7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
367
For Ni 3 Sn 4 , the Mössbauer spectrum obtained at the end of delithiation is close
to that of the pristine material and can be attributed to the reformation of Ni 3 Sn 4 .
Such a back reaction was also observed for Ni 3 Sn 4 /Si/C composite where Si was
introduced to increase the specific capacity [144]. In that case, both Ni 3 Sn 4 and Si
are electrochemically active and operando
119 Sn Mössbauer spectroscopy was used
to follow reactions involving Sn. The spectra obtained in galvanostatic regime clearly
show the back reaction of Ni with Sn to form Ni 3 Sn 4 at the end of delithiation [145].
The present analysis of the Mössbauer results obtained for the first delithiation of
tin intermetallics indicates that, depending on the transition metal M (Mn, Fe, Co, Ni),
different mechanisms take place with different contributions of the back reactions
between Sn and M. This could be due to differences in the bonding energies of M-Sn
and M-M bonds, but also to interfacial and atomic diffusion properties. This aspect
has not been elucidated yet and it is still not clear whether such back reactions that
trap M and Sn atoms improve the electrode performance such as cycle life, which
is a crucial issue for the application of intermetallic electrode materials in Li-ion
batteries.
Finally, the Mössbauer spectra obtained during the second lithiation of MnSn 2 ,
CoSn 2 and Ni 3 Sn 4 differ from the spectra obtained during the first lithiation, in
line with the observed different voltage profiles. This is consistent with the mechanism described for FeSn 2 and is obviously due to differences between the electrode
materials at the beginning of the first (pristine) and second (nanocomposite) lithiations. However, both lithiation processes end with the formation of M/Li 7 Sn 2
composite. The cycle formed by the first delithiation and the second lithiation is
reversible and reflects the electrochemical mechanism (Li-Sn alloying reactions) of
the following reversible cycles. Unfortunately, the poor mechanical stability of the
electrodes during cycling limits the use of such materials and requires the optimization of the electrode formulation. In addition, all the reaction mechanisms described
above can also be affected by parasitic reactions on cycling or for the cell at rest as
discussed in the next subsection.
7.6.3 Aging Phenomena
Li-ion batteries can suffer from energy and power losses, voltage lowering or more
severe issues with time. The performance degradation occurs for batteries at rest
or during cycling [146]. The in situ Mössbauer spectroscopy can be used to study
aging mechanisms if they affect the electronic properties of Mössbauer atoms. Two
mechanisms are described here for tin based intermetallics as negative electrode
materials for Li-ion batteries at rest and during cycling, respectively.
The first aging mechanism concerns the instability of the species formed during the
lithiation or delithiation of electrode materials [147]. The example of a fully lithiated
FeSn 2 based electrode is considered here. Such an aging process is complex since
it depends not only on the macroscopic and microscopic features of the electrode
as the morphology of the FeSn 2 particles or the electrode formulation, but also on
367
For Ni 3 Sn 4 , the Mössbauer spectrum obtained at the end of delithiation is close
to that of the pristine material and can be attributed to the reformation of Ni 3 Sn 4 .
Such a back reaction was also observed for Ni 3 Sn 4 /Si/C composite where Si was
introduced to increase the specific capacity [144]. In that case, both Ni 3 Sn 4 and Si
are electrochemically active and operando
119 Sn Mössbauer spectroscopy was used
to follow reactions involving Sn. The spectra obtained in galvanostatic regime clearly
show the back reaction of Ni with Sn to form Ni 3 Sn 4 at the end of delithiation [145].
The present analysis of the Mössbauer results obtained for the first delithiation of
tin intermetallics indicates that, depending on the transition metal M (Mn, Fe, Co, Ni),
different mechanisms take place with different contributions of the back reactions
between Sn and M. This could be due to differences in the bonding energies of M-Sn
and M-M bonds, but also to interfacial and atomic diffusion properties. This aspect
has not been elucidated yet and it is still not clear whether such back reactions that
trap M and Sn atoms improve the electrode performance such as cycle life, which
is a crucial issue for the application of intermetallic electrode materials in Li-ion
batteries.
Finally, the Mössbauer spectra obtained during the second lithiation of MnSn 2 ,
CoSn 2 and Ni 3 Sn 4 differ from the spectra obtained during the first lithiation, in
line with the observed different voltage profiles. This is consistent with the mechanism described for FeSn 2 and is obviously due to differences between the electrode
materials at the beginning of the first (pristine) and second (nanocomposite) lithiations. However, both lithiation processes end with the formation of M/Li 7 Sn 2
composite. The cycle formed by the first delithiation and the second lithiation is
reversible and reflects the electrochemical mechanism (Li-Sn alloying reactions) of
the following reversible cycles. Unfortunately, the poor mechanical stability of the
electrodes during cycling limits the use of such materials and requires the optimization of the electrode formulation. In addition, all the reaction mechanisms described
above can also be affected by parasitic reactions on cycling or for the cell at rest as
discussed in the next subsection.
7.6.3 Aging Phenomena
Li-ion batteries can suffer from energy and power losses, voltage lowering or more
severe issues with time. The performance degradation occurs for batteries at rest
or during cycling [146]. The in situ Mössbauer spectroscopy can be used to study
aging mechanisms if they affect the electronic properties of Mössbauer atoms. Two
mechanisms are described here for tin based intermetallics as negative electrode
materials for Li-ion batteries at rest and during cycling, respectively.
The first aging mechanism concerns the instability of the species formed during the
lithiation or delithiation of electrode materials [147]. The example of a fully lithiated
FeSn 2 based electrode is considered here. Such an aging process is complex since
it depends not only on the macroscopic and microscopic features of the electrode
as the morphology of the FeSn 2 particles or the electrode formulation, but also on