An extended carbonyl-rich conjugated polymer cathode for high-capacity lithium-ion batteries

Article abstract of DOI:10.1039/d0ta11648c

Organic materials have attracted extensive attention for use in lithium-ion batteries due to the flexible designability of their structures and their high theoretical capacities. However, the high solubilities, low voltages, and poor conductivities of traditional organic materials have impeded their further application. Herein, a novel carbonyl-rich covalent organic polymer (COP), polyphenyl-1,3,5-(pyrene-4,5,9,10-tetraone) (PPh-PTO), was designed and synthesized.In situFourier-transform infrared spectroscopy (FTIR),ex situX-ray photoelectron spectroscopy (XPS), and Raman analysis were used to confirm the carbonyl redox-active centers. Moreover, density functional theory (DFT) calculations were used to verify that the extended conjugated structure can induce electron delocalization and achieve an elevated and slanted voltage plateau in comparison with the pyrene-4,5,9,10-tetraone (PTO) monomer. The extended structure enables PPh-PTO to show a high reversible capacity of 235 mA h g^?1at 0.1 A g^?1, superior cycling abilities (95% capacity retention after 1400 cycles), and excellent rate performance (94 mA h g^?1at 2 A g^?1) in lithium-ion batteries. Our work developed a promising strategy for designing high-energy carbonyl-rich covalent organic polymers for energy storage.

Full text of DOI:10.1039/d0ta11648c

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Materials Chemistry A
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An extended carbonyl-rich conjugated polymer
cathode for high-capacity lithium-ion batteries†
Cite this: J. Mater. Chem. A, 2021, 9,
2700
a
Shibing Zheng,a Licheng Miao,b Tianjiang Sun,a Lin Li,
Tao Ma,a Junquan Bao,a
a
a
*
Zhanliang Tao
and Jun Chen
Received 30th November 2020
Accepted 16th January 2021
DOI: 10.1039/d0ta11648c
rsc.li/materials-a
Organic materials have attracted extensive attention for use in lithium- owing to their high redox reversibility and capacity versus other
ion batteries due to the flexible designability of their structures and materials.5–7 Pyrene-4,5,9,10-tetraone (PTO), with four carbonyl
their high theoretical capacities. However, the high solubilities, low active sites and a high theoretical capacity of 408 mA h gꢀ1, has
voltages, and poor conductivities of traditional organic materials have attracted attention as cathode material for use in high-energy-
impeded their further application. Herein, a novel carbonyl-rich density lithium batteries. However, there are inevitable
covalent organic polymer (COP), polyphenyl-1,3,5-(pyrene-4,5,9,10- intrinsic drawbacks, including high dissolubility, low conduc-
tetraone) (PPh-PTO), was designed and synthesized. In situ Fourier- tivity, and
a low redox potential. Traditional strategies,
transform infrared spectroscopy (FTIR), ex situ X-ray photoelectron including linear polymerization, graing, and tuning the
spectroscopy (XPS), and Raman analysis were used to confirm the molecular polarity, are aimed at suppressing the solubility of
carbonyl redox-active centers. Moreover, density functional theory materials.8 Methods for improving potentials include intro-
(DFT) calculations were used to verify that the extended conjugated ducing electron-withdrawing groups and heteroatoms (N, S),
structure can induce electron delocalization and achieve an elevated which can contribute to an increase in electroconductivity.9–11
and slanted voltage plateau in comparison with the pyrene-4,5,9,10- Recently, a reported extended conjugated system bearing all
tetraone (PTO) monomer. The extended structure enables PPh-PTO these characteristics may have broken the deadlock. Luo et al.
to show a high reversible capacity of 235 mA h gꢀ1 at 0.1 A gꢀ1
superior cycling abilities (95% capacity retention after 1400 cycles), voltage and insoluble properties.12 Stoddart's group further
and excellent rate performance (94 mA h gꢀ1 at 2 A gꢀ1) in lithium-ion conrmed that extended macrocycles can delocalize electrons
,
prepared an extended p-conjugated system showing high-
batteries. Our work developed a promising strategy for designing to homogenize the voltage platform, causing
a voltage
high-energy carbonyl-rich covalent organic polymers for energy improvement.13 Thus, extended conjugated systems seem to
storage.
have the potential to pave the way for the application of organic
materials.
