Patent Description:
Conventionally, a lithium secondary battery in which insulation properties of a positive electrode or a negative electrode are improved by using an insulating tape has been proposed.

PATENT LITERATURE <NUM> describes a lithium secondary battery in which disconnection of a current collector at a part at which the current collector and a lead are in contact with each other is suppressed.

<FIG> is a configurational diagram of a positive electrode of the lithium secondary battery described in PATENT LITERATURE <NUM>, <FIG> is a partial top view viewed from a one principal surface-side of the current collector, and <FIG> is a sectional view taken along the line L1-L1 in <FIG>.

As shown in <FIG>, an insulating tape <NUM> disposed at the one principal surface-side of a positive electrode current collector 40A covers a positive electrode current collector exposed surface 40a at a both-surface non-coated portion 40b on which no positive electrode mixture layer 40B is formed, a positive electrode lead <NUM> above the positive electrode current collector exposed surface 40a, and a protective layer <NUM> interposed between a lower end part of the positive electrode lead <NUM> and the positive electrode current collector exposed surface 40a. This insulating tape <NUM> is provided to prevent heat generation in a battery when a separator or the like splits at the time of battery abnormality and the positive electrode and a negative electrode come into contact with each other, for example.

A further example of the prior art is disclosed in PATENT LITERATURE <NUM>. Therein, the insulating tape includes a first adhesive layer, a second organic layer, a second adhesive layer, and a first organic layer that are disposed in that order from the side on which the positive electrode lead is located. All of these layers have the same width.

However, when a foreign matter adheres onto the insulating tape <NUM> covering the positive electrode lead <NUM> and the insulating tape <NUM> breaks due to the foreign matter or the like, an internal short circuit occurs between the positive electrode lead <NUM> and the negative electrode, for example, which may lead to an increase in battery temperature.

An object of the present disclosure is to provide a secondary battery, an insulating member, and a positive electrode lead capable of suppressing an increase in battery temperature when an insulating tape covering a positive electrode lead breaks due to a foreign matter.

The present invention is defined in the independent claim. Preferred embodiments are referred to in the dependent claims.

According to the present disclosure, an increase in battery temperature which occurs when the insulating tape covering the positive electrode lead breaks due to a foreign matter may be suppressed.

Hereinafter, one example of a secondary battery which is one aspect of the present disclosure will be described. Figures referenced in the following description of embodiments are schematically illustrated, and dimension ratios or the like of components drawn in the figures may be different from actual ones.

<FIG> is a sectional view of a secondary battery according to an embodiment. The secondary battery <NUM> shown in <FIG> is one example of a lithium ion secondary battery. The secondary battery according to the embodiment is not limited to a lithium ion secondary battery and may be other secondary batteries such as an alkaline secondary battery.

The secondary battery <NUM> shown in <FIG> comprises a wound-type electrode assembly <NUM> in which a positive electrode <NUM> and a negative electrode <NUM> are wound with a separator <NUM> interposed therebetween, an electrolyte, insulating plates <NUM>, <NUM> respectively disposed at the upper and lower sides of the electrode assembly <NUM>, a positive electrode lead <NUM> and a negative electrode lead <NUM>, an insulating tape (not shown) covering a part of the positive electrode lead <NUM>, and a battery case <NUM> housing these members described above. Incidentally, the secondary battery <NUM> may comprise an insulating member covering the negative electrode lead <NUM>.

The electrode assembly <NUM> is not limited to such a wound-type electrode assembly, and other forms such as a laminated-type electrode assembly in which a positive electrode and a negative electrode are alternately laminated with a separator interposed therebetween may be employed, for example.

The battery case <NUM> comprises a bottomed cylindrical case body <NUM> having an opening, and a sealing assembly <NUM> sealing the opening of the case body <NUM>, for example. The battery case <NUM> desirably comprises a gasket <NUM> provided between the case body <NUM> and the sealing assembly <NUM>, and sealability inside the battery is secured thereby. The shape of the battery case <NUM> is not limited to a cylindrical shape, and the battery case <NUM> may be a square shape, a laminate-type, and the like, for example.

The case body <NUM> has a projecting portion <NUM> in which a part of a side surface thereof inwardly projects and which supports the sealing assembly <NUM>, for example. The projecting portion <NUM> is preferably formed into an annular shape along a circumferential direction of the case body <NUM> and supports the sealing assembly <NUM> with its upper surface.

The sealing assembly <NUM> has a structure in which a filter <NUM>, a lower vent member <NUM>, an insulator <NUM>, an upper vent member <NUM>, and a cap <NUM> are laminated from the side of the electrode assembly <NUM> in this order. Each member configuring the sealing assembly <NUM> has a disc shape or a ring shape, for example, and the members other than the insulator <NUM> are electrically connected to each other. The lower vent member <NUM> and the upper vent member <NUM> are connected to each other at their respective central parts, and the insulator <NUM> is interposed between respective peripheral parts of the lower vent member <NUM> and the upper vent member <NUM>. When an inner pressure increases due to heat generated by an internal short circuit or the like, the lower vent member <NUM> deforms so as to push up the upper vent member <NUM> toward the side of the cap <NUM> and breaks, and a current path between the lower vent member <NUM> and the upper vent member <NUM> is cut off, for example. When the inner pressure further increases, the upper vent member <NUM> breaks, and gas is discharged from an opening of the cap <NUM>.

