Surface film formation and lithium underpotential deposition on Au(111) surfaces in propylene carbonate. In situ scanning tunneling microscopy study

Article abstract of DOI:10.1149/1.1557966

The formation and the morphological change of surface film on a Au(111) electrode in propylene carbonate solution containing 0.1 M LiClO₄ in the potential region between 0.8 and 2.5 V (Li/Li⁺) were studied by in situ scanning tunneling microscopy. The surface film was observed on a gold electrode at potentials more negative than 1.5 V (Li/Li⁺), and many nuclei appeared on the flat terrace of the electrode at potentials more negative than 0.9 V, where underpotential deposition of lithium on gold was started. Many holes on the surface film were observed after the dissolution of lithium and were thought to be formed as a result of breakdown of the film in nanometer order. The deposition and dissolution of the submonolayer lithium affected the surface morphology in nanometer order.

Full text of DOI:10.1149/1.1557966

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Journal of The Electrochemical Society, 150 ͑4͒ A532-A537 ͑2003͒
0013-4651/2003/150͑4͒/A532/6/$7.00 © The Electrochemical Society, Inc.
Surface Film Formation and Lithium Underpotential
Deposition on Au„111… Surfaces in Propylene Carbonate
In Situ Scanning Tunneling Microscopy Study
,z
*
Toshiya Saito and Kohei Uosaki
Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University,
Sapporo 060-0810, Japan
The formation and the morphological change of surface film on a Au͑111͒ electrode in propylene carbonate solution containing 0.1
M LiClO4 in the potential region between 0.8 and 2.5 V (Li/Li ϩ ) were studied by in situ scanning tunneling microscopy. The
surface film was observed on a gold electrode at potentials more negative than 1.5 V ͑Li/Li ϩ ), and many nuclei appeared on the
flat terrace of the electrode at potentials more negative than 0.9 V, where underpotential deposition of lithium on gold was started.
Many holes on the surface film were observed after the dissolution of lithium and were thought to be formed as a result of
breakdown of the film in nanometer order. The deposition and dissolution of the submonolayer lithium affected the surface
morphology in nanometer order.
© 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1557966͔ All rights reserved.
Manuscript submitted June 7, 2002; revised manuscript received September 9, 2002. Available electronically March 6, 2003.
Introduction
proceeds at a specific site on the surface such as a place where the
film is thinner than other parts. Shiraishi et al. also found that den-
dritic lithium was covered with a thinner surface film than other
places by using surface potential microscopy and suggested that
higher current flow at the site with thinner film promotes the growth
of dendritic lithium.14
The in situ investigations carried out so far were, however, lim-
ited in the order of micrometers or submicrometers at most, and the
mechanism of morphological change of the surface film such as
breakdown in nanometer scale was still unknown. One of the main
reasons for the lack of surface investigation with nanometer resolu-
tion is the difficulty in the preparation of a well-ordered lithium
surface.
Although noble metals and transition metals are less reactive
than alkali metals, the reduction of the solution components leading
to film formation is reported to take place even on noble or transi-
tion metal electrode in the relatively negative potential region.15-17
This process can be considered as model reaction for the surface
film formation on a lithium electrode. The surface films formed on
nickel and gold electrodes in propylene carbonate ͑PC͒ solution
were reported to be composed of several layers, which are roughly
divided into two parts, namely, a compact layer at the metal side and
a porous layer at the solution side.17,18 The compact layer is com-
posed of LiOH and/or Li2CO3 , and the porous layer contains some
organic components.17,18 Since it is much easier to prepare a well-
defined and clean surface of noble metal, it is appropriate to use the
noble meal electrode to investigate the fundamental process of sur-
face film and lithium deposition/dissolution in nonaqueous solu-
tions.
We have already examined the surface morphology of a Au͑111͒
electrode in PC solution containing 0.1 M LiClO4 by in situ STM at
potentials between 1.9 and 2.5 V where no faradaic current flowed.19
We found that the surface of the Au͑111͒ electrode was covered with
an ultrathin film whose structure changed with applied potentials.
An island-like adsorbate was observed at potentials more positive
than the potential of zero charge ͑pzc͒, and a flat smooth surface
having small holes of several nanometers was observed at potentials
more negative than the pzc.
