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Electrochemical intercalation of anions in graphite for high-voltage aqueous zinc battery
Journal of Power Sources 449 (2020) 227594
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Electrochemical intercalation of anions in graphite for high-voltage aqueous zinc battery Huang Zhang a, b, c, Xu Liu b, c, Bingsheng Qin b, c, Stefano Passerini b, c, * a
Xi’an Institute of Flexible Electronics (IFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an, 710072, Shaanxi, China Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081, Ulm, Germany c Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021, Karlsruhe, Germany b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Hybrid “water-in-salt” electrolyte for zinc batteries. � Highly reversible Zn stripping/plating reaction. � Graphite cathode for anion intercalation in aqueous electrolyte. � A novel concept of aqueous dual-ion battery with high operating voltage.
A R T I C L E I N F O
A B S T R A C T
Keywords: Zn/graphite batteries Dual-ion batteries Zn anode Aqueous electrolyte High-voltage
Rechargeable aqueous zinc batteries are ideal for large-scale energy storage due to their low cost and high safety. Here we demonstrate the reversible anion intercalation chemistry in graphite cathode for aqueous zinc battery, thus operating as a high-voltage dual-ion battery system. The use of a hybrid “water-in-salt” electrolyte with zinc and lithium bi-salts can maximize the ionic content, expand the anodic stability, promote dendrite-free zinc plating/stripping, and bring the reversible anion intercalation in graphite cathode. This cell configuration offers a high operating voltage and unprecedented cycling stability with comparable Coulombic efficiency to the organic dual-ion battery, opening new opportunities for the development of high-voltage rechargeable aqueous batteries.
1. Introduction
the large variety of battery chemistries recently proposed, aqueous batteries rise as very promising candidates, offering both the low-cost and safety advantages [4,5]. Metallic zinc has been regarded as an ideal anode material for aqueous batteries, due to its high theoretical capacity (820 mAh g 1), low redox potential ( 0.76 V vs. SHE), high abundance and intrinsic benignity, alongside with the safety of aqueous battery systems [6]. These advantages have driven the development of aqueous zinc batteries using a metal as the negative electrode and a Zn
The search for new rechargeable battery systems with high energy density and safety is triggered by the rapid development of renewable energy resources [1,2]. Although conventional “rocking-chair” lithium-ion batteries have been successfully employed in portable electronic devices and electric vehicles, the large-scale energy storage requires low-cost and safety rather than high energy density [3]. Among
* Corresponding author. Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081, Ulm, Germany. E-mail address: [email protected] (S. Passerini). https://doi.org/10.1016/j.jpowsour.2019.227594 Received 21 October 2019; Received in revised form 23 November 2019; Accepted 8 December 2019 Available online 25 January 2020 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 449 (2020) 227594
insertion compound as the positive electrode [7]. In fact, although alkaline Zn/MnO2 batteries have been shown rechargeable for extended cycles their delivered capacity is limited. Additionally, the Zn electrode in alkaline electrolytes persistently suffers from poor reversibility with low Coulombic efficiency and dendritic plating, which make the further performance improvement rather difficult [8,9]. Although there are some reports on the dendrite-free zinc deposition by using neutral pH electrolytes and innovative anode architectures [10,11], very few pos itive electrode materials, i.e. polymorphs of MnO2 [12–14], hex acyanoferrate [15–17], vanadium oxide [18–20], and organic quinone [21], have been explored as hosts for reversible Zn2þ insertion. How ever, they either show poor cycling stability or low working potential, consequently offering limited energy densities [22]. In contrast to the conventional batteries making use of a single electroactive species (e.g., a metal-ion), another concept, commonly referred to as hybrid or dual-ion battery (DIB), is attracting increasing attention. This battery chemistry operates via both the anions and cat ions in the electrolyte, which are active at the positive (cathode) and the negative (anode) electrodes, respectively [23]. The most investigated DIB system is the dual-graphite battery, employing graphite as both anion host at the cathode and cation host at the anode [24]. In principle, any anode materials in conventional metal-ion batteries, such as inser tion, alloying or conversion-type materials, could work as anodes for DIBs. However, to enable the maximum output of the cell at the lowest cost, metallic anodes based on abundant metals are especially attractive. Indeed, potassium [25,26], sodium [27,28], calcium [29,30], and aluminum [31,32] based dual-ion batteries have been explored as environmentally friendly, high voltage battery systems. Similar to the metal-batteries, the reversibility of metal anode is essential in terms of cycling stability. Meanwhile, very limited materials have been reported as hosts for anion uptake, among which graphite is ideal with its layered structure [33]. Indeed, graphitic carbon has been proved able to inter calate tetrachloroaluminate (AlCl4 ) [34], hexafluorophosphate (PF6 ) [35], trifluoromethanesulfonate (TfO ) [36], bis(tri fluoromethanesulfonimide) (TFSI ) [37], fluorosulfonyl(tri fluoromethanesulfonyl) imide (FTFSI ) [38], bis (pentafluoroethanesulfonyl) imide (BETI ) [39], and bis(fluorosulfonyl) imide (FSI ) [40] anions from ionic liquids and/or organic electrolytes. However, most DIBs suffer from energy density, rooted in their opera tion principle where the electrolyte is the only source of ions, thus becoming an active component in the system. Its weight affects the overall cell performance. The promising approach for increasing the energy density of DIBs is to maximize the ionic content in the electrolyte without compromising the charge storage capacities of the electrodes [33,40]. On the other hand, the use of organic solvent for the anion intercalation into graphite, typically requires an operating potential above 4.5 V vs. Liþ/Li, brings to the safety concern for practical appli cation. Aqueous electrolyte would be an option, however, the operation condition is beyond the anodic stability limit of conventional aqueous electrolyte, inhibiting the dual-ion chemistries in aqueous energy stor age applications. In this work, we present an aqueous dual-ion battery concept using a graphite cathode and a zinc anode in highly concentrated aqueous electrolytes. We employed a hybrid 21 m LiTFSI and 3 m ZnTfO2 “waterin-salt” zinc electrolyte (HWiSE), which maximizes the ionic content, expands the anodic stability window, promotes dendrite-free zinc plating/stripping with high columbic efficiency, and enables the reversible anion intercalation in graphite cathode. The Zn metal anode has been examined in details, showing an excellent reversibility. In spite of the intriguing reversibility with comparable Coulombic efficiency (>95%) to organic system, the cell offered potentially high operating voltage. The structural evolution of graphite cathode was investigated during electrochemical reactions by in-situ X-ray diffraction (XRD) revealing the anion intercalation process in aqueous electrolyte. This study provides direct insights enabling the design of high energy density aqueous rechargeable batteries for alternative energy storage systems,
