An Organic Cathode Based Dual-Ion Aqueous Zinc Battery Enabled

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An Organic Cathode for a Dual-ion Aqueous Zinc Battery Enabled by a Cellulose Membrane Hadrien Glatz, Erlantz Lizundia, Fiona Pacifico, and Dipan Kundu ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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ACS Applied Energy Materials

An Organic Cathode for a Dual-ion Aqueous Zinc Battery Enabled by a Cellulose Membrane Hadrien Glatz,a Erlantz Lizundia,b, c Fiona Pacifico,a and Dipan Kundu*a *E-mail: [email protected] a

Multifunctional Materials, Dept. of Materials, ETH Zürich, Vladimir Prelog Weg 5, 8093 Zürich, Switzerland b Department of Graphic Design and Engineering Projects, University of the Basque Country (UPV/EHU), Bilbao 48103, Spain. c BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain.

ABSTRACT Aqueous zinc batteries (AZBs) have recently garnered considerable interest owing to their potential cost benefit and safety. Use of abundant and high capacity zinc metal anode and inexpensive and safe aqueous electrolytes make them suitable for large-scale energy storage applications. Yet, sluggish solid-state diffusion of divalent zinc put stringent requirements on the choice of inorganic host structures. Organic solids, which are presumably sustainable, offer unique versatility, as they possess soft lattice for facile ionic diffusion and diverse redox functions. Here, we tap into that prospect with a novel organic cathode, namely 1,4 bis(diphenylamino)benzene (BDB), which delivers nearly a 2-electron redox capacity of 125 mAh g-1, at an average voltage of 1.25 V in an AZB. The two tertiary nitrogens reversibly oxidize/reduce in two steps, with accompanying anion insertion/release from/into a highly concentrated aqueous electrolyte possessing a high oxidative stability. Reversible plating/stripping of zinc on the anode side complements the anion (de) insertion on the cathode side, yielding a rechargeable dual-ion system. Paired with a cellulose nanocrystal membrane to suppress the active material diffusion into the electrolyte, the BDB cathode delivers 112 mAh g-1 of capacity with 82% retention after 500 cycles at a 3C rate (1C = 130 mA g-1), and 1000 cycles with 75% capacity retention at a 6C rate, at nearly 100% Coulombic efficiency. Reversible electrochemistry is accompanied by two reversible biphasic transformations and reversible chemical evolution between BDB, BDB+, and BDB2+ species, as evidenced by operando X-ray diffraction and solid-state operando ultraviolet-visible

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spectroscopy studies. These results highlight a new avenue and understanding of organic cathode hosts development for AZBs. Keywords: Aqueous zinc batteries, organic cathode, dual-ion battery, anion insertion cathode, highly concentrated electrolyte, cellulose nanocrystal membrane INTRODUCTION Aqueous rechargeable batteries (ARBs) utilizing near neutral aqueous electrolytes operate by the same principle as commercial Li-ion batteries, i.e., reversible storage of guest ions in host electrodes. However, ARBs can offer significant advantages over their nonaqueous Li-ion analogues in terms of cost, safety, and cycle life, which are critical parameters for large-scale storage applications.1,2 ARBs utilize inexpensive and highly safe water based electrolytes (e.g., 1 M ZnSO4 in H2O), and do not involve the dry atmospheric assembly conditions of nonaqueous batteries, making them attractive candidates for development as stationary and renewable storage devices. Among ARBs, aqueous zinc batteries (AZBs) are unique as they can use a high capacity zinc anode (820 mAh g-1)3 unlike aqueous Li/Na/Mg-ions that are primarily hybrid systems combining an intercalation cathode with a supercapacitor type anode.4-6 A high kinetic overpotential for hydrogen evolution on zinc allows it to be reversibly stripped/deposited from aqueous electrolytes.7 Furthermore, zinc is nontoxic, highly abundant and inexpensive.7 The issues of poor reversibility and morphological instability, observed with alkaline electrolytes, are not a concern with aqueous electrolytes of pH 3 to 7.8 However, severe diffusion limitation imposed by high charge density of Zn2+ (small size: 0.76 Å and double valence) restricts the choice of inorganic hosts that can reversibly store zinc.9 Layered vanadium oxide bronze,10,11 MnO2,12,13,14 and metal hexacyanoferrates15,16 display attractive performance as they possess wide and intersecting Zn2+ migration pathways. Additionally, use of nanostructured materials, which increases the electrode/electrolyte contact area and shortens the ion diffusion length, is crucial for facile solidstate electrochemical kinetics. Organic solids - bound by weak intermolecular forces - are potentially inexpensive and a sustainable alternative to inorganic materials for diverse types of ion storage.17-20 Their soft lattice and ability to accommodate ions through molecular reorientation even allow reversible intercalation of hard divalent ions like Mg2+ and Zn2+.6,17,18,21 So far, only three quinone compounds,21-23 and one organic polymer based on tetrathiafulvalene substituent,24 have been 2 ACS Paragon Plus Environment

