Nonaqueous arylated quinone catholytes for lithium-organic flow batteries

Article abstract of DOI:10.1039/c8ta04720k

Chemically modified organic redox couples have the advantages of tunable redox properties, high solubility, environmental benignity, and cost effectiveness. Inspired by nature, a series of quinone derivatives bearing electron-donating methoxy or electron-withdrawing trifluoromethyl groups are synthesized in moderate to high yields by Pd-catalyzed Suzuki cross-coupling reactions. This study utilizes the synthetic quinones as redox-active organic molecules for nonaqueous lithium-organic flow batteries. The aryl moiety incorporated quinone scaffolds show enhanced electrochemical stability and rate capability. The nonaqueous catholyte, 2-phenyl-1,4-naphthoquinone, reaches a cell voltage of ～2.6 V and a specific capacity of 196 mA h g^-1, while the discharge capacity is retained at ～92% for 150 cycles. Moreover, the tubular lithium-organic flow battery system features stable cycle performance under a continuous circulation without clogging-associated intermittency flow.

Full text of DOI:10.1039/c8ta04720k

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Nonaqueous arylated quinone catholytes for lithium-organic flow
batteries†
a
Dong-Seon Shin,‡ Minjoon Park,‡ab Jaechan Ryu,a Inchan Hwang,a Jeong Kon Seo,c Kwanyong
Seo,*a Jaephil Cho*a and Sung You Hong*a
aSchool of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology
(UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea. E-mail: kseo@unist.ac.kr; jpcho@unist.ac.kr;
syhong@unist.ac.kr
bPresent address: Harvard John A. Paulson School of Engineering and Applied Science, 29 Oxford
Street, Cambridge, MA 02138, USA
cUNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST),
50 UNIST-gil, Ulsan 44919, Republic of Korea
† Electronic Supplementary Informaꢀon (ESI) available.
‡ These authors equally contributed to this work.
1

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Abstract
Chemically modified organic redox couples have the advantages of tunable redox properties,
high solubility, environmental benignity, and cost effectiveness. Inspired from nature, a
series of quinone derivatives bearing electron-donating methoxy or electron-withdrawing
trifluoromethyl groups are synthesized in moderate to high yields by the Pd-catalyzed Suzuki
cross-coupling reactions. This study utilizes the synthetic quinones as redox-active organic
molecules for nonaqueous lithium-organic flow batteries. The aryl moiety incorporated
quinone scaffolds show the enhanced electrochemical stability and rate capability. The
nonaqueous catholyte, 2-phenyl-1,4-naphthoquinone, reaches a cell voltage of ~2.6 V,
specific capacity of 196 mA h g–1, while the discharge capacity is retained at ~92% for 150
cycles. Moreover, the tubular lithium-organic flow battery system features stable cycle
performance under a continuous circulation without clogging-associated intermittency flow.
Introduction
The stringent regulation of greenhouse gas emission accelerates the exploitation of
renewable energy for the progressive replacement of combustion turbines, gasoline vehicles,
or coal power plants.1–5 However, the intermittency, geographical restriction, and output
fluctuation of sustainable energy sources such as wind or solar power impede their stable
power supply.1,3,4 Hence, there is increasing demand for large-scale electrochemical energy
storage systems that can be combined with renewable energy sources. In particular, redox
flow batteries (RFBs) have been regarded as attractive candidates owing to their cost
effectiveness, modularity, and design flexibility.6–11 However, the conventional aqueous RFB
systems including all–vanadium, iron–chromium, and zinc–bromine redox couples operate
under strong acidic and corrosive conditions, which is likely to cause environmental
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problems. The metal-based redox couples are also involved in the issues related to scarcity
and price rise.
Redox-active organic molecules have attracted significant attention as alternative active
materials to replace inorganic compounds for greener and more sustainable RFBs.6–16
Classical or modern organic synthetic methods have provided tools to access a wide range of
redox-active organic scaffolds. And they also have enabled the structure-dependent tunable
(electro)chemical properties.5,15,17–23 In particular, quinone-based molecules have several
advantages such as fast kinetics, sustainability of natural resources, and cost-effective
synthetic protocols.24–28 According to the Deuchert and Hünig classification, quinone is
categorized into the Wurster-type compound undergoing a two-electron transfer process to
furnish hydroquinone skeleton.29 Previously, we unveiled the prototype tubular lithium–
organic batteries by utilizing 5,12-naphthacenequinone (NAQ) and 1,2-benzanthraquinone
(BAQ) as the 1st generation organic catholytes (Fig. 1a).28 Although they showed high power
density, their low solubility in organic electrolytes limited the electrolyte circulation, only
allowing an intermittent flow mode.
