Synthesis and charge-discharge properties of a ferrocene-containing polytriphenylamine derivative as the cathode of a lithium ion battery

Article abstract of DOI:10.1039/c2jm34752k

In this work, a novel linear polytriphenylamine derivative was successfully synthesized by the introduction of ferrocene as a terminating group to the triphenylamine moieties. The morphology, structure and charge-discharge performance of the prepared polymer as the cathode were characterized by fourier transform infrared spectroscopy (FTIR), ultraviolet visible spectroscopy (UV-vis), scanning electron microscopy (SEM), cyclic voltammograms (CV) and galvanostatic charge-discharge testing. The results showed that the introduction of ferrocene to polytriphenylamine obviously improved the specific capacity and rate capability of the resulting PTPAFc cathodes in lithium ion batteries. Under our experimental conditions, the PTPAFc-based electrodes exhibited an initial discharge capacity of up to 100.2 mA h g^-1 at 20 mA g ^-1 between 2.5 and 4.2 V, while PTPAn-based electrodes only presented 70.3 mA h g^-1, comparatively. Also, the PTPAFc cathode specially retained over 89.7% of the initial capacity with a ten times increase of the current from 50 to 500 mA g^-1. These improved electrochemical performances were ascribed to the introduction of ferrocene as the termination couple, which makes the PTPAFc main chain as a chainlike molecule structure and benefits from charge carrier transportation in the molecular polymer.

Full text of DOI:10.1039/c2jm34752k

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PAPER
Synthesis and charge–discharge properties of a ferrocene-containing
polytriphenylamine derivative as the cathode of a lithium ion battery
*
Chang Su, Yinpeng Ye, Lihuan Xu and Cheng Zhang
Received 19th July 2012, Accepted 7th September 2012
DOI: 10.1039/c2jm34752k
In this work, a novel linear polytriphenylamine derivative was successfully synthesized by the
introduction of ferrocene as a terminating group to the triphenylamine moieties. The morphology,
structure and charge–discharge performance of the prepared polymer as the cathode were characterized
by fourier transform infrared spectroscopy (FTIR), ultraviolet visible spectroscopy (UV-vis), scanning
electron microscopy (SEM), cyclic voltammograms (CV) and galvanostatic charge–discharge testing.
The results showed that the introduction of ferrocene to polytriphenylamine obviously improved the
specific capacity and rate capability of the resulting PTPAFc cathodes in lithium ion batteries. Under
our experimental conditions, the PTPAFc-based electrodes exhibited an initial discharge capacity of up
to 100.2 mA h gꢀ1 at 20 mA gꢀ1 between 2.5 and 4.2 V, while PTPAn-based electrodes only presented
70.3 mA h gꢀ1, comparatively. Also, the PTPAFc cathode specially retained over 89.7% of the initial
capacity with a ten times increase of the current from 50 to 500 mA gꢀ1. These improved
electrochemical performances were ascribed to the introduction of ferrocene as the termination couple,
which makes the PTPAFc main chain as a chainlike molecule structure and benefits from charge carrier
transportation in the molecular polymer.
performances of organic-based cathodes of lithium ion batteries
1. Introduction
improve constantly.
Li-ion batteries as a promising power source have received
widespread attention, owing to their convenient and good
charge–discharge capacity.1–4 However, current Li-ion battery
technologies are mostly based on transition-metal cathodes (such
as LiCoO2, LiMnO2 and V2O5), which have serious cost and
resource limitations that hinder their large scale application for
upcoming portable electronic devices.5–8 A variety of efforts have
been undertaken to improve the battery materials, such as using
nanosized and nanostructured materials, doping transition-
metal cathodes, optimizing the electrode components and
developing novel cathodic materials, etc. Among all of the
attempts, the application of organic electronics materials as
cathode materials in Li-ion batteries has been explored because
of easy preparation and high energy density. So far several types
of organic positive-electrode materials have been proposed,
mainly including electroactive conducting polymers,8,9 organo-
sulfur polymers,10 nitroxide radical tetramethylpiperidine-N-oxyl
(TEMPO)-based polymers,11,12 pendant-type polymer based on
ferrocene and carbazole,13,14 aromatic carbonyl derivatives and
quinine-based materials,15 etc. All of these make the
Among the above organic positive-electrode materials,
the polymers with organic redox radicals as the stable backbone
or pendant groups have attracted much interest of
researchers.11,12,16 Among the radicals polymers, polytriphenyl-
amine (PTPAn) and related derivatives are well known as p-type
