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A high-performance flexible aqueous Al ion rechargeable battery with long cycle life
Journal Pre-proof A High-performance Flexible Aqueous Al Ion Rechargeable Battery with Long Cycle Life Panpan Wang, Zhe Chen, Zhenyuan Ji, Yuping Feng, Jiaqi Wang, Jie Liu, Mengmeng Hu, Hua Wang, Jinbo Fei, Wei Gan, Yan Huang PII:
S2405-8297(19)30995-X
DOI:
https://doi.org/10.1016/j.ensm.2019.09.038
Reference:
ENSM 942
To appear in:
Energy Storage Materials
Received Date: 13 June 2019 Revised Date:
25 September 2019
Accepted Date: 28 September 2019
Please cite this article as: P. Wang, Z. Chen, Z. Ji, Y. Feng, J. Wang, J. Liu, M. Hu, H. Wang, J. Fei, W. Gan, Y. Huang, A High-performance Flexible Aqueous Al Ion Rechargeable Battery with Long Cycle Life, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.09.038. 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 Elsevier B.V. All rights reserved.
A High-performance Flexible Aqueous Al Ion Rechargeable Battery with Long Cycle Life Panpan Wanga,b,c, Zhe Chena,b,c, Zhenyuan Jia,b,c, Yuping Fenga,b,c, Jiaqi Wanga,b,c, Jie Liua,b,c, Mengmeng Hua,b,c, Hua Wanga,b,c, Jinbo Feia,b,c, Wei Gana,b,d and Yan Huanga,b,c* a
Flexible Printed Electronic Technology Center, Harbin Institute of Technology,
Shenzhen, 518055, China. b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of
Technology, Shenzhen, 518055, China. c
School of Materials Science and Engineering, Harbin Institute of Technology,
Shenzhen, 518055, China. d
School of Science, Harbin Institute of Technology, Shenzhen, 518055, China.
*E-mail: [email protected]
Declaration of interests The authors declare no competing financial interest.
A High-performance Flexible Aqueous Al Ion Rechargeable Battery with Long Cycle Life Abstract: With the increasing development of flexible and wearable electronic devices, exploration of advanced power supplies with excellent electrochemical performance, safety and flexibility is highly urgent. In light of natural abundance as well as three-electron redox properties, rechargeable Al ion batteries (AIBs) draw much attention while they still face great challenge of limited Al3+ storage material as well as poorly ionic conductive electrolyte. In our work, an intercalation MoO3 anode and VOPO4 cathode are proposed and mechanically robust gelatin-polyacrylamide hydrogel electrolyte is utilized to fabricate a safe and flexible high-performance rechargeable AIB. The as-assembled AIB exhibits high rate capability of 6 A g-1, high discharge capacity of 88 mAh g-1 and long cyclic stability of 86.2% capacity retention even after 2800 cycles. Besides, the AIB possesses high safety and mechanical flexibility properties, which could serve as reliable power source when even being bent, punctured or cut. Additionally, the aqueous AIB could power 1 m long and 100 cm2 electroluminescent panels, demonstrating their good Al storage property. Considering their favourable electrochemical performance, robustness and safety, the flexible aqueous AIB in our work is believed to open up new avenue in flexible and wearable energy. Keywords: flexible, Al ion battery, intercalation compound, long cyclic stability, wearable electronics
1
1. Introduction Nowadays, it is highly urgent to exploit advanced flexible power supplies to keep pace with the increasing development of portable, flexible and wearable electronic devices such as roll-up displays, smart mobile devices.[1-3] However, the traditional rechargeable lithium ion batteries cannot meet aforementioned criteria because of their toxic and flammable organic electrolyte as well as the rigid cell assembly.[4] Thus, exploitation of flexible metal ion battery technologies based on safer aqueous electrolyte and more abundant metal material such as calcium, magnesium, zinc and aluminum is critical to boost the evolution of flexible electronics.[5-8] In particular, rechargeable Al ion batteries (AIB) are intriguing in light of their natural abundance feature as well as three-electron redox properties that are capable of transporting more electrons relative to monovalent ion.[9, 10] However, the investigation of aqueous Al-ion batteries is still at the infancy stage. On one hand, host material capable of Al3+ insertion/extraction reversibly is very limited. The high charge-to-size ratio of Al3+ gives rise to stronger coulombic interaction between the host lattice and Al3+, which sluggishes the Al3+ solid state diffusion kinetics in the lattice.[11] Therefore, exploration of appropriate electrode materials with facilitated Al3+ intercalation kinetics remains a significant and fundamental scientific challenge.[12] The kinetics limitation of Al3+ places restriction on the selection of potential electrode material, which should possess highly opened crystal structure to allow facile ion insertion/extraction and rich redox centers to maintain charge neutrality 2
upon metal ions insertion. To date, the only reported electrode materials in aqueous Al ion system involved prussian blue compound[13, 14] and transition metal oxides (titanium dioxide,[15, 16] vanadium oxides,[17, 18] etc). An aqueous AIB consisted of TiO2 anode and prussian blue cathode was proposed, whereas it exhibits a maximum capacity of only 10.6 mAh g-1 and limited cycle life with low capacity retention of 58% after 2000 cycles.[19] Inspired by the prototype of lithium ion battery based on intercalation chemistry,[20] rechargeable aqueous AIB based on layered transition metal compound should also be feasible because of their adjustable interlayer engineering and rich redox reactions that is ideal for multivalent metal ion intercalation.[21-23] However, there was no related investigation on aqueous Al ion full battery based on intercalation mechanism to the best of our knowledge. In this respect, the construction of aqueous Al ion full cell systems based on layered transition metal compound is highly desirable for the further promotion of multivalent metal ion battery. Meanwhile, a mechanical robust electrolyte with high Al3+ionic conductivity is also an indispensable component for the flexible AIB devices. For the commonly utilized polyvinyl alcohol (PVA)-based gel electrolyte, the limited water retention and poor mechanical strength prevent their further application in flexible AIB.[26] Hydrogel electrolytes are proved to be powerful to fabricate the flexible energy storage devices due to their low cost, nontoxicity, chemical stability as well as attractive mechanical properties.[24-27] Whereas for the hydrogel with mannuronic acid unit, the strong interaction between Al3+ and the functional groups in polymer chains would make the 3
monomer highly crosslinked with Al3+ as ionic crosslinker, resulting in tough hydrogel but poor ionic activity.[28] To this end, the exploration of highly ionic conductive and durable electrolyte for multivalent Al3+cations is fairly tough but vital for the further development of extremely safe and rechargeable high-performance AIBs. In this work, a flexible aqueous Al-ion full battery with long cycling stability, high capacity, good rate capability and safety property was developed based on intercalation
compounds
with
layered
electrode
sandwiched
by
gelatin-polyacrylamide (PAM) hydrogel electrolyte. A capacity retention as high as 86.2% after 2800 cycles at 1 A g-1 was achieved, which was far better than the performance of the AIB reported so far. Additionally, it demonstrated a nice mechanical tolerance against severe working condition with the ability to power electronic watches or LED lights array even after puncturing or cutting. In terms of long-term stability, high storage capability and high safety, the flexible aqueous AIB exhibits great potential application in wearable electronic devices.
2. Results and Discussion A rechargeable aqueous Al ion battery based on intercalation compound is proposed and the working principle of aqueous Al ion battery is illustrated in Fig. 1a. The VOPO4 cathode displays a typical layered structure consisting of vertex-sharing VO6 octahedra linking to PO4 tetrahedra. The adjacent layers were held together through weak hydrogen bonds, which facilitate cation intercalation due to the rich 4
coordination circumstance of V atom. For the MoO3 anode, the layered structure is formed by stacking bilayer sheets of distorted MoO6 octahedra via van der Waals forces, which is also of great interest in electrochemical intercalation chemistry given its unique two dimensional (2D) layer structure and the ease coupling of multivalent-state Mo atom.[29] During the charge process for the full cell, Al ions would extract from the cathode, delivered by Al3+ conductive electrolyte and insert into anode host, accompanied with three electrons traversing in the external circuits to power a load, which is analogous to the ‘rocking-chair’ Li-ion batteries.[30] The XRD pattern of as-prepared MoO3 anode (Fig. 1b) is readily indexed to orthorhombic α-MoO3 phase with space group of Pbnm (JCPDS no. 76-1003). The strong diffraction peaks of (020), (040) and (060) planes implies that MoO3 possess a typical layered crystal structure with highly anisotropic growth orientation. From the FESEM and TEM images in Fig. 1c(i) and (ii), we can find that the synthesized MoO3 exhibits nanobelt morphology with the length around 2-4 µm and width around 300 nm. The HRTEM (Fig. 1c (iii)) image further disclosed clear lattice fringes of 3.6 and 4.0 Å along two different directions, corresponding to the (100) and (001) facet of MoO3, respectively. The selected area electron diffraction (SAED) pattern (Fig. 1c (iv)) can be indexed to the [010] zone, revealing the preferential growth along the c axis as well as the single crystallinity of MoO3, which agrees well with the XRD results.[31] Meanwhile, the XRD pattern of the obtained VOPO4 sample can be readily indexed into the tetragonal VOPO4•2H2O with the space group of P4/nmm (JCPDS No. 84-0111). Noticeably, the main (001) peak located at ~11.9o was correlated to the 5
interlayer distance d value of 7.4 Å, which is identical to the previous report according to Bragg’s equation (2dsinθ = nλ).[32] The SEM image (Fig. 1e) displays that VOPO4 possesses a flat, rectangle microsheet shape with lateral size of ~10 µm, and the TEM image in Fig. 1f further reveals that VOPO4 presents an individual sheet-like rectangle morphology.
