Oxocarbon Salts for Fast Rechargeable Batteries

Article abstract of DOI:10.1002/anie.201607194

Oxocarbon salts (M₂(CO)_n) prepared through one-pot proton exchange reactions with different metal ions (M=Li, Na, K) and frameworks (n=4, 5, 6) have been rationally designed and used as electrodes in rechargeable Li, Na, and K-ion batteries. The results show that M₂(CO)₅/M₂(CO)₆salts can insert two or four metal ions reversibly, while M₂(CO)₄shows less electrochemical activity. Especially, we discover that the K₂C₆O₆electrode enables ultrafast potassium-ion insertion/extraction with 212 mA h g^?1at 0.2 C and 164 mA h g^?1at 10 C. This behavior can be ascribed to the natural semiconductor property of K₂C₆O₆with a narrow band gap close to 0.9 eV, the high ionic conductivity of the K-ion electrolyte, and the facilitated K-ion diffusion process. Moreover, a first example of a K-ion battery with a rocking-chair reaction mechanism of K₂C₆O₆as cathode and K₄C₆O₆as anode is introduced, displaying an operation voltage of 1.1 V and an energy density of 35 Wh kg^?1. This work provides an interesting strategy for constructing rapid K-ion batteries with renewable and abundant potassium materials.

Full text of DOI:10.1002/anie.201607194

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607194
German Edition: DOI: 10.1002/ange.201607194
Energy Storage
Hot Paper
Oxocarbon Salts for Fast Rechargeable Batteries
Qing Zhao, Jianbin Wang, Yong Lu, Yixin Li, Guangxin Liang, and Jun Chen*
Abstract: Oxocarbon salts (M2(CO)n) prepared through one-
pot proton exchange reactions with different metal ions (M =
Li, Na, K) and frameworks (n = 4, 5, 6) have been rationally
designed and used as electrodes in rechargeable Li, Na, and K-
ion batteries. The results show that M2(CO)5/M2(CO)6 salts can
insert two or four metal ions reversibly, while M2(CO)4 shows
less electrochemical activity. Especially, we discover that the
K2C6O6 electrode enables ultrafast potassium-ion insertion/
extraction with 212 mAhgÀ1 at 0.2 C and 164 mAhgÀ1 at 10 C.
This behavior can be ascribed to the natural semiconductor
property of K2C6O6 with a narrow band gap close to 0.9 eV, the
high ionic conductivity of the K-ion electrolyte, and the
facilitated K-ion diffusion process. Moreover, a first example
of a K-ion battery with a rocking-chair reaction mechanism of
K2C6O6 as cathode and K4C6O6 as anode is introduced,
displaying an operation voltage of 1.1 V and an energy density
of 35 WhkgÀ1. This work provides an interesting strategy for
constructing rapid K-ion batteries with renewable and abun-
dant potassium materials.
Oxocarbon, which has been first introduced in 1963, can
be regarded as the polycarbonyl compounds.[8] The carbonyl is
redox center of cation (H+, Li+, Na+, K+, Mg2+) insertion/
extraction.[2c] Thus, the oxocarbon compounds with salted
substituents will process the superiorities of high theoretic
capacity and low dissolution. In 2008, Li2C6O6 was initially
investigated as a high capacity cathode for rechargeable
lithium batteries, displaying a high capacity of 580 mAhgÀ1 in
the first cycle.[5a] After that, other oxocarbon salts such as
Na2C5O5 and Na2C6O6 were also reported in rechargeable
lithium and sodium batteries.[5d–g] These previous studies have
verified the potential interest of oxocarbon salts for energy
storage, while the comprehensive investigation of oxocarbon
salts with various frameworks/substituents is stilled limited.
Meanwhile, the accommodating ability of big cations such as
the K ion with oxocarbon salts is also unexploited.
Benefiting from the widespread K-ion transport, high
abundance of K resource, and low standard electric potential
of K+/K, the rechargeable K-based batteries have been paid
durable attention, while the study of proper hosts for inserting
large K ions (r(K+) = 152 pm, r(Li+) = 73 pm) is still challeng-
ing.[9] The electrochemical redox behavior with organic
carbonyl compounds is based on a conversion reaction,
which is less limited by the cation radius[10] and hence
provides the possibility of affording the bigger K-ion. More-
over, with an elaborate design of substituents, the oxocarbon
salt also offers the opportunity of constructing an all-organic
rocking-chair K-ion battery without using the risky metal
potassium.
