66
5 Combinatorial Studies of the Spinel and Rocksalt Regions . . .
region. However, in oxygen, the lower boundary has moved down. This implies that
the spinel region is favored over the layered materials as oxygen partial pressure
increases. This can be attributed to the higher oxygen content in the spinel structures
as discussed in the introduction (Sect. 1.4.1). The cooling rate seems to have no effect
on the position of this lower boundary in oxygen even though the minimum lattice
parameter near LiNi 0.5 Mn 1.5 O 4 increases from 8.173 (regular cooling) to 8.187 Å
(quenching). Since this increase is not associated with any phase separation, it can
be attributed to oxygen deficiencies arising during heating at 800
◦ C and then being
relieved by oxygen returning into the sample during regular cooling, as observed by
Ma [37].
Figure 5.7 also shows that excess lithium can be added to the stoichiometric spinel
samples. The stoichiometric LiNi x Mn 2−x O 4 line was included in Fig. 5.7 as a green
dashed line. The lower boundary of the single-phase spinel region lies to the right
and below the green dashed line, corresponding to an excess of lithium. This effect
was more significant in oxygen but even in air at 800
◦ C, a small amount of excess
lithium can be added to the spinel structures containing nickel. Therefore, some
spinel–layered tie-lines terminate at points slightly below the LiNi x Mn 2−x O 4 line.
The contour plots also show that the upper spinel phase boundary moves up significantly at high temperature. This can be interpreted as oxygen loss since coexistence
with Mn 2 O 3 requires a higher oxygen content per metal atom (3:2) than that of a
single-phase spinel sample (4:3). The fact that the upper boundary is also higher
for samples made in air is consistent with this interpretation: the lower oxygen content favors spinel structures over manganese oxide. The motion of the boundaries
can therefore be explained using Le Chatelier’s principle and the fact that at high
temperature, oxygen gas is produced giving rise to high entropy.
5.3 Rocksalt Single-Phase Region
Figure 5.8 shows XRD patterns obtained along the C 5 –D 5 line in Fig. 5.4. The
right panel showing the region near 44
◦ suggests a solid solution. For the samples
with 0.4 or 0.5 lithium fraction, all peaks index to the layered structure of LiNiO 2
(JCPDS #89-3601). For lower lithium content, the structures become cubic and will
be discussed in more detail below.
The location of the phase transition from the cubic to hexagonal layered structures
was obtained by fitting all samples as layered and then using the c/a ratio as discussed
in Sect. 1.4. Figure 5.9 shows the c/a ratio as a function of lithium content along
three lines with various manganese contents. In a cubic structure, c/a =
√
24 such
that extrapolating to this value gives the composition where the structures convert
from a cubic to a hexagonal structure. Li et al. found that this transition occurs at a
lithium content of 0.31 along the Li–Ni line [36]. The blue dotted lines in Figs. 5.2,
5.4 and 5.10 show the position of the phase transition obtained by using the four
points generated by this method.
5 Combinatorial Studies of the Spinel and Rocksalt Regions . . .
region. However, in oxygen, the lower boundary has moved down. This implies that
the spinel region is favored over the layered materials as oxygen partial pressure
increases. This can be attributed to the higher oxygen content in the spinel structures
as discussed in the introduction (Sect. 1.4.1). The cooling rate seems to have no effect
on the position of this lower boundary in oxygen even though the minimum lattice
parameter near LiNi 0.5 Mn 1.5 O 4 increases from 8.173 (regular cooling) to 8.187 Å
(quenching). Since this increase is not associated with any phase separation, it can
be attributed to oxygen deficiencies arising during heating at 800
◦ C and then being
relieved by oxygen returning into the sample during regular cooling, as observed by
Ma [37].
Figure 5.7 also shows that excess lithium can be added to the stoichiometric spinel
samples. The stoichiometric LiNi x Mn 2−x O 4 line was included in Fig. 5.7 as a green
dashed line. The lower boundary of the single-phase spinel region lies to the right
and below the green dashed line, corresponding to an excess of lithium. This effect
was more significant in oxygen but even in air at 800
◦ C, a small amount of excess
lithium can be added to the spinel structures containing nickel. Therefore, some
spinel–layered tie-lines terminate at points slightly below the LiNi x Mn 2−x O 4 line.
The contour plots also show that the upper spinel phase boundary moves up significantly at high temperature. This can be interpreted as oxygen loss since coexistence
with Mn 2 O 3 requires a higher oxygen content per metal atom (3:2) than that of a
single-phase spinel sample (4:3). The fact that the upper boundary is also higher
for samples made in air is consistent with this interpretation: the lower oxygen content favors spinel structures over manganese oxide. The motion of the boundaries
can therefore be explained using Le Chatelier’s principle and the fact that at high
temperature, oxygen gas is produced giving rise to high entropy.
5.3 Rocksalt Single-Phase Region
Figure 5.8 shows XRD patterns obtained along the C 5 –D 5 line in Fig. 5.4. The
right panel showing the region near 44
◦ suggests a solid solution. For the samples
with 0.4 or 0.5 lithium fraction, all peaks index to the layered structure of LiNiO 2
(JCPDS #89-3601). For lower lithium content, the structures become cubic and will
be discussed in more detail below.
The location of the phase transition from the cubic to hexagonal layered structures
was obtained by fitting all samples as layered and then using the c/a ratio as discussed
in Sect. 1.4. Figure 5.9 shows the c/a ratio as a function of lithium content along
three lines with various manganese contents. In a cubic structure, c/a =
√
24 such
that extrapolating to this value gives the composition where the structures convert
from a cubic to a hexagonal structure. Li et al. found that this transition occurs at a
lithium content of 0.31 along the Li–Ni line [36]. The blue dotted lines in Figs. 5.2,
5.4 and 5.10 show the position of the phase transition obtained by using the four
points generated by this method.