Page 385 - Handbook of Battery Materials

356 12 Lithium Intercalation Cathode Materials for Lithium-Ion Batteries

2 )
(d x -y 2
e (d 2 , d 2)
2
z
x -y
g
(d z2 )
∆ oct
(d )
xy
t 2g (d xy , d yz , d zx )
(d xz , d yz )
4+
3+
n+
Free Mn ion Mn - Cubic Mn - Tetragonal
Figure 12.11 Illustration of Jahn–Teller distortion in manganese oxides.

cathodes. The extraction/insertion of two lithium ions from/into the LiMn 2 O 4
spinel framework occurs in two distinct steps [9]. The lithium extraction/insertion
from/into the 8a tetrahedral sites occurs around 4 V with the maintenance of
the initial cubic symmetry, while that from/into the 16c octahedral sites occurs
around 3 V by a two-phase mechanism involving the cubic spinel LiMn 2 O 4 and the
tetragonal lithiated spinel Li 2 Mn 2 O 4 . A deep energy well for the 8a tetrahedral Li +
ions and the high activation energy required for the Li ions to move from one 8a
+
tetrahedral site to another via an energetically unfavorable neighboring 16c site lead
to a higher voltage of 4 V. On the other hand, the insertion of an additional lithium
into the empty 16c octahedral sites occurs at 3 V. Thus, there is a 1 V jump on going
from octahedral-site lithium to tetrahedral-site lithium with the same Mn 3+/4+
redox couple, reflecting the contribution of site energy to the lithium chemical
potential and the overall redox energy. The Jahn–Teller distortion associated with
4 3 1
3+
the single electron in the e g orbitals of a high spin Mn :3d (t e ) ion results
2g g
in the cubic-to-tetragonal transition (Figure 12.11) on going from LiMn 2 O 4 to
Li 2 Mn 2 O 4 . The cubic-to-tetragonal transition is accompanied by a 6.5% increase in
unit cell volume, which makes it difficult to maintain structural integrity during
discharge–charge cycling and results in rapid capacity fade in the 3 V region.
Therefore, LiMn 2 O 4 can only be used in the 4 V region with a limited practical
−1
capacity of around 120 mAh g , which corresponds to an extraction/insertion
+
of 0.8 Li ion per formula unit of LiMn 2 O 4 (Figure 12.12). However, LiMn 2 O 4
tends to exhibit capacity fade even in the 4 V region as well, particularly at elevated
◦
temperatures (55 C) (Figure 12.13). Several factors, such as Jahn–Teller distortion
under conditions of nonequilibrium cycling [60, 61], manganese dissolution into
the electrolyte [62, 63], formation of two cubic phases in the 4 V region, loss
of crystallinity [64], and development of micro-strain [65] during cycling, have
been suggested to be the source of capacity fade. Among these, dissolution of
manganese is believed to be the main cause for capacity fade, especially at elevated
temperatures. Manganese dissolution is due to the disproportionation of Mn 3+
into Mn 4+ (remains in the solid) and Mn 2+ (leaches out into the electrolyte) in the