Electrodeposited tin coating as negative electrode material for lithium-ion battery in room temperature molten salt

Article abstract of DOI:10.1149/1.1448501

A new room temperature molten salt (RTMS) [1-methyl-3-ethylimidazolium/AlCl₃/SnCl₂ (3:2:0.5)] was developed for depositing tin on a copper electrode. Different tin crystallites were deposited at different temperatures, giving widely different performances of the assembled lithium cell [Sn (Cu)/LiCl buffered MEICl-AlCl₃ RTMS/lithium]. Tin film deposited at 50°C or higher gave a more desirable crystal structure and an improved performance than films obtained at lower temperatures. Both cyclic voltammetry and galvanostatic cycling show the formation of three major lithium-tin alloy phases corresponding to the phase transition of LiSn/Li₇Sn₃, Li₁₃Sn₅/Li₇Sn₂, and Li₇Sn₂/Li₂₂Sn₅. Increases in the charging and discharging capacities were found with the deposition of higher lithium-rich tin alloys, though at the degradation of the irreversible capacity at the first cycle. The discharging capacity decreased rapidly, producing loose, expanded, and irregular crystallites upon cycling at a high current density (cd) (1.0 mA/cm²). However, an average capacity of 140 mAh/g, coulombic efficiency around 85%, and more than 200 cycles were obtained at a low cd (0.4 mA/cm²). The improvement is attributed to the deposition of small and regular tin crystallites that allows reversible insertion and removal of lithium from a more stable crystal structure without a significant volume change during cycling.

Full text of DOI:10.1149/1.1448501

Electrodeposited Tin Coating as Negative Electrode Material
for Lithium-Ion Battery in Room Temperature Molten Salt
Y. S. Fung and D. R. Zhu
J. Electrochem. Soc. 2002, Volume 149, Issue 3, Pages A319-A324.
doi: 10.1149/1.1448501
Receive free email alerts when new articles cite this article - sign up in the
box at the top right corner of the article or click here
Email alerting
service
To subscribe to Journal of The Electrochemical Society go to:
http://jes.ecsdl.org/subscriptions
© 2002 ECS - The Electrochemical Society
Downloaded on 2012-10-17 to IP 128.143.23.241 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

Journal of The Electrochemical Society, 149 ͑3͒ A319-A324 ͑2002͒
A319
0013-4651/2002/149͑3͒/A319/6/$7.00 © The Electrochemical Society, Inc.
Electrodeposited Tin Coating as Negative Electrode
Material for Lithium-Ion Battery in Room Temperature
Molten Salt
,z
*
Y. S. Fung and D. R. Zhu
Department of Chemistry, The University of Hong Kong, Hong Kong SAR, China
A new room temperature molten salt ͑RTMS͒ ͓1-methyl-3-ethylimidazolium/AlCl3 /SnCl2 ͑3:2:0.5͔͒ was developed for depositing
tin on a copper electrode. Different tin crystallites were deposited at different temperatures, giving widely different performances
of the assembled lithium cell ͓Sn ͑Cu͒/LiCl buffered MEICl-AlCl3 RTMS/lithium͔. Tin film deposited at 50°C or higher gave a
more desirable crystal structure and an improved performance than films obtained at lower temperatures. Both cyclic voltammetry
and galvanostatic cycling show the formation of three major lithium-tin alloy phases corresponding to the phase transition of
LiSn/Li7Sn3 , Li13Sn5 /Li7Sn2 , and Li7Sn2 /Li22Sn5 . Increases in the charging and discharging capacities were found with the
deposition of higher lithium-rich tin alloys, though at the degradation of the irreversible capacity at the first cycle. The discharging
capacity decreased rapidly, producing loose, expanded, and irregular crystallites upon cycling at a high current density ͑cd͒
(1.0 mA/cm2). However, an average capacity of 140 mAh/g, coulombic efficiency around 85%, and more than 200 cycles were
obtained at a low cd (0.4 mA/cm2). The improvement is attributed to the deposition of small and regular tin crystallites that allows
reversible insertion and removal of lithium from a more stable crystal structure without a significant volume change during
cycling.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1448501͔ All rights reserved.
Manuscript submitted June 11, 2001; revised manuscript received October 16, 2001. Available electronically February 5, 2002.
