Biomedical Engineering Reference
In-Depth Information
this material makes possible its combination with a high-voltage
anode material such as TiO
. A cell built according to this concept
belongs to the category of third-generation Li-ion batteries.
For commercial Li-ion cells, the most used anode remains
graphite, which has the advantage of a very good cycle life
and reasonably low cost. A graphite electrode works by the
intercalation/deintercalation of the Li ions between the graphene
layers that correspond to charging/discharging reaction of the Li-
ion cell. Concomitantly, electrons are injected or removed from the
conduction band of the graphite electrode. However, the graphite
and other carbonaceous materials have some major shortcomings.
It has to be noted from the beginning that, after a series of
improvements in the last decade, the graphite negative electrodes
operate very close to their theoretical capacity leaving little place for
further improvement. The theoretical specific capacity of graphite
is 372 mAh g
2
. The very
large majority of commercial systems already operate close to the
theoretical value of the capacity. Thus, if significant further capacity
improvement of a Li-ion battery is desired, a new negative active
material is required to replace graphite.
Another disadvantage of the graphite electrode is the very low
intercalation potential, which is only several tens of mV above the
potential of the Li/Li
-
1
corresponding to a composition of LiC
6
redox couple. As a consequence, the negative
electrode is at a strongly reducing potential. Up to date, there is no
electrolyte and solvent combination that can fully withstand the
strong reducing conditions at the graphite electrode. Nevertheless,
the graphite electrode is stable as a compact passive layer, usually
referred to as the solid electrolyte interphase (SEI), formed on its
surface by the reduction of the electrolyte and the solvent. The
large irreversible capacity of a Li-ion battery in the first cycle is
a direct consequence of this process. Although stable at room
temperature, the SEI passivating abilities degrade rather fast when
the temperature increases, hence making the Li-ion batteries quite
sensitive to temperatures as low as 50°C.
An intercalation potential very close to the reduction of Li
+
+
ions may also lead to metallic Li electrodeposition on the graphite
electrode. The metallic lithium formed in this way is a finely divided
powder that, unsurprisingly, is highly reactive, making the battery
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