Redox Targeting-Based Aqueous Redox Flow Lithium Battery - ACS

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A Redox Targeting-based Aqueous Redox Flow Lithium Battery Juezhi Yu, Li Fan, Ruiting Yan, Mingyue Zhou, and Qing Wang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01420 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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ACS Energy Letters

A Redox Targeting-Based Aqueous Redox Flow Lithium Battery

Juezhi Yu, Li Fan, Ruiting Yan, Mingyue Zhou, Qing Wang * Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576 Corresponding Author Email: [email protected].

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ABSTRACT: Redox flow batteries (RFBs) have been extensively investigated because of their great operation flexibility and scalability for large-scale energy storage, yet they suffer from low energy density and relatively high cost when price per kWh is considered. Here, we report an aqueous redox flow lithium battery (RFLB) system based on the concept of Nernstian potential driven redox targeting reactions of battery materials to address the above issues. With [Fe(CN)6]4-/[Fe(CN)6]3- and S2-/S22- as the redox mediators in the catholyte and anolyte, the cell reveals an anodic and cathodic volumetric capacity up to 305 and 207 Ah L-1 when LiFePO4 and LiTi2(PO4)3 are respectively loaded into the cathodic and anodic tank as energy storage materials. These are 4-6 times as high as that of the vanadium redox-flow battery (VRB). In addition, with water-based electrolytes, the system presents notably enhanced Li+-conductivity in the membrane and consequently much improved power performance as compared to its nonaqueous counterpart. We anticipate this work would be a paradigm and pave the way for the deployment of redox targeting-based flow battery technology for large-scale applications.

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Electrochemical energy storage (EES) technology has been recognized as one of the most effective ways to regulate the power supply and demand for electric grid, and buffer the impact of unstable electricity generation from the waving renewable energy sources such as solar and wind, when integrated to the grid.1-3 Among various EES technologies, redox flow batteries (RFBs) which generate electricity from battery stack while store energy in separated electrolyte tanks, have attracted a great deal of attention for large-scale stationary energy storage owing to their high operation flexibility and system scalability.4-7 Vanadium redox-flow batteries (VRBs), as the most mature RFB technology, have been demonstrated in a wide variety of power/energy from kW/KWh to MW/MWh across the globe.8-11 However, the large-scale deployment of VRBs is hindered by the low energy density (25-30 Wh L-1) and relatively high materials cost. In addition, high concentration acids are generally required in order to enhance the solubility and thermal stability of vanadium species, which however makes the electrolytes corrosive and imposes additional environmental concern for VRBs. To address the issues of VRBs, tremendous efforts have been made to develop alternative battery chemistries in different electrolyte systems. For instance, benefited from the larger electrochemical window of non-aqueous electrolytes, organic RFBs with higher cell voltage than VRB have been extensively explored.12-17 However, the relatively low solubility of redox species in organic solvents results in unsatisfactory volumetric capacity. In addition, the power performance of the cell is severely limited by the poor ionic conductivity of ion-exchange membrane and organic electrolytes. Alternatively, abundant, low-cost organic redox species, such as quinone, pyridine or ferrocene derivatives have been employed to replace vanadium in aqueous electrolyte, which have led to the development of aqueous organic redox flow batteries (AORFBs) in the past few years.18-21 Although AORFBs potentially have lower cost than VRBs,


