A clean process of preparing VO as LIBs anode materials via the reduction of V2O3 powder in a H2 atmosphere: Thermodynamic assessment, isothermal kinetic analysis, and electrochemistry performance evaluation

Article abstract of DOI:10.1016/j.jallcom.2020.156305

VO is one of the most promising anode materials for lithium ion batteries (LIBs). Herein, we present the clean preparation of VO powder via reduction of V₂O₃ powder under high-purity H₂. Thermodynamic calculation and V–O phase diagram analysis were carried out to ensure reasonable oxygen potential conditions. Isothermal reduction experiments and kinetic analysis using thermogravimetric analysis (TGA) were then performed at 1623 K, 1648 K, and 1673 K under a pure H₂ gas flow. The reduction of V₂O₃ powder appears to obey an unreacted shrinking core model and can be divided into two steps. The first step was controlled by a chemical reaction, the kinetics of which can be described as G(α) = [-ln(1-α)]^1/3 with an apparent activation energy of 107.3 kJ·mol^?1. The second stage was controlled by gas diffusion, the kinetics of which can be described as G(α) = [1-(1-α)^1/3]^1/2 with an apparent activation energy of 45.5 kJ·mol^?1. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) also were employed for characterizing the morphology of the prepared VO powder. As a result, when the VO was served as the LIBs anode material, the resulting electrodes exhibited a high specific capacity (843 mAh·g^?1 after 30 cycles) and remarkable cyclic stability.

Full text of DOI:10.1016/j.jallcom.2020.156305

Journal Pre-proof
A clean process of preparing VO as LIBs anode materials via the reduction of
V O powder in a H atmosphere: Thermodynamic assessment, isothermal kinetic
2 3
2
analysis, and electrochemistry performance evaluation
Guishang Pei, Junyi Xiang, Dapeng Zhong, Xin Jin, Xuewei Lv
PII:
S0925-8388(20)32669-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.156305
JALCOM 156305
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 8 November 2019
Revised Date: 1 July 2020
Accepted Date: 4 July 2020
Please cite this article as: G. Pei, J. Xiang, D. Zhong, X. Jin, X. Lv, A clean process of preparing VO
as LIBs anode materials via the reduction of V O powder in a H atmosphere: Thermodynamic
2 3
2
assessment, isothermal kinetic analysis, and electrochemistry performance evaluation, Journal of Alloys
and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.156305.
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Credit Author Statement
Guishang Pei: Original draft preparation,
Junyi Xiang: Methodology,
Dapeng Zhong: Data analysis,
Gang Li: Software,
Xuewei Lv: Supervision

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A clean process of preparing VO as LIBs anode materials via the
reduction of V2O3 powder in a H2 atmosphere: thermodynamic
assessment, isothermal kinetic analysis, and electrochemistry
performance evaluation
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Guishang Peib, c, Junyi Xiangb, c,*, Dapeng Zhongb, c, Xin Jin b, c, Xuewei Lva, b, c, d,*
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a The State Key Laboratory of Mechanical Transmissions, Chongqing University, No.
174 Shazheng Street, Shapingba District, Chongqing, 400044 China.
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b School of Materials Science and Engineering, Chongqing University, No. 174
Shazheng Street, Shapingba District, Chongqing, 400044 China.
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c Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and New Materials,
Chongqing University, Chongqing, 400044, China.
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d State Key Laboratory of Vanadium and Titanium Resources Comprehensive
Utilization.
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* Corresponding author:
E-mail: xiangjunyi126@126.com (Junyi Xiang), lvxuewei@163.com (Xuewei Lv)
Phone: +86-15881297340 (Junyi Xiang); +86-13658335559 (Xuewei Lv)