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reported as cathode host for AZBs. The performance of all these systems is characterized by very low discharge/charge voltage polarization – thus, high energy efficiency, besides good cycling stability. Unlike Zn2+ insertion compounds like quinones, anion insertion systems where electron loss during oxidation is compensated by anion uptake offer an interesting alternative. Pairing such an anion-hosting cathode with a zinc anode in aqueous electrolyte affords a rechargeable dual-ion aqueous zinc battery. Reversible anion incorporation not only makes the cathode bypass the slow solid-state kinetics associated with zinc ion insertion25 but also furnishes relatively high voltage systems.26 Tertiary ammonium chemistries, which are typically recognized for very fast kinetics in the nonaqueous Li/Na batteries,27 have rarely been explored in the aqueous system. In this report, we introduce such a compound – namely, 1,4-bis(diphenylamino)benzene (BDB) for an aqueous battery with a zinc anode. The two tertiary nitrogen centers stepwise oxidize/reduce during charge/discharge when the anions from electrolyte reversibly gets in/out to balance the charge (Fig. 1a). Interestingly, the oxidized BDB was found to catalyze the water oxidation at lower potentials, which has been prevented by employing a highly concentrated aqueous electrolyte, also known as the ‘water-in-salt’ electrolyte.28,29 The concentrated electrolyte enables reversible cycling of BDB at an average voltage of 1.25 V, delivering a reversible capacity of 125 mAh g-1 for a ~2e- redox process. Stable cycling for 500 cycles with 82% retention and 1000 cycles with 75% retention is achieved at a 3C (1C: 130 mA g-1) and a 6C rate, respectively, with a thin cellulose nanocrystal (CNC) membrane on the BDB cathode. As revealed by the ultraviolet-visible (UV-Vis) spectroscopy of the cycled electrolyte, the negatively charged (negative zeta potential) CNC membrane inhibits the diffusion of soluble BDB+/BDB2+ species into the electrolyte, thus ensuring long-term cycling of BDB at nearly 100% Coulombic efficiency. Operando X-ray diffraction (XRD) and solid-state operando UV-Vis spectroscopy, respectively, confirmed the structural and chemical reversibility of the system. EXPERIMENTAL SECTION Preparation of cellulose nanocrystals (CNCs) and CNC membrane. Cellulose nanocrystals (CNCs) were synthesized via a sulphuric acid assisted method followed by a tip sonication as previously reported.30,31 20 g of commercial microcrystalline cellulose (Aldrich) was hydrolyzed with 400 mL of 64 % (w/w) sulphuric acid solution at 45 °C for 30 minutes with constant stirring (400 rpm). The reaction was quenched by adding 10-fold cold deionized water. The suspension was centrifuged several times at 4000 rpm for 10 minutes to concentrate the cellulose and to 3 ACS Paragon Plus Environment


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Figure 1. BDB redox couples and the effect of a highly concentrated electrolyte. (a) Two step oxidation/reduction of BDB accompanied by anion uptake/expulsion; A-: OTf-/TFSI-. (b) Illustration of the catalytic oxidation of free H2O by BDB2+ species in regular electrolytes (1 M ZnSO4 or 2 M Zn(OTf)2 in H2O) vs. the oxidative stability of a highly concentrated electrolyte, devoid of free water.

