Rechargeable Aqueous Zn–V2O5 Battery with High Energy Density

May 16, 2018 - (19,20) Nonetheless, the development of AZBs is plagued by the limitation of advanced cathodes with high energy density and long cycle ...
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Rechargeable Aqueous Zn−V2O5 Battery with High Energy Density and Long Cycle Life Ning Zhang,*,†,§ Yang Dong,† Ming Jia,† Xu Bian,† Yuanyuan Wang,† Mande Qiu,† Jianzhong Xu,† Yongchang Liu,*,‡,§ Lifang Jiao,§ and Fangyi Cheng§ †

College of Chemistry & Environmental Science, Key Laboratory of Medicinal Chemistry and Molecular Diagnosis (Ministry of Education), Hebei University, Baoding 071002, China ‡ Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China § Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: We report an aqueous Zn−V2O5 battery chemistry employing commercial V2O5 cathode, Zn anode, and 3 M Zn(CF3SO3)2 electrolyte. We elucidate the Zn-storage mechanism in the V2O5 cathode to be that hydrated Zn2+ can reversibly (de)intercalate through the layered structure. The function of the cointercalated H2O is revealed to be shielding the electrostatic interactions between Zn2+ and the host framework, accounting for the enhanced kinetics. In addition, the pristine bulk V2O5 gradually evolves into porous nanosheets upon cycling, providing more active sites for Zn2+ storage and thus rendering an initial capacity increase. As a consequence, a reversible capacity of 470 mAh g−1 at 0.2 A g−1 and a long-term cyclability with 91.1% capacity rentention over 4000 cycles at 5 A g−1 are achieved. The combination of the good battery performance, safety, scalable materials synthesis, and facile cell assembly indicates this aqueous Zn−V2O5 system is promising for stationary grid storage applications.

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and long cycle life, mainly attributed to the heavy mass and high polarization of divalent Zn2+. Manganese oxides and Prussian blue analogues have been initially studied for Zn2+ intercalation, but they typically display either poor cycling life or limited specific capacity.21−27 Preaddition of Mn2+ salt in mild aqueous electrolytes is proposed to improve the cycling stability of MnO2 polymorphs,17,28 while the rate capability still cannot meet the application standard. Very recently, layered vanadium-based compounds have been reported as potential host materials for AZBs, due to the natural abundance and multiple oxidation states of vanadium.29−35 For example, Ca0.25V2O5·xH2O nanobelts,7 ZnxV2O5·nH2O nanobelts,16 Zn3V2O7(OH)2· 2H2O nanowires,29 and hydrated V2O5·nH2O/graphene composite30 showed considerable specific capacity and respectable cycling performance. To capitalize on these features, anhydrous vanadium pentoxide (V2O5) holds great potential as a cathode material for AZBs, owing to the highest theoretical Zn-storage capacity of 589 mAh g−1 (based on the two-electron redox center (vanadium)) among various

dvanced battery technologies with high safety and low cost are highly desirable for applications in consumer electronics, electric vehicles, and grid-scale energy storage.1−4 Although lithium-ion batteries (LIBs) possess high energy density, their large-scale application is limited by the safety issues associated with flammable organic electrolytes and the growing concerns regarding the availability of Li resource. Rechargeable aqueous batteries have been considered as promising alternatives for stationary grid-level storage of renewable energies because of their high safety, less rigorous manufacturing conditions, and environmental friendliness.5−10 Furthermore, water-based electrolytes endow much higher ionic conductivity than their nonaqueous counterparts, favoring high rate capability. In this regard, aqueous zinc batteries (AZBs) hold particular promise because Zn features large-scale production, high capacity (820 mAh g−1 ), and good compatibility with water.11−15 Recently developed AZBs employing mild acidic electrolytes (e.g., 1 M ZnSO4, pH ∼ 4.0) highly improve the stability of the metallic Zn electrode,16−18 while it suffers from the dendrite and byproduct issues prevalent in traditional alkaline electrolytes (e.g., KOH).19,20 Nonetheless, the development of AZBs is plagued by the limitation of advanced cathodes with high energy density © XXXX American Chemical Society

Received: April 10, 2018 Accepted: May 16, 2018

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DOI: 10.1021/acsenergylett.8b00565 ACS Energy Lett. 2018, 3, 1366−1372

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Figure 1. (a) Schematic illustration of the rechargeable aqueous Zn−V2O5 battery chemistry. (b) Galvanostatic cycling performance at 0.2, 0.5, and 1.0 A g−1 and the corresponding Coulombic efficiency at 0.2 A g−1.

