Reaction mechanism of a Ag36.4Sb15.6Sn48 nanocomposite electrode for advanced Li-ion batteries

Article abstract of DOI:10.1149/1.1827597

The phase transformations of an Ag_36.4Sb_15.6Sn ₄₈ nanocomposite electrode, Ag₃Sn/Sn/SnSb, were investigated during electrochemical lithium insertion/extraction. The high performance gives 410 mAn/g for 300 cycles. Electrodes were prepared at selected voltage points in the discharge-charge curves and high-energy synchrotron X-ray data was collected in order to study the reaction mechanism in detail. Both Ag₃3Sn and SbSn insert Li stepwise to form compounds with the partial compositions Ag_2-xLi_1+xSn (0 ≤ x ≤ 1) and Li _2+xSn_1-xSb (x ≤ 1). At lower voltages, Li ₇Sn₂, Li₂₂Sn₅, AgLi, and Ag ₅Li₈ could be observed. The intermetallic compounds before and after lithiation have a strong crystallographic relation.

Full text of DOI:10.1149/1.1827597

A152
Journal of The Electrochemical Society, 152 ͑1͒ A152-A157 ͑2005͒
0
013-4651/2004/152͑1͒/A152/6/$7.00 © The Electrochemical Society, Inc.
Reaction Mechanism of a Ag Sb Sn Nanocomposite
3
6.4
15.6
48
Electrode for Advanced Li-Ion Batteries
a, ,d,z
a
b
c
Ewa R o¨ nnebro, * Jingtian Yin, Akiko Kitano, Masahi Wada,
a,
a,
Shigeo Tanase, * and Tetsuo Sakai *
aNational Institute of Advanced Industrial Science and Technology, Kansai Center, Ikeda,
Osaka 563-8577, Japan
b
Japan Synchrotron Radiation Research Institute, Mikazuki, Sayo Hyogo 679-5198, Japan
c
Fukuda Metal Foil and Powder Company, Company, Limited, Yamashina-ku, Kyoto 607-8305, Japan
The phase transformations of an Ag36.4Sb15.6Sn48 nanocomposite electrode, Ag Sn/Sn/SnSb, were investigated during electro-
3
chemical lithium insertion/extraction. The high performance gives 410 mAh/g for 300 cycles. Electrodes were prepared at selected
voltage points in the discharge-charge curves and high-energy synchrotron X-ray data was collected in order to study the reaction
mechanism in detail. Both Ag Sn and SbSn insert Li stepwise to form compounds with the partial compositions Ag2ϪxLi1ϩxSn
3
(
0 р x р 1) and Li2ϩxSn1ϪxSb (x р 1). At lower voltages, Li Sn , Li Sn , AgLi, and Ag Li could be observed. The
7 2 22 5 5 8
intermetallic compounds before and after lithiation have a strong crystallographic relation.
2004 The Electrochemical Society. ͓DOI: 10.1149/1.1827597͔ All rights reserved.
©
Manuscript submitted March 26, 2004; revised manuscript received June 8, 2004. Available electronically December 2, 2004.
In search for second-generation anode materials for Li-ion re-
tail, electrodes were prepared at certain voltage points in the charge-
discharge cycles, similar to an in situ experiment, and analyzed with
synchrotron X-ray diffraction ͑XRD͒.
chargeable batteries, it is important to investigate the reaction
mechanism of Li insertion and extraction during electrochemical
discharge and charge. The commercialized rechargeable batteries
have carbon anodes and a cathode of metal oxide ͑such as LiCoO ).
2
Experimental
Recently, the focus has turned to intermetallic compounds which
have higher theoretical capacities and energy densities, but so far the
cycle life has not been satisfying. The crystal structure is crucial for
the reversible battery performance and therefore more detailed in-
formation is highly desirable. The structural changes that occur
when metals and intermetallic compounds are lithiated have previ-
ously been discussed by Thackeray et al. and Wachtler et al. They
proposed that the strong crystallographic relationship between the
structures contributes to their improved battery performance despite
a large volume expansion.
