JP2002151037A - Separator for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an extremely safe lithium ion secondary battery without having a rupture/inflammation of the battery in overcharge. SOLUTION: In an electrochemical cell in which a film having a thickness of 40 μm or less and 1.5×102 N/m or more of load-bearing point strength in a tensile test is used, a MacMillan number is 10 or less when an electrolytic solution is impregnated into the film, and in which the film is pinched between a metallic foil electrode (electrode A) electrochemically stable in an oxidation- reduction potential of lithium and an electrode (electrode B) capable of electrochemically discharging lithium, when a current is made to flow so as to make a metallic lithium deposited on the electrode A, this is a separator for a lithium ion secondary battery in which a rapid reduction of an absolute value of voltage of the electrochemical cell is observed within 2 mAh/cm2 of supply-current amount of electricity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a separator for a lithium ion secondary battery and a lithium ion secondary battery. In particular, the present invention relates to a lithium ion secondary battery separator for protecting a lithium ion secondary battery from overcharge and a lithium ion secondary battery.

[0002]

2. Description of the Related Art A 4V-class lithium ion secondary using a lithium-containing transition metal oxide represented by lithium cobalt oxide for the positive electrode and a carbon material capable of doping and undoping lithium for the negative electrode Batteries are very important as a power source for portable electronic devices typified by cellular phones because of their high energy density, and the demand for batteries is increasing with the rapid spread of these portable electronic devices.

A major problem in the effective use of a lithium ion secondary battery is safety, particularly, security during overcharge. When the lithium ion secondary battery is overcharged, excessive lithium is extracted from the positive electrode, and metallic lithium is deposited on the surface of the negative electrode. At this time, crystal collapse of the positive electrode and decomposition of the electrolytic solution occur. As a result, an abnormal rise in battery temperature and gas generation inside the battery are induced, and the battery is ruptured and ignited, which is extremely dangerous.

In the current lithium ion secondary battery, in order to prevent overcharging, the voltage is controlled by adopting a constant current / constant voltage charging and mounting a protection circuit. Since these controls are performed by an electronic circuit, they may be broken. Therefore, assuming such a case, a safety valve, a PTC element, and a polyolefin microporous membrane separator having a thermal fuse function are mounted to provide double and triple protection. Nevertheless, in some cases, overcharging may be caused and the user may be exposed to dangers such as ignition. Therefore, there is a problem that these protection circuits and the like do not essentially solve overcharging. Further, these protection devices occupy one third of the cost of a single cell of the lithium ion secondary battery, which is a factor of increasing the cost of the lithium ion secondary battery.

As an approach to such a problem, a technique of adding a redox shuttle called a redox shuttle to an electrolytic solution has been proposed in JP-A-6-338347 and JP-A-7-302614.
This is achieved by adding a reagent having an oxidation-reduction potential of 4.0 to 4.5 V (based on the oxidation-reduction potential of lithium) to an electrolyte and consuming an overcharge current by an oxidation-reduction reaction between the positive and negative electrodes of the reagent. This is a technique for preventing the battery from being overcharged.

[0006] However, considering the electrode reaction rate of the redox reagent and the diffusion in the electrolytic solution, when considering the amount of addition for sufficiently bringing out such a function at a practical charging rate such as 1C, the solubility and the battery performance are reduced. From the viewpoint of characteristic deterioration, overcharge protection by this method is difficult and has not been put to practical use. Further, these redox reagents are not necessarily low-cost reagents, and have no merit in terms of cost.

Japanese Patent Application Laid-Open No. 2000-67917 discloses a technique for improving the safety of a polymer lithium ion secondary battery at the time of overcharging by suppressing lithium dendrite which grows on the surface of the negative electrode at the time of overcharging. . According to this, in this technique, the thicker the gel electrolyte layer, the better the overcharge characteristics. However, this technology also does not completely prevent overcharging and is therefore not inherently safe.

On the other hand, a technique using a thin film nonwoven fabric as a separator for a lithium ion secondary battery is disclosed in, for example,
No. 0-123815. However, there is no description about the overcharge prevention function of the separator using these nonwoven fabrics.

