Reversible Redox Chemistry of Azo Compounds for Sodium-Ion Batteries

Article abstract of DOI:10.1002/anie.201713417

Sustainable sodium-ion batteries (SSIBs) using renewable organic electrodes are promising alternatives to lithium-ion batteries for the large-scale renewable energy storage. However, the lack of high-performance anode material impedes the development of SSIBs. Herein, we report a new type of organic anode material based on azo group for SSIBs. Azobenzene-4,4′-dicarboxylic acid sodium salt is used as a model to investigate the electrochemical behaviors and reaction mechanism of azo compound. It exhibits a reversible capacity of 170 mAh g^?1 at 0.2C. When current density is increased to 20C, the reversible capacities of 98 mAh g^?1 can be retained for 2000 cycles, demonstrating excellent cycling stability and high rate capability. The detailed characterizations reveal that azo group acts as an electrochemical active site to reversibly bond with Na⁺. The reversible redox chemistry between azo compound and Na ions offer opportunities for developing long-cycle-life and high-rate SSIBs.

Full text of DOI:10.1002/anie.201713417

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Accepted Article
Title: Reversible Redox Chemistry of Azo Compounds for Sodium Ion
Batteries
Authors: Chao Luo, Gui-Liang Xu, Xiao Ji, Singyuk Hou, Long Chen,
Fei Wang, Jianjun Jiang, Zonghai Chen, Yang Ren, Khalil
Amine, and Chunsheng Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201713417
Angew. Chem. 10.1002/ange.201713417
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10.1002/anie.201713417
Angewandte Chemie International Edition
COMMUNICATION
Reversible Redox Chemistry of Azo Compounds for Sodium Ion
Batteries
Chao Luo,[a] Gui-Liang Xu,[b] Xiao Ji,[a,c] Singyuk Hou,[a] Long Chen,[a] Fei Wang,[a] Jianjun Jiang,[c]
Zonghai Chen,[b] Yang Ren,[b] Khalil Amine,[b,d] Chunsheng Wang*[a]
Abstract: Sustainable sodium ion batteries (SSIBs) using renewable
organic electrodes are promising alternatives to lithium ion batteries
for large-scale renewable energy storage. However, the lack of high
performance anode material impedes the development of SSIBs.
Here, we report a new type of organic anode material based on azo
group for SSIBs. Azobenzene-4,4’-dicarboxylic acid sodium salt is
used as a model to investigate the electrochemical behaviors and
reaction mechanism of azo compound. It exhibits a reversible capacity
of 170 mAh g-1 at 0.2C. When current density is increased 20C, the
reversible capacities of 98 mAh g-1 can be retained for 2000 cycles,
demonstrating excellent cycling stability and high rate capability. The
detailed characterizations reveal that azo group acts as an
electrochemical active site to reversibly bond with Na+. The reversible
redox chemistry between azo compound and Na-ion offer
opportunities for developing long-cycle-life and high-rate SSIBs.
Although Na-ion can co-intercalate into graphite with diglyme at a
low potential, its low specific capacity of <100 mAh g-1, limits the
energy density of SIBs.[13] Therefore, developing high
performance renewable anode materials becomes a major
challenge for SSIBs.
Organic compounds, which are abundant, inexpensive,
sustainable and recyclable, are promising anode materials for
SSIBs.[14-16] There are three categories of organic electrodes
based on carbonyl (C=O), imine (C=N) and doping reaction in
SIBs (Scheme 1).[17-21] The doping reaction-based organic
materials such as organic radicals display high reaction potentials
−
−
during p-doping process with anions (ClO4 , PF6 ), and are not
suitable for anodes, while the conventional organic anodes are
mainly based on carbonyl and imine compounds, which provide
carbonyl and imine redox centers to reversibly react with Na-ions
at relatively low reaction potentials.[22-25] The typical carbonyl
compounds such as sodium terephthalate[26,27] and benzoquinone
derivatives,[28,29] and imine compounds such as Schiff base
derivatives[30] exhibit high specific capacity (>150 mAh g-1) in SIBs.
However, the conventional organic anodes still suffer from low
power density and short cycle life. Most organic materials cannot
maintain high capacity at current density over 20C and very rare
organic material show a cycle life longer than 1000 cycles in SIBs.
No organic compounds can satisfy the requirements of high
power density and long cycle life for large-scale SSIBs.