The positive electrode lead <NUM> has a first end part (not shown), an extension part 20b, and a second end part 20c; the first end part is connected to the positive electrode <NUM>; the extension part 20b extending from the first end part extends toward the sealing assembly <NUM> through a through hole of the insulating plate <NUM>; and the second end part 20c positioned closer to a leading end than the extension part 20b is connected to a lower surface of the filter <NUM> of the sealing assembly <NUM> as described later. Consequently, the cap <NUM> electrically connected to the filter <NUM> becomes a positive electrode terminal. One end of the negative electrode lead <NUM> is connected to the negative electrode <NUM> and the other end of the negative electrode lead <NUM> is connected to an inner surface of a bottom part of the case body <NUM>, with the negative electrode lead <NUM> extending from the negative electrode <NUM> through an outside of the insulating plate <NUM>. Consequently, the case body <NUM> becomes a negative electrode terminal. Incidentally, each of the second end part of the positive electrode lead <NUM> and the other end of the negative electrode lead <NUM> may be connected at a position opposite to the above described position. For example, the second end part of the positive electrode lead <NUM> may be connected to the case body <NUM>, and the other end of the negative electrode lead <NUM> may be connected to the lower surface of the filter <NUM> of the sealing assembly <NUM>. Incidentally, while the positive electrode lead <NUM> of <FIG> has the second end part 20c to be connected to the battery case <NUM>, in the case of a battery in which the battery case <NUM> is not connected (for example, a square battery, a laminated battery, and the like), the positive electrode lead <NUM> does not have the second end part 20c connected to the battery case <NUM>.

Hereinafter, the positive electrode <NUM>, the positive electrode lead <NUM>, the negative electrode <NUM>, the electrolyte, and the separator <NUM> will be described with reference to <FIG> and <FIG>. The insulating tape <NUM> is illustrated by an alternate long and short dash line as a transmissive image in <FIG>.

The positive electrode <NUM> comprises a positive electrode current collector <NUM> and a positive electrode active material layer <NUM> formed on the positive electrode current collector <NUM>. Foil of a metal stable in an electric potential range of the positive electrode such as aluminum, a film in which said metal is disposed on a surface layer thereof, and the like are used for the positive electrode current collector <NUM>. The positive electrode active material layer <NUM> includes a positive electrode active material. In addition, the positive electrode active material layer <NUM> preferably includes a conductive agent and a binder in addition to the positive electrode active material.

Examples of the positive electrode active material included in the positive electrode active material layer <NUM> include a lithium-transition metal composite oxide. Specifically, lithium cobaltate, lithium manganate, lithium nickelate, a lithium nickel manganese composite oxide, a lithium nickel cobalt composite oxide, and the like can be used as the positive electrode active material, and Al, Ti, Zr, Nb, B, W, Mg, Mo, and the like may be added to these lithium-transition metal composite oxides.

Examples of the conductive agent included in the positive electrode active material layer <NUM> include carbon powder such as carbon black, acetylene black, Ketjen black, and graphite. These conductive agents may be used singly, or two or more kinds thereof may be used in combination.

Examples of the binder included in the positive electrode active material layer <NUM> include a fluorine polymer and a rubber polymer. Examples of the fluorine polymer include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and a modified product thereof, and examples of the rubber polymer include an ethylene-propylene-isoprene copolymer and an ethylene-propylene-butadiene copolymer. These binders may be used singly, or two or more kinds thereof may be used in combination.

The positive electrode current collector <NUM> has an exposed part 32a on which the positive electrode active material layer <NUM> is not formed. Incidentally, the exposed part 32a illustrated in <FIG> is formed at a longitudinally central part of the positive electrode current collector <NUM>. Note that the exposed part 32a may be formed at any position of the positive electrode current collector <NUM> and may be formed at a longitudinally end part of the positive electrode current collector <NUM>, for example.

The positive electrode lead <NUM> comprises a first end part 20a connected to the exposed part 32a of the positive electrode current collector <NUM>, an extension part 20b extending from the first end part 20a toward an outside of a peripheral part 32b of the positive electrode current collector <NUM>. In addition, although not shown in <FIG>, the positive electrode lead <NUM> has the second end part 20c (see <FIG>) in a leading end side of the extension part 20b and is connected to the filter <NUM> of the sealing assembly <NUM> as described above. A method for connecting the first end part 20a of the positive electrode lead <NUM> and the exposed part 32a of the positive electrode current collector <NUM> and a method for connecting the second end part 20c of the positive electrode lead <NUM> and the sealing assembly <NUM> are not particularly limited as long as electrical connection is secured, and examples thereof include ultrasonic welding.

A material for the positive electrode lead <NUM> is not particularly limited and includes a metal such as aluminum and titanium.

The negative electrode <NUM> comprises a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. Foil of a metal stable in an electric potential range of the negative electrode such as copper, a film in which said metal is disposed on a surface layer thereof, and the like are used for the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material layer preferably includes a thickener and a binder in addition to the negative electrode active material.

As the negative electrode active material, a carbon material capable of occluding and releasing lithium ions can be used, and hardly graphitizable carbon, easily graphitizable carbon, fibrous carbon, coke, carbon black, and the like can be used besides graphite. Furthermore, silicon, tin, and an alloy and oxide mainly containing them can be used as a non-carbon-based material.

