Electrochemical performances of Sn anode electrodeposited on porous Cu foam for Li-ion batteries

Article abstract of DOI:10.1016/j.electacta.2012.01.084

A three-dimensional Sn electrode is fabricated by the electrodeposition of Sn on a porous Cu foam substrate that consists of grape-like Cu nanodeposits. It is revealed from the XRD results that a small amount of Cu₆Sn ₅ is formed at the interface of Sn deposits and the Cu foam, thereby forming Sn-Cu₆Sn₅/Cu foam electrode. The electrode shows better cyclic performance and higher charge capacity than the Sn electrode electrodeposited on a smooth Cu foil does. The improved electrochemical performances of the Sn-Cu₆Sn₅/Cu foam electrode is due to the fact that the porous Cu foam accommodates the volumetric expansion of Sn-Cu₆Sn₅, and hence inhibits the pulverization or delamination of the active materials from the substrate. Furthermore, ex situ XRD analysis and SEM observation demonstrate that Sn deposits electrodeposited on the Cu foam are gradually blended with the Cu deposits, causing the transformation of Sn into Cu₆Sn₅. The phase transformation of Sn into Cu₆Sn₅ enhances the bonding force between the Sn and the Cu foam, and reducing the volume changes of active materials during cycling.

Full text of DOI:10.1016/j.electacta.2012.01.084

Electrochimica Acta 66 (2012) 126–132
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical performances of Sn anode electrodeposited on porous Cu foam
for Li-ion batteries
Do Hwan Nam, Ryoung Hee Kim, Dong Wook Han, Hyuk Sang Kwon∗
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A three-dimensional Sn electrode is fabricated by the electrodeposition of Sn on a porous Cu foam sub-
strate that consists of grape-like Cu nanodeposits. It is revealed from the XRD results that a small amount
of Cu6Sn5 is formed at the interface of Sn deposits and the Cu foam, thereby forming Sn–Cu6Sn5/Cu
foam electrode. The electrode shows better cyclic performance and higher charge capacity than the Sn
electrode electrodeposited on a smooth Cu foil does. The improved electrochemical performances of the
Sn–Cu6Sn5/Cu foam electrode is due to the fact that the porous Cu foam accommodates the volumetric
expansion of Sn–Cu6Sn5, and hence inhibits the pulverization or delamination of the active materials
from the substrate. Furthermore, ex situ XRD analysis and SEM observation demonstrate that Sn deposits
electrodeposited on the Cu foam are gradually blended with the Cu deposits, causing the transformation
of Sn into Cu6Sn5. The phase transformation of Sn into Cu6Sn5 enhances the bonding force between the
Sn and the Cu foam, and reducing the volume changes of active materials during cycling.
© 2012 Elsevier Ltd. All rights reserved.
Received 15 November 2011
Received in revised form
30 December 2011
Accepted 17 January 2012
Available online 1 February 2012
Keywords:
Lithium ion battery
Tin anode
Copper foam
Porous structure
Phase transformation
1. Introduction
volumetric expansion of active materials but also enable fast elec-
trochemical reactions due to its high surface area [20–26].
With the increase in the demand for high-capacity power
sources with higher energy densities and power densities, the
development of new anode materials for Li-ion batteries has also
increased because graphite, which is currently used as a commer-
cial anode material, is inadequate for high-capacity power sources
due to its low theoretical capacity of 372 mAh g−1. Sn is one of the
most attractive anode materials that can be used as an alternative to
the graphite, owing to its high theoretical capacity of 991 mAh g−1
[1–6]. However, pure Sn anode has not been put into practical
use because of large volumetric changes (∼300%) during Li+ inser-
tion/extraction, which cause the pulverization and delamination of
active materials from current collector during cycling [3–6].
To improve the cyclic performance of Sn electrode, Sn–M inter-
metallic compounds have been designed to provide an inactive
phase that acts as buffer matrix against volumetric changes in
active materials, such as Sn–Fe [7], Sn–Ni [8,9], Sn–Cu [10–19], and
etc. Consequently, Sn–M alloy electrodes have exhibited improved
cyclic performances compared with pure Sn electrode [10,14]. In
addition, Cu substrates with foam structure have been used as cur-
rent collectors to enhance the electrochemical performance of Sn
electrode. They not only offer pores that can accommodate the
Shin et al. [27] reported the preparation of a thin Cu foam by
electrodeposition using hydrogen bubbles as a dynamic template
without polymer framework. The electrodeposited Cu foam has
several advantages when used as a current collector for electrodes
in Li-ion batteries: simple preparation, easy thickness control, and
large surface area. However, the Cu foam has two major problems
that occurs due primarily to the rapid Cu deposition at extremely
high cathodic overpotential; one is the low mechanical strength
of the Cu foam due to fragile Cu dendrites in the foam walls
[26–28], and the other is the poor surface coverage of electrode
materials results from the current concentration on Cu dendritic
deposits during electrodeposition of active material. Therefore, the
development of electrodeposited Cu foams with both an improved
mechanical strength and a good surface coverage is essential to the
fabrication of Sn electrode deposited on the Cu foam.
Recently, we successfully fabricated a three-dimensionally
porous Cu foam composed of grape-like deposits by electrodeposi-
tion using chemical additives, (NH4)2SO4 and BTA [29]. The porous
Cu foam showed not only much increased porosity with larger sur-
face area but also improved mechanical properties compared with
that having dendrite deposits. Thus, it is anticipated that the porous
Cu foam we have developed would be more appropriate as a sub-
strate for Sn electrodes rather than a smooth Cu foil or the Cu foam
with the dendrite foam walls.
The objective of the present work is to examine the electrochem-
ical performance of a Sn electrode electrodeposited on the porous
∗
Corresponding author. Tel.: +82 423503326; fax: +82 423503310.
E-mail address: hskwon@kaist.ac.kr (H.S. Kwon).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.01.084

