Reverse Dual-Ion Battery via a ZnCl2 Water-in-Salt Electrolyte

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Reverse Dual-Ion Battery via a ZnCl2 Water-in-Salt Electrolyte Xianyong Wu, Yunkai Xu, Chong Zhang, Daniel P. Leonard, Aaron Markir, Jun Lu, and Xiulei Ji J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00617 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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Journal of the American Chemical Society

Reverse Dual-Ion Battery via a ZnCl2 Water-in-Salt Electrolyte Xianyong Wu,#† Yunkai Xu,#† Chong Zhang,† Daniel P. Leonard,† Aaron Markir,† Jun Lu,*‡ and Xiulei Ji*† †Department

of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, United States National Laboratory, Lemont, Illinois, 60439, United States KEYWORDS: Reverse dual-ion batteries; Water-in-salt-electrolytes; Potential shift; Zinc chloride; Ferrocene ‡Argonne

ABSTRACT: Dual-ion batteries are known for anion storage in cathode coupled by cation incorporation in anode. Herein, we flip the sequence of anion/cation-storage chemistries of anode and cathode in DIBs by allowing anode to take in anions and a cationdeficient cathode to host cations, thus operating as a reverse dual-ion battery. The anion-insertion anode is a nanocomposite having ferrocene encapsulated inside a microporous carbon, and the cathode is a Zn-insertion Prussian blue—Zn3[Fe(CN)6]2. This unique battery configuration benefits from the usage of a 30 m ZnCl2 “water-in-salt” electrolyte. This electrolyte minimizes the dissolution of ferrocene; it raises the cation-insertion potential in cathode, and depresses the anion-insertion potential in anode, thus widening the full cell’s voltage by 0.35 V compared to a dilute ZnCl2 electrolyte. Reversible dual-ion batteries provide the configurationbased solution to exploit the practicality of cation-deficient cathode materials in aqueous electrolytes.

Introduction The utilization of renewable-but-intermittent energy sources calls for high-performing storage batteries.1,2 Recently, dualion batteries (DIBs) have engendered increasing attention as an alternative solution for energy storage.3,4 Different from the conventional “rocking-chair” batteries,5 the cathode and anode of DIBs reversibly incorporate electrolyte-born anions and cations, respectively, as Figure 1a illustrates.3,4 The first generation of DIBs—dual-graphite batteries—employs graphite as the anion-hosting cathode.6 Such graphite cathode often operates at oxidizing potentials, oft-maligned for causing the electrolyte decomposition.7-10 To address this issue, efforts have been devoted to developing new anion-storage materials with lower working potentials, where polymers,11-17 organic molecular solids,18-20 and metal-organic frameworks (MOFs)21 have been demonstrated capable of hosting anions at lower potentials. Along this line, if anions can be (de)inserted at sufficiently low potentials, such anion-storing materials may even serve as an anode that couples a cation-deficient cathode. We refer to such a battery configuration as reverse dual-ion batteries (RDIBs), as illustrated in Figure 1b. Herein, we demonstrate the intriguing performance of a RDIB that employs ferrocene encapsulated in microporous carbon as the anode and a Prussian blue of Zn3[Fe(CN)6]2 as the cathode. To promote the energy density of dual-ion batteries, it is pivotal to employ concentrated electrolytes because the electrolyte is the sole source of both cations and anions, thus being considered as active mass.7,16, 22-24 We employed a 30 m ZnCl2 “water-in-salt” electrolyte (WiSE), which maximizes the electrolyte concentration, mitigates the dissolution of ferrocene anode, raises the potential of the cation cathode, and depresses the potential of the anion anode. This cell configuration increases the full-cell voltage and the energy density.

Figure 1. The schematics of dual-ion batteries. a, A conventional dual-ion battery, where the cathode operates on anion insertion, and the anode relies on cation insertion. b, A reverse dual-ion battery, where the cathode takes in cations, and the anode hosts anions.

Results and discussion Albeit a few anion-storage electrode materials were reported in non-aqueous electrolytes, it remains challenging to do so in aqueous electrolytes. Recently, it has been reported that ferrocene-based polymers exhibit moderate potentials for anion (de)doping.11,12 Here we present approaches to enable a commercially available molecular solid of ferrocene as a feasible anion-insertion anode. In a ferrocene molecule, the FeII cation is sandwiched by two cyclopentadienyl rings, and the van der Waals forces stack the Fe(C5H5)2 molecules into a monoclinic crystal structure with a space group of P21/a (a = 10.539 Å, b = 7.612 Å, and c = 5.933 Å), as displayed in Figure 2a.25,26 The ferrocene solid with a low density of 1.51 g cm-3 comprises ample interstitial sites, which facilitates the oxidative incorporation of bulky anions in its structure during the oxidation of ferrocene to ferrocenium.25,26 With one etransferred per molecule, the ferrocene electrode has a theoretical capacity of 144 mAh g-1.