remove excess aqueous acid. Nanocellulose was then obtained by sonicating the concentrated cellulose suspension in a Vibracell Sonicator (Sonicsand Materials Inc., Danbury, CT) at 50% output for 10 min. Finally, the suspension was dialyzed for 7 days using a Visking dialysis membrane with a molecular weight cut-off (MWCO) of 12.000 to 14.000 Da (Medicell Membranes Ltd) to obtain an aqueous CNC dispersion with a pH of 2.3 and a concentration of 1.3 % w/w. CNC films were obtained by casting 100 mg of water-dispersed CNCs onto hexagonal polystyrene weighing dishes and allowing them to dry for 72 h under ambient conditions.32,33 After the evaporation induced self-assembly (EISA), 25 μm thick free-standing films were obtained, which were punched into coins (1.13 cm2) to use as membranes on the cathode. Electrochemical characterization. 1,4-bis(diphenylamino)benzene (BDB) was purchased from Sigma-Aldrich (97%, white crystalline powder) and was assessed by elemental microanalysis, XRD, and infrared spectroscopy (Figure S1). The positive electrode (cathode) was prepared from a ball-milled mixture of 1,4-bis(diphenylamino)benzene (BDB) and Super P carbon (Timcal, Switzerland), and a water based composite binder consisting of carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) (CMC/SBR = 2 : 1) in a 60 : 35 : 5 weight ratio. A small volume of deionized water was added to obtain a smooth and homogeneous slurry, which was casted on a graphite foil (Alfa Aesar, 99.8%) and dried under vacuum for 24 h. The mass loading for each 1cm2 punched coin ranged between 2 and 4 mg. Swagelok® type cells were assembled 4 ACS Paragon Plus Environment

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with a 0.25 mm thick and 11 mm diameter Zn foil as the counter electrode, 100 μL of 19 m LiN(SO3CF3)2 + 1 m Zn(CF3SO3)2 (LiTFSI and ZnOTf, respectively; m: molal) in water (pH 6.5) as the electrolyte, and a 0.26 mm thick 12 mm diameter glass fiber filter (Whatman® GF/A) as the separator. 1 M ZnSO4 (pH 4.2), 2 M Zn(CF3SO3)2 (pH 4), and 10 m LiTFSI + 1 m Zn(OTf)2 were also used as electrolytes. The cellulosic membrane was soaked in the electrolyte for 1 h prior to use. Cyclic voltammetry and electrochemical impedance spectroscopy measurements were carried out in a 3-electrode cell with one zinc foil as the counter and another as the reference electrode. Cyclic voltammetry was performed at various scan rates. Galvanostatic cycling studies were conducted in a 2-electrode configuration against Zn as the counter/reference electrode. All the electrochemical measurements were performed at room temperature (22 ± 1 ˚C) using a VMP3 (biologic) potentiostat/galvanostat. Impedance spectroscopy data were recorded by applying a potentiostatic signal perturbation of 5 mV in a frequency range of 200 kHz and 100 mHz. Galvanostatic charge-discharge cycling were performed in 0.6 - 1.8 V (vs. Zn) window for low rate (0.2 C) studies and 0.4 - 2.1 V window for high rate (3C and 6C) ones. Physicochemical characterization. Powder X-ray diffraction (XRD) was performed on a PANalytical Empyrean diffractometer in Bragg–Brentano geometry using Cu-Kα radiation and a PIXcel1D detector. Operando XRD measurements were conducted using a home-made cell at a rate of 0.2C (1C: 130 mA g-1) with a XRD pattern collection time of 15 min. UV–Vis absorption spectra were recorded using a Jasco V-660 spectrophotometer. For operando spectroscopy the cell was fixed to the integrating sphere setup and charged/discharged at a rate of C/5 between 0.6 and 2 V, and spectra were recorded between 900 nm and 187 nm every 30 min or according to the evolution of the voltage profile. For the ex situ UV-Vis study of the cycled electrolyte, the glass fiber separator recovered from the cell after a certain number of charge-discharge cycle(s) was washed with 2.5 ml of Milli-Q® water to extract the dissolved species. The microscopic structure of the CNC membrane was examined by field-emission scanning electron microscopy (Zeiss LEO 1530) equipped with an energy dispersive X-ray spectroscopy (EDX) attachment. Nitrogen adsorptiondesorption analysis were performed at 77 K on a Quantachrome AUTOSORB-iQ. The sample was outgassed for 12 hours at 60°C under vacuum prior to analysis. Pore size distribution was obtained by the DFT method applied to the desorption branch of the isotherm. Zeta-potential measurements of water-dispersed CNCs were performed on a Malvern Zetasizer Nano-ZS using a particle 5 ACS Paragon Plus Environment