Electrode preparation.—High-purity elemental powders of Ag
99.9 mass %͒ and Sn ͑99.9 mass %͒ ͑Fukuda Metal Foil and Pow-
͑
der Company, Limited, Japan͒ and Sb ͑99.9 mass %͒ were mixed
and mechanically alloyed during 2 h in a planetary mill ͑Fritsch
Pulverisette 05-101͒ to the composition of Ag36.4Sb15.6Sn48 . The
metallic test electrodes were prepared by mixing 85 wt % alloy
powders, 5 wt % carbon black, and 10 wt % polyvinylidene fluoride
PVdF͒ dissolved in N-methylpyrrolidone ͑NMP͒. The slurries of
the mixtures were coated on Cu foil substrate and dried in order to
remove the solvent. The average thickness of the electrodes was 11
1
2
͑
A lot of focus has turned to nanocomposite alloys and the most
␮
m. CR2032-type coin cells were constructed using the metallic
3
promising so far is the Ag-containing ones, such as Ag Sn/Sn,
3
electrodes, a polypropylene microporous separator, an electrolyte ͓1
M LiPF6 dissolved in a mixture of ethylene carbonate ͑EC͒ and
dimethyl carbonate ͑DMC 1:2 volume͔͒, and metallic lithium ͑0.5
mm thick foil͒ as negative counter electrode. Electrodes were pre-
pared at certain voltage points during the first cycles of discharge
and charge, as shown in Fig. 1. The cells were cycled between 0 and
4
2
Ag Sb, and AgSb/Sn alloys. Recently, Ag-Sn composite alloys
3
prepared by mechanical grinding were investigated as new nano-
3
composite anode materials for advanced lithium-ion batteries.
Composite electrodes with high Sn content contribute to a high ini-
tial capacity but a rapid capacity fading. For Ag-rich alloy electrodes
the capacity decreases due to the reduction of Sn content. Optimiz-
ing the composition to Ag Sn , a high capacity ͑350 mAh/g͒ and
2
0
V ͑vs. Li͒ with a constant charge and discharge current of
2
.2 mA/cm . The electrodes with the active material were removed
52
48
from the coin cells in a dry room and placed in thin films of poly-
long cycle life ͑300 cycles͒ were obtained. The reaction mechanism
5
was studied in detail by R o¨ nnebro et al., revealing how the start
sample consists of two phases, Ag Sn and Sn, that upon stepwise
3
lithiation form Ag2ϪxLi1ϩxSn (0 р x р 1) and Li Sn (y р 4.4),
y
respectively. In this present investigation we exchanged an appro-
priate amount of Ag for Sb, i.e., forming an Ag Sn/SnSb/Sn com-
3
posite alloy, in an attempt to improve the battery performance fur-
ther by forming not only Ag2ϪxLi1ϩxSn but also lithiating SbSn.
This reduces the presence of Li Sn phases, which has a high con-
y
tributing theoretical capacity but has been shown to be the reason
for insufficient cycle life due to a large increase in volume, leading
to a rapid decay in capacity. In the following we focus on the crystal
structures and roles of Ag2ϪxLi1ϩxSn and Li2ϩxSn1ϪxSb for improv-
ing the battery performance in the nanocomposite alloy
Ag36.4Sb15.6Sn48 . In order to clarify the reaction mechanism in de-
*
Electrochemical Society Active Member.
Present address: Sandia National Laboratories, Livermore, CA 94550.
E-mail: ecronne@sandia.gov
d
Figure 1. Discharge capacity of Ag36.4Sb15.6Sn48 composite electrode as a
function of cycle number.
z
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Journal of The Electrochemical Society, 152 ͑1͒ A152-A157 ͑2005͒
A153
Figure 2. The charge-discharge curves of the Ag36.4Sb15.6Sn48 composite
electrode in the first cycles.
imide ͑Kapton͒ for protection against decomposition. During trans-
portation to SPring-8 the samples were kept in heat-sealed alumi-
num bags.
XRD experiment.—XRD data was collected at the synchrotron
radiation facility SPring-8, Japan, at beam line BL19B2, which is a
medium-length hard X-ray bending magnet beam line designed for
engineering science research. A large Debye-Scherrer camera is in-
stalled at the second experimental hutch in order to perform accurate
structure analysis of powder diffraction patterns. The camera with a
Figure 4. XRD patterns of the Ag36.4Sb15.6Sn48 composite electrode during
charge in the first cycles.
radius of 286.5 mm has an imaging plate ͑IP͒ on the 2␪ arm as a
detector. The high-energy beam with high flux allows us to collect
much more Bragg reflections with high counting statistics from a
little amount of powder specimen. In the present experiment the
diffraction patterns were recorded in the 2␪ range 3-75° and the
pixel size of the IP was selected to 50 ␮m. The wavelength was
calibrated to ␭ ϭ 0.05 nm ͑room temperature͒ by using CeO as a
2
standard. The exposure time of X-rays was 300 s for each sample.