Herein, we report that synthetic approach coupled with lithium battery technology can
provide three-fold advances for lithium-organic flow batteries: (i) enhanced electrochemical
stability toward lithium metal, (ii) tunable electrochemical properties, and (iii) conformity
and enhanced solubility to the flexible-tube type design. The π-extended quinone derivatives
bearing electron-donating or electron-withdrawing substituents are synthesized by the
palladium-catalyzed Suzuki cross-coupling reactions. Aryl moiety incorporation significantly
affects the electrochemical behavior and cycle stability of molecular engineered catholytes.
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Experimental section
General synthesis of arylated quinones
To a dried Schlenk tube were added 2-bromo-1,4-naphthoquinone (Br-NQ, 100 mg, 0.42
mmol, 1.0 equiv.), arylboronic acid (0.63 mmol, 1.5 equiv.), potassium carbonate (1.26 mmol,
3.0 equiv.), Pd(OAc)2 (3 mol%), and PPh3 (12 mol%). The reaction mixture in tetrahydrofuran
(THF)/water solution (5 mL, 10:1, v/v) was stirred at 80 °C for 8 h under an argon
atmosphere. The residue was filtered through a Celite pad, then repeatedly rinsed with
dichloromethane (DCM). The filtrate was concentrated in vacuo and the residue was purified
by flash column chromatography.
Preparation of organic catholytes
All chemicals were handled inside an Ar-filled glove box filled. Tetraethylene glycol dimethyl
ether (TEGDME, Sigma-Aldrich) was dehydrated by anhydrous CaH2 (Sigma-Aldrich) before
use. Quinone powder was dissolved in 1.0 M lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI)- TEGDME electrolyte.
Lithium metal half-cell test
Synthetic quinone (0.05 M) dissolved in LiTFSI (1.0 M)-TEGDME was used as the catholyte.
Electrochemical performance was measured using 2016 coin cells. Carbon paper with a
thickness of 280 ꢀm (TGP-H 090, Toray) was used as a positive electrode, and microporous
polyethylene (PE) film was used as a separator. A coin cell was assembled in the argon-filled
glove box using lithium metal as a counter electrode. 40 ꢀL of the catholyte was injected into
a coin cell. During the measurement, temperature was maintained at 25 °C, and precyclings
were carried out for the system stabilization, then the following galvanic tests were recorded.
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Results and discussion
Our synthetic approach and redox chemistry are based on the naturally occurring molecules
such as ubiquinone and vitamin K derived from 1,4-naphthoquinone (NQ), and they involve
the biological redox processes using protons as charge carriers (Fig. 1b).25,26,30,31 Inspired
from nature, we designed the 2nd generation organic molecules from Br-NQ and
phenylboronic acid. Fig. 1c shows the Suzuki cross-coupling reactions to provide arylated
quinones bearing electron-donating group (EDG) or electron-withdrawing group (EWG), e.g.,
OCH3 or CF3 moieties. Subsequently, we apply the synthetic compounds in the tubular flow
batteries using lithium-ions as charge carriers (Fig. 1d), which will be discussed vide infra.
Table 1 shows the standard reaction conditions to afford 2-phenyl-1,4-naphthoquinone
(PNQ-1). In this reaction, the active Pd0 species is in situ generated from Pd(OAc)2 precatalyst
in the presence of triphenylphosphine (PPh3) ligand, while producing the corresponding
phosphine oxide.32–35 The optimized reaction conditions (entry 1) showed 86% yield of PNQ-
1
1 by forming C–C bond (see its H NMR spectrum presented in Fig. S1, ESI†). The change in
solvent decreased the yields (entries 2 and 3) owing to the reduced solubility of an inorganic
compound K2CO3. When the reactions were performed at room temperature (entry 4) or in
air (entry 5), the reaction yields significantly decreased to 51% or 37%, respectively. Low
reactivity at room temperature and the deactivation problem of the Pd0 species under air
exposure can impede the catalytic reactions. We used NQ as a negative control sample for
the following electrochemical tests, which does not possess an aryl moiety.
