Electrochemical lithiation of new graphite-nanosized tin particle materials obtained by SnCl2 reduction in organic medium

Article abstract of DOI:10.1016/j.electacta.2005.09.045

SnCl₂ was reduced in the presence of graphite by t-BuONa-activated NaH. The resulting (tin/graphite)-based system was composed of nanosized tin particles deposited on the graphite surface and of free tin aggregates. Lithium electrochemical insertion occurs in graphite and in tin. A reversible specific charge of 500 mAh/g is found stable upon cycling. This value is lower than the maximum theoretical one (650 mAh/g) assuming a Sn/12C molar composition and the formation of the highest lithium content alloy Li₂₂Sn₅. It is suggested that the part of tin responsible for the stable reversible capacity is the one bound to graphite. To the contrary, free tin aggregates could contribute to a capacity which decreases upon cycling in connection with the volume changes accompanying lithium insertion/extraction in/out of these aggregates.

Full text of DOI:10.1016/j.electacta.2005.09.045

Electrochimica Acta 51 (2006) 3385–3390
Electrochemical lithiation of new graphite–nanosized tin particle
materials obtained by SnCl reduction in organic medium
2
a
b
a
c
a,∗
L. Balan , R. Schneider , J. Ghanbaja , P. Willmann , D. Billaud
a
LCSM, UMR CNRS 7555, UHP Nancy I, BP 239, 54506 Vandoeuvre-les-Nancy Cedex, France
LSOR, UMR CNRS 7565, UHP Nancy I, BP 239, 54506 Vandoeuvre-les-Nancy Cedex, France
b
c
CNES, 18 Avenue E. Belin, 31055 Toulouse Cedex, France
Received 9 May 2005; received in revised form 29 September 2005; accepted 29 September 2005
Available online 21 November 2005
Abstract
SnCl was reduced in the presence of graphite by t-BuONa-activated NaH. The resulting (tin/graphite)-based system was composed of nanosized
2
tin particles deposited on the graphite surface and of free tin aggregates. Lithium electrochemical insertion occurs in graphite and in tin. A reversible
specific charge of 500 mAh/g is found stable upon cycling.
This value is lower than the maximum theoretical one (650 mAh/g) assuming a Sn/12C molar composition and the formation of the highest
lithium content alloy Li22Sn . It is suggested that the part of tin responsible for the stable reversible capacity is the one bound to graphite. To the
5
contrary, free tin aggregates could contribute to a capacity which decreases upon cycling in connection with the volume changes accompanying
lithium insertion/extraction in/out of these aggregates.
©
2005 Elsevier Ltd. All rights reserved.
Keywords: Lithium-ion battery; Graphite; Tin; Anodic material; Chemical reduction
1
. Introduction
multi-phase systems or alloys [5–12]. An alternative way for
improving the electrode consists in associating to carbonaceous
materials lithium storage metals of high specific charge [13–17].
In recent papers, we showed that the reduction of some metal
Carbonaceous materials are commonly used as negative elec-
trodes in commercial lithium-ion batteries [1]. With graphite-
based anodes, the maximum theoretical specific charge of
chlorides (SbCl , BiCl3, SnCl2) by the first stage KC8 graphite
5
3
72 mAh/g corresponds to the formation of the stage I LiC6
intercalation compound in THF medium led to the formation
of Sb, Bi and Sn-based graphite materials able to insert lithium
reversibly [18,19].
graphite intercalation compound. Lithium intercalation occurs
at a low voltage, typically lower that 0.3 V versus Li /Li and the
+
volume change accompanying Li intercalation/deintercalation,
as low as 10%, allows the electrode to maintain its integrity
upon cycling. The demand to further increase both electrode
specific charges and long life cycling has stimulated the research
for new storage electrode materials [2–4]. Among them, metals
able to alloy lithium have been proposed as alternative elec-
trodes which can display large theoretical storage capacities.
However, theysufferfrompoorlong-termcyclingstabilitydueto
the large volume changes occurring during lithium insertion and
extraction. It has been demonstrated that these drawbacks can be
partly overcome by reducing the metal particle size and by using
The evolution of their electrochemical performances upon
cycling was discussed in terms of particle size, crystalline or
amorphous character of the lithium storage element and more
likely of their ability to form bonds with graphite [18,20].
Bi and Sb/graphite composites were also obtained by reduc-
tion in the presence of graphite of BiCl3 and SbCl5 with
alkoxide-activated NaH in organic media [16,17,20]. In that
case, graphite-supported nanosized metal particles are embed-
ded in an organic matrix constituted of the alkoxide. The
graphite/Sb system exhibits an improved and stable reversible
capacity upon cycling.
In this communication, we studied the reduction of SnCl2
by t-BuONa-activated NaH with the aim to obtain graphite/Sn-
based composites of high specific charge for their use as negative
electrode materials of Li-ion batteries. Both structure and mor-
∗
Corresponding author. Tel.: +33 3 83 68 46 22; fax: +33 3 83 68 46 23.
E-mail address: billaud@lcsm.uhp-nancy.fr (D. Billaud).
0
013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.09.045

