Blend formed by oxygen deficient MoO3-δ oxides as lithium-insertion compounds

Article abstract of DOI:10.1016/j.jallcom.2016.06.043

Oxygen deficient Mo_xO_y 200 nm-thick particles were synthesized by a citrate sol-gel method using ammonium heptamolybdate tetrahydrate as a source of Mo and heat treated with a small fraction of zirconia under reducing atmosphere. The samples were investigated by X-ray powder diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscope (SEM) and Raman spectroscopy (RS). The structural analyses show that the composite is a blend formed by layered α-MoO₃, orthorhombic oxygen deficient phases MoO_3-δ with δ = 0.25 (γ-Mo₄O₁₁) and α-ZrMo₂O₈. The insertion of lithium into the lattice was performed by electrochemical method. Redox peaks were observed for the MoO_3-δ composite attributed to distinct Li⁺ ion insertion/extraction reactions in the MoO₃ and Mo₄O₁₁ hosts. The cyclic performance revealed improved reversibility, rate capability, and electrochemical stability of the ZrMo₂O₈-decorated composite with respect to the bare molybdenum oxides. At C/10 rate, the composite delivered a stable reversible capacity of 135 mAh g^-1 after the 50^th cycle. At a rate of 2C the reversible capacity is maintained at 118 mAh g^-1, approximately twice the capacity observed for the MoO₃ particles alone.

Full text of DOI:10.1016/j.jallcom.2016.06.043

Accepted Manuscript
Blend formed by oxygen deficient MoO
oxides as lithium-insertion compounds
3−δ
A.M. Hashem, S.M. Abbas, A.E. Abdel-Ghany, A.E. Eid, A.A. Abdel-Khalek, S. Indris,
H. Ehrenberg, A. Mauger, C.M. Julien
PII:
S0925-8388(16)31766-2
10.1016/j.jallcom.2016.06.043
JALCOM 37912
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 17 March 2016
Revised Date: 4 June 2016
Accepted Date: 7 June 2016
Please cite this article as: A.M. Hashem, S.M. Abbas, A.E. Abdel-Ghany, A.E. Eid, A.A. Abdel-
Khalek, S. Indris, H. Ehrenberg, A. Mauger, C.M. Julien, Blend formed by oxygen deficient MoO
oxides as lithium-insertion compounds, Journal of Alloys and Compounds (2016), doi: 10.1016/
j.jallcom.2016.06.043.
3−δ
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Blend formed by oxygen deficient MoO3-δ oxides as lithium-insertion compounds
A.M. Hashem1,2, S.M. Abbas3, A.E. Abdel-Ghany1, A.E. Eid1, A.A. Abdel-Khalek3, S. Indris2,
H. Ehrenberg2, A. Mauger4, C.M. Julien5
1National Research Centre, Inorganic Chemistry Department, 33 El Bohouth St., (former El
Tahrir St.), Dokki-Giza 12622, Egypt
2Karlsruhe Institute of Technology (KIT), Institute for Applied Materials –Energy Storage
Systems (IAM- ESS), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen,
Germany
3Department of Chemistry, Faculty of Science, Beni-Suef University, Beni-Suef 62111,
Egypt
4Sorbonne Universités, Univ. UPMC, Paris-6, PHENIX, UMR 8234, 4 place Jussieu, 75005
Paris, France
5Sorbonne Universités, Univ. UPMC, Paris-6, IMPMC, 4 place Jussieu, 75005 Paris, France
Corresponding author: christian.julien@upmc.fr (C.M. Julien)
Keywords: Molybdenum oxides; Oxygen deficiency; Sol-gel synthesis; Blend; Lithium-
insertion compounds
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Abstract
Oxygen deficient MoxOy 200 nm-thick particles were synthesized by a citrate sol-gel method
using ammonium heptamolybdate tetrahydrate as a source of Mo and heat treated with a
small fraction of zirconia under reducing atmosphere. The samples were investigated by X-
ray powder diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron
microscope (SEM) and Raman spectroscopy (RS). The structural analyses show that the
composite is a blend formed by layered α-MoO3, orthorhombic oxygen deficient phases
MoO3-δ with δ=0.25 (γ-Mo4O11) and α-ZrMo2O8. The insertion of lithium into the lattice was
performed by electrochemical method. Redox peaks were observed for the MoO3-δ composite
attributed to distinct Li+-ion insertion/extraction reactions in the MoO3 and Mo4O11 hosts.
The cyclic performance revealed improved reversibility, rate capability, and electrochemical
stability of the ZrMo2O8-decorated composite with respect to the bare molybdenum oxides.
