7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
359
7.6 Conversion Reactions
7.6.1 FeSn 2 as Negative Electrode Material for Li-Ion
Batteries
Intermetallics composed of an electrochemically active element such as Si, Sn or
Sb and an electrochemically inactive metallic element were investigated as negative
electrode materials for Li-ion batteries in order to reduce the effects of the volume
variations coming with alloying reactions [112–117]. Most of the tin based intermetallics, MSn x , combine Sn with a transition metal element M that does not react
with Li. The first lithiation is expected to extrude M from MSn x to form metallic
nanoparticles that maintain the dispersion of Li x Sn particles and improve the electronic conductivity. It is important to recall that the MSn x active particles are mixed
with carbon additives and a binder to form a porous film coated onto the current
collector. The conductive additives play a key role in the performance of the electrodes during cycling. Carbon can also be introduced during the synthesis process
to form MSn x /C composites that were widely studied in the past in relation with
the commercialization by Sony of the Nexelion Li-ion batteries [118–124]. FeSn 2 is
regarded here as a typical example of the application of Mössbauer spectroscopy to
MSn x intermetallic based electrodes while other transition metals for M are briefly
discussed in Sect. 7.6.2. FeSn 2 contains the highest amount of Sn among the crystalline phases reported in the Fe-Sn binary system, leading to the highest specific
capacity as Fe-Sn based electrode material. Finally, FeSn 2 contains the two Mössbauer isotopes
57 Fe and
119 Sn to probe the electrochemical reactions at the atomic
scale.
FeSn 2 has a tetragonal structure (I4/mcm) and each chemical element occupies
one crystallographic site. Each Fe atom is at the center of a Sn square-based antiprism
while Sn is bonded to four Fe atoms forming a SnFe 4 square-based pyramid. FeSn 2 is
antiferromagnetic below 378 K [125]. Although the magnetic properties of FeSn 2 are
not of real interest for electrochemical applications, they strongly affect the hyperfine structures, leading to rather complex and distinctive
57 Fe and
119 Sn Mössbauer
spectra.
Crystalline microparticles of FeSn 2 were synthesized by solid-state reaction [126].
The
57 Fe Mössbauer spectrum of FeSn 2 at room temperature is formed by a sextet
(Fig. 7.19a) while broad structures in the range 0–5 mm s
−1 are observed for the
119 Sn
Mössbauer spectrum (Fig. 7.19b), reflecting combined quadrupole effect and transferred hyperfine magnetic field [127]. The two sets of Mössbauer parameters: δ =
0.5 mm s
−1 , = 0 mm s
−1 and B = 11 T for
57 Fe and δ = 2.18 mm s
−1 , =
0.83 mm s
−1 , B = 2.4 T for
119 Sn are typical of Fe(0) and Sn(0) oxidation states,
respectively, with a Sn asymmetrical environment.
The voltage profile obtained in galvanostatic regime at low current density consists
of a low voltage plateau at 0.2 V for the first lithiation and voltage hystereses at the
average value of 0.5 V for the subsequent delithiation-lithiation cycles [126]. This
indicates that the first lithiation mechanism differs from the following reversible
359
7.6 Conversion Reactions
7.6.1 FeSn 2 as Negative Electrode Material for Li-Ion
Batteries
Intermetallics composed of an electrochemically active element such as Si, Sn or
Sb and an electrochemically inactive metallic element were investigated as negative
electrode materials for Li-ion batteries in order to reduce the effects of the volume
variations coming with alloying reactions [112–117]. Most of the tin based intermetallics, MSn x , combine Sn with a transition metal element M that does not react
with Li. The first lithiation is expected to extrude M from MSn x to form metallic
nanoparticles that maintain the dispersion of Li x Sn particles and improve the electronic conductivity. It is important to recall that the MSn x active particles are mixed
with carbon additives and a binder to form a porous film coated onto the current
collector. The conductive additives play a key role in the performance of the electrodes during cycling. Carbon can also be introduced during the synthesis process
to form MSn x /C composites that were widely studied in the past in relation with
the commercialization by Sony of the Nexelion Li-ion batteries [118–124]. FeSn 2 is
regarded here as a typical example of the application of Mössbauer spectroscopy to
MSn x intermetallic based electrodes while other transition metals for M are briefly
discussed in Sect. 7.6.2. FeSn 2 contains the highest amount of Sn among the crystalline phases reported in the Fe-Sn binary system, leading to the highest specific
capacity as Fe-Sn based electrode material. Finally, FeSn 2 contains the two Mössbauer isotopes
57 Fe and
119 Sn to probe the electrochemical reactions at the atomic
scale.
FeSn 2 has a tetragonal structure (I4/mcm) and each chemical element occupies
one crystallographic site. Each Fe atom is at the center of a Sn square-based antiprism
while Sn is bonded to four Fe atoms forming a SnFe 4 square-based pyramid. FeSn 2 is
antiferromagnetic below 378 K [125]. Although the magnetic properties of FeSn 2 are
not of real interest for electrochemical applications, they strongly affect the hyperfine structures, leading to rather complex and distinctive
57 Fe and
119 Sn Mössbauer
spectra.
Crystalline microparticles of FeSn 2 were synthesized by solid-state reaction [126].
The
57 Fe Mössbauer spectrum of FeSn 2 at room temperature is formed by a sextet
(Fig. 7.19a) while broad structures in the range 0–5 mm s
−1 are observed for the
119 Sn
Mössbauer spectrum (Fig. 7.19b), reflecting combined quadrupole effect and transferred hyperfine magnetic field [127]. The two sets of Mössbauer parameters: δ =
0.5 mm s
−1 , = 0 mm s
−1 and B = 11 T for
57 Fe and δ = 2.18 mm s
−1 , =
0.83 mm s
−1 , B = 2.4 T for
119 Sn are typical of Fe(0) and Sn(0) oxidation states,
respectively, with a Sn asymmetrical environment.
The voltage profile obtained in galvanostatic regime at low current density consists
of a low voltage plateau at 0.2 V for the first lithiation and voltage hystereses at the
average value of 0.5 V for the subsequent delithiation-lithiation cycles [126]. This
indicates that the first lithiation mechanism differs from the following reversible