Poly(diphenanthrenequinone-substituted norbornene) for long life and efficient lithium battery cathodes

Full text of DOI:10.1246/bcsj.20170420

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Poly(diphenanthrenequinone-substituted norbornene) for Long Life and Efficient
Lithium Battery Cathode
Takuma Kawai, Satoshi Nakao, Hiroyuki Nishide,* and Kenichi Oyaizu*
Advance Publication on the web February 3, 2018
doi:10.1246/bcsj.20170420
© 2018 The Chemical Society of Japan
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Poly(diphenanthrenequinone-substituted norbornene) for Long Life and Efficient
Lithium Battery Cathode
Takuma Kawai, Satoshi Nakao, Hiroyuki Nishide*, Kenichi Oyaizu*
Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan.
E-mail: <oyaizu@waseda.jp>
Kenichi Oyaizu received his Ph.D. from Waseda University in 1995. He was appointed to Lecturer of Waseda University
in 1997, Associate Professor of Tokyo University of Science in 2003, Associate Professor of Waseda University in 2007,
and Professor in 2012. His research interests include synthesis of functional polymers for engineering plastics and
energy-related devices.
Abstract
Redox-active polymers with large charge-storage density are
replacement since the redox reaction does not relate to almost
any morphological changes making the material more stable
and energy efficient.14-15 High-density redox polymers,
candidates for electrode-active materials in next-generation
energy storage devices, due to their swift charge-discharge
capabilities and their inherent characteristics of redox reactions
that undergo without significant structural changes, leading to
their highly energy-efficient and durable performances. Here
we report that poly(di-phenanthrenequinone-substituted
especially
2,2,6,6-tetramethylpiperidin-1-oxyl
those
substituted
(TEMPO)
with
the
as
redox-active pendant group per repeating unit are one of the
promising candidates for its fast charge-discharge, very small
chance of side reactions, light weight, and mechanically
flexible properties characteristic to organic materials.12-13,16-17
We have already reported a highly flexible rechargeable
organic battery with an excellent power density and cycle
performance in which these properties are derived from the
completely reversible and swift redox reaction of the
TEMPO0/+ couple.12-13, 16-21 Not only for the cathode materials,
but many types of redox polymers can also be used as the
anode material such as organic-air secondary batteries.22-23
Quinones or carbonyl compounds are often selected for anode
materials for their negative potentials and structural
robustness.24-29 Quinones are focused for its wide availability
and high charge capacity for its two-electron reaction at the two
carbonyl oxygen atoms.30 Among them, the redox couple of
anthraquinone (AQ2-/0) was found to be particularly useful as
the electrode reaction in organic anodes22-23 for its
exceptionally stable redox performance and chemically robust
structure,31 but as a cathode material against Li0/+, AQ only
exhibits ~2.4 V (vs. Li/Li+) due to its negative potential.32-33
Recently, Shimizu and Yoshida et al. reported that the redox
cyclability in Li-ion battery electrolytes greatly enhanced by
replacing p-quinones with o-quinones, which was ascribed to
the attenuated RO--Li+ interaction as a result of the bidentate
coordination of two adjacent carbonyl (or phenolate) oxygen
atoms to the Li+ ion.34,35 This result inspired a variety of
relevant research employing o-benzoquinones bound to many
types of polymer backbones.36-38 Typically, Patil et al recently
reported polymers bearing o-benzoquinone with excellent cycle
performance of 98% capacity retention after 3400 cycles.36 In
search of organic redox couples with high reversibility,
robustness and positive potentials suitable for organic cathode
in Li-ion batteries, we have focused on phenanthrenequinone
(PQ) which shows a moderately positive potential (~0.4 V
more positive than AQ) and desired chemical inertness due to
the fused aromatic ring structure with the o-quinone or
1,2-diketone framework. PQ showed a better stability than AQ
at the reduced state due to the formation of the chelate Li+
complex similar to that of the o-benzoquinone oxygen
atoms.34-35, 39-41 Here we report the first successful attempt to
incorporate PQ in an ultimate density per repeating unit of an
norbornene) (PQN) as
a novel class of the organic
electrode-active material. The Li coin cell composed of the
PQN/carbon composite electrode as the cathode exhibited 2.8
V (V vs. Li/Li+) and great cycle performance maintaining a
capacity higher than 100 mAh/g for more than 100 cycles at 60
C (i.e. in 1 min charging and discharging). Among many types
of o-quinone-containing polymers for Li-ion batteries reported
so far, the present research provides the first example of
introducing phenanthrenequinone as the pendant group per
repeating unit of polymers, which proved to be especially
advantageous in terms of robustness and cyclability by virtue of
the fused-ring structure to protect the reactive positions of the
o-benzoquinone. We also report that the functional group
tolerance against many types of redox-active groups, which we
have established for the initiator and the propagating end of
norbornene
derivatives,
apply
for
the
phenanthrenequinone-substituted monomer, giving rise to a
reversible redox activity.
