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Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery
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Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery Xinliang Li a, 1, Longtao Ma a, 1, Yuwei Zhao a, Qi Yang a, Donghong Wang a, Zhaodong Huang a, Guojin Liang a, Funian Mo a, Zhuoxin Liu a, Chunyi Zhi a, b, * a b
Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China Shenzhen Research Institute, City University of Hong Kong, High-Tech Zone, Nanshan District, Shenzhen 518057, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2019 Received in revised form 16 October 2019 Accepted 19 October 2019 Available online xxx
Zinc batteries are at the forefront of aqueous energy storage due to their intrinsic safety and low cost. Various vanadium oxides stand out among the few cathode candidates. Herein, we demonstrate a new heterogeneous vanadium oxide nanowire with V2O5$nH2O shell and V3O7$H2O core, named h-VOW, as a promising cathode candidate for Zn batteries. Thanks to the synergies of multivalent states of vanadium elements associated with inherent high conductivity, in an aqueous electrolyte, h-VOW/Zn battery delivers high discharge capacity (455 mAh g1 at 0.1 A g1), energy density (340 Wh kg1) and power density (9105 W kg1). Moreover, the gel electrolyte imparts satisfied deformability and weather resistance to the flexible quasi-solid-state battery, allowing it to withstand extremely harsh conditions without catastrophic failure. © 2019 Elsevier Ltd. All rights reserved.
Keywords: V2O5 Cathode Aqueous battery Zinc anode
1. Introduction The renewable power supply has become an urgent technical obstacle that restricts the application of portable electronic products [1,2]. Especially in the field of wearable devices, more stringent requirements such as extreme safety, flexibility, weather resistance and stable output have been proposed [3e6]. Therefore, aqueous batteries with inherent safety feature have received ever-growing attention compared with traditional organic batteries [7e9]. Out of many reported aqueous metal ions Al-, Na-, K-, Fe-, Mg-based and non-metal ions secondary batteries Hþ, NHþ 4 , aqueous Zn ion battery (AZIB) should be highlighted, when taking their high theoretical capacity, low redox potential, great accessibility into account [10e19]. Especially, unlike counterparts mentioned above, metallic Zn anodes, which does not require any pretreatment, and more importantly, they can be striped/plated in the aqueous electrolyte [1,20]. On the other hand, employment of gel electrolytes satisfies the stringently mechanical requirements of flexible devices on aqueous batteries. The quasi-solid physical structure
* Corresponding author. Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China. E-mail address: [email protected] (C. Zhi). 1 X.L. Li and L.T. Ma contribute equally to this work.
eliminates the arbitrary flow of the liquid electrolyte while imparting flexibility and deformability to the electrolyte films, which is not available in solid electrolytes [5,21e25]. In recent years, significant progress has been achieved in the research of AZIB system based on manganese oxide, Prussian blue, vanadium oxide, and other cathodes [11,13,26,27]. To date, the voltage platforms of the former two can exceed 1.7 V. For example, 1.95 V of MnO2 and 1.75 V of ZnHCF [28]. Nevertheless, high discharge capacity, voltage platform, and significant lifespan are often not achievable simultaneously [27,29]. Despite this, vanadium oxide should be a promising candidate because its superior capacity (>300 mAh g1) makes up for the inadequate voltage platform (about 0.8 V), especially in the wearable electronics field that focuses more on the former. Various types of research have been devoted to them [30,31]. Recently, as Lu et al. reported, V2O5 equipped with the layered structure that facilitated ion shuttle, tended to exhibit a more stable and reversible crystal structure during the repeated Zn2þ insertion/extraction ions, which was decisive for the cycle life [32]. Moreover, Mai and Liang et al. revealed the positive effect of water pre-intercalation on electrochemical performance of V2O5 cathode [30,33]. Benefiting from the proposed “lubricating effect”, crystal water resulted in expanded layers spacing and weakened the strong electrostatic interaction between Zn2þ and V2O5 layers, thereby reducing the ions diffusion resistance and optimizing the reaction kinetics. Furthermore, Wang
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et al. demonstrated that the existing mixed valence states of V (V4þ/ V5þ) benefited electronic transportation and the related rate performance of H2V3O8 cathode [34,35]. These attempts uncover the enormous potential of vanadium oxides as cathode for AZIB. In this work, we synthesized the hydrated hybrid vanadium oxide nanowires with V2O5$nH2O shell and V3O7$H2O core via a facile hydrothermal process. Their electrochemical behaviors were investigated in the high-concentrated salt electrolyte (Zn(CF3SO3)2 þ LiTFSI) system. Consequently, aqueous h-VOW/Zn battery delivered the impressive discharge capacity of about 455 mAh g1 at 0.1 A g1 and capacity retention of 85% over 1200 cycles. Also, the superior specific energy of 340 Wh kg1 and specific power of 9105 W kg1 were achieved. Moreover, by employing a gel electrolyte film that contains high-concentrated salt featured with a low freezing point, quasi-solid-state batteries exhibited satisfying mechanical properties and frost resistance, which could cope with numerous demanding scenarios without catastrophic failure. 