Synthesis, characterization and charge-discharge studies of ferrocene-containing poly(fluorenylethynylene) derivatives as organic cathode materials

Article abstract of DOI:10.1016/j.polymer.2015.01.054

Two new poly(fluorenylethynylene)s with ferrocene moieties PFFAL1 and PFFAL2 were designed and synthesized. Their structures, molar masses, photophysical properties and thermal characteristics were analyzed by ¹H NMR, Fourier transform infrared

Full text of DOI:10.1016/j.polymer.2015.01.054

Polymer xxx (2015) 1e7
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis, characterization and chargeedischarge studies of
ferrocene-containing poly(fluorenylethynylene) derivatives as organic
cathode materials
Jing Xiang a , Ren eꢀ Burges , Bernhard H a€ upler , Andreas Wild d,
,
b
c
,
d
c
,
d
c,
c
,
d,
a,
b,
**
, Wai-Yeung Wong a
, b, **
Ulrich S. Schubert
*, Cheuk-Lam Ho
a
Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road,
Kowloon Tong, Hong Kong, PR China
HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen 518057, PR China
Laboratory of Organic and Macromolecular Chemistry, Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany
Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 September 2014
Received in revised form
Two new poly(fluorenylethynylene)s with ferrocene moieties PFFAL1 and PFFAL2 were designed and
synthesized. Their structures, molar masses, photophysical properties and thermal characteristics were
analyzed by 1H NMR, Fourier transform infrared (FTIR) spectroscopy, size exclusion chromatography
20 January 2015
(
SEC), ultravioletevisible (UVeVis) spectroscopy and thermogravimetric analysis (TGA). The morphol-
Accepted 22 January 2015
Available online xxx
ogies and electrochemical performances of the polymer-composed cathodes were studied by scanning
electron microscopy (SEM), cyclic voltammetry (CV) and galvanostatic chargeedischarge test. The results
showed that PFFAL1-composed electrode retained over 90% of the initial capacity after 100 cycles at 10 C
Keywords:
Ferrocene
Poly(fluorenylethynylene)s
Organic cathode materials
ꢀ
1
and PFFAL2-based cathode exhibited a stable discharge capacity of 73 mAh g at 1 C, which is close to
ꢀ
1
the theoretical value (82.3 mAh g ). The stable capacity as well as the good cycle endurance of these
polymer-based coin cells revealed their great potential as cathode-active materials for rechargeable
lithium batteries.
©
2015 Elsevier Ltd. All rights reserved.
1
. Introduction
Meanwhile, continuous progress in portable electronic applications,
such as touch screens and rollup displays, also places a huge burden
on the implementation of efficient energy-storing devices [3].
Among various electrical storage technologies, rechargeable
lithium-ion batteries are undoubtedly one of the most simple and
reliable devices, which facilitate the conversion between electrical
power and chemical energy by reversible electrochemical redox
reactions [4,5]. However, in the research of efficient and portable
rechargeable lithium-ion batteries, development of good cathode
material is always the bottleneck issue [6]. The ideal features of
organic redox-active molecules, such as structure diversity, device
flexibility, environmental friendliness and solution processability,
make them a promising candidate for next-generation batteries
The ever growing energy demands and global environmental
issues are recognized as two major challenges in today's modern
society. To tackle these problems, two promising sustainable energy
sources, solar power and wind, will play an increasingly important
role in the coming years [1]. While there is a widespread utilization
of solar and wind energy, new problems associated with electro-
chemical energy storage (EES) will be brought out by the increase of
our dependence on these interruptible natural resources [2].
*
Corresponding author. Laboratory of Organic and Macromolecular Chemistry,
Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany. Fax: þ49
641 948202.
* Corresponding authors. Institute of Molecular Functional Materials, Depart-
[7,8]. Extensive efforts have recently been dedicated to develop
3
*
sustainable organic cathode materials including conducting poly-
mers [9e13], organosulfur molecules [14e23], organic radical
polymers [24e29], carbonyl compounds [30e38] and some other
organic structures [39e45].
ment of Chemistry and Institute of Advanced Materials, Hong Kong Baptist Uni-
versity, Waterloo Road, Kowloon Tong, Hong Kong, PR China. Fax: þ852 3411 7348.