In other words, the present invention provides a separator for a lithium ion secondary battery in which overcharging itself does not occur even if an overcharging operation is performed, and a lithium ion secondary battery using the same, based on the above-described current situation. The purpose is to:

In a lithium ion secondary battery, not all lithium contained in a lithium-containing transition metal oxide as a positive electrode active material is used for ordinary charge and discharge. For example, in lithium cobalt oxide, which is currently mainly used as a positive electrode active material, about half of the contained lithium is used for ordinary charge and discharge.

As described above, when the lithium ion secondary battery is overcharged and all the lithium is withdrawn from the positive electrode, the battery eventually explodes or ignites. Conversely, before all the lithium is extracted from the positive electrode, there is little possibility of exposure to danger such as ignition. For example, in a lithium ion secondary battery using lithium cobalt oxide as a positive electrode active material, when overcharging is performed by 1C charging which is usually adopted, the battery bursts and ignites when the charging rate exceeds 200%.

From the above, the present inventors use the metallic lithium deposited on the surface of the negative electrode during overcharging, and establish an electron conduction path between the positive and negative electrodes before all the lithium is extracted from the positive electrode active material. If such a path for consuming the charging current is formed, the charging of the battery does not proceed, the extraction of lithium from the positive electrode stops apparently, and the battery is in an equilibrium state. Even if the charging operation is continued, the charging rate is 200% or more. It is thought that it is possible to provide a lithium-ion secondary battery that is essentially safe from overcharging and does not lead to a dangerous state of explosion or ignition, since the battery does not occur.

In order to bring out such an overcharge protection function, it is not preferable to strongly suppress the surface of the negative electrode by a separator, but it is preferable that the state between the positive electrode and the negative electrode is close to the state of only the electrolytic solution. However, a separator is required to prevent a short circuit between the positive and negative electrodes.

Therefore, the present inventors have focused on nonwoven fabrics,
The non-woven fabric having the above-described overcharge protection function and having sufficient short-circuit resistance and mechanical properties has been intensively studied, and has a film thickness of 40 μm or less and a proof point strength of 1.5 × 10 ⁵ N in a tensile test. / M or more, when impregnated with an electrolytic solution, has a Macmillan number of 10 or less, and forms the film with a metal foil electrode (electrode A) which is electrochemically stable at the oxidation-reduction potential of lithium. In an electrochemical cell sandwiched between electrodes capable of releasing lithium (electrode B), when a current is applied so as to deposit metallic lithium on electrode A, a sharp decrease in the absolute value of the cell voltage causes a decrease in the electric current. and we found that it is possible to amount 2 mAh / cm extremely safe provision of a lithium ion secondary battery against overcharge by ² by using the separator of the lithium ion secondary battery to be observed within, Which resulted in the completion of the invention.

[0015]

That is, the present invention includes the following inventions. 1. A separator used for a lithium ion secondary battery in which a negative electrode is mainly made of a lithium-doped / dedopable carbon material and a positive electrode is mainly made of a lithium-containing transition metal oxide. The separator has a thickness of 40 μm or less and is subjected to a tensile test. Strength of 1.5 × 10 ² N / m
A metal foil electrode (electrode A) containing the above film, having a Macmillan number of 10 or less when the film is impregnated with an electrolytic solution, and electrochemically stable at a redox potential of lithium. In an electrochemical cell sandwiched between electrodes capable of releasing lithium (electrode B), when an electric current is applied so as to deposit metallic lithium on electrode A, the absolute value of the voltage of the electrochemical cell sharply increases. A separator for a lithium ion secondary battery in which a decrease is observed within an electric current of ² mAh / cm ² .

2. The above-mentioned item 1, wherein when the film is sandwiched between positive and negative electrodes and the film is bent 180 degrees so that the R of the bent portion is 0.3 to 1 mm, the resistance of the bent portion is 1 × 10 ⁹ Ω or more. For lithium ion secondary batteries.

