A Microporous Covalent–Organic Framework with Abundant Accessible Carbonyl Groups for Lithium-Ion Batteries

Article abstract of DOI:10.1002/anie.201805540

A key challenge faced by organic electrodes is how to promote the redox reactions of functional groups to achieve high specific capacity and rate performance. Here, we report a two-dimensional (2D) microporous covalent–organic framework (COF), poly(imide-benzoquinone), via in situ polymerization on graphene (PIBN-G) to function as a cathode material for lithium-ion batteries (LIBs). Such a structure favors charge transfer from graphene to PIBN and full access of both electrons and Li⁺ ions to the abundant redox-active carbonyl groups, which are essential for battery reactions. This enables large reversible specific capacities of 271.0 and 193.1 mAh g^?1 at 0.1 and 10 C, respectively, and retention of more than 86 % after 300 cycles. The discharging/charging process successively involves 8 Li⁺ and 2 Li⁺ in the carbonyl groups of the respective imide and quinone groups. The structural merits of PIBN-G will trigger more investigations into the designable and versatile COFs for electrochemistry.

Full text of DOI:10.1002/anie.201805540

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Accepted Article
Title: A Microporous Covalent Organic Framework with Abundant
Accessible Carbonyls for Lithium-Ion Batteries
Authors: Zhiqiang Luo, Luojia Liu, Jiaxin Ning, Kaixiang Lei, Yong Lu,
Fujun Li, and Jun Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805540
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Angewandte Chemie International Edition
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COMMUNICATION
A Microporous Covalent Organic Framework with Abundant Accessible Carbonyls for Lithium-
ion Batteries
Zhiqiang Luo, Luojia Liu, Jiaxin Ning, Kaixiang Lei, Yong Lu, Fujun Li*, and Jun Chen
[
15c]
Abstract: A key challenge faced by organic electrodes is how to
promote the redox reactions of functional groups to achieve high
specific capacity and rate performance. Here, we report a two-
dimensional (2D) microporous covalent organic framework (COF),
poly(imide-benzoquinone), via in-situ polymerization on graphene
compromised by the limited redox centers.
COFs have been
[
16]
applied as electrode materials for LIBs,
but the
electrochemical performance, like capacity, rate capability, and
cycle stability, is still low and needs improvement. Appropriate
design and synthesis of COFs with abundant accessible redox
centers and electron conduction pathways are highly desirable
for organic LIBs.
Herein, a microporous COF, poly(imide-benzoquinone), is
synthesized from tetramino-benzoquinone (TABQ) and
pyromellitic dianhydride (PMDA) with and without graphene
(PIBN-G and PIBN). The as-synthesized PIBN is featured as a
2D framework with micropores of 1.5 nm, which facilitates
diffusion and access of electrolyte to its abundant redox
carbonyls. Favored by graphene, PIBN-G exhibits both good
(PIBN-G) to function as a cathode material for lithium-ion batteries
(
LIBs). Such a structure favors charge transfer from graphene to
+
PIBN and full access of both electrons and Li to the abundant
redox-active carbonyls, which are essential for battery reactions.
This enables large reversible specific capacities of 271.0 and 193.1
-
1
mAh g at 0.1 and 10C, respectively, and retention of >86% after
3
00 cycles. The discharging/charging process successively involves
+
+
8Li and 2Li in carbonyls of the respective imide and quinone
groups. The structural merits of PIBN-G will trigger more
investigations into the designable and versatile COFs for
electrochemistry.
+
electron conduction and Li diffusion. These ensure full
utilization of the carbonyls and hence a large capacity of 271
mAh g with fast kinetics and long cycle stability. It is revealed
that the lithiation and delithiation processes of PIBN involve 8Li
-1
+
Organic lithium-ion batteries (LIBs) are considered as one of the
next-generation energy storage devices because of the tunable
molecular structure, recyclability, and low cost of the organic
+
and 2Li in the carbonyls of the respective imide and quinone
groups of each repeat unit of the COF in two successive steps.
This will encourage more investigations into the designable and
functional COFs.
[
1,2]
electrode materials.
However, small organic molecules are
plagued by their solubility in aprotic electrolytes and intrinsically
[3,4]
low electric conductivity.
One efficient strategy is to stitch
organic molecules to polymeric moieties and inhibit the
[
5,6]
dissolution.
This usually incurs agglomeration of polymer
+
chains, and results in poor access of redox-active groups by Li
or electrons, which are essential for electrochemical reactions,
and hence low utilization efficiency of ˂70% of the polymers,
[7,8]
especially at high rates.
