JP2002015720A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve cycle life performance at high temperature as well as performance under normal temperature environment. SOLUTION: The nonaqueous electrolyte secondary battery utilizes a laminated separator of two or more layers combining an ultrahigh molecular weight polyethylene fine porous film with a million or more weight average molecular weight and a high porosity polyethylene fine porous film with 45% or more porosity and is structured by arranging the ultrahigh molecular weight polyethylene fine porous film layer or a positive electrode plate side and the high porosity polyethylene fine porous film layer at a negative electrode plate side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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 performance and productivity.

[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 requirements is an active material such as lithium metal or lithium alloy, or a host material containing lithium ions (here, the host material refers to a material capable of inserting and extracting lithium ions). A lithium intercalation compound occluded in carbon is used as a negative electrode, an aprotic organic solvent in which a lithium salt such as LiClO ₄ or LiPF ₆ is dissolved is used as an electrolytic solution, and a separator provided between the positive electrode and the negative electrode includes: A non-aqueous electrolyte secondary battery using a polyolefin-based material which is insoluble in an organic solvent and stable to an electrolyte or an electrode active material and is processed into a microporous film or nonwoven fabric.

In particular, lithium-cobalt composite oxide, lithium-nickel composite oxide, spinel-type lithium manganese oxide and the like can be charged and discharged at an extremely noble potential of 4 V (vs. Li ^/ Li ⁺ ) or more. By using this, a battery having a high discharge voltage can be realized.

[0004]

In recent years, non-aqueous electrolyte secondary batteries have been increasingly used in electronic equipment used not only in a normal temperature environment but also in various environments from a low temperature to a high temperature. Is coming. In particular, in a notebook personal computer, the temperature inside the personal computer increases as the speed of the central processing unit increases, and the built-in non-aqueous electrolyte secondary battery is used for a long time in a high-temperature environment. For this reason, among the characteristics of the non-aqueous electrolyte secondary battery, battery characteristics in a high-temperature environment are becoming important.

[0005] However, the conventional non-aqueous electrolyte secondary battery shows very excellent performance under a normal temperature environment,
It has become clear that the cycle life performance at high temperatures is not always sufficient. The present invention is
It is intended to improve cycle life performance at high temperatures.

[0006]

SUMMARY OF THE INVENTION The above-mentioned problems of deterioration in cycle life performance at high temperatures include oxidation deterioration on the surface of the separator facing the positive electrode, clogging of electrolytic decomposition products on the surface of the separator facing the negative electrode, and the like. This is thought to be due to an increase in diffusion resistance accompanying this, and it is necessary to provide the separator with performance that can withstand them. The inventors of the present application have conducted intensive studies in order to solve the above-described problems, and as a result, by using a separator having a specific configuration and disposing it in a specific positional relationship between a positive electrode and a negative electrode, a battery having excellent high-temperature cycle characteristics is provided. Have been found and productivity can be improved, and the present invention has been accomplished.

That is, the present invention provides a microporous ultrahigh molecular weight polyethylene membrane having a weight average molecular weight of 1,000,000 or more,
A porosity of 45% or more, a high porosity polyethylene microporous membrane is used, and two or more laminated separators are used in combination, and an ultrahigh molecular weight polyethylene microporous membrane layer is provided on the positive electrode plate side and a negative electrode plate side is provided. A non-aqueous electrolyte secondary battery characterized in that a high porosity polyethylene microporous membrane layer is disposed in contact therewith.

[0008] The ultra-high molecular weight polyethylene microporous membrane is made of polyethylene having a weight average molecular weight of 1,000,000 to 2.5,000,000 and a porosity of 35% to 60%.
It is preferable to use the following microporous membrane, further, as a high porosity polyethylene microporous membrane, the weight average molecular weight is composed of polyethylene of 200,000 or more and 2.5 million or less,
It is preferable to use a microporous film having a porosity of 45% or more and 80% or less.

