364
P.-E. Lippens
The observed differences in the voltage values of the first lithiation plateau of
MnSn 2 , CoSn 2 , and Ni 3 Sn 4 can be mainly related to the enthalpies of the reactions
(7.33)-(7.35) although significant kinetic effects can affect these values. In the three
cases, the mechanisms of the first lithiation are the same as FeSn 2 and consist in
the transformation of the pristine material into M/Li 7 Sn 2 nanocomposite (M = Mn,
Co, Ni). The Mössbauer spectra obtained at the end of the lithiation of MnSn 2 ,
CoSn 2 and FeSn 2 are similar, reflecting the formation of poorly cristallized Li 7 Sn 2
nanoparticles. For the fully lithiated Ni 3 Sn 4 electrode, the Mössbauer spectrum is
similar to that of the Li 7 Sn 2 crystalline reference, suggesting the formation of a more
stable phase.
Both CoSn 2 and MnSn 2 have the same I4/mcm structure as FeSn 2 while Ni 3 Sn 4
has a monoclinic C2/m structure with two different Sn crystallographic sites in the
primitive cell. MnSn 2 is antiferromagnetic below 325 K and the
119 Sn Mössbauer
spectrum at room temperature was fitted to two sextuplets reflecting two Sn magnetic
sites due to transferred hyperfine magnetic fields with parameters: δ = 2.34 and
2.32 mm s
−1 , = 1.07 and 1.10 mm s
−1 , B = 4.5 and 2.7 T [140]. Nanostructured
MnSn 2 was obtained by ball milling from crystalline MnSn 2 . The Mössbauer spectrum is formed by a doublet showing the absence of transferred hyperfine magnetic
field. The values of the Mössbauer parameters δ = 2.3 mm s
−1 and = 1.1 mm s
−1
are close to those of crystalline MnSn 2 [129]. At room temperature, the Mössbauer
spectrum of CoSn 2 can also be fitted to a doublet with δ = 2.14 mm s
−1 , =
0.77 mm s
−1 [141].
These results show that the isomer shift slightly decreases for the series MnSn 2 -
FeSn 2 -CoSn 2 . According to Eq. (7.5), this reflects the decrease of the number of Sn
5s electrons and the increase of the number of Sn 5p electrons. In the latter case, this
is due to electron transfer from the transition metal, M, to Sn through the Sn 5p–M 3d
bonds. The decrease of the quadrupole splitting for this series reflects the decrease
of the Sn 5p charge anisotropy. The Mössbauer spectrum of Ni 3 Sn 4 is also formed
by a doublet but was fitted by considering two Sn sites, in agreement with the crystal
structure of this compound. The Mössbauer parameters are δ = 2.01 mm s
−1 , =
0.7 mm s
−1 for one site and δ = 2.02 mm s
−1 , = 1.18 mm s
−1 for the other site.
The quadrupole splittings reflect the two different asymmetric environments of the
Sn crystallographic sites [135]. It should be noted that different values of the average
isomer shift of Ni 3 Sn 4 can be found in the range ~1.9–2.1 mm s
−1 , depending on
the synthesis conditions. This reflects variations of the composition from Ni 3 Sn 4
to Ni 3.5 Sn 4 due to the existence of a solid solution [142]. In that case, Mössbauer
spectroscopy can be used to evaluate the composition of Ni 3+x Sn 4 [143].
The values of the isomer shift decrease for the series MnSn 2 -FeSn 2 -CoSn 2 -
Ni 3 Sn 4. This is due to changes in chemical bonds and electronic transfer, leading
to the increase of the number of M 3d electrons. However, all the values are in the
range 2.0–2.3 mm s
−1 , which is typical of Sn(0) formal oxidation state (see Fig. 7.3).
All these results explain why the first lithiation of nanostructured (paramagnetic)
MnSn 2 , nanostructured FeSn 2 , CoSn 2 and Ni 3 Sn 4 , leads to similar changes in the
119 Sn Mössbauer spectra from a well-defined doublet for the pristine material to a
slightly asymmetric doublet for Li 7 Sn 2 .
