JP2001266949A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a lithium ion secondary battery in which a separator is consisted of a heat-resistant porous layer and a shut down layer, and a deterioration due to aging of a heat-resistant porous resin layer is small while charging. SOLUTION: In the lithium ion secondary battery containing a positive electrode sheet, a separator, non-aqueous electrolyte and a negative electrode sheet, the separator is consisted of a heat-resistant porous layer and a shut down layer, and the heat-resistant porous layer is arranged on the negative electrode sheet side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery including a positive electrode sheet, a separator, a non-aqueous electrolyte and a negative electrode sheet.

[0002]

2. Description of the Related Art In recent years, portable information devices including personal computers, portable telephones, portable information terminals and the like have been remarkably spread. Since these devices as multimedia are desired to have multiple functions, secondary batteries used as power sources are required to be small, light, and have large capacity, that is, high energy density. I have. In this regard, conventional aqueous secondary batteries such as lead storage batteries and nickel cadmium storage batteries are not satisfactory, and lithium ion secondary batteries capable of realizing higher energy densities, particularly lithium cobaltate, lithium nickelate, lithium manganese, etc. Research and development of lithium ion secondary batteries using a lithium composite oxide such as spinel as a positive electrode active material and a carbon material capable of doping / dedoping lithium ions as a negative electrode active material have been actively conducted.

Since these lithium ion secondary batteries have large inherent energy, higher safety is required against abnormal situations such as internal short-circuit and external short-circuit.
As a safety measure, a separator made of a polyolefin-based porous film is provided with a function that does not conduct electricity by making the polyolefin layer nonporous during abnormal heat generation (shutdown function) (hereinafter, a layer with a shutdown function is shut down) Layer).

[0004] In order to further improve the heat resistance of the separator to further increase the safety of the lithium ion secondary battery, a separator is disclosed in which a heat-resistant porous body and a porous body mainly composed of polyolefin (shutdown layer) are combined. . For example, JP-A-8-87995 describes a laminated structure of a polyphenylene sulfide and a porous body. Japanese Patent Application Laid-Open No. 9-161775 describes a structure in which each of a positive electrode sheet and a negative electrode sheet is covered with a polyolefin-based separator, and a heat-resistant porous film is arranged between the separators. Further, a heat-resistant porous layer of a battery using metallic lithium for the negative electrode,
For the arrangement of the shutdown layer, positive electrode and negative electrode,
A separator composed of a laminate of a fluororesin porous body and a polyolefin porous body, wherein the polyolefin porous body (shutdown layer) is disposed on the lithium metal (negative electrode) side (Japanese Patent Laid-Open No. 6-76808). Disposing a polyimide nonwoven fabric (heat-resistant porous layer) on the negative electrode side with a separator made of a laminate with a polyolefin nonwoven fabric (Japanese Unexamined Patent Publication No. 62-37871) is disclosed. Is not known about.

[0005] In a lithium ion secondary battery having a separator composed of a heat-resistant porous layer and a shutdown layer, there is a problem that the heat-resistant porous resin layer is significantly deteriorated with time during charging.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium ion secondary battery in which a separator comprises a heat-resistant porous layer and a shutdown layer, and the heat-resistant porous resin layer undergoes little deterioration over time during charging. It is in.

[0007]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that the heat-resistant porous layer is disposed on the negative electrode sheet side by using a separator comprising a heat-resistant porous layer and a shutdown layer. They have found that the deterioration of the porous layer with time can be suppressed, and have completed the present invention.

That is, the present invention relates to a lithium ion secondary battery including a positive electrode sheet, a separator, a non-aqueous electrolyte and a negative electrode sheet, wherein the separator comprises a heat-resistant porous layer and a shutdown layer, and the heat-resistant porous layer comprises the negative electrode sheet. The present invention relates to a lithium ion secondary battery disposed on a seat side.