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the volumetric capacity remains low due to the limited solubility of organic redox-active molecules in aqueous electrolytes, which is especially so for the polymer-based AORFBs.22 To enhance the volumetric capacity and sustain a high output voltage, redox targeting-based organic flow batteries have been proposed,23-26 in which energy is stored in solid battery materials kept in tanks while power is generated in the cell stack as a flow battery. With the redox targeting reactions between the redox mediators and energy storage materials, redox electrolytes that circulate in the system transport charges from the solid materials in tanks to electrodes in cell stack. Since the redox molecules just work as charge carriers commuting between the electrodes and battery materials, the volumetric capacity is no longer determined by the concentration of redox mediators, but the solid materials stored in tanks, which undoubtedly leads to a leap of the capacity. For instance, the redox flow lithium battery (RFLB) demonstrated with FcBr2/Fc as catholyte redox mediators and LiFePO4 as the cathodic energy storage material, while with CoCp2/CoCp*2 as anolyte redox mediators and TiO2 as the anodic energy storage material, has a tank energy density up to 500 Wh L-1.24 Although RFLB has adequately addressed the issue on volumetric capacity, the battery performance, as other organic flow battery systems, is critically compromised by the poor Li+-conductivity of the membrane. In addition, large voltage loss owing to the potential difference between the redox mediators and solid materials during charge and discharge leads to intolerably low voltage efficiency. As such, Nernstian potential driven single molecule redox targeting (SMRT) reactions of redox mediator with matched potential to that of the solid material is desired.27 To sufficiently address the above issues, here we report a Nernstian potential driven redox targeting-based aqueous redox flow lithium battery, which operates upon the SMRT reaction of a single redox molecule with a suitable lithium battery material in water-based electrolytes. As


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ACS Energy Letters

shown in Figure 1, [Fe(CN)6]4-/[Fe(CN)6]3- and S2-/S22- are employed as the redox mediators in catholyte and anolyte, which are paired with LiFePO4 and LiTi2(PO4)3 as the cathodic and anodic energy storage material, respectively. The accessible lithium concentration in LiFePO4 and LiTi2(PO4)3 is 22.5 and 11.5 M equivalent to a volumetric capacity of 603 and 308 Ah L-1 respectively, considerably higher than that of VRB (53 Ah L-1, 2 M VOSO4). More importantly, the cell could operate at significantly enhanced power with water-based electrolytes. We anticipate the redox targeting-based battery chemistry and the flow battery system demonstrated here would provide a credible solution for practical applications.

Figure. 1 Schematic illustration of a Nernst potential driven redox targeting-based aqueous flow lithium battery full cell. (a) Configuration of the cell with LiFePO4 and LiTi2(PO4)3 as cathodic and anodic energy storage materials in tanks, paired with redox mediator [Fe(CN)6]4-/[Fe(CN)6]3and S2-/S22- in catholyte and anolyte, respectively. (b) Potential profiles of [Fe(CN)6]4/[Fe(CN)6]3- and S2-/S22- as a function of state of charge-discharge, and their comparisons with


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the potential of LiFePO4 and LiTi2(PO4)3. The insets show the photograph of LiFePO4 and LiTi2(PO4)3 granules used in this study. They have a diameter of 1.0 and 1.5 mm, respectively.

For an effective SMRT reaction, it is essential to have redox mediators that possess identical redox potential to that of the solid energy storage material. Olivine LiFePO4 and rhombohedral LiTi2(PO4)3 are proven robust cathode and anode materials for aqueous lithium ion battery. Their potentials are relatively flat — ideal for redox targeting reactions and are well within the electrochemical window of water splitting at alkaline conditions. We hence choose these two materials as the cathodic and anodic energy storage materials, respectively. Cyclic voltammograms (CV) in Figure 2 shows the redox potential of LiFePO4 is 0.21 V (vs. Hg/HgO), and that of LiTi2(PO4)3 is -0.69 V (vs. Hg/HgO) in 0.1 M LiOH electrolyte. Benchmarked with the redox potentials of these two materials, redox mediators with matched potentials have been extensively screened. One of those is the widely studied [Fe(CN)6]4/[Fe(CN)6]3-,28,29 whose redox potential is just 100 mV more positive than that of LiFePO4 (Figure S1a) and has excellent reversibility and solubility in water. To tailor it to the potential of LiFePO4, tetra-ethylene glycol dimethyl ether (TEGDME) was used as a co-solvent. Interestingly, the potential of [Fe(CN)6]4-/[Fe(CN)6]3- progressively shifts to more negative values with the addition of TEGDME (Figure S1b), and becomes exactly matched when the quantity of TEGDME is around 30% in volume. Such a solvent effect is attributed to the lower acceptor number of TEGDME than that of H2O, with which ferrocyanide anion would be less stabilized relative to ferricyanide.30 As a result, the oxidation proceeds more easily with a negative shift of formal potential.


