Chemistry: electrical current producing apparatus – product – and – Means externally releasing internal gas pressure from closed...
Reexamination Certificate
1999-08-30
2003-05-13
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Means externally releasing internal gas pressure from closed...
C429S056000, C429S057000, C429S163000, C429S175000
Reexamination Certificate
active
06562508
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to secondary cells which comprise a can and a rolled-up electrode unit or like cell element accommodated in the can and which are adapted to deliver electric power generated by the cell element from a pair of electrode terminals attached to the can.
BACKGROUND OF THE INVENTION
In recent years, attention has been directed to lithium secondary cells or batteries having a high energy density for use as power sources for portable electronic devices, electric motor vehicles, etc. Cylindrical lithium secondary cells of relatively large capacity, for example, for use in electric motor vehicles comprise, as shown in
FIGS. 11 and 12
, a cylindrical can
1
having a cylinder
11
and lids
12
,
12
welded to the respective ends thereof, and a rolled-up electrode unit
2
encased in the can
1
. A pair of positive and negative electrode terminal assemblies
9
,
9
are attached to the lids
12
,
12
, respectively. The rolled-up electrode unit
2
is connected to the terminal assemblies
9
,
9
by a plurality of electrode tabs
3
, whereby the electric power generated by the electrode unit
2
can be delivered to an external device from the pair of terminal assemblies
9
,
9
. Each lid
12
is provided with a pressure-relief gas vent plug
13
.
With reference to
FIG. 13
, the rolled-up electrode unit
2
comprises a positive electrode
21
containing a lithium containing composite oxide, a negative electrode
23
containing a carbon material, and a separator
22
impregnated with a nonaqueous electrolyte and interposed between the electrodes, the assembly of the these components
21
to
23
being rolled up into a cylinder. A plurality of electrode tabs
3
outwardly extend from each of the positive electrode
21
and the negative electrode
23
of the unit
2
, and the outer ends
31
of the electrode tabs
3
of the same polarity are joined to one electrode terminal assembly
9
. For convenience' sake, only some of these tabs are shown as being joined to the terminal assembly
9
in
FIG. 13
, while the connection of the ends of the other tabs to the assembly
9
is omitted from the illustration.
The electrode terminal assembly
9
comprises a screw member
91
extending a hole in the lid
12
of the can
1
and mounted on the lid
12
. The screw member
91
has a flange
92
at its base end. An insulating packing
93
is fitted in the hole of the lid
12
for electrically insulating the screw member
91
from the lid
12
and providing a seal therebetween. The screw member
91
has a washer
94
fitted therearound from outside the cylinder
11
, and a first nut
95
and a second nut
96
screwed thereon similarly. The first nut
95
is tightened up to clamp the insulating packing
93
between the flange
92
of the screw member
91
and the washer
94
and thereby seal off the hole more effectively. The outer ends
31
of the electrode tabs
3
are fixedly joined to the flange
92
of the screw member
91
by spot welding or ultrasonic welding.
FIG. 9
shows a known secondary cell which is provided with a pressure relief gas vent closure
4
fitted in a through bore
14
formed in a lid
12
and operable upon the internal pressure of a cell can
1
exceeding a predetermined value (JP-A No. 68861/1994). As shown in FIG.
10
(
a
), the gas vent closure
4
comprises a ring
41
and a disklike diaphragm
42
fixed thereto. The gas vent closure
4
is fixed to the lid
12
by joining the outer periphery of the ring
41
to an opening edge of the bored portion
14
of the lid
12
by laser welding at
15
as seen in FIG.
10
(
b
).
With large-sized secondary cells, however, the lid
12
is as thick as at least several millimeters and has a very large thickness relative to the thickness of the gas vent closure
4
. Consequently, when the closure
4
is fixed to the lid
12
by laser welding, there arises the problem that the weld develops defects, such as pinholes or cracks, because the heat of welding dissipates markedly, rapidly cooling the metal melted by being irradiated with the laser beam.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a secondary cell wherein a gas vent closure can be welded to a lid without the occurrence of defects such as pinholes or cracks.
The present invention provides a secondary cell comprising a gastight can
1
having a cylinder
11
and a lid
12
fixed to an opening portion of the cylinder, and a secondary cell element accommodated in the can
1
. The lid
12
is fixedly provided with a vent closure
4
in the form of a disk and having a diaphragm
42
openable upon the internal pressure of the can
1
exceeding a predetermined value, the vent closure
4
having an outer peripheral portion welded to an opening edge defining a through bore
14
formed in the lid
12
. The lid
12
has a diminished portion
51
provided around the welded portion of the vent closure
4
for reducing the sectional area of a heat flow path for the heat of welding to pass therethrough.
Since the secondary cell of the invention described has the diminished portion
51
around the welded portion of the vent closure
4
, the heat of welding is transferred first to the diminished portion
51
and then to other portion around the diminished portion
51
for dissipation. The heat capacity of the diminished portion
51
is much smaller than that of the entire lid
12
, while the sectional area of the heat flow path for the heat flux to pass through is also small in the diminished portion
51
, with the result that the diminished portion
51
provides resistance to heat conduction to suppress the dissipation of the heat of welding. Consequently, the weld cools at a suitable rate, solidifying without developing pinholes or cracks.
Stated specifically, the lid
12
is formed in the front surface or rear surface thereof with a circumferential groove
5
surrounding the welded portion of the vent closure
4
, and the diminished portion
51
is formed between the welded portion of the vent closure
4
and a side wall of the lid defining the groove
5
and positioned closer to the vent closure
4
.
Further stated specifically, the diminished portion
51
is alternatively provided by a cylindrical projection
6
jutting from the front surface of the lid
12
and surrounding the through bore
14
, and the vent closure
4
is fixed to a central opening portion of the projection by welding.
Preferably, the circumferential groove
5
has a width in the range of {fraction (1/50)} of the inside diameter of the through bore
14
formed in the lid
12
to a value equal to the inside diameter, and a depth up to ⅔ of the thickness of the lid
12
. If the width of the groove
5
is less than {fraction (1/50)} of the inside diameter of the bore
14
, the diminished portion
51
produces a low effect, whereas if the groove width is greater than the inside diameter, there arises a problem with respect to the strength of the lid
12
. Further when the groove depth is greater than ⅔ of the thickness of the lid
12
, a problem as to the strength of the lid
12
will also result.
The secondary cell is, for example, at least 5 Ah to not greater than 200 Ah in capacity and 1 to 5 mm in the thickness of the lid
12
. With secondary cells having such a large size or capacity, heat is markedly dissipated from the welded portion of the conventional gas vent closure, whereas the diminished portion
51
of the present invention produces an outstanding effect.
With the secondary cell of the present invention, the provision of the diminished portion
51
for the lid
12
suppresses the dissipation of heat that occurs when the vent closure
4
is welded to the lid
12
, permitting the metal melted by welding to cool at an appropriate rate and therefore obviating the likelihood that the welded portion will develop pinholes, cracks or like defects.
REFERENCES:
patent: 3897270 (1975-07-01), Rosansky
patent: 4191806 (1980-03-01), Levy
patent: 4467021 (1984-08-01), Stocchiero
patent: 5080985 (1992-01-01), Wi
Fujiwara Kazuyasu
Nakanishi Naoya
Nohma Toshiyuki
Satoh Kouichi
Yonezu Ikuo
Rader Fishman & Grauer
Sanyo Electric Co., LTD
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