3. The film is formed of fibers composed of an organic polymer and having an average fiber diameter of 5 μm or less.
3. The separator for a lithium ion secondary battery according to the above 1 or 2, which is a nonwoven fabric having a maximum pore size of 20 μm or less and 5 g / m ² or less.

4. A lithium ion secondary battery in which the negative electrode is mainly made of a lithium-doped / dedopable carbon material and the positive electrode is mainly made of a lithium-containing transition metal oxide, wherein the lithium ion secondary battery according to any one of the above 1 to 3 is provided. A lithium ion secondary battery using a battery separator.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a separator for a lithium ion secondary battery and a lithium ion secondary battery of the present invention will be described in detail.

<Separator for lithium ion secondary battery>
The separator for a lithium ion secondary battery of the present invention is mainly for a lithium ion secondary battery in which the negative electrode is mainly made of a lithium-doped / undoped carbon material and the positive electrode is mainly made of a lithium-containing transition metal oxide, Thickness 4
0 μm or less, strength point strength in tensile test is 1.5 × 1
0 ² N / m or more, the film has a Macmillan number of 10 or less when the film is impregnated with an electrolytic solution, and the film is a metal foil electrode (electrode) which is electrochemically stable at the oxidation-reduction potential of lithium. In the electrochemical cell sandwiched between A) and an electrode capable of electrochemically releasing lithium (electrode B), when a current is applied so as to deposit metallic lithium on electrode A, the voltage of the electrochemical cell is reduced. It is characterized in that a sharp decrease in the absolute value is observed within an electric current of ² mAh / cm ² .

The thickness depends on the internal resistance and energy density of the battery. When the film thickness exceeds 40 μm, the internal resistance increases and the energy density decreases, which is not preferable. From such a viewpoint, the film thickness is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less.

The proof stress point is the strength at the elastic limit, and the physical properties indicate how much force the film is pulled. That is, the higher the value, the easier the handling and the higher the productivity.
Considering the productivity of the battery, the yield strength in the tensile test is preferably 1.5 × 10 ² N / m or more, more preferably 3.0 × 10 ² N / m or more.

The Macmillan number is reflected in the characteristics of the battery.
The Macmillan number is a value obtained by dividing the resistance when the film is impregnated with the electrolyte by the resistance when only the electrolyte is used. Considering the practicality of the lithium ion secondary battery, 10 or less is preferable, and 5 or less is more preferable.

In an electrochemical cell having this film sandwiched between a metal foil electrode (electrode A) electrochemically stable at the oxidation-reduction potential of lithium and an electrode capable of electrochemically releasing lithium (electrode B), The feature that when a current is applied so as to deposit metallic lithium on the electrode A, a sharp decrease in the absolute value of the cell voltage is observed within an electric current of ² mAh / cm ^2. is there. The sudden decrease in the absolute value of the cell voltage occurs when the metallic lithium deposited on the electrode A reaches the vicinity of the interface of the electrode B and is inserted, and the amount of electricity until the sudden decrease occurs is determined by the overcharge protection function. Is the amount of the deposited metal lithium required to develop the overcharge protection function. The lower this value is, the more easily the overcharge protection function is developed.

In the current lithium ion secondary battery, lithium cobalt oxide is used as the positive electrode active material, and the capacity per unit area is about ² to 3 mAh / cm ² , so that the overcharge is performed at ² mA / cm ² or less. If a protective function is developed, safety during overcharging can be sufficiently ensured. In consideration of safety, 1 mA / cm ² or less is more preferable,
0.5 mA / cm ² or less is more preferable.

Hereinafter, the present measuring method will be described. Electrode A
Any metal that is electrochemically stable at the oxidation-reduction potential of lithium can be used, and examples thereof include copper foil, lithium foil, nickel foil, and SUS foil. The electrode B may be any electrode capable of electrochemically releasing lithium, and is preferably a positive electrode used for a lithium ion secondary battery using a lithium-containing transition metal oxide as an active material.

