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Accepted Article
Title: Redox-Active 2D Metal-Organic Framework for Efficient Lithium
Storage with Extraordinary High Capacity
Authors: Qiang Jiang, Peixun Xiong, Jingjuan Liu, Zhen Xie, Qinchao
Wang, Xiao-Qing Yang, Enyuan Hu, Yu Cao, Jie Sun, Yunhua
Xu, and Long Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914395
Angew. Chem. 10.1002/ange.201914395
Link to VoR: http://dx.doi.org/10.1002/anie.201914395
http://dx.doi.org/10.1002/ange.201914395
10.1002/anie.201914395
Angewandte Chemie International Edition
COMMUNICATION
Redox-Active 2D Metal-Organic Framework for Efficient Lithium
Storage with Extraordinary High Capacity
Abstract: A novel redox-active 2D copper-benzoquinoid metalorganic framework (Cu-THQ MOF) has been readily synthesized via
a simple solvothermal method. The abundant porosity and intrinsic
redox characters endow the 2D Cu-THQ MOF with promising
electrochemical activity. For example, superior performance is
achieved as a lithium-ion battery cathode with a high reversible
capacity (387 mA h g−1), large specific energy density (775 Wh kg-1),
and good cycling stability. These metrics surpass most MOF-based
cathode materials for rechargeable energy storage. Most importantly,
the reaction mechanism is further unveiled by comprehensive
spectroscopic techniques where a three-electron redox reaction per
coordination unit and one-electron redox reaction per copper ion
mechanism is demonstrated. This elucidatory understanding sheds
new light on future rational design of high-performance MOF-based
cathode materials for efficient energy storage and conversion.
Over the last decade, lithium-ion batteries (LIBs) have been
widely used in a variety of energy storage devices because of
their high power/energy density and long-term cycling stability.[13]
However, commercial LIBs constructed with transition-metal
compound based cathodes and graphite anodes are reaching
their performance limits.[4] Particularly, transition-metal compound
cathodes possess low gravimetric capacity (< 200 mA h g -1), and
thus facing significant challenges in satisfying the demand for high
gravimetric capacity battery systems. [5] Therefore, high capacity
cathode materials are highly desirable to enhance the
performance of LIBs.
Compared with transition-metal compounds, organic materials
possess more appealing features, including low environment
footprint, eco-efficient production and disposal, cost effectiveness
and high specific capacity with appropriate material design. [6]
However, the dissolution in non-aqueous electrolytes and
intrinsically low electronic conductivity of small molecules
severely hinder their use as electrode materials in LIBs. [7]
Recently, as a new class of crystalline porous coordination
[*]
[+]
Q. Jiang,[+] J. Liu, Z. Xie, Prof. L. Chen
Department of Chemistry, Institute of Molecular Science
Tianjin Key Laboratory of Molecular Optoelectronic Science
Tianjin University, Tianjin 300072, China
E-mail: long.chen@tju.edu.cn
P. Xiong,[+] Prof. Y. Xu
School of Materials Science and Engineering
Key Laboratory of Advanced Ceramics and Machining Technology
Tianjin Key Laboratory of Composite and Functional Materials
Tianjin University, Tianjin 300072, China
E-mail: yunhua.xu@tju.edu.cn
Q. Wang, X.-Q. Yang, Dr. E. Hu
Chemistry Division, Brookhaven National Laboratory, Upton, New
York 11973, USA
E-mail: enhu@bnl.gov
Y. Cao, Prof. J. Sun
School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, China.
These authors contributed equally to this work.
polymers constructed by organic linkers and metal nodes, metalorganic frameworks (MOFs) have attracted increasing attention
for electrochemical energy storage, which are not soluble in
organic electrolytes due to the rigid and extended framework. [8]
Besides, benefiting from the varieties of organic bridging ligands
and metal ions/clusters, the electrochemical properties of MOFs
can be effectively tuned.[9] Furthermore, the inherent porous
channels enable the facile ion transportation and electrolyte
penetration.[10] These merits make MOFs very promising as
electrode materials for LIBs.
