Preparation of Cu6Sn5-encapsulated carbon microsphere anode materials for Li-ion batteries by carbothermal reduction of oxides

Article abstract of DOI:10.1149/1.2229276

A novel process was proposed to prepare Cu₆Sn ₅-encapsulated carbon microsphere (CM/Cu₆Sn₅) anode materials for the first time through inverse emulsion polymerization of resorcinol-formaldehyde (RF) performed in the presence of CuO and SnO ₂ powders (1.2:1 molar ratio), followed by carbonization and carbothermal reduction in an inert atmosphere. The oxides powder were encapsulated within the carbon gel microspheres and the elemental Sn and Cu were reduced from their oxides by the carbonized RF gel to form crystalline Cu ₆Sn₅ alloy within carbon microspheres. The CM/Cu ₆Sn₅ presented much better cycleability than that of Cu₆Sn₅ alloy powder. The proposed process paves an effective way to prepare high-performance alloy/C microspheres composite anode materials for lithium-ion batteries.

Full text of DOI:10.1149/1.2229276

Journal of The Electrochemical Society, 153 ͑10͒ A1859-A1862 ͑2006͒
A1859
0
013-4651/2006/153͑10͒/A1859/4/$20.00 © The Electrochemical Society
Preparation of Cu Sn -Encapsulated Carbon Microsphere
6
5
Anode Materials for Li-ion Batteries by Carbothermal
Reduction of Oxides
z
Ke Wang, Xiangming He, Li Wang, Jianguo Ren,
Changyin Jiang, and Chunrong Wan
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
A novel process was proposed to prepare Cu Sn -encapsulated carbon microsphere ͑CM/Cu Sn ͒ anode materials for the first time
6
5
6
5
through inverse emulsion polymerization of resorcinol-formaldehyde ͑RF͒ performed in the presence of CuO and SnO powders
2
͑
1.2:1 molar ratio͒, followed by carbonization and carbothermal reduction in an inert atmosphere. The oxides powder were
encapsulated within the carbon gel microspheres and the elemental Sn and Cu were reduced from their oxides by the carbonized
RF gel to form crystalline Cu Sn alloy within carbon microspheres. The CM/Cu Sn presented much better cycleability than that
6
5
6
5
of Cu Sn alloy powder. The proposed process paves an effective way to prepare high-performance alloy/C microspheres com-
6
5
posite anode materials for lithium-ion batteries.
2006 The Electrochemical Society. ͓DOI: 10.1149/1.2229276͔ All rights reserved.
©
Manuscript submitted April 25, 2006; revised manuscript received May 30, 2006. Available electronically August 4, 2006.
Carbon materials have been commonly used as anodes in
lithium-ion battery systems, but an upper limit in the lithium ion
Experimental
1-5
Preparation of Cu Sn alloy powder.— SnO
2
99.9%͒, and carbon powder ͑Ͼ99.5%͒ were used as raw materials.
Mixtures were prepared in accordance with Reaction 1
͑99.9%͒, CuO
6
5
capacities of such materials exists. For the further improvement of
lithium-ion batteries, the development of alternative anode materials
͑
6
-10
which possess larger capacities is required. In recent studies,
alloy-based materials containing lithium storage metals have shown
a promising application in rechargeable lithium-ion batteries due to
their high-volumetric and mass-energy density. Much research has-
been undertaken to improve the electrochemical performance of
alloy-based materials and to explore their potential use as anodes in
lithium-ion batteries. Among these alloy materials, Cu Sn alloy ex-
5
SnO + 6CuO + 16C → Cu Sn + 16CO
͓1͔
2
6
5
After mixing and ballmilling, the sample was calcined at 1000°C
for 2 h in argon atmosphere at the heating rate of 5°C/min in a tube
furnace and then allowed to cool naturally to room temperature. The
so-obtained product was ground to fine powder in an agate mortar.
In order to investigate carbothermal reduction in the process,
differential scanning calorimetry ͑DSC͒ was performed on the as-
mixed powders ͑SnO + 2C, CuO + C, and 5SnO + 6CuO + 16C,
6
5
hibits excellent electrochemical properties as structural decomposi-
tion would encase the lithium/tin alloy in a conductive copper
11-13
matrix.
2
2
respectively͒, ϳ10 mg of powder was placed in an alumina crucible
and heated to 20°C/min to 1000°C.
Cu Sn alloy powder used for lithium-ion batteries is usually
6
5
synthesized by chemical precipitation from aqueous solutions con-
taining the chlorides of the respective metals and complex agents
8,11
with sodium borohydride as reductive.