conductors and have been widely applied as organic electrolu-
minescence (EL) materials and photo-conduction materials.17
Recently, the triphenylamine-based radical polymer material has
also been explored in the energy storage field, such as lithium ion
batteries16 and super capacitors,18 and exhibits excellent elec-
trochemical property. As reported,16 PTPAn, which is composed
of a p-conjugated triphenylamine substructure, showed good
electrochemical performance and well-defined voltage plateaus
(ꢁ3.6 V) as the cathode of lithium ion batteries. However, the
steric torsion of neighbouring molecules in PTPAn molecular
chains limits effective electron transportation during the redox
process, resulting in poor electron/ion conductivity in the poly-
mer. In addition, the serious crosslinking structure of PTPAn,
due to three polymerizable active sorts of triphenylamine unit,
will make an aggregation of PTPAn particles and this hinders the
electrolyte to infiltrate to the surface of PTPAn. As a result, the
active parts of PTPAn will not be fully utilized, and the high cell
performance cannot be fully reached. As a solution, producing a
linear structure of PTPAn will be an effective way to relieve steric
torsion in PTPAn molecular chains and decrease the crosslinking
density of PTPAn, which, as a result, will promote the electrolyte
State Key Laboratory Breeding Base of Green Chemistry Synthesis
Technology, International Science and Technology Cooperation Base of
Energy Materials and Application, College of Chemical Engineering and
Materials Science, Zhejiang University of Technology, Hangzhou,
310014, P. R. China. E-mail: czhang@zjut.edu.cn; Tel: +86-571-88320253
22658 | J. Mater. Chem., 2012, 22, 22658–22662
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to approach the redox active center of PTPAn and improve the
cell performance of PTPAn-based cathodes.
chromatography as an orange residue with 62.9% yield. 1H
NMR (CDCl3, 400 MHz) d ppmꢀ1: 4.26 (s, 5H, ferrocene), 4.50
(s, 2H, ferrocene), 4.81 (s, 2H, ferrocene), 7.02 (t, 2H), 7.11 (d,
8H), 7.26 (t, 4H), 8.38 (s, 1H). MS (EI): m/z ¼ 456.2 (M+).
Ferrocene as an organometallic compound has been exten-
sively investigated as a standard electrode in redox potential
measurements because of its air stability, excellent electro-
chemical response, redox property, and so on.19–22 It has been
reported13 that polymers carrying ferrocene moieties such as
poly(vinlferrncene), poly(ethynylferrocene) and poly(ferrocene)
have been applied to cathode-active materials in organic lithium
ion batteries, which exhibit promising battery properties, such as
being quickly chargeable, having a high power density and the
stable voltage plateaus (ꢁ3.4 V) as PTPAn. In light of these
considerations, we utilized the ferrocene as the terminating group
to terminate one of three polymerizable active sorts of triphe-
nylamine precursors, then a linear polymer based on the
prepared ferrocene-contained polytriphrylamine monomer was
synthesized by a chemical polymerization method. Thanks to the
un-crosslinking structure of the novel linear polymer and the
introduction of the electroactive ferrocene as the active termi-
nating group, the prepared ferrocene-contained polytriphryl-
amine polymer as the cathode of organic lithium ion battery
exhibited an improved discharge specific capability and superior
rate capability by using the lithium ion half-cell method,
compared to that of the conventional PTPAn, which was
promising for advanced cathode materials.
Polymer. The polymer of TPAFc was prepared by chemical
oxidative polymerization of TPAFc in chloroform (20 ml) using
ferric chloride as the oxidant. The solution was stirred over night
at room temperature under N2. After completion of the solution
polymerization reaction, the reaction mixture was poured into
methanol to deposit the polymer product, which was then filtered
and washed with methanol several times. Finally, the polymer
product was filtered and dried in vacuum at 50 ꢂC for 12 h. The
chemical structure of the polymer is shown in Scheme 1.
Material characterization
FT-IR spectra were carried out on a Nicolet 6700 spectrometer
(Thermo Fisher Nicolet, USA) with KBr pellets. UV-vis spectra
were recorded on a Varian Cary 100 UV-vis spectrophotometer
(Varian, USA), and the measurement was carried out in DMF
solution. 1H NMR spectra of the compounds were recorded on a
Bruker AVANCE III 500 MHz spectrometer (Bruker, Switzer-
land) using CDCl3. The mass spectrometry (MS) analysis was
measured on a GCT Premier spectrometer (Waters, USA) using
the electron impact (EI+) mass spectra technique. Scanning
electron microscopy (SEM) measurements were taken using a
Hitachi S-4800 scanning electron microscope (Hitachi, Japan).