The electrochemical deposition and dissolution of lithium in
aprotic solvents has attracted much attention because of the indus-
trial significance with regard to application in lithium secondary
batteries. Although lithium metal is the ideal material as an anode of
light weight and small-size rechargeable batteries,1 the lower revers-
ibility of the lithium deposition/dissolution cycle than that of the
lithium intercalation/deintercalation cycle at a carbon electrode in-
hibits practical application of lithium metal for secondary batteries.
Due to the high reactivity of lithium metal, the lithium electrode
is known to be always covered by a surface film in an aprotic sol-
vent. The solid electrolyte interphase ͑SEI͒ model2,3 is the most
accepted to describe this film. In this model, the film is thought to
have lithium ion conductivity, and the deposition and dissolution of
lithium take place at the film/electrode interface so that reactions
between lithium and the solvent and/or anion are prevented. Thus,
the physical and chemical properties of the surface film significantly
affect the electrochemical properties of the lithium electrode. For
example, nonuniformity of the surface film such as thickness distri-
bution induces nonuniform current distribution on a lithium elec-
trode and causes growth of dendrites.4,5 The ‘‘breakdown and re-
pair’’ mechanism was proposed by Dey and Schlaikjer, explaining
the behavior of the surface film during the dissolution of lithium.6
According to their proposal, a part of the surface film becomes
cracked during the dissolution of lithium. The exposed lithium at the
bottom of the cracked hole reacts with the solvent and/or anion and
the surface film is formed within the cracked hole. As a result of the
‘‘breakdown and repair,’’ the surface film becomes inhomogeneous
because the original film and the repaired film may have different
chemical and physical properties. Actually, the roughening of the
surface after the dissolution of lithium was confirmed by in situ
atomic force microscopy ͑AFM͒.7 The growth of dendrites and/or
breakdown of the film on the lithium electrode should be responsible
for the low reversibility of the lithium deposition/dissolution cycle
and may cause electric shorts or the isolation of metallic lithium.
Morphological changes of the lithium surface were already observed
under various conditions by scanning electron microscopy ͑SEM͒
and AFM.4,7-12 The control of the chemical and physical properties
of the surface film is an important subject for the development of
lithium secondary batteries.
In this study, we extended the in situ electrochemical scanning
tunneling microscopy ͑STM͒ observation of the Au͑111͒ electrode
in PC solution containing 0.1 M LiClO4 to the more negative po-
tential region where the reduction processes including underpoten-
tial deposition ͑UPD͒ of lithium take place. The surface film forma-
tion and morphological change of the film as a function of the
electrode potential are discussed based on the STM observation as
well as the results of cyclic voltammetry ͑CV͒.
Aurbach et al. reported the heterogeneity in the surface film on
the lithium electrode by in situ AFM and lateral force
microscopy,7,13 suggesting that the lithium deposition preferentially
* Electrochmeical Society Active Member.
z E-mail: uosaki@pcl.sci.hokudai.ac.jp
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Journal of The Electrochemical Society, 150 ͑4͒ A532-A537 ͑2003͒
Experimental
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A gold single-crystal bead was prepared by Clavilier’s method20
from a gold wire ͑Tanaka Precious Metal, 99.99% purity͒ and was
cut parallel to a ͑111͒ facet. The exposed ͑111͒ face was polished
with a diamond slurry down to 0.5 ␮m. The crystal was then an-
nealed in an electric furnace at 850°C for 12 h under an argon
atmosphere. Lithium wire ͑Johnson-Matthey, 99.9% purity͒ was cut
and washed several times with hexane in an argon-filled glove box
͑VAC, Nexus 2000 system͒ before use as a reference electrode.
Platinum wire ͑Tanaka Precious Metal, 99.99% purity͒ was used as
a counter electrode. PC solution containing 0.1 M LiClO4 was pur-
chased from Tomiyama Chemicals ͑lithium battery grade͒. The so-
lution contained less than 20 ppm water and was stored in the glove
box to avoid the dissolution of water or oxygen into the solution and
was used without further purification. The electrode potentials are
referenced with respect to the lithium reference electrode.
The STM measurement was carried out in the argon-filled glove
box using a Nanoscope E control unit ͑Digital Instruments͒ and a
PicoSPM scanning unit ͑Molecular Imaging͒ with a homemade elec-
trochemical cell made of poly͑chlorotrifluoroethylene͒ ͑PCTFE͒.