E-mail addresses: ulrich.schubert@uni.jena.de (U.S. Schubert), clamho@hkbu.
edu.hk (C.-L. Ho), rwywong@hkbu.edu.hk (W.-Y. Wong).
http://dx.doi.org/10.1016/j.polymer.2015.01.054
0
032-3861/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xiang J, et al., Synthesis, characterization and chargeedischarge studies of ferrocene-containing
poly(fluorenylethynylene) derivatives as organic cathode materials, Polymer (2015), http://dx.doi.org/10.1016/j.polymer.2015.01.054

2
J. Xiang et al. / Polymer xxx (2015) 1e7
Ferrocene (Fe(C
5
H
5
)
2
, C
5
H
5
¼ cyclopentadienyl) is a famous
of the polymer and PVDF (SigmaeAldrich) in NMP (10 mg/mL) to
VGCF (SigmaeAldrich) as conducting additives (polymer/VGCF/
PVDF, 10/80/10, w/w/w). These materials were mixed in a montar
for 10 min and more NMP was added to give a paste. The thus-
obtained paste was coated on a graphite foil (Alfa Aesar) by
applying the doctor blading method. Next, NMP was removed by
organometallic compound with a unique sandwich-like structure
that could be oxidized to ferrocenium cation (Fe(C
oxidation reaction was proved to be reversible, and the one-
electron exchange rate constant for Fe(C
þ
5
H
5
)
2
). This
þ
5
5
H )
2
/Fe(C
5
5
H )
2
redox
ꢀ1
ꢀ1
system could reach up to 7 ꢁ 10 cm s [46]. Combined with
other distinguished features such as excellent stability and solvent-
independent redox property, ferrocene is therefore turned into the
most well-known standard reference applied in cyclic voltammetry
measurements [47]. In recent years, several attempts on ferrocene-
based organic molecules for active cathode materials have been
reported. However, most known systems suffer from low molar
masses and therefore unstable electrodes [48e52]. In search of
efficient organic cathode materials, poly(fluorenylethynylene) de-
rivatives with a high degree of polymerization are large organic
molecules which will not dissolve in commonly used electrolytes,
and thus provide a direct and valid way to get stable electrodes.
ꢂ
heating the electrodes at 40 C under high vacuum for 24 h.
2.4. Preparation of coin cells
A solution of the polymer and PVDF in NMP (10 mg/mL) was
added to VGCF as conducting additive (polymer/VGCF/PVDF, 10/80/
10, w/w/w). More NMP was added and the materials were mixed in
a mortar for 10 min. The paste was coated on an aluminum foil
(thickness: 0.015 mm, MTI Corporation) by applying the doctor
ꢂ
blading method. Next, the NMP was removed at 40 C under high
vacuum for 24 h. After drying, the amount of active material on the
electrode was determined on the basis of the weight of the elec-
trodes. The coin cells (type 2032) were manufactured under an
argon atmosphere. Suitable, round composite electrodes (15 mm
diameter) were cut with a MTI Corporation Precision Disc Cutter T-
0.6. This electrode employed as cathode was placed into the bottom
cell case and separated from the lithium anode by a porous poly-
propylene membrane (celgard, MTI Corporation). On top of the
lithium anode (lithium foil, SigmaeAldrich), a stainless steal space
(diameter: 15.5 mm, thickness: 0.3 mm, MTI Corporation) and a
stainless steal wave spring (diameter: 14.5 mm, height 1.2 mm)
were placed. The cell was filled with electrolyte (propylene car-
bonate, 0.5 M lithium perchlorate) and the top cell case was placed
onto the electrode. The cell was sealed with an electric crimper
machine (MTI Corporation MSK-100D). Electrochemical measure-
ments were performed after an equilibration time of 24 h. All ex-
periments were performed at room temperature.
Meanwhile, owing to the overlapping
double- and single-bonds, -electrons in conjugated polymers can
p-orbitals of alternating
p
be easily moved from one bond to the other, which is also beneficial
to the desired fast kinetics for electrode materials [53]. Herein, two
new conjugated poly(fluorenylethynylene) type polymers bearing
ferrocene moieties PFFAL1 and PFFAL2 were designed and syn-
thesized. Their structural determinations, optical properties, ther-
mal characteristics and redox behaviors were systematically
investigated. Moreover, after the fabrication of a composite elec-
trode consisting of the synthesized polymers, vapor-grown carbon
nanofibers (VGCF) and poly(vinylidenefluoride) (PVDF), their
chargeedischarge performances as cathode active materials for
rechargeable lithium batteries were studied.
