Targeted synthesis of ionic liquid-polyoxometalate derived Mo-based electrodes for advanced electrochemical performance

Article abstract of DOI:10.1039/c8ta12562g

Rational design of advanced electrode materials with high capacity and long cycle stability is a great challenge for both lithium and sodium storage. In this work, we report a versatile strategy for the synthesis of N/P-codoped MoO₂@carbon (N/P

Full text of DOI:10.1039/c8ta12562g

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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DOI: 10.1039/C8TA12562G
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Targeted synthesis of ionic liquid-polyoxometalates derived Mo-
based electrodes for advanced electrochemical performance
a
b
a
a
a
a
Guojian Chen,* Lei Zhang, Yadong Zhang, Ke Liu, Zhouyang Long and Ying Wang*
Received 00th January 20xx,
Accepted 00th January 20xx
Rational design of advanced electrode materials with high capacity and long cycle stability is a great challenge for both
lithium and sodium storage. In this work, we report a versatile strategy for the synthesis of N/P-codoped MoO @carbon
(N/P-MoO @C) electrodes via a simple pyrolysis of ionic liquid-based polyoxometalate (IL-POM) molecular precursors. The
contents of C, N, and P, and the pore geometry of N/P-MoO @C networks can be easily tailored by adjusting the position of
cyano groups in the IL-POMs precursor. Benefiting from this novel design, the optimized N/P-MoO @C4 electrode with
cross-linked porous tunnels and abundant defects exhibits excellent lithium storage performance, with a high reversible
DOI: 10.1039/x0xx00000x
2
www.rsc.org/
2
2
2
-1
-1
-1
-1
+
capacity of 1381 mAh g after 100 cycles at 0.5 A , and 346 mAh g after 5000 cycles at 20 A g . The Li storage performance
of this N/P-MoO @C4 is dominated by pseudocapacitance behavior, which attributed to the high reversible capacity and
long cycle stability. Exceptional sodium storage performance is also observed in the N/P-MoO @C4 electrode with 0.02%
2
2
-1
capacity decay per cycle over 1100 cycles at 1.0 A g . The present approach provides some insight into design and synthesis
of task-specific Mo-based materials towards the applications in energy storage and conversion.
MoO
capacity decay. To date, many efforts have been made to address the
above issues (i.e. fabricating nanostructured MoO and hybridizing
/carbon hybrids have
demonstrated an improved Li-insertion kinetics and a well cycle
2
during electrochemical cycling result in large irreversible
Introduction
Lithium ion batteries (LIBs) have been widely used in various portable
devices and smart grids, owing to their high energy density and
environmentally friendly behavior. The constantly growing
requirements arising from high-performance electronic devices are
dependent on the further innovations on LIBs in terms of higher
energy/power density, longer cycling performance, and lower cost.
Traditional anode material of graphite, however, fails to fulfill all
these requirements due to its low theoretical capacity (372 mAh g ).
Recently, sodium ion batteries (SIBs) have also received ever-
increasing attention as alternatives to LIBs due to their similar
chemical mechanism and the Earth’s abundant resources of sodium.
Compared to LIBs, the larger and heavier Na generally results a more
sluggish kinetics in SIBs, which results in poor rate performance and
low utilization of active materials. In both cases, rational design of
electrode materials with high reversible capacity and long cycle-life
is highly urgent.
As a typical conversion-type material, MoO
potential for both LIBs and SIBs due to its high theoretical capacity
and good conductivity (8.8 × 10 Ω cm). Unfortunately, the strong
Mo-O bond and slow electron transport kinetics of bulk MoO
2
5
-8
MoO
2
with carbon).
Especially, MoO
2
1
9
-14
stability. It is still a huge challenge, however, to achieve attractive
high capacity and long-term cycle stability at high current densities.
Herein,
electrochemical performance of MoO
constructing ionic liquid-polyoxometalate derived N/P-codoped
MoO @carbon mesoporous networks (denoted as N/P-MoO @C), as
demonstrated in Scheme 1. To the best of our knowledge, the
present N/P-MoO @C hybrids have never been reported for lithium
a
novel strategy to dramatically improve the
2
,3
2
is proposed, based on
-
1
2
2
4
2
or/and sodium storage. Fortunately, recent reports have revealed
some scientific clues that make this electrode attractive: (1) The
cross-linked conductive networks that formed by MoO skeleton and
2
carbon layers can provide multidimensional channels for efficient
electron/ion transport, thus promoting the electrochemical
+
1
1-14
kinetics.
MoO and the carbon can generate more defects, vacancies, and
bifunctional active sites to facilitate better electrochemical
(2) Synergetic effects between N and P codoping within
2
demonstrates huge
2
-5
5
1
5-18
performance, as has been revealed in other electronic devices.