Covalent organic polymers (COPs), sharing the general
characteristic of an extended topological structure, are prom-
inent structural skeletons with high designability,14,15 porosity,
and stability.16 Their ease of chemical modication extended
their application elds to areas such as proton conduction,17
catalysis,18 electrolyte membranes,19 and gas separation.20
Through introducing rational active sites, COPs can be applied
to energy storage. For instance, Wang's group successfully
prepared covalent organic framework nanosheets, DAAQ-ECOF,
exhibiting an excellent capacity of 104 mA h gꢀ1 at 0.5 A gꢀ1 with
95% capacity retention aer 1800 cycles.21 Xu et al. constructed
a conjugated microporous polymer based on an imine group,
which delivered a capacity of 147 mA h gꢀ1 at 0.1 A gꢀ1 and high
cycling stability.22 However, the reported cathodes were still
plagued by low capacities and voltages due to active-monomer
insufficiencies. Thus, creating an ideal active monomer is of
the essence. Representative studies, such as those constructing
Introduction
Eco-friendly and sustainable organic materials are considered
as promising substitutes for inorganic materials for use in
lithium-ion batteries.1,2 Investigations into various materials
with different active centers (carbonyls, nitriles, radicals, imine
compounds, etc.) have been widely performed.3,4 Among them,
conjugated carbonyl compounds have been intensively studied
aKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
College of Chemistry, Nankai University, Tianjin 300071, China. E-mail: taozhl@
nankai.edu.cn
bCollege of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen
518060, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta11648c
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two-dimensional23 or linear24,25 polymers, have achieved excel-
lent performance. Inspired by this, combining the merits of an
extended p-conjugated structure with high-energy active sites
may be a way to overcome related issues.
Herein, we constructed a novel extended p-conjugated
carbonyl-rich covalent organic polymer (PPh-PTO) based on
a high-energy monomer (PTO) for lithium-ion batteries. Ex/in
situ UV-vis and ex situ FTIR spectroscopy studies were carried
out to verify the insolubility in electrolyte. In situ FTIR and ex
situ XPS studies conrmed that the carbonyl groups were the
lithiation/delithiation sites. As predicted, the extended p-
conjugation lowered the LUMO level and narrowed the energy
gap of PPh-PTO, resulting in voltage and electronic conductivity
enhancements. PPh-PTO showed
a
high capacity of
235 mA h gꢀ1 at 0.1 A gꢀ1, excellent cycling abilities (95%
retention aer 1400 cycles), and good rate capabilities
(95 mA h gꢀ1 at 2 A gꢀ1). Further quantitative kinetic analysis
revealed that the discharge/charge process tends to be a redox
pseudocapacitive process, which accounts for the fast electro-
chemical kinetics of PPh-PTO. Our investigation suggests that
COP materials based on the rational design of extended
conjugated structure can achieve multi-aspect improvement for
rechargeable battery applications.
Fig. 1 Optical photographs of (a) PTO and (b) PPh-PTO electrodes in
the electrolyte and corresponding FTIR spectra of the electrolyte
samples after soaking for different lengths of time. In situ UV-vis
spectra of electrolyte samples from a (c) Li-PTO battery and (d) Li-
PPh-PTO battery (125 mA gꢀ1, collection frequency: every 5 min).