2
. Experimental section
2.1. Materials
2.5. Synthesis of PFFAL1
All the reactions were carried out under a nitrogen atmosphere.
All materials for the chemical synthesis were purchased from Sig-
maeAldrich, J&K or Acros Organics and used without further pu-
rification. Commercially available reagents were used as received
unless otherwise noted. The solvents used in the polymerization
reactions were dried and distilled appropriately prior to use. FFA
To a mixture of FFA (56 mg, 0.136 mmol) and L1 (46 mg,
0.136 mmol) in a mixed solution of triethylamine (10 mL) and dry
THF (10 mL) under nitrogen were added copper iodide (2 mg,
0.0068 mmol) and tetrakis(triphenylphosphine)palladium (8 mg,
0.0068 mmol). The solution was stirred for 30 min at room tem-
perature and was then heated to reflux overnight. After completion
of the reaction, the solvent was removed under reduced pressure.
The residue was chromatographed over a silica gel column by
eluting with dichloromethane and tetrahydrofuran to give a red
solid, which was precipitated in hexane to afford the desired com-
[54,55] and L2 [56] were synthesized according to the methods as
described in the literature.
2.2. General procedures
1H NMR and 13C NMR spectra were measured in deuterated sol-
pound. Anal. Calc. for C32
3.88. H NMR (tetrahydrofuran-d , 400 MHz): 8.48 ~ 8.45 (m, 1H,
H
18SFe: C, 78.37; H, 3.70. Found: C, 78.65; H,
1
vents on a Bruker AV 400 MHz FT-NMR spectrometer, where
chemical shifts ( in ppm) were quoted with tetramethylsilane as the
8
d
d
fluorenyl H), 8.19 ~ 8.12 (m, 1H, fluorenyl H), 7.90 ~ 7.76 (m, 3H,
fluorenyl H), 7.57 ~ 7.56 (m, 2H, fluorenyl H and vinyl H), 7.49 ~ 7.32
(m,1H, thiophenyl H), 7.29 ~ 7.24 (m,1H, thiophenyl H), 4.85 (br, 2H,
1
13
internal standard for H and C nuclei. Size exclusion chromatog-
raphy (SEC) was analyzed by using Agilent 1050 HPLC system with
visible wavelength and fluorescent detector. Fourier transform
infrared (FTIR) spectra were recorded on a Perkin Elmer Paragon
ꢀ
1
Fc), 4.64 (br, 2H, Fc), 4.31 ~ 4.30 (m, 5H, Fc). IR (KBr pellet, cm ):
3086 (CeH stretching, Fc), 2190 (C^C stretching), 1622 (C]C
stretching, Ar),1596 (C]C stretching, Ar),1456 (C]C stretching, Ar),
1411 (C]C stretching, Fc), 1103 (CeC stretching, Fc), 1000 (CeC
stretching, Fc), 815 (CeH bending, Fc), 488 (C-Fc).
1000 PC spectrometer. UV-Vis absorption spectra were obtained on a
HP-8453 diode array spectrophotometer. TGA measurements were
performed with a PerkineElmer TGA6 thermal analyzer.
