Thermodynamic and kinetic analysis for carbothermal reduction process of CoSb alloy powders used as anode for lithium ion batteries

Article abstract of DOI:10.1016/j.jallcom.2011.04.143

Thermodynamic calculation and kinetic analysis were performed on the carbothermal reduction process of Co₃O₄-Sb ₂O₃-C system to clarify the reaction mechanism and synthesize pure CoSb powder for the anode material of secondary lithium-ion batteries. The addition of carbon amount and thus the purity of CoSb powders were critical to the electrochemical property of CoSb anode. It was revealed that in an inert atmosphere, Co₃O₄ was preferentially reduced to CoO, followed by the reduction of Sb₂O₃ and CoO. CO₂ was the gas product for the reduction of Co ₃O₄ and Sb₂O₃, while CO was the gas product for that of CoO. Based on the analysis result, pure CoSb powder without any oxides and residual carbon was synthesized, which showed a higher specific capacity and a lower initial irreversible capacity loss, compared to CoSb sample with residual carbon. This work can be a reference for other carbothermal reduction systems.

Full text of DOI:10.1016/j.jallcom.2011.04.143

Journal of Alloys and Compounds 509 (2011) 7657–7661
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Thermodynamic and kinetic analysis for carbothermal reduction process of CoSb
alloy powders used as anode for lithium ion batteries
a
a
b
a,c,∗
, Ronglin Wang , Jingbo Chena
a
Jianying Yang , Mengwei Wang , Yuntong Zhu , Hailei Zhao
a
School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Tsinghua High School, Beijing 100084, China
Beijing Key Lab of New Energy Material and Technology, Beijing 100083, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermodynamic calculation and kinetic analysis were performed on the carbothermal reduction process
of Co3O4–Sb2O3–C system to clarify the reaction mechanism and synthesize pure CoSb powder for the
anode material of secondary lithium-ion batteries. The addition of carbon amount and thus the purity of
CoSb powders were critical to the electrochemical property of CoSb anode. It was revealed that in an inert
atmosphere, Co3O4 was preferentially reduced to CoO, followed by the reduction of Sb2O3 and CoO. CO2
was the gas product for the reduction of Co3O4 and Sb2O3, while CO was the gas product for that of CoO.
Based on the analysis result, pure CoSb powder without any oxides and residual carbon was synthesized,
which showed a higher specific capacity and a lower initial irreversible capacity loss, compared to CoSb
sample with residual carbon. This work can be a reference for other carbothermal reduction systems.
© 2011 Elsevier B.V. All rights reserved.
Received 15 January 2011
Received in revised form 28 April 2011
Accepted 30 April 2011
Available online 10 May 2011
Keywords:
Carbothermal reduction
Synthesis
CoSb
Anode
Lithium ion battery
1
. Introduction
such as Co [14,15], Se [16], Al [17], Zn [18,19], Ni [20] and Cu
[
21,22]. Active elements such as Sn [23,24] and In [25] can also
Secondary lithium-ion battery plays more and more impor-
make the volume change of the Sb-based electrode take place much
smoothly because the lithiation and delithiation of the two active
components occur at different potentials, the unreacted phase can
accommodate the mechanical strain yielded by the reacted phase.
There are diversified methods for synthesizing Sb-based alloy
materials, including mechanical alloying of metal powders [26,27],
hydrothermal method [28], chemical precipitation [23,29], elec-
trochemical deposition [30,31] as well as carbothermal reduction
of metal oxides [32–35]. Compared to other preparation methods,
carbothermal reduction method is low cost and much more easily
mass-productive, making it a promising method for both labora-
tory scale experiment and industrial production. In carbothermal
reduction processes, the carbon amount is a critical factor because it
determines the purity of the product directly. If the carbon amount
is in excess, there will be a certain amount of carbon remained
in the synthesized anode materials; on the contrary, less carbon
than needed will lead to the incomplete reduction of metal oxides.