With the advent of renewable energy for smart grids, Li-ion
batteries (LIBs), the dominant power supply for most portable
electronics, cannot satisfy the vast demand from the market due
to the lack of lithium resources in the earth crust.[1,2] It is significant
to search for an inexpensive and abundant alternative to LIBs for
the large-scale application of smart grids. Na-ion batteries (SIBs),
which share similar chemistry as LIBs, are promising alternatives,
because sodium is the sixth most abundant element in the earth.
In the last decade, extensive research efforts were devoted to
developing high performance SSIBs for large-scale renewable
energy storage, and significant advance in high performance
cathode materials (such as sodium metal oxides,[3-5] sodium metal
phosphates,[6,7] sulfur[8,9] and selenium[10,11]) has been achieved.
However, the lack of high performance anode material limits the
development of SIBs since the high performance LIB anodes
(such as graphite and silicon) are inactive for SIBs, while sodium
metal suffers from safety issue induced by dendrite growth.[12]
Here, we report a new type of organic electrode materials
based on azo group (N=N) for large-scale SSIBs. The azo group
can function as a redox center to reversibly react with Na-ions.
Three organic compounds (Fig. 1), azobenzene (AB), 4-
(phenylazo)benzoic acid sodium salt (PBASS) and azobenzene-
4,4’-dicarboxylic acid sodium salt (ADASS), were selected as
model azo compounds to evaluate their electrochemical
performance in SIBs. AB is selected as a basic model aromatic
azo compound since it contains only one functional azo group in
the center. However, it has high solubility in the organic electrolyte
as evidenced by fig. S1. The colorless electrolyte becomes
orange after mixing with AB. To suppress the dissolution of AB,
carboxylic salt groups are added in AB to generate PBASS and
ADASS. Among them, ADASS exhibits the best electrochemical
performance, in terms of high-rate capability and long cycle life.
ADASS provides 170 mAh g-1 at 0.2C, and retains 66% and 58%
of the capacity when the current density increases to 10C and
20C, respectively. Moreover, a reversible capacity of 98 mAh g-1
is retained for 2000 cycles at 20C with a slow capacity decay rate
of 0.0067% per cycle. The detailed characterizations using X-ray
diffraction (XRD), Raman spectroscopy and density functional
theory (DFT) calculation confirm azo group functions as an
electrochemical active site to reversibly bond with Na-ions as
[a]
Dr. C. Luo, X. Ji, S. Hou, Dr. L. Chen, Dr. F. Wang, Prof. C. Wang
Department of Chemical and Biomolecular Engineering
University of Maryland
College Park, MD 20740 (USA)
E-mail: cswang@umd.edu
[b]
[c]
[d]
Dr. G. Xu, Dr. Z. Chen, Dr. R. Yang, Dr. K. Amine
Chemical Sciences and Engineering Division
Argonne National Laboratory
Argonne, IL 60439 (USA)
X. Ji, Prof. J. Jiang
School of Optical and Electronic Information
Huazhong University of Science and Technology
Wuhan, Hubei, 430074 (China)
Dr. K. Amine
IRMC
Imam Abdulrahman Bin Faisal University (IAU)
Dammam (Saudi Arabia)
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201713417
Angewandte Chemie International Edition
COMMUNICATION
shown in scheme 1. Therefore, azo compounds are a new
category of anode materials for high performance SIBs.
is added to PBASS to form ADASS. As shown in fig. 2a, ADASS
displays two flat plateaus at 1.2V and 1.26V during discharge and
corresponding two charge plateaus at 1.37V and 1.43V, which is
consistent with one broad cathodic peak at 1.2V and two sharp
anodic peaks at 1.37V and 1.43V in cyclic voltammetry (CV) scan
(Fig. 2b). The single cathodic peak at 1.2V in CV scan
demonstrates that two pairs of sodiation/de-sodiation peaks are
close to each other and overlapped. Adding carboxylic salt group
in azobenzene cannot only reduce the solubility, but also change
the sodiation/desodiation potentials. The low solubility of ADASS
in the electrolyte enables ADASS to stably charge/discharge for
100 cycles with Coulombic efficiency of 100% even at a low rate
of 0.2C (Fig. 2c). Moreover, when the current density increases
from 0.2C to 40C (Fig. 2d), a reversible capacity of 71 mAh g-1 is
retained, demonstrating the robust reaction kinetics. To further
evaluate the long-term cycling stability, ADASS is cycled at 10C
and 20C for 1000 cycles and 2000 cycles (Fig. 2e and 2f), and
ADASS retains reversible capacities of 113 mAh g-1 after 1000
cycles and 98 mAh g-1 after 2000 cycles, further confirming the
high cycling stability and high-rate capability. Therefore, the
exceptional electrochemical performance of ADASS manifests
that azo compounds are promising anode materials for SIBs.