The electrochemical properties of NQ and PNQ-1 were investigated by the galvanostatic
charge and discharge processes, where a lithium metal was used in a half-cell without any
surface modification. Carbon paper as a positive electrode was utilized to infiltrate the
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quinone catholytes. The organic catholytes were prepared based on the TEGDME solution
containing 0.05 M quinone as an active material and 1.0 M LiTFSI as a lithium salt. Fig. 2a
shows the reversible redox processes of NQ and PNQ-1 involving two-electrons and two-
lithium ions. Through these consecutive two-electron transfer reactions, PNQ-1 is reduced
into a lithiated hydroquinone species, redPNQ-1.26,29,30,36 Fig. 2b shows the voltage profiles
of PNQ-1 at a current density of 100 mA g–1 corresponding to ca. 0.5 C-rate in a potential
range of 1.8–3.0 V vs. Li/Li+. PNQ-1 presented two distinctive discharge plateaus at ca. 2.4
and 2.6 V, delivering a specific capacity of 196 mA h g–1, which was comparable to a
theoretical capacity of 229 mA h g–1. These redox plateaus are consistent with the values
evaluated from cyclic voltammograms (Fig. S2 in the ESI†). PNQ-1 also shows good rate
capability delivering 161 mA h g–1 even at a rate of 2000 mA g–1 that corresponds to ca. 10 C-
rate (Fig. 2c; see also Fig. S3 for the concentration-dependent rate capability test). In terms
of long-term cycle stability, PNQ-1 catholyte revealed good capacity retention (92%) over
150 cycles at ca. 0.5 C-rate with the high Coulombic efficiency of near 100% (Fig. 2d). The
open-circuit voltage (OCV) of the PNQ-1 catholyte was moderately maintained in a voltage
range of 2.56–2.71 V upon the prolonged storage time of ~50 h (Fig. S4). The intimate
contact between the redox-active species and carbonaceous materials through π-π
interaction and/or infiltration can suppress the crossover issue. In contrast, the specific
capacity of NQ only reached 115 mA h g–1 with significant capacity fading after 150 cycles.
To investigate the capacity fading mechanism, the cycled half-cell was disassembled, and
the surface of the lithium metal was examined by scanning electron microscopy (SEM)
analysis. The surface of the lithium anode in the NQ cell was severely damaged, showing
cracks and an inhomogeneous morphology after 150 cycles (Fig. 2e). In contrast, the PNQ-1
cell showed a clean lithium surface (Fig. 2f). This result is associated with the enhanced
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chemical stability of the synthetic quinone backbone toward a highly reactive lithium
metal.37–44 The Suzuki cross-coupling reaction generates extended π-conjugated structure,
causing the delocalization effect along with increased steric hindrance. Concurrently, the
olefinic (more localized π-electrons) character of non-arylated NQ could be reduced.45 In
addition, electrochemical impedance spectroscopy (EIS) analyses were carried out to
investigate the changes of internal resistance upon cycling (Fig. S5 in the ESI†). Compared to
the PNQ-1 catholyte, the significant increasement of charge transfer resistance (Rct) of the
NQ catholyte was observed upon cycling, implying the poor electron-transfer kinetics of the
NQ catholyte. To unambiguously demonstrate the chemical stability of PNQ-1 upon cycling,
1
the H NMR spectra were recorded (Fig. S6 in the ESI†). Whereas the decomposition of NQ
was confirmed after cycling implying the unstable redox reaction with lithium metal, PNQ-1
showed relatived well-maintained NMR peaks. It indicates that the Suzuki cross-coupling
chemistry can provide enhanced π-extension and chemical stability toward the quinone
scaffold guiding the rational molecular design.
To evaluate the substituent-dependent electrochemical performances, we further
synthesized a family of arylated quinones bearing methoxy (OCH3) and trifluoromethyl (CF3)
moieties as EDG and EWG (Table 2). The arylboronic acids bearing ortho/meta/para-
methoxy substituents were coupled in high yields of 86%, 80%, and 91% to furnish PNQ-2, 3,
and 4 samples, respectively. In contrast, the yields of quinone derivatives bearing CF3 groups
were lower compared to those of methoxy functionalized quinones, showing the values of
67%, 54%, and 69% in PNQ-5, 6, and 7 samples, respectively (for the NMR spectra of the
prepared compounds, see ESI†, Fig. S7–S14).
Subsequently, we prepared the organic catholytes from the synthetic quinones (PNQ-2 to
PNQ-7), where the same electrochemical conditions were applied as used for the PNQ-1 test.