At C/10 rate, the composite delivered a stable reversible capacity of 135 mAh g-1 after the
50th cycle. At a rate of 2C the reversible capacity is maintained at 118 mAh g-1,
approximately twice the capacity observed for the MoO3 particles alone.
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1. Introduction
MoO3 is currently investigated for many purposes. Plasmonic MoO3-x@MoO3 nanosheets
obtained from surface oxidation of MoO3-x were employed as a surface-enhanced Raman
scattering (SERS) substrate for methylene blue detection [1]. MoO3 also enters the
composition of solar cells and light-emitting diodes [2,3]. MoO3 nanoribbons were proposed
as a reversible chemochromic material [4]. Redox active polyaniline-h-MoO3 hollow
nanorods [5], ordered mesoporous α-MoO3 with iso-oriented nanocrystalline walls [6] and
ultrathin MoO3 nano-crystals self-assembled on graphene nanosheets [7] were proposed as
new supercapacitors. It is also applied in the selective conversion of propylene [8]. But
MoO3 is mostly studied for use as an anode material for Li-ion batteries. Recently, single-
crystalline α-MoO3 nanobelts prepared in 5 M HNO3 delivered a capacity of 175 mAh g-1 at
the first cycle at 2C rate, falling to 150 mAh g-1 after 10 cycles, while commercial MoO3
delivers a stable reversible capacity of 72 mAh g-1 at 2C rate [9]. To overcome the
degradation of MoO3 nanorod anodes in lithium-ion batteries at high-rate cycling, coating of
the nanorods by a protective layer has been proposed. The capacity of α- MoO3–In2O3 core–
shell nanorods maintains 1114 and 443 mAh g-1 after 50 cycles at 0.2C and 2C rate,
respectively [10]. Conformal passivation of the surface of MoO3 nanorods by HfO2 using
atomic layer deposition (ALD) was proposed with success [11]. After 50 charge/discharge
cycles at high current density1500 mA g-1, HfO2-coated MoO3 electrodes exhibited a specific
capacity of 657 mAh g-1. MoO3 nanoparticles were electrodeposited on Ti nanorod arrays
that showed enhanced rate capability (300 mAh g-1 at 100 A g-1) and excellent durability
(retaining 300 mA h g-1 after 1500 cycles at 20 A g-1) [12]. α-MoO3/multiwalled carbon
nanotube (MWCNT) nanocomposite delivered 490 mAh g-1 at current density 20 A g-1 owing
to the high conductivity of the carbon nanotubes [13]. Porous MoO3 grafted on TiO2
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nanotube array was found to improve the specific capacity with respect to MoO3 alone and
the TiO2 nanotubes alone by a factor thre [14]. These results show that MoO3 shows
remarkable electrochemical properties for use as an anode for Li-ion batteries. The price to
be paid to obtain such performance, however, is the reduction of the size of the particles to
the nano-range, and the synthesis of nano-composites. Such synthesis processes, however,
are not scalable, and expensive, which explains that the anode of the commercial Li-ion
batteries is still graphitic carbon, although much better results are obtained since years at the
laboratory scale, not only with MoO3 but also with silicon and many metal oxides.
MoO3 usually exists in three main phases: monoclinic, hexagonal and orthorhombic
structure, the later being the most thermodynamically stable phase. Orthorhombic MoO3 (α-
MoO3, n-type semiconductor) attracts interest due to its unique layered structure, which is
composed of MoO6 octahedra in which the Mo atom is surrounded by a distorted oxygen
octahedron and connected to another MoO6 octahedra through edges and corners by covalent
and ionic bond, to form double layers [15,16]. The lamellar structure contains extension
channels with tetrahedral and octahedral holes suitable for insertion and de-insertion of Li+
ions [6]. Crystalline α-MoO3 has high theoretical discharge capacity of ~370 mAh g-1, based
on a 2e- reduction due a wide range of oxidation states from +4 to +6 [17,18]. The
electrochemical behavior of α-MoO3 crystals can be described by the redox reaction:
xLi+ + MoO3 + xe- ↔ LixMoO3.
(1)
Several methods have been used to prepare nanostructured molybdenum oxides, i.e.
hydrothermal synthesis [19,20] electro-spinning method [21], sol-gel technology [22], thin-
film deposition [23]. Ette et al. [24] prepared lamellar α-MoO3 particles through facile
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polymer assisted solvothermal method, and reported good electrochemical performance due
to the fast lithium ion diffusion into the lamellar-like morphology.
