Novel Lamellar Tetrapotassium Pyromellitic Organic for Robust High-Capacity Potassium Storage

Article abstract of DOI:10.1002/anie.202103052

Redox-active organics are investigation hotspots for metal ion storage due to their structural diversity and redox reversibility. However, they are plagued by limited storage capacity, sluggish ion diffusion kinetics, and weak structural stability, especi

Full text of DOI:10.1002/anie.202103052

Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 11835–11840
International Edition: doi.org/10.1002/anie.202103052
German Edition: doi.org/10.1002/ange.202103052
Batteries
Hot Paper
Novel Lamellar Tetrapotassium Pyromellitic Organic for Robust High-
Capacity Potassium Storage
Qingguang Pan, Yongping Zheng, Zhaopeng Tong, Lei Shi, and Yongbing Tang*
Abstract: Redox-active organics are investigation hotspots for
metal ion storage due to their structural diversity and redox
reversibility. However, they are plagued by limited storage
capacity, sluggish ion diffusion kinetics, and weak structural
stability, especially for K+ ion storage. Herein, we firstly
reported the lamellar tetrapotassium pyromellitic (K4PM) with
four active sites and large interlayer distance for K+ ion storage
based on a design strategy, where organics are constructed with
the small molecular mass, multiple active sites, fast ion
diffusion channels, and rigid conjugated p bonds. The K4PM
electrode delivers a high capacity up to 292 mAhgꢀ1 at
50 mAgꢀ1, among the best reported organics for K+ ion
storage. Especially, it achieves an excellent rate capacity and
long-term cycling stability with a capacity retention of ꢁ 83%
after 1000 cycles. Incorporating in situ and ex-situ techniques,
the K+ ion storage mechanism is revealed, where conjugated
carboxyls are reversibly rearranged into enolates to stably store
K+ ions. This work sheds light on the rational design and
optimization of organic electrodes for efficient metal ion
storage.
challenge consists of searching for suitable electrode materi-
als.
Recently, extensive candidates for K+ ion storage have
been investigated including intercalation-type carbonaceous
materials,[4] and Mxenes,[5] intercalation and conversion-type
chalcogenide,[6] alloy-type metals[7] and phosphide,[8] which all
inevitably suffer from the distinct volume expansion ascribed
to the insertion of large-sized K+ ions, resulting in the battery
capacity fading.[9] Therefore, it remains a challenge to develop
high efficient electrode materials. Recently, organics have
attracted much attention for their structural diversity, flexible
designability, and redox reversibility.[10] For example, conju-
gated carbonyl compounds,[11] nitroaromatics,[12] azo,[13] poly-
mers,[14] covalent organic frameworks,[15] metal-organic frame-
works,[16] etc., have been proved can reversibly manipulate
molecular structures to accommodate large K+ ions without
structural collapse.[17] However, they still need to be further
optimized on account of the finite ion storage capacity,
sluggish diffusion kinetics of K+ ions, and poor structural
robustness.[18] To address the above issues, organics for metal
ion storage should present the following properties. Firstly, to
achieve the high specific capacity, organics should possess
small molecular mass and rich active sites as many as possible.
Secondly, if they could contribute the fast ion diffusion
channels, the ion diffusion barriers for K+ ion storage can be
decreased, resulting in the fast rate capacity. Further, the
structural stability could be enhanced, if the organic mole-
cules contained rigid conjugated p bond groups.