Herein, nine oxocarbon salts of M2(CO)n (M = Li, Na, K,
n = 4, 5, 6) with tailored frameworks and substituents are
rationally designed for rechargeable batteries. With different
metals as anodes (Li2(CO)n with Li anode; Na2(CO)n with Na
anode; K2(CO)n with K anode), the electrochemical proper-
ties of these nine salts are systematically investigated and
compared. The four-membered ring salts have difficulties to
take up metal ions in a selected voltage region. The five/six-
membered ring salts are feasible to take up a certain number
of metal ions. Remarkably, we firstly found that K2C6O6 (or
K2C5O5) can be applied as an ultrafast K-ion insertion/
extraction host with two K-ion reactions per compound. The
discharge capacity of K2C6O6 is 212 mAhgÀ1 at 0.2 C and
164 mAhgÀ1 at a higher rate of 10 C. DFT calculations show
that K2C6O6 is a semiconductor with a narrow band gap close
to 0.9 eV. Moreover, the K-ion electrolyte processes natural
higher ionic conductivity than Li/Na-ion electrolyte. Mean-
while, K-ion in K2C6O6 also shows faster diffusion than Li/Na-
ion in Li2C6O6/Na2C6O6. As a pioneer, the K-ion battery with
K2C6O6 as cathode and K4C6O6 as anode was constructed and
exhibited an energy density of 35 WhkgÀ1.
B
uilding better battery systems with advanced and renew-
able materials is essentially significant for future energy
storage and conversion of human society.[1] Among the
considerable efforts on developing novel electrode materials
with earth-abundant elements, organic electrode materials
have revealed powerful competence owing to their intrinsic
designability, high abundance, and sustainability.[2] A target
issue that needs to be resolved for organic electrode materials
such as quinones is their high solubility in aprotic electrolyte,
which results in fast capacity/cycling decay of rechargeable
batteries.[3] Throughout the various strategies on conquering
this problem, the design of “inorganic/organic” hybrid
materials (metalorganic salts) is intriguing.[4] This type of
hybrid materials endowed with high polarity and intermolec-
ular chelate bonds exhibits less solubility in aprotic electro-
lyte, maintaining an ameliorative cycling life. Up to now,
a series of organic salts with functional groups of metal
enolate (-OLi, -ONa),[5] metal carboxylate (-COOLi,
-COONa)[6] and sulfonate (-SO3Na)[7] have been applied for
rechargeable lithium and sodium batteries.
[*] Dr. Q. Zhao, Dr. J. Wang, Dr. Y. Lu, Dr. Y. Li, Prof. G. Liang,
Prof. J. Chen
Key Laboratory of Advanced Energy Materials Chemistry and
State Key Laboratory of Elemento-Organic Chemistry
College of Chemistry, Nankai University, Tianjin 300071 (China)
E-mail: chenabc@nankai.edu.cn
Prof. J. Chen
Collaborative Innovation Center of Chemical Science and
Engineering, Nankai University Tianjin 300071 (China)
The organic oxocarbon salts with active carbonyl com-
pounds (Scheme 1) were obtained in simple one-pot proton
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607194.
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=
=
ing to C C bonds and C O bonds can be detected in the
FTIR spectra. In general, the particle size of the as-prepared
compounds ranges from several hundred nanometers to
several micrometers. Meanwhile, most oxocarbon salts are
hardly soluble in organic electrolyte (Figure S7).
The batteries were tested with different metals, such as Li,
Na, and K, as anodes and oxocarbon salts as cathodes. The
carbonyl activities with initial discharge capacities are sum-
marized in Figure 1a. The four-membered ring salts show an
ignorable capacity after eliminating the contribution of
Super P conducting additive (Figure S8a–f). This result sug-
gests that the four-membered ring oxocarbon salts are unable
to insert enough metal ions. The steric hindrance or electro-
static repulsion is considered primarily responsible. In
comparison, five-membered oxocarbon salts can insert
about two metal-ions (Li+, Na+, and K+) in the first cycle
(Figure S9a–9d). Among them, the Li/Li2C5O5 battery dis-
plays the highest operation potential due to the lower
standard potential of Li+/Li and less electron-donating
Scheme 1. Preparation, structures, and theoretic reactions of designed
oxocarbon salts.