1 h to 1 month). A distinct colour change and corresponding
peak increase at 1655 cmꢀ1 (the C]O functional group of PTO)
can be observed with an increase in time, while the peaks at
1080 cmꢀ1 (C–O–C stretching vibrations) and 2880 cmꢀ1 (C–H
stretching) pertaining to the tetraethylene glycol dimethyl ether
(G4) electrolyte remain unchanged (Fig. 1a). Furthermore, an
increase in the intensity of the peak at 310 nm can be observed
from the UV-vis spectroscopy analysis of PTO (Fig. S9a†). In
contrast, the colour and spectra of PPh-PTO solutions remained
unchanged (Fig. 1b and S9b†). Also, the morphologies of pris-
tine and soaked (7, 14, and 30 days) PPh-PTO electrodes are very
similar (Fig. S10†), indicating the stability of PPh-PTO in the
electrolyte. Moreover, in situ UV-vis spectroscopy technology
was used to compare the dissolution of active materials during
the electrochemical reaction in G4. For the PTO battery, an
ultraviolet absorption peak from PTO appeared at 310 nm, and
this increased with the length of the discharging/charging
process (Fig. 1c). However, for the PPh-PTO battery no absorp-
tion peak emerged, and the electrolyte remained clear aer
cycling, suggesting that PPh-PTO is indeed insoluble in the
electrolyte (Fig. 1d).
Results and discussion
The PPh-PTO compound was synthesized via a Suzuki coupling
reaction between the prepared 2,7-dibromo-4,5,9,10-
pyrenetetrone (PTO-Br2) (Fig. S1 and S2†) and 1,3,5-benzene-
tribonic acid (Scheme 1). The obtained samples were charac-
terized via FTIR (Fig. S3†), solid-state 13C nuclear magnetic
resonance (NMR) spectroscopy (Fig. S4†), and powder X-ray
diffraction (XRD, Fig. S5†). PPh-PTO was a brownish-black
powder with irregular micron- or sub-micron-sized particles
(Fig. S6†). Energy dispersive spectroscopy (EDS) analysis
conrmed the presence and homogenous distribution of C, O,
and a small amount of Br in as-synthesized PPh-PTO (Fig. S7†).
The thermo-gravimetric analysis (TGA) results show that PPh-
PTO was stable below 250 ꢁC (Fig. S8†), indicating the safety
of PPh-PTO as an active electrochemical material in recharge-
able batteries.
To gain insights into the solubility performance of the
monomer (PTO) and PPh-PTO, dissolution experiments were
performed. We rst collected optical photographs and FTIR
spectra of electrolytes aer different soaking durations (from
Subsequently, the electrochemical activity of the PPh-PTO
electrode was investigated. In situ-analysis-suitable and
normal CR2032 coin-type cells were assembled and tested at
room temperature. In situ FTIR was applied to monitor the
lithium storage mechanism of PPh-PTO during the discharging/
charging process. First, to avoid interference peaks originating
from the organic or inorganic matrix at the electrode/electrolyte
interface, the spectra of the blank electrode, electrolyte, and
PPh-PTO electrode were compared (Fig. S11†). Then we selected
a collection section (1690–1660 cmꢀ1) to detect the change in
the carbonyl characteristic peak. The discharging and charging
Scheme 1 The synthesis route to PPh-PTO.
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Fig. 2 (a) Discharge and charge curves of PPh-PTO during the first three cycles and (b) the corresponding in situ FTIR spectra. High-resolution C
1s (c) and O 1s (f) XPS spectra of pristine PPh-PTO and (d) the corresponding discharge/charge states of the PPh-PTO electrode when discharged
to 2.5 V and 1.5 V and charged to 3.0 V and 3.8 V at a current density of 100 mA gꢀ1. (e) The lithiation/delithiation mechanism of PPh-PTO.
curves of PPh-PTO for the rst three cycles (Fig. 2a) and the further conrmed the reversibility of the lithiation/delithiation
corresponding FTIR spectrum (Fig. 2b) were investigated in process at the carbonyl active sites (Fig. 2e). Moreover, ex situ
real-time. As shown in the colour-map proles, a reduction in Raman analysis was conducted to investigate the cathode in
the intensity of the FTIR absorbance peak at around 1670 cmꢀ1 different discharge/charge states, which further veried the
was observed, which corresponded with the carbonyl peak reversible switch between C]O and C–O during the electro-
reduction, during the discharge process. Meanwhile, the chemical reaction of the PPh-PTO electrode (Fig. S12†).
increase in the intensity of the carbonyl characteristic peak
Compatibility between an electrolyte and electrode material
during the charging process was inverse to the decrease during is important. Thus, we rst investigated the compatibility
the discharge process, which means that carbonyl oxygen atoms between the PPh-PTO electrode and both a popular electrolyte
were the active sites of the PPh-PTO electrode.