[0009]

Next, the present invention will be described in detail.
The lithium ion secondary battery of the present invention is a non-aqueous electrolyte ion battery including a positive electrode sheet, a separator, a non-aqueous electrolyte and a negative electrode sheet, wherein the separator comprises a heat-resistant porous layer and a shutdown layer, and the heat-resistant porous layer Are disposed on the negative electrode sheet side. Here, the heat resistant porous layer may or may not be adjacent to the negative electrode sheet.

As the positive electrode sheet in the present invention, a sheet in which a mixture containing a positive electrode active material, a conductive material and a binder is carried on a current collector is usually used. As the positive electrode active material, a material containing a material capable of doping / dedoping lithium ions, a carbonaceous material as a conductive material, and a thermoplastic resin as a binder can be used. As the material capable of doping / dedoping the lithium ion, V, Mn, Fe,
Examples of the lithium composite oxide include at least one transition metal such as Co and Ni. Among them, preferred is a layered lithium composite oxide having an α-NaFeO ₂ type structure as a base, such as lithium cobalt oxide, lithium nickel oxide in which part of nickel is replaced by another element, or the like, in that the average discharge potential is high. And lithium composite oxides based on a spinel structure such as lithium manganese spinel. Above all, it is more preferable that the amount of oxygen that can be released at the time of thermal decomposition after charging the positive electrode active material is 15% by weight or less based on the positive electrode active material, from the viewpoint of increasing safety. Examples of such a positive electrode active material include lithium cobalt oxide, lithium manganese spinel, and lithium nickel oxide in which part of nickel is replaced with another element. Of these, lithium cobalt oxide is particularly preferable. When a part of nickel of lithium nickelate is replaced with another element, it depends on the element to be replaced. Generally, when 10% or more of nickel is replaced, the amount of oxygen that can be released at the time of thermal decomposition becomes 15% by weight per cathode active material. It is as follows.

The thermoplastic resin as the binder used in the present invention includes polyvinylidene fluoride, a copolymer of vinylidene fluoride, polytetrafluoroethylene,
Tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkylvinyl ether copolymer, ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer , Thermoplastic polyimide, polyethylene, polypropylene and the like.

Examples of the carbonaceous material as the conductive agent used in the present invention include natural graphite, artificial graphite, cokes, and carbon black. As the conductive material, each may be used alone, or for example, a composite conductive material system in which artificial graphite and carbon black are used in combination may be selected.

The heat-resistant porous layer in the present invention is a layer manufactured from a heat-resistant resin, and is a general term for a porous film, a nonwoven fabric, a woven fabric, and the like. preferable. The size of the voids in the heat-resistant porous layer, or when the voids can be approximated to a cylinder, the diameter of the cylinder (hereinafter sometimes referred to as the pore diameter) is preferably 3 μm or less, more preferably 1 μm or less, particularly preferably 0.1 μm or less. It is 1 μm or less. If the average size or pore diameter of the voids exceeds 3 μm, there may be a problem that short-circuit is likely to occur when carbon powder or a small piece of the main component of the positive electrode or the negative electrode falls off. The porosity of the heat-resistant porous layer is preferably 30 to 80% by volume, more preferably 40 to 70% by volume. If the porosity is less than 30% by volume, the retained amount of the electrolyte may be small, and if it exceeds 80% by volume, the strength of the heat-resistant porous layer may be insufficient.
The thickness of the heat-resistant porous layer is preferably 3 to 30 μm, more preferably 5 to 20 μm. If the thickness is less than 3 μm, the effect on safety as a heat-resistant porous layer may be insufficient. If the thickness exceeds 30 μm, the thickness of the non-aqueous electrolyte battery separator including the shutdown layer is too large to have a high electric capacity. May be difficult to achieve.

The heat-resistant resin for forming the heat-resistant porous layer is 18.6 kg / cm ² according to JIS K 7207.
At least one heat-resistant resin selected from resins having a deflection temperature under load of 100 ° C. or higher in the measurement under load is used. In order to be safer even at a high temperature due to severe use, the heat-resistant resin in the present invention is preferably at least one heat-resistant resin selected from resins having a deflection temperature under load of 200 ° C. or higher. The deflection temperature under load is 10
Examples of the resin at 0 ° C. or higher include polyimide, polyamideimide, aramid, polycarbonate, polyacetal, polysulfone, polyetheretherketone, aromatic polyester, polyethersulfone, and polyetherimide. Examples of the resin having a deflection temperature under load of 200 ° C. or higher include polyimide, polyamideimide, aramid, polyethersulfone, and polyetherimide. Further, as the heat resistant resin, polyimide,
It is particularly preferred to select from the group consisting of polyamideimide and aramid.

