6.9 Conclusions Regarding Combinatorial Studies of Li–Mn–Ni–O Materials
93
is required to have a better understanding of the thermodynamics and kinetics of
these phase transformations.
The results obtained here along the lithium rich line between LiNi 0.5 Mn 0.5 O 2 and
Li 2 MnO 3 also require discussion. This composition line is not a solid solution over
its entire length when synthesized in oxygen. This would suggest that the broadening
seen in XRD patterns by Jo et al. [54] was due to multiple phases. Furthermore, the
region near 43.5
◦ in the XRD spectra published by Jo et al. shows that there may be
trace amounts of ordered rocksalt present, which means that this sample obtained
with a slow cooling rate contained three phases, consistent with the phase diagram
produced here.
Furthermore, the layered boundary moved upward in the Gibbs triangle when
heated to higher temperatures (i.e., quenched), especially near the N-layered corner. At 800
◦ C in oxygen, a layered material synthesized at the composition of
LiNi 0.5 Mn 0.5 O 2 is not stable, but instead, the phase separates into two layered structures: the N and M phases. As the boundary moves up (either with temperature or
in air) the endpoints of the tie-line on which the sample lies will approach each
other and ultimately become a single-phase sample. Inversely, during cooling the
boundary moves down such that a single-phase sample near the upper boundary of
the layered region phase separates into two layered structures during slow cooling.
As suggested from the work in the Li–Co–Mn–O system in Chap. 4, these conditions would give rise to layered–layered nano-composites. However, the endpoints
of this coexistence are not Li 2 MnO 3 and LiNi 0.5 Mn 0.5 O 4 as assumed by many authors [43]. The tie-lines obtained here never extend to Li 2 MnO 3 and they include
points to the left of LiNi 0.5 Mn 0.5 O 2 in the Gibbs triangle. Therefore, the so-called
LiNi 0.5 Mn 0.5 O 2 -Li 2 MnO 3 nano-phase separation promoted by many authors needs
to be adapted given the actual endpoints of the coexistence. The layered–layered
region does not include the lithium-rich materials at all at higher temperatures as
the single-phase boundary sweeps upward in the triangle though. This will be discussed in the next chapter. It is also relevant that the unit cell volumes of the M and
N-layered phases differ by approximately 4.0 %; which is relatively large compared
to that observed in Chap. 4 and would most likely result in fracturing of the lattice.
This prediction is supported by the SEM images here where very small particles are
seen in the samples with two layered structures and larger particles in the singlephase layered sample. Thus, the region where nano-scale domains would be found
is further restricted to some portion of the layered–layered region. Clearly, further
study is needed to truly understand the short-range ordering in such samples and
any sample near the boundary of the layered region where XRD peak broadening
is seen. In particular, the consequences of these nano-domains on the lithium layer
must be evaluated given that one of the layered structures in the nano-composites
must be quite close in composition to the N-layered structure which was found to
contain a high fraction of nickel on the lithium layer. Clustering of nickel on the
lithium layer during cooling would interfere significantly with lithium extraction
during electrochemical cycling. This will be explored in Chap. 9 for materials near
LiNi 0.5 Mn 0.5 O 4 .
93
is required to have a better understanding of the thermodynamics and kinetics of
these phase transformations.
The results obtained here along the lithium rich line between LiNi 0.5 Mn 0.5 O 2 and
Li 2 MnO 3 also require discussion. This composition line is not a solid solution over
its entire length when synthesized in oxygen. This would suggest that the broadening
seen in XRD patterns by Jo et al. [54] was due to multiple phases. Furthermore, the
region near 43.5
◦ in the XRD spectra published by Jo et al. shows that there may be
trace amounts of ordered rocksalt present, which means that this sample obtained
with a slow cooling rate contained three phases, consistent with the phase diagram
produced here.
Furthermore, the layered boundary moved upward in the Gibbs triangle when
heated to higher temperatures (i.e., quenched), especially near the N-layered corner. At 800
◦ C in oxygen, a layered material synthesized at the composition of
LiNi 0.5 Mn 0.5 O 2 is not stable, but instead, the phase separates into two layered structures: the N and M phases. As the boundary moves up (either with temperature or
in air) the endpoints of the tie-line on which the sample lies will approach each
other and ultimately become a single-phase sample. Inversely, during cooling the
boundary moves down such that a single-phase sample near the upper boundary of
the layered region phase separates into two layered structures during slow cooling.
As suggested from the work in the Li–Co–Mn–O system in Chap. 4, these conditions would give rise to layered–layered nano-composites. However, the endpoints
of this coexistence are not Li 2 MnO 3 and LiNi 0.5 Mn 0.5 O 4 as assumed by many authors [43]. The tie-lines obtained here never extend to Li 2 MnO 3 and they include
points to the left of LiNi 0.5 Mn 0.5 O 2 in the Gibbs triangle. Therefore, the so-called
LiNi 0.5 Mn 0.5 O 2 -Li 2 MnO 3 nano-phase separation promoted by many authors needs
to be adapted given the actual endpoints of the coexistence. The layered–layered
region does not include the lithium-rich materials at all at higher temperatures as
the single-phase boundary sweeps upward in the triangle though. This will be discussed in the next chapter. It is also relevant that the unit cell volumes of the M and
N-layered phases differ by approximately 4.0 %; which is relatively large compared
to that observed in Chap. 4 and would most likely result in fracturing of the lattice.
This prediction is supported by the SEM images here where very small particles are
seen in the samples with two layered structures and larger particles in the singlephase layered sample. Thus, the region where nano-scale domains would be found
is further restricted to some portion of the layered–layered region. Clearly, further
study is needed to truly understand the short-range ordering in such samples and
any sample near the boundary of the layered region where XRD peak broadening
is seen. In particular, the consequences of these nano-domains on the lithium layer
must be evaluated given that one of the layered structures in the nano-composites
must be quite close in composition to the N-layered structure which was found to
contain a high fraction of nickel on the lithium layer. Clustering of nickel on the
lithium layer during cooling would interfere significantly with lithium extraction
during electrochemical cycling. This will be explored in Chap. 9 for materials near
LiNi 0.5 Mn 0.5 O 4 .