1. Introduction
The improvement of technology has always influenced and
been influenced by the development of energy storage devices,
which started with the non-rechargeable lead-acid battery1-2 and
has now changed its form into various kind of primary and
secondary batteries.3 Recent progress in the chemistry of
reversible charge storage have spawned a variety of organic
electrode-active materials.4-8 To enhance the energy density of
batteries, the charge-storage capacity of the electrode material
is very important.9 For anodes, lithium metal has the highest
theoretical capacity (~3600 mAh/g), but has difficulties in the
matter of cycle life and safety.10-11 Li-ion batteries (LIBs) have
been deeply focused for its excellent power efficiency,12 which
utilize lithium metal oxides as the cathode and graphite or
carbon electrodes as the anode. LIBs are used from handheld
devices to EVs and large energy storage systems, but the rare
metals that are used in the cathode has always been a concern
in the matter of sustainability.13 Compared to the conventional
cathodes, organic electrode materials are
a
choice of
aliphatic
polymer
chain.

Poly(diphenanthrenequinone-substituted norbornene) (PQN)
was synthesized in a sufficiently high molecular weight for
electrode fabrication, by monomer synthesis of simple
reactions starting from 3-acetylphenanthrene followed by
ROMP with the Grubbs 2nd generation catalyst.42-45
Norbornene was chosen as the main chain for its high
polymerization rate, film formability, redox inactivity, and the
functional group tolerance against the quinoid group during
ROMP.23 Indeed, polymers obtained from monomers with the
redox-active pendants have inherent advantages, in terms of
charge-storage capacity, over those having the redox-active
groups incorporated by polymer-analogous reactions that
frequently suffer from the limited degree of incorporation.34,48
Interestingly, almost all of the PQ pendant in the polymer
maintained its redox capability, characterized by the efficient
coupling with the Li+ ions.40-41,46-47,49 These properties were
found to be important for fabricating energy-efficient organic
electrodes and directly effected the battery performance, such
as voltage, capacity, and cyclability. The Li coin cell composed
of PQN as the cathode and Li metal as the anode gave a stable
working voltage of 3 V and maintained over 100 mAh/g of
capacity for more than 100 cycles without capacity decay.49
This work is sure to give a strong positive possibility to the
organic electrode area and should become an influence for
futuristic energy storage systems.
O
O
O
O
NIS
I
CF3COOH
IPQ
n
O
O
O
PdCl2(PPh3)2
TEA
HCOOH
O
Grubbs 2nd cat.
CHCl3
Toluene
PQN1
Scheme 1. Synthesis of IPQ and PQN1.
Synthesis
of
2-Iodophenanthrenequinone
(2.08 g, 10
(IPQ).
mmol),
9,10-Phenanthrenequinone
N-iodosuccinimide (3.38 g, 15 mmol), and trifluoroacetic acid
(7.4 mL, 100 mmol) were added together and stirred for 15 h at
r.t. under N2 or Ar gas atmosphere. The mixture was washed
with CHCl3/sodium thiosulfate aq. to remove excess and
unreacted acid and iodides. After purification by silica gel
column chromatography using CHCl3 as eluent, IPQ was
obtained as a red solid. Yield: 60%. FAB-MS (m/z): M+ 334.1.