Room temperature molten salts ͑RTMS͒ based on 1-methyl-3-
ethylimidazolium chloride ͑MEICl͒ have been shown to provide an
interesting ionic medium for battery applications and for alloy
electrodeposition.1-6 Since the successful introduction of a high con-
centration of lithium ion into the melts7-11 by adding an excess
amount of solid LiCl into the Lewis acidic AlCl3-MEICl melts to
form buffered neutral molten salts, the pace of development of the
MEICl system for the lithium-ion battery has been increased re-
cently. This is attributed to several desirable features of the system
as an electrolyte for a lithium battery such as nonflammability, im-
proved lithium conductivity, and a wide electrochemical window.7-9
Materials intercalated with lithium ions such as LiCoO2 ,
LiMn2O4 , and LiNiO2 have been used successfully as positive elec-
trode materials in RTMS in previous studies.10,11 However, a suit-
able negative electrode material for the lithium battery is more dif-
ficult to find in RTMS. Although carbonaceous materials had been
widely investigated as intercalation anodes for lithium-ion second-
ary battery in the organic electrolyte system, their application as
anode material in RTMS led to a very serious exfoliation problem
due to the irreversible cointercalation of other ions present in the
melt that reduced the subsequent performance of the anode during
cycling. Moreover, as the potential of the lithium-intercalated carbon
electrode is within 10 mV of that of the lithium metal, concerns have
been raised about the safety of the negative electrode. Thus, metals
capable of alloying reversibly with lithium were studied as the nega-
tive electrode material in RTMS. The most common metal chosen is
aluminum which had been used as the negative electrode in our
previous studies.10,11 The problem with aluminum was the rather
large volume expansion during alloying with lithium that led to the
breakdown of the electrode structure upon repeated cycling.10,11
Recently, attention has been given to lithium-tin alloys or oxides
that utilized the reversible reaction of lithium with tin because they
offered high capacity, low capacity fade, and good safety
properties.12-14 A high irreversible capacity was observed during the
first cycle in the use of the tin oxide negative electrode, and this
precluded its commercial use.15 To mitigate the irreversible capacity
associated with tin oxides, Metallic electrodes such as tin alloys or
other tin composites16,17 have been employed directly as the nega-
tive electrode. The metallic tin electrode offered a promising anode
material for lithium batteries due to its good mechanical properties.
Thus, it was selected for studying its performance in RTMS for
lithium battery application in the present work.
As different forms of tin metal had been shown to exhibit a
strong effect on their electrochemical performance in lithium
battery,18 methods for the fabrication of the tin working electrode
were investigated to obtain tin deposits of suitable forms for battery
operation in RTMS. The procedure for the electrochemical deposi-
tion of a thin film of tin on the copper substrate as a current collector
was adopted in the present study to develop an anode with a high
current capacity. As RTMS was used as the medium for the devel-
opment of the lithium battery in the present study, it was chosen as
the electrolyte for the deposition of a tin film on the copper sub-
strate, followed by a detailed study of its electrochemical and cy-
cling behavior in RTMS.
In this paper, the development of a novel procedure for deposit-
ing a suitable tin film in RTMS for lithium-ion battery application is
given. Factors affecting the quality of the tin film deposit have been
investigated, and the optimized working conditions studied. Results
on the electrochemical performance of the tin film electrode for
lithium battery application are presented and discussed. The perfor-
mance of a secondary lithium battery in RTMS incorporating the
thin tin film as one of the electrodes is reported and its suitability as
anode material discussed.
Experimental
Room temperature molten salts.—The MEICl was synthesized as
previously described.1-7 The MEICl/AlCl3 melts ͑RTMS͒ was pre-
pared by slowly adding a calculated amount of AlCl3 ͑Aldrich,
99.99%͒ to the MEICl in a glove box under a positive pressure of
argon. The SnCl2 and LiCl were dehydrated under vacuum at 180°C
for 10 h prior to use. The electrolyte for the electrodeposition of Sn
was prepared by adding calculated amounts of SnCl2 to the basic
RTMS with a mole ratio of 3:2. All compositions of RTMS in this
paper are expressed as mole ratios unless indicated otherwise. The
LiCl buffered neutral RTMS was prepared by adding a calculated
amount of solid LiCl into the acidic RTMS to form a melt with mole
ratios of 1:1.2:0.2 and stirred for several days until all the LiCl had
dissolved. Then about 5% of the excess buffering solid LiCl was
added to the melts to produce the buffered neutral melts. The elec-
trodeposition of Sn was conducted in a RTMS consisting of
MEICl/AlCl3 /SnCl2 in mole ratios of 3:2:0.5, respectively.
