A dicyanobenzoquinone based cathode material for rechargeable lithium and sodium ion batteries

Article abstract of DOI:10.1039/c9ta04869c

Organic quinone materials offer a sustainable approach for electric energy storage, however, their intrinsic electrical insulation and dissolution into the electrolyte during cycling have hampered their wide applications. To tackle these two issues, we have synthesized a novel organic cathode material by anchoring a quinone compound, 2,3-dicyano-p-benzoquinone (DCBQ) with a high redox potential of 3.37 V vs. Li/Li⁺, onto carbon nanotubes (CNTs) (CNTs-DCBQ) through a facile ''grafting to'' method. The elaborate combination of excellent electron conductivity and large surface area of CNTs and stable and reversible redox reaction of DCBQ enables CNTs-DCBQ to deliver high reversible capacities of 206.9 and 175.8 mA h g^-1 at a current density of 10 mA g^-1 and also remarkable capacities of 110.2 and 82.1 mA h g^-1 at a higher current density of 200 mA g^-1 with a capacity retention approaching 100% after 1000 cycles for lithium and sodium ion batteries, respectively.

Full text of DOI:10.1039/c9ta04869c

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Materials for energy and sustainability
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DOI: 10.1039/C9TA04869C.
Volume
Number 12
March 2018
Pages 4883-5230
6
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DOI: 10.1039/C9TA04869C
A Dicyanobenzoquinone based Cathode Material for
Rechargeable Lithium and Sodium Ion Batteries
ad
a
a
b
c
Hailong Lyu, Charl J. Jafta, Ilja Popovs, Harry M. Meyer III, Jordan A. Hachtel,
*
c
c
ad
*a
Jingsong Huang, Bobby G. Sumpter, Sheng Dai, Xiao-Guang Sun
a
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
7831, USA
b
3
c
Center for Nanophase Materials Sciences and Computational Sciences & Engineering Division, Oak
Ridge National Laboratory, Oak Ridge, TN 37831, USA
d
Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
Corresponding Authors: huangj3@ornl.gov; sunx@ornl.gov
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-
0OR22725 with the U.S. Department of Energy. The United States Government retains and the
0
publisher, by accepting the article for publication, acknowledges that the United States
Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or
reproduce the published form of this manuscript, or allow others to do so, for United States
Government purposes. The Department of Energy will provide public access to these results of
1

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federally sponsored research in accordance with the DOE Public Access Plan
http://energy.gov/downloads/doe-public-access-plan).
(
ABSTRACT: Organic quinone materials offer a sustainable approach for electric energy storage,
however, their intrinsic electrical insulation and dissolution into the electrolyte during cycling have
hampered their wide application. To tackle these two issues, we have synthesized novel organic
cathode materials by anchoring a quinone compound, 2,3-dicyano-p-benzoquinone (DCBQ) with
+
a high redox potential of 3.37 V vs. Li/Li , onto carbon nanotubes (CNTs) (CNTs-DCBQ) through
a facile ‘‘grafting to’’ method. The elaborate combination of excellent electron conductivity and
large surface area of CNTs and stable and reversible redox reaction of DCBQ enables CNTs-
-1
DCBQ delivering high reversible capacities of 206.9 and 175.8 mA h g at a current density of 10
-1
-1
mA g and also remarkable capacities of 110.2 and 82.1 mA h g at a higher current density of
-1
2
00 mA g with a capacity retention approaching 100 % after 1000 cycles for lithium and sodium
ion batteries, respectively.
