Structural and morphological modifications of a nanosized 62 atom percent Sn-Ni thin film anode during reaction with lithium

Article abstract of DOI:10.1149/1.1856913

Nanosized electrodeposited 62 atom % Sn-Ni alloy was tested to highlight the effects of volume changes on the cycling life of the electrode during lithiation and delithiation. X-ray diffraction showed that the Ni ₃Sn₄ was the main phase of the as-deposited alloy. A unique feature of the 62 atom % Sn-Ni is that it exhibited a capacity recovery upon cycling. When cycled galvanostatically, the Sn₆₂Ni₃₈ offers low capacity fade while reversibly incorporating lithium up to 600 mAh/g. At the first charge LiSn alloy phases are formed. This led to volume expansion of the electrode causing the formation of cracks. At the following cycles the Ni₃Sn₄, phase was restored and preserved over extensive cycling revealing the reversibility of the reaction between Ni ₃Sn₄, and Li⁺. As to the reasons of the capacity recovery noticed with this alloy, scanning electron microscopy images provided evidence of modifications of the surface condition accompanying a volume change during cycling. The chemical diffusion coefficient (D _Li) value determined from electrochemical impedence spectroscopy measurements during lithium insertion was within 10^-9 to 10 ^-10 cm² s^-1.

Full text of DOI:10.1149/1.1856913

A560
Journal of The Electrochemical Society, 152 ͑3͒ A560-A565 ͑2005͒
0
013-4651/2005/152͑3͒/A560/6/$7.00 © The Electrochemical Society, Inc.
Structural and Morphological Modifications of a Nanosized 62
Atom Percent Sn-Ni Thin Film Anode during Reaction
with Lithium
Hitomi Mukaibo, Toshiyuki Momma,a,b,* Mohamed Mohamedi,a,c,*,z and
a
a,
Tetsuya Osaka **
a
Graduate School of Science and Engineering, Waseda University, Tokyo 169-8555, Japan
CREST, Japan Science and Technology Agency, Tokyo 105-6218, Japan
b
Nanosized electrodeposited 62 atom % Sn-Ni alloy was tested to highlight the effects of volume changes on the cycling life of the
electrode during lithiation and delithiation. X-ray diffraction showed that the Ni Sn was the main phase of the as-deposited alloy.
3
4
A unique feature of the 62 atom % Sn-Ni is that it exhibited a capacity recovery upon cycling. When cycled galvanostatically, the
Sn Ni offers low capacity fade while reversibly incorporating lithium up to 600 mAh/g. At the first charge LiSn alloy phases are
6
2
38
formed. This led to volume expansion of the electrode causing the formation of cracks. At the following cycles the Ni Sn phase
3
4
ϩ
was restored and preserved over extensive cycling revealing the reversibility of the reaction between Ni Sn and Li . As to the
3
4
reasons of the capacity recovery noticed with this alloy, scanning electron microscopy images provided evidence of modifications
of the surface condition accompanying a volume change during cycling. The chemical diffusion coefficient (D ) value determined
Li
Ϫ10
Ϫ9
2
Ϫ1
from electrochemical impedence spectroscopy measurements during lithium insertion was within 10 to 10
2005 The Electrochemical Society. ͓DOI: 10.1149/1.1856913͔ All rights reserved.
cm s .
©
Manuscript submitted May 13, 2004; revised manuscript received August 5, 2004. Available electronically January 26, 2005.
There is a strong incentive to develop and characterize noncar-
impedance spectroscopy ͑EIS͒, and charge-discharge methods, we
focus onto gaining further insight into the behavior of the 62 atom %
Sn-Ni alloy. The cycling was extended up to 150 cycles of charge-
discharge. At some stages of the capacity vs. cycle number profile,
ex situ X-ray and scanning electron microscopy ͑SEM͒ were em-
ployed to evaluate the changes in the structure and for imaging the
evolution of the surface morphology, respectively.