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capacity here is based on the ferrocene mass in the Fc/C composite.

Figure 2. a, The schematic crystal structure of the ferrocene molecular crystal; b, XRD patterns of the pure ferrocene and the ferrocene/activated carbon nanocomposite (Fc/C).

Despite its straightforward redox chemistry, aqueous rechargeable batteries have yet to employ the ferrocene solid as an electrode material due to the solubility of ferrocenium cations (formed after oxidation) in water. 25,26 To mitigate dissolution, we infiltrated ferrocene into the nanopores of activated carbon (AC) by a melt-diffusion method to form a nanocomposite with 50 wt.% of ferrocene (see the thermogravimetric analysis, (TGA), Figure S1), denoted as Fc/C, as described in the experimental section. The preparation of AC is detailed in the Supporting Information. After infiltration, the strong X-ray diffraction (XRD) peaks of the pristine ferrocene are reduced into a broad bump at ~16°, indicative of the complete disruption of the long-range order of the pristine molecular crystal structure (Figure 2b). This suggests the successful infiltration of ferrocene into the activated carbon’s nanopores, which resembles the extensively investigated fabrication of sulfur/carbon nanocomposites.24 Ferrocene is uniformly distributed in the carbon matrix, as demonstrated by the transmission electron microscopy (TEM) and the corresponding energy dispersive X-ray spectra (EDX) (Figure S2).

To find out the optimal WiSE for the ferrocene electrode, we first conducted our ‘survey’ studies in three-electrode cells, which covered 20 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 9 m sodium nitrate (NaNO3), 9 m sodium trifluoromethanesulfonate (NaCF3SO3), and 30 m ZnCl2 (1 m = one mole of solute in one kg of solvent). We discovered that the Fc/C electrode exhibits the most stable cycling performance in 30 m ZnCl2, as shown in Figure S3. As our recent studies reveal, in the 30 m ZnCl2 WiSE it is the anions of [ZnCl4]2- and [Zn(OH2)2Cl4]2 that serve as the charge carriers for anion-storage.27 Of note, recent studies on the LiTFSI WiSE suggested the heterogeneous nano-liquid-structure with 3D percolating Li+/H2O channels would facilitate the fast transport of hydrated Li-ions.28-30 The incomplete solvation shells afforded by the fewer H2O molecules may lower the desolvation energy, a barrier to overcome when anions, i.e., [ZnCl4]2-, transport across the electrode/electrolyte interfaces, similar to what takes place to cations. We worked to understand the impact of the ZnCl2 concentration on the electrochemical performance of the Fc/C anode. As shown in the cyclic voltammetry (CV) tests on Fc/C in 5, 10, and 30 m ZnCl2 electrolytes (Figure 3a), interestingly, the CV curves differ in potentials albeit resembling one another in their shapes, where a higher concentration leads to a lower anion (de)insertion potential. The formal potentials, Ef, which is the arithmetic average of the cathodic (Ec) and anodic potentials (Ea), are calculated as 0.26, 0.20, and 0.05 V vs. Ag/AgCl in 5, 10, and 30 m ZnCl2 electrolytes, respectively. We also found the same trend of the declining redox potentials of the Fc/C electrode in 1, 5, and 20 m LiTFSI electrolytes (Figure S4). To our knowledge, this is the first report on the negative potential shift of anion insertion in WiSE, which contrasts with the positive potential shift of cation insertion in WiSE.22-24 The half-reaction at the anode is expressed as: 2Fc + [ZnCl4]2- – 2e- ↔ 2[Fc]+∙[ZnCl4]2(1) where both the ferrocene (the reduced state) and ferrocenium salt (the oxidized state) are solid-state with a unity activity. The Nernst equation can be written as: E = E° – RT/2F‧ln(a[anion]) = E° – RT/2F‧ln(c[anion]‧γ[anion])

Figure 3. The electrochemical performance of the Fc/C electrode. a, CV curves in 5, 10, and 30 m ZnCl2 electrolytes at a scan rate of 1 mV s-1; b, GCD profiles in 30 m ZnCl2 electrolyte at 1 C rate (1 C = 106 mA g-1); c, A comparison of the operation potentials and capacity values of the reported anion-storage materials, where the Li2DAnt, DARb, and Fe2(dobpdc) represent dilithium 2,5(dianilino)terephthalate,19 5,12-diaminorubicene,20 and iron 4,4’dioxidobiphenyl-3,3’-dicarboxylate,21 respectively; d, Cycling comparison in 5, 10, and 30 m ZnCl2 electrolytes at 1 C rate. The

