An Sn-Fe/carbon nanocomposite as an alternative anode material for rechargeable lithium batteries

Article abstract of DOI:10.1016/j.electacta.2008.11.060

Sn-Fe/carbon nanocomposites were synthesized by the mechanochemical treatment of Sn with various amounts of an Fe/C composite through the pyrolysis of Fe(III) acetylacetonate. The composites were then evaluated as alternative anode materials for rechargea

Full text of DOI:10.1016/j.electacta.2008.11.060

Electrochimica Acta 54 (2009) 2699–2705
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
An Sn–Fe/carbon nanocomposite as an alternative anode material for
rechargeable lithium batteries
Sukeun Yoona, Jae-Myung Leea, Hansu Kimb, Dongmin Imb, Seok-Gwang Doob, Hun-Joon Sohna,∗
a Department of Materials Science and Engineering, Research Center for Energy Conversion and Storage, Seoul National University, Seoul 151-742, Republic of Korea
b Materials Laboratory, Samsung Advanced Institute of Technology, Yongin-si, Gyeounggi-do 449-712, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Sn–Fe/carbon nanocomposites were synthesized by the mechanochemical treatment of Sn with various
amounts of an Fe/C composite through the pyrolysis of Fe(III) acetylacetonate. The composites were then
evaluated as alternative anode materials for rechargeable lithium batteries. Based on the obtained ex
situ X-ray diffraction (XRD) data, X-ray absorption spectroscopy (XAS) results, and differential capacity
plots (DCPs), a reaction mechanism was suggested. It was found that increasing the amounts of the SnFe
phase and pyrolyzed carbon in the composite improved its electrochemical characteristics in terms of its
capacity retention.
Received 29 October 2008
Received in revised form
24 November 2008
Accepted 26 November 2008
Available online 6 December 2008
Keywords:
Lithium-ion batteries
Anode
© 2008 Elsevier Ltd. All rights reserved.
Sn composite
EXAFS
1. Introduction
anode, which consists of an amorphous ternary Sn–Co/carbon
material, has been found to exhibit excellent properties. For
Lithium-ion batteries have been widely used as power supplies
for portable electronic devices such as cellular phones and note-
book PCs. They are also expected to be used in future hybrid cars.
The incorporation of new functions in such devices has made it
necessary to increase the energy density of their batteries. How-
ever, the carbonaceous materials that are currently used as anodes
in commercial rechargeable lithium-ion batteries are not suitable
for the high-capacity batteries that will be required in the future
[1–3].
Silicon (Si) and tin (Sn) compounds that have large theoreti-
cal capacities and exhibit low operating voltages (vs. Li/Li+) have
been considered as alternative anode materials for lithium-ion bat-
teries. However, their use in practice has been hindered due to
their tendency to exhibit capacity fading. Capacity fading is caused
by drastic volume changes occurring during the charge–discharge
reaction [4–8]. A great deal of research has focused on ways to buffer
the volume change experienced by Si and Sn materials. Sugges-
tions have included the use of nano-sized particles, active–inactive
materials, amorphous materials, and composites containing carbon
[5,6,9–11].
instance, it has shown a high capacity and good capacity retention.
These qualities make this type of anode an attractive alternative
to the graphite anodes that are currently used in lithium-ion bat-
teries [12–15]. Despite these merits, an alternative to cobalt is
needed due to its high cost. For this reason, other Sn–M/carbon
(M represents a transition metal) composite materials have gained
attention. Examples include Sn–Fe/carbon [16–18], Sn–Mn/carbon
[19], Sn–Ti/carbon [13], Sn–V/carbon [13], and Ag–Fe–Sn/carbon
[20]. These have all been found to exhibit an enhanced cycling per-
formance compared to their corresponding non-carbon-containing
alloys. However, further studies of these composites are needed in
order to clarify their structures and reactions with lithium.