Microporous polymers with two- or
three-dimensional (2D or 3D) frameworks and controllable pore
size favor permeation of aprotic electrolyte and diffusion of
[
9-11]
Figure1. Synthesis route of PIBN and PIBN-G.
ions.
Structuring microporous polymer framework electrodes
+
with both facile Li diffusion and electron transport is of crucial
importance for organic LIBs.
PIBN and PIBN-G were synthesized via a solvothermal
reaction of TABQ and PMDA in dimethyl sulfoxide (DMSO) at
Covalent organic frameworks (COFs) have attracted extensive
[
12]
[13]
180 °C with and without addition of graphene, as depicted in
interests in gas separation,
electrochemical devices recently.
electrolyte membrane,
and
[
14,15]
Figure 1. The experimental details can be found in Supporting
Information and Scheme S1 and S2. The obtained samples
Rational design of COFs
is effective to realize their versatile functionalities and
applications. By tuning the functional groups, Jiang's group
1
13
were characterized by both liquid H and solid-state C nuclear
magnetic resonance (NMR) spectroscopy (Figure S1), elemental
analysis (Table S1), and Fourier-transform infrared spectroscopy
reported
a
COF
from
hexaazatrinaphthalene
and
A thin
-1 [15a]
diethynylbenzene to deliver a capacity of 147 mAh g .
(
8
FTIR, Figure S2). The average molecular weight of PIBN is
052 (Figure S3). These indicate formation of PIBN of high
film from naphthalene diimide and triformylphloroglucinol was
+
-1
obtained to improve Li diffusion and deliver 200 mAh g with
[
15b]
purity. The content of graphene with ~7 layers (Figure S4) in
PIBN-G is estimated to be 20 wt%, according to
thermogravimetric analysis (Figure S5).
conjugated carbonyls as redox centers.
On the other hand,
carbon nanotubes were incorporated into a donor-acceptor COF
-1
with a capacity of 69 mAh g at 2.4C; although its rate
performance is improved, the achievable capacity is
Figure 2a shows a typical scanning electron microscope (SEM)
image of PIBN as well as Figure S6, in which it is agglomerated
as nanoflakes. The layer structure of PIBN is evident in the
transmission electron microscope (TEM) image in the inset of
Figure 2a. The stacking multiple layers of PIBN can be seen
from the high-resolution TEM image in Figure S7. Of note, the
micropores of ~1.5 nm are visible in PIBN in Figure 2b, which is
consistent with its molecular framework structure in Figure 1 and
[*]
Z. Luo, L. Liu, J. Ning, K. Lei, Y. Lu, Prof. F. Li, Prof. J. Chen
Key Laboratory of Advanced Energy Materials Chemistry (Ministry
of Education), College of Chemistry, Nankai University, Tianjin
300071, China
E-mail: fujunli@nankai.edu.cn
Supporting information for this article is given via a link at the end of
the document.
pore-size distribution from N
With addition of graphene, PIBN-G appears as thin crystalline
2
sorption isotherms in Figure S8.
1
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nanosheets in Figure 2c, Figure S9 and S10. The molecule sizes
of DOL (1,3-dioxolane), DME (dimethoxyethane), and LiTFSI
their two successive lithiation/delithiation steps. The peak
voltage differences of the two redox couples of PIBN-G are
smaller than those of PIBN, and the peak areas of the two redox
couples are larger than those of PIBN. These suggest lower
polarization and higher utilization of redox carbonyls in PIBN-G
than in PIBN. This is consistent with the lower charge transfer
resistance of PIBN-G, as indicated by EIS in Figure 4a.
(
0
lithium bis(trifluoromethanesulfonyl)imide) are calculated to be
.38, and 0.73 nm, 0.72, respectively, in Figure 2d, which are
the electrolyte components. It means that the electrolyte can
facilely transport in the micropores of ~1.5 nm, as displayed in
Figure 1 and 2b, and fully access the abundant redox carbonyls
of PIBN.
Figure 3. (a) Raman spectra of PIBN, graphene, and PIBN-G, (b) Simulated
differential charge density of PIBN-G, and (c,d) XPS spectra of N1s and C1s.
Figure 2. (a) SEM image with an inset of TEM image and (b) High-resolution
TEM image of PIBN, (c) TEM image of PIBN-G, and (d) Optimized geometries
of DOL, DME, and LiTFSI.
Raman spectrum of PIBN-G is shown with PIBN and
graphene in Figure 3a. The pure PIBN presents two Raman
-1
bands at 1344 and 1538 cm , respectively, which are assigned
2
to the sp hybridization of C in phenyl groups and similar to the
-1 [17]
D and G bands of graphene at ~1320 and 1580 cm .
intensity ratio of the D and G bands of PIBN-G is increased to
.45 from 0.38 of graphene, and its G band is positively shifted.