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concentration of 1 mg mL-1. Attenuated total reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR) of CNCs was performed on a Bruker Alpha FT-IR Spectrometer equipped with diamond ATR optics. Scanning electron microscopy (SEM) analyses were performed on a DSM 982 Gemini instrument (Zeiss). RESULTS AND DISCUSSION Theoretically, BDB can deliver a specific capacity of 130 mAh g-1 for a reversible redox between the neutral amine and the amine dication, accompanying the absorption/desorption of anions like sulfate (SO42-), trifluoromethanesulfonate (CF3SO2-/OTf-), and trifluoromethanesulfonimide (TFSI-), etc (Figure 1a). Chemically similar systems, e.g., polytriphenylamines have been previously studied in nonaqueous Li-ion batteries,34 but so far their aqueous electrochemistry remains relatively unexplored. In addition, BDB is highly stable and insoluble in aqueous electrolytes, rendering it functional as a solid electrode material. The electrochemical performance of BDB was first evaluated in conventional zinc electrolytes: 1 M ZnSO4-H2O and 2 M Zn(OTf)2H2O. Both showed a strikingly prolonged charge plateau at a fixed voltage around 1.6 V (vs. Zn), exhibiting charge capacity far exceeding the theoretical value (Figure S2a). Although aqueous ZnSO4 electrolyte is anodically stable up to 2.2 V (vs. Zn),10 the continuous charge suggests electrolyte oxidation. This phenomenon is believed to be caused by the tertiary nitrogen cation of the charged cathode material, catalyzing the oxygen evolution reaction (OER) at lower potentials. Several cationic organic nitrogen compounds were recently found to possess unexpected performance in OER, validating our hypothesis.35 The electrolyte oxidation was averted with a highly concentrated ‘water-in-salt’ electrolyte - 1m Zn(OTf)2 + 19m LiTFSI (m: molality).36 Since the solubility of Zn(OTf)2/Zn(TFSI)2 is limited to ~ 3m, abundantly soluble LiTFSI was used as the secondary salt to prepare the highly concentrated electrolyte, as proposed previously by Wang et al.36 The choice of zinc salt used as the Zn2+ source have no significant influence. Lack of free water molecules due to the high salt concentration suppresses the electrochemical activity of water (Figure 1b) - pushing the anodic and cathodic stability limit to a wide operational window.28 Additionally, the catalytic effect of the oxidized amine for water oxidation is subdued, as evidenced by linear sweep voltammetry (LSV) measurements (Figure S3). The zinc anode shows reversible cycling with a flaky and sheet like deposition morphology (Figure S4) in the concentrated electrolyte, which has been already shown to enable dendrite free zinc plating/stripping at nearly 100% Coulombic efficiency.36 Galvanostatic charge-discharge 6 ACS Paragon Plus Environment

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behaviour of the BDB cathode, studied for different concentrations of LiTFSI in the electrolyte (Figure S2b) further confirmed the quelling of anodic oxidation of water in the highly concentrated electrolyte, which allows the utmost electrochemical reversibility in BDB−Zn system. Therefore, 1m Zn(OTf)2 + 19m LiTFSI in water served as the optimized electrolyte in all subsequent studies. Figure 2a shows the galvanostatic charge/discharge profiles at a current density of 26 mA g-1. The initial charge-discharge capacity of 125-120 mAh g-1 at an average potential of 1.25 V corresponds to a 2-electron redox process (130 mAh g-1) occurring in a two-step plateau, which involves two distinct biphasic electrochemical processes as suggested by the Operando XRD (see below). The first step corresponds to the formation/reduction of BDB+∙A- (A-/anion : TFSI-/OTf-) that oxidizes/forms in the second step to/from BDB2+∙2A- (Figure 1a). This two-step redox behavior can be rationalized by electron donating/pulling effects of the electron pair on the neutral