2. Experimental
Material synthesis
Electrochemical measurements
Monomer. 4-nitro-N,N-diphenylaniline (1) was synthesized as
following: N-phenylaniline (1.69 g) and sodium hydride (0.9 g)
were firstly dissolved in 30 mL of N,N-dimethylacetamide
(DMAC). Then, 4-fluoronitrobenzene (2.11 g) was added into
the above solution. The reaction was carried out under a nitrogen
atmosphere for 4 h. The resulting solution was extracted with
chloroform and dried by anhydrous MgSO4. The obtained
4-nitro-N,N-diphenylaniline (1) was isolated by column chro-
matography with 81% yield as a yellow residue. 1H NMR
(CDCl3, 400 MHz) d ppmꢀ1: 6.94 (d, 2H), 7.20 (d, 4H), 7.25
(d, 2H), 7.39 (t, 4H), 8.05(d, 2H).
N0,N00-diphenylbenzene-1,4-diamine (2) was synthesized as
following: reduced iron powder (0.8 g) and ammonium chloride
(1.4 g) were dispersed in a mixed solution of ethanol (50 mL) and
water (50 mL). To the above dispersed solution, 1.5 g of 4-nitro-
N,N-diphenylaniline was then added. The reaction mixture was
refluxed for 2 h under a nitrogen atmosphere. The reaction
mixture was then cooled, and the excess iron powder was filtered
off. The solution was extracted with chloroform and dried over
anhydrous MgSO4. N0,N00-diphenylbenzene-1,4-diamine (2) was
isolated by column chromatography in 89% yield as a purple
residue. 1H NMR (CDCl3, 400 MHz) d ppmꢀ1: 4.10 (s, 2H,
broad, NH2), 6.72 (d, 2H), 6.94 (t, 2H), 6.99 (d, 2H), 7.05 (d, 4H),
7.22 (t, 4H).
For cathode characterization, CR2032 coin-type cell was used
and assembled in an argon-filled glove box. The cathode elec-
trodes were prepared by coating a mixture containing 50% as
prepared polymers, 40% acetylene black, 10% PVDF binder on
circular Al current collector foils, followed by drying at 60 ꢂC for
10 h. After that, the cells were assembled with lithium foil as the
anode, the prepared electrodes as the cathode and 1 M LiPF6
dissolved in ethylenecarbonate (EC) and dimethylcarbonate
(DMC) (EC/DMC ¼ 1 : 1 v/v) as the electrolyte. The charge–
discharge measurements were carried out on a LAND CT2001A
in the voltage range of 2.5–4.2 V versus Li/Li+, using a constant
current density at room temperature. The cyclic voltammograms
(CV) tests was performed with a CHI 660C electrochemical
(E)-N-(4-(Diphenylamino)phenyl)formimidoyl
ferrocene
(TPAFc) was synthesized as follows: N0,N00-diphenylbenzene-
1,4-diamine (0.69 g) and ferrocenecarboxaldehyde (0.53 g) were
dissolved in 30 mL of ethanol (100 mL) and refluxed for 3 h.
After evaporation of ethanol, TPAFc was isolated by column
Scheme 1 The synthesis route to TPAFc.
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working station in 0.1 M [Bu4N]ClO4–CH2Cl2 versus Ag/AgCl at
a scan rate of 25 mV sꢀ1
.
3. Results and discussion
3.1. Material characterization
FTIR. Fig. 1 shows the FTIR spectra of the as-prepared
PTPAn and PTPAFc. As shown in Fig. 1, the main characteristic
peaks of the triphenylamine moieties can be found in the two
samples for the C]C ring stretching at 1590 cmꢀ1, C–C
stretching at 1494 cmꢀ1 and C–H bending at 1590 and 1494
cmꢀ1. Also, the absorption peaks at 1276 and 820 cmꢀ1 corre-
spond to the C–N stretching from the tertiary amine and a C–H
out-of-plane vibration from 1,4-disubstituted benzene rings.16
Besides, some new bands can be observed clearly in the spectrum
of PTPAFc. The absorption peaks at 1631 cmꢀ1 can be ascribed
to the N]C (Schiff base structure) stretching. The absorption
peaks at 1103, 827 and 474 cmꢀ1 are due to the mono-substituted
ferrocene.23,24 These indicate that the PTPAFc polymer contains
both triphenylamine and ferrocene moieties.