The STM tip was mechanically cut 20% Ir-Pt wire (␾ ϭ 0.3 mm͒
coated with Apiezon wax. The scanning unit and a homemade vi-
bration isolator were placed in the glove box, while the control unit
was placed outside the glove box. The scanning unit was electroni-
cally connected to the control unit through an airtight connector.
Prior to STM measurement, the gold single crystal was annealed by
a gas flame for a few min and cooled under argon gas stream. The
annealed gold substrate was transferred to the glove box as kept in a
hexane-filled glass container. The gold single crystal was then fixed
on the electrochemical STM cell. Argon gas in the glove box was
circulated through catalysts to remove oxygen and water. The oxy-
gen concentration in the glove box was kept below 1 ppm, typically
0.1-0.2 ppm. The dew-point temperature in the glove box was less
than the lower limit of the hygrometer, i.e., below Ϫ100°C. The
blower for argon circulation and a vacuum pump for controlling the
inner pressure of the glove box were stopped during the STM mea-
surement. The oxygen concentration in the glove box was a few
ppm, at most 5 ppm, even 12 h after the STM experiment without
any argon circulation. The water concentration was still under the
lower limit of the hygrometer after the STM measurement.
Cyclic voltammetry was carried out using a three-compartment
glass cell in the glove box while argon was continuously circulated.
The electrochemical potential was controlled by a potentiostat
͑Hokuto Denko, HA-151͒, and the external potential was provided
by a function generator ͑Hokuto Denko, HB-111͒. The potentiostat
and the function generator were placed outside the glove box and
were electronically connected to the electrochemical cell through an
airtight connector. The annealing and transfer procedures of the gold
electrode were the same as for the STM measurement.
Figure 1. Cyclic voltammogram of Au͑111͒ electrode in the PC solution
containing 0.1 M LiClO4 at 50 mV/s. The rest potential of the electrode was
ca. 2.8 V, and ͑a͒ the first potential scan was initiated from this potential at a
scan rate of 50 mV/s. The potential was reversed at 0.2 V and was scanned
between 0.2 and 2.09 V in ͑b͒ the second cycle and ͑c͒ the cycle after
continuous scan for 10 min, i.e., as the steady state.
Results and Discussion
Electrochemical characteristics.—Figure 1 shows a CV of the
Au͑111͒ electrode in PC solution containing 0.1 M LiClO4. The rest
potential of the electrode was about 2.8 V, and the first potential
scan ͑Fig. 1a͒ was initiated from this potential at a scan rate of 50
mV/s. The potential was reversed at 0.2 V and was scanned between
0.2 and 2.09 V, which is the pzc of the Au͑111͒ electrode in this
solution as previously reported,19 in the following cycles. The re-
sults of the second cycle and of a cycle after a continuous scan for
10 min, i.e., of a steady state, are shown in Fig. 1b and c, respec-
tively. The positions of the cathodic peaks and value of the cathodic
current in the first and second negative-going potential scans in the
potential region of 0.5-2.0 V were very similar to those of the pre-
vious result, in which the electrode potential was scanned between
0.5 and 2.5 V.19 In the first scan, three cathodic peaks appeared at
1.5, 0.9, and 0.35 V, and the current rapidly decreased as the poten-
tial was scanned to a more negative value. After reversing the po-
tential scan at 0.2 V, the current became anodic at 0.6 V, reached a
peak at ca. 1.1 V, and then decreased as the potential became more
positive.
In the second negative-going scan, three new peaks were ob-
served at 0.6, 0.4, and 0.25 V. In the following positive-going scan,
the current changed to anodic at 0.4 V, which was more negative
than the value in the first scan. In addition to the anodic peak at ca.
1.1 V, a small shoulder at ca. 0.9 V, which was not observed in the
first scan, was observed. The peak current of the anodic peak at ca.
1.1 V was slightly larger than that of the first scan.
In the steady-state scan ͑Fig. 1c͒, the cathodic current in the
negative-going scan was smaller than those in the first and the sec-
ond scans, and that at 0.4 V was 67 ␮A/cm2. The cathodic peak at
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Journal of The Electrochemical Society, 150 ͑4͒ A532-A537 ͑2003͒
Figure 2. In situ STM images (200
ϫ 200 nm͒ of a Au͑111͒ surface in
PC solution containing 0.1 M LiClO4
at ͑a͒ 2.0, ͑b͒ 1.5, ͑c͒ 1.0, ͑d͒ between
1.0 and 0.91, and ͑e͒ between 0.91 and
0.85 V during the potential sweep.
Cross section of ͑e͒ at broken lines are
shown in ͑e ͒ and arrows in ͑d͒ and ͑e͒
Ј
show the scan direction of the tip. The
tip potential of ͑a͒ is 2.2 V and others
are 2.0 V. The tunneling current is 0.4-
0.8 nA. Scan areas of Fig. 2a-c differ
from each other but scan areas of Fig.
