Note: Descriptions are shown in the official language in which they were submitted.
CA 02578827 2007-03-09
Method of Manufacturing Sealed Battery
and Sealed Battery
This application is a division of Canadian Patent
Application Serial No. 2,517,946, which is a division of
Canadian Patent Application Serial No. 2,308,640, filed
06 November 1998 as a Canadian National Phase Application
corresponding to International Application No.
PCT/JP98/04990, published 20 May 1999 as International
Publication No. W099/25035.
FIELD OF THE INVENTION
With a growing demand for portable electric appliances
such as mobile phone, portable audiovisual device, and
portable computers, demand for high-performance batteries has
grown. Especially, demand for secondary battery such as
nickel-cadmium battery, nickel-metal hydride battery, and
lithium-ion secondary battery has increased.
Generally, those batteries are sealed ones and have the
shape of cylinder or rectangular cylinder. Among them,
rectangular-cylinder sealed batteries draw attention due to a
space-saving advantage. Accordingly, it is assumed that
there is a great demand for higher performance and
reliability of rectangular-cylinder sealed batteries.
A rectangular-cylinder sealed batter is
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generally manufactured as follows. A bottomed,
rectangular-cylinder external casing is formed by
drawing a metal plate. In the external casing, a
generator element composed of positive and negative
electrodes are enclosed. The opening of the external
casing is covered with a cover plate. When covering the
opening with the cover plate, the cover plate is sealed
by caulking or melting in general.
The sealing prevents electrolyte and gas from
leaking out of the external casing when pressure
increases inside of the external casing. The sealing
quality profoundly affects the reliability.and life of a
battery.
Generally, mechanical caulking is often used for
sealing process of battery. For rectangular-cylinder
sealed batteries, however, it is difficult to adopt
caulking in many cases, so that laser welding is also
often used.
Fig. 25 is a conceptual diagram showing a
conventional sealing technology using laser welding that
has been generally adopted for sealed batteries.
Fig. 25 shows that a cover plate and an outer
casing are welded together using a laser as follows. A
flat cover plate 410 is fit into the opening of an
external casing 400 so that the upper surface of the
cover plate 410 has the same level as the top end of the
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. ~ l
rim of the external casing 400. A laser beam is
intermittently aimed at the boundary between the outer
edge of the cover plate 410 and the rim of the external
casing 400 at a certain speed. The sealing technology
using laser welding can realize the complete sealing of
rectangular-cylinder sealed batteries, leading to higher
reliability and longer life of rectangular-cylinder
sealed batteries. As a result, the laser sealing is
regarded as one core technology for higher quality of
rectangular-cylinder sealed batteries.
Conventionally, the external casing and the cover
plate are made of a nickel-plated steel plate or a
stainless steel plate. Recently, however, an aluminum
alloy plate, which is made by doping aluminum with
manganese and the like, has been also used in many cases
for weight reduction.
It is troublesome to use an aluminum alloy plate,
however. When the external casing and the cover plate
are welded using a laser, the welded part is susceptible
to cracking.
Generally, cracking arises in the scanning
direction of a laser beam. It is assumed that a part
which has been welded using a laser beam, i.e., a welded
part is pulled by heat stress that has been generated
around the welded part in the course of cooling. The
welded part is susceptible to cracking in the case of an
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. ,~
aluminum alloy plate since the welded part is rapidly
cooled due to the lower tensile strength and higher heat
conductivity than iron and stainless.
One proposed solution to this problem is to lower
the scanning speed of the laser beam since a lower
scanning speed reduces the incidence of cracking. In
order to curb the incidence of cracking as low as
possible, the scanning speed of the laser beam is now
set as relatively low for the laser welding. In terms
of production efficiency, however, it is not desirable
to lower the laser beam scanning speed since it takes
longer time for the sealing.
DISCLOSURE OF THE INVENTION
Accordingly, the present invention is made about
the sealed battery including the rectangular sealed
battery and takes into account these problems that have
been described. The object of the present invention is
to provide a manufacturing method of sealed battery'that
is able to keep productivity as high as possible while
suppressing cracking incident to welding using an energy
beam such as a laser beam when a material such as an
aluminum alloy is used for the external casing and the
cover plate and to provide the sealed battery.
First of all, inventive design of the closure cap
and the external casing reduce the heat stress at the
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welded part, so that cracking is suppressed. The
result depends on the sizes of the closure cap and
the external casing. When the closure cap and the
external casing are designed so as to satisfy the
equations (Equations 3 to 5, which are described
later) obtained by numerical value analysis, the
heat stress is more significantly reduced.
Then, inventive distribution of the energy of
the laser that is used for welding reduces the heat
stress at the welded part, so that cracking is
suppressed.
In addition, gradual cooling of the molten part
during welding reduces the molten part cooling speed
and the heat stress at the welded part, so that
cracking is suppressed.
In one aspect, the present invention provides a
method of manufacturing a sealed battery,
comprising: an external casing preparing step for
preparing an external casing that is made of an
aluminum alloy, the external casing with an opening
and a rim around the opening, a thickness of the
external casing at the rim being no greater than a
thickness of the external casing at parts of the
external casing other than the rim of the external
casing; a closure cap preparing step for preparing a
closure cap that is made of the aluminum alloy, the
closure cap fitting into the opening, the closure
cap being a plate with a rib along an outer edge of
the plate; a setting step for setting the closure
cap to the external casing, after a generator
CA 02578827 2007-03-09
element has been inserted into the external casing,
so that the rib comes into contact with the rim; a
welding step for welding the rim and the rib
together by radiating an energy beam along a
boundary between the rim and the rib, wherein
the closure cap is prepared at the closure cap
preparing step so as to satisfy equations T2_
(Tl/10+40)um and 50(pm)<_T3_TI, a thickness of the
plate being set as T1(pm), a height of the rib being
set as T2(pm), and a thickness of the rib being set
as T3(pm), and as a result, heat stress at a time of
welding the aluminum alloy is reduced at the welding
step.
In another aspect, the present invention
provides a sealed battery, comprising: an external
casing that is made of an aluminum alloy, the
external casing with an opening and a rim around the
opening, a thickness of the external casing at the
rim being no greater than a thickness of the
external casing at parts of the external casing
other than the rim of the external casing; and a
closure cap that is made of the aluminum alloy, the
closure cap being a plate with a rib along an outer
edge of the plate, wherein the closure cap is
disposed so that the rib comes into contact with the
rim, and the external casing and the closure cap are
welded together by radiation of an energy beam,
wherein the closure cap satisfies equations T2_
(Tl/10+40) um and 50 (um) _T3<_T1, a thickness of the
5a
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plate being set as Tl(pm) , a height of the rib
being set as T2(pm) , and a thickness of the rib
being set as T3(um), and as a result, heat stress at
a time of welding the aluminum alloy is reduced when
the closure cap and the external casing are welded
together.
In another aspect, the present invention
provides a method of manufacturing a sealed battery,
comprising: an inserting step for inserting a
generator element into an external casing that is
made of an aluminum alloy, the external casing with
an opening and a rim around the opening; and a
sealing step for fitting a cover plate that is made
of the aluminum alloy into the opening, for welding
an outer edge of the cover plate and the rim
together by radiating an energy beam along a
boundary between the outer edge and the rim, and for
sealing the external casing, wherein the energy beam
used at the sealing step has an energy distribution
so that a temperature gradient of a sealed part
while a welded material is molten is less steep than
when an energy beam with a Gaussian distribution is
used, whereby the energy beam reduces heat stress at
time of welding the aluminum alloy.