D.H. Nam et al. / Electrochimica Acta 66 (2012) 126–132
127
Fig. 1. (a) SEM image and (b) TEM image of Cu foam prepared by electrodeposition. (c) SEM image of Sn electrode electrodeposited onto the Cu foam and (d) XRD patterns
for Sn electrode and Cu foam used as a substrate.
Cu foam with high porosity and enhanced mechanical strength,
with emphasis on the effects of the Cu foam on the morpho-
logical change and phase transformation of the electrode with
cycling.
the microstructural changes and phase transformation of the elec-
trode with cycling, ex situ SEM observation and ex situ XRD analysis
were performed.
3. Results and discussion
2. Experimental
3.1. Preparation of Sn/Cu foam electrode and its electrochemical
performances
A three-dimensionally porous Cu foam was prepared by elec-
trodeposition at a constant current density of −3 A cm−2 for 4 s from
a copper sulfate electrolytic bath containing 0.2 M CuSO4, 0.7 M
H2SO4, 1.2 M (NH4)2SO4 and 0.4 mM BTA (benzotriazole). The com-
position of the electrolytic bath used for the Cu electrodeposition
is described in the previous study [29]. Electrodeposition of Cu was
performed in a two-electrode cell in which a nodule-type Cu foil
was used as cathode and Cu plate as anode, respectively. The dis-
tance between cathode and anode was 3 cm. The bath solution was
stirred magnetically at 800 rpm, with its temperature being held at
room temperature, 25 ◦C.
Sn was electrodeposited onto the porous Cu foam substrate at a
constant current density of −15 mA cm−2 for 300 s in a tin sulfate
bath containing 0.18 M SnSO4, 1.52 M H2SO4 and 0.4 mM Triton X-
100, and then was used as electrode for half cell of Li-ion batteries.
The electrochemical performance of Sn electrode was investi-
gated using 2016 coin-type cells that had been assembled in an
Ar-filled glove box. The cell was composed of a sheet of the Sn/Cu
foam electrode (1 cm2 in area) and a Li-metal electrode with an
electrolyte consisting of 1 M LiPF6 dissolved in EC:DMC (1:1, v/v).
The charge/discharge characteristics of the electrode were galvano-
statically examined at a current density of 100 mA g−1 between
0.02 V and 1.0 V (vs. Li/Li+) at 30 ◦C.
Fig. 1a shows SEM image on the surface of the porous Cu foam
prepared by electrodeposition. Since the Cu foam has much finer
pores and thinner foam walls with sphere-shaped Cu nanoparti-
cles, it shows higher porosity and improved mechanical strength
[29], compared with those reported previously [23–27]. As clearly
demonstrated in Fig. 1b, TEM analysis revealed that the elec-
trodeposits in the Cu foam walls were not powdery crystals but
grape-like deposits. Interestingly, the small bumps formed on the
grape-like Cu deposits are made up of extremely fine nanocrys-
talline Cu particles less than 5 nm in size. The corresponding
selected area electron diffraction pattern (SAED) for these deposits
shows a clear ring-like pattern, confirming the formation of poly-
crystalline Cu [29]. Fig. 1c shows SEM image on the surface of Sn
electrode electrodeposited onto the Cu foam at a relatively low cur-
rent density of −15 mA cm−2 for 300 s. Evidently, Sn is uniformly
deposited on the porous Cu foam due probably to the relatively uni-
form current concentration on the grape-like Cu deposits during
the electrodeposition of Sn. The XRD pattern for the as-deposited
Sn electrode, shown in Fig. 1d, verifies the formation of Sn phase
and Cu6Sn5 intermetallic compound phase [15,17,23–26] in which
the ␩-Cu6Sn5 phase formed at the interface between Cu deposits
and Sn deposits [10–15]. The Sn electrode electrodeposited onto
the porous Cu foam is termed “Sn/Cu foam electrode”.
The surface morphology and phase structure of the Sn/Cu foam
electrode were examined by scanning electron microscopy (SEM)
and X-ray diffraction (XRD). The composition of the electrode was
analyzed by energy dispersive spectroscopy (EDS). To investigate
Fig. 2 exhibits the cycle performance of the Sn/Cu foam elec-
trode, which is compared with that of the Sn electrode deposited

