Hierarchical Porous NiO/β-NiMoO4 Heterostructure as Superior Anode Material for Lithium Storage

Article abstract of DOI:10.1002/cplu.201800220

Ternary transition metal oxides (TTMOs) have attracted considerable attention for rechargeable batteries because of their fascinating properties. However, the unsatisfactory electrochemical performance originating from the poor intrinsic electronic conductivity and inferior structural stability impedes their practical applications. Here, the novel hierarchical porous NiO/β-NiMoO₄ heterostructure is fabricated, and exhibits high reversible capacity, superior rate capability, and excellent cycling stability in Li-ion batteries (LIBs), which is much better than the corresponding single-phase NiMoO₄ and NiO materials. The significantly enhanced electrochemical properties can be attributed to its superior structural characteristics, including the large surface area, abundant pores, fast charge transfer, and catalytic effect of the intermediate product of metallic nickel. The NiO/β-NiMoO₄ heterostructure delivers a high capacity of 1314 mA h g^?1 at 0.2 A g^?1 after 100 cycles. Furthermore, even after 400 cycles at 1 A g^?1, the reversible capacity remains at around 500 mA h g^?1. These results indicate that the NiO/β-NiMoO₄ heterostructure shows great potential as an anode material for high-performance LIBs.

Full text of DOI:10.1002/cplu.201800220

Accepted Article
Title: Hierarchical porous NiO/β-NiMoO4 heterostrucutre as superior
anode material for lithium storage
Authors: Zhijian Wang, Shilin Zhang, Hai Zeng, Haimin Zhao, Wei Sun,
Meng Jiang, Chuanqi Feng, Jianwen Liu, Tengfei Zhou, Yang
Zheng, and Zaiping Guo
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10.1002/cplu.201800220
Hierarchical porous NiO/β-NiMoO heterostructure as
4
superior anode material for lithium storage
1
†
2†
3
4
4
1
Zhijian Wang , Shilin Zhang , Hai Zeng , Haimin Zhao , Wei Sun , Meng Jiang ,
1
1
2,3,5
2*
1, 2*
Chuanqi Feng , Jianwen Liu , Tengfei Zhou , Yang Zheng , Zaiping Guo
1
Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials,
Ministry of Education Key Laboratory for Synthesis and Applications of Organic
Functional Molecules, Hubei University, Wuhan 430062, China.
2
Institute for Superconducting and Electronic Materials, School of Mechanical,
Materials, and Mechatronics Engineering, University of Wollongong, North
Wollongong, NSW 2500, Australia
3
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission & Ministry of Education, South-Central University for Nationalities,
Wuhan, 430074, P. R. China
4
Tianneng Battery Group Co. Ltd, 18 Baoqiao Road, Huaxi Industrial Functional
zone, Changxing, Zhejiang, 313100, China
5
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
Nankai University, Tianjin 300071, China
*
Corresponding author: zguo@uow.edu.au; yz966@uowmail.edu.au
1
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10.1002/cplu.201800220
ABSTRACT
Ternary transition metal oxides (TTMOs) have attracted considerable attention for
rechargeable batteries because of its fascinating properties. However, the unsatisfied
electrochemical performance originating from the poor intrinsic electronic
conductivity and inferior structural stability impede their practical applications. In this
work, the novel hierarchical porous NiO/β-NiMoO heterostructure has been
4
fabricated and exhibited high reversible capacity, superior rate capability, and
excellent cycling stability towards LIBs, which is much better than the corresponding
single phase NiMoO and NiO materials. The significantly enhanced electrochemical
4
properties can be attributed to its superior structural characteristics, including the
large surface area, abundant pores, fast charge transfer, and catalytic effect of the
intermediate product of metallic nickel. The NiO/β-NiMoO heterostructure delivered
4
−
1
−1
a high capacity of 1314 mA h g at 0.2 A g after 100 cycles. Furthermore, even
−
1
after 400 cycles at 1 A g , the reversible capacity still remained at around 500 mA h
−
1
g . These results indicate that the NiO/β-NiMoO heterostructure shows great
4
potential as anode materials for high-performance LIBs.
