Eutectic Electrolytes for High-Energy-Density Redox Flow Batteries

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Eutectic Electrolytes for High-Energy-Density Redox Flow Batteries Changkun Zhang, Leyuan Zhang, Yu Ding, Xuelin Guo, and Guihua Yu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01899 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Energy Letters

Eutectic Electrolytes for High-Energy-Density Redox Flow Batteries Changkun Zhang,†,§ Leyuan Zhang,†,§Yu Ding,† Xuelin Guo,† and Guihua Yu†,* †Materials

Science and Engineering Program and Department of Mechanical

Engineering, The University of Texas at Austin, Austin, TX 78712, USA. Corresponding Author *[email protected]

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Abstract: Redox flow batteries (RFBs) have attracted immense research interests as one of the most promising energy storage devices for grid-scale energy storage. However, designing cost-effective systems with high energy and power density as well as long cycle life is still a big challenge for the development of RFBs. Eutectic electrolytes as a novel class of electrolytes have been recently explored to enhance the energy density of RFBs, as they offer advantageous features such as low cost, ease of preparation, and tunable high concentration of active materials. In this perspective, we present an overview on recent progress of eutectic-electrolyte-based RFBs and highlight the development of understanding the working mechanism and related coordination chemistry. The challenges and potential research opportunities of eutectic electrolytes for RFBs are also discussed.

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Green, clean and carbon-neutral energy is critical to economic and sustainable development of the modern society. However, the inherently intermittent nature of renewable energy sources, such as solar, tidal, and wind, is challenging for the electric power grid.1-3 Among currently available electrical energy storage (EES) systems, redox flow batteries (RFBs) stand out as a well-suited option for large-scale energy storage (Figure 1a), as they have decoupled electrolyte and electrode components, allowing scalable and flexible modular design.4

Figure 1. (a) Schematic configuration of typical RFBs. (b) Desired merits of advanced RFBs and electrolytes. (c) Representative phase diagram of eutectic electrolyte with two components. (d) Eutectic electrolytes with several advantageous properties for sustainable and high-energy RFBs.

To develop the low-cost and high-performance RFB systems (Figure 1b), significant 3



efforts and progress have been made in the past decades.4 Advanced membranes have been designed for all-vanadium RFBs (VFBs) to inhibit the crossover of V ions and improve the battery efficiencies.5 Novel electrode structures and electrolyte optimization have been applied in Zn-Br2 RFBs to accelerate the reaction rate of Br2/Br− and enhance the Br2-entrapping-capability.6 The neutral Zn-Fe and Zn-I2 RFBs have been demonstrated at the stage of commercialization.7, 8 However, current RFBs still suffer from the relatively high cost of the electroactive materials (vanadium electrolytes and I2), low energy density, and poor cycling stability. Compared to the widely used lithium-ion batteries, the limited energy density of RFBs is another barrier for their wide implementation in the consumer market. The energy density of RFBs is dependent on the working voltage, the concentration of electroactive materials, and number of electrons that participate in the redox reactions.912

A variety of novel RFB systems have been proposed and systematically studied to

extend the working voltage of RFBs and increase the concentration of electroactive materials.13-20 For instance, alkali metals with low stripping/deposition potentials have been applied as the anode in the RFBs in order to increase the working voltage.14-18 However, the dendrite growth in the alkali-metal anode not only limits the operating current density, but also causes potential safety problems.21 On the other hand, by mixing solid active materials such as LiFePO4, sulfur, Li4Ti5O7 with conducting carbons in organic electrolytes to make suspensions, the semi-solid RFBs can break the solubility limitation of active materials in common solvents and thus achieve the high energy density.19,

