Synthesis of CuS@CoS2 Double-Shelled Nanoboxes with Enhanced Sodium Storage Properties

Article abstract of DOI:10.1002/anie.201902583

Metal sulfides have received considerable attention for efficient sodium storage owing to their high capacity and decent redox reversibility. However, the poor rate capability and fast capacity decay greatly hinder their practical application in sodium-ion batteries. Herein, an elegant multi-step templating strategy has been developed to rationally synthesize hierarchical double-shelled nanoboxes with the CoS₂ nanosheet-constructed outer shell supported on the CuS inner shell. Their structure and composition enable these hierarchical CuS@CoS₂ nanoboxes to show boosted electrochemical properties with high capacity, outstanding rate capability, and long cycle life.

Full text of DOI:10.1002/anie.201902583

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Accepted Article
Title: Elegant Synthesis of CuS@CoS2 Double-Shelled Nanoboxes
with Enhanced Sodium Storage Properties
Authors: Xiong-Wen (David) Lou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902583
Angew. Chem. 10.1002/ange.201902583
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http://dx.doi.org/10.1002/ange.201902583

Angewandte Chemie International Edition
10.1002/anie.201902583
Elegant Synthesis of CuS@CoS
2
Double-Shelled Nanoboxes with Enhanced
Sodium Storage Properties
Yongjin Fang, Bu Yuan Guan, Deyan Luan, and Xiong Wen (David) Lou*
[*] Dr. Y. J. Fang, Dr. B. Y. Guan, Dr. D. Y. Luan, Prof. X. W. Lou
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang
Drive, Singapore 637459, Singapore
Email: xwlou@ntu.edu.sg; davidlou88@gmail.com
Webpage: http://www.ntu.edu.sg/home/xwlou/
Abstract
Metal sulfides have received considerable interest for efficient sodium storage owing to their high
capacity and decent redox reversibility. However, the poor rate capability and fast capacity decay
greatly hinder their practical application in sodium-ion batteries. Herein, an elegant multi-step
templating strategy has been developed to rationally synthesize hierarchical double-shelled
nanoboxes with the CoS
the structural and compositional advantages, these hierarchical CuS@CoS
boosted electrochemical properties with high capacity, outstanding rate capability, and long cycle life.
2
nanosheet-constructed outer shell supported on the CuS inner shell. With
2
nanoboxes manifest
Keywords: nanosheets, nanoboxes, CoS , CuS, sodium-ion batteries
2
1
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Sodium-ion batteries (SIBs) have aroused increasing interest as a promising candidate for large-scale
electric energy storage in view of their overwhelming merits of both cost and natural abundance
compared to the established lithium-ion battery systems.[1-8] Developing efficient anode materials is
highly desirable to promote the practical implementation of SIBs.[9, 10] Due to the high capacity and
good electrochemical reversibility, various metal sulfides such as CoS
x
, CuS
x
, SnS
x
, FeS have been
x
reported for sodium storage.[11-19] However, most of them suffer from poor conductivity and large
volume change during electrochemical reactions, which cause limited rate and cycling performance.
Rational design in nanostructure and carbon modification are two effective ways to address these
issues thus to enhance the electrochemical performance. For example, construction of hybrid
nanostructures for electrodes has been proposed as an effective strategy to enhance the sodium storage
properties, owing to the synergetic effects from different components.[20, 21]
Hierarchical hollow configurations assembled by low-dimensional nanoscale building blocks
show promising properties in high-performance SIBs.[22-26] The hollow structure with high surface
area can effectively accommodate the strain during electrochemical reactions and facilitate the full
infiltration of electrolyte. And the low-dimensional nanosized building units can offer significantly
decreased electron/ion transport pathways, leading to better rate performance. Notwithstanding a lot
of progress achieved, fabrication of metal sulfides with complex nanoarchitectures and chemical
compositions with enhanced sodium storage properties is less reported.