As the binder, while PTFE and the like can be used as with the case of the positive electrode, a styrene-butadiene copolymer (SBR) or a modified product thereof and the like may be used. As the thickener, carboxymethyl cellulose (CMC) and the like may be used.

The electrolyte includes a solvent and an electrolyte salt dissolved in the solvent. The electrolyte is not limited to a liquid electrolyte and may be a solid electrolyte using a gelatinous polymer and the like. The solvent is preferably a non-aqueous solvent such as carbonates, lactones, ethers, ketones, esters, and a mixed solvent of two or more kinds thereof, for example. However, the solvent may be an aqueous solvent.

As the electrolyte salt, LiPF<NUM>, LiBF<NUM>, LiCF<NUM>SO<NUM>, and a mixture of two or more kinds thereof can be used, for example. A dissolved amount of the electrolyte salt based on the solvent is <NUM> to <NUM> mol/L, for example.

A porous sheet having ion permeability and insulation properties is used for the separator <NUM>, for example. Specific examples of the porous sheet include a fine porous thin film, a woven fabric, and a nonwoven fabric. As a material of the separator <NUM>, an olefin-based resin such as polyethylene and polypropylene and cellulose are preferable. The separator <NUM> may be a laminated body having a cellulose fiber layer and a fiber layer of a thermoplastic resin such as an olefin-based resin. In addition, the separator <NUM> may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator in which a material such as an aramid-based resin and a ceramic is applied to a surface thereof may be used.

<FIG> is a sectional view taken along the line L1-L1 in <FIG>. An insulating member covering a part of the positive electrode lead <NUM> has an insulating tape <NUM> and an insulating layer <NUM> described later.

As shown in <FIG>, the insulating layer <NUM> is disposed on an outer surface of the first end part 20a of the positive electrode lead <NUM>. In addition, the insulating layer <NUM> is preferably disposed on the exposed part 32a of the positive electrode current collector <NUM>. Here, the outer surface of the first end part 20a of the positive electrode lead <NUM> is a surface other than the part contacting the exposed part 32a of the positive electrode current collector <NUM>. That is, in a case where the positive electrode lead <NUM> has a plate shape as shown in <FIG>, the outer surface of the first end part 20a includes one principal surface opposed to a surface contacting the exposed part 32a of the positive electrode current collector <NUM> and a pair of side surfaces opposed to each other. Incidentally, a shape of the positive electrode lead <NUM> is not limited to a plate shape and may be a columnar shape and the like.

As shown in <FIG>, the insulating layer <NUM> is preferably disposed on the one principal surface and the pair of side surfaces of the first end part 20a of the positive electrode lead <NUM>. However, the insulating layer <NUM> may be disposed on the entirety or a part of the outer surface of the first end part 20a of the positive electrode lead <NUM>. That is, with respect to the positive electrode lead <NUM> having a plate shape, the insulating layer <NUM> may be disposed on a part or the entirety of the one principal surface of the first end part 20a, may be disposed on a part or the entirety of the pair of side surfaces, or may be disposed on a part or the entirety of each of the one principal surface and the pair of side surfaces of the first end part 20a. While the insulating layer <NUM> may be formed on a surface of the first end part 20a of the positive electrode lead <NUM> contacting the exposed part 32a of the positive electrode current collector <NUM>, in this case, contact resistance at a contact between the positive electrode lead <NUM> and the positive electrode current collector <NUM> may increase, and battery performance may deteriorate. Accordingly, in the first end part 20a of the positive electrode lead <NUM>, the insulating layer <NUM> is preferably not disposed on the surface contacting the exposed part 32a of the positive electrode current collector <NUM>.

The insulating layer <NUM> has electrical resistance higher than that of a native oxide film naturally foamed on the positive electrode lead <NUM>. The insulating layer <NUM> preferably has electrical resistance of <NUM> MΩ or more and more preferably has electrical resistance of <NUM> MΩ or more, for example.

A material of the insulating layer <NUM> is not particularly limited as long as the insulating layer <NUM> includes a material having insulation properties and the like. However, the insulating layer <NUM> preferably includes an inorganic material. In addition, the insulating layer <NUM> preferably includes a binder in terms of mechanical strength, adhesiveness, and the like of the insulating layer <NUM>.

The inorganic material included in the insulating layer <NUM> includes at least one selected from the group consisting of a metal oxide, a metal hydroxide, a metal nitride, a metal fluoride, and a metal carbide, for example.

The binder included in the insulating layer <NUM> is preferably a substance dissolvable in a solvent such as NMP and water, for example, and chemically stable in the positive electrode. Examples thereof include polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVdF), polyacrylic acid, polyacrylonitrile, polyisobutylene, and polyisoprene.

As shown in <FIG> and <FIG>, the insulating tape <NUM> is disposed at both of the one principal surface-side and the other principal surface-side of the positive electrode <NUM>. However, the insulating tape <NUM> may be disposed at the one principal surface-side of the positive electrode <NUM> on which the positive electrode lead <NUM> is disposed. The insulating tape <NUM> disposed at the one principal surface-side covers the insulating layer <NUM> disposed on the outer surface of the first end part 20a of the positive electrode lead <NUM>. In addition, the insulating tape <NUM> disposed at the one principal surface-side preferably covers the insulating layer <NUM> disposed on the exposed part 32a of the positive electrode current collector <NUM> and also covers a boundary part between the exposed part 32a and the positive electrode active material layer <NUM>. In addition, the insulating tape <NUM> disposed at the other principal surface-side covers the exposed part 32a and a boundary part between the exposed part 32a and the positive electrode active material layer <NUM>.