Found 334.8. 1H NMR (CDCl3, 500 MHz, ppm, TMS): δ 8.49
(d, 1H, Ph), 8.19 (dd, 1H, Ph), 8.03 (dd, 1H, Ph), 7.98 (d, 1H,
Ph), 7.73 (m, 2H, Ph), 7.50 (t, 1H, Ph).
Synthesis
of
5-Norbornene-2-phenenthrenequinone.
2-Iodophenanthrenequinone (1g, 3 mmol) and PdCl2(PPh3)2
(138 mg, 0.12 mmol) was purged by N2 gas and dissolved in 30
mL of toluene. 2,5-Norbornadiene (0.9 mL, 9 mmol) and
triethylamine (1.38 mL, 9 mmol) were added to the solution
under N2 gas, and the mixture was heated to 80 oC. Formic acid
(0.21 mL, 6 mmol) was added to the mixture and was stirred
2. Experimental
Characterization of prepared materials can be found in the see
Supporting Information.
Electrode Fabrication
o
for 15 h at 80 C. The mixture was washed with CHCl3/brine
In a mortar, 2 mg of PQN (PQN1, PQN2, and PQN3), 2 mg of
PVdF, 16 mg of VGCF-H, and few drops of NMP were added
and mixed. The mixture was coated on a glassy carbon
electrode and vacuum dried at 80 oC for 8 h to yield the
PQN/carbon composite electrode and these electrodes were
used for half-cell experiments (Figure 3-7). For coin cell
experiments (Figure 8-10), the substrate was replaced with Al
foil.
and purified by silica gel column chromatography using CHCl3
as eluent to obtain 5-norbornene-2-phenenthrenequinone as a
red solid. Yield: 65%. FAB-MS (m/z): M+ 300.4. Found 301.1.
1H NMR (CDCl3, 500 MHz, ppm, TMS): δ 8.15 (d, 1H, Ph),
8.08 (d, 1H, Ph), 7.96 (d, 1H, Ph), 7.91 (d, 1H, Ph), 7.68 (t, 1H,
Ph), 7.61 (m, 1H, Ph), 7.43 (t, 1H, Ph), 6.27 (dd, 1H, -HC=),
6.19 (dd, 1H, -HC=), 3.02 (s, 1H, -CH-), 2.95 (s, 1H, -CH-),
2.75 (m, 1H, -CH-), 1.73 (m, 2H, -CH2-), 1.51 (m, 2H, -CH2-).
Polymerization of 5-Norbornene-2-phenenthrenequinone.
The monomer 5-norbornene-2-phenenthrenequinone (100 mg,
0.33 mmol) and the Grubbs 2nd generation catalyst (2.83 mg,
0.01 eq, 0.0033 umol) was added to a round bottom flask and
was charged with nitrogen. Dry CHCl3 (0.5 mL) was added by
syringe and was stirred for 20 h at 60 ºC. After removing the
solvent by evaporation, the polymer solution was centrifuged
with methanol, washed with methanol using soxhlet extraction,
and dried to yield PQN1 as a green solid. The polymer was
insoluble in conventional organic solvents after drying.
Electrochemical Measurements.
Electrochemical measurements were carried out using the
ALS electrochemical analyzer Model 660C, 660D, and 760EW.
All half-cell measurements were performed using a sealed cell
under an atmosphere of dry nitrogen. The three-electrode cell
was composed by a glassy carbon plate working electrode, a
platinum wire counter electrode, and an Ag/AgCl reference
electrode.
Electrolytes
were
prepared
using
gamma-butyrolactone (GBL) and LiClO4 as purchased inside a
glove box. The electrolytes were primary degassed by argon
before use. 2032 type coin cell batteries were fabricated by
polymer/carbon composite as the cathode, Celgard #3501 as the
separator, Li metal as the anode, and 1 M LiClO4/GBL as the
electrolyte.
O
O
NaClO
NaOH
CrO3
18-crown-6
1,4-dioxane
CH3COOH
O
O
O
PCA
HO
HO
Synthesis
n
O
Phenathrenequinone substituted polymers were synthesized by
the following procedures. A total of three polymers with direct
linkage, ester linkage, and di-ester linkage of PQ was
synthesized.
