1.3 The Li–Co–Mn–O Face of the Pyramid
5
on the stoichiometry of the starting material [15]. Although some electrochemical
studies are included in this thesis, the primary focus will be the structures of the
starting materials such that an in-depth description of the electrochemical behavior
of these materials will not be included in this introduction. It is sufficient to point
out that lithium can be removed from the layered materials by way of 2-D transport
along the hexagonal planes.
Figure 1.2c shows that the spinel structures have lithium atoms aligned in rows
within the cubic lattice. Here, every lithium atom is in a tetrahedral site, surrounded
by four nearest neighbor oxygens. In spinel structures, the oxygen content is higher
than for layered materials as there are four oxygen atoms per three metal atoms.
This higher oxygen content in the spinels will be significant in discussing how the
boundaries of the single-phase regions move under various synthesis conditions.
Charging of a cell made with a spinel positive electrode involves one dimensional Liion transport through the 3-D network of tunnels and proceeds by the deintercalation
mechanism described for the layered structures [1].
While no attempt has been made to precisely work out electronic structures of
the layered and spinel materials, simple models considering the oxidation state of
each metallic atom can be very useful in predicting the possible mechanisms taking place during cycling. To illustrate the importance of the oxidation states of the
starting materials, consider LiMn 2 O 4 . The initial average oxidation number of the
manganese is 3.5 + such that all the lithium can be removed while the manganese
transitions to the 4 + state. However, it is possible to synthesize Li 1+x Mn 2−x O 4 materials with more lithium, up to a maximum of Li 4/3 Mn 5/3 O 4 [7, 8]. In this material,
all manganese is in the 4 + state such that no redox transition is possible and thus
no lithium can be removed in the potential window typically used (below 5.0 V vs
Li
+ /Li). This illustrates how very simple models can be useful. This approach will
be used throughout this thesis to discuss the electronic structures of the metals in
these complex structures as they undergo changes during electrochemical cycling.
1.3 The Li–Co–Mn–O Face of the Pyramid
Figure 1.3 shows the single-phase structures known in the Li–Co–Mn–O system
prior to the current work. To demonstrate how these Gibbs triangles are constructed,
consider the sample LiCo 2/3 Mn 4/3 O 4 that has metallic fractions Li 1/3 Co 2/9 Mn 4/9
and is, therefore, plotted at the point (Li, Co, Mn) = (0.333, 0.222, 0.444). The
tick marks on all ternary diagrams are slanted to make the values of the three axes
more apparent. Only the metal atoms obey the rules of a Gibbs triangle and the three
axes are therefore the metal atomic fractions with Li + Co + Mn = 1 for all points. The
labels at the three corners only show the phases present at the corners. Also, since the
cobalt content can be calculated as 1 − Li − Mn, all points will be shown as (Li, Mn),
such that (0.333, 0.444) refers to LiCo 2/3 Mn 4/3 O 4 , (0.333, 0.666) refers to Li 2 MnO 3 ,
and (0.5, 0) refers to LiCoO 2 . It is worth noting that since the oxygen content varies
through the triangle, this pseudo-ternary diagram represents a nonplanar surface
5
on the stoichiometry of the starting material [15]. Although some electrochemical
studies are included in this thesis, the primary focus will be the structures of the
starting materials such that an in-depth description of the electrochemical behavior
of these materials will not be included in this introduction. It is sufficient to point
out that lithium can be removed from the layered materials by way of 2-D transport
along the hexagonal planes.
Figure 1.2c shows that the spinel structures have lithium atoms aligned in rows
within the cubic lattice. Here, every lithium atom is in a tetrahedral site, surrounded
by four nearest neighbor oxygens. In spinel structures, the oxygen content is higher
than for layered materials as there are four oxygen atoms per three metal atoms.
This higher oxygen content in the spinels will be significant in discussing how the
boundaries of the single-phase regions move under various synthesis conditions.
Charging of a cell made with a spinel positive electrode involves one dimensional Liion transport through the 3-D network of tunnels and proceeds by the deintercalation
mechanism described for the layered structures [1].
While no attempt has been made to precisely work out electronic structures of
the layered and spinel materials, simple models considering the oxidation state of
each metallic atom can be very useful in predicting the possible mechanisms taking place during cycling. To illustrate the importance of the oxidation states of the
starting materials, consider LiMn 2 O 4 . The initial average oxidation number of the
manganese is 3.5 + such that all the lithium can be removed while the manganese
transitions to the 4 + state. However, it is possible to synthesize Li 1+x Mn 2−x O 4 materials with more lithium, up to a maximum of Li 4/3 Mn 5/3 O 4 [7, 8]. In this material,
all manganese is in the 4 + state such that no redox transition is possible and thus
no lithium can be removed in the potential window typically used (below 5.0 V vs
Li
+ /Li). This illustrates how very simple models can be useful. This approach will
be used throughout this thesis to discuss the electronic structures of the metals in
these complex structures as they undergo changes during electrochemical cycling.
1.3 The Li–Co–Mn–O Face of the Pyramid
Figure 1.3 shows the single-phase structures known in the Li–Co–Mn–O system
prior to the current work. To demonstrate how these Gibbs triangles are constructed,
consider the sample LiCo 2/3 Mn 4/3 O 4 that has metallic fractions Li 1/3 Co 2/9 Mn 4/9
and is, therefore, plotted at the point (Li, Co, Mn) = (0.333, 0.222, 0.444). The
tick marks on all ternary diagrams are slanted to make the values of the three axes
more apparent. Only the metal atoms obey the rules of a Gibbs triangle and the three
axes are therefore the metal atomic fractions with Li + Co + Mn = 1 for all points. The
labels at the three corners only show the phases present at the corners. Also, since the
cobalt content can be calculated as 1 − Li − Mn, all points will be shown as (Li, Mn),
such that (0.333, 0.444) refers to LiCo 2/3 Mn 4/3 O 4 , (0.333, 0.666) refers to Li 2 MnO 3 ,
and (0.5, 0) refers to LiCoO 2 . It is worth noting that since the oxygen content varies
through the triangle, this pseudo-ternary diagram represents a nonplanar surface