Chemistry: electrical current producing apparatus – product – and – Means externally releasing internal gas pressure from closed... – Blowout type
Reexamination Certificate
1998-07-13
2002-02-05
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Means externally releasing internal gas pressure from closed...
Blowout type
C429S053000
Reexamination Certificate
active
06344292
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a lithium secondary battery which can be easily fabricated, has small internal resistance, and is superior in operational safety.
Attention has been paid to realization of practical use of a lithium secondary battery as a secondary battery which is small, has a large energy density, and can be charged and discharged, and which can be used as a power source for an electric equipment such as a portable communication equipment and a notebook-sized personal computer, the electric equipment being rapidly miniaturized in recent years. Moreover, concerns for resource saving and energy saving are raised on the background of international protection of the earth environment, and that is one of the reasons why the lithium secondary battery is expected as a motor driving battery for an electric vehicle which is under consideration for the positive introduction to the market in the automobile industry, or also as an effective means for using electric power by storing the electric power in the night. Thus, it is eagerly desired to put a large capacity lithium secondary battery, which is suitable for these uses, into early practical use.
In the lithium secondary battery, a lithium transition metal compound oxide or the like is used as a positive active material, while a carbon material such as hard carbon or graphite is used as a negative active material. At charging, lithium ions in the positive active material are transferred to and captured by the negative active material through an electrolyte obtained by dissolving a lithium ion electrolyte in an organic solvent. At discharging, a reverse battery reaction occurs.
Like this, the lithium secondary battery is a secondary battery which can be charged and discharged, and has properties that its voltage is higher than a conventional manganese battery and its energy density is high. Thus, the lithium secondary battery is provided with a safety mechanism for preventing accidents such as a burst of the battery caused from the rise of a battery temperature by abnormalities at the time of charging or discharging, for example, over-discharging due to a short circuit of an output terminal, rapid charging or over-charging due to malfunction of a charging device, application of a reverse connection potential by erroneous use of a user, and the like.
In a small battery as an example, as shown in
FIG. 13
, one end of a positive lead line
11
is connected to a positive electrode (not shown), and the other end thereof is connected to an internal terminal
13
having a communicating hole
12
. The internal terminal
13
is electrically connected to a pressure switch plate
15
having a burst groove
14
through contact B. The pressure switch plate
15
is connected to an output terminal
17
through a PTC element
16
. The internal terminal
13
and the pressure switch plate
15
are isolated from each other through an insulator
18
so that when the contact B is separated by the increase of internal pressure of the battery, the electrical conduction is lost. Reference numeral
19
denotes a battery case.
In the battery having such a safety mechanism, in the case where the temperature of the battery itself is raised due to the occurrence of an abnormality in the state of use of the battery, when the PTC element
16
reaches a predetermined temperature, the resistance thereof is abruptly increased so that current hardly comes to flow. As a result, the battery reaction is restrained and the rise of temperature is suppressed.
Here, as this kind of PTC element
16
, a mixture of conductive particles and polymer is generally used. The mixture has such properties that at room temperature, the mixture has low resistance since the conductive particles form a current path, while at a temperature higher than a certain temperature, the mixture comes to have high resistance close to an insulator since the molecular structure of the polymer is changed so that the current path formed of the conducive particles is cut to pieces, and when the temperature lowers again, the polymer returns to the original structure so that the current path of the conductive particles is again formed and the resistance is returned to a low resistance value.
If the internal pressure of the battery is increased although a current is restricted by the PTC element
16
, and the internal pressure exceeds the welding strength of the contact B between the pressure switch plate
15
and the internal terminal
13
, the contact B separates so that the connection between an internal electrode body and the output terminal
17
is completely cut off and the battery reaction comes not to occur. However, if the internal pressure is increased in spite of this, the burst groove
14
bursts so that the internal pressure of the battery is released to the atmospheric pressure, that is, such a structure is adopted that the pressure release mechanism operates.
In addition to the above described various safety mechanisms, such a safety mechanism is provided that by using a porous macromolecular film, such as polyethylene with a low softening point, as the separator for separating the positive electrode from the negative electrode in the internal electrode body, when the battery temperature is raised, the separator film is softened and micro pores formed in the film are collapsed so that the movement of lithium ions is blocked and the battery reaction is suppressed.
Accordingly, also in a large capacity lithium secondary battery, it is conceivable that such various safety mechanisms as are installed in a small lithium secondary battery as described above must be indispensably installed.
However, since the resistivity of the above-mentioned PTC element made up of the conductive particles and polymer is about 1 &OHgr;·cm at room temperature, the internal resistance of the battery becomes large to cause output loss, and the PTC element may cause the discharge characteristics to lower and the lifetime of the battery to shorten. Especially, in the case where such a PTC element is installed in a large capacity battery, the concentration of current in the inside of the PTC element is apt to occur due to the increase of an area of the PTC element, which causes heat generation, so that the installation to a large capacity battery is difficult. In addition, such a PTC element is generally expensive, and a large one in size is not manufactured, so that a current control element which is more inexpensive, is capable of dealing with a large battery, and has low resistance, is earnestly desired.
According to the Battery Association of Japan, as a mechanical test (erroneous use test) of a lithium secondary battery safety estimation guideline, it is regulated that even if an abnormal discharge current abruptly flows by an internal short circuit of electrodes which is caused by driving a nail (metal rod) in a surface (lamination surface) so that the nail pierces vertically electrode plates of a lithium secondary battery, which is fully charged in the charging capacity, the electrode plates being overlapped with each other on the surface, the battery does not burst, does not fire, and the safety can be secured (such a test will be hereinafter referred to as a nail piercing test).
As a result of the foregoing nail piecing test carried out by the present inventors for a small lithium secondary battery (battery capacity 1.5 Ah or less), it was confirmed that safety was able to be sufficiently secured for the battery in which a pressure releasing mechanism was provided at an electrode side of only one end.
However, in the case where the nail piecing test was carried out for the below-mentioned lithium secondary battery which has a wound cylinder-shape and a large capacity, employs an inner electrode body, and was made an airtight structure and in which a pressure releasing mechanism was provided at one end and the pressure release mechanism was not provided at the other end, it was confirmed that the pressure release mechanism was normally operate
Kitoh Kenshin
Kurokawa Teruhisa
Nemoto Hiroshi
Brouillette Gabrielle
Crepeau Jonathan
NGK Insulators Ltd.
Parkhurst & Wendel L.L.P.
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