JP2002110126A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having high temperature performance, high temperature cycle service life characteristics and excellent standing performance. SOLUTION: The nonaqueous electrolyte secondary battery is characterized in a multilayer separator consisting of the first layer of cross-linked polyethylene minute porous membrane and the second layer of non-cross-linked polyethylene minute porous membrane, in which the second layer is abutted with a positive plate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery excellent in high-temperature cycle life, standing performance and safety.

[0002]

2. Description of the Related Art In recent years, as electronic devices such as mobile phones and portable personal computers have become smaller, lighter, and more sophisticated, lightweight batteries having a high energy density have been adopted as built-in batteries. I have. A typical battery that satisfies such a requirement is an active material such as lithium metal or lithium alloy, or a lithium ion as a host material (here, a host material refers to a material that can occlude and release lithium ions).
A lithium intercalation compound occluded in carbon as a negative electrode, and an aprotic organic solvent in which a lithium salt such as LiClO ₄ or LiPF ₆ is dissolved is used as an electrolyte,
A non-aqueous separator made of a polyolefin-based material that is insoluble in an organic solvent and stable to an electrolyte or an electrode active material processed into a microporous membrane or nonwoven fabric is used as a separator between the positive electrode and the negative electrode. It is an electrolyte secondary battery.

[0003] In addition to the general properties such as the mechanical strength and permeability of a microporous membrane, the separator for a non-aqueous electrolyte secondary battery may be melted when the inside of the battery generates abnormal heat. There is a demand for a "shutdown effect" in which the safety of the battery is ensured by interrupting the current.

[0004] In the case of a polyethylene microporous membrane using polyethylene as a polyolefin-based material, the temperature at which the shutdown effect appears is approximately 130 to 150 ° C.
Even if the inside of the battery generates heat due to an external short circuit or the like for some reason, when the shutdown temperature is reached, the microporous film is melted and the micropores are closed, so that the current is interrupted. And the battery reaction stops. However, when the temperature rise is extremely rapid, the temperature further rises even after shutdown, and as a result, the microporous membrane has
In some cases, the membrane was ruptured or broken, resulting in a short circuit. Development of a microporous polyethylene film having high heat resistance capable of preventing short-circuiting of a battery even under such severe conditions has been an issue, and various techniques have been proposed in recent years.

For example, Japanese Patent Application Laid-Open No. Sho 62-10857 proposes a method of improving heat resistance by laminating a microporous film of polypropylene having a higher melting point than polyethylene and a microporous film of polyethylene. . According to this method, the heat resistance is improved, but the shutdown temperature of the polypropylene layer is high, so that there is a disadvantage that the shutdown performance of the entire film is reduced.

Japanese Patent Application Laid-Open No. Hei 4-206257 proposes a method of improving heat resistance by blending polypropylene with polyethylene.
Although polypropylene is blended, it has not been an essential improvement in terms of improvement in heat resistance due to easy flow after melting due to heat generation and film breakage.
In addition, since the compatibility between polyethylene and polypropylene is low, they are separated from each other in the microporous membrane, so that the strength is reduced.

Japanese Patent Application Laid-Open No. 3-1055851 proposes a method for improving mechanical strength by blending a specific amount of ultra-high-molecular polyethylene with high-molecular polyethylene. Ultra-high molecular weight polyethylene has a considerable viscosity even after melting, that is, has shape retention properties, so the polyethylene microporous membrane according to the method disclosed in the above-mentioned publication makes it difficult to cause secondary fracture after melting. However, under severe conditions, the membrane is still broken, and is not an essential solution like the invention disclosed in the above-mentioned publication.

Japanese Patent Application Laid-Open No. 3-274661 discloses a method for improving mechanical strength, heat resistance and the like by crosslinking a polyolefin microporous membrane. According to this method, the viscosity at the time of melting is increased by the crosslinking, so that it is possible to impart high shape retention. However, when used or left for a long time in a high-temperature environment, the crosslinking is cut. However, there is a disadvantage that the oxidative deterioration such as the above proceeds, and the high-temperature cycle life and the leaving performance deteriorate.

