7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
363
second lithiation are consistent with the reversibility of the mechanism observed
during the first delithiation and strongly differs from the first lithiation (Fig. 7.21b).
These spectra obtained for the first delithiation-lithiation reversible cycle cannot be
unambiguously fitted by considering the Li x Sn references. However, the observed
variations of δ av in the range 1.8–2.3 mm s
−1 can be interpreted from Eq. (7.28) as
the variations of the Li x Sn average composition between Li 7 Sn 2 and LiSn, which is
consistent with the number of Li extracted during the first delithiation and inserted
during the second lithiation.
A deeper delithiation can be obtained by decreasing the current density at the
end of the process, leading to the back reaction of Fe with Sn extruded from Sn-rich
Li x Sn and βSn to give FeSn 2 small particles [27]. However, for current densities
commonly used in electrochemical tests, such a back reaction was not observed as
confirmed by the
57 Fe Mössbauer spectra that do not change significantly during
this first reversible cycle formed by the first delithiation and second lithiation. In
that case, the reversible mechanism is based on alloying-dealloying Li-Sn reactions
while α-Fe nanoparticles remain spectators and help to buffer the volume variations
resulting from these reactions. The absence of back reaction between Fe and Sn during
cycling was confirmed by electron paramagnetic resonance [128]. However, the existence of such reactions clearly depends on the experimental conditions used for the
electrochemical tests.
7.6.2 Other Tin Based Intermetallic Compounds
The performance and electrochemical mechanisms of different tin based transition metal intermetallics were previously investigated, including MnSn 2 [129, 130],
CoSn 2 [67, 131, 132], Ni 3 Sn 4 [133–135] and Cu 6 Sn 5 [136–139]. The voltage profiles
obtained for the first three compounds are similar to that of FeSn 2 . They are formed
by a plateau for the first lithiation process and reversible voltage hystereses for the
following delithiation-lithiation cycles. The main differences concern the voltage of
the first-lithiation plateau and the existence of an additional plateau observed for
some of these compounds at the end of delithiation. The
119 Sn Mössbauer spectra
obtained during the first lithiation of MnSn 2 , CoSn 2 , and Ni 3 Sn 4 based electrodes
with similar galvanostatic regimes show the formation of Li 7 Sn 2 according to the
conversion reactions
MnSn 2 + 7 Li → Li 7 Sn 2 + Mn
(7.33)
CoSn 2 + 7 Li → Li 7 Sn 2 + Co
(7.34)
Ni 3 Sn 4 + 14 Li → 2 Li 7 Sn 2 + 3 Ni
(7.35)
363
second lithiation are consistent with the reversibility of the mechanism observed
during the first delithiation and strongly differs from the first lithiation (Fig. 7.21b).
These spectra obtained for the first delithiation-lithiation reversible cycle cannot be
unambiguously fitted by considering the Li x Sn references. However, the observed
variations of δ av in the range 1.8–2.3 mm s
−1 can be interpreted from Eq. (7.28) as
the variations of the Li x Sn average composition between Li 7 Sn 2 and LiSn, which is
consistent with the number of Li extracted during the first delithiation and inserted
during the second lithiation.
A deeper delithiation can be obtained by decreasing the current density at the
end of the process, leading to the back reaction of Fe with Sn extruded from Sn-rich
Li x Sn and βSn to give FeSn 2 small particles [27]. However, for current densities
commonly used in electrochemical tests, such a back reaction was not observed as
confirmed by the
57 Fe Mössbauer spectra that do not change significantly during
this first reversible cycle formed by the first delithiation and second lithiation. In
that case, the reversible mechanism is based on alloying-dealloying Li-Sn reactions
while α-Fe nanoparticles remain spectators and help to buffer the volume variations
resulting from these reactions. The absence of back reaction between Fe and Sn during
cycling was confirmed by electron paramagnetic resonance [128]. However, the existence of such reactions clearly depends on the experimental conditions used for the
electrochemical tests.
7.6.2 Other Tin Based Intermetallic Compounds
The performance and electrochemical mechanisms of different tin based transition metal intermetallics were previously investigated, including MnSn 2 [129, 130],
CoSn 2 [67, 131, 132], Ni 3 Sn 4 [133–135] and Cu 6 Sn 5 [136–139]. The voltage profiles
obtained for the first three compounds are similar to that of FeSn 2 . They are formed
by a plateau for the first lithiation process and reversible voltage hystereses for the
following delithiation-lithiation cycles. The main differences concern the voltage of
the first-lithiation plateau and the existence of an additional plateau observed for
some of these compounds at the end of delithiation. The
119 Sn Mössbauer spectra
obtained during the first lithiation of MnSn 2 , CoSn 2 , and Ni 3 Sn 4 based electrodes
with similar galvanostatic regimes show the formation of Li 7 Sn 2 according to the
conversion reactions
MnSn 2 + 7 Li → Li 7 Sn 2 + Mn
(7.33)
CoSn 2 + 7 Li → Li 7 Sn 2 + Co
(7.34)
Ni 3 Sn 4 + 14 Li → 2 Li 7 Sn 2 + 3 Ni
(7.35)