Although this approach
can yield nanoscaled Cu Sn particles which are deemed to be fa-
Preparation and characterization of Cu Sn -encapsulated car-
6
5
6
5
vorable to the cycling stability of anodes, a certain amount of chloric
ions and hydroxyl groups adhered on the surface of particles is
difficult to be removed completely, and that will certainly affect the
electrochemical performance of the Cu Sn anode. Cu Sn powder
bon microspheres.— RF solution containing sodium carbonate as a
basic catalyst was prepared according to the conventional method.
1
5
In this work, the mixture of CuO and SnO powders ͑1.2:1 molar
2
6
5
6
5
ratio͒ ballmilled for 24 h in an argon atmosphere was added to the
is also prepared by melting the mixture of Sn and Cu metal powders
RF solution. The molar ratios of resorcinol to catalyst and resorcinol
12
3
and then mechanically milling or by electrochemically depositing
to formaldehyde were, respectively, fixed to 100 and 0.5 g/cm , and
14
Sn directly on the Cu foil. These methods are either complicated in
controlling the processing parameters or costly in raw materials.
In this study, the carbothermal reduction method was employed
to synthesize the Cu Sn alloy powders from oxides. There is no
the value of pH was adjusted 9–10. The weight ratio of mixture of
oxides to resorcinol was fixed to 1:2.
First, the RF solution was prepared in a glass vial and was thor-
oughly stirred at 25°C until the RF solution almost lost its fluidity.
Then, it was dispersed into kerosene containing surfactant SPAN80
͑9:1 volume ratio͒, and an inverse emulsion which consists of mi-
cromicelles of RF solution was formed. The emulsion was stirred at
25°C until the dispersed RF solution gelled. The obtained RF hy-
drogel microspheres were filtered out and subsequently immersed
into t-butanol in order to exchange the water included within them
with t-butanol. After drying in a convection oven, the product was
heat-treated at 1000°C for 2 h under argon atmosphere to carbonize
the RF hydrogel microspheres. During the heat-treatment, the SnO2
and CuO in carbon gel microspheres were deoxidized by the car-
bonized RF polymer to form crystalline Cu Sn alloy powder, as
6
5
report on the synthesis of Cu Sn alloy powder from its correspond-
6
5
ing oxides.
Furthermore, we tried to improve the cycling performance of
Cu Sn alloy particles by encasing them within carbon gel micro-
6
5
spheres. Carbon gel microspheres containing Cu Sn alloy powders
6
5
͑
CM/Cu Sn ͒ were prepared by adding CuO and SnO powder to
6
5
2
the water phase during the inverse emulsion polymerization of
resorcinol-formaldehyde ͑RF͒, followed by carbonization in an inert
atmosphere. The elemental Sn and Cu were reduced from their ox-
ides by the carbonized RF gel to form crystalline Cu Sn alloy. It is
6
5
assumed that the mesoporous structure of the carbon gel micro-
spheres acted as a buffering matrix which relieves the morphological
6
5
represented in Reaction 1. Finally, carbon gel microspheres contain-
changes of Cu Sn alloy powders during charging/discharging and
6
5
ing Cu Sn alloy particles were obtained. In this process, the Cu Sn
6
5
6
5
that the carbon gel microspheres themselves act as an active material
alloy content in the composite was 69.5 wt %. This was estimated
from the thermographic analysis ͑TGA͒ carried out in air, as only
SnO and CuO should remain after the RF gel is oxidized to CO
+
for additional Li storage.
2
2
and H O. The Cu Sn alloy content could be calculated based on the
initial and final weight of sample as represented in Eq. 2
2
6
5
z
E-mail: hexm@tsinghua.edu.cn
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Journal of The Electrochemical Society, 153 ͑10͒ A1859-A1862 ͑2006͒
Figure 1. DSC curves for mixture powders of CuO–C, SnO –C, and
2
SnO –CuO–C. Powders were heated at the rate of 10°C/min in nitrogen.
2
molecular weight of Cu Sn
6
5
Cu Sn content͑%͒ = 100 ϫ
6
5
molecular weight of Cu Sn O
16
6
5
final weight
ϫ
͓2͔
initial weight
The existence and crystallinity of Cu Sn in the carbon gel mi-
6
5
crospheres were analyzed by X-ray diffractometry ͑XRD͒. The mor-
phology of carbon microspheres was observed using a scanning
electron microscope ͑SEM͒ ͑JEM-7401͒.
Figure 3. SEM images of ͑a͒ Cu Sn alloy particles and ͑b͒ Cu Sn /C mi-
6
5
6
5
crospheres ͑CM/Cu Sn ͒ synthesized through carbonization at 1000°C.