Fig. 1. (a) Working principle schematic of aqueous Al ion battery based on intercalation electrtode. Structural and morphological characterization of MoO3 samples: (b) XRD patterns, (c) (i) SEM image; (ii) TEM image; (iii) HRTEM image and (iv) SAED pattern. Structural and morphological characterization of VOPO4 samples: (d) XRD patterns, (e) SEM image and (f) TEM images.
Herein, a gelatin-polyacrylamide hydrogel was utilized as flexible Al ion electrolyte with good ionic conductivity. The typical synthesis route of the 6
gelatin-PAM polymer electrolyte is illustrated in Fig. 2a. First, the pristine gelatin and potassium persulfate were added into the Al(NO3)3 solution. Sequentially, acrylamide (AM) monomers and N,N-methylenebisacrylamide (BIS) were added to the above solution. During the initial stage, the polyacrylamide (PAM) was grafted onto the gelatin chains via free radical polymerization. Afterwards, the mixture solution was injected into a cellulose membrane with heat treatment at 60 oC for 2.5 h. Thus, a covalent crosslinked gelatin-PAM hydrogel was formed in the cellulose fiber membrane. As shown in Fig. 2b, the achieved ionic conductivity of Al3+ gelatin-PAM polymer electrolyte is calculated to be over 20 mS cm-1, which were higher than those reported gel electrolyte including typical polyvinyl alcohol (PVA), gelatin, and poly(propylsulfonate
dimethylammonium
propylmethacrylamide)
(PPDP)
gels,[33-36] endowed the AIB with desired electrochemical performance. As a demonstration, the hydrogel electrolyte could connect a light-emitting diode (LED) circuit successfully as shown inset in Fig. 2b, further manifesting the favorable ionic conductivity of gelatin-PAM hydrogel electrolyte. The cross-section scanning electron microscopy (SEM) image of the freeze dried gelatin-PAM polymer was provided in Fig. 2c, the gelatin-PAM hydrogel is very thin with a thickness of 100 µm, which is also beneficial for the assembly and realization of flexible energy storage devices.
7
Fig. 2. (a) Schematic of the synthesis route to the gelatin-PAM electrolyte. (b) Ionic conductivity of our gelatin-PAM electrolyte compared with other gel electrolytes. Inset: photograph demonstration of the electrolyte as ionic conductor connected in circuit to light up LED successfully. (c) Cross-section SEM image of gelatin-PAM hydrogel electrolyte. Scale bars: 50 µm.
The electrochemical behavior of the MoO3 anode and VOPO4 cathode were examined by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) test in 1 M Al(NO3)3 electrolyte in a three-electrode configuration. Fig. 3a shows the CV curves of the MoO3 anode at different scan rates in a potential window of -0.6 to 0.4 V (vs. Ag/AgCl). A pair of well-defined redox peak located at -0.3 V/0.2 V (vs. Ag/AgCl)
was
detected
obivously,
corresponding
to
the
reversible
Al3+
deintercalation from and intercalation into the layered host structure that similar with the topotactic redox reaction mechanism.[37, 38] The high peak current of the 8
symmetrical cathodic and anodic redox peaks for all the CV curves indicate that the MoO3 nanobelt electrode possesses high electrochemical activity and good reversibility for Al3+ intercalation. Meanwhile, GCD curves of MoO3 anode under different current density were depicted in Fig. 3b. There exists a long and flat discharge plateau located around -0.2 V (vs. Ag/AgCl), and a slanted charge plateau centered from -0.1 V to 0.2 V (vs. Ag/AgCl), which corresponds well with the strong redox peaks observed in the CV curves in Fig. 3a. Importantly, the MoO3 electrode delivered a high specific discharge capacity of 308, 270, 251, 242 and 232 mAh g-1 at current densities of 1, 2, 4, 6 and 8 A g-1, respectively, further suggesting the good Al storage capability of the obtained MoO3 nanobelts. The high Al storage capacity was probably related to the more active crystallographic (010) planes, which is favourable for ion intercalation as described in the case of Li+ intercalation preferentially in the van der Waals spacing at the (010) plane.[39, 40] Herein, the cutting off potential of charge/discharge curves was set to be -0.3 V rather than -0.6 V (vs. Ag/AgCl), since the H2O decomposition phenomenon with H2 evolution might appear under more negative potential demonstrated by an ultralong discharge plateau located around -0.4 V as shown in Fig. S1. Fig. 3c shows the CV curves of the VOPO4 cathode in the voltage range of 0-1 V (vs. Ag/AgCl) at different scan rates. Two cathodic peaks centered around 0.1 V and 0.34 V (vs. Ag/AgCl) were observed obviously, which were probably related with the V5+/V4+ and V4+/V3+ redox induced by Al3+ insertion. Subsequently, two anodic peaks centered at 0.25 V and 0.55 V (vs. Ag/AgCl) were observed on the anodic process, 9
which was attributed the reduction reaction induced by the Al3+ extraction from the VOPO4. Additionally, the typical charge and discharge profiles of VOPO4 cathode at different current densities were displayed in Fig. 3d. During discharge, a slanted discharge plateau around 0.5 V that ranged from 0.6 V to 0.2 V was observed. Correspondingly, a charge voltage slope varied from 0.5 V to 0.8 V was exhibited, which comes from the insertion/extraction of Al3+ ions from VOPO4 host accomponied with the reversibly redox process for V5+/V4+ and V4+/V3+. Meanwhile, a high specific discharge capacity of 115, 92, 79, 69, 64 mAh g-1 is obtained at the current density of 1, 1.5, 2, 3, 4 A g-1, respecitvely, further suggesting the favourable kinetics of reversible Al3+ ion intercalation chemistry in VOPO4 host. Based on the results, the appropriate discharge potential of 0.54 V and -0.2 V (vs. Ag/AgCl) for VOPO4 and MoO3, respectively, making them suitable as the cathode and anode materials for aqueous Al ion full cell.
10
Fig. 3. Electrochemical performance of MoO3 and VOPO4 in the three-electrode system. (a) CV curves of the MoO3 at different scan rates. (b) Galvanostatic charge/discharge plots of the MoO3 anode at different current densities. (c) CV curves of the VOPO4 cathode at different scan rates. (d) Galvanostatic charge/discharge plots of the VOPO4 cathode at different current densities.