exchange reactions. In brief, M2C4O4, M2C5O5, and M2C6O6
were prepared by reactions of square acid (H2C4O4), croconic
acid (H2C5O5), and rhodizonic acid (H2C6O6·2H2O) and the
corresponding hydroxide (MOH) or carbonates (M2CO3) in
water as solvent, respectively (experiment details are given in
the Supporting Information). In addition, the excess anneal-
ing process is necessary to remove crystal water from some
products (Li2C5O5 and Li2C6O6). The structures, morpholo-
gies, thermostabilities of the prepared composites were
confirmed and characterized with Fourier Transform infrared
spectroscopy (FTIR), X-ray powder diffraction (XRD),
scanning electron microscope (SEM), and thermogravimetry
analysis (TG) (see Figures S1–S6 in the Supporting Informa-
tion). All prepared oxocarbon salts exhibit high crystallinity
and thermostability (up to 3008C). Typical vibrations belong-
À
properties of the OLi group. The six-membered ring cathode
reveals high reversibility. The Li/Li2C6O6 battery is able to
take up four Li ions with an initial capacity close to
580 mAhgÀ1. The capacity fades fast with cycling and only
a capacity of less than 300 mAhgÀ1 is obtained after 10 cycles
owing to the exfoliation of the C6O6 layers (Figure S10a).[5b]
As a comparison, Na/Na2C6O6 (Figure S10b) and K/K2C6O6
(Figure S10c) batteries can take up two electrons and metal
ions. In addition, the capacity of K2C6O6 decays only by 0.4%
per cycle from the 2nd to 100th cycling (Figure S11). When
the operation voltage is widened, the use of carbonyl
compounds in Na2C6O6 and K2C6O6 is also up to 4 (Fig-
Figure 1. Electrochemical performance of oxocarbon salts in rechargeable batteries. a) Initial discharge capacity and corresponding use of carbonyl
groups in the oxocarbon salts. The results were obtained with M/M2(CO)n batteries (M=Li, Na, K; n=4, 5, 6). The current density is 0.2 C
(1 C=200 mAgÀ1). The inset is a SEM image of K2C6O6. b) Comparison of the capacity retention of M/M2(CO)n batteries at different current
density. c) Characterization of the ionic conductivity of different concentrations of Li, Na, and K-ion electrolytes. d) Density of states (DOS) with
crystal structures of K2C6O6. e) Cyclic voltammetry (CV) curves of K/K2C6O6 battery at different scan rates. f) Comparison of diffusion coefficient of
Li/Li2C6O6, Na/Na2C6O6 and K/K2C6O6 batteries. The electrodes were prepared with the active materials, super P and polyvinylidene fluoride
(PVdF) with a mass ratio of 6:3:1. The mass loading of each electrode is about 1.5–2.0 mgcmÀ2
.
2
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ure S12a,b), while the use in five-membered oxocarbon salts
with the increase of scan rate, ascribing to the enhanced
is nearly unchanged (Figure S13a–c). In general, the oper-
ation voltage of the oxocarbon salts increase from the four- to
five- and six-membered ring salts (Figure S14), which is
attributed to the gradually reduced energy of the lowest
unoccupied molecular orbital (LUMO; Table S1) and is in
accordance with the reported rules on dicarbonyl com-
pounds.[11] As a short summary of preliminary test on the
nine oxocarbon salts, the four-membered ring oxocarbon salts
reveal less activity for rechargeable batteries and M2(CO)5 is
shown as the smallest ring for inserting metal ions.
Specially, the batteries with oxocarbon K-salts (K2C5O5
and K2C6O6) show fast K-ion reaction activities. The dis-
charge capacities of K/K2C6O6 battery are 210, 205, 192, 177,
164 mAhgÀ1 at current densities of 0.5 C, 1 C, 2 C, 5 C and
10 C, respectively (Figure S15c). The capacity retention of
10 C is close to 80% of that at 0.2 C. Meanwhile, this superior
high-rate performance is remained in the following cycling
(Figure S16). However, the capacity retention of 10 C to 0.2 C
for Na/Na2C6O6 battery is 49% and it is only 11% for Li/
Li2C6O6 battery (Figure 1b). In addition, the K/K2C6O6
battery also reveals less voltage decrease at high current
density than Na/Na2C6O6 and Li/Li2C6O6 (Figure S15a,b).
