7 Application of Mössbauer Spectroscopy to Li-Ion and Na-Ion …
325
Conversion reactions: Li atoms react with AB (intermetallics, oxides, sulfides,
etc.) to give A and Li x B nanoparticles that usually form a composite. For example,
the lithiation of SnO produces Sn and Li 2 O nanoparticles. Conversion reactions can
be reversible, irreversible and partially reversible as observed for transition metal
oxides, tin intermetallics and tin oxides, respectively. The voltage profile is formed
by two plateaus for lithiation and delithiation, respectively, showing a large voltage
polarization. Some conversion reactions are followed by alloying reactions.
It should be noted that the number of Li (or Na) per electrochemically active
element involved in alloying and conversion reactions is generally higher than for
insertion reactions, leading to high capacity electrode materials.
7.2.3 Characterization of Electrochemical Reactions
In Li-ion batteries using a lithium salt and organic solvent as electrolyte, the electrodes consist of metallic foils as current collectors that are coated with a film of
several tens to hundreds micrometers obtained from a slurry containing powdered
electrochemically active materials, electronic conductive additives and binder. All
these components should be optimized for better performance. For instance, the electrode formulation is essential to form a mechanically and chemically stable composite
film containing the active particles with electron and lithium conductive pathways. Concerning the electrochemically active materials, not only their intrinsic
properties should be improved, but also their interfaces with the electrolyte and the
other components of the film. Thus, the characterization of pristine electrode materials, as obtained after synthesis, should be combined with the analysis of the changes
in these materials during charge-discharge cycles.
A lot of techniques used in solid-state chemistry and electrochemistry can be
considered for the characterization of electrode materials and reaction mechanisms
at different length and time scales. Electrode performance, such as volumetric and
specific capacities, operating voltage, cycling and rate capabilities, and cycle life,
can be evaluated from the measurements of the cell voltage or the electric current
with different experimental protocols based on galvanostatic (constant current) or
potentiostatic (constant voltage) regimes. Electrochemical experiments also provide
useful information about the reactions taking place in the cell from the voltage profile,
current or voltage relaxations, impedance, etc. For such experiments, but also for
in situ characterizations, Li half-cells are used instead of Li-ion full-cells. In a Li
half-cell, the negative electrode is lithium metal that should be considered as the
reservoir of lithium while the positive electrode contains the active material under
study. The latter material can be the positive or negative electrode material in a Li-ion
full-cell. Thus, the discharge and charge of a Li half-cell correspond to the lithiation
and delithiation of the electrode material under study, respectively, while the situation
is somewhat more ambiguous for a Li-ion full-cell that contains the two types of
electrodes. To avoid confusion, the terms lithiation and delithiation (sodiation and
desodiation) are used along this chapter instead of charge and discharge, respectively.
325
Conversion reactions: Li atoms react with AB (intermetallics, oxides, sulfides,
etc.) to give A and Li x B nanoparticles that usually form a composite. For example,
the lithiation of SnO produces Sn and Li 2 O nanoparticles. Conversion reactions can
be reversible, irreversible and partially reversible as observed for transition metal
oxides, tin intermetallics and tin oxides, respectively. The voltage profile is formed
by two plateaus for lithiation and delithiation, respectively, showing a large voltage
polarization. Some conversion reactions are followed by alloying reactions.
It should be noted that the number of Li (or Na) per electrochemically active
element involved in alloying and conversion reactions is generally higher than for
insertion reactions, leading to high capacity electrode materials.
7.2.3 Characterization of Electrochemical Reactions
In Li-ion batteries using a lithium salt and organic solvent as electrolyte, the electrodes consist of metallic foils as current collectors that are coated with a film of
several tens to hundreds micrometers obtained from a slurry containing powdered
electrochemically active materials, electronic conductive additives and binder. All
these components should be optimized for better performance. For instance, the electrode formulation is essential to form a mechanically and chemically stable composite
film containing the active particles with electron and lithium conductive pathways. Concerning the electrochemically active materials, not only their intrinsic
properties should be improved, but also their interfaces with the electrolyte and the
other components of the film. Thus, the characterization of pristine electrode materials, as obtained after synthesis, should be combined with the analysis of the changes
in these materials during charge-discharge cycles.
A lot of techniques used in solid-state chemistry and electrochemistry can be
considered for the characterization of electrode materials and reaction mechanisms
at different length and time scales. Electrode performance, such as volumetric and
specific capacities, operating voltage, cycling and rate capabilities, and cycle life,
can be evaluated from the measurements of the cell voltage or the electric current
with different experimental protocols based on galvanostatic (constant current) or
potentiostatic (constant voltage) regimes. Electrochemical experiments also provide
useful information about the reactions taking place in the cell from the voltage profile,
current or voltage relaxations, impedance, etc. For such experiments, but also for
in situ characterizations, Li half-cells are used instead of Li-ion full-cells. In a Li
half-cell, the negative electrode is lithium metal that should be considered as the
reservoir of lithium while the positive electrode contains the active material under
study. The latter material can be the positive or negative electrode material in a Li-ion
full-cell. Thus, the discharge and charge of a Li half-cell correspond to the lithiation
and delithiation of the electrode material under study, respectively, while the situation
is somewhat more ambiguous for a Li-ion full-cell that contains the two types of
electrodes. To avoid confusion, the terms lithiation and delithiation (sodiation and
desodiation) are used along this chapter instead of charge and discharge, respectively.