2. Results and discussions h-VOW was synthesized by a facile hydrothermal process at 200 C with the acetic acid additive, as depicted in the proposed schematic (Fig. 1a). The precursor commercial V2O5 featured with particle morphology (Fig. 1b) ultimately evolves into uniform onedimensional nanowire under high temperature and pressure conditions. The resulted h-VOW holds a high-aspect-ratio, which is precisely 10e200 nm in diameter and over 100 mm in length, as shown in Fig. 1c and d. After annealing at 250 C to decrease the crystal water, the morphology of h-VOW-250 is almost unchanged (Fig. S1). TEM images reveal that h-VOW is comprised of two heterogeneous phases that are uniformly distributed along the axis and formed a typical coreeshell structure, as displayed in Fig. 1e. Their diameters are about 31 and 39 nm, respectively. The novel heterogeneous interface derives from V atoms rearrangement and water intercalation, similar to previous reports [34,36]. The EDS mapping images show the homogeneous distribution of V and O elements in h-VOW. Moreover, HRTEM image representing the white circle region displays two lattice fringes of 0.63 and 0.29 nm (Fig. 1f). XRD pattern further qualitatively determines that the above two lattice spacings correspond to (210) crystal plane of V3O7$H2O (Orthorhombic phase; PDF: 85-2401; cell: 16.93 9.36 3.64) and (601) crystal plane of V2O5$nH2O (Monoclinic phase; PDF: 07-0332; cell: 17.43 3.65 12.25), respectively. Furthermore, the diffraction peaks at 2q ¼ 6.52 , 8.22 and 30.83 are indexed into (001), (101) and (601) crystal planes of V2O5$nH2O, while the peaks at 2q ¼ 10.62 and so on stand for V3O7$H2O (Fig. 1g) [37e41]. Obviously, the corresponding lattice spacings of h-VOW far exceed those of precursor V2O5 particle, especially for (001) plane. In this case, the larger interlayer distance favors ions shutting and thus enhance diffusion kinetics [33]. Based on the K-value method, the calculated content of V3O7$H2O in hVOW is about 72% [42,43]. Fig. 1h gives the TG curve of h-VOWs between 30 and 450 C that indicates the whole crystal water content in h-VOWs is about 3.2e3.5%. As a result, each V2O5 molecule contains 0.25e0.55 water molecule, meaning that parameter n is 0.25e0.55. Raman spectrum is given in Fig. 1i, in which the peaks at 141.9 cm1 corresponds to V2O2 stretching, 284.2 and 408.4 cm1 correspond to V]O vibration, 690.7 cm1 corresponds to V2eO stretching, and 993.7 cm1 corresponds to V]O stretching originated from terminal oxygen, respectively [44e46]. Besides, the signals of V and O elements are detected by XPS technique, as shown in Fig. 1j. Specifically, the multivalent V is constituted by the V4þ located at 515.9 and 523.7 eV and V5þ located at 517.4 and 525.0 eV [34,36,40]. More importantly, after
the structural evolution, h-VOW holds a much larger specific surface area of about 21.7 m2 g1 and almost three times that of V2O5 particles, suggesting more active sites associated with superior reaction kinetic (Fig. 1k). The electrochemical performance is conducted using coin type 2032 batteries, and Zn metal plate is directly employed as the anode. Fig. 2a displays the prolonged cyclic performance. Noted that h-VOW exhibits the decent discharge capacity of about 250 mAh g1 at 0.5 A g1 and capacity retention of 85% after 1200 cycles, whereas the discharge capacity of V2O5 particle is only 100 mAh g1. Besides, rate performance curves at different current densities ranging from 0.1 to 10 A g1 of h-VOW (Fig. 2b) reflect the specific discharge capacity up to 455 mAh g1 is realized at 0.1 A g1 with a discharge platform of about 0.85 V. Even at the ultrahigh current density of 10 A g1, 75 mAh g1 can also be delivered. Detailed rate data given in Fig. 2c further reveal the significant electrochemical activity enhancement of h-VOW that is more obvious in low current density, compared to V2O5 particle, which should be ascribed to the optimized crystal structure as well as water pre-intercalation. After moving the crystal water by calcining at 250 C, the attenuated capacity of 98 mAh g1 at 0.5 A g1 of h-VOW-250 also verifies this point (Fig. S2). The cyclic voltammetry (CV) results of h-VOW collected in a voltage window of 0.4e1.6 V are given in Fig. 3d. There are two pairs redox peaks, 0.75/0.81 V and 0.98/1.15 V, which is in conformity with the above discharge curves [33]. Besides, stable cyclic performance is also elaborated by the almost overlapping first four CV curves, resulting from the stable crystal structure. Apparently, the redox peaks of CV curves are constant except for a tiny change in position when scan rates are set as 0.1e0.4 mV s1, echoing the excellent rate capability (Fig. 2e). Calculated b values corresponding to peak 1 and 2 are 0.68 and 0.80, respectively, and they reflect the dominated diffusioncontrolled battery behaviors [47]. On the other hand, for Zn anode, its electrochemical behavior during cycling was further investigated using a symmetric battery. The stable polarization voltage less than 0.2 V (Fig. S3) indicates the stability of Zn plating/ striping during the whole cyclic process, except for the initial several cycles [48,49]. Notably, after a long period of more than 310 h, Zn anode surface remains smooth, and no zinc dendrites are observed, as depicted in insets, agreeing well with the corresponding XRD patterns (Fig. S4). Consequently, the h-VOWs can deliver the high energy density of 340 Wh kg1 and power density of 9105 W kg1, which are highly competitive among the reported V-based counterparts including Zn0.25V2O5$nH2O, Ca0.25V2O5$nH2O, Na2V6O16$3H2O, V2O5$nH2O, and place decent locations in Ragone plots in Fig. 2g (The details are given in Table S1, Supporting information). Ex-XRD, XPS, and TEM-EDS technologies were conducted to investigate the ions intercalation/deintercalation behavior during the cyclic process. Fig. 3a shows the galvanostatic discharge/ charge profiles of h-VOW at 0.1 A g1 of the second cycle and the marked points stand for the different selected voltage states for phase and structure analysis, corresponding to XRD patterns in Fig. 3b. First, the reversible and stable crystal structure of h-VOW is verified by the inverse trends of XRD patterns during discharge and charge processes. More importantly, it can be observed that the peak representing (200) plane, shift positively from 10.69 to 10.85 during discharging, and then does the opposite in the subsequent charge process. Actually, decreased lattice spacing occurred during discharge can be elaborated by the significant electrostatic interaction between inserted cations (Zn2þ and Liþ), as previously reported [38,39]. Also, more direct evidence of Zn2þ and Liþ insertion is supplied by XPS spectra (Figs. 3d and S5). As seen from the survey XPS spectrum and detailed Zn 2p and Li 1s spectra of h-VOW at discharging state (0.4 V) in Fig. 3d, the new
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Fig. 1. Synthetic process, phase composition, and morphology analysis of h-VOW. (a) Schematics of the fabrication of h-VOW. SEM images of (b) commercial V2O5 particles, (c, d) hVOW. (e) TEM image with EDS mapping, including V and O elements. (f) HRTEM image. (g) XRD pattern. (h) TG curves. Inset is the calculated phase composition using the K-value method. (i) Raman spectra. (j) XPS spectra. (k) N2 adsorptionedesorption isotherms of V2O5 and h-VOW.