In this study, an Sn–Fe/carbon nanocomposite was synthesized
by the mechanochemical treatment of metallic Sn and an Fe/carbon
composite by the pyrolysis of Fe(III) acetylacetonate. Various ana-
lytical techniques were used to investigate the electrochemical
characteristics and structural changes of this composite material
when it is used as an anode material for lithium secondary batteries.
2. Experimental
Sony Corporation has recently launched a new lithium-ion bat-
tery that is marketed under the trade name Nexelion. Nexelion’s
The Sn–Fe/carbon nanocomposite was prepared by the follow-
ing procedure. First, the Fe/C composite was produced through the
pyrolysis of Fe(III) acetylacetonate powder (C15H21FeO6, 99.9+%,
Aldrich) at 900 ◦C under an Ar atmosphere for 1 h. The final prod-
uct was ground and sieved in order to obtain particles smaller than
∗
Corresponding author. Tel.: +82 2 880 7226; fax: +82 2 885 9671.
E-mail address: hjsohn@snu.ac.kr (H.-J. Sohn).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.11.060

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S. Yoon et al. / Electrochimica Acta 54 (2009) 2699–2705
200 mesh. The thermal decomposition behavior of the precursor,
viz., the Fe(III) acetylacetonate powder, was studied by thermo-
gravimetric analysis (TGA, Rheometric Scientific). The compositions
of the synthesized powders were analyzed by energy dispersive
spectroscopy (EDS). Sn powder (99.8%, <45 ␮m, Aldrich) and the
Fe/carbon composite powder, in a weight ratio of 50:50 (A), 40:60
(B), or 30:70 (C), were put into a hardened steel vial having a capac-
ity of 80 cm3. Weight ratios of Fe and carbon in each composite are
33:17 for A, 40:20 for B, and 47:23 for C, respectively. Stainless steel
balls were also added and the ball-to-powder ratio was 20:1. The
vial, which was assembled in an Ar-filled glove box, was installed on
a vibratory mill. Mechanochemical synthesis was performed using
a rotation speed of 500 rpm. Preliminary tests showed that the opti-
mum milling time considering the first cycle capacity, the first cycle
efficiency and cycle performance for 50 cycles was 40 h. To clar-
ify the mechanism of the reaction of the composite with lithium,
Sn2Fe and SnFe alloy samples were prepared through heat treat-
ment with stoichiometric amount of Sn and Fe (99.9+%, <10 ␮m,
Aldrich) powders at 450 ◦C in an Ar atmosphere for 12 h.
Fig. 1. Thermogravimetric analysis and XRD data of the pyrolyzed Fe(III) acetylace-
tonate powder.
The test electrodes consisted of the active powder material
(70 wt.%), carbon black (Super P, 15 wt.%) as a conducting agent,
and polyvinylidene fluoride (PVdF, 15 wt.%) dissolved in N-methyl
pyrrolidinone (NMP) as a binder. All of these components were
mixed together thoroughly to form a slurry, which was coated onto
copper foil substrates. These samples were then pressed and dried
under vacuum at 120 ◦C for 4 h. Laboratory-made coin-type elec-
trochemical cells were assembled in an Ar-filled glove box using
Celgard 2400 as a separator, lithium foil as the counter and refer-
ence electrodes, and 1 M LiPF6 in ethylene carbonate (EC)/diethyl
carbonate (DEC) (1:1 (v/v), Cheil Industries, Korea) as an electrolyte.
The charge–discharge experiments were performed galvanostati-
cally using a Maccor cycler at a constant current of 100 mA g−1 of
active material over a voltage range of 0.0–1.5 V (vs. Li/Li+).
To investigate any structural changes of the active material dur-
ing the electrochemical reaction, both ex situ X-ray diffraction
(XRD, MacScience, M18XHFSRA) with Cu K␣ radiation (ꢀ = 1.5418 Å)
and high-resolution transmission electron microscopy (HRTEM,
JEOL, JEM-3010) were carried out. Fe K-edge X-ray absorption
spectra (XAS) of the Sn–Fe/carbon nanocomposite powders were
recorded during the first two cycles using a BL7C1 (Electrochem-
istry) beamline, in a storage ring of 2.5 GeV, with a ring current
of 120–170 mA, at Pohang Light Source (PLS), Korea. A Si (1 1 1)
double-crystal monochromator was employed to monochromatize
the X-ray photon energy. Higher-order harmonic contaminations
were eliminated by detuning the monochromator to reduce the
incident X-ray intensity by ∼30%. The incident X-ray intensity
was monitored using ionization chambers filled with pure N2
gas. The spectroscopic data were collected in transmittance mode.