(2)

In 30 m ZnCl2 WiSE, the high concentration of c[anion] and increased activity coefficient of γ[anion] would dramatically raise the anion activity of a[anion],31-34 which thus decreases the insertion potential accordingly. The detailed analysis by the Nernst equation is given in the Supporting Information and Figure S5. Figure 3b shows the sloping galvanostatic charge/discharge (GCD) profiles of the Fc/C electrode with polarization of only ca. 0.1 V. By excluding the capacity contribution from the carbon host (Figure S6), ferrocene delivers a high capacity of 106 mAh g-1, which corresponds to ~73% of the theoretical capacity. Note that based on the total mass of the composite, the Fc/C electrode exhibits a capacity of 60 mAh g-1 (ferrocene: 50 wt.% in the composite, and 45 wt.% in the entire electrode). The encapsulation by the microporous carbon helps Fc/C exhibit much better cycling stability than the pure ferrocene electrode (Figure S7), presumably due to the physical absorption and the consequently mitigated activemass dissolution.24


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The low potential for anion storage in Fc/C is quite intriguing. To put the system of ferrocene and ZnCl2 WiSE in the context of DIBs, we list the operation potentials and reversible capacities of most anion-storage materials reported in the literature (Figure 3c),11-21 where the insertion potential of 0.05 V vs. Ag/AgCl of Fc/C is among the lowest. Thus, Fc/C is contemplated highly suitable as an anode in aqueous reverse DIBs. Note that some data were retrieved from nonaqueous systems, and it remains elusive whether their potentials would be identical if translating to aqueous electrolytes. Besides the appealing potential shift, the 30 m ZnCl2 WiSE also enhanced the cycling performance of Fc/C. As shown in Figure 3d, Fc/C exhibits ~80% capacity retention over 100 cycles at 1 C rate, which is much greater than 5 m (22%) and 10 m ZnCl2 (32%). When evaluated at 5 C rate, Fc/C retains ~70% of its original capacity after 2000 cycles (Figure S8). The more stable cycling by WiSE stems from its ionic liquid nature, where most water molecules are strongly bound with ions,27, 35-37 thus leaving few free water to solvate ferrocenium cations. Recently, Wang et al. reported the suppressed dissolution of Li2S in 21 m LiTFSI WiSE, which supported reversible Li+ storage in the sulfur electrode.24 The different extents of active mass dissolution shown by the beaker cells after five GCD cycles at 0.5 C also support the suppressed solvation of ferrocenium cations in the 30 m ZnCl2 electrolytes (Figure S9). We performed ex situ XRD to understand the impacts of anion insertion on the structure of the ferrocene solid, where we used the pure ferrocene electrode instead of the amorphous Fc/C nanocomposite. As shown in Figure S10, after anion incorporation, most XRD peaks vanish, which suggests the annihilation of crystallinity to accommodate the bulky anions. This is akin to our prior study of the coronene molecule for PF6- storage.18 However, most interestingly, after anion extraction, the ferrocene structure is restored to show its initial XRD pattern, where such an elastic transition is extremely rare. To couple the low-potential Fc/C anode in ZnCl2 WiSE, we selected a high-potential cathode of a Prussian blue analogue, ZnII3[FeIII(CN)6]2, for Zn2+ storage. Liu et al. investigated this compound in a diluted ZnSO4 electrolyte, which shows an insertion potential of 0.8 V vs. Ag/AgCl.38 Figure 4a shows the XRD pattern of Zn3[Fe(CN)6]2, which is indexed to a — rhombohedral phase with the space group of R3c (JCPDS no. 38-0688). Figure 4b schematically shows its crystal structure, where the cyanide groups connect the alternating FeC6 octahedra and ZnN4 tetrahedra, thus constituting an open framework with roomy interstitial sites to accommodate the inserted cations.38 Zn3[Fe(CN)6]2 has a theoretical capacity of ~86 mAh g-1 for one Zn2+ insertion. Note that the three Zn2+ ions in the formula are not extractable. Scanning electron microscopy (SEM) reveals its morphology comprising micronsized quasi-cubes (Figure 4a inset). Thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectrum of this compound are shown in Figure S11. Figure 4c and Figure S12 show the CV curves of Zn3[Fe(CN)6]2 in 5, 10, and 30 m ZnCl2 electrolytes. As marked in Figure S12, there exist two pairs of major CV peaks, which are presumably related to the occupancy of inserted Zn2+ ions in the very large voids of Zn3[Fe(CN)6]2 framework that are even larger than those in cubic Prussian

blue analogues.38-40 Na-ion insertion in Na2Zn3[Fe(CN)6]2 also showed similar multiple CV peaks.39 Additionally, the hydration number change of the Zn2+ ions in 5, 10, and 30 m ZnCl2 electrolytes also affects the CV shapes and intensities.27,35,36