Results and Discussion
Structural analysis by the Rietveld method.—In conventional
XRD spectra, the atomic scattering factor for Li atoms is very low.
However, with high-energy synchrotron data it was possible here to
determine the atomic parameters for lithium in these high-symmetry
intermetallic compounds. We present the reaction mechanism con-
cluded from structural analysis with the Rietveld method using the
6
program FullProf. In the Rietveld method, which is a structure
refinement method, the least-squares refinements are carried out un-
til the best fit is obtained between the entire observed powder dif-
fraction pattern and the entire calculated pattern based on the simul-
taneously refined models for the crystal structures.7 In order to
evaluate the proceeding of the refinements, a set of numerical crite-
ria of fit, or R values, are reported after every least-squares refine-
ment. The Rwp best reflects the progress of the fitting of the calcu-
lated pattern to the observed pattern. The Bragg agreement index,
R , reflects the fit of the calculated crystal-structural model to the
observations. For single-phase, crystalline powder diffraction pat-
terns, R values below 10% reflect a satisfying structural model.
Figure 3. XRD patterns of the Ag36.4Sb15.6Sn48 composite electrode during
discharge in the first cycles. The Cu peaks from the substrate have been cut
off.
B
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A154
Journal of The Electrochemical Society, 152 ͑1͒ A152-A157 ͑2005͒
Table I. Summary of the crystallographic parameters for the unlithiated compounds at a discharge voltage of 1.687 V determined from Rietveld
refinements. Standard deviations from FullProf are given in parentheses. Content is calculated after normalizing the integrated intensity to the
strongest peak attributed to the composite material.
Volume/Z
(Å )
Content
͑%͒
3
Voltage
.687 V
Phase
Space group
Cell parameter ͑Å͒
1
Ag Sn
Pmmn
a ϭ 6.0121(7)
b ϭ 4.7950(4)
c ϭ 5.1802(6)
a ϭ 5.849(2)
a ϭ 4.3392(4)
74.3
51
3
Sn
SbSn
I4 /amd
27.2
58.3
11
37
1
¯
R3m
b ϭ 5.361(1)
Here, the composite alloy is in minority in the XRD patterns, and
the peaks from the lithiated phases have a more or less amorphous
character. However, with the high-quality SPring-8 data, it was pos-
sible to extract valuable structural information about the phases of
interest. Cu peaks from the Cu foil substrate constitutes about 80-
lattice parameters, and occupancy factors ͑for Ag2ϪxLi1ϩxSn,
Li2ϩxSn1ϪxSb). The parameters refined for the amorphous phases
were scale factors and lattice parameters. The background was de-
scribed by linear interpolation between adjacent points on the 2␪
scale.
85% of the total integrated intensity. The RB values were in the
Reaction mechanism.—The nanocomposite of interest,
Ag36.4Sb15.6Sn48 , is a new high-capacity alloy ͑410 mAh/g͒ for re-
versible Li-ion batteries and has a long cycle life of 300 cycles as
range of 10-15% for the more crystalline phases (Ag LiSn,
2
Li2ϩxSn1ϪxSb, AgLi, and Ag Li ) and 20-30% for the amorphous
5
8
phases (Li Sn, Li Sb, and AgLi Sn). The R values were about
y
3
2
wp
2
can be seen in Fig. 1. The loading mass is 2.5-3.0 mg/cm , that
10% for all the patterns. The structure of Cu was refined to RB
2
gives 1.2 mAh/cm . The electrolyte was a mixture of 1 M LiPF and
Ͻ 10%. However, it is also important to use graphical criteria of
fit, that is, obtaining an agreement between the plots of the observed
and calculated patterns. Because the lithiated phases are all of high
symmetry and the atoms are located in fixed positions, refinements
could after all be performed to a relatively satisfying level. The
parameters refined for the crystalline phases were scale factors, half-
width parameters ͑U,V,W͒, isotropic displacement parameters ͑B͒,
6
EC:DEC ͑1:1͒. The irreversible capacity loss ͑ICL͒ is illustrated by
representing curves of both Li insertion and Li extraction. There are
many factors contributing to ICL: oxide impurities ͑i.e., SnO2) that
are mainly formed during the mechanical alloying; the residual lithi-
ated phases in the host structures during cycling because of kinetic
and thermodynamic reasons and high contact resistance caused by
Table II. Summary of the crystallographic parameters for the lithiated phases during discharge in the 1st cycle of Ag36.4Sb15.6Sn48 determined
by the Rietveld method. Standard deviations from FullProf are given in parentheses. Contents are calculated after normalizing the integrated
intensity to the strongest peak attributed to the composite material.
