7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
357
A
B
C
(a)
End of
plateau A
End of
plateau B
End of
plateau C
(b)
Fig. 7.18 a Voltage curves for the first sodiation of βSn in Na half-cells, showing three plateaus
A, B and C at about 0.2 V, 0.1 V and 0.05 V. b Ex situ 119 Sn Mössbauer spectra recorded at the
end of the plateaus A, B and C. Reprinted from Ref. [106]. Copyright 2014, with permission from
Elsevier
electrodes also suffer from large volume variations as observed during the lithiationdelithiation cycles.
The voltage curve obtained for the first galvanostatic sodiation of βSn foil in a Na
half-cell at low current rate shows three plateaus at about 0.2 V, 0.1 V and 0.05 V,
respectively (Fig. 7.18a). These values are lower than those of the lithiation of βSn
in a Li half-cell, in line with the difference between the standard electrode potentials
of Li
+ /Li (−3.04 V) and Na
+ /Na (−2.71 V). The observed number of plateaus is
lower than that expected from the formation of the eight equilibrium crystalline
phases reported in the Na-Sn phase diagram [105].
The electrochemical reactions were characterized by ex situ
119 Sn Mössbauer
spectroscopy, following the same approach as described for Li-ion batteries in
Sect. 7.5.3 [106]. The room temperature spectra reported for crystalline NaSn,
Na 9 Sn 4 and Na 15 Sn 4 , and considered as reference materials, are all formed by a
single peak. According to their number of Sn crystalline sites, the spectra of NaSn,
Na 9 Sn 4 and Na 15 Sn 4 were successfully fitted to one doublet, two doublets and a
single line, respectively. As observed for Li x Sn, the variations of the average isomer
shift of Na x Sn, δ av (x), can be used to distinguish Sn-rich from Na-rich Na x Sn phases.
The isomer shift of NaSn (2.44 mm s
−1 ) is close to that of βSn, which corresponds
to a Sn-rich Na x Sn phase. The isomer shifts of Na 9 Sn 4 (2.3 mm s
−1 ) and Na 15 Sn 4
(2.17 mm s
−1 ) are significantly lower than that of βSn and correspond to Na-rich
Na x Sn phases. This behavior can be correlated to the number of Sn-Sn bonds as for
Li x Sn. The isomer shifts of the Na x Sn phases are typical of Sn(0) oxidation state.
357
A
B
C
(a)
End of
plateau A
End of
plateau B
End of
plateau C
(b)
Fig. 7.18 a Voltage curves for the first sodiation of βSn in Na half-cells, showing three plateaus
A, B and C at about 0.2 V, 0.1 V and 0.05 V. b Ex situ 119 Sn Mössbauer spectra recorded at the
end of the plateaus A, B and C. Reprinted from Ref. [106]. Copyright 2014, with permission from
Elsevier
electrodes also suffer from large volume variations as observed during the lithiationdelithiation cycles.
The voltage curve obtained for the first galvanostatic sodiation of βSn foil in a Na
half-cell at low current rate shows three plateaus at about 0.2 V, 0.1 V and 0.05 V,
respectively (Fig. 7.18a). These values are lower than those of the lithiation of βSn
in a Li half-cell, in line with the difference between the standard electrode potentials
of Li
+ /Li (−3.04 V) and Na
+ /Na (−2.71 V). The observed number of plateaus is
lower than that expected from the formation of the eight equilibrium crystalline
phases reported in the Na-Sn phase diagram [105].
The electrochemical reactions were characterized by ex situ
119 Sn Mössbauer
spectroscopy, following the same approach as described for Li-ion batteries in
Sect. 7.5.3 [106]. The room temperature spectra reported for crystalline NaSn,
Na 9 Sn 4 and Na 15 Sn 4 , and considered as reference materials, are all formed by a
single peak. According to their number of Sn crystalline sites, the spectra of NaSn,
Na 9 Sn 4 and Na 15 Sn 4 were successfully fitted to one doublet, two doublets and a
single line, respectively. As observed for Li x Sn, the variations of the average isomer
shift of Na x Sn, δ av (x), can be used to distinguish Sn-rich from Na-rich Na x Sn phases.
The isomer shift of NaSn (2.44 mm s
−1 ) is close to that of βSn, which corresponds
to a Sn-rich Na x Sn phase. The isomer shifts of Na 9 Sn 4 (2.3 mm s
−1 ) and Na 15 Sn 4
(2.17 mm s
−1 ) are significantly lower than that of βSn and correspond to Na-rich
Na x Sn phases. This behavior can be correlated to the number of Sn-Sn bonds as for
Li x Sn. The isomer shifts of the Na x Sn phases are typical of Sn(0) oxidation state.