Synthesis and properties of novel TEMPO-contained polypyrrole derivatives as the cathode material of organic radical battery

Article abstract of DOI:10.1016/j.electacta.2014.03.017

Two 2,2,6,6-tetramethylpiperidinyl-N-oxy (TEMPO) contained polypyrrole (PPy) derivatives with the different side-chain length were synthesized by esterification of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl free radial with pyrrole butyric acid and pyrrole caproic acid. Then the homopolymers of 4-(3-(Pyrrol-1-yl)butyryloxy)-2,2,6,6-tetramethylpiperidin-1-yloxy (PPy-B-TEMPO) and 4-(3-(Pyrrol-1-yl)hexanoyloxy)-2,2,6,6-tetramethylpiperidin-1-yloxy (PPy-C-TEMPO) were prepared by chemical oxidative polymerization. The structure, morphology, electrochemical properties of prepared polymers were characterized by fourier transform infrared spectroscopy (FTIR), ultraviolet visible spectroscopy (UV-vis), scanning electron microscopy (SEM), cyclic voltammograms (CV) and electrochemical impedance spectra (EIS), respectively. Also, the charge-discharge properties of the prepared polymers were studied by galvanostatic charge-discharge testing. The results demonstrated that the as-synthesized nitroxide radical polymers showed a reversible two-electron redox reaction process in the potential limits of 2.5-3.0 V and 3.4-3.8 V vs Li/Li⁺, respectively. Under our experimental conditions, PPy only presented the discharge capacity of 16.5 mAh·g^-1 at 20 mA·g^-1 between 2.5 and 4.2 V, while PPy-B-TEMPO with the short side-chain linked to TEMPO groups exhibited an initial discharge capacity of up to 86.5 mAh·g^-1 with two well-dened plateaus. Furthermore, the PPy-C-TEMPO with the longer linking side-chain even displayed a discharge capacity of 115 mAh·g^-1. These superior electrochemical performances were ascribed to the flexible linking side-chain and the introduction of stable conductive PPy main chain, which benefits the improvement of charge carrier transportation in the aggregated polymer bulk.

Full text of DOI:10.1016/j.electacta.2014.03.017

Electrochimica Acta 130 (2014) 148–155
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and properties of novel TEMPO-contained polypyrrole
derivatives as the cathode material of organic radical battery
Lihuan Xua,b, Fang Yanga, Chang Sua, Lvlv Jia, Cheng Zhanga,∗
a 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
b College of Chemical Engineering, Shenyang University of Chemical Technology
a r t i c l e i n f o
a b s t r a c t
Article history:
Two 2,2,6,6-tetramethylpiperidinyl-N-oxy (TEMPO) contained polypyrrole (PPy) derivatives with the dif-
ferent side-chain length were synthesized by esterification of 4-hydroxy-2,2,6,6-tetramethylpiperidine-
1-oxyl free radial with pyrrole butyric acid and pyrrole caproic acid. Then the homopolymers of
4-(3-(Pyrrol-1-yl)butyryloxy)-2,2,6,6-tetramethylpiperidin-1-yloxy (PPy-B-TEMPO) and 4-(3-(Pyrrol-
1-yl)hexanoyloxy)-2,2,6,6-tetramethylpiperidin-1-yloxy (PPy-C-TEMPO) were prepared by chemical
oxidative polymerization. The structure, morphology, electrochemical properties of prepared polymers
were characterized by fourier transform infrared spectroscopy (FTIR), ultraviolet visible spectroscopy
(UV-vis), scanning electron microscopy (SEM), cyclic voltammograms (CV) and electrochemical
impedance spectra (EIS), respectively. Also, the charge-discharge properties of the prepared polymers
were studied by galvanostatic charge-discharge testing. The results demonstrated that the as-synthesized
nitroxide radical polymers showed a reversible two-electron redox reaction process in the potential limits
of 2.5–3.0 V and 3.4–3.8 V vs Li/Li+, respectively. Under our experimental conditions, PPy only presented
the discharge capacity of 16.5 mAh·g−1 at 20 mA·g−1 between 2.5 and 4.2 V, while PPy-B-TEMPO with the
short side-chain linked to TEMPO groups exhibited an initial discharge capacity of up to 86.5 mAh·g−1 with
two well-defined plateaus. Furthermore, the PPy-C-TEMPO with the longer linking side-chain even dis-
played a discharge capacity of 115 mAh·g−1. These superior electrochemical performances were ascribed
to the flexible linking side-chain and the introduction of stable conductive PPy main chain, which benefits
the improvement of charge carrier transportation in the aggregated polymer bulk.
Received 19 January 2014
Received in revised form 6 March 2014
Accepted 7 March 2014
Available online 16 March 2014
Keywords:
Cathode materials
Nitroxide radical derivative
Polypyrrole
Electrochemical characteristic
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
upcoming electric vehicles and renewable power stations. [9–11]
As a result, to exploit novel materials for secondary batteries
Nowadays, high-performance lithium ion batteries have been
considered to be the most promising technology for satisfying
the demands of communication, portable electronic devices, elec-
tric vehicles (EVs) and hybrid EVs (HEVs). [1–5] Whereas, the
wide use of this technology is mainly restricted by the posi-
tive electrode, which usually shows a much lower capacity than
that of the negative one. The cathode materials generally used in
lithium ion batteries are inorganic materials (e.g., LiCoO2, LiMn2O4,
and LiFePO4), [6–8] which, however, suffer from some drawbacks
involving the limited theoretical capacities, the limited mineral
resources and the large energy consumption during synthesis.
In addition, the high energy demand and CO2 emissions during
the producing process also limit their large scale applications for
has been widely developed by many groups. Particularly, the
organic functional polymers have been investigated as potential
alternatives to inorganic-based electrode-active material, although
they have some well-known problems, such as relatively poor per-
formances in capacity and stability. So far, many organic materials
have been developed for lithium rechargeable batteries, including
organic conductive polymers, [12,13] organosulfur polymers, [14]
nitroxide radical tetramethylpiperidine-N-oxyl (TEMPO)-based
polymers, [15] pendant-type polymer based on ferrocene and car-
bazole, [16,17] aromatic carbonyl derivatives and quinine-based
materials, [18] etc.
