Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-09-30
2001-02-13
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231900, C429S218100, C429S199000, C429S062000
Reexamination Certificate
active
06187477
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithium secondary battery which may be used as a power source for retaining data in a memory of an electronic apparatus (e.g., a personal computer) or for driving a portable electronic apparatus (e.g., a portable telephone receiver). The present invention also relates to a cathode composition used for such a battery.
2. Description of the Related Art
As is well known, a lithium secondary battery comprises a cathode dischargeably charged with lithium ions, an anode and an electrolyte which allows migration of lithium ions between both electrodes. The anode may consist of lithium metal, a lithium alloy or any other material which can be releasably doped with lithium ions. Typically, the electrolyte may be a nonaqueous electrolytic solution which is prepared by dissolving a lithium salt in an organic solvent.
Due to the high energy density and the use of an organic solvent, a lithium secondary battery is known to have a problem of generating a large amount of heat under severe conditions. For example, the lithium battery generates heat at the time of compression (e.g., battery crushing under a heavy object), nail piercing (e.g., when erroneously driving a nail into the battery at the time of packaging), internal shorting, exposure to high temperature, or external shorting.
One way to solve such a problem is to provide a porous separator between the cathode and the anode, as disclosed in JP-A-54(1979)-52157 or JP-A-59(1984)-207230 for example. According to this solution, the pores of the separator are closed at the melting point of the separator material due to the fusion thereof, thereby interrupting the ion migration between the cathode and the anode. As a result, the current flow terminates to stop the temperature rise.
As an improvement to a lithium secondary battery incorporating a porous separator, JP-A-5(1993)-74443 discloses an arrangement wherein the separator has an excess portion projecting beyond the edge faces of the cathode and the anode, and wherein the excess portion of the separator is pressed down against the edge faces of both electrodes by an insulating plate which is thermally fusible to the separator. Such an arrangement prevents excess heat generation or thermal runaway which may occur through shorting between the cathode and the anode due to a shrinkage of the separator near the edge faces of both electrodes after the pores of the separator are thermally closed.
However, the prior art lithium secondary battery incorporating the porous separator operates properly for the prevention of excessive heat generation only when the separator is kept in its appropriate state. Therefore, the battery is incapable of preventing excessive heat generation if the cathode comes into direct contact with the anode upon rupture of the separator under crushing of the battery or if both electrodes are shorted via a nail which has penetrated through the separator. It should be noted that excessive heat generation in a lithium secondary battery occurs because the Joule heat generated at the time of shorting causes oxygen to separate from the cathode active substance for reacting with active lithium.
On the other hand, JP-A-7(1995)-78635 proposes the use, in a lithium secondary battery, of an electrolytic solution which contains LiAsF
6
/1,3-dioxolane+tertiary amine. Normally, the tertiary amine prevents polymerization of 1,3-dioxolane. Conversely, when the temperature of the battery rises due to high-temperature exposure or shorting for example, 1,3-dioxolane starts polymerizing to increase the internal resistance of the battery, whereby the current flow decreases and the temperature of the battery drops.
However, the above-described electrolytic solution contains As in LiAsF
6
. Therefore, sufficient care needs to be taken in handling the battery for preventing environmental pollution. Further, the electrolytic solution is known to decompose when the battery voltage increases to no less than 4V, so that the candidate materials for the cathode active substance are limited to those which make the charge terminating voltage of the battery below 4V. This is critically disadvantageous in increasing the energy density of the battery.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention is to provide a lithium secondary battery which is capable of reliably preventing excessive heat generation even under severe conditions such as battery crushing, nail piercing, internal shorting, high-temperature exposure or external shorting without entailing the problems of the prior art lithium batteries described above.
Another object of the present invention is to provide a cathode composition which can be advantageously used for such a battery.
According to a first aspect of the present invention, there is provided a lithium secondary battery comprising: a cathode which can be dischargeably charged with lithium ions; an anode selected from a group consisting of lithium metal, a lithium alloy and any other anode material which can be releasably doped with lithium ions; and an electrolyte which allows migration of lithium ions between both electrodes; wherein the cathode contains a halogen compound which releases halogen atoms, halogen ions or a reactive halogen-containing substance for reacting with the anode and for thereby deactivating the anode to prevent excessive heat generation before oxygen released from the cathode due to a temperature rise reacts with the anode.
With the structure, when the temperature of the battery rises due to battery crushing, nail piercing, internal shorting, high-temperature exposure or external shorting, the halogen compound contained in the cathode decomposes to generate a halogen-family gas (halogen atoms, halogen ions or halogen-containing substance in gaseous phase) before oxygen released from the cathode reacts with the anode. The halogen-family gas thus generated reacts with active lithium of the anode to form a layer of lithium halide which is thermodynamically stable (i.e., inert or inactive). As a result, the lithium halide layer serves as an oxidation preventing layer even if oxygen is later released from the cathode, thereby preventing excessive heat generation which would be otherwise caused by oxidation of lithium.
Further, in case where a temperature rise is caused by internal shorting of the battery, the lithium halide layer formed on the anode interrupts current flow at the shorted portion because it has electrical insulation. Thus, the lithium halide layer has a dual function of serving as an oxidation preventing layer and as an insulating layer for effectively preventing excessive heat generation.
In general, the halogen compound contained in the cathode, if it thermally decomposes at a temperature of 100~380° C., can release a halogen-family gas (i.e., halogen atoms, halogen ions or a reactive halogen-containing substance in gaseous phase) before oxygen released from the cathode reacts with the anode. Examples of the halogen compound include iodine compounds, bromine compounds, chlorine compounds, fluorine compounds and any other compounds which contain two different kinds of halogens.
Examples of iodine compounds include 4,4′-diiodobiphenyl and p-iodotoluene.
Examples of bromine compounds include tetrabromobisphenol S represented by the following formula (1), hexabromobenzene, brominated amides, carbonate oligomers of tetrabromobisphenol A represented by the following formula (2), polymers of an derivative of tetrabromobisphenol A represented by the following formula (3), pentabromotoluene, p-bromobenzoic acid, 1,3,5-tribromobenzene, tetrabromodiphenyl ether, and 2,4,6-tribromophenol.
Examples of chlorine compounds include chlorinated paraffins, chlorinated polyethylene, tetrachlorophthalic anhydride, perchlorocyclopentadecane, chlorend acid, and poly(vinylidene chloride).
Examples of fluorine compounds include fluororesins such as poly(vinylidene fluoride) (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-perf
Horiuchi Hiroshi
Miyashita Tsutomu
Tsutsumi Masami
Watanabe Isao
Yamamoto Tamotsu
Alejandro Raymond
Armstrong, Westerman Hattori, McLeland & Naughton
Fujitsu Limited
Kalafut Stephen
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