If the weight average molecular weight of the polyethylene microporous membrane is smaller than the above range, the oxidation resistance and mechanical strength of the separator will be reduced and the separator will not be suitable for practical use. Since the viscosity is high and the fluidity is low, it is difficult to form a film. Therefore, it is important to use the ultra-high molecular weight polyethylene microporous membrane layer and the high porosity polyethylene microporous membrane layer each having the above-mentioned molecular weight. In addition, the porosity of the microporous polyethylene membrane is an important factor that determines the electrolyte retention and mechanical strength of the separator. If the porosity is small beyond the above range, the amount of retentate decreases and the diffusion resistance becomes large, so that the battery performance becomes unsuitable for practical use. The strength is low, and the productivity at the time of film formation and at the time of battery production decreases. Therefore, it is important to use the ultra-high molecular weight polyethylene microporous membrane layer and the high porosity polyethylene microporous membrane layer each having the above porosity range.

In the separator comprising two or more ultra-high molecular weight polyethylene microporous membrane layers and high porosity polyethylene microporous membrane layers, oxidation resistance is regarded as important on the positive electrode side, and on the negative electrode side. Since electrolyte retention and clogging resistance are emphasized, correspondingly, the weight average molecular weight of the ultrahigh molecular weight polyethylene microporous membrane layer is larger than that of the high porosity polyethylene microporous membrane layer, The porosity of the high porosity polyethylene microporous membrane layer is set to be larger than that of the ultrahigh molecular weight polyethylene microporous membrane layer.

On the other hand, if the porosity is set too high, the mechanical strength of the separator, typically puncture strength, is lowered, and the separator is ruptured or deformed at the time of battery production, resulting in defective products. In the present invention, the porosity of the high porosity polyethylene microporous layer is set to a high value of 80% because the microporous ultrahigh molecular weight polyethylene layer is provided with a porous membrane having sufficient mechanical strength. Can be
In addition, since the strength of the entire laminated separator can be sufficiently secured, the occurrence of product defects does not occur.

[0012]

Embodiments of the present invention will be described below. The laminated separator used in the present invention can be formed by forming microporous polyethylene membranes having different ultra-high molecular weights and porosity and then bonding them by a method such as pressure bonding and adhesion.

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. In addition, those containing appropriate amounts of additives such as various plasticizers, antioxidants, and flame retardants may be used.

When manufacturing the non-aqueous electrolyte secondary battery according to the present invention, a microporous ultrahigh molecular weight polyethylene layer having a weight average molecular weight of 1,000,000 or more is used by using the separator formed as described above. The porosity is 45 on the positive electrode plate side.
% Or more of a high porosity polyethylene microporous layer is disposed on the negative electrode plate side, and may be manufactured by a usual method.

[0015] positive electrode plate is constituted by using a positive electrode active material, for example, as the positive electrode active material in the case of manufacturing a lithium secondary battery, is capable of absorbing and desorbing lithium compound, composition formula Li _x MO ₂ or Li _y M ₂ O ₄ (where M
Is a transition metal, represented by 0 ≦ x ≦ 1, 0 ≦ y ≦ 2),
A composite oxide, an oxide having tunnel-like holes, and a metal chalcogenide having a layered structure can be used. Specific examples thereof include LiCoO ₂ , LiNiO ₂ , and LiMn ₂
O ₄ , Li ₂ Mn ₂ O ₄ , MnO ₂ , FeO ₂ , V ₂ O ₅ , V
₆ O ₁₃ , TiO ₂ , TiS _{2 and the} like. Further, an organic compound such as a conductive polymer such as polyaniline can be used, and these may be used as a mixture. When a granular active material is used, it can be produced, for example, by forming a mixture of active material particles, a conductive additive, and a binder on a metal current collector such as aluminum.

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. 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 electrolytic solution and the like can be used. When a non-aqueous electrolyte lithium secondary battery is manufactured, for example, ethylene carbonate, propylene carbonate is used as the electrolytic 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.

