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P.-E. Lippens
transition. The success of Li-ion batteries is mainly due to their high energy density,
arising from the low atomic mass of lithium and its high electropositivity that give
high electrode capacity and high cell voltage, respectively. But such lightweight,
rechargeable and powerful batteries are also the result of a long series of fundamental
research and technology advances [5].
The early use of lithium metal as negative electrode for lithium batteries encountered severe safety issues due to Li dendritic growth, leading to internal short-circuits
[6]. In a Li-ion battery, the negative electrode is usually made from an intercalation material like graphite, while Li
+ ions are provided by the positive electrode
mostly comprising a layered lithium transition metal oxide. The Li
+ ions move
between the two electrodes through an electrolyte that usually consists of a lithium
salt and a liquid organic solvent, although solid electrolytes should be a promising
alternative to enhance safety and energy density [7–9].
Depending on their applications, Li-ion batteries should have high power and/or
high energy density, but in all cases they should be safe, with long calendar and
cycle life, environmentally friendly, and as cheap as possible. Everyone would
like to use small and light batteries that can be charged fast and do not explode!
The performance of conventional Li-ion batteries based on the “negative electrodeliquid electrolyte-positive electrode” configuration depends on the properties of each
component and on their combination. It is a real challenge to improve significantly
such complex systems, which not only requires to optimize the electrode materials
and electrolyte, but also the interfaces, the design of electrodes and cells, the current
collectors, etc. [10–16]. Such a task needs a multidisciplinary approach combining
different fields of chemistry and physics, but above all, the application of different
techniques of characterization [17–23].
Mössbauer spectroscopy can be used in different ways to improve the performance of Li-ion batteries [24–27]. The most obvious application concerns the characterization of the structural and electronic properties of pristine electrode materials, following the methodologies developed in solid-state chemistry. The technique
provides a lot of information about the local environment of atoms through electric
and magnetic interactions. Mössbauer spectroscopy is also of high interest for the
characterization of poorly crystalline materials, amorphous phases and nanoparticles since X-ray diffraction cannot be used due to the loss of long-range order. This
aspect is not only important for pristine electrode materials but also for the products of electrochemical reactions that can be, under certain experimental conditions,
studied ex situ, i.e. by extracting the electrode material out of the electrochemical
cell at a certain stage of lithiation or sodiation.
Most importantly, Mössbauer spectroscopy is a powerful technique to investigate
reaction mechanisms under operating conditions as shown, for example, in catalysis
[28]. By considering specific electrochemical cells that make γ-ray transmission
possible, the technique can be used for in situ or operando studies of electrochemical
reactions. Although these two terms are often used indistinctly in the literature, one
can consider that operando measurements refer to batteries in charge or discharge
operations while in situ characterizations concern all studies where batteries are not
P.-E. Lippens
transition. The success of Li-ion batteries is mainly due to their high energy density,
arising from the low atomic mass of lithium and its high electropositivity that give
high electrode capacity and high cell voltage, respectively. But such lightweight,
rechargeable and powerful batteries are also the result of a long series of fundamental
research and technology advances [5].
The early use of lithium metal as negative electrode for lithium batteries encountered severe safety issues due to Li dendritic growth, leading to internal short-circuits
[6]. In a Li-ion battery, the negative electrode is usually made from an intercalation material like graphite, while Li
+ ions are provided by the positive electrode
mostly comprising a layered lithium transition metal oxide. The Li
+ ions move
between the two electrodes through an electrolyte that usually consists of a lithium
salt and a liquid organic solvent, although solid electrolytes should be a promising
alternative to enhance safety and energy density [7–9].
Depending on their applications, Li-ion batteries should have high power and/or
high energy density, but in all cases they should be safe, with long calendar and
cycle life, environmentally friendly, and as cheap as possible. Everyone would
like to use small and light batteries that can be charged fast and do not explode!
The performance of conventional Li-ion batteries based on the “negative electrodeliquid electrolyte-positive electrode” configuration depends on the properties of each
component and on their combination. It is a real challenge to improve significantly
such complex systems, which not only requires to optimize the electrode materials
and electrolyte, but also the interfaces, the design of electrodes and cells, the current
collectors, etc. [10–16]. Such a task needs a multidisciplinary approach combining
different fields of chemistry and physics, but above all, the application of different
techniques of characterization [17–23].
Mössbauer spectroscopy can be used in different ways to improve the performance of Li-ion batteries [24–27]. The most obvious application concerns the characterization of the structural and electronic properties of pristine electrode materials, following the methodologies developed in solid-state chemistry. The technique
provides a lot of information about the local environment of atoms through electric
and magnetic interactions. Mössbauer spectroscopy is also of high interest for the
characterization of poorly crystalline materials, amorphous phases and nanoparticles since X-ray diffraction cannot be used due to the loss of long-range order. This
aspect is not only important for pristine electrode materials but also for the products of electrochemical reactions that can be, under certain experimental conditions,
studied ex situ, i.e. by extracting the electrode material out of the electrochemical
cell at a certain stage of lithiation or sodiation.
Most importantly, Mössbauer spectroscopy is a powerful technique to investigate
reaction mechanisms under operating conditions as shown, for example, in catalysis
[28]. By considering specific electrochemical cells that make γ-ray transmission
possible, the technique can be used for in situ or operando studies of electrochemical
reactions. Although these two terms are often used indistinctly in the literature, one
can consider that operando measurements refer to batteries in charge or discharge
operations while in situ characterizations concern all studies where batteries are not