Recently, organic radical species, particularly pendant nitrox-
ide radical polymers, have been explored as a cathode-active,
charge-storage material for secondary Li-ion batteries. A typ-
ical example of
tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), which con-
tains 2,2,6,6-tetramethylpiperidinyl-N-oxy (TEMPO) radical
a nitroxide radical polymer is poly(2,2,6,6-
∗
Corresponding author. Tel.: +86 571 88320253; fax: +86 571 88320253.
E-mail address: chengzhang@zjut.edu.cn (C. Zhang).
a
http://dx.doi.org/10.1016/j.electacta.2014.03.017
0013-4686/© 2014 Elsevier Ltd. All rights reserved.

L. Xu et al. / Electrochimica Acta 130 (2014) 148–155
149
Table 1
BET of powder samples (a) PPy, (b) PPy-B-TEMPO (c) PPy-C-TEMPO.
Sample
PPy
PPy-B-TEMPO
17.03
PPy-C-TEMPO
24.61
A (m2·g−1
)
9.49
moieties. [15] When PTMA is applied to the cathode in lithium ion
batteries, it exhibits the outstanding cathode performance: high
capacity, high charging and discharging rate performance, long
cycle life and processing compatibility. Following its exploration,
various polymers carrying nitroxide radicals was designed, in order
to increase the energy density of radical batteries and improve
the cell-performance of the nitroxide radical based cathode. Sev-
eral groups have pursued this challenging target and reported new
nitroxide bearing polymers. In their efforts, nitroxide radicals are
attached mostly to polymers with the aliphatic or the noncon-
ducting type backbone, such as polymethacrylate, polynorbornene,
celluse, poly(p-tert-butylaminoxy-styrene) and poly(vinyl ether),
et al. However, above nitroxide radical contained polymers are
derivatives of conventional plastics, which are insulator, result-
ing in the electron transfer among the polymer by way of hopping
between the pendent redox sites (short-range conductivity) which
are isolated in the polymer and the long-range conductivity limited.
[19] As a result, a very large amount of conducting agents (such
as carbon black, graphite, graphene and carbon fibers), even up to
60-80 wt% of the total electrode, is required in the fabrication of
the composite electrode, in order to improve the long-range con-
ductivity and the utilization rate of nitroxide radical contained the
electro-active polymer. This leads to the actual redox capacity of
the composite electrode to decrease seriously and the extensive
studies to use them as active materials for batteries to be impeded.
As the intrinsic conductivity for most of radical polymers are rather
low due to the insular backbone, so a combination of a stable radical
with a conductive polymer is regard as an effective way to produce
a new radical polymer with an improvement of electron migra-
tion. A few of TEMPO-carrying polyacetylene and polythiophene
have been synthesized and used as a cathode-active material in
a rechargeable battery. [20,21] Unfortunately, degradation of the
unstable conducting polymer backbone enforced by the presence
of the radical moiety is observed, which hinders the improvement
of the electrochemical properties of TEMPO-based active polymer,
as well the further exploration on the detail investigation for the
charge-discharge process or the influence of conductive backbone
on the electrochemical properties of TEMPO.
Polypyrrole (PPy) is one of the most popular conductive poly-
mers because of its mostly environmental stability among the
present conducting polymers and high conductivity at room tem-
perature, which has also been used as a cathode with high specific
energy (ranging from 80 to 390 Whkg−1) [7] and theoretical capac-
ity (∼400mAh·g−1). Recently, it has been reported that polypyrrole
derivatives with pending quinone groups have been proposed as
cathode-active materials in organic lithium ion batteries, and the
resulting polymer with high redox active groups provided a con-
ducting network for charge transport which decreased the need for
a conductive additive when designing electric energy storage mate-
rials based on sustainable and renewable components. [22–24]
However, the active pendent quinone groups usually exhibits a
lower redox potential of 2.0 V, [25] which limits its application as
the cathode material with high discharge voltage. Therefore, it is
a promising strategy to design the novel polypyrrole derivatives
pending with high redox potential, in order to explore the high
performance organic lithium batteries.
Scheme 1. The synthesis route to PPy-B-TEMPO.
aiming to prepare the structure stable and conductive radical poly-
mers. So, in this work, two novel TEMPO-containing polypyrrole
derivates with different lengths of the side-chain linked to the
TEMPO groups were synthesized by chemical oxidation polymer-
ization. And the structure and electrochemical properties of the
prepared polymer as cathode materials for Li-ion battery were
systematically investigated. Also the possible relationship of both
the introduction of stable conjugated PPy with the improved elec-
trochemical properties was discussed. Moreover, the effect of the
side-alkyl chain length on the cell performance was further inves-
tigated.
2. Experimental
2.1. Materials
4-Aminobutyric acid (98%), 6-Aminocaproic acid (98%), 2,5-
dimethoxytetrahydrofuran (98%), pyrrole were purchased from
Aladdin. 4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-Oxyl Free
Radial (99%) was purchased from Energy Chemical Reagent Co. All
other reagents were received as analytical grade and used without
further purification.
2.2. Synthesis of PPy-B-TEMPO
The synthesis of the polymer (PPy-B-TEMPO) is shown in
Scheme 1.