(1 M LiTFSI in DOL/DME (v/v: 1/1)) and the selected electrolyte
Moreover, ex situ XPS analysis was carried out to further (1 M LiClO4/G4). PPh-PTO showed a sharp decay from dissolu-
investigate the lithiation/delithiation process during electro- tion and greater polarization in the popular electrolyte than in
chemical cycling. Different discharge/charge states of PPh-PTO the 1 M LiClO4/G4 system (Fig. S13†). Therefore, the further
electrodes including discharged to 2.5 V and 1.5 V and charged electrochemical testing of PTO and PPh-PTO was carried out in
to 3.0 V and 3.8 V at a current density of 100 mA gꢀ1, were 1 M LiClO4/G4. Both PTO and PPh-PTO displayed similar cyclic
measured (Fig. 2d). The XPS spectra of as-prepared PPh-PTO voltammogram (CV) curves, indicating their consistent two-step
showed two distinct peaks at 284.6 and 531.7 eV in the C 1s
and O 1s spectra (Fig. 2c and f), respectively. The C 1s spectrum
can be divided into three peaks, located at 284.29, 286.28, and
289.68 eV, which represent sp2-hybridized C–C, C–O, and C]O
bonds, respectively (Fig. 2c). The C]O peak (289.68 eV) is
weakened during discharging to 2.5 V, and it nearly disappears
upon discharging to 1.5 V. Conversely, the C–O peak (286.28 eV)
emerged and was enhanced during the discharge process,
which can be attributed to the evolution of C]O to C–O–Li as
a result of Li+ insertion. It is worth noting that the C–O binding
peak shied to a lower binding energy upon discharging from
2.5 to 1.5 V, and it returned to its initial position upon charging
to 3.0 V. This phenomenon may be ascribed to the synergistic
effect of the adjacent carbonyl. Simultaneously, the changes in
the O 1s spectrum peaks at 531.1 eV, 531.76 eV, and 532.42 eV
Fig. 3 (a) CV curves of PTO and PPh-PTO at a scan rate of 0.3 mV sꢀ1
(b) The HOMO/LUMO energy levels and orbital plots; Eg is the HOMO–
.
(Fig. 2f), corresponding to O–Li, C–O, and C]O, respectively,
LUMO gap.
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lithiation/delithiation processes.26 The PPh-PTO electrode and it therefore exhibited a slanted and elevated voltage
showed two obvious pairs of redox peaks at 2.87/2.18 V for plateau. Furthermore, the HOMO–LUMO gap of PPh-PTO (3.09
reduction and at 3.28/2.66 V for oxidation at 0.3 mV sꢀ1 (Fig. 3a). eV) was narrower than that of PTO (3.49 eV), indicating that the
As predicted, PPh-PTO has a higher redox potential than PTO electronic conduction of PPh-PTO will be better.
due to the extended p-conjugated structure. To verify this,
The galvanostatic discharge–charge prole of PPh-PTO
density functional theory (DFT) calculations were performed. showed an inclined voltage prole due to the delocalization of
PPh-PTO shows a lower LUMO (lowest unoccupied molecular electrons and it could deliver a high reversible capacity of
orbital) level (ꢀ4.04 eV) compared with that of the PTO mono- 220 mA h gꢀ1 (at 0.1 A gꢀ1 and 0.4C) aer 1000 cycles (Fig. S13†),
mer (ꢀ3.94 eV) (Fig. 3b). For molecules that are closely related being more stable than PTO (Fig. S15†). The rate performance of
in structure, reduction potentials are negatively correlated in the PPh-PTO electrode was estimated at various current densi-
a linear fashion with the LUMO energy.27 Therefore, PPh-PTO ties. As the current density was gradually increased from 20 to
can achieve a higher reduction potential due to its better elec- 100 to 1000 to 2000 mA gꢀ1, it delivered superior rate capabil-
tron affinity. Besides, the more extended p-conjugated skeleton ities of 243, 224, 191, and 95 mA h gꢀ1, respectively. When the
of PPh-PTO compared with PTO could induce electron delo- current density was returned to 20 mA gꢀ1, the PPh-PTO elec-
calization (see the aromaticity calculations, which included trode recovered a reversible capacity of 244.5 mA h gꢀ1 (Fig. 4a).