* Electrochemical Society Active Member.
z E-mail: ysfung@hkucc.hku.hk
Downloaded on 2012-10-17 to IP 128.143.23.241 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

A320
Journal of The Electrochemical Society, 149 ͑3͒ A319-A324 ͑2002͒
Instrumentation.—All electrochemical experiments were per-
formed by an EG&G software-controlled ͑EG&G model 270͒ PAR
model 263 potentiostat/galvanostat and conducted inside an argon-
filled glove box. The specific conductivity was measured by the
EG&G model 5210 lock-in amplifier and 398 software at 100 kHz
using a DJS-1 conductivity electrode. The Cambridge model 360
Stereoscan scanning electron microscope ͑SEM͒ and the Perkin
Elmer model 3110 atomic absorption spectrometer ͑AAS͒ were used
for analysis of the morphology and chemical composition of the tin
deposit on the copper electrode. The charging-discharging experi-
ments were conducted by a homemade constant current device con-
trolled by a microcomputer with software written in the Basic pro-
gram.
Cells and electrodes.—All electrochemical experiments were
conducted in an electrochemical cell using an Al wire in the acidic
RTMS (AlCl3 :MEICl ϭ 66.7:33.3) as the reference electrode and
an aluminum foil (6 ϫ 5 mm) as the counter electrode. Copper foil
͑Aldrich, 0.127 mm thick, 99%͒ was used as the working electrode
with one side of the surface covered by a Teflon adhesive tape so as
to expose a known area (6 ϫ 5 mm) to the RTMS. The electro-
chemical cell was placed on a copper furnace inside the glove box
with a controlled temperature from 22 to 120°C by a Eurotherm
model 1428 temperature controller.
The copper working electrode was polished using abrasive sili-
con carbide papers P 220, P 1000, and P 1600 in sequence before
washing with acetone and drying in vacuum prior to use. After coat-
ing a thin film of Sn without stirring under the selected potential or
constant current in the MEICl/AlCl3 /SnCl2 , the copper electrodes
were rinsed immediately in acetonitrile and dried in the dry box
directly. It was then assembled as a negative electrode in the lithium
cell, Sn͑Li͒/LiCl buffered RTMS/Lithium foil, for performing the
cycling experiments.
Figure 1. The variation of the specific conductivity of the ternary molten
salts MEICl/AlCl3 /SnCl2(3:2:x) on the mole fraction x of SnCl2 at differ-
ent temperatures.
composition of the ternary melts at mole fractions of 3:2:0.5. The
conductivity is in general increased with the mole fraction of SnCl2
until 0.5 and gradually declined at higher mole fractions. This is in
agreement with the previous observation that the highest specific
conductivity was obtained at neutral melts.19 Thus, a ternary melt of
MEICl/AlCl3 /SnCl2 ͑3:2:0.5͒ was selected as the medium for the
electrodeposition of tin.
The widely used copper foil was chosen as the substrate for the
electrodeposition of tin due to its availability, good electric conduc-
tance, and inertness for alloying with tin. The electrochemical be-
havior of the copper electrode in MEICl/AlCl3 /SnCl2 ͑3:2:0.5͒ was
studied using cyclic voltammetry ͑CV͒. The cyclic voltammogram
shown in Fig. 2 exhibits a single reduction wave near Ϫ1.0 V fol-
lowed by a broad oxidation wave at Ϫ0.65 V. The reduction current
starts to increase from the baseline at Ϫ0.72 V, and when the copper
electrode was controlled at a value more negative than this potential,
a gray film was found to appear at the copper substrate. Results of
XRD analysis of the film indicated the existence of tin metal by
Chemical composition analysis.—After the deposition of Sn, the
copper electrodes were immediately rinsed thoroughly in acetoni-
trile and dried in the dry box. The morphology of the deposits was
investigated using the SEM and their chemical composition deter-
mined by the energy dispersive X-ray ͑EDX͒ microprobe. The total
amount of Sn in the coating was determined by dissolving the de-
posits in 10 mL 1% HNO3 solution prior to chemical analysis using
an AAS.