In another aspect, the present invention
provides a method of manufacturing a sealed battery,
comprising: an inserting step for inserting a
generator element into an external casing that is
made of an aluminum alloy, the external casing with
an opening and a rim around the opening; and a
5b
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sealing step for fitting a cover plate that is made
of the aluminum alloy into the opening, for welding
an outer edge of the cover plate and the rim
together by radiating an energy beam along a
boundary between the outer edge and the rim, and for
sealing the external casing, wherein the energy beam
used at the sealing step has an energy distribution
so that a diameter of a flat area that satisfies a
condition dP/Pc<0.05 is no smaller than 0.2W, a
diameter of a beam spot being defined as W, energy
at a predetermined point of the beam spot being
defined as Pc, and a finite difference between the
Pc and energy at a given point of the beam spot
being defined as dP, and the energy beam is radiated
onto the flat area along the boundary, whereby the
energy beam reduces heat stress at a time of welding
the aluminum alloy.
In still another aspect, the present invention
provides a sealed battery, comprising: an external
casing that is made of an aluminum alloy, the
external casing with an opening and a rim around the
opening; a generator element that has been inserted
into the external casing; and a cover plate that is
made of the aluminum alloy for sealing the external
casing by being welded onto the rim, wherein a
welded mark having a shape of silk-hat is formed at
a place where the rim and the cover plate are welded
together so as to reduce heat stress at a time of
welding the aluminum alloy.
5c
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In still another aspect, the present invention
provides a method of manufacturing a sealed battery,
comprising: an external casing preparing step for
preparing an external casing that is a bottomed
cylinder made of an aluminum alloy, the external
casing with an opening and a rim around the opening
and a cover plate that is made of the aluminum
alloy, the'cover plate sealing the external casing;
an inserting step for inserting a generator element
into the external casing; a setting step for fitting
the cover plate into the opening; a melting step for
melting a part of an outer edge of the cover plate
and a part of the rim by radiating an energy beam
along a boundary between the outer edge and the rim;
and a gradual cooling step for locally and gradually
cooling the parts that have been molten so that heat
stress at a time of welding is no greater than
breaking strength.
In a further aspect, the present invention
provides a sealed battery, comprising: an external
casing that is a bottomed cylinder that is made of
an aluminum alloy, the external casing with an
opening and a rim around the opening; a generator
element that has been inserted into the external
casing; and a cover plate that is made of the
aluminum alloy for sealing the external casing,
wherein an outer edge of the cover plate and the rim
are welded together by melting a part of the outer
edge and a part of the rim with laser radiation and
by locally and gradually cooling the parts that have
5d
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1
been molten so that heat stress at a time of welding
is no greater than breaking strength.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, advantages and features
of the invention will become apparent from the
following description thereof taken in conjunction
with the accompanying drawings that illustrate a
specific embodiment of the invention. In the
Drawings:
Fig. 1 is a perspective view, partly broken away
to show the interior construction, of a sealed
battery according to the first embodiment;
Fig. 2 is an enlarged sectional view of the main
structure of the sealed battery in Fig. 1;
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Fig. 3 shows the manufacturing process of the
sealed battery in Fig. 1;
Fig. 4 is an enlarged sectional view of the main
structure of the sealed battery in Fig. 1 when the cover
plate is fit into the opening of the external casing;
Fig. 5 is a perspective view showing how the
external casing is sealed with laser welding;
Fig. 6 is a conceptual diagram showing a model,
in which one piece of the shielded part (welded part)
with laser welding is divided in a mesh form , for the
analysis of heat stress;
Fig. 7 is a plot showing the relationship between
the height of the rib of the cover plate and the heat
stress;
Fig. 8 is a plot showing the relationship between
the thickness of the flat part of the cover plate and
the height of the rib of the cover plate;
Fig. 9 is a plot showing the relationship between
the thickness of the rib of the cover plate and the-heat
stress;
Fig. 10 is a plot showing the relationship
between the rim of the external casing and the heat
stress;
Fig. 11 is a sectional view of a possible
modification of the sealed battery shown in Fig. 1;
Fig. 12 is a plot showing the relationship
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I
between the number of laser radiations and heat stress;
Fig. 13 is a perspective view, partly broken away
to show the interior construction, of a sealed battery
according to the second embodiment;
Fig. 14 is an enlarged sectional view of the main
structure of the sealed battery in Fig. 13;
Fig. 15 is a plan view showing how the external
casing is sealed with laser welding;
Fig. 16 shows energy distribution of laser and
sectional views of molten pools;
Fig. 16(a) shows the energy distribution of the
laser used in the present invention;
Fig. 16(b) shows a sectional view of a molten
pool in the present invention;
Fig. 16(c) shows the energy distribution of a
conventional laser;
Fig. 16(d) shows a sectional view of a molten
pool that has been radiated with the conventional laser;
Fig. 17 shows a form of welded mark;
Fig. 17(a) is a schematic view of a welded mark;
Fig. 17(b) is a vertical sectional view of the
central part of the welded mark in Fig. 17(a);
Fig. 18 shows an example of shape of welded mark
that is formed at the boundary between the external
casing and the cover plate;
Fig. 19 is a perspective view, partly broken away
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to show the interior construction, of a sealed battery
according to the third embodiment;
Fig. 20 is a perspective view showing how the
external casing is sealed with laser welding;
Fig. 21 is a plot showing the temperature history
of the molten part when the temperature of assist gas is
changed;
Fig. 22 is a plot showing the heat stress in the
molten part for each number of laser radiations when the
temperature of assist gas is changed;
Fig. 23 is a plot showing the relationship
between the temperature of the assist gas and the yield
ratio;
Fig. 24 shows how the external casing is sealed
with laser welding when a sealed battery according to
the fourth embodiment is manufactured; and
Fig. 25 is a conceptual diagram showing the
method of manufacturing a conventional sealed battery
and a plan view showing how the outer casing is sealed
with laser welding.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A specific explanation of a rectangular-cylinder
sealed battery according to the present invention will
be given below in accordance with figures.
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~
[The First Embodiment]
Fig. 1 is a perspective view of a rectangular-
cylinder sealed battery 1 (referred to a "battery 1"
hereinafter) according to the first embodiment. Fig. 2
is an enlarged sectional view taken on line X-X of Fig.
1 of the main structure of the sealed battery in Fig. 1.
The battery 1 is a lithium-ion secondary battery
and has a structure as follows. Into an external casing
10, which is a bottomed rectangular cylinder, an
electrode group 20, which is formed by sandwiching a
separator between a positive and negative electrodes,
and nonaqueous electrolyte are inserted. The opening of
the external casing 10 is sealed with a closure cap 30.
The external casing 10 is a bottomed rectangular
cylinder made from an Al-Mn alloy plate.
Including aluminum (Al) as its major constituent,
Al-Mn alloy is light in weight. At the same time, doped
with manganese, Al-Mn alloy possesses high tensile
strength. Note that too high content of manganese
lowers the suitability of Al-Mn alloy to be worked and
welded at the time of forming external casings. As a
result, appropriate content of manganese is 1.0 to
1.5wt%.
As shown in Figs. 1 and 2, the closure cap 30 is
formed by attaching a negative electrode terminal 32,
which is nail-shaped, to the cover plate 31 via an
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insulating packing 33 so as to go through the cover
plate 31 at the center, which has been formed so as to
be fit into the opening of the external casing 10.