In order to verify the reversible intercalation/deintercalation behavior of Al3+ from the layered host structure, ex situ XRD and Raman was utilized to investigate the structural evolution of MoO3 anode and VOPO4 cathode at different Al ion insertion states. Fig. 4a provides the charge and discharge curves of MoO3 anode in 1 M Al(NO3)3 solution at current density of 1 A g-1, which exhibits a flat discharge plateau 11
and slanted charge plateau located around -0.2 V/0 V vs. Ag/AgCl. The selected states marked with A-H points were collected for XRD and Raman analysis. The peak at 12.9° corresponds to the (020) planes with a spacing of 0.693 nm. It should be noted that all of the diffraction peaks of (020), (040), and (060) shift to higher angle upon charging, which are induced by the Al3+ extraction from the layered structure. During the discharge process, the three main crystallographic plane reflection shift to lower angle, implying the interlayer d-spacing increased along with Al3+ ions intercalating. This phenomenon is typical for the intercalation electrode possessing the ability to insert or extract gust ion along with the expansion or shrink of lattice.[41] Meanwhile, ex situ Raman spectra are recorded as shown in Fig. 4c. The observed bands of 284 cm-1 are typically ascribed to the wagging mode for the double bond (O=Mo=O).[42] The bands located at 660, 816 and 991 cm-1 were assigned to the stretching vibration mode of oxygen atoms resulting from edge-shared, corner-shared oxygen (Mo-O) and unshared oxygen (Mo=O), respectively. As observed, no obvious peak shift or new Raman peak are detected upon the whole charge and discharge process, suggesting that no structural transition is involved during the Al3+ ion insertion and extraction process. For the case of VOPO4 cathode, the characteristic XRD diffraction peak of (001) plane shifted from 12.3° (initial state O point) to the high angle of 13.0° (discharged state A point) during the Al3+ pre-insertion process from the open circuit potential to 0 V (Fig. S2), corresponding to a slight decrease in the interlayer spacing of (001) plane. Specially, the interlayer distance decreased rather than increased upon guest ion 12
insertion, which was probably due to the strong electrostatic attraction between positively Al3+ and negatively oxygen atoms in the host materials. These adjacent layers are then brought closer by electrostatic interaction with the intercalated cation, which resulted in the decrease of the interlayer distance.[43] During the subsequent charge stage (A-E points in Fig. 4d), the attraction between Al3+ and oxygen as well as the repulsion between oxygen atoms in neighboring layers are weakened since Al3+ extracted from the host. Expectedly, the primary diffraction peak of (001) plane at the fully charged state shifts back to the pristine position, further indicating the reversible Al3+ intercalation chemistry into layered VOPO4 cathode. In addition, ex situ Raman was utilized to reveal the structural evolutions of VOPO4 during Al3+ intercalation process. As shown in Fig. 4e, the bands centered at 531, 560 and 920 cm-1 are typically ascribed to the symmetric bending and stretching vibrations of O-P-O modes, and the bands located at 687 and 1021 cm-1 can be attributed to bending vibrations of V-O and stretching vibrations of V=O modes, respectively.[44] During the Al3+ pre-insertion process (Fig. S2c), the blue shift of Raman peaks for O-P-O and V=O stretching mode was observed obviously, whereas in the charging process (Fig. 4e points A to E), the red shift of Raman peaks for O-P-O and V=O stretching mode was also noticeable. Obviously, the O-P-O and V=O stretching mode was strongly correlated with the interaction between the oxygen atoms from the P-O and V=O bond with the intercalated Al3+ guest cation. During the following discharge process, the blue shift of the characteristic Raman bands was detected once again with the intercalation of Al3+ into VOPO4 interlayers, which 13
further confirmed the fact that Al3+ could be intercalated into VOPO4 layers reversibly.
Fig. 4. The structural evolution of MoO3 anode and VOPO4 cathode in 1 M Al(NO3)3 electrolyte during Al ion insertion/extraction process. The points A-I represents the states where spectra are collected for XRD and Raman analysis: (a) Charge/discharge curve for MoO3 at current density of 1 A g-1. (b) Ex situ XRD pattern and (c) Ex situ Raman spectrum of the MoO3 anode at selected Al ion insertion/extraction states. (d) Charge/discharge curves for VOPO4 at current density of 1 A g-1. (e) Ex situ XRD pattern and (f) Ex situ Raman spectrum of the VOPO4 cathode at selected Al ion insertion/extraction states.
Moreover, the elemental mapping analysis was also employed to inspect the Al3+ intercalation behavior (Fig. 5). At fully Al3+ insertion state of MoO3, the distribution of Al element matches well with the Mo and O element since Al3+ was intercalated 14
into MoO3 host. Whereas not all the inserted Al could be extracted from the MoO3, the Al element could be detected but with much weaker signals at the fully extraction state, meaning that the Al ions could extract from the MoO3 as well during the subsequent charge process. For the VOPO4 cathode, the uniformly distributed Al element with V, O and P element at fully insertion state reveals that Al3+ was also stepped into the structural interlayer of VOPO4 cathode. And the sparse Al concentration at fully extraction state indicates Al3+ was escaped from the layered structure. The reversible Al3+ intercalation/deintercalation mechanism from the layered host structure would guarantee the rechargeable Al ion battery working functionally.
Fig. 5. TEM micrographs of MoO3 anode at Al ion fully insertion (a) and extraction (b) state with the related elemental mapping images of Mo, O and Al species. TEM 15
micrographs of VOPO4 cathode at fully insertion (c) and extraction (d) state with the related elemental mapping images of V, O, P and Al species.
To acquire a flexible solid-state Al ion battery, a piece of gelatin-PAM hydrogel electrolyte was sandwiched between the MoO3 anode and VOPO4 cathode with conductive carbon cloth. The high adhesion of gel electrolyte would pack the soft and bendable electrodes together with a solid state flexible Al ion battery achieved as illustrated in Fig.6a. To achieve the optimal degree of matching for the full cell test, the balancing weight ratio of VOPO4 cathode to MoO3 anode is designed to be 3:1 with mass loading around 1.5 mg and 0.5 mg, respectively, which was based on the reversible capacity of 115 mAh g-1 and 308 mAh g-1 delivered in 1 M Al(NO3)3 aqueous solution at a current density of 1 A g-1 in Fig. 3. Further, the electrochemical performance of the flexible full Al-ion cell was evaluated by CV and GCD tests. Fig. 6b shows the representative CV profiles of the full Al-ion cell at different scan rates. Two pair of redox peaks located at 0.8 V/1.0 V and 0.3 V/0.5 V was observed under different scan rates, which is ascribed to the Al ion insertion/extraction reaction from the host structure of VOPO4 and MoO3 electrode. The charge/discharge profiles at different current density were also presented in Fig. 6c. A long slop was observed on the discharge curves with discharge capacity of 73 mAh g-1 delivered at the current density of 1 A g-1. When the current density increased to 6.0 A g-1, the AIB full cell could also deliver a specific discharge capacity of 35 mAh g-1. Moreover, the rate capability of AIBs was also investigated by cycling it for 15 times at each rate as 16
shown in Fig. 6d. The AIB could deliver a high discharge capacity of 88, 77, 56, 46, 34, 27 and 22 mAh g-1 at rate of 0.8, 1, 1.6, 2, 3, 4, and 6 A g-1, respectively. When the rate returned back to 0.8 A g-1, the capacity was recovered to 69 mAh g-1, which is equivalent to 80% of the initial average capacity. Noticeably, the capacity of the AIB cycled under every current density exhibited a rather high stability with almost no decay, which could be ascribed to the favourable Al3+ intercalation reversibility of the layered electrode as well as the high ionic conductivity of the hydrogel electrolyte. In addition, the long-term cycle stability of Al ion battery is investigated under current density of 1 A g-1 as shown in Fig. 6e. Obviously, the reversible capacity retains 80.1 mAh g-1 with a capacity retention of 86.2% even after 2800 cycles and the coulombic efficiency remains close to 100% during cycles. It is worth noting that the capacity increased at the initial stage, which was mainly derived from the electrolyte fully wetting into the electrode because of the physical contact interface between the electrode and hydrogel electrolyte during initial cycles. The following gradual capacity decay was mainly because the inserted Al3+ in layered host cannot be completely escaped during every cycles. However, the capacity increased gradually with the subsequent charge/discharge process proceeding. For better explanation, the XRD and SEM of the cathode and anode before and after cycling were carried out as shown in Fig. S3 and Fig. S4. From the XRD results, we can find that the main diffraction peak for the MoO3 and VOPO4 were still maintained after cycling with minor peak position shift due to the trapped Al3+ in the host interlayer. From the SEM, we can find that the MoO3 nanobelts almost remained intact and the VOPO4 17
microsheets became nanosheets that probably derived from the volume change, which might play beneficial effects on the cycling capacity since nanosized grain would be beneficial for the ion diffusion. To the best of our knowledge, the long cyclic stability of AIB in our work was appealing compared to all the aqueous AIBs and most related aqueous multivalent batteries reported to date.[19, 45-49] To make an intuitionistic observation of the long cycle life of our flexible aqueous Al ion battery, the electrochemical performance of the ever-reported aqueous multivalent metal ion battery were schematically provided in Fig. 6f. The AIBs in our work exhibited a long cyclic life span of 2800 cycles, which was profited not only from the reversible intercalation electrode but also from the hydrogel electrolyte with the ability to avoid electrode dissolving. The polyanion-type layered VOPO4 materials were a typical intercalation electrode for multivalent metal ion battery, which was investigated as the cathode for Al ion battery in this work, with a benign capacity that profited from the multivalent vanadium redox chemistry upon Al3+ insertion. For the anode, the MoO3 nanobelts delivered a good Al3+ storage performance, which was mainly benefited from the nanostructure with a preferential growth along the c axis, facilitating the Al3+ diffusion dynamics that enabled a short diffusion path on (010) plane. Coupled with hydrogel electrolyte as well as the pleased performance of cathode and anode, the aqueous Al ion battery exhibited good cycling performance expectedly. It was worth to mention that this is the first attempt to fabricate a flexible aqueous rechargeable Al-ion full cell with good cyclic stability and high capacity to the best of 18
our knowledge, posing promising application in flexible electronic devices market. For convenience, the detailed comparison of our work with the related aqueous multivalent metal ion battery were summarized in Table S1.
