- Email: [email protected]
A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents
Article
A Low-Cost and High-Energy Hybrid IronAluminum Liquid Battery Achieved by Deep Eutectic Solvents Leyuan Zhang, Changkun Zhang, Yu Ding, Katrina Ramirez-Meyers, Guihua Yu [email protected]
HIGHLIGHTS A low-cost and high-energy Fe-Al hybrid-flow battery New reaction mechanism in FeCl3$6H2O/urea/EG DES Stable cycling achieved by dissociating iron complexes
A low-cost and high-energy Fe-Al RFB is established for large-scale energy storage. Using Fe catholyte at a concentration of 5 M, the Fe-Al battery can deliver a high energy density of 166 Wh L 1. This study also furthers our fundamental understanding about the working mechanism of Fe-urea DESs. By dissociating the complex ions in Fe DES, the Fe-Al battery can achieve the full charge and discharge over 1,500 hr without any capacity fading.
Zhang et al., Joule 1, 1–11 November 15, 2017 ª 2017 Elsevier Inc. https://doi.org/10.1016/j.joule.2017.08.013
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Article
A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents Leyuan Zhang,1 Changkun Zhang,1 Yu Ding,1 Katrina Ramirez-Meyers,1 and Guihua Yu1,2,*
SUMMARY
Context & Scale
This work demonstrates a low-cost, high-energy Fe-Al hybrid liquid battery that takes advantage of the desirable properties of deep eutectic solvents (DESs). The strategy of additive enables the full charging and discharging of the Fe-Al battery with long cycle life while the stable stripping and deposition of Al is achieved. Using Fe(210) catholyte at a concentration of 5 M, the Fe-Al battery can deliver a high energy density of approximately 166 Wh L 1 with an average operating voltage of 1.41 V. Furthermore, by dissociating the iron complexes in Fe(126) DES, the Fe-Al battery can achieve the full charge and discharge over 60 cycles without degradation. Here, an all-DES-based liquid battery is proposed with an ultrahigh concentration of redox species, resulting in high energy density. The DES maintains reduced lattice energy and depressed freezing point and provides a new platform for developing green redox species based on new chemistry.
Given advantages of low cost, high concentration, and potential biodegradability, the concept of deep eutectic solvents (DESs) is beneficial to developing costeffective and sustainable batteries with high energy density. Combining environmentally friendly Al DES and Fe DES, a green Fe-Al hybrid liquid battery was designed. The stable deposition and stripping of Al and long cycling of Fe DES with high utilization were achieved using 1,2-dichloroethane (DCE) and ethylene glycol (EG) as additives, respectively. The Fe-Al hybrid battery delivered a high energy density of 166.2 Wh L 1. Furthermore, in-depth chemical characterizations reveal that the excellent cycling performance of Fe DES should be ascribed to the dissociation of complex ions by the EG additive. Therefore, the strategy of additives and the influence of coordination environment on electrochemical performance provide a new insight into the design of novel catholytes and anolytes using DESs for redox-flow batteries.
INTRODUCTION Considering the resource limitations and environmental concerns of conventional fossil fuels, energy conversion from ecologically sustainable sources, such as sunlight and wind, can provide green power sources in modern society. However, the intermittency and unreliable stability of renewable energy systems limit their extensive application in direct power supply. Therefore, the development of efficient large-scale energy storage systems is necessary to make full use of the renewable energy resources. Among various energy storage technologies, rechargeable batteries have been one of the most dominant technologies for many years.1–3 In particular, redox-flow batteries (RFBs) are considered as a promising technology for large-scale energy storage.4–7 Conventional inorganic RFBs, such as vanadium RFBs or Zn-Br2 RFBs, have been commercialized but still suffer from intrinsic disadvantages, such as the use of scarce and expensive elements, corrosive electrolytes that raise safety concerns, and dendrite formation (zinc).8–11 On the other hand, to improve the energy density of RFBs, lithium-based hybrid-flow batteries (HFBs) with high operating voltage are being developed.12–15 However, limited resources, serious dendrite problems, and flammable electrolytes still impede the extensive implementation of lithium in HFBs. As an alternative to lithium, aluminum is attracting increasing interest due to its low cost, high abundance, and high theoretical capacity (2,980 mAh g 1).16,17 Moreover, recent research on Al-ion batteries has shown stable stripping and deposition of aluminum in Al ionic liquids (ILs) and Al DESs without serious dendrite formation.18–20 These characteristics make aluminum a promising potential anode in HFBs.
Joule 1, 1–11, November 15, 2017 ª 2017 Elsevier Inc.
1
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
In addition to broadening the operating voltage, increasing the concentration of redox species is another strategy for enhancing the energy density of RFBs.21–23 In traditional aqueous solutions, a solute molecule is surrounded by large amounts of water molecules to form a solvation complex, which becomes a barrier for achieving high concentration and, hence, high energy density in an aqueous system.24 Recently, many metal-based DESs, such as Fe, Co, Cr, Al, Mg, and Zn, have been easily synthesized in ambient conditions with a high concentration of active species, which offers a potential opportunity for RFBs.25–28 Fe-based DESs, in particular, are especially promising as cathode-active materials due to their stability, high crust abundance, and high redox potential (Fe2+/Fe3+ at 0.77 V versus normal hydrogen electrode).29 Meanwhile, an Al-based DES, encompassing the advantages of both Al metal and DESs, would be a better choice for anolyte compared with Al ILs. However, due to the intrinsic properties of DESs or organic electrolytes, the rate capability is currently not yet comparable with that of aqueous systems. Herein, a proof-of-concept Fe-Al HFB was proposed based on Fe DES and Al DES. A series of FeCl3$6H2O/urea/EG DESs with various molar ratios, simplified as a combination of three numbers (Fe(xxx) DESs), are obtained and the influence of EG as the additive is explored. For Al DES, 1,2-dichloroethane (DCE) is added to decrease the viscosity and then increase the ionic conductivity. Finally, paired with Al-DES/DCE, Fe(126) (1.8 M) and Fe(210) (5.5 M) DESs with EC/DMC (ethylene carbonate/dimethyl carbonate) were selected as catholytes to test the performance of Fe-Al HFBs. This battery has an average operating voltage of approximately 1.41 V, delivering a high energy density of 166.2 Wh L 1 when using the Fe(210) catholyte. Moreover, we find that adding EG can change the coordination environment of Fe3+ and enable the dissolution of LiCl. The dissociation of complex cations and anions in Fe(126) catholyte from adding EG thus improves the full charging and discharging behavior and cycling stability. Furthermore, stable stripping and deposition of the Al anode is demonstrated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterization.
RESULTS AND DISCUSSION The Proof-of-Concept Fe-Al Hybrid Liquid Battery from the Concept of Fe DES and Al DES As reported in the previous work, DESs can be technically and economically favorable for developing low-cost and high-energy flow batteries since they form a liquid with high concentration by simply mixing the cheap raw materials at room temperature. Considering the high abundance, low cost, and appropriate potentials, Fe and Al DESs were selected as the catholyte and anolyte, respectively. The scheme and photograph of the proof-of-concept prototype for the Fe-Al hybrid battery system are displayed in Figure 1. An aluminum strip attached to Cu foil serves as the anode. According to the literature,18,19 the reaction that occurs in Al DESs is different from the one that occurs in Al ILs, due to different Al3+ coordination environments. The Al-urea cations and Al-chloride anions in Al DES result in two possible reactions of Al deposition and stripping as follows. Anode reaction: 4Al2Cl7 + 3e / Al + 7AlCl4
(Equation 1)
Anode reaction: 2[AlCl2$(urea)2]+ + 3e / Al + AlCl4 + 4(urea) (Equation 2)
1Materials
Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA
2Lead
As similarly discussed in our recent work,30 DCE with a low viscosity was added into Al DES to further decrease the viscosity and enhance the ionic conductivity of
2
Joule 1, 1–11, November 15, 2017
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.joule.2017.08.013
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 1. The Proof-of-Concept Fe-Al Hybrid Liquid Battery (A) Schematic of an Fe-Al battery based on DESs. (B) The photograph of a working Fe-Al battery (top) and its schematic of device configuration (bottom).