The cover plate 31 is a rectangular plate made by
punching out an Al-Mn alloy plate, which is the same
kind of Al-Mn alloy plate used for the outer casing 10.
The thickness of the outer casing 10 and the
cover plate 31 is set as thin as possible so as to keep
required strength. Generally, the thickness is set at
about 500/1m.
To the bottom of the negative electrode terminal
32 (inside of the battery), an electrode collector plate
34 is attached and to the top (outside of the battery),
a washer 35. The electrode terminal 32, the electrode
collector plate 34, and the washer 35 are crimped onto
the cover plate 31 by caulking so as to be insulated
from the cover plate 31 by the insulating packing 33.
The negative electrode 21 is formed by applying
carbon with a layer structure (graphite powders) onto a
plate and is covered by a separator 23. The plate of
the negative electrode 21 is connected to the electrode
collector plate 34 by a lead plate 25.
On the other hand, the positive electrode 22 in
the electrode group 20 is formed by applying positive
active material of oxide containing lithium (for
instance, lithium cobaltate) and conductive material
CA 02578827 2007-03-09
(for instance, acetylene black) onto a plate (not
illustrated in detail). The positive electrode 22
directly comes into contact with and is electrically
connected to the external casing 10, which is also the
positive pole.
The nonaqueous electrolyte is made by dissolving
LiPF6 as the solute in the mixed solvent of ethylene
carbonate and dimethyl carbonate.
The external casing 10 is sealed by welding a rim
l0a of the external casing 10 and the outer edge of the
cover plate 31 together using laser.
Not illustrated in Fig. 1, an insulating sleeve
26 made of an insulating resin is disposed between the
electrode group 20 and the cover plate 31 (refer to Fig.
5). By doing so, the electrode group 20 is fixed in a
home position inside of the external casing 10 and is
prevented from coming into contact with the closure cap
30.
The following is an explanation of the method of
manufacturing the battery 1.
Fig. 3 is a diagrammic sketch of the
manufacturing process of the battery 1.
First, the bottomed external casing 10 is made by
drawing an aluminum alloy plate using a punch and a die.
Fig. 4 shows how the cover plate 31 is fit into
the opening of the external casing 10 before the outer
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. =~
edge of the cover plate 31 and the rim 10a are welded
together.
The thickness L1 (J1m) of the rim 10a of the
external casing 10 is adjusted so as to be smaller than
the thickness L2 (J,lm) of the main body lOb of the
external casing 10. More specifically, as shown in Fig.
4, the thickness L1 of the external casing 10 at the rim
l0a is set smaller than the thickness L2 of the main
body lOb so that the external diameter R at the rim l0a
is the same as that at the main body lOb and the
internal diameter r at the rim 10a is greater than that
at the main body 10b. The thickness difference can be
easily adjusted by making the part of the punch
corresponding to the rim 10a greater.
Then, the cover plate 31 on which a rib 31b is
formed along the outer edge is made by drawing an
aluminum alloy plate so as to be fit into the opening of
the external casing 10.
Here, the thickness T3 (Mm) of the rib 31b is
adjusted so as to be smaller than the thickness Ti (Jim)
of the flat part 31a. The thicknesses T1 of the flat
part 31a and the thickness T3 of the rib 31b can be
easily adjusted by adjusting the size of the part of the
punch corresponding to the rib 31b as in the case of the
adjustment of the thicknesses L1 and L2. Here, the
distance between the top end of the rib 31b and the
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upper surface of the flat part 31a is defined as the
height T2 (u m) of the rib 31b.
Next, the closure cap 30 is made by attaching the
negative terminal and the like at the center of the
cover plate 31.
Then, the electrode group 20 composed of the
positive and negative electrodes and the separator is
inserted into the external closing 10. The negative
electrode 21 is electrically connected to the electrode
collector plate 34 using a lead plate 25. Next, the
electrolyte is poured into the external casing 10, and
the closure cap 30 is press-fit into the external casing
10 so that the top end lOc of the external casing 10 and
the rib top end 31c of the closure cap 30 have almost
the same levels.
Then, a laser beam is intermittently aimed at the
part (boundary) 40 between the external casing 10 and
the closure cap'30, i.e., the boundary between the top
end lOc of the external casing 10 and the rib top end
31c to perform laser welding.
A more detailed explanation of the laser welding
will be given below. Fig. 5 is a perspective view
showing how the external casing 10 is sealed with laser
welding.
In a device shown in Fig. 5, a condenser lens 51
can drive the optical axis in any direction in a plane
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. .1
l
parallel with the cover plate 31. A laser is guided to
the condenser lens 51 from a laser oscillator (not
illustrated) via an optical fiber.
The laser oscillator is a device that has
yttrium-aluminum-garnet (YAG) emit light and outputs a
pulsed laser 50 (for instance, laser pulse repetition
rate: 50pps). The laser 50 converges on the boundary
40 between the cover plate 31 and the rim l0a of the
external casing 10 by passing through the condenser lens
51 to form a small round spot 52 (the spot diameter:
few hundred jim) .
By doing so, the part corresponding to the spot
52 is locally molten without inflicting any heat damage
on the elements (for instance, the insulating sleeve 26)
surrounding the molten part.
In the parts corresponding to the spot 52 on
which the laser has been radiated, the outer edge (the
rib 31b) of the cover plate 31 and the rim l0a of the
external casing 10 are molten to form a molten pool:
The molten pool solidifies in a short period of time.
In Fig. 5, the reference number 60 indicates a welded
part where a molten pool has solidified.
Note that a jet of inert gas (nitrogen gas) is
emitted to the part surrounding the spot 52 of the laser
50 to prevent the welded part from oxidizing.
The laser pulse repetition rate of the oscillator
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and the scan rate of the condenser lens 51 are adjusted
so that the spot 52 of the laser 50 and an immediately
preceding spot, a spot 52a have a moderate overlap
(generally, 40 to 60% overlap ratio).
As has been described, the laser 50 is converged
on and projected onto the boundary 40 by the condenser
lens 51 and the boundary 40 is scanned by moving the
condenser lens 51 along the boundary 40 (in the
direction of an arrow "A" in Fig. 5) to consecutively
form welded parts 60. When the welding of the outer
edge of the cover plate 31 and the rim l0a along the
boundary 40 is completed, the sealing is finished.
Unlike in the conventional method, in which the
outer edge of the flat cover plate is fixed to the rim
of the external casing by welding, the cover plate 31
with rib 31b is welded in the manufacturing method of
the battery 1. As a result, the battery 1 has the rib
31b on the outer edge of the cover plate 31 as shown in
Fig. 2 in the finished form. Some batteries, however,
have no rib in the finished form due to the laser beam
energy or the height of the rib.
While a more detailed explanation will be given
later, the top end of the rib is welded by a laser beam
according to the manufacturing method of sealed battery,
so that it is assumed that smaller amount of heat energy
is transmitted from a molten pool toward the center of
CA 02578827 2007-03-09
the closure cap and a molten pools is difficult to be
cooled compared with the case in which a conventional
flat cover plate is used.
Meanwhile, the thickness T3 is set smaller
compared with the thickness of the cover plate, so that
the area to which the heat energy of a molten pool is to
be transmitted is much more small. As a result, it is
assumed that a molten pool is more difficult to be
cooled.
In addition, the thickness Li of the external
casing 10 at the rim l0a is set smaller than the
thickness L2 of the main body lOb, so that it is much
more difficult to transmit the heat energy of the laser
beam from a molten pool.
As has been described, it is difficult to
transmit heat energy from a molten pool, so that it is
assumed that the heat energy given by the laser beam is
accumulated in a molten pool and the temperature of a
molten pool decreases extremely slow compared with the
case of the conventional sealing method. As a result,
the heat stress during the sealing process can be
reduced. This leads to less incidence of cracking in
the molten pools, so that productivity is expected to be
improved.
[Effectiveness of the Shapes of the External Casing 10
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and the Closure Cap 30 before Welding]
Here, the effectiveness of the shapes of the
external casing 10 and the closure cap 30 before welding
will be examined in detail.
In order to study the relationship between the
heat stress at laser welding and the size of the rib 31b
before welding and between the heat stress and the
thickness of the external casing 10 before welding, the
inventors carried out the experiment by simulation as
follows, considering the laser welding is a heat
processing method by absorbing laser beam.
An analytical model as shown in Fig. 6 that is a
piece of shielded part (welded part) with laser welding
and is divided in a mesh form is used. As a result of
analysis using this analytical model, according to the
finite-element method (reference; Japan Society of
Mechanical Engineers, "Computer Analysis of Heat and
Flow (Netsu to Nagare no Komputa Anarisisu)" (Corona
Publishing Co., Ltd., 1986), and in accordance with-
Equation 1 (a three-dimensional nonlinear non-stationary
heat conduction equation) and Equation 2 that are given
below, the heat stress arising from the temperature
distribution at the laser shielded part is calculated.
In order to improve the analysis precision, the mesh is
especially fine around the laser beam spots, where the
temperature gradient is steep, as shown in Fig. 6.
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Equation 1
p~~ T - 8 8 x(KS S x T )+ S 8T ~(K T)+Q
8 (Kc~
y y )+Sz Sz
p : density
c : specific heat
T :temperature
k : heat conduction coefficient
LQ: heat input amount
Equation 2
a = -E a(t-to)
t : temperature
J t0 : initial temperature
J E : Young's modulus
L v : heat stress
17a
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~
Main analytical conditions are as follows. The
laser is a YAG laser. The wavelength is 1.0611m. The
laser power is 9.3xi0W. The beam diameter is 450u m.
The pulse width is 12.Oms. The analytical time is set
at 15.Oms, considering the period of time required from
laser beam radiation via temperature increase, melting,
solidification, to returning to a low temperature.
Fig. 7 shows the result calculated by the
analysis. Fig. 7 is a plot showing the relationship
between the height T2 of the rib 31b and the heat stress
at the center of the laser spot (N/cm2). Here, being
greatest at the center of the laser spot, the heat
stress at the center of the laser spot is calculated.
The sizes excluding the numerical value that
changes (the height T2), i.e., the thicknesses T3, L1,
and L2, are set at 500u m. Under this condition, the
heat stress is likely to be great.
As shown in Fig. 7, the higher the height T2 of
the rib 31b, the less the heat stress, and the smaller
the thickness T1 of the flat part 31a, the less the heat
stress.
When the heat stress at a molten pool exceeds the
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limit of the tensile strength of the external casing and
the closure cap (the tensile strength limit of the
material used here is 4x103N/cm2), cracking arises. As a
result, it is necessary to design the closure cap so
that the heat stress of a molten pool is no greater than
the tensile strength. Fig. 7 shows that it is desirable
to determine the thickness T1 of the flat part 31a and
the height T2 of the rib 31b so as to satisfy Equation 3
given below.
[Equation 3)
T2L"T1/10+40
Equation 3 is illustrated by Fig. 8.
The plot in Fig. 8 shows that in order to keep
the heat stress less than the tensile strength limit,
i.e., in order to satisfy Equation 3, it is necessary to
design the battery 1 so as to satisfy the conditions
within the diagonally shaped area in Fig. 8.
Then, the relationship between the thickness T3
of the rib 31b and the heat stress at the center of the
laser spot is calculated when the height T2 of the rib
31b is the same, the heat stress is great, and the
thickness T1 is 500,U m according to the analysis result.
Here, the height T2 is set at 90,u m, the thicknesses Li
and L2 are set at 500j,Im. Fig. 9 is a plot showing the
19
CA 02578827 2007-03-09
~
result of the calculation.
As shown in Fig. 9, when the thickness T3 of the
rib 31b is the same as the thickness Ti of the flat part
31a, the heat stress at a molten pool is almost the same
as the tensile strength limit (here, 4xl03N/cm2). On the
other hand, when the thickness T1 of the flat part 31a
is fixed and the thickness T3 of the rib 31b of the
cover plate 31 is set as smaller, the heat stress
becomes less. This shows that it is effective to set
the thickness T3 of the rib 31b as no greater than the
thickness Tl of the flat part 31a in order to have the
heat stress at a molten pool be less than the tensile
strength limit and to prevent cracking.
As a result, considering the circumstances under
which the thickness T3 of the rib 31b, which is decided
according to the mechanical strength of the aluminum
alloy plate, should be set as at least 50/1m, it is
preferable to set the thicknesses T3 of the rib 31b and
the thickness T1 of the flat part 31a so as to satisfy
Equation 4 given below.
[Equation 4]
50 [ um] ST3ST1
Further, the relationship between the thickness
L1 of the external casing 10 at the rim l0a and the heat
CA 02578827 2007-03-09
~
stress at the center of the heat spot is calculated when
the thickness of the external casing 10 at the main body
lOb is set as 500jim. Here, the height T2 is set as
90j.lm, the thicknesses T1, T3, and L2 are set as 500j,1m.
Fig. 10 is a plot showing the result.
As shown in Fig. 10, when the thickness L1 of the
external casing 10 at the rim l0a is the same as the
thickness L2 of the main body lOb, the heat stress at a
molten pool is almost the same as the tensile strength
limit (4x103N/cm2) . On the other hand, when the
thickness L2 of the main body lOb of the external casing
is fixed and the thickness L1 at the rim 10a is set as
smaller, the heat stress becomes less. This shows that
it is effective to set the thickness Ti of the external
casing 10 at the rim l0a as no greater than the
thickness T2 of the main body lOb in order to have the
heat stress at a molten pool be less than the tensile
strength limit and to prevent cracking.
As a result, considering the circumstances under
which the thickness L1 of the external casing 10 at the
rim l0a , which is decided according to the mechanical
strength of the aluminum alloy plate, should be set as
at least 50Mm, it is preferable to set the thicknesses
L1 at the rim 10a and L2 at the main body lOb of the
external casing 10 so as to satisfy Equation 5 given
below.
21
CA 02578827 2007-03-09
~
[Equation 5]
50 [ Jim] SL1SL2
When the thickness L1 of the external casing 10
at the rim l0a is determined so as to satisfy the
equation, the rim 10a and the rib 31b of the cover plate
31 are welded together more firmly. In this respect, it
is preferable to determined the thickness L1 at the rim
l0a so as to Equation 5.
In Fig. 4, the thickness L1 of the external
casing 10 at the rim l0a is set smaller than the
thickness L2 of the main body lOb so that the external
diameter at the rim l0a is the same as that at the main
body lOb and the internal diameter at the rim l0a is
greater than that at the main body lOb. The thickness
Li may be set smaller than the thickness L2 so that the
internal diameter r at the rim 10a is the same as that
at the main body lOb and the external diameter at the
rim 10a is smaller than that at the main body lOb as
shown in Fig. 11. The thickness difference can be
easily adjusted by making the diameter of the part of
the die corresponding to the rim 10a smaller.
