358
P.-E. Lippens
For a given value of x, the isomer shift of Na x Sn is higher than that of Li x Sn, which
can be explained by the lower electronegativity of Na compared to Li.
The ex situ Mössbauer spectra obtained at the end of the three different voltage
plateaus A, B and C are similar to those of NaSn, Na 9 Sn 4 and Na 15 Sn 4 reference
materials, respectively (Fig. 7.18b). This indicates that the electrochemical sodiation
of βSn at a very low Na rate leads to the formation of crystalline phases following
the alloying reactions
βSn + Na → NaSn
(7.29)
2 NaSn + 3 Na → Na 5 Sn 2
(7.30)
2 Na 5 Sn 2 + 5 Na → Na 15 Sn 4
(7.31)
Thus, the lithiation and sodiation of βSn in half-cells produce Li x Sn and Na x Sn
crystalline phases, respectively, for x < 2.5. For x > 2.5, the formation of the Na 15 Sn 4
equilibrium phase is observed instead of disordered Li-rich Li x Sn phases for lithiation. This could be due to differences in the diffusion of Li and Na atoms. The diffusion mechanisms could also explain the observed differences between the voltage
profiles obtained for Sn foil and sputtered films in Na half-cells [107]. In the latter
case, the mechanism is more complex, with the formation of Na x Sn amorphous
and metastable phases. However, the
119 Sn Mössbauer spectrum of the fully sodiated electrode material is a single peak with the same Mössbauer isomer shift, δ =
2.14 mm s
−1 , as the Na 15 Sn 4 reference crystalline phase. Further investigations on
the phase transformations observed during the sodiation of βSn with other characterization tools led to somewhat different mechanisms but the fully sodiated phase
is always Na 15 Sn 4 [108–111]. This shows that, irrespective of the structure of the
pristine material and the composition of the reaction intermediates, the same phase
Na 15 Sn 4 is obtained at the end of the sodiation of βSn, confirming the high capacity of
this negative electrode material for Na-ion batteries. The observed different mechanisms suggest that, depending on the microstructure of the pristine material and
the experimental conditions for electrochemical sodiation, the phase transformations can produce metastable and amorphous intermediate phases that differ from
the crystalline references. For the lithiation of βSn, such a mechanism is mainly
observed for Li-rich Li x Sn phases. This analysis shows that the electrochemical
reactions taking place in βSn based negative electrodes of Li-ion or Na-ion batteries
are rather complex due to both thermodynamic and kinetic effects. There are still
some unclear aspects in these mechanisms that should be investigated by combining
119 Sn Mössbauer spectroscopy and other characterization tools.
P.-E. Lippens
For a given value of x, the isomer shift of Na x Sn is higher than that of Li x Sn, which
can be explained by the lower electronegativity of Na compared to Li.
The ex situ Mössbauer spectra obtained at the end of the three different voltage
plateaus A, B and C are similar to those of NaSn, Na 9 Sn 4 and Na 15 Sn 4 reference
materials, respectively (Fig. 7.18b). This indicates that the electrochemical sodiation
of βSn at a very low Na rate leads to the formation of crystalline phases following
the alloying reactions
βSn + Na → NaSn
(7.29)
2 NaSn + 3 Na → Na 5 Sn 2
(7.30)
2 Na 5 Sn 2 + 5 Na → Na 15 Sn 4
(7.31)
Thus, the lithiation and sodiation of βSn in half-cells produce Li x Sn and Na x Sn
crystalline phases, respectively, for x < 2.5. For x > 2.5, the formation of the Na 15 Sn 4
equilibrium phase is observed instead of disordered Li-rich Li x Sn phases for lithiation. This could be due to differences in the diffusion of Li and Na atoms. The diffusion mechanisms could also explain the observed differences between the voltage
profiles obtained for Sn foil and sputtered films in Na half-cells [107]. In the latter
case, the mechanism is more complex, with the formation of Na x Sn amorphous
and metastable phases. However, the
119 Sn Mössbauer spectrum of the fully sodiated electrode material is a single peak with the same Mössbauer isomer shift, δ =
2.14 mm s
−1 , as the Na 15 Sn 4 reference crystalline phase. Further investigations on
the phase transformations observed during the sodiation of βSn with other characterization tools led to somewhat different mechanisms but the fully sodiated phase
is always Na 15 Sn 4 [108–111]. This shows that, irrespective of the structure of the
pristine material and the composition of the reaction intermediates, the same phase
Na 15 Sn 4 is obtained at the end of the sodiation of βSn, confirming the high capacity of
this negative electrode material for Na-ion batteries. The observed different mechanisms suggest that, depending on the microstructure of the pristine material and
the experimental conditions for electrochemical sodiation, the phase transformations can produce metastable and amorphous intermediate phases that differ from
the crystalline references. For the lithiation of βSn, such a mechanism is mainly
observed for Li-rich Li x Sn phases. This analysis shows that the electrochemical
reactions taking place in βSn based negative electrodes of Li-ion or Na-ion batteries
are rather complex due to both thermodynamic and kinetic effects. There are still
some unclear aspects in these mechanisms that should be investigated by combining
119 Sn Mössbauer spectroscopy and other characterization tools.