356
P.-E. Lippens
[101]. All the spectra were successfully fitted to a doublet and the Mössbauer parameters strongly change during lithiation (Fig. 7.17). Similar variations are obtained
for delithiation, showing the reversibility of the mechanism.
From x = 0 to x = 1, the isomer shift increases from 1.8 to 2 mm s
−1 and the
quadrupole splitting increases from 0.4 to 1.2 mm s
−1 . The value δ = 1.8 mm s
−1
is close to that of αSn and characteristic of Sn(0) in tetrahedral environment, as
expected for sp
3 hybridization in a-Si. This confirms the substitution of Sn for Si in
a-Si and the absence of βSn. The values of isomer shift obtained for x < 1 are lower
than 2 mm s
−1 , which excludes the formation of Sn-rich Li x Sn alloys. The increase
of δ from 1.8 to 2 mm s
−1 is mainly due to the increase of N 5s (see Eq. 7.5) arising
from the electronic transfer from Li to Sn, which indicates the substitution of Li for Si
as first nearest neighbors of Sn. The progressive increase in the number of Li atoms
around Sn makes the Sn tetrahedral environments formed by Li and Si atoms more
asymmetrical, in agreement with the observed increase of the quadrupole splitting.
From x = 1 to x = 2.3, the isomer shift and quadrupole splitting progressively
decrease until δ = 1.8 mm s
−1 and = 0.6 mm s
−1 , respectively. According to
Eq. (7.28), the value δ = 1.8 mm s
−1 is characteristic of Li-rich Li x Sn alloys and
shows that Sn atoms are mainly bonded to Li atoms. This is consistent with the small
values of the quadrupole splitting that reflect rather symmetrical Sn environments.
From x = 2.3 to x = 3.5, both isomer shift and quadrupole splitting are almost
constant, δ ≈ 1.8 mm s
−1 and ≈ 0.6 mm s
−1 , showing there is no change in the
local environment of the Sn atoms that are only surrounded by Li. The additional
inserted Li atoms are expected to be located further from Sn atoms. This differs from
the lithiation of βSn whose average isomer shift decreases continuously in the range
x = 0–3.5 (Fig. 7.15). These different behaviors of βSn and a-Si 0.87 Sn 0.13 can be
related to the strong differences between the Sn local environments in the lithiated
species. In lithiated Li x Si 0.87 Sn 0.13 , the Sn atoms are tetrahedrally bonded to four
first-nearest neighbors, whereas the Sn atoms in the Li x Sn alloys resulting from the
lithiation of βSn are surrounded by more Li and Sn atoms. The variations of the
Mössbauer parameters during the delithiation from x = 3.5 to x = 1 confirm the
reversibility of the mechanism.
To conclude, the two successive sloping plateaus observed in the voltage curve of
the lithiation of a-Si y Sn 1-y reflect the insertion of Li, first, close to Sn/Si atoms and
then, close to Li atoms. This mechanism is reversible for delithiation.
7.5.5 βSn as Negative Electrode Material for Na-Ion
Batteries
In contrast to Li-ion batteries, graphite cannot be used as negative electrode material
for Na-ion batteries [104] and should be replaced by amorphous carbon. Tin could be
an interesting alternative with a theoretical capacity of 850 mA h g
−1 corresponding
to the formation of Na 15 Sn 4 . However, the sodiation and desodiation of tin based
P.-E. Lippens
[101]. All the spectra were successfully fitted to a doublet and the Mössbauer parameters strongly change during lithiation (Fig. 7.17). Similar variations are obtained
for delithiation, showing the reversibility of the mechanism.
From x = 0 to x = 1, the isomer shift increases from 1.8 to 2 mm s
−1 and the
quadrupole splitting increases from 0.4 to 1.2 mm s
−1 . The value δ = 1.8 mm s
−1
is close to that of αSn and characteristic of Sn(0) in tetrahedral environment, as
expected for sp
3 hybridization in a-Si. This confirms the substitution of Sn for Si in
a-Si and the absence of βSn. The values of isomer shift obtained for x < 1 are lower
than 2 mm s
−1 , which excludes the formation of Sn-rich Li x Sn alloys. The increase
of δ from 1.8 to 2 mm s
−1 is mainly due to the increase of N 5s (see Eq. 7.5) arising
from the electronic transfer from Li to Sn, which indicates the substitution of Li for Si
as first nearest neighbors of Sn. The progressive increase in the number of Li atoms
around Sn makes the Sn tetrahedral environments formed by Li and Si atoms more
asymmetrical, in agreement with the observed increase of the quadrupole splitting.
From x = 1 to x = 2.3, the isomer shift and quadrupole splitting progressively
decrease until δ = 1.8 mm s
−1 and = 0.6 mm s
−1 , respectively. According to
Eq. (7.28), the value δ = 1.8 mm s
−1 is characteristic of Li-rich Li x Sn alloys and
shows that Sn atoms are mainly bonded to Li atoms. This is consistent with the small
values of the quadrupole splitting that reflect rather symmetrical Sn environments.
From x = 2.3 to x = 3.5, both isomer shift and quadrupole splitting are almost
constant, δ ≈ 1.8 mm s
−1 and ≈ 0.6 mm s
−1 , showing there is no change in the
local environment of the Sn atoms that are only surrounded by Li. The additional
inserted Li atoms are expected to be located further from Sn atoms. This differs from
the lithiation of βSn whose average isomer shift decreases continuously in the range
x = 0–3.5 (Fig. 7.15). These different behaviors of βSn and a-Si 0.87 Sn 0.13 can be
related to the strong differences between the Sn local environments in the lithiated
species. In lithiated Li x Si 0.87 Sn 0.13 , the Sn atoms are tetrahedrally bonded to four
first-nearest neighbors, whereas the Sn atoms in the Li x Sn alloys resulting from the
lithiation of βSn are surrounded by more Li and Sn atoms. The variations of the
Mössbauer parameters during the delithiation from x = 3.5 to x = 1 confirm the
reversibility of the mechanism.
To conclude, the two successive sloping plateaus observed in the voltage curve of
the lithiation of a-Si y Sn 1-y reflect the insertion of Li, first, close to Sn/Si atoms and
then, close to Li atoms. This mechanism is reversible for delithiation.
7.5.5 βSn as Negative Electrode Material for Na-Ion
Batteries
In contrast to Li-ion batteries, graphite cannot be used as negative electrode material
for Na-ion batteries [104] and should be replaced by amorphous carbon. Tin could be
an interesting alternative with a theoretical capacity of 850 mA h g
−1 corresponding
to the formation of Na 15 Sn 4 . However, the sodiation and desodiation of tin based