Green molten salt synthesis and Li-ion storage performance of sodium dimolybdate

Article abstract of DOI:10.1016/j.jallcom.2020.154781

A facile and fast molten salt strategy is developed, for the first time, to synthesis sodium dimolybdate (Na₂Mo₂O₇). The process is based on a thermodynamically favorable reaction between naturally available precursors MoS₂ and NaCl to produce Na₂Mo₂O₇ as well as a gas phase that leaves the system. The product is characterized by electron microscopy and energy-dispersive X-ray spectroscopy (EDX) as well as X-ray diffraction (XRD) and Raman spectroscopy. The prepared Na₂Mo₂O₇ in combination with molten salt-synthesized graphene nanosheets is examined as anode material for Li-ion batteries, exhibiting an interesting performance, with the first cycle discharge and charge capacities of 1363 and 857 mAh g^?1, respectively, at 100 mA g^?1. A reversible Li storage capacity of 506 mAh g^?1 can still be observed after 100 cycles. The results are rather interesting towards the development of low-cost and high-performance Na₂Mo₂O₇ anode materials.

Full text of DOI:10.1016/j.jallcom.2020.154781

Journal of Alloys and Compounds 831 (2020) 154781
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Green molten salt synthesis and Li-ion storage performance of sodium
dimolybdate
Wenhui Zhu , Ali Reza Kamali *
Energy and Environmental Materials Research Centre (E2MC), School of Metallurgy, Northeastern University, Shenyang, 110819, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile and fast molten salt strategy is developed, for the first time, to synthesis sodium dimolybdate
(Na2Mo2O7). The process is based on a thermodynamically favorable reaction between naturally available
precursors MoS2 and NaCl to produce Na2Mo2O7 as well as a gas phase that leaves the system. The
product is characterized by electron microscopy and energy-dispersive X-ray spectroscopy (EDX) as well
as X-ray diffraction (XRD) and Raman spectroscopy. The prepared Na2Mo2O7 in combination with molten
salt-synthesized graphene nanosheets is examined as anode material for Li-ion batteries, exhibiting an
Received 3 February 2020
Received in revised form
10 March 2020
Accepted 12 March 2020
Available online xxx
interesting performance, with the first cycle discharge and charge capacities of 1363 and 857 mAh gꢀ1
,
Keywords:
Sodium dimolybdate
MoS2
respectively, at 100 mA gꢀ1. A reversible Li storage capacity of 506 mAh gꢀ1 can still be observed after 100
cycles. The results are rather interesting towards the development of low-cost and high-performance
Na2Mo2O7 anode materials.
Molten salt
© 2020 Elsevier B.V. All rights reserved.
Reaction mechanism
Lithium ion battery
1. Introduction
involved, as well as complicated and/or long processing periods.
Table 1 summarizes these techniques, outlining the limitations
Among the candidate materials for use as the anode material in
lithium ion batteries (LIBs), Mo-based ceramics including MoO3
[1,2] and metal molybdates such as Li2MoO4 [3] and Na2Mo2O7 [4]
have attracted attentions. This is because during the battery cycling,
the valence state of Mo in the compounds can vary between þ6 and
0, resulting in a multi-electron transfer in the corresponding redox
reactions, providing a high specific capacity. Sodium dimolybdate
(Na2Mo2O7) is an interesting member of molybdates which has
extensively been studied for basic research [5], luminescence and
light emitting applications [6e8], and recently, for application as
the anode material in LIBs [4].
So far, Na2Mo2O7 has been produced by variety of techniques,
including sol gel [4], solid state [9e11], ultrasonic spray [12,13] and
Czochralski [14]. As the Mo source, these methods often use mo-
lybdenum oxides, which are fabricated as intermediate materials in
the production of molybdenum metal. On the other hand, these
methods use expensive resources such as Na2CO3 [4,9,10], or haz-
ardous materials such as (NH4)2Mo7O24 [11] or NaClO4 [12,13] as
the Na source. Such methods, therefore, have fundamental limita-
tions associated with the expensive and/or hazardous precursors
involved. It should be mentioned that the facile, simple and envi-
ronmentally friendly production of Na2Mo2O7 should be interesting
from both scientific and also technological points of view, in order
to develop its applications. Here, we present a new molten salt-
based approach to synthesize Na2Mo2O7 using MoS2 and NaCl.