showing two peaks at diffraction angles ͑2␪͒ of 30.6 and 32.0°.20
Thus, the reduction current starting at Ϫ0.72 V was attributed to the
reduction of Sn͑II͒ to tin metal. It was interesting to note that a
Results and Discussion
Deposition of a thin film of tin on copper in RTMS.—The RTMS
system based on MEICl/AlCl3 provides a suitable medium for the
electrodeposition of tin, as its amphoteric property can be utilized to
assist its dissolution in RTMS by adjusting the acidity of the RTMS
via the acidic AlCl3 and basic MEICl interaction. When SnCl2 or
other acidic transition metal chloride is added to the basic melts, it
can undergo the following coordination reaction with the free ClϪ
present in RTMS as shown in
pϪ
SnCl2 ϩ pClϪ
→
SnCl
͓1͔
͓
͔
pϩ2
Thus, SnCl2 can be dissolved into the ternary
MEICl/AlCl3 /SnCl2 melt by adding SnCl2 to the basic binary melt
MEICl/AlCl3 with the excess free ClϪ coming from MEICl. A con-
siderable amount of SnCl2 were found to be incorporated in the
ternary melt using this procedure. To provide a suitable medium for
the electrodeposition of tin, the ternary melt must contain a high
concentration of SnCl2 and show good conductivity at a suitable
temperature. The basic molten salt MEICl/AlCl3 with a mole ratio
of 3:2 was chosen as the base melt for the incorporation of SnCl2 to
form the ternary melt. The variation of the specific conductivity
of the melts at different mole fractions of SnCl2 in the 3:2
MEICl/AlCl3 melt is shown in Fig. 1. The lowest conductivity had
been observed at 5 mS/cm for the binary melt at 22°C and the
highest conductivity at 43 mS/cm was obtained near the neutral
Figure 2. Cyclic voltammogram for
MEICl/AlCl3 /SnCl2(3:2:0.5) melts at a scan rate of 20 mV/s. Temperature:
50°C.
a
copper electrode in the
Downloaded on 2012-10-17 to IP 128.143.23.241 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

Journal of The Electrochemical Society, 149 ͑3͒ A319-A324 ͑2002͒
A321
Figure 4. The effect of the deposition time at a constant current density of
3.3 mA/cm2 on the coulombic efficiency for the electrodeposition of tin in
RTMS at 50°C.
of the formation of large, regular crystallites, temperature at 50°C
was selected for the deposition of tin on copper to produce the tin
electrode for subsequent studies.
As the thickness of the tin film was controlled by the deposition
time, i.e., the total coulombs passed during electrodeposition, the
effect of the deposition time on the coulombic efficiency was stud-
ied in RTMS at 50°C under a constant current density of
3.3 mA/cm2 with an average deposition potential at about Ϫ0.85 V.
The coulombic efficiency for Sn deposition was calculated as
Figure 3. SEM micrographs of tin film deposited at different temperatures at
a constant current density of 3.3 mA/cm2.
Q
Sn /QTotal ϫ 100%. QSn was the charge estimated from the depos-
ited mass of Sn by the results of AAS analysis, and QTotal was the
total quantity of charge passed during the electrodeposition. The
results are shown in Fig. 4. The coulombic efficiency was kept
larger than 90% up to a deposition time of 120 s or 400 mC/cm2 and
decreased rapidly when more than 400 mC/cm2 were deposited. A
lot of dendrites were observed when more coulombs, due to tin
deposition, were passed. Thus, a deposition time of 120 s at
3.3 mA/cm2 was chosen for the preparation of the tin film electrode
for subsequent studies.
marked increase in the reduction current was observed at potentials
more negative than Ϫ0.82 V. Moreover, dendrites were found to
deposit easily near the edge of the substrate when the potential was
kept more negative than Ϫ0.9 V. Thus, it can be concluded that the
marked increase in current was caused by a rapid increase in the
electrode area at a negative potential as the result of a dendritic
outgrowth of the tin deposits. To avoid uncontrollable deposition
and the formation of an unstable electrode structure, the deposition
potential was set to be more positive than Ϫ0.9 V to prevent the
formation of dendrites. Therefore, a constant current density of
3.3 mA/cm2 was chosen to keep the average deposition potential
near Ϫ0.85 V.