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Figure 2. Cyclic voltammograms (CV) of redox mediators and the paired solid Li+-storage materials used in the aqueous RFLB. The CV curves of Li2S2 and LiTi2(PO4)3 were measured in 0.1 M LiOH aqueous solution, while those of K4[Fe(CN)6] and LiFePO4 were obtained in 0.1 M LiOH solution with a mixed solvent of H2O and TEGDME (30 vol.%). The concentration of the respective redox species is 10 mM and the scan rate is 5 mV s-1.

For the anodic side, polysulfides which have been broadly employed as anolyte in various redox flow batteries,31,32 are noticed to have comparable redox potential to that of LiTi2(PO4)2. As shown in Fig. 2, the redox potential of S2-/S22- is -0.69 V (vs. Hg/HgO), just consistent with that of LiTi2(PO4)2. In view of the high solubility (up to 2.63 M for Li2S2), polysulfide itself possesses excellent volumetric capacity even in the absence of LiTi2(PO4)2. For redox targeting-based flow batteries, there are two sets of reactions concomitantly taking place


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at the electrodes and tanks. Equation (1) and (2) show the electrochemical reactions on the cathode and anode respectively, which are identical to the conventional flow batteries. [ ] ⇋ [ ] + e S + 2e ⇋ 2S

(1) (2)

The other set is the redox targeting reactions in the cathodic and anodic tanks, as shown by equation (3) and (4), respectively. [ ] + LiFePO ⇋ [ ] + FePO + Li (3) 2S + LiTi PO + 2Li ⇋ S + Li Ti PO

(4)

In the charge process, [Fe(CN)6]3- is generated on the cathode by the oxidation of [Fe(CN)6]4- in the catholyte (eq.1), which then flows into the cathodic tank and oxidizes LiFePO4 to form FePO4 (delithiation, eq.3). The reduced species is regenerated when circulates back to the cell, starting a new round of reactions. Meanwhile, S2- is formed on the anode by the reduction of S22in the anolyte (eq.2), which then flows into the anodic tank and reduces LiTi2(PO4)3 to form Li3Ti2(PO4)3 (lithiation, eq.4). During this process, the overall reaction is that the Li+ ions extracted from LiFePO4 in the cathodic tank transport through the membrane and are stored in LiTi2(PO4)3 in the anodic tank. In the discharge process, all the above processes proceed conversely. Given both the redox mediator and material start with the same equilibrium redox potential, a driving force for the redox targeting reaction would then be induced by the activity changes of redox species and Li+ as described by Nernst equation. Equation (5) and (6) show the potential difference (∆E) between the redox mediator and solid material as a function of activities.


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∆

!"#

$

%& '

%&

ln

∆5!"# $ ln '

[*+,- ]/0 .

123 [*+,-. ]40

6 6 12 3 60 7

760

(5)

(6)

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During the charge process, with the formation of [Fe(CN)6]3- in the catholyte and S2- in the anolyte (consumption of [Fe(CN)6]4- and S22- concurrently), the equilibrium potential of [Fe(CN)6]4-/[Fe(CN)6]3- becomes higher than LiFePO4 (eq.5), while that of S2-/S22- becomes lower than LiTi2(PO4)3 (eq.6) — a favorable potential difference is thus generated for the delithiation of LiFePO4 and lithiation of LiTi2(PO4)3. Similar analysis applies to the discharge process.