Figure 2. (a) Rate capability and (b) the corresponding discharge−charge profiles at various current densities. (c) Ragone plots of this aqueous Zn−V2O5 battery and aqueous zinc batteries using other reported cathode materials. Energy density values are based on the cathodes only. (d) Long-cycling performance at 5.0 A g−1. Inset shows the capacity evolution in the initial 19 cycles. (e) The corresponding dQ/dV curves of the selected cycles at 5.0 A g−1.

vanadium-based oxides. However, vanadium-based oxides for Zn2+ intercalation generally display an inadequate lifespan in diluted aqueous electrolytes. Recent studies show that the highconcentration aqueous electrolyte can highly improve the cycling stability and expand the operation potential window of aqueous ion batteries.36−40 Accordingly, extending this strategy to an aqueous Zn−V2O5 battery system would be interesting. In addition, although V2O5 has been widely tested as electrodes in LIBs and the charge storage mechanism is wellestablished,41−44 the intercalation chemistry of multivalent cations (i.e., Zn2+) in this host lattice has scarcely been reported and remains elusive. Herein, we report a rechargeable aqueous Zn−V2O5 battery system with high energy density and long cycle life, based on ball-milled commercial V 2 O 5 cathode, Zn anode, and concentrated 3 M Zn(CF3SO3)2 electrolyte. The layered V2O5 cathode allows a high reversibility for hydrated Zn2+

(de)intercalation (Figure 1a), as evidenced by electrochemical measurements, X-ray diffraction (XRD) analysis, scanning/ transmission electron microscopy (SEM/TEM), and X-ray photoelectron spectroscopy (XPS). The co-intercalated H2O molecules have been shown to buffer the high charge density of Zn2+, rendering fast cation transfer and high rate capability. In addition, the morphological evolution from the original bulk V2O5 cathode to the complete porous nanosheets is revealed, accounting for the initial capacity increase and enabling a high utilization of active materials. In the concentrated aqueous electrolyte, the V2O5 cathode delivers a high capacity of 470 mAh g−1 at 0.2 A g−1 (Figure 1b) and simultaneously exhibits a high energy density (274 Wh kg−1 at 7100 W kg−1) with a longterm cyclability (91.1% capacity retention over 4000 cycles at a high current rate up to 5 A g−1). The V2O5 cathode is simply prepared by ball-milling commercial V2O5 powder with 20 wt% graphite for 180 min 1367

DOI: 10.1021/acsenergylett.8b00565 ACS Energy Lett. 2018, 3, 1366−1372

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Figure 3. (a) CV curves of the V2O5 cathode at different sweep rates, (b) log i vs log v plots based on the CV profiles at the different oxidation/ reduction states, and (c) CV profile at 1.2 mV s−1 showing the capacitive contribution (blue region) to the total current.