The effect of temperature on the deposition of tin on the copper
electrode in the MEICl/AlCl3 /SnCl2 ͑3:2:0.5͒ melt was investigated
using a constant current density of 3.3 mA/cm2 at temperatures
ranging from 22 to 70°C. SEM was used to study the particle size
and surface morphologies of the deposited Sn film, and the results
are shown in Fig. 3. At low deposition temperatures ͑22 and 35°C͒,
dendritic and large elongated tin crystallites were found. As the tem-
perature was increased, the deposited tin crystals became more regu-
lar. The grain size of the Sn crystallites was found to decrease at a
higher temperature, showing an average size of 4 ␮m at 50°C and 2
␮m at 70°C. The distribution of the Sn crystallites at the copper
electrode was found to be homogeneous as shown by the dot map-
ping and results of the EDX elemental analysis of tin. The results
indicate that the morphology of the tin crystals is highly dependent
on the temperature of the melt during electrodeposition, and the
electrochemical performance of the thin film tin electrode is highly
affected by its morphology. Controlling the melt at a temperature of
50°C during tin deposition was found to produce a more uniform
particle distribution and round-shaped crystallites that have been
found suitable for the insertion of lithium as shown by results given
in the next section. However, too high a temperature could lead to
the formation of smaller, though still regular, crystallites. This may
be the balancing effect of crystal growth against nucleation. In view
Deposition of lithium on tin film electrode in RTMS.—To study
the electrochemical insertion of lithium into the tin film electrode, a
tin film electrode was prepared by procedures as described in the
previous section. After Sn deposition, the electrode was rinsed rig-
orously by anhydrous acetonitrile and dried for a few minutes in the
dry box before use as the negative electrode for the RTMS lithium
battery. It was then used directly for the electrochemical
investigation in the LiCl buffered neutral molten salts
(MEICl:AlCl3 :LiCl ϭ 1:1.2:0.2) that was used as the electrolyte
for battery application in previous studies.10,11 First, CV was used to
study the kinetics for the deposition of lithium on a pure copper
electrode in LiCl buffered RTMS with results shown in Fig. 5. For
lithium deposition at the copper electrode, a large cathodic peak was
found to appear at a potential of Ϫ2.0 V followed by a couple of
small anodic peaks during stripping at the first scan ͑Fig. 5͒. At the
second scan, the cathodic current was greatly suppressed, and the
stripping peaks almost disappeared. A black film was found at the
surface of the copper electrode after several scans. Results of the
EDX microanalysis of the film showed the existence of a Cl element
in addition to the copper peak. This indicates that lithium has been
deposited during the first CV scan with a low stripping efficiency. It
is possible that the highly reactive lithium deposit could react with
components in the melt to form a passivated LiCl film that prevents
further lithium deposition in subsequent CV scans.
The cyclic voltammograms for the Sn film electrode were found
to be highly dependent on the temperature for the deposition of the
tin film. For a tin film deposited at 22°C, its CV scan in the LiCl
Downloaded on 2012-10-17 to IP 128.143.23.241 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

A322
Journal of The Electrochemical Society, 149 ͑3͒ A319-A324 ͑2002͒
Figure 7. Cyclic voltammograms for tin film electrodes in the LiCl buffered
RTMS at a scan rate of 20 mV/s. Tin films were plated at 50°C. Switching
potentials are: ͑a͒ Ϫ1.62, ͑b͒ Ϫ2.1, and ͑c͒ Ϫ2.3 V. Temperature: 22°C.
Figure 5. Cyclic voltammogram of a copper electrode in the LiCl buffered
RTMS at a scan rate of 20 mV/s. The second scanning was conducted after
a rest time of 1 min. Temperature: 22°C.
deposition at 22°C, the tin deposits were dendritic in appearance,
and the crystallites produced could not form Li/Sn alloys during
lithium deposition, possibly due to the restriction in the crystal lat-
tice. Thus, the tin film electrode should be formed at 50°C, if it is
going to be used as a negative electrode in lithium-ion battery and
the high temperature Sn electrodes were used in subsequent cycling
studies.
buffered RTMS electrolyte is shown in Fig. 6. Only a rising current
for cathodic deposition was observed without any trace of stripping
current during the reverse scan. However, when the deposition tem-
perature was increased to 50°C, stripping peaks were found to ap-
pear in the voltammograms as shown in Fig. 7. When the scan
potential was reversed at Ϫ1.65 V, a deposition peak appeared at
Ϫ1.55 V together with a stripping peak at Ϫ1.3 V. When the
switching potential was further extended to Ϫ2.1 V, two more depo-
sition peaks appeared at Ϫ1.9 and Ϫ2.0 V in addition to the existing
deposition peak at Ϫ1.65 V. Three corresponding stripping peaks
were found at Ϫ1.5, Ϫ1.4, and Ϫ1.3 V. The results indicated the
formation of three different types of lithium-tin alloys corresponding
to the three couples of deposition and stripping peaks. This was in
agreement with results reported in the literature21 on the phase dia-
gram of the lithium-tin system that showed three major lithium-
tin alloy phases corresponding to the two-phase transition for
LiSn/Li7Sn3 , Li13Sn5 /Li7Sn2 , and Li7Sn2 /Li22Sn5 among the six
intermediate phases.