19
Fig. 6. Electrochemical performance of the rechargeable solid-state aqueous AIB. (a) The structure schematic illustration of flexible aqueous Al ion battery. (b) CV curves of the solid-state AIB at various scan rates. (c) Galvanostatic charge/discharge profiles at differernt current density. (d) Rate capability of the solid-state AIB at various rates. (e) Long-term cycling performance and the corresponding coulombic efficiency at 1 A g-1. (f) Performance comparison of the aqueous Al ion battery in our work with the related aqueous multivalent metal ion batteries.
To meet the energy and power requirement for practical application, multiple AIBs can be connected in series or parallel to amplify their output voltage or current. As shown in Fig. 7a, three aqueous Al ion batteries connected in series can achieve an output voltage of 3.6 V since a single AIB exhibit an operating voltage of 1.2 V. Similarly, three Al ion batteries in-parallel could deliver a three-fold higher capacity that of a single device (Fig. 7b). As shown in Fig. S5, two AIB in series could power a LED array pattern comprised of 15 lamps. Additionally, the flexible AIB in our work displayed unique safety feature and nice mechanical reliability, which could deliver a specific capacity of 55 mAh g-1 with well-defined charge/discharge curves even after 8 times puncturing operation as shown in Fig. 7c. The AIB could also work properly to light up a commercial electronic watch even after punching a number of large holes (inset in Fig. 7c), which pose great advantage over conventional organic batteries. To demonstrate its flexibility, the Al-ion battery could be squeezed, twisted and folded to any angle as shown in Fig. 7d, which has no effect on their ability to power an 20
electronic watch. Moreover, the Al ion battery can be tailored arbitrarily by ceramic scissor as shown in the Fig. 7e. The tailored AIB can still work well to power the electronic watch, which can also light up the 15 LED lamps array by connecting in series. The voltage of AIB after different cutting times was provided in Fig. 7f, which was almost unchanged during the whole cutting test process. As a valid demonstration of the aqueous AIB with good Al storage performance, two to four AIBs (size 4.0 cm × 6.0 cm) in series could power a 1 m long and a 100 cm2 electroluminescent panel (size 10.0 × 10.0 cm) as shown in Fig. 7g and 7h. With the attractive electrochemical performance, high robustness and safety, our flexible aqueous Al-ion battery can serve as a potential energy storage device for various wearable electronics.
21
Fig. 7. Electrochemical performance of multiple Al ion battery and demonstration of the AIB under different states. (a) Charge/discharge curves of three-in-series batteries. (b) Charge/discharge curves of three-in-parallel batteries. (c) Galvanostatic charge/discharge curve of the flexible AIB after drilling tests. (d) Demonstration of flexible AIB to power an electronic watch when being (i) flat, (ii) and (iii) folded, (iv) squeezed. (e) Demonstration of flexible AIB to power an electronic watch after cuting test. (i) the first cut. (ii) the second cut. (iii) the third cut. (iv) Photograph of two halves of the cutted battery connected in series to power a LED array. (f) Voltages of the AIB after different cutting times. (g) Four Al ion batteries were connected in series to power a 1 m long electroluminescent panel (size 1.0 × 100 cm) under bending condition. (h) Two Al ion batteries were connected in series to power a 100 cm2 electroluminescent panel (size 10.0 × 10.0 cm) under bending condition.
3. Conclusion In summary, a flexible aqueous rechargeable AIB with long cycle life and high safety feature was fabricated successfully based on layered VOPO4 cathode and MoO3 anode with conductive polymer electrolyte. The as-assembled Al-ion battery exhibits long cyclic stability with a capacity retention of 86.2% even after 2800 cycles. It delivers a high specific capacity of 88 mAh g-1 and shows high rate capability of 6 A g-1. Besides, the solid state Al-ion battery exhibits favorable mechanical flexibility and safety properties, which could serve as a reliable power source even under severe conditions including being bent, punctured or cut. Moreover, the aqueous Al-ion 22
battery could power 1 m long and 100 cm2 electroluminescent panels, demonstrating their pleased Al storage performance. Equipped with the appealing electrochemical performance, high robustness and safety properties, our flexible aqueous Al-ion battery is believed to open up a new avenue for the application in flexible and wearable electronics.
4. Experiemetal Section: 4.1 Preparation of electrode materials and polymer electrolyte Preparation of VOPO4•2H2O sheets: The VOPO4•2H2O sheets were obtained by hydrothermal method.[50] Typically, 2.4 g V2O5 powders was dispersed into 60 mL deionized water with the addition of 13.3 mL concentrated H3PO4 under magnetic stirring for 1 h. Afterwards, the bright-yellow solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 16 h. Finally, the product was collected and washed by deionized water and acetone alternatively via centrifugation for several times, which was then dried at 60 °C for 24 h under vacuum. Preparation of MoO3 nanobelts: The MoO3 nanobelts were synthesized by hydrothermal route [39]. 5 mmol Na2MoO4 and 10 mmol NaCl were dissolved into 40 mL deionized water, and a small amout of 3 M HCl was added dropwise to adjust the pH of 1~2 under magnetic stirring. Then the mixture solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. Finally,
23
the obtained suspension was centrifugated and washed by deionized water and ethanol for several times before dried at 60 °C for 24 h. Preparation of gelatin-PAM hydrogel electrolyte: The gelatin-PAM electrolyte was prepared by a free-radical polymerization approach. 2 g gelatin and 15 mg potassium persulfate (initiator) were fully dissolved into 20 mL 1 M Al(NO3)3 solution at 80 °C with
stirring.
Then,
3
g
acrylamide
monomer
and
2
mg
cross-linker
N,N'-methylenebisacrylamide (cross linker) was added sequentially into the above solution with stirring at 40 °C for 2 h. Afterwards, the mixed solution was degassed and injected into a piece of cellulose (CMC) fiber membrane, which was polymerized at 60 °C for 2~3 h. Finally, a crosslinked gelatin-PAM based Al ion electrolyte was formed in the CMC membrane. 4.2 Materials characterization The crystal structure of the obtained sample was characterized by X-ray diffraction (XRD, Rigaku D/Max2500 with Cu Kα radiation, λ = 1.54 Å) and Raman spectra (Jobin Yvon Horiba with He-Ne laser, 632.9 nm). The morphology of the powder samples and the as-prepared solid electrolyte films were characterized by field-emission scanning electron microscope (FE-SEM, FEI Quanta 200F). The high-resolution morphology and microstructure of the samples were characterized by transmission electron microscope (TEM, JEM 2100, 200 kV). 4.3 Electrodes preparation and cell assembly The working electrode was fabricated by dispersing the active material, acetylene black (AB, conductive agent) and polyvinylidene fluoride (PVDF, binder) with a 24
mass ratio of 8:1:1 into N-Methyl pyrrolidone (NMP) solvent. The formed slurry was coated uniformly onto carbon cloth current collectors followed by oven-drying at 80 °C overnight. In the three-electrode tests, the mass loading of cathode and anode active material were around 0.5 mg on carbon cloth with size of 1 cm × 2 cm. The flexible Al ion battery was acquired by packing the cathode and anode in mass loading ratio of 3:1 with the polymer electrolyte. Prior to assembling, the VOPO4 cathode was pre-inserted Al3+ by discharging to 0 V vs. Ag/AgCl at the current density of 30 mA g-1 in 1 M Al(NO3)3 solution. 4.4 Electrochemical measurement Electrochemical performance of VOPO4 cathode and MoO3 anode were investigated in a three-electrode system in 1 M Al(NO3)3 aqueous solution. The Pt sheet and Ag/AgCl electrode were used as counter and reference electrode, respectively. The cyclic stability and rate capability of the prepared AIB were conducted by LandCT 2001A battery testing system. Galvanostatic charge/discharge measurements and cyclic voltammetry were collected on an electrochemical workstation (CHI 760e).
Acknowledgements This research was supported by the National Natural Science Foundation of China (No.21805063), the Natural Science Foundation of Guangdong Province for Distinguished Young Scholars (No. 2018B030306022), and the Economic, Trade and Information Commission of Shenzhen Municipality through the Graphene 25
Manufacture Innovation Center (No.201901161514). The authors also acknowledge the support from China Postdoctoral Science Foundation (2018M641823).
