Page 373 - Handbook of Battery Materials

344 12 Lithium Intercalation Cathode Materials for Lithium-Ion Batteries

exactly the reverse reaction occurs. The layered structure of TiS 2 is maintained
during the charge–discharge (lithium extraction/insertion) process, resulting in
good reversibility. Following this, several other sulfides and chalcogenides with
high capacities were investigated as cathodes during the 1970s and 1980s [3].
However, most of these exhibited a low cell voltage of <2.5 V versus a metal-
lic lithium anode. This limitation in cell voltage is due to the overlap of the
n+
higher-valent M :d band with the top of the nonmetal:p band. Figure 12.1, for
2−
3+
example, illustrates the overlap of the Co :3d band with the top of the S :3p
band in cobalt sulfide. Such an overlap results in an introduction of holes or
2−
removal of electrons from the S :3p band and the formation of molecular ions
2− . This results in an inaccessibility of the higher oxidation states of
such as S 2
the M n+ ions in a sulfide like M y S z , leading to a limitation in cell voltage to
<2.5 V.
Recognizing this difficulty with chalcogenides, Goodenough’s group at the
University of Oxford focused on oxide cathodes during the 1980s [4–6]. The larger
Madelung energy in an oxide compared to that in a sulfide as well as the positioning
2−
of the top of the O :2p band below that of the S :3p band make the higher-valence
2−
states accessible in oxides. For example, while Co 3+ can be readily stabilized in an
oxide, it is difficult to stabilize Co 3+ in a sulfide since the Co 2+/3+ redox couple
2−
lies within the S :3p band, as seen in Figure 12.1. Accordingly, several transition
+
metal oxide hosts (e.g., LiCoO 2 and LiMn 2 O 4 ) providing ∼4V vs Li/Li have been
identified as lithium intercalation cathodes during the past three decades. Although
the cell voltage could be raised significantly with the oxide cathodes, rechargeable
lithium cells based on a metallic lithium anode could not be commercialized
because of the safety problems associated with metallic lithium [7, 8]. The inherent
safety problem of the metallic lithium anode and the dendrite formation during
the charge–discharge cycling eventually forced the use of intercalation compounds
as anodes. This led to the commercialization of the lithium-ion battery technology
by Sony in 1990, with LiCoO 2 as the cathode and graphite as the anode.

E E

2+/3+
Co :3d
2+/3+
Co :3d 3+/4+
Co :3d
2−
S :3p
2−
O :2p
N(E) N(E)
Figure 12.1 Relative energies of metal:d (e.g., Co:3d) and
nonmetal:p in a sulfide and an oxide.