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D.H. Nam et al. / Electrochimica Acta 66 (2012) 126–132
the morphological change of the electrode during cycling was
examined by ex situ SEM.
Fig. 3 represents SEM image on the microstructural change of
Sn/Cu foam electrode with cycling. As shown in Fig. 3b, the con-
nection between the active material of Sn and Cu foam substrate
was maintained after the insertion/extraction of Li+ into/from the
Li–Sn alloy; that is, the backbone of the foam structure remained
intact after the 1st cycle despite severe volume expansion and con-
traction during cycling, indicating that the electrode degradation
caused by the delamination of active materials from the substrate
was inhibited. Although the surface morphology of the electrode
did not sustain the “foam structure” after 5 cycles, the pores were
changed to a sufficient void space for accommodating the volume
expansion of the Sn–Cu6Sn5 active materials during cycling. Thus,
the Sn/Cu foam electrode did not show obvious delamination of the
active materials from the substrate even after 50 cycles, as depicted
in Fig. 3f.
Fig. 4 shows the internal microstructural changes of the elec-
trode with cycling. As shown in Fig. 4a, the cross-section of the
as-deposited Sn/Cu foam electrode clearly exhibits that the Sn
deposits (dark part) fully covered the Cu foam (bright part). After 1
cycle, although numerous nanopores were generated over the elec-
trode due to the severe volumetric changes of active materials, Sn
deposits still fully cover the Cu foam (Fig. 4b). This confirms that
the Cu foam not only accommodates the volumetric expansion of
Sn anode but also effectively inhibits the pulverization and delam-
ination of the Sn anode from the substrate. The most interesting
feature obtained from the cross-sectional observation is that the
Cu deposits and Sn deposits were gradually blended during cycling.
After 10 cycles, the further blending makes it difficult to discrim-
inate between the Cu deposits (bright one) and the Sn deposits
(dark one), but the mixed part of the electrode was identified as
a Sn–Cu alloy phase (molar ratio of Sn/Cu is 0.82) by EDS anal-
ysis. In the following cycles, the internal microstructure was not
changed further whereas the surface morphology of the Sn/Cu foam
electrode was transformed into a more micro-porous structure
(Fig. 4d).
Fig. 2. Cycle performance of the Sn/Cu foam electrode and that of a Sn electrode
deposited onto Cu smooth foil [30] at galvanostatic current densities of 100 mA g−1
potential range between 0.02 and 1.0 V (vs. Li/Li+).
,
onto a smooth Cu foil [30]. In this paper, the Li+ insertion into the
Sn/Cu foam electrode is termed discharging, and the Li+ extraction
from that as charging, based on the half-cell reaction. The initial
charge capacity (delithiation capacity) of the Sn/Cu foam electrode
(821.5 mAh g−1) is much higher than that of the Sn/smooth Cu
foil (655.5 mAh g−1). It is well known that the reversible charge
capacity of pure Sn electrode is abruptly degraded in a few cycles
due to the electrical isolation of the Sn active materials from
the substrate that is caused by extreme volume changes during
lithiation/delithiation [2–6]. The Sn/Cu foam electrode, however,
retains a charge capacity of 686.1 mAh g−1 after 20 cycles and
476.4 mAh g−1 after the 50th cycle that are corresponding to 83.5%
and 58.0% of its initial charge capacity, respectively. The results con-
firm that the porous Cu foam is more suitable as a substrate than a
smooth Cu foil for improving the cyclic performance and reversible
charge capacity of a Sn electrode.
Therefore the improvement in the cyclic performance of the
Sn/Cu foam electrode was attributed to the specific microstruc-
ture of the Cu foam with high mechanical strength and large
porosity that not only accommodates the volumetric expansion
of Sn–Cu6Sn5 active materials, but also effectively relieves the
internal stress during cycling [22–26]. In addition, the contact
area between Sn deposits and Cu foam gradually increased by the
3.2. Morphological change of Sn/Cu foam electrode with cycling
To investigate the effects of the porous Cu foam on the improve-
ment in the electrochemical performance of Sn/Cu foam electrode,
Fig. 3. Surface morphology of Sn/Cu foam electrode (a) as deposited, (b) after 1 cycle, (c) after 5 cycles, (d) after 10 cycles, (e) after 20 cycles, and (f) after 50 cycles.

