/m (JCPDs No. 80-1170, a =10.61 Å, b = 5.86 Å, c = 7.21 Å). Meanwhile, these diffraction peaks are shape and intense, indicating a high degree of crystallinity. Raman spectrum displayed in Fig. 1c further proves the phase composition of CuVO. Evidently, all bands present the same characteristics as the standard data of volborthite achieved from the RRUFF Raman database (R070710, 532 nm). Fig. 1d shows SEM micrographs of as-synthesized CuVO, from which we can observe that the sample adopt a typical nanoribbon-like appearance with length of several microns and diameters of approximate 150-200 nm. These nanoribbons adhere to each other, forming a substantially continuous network. In addition, TEM investigations were also conducted for evaluating the appearance and lattice structure of CuVO. The low-magnification TEM observation in Fig. 1e reveals the ribbon-like architecture again, and the high-resolution TEM image in Fig. 6
1f presents clearly resolved lattice fringe with a distance of 0.256 nm, which agrees well with the (220) plane of CuVO.
Systematic electrochemical measurements are executed to demonstrate the potential of zinc ion storage by using CuVO electrodes. Fig. 2a displays the first five CV profiles of CuVO at a scan speed of 0.5 mV s-1 within the potential window of 0.2 - 1.6 V. It can be noticed that the first CV profiles display a different shape compared to the following cycle scans, especially the cathodic peak at low potential, which is usually thought to be connected with some irreversible reactions occurred in the initial process. During the subsequent CV scans, there are three couples of well-defined redox peaks situated at 1.39/1.33, 1.10/0.94, and 0.59/0.44 V, implying the stepwise electrochemical insertion/extraction of Zn2+ into/from the CuVO structure. The first several GCD curves of CuVO cathode at 0.2 A g-1 are shown in Fig. 2b. Interestingly, a discharge platform appears at ca. 0.25V in the second discharge process and gradually disappears in the following cycles, which may be related to the pH evolution of electrolyte and the disappearance of some side reactions [21,22]. A fairly high initial discharge capacity of 299.8 mAh g-1 with a columbic efficiency of 78.2% is achieved, while it tardily decreases to 157.7 mAh g-1 after three cycles. Such rapid capacity attenuation is due to an electrochemically irreversible reaction, which will be discussed later. The specific discharge capacity tends to stabilize after the 3rd cycle, and about 100.1 mAh g-1 (63.5% of the value in the 3rd cycle) could be retained undergo 130 cycles (Fig. 2c). Meanwhile, the EIS Nyquist plots during the cycling 7
were also studied, as displayed in Fig. S2. The charge transfer resistance (semicircle in the high frequency area) and the Warburg impedance (oblique line in the low frequency range) decrease significantly undergoing three initial cycles, implying improved electron transfer at the electrode/electrolyte interface and accelerated zinc ion diffusion rate. After 5 cycles, the impedance of these two parts remains basically unchanged, and both remain at a very impressive low value, which could be attributed to the electrochemical performance. The rate capability of the CuVO cathode is displayed in Fig. 2d. The average specific capacities of 216, 159, 148, 133, 127, and 105 mAh g-1 are attained under varying current densities of 0.1, 0.3, 0.5, 0.8, 1.0, and 2.0 A g-1, respectively. As the current rate gradually reduces to 0.1 A g-1, an impressive capacity of 198 mAh g-1 could be achieved, demonstrating superior structural stability of the CuVO electrode under a high rate charge and discharge. Meanwhile, the electrode delivers a very small changes in capacity over a wide current range studied. The corresponding GCD profiles under different applied current densities are presented in Fig. 2e. It must be mentioned that the charge/discharge plateaus can be well discerned even at a high rate of 2.0 A g-1. Notably, a high discharge capability of 92 mAh g-1 with about 89.3% capacitance retention can be achieved after 500 cycles at 0.5 A g-1 (Fig. 2f). The increasing specific capacities in the initial 100 cycles was considered to be due to the electrode activation. At first, it needs time for cathode materials to be fully infiltrated by electrolyte; secondly, with the repeated Zn2+ insertion/extraction and phase transition, more active sites would be generated, resulting in the enhancement of the utilization of active materials. Similar activation 8
phenomena are often prevalent in previously reported vanadium oxide electrodes [13,17,23]. Moreover, the coulomb efficiency stabilizes at around 100% throughout the repetitive cyclic testing, and these cycles reflect a good potential-time response (inset of Fig. 2f), suggesting the excellent reversibility and superior cyclic durability of the CuVO electrode.