2.3. Electrochemical analysis
2.6. Synthesis of PFFAL2
A three electrode set-up was used (WE: glassy carbon disk, RE:
PFFAL2 was synthesized similar to PFFAL1 except that L2
(68 mg, 0.136 mmol) was used instead of L1. Anal. Calc. for
AgNO
3
/Ag in THF 0.1 M n-Bu
4
NPF
6
, CE: Pt wire) for cyclic voltam-
þ
metry. The redox couple of ferrocenium/ferrocene (Fc /Fc) was
utilized as the internal standard. All electrolytes were degassed
with dry argon and all measurements were performed under an
argon atmosphere. Electrodes were prepared by adding a solution
46
C H28NFe: C, 84.92; H, 4.34; N, 2.15. Found: C, 85.10; H, 4.56; N,
1
8
2.24. H NMR (tetrahydrofuran-d , 400 MHz): d 8.47 ~ 8.45 (m, 1H,
fluorenyl H), 8.20 ~ 8.07 (m, 1H), 7.86 ~ 7.76 (m, 4H), 7.62 ~ 7.56 (m,
2H, Ar and vinyl CH), 7.53 ~ 7.47 (m, 2H), 7,46 ~ 7.38 (m, 2H),
Please cite this article in press as: Xiang J, et al., Synthesis, characterization and chargeedischarge studies of ferrocene-containing
poly(fluorenylethynylene) derivatives as organic cathode materials, Polymer (2015), http://dx.doi.org/10.1016/j.polymer.2015.01.054

J. Xiang et al. / Polymer xxx (2015) 1e7
3
ꢀ
1
7
2
.35 ~ 7.27 (m, 2H), 7.18 ~ 7.01 (m, 5H), 6.92 ~ 6.88 (m, 1H), 4.83 (br,
2197 cm . The transmission peaks at 3032, 1413, 1106, 1001, 817
ꢀ1
H, Fc), 4.63 ~ 4.59 (m, 2H, Fc), 4.31 ~ 4.28 (m, 5H, Fc). IR (KBr pellet,
and 500 cm are ascribed to ferrocene. These FTIR results could
further confirm the completeness of the coupling polymerization.
The molar masses of PFFAL1 and PFFAL2 were estimated via
SEC (Table 1). Notably, PFFAL1 with the thiophene moiety gets the
highest molar mass and the largest polydispersity index among the
obtained polymers.
ꢀ
1
cm ): 3032 (CeH stretching, Fc), 2197 (C^C stretching), 1624 (C]
C stretching, Ar), 1588 (C]C stretching, Ar), 1503 (C]C stretching,
Ar),1484 (C]C stretching, Ar),1413 (C]C stretching, Fc),1106 (CeC
stretching, Fc), 1001 (CeC stretching, Fc), 817 (CeH bending, Fc),
5
00 (CeFe).
3
. Results and discussion
3.2. Photophysical properties
3
.1. Synthesis and characterization
The optical features of PFFAL1 and PFFAL2 were investigated by
UVeVis spectroscopy (Fig. 2). It can be seen that PFFAL1 shows five
absorption peaks at 239, 265, 288, 389 and 512 nm. In comparison
with the related monomers, the absorption signals at 239 and
288 nm are likely to result from the electronic transitions of the
thiophene moiety, and the peaks at 265, 389 and 512 nm possibly
correspond to the fluorene unit of PFFAL1. The conjugated polymer
PFFAL2 exhibits five absorption peaks at 239, 267, 292, 381 and
505 nm. The absorptions at 239 and 292 nm correspond to the
electron transitions of the triphenylamine moiety, while the signals
at 267, 381 and 505 nm are assigned to the ferrocene unit. These
results indicate that PFFAL2 contains both ferrocene and triphe-
nylamine moieties. The absorption peaks of all conjugated poly-
mers are obviously red-shifted, which is attributed to the extension
Two comonomers were utilized for polymerization to enable on
one hand the straightforward synthesis of a polymer with only one
redox center (2,5-diiodothiophene L1) and on the other hand an
advanced system with higher capacity (4,4 -diiodotriphenylamine
L2). L2 was synthesized through direct iodination of triphenyl-
amine under an acidic condition by carefully controlling the
3
equivalent amounts of KIO and KI as well as the reaction time [56].
In the presence of triethylamine, tetrakis(triphenylphosphine)
palladium and copper(I) iodide, polymers PFFAL1 and PFFAL2 were
synthesized via a Sonogashira cross-coupling polycondensation of
ferrocene-based fluorene derivative FFA and diiodo-substituted
monomers L1 and L2, respectively (Scheme 1).
As the conjugated polymers PFFAL1 and PFFAL2 are almost
insoluble in commonly used chlorinated organic solvents, deuter-
ated tetrahydrofuran (THF-d
Figs. S1eS3). All signals in the H NMR spectra are consistent with
their structures. Protons at
¼ 4.20 ~ 4.87 ppm are assigned to the
ferrocene unit, while peaks at
0
of the p-conjugated systems after the polymerization. As a result,
the transport of the active charged species along the main chain of
the conjugated polymers is improved, which is very important for
high-performance rechargeable lithium-ion batteries [51].