20

Nevertheless, the high viscosity of semi-solid suspensions and 4


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ACS Energy Letters

difficulty of scale-up lead to the limited demonstration of this kind of RFBs. On the other hand, organic electroactive molecules are superior to inorganic materials, thanks to the high tunability of redox potential, solubility and electrochemical stability.22-25 For now, the development of organic-based RFBs is still in the infancy, and more efforts are required to address the challenges of organic-based RFBs before they are practically viable to the industry.22, 26 Eutectic electrolyte, as an emerging concept in RFBs, has been proposed to boost the energy density of RFBs due to the high concentration of electroactive materials.27-30 Generally, eutectic electrolytes are systems formed by mixing the electroactive materials and organic/inorganic compounds. The interactions between those components reduce the lattice energy and lower the melting point (Tm), which thereby allows the raised concentration of electroactive materials (Figure 1c). Considering the complexity and time consumption of the organic synthetic methods,9, 31 the eutectic electrolyte can be a better choice for energy density enhancement (Figure 1d). The most intensively studied eutectic electrolyte is the room-temperature liquidus deep eutectic solvents (DESs), which are typically prepared by mixing the Lewis or Brønsted acids and bases.28 Interacted with quaternary ammonium or some inorganic salts, a variety of metal salts and hydrogen bond donor (HDB) such as alcohol, amide, and carboxylic acid can be used to form DESs. Currently, eutectic electrolytes have been explored in electrical energy storage systems such as lithium-ion batteries and RFBs.30, 32 Lithiumbased eutectic electrolytes prepared by mixing lithium salts with urea or Nmethylacetamide show potential to replace conventional electrolytes in battery 5



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applications due to non-toxicity, low-cost, and non-flammability.32 Eutectic electrolytes with high concentration of electroactive materials have also attracted much attention for high-energy-density RFBs. In this perspective, we will introduce the application of eutectic electrolytes in RFBs and discuss the potential challenges of eutectic electrolytes for enabling high-performance RFBs. Based on the characteristics of electroactive materials, eutectic electrolytes can be classified into two types: metalbased and organic-based eutectic electrolytes. The earlier studies on the DESs as inactive solvent media for RFBs are also summarized here to give an insight into designing suitable eutectic electrolytes for high-energy-density RFBs.

Table 1. Physicochemical properties of ammonium-based DESs.33, 34

DESs Formulae salt

HBD

Molar ratio

ρ/g cm-3

η/ mPa s

σ / mS cm-1

#1

ChCl (Choline chloride)

Malonic acid

1:1

1.37

828.7

0.91

#2

ChCl

Oxalic acid

1:1

1.24

458.4

1.88

#3

ChCl

Triethanolamine

1:2

1.33

838.82

0.65

#4

ChCl

Zn(NO3)2 6H2O

1:1

1.46

106.7

9.28

#5

ChCl

Trifluoroacetamide

1:2

1.17

77.3

2.48

#6

ChCl

Urea

1:2

1.24

632

0.75

#7

ChCl

Ethylene glycol

1:2

1.12

36

7.61

#8

N,N-diethylethanol

Malonic acid

1:1

1.23

541.1

1.13

Zn(NO3)2 6H2O

1:1

1.20

163.4

7.05

ammonium chloride #9

N,N-diethylethanol ammonium chloride

DESs as reaction media in RFBs. DESs are classified as a type of ionic conducting solvents, containing a variety of anionic and cationic species, which share many similar characteristics with ionic liquids (ILs) such as good stability, negligible volatility and biodegradability. Compared with aqueous or organic electrolytes, the high viscosity 6


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ACS Energy Letters

and insufficient conductivity of DESs still limit their application in the energy storage field.28 Many studies have been carried out to optimize the physicochemical properties of DESs by selecting different HDBs and halide salts. As shown in Table 1, DESs with the amide HBD exhibit the lowest viscosities among these ammonium-based DESs. Moreover, nitrate based DESs show lower viscosities than acidic and amine based DESs leading to the higher conductivities.33 In Figure 2a, it can be seen that DES 5 and DES 6 lie below the “ideal” Walden line, while the other DESs examined from Walden plots lie on or above the ideal line, indicating that they exhibited good properties similar to ILs. According to the hole theory, the charge transport in DESs is limited by the holes that arise from thermally generated fluctuations in local density.35 Therefore, DES must contain large holes and small ions to enhance ion mobility. In addition, evaluation on the electrochemical potential window of DESs revealed that DESs have similar potential ranges as compared to some typical ILs (Figure 2b).33 The screened electrochemical potential window may assist the selection of proper DESs as solvents in various spectroscopic and electrochemical applications.36

Figure 2. (a) Walden plot for various DESs (the dotted line indicates the data for aqueous KCl solution to fix the position of the “ideal” Walden line). (b) Electrochemical potential windows of 7



seven DESs using GC as the working electrode. The properties of DESs are shown in Table 1. Reprinted with permission from ref 33. Copyright 2013 Elsevier.