Declaration of interests The authors declare no competing financial interest.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version.
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Declaration of interests The authors declare no competing financial interest.
S2405-8297(19)30995-X
DOI:
https://doi.org/10.1016/j.ensm.2019.09.038
Reference:
ENSM 942
To appear in:
Energy Storage Materials
Received Date: 13 June 2019 Revised Date:
25 September 2019
Accepted Date: 28 September 2019
Please cite this article as: P. Wang, Z. Chen, Z. Ji, Y. Feng, J. Wang, J. Liu, M. Hu, H. Wang, J. Fei, W. Gan, Y. Huang, A High-performance Flexible Aqueous Al Ion Rechargeable Battery with Long Cycle Life, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.09.038. 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 Elsevier B.V. All rights reserved.
A High-performance Flexible Aqueous Al Ion Rechargeable Battery with Long Cycle Life Panpan Wanga,b,c, Zhe Chena,b,c, Zhenyuan Jia,b,c, Yuping Fenga,b,c, Jiaqi Wanga,b,c, Jie Liua,b,c, Mengmeng Hua,b,c, Hua Wanga,b,c, Jinbo Feia,b,c, Wei Gana,b,d and Yan Huanga,b,c* a
Flexible Printed Electronic Technology Center, Harbin Institute of Technology,
Shenzhen, 518055, China. b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of
Technology, Shenzhen, 518055, China. c
School of Materials Science and Engineering, Harbin Institute of Technology,
Shenzhen, 518055, China. d
School of Science, Harbin Institute of Technology, Shenzhen, 518055, China.
*E-mail: [email protected]
Declaration of interests The authors declare no competing financial interest.
A High-performance Flexible Aqueous Al Ion Rechargeable Battery with Long Cycle Life Abstract: With the increasing development of flexible and wearable electronic devices, exploration of advanced power supplies with excellent electrochemical performance, safety and flexibility is highly urgent. In light of natural abundance as well as three-electron redox properties, rechargeable Al ion batteries (AIBs) draw much attention while they still face great challenge of limited Al3+ storage material as well as poorly ionic conductive electrolyte. In our work, an intercalation MoO3 anode and VOPO4 cathode are proposed and mechanically robust gelatin-polyacrylamide hydrogel electrolyte is utilized to fabricate a safe and flexible high-performance rechargeable AIB. The as-assembled AIB exhibits high rate capability of 6 A g-1, high discharge capacity of 88 mAh g-1 and long cyclic stability of 86.2% capacity retention even after 2800 cycles. Besides, the AIB possesses high safety and mechanical flexibility properties, which could serve as reliable power source when even being bent, punctured or cut. Additionally, the aqueous AIB could power 1 m long and 100 cm2 electroluminescent panels, demonstrating their good Al storage property. Considering their favourable electrochemical performance, robustness and safety, the flexible aqueous AIB in our work is believed to open up new avenue in flexible and wearable energy. Keywords: flexible, Al ion battery, intercalation compound, long cyclic stability, wearable electronics
1
1. Introduction Nowadays, it is highly urgent to exploit advanced flexible power supplies to keep pace with the increasing development of portable, flexible and wearable electronic devices such as roll-up displays, smart mobile devices.[1-3] However, the traditional rechargeable lithium ion batteries cannot meet aforementioned criteria because of their toxic and flammable organic electrolyte as well as the rigid cell assembly.[4] Thus, exploitation of flexible metal ion battery technologies based on safer aqueous electrolyte and more abundant metal material such as calcium, magnesium, zinc and aluminum is critical to boost the evolution of flexible electronics.[5-8] In particular, rechargeable Al ion batteries (AIB) are intriguing in light of their natural abundance feature as well as three-electron redox properties that are capable of transporting more electrons relative to monovalent ion.[9, 10] However, the investigation of aqueous Al-ion batteries is still at the infancy stage. On one hand, host material capable of Al3+ insertion/extraction reversibly is very limited. The high charge-to-size ratio of Al3+ gives rise to stronger coulombic interaction between the host lattice and Al3+, which sluggishes the Al3+ solid state diffusion kinetics in the lattice.[11] Therefore, exploration of appropriate electrode materials with facilitated Al3+ intercalation kinetics remains a significant and fundamental scientific challenge.[12] The kinetics limitation of Al3+ places restriction on the selection of potential electrode material, which should possess highly opened crystal structure to allow facile ion insertion/extraction and rich redox centers to maintain charge neutrality 2
upon metal ions insertion. To date, the only reported electrode materials in aqueous Al ion system involved prussian blue compound[13, 14] and transition metal oxides (titanium dioxide,[15, 16] vanadium oxides,[17, 18] etc). An aqueous AIB consisted of TiO2 anode and prussian blue cathode was proposed, whereas it exhibits a maximum capacity of only 10.6 mAh g-1 and limited cycle life with low capacity retention of 58% after 2000 cycles.[19] Inspired by the prototype of lithium ion battery based on intercalation chemistry,[20] rechargeable aqueous AIB based on layered transition metal compound should also be feasible because of their adjustable interlayer engineering and rich redox reactions that is ideal for multivalent metal ion intercalation.[21-23] However, there was no related investigation on aqueous Al ion full battery based on intercalation mechanism to the best of our knowledge. In this respect, the construction of aqueous Al ion full cell systems based on layered transition metal compound is highly desirable for the further promotion of multivalent metal ion battery. Meanwhile, a mechanical robust electrolyte with high Al3+ionic conductivity is also an indispensable component for the flexible AIB devices. For the commonly utilized polyvinyl alcohol (PVA)-based gel electrolyte, the limited water retention and poor mechanical strength prevent their further application in flexible AIB.[26] Hydrogel electrolytes are proved to be powerful to fabricate the flexible energy storage devices due to their low cost, nontoxicity, chemical stability as well as attractive mechanical properties.[24-27] Whereas for the hydrogel with mannuronic acid unit, the strong interaction between Al3+ and the functional groups in polymer chains would make the 3
monomer highly crosslinked with Al3+ as ionic crosslinker, resulting in tough hydrogel but poor ionic activity.[28] To this end, the exploration of highly ionic conductive and durable electrolyte for multivalent Al3+cations is fairly tough but vital for the further development of extremely safe and rechargeable high-performance AIBs. In this work, a flexible aqueous Al-ion full battery with long cycling stability, high capacity, good rate capability and safety property was developed based on intercalation
compounds
with
layered
electrode
sandwiched
by
gelatin-polyacrylamide (PAM) hydrogel electrolyte. A capacity retention as high as 86.2% after 2800 cycles at 1 A g-1 was achieved, which was far better than the performance of the AIB reported so far. Additionally, it demonstrated a nice mechanical tolerance against severe working condition with the ability to power electronic watches or LED lights array even after puncturing or cutting. In terms of long-term stability, high storage capability and high safety, the flexible aqueous AIB exhibits great potential application in wearable electronic devices.
2. Results and Discussion A rechargeable aqueous Al ion battery based on intercalation compound is proposed and the working principle of aqueous Al ion battery is illustrated in Fig. 1a. The VOPO4 cathode displays a typical layered structure consisting of vertex-sharing VO6 octahedra linking to PO4 tetrahedra. The adjacent layers were held together through weak hydrogen bonds, which facilitate cation intercalation due to the rich 4
coordination circumstance of V atom. For the MoO3 anode, the layered structure is formed by stacking bilayer sheets of distorted MoO6 octahedra via van der Waals forces, which is also of great interest in electrochemical intercalation chemistry given its unique two dimensional (2D) layer structure and the ease coupling of multivalent-state Mo atom.[29] During the charge process for the full cell, Al ions would extract from the cathode, delivered by Al3+ conductive electrolyte and insert into anode host, accompanied with three electrons traversing in the external circuits to power a load, which is analogous to the ‘rocking-chair’ Li-ion batteries.[30] The XRD pattern of as-prepared MoO3 anode (Fig. 1b) is readily indexed to orthorhombic α-MoO3 phase with space group of Pbnm (JCPDS no. 76-1003). The strong diffraction peaks of (020), (040) and (060) planes implies that MoO3 possess a typical layered crystal structure with highly anisotropic growth orientation. From the FESEM and TEM images in Fig. 1c(i) and (ii), we can find that the synthesized MoO3 exhibits nanobelt morphology with the length around 2-4 µm and width around 300 nm. The HRTEM (Fig. 1c (iii)) image further disclosed clear lattice fringes of 3.6 and 4.0 Å along two different directions, corresponding to the (100) and (001) facet of MoO3, respectively. The selected area electron diffraction (SAED) pattern (Fig. 1c (iv)) can be indexed to the [010] zone, revealing the preferential growth along the c axis as well as the single crystallinity of MoO3, which agrees well with the XRD results.[31] Meanwhile, the XRD pattern of the obtained VOPO4 sample can be readily indexed into the tetragonal VOPO4•2H2O with the space group of P4/nmm (JCPDS No. 84-0111). Noticeably, the main (001) peak located at ~11.9o was correlated to the 5
interlayer distance d value of 7.4 Å, which is identical to the previous report according to Bragg’s equation (2dsinθ = nλ).[32] The SEM image (Fig. 1e) displays that VOPO4 possesses a flat, rectangle microsheet shape with lateral size of ~10 µm, and the TEM image in Fig. 1f further reveals that VOPO4 presents an individual sheet-like rectangle morphology.