anolytes. Compared with the conductivity of Al DES (0.36 mS cm 1), the diluted Al DES (denoted as Al-DES/DCE) showed a 5-fold increase in ionic conductivity to 1.9 mS cm 1. In the diluted state, the concentration of all redox species ([AlCl2(urea)n]+, Al2Cl7 , and AlCl4 ) in Al-DES/DCE was still as high as 3.2 M. In addition, the reaction that occurred in Al-DES/DCE was similar to the Al DES reaction, already reported in other literature. For the cathode, Fe-urea DES paired with Li metal was recently demonstrated in a high-energy green battery, but the initial coulombic efficiency and cycling performance based on fully charging/discharging was not satisfactory. In this work, EG was added to adjust the physical properties of FeCl3$6H2O/urea/EG DESs for the following reasons: (1) the viscosity of EG is about 16.1 cP at 25 C, suggesting that it may decrease the viscosity of Fe DESs; (2) the freezing point of EG is about 10 C, giving it the potential to maintain the liquid form of Fe DESs at relatively low temperatures; (3) it can dissolve LiCl salt, implying it will improve the reversibility of redox reaction of Fe DESs. The various compositions of Fe DESs are named by using three numbers to represent the molar ratios of FeCl3$6H2O, urea, and EG, respectively. For example, Fe(123) DESs means the molar ratios of FeCl3$6H2O/urea/EG is 1:2:3. Based on the knowledge of Fe-based DESs,28 three original Fe-urea DESs are synthesized in the first step: Fe(120), Fe(110), and Fe(210) DESs. The last number represents the molar ratio of the EG component, meaning that EG does not exist in these three original Fe DESs. As shown in Figure 1B, Fe DESs were injected into the quartz shell through the predrilled hole in the Ti foil as catholyte. To improve the current collection efficiency, Super P is mixed with a small amount of polyvinylidene fluoride binder coated on the surface of Ti foil. The Viscosity, Conductivity, and Reversibility of Various Fe DESs As displayed in Figure 2A, various Fe DESs can be easily synthesized at ambient environment, which also show long-term stability during storage in air (Figure S1). With the increase of EG component in Fe DESs, the Fe3+ concentration will
Joule 1, 1–11, November 15, 2017
3
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 2. The Physical Properties and CVs of Various Fe DESs (A) Photograph of various Fe DESs. (B) Ionic conductivity of FeCl3 $6H2 O/urea/EG as a function of composition. (C) CV curves of Fe DESs at the sweeping rate of 1 mV s 1 . (D) Viscosity data of FeCl 3 $6H 2 O/urea/EG as a function of temperature and composition.
decrease, leading to a color change from dark to light. Fe(126) DES has a relatively low concentration and thus forms a light-brown and transparent liquid, implying a homogeneous and stable state. Generally, Fe DESs are more viscous than aqueous solutions, so the ionic conductivity may be a concern in practical applications. Electrochemical impedance spectroscopy (EIS) is utilized to measure the ionic conductivities of the different Fe DESs. As seen in Figure 2B, Fe(210) DES has the highest conductivity and 120-based Fe DESs have a lower conductivity. The conductivities of these Fe DESs (>10 mS cm 1) are much larger than that of the lithium aluminum titanium phosphate (LATP) separator (0.1 mS cm 1), indicating that the conductivity would not limit the electrochemical performance. Cyclic voltammetry (CV) and viscosity tests were conducted to observe the reaction reversibility of various Fe DESs and select the appropriate catholyte for the Fe-Al battery. From the CV curves at different sweeping rates (Figure S2) and analysis of the CV curves at 1 mV s 1 (Figure 1C and Table S1), we find that Fe(126) DES exhibits a highly reversible reaction compared with other samples. Differences in reversibility should be ascribed to the different molar ratios and the influence of EG component, which also affects viscosity. As shown in Figures 2D and S3, as the amount of EG increases, the viscosity of 120-based Fe DESs initially increases and then decreases. In contrast, the viscosity of Fe(210)-derived DESs continuously increases with the addition of EG. Through consideration of the concentration, reversibility, and viscosity of various Fe DESs, Fe(126) and Fe(210) DESs are chosen as catholytes for the electrochemical test of the Fe-Al hybrid battery due to the high reversibility and high concentration, respectively. To further decrease the viscosity, a small amount of EC/DMC could be added into these two Fe DESs (denoted as Fe-DES/EC-DMC). CV tests were conducted to assess the electrochemical stability of EG and EC/DMC (Figure S4) and showed that no decomposition or other side reactions occurred in the working voltage range.
4
Joule 1, 1–11, November 15, 2017
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 3. The Electrochemical Performance of Fe(126) DES (A) CV curves of Fe(126) DES at different sweeping rates. (B) Linear relationships between the oxidation and reduction peak current with the square root of the sweeping rate. (C) The charge and discharge capacity with corresponding coulombic efficiency of Fe(126) catholyte when paired with Li at the current density of 0.1 mA cm 2 . (D) Polarization curve (black) and corresponding specific power density (red) at room temperature.
The Electrochemical Performances of Fe-Li and Fe-Al Hybrid Batteries For Fe(126) DES, CV tests were conducted at different sweeping rates using the three-electrode system (Figure 3A). By comparing the anodic and cathodic peak currents, we find that the redox reaction of Fe(126) DES is highly reversible. Given the high concentration of redox active material, the notable potential hysteresis, especially at high scan rates, should be attributed to the electromigration and concentration polarization. The linear relationship between the peak currents and the square root of sweeping rate, displayed in Figure 3B, implies that the reaction in Fe(126) DES is a diffusion-controlled process. Using the Randles-Sevcik equation below, the diffusion coefficient was calculated to be about 2 3 10 8 cm2 s 1. Ip = 26,900 3 n1.5AD0.5v0.5C,
(Equation 3)
where Ip is peak current, n is the number of electrons involved, A is the active surface area, D is the diffusion coefficient, v is sweeping rate, and C is the concentration of redox species. While the derived coefficient is not comparable with those of conventional redox species in water, the value is reasonable at such a high concentration, and the diffusion process can be further facilitated by exploiting 3D current collector in the practical battery test. The Fe(126)-Li hybrid battery was constructed to evaluate the charging and discharging behavior. Figure 3C shows the cycling performance of the Fe(126)-Li battery, and the full charging/discharging curves are displayed in Figure S5. Under the operating voltage of 3.45 V, a stable, long-life cycling with a high coulombic efficiency (100%–103%) is achieved. At a current density of 0.1 mA cm 2, the battery has a volumetric capacity of about 41 Ah L 1 along with the high initial coulombic
Joule 1, 1–11, November 15, 2017
5
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
efficiency of 100%. Benefiting from relatively low viscosity, about 90% of the theoretical capacity can be reversibly utilized and the average voltage difference between charging and discharging curves is only 0.15 V. Based on the high concentration of 1.7 M and high utilization, an energy density of 142.4 Wh L 1 was achieved, which is among the highest energy densities of RFBs reported so far. The polarization curve of the Fe(126)-Li hybrid battery was obtained at room temperature (Figure 3D). The discharging potential and current density showed a linear relationship and a high power density of over 20 mW cm 2 was obtained, which is the highest among liquid batteries using DESs as catholytes. To demonstrate the stable stripping/deposition of Al, ex situ XRD and scanning EM characterizations were taken along with the CV test at different scan rates (Figure S6). Using Al rods as the counter and reference electrodes, we obtained the CV curves from 10 to 100 mV s 1. As seen in Figure S6A, the stripping and deposition of Al is a highly reversible reaction in this Al-DES/DCE and the dissolution of Al occurred at about 0 V versus Al. Figure S6B shows the XRD patterns of Al anodes before and after electrochemical tests, which do not show any indication of impurities (such as Al2O3 or AlCl3). Furthermore, based on the SEM images of Al anodes (Figures S6C and S6D), we believe that the dendrite problem, existing in the Li anode, was not a serious issue in Al DESs. Above all, the stripping and deposition of Al in Al-DES/DCE is stable and highly reversible, which makes it possible to design advanced Al-based batteries. The stable deposition/stripping of aluminum in the Al DES and the low-cost Fe DES with high concentration and low viscosity enable the design of an advanced Fe-Al HFB. Figure 4A shows the charging and discharging curves of the Fe(126)-Al hybrid battery. Similarly, a high utilization of about 90% can be achieved, delivering a discharge capacity of 41.4 Ah L 1. The average operating charge and discharge potentials are 1.57 V and 1.41 V, respectively, resulting in a voltage gap of 0.16 V. Due to the limitation of operating voltage, the energy density decreases to 58.1 Wh L 1, which is lower than that of an Fe(126)-Li hybrid battery. Figure 4B shows the relationship between current density and power density in the range of 0.05–1 mA cm 2. Due to the internal resistance of the battery, the discharge potential linearly decreases as current density increases. Moreover, the power density turned out to be over 1 mW cm 2 at the current density of 1 mA cm 2 associated with a discharge voltage of roughly 1.1 V. The potential-time profile of the Fe(126)-Al hybrid battery is presented in Figure 4C, showing that the full charging and discharging potential can be maintained during a long operation time of 500 hr. This is consistent with the aforementioned result of the Fe(126)-Li battery. The problem of crossover was eliminated by using an LATP ceramic separator, resulting in a high coulombic efficiency of over 96% during cycling (Figure 4D). During continuous charging and discharging, no capacity fading was found over 60 cycles with 90% utilization. Both the steady potential profile and stabilized cycling performance confirmed that the redox reactions in Fe(126)-DES/EC-DMC and Al-DES/DCE were highly reversible and stable. To further improve the energy density of this proof-of-concept Fe-Al hybrid battery, the Fe(210) catholyte was prepared by adding 50 mL of EC/DMC into 0.5 mL of Fe(210) DES. The EC/DMC is meant to decrease the viscosity and is confirmed to be electrochemically stable in the potential window. Since the concentration of Fe(210) DES is 5.5 M, the Fe(210) catholyte still maintains a concentration of 5 M. Paired with Li metal, the Fe(210) catholyte can also show a high utilization of about 90% in the first cycle (Figure S7A) and the power density approaches 22 mW cm 2 (Figure S7B), which is much higher than reported in previous work.29 Figure 5A shows the initial discharging and charging profile of the Fe(210)-Al battery. Due to
6
Joule 1, 1–11, November 15, 2017
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 4. The Electrochemical Performance of an Fe(126)-Al Battery (A) Charge/discharge profile of Fe(126)-DES/EC-DMC (1.7 M, 0.05 mL) paired with Al-DES/DCE (3.2 M, 0.1 mL) at current density of 0.1 mA cm 2 . (B) The discharge voltage and corresponding power density of an Fe(126)-Al battery at various current densities. (C) Representative charge and discharge profiles over time at the current density of 0.1 mA cm 2 . (D) The charge and discharge capacity with corresponding coulombic efficiency of an Fe(126)-Al battery at room temperature over cycling.