Further, the heat-resistant resin in the present invention includes:
The limiting oxygen index is preferably 20 or more. The limiting oxygen index is the minimum oxygen concentration at which a specimen placed in a glass tube can keep burning. That is, the heat-resistant porous layer is preferably flame-retardant in consideration of oxygen generated from the positive electrode material at high temperature, in addition to heat resistance. Specific examples of such a resin include the above-described heat-resistant resin.

The shutdown layer according to the present invention is a layer having a shutdown function, that is, a function of substantially disabling the layer due to a rise in temperature and preventing electricity from passing between the positive electrode and the negative electrode. It is a generic term for woven fabric and the like, and among them, a porous film is preferable because it can effectively provide a shutdown function. The shutdown layer is usually a porous layer made of a thermoplastic resin, and the size, porosity, and thickness of the voids are the same as those in the heat-resistant porous layer. Specifically, the size of the gap, or the diameter of the cylinder when the gap can be approximated to a cylinder (hereinafter, referred to as
(May be referred to as a pore size) is preferably 3 μm or less.
μm or less is more preferable. Furthermore, preferably 0.1 μ
m or less. The average size or pore size of the voids is 3μ.
If it exceeds m, there is a possibility that problems such as easy short-circuiting may occur when carbon powder or a small piece thereof, which is a main component of the positive electrode or the negative electrode, falls off. The porosity of the shutdown layer is
30-80 volume% is preferable, More preferably, 40-80 volume%.
70% by volume. If the porosity is less than 30% by volume, the holding amount of the electrolyte is small, and if it exceeds 80%, the strength of the film becomes insufficient and the shutdown function decreases. The thickness of the film is preferably from 3 to 30 μm, more preferably from 5 to 20 μm. If the thickness is less than 3 μm, the shutdown function is insufficient, and
If it exceeds 300, the thickness of the separator for a non-aqueous electrolyte battery including the heat-resistant porous layer is too large, and it is difficult to achieve a high electric capacity. However, as for the size of the void, if one of the heat-resistant porous layer and the shutdown layer satisfies the above condition, the other may be larger than 3 μm.

The thermoplastic resin forming the shutdown layer softens at 80 to 180 ° C., and closes the porous voids.
A thermoplastic resin that does not dissolve in the electrolytic solution is preferable. Specific examples include polyolefin and thermoplastic polyurethane, and polyolefin is more preferable. More specifically, the polyolefin includes at least one polyolefin selected from polyethylene, such as low-density polyethylene, high-density polyethylene, and ultrahigh-molecular-weight polyethylene, and polypropylene.

In the present invention, any of the heat-resistant resin for the heat-resistant porous layer and the thermoplastic resin for the shutdown layer can contain inorganic or organic fine particles.

Used in the lithium ion secondary battery of the present invention
As the non-aqueous electrolyte solution, for example, lithium salt is added to an organic solvent
Can be used. Re
As the lithium salt, LiClO_Four, LiPF₆, LiAs
F₆, LiSbF₆, LiBF _Four, LiCF_ThreeSO_Three, Li
N (SO_TwoCF_Three)_Two, LiC (SO_TwoCF_Three)_Three, Li_TwoB
_TenCl_Ten, Lower aliphatic carboxylic acid lithium salt, LiAl
Cl_FourOne or a mixture of two or more of these
It is. As lithium salt, contains fluorine among them
LiPF₆, LiAsF₆, LiSbF₆, LiBF_Four, L
iCF_ThreeSO_Three, LiN (CF_ThreeSO_Two)_Two, And LiC
(CF_ThreeSO_Two)_ThreeAt least one selected from the group consisting of
It is preferable to use those containing seeds.