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Figure 3. Characterizations of LiFePO4 and LiTi2(PO4)3 at different stages of redox targeting reactions and voltage profiles of the symmetric flow cells. (a) XRD patterns of LiFePO4 after delithiation by [Fe(CN)6]3- and re-lithiation by [Fe(CN)6]4-. (b) XPS spectra of Ti2p in LiTi2(PO4)3 after lithiation by S2- and delithiation by S22-. (c) Voltage profiles of a catholyte symmetric cell before and after loading LiFePO4. The electrolyte consisted of 0.3 M K4Fe(CN)6, 1 M LiNO3 and 0.1 M LiOH in mixed solvents with 30 vol.% TEGDME. (d) Voltage profiles of an anolyte symmetric cell before and after loading LiTi2(PO4)3. The electrolyte consisted of 1 M LiNO3 and 0.1 M LiOH in deionized water.

The redox targeting reactions of LiFePO4 with [Fe(CN)6]4-/[Fe(CN)6]3- were monitored by X-ray diffraction (XRD). As the XRD patterns shown in Figure 3a, after immersing LiFePO4 powder in 0.3 M K3[Fe(CN)6] solution (with 30% TEGDME) for 12 hours, the fingerprint diffraction of (210), (211), (301) and (311) planes disappeared and the new pattern is indexed to orthorhombic FePO4, indicating the oxidation of LiFePO4 by [Fe(CN)6]3- which accompanies delithiation. Interestingly, after transferring the obtained FePO4 into K4[Fe(CN)6] solution (1 M Li+ with 30% TEGDME) for 12 hours, the XRD pattern changed back to that of LiFePO4, suggesting the reduction of FePO4 by [Fe(CN)6]4- which accompanies lithiation. The reactions between LiTi2(PO4)3 and S2-/S22- were investigated by X-ray photoelectron spectroscopy (XPS). As shown in Figure 3b, the valence state of Ti in LiTi2(PO4)3 changed from Ti4+ to Ti3+ after immersing in 1 M Li2S solution for 12 hours, implying the material have been reduced by S2-. Similar reaction has also been confirmed by the color changes of electrolyte — the colorless sulfide changes to yellowish brown after reaction indicating the formation of polysulfides (Figure S2). The formed Li3Ti2(PO4)3 was subsequently mixed with Li2S2 solution in N2 for 12


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hours, after which the binding energy of Ti was changed back to Ti4+ (Figure 3b) suggesting Li3Ti2(PO4)3 have been oxidized by S22-. The reversible SMRT reactions of LiFePO4 and LiTi2(PO4)3 have also been verified by Fourier-transform infrared spectroscopy and Raman spectroscopy, respectively. The details are shown in Figure S3. To verify the redox targeting reactions and contributions of the energy storage materials during charge/discharge, “capacity-unbalanced, compositionally-symmetric” flow cells with electrolyte of the same composition while different volume in both cell compartments were fabricated. In the case of LiFePO4, a catholyte symmetric flow cell with 10 mL 0.3 M K4Fe(CN)6 on one side and 30 mL 0.3 M K3Fe(CN)6 on the other side was tested. As shown in Figure 3c, after 0.90 g LiFePO4 granules were loaded into the capacity-limiting side, the cell capacity instantly extended from 78 to 138 mAh as a result of the contribution of LiFePO4 via redox targeting reactions with [Fe(CN)6]4-/[Fe(CN)6]3- corresponding to a utilization of 37.5 % of LiFePO4. The cycling performance of the cell is shown in Figure S4. The capacity retention is nearly 100% for 68 cycles indicating great robustness of the system. In the case of LiTi2(PO4)3, a flow cell with 10 mL 0.3 M Li2S on one side and 7 mL 0.1 M Li2S2 on the other side was fabricated. To enhance the kinetics of polysulfides, nanoparticulate Cu2S was coated onto carbon felt to catalyze the reaction of S2-/S22- on the electrode.33,34 As revealed in Figure 3d, the capacity extended from 35 to 48 mAh after loading 1.0 g LiTi2(PO4)3 granules into the capacity-limiting tank, indicative of the participation and contribution of LiTi2(PO4)3 to the cell capacity via redox targeting reactions with S2-/S22-. In comparison, the utilization of LiTi2(PO4)3 is only 14%, notably lower than that of LiFePO4 presumably due to the sluggish reaction kinetics of polysulfide. To enhance the reaction yield, higher concentration polysulfide and high surface area LiTi2(PO4)3 are desired. We notice the solubility of sulfide (Li2S & Li2S2)


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is as high as 2.63 M (See experimental section in supporting information), which corresponds to a volumetric capacity of 141 Ah L-1. This makes the polysulfide-based anolyte promising for high density energy storage, even in the absence of solid materials.