Non-stoichiometric MoO3- oxides with oxygen deficiency [25] are attractive cathode
δ
materials for lithium battery application because the excess of metal atoms acts as doping
centers, which control the electrical and optical properties [18,26-33]. MoO3 (Mo as Mo6+) is
insulating and transparent with an electronic conductivity σ < 10-8 S cm-1 at 25 °C [32], while
the conductivity of MoO2 is σ = 5.0×103 S cm-1 [34]. The poor electrical conductivity of
α-MoO3, due to the almost ideal stoichiometry, is the major drawback, which severely limits
the charge transfer during Li insertion. On another hand, the oxygen deficient Mo4O11
(MoO2.75) exhibits better transport properties with a small electrical resistivity ρ=10-4 ꢀ cm
and larger average lithium chemical diffusion coefficient of ~4×10-12 cm2 s-1. In addition, it
can accommodate up to 2.1 Li+/Mo in its tunnels without change of the lattice providing a
theoretical specific capacity of 401 mAh g-1 [27]. However, the reversible capacity becomes
stable at the level of 170-200 mAh g-1 [21].
Recent advances to develop highly effective electrode materials for Li batteries derived
from composite or blended architectures, which possess at least two percolating frameworks,
for ions and electrons [35]. Numerous examples are cited in the literature:
xLi2MnO3•(1-x)LiMn2O4 [36,37], Si/PAN-C [38], xLiMnPO4•(1-x)C-LiFePO4 [39],
Li1.1Mn1.9O4•LiNi0.4Mn0.3Co0.3O2 [40], LiCoO2-LiFePO4 [41]. Depending on the nature and
the combination of materials, the blending resulted in different advantages such as: (i)
inhibition of overheating effects during overcharge conditions, (ii) reduction of the
irreversible capacity decay, (iii) improvement of cycling life and (iv) rise of the accessible
discharge C-rate. In addition to the blends, which are physical mixtures, surface modification
has been demonstrated to improve the stability and integrity of cathode particles that
complement the blending effect [42].
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As an effort to alleviate the fading capacity during cycling of the α-MoO3 phase, this
work present the study of as-grown blend containing stoichiometric MoO3 and oxygen
deficient Mo4O11 fabricated through a simple heat treatment of MoO3 coated with zirconia in
an inert atmosphere. The Mo4O11 phase of the composite provides better transport properties
by solving the problem of the poor electronic conductivity of MoO3 framework, and by
buffering effectively the volume change during cycling. The ZrMo2O8 decoration is an
effective method to preserve the side reactions occurring at the electrolyte-electrode interface
that degrade the electrode performance. The samples are investigated by X-ray powder
diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscope (SEM)
and Raman spectroscopy (RS). The structural analyses show that the composite is formed of
stoichiometric MoO3 and oxygen-deficient phases Mo4O11+MoxOy. The insertion of lithium
into the composite is performed by electrochemical methods and compared with that of
pristine MoO3 and Mo4O11 hosts. Cyclic voltammetry (CV), galvanostatic and
electrochemical impedance spectroscopy (EIS) measurements have shown the enhanced
transport properties of the composite electrode at the origin of the improvement of the
reversible capacity and rate capability.
2. Experimental
2.1. Synthesis of the composite
Layered MoO3 (α-MoO3) was prepared by a citrate sol–gel method using ammonium
molybdate tetrahydrate (NH4)6Mo7O24•4H2O) as precursor. First, a stoichiometric amount of
ammonium molybdate was dissolved in de-ionized water to form an aqueous solution, which
was added in a second step to an aqueous solution of citric acid with a 1:1 ratio for
metal/chelating agent while stirring gently. The pH values was adjusted to pH=7 using
ammonium hydroxide. Then, the sol was obtained by stirring the homogenous mixture for 3 h
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in order to favor the complex reaction between the metal ion and the chelating agent. The
transparent gel was formed after the slow evaporation of the solution heated at 70 °C.
