Chapter 8
Layered Materials with Metal Site Vacancies
8.1 Motivation for the Study of Samples near Li 2 MnO 3
In the Li–Mn–Ni–O system, studies of the lithium-rich layered materials have generally been limited to the composition line between Li 2 MnO 3 and LiNi 0.5 Mn 0.5 O 2
(referred to here as “stoichiometric lithium-rich”) where reversible cycling above
250 mAh/g has been achieved [2, 3, 43]. These materials take the O3-type structure
with lithium layers containing a small amount of nickel and transition metal (TM)
layers being made up of manganese, nickel, and some lithium. The oxidation states
of nickel and manganese are typically 2+ and 4+ in these structures [90]. In the previous chapters, it was found that the solid-solution layered structures can be made
with either more or less lithium than the stoichiometric lithium rich line.
Figure 8.1 shows the phase diagram obtained by quenching combinatorial samples
in oxygen. Also shown are composition lines obtained by keeping nickel and manganese oxidation numbers constant. This diagram shows that all spinel samples are
consistent with Ni
2+ and Mn
3,4+ . The rocksalt/layered lines show that the majority
of samples in this region have Mn
4+ with Ni
2,3+ . That excess lithium can be added to
the stoichiometric lithium-rich materials while keeping the structures single-phase
has been well known for some time and has been used extensively to make high
capacity materials [51, 52]. These structures can be understood as being identical to
the usual Li-rich materials with some of the nickel oxidized to 3+ during synthesis,
which is quite feasible given that LiNiO 2 can be made under the same conditions.
However, there are two exceptions in Fig. 8.2 where layered materials cannot be
made without reducing some manganese to 3+ or having metal site vacancies: the
“bump” region near the Li–Mn line (to be discussed here) and the top of the ordered
rocksalt region. Structures in the ordered rocksalt region have already been shown
to contain metal site vacancies which allow a higher manganese oxidation state (perhaps even keeping it in the 4+ state), as discussed in Chaps. 5 and 6. However, for
the bump region, the ambiguity remains: there may be metal site vacancies and/or
some manganese reduced to 3+. This ambiguity requires further study and will be
resolved in this chapter.
Data in this chapter are reprinted from Ref. [89] with permission from the American Chemical
Society.
105
E. McCalla, Consequences of Combinatorial Studies of Positive Electrodes
for Li-ion Batteries, Springer Theses, DOI 10.1007/978-3-319-05849-8_8,
© Springer International Publishing Switzerland 2014
Layered Materials with Metal Site Vacancies
8.1 Motivation for the Study of Samples near Li 2 MnO 3
In the Li–Mn–Ni–O system, studies of the lithium-rich layered materials have generally been limited to the composition line between Li 2 MnO 3 and LiNi 0.5 Mn 0.5 O 2
(referred to here as “stoichiometric lithium-rich”) where reversible cycling above
250 mAh/g has been achieved [2, 3, 43]. These materials take the O3-type structure
with lithium layers containing a small amount of nickel and transition metal (TM)
layers being made up of manganese, nickel, and some lithium. The oxidation states
of nickel and manganese are typically 2+ and 4+ in these structures [90]. In the previous chapters, it was found that the solid-solution layered structures can be made
with either more or less lithium than the stoichiometric lithium rich line.
Figure 8.1 shows the phase diagram obtained by quenching combinatorial samples
in oxygen. Also shown are composition lines obtained by keeping nickel and manganese oxidation numbers constant. This diagram shows that all spinel samples are
consistent with Ni
2+ and Mn
3,4+ . The rocksalt/layered lines show that the majority
of samples in this region have Mn
4+ with Ni
2,3+ . That excess lithium can be added to
the stoichiometric lithium-rich materials while keeping the structures single-phase
has been well known for some time and has been used extensively to make high
capacity materials [51, 52]. These structures can be understood as being identical to
the usual Li-rich materials with some of the nickel oxidized to 3+ during synthesis,
which is quite feasible given that LiNiO 2 can be made under the same conditions.
However, there are two exceptions in Fig. 8.2 where layered materials cannot be
made without reducing some manganese to 3+ or having metal site vacancies: the
“bump” region near the Li–Mn line (to be discussed here) and the top of the ordered
rocksalt region. Structures in the ordered rocksalt region have already been shown
to contain metal site vacancies which allow a higher manganese oxidation state (perhaps even keeping it in the 4+ state), as discussed in Chaps. 5 and 6. However, for
the bump region, the ambiguity remains: there may be metal site vacancies and/or
some manganese reduced to 3+. This ambiguity requires further study and will be
resolved in this chapter.
Data in this chapter are reprinted from Ref. [89] with permission from the American Chemical
Society.
105
E. McCalla, Consequences of Combinatorial Studies of Positive Electrodes
for Li-ion Batteries, Springer Theses, DOI 10.1007/978-3-319-05849-8_8,
© Springer International Publishing Switzerland 2014