A hyperbranched conjugated Schiff base polymer network: A potential negative electrode for flexible thin film batteries

Article abstract of DOI:10.1039/c5cc09662f

A hyperbranched conjugated Schiff base polymer network was synthesized by condensation between 4,4′,4′′-nitrilotribenzaldehyde and p-phenylenediamine. The material exhibits excellent rate capability and long cycle life for lithium storage. Coupled with lo

Full text of DOI:10.1039/c5cc09662f

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A hyperbranched conjugated Schiff base polymer
network: a potential negative electrode for
flexible thin film batteries†
Cite this:DOI: 10.1039/c5cc09662f
Received 23rd November 2015,
Accepted 13th January 2016
Yubao Sun,* Yahui Sun, Qiyun Pan, Gai Li, Bo Han, Danli Zeng, Yunfeng Zhang and
Hansong Cheng*
DOI: 10.1039/c5cc09662f
www.rsc.org/chemcomm
A hyperbranched conjugated Schiff base polymer network was with a substantially lower electrode potential is the key to the
synthesized by condensation between 4,40,400-nitrilotribenzaldehyde development of high-voltage fully polymeric secondary batteries.
and p-phenylenediamine. The material exhibits excellent rate capability
Recently, a Schiff base (–CHQN–) was reported to exhibit a
and long cycle life for lithium storage. Coupled with lower electrode relatively low electrode potential from 0 to 1.5 V (vs. Na+/Na)
potential (0.7 V vs. Li+/Li), it may be well suited for fully flexible thin film for sodium storage.22 The work inspired us to speculate that
polymeric batteries as the negative electrode.
the Schiff base might be an appropriate organo functional
group to be used in a redox polymeric anode. With the idea
The emerging market of wearable electronics presents an increasing in mind, we designed and synthesized a hyperbranched con-
demand for flexible thin film batteries, which has stimulated jugated Schiff base polymer through polycondensation between
renewed interest in new electrode and electrolyte materials and 4,40,400-nitrilotribenzaldehyde and p-phenylenediamine. The
new battery structures to fulfil the requirement for higher energy reaction scheme is shown in Scheme 1. The p conjugation in
density, longer device lifespan and faster charging rate. The the precursor molecules is substantially enhanced upon poly-
required flexibility represents a significant technical challenge merization, which subsequently gives rise to strong structural
because the physical connection among the inorganic particles, stability. Detailed information on the synthesis and characterization
carbon supports and current collectors in the electrode coating can be found in the ESI.†
deteriorates upon bending and thus degrades the battery per-
The synthesized hyperbranched conjugated Schiff base polymer
formance. Redox polymeric substances combine the advantage is a bright orange powder. The morphology characterized by FE-
of the intrinsic stretchability of polymers and the electrochemical SEM is shown in Fig. 1(a), which reveals significant agglomeration
activity of the redox sites, such as carboxyl,1–3 quinone,4–9 of irregular shaped particles. The FTIR spectra of the reactants and
nitroxide radicals,10–13 conjugated aromatics,14,15 organosulfur16 the product are depicted in Fig. 2(b). For p-phenylenediamine,
and polyaromatic amine.17 Therefore, these materials may be 3409 cmꢀ1 (nN–H, as, –NH2), 3372 cmꢀ1 (nN–H, s, –NH2), 1632 cmꢀ1
suited as electrodes for flexible thin film batteries. Furthermore, (dN–H, –NH2), 1261 cmꢀ1 (nC–N, Ar–NH2), 832 cmꢀ1 (dAr–H
,
organic polymeric electrode materials are potentially more cost 1,4-substitued benzene), and 717 ꢀc1mꢀ1 (gN–H, –NH2). For
effective and more environmentally sustainable because they can 4,40,400-nitrilotribenzaldehyde, 2832 cm (nC–H, –CHO), 2734 cmꢀ1
be derived from renewable resources.18 Therefore, secondary (dC–H, –CHO), 1688 cmꢀ1 (nCQO, –CHO), 1275 cmꢀ1 (nC–N, Ar–NH2),
batteries made largely of polymeric materials with high energy and 816 cmꢀ1 (dAr–H, 1,4-substitued benzene). For the hyper-
density are well suited for flexible electronic applications.
Unfortunately, the electrode potentials of the majority of organic
electrodes reported to date lie between 2.5 and 4.0 V (vs. Li+/Li).
A pair of electrode materials for the construction of a full polymeric
secondary battery with a cell voltage higher than 2.0 V has not yet
been discovered.19–21 Indeed, design of an organic electrode material
branched conjugated Schiff base polymer, 1688 cmꢀ1 and 1619 cmꢀ1
Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan),
Lumo Road No. 388, Wuhan, 430074, China. E-mail: sunyubao@gmail.com,
chghs2@gmail.com
† Electronic supplementary information (ESI) available: Detailed information on
4,40,400-nitrilotribenzaldehyde
& polymer synthesis, characterization, battery
Scheme 1 Synthesis of a conjugated Schiff base polymer.
assembly and bending test. See DOI: 10.1039/c5cc09662f
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to the electrochemical oxidation and reduction processes of the
Schiff base. Above 1.5 V, it is a non-Faradic process. Further-
more, both the oxidation and the reduction current curves are
nearly symmetric, indicating that the electrochemical processes
with the hyperbranched conjugated Schiff base polymer are