[0009]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems, and is excellent in high-temperature performance, that is, excellent in oxidation resistance. An object of the present invention is to provide a non-aqueous electrolyte secondary battery having extremely high heat resistance capable of preventing short-circuit of the battery, and having excellent high-temperature cycle life characteristics and standing performance.

[0010]

The short-circuiting of the battery is caused by internal short-circuiting due to film breakage or rupture when the temperature further rises after shutdown, and the problems of cycle life at high temperature and deterioration of storage performance are as follows. It is thought to be due to oxidative degradation on the surface of the separator facing the positive electrode, and it is necessary to give the separator a performance that can withstand them.

The inventor of the present invention has conducted intensive studies in order to solve the above-mentioned problems, and as a result, by using a separator having a specific configuration and arranging it in a specific positional relationship between the positive electrode and the negative electrode, even when abnormal, It has been found that a battery having no short circuit, excellent in high-temperature cycle life, and excellent in storage performance can be obtained, and the present invention has been accomplished.

That is, the present invention provides a multilayer separator having a first layer of a crosslinked polyethylene microporous membrane and a second layer of a non-crosslinked polyethylene microporous membrane, wherein the positive electrode plate has the above-mentioned structure. A nonaqueous electrolyte secondary battery, wherein the second layer is in contact with the nonaqueous electrolyte secondary battery.

Further, the non-crosslinked polyethylene microporous membrane has a weight average molecular weight of 1,000,000 or more.
Microporous membranes of less than 500,000 polyethylene are preferred. The crosslinked polyethylene microporous membrane preferably has a weight average molecular weight in the range of 200,000 to 2.5 million. More preferably, the first layer is in contact with a negative electrode plate.

The above-mentioned crosslinked polyethylene microporous membrane layer and the non-crosslinked polyethylene microporous membrane layer are two
In the separator formed by laminating more than one layer, the positive electrode side is exposed to a very strong oxidizing atmosphere. However, in the present invention, the positive electrode side has a correspondingly higher oxidation resistance than the crosslinked polyethylene microporous membrane layer. Since the non-crosslinked polyethylene microporous membrane layer having the property is provided, it is possible to suppress a decrease in performance due to oxidative deterioration of the separator.

Further, even when the temperature rise is extremely rapid, the present invention includes a crosslinked polyethylene microporous membrane layer having excellent shape retention even after melting in the membrane. Can be prevented, and a battery maintaining excellent heat resistance even under severe conditions can be obtained.

[0016]

Embodiments of the present invention will be described below.

The laminated separator used in the present invention is a crosslinked polyethylene microporous membrane and a non-crosslinked polyethylene microporous membrane, each of which is made of, for example, T
After molding by a known method such as a die extrusion method or an inflation method, it can be molded by bonding by a method such as pressure bonding or adhesion.

Examples of the method of crosslinking polyethylene include ionizing radiation typified by ultraviolet rays, electron beams, and gamma rays, and chemical crosslinking by adding a crosslinking agent or a crosslinking assistant. Among these, the method of electron beam irradiation is used. More desirable.

As polyethylene, high density, medium density,
Any polyethylene, such as various types of low-density branched polyethylene, linear polyethylene, high molecular weight and ultrahigh molecular weight polyethylene, can be used. Further, those containing appropriate amounts of additives such as various plasticizers, antioxidants and flame retardants may be used.

In producing the non-aqueous electrolyte secondary battery according to the present invention, a separator formed as described above is used, and a non-crosslinked polyethylene microporous membrane layer is disposed on the positive electrode plate side. What is necessary is just to manufacture by a normal method.