Voltage
͑V͒
Phase
Space group
Cell parameter ͑Å͒
Volume/Z (Å3)
Content ͑%͒
0
.830
.672
Li2.6Sn0.4Sb
Ag1.6Li1.4Sn
Li2.6Sn0.4Sb
¯
a ϭ 6.596(1)
a ϭ 6.6227(6)
a ϭ 6.5922(9)
a ϭ 6.618(1)
a ϭ 6.589(2)
71.7
72.6
71.6
72.4
71.5
35
38
50
22
72
Fm3m
0
¯
Fm3m
¯
Fm3m
0
.405
.090
AgLi Sn
¯
Fm3m
¯
Fm3m
P6 /mmc
2
Li2.6Sn0.4Sb
␤
-Li Sb
a ϭ 4.705(9), c ϭ 8.25(3)
a ϭ 6.633(2)
79.1
72.9
4
28
3
3
0
AgLi Sn
¯
Fm3m
2
␣
␤
-Li Sb
¯
a ϭ 6.579(4)
71.3
15
3
Fm3m
-Li Sb
P6 /mmc
Pm3m
a ϭ 4.699(9), c ϭ 8.48(3)
a ϭ 3.1824(8)
80.1
32.2
221.6
17
17
6
3
3
AgLi
Ag Li
¯
a ϭ 9.610(8)
5
8
I43m
Li Sn
Cmmm
a ϭ 9.77(3), b ϭ 13.93(3)
c ϭ 4.668(9)
158.2
10
7
2
Li Sn
¯
a ϭ 19.72(2)
476.8
72.9
71.4
7
20
12
2
2
5
F43m
0
0
.052
AgLi Sn
¯
Fm3m
¯
Fm3m
a ϭ 6.630(2)
a ϭ 6.585(6)
2
␣
␤
-Li Sb
3
-Li Sb
P6 /mmc
a ϭ 4.70(1), c ϭ 8.37(4)
a ϭ 3.182(8)
80.2
32.2
222.0
9
23
10
3
3
AgLi
Pm3m
¯
I43m
¯
F43m
¯
Fm3m
¯
Fm3m
Ag Li
a ϭ 9.612(6)
5
8
Li Sn
a ϭ 19.67(2)
a ϭ 6.633(3)
a ϭ 6.595(4)
475.4
72.9
71.7
27
19
8
2
2
5
.0
AgLi Sn
2
␣
-Li Sb
3
␤
-Li Sb
P6 /mmc
¯
F43m
a ϭ 4.709(7), c ϭ 8.36(3)
a ϭ 19.692(10)
80.3
477.0
9
35
3
3
Li Sn
2
2
5
AgLi
Pm3m
¯
I43m
a ϭ 3.187(1)
a ϭ 9.620(3)
32.3
222.6
15
14
Ag Li
5
8
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Journal of The Electrochemical Society, 152 ͑1͒ A152-A157 ͑2005͒
A155
observed and calculated patterns from the Rietveld refinements are
shown in Fig. 5. For the following lithiated phases, the plots from
the Rietveld refinements are shown in Fig. 6. At 0.830 V ͑2͒ SbSn
has extruded Sn upon insertion of Li to form Li2.6Sn0.4Sb, which has
a face-centered cubic ͑fcc͒ structure smiliar to ␣-Li Sb ͑PDF no.