L
ithium-ion batteries (LIBs) have dominated the global
energy storage market for portable electronics, electric
vehicles and grid-scale storage in the past decades.[1] Never-
theless, the resource scarcity in the Earthꢀs crust
(0.0017 wt%) and maldistribution of lithium have become
significant obstacles to impede the sustainable advance of
LIBs.[2] Noteworthily, potassium-ion batteries (PIBs) with the
similar energy storage mechanism and adjacent redox couple
to LIBs (ꢀ2.93 V for K+/K vs. ꢀ3.04 V for Li+/Li) have been
one of the research hotspots benefiting from the abundant
reserves of potassium resource (2.09 wt%).[3] Nevertheless,
the exploration of PIBs is still in its infancy, and the key
Based on the above strategy, we firstly design and
synthesize a lamellar tetrapotassium pyromellitic (K4PM)
via a one-step and scalable method. In this case, we choose
pyromellitic acid (PMA) as the precursor owing to its small
molecular mass, four K+ ion storage sites, convenient ion
diffusion routes, and rigid conjugated p bonds from the
benzene ring. Accordingly, the as-prepared layered K4PM
achieves a high theoretical capacity, fast K+ ion diffusion
kinetics and excellent robustness for K+ ion storage due to its
four conjugated carboxylate groups as active sites and a large
interlayer distance than conventional graphite materials.
Eventually, the above contributions synergistically boost the
[*] Dr. Q. G. Pan, Dr. Y. P. Zheng, Z. P. Tong, L. Shi, Prof. Y. B. Tang
Functional Thin Films Research Center, Shenzhen Institute of
Advanced Technology, Chinese Academy of Sciences
Shenzhen 518055 (China)
E-mail: tangyb@siat.ac.cn
potassium storage with
a high specific capacity of
Prof. Y. B. Tang
School of Chemical Science, University of Chinese Academy of
Sciences
Beijing 100049 (China)
and
Key Laboratory of Advanced Materials Processing & Mold, Ministry
of Education, Zhengzhou University
Zhengzhou 450002 (China)
292 mAhgꢀ1, good rate performance and excellent long
cycling stability with a capacity retention of ꢁ 83% after
1000 cycles, among the best results of K-organic batteries.
Typically, the four carboxylic acids in the precursor react
with KOH through a neutral reaction, where four H+ ions of
carboxylic acid in PMA can be substituted by four K+ ions,
forming the K4PM, as displayed in Figure 1a. X-ray diffrac-
tion (XRD) characterizations of the as-prepared sample and
precursor were performed, where peaks of as-prepared
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202103052.
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Figure 1. a) Synthesis route of K4PM. b) XRD pattern of K4PM. Vertical dark lines indicate the simulation diffraction peaks. c) FT-IR and d) Raman
spectra of PMA and K4PM. e) The crystalline structure of K4PM. f, g) SEM images of K4PM and h) corresponding EDS mapping images.
sample match well with simulation results of K4PM and are
obviously different PMA (Figure 1b and Figure S1 in Sup-
porting Information), especially for the main characteristic
peaks of 10.98, 13.48, 18.48, 21.88, 24.08, and 27.08, indicating
the high crystallographic forms of a triclinic lattice. Figure 1c
manifests the Fourier transform infrared (FT-IR) spectra of
as-prepared K4PM and PMA. The PMA exhibits the asym-
identify the uniform distribution of C, O, and K elements
(Figure 1h). According to the Brunauer-Emmett-Teller
(BET) measurement, the synthesized K4PM has a specific
surface area (31.90 m2 gꢀ1) and a large mesoporous volume
(Figure S3), in favor of the fast ion diffusion.[22]
The electrochemical kinetics and performances of K4PM
were investigated with a K foil as the counter electrode within
coin-type cells. Based on the relationship: i ¼ avb, the
contributions of the surface pseudocapacitive and diffusion
behaviors can be assessed by CV curves at different scan rates
(Figure 2a), where a and b are tunable values, when the value
of b is 0.5, representing the diffusion-controlled process, once