Energy calibration was carried out using standard metallic Fe
foil.
Fe3C [11,13,14]. The lack of Bragg peaks corresponding to a carbon
phase is probably due to the low crystallinity that results from the
mechanical milling. As the Fe/carbon composite content of the reac-
tant is increased, the intensity of the peak relating to the Sn2Fe
phase decreases while that of the SnFe phase increases.
Fig. 3 shows HRTEM images of the Fe/carbon composite (Fig. 3a)
and the Sn–Fe/carbon nanocomposite A (Fig. 3b), along with fast
Fourier transformed images corresponding to the selected regions.
The individual Fe particles are approximately 50–150 nm in size
(Fig. 3a). The fast Fourier transformed image of Fig. 3a corresponds
to crystalline Fe. The catalytic pyrolysis causes graphene layers to
adhere tightly to the surface of the Fe particles, as clear in Fig. 3a
[25]. However, these layers are easily broken during the subsequent
mechanochemical step. Fig. 3b shows images of the Sn–Fe/carbon
nanocomposite A. The Sn2Fe and SnFe phases that were previously
observed in the X-ray diffraction data are also shown to be present
by the fast Fourier transformed image. The agglomerated Sn2Fe and
SnFe nanoparticles are surrounded by carbon. The Sn2Fe and SnFe
crystallites are approximately 10 nm in size.
The first charge–discharge curves of the various Sn–Fe/carbon
nanocomposite electrodes, at a constant current of 100 mA g−1, over
a voltage range of 0.0–1.5 V (vs. Li/Li+), are shown in Fig. 4. The
first charge (Li-insertion) and discharge (Li-extraction) capacities of
3. Results and discussion
Thermogravimetric data for the decomposition of Fe(III) acety-
lacetonate in an Ar atmosphere at a heating rate of 5 ◦C min−1 and
the X-ray diffraction patterns of the pyrolyzed Fe(III) acetylaceto-
nate at different temperatures are given in Fig. 1. The weight loss
during the pyrolysis is approximately 85% at 480 ◦C, and carbon and
Fe present in the product at 33 and 67 wt.%, respectively. As shown
in the inset graph, the Fe3O4 phase starts to form at 235 ◦C, and this
phase is completely transformed into Fe and Fe3C phases when the
temperature reaches 900 ◦C [21–24].
Fig. 2 shows the X-ray diffraction patterns of the Sn-Fe/carbon
nanocomposites produced by mechanochemical synthesis using
three different weight ratios of Sn and Fe/carbon composite. The
diffraction peaks can be indexed to Sn2Fe, SnFe, unreacted Fe, and
Fig. 2. XRD patterns of the Sn–Fe/carbon nanocomposites obtained by
mechanochemical synthesis with Sn and the Fe/carbon composite of different
weight ratios; (a) A (50:50), (b) B (40:60), and (c) C (30:70), respectively.

S. Yoon et al. / Electrochimica Acta 54 (2009) 2699–2705
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Fig. 3. HRTEM images of (a) the Fe/carbon composite and (b) the Sn–Fe/carbon nanocomposite powders of A, also shown are fast Fourier transformed images corresponding
to the selected regions.
composite A are approximately 759 and 535 mA h g−1, respectively,
showing an initial cycling efficiency of 71%. Also, the first charge
capacities of composites (B and C) are 751 and 702 mA h g−1, respec-
decrease approximately to one-third in the next cycles. It is clear
that the capacity decreases as the amount of SnFe in the composite
increases.
tively. Their first discharge capacities are 494 and 456 mA h g−1
,
Since the composite contains both Sn2Fe and SnFe phases as
active materials, the electrochemical characteristics of pure Sn2Fe
and SnFe alloy electrodes were investigated. Fig. 5a and b shows
the voltage profiles of the pure Sn2Fe and SnFe electrodes, respec-
tively. The capacity fades rapidly in the case of the Sn2Fe electrode.