bonaceous materials for use as negative electrodes that deliver ca-
pacities higher than carbon. In 1994, Fuji Film Co., Ltd., filed a
patent for Sn oxides as a novel anode material for lithium ion bat-
1
teries with a theoretical capacity exceeding that of carbon. How-
ever, the tin oxide material showed high irreversible capacity during
the first cycle, which has precluded its commercial success as an
anode in lithium ion secondary batteries. The large irreversible ca-
pacity is caused by the reduction of the tin oxides and the formation
Experimental
2
of lithium oxide during the first cycle. Nonetheless, the studies on
A thin film of pure tin was synthesized on a copper foil from an
Ϫ1
Ϫ1
tin oxide material demonstrated the possible performance given a
tin-based negative electrode material. To mitigate the irreversible
capacity associated with tin oxides, several attempts have been made
to develop nonoxide tin materials with both high capacity and long
electrolytic bath containing SnSO (40 g L ), H SO (60 g L ),
4 2 4
Ϫ1
o-Cresolsulfonic acid (40 g L ), and polyethylene glycol ͑100
ppm͒. The electrodeposition was carried out at room temperature
with a current density of 10 mA cm for 5 min.
2
8
Ϫ2
3
-26
cycle life.
From these studies, it may be seen that the selection of
The Sn Ni was prepared by electrodeposition on a copper
6
2
38
an adequate matrix that can accommodate the volume change of tin
during cycling is crucial to a successful tin anode compound that
can deliver both high capacity and long cycle life. Elements that are
inactive against lithium are assumed to suppress the volume change
effectively without appreciable irreversible capacity. Alloying Sn
Ϫ1
foil using an electrolytic bath of NiCl •6H O (0.075 mol L ),
2
2
Ϫ1
Ϫ1
SnCl •2H O (0.175 mol L ), K P O (0.5 mol L ), glycine
2
2
4
2
7
Ϫ1
Ϫ1
(
0.125 mol L ), and NH OH (5 ml L ); the mixture was stirred
4
29
at 50°C. The electrodeposition time was 5 min with a current
density of 5 mA cm . The pH values of solutions were Ӷ1 for pure
Ϫ2
with elements such as Fe,1
5-19
20-23
19,24
19
Cu,
Mn,
and Co, has been
Sn solution ͑a very acidic bath containing 0.61 M of H SO ) and 8
2
4
investigated based on this assumption. Nickel is a typical element,
which does not react with lithium and can be expected to serve as an
adequate matrix for improving the cycleability of the electrode with-
out high initial irreversible capacity. Studies on Sn-Ni alloys pre-
for Sn-Ni alloys. No alkali or acidic solution were used to adjust the
pH. The sample dimension of the electrochemical measurements
2
was 4 cm . The thickness of the film was 14 ␮m for Sn and 1.2 ␮m
7
,9,11
10,12
14
for Sn Ni .
pared by ballmilling,
electroplating
sintering,
E-beam evaporating, and
62 38
8
,10,13
X-ray diffraction ͑XRD͒ was performed on Rigaku RAD-1C
using Cu K␣ radiation. XRD conditions of voltage and current
measurements were 40 kV and 20 mA, respectively. Morphology of
the film was imaged using a scanning electron microscope ͑SEM,
HITACHI S2500CX͒. For SEM observations, a Pt-Pd layer was
sputtered on the sample surface. The samples are designated in
terms of the atomic percentage of Sn in the deposited film, which
was determined by inductively coupled plasma atomic emission
spectroscopy instrument ͑ICP-AES: Thermo Electron K.K., IRIS-
AP͒. Electrochemical measurements were performed in a conven-
tional glass cell using two lithium foils as reference and counter
have been reported.
In our recent Letter,27 by means of the electrodeposition route,
we have synthesized Sn-Ni thin-film alloys of various compositions
and demonstrated their lithium storage capacities. Among the alloys
fabricated, we have shown that the 62 atom % Sn-Ni alloy can store
lithium with capacities well in excess of a typical carbon electrode.
An intriguing feature of the 62 atom % Sn-Ni is that it exhibited a
capacity recovery upon cycling.27 The capacity vs. cycle number
profile demonstrated two stages: (i) a rapid decline in the capacity
followed by (ii) a significant recovery in the capacity that sustained
up to 70 cycles tested.
electrode. The electrolyte was 1 M LiClO ͑Tomiyama Pure Chemi-
4
In this work, by means of cyclic voltammetry, electrochemical
cal Industries, battery grade͒/ethylene carbonate ͑EC͒/propylene car-
bonate ͑PC͒ 1:1 volume ͑Mitsubishi Chemicals, battery grade͒.
Cyclic voltammetry ͑CV͒ was carried out with a function gen-
erator ͑Hokuto Denko, HB104͒ and a potentiostat ͑Hokuto Denko,
HA501G͒ at the scan rate of 0.02 mV/s. The cycleability of the cells
was evaluated by galvanostatic charge-discharge tests ͑Hokuto
Denko, HJR-110mSM6͒. Both charge and discharge were carried
*
Electrochemical Society Active Member.
