JP3236796B2 - Non-aqueous electrolyte lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte lithium secondary battery, and more particularly to a high-capacity non-aqueous electrolyte lithium secondary battery having excellent charge / discharge cycle characteristics.

[0002]

2. Description of the Related Art Non-aqueous electrolyte lithium secondary batteries are gaining more and more attention because of their excellent electromotive force and battery capacity. In the art, various improvements and researches have been made to develop more practical products. Has been eager. Improvement research of positive and negative electrode active materials is also one of the important things. We have:
From research to develop a nonaqueous electrolyte lithium secondary battery that is easy to manufacture and has high performance, conductive graphite fibers are used as a negative electrode active material, while a specific Li-
When a transition metal-based composite oxide is adopted and both are combined, a battery having a high discharge capacity can be obtained, but there is a problem that the performance of the product becomes unstable as described later.

When a conductive graphite fiber is used as a negative electrode active material, it is usually applied and formed on a negative electrode current collector as a mixture with an organic polymer binder such as a fluororesin. Therefore, the negative electrode active material layer on the negative electrode current collector exists in a state where the conductive graphite fibers are bound with a fluororesin or the like. By the way, the conductive graphite fiber is usually used in the form of a short fiber having a length of about several tens to several tens of μm. The graphite graphite may protrude vertically from the surface of the negative electrode active material layer. In the normal negative electrode manufacturing process, the negative electrode active material layer is rolled and flattened after applying and drying the mixture containing the negative electrode active material on the negative electrode current collector. A part of the protruding portion may remain as protruding, or a piece of the protruding portion broken by the planarization treatment may adhere to the surface of the negative electrode active material layer and remain. Further, a part of the conductive graphite fiber newly protrudes or breaks the new protruding portion due to the entrance and exit of lithium ions in the course of the charge and discharge cycle of the battery. In particular, when a Li-Co-based composite oxide, a Li-Ni-based composite oxide, or a Li-Mn-based composite oxide is used as the Li-transition metal-based composite oxide serving as the positive electrode active material, the above phenomenon occurs. Greatly affects the charge-discharge cycle characteristics.

[0004] A non-aqueous electrolyte lithium secondary battery has a structure in which a non-aqueous electrolyte is impregnated between a negative electrode and a positive electrode with a separator interposed therebetween. Generally, since the separator is formed of an extremely thin porous electric insulating sheet, the protruding portions of the conductive graphite fibers on the surface of the negative electrode active material layer or fragments of the remaining protruding portions adhere to the separator and reach the positive electrode to reach the battery. Is partially short-circuited, and the life is shortened by self-discharge inside the battery even if the battery is left unused. Since such a battery is treated as a defective product, it has been difficult to produce a nonaqueous electrolyte lithium secondary battery at a high yield when using conductive graphite fibers as a result. Furthermore, from the research of the present inventors, the above problem can be solved by using a separator made of an organic polymer having high mechanical strength such as polypropylene, but in that case, the battery has an abnormal abnormality due to the high melting point of polypropylene. Even if a situation occurs, there is a problem that it is difficult to cause electrical interruption between the positive and negative electrodes due to the melt deformation of the polypropylene separator, which is dangerous.

[0005]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a non-aqueous battery having a stable performance and a high discharge capacity even when conductive graphite fibers are used as the negative electrode active material. It is an object to provide an electrolyte lithium secondary battery.

[0006]

The present invention has the following features. (1) a negative electrode using conductive graphite fibers as a negative electrode active material;
Co-based composite oxide, Li-Ni-based composite oxide, and L
A nonaqueous electrolyte lithium ion battery comprising a composite separator comprising a polyethylene layer and a polypropylene layer between a positive electrode having at least one selected from the group consisting of i-Mn-based composite oxides as a positive electrode active material. Next battery. (2) The nonaqueous electrolyte lithium secondary battery according to (1), wherein the conductive graphite fiber has an average outer diameter of 2 to 20 μm and an average length of 10 to 200 μm. (3) The nonaqueous electrolyte lithium secondary battery according to the above (1) or (2), wherein the polypropylene layer of the composite separator faces the negative electrode.

[0007]

According to the present inventors' new findings, a separator having a composite structure comprising a polyethylene layer and a polypropylene layer has excellent penetration resistance to conductive graphite fibers. Thus, even if a protruding portion of the conductive graphite fiber or a broken fragment thereof exists on the surface of the negative electrode active material layer, the composite separator has resistance to piercing by the composite graphite fiber. The problem of short circuit between them is improved, and as a result, a battery with stable performance can be manufactured with high yield. In addition, since the separator has a polyethylene layer, even if an abnormal situation occurs in the battery, the electrical deformation between the positive and negative electrodes is satisfactorily performed by melting deformation due to the low melting point of the polyethylene layer, and the safety of the battery is also ensured.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The composite separator used in the present invention comprises a polyethylene layer and a polypropylene layer, as described later. The composite separator itself, that is, the composite separator itself, operates normally. In a state where it is present, it has porosity in which lithium ions can move. The degree may be a degree well-known in the field of non-aqueous electrolyte lithium secondary batteries.
The air tightness measured by IS-P8117 is 100 to 2
About 000 Gurley (sec / 100cc), with a pore size of 0.
It is about 08 to 0.25 μm, and the aperture ratio is about 30 to 60%.