3
4
0
-0438͒. For the Ag Sn composite, Ag LiSn started to form at
52 48 2
.680 V, and thereafter the Li content increases to form
5
Ag2ϪxLi1ϩxSn (x р 1). Here, at 0.672 V ͑3͒, the ternary com-
pound Ag2ϪxLi1ϩxSn has inserted Li to the composition of
Ag1.6Li1.4Sn according to the Rietveld refinements. The XRD pat-
tern consists of Ag1.6Li1.4Sn, Li2.6Sn0.4Sb, and remainings of Ag Sn
3
and Sn. A small amount of Sb was unexpectedly observed which
Figure 5. Observed and calculated patterns from Rietveld refinement of the
unlithiated composite material, no. 1 during discharge.
might be attributed to nonequilibrium conditions. At 0.405 V ͑4͒,
more Li has been inserted to form the end-compound AgLi Sn, and
2
the XRD peaks belong to AgLi Sn and Li2.6Sn0.4Sb. A small amount
2
particle pulverization; the trapped Li/Liϩ inside defects and lattice
distortions in the alloy structures; and the decomposition of electro-
lyte and formation of solid-electrolyte interphase ͑SEI͒ films. Even
though the irreversible capacity loss is large at present, this material
is promising for further development. In Fig. 2, typical charge-
discharge curves during the 1st cycle are shown. In this section we
present the reaction mechanism for the nanocomposite anode mate-
rial Ag36.4Sb15.6Sn48 concluded from structural analysis of the high-
energy XRD data with the Rietveld method using the program
of ␣-Li3Sb has transformed into ␤-Li3Sb ͑hexagonal, PDF no.
4-0791͒. Moreover, the extruded Ag reacted with Li upon forming
AgLi. At 0.090 V ͑5͒, the strongest peaks belong to AgLi Sn and
2
Li Sb ͑␣ and ␤͒. The amount of AgLi has increased and also a small
3
amount has inserted more Li to form Ag Li . Binary Li Sn alloys
5
8
y
were observed for the first time, i.e., Li Sn and Li Sn as con-
7
2
22
5
cluded from the Rietveld refinements. At 0.052 V ͑6͒, the strongest
peaks belong to Li Sn , but also AgLi Sn and AgLi can be ob-
2
2
5
2
6
served. Moreover, AgLi is continuing to transform into Ag5Li8. At
.0 V ͑7͒, the sample consists of a large amount of binary materials:
Li Sn , AgLi, and Ag Li but also AgLi Sn, ␣-Li Sb and
FullProf. In the following section we discuss the structures in more
0
detail. The phase transformations during the first discharge-charge
cycles are shown in Fig. 3 and 4, in a 2␪ range of 6-20°, zooming in
the peaks from the composite Ag36.4Sb15.6Sn48 alloy upon Li
insertion/extraction, cutting off the Cu peaks. Structural data deter-
mined from the Rietveld refinements of the interesting phases found
during discharge in the lithiated Ag-Sb-Sn system are summarized in
Table I and II.
2
2
5
5
8
2
3
␤
-Li Sb. It is noteworthy that substantial amounts of AgLi Sn and
3
2
Li Sb remain at full discharge, unlike the Ag Sn nanocomposite
3
52
48
5
material where only Ag Li and Li Sn remained: as Li insertion
5
8
22
5
increased, AgLi2Sn disintegrated to form AgLiz and LiySn of low
densities and large unit cell volumes. The preservation of the flex-
ible structures of the ternary compounds, with higher densities and
smaller volume increase, is expected to benefit a longer cycle life.
The conclusions from the Rietveld refinements at selected volt-
age points during discharge can be summarized in three reaction
mechanisms as follows:
The initial composite consists of three intermetallic compounds:
Ag Sn, SbSn, and Sn. The phase transformations during the 1st
3
cycle of discharge, progressing toward lower voltages, are evalu-
ated. The voltage points can be found in Fig. 2. Hereafter, the volt-
age points are referred to with both voltage and number. At 1.687 V
͑
1͒, the XRD peaks belong to Ag Sn, SbSn, and Sn, and the plots of
The reaction mechanism for lithium insertion into Ag Sn—
3
3
Figure 6. Observed and calculated patterns from Rietveld refinement of the lithiated composite materials, no. 2-7 during discharge.
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A156
Journal of The Electrochemical Society, 152 ͑1͒ A152-A157 ͑2005͒
Table III. The atomic parameters for Li2.6Sn0.4Sb, ␣-Li Sb, and ␤-Li Sb. Standard deviations from FullProf are given in parentheses.
3
3
N ϭ normalized occupancy.