O
O
O
OH
DMT-MM
DMAP
Grubbs 2nd cat.
CHCl3
O
DMF
O
O
O
PQN2
Scheme 2. Synthesis of PCA and PQN2.

Synthesis
of
Phenanthrene-3-carboxylic
acid.
O
OH
3-Acetylphenanthrene (1 g, 4.54 mmol) was dissolved in 20
mL of 1,4-dioxane. NaClO solution (64 mL, available chlorine
5 %) and 1 M NaOH aq. (90 mL) was added to the solution and
n
O
O
O
O
OH
DMT-MM
O
O
O
O
O
O
Grubbs 2nd cat.
CHCl3
DMAP
O
DMF
o
O
stirred for 6 h at 65 C. After cooling down to r.t., sodium
O
HO
O
O
thiosulfate (26 g, 0.16 mmol) was added and stirred for
neutralization. The solution was washed with water and diethyl
O
O
O
PQN3
ether,
followed
by
vacuum
drying
to
yield
phenanthrenequinone-3-carboxylic acid as a white solid. Yield:
97%. FAB-MS (m/z): M+ 222.1. Found 222.9. 1H NMR
(DMSO-d6, 500 MHz, ppm, TMS): δ 13.1 (s, 1H, -OH), 9.33
(s, 1H, Ph), 8.82 (d, 1H, Ph), 8.12 (d, 1H, Ph), 8.07 (d, 1H Ph),
8.02 (d, 1H, Ph), 7.97 (d, 1H, Ph), 7.90 (d, 1H, Ph), 7.74 (t, 1H,
Ph), 7.68 (t, 1H, Ph). 13C NMR (DMSO-d6, 500 MHz, ppm,
TMS): δ 168.1, 134.8, 132.3, 130.3, 129.8, 129.6, 129.4,
129.3, 129.2, 128.1, 127.9, 127.1, 126.3, 125.0, 123.3.
Scheme 3. Synthesis of PQN3.
of 5-Norbornen-2,3-diylmethyl
Synthesis
bis(9,10-phenanthrenequinone-3-carboxylate).
Phenanthrenequinone-3-carboxylic acid (504 mg, 2 mmol),
5-norbornene-2,3-dimethanol (154 mg, 1 mmol), DMT-MM
(1.33 g, 4.8 mmol), and DMAP (200 mg, 1.6 mmol) were
dissolved in 10 mL of DMF and was stirred for 15 h at r.t.
Water was added to the reaction mixture for precipitation of the
product, and the precipitate was washed by CHCl3/brine
followed by column chromatography with CHCl3/ethyl acetate
Synthesis of Phenanthrenequinone-3-carboxylic acid
(PCA). Phenanthrene-3-carboxylic acid (489 mg, 2.20 mmol)
was dissolved in 20 mL of acetic acid. In a separate flask,
chromium(VI) oxide (808 mg, 8.80 mmol) and 18-crown-6
(6.90 mg, 0.367 mmol) was dissolved in 11 mL of acetic
acid/water (v/v = 10/1). After complete dissolution, the latter
solution was added to the first solution and was stirred for 16 h
at 60 oC. Water was added to the reaction mixture for
precipitation and the precipitate was purified by washing with
acetic acid/water (v/v = 1/1) and diethyl ether to obtain
phenanthrenequinone-3-carboxylic acid as a yellow solid.
to
obtain
5-norbornen-2,3-diylmethyl
bis(9,10-phenanthrenequinone-3-carboxylate) as a yellow solid.
1
Yield: 45%. FAB-MS (m/z): M+ 622.2. Found 623.4. H NMR
(CDCl3, 500 MHz, ppm, TMS): δ 8.64 (s, 2H, Ph), 8.20 (m,
4H, Ph), 8.08 (m, 4H, Ph), 7.74 (t, 2H, Ph), 7.51 (t, 2H, Ph),
6.30 (m, 2H, -HC=), 4.73 (m, 2H, -CH2-), 4.46 (m, 2H, -CH2-),
2.92 (m, 2H, -CH-), 2.20 (m, 2H, -CH-), 1.70 (d, 1H, -CH2-).