Keywords：Ternary transition metal oxides, heterostructure, β-NiMoO , NiO, Li-ion
4
batteries
2
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Introduction
High-performance rechargeable lithium-ion batteries (LIBs) are gradually becoming
indispensable units in portable commercialized electronics, electric vehicles, and
[
1,2]
renewable energy storage.
Nevertheless, the conventional LIBs based on the
prevailing carbon-based intercalation materials are already reaching their energy
capacity limits. Hence, tremendous efforts have been devoted to exploring novel
anode materials with higher energy density, sustainable lifespan, cost-effectiveness,
and safe working potential to make these materials applicable for the advanced LIBs
[3–5]
to meet the ever-growing demands for electrical applications.
Ternary transition metal oxides (TTMOs) are emerging as promising candidate
[
6,7]
electrode materials for high-energy LIBs due to their distinctive properties.
It is
widely demonstrated that TTMOs generally exhibit superior reversible capacity, good
structural stability, and high electronic conductivity. Specifically, the synergetic
effects associated with their complex chemical compositions typically contribute to
remarkable electrochemical activities and high specific capacity, while the multiple
[
8,9]
valences of the cations are beneficial to achieve more versatile redox reactions.
More importantly, because of the lower activation energy for electron transfer
between different cations, it normally possess higher electrical conductivity than their
[
10]
corresponding binary metal oxide counterparts. Among various potential TTMOs,
spinel structure nickel molybdate (NiMoO ) has been regarded as an attractive
4
−
1
material in view of its high theoretical capacity (about 984 mA h g ), good electrical
[11,12]
conductivity, environmental friendliness, and low cost.
3
At atmospheric pressure,
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10.1002/cplu.201800220
there are two phases of NiMoO both with monoclinic structure: the α-NiMoO (low
4
4
temperature stable phase) and β-NiMoO (high temperature stable phase). The most
4
6
+
obvious difference between those two polymorphs is the coordination of the Mo
ions, with pseudo-octahedral coordination in the α-NiMoO phase and distorted
4
[13]
tetrahedral coordination in the β-NiMoO phase. Generally, β-NiMoO has higher
4
4
[14]
intrinsic conductivity and reactivity than that of α-NiMoO4,
but it is very difficult
to be prepared and maintained. In addition, NiMoO electrode materials suffer from
4
some drawbacks, such as the huge volume change, low electrochemical reversibility,
[
15,16]
leading to inferior electrochemical performance.
Therefore, rational design and
facile synthesis of β-NiMoO materials for high energy LIBs still remains a
4
significant challenge.
Constructing heterostructure by combining active materials with metal oxides or
[
17,18]
sulfides has proven to be an effective way to conquer above-mentioned obstacles.
It is believed that such constructed heterostructure usually enables abundant
electrochemical active sites, short ion diffusion path, and good structural stability
originating from the synergistic effect of different components, thus exhibiting better
[
19]
electrochemical performance. For example, Yuan et.al, developed the hierarchical
mesoporous ZnO/ZnFe O sub-microcubes which exhibited better electrochemical
2
4
Li-storage properties than the single-phase ZnFe O , owning to the striking
2
4
synergistic effect between the bi-component active, well-dispersed ZnO and ZFO
[
20]
phases at the nanoscale.
Mai et al. found that hierarchical MnMoO /CoMoO4
4
heterostructured nanowires shown enhanced specific capacitance and reversibility
4
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10.1002/cplu.201800220
[
21]
when compared to the MnMoO and CoMoO electrodes. Similarity, other hybrid
4
4
[
22]
heterostructures such as the ZnCo O /NiO core/shell nanowires,
NiO-Co O4
2
4
3
[
23]
[24]
[25]
nanoplates, CoO/CoFe O nanocomposites, and CoMoO /Co O composites,
2
4
4
3
4
have been employed as LIB electrodes and demonstrated improved reversible
capacity and cycling performance.
Besides the favorable component, the structure also plays important roles in
determining the electrochemical property of the developed electrode materials. It is
considered that the hierarchical architecture constructed from 2D building blocks
possess superior structural characteristics featuring in large specific surface area, more
accessible pores, high structural stability, endowing them promising prospects for
[26]
energy-related applications. Undoubtedly, the large specific surface would provide
rich Li storage active sites, while the generated numerous pores may offer additional
space to accommodate the volume expansion, consequently maintaining the structural
stability and electrode integrity. Lou et al, synthesized the hierarchical tubular
structure of Co-Mn mixed metal oxides manifesting excellent electrochemical
properties when employed as electrodes in Li-ion batteries and hybrid
[
27]
supercapacitors.