2.2.1. Synthesis of pyrrole butyric acid [26]
2,5-dimethoxytetrahydrofuran (6.61 g, 0.05 mol) and 4-Amino
butyric acid (4.12 g, 0.04 mol) were dissolved in a mixture of 30 ml
deionized water and 20 ml glacial acetic acid and heated reflux
(60 ◦C) for 1 h until the solution turned jacinth. The reaction mixture
was then cooled and extracted with CH2Cl2, and the organic frac-
tion was washed with saturated brine for three times. The organic
phase was separated, dried and evaporated to dryness. Flash chro-
matography of the residue (silica gel, petrol ether/ethyl acetate
20:1) afforded the title compound (4.34 g, 71.1% yield) as a white
liquid. 1H NMR (CDCl3, 500 MHz) d/ppm: 2.15 (t, J = 7.05 Hz, 2H),
2.39 (t, J = 7.3 Hz, 2H), 4.01 (t, J = 6.85 Hz, 2H), 6.24 (t, J = 2 Hz, 2H),
6.72 (t, J = 2.05 Hz, 2H), 11.42(s, 1H).
2.2.2. Synthesis of Py-B-TEMPO
4-Hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl free radial
(2.1 g, 0.012 mol) and pyrrole butyric acid (2.4 g, 0.015 mol) were
dissolved in 40 ml dichloromethane in the pre-dried three-necked
flask,
then
1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (2.76 g, 0.015 mol) as dehydrating agent and
4-dimethylaminopyridine (0.73 g, 0.006 mol) were added as acy-
lating catalyst, and stirring for 24 h at room temperature. The
reaction mixture was then separated by vacuum filtration. The
filtrate was washed with saturated brine for three times. Organic
phase was dried over sodium sulfate. The ester was purified by
column chromatography using silica gel and petroleum ether/ethyl
Table 1.
In light of these considerations, we anticipate to use PPy as
the main chain to prepare the TEMPO-containing PPy derivates,

150
L. Xu et al. / Electrochimica Acta 130 (2014) 148–155
acetate afforded the title compound as orange red flakes like crys-
tals. MS (EI): calculated for C17H27N2O3 m/z: 307.41, found m/z:
307.4.
2.2.3. Synthesis of pyrrole caproic acid
This
compound
was
synthesized
using
2,5-
dimethoxytetrahydrofuran and 6-Amino caproic acid with
a
similar synthesis process of pyrrole butyric acid. Yield 48.3%, white
liquid. 1H NMR (CDCl3, 500 MHz) d/ppm: 1.43 (m, 2H), 1.74 (m,
2H), 1.86 (m, 2H), 2.43 (t, J = 7.4 Hz, 2H), 3.95 (t, J = 7.1 Hz, 2H), 6.22
(t, J = 2.05 Hz, 2H), 6.72 (t, J = 2.05 Hz, 2H), 11.55(s, 1H).
2.2.4. Synthesis of Py-C-TEMPO
This compound was synthesized from 4-Hydroxy-2,2,6,6-
tetramethylpiperidine-1-oxyl free radial and pyrrole caproic acid
in a manner similar to that of Py-B-TEMPO. Yield 64.8%, crimson
liquid. As a result of the presence of free radicals, it was impossible
to measure the NMR spectra of the monomers (Py-B-TEMPO, Py-C-
TEMPO). The structures of the monomers were confirmed by Mass
spectrum. MS (EI): calculated for C19H31N2O3 m/z: 335.46, found
m/z: 335.2.
Fig. 1. FTIR spectrum of the (a) PPy, (b) PPy-B-TEMPO and (c) PPy-C-TEMPO samples.
were performed with a CHI 660 C electrochemical working station
in 0.1 M LiClO4/CH3CN versus Ag/AgCl at a scan rate of 5 mV·s−1
.
2.2.5. Chemical polymerization of PPy, PPy-B-TEMPO and
PPy-C-TEMPO [21]
3. Results and discussion
All of PPy, PPy-B-TEMPO and PPy-C-TEMPO were prepared by
the same methods. The samples of Py, Py-B-TEMPO or Py-C-TEMPO
(0.3 g) were firstly dissolved in CCl4 (20 ml), and then was added
into a 250 ml boiling flask-3-neck. To this solution, 10 ml of CH3NO2
containing 0.63 g of FeCl3 was added drop wise for about 0.5 h at
-10 ◦C. With increasing of density of FeCl3 in the reactor, the mix-
ture turned black gradually. After the addition was completed, the
mixture was stirring at 0 ◦C under a flux of dry nitrogen for 24 h. The
molar ratio of the oxidant to monomer was 4:1. After completion of
the reaction, 100 ml methanol was added to deposit the obtained
polymer, which was then filtered and washed with methanol and
water alternately for several times to remove the residual FeCl3,
until the filtrate becoming colorless. Finally, the black powder was
dried in vacuum at 60 ◦C for 24 h, and all the obtained polymers are
black powder.
3.1. Material characterization
Fig. 1 showed the FTIR spectra of the as-prepared PPy, PPy-B-
TEMPO and PPy-C-TEMPO. Compared with typical FTIR spectrum
of PPy, [27] the main characteristic peaks of PPy can be found in
all of the samples, involving the pyrrole ring fundamental vibra-
tion at 1545 cm−1 and 1459 cm−1, the C-H in-plane vibration at
1286 cm−1 and 1045 cm−1 and the C-N stretching vibration at 1161-
1172 cm−1. In Fig. 1 (b) and Fig. 1(c), the similar characteristic
bands of PPy are presented in PPy-B-TEMPO and PPy-C-TEMPO.
In addition, some new bands can be observed clearly in the spec-
trum of PPy-B-TEMPO and PPy-C-TEMPO, in which the absorption
peaks of 2860-2980 cm−1 were ascribed to -CH3 and -CH2 stretch-
ing vibration containing in TEMPO and alkyl chain, while the
absorption peaks of 1732 cm−1 is the stretching of C = O (ester
carbonyl). Specially, the absorption peak at 1365 cm−1, which is
attributed to nitroxyl radical groups in the polymers, [28] indi-
cates the 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yloxy group
has been successfully introduced into both pyrrole butyric acid
and pyrrole caproic acid and has not been destroyed during the
polymerization process.
