Chemistry: electrical current producing apparatus – product – and – Plural concentric or single coiled electrode
Reexamination Certificate
1999-04-15
2001-12-11
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Plural concentric or single coiled electrode
C429S060000, C429S129000, C429S144000
Reexamination Certificate
active
06329097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a secondary battery and, more particularly, to an electrode assembly of a secondary battery, which is designed to enhance battery safety, and a method for making the same.
2. Description of Prior Art
A secondary battery comprises an electrode assembly disposed within a can containing an electrolyte. The electrode assembly comprises positive and negative electrodes on which positive and negative active materials are deposited respectively. Therefore, the charging and discharging of the second battery is realized by the physical and chemical reactions occurring between active materials and the electrolyte.
In such a secondary battery, a capacity ratio of the negative active material to the positive active material (N/P ratio) is usually maintained within a range of 1.2-1.4 to obtain a reserve for sufficient absorption of ions released from the positive electrode into the negative electrode.
If the N/P ratio is less than 1, a metal oxide material is precipitated within the secondary battery or the electrolyte may leak since ions discharged from the positive electrode cannot be completely absorbed into the negative electrode. This result in the deterioration of the charging and discharging performance or in the explosion of the battery by increased internal pressure.
Secondary batteries are classified into several types depending on their shape: a cylindrical type, a package type and a prism type. For example, the prism type secondary battery, since the electrode assembly is rolled having an oval section, it is very difficult to provide the suitable N/P ratio throughout the entire surface of the electrode assembly.
This will be described more in detail with reference to
FIGS. 1-5
.
FIG. 1
shows a conventional prism type secondary battery.
The conventional prism type secondary battery comprises a can
4
, a roll electrode assembly
2
disposed within the can
4
, and a cap assembly
6
close-tightly mounted on an upper end of the can
4
.
Rolling a group of plates makes the roll electrode assembly
2
. That is, a separator plate, a negative electrode plate, another separator plate, and a positive electrode plate are, in this order, stacked to provide a group of stacked plates. The plate group is then rolled into the roll electrode assembly as shown in FIG.
2
.
The cross section of the roll electrode assembly
2
is track-shaped so that it can be inserted into the prismatic rectangular can
4
. As shown in
FIG. 2
, the roll electrode assembly
2
comprises positive and negative electrodes
2
a
and
2
b,
and separators
2
c
disposed between the positive and negative electrodes
2
a
and
2
b.
The track-shaped roll electrode assembly
2
has a straight section St and a curved section C.
The N/P ratio is determined depending on which electrode is disposed on the outer surface of the separator
2
c.
That is, when the curved section of the roll electrode assembly
2
is unrolled in a flat state, as shown in
FIG. 3
, there is a length difference &Dgr;L between the positive and negative electrodes
2
a
and
2
b.
Therefore, a difference between the amount of the positive active material and the amount of the negative active material can be obtained according to the following equation:
A=&Dgr;LXT
where,
A is the difference between the amount of the positive active material and the amount of the negative active material;
&Dgr;L is the length difference between the positive and negative electrodes; and
T is a thickness of the active material of the electrode.
The difference between the amount of the negative and positive active material causes variations in the capacity ratio difference between the negative and positive electrodes adjacent to each other with the separator disposed there between. That is, a region C where the N/P ratio is less than 1 appears on the roll electrode assembly
2
as shown in FIG.
4
.
In the region C, the capacity of the positive electrode
2
a
is larger than that of the negative electrode
2
b,
as shown in FIG.
5
. Therefore, the ions released from the positive electrode
2
a
are not be completely absorbed into the negative electrode
2
b
but precipitated as a metal oxide material
32
on a surface of the negative electrode
2
b.
However, in the conventional battery, since the thickness of the separator
2
c
is uniform, the precipitated metal oxide material
10
continues to grow, then may penetrate the separator
2
c,
and contact the positive electrode
2
a
as charging and discharging are repeated. This may cause the battery to explode, deteriorating the safety of the secondary battery.
SUMMARY OF THE INVENTION
Therefore, the present invention has been made in an effort to solve the above-described problems of the conventional secondary battery.
It is an objective of the present invention to provide an electrode assembly for a secondary battery in which any hazardous problems, which may occur by the precipitation of the metal oxide, can be prevented, thereby improving safety of the secondary battery.
To achieve the above objective, the present invention provides a roll electrode assembly of a secondary battery including a positive electrode, a negative electrode, and first and second separators rolled a plurality of turns.
The first separator is disposed between outer surface of the turns of the negative electrode and inner surface of the turns of the positive electrode. The second separator is disposed between outer surface of the turns of the positive electrode and inner surface of the turns of the negative electrode. The thickness of the second separator is less than that of the first separator.
Preferably, the thickness of the first separator is 2-3 times that of the second separator.
More preferably, the thickness of the first separator is about 30-45 &mgr;m, while the thickness of the second separator is about 15-23 &mgr;m.
When the roll electrode assembly is for a prism type battery, thus having, in a cross section, a straight portion and a curved portion, the thickness of the first and second separators are equal in the straight portion.
REFERENCES:
patent: 5746780 (1998-05-01), Narukawa et al.
patent: 5925482 (1999-07-01), Yamashita
Bushnell , Esq. Robert E.
Crepean Jonathan
Kalafut Stephen
Samsung SDI & Co., Ltd.
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