Readily useable bulk phenoxazine-based covalent organic framework cathode materials with superior kinetics and high redox potentials

Article abstract of DOI:10.1039/d0ta10785a

Redox-active covalent organic frameworks (COFs) with dense redox sites are promising electrical energy storage materials with robust architectures, high surface areas, insolubility in electrolytes, and open pores for electrolyte transportation. However, low redox potentials and poor electrical conductivity of pristine COFs often result in low accessibilities of redox-active sites and slow redox kinetics, greatly limiting their practical applications. Herein, we report the design and synthesis of two novel p-type phenoxazine-based COFs (DAPO-COFs) with high redox potentials (～3.6 V vs. Li/Li+) and excellent electrical conductivities. Simply blended with conductive additives (CAs) and binders, pristine bulk DAPO-COFs without pre-composition with CAs or extra exfoliation are readily useable as cathode materials for lithium-ion batteries. Both as-synthesized DAPO-COF powders displayed superior active-site accessibility, ultrafast redox kinetics, and remarkable cycling stability. This work provides new perspectives on the development of readily useable COF-based cathode materials, and contributes to the advancement of eco-friendly and sustainable organic-based energy storage devices. This journal is

Full text of DOI:10.1039/d0ta10785a

Highlighting a study by a group of researchers led
by Prof. Ya Du from Nanjing Tech University and
Prof. Haishen Yang from Shanghai University
of Electric Power.
As featured in:
Readily useable bulk phenoxazine-based covalent organic
framework cathode materials with superior kinetics
and high redox potentials
Two distinctive p-type phenoxazine-based COFs
(DAPO-COFs) with high discharge potentials (up to
3.6 V vs. Li/Li+) are introduced as directly usable lithium ion
battery cathode materials, i.e. without pre-composition
with conductive additives or extra exfoliation. Pristine bulk
DAPO-COFs are demonstrated as excellent cathode
materials with superior active-site accessibility, ultrafast
redox kinetics, and remarkable cycling stability.
See Haishen Yang, Ya Du et al.,
J. Mater. Chem. A, 2021, 9, 10661.
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Readily useable bulk phenoxazine-based covalent
organic framework cathode materials with superior
kinetics and high redox potentials†
Cite this: J. Mater. Chem. A, 2021, 9,
10661
Zhiying Meng,a Ying Zhang,b Mengqing Dong,a Yue Zhang,a Fengmin Cui,a
Received 5th November 2020
Accepted 22nd February 2021
a
Teck-Peng Loh,ac Yinghua Jin, d Wei Zhang, d Haishen Yang
b and Ya Du
*
*
DOI: 10.1039/d0ta10785a
rsc.li/materials-a
Redox-active covalent organic frameworks (COFs) with dense redox of sustainable organic electrode materials. To date, a number of
sites are promising electrical energy storage materials with robust COFs with different redox-active groups have been explored as
architectures, high surface areas, insolubility in electrolytes, and open electrode materials.9–24 However, most as-prepared bulk COF
pores for electrolyte transportation. However, low redox potentials powders exhibit rather low active-site accessibility and slow
and poor electrical conductivity of pristine COFs often result in low redox kinetics because of their extremely low bulk electrical
accessibilities of redox-active sites and slow redox kinetics, greatly conductivities.11,25–27 Signicant efforts have been devoted to
limiting their practical applications. Herein, we report the design and improving their electrical conductivities through sophisticated
synthesis of two novel p-type phenoxazine-based COFs (DAPO-COFs) thin-lm formation or exfoliation strategies or in situ prepara-
with high redox potentials (ꢀ3.6 V vs. Li/Li+) and excellent electrical tion of COF composites.15,28–33 However, numerous drawbacks of
conductivities. Simply blended with conductive additives (CAs) and those approaches, such as complicated manipulations, poor
binders, pristine bulk DAPO-COFs without pre-composition with CAs scalability, energy capacity deductions, etc., limit their practical
or extra exfoliation are readily useable as cathode materials for applications. Therefore, readily useable bulk COFs with high
lithium-ion batteries. Both as-synthesized DAPO-COF powders dis- electrical conductivities are highly desired.34 Moreover, most
played superior active-site accessibility, ultrafast redox kinetics, and known COFs have operating voltages lower than 3.0 V,15,27,35,36
remarkable cycling stability. This work provides new perspectives on and thus COFs with high redox potentials are also highly
the development of readily useable COF-based cathode materials, and desired. Herein, we successfully address these problems by
contributes to the advancement of eco-friendly and sustainable construction of novel p-type phenoxazine-based COFs (DAPO-
organic-based energy storage devices.