Scheme 1. Reaction mechanisms for organic electrode materials.
The structure and morphology of PBASS and ADASS were
characterized by XRD, Raman spectroscopy, Fourier-transform
infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA)
and scanning electron microscopy (SEM). As shown in fig. S2-3,
PBASS and ADASS have crystal structures with typical Raman
peak for azo group in 1400-1450 cm-1 range,[31] and typical IR
peaks for azo group in 1575-1630 cm-1 range.[32] In addition, they
display increasing baseline intensity in Raman spectra (Fig. S2b
and S3b) due to the fluorescence emitted by azo compounds. TG
analysis (Fig. S2d, S3d) shows PBASS and ADASS are stable up
o
o
to 405 C and 325 C, respectively. The SEM images (fig. S2e,
S3e) show that both PBASS and ADASS consist of micro-sized
particles. Therefore, chemical characterizations confirm the
molecular structure and particle size of PBASS and ADASS.
Figure 1. Molecular structure of (a) AB, (b) PBASS and (c) ADASS.
The electrochemical performance of azo compounds was
measured in coin cells with sodium metal as counter electrode. In
fig. S4a, AB exhibits two long and flat plateaus at 2.0V and 1.35V
with ultrahigh capacity of 929 mAh g-1 during the first discharge.
However, the high solubility of AB in the electrolyte significantly
reduces the corresponding charge capacity to 15 mAh g-1. To
reduce the solubility of AB, a carboxylic salt group is added into
AB to form PBASS. As shown in fig. S4b, PBASS delivers two
pairs of charge/discharge plateaus centered at 1.1V and 1.4V with
discharge capacities ~162 mAh g-1. However, the Coulombic
efficiency of PBASS is ~115% in the charge/discharge cycling
(Fig. S4c), demonstrating that PBASS still dissolves into
Figure 2. The electrochemical performance of ADASS in Na-ion batteries. (a)
The galvanostatic charge-discharge curves at 0.2 C; (b) Cyclic voltammograms
at 0.1 mV s-1; (c) De-sodiation capacity and Coulombic efficiency versus cycle
number at the current density of 0.2 C; (d) Rate performance at various C-rates;
De-sodiation capacity and Coulombic efficiency versus cycle number at 10 C
(e) and 20 C (f).
electrolyte, resulting
a severe shuttle reaction. To further
suppress the shuttle reaction, an additional carboxylic salt group
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peak current and scan rate (Fig. 3f) displays that the slopes of
both anodic and cathodic peaks are close to 0.5, demonstrating
the reaction kinetics of ADASS is determined by Na-ion
diffusion.[33] The extended π-conjugated structure in ADASS and
strong Na+ adsorption by nitrogen in azo group contribute to the
fast Na-ion diffusion.[34] Therefore, the small interphase
resistance/overpotential/potential hysteresis and fast Na-ion
diffusion result in the high-rate capability of ADASS.
Figure 3. Potential response of ADASS electrode in the (a) first cycle, (b)
second cycle, (c) third cycle during GITT measurements; (d) Equilibrium
potential versus specific capacity during GITT measurement; (e) CV curves of
ADASS at various scan rate; (f) The ln relationship of peak current and scan
rate for ADASS.