A separator is sandwiched between the electrodes A and B and joined, and an electrolytic solution is injected to produce an electrochemical cell. Here, there is no problem if the cell is assembled after the separator is impregnated with the electrolytic solution. The electrolytic solution can be used for the measurement as long as it is an organic electrolytic solution in which a lithium salt used for an ordinary lithium ion secondary battery is used as an electrolyte.

After the cell is assembled, an electric current is applied to the electrode A so as to deposit metallic lithium. If the current density is too low, it is difficult to observe a sharp drop in voltage, which is not preferable. It is preferably about 0.5 to ⁵ mA / cm ² ,
2-3 mA / cm ² is more preferred.

The rapid decrease in the absolute value of the cell voltage means an instantaneous decrease of 5 mV or more, and the instant refers to a time within 5 seconds. Depending on how the electrodes and cells are made, a sharp drop in voltage may be observed immediately after energization, but this is not because the metallic lithium deposited at the electrode A has reached and inserted near the interface of the electrode B. It shall be different from a sudden voltage drop.

When the film is sandwiched between positive and negative electrodes and bent 180 degrees so that the R of the bent portion is 0.3 to 1 mm, the resistance of the bent portion is 1 × 109.
It is preferably at least Ω. This is, for example, sandwiching the film between a positive electrode and a negative electrode having a thickness of 75 to 150 μm,
This is a jig (1) having an uneven shape as shown in FIG.
Jig (1) with a load of 1 kg sandwiched between the uneven surfaces of (2)
Or when it is applied from the flat surface of (2), the jig (1),
(2) This is related to short-circuit resistance in which the resistance between them becomes 1 × 10 ⁹ Ω or more. This measurement method is a model of a bent portion where a short circuit is most likely to occur in battery production. 1 × 10 ⁹ Ω or more, more preferably 1 × 10 ⁹ Ω
If a value of × 10 ¹⁰ Ω or more can be obtained, a battery can be manufactured without short-circuiting even in a bending process of a rectangular battery or the like.

Hereinafter, the present measuring method will be described. The material of the jigs (1) and (2) is not particularly limited, but is preferably made of a hard metal such as SUS. The positive electrode and the negative electrode used for this measurement are not particularly limited as long as they are those used in a normal lithium ion secondary battery. The most important thing for the electrode used in this measurement is the strength of the active material layer,
When sandwiched between the jigs (1) and (2), a crack or the like is generated in the electrode active material layer, and the active material layer must not be detached from the current collector.

The thickness of the positive electrode and the negative electrode used in this measurement is preferably in the range of 75 to 150 μm in consideration of the structure of the jig. Here, the thickness of the electrode is the total thickness of the active material layer and the current collector. A separator is sandwiched between the positive electrode and the negative electrode. This is sandwiched between the uneven surfaces of the jigs (1) and (2), and a load of 1 kg is applied to the flat surface of the jig (1) or (2). It is made. At this time, the electrolyte is not injected, and the measurement is performed with the separator and the electrode in a dry state. The measurement is performed with a 15 mm diameter circle as the electrode. If the jigs do not come into contact with each other, the measurement will not be performed. Therefore, it is preferable that the separator be used with an area that protrudes from the jigs.

The resistance of the cell is determined by measuring the current flowing after one minute when a voltage of 1 V is applied to the cell and calculating from Ohm's law. A separator that satisfies all of the above characteristics is formed of fibers of an organic polymer having an average fiber diameter of 5 μm or less, and can be realized by a nonwoven fabric having a basis weight of 25 g / m ² or less and a maximum pore diameter of 20 μm or less.

For the purpose of preventing a short circuit, the maximum pore size is preferable.
20 μm or less, more preferably 10 μm or less.
Suitable. Such a film thickness 40 having a maximum pore diameter of 20 μm or less.
In order to obtain a non-woven fabric of μm or less,
The average fiber diameter of the fibers is preferably 5 μm or less, and 2 μm
The following are further preferred. The basis weight is 25 g / m ^Two
The following are preferred. The above maximum pore diameter and average fiber diameter are
It can be measured with a scanning microscope.