2c-e are the same.
around 0.6 V was significantly decreased and was observed only as
a shoulder. The cathodic peak at 0.25 V observed in the second scan
shifted negatively as the scan was repeated and was not observed in
the steady state. The anodic peak at ca. 1.1 V became larger with the
peak current of 36 ␮A/cm2. The cathodic shoulder at ca. 0.8 V
became smaller.
slower. Cathodic current started to flow even at ca. 0.9 V when the
scan rate was 1 mV/s. This result supports that lithium deposition
can take place as positive as 0.8 V.
Morphological change of the surface during first potential
scan.—Figures 2 and 3 show STM images of the Au͑111͒ surface
captured at various potentials. Figure 2a shows an STM image cap-
tured after the electrolyte solution was injected into the STM cell
while keeping the potential of the gold electrode at 2.5 V and then
the potential was stepped to 2.0 V. Two flat terraces separated by ca.
0.25 nm, i.e., a monoatomic step of Au͑111͒, were observed. No
faradaic current flowed, indicating that a reduction reaction had not
occurred yet. Thus, the surface film containing reduced products did
not form yet, but a very thin adsorbate layer of the solvent was
expected at this potential, as previously reported.19
The electrode potential was then swept negatively to 1.5 V at a
scan rate of 1 mV/s. A cathodic current flowed and STM images
became noisy during the potential sweep. Figure 2b shows an STM
image captured at 1.5 V after the cathodic current significantly de-
creased. Holes ca. 0.5 nm deep were observed on the flat terraces.
The potential was swept negatively again from 1.5 to 1.0 V at 1
mV/s. The cathodic current also flowed and the STM image became
noisy during the potential scan. Fig. 2c shows an STM image cap-
tured after the cathodic current significantly decreased. The terraces
separated by a monoatomic, i.e., 0.25 nm, step were still observed,
but many holes of typically 0.5 nm deep and various diameters
between several nanometers and several tens of nanometers were
found on the terraces. The formation of the surface film on a noble
As we have already reported,19 the cathodic current at ca. 1.5 V
was thought to be due to the reduction of water, which was con-
tained in the electrolyte solution as an impurity, and the absence of
the cathodic peak at around 2.0 V shows that no oxygen was con-
tained in the solution. The cathodic peaks between 0.4 and 0.6 V
were assigned to UPD of lithium, although the reason why two
peaks appeared is unclear.15,21 The cathodic peak at more negative
values than 0.3 V was considered to be due to alloy formation be-
tween lithium and gold.15,21 The reduction of solvent and/or anion
should also contribute to the cathodic current.15 The anodic peak at
ca. 1.1 V and a small shoulder at ca. 0.9 V were reported to be due
to the dissolution of UPD lithium and lithium-gold alloy,
respectively.15,21
The anodic peak at ca. 1.1 V was not observed when the
negative-going potential scan was reversed at 0.8 V. The charge
corresponding to the anodic peak, however, grew with the holding
time at 0.8 V, and it took 2 min to reach a limiting value of ca. 55
␮C/cm2, which is about half the anodic charge calculated from the
CV shown in Fig. 1c of 96 ␮C/cm2. This result shows that lithium
UPD on the Au͑111͒ surface took place even at 0.8 V, although the
deposition rate was rather slow. Scan rate dependence of CV showed
that the cathodic peak shifted positively as the scan rate became
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Journal of The Electrochemical Society, 150 ͑4͒ A532-A537 ͑2003͒
A535
Figure 3. In situ STM images (200
ϫ 200 nm͒ captured during second
potential scan following Fig. 2 at ͑a͒
2.5, ͑b͒ 1.0, and ͑c͒ between 1.0 and
0.85 V during potential sweep, and ͑d͒
at 2.50 V. Magnified image (30 ϫ 30
nm͒ of ͑d͒ is shown in ͑e͒. Cross sec-
tions of ͑a͒ and ͑b͒ at broken lines are
shown in ͑a ͒ and ͑b ͒, respectively.