Tarascon and co-workers first reported a MOF cathode
material for LIBs, MIL-53(Fe), in which Fe centres function as
redox active sites by Mössbauer spectroscopic technique. [11]
Thereafter, Férey et al. constructed a tetrathiafulvalene
tetracarboxylic acid-based MOF as cathode with organic linkers
as active sites.[12] Hence, to improve specific capacity, an idea
approach is to combine both metal cations and redox-active
organic linkers into conjugated skeletons. Awaga’s group carried
out the first attempt from this perspective through the utilization of
redox copper ions and anthraquinone based organic moieties to
construct cupric-based MOF (Cu(2,7-AQDC)). Unfortunately, a
low capacity of 147 mA h g-1 was obtained, which is even lower
than commercial LiCoO2.[13]
Herein, we synthesized a two-dimensional copperbenzoquinoid metal-organic framework (2D Cu-THQ MOF) under
very mild and green conditions. This material is cost effective and
environmentally friendly because the organic linker can be
obtained from natural resources and copper is an abundant metal
element.[14] In addition, the porosity and semiconducting features
of 2D conjugated Cu-THQ MOF are favourable to charge
transport and energy storage. To our delight, 2D Cu-THQ MOF
shows an unprecedentedly high reversible capacity (> 350 mA h
g-1) and good cycling stability. Comprehensive spectroscopic
studies reveal that such high capacity is benefited from a new
lithium storage mechanism in which the redox process of both
metal ions and ligands are involved.
2D Cu-THQ MOF was synthesized via a simple modified
hydrothermal method.[15] The ultra-small tetrahydroxy-1,4quinone (THQ) linker coordinates with copper ions, which both
increases the density of redox active centres and creates nanosized pores. As shown in Figure 1a, a 2D honeycomb layered
framework was formed via the topological combination between
the THQ and copper ions. The crystalline structure of 2D Cu-THQ
MOF was resolved by the powder X-ray diffraction (PXRD)
measurement in conjunction with structural simulation. The AA
stacking mode after Pawley refinement agrees well with the
experimental data with low Rwp of 1.49% and Rp of 1.14% (Figure
1c). The unit cell belongs to a Cmcm space group with lattice
parameters a = 20.763 Å, b = 12.535 Å, c = 6.382 Å, and α = β =
γ = 90°. The crystallite size of 2D Cu-THQ MOF is around 10~30
nm (Figure 1e). The microscopic characterization was conducted
by high-solution transmission electron microscopy (HRTEM,
This article is protected by copyright. All rights reserved.
Accepted Manuscript
Qiang Jiang,+ Peixun Xiong,+ Jingjuan Liu, Zhen Xie, Qinchao Wang, Xiao-Qing Yang, Enyuan Hu,* Yu
Cao, Jie Sun, Yunhua Xu,* and Long Chen*
10.1002/anie.201914395
Angewandte Chemie International Edition
Figure 1f, Figure S1). The hexagonal pores are ~1.1 nm which
agrees well with the expected structure of 2D Cu-THQ MOF
(Figure 1a). Furthermore, the N2 adsorption isotherm of 2D CuTHQ MOF shows apparent BET surface area and pore volume of
∼137 m2 g−1 and 0.43 cm3 g-1, respectively (Figure 1d). Elemental
analysis (EA) results indicate that the contents of C, H, N and O
are close to theoretical values in the proposed formula (Table S1).
The X-ray photoelectron spectroscopy (XPS) spectrum also
reveals that most copper ions are monovalent (Figure S2b). The
2D Cu-THQ MOF exhibits temperature dependent electrical
conductivities of 2.15×10-3~0.16 μS cm-1 from 30 to 110 °C,
indicating a typical semiconductor characteristic (Figure S3).