The electrochemical properties of several electroactive materials such as ferrocene, V(acac)336, FeCl337, CuCl229, and ZnCl237 have been evaluated in the DES-based solvents for RFBs. Lloyd et al. investigate the electrochemical activity of CuCl2 in a mixture of choline chloride and ethylene glycol (known as ethaline). They found that copper could exist in three different stable states: Cu(s), [Cu(I)Cl3]2−, and [Cu(II)Cl4]2− which dominated the redox reactions in electrolytes.29 They also proposed an iron RFB based on the chloro-complexes of iron.37 However, the DESs used above only act as solvents to dissolve metal chloride salts, and the low solubility of the electroactive materials and large overpotential of the cell lag behind the requirement of RFBs. Therefore, exploring new types of DESs with high concentration of electroactive materials is critical for the RFB development. Metal-based eutectic electrolytes. Metal-based RFBs have been proposed several decades ago and now the most representative is the VFB system. As mentioned above, vanadium electrolyte is costly and the crossover of vanadium ions limits capacity stability of VFBs. Although many other metal salts have been used in RFBs (Figure 3a), developing low-cost, high-energy, and noncorrosive electrolytes is still a big challenge for advanced RFB systems.

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Figure 3. (a) Redox potential of various metal-based electroactive materials. (b) Redox potentials and concentrations of Zn, Al and Fe-based DESs. (c) Full charge and discharge profile of the Li-Fe cell at room temperature. Reprinted with permission from ref. 30 Copyright 2016 Royal Society of Chemistry. (d) Schematic of the proof-of-concept Fe-Al hybrid flow battery based on eutectic electrolytes. Reprinted with permission from ref 38. Copyright 2017 Elsevier. (e) NMR analysis of Al-DES/DCE eutectic electrolyte at various charging states. Reprinted with permission from ref 40. Copyright 2017 Wiley. (f) Photographs and optimized coordination geometries of Zn-based eutectic electrolyte. Reprinted with permission from ref 43. Copyright 2018 Nature Publishing Group.

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Metal-based eutectic electrolytes have been investigated for metal electrodeposition since they have high solubility of metal salts, the absence of water, and a great potential to form uniform coating on various substrates.28 Benefited from the high concentration, metal-based eutectic electrolytes can also be applied for developing high-energydensity RFBs. The metal-based eutectic electrolytes are mainly formed by mixing anhydrous/hydrated metal halides with HBDs, such as urea or acetamide. The metal chloride works as both eutectic component and redox species in the eutectic electrolyte. The low cost and non-toxic Fe, Al and Zn metal-based eutectic electrolytes with the concentration of elctroactive species above 2 M have been systematically exploited as potential high-energy-density electrolytes (Figure 3b). For instance, Zhou et al. reported a rechargeable RFB based on the Fe-based eutectic electrolyte, which was formed by a mixture of FeCl3·6H2O and urea in a molar ratio of 2:1.30 The obtained Febased eutectic electrolyte had a high concentration of electroactive species (5.4 M), and was stable at ambient environment. The redox reaction involved the valence change of metal centers in two iron complex species, which were described as [FeCl2(OD)4]+ and [Fe(OD)5]2+. When coupled with Li metal, the Li-Fe cell exhibited a high volumetric capacity of ~95 Ah L-1 at 0.5 mA cm-2 (Figure 3c). The redox reactions and physicochemical properties of metal-based eutectic electrolytes can be tailored by tuning the coordination environment of metal ions. One possible means is changing organic salts or HBDs or adding functional additives. Yu et al. investigated the effect of ethylene glycol (EG) as the additive on the coordination environment of Fe3+ and physicochemical properties including concentration, viscosity 10


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ACS Energy Letters

and freezing point.38 EG could enhance the dissolution of supporting salt LiCl, thereby achieving high Coulombic efficiency (CE). Moreover, adding EG also changed the reaction mechanism of Fe-based eutectic electrolyte. According to the Raman analysis, the iron complex species, [FeCl2(OD)4]+ & [FeCl4]-, dissociated to be [Fe(OD)6]3+ after adding a certain amount of EG. As displayed in Figure 3d, the all-eutectic-based RFB was designed based on the FeCl3·6H2O/urea/EG and Al eutectic electrolytes. The FeAl hybrid cell delivered a high energy density of 166.2 Wh L-1 with an average operating voltage of 1.41 V, and there was no capacity fade even after fully charging and discharging for over 1500 h. Another type of Fe eutectic electrolyte, the FeCl3/ChCl/EG electrolyte, has also been explored for RFB application. In this system, the physical properties of the electroplated iron, are greatly influenced by electrolyte composition.39 When the chloride to iron ratio was ≥4:1, the dominant species were [FeCl4]- and [FeCl4]2- complexes. However, EG was found to be capable of forming iron complex below 4:1, and the presence of this complex hindered fluid properties as shown by an order of magnitude decrease in solution conductivity as well as alter the iron deposition mechanism. Yu et al. also explored the use of AlCl3/urea eutectic electrolyte as anolyte in the Alhalogen RFB.40 As the most abundant metal, Al is considered as one of the most promising alternatives to replace the lithium metal for energy storage. However, it is difficult to achieve the reversible stripping/deposition of Al in common electrolytes. The reversible reaction of Al has been recently achieved in both AlCl3/[EMIm]Cl IL and Al DES, respectively.40 Since Al DES is significantly cheaper than Al IL, it would 11