Ј
Ј
Arrows show the scan direction of the
tip. The tip potential is 2.0 V, and the
tunneling current is 0.4-0.8 nA. Scan
areas of Fig. 3a, b, and d differ from
each other but scan areas of Fig. 2b
and d are the same.
metal electrode at potentials more negative than 1.5 V has been
confirmed in several reports by using Fourier transform infrared
͑FTIR͒, X-ray photoelectron spectroscopy ͑XPS͒, electrochemical
quartz crystal microbalance ͑EQCM͒, and surface-enhanced Raman
scattering ͑SERS͒.15,21-26 The dominant compositions of the surface
film were considered as ROCO2Li and/or Li2CO3, which were pro-
duced as a result of the one- or two-electron reduction of the
solvent.17,18 Actually, the cathodic charge between 2.0 and 1.0 V in
the first scan of CV ͑Fig. 1a͒ was ca. 400 ␮C/cm2 and is sufficient
to form the surface film covering the electrode surface of the gold
electrode with the one- or two-electron reduction process. The sur-
face shown in Fig. 2b should be covered by a surface film with a
uniform thickness of ca. 0.5 nm containing the reduction product of
the solvent, and the holes on the terraces should be defects in the
surface film. It must be noted that although it was reported that the
overlayer of the surface film formed on gold in PC solution contain-
ing LiClO4 was porous by several groups,17,27 the surface observed
in the present study was rather smooth. This may be because the
surface film of the present study was only 1 or 2 monolayers, while
the films in the previous reports were much thicker due to a pro-
longed reduction.
before, the nuclei height is too large for a lithium atom whose di-
ameter is only 0.3 nm. Thus, the nuclei should be a morphological
change of the surface film accompanied by lithium deposition. No
clear image was obtained at potentials more negative than 0.85 V.
Surface morphology after dissolution of lithium.—The electrode
potential was further scanned negatively and when the potential be-
came 0.6 V, the potential was stepped to 2.5 V. Figure 3a shows an
STM image captured at 2.5 V after the anodic current sufficiently
decreased so that noise in the STM image became negligible. Many
holes of several tens of nanometers in diameter were observed on
the terraces. The surface became very rough compared with the
surface before the lithium deposition ͑Fig. 2a͒. Worm-eaten holes of
ca. 10 nm diam were observed everywhere on the terrace, and the
number of steps increased compared to the surface shown in Fig. 2a.
The step height and depth of the holes were roughly a multiple of
0.5 nm, as shown in the cross section at the broken line ͑Fig. 3a ͒.
Ј
The EQCM measurement showed that the surface film formed on a
gold electrode in PC solution containing LiClO4 or LiAsF6 at po-
tentials more negative than 1.5 V did not dissolve even when the
potential became as positive as 3.0 V.23-25 The gold surface shown in
Fig. 3a captured at 2.5 V is expected to be still covered with the
surface film. There are two plausible reasons for the roughening of
the surface after the dissolution of lithium. One is the morphological
change of the surface film itself. In this case, the surface film was
roughened during lithium dissolution, while the gold electrode be-
neath the film still remained flat. The other is the roughening of the
gold surface covered with a film of uniform thickness.
The electrode potential was then scanned more negatively at 1
mV/s. Fig. 2d and e were captured during a negative potential scan
from 1.0 to 0.91 V and from 0.91 to 0.85 V, respectively. Many
nuclei appeared on the terrace when the electrode potential became
more negative than ca. 0.9 V ͑Fig. 2e͒. The nuclei were several tens
of nanometers in diameter and ca. 0.5-1 nm in height as presented in
the cross section of the terrace shown by the broken line ͑Fig. 2e ͒.