Figure 2. Electrochemistry of the BDB against a Zn anode in 1m Zn(OTf)2 + 19m LiTFSI. (a) Galvanostatic discharge-charge profiles at a current density of 26 mA g-1 (0.2 C); (b) a cyclic voltammogram at a scan rate of 0.1 mV S-1; (c) cyclic voltammograms obtained at variable scan rates; (d) the logarithmic relationship of the peak current with the scan rate for the anodic and cathodic peaks labelled in Figure 1c. 7 ACS Paragon Plus Environment


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amine vis-à-vis the radial cation, tuning the energy level of the molecular orbitals.37 This is also what we observe in the cyclic voltammograms (CV) with two pairs of peaks at 0.82 V/1.19 V and 1.27 V/1.57 V during the anodic/cathodic processes (Figure 2b). In addition, CV at different scan rates, as shown in Figure 2c, and the corresponding linear fit of the logarithmic variations of peak currents (i) with scan rates (ν) (Figure 2d), reveal the nature of the electrochemical mechanism. With an increasing scan speed, the cathodic (reduction) peaks shift to a lower potential and the anodic (oxidation) peaks to a higher potential as a result of worsened polarization. The nearly 0.5 gradient of log i vs. log ν plots for all the peaks suggests a dominant faradic anion insertion mechanism controlled by diffusion kinetics. The neutral BDB is insoluble in water or in the electrolyte, but the oxidized forms BDB+∙A/BDB2+∙2A- were found to have some solubility in the concentrated electrolyte, as indicated by the strong blue coloration of the glass fibre separator (Figure 3ai) after a few charge-discharge cycles. Those ionic complexes could easily dissociate and get solvated in polar as well as hydrophobic media resulted from the large concentration of salts with perfluorinated complex anion in water. Such dissolution of BDB cations, which could further shuttle to the Zn electrode and reduce, led to a progressive capacity loss in every cycle, as evident from Figure 3b (red shaded area). Therefore, a cellulose nanocrystal membrane (thickness: 25 μm; Figures. 3aii and 3aiii) was introduced in front of the cathode to suppress the dissolution of the oxidized active material in the electrolyte. Galvanostatic cycling with the protective membrane barely showed any coloration of the separator (Figure 3aiv), visually confirming the inhibition of the dissolution mechanism. As expected, the use of the membrane improved the electrochemical reversibility immensely (Figure 3b: black shaded area). Cellulose nanocrystals (CNCs), derived from the most abundant natural polymer, cellulose, can be surface functionalized to address various challenging requirements.38 Here, the nanoporous CNC membranes (Figure S5) were casted from a water dispersion of CNCs, obtained by sulphuric acid hydrolysis of cellulose (see experimental section). The acid treatment decorates the CNC surface with sulfate half-ester groups, which render the CNCs with a highly negative zeta potential (-47.5 mV). While strong hydrophilicity of the CNC film ensures excellent wettability in the aqueous electrolyte - necessary for smooth ion transport, the negative surface charge acts like an electrostatic filter - retaining positively charged BDB cations on the cathode side. The role of the CNC membrane is depicted schematically in Figure 3c. Further insight into the role of the CNC membrane was obtained from ex-situ UV-Vis studies of the electrolyte, 8 ACS Paragon Plus Environment

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extracted from cells electrochemically cycled with and without the membrane. For cells without the membrane, multiple absorption peaks appear and their intensities gradually increase within first few cycles (Figure 3d), which can be attributed to the charged BDB species dissolved in the electrolyte. Similar absorption features have been noted in literature for BDB2+∙Al[OC(CF3)3]4− in CH2Cl2 solution.39 However, dissolution of the BDB+ species can not be ruled out which is expected to display a broad absorption band centred at 910 nm,40 as we observe here partially.