Figure 4. Cell performance of single molecule redox targeting-based aqueous RFLBs. (a) Voltage profiles of a full cell containing 10 mL 0.3 M K4Fe(CN)6 catholyte and an excess amount of 1 M Li2S2 anolyte before and after 0.90 and 2.69 g LiFePO4 granules were loaded into


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the cathodic tank. The current density was 5 mA cm-2. (b) Cycling performance of a full cell containing 7 mL 0.3 M K4Fe(CN)6 catholyte and 4 mL 1 M Li2S2 anolyte in conjunction with 0.90 g LiFePO4 and 1.0 g LiTi2(PO4)3 granules loaded into the cathodic and anodic tanks, respectively. The current density was 5 mA cm-2. (c) Comparison of volumetric capacities of redox systems in this work with those reported in various aqueous redox flow batteries [19~20,35~40]

.

The above results have unambiguously confirmed the Nernstian potential driven redox targeting reactions in both the cathodic and anodic sides. To demonstrate single molecule redox targetingbased aqueous RFLBs, a cell with 10 mL 0.3 M K4Fe(CN)6 catholyte and an excess amount of 1 M Li2S2 anolyte was constructed. The cell was charged/discharged for three cycles before LiFePO4 granules were loaded into the cathodic tank. As shown in Figure 4a, the cell has a discharge voltage of around 0.70 V at a current density of 5 mA cm-2. After 0.90 g LiFePO4 was loaded the charge capacity of the cell extended from 82 to 152 mAh. The utilization of LiFePO4 is 43.7 %. When 2.69 g LiFePO4 granules with higher porosity were loaded into the cathodic tank, the cell presented a capacity of 420 mAh with a utilization of 73.3 % (Figure 4a). XRD patterns of LiFePO4 after fully charge and discharge were shown in Figure S5. A simple calculation based on the volumes of catholyte and LiFePO4 granules in the tank shows a volumetric capacity of 76 Ah L-1, which can primarily be further enhanced by optimizing the loading of solid materials in the tank.27 The cycling and power performance of the full cell were evaluated with 7 mL 0.3 M K4Fe(CN)6 catholyte and 4 mL 1 M Li2S2 anolyte in conjunction with 0.9 g LiFePO4 and 1.0 g LiTi2(PO4)3


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granules in the cathodic and anodic tanks, respectively. The capacity of anolyte was set in excess of catholyte in order to balance the capacity loss of anolyte due to crossover (yellowish elementary sulfur which is oxidized at the cathodic side by ferricyanide was observed on the membrane, Figure S6). The cell exhibited reasonably good cycling stability with 99.1% capacity retention after 55 cycles at a current density of 5 mA cm-2 (Figure 4b), and a maximum power density of 8 mW cm-2 at a current density of 20 mA cm-2 when tested at 100% SOC (Figure S7). Impedance spectroscopic measurements (Figure S8) show a total areal resistance of 26 Ω cm2 for the full cell, of which 9.2 Ω cm2 is contributed by the electrolytes and membrane, 10.4 and 6.4 Ω cm2 from anodic and cathodic charge transfer impedance, respectively. To further improve the power density, a better anodic electrocatalyst for polysulfide is desired. The achieved volumetric capacity of this work surpasses the volumetric capacity of anolyte and catholyte reported recently (Figure 4c). To gauge the reachable volumetric tank capacity, an optimized packing of solid materials in the tank is essential. In the presence of both electrolyte and solid material, the tank volumetric capacity Ctank is written as: 58 $ 91 − ?" + #@!>A#