(b)
(a)
1.1 nm
THQ-Cu-MOF
(c)
3.2 Å
(d) 400
5
10
15
20
25
30
3
-1
Volume @STP (cm g )
Intensity (a.u.)
Experimental
Calculated
Differential
300
100
0
0.0
35
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
2θ / Deg.
(e)
Adsorption
Desorption
200
first three cycles at 50 mA g-1. A large reversible capacity of 387
mA h g−1 and high specific energy density of 775 Wh kg-1 is
achieved in the second cycle with a Coulombic efficiency of 90%.
Three sloping discharge plateaus are presented at 3.2 to 2.6 V,
2.2 to 1.8 V, and 1.7 to 1.2 V, respectively, which are consistent
with CV results. The cycling performance was evaluated at a
current density of 50 mA g-1. Exceptional cycling stability is
demonstrated with a reversible capacity of 340 mA h g−1 after 100
cycles, and a capacity retention of 85% (Figure 2c). Compared to
the materials synthesized at other dosages of ethylenediamine
based on THQ, the higher crystallinity of Cu-THQ MOF ensures
better cycling stability, thus enhancing the electrochemical
performance (Figure S4). The rate performance was also tested
at current densities varied from 50 to 1000 mA g−1. A capacity of
159 mA h g−1 is achieved at 500 mA g-1 (Figures 2d and S5). Even
at a higher current density of 1000 mA g−1, 2D Cu-THQ MOF
electrode still delivers a high capacity of 93 mA h g −1, and a high
capacity of 310.5 mA h g-1 was obtained at 50 mA g−1 after the
large current test, indicating a good rate performance. The
reaction kinetics was also investigated by collecting the CV curves
at different scan rates and galvanostatic intermittent titration
technique (GITT) (Figures S6-S7). The open-circuit voltage was
monitored and shows almost no change for 50 days, indicating no
obvious self-discharge occurred for 2D Cu-THQ MOF (Figure S8).
These results indicate that the 2D Cu-THQ MOF electrodes
feature one of the highest capacity and the superior
electrochemical performance among MOF-based cathodes
reported up to date (Table S2).
(f)
(a)
(b) 4.0
3.5
Current (μA)
+
Voltage (V vs Li /Li)
20
2 nm
100 nm
20 nm
0
-20
1st
2nd
3rd
-40
-60
1.0
1.5
2.0
2.5
3.0
3.5
100
200
300
-1
400
500
Capacity (mA h g )
(d)
800
600
600
80
400
60
-1
50 mA g
40
200
20
0
0
20
40
60
Cycle number
80
100
CE. (%)
-1
Capacity (mA h g )
100
-1
Capacity (mA h g )
Insertion/extraction behaviours of Li-ions into/from the 2D CuTHQ MOF electrodes were evaluated in coin cells using Li metal
anodes. The cyclic voltammogram (CV) test of the 2D Cu-THQ
MOF cathode was performed in the potential range from 1.2 to
4.0 V with a scan rate of 0.1 mV s-1 (Figure 2a). In the first cycle,
the reductive peak at 2.6 V may be related to the reduction from
Cu(II) to Cu(I). The other two peaks appeared at 2.1 and 1.6 V,
which are attributed to the reduction of the O atoms in organic
ligands. Three anodic peaks centred at 1.9, 2.9 and 3.4 V,
corresponding to the oxidation of O atoms and Cu(I), respectively.
In the subsequent cycles, the CV curves are well overlapped
except that the peak from Cu(II) to Cu(I) became broad and the
first reduction peak of O atoms shifted to 2.2 V, indicating the high
reversibility of Cu-THQ MOF. The difference between the first and
second cathodic scan is probably due to the synergistic effect
created by the formation of solid-electrolyte interface (SEI) film
and the activation process of the crystalline MOF.[9] Figure 2b
shows the charge/discharge profiles of 2D Cu-THQ MOF in the
2.0
0
+
Figure 1. (a, b) Synthetic scheme and unit-cell structure, (c) PXRD patterns, (d)
N2 sorption isotherm, (e) SEM image, (f) HRTEM image of 2D Cu-THQ MOF.