Figure 3. BDB−Zn cells with/without a CNC membrane. (a) i: Blue coloration of the glass fibre separator upon electrochemical cycling, showing dissolution of the charged BDB in the electrolyte; ii: a 25 μm thick and dry CNC membrane; iii: a CNC membrane recovered from a cell after 100 cycles; iv: the separator after 100 cycles for a cell with a membrane placed on the BDB cathode. (b) The charge-discharge capacity imbalance, as indicated by the shaded area, with/without the CNC membrane. (c) The role of the CNC membrane shown schematically; while without a membrane the BDB+/BDB2+ species can diffuse into the electrolyte, a CNC membrane retains those near the cathode. (d) Enhanced dissolution of oxidized BDB with cycling as indicated by the increase in the UV-Vis absorption band (arrows) intensity of the electrolyte without a CNC membrane. (d) No change in the UV-Vis absorption of the electrolyte over 120 cycles for a cell with a CNC membrane. 9 ACS Paragon Plus Environment


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With the membrane closely stuck onto the positive electrode, no absorption peaks emerge, even after 100 cycles (Fig. 3e), affirming the visual observation. The lack of an absorption feature also underlines the stability of the electrolyte with respect to its concentration and the composition during cycling. The efficacy of the CNC membrane in trapping the BDB+/BDB2+ was further confirmed by immersing a membrane into a blue colored cycled electrolyte solution in water. UVVis signals of the soluble species disappeared soon after the membrane immersion (Figure S6). Suppression of diffusion of the oxidized BDB from the cathode into the electrolyte by a CNC membrane led to high electrochemical reversibly and stable cycling, which is evident from the charge-discharge voltage profiles shown in Figure 4a, obtained at a current rate of 3C. However, a preceding ‘breathing’ step - involving charge-discharge cycling at a slower current (0.2C) - was crucial to perform high rate (> 2C) cycling studies (Figures S7a and S7b). To investigate the origin, impedance of the BDB electrode was probed as a function of cycling (Figure S6c). All the impedance profiles consist of two partially overlapped semicircles in the highmedium frequency region followed by a low frequency slopping tail characteristic of the solidstate diffusion. Although it is difficult to assign the two semicircles to particular electrochemical processes, the overall impedance - as indicated by the width of the semicircles - decreased significantly with cycling, before stabilizing at around the 15th cycle. The nearly nine times impedance drop from the 1st to the 15th cycle can be attributed to the ‘breathing’ of the CNC membrane, during which the extremely viscous concentrated electrolyte effectively percolates into the nanoporous membrane, creating continuous and facile pathways for ionic conduction. During this ‘breathing’ period, the capacity increased steadily to a value obtained without the CNC membrane (Figure S7). Note that, due to slightly increased polarization at higher rates, a little wider voltage window of 0.4 - 2.1 V was applied compared to 0.6 - 2 V window used for the preceding slow rate cycling. The ‘breathing’ mediated activation resulted in a high rate capability of the membrane protected BDB electrode (Figure 4b), as manifested by an impressive capacity of > 80 mAh g-1 at a 6C rate and the recovery of the starting 0.2C rate capacity after 50 cycles at variable rates. Apart from stable cycling at a 0.2 C rate (Figure S8), a high starting capacity of 112 mAh/g is delivered at a 3C rate, of which an impressive 101 mAh g-1 (>90%) is retained after 300 cycles, and more than 82% is still available after 500 cycles (Figures 4a and 4c). Favourable soft crystal structure, compatibility of the oxidized BDB with the concentrated electrolyte, and inhibition of the dissolution mechanism by the CNC membrane act in concert to render long-term 10 ACS Paragon Plus Environment

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Figure 4. Electrochemistry of the BDB−Zn system with the CNC membrane placed on the BDB cathode. (a) Charge-discharge profiles; (b) cycling behavior under variable C rates; electrochemical cyclability and the corresponding Coulombic efficiency at a (c) 3C and (d) 6C rate.

cycling even at a 6C rate, retaining 75% of the highest capacity after 1000 cycles. A very high Coulombic efficiency of nearly 100% is delivered at all current rates. Structural and chemical reversibility of BDB during charge-discharge were further confirmed by operando XRD and solid-state UV-Vis studies. The XRD revealed two distinct phase transitions marked by the appearance of a new non-moving set of peaks, involving BDB/BDB+∙A− and BDB/BDB2+∙2A− transformations, coinciding with lower and higher voltage plateau, respectively (Figs. 5a and 5b). Although structural identification was not possible due to inferior quality of the data, reversible (dis)appearance of the diffraction peaks (phases) with electrochemical cycling confirms the reversibility of the anion insertion mediated structural transformation of BDB. Attempts to identify the inserting anion (between OTf- and TFSI-) by infrared spectroscopic studies of the charged electrode proved to be unsuccessful as both anions 11 ACS Paragon Plus Environment


