Page 384 - Handbook of Battery Materials

12.11 Spinel LiMn 2 O 4 355
making it an unattractive cathode material. LiCrO 2 can also be prepared in the
O3 structure, but it has been shown to be electrochemically inactive for lithium
insertion/extraction. Layered LiFeO 2 , like LiMnO 2 , is thermodynamically unstable
at high temperatures and has to be prepared by an ion exchange of layered NaFeO 2
+
[59] with Li . However, the O3-type LiFeO 2 also exhibits poor electrochemical
5
3+
performance due to structural instabilities since the high-spin Fe :3d , with an
OSSE value of zero, can readily migrate from the octahedral sites to the tetrahedral
sites.

12.10
Spinel Oxide Cathodes

Oxides with the general formula LiM 2 O 4 (M = Ti, V, and Mn) crystallize in
the normal spinel structure (Figure 12.10), in which the Li + and the M 3+/4+
ions occupy, respectively, the 8a tetrahedral and 16d octahedral sites of the
cubic close-packed oxygen array. A strong edge-shared octahedral [M 2 ]O 4 array
permits reversible extraction of the Li + ions from the tetrahedral sites without
collapsing the three-dimensional [M 2 ]O 4 spinel framework. While an edge-shared
MO 6 octahedral arrangement with direct M–M interaction provides good hopping
electrical conductivity, the interconnected interstitial (lithium) sites via the empty
16c octahedral sites in the three-dimensional structure provide good lithium-ion
conductivity.

12.11
Spinel LiMn 2 O 4
Spinel LiMn 2 O 4 has become an attractive cathode, as Mn is inexpensive and
environmentally benign compared to Co and Ni involved in the layered oxide

Figure 12.10 Crystal structure of spinel LiMn 2 O 4 .