P.-E. Lippens
The observed differences in the voltage values of the first lithiation plateau of
MnSn 2 , CoSn 2 , and Ni 3 Sn 4 can be mainly related to the enthalpies of the reactions
(7.33)-(7.35) although significant kinetic effects can affect these values. In the three
cases, the mechanisms of the first lithiation are the same as FeSn 2 and consist in
the transformation of the pristine material into M/Li 7 Sn 2 nanocomposite (M = Mn,
Co, Ni). The Mössbauer spectra obtained at the end of the lithiation of MnSn 2 ,
CoSn 2 and FeSn 2 are similar, reflecting the formation of poorly cristallized Li 7 Sn 2
nanoparticles. For the fully lithiated Ni 3 Sn 4 electrode, the Mössbauer spectrum is
similar to that of the Li 7 Sn 2 crystalline reference, suggesting the formation of a more
stable phase.
Both CoSn 2 and MnSn 2 have the same I4/mcm structure as FeSn 2 while Ni 3 Sn 4
has a monoclinic C2/m structure with two different Sn crystallographic sites in the
primitive cell. MnSn 2 is antiferromagnetic below 325 K and the
119 Sn Mössbauer
spectrum at room temperature was fitted to two sextuplets reflecting two Sn magnetic
sites due to transferred hyperfine magnetic fields with parameters: δ = 2.34 and
2.32 mm s
−1 , = 1.07 and 1.10 mm s
−1 , B = 4.5 and 2.7 T [140]. Nanostructured
MnSn 2 was obtained by ball milling from crystalline MnSn 2 . The Mössbauer spectrum is formed by a doublet showing the absence of transferred hyperfine magnetic
field. The values of the Mössbauer parameters δ = 2.3 mm s
−1 and = 1.1 mm s
−1
are close to those of crystalline MnSn 2 [129]. At room temperature, the Mössbauer
spectrum of CoSn 2 can also be fitted to a doublet with δ = 2.14 mm s
−1 , =
0.77 mm s
−1 [141].
These results show that the isomer shift slightly decreases for the series MnSn 2 -
FeSn 2 -CoSn 2 . According to Eq. (7.5), this reflects the decrease of the number of Sn
5s electrons and the increase of the number of Sn 5p electrons. In the latter case, this
is due to electron transfer from the transition metal, M, to Sn through the Sn 5p–M 3d
bonds. The decrease of the quadrupole splitting for this series reflects the decrease
of the Sn 5p charge anisotropy. The Mössbauer spectrum of Ni 3 Sn 4 is also formed
by a doublet but was fitted by considering two Sn sites, in agreement with the crystal
structure of this compound. The Mössbauer parameters are δ = 2.01 mm s
−1 , =
0.7 mm s
−1 for one site and δ = 2.02 mm s
−1 , = 1.18 mm s
−1 for the other site.
The quadrupole splittings reflect the two different asymmetric environments of the
Sn crystallographic sites [135]. It should be noted that different values of the average
isomer shift of Ni 3 Sn 4 can be found in the range ~1.9–2.1 mm s
−1 , depending on
the synthesis conditions. This reflects variations of the composition from Ni 3 Sn 4
to Ni 3.5 Sn 4 due to the existence of a solid solution [142]. In that case, Mössbauer
spectroscopy can be used to evaluate the composition of Ni 3+x Sn 4 [143].
The values of the isomer shift decrease for the series MnSn 2 -FeSn 2 -CoSn 2 -
Ni 3 Sn 4. This is due to changes in chemical bonds and electronic transfer, leading
to the increase of the number of M 3d electrons. However, all the values are in the
range 2.0–2.3 mm s
−1 , which is typical of Sn(0) formal oxidation state (see Fig. 7.3).
All these results explain why the first lithiation of nanostructured (paramagnetic)
MnSn 2 , nanostructured FeSn 2 , CoSn 2 and Ni 3 Sn 4 , leads to similar changes in the
119 Sn Mössbauer spectra from a well-defined doublet for the pristine material to a
slightly asymmetric doublet for Li 7 Sn 2 .