Keywords: Carbon nanotubes, dicyanobenzoquinone (DCBQ), organic cathode, lithium-ion
batteries, sodium-ion batteries
2

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1
. Introduction
Currently, lithium ion batteries (LIBs) and sodium ion batteries (SIBs) mainly depend on
inorganic cathode materials, which still need improvement in power density, sustainability, as well
1
-3
as safety. As alternatives, organic electrode materials have been intensively investigated for both
LIBs and SIBs due to their advantages of sustainability, flexibility of structure design and
environmental friendliness. 4-16 Among various organic materials, benzoquinone (BQ) and its
derivatives have emerged as one of the most promising materials owing to their multiple electron-
redox reactions with high theoretical capacities and high redox potentials.17, 18 Unfortunately,
intrinsic electrical insulation and dissolution in the electrolytes are the two major drawbacks
obstructing the wide applications of the quinone based materials.8, 19, 20 So far, different strategies
have been utilized to surmount those two drawbacks, 8, 9, 21-32 however, limited success has been
achieved in enhancing electronic conductivity and suppressing the solubility of the quinone based
cathode materials.17, 33, 34
Herein, we report a new strategy to surmount those two drawbacks by grafting quinone
derivatives on the carbon nanotubes (CNTs) that have been extensively investigated for high-
performance LIBs due to their advantages of high electronic conductivity, excellent mechanical
and chemical stability, facile chemical functionalization, and large specific surface areas.28, 35-39
Our selection of 2,3-dicyano-p-benzoquinone (DCBQ) is prompted by the knowledge that cyano-
containing species such as tetracyanoethylene (TCNE) and tetracyanoquiondimethane (TCNQ) are
commonly used as strong electron acceptors that can elevate the redox potentials.40, 41 Indeed,
calculations using the validated G3MP2B3 composite method predict that 2,3-dicyano-p-
benzoquinone (DCBQ) has an electron affinity (EA) of 3.21 eV, which is even higher than that of
2
.69 eV for tetrafluoro-p-benzoquinone (TFBQ) and 2.79 eV for tetrachloro-p-benzoquinone
3

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(
TCBQ), 42, 43 despite the substitution number of 2 vs. 4 (validations and results are summarized in
Table S1 with their structures shown in Figure S1).44 The obtained new cathode materials
demonstrate outstanding reversible capacity and rate performance, as well as long cycling stability
in both LIBs and SIBs.
2
2
. Experimental
.1. Materials
2
,3-Dicyanohydroquinone (DCHQ, 95%) was purchased from Oakwood Chemical.
Iodobenzene diacetate (IBDA, 95%) was purchased from AK Scientific. 2-Aminoethanethiol
C H NS, >95.0%) was purchased from TCI. Multi-walled carbon nanotube (CNTs, carbon >95%),
(
2
7
acetic acid (AcOH, ≥99%), nitric acid (HNO , 70%), thionyl chloride (SOCl , ≥99%), N,N-
3
2
Dimethylformamide (DMF, anhydrous, 99.8%), tetrahydrofuran (THF, anhydrous, ≥99.9%),
potassium carbonate (K CO , ≥99.0%), p-Benzoquinone (BQ, ≥98%), ferrocene (98%) and
2
3
tetrabutylammonium hexafluorophosphate (TBAPF , ≥99.0%) were purchased from Sigma-
6
Aldrich. Dichloromethane (CH Cl , 99.5 %), chloroform (CHCl , 99.8 %) and sulfuric acid
2
2
3
(
H SO , 95-98%) were purchased from BDH. Battery grade ethylene carbonate (EC), diethyl
2 4
carbonate (DEC), dimethyl carbonate (DMC), propylene carbonate (PC), lithium
hexafluorophosphate (LiPF ) and sodium hexafluorophosphate (NaPF ) were obtained from BASF.
6
6
All the chemicals and solvents were used directly without further treatment.
2
2
.2. Synthesis
.2.1. Synthesis of 2,3-dicyano-p-benzoquinone (DCBQ)
The redox-active DCBQ was synthesized from DCHQ by a novel liquid-phase oxidation
process. In a typical synthesis, DCHQ (5 g) was first dispersed homogeneously in CH Cl (80 ml)
2
2
with ultrasound for 30 mins, followed by addition of IBDA (22 g) and AcOH (2 ml) with stirring.
4

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After refluxing at 40 ̊C for 12 hrs, a transparent dark orange solution was obtained. The solvent
and AcOH were then removed thoroughly by rotary evaporation to obtain an orange residue.
DCBQ was purified by mixing the above residue with CHCl (30 ml) and heated to boiling. It was
3
then cooled to room temperature and placed in a refrigerator at 5 °C for 12 hrs. The solid was
washed with CHCl , filtered and dried at room temperature under vacuum to obtain 3 g golden
3
1
DCBQ powder in ~ 60% yield. H NMR (400 MHz, CDCl , δ): 7.10 (s, 2H, Ar H).