[0020]

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 nonaqueous 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 is brought into contact with 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.
% By weight, and 5% by weight of acetylene black as a conductive additive,
After mixing with 5% by weight of polyvinylidene fluoride as a binder, N-methyl-2-pyrrolidone was appropriately added and dispersed to prepare a slurry. This slurry has a thickness of 20 μm.
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% by weight of a carbon material that absorbs and releases lithium ions and 10% by weight of polyvinylidene fluoride, and then appropriately adding N-methyl-2-pyrrolidone to disperse the mixture. 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 electrolyte 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 of a polyethylene microporous membrane having a weight molecular weight of about 1.5 million and a porosity of 40%, made porous by a stretching method, and a weight average molecular weight of about 100.
One microporous membrane having a porosity of 50% is laminated and pressed using a roll press, and a microporous membrane layer having a weight average molecular weight of about 1.5 million is provided on the positive electrode plate side. Was wound so that a microporous membrane layer having a weight average molecular weight of about 1,000,000 was disposed. The thickness of this separator was 25 μm, and the piercing strength was 650 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) A battery was produced in exactly the same manner as in Example 1 except that a polyethylene microporous membrane layer having a weight average molecular weight of about 2,000,000 was disposed on the positive electrode plate side. The thickness of the separator was 25 μm, and the piercing strength was 680.
g.

Example 3 A battery was produced in exactly the same manner as in Example 1 except that a polyethylene microporous membrane layer having a weight average molecular weight of about 1,000,000 was disposed on the positive electrode plate side. The thickness of this separator was 25 μm, and the piercing strength was 630.
g.

Example 4 The porosity was 45% on the negative electrode plate side.
Example 1 except that a polyethylene microporous membrane layer of
A battery was produced in exactly the same manner as in Example 1. The thickness of this separator was 25 μm, and the piercing strength was 660 g.

Example 5 A microporous polyethylene membrane having a weight average molecular weight of about 700,000 was disposed on the negative electrode plate side.
A battery was manufactured in exactly the same manner as in Example 1. The thickness of this separator was 25 μm, and the piercing strength was 640 g.

Comparative Example 1 A microporous polyethylene layer having a weight average molecular weight of about 700,000 was disposed on the positive electrode plate side.
A battery was manufactured in exactly the same manner as in Example 1. The thickness of this separator was 25 μm, and the piercing strength was 530 g.

Comparative Example 2 A polyethylene microporous membrane layer having a weight average molecular weight of about 700,000 was provided on the positive electrode plate side, and a polyethylene microporous film layer was provided on the negative electrode plate side.
A battery was produced in exactly the same manner as in Example 1, except that a polyethylene microporous membrane layer having a porosity of 40% was provided. The thickness of the separator was 25 μm, and the piercing strength was 55 μm.
It was 0 g.

Comparative Example 3 A battery was produced in exactly the same manner as in Example 1, except that a polyethylene microporous membrane layer having a porosity of 40% was provided on the negative electrode plate side. The thickness of the separator was 25 μm, and the piercing strength was 670 g.

Comparative Example 4 A battery was produced in exactly the same manner as in Example 1, except that a separator composed of a single layer of polyethylene having a weight average molecular weight of polyethylene of about 1.5 million and a porosity of 50% was used. The thickness of this separator was 25 μm.
And the piercing strength was 320 g.

Comparative Example 5 A battery was manufactured in exactly the same manner as in Example 1, except that a polyethylene single-layer separator having a weight average molecular weight of polyethylene of about 700,000 and a porosity of 40% was used. The thickness of this separator is 25
In μm, the piercing strength was 560 g.

Table 1 shows the structures of the separators used in the batteries of Examples and Comparative Examples.

[0037]

[Table 1]

(Comparative test) High temperature cycle life test: The above-mentioned battery was charged at a constant current and a constant current of 3 V at a current of 1 CA to 4.2 V in an atmosphere of a temperature of 45 ° C., and then at a constant current of 1 CA. 300 charge / discharge cycles
Repeated times. Then, the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle was determined, and those having a capacity of 80% or more of the discharge capacity at the first cycle were evaluated as good.

Investigation of defective manufacturing rate: A flat electrode group produced by winding a positive electrode and a negative electrode through a separator is inserted into a battery case, and an insulation test is performed using a 250 V, 50 MΩ insulation resistance meter. The incidence of defects was investigated.

Table 2 shows the results of the high-temperature cycle life test and the production failure rate of the batteries of Examples and Comparative Examples.

