348
P.-E. Lippens
Fig. 7.12 Variations of the
relative amounts of LiFePO 4
and FePO 4 obtained from
operando 57 Fe Mössbauer
spectra for the
first delithiation and
lithiation of LiFePO 4
0.0
0.2
0.4
0.6
0.8
1.0
0
20
40
60
80
100
FePO 4
LiFePO 4
Delithiation
Lithiation
Relative amount (%)
x in Li 1-x FePO 4
The substitution of Mn or Co for Fe was proposed to increase the electrode potential and, consequently, the energy density of the battery. The mechanism is more
complex than that of LiFePO 4 since it involves not only the Fe
3+ /Fe
2+ redox couple but
also Mn
3+ /Mn
2+ or Co
3+ /Co
2+ . For example, the voltage curves of LiFe 0.75 Mn 0.25 PO 4
and LiFe 0.25 Mn 0.75 PO 4 are both formed by two successive plateaus at about 3.4 and
4 V. The analysis of the
57 Fe Mössbauer spectra shows that these two plateaus can be
attributed to two-phase reactions corresponding to the redox couples Fe
3+ /Fe
2+ and
Mn
3+ /Mn
2+ , respectively. The observed transition region between the two plateaus
was attributed to a solid-solution reaction [83, 84]. Other studies of two-phase
reactions in batteries by Mössbauer spectroscopy can be found in Ref. [85–88].
7.5 Alloying Reactions
7.5.1 Negative Electrode Materials for Li-Ion Batteries
Negative electrode materials containing elements that form alloys with lithium have
been proposed to increase the electrode capacity and avoid the formation of lithium
dendrites. Many chemical elements of the groups 13–15 of the periodic table can
reversibly react with lithium at room temperature in liquid organic electrolyte based
cells [89]. The two most interesting and studied elements are Si and Sn that can be
used as high capacity and low voltage electrode materials. They are rather abundant
and environmentally benign. The lithiation-delithiation reactions of Si and βSn lead
to the reversible formation of Li x Si and Li y Sn alloys or compounds, respectively,
with the usually accepted maximum values x = 3.75 and y = 4.4. By considering
these values, the volumetric capacities evaluated at the state of full lithiation are of
about 2200 mA h cm
−3 (Si) and 2100 mA h cm
−3 (βSn) and the specific capacities
P.-E. Lippens
Fig. 7.12 Variations of the
relative amounts of LiFePO 4
and FePO 4 obtained from
operando 57 Fe Mössbauer
spectra for the
first delithiation and
lithiation of LiFePO 4
0.0
0.2
0.4
0.6
0.8
1.0
0
20
40
60
80
100
FePO 4
LiFePO 4
Delithiation
Lithiation
Relative amount (%)
x in Li 1-x FePO 4
The substitution of Mn or Co for Fe was proposed to increase the electrode potential and, consequently, the energy density of the battery. The mechanism is more
complex than that of LiFePO 4 since it involves not only the Fe
3+ /Fe
2+ redox couple but
also Mn
3+ /Mn
2+ or Co
3+ /Co
2+ . For example, the voltage curves of LiFe 0.75 Mn 0.25 PO 4
and LiFe 0.25 Mn 0.75 PO 4 are both formed by two successive plateaus at about 3.4 and
4 V. The analysis of the
57 Fe Mössbauer spectra shows that these two plateaus can be
attributed to two-phase reactions corresponding to the redox couples Fe
3+ /Fe
2+ and
Mn
3+ /Mn
2+ , respectively. The observed transition region between the two plateaus
was attributed to a solid-solution reaction [83, 84]. Other studies of two-phase
reactions in batteries by Mössbauer spectroscopy can be found in Ref. [85–88].
7.5 Alloying Reactions
7.5.1 Negative Electrode Materials for Li-Ion Batteries
Negative electrode materials containing elements that form alloys with lithium have
been proposed to increase the electrode capacity and avoid the formation of lithium
dendrites. Many chemical elements of the groups 13–15 of the periodic table can
reversibly react with lithium at room temperature in liquid organic electrolyte based
cells [89]. The two most interesting and studied elements are Si and Sn that can be
used as high capacity and low voltage electrode materials. They are rather abundant
and environmentally benign. The lithiation-delithiation reactions of Si and βSn lead
to the reversible formation of Li x Si and Li y Sn alloys or compounds, respectively,
with the usually accepted maximum values x = 3.75 and y = 4.4. By considering
these values, the volumetric capacities evaluated at the state of full lithiation are of
about 2200 mA h cm
−3 (Si) and 2100 mA h cm
−3 (βSn) and the specific capacities