The porous geometry of the designed networks can not only tolerate
the volume changes of MoO , but also provide better contact
between the electrolyte and electrode, thus enhancing the structural
(3)
2
-1
generally lead to low specific capacity (210 mAh g in LIBs) and poor
5
2
rate capability. Moreover, the large volume expansion/shrinkage of
5
,11,13,14
stability and shortening ion diffusion pathways.
Therefore,
seeking a suitable method to synthesize N/P-MoO
great scientific and practical importance.
2
@C networks is of
a. School of Chemistry and Materials Science, Jiangsu Key Laboratory of Green
Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou
2
21116, China. E-mail: gjchen@jsnu.edu.cn; yingwang@jsnu.edu.cn
b.
Centre for Clean Environment and Energy, Griffith University, Queensland 4222,
Australia
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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nuclear magnetic resonance (NMR), 13C NMR, elemental analysis,
View Article Online
DOI: 10.1039/C8TA12562G
and Fourier transform infrared (FTIR) spectroscopy (see Fig. S1-S4,
ESI). The most obvious distinction between these IL-POMs depends
on the different molecular structures related to the positions (3 or 4)
of cyano-groups in the IL cations or whether the material is cyano-
free (denoted by 0), which leads to the differences in pore geometry
and thermal stability between the IL-POMs presented later.
The scanning electron microscopy (SEM) image (Fig. 1a) reveals
that D[4-CNBzBPy]1.5PMo consists of fluffy sponge-like three-
dimensional (3D) mesoporous networks. Similar morphologies are
also observed in the D[3-CNBzBPy]1.5PMo and D[BzBPy]1.5PMo (Fig.
S5, ESI). The uniformly distributed cyano-tethered viologen organic
cations and PMo nanoclusters in IL-POM hybrids allow in-situ
generation of N/P-MoO
the sponge-like 3D mesoporous morphology is well preserved in the
resulting N/P-MoO @C4 network. Meanwhile, the decomposition
2
@C networks. After the annealing process,
2
and release of IL cations leads to the formation of an intercross-
linked structure that is composed of some tightly packed nanorods
Scheme 1 (a) Schematic illustration of the bottom-up preparation of the N/P-
MoO
CNBzBPy]1.5PMo, that is self-assembled from 4-cyano-tethered viologen IL
precursor designed on the molecular scale with H PMo12 40. (b) Other control IL-
2
@C4 network by direct pyrolysis of the IL-POM precursor, D[4-
(
Fig. 1b). The cross-linked structure of N/P-MoO
confirmed by transmission electron microscopy (TEM) image in Fig.
c, which shows abundant mesopores originated from nanovoids
2
@C4 networks is
3
O
1
POM precursors with the adjustable 3-cyano-groups or cyano-free IL cations.
between interconnected nanoparticles, in accordance with the SEM
image. Typical lattice fringes with spacing of 0.34 nm for the (-111)
Ionic liquid-based polyoxometalates (IL-POMs) are organic-
inorganic hybrid materials that combine diverse IL cations with POM
plane of MoO
high-resolution TEM (HR-TEM) image (Fig. 1d). Compared to N/P-
MoO @C4, the network structure of N/P-MoO @C0 is relatively
2
and some amorphous carbons are observed in the
1
9-21
22
anions,
which have been used as heterogeneous catalysts. IL-
2
2
POM-derived Mo-based electrode materials have never been
reported, however. In this work, for the first time, well-designed IL-
POMs are used as molecular precursors to synthesize N/P-MoO @C
2
electrodes. Novel mesoporous IL-POM ionic hybrids were self-
assembled between newly-designed cyano-tethered viologen IL
loose (see SEM image in Fig. S5 and TEM image in Fig. S6).
A comprehensive analysis of the pore structures of D[4-
CNBzBPy]1.5PMo and N/P-MoO @C4 was carried out by N sorption
2 2
experiments. Fig. 1f and Fig. S7 show that the adsorption isotherms
are all type IV with H1-type hysteresis loops in the high relative
cations and commercial H
3
PMo12O40 (Scheme 1). After the pyrolysis,
pressure (P/P
mesoporous materials.
0
) range from 0.5 to 0.9, indicative of typical
3
-
a MoO skeleton derived from PMo12O40 anions is generated. In
2
2
0,21
All the textural properties of IL-POMs and
addition, the introduced N/P heteroatoms and carbon layers
originated from the cyano-tethered viologen IL cations and the P-
centered POM anions. By simply adjusting the positions of cyano
their derived networks are listed in Table S1. The typical D[4-
CNBzBPy]1.5PMo has a desirable Brunauer-Emmett-Teller (BET)
2
-1
3
-1
surface areas of 63 m g with a large pore volume of 0.37 cm g .