the value of b attains 1.0, meaning the surface capacitive-
controlled process.[23] Figure 2b shows that the value of b at
peak 1 and peak 2 are 0.71 and 0.73, respectively; therefore,
the kinetics for K4PM are the combined effect of surface
pseudocapacitive and diffusion behaviors. Further, the con-
tribution of the above behaviors can be differentiated
referring to the equation: i ¼ k1v þ k2v1=2, where k1v and
k2v1/2 denote the current from the surface capacitive effect and
diffusion-controlled process, respectively.[24] As the typical
example at the scan sweep of 0.2 mVsꢀ1 (Figure S4), the
surface capacitive-controlled contribution of K4PM electrode
is 28.1% for the K+ ion storage. Correspondingly, as depicted
in Figure 2c, the diffusion-controlled processes are still the
=
metric and symmetric stretching vibrations of -C O (i.e., uas(-
ꢀ1
C O) at 1690 cm and us(-C O) at 1406 cmꢀ1), which belong
to the characteristic of the carboxylic acid group.[19] As for the
synthesized K4PM, the emerged peaks at 1561, 1367, and
522 cmꢀ1 match well with uas(-COO), us(-COO), and the -OK
out-of-plane bending vibration (i.e., u(-OK)), while the peaks
of PMA at 1690 and 1406 cmꢀ1 disappear.[19] Raman spectra
(Figure 1d) also show two peaks at 1567 and 1370 cmꢀ1
corresponding to the uas(-COO) and us(-COO) of K4PM,
respectively, obviously different from the PMA.[19,20] Accord-
ingly, the crystalline structure of K4PM is simulated as
demonstrated in Figures 1e and S2, which features the
layered structure with an interlayer space of 4.71 ꢁ, much
larger than that of graphite materials (3.35 ꢁ),[21] possibly
beneficial for the large ion storage. Moreover, the typical
scanning electron microscope (SEM) images of K4PM exhibit
the lamellar structure (Figures 1 f and g) and corresponding
energy disperse spectroscopy (EDS) mapping images further
=
=
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Figure 2. a) CV curves of K4PM electrode for PIBs at different scan rates from 0.2–1 mVsꢀ1, b) Calculation of b values from the relationship
between the scan rate and peak currents. c) The percent of pseudocapacitive contribution at different scan rates. d) Discharge GITT pattern of the
K4PM electrode, inset: the corresponding ion diffusion coefficient. e) Typical charge/discharge curves of different current densities. f) Rate
performance. g) Comparison of rate performances between other K-organics cells and this work. () Long-term cycling performance of K4PM
electrode at a current density of 500 mAgꢀ1
.
main behaviors, although surface capacitive-controlled con-
tributions increase up to 43.7% with the rise of scan sweeps.
Further, the diffusion coefficient (D) of K+ can be obtained
by the galvanostatic intermittent titration technique (GITT in
Figure 2d) based on Fickꢀs second law:
than the reported organic electrodes,[13,28] further indicating
the fast ion transfer in K4PM. Meanwhile, the EIS plot of
K4PM after 1000 cycles was also tested, where the RCT value is
lower than that of the fresh K4PM. The reduction of resistance
after cycles is mainly ascribed to the activation process of
K4PM for potassium ion storage.
The discharge/charge profiles of K4PM electrodes were
depicted under the enhancive current densities from 0.5 C to
5 C (Figure 2e, 1 C = 100 mAgꢀ1), where the K4PM electrode
displays two symmetric discharge/charge plateaus at 0.31/
0.63 V and 0.54/0.89 V, respectively. The difference between
discharge and charge plateaus is due to the battery polar-
izations. Meanwhile, it delivers the discharge specific capaci-
ties of ꢁ 292, 265, 245, and 223 mAhgꢀ1 at the current
densities of 0.5 C, 1 C, 2 C, and 5 C with ꢁ 100% Coulombic
efficiencies. Subsequently, the corresponding rate perform-
ances were evaluated (Figure 2 f), where it regains a reversible
capacity of ꢁ 275 mAhgꢀ1 after the current density is
returned to 0.5 C, indicating the good reversibility, which is
better than previously reported results of K-organic batteries