The UV-vis spectra (normalized absorbance) were further mea-
sured in DMF (10−3g/L) to explore on characteristics of PPy, PTMA,
PPy-B-TEMPO and PPy-C-TEMPO. The curve (a) shows the UV-vis
spectra of PPy, in which the absorption peaks at ∼383 nm are cor-
responding to the ␲-␲* electron transition from the pyrrole units
of PPy backbone, while the curve (d) displays an absorption of
450-500 nm (n∼␲*), due to the pendants of the nitroxyl radical
along the polymer chain. [29] For the PPy-B-TEMPO and PPy-C-
TEMPO, the characteristic absorption peaks of both pyrrole units
and nitroxyl radical moieties coexist in the absorption spectra, in
which the broad absorption peaks at ∼341 nm and 371 nm are still
attributed to the ␲-␲* electron transition from the pyrrole units
of PPy-B-TEMPO and PPy-C-TEMPO, respectively, and the absorp-
tion peaks at ∼470 nm nm and ∼487 nm is due to the present of the
nitroxyl radical moieties. In comparison to PPy, the ␲-␲* electron
transition of pyrrole units for the TEMPO-contained PPy deriva-
tives has an obviously blue shift, and moreover, the blue-shift of
the characteristic absorption peaks is becoming larger with the
decrease of the length of the side-chain linked to the TEMPO groups.
The blue-shift indicates that the electron delocalization in the PPy
2.3. Characterization and electrochemical measurements
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 (Var-
ian, USA). 1H NMR spectra of the compounds were recorded on
a Bruker AVANCE III 500 MHz spectrometer (Bruker, Switzer-land)
using CDCl3. Scanning electron microscopy (SEM) measurements
were taken using a Hitachi S-4800 scanning electron microscope
(Hitachi, Japan). BET were carried out on a Surface Area and Porosity
Analyzer (micromeritics, ASAP2020).
For cathode characterization, CR2032 coin-type cell was used
and assembled in an argon-filled glove box. The cathode electrodes
were prepared by coating a mixture containing 50% the prepared
polymers, 40% acetylene black and 10% PVDF binder on Al current
collector foils, followed by dried at 60 ◦C for 10 h. After that, the
cells were assembled with lithium foil as the anode, the prepared
electrodes as cathode and 1 M LiPF6 dissolved in ethylene carbonate
(EC) and dimethyl carbonate (DMC) (EC/DMC = 1:1 v/v) as the elec-
trolyte. The charge-discharge measurements were carried out on a
LAND CT2001 A in the voltage range of 2.5-4.2 V versus Li/Li+, using
a constant current density at room temperature. Electrochemical
Impedance Spectroscopy (EIS) experiments were carried out at
open circuit voltage (OCV) of frequency ranges from 0.1Hz-1 MHz in
CHI 660 C electrochemical working station by the assembled stim-
ulant lithium ion half-cells. The cyclic voltammograms (CV) tests

L. Xu et al. / Electrochimica Acta 130 (2014) 148–155
151
backbone is weaken due to the introduction of the large steric
TEMPO pendant groups, which hinders polymerization process and
makes the neighboring pyrrole units in the resulting polymer be
seriously torsion. As a result, the electron delocalization along the
polymer backbone becomes less effective compared with PPy and
the blue-shift phenomenon occurs. For the TEMPO-contained PPy
derivatives with different the side-chain length, the shorten side-
chain length linked to the big TEMPO group tends to cause the even
more seriously torsion between the neighboring pyrrole units in the
polymer and to make the decrease of the conjugation degree along
the PPy chain compared to the longer side-chain one, resulting in
the UV-vis spectra of PPy-B-TEMPO exhibits the larger blue-shift
than that of PPy-C-TEMPO.
Oppositely, with increasing the length of alkyl side-chain,
the absorption peaks of nitroxyl radical moieties in the TEMPO-
contained PPy derivatives red-shift from 460 nm of PTMA to
470 nm (PPy-B-TEMPO) and 487 nm (PPy-C-TEMPO), respectively.
The obviously red-shift of absorption position might imply the site-
to-site electron hopping of the nitroxyl radical by self-exchange
reactions becoming easier with the introduction of the flexible
alkyl side-chain between the polymer main chain and the radi-
cal pendant groups (TEMPO), which is benefit to the charge carrier
transportation type changing from the short-range conduction to
the long-range conduction, which is crucial for high-performance
lithium battery.
Fig. 2. UV-vis spectra of (a) PPy, (b) PPy-B-TEMPO (c) PPy-C-TEMPO and (d) PTMA
(10−3 g/L) in DMF.
method, and the results of PPy, PPy-B-TEMPO and PPy-C-TEMPO
were found to be 9.49, 17.03 and 24.61 m2·g−1, respectively. The
electrode material with a loose assemble particle and large special
surface area is in favor of the utilization of electrode-active mate-
rial as well the improvement of the electrochemical properties. All
of those are very important to prepare a good cathode material for
Li-ion batteries.
Fig. 2.
The morphologies of the as-obtained polymers have also been
investigated by SEM, and the morphology changes for PPy, PPy-B-
TEMPO and PPy-C-TEMPO were shown in Fig. 3. As can be seen in
Fig. 3 (a), PPy has a dense packing plate-like structure and the aggre-
gation of PPy particles can be noticed obviously. Comparatively, as
the TEMPO-based moieties are introduced into PPy, the morpholo-
gies of PPy-B-TEMPO and PPy-C-TEMPO change obviously from that
of PPy. Fig. 3 (c) shows a low-magnification SEM image of PPy-C-
TEMPO, it can be clearly seen that the sample has a looser stacking
structure compared with PPy-B-TEMPO. Specially, as shown in the
magnified SEM image (Fig. 3 (d)), the obtained PPy-C-TEMPO sam-
ple exhibits a regularly spherical structure assembled by many
small particles of about 100 nm with well dispersion. We also mea-
sured the specific surface areas of three polymers by BET testing
3.2. Electrochemical performance
Fig. 4 showed the cyclic voltammetry (CV) profile of the PTMA,
PPy-B-TEMPO and PPy-C-TEMPO measured in 0.1 M lithium per-
chlorate/acetonitrile solution. As depicted in the figure, the PTMA
displays a pair of redox potentials at 0.90 and 0.70 V vs Ag/AgCl,
which is assigned to the oxidation of the nitroxide radical to
the corresponding oxoammonium cation or the p-type doping of
the material. [30] The difference of separation of the anodic and
cathodic peak potentials is estimated as 0.20 V. Comparatively, the
Fig. 3. SEM images of powder samples (a) PPy, (b) PPy-B-TEMPO (c) PPy-C-TEMPO (d) partially enlarged SEM image in (c).