Continuous heating with stirring led to the transformation of the gel to a xerogel, which
converted to powder by further drying at ca. 120 °C. The resulting dried precursor was
ground and calcined at 500 °C for 5 h in air to obtain MoO3 single crystals (α-MnO3). The
zirconia-coated composite, i.e. oxygen deficient MoxOy, was obtained by mixing α-MnO3
with Zr(OH)4 prepared by precipitation of ca. ~4% ZrCl4 and NaOH dissolved in bi-distilled
water. The mixture Zr(OH)4+( α-MoO3) was stirred at room temperature for 3 h, then
filtrated, and dried overnight at 60 °C. The final stage to obtain oxygen-deficient blend was a
calcination under reduced atmosphere at 500 °C for 5 h in a 5% H2/Ar flow
2.2. Characterization
Thermogravimetric (TG) measurements were carried out using a thermal gravimetric
analyzer (Perkin Elmer, TGA 7 series) in the temperature range of 30–1000 °C in air at a
heating rate of 10 °C min−1. The structure of the samples was analyzed by X-ray diffraction
using a Philips X’Pert apparatus equipped with a CuKα X-ray source (λ = 1.54056 Å). Data
were collected in the 2θ-range 10–80°. XRD pattern was analyzed by the Rietveld structure
refinement program, FULLPROF [43]. The particle morphology of the samples was
examined by scanning electron microscope (SEM, JEOL, Japan, JXA-840A). High resolution
transmission electron microscopy (HRTEM) analysis was performed using a JEOL
TEM2010F at a 200 kV acceleration voltage. For TEM analysis, the powder sample was
dispersed on a 3-mm Cu-grid with a hole size of 1 × 2 mm2. HRTEM and elemental mapping
were completed by high-angle annular dark field in a scanning transmission electron
microscope (HAADF/STEM) using Emispec ES vision, version 4.0, of the STEM-EDX
mapping system. Raman spectra were collected with a double monochromator (Jobin-Yvon
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model U1000) using the 514.5 nm line from the Spectra-Physics 2020 Ar-ion laser. The
spectra were recorded using a backscattering geometry and the laser power was kept below
25 mW to prevent the degradation of materials upon data acquisition. All spectroscopic data
were collected with a spectral resolution of 2 cm−1.
2.3. Electrochemical tests
Electrode materials were prepared by mixing 80% (w/w) of the active material, 10% (w/w)
super C65 carbon (TIMCAL) and 10% (w/w) polyvinylidene fluoride (Solef PVdF 6020
binder, Solvay), in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) to get a slurry. A 90 µm
thick film was obtained by coating this slurry on aluminum foil that gives an electrode
loading ~7.7 mg cm-2. This film was dried overnight at 80 °C, then punched out with the
diameter of 1.4 cm. Coin-type cells were assembled in an argon-filled glove box using
lithium foil (Alfa Aesar) as anode, LP30 (1 mol L-1 LiPF6 in (1:1) ethylene carbonate (EC) -
dimethyl carbonate (DMC) as electrolyte and glass microfiber filters (Whatmann®-GF/D 70
mm Ø) as separator. VMP3 multi-channel potentiostat (Bio-Logic, France) was used to
electrochemically characterize the electrodes at 25 °C in the voltage range of 1.5-2.8 V vs.
Li0/Li+ in galvanostatic test and cyclic voltammetry modes.
3. Results and discussion
3.1. Structure and morphology
Fig. 1a shows the X-ray powder diffraction (XRPD) patterns for the as-prepared pure MoO3
(α-MoO3) and the decorated MoO3-Mo4O11 composite sample synthesized under reduced
atmosphere. The XRD spectrum of α-MoO3 exhibits distinct and sharp peaks attributed to the
α-MoO3 phase indexed in the orthorhombic structure, Pbnm space group (JCPDS card 05-
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0508). The lattice parameters a=3.993 Å, b=13.575 Å and c=3.713 Å were determined using
12 Bragg lines that are in good agreement with values of the literature [15,16]. The XRD
diagram of the composite (blend) material shows complex features with the superposition of
three sets of Bragg lines. XRD analysis was carried out by Rietveld structure refinement (Fig.
1b), which matches well with the experimental data and gives the composition of the blend
sample. The structural data for the modelling were taken from previously reported articles for
α-MoO3 [15], γ-Mo4O11 [45] and α-ZrMo2O8 [46]. The three phases of low symmetry appear
to be with the proportion as follows: (1) an orthorhombic α-MoO3 (~65 mol.%) (2) an
orthorhombic γ-Mo4O11 (30.2 mol.%) and (3) a trigonal α-ZrMo2O8 (4.8 mol.%). Almost no
change is observed in the value lattice parameters of the MoO3 component after inert heat-
treatment. The appearance of the oxygen deficient oxide MoO3-δ with δ=0.25 is attributed to
a slight reduction of MoO3 in hydrogen containing atmosphere [44] that takes part in creation
of oxygen vacancies. The non-stoichiometric molybdenum oxide, Mo4O11, is clearly detected
by the presence of XRD peaks located at 2θ position 20.9, 22.04, 31.9, 34.4 and 48.2°, which
match well with database values (JCPDS card 89-6725). The suboxide Mo4O11 can also be
written (Mo36+Mo4+)O11 to outline the two valence states of Mo in it. Kihlborg demonstrated
its three-dimensional network based on the ReO3-type structure characterized by the presence
of empty channels [25]. The lattice parameters of the γ-Mo4O11 phase (Pnma S.G.) are
determined as a=24.397 Å, b=5.443 Å, c=6.732 Å that are in agreement with the data in Ref.
