10.2 Resolving Points of Confusion
135
with oxygen. Samples heated in oxygen-free atmospheres showed solid lithium oxide
in the XRD while samples heated in oxygen-containing atmospheres did not. This
implies that LiO 2 does not evaporate under the conditions used to make electrode
materials, but rather it must react with oxygen to leave the sample.
IV A number of researchers have found a rocksalt contaminant in either spinel or
layered-spinel materials in the Li–Mn–Ni–O system [34, 37, 38]. The composition
and structure of this contaminant was under debate prior to the current thesis. The
results of the combinatorial study show that the contaminant, when slow cooled
in oxygen, had metallic composition Li 0.22 Mn 0.16 Ni 0.62 which lies well inside the
Gibbs triangle and not on the Li–Ni line as previously believed. The structure of this
material is a cubic rocksalt with a 2 x 2 x 2 superlattice with ordering of manganese
on one site and lithium, nickel, and vacancies on another.
V The LiNi 0.5 Mn 1.5 O 4 spinel material phase separates in air forming the rocksalt
material already mentioned and a spinel lying higher in the Gibbs triangle [34].
However, LiNi 0.5 Mn 0.5 O 4 may also accommodate oxygen vacancies [39, 40]. It is
difficult to distinguish these two processes in air because the sample is two-phase and
both the phase separation and the formation of oxygen vacancies involve the loss of
oxygen and an increase in lattice parameter. However, in oxygen, this composition is
single-phase and the lattice parameter contour plots in Chap. 5 clearly show a larger
lattice parameter for the quenched sample. This shows that the Li–Mn–Ni–O spinel
materials do sustain oxygen vacancies at high temperature, whereas the Li–Mn–O
spinels do not as the lattice parameters do not change dramatically here.
VI The lithium-rich layered line from LiNi 0.5 Mn 0.5 O 2 to Li 2 MnO 3 was considered to be either solid solutions [2] or layered–layered nano-composites [43] in
the literature. Once again, the results from this project showed that the structure
of these materials is strongly affected by cooling rate. If quenched from above
800
◦ C in air, the entire composition line is single-phase. If cooled more slowly,
the samples near the LiNi 0.5 Mn 0.5 O 2 end of the composition line phase separate
into layered–layered nano-composites. If cooled even more slowly, these samples
will form layered–layered composites or even layered–layered rocksalt materials.
In terms of electrochemistry, the layered–layered composites were found to have
lower capacity and a higher impedance than single-phase samples synthesized with
the same metallic composition. This poor performance of the layered–layered composites may be attributed to the fact that one of the layered materials must have
more nickel on the lithium layer than LiNi 0.5 Mn 0.5 O 2 (i.e., more than 10 %). When
this nickel clusters during phase separation, lithium diffusion may be severely hindered. It is also of note that the material over which there has been the most debate,
Li 1.2 Ni 0.2 Mn 0.6 O 2 [44, 46], is very near to the boundary of the layered region when
regular cooled. As such, differing opinions as to whether or not this material is
single-phase may simply be due to slight differences in synthesis conditions and
compositions.
VII Materials along the lithium-rich layered line from LiNi 0.5 Mn 0.5 O 2 to
Li 2 MnO 3 are generally considered to contain Ni
2+ and Mn
4+ [49, 50]. However,
Simonin et al. [48] recently found that the magnetic moment in one such layered
material was too low and this was attributed to some manganese being in the 3+ state.
135
with oxygen. Samples heated in oxygen-free atmospheres showed solid lithium oxide
in the XRD while samples heated in oxygen-containing atmospheres did not. This
implies that LiO 2 does not evaporate under the conditions used to make electrode
materials, but rather it must react with oxygen to leave the sample.
IV A number of researchers have found a rocksalt contaminant in either spinel or
layered-spinel materials in the Li–Mn–Ni–O system [34, 37, 38]. The composition
and structure of this contaminant was under debate prior to the current thesis. The
results of the combinatorial study show that the contaminant, when slow cooled
in oxygen, had metallic composition Li 0.22 Mn 0.16 Ni 0.62 which lies well inside the
Gibbs triangle and not on the Li–Ni line as previously believed. The structure of this
material is a cubic rocksalt with a 2 x 2 x 2 superlattice with ordering of manganese
on one site and lithium, nickel, and vacancies on another.
V The LiNi 0.5 Mn 1.5 O 4 spinel material phase separates in air forming the rocksalt
material already mentioned and a spinel lying higher in the Gibbs triangle [34].
However, LiNi 0.5 Mn 0.5 O 4 may also accommodate oxygen vacancies [39, 40]. It is
difficult to distinguish these two processes in air because the sample is two-phase and
both the phase separation and the formation of oxygen vacancies involve the loss of
oxygen and an increase in lattice parameter. However, in oxygen, this composition is
single-phase and the lattice parameter contour plots in Chap. 5 clearly show a larger
lattice parameter for the quenched sample. This shows that the Li–Mn–Ni–O spinel
materials do sustain oxygen vacancies at high temperature, whereas the Li–Mn–O
spinels do not as the lattice parameters do not change dramatically here.
VI The lithium-rich layered line from LiNi 0.5 Mn 0.5 O 2 to Li 2 MnO 3 was considered to be either solid solutions [2] or layered–layered nano-composites [43] in
the literature. Once again, the results from this project showed that the structure
of these materials is strongly affected by cooling rate. If quenched from above
800
◦ C in air, the entire composition line is single-phase. If cooled more slowly,
the samples near the LiNi 0.5 Mn 0.5 O 2 end of the composition line phase separate
into layered–layered nano-composites. If cooled even more slowly, these samples
will form layered–layered composites or even layered–layered rocksalt materials.
In terms of electrochemistry, the layered–layered composites were found to have
lower capacity and a higher impedance than single-phase samples synthesized with
the same metallic composition. This poor performance of the layered–layered composites may be attributed to the fact that one of the layered materials must have
more nickel on the lithium layer than LiNi 0.5 Mn 0.5 O 2 (i.e., more than 10 %). When
this nickel clusters during phase separation, lithium diffusion may be severely hindered. It is also of note that the material over which there has been the most debate,
Li 1.2 Ni 0.2 Mn 0.6 O 2 [44, 46], is very near to the boundary of the layered region when
regular cooled. As such, differing opinions as to whether or not this material is
single-phase may simply be due to slight differences in synthesis conditions and
compositions.
VII Materials along the lithium-rich layered line from LiNi 0.5 Mn 0.5 O 2 to
Li 2 MnO 3 are generally considered to contain Ni
2+ and Mn
4+ [49, 50]. However,
Simonin et al. [48] recently found that the magnetic moment in one such layered
material was too low and this was attributed to some manganese being in the 3+ state.