After the pyrolysis, N/P-MoO @C4 shows a higher BET surface area
groups in the IL-POMs, we constructed a series of N/P-MoO
samples with controllable contents of C, N, and P, and controllable
pore geometry. The optimized sample N/P-MoO @C4 exhibited
2
@C
2
2
-1
3
-1
of 89 m g with a declining pore volume of 0.16 cm g that is due
to its compact cross-linked network. Besides, both D[4-
2
outstanding electrochemical performance for both LIBs and SIBs,
with long-life cycling stability, high rate capability, and high
reversible capacity.
2
CNBzBPy]1.5PMo and N/P-MoO @C4 have some micropore surface
areas (Table S1, ESI), indicating the existence of a small amount of
micropores. The Barrett-Joyner-Halenda (BJH) pore size distribution
curves (Fig. 1g) also show that the most probable pore diameters
have changed from 19.0 nm for D[4-CNBzBPy]1.5PMo to smaller
Results and discussion
Scheme 1a depicts the synthetic procedure of the typical N/P-
2
mesopores centered at 3.5 nm for N/P-MoO @C4, demonstrating its
small-size mesoporous structure. As shown in Fig. S8, the NLDFT pore
size distribution further confirms that a small amount of micropores
appear at 1.4 nm and enriched small mesopores appear at 4.3 nm,
which is consistent with the results in Table S1 and Fig. 1g. The
control samples D[3-CNBzBPy]1.5PMo and D[BzBPy]1.5PMo possess
similar surface areas and pore volumes with D[4-CNBzBPy]1.5PMo,
MoO
CNBzBPy]Br
2
@C4 network. First, cyano-tethered viologen IL (D[4-
) is prepared by quaternization reaction between 4,4'-
2
bipyridine and 4-cyanobenzyl bromide, and then D[4-
CNBzBPy]1.5PMo is synthesized by the ionic self-assembly of D[4-
CNBzBPy]Br
2 3
with equivalent H PMo12O40 in aqueous solution. The
targeted N/P-MoO
D[4-CNBzBPy]1.5PMo at 700 C for 2 h under a N
2
@C4 network is fabricated by direct pyrolysis of
2 2
but their pyrolyzed products N/P-Mo C@C3 and N/P-MoO @C0
o
2
atmosphere. By
2 -1
have obviously different surface area of 28 and 51 m g , respectively,
with low pore volumes and larger mesopore sizes (Table S1). The
above difference is attributed to the tailored molecular structures of
the IL cations, which were achieved by
varying the IL-POM molecular precursors D[3-CNBzBPy]1.5PMo and
D[BzBPy]1.5PMo (Scheme 1b), distinct N/P-Mo C@C3 and N/P-
MoO @C0 networks were prepared. The chemical structures and
compositions of the above ILs and IL-POMs were confirmed by H
2
2
1
2
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2 2
Fig. 1 SEM images of (a) D[4-CNBzBPy]1.5PMo and (b) N/P-MoO @C4. (c) TEM and (d) HR-TEM images of N/P-MoO @C4. (e) STEM EDS elemental mapping of Mo, O, C,
N and P in the N/P-MoO
2
@C4 at the nanoscale (50 nm). (f) N
2
sorption isotherms and (g) BJH pore size distributions of D[4-CNBzBPy]1.5PMo and N/P-MoO
@C4.
2
@C4. (h)
Illustration of ion/electron diffusion paths of the N/P-MoO
2
@C4. (i) N 1s, (j) C 1s, and (k) P 2p XPS spectra of N/P-MoO
2
varying the positions of cyano groups. The structural advantage of in multiple directions, which allows more electrons to pass in a given
this cross-linked N/P-MoO @C4 network is illustrated in Fig. 1h. The time. Compared to N/P-MoO @C4, N/P-MoO @C0 has a relatively
tightly-packed cross-linked porous structure not only enables shorter loose network, which indicates longer ion diffusion pathways and
ion diffusion pathways, but also makes the electron diffusion happen less directions for electron diffusion. Therefore, N/P-MoO @C4
2
2
2
2
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provides multiple channels for faster electron/ion transport, as results collected in air (Fig. S17, ESI). The C and N content in each
View Article Online
DOI: 10.1039/C8TA12562G
sample is obtained based on elemental analysis, and P content is
confirmed by a lower charge transfer resistance in Fig. S9.