The positive electrode plate is constituted by using a positive electrode active material.
However, for example, when producing a lithium secondary battery, the positive electrode active
The substance is a compound capable of inserting and extracting lithium.
The composition formula Li_xMO_TwoOr Li_yM_TwoO_Four(However, M
Is a transition metal, represented by 0 ≦ x ≦ 1, 0 ≦ y ≦ 2),
Compound oxides, oxides with tunnel-like vacancies, layered structure
Metal chalcogenide can be used. Its concrete
For example, LiCoO _Two, LiNiO_Two, LiMn
_TwoO_Four, Li_TwoMn_TwoO_Four, MnO_Two, FeO_Two, V_TwoO_Five, V₆
O₁₃, TiO_Two, TiS_TwoEtc. Also, polyaniline
Organic compounds such as conductive polymers can also be used.
And may be used in combination. Also grain
When an active material in the form of particles is used, for example, the active material particles
A mixture consisting of an electric assistant and a binder is made of a metal such as aluminum.
It can be manufactured by forming it on a current collector.

The negative electrode plate is formed using a negative electrode active material. For example, when a lithium secondary battery is manufactured, the negative electrode active material includes Al, Si, Pb, Sn, Zn, Cd and the like and lithium. Alloys, transition metal oxides such as LiFe ₂ O ₃ , WO ₂ and MoO ₂ , carbonaceous materials such as graphite and carbon, lithium nitride such as Li ₅ (Li ₃ N), or metallic lithium foil, or a mixture thereof May be used. When a granular carbonaceous material is used, it can be produced, for example, by forming a mixture of active material particles and a binder on a metal current collector such as copper.

As the electrolyte, an inorganic solid electrolyte, a polymer solid electrolyte, an electrolyte and the like can be used. When a non-aqueous electrolyte lithium secondary battery is manufactured, for example, ethylene carbonate, propylene carbonate, or propylene carbonate is used as the electrolyte solvent. Dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2
Polar solvents such as diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, methyl acetate, or mixtures thereof;

Lithium salts dissolved in these electrolyte solvents include LiPF ₆ , LiClO ₄ , LiBF
₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , L
iN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ ,
LiN (COCF ₃ ) ₂ and LiN (COCF ₂ CF ₃ )
Salts such as ₂ , or mixtures thereof, can be used.

The shape of the battery is not particularly limited, and the present invention is applicable to non-aqueous electrolyte batteries having various shapes such as a square, cylindrical, long cylindrical, coin, button, and sheet batteries. Applicable to secondary batteries.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments to which the present invention is applied will be described. However, the present invention is not limited to the present embodiments at all, and may be appropriately changed within a scope not changing the gist of the present invention. Can be implemented.

Embodiment 1 FIG. 1 is a sectional view showing the configuration of a prismatic nonaqueous electrolyte secondary battery according to this embodiment.

The prismatic nonaqueous electrolyte secondary battery 1 of the first embodiment
Are composed of a positive electrode 3 formed by applying a positive electrode mixture containing a substance capable of absorbing and releasing lithium ions to an aluminum current collector, and a substance capable of absorbing and releasing lithium ions to a copper current collector. A flat electrode group 2 in which a negative electrode 4 coated with a negative electrode mixture is wound via a separator 5 and a non-aqueous electrolyte containing an electrolyte salt are housed in a battery case 6. .

A battery lid provided with a safety valve 8 is attached to the battery case 6 by laser welding, a positive electrode terminal 9 is connected to the positive electrode 3 via a positive electrode lead 10, and the negative electrode 4 contacts the inner wall of the battery case 6. It is electrically connected.

The positive electrode mixture was made of LiCoO ₂ : 90 as an active material.
Parts by weight, 5 parts by weight of acetylene black as a conductive additive,
After mixing with 5 parts by weight of polyvinylidene fluoride as a binder, N-methyl-2-pyrrolidone was appropriately added and dispersed to prepare a slurry. This slurry is 20 μm thick
After uniformly applying and drying the aluminum current collector of Example 1, the positive electrode 3 was produced by compression molding with a roll press.

The negative electrode mixture is prepared by mixing 90 parts by weight of a carbon material that absorbs and releases lithium ions and 10 parts by weight of polyvinylidene fluoride, and then appropriately adding N-methyl-2-pyrrolidone to disperse the slurry. Prepared. The slurry was uniformly applied to a 10 μm-thick copper current collector, dried, and then compression-molded by a roll press to produce a negative electrode 4.

The electrolytic solution is ethylene carbonate (EC)
/ Diethyl carbonate (DEC) = 1/1 (vol /
vol.) in which 1 mol of LiPF ₆ is dissolved.