19 ABSTRACT
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VO is one of the most promising anode materials for lithium ion batteries (LIBs).
Herein, we present the clean preparation of VO powder via reduction of V2O3 powder
under high-purity H2. Thermodynamic calculation and V-O phase diagram analysis
were carried out to ensure reasonable oxygen potential conditions. Isothermal
reduction experiments and kinetic analysis using thermogravimetric analysis (TGA)
were then performed at 1623 K, 1648 K, and 1673 K under a pure H2 gas flow. The
reduction of V2O3 powder appears to obey an unreacted shrinking core model and can
be divided into two steps. The first step was controlled by a chemical reaction, the
kinetics of which can be described as G(α)=[-ln(1-α)]1/3 with an apparent activation
energy of 107.3 kJ·mol-1. The second stage was controlled by gas diffusion, the
kinetics of which can be described as G(α)=[1-(1-α)1/3]1/2 with an apparent activation
energy of 45.5 kJ·mol-1. Powder X-ray diffraction (XRD) and scanning electron
microscopy (SEM) also were employed for characterizing the morphology of the
prepared VO powder. As a result, when the VO was served as the LIBs anode
material, the resulting electrodes exhibited a high specific capacity (843 mAh·g-1 after
30 cycles) and remarkable cyclic stability.
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Keywords: Thermodynamic calculation; Thermogravimetric analysis; Apparent
activation energy
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1. Introduction
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Since lithium ion batteries (LIBs) were first commercialized by Sony
Corporation in 1991 [1-3], they have been widely used in portable electronics.
High-energy-density electrode materials for LIBs are now increasingly required to
fulfill the growing demand for portable energy sources in modern information society.
As a substitute for the traditional one-electron-transfer electrode materials, e.g.
LiCoO2 [4, 5], LiMn2O4 [6], and LiFePO4 [7], vanadium oxides (VxOy) show good
host capacity for reversible lithium intercalation and may accommodate
multi-electron transfer processes. Despite the high theoretical capacity of V2O3 (1070
mAh·g-1) compared to V2O5 and VO2 [8-10], reported capacity values for V2O3
nanostructures cycled as an anode material can vary wildly. V2O3 nanorods delivered
a capacity of ~50 mAh·g-1 after 100 cycles at 0.1 C [11], whereas composites of V2O3
nanoparticles with carbon have been reported to deliver a capacity of ~780 mAh·g-1
after 100 cycles at 0.2 C, and carbon is electrochemically active, so it may also be
contributing to the increased capacity [12]. It is difficult for V2O3 or its composites to
exhibit high reversible capacity in previous research [13, 14]. Therefore, it’s hard to
commercialize. Thus, exploring new anode materials for LIBs is highly desirable.
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Vanadium monoxide (VO) is an important member of the vanadium oxides
family, and has a theoretical capacity (800 mAh·g-1) comparable to that of V2O3;
therefore, VO is one of the most promising anode materials for LIBs. VO has been of
a material of interest in a long time, science its existence was verified and it was
synthesized in 1952 by Nikkel Frands [15]. However, there are only few reports on

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the preparation of VO powder via simple methods and the application of VO as an
anode material for lithium-based storage systems.
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It is critical that any investigation of the potential application into VO as a novel
anode materials for LIBs start with the development of an economically viable
synthetic approach. There are two main methods reported in the literatures for the
preparation of VO powder: solid-state reaction [16, 17] and single-collision oxidation
of vanadium metal [18-20]. When using the solid-state reaction, VO powder is
prepared by prolonged heating of a mixture of vanadium metal and vanadium trioxide
in hydrogen atmosphere at 1623 K, as reported by Todd et al [16]. However, the
solid-state reaction proceeded slowly in the latter stages and obtaining high-purity
products was difficult. Roger et al. [18] investigated the preparation of VO powder
using a “single-collision conditions” method, in which the VO is formed via the
gas-phase reaction of 2V + NO2 = 2VO + N2. An effusive beam of element vanadium
at 2600 K is intersected by an oxidant gas at 300 K at low pressures. Implementing
this method is difficult under steady-state condition, which impeded further
development. Hydrogen can be a clean reducing agent due to the absence of
by-products and is commonly employed in the production of low-valent metal oxide
and metals [21-23]. Therefore, a method to prepare VO powder by thermal reduction
of V2O3 powder in a hydrogen atmosphere was proposed in this study.
Thermodynamic calculations and V-O system assessment were first preformed to
explore whether the reduction potential was favorable. The changes in phase
composition and morphology of the product were also characterized by using powder