The insulating tape <NUM> includes a substrate layer and an adhesive layer on the substrate layer, for example. The substrate layer is a layer mainly containing an organic material, for example, and examples thereof include polyphenylene sulfide (PPS), polyetheretherketone (PEEK), polyimide (PI), polypropylene (PP), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT). The adhesive layer is a layer adhering to the insulating layer <NUM>, the exposed part 32a, and the like and preferably includes an adhesive such as a rubber-based resin, an acrylic resin, and a silicone-based resin, for example.

<FIG> is a diagram showing one example of a method for arranging the insulating layer and the insulating tape. First, slurry including inorganic particles, a binder, and the like is applied on the outer surface of the first end part 20a of the positive electrode lead <NUM> and on the exposed part 32a to dispose the insulating layer <NUM> on the outer surface of the first end part 20a and on the exposed part 32a (see <FIG>). Thereafter, the insulating tape <NUM> having the substrate layer 30a and the adhesive layer 30b on the substrate layer 30a is prepared, and the insulating tape <NUM> is attached on the insulating layer <NUM>, with the adhesive layer 30b facing the insulating layer <NUM> (see <FIG>). The insulating layer <NUM> prepared according to such a procedure is in a state of adhering to the first end part 20a and the exposed part 32a.

<FIG> is a diagram showing another example of the method for arranging the insulating layer and the insulating tape. First, the insulating layer <NUM> including inorganic particles, a binder, and the like is formed on the adhesive layer 30b of the insulating tape <NUM> having the substrate layer 30a and the adhesive layer 30b on the substrate layer 30a to prepare an insulating member <NUM>. (See <FIG>). Thereafter, the insulating member <NUM> is attached so that the insulating layer <NUM> of the insulating member <NUM> is disposed on the outer surface of the first end part 20a and on the exposed part 32a (see <FIG>). The insulating layer <NUM> prepared according to such a procedure is in a state of not adhering to the first end part 20a and the exposed part 32a.

In this manner, by disposing the insulating layer <NUM> on the outer surface of the first end part 20a of the positive electrode lead <NUM> connected to the exposed part 32a of the positive electrode current collector <NUM> and covering the insulating layer <NUM> with the insulating tape <NUM>, an internal short circuit between the positive electrode lead <NUM> and the negative electrode <NUM> is suppressed even if a foreign matter having entered a battery breaks through the insulating tape <NUM> and the insulating tape <NUM> breaks because the positive electrode lead <NUM> on which the insulating layer <NUM> is disposed is exposed. Even in a case where a foreign matter penetrates the insulating layer <NUM> and reaches the positive electrode lead <NUM> and an internal short circuit occurs between the positive electrode lead <NUM> and the negative electrode <NUM>, since the insulating layer <NUM> existing around the foreign matter acts as a large short-circuit resistor, heat generation in the battery due to an internal short circuit is suppressed and an increase in battery temperature is suppressed.

While the insulating layer <NUM> may be only disposed on the outer surface of the first end part 20a of the positive electrode lead <NUM>, the insulating layer <NUM> is preferably disposed on a part or the entirety of the exposed part 32a as described above. Consequently, a foreign matter having entered a battery hardly comes into contact with the exposed part 32a, and occurrence of an internal short circuit in the battery is further suppressed.

A thickness of the insulating layer <NUM> is preferably within a range of <NUM> to <NUM>, for example. By setting the thickness of the insulating layer <NUM> to <NUM> or more, occurrence of an internal short circuit or an increase in battery temperature, which arises when the insulating tape <NUM> breaks due to a foreign matter, is suppressed compared with a case where the thickness of the insulating layer <NUM> is less than <NUM>. An insulating layer <NUM> having a thickness exceeding <NUM> may require other components to have reduced volumes so as to allow the case body <NUM> with a predetermined size to house the electrode assembly <NUM>.

Examples of the inorganic material in the insulating layer <NUM> include a metal oxide such as aluminum oxide, titanium oxide, zirconium oxide, silicon oxide, manganese oxide, magnesium oxide, and nickel oxide; a metal hydroxide such as aluminum hydroxide and magnesium hydroxide; a metal nitride such as titanium nitride, boron nitride, aluminum nitride, magnesium nitride, and silicon nitride; a metal fluoride such as aluminum fluoride, lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, and barium fluoride; and a metal carbide such as silicon carbide, boron carbide, titanium carbide, and tungsten carbide. The inorganic material preferably includes at least one of the group consisting of aluminum oxide, aluminum hydroxide, titanium oxide, magnesium oxide, and magnesium hydroxide in terms of chemical stability against the electrolyte and the like.

A basis weight of the inorganic material in the insulating layer <NUM> is preferably <NUM> to <NUM>/cm<NUM>. When the basis weight of the inorganic material is <NUM>/cm<NUM> or more, an increase in battery temperature due to an internal short circuit is suppressed compared to a case where the basis weight of the inorganic material is less than <NUM>/cm<NUM>. An insulating layer <NUM> in which a basis weight of the inorganic material exceeds <NUM>/cm<NUM> is undesirable because costs may increase.