the increased concentration of redox species, the specific capacity soared to about 120 Ah L 1 at 0.1 mA cm 2. However, at the current density of 0.2 mA cm 2, the utilization and coulombic efficiency of the initial cycling decreased (Figure S8A). The EIS result shows that the total resistance is not very high (Figure S8B), indicating that the performance decay at a high current density may be caused by slow mass/ions transport. With an average voltage of approximately 1.41 V, the high energy density of 166.2 Wh L 1, based on the volume of Fe catholyte, was delivered by this low-cost Fe(210)-Al hybrid battery. It appears that high concentration contributes to boosting the energy density, but may negatively affect the cycling performance. Figure S8C displays the full charging and discharging of the Fe(210)-Al battery over time and suggests that the cycling performance is related to the working mechanism of Fe(210) catholyte. To further clarify the calculation of energy density, an example of an Fe(210)-Al battery is provided in Figure S9. The polarization curve was measured at room temperature, as presented in Figure 5B. Similar to the Fe(126)-Al battery, the Fe(210)-Al battery shows a limited power density of about 1.2 mW cm 2 at a current density of 1.5 mA cm 2. The potential profile with the increase of current density was also measured (Figure S8D) and was consistent with the polarization curve. To improve the power performance, we used the flow electrode with carbon felt as current collector in the Fe side (Figure S10). As displayed in Figures S10B and S10C, power densities of 4.5 and 5 mW cm 2 were achieved for Fe(126)- and Fe(210)-DES/EC-DMC catholytes, respectively, both of which were significantly higher than the static one, although still not yet comparable with the aqueous systems. The possible reasons for the limited power density are the low ionic conductivity of LATP membrane and the high concentration of redox species in DESs. Therefore, modifications of the device configuration31 and
Joule 1, 1–11, November 15, 2017
7
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 5. The Electrochemical Performance of an Fe(210)-Al Battery (A) The initial charge/discharge profile of Fe(210)-DES/EC-DMC (5 M, 0.03 mL) paired with Al-DES/ DCE (3.2 M, 0.18 mL) at current density of 0.1 mA cm 2 . (B) Polarization graph showing the discharging potential and power density at room temperature.
exploitation of advanced membranes may provide a forward path to push the limit of power density. However, considering the advantageous low cost and high energy density, the Fe-Al battery has great potential to be applied in large-scale energy storage. The Working Mechanism of Fe DESs The coordination of Fe3+ in the Fe-urea eutectic system is complicated and not well understood, which limits the elucidation of the working mechanism of Fe-urea DESs. Possible cathode reactions were proposed in previous reports,29,32 such as: [FeCl2(OD)4]+ + OD + e / [Fe(OD)5]2+ + 2Cl
(Equation 4)
[FeCl4] + e / [FeCl4]2 ,
(Equation 5)
where OD is the oxygen donor. In this work, to understand the working mechanism the Raman spectra of various Fe DESs were measured, as shown in Figures 6A and 6B. Interestingly, in the absence of EG, Fe(120), Fe(110), and Fe(210) DESs had similar Raman spectra, indicating the same coordination environment for Fe3+. The strongest peak at around 328 cm 1 corresponded to the vibration of [FeCl4] ,33–36 while the influence of EG additives on the three original Fe DESs had a slight variation. For 120-based Fe DESs (121 and 126), adding EG immediately caused the disappearance of the [FeCl4] peak, while the peaks corresponding to urea and EG (828, 1,020 and 1,080 cm 1) increased (Figure S11A). For Fe(221) DES, originating from Fe(110) DES, the weakened peak of [FeCl4] still existed along with the strengthened peak at 1,020 cm 1, which could be attributed to urea. For 210-based Fe DESs, a small amount of EG did not affect the coordination of Fe3+ (Fe(421)). When the molar ratio became 2:1:2, the peak of [FeCl4] eventually decreased considerably. However, the feature peaks related to urea and EG did not increase strongly, as seen in Figure 6B. To understand the redox reaction of Fe DESs, we conducted a Raman test of Fe(126) and Fe(210) catholytes after discharging and charging. As expected, the Raman spectra of Fe(126) catholyte had negligible changes during charging and discharging, as shown in Figure 6C, implying that the coordination of Fe3+ or Fe2+ did not change. Since a large amount of EG caused the dissociation of [FeCl4] , we thought Fe3+ might not have the chloride-related coordination in Fe(126) DES, merely
8
Joule 1, 1–11, November 15, 2017
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 6. Ex Situ Raman Evidence for the Working Mechanism of Fe DESs (A) The Raman spectra of 120- and 110-based Fe DESs. (B–D) The Raman spectra of 210-based Fe DESs (B); the Raman spectra of original, discharged, and charged (C) Fe(126) catholyte and (D) Fe(210) catholyte.
forming solvated ions such as [Fe(OD)6]3+ instead.27 For Fe(210) catholyte, shown in Figure 6D, an obvious change could be observed at the discharging state that the characteristic peaks of FeCl2$4H2O rather than FeCl3$6H2O (Figure S11B) occurred, suggesting that [FeCl2(OD)4]+ reduced to be insoluble FeCl2$xH2O. During the charging process, the solid FeCl2$xH2O was oxidized to reform [FeCl2(OD)4]+ and the Raman spectrum recovered its original status. Figure S12 displays the chemical titration using K3Fe(CN)6 and K4Fe(CN)6 solutions and physical changes of Fe(126) and Fe(210) catholytes. The phenomenon is consistent with Raman and electrochemical results. The different coordination environments of Fe3+ in Fe(210) and Fe(126) DESs result in different working mechanisms and will play an important role in their respective electrochemical performance. Therefore, we propose a new perspective for redox reactions in Fe-urea DESs: Cathode reaction: [FeCl2(OD)4]+ + e / [FeCl2(OD)4]
(Equation 6)
Cathode reaction: [FeCl4] + e / [FeCl4]2
(Equation 7)
Cathode reaction: [Fe(OD)6]3+ + e / [Fe(OD)5]2+ + OD,
(Equation 8)
where [FeCl2(OD)4] appears to be a precipitate, causing the coagulation of Fe(210) DES during discharging. In conclusion, considering the results from Raman spectra and electrochemical tests, we offer a new principle regarding Fe-urea DESs whereby the dissociation of
Joule 1, 1–11, November 15, 2017
9
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
[FeCl4] and [FeCl2(OD)4]+ by the EG additive, which can help avoid the formation of solid, facilitates the full cycling performance, increases the utilization, and improves reversibility. Following this principle, we obtained the preliminary result, as shown in Figure S13. Dissociating the complex ions in Fe(210) DES, Fe(213) catholyte (3.6 M) had the potential to reduce the charging/discharging potential gap and achieve the stable cycling under full charge and discharge.
EXPERIMENTAL PROCEDURES The details of DES preparation, cell assembly, and other characterizations are provided in Supplemental Experimental Procedures.
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, 13 figures, and 1 table and can be found with this article online at https://doi.org/10.1016/ j.joule.2017.08.013.
AUTHOR CONTRIBUTIONS L.Z., C.Z., and Y.D. contributed equally to this work. G.Y., L.Z., C.Z., and Y.D. conceived the idea and designed the experiments. L.Z., C.Z., and Y.D. conducted the experiments and data analysis. K.R.-M. assisted with some experiments and paper editing. All authors co-wrote the manuscript and discussed the results.