COFs) with high working potentials (ꢀ3.6 V vs. Li/Li+), fast
redox kinetics, and ready bulk usability (Scheme 1).
DAPO–TFB-COF and DAPO–TpOMe-COF (DAPO-COFs) were
synthesized through the condensation of 2,7-diamine-10-
methyl-phenoxazine (DAPO) with 1,3,5-triformylbenzene (TFB)
and 2,4,6-trimethoxy-1,3,5-triformylbenzene (TpOMe), respec-
tively (Scheme 1). Both DAPO-COFs were characterized by FT-IR,
Two-dimensional covalent organic frameworks (2D COFs)
represent a novel class of predesignable porous crystalline
polymers with various applications. With redox-active mole-
cules as building blocks, robust redox-active COFs could be
precisely achieved with dense redox sites, enhanced electro-
chemical stability, insolubility in organic solvents (i.e. electro-
lytes), and tunable open pores for mass transportation.1–8
Redox-active COFs have many advantages as an emerging type
aInstitute of Advanced Synthesis, School of Chemistry and Molecular Engineering,
Nanjing Tech University, Nanjing 211816, China. E-mail: teckpeng@njtech.edu.cn;
ias_ydu@njtech.edu.cn
bShanghai Key Laboratory of Materials Protection and Advanced Materials in Electric
Power, Shanghai University of Electric Power, Shanghai 200090, China. E-mail:
yanghsh@shiep.edu.cn
cDivision of Chemistry and Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore 637616, Singapore
dDepartment of Chemistry, University of Colorado, Boulder, CO 80309, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta10785a
Scheme 1 Design and synthesis of DAPO-COFs.
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13C CP/MAS NMR, elemental analysis (EA), TGA, SEM and PXRD
analysis. FT-IR spectra of DAPO–TFB-COF and DAPO–TpOMe-
COF show the presence of the stretching bands of C]N at
1514 cmꢁ1 and 1592 cmꢁ1, respectively, and the disappearance
of the typical stretching bands of carbonyl (pC]O) and amino
groups (–NH2) of starting materials, indicating the successful
formation of imine bonds (Fig. S2†). In the solid-state cross-
polarization magic-angle spinning 13C NMR spectra, we
observed the peak corresponding to the imine carbons (C]N)
at 153 ppm for DAPO–TFB-COF, and 152 ppm for DAPO–
TpOMe-COF (Fig. S3†). Thermogravimetric analyses show that
both DAPO–TFB-COF and DAPO–TpOMe-COF have excellent
thermal stability, losing only 8% and 12%, respectively, at the
temperature of 450 ꢂC (Fig. S6†). High thermal stability of
electrode materials is critical for electrochemical energy storage
devices (EESDs) under high-temperature conditions. Scanning
electron microscopy (SEM) images of both DAPO-COFs show
uniform granular aggregates with diameters of several hundred
nanometers (Fig. 1c and d).
The crystallinity of DAPO–TFB-COF and DAPO–TpOMe-COF
was examined by powder X-ray diffraction (PXRD) measure-
ment (Fig. 1b). The diffraction peaks at 3.49ꢂ, 5.86ꢂ, 6.82ꢂ, 9.26ꢂ
and 12.22ꢂ for DAPO–TFB-COF correspond to the reections of
(100), (120), (200), (310) and (140) diffractions, and the diffrac-
tion peaks at 3.54ꢂ, 6.04ꢂ, 7.06ꢂ, 9.16ꢂ and 12.13ꢂ for DAPO–
TpOMe-COF are attributed to the reections of (100), (120),
(200), (310) and (140) facets. The experimental PXRD patterns of
both frameworks are consistent with the simulated AA stacking
models calculated using Materials Studio soware. Pawley
renement results show that the differences between experi-
mental and simulated data are negligible with reasonable Rwp
and Rp values (6.43% and 5.05% for DAPO–TFB-COF; 4.49% and
3.37% for DAPO–TpOMe-COF). The slightly broad PXRD peaks
might be due to the rotational freedom of DAPO with a rather
low symmetry, which likely causes imperfection in crystallinity.