A hierarchical fascicular structure of Zn GeO anode obtained
2 4
through a wet chemistry method also exhibited remarkable Li-ion storage
performance, since the hierarchical structure could alleviate the volume changes and
retain the whole configuration, while the small building units will enlarge the contact
area for electrode/electrolyte and construct the integrated interlaced conductive
[
28]
network.
5
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Herein, we have designed and constructed a novel hierarchical porous
NiO/β-NiMoO4 (NNMO) heterostructure via a facile hydrothermal technique
followed by a calcination process. Interestingly, the NiO phases in the NNMO
heterostructure could be regarded as a multifunctional layer, which can not only
contribute to high theoretical capacity, but also function as the mechanical buffer to
alleviate the huge volume changes of NiMoO matrix, thus offering high capacity and
4
good structural stability. Furthermore, the generated metallic nickel (Ni) nanoparticles
from the lithiation reaction of NiO nanosheets with Li are able to promote the
reversibly decompose of Li O during delithiation process, leading to high initial
2
columbic efficiency and enhanced reversible capacity of the NiMoO , while the
4
abundant pores produced from the surface interconnected nanosheets and the
self-assembled microspheres with hierarchical structure will provide more space to
relieve the induced-stress during lithiation/delithiation processes and preserve the
structural integrity. As expected, the as-prepared NiO/β-NiMoO heterostructured
4
anode exhibited much better electrochemical Li storage properties than those of
single-phase NiO or NiMoO4 counterparts. Such significantly enhanced
electrochemical performance can be attributed to its superior structural characteristics
achieved by rational structure engineering of the electrode.
Results and discussion
The synthesis of hierarchical porous NiO/β-NiMoO (NNMO) heterostructure by a
4
novel and facile strategy is schematically illustrated in Scheme 1. First, appropriate
6
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10.1002/cplu.201800220
2
+
2−
proportions of Ni and MoO4 precursors were homogeneously mixed into
deionized water under magnetic stirring for certain time, so that the acetate could be
2
−
adsorbed on the surface of MoO4 by electrostatic interaction, and after that, the
NNMO was obtained by oxidation of the as-prepared precursor mixture at 550ꢀ°C
under oxygen atmosphere. For further comparison, single phase NiO (NO) and
NiMoO (NMO) were also fabricated by similar processes, with the exception of the
4
different precursors that were chosen.
The purity and crystal structure of fabricated samples were analyzed by powder
X-ray diffraction (XRD). The XRD patterns in Figure 1a confirm that the as-prepared
NNMO materials are composed of pure β-phase NiMoO (JCPDS Card No. 12-0348)
4
and cubic bunsenite NiO (JCPDS Card No. 47-1049), indicating the coexistence of
pure β-NiMoO and NiO phase in the obtained NNMO composite. Furthermore, the
4
formation of cubic NiO phase in the hybrid heterostructure is revealed by the
diffraction peaks centered at 37.25°, 43.28°, and 62.89°, corresponding to the (111),
(200), and (220) crystal planes, respectively. The remarkable diffraction peak at the
value of 26.65° can be indexed to the characteristic (220) planes of β-NiMoO4,
suggesting the high purity of the β-phase NiMoO , without any trace/residual of the
4
[
29]
α-phase. As shown in Figure S1a (Supporting Information), the diffraction patterns
of prepared single-phase NO are well matched with pure NiO (JCPDS Card No.
4
7-1049), while all the diffraction peaks in Figure S1b (Supporting Information) can
be assigned to the pure monoclinic α-NiMoO (JCPDS Card No. 33-0948). Notably,
4
no diffraction peaks can be indexed to β-phase NiMoO4.