An average particle diameter of the inorganic material is preferably within a range of <NUM> to <NUM> because an increase in battery temperature due to an internal short circuit can be further suppressed thereby and other reasons. Here, the average particle diameter is a volume average particle diameter measured by a laser diffraction method and means a median diameter at which a volumetric integrated value becomes <NUM>% in a particle size distribution. The average particle diameter can be measured using a laser diffraction and scattering-type particle size distribution measuring device (manufactured by HORIBA, Ltd. ), for example.

While the insulating tape <NUM> may only cover the insulating layer <NUM> disposed on the first end part 20a of the positive electrode lead <NUM>, the insulating tape <NUM> preferably further covers the insulating layer <NUM> disposed on the exposed part 32a and more preferably covers a part or the entirety of the boundary part between the exposed part 32a and the positive electrode active material layer <NUM>. Consequently, a foreign matter having entered a battery hardly comes into contact with the exposed part 32a or the boundary part, and occurrence of an internal short circuit in the battery is further suppressed. Furthermore, the insulating tape <NUM> may cover a part or the entirety of the extension part 20b of the positive electrode lead <NUM>. Consequently, deterioration of battery performance due to contact of the positive electrode lead <NUM> with other members is suppressed.

A thickness of the insulating tape <NUM> is not particularly limited but is preferably within a range of <NUM> to <NUM>, for example. When the thickness of the insulating tape <NUM> is less than <NUM>, breakage is easily caused by a foreign matter having entered a battery. In addition, when the thickness of the insulating tape <NUM> exceeds <NUM>, other components may be required to have reduced volumes so that the case body <NUM> with a predetermined size is allowed to house the electrode assembly <NUM>.

As a positive electrode active material, <NUM> parts by weight of a lithium nickel cobalt aluminum composite oxide represented by LiNi<NUM>Co<NUM>Al<NUM>O<NUM>, one part by weight of acetylene black (AB), and one part by weight of polyvinylidene difluoride (PVDF) were mixed; and an appropriate amount of N-methyl-<NUM>-pyrolidone (NMP) was further added thereto to prepare positive electrode mixture slurry. Thereafter, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector made from aluminum foil and dried. The resultant product was cut into a predetermined electrode size and rolled by using a roller to prepare a positive electrode with a positive electrode active material layer formed on the both sides of the positive electrode current collector. An exposed part (width: <NUM>) on which no positive electrode active material layer was formed and the positive electrode current collector was exposed was formed approximately at a central part in the longitudinal direction of the positive electrode. An aluminum positive electrode lead having a thickness of <NUM> and a width of <NUM> was bonded to the formed exposed part by ultrasonic welding.

Slurry in which <NUM> parts by weight of aluminum oxide (average particle diameter: <NUM>) and <NUM> parts by weight of PVDF were dispersed in NMP was applied on a PET film and dried to form an insulating layer. The basis weight of aluminum oxide in the formed insulating layer was <NUM>/cm<NUM>.

An insulating tape in which an adhesive layer (thickness: <NUM>) made from an acrylic adhesive was formed on a substrate layer made from a polyimide film having a width of <NUM> and a thickness of <NUM> was prepared. The insulating layer formed on the PET film was attached to the adhesive layer of the insulating tape, the PET film was subsequently peeled, and the insulating layer (width: <NUM>) was transferred on the adhesive layer to prepare an insulating member.

The insulating member was attached so that the insulating layer of the insulating member was disposed on the outer surface of first end part of the positive electrode lead and on the exposed part. That is, the insulating layer disposed on the outer surface of the first end part of the positive electrode lead and on the exposed part was covered with the insulating tape.

Thereafter, a thin plate of copper foil was used as a negative electrode current collector; negative electrode mixture slurry was prepared by dispersing a graphite powder, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene rubber (SBR) as a binder in water at a ratio of <NUM>:<NUM>:<NUM> in terms of respective mass ratios; the negative electrode mixture slurry was applied to both sides of the current collector and dried; and the resultant product was compressed by roll pressing so as to have a predetermined thickness. An exposed part on which no negative electrode active material layer was formed and the negative electrode current collector was exposed was formed at an end part of the negative electrode in the longitudinal direction, and a nickel negative electrode lead was bonded to the exposed part by ultrasonic welding.

Then, the negative electrode lead on the exposed part and the exposed part were covered with an insulating tape. The insulating tape was obtained by forming an adhesive layer made from a rubber-based resin on a substrate layer made from a polypropylene film having a thickness of <NUM>.

A wound-type electrode assembly was prepared by spirally winding the prepared positive electrode and negative electrode with a separator interposed therebetween. A polyethylene fine porous film with a heat resistant layer in which a filler including polyamide and alumina was dispersed formed on one surface thereof was used as the separator.

The above-described electrode assembly was housed in a bottomed cylindrical case body having an outer diameter of <NUM> and a height of <NUM>. At this time, the second end part of the positive electrode lead was welded to a sealing assembly, and the other end of the negative electrode lead was welded to the case body. Then, a non-aqueous electrolytic solution in which LiPF<NUM> was added to a mixed solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) at a volume ratio of <NUM>:<NUM>:<NUM> so that a concentration of LiPF<NUM> became <NUM> mol/L was injected to the case body, and the opening of the case body was subsequently sealed by a gasket and the sealing assembly to prepare a <NUM> cylindrical non-aqueous electrolyte secondary battery.