(Figure 2g).[13,14b,17b,18c,29] Further, the long-term cycling per-
2
2
ð1Þ
D ¼ 4=pt ððmBVMÞ=ðMBSÞÞ =ððDESÞ=ðDEt ÞÞ
where t is the pulse duration, mB, MB, and VM are the active
mass, molar mass, and molar volume of K4PM, respectively, S
is the surface area of the electrode, DES and DEt are the
voltage change caused by current pulse and by galvanostatic
discharging, respectively.[25] Here, the K4PM electrode ach-
ieves
a
large diffusion coefficient (D) of K+ ( ꢁ 4 ꢂ
10ꢀ11 cm2 sꢀ1, inset of Figure 2d),[26] which derives from the
layered structure and large mesoporous volume. Next, the
charge transfer kinetics between the electrolyte and electro-
des can be tested by the electrochemical impedance spec-
troscopy (EIS) plots.[27] As displayed in Figure S5, the charge
transfer resistance (RCT) of K4PM electrode from the semi-
circle in the low frequency range is ꢁ 110 W, which is smaller
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formances of K4PM electrode was tested at a high current
density of 500 mAgꢀ1 (Figure 2h), where its discharge
capacities maintain over 205 mAhgꢀ1 with the Coulombic
efficiency of ꢁ 100% and a capacity retention of ꢁ 83% after
1000 cycles, among the best reported K-organics batteries
(Table S1). And the discharge/charge plots under different
cycles indicate the robust stability with nearly overlapped
curves and voltage platforms, especially from 400th to 1000th
cycles (Figure S6), which reveals the low electrode polar-
ization and reversible redox behavior.
ascribed to u(-KO) is split into two peaks at 528 and 515 cmꢀ1
(Figure S7), which should be originated from the formation of
ꢀ
new K O bond, indicating that the transition from conjugated
carboxyls to enolates.[11,17b] As for the depotassiation process,
the peak at 1656 cmꢀ1 weakened gradually, assigned to the
=
ꢀ
recovery of non-benzene C C bonds into C C bonds with the
enhancement of uas(-COO) and us(-COO), and vibrations at
528 and 515 cmꢀ1 receded with the increase of the peak at
522 cmꢀ1 (Figure S7), suggesting the reversible transforma-
tion of the functional groups. Raman characterizations during
the discharge/charge process also verified that the reversible
evolution of conjugated carboxyls into enolates referring to
To explore the K+ ion storage mechanism of K4PM, we
incorporated in situ and ex-situ characterizations to inves-
tigate the variations of functional groups and stability of
crystal structure during the discharge/charge process. The in
situ FT-IR spectra were measured in the range of 0.01–3 V to
detect the evolution of functional groups of K4PM during the
potassiation/depotassiation process (Figures 3a and S6),
where the background of data was based on the electrode
and electrolyte (details in Supporting Information). During
the reversible decrease of uas(-COO) at 1567 cmꢀ1 and us(-
ꢀ1
=
COO) at 1370 cm , and the reversible increase of u(C C) at
ꢁ 1656 cmꢀ1 (Figure 3c).[26,30] XPS spectra further confirmed
the reversible enolization reaction of the carbonyl group
(Figures 3d,e, and S8), where the peak intensities of C-O
=
(peaks at 282.8 eV for C 1s and 532.5 eV for O 1s) and C C
(peak at 284.5 eV for C 1s) bonds are reversibly enhanced and
=
=
the potassiation process, the non-benzene C C vibration
peak intensity of C O (peaks at 288.2 eV for C 1s and
(u(C C)) at ꢁ 1656 cmꢀ1 emerges with the gradually
530.9 eV for O 1s) bonds are reversibly weakened according
to deconvolution results of C1s and O1s spectra[31] during the
discharge/charge process, which match well with the FT-IR
=
enhanced intensities, along with the decrease of uas(-COO)
and us(-COO) (Figure 3b),[17b] while the peak at 522 cmꢀ1
Figure 3. a) Typical discharge/charge curves. b) Corresponding spectra of in situ FT-IR, c) ex-situ Raman spectra, d) C 1s and e) O 1s spectra of
ex-situ XPS spectra, and f) in situ XRD patterns. g) Proposed energy storage mechanism of K4PM and corresponding variation of molecular
structure during discharge/charge process.