MoS2 is naturally available in abundance [15,16], particularly in
China [17]. Also, NaCl, is the most abundant and the least expensive
among salts, making the proposed molten salt approach attractive.
Ir should be mentioned that fantastic processes have recently
been developed to utilize MoS2 to prepare efficient photocatalysts
[18], antibiotics [19], structural composites [20], electronics [21]
and anode materials for LIBs [22,23]. Here, for the first time, we
propose the molten salt conversion of MoS2 into Na2Mo2O7.
Molten salts form a large family of ionic melts with enhanced
diffusion coefficients which can be utilized as the reaction medium
to accelerate the chemical reactions. Molten salt based methods
have recently been used for the facile production of various
commercially valuable materials such as SnO2 nanostructures
[24,25], carbon nanotubes [26], nanodiamonds [27], iron [28] and
graphene [29]. The latter is based on the intercalation of hydrogen
into cathodically polarized graphite electrodes, leading to the
exfoliation of graphite into high-quality, but low-cost, graphene
nanosheets.
* Corresponding author.
E-mail addresses: ali@smm.neu.edu.cn, a.r.kamali@cantab.net (A.R. Kamali).
This article, presents a novel strategy for the preparation of
https://doi.org/10.1016/j.jallcom.2020.154781
0925-8388/© 2020 Elsevier B.V. All rights reserved.

2
W. Zhu, A.R. Kamali / Journal of Alloys and Compounds 831 (2020) 154781
Na2Mo2O7 using molten NaCl. This strategy is fast and efficient, and
doesn’t require the use of environmentally problematic or expen-
sive precursors. Furthermore, the Li storage performance of
Na2Mo2O7 is discussed. For this, molten salt-synthesized graphene
nanosheets were used as the conductive additive. The mixture
exhibited a remarkable electrochemical performance.
laser with the wavelength of 633 nm was employed to evaluate
molecular vibrational modes. Thermodynamic data of selected re-
actions were calculated using a commercial software package [32].
2.4. Electrochemical characterizations
The electrochemical properties of the molten salt-synthesized
Na2Mo2O7 as the anode material for LIBs were evaluated with
two-electrode electrochemical half-cell. In this research, graphene
nanosheets produced in molten salts were used as the conductive
additive to prepare the coin cell. The graphene material has a high
bulk specific electrical conductivity of 26.4 S cm2 gꢀ1 [33]. For the
fabrication of Na2Mo2O7 e graphene anode material, the mixture of
Na2Mo2O7, graphene nanosheets and polyvinylidene fluoride in the
weight ratio of 7:2:1 was ground homogenously using N-methyl-2-
pyrrolidinone solvent. The resultant slurry was coated on a copper
2. Experimental
2.1. Synthesis of Na2Mo2O7
A simple and scalable molten salt strategy was used to synthe-
size Na2Mo2O7. In a typical experiment, molybdenum disulfide
(MoS2 99.5%, Aladdin CAS-1317-33-5) was thoroughly mixed with
sodium chloride (NaCl analytical purity, Tianjin Zhiyuan Chemical
Reagent）in the mass ratio of 1:10 in a corundum mortar to obtain
a homogenous mixture. The mixture was then placed in 50 ml
corundum crucibles and heated from room temperature to 1300 ꢁC
(above the melting point of NaCl, 800 ꢁC) in a muffle furnace in air
with a heating rate of 5 ꢁC minꢀ1, and a dwell time at the maximum
temperature of 20 min. After the completion of the heating process,
the furnace was cooled down to the room temperature, and the
materials remained in the crucible was washed with distilled water
to remove the remaining salt, followed by vacuum filtration. The
filtrate was then dried at 80 ꢁC overnight, and characterized with
no further processing.