using the dual-ion battery concept. 2. Experimental methods 2.1. Materials Commercial graphite (SLP-30, IMERYS) was used as received without any further treatment. The positive electrodes were fabricated by casting slurries composed of 90 wt% graphite and 10 wt% poly vinylidenefluoride (PVDF, Solef 6020, Solvay) in N-methyl-2-pyrroli done (NMP, anhydrous, Sigma-Aldrich) on stainless foil (type 316; thickness: 0.050 mm). After pre-drying at 80 � C in air, electrodes with a diameter of 14 mm were punched and further dried at 120 � C under vacuum. The average active material (graphite) mass loading was ~3.0 mg cm 2. Zinc foil (thickness: 0.025 mm; purity: 99.95þ%) was pur chased from Goodfellow (Germany) and punched into 16mm discs to be used as negative electrodes. Before use, the metallic zinc discs were rinsed with acetone. 2.2. Electrolyte The water-in-salt-electrolyte (WiSE) was prepared by dissolving 21 mol of LiTFSI (3 M, battery grade) and 3 mol of ZnTfO2 (99.5%, Sol vionic) in 1 kg of de-ionized water (Millipore). 2.3. Characterizations X-ray diffraction (XRD) was conducted on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ ¼ 0.154 nm). In-situ XRD spectra were collected with a scan duration of 15 min in the 2θ range of 20–60� with a step size of 0.02� . The morphological characterizations were conducted with a ZEISS 1550VP field-emission scanning electron microscopy (FESEM) equipped with an Oxford AZtec XMax50 SDD energy-dispersive detector. Liquid Raman spectra were collected with a RAM II FT-Raman module of a Bruker Vertex70 FTIR spectrometer, with a laser wavelength of 1064 nm for 512 scans with a resolution of 2 cm 1 and were analyzed by using PeakFit software (version 4.12, Sea-Solve Software. Inc.). The ionic conductivity of the electrolyte was measured in sealed glass conductivity cells (Materials Mates 192/K1) equipped with two porous platinum electrodes (cell constant of (1.0 � 0.1 cm)) using a Bio-Logic conductivity meter. Aqueous, three-electrode cells for the electrolyte test were fabricated using zinc metal as both counter and reference electrodes, and stainless steel disc (2 cm 2) as the working electrode. Symmetric cell for strip ping/plating tests were assembled into CR2032-type coin cells using zinc metal foil as both the positive and negative electrodes. Full Zn/ graphite cells were fabricated into CR2032-type coin cells using graphite positive electrode, zinc metal disc as the negative electrode and 21 m LiTFSI þ 3 m ZnOTf2 as the electrolyte (here the “m” is molality standing for mol of salt per kg of solvent). All the cells were assembled using glass fiber discs (GF/D, Waterman) separators. Galvanostatic char ge discharge (GCD) tests were performed using battery tester (Auto mated Test System SERIES 4000, Maccor). The current density for the cell test are calculated based on the positive electrode, graphite, weight. Cyclic voltammetries (CV) were conducted on a multichannel poten tiostat (VMP3, Bio-Logic Science Instruments). All measurements were carried out in climatic chambers at 20 � 1 � C. 3. Results and discussion Firstly, the reversibility of metallic zinc anode in the hybrid aqueous electrolyte (21 m LiTFSI þ 3 m ZnTfO2) was investigated. Typically, a Zn/Zn symmetric cell was fabricated for the galvanostatic stripping/ plating test, which time-dependent voltage profile is presented in Fig. 1a (each cycle starts with 1-h zinc dissolution followed by 1-h deposition for 75 cycles at 0.1 mA cm 2). The cell using the hybrid aqueous 2
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electrolyte exhibits very stable voltage profiles over 150 h of operation without obvious polarization as indicated by the stable value at the end of step voltage although some changes are observed in the voltage profile between the initial and final cycles. Furthermore, the stability of Zn stripping/plating in this hybrid aqueous electrolyte was also inves tigated by cyclic voltammetry (CV) in coin cell using a stainless steel disc (area of 2 cm2) as the working electrode and zinc foil as the counter. Fig. 1b depicts the 1st cycle of voltammetry curve, showing the suc cessful deposition of zinc on the stainless steel current collector. The chronoamperometry curves (Fig. 1b, inset) clearly demonstrate the stable and reversible stripping/plating of Zn after several cycles, in good agreement with previous results achieved with the hybrid (LiTFSI and ZnTfO2) aqueous electrolyte [41]. The 5th, 50th and 100th CV curves (Fig. S1, Supplemental Information) confirm the Zn plating/stripping reversibility on the stainless steel after the initial cycles. To reveal the stability of zinc anode in the hybrid aqueous electrolyte, the Zn foil of a symmetric Zn/Zn cell was collected from the cell after 400 stripping/ plating cycles. The XRD pattern (Fig. 1c) shows the hexagonal crystal line phase of zinc metal (PDF#87–0713), without any ZnO and other impurity phases being detected. The SEM image of the cycled Zn elec trode surface (Fig. 1d) exhibits a rather dense and homogeneous morphology without obvious dendrite formation. As reported previ ously, the ZnO formation and Zn-dendrite formation, especially in the alkaline electrolyte, would induce to the low reversibility and even short-circuit the cell [12]. In these tests, both the high concentration of TFSI and TfO anions in the electrolyte could be responsible for the high reversibility of the Zn plating/stripping. In the approach herein described for hybrid aqueous zinc electrolyte, the LiTFSI and ZnTfO2 salts were selected because the former has an extremely high solubility in water (enabling the formation of HWiSE) and the latter has been proved to enable efficient Zn plating/stripping
[42,43]. The electrochemical stability window (ESW) of the hybrid concentrated electrolyte (HWiSE) was evaluated on the stainless steel current collector using metallic Zn as both the counter and reference electrodes. As shown in Fig. 2a, an extended anodic stability potential is achieved (i.e., up to 2.6 V vs. Zn or about 4.9 V vs. Li), which is higher than that reported for 3 M ZnTfO2 and certainly attributable to the rather low fraction of free water molecules with higher decomposition potential [43]. Fig. 2b displays the temperature-dependent conductivity of the hybrid electrolyte. The conductivity strongly depends on the temperature, which is typical behavior for highly concentrated LiTFSI aqueous electrolyte [42]. At room-temperature (20 � C), the electrolyte offers a conductivity of 7 mS cm 1, however, a sudden decrease is observed below 0 � C, which could be ascribed to the salt crystallization. The solvation structure and ion association of the HWiSE were investi gated by Raman spectroscopy. The full range spectrum from 50 to 4000 cm 1 is shown in Fig. S3 (Supplemental Information). To evidence the cation-anion interactions in the hybrid aqueous electrolyte, the S–N–S bending and S–O3 stretching modes were examined (Fig. 2c and d). According to the Gaussian deconvolution, the S–N–S bending peak re veals two main cation-anion (TFSI ) clusters, i.e. loose ion pairs (LIP s)/intimate ion pairs (IIPs) at 743 cm 1 and aggregated ion pairs (AGIPs) at 750 cm 1 [44]. Moreover, the S–O3 stretching peak only demon strates the contact ion pairs (CIPs)/aggregated ion pairs (AGIPs) at 1047 cm 1 of the cation-anion (TfO ) clusters, indicating no free TfO anions in the electrolyte as the typical characteristic of water-in-salt electrolyte [45]. The multiple cation coordination by the TFSI and TfO anions provides the basis for the higher oxidation potential. Commercial graphite powder without any additional treatment was used to make the positive electrodes. Such electrodes were tested in Zn/ graphite dual-ion cells, where zinc foil served as the counter electrode. As shown in Figure S4a (Supplemental Information), the graphite
Fig. 1. Reversibility and stability of the Zn anode in the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) Galvanostatic Zn stripping/plating in a Zn/Zn symmetric cell at 0.1 mA cm 2. (b) Cyclic voltammogram of Zn plating/stripping in a two-electrode cell using a stainless steel disc (2 cm2) as the working and Zn as the counter electrode at a scan rate of 1 mV s 1. Inset: The chronoamperometry curves of the stainless steel/Zn cell over stripping/plating test. (c) XRD pattern and (d) SEM image of the Zn electrode after 400 stripping/plating cycles. 3
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Fig. 2. Characterization of the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) Linear sweep voltammetry recorded at 10 mV s 1 using a stainless steel disc as current collector. (b) Temperature dependence of the ionic conductivity. The Raman spectral regions highlighting the (c) S–N–S bending and the (d) S–O3 stretching features of the two anions in the aqueous electrolyte at 20 � C.
