Biomedical Engineering Reference
In-Depth Information
μ
reversible capacity (89
) are higher than for crystalline
structure. From these results, one can admit that the irreversible
capacity is strongly dependent on the structure of ntTiO
Ahcm
−2
. The
relatively high irreversible capacity can be attributed to different
phenomena: first, the irreversible reaction of Li
2
+
with adsorbed
electrode [84] and, second, the
formation of a very thin, disordered layer at the electrode surface
that may appear on both amorphous and crystalline electrodes
[35, 85-86]. Water is certainly still present in some proportion in
annealed samples, because the annealing temperature is not high
enough to completely remove strongly chemisorbed water [35] or
bound water [40]. The difference in the irreversible capacity can be
explained by the fact that an annealing treatment removes structural
and chemical defects in the amorphous phase that act as Li
water molecules onto the ntTiO
2
ion traps.
A cycling life study at two different kinetics was carried out
(Fig. 5.13c). The lithium intercalation can be viewed as a filling of
titanium
+
levels by neglecting the influence of Li levels. The partially
filled titanium levels are the t
d
set in an ideal octahedral coordination
as mention above. Such behavior is expected for compounds that
are able to incorporate lithium ions easily and reversibly [83]. At
5
2g
μ
A cm
−2
, the capacity for amorphous ntTiO
2
onto Si is about
μ
56
Ah cm
−2
after 50 cycles leading to an efficiency of about 63%.
The ntTiO
annealed at 450ºC presented a maximum capacity of
2
μ
μ
76
resulting in an efficiency
of about 53%. These results suggest that amorphous nanotubes
can accommodate extra Li into its structure compared with heated
sample. Part of this enhancement is due to the amorphous structure
of titania nanotubes, which facilitates an extra lithium insertion.
Ah cm
−2
and after 50 cycles 40
Ah cm
−2
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