*
* Electrochemical Society Fellow.
c
`
Present address: INRS-Energie, Mat e´ riaux et T e´ l e´ communications, University of
bec, Canada J3X 1S2.
Quebec, Varennes, Que
E-mail: mohamedi@emt.inrs.ca
´
z
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Journal of The Electrochemical Society, 152 ͑3͒ A560-A565 ͑2005͒
A561
Figure 2. Voltage vs. capacity profiles in 1 M LiClO /EC-PC for the elec-
4
Figure 1. Voltage vs. capacity profiles in 1 M LiClO /EC-PC for the elec-
4
trodeposited 62 atom % Sn-Ni on Cu substrate. The electrode was charged
and discharged with currents of 50 mA/g. The letters C and D in the figure
stand for charge and for discharge, respectively.
trodeposited Sn electrode on Cu substrate. The electrode was charged and
discharged with currents of 50 mA/g. The insert shows the second voltage
charge ͑lithiation͒ curve illustrating the presence of the abnormal high-
voltage plateau. The letters C and D in the figure stand for charge and for
discharge, respectively.
ϩ
electrochemical reduction of a tin electrode in a Li -containing elec-
trolyte leads to the subsequent formation of a number of intermetal-
2
out at the current density of 50 mA/g for both pure Sn and Sn-Ni
alloy. Electrochemical impedance spectroscopy ͑EIS͒ measurements
were carried out using a frequency response analyzer ͑NF Electronic
Instruments, S-5720C͒ combined with a potentiostat ͑Hokuto
Denko, HA501G͒. The alternating voltage signal was Ϯ10 mV rms,
and the frequency range was from 20 kHz to 10 mHz. EIS was
performed according to the following sequences: galvanostatic
lic phases Li Sn. On the other hand, the tin pulverizes rapidly when
x
it experiences full lithiation due to the large volume increase leading
to the electrode failure in the subsequent cycles. Indeed, only a
visual observation of our electrode showed that the electrodeposited
tin has detached from the copper substrate.
A point we would like to briefly discuss here is the appearance of
the abnormal high-voltage plateau during the charge of pure tin
electrode. This phenomenon has not been often investigated and was
reported only by Tamura et al. and by Beaulieu et al. In our
experiments, when the tin electrode was charged from 2.5 to 0 V, a
voltage plateau close to 1.5 V could be seen as shown in the insert of
Fig. 1. The occurrence of this plateau is believed to be due to the
formation of a thin film resulting from a catalytic decomposition
of electrolyte by the pure tin electrode.
that the occurrence of the high-voltage plateau is not an experi-
mental artifact in agreement with Tamura et al. and Beaulieu
et al. In conclusion, the abnormal high-voltage behavior of Sn
certainly deserves further investigations onto the nature of the cata-
lytic Sn surface.
ϩ
charge ͑lithium insertion͒ to 0.01 V vs. Li /Li→ impedance
2
3
31
→
galvanostatic discharge ͑lithium extraction͒ to 2.00 V → imped-
ance. This cycle was repeated ten times. Impedance spectra were
modeled using complex nonlinear least-squares ͑CNLS͒ fitting pro-
gram EQUIVCRT developed by B. A. Boukamp. Then, to get an
indication on how well the modeling function reproduces the actual
data set, we first observed the parameter values and their relative
2
3,31
We thus confirm here,
2
error estimate ͑in %͒. The Chi-square (␹ ) function also gives a
good indication of the quality of the fit; a value of 10 -10 or less
for ␹ indicates a reasonably good fit.
23
Ϫ4
Ϫ5
3
1
2
Results and Discussion
Electrochemical characterization of a pure tin electrode.—The
charge ͑lithiation͒ and discharge ͑delithiation͒ profiles of the elec-
trodeposited tin anode in the first two cycles are shown in Fig. 1.
During the first charge, the potential of the tin anode dropped rap-
Electrochemical characterization of Sn62Ni38 electrode.—Figure
2 shows the charge-discharge curves of electrodeposited Sn Ni
62
38
alloy in the first, second, tenth, and fiftieth cycles against Li metal.
The first charge and discharge capacities are both the same, ca.