XRD patterns of the IL-POM precursors and their derivatives are calculated based on (100-wtMoO2-wt
C
-wt
@C4 and N/P-MoO
@C4 shows a higher content of
N
)%. Detailed elemental
shown in Fig. S10. The D[4-CNBzBPy]1.5PMo shows a broad Bragg compositions of N/P-MoO
peak that appears at 8.30 with a d-spacing of 1.06 nm, indicating Table S2. Apparently, the N/P-MoO
2
2
@C0 are shown in
o
2
that PMo clusters (~1.0 nm) are uniformly dispersed in the IL-POM C (5.2 wt%), N (1.8 wt%), and P (6.9 wt%), compared with that of
2
0,21
networks in a certain regular ion-pair array.
S10b, ESI) of the as-synthesized N/P-MoO @C4 matches well with levels in the resultant samples can be elaborately tailored by
standard monoclinic MoO peaks (JCPDS No. 32-671), demonstrating adjusting the molecular structures of IL cations.
the formation of high purity MoO only with small amount of
amorphous carbons by the degradation of IL-POMs.As for the control
samples, D[BzBPy]1.5PMo gives a similar N/P-MoO @C0 network, but
D[3-CNBzBPy]1.5PMo changes to a N/P-Mo C@C3 network (Fig. S10c),
2
XRD pattern (Fig. N/P-MoO @C0. The above results suggest that the N, P and C doping
2
2
2
2
2
which caused by their distinct thermal stabilities (Fig. S11, ESI). The
above results imply that IL-POM derived materials can be easily
tailored on molecular-scale by simply varying the positions of cyano
groups in the IL cations. Raman spectroscopy (Fig. S12, ESI) was
further employed to clarify the chemical components of the N/P-
MoO
for monoclinic MoO
2
@C4 and N/P-MoO
2
@C0 networks. The characteristic peaks
-
2
are observed at 282, 337, 662, 819, and 992 cm
1
7,13
-1
.
The broad band at 1348 cm is the D band that associates with
3
-1
sp -type disordered carbon, and the band at 1606 cm is assigned to
2
7,13
the G band derived from sp -type graphitic carbon. The intensity
ratios of the D to the G band (I /I ) are 1.07 and 1.05 for N/P-
MoO @C4 and N/P-MoO @C0, respectively, which are higher than
other reported carbon materials.
these N/P-MoO @C networks not only confirm the existence of
D
G
2
2
1
3,23-25
D G
The higher I /I values in
2
amorphous carbons in both samples, but also indicate more defects
and disorders, which is favorable for enhancing the electrical and
5
ionic transport of the electrode. The I
D
/I
G
value of N/P-MoO
2
@C4 is
2
higher than that of N/P-MoO @C0, demonstrating more structural
defects (e.g. voids and holes) and disorders in the carbon layers.
Further investigation about the electronic states of N/P-
2
MoO @C4 was performed by X-ray photoelectron spectroscopy (XPS,
Fig. 1i-1k). Besides the Mo, O, and C, the XPS survey (Fig. S13, ESI)
shows obvious signals for N and P, indicating that N and P were
successfully introduced into the networks. The N 1s plot shows four
2
6
typical peaks for the N-Mo bonds (N1 at 397.4 eV), pyridinic N (N2
at 399.0 eV), pyrrolic N (N3 at 400.7 eV), and graphitic N (N4 at 401.3
1
7,27
eV), respectively.
1
The P 2p plot shows two dominant peaks at
2
Fig. 2 Lithium storage performances of the synthesized N/P-MoO @C networks. (a,
33.4 and 134.1 eV, corresponding to the P-O and P-C bonds.16-18
Since the P-C bond (1.77 Å) is much larger than the C-C sp bond (1.42
@C4 at 0.5 A g-1,
b) Charge-discharge profiles of N/P-MoO @C0 and N/P-MoO
2 2
2
respectively. (c) Rate performance at various current densities, from 0.05 to 20 A
Å), the doped-P can induce structural distortion and introduce more
g-1 and then back to 0.5 A g , and (d) long-life cycling performance at 20 A g . (e)
Electrochemical performance comparison of N/P-MoO @C4 with other high
quality MoO -based electrode.
-1
-1
1
7,18
defects into the carbon matrix,
in accordance with the Raman
2
results. The P-Mo bond is observed at 130.8 eV, confirming partial P
2
1
7,18
doping within the MoO
2
structure.
The C 1s spectrum contains
three peaks at 283.5, 284.7, and 286.9 eV, corresponding to sp3-C,
Lithium storage performance of the N/P-MoO @C networks is
2
1
8,27
sp2-C, and C-O bond, respectively.
and P are codoped into both MoO
state is also observed in the N/P-MoO
The XPS results confirm that N determined by assembling 2032 half-cells. Fig. 2a and 2b compares
and carbon. A similar chemical the discharge/charge profiles of N/P-MoO @C4 and N/P-MoO @C0
2
2
2
-1
2
@C0 sample (Fig. S14, ESI).
