7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
351
7.5.3 Li x Sn Reference Materials
To identify the lithiated products resulting from the Li-Sn alloying reactions by
119 Sn
Mössbauer spectroscopy, it is convenient to determine the Mössbauer parameters of
the equilibrium crystalline phases considered as references. Different experimental
Li-Sn phase diagrams were reported and the currently accepted one shows the existence of seven crystalline phases: Li 2 Sn 5 , LiSn, Li 7 Sn 3 , Li 5 Sn 2 , Li 13 Sn 5 , Li 7 Sn 2 and
Li 22 Sn 5 , [91], although the latter phase should be better described by Li 17 Sn 4 [92].
Li 8 Sn 3 was also observed and should be included in the Li-Sn phase diagram but is
not considered here [93]. Finally, other compositions or structures were predicted
by DFT calculations but have not been experimentally observed yet [94–96]. In line
with previously reported works [97, 98], only the seven crystalline phases given
above are considered here as references. The crystal structures of all the phases
were experimentally determined by XRD and show the existence of one (Li 5 Sn 2 ),
two (Li 2 Sn 5 , LiSn, Li 7 Sn 2 ), three (Li 7 Sn 3 , Li 13 Sn 5 ) and four (Li 22 Sn 5 ) Sn crystallographic sites. The Mössbauer spectra were reported by two groups using different
synthesis methods: high temperature solid-state reactions [97] and mechanosynthesis
followed by annealing [98]. The contributions of the different Sn crystallographic
sites cannot be easily distinguished in the spectra that are unresolved and contain the
contributions of tin based impurities (Fig. 7.14).
The comparison between the values of the Mössbauer parameters obtained by the
two groups shows some small variations that can be attributed to differences in the
purity and crystallinity of the materials (Table 7.1).
The relative contributions of the subspectra, when not fixed, are consistent with
the crystallographic site multiplicities. The values of the quadrupole splitting reflect
the different Sn local environments due to the existence of different nearest neighbors
(Li, Sn), polyhedral geometries and bond lengths. Finally, the values of the isomer
shift averaged over the different Sn crystallographic sites, δ av , are of about 2.4 mm s
−1
for the two Sn-rich Li x Sn phases and decrease from 2.1 to 1.8 mm s
−1 with increasing
relative amount of Li for the Li-rich Li x Sn phases (Fig. 7.15). These values are all in
the range of the Sn(0) oxidation state and can be correlated to the number of Sn-Sn
bonds. The two ranges of isomer shifts reflect the existence of Sn based sub-lattices
for the Sn-rich Li x Sn phases and Sn single atoms or clusters of two or three Sn atoms
bonded to Li atoms for the Li-rich Li x Sn phases.
It is also interesting to plot the values of the average isomer shift of the Li x Sn
references as a function of the number of Li per Sn (Fig. 7.15). The observed linear
correlation suggests that δ av (x) strongly depends on the average composition of Li x Sn
but not on crystal structure. This means that such a correlation can be used to evaluate
x from the measurement of δ av for Li x Sn amorphous phases or small particles as often
encountered during the lithiation-delithiation processes of Sn based electrodes.
The linear variations of δ av (x) can be related to changes in the numbers of Sn 5s
(N 5s ) and Sn 5p (N 5p ) electrons. The values of ρ(0), N 5s and N 5p were evaluated
with the DFT-LAPW method. Following the procedure described in Sect. 7.3.2,
the theoretical values of δ were obtained from ρ(0) and the calibration constant
351
7.5.3 Li x Sn Reference Materials
To identify the lithiated products resulting from the Li-Sn alloying reactions by
119 Sn
Mössbauer spectroscopy, it is convenient to determine the Mössbauer parameters of
the equilibrium crystalline phases considered as references. Different experimental
Li-Sn phase diagrams were reported and the currently accepted one shows the existence of seven crystalline phases: Li 2 Sn 5 , LiSn, Li 7 Sn 3 , Li 5 Sn 2 , Li 13 Sn 5 , Li 7 Sn 2 and
Li 22 Sn 5 , [91], although the latter phase should be better described by Li 17 Sn 4 [92].
Li 8 Sn 3 was also observed and should be included in the Li-Sn phase diagram but is
not considered here [93]. Finally, other compositions or structures were predicted
by DFT calculations but have not been experimentally observed yet [94–96]. In line
with previously reported works [97, 98], only the seven crystalline phases given
above are considered here as references. The crystal structures of all the phases
were experimentally determined by XRD and show the existence of one (Li 5 Sn 2 ),
two (Li 2 Sn 5 , LiSn, Li 7 Sn 2 ), three (Li 7 Sn 3 , Li 13 Sn 5 ) and four (Li 22 Sn 5 ) Sn crystallographic sites. The Mössbauer spectra were reported by two groups using different
synthesis methods: high temperature solid-state reactions [97] and mechanosynthesis
followed by annealing [98]. The contributions of the different Sn crystallographic
sites cannot be easily distinguished in the spectra that are unresolved and contain the
contributions of tin based impurities (Fig. 7.14).
The comparison between the values of the Mössbauer parameters obtained by the
two groups shows some small variations that can be attributed to differences in the
purity and crystallinity of the materials (Table 7.1).
The relative contributions of the subspectra, when not fixed, are consistent with
the crystallographic site multiplicities. The values of the quadrupole splitting reflect
the different Sn local environments due to the existence of different nearest neighbors
(Li, Sn), polyhedral geometries and bond lengths. Finally, the values of the isomer
shift averaged over the different Sn crystallographic sites, δ av , are of about 2.4 mm s
−1
for the two Sn-rich Li x Sn phases and decrease from 2.1 to 1.8 mm s
−1 with increasing
relative amount of Li for the Li-rich Li x Sn phases (Fig. 7.15). These values are all in
the range of the Sn(0) oxidation state and can be correlated to the number of Sn-Sn
bonds. The two ranges of isomer shifts reflect the existence of Sn based sub-lattices
for the Sn-rich Li x Sn phases and Sn single atoms or clusters of two or three Sn atoms
bonded to Li atoms for the Li-rich Li x Sn phases.
It is also interesting to plot the values of the average isomer shift of the Li x Sn
references as a function of the number of Li per Sn (Fig. 7.15). The observed linear
correlation suggests that δ av (x) strongly depends on the average composition of Li x Sn
but not on crystal structure. This means that such a correlation can be used to evaluate
x from the measurement of δ av for Li x Sn amorphous phases or small particles as often
encountered during the lithiation-delithiation processes of Sn based electrodes.
The linear variations of δ av (x) can be related to changes in the numbers of Sn 5s
(N 5s ) and Sn 5p (N 5p ) electrons. The values of ρ(0), N 5s and N 5p were evaluated
with the DFT-LAPW method. Following the procedure described in Sect. 7.3.2,
the theoretical values of δ were obtained from ρ(0) and the calibration constant