This behavior is similar to that observed for a pure Sn electrode.
The Sn2Fe phase has a tetragonal structure (space group I4/mcm)
with Sn atoms occupying 8 h sites and Fe atoms occupying 4a
sites. The tunnels that exist among the Sn atoms of the Sn2Fe
phase enable the Li to penetrate into the bulk of large grains,
thereby allowing more extensive reactions to take place [26]. Dur-
ing the first charge, the Sn2Fe phase is decomposed and a Li4.4Sn
(2ꢁ = 39◦) phase is formed, as shown by the X-ray diffraction data
(inset graph of Fig. 5a). When the Li is extracted during the dis-
charge, the reaction sequence is reversed. The reaction with Li
during the second cycle is similar to that observed during the
first cycle. On the other hand, the SnFe electrode exhibits a good
cycle performance, as shown in Fig. 5b. The SnFe phase has a
hexagonal structure (space group P6/mmm) with Sn atoms occu-
pying 1a and 2d sites and Fe atoms occupying 3f sites. However,
no tunnels exist among the Sn atoms of the SnFe phase [27].
Therefore, the Li only reacts with Sn atoms on the surfaces of the
grains and/or particles of the materials. Excluding the capacity con-
tributed by the conducting agent (carbon black, 37 mA g−1), the
fraction of the SnFe phase that is reacted is approximately 0.45.
with initial cycling efficiencies of 66 and 65%, respectively. The
contributions to the first charge capacity due to the presence
of carbon in the Sn–Fe/carbon nanocomposites are 68 mA h g−1
for A, 80 mA h g−1 for B and 92 mA h g−1 for C, and these values
Fig. 4. Voltage profiles for the Sn–Fe/carbon nanocomposites electrodes of A, B and
C.

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S. Yoon et al. / Electrochimica Acta 54 (2009) 2699–2705
To investigate the reaction mechanism of the Sn–Fe/carbon
nanocomposite A, ex situ X-ray diffraction analyses were performed
at selected potentials during the first and second cycles, as indi-
cated in the DCP (Fig. 6a). The results are shown in Fig. 7a. The
Bragg peaks of the Sn2Fe phase almost disappear at 0.12 V as the
charge proceeds, indicating that a reaction between the Sn2Fe phase
and Li is occurring. After the electrode has been fully charged,
except for the SnFe peak, all of the peaks in the XRD pattern
almost disappear. This indicates the decomposition of the Sn2Fe
phase at 0.0 V. The formation of the Li4.4Sn phase (2ꢁ = 38.8◦) can
be confirmed at 30 cycles. However, it is difficult to identify the
Fig. 5. Voltage profiles and ex situ XRD data for (a) Sn2Fe and (b) SnFe alloy elec-
trodes.
The unreacted residual SnFe phase (as shown by the X-ray diffrac-
tion data (inset graph of Fig. 5b)) and the liberated Fe atoms on
the surfaces of the grains and/or particles act as a stable, con-
ducting, inactive matrix that suppresses the aggregation of Sn
atoms.
Fig.