2
at 0.5 A g . Clearly, the electrode N/P-MoO @C4 delivers a higher
-1
The energy dispersive spectroscopy (EDS) elemental mapping initial discharge/charge capacity of 1094/801 mAh g , relating to an
images performed both in the SEM mode (Fig. S15 and S16, ESI) and initial Coulombic efficiency (ICE) of 73.2%. Meanwhile, N/P-
the scanning TEM (STEM) mode (Fig. 1e and Fig. S6c) display uniform MoO @C0 shows an initial discharge/charge capacity of 890/595
2
-1
distributions of Mo, O, C, N, and P elements for the N/P-MoO
and N/P-MoO
contents of MoO
determined to be 86.1 and 88.2 wt%, calculated based on the TGA works, such as electrolyte optimization, need to be done to improve
2
@C4 mAh g , corresponding to an ICE value of 66.8%. The low ICE may
+
2
@C0, from the micrometer scale to nanoscale. The due to the irreversible trapping of Li in the MoO lattice and the
2
1
2,33
2
in N/P-MoO
2
@C4 and N/P-MoO
2
@C0 are formation of the solid electrolyte interphase (SEI) film.
Further
4
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the ICE value of N/P-MoO
practical application. After the first cycle, the capacity of N/P-
MoO @C4 continues to grow, which is related to a gradual activation
process, and a change in the lithiation mechanism form intercalation
2
@C networks to make it more suitable for
View Article Online
DOI: 10.1039/C8TA12562G
2
1
2,34,35
to conversion dominate the process.
increasing capacity of N/P-MoO @C4, which is not obvious in the
N/P-MoO @C0. After 100 cycles, the N/P-MoO @C4 still exhibits a
high reversible capacity of 1381 mAh g , which is much higher than
that of N/P-MoO @C0. The rate capabilities of these N/P-MoO @C
networks are shown in Fig. 2c, where the current density increases
Fig. S18 confirms the
2
2
2
-1
2
2
-
1
-1
step-wisely from 0.05 to 20 A g and then returns to 0.5 A g . The
specific discharge capacity of N/P-MoO @C4 at high current density
2
-1
of 1.0, 3.0, 5.0, 8.0, 10, 15, and 20 A g is 749, 557, 477, 417, 383,
34, and 321 mAh g , respectively. Excellent capacity retention of
3.5% is observed in the N/P-MoO
density from 1.0 to 20 A g . After a deep cycling at 20 A g , a very
-1
3
4
2
@C4 on increasing the current
-1
-1
Fig. 3 CV curves at various scan rates and the corresponding logi versus logv plot
-1
of (a, b) N/P-MoO @C4, and (c, d) N/P-MoO @C0, respectively.
high capacity of 1023 mAh g could be retained after switching the
2
2
-
1
th
scan rates and the corresponding logi versus logv plot plots are given
superior structural stability of this novel electrode. The long-life in Fig. 3. Typical peak A and peak B are selected to investigate the b
cycling performance of these N/P-MoO
@C networks was values during the cathodic and anodic process. As shown in Fig. 3b,
the b-values are calculated to be 0.925 and 0.939 for the cathodic
@C4 exhibit good capacity retention over 5000 cycles at a and anodic process of N/P-MoO @C4, reflecting its remarkable
high current density. In particular, the N/P-MoO
@C4 still delivers pseudocapacitive effect in enhancing electrochemical performance.
For comparison, the b-values are calculated to be 0.793 and 0.601
to a near 100% capacity retention. As far as we know, this is the best for N/P-MoO @C0, which are also larger than 0.5, but are much
lower than those of N/P-MoO @C4, indicating a moderate
current density back to 0.5 A g at the 50 cycle, indicating the
2
-1
investigated at 20 A g (Fig. 2d). Clearly, both the N/P-MoO
N/P-MoO
2
@C0 and
2
2
2
-1
th
capacity of 346 mAh g at the end of the 5000 cycle, corresponding
2
1
1,13,14,24,28-32,36
Li-storage capacity among the reported MoO
and many other Mo-electrodes shown in Fig. 2e.