Herein, we demonstrate the rational design and construction of hierarchical CuS@CoS
shelled nanoboxes (DSNBs) by taking advantage of the unique reactivity of Cu O. The multi-step
synthesis process of the CuS@CoS DSNBs is schematically depicted in Figure 1. First, a shell of
Co(OH) nanosheets is grown on Cu O nanocubes by carefully manipulating the balance of the
coordinating etching rate toward the Cu O template and precipitating rate of the Co(OH) nanosheets.
Afterward, the obtained Cu O@Co(OH) core-shell nanocubes are selectively sulfurized by a proper
amount of Na S solution at room temperature to get the Cu O@CuS@Co(OH) nanocubes. Then, the
2
double-
2
2
2
2
2
2
2
2
2
2
2
2
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Cu
hierarchical CuS@Co(OH)
transformed to CuS@CoS
CuS@CoS DSNBs simultaneously combine the structural and compositional design rationales as
advanced anodes for SIBs. As expected, the as-synthesized CuS@CoS DSNBs demonstrate
2
O core in the Cu
2
O@CuS@Co(OH)
nanoboxes. Lastly, the resultant CuS@Co(OH)
DSNBs via a solvothermal sulfidation process. The as-obtained
2
is selectively removed with a Na
2
S
2
O
3
solution to obtain the
2
2
nanoboxes are further
2
2
2
improved electrochemical performance with high reversible capacity, decent rate performance and
long cyclability.
Cu
2
O nanocubes synthesized through a reduction method are employed as the starting templates.
Field-emission scanning electron microscopy (FESEM) images show that the as-synthesized Cu
2
O
nanocubes are quite homogeneous with an average particle size of around 650 nm (Figure 2a and
Figure S1a,b, Supporting Information (SI)). The transmission electron microscopy (TEM) images
(
Figure 2e and Figure S1c, SI) indicate the solid nature of these nanocubes. X-ray diffraction (XRD)
Figure S1d, SI) and energy-dispersive X-ray spectroscope (EDX) analysis (Figure S2, SI) further
(
confirm the crystalline structure and phase purity of the Cu
a proper amount of CoCl and Na for 1.5 min, a shell of Co(OH)
Cu O nanocubes. A typical FESEM image (Figure 2b) of the as-derived sample demonstrates that the
particles still maintain the cubic shape but the surface become extremely rough. TEM images (Figure
f and Figure S3, SI) reveal that the shell consists of random nanosheets. The evolution of the
2
O. When the Cu
2
O nanocubes react with
2
2
S O
2 3
2
nanosheets is grown on the
2
2
Co(OH)
2
nanosheets on the Cu O nanocubes can be attributed to the controlled coordinating etching
2
and precipitating reaction.[27]
A subsequent sulfidation process is conducted by the mild treatment of core-shell
O@Co(OH) nanocubes with an appropriate amount of Na S at room temperature. FESEM
Figure 2c) and TEM (Figure 2g and Figure S4, SI) images reveal that the morphology of the as-
formed product exhibits small variation compared to the core-shell Cu O@Co(OH) nanocubes.
Cu
2
2
2
(
2
2
However, the EDX result of the as-derived product (Figure S5, SI) reveals the existence of S element,
3
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Angewandte Chemie International Edition
10.1002/anie.201902583
which should come from CuS formed through the reaction between surficial oxidized CuO and S2-
ions.[28, 29] The selective sulfidation of copper oxide cores in the Cu
2
O@Co(OH) nanocubes is
2
because of the ultralow solubility product constant (Ksp) of CuS, leading to remarkably high binding
2
+
2-
force between Cu and S ions in solution. The as-prepared Cu
then transformed into CuS@Co(OH) nanoboxes by selectively etching Cu
is well retained after the elimination of the Cu O core (Figure 2d). And a distinct interior cavity can
2
O@CuS@Co(OH)
2
nanocubes are
2
2
O out. The cubic structure
2
be obviously discerned in the nanocube (Figure 2h and Figure S6a,b, SI), and the ultrathin nanosheets
on the outer shell are unaltered during the etching process (Figure S6c, SI). The EDX spectrum
(
Figure S7, SI) shows the presence of Cu, Co, S, and O elements, confirming the composition of
CuS@Co(OH)
Through a sulfidation treatment in thioacetamide solution, the CuS@Co(OH)
converted to CuS@CoS DSNBs. All the diffraction peaks in the XRD pattern are readily identified
as cubic CoS and hexagonal CuS (Figure S8a, SI), and the EDX result indicates that these
CuS@CoS DSNBs have a Co/Cu molar ratio of 1:0.84 (Figure S8b, SI). The FESEM images (Figure
a,b and Figure S9a, SI) demonstrate that the CuS@CoS DSNBs inherit the hierarchical cubic
2
.