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X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, the
isothermal reduction kinetics were investigated.
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2. Theoretical basis
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Thermogravimetric-analysis (TGA) can be used to monitor real-time mass
changes in a chemical reaction under isothermal conditions, and is widely utilized for
restudy of gas-solid reduction kinetics [24, 25]. To explore the mechanism of
reducing powdered V2O3 to VO powder via treatment with H2, the commonly
employed ln-ln method was performed and is presented herein. A linear fit of the
thermodynamics curve was obtained with the Avrami–Erofeev model; the
thermodynamics models are shown in the logarithmic-type curves [26-29]. Taking
natural logarithm of the Avrami–Erofeev model,
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ln[−ln(1−α)] = nln(t)+ln(k)
(1)
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where α is conversion ratio (%), t is time (min), k is reaction rate constant and n is a
criterion for distinguishing the model fitting function, respectively. ln[−ln(1−α)] is
plotted versus ln t. Eq. (1) is generally used to evaluate practical data. The slope and
intercept of the resulting straight line are n and ln(k). The algebraic expressions of
g(α), used to describe the solid-state reactions are also summarized in detail elsewhere
[30].
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Besides the mechanistic analysis for the gas-based reduction of V2O3 powder, the
apparent activation energy is another key kinetic parameter, which represents the

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additional energy required for an atom to create an activated cluster. In this study, the
apparent activation energy was obtained by an iso-conversional method and by a
model function method based on dynamic data. The iso-conversional method involves
carrying out a series of experiments at different temperatures. To obtain the apparent
activation energy of the reduction reaction, the Arrhenius equation was used herein
[31]:
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ln(k) = ln(A) - Ea/RT
(2)
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where k is the rate constant, A is the pre-exponential factor, Ea is the apparent
activation energy, R is the universal gas constant, and T is the absolute temperature.
The apparent activation energy can be obtained by plotting ln(k) against 1/T.
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3. Experimental
3.1 Materials and characterization
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The V2O3 (99.95%, metals basis) powders was provided by
Aladdin Industrial Corporation, which was first roasted in a tube furnace under argon
to remove any additional moisture, and then grinded down a much smaller particle
size. The particle size of the V2O3 powder, measured via laser particle testing using
water as a dispersant (Fig. 1), was smaller than ~2.0 μm. X-ray powder diffraction
(XRD, Cu Kα radiation, PANalytical X'Pert Powder, Panalytical B.V.) was carried
out to investigate the phase composition and chemical composition of both raw
materials and products. A scanning range of 5-90° was employed at a scanning step of

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4°/min. The diffraction intensity data from the sample and standard PDF card analysis
was compared using MDI Jade 6.5® (Fig. 2). The diffraction peak of V2O3 powder
was shown to be highly consistent the standard (PDF card; #85-1411), which
indicated that the main composition of the sample were V2O3. Scanning electron
microscopy (TESCAN VEGA 3 LMH, Czech Republic) was applied to study the
surface morphology and determine particle size of the reduced powder. Polished to
cross sections of the sample embedded in the resin were observed by backscattered
electron (BSE) diffraction, and energy dispersive spectroscopy (EDS) microanalysis.
The accelerating voltage used for the EDS analysis was 10 kV.
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Fig. 1 Particle size distribution of the V2O3 powder.
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Fig. 2 XRD patterns of V2O3 powder.
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3.2 Method
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Isothermal reduction of the V2O3 powder was carried out at various temperatures
using a Setsys Evo 1750 TGA apparatus (SETARAM, France). The roasted V2O3
powder (40 mg ± 1 mg) was heated from room temperature to the specific
temperature (1623 K, 1648 K, and 1673 K) in a furnace at a heating rate of 10 K/min
in an argon atmosphere. When the temperature reached the target value, the reactor
was charged with H2 at a flow rate of 20 mL/min. The mass change over time of the
samples was accurately recorded during the entire isothermal reduction section. The
as-prepared sample were applied as lithium storage materials and their
electrochemical performances were systematically evaluated through assembling half
cell in 2302 coin type cells.
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3.3 Thermodynamics analysis

