Flat cells

Details Flat cells Flat cells

1999-07-21

2001-03-20

Maples, John S. (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Plural concentric or single coiled electrode

C429S162000, C429S218100, C429S245000

Reexamination Certificate

active

06203938

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a thin-type flat cell in which an electrode group having an oval cross section configured by spirally winding, one each of a strip-shaped positive plate and a strip-shaped negative plate with a separator interposed, is housed. In particular, the invention relaters to a flat cell which is free from failure, due to breakage of electrode plates and/or peeling off or dropping of the active material when assembling the electrode group, by a preliminary processing of the portion of the electrode plates to be folded.
BACKGROUND OF THE INVENTION
In association with the trend toward smaller-size, lighter-weight, and thinner design of portable equipment, as represented by portable telephone, note-book type personal computers, and camcorders, there has been remarkable progress in recent years in the small-size secondary batteries to be used as the power source of such equipment. Following the initially commercialized conventional type lead-acid system and nickel-cadmium system, new types of nickel-metal hydride systems and lithium-ion systems with higher energy densities have recently been commercialized.
With small-size sealed lead-acid batteries, rectangular batteries are the generally adopted configuration. In such batteries, the electrode groups are constructed by alternately stacking a plurality of positive plates and negative plates with a separator interposed and connecting together the electrode plates of the same polarity. The groups of electrodes then are encased in a 3-cell or 6-cell mono-block type plastic battery container, and are sealed after being connected in series.
Nickel-cadmium batteries and nickel-metal hydride batteries are used in the form of a battery pack in which a plurality of either cylindrical cells, obtained by encasing and sealing an electrode group constructed by spirally winding one each of strip-shaped positive plate and strip-shaped negative plate with a separator interposed in a metallic cell container, or rectangular cells, obtained by encasing and sealing an electrode group constructed by alternately stacking a plurality of positive plates and negative plates with a separator interposed and connecting together the electrode plates of the same polarity in a metallic container, are connected in series and/or in parallel in order to obtain a required voltage and capacity.
With lithium-ion secondary batteries, cylindrical cells are constructed in basically the same way as the nickel-cadmium system batteries and nickel-metal hydride system batteries. In contrast to this, with thin type flat cells having a rectangular or oval cross section and which are regarded important from the standpoints of thinning appliances and reducing dead space of the power source, because the thicknesses of the positive and negative plates and the separator are extremely small, a cell configuration is adopted in which an electrode group is constructed by spirally winding one each of strip-shaped positive and negative plates with a separator interposed in such a manner that they are folded to make the cross section oval in shape. The electrode groups are encased in a container, and sealed after pouring and impregnating with an electrolyte.
In order to further make this type of a cell thinner or higher in capacity, the core material of the electrode plates is made as thin as possible and the amount of the separator in the core portion and the outermost portion of the electrode group is reduced, or the thickness of the electrode group is made smaller by applying a higher pressure to the electrode group when it is inserted and encased in a cell container.
On the other hand, in order to fabricate such an electrode group with a high productivity, a core portion is formed as a first step by winding a separator while keeping it firmly held on a mandrel, a coil one each of strip-shaped positive plate and negative plate is spirally wound with the separator interposed, and the outer surface is fixed by wrapping with an extra length of the separator, thereby completing an electrode group having an elliptical cross section. As a second step, a process is carried out in which the electrode group is pressed and deformed while being sandwiched between a pair of flat plates which are parallel to the major axis of the electrode group and have an elliptical cross section, thereby producing an electrode group having an oval cross section as shown in FIG.
1
.
FIG. 1
is an enlarged cross-sectional view of an essential part of an electrode group
1
of a thin-type lithium-ion secondary flat cell. In
FIG. 1
, the electrode group
1
is constructed by spirally winding one each of strip-shaped positive plate
11
and negative plate
12
in such a way that they are folded with a separator
13
interposed. The electrode group
1
is made by first spirally winding the positive plate
11
and the negative plate
12
with the separator
13
interposed enwrapping a core portion
13
a
which is made by folding over the tip of one end of the separator
13
. The separator
13
enwraps the outside of the electrode group
1
, and the tip
13
b
of the other end of the separator is tightly fastened to the electrode group
1
by a heat-sealing method or the like.
In the final process of pressing and deforming in the second step of constructing an electrode group, breakage of an electrode plate sometimes takes place because of tearing off of the core material of the electrode plate at the innermost folded portion
11
a
of the positive plate
11
close to the core portion
13
a
and the subsequent folded portion
11
a
′, and at the innermost folded portion
12
a
and subsequent folded portion
12
a
′ of the negative plate
12
, thus causing a reduction in the cell capacity. Also, even when an electrode plate is not broken, the active material layer sometimes peels off from the core material, leading to dropping of small portions of the active material layer, thus damaging the separator and causing internal short circuits between the positive and negative electrodes, resulting in a loss of reliability in the lithium-ion secondary batteries employing an organic electrolyte. Consequently, it is critically important to totally prevent such breakage of electrode plates and/or peeling off and dropping of the active material layer. Many of such failures tend to take place with the positive plate which has a relatively higher filling density of the active material layer than the negative plate and which uses as the core material of the electrode plate aluminum which has a relatively lower tensile strength and repetitive bending strength than the copper foil used as the core material of the negative plate. Also, the failures tend to take place on the innermost folded portion which is close to the core portion.
FIG. 2
is a plan view of a folded portion of a strip-shaped positive plate of an electrode group having an oval cross section as constructed in the final step. In
FIG. 2
, breakage and/or peeling off and dropping of the active material layer tends to take place at the innermost folded portion
11
a
of the positive plate
11
which is close to the core portion. Occasionally, breakage of the positive plate and/or peeling off and dropping of the active material layer takes place at the folded portion
11
a
′ which is next to the innermost folded portion
11
a
. No breakage of the positive plate or peeling off and dropping of the active material layer takes place in the subsequent folded portion
11
a
″ (corresponding to the outer region of the innermost folded portion
11
a
). Reference numeral
11
b
is a positive plate lead tab made of aluminum onto which an insulating tape
11
c
is affixed.
FIG. 3
is an enlarged cross-sectional view of the core portion and its outer regions only of an electrode group having an oval cross section. In
FIG. 3
, the negative plate
12
, which is adjacent to the core portion
13
a
consisting of a separator
13
that separates the positive plate
11
and the negative plate
12
, comprises a core mater

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