foil with a thickness of 200 mm using doctor blade and subsequently
dried at 60 ꢁC for 12 h in an oven. The mass loading of the active
material was around 2 mg cmꢀ2. Typical CR 2032 coin half-cells
were prepared, in which metallic Li was used as the counter elec-
trode. In this setup, 1.0 M LiPF6 in EC: DEC: EMC (1:1:1 wt%) and
Celgard 2400 polypropylene films were used as the electrolyte and
separator, respectively. In order to evaluate the Li storage perfor-
mance of Na2Mo2O7 e graphene anode material, half-cells were
assembled in an argon-filled glove-box (Mikrouna) with the O2 and
H2O level of less than 0.1 ppm. The cells were allowed to equilibrate
for 12 h at room temperature, and then galvanostatically tested in
the voltage window of 0.01e3.0 V at the current density of
100 mA gꢀ1 using a Land CT2001A battery testing system. Cyclic
voltammetry (100 V sꢀ1) and electrochemical impedance spec-
troscopy (0.01 Hz-100 kHz) were carried out on a CHI-660E
workstation. Selected discharged/charged cells were dis-
assembled inside an Ar gas protected glove-box to extract cycled
materials for electron microscopy.
2.2. Synthesis of graphene nanosheets
Graphene nanosheets were synthesized according to the
method discussed in Ref. [30,31]. Briefly, a high purity grade
graphite rod (diameter ¼ 13 mm, 99.997%) was cathodically
polarized in molten LiCl at 800 ꢁC and for 60 min under a flow of
Ar ꢀ 4%H2, delivering a constant direct electric current of 40 A,
corresponding to an initial cathode current density of about
1 A cmꢀ2. During the process, hydrogen cations from the molten
salt are reduced on the graphite electrodes, followed by intercala-
tion of reduced hydrogen species into the lattice structure of the
graphite, leading to the exfoliation of the graphite structure into
graphene nanosheets. After the molten salt process, the reactor is
cooled down to the room temperature, and graphene nanosheets
produced are extracted by washing of the graphene-salt mixture by
distilled water, followed by filtration and drying at 150 ꢁC for 2 h.
3. Results and discussion
3.1. Molten salt conversion of MoS2 into Na2Mo2O7
MoS2 precursor was treated with NaCl at 1300 ꢁC in air. This
temperature is greater than the melting point of NaCl (~800 ꢁC).
This simple molten salt method led to the formation of Na2Mo2O7,
as can be realized from the XRD patterns exhibited in Fig. 1(a). The
diffraction peaks of the MoS2 precursor can all be attributed to
those of MoS2 (JCPDS#01-073-1508), indicating that the commer-
cial MoS2 received had no detectable impurity. On the other hand,
the diffraction peaks observed in the pattern of the product can be
fully indexed to those of orthorhombic Na2Mo2O7, indicating the
formation of single phase Na2Mo2O7. Table S1 compares the X-ray
diffraction data of the molten salt synthesized Na2Mo2O7 with
those of standard Na2Mo2O7 (JCPDS#01-073-1797). It should be
2.3. Morphological and structural characterizations
The microscopy studies were conducted using a field emission
scanning electron microscope (SEM, Zeiss Ultra Plus) equipped
with an energy dispersive X-ray detector. The phase analysis was
performed on an X-ray diffractometer (Malvern Panalytical). A
Raman spectrometer (Horiba Jobin-Yvon LabRam HR800) using red
Table 1
Methods used for the preparation of Na2Mo2O7, and the corresponding Li-storage performance comprising capacity (CA, mAh g ꢀ1), cycle number (CN) and current density (CD,
mA gꢀ1) where applicable.
