116
8 Layered Materials with Metal Site Vacancies
8.6 Conclusions Regarding Metal Site Vacancies
in Li–Mn–Ni–O Materials
Layered Li–Mn–Ni–O materials were analyzed in a region of the phase diagram
where a strange bump was seen in the boundary of the layered region. These structures
were found to contain a significant amount of metal site vacancies. The maximum
vacancy content was found to result in highly ordered monoclinic structures where
manganese occupies two of the
√
3 ×
√
3 superlattices on the transition metal layers
while the third was randomly filled with nickel, lithium and vacancies. The resulting
ordering predicted by a Monte Carlo simulation was consistent with the sharp ordering peaks seen in the XRD patterns. The vacancy concentrations were confirmed by
Rietveld refinement, density measurements and redox titration; all of which were in
good agreement. The role of the vacancies during electrochemical cycling, if there is
one, remains unclear. The material with the greatest possible vacancy concentration,
Li[Ni 1/6 P 1/6 Mn 2/3 ]O 2 , showed electrochemical behavior consistent with lithiumrich layered materials, namely high irreversible capacity associated with the high
voltage plateau and voltage fade associated with conversion to spinel. However, this
material was not, in fact, lithium-rich given that the vacancies result in lithium occupying only 50 % of metal sites. The vacancy results also demonstrated that there were
roughly 1 % vacancies in a stoichiometric lithium-rich material lying along the line
from Li 2 MnO 3 and LiNi 0.5 Mn 0.5 O 2 . The Monte Carlo simulation suggested that this
allows Ni
3+ to substitute for Mn
4+ on two of the superlattices. This has never been
recognized before and a complete understanding of the starting material is crucial to
fully understand the complex electrochemical behavior of the lithium-rich positive
electrode materials. The exact shape of the other side of the bump (to the left of
Li[Ni 1/6 P 1/6 Mn 2/3 ]O 2 in the Gibbs triangles) has not been determined. It is also
unclear as to why the bump would be so sharp on both sides and this is worthwhile
for further study.
8 Layered Materials with Metal Site Vacancies
8.6 Conclusions Regarding Metal Site Vacancies
in Li–Mn–Ni–O Materials
Layered Li–Mn–Ni–O materials were analyzed in a region of the phase diagram
where a strange bump was seen in the boundary of the layered region. These structures
were found to contain a significant amount of metal site vacancies. The maximum
vacancy content was found to result in highly ordered monoclinic structures where
manganese occupies two of the
√
3 ×
√
3 superlattices on the transition metal layers
while the third was randomly filled with nickel, lithium and vacancies. The resulting
ordering predicted by a Monte Carlo simulation was consistent with the sharp ordering peaks seen in the XRD patterns. The vacancy concentrations were confirmed by
Rietveld refinement, density measurements and redox titration; all of which were in
good agreement. The role of the vacancies during electrochemical cycling, if there is
one, remains unclear. The material with the greatest possible vacancy concentration,
Li[Ni 1/6 P 1/6 Mn 2/3 ]O 2 , showed electrochemical behavior consistent with lithiumrich layered materials, namely high irreversible capacity associated with the high
voltage plateau and voltage fade associated with conversion to spinel. However, this
material was not, in fact, lithium-rich given that the vacancies result in lithium occupying only 50 % of metal sites. The vacancy results also demonstrated that there were
roughly 1 % vacancies in a stoichiometric lithium-rich material lying along the line
from Li 2 MnO 3 and LiNi 0.5 Mn 0.5 O 2 . The Monte Carlo simulation suggested that this
allows Ni
3+ to substitute for Mn
4+ on two of the superlattices. This has never been
recognized before and a complete understanding of the starting material is crucial to
fully understand the complex electrochemical behavior of the lithium-rich positive
electrode materials. The exact shape of the other side of the bump (to the left of
Li[Ni 1/6 P 1/6 Mn 2/3 ]O 2 in the Gibbs triangles) has not been determined. It is also
unclear as to why the bump would be so sharp on both sides and this is worthwhile
for further study.