Examples of the organic solvent used in the non-aqueous electrolyte of the present invention include propylene carbonate, ethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolane-solvent.
Carbonates such as 2-one and 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropylmethyl ether, 2,2,3,3-tetrafluoro Ethers such as propyldifluoromethyl ether, tetrahydrofuran and 2-methyltetrahydrofuran;
Esters such as methyl formate, methyl acetate and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N, N-dimethylformamide and N, N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone And sulfur-containing compounds such as sulfolane, dimethylsulfoxide, and 1,3-propanesultone, or those obtained by introducing a fluorine substituent into the above-mentioned organic solvent. Usually, two or more of these are used. Mix and use.

Among these, a mixed solvent containing a carbonate is preferable, and a mixed solvent of a cyclic carbonate and an acyclic carbonate, or a mixed solvent of a cyclic carbonate and an ether is more preferable. The mixed solvent of cyclic carbonate and acyclic carbonate has a wide operating temperature range, excellent load characteristics, and is hardly decomposable even when a graphite material such as natural graphite or artificial graphite is used as a negative electrode active material. And a mixed solvent containing ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is preferred.

As the negative electrode sheet in the present invention, for example, a material capable of doping / de-doping lithium ions, lithium metal or lithium alloy can be used. Materials that can be doped or undoped with lithium ions include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds, and a lower potential than the positive electrode. And chalcogen compounds such as oxides and sulfides for doping / dedoping lithium ions. As a carbonaceous material, a carbonaceous material mainly composed of a graphite material such as natural graphite and artificial graphite, in that a high energy density can be obtained when combined with a positive electrode because the potential flatness is high and the average discharge potential is low. Is preferred.

When the electrolyte is used in combination with a liquid electrolyte and the liquid electrolyte does not contain ethylene carbonate, it is preferable to use a polyethylene carbonate-containing negative electrode because cycle characteristics and large current discharge characteristics are improved. The shape of the carbonaceous material may be any of, for example, a flaky shape such as natural graphite, a spherical shape such as mesocarbon microbeads, a fibrous shape such as graphitized carbon fiber, or an aggregate of fine powder. In addition, a thermoplastic resin as a binder can be added. Examples of the thermoplastic resin include polyvinylidene fluoride, a copolymer of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene-terafluoroethylene, thermoplastic polyimide, polyethylene, and polypropylene. Examples of chalcogen compounds such as oxides and sulfides used as the negative electrode include crystalline or non-crystalline compounds mainly composed of groups 13, 14, and 15 of the periodic table, such as amorphous compounds mainly composed of tin oxide. And crystalline oxides. Also for these, a carbonaceous material as a conductive material and a thermoplastic resin as a binder can be added as necessary.

As the negative electrode current collector used in the lithium ion secondary battery of the present invention, Cu, Ni, stainless steel, or the like can be used. However, Cu is difficult to form an alloy with lithium and is easily processed into a thin film. Is preferred. As a method of supporting the mixture containing the negative electrode active material on the negative electrode current collector, a method of pressure molding, or a method of pasting using a solvent or the like, applying a paste on the current collector, drying and pressing, or the like, is used. Is mentioned.

In the separator of the present invention, various methods can be used for laminating the heat-resistant porous layer and the shutdown layer. Although the two layers may not be joined and may be simply overlapped, it is preferable to stack and fix them from the viewpoint of handleability. Examples of the fixing method include a method using an adhesive and a method using heat fusion, but the present invention is not limited to these.

Further, as a preferred laminating method, a porous film composed of a heat-resistant porous layer or a shutdown layer is used as a substrate, and the other is coated in a solution state to form a solution layer. This is a manufacturing method for forming a laminated film.

