324
P.-E. Lippens
The negative electrodes of Li-ion batteries are mainly based on carbon. Graphite
can intercalate Li
+ ions, has a low cost and can be easily found or produced. Graphite
can intercalate between the carbon layers up to 1 Li per 6 C, leading to specific and
volumetric capacities of 372 mA h g
−1 and ∼800 mA h cm
−3 , respectively. The
voltage profile of a graphite based electrode, i.e., the electrode potential vs. Li/Li
+
as a function of the number of inserted Li, shows well-defined plateaus and a good
reversibility. The mechanism is more complex for disordered forms of carbon such as
hard carbons, soft unorganized carbons or natural graphite that contain a lot of defects,
leading to different voltage profiles and electrode performances [34–36]. One issue
with carbon is the possible formation of lithium dendrites under certain conditions
due to its operating potential close to that of metallic lithium. In addition, specific
and volumetric capacities should be increased. This is obtained by the addition of
a small amount of silicon since this element has a specific capacity about ten times
higher than carbon, but the increase of capacity is still too low for future applications.
Titanates (TiO 2 , Li 4 Ti 5 O 12 ) have been commercialized as negative electrode materials for high power batteries. The high operating potential of titanate based electrodes
provides a high level of safety but a lower energy density than carbon. Finally, other
families of materials with higher operating potentials and capacities than carbon
have been proposed but encountered different critical issues. For example, tin based
materials were previously regarded as negative electrode materials for high energy
density batteries with the trade names STALION by Fuji in 1997 [37] and NEXELION by Sony in 2005 [38]. Tin is of particular interest within the framework of
this chapter since it is involved in different electrochemical mechanisms that can be
studied by
119 Sn Mössbauer spectroscopy as shown in Sects. 7.5 and 7.6.
7.2.2 Electrochemical Mechanisms
Three main types of electrochemical reactions that take place in electrode materials
are usually distinguished: insertion, alloying and conversion reactions.
Insertion or intercalation reactions: Li
+ ions are intercalated between layers
(graphite, LiMO 2 ) or inserted into diffusion channels (Nasicon, LiFePO 4 , Li 4 Ti 5 O 12 )
of a host material to form a solid solution without strong structural changes or a new
phase (two-phase reaction). In the first case, the voltage profile has a typical S-shape
whereas a two-phase reaction often shows a voltage plateau reflecting first-order
phase transition. These reactions are often highly reversible.
Alloying reactions: Li atoms form chemical bonds with an element A (Si, Sn,
etc.) leading to Li x A alloys or compounds. Such reactions come along with strong
volume variations, causing mechanical and chemical instabilities of the electrode.
The formed species are often nanoparticles but they can coalesce during cycling.
The generally observed staircase voltage profile reflects the succession of two-phase
alloying reactions but kinetic limitations or the formation of metastable phases often
smooth the voltage curve. These reactions are reversible but reducing the effect of
volume variations is still a challenge for long-term cycling.
P.-E. Lippens
The negative electrodes of Li-ion batteries are mainly based on carbon. Graphite
can intercalate Li
+ ions, has a low cost and can be easily found or produced. Graphite
can intercalate between the carbon layers up to 1 Li per 6 C, leading to specific and
volumetric capacities of 372 mA h g
−1 and ∼800 mA h cm
−3 , respectively. The
voltage profile of a graphite based electrode, i.e., the electrode potential vs. Li/Li
+
as a function of the number of inserted Li, shows well-defined plateaus and a good
reversibility. The mechanism is more complex for disordered forms of carbon such as
hard carbons, soft unorganized carbons or natural graphite that contain a lot of defects,
leading to different voltage profiles and electrode performances [34–36]. One issue
with carbon is the possible formation of lithium dendrites under certain conditions
due to its operating potential close to that of metallic lithium. In addition, specific
and volumetric capacities should be increased. This is obtained by the addition of
a small amount of silicon since this element has a specific capacity about ten times
higher than carbon, but the increase of capacity is still too low for future applications.
Titanates (TiO 2 , Li 4 Ti 5 O 12 ) have been commercialized as negative electrode materials for high power batteries. The high operating potential of titanate based electrodes
provides a high level of safety but a lower energy density than carbon. Finally, other
families of materials with higher operating potentials and capacities than carbon
have been proposed but encountered different critical issues. For example, tin based
materials were previously regarded as negative electrode materials for high energy
density batteries with the trade names STALION by Fuji in 1997 [37] and NEXELION by Sony in 2005 [38]. Tin is of particular interest within the framework of
this chapter since it is involved in different electrochemical mechanisms that can be
studied by
119 Sn Mössbauer spectroscopy as shown in Sects. 7.5 and 7.6.
7.2.2 Electrochemical Mechanisms
Three main types of electrochemical reactions that take place in electrode materials
are usually distinguished: insertion, alloying and conversion reactions.
Insertion or intercalation reactions: Li
+ ions are intercalated between layers
(graphite, LiMO 2 ) or inserted into diffusion channels (Nasicon, LiFePO 4 , Li 4 Ti 5 O 12 )
of a host material to form a solid solution without strong structural changes or a new
phase (two-phase reaction). In the first case, the voltage profile has a typical S-shape
whereas a two-phase reaction often shows a voltage plateau reflecting first-order
phase transition. These reactions are often highly reversible.
Alloying reactions: Li atoms form chemical bonds with an element A (Si, Sn,
etc.) leading to Li x A alloys or compounds. Such reactions come along with strong
volume variations, causing mechanical and chemical instabilities of the electrode.
The formed species are often nanoparticles but they can coalesce during cycling.
The generally observed staircase voltage profile reflects the succession of two-phase
alloying reactions but kinetic limitations or the formation of metastable phases often
smooth the voltage curve. These reactions are reversible but reducing the effect of
volume variations is still a challenge for long-term cycling.