Biomedical Engineering Reference
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coordination. Such behavior is expected for the compounds that are
capable of incorporating lithium simply and reversibly.
In an ideal octahedral symmetry, the t
position of the titanium
“d” orbitals is not degenerated, but this symmetry is split in the case
of anatase and Li
2g
TiO
because the local TiO
6
geometry is lowered
0.5
2
from O
in an idealized framework to D
and C
, respectively. Figure
h
2d
2v
5.12 shows in short the splitting of titanium
d
orbitals. This splitting
of the t
set can be easily derived by using arguments of ligand-field
theory [72]. This phase transition is reflected in the electrochemical
charge and discharge curves by the appearance of a large plateau.
The continuous structural changes on charge/discharge cycling are
responsible for the capacity fade [73]. Instead, lithium insertion in
nanostructured anatase revealed a continuous voltage decrease on
discharge, which is related to a solid solution process, overcoming
the detrimental effect of the phase transition [74].
Size is not the only factor that matters in preparing new
nanostructured anodes for Li-ion batteries. The control of shape
may provide interesting advantages. The improved electrochemical
behavior of TiO
2g
-B nanowires with diameters similar to that of
2
anatase and TiO
2
-B nanoparticles has been recently reported [75].
A number of reports have been published related to the
improved electrochemical properties of TiO
2
nanotubes. Zhou et al.
used a hydrothermal process to obtain uniform anatase nanotubes
with a rate capacity of 182 mAh g
and a minimal electrochemical
resistance although the discharge current density was low (ca.
80 mA g
-
1
) [76]. Li et al. prepared anatase nanotubes by calcining
protonic titanate nanotubes precursors. The lithium test cell
assembled with this electrode exhibited high discharge capacity with
314.4 mAh g
-
1
and good capacity retention [77]. The hydrothermal
process has been also used to prepare TiO
-
1
nanorods with 40-80 nm
in diameters and 400-1500 nm lengths. This active material was
able to deliver 280 mAh g
2
after 40 cycles [78].
Nevertheless, the hydrothermal method provides nanotubes
with different sizes and random orientations, thus hindering the
electronic and ionic transport. Hence, it makes necessary the
presence of a conductive second phase mix uniformly even if it is
present as a nanostructured material. The electron-conductive
additive must be intimately mixed or even better, as nanostructured
−1
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