highly reversible. The reversibility is further evidenced from the
rate and cycle performance as shown in Fig. 2(b) and (c). From
the galvanostatic charge and discharge curves at 0.4C in
Fig. 2(b), we estimate that the reversible electrode potential for
the conjugated Schiff base polymer electrode is around 0.7 V
(vs. Li+/Li) and its reversible capacity is approximately 140 mA h gꢀ1
.
Fig. 1 (a) FE-SEM image; (b) FTIR spectra: (b.1) conjugated Schiff base polymer,
(b.2) 4,40,400 -nitrilotribenzaldehyde, (b.3) p-phenylenediamine; (c) TGA analysis.
Indeed, the half cell was found to deliver 140 mA h gꢀ1 at 0.4C,
129 mA h gꢀ1 at 0.8C, 127 mA h gꢀ1 at 1C, 118 mA h gꢀ1 at 2C,
104 mA h gꢀ1 at 4C, 86 mA h gꢀ1 at 8C and 60 mA h gꢀ1 at 16C.
The discharge capacity remains nearly a half of the initial capacity
value at 16.0C, exhibiting excellent reversibility. Subsequently, the
half cell was charged and discharged for additional 400 cycles at
1C. The discharge capacity declines only marginally and the
coulombic efficiency remains above 99%, exhibiting outstanding
electrochemical stability.
The redox reactions occur in the polymer matrix with the
functional groups exposed to the liquid electrolyte, which
enables lithium ions to diffuse readily throughout the polymer
network. The process is still Faradic. Indeed, the Schiff base
functional groups are reduced to radical anions by accepting
electrons from lithium upon discharge and, in the reverse
process, the radical anions are oxidized to the Schiff base by
losing electrons upon charge. Structurally, the polymer matrix
differs significantly from the conventional crystalline based
inorganic electrode materials where redox reaction takes place
via lithium insertion/desertion. Furthermore, the oxidation and
reduction current curves are essentially symmetric (Fig. 2(a)). As
a result, the p conjugated Schiff base polymer undergoes rapid
Fig. 2 Electrochemical measurement of the conjugated Schiff base polymer
in a half cell. (a) Cyclic voltammetry; (b) charge and discharge plateaus at
different C-rates; and (c) rate capability and cycle performance.
(nCQN, Ar–CHQN–Ar), 1269 cmꢀ1 (nC–N, Ar–CHQN–Ar), and charge/discharge without a significant loss of discharge capacity.
823 cmꢀ1 (dAr–H, 1,4-substitued benzene). Upon the polycon-
A flexible battery is required to resist stretching and bending.
densation, the characteristic absorption peaks of the aromatic To test the mechanical properties of the electrode film, we
amine as well as the aldehyde disappeared, indicating a rather prepared a pouch cell and conducted a series of tests on the
complete reaction of the cross condensation. The thermal tensile strength of the hyperbranched Schiff base polymer and
gravimetric analysis is shown in Fig. 1(c) indicating that the the electrochemical properties of the cell upon bending with
conjugated Schiff base polymer maintains roughly 95 wt% up careful characterization. The measurement details are presented
to 400 1C and a significant weight loss occurs at around 460 1C. in the ESI.† The tensile strength and the elongation rate of the
The results reflect the strong thermal stability of the material. electrode film were found to be 0.28 MPa and 13%, respectively,
Cyclic voltammetry was performed with a half cell using a in a dry state. The EIS data were collected at various bending
lithium metal foil as the counter electrode as well as the radii and bending degrees as shown in Fig. 3. Clearly, the charge
reference electrode (Fig. 2(a)). The open circuit potential of transfer resistances change only marginally in comparison with
the half cell was found to be 2.7 V (vs. Li+/Li) corresponding to the unbent state. The results indicate that the hyperbranched
the oxidation state of the hyperbranched conjugated Schiff base conjugated Schiff base electrode film is capable of accommodating
polymer. The electrode was scanned from the open circuit mechanical bending.
potential to 0.2 V and then reverse-scanned to 2.3 V, recording
In conclusion, a hyperbranched conjugated poly(4,40,400-nitrilo-
fifteen cycles. In the first cycle, there is an irreversible electro- tribenzaldehyde-co-p-phenylenediamine) Schiff base polymer was
chemical reduction current peak, which is ascribed to the successfully synthesized and well characterized. Due to the high
formation of a solid electrolyte interfacial (SEI) film and/or degree of p conjugation, the hyperbranched conjugated Schiff
reduction of organic residues. In the subsequent four cycles, base polymer exhibits outstanding electrochemical reversibility
the I–V curves vary only modestly. Another ten cycle curves also and stability for lithium storage. In addition, the hyperbranched
almost overlap, which can be found in the ESI.† Notably, there network is benefit for structural integrity and electrochemical
are two current peaks between 0.2 V and 1.5 V, corresponding stability as verified by the bending measurements on the fabricated
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This work was supported by the National Natural Science
Foundation of China (No. 21233006, 21403202, and 21504084),
the Natural Science Foundation of Hubei province of China
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the Central Universities (No. CUGL140812).
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