Either of them will affect the electrochemical properties of materi-
als. Most researches in this field [33,36–40] have been carried out
based on the hypothesis that all the carbon is oxidized to CO dur-
ing the carbothermal process. Nevertheless, the generation of CO2
cannot be excluded in some systems. Whether CO or CO2 is possi-
bly produced depends mainly on the chemical features of different
metal oxides. As a matter of fact, in a multiple metal oxides carboth-
ermal reduction process, the gas products might be the mixture of
tant roles in modern society as an energy storage device and is
widely used in cell phones, portable electronic devices and elec-
tric vehicles. Graphite is the commonly used anode materials for
lithium-ion batteries due to its long cycle life, abundant material
supply and relatively low cost [1]. However, the relatively low
theoretical capacity (372 mAh/g) and the safety issue related to
lithium deposition have limited the development of higher perfor-
mance lithium-ion batteries. Meanwhile, alloy anodes have been
considered as one of the most promising electrode materials for
the next-generation lithium-ion batteries due to their high energy
densities, relatively low cost, environmental compatibility and safe
operation potential [2]. They have been attracting lots of atten-
tions and various endeavors have been made aiming to synthesize
high performance alloy anode materials [3–13]. Among those dif-
ferent types of candidates for anode materials, Sb-based alloy anode
materials have high specific capacity and elevated operating volt-
age, which can overcome the drastic volume change of metallic
antimony during the lithiation and delithiation cycles by intro-
ducing less active or inactive component as matrix for active Sb,
∗ Corresponding author at: School of Materials Science and Engineering, Univer-
sity of Science and, Technology Beijing, Beijing 100083, China.
E-mail address: hlzhao@ustb.edu.cn (H. Zhao).
0
925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.04.143

7
658
J. Yang et al. / Journal of Alloys and Compounds 509 (2011) 7657–7661
0
Fig. 1. Gibbs free energies (ꢀG ) of reactions (1)–(4) as a function of temperature.
0
Fig. 3. Gibbs free energies (ꢀG ) of reactions (7) and (8) as a function of temperature.
CO and CO2 due to various factors. It is of great importance to get
further understandings about the carbothermal reduction process
peratures for different species were looked up from handbooks
0
[41,42], then the enthalpy change (ꢀH ) and the entropy change
T
details such as the reduction order and the CO and CO composition
0
T
2
(ꢀS ) at different temperatures for a certain reaction could be
in the gas product, which can help to determine the precise carbon
amount actually required, leading to products of high purity.
In present work, CoSb alloy was selected as an example to clarify
derived. Accordingly, the standard Gibbs free energies at differ-
ent temperatures for a certain reaction could be calculated by the
equation of ꢀG 0T = ꢀHT − TꢀS .
0
0
T
3
the actual kinetic process of carbothermal reduction from Co O4
From the perspective of thermodynamics, reactions (2) and (4)
are more feasible compared to reactions (1) and (3) below 707.9 C,
and Sb O , based on the thermodynamic analysis of all possible
◦
2
3
reactions in this system, the SEM observation and chemical compo-
sition analysis of the products. With the calculated carbon amount,
high purity CoSb alloy were synthesized and its electrochemical
property was evaluated and compared with that containing a cer-
tain amount of remaining carbon.
i.e. CO2 will be produced as gas product. With respect to reactions
(2) and (4), the latter is even easier to occur at low temperature.
According to Fig. 2, CoO can be reduced by carbon to metal Co via
◦
reactions (6) or (5) at the temperature below or above 707.9 C,
respectively.