6. Therefore, Mo4O11 can be considered as an intermediate phase during the structural change
from MoO3 to MoO2 [44]. The third phase characterized by an additional peak at 2θ = 30.54°
related to trigonal α-ZrMo2O8 phase (P-3c S.G.). The structure of the zirconium molybdate is
a two-dimensional network with layers perpendicular to the c axis that is built from ZrO6
octahedra linked together by MoO4 tetrahedra with unit-cell parameters a=10.1391 Å and
c=11.7084 Å, which match well with the literature data [47]. The presence of α-ZrMo2O8
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comes from the use of zirconium hydroxide in the synthesis process, which has generated a
decoration on the surface of particles. Note that (i) the formation of two molybdate
components, i.e. Mo4O11 and ZrMo2O8 mixed into the blend does alter significantly the
lattice parameters of the layered α-MoO3 phase, (ii) ZrO2 cannot be detected via XRD and
(iii) the overall oxygen deficiency of the blend sample was determine by Raman
spectroscopy as shown below.
Fig. 2a shows the gravimetric thermogram and the corresponding differential scanning
curve (DTG) of the precursor ammonium heptamolybdate tetrahydrate used as the source of
Mo and the citric acid used as the chelating agent for the synthesis of α-MoO3 by sol-gel
method. The thermal behavior in the range 25-950 °C shows stepwise decomposition of the
mixture with a total weight loss 71.5 % until the formation of crystalline α-MoO3 at 550 °C.
The large weight loss occurs in two steps: (i) the loss of water (152 °C) and (ii) the loss of
ammonia, combustion of carboxylic acid and acetate xerogel (429 °C) [30]. The
thermodynamically stable anhydrous α-MoO3 is formed at Tc~525 °C, which is in good
agreement with reported values [48]. No change in the weight is observed during heating
until the melting point of MoO3 at about 795 °C, followed by the decomposition observed as
a broad peak in the DTG curve. Fig. 2b displays the TG curve of the Mo-O blend prepared
under reduced atmosphere at 500 °C, recorded in air at a rate 10 °C/min. A pronounced
increase of weight is observed at T>580 °C, which is attributed to the oxidation process (gain
of oxygen) transforming the blend into the stoichiometric MoO3 phase at T≈600 °C. Upon
further heating, we observed the melt at 780 °C followed by the decomposition of the
molybdenum trioxide. Here, the fraction 35% of Mo4O11 is detected from the gain of mass
providing the blend composition 0.65MoO3-0.35Mo4O11, a result in agreement with that of
Rietveld refinement. Therefore, the presence of a thin ZrMo2O8 coating is not detected in TG
curve, as expected since its melting temperature is known to be as high as 950 °C.
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Fig. 3 presents the SEM images of the α-MoO3 (a,b) and of the MoO3-Mo4O11 blend.
The morphology shown in images (3a,3b) reveals well-developed regular slabs of the α-
MoO3 structure with particles composed of dense packed layers. Image 3a presents the MoO3
slabs, which form terrace at the edge of particles. The surface morphology of blended sample
(images 3c,3d) shows different shapes with irregular rod-like particles typically 10 ꢁm thick,
decorated with smaller ZrMo2O8 particles (~50 nm), which consists of discontinuous lumps
attached to the particle surface.
Fig. 4 illustrates typical HRTEM images and HAADF/STEM mapping of Mo-O blend
material. The patterns provide evidence for the structural quality of the sample examined.
From the HRTEM image (Fig. 4a) crystal defects (marked by red arrows) of a blend particle
are identified, which are attributed to the presence of the two Mo-O phases, i.e. the
stoichiometric orthorhombic MoO3 and the oxygen-deficient orthorhombic Mo4O11 phase.
The lattice fringes observed in the HRTEM image are an indicative of orthorhombic
structure. The measured lattice fringe spacing 0.37(5) nm corresponds to the (001)
crystallographic plane of MoO3. The lattice fringes are not uniform and exhibit some
discontinuities or misalignment due to the blend architecture of the particle. To determine the
local chemical composition, tomography study of the blend phase is shown in the
HAADF/STEM image (Fig. 4b). The image of the representative particle used to obtain the
elemental analysis is also shown. The images labeled Mn and O, which represent the
elemental mapping of the respective elements, are uniform along the shape of the particle
examined. Finally, the STEM image (Fig. 4c) displays the MoO3-Mo4O11 particles decorated
by ZrMo2O8. The lighter part surrounding the darker particles is the ZrMo2O8 layer, few nm-
thick.