Figure 4. Physical and electrochemical characterization of the Prussian blue cathode of Zn3[Fe(CN)6]2. a, The XRD pattern, where the inset is a SEM image; b, The schematic crystal structure, where the cation insertion sites are marked as yellow circles; c, CV curves measured in 5, 10, and 30 m ZnCl2 electrolytes at a scan rate of 1 mV s-1; d, GCD profiles at a current density of 65 mA g-1.

Notably, a higher concentration of the ZnCl2 electrolyte raises the working potential for the Zn3[Fe(CN)6]2 cathode. This is in accordance with recent reports on WiSE, where a higher cation activity results in a higher potential according to the Nernst equation:22-24 E = E° + RT/2F‧ln(a[cation]) = E° + RT/2F‧ln(c[cation]‧γ[cation]) (3) for the half-reaction:

Zn2+ + 2e- + Zn3[FeIII(CN)6]2 ↔ Zn2+‧Zn3[FeII(CN)6]2. (4) Thus, an increased cation activity would elevate the overall insertion potential. To put the shift in the context, we evaluated the potential shift of a model experiment—the plating of zinc: Zn2+ + 2e-  Zn by cyclic voltammetry (CV) in different electrolytes. Zn plating occurs at the potentials of 0.92, -0.84, and -0.69 V vs. Ag/AgCl in 5, 10, and 30 m ZnCl2, respectively, where the potential shift from 5 to 30 m is 0.23 V (See Supporting Information and Figure S13 for detailed analysis). However, the potential increase is only ~0.1 V for the formation of Zn2+‧Zn3[FeII(CN)6]2. Here, the different potential shift in the Prussian blue electrode is likely due to the number of lattice water molecules (x) incorporated into the cathode structure, which may vary depending on the concentrations of the ZnCl2 electrolytes. Figure 4d shows the GCD profiles, where a reversible capacity of 65 mAh g-1 is delivered after the initial conditioning, corresponding to 0.75 moles of Zn2+ inserted per formula. The lower capacity in the first discharge is due to the presence of “impurity” FeII ions in the structure. The XPS analysis confirms the presence of both FeII and FeIII oxidation states in the pristine Zn3[Fe(CN)6]2 structure (Figure S14).



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Importantly, the average Zn2+ insertion potential in 30 m ZnCl2 is as high as ~1.0 V vs. Ag/AgCl, higher than in a diluted ZnSO4 electrolyte by ~0.2 V.38 To our knowledge, this is the highest Zn-ion insertion potential among all cathode materials reported for Zn-ion batteries (See Figure S15 for comparison).31-45 The Zn3[Fe(CN)6]2 electrode also shows stable cycling performance with capacity retention of 70% over 1000 cycles in the ZnCl2 WiSE electrolyte, where the GCD slope profiles are well retained (Figure S16). Due to the opposite directions of potential shifts of the cation and anion insertions, the potential gap between the Zn3[Fe(CN)6]2 and the Fc/C electrodes widens from 0.60 to 0.95 V from 5 m to 30 m ZnCl2 electrolytes (Figure 5a). To demonstrate the proof-of-concept RDIB, we fabricated full cells with the Zn3[Fe(CN)6]2 cathode and the Fc/C anode. Different from the conventional DIBs that begin cycling with a charge process,3,4 the as-assembled RDIB proceeds by an initial discharge, where the cation-deficient Zn3[Fe(CN)6]2 gets reduced and incorporates Zn2+ cations, whereas the ferrocene anode gets oxidized and accommodates anions, e.g., [ZnCl4]2-, as shown in Figure 5b. Importantly, the cell configuration of RDIBs frees the cation-deficient cathode from the necessity of coupling with a metal anode or being pre-reduced chemically or electrochemically.