Co O + 4C = 3Co + 4CO(g)
(1)
(2)
(3)
(4)
(5)
(6)
3
4
2
. Thermodynamics analysis
Co O + 2C = 3Co + 2CO (g)
3
4
2
2.1. Co O –C system
3
4
Co O + C = 3CoO + CO(g)
3
4
For the carbothermal reduction of Co O , there is a series of
3
4
Co O + 0.5C = 3CoO + 0.5CO (g)
3
4
2
possible reactions. Co O4 can be reduced by carbon directly to
metal Co, as described by reactions (1) and (2), or to CoO first and
then to Co, as presented by reactions (3)–(6). For each route, the
3
CoO + C = Co + CO(g)
CoO + 0.5C = Co + 0.5CO (g)
2
gas product could be CO or CO . The standard Gibbs free energies
2
of reactions (1)–(6) as a function of temperature were calculated
and plotted in Figs. 1 and 2. With respect to the thermodynamic
calculation, the standard enthalpies and entropies at different tem-
2
.2. Sb O –C system
2 3
The possible reactions for the carbothermal reduction of Sb O3
2
are reactions (7) and (8), corresponding to the generation of CO
and CO2 gases, respectively. Fig. 3 shows the Gibbs free energies of
reactions (7) and (8) as a function of temperature [41,42], indicating
◦
their theoretical feasibility when temperature is above 420.1 C and
◦
◦
1
49.5 C, respectively. At temperature lower than 707.9 C, reaction
(8) has a lower Gibbs free energy value.
Sb O + 3C = 2Sb + 3CO(g)
(7)
(8)
2
3
Sb O + 3/2C = 2Sb + 3/2CO (g)
2
3
2
According to the above analyses, it can be concluded that CO2
gas will be the reduction product if the temperature is lower than
◦
7
07.9 C, otherwise CO gas will be.
2.3. Co–Sb system
Our previous work on the thermodynamic calculation [33] indi-
cated that the alloying reaction of Co with Sb (reaction (9)) could
◦
occur in the temperature range from 0 to 1000 C. It is worth to
note that the alloying reaction (9) is an exothermic reaction, which
0
Fig. 2. Gibbs free energies (ꢀG ) of reactions (5) and (6) as a function of temperature.

J. Yang et al. / Journal of Alloys and Compounds 509 (2011) 7657–7661
7659
Fig. 4. Schematic drawing of the setup for CO2 analyzing.
Fig. 5. TG-DTA curves of Co3O4–Sb2O3–C system with the molar ratio of 3:2:17.
has a positive effect for the endothermic carbothermal reduction
process of the metal oxides.
To determine the reduction sequence of these oxides or the
actual kinetic process of the carbothermal reduction, the theoret-
ical mass losses of the carbothermal reduction of different oxides
via the generation of CO2 or CO gases, respectively, were calcu-
lated based on the total loaded material weight. The results were
Co + Sb = CoSb
(9)
3.
Experimental
listed in Table 1. It was apparent that the weight loss of 3.10 wt%
Co3O4, Sb2O3 and carbon were used as raw materials in present work. Those
◦
between 420 and 550 C was ascribable to the reduction of Co O
three different powders were mixed and ground in an agate mortar following
different stoichiometric ratios. The mixed powders were put into a temperature-
3
4
to CoO, as described in reactions (3) or (4). Considering the ther-
◦
controlled tubular furnace equipped with flowing argon and calcined at 800 C for
modynamic analysis in Section 2, reaction (4) was preferable below
2
h. The products were removed from the off-powered furnace after cooling down
naturally to room temperature.
◦
7
5
07.9 C. With respect to the weight loss of 21.82 wt% produced in
◦
50–720 C, it should be associated with the reductions of Sb O
2
3
Thermo-gravimetric/differential thermal analysis (TG-DTA, NETZSCH STA 449 C)
◦
and CoO. The thermodynamic calculation (Figs. 2 and 3) suggested
was conducted with a heating rate of 5 C/min under a N2 flux of 40 ml/min. The
phases of carbothermal reduction product were analysed by X-ray Diffraction (XRD)
over the 2ꢁ range from 10 to 90 using a Rigaku D/max-A diffractometer with Cu
K␣ radiation (40 kV, 100 mA). The microstructure of synthesized alloy powders was
observed by scanning electron microscope (SEM, LEO-1450).
