7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
373
be explained by the formation of Sn-O bonds. Since all the Li atoms are extracted
from Li x Sn at this stage, this is due to the partial delithiation of Li 2 O and back
reaction of Sn with O. The in situ spectrum obtained for the delithiated electrode at
2.5 V shows the existence of SnO 2 , SnO and βSn, confirming such a back reaction.
The same results are obtained for ex situ measurements at 3 V (Fig. 7.29) [148].
These reactions operate at a rather high voltage and are not expected to occur for
SnO based negative electrodes in Li-ion full-cells.
To summarize, the electrochemical mechanism of SnO in a Li half-cell consists
in the irreversible transformation of Sn(II)O into βSn(0) and Li 2 O small particles,
followed by reversible Li x Sn alloying and dealloying reactions for the first delithiation and the second lithiation, respectively, if the upper voltage cutoff is limited to
about 1 V. In that case, the Li 2 O particles form an electrochemically inactive matrix
that maintains the dispersion of the Li x Sn particles, improving the cycling properties of the electrode compared to βSn. This is the same basic concept as described
in Sect. 7.6.1 for tin intermetallic based electrodes whose metallic nanoparticles
formed during the first lithiation buffer the volume variations, as the Li 2 O particles
in the present case. However, there is a large capacity loss at first cycle due to the
transformation of SnO into βSn/Li 2 O. In Li-ion full-cells, such a reaction requires
the prelithiation of the negative electrode or the increase of the positive/negative
electrode ratio.
The situation is even more critical for SnO 2 since 4 Li per SnO 2 are required for
the conversion reaction ending with βSn/Li 2 O composite. The in situ
119 Sn Mössbauer spectroscopy was used for SnO 2 in a plastic bag half-cell showing a three-step
mechanism for the first lithiation including intercalation, conversion and alloying
reactions [150]. However, the analysis of the conversion reaction is more ambiguous
due to overlaps between the Sn(IV)O 2 subspectrum and broad lines due to the Sn-O
bonds between βSn and Li 2 O. The additional initial step reflecting Li intercalation
into SnO 2 only involves 0.4 Li per SnO 2 . The voltage curve of SnO 2 corresponding
to Li x Sn alloying reactions (R2 for SnO) does not show well-defined plateaus and
the Mössbauer spectra cannot be fitted to Li x Sn references. However, the variations
of the average isomer shift are linear in this region, as expected from Eq. (7.28),
reflecting reversible Li x Sn alloying reactions.
As an example of TCO, a tin borophosphate glass with the composition Sn 2 BPO 6
is considered here since the mechanism was studied by operando
119 Sn Mössbauer
spectroscopy [149]. The voltage profile of the first cycle of Sn 2 BPO 6 in a Li half-cell
is similar to that of SnO, except that the first plateau (R1) has a higher voltage. The
Mössbauer spectrum of the pristine material consists of an asymmetric doublet that
can be attributed to Sn(II) while a broad spectrum of weak intensity was obtained at
the end of the first plateau. This reflects the strong decrease of the average recoil-free
fraction in R1, which was attributed to the transformation of Sn 2 BPO 6 into βSn small
clusters embedded in a mixed borophosphate and Li 2 O matrix. Such a decrease can
be related to the variations of the recoil-free fraction of βSn as particle size decreases
below ~10 nm when the interface with the matrix is not too constrained [149]. The
proposed conversion reaction is
373
be explained by the formation of Sn-O bonds. Since all the Li atoms are extracted
from Li x Sn at this stage, this is due to the partial delithiation of Li 2 O and back
reaction of Sn with O. The in situ spectrum obtained for the delithiated electrode at
2.5 V shows the existence of SnO 2 , SnO and βSn, confirming such a back reaction.
The same results are obtained for ex situ measurements at 3 V (Fig. 7.29) [148].
These reactions operate at a rather high voltage and are not expected to occur for
SnO based negative electrodes in Li-ion full-cells.
To summarize, the electrochemical mechanism of SnO in a Li half-cell consists
in the irreversible transformation of Sn(II)O into βSn(0) and Li 2 O small particles,
followed by reversible Li x Sn alloying and dealloying reactions for the first delithiation and the second lithiation, respectively, if the upper voltage cutoff is limited to
about 1 V. In that case, the Li 2 O particles form an electrochemically inactive matrix
that maintains the dispersion of the Li x Sn particles, improving the cycling properties of the electrode compared to βSn. This is the same basic concept as described
in Sect. 7.6.1 for tin intermetallic based electrodes whose metallic nanoparticles
formed during the first lithiation buffer the volume variations, as the Li 2 O particles
in the present case. However, there is a large capacity loss at first cycle due to the
transformation of SnO into βSn/Li 2 O. In Li-ion full-cells, such a reaction requires
the prelithiation of the negative electrode or the increase of the positive/negative
electrode ratio.
The situation is even more critical for SnO 2 since 4 Li per SnO 2 are required for
the conversion reaction ending with βSn/Li 2 O composite. The in situ
119 Sn Mössbauer spectroscopy was used for SnO 2 in a plastic bag half-cell showing a three-step
mechanism for the first lithiation including intercalation, conversion and alloying
reactions [150]. However, the analysis of the conversion reaction is more ambiguous
due to overlaps between the Sn(IV)O 2 subspectrum and broad lines due to the Sn-O
bonds between βSn and Li 2 O. The additional initial step reflecting Li intercalation
into SnO 2 only involves 0.4 Li per SnO 2 . The voltage curve of SnO 2 corresponding
to Li x Sn alloying reactions (R2 for SnO) does not show well-defined plateaus and
the Mössbauer spectra cannot be fitted to Li x Sn references. However, the variations
of the average isomer shift are linear in this region, as expected from Eq. (7.28),
reflecting reversible Li x Sn alloying reactions.
As an example of TCO, a tin borophosphate glass with the composition Sn 2 BPO 6
is considered here since the mechanism was studied by operando
119 Sn Mössbauer
spectroscopy [149]. The voltage profile of the first cycle of Sn 2 BPO 6 in a Li half-cell
is similar to that of SnO, except that the first plateau (R1) has a higher voltage. The
Mössbauer spectrum of the pristine material consists of an asymmetric doublet that
can be attributed to Sn(II) while a broad spectrum of weak intensity was obtained at
the end of the first plateau. This reflects the strong decrease of the average recoil-free
fraction in R1, which was attributed to the transformation of Sn 2 BPO 6 into βSn small
clusters embedded in a mixed borophosphate and Li 2 O matrix. Such a decrease can
be related to the variations of the recoil-free fraction of βSn as particle size decreases
below ~10 nm when the interface with the matrix is not too constrained [149]. The
proposed conversion reaction is