The fibers forming such a nonwoven fabric are preferably made of an organic polymer from the viewpoint of moldability. However, since the overcharge protection function of the present invention is realized by the shape of the separator, the material constituting the separator is not particularly limited. For example, a derivative of polyolefin, polyester, polyphenylene sulfide, polysulfone, polyether sulfone, polytetrafluoroethylene, polyamide, or a mixture thereof is preferably used.

Examples of the method for producing the nonwoven fabric include a needle punch method in which carding and needle processing are combined, a chemical bonding method in which fibers are bonded with an adhesive, a thermal bonding method using a thermoplastic resin, an air-lay method using air, Various methods are known, such as a spanning lace method in which fibers are entangled with a high-pressure jet stream, a melt blowing method in which a nonwoven fabric is directly produced while blowing fibers, and a wet papermaking method. In order to obtain a nonwoven fabric as in the present invention, a melt blow method is suitable. <Lithium ion secondary battery> The lithium ion secondary battery of the present invention is characterized by using the lithium ion secondary battery separator of the present invention as described above,
As the electrolyte and the electrode, those used in conventional lithium ion secondary batteries can be used.

The electrode used in the lithium secondary battery of the present invention includes an active material for doping and undoping lithium ions, a binder polymer for binding the active material and swelling in an electrolyte, and a conductive agent for improving electron conductivity. And a current collector. The electrode may be gelled to have a structure capable of holding the electrolytic solution.

Examples of the positive electrode active material include various lithium-containing transition metal oxides, but are not particularly limited thereto, and may be any active material used in a so-called 4V class lithium secondary battery. . Lithium-containing cobalt oxide such as LiCoO ₂ as an example of a lithium-containing transition metal oxide, lithium-containing nickel oxides such as LiNiO _2, and the like lithium-containing manganese oxides such as LiMn ₂ O _4.

As the negative electrode active material, a carbon material that stores and releases lithium ions is used. Examples of the carbon material include polyacrylonitrile, a phenol resin, a phenol novolak resin, a material obtained by sintering an organic polymer compound such as cellulose, artificial graphite, and natural graphite.

The binder polymer that binds the active material and swells in the electrolyte is polyvinylidene fluoride (PVd).
F), PVdF and hexafluoropropylene (HFP)
Copolymer resins such as styrene and perfluoromethylvinylether (PFMV) and tetrafluoroethylene; fluororesins such as polytetrafluoroethylene and fluororubber; styrene-butadiene copolymer; styrene-acrylonitrile copolymer A hydrocarbon polymer such as a polymer, carboxymethylcellulose, a polyimide resin, or the like can be used, but is not limited thereto. These may be used alone or in combination of two or more.

As the current collector, a foil or mesh made of a material having excellent oxidation stability is suitably used for the positive electrode, and a foil or mesh made of a material having excellent reduction stability is preferably used for the negative electrode. Specifically, aluminum, stainless steel, nickel, carbon and the like can be used for the positive electrode, and metallic copper, stainless steel, nickel and carbon can be used for the negative electrode. In particular, for the positive electrode, aluminum foil or mesh,
Copper foil or mesh is suitably used for the negative electrode.

As the conductive aid, artificial graphite, carbon black (acetylene black), nickel powder and the like are preferably used. In the negative electrode, the conductive auxiliary may not be included.

For the lithium secondary battery of the present invention, an electrolyte obtained by dissolving a lithium salt as an electrolyte in a polar organic solvent is preferably used.

The organic solvent used is not particularly limited as long as it is a polar organic solvent having 10 or less carbon atoms generally used for lithium secondary batteries. For example, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DM
C), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), γ
-Butyrolactone (γ-BL), sulfolane, acetonitrile and the like. These polar organic solvents may be used alone or as a mixture of two or more. In particular, PC, EC, γ-BL, DMC, DEC, M
At least one or more organic solvents selected from EC and DME are preferably used.