3
2
.2.2. Oxidation of CNTs (CNTs-COOH)
CNTs were oxidized in strong and oxidative acid medium to form oxygen-containing
functional groups. Thus, CNTs (1.0 g) was dispersed in an acid mixture of H SO /HNO (3:1, vol)
2
4
3
for 30 mins under sonication. The acid suspension of CNTs was stirred for 24 hrs at room
temperature in a sealed flask. The product was filtered and washed thoroughly with DI-water until
pH = 7. The oxidized CNTs were obtained by freeze-drying under vacuum and designated as
CNTs-COOH.
2
.2.3. Thiol-functionalization of CNTs (CNTs-SH)
The thiol-modified CNTs were obtained by a two-step procedure. First, CNTs was chlorinated
by reacting CNTs-COOH (1 g) in a mixture of SOCl (100 ml) and anhydrous DMF (1 ml) at
2
7
0 °C for 24 hrs under nitrogen protection. Then, SOCl was removed by vacuum distillation to
2
obtain the chlorinated CNTs. Hereafter, THF (25 ml) and K CO (1.35 g) were mixed with the
2
3
chlorinated CNTs under sonication for 45 mins, followed by addition of 2-aminoethanethiol (0.75
g). The suspension was heated to reflux at 65 C for 24 hrs under nitrogen. After reaction, it was
̊
cooled to room temperature and the solid was filtered and washed with THF and methanol to
remove the excess reactants. The CNTs-SH product was obtained by drying under vacuum at 80 °C.
2
.2.4. Grafting of DCBQ onto CNTs (CNTs-DCBQ)
5

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The DCBQ functionalized CNTs were prepared by the conjugate addition reaction of mercapto
groups to DCBQ. 45 Briefly, CNTs-SH (1 g) was dispersed in a solution of methanol (50 ml) and
DCBQ (1.58 g) for 30 mins under sonication. After refluxing at 65 ̊C for 12 hrs, IBDA (6.44 g)
was added to oxidize the anchored DCBQ. The suspension was refluxed for another 12 hrs with
stirring. The final product of CNTs-DCBQ was obtained by filtration, washing with methanol, and
drying at 60 °C under vacuum. Similarly, the BQ functionalized CNTs (CNTs-BQ) were
synthesized following the above procedure.
2
.3. Characterization
1H nuclear magnetic resonance (NMR) spectra was obtained on a Bruker Advance 400 MHz
spectrometer using CDCl as the solvent. The chemical structures of the fabricated products were
3
characterized by Fourier transform infrared (FTIR) spectroscopy, whereas their thermal properties
were evaluated by thermogravimetric analysis (TGA). TGA was carried out on a Perkin Elmer
instrument from the ambient temperature to 800 °C in the nitrogen atmosphere at a heating rate of
-1
1
0 °C min . The morphologies and microstructures of the samples were observed by a Zeiss
Merlin VP Scanning Electron Microscope (SEM) operating at 10 kV. The elemental compositions
of the products were analyzed by energy dispersive X-ray spectroscopy (EDS) on a SEM
instrument with an EDAX accessory. Nitrogen adsorption isotherms were measured at -196 ºC
using TriStar 3000 volumetric adsorption analyzer manufactured by Micromeritics Instrument
Corp. (Norcross, GA). Before adsorption measurements, the as-prepared samples were degassed
in flowing nitrogen at 150 ºC for 12 h. The specific surface area of the samples was calculated
using the Brunauer-Emmett-Teller (BET) method within the relative pressure range of 0.05 to
0
.20. The X-ray photoelectron spectrometer (XPS) used for obtaining the XPS spectra is a Thermo
Scientific (Waltham, MA, USA) Model K-Alpha instrument. This anode utilized is a
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monochromated, micro-focusing Al Kα X-ray source (1486.6 eV). The XPS samples of cycled
electrodes were prepared by dissembling the coin cells after cycling in an argon-filled glovebox.
The obtained electrodes were washed several times with N-methylpyrrolidone (NMP) to dissolve
the binder and obtain the powder samples for XPS measurements. The STEM measurement was
carried out using the aberration-corrected (equipped with fifth-order aberration corrector) Nion
UltraSTEM 100 (operated at 60 kV). The samples were deposited on a carbon support TEM grid
by solvent casting.
2
.4. Electrochemical measurements
The electrodes of modified CNTs were obtained by casting well homogenized slurries of active
material and polyvinylidene fluoride (PVDF) binder with a weight ratio of 94:6 in N-
methylpyrrolidone (NMP) on aluminum foils. After solvent evaporation, the electrodes were cut
into discs with a diameter of 12 mm and dried thoroughly in a vacuum oven at 120 °C for 12 h.
2
The as-prepared electrodes have similar active material loading of 1.6 mg/cm . The electrodes of
DCBQ were papered following the same procedure with a mass ratio of DCBQ, C45, and PVDF
of 6:3:1. All the coin cells for electrochemical measurement were assembled in an argon filled
glove-box with the oxygen and moisture levels below 0.5 ppm. LIB half-cells were assembled
with the modified CNTs electrodes as the cathode, lithium metal foils as both counter and reference
electrodes, Celgard 2320 as separator and 1 M LiPF dissolved in EC, DEC, and DMC (1:1:1, vol.)
6
as the electrolyte. SIB half-cells were assembled with the modified CNTs electrodes as the
cathode, sodium metal foil as both counter and reference electrodes, and Celgard 3401 as separator
and 1 M NaPF dissolved in PC as the electrolyte. The coin cells were galvanostatically cycled on
6
a LAND CT2001A battery test system between 1.5 and 4.0 V at various current rates. Cyclic
voltammogram (CV) data of the coin cells were recorded on a Biologic VSP instrument at a scan
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-1
rate of 0.5 mV s in the voltage range of 1.5 - 4.0 V. CVs of BQ and DCBQ were obtained at a
-1
scan rate of 20 mV s in the voltage range of -2.2 - 0.8 V vs. SCE by using two glassy carbon
electrodes as the working and counter electrodes, a platinum wire as the reference electrode, and
0
.05 M BQ or DCBQ in 0.5 M TBAPF in DCM as the electrolyte. Ferrocene (0.48 V vs. SCE in
6
DCM) was used to calibrate the relative potentials vs. SCE reference. Electrochemical impedance
spectra (EIS) were measured on the same instrument with a voltage bias of 10 mV in the frequency
range from 200 kHz to 10 mHz.
2
.5. Theoretical calculations
Electron affinities (EAs) were calculated by using the G3MP2B3 composite method, which
4
6
was designed for the reliable predictions of thermochemistry. Using this method, B3LYP/6-31G*
model chemistry was first used to optimize the geometries, and then to calculate the thermal energy
corrections to Gibbs free energies. Afterwards, high level UQCISD(T)/BTBas1 theory was used
to calculate the electron correlation corrections. Finally, the basis set extension corrections were
calculated using UMP2/GTMP2Large method. Following these calculations, Gibbs free energies
G° at 298.15 K and 1 atm were obtained for both neutral (q = 0) and anionic (q = -1) species of
BQ, TFBQ, TClBQ, DCBQ, and DCBQ-SCH . EAs were obtained by taking the Gibbs energy
3
difference between neutral and anionic species of each molecule. Infrared vibrational frequencies
were calculated for various conformations of DCHQs and DCBQ using B3LYP/6-31G* model
chemistry, following geometry optimizations. Frequencies reported were scaled using the
4
7
recommended scaling factor of 0.96 for the corresponding model chemistry. Frontier orbital
energies were calculated for BQ, DCBQ, and DCBQ-SCH using B3LYP, BHandHLYP and LC-
3
wPBE functionals, along with 6-31G* basis set, all at the B3LYP/6-31G* optimized geometries.
All calculations were done using the Gaussian 16 suite of programs.48
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3. Results and Discussion
Scheme 1. Synthetic scheme for CNTs-DCBQ.
Small molecule DCBQ was synthesized by a facile liquid-phase oxidation of 2,3-
dicyanohydroquinone (DCHQ) with iodobenzene diacetate (IBDA) (Scheme S1), instead of using
4
9-
highly toxic and insoluble heavy metal oxides such as MnO , Ag O, and Pb O as the oxidant
.
2
2
3
4
5
1
The structure of DCBQ was confirmed by FTIR spectra (Figure S2a) with all peaks assigned
with the assistance of density functional theory (DFT) calculations (Table S2 and Figure S1) and
1
H NMR spectra [(400 MHz, CDCl , δ): 7.10 (s, 2H, Ar H)].