Fig. 1. (a) Working principle schematic of aqueous Al ion battery based on intercalation electrtode. Structural and morphological characterization of MoO3 samples: (b) XRD patterns, (c) (i) SEM image; (ii) TEM image; (iii) HRTEM image and (iv) SAED pattern. Structural and morphological characterization of VOPO4 samples: (d) XRD patterns, (e) SEM image and (f) TEM images.
Herein, a gelatin-polyacrylamide hydrogel was utilized as flexible Al ion electrolyte with good ionic conductivity. The typical synthesis route of the 6
gelatin-PAM polymer electrolyte is illustrated in Fig. 2a. First, the pristine gelatin and potassium persulfate were added into the Al(NO3)3 solution. Sequentially, acrylamide (AM) monomers and N,N-methylenebisacrylamide (BIS) were added to the above solution. During the initial stage, the polyacrylamide (PAM) was grafted onto the gelatin chains via free radical polymerization. Afterwards, the mixture solution was injected into a cellulose membrane with heat treatment at 60 oC for 2.5 h. Thus, a covalent crosslinked gelatin-PAM hydrogel was formed in the cellulose fiber membrane. As shown in Fig. 2b, the achieved ionic conductivity of Al3+ gelatin-PAM polymer electrolyte is calculated to be over 20 mS cm-1, which were higher than those reported gel electrolyte including typical polyvinyl alcohol (PVA), gelatin, and poly(propylsulfonate
dimethylammonium
propylmethacrylamide)
(PPDP)
gels,[33-36] endowed the AIB with desired electrochemical performance. As a demonstration, the hydrogel electrolyte could connect a light-emitting diode (LED) circuit successfully as shown inset in Fig. 2b, further manifesting the favorable ionic conductivity of gelatin-PAM hydrogel electrolyte. The cross-section scanning electron microscopy (SEM) image of the freeze dried gelatin-PAM polymer was provided in Fig. 2c, the gelatin-PAM hydrogel is very thin with a thickness of 100 µm, which is also beneficial for the assembly and realization of flexible energy storage devices.
7
Fig. 2. (a) Schematic of the synthesis route to the gelatin-PAM electrolyte. (b) Ionic conductivity of our gelatin-PAM electrolyte compared with other gel electrolytes. Inset: photograph demonstration of the electrolyte as ionic conductor connected in circuit to light up LED successfully. (c) Cross-section SEM image of gelatin-PAM hydrogel electrolyte. Scale bars: 50 µm.
The electrochemical behavior of the MoO3 anode and VOPO4 cathode were examined by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) test in 1 M Al(NO3)3 electrolyte in a three-electrode configuration. Fig. 3a shows the CV curves of the MoO3 anode at different scan rates in a potential window of -0.6 to 0.4 V (vs. Ag/AgCl). A pair of well-defined redox peak located at -0.3 V/0.2 V (vs. Ag/AgCl)
was
detected
obivously,
corresponding
to
the
reversible
Al3+
deintercalation from and intercalation into the layered host structure that similar with the topotactic redox reaction mechanism.[37, 38] The high peak current of the 8
symmetrical cathodic and anodic redox peaks for all the CV curves indicate that the MoO3 nanobelt electrode possesses high electrochemical activity and good reversibility for Al3+ intercalation. Meanwhile, GCD curves of MoO3 anode under different current density were depicted in Fig. 3b. There exists a long and flat discharge plateau located around -0.2 V (vs. Ag/AgCl), and a slanted charge plateau centered from -0.1 V to 0.2 V (vs. Ag/AgCl), which corresponds well with the strong redox peaks observed in the CV curves in Fig. 3a. Importantly, the MoO3 electrode delivered a high specific discharge capacity of 308, 270, 251, 242 and 232 mAh g-1 at current densities of 1, 2, 4, 6 and 8 A g-1, respectively, further suggesting the good Al storage capability of the obtained MoO3 nanobelts. The high Al storage capacity was probably related to the more active crystallographic (010) planes, which is favourable for ion intercalation as described in the case of Li+ intercalation preferentially in the van der Waals spacing at the (010) plane.[39, 40] Herein, the cutting off potential of charge/discharge curves was set to be -0.3 V rather than -0.6 V (vs. Ag/AgCl), since the H2O decomposition phenomenon with H2 evolution might appear under more negative potential demonstrated by an ultralong discharge plateau located around -0.4 V as shown in Fig. S1. Fig. 3c shows the CV curves of the VOPO4 cathode in the voltage range of 0-1 V (vs. Ag/AgCl) at different scan rates. Two cathodic peaks centered around 0.1 V and 0.34 V (vs. Ag/AgCl) were observed obviously, which were probably related with the V5+/V4+ and V4+/V3+ redox induced by Al3+ insertion. Subsequently, two anodic peaks centered at 0.25 V and 0.55 V (vs. Ag/AgCl) were observed on the anodic process, 9
which was attributed the reduction reaction induced by the Al3+ extraction from the VOPO4. Additionally, the typical charge and discharge profiles of VOPO4 cathode at different current densities were displayed in Fig. 3d. During discharge, a slanted discharge plateau around 0.5 V that ranged from 0.6 V to 0.2 V was observed. Correspondingly, a charge voltage slope varied from 0.5 V to 0.8 V was exhibited, which comes from the insertion/extraction of Al3+ ions from VOPO4 host accomponied with the reversibly redox process for V5+/V4+ and V4+/V3+. Meanwhile, a high specific discharge capacity of 115, 92, 79, 69, 64 mAh g-1 is obtained at the current density of 1, 1.5, 2, 3, 4 A g-1, respecitvely, further suggesting the favourable kinetics of reversible Al3+ ion intercalation chemistry in VOPO4 host. Based on the results, the appropriate discharge potential of 0.54 V and -0.2 V (vs. Ag/AgCl) for VOPO4 and MoO3, respectively, making them suitable as the cathode and anode materials for aqueous Al ion full cell.
10
Fig. 3. Electrochemical performance of MoO3 and VOPO4 in the three-electrode system. (a) CV curves of the MoO3 at different scan rates. (b) Galvanostatic charge/discharge plots of the MoO3 anode at different current densities. (c) CV curves of the VOPO4 cathode at different scan rates. (d) Galvanostatic charge/discharge plots of the VOPO4 cathode at different current densities.