LOL-p (localized orbital locator-p)28,29 and HOMA (harmonic The sharp decrease in the capacity upon increasing the current
oscillator measure of aromaticity)30 calculations, in Fig. S14†), from 1000 to 2000 mA gꢀ1 was ascribed to the viscosity of G4.26,31
Fig. 4 Quantitative analysis of the charge storage behavior. (a) The rate performances of PTO and PPh-PTO. (b) Discharge and charge profiles of
PPh-PTO at different current rates. (c) CV curves from a PPh-PTO electrode at different scan rates from 0.4–2.8 mV sꢀ1 and (d) the relationship
between log i and log v. (e) Capacitive and diffusion-controlled contributions at a scan rate of 2 mV sꢀ1. (f) The pseudocapacitive and diffusion-
controlled charge storage contributions at different scan rates. (g) The long-term cycling performances of PPh-PTO and PTO at 0.1 A gꢀ1 (0.4C).
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However, the PTO monomer with poor electronic conduction HOMO–LUMO gap. Thus, carbonyl-rich PPh-PTO delivered
delivered inferior rate performance to the polymer, although it high reduction potential, excellent rate performance
a
showed a high initial capacity. This superior rate performance (94 mA h gꢀ1 at 2 A gꢀ1), and a stable cycling capacity of
of PPh-PTO can also be attributed to the specic surface area of 224 mA h gꢀ1 (0.1 A gꢀ1), with 95% capacity retention aer 1400
the annealed PPh-PTO (39.99 m2 gꢀ1) and its wide pore-size cycles. Moreover, proting from its conjugated character, PPh-
distribution, which provides sufficient reaction space and ion PTO manifested dominant pseudocapacitive behavior. Quanti-
migration paths aer the annealing treatment (Fig. S16†). In tative kinetic analysis veried that the pseudocapacitive redox
addition, the good wettability of LiClO4/G4 on the electrode behavior facilitates electrochemical reactions. This work could
provided another reason for the excellent rate capabilities inspire the design and application of extended p-conjugated
compared with the PTO monomer (Fig. S17†). Only slight COPs for use in high-energy and high-power lithium-ion
polarization can be observed with an increase in the current for batteries.
the extended conjugated material (Fig. 4b). The high reversible
capacity and coulombic efficiency signied the excellent
extended structural and electrochemical stability of PPh-PTO.
Conflicts of interest
To further elucidate the electrochemical kinetics of the PPh-
PTO electrode, cyclic voltammetry (CV) measurements were
There are no conicts to declare.
conducted. The CV curves were almost identical upon
increasing the sweep rate (0.4–2.8 mV sꢀ1), except for slight Acknowledgements
shis in the cathodic and anodic peaks (Fig. 4c). Theoretically,
This study was supported by the National Key R&D Program of
China (2016YFB0901500), National Natural Science Foundation
of China (51771094 and 21835004), Ministry of Education of
the peaks can be divided into capacitive and diffusion compo-
nents starting from the equation:
i ¼ anb
China (B12015 and IRT13R30), and Tianjin Natural Science
Foundation (No. 18JCZDJC31500).
where i is the peak current, n is the scan rate, and a and b are
parameters. The b value is the slope of the linear t of a log(n) vs.
log(i) plot (Fig. 4d). The cathodic and anodic slope values were
Notes and references
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