ACKNOWLEDGMENTS G.Y. acknowledges the financial support from the National Science Foundation award (NSF-CMMI-1537894), Sloan Research Fellowship, and Camille Dreyfus Teacher-Scholar award. Received: May 6, 2017 Revised: July 12, 2017 Accepted: August 17, 2017 Published: October 11, 2017
REFERENCES 1. Yang, Z., Zhang, J., Kintner-Meyer, M.C., Lu, X., Choi, D., Lemmon, J.P., and Liu, J. (2011). Electrochemical energy storage for green grid. Chem. Rev. 111, 3577–3613. 2. Liu, J., Zhang, J., Yang, Z., Lemmon, J.P., Imhoff, C., Graff, G.L., Li, L., Hu, J., Wang, C., Xiao, J., et al. (2013). Materials science and materials chemistry for large scale electrochemical energy storage: from transportation to electrical grid. Adv. Funct. Mater. 23, 929–946. 3. Dunn, B., Kamath, H., and Tarascon, J.-M. (2011). Electrical energy storage for the grid: a battery of choices. Science 334, 928–935. 4. Soloveichik, G.L. (2015). Flow batteries: current status and trends. Chem. Rev. 115, 11533– 11558. 5. Zhao, Y., Ding, Y., Li, Y., Peng, L., Byon, H.R., Goodenough, J.B., and Yu, G. (2015). A chemistry and material perspective on lithium redox flow batteries towards high-density electrical
10
Joule 1, 1–11, November 15, 2017
energy storage. Chem. Soc. Rev. 44, 7968– 7996.
using a sulfuric/chloric mixed-acid supporting electrolyte. Adv. Energy Mater. 2, 487–493.
6. Noack, J., Roznyatovskaya, N., Herr, T., and Fischer, P. (2015). The chemistry of redox-flow batteries. Angew. Chem. Int. Ed. 54, 2–36.
11. Biswas, S., Senju, A., Mohr, R., Hodson, T., Karthikeyan, N., Knehr, K.W., Hsieh, A.G., Yang, X., Koel, B.E., and Steingart, D.A. (2017). Minimal architecture zinc-bromine battery for low cost electrochemical energy storage. Energy Environ. Sci. 10, 114–120.
7. Wang, W., Luo, Q., Li, B., Wei, X., Li, L., and Yang, Z. (2013). Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986. 8. Winsberg, J., Hagemann, T., Janoschka, T., Hager, M.D., and Schubert, U.S. (2016). Redoxflow batteries: from metals to organic redoxactive materials. Angew. Chem. Int. Ed. 55, 2–28. 9. Li, L., Kim, S., Wang, W., Vijayakumar, M., Nie, Z., Chen, B., Zhang, J., Xia, G., Hu, J., Graff, G., et al. (2011). A stable vanadium redox-flow battery with high energy density for large-scale energy storage. Adv. Energy Mater. 1, 394–400. 10. Wang, W., Nie, Z., Chen, B., Chen, F., Luo, Q., Wei, X., Xia, G., Skyllas-Kazacos, M., Li, L., and Yang, Z. (2012). A New Fe/V redox flow battery
12. Wang, Y., He, P., and Zhou, H. (2012). Li-Redox flow batteries based on hybrid electrolytes: at the cross road between Li-ion and redox flow batteries. Adv. Energy Mater. 2, 770–779. 13. Zhao, Y., Ding, Y., Song, J., Dong, G., Goodenough, J.B., and Yu, G. (2014). Sustainable electrical energy storage through the ferrocene/ferrocenium redox reaction in aprotic electrolyte. Angew. Chem. Int. Ed. 53, 11036–11040. 14. Zhao, Y., Ding, Y., Song, J., Peng, L., Goodenough, J.B., and Yu, G. (2014). Reversible Br2/Br- redox couple in aqueous phase as high-performance catholyte for alkaliion batteries. Energy Environ. Sci. 7, 1990– 1995.
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
15. Ding, Y., Li, Y., and Yu, G. (2016). Exploring bioinspired quinone-based organic redox flow batteries: a combined experimental and computational study. Chem 1, 790–801. 16. Ambroz, F., Macdonald, T.J., and Nann, T. (2017). Trends in aluminium-based intercalation batteries. Adv. Energy Mater. 7, 1602093.
22. Li, B., Nie, Z., Vijayakumar, M., Li, G., Liu, J., Sprenkle, V., and Wang, W. (2015). Ambipolar zinc-polyiodide electrolyte for a high-energy density aqueous redox flow battery. Nat. Commun. 6, 6303–6310. 23. Orita, A., Verde, M.G., Sakai, M., and Meng, Y. (2016). A biomimetic redox flow battery based on flavin mononucleotide. Nat. Commun. 7, 13230–13237.
17. Elia, G.A., Marquardt, K., Hoeppner, K., Fantini, S., Lin, R., Knipping, E., Peters, W., Drillet, J.F., Passerini, S., Hahn, R., et al. (2016). An overview and future perspectives of aluminum batteries. Adv. Mater. 28, 7564–7579.
24. Gong, K., Fang, Q., Gu, S., Li, S., and Yan, Y. (2015). Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs. Energy Environ. Sci. 8, 3515–3530.
18. Lin, M., Gong, M., Lu, B., Wu, Y., Wang, D.Y., Guan, M., Angell, M., Chen, C., Yang, J., Hwang, B.J., and Dai, H. (2015). An ultrafast rechargeable aluminium-ion battery. Nature 520, 324–328.
25. Abbott, A.P., Capper, G., Davies, D.L., and Rasheed, R. (2004). Ionic liquids based upon metal halide/substituted quaternary ammonium salt mixtures. Inorg. Chem. 43, 3447–3452.
19. Angell, M., Pan, C.J., Rong, Y., Yuan, C., Lin, M.C., Hwang, B.J., and Dai, H. (2017). High Coulombic efficiency aluminum-ion battery using an AlCl3-urea ionic liquid analog electrolyte. Proc. Natl. Acad. Sci. USA 114, 834–839.
26. Abbott, A.P., Al-Barzinjy, A.A., Abbott, P.D., Frisch, G., Harris, R.C., Hartley, J., and Ryder, K.S. (2014). Speciation, physical and electrolytic properties of eutectic mixtures based on CrCl3 $ 6H2O and urea. Phys. Chem. Chem. Phys. 16, 9047–9055.
20. Zhang, L., Chen, L., Luo, H., Zhou, X., and Liu, Z. (2017). Large-sized few-layer graphene enables an ultrafast and long-life aluminum-ion battery. Adv. Energy Mater. 7, 1700034.
27. Hartley, J.M., Ip, C.M., Forrest, G.C., Singh, K., Gurman, S.J., Ryder, K.S., Abbott, A.P., and Frisch, G. (2014). EXAFS study into the speciation of metal salts dissolved in ionic liquids and deep eutectic solvents. Inorg. Chem. 53, 6280–6288.
21. Hu, B., DeBruler, C., Rhodes, Z., and Liu, T. (2017). Long-cycling aqueous organic redox flow battery (AORFB) toward sustainable and safe energy storage. J. Am. Chem. Soc. 139, 1207–1214.
28. Smith, E.L., Abbott, A.P., and Ryder, K.S. (2014). Deep eutectic solvents (DESs) and their applications. Chem. Rev. 114, 11060–11082.
29. Wang, Y., and Zhou, H. (2016). A green and cost-effective rechargeable battery with high energy density based on a deep eutectic catholyte. Energy Environ. Sci. 9, 2267–2272. 30. Zhang, C., Ding, Y., Zhang, L., and Yu, G. (2017). A sustainable redox-flow battery with an aluminum-based, deep-eutectic-solvent anolyte. Angew. Chem. Int. Ed. 56, 7454–7459. 31. Hoyt, N.C., Hawthorne, K.L., Savinell, R.F., and Wainright, J.S. (2016). Plating utilization of carbon felt in a hybrid flow battery. J. Electrochem. Soc. 163, A5041–A5048. 32. Lloyd, D., Vainikka, T., Ronkainen, M., and Kontturi, K. (2013). Characterisation and application of the Fe(II)/Fe(III) redox reaction in an ionic liquid analogue. Electrochim. Acta 109, 843–851. 33. Sharma, S. (1974). Raman study of ferric chloride hexahydrate and ferric chloride hexadeutrate in crystalline, molten and glassy states. J. Noncryst. Sol. 15, 83–95. 34. Sharma, S. (1974). Raman study of ferric chloride solutions and hydrated melts. J. Chem. Phys. 60, 1368–1375. 35. Cariati, F., Masserano, F., Martini, M., and Spinolo, G. (1989). Raman studies of NiX2$6H2O and FeCl2$4H2O. J. Raman Spectrosc. 20, 773–777. 36. Sitze, M.S., Schreiter, E.R., Patterson, E.V., and Freeman, R. (2001). Ionic liquids based on FeCl3 and FeCl2. Raman scattering and ab initio calculations. Inorg. Chem. 40, 2298–2304.