¯
Site
¯
Site
Li2.6Sn0.4Sb
Atom
Fm3m
␣-Li Sb
Atom
Fm3m
␤-Li Sb
Atom
P6 /mmc
3
3
3
N
N
Site
N
Sb
Li1
4a: 0 0 0
1.0
1.0
Sb
Li1
4a: 0 0 0
1.0
1.0
Sb
Li1
2c: 1/3 2/3 1/4
1.0
1.0
1
2
1
4
1
4
1
2
1
4
1
4
1
2
1
4
1
4
1
2
1
4
1
4
1
2
1
4
1
4
1
2
1
4
1
4
1
4
8
8
b:
c:
c:
4b:
8c:
8c:
2b: 0 0 4
Li2
Sn
0.74͑1͒
0.26͑1͒
Li2
Sn
0.5
0.5
Li2
4f: 1/3 2/3 0.583
1.0
1
2
3
4
5
6
. Ag Sn ϩ Li → Ag LiSn ϩ Ag ͑0.68 V͒
Sb randomly occupy the same site (6c;00z), forming a slightly
distorted cubic close packed ͑ccp͒ structure. According to our
knowledge, the atomic sites have previously not been reported in the
literature. Upon Li insertion, the Sb and Sn atoms have to rearrange
to form the fcc structure. Upon extrusion of Sn, the small Li ion
easily diffuses in between the Sb layers, finding interstitial space
between the former Sb/Sn layers. The crystal structure of
3
2
. Ag LiSn ϩ xLi → Ag Li Sn ϩ Ag; (0 Ͻ x Ͻ 1)
2
2Ϫx 1ϩx
. Ag Li Sn ϩ xLi → AgLi Sn ϩ Ag ͑0.40 V͒
2
Ϫx 1ϩx
2
. AgLi Sn ϩ yLi → Li Sn ϩ AgLi; (y р 4.4)
2
y
. Ag ϩ Li → AgLi (Ͻ0.1 V)
. AgLi ϩ Li → Ag Li (Ͻ0.1 V)
5
8
Into SbSn—
¯
Li2ϩxSn1ϪxSb can be described in Fm3m ͑no. 225͒. Because Sn and
7
8
9
. SbSn ϩ Li → Li2ϩxSn1ϪxSb ϩ Sn ͑0.83 V͒
Sb have similar atomic scattering factors, it was impossible to dis-
tinguish between them in the Rietveld refinements. However, from a
crystallographic point of view it is reasonable that the larger Sb
atoms are forming a rigid host structure, and Sn and Li are diffusing
in and out of the framework similar to the diffusion of Ag and Li in
the Ag2ϪxLi1ϩxSn structure. The Sb atoms are thus located in 4a;
. Li Sn Sb ϩ Li → ␣-Li Sb ϩ Sn ͑0.4 V͒
2
ϩx
1Ϫx
3
. ␣-Li Sb → ␤-Li Sb ͑0.4 V͒
3
3
And into Sn—
1
0. Sn ϩ yLi → Li Sn; (y р 4.4); Li Sn and Li Sn (Ͻ0.2 V)
y
7
2
22
5
During charging in the 1st cycle the reversed process was ob-
served. The XRD patterns can be found in Fig. 4. The observed and
calculated plots from the Rietveld refinements have similar appear-
ance to the corresponding voltages during discharge and are there-
fore not shown here. At 0.105 V ͑8͒ Li has not yet been extruded and
the XRD pattern thus consists of: AgLi Sn, ␣-Li Sb, ␤-Li Sb,
AgLi, Ag Li , and Li Sn . At 0.489 V ͑9͒, some Li has been
extruded from ␣-Li Sb to form Li2.6Sn0.4Sb. AgLi Sn and ␤-Li Sb
were also observed. At 0.804 V ͑10͒ the Li content has decreased
further to form Ag0.2Li1.8Sn, Li2.6Sn0.4Sb, and ␤-Li Sb. Also Sn and
Ag Sn were observed. At 1.081
Ag2ϪxLi1ϩxSn and Li2ϩxSn1ϪxSb are small and the amounts of
Ag Sn, Sn, and SbSn have therefore increased. Above 1.243 V
almost all Li has been extruded upon insertion of Ag and Sn to form
the end-compounds, i.e., Ag Sn and SbSn, and almost no
Ag2ϪxLi1ϩxSn and Li2ϩxSn1ϪxSb were observed, thus showing the
reversability of this composite electrode.