The CO2 amount in the gas product of the carbothermal reduction was deter-
mined with a self-fabricated device as depicted in Fig. 4. The sample was calcined
in a tube furnace under flowing argon. The produced gases with the flowing argon
were vented into Ca(OH)2 solution, where CO2 gas could be absorbed and CaCO3 pre-
cipitation was formed. After washing, filtering and drying, the CaCO3 precipitation
was weighed and accordingly the CO2 amount could be derived.
Li/LiPF6 (EC + DMC)/CoSb two-electrode half-cells were assembled for cycling
tests. Firstly, CoSb electrode was prepared using the synthesized CoSb powders
mixed with acetylene black as conductive additive and polyvinylidene fluoride
that Sb O3 was relatively easier to be reduced by carbon com-
2
◦
pared to CoO. Because the second weight loss process started below
◦
7
07.9 C, it was reasonable to state that the reduction of Sb O was
2
3
via the route of reaction (8). For the reduction of CoO, either reaction
(5) or reaction (6) was possible, as the reduction process lasted to
◦
7
20 C. If CoO was reduced via reaction (5), then the whole weight
loss of the second step was (12.50 + 10.37) = 22.87 wt%, theoreti-
cally. Meanwhile, if it was reduced via reaction (6), then the whole
weight loss should be (12.50 + 8.46) = 20.96 wt%. The actual weight
loss in this period (21.82 wt%) was just in between.
(
PVDF) as binder, which was dissolved in N-methyl-pyrrolidinone in the weight
3Sb2O3 + 2Co3O4 + 17 C → 6CoSb + 17CO(g)
(10)
ratio of 80:10:10 and then painted on a copper foil used as current collector. Then
the electrode was pressed and dried at 120 C under vacuum for 24 h. Lastly, cells
were assembled in an argon-filled glove box using Celgard 2400 polyethylene as
separator, 1 M LiPF6 in a mixture of ethyl carbonate (EC) and dimethyl carbonate
◦
Then, we can deduce that the reduction process could carry out
via the routes of reactions (4), (5) and (8) or (4), (6) and (8). The for-
mer route corresponded to the whole reaction (11), while the latter
to the reaction (12). Compared to the required amount of carbon
for reaction (10), where all oxides were supposed to be oxidized
via the generation of CO gases, the carbon amount required for the
accomplishment of reactions (11) and (12) were only 67.7% and 50%
of the full carbon, respectively. With the help of the setup described
(
DMC) (1:1 in vol. ratio) as electrolyte, and metallic lithium foil as counter elec-
trode. The cycling tests were carried out at current density of 100 mA/g in voltage
range of 0.01–1.5 V versus Li/Li+ by LAND BT-10 tester (Wuhan, China).
4
. Results and discussion
in Fig. 4, the produced CO amount was calculated, which was much
2
In order to gain an insight into the kinetic process of CoSb
closer to the theoretical result predicted by reaction (11). Accord-
ingly, reaction (11), consisting of reactions (4), (5) and (8), was the
most possible route for the carbothermal reduction synthesis of
CoSb alloy powders.
alloy via carbothermal reaction, TG-DTA analysis was performed on
Sb O –Co O –C powders, which was prepared in the molar ratio of
2
3
3
4
Sb O :Co O :C = 3:2:17 according to reaction (10), where carbon
2
3
3
4
monoxide (CO) was supposed to generate during the carbother-
mal reduction process, and the carbon amount in reaction (10) was
3
Sb O + 2Co O + 23/2 C → 6CoSb + 6CO(g) + 11/2CO (g)(11)
2
3
3
4
2
defined as full carbon. The TG-DTA result was shown in Fig. 5. Before
◦
4
20 C, the system had a little mass loss, which corresponded to the
Table 1
*
Theoretical mass loss corresponding to the carbothermal reduction of different
oxides via the generation of CO2 or CO gases, respectively.