signals of Zn2þ and Li1þ are found simultaneously, as the consequence of two above ions insertion in this stage. Moreover, the intensity of Zn signal is much stronger than the Li, meaning the Zn2þ-dominated insertion mechanism, although Liþ concentration in the electrolyte is more advantageous. Specifically, the calculated Li/Zn atomic ration is only about 0.01 according to XPS data. This can be explained by the larger radius of Li (0.76 Å) associated with weak kinetic than that of Zn (0.74 Å) [50]. More importantly, Liþ ions are severely confined with the surrounding water molecular
and thus release the spacing and diffusion resistance of Zn2þ, resulting in that Zn2þ precede Liþ insertion in the unique electrolyte [8,48]. What's more, HRTEM associated with EDS data collected at 0.4 V reveal that the Zn2þ ions distribute in both outside and inside of coreeshell h-VOW (Fig. 3e). Consequently, after ions intercalation, the lattice spacing corresponding to (200) plane at 2q ¼ 10.62 of V3O7$H2O, which should be 0.833 nm, reduces to 0.815 nm and echoes the forward shift of the above XRD pattern (Fig. 3f).
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Fig. 2. Electrochemical performance of Zn/h-VOW batteries. (a) Prolonged cyclic performance of V2O5 and h-VOW. (b) Galvanostatic chargeedischarge (GCD) curves of h-VOW at different current densities of 0.1e10 A g1. (c) Rate performance of V2O5 and h-VOW at different current densities of 0.1e10 A g1. (d). Cyclic voltammetry (CV) curves of h-VOW at 0.2 mV s1. (e) CV curves of h-VOW at different scan rates of 0.1e0.4 mV s1. (f) The fit curves of log (current, i) vs log (scan rate, v). (g) The Ragone plots in comparison with other reported aqueous Zn/VXOY counterparts. The details are given in Table S1 in supporting information.
The complexity of the actual application scenario has spawned the popularity of quasi-solid-state batteries. Among them, hydrogels, which hold excellent mechanical properties and ionic conductivity, stand out as the promising electrolyte substrate but are limited by the freezing problem at a sub-zero temperature [51,52]. Interestingly, the high-concentrated salt electrolyte can reduce the hydrogel freezing point and thus solve the above issues [23]. Thus, quasi-solid-state battery with anti-freezing feature was prepared using hydrogel-based electrolyte, as shown in the schematic in Fig. 4a. The resulted sandwich-like battery shows excellent flexibility and can be bent (Fig. S6), benefiting from the flexible anode that is prepared by electrodepositing Zn metal on carbon cloth (Fig. S7), and sticky PAM electrolyte that is synthesized by in-situ polymerization of high-concentrated salts and acrylamide monomers (Fig. S8a). The PAM electrolyte can be up to 18 18 cm2 and appears porous feature after freeze-drying treatment. Also, the PAM electrolyte shows significant tensile performance and can stretch to more than 1300% of its original length, as displayed in Fig. S8bed and Movie S1. Enormously differing from traditional counterparts, quasi-solid-state batteries are tolerant to severe damage and could be tailored to any shape without failure, as shown in Fig. 4bed and Movie S2. Actually, the cut triangle is a separate battery with the same voltage as the parent. Putting their cathode and anode together can achieve a tandem effect and illuminate a LED light (Fig. 4eeg and Movie S3). Additionally, the flexibility feature enables the
battery to directly act as the straps with energy storage function to power a bracelet (Fig. 4h). Nevertheless, considering the increase in thickness due to excessive use of anode and electrolyte, it is expected that the volumetric capacity and energy are inferior to those of organic counterparts [53,54]. On the other hand, the ultra-low freezing point, which stems from the high-concentrated electrolyte, allows the battery to withstand temperature below 23 C (Fig. 4i). The impaired ionic conductivity of PAM electrolyte due to the decreasing temperatures should be responsible for the decaying capacity, which is calculated to decrease from 9.1 mS cm1 at 25 C to 1.5 mS cm1 at 15 C (Fig. S9). Specifically, the discharge capacity loss at 25 C is almost negligible compared to liquid electrolyte system, about 60 mAh g1 capacity can be delivered at 15 C and remains stable during further cycling (Fig. 4j). The detailed GCD curves given in Fig. 4k shows the discharge platform remains unchanged. Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2019.100361. 3. Conclusion In summary, we have studied the electrochemical performance of the h-VOW/Zn battery in an aqueous electrolyte. The h-VOW cathode holds a distinguished discharge capacity of more than 455 mAh g1 at 0.1 A g1. After 1200 cycles, 85% capacity retention
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Fig. 3. Ions intercalation/deintercalation mechanism of h-VOW during cycling. (a) GCD curves at 0.1 A g1 of the second cycle and the marked points represent the selected voltage states for phase composition analysis. (b, c) Ex-XRD patterns. (d) Zn 2p and Li 1s XPS spectra at 0.4 V. (e) TEM image associated with EDS mapping of h-VOW at 0.4 V. (f) HRTEM image corresponding to (002) plane of V3O7$H2O.