7
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N adsorption/desorption measurement were carried out to analyze the physical
2
properties of the obtained samples (Figure 1b-1c). As presented in Figure 1b, the
isotherm curves of all three samples show distinct type IV adsorption isotherm with
H1-type hysteresis loops within a relative pressure P/P range from 0.05 to 0.95 in the
0
desorption isotherm, implying the existence of numerous pores. Based on the BET
method analysis, it was found that the NNMO has a much larger specific surface area
2
−1
3
−1
of 23.20 m g with a total pore volume of 2.4 cm g , compared to the single-phase
2
−1
2
−1
NO (10.45 m g ) and NMO (18.40 m g ) (Inset in Figure1b). Accordingly, the BJH
pore size distribution profiles derived from the adsorption branches of isotherms were
also calculated and are shown in Figure 1c. The pore size distribution curves for
NNMO reveal that most of the pores range between 20 and 50 nm, markedly
confirming the existence of a kind of mesoporous nanostructure. In addition, the
narrow pore size distributions of NO and NMO show that the numerous pores are
mainly macropores. Impressively, the unique structural features including the high
specific surface area and abundant porous structure of NNMO heterostructure will not
only provide many electroactive sites, large electrode/electrolyte contact area as well
as easy access of electrolyte, but also accommodate the huge volume changes of the
[
8,30,31]
electrode during repeated lithiation/delithiation processes,
thus making it
extremely beneficial to high-rate performance and long-cycle stability for Li ions
storage.
The chemical states and elemental composition of the as-synthesized sample
were further characterized by the X-ray photoelectron spectroscopy (XPS) technique.
8
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As displayed in Figure 2a, the whole survey scan of the NNMO primarily
demonstrates the presence of the elements Ni, Mo, and O. The high-resolution
spectrum of Ni 2p (Figure 2b) can be deconvoluted into two main peaks by using the
Gaussian fitting method, with the binding energies centered at 855.9 eV and 873.8 eV,
corresponding to two spin-orbit doublets (Ni 2p3/2 and Ni 2p ) and two weak
1/2
[
32,33]
shake-up satellite peaks of nickel located at 861.5 eV and 880.1 eV, respectively.
2
+ [34]
The spin-energy separation of 17.9 eV is characteristic of the presence of Ni . On
the other hand, the binding energies of Mo 3d5/2 and Mo 3d3/2 (Figure 2c) are located
at 232.0 eV and 235.1 eV, respectively. The binding energies and the energy
6
+
[35]
separation (3.1 eV) are characteristics of the Mo oxidation state in NiMoO4.
Figure 2d clearly shows the high-resolution spectrum in the O 1s region, which
consists of a main peak located at 531.7 eV coupled with a shoulder at 532.7 eV.
Therefore, the XPS results indicate the presence of both β-NiMoO and NiO in the
4
composite, which is consistent with the XRD result (Figure 1a).
The detailed morphology and microstructure of the as-prepared hierarchical
porous NiO/β-NiMoO heterostructure were investigated by the scanning electron
4
microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM).
Figure 3a,b shows the representative SEM images of the NNMO at low and high
magnification, respectively. It can be seen that the NNMO shows typical hierarchical
porous microspheres, constructed from large amount of nanosheets with a thickness of
1
0~20 nm. Moreover, close observation can reveal that there are numerous nanopores
generated on the nanosheets (Figure 3b) and the assembled nanosheets also formed
9
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10.1002/cplu.201800220
abundant large pores, which is also consistent with the BET results. The TEM images
further confirm the hierarchical porous structure of the NNMO sample. (Figure 3c,d)
These pores could promote the electrolyte to access the interior portion of material
and offer enough additional space to accommodate the volume changes of the
electrodes, thus improving electrochemical performance for Li-ion storage.
Furthermore, the marked interplanar distances in the high resolution TEM image are
0
.336 and 0.241 nm (Figure 3e), corresponding to the (220) and (111) lattice planes of
β-NiMoO and NiO, respectively, certifying the presence of the two phases in the
4
composite. More interestingly, the β-NiMoO and the NiO nanosheets are in tight
4
contact with each other and form the heterostructure, which may promote rapid
[
18]
ion/electron transfer. In addition, the introduced NiO phase could function as the
buffer layer to accommodate the volume expansion and effectively maintain the
structural integrity and stability. In the selected area electron diffraction (SAED)
pattern of the NNMO (Figure 3f), the diffraction rings can be assigned to the (111),
(200), (220), and (222) planes of NiO phase, as well as to the (220) planes of
β-NiMoO , respectively, which also indicates the coexistence of the two phases in the
4
NNMO composite. This novel architecture and its composition were further
researched by the corresponding energy dispersive X-ray spectroscopy (EDX)
elemental mapping images (Figure 3g-3j), which confirm the presence of Ni, Mo, and
O elements in the NNMO. Moreover, the Ni, Mo and O elements are homogeneously
distributed in the entire microsphere, indicating the intimate contact between the two
components and the formation of heterostructures. On the contrary, the SEM images
1
0
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of the NO (Figure S2a,S2b) show networks constructed from many disordered and
aggregated rods. The length of the rods is between one and tens of micrometers.