In order to verify the reversible intercalation/deintercalation behavior of Al3+ from the layered host structure, ex situ XRD and Raman was utilized to investigate the structural evolution of MoO3 anode and VOPO4 cathode at different Al ion insertion states. Fig. 4a provides the charge and discharge curves of MoO3 anode in 1 M Al(NO3)3 solution at current density of 1 A g-1, which exhibits a flat discharge plateau 11
and slanted charge plateau located around -0.2 V/0 V vs. Ag/AgCl. The selected states marked with A-H points were collected for XRD and Raman analysis. The peak at 12.9° corresponds to the (020) planes with a spacing of 0.693 nm. It should be noted that all of the diffraction peaks of (020), (040), and (060) shift to higher angle upon charging, which are induced by the Al3+ extraction from the layered structure. During the discharge process, the three main crystallographic plane reflection shift to lower angle, implying the interlayer d-spacing increased along with Al3+ ions intercalating. This phenomenon is typical for the intercalation electrode possessing the ability to insert or extract gust ion along with the expansion or shrink of lattice.[41] Meanwhile, ex situ Raman spectra are recorded as shown in Fig. 4c. The observed bands of 284 cm-1 are typically ascribed to the wagging mode for the double bond (O=Mo=O).[42] The bands located at 660, 816 and 991 cm-1 were assigned to the stretching vibration mode of oxygen atoms resulting from edge-shared, corner-shared oxygen (Mo-O) and unshared oxygen (Mo=O), respectively. As observed, no obvious peak shift or new Raman peak are detected upon the whole charge and discharge process, suggesting that no structural transition is involved during the Al3+ ion insertion and extraction process. For the case of VOPO4 cathode, the characteristic XRD diffraction peak of (001) plane shifted from 12.3° (initial state O point) to the high angle of 13.0° (discharged state A point) during the Al3+ pre-insertion process from the open circuit potential to 0 V (Fig. S2), corresponding to a slight decrease in the interlayer spacing of (001) plane. Specially, the interlayer distance decreased rather than increased upon guest ion 12
insertion, which was probably due to the strong electrostatic attraction between positively Al3+ and negatively oxygen atoms in the host materials. These adjacent layers are then brought closer by electrostatic interaction with the intercalated cation, which resulted in the decrease of the interlayer distance.[43] During the subsequent charge stage (A-E points in Fig. 4d), the attraction between Al3+ and oxygen as well as the repulsion between oxygen atoms in neighboring layers are weakened since Al3+ extracted from the host. Expectedly, the primary diffraction peak of (001) plane at the fully charged state shifts back to the pristine position, further indicating the reversible Al3+ intercalation chemistry into layered VOPO4 cathode. In addition, ex situ Raman was utilized to reveal the structural evolutions of VOPO4 during Al3+ intercalation process. As shown in Fig. 4e, the bands centered at 531, 560 and 920 cm-1 are typically ascribed to the symmetric bending and stretching vibrations of O-P-O modes, and the bands located at 687 and 1021 cm-1 can be attributed to bending vibrations of V-O and stretching vibrations of V=O modes, respectively.[44] During the Al3+ pre-insertion process (Fig. S2c), the blue shift of Raman peaks for O-P-O and V=O stretching mode was observed obviously, whereas in the charging process (Fig. 4e points A to E), the red shift of Raman peaks for O-P-O and V=O stretching mode was also noticeable. Obviously, the O-P-O and V=O stretching mode was strongly correlated with the interaction between the oxygen atoms from the P-O and V=O bond with the intercalated Al3+ guest cation. During the following discharge process, the blue shift of the characteristic Raman bands was detected once again with the intercalation of Al3+ into VOPO4 interlayers, which 13
further confirmed the fact that Al3+ could be intercalated into VOPO4 layers reversibly.
Fig. 4. The structural evolution of MoO3 anode and VOPO4 cathode in 1 M Al(NO3)3 electrolyte during Al ion insertion/extraction process. The points A-I represents the states where spectra are collected for XRD and Raman analysis: (a) Charge/discharge curve for MoO3 at current density of 1 A g-1. (b) Ex situ XRD pattern and (c) Ex situ Raman spectrum of the MoO3 anode at selected Al ion insertion/extraction states. (d) Charge/discharge curves for VOPO4 at current density of 1 A g-1. (e) Ex situ XRD pattern and (f) Ex situ Raman spectrum of the VOPO4 cathode at selected Al ion insertion/extraction states.
Moreover, the elemental mapping analysis was also employed to inspect the Al3+ intercalation behavior (Fig. 5). At fully Al3+ insertion state of MoO3, the distribution of Al element matches well with the Mo and O element since Al3+ was intercalated 14
into MoO3 host. Whereas not all the inserted Al could be extracted from the MoO3, the Al element could be detected but with much weaker signals at the fully extraction state, meaning that the Al ions could extract from the MoO3 as well during the subsequent charge process. For the VOPO4 cathode, the uniformly distributed Al element with V, O and P element at fully insertion state reveals that Al3+ was also stepped into the structural interlayer of VOPO4 cathode. And the sparse Al concentration at fully extraction state indicates Al3+ was escaped from the layered structure. The reversible Al3+ intercalation/deintercalation mechanism from the layered host structure would guarantee the rechargeable Al ion battery working functionally.
Fig. 5. TEM micrographs of MoO3 anode at Al ion fully insertion (a) and extraction (b) state with the related elemental mapping images of Mo, O and Al species. TEM 15
micrographs of VOPO4 cathode at fully insertion (c) and extraction (d) state with the related elemental mapping images of V, O, P and Al species.
To acquire a flexible solid-state Al ion battery, a piece of gelatin-PAM hydrogel electrolyte was sandwiched between the MoO3 anode and VOPO4 cathode with conductive carbon cloth. The high adhesion of gel electrolyte would pack the soft and bendable electrodes together with a solid state flexible Al ion battery achieved as illustrated in Fig.6a. To achieve the optimal degree of matching for the full cell test, the balancing weight ratio of VOPO4 cathode to MoO3 anode is designed to be 3:1 with mass loading around 1.5 mg and 0.5 mg, respectively, which was based on the reversible capacity of 115 mAh g-1 and 308 mAh g-1 delivered in 1 M Al(NO3)3 aqueous solution at a current density of 1 A g-1 in Fig. 3. Further, the electrochemical performance of the flexible full Al-ion cell was evaluated by CV and GCD tests. Fig. 6b shows the representative CV profiles of the full Al-ion cell at different scan rates. Two pair of redox peaks located at 0.8 V/1.0 V and 0.3 V/0.5 V was observed under different scan rates, which is ascribed to the Al ion insertion/extraction reaction from the host structure of VOPO4 and MoO3 electrode. The charge/discharge profiles at different current density were also presented in Fig. 6c. A long slop was observed on the discharge curves with discharge capacity of 73 mAh g-1 delivered at the current density of 1 A g-1. When the current density increased to 6.0 A g-1, the AIB full cell could also deliver a specific discharge capacity of 35 mAh g-1. Moreover, the rate capability of AIBs was also investigated by cycling it for 15 times at each rate as 16
shown in Fig. 6d. The AIB could deliver a high discharge capacity of 88, 77, 56, 46, 34, 27 and 22 mAh g-1 at rate of 0.8, 1, 1.6, 2, 3, 4, and 6 A g-1, respectively. When the rate returned back to 0.8 A g-1, the capacity was recovered to 69 mAh g-1, which is equivalent to 80% of the initial average capacity. Noticeably, the capacity of the AIB cycled under every current density exhibited a rather high stability with almost no decay, which could be ascribed to the favourable Al3+ intercalation reversibility of the layered electrode as well as the high ionic conductivity of the hydrogel electrolyte. In addition, the long-term cycle stability of Al ion battery is investigated under current density of 1 A g-1 as shown in Fig. 6e. Obviously, the reversible capacity retains 80.1 mAh g-1 with a capacity retention of 86.2% even after 2800 cycles and the coulombic efficiency remains close to 100% during cycles. It is worth noting that the capacity increased at the initial stage, which was mainly derived from the electrolyte fully wetting into the electrode because of the physical contact interface between the electrode and hydrogel electrolyte during initial cycles. The following gradual capacity decay was mainly because the inserted Al3+ in layered host cannot be completely escaped during every cycles. However, the capacity increased gradually with the subsequent charge/discharge process proceeding. For better explanation, the XRD and SEM of the cathode and anode before and after cycling were carried out as shown in Fig. S3 and Fig. S4. From the XRD results, we can find that the main diffraction peak for the MoO3 and VOPO4 were still maintained after cycling with minor peak position shift due to the trapped Al3+ in the host interlayer. From the SEM, we can find that the MoO3 nanobelts almost remained intact and the VOPO4 17
microsheets became nanosheets that probably derived from the volume change, which might play beneficial effects on the cycling capacity since nanosized grain would be beneficial for the ion diffusion. To the best of our knowledge, the long cyclic stability of AIB in our work was appealing compared to all the aqueous AIBs and most related aqueous multivalent batteries reported to date.[19, 45-49] To make an intuitionistic observation of the long cycle life of our flexible aqueous Al ion battery, the electrochemical performance of the ever-reported aqueous multivalent metal ion battery were schematically provided in Fig. 6f. The AIBs in our work exhibited a long cyclic life span of 2800 cycles, which was profited not only from the reversible intercalation electrode but also from the hydrogel electrolyte with the ability to avoid electrode dissolving. The polyanion-type layered VOPO4 materials were a typical intercalation electrode for multivalent metal ion battery, which was investigated as the cathode for Al ion battery in this work, with a benign capacity that profited from the multivalent vanadium redox chemistry upon Al3+ insertion. For the anode, the MoO3 nanobelts delivered a good Al3+ storage performance, which was mainly benefited from the nanostructure with a preferential growth along the c axis, facilitating the Al3+ diffusion dynamics that enabled a short diffusion path on (010) plane. Coupled with hydrogel electrolyte as well as the pleased performance of cathode and anode, the aqueous Al ion battery exhibited good cycling performance expectedly. It was worth to mention that this is the first attempt to fabricate a flexible aqueous rechargeable Al-ion full cell with good cyclic stability and high capacity to the best of 18
our knowledge, posing promising application in flexible electronic devices market. For convenience, the detailed comparison of our work with the related aqueous multivalent metal ion battery were summarized in Table S1.