is retained, indicating a capacity decay rate of only 0.0125% per cycle. Meanwhile, the resulting high specific energy (340 Wh kg1) and power (9105 W kg1) highlight its energy storage potential. In addition, the mechanism by which the layered heterostructure and pre-intercalation water are responsible is elucidated. Furthermore, with excellent deformability and weather resistance, flexible quasisolid-state batteries based on PAM electrolyte perform well in a variety of simulated demanding scenarios. Even in the extreme conditions of cutting, freezing, and burning, flexibility can be well maintained and catastrophic failure does not occur, demonstrating their great potential for application in flexible energy storage devices. 4. Experiment 4.1. Preparation of vanadium oxide nanowires To synthesis vanadium oxide nanowires (h-VOW), 0.712 g of commercial V2O5 (AR grade, Aladdin), 4 ml acetic acid was added in 56 ml DI water and stirred continuously for 2 h. Then the above mixture was transferred into a stainless-steel reactor with 100 ml volume. After being kept at 200 C for 72 h, the green precipitates were washed with ethanol for several times and dried in a vacuum oven at room temperature for 24 h. The obtained green powder was
h-VOW. Moreover, the h-VOW was thermally treated at 250 C for 120 min under Ar atmosphere named h-VOW-250 as a control sample. 4.2. Preparation of flexible electrodes and quasi-solid-state hydrogel electrolyte Carbon cloth was selected as the flexible substrate for both cathode and anode. For the cathode, active materials including V2O5, h-VOW, h-VOW-250, were directly mixed with conductive carbon black and polyvinylidene fluoride (PVDF; AR grade, Aladdin) according to the mass ratio of 7:2:1. After stirring in Nmethyl-2-pyrrolidone (NMP; AR grade, Aladdin) solvent for 5 h. The homogenous liquid slurry was transferred to carbon cloth substrates, followed by vacuum drying at 50 C for 36 h. Flexible Zn anode was prepared by an electrodeposition method. Specifically, electrodeposition voltage and time were set as 0.9 V (vs Zn) and 4000 s, while employing Zn metal plate as the reference and counter electrodes, carbon cloth as the working electrode, and 2 M ZnSO4 solution as the electrolyte, respectively. For quasisolid-state electrolyte, polyacrylamide (PAM) hydrogel was employed as the flexible substrate and high concentrated salt as the electrolyte. Typically, the aqueous mixture solution containing 21 M LiTFSI þ 1 M Zn(CF3SO3)2 was first prepared by stirring
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Fig. 4. The quasi-solid-state batteries. (a) Schematic illustration of the quasi-solid-state battery. (b) Photographs of the quasi-solid-state battery before the cut, (c) after two cuts, (d) after four cuts. (e) Voltage measurement of the triangle batteries in individual and (f) in series. (g) Photographs of a LED light powered by two triangle batteries in series. (h) Photographs of a sports bracelet powered by two flexible batteries in series. (i) Demo of the quasi-solid-state battery operating in solid ice at <23 C. (j) Cyclic performance of the quasi-solid-state battery at different temperatures of 15 to 25 C at 0.5 A g1. (k) Galvanostatic chargeedischarge curves of the quasi-solid-state battery at different temperatures of 15 to 25 C at 0.5 A g1.
at room temperature. After that, acrylamide (1320 mg), ammonium persulfate (16 mg) and N,N0 -methylenebisacrylamide (0.52 mg) were successively into the above mixture solution (6 ml) and stirred for 3 h at room temperature. Then, injecting the solution into a flat glass or PTFE mold and holding it at about 70 C for 90 min. The resulted thickness of hydrogel electrolyte film could be controlled by adjusting the spacing between the mold plates.
4.3. Materials characterization Phase composition transition was confirmed by X-ray diffraction equipment (XRD, Bruker D2 Phaser) with Cu Ka radiation at 30 kV, and the scan rate and range used were 0.01 s per step and 5e50 , respectively. Moreover, the weight ratio of the two phases in hVOW was calculated by the K-value method based on the XRD pattern [42]. Microstructure information and elements distribution of cathodes and Zn anode were revealed by the scanning electron microscope coupled with (SEM, FEI Quanta 450 FEG) and transmission electron microscopy (TEM; JEOL-2001F) and their EDX components, respectively. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was employed to further analyze the surface elements changes of the electrodes at different states. Nitrogen sorption and desorption isotherms associated with the specific surface area were derived from Micromeritics ASAP 2460 system.
The pre-treatment process, drying the samples at 50 C for 24 h in a vacuum oven, was conducted before the test. 4.4. Electrochemical measurements Both coin-2032 and quasi-solid-state batteries were assembled for electrochemical measurements, with the former using a zinc sheet as the anode electrode. For the latter, flexible electrodes were bonded to the outer surfaces of the sticky hydrogel electrolyte film without any encapsulation. The cyclic performance, electrochemical properties of batteries were characterized by the LAND CT2001A device and electrochemical workstation CHI 760D. Multifunctional oven provided a low-temperature environment. AC impedance curves of electrolyte film at different temperatures were measured using CHI 760D workstation. Two stainless steel plates with 1 cm 1 cm size were bonded to the outer surfaces of the sticky hydrogel electrolyte film with 1 cm 1 cm 1 mm and formed a sandwich-like structure without any encapsulation. By adjusting temperature through multi-functional oven, all data were collected. 4.5. b values The parameter b was calculated according to the equation as follows: log(i) ¼ b log(v) þ log(a). Where v stands for the
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voltammetric scan rate, i represents peak current response, a is stand for constants. 4.6. The ionic conductivity of hydrogel electrolyte at different temperatures The ionic conductivity (s) at different temperatures of hydrogel electrolyte were calculated based on the corresponding AC impedance data according to the following equation: s ¼ L/A$R. Where A is the area (10 mm 10 mm), L stands for the thickness (1 mm), and R represents ohmic resistance, respectively. Declaration of competing interest The authors declare no conflict of interest. Acknowledgment This research was supported by GRF under Project N_CityU11305218. The work was also partially sponsored by the Science Technology and Innovation Committee of Shenzhen Municipality (the Grant No. JCYJ20170818103435068) and a grant from City University of Hong Kong (9667165). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2019.100361. References [1] [2] [3] [4] [5] [6] [7]
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Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery Xinliang Li a, 1, Longtao Ma a, 1, Yuwei Zhao a, Qi Yang a, Donghong Wang a, Zhaodong Huang a, Guojin Liang a, Funian Mo a, Zhuoxin Liu a, Chunyi Zhi a, b, * a b
Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China Shenzhen Research Institute, City University of Hong Kong, High-Tech Zone, Nanshan District, Shenzhen 518057, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2019 Received in revised form 16 October 2019 Accepted 19 October 2019 Available online xxx
Zinc batteries are at the forefront of aqueous energy storage due to their intrinsic safety and low cost. Various vanadium oxides stand out among the few cathode candidates. Herein, we demonstrate a new heterogeneous vanadium oxide nanowire with V2O5$nH2O shell and V3O7$H2O core, named h-VOW, as a promising cathode candidate for Zn batteries. Thanks to the synergies of multivalent states of vanadium elements associated with inherent high conductivity, in an aqueous electrolyte, h-VOW/Zn battery delivers high discharge capacity (455 mAh g1 at 0.1 A g1), energy density (340 Wh kg1) and power density (9105 W kg1). Moreover, the gel electrolyte imparts satisfied deformability and weather resistance to the flexible quasi-solid-state battery, allowing it to withstand extremely harsh conditions without catastrophic failure. © 2019 Elsevier Ltd. All rights reserved.