8
) was employed for NMR analysis (SI,
1
d
d
¼ 6.50 ~ 8.80 ppm are attributed to
3.3. Thermal characteristics
the aromatic rings and the vinyl protons. The near completeness of
the polymerizaion could be confirmed because of the disappear-
As a high thermal stability is crucial for lithium-ion batteries
from the viewpoint of safety, the thermogravimetric analyses of the
ferrocene-containing conjugated polymers PFFAL1 and PFFAL2
were performed (SI, Fig. S4), and the results show that these pre-
pared polymers are thermally stable up to 350 C without
decomposition.
ance of the terminal acetylenic protons at
FFA. Compared with the corresponding signals in monomers, all
protons in polymers are downfield shifted, which is possibly caused
d
¼ 3.53 and 3.61 ppm in
ꢂ
by the increased deshieding effect of the enlarged conjugated
1
systems. Meanwhile, the peaks of the polymers shown in the
NMR spectra are broadened as expected.
H
The comparative FTIR spectra of the synthetic poly(-
fluorenylethynylene)s and the corresponding monomer FFA were
analyzed (Fig. 1). As for PFFAL1, the alkyne C^C vibration is shifted
3.4. Electrochemical performances
The electrochemical stability of the two ferrocene-containing
polymers was studied initially in solution. Fig. 3 displays the cy-
clic voltammograms of PFFAL1 and PFFAL2 in THF solution. The
cyclic voltammogram of PFFAL1 exhibits only the oxidation of
ꢀ1
ꢀ1
from 2099 cm to 2191 cm , and the peaks at 3086, 1411, 1103,
ꢀ
1
1
000, 815 and 488 cm are the characteristic signals of ferrocene.
The alkyne C^C vibration signal of PFFAL2 is also shifted to
Scheme 1. Schematic representation of the synthesis of PFFAL1 and PFFAL2.
Please cite this article in press as: Xiang J, et al., Synthesis, characterization and chargeedischarge studies of ferrocene-containing
poly(fluorenylethynylene) derivatives as organic cathode materials, Polymer (2015), http://dx.doi.org/10.1016/j.polymer.2015.01.054

4
J. Xiang et al. / Polymer xxx (2015) 1e7
ferrocene at the half-wave potential of (Epa þ Epc)/2 ¼ 0.097 V,
while PFFAL2 displays two quasi-reversible oxidations: the ferro-
cene moiety is oxidized at 0.101 V and the triarylamine at 0.573 V
þ
vs. Fc /Fc.
To further prove the stability of these polymers as cathode
materials in Li-organic batteries, electrodes were built. Due to the
low intrinsic conductivity of the polymeric backbone, the electro-
chemical behavior of the polymer was investigated in a composite
electrode. PFFAL1-based composite electrodes were measured in
4
0.1 M LiBF in propylene carbonate solution (Fig. 4). As can be seen,
owing to the presence of ferrocene moiety, cyclic voltammograms
reveal reversible reactions. The potential separation between the
oxidation and reduction peak is about 350 mV, which is obviously
higher than that in solution and indicates the limited charge
transfer within the electrode. The approximately symmetrical
peaks indicate high coulomb efficiency for the chargeedischarge
processes and the electrochemical stability of the electroactive
polymers is examined by CV measurements over 20 cycles. On
repeated cycling, there is no significant change in the oxidation and
reduction peak potentials or currents. Notably, PFFAL1-composed
electrode exhibits excellent reversible redox performance in
different electrolytes (SI, Fig. S5). The composite electrodes of the
triarylamine-containing polymer PFFAL2 possess similar stabil-
ities, when the cycling is stopped at a potential of 0.5 V vs. Ag/
3
AgNO (Fig. 4). However, when cycling is extended to 1.2 V, the
triphenylamine moiety is oxidized which leads to a positive charge
at the free para-position of one phenyl ring attached to the triar-
ylamine and enables oxidative coupling. This leads, according to the
literature, to a shift and decrease of the triarylamine signal [57].
These observations demonstrate that these synthetic conjugated
poly(fluorenylethynylene) type polymers could be further investi-
gated as cathode-active materials for rechargeable lithium
batteries.