potentials, however, differ by as large as ~1.0 V due to the different chemical environments of the redox couple as well as the impact of WiSE. The RDIB demonstrates relatively stable cycling performance with capacity retention of 58% after 1000 cycles, and the GCD slope profiles retain the discharge voltage well (Figure S18). We have to point out that the electrolyte’s concentration in DIBs would decrease continuously during the discharge, where the associated impacts would not be revealed in a flooded cell like what we use here (with a ratio of electrolyte/electrode (cathode and anode) of 0.1 mL/1.0 mg). However, we propose that additional electrolyte salt can be pre-mixed with the cathode in practical cells, which will be dissolved during the battery operation to maintain the electrolyte concentration constant. By having 200% the salt needed for the operation cathode and anode, the mass percentage of the electrolyte to the total mass of electrolyte plus electrodes increases from 20% to 26%. We need to emphasize that a higher concentration of electrolyte in conventional DIBs inevitably decreases the cell voltage due to the potential shift of cation and anion insertions. We assembled a dual-ion battery with Zn metal anode and Fc/C cathode, which exhibits the lowest voltage in the 30 m ZnCl2 electrolyte compared to dilute ones (Figure S19). On the other hand, the rocking-chair batteries do not see a voltage increase either, as the cation-potential shifts on both electrodes cancel out at the full cell scale. We tested a Znmetal battery with the Zn3[Fe(CN)6]2 cathode, which exhibits slightly decreasing cell voltages in all 2, 5, 10, and 30 m ZnCl2 electrolytes (Figure S20). Figure 5d summarizes the extents of voltage change of these full cells in ZnCl2 electrolytes, which clearly reveals the advantage of the reverse DIB design in concentrated electrolytes. Table S1 compares the metrics of electrochemical properties of the RDIB in this study and other aqueous battery systems. The full cell performance of the RDIB is comparable to most rocking-chair aqueous batteries. However, when comparing to most reported zinc metal batteries, caution should be taken because these zinc cells used an excessive amount of zinc metal anode, which represents the cathode’s capacity and cycle life in the half cell setting instead of the full cell chemistries.

Figure 5. Electrochemical performance of the full-cell RDIB. a, The comparison of CV curves of the Fc/C anode and Zn3[Fe(CN)6]2 cathode in ZnCl2 electrolytes; b, The schematic of the working mechanism of the RDIB; c, GCD profiles of the RDIB full cell at 1 C rate (1 C = 30 mA g-1); The current density and capacity are based on the total mass of the Zn3[Fe(CN)6]2 cathode and the Fc/C composite anode. The Zn3[Fe(CN)6]2 cathode was discharged and charged for one cycle of “conditioning” before being coupled with the Fc/C anode to assemble a full cell. d, The voltage change of full cells of rockingchair batteries (RCBs), DIBs, and RDIBs in ZnCl2 electrolytes.

Conclusion

Figure 5c shows the GCD profiles of a full cell with the N/P ratio of 1:1, where a reversible capacity of ~30 mAh g-1 can be achieved based on the total active mass of both the Zn3[Fe(CN)6]2 cathode and the Fc/C anode, and the average voltage is around 0.90 V, compared to 0.61 V and 0.76 V of full cells when 5 and 10 m ZnCl2 electrolytes are employed, respectively (Figure S17). Notably, both electrodes in the RDIB employ the FeIII/FeII redox couple, where their

In summary, we introduce an alternative battery configuration—reversible dual-ion batteries, where during discharge the low-potential anode operates on anion insertion and the cation-deficient cathode functions by cation-insertion. The RDIB exploits the potential shifts of electrodes in the emerging WiSE. We investigated a Prussian blue analogue of Zn3[Fe(CN)6]2 as the cathode and the encapsulated ferrocene in microporous carbon as the anion-storage anode. We found that the 30 m ZnCl2 WiSE electrolyte pushes the cationinsertion potential higher and drags the anion-insertion potential lower, which enlarges the cell voltage by as much as 0.35 V compared to a dilute ZnCl2 electrolyte (5 m). The reverse dual-ion battery design also tackles the long-standing challenge of making use of a plethora of cation-deficient electrode materials as cathodes in metal-anode-free full cells. The intriguing potential shift and the underlying mechanism may call on more attention about the role of ion charge carriers in redox reactions. This perspective may be relevant to some existing battery technologies for grid storage. For


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instance, some redox flow batteries (RFBs) such as zinccerium and sulfur-iron systems adopt a similar working mechanism of using anion-based redox as anolytes and cationbased redox as catholytes,47,48 where a higher electrolyte concentration would also elevate the cell voltage.

ASSOCIATED CONTENT Supporting Information. Experimental details, TGA, TEM, XRD, XPS, surface area analysis, and electrochemical characterization.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes Xianyong Wu and Yunkai Xu contributed equally to this work.

ACKNOWLEDGMENT X. J. is grateful for the ﬁnancial support from the U.S. National Science Foundation, Award No.1551693. J. L. acknowledges the support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357.

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Reverse Dual-Ion Battery via a ZnCl2 Water-in-Salt Electrolyte

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