Insert: magnified picture of the selected area.
1st
2nd
3rd
2.5
1.5
4.0
Voltage (V vs. Li /Li)
(c)
3.0
-1
400 50
mA g
50
100
200
200
500
750 1000
0
0
5
10
15
20
25
30
35
Cycle number
Figure 2. Electrochemical performance of 2D Cu-THQ MOF electrodes. (a) CV
curve at 0.1 mV s-1. (b) Charge/discharge profiles in the first three cycles at 50
mA g−1. (c) Cycling performance at 50 mA g−1. (d) Rate performance at various
current densities from 50 to 1000 mA g-1.
To gain more insights into the lithium storage mechanism of
the 2D Cu-THQ MOF, ex-situ electron paramagnetic resonance
(EPR) measurement was conducted upon redox processes
(Figure 3a). The EPR signal exhibits the strongest intensity when
the electrode is discharged to 1.2 V, which can be ascribed to the
increase of radicals and the transformation from C=O double
bond to C-O single bond due to the acceptance of electrons in the
ligands.[16] The intensity of EPR becomes obviously weak when
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Accepted Manuscript
COMMUNICATION
10.1002/anie.201914395
Angewandte Chemie International Edition
charged to 2.4 V and then significantly decreases with further
charge to 3.2 V, suggesting the transformation of C-O• radical to
C=O double bond in the first two anodic steps. Interestingly, the
EPR signal displays an obvious increase when fully charged to
4.0 V, which could be ascribed to the oxidation of Cu(I) to Cu(II).[17]
Furthermore, ex-situ Fourier transformed infrared (FT-IR)
spectra recorded at different charge/discharge states provide
further evidence for the redox mechanism of the 2D Cu-THQ MOF
(Figure 3b). At oxidized state, a new peak appears at ~832 cm -1,
which could be assigned to PF 6− anions,[18] indicating that PF6−
are embedded in the framework of 2D Cu-THQ MOF to maintain
the charge balance when Cu(I) cations are oxidized to Cu(II). In
addition, both oxidized and reduced states show a peak of the
C=O band at 1600 cm-1, confirming the existence of carbonyl
groups.[19] The peak at 1530 cm-1 (attributed to C=C bond from
benzene ring) almost vanished from reduced state to oxidized
state, confirming the disappearance of conjugation of C 6 ring.[20]
Ex-situ XPS and TEM analysis were also performed for the fully
discharged and charged Cu-THQ MOF electrode, which also
proves that PF6- anions are involved in the redox process of CuTHQ MOF (Figure S13-S15).
3270
3300
3330
3390
-
PF6
Reduced
Pristine
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
500
CuO
4.0 V
3.2 V
3.0 V
2.8 V
2.4 V
1.2 V
charging
Normalized  (a.u.)
Cu2O
CuO
1.2 V
2.4 V
3.2 V
3.4 V
4.0 V
8970 8980 8990 9000
528
4.0 V
2.8 V
2.3 V
1.9 V
1.2 V
534.1
532.7
531.3
532
charging
536
Energy (eV)
Charge
(a)
Ⅲ
Energy (eV)
528
Discharge
discharging
8970 8980 8990 9000
Energy (eV)
(d)
Normalized  (a.u.)
C=C
C=O
Oxidized
Intensity (a.u.)
3360
Magnetic field (G)
(b)
Cu2O
Normalized  (a.u.)
Intensity (a.u.)
C-1.2 V
C-2.4 V
C-3.2 V
C-4.0 V
Normalized  (a.u.)
(c)
(a)
region. Remarkably, no further change is observed in the Cu Kedge X-ray absorption spectra when the cell voltage is lower than
2.4 V, suggesting the rest of the redox reactions are not attributed
to the metal, but more likely to the ligands as discussed below.