Keywords: V2O5 Cathode Aqueous battery Zinc anode
1. Introduction The renewable power supply has become an urgent technical obstacle that restricts the application of portable electronic products [1,2]. Especially in the field of wearable devices, more stringent requirements such as extreme safety, flexibility, weather resistance and stable output have been proposed [3e6]. Therefore, aqueous batteries with inherent safety feature have received ever-growing attention compared with traditional organic batteries [7e9]. Out of many reported aqueous metal ions Al-, Na-, K-, Fe-, Mg-based and non-metal ions secondary batteries Hþ, NHþ 4 , aqueous Zn ion battery (AZIB) should be highlighted, when taking their high theoretical capacity, low redox potential, great accessibility into account [10e19]. Especially, unlike counterparts mentioned above, metallic Zn anodes, which does not require any pretreatment, and more importantly, they can be striped/plated in the aqueous electrolyte [1,20]. On the other hand, employment of gel electrolytes satisfies the stringently mechanical requirements of flexible devices on aqueous batteries. The quasi-solid physical structure
* Corresponding author. Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China. E-mail address: [email protected] (C. Zhi). 1 X.L. Li and L.T. Ma contribute equally to this work.
eliminates the arbitrary flow of the liquid electrolyte while imparting flexibility and deformability to the electrolyte films, which is not available in solid electrolytes [5,21e25]. In recent years, significant progress has been achieved in the research of AZIB system based on manganese oxide, Prussian blue, vanadium oxide, and other cathodes [11,13,26,27]. To date, the voltage platforms of the former two can exceed 1.7 V. For example, 1.95 V of MnO2 and 1.75 V of ZnHCF [28]. Nevertheless, high discharge capacity, voltage platform, and significant lifespan are often not achievable simultaneously [27,29]. Despite this, vanadium oxide should be a promising candidate because its superior capacity (>300 mAh g1) makes up for the inadequate voltage platform (about 0.8 V), especially in the wearable electronics field that focuses more on the former. Various types of research have been devoted to them [30,31]. Recently, as Lu et al. reported, V2O5 equipped with the layered structure that facilitated ion shuttle, tended to exhibit a more stable and reversible crystal structure during the repeated Zn2þ insertion/extraction ions, which was decisive for the cycle life [32]. Moreover, Mai and Liang et al. revealed the positive effect of water pre-intercalation on electrochemical performance of V2O5 cathode [30,33]. Benefiting from the proposed “lubricating effect”, crystal water resulted in expanded layers spacing and weakened the strong electrostatic interaction between Zn2þ and V2O5 layers, thereby reducing the ions diffusion resistance and optimizing the reaction kinetics. Furthermore, Wang
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Please cite this article as: X. Li et al., Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery, Materials Today Energy, https://doi.org/10.1016/j.mtener.2019.100361
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et al. demonstrated that the existing mixed valence states of V (V4þ/ V5þ) benefited electronic transportation and the related rate performance of H2V3O8 cathode [34,35]. These attempts uncover the enormous potential of vanadium oxides as cathode for AZIB. In this work, we synthesized the hydrated hybrid vanadium oxide nanowires with V2O5$nH2O shell and V3O7$H2O core via a facile hydrothermal process. Their electrochemical behaviors were investigated in the high-concentrated salt electrolyte (Zn(CF3SO3)2 þ LiTFSI) system. Consequently, aqueous h-VOW/Zn battery delivered the impressive discharge capacity of about 455 mAh g1 at 0.1 A g1 and capacity retention of 85% over 1200 cycles. Also, the superior specific energy of 340 Wh kg1 and specific power of 9105 W kg1 were achieved. Moreover, by employing a gel electrolyte film that contains high-concentrated salt featured with a low freezing point, quasi-solid-state batteries exhibited satisfying mechanical properties and frost resistance, which could cope with numerous demanding scenarios without catastrophic failure. 2. Results and discussions h-VOW was synthesized by a facile hydrothermal process at 200 C with the acetic acid additive, as depicted in the proposed schematic (Fig. 1a). The precursor commercial V2O5 featured with particle morphology (Fig. 1b) ultimately evolves into uniform onedimensional nanowire under high temperature and pressure conditions. The resulted h-VOW holds a high-aspect-ratio, which is precisely 10e200 nm in diameter and over 100 mm in length, as shown in Fig. 1c and d. After annealing at 250 C to decrease the crystal water, the morphology of h-VOW-250 is almost unchanged (Fig. S1). TEM images reveal that h-VOW is comprised of two heterogeneous phases that are uniformly distributed along the axis and formed a typical coreeshell structure, as displayed in Fig. 1e. Their diameters are about 31 and 39 nm, respectively. The novel heterogeneous interface derives from V atoms rearrangement and water intercalation, similar to previous reports [34,36]. The EDS mapping images show the homogeneous distribution of V and O elements in h-VOW. Moreover, HRTEM image representing the white circle region displays two lattice fringes of 0.63 and 0.29 nm (Fig. 1f). XRD pattern further qualitatively determines that the above two lattice spacings correspond to (210) crystal plane of V3O7$H2O (Orthorhombic phase; PDF: 85-2401; cell: 16.93 9.36 3.64) and (601) crystal plane of V2O5$nH2O (Monoclinic phase; PDF: 07-0332; cell: 17.43 3.65 12.25), respectively. Furthermore, the diffraction peaks at 2q ¼ 6.52 , 8.22 and 30.83 are indexed into (001), (101) and (601) crystal planes of V2O5$nH2O, while the peaks at 2q ¼ 10.62 and so on stand for V3O7$H2O (Fig. 1g) [37e41]. Obviously, the corresponding lattice spacings of h-VOW far exceed those of precursor V2O5 particle, especially for (001) plane. In this case, the larger interlayer distance favors ions shutting and thus enhance diffusion kinetics [33]. Based on the K-value method, the calculated content of V3O7$H2O in hVOW is about 72% [42,43]. Fig. 1h gives the TG curve of h-VOWs between 30 and 450 C that indicates the whole crystal water content in h-VOWs is about 3.2e3.5%. As a result, each V2O5 molecule contains 0.25e0.55 water molecule, meaning that parameter n is 0.25e0.55. Raman spectrum is given in Fig. 1i, in which the peaks at 141.9 cm1 corresponds to V2O2 stretching, 284.2 and 408.4 cm1 correspond to V]O vibration, 690.7 cm1 corresponds to V2eO stretching, and 993.7 cm1 corresponds to V]O stretching originated from terminal oxygen, respectively [44e46]. Besides, the signals of V and O elements are detected by XPS technique, as shown in Fig. 1j. Specifically, the multivalent V is constituted by the V4þ located at 515.9 and 523.7 eV and V5þ located at 517.4 and 525.0 eV [34,36,40]. More importantly, after
the structural evolution, h-VOW holds a much larger specific surface area of about 21.7 m2 g1 and almost three times that of V2O5 particles, suggesting more active sites associated with superior reaction kinetic (Fig. 1k). The electrochemical performance is conducted using coin type 2032 batteries, and Zn metal plate is directly employed as the anode. Fig. 2a displays the prolonged cyclic performance. Noted that h-VOW exhibits the decent discharge capacity of about 250 mAh g1 at 0.5 A g1 and capacity retention of 85% after 1200 cycles, whereas the discharge capacity of V2O5 particle is only 100 mAh g1. Besides, rate performance curves at different current densities ranging from 0.1 to 10 A g1 of h-VOW (Fig. 2b) reflect the specific discharge capacity up to 455 mAh g1 is realized at 0.1 A g1 with a discharge platform of about 0.85 V. Even at the ultrahigh current density of 10 A g1, 75 mAh g1 can also be delivered. Detailed rate data given in Fig. 2c further reveal the significant electrochemical activity enhancement of h-VOW that is more obvious in low current density, compared to V2O5 particle, which should be ascribed to the optimized crystal structure as well as water pre-intercalation. After moving the crystal water by calcining at 250 C, the attenuated capacity of 98 mAh g1 at 0.5 A g1 of h-VOW-250 also verifies this point (Fig. S2). The cyclic voltammetry (CV) results of h-VOW collected in a voltage window of 0.4e1.6 V are given in Fig. 3d. There are two pairs redox peaks, 0.75/0.81 V and 0.98/1.15 V, which is in conformity with the above discharge curves [33]. Besides, stable cyclic performance is also elaborated by the almost overlapping first four CV curves, resulting from the stable crystal structure. Apparently, the redox peaks of CV curves are constant except for a tiny change in position when scan rates are set as 0.1e0.4 mV s1, echoing the excellent rate capability (Fig. 2e). Calculated b values corresponding to peak 1 and 2 are 0.68 and 0.80, respectively, and they reflect the dominated diffusioncontrolled battery behaviors [47]. On the other hand, for Zn anode, its electrochemical behavior during cycling was further investigated using a symmetric battery. The stable polarization voltage less than 0.2 V (Fig. S3) indicates the stability of Zn plating/ striping during the whole cyclic process, except for the initial several cycles [48,49]. Notably, after a long period of more than 310 h, Zn anode surface remains smooth, and no zinc dendrites are observed, as depicted in insets, agreeing well with the corresponding XRD patterns (Fig. S4). Consequently, the h-VOWs can deliver the high energy density of 340 Wh kg1 and power density of 9105 W kg1, which are highly competitive among the reported V-based counterparts including Zn0.25V2O5$nH2O, Ca0.25V2O5$nH2O, Na2V6O16$3H2O, V2O5$nH2O, and place decent locations in Ragone plots in Fig. 2g (The details are given in Table S1, Supporting information). Ex-XRD, XPS, and TEM-EDS technologies were conducted to investigate the ions intercalation/deintercalation behavior during the cyclic process. Fig. 3a shows the galvanostatic discharge/ charge profiles of h-VOW at 0.1 A g1 of the second cycle and the marked points stand for the different selected voltage states for phase and structure analysis, corresponding to XRD patterns in Fig. 3b. First, the reversible and stable crystal structure of h-VOW is verified by the inverse trends of XRD patterns during discharge and charge processes. More importantly, it can be observed that the peak representing (200) plane, shift positively from 10.69 to 10.85 during discharging, and then does the opposite in the subsequent charge process. Actually, decreased lattice spacing occurred during discharge can be elaborated by the significant electrostatic interaction between inserted cations (Zn2þ and Liþ), as previously reported [38,39]. Also, more direct evidence of Zn2þ and Liþ insertion is supplied by XPS spectra (Figs. 3d and S5). As seen from the survey XPS spectrum and detailed Zn 2p and Li 1s spectra of h-VOW at discharging state (0.4 V) in Fig. 3d, the new
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Fig. 1. Synthetic process, phase composition, and morphology analysis of h-VOW. (a) Schematics of the fabrication of h-VOW. SEM images of (b) commercial V2O5 particles, (c, d) hVOW. (e) TEM image with EDS mapping, including V and O elements. (f) HRTEM image. (g) XRD pattern. (h) TG curves. Inset is the calculated phase composition using the K-value method. (i) Raman spectra. (j) XPS spectra. (k) N2 adsorptionedesorption isotherms of V2O5 and h-VOW.