ϩ
Ϫ1
idly to about 0.4 V vs. Li/Li , stabilized, and then gradually
660 mAh g , which shows that the initial cycling efficiency is
changed toward cathodic values again. The plateau at 0.4 V is con-
sidered to result from the reaction of LiSn with metallic lithium.
The gradual decrease of the voltage results from the coexistence
with several LiSn alloy phases in the active material. During dis-
charging, multiple plateaus were clearly observed. By the second
100%. A very interesting feature of Fig. 2 is that the discharge
capacity decreased as cycling extended to ten cycles and started to
increase for higher cycles. The 50th cycle is illustrated in the figure
for clarity, indeed the recovery of the capacity was observed by the
10th cycle as shown later in this paper.
3
0
charge, the capacity dramatically dropped to values less than
As to the anomalous high-voltage, one expects that alloying
other metal with Sn would likely avoid the occurrence of the abnor-
mal high-voltage irreversible capacity. Figure 3, illustrates the sec-
Ϫ1
40 mAh g and was worse in the subsequent cycles. The profiles
3
7
are similar to these reported by Tamura et al. for an electrodeposited
2
3
tin on copper foil using different bath compositions. The signifi-
cant drop in the capacity was considered by Tamura et al. to result
from a lack of interface strength between the entire part of the active
material and the copper current collector.23 It is well known that the
ond and third charge curves related to the Sn Ni alloy electrode. It
6
2
38
clearly shows the high-voltage plateau at the second-charge process,
whereas the plateau disappeared at the third charge process. Al-
though we do not show the data here, we have tested three other
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A562
Journal of The Electrochemical Society, 152 ͑3͒ A560-A565 ͑2005͒
Figure 3. The second and third charge vs. capacity profiles in
M LiClO /EC-PC for the electrodeposited 62 atom % Sn-Ni on Cu sub-
strate. The figure illustrates the presence of the abnormal high-voltage pla-
teau at the second charge only.
Figure 5. Nyquist plots at 62 atom
% Sn-Ni on Cu substrate in
1
4
1
M LiClO /EC-PC solution. ͑a͒ Full scale, ͑b͒ enlarged view of the high
4
frequency domain of the impedance spectra. ͑᭺͒ 1st, ͑ᮀ͒ 2nd, ͑•͒ 3rd, and
᭿͒ 10th charge. ͑c͒ Modified Randles-Ershler model used to simulate the
͑
impedance spectra. Solid lines in Fig. 5a and 5b represent the simulation
results.
SnNi alloys by changing the Sn content ͑in atom %͒, namely,
Sn Ni , Sn Ni , and Sn Ni . The high-voltage plateau was ob-
54
46
84 16
92
8
served with Sn Ni and Sn Ni but not with the Sn Ni alloy. It
84
16
92
8
54 46
is quite clear from Fig. 3 that Sn alloyed with another metal ͑for
instance, nickel͒ does exhibit a high-voltage plateau for an Sn con-
tent higher than 62 atom %. As for the pure tin electrode, these
phenomena require further investigations.
0
.65 V was found to depend on the Sn content, i.e., its maximum
27
current increased when the Sn content in the alloy increased. The
peak at 0.65 V has been observed during the second and subsequent
cycles with SnO materials, however, it has not been observed dur-
ing the first reduction of SnO2 , nor with tin-iron based alloys,
thus it may be associated with the Li-Sn alloying process.
The cyclic voltammograms ͑CVs͒ for the Sn Ni film for the
32
6
2
38
Ϫ1
first three cycles at 0.02 mV s are shown in Fig. 4. The samples
33
16-18
were cycled between the open circuit potential ͑OCP͒ and 0.0 V vs.
ϩ
Li/Li . During the first reduction process, the voltammograms are
Figure 5a shows a family of four Nyquist plots of Sn Ni thin
62
38
characterized by two reduction peaks at 0.65 and 0.34 V. The peak at
film electrode subjected to up to ten cycles of lithium insertion.
Figure 5b displays an enlarged view of their high frequency domain.
The impedance spectra of Fig. 5 are basically the typical Nyquist
plot of an ion insertion thin-film electrode. There is one semicircle
in the high-frequency region, and a steep sloping line at the lower
frequencies due to the onset of the finite length effect. The high
frequency semicircle was found to be dependent upon cycling, i.e., it
decreased in size from the first to the tenth cycle. It is interesting to
note that no diffusion impedance is clearly seen at the first three
charging processes. However, Warburg diffusion impedance ap-
peared at the tenth cycle in the middle frequency region ͑see insert
in Fig. 5b͒. This is naturally ascribed to solid-state diffusion of
lithium-ion into the bulk anode material. We explain the appearance
of the Warburg impedance by an increase in the electrode thickness.