Moreover, this aqueous Zn−V2O5 battery demonstrates superior rate performance, as shown in Figure 2a. After an initial capacity increase, reversible capacities of 460, 453, 445, 424, and 396 mAh g−1 are characterized at 0.5, 1.0, 2.0, 4.0, and 8.0 A g−1, respectively, with 10 cycles staying at each rate. Even at 10.0 A g−1, a significantly high capacity of 386 mAh g−1 is achieved, accompanied by a high capacity retention (83.7% compared to 460 mAh g−1 at 0.5 A g−1). More impressively, when the current density shifts back to 0.2 A g−1 after such a high-rate cycling (120 cycles), the reversible capacity recovers to 465 mAh g−1 immediately, showing a strong tolerance for rapid Zn2+ (de)intercalation. The corresponding discharge− charge curves at different rates are given in Figure 2b, delivering an average discharge voltage of 0.72 V. The excellent rate capability is further reflected in the Ragone plots (specific energy vs specific power) by comparison with the reported αMnO2,17 β-MnO2,28 δ-MnO2,45 todorokite,46 Zn1.86Mn2O4,39 Zn3[Fe(CN)6]2 (ZnHCF),25 KCuFe(CN)6 (CuHCF),27 V2O5· nH2O/GN,30 and Zn0.25V2O5·nH2O16 cathodes for AZBs (Figure 2c). To the best of our knowledge, the V2O5 electrode exhibits the highest energy density at high power (322 Wh kg−1 at 710 W kg−1; 274 Wh kg−1 at 7100 W kg−1) among the reported cathodes. To evaluate the long-term cycling stability, this Zn−V2O5 battery is galvanostatically discharged−charged at a high current density of 5.0 A g−1 (Figure 2d). The inset displays the initial capacity evolution (19 cycles) at slow rates. Remarkably, a high capacity retention of 99.3% with ∼100% CE is achieved over 1000 cycles, demonstrating a high durability. Even after 4000 repeated cycles, the reversible capacity sustains 372 mAh g−1 with 91.1% capacity retention. Such a long cycling life for the cathode material of AZBs has scarcely been reported previously. Figure 2e presents the corresponding differential capacity (dQ/dV) curves at selected states. The two couples of broad peaks, attributed to the insertion−extraction reactions, are well overlapped with tiny shift. This result further manifests the high reversibility of the V2O5 cathode. In addition, it is particularly noteworthy that this aqueous Zn−V2O5 battery can also operate at rough conditions. At a low temperature (−10 °C), the capacity gradually increases up to 220 mAh g−1 after 100 cycles with ∼100% CE (Figure S11). When working at a high temperature (50 °C), the cell can realize a higher initial discharge capacity of 378 mAh g−1 compared with the capacity at room temperature (323 mAh g−1, Figure 1b) and exhibits a favored activation process with a stable capacity of 476 mAh g−1 only after the initial 7 cycles (Figure S12). These results suggest that this aqueous Zn−V2O5 system can meet the commercial requirements for wide applications in special circumstances.

(the details are described in Experimental Section in the Supporting Information). The XRD pattern (Figure S1) reveals the high purity of ball-milled V2O5 powder, in line with the standard values of orthogonal phase (space group Pmmn (59), JCPDS no. 41-1426). SEM image (Figure S2) shows the uniform distribution of conductive graphite and V2O5, and the particle size of V2O5 remains around 1−5 μm. To evaluate the electrochemical performance, coin-type Zn−V2O5 cells are safely assembled in ambient air, employing the ball-milled V2O5 cathode, Zn foil anode, 3 M Zn(CF3SO3)2 aqueous electrolyte, and glass fiber separator. All the specific capacity values in this study are calculated based on the pristine V2O5 mass. Figure 1b shows the highly reversible and durable cycling performance of the aqueous Zn−V2O5 cells at 0.2, 0.5, and 1.0 A g−1. An initial discharge capacity of 323 mAh g−1 at 0.2 A g−1 with a high Coulombic efficiency (CE) of 92.3% is delivered. After 15 cycles, the reversible capacity stabilizes at a high value of 470 mAh g−1, corresponding to a 3.2-electron redox process. The capacity rise within the first 15 cycles indicates the gradually increased utilization of active materials (to be discussed below). Furthermore, the electrode polarization decreases with cycling (Figure S3) because of the reduced charge-transfer resistance as viewed from the electrochemical impedance spectroscopy (EIS, Figure S4). This kind of capacity increase disappears at around 25 cycles and 50 cycles for higher current densities of 0.5 and 1.0 A g−1, respectively. Finally, the reversible discharge capacities maintain 465 mAh g−1 at 0.5 A g−1 and 455 mAh g−1 at 1.0 A g−1 (Figure 1b). The corresponding Coulombic efficiencies at 0.5 A g−1 and 1.0 A g−1 are shown in Figure S5. In addition, the V2O5 cathodes with higher loadings of 5 and 10 mg cm−2 can deliver reversible capacities of 455 mAh g−1 (after 25 cycles) and 433 mAh g−1 (after 35 cycles) at 0.2 A g−1, respectively, corresponding to areal capacities of 2.28 mAh cm−2 and 4.33 mAh cm−2 (Figure S6). Note that the concentrated 3 M Zn(CF3SO3)2 electrolyte supports a better cyclic stability than the diluted electrolyte (e.g., 1 M Zn(CF 3 SO 3 ) 2 , Figure S7). This is because the highconcentration electrolyte can effectively decrease the water activity and water-induced side reactions.37−39 In addition, the Zn anode has been investigated to understand the highperformance of Zn−V2O5 cell chemistry (Figure S8,9). Neither dendritic morphology nor formation of byproducts such as Zn(OH)2 or ZnO is detected in the post-mortem examination of the cells after 100 cycles, favoring the cycling stability of Zn− V2O5 batteries. The graphite electrode affords negligible Znstorage capacity of ∼3 mAh g−1 in aqueous electrolyte at 0.05 A g−1 (Figure S10). 1368