6
5
Electrochemical measurements.— The coin-type test cells were
assembled in a glove box filled with dry argon. The working elec-
trodes were made from as-prepared products ͑80 wt %͒, carbon
black ͑10 wt %͒, and polytetrafluoroethylene ͑PTFE, as binder,
charging/discharging cycling performance was conducted at a con-
stant current density of 25 mA g− between 0.0 and 1.5 V at room
temperature.
1
1
0 wt %͒. Lithium foil was used as the counter electrode and the
electrolyte was a solution of 1 M LiPF dissolved in a 1:1 ͑v/v͒
6
mixture of ethylene carbonate ͑EC͒ and diethyl carbonate ͑DEC͒.
Celgard 2400 porous membrane was used as the separator. The
Results and Discussion
DSC analysis.— DSC was performed on mixtures of SnO –C,
2
CuO–C, and SnO –CuO–C, respectively, and the results are shown
2
in Fig. 1. It is found that the carbothermal reduction is endothermic
for all investigated systems. An endothermic peak at 809°C is pre-
sented in the SnO –C system, attributable to the formation of Sn
2
metal via the carbothermal reduction of SnO . The reduction of CuO
2
by carbon occurs at relatively higher temperatures, as indicated by a
broad endothermic peak at 910°C in the DSC curve. For the ternary
mixture of SnO , CuO, and C in a molar ratio of 5:6:16, however,
2
the DSC result shows some changes in reduction peaks of SnO and
2
CuO. Two overlapped endothermic peaks centered at 817 and
8
29°C exhibit in DSC curves. The first peak at 817°C should be
ascribed to the reduction of SnO by carbon and the overlapped peak
2
at 829°C is due to the carbothermal reduction of CuO. It is likely
that the reduction of CuO by carbon is facilitated with the presence
of SnO , compared with the DSC result of CuO–C where CuO is
2
reduced to Cu metal at high 910°C. So, the reduction temperatures
of SnO and CuO are close, which is favorable to form crystalline
2
Cu Sn alloy.
6
5
XRD results.— Figure 2 shows the XRD patterns of the ball-
milled mixture of SnO –CuO–C ͑5:6:16͒ and CM/Cu Sn before
2
6
5
and after heat-treatment at 1000°C. The XRD reflection of mixture
before heat-treatment ͑a͒ shows no evidence of reaction; only SnO2
and CuO phases are present. After heat-treatment at 1000°C, the
Figure 2. XRD patterns of ballmilled mixture of ͑a͒ SnO /CuO/C before
2
heat-treatment, ͑b͒ the mixture calcined at 1000°C, and ͑c͒ CM/Cu Sn cal-
6
5
cined at 1000°C.
XRD pattern ͑b͒ includes sharp reflection lines which can be as-
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Journal of The Electrochemical Society, 153 ͑10͒ A1859-A1862 ͑2006͒
A1861
Table I. Description of the samples used for the electrochemical
characterization.
Table II. Summary of electrochemical performance of the
samples.
Cu Sn content
Capacity retained at cycle ͑%͒
6
5
Sample
Description
͑wt %͒
Second discharging
capacity
Cu Sn alloy
Crystalline Cu Sn alloy powder
100a
69.5
6
5
6
5
6
5
6 5
Sample
͑mAh g 1͒
−
10th
20th
50th
CM/Cu Sn
Cu Sn /C microspheres composite
a
Cu Sn alloy
433
372
66.4
96.5
37.8
94.5
28.9
86.9
Determined from TGA profile.
6
5
CM/Cu Sn
6
5
signed to Cu Sn ͑JCPDS no. 45-1488͒. The lines corresponding to
6
5
any oxide phase are not found, implying that the elemental Sn and
Cu have been reduced from their oxides to form crystalline Cu Sn
alloy by carbothermal reduction. A similar profile has been observed
The profile taken with the Cu Sn /C microspheres ͑CM/Cu Sn ͒
6
5
6
5
6
5
is shown in Fig. 4b, where the 0.4 V voltage slope assigned to the
+
in the XRD pattern of CM/Cu Sn5 ͑c͒, indicating that the same
reaction between Li and Sn could also be observed. Moreover, the
6
+
reduced reaction occurs in carbon gel microspheres during heat-
treatment.
