4.5 Conclusions Regarding the Formation of Layered–Layered Composites . . .
59
4.5 Conclusions Regarding the Formation of Layered–Layered
Composites in the Li–Co–Mn–O System
The layered single-phase region in the Li–Co–Mn oxide pseudo-ternary system was
explored by a solution-based combinatorial approach. The results showed that the
layered region is a single composition line corresponding to cobalt being constrained
to the 3+ oxidation state only. This composition line, joining LiCoO 2 to Li 2 MnO 3 ,
was a solid solution over its entire length when samples were quenched from either
800 or 900
◦ C. Upon slow cooling, the structures phase separated near the center of
the line with the maximum phase separation occurring over the range x = 0.2–0.8 in
Li[Li (1−x)/3 Co x Mn (2−2x)/3 ]O 2 when samples were cooled from 900
◦ C at a rate of
1
◦ C/min. These endpoints correspond to Co and Mn 2 Li domains with approximately
20 % disorder on the transition metal layers. Such disorder was also found over a
range of temperatures during cooling using a Monte Carlo simulation.
The phase separation upon slow cooling helps explain results from previous studies [18] performed with an intermediate cooling rate where peak broadening seen in
the XRD patterns can now be attributed to phase separation on the 2–10 nm length
scale with both domains lying on the same lattice as shown in Ref. [22]. The results here show that the nano-scale phase separation occurs when the system has
insufficient time to make large scale crystallites of each phase, and the unit cells
are close enough in size that the lattice does not fracture upon phase separation. It
was also demonstrated that this partial phase separation can be detected by way of
careful peak width analysis in the XRD patterns. As such, nano-domain composites
can be expected when samples are regular cooled at composition points on the phase
diagram that are single-phase when quenched and show layered–layered phase separation when slow cooled. This condition will be used in Chap. 9 to determine the
location of nano-composites in the Li–Ni–Mn oxide system where there continues
to be considerable debate regarding the structures of the Li-rich layered materials [2,
43–46].
59
4.5 Conclusions Regarding the Formation of Layered–Layered
Composites in the Li–Co–Mn–O System
The layered single-phase region in the Li–Co–Mn oxide pseudo-ternary system was
explored by a solution-based combinatorial approach. The results showed that the
layered region is a single composition line corresponding to cobalt being constrained
to the 3+ oxidation state only. This composition line, joining LiCoO 2 to Li 2 MnO 3 ,
was a solid solution over its entire length when samples were quenched from either
800 or 900
◦ C. Upon slow cooling, the structures phase separated near the center of
the line with the maximum phase separation occurring over the range x = 0.2–0.8 in
Li[Li (1−x)/3 Co x Mn (2−2x)/3 ]O 2 when samples were cooled from 900
◦ C at a rate of
1
◦ C/min. These endpoints correspond to Co and Mn 2 Li domains with approximately
20 % disorder on the transition metal layers. Such disorder was also found over a
range of temperatures during cooling using a Monte Carlo simulation.
The phase separation upon slow cooling helps explain results from previous studies [18] performed with an intermediate cooling rate where peak broadening seen in
the XRD patterns can now be attributed to phase separation on the 2–10 nm length
scale with both domains lying on the same lattice as shown in Ref. [22]. The results here show that the nano-scale phase separation occurs when the system has
insufficient time to make large scale crystallites of each phase, and the unit cells
are close enough in size that the lattice does not fracture upon phase separation. It
was also demonstrated that this partial phase separation can be detected by way of
careful peak width analysis in the XRD patterns. As such, nano-domain composites
can be expected when samples are regular cooled at composition points on the phase
diagram that are single-phase when quenched and show layered–layered phase separation when slow cooled. This condition will be used in Chap. 9 to determine the
location of nano-composites in the Li–Ni–Mn oxide system where there continues
to be considerable debate regarding the structures of the Li-rich layered materials [2,
43–46].