Page 380 - Handbook of Battery Materials

12.6 Layered LiNiO 2 351
Ni being more electronegative than Co and lying to the right of Co in the
3+ 2
1
Periodic Table. This is because while the redox reaction with Ni :t e involves
2g g
3+ 2
0
the upper-lying, σ-bonding e g band, that with Co :t e involves the lower-lying,
2g g
π-bonding t 2g band. However, it is difficult to synthesize LiNiO 2 as a well-ordered
stoichiometric material with all Ni 3+ because of the difficulty of stabilizing Ni 3+
at the high synthesis temperatures and the consequent volatilization of lithium
2+
[27–29]. It invariably forms as Li 1−x Ni 1+x O 2 with some Ni , which results in a
disordering of the cations in the lithium and nickel planes due to smaller charge
+
and size differences between Li and Ni 2+ and consequently poor electrochemical
performance. In addition, charged Li 1−x NiO 2 suffers from a migration of Ni 3+ ions
from the octahedral sites of the nickel plane to the octahedral sites of the lithium
plane via the neighboring tetrahedral sites, particularly at elevated temperatures,
3+ 2
1
due to a lower OSSE associated with the low-spin Ni :t e ions compared to
2g g
0
3+ 2
that of the low-spin Co :t e ions (Table 12.1) [30, 31]. While a moderate OSSE
2g g
allows the Ni 3+ ions to migrate through the tetrahedral sites under mild heat, the
stronger OSSE of Co 3+ hinders such a migration. Moreover, LiNiO 2 also suffers
from Jahn–Teller distortion (tetragonal structural distortion) associated with the
7
2
1
low-spin Ni :3d (t e )ion. AlsoLi 1−x NiO 2 electrodes in their charged state are
3+
2g g
thermally less stable than the charged Li 1−x CoO 2 electrodes, an indication that
Ni 4+ ions are reduced more easily than Co 4+ ions [32, 33]. As a result, LiNiO 2 is
not a promising material for lithium-ion cells.
However, partial substitution of Co for Ni has been shown to suppress the
cation disorder and Jahn–Teller distortion. For example, LiNi 0.85 Co 0.15 O 2 has been
found to show a reversible capacity of ∼180 mAh g −1 (Figure 12.5) with excellent
cyclability [34, 35]. The increase in the capacity of LiNi 0.85 Co 0.15 O 2 compared to
that of LiCoO 2 can be understood by considering the qualitative band diagrams for
the Li 1−x CoO 2 and Li 1−x NiO 2 systems, as shown in Figure 12.6. With a low-spin
6
Co :3d configuration, the t 2g band is completely filled and the e g band is empty
3+
2
0
(t e )in LiCoO 2 . Since the t 2g band overlaps with the top of the O :2p band,
2−
2g g
deep lithium extraction with (1 − x) < 0.5inLi 1−x CoO 2 results in the removal of
a significant amount of electron density from the O :2p band and consequent
2−
chemical instability, limiting its practical capacity. In contrast, the LiNiO 2 system
3+ 2
1
with a low-spin Ni :t e configuration involves the removal of electrons only
2g g
from the e g band. Since the e g band barely touches the top of the O :2p b and,
2−
Li 1−x NiO 2 , and LiNi 1−y Co y O 2 exhibit better chemical stability [15] than LiCoO 2 ,
resulting in higher capacity values. In addition, nanocoating of AlF 3 on doped
LiNi 0.85 Co 0.15 Al 0.05 O 2 has been shown to improve the cycle performance and
thermal stability of the cathodes in its oxidized (charged) state. This benefit is
ascribed to the AlF 3 coating layer protecting the oxidized cathode from attack by
hydrogen fluoride in the electrolyte [36].
Recent studies have shown that partial substitution of manganese in
−1
LiNi 0.5 Mn 0.5 O 2 not only provides high capacities (∼200 mAh g ), but also results
in a significant improvement in thermal stability compared to LiNiO 2 [37]. The
increase in capacity and thermal stability is associated with the substitution of
3+
chemically more stable Mn 4+ ions for Ni . Recently, the mixed layered oxide