Biomedical Engineering Reference
In-Depth Information
The cationic transport number in amorphous inorganic and polymeric
electrolytes can be approximated through the use of the decoupling
constant, R. The larger the value of R, the greater is the structural
relaxation time and less is the transport of ions (both anions and cations)
mediated by the structure. In polymer-ceramic composite electrolytes, the
glass transition temperature, T
g
, is proportional to the volume fraction of
the ceramic phase. Angell (1986) has shown that for an R value of
10
2
, the
cationic transport number could be over 0.9, as the transport of ions
mediated by the structure would diminish. An increase of 50
~
8
C in the T
g
in
most polymer electrolytes resulting from the addition of ceramic phase will
bring the T/T
g
ratio to 1.2
^
10
2
ð
R
Þ
and the cationic transport number to
around 0.9.
The conductivity of a polymer electrolyte originates from two distinct
processes: ion hopping and ion transport assisted by polymer chain motion.
The addition of a ceramic phase suppresses the chain motion-mediated
contribution and thus must increase the contribution associated with ion
hopping if the conductivity remains the same (the worst case scenario). The
ion hopping process is more favorable for cationic species than anionic
species because of their small size and mass. This scenario suggests an
enhanced cationic transport number as the volume fraction of ceramic phase
increases in the polymer matrix of the polymer-ceramic nanocomposites.
The conductivity and transport number of nanocomposities comprising
LiI, PEO, SiO
2
, MgO, and Al
2
O
3
have been measured and reported by
Nagasubramanian et al. (1993) and Peled et al. (1993). They calculated the
conductivity from bulk resistance, R
b
, measured at high frequency and
transport number, t
+
, using:
t
þ
¼
R
b
=
ð
R
b
þ
Z
d
Þ
½
15
:
16
where Z
d
is diffusional impedance as measured from a Nyquist plot. For a
composite electrolyte film containing 0.05
m alumina, the bulk conductiv-
ity is around 10
4
Scm
1
and the lithium ion transport number is close to
unity at 104
μ
C. Cho and Lin (1997) report a transport number of 0.98 for a
glass-polymer composite electrolyte containing 13 vol% PEO:LiN
(CF
3
SO
2
)
2
and 87% 0.56Li
2
S.0.19B
2
S
3
.0.25LiI. Croce et al. (1998) reported
a transport number of 0.6 in a PEO:LiCO
4
-TiO
2
(10 wt%) composite
electrolyte in the 45-90
8
8
C temperature range.
15.3.2 Ceramic-ceramic nanocomposite
Lithium ion conductors
This subclass of nanocomposites contains only ceramic components. The
matrix is generally an ionic conducting material, whereas the dopant is a