The separator 5 is made porous by a stretching method.
Crosslinked after extraction of plasticizer, weight average molecular weight of about 80
10,000 polyethylene microporous membranes, and a polyethylene microporous membrane having a weight average molecular weight of about 800,000 formed into a porous form by stretching and extracting without crosslinking after extracting a plasticizer,
Using what was press-bonded by lamination using a roll press, it was wound so that the non-crosslinked polyethylene microporous membrane layer was arranged on the positive electrode plate side and the crosslinked polyethylene microporous membrane layer was arranged on the negative electrode plate side. The thickness of this separator was 25 μm.
And the piercing strength was 660 g. According to the above-described configuration and procedure, a battery of the present invention having a design capacity of 600 mAh was manufactured.

Example 2 The weight average molecular weight of the non-crosslinked polyethylene microporous membrane disposed on the positive electrode plate side was about 15
A battery was fabricated in exactly the same manner as in Example 1, except that the battery was set at 100,000. The thickness of the separator was 25 μm, and the piercing strength was 700 g.

Comparative Example 1 A crosslinked polyethylene microporous membrane layer having a weight average molecular weight of about 800,000 was provided on the positive electrode plate side, and a noncrosslinked polyethylene microporous polyethylene having a weight average molecular weight of about 800,000 was provided on the negative electrode plate side. A battery was fabricated in exactly the same manner as in Example 1 except that the membrane layer was provided. The thickness of this separator was 25 μm, and the piercing strength was 660 g.

Comparative Example 2 A non-crosslinked polyethylene microporous membrane layer having a weight average molecular weight of about 1.5 million was provided on the positive electrode plate side, and a non-crosslinked weight average molecular weight of about 80 was provided on the negative electrode plate side.
A battery was produced in exactly the same manner as in Example 1 except that 10,000 polyethylene microporous membrane layers were provided. The thickness of this separator was 25 μm, and the piercing strength was 640 g.

(Comparative Example 3) Crosslinked weight average molecular weight of about 8
A battery was fabricated in exactly the same manner as in Example 1, except that a polyethylene single-layer microporous membrane of 100,000 was used. The thickness of this separator was 25 μm, and the piercing strength was 720 g.

Comparative Example 4 A battery was produced in exactly the same manner as in Example 1, except that a non-crosslinked polyethylene single-layer microporous membrane having a weight average molecular weight of about 800,000 was used. The thickness of this separator was 25 μm, and the piercing strength was 550.
g.

(Comparative test) High-temperature cycle life test: The above battery was subjected to a constant voltage up to 4.2 V at a current of 1 CA under an atmosphere of a temperature of 45 ° C.
A charge / discharge cycle of charging at a constant current for 3 hours and then discharging at a constant current of 1 CA was repeated 300 times. And 1
The ratio of the discharge capacity at the 300th cycle to the discharge capacity at the cycle was determined, and 8 times the discharge capacity at the first cycle.
Those holding a capacity of 0% or more were evaluated as good.

High temperature storage test: The above battery was charged at a current of 1 CA to 4.2 V at a constant voltage and a constant current for 3 hours, and left in a charged state at 60 ° C. for 30 days. After storage, the discharge capacity with respect to the initial capacity after the battery was discharged at a constant current of 1 CA was determined. Then, the ratio of the discharge capacity after being left to the discharge capacity before being left was determined, and those having a capacity of 80% or more of the discharge capacity before being left were evaluated as good.

Heat resistance test: A battery in a discharged state was placed in an oven, and the temperature was raised to 180 ° C. at a rate of 5 ° C./min. After the temperature was raised, the battery was taken out of the oven and disassembled. Observation of the presence or absence of contraction and film breakage of the separator taken out, the degree of blockage, the battery short-circuit, that is, those in which the separator contraction and film breakage occurred ×, those in which the battery did not short-circuit, that is, the separator When no shrinkage or rupture of the film occurred, it was evaluated as ○.

Table 1 (Examples and Comparative Examples) shows the results of the high-temperature cycle life, the standing test, and the heat resistance investigation.