A non-aqueous electrolyte secondary battery was prepared in the same manner as Example A1 except that the basis weight of aluminum oxide in the insulating layer was changed to <NUM>/cm<NUM>.

A non-aqueous electrolyte secondary battery was prepared in the same manner as Example A1 except that the average particle diameter of aluminum oxide in the insulating layer was changed to <NUM>.

A non-aqueous electrolyte secondary battery was prepared in the same manner as Example A1 except that the average particle diameter of aluminum oxide in the insulating layer was changed to <NUM> and the basis weight of aluminum oxide in the insulating layer was changed to <NUM>/cm<NUM>.

An insulating layer was formed by applying slurry in which <NUM> parts by weight of aluminum oxide (average particle diameter: <NUM>) and <NUM> parts by weight of PVDF were dispersed in NMP to the outer surface of a first end part of a positive electrode lead and an exposed part and drying the slurry. The basis weight of aluminum oxide in the formed insulating layer was <NUM>/cm<NUM>. An insulating tape was attached so as to cover the formed insulating layer. The insulating tape was obtained by forming an adhesive layer (thickness: <NUM>) made from an acrylic adhesive on a substrate layer made from a polyimide film having a width of <NUM> and a thickness of <NUM>.

A non-aqueous electrolyte secondary battery was prepared in the same manner as Example A1 except that the insulating layer was not disposed on the outer surface of the first end part of the positive electrode lead and the exposed part.

Battery temperature at the time of short circuit due to a foreign matter was measured with respect to each of the non-aqueous electrolyte secondary batteries of Examples and Comparative Example. The battery temperature at the time of short circuit due to a foreign matter is the highest arrival temperature obtained by putting a foreign matter (a small nickel piece) on an insulating tape and measuring a temperature of the side surface of a battery with a thermocouple at the time of forcibly causing a short circuit according to JIS C <NUM>. Results of respective Examples and Comparative Example are shown in Table <NUM>.

In each of Examples A1 and A2 and Comparative Example <NUM>, an internal short circuit due to contamination of the foreign matter occurred. However, in each of Examples A1 and A2 in which the insulating layer was disposed on the outer surface of the first end part of the positive electrode lead and the insulating layer was covered with the insulating tape, battery temperature at the time of internal short circuit significantly decreased and an increase in battery temperature was suppressed compared with Comparative Example in which no insulating layer was disposed on the outer surface of the first end part of the positive electrode lead and the insulating tape merely covered thereon. In addition, in Examples A3 to A10, an increase in battery temperature was suppressed more than Examples A1 and A2. Especially, in Examples A6, A8, A9, and A10, no internal short circuit occurred, and no increase in battery temperature was observed.

Hereinafter, a second embodiment will be described. However, description overlapping with that of the first embodiment is appropriately omitted. As shown in <FIG>, an insulating layer <NUM> is formed on the outer surface of the first end part 20a of the positive electrode lead <NUM>.

While the insulating layer <NUM> is formed on one principal surface of the first end part 20a of the positive electrode lead <NUM> in <FIG>, its configuration is not limited thereto, and the insulating layer <NUM> may be formed on the entirety or a part of the outer surface of the first end part 20a of the positive electrode lead <NUM>. That is, in a plate-shaped positive electrode lead <NUM>, the insulating layer <NUM> may be formed on a part or the entirety of the one principal surface of the first end part 20a, may be formed on a part or the entirety of a pair of side surfaces, or may be formed on a part or the entirety of the one principal surface and the pair of side surfaces of the first end part 20a.

The insulating layer <NUM> is different from a native oxide film naturally foamed on the positive electrode lead <NUM> and has electrical resistance higher than that of a native oxide film. The insulating layer <NUM> preferably has electrical resistance of <NUM> MΩ or more and more preferably has electrical resistance of <NUM> MΩ or more, for example. While examples of the insulating layer <NUM> include an oxide film such as an anodized film, a phosphate film, a chromate film (chromic acid salt film), a stannate film, and a fluoride film, the insulating layer <NUM> is preferably an oxide film such as an anodized film or a phosphate film in terms of insulating properties, denseness, and the like.

The phosphate film is formed by immersing the positive electrode lead <NUM> including aluminum or an alloy mainly containing aluminum in a phosphate solution for a predetermined time, for example. The positive electrode lead <NUM> after being immersed in the solution is desirably subjected to drying treatment at a temperature ranging from <NUM> to <NUM>, for example. A solution including phosphoric acid, chromic anhydride, and a fluoride or a solution including phosphoric acid, zinc phosphate monobasic, and a fluoride is preferably used as the phosphate solution in terms of obtaining an insulating layer <NUM> with higher insulation properties and denseness, and the like, for example. A phosphoric chromate film (specific compositions thereof are Al<NUM>O<NUM>, CrPO<NUM>, and the like, for example) is formed by immersing the positive electrode lead <NUM> in a solution including phosphoric acid, chromic anhydride, and a fluoride. In addition, a zinc phosphate film (specific compositions thereof are Zn<NUM>(PO<NUM>)<NUM>, AlPO<NUM>, and the like, for example) is formed by immersing the positive electrode lead <NUM> in a solution including phosphoric acid, zinc phosphate monobasic, and a fluoride.