Method
Precursors
Processing
Performance
CA
CN
CD
Sol gel [4]
Solid state [9e11]
MoO3 and Na2CO3
MoO3 and Na2CO3
(NH4)2Mo7O24 and NaNO3
MoO3, HClO3, and NaClO4
MoO3 and Na2CO3
Heating at 80 ꢁC for 6 h þ calcination at 550 ꢁC for 6 h in argon, and 12 h in air
Calcination at 550 ꢁC for 6 h [9,10]
470
200
e
50
50
e
800
25
e
Calcination at 300e700 ꢁC for 5 h [11]
Heating up to 200e400 ꢁC
Ultrasonic spray [12,13]
Czochralski [14]
Molten salt [Current study]
e
e
e
Heating at 650 ꢁC for 5 h
Heating at 1300 ꢁC for 20 min
e
e
e
NaCl and MoS2
506
312
100
500
100
100

W. Zhu, A.R. Kamali / Journal of Alloys and Compounds 831 (2020) 154781
3
Among these, A1g, E1g, E21g and E22g are Raman active; although
the vibration intensity of the E1g mode is rather weak. In single-
layer MoS2, E12g (the in-plane vibration mode of Mo and S atoms)
and A1g (out-of-plane vibration mode
S atoms) appear at
384.8 cmꢀ1 and 404.8 cmꢀ1, respectively. As the number of MoS2
layers in the crystal increases, the frequencies of E22g and A1g are
red-shifted, whilst that of the E12g moves to a lower frequency
[37,38]. Based on this information, the Raman peak appeared in the
spectrum of the as-received MoS2 can be assigned to Eg1
(285.8 cmꢀ1), A1g (381.5 cmꢀ1), E21g (405.9 cmꢀ1
(451.5 cmꢀ1).
) and B1g
The Raman spectrum of the Na2Mo2O7 product can also be seen
in Fig. 1(b). Na2Mo2O7 has an orthorhombic structure, belonging to
the Cmca (D18 h) space group, with eight formula units per unit
2
cell. According to the literature [39,40], the peaks in the range of
741e938 cmꢀ1 are related to Mo-Oi (i ¼ 1e6) stretching vibrations,
whereas those at 413e524 cmꢀ1 are attributed to MoO6 octahedral
units. Peaks in the range of 226e369 cmꢀ1 are combined bending
vibrations in MoO4 tetrahedral and MoO6 octahedral units. More-
over, peaks at lower wave numbers are associated with MoO4 and
MoO6 unit motions. Based on the XRD and Raman spectra exhibited
in Fig. 1, the formation of Na2Mo2O7 upon the molten salt treatment
of MoS2 is evident.
Fig. 2 exhibits morphological characteristics of the MoS2 pre-
cursor and the molten salt-synthesized Na2Mo2O7. Accordingly,
MoS2 precursor had a fine morphology with particles typically less
than 2
mm with a characteristic layer structure (Fig. 2(a)). In
contrast, the morphology of the Na2Mo2O7 product (Fig. 2(b) and
(c)) can be characterized by the presence of substantially larger
crystals. It can clearly be observed that Na2Mo2O7 particles have a
nearly uniform rod-like morphology with a diameter of around
5 mm and length of up to 30 mm. This morphology indicates that
Na2Mo2O7 crystals have grown from MoS2 particles during the
molten salt process. Since this process was conducted in air, the
reaction involved in the phase transformation of fine MoS2 particles
into rod-shaped Na2Mo2O7 crystals can be assumed to be as
follows:
Fig. 1. (a) The XRD and (b) Raman spectra of the MoS2 precursor and the molten salt-
synthesized Na2Mo2O7.
mentioned that while all XRD peaks in the molten salt - synthe-
sized sample are easily assignable, the peaks have slightly shifted
toward higher scattering angles, corresponding to slightly lower
values of d-spacing, compared to those of standard Na2Mo2O7. This
observation can be explained considering the influence of stress on
the position of XRD peaks [34,35]. What is noticeable is that the
standard Na2Mo2O7 was prepared by prolonged heating an inti-
mate mixture of sodium molybdate and molybdenum trioxide at
700 ꢁC in a platinum crucible for two weeks [36]. In contrast,
Na2Mo2O7 crystals in our case were prepared by fast heating of NaCl
and MoS2 at 1300 ꢁC for only 20 min. Therefore, the appearance of
XRD peaks of standard Na2Mo2O7 at slightly lower diffraction an-
gles can be assigned to the reduced internal stress of Na2Mo2O7
crystals caused by the prolonged heat treatment. Nevertheless, the
superior effectiveness of the molten salt process in producing
Na2Mo2O7 crystals in such a short time is remarkable. Moreover,
except for the characteristic peaks of Na2Mo2O7, no other peaks can
be observed in the XRD profile of Fig. 1(a), indicating that the
product is pure Na2Mo2O7.
2MoS2 þ 2NaCl þ 7:5O2 ðgÞ ¼ Na2Mo2O7 þ 4SO2 ðgÞ
þ Cl2 ðgÞ
(2)
The attractive feature of the reaction (2) is that the by-products
(SO2 and Cl2) are in the gas form and leave the system upon for-
mation; hence the product is pure Na2Mo2O7, as can be realized
from the X-ray diffraction pattern of Fig. 1(a). Fig. 3 exhibits the
changes of standard Gibbs free energy and enthalpy of reaction (2)
at different temperatures up to 1300 ꢁC. As can be seen, the reaction
is exothermic and thermodynamically favorable at all tempera-
tures. Upon melting of NaCl at around 800 ꢁC, the enthalpy of the
reaction reduces from ꢀ2173.8 to ꢀ2229.3 kJ. This value increases
to ꢀ2198.5 at 1300 ꢁC. The change of Gibbs free energy experiences
a constant increasing, and reaches the value of ꢀ1600.1 kJ at
1300 ꢁC. According to the reaction (2), the MoS2: NaCl mass ratio of
around 5.5:1 contributes to the chemical reaction. In our experi-
ment, a ratio of 1:10 was employed, and therefore, the excess NaCl
provided an ionically conductive medium to facilitate the chemical
reaction, exhibiting a molten salt reactive process [41,42] with
enhanced kinetics. Such favorable thermodynamic features result
in the rapid formation of Na2Mo2O7 crystals with a uniform dis-
tribution of Na, Mo, and O, as observed in EDX mapping analyses
shown in Figs. 2(d)e(f).
Raman spectroscopy is a powerful method to evaluate the
vibrational and rotational characteristics of Mo-based compounds
[10,37e40]. MoS2 belongs to the D46h (p63/mmc) space group, and
since it is a nonlinear molecule, each unit cell contains 6 atoms,
providing several vibrational modes as follows:
It should also be mentioned that the by-products of reaction (2)
are SO2 and Cl2. SO2 can be utilized to produce useful chemicals
G
¼ A1 þ 2A2 þ 2B2 þ B1 þ E1 þ 2E1 þ 2E2 þ E2
(1)
g
u
g
u
g
u
g
u

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W. Zhu, A.R. Kamali / Journal of Alloys and Compounds 831 (2020) 154781
Fig. 2. SEM micrographs of (a) MoS2 precursor, and (b and c) the molten salt-synthesized Na2Mo2O7. (def) Corresponding elemental mapping analysis of Na2Mo2O7. (g) SEM
micrograph of graphene nanosheets, and (h and i) Na2Mo2O7 e graphene electrode after 500 cycles of Li insertion/extraction at the current density of 100 mA gꢀ1
.
devices is scarce. In 2016, Verma et al. fabricated two types of
Na2Mo2O7-based materials to be evaluated as the anode for LIBs
[4]: (a) Micrometer-sized Na2Mo2O7 with particle sizes of 3e10 mm,
produced by the solid state reaction between Na2CO3 and MoO3 in
air at 550 ꢁC for 12 h; and (b) carbon coated nanocrystalline
Na2Mo2O7 with particle sizes of less than 100 nm, produced using a
sol gel method. For the latter, a mixture of Na2CO3, MoO3, C6H8O7
and ethanol was heated to 80 ꢁC for 6 h to form a gel, which was
dried completely overnight. The dried substance was heated at
550 ꢁC for 6 h in argon. The micrometer-sized Na2Mo2O7 synthe-
sized by the method (a) exhibited a Li-storage capacity of 294 mAh
gꢀ1 after 50 cycles. The capacity of the carbon-coated nano-
crystalline Na2Mo2O7 synthesized by the method (b) after 50 cycles
was reported to be 600 mAh gꢀ1 at the current density of 460 mA
g
ꢀ1. The performance of neither electrodes at further cycles were
reported.