Fig. 3. Electrochemical performance of the Zn/graphite dual-ion battery in the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) First cycle voltage profile of the Zn/graphite dual-ion cell at a constant current of 20 mA g 1. (b) The magnified dQ/dV curve of the 2nd charge–discharge voltage profile in the 0.5–2.5 V range. (c) The charge–discharge performances at different current densities of 20 and 200 mA g 1 in the voltage range of 0.5–2.5 V. (d) The capacities and corresponding Coulombic efficiencies at 200 mA g 1 over 600 cycles. 4
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powder shows its typical XRD diffraction pattern. The (002) feature appears at 26.4� (2θ) with the corresponding d-space being 0.334 nm. The average particle size of the graphite powder is about 10 μm as from the SEM image in Figure S4b. According to the electrochemical stability window of the aqueous electrolyte, we evaluated the electrochemical intercalation of anions into graphite in a Zn/graphite cell. The cell was firstly charged/dis charged in the voltage range of 0.5–2.5 V at 20 mA g 1 (Fig. 3a). The suddenly increasing and then sloping voltage profile upon the 1st gal vanostatic charge discloses a sort of activation process at high voltage. In fact, the first discharge appears rather different, showing a multistep discharge process. The dQ/dV curve of the 2nd charge–discharge voltage profile in the 0.5–2.5 V range as shown in Fig. 3b, which are clearly indicative of the typical anion intercalation into graphite [31]. In particular, three dQ/dV peaks are observed upon charge at about 1.58 V, 2.1 V and above 2.3 V, corresponding to the formation of stage III0 , stage II0 , and stage I0 , respectively. We note that the peak of stage I’ is not obvious, due to cut-off voltage (2.5 V versus Zn2þ/Zn or ~4.8 V versus Liþ/Li). Upon discharge, again three peaks are observed, corresponding to the different stages of the anion deintercalation from graphite. Figure S5a shows the current-voltage curves during the first, second and tenth CV cycles of the dual-ion battery at 0.1 mV s 1 scan rate. During the 1st anodic scan, a sharp peak at 2.25 V appears due to the anion intercalation into the graphite layered structure. Actually, two pairs of redox peaks located at 2.25/2.15 V, 2.38/1.70 V are observed during the first cycle, however, only the redox peaks at 2.38/1.70 V are observed over the following ten cycles. It is highly likely that a pre-activation step of anions intercalating into graphite layer is needed. However, the overlapping redox peaks in the following cycle indicate the high reversibility of the cell after the first cycle. To understand the anion intercalation mechanisms in aqueous electrolyte and the resulting rate capability performance, CV tests at higher scan rates (1.0 mV s 1 and 10 mV s 1) were performed, which results are presented in Fig. S5b. Interestingly, the CV curves resemble one another in their shapes, but higher scan rates lead to lower anion intercalation potential. For example, at 10 mV s 1, one pair of redox peaks at 2.08/1.58 V is found, in which both the cathodic and anodic potentials are lower than those detected at 0.1 mV s 1. As reported, the negative shift of anion inter calation in highly concentrated aqueous electrolyte, i.e., water-in-salt electrolyte (WiSE), contrasts with the typically positive potential shift of cation insertion but consists with the anion insertion behavior of the ferrocene negative electrode in WiSE [42,46]. Fig. 3c depicts the galvanostatic charge/discharge voltage profiles of the Zn/graphite aqueous DIB. During the 1st cycle at low current den sity, 20 mA g 1, the cell shows a charge capacity of 70 mAh g 1 with the voltage plateaus above 2.2 V, i.e., consistent with the CV results. How ever, the capacity of only 27 mAh g 1 is achieved in the following discharge. Nonetheless, we would like to stress that this is the first report of reversible anion intercalation into graphite using aqueous electrolyte. Even at 200 mA g 1, the cell can maintain 20 mAh g 1 capacity with improved Coulombic efficiency. Fig. 3d demonstrates the cycling sta bility test of the aqueous DIB at 200 mA g 1. The initial capacity fading observed in the first 50 cycles is mainly attributed to the electrolyte decomposition at the graphite surface prior the establishment of a stable SEI, the partial degradation of graphite itself due to, e.g., exfoliation [27], and the activation of the zinc surface for reversible plating/strip ping. However, the Zn/graphite DIB exhibits Coulombic efficiency values higher than 95% over 600 cycles, with comparable Coulombic efficiency to the organic dual-ion battery system, for example, Coulombic efficiency value of 94% at 200 mA g 1 for Al/graphite dual-ion battery system within 4 mol L 1 LiPF6 in ethyl–methyl car bonate (EMC) electrolyte [31]. The successful anion intercalation in graphite by WiSE could be attributed to the expanded electrochemical stability window of the aqueous electrolyte due to reduced free water, yielding to rather high CEs, at least comparable to those recorded in organic electrolytes [31,40]. Although only very limited discharge
capacities were achieved in this battery system (Fig. 3c), quite typically stepwise anion intercalation behavior in the graphite similar to organic system was observed [31]. It should be mentioned that cycling aqueous batteries at low rate is often challenging, because the extent of the parasitic reactions, mainly water decomposition, increases during the longer exposure to the extreme potentials [47]. On the other hand, the common metals, such as Al or stainless steel can easily incur into anodic dissolution at the high voltages (e. g., 2.2 V vs. Zn or 4.5 V vs. Li) occurring in organic electrolyte-based dual-ion batteries [38]. Similarly, the anodic dissolu tion of the current collector and parasitic reactions of the electrolyte could contribute to the capacity loss in the concentrated aqueous electrolyte. In situ XRD was used to investigate the structural evolution occurring in the graphite electrode upon the 1st cycle of the Zn/graphite cell (Fig. 4a). The XRD patterns obviously exhibit the stage changes in the interlayer distance of graphite upon anion intercalation. During the 1st charge, the (002) reflection remains the intensity at beginning, however, gradually decreases in intensity, which may result from the initial intercalation of anions into the layered structure. Upon deep charging, the splitting of the (002) reflection of the pristine graphite upon charging evidences the further intercalation process. It has been already clarified that the appearance of the two dominant peaks corresponding to the (00nþ1) and (00nþ2) reflections also indicates the staging of intercalated graphite compounds with all stages from 1 to 4 [40]. The (00nþ1) gradually shifts to