19
Fig. 6. Electrochemical performance of the rechargeable solid-state aqueous AIB. (a) The structure schematic illustration of flexible aqueous Al ion battery. (b) CV curves of the solid-state AIB at various scan rates. (c) Galvanostatic charge/discharge profiles at differernt current density. (d) Rate capability of the solid-state AIB at various rates. (e) Long-term cycling performance and the corresponding coulombic efficiency at 1 A g-1. (f) Performance comparison of the aqueous Al ion battery in our work with the related aqueous multivalent metal ion batteries.
To meet the energy and power requirement for practical application, multiple AIBs can be connected in series or parallel to amplify their output voltage or current. As shown in Fig. 7a, three aqueous Al ion batteries connected in series can achieve an output voltage of 3.6 V since a single AIB exhibit an operating voltage of 1.2 V. Similarly, three Al ion batteries in-parallel could deliver a three-fold higher capacity that of a single device (Fig. 7b). As shown in Fig. S5, two AIB in series could power a LED array pattern comprised of 15 lamps. Additionally, the flexible AIB in our work displayed unique safety feature and nice mechanical reliability, which could deliver a specific capacity of 55 mAh g-1 with well-defined charge/discharge curves even after 8 times puncturing operation as shown in Fig. 7c. The AIB could also work properly to light up a commercial electronic watch even after punching a number of large holes (inset in Fig. 7c), which pose great advantage over conventional organic batteries. To demonstrate its flexibility, the Al-ion battery could be squeezed, twisted and folded to any angle as shown in Fig. 7d, which has no effect on their ability to power an 20
electronic watch. Moreover, the Al ion battery can be tailored arbitrarily by ceramic scissor as shown in the Fig. 7e. The tailored AIB can still work well to power the electronic watch, which can also light up the 15 LED lamps array by connecting in series. The voltage of AIB after different cutting times was provided in Fig. 7f, which was almost unchanged during the whole cutting test process. As a valid demonstration of the aqueous AIB with good Al storage performance, two to four AIBs (size 4.0 cm × 6.0 cm) in series could power a 1 m long and a 100 cm2 electroluminescent panel (size 10.0 × 10.0 cm) as shown in Fig. 7g and 7h. With the attractive electrochemical performance, high robustness and safety, our flexible aqueous Al-ion battery can serve as a potential energy storage device for various wearable electronics.
21
Fig. 7. Electrochemical performance of multiple Al ion battery and demonstration of the AIB under different states. (a) Charge/discharge curves of three-in-series batteries. (b) Charge/discharge curves of three-in-parallel batteries. (c) Galvanostatic charge/discharge curve of the flexible AIB after drilling tests. (d) Demonstration of flexible AIB to power an electronic watch when being (i) flat, (ii) and (iii) folded, (iv) squeezed. (e) Demonstration of flexible AIB to power an electronic watch after cuting test. (i) the first cut. (ii) the second cut. (iii) the third cut. (iv) Photograph of two halves of the cutted battery connected in series to power a LED array. (f) Voltages of the AIB after different cutting times. (g) Four Al ion batteries were connected in series to power a 1 m long electroluminescent panel (size 1.0 × 100 cm) under bending condition. (h) Two Al ion batteries were connected in series to power a 100 cm2 electroluminescent panel (size 10.0 × 10.0 cm) under bending condition.
3. Conclusion In summary, a flexible aqueous rechargeable AIB with long cycle life and high safety feature was fabricated successfully based on layered VOPO4 cathode and MoO3 anode with conductive polymer electrolyte. The as-assembled Al-ion battery exhibits long cyclic stability with a capacity retention of 86.2% even after 2800 cycles. It delivers a high specific capacity of 88 mAh g-1 and shows high rate capability of 6 A g-1. Besides, the solid state Al-ion battery exhibits favorable mechanical flexibility and safety properties, which could serve as a reliable power source even under severe conditions including being bent, punctured or cut. Moreover, the aqueous Al-ion 22
battery could power 1 m long and 100 cm2 electroluminescent panels, demonstrating their pleased Al storage performance. Equipped with the appealing electrochemical performance, high robustness and safety properties, our flexible aqueous Al-ion battery is believed to open up a new avenue for the application in flexible and wearable electronics.
4. Experiemetal Section: 4.1 Preparation of electrode materials and polymer electrolyte Preparation of VOPO4•2H2O sheets: The VOPO4•2H2O sheets were obtained by hydrothermal method.[50] Typically, 2.4 g V2O5 powders was dispersed into 60 mL deionized water with the addition of 13.3 mL concentrated H3PO4 under magnetic stirring for 1 h. Afterwards, the bright-yellow solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 16 h. Finally, the product was collected and washed by deionized water and acetone alternatively via centrifugation for several times, which was then dried at 60 °C for 24 h under vacuum. Preparation of MoO3 nanobelts: The MoO3 nanobelts were synthesized by hydrothermal route [39]. 5 mmol Na2MoO4 and 10 mmol NaCl were dissolved into 40 mL deionized water, and a small amout of 3 M HCl was added dropwise to adjust the pH of 1~2 under magnetic stirring. Then the mixture solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. Finally,
23
the obtained suspension was centrifugated and washed by deionized water and ethanol for several times before dried at 60 °C for 24 h. Preparation of gelatin-PAM hydrogel electrolyte: The gelatin-PAM electrolyte was prepared by a free-radical polymerization approach. 2 g gelatin and 15 mg potassium persulfate (initiator) were fully dissolved into 20 mL 1 M Al(NO3)3 solution at 80 °C with
stirring.
Then,
3
g
acrylamide
monomer
and
2
mg
cross-linker
N,N'-methylenebisacrylamide (cross linker) was added sequentially into the above solution with stirring at 40 °C for 2 h. Afterwards, the mixed solution was degassed and injected into a piece of cellulose (CMC) fiber membrane, which was polymerized at 60 °C for 2~3 h. Finally, a crosslinked gelatin-PAM based Al ion electrolyte was formed in the CMC membrane. 4.2 Materials characterization The crystal structure of the obtained sample was characterized by X-ray diffraction (XRD, Rigaku D/Max2500 with Cu Kα radiation, λ = 1.54 Å) and Raman spectra (Jobin Yvon Horiba with He-Ne laser, 632.9 nm). The morphology of the powder samples and the as-prepared solid electrolyte films were characterized by field-emission scanning electron microscope (FE-SEM, FEI Quanta 200F). The high-resolution morphology and microstructure of the samples were characterized by transmission electron microscope (TEM, JEM 2100, 200 kV). 4.3 Electrodes preparation and cell assembly The working electrode was fabricated by dispersing the active material, acetylene black (AB, conductive agent) and polyvinylidene fluoride (PVDF, binder) with a 24
mass ratio of 8:1:1 into N-Methyl pyrrolidone (NMP) solvent. The formed slurry was coated uniformly onto carbon cloth current collectors followed by oven-drying at 80 °C overnight. In the three-electrode tests, the mass loading of cathode and anode active material were around 0.5 mg on carbon cloth with size of 1 cm × 2 cm. The flexible Al ion battery was acquired by packing the cathode and anode in mass loading ratio of 3:1 with the polymer electrolyte. Prior to assembling, the VOPO4 cathode was pre-inserted Al3+ by discharging to 0 V vs. Ag/AgCl at the current density of 30 mA g-1 in 1 M Al(NO3)3 solution. 4.4 Electrochemical measurement Electrochemical performance of VOPO4 cathode and MoO3 anode were investigated in a three-electrode system in 1 M Al(NO3)3 aqueous solution. The Pt sheet and Ag/AgCl electrode were used as counter and reference electrode, respectively. The cyclic stability and rate capability of the prepared AIB were conducted by LandCT 2001A battery testing system. Galvanostatic charge/discharge measurements and cyclic voltammetry were collected on an electrochemical workstation (CHI 760e).
Acknowledgements This research was supported by the National Natural Science Foundation of China (No.21805063), the Natural Science Foundation of Guangdong Province for Distinguished Young Scholars (No. 2018B030306022), and the Economic, Trade and Information Commission of Shenzhen Municipality through the Graphene 25
Manufacture Innovation Center (No.201901161514). The authors also acknowledge the support from China Postdoctoral Science Foundation (2018M641823).
Declaration of interests The authors declare no competing financial interest.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version.
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Declaration of interests The authors declare no competing financial interest.