7.4 Electrochemistry of the R, M and N Phases
101
The two peaks seen in the quenched C 7 and D 7 samples correspond to the Kα 1,2
splitting visible due to large crystallites giving sharp peaks and do not indicate phase
separation. The XRD results were used to create the approximate boundaries shown
in Fig. 7.5. The upward movement of the layered boundary with temperature can be
attributed to the reaction equilibrium favoring the layered phase over the spinel phase
that contains more oxygen per metal atom. This again occurs as the high entropy
of the oxygen gas wins out at elevated temperatures. The improved performance of
the Li-rich materials synthesized at high temperature [2, 86] may therefore simply
be due to the samples remaining single-phase, thereby avoiding the layered–layered
coexistence region. This will be examined in detail in Chap. 9.
Furthermore, the layered region extends quite low in the Gibbs triangle compared
to solid-solutions known prior to this thesis (Fig. 1.8b in the introduction). This
implies that some published results must be reexamined. For example, a sample
reported by Ohzuku et al. [52] (blue point in Fig. 7.5) had a very high capacity
and is reported as being LiNi 0.5 Mn 0.5 O 2 which is very near to point G 7 . However,
the contour plots were used along with the published lattice parameter values of
a = 2.883 Å and c = 14.269 Å to position this sample on the phase diagram in
Fig. 7.5. This result suggests that the sample lost very little of the 25 % excess lithium
used during synthesis such that the material was in fact a lithium-rich layered material,
and thus its improved electrochemistry over other published LiNi 0.5 Mn 0.5 O 2 samples
can be attributed to the extra lithium after nickel oxidation. Similarly, the open green
symbol in Fig. 7.5 represents a sample from Ref. [51] that was reported to lie on the
Li-rich line joining Li 2 MnO 3 to LiNi 0.5 Mn 0.5 O 2 . This point was positioned using
the published lattice parameter, a = 2.860 Å and c = 14.246 Å, again suggesting
that it too was below the Li-rich line such that not all of the 5 % excess lithium
was lost during synthesis. These two points were used to sketch the curved line
joining LiNiO 2 to Li 2 MnO 3 . The electrochemistry of single-phase layered materials
below the lithium-rich layered line must be carefully studied and elemental analysis
is required to confirm final compositions of samples in this region of the Gibbs
triangle.
7.4 Electrochemistry of the R, M and N Phases
Figure 7.6 shows the electrochemical data obtained for the three new materials: N,
M and R made using the one-pot synthesis approach and heated to 800
◦ C in oxygen
for 5 h before being quenched. The only material with promising electrochemical
properties was the M-layered phase. This is not surprising, since the XRD patterns
in Chap. 6 showed this material to be a highly ordered lithium-rich material. While
some of the lithium was removed by oxidizing the nickel from 2 + to 4 + up to 4.45 V
[2], the remaining lithium was removed at about 4.5 V via the high voltage plateau
discussed in the introduction. This plateau has long been thought to involve lithium
being removed along with oxygen gas via the so-called oxygen release process [2,
17]. This process would leave the oxidation states of the transition metals unchanged
101
The two peaks seen in the quenched C 7 and D 7 samples correspond to the Kα 1,2
splitting visible due to large crystallites giving sharp peaks and do not indicate phase
separation. The XRD results were used to create the approximate boundaries shown
in Fig. 7.5. The upward movement of the layered boundary with temperature can be
attributed to the reaction equilibrium favoring the layered phase over the spinel phase
that contains more oxygen per metal atom. This again occurs as the high entropy
of the oxygen gas wins out at elevated temperatures. The improved performance of
the Li-rich materials synthesized at high temperature [2, 86] may therefore simply
be due to the samples remaining single-phase, thereby avoiding the layered–layered
coexistence region. This will be examined in detail in Chap. 9.
Furthermore, the layered region extends quite low in the Gibbs triangle compared
to solid-solutions known prior to this thesis (Fig. 1.8b in the introduction). This
implies that some published results must be reexamined. For example, a sample
reported by Ohzuku et al. [52] (blue point in Fig. 7.5) had a very high capacity
and is reported as being LiNi 0.5 Mn 0.5 O 2 which is very near to point G 7 . However,
the contour plots were used along with the published lattice parameter values of
a = 2.883 Å and c = 14.269 Å to position this sample on the phase diagram in
Fig. 7.5. This result suggests that the sample lost very little of the 25 % excess lithium
used during synthesis such that the material was in fact a lithium-rich layered material,
and thus its improved electrochemistry over other published LiNi 0.5 Mn 0.5 O 2 samples
can be attributed to the extra lithium after nickel oxidation. Similarly, the open green
symbol in Fig. 7.5 represents a sample from Ref. [51] that was reported to lie on the
Li-rich line joining Li 2 MnO 3 to LiNi 0.5 Mn 0.5 O 2 . This point was positioned using
the published lattice parameter, a = 2.860 Å and c = 14.246 Å, again suggesting
that it too was below the Li-rich line such that not all of the 5 % excess lithium
was lost during synthesis. These two points were used to sketch the curved line
joining LiNiO 2 to Li 2 MnO 3 . The electrochemistry of single-phase layered materials
below the lithium-rich layered line must be carefully studied and elemental analysis
is required to confirm final compositions of samples in this region of the Gibbs
triangle.
7.4 Electrochemistry of the R, M and N Phases
Figure 7.6 shows the electrochemical data obtained for the three new materials: N,
M and R made using the one-pot synthesis approach and heated to 800
◦ C in oxygen
for 5 h before being quenched. The only material with promising electrochemical
properties was the M-layered phase. This is not surprising, since the XRD patterns
in Chap. 6 showed this material to be a highly ordered lithium-rich material. While
some of the lithium was removed by oxidizing the nickel from 2 + to 4 + up to 4.45 V
[2], the remaining lithium was removed at about 4.5 V via the high voltage plateau
discussed in the introduction. This plateau has long been thought to involve lithium
being removed along with oxygen gas via the so-called oxygen release process [2,
17]. This process would leave the oxidation states of the transition metals unchanged