[0041]

[Table 2]

In the high temperature cycle life test, the positive electrode plate side of the separator was placed in a strong oxidizing atmosphere, but as shown in Examples 1 to 3, the high temperature cycle life performance was improved. This is because deterioration of the separator due to oxidation is suppressed by disposing a polyethylene microporous membrane layer having a large molecular weight on the positive electrode plate side, and further, a polyethylene microporous membrane layer having a large porosity is arranged on the negative electrode plate side. As a result, the electrolyte retention of the separator was maintained, and clogging of the microporous membrane due to the decomposition product of the electrolyte was suppressed, thereby preventing a decrease in discharge performance without increasing internal resistance. It is thought that it was done. Further, from a comparison between Examples 1 and 4, it was found that this effect was particularly prominent as the porosity on the negative electrode side of the separator was higher. Further, from the comparison between Examples 1 and 5, it was found that the high-temperature cycle performance and the failure rate due to film breakage during the process did not depend on the molecular weight of the polyethylene microporous membrane layer disposed on the negative electrode plate side. Was.

On the other hand, in Comparative Examples 1, 2, and 5, since the polyethylene microporous membrane layer disposed on the positive electrode plate side has a small molecular weight, deterioration due to oxidation of the separator proceeds during a charge / discharge cycle, and the cycle life performance is reduced. It is considered to have decreased. In particular, in Comparative Examples 2 and 5, the porosity of the polyethylene microporous membrane layer disposed on the negative electrode plate side was also small, so it is considered that the performance was significantly reduced.

In Comparative Example 3, although the polyethylene microporous membrane layer disposed on the positive electrode plate side has a large molecular weight, no deterioration occurs on the positive electrode side, but the polyethylene microporous membrane layer disposed on the negative electrode plate side has no empty space. It is considered that the microporous film was clogged with the progress of the charge / discharge cycle due to the small porosity, and the cycle performance was reduced.

In Comparative Example 4, a separator composed of a single layer of a polyethylene microporous membrane having a large molecular weight was used, and its porosity was large, so that deterioration and clogging of the microporous membrane did not occur. Lifetime performance was excellent. However, due to the high porosity, mechanical strength (piercing strength)
In the production of batteries, poor insulation occurred due to a separator rupture or the like, resulting in reduced productivity.

From the above results, the ultra-high molecular weight polyethylene microporous membrane in which the polyethylene has a weight average molecular weight of 1,000,000 to 2.5 million, the polyethylene has a weight average molecular weight of 200,000 to 2.5 million and the porosity Of a microporous polyethylene membrane having a porosity of 45% or more, using a laminated separator of two or more layers, an ultrahigh molecular weight polyethylene microporous membrane layer on the positive electrode plate side, and a high porosity polyethylene microporous film layer on the negative electrode plate side. By arranging the porous membrane layer, it was found that it was possible to provide a battery having excellent productivity and excellent high-temperature cycle life performance.

[0047]

According to the present invention, a battery excellent in high-temperature cycle life performance and productivity can be manufactured, and it is possible to measure the high performance of electronic equipment 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

Continued on front page F-term (reference) 5H021 AA06 CC04 EE04 HH02 HH07 HH10 5H029 AJ05 AK02 AK03 AL01 AL02 AL06 AL12 AM03 AM04 AM05 AM07 BJ02 BJ14

Claims (3)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein, as the separator, a microporous ultrahigh molecular weight polyethylene membrane having a weight average molecular weight of 1,000,000 or more; A separator comprising a combination of two or more layers of a high porosity polyethylene microporous membrane having a porosity of 45% or more is used. An ultrahigh molecular weight polyethylene microporous membrane layer is provided on the positive electrode plate side, and a high porosity is provided on the negative electrode plate side. A non-aqueous electrolyte secondary battery, wherein a polyethylene microporous membrane layer is disposed in contact with the non-aqueous electrolyte secondary battery. 2. A microporous ultrahigh molecular weight polyethylene microporous membrane comprising polyethylene having a weight average molecular weight of 1,000,000 to 2.5,000,000 and a porosity of 35% to 60%. The non-aqueous electrolyte secondary battery according to claim 1, wherein: 3. A high-porosity polyethylene microporous membrane comprising a polyethylene having a weight-average molecular weight of 200,000 to 2.5 million and a porosity of 45% to 80%. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein: