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P.-E. Lippens
One of the most commonly used techniques for studying electrode materials is
X-ray diffraction (XRD). XRD has often been considered for the characterization
of crystalline pristine materials and their evolution during the lithiation-delithiation
cycles. This is of particular interest to follow changes in the lattice parameters and
atomic positions of crystalline insertion materials, resulting from the insertion of
ions, or to identify new formed crystalline phases. However, this technique fails for
poorly crystalline, nanosized and amorphous phases as often encountered in pristine electrode materials or detected as products of alloying and conversion reactions.
Such characterizations require local probe techniques, including X-ray photoelectron spectroscopy (XPS), which is a surface sensitive technique, X-ray absorption
spectroscopy (XAS), nuclear magnetic resonance (NMR), electron paramagnetic
resonance (EPR) or Mössbauer spectroscopy. These techniques not only give information about the local environment of the probe elements, but also on the electronic,
magnetic and dynamic properties of the electrode materials. For instance, they are
used to determine the oxidation and spin states, charge anisotropy and bond properties of atoms in the electrode particles, but they are also used to identify the products
formed during insertion-deinsertion processes within the electrode and at the interface with the electrolyte. All these techniques go along with imaging techniques, such
as transmission electron microscopy (TEM), scanning electron microscopy (SEM)
or atomic force microscopy (AFM), that provide direct visualized information about
changes in the size, morphology and aggregation of the electrode particles caused
by electrochemical reactions.
7.3 Basic Aspects of Mössbauer Spectroscopy
7.3.1 The Mössbauer Effect
The Mössbauer effect is briefly described here, while more details can be found in
many textbooks [39–42]. This effect is based on transitions between excited and
ground nuclear states at energy E γ (~keV). An excited free nucleus can reach equilibrium by emitting a γ-photon (γ decay) of energy E γ − E R , where E R (~meV)
is the nuclear recoil energy, while a nucleus of the same isotope can be excited by
absorbing a γ-photon of energy E γ + E R . The energy distribution of γ-rays is a
Lorentzian curve whose the linewidth (~neV) is related to the mean lifetime of the
nuclear excited state by the Heisenberg uncertainty principle. The perfect overlap of
the emission and absorption Lorentzian curves (resonance) is not possible for free or
weakly bound atoms because the nuclear recoil energy is several orders of magnitude
higher than . However, R.L. Mössbauer observed such a resonance in solids that can
be explained as follows. In a solid, the recoil energy of nuclei during the emission of
γ-rays should be transferred to the lattice, but the quantization of atomic vibrations
requires that such transfer is only possible for energy values corresponding to the
creation of phonons. Thus, a fraction of the excited nuclei can emit γ-photons without
P.-E. Lippens
One of the most commonly used techniques for studying electrode materials is
X-ray diffraction (XRD). XRD has often been considered for the characterization
of crystalline pristine materials and their evolution during the lithiation-delithiation
cycles. This is of particular interest to follow changes in the lattice parameters and
atomic positions of crystalline insertion materials, resulting from the insertion of
ions, or to identify new formed crystalline phases. However, this technique fails for
poorly crystalline, nanosized and amorphous phases as often encountered in pristine electrode materials or detected as products of alloying and conversion reactions.
Such characterizations require local probe techniques, including X-ray photoelectron spectroscopy (XPS), which is a surface sensitive technique, X-ray absorption
spectroscopy (XAS), nuclear magnetic resonance (NMR), electron paramagnetic
resonance (EPR) or Mössbauer spectroscopy. These techniques not only give information about the local environment of the probe elements, but also on the electronic,
magnetic and dynamic properties of the electrode materials. For instance, they are
used to determine the oxidation and spin states, charge anisotropy and bond properties of atoms in the electrode particles, but they are also used to identify the products
formed during insertion-deinsertion processes within the electrode and at the interface with the electrolyte. All these techniques go along with imaging techniques, such
as transmission electron microscopy (TEM), scanning electron microscopy (SEM)
or atomic force microscopy (AFM), that provide direct visualized information about
changes in the size, morphology and aggregation of the electrode particles caused
by electrochemical reactions.
7.3 Basic Aspects of Mössbauer Spectroscopy
7.3.1 The Mössbauer Effect
The Mössbauer effect is briefly described here, while more details can be found in
many textbooks [39–42]. This effect is based on transitions between excited and
ground nuclear states at energy E γ (~keV). An excited free nucleus can reach equilibrium by emitting a γ-photon (γ decay) of energy E γ − E R , where E R (~meV)
is the nuclear recoil energy, while a nucleus of the same isotope can be excited by
absorbing a γ-photon of energy E γ + E R . The energy distribution of γ-rays is a
Lorentzian curve whose the linewidth (~neV) is related to the mean lifetime of the
nuclear excited state by the Heisenberg uncertainty principle. The perfect overlap of
the emission and absorption Lorentzian curves (resonance) is not possible for free or
weakly bound atoms because the nuclear recoil energy is several orders of magnitude
higher than . However, R.L. Mössbauer observed such a resonance in solids that can
be explained as follows. In a solid, the recoil energy of nuclei during the emission of
γ-rays should be transferred to the lattice, but the quantization of atomic vibrations
requires that such transfer is only possible for energy values corresponding to the
creation of phonons. Thus, a fraction of the excited nuclei can emit γ-photons without