340
P.-E. Lippens
that often produce poorly crystalline and small particles with different compositions. In such cases, simplified fitting procedures are often used in order to
highlight some trends in the variations of the Mössbauer parameters as described
in Sect. 7.5.3.
7.3.7 In Situ Experiments
The application of ex situ Mössbauer spectroscopy to study electrode materials
is similar to the application of this technique in solid-state chemistry as widely
described in textbooks [39, 40–42]. However, the products of electrochemical reactions extracted from the electrochemical cells are in powder or film form and can
be chemically unstable. Samples must be prepared and handled with care to avoid
oxidation or other parasitic reactions. They must be characterized quickly after the
electrochemical experiments to reduce a possible evolution due to chemical reactions that could still operate after extraction from the cell even after washing. The
low concentration or the low recoil-free fractions of the Mössbauer atoms in the
electrode material can be responsible for long-term experiments. In that case, it is
possible to use Swagelok-type cells with higher amounts of powdered electrode
material, but the measurements are often limited to few cycles. Finally, it should be
noted that in many cases, such post mortem characterizations of the electrochemical
mechanisms provide rather reliable results that can be favorably compared to in situ
measurements as shown in Sects. 7.5.3 and 7.6.4.
The in situ Mössbauer experiments are conducted in the same way as the electrochemical measurements except that the commonly used coin cells or Swagelok-type
cells are replaced by an electrochemical “in situ cell” for γ-ray transmission. The
in situ cells are also based on the “negative electrode-electrolyte-positive electrode”
configuration including a separator, but all the components in the path of the γrays are optimized to enhance the signal-to-noise ratio and avoid parasitic resonance
absorption. This is of particular importance for electrochemical reactions since the
time required to record a spectrum, typically several hours, should be consistent
with the evolution of the system due to insertion/extraction of typically 0.1 Li or Na
per active element and per hour. The commonly encountered difficulties are due to the
electrode thickness and the existence of metals or heavy elements in the composition
of the cell components.
The in situ Mössbauer measurements are usually performed at room temperature and no special cell designs are required for low or high temperatures. Dunlap
et al. used a modified 2325-type coin-cell with thick (1 mm) and thin (250 μm) Be
windows for γ-ray transmission (Fig. 7.6) [27, 64]. These windows are electronically conductive and act as current collectors. The thin Be window is coated with
the electrode material under study while a lithium foil is placed against the thick
window (half-cell configuration). The
57 Fe Mössbauer spectra were corrected from
the presence of iron in Be windows [27]. A similar cell was proposed by Wattiaux
et al. but with Mylar instead of Be windows [65].
P.-E. Lippens
that often produce poorly crystalline and small particles with different compositions. In such cases, simplified fitting procedures are often used in order to
highlight some trends in the variations of the Mössbauer parameters as described
in Sect. 7.5.3.
7.3.7 In Situ Experiments
The application of ex situ Mössbauer spectroscopy to study electrode materials
is similar to the application of this technique in solid-state chemistry as widely
described in textbooks [39, 40–42]. However, the products of electrochemical reactions extracted from the electrochemical cells are in powder or film form and can
be chemically unstable. Samples must be prepared and handled with care to avoid
oxidation or other parasitic reactions. They must be characterized quickly after the
electrochemical experiments to reduce a possible evolution due to chemical reactions that could still operate after extraction from the cell even after washing. The
low concentration or the low recoil-free fractions of the Mössbauer atoms in the
electrode material can be responsible for long-term experiments. In that case, it is
possible to use Swagelok-type cells with higher amounts of powdered electrode
material, but the measurements are often limited to few cycles. Finally, it should be
noted that in many cases, such post mortem characterizations of the electrochemical
mechanisms provide rather reliable results that can be favorably compared to in situ
measurements as shown in Sects. 7.5.3 and 7.6.4.
The in situ Mössbauer experiments are conducted in the same way as the electrochemical measurements except that the commonly used coin cells or Swagelok-type
cells are replaced by an electrochemical “in situ cell” for γ-ray transmission. The
in situ cells are also based on the “negative electrode-electrolyte-positive electrode”
configuration including a separator, but all the components in the path of the γrays are optimized to enhance the signal-to-noise ratio and avoid parasitic resonance
absorption. This is of particular importance for electrochemical reactions since the
time required to record a spectrum, typically several hours, should be consistent
with the evolution of the system due to insertion/extraction of typically 0.1 Li or Na
per active element and per hour. The commonly encountered difficulties are due to the
electrode thickness and the existence of metals or heavy elements in the composition
of the cell components.
The in situ Mössbauer measurements are usually performed at room temperature and no special cell designs are required for low or high temperatures. Dunlap
et al. used a modified 2325-type coin-cell with thick (1 mm) and thin (250 μm) Be
windows for γ-ray transmission (Fig. 7.6) [27, 64]. These windows are electronically conductive and act as current collectors. The thin Be window is coated with
the electrode material under study while a lithium foil is placed against the thick
window (half-cell configuration). The
57 Fe Mössbauer spectra were corrected from
the presence of iron in Be windows [27]. A similar cell was proposed by Wattiaux
et al. but with Mylar instead of Be windows [65].