7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
347
Fig. 7.11 Operando 57 Fe
Mössbauer spectra (fitted
curves) obtained for the first
delithiation-lithiation cycle
of Li x FePO 4 in Cell 2
-4
-2
0
2
4
x = 0,70
x = 0,59
x = 0,50
x = 0,87
x = 0,26
x = 0,61
x = 0,78
x = 0,41
x = 0,25
x = 0,11
x = 0,35
x = 0,51
x = 0,69
x = 0,89
x = 1
Fe
III
Fe
II
Lithiation
Delithiation
Velocity (mm/s)
P-O bonds. This structural distortion in FePO 4 has a smaller effect on the value of
V ZZ than the Fe 3d ↓ valence electron anisotropy in LiFePO 4 , which explains that the
quadrupole splitting of FePO 4 is lower than that of LiFePO 4 , as often encountered
for high spin Fe
3+ and Fe
2+ in iron phosphates and oxides.
All the Mössbauer spectra recorded during the delithiation and lithiation of
LiFePO 4 were succesfully fitted to a linear combination of the doublets of LiFePO 4
and FePO 4 [81]. Since the recoil-free fractions are similar for these two compounds
[82], the relative amounts of LiFePO 4 and FePO 4 are given by the relative contributions of the two subspectra. The observed variations are linear (Fig. 7.12), in line
with the reversible two-phase reaction
FePO 4 + Li
+
+ e
−
= LiFePO 4
(7.21)
This shows that Mössbauer spectroscopy can be used as a quantitative tool to evaluate the relative amounts of reactants and products of a two-phase electrochemical
reaction.