Examples of the lithium salt dissolved in the organic solvent include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borotetrafluoride (LiBF ₄ ), and arsenic hexafluoride. Lithium (LiAs
F ₆ ), lithium trifluorosulfonate (LiCF ₃ SO
₃ ), lithium perfluoromethylsulfonylimide [L
iN (CF3SO _{2) 2]} and lithium perfluoro ethyl sulfonylimide _{[LiN (C 2 F 5 SO} 2) 2] , and the like. These may be used as a mixture. The concentration of the lithium salt to be dissolved is preferably in the range of 0.2 to 2M.

[0046]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. However, the following examples do not limit the present invention. <Evaluation Method of Separator> Hereinafter, an evaluation method of the separator in this example will be described.

[Evaluation of Overcharge Protection Function Characteristics] In this example, the overcharge protection function was evaluated by using a copper foil (18 μm thick) on the electrode A.
Rolled copper foil) and the positive electrode as the electrode B. The positive electrode used in this example was lithium cobalt oxide powder 89.5.
A positive electrode material paste was prepared using a 5% by weight solution of polyvinylidene fluoride in N-methyl-2-pyrrolidone so that the dry weight of carbon black, 4.5 parts by weight and polyvinylidene fluoride was 6 parts by weight. The obtained paste is applied on an aluminum foil having a thickness of 20 μm, dried and pressed. The thickness of the positive electrode was 100 μm, and the weight of the active material per unit area was 23 g / cm ² .

The separator was impregnated with an electrolytic solution, and sandwiched between electrodes A and B to produce an electrochemical cell. At this time, the electrode was punched out into a circle having a diameter of 15 mm (electrode area: 1.77 cm ² ). The electrolyte used was a solution obtained by dissolving LiBF ₄ at a concentration of 1 M in a mixed solvent of ethylene carbonate and diethyl carbonate in a weight ratio of 1: 1.
A current density of 0.56 mA /
An electric current was applied so that metallic lithium was deposited on the electrode A in cm ² , and the amount of electricity required for a sharp decrease in the absolute value of the cell voltage to occur was measured. The amount of electricity measured here is defined as an overcharge protection function characteristic value in this specification.

[Evaluation of short-circuit resistance] In the evaluation of short-circuit resistance in this example, evaluation was performed using a positive electrode and a negative electrode prepared as described below. 5.5% by weight of PVd so that the dry weight of lithium cobaltate powder 85 parts by weight, carbon black 5 parts by weight, and polyvinylidene fluoride becomes 10 parts by weight.
A cathode material paste was prepared using an N-methylpyrrolidone solution of F. The obtained paste was applied onto an aluminum foil having a thickness of 20 μm, dried and pressed, and a positive electrode having a thickness of 120 μm was produced. The average particle size of lithium cobaltate as an active material is 10 μm.

The negative electrode was used as a carbonaceous negative electrode material so that 90 parts by weight of mesophase carbon microbead powder and 10 parts by weight of the dry weight of polyvinylidene fluoride became 10 parts by weight of N-methyl-2-vinyl-2-fluorovinylidene fluoride. A negative electrode material paste was prepared using a pyrrolidone solution, and the obtained paste was applied on a copper foil having a thickness of 18 μm, dried, and then pressed. The thickness of the negative electrode is 100 μm. The average particle size of the mesophase carbon microbeads as an active material is 6 μm.

Each of the positive and negative electrodes as described above was 15 m in diameter.
m, and joined via a separator punched into a circle having a diameter of 30 mm such that the positive and negative electrode active material layers face each other. This is sandwiched between the uneven surfaces of the jigs (1) and (2) made of SUS316 as shown in FIG. 1 and a load of 1 kg is applied from the flat surface of the jig (2) to the jigs (1) and (2). )
Was measured for resistance. The resistance was measured by applying a voltage of 1V to both jigs and measuring the current flowing one minute after applying the voltage.
Calculated according to Ohm's law. In this specification, the resistance value measured by the above method is defined as a short-circuit resistance characteristic value.