Cycling study of a RTMS cell with a Sn film negative
electrode.—The first five galvanostatic cycles of a RTMS cell
͑Sn͑Li͒/RTMS (MEICl:AlCl3 :LiCl ϭ 1:1.2:0.2)/Li) at room tem-
perature ͑22°C͒ are shown in Fig. 8. The cycling was controlled at
cell voltage limits between 0.1 and 1.0 V vs. lithium. The potentials
for the first cycle show plateaus at 0.38, 0.23, and 0.15 V in the
charging ͑or lithium deposition͒ curve corresponding to the three
major peaks for the two-phase regions as observed in the CV
voltammogram ͑Fig. 7͒. The subsequent cycles showed a reduced
The reason for the appearance of Li/Sn alloys only on Sn crys-
tallites produced at high temperature may due to the difference of
the Sn crystals formed. Bigger and more regular tin crystallites de-
posited on the copper electrode at temperatures higher than 50°C
allowed the diffusion of lithium into its crystal lattice to form vari-
ous Li-Sn alloys as shown by the results given in Fig. 3. For the
Figure 6. Cyclic voltammogram for a tin film electrode deposited at 22°C in
the LiCl buffered RTMS at a scan rate of 20 mV/s. Temperature: 22°C.
Figure 8. The first five galvanostatic cycles under a constant cd at
0.4 mA/cm2 for a RTMS cell ͓Sn͑Cu͒/LiCl buffered RTMS/lithium foil͔.
Downloaded on 2012-10-17 to IP 128.143.23.241 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

Journal of The Electrochemical Society, 149 ͑3͒ A319-A324 ͑2002͒
A323
Table I. The irreversible capacity obtained in the first cycle for the LixSnÕRTMSÕLi cells charging to different depths to deposit various
lithium-tin alloys.
a
First cycle capacity ͑mAh/g͒
Irreversible capacity, Qirrev.
Cell
LixSnb
Charging
Discharging
͑mAh/g͒
% Qirrev.
1
2
3
4
LiSn
225
541
790
993
160
271
289
305
65
270
501
688
28.9
49.9
63.4
69.3
Li2.4Sn
Li3.5Sn
Li4.4Sn
a Charging-discharging current density ϭ 0.4 mA/cm2.
b Deposited at constant current charging to different depths in the first cycle to form specified lithium-tin alloys according to the coulombic calculation.
charging/discharging curve ͑Fig. 8͒ as compared to the first cycle,
indicating that lithium was taken up during the initial development
of the lithium-tin alloy electrode during cycling. The loss of capacity
during cycling may due to the following reasons: ͑i͒ corrosion of the
reactive alloys with impurities present in the melt, ͑ii͒ some lithium
metals are needed in order to develop the electrode initially by al-
loying with tin, and ͑iii͒ there may be side reactions occurring at the
surface of the tin electrode.
upon cycling. Moreover, the tin alloy crystallites were found to ex-
pand and became irregular and loose when charging at a high cd of
1.0 mA/cm2 for 20 cycles as shown by the SEM micrographs of the
spent electrode ͑Fig. 10͒. However, when cycling at a low cd of
0.4 mA/cm2, no obvious change of the morphology of the tin crys-
tallites were obtained after 20 cycles, giving a more intact and regu-
lar crystal structure as shown in the SEM micrograph after 20
cycles. The difference in the crystal structures may account for the
large difference observed in the rapid degradation in capacity when
charging at a high cd ͑Fig. 9͒. Thus, the charging cd is recom-
mended to be kept below 0.4 mA/cm2.