In order to reveal the zinc ion storage mechanism, ex-situ XRD data of CuVO under varied states during the first cycle were analysed, as displayed in Fig. 3a. During the discharge process, some new phases located at 43.3° and 50.4°were detected, which is indicative of the formation of the reduced products Cu. Meantime, the characteristic peaks attributed to CuVO were obviously weakened, especially the strong peak at 12.3°, suggesting the decrease of active component CuVO content owing to phase transition. In addition, some new peaks appear at 9.8°, 10.9°and 16.4°, and their peak intensity gradually become stronger with increasing depth of discharge. The new peaks present a structure analogous to zinc chlorate hydroxide (Zn4ClO4(OH)7·nH2O, PDF#41-0715) [24], so we infer that its appearance may be related to the electrolyte byproduct of zinc hydroxide trifluoromethanesulfonate produced on the electrode surface. Ex-situ Raman spectra of the CuVO electrode at different states (Fig. S3) further reveal the formation of the electrolyte byproduct. Meanwhile, SEM images of the CuVO electrode after fully discharging were also investigated, as shown in Fig. S4. It can be seen that many large flakes appear on the electrode surface, showing a similar morphology to zinc hydroxide sulfate hydrate 9
reported in the literature [25]. Energy-dispersive spectroscopy (EDS) mapping (Fig. S4b) presents that these flake-like product contains abundant Zn, C, F, and S elements, which confirm that the formed flakes are zinc hydroxide trifluoromethanesulfonate. Surprisingly, some bright particles that covered by some thin flakes are observed from the high-magnification SEM images (Fig. S4c,d), presumably to be in-situ reduced highly conductive Cu0 nanoparticles. Conversely, this electrolyte byproduct and the new phase of Cu disappear during the subsequent charging process, accompanied by the enhancement of the diffraction peak of the main active component. Surprisingly, the charging product can match well with the Zn3(OH)2V2O7.2H2O ( defined as ZnVO, PDF#87-0417), not CuVO, implying that CuVO has been transformed into ZnVO through a complete charge-discharge cycle. This phenomenon can be explained by the following: during the discharge process, the Cu-O bond breaks to form a reduced metallic Cu0, and the embedded zinc ions can be bonded with O atoms to readjust the interlayer structure, eventually forming a frame structure constructed through alternating Zn–O with tetrahedral layer and V–O with octahedral layer, namely ZnVO. Since the electronegativity of Zn is stronger than that of Cu, the formed Zn-O bond is much more stable than the Cu-O bond [26], so the resultant ZnVO can be stably present. The irreversible phase transition in the first cycle can be recognized as an important factor for the unsatisfactory columbic efficiency and subsequent capacity decay. As shown in Fig. 3b, this ZnVO phase dominates the subsequent insertion and extraction process, and only the expansion and contraction of the crystal lattice is noticed. Compared with the discharge state, the peak in the charged state shift towards 10
lower 2θ angles (partial enlargement of Fig. 3b), corresponding to the increase of the lattice distance in view of Bragg's equation, which can be attributed to the extraction of Zn2+. Furthermore, the generation of Cu0 can still be detected in the discharge state, which can effectively enhance the electrical conductivity of electrode materials. In addition, a discharge byproduct of Cu(OH)2·H2O occurs in the 2nd and 3rd cycles, which is ubiquitous and difficult to avoid in aqueous electrolyte [27]. The illustration of the reaction mechanism of the CuVO electrode is depicted in Fig. S5.