left (larger d-spacing) but the (00nþ2) reflection moves inversely. After full charge, i.e., up to the potential limit of 2.5 V vs. Zn, the positions of the two strongest peaks (23.7� and 30.9� in 2θ-units, or 3.75 and 2.90 Å in d-spacing units) match well with the stage 3 of the anion intercalated graphite [48,49]. During the following discharge process, the (n ¼ 3) reflection decreases corre sponding to the anion removal from the interlayers. Finally, the graphite material re-gains its original structure, at least partially, as indicated by the typical (002) reflection re-appearance. This clearly confirms the overall reversibility of the aqueous Zn/graphite dual-ion cell even at the high working voltage. The selected XRD patterns at various charged/ discharged states (Fig. 4b and Fig. S6 in Supplemental Information) clearly illustrate the permanent structural changes occurring during the 1st cycle, mainly the permanence of the (00nþ1) and (00nþ2) re flections. This indicates that a fraction of the anions (TFSI and TfO ) could not be removed explaining the capacity difference between the first charge and the first discharge (Fig. 3c). However, the discharged electrode after the 2nd cycle maintained the same structural imprint with the one after the 1st discharge, indicating the good reversibility of the structural changes after the 1st cycle, in good agreement with the electrochemical performance. Fig. 4c presents a schematic illustration of TFSI and TfO anions intercalated in graphite (stage-3) with a repeating distance (i.e., the distance between two neighboring interca lated layers) of Ic¼(nþ1)d00nþ1¼(nþ2)d00nþ2 ¼ 4 � 3.75 ¼ 5 � 2.90, i. e., ranging from 14.5 nm to 15.0 Å. Overall, the reversible intercalation of the anions, i.e. TFSI and TfO , is well tracked by in situ XRD investigation also showing the irreversible structural changes resulting from the anions trapped in the graphitic layers upon the first interca lation. However, after the first charge/discharge cycle, the subsequent anions (de-)intercalation in graphite becomes relatively reversible due to the opened space in the graphite structure [40,50]. On the basis of the above results, the operating mechanism of Zn/ graphite aqueous DIB can be concluded as follows. During charging, TFSI and TfO anions from electrolyte intercalate into the graphite cathode, while the Zn2þ ions are reduced to metallic zinc on the negative electrode, as schematically shown in Fig. 5a. Rooted from the working mechanism of DIB, the electrolyte’s concentration would decrease gradually during the charge, but, here comes the beneficial impact from the highly concentrated electrolyte, without a substantial decrease of the ESW. Thus, the HWiSE enables the high voltage operation for the successful achievement of aqueous, dual-ion batteries. Compared with 5
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Fig. 4. Electrochemical reaction mechanism of Zn/graphite dual-ion battery in the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) in situ XRD measurements of graphite during charging and discharging. Left: galvanostatic voltage profile measured at a current density of 5 mA g 1 for the first cycle. Right: contour plot of the XRD patterns of graphite during cycling. (b) Selected XRD patterns at various charge and discharge states. (c) Schematic of TFSI and TfO anions intercalated into graphite. Fluorine, oxygen, sulfur, carbon, and nitrogen atoms representing anions are shown in green, red, yellow, black, and blue colors, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Schematic and state-of-the-art of the aqueous Zn/graphite DIB. (a) Aqueous dual-ion battery, where positive electrode operates with the anion inter calation and the negative feature zinc plating/stripping. (b) Comparison of specific capacities and operating voltages of typical cathode materials in various aqueous zinc battery chemistries compared with the theoretical (filled star) and actual (open star) voltage values of the herein developed Zn/graphite DIB.
other Zn battery systems (Fig. 5b), the Zn/graphite configuration shows relatively high cell voltage. Our proof-of-concept aqueous Zn/graphite already demonstrates an output of 1.7 V, which is still far away from the theoretical value of ~2.2 V (in which the graphite would achieve the cathodic voltage equivalent to 4.5 V vs. Liþ/Li). Therefore, the aqueous dual-ion battery demonstrates very promising avenue to enlarge the aqueous battery chemistries and enables the potentially higher working voltage in terms of energy density improvement (in a theoretical value of 40 Wh kg 1 considering the graphite cathode and electrolyte mass).
Meanwhile, a pending challenge on the aqueous electrolyte design would enable the further improvement on the performance of aqueous DIB. Overall, this study proves the fundamental feasibility of graphite cathode for the dual-ion battery concept in aqueous batteries. 4. Conclusions In summary, the presented work demonstrates the feasibility of graphite as a high voltage cathode material for aqueous dual-ion 6
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batteries, which has never been reported before. The proof-of-concept Zn/graphite dual-ion battery shows relatively high reversibility through the metallic Zn stripping/plating on the negative electrode and anions storage in the graphite positive electrode. The proposed HWiSE composed of 21 m LiTFSI and 3 m ZnTfO2 enables the electrochemical stability window up to 2.6 V vs. Zn, and the reversible and dendrite-free Zn stripping/plating. The aqueous Zn/graphite DIB exhibits an excellent cycling stability with a Coulombic efficiency of around 95%, which is comparable to the organic electrolyte-based dual-ion batteries. The structural evolution of the graphite electrode has been comprehensively investigated by in situ X-ray diffraction, showing the typical intercala tion behavior of anions in graphite during charge/discharge process and explaining the observed 1st cycle irreversibility as due to anions trap ping in the electrode structure. This research clearly shows the potential of graphite to act as a positive electrode material in aqueous dual-ion batteries for the next-generation of low-cost and environmentally friendly rechargeable battery assuming that an appropriate replacement is found for the TFSI- anion. Future work should address this and other aspects, to design HWiSE and new electrode materials enabling the more efficient anion storage for aqueous electrolytes. H.Z. conceived the research idea, designed the experiments, analyzed the results, and prepared the first draft of the manuscript. X.L. and B.Q. synthesized the electrode materials and prepared the elec trodes. S.P. supervised and coordinated the work. All authors contrib uted to the writing and have given approval to the final version of the manuscript.
