Biomedical Engineering Reference
In-Depth Information
very unstable. This situation may occur in the event of an accidental
overcharge of the battery. Practically, today all the commercial
battery packs include electronics that monitor the battery and
prevent overcharging.
Needless to say, that an insufficient thermal design of the
battery assembly or an accidental slight overcharge may lead to the
catastrophic destruction of the battery by the phenomenon of thermal
runaway. The losses suffered recently by some battery and laptop
computers manufacturers that needed to recall a large number of
faulty batteries constitute an eloquent illustration, in economic terms,
of the importance that this phenomenon may acquire.
From an economical point of view, graphite will probably never
be surpassed by other materials, but the fabrication of electrodes
showing better properties is still required. Therefore, other active
materials that can serve as anodes have to be investigated. For
instance, titanium dioxide has been known for a relatively long time
to be able to insert Li ions. Among the known polymorphs of this
oxide, it has been reported that the anatase structure is potential
candidate for the fabrication of anode in lithium-ion batteries due to
the very good lithium intercalation behavior [21].
A clear advantage of the TiO
2
on graphite negative electrodes
emerges from the relatively high insertion potential of TiO
2
(around
1.75 V vs. Li/Li
+
) with respect to the intercalation potential of Li
+
into
). Such a high potential eliminates
the risk of overcharging that may lead to growth of metallic lithium
dendrites. Hence, the irreversible capacity due to the formation of
the passive layer can be avoided as the passive layer (SEI) would be
very thin or inexistent. Without the passive layer, the overall kinetics
of the electrode should increase as there are no diffusion penalties
introduced by a thick solid SEI.
It can be noted that the use of a high-potential negative electrode
may also have an impact on the design of positive electrode active
materials. Due to the generalized use of graphite, the large majority
of commercial positive electrodes belong to the so-called “4 Volts”
(4 V) category, among which LiCoO
graphite (less than 0.1 V vs. Li/Li
+
is the most used. A cell formed
between a graphite electrode and LiCoO
2
has a nominal voltage
of 3.7 V. Although positive electrode active materials reaching a
potential of 5 V vs. Li/Li
2
+
exist, they cannot be used in functional
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