The evolution of chemical composition and element valence states of CuVO upon cycling was investigated by utilizing ex-situ XPS measurements. As can be seen from the core level Zn 2p XPS spectrum displayed in Fig. 3c, the pristine CuVO does not contain Zn element because no signal for zinc is captured. While discharging to 0.2 V, a pair of distinct peaks at binding energies of 1044.8 (Zn 2p1/2) and 1021.6 eV (Zn 2p3/2) are observed, confirming the successful embedding of Zn2+ into CuVO. It is noteworthy that the peak of Zn2p still exists in the subsequent full-charge state but reduce in intensity, which is related to the phase transition from CuVO to ZnVO and the incomplete extraction of Zn2+. Fig. 3d displays the core level XPS spectra of Cu element, in which two main peaks situated at 955.2 and 935.5 eV can be ascribed to the typical Cu2+ line shape of Cu 2p1/2 and Cu 2p3/2, respectively, and the peak in the range of 940-946 eV corresponds to the Cu2+ satellite response [28,29]. Note that a small amount of Cu0 might be generated during the hydrothermal process [11]. Relative to pristine state, most of Cu2+ is reduced to Cu+ and Cu0, and the 11
characteristic satellite peak representing Cu2+ disappears in the discharge state. After recharging to 1.6 V, a Cu2+ feature appears, implying the partial oxidation of Cu0 and Cu+ accompanied by de-intercalation of Zn2+. The V2p spectrum in Fig. 3e also reveals a similar phenomenon that the insertion and extraction of Zn2+ can drive the redox reaction between V5+ and V4+.
For electrode materials involving the (de)-intercalation mechanism, it is necessary and meaningful to have a comprehensive insight into the electrochemical kinetics. Fig. 4a exhibits the CV curves of CuVO at various sweep speeds ranging from 0.1 to 1.0 mV s-1. It should be pointed out that the peak current intensity gradually enhances as the sweep speed increases. Generally speaking, the peak current (i) versus the sweep speed (v) satisfy a power exponential relationship: i=avb, where a and b are alterable parameters [30]. Based on the formula, the value of b can be determined by the fitted slopes of log (i) vs. log (v), which can be used to estimate the electrochemical rate-limiting step and thus reveal the charge storage mechanism. In particular, the b-value closer to 1.0 suggests a surface-dominated capacitive behavior, whereas b=0.5 represents a total diffusion driven process. As can be seen in Fig. 4b, the calculated b values of peak 1, 2, 5, 6 are 0.81, 0.90, 0.95, and 0.85, respectively, implying that the capacitive effect contributes most to the capacity. To be more exact, the specific capacitance contribution percentages can be quantified through the following formula: i=k1v+k2v1/2, in which the first half (k1v) represents the fraction of current derived from the surface capacitive behavior, and the latter 12
(k2v1/2) stands for the contribution determined by the diffusion-induced insertion process [31]. At a fixed sweep speed of 0.8 mV s-1, about 77.2% of the current is provided by the capacitive properties (Fig. 4c). In addition, the capacitance contribution at other scan speeds is also calculated, as presented in Fig. 4d. The results show that within the investigated range, the capacitive effect holds dominant position throughout the entire electrochemical process, and its contribution proportion gradually increases with improvement of the scan rates. It is generally believed that a high capacitance contribution is helpful for executing high rate capability. To further inquire into the Zn2+ diffusion and transport ability in the CuVO electrode, galvanostatic intermittent titration technique (GITT) measurement was carried out based on the previous method. During a single GITT process, the voltage variation under constant current display a good linear relationship with the root of pulse time τ1/2 (Fig. S6). Fig. 4e exhibits the GITT profiles and corresponding calculated Zn2+ diffusion coefficients (DZn). As expected, the CuVO cathode presents a high DZn values of 10-9-10-12 cm2 s−1, which is more competitive than those of other cathodes including V10O24·12H2O [32], KV3O8 [33], VO2 (B) nanofibers [8], Z0.5N2VPF [34], and K2V6O16·1.57H2O [33]. Excellent diffusion kinetics is the guarantee of high specific capacity and outstanding rate performance. It's worth noting that the Zn2+ ion diffusivity is apparently higher on charge than on discharge at early stage, which can be explained by the following reasons: firstly, some in-situ formed metallic Cu0 has not been oxidized at the initial stage of charging, which endow the electrode high electrical conductivity and fast electron transfer, leading to rapid extraction of Zn2+; 13
secondly, at the initial stage of discharge, Zn2+ will be embedded in the bulk of the electrode active particles through a long diffusion distance and a large resistance, while the charging process is reversed, and Zn2+ at the surface and interface will be preferentially and rapidly extracted. In addition, a large fluctuation of Zn2+ diffusion coefficient is observed during the charging process. The D value decreases on the charge platform, while it rises suddenly when reaching the next platform, and then decreases. The diffusion of zinc ions between two platforms is relatively easier, but it is more difficult on the platforms due to the redox reactions [35].