Fig. 2 UV-vis spectra of PTPAn and PTPAFc (10ꢀ3 g Lꢀ1) in DMF.
steric torsion of the main chains by the big triphenylamine units.
However, as the ferrocene couple is introduced to the triphe-
nylamine, the morphology of PTPAFc is very different from that
of PTPAn. As shown in Fig. 3(b), PTPAFc shows a loose
assembled particle structure with a size of about 100 nm. This
feature may provide sufficient surface area and ionic channels for
redox reactions of a polymeric electrode.
UV-vis. The UV-vis spectra (normalized absorbance) are
further utilized to explore the characteristics of the TPAFc
monomer and PTPAFc, comparatively. As shown in Fig. 2, the
TPAFc monomer exhibits two pairs of absorption peaks, at
293 nm and 362 nm, respectively. The maximum at 362 nm is
assigned to the p–p* electron transition from the triphenylamine
units of TPAFc,16,23 while the absorption peaks at ꢁ293 nm
correspond to the p–p* electron transition of the ferrocene
moieties. After the chemical polymerization, it can be seen
that the corresponding absorption peaks for the obtained
PTPAFc are obviously red-shifted to 305 nm and about 450 nm,
respectively, in comparison with that of the TPAFc monomer,
which is due to the expansion of the p-conjugated systems by the
produced conjugated polymer system. As a result, the charge
carrier transportation alone in the polymer main chain of PTPAn
is improved, which is crucial for high-performance lithium
batteries.
3.2. Electrochemical performance
Fig. 4 shows the cyclic voltammetry (CV) profiles of both PTPAn
and PTPAFc measured in 0.1 M [Bu4N]ClO4–CH2Cl2 solution.
As can be seen, the electrode of PTPAn shows a couple of anodic
and cathodic peaks corresponding to the charge–discharge
reaction of the radical redox couple of PTPAn at about 1.05 and
0.8 V respectively. The potential separation between the oxida-
tion and reduction peaks is about 0.25 V. The approximately
symmetrical peaks suggest a good insertion/extraction revers-
ibility of the produced PTPAn cathode active material.
Comparably, there is some difference between the CV curves of
PTPAn and PTPAFc. For PTPAFc, it exhibits one pair of well-
overlapped and broad redox peaks, which can be explained as the
organometallic ferrocene, the activity compound, possesses
similar charge–discharge voltage plateaus as that of PTPAn,
which may stack up to give broad redox peaks. Correspondingly,
the oxidation redox peaks of the PTPAFc shift to 1.03 and
1.16 V, respectively. The peak separation exhibited by the
PTPAFc electrode is about 0.13 V, which is about 0.12 V smaller
than that of the PTPAn electrode, suggesting the lower degree of
polarization of the PTPAFc material during the electrochemical
reaction. These results are in good agreement with the electro-
chemical cycling performance of the PTPAFc cathode as
described below.
SEM. Fig. 3 shows the morphology of the PTPAn and
PTPAFc. As can be seen, the PTPAn polymer has a dense
packing structure and the aggregation of PTPAn particles can be
noticed obviously. This structural feature may be caused by the
Fig. 1 FTIR spectrum of the PTPAn and PTPAFc sample.
22660 | J. Mater. Chem., 2012, 22, 22658–22662
Fig. 3 SEM images of powder samples (a) PTPAn and (b) PTPAFc.
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differences in the voltage plateau of PTPAFc and PTPAn. There
are two voltage plateaus observed during the charge process in
the voltage range of 3.5–4.2 V, while in the discharge process, the
two voltage plateaus become almost combined. This phenom-
enon can be illustrated by the existence of the electroactive
ferrocene couple, charge–discharge behaviours of which are not
in complete consistency with the triphenylamine units.
The cycling stability of the as-prepared polymers as cathode
materials of Li-ion batteries is also examined, as shown in Fig. 6.
It is found that both PTPAFc and PTPAn electrodes show a
similar cycling performance after 30 cycles, which is decided by
the nature of the organic-based material electrode. Moreover, the
cycling stability of PTPAFc is even poorer than that of PTPAn at
initial cycles, which is possibly due to the unstable nature of the
prepared linear polymer compared to the crosslinking PTPAn
during the charge–discharge process.
Fig. 4 Cyclic voltammograms (CV) of (a) PTPAn and (b) PTPAFc in
0.1 M [Bu4N]ClO4–CH2Cl2 versus Ag/AgCl at a scan rate of 25 mV sꢀ1
.