Figure S2c,S2d further reveal the general microsphere architecture of α-phase
NiMoO with several micrometers in average diameters. Although the morphology of
4
α-phase and β-phase NiMoO are quite similar, without the introduced NiO phase, the
4
nanosheets of NMO microsphere aggregated severely, resulting in less pores and
inferior structural stability.
To further investigate the electrochemical behavior of the synthesized materials
for Li-ion batteries, cyclic voltammetry (CV) measurements were conducted. Figure
4
a shows the first five consecutive CV curves of NNMO electrode recorded at a scan
+
rate of 0.1 mV/s in the potential range of 0.01–3.00 V versus Li/Li . It is evidently
observed that from the second to fifth cycles, these CV profiles overlap well,
implying the good reversibility and cycling stability of the NNMO electrode during
electrochemical reactions. In the first cycle, two intense reduction peaks in the
cathodic scan occurred at around 0.59 V and 0.42 V, corresponding to the initial
+
−
reduction of β-NiMoO to Ni and Mo (NiMoO + 8Li + 8e → Ni + Mo + 4Li O)
4
4
2
+
−
and NiO to Ni (NiO + 2Li + 2e → Ni + Li O), combined with the formation of
2
[
16,36,37]
amorphous Li O and SEI layers.
Meanwhile, the broad peaks in the voltage
2
range from 1.4 V to 1.8 V can be further ascribed to the oxidation process of Mo to
6
+
+
− [38]
Mo (Mo + 3Li O → MoO + 6Li + 6e ), whereas the oxidation of metallic Ni to
2
3
[
39]
NiO contributes to the anodic peak located at approximately 1.83 V.
In the
x
following cycles, the different reduction/oxidation peak pairs of 0.75/1.45 V and
11
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10.1002/cplu.201800220
1
.42/1.79 V are reversible because of the lithium insertion/extraction process with
[39,40]
MoO3.
Notably, an anodic peak near 2.25 V is observed in the NNMO electrode,
+
− [36]
owing to the conversion reaction of Ni to NiO (Ni + Li O → NiO + 2Li + 2e ).
2
On the other hand, the CV curves of NO and NMO electrodes show different behavior
in comparison with that of the NNMO. In the first cathodic process of NO electrode,
two sharp peaks are clearly observed at 0.437 V and 0.398 V, respectively, because of
the reduction reactions of NiO and the formation of amorphous Li O, as well as a
2
partially irreversible SEI layer. The oxidation of metallic Ni to NiO contributes to the
anodic peak located at approximately 2.26 V, whereas the anodic peaks located at
about 1.805 V are attributed to the formation of Mo. (Figures S3).
Figure 4b shows the typical galvanostatic discharge/charge voltage profiles of
−
1
NNMO anode from 0.01 to 3.0 V at a current density of 0.2 A g . In the first cycle,
−
1
the NNMO electrode delivered an initial discharge capacity of 1538 mA h g and
−
1
charge capacity of 1282 mA h g , corresponding to an initial coulombic efficiency
(
ICE) of 83%, which is higher than those of the NO (80%) and NMO electrodes (78%)
Figure S4), demonstrating the high reversibility and good stability of the NNMO
(
electrode during the electrochemical reactions. The higher reversible capacity of the
NNMO can be explained by the further delithiation reaction observed above 2.0 V
(Figure 4a), which is attributed to the catalytic behavior of the metallic nickel
nanocrystals and its unique porous hierarchical structure that can shorten ion diffusion
[
41]
pathways. Furthermore, the NNMO anode exhibited excellent cycling stability. As
shown in Figure 4c, the NNMO electrode delivered an initial reversible capacity of
1
2
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10.1002/cplu.201800220
−
1
−1
1
282 mA h g , which then gradually increased to 1314 mA h g , with the coulombic
efficiency nearly 100% after 100 cycles. The feature of increased capacity after the
initial cycle is not surprising for a metal oxide-based material, as it always ascribed to
the reversible growth of polymeric gel-like layers and capacitive interface storage
including electric double layer capacitance, as well as an activation process in the
[
37]
nanostructures.