Joule 1, 1–11, November 15, 2017
11
A Low-Cost and High-Energy Hybrid IronAluminum Liquid Battery Achieved by Deep Eutectic Solvents Leyuan Zhang, Changkun Zhang, Yu Ding, Katrina Ramirez-Meyers, Guihua Yu [email protected]
HIGHLIGHTS A low-cost and high-energy Fe-Al hybrid-flow battery New reaction mechanism in FeCl3$6H2O/urea/EG DES Stable cycling achieved by dissociating iron complexes
A low-cost and high-energy Fe-Al RFB is established for large-scale energy storage. Using Fe catholyte at a concentration of 5 M, the Fe-Al battery can deliver a high energy density of 166 Wh L 1. This study also furthers our fundamental understanding about the working mechanism of Fe-urea DESs. By dissociating the complex ions in Fe DES, the Fe-Al battery can achieve the full charge and discharge over 1,500 hr without any capacity fading.
Zhang et al., Joule 1, 1–11 November 15, 2017 ª 2017 Elsevier Inc. https://doi.org/10.1016/j.joule.2017.08.013
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Article
A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents Leyuan Zhang,1 Changkun Zhang,1 Yu Ding,1 Katrina Ramirez-Meyers,1 and Guihua Yu1,2,*
SUMMARY
Context & Scale
This work demonstrates a low-cost, high-energy Fe-Al hybrid liquid battery that takes advantage of the desirable properties of deep eutectic solvents (DESs). The strategy of additive enables the full charging and discharging of the Fe-Al battery with long cycle life while the stable stripping and deposition of Al is achieved. Using Fe(210) catholyte at a concentration of 5 M, the Fe-Al battery can deliver a high energy density of approximately 166 Wh L 1 with an average operating voltage of 1.41 V. Furthermore, by dissociating the iron complexes in Fe(126) DES, the Fe-Al battery can achieve the full charge and discharge over 60 cycles without degradation. Here, an all-DES-based liquid battery is proposed with an ultrahigh concentration of redox species, resulting in high energy density. The DES maintains reduced lattice energy and depressed freezing point and provides a new platform for developing green redox species based on new chemistry.
Given advantages of low cost, high concentration, and potential biodegradability, the concept of deep eutectic solvents (DESs) is beneficial to developing costeffective and sustainable batteries with high energy density. Combining environmentally friendly Al DES and Fe DES, a green Fe-Al hybrid liquid battery was designed. The stable deposition and stripping of Al and long cycling of Fe DES with high utilization were achieved using 1,2-dichloroethane (DCE) and ethylene glycol (EG) as additives, respectively. The Fe-Al hybrid battery delivered a high energy density of 166.2 Wh L 1. Furthermore, in-depth chemical characterizations reveal that the excellent cycling performance of Fe DES should be ascribed to the dissociation of complex ions by the EG additive. Therefore, the strategy of additives and the influence of coordination environment on electrochemical performance provide a new insight into the design of novel catholytes and anolytes using DESs for redox-flow batteries.
INTRODUCTION Considering the resource limitations and environmental concerns of conventional fossil fuels, energy conversion from ecologically sustainable sources, such as sunlight and wind, can provide green power sources in modern society. However, the intermittency and unreliable stability of renewable energy systems limit their extensive application in direct power supply. Therefore, the development of efficient large-scale energy storage systems is necessary to make full use of the renewable energy resources. Among various energy storage technologies, rechargeable batteries have been one of the most dominant technologies for many years.1–3 In particular, redox-flow batteries (RFBs) are considered as a promising technology for large-scale energy storage.4–7 Conventional inorganic RFBs, such as vanadium RFBs or Zn-Br2 RFBs, have been commercialized but still suffer from intrinsic disadvantages, such as the use of scarce and expensive elements, corrosive electrolytes that raise safety concerns, and dendrite formation (zinc).8–11 On the other hand, to improve the energy density of RFBs, lithium-based hybrid-flow batteries (HFBs) with high operating voltage are being developed.12–15 However, limited resources, serious dendrite problems, and flammable electrolytes still impede the extensive implementation of lithium in HFBs. As an alternative to lithium, aluminum is attracting increasing interest due to its low cost, high abundance, and high theoretical capacity (2,980 mAh g 1).16,17 Moreover, recent research on Al-ion batteries has shown stable stripping and deposition of aluminum in Al ionic liquids (ILs) and Al DESs without serious dendrite formation.18–20 These characteristics make aluminum a promising potential anode in HFBs.
Joule 1, 1–11, November 15, 2017 ª 2017 Elsevier Inc.
1
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
In addition to broadening the operating voltage, increasing the concentration of redox species is another strategy for enhancing the energy density of RFBs.21–23 In traditional aqueous solutions, a solute molecule is surrounded by large amounts of water molecules to form a solvation complex, which becomes a barrier for achieving high concentration and, hence, high energy density in an aqueous system.24 Recently, many metal-based DESs, such as Fe, Co, Cr, Al, Mg, and Zn, have been easily synthesized in ambient conditions with a high concentration of active species, which offers a potential opportunity for RFBs.25–28 Fe-based DESs, in particular, are especially promising as cathode-active materials due to their stability, high crust abundance, and high redox potential (Fe2+/Fe3+ at 0.77 V versus normal hydrogen electrode).29 Meanwhile, an Al-based DES, encompassing the advantages of both Al metal and DESs, would be a better choice for anolyte compared with Al ILs. However, due to the intrinsic properties of DESs or organic electrolytes, the rate capability is currently not yet comparable with that of aqueous systems. Herein, a proof-of-concept Fe-Al HFB was proposed based on Fe DES and Al DES. A series of FeCl3$6H2O/urea/EG DESs with various molar ratios, simplified as a combination of three numbers (Fe(xxx) DESs), are obtained and the influence of EG as the additive is explored. For Al DES, 1,2-dichloroethane (DCE) is added to decrease the viscosity and then increase the ionic conductivity. Finally, paired with Al-DES/DCE, Fe(126) (1.8 M) and Fe(210) (5.5 M) DESs with EC/DMC (ethylene carbonate/dimethyl carbonate) were selected as catholytes to test the performance of Fe-Al HFBs. This battery has an average operating voltage of approximately 1.41 V, delivering a high energy density of 166.2 Wh L 1 when using the Fe(210) catholyte. Moreover, we find that adding EG can change the coordination environment of Fe3+ and enable the dissolution of LiCl. The dissociation of complex cations and anions in Fe(126) catholyte from adding EG thus improves the full charging and discharging behavior and cycling stability. Furthermore, stable stripping and deposition of the Al anode is demonstrated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterization.
RESULTS AND DISCUSSION The Proof-of-Concept Fe-Al Hybrid Liquid Battery from the Concept of Fe DES and Al DES As reported in the previous work, DESs can be technically and economically favorable for developing low-cost and high-energy flow batteries since they form a liquid with high concentration by simply mixing the cheap raw materials at room temperature. Considering the high abundance, low cost, and appropriate potentials, Fe and Al DESs were selected as the catholyte and anolyte, respectively. The scheme and photograph of the proof-of-concept prototype for the Fe-Al hybrid battery system are displayed in Figure 1. An aluminum strip attached to Cu foil serves as the anode. According to the literature,18,19 the reaction that occurs in Al DESs is different from the one that occurs in Al ILs, due to different Al3+ coordination environments. The Al-urea cations and Al-chloride anions in Al DES result in two possible reactions of Al deposition and stripping as follows. Anode reaction: 4Al2Cl7 + 3e / Al + 7AlCl4
(Equation 1)
Anode reaction: 2[AlCl2$(urea)2]+ + 3e / Al + AlCl4 + 4(urea) (Equation 2)
1Materials
Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA
2Lead
As similarly discussed in our recent work,30 DCE with a low viscosity was added into Al DES to further decrease the viscosity and enhance the ionic conductivity of
2
Joule 1, 1–11, November 15, 2017
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.joule.2017.08.013
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 1. The Proof-of-Concept Fe-Al Hybrid Liquid Battery (A) Schematic of an Fe-Al battery based on DESs. (B) The photograph of a working Fe-Al battery (top) and its schematic of device configuration (bottom).