In summary, the tin film electrode offers a better cycling perfor-
mance at a low cd as compared to other metal alloy electrodes such
as lithium-aluminum electrode that showed an appreciable problem
of volume changing upon cycling.10 This may be due to the special
texture of the tin deposit film as shown in Fig. 3 that allows suffi-
cient space between the tin crystal for the incorporation of lithium,
To investigate the effect of the appearance of different phases of
Li-Sn at the electrode surface on the development of the lithium-tin
alloy electrode, the first cycle charging was performed at different
depths corresponding to the appearance of selected Li-Sn phases by
coulombic calculation. The results are shown in Table I. The charg-
ing and discharging capacity were calculated as the total amount of
coulombs delivered to or extracted from the tin film electrode during
the formation of the Sn-Li alloy electrode. In general, both the
charging and discharging capacities were found to increase with the
deposition of Li-Sn alloys with higher lithium activities. However,
the irreversible capacity at the first cycle is also increased at the
same time, indicating that more lithium is needed for the develop-
ment of the tin film electrode for initial cycling at a higher voltage.
As a higher cell voltage is desired for the lithium battery, cycling
up to the formation of Li22Sn5 was investigated for extended cycling
with results given in Table II. The coulombic efficiency was calcu-
lated based on the fraction of charges recovered during the stripping
of lithium from the Li-Sn electrode. For cycling at a low current
density of 0.4 mA/cm2, the first two cycles showed coulombic effi-
ciencies lower than 70% and the rest from 83 to 89%. Thus, lithium
was stored in the Li-Sn electrode during the first two cycles for
electrode development prior to cycling at a more or less constant
efficiency at around 85% during most of the cycles. More than 200
cycles had been obtained with an average capacity of 140 mAh/g.
To study the effect of current density, the tin film electrode was
cycled at constant current densities from 0.4 to 1.0 mA/cm2. The
change in the discharging capacity upon cycling at different current
densities is shown in Fig. 9. The use of a current density higher than
0.4 mA/cm2 leads to a rapid degradation of the discharging capacity
thus mitigating
deposition.
a
significant volume change during lithium
Conclusions
In the search for a suitable negative electrode material for the
lithium-ion battery in room temperature molten salt ͑RTMS͒, a new
RTMS system containing 1-methyl-3-ethylimidazolium/AlCl3 /
SnCl2 ͑3:2:0.5͒ has been developed for the electrodeposition of tin
on copper electrode. The tin film deposited at a potential more nega-
tive than Ϫ0.72 V under different temperatures produced tin crys-
tallites with different surface morphologies and particle sizes, and
showed widely different performances of the lithium cell ͑Sn ͑Cu͒/
LiCl buffered RTMS/lithium͒ assembled using LiCl buffered
MEICl-AlCl3 melts as the electrolyte. It was found that the tin film
Table II. The performance of a tin film electrode cycled at a
constant current density of 0.4 mAÕcm2.
Charging
capacity
͑mAh/g͒
Discharging
capacity
͑mAh/g͒
Coulombic
efficiency
͑%͒
Cycle
number
1
2
3
4
5
10
20
30
40
541
206
170
169
165
156
157
150
148
271
139
149
140
137
133
139
128
122
50.1
67.5
87.6
82.8
83.0
85.3
88.5
85.3
82.4
Figure 9. The discharging capacity for the tin film electrode cycling at dif-
ferent current densities.
Downloaded on 2012-10-17 to IP 128.143.23.241 address. Redistribution subject to ECS license or copyright; see www.esltbd.org

A324
Journal of The Electrochemical Society, 149 ͑3͒ A319-A324 ͑2002͒
charging capacities were found to increase with the deposition of
Li-Sn alloys with higher lithium activities. However, the irreversible
capacity at the first cycle is also increased at the same time, indicat-
ing that more lithium was needed for the initial development of the
tin film electrode when cycling at a higher voltage. When cycling at
a high cd, the discharging capacity was found to decrease rapidly,
producing loose, expanded, and irregular crystallites at the tin film
electrode surface after 20 cycles at 1.0 mA/cm2. When cycling at a
low cd of 0.4 mA/cm2, the performance of the tin film electrode was
greatly improved, showing an average capacity of 140 mAh/g, a
coulombic efficiency around 85%, and more than 200 cycles. The
improvement is attributed to the deposition of small, regular, round
tin crystallites that allows reversible insertion and removal of
lithium from a more stable crystal structure deposited at high tem-
peratures in the 1-methyl-3-ethylimidazolium/AlCl3 /SnCl2 melt.