On the contrary, rapid capacity fading and inferior capacity
retention are clearly observed in the NO and NMO materials. The high
electrochemical reversibility and good cycling stability of the NNMO material may be
attributed to the following factors: 1) the in-situ generated Ni nanocrystals from the
NiO phase could function as a direct catalyst for the reversible decomposition of
intermediate Li O and the oxidation of metallic Ni and Mo from β-NiMoO ; 2) the
2
4
NiO nanosheets can act as the mechanical buffer to effectively stabilize the NiMoO4
phase, thus reducing the self-agglomeration of NiMoO nanosheets and maintaining
4
good structural stability; 3) the unique hierarchical porous architecture constructed
+
from the small nanosheets will provide more active sites for Li storage, short ion
diffusion pathways, as well as abundant additional space to release the internal stress
induced by volume changes during discharge/charge process.
Additionally, the NNMO electrode also exhibited remarkable rate capability at
different current densities. As can be seen from Figure 4d, the NNMO electrode is
capable of delivering the specific capacities of 1360, 1003, 631, 469, and 308 mA h
−
1
−1
g
at the current densities of 0.2, 0.5, 1.0, 3.0, and 5.0 A g , respectively. More
−
1
importantly, the capacity can recover to the initial value (about 1339 mA h g ) when
1
3
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10.1002/cplu.201800220
−
1
the current rate is set back to 0.2 A g after 60 cycles, indicating the excellent rate
capability and good reversibility of the NNMO electrode. On the contrary, both the
NO and the NMO electrodes show much inferior capacity retention and severe
−
1
capacity decay between the current densities of 1 and 5 A g . The excellent rate
capability can be attributed to the synergistic effects of the binary composition and the
short lithium diffusion distance in the porous hierarchical NNMO heterostructure. The
long cycling performance is also an important parameter of batteries in practical
applications. Hence, we further examined the long-term cycling property of the
−
1
NNMO electrode at a high current density of 1 A g (Figure 4e). After 400 cycles, the
−
1
NNMO anode still retained a high reversible capacity around 500 mA h g , and the
average coulombic efficiency is close to 100%. More importantly, the NNMO
electrode can maintain the hierarchical porous structure after long-term cycles,
demonstrating its good structural stability. The superior structural characteristics, such
as the 2D building blocks, large surface area and abundant pores enable its good
structural integrity (Figure S5).
To reveal the boosted charge transfer behavior in the NNMO heterostructure, the
kinetic differences of lithium ion diffusion in these anodes have been investigated as
well. As shown in Figure 5a and 5b, the NNMO sample exhibits larger peak
intensities and smaller polarization at all scan rates, indicating the greatly enhanced
electrochemical kinetics in the NNMO electrode, which are favorable for achieving
better rate capability. Furthermore, the significantly promoted charge transfer process
in the NNMO heterostructure also can be proved by the higher diffusion coefficient of
1
4
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10.1002/cplu.201800220
+
Li compared to the NMO sample (Figure 5c). Meanwhile, the electrochemical
impedance spectroscopy (EIS) measurements have been conducted to research the
kinetic differences between various electrodes (Figure S6). It is noticed that the
diameter of the semicircle in the high–medium–frequency region for the NNMO
electrode is much smaller than for the NMO and NO electrodes, demonstrating the
highly improved electron/ion transport dynamic in the heterostructure of the NNMO
[
42,43]
sample.
Conclusion
In summary, we have developed a facile strategy to synthesize the novel hierarchical
porous NiO/β-NiMoO heterostructure constructed from small nanosheets, featuring
4
in large surface area, short ions diffusion paths, and good structural stability. By
means of synergistically active ternary metal oxides, boosted charge transfer dynamic,
and enhanced electrochemical reversibility, the NNMO heterostructure achieved high
reversible capacity, good rate capability and superior long-term cyclability for Li-ion
batteries. Our material design strategy presented here could be a promising approach
for designing novel heterostructure nanomaterials for high-performance rechargeable
batteries.