6 displays the differential capacity plots (DCPs) for
the various Sn–Fe/carbon nanocomposite electrodes. The alloy-
ing/dealloying of lithium with Sn and carbon over a potential range
of 1.5–0.0 V (vs. Li/Li+) would yield various LixSn (x ≤ 4.4) alloys and
LizC phases. During the charge in the first cycle, the electrolyte may
decompose at approximately 0.75 V to form a solid electrolyte inter-
face (SEI) on the surfaces of the active materials [28]. In the case of
composites A (Fig. 6a) and B (Fig. 6b), after the first cycle, the DCPs
become similar for subsequent cycles. In general, the intermetallic
structure (Sn–M, M indicates a transition metal) is broken down to
form Li–Sn alloys.The original crystal structure decomposes and ␤-
Sn is formed. During the discharge near 0.75 V, the ␤-Sn aggregates
into large tin clusters as the number of cycles increases. This has
previously been suggested by Courtney and Dahn [29] and identi-
fied by Retoux et al. [30] and Kim et al. [6]. The cycleability of the
electrode decreases because the large tin regions are more sensitive
to cracking and crumbling. On the other hand, for the composite C
(Fig. 6c), the DCPs are rather smooth due to the presence of large
amounts of SnFe phase and pyrolyzed carbon. After the tenth cycle,
the DCP profiles overlap each other, reflecting the reversible nature
of the process. This may be connected with the presence of unre-
acted residual SnFe, liberated Fe atoms, and carbon. Together, these
form an inactive matrix that suppresses the aggregation of the Sn
atoms, as mentioned above.
Fig. 6. DCPs of the Sn–Fe/carbon nanocomposites obtained by mechanochemical
synthesis with Sn and the Fe/carbon composite of different weight ratios: (a) A, (b)
B, and (c) C, respectively.

S. Yoon et al. / Electrochimica Acta 54 (2009) 2699–2705
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Fig. 7. (a) Ex situ XRD data, (b) and (c) are Fourier transform (FT) of the EXAFS data of the Sn–Fe/carbon nanocomposite electrode of A during the first and second cycles,
respectively.
Li4.4Sn phase from the ex situ XRD patterns obtained during the
first few cycles. The delithiation of the Li4.4Sn phase and the ref-
ormation of the Sn2Fe phase occur during the discharge. After 30
cycles, however, the Sn2Fe phase is not recovered due to the entire
collapse of the original structure as a result of the volume expan-
sion/contraction and the aggregation of ␤-Sn that occur during the
charge–discharge process. This can be expected considering the
presence of the peak near 0.75 V during the discharge, as shown in
Fig. 6a. The intensity of the peak corresponding to the SnFe phase is
reduced but remains during the cycle, indicating that the SnFe par-
tially reacts with lithium, as mentioned previously. Based on the
above discussion, the electrochemical reaction of the Sn–Fe/carbon
nanocomposite with lithium during the cycle can be expressed as
follows (reaction between carbon present in the composite with Li
is not included):
Charge: (0 < x < 1, 0 < y < 1)
8.8Li + Sn2Fe → 2Li4.4Sn + Fe
4.4xLi + SnFe → xLi4.4Sn + xFe + (1 − x)SnFe(residualphase)
Discharge:
2Li4.4Sn + Fe → ySn2Fe + 2(1 − y)ˇ-Sn + (1 − y)Fe
xLi4.4Sn + xFe + (1 − x)SnFe(residualphase)
→ xSn + xFe + (1 − x)SnFe(residualphase)

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in Table 1) were calculated from the electrode thickness and the
weight of the active material. The first volumetric charge capaci-
ties of the Sn–Fe/carbon nanocomposite electrodes of A, B and C
are 1548, 1411 and 1277 mA h cm−3, respectively, while their dis-
charge capacities are 1091, 930 and 829 mA h cm−3, respectively.
These values demonstrate that the Sn–Fe/carbon nanocomposite
electrodes possess a relatively large volumetric capacity compared
to the graphite electrodes (∼500 mA h cm−3) currently used in
commercially available lithium-ion batteries. Among the compos-
ites tested, the enhanced reversible lithium-storage capability and
the improved cycle performance of the Sn–Fe/carbon nanocom-
posite electrode C are related to the presence of large amounts
of SnFe phase and pyrolyzed carbon. This is because the resid-
ual SnFe phase, the Fe atoms liberated during the lithiation, and
the pyrolyzed carbon should all act as a stable, conducting, inac-
tive matrix that suppresses the growth of the Sn grains and
buffers against the volume expansion–contraction that occurs dur-
ing cycling.