1
1 1
000. Further, one of the Li layers are located in 4b; 2 2 2 and a second
1 1 1
4 4
layer of Li is located in 8c; 4 together with a small amount of Sn.
Upon full Li insertion, the Sn atoms are extruded to form the end-
2
3
3
compound ␣-Li Sb. At lower voltages (Ͻ0.405 V) cubic ␣-Li Sb
3
3
5
8
22
5
is transformed to hexagonal ␤-Li Sb, which leads to a higher de-
3
3
2
3
gree of close packing, i.e., lower density. A hexagonal structure is
more close-packed with shorter bonding distances and therefore it is
preferrably formed. The atomic paramters can be found in Table III.
The reaction mechanism is illustrated in Fig. 7.
3
V
͑11͒, the amounts of
3
The structure of Ag Sn is orthorhombic with alternating layers of
3
1
2,13
3
Ag and Sn. Upon Li insertion into Ag Sn, the ternary compound
3
Ag2ϪxLi1ϩxSn (0 р x р 1) is formed at 0.7 V. In the literature
3
three different space groups for Ag LiSn and AgLi Sn have been
2
2
8,9
but R o¨ nnebro et al.5
¯
¯
¯
suggested, i.e., Fm3m, F43m, and Fd3m,
could give a definite answer; Ag LiSn can best be described in
2
Structural considerations.—Li insertion into SbSn was previ-
ously described by Thackeray et al. to simply be an exchange of Sn
for Li in the host matrix of Sb atoms, presuming that SnSb and
Li Sb have cubic fcc structures. We could here conclude the reaction
¯
¯
F43m and AgLi Sn in Fm3m when formed during electrochemical
2
1
lithiation. Previously it was suggested that Ag LiSn and AgLi Sn
coexist, but here refinement of Ag LiSn and AgLi Sn as coexisting
alloys did not give a good calculated representation of the observed
pattern, and it was concluded that inhomogenous compositions of
Ag2ϪxLi1ϩxSn (0 р x р 1) exist between these two end com-
2
2
3
2
2
3
mechanism in detail. SbSn can be described in the trigonal space
¯
group R3m ͑no. 166͒ with hexagonal axes ͑PDF 33-0118͒. Sn and
¯
pounds. The crystal structure of Ag LiSn can be described in F43m
2
͑no. 216͒ with the atoms distributed between four different sites: Ag
in 4a and 4b, Li in 4c, and Sn in 4d. During discharge the Li
Figure 7. The structures of the unit cells of SbSn and cubic Li2ϩxSn1ϪxSb
(
(
␣-Li Sb) upon Li insertion and transformation into hexagonal Li Sb
3 3
␤-Li Sb). Dark gray atoms in site 4a contain Sb, light gray atoms in site
Figure 8. The structures of the unit cells of Ag Sn, Ag LiSn, and AgLi Sn
3 2 2
3
8
c contain Li and Sn, white atoms in site 4b contain Li.
upon lithium insertion.
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Journal of The Electrochemical Society, 152 ͑1͒ A152-A157 ͑2005͒
A157
Table IV. The atomic parameters for Ag LiSn, Ag1.4Li1.6Sn, and AgLi Sn. Standard deviations from FullProf are given in parentheses.
2
2
N ϭ normalized occupancy.
¯
Site
¯
Site
¯
Site
Ag LiSn
Atom
F43m
Ag1.4Li1.6Sn
Atom
Fm3m
AgLi Sn
Atom
Fm3m
2
2
N
N
N
Ag1
Ag2
4a: 0 0 0
1.0
1.0
Ag1
Li1
4a: 0 0 0
1.0
1.0
Ag
Li1
4a: 0 0 0
1.0
1.0
1
2
1
1
2
1
4
3
4
1
2
1
4
3
4
1
2
1
1
2
1
1
2
1
4
1
4
1
4
1
2
1
4
1
4
1
2
1
4
1
4
1
2
1
4
1
4
4
4
4
b:
4b:
4b:
8c:
8c:
Li
1.0
1.0
Ag2
Li2
Sn
0.46͑2͒
0.34͑2͒
0.5
Li2
Sn
1.0
1.0
c: 4
8c: 4 4
Sn
3
4
1 1
d:
8c: 4 4
1
1
4
8
c: 4
upon displacement of Sb and Sn and also extrusion of Sn forming
fcc Li2ϩxSn1ϪxSb (x р 1). At lower voltages the end compound
content increases until the end compound AgLi Sn forms at 0.4 V.