3
DCBQ-functionalized CNTs (CNTs-DCBQ) were prepared for the first time using a ‘‘grafting
to’’ approach with a thioether linkage as illustrated in Scheme 1. 45 The content of grafted DCBQ
on the CNTs is estimated to be 14.62 wt%, based on the TGA plots of pristine CNTs, CNTs-
COOH, CNTs-SH, CNTs-DCBQ and pure DCBQ (Figure S3). Figure 1a shows the scanning
9

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electron microscopy (SEM) image of CNTs-DCBQ and the corresponding energy dispersive
spectroscopy (EDS) elemental mapping. The nanotube morphologies in CNTs-DCBQ are almost
identical to those in pristine CNTs and those in CNTs-COOH and CNTs-SH (Figure S4),
indicating the structural integrity and dispersibility of the CNTs are barely affected by the multi-
(a)
step modifications. This is also confirmed by the unaffected specific surface area of the
2
-1
functionalized CNTs, that is, 218.3, 226.8, 204.7, and 215.3 m g for CNTs, CNTs-COOH,
CNTs-SH and CNTs-DCBQ, respectively (Figure S5). In addition, the EDS mapping of CNTs-
DCBQ shows uniform distribution of the key elements of C, N, O and S (Figure 1a). To further
confirm the successful graft of DCBQ on CNTs, X-ray photoelectron spectra (XPS) of DCBQ,
CNTs-SH and CNTs-DCBQ were also measured. As shown in Figure 1, both C1s and N1s spectra
confirms the existence of C≡N in CNTs-DCBQ,52, 53 while the S2p spectra confirms the linkage
between CNTs and DCBQ.
(a)
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 C=O C≡N C-O
C-C
S 2p3/2
C-N
C≡N
SOx
S 2p1/2
(
b)
C 1s
(c)
N 1s
(d)
S 2p
Cycled CNTs-DCBQ
Cycled CNTs-DCBQ
CNTs-DCBQ
CNTs-SH
Cycled CNTs-DCBQ
CNTs-DCBQ
CNTs-SH
CNTs-DCBQ
CNTs-SH
DCBQ
DCBQ
DCBQ
2
92
290
288
286
284
282
406
404
402
400
398
396
172
170
168
166
164
162
160
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Figure 1. a) SEM image of CNTs-DCBQ (top left frame), as well as its total and individual EDS
mapping of elements C, N, O and S; X-ray photoelectron spectra (XPS) of b) C1s, c) N1s and d)
S2p for DCBQ, CNTs-SH, pristine and cycled CNTs-DCBQ.
Figure 2a shows the cyclic voltammograms (CVs) of the CNTs, CNTs-COOH, CNTs-SH and
-1
+
CNTs-DCBQ electrodes at a scan rate of 0.5 mV s in the voltage range of 1.5-4.0 V vs. Li/Li .
Nearly rectangular CV shapes are observed for CNTs and CNTs-COOH, indicating typical
electrochemical capacitor behavior. Meanwhile, the CV of CNTs-COOH shows a much larger
loop area than that of CNTs, due to the pseudo-capacitance through the Faradaic reactions of the
surface oxygen-containing functional groups of CNTs-COOH. For CNTs-SH, in addition to the
rectangular CV curve, a small oxidation peak at 2.65 V is observed, which may be attributed to
the oxidation of the thiol groups to disulfide bonds. However, the absence of the corresponding
reduction peak reveals poor reversibility of this redox reaction. For CNTs-DCBQ, two reduction
peaks at 2.65 and 2.30 V and two corresponding oxidation peaks at 2.80 and 3.15 V are observed,
though the latter two peaks are not well separated as the former ones. Figure 2b shows the typical
-1
charge–discharge profiles of the LIB half-cells at a current density of 10 mA g based on the total
11

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weight of the electrode. The overall slope profiles of all the curves are consistent with the
electrochemical capacitor behavior of the CNTs based materials, whereas the plateaus of the
CNTs-DCBQ material are consistent with the redox peaks shown in Figure 2a and Figure S6. It
-1
is noted that the specific capacity of CNTs-DCBQ is 170 mA h g based on the total weight of the
electrode loading, which is combination of the redox reactions of DCBQ, the Faradaic reactions
of the surface oxygen functional groups, and the super capacitance adsorption on the surface of
the CNTs.35, 54 It is calculated that 28.2 % (47.9 mAh g ), 55.4 % (94.2 mAh g ) and 16.4 % (27.9
-1
-1
-1
mAh g ) of the total capacity of CNTs-DCBQ is from the redox reactions of DCBQ, the Faradaic
reactions of the surface oxygen functional groups and the super capacitance adsorption on the
surface of the CNTs, respectively (see ESI for details). According to 14.62 wt.% of DCBQ in
-1
CNTs-DCBQ, the specific capacity from DCBQ is calculated to be 328 mAh g , which is close to
-1
its theoretical capacity of 339 mAh g .