Fig. 5 displays the Raman spectra of the α-MoO3 and MoO3-Mo4O11 blend sample. In
the high wavenumber region, the Raman bands observed at 776, 848, and 902 cm−1 are the
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unique spectral features of the α-MoO3 orthorhombic phase, especially the 902-cm-1 peak is
assigned as the molybdyl mode, which is the vibration of the terminal Mo=O in the layered
structure [38]. The deviation from stoichiometry (i.e. the value of x in MoO3-x) can be
determined from the intensity ratio (I185/I295) of the Raman bands, i.e. the wagging modes of
the terminal Mo=O groups located at 285 and 295 cm-1 as a function of x in Fig. 11 of ref.
[50]. This deviation from stoichiometry, however is found to be small (the order of 0.5%) for
α-MoO3. The much more oxygen-deficient phase in the blend MoO3-Mo4O11 is x=6%. The
.
Raman pattern of the Mo-O blend displays superposition of both MoO3 and Mo4O11
components of the blended samples. Seven bands located at 148, 423, 441, 743, 781, 897 and
942 cm-1 are assigned to the internal modes of the orthorhombic Mo4O11 phase [51]. In
addition, small peaks/shoulders pointed out at 177, 326, 360, 751, 885, 943 and 1003 cm–1
reveal the presence of a small amount of ZrMo2O8, in good agreement with the XRD results.
These Raman peaks are respectively assigned to the lattice mode, the asymmetric and
symmetric bending, the asymmetric stretching and the symmetric stretching of MoO4
tetrahedra [46].
3.2. Electrochemical features
Fig. 6 shows the cyclic voltammograms (CV) of α-MoO3 (a) and the Mo-O blend (b) samples
as positive electrodes in Li coin-type cells. Cycles were performed in the potential range 1.5-
3.5 V vs. Li0/Li+ at a scan rate 0.05 mV s-1. The CV curve of α-MoO3 (Fig. 5a) in the first
cycle shows only one anodic peak at 2.55 V, but two cathodic peaks at potentials of 2.70 and
2.30 V vs. Li0/Li+. These two reduction peaks correspond to consecutive Li+ ion intercalation
processes into the MoO3 framework: (i) onto sites into the interlayer spacing between the
[MoO6] octahedral layers (named van der Waals gap) and (ii) onto sites into the slabs, i.e.
octahedral intra-layers [52] as illustrated in Fig. 10 of Ref. [53]. The intercalation process
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associated with the peak observed at 2.7 V is irreversible, as it is observed only during the
first intercalation reaction; this peak vanishes in subsequent cycles. Tsumura and Ingaki [54]
reported the irreversible reaction during the first lithiation, which proceeds in a two-phase
reaction up to Li0.25MoO3. The appearance of the α-MoO3/LixMoO3 system causes a
pronounced expansion of the interlayer spacing [55]. The first cycle CV curve for the MoO3-
Mo4O11 blend (Fig. 5b) appears to be more complex showing the superposition of redox
peaks due to each component. The three peaks of MoO3 are observed at the same potentials
as in the parent material, while three sets of additional peaks are attributed to the Mo4O11
phase: (i) the reversible cathodic peaks at 3.28 and 1.91 V, (ii) reversible anodic peaks at 3.33
and 1.88 V and (iii) irreversible cathodic peaks at 2.45 and 2.20 V vs. Li0/Li+. These redox
peaks assigned to Mo4O11 are in good agreement with data reported by Besenhard and
Schollhorn [56], who noticed trace of reduced Mo suboxide during the reduction process of
molybdenum trioxide.
Fig. 7 presents the galvanostatic discharge–charge profiles of the Li coin-type cells
cycled at C/10 rate for 50 times, while Figs. 8 (a-b) display the electrochemical performances
of Mo-O electrodes. In the first discharge process (lithium intercalation), the discharge–
charge curves for α-MoO3 (Fig. 7a) and MoO3-Mo4O11 blend (Fig. 7b) show two plateaus
that correspond to the redox reaction recorded in CV curves. The first irreversible plateau
corresponds to an irreversible capacity of ~50 mAh g-1. It is observed at about 2.75 V vs.