2
nanoboxes are
2
2
2
3
2
structure. Magnified FESEM image clearly shows the hierarchical shells, which are composed of
randomly assembled nanosheets (Figure 3c). The well-defined hollow configuration of the
CuS@CoS
hierarchical nanosheets show some shrink (Figure 3e and Figure S9c, SI) compared to those of the
CuS@Co(OH) nanoboxes. A closer examination on the shell of an individual CuS@CoS DSNB
reveals that no visible inter-shell gap can be discerned (Figure S9d, SI), suggesting that the CoS
2
DSNBs is verified by TEM (Figure 3d) and FESEM (Figure S9b, SI) studies. The
2
2
2
nanosheets are strongly anchored on the CuS inner shell. The typical lattice fringe (Figure 3f) of the
outermost nanosheet has an interplanar distance of 0.32 nm, which is consistent with the d-value of
(
111) plane of cubic CoS .
2
4
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In addition, by controlling the coordinating etching and precipitating reaction time to 6 min,
yolk-shelled Cu O-Co(OH) nanocubes can be synthesized (Figure S10a,b, SI). Accordingly, well-
defined double-shelled CuS-Co(OH) nanoboxes (Figure S10c,d, SI) and their derived double-shelled
nanoboxes (here denoted as CuS-CoS DSNBs) can be obtained through similar sulfidation and
etching processes. The CuS-CoS DSNBs exhibit similar composition (Figure S11, SI) and preserve
2
2
2
2
2
the hierarchical nanobox structure (Figure 4a and Figure S12a, SI). TEM images (Figure 4d and
Figure S10e,f, SI) and FESEM image (Figure S12b, SI) further confirm that the as-obtained CuS-
CoS
the outer and inner shells can be observed. The enlarged view of one CuS-CoS
SI) reveals that the outer shell is constructed by CoS nanosheets, while the inner shell consists of
CuS nanoparticles. Moreover, single-shelled CoS nanoboxes (denoted as CoS SSNBs) (Figure S13,
SI) and single-shelled CuS nanoboxes (denoted as CuS SSNBs) (Figure S14, SI) can be obtained
through the sulfidation of Co(OH) nanoboxes (Figure S15 and Figure S16a,b, SI) and Cu
nanocubes (Figure S1, SI), respectively. The CoS SSNBs exhibit hierarchical nanosheet-assembled
2
DSNBs hold a unique shell-in-shell hollow configuration, in which an apparent gap between
2
DSNB (Figure S12c,
2
2
2
2
2
O
2
single-shelled nanoboxes (Figure 4b,e and Figure S16c,d, SI), while the CuS SSNBs are constructed
by nanoparticles with well-defined cube-like morphology (Figure 4c,f and Figure S17, SI).
The electrochemical performance of the CuS@CoS DSNBs are investigated by assembling coin
2
cells. Figure 5a shows the representative cyclic voltammetry (CV) curves of the CuS@CoS DSNBs.
