NANOSTRUCTURED TiO2 MATERIALS FOR NEW-GENERATION Li-ION BATTERIES - Electrochemical Nanofabrication: Principles and Applications - page 211

Anatase Anatase Li 0.5 TiO 2 Li 0.5 TiO 2

Ideal Oh Ideal Oh

d xz d xz

d xz d xz d yz d yz

d xy d xy d xz d xz d yz d yz d yz d yz

d xy d xy

d xy d xy

Figure 5.12 Splitting of titanium d orbitals when lithium intercalation takes

place into TiO . Adapted from Ref. [72].

2

5.5 BATTERY APPLICATIONS

5.5.1 Electrochemical Behavior of Samples in Lithium Cells

Because of its relevance to batteries, the mechanism of lithium
insertion into anatase TiO
2

has been extensively studied. The
electrochemical insertion/extraction of Li is believed to be driven
by the accumulation of electrons in TiO

electrodes in contact with

2

Li -containing electrolytes, and the overall cell reaction can be

written as

+

TiO + xLi

+

+ xe

-

� Li TiO (5.4)

2 x 2

The crystalline structure of anatase is tetragonal (s.g. I4 /amd)

1

and contains distorted TiO
6

octahedra, which define a series of
octahedral and tetrahedral vacant sites. These sites allow lithium
uptake of 0.5 Li per formula unit, corresponding to a theoretical
capacity of 168 mAh g [81]. A two-phase mechanism has been

suggested to describe the electrochemical insertion of lithium into
anatase, involving equilibrium between Li-poor (tetragonal) and Li-
rich (orthorhombic) phases [82] as we mentioned above. The latter
phase results from a structural distortion caused by a cooperative
Jahn-Teller effect, as the incoming electrons increase the

−1

d electron

density in localized Ti- orbitals above a critical intercalation

concentration. The maximum theoretical capacity for

d

x = 1 should

be 336 mAh g

- 1

, in which lithium transfers its valence electron to