Fig. 8. Comparison of the cycle performances of the Sn–Fe/carbon nanocomposite
electrodes of A, B and C.
4. Conclusions
Sn–Fe/carbon nanocomposite materials were synthesized by a
mechanochemical method using Sn and Fe/carbon composite pow-
ders. The Fe/carbon composite was produced by the pyrolysis of
Fe(III) acetylacetonate. The Sn–Fe/carbon nanocomposites were
investigated as anode materials for lithium rechargeable batter-
ies. It was found that increasing the amounts of the SnFe phase
and pyrolyzed carbon in the Sn–Fe/carbon nanocomposite elec-
trode improved its electrochemical characteristics in terms of its
cycleability. The superior cycling performance of this type of com-
posite electrode is attributed to the suppressed Sn grain growth and
the reduced volume expansion–contraction during cycling.
XAS analyses were performed on the Fe atoms in the electrode
material at selected potentials during the first and second cycles
in order to gain further insight into the local structural variations
and to confirm the preceding reaction scheme. Representative Fe
K-edge EXAFS spectra of the Sn–Fe/carbon nanocomposite A elec-
trode at various stages of lithiation/delithiation are shown in Fig. 7b
and c. The symmetric FT peaks at ∼1.92 and 2.04 Å correspond to
the Fe–Sn bonds, which are the first nearest neighbors of the SnFe
and Sn2Fe phases, respectively, and these two peaks are located
very close to each other. At the fully charged state, the peak of the
Fe–Sn bond of the Sn2Fe phase disappears, indicating the structural
decomposition of the Sn2Fe phase. During discharge, the spectral
recovery of the peaks related to the Sn2Fe phase indicates that the
structural variation of Sn2Fe is reversible during the first cycle. Sim-
ilar behavior can be observed during the second cycle, as shown in
Fig. 7c. These results are in good agreement with the previous XRD
data.
The cycle performances of the various Sn–Fe/carbon nanocom-
posite electrodes can be compared by looking at Fig. 8. These data
were obtained by testing the electrodes over a voltage range of
0.0–1.5 V, at a constant current of 100 mA g−1. Drastic capacity fad-
ing is observed for the Sn–Fe/carbon nanocomposite A electrode
after 40 cycles. On the other hand, the Sn–Fe/carbon nanocom-
posite electrodes of B and C deliver lithium storage capacities of
751 and 702 mA h g−1, respectively. And 64 and 83% of their initial
discharge capacities of 494 and 456 mA h g−1 are retained, respec-
tively, after 100 cycles. The gravimetric and volumetric capacities
of the Sn–Fe/carbon nanocomposite electrodes are compared in
Table 1. In each case, the volumetric capacity on the composite
electrode does not include the capacity from the conducting agent
(Super P) which reacted with Li. The electrode densities (also given
Acknowledgments
The authors wish to thank to Pohang Light Source(PLS) for the
XAS measurements. This research was financially supported by
Samsung Advanced Institute of Technology and the Korea Science
and Engineering Foundation (KOSEF), through the Research Cen-
ter for Energy Conversion and Storage at Seoul National University
(Grant No. R11-2002-102-02001-0).
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Table 1
Gravimetric and volumetric capacities, and the densities of the Sn–Fe/carbon
nanocomposite electrodes.
Weight ratio (%)
Sn–Fe/carbon
Sn–Fe/carbon
Sn–Fe/carbon
(A) (30-47/23) (B) (40-40/20) (C) (50-33/17)
Electrode density
2.14 2.21 2.40
702 751 759
Charge capacity (mA h g−1
)
Discharge capacity (mA h g−1
)
456
1277
829
65
494
1411
930
66
535
1548
1019
71
Charge capacity (mA h cm−3
)
Discharge capacity (mA h cm−3
1st cycle efficiency (%)
)
[21] JCPDS, File No. 39-1346.
[22] JCPDS, File No. 87-0722.
[23] JCPDS, File No. 75-1621.
After 100th cycles efficiency (%)
83
64
23

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