(7)

where Csolid and Celectrolyte are the theoretical capacity of solid material and electrolyte, 9 and < are the utilization ratio and porosity of solid material in the tank, respectively. Here, Csolid of LiFePO4 and LiTi2(PO4)3 are 603 and 308 Ah L-1, and Celectrolyte of catholyte (0.3 M K4Fe(CN)6) and anolyte (2.63 M Li2S2) are 8 and 141 Ah L-1, respectively. Thus, the attainable cathodic and anodic tank volumetric capacity would be 305 and 207 Ah L-1 respectively, when 9 is unity and < is 50% as achieved in our previous work.27


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ACS Energy Letters

In summary, this work demonstrates a novel Nernstian potential driven redox targeting-based aqueous redox flow lithium battery based on a pair of robust aqueous redox electrolyte systems with K4Fe(CN)6 and Li2S2 as the cathodic and anodic redox mediators, respectively. Compared with the organic counterparts, the water-based electrolytes have superior ionic conductivity in the membrane and reaction kinetics on the electrodes, which are translated into enhanced power performance of the cell. In conjunction with a pair of cathodic and anodic energy storage materials — LiFePO4 and LiTi2(PO4)3 statically stored in the tanks, the battery system reveals considerably improved energy density with the redox targeting reactions between the redox mediator and respective Li+-storage material. The achieved volumetric capacity for the cathodic and anodic tank is 76 and 141 Ah L-1 respectively, which can be further improved to 305 and 207 Ah L-1 by optimizing the utilization ratio and loading of energy storage materials in the tanks. This study adequately validates the approach of Nernstian potential driven single molecule redox targeting reactions by simply incorporating solid-state energy storage materials into a conventional redox flow cell — to substantially promote the energy density without compromising other properties of the cell. We envisage that with the combination of low-cost, noncorrosive electrolytes and energy storage materials, the aqueous redox flow lithium battery system demonstrated here present an important step towards its eventual deployment for largescale energy storage.

ASSOCIATED CONTENT Supporting information Detailed experimental methods; Redox potential of LiFePO4 and Fe(CN)63-/Fe(CN)64- in different solvents; UV-Vis spectra and photographs of Li2S solution before and after adding


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LiTi2(PO4)3; FTIR spectra of LiFePO4 and Raman spectra of LiTi2(PO4)3 at different stages of redox targeting reactions; XRD patterns of LiFePO4 after fully charge and discharge; Photograph of a membrane faced to catholyte after cycling; Power performance of the cell; Electrochemical impedance spectroscopic analysis. Abbreviations DHPS: 7,8-dihydroxyphenazine-2-sulfonic acid[35]; 2,6-DHAQ: 2,6-dihydroxyanthraquinone[20]; ACA: alloxazine 7/8-carboxylic acid[19]; FMN-Na: flavin mononucleotide[36]; BTMAP-Vi: bis(3trimethylammonio) propyl viologen tetrachloride[37]; 2,7-AQDS: 9,10-anthraquinone-2,7disulphonic acid[21]; BTMAP-Fc: bis((3-trimethylammonio) propyl) ferrocene dichloride[37]; OHTEMPO:

4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl[38];

TEMPTMA:

N,N,N-2,2,6,6-

heptamethyl piperidinyl oxy-4-ammonium chloride[39]; I2Br-: iodine-bromide anions[40]. AUTHOR INFORMATION Corresponding author information Email: [email protected]. Author contribution Q. W. conceived the project. J. -Z. Y performed and analyzed most of the experiment. L. F initiated the study of the cathodic side, and conducted cyclic voltammetry experiment of Fe(CN)63-/Fe(CN)64-. R. Y provide LiFePO4 granules. Q. W. and J. -Z. Y wrote the manuscript. All authors edited the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This research was supported by the Energy Market Authority, Singapore under its Energy Innovation Research Program – Energy Storage (NRF2015EWT-EIRP002).

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ACS Energy Letters

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Redox Targeting-Based Aqueous Redox Flow Lithium Battery - ACS

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