background voltage profile associated with the Li insertion/removal
into/from carbon also appears, where the different voltage slopes at
three different potential regions ͑0–0.12, 0.12–0.8, and above
SEM observation.— Figure 3 shows the SEM images of as-
5
0
.8 V͒ in the discharge profile are observed. This indicates the pres-
obtained materials in this work. In Fig. 3a of Cu Sn particles, mul-
6
5
+
ence of three different Li storage sites in carbon matrix, which is
one of the characteristic features found in hard carbons that are
prepared from cross-linked polymeric precursors or isotropic
pitches. In the first cycle, the charging and discharging capacities
are 509 and 360 mAh g , respectively, from which a coulombic
tiangular particles ͑5–10 ␮m͒ are observed, due to mechanical
crushing. The SEM image of CM/Cu Sn presents a spherical shape
6
5
͑
ϳ10 ␮m͒, as indicated in Fig. 3b. Because SnO and CuO powders
5
2
were not observed around the microspheres in the image, it is sug-
gested that most the oxides powders were encased in the carbon gel
microspheres. The SEM image and XRD analysis confirm that car-
−1
efficiency of 70.7% is calculated. It is well known that hard carbons
show a lower coulombic efficiency compared to graphites. The irre-
versible capacity of hard carbons at the first cycle is attributed to the
solid electrolyte interface ͑SEI͒ formation, the degree of which is
proportional to the surface area of carbon and the irreversible stor-
age ͑trapping͒ of Li+ ion at void or cavity sites that are highly
bon gel microspheres containing Cu Sn5 alloy particles were
6
straightforwardly prepared using this method, which is based on
inverse emulsion polymerization and carbothermal reduction from
oxides.
1
6,17
Electrochemical properties.— Two samples ͑Table I͒ were elec-
trochemically characterized by intercalation/deintercalation of
lithium. Figure 4 shows the charging/discharging curves of ͑a͒
Cu Sn alloy powder and ͑b͒ CM/Cu Sn . The Li/Cu Sn cells were
populated in this type of carbons.
Figure 5 compares the cycling performance of CM/Cu6Sn5 to
that of the crystalline Cu Sn alloy powder sample. The results are
6
5
summarized in Table II, along with some initial performance data.
The discharging capacity of Cu Sn alloy powder shows significant
6
5
6
5
6
5
cycled between 0 and 1.2 V at a constant current density of
6
5
−
1
2
5 mA g . In Fig. 4a, the voltage plateau around 0.4 V in the dis-
fading during cycling, because the nickel–arsenide to zinc–blende
charging curve corresponds to the electrochemical alloying reaction
phase transition of Cu Sn is accompanied by a 59% expansion in
6
5
+
11
between the Li ion and Sn component. In the first discharge, the
crystalline Cu Sn5 alloy electrode delivers capacity of
55 mAh g , which is more than that of maximum lithium inser-
crystallographic volume and some extruded copper atoms cannot
insert the structure reversibly, resulting in the aggregation of Sn
a
6
−
1
12
6
component. The capacity retention of CM/Cu Sn is much better
6
5
−
1
tion achievable theoretically with a capacity of 608 mAh g . A
reversible capacity of 439 mAh g−1 was achieved in the first charg-
ing, which is 67% of the coulombic efficiency in the first cycle. The
lost capacity of 216 mAh g−1 is most likely associated with irrevers-
ible Li loss as a result of the reduction of the electrolyte solvent to
form a solid electrolyte interphase on the particle surface.
than that of crystalline Cu Sn alloy powder with an 86.9% of ca-
6 5
pacity retention after 50 cycles, while the coulombic efficiency re-
mains above 98% during the following cycling. This indicates that
the spherical carbon can probably act as a barrier to prevent the
aggregation of Cu Sn particles and a buffer to relieve the morpho-
6
5
logical expansion of Cu Sn particles inside.
6
5
Figure 4. The gal-
vanostatic charge–
discharge voltage pro-
files of ͑a͒ Cu Sn
6
5
alloy powder and
b͒ CM/Cu Sn elec-
͑
6
5
trodes in the first
cycle.
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Journal of The Electrochemical Society, 153 ͑10͒ A1859-A1862 ͑2006͒
Figure 5. The ͑a͒ cycleability and ͑b͒ coulombic efficiency of Cu Sn alloy and CM/Cu Sn composite electrodes.
6
5
6
5
Tsinghua University assisted in meeting the publication costs of this ar-
ticle.
Conclusion
Cu Sn -encapsulated carbon microsphere ͑CM/Cu Sn ͒ can be
prepared through an inverse emulsion polymerization of RF per-
formed in the presence of CuO and SnO2 powders ͑1.2:1 molar
ratio͒, followed by carbonization and carbothermal reduction in an
inert atmosphere. The oxides powder can be successfully encapsu-
lated within the carbon gel microspheres and the elemental Sn and
Cu can be reduced from their oxides by the carbonized RF gel to
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1
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This work was financially supported by “985” Basic Research
Fund ͑project no. 985-04-202-1͒ from Tsinghua University.
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