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Figure 4. SEM images of V2O5 cathodes (a) at the pristine state and after the (b) 1st, (c) 10th, and (d) 100th cycles. Scale bars are 1 μm. (e, f) TEM images of the V2O5 cathode after 100 cycles. Scale bars are 500 and 100 nm, respectively.

Figure 5. (a) Typical charge−discharge curves of the aqueous Zn−V2O5 cell in the 1st, 2nd, and 20th cycles at 0.1 A g−1. The points A−J mark the states where data are collected for XRD analysis. (b) XRD patterns of the cathode materials at selected states. (c) XPS spectra of the cathodes at pristine, fully discharged, and charged states in the first cycle. (d) STEM image with the corresponding (e) EDS line scanning curves and (f−h) elemental mapping images of the first fully discharged electrode.

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deintercalation processes, accounting for the initial capacity increase and the high-rate capability of the V2O5 cathode. The reaction mechanism of the V2O5 electrode has been elucidated by a combined study of electrochemical measurements, XRD, TEM, and XPS analyses. Figure 5a displays the representative discharge−charge profiles at the 1st, 2nd, and 20th cycles. Upon cycling, the capacity increases and the overpotential gradually reduces. This is attributed to an activation process of the V2O5 electrode, as evidenced by the SEM/TEM (Figure 4) and EIS observations (Figure S4). To probe the structural evolution of V2O5, ex situ XRD patterns (Figure 5b) are performed at selected states (marked points in Figure 5a). On the first discharging (A → D), the characteristic peaks of V2O5 gradually weaken, and new phase peaks arise. After full discharge, the peaks emerging at 6.60°, 13.19°, 19.76°, and 33.14° can be assigned to the set of (00l) reflections from a layered ZnxV2O5·nH2O phase.16,30 This result indicates that the pristine layered V2O5 undergoes structural evolution and transforms into a layered ZnxV2O5·nH2O with the interlayer spacing ((001) plane) enlarged from 4.4 to 13.4 Å, due to the co-insertion of Zn2+ and H2O. Furthermore, the subsequent charging process (E → F) allows the reverse structural evolution of discharging, accompanied by the extraction of hydrated Zn2+. The reversible reaction of the Zn−V2O5 battery is schematically illustrated in Figure 1a, corresponding to an equation of V2O5 + xZn2+ + 2xe− + nH2O ↔ ZnxV2O5·nH2O, where x denotes the number of reversibly inserted Zn2+ and is up to 1.6 in this study based on the electrochemical behavior. In the 2nd and 20th cycles, the signals of layered compounds can reversibly strengthen/weaken upon hydrated Zn2+ insertion/ extraction. In the initial several cycles, the presence of V2O5 can be observed at discharge states but is not discernible over 20 cycles, confirming the increased utilization of active materials. In addition, the charge-shielding function of the co-intercalated water is revealed by the fact that the V2O5 cathode delivers only limited capacity with large overpotential and a low initial CE (75.9%) in nonaqueous electrolyte (Zn(CF3SO3)2 in acetonitrile), as shown in Figure S17. In the hybrid electrolyte [Zn(CF3SO3)2 in acetonitrile−water mixed solvent (volume ratio, 1:1)], the cathode allows much improved Zn-storage capacity with a higher initial CE of 79.8% (Figure S18). The facilitated charge transfer in aqueous electrolyte is also evidenced by EIS analysis (Figure S19). In addition, despite the divalent nature of Zn2+, the decent Zn2+ diffusion coefficient in the V2O5 cathode (Figure S13) is revealed, further demonstrating that the co-intercalated water can work as a charge-screening media during the redox reactions. Similar observation of the screened charge behavior induced by hydrated compounds has also been recorded for other multivalent ions (e.g., Mg2+) insertion into layered oxides.50 To gain insight into the variation of V oxidation states during the (de)intercalation process, we perform the XPS characterization as displayed in Figure 5c. At the pristine state, the V5+ signal (2p 3/2 : 517.1 eV) accompanied by a weak V 4+ component (2p3/2: 515.4 eV) is observed in the V 2p XPS region. This is because the surface of the original V2O5 is slightly reduced by the graphite during a ball-milling synthesis process. After the first discharge, the intensity of the V4+ signal increases and the V3+ component apparently appears, as a consequence of Zn2+ intercalation. Upon charging, the pristine V 2p spectrum could be recovered, indicating the high reversibility. In addition, to eliminate the impact of precipitated electrolyte salt, the discharged electrode was rinsed with water