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Phase diagram assessment and thermodynamic calculations were carried out
before the experiments. The V-O system was investigated extensively and compared
to the literature to establish the stable regions of various vanadium oxides. The main
objective of the thermodynamic calculation herein was to predict the effect of
temperature and p(H2)/p(H2O) on isothermal reduction. FactSage®, was applied herein,
using the databases FACT53, FToxide, Reaction module, and Equilib module were to
cover the entire pure substance and available gas-phase and solid-state compounds in
the databases for a given gas-solid mixture system.
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4. Results and discussion
4.1 Thermodynamics calculation results
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The V-O phase diagram has been surveyed by many researchers, mainly
focusing on the experimental phase equilibrium corresponding to different
components. This system has been reviewed by Wriedt [32], Alcock [33], and
Stringer [34]. Recently, the thermodynamics of the V-O system was reassessed by
Cao et al. [35] using a modified quasichemical model. Yang et al. [36] used a
compound energy formalism model, which gives a Gibbs free energy formula for
every phase, with the most stable phase corresponding to the minimum Gibbs free
energy being selected and presented in a V-O phase diagram. From this reassessed
V-O phase diagram, it was indicated that the VO phase (δ-phase) exists on a wide
range of compositions (approximately VO0.80-VO1.20) rather than the ideal 50 % mole

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ratio. No additional phase exists on the VO phase (δ-phase), while a V2O3 phase
(corundum phase) exists on the specific temperatures of 1623 K, 1648 K and 1673 K.
Therefore, the predicted pathway for the isothermal reduction process can be
described theoretically as V2O3-VO.
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As for the VO solid-solution phase, the homogeneity range, lattice parameters,
and defects had been investigated in previous studies. Schonberg [37] and Andersson
reported that the VO phase is stable between the limits VO0.80 and VO1.30. The results
of Vol’f et al [38] shows that the lattice parameters of the vanadium-rich phase
increase with temperature decrease and subsequent equilibrations. Lattice parameters
were also reported by Andersson et al. [39] and Gel’d [40], Westman et al. [41] with
significantly variation in comparison to other reports. According to their
measurements, the limits of the field are VO0.87-VO1.20. Lattice parameters are
reported on the range 4.060 Å ≤ a ≤ 4.123 Å, and the relationship between lattice
parameter and oxygen content is expressed as: Å = 0.276X + 3.787 (X ranges from
0.87 to 1.20).
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The variation of the defect concentration on the cationic and anionic lattices
through the phase field has been examined by Westman and Nordmark [41], Arlya
and Popov [42] and Gel’d et al [40], their results, which are in substantial agreement,
are shown in Fig. 3. It can be seen that V2+ and O2- are the dominant species in the
homogeneity range, with increases in O content being in the range of x(O)>1.0, e.g.
more O is dissolved into the VO halite increasing the oxidative environment and
resulting in a rapid conversion of V2+ to V3+. On the other hand, with the decrease in

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O content in the range of x(O)<1.0, more V will be dissolved into the VO halite and
V2+ will be predominately replaced by V, which is reasonably well reflected by the
reducing environment found under O-deficient conditions. Site fractional vacancies of
the VO phase reached 11.6 % of the equiatomic composition. Such a vacancy content
is consistent with Davydov [43], which found that cubic vanadium monoxide contains
between 10-15 % structural vacancies at the vanadium and oxygen sub lattices
simultaneously. Therefore, at the final maximum mass loss for various reduction
temperatures there probably exists a small difference due to defects in the VO phase.
In this study, the theoretical maximum mass loss occurring under an isothermal
kinetic process was taken to be at a reaction temperature of 1673 K.
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Fig. 3 Lattice-parameter (A) and defect (B) in VOx phase.
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There are many vanadium oxides in V-O phase diagram, and only V2O5, VO2,
V2O3, and VO were considered in this study. The reduction process of vanadium
oxide under H2 from high valent oxides to lower valent compounds occurs step-wise.