Note that it is preferable to set the internal
and external diameters of the external casing as shown
in Fig. 4 so that the smaller thickness of the external
casing at the rim works effectively to fix the cover
22
CA 02578827 2007-03-09
~
plate that is fit into the opening of the external
casing.
(Practical Example)
A plurality of the batteries 1 are manufactured
on condition that the thickness L1 of the external
casing 10 at the rim 10a is set at 400j.1m, the thickness
L2 of the main body lOb at 550/1m, the thickness Tl of
the flat part 31a of the cover plate 30 surrounded by
the rib 31b at 500jim, the height T2 of the rib 31b at
500,t.1m, the thickness T3 of the rib 31b at 400/im, and
the thickness of the insulating sleeve 26 as 800i1,lm.
The laser radiation conditions are as described below.
The incidence of cracking is checked. Note that the
sizes of the elements used for manufacturing the
batteries satisfy Equations 3 to 5.
Laser Radiation Conditions
Laser Wavelength: 1.064jim (YAG laser)
Laser Pulse Repetition Rate: 50pps (pulse/second)
Laser Spot Diameter: 500j1m
Laser Beam Scanning Speed: 15mm/s
(Comparative Example)
As the comparative example of the present
invention, a plurality of sealed batteries are
manufactured as follows. The conventional flat cover
23
CA 02578827 2007-03-09
plate without rib and the conventional external casing
the thickness at the rim is the same at the main body
are used. The cover plate is fit into the opening of
the external casing so that the top end of the rim and
the upper surface of the cover plate have the same
level. The cover plate and the external casing are
welded together using a laser on the same laser
radiation condition as described above. The incidence
of cracking for the sealed batteries is checked. Note
that the thickness of the external casing is set at
500/.1m and that of the cover plate at 800jim.
The result of the experiment is shown in Table 1.
[Table 1]
incidence of cracking
present invention <1.0%(less than 10%)
comparative example 27.0%
As shown in Table 1, as high as 27% for the
comparison example, the incidence of cracking can be
decreased as low as 1% for the practical example by
manufacturing batteries using the cover plate and the
external casing that have inventively-changed shapes.
This proves that the heat stress in the welded
part can be reduced and eventually the cracking can be
suppressed for a sealed battery that has been
24
CA 02578827 2007-03-09
~
manufactured according to the first embodiment, i.e.,
proves the practical effectiveness of the manufacturing
method.
Fig. 12 is a plot showing the heat stress in the
molten part for each number of laser radiations.
The plot in Fig. 12 shows that the heat stress
becomes slightly great at the second radiation but
remains almost the same after that.
While the maximum value of the heat stress is
about 6.Ox103N/cm2 for the conventional manufacturing
method, the maximum value for the manufacturing method
according to the practical example is less than
4. Ox103N/cm2.
Considering the fact that the tensile strength of
the aluminum alloy used is about 4.Ox103N/cm2 according
to the manufacturing method of rectangular sealed
battery of the practical example, it is concluded that
this manufacturing method is effective to set the
maximum heat stress as less than the tensile strength.
[The Second Embodiment]
A specific explanation of another embodiment will
be given below in accordance with figures.
[Battery Structure]
Fig. 13 is a perspective view of a rectangular
CA 02578827 2007-03-09
sealed battery 100 according to another embodiment
(referred to the "battery 100" hereinafter). In Fig.
13, the same elements as in Fig. 1 have the same
reference numbers.
[Manufacturing Method of Battery]
An explanation of the method of manufacturing the
battery 100. Note that the shapes of the cover plate 31
and the external casing are the same as in the
conventional rectangular sealed battery. More
specifically, the cover plate 31 is a flat plate and the
thickness of the external casing is designed to be the
same at the rim 10a and at the main body.
Fig. 14 is an enlarged sectional view of the main
structure of the battery 100 when the cover plate 31 is
fit into the opening of the external casing 10. Fig. 14
shows a part of the battery 100 around the outer edge of
the cover plate 31 and the rim of the external casing.
First, the external casing 10 is made by forming
an Al-Mn alloy plate into a bottomed rectangular
cylinder. Meanwhile, the cover plate 31 is made by
punching out an Al-Mn alloy plate.
More specifically, the external casing 10 is
formed by transfer drawing an aluminum alloy flat plate
using a punch or an ironing die. On the other hand, the
cover plate 31 is made by punching out an aluminum alloy
26
CA 02578827 2007-03-09
flat plate using a punch.
Then, a group of predetermined elements
(including the insulating packing, the negative
electrode terminal, and the electrode collector plate)
is attached to the cover plate 31 and the washer is
attached to the negative electrode terminal at the top
by caulking to form the closure cap 30.
Next, into the external casing 10, the electrode
group, which has been made, is inserted, and the
negative electrode and the collector plate are
electrically connected. Then, the electrolyte is poured
into the external casing 10, and the closure cap 30 is
press-fit into the external casing 10 so that the top
end lOc of the external casing 10 and the upper surface
31d of the closure cap 31 have the same level.
The outer edge of the cover plate 31 and the rim
of the external casing 10 are welded together by aiming
a laser at the boundary 40 between the outer edge and
the rim. By doing so, the battery 100 is completed:
[Sealing with Laser Welding]
Fig. 15 is a plan view showing how the external
casing is sealed with laser welding.
The device shown in Fig. 15 includes beam
homogenizers 120= =. and a projection lens 130 for
projecting light pencils generated by the beam
27
CA 02578827 2007-03-09
homogenizers 120= ==. The beam homogenizers 120= ==
and the projection lens 130 are integrally driven in any
direction in a plane parallel with the cover plate 31.
The beam homogenizers 120= =- are process
lenses for dividing a laser 140, which has been guided
from the laser oscillator (not illustrated) via the
optical fiber, into finer light pencils 141 ===.
The projection lens 130 projects the light
pencils 141= =. onto a place including the boundary 40,
which is the part to be welded, so that the centers and
the diameters of the light pencils 141= =- overlap one
another to form a round laser spot.
As has been described, the laser 140 is divided
into finer light pencils and the divided laser is
radiated so as to overlap to generate a laser spot 150,
which has the highest energy at the center and the
energy does not substantially change around the center
(a detailed explanation of the influence will be given
later).
The laser oscillator has yttrium-aluminum-garnet
(YAG) emit light and outputs the pulsed laser 140 (for
instance, laser pulse repetition rate: 50pps).
In the part corresponding to the laser spot 150
on which the laser has been radiated, the outer edge of
the cover plate 31 and the rim of the external casing 10
melt to form a molten pool. The molten pool solidifies
28
CA 02578827 2007-03-09
~
in a short period of time. In Figs. 14 and 15, the
reference number 110 indicates a welded part, which is a
solidified molten pool.
Note that inert gas (nitrogen gas, not
illustrated) is emitted to the part surrounding the
laser spot 150 to prevent the welded part from
oxidizing.
As in the case of the first embodiment, the laser
pulse repetition rate of the laser oscillator and the
scan speed of the laser spot 150 are adjusted so that
the laser spot 150 and an immediately preceding spot
have a moderate overlap (generally, 40 to 60% overlap
ratio ) .
The laser spot 150 is radiated so that the center
of the flat part of the energy distribution is
positioned at the boundary 40. The center of the laser
spot, where the energy is greatest, is positioned at the
boundary 40 in order to weld the external casing and the
cover plate together most firmly where the external-
casing and the cover plate come into contact.
Certainly, this welding causes no heat damages to the
surrounding elements (for instance, the insulating
sleeve 26). Only the part corresponding to the laser
spot 150 is locally molten.
