138
10 Conclusions and Future Works
out the single-phase regions in the entire pyramid is therefore the main objective for
on-going combinatorial work and this project has also already begun.
The robot can also be used to study any number of systems where a few singlephase structures of interest for positive electrode materials are already known. For
example, LiFePO 4 has been used commercially as a relatively high power electrode
material [96] and many efforts have been made to substitute a variety of atoms for
either the iron or the phosphorous [97–99]. Efforts are now being made to develop
a Li 2 MSiO 4 material with M = (Fe, Mn) to cycle two lithium ions per formula unit
[100]. All these efforts can be helped significantly by complete phase diagrams.
Likely compositions where two-lithium cycling is possible could then be identified
and tested.
The combinatorial approach discussed at length here for lithium containing layered oxides could also be of use in sodium-ion battery research where commercially
viable electrode systems are still under development. Promising sodium layered oxide materials with varying amounts of iron and manganese on the transition metal
layer are being studied [101, 102]. Making these materials with the combinatorial
method is not trivial given that sodium loss during synthesis is more severe than
lithium loss in layered materials. Assuming sodium loss can be reduced to a manageable level as was lithium loss in this thesis, a rapid screening of potential positive
electrode materials for sodium-ion batteries could lead to new materials permitting
sodium-ion batteries to become a viable alternative to lithium-ion batteries.
In terms of optimizing the search for promising positive electrode materials with
the highest energy density, the current research strongly suggests that the best approach is to ensure that the samples lie within the single-phase layered region and in
some cases they should be made as near as possible to the boundaries of these regions.
The phase diagrams generated here should therefore be of significant value to Li-ion
battery researchers. Two factors favoring the layered structures have been identified:
increasing the synthesis temperature and lowering the oxygen partial pressure. Both
of these conditions result in a larger single-phase layered region such that a greater
range of compositions remain single-phase during regular cooling. There is therefore
still a considerable amount of work that can be done in the Li–Mn–Ni–O pseudoternary system. Much of this requires synthesis of bulk samples to perform a variety
of tests to find the optimum compositions and synthesis conditions for lithium-rich
layered oxides.
There are also opportunities to make interesting core-shell materials in the Li–Mn–
Ni–O system. One challenge with core-shell particles has always been to maintain
the core-shell structure while heating at sufficiently high temperatures to make the
desired materials. The quenched phase diagram shows the equilibrium phases such
that tie-lines show which core-shell materials are possible during extended heating
periods at that particular temperature. This therefore allows for careful selection
of compositions and heating temperature to maintain the desired coexistence. This
challenging project is also underway. Furthermore, as the combinatorial project in
the Gibbs pyramid progresses, there will be opportunities to study new materials
meeting the criteria listed in the introduction, namely high energy density and as
little cobalt as possible to minimize cost.
10 Conclusions and Future Works
out the single-phase regions in the entire pyramid is therefore the main objective for
on-going combinatorial work and this project has also already begun.
The robot can also be used to study any number of systems where a few singlephase structures of interest for positive electrode materials are already known. For
example, LiFePO 4 has been used commercially as a relatively high power electrode
material [96] and many efforts have been made to substitute a variety of atoms for
either the iron or the phosphorous [97–99]. Efforts are now being made to develop
a Li 2 MSiO 4 material with M = (Fe, Mn) to cycle two lithium ions per formula unit
[100]. All these efforts can be helped significantly by complete phase diagrams.
Likely compositions where two-lithium cycling is possible could then be identified
and tested.
The combinatorial approach discussed at length here for lithium containing layered oxides could also be of use in sodium-ion battery research where commercially
viable electrode systems are still under development. Promising sodium layered oxide materials with varying amounts of iron and manganese on the transition metal
layer are being studied [101, 102]. Making these materials with the combinatorial
method is not trivial given that sodium loss during synthesis is more severe than
lithium loss in layered materials. Assuming sodium loss can be reduced to a manageable level as was lithium loss in this thesis, a rapid screening of potential positive
electrode materials for sodium-ion batteries could lead to new materials permitting
sodium-ion batteries to become a viable alternative to lithium-ion batteries.
In terms of optimizing the search for promising positive electrode materials with
the highest energy density, the current research strongly suggests that the best approach is to ensure that the samples lie within the single-phase layered region and in
some cases they should be made as near as possible to the boundaries of these regions.
The phase diagrams generated here should therefore be of significant value to Li-ion
battery researchers. Two factors favoring the layered structures have been identified:
increasing the synthesis temperature and lowering the oxygen partial pressure. Both
of these conditions result in a larger single-phase layered region such that a greater
range of compositions remain single-phase during regular cooling. There is therefore
still a considerable amount of work that can be done in the Li–Mn–Ni–O pseudoternary system. Much of this requires synthesis of bulk samples to perform a variety
of tests to find the optimum compositions and synthesis conditions for lithium-rich
layered oxides.
There are also opportunities to make interesting core-shell materials in the Li–Mn–
Ni–O system. One challenge with core-shell particles has always been to maintain
the core-shell structure while heating at sufficiently high temperatures to make the
desired materials. The quenched phase diagram shows the equilibrium phases such
that tie-lines show which core-shell materials are possible during extended heating
periods at that particular temperature. This therefore allows for careful selection
of compositions and heating temperature to maintain the desired coexistence. This
challenging project is also underway. Furthermore, as the combinatorial project in
the Gibbs pyramid progresses, there will be opportunities to study new materials
meeting the criteria listed in the introduction, namely high energy density and as
little cobalt as possible to minimize cost.