The
1
This indicates existence of strong interaction between PIBN and
graphene in PIBN-G. Charge differential density of PIBN-G from
density functional theory (DFT) calculations is depicted in Figure
3b, where charge re-distribution exists between the PIBN
molecule and graphene. The blue regions on graphene and
yellow regions on PIBN indicate charge transfer from graphene
[
18]
to PIBN via conjugation of π-π stacking, which enhances the
stability of PIBN molecules and promotes electron conduction of
PIBN-G. Furthermore, X-ray photoelectron spectroscopy (XPS)
spectra of PIBN-G, PIBN, and graphene are displayed in Figure
3c and d. Their elemental compositions are exhibited in wide-
scan XPS spectra in Figure S11. In the N1s spectra of Figure 3c,
the N-C peak of PIBN-G shifts to lower binding energy, implying
π-π stacking and electron transfer between the phenyl groups of
PIBN and six-member carbon rings of graphene. This is in good
agreement with the DFT calculations in Figure 3b and Figure
S12. The emergence of π-π* satellite peak in Figure 3d further
confirms that PIBN molecules are strongly interacting with
graphene in PIBN-G.
-1
Figure 4. (a) EIS, (b) CV curves at 0.5 mV s , and (c) Rate performance of
PIBN-G and PIBN, and (d) Discharge/charge profiles and (e) Cycle stability of
PIBN-G. The current and capacity are based on the mass of PIBN.
The as-synthesized PIBN-G and PIBN are studied as cathode
materials of LIBs in coin cells. The applied electrolyte is 1.0 M of
LiTFSI in DOL and DME (v։v, 1։1). Figure 4a shows the
electrochemical impedance spectroscopy (EIS) of PIBN-G and
PIBN. The semicircles in high frequency regions denote charge
transfer resistance (Rct), which is related to reaction kinetics, and
The discharge capacities of PIBN-G are obtained at varied
current rates of 0.1, 0.2, 0.5, 1, 2, 5, and 10C, together with
PIBN in Figure 4c. At 0.1C, a high discharge capacity of 271.0
-1
-1
mAh g (601 Wh kg ) is achieved at PIBN-G, which is close to
-1
the theoretical value of 280.0 mAh g and corresponds to a
utilization of 96.8%. At the same condition, the specific capacity
+
-1
the straight line in low frequency regions represents Li diffusion
of PIBN is only 244.8 mAh g , leading to a lower utilization of
resistance in the electrolyte. It is clear that Rct of PIBN-G is
much smaller than that of PIBN (169.4 v.s. 280.2 Ω), suggesting
faster reaction kinetics of PIBN-G. Cyclic voltammetry (CV)
86.1%. It is more evident to obtain larger specific capacities on
PIBN-G over PIBN at high rates in Figure 4c. This is favored by
the fast Li and electron transport in PIBN-G, which is rewarded
by the strong interaction and charge transfer between PIBN and
graphene. Figure 4d presents the discharge/charge curves of
+
-1
curves of PIBN and PIBN-G are obtained at 0.5 mV s in Figure
b. The two distinctive couples of redox peaks are ascribed to
4
2
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COMMUNICATION
PIBN-G at different rates. They are composed of two distinctive
discharge/charge plateaus, which suggest two successive
lithiation/delithiation steps and are in good agreement with the
two couples of redox peaks in the CV curves in Figure 4b.
Moreover, the operando EIS of PIBN-G and PIBN during 100
cycles are recorded and summarized in Figure S13. The
impedance of PIBN-G is gradually increased in initial cycles and
becomes stable beyond 10 cycles, while in PIBN it is increasing
with cycles. The cycle stability is also revealed by almost the
and depicted in Figure S17. During the test, the PIBN-G cathode
is closely contacted with an attenuated total reflection (ATR)
crystal, the setup of which is depicted in Figure S18. The
characteristic absorbance peaks of PIBN-G in two regions in
Figure 5b are of interest: the light blue region represents the
breathing vibration of the C=O bonds of imide groups at ~1720
-1
cm , and the gray region is ascribed to the vibration of the C=O
-1 [1b]
bonds of quinone groups at ~1570 cm . Both of the two kinds
of carbonyls are the active redox centers. In a discharging
process, the signals of the C=O bonds of imide groups at ~1720
1
same SEM image of in Figure S14, H NMR spectra, FTIR
-1
spectra in Figure S15, and Figure S16 during cycles,
respectively. In Figure 4e, the PIBN-G delivers initial capacities
cm become weak and almost disappear beyond the first
plateau; after fully discharged to 1.5 V, the signals of the C=O
-1
-1
of 242.3 and 206.7 mAh g at 1 and 5C, respectively, and
bonds of quinone groups at ~1570 cm disappear in the
-1
maintains at 208.1 and 182.3 mAh g after 300 cycles. The high
specific capacity, capacity retention, and good rate capability of
PIBN-G are attributed to the 2D microporous framework with
abundant accessible carbonyls, and strong interaction between
PIBN and graphene.
