Page 385 - Book Hosokawa Nanoparticle Technology Handbook

6.5 ELECTROCHEMICAL PROPERTIES FUNDAMENTALS
shown here. These fine powder materials, prepared by
the mist-dry method, have a large surface area and uni-
form spherical shape, which may improve the cycling
efficiency, high rate performance and so on. Fig. 6.5.4
shows the scanning electron micrographs (SEM) of
LiNiO prepared by the mist-dry method [3]. The aver-
2
age diameter of the particle was 0.8 m, with relatively
narrow dispersion. It was found that the particle was
secondly agglomerated particle by SEM observation in
high magnification. This fact was reflected in the large
specific surface area, measured using the BET method
2
1
of 10 m g , compared to that of LiNiO 2 prepared by
1
2
the conventional method of several m g . The results
of the charge/discharge cycle test of LiNiO at 0.1, 0.3
2
and 0.5 mA cm 2 respectively were shown in Fig.
6.5.5. After 10 cycles, constant charge/discharge could
be carried out for more than 50 cycles for the LiNiO
Figure 6.5.1 fine particle sample, prepared here by the mist-dry 2
The experimental cell for the electrochemical method. Recently it has been reported that LiMn O
measurement concerning the electrode process. 2 4
fine particles can also be prepared using the mist-dry
method. LiMn O fine particles were prepared with
2
4
varying size and shape and their large specific area was
OP amp
found to be potentially capable of helping improve the
+ Counter Electrode properties at a high rate of charge/discharge [4,5].
− Comparing the discharge capacities is the simple
Controlling and convenient method used to discuss the electro-
voltage cell chemical properties of various samples, while a
Reference Electrode Peukert equation can also be used to discuss the rate
dependence of the charge/discharge capacity:
• Working Electrode
x
t
i const (6.5.1)
Figure 6.5.2 where i, t and x correspond to the charge/discharge
An electric circuit with a three electrode cell for current density, charge/discharge time and the factor
electrochemical measurement. which reflects the effect of the charge/discharge rate
on the capacity respectively. Although this equation
(6.5.1) is empirical, it is often used to quantitatively
Conversely, the counter electrode used is so large that express the effect of the reaction rate at the electrode
the electrochemical process on it never limits the total on the charge/discharge capacity. The following equa-
process. The potential difference between the working tion (6.5.2) can be derived from equation (6.5.1) by
and counter electrodes is relatively unimportant in this using discharge capacity C, as follows:
case. Lithium metal with a sufficiently large area can
be used to investigate the electrochemical properties of ) logi logC const (6.5.2)
x
1
(
the anode for a lithium ion battery. Fig. 6.5.3 shows
the cells which are practically used to evaluate the Thus, the effect of the electrode reaction on the dis-
electrochemical properties of the electrodes for nickel– charge capacity can be quantitatively expressed by the
hydrogen and lithium ion batteries. When using elec- gradient of the log i vs. log C plot. The Peukert plots
trolyte with high resistivity, like the nonaqueous for polyaniline, prepared in various organic solvents
solution for the electrolyte solution of a lithium ion bat- such as propylene carbonate,
-butylolactone and
tery, the arrangement of the electrodes, especially that acetonitrile by electropolymerization, were shown in
of the reference electrode, can affect the results. In the Fig. 6.5.6. The nanostructure of the polyaniline was
case of a lithium ion battery, the water must be removed influenced by the kind of solvent used for electropoly-
from the system. Therefore the cell is constructed in a merization. A fine fibril structure was observed for
globe box filled with inert gas, such as Ar and N . A polyaniline prepared in acetonitrile solution, which
2
simple sealed test cell for a lithium ion battery, which reflects the larger x value. This also means polyaniline
only requires an inert gas atmosphere during the cell with a fine porous structure has an advantage as an
construction, has been developed as shown in Fig. 6.5.3. electrode material for frequent use.
As an example of the charge/discharge tests for The experimental techniques of a charge/discharge
material with a nanostructure, the results for LiNiO 2 test at a constant current, as described above, are so
fine particles prepared by the mist-dry method were basic that they can be applied to other methods, such
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