signals of Zn2þ and Li1þ are found simultaneously, as the consequence of two above ions insertion in this stage. Moreover, the intensity of Zn signal is much stronger than the Li, meaning the Zn2þ-dominated insertion mechanism, although Liþ concentration in the electrolyte is more advantageous. Specifically, the calculated Li/Zn atomic ration is only about 0.01 according to XPS data. This can be explained by the larger radius of Li (0.76 Å) associated with weak kinetic than that of Zn (0.74 Å) [50]. More importantly, Liþ ions are severely confined with the surrounding water molecular
and thus release the spacing and diffusion resistance of Zn2þ, resulting in that Zn2þ precede Liþ insertion in the unique electrolyte [8,48]. What's more, HRTEM associated with EDS data collected at 0.4 V reveal that the Zn2þ ions distribute in both outside and inside of coreeshell h-VOW (Fig. 3e). Consequently, after ions intercalation, the lattice spacing corresponding to (200) plane at 2q ¼ 10.62 of V3O7$H2O, which should be 0.833 nm, reduces to 0.815 nm and echoes the forward shift of the above XRD pattern (Fig. 3f).
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Fig. 2. Electrochemical performance of Zn/h-VOW batteries. (a) Prolonged cyclic performance of V2O5 and h-VOW. (b) Galvanostatic chargeedischarge (GCD) curves of h-VOW at different current densities of 0.1e10 A g1. (c) Rate performance of V2O5 and h-VOW at different current densities of 0.1e10 A g1. (d). Cyclic voltammetry (CV) curves of h-VOW at 0.2 mV s1. (e) CV curves of h-VOW at different scan rates of 0.1e0.4 mV s1. (f) The fit curves of log (current, i) vs log (scan rate, v). (g) The Ragone plots in comparison with other reported aqueous Zn/VXOY counterparts. The details are given in Table S1 in supporting information.
The complexity of the actual application scenario has spawned the popularity of quasi-solid-state batteries. Among them, hydrogels, which hold excellent mechanical properties and ionic conductivity, stand out as the promising electrolyte substrate but are limited by the freezing problem at a sub-zero temperature [51,52]. Interestingly, the high-concentrated salt electrolyte can reduce the hydrogel freezing point and thus solve the above issues [23]. Thus, quasi-solid-state battery with anti-freezing feature was prepared using hydrogel-based electrolyte, as shown in the schematic in Fig. 4a. The resulted sandwich-like battery shows excellent flexibility and can be bent (Fig. S6), benefiting from the flexible anode that is prepared by electrodepositing Zn metal on carbon cloth (Fig. S7), and sticky PAM electrolyte that is synthesized by in-situ polymerization of high-concentrated salts and acrylamide monomers (Fig. S8a). The PAM electrolyte can be up to 18 18 cm2 and appears porous feature after freeze-drying treatment. Also, the PAM electrolyte shows significant tensile performance and can stretch to more than 1300% of its original length, as displayed in Fig. S8bed and Movie S1. Enormously differing from traditional counterparts, quasi-solid-state batteries are tolerant to severe damage and could be tailored to any shape without failure, as shown in Fig. 4bed and Movie S2. Actually, the cut triangle is a separate battery with the same voltage as the parent. Putting their cathode and anode together can achieve a tandem effect and illuminate a LED light (Fig. 4eeg and Movie S3). Additionally, the flexibility feature enables the
battery to directly act as the straps with energy storage function to power a bracelet (Fig. 4h). Nevertheless, considering the increase in thickness due to excessive use of anode and electrolyte, it is expected that the volumetric capacity and energy are inferior to those of organic counterparts [53,54]. On the other hand, the ultra-low freezing point, which stems from the high-concentrated electrolyte, allows the battery to withstand temperature below 23 C (Fig. 4i). The impaired ionic conductivity of PAM electrolyte due to the decreasing temperatures should be responsible for the decaying capacity, which is calculated to decrease from 9.1 mS cm1 at 25 C to 1.5 mS cm1 at 15 C (Fig. S9). Specifically, the discharge capacity loss at 25 C is almost negligible compared to liquid electrolyte system, about 60 mAh g1 capacity can be delivered at 15 C and remains stable during further cycling (Fig. 4j). The detailed GCD curves given in Fig. 4k shows the discharge platform remains unchanged. Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2019.100361. 3. Conclusion In summary, we have studied the electrochemical performance of the h-VOW/Zn battery in an aqueous electrolyte. The h-VOW cathode holds a distinguished discharge capacity of more than 455 mAh g1 at 0.1 A g1. After 1200 cycles, 85% capacity retention
Please cite this article as: X. Li et al., Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery, Materials Today Energy, https://doi.org/10.1016/j.mtener.2019.100361
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Fig. 3. Ions intercalation/deintercalation mechanism of h-VOW during cycling. (a) GCD curves at 0.1 A g1 of the second cycle and the marked points represent the selected voltage states for phase composition analysis. (b, c) Ex-XRD patterns. (d) Zn 2p and Li 1s XPS spectra at 0.4 V. (e) TEM image associated with EDS mapping of h-VOW at 0.4 V. (f) HRTEM image corresponding to (002) plane of V3O7$H2O.