vaporization of water contained in the slightly damped raw mate-
◦
rials. From 420 to 550 C, the system underwent a process with a
mass loss of 3.10 wt% (first step), accompanied with an endother-
Co O → CoO
CoO → Co
Sb O → Sb
3
4
2
3
◦
◦
mic peak centered at ca. 507.5 C. After 550 C, there was a slow
CO2
2.82
CO
CO2
CO
CO2
CO
◦
weight loss and then followed by a rapid loss from 650 to 720 C
◦
Weight loss (wt%)
3.59
8.46
10.37
12.50
16.15
with an endothermic peak at ca. 721.2 C. The total weight loss in
◦
*
5
50–720 C was 21.82 wt% (second step).
Based on the total loaded material weight as expressed in reaction (10).

7
660
J. Yang et al. / Journal of Alloys and Compounds 509 (2011) 7657–7661
Fig. 6. XRD patterns of the samples with 60% and 65% of full carbon amount.
3
Sb O + 2Co O + 17/2 C → 6CoSb + 17/2CO (g)
(12)
2
3
3
4
2
In order to verify the theoretically predicted result, two
Sb O –Co O –C samples with 60% and 65% of the full carbon were
2
3
3
4
◦
prepared and calcined at 800 C for 2 h in flowing Ar atmosphere,
respectively. The XRD patterns of the synthesized materials are
shown in Fig. 6. For sample with 60% C, besides the peaks to CoSb, a
little amount of impurity was detected, which could be assigned to
CoO, indicating that the reduction of oxides was not complete and
the carbon amount was insufficient. Fortunately, all the peaks of
sample with 65% C were identified as CoSb and no other impurity
or residual carbon was detected. This is well consistent with the
theoretically predicted result.
Fig. 8. Electrochemical performance of CoSb powder synthesized from 65% carbon
and full carbon prescription: (a) potential profiles; (b) cycling performance.
SEM observation was performed on the synthesized CoSb pow-
ders prepared from 65% and 100% of full carbon, respectively.
Fig. 7 presents the SEM images of two samples. Sample with 65%
C showed worm-like particle morphology without observable car-
bon component, i.e. pure CoSb alloy powders, while sample with
1
00% C displays mixture morphology with relatively smaller CoSb
alloy particles and irregular carbon particles, i.e. CoSb/C composite
powders.
The electrochemical property of the two samples as anode mate-
rial for lithium ion battery was evaluated by two-electrode method.
The samples were cycled at 100 mA/g between 0.01 and 1.5 V versus
+
Li/Li . As illustrated in Fig. 8(a) and (b), CoSb electrode presented
a higher reversible capacity and a lower initial irreversible capac-
ity loss than CoSb/C electrode, which can be attributed to the high
purity of the CoSb sample. The residual carbon in the CoSb/C sample
should be responsible for its lower specific capacity. The synthe-
sized pure CoSb alloy exhibited a specific capacity of ca. 400 mAh/g,
which was much higher than that of the commonly used graphite
anode materials, and thus being a promising anode material for
lithium ion battery.
5. Conclusions
The kinetic process of carbothermal reduction of
Co O –Sb O –C system was investigated with the aim of under-
3
4
2
3
standing the detailed process and synthesizing pure CoSb alloy
powders. The thermodynamic calculation and the TG-DTA analysis
revealed that Co3O4 was preferentially reduced to CoO at relatively
Fig. 7. SEM images of the synthesized powders. (A): 65% carbon; (B): full carbon.

J. Yang et al. / Journal of Alloys and Compounds 509 (2011) 7657–7661
7661
lower temperature, then followed by the simultaneous reduction
of Sb O and CoO by carbon. The former is reduced by carbon via
the generation of CO2 gases, while the latter is reduced via the
formation of CO gases. The overall reaction for the synthesis of
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CoSb alloy from Co O –Sb O –C system could be expressed as:
3
4
2
3
[
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3
4
2
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with the carbon-containing CoSb samples. The kinetic analysis is
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