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L. Balan et al. / Electrochimica Acta 51 (2006) 3385–3390
phology of these materials were characterized by X-ray diffrac-
tion (XRD) and transmission electron microscopy (TEM).
Their suitability as anodes materials in Li-ion batteries was
evaluated by classical galvanostatic and voltammetry tech-
niques.
activated NaH as the reducing agent [21]. The chemical reaction
that we employed for the preparation of the graphite/tin-based
material can be formulated as:
SnCl2 + 2t-BuOH + 4NaH + graphite
THF
−
→◦ Sn(0)/2t-BuONa/graphite + 2NaCl + 2H2 ↑
2
. Experimental
65 C
Anhydrous tin(II) chloride (Aldrich) was used without purifi-
A slight excess of NaH is added to the reaction medium to
cation. THF was distilled from sodium benzophenone adduct
and stored over Na wires. Tert-butyl alcohol (t-BuOH) was dis-
tilled over Na. Sodium hydride (65%, NaH Fluka) in mineral oil
was used after two washings with THF.
insure the complete reduction of SnCl . The C/Sn atomic ratio
is equal to 12.
Our technique to synthesize Sn(0) nanoparticles takes advan-
tage of the weak coordinating properties of t-BuONa which acts
as a stabilizer and avoids aggregation of the particles generated
in the course of the reduction.
2
The synthesis was handled under standard airless techniques
in a nitrogen atmosphere according to a procedure described in a
previous paper [21]. In a Schlenk tube, t-BuONa-activated NaH
2
and graphite (UF4 Carbone Lorraine, surface area of 10 m /g)
3.2. X-ray characterization
were stirred in anhydrous THF for 5 min. SnCl2 is then added
to the reaction medium in one portion. The Schlenk contents
Fig. 1 presents the XRD patterns of the crude material result-
◦
were further stirred at 65 C for 15 min. After completion of the
ing from the reduction of SnCl2 by t-BuONa-activated NaH, in
the presence of graphite (a), after ethanol washing (b) and after
ethanol and water washings (c). The SnCl2 reduction is effective
since reflections of both Sn(0) and NaCl are present while those
of SnCl2 are no more visible. The excess of NaH was removed
by ethanol washing (Fig. 1b) while NaCl can be eliminated by
further washing with water (Fig. 1c). Low intensity reflections
of Na2SnO2·3H2O are present in Fig. 1c indicating the presence
of small amounts of this oxidized product after washing with
water. Graphite reflections can also been seen. The graphite
reaction, the solution was rotary evaporated under air to remove
volatile organic, which led to the obtention of a grey powder of
t-BuONa-stabilized Sn(0) particles and graphite. Afterward, all
handling were performed under nitrogen atmosphere.
The reduction products were characterized by X-ray diffrac-
tion (XRD) using the Mo K␣ radiation of an automated
powder diffractometer (Rotaflex Ru-200B, Rigaku generator
and CPS 120 INEL detector transmission assembly). Electron
microscopy studies (imaging, selected area electron diffraction
(
SAED)) were carried out by using a Philips CM20 microscope
0
0 2 reflection at 336 pm indicates that graphite is not modified
operated at 200 kV. Elemental analysis was performed by energy
dispersive X-ray spectrometry (EDXS) using an EDAX spec-
trometer.
For TEM characterizations, the samples were previously dis-
persed in THF by sonication. A drop of the obtained suspension
was deposited on a carbon observation grid which was further
introduced into the microscope column.
during the reduction process. The coherence length along the
c-axis of graphite, Lc, was estimated from the value of the
full width at half height of the 0 0 2 reflection, according to
the Scherrer formula (Lc = 0.9λ/β cosθ). A Lc value of 20 nm
was found. Tin reflections can be indexed in the tetragonal
structure of tin. The average size of the tin crystalline domains
is equal to 10 nm. Subsequent ethanol and water washings
Electrochemical insertion of lithium was performed in half
cells. A lithium ribbon was used both as reference and aux-
iliary electrode. The electrolyte was a LiClO4 (1.5 mol/kg)-
ethylene carbonate (EC) solution. A 1-methyl-2-pyrrolidinone
slurry of the graphite/Sn-based composite (95 wt.%) and of the
polyvinylidene fluoride (PVDF) binder (5 wt.%) was used to
coat a copper current collector. A Mac Pile II system operating
either in galvanostatic or voltammetry modes was used. In the
voltammetry mode, the current was monitored when the voltage
was scanned by steps at a low rate (2.5 mV/2 min) between 0 and
+
2
.5 V versus Li /Li. In the galvanostatic mode, the output volt-
age was recorded for a specific current cell of 7 ␮A/mg applied
for 6 min followed by a 10 s relaxation period.
3
. Results and discussion
3
.1. Elaboration of the graphite/tin-based material
We have recently reported a low temperature method for the
synthesis of nanoscale tin(0) particles which uses t-BuONa-
Fig. 1. XRD patterns of the Sn/graphite-based system (a) as-synthesized, (b)
after EtOH washing and (c) after H2O washing.