Ј
Although UPD of lithium took place at this potential as mentioned
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Journal of The Electrochemical Society, 150 ͑4͒ A532-A537 ͑2003͒
Morphological change of the surface during the second negative
potential scan.—The electrode potential was swept again to 1.0 V
with 10 mV/s. Figure 3b shows an STM image captured at 1.0 V
after the cathodic current significantly decreased. The hole density
significantly decreased after the negative potential scan. The step
heights of these terraces are ca. 0.25 nm as shown in Fig. 3b, and
this value is close to the monoatomic step height of the Au͑111͒
surface. Since no lithium deposition is expected in this potential
region, the decrease in the hole density should be the result of the
surface film formation. Actually, the surface film formation was re-
ported to take place in this potential region during the several initial
scans.23-25 Thus, the worm-eaten holes observed in Fig. 3a are con-
cluded to be the holes in the surface film, not in the gold. The film
is probably broken by intensive flux of the lithium ion. Since the
electrode surface is exposed to the electrolyte at the hole, the surface
at the holes should be electrochemically more active than other
places and therefore the reduction reaction of solvent and/or anion
should selectively proceed at the bottom of the holes, resulting in
filling of the holes by the reaction products.
As compared with Fig. 2, the number of step lines increased in
Fig. 3b. This cannot be explained by the hole formation. The anodic
shoulder of stripping lithium from the lithium-gold alloy was ob-
served at 0.8 V, and the alloy formation dissolution cycle of lithium
on gold is the probable reason for the generation of new steps on the
gold.
Figure 3c shows an STM image during a potential sweep from
1.0 to 0.85 V with 5 mV/s. The nucleation on the terraces was
observed and the nucleation density seems to become larger as the
potentials became negative. Compared to the result of the first po-
tential scan ͑Fig. 2e͒, the nucleation density in the same potential
region ͑lower 70% of Fig. 3c͒ was smaller than that of the first
potential cycle, but no obvious difference in size of the nuclei was
found. The difference in the nucleation density between the first and
the second potential cycles may reflect the difference in the densities
of the surface defect.
The electrode potential was swept to 0.6 V and then stepped to
2.5 V. Figure 3d and e shows STM images of 200 ϫ 200 nm and
30 ϫ 30 nm, respectively, at 2.5 V after the second potential cycle.
The surface structure was very similar to the one observed after the
first lithium dissolution ͑Fig. 3a͒. The worm-eaten surface should be
the result of the breakdown of the surface film as mentioned before.
The surface structure did not change even after the potential was
kept at 2.5 V for 30 min, in contrast to the result observed before the
lithium deposition where the growth of the island structure was
observed.19 This result shows that the adsorbate layer on the bare
gold surface was replaced by the surface film containing reduced
products of the solution, and no adsorbate layer remained on the
electrode surface based on the STM observation.
Figure 4. The schematic model of the formation and the change of the
surface film on a gold electrode in PC solution containing 0.1 M LiClO4
during two potential scan cycles: ͑a͒ at 2.0 V; ͑b͒ between 1.5 and 1.0 V; ͑c͒
at potentials more negative than 0.9 V; ͑d͒ during anodic stepping to 2.5 V;
͑e͒ at 2.5 V of the second potential scan cycle; ͑f͒ at 1.0 V; and ͑g͒ at
potentials more negative than 0.9 V.
tion of the surface film simultaneously start again from ca. 0.9 V
͑Fig. 4g͒. The ‘‘breakdown and repair’’ process in nanometer order
is repeated during the potential cycle of the electrode between 0.8
and 2.5 V.
Acknowledgment
This work was partially supported by a Grant-in-Aid for Scien-
tific Research ͑13554026͒ from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Hokkaido University assisted in meeting the publication costs of this
article.
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Conclusion
The surface film formation and lithium deposition process on a
gold electrode can be schematically summarized in Fig. 4. At 2.0 V,
no faradaic current flows and there is an adsorbate layer on the gold
surface as previously reported ͑Fig. 4a͒.19 A cathodic current flows
and a surface film of uniform thickness is formed on the gold sur-
face as the potential becomes more negative ͑Fig. 4b͒. When the
electrode potential becomes more negative than 0.9 V, lithium depo-
sition and further formation of the surface film simultaneously take
place ͑Fig. 4c͒. The lithium deposition seems to proceed at the de-
fect of the film and the film formation is further induced by the
lithium deposition. When the electrode potential is stepped to 2.5 V,
the dissolution of lithium takes place. The breakdown of the surface
film in nanometer order is induced by the intensified flux of the
lithium ion resulting in many worm-eaten holes of ca. 10 nm diam
after the dissolution ͑Figs. 4d-e͒. The step density on the gold is
increased maybe due to the alloy formation dissolution cycle. When
the electrode potential is swept negatively again, the surface film
formation predominantly takes place at the holes, resulting in the
filling of the holes ͑Fig. 4f͒. Lithium deposition and further forma-
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