Biomedical Engineering Reference
In-Depth Information
[63, 64]. Thus, rutile, being the most thermodynamically stable
polymorph of TiO
, insert less than 0.1 Li ions per formula unit in its
bulk crystalline [65] Li diffusion in rutile is highly anisotropic. While
lithium diffuses rapidly through
2
c
-axis channels (ca. 10-6 cm
2
s
-
1
),
a very slow diffusion proceed through the
ab
plane (ca. 10-15 cm
2
). Consequently, the repulsive Li-Li interaction in c channels and
the lithium blocking in ab plane significantly restrict the insertion
reaction in crystalline bulk materials [66]. Similarly, brookite
inserts only small amounts of lithium [67]. However, it has been
recently reported that lithium can be efficiently inserted in both
phases when produced as nanometric particles. It can be ascribed
to the stability of the morphology and to better accommodation
of the structural changes at nanometer size. [68, 69]. Contrarily,
bulk anatase [70] and the synthetic polymorph TiO
2
s
-
1
(B) [71] react
widely with stoichiometries up to 0.8-1 Li per titanium.
Natural mineral anatase has a three-dimensional network that
consists of strongly distorted edge sharing TiO
6
octahedra as shown
in Fig. 5.11. It can be considered a stacking of one-dimensional
zigzag, equivalent in [100] and [010] directions, leading to empty
zigzag channels in the anatase framework. When reacting with
lithium, the ions are accommodated on highly distorted octahedral
interstices and consequently undergo an orthorhombic distortion
which is better described as a five-fold coordination of lithium
with oxygen. The overall distortion of the atomic positions in the
change from TiO
(anatase) to Li
x
TiO
2
(Li-titanate) is small and
2
leads to a more regular TiO
6
octahedra in the inserted compound.
A volume increase of the unit cell is supposed to be about 4% as
Nuspl et al. reported. Thus, a decrease of unit cell along the
c
-axis
and an increase along
-axis takes place. The structural change
can be explained as occupation of Ti-Ti bonding atomic orbitals
by the electron that enters the TiO
b
lattice with each Li-ion to
maintain charge neutrality. As a result of lithium intercalation, the
symmetry of anatase is lowered from I41/amd to Imma in Li
0.5
2
,
where lithium is statistically distributed over one four-fold site in
Imma since only one half of the crystallographic sites are occupied.
Consequently, the lithium intercalation can be viewed as a filling of
titanium
TiO
2
levels by neglecting the influence of lithium levels. The
partially filled titanium levels are the t
d
set in an ideal octahedral
2g
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