Oxygen K-edge sXAS were conducted to understand the role
of O in charge compensation mechanism in 2D Cu-THQ MOF.
Pre-edge region of O XAS is related to the unoccupied states in
the neighbourhood of the Fermi level, corresponding to the lowest
unoccupied molecular orbital (LUMO) where the redox reaction
takes place.[21-23] Figure 3d shows the pre-edge region of O XAS
at different charge/discharge states to elucidate the redox
mechanism involving O. Upon discharge process, the absorption
peak at 534.1 eV, which can be attributed to π*C=O, decreases at
1.2 V.[24,25] Meanwhile, a new absorption peak at 531.3 eV
emerges with Li+ insertion. These evolutions indicate that Li + are
captured by the carbonyl O-atoms and the transformation of Oatoms from C=O state to Li-O state. Conversely, during the
charge process, the absorption peak at 531.3 eV gradually
disappears and the absorption peak at 534.1 eV recovers
simultaneously, indicating the insertion/extraction of Li + are highly
reversible. In addition, the peak at 532.7 eV attributed to the
hybridized orbital of O and Cu shows almost no change during the
charge/discharge processes,[26] implying the excellent stability of
the whole coordination framework.
Ⅱ
4.0 V
2.6 V
1.8 V
1.2 V
Ⅳ
discharging
Ⅴ
Ⅰ
532.7
531.3
532
Ⅵ
534.1
536
(b)
(II)
Energy (eV)
Figure 3. (a) EPR spectra, (b) FTIR spectra, (c) Cu hXAS, and (d) O sXAS
analysis of 2D Cu-THQ MOF electrodes at different charge/discharge states.
Hard X-ray absorption spectroscopy (hXAS) and soft X-ray
absorption spectroscopy (sXAS) were carried out to probe the
electronic structure around Cu and O, respectively. And these
results are analysed for further clarifying the redox mechanism in
2D Cu-THQ MOF (Figure 3c and 3d). The Cu K-edge spectra for
samples at various state-of-charge (SOC) clearly demonstrate the
reversible redox couple involving cuprous copper and cupric
copper. During the charge process, the Cu spectrum for the 1.2 V
sample is very similar to the reference Cu2O. Upon charging, the
spectra are nearly unchanged between 1.2~3.4 V but shift to
higher energy for the 3.4~4.0 V region. The overall Cu K-edge
spectrum of the full-charged Cu-THQ MOF sample is close to that
of CuO, suggesting an oxidation of cuprous cation to cupric cation
between 3.4~4.0 V.[13] During the discharge process, the Cu Kedge spectra show no obvious change until 2.8 V but obviously
shift back toward the spectrum of Cu2O, with the spectrum at 2.4
V being very close to that of the cuprous oxide reference. This
result indicates the reduction of Cu(II) to Cu(I) in the 2.8~2.4 V
Ⅳ
Ⅲ
(I)
（I）
PF6-
-e-, -Li+
Ⅱ
Discharge
Charge
Ⅴ-
+e , +Li+
Li +
(I)
(I)
Ⅵ
Ⅰ
(I)
Figure 4. (a) Galvanostatic charge/discharge curves of 2D Cu-THQ MOF
electrode at 50 mA g-1. The six areas marked by the resent the various
charge/discharge processes of 2D Cu-THQ MOF marked in (b). (b) The
evolution of electronic states of the repeating coordination unit of 2D Cu-THQ
MOF during the charge/discharge process. The binding sites between Li and O,
and variation of valence states of Cu are indicated by blue and gray circles.