The porosity of DAPO–TFB-COF and DAPO–TpOMe-COF was
investigated by N2 adsorption–desorption isotherms as shown
in Fig. 1a. The BET surface areas were calculated to be 1151 m2
gꢁ1 and 1662 m2 gꢁ1 for DAPO–TFB-COF and DAPO–TpOMe-
COF, respectively. The simulated surface areas based on the
crystal structure are 2434 and 2033 m2 gꢁ1, respectively. The
lower experimental BET surface area than the theoretical values
is likely due to the imperfect crystalline structures. Calculations
based on the quenched solid density functional theory (QSDFT)
show narrow pore-size distribution (PSD) centered around
1.9 nm for DAPO–TFB-COF and 2.1 nm for DAPO–TpOMe-COF,
which matches reasonably well with the theoretical values
(2.3 nm for DAPO–TFB-COF and 2.3 nm for DAPO–TpOMe-COF)
of the eclipsed stacking structures. High specic surface areas
would be benecial for the active-site accessibility, while
uniform large pores render DAPO-COFs with the ability to hold
sufficient electrolytes and the pathway for ion transport.37
The electrochemical performance of DAPO-COFs as cathode
active materials was evaluated in 2016 coin-type cells. The cyclic
voltammetry (CV) curves are shown in Fig. 2a. DAPO–TpOMe-
COF (red) shows two distinctive oxidation peaks I and II in
the potential region of 3.4–4.0 V, while DAPO–TFB-COF (blue)
Fig. 1 (a) Nitrogen adsorption–desorption isotherms of DAPO-COFs
and QSDFT pore size distributions of DAPO-COFs (inset); (b) experi-
mental PXRD patterns (blue), Pawley refined patterns (green), differ-
ence plots (cyan), simulated AA-stacking patterns (pink), and simulated
AB-stacking patterns (violet) for DAPO–TFB-COF and experimental
PXRD patterns (red), Pawley refined patterns (black), difference plots
(grey), simulated AA-stacking patterns (purple), and simulated AB-
stacking patterns (orange) for DAPO–TpOMe-COF; (c) SEM image of
DAPO–TFB-COF at 3 mm scale; (d) SEM image of DAPO–TpOMe-COF
at 3 mm scale.
10662 | J. Mater. Chem. A, 2021, 9, 10661–10665
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shows one oxidation peak with a very gentle slope in the
potential region of 3.6–4.0 V, indicating that an overlapped peak
is present.
The broad reduction peaks of both DAPO-COFs imply the
presence of overlapped peaks. The CV curves, which are quite
different from that of the parent phenoxazine, suggest a step-
wise charge storage mechanism.38,39 This observation could be
reasonably explained by the strong interlayer p–p interaction
and the formation of a stabilized intermediate state B upon
oxidation, in which the phenoxazine radical cation formed in
one layer can associate with the adjacent neutral phenoxazine
unit in the neighboring layer (Fig. 2b).38 The peak pair I between
3.38 and 3.62 V corresponds to the transition between states A
and B, while the peak pair II between 3.68 and 3.85 V corre-
sponds to the transition between states B and C. The result also
indicates the efficient interlayer charge-carrier transportation.
The more intensive peak pair II in DAPO–TpOMe-COF (red)
than that in DAPO–TFB-COF (blue) might indicate the more
efficient transition between states B and C and more intense
interlayer interactions of the oxidized states. This difference
could be attributed to the presence of methoxy groups on the
linker, which also accounts for the redox peaks at the potential
regions of 1.5–3.2 V, providing extra capacity for the framework.