The anodized film is formed by methods defined in JIS H <NUM> and JIS H <NUM>, for example. An anodized film (alumite) is formed by immersing the positive electrode lead <NUM> including aluminum or an alloy mainly containing aluminum in a <NUM>% sulfuric acid solution followed by anodization with an applied voltage of <NUM> V, for example. A specific composition of the anodized film is aluminum oxide or the like, for example. The solution is not limited to sulfuric acid and may be phosphoric acid, oxalic acid, or the like, for example.

Examples of the oxide film other than the anodized film include a boehmite film, and the like. These oxide films are formed by hydrothermal treatment and the like, for example. Among the oxide films, the anodized film is preferable in terms of obtaining an oxide film with high insulation properties and denseness.

The insulating layer <NUM> may have a monolayer structure including the oxide film such as the anodized film, the phosphate film, or the like or may have a laminated structure in which the oxide film such as the anodized film and the phosphate film or the like are laminated.

While the insulating tape <NUM> shown in <FIG> and <FIG> is disposed on both of the one principal surface-side and the other principal surface-side of the positive electrode <NUM>, the insulating tape <NUM> may be disposed at the one principal surface-side of the positive electrode <NUM> on which the positive electrode lead <NUM> is disposed. The insulating tape <NUM> disposed on the one principal surface-side covers the insulating layer <NUM> formed on the first end part 20a of the positive electrode lead <NUM>, the exposed part 32a, and the boundary part between the exposed part 32a and the positive electrode active material layer <NUM>. In addition, the insulating tape <NUM> disposed on the other principal surface-side covers the exposed part 32a and the boundary part between the exposed part 32a and the positive electrode active material layer <NUM>.

The insulating tape <NUM> may include at least the substrate layer, and the adhesive layer is not an essential component. When an insulating tape provided with no adhesive layer is used, an adhesive may be applied to a part for attachment, and the insulating tape may be attached on, for example.

In this manner, by forming the insulating layer <NUM> on the outer surface of the first end part 20a of the positive electrode lead <NUM> connected to the exposed part 32a of the positive electrode current collector <NUM> and covering the insulating layer <NUM> with the insulating tape <NUM>, an internal short circuit between the positive electrode lead <NUM> and the negative electrode <NUM> is suppressed even if a foreign matter having entered a battery breaks through the insulating tape <NUM> and the insulating tape <NUM> breaks because the positive electrode lead <NUM> on which the insulating layer <NUM> is formed is exposed. Even in a case where a foreign matter penetrates the insulating layer <NUM> and reaches the positive electrode lead <NUM> and an internal short circuit occurs between the positive electrode lead <NUM> and the negative electrode <NUM>, since the insulating layer <NUM> existing around the foreign matter acts as a large short-circuit resistor, heat generation in the battery due to the internal short circuit is suppressed and an increase in battery temperature is suppressed.

While the insulating layer <NUM> may be formed on the outer surface of the first end part 20a of the positive electrode lead <NUM>, the insulating layer <NUM> is preferably further formed on the extension part 20b of the positive electrode lead <NUM>. Consequently, conduction between the positive electrode lead <NUM> and other members due to contact therebetween is suppressed, and deterioration of battery performance is suppressed. The insulating layer <NUM> may be formed on a part of the extension part 20b or may be formed on the entirety of the extension part 20b.

The insulating layer <NUM> may be formed on the second end part 20c (see <FIG>) of the positive electrode lead <NUM>. However, when the insulating layer <NUM> is formed on the second end part 20c of the positive electrode lead <NUM>, contact resistance against the sealing assembly <NUM> shown in <FIG> increases, and battery performance may deteriorate. Accordingly, the insulating layer <NUM> is preferably not formed on the second end part 20c of the positive electrode lead <NUM>.

A thickness of the insulating layer <NUM> is preferably within a range of <NUM> to <NUM>, for example. By setting the thickness of the insulating layer <NUM> to <NUM> or more, occurrence of an internal short circuit or an increase in battery temperature, which arises when the insulating tape <NUM> breaks due to a foreign matter, is suppressed compared with a case where the thickness of the insulating layer <NUM> is less than <NUM>. An insulating layer <NUM> having a thickness exceeding <NUM> is not desirable because time and costs for film formation increase.

Puncture breaking strength of the insulating layer <NUM> is preferably higher than puncture breaking strength of the insulating tape <NUM>. Consequently, occurrence of an internal short circuit in a battery can be efficiently suppressed. A measurement method of puncture breaking strength will be described in the examples section.

While the insulating tape <NUM> may cover the insulating layer <NUM> formed on the first end part 20a of the positive electrode lead <NUM>, the insulating tape <NUM> desirably further covers a part or the entirety of the exposed part 32a and a part or the entirety of the boundary part between the exposed part 32a and the positive electrode active material layer <NUM>. Consequently, a foreign matter having entered a battery hardly comes into contact with the exposed part 32a or the boundary part, and occurrence of an internal short circuit in a battery is further suppressed. Furthermore, the insulating tape <NUM> may cover a part or the entirety of the extension part 20b of the positive electrode lead <NUM>. Consequently, an internal short circuit due to contact of the positive electrode lead <NUM> with other members is suppressed.