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We found the electronic effect of the substituents including OCH3 and CF3 on
electrochemical performances. The effect of the ortho/meta/para-substituent positions
were investigated through galvanic tests (Fig. 3). We observed that para-OCH3 (PNQ-4) and
para-CF3 (PNQ-7) arylated quinones have low charge overpotentials, and stable discharge
voltage properties. Thus, PNQ-4 and PNQ-7 were selected as representative electron-rich
and electron-deficient redox molecules, and their rate capability and cycle stability were
compared (Fig. 4). The para-OCH3 bearing PNQ-4 showed two distinct plateaus at 2.37 and
2.61 V with an initial discharge capacity of 187 mA h g–1 at a current rate of 100 mA g–1 (Fig.
4a). As the current density increased, the discharge capacity gradually decreased, and the
capacity retention at 2000 mA g–1 corresponding to ca. 10 C-rate was 81% of the initial
specific capacity at 100 mA g–1. The electron-deficient PNQ-7 also showed two distinct
discharge plateaus at 2.45 and 2.68 V. However, its initial specific capacity was 93 mA h g-1 at
a current rate of 100 mA g–1, which was much lower than that in PNQ-4 because of its higher
molecular weight along with potential electronic effect (Fig. 4b). Nevertheless, it showed
good rate capability, with 70% of the initial specific capacity at a current rate of 2000 mA g–1.
Remarkably, both PNQ-4 and PNQ-7 showed superior cyclability for 95 cycles at a current
rate of 100 mA g–1 with the Coulombic efficiency of ca. 100%. Morphological characteristics
of metallic lithium surface were also investigated by SEM analyses, and the interfacial
structures of the cycled lithium anode with PNQ-4 and PNQ-7 were remarkably intact
without showing serious crack or dendritic lithium growth (ESI†, Fig. S15). With the respect
to molecular structure, the arylation by the Pd-catalyzed Suzuki coupling can provide
improved electrochemical stability from the enhanced delocalization of PNQ derivatives.
For practical applications of the organic catholyte, we fabricated a tubular lithium-organic
redox battery architecture based on a spiral lithium foil anode, fully soluble PNQ-1 catholyte,
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and carbon cloth electrode (see also Fig. S16 and S17 in the ESI†). To prevent a direct contact
between the lithium foil and carbon cloth, a porous separator was used. Then, we
performed the galvanostatic charge and discharge tests using 0.1 M of PNQ-1 catholyte at a
current density of 0.25 mA cm–2 in a voltage range from 1.8 to 3.0 V (Fig. 5a). The voltage
profile showed a distinct two-electron transfer reaction, which agrees with the half-cell tests.
The initial volumetric capacity of PNQ-1 was 1.6 A h L–1, and it retained 75% of the initial
capacity after 30 cycles corresponding to 140 h (Fig. 5b). We increased the concentration of
the PNQ-1 catholyte from 50 to 150 mM to improve its energy density, and these values
were compared to those of the BAQ catholyte that our group reported previously.28 The
energy density was proportional to the concentration of catholyte, and the 150 mM of PNQ-
1 reached up to the value of 6.0 W h L–1 (Fig. 5c).
The solubility of PNQ-1 was then quantitatively determined by UV/Vis spectroscopy
(Table S1 and Fig. S18 in the ESI†). Based on the Beer–Lambert law, the maximum solubility
of PNQ-1 in the TEGDME electrolyte was 0.31 M, which was 16 times higher than that of the
BAQ catholyte. This highly improved concentration allows for the continuous electrolyte
circulation. The tubular lithium-organic flow battery was connected with an external
electrolyte tank to provide the catholyte upon cycling (Fig. S17). The PNQ-1 catholyte from
an external reservoir was circulated using a peristaltic pump (a flow rate of 25 mL min-1) that
was purged with nitrogen gas to maintain the inert conditions and to avoid air exposure. Fig.
S19 shows the charge−discharge behavior of the tubular flow battery with 0.1 M of fully
soluble PNQ-1 catholyte. Remarkably, no abrupt capacity drop occurred for 20 h that can be
caused by the flow channel clogging. These enhanced circulation and performance are
associated with the high solubility and chemical stability of PNQ-1. The charge–discharge
capacity of PNQ-1 in the tubular flow battery was moderately stable while showing an
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average energy efficiency of ~70% (Fig. 5d). Further, as a proof-of-concept demonstration,
the tubular lithium-organic flow battery was connected to a silicon photovoltaic (PV)
module46,47 (see also the photocurrent−voltage curve presented in Fig. S20). The tubular
battery was photocharged by the PV module with a current of 20 mA (Fig. 5e), and was
electrochemically discharged with a current of 10 mA. To demonstrate the module operation
under practical conditions, the tubular battery integrated with a solar cell panel was
connected to a fan (Fig. 5f). The discharged tubular battery was readily charged by the solar
cell, and allowed to operate a fan.
Conclusions
We designed and synthesized an arylated quinone family for nonaqueous organic catholytes.
The chemical modification of quinone catholytes leads to enhanced electrochemical stability,
high solubility, and reversible redox processes. In addition, the substituent-dependent
electrochemical performance of arylated quinones has been systemically investigated using
OCH3 and CF3 functionalities. The excellent performance can be attributed to the lowered
reactivity toward lithium metal, owing to the enhanced π-electron delocalization effect and
reduced olefinic character. In the tubular battery system, the arylated quinone PNQ-1
provides an energy density of up to 6.0 W h L–1 at a concentration of 150 mM. We anticipate
that this single-flow lithium battery architecture assisted by the molecular design can pave
the way to provide sustainable and reliable lithium battery systems.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
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Authors acknowledge the financial supports from the National Research Foundation of Korea
(NRF-2016R1A2B4015497) and the UNIST Research Fund (1.170048.01).
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H.-D. Um, K.-H. Choi, I. Hwang, S.-H. Kim, K. Seo and S.-Y. Lee, Energy Environ. Sci., 2017, 10,
931–940.
S. Liao, X. Zong, B. Seger, T. Pedersen, T. Yao, C. Ding, J. Shi, J. Chen and C. Li, Nat. Commun.,
2016, 7, 11474.
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Page 13 of 19
Journal of Materials Chemistry A
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DOI: 10.1039/C8TA04720K
Fig. 1 (a) Organic redox-active molecules of NAQ and BAQ for the 1st generation lithium-organic batteries (Ref.
28). (b) Biochemical redox processes. (c) Synthesis of the 2nd generation quinone derivatives by the Suzuki
cross-coupling reactions. (d) A schematic of tubular lithium-organic flow battery configuration.
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Journal of Materials Chemistry A
Page 14 of 19
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DOI: 10.1039/C8TA04720K
Fig. 2 (a) Two-electron electrochemical lithiation and delithiation. (b) Voltage profiles of PNQ-1 at a current
rate of 100 mA g–1. (c) Rate capability tests of the PNQ-1 catholyte at different current densities: 100, 200, 400,
1000, and 2000 mA g–1. (d) Cyclabilities of NQ and PNQ-1 for 150 cycles at 100 mA g–1. SEM images of lithium
metal anodes for (e) NQ and (f) PNQ-1 after 150 cycles.
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Page 15 of 19
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DOI: 10.1039/C8TA04720K
Fig. 3 Normalized voltage profiles of (a) PNQ-1, PNQ-2, PNQ-3, and PNQ-4 and (b) PNQ-1, PNQ-5, PNQ-6, and
PNQ-7 at a current rate of 100 mA g–1, and (c) their cyclability at 100 mA g–1.
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA04720K
Fig. 4 (a,b) Rate capability tests of PNQ-4 and PNQ-7 catholytes at different current densities from 100 to 2000
mA g–1. (c,d) The cycle performance and Coulombic efficiency of PNQ-4 and PNQ-7 catholytes for 95 cycles at a
current density of 100 mA g–1.
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Page 17 of 19
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DOI: 10.1039/C8TA04720K
Fig. 5 (a) The charge–discharge cycle behavior and (b) the cycling performance of fully soluble PNQ-1 catholyte
for 30 cycles at a current rate of 0.25 mA cm−2 and voltage window from 1.8 to 3.0 V. (c) Volumetric energy
densities of PNQ-1 and BAQ. (d) The charge–discharge capacity of 0.1 M PNQ-1 in a tubular flow battery with
the energy efficiency. (e) The photocharge and electrochemical discharge behavior, and (f) its digital
photograph of a tubular battery equipped with a photovoltaic module.
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Journal of Materials Chemistry A
Page 18 of 19
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DOI: 10.1039/C8TA04720K
Table 1 Optimization of the Suzuki cross-coupling reactiona
Entry
Deviations from standard conditions
Yieldb (%)
1
2
3
4
5
Nonea
86
1,4-dioxane and water, 80 °C
71
Only THF, 80 °C
80
51c
37
THF and water, room temperature
THF and water, 80 °C in the air
a The standard conditions: Br-NQ (1.0 equiv.), phenylboronic acid (1.5 equiv.), Pd(OAc)2 (3 mol%), PPh3 (12 mol%) and K2CO3
(3.0 equiv.) in THF/water (5 mL, 10:1, v/v) at 80 °C for 8 h under an argon atmosphere. b Isolated yields. c 20 h.
Table 2 Reaction scope using substituted boronic acidsa
b
a The standard conditions and isolated yields. The changes from the standard conditions: Pd(OAc)2 (5 mol%), PPh3 (20
mol%), 1,4-dioxane instead of THF, 100 °C for 12 h.
18

tꢈꢊꢇ ꢊ #ꢃ ꢆꢃꢍ ꢇ#ꢂꢄ ꢍ ꢏꢇꢅ$ꢁꢆ
Journal of Materials Chemistry A
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DOI: 10.1039/C8TA04720K
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