We further investigate the charge–discharge properties for the
PTPAFc and PTPAn electrodes at various rates and Fig. 7 shows
the rate and cycling performances of the polymer electrodes at
different current rates of 50, 100, 300 and 500 mA gꢀ1, respec-
tively, during the whole test of 50 cycles. Compared with parent
PTPAn, PTPAFc electrodes display an obviously improved rate
capability and the flat plateaus curve of charge–discharge. As
shown in Fig. 7(a), with an enhanced current rate from 50 to
500 mA gꢀ1, the corresponding reduction capacity for PTPAFc
is only about 10.3%. Comparatively, the discharge capacities of
PTPAn decreases seriously with an increase of current rate. The
improved rate capability can be ascribed to the serious molecular
torsion between neighbouring triphenylamines in the PTPAn
molecule, which is effectively relieved by the introduction of
ferrocene as the termination couple, which makes the PTPAFc
main chain as a chainlike molecular structure and benefits from
charge carrier transportation in the polymer molecule. In addi-
tion, the reduced crosslinking density of PTPAn, as well as a tiny
particle morphology, will be convenient for the Li+ ion insertion–
extractraction process during the charge–discharge process,
which is also responsible for the high rate capability.
3.3. Charge–discharge performance
The charge–discharge behaviours of the as-prepared polymers as
cathode materials of Li-ion batteries are examined, respectively.
The initial charge–discharge profiles of the polymers at 20 mA
gꢀ1 between 2.5 and 4.2 V in LiPF6 EC/DMC (v/v, 1 : 1) elec-
trolyte are shown in Fig. 5. As it can be seen in the figure, the
PTPAn shows an initial discharge capacity of 70.3 mA h gꢀ1 with
a typical voltage plateau in the voltage range of 3.5–4.1 V at the
initial cycle. However, under the same conditions, the PTPAFc
electrode exhibits an initial discharge capacity of up to 100.2 mA
h gꢀ1. The improvement of the PTPAFc electrode capacity
can be ascribed to higher theoretical capacities of PTPAFc
(120.5 mA.gꢀ1) than that of PTPAn (109.4 mA.gꢀ1), and the
produced linear structure of PTPAn by an introduction of
ferrocene as the terminating group to one of three polymerizable
active sorts of triphenylamine precursors, which will decrease the
crosslinking density of PTPAn and promote the electrolyte to
approach the redox active center of PTPAn, as a result,
improving the availability of the redox active material and the
cell specific capability of the PTPAn-based cathode. Further-
more, the tiny particle morphology of PTPAFc compared to that
of PTPAn further facilitates the diffusion of the electrolyte
solution to the active-polymer center and improves the PTPAFc
performance to some degree. Otherwise, there are some
Fig. 7(b) further presents the charge–discharge profiles of the
PTPAFc electrode at various current rates of 50, 100, 300 and
500 mA gꢀ1. Even at a rate as high as 500 mA gꢀ1, the PTPAFc
electrode still displays an obviously stable voltage plateau,
Fig. 5 Initial charge–discharge profiles of the polymer electrodes
material at a constant current of 20 mA gꢀ1 between 2.5 and 4.2 V in
LiPF6 EC/DMC (v/v, 1 : 1) electrolyte versus Li/Li+.
Fig. 6 Cycling stability of the polymer electrodes material at a constant
current of 20 mA gꢀ1 between 2.5 and 4.2 V in LiPF6 EC/DMC (v/v, 1 : 1)
electrolyte versus Li/Li+.
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PTPAFc cathode retained over 89.7% of the initial capacity with
a ten times increase in the current from 50 to 500 mA gꢀ1, which
was obviously improved compared to that of the PTPAn. The
improved performance of the PTPAFc-based composite cath-
odes made it a good candidate for the potential applications in
organic lithium ion batteries.
Acknowledgements
The authors gratefully thank the National Natural Science
Foundation of China (NSFC, no. 51003095, no. 51103132) and
the Research on Public Welfare Technology Application Projects
of Zhejiang Province, China (2010C31121) for financial support.
This work also was supported by the analysis and testing foun-
dation of Zhejiang University of Technology.
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4. Conclusions
A novel linear triphenylamine derivative (TPAFc) has been
successfully synthesized by the introduction of a ferrocene couple
and the polymers were then prepared by chemical oxidation
polymerization. Using a lithium ion half-cell method, we
comparably investigated the electrochemical properties for the
PTPAn and PTPAFc-based cathode. Both the introduction of
the electroactive ferrocene group and the relieving of the cross-
linking structure by the produced linear structure of the con-
ducting PTPAFc were attributed to the enhancement of the
capacity and rate capability of the electrode. Especially, the
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