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L. Xu et al. / Electrochimica Acta 130 (2014) 148–155
Fig. 5. Initial charge/discharge profiles of (a) PPy, (b) PTMA, (c) PPy-B-TEMPO (d)
PPy-C-TEMPO electrodes material at a constant current of 20 mA·g−1 between 2.5
and 4.2 V in LiPF6 EC/DMC (v/v, 1:1) electrolyte versus Li/Li+.
and 2.7 V, respectively. It is reasonable to assume that the redox
characteristics of TEMPO are exhibited by a one by one two-stage
redox reaction from anions 1 via nitroxyl radicals 2 to cations 3 (as
shown in the inset in the Fig. 5), [31] which is consistent with the
CV measurements which shows two pairs of reduction and oxida-
tion characteristic peaks. Compared to PTMA, we can also observe
from the plots, that the initial discharge capacity of PPy-B-TEMPO
exhibits an improved capacity of 86.5 mAh·g−1 at the initial cycle,
while the capacity of the PPy-C-TEMPO is further increased to
115 mAh·g−1 on the same conditions which is higher than that
of the PPy-B-TEMPO. The improved capacity can be attributed to
the delicate molecular structures design (e.g., the introduction
of the conjugated conductive polymer backbone and the spacial
arrangement of the TEMPO radicals), which improves the electron
migration and the convenient radical pendant interaction and/or
reaction in the TEMPO-based polymer bulk, leading to the full
utilization of the electroactive material and the increased capacity
of the cathode during the charge/discharge process. Specially, for
PPy-C-TEMPO, a suitable side-chain length linked to the TEMPO
group can availably alleviate torsion between the neighboring
pyrrole units in PPy backbone and enhances the interaction among
the pendant radical groups (as proved by UV-vis spectra), resulting
to an increased capacity compared to PPy-B-TEMPO. In addition,
the loose aggregation structure of the PPy-C-TEMPO particles
[13] is in favor of the contact of electrode-active material and
electrolyte, which can also attribute to the utilization of electrode-
active material and the increase of specific capacity. These results
demonstrate that the PPy-C-TEMPO has an excellent performance
with the help of both the conductive PPy with ␲-conjugated
structure and the longer alkyl side-chain. It is worth mentioning
that the coulombic efficiencies of the three samples are different
from 100%, 85% to 80% for PPy, PPy-B-TEMPO and PPy-C-TEMPO,
respectively. For PPy-B-TEMPO and PPy-C-TEMPO, the coulombic
efficiencies are relatively low, which may be due to a volumetric
change of the polymer during the initial charging process which
results in degradation of the electrode contact and reduction of
coulomb efficiency. [32] It should also be noted, however, that the
coulombic efficiency of PPy-B-TEMPO and PPy-C-TEMPO improves
gradually with the increase of cycling times, rising up to 92%
and 85% after 50 times, respectively, which indicates that the
structures of the two electrodes are tending towards stability.
The cycling stability of the as-prepared polymers cathode was
also examined and the results were shown in Fig. 6(a) and (b). It is
found that both PPy-B-TEMPO and PPy-C-TEMPO electrodes show
Fig. 4. Cyclic voltammograms (CV) of (a) PTMA, (b) PPy-B-TEMPO (c) PPy-C-TEMPO
in 0.1 M LiClO4/CH3CN versus Ag/AgCl at the scan rate of 5 mV·s−1
.
CV feature of the PPy-B-TEMPO shows one pair of oxidation and
reduction peaks appearing at 0.97 and 0.83 V, and the PPy-C-TEMPO
displays one pair of redox peaks at 0.91 and 0.82 V, which can all be
assigned to the p-type doping of nitroxide radical (from the nitroxyl
radicals to the oxoammonium cations). The relatively narrow peak-
to-peak separations for both PPy-B-TEMPO (about 0.14 V) and
PPy-C-TEMPO (about 0.09 V) imply the good oxidation/reduction
reversibility of the produced active materials as cathode. It’s worth
noting that another couple of redox peaks appears at near -0.1 V
for PPy-B-TEMPO and PPy-C-TEMPO, which can be attributed to
the redox couple of the n-type doping of nitroxide radical (from
the aminoxy anion to the nitroxyl radicals). However, the redox
peak for PTMA at the same position is not observed. We assume
that the stable conjugated PPy being introduced as the main chain
of TEMPO-contained polymer works as an electro-catalysis, which
promotes the redox reaction between the radical and aminoxy
anion (n-type doped state), leading to the appearance of the sec-
ond pair of redox peaks. Furthermore, the increased flexibility of the
pendant nitroxide radical (TEMPO) by the longer side-chain results
in the radical site-to-site electron hopping by self-exchange reac-
tions in the polymer more easier, which is also considerate to as a
factor to facilitate the change of the electrochemical characteristics.