D.H. Nam et al. / Electrochimica Acta 66 (2012) 126–132
129
Fig. 4. Cross-sectional images of the Sn/Cu foam electrode (a) as deposited, (b) after 1 cycle, (c) after 10 cycles, and (d) after 20 cycles.
repeated severe volumetric changes, and hence the Sn–Cu alloy
phase of Cu6Sn5 could be newly formed inside the electrode.
and then gradually decreased toward 0.02 V. Contrary to the rapid
potential drop during the first cycle, three potential plateaus at
0.75 V, 0.6 V, and 0.4 V were observed during the discharge process
of the 2nd cycle, which are in accordance with the lithiation poten-
tials of the Li–Sn cell [2–5]. In addition, a small plateau around 0.1 V
was observed in the second discharge curve, which is attributed to
the lithiation of Cu6Sn5 formed along the interface between the Sn
deposits and the Cu substrate [10–12]. It is well known that Cu6Sn5
could be transformed into an intermediate phase of Li2CuSn at 0.4 V,
and then eventually to the formation of the Li22Sn5 phase at poten-
tials below 0.1 V in Cu6Sn5/Li cell [15–19]. Therefore, the electrode
potential plateaus at 0.75 V, 0.6 V, 0.4 V, and 0.1 V in the discharge
curve of the Sn/Cu foam electrode indicate that the Sn and Cu6Sn5
simultaneously reacted with Li after the second cycle [15].
3.3. Phase transformation of Sn with cycling
Fig. 5 shows the discharge–charge curves of the Sn/Cu foam elec-
trode at a constant current density of 100 mA g−1 between 0.02
and 1.0 V (vs. Li/Li+). During the initial discharge process, the elec-
trode potential immediately dropped to 0.4 V after starting the test,
To examine the change in the lithiation/delithiation character-
istics of the Sn/Cu foam electrode, differential capacity plots of the
electrode were displayed in Fig. 6. Whereas the differential capac-
ity plot shows the peaks associated with the lithiation/delithiation
reactions of Sn with Li+ at the first cycle (Fig. 6a), the differential
capacity plot at the 5th cycle exhibits the similar profile to that of
the Cu6Sn5 electrode [16]. Furthermore, as shown in Fig. 6c and d,
the differential capacity plot of the electrode became more sim-
ilar to that of Cu6Sn5 during the following 15 cycles where the
peaks related to the reaction of Sn with Li+ were almost dimin-
ished while the peaks attributed to the reaction between Cu6Sn5
and Li+ were retained. Since the lithiation/delithiation character-
istics of the electrode is greatly dependent on the phase structure
of the electrode, the changes in the charge–discharge properties of
the Sn/Cu foam electrode demonstrate that crystalline ␤-Sn might
be amorphized [31] and a substantial amount of ␤-Sn was trans-
formed into the Cu6Sn5 phase with cycling. It is noteworthy that
Fig. 5. Charge–discharge curves of the Sn/Cu foam electrode at galvanostatic current
densities of 100 mA g−1 and a potential range between 0.02 and 1.0 V (vs. Li/Li+)
during initial cycle, 2nd cycle, 10th cycle, and 20th cycle.