2
+
In the first cathodic scan, a small broad peak at 1.49 V corresponds to the Na ion insertion into the
CuS structure,[30, 31] while the sharp reduction peak located at 1.25 V can be assigned to Na ion
+
insertion into CoS
2
.[32, 33] Another small peak at 0.95 V may be associated with the structural
CuS phases.[34, 35] And the peaks at 0.85 and 0.50 V are ascribed to the
transformation of the Na
conversion of Na CoS
x
x
2
to Co and Na S and the formation of a solid-electrolyte interphase (SEI)
2
film.[36, 37] In the second cathodic scan, two new peaks at 1.91 and 1.49 V emerge, which are ascribed
+
, respectively.[32, 34-36, 38] During the anodic
to the Na ion intercalation processes of CuS
x
and CoS
x
5
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Angewandte Chemie International Edition
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+
scans, two peaks at about 1.58 and 2.15 V are associated with deintercalation of Na ions from the
Na
x
CuS structure,[34, 35, 39] and three peaks at 1.87, 1.96 and 2.07 V represent the reverse processes
.[32, 36, 38] Figure 5b displays the representative galvanostatic discharge-
from Co and Na
2
S to the CoS
x
-1
charge curves of the CuS@CoS
2
DSNBs at 0.1 A g . Remarkably, the CuS@CoS DSNBs deliver a
2
-1
stable reversible capacity of about 625 mAh g , which is higher than those of the CoS
2
SSNBs and
CuS SSNBs (Figure S18, SI), and most CoS
x
-based anodes.[33, 40-44] The initial CV and discharge-
charge curves of CuS@CoS DSNBs show some difference with that of subsequent cycles, which
2
can be ascribed to the structural rearrangement and formation of SEI film in the initial sodiation
process.[36, 38, 41]
The rate performance of the CuS@CoS
2
DSNBs, CuS@CoS
2
DSNBs, CoS
2
SSNBs and CuS
SSNBs is examined at various current densities. As presented in Figure 5c, the average reversible
-1
capacities of CuS@CoS
2
DSNBs are 612, 570, 535, 483, 416, 360 mAh g at 0.1, 0.2, 0.3, 0.5, 1, 2
-1
-1
A g , respectively. Even at a large current density of 5 A g , the CuS@CoS
2
DSNBs can also deliver
DSNBs
SSNBs decrease quickly when the current density is increased. Notably, the CuS SSNBs
exhibit superior rate capability without obvious capacity decay at high current densities but with
relatively low specific capacities. The outstanding rate performance of the CuS@CoS DSNBs can
also be confirmed by the discharge-charge curves at different current densities (Figure 5d).
Furthermore, the CuS@CoS DSNBs also show good cycling performance. Of note, 79% of the initial
capacity is retained after 500 cycles at a current density of 0.5 A g (Figure 5e). Impressively, the
-1
a high capacity of 304 mAh g . In stark contrast, the reversible capacities of the CuS-CoS
2
and CoS
2
2
2
-1
-
1
CuS@CoS
CuS@CoS
2
2
DSNBs also exhibit excellent cyclic stability at 0.3 A g (Figure S19, SI). Moreover,
DSNBs still largely retain their nanobox structures after cycling (Figure S20, SI). For
comparison, the capacities of the CuS-CoS
2
DSNBs and CoS SSNBs drop rapidly, while the CuS
2
SSNBs exhibit superior cyclability (Figure 5e). The stable cyclability of the CuS@CoS
2
DSNBs is
also better than most CoS
x
-based electrodes.[32, 38, 41, 43-45] To explain the outstanding rate performance
6
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Angewandte Chemie International Edition
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of the CuS@CoS
2
DSNBs, a kinetics analysis based on CV test is performed (Figure S21, SI). The
DSNBs. The
results suggest the characteristics of capacitance-controlled kinetics of the CuS@CoS
2
high capacitive contribution of CuS@CoS
2
DSNBs is mainly due to the hierarchical nanosheet-on-
+
nanobox configuration, which provides a large surface for the adsorption and desorption of Na ions
during the repeated (de)sodiation processes.[46, 47]
The outstanding sodium storage properties of these CuS@CoS
2
DSNBs may be ascribed to the
nanosheets supported
distinctive hierarchical hollow configuration and synergistic effect of the CoS
2
on CuS nanoboxes. Specifically, the primary nanosheet building blocks could decrease the diffusion
+
path of electrons/Na ions to accelerate the reaction kinetics. Both the hollow architecture and
nanosheet subunits could efficiently accommodate the mechanical strain during repeated discharge-
charge processes. The CuS backbone can enhance the electronic conductivity of the CuS@CoS
2
DSNBs thus to effectively boost the charge transfer process (Figure S22, SI). Besides, the double-
shelled structure could improve the structural integrity of the active materials, leading to remarkable
enhanced cycling performance.