The composite separator comprises a porous polyethylene layer and a polypropylene layer, and the thickness ratio is such that the thickness of the polypropylene layer is about 10 to 900, especially about 20 to 800, with respect to the thickness of the polyethylene layer of 100. is there. The thickness of the composite separator, that is, the total thickness of the polyethylene layer and the polypropylene layer may be the same as that of a normal separator, and is, for example, about 10 to 100 μm. Thus, the composite separator may have a structure in which one or more polyethylene layers and one or more polypropylene layers are variously laminated as long as the thickness ratio is within the above range and within the thickness range. For example, a two-layer structure consisting of one polyethylene layer and one polypropylene layer, a three-layer structure in which one polyethylene layer is sandwiched by two polypropylene layers, and one polypropylene layer is sandwiched by two polyethylene layers It has a three-layer structure or the like. The polyethylene layer and the polypropylene layer may be joined by a method such as adhesion with an adhesive such as a polyolefin-based hot melt resin or partial heat welding.

[0010] The higher the mechanical strength, particularly the tensile strength, of both polyethylene and polypropylene constituting the composite separator, the better the penetration resistance to the conductive graphite fiber. Therefore, it is preferable that the tensile strength measured according to JIS-K7113 is at least about 30 MPa, particularly at least about 33 MPa. In general, polypropylene has better mechanical strength than polyethylene, so if the composite separator has a polypropylene layer on one side facing the negative electrode active material, the composite separator has a puncture resistance to conductive graphite fibers. The permeability is further improved. Therefore, from the viewpoint of piercing resistance, the composite separator preferably has a polypropylene layer on at least one surface thereof, and particularly has a three-layer structure in which one polyethylene layer is sandwiched between two polypropylene layers, that is, a two-layer structure. Those having a polypropylene layer on both surfaces are preferable from the viewpoint of handling during battery production. Further, in such a three-layer structure, the thickness of the polyethylene layer is about 3 to 20 μm, and the thickness of each polypropylene layer on both sides is 5 to 20 μm.
m is preferable.

The conductive graphite fibers used as the negative electrode active material in the present invention may be various types of graphite fibers. For example, the distance between the basal planes (D002) of the crystal lattice is 0.3.
About 35 to 0.400 nm, especially 0.335 to 0.34
About 0 nm, crystallite size (Lc) in the c-axis direction is 1 to 10
It is preferably made of graphite having a high crystallinity of about 0 nm, particularly about 20 to 100 nm. The conductive graphite fiber has an average outer diameter of about 2 to 20 μm, particularly about 5 to 10 μm,
Average length is about 10 to 200 μm, especially 20 to 40 μm
A degree is appropriate.

The conductive graphite fiber is used by being mixed with a usual binder such as polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, ethylene-propylene-diene-based polymer, and the like. The amount used is about 80 to 96 parts by weight per 100 parts by weight of the total amount of the graphite fiber and the binder.

The negative electrode sheet is formed by applying a mixed composition comprising the above-mentioned conductive graphite fiber and a binder to one or both surfaces of a negative electrode current collector, drying it sufficiently, and rolling it to form a negative electrode active material layer. Obtained. The thickness of the negative electrode active material layer is 20 to 200 μ
m, especially about 50 to 150 μm. As the negative electrode current collector, a conductive metal such as copper, nickel, silver, and SUS may have a thickness of about 5 to 100 μm, particularly 8 to 50 μm.
The preferred thickness is about 20 to 300 [mu] m, particularly about 25 to 100 [mu] m of expanded metal.

In the present invention, Li is used as the positive electrode active material.
At least one selected from the group consisting of -Co-based composite oxide, Li-Ni-based composite oxide, and Li-Mn-based composite oxide is used, all of which are well known in the art or are commercially available. It may be what is. The following are some typical examples of these composite oxides.

As the Li—Co-based composite oxide, for example, Li _x Co _y O _z (0.3 ≦ x ≦ 1.0, 0.8 ≦
y ≦ 1.2, 1.8 ≦ z ≦ 2.2), Li _x Co _y O _z
(0.3 ≦ x ≦ 1.0, 0.8 ≦ y ≦ 1.2, 1.8 ≦
In the case where z ≦ 2.2), a part of Co is replaced by an element such as Al, B, P or the like.