Li0/Li+ and vanishes in the second and forthcoming cycles. This suggests that part of the Li+
ions, first introduced during the reduction reaction (discharge), cannot be extracted. Such an
effect has also been observed in V2O5 [57]. Thus the total intercalation process up to
1.5Li+/Mo can be described as the formation of three phases separated by the bi-phase
domain MoO3-Li0.25MoO3 that is responsible for voltage plateaus in the first discharge at 2.7
V. The charge-discharge profiles of the Li cell with the MoO3-Mo4O11 positive electrode
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shows multiple stepwise plateaus at different voltages, associated to the different site
occupancies for Li+ ions into the framework of each component of the blend. These curves
are in good agreement with the data of the cyclic voltammetry. The specific capacity of the
α-MoO3 in the first discharge is 250 mAh g-1 and decreases to ~100 mAh g-1 at the 50th
cycle, which represents a capacity retention of 37% at rate C/10 (see Figs 7 and 8). The
presence of the Mo4O11 phase in the blend improves significantly the electrochemical
performance of the electrode. The specific capacity is raised to 275 mAh g-1 at the first cycle,
135 mAh g-1 after the 50th cycle, both of them being higher than for the MoO3 electrode. Our
results can be compared with those reported for polyaniline (PANI)/MoO3 nanobelt
composites synthesized by a hydrothermal method, which exhibited an initial specific
capacity 228 mAh g-1, but only ~90 mAh g-1 after 40 cycles at a current density of 30.7 mA
g-1 (~C/12 rate) [58]. The rate capability presented by the modified Peukert plots (Fig. 8a) for
the 2nd cycle (after the cell formation process and avoid the irreversible loss of capacity
associated to the vanishing of the plateau at 2.7 V in Fig. 7) confirms the enhancement in the
discharge specific capacity at high C-rate for the MoO3-Mo4O11 blended electrode. Typically,
at a rate of 2C the capacity is twice that of pure MoO3; the MoO3-Mo4O11 blend electrode
retains a capacity of 118 mAh g-1 against 60 mAh g-1 for the pure MoO3 cathode material.
Fig. 9 presents the EIS results for the MoO3-Mo4O11 blended electrode compared with
the pristine MoO3 material, measured in the frequency range 100 kHz-0.1 Hz with a 5 mV
bias. These Nyquist plots show clearly the enhanced transport property of the composite
electrode including the Mo4O11 phase. Figs. 9 (a-b) display the electrochemical performances
of Mo-O electrodes. This increase in the discharge capacity is attributed to the enhancement
of the electronic conductivity of the electrode materials, which can accept higher current
densities. These results support the advantage of the Mo-O blend containing the Mo4O11
phase. Stoichiometric MoO3 is an insulator with a band gap of 3.1 eV. Oxygen vacancies acts
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as doping centers and restores a semiconducting behavior with a conductivity that, however,
remains small in MoO3-δ. On another hand, Mo4O11 is a metal (resistivity 1.66 10-4 ꢀ cm)
[59]. Since Mo4O11 can be viewed as MoO3 that has lost ¼ oxygen per chemical formula, the
metallic character of Mo4O11 can be viewed as the result of an insulator to metal Mott
transition associated to the increasing concentration of oxygen vacancies. The increase of the
electrical conductivity in the blend, as evidenced in Fig. 8, simply gives evidence that the
Mo4O11 phase forms an electric conductive network. The increased conductivity and the
access to the oxidation states +6, +5 of Mo due to Mo4O11 explains the improvement of the
rate capability at C>0.2 in the blend evidenced in Fig. 8b. The better cycling stability can be
attributed to the combination of two effects: (a) the increase of electronic conductivity also
makes possible the extraction of a larger amount of lithium. (b) the buffering effect of the
non-stoichiometric Mo4O11 component into the blend. (c) the presence of the ZrMo2O8
decoration protects the electrode from side reaction at the electrolyte-electrode interface.
To our knowledge, this blend is the best result published for molybdenum oxide micron-
sized particles so far. As usual better results can be obtained by decreasing the size of the
particles to the nano-range. In particular C-MoO3 porous nanofibers, could deliver a stable
reversible capacity of circa 200 mAh g-1 [21]. This performance, however, is due to the
contribution of the carbon to the electrochemical process, and also the contribution of active
sites at the porous surface, evidenced by the fact that the capacity at the first cycles was much
larger than the theoretical value. Similar remarkable improvements of the electrochemical
properties have been obtained by decreasing the size of ZrMo2O8-coated particles, such as
ZrO2-coated Li4Ti5O12 anodes [60], or ZrO2-coated LiMn2O4 cathodes [61]. The drawback is
that such nano-structures have a low density, and their synthesis process is also less scalable
than the simple sol-gel process. Nevertheless, the present results suggest that decreasing the
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size of the ZrMo2O8-decorated MoO3-Mo4O11 blend to the nano-range might be promising
for applications as electrode of lithium-ion batteries.