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have similar vibrational signatures.41,42 Solid-state operando UV-Vis of the cathode showed reversible evolution of two bands centered at 595 nm and 670 nm (Figures 5c and 5d; indicated by arrows) during the second voltage step, i.e., accompanying the formation/disappearance of BDB+2A-. The first step corresponding to the BDB/BDB+∙A- transformation shows no change in the UV-Vis signal, which could be simply because the band associated with the solid-state BDB+Aappears beyond the measured window (187 nm – 900 nm). While a red shift of the solid-state absorption bands of BDB2+∙2A- relative to the solution (in electrolyte; Figures 3d and S4) indicates an intermolecular π-π interaction in the solid phase, reversibility of their evolution as a function of charge/discharge reassert the reversibility of solid-state electrochemistry of BDB. CONCLUSION In summary, we introduce a novel organic cathode - 1,4-bis(diphenylaminobenzene) – alias, BDB for a dual-ion AZB that operates through reversible oxidation/reduction of BDB/BDB2+, while electrolyte anions (OTf-/TFSI-) insert/deinsert to maintain the charge neutrality. A reversible capacity of 120-125 mAh g-1 at an average voltage of 1.25 V leads to an energy density (cathode only) of ~155 Wh kg-1 (60 - 80 Wh kg-1 for cathode and anode mass combined, as calculated considering the intercalation of anions; see supporting information), which is higher than the cyanoferrates and comparable to some of the layered vanadium oxide cathodes that have been reported for AZBs. Application of a highly concentrated aqueous electrolyte to improve the stability at high potentials and a CNC membrane to inhibit the active material dissolution during cell cycling enable long-term cyclability with 82% retention after 500 cycles and 75% after 1000 cycles, besides excellent rate capability at nearly 100% Coulombic efficiency. These strategies are simple and versatile enough to be extendable for similar organic cathode systems. Even though the zinc deposition during the charge depletes the Zn2+ concentration in the electrolyte, the diminution is only 5-7% for the non-optimized BDB loading (2-4 mg cm2) used here for 100 μL of the electrolyte, which would be ~20% for a more practical loading of 10 mg cm-2 (of BDB). However, this can be avoided by assembling the cell in the charged state with BDB2+2A- (A-: OTf/TFSI-) as the cathode. Thus, we propose BDB not only as a mere sustainable alternative to inorganic hosts, it’s energy density and cycling stability in aqueous zinc system is competitive to other AZBs and have the potential to lead to disruptive large-scale storage technologies.


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Figure 5. Operando XRD and solid-state operando UV-Vis studies. Reversible evolution of the (a, b) XRD pattern and (c, d) solid-state UV-Vis spectra with charge-discharge. ACKNOWLEDGEMENTS D.K. acknowledges the Swiss National Science Foundation (SNSF) for the financial support for this work through their Ambizione grant. We also thank Professor Markus Niederberger for hosting H.G. and D.K. and providing all the research facilities in his lab. NOTES The authors declare no competing financial interest ASSOCIATED CONTENT Supporting information contains characterization of the purchased BDB, electrochemical analysis of the highly concentrated electrolyte, CV of the Zn anode and SEM of the Zn deposit in the concentrated electrolyte, physical characterization of the CNC membrane, UV-Vis spectra of the cycled electrolyte, and galvanostatic profiles and impedance data for the BDB cell with the CNC membrane. REFERENCES (1) Kim, H.; Hong, J.; Park, K-Y.; Kim, H.; Kim, S-W.; Kang, K. Aqueous Rechargeable Li and Na Ion Batteries. Chem. Rev. 2014, 114, 11788-11827.



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TOC FIGURE


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An Organic Cathode Based Dual-Ion Aqueous Zinc Battery Enabled

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