As has been described, while the laser is
projected onto the boundary 40 by the projection lens
29
CA 02578827 2007-03-09
130, the boundary 40 is scanned by moving the beam
homogenizers 120= =. and the projection lens 130
integrally along the boundary 40 (in the direction of an
arrow "Al" in Fig. 14) to consecutively form welded
parts 110 along the boundary 40. When the welding of
the external casing and the cover plate along the
boundary 40 is completed, the sealing is finished.
[Laser Beam Energy Distribution and the Influence on
Welding]
The energy distribution of the laser used in the
sealing process is shown in Fig. 16(a). For reference
purposes, the energy distribution of the laser that has
been conventionally used is shown in Fig. 16(c).
Figs. 16(a) and 16(c) show the energy
distribution from the center to the edge of a round
laser spot with reference to the energy at the center.
As shown in Fig. 16(a), the energy distribution
of the laser spot is different from a general Gaussian
distribution.
More specifically, as shown in Fig. 16(a), there
is a flat part around the center of the laser spot where
the energy does not substantially change.
This energy distribution restricts the phenomena
of molten metal flowage, i.e., the Marangoni convection.
The lower the temperature, the higher the surface
CA 02578827 2007-03-09
tension of molten metal. As a result, molten metal with
higher temperature is pulled by molten metal with lower
temperature. This leads to the flowage of the molten
metal. In the case of the laser with the energy
distribution as the Gaussian distribution shown in Fig.
16(c), the energy is greatest at the center of the laser
spot and smallest at the edge. As a result, the
temperature of a molten pool is highest at the center of
the laser spot and lowest at the edge. This temperature
gradient generates a flowage (Marangoni convection) of
molten metal that circulates in the order of the top
center of the molten pool, the edge, the bottom, and the
top center (as indicated by arrows "B" in Fig. 16(d)).
As a result, dome-shaped welded mark is formed.
On the other hand, the laser in the present
embodiment has the energy distribution with a flat part
as has been described. In the molten pool corresponding
to the flat part has no substantial temperature
gradient. As a result, the Marangoni convection due to
temperature gradient can be restricted in the molten
pool.
The local restriction of the Marangoni convection
leads to effective transmission of laser energy to the
bottom of the molten pool, so that the boundary 40 can
be molten more deeply (refer to Fig. 16(b)). As a
result, a battery with improved sealing can be obtained.
31
CA 02578827 2007-03-09
~
Note that some degree of Marangoni convection (as
indicated by arrows "C" in Fig. 16(c)) arises at the
edge of a laser spot, so that a so-called silk-hat-
shaped welded mark (which will be described later) is
formed.
In addition, the restriction of the Marangoni
convection can reduce the heat stress. The heat stress
arises when a molten pool is rapidly cooled and the
degree of heat stress depends on the fluidity of the
molten metal. More dynamically the molten metal flows,
more strongly the molten metal is pulled, so that the
heat stress becomes greater. As a result, the
restriction of the Marangoni convection can reduce the
heat stress that arises from the molten metal flowage.
In addition, the heat stress is expressed by a function
of temperature change as indicated by Equation 2, so
that easy temperature gradient can reduce the heat
stress. As has been described, the heat stress can be
reduced in the welded part, so that cracking can be~
suppressed. .
Note that the effect can be obtained only when
energy is distributed so that the temperature gradient
of the molten metal is easier than the conventional beam
having the Gaussian distribution.-
Meanwhile, the effect of heat stress reduction
depends on the ratio of the flat part of the energy
32
CA 02578827 2007-03-09
distribution of the beam to the laser spot. More
specifically, if the flat part proportion is too small,
the temperature gradient is seen in a relatively wide
range, so that the Marangoni convection cannot be
restricted. As a result, the flat part proportion needs
to be determined in consideration of the diameter of the
beam spot.
In addition, the energy change in the flat part
is allowable as long as the effects of restricting the
Marangoni convection and cracking are obtained. In this
respect, the flat part does not indicate that the energy
never changes in this part.
Especially, the effect is profound when Equation
6 given below is satisfied.
Equation 6
dP < 0.05
Pc
area occupied by the flat part ~ 0.2W
provided
W: diameter of laser spot
Pc : energy at the center of laser spot
dp : finite difference between Pc and energy at predetermined
position of laser spot
Notes: Not explained in detail, the grounds are
derived from the simulation analysis under the
conditions described below.
33
CA 02578827 2007-03-09
Main conditions of the simulation are as follows.
The thickness of the external casing 10 and the
cover plate 31; 5001.1m
The YAG laser wavelength; 1.064/1m
The diameter of the radiation spot; 60011m
The radiation energy; 60W, 70W, or 80W per spot
Note that the energy distribution satisfying
Equation 6 cannot be obtained by the laser having the
Gaussian distribution. As is evident from the fact that
the energy I(x) of the Gaussian distribution is
indicated in Equation 7 described below, the part in
which the energy fluctuation from the energy at the
center is less than 5% accounts for less than 20% of the
laser spot.
Equation 7
I(x) = exp l- 2 2 X x2~
provided
cv : radius of laser spot
x : distance from center of laser spot
34
CA 02578827 2007-03-09
. '~
[Shape of Welded Mark]
Welding by the laser having the energy
distribution that has been described, a welded mark 110
15 has a unique shape different from the conventional
welded mark.
Fig. 17(a) is a perspective view of a welded mark
160 formed by a laser spot for welding the external
casing and the cover plate and Fig. 17(b) a sectional
20 view of the central part of the welded mark.
Figs. 17(a) and 17(b) show that the welded mark
160 is formed as follows. In the part corresponding to
the diameter of the flat part at the center of the laser
spot, a molten pool is formed by going straight. In the
25 upper part of the molten pool (the outer regions of the
laser spot), some amount of Marangoni convection arises.
34a
CA 02578827 2007-03-09
As a result, a so-called silk-hat-shaped welded mark is
formed by a planiform dome-shaped first welded mark 161
that has a gently-curved boundary between the non-molten
part and a second welded mark 162 that has an upper
diameter R2, which is smaller than an upper diameter Ri
of the first welded mark 162.
Here, formed by the go-straight energy of the
laser spot corresponding to the flat part of the laser
spot energy distribution, so that the second welded mark
162 has a boundary between the non-molten part that is
steeper than that of the first welded mark 161.
On the other hand, when the external casing and
the cover plate are welded with a laser having the
Gaussian distribution, the Marangoni convection arises
in the molten pool as a whole, so that a dome-shaped
welded mark is formed.
As has been described, the shape of the welded
mark formed by a laser spot according to the welding
method of the present embodiment is quite different' from
that in a conventional method. Due to the unique shape
of the welded mark, when the total amount of the energy
of the laser spot is set the same, the degree of the
progress (depth) of the welded part in the direction of
laser radiation (in the perpendicular direction) is
greater in the case of the present embodiment, so that
better sealing quality can be obtained according to the
CA 02578827 2007-03-09
1
present embodiment.
In reality, however, laser is consecutively
radiated and welding is consecutively performed, so that
the form of the welded part 110 that is formed in the
boundary 40 is different according to the laser pulse
repetition speed of the laser oscillator and the scan
speed of the laser spot. More specifically, the overlap
ratio of the laser spot 150 differs according to the
laser pulse repetition rate and the laser spot scan
speed, and the shape of the welded mark differs
according to the overlap ratio.