Specific examples are shown below, but the present invention is not limited to these. That is, the following (a)
To (e). (A) A solution comprising a heat-resistant resin and an organic solvent is prepared. When using inorganic fine powder, a slurry solution is prepared by dispersing 1 to 200 parts by weight of inorganic fine powder with respect to 100 parts by weight of heat-resistant resin. (B) applying the solution or slurry solution to a polyolefin-based porous film to form a coating film. (C) depositing the heat-resistant resin on the coating film. (D) removing the organic solvent from the coating film; (E) drying the coated film;

Further, the case of using a para-oriented aromatic polyamide (hereinafter sometimes referred to as para-aramid) and a polyimide will be specifically described. When para-aramid is used, for example, in a polar organic solvent in which 2 to 10% by weight of an alkali metal or alkaline earth metal chloride is dissolved, 1.00 mol of para-oriented aromatic diamine and para-oriented aromatic dicarboxylic acid are used. Acid dihalide 0.94-0.
A para-aramid having a concentration of 1 to 10% and an intrinsic viscosity of 1.0 to 2.8 dl / g, and an organic solvent, are added by adding 99 moles and performing condensation polymerization at a temperature of -20 ° C to 50 ° C. A solution consisting of Using this solution, the separator of the present invention in which a para-aramid porous layer is laminated on a polyolefin-based porous film by the above-mentioned production method can be produced. In the case of para-aramid, to remove the solvent and the chloride, it can be washed with the same solvent as a coagulating liquid such as water or methanol, but a part or all of the solvent was evaporated and a polymer was precipitated at the same time. Thereafter, the chloride may be removed by a method such as washing with water.

Examples of the solution comprising a polyimide and an organic solvent include 3,3 ', 4,4'-diphenylsulfonetetracarboxylic dianhydride and 4,4'-bis (p-aminophenoxy) diphenylsulfone. An example of the N-methyl-2-pyrrolidone solution of imidized polyimide obtained by a polycondensation reaction with an aromatic diamine is shown. In the solution, the inorganic fine powder is preferably 1 to 200 parts by weight, more preferably 5 to 10 parts by weight, based on 100 parts by weight of the polyimide.
0 parts by weight are sufficiently dispersed to prepare a slurry solution.
If the amount of the inorganic fine powder is less than 1 part by weight, the ion permeability and battery characteristics are insufficiently improved.
The separator is not preferred because it becomes brittle and difficult to handle. Next, the slurry solution is applied to the polyolefin-based porous film to form a solution layer. Examples of the coating method include a coating method using a knife, a blade, a bar, a gravure, a die, or the like. The obtained coating film is preferably precipitated in an atmosphere controlled at a temperature of 20 ° C. or higher and a constant humidity, and then dipped in a coagulation liquid to obtain a wet coating film. Next, after washing with water to remove the solvent and drying, a separator in which a polyimide porous layer is laminated on a polyolefin-based porous film can be produced.

The shape of the lithium ion secondary battery of the present invention is not particularly limited, and may be any of a paper type, a coin type, a cylindrical type, a square type, and the like.

[0031]

The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the invention thereto. (1) Intrinsic Viscosity In the present invention, the intrinsic viscosity is defined by the following measuring method. For a solution in which 0.5 g of the para-aramid polymer was dissolved in 100 ml of 96-98% sulfuric acid and for a 96-98% sulfuric acid, the flow time was measured at 30 ° C. using a capillary viscometer. Was used to determine the intrinsic viscosity. Intrinsic viscosity = ln (T / T ₀ ) / C [unit: dl / g] where T and T ₀ are the flow times of the para-aramid sulfate solution and sulfuric acid, respectively, and C is the concentration of para-aramid in the para-aramid sulfate solution ( dl / g).

(2) Measurement of Thickness of Coated Film and the Like The thickness of the obtained coated film and the like is measured in accordance with JIS K7130-
1992.

(5) Measurement of the amount of oxygen that can be released during pyrolysis after charging the positive electrode active material The positive electrode active material powder was press-molded to produce a 1 cm 2 pellet electrode. A test battery was prepared using lithium metal as a negative electrode and a porous polyethylene film as a separator. The electrolytic solution had a volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate of 1
A solution obtained by dissolving LiPF6 in a 6:10:74 mixture at a concentration of 1 mol / liter was used. As the positive electrode active material, lithium nickelate and lithium cobaltate were used. After the prepared test battery was fully charged at 4.3 V, the charged positive electrode active material powder was taken out in an argon atmosphere. The thermal decomposition behavior of the charged positive electrode was measured by a thermal analyzer. Both lithium nickelate and lithium cobaltate lost weight between 200 ° C and 500 ° C. The gas generated during the thermal decomposition was subjected to mass spectrum measurement, and it was confirmed that the gas was oxygen gas. When lithium nickelate was used as the active material, the weight loss due to oxygen release was 16% by weight per active material. When lithium cobaltate was used as the active material, the weight loss due to oxygen release was 6% by weight per active material.