This high rate performance is considered as the natural
character of K2C6O6. When we reduce the ratio of conduc-
tivity carbon (only 10%), it also remains a capacity of
171 mAhgÀ1 at 5 C (Figure S17). In addition, the high-rate
performance of K/K2C5O5 battery is also much more out-
standing than Li/Li2C5O5 and Na/Na2C5O5 batteries (Fig-
ure S18a–c). Furthermore, this rapid K-ion insertion activity
is propagable for other organic electrodes (For instance, 1,4-
anthraquinone, AQ). K-AQ battery also unfolds the best
high-rate performance among metal-AQ batteries (Fig-
ure S19a–e).
polarization by increasing scan rate. The diffusion coefficients
of metal ion in M2C6O6 were calculated with the Randles–
Sevcik equation (details are given in the Supporting Infor-
mation).[13] We estimate two oxidation peaks and two
reduction peaks of K/K2C6O6 batteries. The peak currents
are linear with the square roots of scan rate (Figure S22). The
diffusion coefficients of K ion calculated by the four peaks are
about 5 ꢀ 10À10 cm2 sÀ1 (Figure 1 f), which exceeds the diffu-
sion coefficients of Li/Li2C6O6 (Figure S23a,b) and Na/
Na2C6O6 (Figure S24a,b) batteries. The higher diffusion
coefficients also imply the fast rate performance of the K/
K2C6O6 battery. The intrinsic semiconductor character, high
ionic conductivity K-ion electrolyte, and fast K-ion diffusion
synergistically contribute to the fast rate performance of the
K/K2C6O6 battery.
According to the discharge capacity, K2C6O6 is able to
undergo two-electron transfer reactions with K4C6O6 as the
discharge product (Figure 2a). This reaction mechanism is
further confirmed by in situ Raman measurement (Fig-
ure 2b). For fresh electrodes (points 1), two major peak
regions can be observed. The yellow region stands for the
breathing vibration of the ring skeleton and the blue region
stands for the vibration of the carbonyl compounds.[14] The
carbonyl compounds are the active groups because their
vibration signals gradually fade with increasing discharge
depth. Meanwhile, the vibration of ring breathing is well
remained, which proves the molecular structural stability of
K2C6O6. During the charging process, the vibration of the
carbonyl compound recurs to the original states, demonstrat-
To unravel the reasons of fast rate performance, the ionic
conductivity of a different electrolyte concentration (Fig-
ure 1c and Table S2) was tested. Finally, we found that the
1.25m KPF6/DME (dimethoxyethane) electrolyte showed the
highest ionic conductivity, exceeding the highest conductivity
of Li-ion (1m LiPF6/EC (ethylene carbonate), DEC (diethyl
carbonate)) and Na-ion (1.25m NaPF6/DME) electrolytes.
Although the K ion has a larger ionic radius than Li and Na
ions, the K ion is considered slightly soluble, which enables
the fast motion of the K ion.[9g] DFT calculations on the
density of states (DOS) of K2C6O6 were carried out. As
illustrated in Figure 1d, the band gap of K2C6O6 is 0.9 eV,
which is narrower than that of Na2C6O6 (Figure S20). This
proves the inherent semiconductor feature of K2C6O6. In
addition, the order of electric conductivity is K2C6O6 >
Na2C6O6 > Li2C6O6, which agrees well with the rate perfor-
mance (Figure S21). K2C6O6 presents a typical layered
structure.[12] The K ion can diffuse between the layers and
the electrons can easily transfer along the conjugated benzene
ring layers. Furthermore, cyclic voltammetry (CV) measure-
ment was applied to record the redox properties of the K/
K2C6O6 battery from the second cycle. Two reduction peaks at
2.4 V/1.2 V and two oxidation peaks at 2.8 V/1.3 V can be
observed (Figure 1e). The reduction peaks shift to lower
potentials and the oxidation peaks shift to higher potentials
Figure 2. Characterization of the reaction mechanism of the K/K2C6O6
battery. a) Discharge/charge curves with marked points for in situ
Raman tests. b) Corresponding Raman spectra. c) The most stable
structure of the discharge product (K4C6O6). d) HOMO plots of
4À
C6O6
.