For instance, in the case of laser radiation in
which the outer regions of laser spots slightly overlap
one another (in case 1), a series of welded marks having
almost the same shape as the welded mark 160 are formed.
On the other hand, in the case in which the
regions of laser spots closer to the centers overlap one
another (in case 2), since the parts corresponding to
the flat part of the energy distribution have overlaps,
the vertical sectional view of the welded part in the
direction of the laser is the same as the shape shown in
Fig. 17(b). The vertical sectional view is shown in
Fig. 18. The welded mark having so-called silk-hat-
shaped vertical sectional view is formed along the outer
regions of the upper part of the battery.
As in the case of the conventional method, the
36
CA 02578827 2007-03-09
welded part has different shape according to the overlap
ratio of the laser spot. In the case 1, however, a
series of dome-shaped welded marks is formed as a whole,
and in the case 2, the welded mark has the dome-shaped
vertical sectional view in the direction of the laser
radiation.
As has been described, the shape of the welded
part that is finally formed along the boundary between
the external casing and the cover plate in the present
embodiment is totally different from that in the
conventional method. More specifically, in the case of
the sealed battery of the present embodiment, the second
welded part that goes straight in the direction the
height of the molten pool is formed, so that the welded
mark has the shape so as to secure larger welded area of
the external casing and the cover plate compared with
the conventional welding method. As a result, the
battery has an improved sealing effect of separating the
generator elements from outside of the battery, i.e:,
the battery is resistant to electrolyte leakage and has
higher security and reliability.
Note that it is preferable that the boundary is
scanned so that the flat parts of the welded marks
overlap one another in the respect of sealing quality
improvement since the welded regions become larger.
37
CA 02578827 2007-03-09
(Practical Example)
Rectangular sealed batteries are manufactured
according to the present embodiment.
The thickness of the external casing 10 and the
cover plate 31 is 500it1m.
At the time of welding, the YAG laser wavelength
is 1.064/1m, the diameter of the radiation spot is 50011
m, and the radiation energy is 60W, 70W, or 80W per
spot.
(Comparative Example)
Rectangular sealed batteries are manufactured in
the same manner as the practical example except for the
energy distribution of the laser.
(Experiment 1)
Sealed batteries are manufactured in the method
of the practical and experimental examples, and the heat
stress is analyzed. The result of the analysis is shown
in Table 2. Note that the greatest heat stress at the
part where the laser has been radiated is written in
Table 2.
38
CA 02578827 2007-03-09
Table 2
beam shape power(W) heat stress(X 103N/cm2)
conventional 60 5.5
example
70 6.2
80 6.9
practical 60 3.3
example 70 4.1
80 4.6
As is evident from Table 2, in the welding
38a
CA 02578827 2007-03-09
~
according to the practical example, the heat stress is
considerably reduced.
(Experiment 2)
Batteries are manufactured in the method of the
practical and comparative examples, and the yield ratio
(the ratio of batteries without cracking) is checked.
As a result, in the welding according to the
practical example, the incidence of cracking is
considerably reduced compared with the comparative
example. The yield ratio of the practical example in
the manufacturing method described above is no less than
95%. -
While the shape of the laser is changed with the
beam homogenizers and the projection lens in the
explanation, the devices are not limited to the beam
homogenizers and the projection lens. For instance, the
laser shape can be changed with an expander, a mask, and
a projection lens. More specifically, the laser from
the laser oscillator is expanded with the expander and
the central part is projected onto the welded part with
the mask and the projection lens to form the flat part.
While the laser spot has the shape of circle in
the present embodiment, the shape of the laser spot is
not limited to a circle. The laser spot can be an oval
figure or a polygon. Note that when the laser spot has
39
CA 02578827 2007-03-09
a shape other than a circle, the spot diameter W can be
indicated by the maximum diameter passing the barycenter
of the laser spot, for instance.
[The Third Embodiment]
An explanation of a further embodiment will be
given below.
Fig. 19 is a perspective view of a rectangular
sealed battery 200 (referred to the "battery 200"
hereinafter) according to the present embodiment. In
Fig. 19, the same elements as in Fig. 13 have the same
reference numbers.
In respect of manufacturing the battery 200, the
methods of making the cover plate, the external casing,
and the other elements are the same as in the second
embodiment, so that any more explanation of the
manufacturing method of these elements will not be
given.
Here, the method of welding the cover plate 'and
the external casing is different from that in the second
embodiment. A detailed explanation of the welding
method will be given below.
[Sealing with Laser Welding]
Fig. 20 shows how the external casing is sealed
with laser welding.
CA 02578827 2007-03-09
Among the devices shown in Fig. 20, a condenser
lens 210 can drive the optical axis in any direction in
a plane parallel to the cover plate 31. To the
condenser lens 210, a laser 220 from the laser
oscillator (not illustrated) is guided via an optical
fiber.
The laser oscillator has yttrium-aluminum-garnet
(YAG) emit light and outputs the pulsed laser 220 (for
instance, laser pulse repetition rate: 50pps).
The laser 220 converges on the boundary 40
between the cover plate 31 and the rim l0a of the
external casing 10 by passing through the condenser lens
210 to form a small round spot 230 (the spot diameter:
few hundred rum) .
By doing so, the part corresponding to the spot
230 is locally molten without inflicting any heat damage
on the elements (for instance, the insulating sleeve 26)
surrounding the molten part.
In the part corresponding to the spot 230 ori
which the laser 220 has been radiated, parts of the
cover plate 31 and the rim 10a of the external casing 10
are molten to form a molten pool. The molten pool
solidifies in a short period of time. In Fig. 20, the
reference number 240 indicates a welded part where a
molten pool has solidified.
The laser pulse repetition rate of the laser
41
CA 02578827 2007-03-09
oscillator and the scan speed of the condenser lens 210
are adjusted so that the laser spot 230 of the laser 220
and an immediately preceding spot have a moderate
overlap (generally, 40 to 60% overlap ratio).
With the radiation of the laser 220, N2 gas
supplied from an N2 gas container 250 is emitted onto
the spot 230 of the laser 220 as the assist gas (amount
of flow: 5 liters/min, for instance). Welding in the
atmosphere of the emitted assist gas is to prevent the
oxidation of the welded part.
The assist gas is supplied to the part to be
welded after heated to a high temperature by a heating
device 260 that is provided with a heater.
As a result, the molten pool that is formed at
the part corresponding to the spot 230 is gradually
cooled in the atmosphere of the high gas to ease the
heat stress and decrease the incidence of cracking in
the welded part. Note that even the assist gas is
heated, the welded part is locally heated, so that no
heat damage is inflicted on the generator element and
the like in the battery.
While a more detailed explanation will be given
later, it is preferable to set the heat temperature of
the assist gas at no lower than 400K in order to obtain
enough effects of restricting the cracking in the welded
part.
42
CA 02578827 2007-03-09
Note that apart from nitrogen gas, hydrogen gas,
oxygen, and inert gas such as argon gas can be used as
the assist gas.
As has been described, by moving the condenser
lens 210 along the boundary 40 (in the direction of an
arrow "A2" in Fig. 20) while the laser 220 is converged
on and projected onto the boundary 40 and the heated
assist gas is emitted onto the boundary 40, welded parts
240 are consecutively formed along the boundary 40.
When the welding of the external casing and the cover
plate along the boundary 40 is completed, the sealing is
finished.
[Relationship between Assist Gas Temperature and Heat
Stress]
The relationship between the temperature of the
assist gas that is emitted onto the welded part at the
time of welding, the temperature history of the welded
part, and the heat stress in the welded part (the heat
stress at the center of the spot) is analyzed according
to the finite-element method.