The mechanism for excellent electrochemical kinetics of
ADASS was investigated using electrochemical impedance
spectroscopy (EIS), galvanostatic intermittent titration technique
(GITT) and CV. Fig. S5 shows the EIS evolution of ADASS
electrode upon cycling. The interphase resistance of pristine
ADASS electrode, represented by the depressed semi-circle, is
very small (~20 Ohm), and only slightly increases to ~30 Ohm
after 5 cycles due to the formation of solid electrolyte interface
(SEI) layer. The small interphase resistance enables the high-rate
capability of ADASS electrode. GITT measurement was
conducted to check the overpotential and equilibrium potential of
ADASS upon cycling. As shown in fig. 3a-c, the qusi-equilibrium
potential in the first three charge/discharge cycles is stable
(Fig.3c). However, the overpotential of ADASS during the first
discharge is larger than the subsequent charge/discharge cycles
due to large strain/stress in the pristine ADASS (Fig.3a-c). After
the first discharge, the overpotential of ADASS at the
charge/discharge plateaus is small (~0.03V) and stable,
coincident with the small interphase resistance in fig. S5. The
equilibrium potentials obtained from GITT in fig. 3d show that the
charge/discharge equilibrium potentials of ADASS is at
1.37V/1.25V, and the potential hysteresis is only 0.12V. The small
overpotential and potential hysteresis validate the high-rate
capability of ADASS electrode. The reaction kinetics was further
studied using CV at various scan rate (Fig. 3e). With elevated
scan rate, the cathodic peaks shift to lower potential, while the
anodic peaks shift to higher potential, ascribing to the enhanced
polarization. The linear fit of natural logarithm ln relationship of
Figure 4. Reaction mechanism. (a) In situ synchrotron XRD patterns and color-
mapped profile of ADASS electrode, and the corresponding voltage profile; (b)
XRD spectra of ADASS electrodes before and after 5 cycles; (c) Raman spectra
of ADASS electrodes before and after 5 cycles and 20 cycles; (d) DFT
calculation results for the relative energies and optimized structures of AB,
PBASS and ADASS (The right axis aligns the relative energies in vacuum to the
relative potential versus SHE).
The phase and structure change of ADASS during
sodiation/desodiation were investigated using XRD, Raman
spectroscopy and DFT calculation. As shown in the in situ XRD
patterns (Fig. 4a), during the discharge from 2.5V to 0.5V, the
typical XRD peaks of ADASS at 16.6o, 17.3o and 18.2o gradually
decrease, while two new peaks at 19o and 22o appear after
ADASS is sodiated to below 1.2V, demonstrating the formation of
a new phase at 1.2V. The color-map clearly shows that the red
and yellow areas for XRD peaks of pristine ADASS become very
weak after ADASS is discharged from 2.5V to 1.2V, while the new
yellow areas for XRD peaks at 19o and 22o show up at 1.2V and
become stronger from 1.2V to 0.5V, confirming the phase
transformation during initial sodiation. Fig. 4b compares the XRD
patterns of pristine ADASS and fully charged ADASS electrodes
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10.1002/anie.201713417
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COMMUNICATION
after 5 cycles. The fully recovered XRD peaks of cycled ADASS
demonstrate excellent phase reversibility of ADASS in SIBs. In
the Raman spectra (Fig. 4c), when ADASS is fully discharged to
0.5V, the characteristic Raman peak at 1450 cm-1 for azo group
disappears. Instead, a new peak at 1295 cm-1, representing
sodiated azo group (Na-N-N-Na), appears, demonstrating the azo
group reacts with Na-ion during sodiation process. When ADASS
electrode is charged to 2 V, the characteristic Raman peak at
1450 cm-1 for azo group recovers, demonstrating reversible
electrochemical reaction between azo group and Na-ion.
Additionally, the Raman peak at 1600 cm-1 for carbonyl group in
ADASS does not change upon cycling, indicating carbonyl group
does not participate in the reaction with Na-ion. The reaction
mechanism of azo compounds was further confirmed using DFT
calculations. The energy levels of the lowest unoccupied
molecular orbital (LUMO) and the highest occupied molecular
orbital (HOMO) for AB, PBASS and ADASS are visualized in fig.
4d. The charge density isosurfaces of LUMO states for AB,
PBASS and ADASS demonstrate that electron localizes in the azo
group during sodiation, confirming azo group is the
electrochemical active site for the reduction of azo compound.
Therefore, the detailed characterizations and DFT calculations
confirm that azo group is the electrochemical active site to
reversibly react with Na-ions.
Keywords: Sodium ion batteries • Anode • Azo compound •
Organic electrode material • Fast charge and discharge
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Acknowledgements
This work was supported by the US National Science Foundation
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10.1002/anie.201713417
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A new category of organic electrode
material, azo compound, is reported
and exhibited one of the best
performances in organic sodium ion
batteries. The extended π-conjugated
structure in the aromatic azo
compound and strong adsorption
toward Na+ by nitrogen in azo group
enables the long cycle life and high-
rate performance of azo compound.
Chao Luo, Gui-Liang Xu, Xiao Ji, Singyuk
Hou, Long Chen, Fei Wang, Jianjun
Jiang, Zonghai Chen, Ren Yang, Khalil
Amine, Chunsheng Wang*
Page No. – Page No.
Reversible Redox Chemistry of Azo
Compounds for Sodium Ion Batteries
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