To explain the high-rate performance, we analyze the redox pseudocapacitance-like contribution in the V2O5 cathode by investigating the kinetics to separate the capacitive-controlled and diffusion-controlled capacities. Figure 3a shows the cyclic voltammetry (CV) curves of the V2O5 cathode at various scan rates from 0.4 to 2.0 mV s−1. Two couples of redox peaks in CV curves are in consistent with the stable discharge−charge profiles as shown in Figure 2b. The capacitive effect of the battery system can be calculated using the relation i = avb, where i is the current density, v is the scan rate, and a and b are adjustable parameters.47−49 For analytical purposes, we rearrange the aforementioned relation slightly to log i = b × log v + log a. When b approaches 0.5, a faradic intercalation dominates the process; when the b value is close to 1.0, a capacitive response is indicated. Based on the log i versus log v plots (Figure 3b), the b values of peaks 1−4 (shown in Figure 3a) are 0.71, 0.98, 0.75, and 0.88, respectively, suggesting that the charge storage process is synergistically controlled by the capacitive and diffusion behaviors. This leads to a fast Zn2+ diffusion kinetics, enabling the high-rate performance. Furthermore, the capacitive contribution can be quantified by separating the current (i) at a fixed potential (V) into capacitive effect (k1v) and diffusion-controlled insertion (k2v1/2), according to the equation of i(V) = k1v + k2v1/2.48 Figure 3c depicts the typical CV profile at 1.2 mV s−1 for the capacitive current (blue region) compared with the total current. Around 48.5% of the total charge comes from the capacitive contribution, which accounts for the unprecedentedly high rate capability of the V2O5 cathode. Compared with the recently reported layered vanadium-based electrodes [e.g., Ca0.25V2O5·xH2O nanobelts (76% capacitive contribution)7 and Zn3V2O7(OH)2·2H2O nanowires (62% capacitive contribution)29], the capacitive contribution in this work is lower (48.5%), which is mainly attributed to the larger particle size of the V2O5 cathode (Figure S2). In addition, the galvanostatic intermittent titration technique (GITT) was carried out to determine the diffusion coefficient of Zn2+ (DZn) in the V2O5 cathode (see details in Figure S13). The DZn value is estimated to be around 10−10−10−11 cm−2 s−1, demonstrating the decent kinetics. The morphological and structural evolution of the V2O5 cathode during cycling is also recorded. Figure 4a−d shows the SEM images of V2O5 electrodes at selected states of pristine and after the 1st, 10th, and 100th cycles, respectively. Obviously, the initial bulk V2O5 morphology gradually develops into the complete thin and smooth nanosheets upon cycling, which is further revealed in the low-magnification SEM image (Figure S14). In addition, TEM analysis indicates that the pristine solid V2O5 cathode (Figure S15) is expanded and exfoliated and turns into a porous structure after 100 cycles (Figure 4e,f). Figure S16 presents the N2 adsorption− desorption isotherms of the cathode material after 100 cycles, exhibiting a typical type IV behavior with a distinct hysteresis loop in the relative pressure (P/P0) region from 0.45 to 1, implying its porous nature. According to the Brunauer− Emmett−Teller (BET) method, the specific surface area of the cathode is significantly increased from pristine 13.6 to 118.4 m2 g−1 after 100 cycles. This result is mainly attributed to the repeated Zn2+ insertion−extraction through the layered V2O5 framework. The unique structure evolution with porous structure generation could provide more active Zn-storage sites and low energy barrier for the Zn2+ intercalation− 1370