anolytes. Compared with the conductivity of Al DES (0.36 mS cm 1), the diluted Al DES (denoted as Al-DES/DCE) showed a 5-fold increase in ionic conductivity to 1.9 mS cm 1. In the diluted state, the concentration of all redox species ([AlCl2(urea)n]+, Al2Cl7 , and AlCl4 ) in Al-DES/DCE was still as high as 3.2 M. In addition, the reaction that occurred in Al-DES/DCE was similar to the Al DES reaction, already reported in other literature. For the cathode, Fe-urea DES paired with Li metal was recently demonstrated in a high-energy green battery, but the initial coulombic efficiency and cycling performance based on fully charging/discharging was not satisfactory. In this work, EG was added to adjust the physical properties of FeCl3$6H2O/urea/EG DESs for the following reasons: (1) the viscosity of EG is about 16.1 cP at 25 C, suggesting that it may decrease the viscosity of Fe DESs; (2) the freezing point of EG is about 10 C, giving it the potential to maintain the liquid form of Fe DESs at relatively low temperatures; (3) it can dissolve LiCl salt, implying it will improve the reversibility of redox reaction of Fe DESs. The various compositions of Fe DESs are named by using three numbers to represent the molar ratios of FeCl3$6H2O, urea, and EG, respectively. For example, Fe(123) DESs means the molar ratios of FeCl3$6H2O/urea/EG is 1:2:3. Based on the knowledge of Fe-based DESs,28 three original Fe-urea DESs are synthesized in the first step: Fe(120), Fe(110), and Fe(210) DESs. The last number represents the molar ratio of the EG component, meaning that EG does not exist in these three original Fe DESs. As shown in Figure 1B, Fe DESs were injected into the quartz shell through the predrilled hole in the Ti foil as catholyte. To improve the current collection efficiency, Super P is mixed with a small amount of polyvinylidene fluoride binder coated on the surface of Ti foil. The Viscosity, Conductivity, and Reversibility of Various Fe DESs As displayed in Figure 2A, various Fe DESs can be easily synthesized at ambient environment, which also show long-term stability during storage in air (Figure S1). With the increase of EG component in Fe DESs, the Fe3+ concentration will
Joule 1, 1–11, November 15, 2017
3
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 2. The Physical Properties and CVs of Various Fe DESs (A) Photograph of various Fe DESs. (B) Ionic conductivity of FeCl3 $6H2 O/urea/EG as a function of composition. (C) CV curves of Fe DESs at the sweeping rate of 1 mV s 1 . (D) Viscosity data of FeCl 3 $6H 2 O/urea/EG as a function of temperature and composition.
decrease, leading to a color change from dark to light. Fe(126) DES has a relatively low concentration and thus forms a light-brown and transparent liquid, implying a homogeneous and stable state. Generally, Fe DESs are more viscous than aqueous solutions, so the ionic conductivity may be a concern in practical applications. Electrochemical impedance spectroscopy (EIS) is utilized to measure the ionic conductivities of the different Fe DESs. As seen in Figure 2B, Fe(210) DES has the highest conductivity and 120-based Fe DESs have a lower conductivity. The conductivities of these Fe DESs (>10 mS cm 1) are much larger than that of the lithium aluminum titanium phosphate (LATP) separator (0.1 mS cm 1), indicating that the conductivity would not limit the electrochemical performance. Cyclic voltammetry (CV) and viscosity tests were conducted to observe the reaction reversibility of various Fe DESs and select the appropriate catholyte for the Fe-Al battery. From the CV curves at different sweeping rates (Figure S2) and analysis of the CV curves at 1 mV s 1 (Figure 1C and Table S1), we find that Fe(126) DES exhibits a highly reversible reaction compared with other samples. Differences in reversibility should be ascribed to the different molar ratios and the influence of EG component, which also affects viscosity. As shown in Figures 2D and S3, as the amount of EG increases, the viscosity of 120-based Fe DESs initially increases and then decreases. In contrast, the viscosity of Fe(210)-derived DESs continuously increases with the addition of EG. Through consideration of the concentration, reversibility, and viscosity of various Fe DESs, Fe(126) and Fe(210) DESs are chosen as catholytes for the electrochemical test of the Fe-Al hybrid battery due to the high reversibility and high concentration, respectively. To further decrease the viscosity, a small amount of EC/DMC could be added into these two Fe DESs (denoted as Fe-DES/EC-DMC). CV tests were conducted to assess the electrochemical stability of EG and EC/DMC (Figure S4) and showed that no decomposition or other side reactions occurred in the working voltage range.
4
Joule 1, 1–11, November 15, 2017
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 3. The Electrochemical Performance of Fe(126) DES (A) CV curves of Fe(126) DES at different sweeping rates. (B) Linear relationships between the oxidation and reduction peak current with the square root of the sweeping rate. (C) The charge and discharge capacity with corresponding coulombic efficiency of Fe(126) catholyte when paired with Li at the current density of 0.1 mA cm 2 . (D) Polarization curve (black) and corresponding specific power density (red) at room temperature.
The Electrochemical Performances of Fe-Li and Fe-Al Hybrid Batteries For Fe(126) DES, CV tests were conducted at different sweeping rates using the three-electrode system (Figure 3A). By comparing the anodic and cathodic peak currents, we find that the redox reaction of Fe(126) DES is highly reversible. Given the high concentration of redox active material, the notable potential hysteresis, especially at high scan rates, should be attributed to the electromigration and concentration polarization. The linear relationship between the peak currents and the square root of sweeping rate, displayed in Figure 3B, implies that the reaction in Fe(126) DES is a diffusion-controlled process. Using the Randles-Sevcik equation below, the diffusion coefficient was calculated to be about 2 3 10 8 cm2 s 1. Ip = 26,900 3 n1.5AD0.5v0.5C,
(Equation 3)
where Ip is peak current, n is the number of electrons involved, A is the active surface area, D is the diffusion coefficient, v is sweeping rate, and C is the concentration of redox species. While the derived coefficient is not comparable with those of conventional redox species in water, the value is reasonable at such a high concentration, and the diffusion process can be further facilitated by exploiting 3D current collector in the practical battery test. The Fe(126)-Li hybrid battery was constructed to evaluate the charging and discharging behavior. Figure 3C shows the cycling performance of the Fe(126)-Li battery, and the full charging/discharging curves are displayed in Figure S5. Under the operating voltage of 3.45 V, a stable, long-life cycling with a high coulombic efficiency (100%–103%) is achieved. At a current density of 0.1 mA cm 2, the battery has a volumetric capacity of about 41 Ah L 1 along with the high initial coulombic
Joule 1, 1–11, November 15, 2017
5
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
efficiency of 100%. Benefiting from relatively low viscosity, about 90% of the theoretical capacity can be reversibly utilized and the average voltage difference between charging and discharging curves is only 0.15 V. Based on the high concentration of 1.7 M and high utilization, an energy density of 142.4 Wh L 1 was achieved, which is among the highest energy densities of RFBs reported so far. The polarization curve of the Fe(126)-Li hybrid battery was obtained at room temperature (Figure 3D). The discharging potential and current density showed a linear relationship and a high power density of over 20 mW cm 2 was obtained, which is the highest among liquid batteries using DESs as catholytes. To demonstrate the stable stripping/deposition of Al, ex situ XRD and scanning EM characterizations were taken along with the CV test at different scan rates (Figure S6). Using Al rods as the counter and reference electrodes, we obtained the CV curves from 10 to 100 mV s 1. As seen in Figure S6A, the stripping and deposition of Al is a highly reversible reaction in this Al-DES/DCE and the dissolution of Al occurred at about 0 V versus Al. Figure S6B shows the XRD patterns of Al anodes before and after electrochemical tests, which do not show any indication of impurities (such as Al2O3 or AlCl3). Furthermore, based on the SEM images of Al anodes (Figures S6C and S6D), we believe that the dendrite problem, existing in the Li anode, was not a serious issue in Al DESs. Above all, the stripping and deposition of Al in Al-DES/DCE is stable and highly reversible, which makes it possible to design advanced Al-based batteries. The stable deposition/stripping of aluminum in the Al DES and the low-cost Fe DES with high concentration and low viscosity enable the design of an advanced Fe-Al HFB. Figure 4A shows the charging and discharging curves of the Fe(126)-Al hybrid battery. Similarly, a high utilization of about 90% can be achieved, delivering a discharge capacity of 41.4 Ah L 1. The average operating charge and discharge potentials are 1.57 V and 1.41 V, respectively, resulting in a voltage gap of 0.16 V. Due to the limitation of operating voltage, the energy density decreases to 58.1 Wh L 1, which is lower than that of an Fe(126)-Li hybrid battery. Figure 4B shows the relationship between current density and power density in the range of 0.05–1 mA cm 2. Due to the internal resistance of the battery, the discharge potential linearly decreases as current density increases. Moreover, the power density turned out to be over 1 mW cm 2 at the current density of 1 mA cm 2 associated with a discharge voltage of roughly 1.1 V. The potential-time profile of the Fe(126)-Al hybrid battery is presented in Figure 4C, showing that the full charging and discharging potential can be maintained during a long operation time of 500 hr. This is consistent with the aforementioned result of the Fe(126)-Li battery. The problem of crossover was eliminated by using an LATP ceramic separator, resulting in a high coulombic efficiency of over 96% during cycling (Figure 4D). During continuous charging and discharging, no capacity fading was found over 60 cycles with 90% utilization. Both the steady potential profile and stabilized cycling performance confirmed that the redox reactions in Fe(126)-DES/EC-DMC and Al-DES/DCE were highly reversible and stable. To further improve the energy density of this proof-of-concept Fe-Al hybrid battery, the Fe(210) catholyte was prepared by adding 50 mL of EC/DMC into 0.5 mL of Fe(210) DES. The EC/DMC is meant to decrease the viscosity and is confirmed to be electrochemically stable in the potential window. Since the concentration of Fe(210) DES is 5.5 M, the Fe(210) catholyte still maintains a concentration of 5 M. Paired with Li metal, the Fe(210) catholyte can also show a high utilization of about 90% in the first cycle (Figure S7A) and the power density approaches 22 mW cm 2 (Figure S7B), which is much higher than reported in previous work.29 Figure 5A shows the initial discharging and charging profile of the Fe(210)-Al battery. Due to
6
Joule 1, 1–11, November 15, 2017
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 4. The Electrochemical Performance of an Fe(126)-Al Battery (A) Charge/discharge profile of Fe(126)-DES/EC-DMC (1.7 M, 0.05 mL) paired with Al-DES/DCE (3.2 M, 0.1 mL) at current density of 0.1 mA cm 2 . (B) The discharge voltage and corresponding power density of an Fe(126)-Al battery at various current densities. (C) Representative charge and discharge profiles over time at the current density of 0.1 mA cm 2 . (D) The charge and discharge capacity with corresponding coulombic efficiency of an Fe(126)-Al battery at room temperature over cycling.