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D.H. Nam et al. / Electrochimica Acta 66 (2012) 126–132
Fig. 6. Differential capacity plot of the Sn/Cu foam electrode (a) during the initial cycle, (b) 5th cycle, (c) 10th cycle, and (d) 20th cycle.
this additional formation of Cu6Sn5 (even after 10 cycles), which
has not been reported for the Sn anode, would be positive influ-
ence on the structural stability and electrochemical performance
of the electrode.
decrease in the peak intensity of the Sn phase, and this tendency
persists until the 20th cycle. In accordance with the differential
capacity plot discussed in the previous section, the ex situ XRD
analysis identifies that Sn was gradually amorphized and parts of
Sn were transformed to the Cu6Sn5 concurrently, which is magni-
fied by an increase in the contact area of between the Sn deposits
and Cu foam substrate.
As mentioned above, the most important reason for paying
attention to the transformation of Sn phase with cycling is that the
improved cyclic performance and the increased reversible charge
capacity of the Sn/Cu foam electrode may be owing to the phase
transformation of Sn into Cu6Sn5. Since the formation of Cu6Sn5
phase can inhibit the delamination of active materials by enhancing
the bond between active materials and Cu substrates. In addition, it
has been reported that the phase transition from Cu6Sn5 to Li22Sn5
To validate the phase transformation of Sn into Cu6Sn5 with
cycling, ex situ X-ray diffraction analysis for the Sn/Cu foam elec-
trode were performed, and the results are shown in Fig. 7. After
the 1st discharge process, most of the peaks relevant to the Sn
and Cu6Sn5 phases disappeared due to the transformation of the
active phases into Li–Sn alloys (Fig. 7b). Thence, the peaks for
Sn and Cu6Sn5 phases reappeared after the following charge pro-
cess, which demonstrates that the Sn–Cu6Sn5 composite electrode
reversibly reacted with Li+ [16–19]. One important consequence of
the ex situ XRD pattern for the Sn/Cu foam electrode after 5 cycles
is that the peak intensity of the Cu6Sn5 phase increased despite the
Fig. 7. XRD patterns for the Sn/Cu foam electrode (a) as-deposited, (b) after 1st discharge, (c) after 1 cycle, (d) after 5 cycles, (e) after 10 cycles, (f) after 20 cycles, and (g)
after 50 cycles.

D.H. Nam et al. / Electrochimica Acta 66 (2012) 126–132
131
Fig. 8. Schematic illustration of the phase transformation mechanism of the Sn/Cu foam electrode with cycling.
shows better capacity retention than that from Sn to Li22Sn5 owing
to the reduced volumetric change during cycling [10,14]. Therefore
the improvement in the cyclic performance of the Sn/Cu foam elec-
trode was attributed to the phase transformation of Sn into Cu6Sn5
as well as the specific microstructure of the Cu foam.
foam accommodates the volumetric expansion of Sn–Cu6Sn5 active
materials and effectively inhibits the pulverization and delamina-
tion of the materials. The other is the transformation of Sn into
Cu6Sn5 with cycling. Cross-sectional observation and ex situ XRD
analysis under cycling demonstrate that Sn deposits were gradually
blended with Cu deposits; hence, a substantial amount of Sn was
transformed into the Cu6Sn5. The electrochemical performance of
the Sn electrode is positively affected by the additional formation of
Cu6Sn5 because Cu6Sn5 not only enhances the bond between active
materials and substrates but also reduces the volumetric changes
in the active materials during cycling.
3.4. Phase transformation mechanism of Sn/Cu foam electrode
with cycling
Based on the results of cross-sectional observation, voltage pro-
file studies, and ex situ XRD analysis, the phase transformation of
the Sn/Cu foam electrode under cycling is suggested. The phase
transformation mechanism proposed here is distinguished from
that of the thin-film Sn anode deposited on the Cu smooth foil due
to the specific structure of the porous Cu foam.
Acknowledgments
This work was supported by the Growth Engine Technology
Development Program and the BK21 program funded by Korea
Ministry of Knowledge Economy.
Fig. 8 shows a schematic illustration of the microstructural
changes proposed for the Sn/Cu foam electrode with cycling. As
was previously mentioned in Section 3.1, the as-deposited elec-
trode is mainly composed of Sn/Cu6Sn5/Cu foam in which minor
quantity of Cu6Sn5 is formed at the interface of Sn and Cu foam, as
presented in Fig. 8a. After the 1 cycle, the backbone of the Cu foam
almost remained intact despite the severe volume expansion and
contraction of the active materials (Fig. 8b). During several cycles,
however, the Cu foam was gradually collapsed by repeated severe
volume changes of the active materials, which caused an increase
in the surface area of the Cu deposits, where the dealloyed Sn atoms
from the Li–Sn alloy could be absorbed (Fig. 8c). With the further
increase in cycle number, the Cu deposits and Sn deposits were
thoroughly blended, which leads to the transformation of the Sn
phase into Cu6Sn5 phase during the charge. Subsequently, after fur-
ther cycles, the Sn phase would be consisted of a Cu6Sn5 phase and
amorphous Sn phase (Fig. 8d).
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