0
0
0
0
.06
.04
.02
.00
4
3
3
2
2
1
.0
.5
.0
.5
.0
.5
(
a)
(b)
CNTs
CNTs-COOH
CNTs-SH
CNTs-DCBQ
-
-
-
-
0.02
0.04
0.06
0.08
CNTs
CNTs-COOH
CNTs-SH
CNTs-DCBQ
0
30
60
90
120
150
-1
180
1
.5
2.0
2.5
3.0
3.5
4.0
+
Capacity/total weight (mAh g )
E (V) vs. Li/Li
Figure 2. Cyclic voltammograms (CVs) of LIB half-cells of a) CNTs, CNTs-COOH, CNTs-SH
-1
and CNTs-DCBQ at a scan rate of 0.5 mV s ; Charge–discharge profiles of b) CNTs, CNTs-
-1
COOH, CNTs-SH and CNTs-DCBQ at a current density of 10 mA g .
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To better understand the electrochemical redox behavior of the CNTs-DCBQ, CVs of DCBQ
and BQ monomers were also measured. As shown in Figure S7, two pairs of redox peaks are
observed for DCBQ, centering at 0.08 and -0.83 V vs. SCE (equivalent to 3.37 and 2.46 V vs.
+
Li/Li ) with a peak separation of 0.21 V. In comparison, the redox peaks of BQ are located at
+
lower potentials of -0.64 and -1.41 V vs. SCE (equivalent to 2.65 and 1.88 V vs. Li/Li ) with a
peak separation of 0.25 V. The average potential difference of 0.65 V between the redox peaks of
BQ and DCBQ confirms the exceptional redox potential enhancing effect of the cyano groups. We
further examined the frontier orbitals, especially the lowest unoccupied molecular orbitals
(
LUMOs) of BQ, DCBQ, and DCBQ-SCH . DCBQ-SCH was adopted as a simplified model of
3 3
CNT-DCBQ, as DCBQs were anchored on CNTs through thioether bonds. Figure 3 shows the
LUMOs for BQ, DCBQ, and DCBQ-SCH , all of which display similar features except at the
3
substitution sites by the cyano and methylsulfanyl groups. It is noteworthy that there is no orbital
node between the cyano group and the BQ ring (not to be confused with the node within the cyano
group), indicating an in-phase and bonding relationship between the cyano group and the BQ ring.
In contrast, when DCBQ is further substituted by a methylsulfanyl group, the corresponding
LUMO displays an orbital node between the -SCH group and the BQ ring, meaning an out-of-
3
phase and antibonding relationship. The absence and presence of orbital nodes suggest that the
LUMO orbital enegies should be decreased for DCBQ but increased for DCBQ-SCH , in line with
3
the expected electron-withdrawing effect of the cyano groups and the electron-donating effect of
the methylsulfanyl group. Indeed, DFT calculated LUMO energies by representative exchange-
correlation functionals confirm the expected trend, showing that the LUMO of BQ is shallow,
significantly deepened by the substitution of the cyano groups, and slightly increased by the further
5
5
substitution of the methylsulfanyl group. By applying Koopmansʼ theorem, the LUMO energies
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correlate well with the electron affinities calculated by the validated G3MP2B3 composite method
Figure S8).
(
Figure 3. Top view (first row) and side view (second row) of the lowest unoccupied molecular
orbital (LUMO) for a) BQ, b) DCBQ, and c) DCBQ-SCH at an isosurface value of ±0.02
3
calculated with B3LYP/6-31G. The red and blue arrows indicate the absence and presence of
orbital nodes.
Figure 4a shows the rate performance of the LIBs based on different functionalized CNTs.