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Therefore, the reduction routes of high valent vanadium oxide can be simply
presented as: V2O5-VO2-V2O3-VO-V, which is consistent with the majority of metal
oxide reduction processes. For each step, temperature (T) and p(H2)/p(H2O) are
critical factors under a H2 atmosphere. Fig. 4 shows the standard Gibbs free energy
variation (oxidation potential) for select vanadium oxide and hydrogen
stoichiometries, which indicates that V2O5 and VO2 can be reduced to sub-vanadium
oxide, while the reduction of V2O3 and VO powder hardly proceeds when the
hydrogen partial pressure is equal to the partial pressure of water, i.e. p(H2)/p(H2O)=1.
Variations in standard Gibbs free energy verses p(H2)/p(H2O) for V2O3 (Fig. 4)
clearly shows an increasing propensity for reduction at partial pressures of H2,
p(H2)/p(H2O). In addition, the reduction can proceeded with p(H2)/p(H2O) 1/10-2
when temperature is beyond near 1700 K (point a in the Fig. 4). When p(H2)/p(H2O)
is equal to 1/10-3, reduction occurs only when temperature exceeds 1300 K (point b in
the Fig. 4). Equilibrium calculations for the reduction of V2O3 powder at different
temperatures (1623 K, 1648 K, and 1673 K) were carried out by FactSage® Equilib
module. As shown in Fig. 5, the H2/V2O3 molar ratio markedly decreases with
increasing the reduction temperature, and the reduction significantly accelerates when
the molar ratio of H2/V2O3 is increased. It was found that the reduction rates and
deoxidation extent can be greatly improved by increasing both the reaction
temperature and H2/V2O3 ratio. Therefore, we took high purity hydrogen as the
reducing gas instead of a hydrogen and argon mixture to lower partial-pressure of

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water in the system. In addition, the reducing gas was deoxygenated and dried before
entering the reaction tank.
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Fig. 4 Oxygen potential diagram of vanadium oxides.
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Fig. 5 Equilibrium calculation of the V2O3 powder reduction process at different
temperature.

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4.2 Isothermal kinetics analysis
4.2.1 Reduction reaction pathway
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The reduction routes for V2O3 under H2 were firstly predicted via
thermodynamic assessment of the V-O system. This clearly demonstrates that no
another phase exists between the Corundum phase (V2O3) and Halite phase (VO) at
reduction temperature ranging from 1623 K to 1673 K. The isothermal reduction of
V2O3 powder at 1673 K, was monitored by powder XRD. Fig. 6 shows diffraction
patterns corresponding to the conversion ratio 0 %, 10 %, 50 %, and 100 %. The
diffraction peak of V2O3 gradually weakens with increasing in conversion percentage,
indicating that the content of V2O3 has decreased. No other phases exists during the
isothermal reduction process of V2O3 powder, which was confirmed by the phase
region in V-O phase diagram. As a result, the reduction pathway, including the phase
diagram reassessment and experimental verification gave very good agreement with
each other, it shows that the reduction process can be actually presented as V2O3 –VO.
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Fig. 6 XRD patterns of the isothermal reduction during the different reaction stage at
1673 K.
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4.2.2 Identification of the reaction mechanism
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The various TG and dTG curves for the isothermal reduction of V2O3 powder,
with relationship to time at different temperature (1623 K, 1648 K and 1673 K), are
presented in Fig. 7. There are two obvious stages during the present reduction process
as shown by the TG curves, indicating that mass loss decreases rapidly in the first
stage (about the time ranging 0-11 min) and then decreases slowly in the final stage
(about the time ranging 11-105 min), which is confirmed by the actual gas-solid
reduction situation. The mass loss curves corresponding to various reduction
temperature almost overlap in the first stage, while the final mass loss (10.3 %, 10.6 %
and 11.3 %) slightly increases with increasing isothermal reduction temperatures
(1623 K, 1648 K and 1673 K). This indicates that a rise in reduction temperature
increases the mass loss rate, most likely due to the partial vacancies which allow

