Chemistry: electrical current producing apparatus – product – and – Means externally releasing internal gas pressure from closed... – Blowout type
Reexamination Certificate
2000-11-16
2003-03-25
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Means externally releasing internal gas pressure from closed...
Blowout type
C429S174000, C029S623400
Reexamination Certificate
active
06537693
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a secondary battery having a function to cut off a current path when the pressure in the battery rises, and a method of manufacturing such a secondary battery.
2. Description of the Related Art
In recent years, there has been a demand for secondary batteries, e.g., lithium ion secondary batteries, of high energy density which can be used for a long period of time through repeated charging and discharging cycles. It is more important than ever to take sufficient safety measures for those secondary batteries. Generally, secondary batteries are of a sealed structure. Therefore, for example, when a secondary battery is overly charged, the electrolytic large amount of gas in the battery, resulting in a sharp rise of the pressure in the battery. If such a condition continues, the battery itself tends to rupture, causing a dangerous situation. One of the safety measures which have been used in secondary batteries is a current cutting-off component hereinafter referred to as “disconnect device”) for cutting of a current path when the pressure in the battery rises.
One conventional disconnect device for use in secondary batteries will be described in detail below. As shown in
FIGS. 1A and 1B
of the accompanying drawings, a conventional secondary battery has a disconnect device disposed in an open end of outer casing
16
. The disconnect device comprises rupture disk
11
having downward protrusion
11
a
with a flat tip end and closing the open end of outer casing
16
, cap
10
electrically connected to an upper surface of rupture disk
11
by ring plate
12
and serving as an outer terminal of the battery, weld plate
14
serving as an inner terminal of the battery joined to the tip end of downward protrusion
11
a
and having vent holes
14
a
, and insulating gasket
13
sealing the gap between rupture disk
11
and outer casing
16
and holding rupture disk
11
and weld plate
14
in an electrically insulated fashion, except for a space around junction
15
between the weld plate and the rupture disk.
Insulating gasket
13
has a recess in which rupture disk
11
and cap
10
are mounted, an opening defined therein at the bottom of the recess, and a mount disposed in an inner peripheral surface of the opening and holding weld plate
14
.
Weld plate
14
and rupture disk
11
are not electrically connected to each other except for junction
15
.
Outer casing
16
holds therein a coiled assembly of an anode sheet, a cathode sheet, and a separator, and an electrolytic solution.
Weld plate
14
is of a circular shape. Electrically conductive tab
17
extending from the coiled assembly is welded to weld plate
14
.
In the secondary battery constructed as the above, when the pressure in the battery rises, a stress is applied to rupture disk
11
in a direction to displace protrusion
11
a
toward cap
10
, with a tensile force concentrated on junction
15
between weld plate
14
and rupture disk
11
. When the tensile force exceeds the strength of junction
15
, the tip end of protrusion
11
a
of rupture disk
11
disengages from weld plate
14
, and protrusion
11
a
is deformed toward cap
10
. The electric connection between the exterior and interior of the battery is broken, stopping the generation of a gas in the battery. As a result, the pressure in the battery is prevented from increasing.
The disconnect device should be designed such that when the pressure in the battery reaches a level tending to break the battery itself, the rupture disk will be peeled off, reliably cutting off the current path. The rupture disk is not permitted to remain undetached when the pressure in the battery reaches the level tending to break the battery itself. The rupture disk is not permitted to be detached easily due to shocks imposed when the battery drops onto a hard object.
Therefore, the mechanical strength of the junction between the weld plate and the rupture disk is an important factor for ensuring the desired reliability of the battery, and needs to be controlled at a certain value.
Japanese laid-open patent publication No.199106/1997 discloses a technique for reducing and stabilizing variations of welding strength. The publication describes that when a first plate (inner terminal plate
6
) corresponding to a weld plate and a second plate (explosion-resistant valve body
3
) corresponding to a rupture disk are welded, with an ultrasonic welding process, at a flat surface of the tip end of a protrusion of the first plate, variations of welding strength can be reduced and stabilized by limiting the area of the flat surface and the height of the protrusion which affect the welding strength.
However, the above publication only discloses that variations of welding strength can be reduced and stabilized by welding with an ultrasonic welding process joint surfaces whose area and shape are limited, but fails to reveal how a junction interface should be to control the peeling strength at a constant level.
When metal joint surfaces abut against each other and then the ultrasonic welding is performed, the area of intimate contact of convexities of all minute concavities and convexities on the metal joint surfaces increases, and the joining between the metal joint surfaces progresses due to diffusion of atoms at the surfaces that are held in intimate contact with each other. Thereafter, the diffusion of atoms activates the metal joint interface, eliminating voids to form molten metal regions (hereinafter referred to as “nuggets”) in the metal joint surfaces. When nuggets are formed by the ultrasonic welding process, the peeling strength between the metal joint surfaces becomes equivalent to the material strength (shearing strength) of the regions where the nuggets are formed. Even with the technique disclosed in the above publication, when the diffusion of atoms progresses to the stage where nuggets are formed by the ultrasonic welding process, the peeling strength of the junction greatly depends on the material strength of the regions where the nuggets are formed, and is difficult to control merely by adjusting setting conditions for the ultrasonic welding process.
The peeling strength of the junction increases in proportion to the joined area. Therefore, when the joined area (current passing area) is increased in order to extract a large current from the secondary battery, if the metal joint surfaces are joined to achieve the same joined strength as the material strength, then the peeling strength of the junction is too large to cut off the current path.
As shown in
FIGS. 1A and 1B
, the electrically conductive tab extending from an electrode sheet is connected to the circular weld plate in the secondary battery. If forces are applied to the electrically conductive tab when the secondary battery is assembled or subjected to vibrations or shocks, the weld plate is liable to rotate in the joined surfaces. Since the weld plate is joined only in the junction, it produces torsional forces in the junction. Therefore, there is the problem in that the junction is unexpectedly peeled off.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of manufacturing a secondary battery so as to achieve a current passing area for extracting a large current from the secondary battery and also to easily control, at a constant level, the peeling strength of a junction between a rupture disk and a weld plate.
Another object of the present invention is to provide a secondary battery which has a highly reliable disconnect device including a junction that will not be detached when the secondary battery is assembled or subjected to vibrations or shocks, except when the pressure in the battery becomes abnormally high.
As a result of the inventor's study conducted to achieve the above objects, it has been found that the peeling strength of a junction can be controlled by only setting conditions of an ultrasonic welding machine if the junction is produced by the diffusion of atoms in an initial states
Morita Shigetomi
Suzuki Hiromi
Hayes & Soloway PC
Kalafut Stephen
NEC Tokin Corporation
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