3.5. Coin cell studies
The chargeedischarge behavior of the prepared polymers
PFFAL1 and PFFAL2 were further examined as cathode materials
for lithium coin cells. For that reason, coin cells were prepared
under inert atmosphere using a composite electrode (polymer/
VGCF/PVDF, 10/80/10, w/w/w) as cathode and a lithium foil as
Fig. 1. Comparative IR spectra of PFFAL1 (top) and PFFAL2 (bottom).
Table 1
anode material. A solution of 0.5 M LiClO
served as the electrolyte.
4
in propylene carbonate
Molar masses of PFFAL1 and PFFAL2, determined by SEC with poly(styrene) cali-
bration and THF as eluent.
To prove the homogeneity of the composite electrodes, SEM
images were taken (Fig. 5). Electrodes of both polymers show a
loose, homogenous and porous morphology, which may provide
sufficient surfaces for realizing efficient redox reactions of the
polymeric electrodes.
ꢀ1
Polymer
Mn (g molꢀ1)
Mw (g mol
)
PDI (Mw/Mn)
4
4
PFFAL1
PFFAL2
2.3 ꢁ 10
3.0 ꢁ 10
1.3
1.1
4
4
1.4 ꢁ 10
1.5 ꢁ 10
Fig. 2. Comparative absorption spectra of PFFAL1 (left) and PFFAL2 (right) in THF solution.
Please cite this article in press as: Xiang J, et al., Synthesis, characterization and chargeedischarge studies of ferrocene-containing
poly(fluorenylethynylene) derivatives as organic cathode materials, Polymer (2015), http://dx.doi.org/10.1016/j.polymer.2015.01.054

J. Xiang et al. / Polymer xxx (2015) 1e7
5
theoretical capacity) after 100 cycles. At 10 C the initial capacity
ꢀ1
notably decreases to 39 mAh g (71% of theoretical capacity), but
still the electrode displays stable charge/discharge curves, resulting
ꢀ
1
in a discharge capacity of the 100th cycle of 36 mAh g (66% of
theoretical capacity). However, at such high currents, the mass
transport is too slow, leading to polarized electrodes and conse-
quently to a large gap between charge and discharge curves (Fig. 6).
The chargeedischarge behavior of PFFAL2 is much more com-
plex due to the incorporated triphenylamine building block. The
initial charging curve at 1 C (Fig. 6, black line) displays one plateau
at about 3.5 V owing to the oxidation of the ferrocene moiety, while
at about 3.8 V the oxidation of the triphenylamine building block is
visible as another broad plateau. The initial charge and discharge
ꢀ1
capacity of PFFAL2 is 100.7 and 88.5 mAh g , respectively, which
ꢀ
1
slightly exceeds the theoretical capacity (82.3 mAh g ) based on a
two-electron reaction. Both the low columbic efficiency as well as
the increased capacity of the first charge cycle, can be explained by
the irreversible side reaction of the oxidative coupling of the tri-
phenylamine moiety, as mentioned before [57]. During cycling, the
coulomb efficiency increases to 95% (20th), and finally 98% (75th).
