6
1 Introduction
Fig. 1.3 The Li–Co–Mn–O
pseudo-ternary system with
single-phase materials known
prior to the combinatorial
studies discussed here. The
red symbols indicate layered
oxides while the green and
blue are cubic and tetragonal
spinels, respectively, and the
black circle represents
bixbyite Mn 2 O 3 . The lithium
corner is labeled as Li as no
stable solid phases are seen
here as lithium is gradually
lost during synthesis
in the Li–Mn–Ni–O quaternary system. As such, the oxygen content at any point
is not only a function of the position within the triangle but also depends on the
synthesis conditions. The atmosphere and temperature profiles used during heating
are particularly important.
Figure 1.3 also shows that only a few spinel structures were known in this triangle
prior to this research [19–21]. However, the fact that the cobalt corner takes a cubic
spinel structure, Co 3 O 4 , and the placement of the other single-phase spinel materials,
suggests that there may be a large spinel region spanning from LiMn 2 O 4 to Co 3 O 4 .
The layered structures in the Li–Co–Mn oxide system have been studied extensively along the composition line Li[Li (1−x)/3 Co x Mn (2−2x)/3 ]O 2 ; 0 ≤ x ≤ 1, which
joins LiCoO 2 to Li 2 MnO 3 [18, 22, 23]. Figure 1.4 shows X-ray diffraction (XRD)
patterns along this composition line reported by Kim et al. [18]. The XRD pattern for
LiCoO 2 can be attributed to the layered structure described in the previous section
where nearly every site on the TM layer is occupied by cobalt. All scattering peaks
can be indexed to the R-3m space group, which has a hexagonal lattice with every
unit cell being made up of three TM layers and three lithium layers (this stacking is
referred to as O3-type and is illustrated in Fig. 1.2b). Figure 1.5 shows two possible
arrangements of atoms on TM layers. The first, shown in Fig. 1.5a, assumes random
occupation of all sites as would be obtained for LiCoO 2 . For Li 2 MnO 3 , however,
there are extra peaks seen in the scattering angle range 20–30
◦ which can be attributed to ordering on the TM layer [24]. Figure 1.5b shows such an ordered layer
where the red atoms represent lithium while the blue atoms represent manganese.
The red atoms all lie on a lattice that is
√
3 times larger in each direction than the
hexagonal lattice shown in Fig. 1.5a. As such, this ordered lattice will be referred to
as a
√
3 ×
√
3 superlattice. The ordering on the TM layer is typically accompanied
by a distortion in the stacking of the layers such that the structure can no longer
1 Introduction
Fig. 1.3 The Li–Co–Mn–O
pseudo-ternary system with
single-phase materials known
prior to the combinatorial
studies discussed here. The
red symbols indicate layered
oxides while the green and
blue are cubic and tetragonal
spinels, respectively, and the
black circle represents
bixbyite Mn 2 O 3 . The lithium
corner is labeled as Li as no
stable solid phases are seen
here as lithium is gradually
lost during synthesis
in the Li–Mn–Ni–O quaternary system. As such, the oxygen content at any point
is not only a function of the position within the triangle but also depends on the
synthesis conditions. The atmosphere and temperature profiles used during heating
are particularly important.
Figure 1.3 also shows that only a few spinel structures were known in this triangle
prior to this research [19–21]. However, the fact that the cobalt corner takes a cubic
spinel structure, Co 3 O 4 , and the placement of the other single-phase spinel materials,
suggests that there may be a large spinel region spanning from LiMn 2 O 4 to Co 3 O 4 .
The layered structures in the Li–Co–Mn oxide system have been studied extensively along the composition line Li[Li (1−x)/3 Co x Mn (2−2x)/3 ]O 2 ; 0 ≤ x ≤ 1, which
joins LiCoO 2 to Li 2 MnO 3 [18, 22, 23]. Figure 1.4 shows X-ray diffraction (XRD)
patterns along this composition line reported by Kim et al. [18]. The XRD pattern for
LiCoO 2 can be attributed to the layered structure described in the previous section
where nearly every site on the TM layer is occupied by cobalt. All scattering peaks
can be indexed to the R-3m space group, which has a hexagonal lattice with every
unit cell being made up of three TM layers and three lithium layers (this stacking is
referred to as O3-type and is illustrated in Fig. 1.2b). Figure 1.5 shows two possible
arrangements of atoms on TM layers. The first, shown in Fig. 1.5a, assumes random
occupation of all sites as would be obtained for LiCoO 2 . For Li 2 MnO 3 , however,
there are extra peaks seen in the scattering angle range 20–30
◦ which can be attributed to ordering on the TM layer [24]. Figure 1.5b shows such an ordered layer
where the red atoms represent lithium while the blue atoms represent manganese.
The red atoms all lie on a lattice that is
√
3 times larger in each direction than the
hexagonal lattice shown in Fig. 1.5a. As such, this ordered lattice will be referred to
as a
√
3 ×
√
3 superlattice. The ordering on the TM layer is typically accompanied
by a distortion in the stacking of the layers such that the structure can no longer