L. Balan et al. / Electrochimica Acta 51 (2006) 3385–3390
3387
Fig. 2. TEM studies of the as-prepared graphite/Sn material: (a) bright field micrograph and (b) EDXS analysis.
do not modify the structural parameters of both graphite and
tin.
the micrograph presented in Fig. 3a. Further washing with water
causes a strong agglomeration of tin particles due to the com-
plete removal of the organic matrix surrounding Sn(0) particles
(
5
Fig. 3b). The average size of these tin aggregates is close to
0 nm. The corresponding SAED pattern insert of Fig. 3b dis-
plays dotted rings characteristic of the tetragonal structure of tin
space group: I41/amd).
3
.3. TEM characterization
Fig. 2ashowsthebrightfieldmicrographofthecrudematerial
(
obtained after reduction of SnCl2 by t-BuONa-activated NaH
and deposited on the TEM observation grid after its dispersion
in THF by sonication. Therefore, it must be pointed out that the
TEM micrographies cannot represent exactly the texture and
organization. They give however an idea of the nature and the
size of the observed particles. The average diameter of tin par-
ticles is 10.0 nm. Examination of the organic phase by EDXS
confirms the presence of carbon, oxygen, sodium and tin in the
material in agreement with the presence of the likely excess
NaH, NaCl and t-BuONa (Fig. 2b). Washing the crude material
with ethanol results in an aggregation of tin particles. The aver-
age diameter of these aggregates is close to 16 nm as shown in
3.4. Electrochemical study
Fig. 4 presents selected current versus potential curves
obtained for the Sn/graphite-based system, at a rate of
2.5 mV/2 min, after ethanol and water washings. The working
electrode was prepared by dispersing the material (95 wt.%)
and the polyvinylidene fluoride (PVDF) binder (5 wt.%) in 1-
methyl-2-pyrrolidinone. The negative current observed between
+
2.5 and 1 V versus Li /Li during the first reduction can be
attributed to the irreversible reduction of tin oxides formed
Fig. 3. TEM micrographs of the graphite/Sn material after washing with EtOH (a) and with H2O (b). The inset shows the related SAED pattern.