3.3. Charge-discharge performance
The charge-discharge behaviors of the as-prepared polymers
as cathode of lithium batteries have been systematically inves-
tigated by simulated lithium ion half-cell method. The initial
charge-discharge profiles of the polymers at 20 mA·g−1 between
2.5 and 4.2 V were shown in Fig. 5. As we can see from Fig. 5
(a), the PPy shows an initial discharge capacity of 16.9 mAh·g−1
at initial cycle, and has no obvious discharge platform at all,
which is corresponding to the conventional reported results.[13]
Comparatively, the charge-discharge curves of PPy-B-TEMPO and
PPy-C-TEMPO present two obvious voltage platforms at about 3.6 V

L. Xu et al. / Electrochimica Acta 130 (2014) 148–155
153
Fig. 7. EIS of PPy, PPy-B-TEMPO and PPy-C-TEMPO sample in Li/Electrolyte/Sample
−1/2
configuration [Inset: Plot between Zre and ␻
for PPy, PPy-B-TEMPO and PPy-C-
TEMPO].
electrode. Specially, the Rct of PPy-C-TEMPO is only 547.3 ꢀ, which
is the lowest. This may be ascribed to both the introduction of PPy
backbone in the TEMPO-based polymer and the suitable side-chain
length linked to TEMPO pendant groups, which enhances the radi-
cal pendant interaction and/or reaction and the long-range electron
migration in the aggregated TEMPO-based polymer bulk, leading
to the smaller charge-transfer reaction resistance (Rct). In addi-
tion, the smaller particles and the loose structure of the polymer,
providing a higher specific surface area, make the electrolyte pen-
etration easily during the redox reaction, which do also facilitate to
the decrease of the charge-transfer reaction resistance. In the radi-
cal polymer layer, an electron is hopping or a charge is transferred
(or propagated) via the redox reaction of the neighboring radical
moieties on the polymer (or the geared cycles of the radicals’ redox
reactions). The redox process is accompanied by incorporation and
transfer of a counter ion (here an anion: PF6−) to compensate the
charge, which would be the rate-determining step of the charge
propagation. [36] The diffusion co-efficient of the electron- or the
charge-propagation in the radical polymer layer could also be cal-
culated by following relation (Equation 1) at low frequency [37,38]:
Fig. 6. (a) Cycling stability of the polymer electrodes material at a constant current
of 20 mA·g−1 between 2.5 and 4.2 V in LiPF6 EC/DMC (v/v, 1:1) electrolyte versus
Li/Li+, (b) discharge profiles of PPy, PPy-B-TEMPO and PPy-C-TEMPO.
a similar cycling performance after 50 cycles, which is decided by
the nature of organic-based material electrode. [33] The discharge
capacity of PPy-B-TEMPO decreases from its initial 86.5 mAh·g−1
to 48 mAh·g−1 after 50 cycles, with about 45% loss of capacity.
Comparatively, the discharge capacity of PPy-C-TEMPO drops from
its initial 115 mAh·g−1 to 86 mAh·g−1 after 50 cycles, with about
25% loss of capacity. Above results demonstrate that the struc-
ture of the PPy-C-TEMPO electrode is relatively stable. Fig. 6(b)
further presents the charge-discharge profile of the as-prepared
polymers after 1th, 25th and 50th cycles. Even at 50th cycles, two
electrodes still display obviously stable voltage plateau, implying
that PPy-based electrodes have a lower polarization during the
charge-discharge process.
Fig. 7 showed electrochemical impedance spectra of pristine
PPy, PPy-B-TEMPO and PPy-C-TEMPO. The impedance spectra can
be explained on the basis of an equivalent circuit with elec-
trolyte resistance (Re), charge transfer resistance (Rct), double
layer capacitance and passivation film capacitance (CPE) and War-
burg Impedance (Zw) [34,35]. In these impedance plots, the initial
intercept of the spectrum at the Zre axis in high frequency corre-
sponds to the resistance of the electrolyte (Re). The semicircles at
low impedance frequencies represent the charge-transfer reaction
resistance (Rct), meanwhile the straight lines at low frequen-
cies indicates the Warburg impedance (Zw), which displays the
diffusion-controlled process. As shown in the Figure, the Re is
almost same for the cells with different cathode material, indicating
that no significant change in ionic conductivity of the electrolyte
or mobility of ions with the different cathode-based cell during
the cycling process. However, Rct varied with different cathode:
2510 ꢀ for PPy-based electrode and 1531.9 ꢀ for PPy-B-TEMPO
D = R2T2/(2A2n4F4C2ꢁ2)
Zim ∝ ıω1/2
(1)
(2)
where, R is the universal gas constant (8.314 J·mol−1k−1), T is the
temperature in Kelvin (K), A is the surface area of the cathode
(cm2), n is the number of electron per molecule during oxidation or
reduction, F is the Faraday constant (96485 C·mol−1), C is the con-
centration of electrolyte in the cathode material (mol·cm−3) and ␴
is the Warburg factor (ꢀ s1/2); which can be determined by plot-
−1/2
ting Zre vs. ␻
in the lower frequency region (Equation 2).−T1h/2e
inset of Fig. 7 shows the relation between Zre and frequency (␻
)
for PPy, PPy-B-TEMPO and PPy-C-TEMPO samples respectively. The
slope of the curve determines ␴ and the intercept of the real com-
ponent of impedance (Zre) will produce Rct value. The diffusion
coefficient (D) in PPy, PPy-B-TEMPO and PPy-C-TEMPO are found
to be 1.34 × 10−15 cm2·s−1, 2.89 ×10−15 cm2·s−1 and 5.71 × 10−15
cm2·s−1 respectively. This suggests that the radical polymer of PPy-
C-TEMPO in the electrode ensures fast counter ion mobility during
the charge-transfer process which is according with the observa-
tion of both UV-vis spectra and CV testing.

154
L. Xu et al. / Electrochimica Acta 130 (2014) 148–155
was supported by the analysis and testing foundation of Zhejiang
University of Technology.
References
[1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. Van Schalkwijk, Nanostruc-
tured materials for advanced energy conversion and storage devices, Nat.
Mater. 4 (2005) 366–377.
[2] J. Maier, Nanoionics: ion transport and electrochemical storage in confined
systems, Nat. Mater. 4 (2005) 805–815.
[3] Y.G. Guo, J.S. Hu, L.J. Wan, Nanostructured Materials for Electrochemical Energy
Conversion and Storage Devices, Adv. Mater. 20 (2008) 2878–2887.