A molecular model compound (TpOMe-Ami) was prepared to
explore the redox mechanism (Section B, ESI†). TpOMe-Ami
showed small redox peaks at the potential regions of 1.5–
3.2 V, as well as the main redox peaks between 3.2 V and 4.0 V
(Fig. S7†). Furthermore, the introduction of electron-donating
groups on the linker also provides the COF material with
superior kinetics of charge transfer, as evidenced by electro-
chemical impedance spectroscopy (EIS). The charge transfer
resistance (Rct) of DAPO–TpOMe-COF is 161 ohm, which is less
than that of DAPO–TFB-COF (201 ohm) (Fig. S8†). Both COF
powders were pressed into disc-shaped pellets (Ø ¼ 1 cm) with
a thickness of 0.22 mm, and the conductivity test was per-
formed at room temperature. The conductivities of DAPO–TFB-
COF and DAPO–TpOMe-COF bulk pellets were measured to be
5.35 ꢃ 10ꢁ7 S cmꢁ1 and 1.28 ꢃ 10ꢁ5 S cmꢁ1, respectively.
Both COFs exhibit high operating potentials and high redox
reversibility. The gaps between each pair of oxidation and
reduction peaks of DAPO-COFs are pretty narrow, especially for
the peak pair I (ꢀ0.1 V), indicating the excellent electrical
conductivity.30 Also, the narrow redox peak separations reect
the rapid redox kinetics during charge and discharge processes.
These observations are attributed to the synergistic effect of the
peculiar properties of the N-substituted phenoxazine motif and
the unique COF architectures.
The charge-storage capabilities of DAPO-COFs were further
evaluated with galvanostatic charge–discharge measurements
in the potential range of 1.5–4.2 V (vs. Li/Li+) at a current density
of 100 mA gꢁ1 (Fig. 2c). The charge–discharge curves of DAPO–
TFB-COF and DAPO–TpOMe-COF are in good accordance with
their CV results. Pristine DAPO–TpOMe-COF provides high rst
discharge specic capacity up to 81.9 mA h gꢁ1, including the
capacity below 3.1 V (ꢀ10 mA h gꢁ1), possibly originating from
the TpOMe linker. Pristine DAPO–TFB-COF delivered a capacity
of 64.4 mA h gꢁ1, which corresponds to 72% utilization of the
Fig. 2 (a) CV curves with a scan rate of 1 mV sꢁ1; (b) p–p interaction
and redox processes of DAPO-COFs; (c) charge (filled) and discharge
(empty) curves at 100 mA g; (d) rate performance at different applied
discharge C-rates; (e) cycling stability and coulombic efficiency at
100 mA gꢁ1
.
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theoretical redox-active sites. The utilization efficiency of active- to the DAPO building blocks with relatively low symmetry.
sites of both pristine DAPO-COFs is higher than that of most of Further improvement of electrochemical performance of DAPO-
the known COFs used as cathode materials, and comparable to COFs as cathode materials is anticipated if higher crystallinity
or even better than their composites prepared in situ with of the samples can be achieved.
conductive additives.16,18,27 The rapid decay of charge capacities
In order to have a better understanding of their excellent
of DAPO-COFs in the initial cycles is possibly due to the SEI lm performance, the surface morphologies of the electrode mate-
formation and growth, and/or the dissolution of trace amounts rials were examined using SEM images before and aer cycling
of electrode materials.40,41
(Fig. 3). As shown in the SEM images before cycling, the
Both pristine DAPO-COFs demonstrate excellent rate conductive carbons and COF aggregates distribute irregularly
performances. As shown in Fig. 2d, when the current densities aer the insufficient ball-milling. The COFs almost retained
increased from 100 mA gꢁ1 to 800 mA gꢁ1, DAPO–TFB-COF and their original sizes and morphologies and there are clear
DAPO–TpOMe-COF retain
a
reversible capacity up to interfaces between neighboring particles (Fig. 3a and c).