13C NMR (CDCl3, 500 MHz, ppm, TMS): δ 179.9, 179.4,
165.1, 137.5, 136.3, 136.0, 134.9, 133.5, 131.1, 130.6, 130.3,
129.9, 125.5, 124.4, 67.0, 45.3, 43.1, 40.1.
1
Yield: 79%. FAB-MS (m/z): M+ 252.0. Found 253.4. H NMR
(DMSO-d6, 500 MHz, ppm, TMS): δ 13.5 (s, 1H, -OH), 8.66
(s, 1H, Ph), 8.30 (d, 1H, Ph), 8.09 (d, 1H, Ph), 8.03 (d, 1H, Ph),
7.99 (d, 1H, Ph), 7.76 (t, 1H, Ph), 7.54 (t, 1H, Ph). 13C NMR
(DMSO-d6, 500 MHz, ppm, TMS): δ 179.0, 178.9, 167.1,
136.8, 136.0, 135.9, 135.0, 134.6, 132.0, 130.2, 130.0, 129.9,
129.7, 125.3, 125.0.
Polymerization
of
5-Norbornen-2,3-diylmethyl
bis(9,10-phenanthrenequinone-3-carboxylate).
The
monomer
5-norbornen-2,3-diylmethyl-bis(9,10-phenanthrenequinone-3-c
arboxylate) (124 mg, 0.2 mmol) and the Grubbs 2nd generation
catalyst (1.72 mg, 0. 01 eq, 2 mol) were added to a round
bottom flask and were charged with nitrogen. Dry CHCl3 (2
mL) was added by syringe and was stirred for 16 h at 40 ºC.
After removing the solvent by evaporation, the polymer
solution was centrifuged with methanol, washed with methanol
using soxhlet extraction, and dried to yield PQN3 as a green
solid. The polymer was insoluble in conventional organic
solvents after drying.
Synthesis
of
5-Norbornen-2-ylmethyl
9,10-phenanthrenequinone-3-carboxylate.
Phenanthrenequinone-3-carboxylic acid (504 mg, 2 mmol),
5-norbornene-2-methanol
4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium
chloride (DMT-MM) (1.1 g, mmol), and
(300
mg,
2.4
mmol),
4
4-dimethylaminopyridine (DMAP) (200 mg, 1.6 mmol) were
dissolved in 20 mL of DMF and were stirred for 14 h at r.t.
Water was added to the reaction mixture for precipitation and
the precipitate was washed by CHCl3/brine followed by column
chromatography with CHCl3/ethyl acetate to obtain
5-norbornen-2-ylmethyl 9,10-phenanthrene-3-carboxylate as a
yellow solid. Yield: 40%. FAB-MS (m/z): M+ 358.1. Found
359.3. 1H NMR (CDCl3, 500 MHz, ppm, TMS): δ 8.69 (d, 1H,
Ph), 8.23 (m, 2H, Ph), 8.10 (m, 2H, Ph), 7.77 (m, 1H, Ph), 7.51
(m, 1H, Ph), 6.25-6.00 (br, 2H, -HC=), 4.51-3.97 (br, 2H,
-CH2-), 3.03-2.55 (br, 2H, -CH2-), 1.94 (m, 1H, -CH-),
1.55-0.65 (br, 4H, -CH2-).
3. Results and Discussion
Quinones are one of the representative organic redox active
units for their 2e- reaction giving higher charge-storage density
compared to 1e- materials.24-31 Benzoquinone is the most ideal
structure which gives a theoretical capacity of 460 mAh/g, but
due to the inherent reactivity of the reduced dianion state and
the concurrent formation of quinhydrones, it is usually difficult
to accomplish the reversible 2e- reaction without the undesired
side-reactions that lead to capacity decay.50 In comparison, the
fused aromatic rings of anthraquinone (AQ) and
phenanthrenequinone (PQ) give durability or robustness that is
critical for long cyclability. Also, since the cation involved in
the redox reaction of quinones in the battery electrolytes will be
only Li+, the battery reaction becomes the rocking-chair
type.51-53
Polymerization
of
5-Norbornen-2-ylmethyl
9,10-phenanthrenequinone-3-carboxylate. The monomer
5-norbornen-2-ylmethyl-9,10-phenanthrene-3-carboxylate (179
mg, 0.5 mmol) and the Grubbs 2nd generation catalyst (4.2 mg,
0. 01 eq, 5 umol) was added to a round bottom flask and was
charged with nitrogen. Dry CHCl3 (0.5 mL) was added by
syringe and was stirred for 12 h at 40 ºC. After removing the
solvent by evaporation, the polymer solution was centrifuged
with methanol, washed with methanol using soxhlet extraction,
and dried to yield PQN2 as a yellow-green solid. GPC (CHCl3,
40 oC, RID): Mw = 1.1 × 104, Mw/Mn = 1.8.