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Declaration of competing interest The authors declare no competing interests. Acknowledgements The authors would like to acknowledge the support Initiative and Networking Fund of the Helmholtz Association within the Network of Excellence on post-Lithium batteries (ExNet-0035). H.Z. acknowledges the financial support from the Fundamental Research Funds for the Central Universities (31020190QD029). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227594. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928–935. [3] A. Manthiram, An outlook on lithium ion battery technology, ACS Cent. Sci. 3 (2017) 1063–1069. [4] J.O.G. Posada, A.J.R. Rennie, S.P. Villar, V.L. Martins, J. Marinaccio, A. Barnes, C. F. Glover, D.A. Worsley, P.J. Hall, Aqueous batteries as grid scale energy storage solutions, Renew. Sustain. Energy Rev. 68 (2017) 1174–1182. [5] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19–29. [6] H. Kim, G. Jeong, Y.-U. Kim, J.-H. Kim, C.-M. Park, H.-J. Sohn, Metallic anodes for next generation secondary batteries, Chem. Soc. Rev. 42 (2013) 9011–9034. [7] C. Xu, B. Li, H. Du, F. Kang, Energetic zinc ion chemistry: the rechargeable zinc ion battery, Angew. Chem. Int. Ed. 51 (2012) 933–935. [8] F.R. McLarnon, E.J. Cairns, The secondary alkaline zinc electrode, J. Electrochem. Soc. 138 (1991) 645–656. [9] Y. Shen, K. Kordesch, The mechanism of capacity fade of rechargeable alkaline manganese dioxide zinc cells, J. Power Sources 87 (2000) 162–166. [10] F.W. Thomas Goh, Z. Liu, T.S.A. Hor, J. Zhang, X. Ge, Y. Zong, A. Yu, W. Khoo, A near-neutral chloride electrolyte for electrically rechargeable zinc–air batteries, J. Electrochem. Soc. 161 (2014) A2080–A2086. [11] J. Yu, F. Chen, Q. Tang, T.T. Gebremariam, J. Wang, X. Gong, X. Wang, Agmodified Cu foams as three-dimensional anodes for rechargeable zinc–air batteries, ACS Appl. Nano Mater. 2 (2019) 2679–2688.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electrochemical intercalation of anions in graphite for high-voltage aqueous zinc battery Huang Zhang a, b, c, Xu Liu b, c, Bingsheng Qin b, c, Stefano Passerini b, c, * a
Xi’an Institute of Flexible Electronics (IFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an, 710072, Shaanxi, China Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081, Ulm, Germany c Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021, Karlsruhe, Germany b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Hybrid “water-in-salt” electrolyte for zinc batteries. � Highly reversible Zn stripping/plating reaction. � Graphite cathode for anion intercalation in aqueous electrolyte. � A novel concept of aqueous dual-ion battery with high operating voltage.
A R T I C L E I N F O
A B S T R A C T
Keywords: Zn/graphite batteries Dual-ion batteries Zn anode Aqueous electrolyte High-voltage
Rechargeable aqueous zinc batteries are ideal for large-scale energy storage due to their low cost and high safety. Here we demonstrate the reversible anion intercalation chemistry in graphite cathode for aqueous zinc battery, thus operating as a high-voltage dual-ion battery system. The use of a hybrid “water-in-salt” electrolyte with zinc and lithium bi-salts can maximize the ionic content, expand the anodic stability, promote dendrite-free zinc plating/stripping, and bring the reversible anion intercalation in graphite cathode. This cell configuration offers a high operating voltage and unprecedented cycling stability with comparable Coulombic efficiency to the organic dual-ion battery, opening new opportunities for the development of high-voltage rechargeable aqueous batteries.
1. Introduction
the large variety of battery chemistries recently proposed, aqueous batteries rise as very promising candidates, offering both the low-cost and safety advantages [4,5]. Metallic zinc has been regarded as an ideal anode material for aqueous batteries, due to its high theoretical capacity (820 mAh g 1), low redox potential ( 0.76 V vs. SHE), high abundance and intrinsic benignity, alongside with the safety of aqueous battery systems [6]. These advantages have driven the development of aqueous zinc batteries using a metal as the negative electrode and a Zn
The search for new rechargeable battery systems with high energy density and safety is triggered by the rapid development of renewable energy resources [1,2]. Although conventional “rocking-chair” lithium-ion batteries have been successfully employed in portable electronic devices and electric vehicles, the large-scale energy storage requires low-cost and safety rather than high energy density [3]. Among
* Corresponding author. Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081, Ulm, Germany. E-mail address: [email protected] (S. Passerini). https://doi.org/10.1016/j.jpowsour.2019.227594 Received 21 October 2019; Received in revised form 23 November 2019; Accepted 8 December 2019 Available online 25 January 2020 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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insertion compound as the positive electrode [7]. In fact, although alkaline Zn/MnO2 batteries have been shown rechargeable for extended cycles their delivered capacity is limited. Additionally, the Zn electrode in alkaline electrolytes persistently suffers from poor reversibility with low Coulombic efficiency and dendritic plating, which make the further performance improvement rather difficult [8,9]. Although there are some reports on the dendrite-free zinc deposition by using neutral pH electrolytes and innovative anode architectures [10,11], very few pos itive electrode materials, i.e. polymorphs of MnO2 [12–14], hex acyanoferrate [15–17], vanadium oxide [18–20], and organic quinone [21], have been explored as hosts for reversible Zn2þ insertion. How ever, they either show poor cycling stability or low working potential, consequently offering limited energy densities [22]. In contrast to the conventional batteries making use of a single electroactive species (e.g., a metal-ion), another concept, commonly referred to as hybrid or dual-ion battery (DIB), is attracting increasing attention. This battery chemistry operates via both the anions and cat ions in the electrolyte, which are active at the positive (cathode) and the negative (anode) electrodes, respectively [23]. The most investigated DIB system is the dual-graphite battery, employing graphite as both anion host at the cathode and cation host at the anode [24]. In principle, any anode materials in conventional metal-ion batteries, such as inser tion, alloying or conversion-type materials, could work as anodes for DIBs. However, to enable the maximum output of the cell at the lowest cost, metallic anodes based on abundant metals are especially attractive. Indeed, potassium [25,26], sodium [27,28], calcium [29,30], and aluminum [31,32] based dual-ion batteries have been explored as environmentally friendly, high voltage battery systems. Similar to the metal-batteries, the reversibility of metal anode is essential in terms of cycling stability. Meanwhile, very limited materials have been reported as hosts for anion uptake, among which graphite is ideal with its layered structure [33]. Indeed, graphitic carbon has been proved able to inter calate tetrachloroaluminate (AlCl4 ) [34], hexafluorophosphate (PF6 ) [35], trifluoromethanesulfonate (TfO ) [36], bis(tri fluoromethanesulfonimide) (TFSI ) [37], fluorosulfonyl(tri fluoromethanesulfonyl) imide (FTFSI ) [38], bis (pentafluoroethanesulfonyl) imide (BETI ) [39], and bis(fluorosulfonyl) imide (FSI ) [40] anions from ionic liquids and/or organic electrolytes. However, most DIBs suffer from energy density, rooted in their opera tion principle where the electrolyte is the only source of ions, thus becoming an active component in the system. Its weight affects the overall cell performance. The promising approach for increasing the energy density of DIBs is to maximize the ionic content in the electrolyte without compromising the charge storage capacities of the electrodes [33,40]. On the other hand, the use of organic solvent for the anion intercalation into graphite, typically requires an operating potential above 4.5 V vs. Liþ/Li, brings to the safety concern for practical appli cation. Aqueous electrolyte would be an option, however, the operation condition is beyond the anodic stability limit of conventional aqueous electrolyte, inhibiting the dual-ion chemistries in aqueous energy stor age applications. In this work, we present an aqueous dual-ion battery concept using a graphite cathode and a zinc anode in highly concentrated aqueous electrolytes. We employed a hybrid 21 m LiTFSI and 3 m ZnTfO2 “waterin-salt” zinc electrolyte (HWiSE), which maximizes the ionic content, expands the anodic stability window, promotes dendrite-free zinc plating/stripping with high columbic efficiency, and enables the reversible anion intercalation in graphite cathode. The Zn metal anode has been examined in details, showing an excellent reversibility. In spite of the intriguing reversibility with comparable Coulombic efficiency (>95%) to organic system, the cell offered potentially high operating voltage. The structural evolution of graphite cathode was investigated during electrochemical reactions by in-situ X-ray diffraction (XRD) revealing the anion intercalation process in aqueous electrolyte. This study provides direct insights enabling the design of high energy density aqueous rechargeable batteries for alternative energy storage systems,