the increased concentration of redox species, the specific capacity soared to about 120 Ah L 1 at 0.1 mA cm 2. However, at the current density of 0.2 mA cm 2, the utilization and coulombic efficiency of the initial cycling decreased (Figure S8A). The EIS result shows that the total resistance is not very high (Figure S8B), indicating that the performance decay at a high current density may be caused by slow mass/ions transport. With an average voltage of approximately 1.41 V, the high energy density of 166.2 Wh L 1, based on the volume of Fe catholyte, was delivered by this low-cost Fe(210)-Al hybrid battery. It appears that high concentration contributes to boosting the energy density, but may negatively affect the cycling performance. Figure S8C displays the full charging and discharging of the Fe(210)-Al battery over time and suggests that the cycling performance is related to the working mechanism of Fe(210) catholyte. To further clarify the calculation of energy density, an example of an Fe(210)-Al battery is provided in Figure S9. The polarization curve was measured at room temperature, as presented in Figure 5B. Similar to the Fe(126)-Al battery, the Fe(210)-Al battery shows a limited power density of about 1.2 mW cm 2 at a current density of 1.5 mA cm 2. The potential profile with the increase of current density was also measured (Figure S8D) and was consistent with the polarization curve. To improve the power performance, we used the flow electrode with carbon felt as current collector in the Fe side (Figure S10). As displayed in Figures S10B and S10C, power densities of 4.5 and 5 mW cm 2 were achieved for Fe(126)- and Fe(210)-DES/EC-DMC catholytes, respectively, both of which were significantly higher than the static one, although still not yet comparable with the aqueous systems. The possible reasons for the limited power density are the low ionic conductivity of LATP membrane and the high concentration of redox species in DESs. Therefore, modifications of the device configuration31 and
Joule 1, 1–11, November 15, 2017
7
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 5. The Electrochemical Performance of an Fe(210)-Al Battery (A) The initial charge/discharge profile of Fe(210)-DES/EC-DMC (5 M, 0.03 mL) paired with Al-DES/ DCE (3.2 M, 0.18 mL) at current density of 0.1 mA cm 2 . (B) Polarization graph showing the discharging potential and power density at room temperature.
exploitation of advanced membranes may provide a forward path to push the limit of power density. However, considering the advantageous low cost and high energy density, the Fe-Al battery has great potential to be applied in large-scale energy storage. The Working Mechanism of Fe DESs The coordination of Fe3+ in the Fe-urea eutectic system is complicated and not well understood, which limits the elucidation of the working mechanism of Fe-urea DESs. Possible cathode reactions were proposed in previous reports,29,32 such as: [FeCl2(OD)4]+ + OD + e / [Fe(OD)5]2+ + 2Cl
(Equation 4)
[FeCl4] + e / [FeCl4]2 ,
(Equation 5)
where OD is the oxygen donor. In this work, to understand the working mechanism the Raman spectra of various Fe DESs were measured, as shown in Figures 6A and 6B. Interestingly, in the absence of EG, Fe(120), Fe(110), and Fe(210) DESs had similar Raman spectra, indicating the same coordination environment for Fe3+. The strongest peak at around 328 cm 1 corresponded to the vibration of [FeCl4] ,33–36 while the influence of EG additives on the three original Fe DESs had a slight variation. For 120-based Fe DESs (121 and 126), adding EG immediately caused the disappearance of the [FeCl4] peak, while the peaks corresponding to urea and EG (828, 1,020 and 1,080 cm 1) increased (Figure S11A). For Fe(221) DES, originating from Fe(110) DES, the weakened peak of [FeCl4] still existed along with the strengthened peak at 1,020 cm 1, which could be attributed to urea. For 210-based Fe DESs, a small amount of EG did not affect the coordination of Fe3+ (Fe(421)). When the molar ratio became 2:1:2, the peak of [FeCl4] eventually decreased considerably. However, the feature peaks related to urea and EG did not increase strongly, as seen in Figure 6B. To understand the redox reaction of Fe DESs, we conducted a Raman test of Fe(126) and Fe(210) catholytes after discharging and charging. As expected, the Raman spectra of Fe(126) catholyte had negligible changes during charging and discharging, as shown in Figure 6C, implying that the coordination of Fe3+ or Fe2+ did not change. Since a large amount of EG caused the dissociation of [FeCl4] , we thought Fe3+ might not have the chloride-related coordination in Fe(126) DES, merely
8
Joule 1, 1–11, November 15, 2017
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
Figure 6. Ex Situ Raman Evidence for the Working Mechanism of Fe DESs (A) The Raman spectra of 120- and 110-based Fe DESs. (B–D) The Raman spectra of 210-based Fe DESs (B); the Raman spectra of original, discharged, and charged (C) Fe(126) catholyte and (D) Fe(210) catholyte.
forming solvated ions such as [Fe(OD)6]3+ instead.27 For Fe(210) catholyte, shown in Figure 6D, an obvious change could be observed at the discharging state that the characteristic peaks of FeCl2$4H2O rather than FeCl3$6H2O (Figure S11B) occurred, suggesting that [FeCl2(OD)4]+ reduced to be insoluble FeCl2$xH2O. During the charging process, the solid FeCl2$xH2O was oxidized to reform [FeCl2(OD)4]+ and the Raman spectrum recovered its original status. Figure S12 displays the chemical titration using K3Fe(CN)6 and K4Fe(CN)6 solutions and physical changes of Fe(126) and Fe(210) catholytes. The phenomenon is consistent with Raman and electrochemical results. The different coordination environments of Fe3+ in Fe(210) and Fe(126) DESs result in different working mechanisms and will play an important role in their respective electrochemical performance. Therefore, we propose a new perspective for redox reactions in Fe-urea DESs: Cathode reaction: [FeCl2(OD)4]+ + e / [FeCl2(OD)4]
(Equation 6)
Cathode reaction: [FeCl4] + e / [FeCl4]2
(Equation 7)
Cathode reaction: [Fe(OD)6]3+ + e / [Fe(OD)5]2+ + OD,
(Equation 8)
where [FeCl2(OD)4] appears to be a precipitate, causing the coagulation of Fe(210) DES during discharging. In conclusion, considering the results from Raman spectra and electrochemical tests, we offer a new principle regarding Fe-urea DESs whereby the dissociation of
Joule 1, 1–11, November 15, 2017
9
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
[FeCl4] and [FeCl2(OD)4]+ by the EG additive, which can help avoid the formation of solid, facilitates the full cycling performance, increases the utilization, and improves reversibility. Following this principle, we obtained the preliminary result, as shown in Figure S13. Dissociating the complex ions in Fe(210) DES, Fe(213) catholyte (3.6 M) had the potential to reduce the charging/discharging potential gap and achieve the stable cycling under full charge and discharge.
EXPERIMENTAL PROCEDURES The details of DES preparation, cell assembly, and other characterizations are provided in Supplemental Experimental Procedures.
SUPPLEMENTAL INFORMATION Supplemental Information includes Supplemental Experimental Procedures, 13 figures, and 1 table and can be found with this article online at https://doi.org/10.1016/ j.joule.2017.08.013.
AUTHOR CONTRIBUTIONS L.Z., C.Z., and Y.D. contributed equally to this work. G.Y., L.Z., C.Z., and Y.D. conceived the idea and designed the experiments. L.Z., C.Z., and Y.D. conducted the experiments and data analysis. K.R.-M. assisted with some experiments and paper editing. All authors co-wrote the manuscript and discussed the results.