A typical discharge and charge profile of PIBN-G at 0.1C is
displayed in Figure 5a. The discharging and charging processes
are monitored by in-situ FTIR. The testing cell is home-made
following step. This indicates that in the discharging process the
C=O bonds of imide and quinone groups are successively
lithiated in two steps. In the charging process, the signals of the
C=O bonds of the imide and quinone groups emerge gradually,
which is the reverse of the discharging process. This is in good
agreement with the two separate couples of redox peaks and
the two distinctive plateaus of the discharge/charge profiles in
Figure 4b and d, respectively.
13
Figure 5. (a) Discharge/charge profile at 0.1 C, (b) In-situ FTIR spectra, (c) Ex-situ solid-state C NMR spectra collected at the indicated states in (a), and (d)
Schematic gradual lithiation and delithiation of the different kinds of carbonyls of PIBN in a discharge and charge cycle.
The lithiation/delithiation of PIBN-G is further elucidated by
reverse process at Stage (IV) and (V). It is believed that the
lithiation/delithiation of PIBN occurs in two distinctive steps: the
C=O bonds of imide groups are firstly inserted/extracted by 8Li
1
3
solid-state C NMR, which is powerful to identify the different
C=O bonds. Figure 5c presents the NMR spectra of PIBN-G at
the selected discharged and charged states in Figure 5a. In the
pristine Stage (I), PIBN shows six peaks at 171.7 (a) 165.1 (b),
+
and then the C=O bonds of quinone groups accept/release
+
another 2Li . The reversible process is schematically depicted in
1
50.4 (c), 118.9 (d), 138.5 (e), and 99.8 ppm (f), which are
assigned to the various C atoms of PIBN, as marked in Figure
d. When PIBN is discharged to 2.05 V at Stage (II), a new peak
at 75.7 ppm (g) appears with disappearance of (d) and (f). It
Figure 5d.
In conclusion, we have successfully synthesized
microporous COF in the presence of graphene, PIBN-G. It is
featured as a 2D framework with micropores of 1.5 nm. This
a
5
+
suggests lithiation of the C=O bonds of imide groups. In this step, favors diffusion and access of Li to the abundant redox
+
8Li are inserted into PIBN, half of which are active for steric
carbonyls, and promotes the reversible electrochemical
reactions. Rewarded by the strong interaction and charge
transfer between PIBN and graphene, PIBN-G exhibits high
hindrance. At Stage (III) of full discharge, another new peak
emerges at 92.0 ppm (e’) with loss of (a) and (e), which
indicates insertion of another 2Li into the quinone groups in the
+
+
electron conduction and Li diffusion, which benefit the redox
second step. Upon subsequent charging, PIBN experiences a
reactions of carbonyls. It presents excellent rate capability with
3
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COMMUNICATION
-
1
large reversible capacities of 271.0 and 193.1 mAh g at 0.1 and
0C, respectively, and long cycle life of 300 cycles with
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1
Coulombic efficiencies of ~100%. The gradual lithiation and
delithiation of PIBN-G involves 8Li in the carbonyls of imide
groups in the first step and another 2Li in the carbonyls of
quinone groups in the following step. Such high performance will
trigger more investigations into the designable and versatile
COFs for electrochemistry and promote the development of
functional COFs.
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4
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COMMUNICATION
COMMUNICATION
A
two-dimensional
covalent organic
poly(imide-benzoquinone)
microporous
Zhiqiang Luo, Luojia Liu, Jiaxin Ning,
Kaixiang Lei, Yong Lu, Fujun Li*, and
Jun Chen
framework
of
was
synthesized via in-situ polymerization
on graphene to function as a cathode
material for lithium-ion batteries. It
exhibits excellent electrochemical
performance owing to the facile
Page No. – Page No.
A Microporous Covalent Organic
Framework
with
Abundant
Accessible Carbonyls for Lithium-
Ion Batteries
+
access of Li and electron to its
abundant carbonyls.
5
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