Biomedical Engineering Reference
In-Depth Information
The electrode material made of titania nanotubes shows a high
rate of charge-discharge, which is probably due to the 3D network
structure of porous titania nanotubes. It worth noting that we are
considering the nanotubes as a perfect cylinder, as is represented in
Fig. 5.16 with the dimension of wall thickness, nanotube diameter,
and length. Moreover, Fig. 5.16 shows the structure of a porous
nanotube configuration and transport path of both lithium ions and
electrons. The lithium ions and electrolyte are easily transported in
the uniform channels of the pores and electrons transport quickly
through the arrangement nanotubular structure.
From these results, the irreversible capacity is strongly
dependent on the structure of ntTiO
2
. To explain the relatively
high irreversible capacity, the influence of the reaction between
adsorbed water molecules onto the ntTiO
has
been considered [84]. From the SEM images (Figs. 5.7 and 5.8),
and assuming that the nanotube is a perfect cylinder (Fig. 5.16), it
is apparent that the nanotubes are not connected along the vertical
axis, leading to a specific area of about 40 cm
electrode and Li
+
2
2
. Assuming that 1 ×
10
15
H
O molecules/cm
2
can be adsorbed onto the nanotubes, the
2
number of H
O molecules present onto the ntTiO
2
electrode should
2
be around 4 × 10
16
, which can react with Li
+
ions according to the
following reaction:
LiOH +
1
↔
H
O + Li
+
+
x
e
-
2
H
(5.5)
2
2
However,
the
number
of
Li
+
/cm
2
corresponding
to
the
irreversible capacity of 55 and 12 μAh cm
for both amorphous and
crystalline materials is significantly higher, i.e., 1.26 × 10
-
2
18
and 2.7 ×
10
, respectively. Therefore, the large irreversible capacity
obtained for both samples can be attributed not only to the presence
of adsorbed water but also to the formation of a very thin disordered
layer at the electrode surface. Considering the numbers given above
for irreversibly inserted Li ions and available sites per unit area,
this would correspond to insertion in about 30 atomic planes for
amorphous tubes and 7 atomic planes for crystalline nanotubes.
17
Li
+
/cm
2
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