Biomedical Engineering Reference
In-Depth Information
the host lattice and enters the one-dimensional channels as Li
+
ion
denoted by (Li
x
. The average net charge of lithium (+1.071)
determined by a Mulliken population analysis supports a complete
ionic formulation [83].
All the electrochemical performance of the ntTiO
2
)
x
+
(TiO
)
x
-
2
layers was
studied by experiments carried out in Li/LiPF
cells.
The electrolyte supplied by Merck was embedded in a Whatman glass
microfiber acting as a separator and the current collector for the
ntTiO
2
(EC:DEC)/ntTiO
6
2
was a copper foil (99.99% purity). For these experiments, no
additives such as poly(vinyl difluoride), which acts as binder agent,
and carbon black (conductive agent) were used. Assembling of the
cells was performed in a glove box filled with purified argon in which
moisture content and oxygen level were less than 2 ppm. Lithium cells
were galvanostatically cycled using an Arbin potentiostat/galvanostat
multichannel system. For the discharge/charge reaction, a constant
current density of 100, 20, and 5 μA cm
-
2
was applied to the assembled
cells in the range between 2.6 and 1.0 V.
In Table 5.1, all capacity values for ntTiO
with about 600 nm
length are expressed per electrode area to allow better comparison
with literature data for thin-film microbatteries. Figures 5.13 and 5.14
show the galvanostatic discharge/charge curves versus composition
of the electrodes grown on Si and Ti foils (Si-free) substrates using
a current of 5
2
μ
(C/8), respectively. Galvanostatic curves
are helpful to identify the behavior for lithium intercalation into
the electroactive materials. As we have materials with different
morphology and crystalline structure, we will study the different
behavior in curves. For instance, the crystallized electrode exhibits
a pseudo-plateau at 1.75 V during the discharge, and during the
charge, a potential plateau at 1.95 V is visible. These two potential
plateaus (Fig. 5.13b) correspond to Li
A cm
−2
insertion and deinsertion
from interstitial and octahedral sites of crystalline anatase TiO
2
+
μ
nanotubes. A total capacity of about 165
was obtained in
the first discharge that includes reversible and irreversible reactions.
The first reversible capacity obtained in the second discharge was
76
Ah cm
−2
μ
. For amorphous nanotubes, no plateau is observed
during the discharge/charge profile of the curve. Only one pseudo-
plateau at about 1.15 V contributes to a large irreversible capacity.
The total capacity in the first discharge (196
Ah cm
−2
μ
Ah cm
−2
) and the first
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