[Measurement of Film Thickness] The film thickness was measured by an ordinary thickness gauge. [Measurement of Ion Conductivity] The separator was cut into a circle having a diameter of 20 mm, impregnated with an electrolytic solution, sandwiched between two SUS electrodes, and the resistance was calculated from the AC impedance at 10 kHz. In addition, the resistance of only the electrolyte was calculated using a conductivity meter. The Macmillan number was determined by dividing the resistance of the separator impregnated with the electrolyte by the resistance of the electrolyte alone. At this time, in the present embodiment, propylene carbonate and ethylene carbonate were used as electrolytes in a weight ratio of 1%.
The concentration of LiBF ₄ in the mixed solvent of 1 is 1
An electrolytic solution in which M was dissolved was used.

[Measurement of proof stress] Separator 1 cm
A 3 cm strip is cut out and a tensile test is performed using Tensilon. The tensile speed was 20 mm / min. The yield strength was calculated from the obtained load-elongation curve.

<Preparation and Evaluation of Separator> [Example 1] Using polyethylene terephthalate as a raw material, a basis weight of 1
A nonwoven fabric having 0 g / m ² and an average fiber diameter of 1.5 μm was produced by a melt blow method. This non-woven film is applied to a
Heat calendering was performed to a thickness of μm to obtain a separator for a lithium ion secondary battery. As a result of observing the obtained lithium ion secondary battery separator with an electron microscope,
The maximum pore size was 9 μm. As a result of evaluating this separator, the overcharge protection function characteristic value was 0.05 mA / cm ² , and the short-circuit resistance characteristic value was 3.5 × 10 ⁹ Ω. The Macmillan number is 3.0 and the proof stress is 2.5 × 1.
It was 0 ² N / m.

Example 2 Using a fiber composed of polyethylene terephthalate having an average fiber diameter of 3.2 μm as a main fiber and a fiber having an average fiber diameter of 4.5 μm as a binder, the main fiber and the binder fiber are in a weight ratio of 1: 1. To prepare a nonwoven fabric made of polyethylene terephthalate fiber by a wet method. This nonwoven fabric film was subjected to a heat calendering treatment so as to have a thickness of 18 μm to obtain a separator for a lithium ion secondary battery. As a result of observing this lithium ion secondary battery separator with an electron microscope, the maximum pore size was 10
μm. As a result of evaluating this separator, the overcharge protection function characteristic value was 0.04 mAh / cm ² and the short-circuit resistance characteristic value was 2.1 × 10 ⁹ Ω. The Macmillan number was 3.2 and the proof stress was 3.3 × 10 ² N / m ² .

<Evaluation in Button Battery> Hereinafter, the lithium ion secondary battery of the present invention will be described with reference to examples using a button battery.

[Preparation of Button Battery] The weight of lithium cobaltate powder was 85 parts by weight, carbon black was 5 parts by weight, and polyvinylidene fluoride was dried so that the dry weight was 10 parts by weight.
A cathode material paste was prepared using a 5% by weight solution of PVdF in N-methylpyrrolidone. The obtained paste was applied onto an aluminum foil having a thickness of 20 μm, dried, and then pressed to produce a positive electrode.

The negative electrode was made of N-methyl-2-polyvinylidene fluoride of 6% by weight so that 90 parts by weight of mesophase carbon microbead powder and 10 parts by weight of polyvinylidene fluoride were dried as a carbonaceous negative electrode material. A negative electrode material paste was prepared using a pyrrolidone solution, and the obtained paste was applied on a copper foil having a thickness of 18 μm, dried, and then pressed.

The above positive and negative were punched into a circle having a diameter of 15 mm and used for producing a button battery. As the electrolytic solution, an electrolytic solution in which LiBF ₄ was dissolved at a concentration of 1 M in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a weight ratio of 1: 1 was used. The size of the battery was CR2032.
0.17C, 4.2V constant current constant voltage charging, 0.17
C, the capacity of the battery in constant current discharge up to 2.5 V is 3 mAh.