is retained, indicating a capacity decay rate of only 0.0125% per cycle. Meanwhile, the resulting high specific energy (340 Wh kg1) and power (9105 W kg1) highlight its energy storage potential. In addition, the mechanism by which the layered heterostructure and pre-intercalation water are responsible is elucidated. Furthermore, with excellent deformability and weather resistance, flexible quasisolid-state batteries based on PAM electrolyte perform well in a variety of simulated demanding scenarios. Even in the extreme conditions of cutting, freezing, and burning, flexibility can be well maintained and catastrophic failure does not occur, demonstrating their great potential for application in flexible energy storage devices. 4. Experiment 4.1. Preparation of vanadium oxide nanowires To synthesis vanadium oxide nanowires (h-VOW), 0.712 g of commercial V2O5 (AR grade, Aladdin), 4 ml acetic acid was added in 56 ml DI water and stirred continuously for 2 h. Then the above mixture was transferred into a stainless-steel reactor with 100 ml volume. After being kept at 200 C for 72 h, the green precipitates were washed with ethanol for several times and dried in a vacuum oven at room temperature for 24 h. The obtained green powder was
h-VOW. Moreover, the h-VOW was thermally treated at 250 C for 120 min under Ar atmosphere named h-VOW-250 as a control sample. 4.2. Preparation of flexible electrodes and quasi-solid-state hydrogel electrolyte Carbon cloth was selected as the flexible substrate for both cathode and anode. For the cathode, active materials including V2O5, h-VOW, h-VOW-250, were directly mixed with conductive carbon black and polyvinylidene fluoride (PVDF; AR grade, Aladdin) according to the mass ratio of 7:2:1. After stirring in Nmethyl-2-pyrrolidone (NMP; AR grade, Aladdin) solvent for 5 h. The homogenous liquid slurry was transferred to carbon cloth substrates, followed by vacuum drying at 50 C for 36 h. Flexible Zn anode was prepared by an electrodeposition method. Specifically, electrodeposition voltage and time were set as 0.9 V (vs Zn) and 4000 s, while employing Zn metal plate as the reference and counter electrodes, carbon cloth as the working electrode, and 2 M ZnSO4 solution as the electrolyte, respectively. For quasisolid-state electrolyte, polyacrylamide (PAM) hydrogel was employed as the flexible substrate and high concentrated salt as the electrolyte. Typically, the aqueous mixture solution containing 21 M LiTFSI þ 1 M Zn(CF3SO3)2 was first prepared by stirring
Please cite this article as: X. Li et al., Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery, Materials Today Energy, https://doi.org/10.1016/j.mtener.2019.100361
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Fig. 4. The quasi-solid-state batteries. (a) Schematic illustration of the quasi-solid-state battery. (b) Photographs of the quasi-solid-state battery before the cut, (c) after two cuts, (d) after four cuts. (e) Voltage measurement of the triangle batteries in individual and (f) in series. (g) Photographs of a LED light powered by two triangle batteries in series. (h) Photographs of a sports bracelet powered by two flexible batteries in series. (i) Demo of the quasi-solid-state battery operating in solid ice at <23 C. (j) Cyclic performance of the quasi-solid-state battery at different temperatures of 15 to 25 C at 0.5 A g1. (k) Galvanostatic chargeedischarge curves of the quasi-solid-state battery at different temperatures of 15 to 25 C at 0.5 A g1.
at room temperature. After that, acrylamide (1320 mg), ammonium persulfate (16 mg) and N,N0 -methylenebisacrylamide (0.52 mg) were successively into the above mixture solution (6 ml) and stirred for 3 h at room temperature. Then, injecting the solution into a flat glass or PTFE mold and holding it at about 70 C for 90 min. The resulted thickness of hydrogel electrolyte film could be controlled by adjusting the spacing between the mold plates.
4.3. Materials characterization Phase composition transition was confirmed by X-ray diffraction equipment (XRD, Bruker D2 Phaser) with Cu Ka radiation at 30 kV, and the scan rate and range used were 0.01 s per step and 5e50 , respectively. Moreover, the weight ratio of the two phases in hVOW was calculated by the K-value method based on the XRD pattern [42]. Microstructure information and elements distribution of cathodes and Zn anode were revealed by the scanning electron microscope coupled with (SEM, FEI Quanta 450 FEG) and transmission electron microscopy (TEM; JEOL-2001F) and their EDX components, respectively. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was employed to further analyze the surface elements changes of the electrodes at different states. Nitrogen sorption and desorption isotherms associated with the specific surface area were derived from Micromeritics ASAP 2460 system.
The pre-treatment process, drying the samples at 50 C for 24 h in a vacuum oven, was conducted before the test. 4.4. Electrochemical measurements Both coin-2032 and quasi-solid-state batteries were assembled for electrochemical measurements, with the former using a zinc sheet as the anode electrode. For the latter, flexible electrodes were bonded to the outer surfaces of the sticky hydrogel electrolyte film without any encapsulation. The cyclic performance, electrochemical properties of batteries were characterized by the LAND CT2001A device and electrochemical workstation CHI 760D. Multifunctional oven provided a low-temperature environment. AC impedance curves of electrolyte film at different temperatures were measured using CHI 760D workstation. Two stainless steel plates with 1 cm 1 cm size were bonded to the outer surfaces of the sticky hydrogel electrolyte film with 1 cm 1 cm 1 mm and formed a sandwich-like structure without any encapsulation. By adjusting temperature through multi-functional oven, all data were collected. 4.5. b values The parameter b was calculated according to the equation as follows: log(i) ¼ b log(v) þ log(a). Where v stands for the
Please cite this article as: X. Li et al., Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery, Materials Today Energy, https://doi.org/10.1016/j.mtener.2019.100361
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voltammetric scan rate, i represents peak current response, a is stand for constants. 4.6. The ionic conductivity of hydrogel electrolyte at different temperatures The ionic conductivity (s) at different temperatures of hydrogel electrolyte were calculated based on the corresponding AC impedance data according to the following equation: s ¼ L/A$R. Where A is the area (10 mm 10 mm), L stands for the thickness (1 mm), and R represents ohmic resistance, respectively. Declaration of competing interest The authors declare no conflict of interest. Acknowledgment This research was supported by GRF under Project N_CityU11305218. The work was also partially sponsored by the Science Technology and Innovation Committee of Shenzhen Municipality (the Grant No. JCYJ20170818103435068) and a grant from City University of Hong Kong (9667165). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2019.100361. References [1] [2] [3] [4] [5] [6] [7]
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Please cite this article as: X. Li et al., Hydrated hybrid vanadium oxide nanowires as the superior cathode for aqueous Zn battery, Materials Today Energy, https://doi.org/10.1016/j.mtener.2019.100361