Experiment Section
Synthesis of hierarchical porous NiO/β-NiMoO heterostructure
4
In a typical synthesis process, 2 mmol nickel acetate tetrahydrate (0.4978 g) and 1
1
5
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10.1002/cplu.201800220
mmol sodium molybdate dehydrate (0.3629 g) with a Ni/Mo molar ratio of 2:1 were
slowly added into 30 mL deionized water under magnetic stirring for 30 min to form a
transparent solution. The above resultant solution was then transferred to a 50 mL
Teflon-lined autoclave, and then it was kept in an oven at 180 °C for 12 h. After the
hydrothermal reaction, the obtained green product was separated by centrifugation
and washed several times with de-ionized water and ethanol. The resultant composite
was then dried at 80 °C overnight, followed by heating in a tube furnace under
−
1
oxygen atmosphere at 550 °C for 2 h with a ramp rate of 2 °C min , yielding the
hierarchical NNMO electrode with a Ni : Mo ratio of 2.3:1, as determined by
inductively coupled plasma (ICP) (Table S1).
Synthesis of single-phase NiO (NO) materials
The synthesis process is similar to that for NNMO. First, 1.2 mmol nickel chloride
hexahydrate (0.2852 g), 3 mmol urea (0.1802 g), and 0.33 mmol cetyltrimethyl
ammonium bromide (CTAB, 0.12 g) were added into 30 ml deionized water under
magnetic stirring for 30 minutes to form a transparent solution. Then, the above
solution was transferred to a 50 mL Teflon-lined autoclave and kept in an oven at
1
80 °C for 12 h. The obtained slurry was centrifuged, washed with deionized water,
and then dried at 80 °C for 24 h. After thermal oxidation at 550 °C for 2 h with a ramp
−
1
rate of 2 °C min , the single phase NiO material was obtained.
Synthesis of single-phase α-NiMoO (NMO) materials
4
In a typical procedure, 1.3 mmol nickel chloride hexahydrate (0.3090 g) and 1.3
1
6
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10.1002/cplu.201800220
mmol sodium molybdate dehydrate (0.3527 g) with a Ni/Mo molar ratio of 1:1 were
added into a given amount (30 mL) of distilled water. The resulting solution was then
transferred into a Teflon-lined stainless-steel autoclave and maintained at 180 °C for
1
2 h. After cooling to room temperature, the slurry was washed several times with
deionized water and dried at 80 °C for 24 h in an oven. Finally, the dried material,
which had a green color, was thermally treated at 550 °C for 2 h in a tube furnace
under oxygen atmosphere to yield the α-NiMoO4.
Material characterization
The crystalline structure of the materials was investigated by X-ray diffraction (XRD，
−
1
Bruker AXS, D8 diffractometer) with Cu Kα radiation at a scanning rate of 1° min .
The morphology and energy dispersive spectroscopy (EDS) mapping of the
as-prepared products were observed by scanning electron microscopy (SEM; JEOL
JSM, 6510V), and the details of the crystal structure were further examined by
transmission electron microscopy (TEM; FEI TF20). X-ray photoelectron
spectroscopy (XPS) was then conducted on an ESCALAB 250XI photoelectron
−
6
spectrometer using monochromatic Al Kα radiation under vacuum at 2 × 10 Pa. All
of the binding energies are referenced to the C 1s peak at 284.8 eV of the surface
adventitious carbon. The actual contents of the Ni and Mo elements were measured by
inductively coupled plasma (ICP) emission spectroscopy using a Perkin Elmer
Optimal 8000 instrument (detection limit: 0.01 ppm for Ni, 0.01 ppm for Mo).
Specific surface areas and pore size distributions were also obtained from N2
1
7
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10.1002/cplu.201800220
adsorption/desorption isotherms recorded at 77 K with a Micromeritics ASAP 2020
porosimeter. Prior to surface area measurements, the sample was dried under vacuum
at 393 K for 8 h. The surface area was computed using the Brunauer–Emmett–Teller
(BET) method, whereas the pore size distribution was calculated in accordance with
the Barrett–Joyner–Halenda (BJH) method.