43.1 mA h gꢁ1 and 61.0 mA h gꢁ1, respectively, which corre- Surprisingly, aer cycling, the boundaries between the particles
spond to 65% and 76% of their initial capacity values of disappeared, and all the particles were “concreted” together. It
66.8 mA h gꢁ1 and 80.0 mA h gꢁ1, respectively, at 100 mA gꢁ1
.
appears that the COF spherical particles are “solidied”, and
When the current density was set back to 100 mA gꢁ1, high conductive carbon particles are wrapped up and indistin-
specic capacities of 58.0 mA h gꢁ1 and 75.5 mA h gꢁ1 were guishable. The cross-section electrode SEM images conrmed
attained for DAPO–TFB-COF and DAPO–TpOMe-COF, respec- similar observations (Fig. S9 and S10†). Previously, a similar
tively, corresponding to 87% and 94% of their respective initial morphology change was observed for
capacities. DAPO–TpOMe-COF is superior to DAPO–TFB-COF in poly(vinylphenothiazine)-based cathode material, which was
rate capability.
affirmed to be through a deposition/dissolution mechanism.42
a p-type linear
Remarkably, both pristine DAPO-COFs demonstrate robust However, since DAPO-COFs are insoluble in the electrolytes,
long-term cycling stabilities, as shown in Fig. 2e and S1a.† even in their oxidized (charged) states, such a mechanism is not
When evaluated at a current density of 100 mA gꢁ1, the capac- applicable in the present case. Although disappearance of the
ities of DAPO–TpOMe-COF and DAPO–TFB-COF are boundaries of cathode material particles was observed for the
68.0 mA h gꢁ1 and 51.0 mA h gꢁ1 with high initial capacity rst time for COF-based electrode materials and a clear mech-
retentions of 79% and 87%, respectively, aer 200 cycles. Even anism remains elusive, such a boundary self-bridging property
aer 1000 cycles at the current density of 100 mA gꢁ1, DAPO– would be an efficient approach to improve the performance of
TFB-COF still retains 72% of its initial capacity. The coulombic electrode materials.
efficiencies for both DAPO-COFs are higher than 99%, indi-
In conclusion, we have successfully developed two novel p-
cating that there are almost no capacity losses during the charge type redox-active COFs (DAPO-COFs) with high discharge
and discharge processes. We found that the crystallinity of the potentials (ꢀ3.6 V vs. Li/Li+), high crystallinity, regular pore
COF samples has a noticeable effect on their energy storage channels and high surface areas. The pristine DAPO-COFs are
performance. As shown in Fig. S1b and c,† the amorphous readily useable in bulk as excellent cathode materials by simply
sample of DAPO–TFB-COF exhibits much lower specic capacity mixing with conductive agents and binders as usual for lithium-
compared to the crystalline sample. In our hands, the COF ion batteries. Both pristine DAPO-COFs exhibit remarkable rate
samples were obtained only with decent crystallinity likely due capabilities, ultra-high active-site utilization, and outstanding
cycling stabilities. DAPO–TpOMe-COF with an electron-
donating methoxy group functionalized linker (TpOMe)
exhibits superior above-mentioned performances to those of
DAPO–TFB-COF. Our study indicates that, in addition to the
redox functional motifs, the performance of COFs could be
nely tuned by linker modications. This work provides
a general approach to develop bulk useable COF-based cathode
materials of high performance for the advancement of eco-
friendly and sustainable organic-based energy storage devices.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Y. D. acknowledges nancial support from the National Natural
Science Foundation of China (21805134), Natural Science
Fig. 3 SEM images of (a) electrode with DAPO–TFB-COF before
cycling; (b) electrode with DAPO–TFB-COF after cycling; (c) electrode
Foundation of Jiangsu Province, China (BK20191363), Science
and Technology Innovation Project for Overseas Students from
with DAPO–TpOMe-COF before cycling; (d) electrode with DAPO–
TpOMe-COF after cycling.
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Nanjing City, and Start-up Grant No. 39837141 from NJTECH. 21 C. R. DeBlase, K. E. Silberstein, T. T. Truong, H. D. Abruna
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