using the dual-ion battery concept. 2. Experimental methods 2.1. Materials Commercial graphite (SLP-30, IMERYS) was used as received without any further treatment. The positive electrodes were fabricated by casting slurries composed of 90 wt% graphite and 10 wt% poly vinylidenefluoride (PVDF, Solef 6020, Solvay) in N-methyl-2-pyrroli done (NMP, anhydrous, Sigma-Aldrich) on stainless foil (type 316; thickness: 0.050 mm). After pre-drying at 80 � C in air, electrodes with a diameter of 14 mm were punched and further dried at 120 � C under vacuum. The average active material (graphite) mass loading was ~3.0 mg cm 2. Zinc foil (thickness: 0.025 mm; purity: 99.95þ%) was pur chased from Goodfellow (Germany) and punched into 16mm discs to be used as negative electrodes. Before use, the metallic zinc discs were rinsed with acetone. 2.2. Electrolyte The water-in-salt-electrolyte (WiSE) was prepared by dissolving 21 mol of LiTFSI (3 M, battery grade) and 3 mol of ZnTfO2 (99.5%, Sol vionic) in 1 kg of de-ionized water (Millipore). 2.3. Characterizations X-ray diffraction (XRD) was conducted on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ ¼ 0.154 nm). In-situ XRD spectra were collected with a scan duration of 15 min in the 2θ range of 20–60� with a step size of 0.02� . The morphological characterizations were conducted with a ZEISS 1550VP field-emission scanning electron microscopy (FESEM) equipped with an Oxford AZtec XMax50 SDD energy-dispersive detector. Liquid Raman spectra were collected with a RAM II FT-Raman module of a Bruker Vertex70 FTIR spectrometer, with a laser wavelength of 1064 nm for 512 scans with a resolution of 2 cm 1 and were analyzed by using PeakFit software (version 4.12, Sea-Solve Software. Inc.). The ionic conductivity of the electrolyte was measured in sealed glass conductivity cells (Materials Mates 192/K1) equipped with two porous platinum electrodes (cell constant of (1.0 � 0.1 cm)) using a Bio-Logic conductivity meter. Aqueous, three-electrode cells for the electrolyte test were fabricated using zinc metal as both counter and reference electrodes, and stainless steel disc (2 cm 2) as the working electrode. Symmetric cell for strip ping/plating tests were assembled into CR2032-type coin cells using zinc metal foil as both the positive and negative electrodes. Full Zn/ graphite cells were fabricated into CR2032-type coin cells using graphite positive electrode, zinc metal disc as the negative electrode and 21 m LiTFSI þ 3 m ZnOTf2 as the electrolyte (here the “m” is molality standing for mol of salt per kg of solvent). All the cells were assembled using glass fiber discs (GF/D, Waterman) separators. Galvanostatic char ge discharge (GCD) tests were performed using battery tester (Auto mated Test System SERIES 4000, Maccor). The current density for the cell test are calculated based on the positive electrode, graphite, weight. Cyclic voltammetries (CV) were conducted on a multichannel poten tiostat (VMP3, Bio-Logic Science Instruments). All measurements were carried out in climatic chambers at 20 � 1 � C. 3. Results and discussion Firstly, the reversibility of metallic zinc anode in the hybrid aqueous electrolyte (21 m LiTFSI þ 3 m ZnTfO2) was investigated. Typically, a Zn/Zn symmetric cell was fabricated for the galvanostatic stripping/ plating test, which time-dependent voltage profile is presented in Fig. 1a (each cycle starts with 1-h zinc dissolution followed by 1-h deposition for 75 cycles at 0.1 mA cm 2). The cell using the hybrid aqueous 2
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electrolyte exhibits very stable voltage profiles over 150 h of operation without obvious polarization as indicated by the stable value at the end of step voltage although some changes are observed in the voltage profile between the initial and final cycles. Furthermore, the stability of Zn stripping/plating in this hybrid aqueous electrolyte was also inves tigated by cyclic voltammetry (CV) in coin cell using a stainless steel disc (area of 2 cm2) as the working electrode and zinc foil as the counter. Fig. 1b depicts the 1st cycle of voltammetry curve, showing the suc cessful deposition of zinc on the stainless steel current collector. The chronoamperometry curves (Fig. 1b, inset) clearly demonstrate the stable and reversible stripping/plating of Zn after several cycles, in good agreement with previous results achieved with the hybrid (LiTFSI and ZnTfO2) aqueous electrolyte [41]. The 5th, 50th and 100th CV curves (Fig. S1, Supplemental Information) confirm the Zn plating/stripping reversibility on the stainless steel after the initial cycles. To reveal the stability of zinc anode in the hybrid aqueous electrolyte, the Zn foil of a symmetric Zn/Zn cell was collected from the cell after 400 stripping/ plating cycles. The XRD pattern (Fig. 1c) shows the hexagonal crystal line phase of zinc metal (PDF#87–0713), without any ZnO and other impurity phases being detected. The SEM image of the cycled Zn elec trode surface (Fig. 1d) exhibits a rather dense and homogeneous morphology without obvious dendrite formation. As reported previ ously, the ZnO formation and Zn-dendrite formation, especially in the alkaline electrolyte, would induce to the low reversibility and even short-circuit the cell [12]. In these tests, both the high concentration of TFSI and TfO anions in the electrolyte could be responsible for the high reversibility of the Zn plating/stripping. In the approach herein described for hybrid aqueous zinc electrolyte, the LiTFSI and ZnTfO2 salts were selected because the former has an extremely high solubility in water (enabling the formation of HWiSE) and the latter has been proved to enable efficient Zn plating/stripping
[42,43]. The electrochemical stability window (ESW) of the hybrid concentrated electrolyte (HWiSE) was evaluated on the stainless steel current collector using metallic Zn as both the counter and reference electrodes. As shown in Fig. 2a, an extended anodic stability potential is achieved (i.e., up to 2.6 V vs. Zn or about 4.9 V vs. Li), which is higher than that reported for 3 M ZnTfO2 and certainly attributable to the rather low fraction of free water molecules with higher decomposition potential [43]. Fig. 2b displays the temperature-dependent conductivity of the hybrid electrolyte. The conductivity strongly depends on the temperature, which is typical behavior for highly concentrated LiTFSI aqueous electrolyte [42]. At room-temperature (20 � C), the electrolyte offers a conductivity of 7 mS cm 1, however, a sudden decrease is observed below 0 � C, which could be ascribed to the salt crystallization. The solvation structure and ion association of the HWiSE were investi gated by Raman spectroscopy. The full range spectrum from 50 to 4000 cm 1 is shown in Fig. S3 (Supplemental Information). To evidence the cation-anion interactions in the hybrid aqueous electrolyte, the S–N–S bending and S–O3 stretching modes were examined (Fig. 2c and d). According to the Gaussian deconvolution, the S–N–S bending peak re veals two main cation-anion (TFSI ) clusters, i.e. loose ion pairs (LIP s)/intimate ion pairs (IIPs) at 743 cm 1 and aggregated ion pairs (AGIPs) at 750 cm 1 [44]. Moreover, the S–O3 stretching peak only demon strates the contact ion pairs (CIPs)/aggregated ion pairs (AGIPs) at 1047 cm 1 of the cation-anion (TfO ) clusters, indicating no free TfO anions in the electrolyte as the typical characteristic of water-in-salt electrolyte [45]. The multiple cation coordination by the TFSI and TfO anions provides the basis for the higher oxidation potential. Commercial graphite powder without any additional treatment was used to make the positive electrodes. Such electrodes were tested in Zn/ graphite dual-ion cells, where zinc foil served as the counter electrode. As shown in Figure S4a (Supplemental Information), the graphite
Fig. 1. Reversibility and stability of the Zn anode in the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) Galvanostatic Zn stripping/plating in a Zn/Zn symmetric cell at 0.1 mA cm 2. (b) Cyclic voltammogram of Zn plating/stripping in a two-electrode cell using a stainless steel disc (2 cm2) as the working and Zn as the counter electrode at a scan rate of 1 mV s 1. Inset: The chronoamperometry curves of the stainless steel/Zn cell over stripping/plating test. (c) XRD pattern and (d) SEM image of the Zn electrode after 400 stripping/plating cycles. 3
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Fig. 2. Characterization of the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) Linear sweep voltammetry recorded at 10 mV s 1 using a stainless steel disc as current collector. (b) Temperature dependence of the ionic conductivity. The Raman spectral regions highlighting the (c) S–N–S bending and the (d) S–O3 stretching features of the two anions in the aqueous electrolyte at 20 � C.