In summary, the tin film electrode offers a better cycling perfor-
mance at a low cd as compared to other conventional metal alloy
electrodes such as lithium-aluminum electrode. The improvement is
attributed to the special texture of the tin deposited film that allows
sufficient space between the tin crystals for the incorporation of
lithium without a significant volume change during cycling.
Acknowledgments
We acknowledge the support of the Committee of the Hong
Kong Research and Conference Grants and the Hong Kong Re-
search Grants Council of the Hong Kong Special Administrative
Region, China ͑HKU 302/93E͒ for this project.
The University of Hong Kong assisted in meeting the publication costs of
this article.
References
1. Y. S. Fung and S. M. Chau, J. Appl. Electrochem., 23, 346 ͑1993͒.
2. Y. S. Fung and S. M. Chau, Mater. Sci. Forum, 73, 677 ͑1991͒.
3. Y. S. Fung, Trends Inorg. Chem., 5, 117 ͑1998͒.
4. J. Fuller and R. A. Osteryoung, J. Electrochem. Soc., 142, 3632 ͑1995͒.
5. R. J. Gale and R. A. Osteryoung, Inorg. Chem., 19, 2240 ͑1980͒.
6. T. L. Riechel and J. S. Wilkes, J. Electrochem. Soc., 139, 977 ͑1992͒.
7. C. Scordilis-Kelly, J. Fuller, and J. S. Wilkes, J. Electrochem. Soc., 139, 694
͑1992͒.
8. J. S. Wilkes, J. A. Levisky, R. A. Wilson, and C. L. Hussey, Inorg. Chem., 21, 1263
͑1982͒.
9. C. Scordilis-Kelly and R. T. Carlin, J. Electrochem. Soc., 140, 1606 ͑1993͒.
10. Y. S. Fung and R. Q. Zhou, J. Power Sources, 81-82, 891 ͑1999͒.
11. Y. S. Fung and R. Q. Zhou, in Proceedings of 7th China-Japan Bilateral Confer-
ence on Molten Salts Chemistry and Technology, p. 86 ͑1998͒.
12. Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, and T. Miyasaka, Science, 276,
1395 ͑1997͒.
13. I. A. Courtney and J. R. Dahn, J. Electrochem. Soc., 144, 4045 ͑1997͒.
14. G. M. Ehrlich, C. Durand, X. Chen, and S. L. Suib, J. Electrochem. Soc., 147, 886
͑2000͒.
Figure 10. Scanning electron micrographs showing the morphology of the
tin films after 20 cycles of charging-discharging at different current densities
͑a, top͒ 0.4 and ͑b, bottom͒ 1.0 mA/cm2.
15. Y. W. Xiao, J. W. Lee, A. S. Yu, and Z. L. Liu, J. Electrochem. Soc., 146, 3623
͑1999͒.
16. G. M. Ehrlich, C. Durand, X. Chen, T. A. Hugener, F. Spiess, and S. L. Suib, J.
Electrochem. Soc., 147, 886 ͑2000͒.
crystallites deposited at a temperature of 50°C or higher gave a
desirable crystal structure and offered the best electrochemical per-
formance as compared to those obtained at lower temperatures.
The results of cyclic voltammetry and galvanostatic cycling
show the deposition of three major lithium-tin alloy phases
corresponding to the two-phase transition for LiSn/Li7Sn3 ,
Li13Sn5 /Li7Sn2 , and Li7Sn2 /Li22Sn5 among the six intermediate
phases identified in the Li/Sn system. Both the charging and dis-
17. O. Mao and J. R. Dahn, J. Electrochem. Soc., 146, 414 ͑1999͒.
18. E. Peled and A. Ulus, in Materials for Lithium-Ion Batteries, C. Julien and Z.
Stoynov, Editor, p. 585, Kluwer Academic Publishers, Norwell, MA ͑2000͒.
19. A. Armand, D. A. Fannin, Floreani, L. A. King, J. S. Wilkes, and J. L. Williams, J.
Phys. Chem., 88, 2614 ͑1984͒.
20. Joint Committee on Powder Diffraction Standards, Powder Diffraction Data: from
the Joint Committee on Powder Diffraction Standards Associateship at the National
Bureau of Standards, 1st ed., 4-673, Washington, DC ͑1976͒.
21. C. J. Wen, and R. A. Huggins, J. Solid State Chem., 35, 376 ͑1980͒.
Downloaded on 2012-10-17 to IP 128.143.23.241 address. Redistribution subject to ECS license or copyright; see www.esltbd.org