Electrochemical measurements
Electrochemical tests were further conducted on CR 2025 coin-type cells that were
assembled in an argon-filled glove box. The working electrodes were prepared by
mixing the active materials, Super P, and Na-carboxymethyl cellulose (CMC) in a
weight ratio of 60:30:10 then pasted on Cu foil and dried at 80 °C for 12 h in a
2
vacuum oven, which was followed by pressing at 300 kg/cm . The mass loading of
−
2
the materials on individual electrodes was about 1.15 mg cm . Electrochemical
measurements were further carried out using two-electrode coin cells with Li metal as
the counter and reference electrode and Celgard (product 2300) film as the separator.
The electrolyte consisted of a solution of 1 M LiPF in ethylene carbonate/diethyl
6
carbonate/ethyl methyl carbonate (1:1:1; v/v/v). Electrochemical impedance
spectroscopy and cyclic voltammetry were conducted on a CHI 660E electrochemical
workstation at different scan rates. The cells were galvanostatically
+
charged/discharged in the voltage range of 0.01–3.00 V versus Li/Li at different
current densities on a Land CT2001A battery tester.
The lithium-ion diffusion coefficient (D ) was further calculated based on the
Li+
1
8
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10.1002/cplu.201800220
[3]
Randles-Sevcik Equation, as in previously reported methods:
5
3/2
1/2
1/2
i = (2. 69 × 10 ) n SD Cν (1)
p
Where i is the peak current (A), n is the number of transferred electrons, DLi+ is the
p
2
-1
diffusion coefficient of Li ions (cm s ), S is the electrode surface area (S = 70.85
2
+
mm ), C is the concentration of Li in the electrolyte, and ν is the potential scan rate
−
1
(
V s ). Since the electrodes were prepared and tested by the same procedure, the
Randles-Sevchik equation can be further simplified as follows:
1
/2 1/2
i = AD ν (2)
p
1
/2
Where A is supposed to be a constant and AD is defined as the apparent diffusion
+
coefficient of Li in the coin cells, which can be calculated by fitting the linear curves.
Acknowledgments
Z. Wang and S. Zhang contributed equally to this work. Financial support provided by
the grant from the National Natural Science Foundation of China (No. 21476063) and
the Hubei Provincial Natural Science Foundation of China (2018CFB237) is
gratefully acknowledged. The authors also thank Dr. T. Silver for critical reading of
the manuscript.
Conflict of interest
The authors declare no conflict of interest.
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Scheme 1. Schematic illustration of the novel hierarchical porous NiO/β-NiMoO4
NNMO) heterostructure prepared via a facile hydrothermal method followed by a
calcination process.
(
2
4
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10.1002/cplu.201800220
Figure 1. (a) General XRD pattern of as-prepared NNMO; (b) N2
adsorption/desorption isotherms (inset: comparison of the specific surface areas of
NO, NMO, and NNMO) and (d) pore size distributions of those three samples.
2
5
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Figure 2. (a) XPS survey spectrum, (b) high resolution Ni 2p spectrum, (c) Mo 3d
spectrum, and (d) O 1s region of the obtained NNMO material.
2
6
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Figure 3. (a,b) Representative SEM images at different magnifications of NNMO; (c–
e) TEM and HRTEM images of as-prepared NNMO; (f) SAED pattern and (g–j)
Corresponding EDS elemental mapping images of the NNMO material.
2
7
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Figure 4. Electrochemical evaluation of NNMO, showing (a) cyclic voltammograms
for the first 5 cycles from 0.01 V to 3.00 V; (b) galvanostatic charge/discharge voltage
profiles versus capacity of NNMO electrode from for selected cycles at a current rate
−
1
of 0.2 A g ; (c) cycling performances of the obtained NO, NMO, and NNMO
−
1
−1
electrodes at 0.2 A g and coulombic efficiency of the NNMO electrode at 0.2 A g ;
(d) rate performances of those three electrodes at varying current rates of 0.2, 0.5, 1, 2,
−
1
−1
5
, and 0.2 A g ; (e) long-term cycling performance at 1 A g of the obtained NNMO
electrode and the corresponding coulombic efficiency.
2
8
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Figure 5. CV curves at different scan rates for LIB cells with (a) NNMO and (b)
NMO materials, and (c) the linear relationship between the peak current (i ) and the
p
1
/2
square root of the scan rate (ν ) for NNMO electrode.
2
9
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