Example 1 (1) Preparation of positive electrode sheet electrode One part by weight of carboxymethylcellulose was dissolved in water, and polytetrafluoroethylene was dispersed in 60% by weight of water so that the amount of polytetrafluoroethylene was 4.5 parts by weight. Liquid and
2.5 parts by weight of acetylene black and 92 parts by weight of lithium cobalt oxide powder as a positive electrode active material were dispersed and kneaded to prepare a positive electrode mixture paste. This paste is used as a current collector to a thickness of 20 μm.
m was applied to predetermined portions on both sides of an Al foil, dried and roll-pressed to obtain a positive electrode sheet electrode.

(2) Preparation of heat-resistant porous film 2.1) Synthesis of para-aramid solution 5 having a stirring blade, a thermometer, a nitrogen inlet tube and a powder addition port
Using a liter (l) separable flask, poly (paraphenylene terephthalamide) (hereinafter referred to as PPTA)
Abbreviation). Dry the flask thoroughly,
Methyl-2-pyrrolidone (hereinafter abbreviated as NMP) 420
After adding 0 g, 272.65 g of calcium chloride dried at 200 ° C for 2 hours was added, and the temperature was raised to 100 ° C. After the calcium chloride was completely dissolved, the temperature was returned to room temperature, and 132.91 g of paraphenylenediamine (hereinafter abbreviated as PPD) was obtained.
And completely dissolved. While maintaining this solution at 20 ± 2 ° C., 243.32 g of terephthalic acid dichloride (hereinafter, abbreviated as TPC) was added in about 10 minutes in 10 divided portions. Thereafter, the solution was aged for 1 hour while maintaining the solution at 20 ± 2 ° C., and stirred for 30 minutes under reduced pressure to remove bubbles. The obtained polymerization liquid showed optical anisotropy. A part was sampled, reprecipitated with water, taken out as a polymer, and the obtained PPT
When the intrinsic viscosity of A was measured, it was 1.97 dl / g. Next, 100 g of this polymerization solution was weighed into a 500 ml separable flask having a stirring blade, a thermometer, a nitrogen inlet tube and a liquid addition port, and an NMP solution in which 5.8% by weight of calcium chloride was dissolved was added. It was added slowly. Finally, a PPTA solution having a PPTA concentration of 2.0% by weight was prepared and used as solution A.

2.2) Preparation of Heat-Resistant Porous Film On a porous polyethylene film (thickness: 25 μm), a film-form material of Liquid A is applied by a bar coater, and the coating is performed in an atmosphere at a temperature of 60 ° C. and a relative humidity of 70%. Was left to precipitate para-aramid. After removing NMP and calcium chloride by washing with water, the film was dried to obtain a film A in which a para-aramid porous layer was laminated on a porous polyethylene film. The thickness of the film A is 28 μm.

(3) Preparation of negative electrode sheet electrode 2 parts by weight of carboxymethylcellulose were dissolved in water, and 98 parts by weight of natural graphite were dispersed and kneaded to prepare a negative electrode mixture paste. The paste was applied to predetermined portions on both sides of a 10-μm-thick Cu foil as a current collector, dried, and roll-pressed to obtain a negative electrode sheet electrode.

(4) Manufacture of Cylindrical Battery The positive electrode sheet, the negative electrode sheet and the film A as a separator prepared as described above were combined with a positive electrode sheet, a porous polyethylene film / para-aramid porous layer, and a negative electrode sheet in the order of para-aramid. The porous layer was laminated so as to be in contact with the negative electrode sheet, and wound from one end to obtain a spiral electrode element.

The above-mentioned electrode element was inserted into a battery can, and the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate was 16: 1 as a non-aqueous electrolyte solution.
A 0:74 mixture was impregnated with a solution prepared by dissolving LiPF ₆ at a concentration of 1 mol / liter, and a battery cover also serving as a positive electrode terminal was caulked via a gasket to obtain a 18650-size cylindrical battery.

After leaving this battery for one week in a fully charged state of 4.2 V, after disassembling the battery and taking out film A,
The infrared absorption spectrum of the para-aramid porous layer was measured. The spectrum of para-aramid was almost the same as that at the time of producing the film A, and there was almost no change in the spectrum.

A nail penetration test was performed on the two cylindrical batteries thus obtained in an overcharged state of 4.3 V. As a result, the battery subjected to the test showed only a gradual temperature rise despite the severe condition of overcharging.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the film A was laminated in the order of the positive electrode sheet, the para-aramid porous layer / porous polyethylene film, and the negative electrode sheet, that is, so that the paraamide porous layer was in contact with the positive electrode sheet. A battery was prepared. After the obtained battery was left for one week in a fully charged state of 4.2 V in the same manner as in Example 2, the battery was disassembled and the film A was taken out.
The infrared absorption spectrum of the para-aramid porous layer was measured. The spectrum of para-aramid at the time of producing the film A showed a change in the spectrum, and the quality changed with time.

Comparative Example 2 A cylindrical battery of 18650 size was obtained in the same manner as in Example 1 except that only a porous polyethylene film was used as the separator. The cylindrical battery was subjected to a nail penetration test in the same manner as in Example 1, and as a result, a remarkable temperature rise was observed after the nail penetration.

[0044]

According to the lithium ion secondary battery of the present invention, the separator is composed of a heat-resistant porous layer and a shutdown layer.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Yasuo Shinohara 6 Kitahara, Tsukuba, Ibaraki Sumitomo Chemical Co., Ltd. F-term (reference) 5H021 AA06 BB17 CC08 EE02 EE07 EE08 EE09 EE15 EE18 HH01 HH06 5H029 AJ12 AK03 AL02 AL04 AL06 AL07 AL08 AM02 AM03 AM04 AM05 AM07 CJ16 DJ04 DJ13 EJ11 EJ12 EJ14 HJ00 HJ01 HJ14 5H050 AA15 BA17 CA08 CA09 CB02 CB05 CB07 CB08 CB09 DA02 DA19 FA13 GA18 HA01 HA14

Claims (6)

[Claims] 1. A lithium ion secondary battery comprising a positive electrode sheet, a separator, a non-aqueous electrolyte and a negative electrode sheet,
A lithium ion secondary battery, wherein the separator comprises a heat-resistant porous layer and a shutdown layer, and the heat-resistant porous layer is disposed on the negative electrode sheet side. 2. The positive electrode active material contained in the positive electrode sheet is a composite oxide containing lithium and a transition metal, and after charging the positive electrode active material, the amount of oxygen that can be released at the time of thermal decomposition is 15% per positive electrode active material. 2. The composition according to claim 1, wherein the content is not more than weight%.
The lithium ion secondary battery according to the above. 3. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material contained in the positive electrode sheet is lithium cobalt oxide. 4. The heat-resistant porous layer has a deflection temperature under load of 100.
The lithium ion secondary battery according to any one of claims 1 to 3, wherein the lithium ion secondary battery is made of a heat-resistant resin having a critical oxygen index of 20 ° C or higher. 5. The shutdown layer according to claim 1, wherein the shutdown layer is a porous layer made of a thermoplastic resin, and becomes a substantially non-porous layer at a temperature of 80 ° C. to 180 ° C. A lithium ion secondary battery according to any one of the above. 6. The heat-resistant resin according to claim 1, wherein the heat-resistant resin is polyimide, polyamideimide, aramid, polycarbonate, polyacetal, polysulfone, polyetheretherketone, aromatic polyester, polyethersulfone or polyetherimide. 6. The lithium ion secondary battery according to any one of 5.