In contrast with this research, our current work is interesting
Fig. 3. Changes of standard Gibbs free energy (dark color circles), and changes of
enthalpy (light color circles) associated with the reaction (2), calculated using
commercial database [26]. (For interpretation of the references to color in this figure
a
since, first it fabricates micrometer sized crystalline Na2Mo2O7
using a novel rapid and substantially low cost molten salt approach.
The particle sizes of our product are slightly larger than that
fabricated by Verma et al. [4]. Here, we evaluate the performance of
the molten salt-synthesized Na2Mo2O7 combined with graphene
nanosheets, added as the conductive carbon, during 500 cycles, ten
times greater than that of [4]. Interestingly, we show that the
performance of the molten salt-synthesized sample is considerably
greater than that of the solid state-synthesized microcrystalline
Na2Mo2O7, and compatible with the sol gel-synthesized nano-
crystalline Na2Mo2O7 coated with carbon (Table 1).
legend, the reader is referred to the Web version of this article.)
such as H2SO4 [43,44] and sulfones [45]. It is interesting to notice
that the presence of Cl2 can facilitate the conversion of SO2 to H2SO4
[46].
Fig. 2(g) shows a typical SEM micrograph of the molten salt-
synthesized graphene nanosheets. The material exhibits
morphology comprising of single or few-layered graphene nano-
sheets configured in a 3D structure, providing a high specific con-
ductivity with a BET surface area of around 200 m2 gꢀ1. More details
about the production and characterization of the graphene material
can be found elsewhere [29e31,47].
a
To illuminate the reaction mechanism occurred during the Li
insertion/extraction cycles into/out of the Na2Mo2O7 electrode,
cyclic voltammetry (CV) was employed at a scan rate of 100 V sꢀ1
using the Na2Mo2O7 electrode as the working electrode and Li as
3.2. Electrochemical evaluations
The fabrication and evaluation of Na2Mo2O7 in energy storage

W. Zhu, A.R. Kamali / Journal of Alloys and Compounds 831 (2020) 154781
5
Fig. 4. Electrochemical performance of the electrode made of the molten salt-synthesized Na2Mo2O7. (a) the CV profile, (b) the cycling performance up to 500 cycles and (c) the
charge-discharge profile at first three cycles at 100 mA gꢀ1. (d) the rate performance, and (e) the Nyquist plot of the electrode.
both the reference and counter electrode; and the result is pre-
Na2Mo2O7 þ 12Liþ þ 12e 42Mo þ Na2O þ 6Li2O
(3)
sented in Fig. 4(a). Three cathodic peaks can be identified during
the first cathodic scan at 1.55, 1.16 and 0.51 V. The first two peaks
can be attributed to the reduction of Mo6þ to Mo4þ and Mo4þ to
Mo0, respectively. The last cathodic peak centered at 0.51 V in the
first scan can be due to the formation of solid electrolyte interface
(SEI) layer. This peak is absent in the subsequent cycles, confir-
mation the stability of formed SEI. Three peaks can be recognized at
the first anodic scan at 0.55, 1.16 and 1.53 V. These peaks can be
attributed to the subsequential oxidation of zero-valent Mo to form
various oxidation states up to Moþ6 [4,48,49]. The CV profile, which
is in agreement with the pervious report [4], implies that the
cycling of Na2Mo2O7 may be based on the overall electrochemical
Fig. 4(c) shows the galvanostatic discharge/charge cycling pro-
file of the synthesized Na2Mo2O7 for the first, second and third
cycles recorded at 100 mA gꢀ1 in a potential range of 0.01e3 V vs.
Liþ/Li. The first Li-insertion and extraction capacities can be found
to be 1362.7 and 857.2 mAh gꢀ1, corresponding to a first discharge
coulombic efficiency (CE) of 62.9%. The first cycle capacity loss is
due to the irreversible formation of SEI as observed in Fig. 4(a).