[4] P.G. Bruce, B. Scrosati, J.M. Tarascon, Nanomaterials for Rechargeable Lithium
Batteries, Angew. Chem. Int. Ed. 47 (2008) 2930–2946.
[5] C.X. Zu, H. Li, Thermodynamic analysis on energy densities of batteries, Energy
Environ. Sci. 4 (2011) 2614–2624.
[6] H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Research on Advanced Materials for
Li-ion Batteries, Adv. Mater. 21 (2009) 4593–4607.
[7] X.L. Wu, L.Y. Jiang, F.F. Cao, Y.G. Guo, L.J. Wan, LiFePO4 Nanoparticles Embedded
in a Nanoporous Carbon Matrix: Superior Cathode Material for Electrochemical
Energy-Storage Devices, Adv. Mater. 21 (2009) 2710–2714.
Fig. 8. Cross-sectional profile of the composite electrode, showing the four signifi-
cant charge transfer processes of the radical battery.
[8] J.W. Fergus, Recent developments in cathode materials for lithium ion batteries,
J.Power Sources. 195 (2010) 939–954.
[9] J.W. Park, J.Y. Eom, H.S. Kwon, Fabrication of Sn–C composite electrodes by elec-
trodeposition and their cycle performance for Li-ion batteries, Electrochemistry
Communications. 11 (2009) 596–598.
[10] L. Taberna, S. Mitra, P. Poizot, P. Simon, J.M. Tarascon, High rate capabilities
Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery appli-
cations, Nature Materials. 5 (2006) 567–573.
[11] H.Y. Chen, M. Armand, G. Demailly, F. Dolhem, P. Poizot, J.M. Tarascon, From
Biomass to a Renewable LiXC6O6 Organic Electrode for Sustainable Li-Ion Bat-
teries, ChemSusChem. 1 (2008) 348–355.
[12] P. Novák, K. Müller, K.S.V. Santhanam, O. Haas, Electrochemically Active Poly-
mers for Rechargeable Batteries, Chemical Reviews. 97 (1997) 207–282.
[13] K.S. Park, S.B. Schougaard, J.B. Goodenough, Synthesis of Nanowire and Hollow
LiFePO4 Cathodes for High-Performance Lithium Batteries, Advanced Materials.
19 (2007) 848–851.
[14] Y. Su, Y. Niu, Y. Xiao, M. Xiao, Z. Liang, K. Gong, Novel conducting polymer
poly[bis(phenylamino)disulfide]: Synthesis, characterization, and properties,
Journal of Polymer Science Part A: Polymer Chemistry 42 (2004) 2329–2339.
[15] K. Nakahara, S. Iwasa, M. Satoh, Y. Morioka, J. Iriyama, M. Suguro, E. Hasegawa,
Rechargeable batteries with organic radical cathodes, Chemical Physics Letters.
359 (2002) 351–354.
[16] K. Tamura, N. Akutagawa, M. Satoh, J. Wada, T. Masuda, Charge/Discharge
Properties of Organometallic Batteries Fabricated with Ferrocene–Containing
Polymers, Macromolecular Rapid Communications. 29 (2008) 1944–1949.
[17] M. Yao, H. Senoh, T. Sakai, T. Kiyobayashi, Redox active poly(N-vinylcarbazole)
for use in rechargeable lithium batteries, Journal of Power Sources 202 (2012)
364–368.
3.4. Possible electrode reaction mechanism
A schematic representation of the cross-sectional profile of the
composite electrode was illustrated in Fig. 8. We presume that
the charge-discharge process of the composite electrode is deter-
mined by four processes: [39] (1) the radical site-to-site electron
hopping by self-exchange reaction in the polymer layer, (2) the
electron transfer of the conductive polymer main (PPy), (3) the elec-
tric conduction between PPy and acetylene black, (4) the interfacial
electron transfer from the acetylene black to the current collector.
The conductive polymer main chain (PPy) works as the electric con-
duction path to assist the repeatable oxidation/reduction process
throughout the aggregated radical polymer bulks, while the flexi-
ble alkyl side-chain can work as cantilever to increase the mobility
of TEMPO pendant, which leads to the enhanced radical pendant
interaction and/or reaction and the improved cell performances.
4. Conclusion
[18] M. Yao, H. Senoh, S. Yamazaki, Z. Siroma, T. Sakai, K. Yasuda, High-capacity
organic positive-electrode material based on a benzoquinone derivative for
use in rechargeable lithium batteries, Journal of Power Sources. 195 (2010)
8336–8340.
[19] K. Oyaizu, T. Suga, K. Yoshimura, H. Nishide, Synthesis and Characterization
of Radical-Bearing Polyethers as an Electrode-Active Material for Organic Sec-
ondary Batteries, Macromolecular. 41 (2008) 6646–6652.
[20] Qu. Jinqing, K. Toru, S. Masaharu, J. Wada, J. Igarashi, M. Kenji, M. Toshio,
Synthesis and Charge/Discharge Properties of Polyacetylenes Carrying 2,2,6,6-
Tetramethyl-1-piperidinoxy Radicals, Chem. Eur. J. 13 (2007) 7965–7973.
[21] M. Aydın, B. Esat, C¸ . Kılıc¸ , M.E. Köse, A. Ata, F. Yılmaz, A polythiophene deriva-
tive bearing TEMPO as a cathode material for rechargeable batteries, European
Polymer Journal. 47 (2011) 2283–2294.
[22] C. Karlsson, E. Ja¨mstorp, M. Strømme, M. Sjo¨din, Computational Electrochem-
istry Study of 16 Isoindole-4,7-diones as Candidates for Organic Cathode
Materials, J. Phys. Chem. C. 116 (2012) 3793–3801.