Indeed, the theoretical analysis of the complex impedance for inser-
tion electrodes predicts that the diffusion control expands with in-
3
4
creasing film thickness. For very thin films, the kinetic control
region may overlap with the charge saturation region and the diffu-
sion region may appear in a very narrow range of frequencies. As
the film thickness gets bigger, charge saturation dominates only at
the very low frequencies, and diffusional control is observed over a
large frequency range. The steep sloping line reflects a capacitive
behavior, which is due to accumulation of the intercalant ͑Li͒ into
the bulk anode.
According to the solid electrolyte interphase ͑SEI͒ model, one-
would expect to observe two semicircles in the impedance spectrum,
one semicircle reflecting the charge-transfer process at the
Sn Ni /electrolyte interface, and one related to the formation of
6
2
38
SEI. However, if their time constants are similar, the semicircles
could overlap one another and become irresolvable. Moreover, it has
been demonstrated with mesocarbon microbeads single-particle an-
ode ͑no binder and no conductive agent͒ that the SEI has to be aged
or forcibly grown for long hours at its formation potential to be able
to see two separate semicircles in the impedance spectra.3
Figure 4. The first three cyclic voltammograms in 1 M LiClO /EC-PC for
4
electrodeposited 62 atom % Sn-Ni on Cu substrate. The voltammograms
Ϫ1
were recorded at the scan rate of 0.02 mV s . The thin line represents the
5,36
first cycle, the bold lines represent the second and third cycles.
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Journal of The Electrochemical Society, 152 ͑3͒ A560-A565 ͑2005͒
A563
Table I. Parameters of interest determined from fitting the ex-
perimental impedance spectra of Fig. 5a to the equivalent circuit
of Fig. 5c.
Vm dE
l
ZЈ ϭ R ϭ
͓1͔
L
nFA dx 3DLi
R (⍀ cm ) R (⍀ cm ) C (␮F cmϪ2) D (cm sϪ1)
2
2
2
s
ct
dl
Li
Vm the molar volume of the host structure, dE/dx the slope of the
coulometric titration curve vs. mobile ion concentration x at each x
value, l the maximum length of the diffusion pathway (l can be
assimilated to the electrode film thickness͒, and n the number of
equivalent.
Ϫ10
Ϫ10
Ϫ10
Ϫ10
1st charge
2nd charge
3rd charge
10th charge
80.7
80.1
79.0
83.5
18.6
11.2
8.3
22.0
25.0
32.0
6.7 ϫ 10
6.1 ϫ 10
6.5 ϫ 10
7.7
140
8.04 ϫ 10
The imaginary part of the impedance as
R : solution resistance; R : charge-transfer resistance; Cdl : double-
s
ct
layer capacitance; and DLi : diffusion coefficient of lithium.
1
Vm dE 1
ZЉ ϭ
ϭ
͓2͔
͓3͔
␻
Cin
nFA dx ␻l
Features of the impedance spectra of Fig. 5a may be modeled by
a simple modified Randles-Ershler equivalent circuit shown in Fig.
1
dZЉ
d␻Ϫ1
ϭ
5
c. In this model, R is the solution resistance, and R is the charge-
s
ct
Cin
transfer resistance at the electrode/electrolyte interface. A constant
phase element ͑CPE͒ was used instead of a double-layer capacitance
to take into account the surface roughness of the particle. Cin is the
insertion capacitance, and ZW is the Warburg impedance that corre-
sponds to the solid-state diffusion of the Li-ion into the bulk anode.
The Warburg element was used only for impedance data obtained at
the tenth charge. The electrical components of the surface film
which is likely formed on the electrode were disregarded, because
no time constant related to this process could be seen in the EIS
spectra. We also checked that their inclusion in the model of Fig. 5c
does not improve the fit.
Thus, in the range where the impedance phase angle approaches
␲/2, the value of the intercalation capacitance, Cin can be deter-
mined graphically from the slope of Eq. 3.