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the Central Universities (FRF-TP-16-078A1); and 111 project (B12015).

for elemental dispersive spectroscopy (EDS) analysis. The scanning transmission electron microscopy (STEM, Figure 5d) and the corresponding line-scanning curves (Figure 5e) along with elemental mapping images (Figure 5f−h) reveal the uniform distribution of Zn, V, and O elements, further confirming the Zn2+ intercalation reaction. In conclusion, a high-performance aqueous Zn−V2O5 battery system composed of commercial V2O5 cathode, Zn anode, and 3 M Zn(CF3SO3)2 electrolyte is elaborately designed. Upon cycling, hydrated Zn2+ can reversibly insert/extract into/from the layered V2O5 structure, and the co-intercalated H2O can effectively shield the electrostatic reactions between Zn2+ ions and the host anions. In the meantime, the pristine solid morphology is gradually developed into porous nanosheets with repeated Zn2+ (de)intercalation. In addition, the concentrated 3 M Zn(CF3SO3)2 electrolyte is demonstrated to favor the cyclic stability. These merits coupled with the pseudocapacitive behavior of the cathode can synergistically accelerate the mass diffusion of electrons and ions, buffer the strain/stress generated during Zn2+ diffusion, and enable a high utilization of the active materials. Thus, this aqueous Zn−V2O5 battery exhibits a high reversible capacity of 470 mAh g−1 at 0.2 A g−1, a high energy density of 274 Wh kg−1 at 7100 W kg−1 (based on the cathode), and a long-term cycling stability with 91.1% capacity retention after 4000 cycles at 5 A g−1. More impressively, it can work well in rough conditions at both low (−10 °C) and high (50 °C) temperatures. As a result of the exceptional electrochemical performance, the high safety of aqueous electrolyte, the low-cost of source materials, and the facile cell assembly, this rechargeable aqueous Zn−V2O5 battery chemistry holds great potential for large-scale energy storage applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00565. Experimental section, additional characterization of materials (XRD patterns, SEM images, TEM images, BET data, etc.), additional electrochemical performance (charge−discharge curves, GITT analysis, EIS profiles, cycling performance in diluted aqueous electrolyte and organic electrolyte, cyclability data at high and low temperatures (−10 and 50 °C), etc.) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ning Zhang: 0000-0002-6176-7278 Lifang Jiao: 0000-0002-4676-997X Fangyi Cheng: 0000-0002-9400-1500 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Project of One Province, One University, National Postdoctoral Program for Innovative Talents (BX201600014); Fundamental Research Funds for 1371

DOI: 10.1021/acsenergylett.8b00565 ACS Energy Lett. 2018, 3, 1366−1372

Letter

ACS Energy Letters

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DOI: 10.1021/acsenergylett.8b00565 ACS Energy Lett. 2018, 3, 1366−1372