Biomedical Engineering Reference
In-Depth Information
the intercalation/deintercalation rates of Li ions, hence enabling
the high-power characteristic of the battery. However, using
nanomaterials does not necessarily mean taking a panacea. Before
using nanomaterials, it is extremely important to understand effects,
both positive and negative, of nanomaterials on the performance of
lithium-ion batteries.
6.4.1 Advantages of Nanomaterials
Generally, the greatest obstacle to the use of lithium-ion batteries
in HEVs or EVs lies in its relatively low power density, which results
from kinetic problems in solid-state electrode materials, i.e., the
slow Li
+
and e
-
diffusion rate. And the mean diffusion (or storage)
time,
τ
, can be expressed by the diffusion coefficient,
D
, and the
eq
diffusion length,
L
, as shown in Eq. 6.3:
2
L
D
τ =
(6.3)
eq
2
Clearly, the value of
can be reduced through two approaches:
One is to increase the value of
τ
eq
, and this can be achieved through
doping foreign atoms; the other approach is to reduce the value
of
D
, and this is what nanomaterials are for [53]. Although the
first method can improve the mixed conduction, only limited
rate-performance enhancement can be observed, and sometimes
this method may even bring about unstable crystal structures
[14]. However, through nanostructuring, the crystal structure of
electrode material remains the same, while the diffusion distance
of Li ions can be greatly shortened. Thus,
L
can be reduced
dramatically. In this way, fast insertion/extraction of lithium ions
in the electrode material can be realized on the condition that no
deterioration will come to the structure or performance of the
material. Moreover, it has been reported that electrode materials
inactive toward Li insertion may become active when ''going nano.”
For example, low Li diffusion rate of rutile TiO
τ
eq
along the
ab
-plane
2
(
) often makes Li insertion into rutile extremely
hard. However, nano-sized rutile TiO
2
D
ab
~
10
-
15
cm
2
s
-
1
(10-40 nm) is able to
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