Therefore, DLi may be deduced from Eq. 1 through Eq. 3
l2
DLi
ϭ
͓4͔
3
R C
L in
The solid lines in Fig. 5a, b represent the simulation results, and
these show an excellent fitting obtained with the simple model of
Fig. 5c over a six-decade range of frequency. The chi-square quality
of fitting parameter was within 10Ϫ6-10 range, and the estimated
standard deviations of the individual electrical components were less
than 10%. Parameters of interest obtained from the nonlinear least
square fitting procedure are listed in Table I. The solution resistance
was more or less constant during the first three cycles of charge,
whereas it slightly increased at the tenth cycle. The charge-transfer
resistance, Rct decreased as the cycling increased indicating an im-
provement in the performance of the electrode. Moreover, it points
out that the contribution of the SEI to the lithium insertion and
extraction kinetics is not significant at these first ten Li insertion
stages, otherwise the charge-transfer process would become diffi-
cult, and one would observe an increase in its resistance value, since
the growth of a dense SEI layer would causes an electronic insula-
tion and the lithium ions would have first to migrate through the SEI
layer to reach the anode. Nonetheless, if a pinhole is present in the
SEI layer, electrons will flow relatively unimpeded from the SEI
layer to the anode.
Although Cin can be graphically determined with good precision, the
determination of RL by extrapolation is not precise and an inaccu-
racy is introduced in the calculation of the diffusion coefficient us-
ing Eq. 3. Consequently, RL and Cin were determined by using the
complex nonlinear least-squares ͑CNLS͒ fitting program. The diffu-
sion coefficient value calculated thus at various cycle of charge are
listed in Table I. There is no significant change in the DLi value from
the first to the third cycle of lithium insertion and sustained at an
Ϫ5
Ϫ10
2
Ϫ1
average value of 6.43 ϫ 10
cycle, DLi increased to 8.04 ϫ 10
cm s . However, at the tenth
Ϫ10
2
Ϫ1
cm s
.
The impedance data reported here are very preliminary. A de-
tailed investigation with EIS on the reaction mechanism of lithium
insertion/extraction into/from an aged Sn Ni anode ͑long-term cy-
62 38
cling and potentiostatic conditions͒ is underway.
Structural and morphological changes of Sn62Ni38 electrode
upon cycling.—Ex situ XRD analysis and morphological changes by
SEM were performed at the Sn Ni subjected to several cycles of
6
2
38
charge and discharge. Figure 6 displays the progression of X-ray
The CPE exponent, n, ranged between 0.85 and unity and thus
may be assimilated to the double-layer ͑dl͒ capacitance, C . C
diffraction ͑XRD͒ patterns for the Sn Ni at the first charge and
6
2
38
after further cycling up to ten cycles. X-ray peaks for the as-
deposited electrode are also reported for reference. Shown in Fig. 7
is the evolution of the surface morphology of the electrode at some
dl
dl
Ϫ2
was found in the range of 22-32 ␮F cm for the first three charges
Ϫ2
but increased to about 140 ␮F cm
at the tenth charge, which
stages of cycling. The as-deposited Sn Ni had an average particle
clearly indicates changes in the electrode surface condition. The
insertion capacitance Cin remained practically unchanged from the
first to the third charge as follows from the fitting results. This
means that the same level of Li insertion is obtained. In contrast, Cin
62 38
size ͑for 100 particles͒ of 30 nm ͑Fig. 7a͒.
Figure 6 shows that the Ni Sn was the main phase confirmed
3
4
from the as-deposited state. The Bragg peaks from the Ni Sn alloy
3
4
disappeared at the first charge, indicating that that a reaction be-
tween the NiSn alloy and lithium is occurring. This proves that
increased at the tenth charge. The element R should not only reflect
s
the solution resistance but also electronic extendion from the current
collector to the electrode bulk. The electronic resistance of the elec-
trode active material may depend on cycling owing to changes in its
composition; the resistance of the other passive component ͑current
collectors͒ is expected to be a invariant with state of charge.
during the first charge Ni Sn is active. The reaction product of the
3
4
first charge shows two broad X-ray peaks at 2␪ Ϸ 24 and 38°,
respectively. Courtney and Dahn identified these two peaks as those
2
of LiSn alloy phases. At the following cycle, peaks of Ni Sn re-
3
4
2
If the homogeneous diffusion (␻ ӷ 2DLi /l ) is observed ͑last
appeared, whereas those of Li-Sn alloy phases disappeared. The
formation of Li-rich alloy phases leads to fast mechanical degrada-
tion of the electrode due to a large volume change during cycling.