The excellent cycle stability and high reversible capacity of the associated with superior pseudocapacitance contribution for the
N/P-MoO
@C4 make it stand out from other competitors as listed in N/P-MoO @C4 can be attributed to its more tightly cross-linked
Table S3. Especially, the reversible capacity of N/P-MoO
2
-based
2
2
pseudocapacitance behavior of N/P-MoO @C0. The high b-values
2
2
@C4 is high nanostructure, higher surface area with abundant smaller
mesopores and some micropores,14,40-43 and a bit more N and P
2
-1
than its theoretical value (838 mAh g based on four electron
transfer). In order to understand such outstanding Li storage
+
codoped carbon coating,38,41,42 with respect to the N/P-MoO @C0. As
2
performance, further studies are carried out. Generally, the total a result, the enhanced pseudocapacitance behavior for N/P-
capacity of conversion-type electrode comes from the following MoO @C4 plays a vital role in contributing to the superior
2
three parts: redox reaction, faradaic contribution from the charge performance in the total electrochemical process.
transfer with surface atoms, and pseudocapacitance contribution
푖 = 푎푣푏
3
7,38
(1)
from the double-layer effect.
pseudocapacitance behavior will also explain the long cycle-life of
this N/P-MoO @C4 electrode. Therefore, the contribution of
pseudocapacitance behavior in the N/P-MoO @C networks is
investigated, and the power-law relationship is used, as shown in
In addition, the existence of
푙표푔푖 = 푏 × 푙표푔푣 + 푙표푔푎 (2)
2
To further confirm the structural advantages of the N/P-
2
@C4 for electrochemical energy storage, the sodium storage
performance was also explored (Fig. 4). The discharge/charge plots
2
of N/P-MoO @C4 at 0.1 A g (Fig. 4a) demonstrate a high initial
discharge capacity of 910 mAh g , with 300 mAh g retained during
the charge process. At the 5 cycle, the discharge and charge
capacities are stabilized at high values of 312 and 299 mAh g ,
corresponding to coulombic efficiency (CE) of 95.8%. As expected,
2
MoO
3
9-41
-1
equation (1) and (2).
Herein, i is current (A), v is scan rate (V s ),
-1
a and b are adjustable parameters. The b values can be calculated
according to the slop of logi versus logv plot. When b approaches to
-
1
-1
th
0
.5, the electrode is dominated by a faradic reaction. When b is close
-1
to 1, the electrochemical reaction is dominated by
pseudocapacitance behavior. The CV curves that tested at various
a
the N/P-MoO
2
@C4 also exhibits excellent rate performance in SIBs
-
1
(
Fig. 4b), with high capacities ranging from 312 to 175 mAh g at
current densities between 0.1 and 0.8 A g . Even at 1.0 A g , a
capacity of 151 mAh g can still be retained, indicating good
electrochemical kinetics. Generally, the cycling performance of
-1
-1
-1
MoO
2
-based electrodes is not satisfactory for SIBs, due to server
@C4 electrode can
volume expansion. Surprisingly, the N/P-MoO
maintain 111 mAh g after 1100 cycles at 1.0 A g (Fig. 4c), indicating
good capacity retention of 73.5% and a CE of ~100%.
2
-1
-1
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Page 6 of 9
ARTICLE
Journal Name
the synthesis more Mo- or other metal-based materials towards
View Article Online
DOI: 10.1039/C8TA12562G
diverse applications.
Experimental
Materials
4
,4´-bipyridine, 4-cyanobenzyl bromide, 3-cyanobenzyl bromide,
benzyl bromide, H 40 and the required solvents were
3
PMo12O
commercially available and used as received.
Methods
Liquid-state H and 13C NMR spectra were measured with a Bruker
1
2
DPX 500 spectrometer at ambient temperature in the solvent D O
using TMS as internal reference. Fourier transform infrared (FTIR)
Fig.
4
Sodium storage performance of the N/P-MoO
2
@C4 electrode. (a) spectra were recorded on a Bruker Vertex 80V instrument (KBr disks)
-1
-1
Galvanostatic charge/discharge profiles at 100 mAh g , (b) rate performance at in the 4000-500cm region. XRD patterns were collected on the
different current densities from 0.1 to 1.0 A g , and (c) long-life cycling Bruker D8 Advance powder diffractometer using Ni-filtered Cu Ka
-1
-1
o
performance and Coulombic efficiency at 1.0 A g .
radiation source at 40 kV and 20 mA, from 5 to 80 with a scan rate
o
-1
The unprecedented energy storage performance of this novel of 0.2 s . The morphologies of the samples were characterized using
N/P-MoO @C4 electrode can attribute to the following essentials: (a) Field-emission scanning electron microscopy (FESEM, Hitachi
The tightly cross-linked tunnels in the N/P-MoO @C4 constitute SU8010), transmission electron microscopy (TEM) and high-
2
2
abundant smaller mesopores and micropores within the internal resolution TEM (HRTEM, JEOL JEM-2010FEF). N2 adsorption-
structure, which shortens the ion diffusion pathways and allows desorption isotherms were performed on a Quantachrome autosorb
more ions to pass through in a given time, leading to a much higher iQ2 at 77 K, and the surface area of samples was calculated using the
efficiency for ion transfer. Meanwhile, these tunnels also offer Brunauer-Emmett-Teller (BET) method and the pore size distribution
multiple channels for electron diffusion, which is beneficial for was determined by the Barrett-Joyner-Halenda (BJH) model and the
5
,11,13,14,44
accelerating the electrochemical kinetics of the electrode.