[0043]

[Table 1]

In the high-temperature cycle life and standing test,
Although the positive electrode plate side of the separator was placed in a strong oxidizing atmosphere, as shown in Examples 1 and 2 in Table 1, a non-crosslinked polyethylene microporous membrane layer having excellent oxidation resistance was brought into contact with the positive electrode plate side. Thus, it is considered that the deterioration due to oxidation was suppressed as compared with the case where the crosslinked polyethylene microporous membrane layer was brought into contact with the positive electrode side, so that the high-temperature cycle life and the leaving performance were improved. Further, from a comparison between Examples 1 and 2, it was found that this effect was particularly prominent as the molecular weight of the non-crosslinked polyethylene microporous membrane in contact with the positive electrode plate side of the separator was larger. Further, in the batteries of Examples 1 and 2, the short-circuit of the batteries was prevented even at a very high temperature equal to or higher than the melting point of polyethylene by contacting the crosslinked polyethylene microporous membrane layer having excellent heat resistance to the negative electrode plate side. Since this does not occur, it is considered that a battery having high heat resistance can be obtained even when some abnormality such as an external short circuit occurs and the battery generates heat.

On the other hand, in Comparative Examples 2 and 4, since the non-crosslinked polyethylene microporous membrane layer was in contact with the positive electrode plate side,
High temperature performance is good, but due to the low heat resistance of the separator, if the battery is heated above the melting point of polyethylene, the separator will break and break due to the flow accompanying the melting of polyethylene, causing a short circuit in the battery. Was.

In Comparative Examples 1 and 3, since a crosslinked polyethylene microporous film having excellent heat resistance was used, even when the battery was heated to a temperature higher than the melting point of polyethylene, the separator did not break, and the battery was not damaged. Although no short circuit occurred, the high-temperature cycle and standing performance were very poor. This is because, in these tests, the positive electrode plate side was exposed to a strong oxidizing atmosphere, so that the cross-linked portion of the polyethylene was cut, and the generated radicals caused the degradation reaction of the polyethylene (separator) that proceeded in a chain and likewise generated. It is considered that the reason is that the decomposition reaction of the electrolytic solution by the radical was promoted. In addition, there was also a problem that the mechanical strength of the separator was significantly reduced due to the cutting of the cross-link.

From the above results, it is a multilayer separator having a first layer of a crosslinked polyethylene microporous membrane and a second layer of an uncrosslinked polyethylene microporous membrane, and the positive electrode plate has the above-mentioned structure. By arranging the second non-crosslinked polyethylene microporous membrane layer in contact, it is possible to provide a battery having excellent high-temperature cycle life, high-temperature storage performance, and excellent heat resistance. I understood. Further, as the non-crosslinked polyethylene microporous membrane which is in contact with the positive electrode plate of the separator, the weight average molecular weight is 1
It has been found that this effect is more remarkably exhibited by using a non-crosslinked polyethylene microporous membrane having a size of from 0,000,000 to 2.5,000,000.

[0048]

According to the present invention, a battery having excellent high-temperature cycle life, standing performance and heat resistance can be produced.
It is possible to measure the high performance of electronic devices used at high temperatures.

[Brief description of the drawings]

FIG. 1 is a configuration sectional view of a prismatic nonaqueous electrolyte secondary battery of the present embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Non-aqueous electrolyte secondary battery 2 Electrode group 3 Positive electrode 4 Negative electrode 5 Separator 6 Battery case 7 Lid 8 Safety valve 9 Positive electrode terminal 10 Positive electrode lead

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the separator is cross-linked with a first layer of a cross-linked polyethylene microporous membrane. And a second layer of a polyethylene microporous membrane, wherein the positive electrode plate has the second layer.
A non-aqueous electrolyte secondary battery characterized by being contacted with a layer of: 2. The non-aqueous electrolyte according to claim 1, wherein the non-crosslinked polyethylene microporous membrane is a microporous membrane made of polyethylene having a weight average molecular weight of 1,000,000 to 2.5,000,000. Rechargeable battery.