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vanadium atoms dissolve widely across the VO solid solution, which creating an
extremely reducing environment as suggested by calculation on O-deficient
conditions reported out by Yang et al [36]. Derivative thermogravimetry (dTG) results
are shown in Fig. 7, it increased with time, reached a maximum value after 8 min,
then decreased approximately to zero and remained constant. An obvious concave
peak was obtained for the V2O3-VO reduction, which strongly supports with the
reaction pathway suggested by both the thermodynamics calculation and XRD
analysis. A small concave peak is observed during the reduction at approximately
time 12~15 min. This was taken to indicate a slight acceleration in reduction. Based
on the SEM analysis of the prepared VO powder (Fig. 11), some cracks can clearly be
seen on the surface at particles at the (f)×2000 image. This is caused by stress from
lattice distortion, which probably accelerated reduction during this time.
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Fig. 7 TG and dTG curves for isothermal reduction V2O3 powder.
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The reduction degree (α) and derivative reduction degree (dα), calculated
according to Eq. (1) and it’s differential, were obtained from the TG curves as shown
in Fig. 8. In the present study, the final mass loss at 1673 K and maximal theoretical
weight loss were used to maintain consistency with the previous reports. Very similar
to the TG curves, the reduction completed about 80-82 % in the first stage about 0-11
min, the second stage 18-20 % in the rest time.
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Fig. 8 α and dα/dt curves for isothermal reduction V2O3 powder.
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The reaction mechanism and kinetics equation were deduced and analyzed by
plotting ln[-ln(1-α)] against ln(t). Two distinguished stages could be found in both
types of isothermal reduction processes as shown in Fig. 9. The fitted parameters,
slope (n), intercept (ln(k)), and coefficient correlation (R2), were summarized in
Table.1. For the first stage, the slopes n was 3.21-3.29 for the reduction of V2O3
powder. All the n values were around 3.0, which indicates that the reduction reaction

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in the first stage is likely controlled by chemical reaction. The kinetic equation in the
second stage is represented as follows:
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G(α)=[-ln(1-α)]1/3
(3)
Table.1 The summary of model fitting results.
Type
Stage 1
Stage 2
1648
0.24
Temperature /K
1623
3.29
-7.38
0.99
1648
3.25
-7.19
0.99
1673
3.21
-7.14
0.99
1623
0.17
-0.13
0.99
1673
n
0.28
-0.03
0.98
lnk
R2
-0.06
0.99
Note: R2 is the goodness of linear fitting.
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Fig. 9 Plots of ln(1-ln(1-α)) vs lnt in two stages of gas-based reduction process.
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In the second stage, the obtained slopes (n) were between 0.17-0.28 for the
reduction process, all of the slopes were around 0.54, which indicates that the
reduction of the V2O3 powders is most probably controlled by a 3D diffusion model.
The kinetic equation in the second stage is represented as follows:
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G(α)=[1-(1-α)1/3]1/2
(4)
4.2.3 Calculation of the apparent activation energy
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The apparent activation energy can be obtained by plotting ln(k) against 1/T (Fig.
10). Form the gradient of those plots, the apparent activation energy of the isothermal
reduction process of different reduction stages was determined. The apparent
activation energy values of the first and the second stage are equal to 107.3 kJ·mol-1
and 45.5 kJ·mol-1, respectively. This indicates that the first stage proceeds more
difficultly than the second stage.
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Fig. 10 Fitting curves of lnk against 1/T at different temperature during two stages.

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4.3 Characterize the reduction products
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Phase composition and micro morphology were investigated and analyzed during
the isothermal reduction process using XRD and SEM-EDS. The observed XRD
pattern (Fig. 6) showed typical reflections of VO phase (PDF#75-0048). No
additional diffraction peak of other phases, e.g., initial reactant and other vanadium
oxides, were observed. Narrow and strong reflection clearly confirmed good
crystallinity of the as-prepared sample. The refinement of the cell parameters showed
that the VO phase was crystallized in isometric system with space group Fm-3m (225),
the lattice parameters of the VO phase being a = 4.12 Å, and Z = 4, very close to the
reference values.
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SEM-EDS analysis was performed to investigate surface morphology and
microanalysis of the prepared VO powder. Characteristic micrographs were obtained
with six different magnifications (A) ×100, (B) ×200, (C) ×400, (D) ×500, (E) ×1000,
and (F) ×2000, (Fig. 11). These micrographs reveal agglomerates of particles shaped
in small chunks of approximately100 microns in size. The grain size can be estimated
at image of ×1000 magnification. The major particle morphology was that of a regular
polygon, with a corresponding average size of 100 μm. Surface cracks are clearly
observable at (F) ×2000, and are postulated to occur due to lattice distortions, and are
expected to accelerate reduction in the final stage. Microanalysis using EDS ( (F)