be a better alternative for RFB applications. The Al-based eutectic electrolyte was prepared by mixing urea (served as HBD) and AlCl3 at room temperature in a molar ratio of 1:1.3, and DCE was then added to decrease the viscosity and improve the conductivity. The reaction of Al-based eutectic electrolyte involves Al complex cations ([AlCl2(urea)n]+) and anions ([AlCl4]-) and DCE did not change the redox reaction mechanism (Figure 3e). Furthermore, the Al-based eutectic electrolyte exhibited a 5fold increase of the ionic conductivity to 1.9 mS cm-1 than the pristine Al-DES. With the concentration of ca. 3.2 M and a low redox potential, the Al-based eutectic electrolyte demonstrated a reversible volumetric capacity of 145 Ah L-1 and an energy density of 189 Wh L-1 when coupled with an I3-/I- catholyte. In addition to Al, Zn is another promising anode material for designing hybrid RFBs. Although the stripping/deposition of Zn in aqueous solutions likely causes the problem of dendrite growth and side reactions, Zn DES without the involvement of water may be a good solution to prevent these problems. Zn DES can be prepared by mixing ZnCl2 and ChCl, EG, urea, acetamide or hexanediol.41 Abbott et al. observed the effect of additives on zinc electrodeposition from DES.42 Three polar additives (acetonitrile, ammonia and ethylene diamine) were observed to have different effects on the Zn deposition mechanism. Usually, most metals electrodeposited from DES are amorphous and nano-crystalline. The additives used in this study helped produce large crystalline deposits, analogous to those obtained from aqueous solutions. Recently, Zhao et al. reported an aprotic Zn-based DES, formed by ZnCl2 and acetamide, as anolyte for hybrid RFBs.43 The transparent liquid was quickly formed under constant 12


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ACS Energy Letters

rate stirring at 60 °C and the reasonable molar fraction of ZnCl2 for stable DES was located in the range of 13.7~19.4% (Figure 3f). Three kinds of zinc complex species existed

in

the

as-prepared

eutectic

electrolyte:

[ZnCl(acetamide)]+,

[ZnCl(acetamide)2]+ and [ZnCl(acetamide)3]+, which were stabilized by the oxygen bond between the acetamide and Zn2+ center (Figure 3f). EC/DMC was applied as the additive to decrease viscosity and improve conductivity through weakening molecular interactions of solvents. The reaction mechanism of Zn-based eutectic electrolyte was proposed by DFT simulation, involving transformation of two coordination geometries: [ZnCl(acetamide)]+ and [ZnCl(acetamide)3]+, during electroplating/electrostripping process. When coupled with concentrated LiI aqueous solution, the Zn-based eutectic electrolyte was able to utilize 1.7 M electroactive material and delivered a volumetric capacity of 90 Ah L-1. Besides, Al-based and Zn-based DES can be also employed in the hybrid ion RFBs with quinone-based derivatives as catholytes.44 The hybrid RFBs maintained a capacity retention of 99.9% per cycle with CE of ~ 99% and energy efficiency of 89%, both of which remarkably outperform the reported organic RFBs. In addition to Fe, Al and Zn, other metal salts such as Mg and Sn-based can also form eutectic mixtures. However, the high Tm (116 oC) of Mg-based eutectic mixture derived from MgCl2·6H2O and ChCl undoubtedly limits its application.28 While Sn-based eutectic mixture has low Tm of 13-15 oC, the redox potential of Sn is nearly 0 V (Figure 3a) which would lower the working voltage of full cells.45 To realize the practical application of metal-based eutectic electrolytes in RFBs, more efforts are still needed to study formation mechanisms and improve their physicochemical properties. 13



Organic molecules-based eutectic electrolytes. Organic active molecules have attracted much attention as promising redox species for RFBs. The flexible molecular structures and abundant sources provide great opportunities to design advanced RFBs.46, 47

Unfortunately, many organic-active molecules have a limited solubility. Although

molecular engineering is one of the most effective strategies to improve the solubility of molecules,9,