[Evaluation of Overcharge in Button Battery] In the evaluation of overcharge in the button battery, overcharge was performed under the condition that a battery having a charge rate of 100% was charged to a charge rate of 2100%. The battery is then left open for 24 hours,
The voltage measured at that time was defined as the open circuit voltage after overcharge.
When the open circuit voltage was in the range of 4.2 to 4.5 V, charging was not proceeding, and it was determined that the overcharge protection function was sufficiently operated. Further, after overcharging, constant current discharge was performed to 0.17C and 2.5V, and the discharge capacity was measured. When this discharge capacity was in the range of 1 to 1.5 times the battery capacity, it was determined that the overcharge protection function worked sufficiently.

Example 3 A button battery was manufactured using the separator manufactured in Example 1. The overcharge evaluation of this battery was performed at a charging current of 1 C. As a result, the maximum battery voltage was 4.65 V, and the battery voltage vibrated finely around 4.5 V during overcharge and became steady. Open circuit voltage after overcharge is 4.2
It was 5 V, and a discharge capacity 1.05 times the battery capacity after overcharge was obtained.

Example 4 A button battery was produced using the separator produced in Example 2. The overcharge evaluation of this battery was performed at a charging current of 1 C. As a result, the maximum battery voltage was 4.62 V, and the battery voltage vibrated finely around 4.5 V during overcharge and became steady. Open circuit voltage after overcharge is 4.2
It was 4 V, and the discharge capacity after overcharge was 1.04 times.

[0063]

As described in detail above, according to the present invention, it is possible to provide a lithium ion secondary battery in which overcharging hardly occurs.

[Brief description of the drawings]

FIG. 1 shows a cross-sectional view of a representative example of two jigs used for short-circuit resistance evaluation in the present invention. FIG. 1A is a sectional view of a convex jig, and FIG. 2B is a concave jig. The unit of the numerical value in the following figure is mm.

Continuing from the front page (72) Inventor Hiromasa Minematsu 2-1 Hinodecho, Iwakuni-shi, Yamaguchi Prefecture F-term in the Iwakuni Research Center, Teijin Limited 5H021 AA06 CC01 CC02 EE02 HH00 HH03 HH06 5H029 AJ12 AK03 AL06 AL07 AM02 AM03 AM04 AM05 AM07 DJ04 DJ15 EJ12 HJ00 HJ04 HJ05 HJ06 HJ16 HJ20 5H050 AA03 BA17 CA08 CA09 CB07 CB08 CB12 DA19 HA04 HA05 HA06 HA17 HA19

Claims (4)

[Claims] 1. A separator for use in a lithium ion secondary battery in which a negative electrode is mainly made of a lithium-doped / dedopable carbon material and a positive electrode is mainly made of a lithium-containing transition metal oxide. 40
The strength point strength in the tensile test is 1.5 × 10
² N / m or more, and the film has a Macmillan number of 10 or less when impregnated with the electrolytic solution, and the film is a metal foil electrode (electrode A) which is electrochemically stable at the oxidation-reduction potential of lithium. ) And an electrode capable of electrochemically releasing lithium (electrode B), when a current is applied to electrode A to deposit metallic lithium, the absolute voltage of the electrochemical cell is A separator for a lithium ion secondary battery in which a sharp decrease in the value is observed within an electric current of ² mAh / cm ² . 2. The film is sandwiched between a positive electrode and a negative electrode, and the film is bent so that the R of the bent portion is 0.3 to 1 mm.
When bent at 80 degrees, the resistance of the bent portion is 1
The separator for a lithium ion secondary battery according to claim 1, which has a resistance of 10 ⁹ Ω or more. 3. The film is formed of fibers of an organic polymer having an average fiber diameter of 5 μm or less, and has a basis weight of 25 g.
/ M ² or less, according to claim 1 or 2 for a lithium ion secondary battery separator, wherein the maximum pore size 20μm or less of the nonwoven fabric. 4. A lithium ion secondary battery in which the negative electrode is mainly composed of a carbon material capable of doping and undoping lithium and the positive electrode is mainly composed of a lithium-containing transition metal oxide. A lithium ion secondary battery using the separator for a lithium ion secondary battery described in the above.