Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-03-15
2002-11-05
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S330000, C429S336000
Reexamination Certificate
active
06475680
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a lithium secondary battery; and, in particular, the invention relates to a rechargeable lithium secondary battery, which is improved in safety by having a self imposed safety function, to an electrolyte for the lithium secondary battery, and to an electric apparatus using the same.
A lithium secondary battery has a high voltage, a high energy density, and superior storage performance and repeat charge-discharge characteristics. Therefore, the lithium secondary battery is being used widely for portable electric consumer products. Furthermore, research and development for utilizing lithium secondary batteries as power sources, such as for electric vehicles and home power storage devices which provide power during the night, by developing batteries of increased size is being performed intensely. The lithium secondary battery is a product which is expected to be used widely in daily life as a clean energy source, and which can be expected to have a significant advantage in preventing environmental pollution and the warming-up of the earth from the release of carbon dioxide.
However, a flammable organic solvent is currently used in the battery in view of its reactivity with lithium and a restriction of the potential window. Therefore, if the temperature of the battery is elevated by any means, such as overcharging or exterior heating, the electrolyte causes a thermal runaway reaction and generates a flammable gas causing an increase in the internal pressure of the battery. The gas is released to the outside the battery can and causes an ignition or, in the worst case, an explosion. Therefore, it can not be emphasized too much that how widely the battery is used in the above objects depends on the extent its safety can be ensured. A carbonate group is generally used for the lithium battery, which uses carbon material for its negative electrode, because the carbonate group exhibits preferable battery characteristics. In particular, five membered ring compounds, such as ethylene carbonate and 1,2-propylene carbonate, are employed as a main solvent and are utilized as an indispensable solvent, because these compounds have a high dielectric constant, and readily dissociate lithium salts. These compounds cause a degradation reaction indicated by the following chemical equation (Equation 1), and generate a combustible gas, when they are heated or overcharged.
The internal pressure of the battery is increased by the combustible gas, the combustible gas is released from the battery can, and, in the worst case, an ignition and explosion are caused.
A method of preventing the ignition and explosion of the battery has been disclosed in JP-A-6-290793 (1994); wherein a solvent, which causes a polymerization reaction with LiPF
6
, i.e., a lithium salt, is mixed as an electrolyte solvent, in order to make sure that the electrolyte will cause no decomposition reaction, but will produce a polymerization reaction when the temperature of the battery is elevated. JPA-6-283206 (1994) and JP-A-9-45369 (1997) disclose methods for solidifying the electrolyte by providing microcapsules, which contain a polymerization initiator and polymerizable material therein, in the electrolyte, in a separator, and the like, whereby these materials are released from the microcapsules to cause a polymerization reaction when the temperature of the battery is elevated.
In accordance with JP-A-6-290793 (1994), the solvent, which causes a polymerization reaction with LiPF
6
is restricted, and mixing one of the compounds in a cyclic ether group is indispensable. However, if the battery is composed of a system wherein the use of the compound in the cyclic ether group is not desirable in view of the battery characteristics, the compound in the cyclic ether group can not be used. A result of analyzing the heat generating behavior of an electrolyte solvent, made by mixing ethylene carbonate (EC) and ethylmethyl carbonate (EMC) in a 1:1 ratio, which is one of the carbonate group solvents exhibiting desirable battery characteristics with a carbon negative electrode, using differential scanning calorimetry (DSC), indicates that the solvent alone does not exhibit a large heat generation. However, a rapid reaction is indicated near 250° C. for the electrolyte dissolving LIPF
6
at one mol/liter, the carbonate solvent is decomposed, and a combustible gas is generated. As a result of analyzing an infrared spectrum of the specimens after the above test, it was found that an absorption based on a carbonyl radical of the carbonate molecule still remained at 1700 cm
−1
with the specimen of the solvent alone. On the contrary, the absorption disappeared with the specimen of the electrolyte dissolving LIPF
6
at one mol/liter. That means that the reaction indicated previously by the equation 1 has proceeded, and generation of lithium carbonate and ethylene gas could be observed. Accordingly, LIPF
6
can not be used effectively as the polymerization initiator in a system using a carbonate solvent as a main solvent.
In a case where microcapsules are used, as disclosed in JP-A-6-283206 (1994) and JP-A-9-45369 (1997), the temperature at which the polymerization initiator and the polymerizable material are released can be controlled based on the material forming the wall of the capsule. However, using a large amount of the microcapsules in a battery is difficult in view of the need to maintain desirable battery characteristics. It is difficult to interrupt propagation of the thermal runaway reaction with dispersed capsules, if the polymerization reaction does not proceed with a significantly rapid reaction rate, because the reaction will be generated locally with a microscale.
SUMMARY OF THE INVENTION
One of the objects of the present invention is to provide a lithium secondary battery, which is capable of terminating functions of the battery safely when any of an overcharge, an overdischarge, or an abnormal temperature rise condition occurs, without an accompanying rapid change in appearance, gas generation, or pressure change, and to provide its electrolyte and an electric apparatus using the same as a power source.
The present invention is characterized by the provision of a lithium secondary battery comprising a negative electrode which is capable of absorbing and desorbing lithium; a positive electrode which is capable of absorbing and desorbing lithium; and an aprotic organic electrolyte, wherein the aprotic organic electrolyte can be solidified by a thermal reaction at a designated temperature. The aprotic organic electrolyte comprises a lithium salt and a non-aqueous solvent; and, the non-aqueous solvent is provided in an amount sufficient to dissolve the lithium salt, and comprises a thermally polymerizable non-aqueous solvent. The content of the non-aqueous solvent, which can dissolve the lithium salt, is in the range of 50-95% by volume, desirably in the range of 65-90% by volume; and, the content of the thermally polymerizable solvent is in the range of 5-50% by volume, and, desirably, it is in the range of 10-35% by volume. The aprotic organic electrolyte can be solidified by a thermal reaction at a designated temperature.
The present invention relates to a lithium secondary battery comprising a negative electrode which is capable of absorbing and desorbing lithium; a positive electrode which is capable of absorbing and desorbing lithium; and an aprotic organic electrolyte, wherein its functions can be terminated safely in a non-returned condition without an accompanying rapid change in appearance, gas generation, or pressure change, particularly a pressure increase, when any of an overcharge, an overdischarge, or an abnormal temperature rise condition occurs.
The present invention also relates to an electrolyte for lithium secondary batteries, the electrolyte being characterized as comprising a lithium salt and a non-aqueous solvent, which pan dissolve the lithium salt, which electrolyte can be solidified by a thermal reaction at a designated temperature.
The prese
Akahoshi Haruo
Arai Juichi
Iwayanagi Takao
Katayama Hideaki
Takamura Tomoe
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