-1
The initial capacity of the CNTs-DCBQ based LIB is over 200 mA h g at a current density of 10
-1
mA g . After capacity fading in the first 10 cycles due to the formation of solid electrolyte (SEI)
on the surface of the lithium anode and the cathode, the CNTs-DCBQ LIB exhibits reversible
-1
capacities of 151.3, 137.7, 125.0, 115.5, 102.8 and 87.3 mA h g at current densities of 10, 20, 50,
-1
1
00, 200 and 500 mA g , respectively, with high coulombic efficiencies of 100% except in the
rate changing cycles. In addition to good rate performance, the CNTs-DCBQ LIB exhibits superior
cycling stability, as shown in Figure 4b. After a gradual capacity fading within the initial 50 cycles,
-1
-1
the reversible capacity stabilizes at 110.2 mA h g under a high current density of 200 mA g and
-1
is still as high as 108.6 mA h g after 1000 cycles, resulting in a capacity retention of 98.5 %. On
14

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the other hand, the reversible capacities of the CNTs-COOH and CNTs-SH LIBs are 76.4 and 81.8
-1
-1
mA h g after the initial stabilization cycles, and gradually decrease to 60.6 and 49.2 mA h g
after 1000 cycles, resulting in capacity retentions of 79.3% and 60.1%, respectively. The superior
cycling performance of the CNTs-DCBQ LIB can be attributed to its unique structure that
combines good electron conductivity of CNTs and excellent redox activity of the anchored DCBQ,
preventing side reactions and dissolution into the liquid electrolyte during cycling. This is
confirmed by the XPS spectra of the cycled CNTs-DCBQ sample, which is almost identical to that
of the pristine CNTs-DCBQ (Figure 2 & Figure S9). As a comparison, the spectra of its close
analog, the CNTs-BQ sample, change significantly before and after cycling, indicating severe
structure damage taking place during cycling (Figure S9). 30 This result clearly indicates that the
two cyano groups enhance the stability of CNTs-DCBQ.
2
2
1
1
50
00
50
00
(
a)
(b)
CNTs
1
50
CNTs-COOH
CNTs-SH
CNTs-DCBQ
CNTs
CNTs-COOH
CNTs-SH
CNTs-DCBQ
10
120
1
0
20
50
9
6
3
0
0
0
0
1
00
200
500
5
0
0
0
20
40
60
80
100
0
200
400
600
800
1000
Cycle number
Cycle number
200
150
100
(
c)
120
(d)
CNTs
CNTs-COOH
CNTs-SH
CNTs-DCBQ
CNTs
CNTs-COOH
CNTs-SH
CNTs-DCBQ
10
10
90
20
50
100
200
60
500
50
30
0
0
0
20
40
60
80
100
0
200
400
600
800
1000
Cycle number
Cycle number
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400
350
300
250
200
150
100
450
(f)
(e)
4
00
50
CNTs
CNTs
CNTs-COOH
CNTs-SH
3
CNTs-COOH
CNTs-SH
CNTs-DCBQ
300
CNTs-DCBQ
250
200
150
100
50
50
0
0
0
50 100 150 200 2250 300 350 400
0
50 100 150 200 250 300 350 400 450
2
Z' ( cm )
Z' ( cm )
Figure 4. Charge–discharge capacities of the LIB (a, b) and SIB (c, d) half-cells based on CNTs,
-1
CNTs-COOH, CNTs-SH and CNTs-DCBQ at different current densities (mA g ) (a, c) and at a
-1
current density of 200 mA g (b, d). (Hollow circles  and solid spheres  represent charge and
discharge capacities, respectively). Electrochemical impedance spectra (EIS) of the LIB half-cells
based on CNTs, CNTs-COOH, CNTs-SH and CNTs-DCBQ (e) before cycling and (f) after 1000
-1
cycles at 200 mA g .
Figure 4c & 4d show the rate and cycling performance of the SIB half-cells, with their CVs
shown in Figure S10. Generally, the SIBs exhibit similar trends as those of the LIBs (Figure 4a
&
4b), except that lower specific capacities are observed at the same current rates. For example,
-1
the initial capacity of the CNTs-DCBQ SIB is 175.8 mA h g by the total weight of the electrode
-1
at a current density of 10 mA g . The lower capacities of the SIBs than those of the LIBs under
the same current rates can be attributed to the lower ionic conductivity of the electrolyte and slower
+
reaction kinetics of the larger Na ion (Figure S11). Meanwhile, the CNTs-DCBQ SIB also
exhibits a very stable cycling performance with an almost indiscernible capacity decay after 1000
-1
cycles at 200 mA g and retains nearly 100% of the 50th cycle’s capacity. One noted common
feature of the SIBs in Figure 4d is the gradual capacity increase after the initial capacity drop,
which is absent for the LIBs in Figure 4b. This feature might be related to the high reactivity and
soft nature of the sodium anode electrode.
