Biomedical Engineering Reference
In-Depth Information
reversibly accommodate Li up to Li
TiO
(168 mAh g
-
1
) at 1-3 V
0.5
2
/Li with excellent capacity retention on cycling compared
with its micrometer-sized counterparts [54]. The difference in
rutile TiO
vs. Li
+
is mainly related to the drastic decrease in the diffusion
2
time
, which is evidently caused by the shortening of Li diffusion
path. This is the same for
τ
eq
, which, when made mesoporous,
will allow reversible lithium intercalation without destruction of
the rutile structure [55]. In addition, with a large surface area,
there will be a higher contact area between the material and the
electrolyte. Hence, more lithium ions can be quickly absorbed
onto and stored in the fine particles; meanwhile, the specific
current density of active material can be significantly reduced,
which will then enable a high lithium-ion flux across the interface.
As a result, material performance such as capacity and high rate
performance (or high power) can be improved. It can also be
concluded from the above example that nano-sized rutile TiO
2
β
-MnO
2
with
a specific surface area of ca. 110 m
2
g
-
1
also exhibits an excellent
high rate performance (100 mAh g
-
1
at 10 C and 70 mAh g
-
1
) [54].
Another advantage brought about by nanomaterials is the
enhanced structural stability. When material has a particle radius
r
at 30 C, where 1 C = 336 mA g
-
1
for that phase,
structural transition to thermodynamically undesirable structures
may take place. Therefore, by using nanoparticles with
larger than the critical nucleation radius
r
p
c
, it
is possible to eliminate such transitions. For example, layered
LiMnO
r
<
r
p
c
suffers from serious structural instability during the
Li insertion/extraction process, which is responsible for its
cycling capacity fade. In order to overcome such difficulties,
materials with nanocrystalline structures are introduced,
for a much higher Li-intercalation capacity than convention
materials resulting from an easily accommodated lattice
stress due to Jahn-Teller distortion [56]. In nanoparticles, it is
widely accepted that the smaller the particles are, the more
atoms these particles will have at the surface. Since the charge
accommodation occurs mainly at or very near the surface, the
need for diffusion of Li
2
in the solid phase of nanomaterials may
be reduced, which will then account for an enhancement in the
charge and discharge rate of the electrode as well as a reduction
+
Search WWH ::
Custom Search