b) The robust cross-linked porous structure of N/P-MoO
can effectively tolerate volume changes of MoO
non-local density function theory (NLDFT), while the samples were
o
(
2
@C4 also degassed at 150 C for 6 h in a high vacuum before analysis.
during the Thermogravimetric analysis (TGA) was carried out with a TA-Q50
2
2
9,45,46
o
charging/discharging processes.
MoO
of MoO
but also facilitates better electrochemical kinetics due to the inVia with laser excitation at 514.5 nm. X-ray photoelectron
(c) This N/P codoped instrument in nitrogen or air atmosphere at a heating rate of 10 C
-1
2
@C4 network can not only accommodate the volume changes min . The CHN elemental analysis was performed on an elemental
as none of the doped or single doped MoO @C hybrids do, analyzer Vario EL cube. Raman spectra were recorded on Renishaw
2
2
1
5-18
synergistic effects between C, N and P.
In a word, these features spectroscopy (XPS) was recorded on the Thermo ESCALAB 250XI.
dramatically enhance the pseudocapacitance behavior and the
kinetics of N/P-MoO @C4 for Li storage/release.
2
+
Synthesis of cyano-tethered viologen ionic liquid precursors
In a typical synthesis, D[4-CNBzBPy]Br , 1,1'-bis(4-cyanobenzyl)-[4,4'-
2
bipyridiium] dibromide, was prepared by treating a homogeneous
Conclusions
CH CN solution (30 mL) of 4,4'-bipyridine (10 mmol, 1.56 g) and 4-
cyanobenzyl bromide (20 mmol, 3.92 g) in a 50 mL Teflon-lined
autoclave at 100 °C for 48 h. After reaction, the formed solid was
3
dispersed into CH CN solution with vigorous stirring for 2 h. The
3
In summary, ionic liquid-based polyoxometalates (IL-POMs) are
regarded as molecular precursors for precise synthesis of Mo-based
electrodes. By simply adjusting the position of cyano groups in the IL-
POMs, we obtain a series of N/P-codoped MoO
distinct N, P and C contents and pore geometry. Excellent energy
storage performance is achieved in the N/P-MoO @C networks,
which synergistically combine the advantages of heteroatom doping,
carbon coating, cross-linked conductive mesoporous networks and
high pseudocapacitance contributions. In particular, the optimized
electrode N/P-MoO
capacity of 1381 mAh g at 0.5 A g , and there is almost 100%
capacity retention at 20 A g for 5000 cycles in LIBs. Considerable
sodium storage performance is also observed in N/P-MoO @C4, in
terms of high capacity, good rate capability, and exciting cycling
suspension was filtrated, washed with CH
times, and dried at 80 C for 12 h in vacuum to give a yellow solid
with a yield of 85%.
3
CN and ethanol several
2
@C networks with
o
2
In a similar process, D[3-CNBzBPy]Br
4,4'-bipyridiium] dibromide; D[BzBPy]Br
2
, 1,1'-bis(3-cyanobenzyl)-
, 1,1'-bis(benzyl)-[4,4'-
[
2
bipyridiium] dibromide, were obtained as high-yield (ca. 90%) yellow
solids by the replacement of 4-bromobenzyl bromide with 3-
bromobenzyl bromide or benzyl bromide.
2
@C4 demonstrates the highest reversible
-1
-1
, 1H NMR (300 MHz, DMSO-d
, δ) (Fig. S1A):
-1
D[4-CNBzBPy]Br
.56~9.58 (CH, 4H), 8.79~8.81 (CH, 4H), 7.68~7.70 (CH, 4H),
.62~7.64 (CH, 4H), 5.98 (CH , 4H); C NMR (75.5 MHz, DMSO-d , δ)
2 6
2
6
9
7
2
1
3
(
Fig. S1B): 149.63, 146.16, 133.89,132.64 131.81, 127.69, 123.55,
62.87. Anal. calcd for C26 Br (MW: 548.27): C 56.96, H 3.68, N
10.22; found: C 56.95, H 3.72, N 10.21.