ACKNOWLEDGMENTS G.Y. acknowledges the financial support from the National Science Foundation award (NSF-CMMI-1537894), Sloan Research Fellowship, and Camille Dreyfus Teacher-Scholar award. Received: May 6, 2017 Revised: July 12, 2017 Accepted: August 17, 2017 Published: October 11, 2017
REFERENCES 1. Yang, Z., Zhang, J., Kintner-Meyer, M.C., Lu, X., Choi, D., Lemmon, J.P., and Liu, J. (2011). Electrochemical energy storage for green grid. Chem. Rev. 111, 3577–3613. 2. Liu, J., Zhang, J., Yang, Z., Lemmon, J.P., Imhoff, C., Graff, G.L., Li, L., Hu, J., Wang, C., Xiao, J., et al. (2013). Materials science and materials chemistry for large scale electrochemical energy storage: from transportation to electrical grid. Adv. Funct. Mater. 23, 929–946. 3. Dunn, B., Kamath, H., and Tarascon, J.-M. (2011). Electrical energy storage for the grid: a battery of choices. Science 334, 928–935. 4. Soloveichik, G.L. (2015). Flow batteries: current status and trends. Chem. Rev. 115, 11533– 11558. 5. Zhao, Y., Ding, Y., Li, Y., Peng, L., Byon, H.R., Goodenough, J.B., and Yu, G. (2015). A chemistry and material perspective on lithium redox flow batteries towards high-density electrical
10
Joule 1, 1–11, November 15, 2017
energy storage. Chem. Soc. Rev. 44, 7968– 7996.
using a sulfuric/chloric mixed-acid supporting electrolyte. Adv. Energy Mater. 2, 487–493.
6. Noack, J., Roznyatovskaya, N., Herr, T., and Fischer, P. (2015). The chemistry of redox-flow batteries. Angew. Chem. Int. Ed. 54, 2–36.
11. Biswas, S., Senju, A., Mohr, R., Hodson, T., Karthikeyan, N., Knehr, K.W., Hsieh, A.G., Yang, X., Koel, B.E., and Steingart, D.A. (2017). Minimal architecture zinc-bromine battery for low cost electrochemical energy storage. Energy Environ. Sci. 10, 114–120.
7. Wang, W., Luo, Q., Li, B., Wei, X., Li, L., and Yang, Z. (2013). Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986. 8. Winsberg, J., Hagemann, T., Janoschka, T., Hager, M.D., and Schubert, U.S. (2016). Redoxflow batteries: from metals to organic redoxactive materials. Angew. Chem. Int. Ed. 55, 2–28. 9. Li, L., Kim, S., Wang, W., Vijayakumar, M., Nie, Z., Chen, B., Zhang, J., Xia, G., Hu, J., Graff, G., et al. (2011). A stable vanadium redox-flow battery with high energy density for large-scale energy storage. Adv. Energy Mater. 1, 394–400. 10. Wang, W., Nie, Z., Chen, B., Chen, F., Luo, Q., Wei, X., Xia, G., Skyllas-Kazacos, M., Li, L., and Yang, Z. (2012). A New Fe/V redox flow battery
12. Wang, Y., He, P., and Zhou, H. (2012). Li-Redox flow batteries based on hybrid electrolytes: at the cross road between Li-ion and redox flow batteries. Adv. Energy Mater. 2, 770–779. 13. Zhao, Y., Ding, Y., Song, J., Dong, G., Goodenough, J.B., and Yu, G. (2014). Sustainable electrical energy storage through the ferrocene/ferrocenium redox reaction in aprotic electrolyte. Angew. Chem. Int. Ed. 53, 11036–11040. 14. Zhao, Y., Ding, Y., Song, J., Peng, L., Goodenough, J.B., and Yu, G. (2014). Reversible Br2/Br- redox couple in aqueous phase as high-performance catholyte for alkaliion batteries. Energy Environ. Sci. 7, 1990– 1995.
Please cite this article in press as: Zhang et al., A Low-Cost and High-Energy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents, Joule (2017), https://doi.org/10.1016/j.joule.2017.08.013
15. Ding, Y., Li, Y., and Yu, G. (2016). Exploring bioinspired quinone-based organic redox flow batteries: a combined experimental and computational study. Chem 1, 790–801. 16. Ambroz, F., Macdonald, T.J., and Nann, T. (2017). Trends in aluminium-based intercalation batteries. Adv. Energy Mater. 7, 1602093.
22. Li, B., Nie, Z., Vijayakumar, M., Li, G., Liu, J., Sprenkle, V., and Wang, W. (2015). Ambipolar zinc-polyiodide electrolyte for a high-energy density aqueous redox flow battery. Nat. Commun. 6, 6303–6310. 23. Orita, A., Verde, M.G., Sakai, M., and Meng, Y. (2016). A biomimetic redox flow battery based on flavin mononucleotide. Nat. Commun. 7, 13230–13237.
17. Elia, G.A., Marquardt, K., Hoeppner, K., Fantini, S., Lin, R., Knipping, E., Peters, W., Drillet, J.F., Passerini, S., Hahn, R., et al. (2016). An overview and future perspectives of aluminum batteries. Adv. Mater. 28, 7564–7579.
24. Gong, K., Fang, Q., Gu, S., Li, S., and Yan, Y. (2015). Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs. Energy Environ. Sci. 8, 3515–3530.
18. Lin, M., Gong, M., Lu, B., Wu, Y., Wang, D.Y., Guan, M., Angell, M., Chen, C., Yang, J., Hwang, B.J., and Dai, H. (2015). An ultrafast rechargeable aluminium-ion battery. Nature 520, 324–328.
25. Abbott, A.P., Capper, G., Davies, D.L., and Rasheed, R. (2004). Ionic liquids based upon metal halide/substituted quaternary ammonium salt mixtures. Inorg. Chem. 43, 3447–3452.
19. Angell, M., Pan, C.J., Rong, Y., Yuan, C., Lin, M.C., Hwang, B.J., and Dai, H. (2017). High Coulombic efficiency aluminum-ion battery using an AlCl3-urea ionic liquid analog electrolyte. Proc. Natl. Acad. Sci. USA 114, 834–839.
26. Abbott, A.P., Al-Barzinjy, A.A., Abbott, P.D., Frisch, G., Harris, R.C., Hartley, J., and Ryder, K.S. (2014). Speciation, physical and electrolytic properties of eutectic mixtures based on CrCl3 $ 6H2O and urea. Phys. Chem. Chem. Phys. 16, 9047–9055.
20. Zhang, L., Chen, L., Luo, H., Zhou, X., and Liu, Z. (2017). Large-sized few-layer graphene enables an ultrafast and long-life aluminum-ion battery. Adv. Energy Mater. 7, 1700034.
27. Hartley, J.M., Ip, C.M., Forrest, G.C., Singh, K., Gurman, S.J., Ryder, K.S., Abbott, A.P., and Frisch, G. (2014). EXAFS study into the speciation of metal salts dissolved in ionic liquids and deep eutectic solvents. Inorg. Chem. 53, 6280–6288.
21. Hu, B., DeBruler, C., Rhodes, Z., and Liu, T. (2017). Long-cycling aqueous organic redox flow battery (AORFB) toward sustainable and safe energy storage. J. Am. Chem. Soc. 139, 1207–1214.
28. Smith, E.L., Abbott, A.P., and Ryder, K.S. (2014). Deep eutectic solvents (DESs) and their applications. Chem. Rev. 114, 11060–11082.
29. Wang, Y., and Zhou, H. (2016). A green and cost-effective rechargeable battery with high energy density based on a deep eutectic catholyte. Energy Environ. Sci. 9, 2267–2272. 30. Zhang, C., Ding, Y., Zhang, L., and Yu, G. (2017). A sustainable redox-flow battery with an aluminum-based, deep-eutectic-solvent anolyte. Angew. Chem. Int. Ed. 56, 7454–7459. 31. Hoyt, N.C., Hawthorne, K.L., Savinell, R.F., and Wainright, J.S. (2016). Plating utilization of carbon felt in a hybrid flow battery. J. Electrochem. Soc. 163, A5041–A5048. 32. Lloyd, D., Vainikka, T., Ronkainen, M., and Kontturi, K. (2013). Characterisation and application of the Fe(II)/Fe(III) redox reaction in an ionic liquid analogue. Electrochim. Acta 109, 843–851. 33. Sharma, S. (1974). Raman study of ferric chloride hexahydrate and ferric chloride hexadeutrate in crystalline, molten and glassy states. J. Noncryst. Sol. 15, 83–95. 34. Sharma, S. (1974). Raman study of ferric chloride solutions and hydrated melts. J. Chem. Phys. 60, 1368–1375. 35. Cariati, F., Masserano, F., Martini, M., and Spinolo, G. (1989). Raman studies of NiX2$6H2O and FeCl2$4H2O. J. Raman Spectrosc. 20, 773–777. 36. Sitze, M.S., Schreiter, E.R., Patterson, E.V., and Freeman, R. (2001). Ionic liquids based on FeCl3 and FeCl2. Raman scattering and ab initio calculations. Inorg. Chem. 40, 2298–2304.
Joule 1, 1–11, November 15, 2017
11