The inhomogeneous composition Ag2ϪxLi1ϩxSn can be better de-
2
Li Sb forms, which further transforms into hexagonal Li Sb with
¯
3
3
scribed in Fm3m ͑no. 225͒ with Ag, Li, and Sn distributed between
shorter bonding distances and therefore lower density. At 0.68 V,
three different sites: Ag in 4a and 8c, Li in 4b and 8c, and Sn in
upon extrusion of Ag in Ag Sn, Li diffuses in between the layers of
3
8
c. Thus, the disorder increases upon Li insertion, also leading to
the remaining Ag, thus forming Ag2ϪxLi1ϩxSn (0 р x р 1). At
diffraction peaks with amorphous character. The atomic parameters
can be found in Table IV. The reaction mechanism is illustrated in
Fig. 8. The fcc-structures of Ag2ϪxLi1ϩxSn and Li2ϩxSn1ϪxSb seem
to be flexible and allow the smaller ions to easily diffuse in and out
of a more rigid framework of the bigger atoms.
lower voltages also binary Li Sn (y р 4.4) and AgLi (z р 1.6)
y
z
compounds form. Upon charging, the reversed process occurs, ex-
truding Li from AgLi Sn and Li Sn to form Ag2ϪxLi1ϩxSn and Sn.
2
y
Finally, Ag is inserted upon extrusion of Li, thus forming Ag Sn.
3
At lower voltages, Sn reacts with Li to form binary compounds
Morover, Li is extruded from ␣-Li3Sb and ␤-Li3Sb forming
Li2ϩxSn1ϪxSb, and upon full Li extrusion, SbSn is formed upon
insertion of Sn in the Sb-host structure.
of Li Sn (y р 4.4). Below 0.2 V both Li Sn and Li Sn with
y
7
2
22
5
amorphous character were observed. There are at least three differ-
ent compositions proposed for ‘‘Li4.4Sn’’ in the literature. The com-
position of this structure was first given as Li Sn in space group
Acknowledgment
2
2
5
F23 by Gladyeshevskii et al.10 Nesper et al. proposed Li Sn of
11
2
1
5
This work was financially supported by the New Energy and
Industrial Technology Development Organization ͑NEDO͒ and Ja-
pan Synchrotron Radiation Research Institute ͑JASRI͒.
1
2
¯
Li Si structure type in space group F43m. Both Goward et al.
and Lupu et al. determined the composition to Li Sn in space
2
1
5
13
1
7
4
¯
group F43m from single-crystal and neutron powder diffraction. In
The National Institute of Advanced Industrial Science and Technology
assisted in meeting the publication costs of this article.
the present investigation, it was unfortunately impossible to distin-
¯
guish between Li Sn (F23) and Li Sn (Fm3m) in the Rietveld
22
5
17
4
References
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1
2
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22
5
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During discharge, the extruded Ag reacts with Li to form AgLi at
lower voltages (Ͻ0.2 V) which thereafter inserts more Li to form
Ag Li . AgLi has a primitive ccp structure of ClCs-type in space
4
5
8
¯
group Pm3m ͑PDF no. 04-0805͒. Upon further Li insertion, the
atoms have to diffuse to make room for more Ag and Li atoms. The
structure is thereupon rearranged to form an I-centered cubic-close
packing for Ag Li8 with Cu Zn8 structure type in space group
6
7
8
5
5
1
60 ͑1969͒.
¯
I43m.
9. H. Pauly, A. Weiss, and H. Witte, Z. Metallkd., 59, 47 ͑1968͒.
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9
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2 ͑2001͒.
Conclusion
1
In summary, we could conclude from the present high-resolution
XRD data how the mechanism in the Ag36.4Sb15.6Sn48 nanocompos-
8
1
3. C. Lupu, J-G. Mao, J. W. Rabalais, and A. M. Guloy, Inorg. Chem., 42, 3765
͑2003͒.
ite material proceeds via sequential Li insertion into Ag Sn, SbSn,
3
and Sn. At 0.83 V, Li is inserted into the Sb-host structure in SbSn
14. J. R. Dahn, I. A. Courtney, and O. Mao, Solid State Ionics, 111, 289 ͑1998͒.
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