Fig. 4(b) shows the capacity retention vs. cycle number measured at
100 mA gꢀ1. As it can be observed, after 100 and 500 cycles, the
reversible capacities of 505.9 and 312 mAh gꢀ1, could be recorded,
respectively. The results demonstrate that not only the Li-storage
capacity of the molten salt-synthesized Na2Mo2O7 is compatible
with carbon coated nanostructured Na2Mo2O7 synthesized by the
sol get method (and greater than that of the solid-state-synthesized
reaction (3), which provides
a theoretical capacity value of
920 mAh gꢀ1
.

6
W. Zhu, A.R. Kamali / Journal of Alloys and Compounds 831 (2020) 154781
sample [4]), but also its prolonged cycling performance is prom-
ising. The capacity obtained is compatible with that of highly
crystalline pure battery grade graphite, which is commercially used
as the anode material for LIBs [50]. It should be noticed that the
increasing demand for LIBs worldwide, has created new re-
quirements to develop alternative anode materials, with the per-
formance compatible with graphite, and low cost [51]. With this
approach, the green preparation of low-cost anode materials
derived from polymers [52,53], biomass [54] and abundant inor-
ganic materials [55] has gained increasing attention. Molten salt-
synthesized Na2Mo2O7 might be considered as an alternative
low-cost anode material for LIBs.
was developed to synthesize sodium dimolybdate (Na2Mo2O7),
which was characterized by various techniques, including XRD,
Raman and SEM analysis. The process is based on a thermody-
namically favorable reaction between naturally available MoS2 and
NaCl in air atmosphere. The product, in combination with molten
salt-synthesized graphene nanosheets, was found to exhibit a
promising Li-storage performance, with a Li charge transfer resis-
tance of 225
U
, and a reversible capacity of 506 and 312 mAh gꢀ1
after 100 and 500 cycles, respectively, at the current density of
100 mA gꢀ1. The results obtained propose a sustainable method for
the preparation of Na2Mo2O7, and advance the current under-
standing on its electrochemical performance.
After 500 discharge and charge cycles at 100 mA gꢀ1, the half-
cell was disassembled and the active material was extracted, and
subjected to electron microscopy. Figs. 2(h) and (i) exhibit the
morphology of the cycled Na2Mo2O7. As can be observed, the
micrometer-sized Na2Mo2O7 crystals (Figs. 2(b) and (c)) are pul-
verized into rather agglomerated nanometer-sized particles due to
the volume changes involved in the prolonged cycling process.
However, the particles remain coalesced without disintegration,
due to the presence of graphene nanosheets. The nanostructured
material formed by cycling of micrometer-sized Na2Mo2O7 particles
has similar morphology to that fabricated by Verma et al. [4] by the
sol gel process, and this can explain their more or less similar
electrochemical performance.
Declaration of competing interests
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
CRediT authorship contribution statement
Wenhui Zhu: Investigation, Formal analysis. Ali Reza Kamali:
Conceptualization, Investigation, Formal analysis, Writing - review
& editing, Supervision, Funding acquisition.
We further examined the rate capability of the molten salt-
synthesized Na2Mo2O7 at different discharge and charge current
densities. As exhibited in Fig. 4(d), a capacity of 110 mAh gꢀ1 could
be recorded after 50 cycles at the high current density of
2000 mA gꢀ1. The cell capacity could be recovered to a high value of
580 mAh gꢀ1 by reducing the current density to 100 mA gꢀ1 after
62 cycles and becomes stable at 520 mAh gꢀ1 at the 70th cycle.
The electrochemical impedance spectroscopy was used to re-
cord the Nyquist plot of the molten salt-synthesized Na2Mo2O7,
using the half-cell electrode and the result is shown in Fig. 4(e). The
value of charge transfer resistance (Rct) can be evaluated from the
diameter of the semicircle observed in the lower frequency region
of the Nyquist plot. In the low frequency region, the inclined line
can represent the lithium ion diffusion into the crystalline structure
of Na2Mo2O7. The values of Rct for the molten salt-synthesized
Acknowledgements
This work was supported by the Fundamental Research Funds
for the Central Universities (Grant Number N2025001) and the
Natural Science Foundation of China (Grant Number 51750110513).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.154781.
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