performance. These encouraging results confirm the structural
+
20
H N
4
2
superiority of N/P-MoO
2
@C networks as electrodes for both Li and
+
Na storage. It is reasonable to deduce that by using IL-POMs as
precursors, this facile and large-scale method can pave the way to
6
| J. Name., 2012, 00, 1-3
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
Journal Name
ARTICLE
View Article Online
2 6 2
D[3-CNBzBPy]Br , δ) (Fig. S2A): overnight. The loading amount of the active materials N/P-MoO @C
, 1H NMR (300 MHz, DMSO-d
−
2
DOI: 10.1039/C8TA12562G
9
2
1
1
5
.54~9.56 (CH, 4H), 8.15 (CH, 2H), 7.94-8.00 (CH, 4H), 7.68~7.72 (CH, networks is about 0.55 mg cm . Lithium discs (MTI Corporation)
1
3
H), 5.96~6.03 (CH
49.72, 146.43, 135.86, 134.45, 133.65, 133.29, 130.92, 127.72, ethylene carbonate (EC, Sigma Aldrich), diethyl carbonate (DEC, Alfa
18.71, 112.54, 62.85. Anal. calcd for C26 Br (MW: 548.27): C Aesar), and fluorinated ethylene carbonate (FEC, Sigma Aldrich)
2 6 6
, 4H); C NMR (75.5 MHz, DMSO-d , δ) (Fig. S2B): were used as the counter electrode. 1 M LiPF (Sigma Aldrich) in
20
H N
4
2
6.96, H 3.68, N 10.22; found: C 56.52, H 3.86, N 10.29.
(volume ratio 6:3:1) was used as the electrolyte. Polypropylene (PP,
, δ) (Fig. S3A): MTI Cooperation) was used as the separator. The cells were
1
D[BzBPy]Br
2
,
H NMR (300 MHz, DMSO-d
6
9
7
.60~9.61 (CH, 4H), 8.80~8.82 (CH, 4H), 7.66~7.68 (CH, 4H), assembled in an argon-filled glove box with the oxygen and water
.46~7.48 (CH, 4H), 7.44~7.45 (CH, 2H), 6.01 (CH , 4H); C NMR (75.5 content below 0.1 ppm. Galvanostatic charge-discharge tests were
2
1
3
MHz, DMSO-d
6
, δ) (Fig. S3A): 149.64, 146.12, 134.67, 129.99, 129.73, carried out at room temperature on a battery testing system (LAND
+
1
5
29.46, 127.72, 63.69. Anal. calcd for C24
7.85, H 4.45, N 5.62; found: C 56.64, H 4.52, N 5.56.
22
H N
Br
2 2
(MW: 498.25): C Wuhan, China) in a potential range of 0.01~3.00 V (vs. Li/Li ). Cyclic
voltammetry (CV) tests and electrochemical impedance
spectroscopy (EIS) measurements were performed on a CHI660E
Preparation of ionic liquid-polyoxometalte (IL-POM) nanohybrids
IL-POM nanohybrids were prepared by the ionic self-assembly of
water-soluble viologen ionic liquid precursors with POM such as
electrochemical work station.
H
H
3
PMo12
PMo12
O
O
40. In a typical synthesis by using D[4-CNBzBPy]Br
40, D[4-CNBzBPy]Br (0.50 g, 0.912 mmol) was dissolved in
2
and Conflicts of interest
3
2
There are no conflicts to declare
deionized water (20 mL) and formed a homogeneous solution with a
-1
mass concentration of 5 mg mL , and then the equivalent
-1
3
H PMo12O40 (1.11 g, 0.608 mmol) aqueous solution (10 mg mL ) was
Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (21603089, 21805117 and 21503098),
the Natural Science Foundation of Jiangsu Province for Youths
slowly added into the above IL aqueous solution within 15 minutes.
The transparent solution gradually become a yellow green liquid-
solid suspension with continuously stirred for 24 h at room
temperature. Finally, the yellow green product named as D[4-
CNBzBPy]1.5PMo was obtained with a yield of 88% after the
consecutive basic operations including filtration, washing and drying.
Anal. calcd for D[4-CNBzBpy]1.5PMo: C 19.47, H 1.26, N, 3.49; found:
C 20.27, H 1.94, N 3.37.
(
BK20160209 and BK20181014), the Natural Science Foundation of
the Jiangsu Higher Education Institutions of China (16KJB150014 and
8KJB150015), TAPP, and PAPD.
1
In a similar process, 3-cyano-contained D[3-CNBzBPy]1.5PMo
and cyano-free IL-POM hybrid D[BzBPy]1.5PMo was prepared by the
Notes and references
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D[BzBPy]1.5PMo: C 18.56; H 1.43; N 1.80; found: C 20.00, H 1.34, N
.92.
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA12562G
The Table of Contents Entry
Targeted synthesis of ionic liquid-based polyoxometalates derived N/P-codoped MoO @carbon
2
porous electrodes with superior lithium and sodium storage performance.