Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-11-04
2001-06-19
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
Reexamination Certificate
active
06248474
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a composite electrode for a secondary battery and a production method thereof and more particularly, to a composite electrode for a rechargeable battery that improves the capacity, cycling performance, and power density, and a production method of the electrode.
2. Description of the Prior Art
In recent years, various research and development have been vigorously performed for the purpose of realizing a light-weight, high output-density secondary battery using the electrochemical doping and dedoping property of a conducting polymer such as polyacetylene and polyaniline.
An example of the conventional secondary batteries of this sort uses polyaniline (Pan) as a positive electrode and metallic lithium (Li) as a negative electrode. In this secondary battery, the reactions at the positive and negative electrodes (i.e., electrode reactions) during the charging process are expressed in the following chemical equations (1a) and (2a) respectively.
Pan+
n
ClO
4
−
+n
H
+
→Pan(ClO
4
)
n
+ne
−
+n
H
+
(1a)
Li
+
+e
−
→Li (2a)
During the discharging process, the electrode reactions are expressed in the following chemical equations (1b) and (2b), respectively, which are reverse of the equations (1a) and (2a).
Pan+
n
ClO
4
−
+n
H
+
←Pan(ClO
4
)
n
+ne
−
+n
H
+
(1b)
Li
+
+e
−
←Li (2b)
As seen from the equations (1a) and (1b), in the reactions at the positive electrode, the doping and dedoping processes of a dopant anion (here, ClO
4
31
) are used as the battery reaction, i.e., redox reaction.
In the equations (1a) and (1b), n is the doping rate of the dopant anion, ClO
4
−
. It has been known that the doping rate n is equal to 0.5 or less, in other words, the maximum value of n is 0.5 (=50%) for polyaniline.
The capacity C (mAh/g) of the active material (i.e., polyaniline) of the positive and negative electrodes are calculated by the following equation (3)
C
=
26950
×
(
N
elec
M
r
)
(
3
)
where N
elec
is the number of reaction-participant electrons and M
r
is the molecular weight of the active material (i.e., polyaniline). Since polyaniline is a polymer material, the molecular weight of the monomer unit of polyaniline is used as the molecular weight M
r
.
The monomer unit of polyaniline has a molecular weight M
r
of 92g. If the doping rate n of polyaniline is supposed to be 0.5 (=50%), the number N
elec
of reaction-participant electrons is equal to 0.5 In this case, the capacity C (mAh/g) of the active material (i.e., polyaniline) is given as 144 mAh/g from the equation (3).
In general, the doping and dedoping reactions (i.e., redox reaction) of the conducting polymers occur with good reversibility, the reason of which is as follows.
Specifically, the matrix of the conducting polymers has a more flexible structure than that of the inorganic materials. Therefore, the volumetric increase and decrease of the matrix will occur with good reversibility during the doping and dedoping processes of the dopant into the matrix.
The conventional secondary batteries using a conducting polymer such as polyaniline as the active material of the positive electrode have the following problems.
A first problem is that even if the active material (i.e., conducting polymer such as polyaniline) has a sufficient reversibility, the cycling property of the battery or positive electrode itself tends to disappear.
The reason of the first problem is explained below with reference to
FIG. 1
illustrating a partial cross-section of the positive electrode of the conventional battery.
In
FIG. 1
, the positive electrode
106
is comprised of a plate-shaped conductive collector
101
and an active material layer
105
. The active material layer
105
is fixed in contact with an opposing surface of the collector
101
. The active material layer
105
contains a particulate active material (i.e., conducting polymer such as polyaniline)
102
and a particulate conductivity-imparting agent (e.g., carbon)
103
, both of which are combine with a binder (not shown) to have a layer-shaped structure. A liquid electrolyte
104
containing LiClO
4
is permeated into the miniaturized pores of the active material layer
105
.
The battery or redox reaction occurs between the active material
102
and the electrolyte
104
and thus, electrons are transferred between active material
102
and the electrolyte
104
through the collector
101
or the conductivity-imparting agent
103
. Therefore, to realize a satisfactory cycling property of the electrode
106
, no only the reversibility of the redox reaction of the active material (i.e., conducting polymer)
102
but also the electron conductivity among the conductivity-imparting agent
103
, the collector
101
, and the active material
102
need to be ensured.
When one of the known conducting polymers (e.g., polyaniline) is used as the active material
102
, some volumetric increase or decrease (i.e., expansion or shrinkage) of the active material
102
will occur together with the battery reaction or the charging/discharging reactions. Specifically, when the dopant anion (e.g., ClO
4
−
) is doped into the matrix of the active material (e.g., polyaniline)
102
to charge the battery, the volume of the active material
102
tends to expand. When the dopant anion (e.g., ClO
4
−
) is dedoped from the matrix of the active material (i.e., polyaniline)
102
to discharge to battery, the volume of the active material
102
tends to shrink.
On the other hand, no expansion nor shrinkage occurs in the collector
101
during the charging and discharging processes. Accordingly, the contact resistance tends to increase at the contact areas of the active material layer
105
and the collector
101
, thereby degrading or preventing the electron conductivity.
As a consequence, the cycling property of the battery or positive electrode
106
itself tends to disappear while the reversibility in the redox reaction of the active material
102
is kept sufficiently.
A second problem is that the capacity per volume is small. The reason is that the active material layer
105
using the conducting polymer such as polyaniline has a low density and therefore, the capacity per volume of the layer
105
becomes smaller than that of the known inorganic active material layers.
The capacity per weight and the capacity per volume of polyaniline, and LiCoO
2
and LiMn
2
O
4
as examples of the inorganic active materials are listed in the following Table 1.
TABLE 1
CAPACITY PER
CAPACITY PER
ACTIVE
DENSITY
WEIGHT
VOLUME
MATERIAL
(g/dm
3
)
(mAh/g)
(mAh/dm
3
)
LiCoO
2
5.1
137
698
LiMn
2
O
4
4.3
104
447
POLYANILINE
1.3
144
187
In Table 1, the following relationship is established as
(CAPACITY PER VOLUME)=(CAPACITY PER WEIGHT)×DENSITY.
As seen from Table 1, although the capacity per weight of polyaniline is approximately equal to that of LiCoO
2
and LiMn
2
O
4
, the capacity per volume of polyaniline is smaller than that of LiCoO
2
and LiMn
2
O
4
due to smallness of the density.
A third problem is that the power density is low.
The power density of the battery or electrode
106
is determined by the reaction rate of the redox reaction of the active material
102
and the diffusion rate of the reaction-participant ion in the liquid electrolyte
104
. Since the diffusion rate of the reaction-participant ion is typically lower than the reaction rate of the redox reaction of the active material
102
, the power density is dominated by the diffusion rate of the ions.
The diffusion rate of the reaction-participant ion in the electrolyte
104
increases with the decreasing radius of the ions. Therefore, it is preferred that the reaction-participant ion has a radius as small as possible.
In the previously-described equations (1a), (1b), (2a), and (2b) where the ClO
4
31
ions with a comparatively large radius
Harada Gaku
Kurihara Junko
Nishiyama Toshihiko
Okada Shinako
Sakata Koji
Chaney Carol
NEC Corporation
Tsang Susy
Young & Thompson
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