In summary, we have proposed a delicate templating strategy for the effective synthesis of
hierarchical double-shelled nanoboxes with the CoS nanosheet-assembled outer shell supported on
2
the CuS inner nanobox for efficient sodium storage. The rational synthetic strategy, involving a
controlled coordinating etching and precipitating reaction and subsequent sulfidation and etching
processes, can effectively tailor the structures and compositions of the final nanoarchitectures. Owing
to the distinctive structural and compositional advantages, the CuS@CoS
2
double-shelled nanoboxes
demonstrate boosted electrochemical performance for sodium storage with high reversible capacity
-
1
-1
-1
(
(
625 mAh g ), outstanding rate performance (304 mAh g at 5 A g ), and excellent cycling stability
79% capacity retention after 500 cycles). This work may inspire further potentials for the design and
construction of hierarchical hollow configurations with great complexity for diverse applications.
7
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Acknowledgments
X. W. L. acknowledges the funding support from the National Research Foundation (NRF) of
Singapore via the NRF investigatorship (NRF-NRFI2016-04).
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1
0
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Figures and Captions
Figure 1. Schematic illustration of the synthetic process of CuS@CoS
2
DSNBs.
1
1
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Angewandte Chemie International Edition
10.1002/anie.201902583
Figure 2. (a-d) FESEM images and (e-h) TEM images of the (a, e) Cu
2
O nanocubes, (b, f) core-shell
Cu
2
O@Co(OH)
2
nanocubes, (c, g) Cu
2
O@CuS@Co(OH)
2
nanocubes, (d, h) CuS@Co(OH)
2
nanoboxes.
1
2
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Figure 3. (a-c) FESEM images, (d, e) TEM images, and (f) HRTEM image of the CuS@CoS DSNBs.
2
1
3
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Angewandte Chemie International Edition
10.1002/anie.201902583
Figure 4. (a-c) FESEM images and (d-f) TEM images of the (a, d) CuS-CoS
2
DSNBs, (b, e) CoS
2
SSNBs, and (c, f) CuS SSNBs.
1
4
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Angewandte Chemie International Edition
10.1002/anie.201902583
st
rd
Figure 5. (a) The 1 to 3 cyclic voltammograms of the CuS@CoS
2
DSNBs at a scan rate of 0.1 mV
DSNBs at a current density of
-1
s . (b) Typical discharge-charge voltage profiles of the CuS@CoS
2
-
1
0
.1 A g . (c) Rate performance of the CuS@CoS
SSNBs, and (d) The corresponding discharge-charge voltage profiles of the CuS@CoS
various current densities. (e) Cycling performance of the CuS@CoS DSNBs, CuS-CoS
SSNBs and CuS SSNBs at a current density of 0.5 A g , and the corresponding Coulombic
efficiency of the CuS@CoS DSNBs.
2
DSNBs, CuS-CoS
2
DSNBs, CoS
2
SSNBs and CuS
DSNBs at
DSNBs,
2
2
2
-1
CoS
2
2
1
5
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Angewandte Chemie International Edition
10.1002/anie.201902583
for Table of Content Entry
Double-shelled CuS@CoS
nanosheets supported on the inner CuS nanobox are synthesized by a delicate template-engaged
strategy using Cu O nanocubes as the starting material. With the structural and compositional merits,
these hierarchical CuS@CoS nanoboxes exhibit attractive electrochemical properties as an anode
material for sodium-ion batteries.
2
nanoboxes composed of an outer shell of hierarchical CoS
2
2
2
1
6
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