This is supported by the SEM image of the electrode, which clearly
part of the diagram͒, the current is approximately 90° out of phase
with the voltage. Under this condition, the real part of the imped-
ance is independent of the frequency. The extrapolated real intercept
of the straight-line region gives rise to the limiting low-frequency
ϩ
shows expansion in the grains resulting in a bumpy surface with Li
34
ϩ
resistance, R , defined by Ho et al.
insertion, and formation of surface cracks with Li elimination ͑Fig.
L
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A564
Journal of The Electrochemical Society, 152 ͑3͒ A560-A565 ͑2005͒
Figure 8. Cycle life performance in 1 M LiClO /EC-PC of electrodeposited
4
Figure 6. Ex situ XRD pattern taken at different stages during cycling of the
62 atom % Sn-Ni on a Cu substrate. The electrode was charged and dis-
6
2 atom % Sn-Ni electrode in 1 M LiClO /EC-PC solution. The XRD pat-
4
charged with currents of 50 mA/g.
tern related to the as-electrodeposited 62 atom % Sn-Ni is also reported.
of the Sn Ni alloy, i.e., after a drop in the capacity to around
6
2
38
7
c͒. It should be noted that the Ni Sn phase is maintained even
3 4
Ϫ1
4
60 mAh g , this one has markedly recovered and sustained at
after extended cycling indicating the reversibility of the reaction
between Ni Sn and Li .
Ϫ1
ϩ
around an average value of 600 mAh g over the 150 cycles tested.
We have to point out with other Sn-Ni compositions tested
3
4
An additional study was made to evaluate the structure and mor-
(
Sn Ni , Sn Ni , and Sn Ni ), their capacities, although stable,
phology changes of Sn Ni cycled for a longer period. Figure 8
displays the capacity vs. cycle number profile upon 150 cycles of
charge and discharge. The profile shows an outstanding performance
54 46 84 16 92 8
62
38
Ϫ1
were very much inferior and were less than 400 mAh g upon
cycling. With regard to the reproducibility of the remarkable per-
2
7
formance of the Sn Ni , the average charge for various experi-
6
2
38
Ϫ1
mental trials we have performed was about 625 mAh g . The re-
covery in the capacity observed with this material is a very
interesting phenomenon and has been observed by Mao and Dahn
with the FeSn system, which showed an increase in capacity with
1
8
cycle number. The authors postulated that this is caused by the
breakdown of the Fe-skin that impedes access to the interior of the
grains of this phase by the repeated volume changes in the electrode.
Figure 9 also reports few SEM images taken at the 50th and
100th cycles. The cracks observed in the tenth cycle ͑Fig. 7d͒ have
coalesced in the 50th and the 100th cycle, and higher roughness can
be noticed from the surface morphology. These images are evidence
that alterations in the surface condition accompanying volume
changes during cycling were responsible of the Sn Ni behavior.
62
38
Figure 7. ͑a͒ High resolution FE-SEM image of the as-deposited 62 atom %
Sn-Ni sample. ͑b͒, ͑c͒, and ͑d͒ are ex situ SEM images taken at 1st charge,
Figure 9. Ex situ SEM images taken at ͑a͒ 50th cycle and ͑b͒ at the 100th
1st discharge, and 10th discharge, respectively.
cycle.
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Journal of The Electrochemical Society, 152 ͑3͒ A560-A565 ͑2005͒
A565
It is obvious that the electrode does not return to the original
volume upon lithium extraction. The major increase in volume hap-
pens in the first alloying. This volume enlargement of the host ma-
trix causes an increase in the electrode/electrolyte interface area.
The repeated formation of cracks and pores has expanded the vol-
ume of the electrode so much that it has covered up the cracks
formed during the earlier cycling. The resulting expanded electrode
offers a higher porosity than the original material, and provided that
there is still electronic contact within the host after this activation,
lithium diffusion is facilitated with lower volume expansions in the
following cycles. This might be one of the reasons for the rise in the
capacity seen in the cycle performance of each sample.
Waseda University assisted in meeting the publication costs of this
article.
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Acknowledgments
This work is supported in part by a Grant-in-Aid from the Center
of Excellence ͑COE͒ Research Molecular Nano-Engineering, and
the 21st Century COE Program Practical Nano-Chemistry from the
Ministry of Education, Culture, Sports, Science and Technology. H.
M. acknowledges a research fellowship from the Japan Society for
the Promotion of Science.
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