Our concept is to produce a cathode material to be coupled
with the lithium metal anode in the battery, focusing on PQ as
the redox active unit. First, we investigated the redox potential
of PQ and AQ in lithium-based electrolyte to observe the effect
of the o-quinone structure on the half-wave potential (E1/2
(Figure 1). When compared to AQ (E1/2 = 2.47, 2.16 V vs.
Li/Li+), a positive shift of ~0.3 V was observed for PQ (E1/2
2.75, 2.48 V). This shift has been ascribed to presence of two
)
=

adjacent carbonyl groups34-35,40-41 in the o-quinone structure in
which the two oxygen atoms serve as the bidentate ligand to
coordinate to Li+ (Figure 2), resulting in a higher potential
1
PQN1
2
PQN2
compared to AQ.34 Also, according to Yoshida et al.,34
a
n
n
O
O
six-membered ring 1,2-diketone group serves as the best
candidate for the cathode materials due to the significant
stabilization of the reduced state by the increase in
aromaticity.34 The positive E1/2 for PQ suggested that it should
act as a cathode working at 3 V.
O
O
O
O
PQN1
PQN2
0
0
.
.2
2
m
m
A/c
m2
A
Li
O
Li Li
-1
-0.5
3.5
0
0.5
2.5
O
O
O
O
O
4
2.2
3
2.6
3
3.4
+e-
-e-
+e-
-e-
Potential (V vs. Ag gCl)
Potential (V vs. L i )
/Ai/L +
Figure 3. Cyclic voltammograms of PQN1 (dashed line) and
PQN2 (solid line)/carbon composite electrode in 1 M LiClO4
GBL electrolyte at a scan rate of 10 mV/s. The electrode
potential was recorded as in Figure 2.
Anion
Dianion
Neutral
Figure 1. Redox reaction of PQ with chelate complex
formation with Li ions.34
2
5
Li Li
PQN1
1
O
O
O
O
Li
O
O
+e-
2
1
4
PQN2
O
O
(+Li+)
3
0
150uAA/cm2
Li
2
-1
(i)
Charge
(ii)
Discharge
Li Li
O
O
O
O
-e-
(-Li+)
1
-2
0
0
50
50
100
100
150
150
200
200
Capacityity(m(mAh/g)
Capac Ah/g)
-1 -0.8 -0.6 -0.4 -0.2
2.2 2.4 2.6 2.8
0
0.2
3.2
Figure 4. Charge-discharge curves of PQN1 (dashed line) and
PQN2 (solid line)/carbon composite electrode in 1 M LiClO4
GBL electrolyte at 10 C. (i) demonstrates the reduction of the
polymer (i.e. negative charge being stored at the carbonyl of
phenanthrenequinone units), which corresponds to the
discharging of the polymer in Li batteries, and (ii) the oxidation.
The electrode potential was recorded as in Figure 2.
2
3
Potential(V vs. Li/Li+)
Potential (V vs. Ag/AgCl)
Figure 2. Cyclic voltammogram of 1mM PQ in 0.1 M LiClO4
GBL electrolyte at a scan rate of 50 mV/s. The potential was
recorded with an Ag/AgCl reference electrode and was
converted to that against Li/Li+ using E (V vs Li/Li+) = E (V vs.
Ag/AgCl) + 3.02 V.
Polymer concentration of the electrode was kept low at 10
wt% and the conductive carbon additive, VGCF-H, was set to
be 80 wt% to offer sufficient electrical conductivity to the
electrode while maintaining the electroactivity of the polymer.
This composition does not effect the total capacity of the
electrode due to the low capacitance of VGCF-H (lower than 2
F/g)54 so that the capacity obtained is derived only from the
redox-active polymer. The half-cell performance of the
PQN/carbon composite electrode was investigated as follows.
00..22 mmAA
/cm2
Three
types
of
poly(phenathrenequinone-substituted
-0.8
2.2
-0.4
2.6
0
0.4
3.4
norbornene) (PQN) were synthesized without unfavorable
side-reactions. PQN substituted with direct linkage to
norbornene (PQN1) and that substituted via methylester linkage
(PQN2) was initially prepared to compare the effect of the
electron-withdrawing methyl ester linkage against the redox
potential and the charge capacity. Huang et al. has reported the
positive potential shift by introduction of ester groups to
fluorene-based polymers.55 Figure 3 demonstrated that PQN2
exhibited a potential shift slightly to the positive side and from
Figure 4, the charge capacity became almost twice of PQN1.
These results clarified the effect of the methyl ester linkage
between norbornene and PQ to improve the electron
withdrawing property and electrolyte affinity for positive
potential and larger capacity.
3
Potential (V vs. Ag/AgCl)
Potential(V vs. Li/Li+)
Figure 5. Cyclic voltammogram of PQN3/carbon composite
electrode in 1 M LiClO4 GBL solution at a scan rate of 10
mV/s. The electrode potential was recorded as in Figure 2.
From cyclic voltammetry, we observed two reversible redox
potentials at 3.02 V and 2.60 V (vs. Li/Li+) (Figure 5). The
galvanostatic charge-discharge curves showed a capacity of
124 mAh/g at 10 C and the capacity was maintained after long
time cycling (Figure 6). PQN3 exhibited the best properties of
all three PQN polymers and this was accomplished by the two
PQ units per repeating unit for high capacity and by the methyl
ester linkage introduced to increase the stability of reduced

state along with positive shift of the potential. Note that the
term 1 C rate is defined as the current density for the material
to undergo charging or discharging in 1 h. We investigated the
rate performance of the electrode by discharging at various
rates after charging the electrode at 10 C. The polymer
electrode showed a slight capacity decay but maintained 70 %
of the capacity even at 50 C compared to 10 C (Figure 7). As
reported previously,32-33 at high rates of over 10 C, it was
difficult to maintain the charge-storage capacity for
pendant-type quinone polymers without addition of ionic
side-chains for charge compensation. It may be stated that this
improved rate performance was accomplished by the enhanced
Li+ diffusivity derived from the o-quinone structure and
electron-withdrawing property and electrolyte affinity derived
from the ester linkage. Further improvements by optimizing
electrode fabrication process and materials are our future
topics.
electromotive force of PQ/Li batteries 17% higher than the
AQ/Li batteries. The charge/discharge curve of the 3rd cycle
cell exhibited a high capacity of >100 mAh/g at both charge
and discharge (Figure 9), and maintained >90% of its capacity
for over 100 cycles (Figure 10). Not only the voltage but also
with the capacity, AQ polymers usually end up losing 60% of
its initial capacity at around 50 cycles due to the irreversibility
of the quinone-Li interaction (vide supra). These prospects
demosntrated that PQN and future PQ electrode materials are
true candidates in the field of organic electrode for Li
secondary batteries.
40 A
5.4
2.4
Li Li
O
O
O
O
+e-
4.2
1.2
(+Li+)
3
0
1.6
2
2.4 2.8 3.2 3.6
4
Voltage (V vs. Li / Li+)
(i)
Charge
(ii)
Discharge
Figure 8. Cyclic voltammogram of the Li coin cell battery with
PQN3/carbon composite electrode as cathode at a scan rate of 1
mV/s.
1.8
-1.2
124 mAh/gLi
108 mAh/g
-e-
Li
O
O
O
O
(-Li+)
0.6
-2.4
0
0
50 100 150 200 250
50 100 150 200 250
6
Li Li
O
O
O
O
-e-
(-Li+)
Capacity (mAh/g)
Capacity (mAh/g)
Figure 6. Charge-discharge curves of PQN3/carbon composite
electrode in 1 M LiClO4 GBL solution at 10 C. (i) demonstrates
the reduction of the polymer (i.e. negative charge being stored
at the carbonyl of phenanthrenequinone units), which
corresponds to the discharging of the polymer in Li batteries,
and (ii) the oxidation. The electrode potential was recorded as
in Figure 2.
Charge
Discharge
4
2
0
108 mAh/g 104 mAh/g
Li Li
O
O
O
O
+e-
(+Li+)
0.6
3.6
0.4
3.4
0
50 100 150 200 250
Capacity (mAh/g)
0.2
3.2
Figure 9. Charge-discharge curves of the 3rd cycle of Li coin
cell battery with PQN3/carbon composite electrode as cathode
at 1 C.
0
3
-0.2
2.8
50 C
10 C
-0.4
2.6
-0.6
2.4
1
-0.8
2.2
0.8
0
0
2
2
0
0
4
4
0
0
6
6
0
0
80 100 120
80 100 120
Capacity (mAh/g)
Capacity (mAh/g)
Li Li
O
O
O
O
0.6
0.4
0.2
0
discharge
charge
Figure 7. Discharging performance of PQN3/carbon composite
at various discharging rates of 10-50 C in 1 M LiClO4 GBL
solution. The electrode potential was recorded as in Figure 2.
The polymer electrode was employed as the cathode of a coin
cell with lithium metal as the anode. For the coin cells, the
PQN/carbon composite electrode was fabricated in the same
way as the above process, except that the substrate (i.e. current
collector) was an Al foil instead of the glassy carbon electrode.
From cyclic voltammetry, we observed two reversible redox
peaks and the overall cell voltage was 2.81 V (vs. Li/Li+)
(Figure 8). These peaks were consistent with the two redox
peaks observed for the half-cell. With the AQ polymers, the
voltage ends up around 2.4 V (vs. Li/Li+) making the
0
20
40
60
80 100
Cycle number (-)
Figure 10. Cycle performance of the Li coin cell battery with
PQN3/carbon composite electrode as cathode at 60 C. Inset:
Charge/discharge diagram of PQ.

4. Conclusion
15. R. Ponnapati, M.J. Felipe, V. Muthalagu, K. Puno, B.
Wolff, R. Advincula, ACS Appl. Mater. Interfaces, 2012,
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To invent a powerful and energy efficient electrode material,
specific structural design for high potential and capacity is
important. The o-quinone structure (PQ) was chosen for its
suitability in term of coordination to Li+ upon reduction which
led to a positive redox potential shift compared to p-quinones
(e.g. AQ) by 0.3 V. Norbornene was chosen for the polymer
backbone for its high rate of polymerization, linearity of the
chain, and the functional group tolerance of the propagating
end against the quinoid unit. An ester linkage between
norbornene and PQ was introduced for improved stability and
capacity which were accomplished by the electron-withdrawing
property and electrolyte affinity. PNQ was synthesized by few
steps of monomer synthesis and ROMP. The earned polymer
was insoluble in conventional organic solvents suggesting a
high robustness for long cyclability without exfoliation from
the current collector. A coin cell composed of PNQ as the
cathode and lithium metal as the anode exhibited an operating
voltage of 2.81 V and a capacity of 110 mAh/g. The battery
was fully discharged in ~ 1 min and maintained its initial
capacity for over 100 cycles at the fast C rate of 60 C. These
characteristics were accomplished thanks to the efficient and
swift Li+ diffusion in the polymer layer during the redox
reaction. Further work on electrode fabrication is in progress to
enhance the performance of the polymer.
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This work was partially supported by Grants-in-Aid for
Scientific Research (Nos. 16K14010, 17H03072) from MEXT,
Japan. This work was also partially supported by TEPCO
Memorial Foundation 2017. T. K. acknowledges the Leading
Graduate Program in Science and Engineering, Waseda
University from MEXT, Japan.
Supporting Information
Device and material used for electrochemical measurements,
equation for formula weight-based theoretical redox capacity,
1H and 13C NMR spectra of the reported molecules are listed in
the supporting information.
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