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The outstanding cycling stability of the CNTs-DCBQ cathode is also supported by the
electrochemical impedance spectra (EIS) data shown in Figure 4e & 4f. The total cell impedances
of the fresh LIBs increase with functionalization of the CNTs, that is, 142.2, 226.7, 268.1 and
2
2
72.3  cm for CNTs, CNTs-COOH, CNTs-SH and CNTs-DCBQ, respectively, suggesting
adverse effects of the insulating functional groups to the highly conductive CNTs. After 1000
cycles, the corresponding cell impedances change to 234.8, 311.3, 318.8, and 270.9  cm2,
respectively he stable impedance of the CNTs-DCBQ LIB confirms its remarkable structural
stability and redox reversibility, consistent with the stable cycling performance in Figure 4b.
Comparatively, the total impedances of the SIBs are several times higher than those of the
corresponding LIBs (Figure S12), attributed to the lower ionic conductivity of the former
electrolyte (Figure S11).
To figure out the origin of the capacity decrease of the first 50 cycles in Figure 4, both fresh
half cells and symmetric cathode cells based on CNTs, CNTs-COOH and CNTs-DCBQ were
-1
assembled and evaluated by CVs at a scan rate of 2 mV s in the voltage range of 1.5 – 4.0 V and
- 2 V for the half cells and the symmetric cathode cells, respectively. The EIS of the cells was
0
monitored during cycling. As shown in Figure S13, the EIS of the symmetric cathode cells
continually increases with cycling, whereas that of the half cells stabilizes after 50 cycles, and then
it tends to decrease with cycling. The gradual increase of the EIS of the half cells correlates well
with the gradual decrease of the capacity within the first 50 cycles. As there are no surface
functional groups on the pristine CNTs, the EIS increase of the symmetric CNT cell can only be
attributed to the side reactions on the surfaces of the CNTs, resulting in SEI formation and the
accompanied reduction of surface area, of which the latter is well correlated with the reduction of
the capacities (Figure S13g). Based on the above observations, the gradual capacity decrease
17

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within the first 50 cycles in Figure 4 can only be attributed to the SEI formations at both lithium
anode and CNTs-DCBQ cathode.
As shown Table S4 and Figure S14, the electrochemical performance of CNTs-DCBQ is
much better than those of the quinone/CNTs composite materials such as the quinone encapsulated
SWCNTs cathodes, 56, 57 the emodin/SCNT hybrid, 58 P(4VC -stat-LiSS14)/CNT buckypaper
8
6
composite electrodes 59 and PBDTD@CNT cathodes. However, it should be noted that the area
60
-2
capacity of CNTs-DCBQ is relatively low (0.11 mAh cm , Figure S15), which can be improved
by grafting more functional groups and by increasing the loading of the active materials.
4
. Conclusion
In summary, cathode materials based on DCBQ grafted CNTs (CNTs-DCBQ) have been
successfully synthesized via a multi-step functionalization approach for application in LIBs and
SIBs. The CNTs-DCBQ cathode exhibits a remarkable high-rate cycling stability, achieving high
-1
-1
capacities of 110.2 and 82.1 mAh g at a high current density of 200 mA g for LIB and SIB,
respectively. The superior electrochemical properties of CNTs-DCBQ result from the elaborate
synergy of the high electronic conductivity and large surface area of the CNTs and the stable redox
reversibility of the grafted DCBQ, providing an avenue to bridge the performance gap between
batteries and electrochemical capacitors. Therefore, CNTs-DCBQ is a promising cathode material
for the next-generation LIBs and SIBs.
Acknowledgements
This work was supported by the ORNL laboratory-directed research and development (LDRD)
program. This work was also supported by the U.S. Department of Energy, Office of Science,
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Basic Energy Sciences, Materials Sciences and Engineering Division. The electron microscopy
work was performed through a user project at ORNL's Center for Nanophase Materials Sciences,
which is a US Department of Energy Office of Science User Facility. Calculations used the
resources of the Compute and Data Environment for Science (CADES) at ORNL and of the
National Energy Research Scientific Computing Center, which are supported by the Office of
Science of the U.S. DOE under Contract Nos. DE-AC05-00OR22750 and DE-AC02-05CH11231,
respectively.
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