Due to this cross-linking effect, the stability of the electrode in-
Fig. 3. Cyclic voltammograms of PFFAL1 (black line) and PFFAL2 (red line) in THF with
.1 M tetrabutylammonium perchlorate as the supporting electrolyte at a scan rate of
0
ꢀ
1
100 mV s . (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
ꢀ1
creases, resulting in a specific capacity of 73 mAh g
(89% of
As shown in Fig. 6, PFFAL1 exhibits at 1 C charging rate an initial
theoretical capacity) after 100 cycles. Comparable to PFFAL1, the
composite electrode of PFFAL2 displays at 5 C a similar initial
ꢀ
1
discharge capacity of 52 mAh g with a stable voltage plateau at
about 3.50 V, which corresponds to 95% of the theoretical capacity
of this polymer. After 100 cycles, still a discharge capacity of
ꢀ
1
charge capacity as at 1 C (99.5 mAh g ), however, also here the
initial coulomb efficiency is very low at 82%. After 20 cycles, it in-
creases to 93%, and finally reaches 99% (50th). Again, after the first
20 cycles, the electrode is stabilized, leading to a specific discharge
ꢀ
1
4
8 mAh g
(88% of theoretical capacity) could be obtained,
underlining the stability of this composite electrode. Even at 5 C
ꢀ
1
charging rate, the discharge capacity of the first cycle is nearly
capacity of 54.5 mAh g (66% of theoretical capacity) after 100
cycles. Finally, the coin cells were charged at 10 C. As also noticed
for the electrodes of polymer PFFAL2, 10 C leads to a notable
ꢀ
1
identical with the one obtained at 1 C (52 mAh g , 95% of theo-
retical capacity), indicating the beneficial charge transport/mobility
properties of such conjugated ferrocene-containing polymers. Also
ꢀ
1
decrease of the initial charge capacity (76.4 mAh g , 93% of
theoretical capacity). Due to the higher charging speed, the
ꢀ
1
here the electrode is quite stable with 46 mAh g
(84% of
Fig. 4. Cyclic voltammograms of a polymer-composite electrode (polymer/VGCF/PVDF, 10/80/10, w/w/w) at a scan rate of 5 mV sꢀ1 in 0.1 M LiBF
PFFAL1 (left), PFFAL2 (right).
in propylene carbonate, 20 cycles;
4
Fig. 5. SEM images of PFFAL1- (left) and PFFAL2-based (right) electrode.
Please cite this article in press as: Xiang J, et al., Synthesis, characterization and chargeedischarge studies of ferrocene-containing
poly(fluorenylethynylene) derivatives as organic cathode materials, Polymer (2015), http://dx.doi.org/10.1016/j.polymer.2015.01.054

6
J. Xiang et al. / Polymer xxx (2015) 1e7
Fig. 6. 1st and 100th charging/discharging capacities of coin cells with PFFAL1 (left) and PFFAL2 (right) based cathode under varying charging rates.
stabilization of the electrode, that is most properly caused by cross-
4. Conclusion
linking of the triphenylamine moieties, is finished not before the
4
5
0th charge-discharge cycle (Fig. 7). After 100 cycles, still
3.9 mAh g (65% of theoretical capacity) could be discharged.
Two new conjugated poly(fluorenylethynylene) type polymers
PFFAL1 and PFFAL2 have been successfully synthesized via Sono-
gashira cross-coupling polycondensation for charging-discharging
studies as cathode materials for rechargeable lithium batteries.
The ferrocene-based moiety, the conjugated redox-active poly
(fluorenylethynylene) type skeleton and the homogeneous elec-
trode surface work together and facilitate the specific capacity, rate
capability and cycle endurance of the coin cells. Especially, PFFAL1-
composed electrode retains over 90% of the initial capacity after
ꢀ1
This excellent cycle performance of the polymer-based coin cells
may result from a combination of different favorable factors such as
the stable electrochemical property of the ferrocene moiety, the
poor solubility of conjugated poly(fluorenylethynylene) type
structure and the homogeneous morphology of the composed
electrode.
100 cycles at 10 C and PFFAL2-based cathode exhibited high utili-
zation of the active polymer with a stable discharge capacity of
ꢀ1
7
3 mAh g at 1 C. These remarkable results demonstrate the po-
tential of these synthetic polymers as organic cathode materials for
rechargeable lithium batteries.
Acknowledgments
We are grateful to the financial support from the National Nat-
ural Science Foundation of China (project number 51373145), Hong
Kong Research Grants Council (HKBU203313), Areas of Excellence
Scheme, University Grants Committee of HKSAR (project No. AoE/P-
03/08), Hong Kong Baptist University (FRG2/12-13/083) and the
Science, Technology and Innovation Committee of Shenzhen Mu-
nicipality (JCYJ20120829154440583). We also thank the Bundes-
ministerium für Bildung und Forschung, the European Social Fund
(
ESF), the Thüringer Aufbaubank (TAB 2011FGR0088), and the
Thuringian Ministry of Economy, Employment and Technology
TMWAT), for financial support.
(
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2015.01.054.
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Please cite this article in press as: Xiang J, et al., Synthesis, characterization and chargeedischarge studies of ferrocene-containing
poly(fluorenylethynylene) derivatives as organic cathode materials, Polymer (2015), http://dx.doi.org/10.1016/j.polymer.2015.01.054