To visually demonstrate the potential of our assembled Zn//CuVO batteries, the Ragone plots showed in Fig. 5a were proposed to compare with other ZIBs systems reported in the literature (see Fig. S7 for detailed calculation process). As can be observed, our Zn//CuVO cell delivers a high energy density of 228 Wh kg−1 at a power density of 96 W kg−1, and still achieves an energy density of 109 Wh kg−1 at an extra high power density of 2075 W kg−1 based on the mass of the cathode material, which is obviously superior to the previously recorded cell system based on VS2 [36], Zn3[Fe(CN)6]2
[37],
Mn2O3
[38],
ZnHCF@MnO2
[39],
Todorokite
[40],
Zn3V2O7(OH)2.2H2O [18], and ZnMn2O4 cathodes [41]. The actual application value of our proposed Zn//CuVO cells was further confirmed by driving a 1.5 V temperature sensor through a prototype device (two coin batteries in series, Fig. 5b). Impressively, it can supply power to maintain the temperature sensor for working about 6 h. 14
4. Conclusion In this work, CuVO nanoribbon was prepared via a simple hydrothermal procedure, and evaluated as a cathode material for ZIBs. After the first GCD cycling, the CuVO electrode undergoes an irreversible phase transition to form a stable ZnVO phase, and then zinc ions are repeatedly inserted / extracted into / from the ZnVO lattice. Meanwhile, the inserted Zn2+ could drive the valence change of Cu element to form a highly conductive metallic Cu0 matrix, which is beneficial to electron transport. In addition, the assembled Zn//CuVO cell exhibits impressive EIS results, which could be attributed to the electrochemical performance. Benefiting from the above findings, the CuVO cathode displays a superior specific capacity of 216 mAh g−1 at 0.1A g−1, outstanding cycle stability (89.3% capacity retention after 500 cycles at 0.5 A g−1), and excellent rate capability. This research not only expands the choice of cathode materials for ZIBs, but also stimulates the utilization of the CuVO electrode in other aqueous metal ion batteries.
Declarations of interest None.
Acknowledgements This work was supported by the Natural Science Foundation of China (no. 21371180) and Hunan Provincial Science and Technology Plan Project (no. 15
2017TP1001)
Appendix A. Supplementary data Supplementary data related to this article can be found at…
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23
Fig. 1. (a) Crystal structure, (b) XRD patterns, (c) Raman spectra, (d) SEM, (e) TEM, and (f) HRTEM images of CuVO.
24
Fig. 2. Electrochemical performance of the CuVO electrode. (a) CV profiles at 0.5 mV s-1, (b) GCD curves for the initial ten cycles at 0.2 A g-1, (c) cycling stability at 0.2 A g-1, (d) rate performance, (e) GCD curves obtained at different current densities, (f) cycle capability at 0.5 A g-1, the inset is the GCD curves between 200th to 220th cycles.
25
Fig. 3. Ex-situ XRD patterns of CuVO obtained during the (a) 1st, and (b) 2nd and 3rd cycles at different states. Ex-situ XPS spectra of (c) Zn 2p, (d) Cu 2p, and (e) V 2p in pristine, fully discharged, and charged states.
26
Fig. 4. (a) CV profiles of CuVO at varied scan rates, (b) Logarithmic plots of current and scan rates, (c) capacitive contribution at 0.8 mV s-1, (d) contribution ratios arising from capacitance or diffusion-limited process at various sweep speeds, (e) GITT curves and calculated DZn values in the charge/discharge process.
27
Fig. 5. (a) Comparison of the Ragone plots of Zn//CuVO battery with other previously recorded Zn-based cell system, (b) Digital photos of our assembled Zn//CuVO batteries powering a 1.5 V temperature sensor.
28
A new cathode material of CuVO is demonstrated to be potential in ZIBs. A hybrid mechanism consisting of conversion and insertion/extraction is proposed. Cu0 matrix generated upon cycling can provide high electronic conductivity.
Author Contribution Statement
CRediT author statement: Linlin Chen: Data curation, Roles/Writing - original draft, Writing - review & editing. Zhanhong Yang: Conceptualization, Methodology, Funding acquisition, Supervision. Jian Wu: Investigation. Hongzhe Chen: Formal analysis, Visualization. Jinlei Meng: Validation.
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships □
that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: