Biomedical Engineering Reference
In-Depth Information
in the volumetric change and lattice stress caused by repeated Li
insertion and expulsion.
Besides, using nanomaterials will enable some lithium-storage
mechanisms available for mass storage. One such new mechanism
is the so-called “conversion” mechanism [32], which is first found in
transition metal oxides, then in fluorides, sulfides, and nitrides [33,
57, 58], and the mechanisms can described by Eq. 6.4:
MX
+
y
Li
+
+
y
e
-
Li
X M
+
,
(6.4)
y
where
= Fe, Co, Ni, Cr, Mn, Cu, and so on.
As can be seen, the mechanism is mainly related to the reversible
in situ formation and decomposition of Li
X
= O, S, F, or N and
M
upon Li uptake and
release, and the reversible capacities in these systems are usually in
the range of 400-1100 mAh g
y
X
. It is reported that electrodes made
of CoO nanoparticles can achieve a specific capacity of 700 mAh g
-
1
-
1
with almost 100% capacity retention for up to 100 charge/discharge
cycles [32], In addition to conversion mechanism, other mechanisms
such as interfacial Li storage [59] and nanopore Li storage [24]
have also been proposed, and more research work will follow this
direction in the future.
Nanostructured electrode materials also possess other merits,
such as the change of electrode potential (or the thermodynamics
of
the
reaction)
[60],
and
the
more
extensive
range
of
solid-solution-existing composition [61];
these merits, along with
other above-mentioned advantages, provide infinite possibilities
to nanomaterials.
6.4.2 Disadvantages of Nanomaterials
For nanomaterials, the excess surface free energy should be taken
into consideration for the chemical potential, as shown in Eq. 6.5:
( )
(
)
(
)
m
r
=
m
r
= ∞ +
2
γ
/
r V
, (6.5)
0
0
where 2(
γ
/
r
)
V
gives the excess surface free energy,
γ
is the effective
surface tension,
V
is the partial molar volume, and
r
is the effective
grain radius.
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