Biomedical Engineering Reference
In-Depth Information
It can be seen from Fig. 6.1a that during the charging process,
lithium ions are extracted from the cathode and flow through the
electrolyte (usually they need to penetrate the battery separator)
to reach and move into the anode; meanwhile, electrons flow in
the same direction from the cathode to the anode in the external
circuit. As a result, external work can be applied on the battery.
When discharging, the reverse process occurs, and the battery
does electrical work to the external circuit. Figure 6.1b shows the
comparison between the schematic diagram of a lithium-ion battery
and a rocking chair. As can be seen, the back-and-forth movement
builds the common ground for the two seemingly unrelated objects.
For a typical lithium-ion battery made from the cathode of
LiCoO
2
and the anode of graphite, the cathode half reaction can be
expressed as
+
-
LiCoO
Li
CoO
+
x
Li
+
x
e ,
(6.1)
2
1
-
x
2
while the anode half reaction can be expressed as
+
-
x
Li
+
x
e
+
6C
Li C
x
(6.2)
6
Meanwhile, the electrons flow through the external circuit;
in this way, devices can be powered. The movements of lithium
ions in both electrodes of the battery are generally called
“lithium
insertion”
(Li
moving
in)
and
“lithium
extraction”
+
(Li
moving out), respectively [11, 12].
Although they have many merits, lithium-ion batteries have
suffered from some limitations. For example, their power density
is relatively low at high charge-discharge rates due to a large
polarization caused by slow lithium diffusion in the active material
[13]. Also, when developing and using Li-ion batteries, safety
issues should be taken into consideration since the introduction
of excessive active materials may lead to a serious consequence
such as explosion. So far, extensive work has been done on further
improving the performance of lithium-ion batteries, most of which
are related to improvements on the cathode, the anode, and the
electrolyte through various methods, including the introduction of
new materials, the chemical modification on the known compounds
+
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