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and Raman results. Moreover, the diffraction peaks at ꢁ 21.88
dispersed on the benzene ring, similar to that of graphite with
and 248 reversibly shift to small angles and large angles in in
situ XRD patterns during the discharge/charge process,
indicating the expansion and shrinkage of lattice planes
owing to the accommodation of K+ according to the Bragg
equation: 2dsinq = nl. After the charging process, the above
two peaks recover gradually to the original states demon-
strating remarkable structural stability. Based on the above
investigations, the possible storage mechanism of K4PM is
illustrated in Figures 3g and h, where the conjugated carbox-
yls are reversibly rearranged into enolates to accommodate
K+ ions with the structural reversibility during the discharge/
charge process.
delocalized p bonds,[35] manifesting potential structural sta-
bility and electronic conductivity. All these simulations
demonstrate that K4PM can accommodate K+ ions with
good ion diffusivity, fine electronic conductivity and robust
stability, contributing excellent electrochemical performan-
ces.
In summary, we synthesized a laminar K4PM electrode for
potassium ion storage based on a design strategy, where
organics are devised with small molecular mass, rich active
sites, fast ion diffusion channels, and rigid conjugated p bonds.
The K4PM delivered expected performances for its abundant
K+ ion storage sites and large interlayer distance. It exhibited
a high capacity of ꢁ 290 mAhgꢀ1, good rate performance and
excellent cycling stability with a capacity retention of ꢁ 83%
after 1000 cycles. Further, DFT simulations incorporating
with in situ and ex-situ techniques revealed that the K4PM
demonstrates a robust structural stability and the conjugated
carboxyls are reversibly rearranged into enolates to accept K+
ions during the discharge/charge process. We believe that this
study may pave the way for rational design and synthesis of
ideal organics for high-performance PIBs.
The first-principles density functional theory (DFT)
calculations were further performed to understand the
structural characteristic of K4PM for potassium-ion storage.
Figure 4a displays the diffusion processes of a K atom in the
interlayer channels of K4PM, where the highest diffusion
energy barrier of K atom in K4PM is only 0.322 eV (Fig-
ure 4b), which manifests that the K4PM delivers a small
diffusion resistance of K+ ions[32] due to the expanded
interlayer distance (Figure 1e) than traditional graphite
materials, in line with the EIS results (Figure S5) and
contributing to the good rate performance (Figure 2e).[33]
Subsequently, the electronic properties of K4PM were inves-
tigated by evaluating the variations of total density of states
associated with K-atom intercalation into K4PM. As demon-
strated in Figure 4c, the pristine K4PM possesses typical
organic semiconductor properties with large band gaps. After
introducing a single K-atom, its Fermi level (Ef) shifts to the
conduction band edge, indicating the transition to the n-type
semiconductor state upon K intercalation, conducive to the
structural conductivity and the decrease of electrode impe-
dance.[34] Further, the charge density difference of K4PM on K
intercalation was investigated as shown in Figure 4d. The
charge depletion centers (cyan) are the intercalated K atoms,
and the partial charge accumulation centers (yellow) could be
Acknowledgements
The authors gratefully acknowledge financial support from
the Key Area Research and Development Program of
Guangdong Province (2019B090914003), National Natural
Science Foundation of China (51822210, 51972329,
52061160484, 22005330), China Postdoctoral Science Foun-
dation (2020M672871), Shenzhen Science and Technology
Planning
Project
(JCYJ20190807171803813,
JCYJ20200109115624923), and Science and Technology Plan-
ning Project of Guangdong Province (2019TX05L389,
2018A050506066).
Conflict of interest
The authors declare no conflict of interest.
Keywords: battery · carboxyls · organic electrode · potassium ·
tetrapotassium pyromellitic
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Manuscript received: March 1, 2021
Accepted manuscript online: March 15, 2021
Version of record online: April 13, 2021
11840 www.angewandte.org
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Angew. Chem. Int. Ed. 2021, 60, 11835 –11840