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typical organic synthesis is a time-consuming and complicated

process. The adoption of eutectic electrolyte emerges as an effective alternative strategy to maximize the molar fraction of active species in organic-based eutectic electrolytes. Through the hydrogen bond interaction between EG/malonic acid and redox-active viologen-based ammonium salts, viologen-based eutectic electrolyte was formed with viologen concentrations of 4.2 M.48 This viologen-based eutectic electrolyte exhibited reversible electrochemistry (Figure 4a), however, the Tm was above 30oC which limited its application in RFBs. In addition, organic-based eutectic electrolytes can be also achieved by mixing alkali ions with organic electroactive materials, in which the interaction between components is different from that in typical DESs. Researchers found that glymes (CH3(CH2CH2O)nCH3) could integrate with certain Li salts (LiX) to form a eutectic mixture (also called “solvate IL”) which has been applied in lithium-ion batteries.49, 50 The strong complexation occurred between glyme molecules and Li+ ions, resulting in [Li(glyme)1]+. It should be noted that the formation of [Li(glyme)]+ was governed by competitive interactions between the glymes-Li+ interaction and the counter (X)--Li+ interaction, only [Li(glyme)]X with stronger glyme-Li+ interaction can form 14


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[Li(glyme)]+ complex. In contrast, the ionicity of [Li(glyme)]X became significantly low when the anionic interaction with Li+ was stronger than the glyme-Li+ interaction, and the mixtures were considered as concentrated solutions in such situation.50 In addition to liquidic glymes, some organic-based electroactive molecules could also interact with Li+ salts to yield eutectic electrolytes.27, 51 In these organic-based eutectic electrolytes, organic molecules participate in the redox reaction and Li+ salts act as the charge carriers. In other words, they not only provide highly concentrated electroactive materials, but also circumvent the addition of extra supporting salts, which simplify the preparation of electrolytes and is beneficial to maximizing energy density of RFBs.

Figure 4. (a) CV profiles of 4:1 EG/BVCl2 at 40 oC. Reprinted with permission from ref 48. 15



Copyright 2018 Royal Society of Chemistry. (b) Phase diagram of the MTLT mixture based on DSC measurements. Reprinted with permission from ref 27. Copyright 2015 Wiley. (c) Spatial distribution functions (SDFs) of species surrounding MT molecules (side views for each case). (d) Initial charge-discharge performance of the MT/TL catholytes. (e) Cycling performance of the catholytes with the x indicated in the plots. Reprinted from reference 52. Copyright 2018 American Chemical Society. (f) Ternary phase diagram of LiTFSI-NMePh-Urea electrolyte at room temperature. (g) The MEP of LiTFSI-NMePh-Urea electrolyte with molar ratio of LiTFSI: NMePh: Urea = 2: 1: 3. (h) CV profiles of different NMePh anolyte in DME/DCE solvent. (i) Charge/discharge profiles of Li | NMePh cell at different NMePh concentrations. Reprinted with permission from ref 53. Copyright 2018 Elsevier.

An organic eutectic electrolyte based on 4-methoxy-2,2,6,6-tetra-methypiperidine 1-oxyl (MT or MeOTEMPO) has been reported.27, 52 MT was mixed with the lithium bis(trifluoromethanesulfonyl) imide (LT or LiTFSI) to yield orange-colored and smooth liquids at room temperature (Figure 4b-e). The obtained phase diagram indicated the Tm of the MT/LT composition can be lower than -70 oC (Figure 4b). Compared to the solubility of MeO-TEMPO in water (~0.3 M), electrolytes above 2.0 M can be prepared using this facile method. The calculated spatial distribution functions (SDFs) for species surrounding MT and TFSI- revealed that Li+ cation was adjacent to the MT ring and TFSI- anion was located under the N-O• group surrounding (Figure 4c). In pure MT/LT eutectic electrolytes, the maximum probability of finding Li+ and TFSI− molecules in these locations is 1.7 and 11.6 times than their bulk densities. With the addition of water, Li+ did not have a well-defined preferred location but tended to 16


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be uniformly distributed at its bulk density. A small amount of water can break up MTTFSI- and MT-Li+ associations, and thus the ions can move freely. The maximum amount of water held up by MT molecules was found to be 5-6 to MT/LT mixture. Intriguingly, the electrolyte still maintained eutectic properties at low water contents (water molar ratio

Eutectic Electrolytes for High-Energy-Density Redox Flow Batteries

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