4. Conclusions
In this work, we have investigated the structural and electrochemical properties of a blend
electrode prepared by self-synthesis of molybdenum oxides creating a suboxide component,
which enhances the properties of the physical mixture. The synthetic procedure performed in
inert atmosphere with addition of Zr(OH)4 provides particles decorated with nanometric
zirconium molybdate particles. The structure and morphology of samples were characterized
by XRD, TGA, SEM, HRTEM, HAADF/STEM mapping and Raman analyses that show a
blend composition 0.65MoO3-0.35Mo4O11 decorated with ZrMo2O8 nanoparticles. It is found
that both components of the blend are electrochemically active, providing an initial discharge
capacity of ~282 mAh g-1 in the voltage range 1.5-2.8 V vs. Li0/Li+ larger than that of α-
MoO3. We have shown that blending cathode materials results in the minimization of the
shortcomings of the parent material, MoO3. We solved the problem of the poor electronic
conductivity of MoO3 by the fabrication of a Mo-O composite including an oxygen deficient
MoO3-δ (δ=0.25) component. Due to the oxygen vacancies, which act as doping centers, the
MoO3-δ phase forms an electric conductive network so that the electrochemical performance
is enhanced at high C rates. Therefore, the improved electrochemical properties in the MoO3-
Mo4O11 cell is attributed to two factors: (i) the presence of oxygen vacancies in the Mo4O11
phase that enhances the electrical conductivity of the blend and serves as buffering
component to absorb the volume expansion and (ii) the ZrMo2O8-decoration that preserves
the electrode against side reaction with the electrolyte.
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Acknowledgments
The authors are grateful to Mr. René Villette for his assistance in collecting the HRTEM
images and HAADF/STEM mapping.
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Figure captions
Fig. 1. (a) X-ray powder diffraction for pure MoO3 prepared by sol-gel method and zirconia
coated composite calcined in reduced atmosphere. (b) Rietveld refinement of the XRD data
of the ZrMo2O8 decorated MoO3-Mo4O11 composite.
Fig. 2. (a) TGA and DTG curves of the precursor showing the crystallization and
decomposition of α-MoO3 in air with heating rate 10 °C/min. (b) TGA curve of the blend
MoO3-Mo4O11 showing the oxidation of the suboxide component at 580 °C.
Fig. 3. SEM images of α-MoO3 (a,b) and MoO3-Mo4O11 blend decorated by ZrMo2O8 (c,d).
Fig. 4. (a) HRTEM image of Mo-O blend material showing the crystal defects (marked by
red arrows) that are induced by the presence of stoichiometric MoO3 and oxygen-deficient
MoxOy phases in the particle. (b) HAADF/STEM mapping and (c) STEM image showing the
ZrMo2O8 phase surrounding the core of blend Mo-O particles.
Fig. 5. Raman spectra of the orthorhombic α-MoO3 and ZrMo2O8-decorated Mo-O blend.
Samples were excited at a laser wavelength of 514.5 nm.
Fig. 6. Cyclic voltammograms of α-MoO3 (a) and MoO3-Mo4O11 blend (b) recorded at 0.05
mV s-1 in the potential range 1.5-3.5 V vs. Li0/Li+ in Li cell using 1 mol L-1 LiPF6 in EC:DEC
electrolyte.
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Fig. 7. Charge-discharge profiles as a function of cycles of Li coin-type cells including (a) α-
MoO3 and (b) MoO3-Mo4O11 blend as cathode materials. Measurements were carried out at
C/10 rate in the voltage range 1.5-2.8 V vs. Li0/Li+.
Fig. 8. Electrochemical performance of Li cells with α-MoO3 and MoO3-Mo4O11 blend
electrodes: (a) rate capability at the second cycle (the rate is denoted C/n, where C is the
theoretical cathode capacity and a full discharge occurs in n hours) and (b) cycling
performance at rate C/10.
Fig. 9. Nyquist plots of (a) α-MoO3 and (b) MoO3-Mo4O11 blend as cathode materials.
Measurements were carried out in the frequency range 100 kHz-0.1 Hz with a 5 mV bias.
Values are frequencies in Hz.
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Figure 1. (a) X-ray powder diffraction for α-MoO3 prepared by sol-gel method and zirconia
coated composite calcined in reduced atmosphere. (b) Rietveld refinement of the XRD data of
the ZrMo2O8 decorated MoO3-Mo4O11 composite.
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Figure 2. (a) TGA and DTG curves of the precursor showing the crystallization and
decomposition of α-MoO3 in air with heating rate 10 °C/min. (b) TGA curve of the blend
MoO3-Mo4O11 showing the oxidation of the suboxide component at 580 °C.
Figure 3. SEM images of α-MoO3 (a,b) and MoO3-Mo4O11 blend decorated by ZrMo2O8
(c,d).
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Figure 4. (a) HRTEM image of Mo-O blend material showing the crystal defects (marked by
red arrows) that are induced by the presence of α-MoO3 and oxygen-deficient MoxOy phases
in the particle. (b) HAADF/STEM mapping and (c) STEM image showing the ZrMo2O8
phase surrounding the core of blend Mo-O particles.
3