Chemistry: electrical current producing apparatus – product – and – With nonbattery electrical component electrically connected...
Reexamination Certificate
1999-06-01
2004-06-22
Walker, W. L. (Department: 1723)
Chemistry: electrical current producing apparatus, product, and
With nonbattery electrical component electrically connected...
C429S211000
Reexamination Certificate
active
06753104
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a lithium secondary battery which maintains a good charge-discharge cycle and in which safety may be secured with electricity being cut off when an excess current has occurred due to external short circuit, etc. so that the battery may not be exploded nor be ignited, and in particular, to a lithium secondary battery which may be preferably used for driving a motor of an electric vehicle, etc.
In recent years, in midst that it is eagerly desired to regulate the emission of exhaust gas including carbon dioxide and other harmful substances with the elevation of the environment protection campaign as a background, the campaign to promote an introduction of an electric vehicle (EV) and a hybrid electric vehicle (HEV) has become active in replacement of automobiles using fossil fuels such as a vehicle driven by gasoline in the automobile industry. A lithium secondary battery as a motor-driving battery acting as a key for putting such EV as well as HEV into practical use, is required to have not only huge battery capacity but also a huge battery output much affecting acceleration performance as well as gradeability of a vehicle, and on the other hand, however, a strict safe standard has been established from the point of view of securing safety since the battery is provided with high energy density.
In general, the internal electrode body of a lithium secondary battery comprises a positive electrode, a negative electrode and a separator made of porous polymer film, the positive electrode and the negative electrode being wound or laminated via the separator so that the positive electrode and negative electrode are not brought into direct contact with each other. For example, as shown in
FIG. 1
, an internal electrode body
1
of winding type is formed by winding a positive electrode
2
and a negative electrode
3
, having a separator
4
in between, and tabs
5
are provided for each of positive and negative electrodes
2
,
3
(hereafter referred to “electrodes
2
,
3
”) respectively. And, the ends opposite to the ends connected with electrodes
2
,
3
of each tab
5
are attached to an external terminal (not shown) or an electric current extracting terminal (not shown) being conductive to the external terminal. That is, the tab
5
serves to act as a lead line (a current path) being conductive to the external terminal, etc. together with conducting current collecting from electrodes
2
,
3
.
Here, a plan view of the electrodes
2
,
3
when the internal electrode body
1
is spread out is shown in FIG.
2
. The electrodes
2
,
3
are formed with an electrode active material being coated respectively onto metal foils
15
made of aluminum, etc. for the positive electrode
2
and made of copper for the negative electrode
3
respectively as current collecting bodies, thus forming an electrode active material layer
16
.
The tab
5
is provided on one side of such a metal foil
15
, and those having thin band shape are preferably used so that the portion where the tab
5
of the electrodes
2
,
3
are attached may not swell to the direction of a periphery when the internal electrode body
1
was formed. In addition, they are preferably disposed in approximately uniform distance so that one tab
5
conducts current collecting from a constant area in the electrodes
2
,
3
. Incidentally, in general, a material to be used for the tab
5
is the same as a material of the metal foil
15
to which the tab
5
is attached.
Incidentally, with respect to a lithium secondary battery for an EV or an HEV, it is necessary to use lithium secondary batteries with a voltage of around 4 V at the highest for a single battery, such single batteries in plurality being connected in series, since a constant voltage is required to drive a motor, and, however, there is a case where discharge of a large current not less than 100 A is required to obtain the herein desired acceleration performance or gradeability. For example, maintaining that 200 V with 100 A be required and 3.6 V be an average terminal voltage at the time of discharge thereof, 56 units of single batteries are required to be connected in series, resulting in 100 A current flowing at each single battery at this time.
The internal configuration of a battery must be designed so that also in the case where such a huge current flows, the battery may normally operate while the output loss is suppressed as low as possible. Therefore, paying attention to the current path from the above described internal electrode body
1
and the external terminal, it is deemed preferable that the resistance of members themselves of the electrodes
2
,
3
as well as the metal foils
15
, or the tabs
5
and the external terminals, etc. all configuring the electrodes
2
,
3
is small.
However, judging from the point of view of securing battery capacity as well as securing mechanical strength of electrodes, few degrees of freedom in setting quantity of the electrode active material layer
16
configuring the electrodes
2
,
3
and sizes of the metal foils
15
are permitted while as concerns the electric current extracting terminal
13
, normally considering the shape of batteries, or the energy density thereof, the quantity of the maximum discharge current, light-weight low-resistance members with resistance values not more than a predetermined value within a range which is possible to set are used.
On the other hand, the tab
5
has an allowable range to set up a resistance value on a point of view of feasibility to set up its shape freely as far as the shape of the tab
5
is to be housed in the space between the battery case housing the internal electrode body
1
therein and the internal electrode body
1
. Metal members are used for the tab
5
, whose resistance value is generally made smaller, nevertheless, the rate of the resistance value of the tab occupying the total internal resistance of a lithium secondary battery is not necessarily small, and cannot be ignored.
On condition that a plurality of above described tabs
5
in the shape of foil band are used, a tab
5
adopting a larger cross-sectional area to make the resistance value smaller will result in introducing a situation where energy density of a battery gets decreased since the total weight of the tabs
5
will become heavier in spite that effective decrease in the internal resistance and effective decrease in the output loss is expected.
On the contrary, making the cross-sectional area of the tabs
5
smaller decreases the total weight of the tabs
5
and increases the battery's energy density, but on the other hand there will occur such problems that the resistance value of the tabs
5
increases, the tabs
5
the fuse due to increase in output loss because of increase of internal resistance or heat generated by current, and thus functioning as a battery will disappear. Accordingly, from the standpoint to avoid such problems and to do well both in reducing output loss and in increasing energy density, cross-sectional area of not less than a certain value is required for the tabs
5
.
On the other hand, with respect to the above described problems, it is feared that an accident involving an explosion or an ignition may occur when a great current has been discharged at a time due to an external short circuit, etc. since a lithium secondary battery has higher energy density, and for the purpose of avoiding such situation in advance, “Guideline for safety evaluation on secondary lithium cells (hereafter referred to SBA Guidelines)” published by Battery Association of Japan regulates that a lithium secondary battery should be free of burst or ignition to be evidenced by an external short circuit test. To fulfill such a standard, in a lithium secondary battery various safety devices such as a current-limiting mechanism comprising a PTC element, a release mechanism of battery's internal pressure involving safety valves, and pressure joints, etc. are incorporated or proposed.
Here, a current fuse is
Burr & Brown
NGK Insulators Ltd.
Sorkin David L.
Walker W. L.
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