The heat stress at the center of the spot is
analyzed since the heat stress in the laser welding is
greatest at the center of the spot and is supposed to be
responsible for the cracking as has been described.
The analysis according to the finite-element
43
CA 02578827 2007-03-09
~
method is performed under the conditions described
below.
The thickness of the external casing 10 and the
cover plate 31: 5001im
The laser wavelength: 1.064u m
The power density of the laser: 1.5x106W/cm2 per
spot
The spot diameter of the laser: 450u m
The pulse width: 3.Oms
The analytical time from the laser radiation:
5.Oms
The cover plate melting point: 930K
The temperature at which heat starts to effect
the resin of the insulating sleeve 26: 600K
Figs. 21 and 22 are plots showing results of the
analysis. Fig. 21 shows the temperature history of the
molten part (molten pool) when the temperature of the
assist gas is set at 300K, 350K, 400K, and 800K.
The plot in Fig. 21 shows that the temperatures
of the molten part are the same until about lms has
elapsed since the start of the laser radiation (until
just after the molten part reaches at the maximum
temperature) regardless of the difference of the assist
gas temperature. After that, however, the molten part
temperatures is quite different according to the assist
gas temperature. More specifically, the speed at which
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CA 02578827 2007-03-09
the molten part cools is quite different when the assist
gas temperature is equal to or lower than 350K and the
temperature is equal to 400K or higher than 400K.
For instance, when the assist gas temperature is
300K and 350K, the temperature of the molten part
decreases to 1000K after about 1.5ms have elapsed. On
the other hand, when the assist gas temperature is 400K
and 800K, the molten part temperature decreases to 1000K
after about 3ms have elapsed.
The result shows that when the assist gas is set
at a high temperature no lower than 400K, the molten
part temperature can be kept at no lower than the
melting point (about 930K) for at least 3ms.
Fig. 22 is a plot showing the heat stress in the
molten part for each number of laser radiations when the
assist gas temperature is set at 300K, 350K, 400K, and
800K.
The plot in Fig. 22 shows that the heat stress
becomes slightly great at the second radiation but
remains almost the same after that.
Meanwhile, when the assist gas temperature is
300K and 350K, the maximum heat stress is about
4.2x103N/cm2. On the other hand, when the assist gas
temperature is 400K and 850K, the maximum heat stress is
less than 4. Ox103N/cm2 .
As a result, when considering the tensile
CA 02578827 2007-03-09
strength of the aluminum alloy used for the rectangular
sealed battery of the present embodiment is about
4.3xl03N/cm2, it is understood that the assist gas is
preferably set at no lower than 400K in order to set the
maximum heat stress as less than the tensile strength.
According to the results shown in Figs. 21 and
22, it is assumed that the maximum heat stress in the
welded part should be set as no greater than 4.3x103N/cm2
at the time of laser radiation for the aluminum-
manganese alloy in order to keep the cracking incidence
low and set the maximum heat stress in the welded part
as lower than the tensile strength of the material of
the external casing. It is understood, however, that
the welded part temperature can be kept at no lower than
the melting point for at least 3ms and the maximum heat
stress in the welded part can be kept as no greater than
4.3xl03N/cm2 by setting the assist gas at no lower than
400K.
[Experiment]
An experiment is performed as follows. The
assist gas temperature is set at 300K, 350K, 400K, and
800K, batteries are manufactured with laser sealing (at
the scan speed of 18m/sec), and the yield ratio is
checked.
Fig. 23 shows the result of the experiment. Fig.
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CA 02578827 2007-03-09
~
23 is a plot showing the relationship between the assist
gas temperature and the yield ratio.
According to the result, while the yield ratio is
96% when the assist gas temperature is 300K and 350K,
the yield ratio is 99% when the assist gas temperature
is 400K and 800K.
This proves that it is preferable to set the
assist gas temperature at no lower than 400K in order to
suppress cracking.
[The Fourth Embodiment]
Fig. 24 shows the laser sealing according to the
present embodiment.
The molten part is kept warm by the heated assist
gas when being cooled during the laser sealing in the
third embodiment. On the other hand, the assist gas is
not heated in the present embodiment. Instead, the
corners of the battery are heated by semiconductor
lasers 301 to 304 during laser sealing.
More specifically, four semiconductor lasers 301
to 304 are disposed so as to face the four corners lOd
and to radiate the laser at the corners 10d. As a
specific example of the semiconductor lasers 301 to 304,
an AlGaAs laser diode or an InGaAsP laser diode is used.
The laser welding is performed as follows as in
the case of the third embodiment. The laser 220 from
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CA 02578827 2007-03-09
the laser oscillator is converged on the boundary 40
between the cover plate and the external casing with the
condenser lens. The laser 220 is radiated onto the
boundary 40 to continuously mark a laser spot along the
boundary 40. At the corners, the laser from the
semiconductor lasers 301 to 304 is also radiated onto
the boundary 40 so as to heat the molten part. By doing
so, the molten part is gradually cooled.
When the external casing 10 of the rectangular
battery is made by forming a metal plate into a bottomed
rectangular cylinder, the residual stress is greater at
the corners 10d, where the degree of unfractuosity is
great, than in the straight-line parts 10e, where the
unfractuosity degree is small, at the rim of the
external casing 10. As a result, the corners lOd are
susceptible to cracking. When the corners 10d are
heated by the semiconductor lasers 301 to 304 during the
laser sealing, however, the corners lOd are gradually
cooled, so that the heat stress in the welded part at
the corners lOd can be decreased and the cracking can be
suppressed.
Meanwhile, the semiconductor lasers 301 to 304
can locally heat the corners lOd, so that no heat damage
is inflicted on the generator element in the battery.
According to the present embodiment, the yield
ratio can be improved as in the case of the first
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CA 02578827 2007-03-09
)
embodiment by gradually cooling the molten part by the
heating with the semiconductor lasers 301 to 304 during
the laser sealing.
While explanations of the embodiments of the
present invention have been given taking the lithium
secondary battery as the example, the present invention
can be adopted for the primary cell and the secondary
battery such as the nickel-metal hydride battery.
In addition, an aluminum alloy, which is
susceptible to cracking, is used as the material of the
external casing and the cover plate in the embodiments.
The present invention, however, can be adopted for the
case in which stainless is used.
Furthermore, explanations of the rectangular
sealed battery, which is high in practically utility,
have been given in the embodiments. The manufacturing
method of the present invention, however, can be adopted
for not only the rectangular sealed battery but also to
the cell and battery using the bottomed external casing.
Note that while no explanation has been given
about the energy distribution of the laser in the first,
third, and fourth embodiment, the energy distribution of
the laser is the Gaussian distribution.
In addition, it may be difficult to observe the
welded part, which is made when the molten pool
solidifies, as it is. In this case, however, the metal
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CA 02578827 2007-03-09
~
is molten at a high temperature and the crystal
structure of the metal changes, so that the welded part
can be observed with the electron microscope after a
predetermined electropolishing process.
When separately adopted, the embodiments are
highly effective in reducing the heat stress in the
molten part. The embodiments, however, can be adopted
in any combination and it is assumed that the
embodiments have greater effects when adopted in
combination than adopted separately.
INDUSTRIAL USE POSSIBILITY
The manufacturing method of sealed battery of the
present invention is used for the manufacturing of the
battery as the power source of a variety of electronic
equipment including portable electric appliances such as
the mobile phone, the audiovisual device, and the
computer.
