Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
1998-09-14
2003-01-28
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S249000, C429S142000
Reexamination Certificate
active
06511774
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a separator for non-aqueous electrolyte batteries which provides superior processability in fabrication of batteries, does not shrink or burn even when heat is generated due to external short-circuit of electrodes, causes no internal short-circuit owing to contact between electrodes, can inhibit ignition of batteries, and can provide high energy density and excellent cycle life; a non-aqueous electrolyte battery using the same; and a method for manufacturing the separator for non-aqueous electrolyte batteries.
BACKGROUND ART
Hitherto, porous materials comprising polyolefins such as polypropylene and polyethylene have been used as separators for non-aqueous electrolyte batteries such as lithium secondary batteries. For example, JP-A-6-325747 discloses a microporous film comprising a high-molecular weight polyethylene having an intrinsic viscosity (&eegr;) of 5 dl/g or higher as a separator for non-aqueous electrolyte batteries. JP-A-6-163023 discloses a microporous film comprising a mixture of polyethylene and ethylene-propylene rubber as separators of lithium primary and secondary batteries.
These separators have shutdown function for the prevention of ignition of batteries. The shutdown function is a function to prevent melting and ignition of Li upon the battery temperature reaching 180° C. in case the electrodes are short-circuited and a great electric current flows to generate heat. Specifically, before occurrence of the ignition of Li, separators are molten and openings thereof are clogged, whereby the battery reaction is stopped to inhibit generation of heat.
For example, the batteries are designed so that when a porous material of polyethylene is used as separators, the shutdown takes place at about 120° C., and when a porous material of polypropylene is used as separators, the shutdown takes place at about 140° C., whereby generation of heat in the battery is stopped to inhibit increase of temperature. However, in the case of generation of excess heat which cannot be suppressed by the shutdown function, melting of the separators proceeds to cause cracking due to complete melting or fusion of the separators, resulting in contact between electrodes, and, as a result, the short-circuit current again flows to cause generation of heat and ignition of battery.
Furthermore, since these separators are low in tear strength and penetration strength, they may be ruptured or broken through by projections of electrodes or by accidents at the time of fabrication of batteries.
JP-A-5-151949 discloses multi-layer separators for batteries which comprise a laminate of a polyolefin microporous thin film and a polyolefin nonwoven fabric, the laminate being hot pressed at a temperature lower than the melting points of the materials constituting the thin film and the nonwoven fabric. However, since the materials of the laminate are all polyolefins, they are inferior in heat resistance and cannot prevent internal short-circuit in case the temperature rises to such an extent as cannot be suppressed by the shutdown function.
For the improvement of heat resistance of separators, it is effective to add inorganic materials such as glass, alumina and ceramics, or resins and fibers superior in heat resistance. However, these materials generally contain polar groups such as hydroxyl group, silanol group and carboxyl group which adversely affect battery characteristics and, thus, these materials cannot be used as they are.
For increasing the strength of separators, the following methods are effective: a method of mixing with pulp to utilize the interlocking of pulp; a method of bonding with polyvinyl alcohol, ethylene-vinyl alcohol copolymer or the like to increase the strength; and a method of making composites with woven fabric, nonwoven fabric, paper or the like. However, pulp, polyvinyl alcohol and ethylene-vinyl alcohol copolymer contain hydroxyl group which adversely affects battery characteristics, and if woven fabric, nonwoven fabric and paper comprises materials containing polar groups such as hydroxyl group, silanol group and carboxyl group which adversely affect battery characteristics, when these are used as separators for non-aqueous electrolyte batteries, the battery characteristics such as energy density and cycle life are considerably deteriorated.
For example, JP-A-7-220710 discloses a separator for batteries, characterized by comprising a paper mainly composed of cellulose fibers and a polyethylene microporous film having micropores, for the purpose of providing a separator for batteries which blockades between positive electrode and negative electrode before the rise of temperature inside the battery reaches a dangerous zone and, besides, is diminished in the danger of rupture when the temperature further rises, and keeps insulation between positive electrode and negative electrode.
For providing a separator for batteries having good shutdown properties and heat resistance, JP-A-9-213296 discloses a battery separator, characterized by a sheet of laminate structure comprising a heat-unfusible microporous layer of a sheet made of a mixture of cellulose fibers and heat-unfusible synthetic fiber fibrils finely divided to a water retentivity of 210-450% and a heat-fusible micro-porous layer comprising a polyolefin resin, these layers being superposed.
However, the separators of JP-A-7-220710 and JP-A-9-213296 suffer from the problem that since hydroxyl group contained in cellulose fibers adversely affects the battery characteristics, energy density and cycle life are considerably deteriorated when used as separators for non-aqueous electrolyte batteries.
For providing a separator which prevents internal short-circuit caused by the contact between positive and negative electrodes, JP-A-7-302584 discloses a separator for batteries, characterized by comprising a nonwoven fabric containing at least 50% by weight of micro-fibrillated fibers of an organic synthetic polymer which have an average fiber length of 0.2-1.5 mm and an average fiber diameter of 0.05-1 &mgr;m.
However, since the microfibrillated fibers comprise an organic synthetic polymer, binding force of the fibers per se is small, and when a porous base is made using at least 50% by weight, especially 100% of the fibers, the fibers fall off from the porous base or the base is considerably low in tear strength and penetration strength to cause problem in rollability together with electrodes.
JP-A-2-170346 discloses an inorganic non-aqueous electrolyte battery having a negative electrode of an alkali metal, a positive electrode comprising a porous molded body mainly composed of carbon, a separator interposed between the negative electrode and the positive electrode and an electrolyte containing a solvent of an oxyhalide which is a positive electrode active material, wherein said separator comprises a glass fiber nonwoven fabric made using a binder mainly composed of polyethyl acrylate or a copolymer of ethyl acrylate and acrylonitrile and containing an organosilane compound.
In this case, the organosilane compound is used in order to improve binding force between the binder and the glass fibers and further increase tensile strength of the glass fiber nonwoven fabric by adding to the binder which is used for increasing tensile strength of the glass fiber nonwoven fabric.
The glass fiber nonwoven fabric here is a nonwoven fabric mainly composed of glass fibers. Therefore, when the tensile strength is increased with binder, rigidity is also increased, and, hence, the nonwoven fabric is readily broken and rollability together with electrodes is inferior, resulting in inferior battery fabricability.
Furthermore, the effect of the organosilane compound is that a part of the organosilane compound dissolves into the electrolyte and prevents densification of the alkali metal halide film which is produced on the surface of negative electrode by the reaction of oxyhalide as a positive electrode active material with alkali metal of the negative electrode and, as a result, reduction of voltage at
Funae Haruyoshi
Tsukuda Takahiro
Alejandro R
Kalafut Stephen
Manelli Denison & Selter PLLC
Mitsubishi Paper Mills Limited
White, Jr. Paul E.
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