Overall, the Li+ insertion/extraction mechanism of 2D Cu-THQ
MOF can be illustrated in Figure 4. During the charge process,
one repeating coordination unit in fully lithiated 2D Cu-THQ MOF
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Accepted Manuscript
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10.1002/anie.201914395
Angewandte Chemie International Edition
skeleton reversibly extract two Li+, accounting for the reversible
charge capacity and CV profiles with two distinct redox peaks in
the voltage window of 1.2~3.2 V (steps I and II). Then copper ions
are oxidized from Cu(I) to Cu(II) until charged to 4.0 V
accompanying by the insertion of PF6− (step III). First-principle
calculations are performed within the density functional theory
(DFT) framework to confirm the interaction between PF6- and
copper ions, the adsorption energy of PF6- onto Cu-THQ MOF is
calculated to be 3.55 eV, indicative of the interaction between PF6and copper ions (Figure S16). During the discharge process,
Cu(II) was reduced to Cu(I) while PF6− was extracted from the 2D
Cu-THQ MOF framework simultaneously between 4.0~2.3 V
(step IV). Finally, one coordination unit in 2D Cu-THQ MOF
accepted two Li+ from 2.3 V to 1.2 V (steps V and VI). The
repeating coordination unit can gain or lose one electron for each
step. Hence, the capacity for each step is 152.7 mA h g-1 and the
total theoretical capacity for Cu-THQ MOF is 458 mA h g-1. As far
as we know, such comprehensive understanding on the
electrochemical Li+ insertion/extraction mechanism has never
been elucidated before in MOF-based cathodes. It clearly
explains the origin of such high reversible capacity of 2D Cu-THQ
MOF and might pave the way for further development of efficient
and practical energy storage devices.
In summary, we have demonstrated that the 2D copperbenzoquinoid MOF opened up new opportunities to realize high
energy density LIBs. Both copper ions and organic ligands
exhibited prominent redox activity during the charge/discharge
processes, which was revealed by comprehensive spectroscopic
approaches of hXAS, sXAS, EPR, FT-IR, thus resulting in an
extraordinary high specific capacity. 2D Cu-THQ MOF delivers a
reversible capacity as high as 387 mA h g−1 with high specific
energy density of 775 Wh kg-1, and good cycling stability retaining
340 mA h g−1 after 100 cycles. Our findings enable an effective
strategy to rationally design and develop 2D conductive MOFbased cathode materials for next-generation LIBs.
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Acknowledgements
This work was financially supported by the National Key Research
and Development Program of China (2017YFA0207500),
National Natural Science Foundation of China (51973153,
51672188) and Natural Science Foundation of Tianjin City
(16JCYBJC40900, 17JCJQJC44600). The synchrotron radiation
experiments were performed at Brookhaven National Laboratory
supported by the Assistant Secretary for Energy Efficiency and
Renewable Energy, Vehicle Technology Office of the U.S. DOE
through the Advanced Battery Materials Research (BMR)
Program, including Battery500 Consortium under contract No.
DESC0012704.
Keywords: copper-benzoquinoid complex • 2D metal-organic
framework • cathode • lithium ion battery • energy storage
This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/anie.201914395
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
(Ⅱ)
Q. Jiang,+ P. Xiong,+ J. Liu, Z. Xie, Q.
Wang, X.-Q. Yang, E. Hu,* Y. Cao, J.
Sun, Y. Xu,* and L. Chen*
Ⅳ
Ⅲ
(Ⅰ)
(Ⅰ)
PF6-
Charge
Ⅱ
Ⅴ
-e-,-Li+
Discharge +e-,+Li+
Li +
(Ⅰ)
(Ⅰ)
Ⅵ
Ⅰ
(Ⅰ)
This article is protected by copyright. All rights reserved.
Page No. – Page No.
Redox-Active 2D Metal-Organic
Framework for Efficient Lithium
Storage with Extraordinary High
Capacity
Accepted Manuscript
Metal-organic framework cathodes
usually exhibit low capacity and
poor electrochemical performance
for Li-ion storage due to their
intrinsic low conductivity and inferior
redox activity. Herein, we report a
novel conductive and redox-active
copper-benzoquinoid 2D MOF with
high capacity for Li-ion batteries and
a new Li-ion storage mechanism
was unveiled by comprehensive
spectroscopic analysis.