As a positive electrode active material, <NUM> parts by weight of a lithium nickel cobalt aluminum composite oxide represented by LiNi<NUM>Co<NUM>Al<NUM>O<NUM>, one part by weight of acetylene black (AB), and one part by weight of polyvinylidene difluoride (PVdF) were mixed; and an appropriate amount of N-methyl-<NUM>-pyrolidone (NMP) was further added thereto to prepare positive electrode mixture slurry. Thereafter, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector made from aluminum foil and dried. The resultant product was cut into a predetermined electrode size and rolled by using a roller to prepare a positive electrode, with a positive electrode active material layer formed on the both sides of the positive electrode current collector. An exposed part on which no positive electrode active material layer was formed and the positive electrode current collector was exposed was formed approximately at a central part in the longitudinal direction of the positive electrode.

An aluminum positive electrode lead having a thickness of <NUM> and a width of <NUM> was cut to have a predetermined length. Parts of this positive electrode lead to which the exposed part of the positive electrode current collector and a sealing assembly are respectively welded were masked, and the positive electrode lead was subsequently immersed in a <NUM>% sulfuric acid solution followed by anodization with an applied voltage of <NUM> V to form an anodized film having a thickness of <NUM> on the positive electrode lead.

Puncture breaking strength of the anodized film was <NUM> N. Puncture breaking strength of the anodized film was measured as follows. The anodized film on the positive electrode lead is pressed by a stainless-steel nail (diameter: <NUM>, tip angle: <NUM>°), and electrical resistance between the stainless-steel nail and the positive electrode lead is measured while increasing the load of the stainless-steel nail. Then, the load applied at a time when electrical resistance indicating dielectric breakdown is observed is taken as puncture breaking strength.

A part of the positive electrode lead on which no anodized film was formed was brought into contact with the exposed part of the positive electrode current collector and bonded to the exposed part by ultrasonic welding. That is, the positive electrode lead had a first end part connected to the exposed part, an extension part extending toward an outside of a peripheral part of the positive electrode current collector, and a second end part connected to a sealing assembly at a leading end side of the extension part and became a positive electrode lead with the anodized film formed on an outer surface of the first end part and the extending part thereof.

The anodized film formed on an outer surface of the first end part of the positive electrode lead and the exposed part were covered with an insulating tape. With respect to the negative electrode side, the negative electrode lead on the exposed part and the exposed part were covered with the insulating tape. The insulating tape used included a substrate layer made from a polyimide film having a thickness of <NUM> and an adhesive layer made from a rubber-based resin.

Puncture breaking strength of the insulating tape was <NUM> N. Puncture breaking strength of the insulating tape was measured as follows. The insulating tape attached to an aluminum plate having a thickness of <NUM> is pressed by a stainless-steel nail (diameter: <NUM>, tip angle: <NUM>°), and electrical resistance between the stainless-steel nail and the aluminum plate is measured while increasing the load of the stainless-steel nail. Then, the load applied at a time when electrical resistance indicating dielectric breakdown is observed is taken as puncture breaking strength.

A non-aqueous electrolyte secondary battery was prepared in the same manner as Example B1 except that an anodized film having a thickness of <NUM> was formed on the positive electrode lead by adjusting the electric quantity during anodization treatment. Puncture breaking strength of the anodized film was <NUM> N.

A non-aqueous electrolyte secondary battery was prepared in the same manner as Example B1 except that no anodized film was formed on the positive electrode lead.

Claim 1:
A secondary battery (<NUM>), comprising:
a positive electrode (<NUM>);
a negative electrode (<NUM>);
a battery case (<NUM>) housing the positive electrode (<NUM>) and the negative electrode (<NUM>);
a positive electrode lead (<NUM>) electrically connected to the positive electrode (<NUM>); and
an electrical insulating tape (<NUM>) covering a part of the positive electrode lead (<NUM>), wherein
the positive electrode (<NUM>) has a positive electrode current collector (<NUM>) and a positive electrode active material layer (<NUM>) formed on the positive electrode current collector (<NUM>),
the positive electrode current collector (<NUM>) has, at a longitudinally central part of the positive electrode active material layer (<NUM>) on the positive electrode current collector (<NUM>), an exposed part (32a) on which no positive electrode active material layer (<NUM>) is formed,
the positive electrode lead (<NUM>) has a first end part (20a) connected to the exposed part (32a) and an extension part (20b) extending from the first end part (20a) toward an outside of a peripheral part (32b) of the positive electrode current collector (<NUM>),
an electrical insulating layer (<NUM>) is disposed on an outer surface of the first end part (20a) of the positive electrode lead (<NUM>),
the electrical insulating layer (<NUM>) is covered with the electrical insulating tape (<NUM>), and
the electrical insulating tape (<NUM>) covers a boundary part between the exposed part (32a) and the positive electrode active material layer (<NUM>),
characterized in that the insulating tape (<NUM>) has a larger width than that of the electrically insulating layer (<NUM>) as measured in the longitudinal direction of the positive electrode (<NUM>), and
in that the insulating tape (<NUM>) includes a substrate layer (30a) containing an organic material and an adhesive layer (30b) adhering to the electrical insulating layer (<NUM>) and the exposed part (32a).