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ing the high reversibility of the K/K2C6O6 battery. In addition,
this reversible property was maintained in the following
discharge/charge process (Figure S25). Two possible struc-
tures of K4C6O6 exists after discharge. The inserted K ion can
occupy the para- (Figure 2c) or ortho-positions (Figure S26)
toward the original K-enolate groups (-OK). Finally, DFT
calculation results reveal that the para-position is more
favorable, which is 0.01057 a.u. lower than the ortho one.
Plots of the highest occupied molecular orbital (HOMO)
express the location of the valence-shell electron, which can
be used to evaluate the stability of the organic compounds
improvement, a state-of-the-art rocking-chair K-ion battery
was obtained from inorganic–organic hybrid materials, which
processes the advantages of high renewability and wide
abundance. Moreover, K2C6O6 can be directly prepared from
the natural product myo-inositol and K4C6O6 can be directly
obtained through reaction of tetrahydroxy-quinone and
KOCH3.[5a,16] Thus, the K-ion battery with a rocking-chair
reaction mechanism can be obtained without using K metal.
In summary, oxocarbon salts with different frameworks
and metal ions have been used as electrodes in rechargeable
(Li, Na, K) batteries. The working voltage and capacity are
visually compared. As an intriguing result, we initially
discovered the fast K-ion insertion/extraction properties at
the K2C6O6 cathode, which shows capacities of 212 mAhgÀ1
at 0.2 C and 164 mAhgÀ1 at 10 C. The narrow band-gap, high
ionic conductivity of the K-ion electrolyte and the high
diffusion K-ion coefficient synergistically contribute to these
results. In addition, this high-rate performance can propagate
towards other K-ion insertion materials. Moreover, a K-ion
battery with rocking-chair reaction mechanism has been
demonstrated. Considering the high abundance of K resource
and the low standard electric potential of K+/K, our finding
not only provides an appealing chance for K-ion batteries but
also opens a new way of developing sustainable materials for
energy storage.
after gaining the electrons.[15] The HOMO structure of C6O6
4À
does not alter, indicating the stability of K2C6O6 after two-
electrons reactions (Figure 2d). In summary, K2C6O6 can
insert two K-ions by forming K4C6O6 in a discharge process.
After charging, these two K ions can be reversibly extracted
from the structures of K4C6O6 to give K2C6O6.
Inspired by the large interval between the initial and the
second K-ion insertion, a first example of rocking-chair K-ion
batteries with inorganic–organic electrode materials was
realized. The K/K2C6O6 battery can separately operate at
different regions with suggested reaction of K3C6O6/K4C6O6
at a discharge platform of 1.3 V (anode reaction, Figure 3a)
and K2C6O6/K3C6O6 at a discharge platform of 2.4 V (cathode
reaction, Figure 3b). The K3C6O6 is considered as a compound
with radical, which has been confirmed by electron spin
resonance (ESR) spectra (Figure S27). As a result, the
constructed K4C6O6/K2C6O6 battery displays an operation
voltage of 1.1 V (Figure 3c), corresponding to energy density
of 35 WhkgÀ1 (based on the mass of both cathode and anode).
Although the capacity and working voltage still need
Acknowledgements
This work was supported by National NSFC (grant numbers
21231005 and 51231003), and MOE (grant numbers B12015
and IRT13R30).
Keywords: batteries · electrodes · molecular engineering ·
renewable materials · sustainable chemistry
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Received: July 25, 2016
Revised: August 25, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Energy Storage
Organic electrode material: Oxocarbon
salts (M2(CO)n) with different metal ions
(M=Li, Na, K) and frameworks (n=4, 5,
6) were rationally designed and used as
electrodes for rechargeable Li, Na, and K-
ion batteries. A first example of a renew-
able and sustainable K-ion battery based
on K2C6O6 and K4C6O6 with a rocking-
chair reaction mechanism is shown.
Q. Zhao, J. Wang, Y. Lu, Y. Li, G. Liang,
J. Chen*
&&&& — &&&&
Oxocarbon Salts for Fast Rechargeable
Batteries
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!