Page 392 - Handbook of Battery Materials

12.15 Phospho-Olivine LiMPO 4 363
a simple oxide such as Fe 2 O 3 to polyanion hosts like Fe 2 (XO 4 ) 3 and a difference
of 0.6 V between the isostructural Fe 2 (SO 4 ) 3 and Fe 2 (MoO 4 ) 3 polyanion hosts, all
operating with the same Fe 2+/3+ couple, were attributed to the influence of inductive
effect and consequent differences in the location of the Fe 2+/3+ redox levels relative
+
to the Li/Li redox level, as seen in Figure 12.17. In the Nasicon-related Fe 2 (SO 4 ) 3
and Fe 2 (MoO 4 ) 3 hosts with corner-shared FeO 6 octahedra, XO 4 tetrahedra, and
Fe–O–X–O–Fe (X = S, Mo, or W) linkages, the strength of the X–O bond can
influence the Fe–O covalence and thereby the relative position of the Fe 2+/3+
redox energy. The stronger the X–O bonding, the weaker the Fe–O bonding, and
consequently the lower the Fe 2+/3+ redox energy relative to that in a simple oxide
like Fe 2 O 3 . The net result is a higher cell voltage on going from Fe 2 O 3 to Fe 2 (MoO 4 ) 3
or Fe 2 (SO 4 ) 3 . Comparing Fe 2 (MoO 4 ) 3 and Fe 2 (SO 4 ) 3 , the S–O covalent bonding
in Fe 2 (SO 4 ) 3 is stronger compared to the Mo–O bonding in Fe 2 (MoO 4 ) 3 , leading
to a weaker Fe–O covalence in Fe 2 (SO 4 ) 3 than that in Fe 2 (MoO 4 ) 3 , resulting in a
lowering of the Fe 2+/3+ redox energy in Fe 2 (SO 4 ) 3 compared to that in Fe 2 (MoO 4 ) 3
and a consequent increase in cell voltage by 0.6 V. Thus, the replacement of simple
O 2− ions by XO 4 n− polyanions was recognized as a viable approach to tune the
position of redox levels in solids and consequently to realize higher cell voltages
with chemically more stable, lower-valent redox couples like Fe 2+/3+ .

12.15
Phospho-Olivine LiMPO 4

Although the above findings in the late 1980s demonstrated an important funda-
mental concept in tuning the redox energies in solids, the cathode hosts pursued
did not contain any lithium, so they could not be combined with the carbon anode
in a lithium-ion cell. Following this initial concept, several phosphates have been
investigated [94–96], and, in 1997, Goodenough’s group identified LiFePO 4 ,crystal-
lizing in the olivine structure (Figure 12.18), as a facile lithium extraction/insertion
host that could be combined with a carbon anode in lithium-ion cells [94]. They
also identified other olivines of the general formula LiMPO 4 (M = Mn, Co, and
Ni) as lithium insertion/extraction hosts. Since its identification as a potential
cathode, LiFePO 4 has been the subject of extensive studies from both scientific and
technological points of view.

Li
Fe
P
O

a
c
Figure 12.18 Crystal structure of olivine LiFePO 4 .