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×2000) affirmed that the atomic percent of V is approximately equals to O, which is
in good accord with the XRD result.
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Fig. 11 SEM-EDS analysis recorded for the VO powder at different magnifications
(a)×100, (b)×200, (c)×400, (d)×500, (e)×1000, (f)×2000
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4.3 Electrochemical performances
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The as-prepared VO were applied as lithium storage materials and their
electrochemical performances were systematically evaluated through assembling half
cell in 2302 coin type cells. As shown in Fig. 12-13, the galvanostatic
charge/discharge process was tested at a current density of 0.2 C (1 C=372 mA/g),
resulting that the initial specific capacity of VO sample was achieved as high as 1159
mAh/g, and a stable capacity could maintain 843 mAh/g after 30 cycle). With further
increasing the current density from 0.2C to 1C (1 C=372 mA/g), the cell with VO

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exhibited a high capacity of 351 mAh/g as shown in Fig. 12, manifested that the
prepared VO cell displayed a good rate performance.
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Fig. 12 Voltage profiles of VO at a current density of 0.2C.
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Fig. 13 Rate performance of VO at different currents.
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Electrochemical impedance spectra (EIS) measurement is a useful method to
understand the internal resistance of cells assembled by introducing these cathode
materials, and small resistance is beneficial for extraction/reinsertion of Li+ ions
during the discharge-charge process. The EIS of the cell with VO sample was
conducted after 30 cycles at 0.2 C (1 C=372 mA/g) as shown in Fig. 14. The sum of

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the charge transfer resistance (Rct) and ion transportation resistance (Rf) for the
prepared VO sample were calculated to be 57.8Ω, respectively, suggesting that the
VO sample had the better kinetics for Li+ ion insertion/extraction
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Fig. 14 EIS of VO at a current density of 0.2C after 30 cycles.
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Compare with the traditional solid-state reaction and single collision oxidation of
vanadium metal, H2-based reduction may provide an environmentally friendly and
relatively simple method for preparing the VO powder and applying it as LIBs anode
materials for high-performance lithium storage. In general, this study provides a clean
and simple way to prepare low-valence transition oxides metal oxides.
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5. Conclusion

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In this study, VO was prepared by the reduction of V2O3 with H2 and the reaction
mechanism was studied via thermodynamic calculation. Based on the results of this
study, the following conclusions are presented:
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1. The reduction pathway of V2O3 powder to VO powder was investigated by
thermogravimetric reduction experiments, thermodynamic modelling and V-O
phase diagram analysis, giving V2O3 (Corundum)-VO (δ phase) as the proposed
path. The reasonable temperature and oxygen partial pressure during isothermal
reduction of V2O3 powder were established from a thermodynamic viewpoint.
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2. Two apparent stages exist in the preparation VO by isothermal reduction reaction.
The first stage was controlled by chemical reaction, the kinetics of which can be
described as G(α)=[-ln(1-α)]1/3, and the apparent activation energy was 107.3
kJ·mol-1. The second stage was controlled by gas diffusion, the kinetics of which
can be described as G(α)=[1-(1-α)1/3]1/2, and the apparent activation energy was
45.5 kJ·mol-1.
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3. Based on the XRD result, the VO phase was crystallized in an isometric system
with space group Fm-3m (225), (a = 4.120 Å and Z = 4). The as-prepared VO
powder was also characterized by SEM-EDS; most of these particles were shown
to have regular polygonal shapes, corresponding to an average size of 100 μm.
The atomic percentages of V and O approximately conform to the
stoichiometric ratio of VO.

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4. As a result, when the VO was served as the LIBs anode material, the resulting
electrodes exhibited a high specific capacity (843 mAh·g-1 after 30 cycles) and
remarkable cyclic stability.
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5. H2-based reduction may provide an environmentally friendly and relatively simple
method of preparing the VO powder, and this method is also applicable to
industrial production.
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419 Acknowledgements
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This work was supported by the National Key R&D Program of China
(2018YFC1900500), and Graduate Scientific Research and Innovation Foundation of
Chongqing, China (Grant No.CYS19001).
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