Fig. 3. Electrochemical performance of the Zn/graphite dual-ion battery in the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) First cycle voltage profile of the Zn/graphite dual-ion cell at a constant current of 20 mA g 1. (b) The magnified dQ/dV curve of the 2nd charge–discharge voltage profile in the 0.5–2.5 V range. (c) The charge–discharge performances at different current densities of 20 and 200 mA g 1 in the voltage range of 0.5–2.5 V. (d) The capacities and corresponding Coulombic efficiencies at 200 mA g 1 over 600 cycles. 4
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powder shows its typical XRD diffraction pattern. The (002) feature appears at 26.4� (2θ) with the corresponding d-space being 0.334 nm. The average particle size of the graphite powder is about 10 μm as from the SEM image in Figure S4b. According to the electrochemical stability window of the aqueous electrolyte, we evaluated the electrochemical intercalation of anions into graphite in a Zn/graphite cell. The cell was firstly charged/dis charged in the voltage range of 0.5–2.5 V at 20 mA g 1 (Fig. 3a). The suddenly increasing and then sloping voltage profile upon the 1st gal vanostatic charge discloses a sort of activation process at high voltage. In fact, the first discharge appears rather different, showing a multistep discharge process. The dQ/dV curve of the 2nd charge–discharge voltage profile in the 0.5–2.5 V range as shown in Fig. 3b, which are clearly indicative of the typical anion intercalation into graphite [31]. In particular, three dQ/dV peaks are observed upon charge at about 1.58 V, 2.1 V and above 2.3 V, corresponding to the formation of stage III0 , stage II0 , and stage I0 , respectively. We note that the peak of stage I’ is not obvious, due to cut-off voltage (2.5 V versus Zn2þ/Zn or ~4.8 V versus Liþ/Li). Upon discharge, again three peaks are observed, corresponding to the different stages of the anion deintercalation from graphite. Figure S5a shows the current-voltage curves during the first, second and tenth CV cycles of the dual-ion battery at 0.1 mV s 1 scan rate. During the 1st anodic scan, a sharp peak at 2.25 V appears due to the anion intercalation into the graphite layered structure. Actually, two pairs of redox peaks located at 2.25/2.15 V, 2.38/1.70 V are observed during the first cycle, however, only the redox peaks at 2.38/1.70 V are observed over the following ten cycles. It is highly likely that a pre-activation step of anions intercalating into graphite layer is needed. However, the overlapping redox peaks in the following cycle indicate the high reversibility of the cell after the first cycle. To understand the anion intercalation mechanisms in aqueous electrolyte and the resulting rate capability performance, CV tests at higher scan rates (1.0 mV s 1 and 10 mV s 1) were performed, which results are presented in Fig. S5b. Interestingly, the CV curves resemble one another in their shapes, but higher scan rates lead to lower anion intercalation potential. For example, at 10 mV s 1, one pair of redox peaks at 2.08/1.58 V is found, in which both the cathodic and anodic potentials are lower than those detected at 0.1 mV s 1. As reported, the negative shift of anion inter calation in highly concentrated aqueous electrolyte, i.e., water-in-salt electrolyte (WiSE), contrasts with the typically positive potential shift of cation insertion but consists with the anion insertion behavior of the ferrocene negative electrode in WiSE [42,46]. Fig. 3c depicts the galvanostatic charge/discharge voltage profiles of the Zn/graphite aqueous DIB. During the 1st cycle at low current den sity, 20 mA g 1, the cell shows a charge capacity of 70 mAh g 1 with the voltage plateaus above 2.2 V, i.e., consistent with the CV results. How ever, the capacity of only 27 mAh g 1 is achieved in the following discharge. Nonetheless, we would like to stress that this is the first report of reversible anion intercalation into graphite using aqueous electrolyte. Even at 200 mA g 1, the cell can maintain 20 mAh g 1 capacity with improved Coulombic efficiency. Fig. 3d demonstrates the cycling sta bility test of the aqueous DIB at 200 mA g 1. The initial capacity fading observed in the first 50 cycles is mainly attributed to the electrolyte decomposition at the graphite surface prior the establishment of a stable SEI, the partial degradation of graphite itself due to, e.g., exfoliation [27], and the activation of the zinc surface for reversible plating/strip ping. However, the Zn/graphite DIB exhibits Coulombic efficiency values higher than 95% over 600 cycles, with comparable Coulombic efficiency to the organic dual-ion battery system, for example, Coulombic efficiency value of 94% at 200 mA g 1 for Al/graphite dual-ion battery system within 4 mol L 1 LiPF6 in ethyl–methyl car bonate (EMC) electrolyte [31]. The successful anion intercalation in graphite by WiSE could be attributed to the expanded electrochemical stability window of the aqueous electrolyte due to reduced free water, yielding to rather high CEs, at least comparable to those recorded in organic electrolytes [31,40]. Although only very limited discharge
capacities were achieved in this battery system (Fig. 3c), quite typically stepwise anion intercalation behavior in the graphite similar to organic system was observed [31]. It should be mentioned that cycling aqueous batteries at low rate is often challenging, because the extent of the parasitic reactions, mainly water decomposition, increases during the longer exposure to the extreme potentials [47]. On the other hand, the common metals, such as Al or stainless steel can easily incur into anodic dissolution at the high voltages (e. g., 2.2 V vs. Zn or 4.5 V vs. Li) occurring in organic electrolyte-based dual-ion batteries [38]. Similarly, the anodic dissolu tion of the current collector and parasitic reactions of the electrolyte could contribute to the capacity loss in the concentrated aqueous electrolyte. In situ XRD was used to investigate the structural evolution occurring in the graphite electrode upon the 1st cycle of the Zn/graphite cell (Fig. 4a). The XRD patterns obviously exhibit the stage changes in the interlayer distance of graphite upon anion intercalation. During the 1st charge, the (002) reflection remains the intensity at beginning, however, gradually decreases in intensity, which may result from the initial intercalation of anions into the layered structure. Upon deep charging, the splitting of the (002) reflection of the pristine graphite upon charging evidences the further intercalation process. It has been already clarified that the appearance of the two dominant peaks corresponding to the (00nþ1) and (00nþ2) reflections also indicates the staging of intercalated graphite compounds with all stages from 1 to 4 [40]. The (00nþ1) gradually shifts to left (larger d-spacing) but the (00nþ2) reflection moves inversely. After full charge, i.e., up to the potential limit of 2.5 V vs. Zn, the positions of the two strongest peaks (23.7� and 30.9� in 2θ-units, or 3.75 and 2.90 Å in d-spacing units) match well with the stage 3 of the anion intercalated graphite [48,49]. During the following discharge process, the (n ¼ 3) reflection decreases corre sponding to the anion removal from the interlayers. Finally, the graphite material re-gains its original structure, at least partially, as indicated by the typical (002) reflection re-appearance. This clearly confirms the overall reversibility of the aqueous Zn/graphite dual-ion cell even at the high working voltage. The selected XRD patterns at various charged/ discharged states (Fig. 4b and Fig. S6 in Supplemental Information) clearly illustrate the permanent structural changes occurring during the 1st cycle, mainly the permanence of the (00nþ1) and (00nþ2) re flections. This indicates that a fraction of the anions (TFSI and TfO ) could not be removed explaining the capacity difference between the first charge and the first discharge (Fig. 3c). However, the discharged electrode after the 2nd cycle maintained the same structural imprint with the one after the 1st discharge, indicating the good reversibility of the structural changes after the 1st cycle, in good agreement with the electrochemical performance. Fig. 4c presents a schematic illustration of TFSI and TfO anions intercalated in graphite (stage-3) with a repeating distance (i.e., the distance between two neighboring interca lated layers) of Ic¼(nþ1)d00nþ1¼(nþ2)d00nþ2 ¼ 4 � 3.75 ¼ 5 � 2.90, i. e., ranging from 14.5 nm to 15.0 Å. Overall, the reversible intercalation of the anions, i.e. TFSI and TfO , is well tracked by in situ XRD investigation also showing the irreversible structural changes resulting from the anions trapped in the graphitic layers upon the first interca lation. However, after the first charge/discharge cycle, the subsequent anions (de-)intercalation in graphite becomes relatively reversible due to the opened space in the graphite structure [40,50]. On the basis of the above results, the operating mechanism of Zn/ graphite aqueous DIB can be concluded as follows. During charging, TFSI and TfO anions from electrolyte intercalate into the graphite cathode, while the Zn2þ ions are reduced to metallic zinc on the negative electrode, as schematically shown in Fig. 5a. Rooted from the working mechanism of DIB, the electrolyte’s concentration would decrease gradually during the charge, but, here comes the beneficial impact from the highly concentrated electrolyte, without a substantial decrease of the ESW. Thus, the HWiSE enables the high voltage operation for the successful achievement of aqueous, dual-ion batteries. Compared with 5
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Fig. 4. Electrochemical reaction mechanism of Zn/graphite dual-ion battery in the hybrid aqueous zinc electrolyte (21 m LiTFSI þ 3 m ZnTfO2). (a) in situ XRD measurements of graphite during charging and discharging. Left: galvanostatic voltage profile measured at a current density of 5 mA g 1 for the first cycle. Right: contour plot of the XRD patterns of graphite during cycling. (b) Selected XRD patterns at various charge and discharge states. (c) Schematic of TFSI and TfO anions intercalated into graphite. Fluorine, oxygen, sulfur, carbon, and nitrogen atoms representing anions are shown in green, red, yellow, black, and blue colors, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Schematic and state-of-the-art of the aqueous Zn/graphite DIB. (a) Aqueous dual-ion battery, where positive electrode operates with the anion inter calation and the negative feature zinc plating/stripping. (b) Comparison of specific capacities and operating voltages of typical cathode materials in various aqueous zinc battery chemistries compared with the theoretical (filled star) and actual (open star) voltage values of the herein developed Zn/graphite DIB.
other Zn battery systems (Fig. 5b), the Zn/graphite configuration shows relatively high cell voltage. Our proof-of-concept aqueous Zn/graphite already demonstrates an output of 1.7 V, which is still far away from the theoretical value of ~2.2 V (in which the graphite would achieve the cathodic voltage equivalent to 4.5 V vs. Liþ/Li). Therefore, the aqueous dual-ion battery demonstrates very promising avenue to enlarge the aqueous battery chemistries and enables the potentially higher working voltage in terms of energy density improvement (in a theoretical value of 40 Wh kg 1 considering the graphite cathode and electrolyte mass).
Meanwhile, a pending challenge on the aqueous electrolyte design would enable the further improvement on the performance of aqueous DIB. Overall, this study proves the fundamental feasibility of graphite cathode for the dual-ion battery concept in aqueous batteries. 4. Conclusions In summary, the presented work demonstrates the feasibility of graphite as a high voltage cathode material for aqueous dual-ion 6
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batteries, which has never been reported before. The proof-of-concept Zn/graphite dual-ion battery shows relatively high reversibility through the metallic Zn stripping/plating on the negative electrode and anions storage in the graphite positive electrode. The proposed HWiSE composed of 21 m LiTFSI and 3 m ZnTfO2 enables the electrochemical stability window up to 2.6 V vs. Zn, and the reversible and dendrite-free Zn stripping/plating. The aqueous Zn/graphite DIB exhibits an excellent cycling stability with a Coulombic efficiency of around 95%, which is comparable to the organic electrolyte-based dual-ion batteries. The structural evolution of the graphite electrode has been comprehensively investigated by in situ X-ray diffraction, showing the typical intercala tion behavior of anions in graphite during charge/discharge process and explaining the observed 1st cycle irreversibility as due to anions trap ping in the electrode structure. This research clearly shows the potential of graphite to act as a positive electrode material in aqueous dual-ion batteries for the next-generation of low-cost and environmentally friendly rechargeable battery assuming that an appropriate replacement is found for the TFSI- anion. Future work should address this and other aspects, to design HWiSE and new electrode materials enabling the more efficient anion storage for aqueous electrolytes. H.Z. conceived the research idea, designed the experiments, analyzed the results, and prepared the first draft of the manuscript. X.L. and B.Q. synthesized the electrode materials and prepared the elec trodes. S.P. supervised and coordinated the work. All authors contrib uted to the writing and have given approval to the final version of the manuscript.
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