Two TEMPO-based PPy derivatives with the different side-chain
length linked to TEMPO groups had been synthesized. A series
of the electro-active homopolymers, such as PPy, PTMA, PPy-B-
TEMPO and PPy-C-TEMPO, were obtained by chemical oxidation
polymerization. Compared to PTMA, PPy-B-TEMPO and PPy-C-
TEMPO both exhibited two obviously voltage plateaus during the
charge/discharge process, which is ascribed to the electro-catalysis
of the introduced PPy backbone. And PPy-B-TEMPO presented the
discharge capacity of 86.5 mAh·g−1, while the charge-discharge
properties of PPy-C-TEMPO obtained by the same process displayed
an obviously improved discharge capacity of 115 mAh·g−1 as well as
two more obviously lengthen voltage plateau. This may be ascribed
to both the introduction of PPy backbone in the TEMPO-based poly-
mer and the suitable side-chain length linked to TEMPO pendant
groups, which enhances the radical pendant interaction and/or
reaction and the long-range electron migration in the aggregated
TEMPO-based polymer bulk, leading to the smaller charge-transfer
reaction resistance. In addition, the smaller particles and the loose
structure of the polymer are also benefit to the enhancement of the
electrochemical properties.
[23] C. Karlsson, A. Gogoll, M. Strømme, M. Sjo¨din, Investigation of the Redox Chem-
istry of Isoindole-4,7-diones, J. Phys. Chem. C. 117 (2013) 894–901.
[24] C. Karlsson, H. Huang, M. Strømme, A. Gogoll, M. Sjo¨din, Polymer-Pendant Inter-
actions in Poly(pyrrol-3-ylhydroquinone): A Solution for the Use of Conducting
Polymers at Stable Conditions, J. Phys. Chem. C. 117 (2013) 23558–23567.
[25] L. Kai, Zh. Jianming, Zh. Guiming, Y. Yong, Poly(2,5-dihydroxy-1,4-
benzoquinonyl sulfide) (PDBS) as
a cathode material for lithium ion
batteries, J. Mater. Chem. 21 (2011) 4125–4131.
[26] S. Thomas, M. Eva, P. Fritz, Electrochemical glucose sensors on permselective
non-conducting substituted pyrrole polymers, Electrochemical Glucose Sen-
sors on Permselective Non-conducting Pyrrole Polymers, Sensors and Actuators
B. 4 (1991) 273.
[27] Liang Xiao, Liu Yu, A nano-structured and highly ordered polypyrrole-sulfur
cathode for lithium–sulfur batteries, Journal of Power Sources. 196 (2011)
6951–6955.
Acknowledgements
[28] Wei Guo, Yin Ya-xia, Superior radical polymer cathode material with a two-
electron process redox reaction promoted by graphene, Energy Environ. Sci. 5
(2012) 5221–5225.
[29] T. Kurosaki, O. Takahashi, M. Okawara, Polymers having stable radi-
cals. II. Synthesis of nitroxyl polymers from 4-methacryloyl derivatives of
The authors gratefully thank the National Natural Science Foun-
dation of China (NSFC, No.51003095, No.51103132) and Research
on Public Welfare Technology Application Projects of Zhejiang
Province, China (2010C31121) for financial support. This work also

L. Xu et al. / Electrochimica Acta 130 (2014) 148–155
155
1-hydroxy-2,2,6,6-tetramethylpiperidine, Journal of polymer science: Polymer
Chemistry Edition. 12 (1974) 1407–1420.
[30] E. Pamela, H. John, H. Julrgen, E. Pamela, H. John, H. Ju1rgen, Relationship
between Chain Length, Disorder, and Resistivity in Polypyrrole Films, J, Phys.
Chem. B. 109 (2005) 19191–19199.
[34] S. Rodrigues, N. Munichandraiah, A.K. Shukla, Electroluminescence in conju-
gated polymers, Journal of Solid State Electrochemistry. 3 (1999) 397–405.
[35] F. Nobili, F. Croce, B. Scrosati, R. Marassi, Electronic and Electrochemical Proper-
ties of LixNi1-yCoyO2 Cathodes Studied by Impedance Spectroscopy, Chemistry
of Materials. 13 (2001) 1642–1646.
[31] J. Qu, R. Morita, M. Satoh, J. Wada, F. Terakura, K. Mizoguchi, N. Ogata, T. Masuda,
Synthesis and Properties of DNA Complexes Containing 2,2,6,6-Tetramethyl-
1-piperidinoxy (TEMPO) Moieties as Organic Radical Battery Materials, Chem.
Eur. J. 14 (2008) 3250–3259.
[32] Ch. Su, L. Wang, Li. Xu, Ch. Zhang, Synthesis of a novel ferrocene-contained
polypyrrole derivative and its performance as a cathode material for Li-ion
batteries, Electrochimica Acta 104 (2013) 302–307.
[33] W. Guo, S. Jing, Li. Yanhua, Wan. Li-Jun, Yu. Guoguo, Nitroxide rad-
ical polymer/graphene nanocomposite as an improved cathode mate-
rial for rechargeable lithium batteries, Electrochimica Acta. 72 (2012)
81–86.
[36] H. Nishide, K. Koshika, K. Oyaizu, Environmentally benign batteries based on
organic radical polymers, Pure Appl. Chem. 81 (2009) 1961.
[37] X. Qin, X. Wang, J. Xie, L. Wen, Hierarchically porous and conductive LiFePO4
bulk electrode: binder-free and ultrahigh volumetric capacity Li-ion cathode,
Journal of Materials Chemistry. 21 (2011) 12444–12448.
[38] Y. Zhou, J. Wang, Y. Hu, R. O’Hayre, Z. Shao, A porous LiFePO4 and carbon
nanotube composite, Chemical Communications 46 (2010) 7151–7153.
[39] Y. Sunao, H. Isozumi, K. Masanori, Y. Hisatomo, Y. Ando, K. Oyaizu, H. Nishide,
Improving Charge/Discharge Properties of Radical Polymer Electrodes Influ-
enced Strongly by Current Collector/Carbon Fiber Interface, J. Phys. Chem. B.
114 (2010) 8335–8340.