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L. Balan et al. / Electrochimica Acta 51 (2006) 3385–3390
Fig. 4. Selected current vs. potential curves of the graphite/Sn-based system obtained after washing with EtOH and H2O.
during the reduction and washing processes according to the
following reactions:
volume variations accompanying the alloying/dealloying pro-
cesses, are likely responsible for the pulverization of these
aggregates in smaller particles which are electrically discon-
nected with consequently a decay of the electrode activity.
Fig. 5 shows selected charge/discharge curves of an electrode
containing the active Sn/graphite-based composite (95 wt.%)
washed with ethanol and water and the PVDF binder (5 wt.%).
A specific current of 20 ␮A/mg is applied in a 0–2.4 V potential
range. The potential plateaux correspond to the peaks observed
on the related voltammograms. The C/Sn atomic ratio is equal
to 12, according to the quantities of reactants and to elemental
analysis results.
+
−
SnO2 + 2Li + 2e → SnO + Li2O
+
−
SnO + 2Li + 2e → Sn + Li2O
In the literature, the reductions of SnO2 and SnO occur at
1
.88 and 1.58 V, respectively [22,23]. Below 1.0 V, the negative
currents can first be related to the likely formation of the solid
electrolyte interphase (SEI) on graphite and Sn surfaces [5,24],
then to the LixSn alloys formation [22,25–27] and finally to
lithium intercalation into graphite [28].
According to previous data, lithium tin alloys (Li2Sn5,
LiSn, Li7Sn3, Li Sn2, Li13Sn and Li22Sn ) are formed in a
The expected theoretical maximum capacity of the
active12C/Sn system is equal to 650 mAh/g, assuming the for-
mation of the highest lithium content alloy, Li22Sn5. During
the first reduction, a capacity of 993 mAh/g is found, in which
about 345 mAh/g are connected to irreversible reactions related
to the SEI formation and to tin oxide reduction. This irreversible
capacity decreases with the cycle number.
The reversible capacity decreases from 650 mAh/g (1st cycle)
to a stabilized value of 500 mAh/g (10th cycle) as shown in
Fig. 6.
The decrease of the irreversible capacity with the cycle num-
ber observed in the galvanostatic curves is related to a shortening
of the reversible capacity in the 0.5–0.2 V potential range in
which lithium tin alloy formation occurs. As discussed in our
previous papers, the decrease of the reversible capacity could be
due to lithium insertion into free tin particles and aggregates not
5
5
5
0
.76–0.38 V potential range in agreement with our experimen-
tal data. Peaks are also present in the oxidation process. Between
.4 and 0.8 V, they correspond to the reversible lithium dealloy-
0
ing. Around 0.2 V, they are related to lithium deintercalation out
of graphite. Profiles of the first voltammograms and the follow-
ing ones differ significantly. Tin oxide reduction which occurs
during the first reduction leads to the formation of the insulating
Li2O that likely influences the kinetics of lithium insertion in tin
and graphite.
Upon cycling, the intensity of the peaks related to lithium
insertion into tin decreases before being stable. As previously
discussed in a previous paper, the presence of tin aggregates
not bound to graphite could contribute to the decrease of the
reversible capacity of tin [19]. In such conditions, the large

L. Balan et al. / Electrochimica Acta 51 (2006) 3385–3390
3389
Fig. 5. Selected charge and discharge curves of graphite/Sn composite washed with ethanol and water.
of Sn(0) particles, graphite, an organic matrix and NaCl. Tin
crystallized in the expected tetragonal structure is present either
as nanosized particles dispersed on the graphite surface or as
large free aggregates.
Electrochemical lithium insertion in such tin–graphite sys-
tems gives evidence for lithium intercalation into graphite and
for lithium–tin alloys formation. A stable reversible capacity of
ca. 500 mAh/g was found. This value is lower than the maxi-
mum theoretical capacity of 650 mAh/g assuming the formation
of the highest lithium content alloy, Li22Sn . It appears that only
5
apartoftinisinvolvedintheinsertion/deinsertionreversiblepro-
cess. It is hypothesized that this reversible alloying/dealloying is
related to the fraction of tin bound to graphite. The other part of
free tin aggregates is supposed to behave as massive tin in which
lithium insertion generates cracks and pulverization of the elec-
trode with consequently a loss of electrical contact between tin
particles and a capacity decay upon cycling.
Fig. 6. Evolution of the reversible capacity upon cycling of the graphite/Sn
composite.
bound to graphite [21]. As a matter of fact, lithium insertion in
such free aggregates leads to dramatic volume expansions gen-
erating cracks and crumbling of the electrode with consequently
the electrical insulation of tin particles leading to the electrode
capacity fade. The stable experimental reversible capacity of
Modification of the experimental conditions of SnCl reduc-
2
tion affects the tin particle size, the larger one being obtained
5
00 mAh/g can therefore be related to graphite and to the part
by increasing the reduction duration [19]. Thus, a better contact
between graphite and well dispersed tin particles of optimum
size would contribute to obtain a reversible capacity closer to
the theoretical value. The determination of free and graphite
bound tin could be obtained by using suitable characterization
techniques like M o¨ ssbauer and electron energy loss spectro-
copies. Such studies are being currently in progress to support
the hypothesis that the improvement of the cycling performances
of tin bound to graphite.
4
. Conclusion
SnCl2 can be reduced in Sn(0) by sodium tert-butyl alkoxide-
◦
activated NaH in THF at 65 C. When the reduction is carried out
in the presence of graphite, the resulting material is composed

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L. Balan et al. / Electrochimica Acta 51 (2006) 3385–3390
of the Sn/graphite system are related to the formation of bonds,
likely covalent, between graphite and tin. Although their large
and stable reversible capacities, the use of such materials as neg-
ative electrodes of Li-ion battery is strongly dependent on the
possibility to reduce their large irreversible capacities.
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