Examples of the Li—Ni-based composite oxide include Li _x Ni _y O _z (0.3 ≦ x ≦ 1.0, 0.8 ≦
y ≦ 1.2, 1.8 ≦ z ≦ 2.2), Li _x Co _y O _z
(0.3 ≦ x ≦ 1.0, 0.8 ≦ y ≦ 1.2, 1.8 ≦
For example, those in which a part of Ni in z ≦ 2.2) is replaced with an element such as Al, B, or P are exemplified.

Examples of the Li—Mn-based composite oxide include those having a spinel crystal structure or a non-spinel crystal structure represented by the following general formulas (1) and (2). _{Li x MnO 2 (1) Li} x M3 2 O 4 (2) Here, the general formula (1) in 0.05 ≦ x ≦ 1.2,
In particular, 0.1 ≦ x ≦ 1.1, and in general formula (2), 0.01 ≦ x ≦ 1.5, particularly 0.05 ≦ x ≦ 1.1.
Of each range. In the general formula (2), M3 is at least Mn. That is, M3 may be Mn alone or a composite of Mn and an element other than Mn. M
The elements other than n include Li, Mg, Al, Ni, S
n is preferred. Further, the Li-Mn-based composite oxide may be a single compound or a mixture of two or more.

Examples of the binder for the positive electrode active material include polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, and ethylene-propylene-diene polymers. As the conductive agent, various types of conductive graphite, conductive graphite black, and the like may be used. The amount of the positive electrode active material used is about 80 to 95 parts by weight per 100 parts by weight of the total amount of the positive electrode active material, the binder, and the conductive agent, and the amount of the binder used is 1 to 100 parts by weight of the positive electrode active material. About 10 to 10 parts by weight, and the amount of the conductive agent used is 3 to 10 parts by weight per 100 parts by weight of the positive electrode active material.
It is about 15 parts by weight.

The positive electrode sheet can be formed by applying a mixed composition comprising a positive electrode active material, a binder, and a conductive agent to one or both surfaces of a positive electrode current collector, sufficiently drying, and rolling. One having a positive electrode active material layer having a thickness of about 20 to 500 μm, particularly about 50 to 200 μm on one or both sides is exemplified. As the positive electrode current collector, a conductive metal such as aluminum, an aluminum alloy,
About 100 μm, especially about 15 to 50 μm foil or perforated foil, about 25 to 300 μm thickness, especially about 30 to 150 μm
A certain degree of expanded metal is preferred.

Examples of the non-aqueous electrolyte include an electrolyte in which salts are dissolved in an organic solvent. The salts include LiC
10 ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , Li
Examples thereof include AlCl ₄ and Li (CF ₃ SO ₂ ) ₂ N, and one or a mixture of two or more thereof is used. Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone, and 1,2-
Examples thereof include dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolan, 2-methyltetrahydrofuran, diethyl ether, and the like, and one or a mixture of two or more thereof is used. In particular, a mixed solvent of ethylene carbonate, propylene carbonate, and diethyl carbonate is preferable from the viewpoint of the charge and discharge characteristics of the battery. The appropriate concentration of the salts in the electrolyte is about 0.1 to 3 mol / l.

[0021]

EXAMPLES The present invention will be described in more detail with reference to the following examples, and comparative examples will also be described to show the remarkable effects of the present invention.

Example 1 90 parts by weight of a negative electrode active material made of graphite fiber having a D002 of 0.336 nm, an Lc of 50 nm, an average outer diameter of 5.15 μm, and an average length of 30 μm, 10 parts by weight of polyvinylidene fluoride, and N -Methyl 2-pyrrolidone (90 parts by weight) was mixed with stirring to form a slurry. Then, the slurry was applied to both sides of a copper foil having a thickness of 10 μm and dried, and then 1 ton f /
Rolling was performed with a linear load of cm, thus obtaining a negative electrode sheet having a negative electrode active material amount of about 20 mg / cm ² . On the other hand, finely divided LiCoO ₂ passed through a 330 mesh Tyler sieve was used as a positive electrode active material, 90 parts by weight of the positive electrode active material, 5 parts by weight of acetylene black, 5 parts by weight of polyvinylidene fluoride, and 40 parts by weight of N-methyl-2-pyrrolidone And the mixture was mixed to form a slurry. This slurry was applied to both sides of a 20 μm-thick aluminum foil, dried, and dried at 20 mg / cm.
^{2 A} positive electrode sheet having a positive electrode active material was obtained. Further, the separator has a three-layer structure in which a 10-μm-thick porous polyethylene sheet is joined to both surfaces of a 10-μm-thick porous polyethylene sheet by partial heat welding, and has an air density of 600 Gurley (sec / 100 cc). I prepared something. The above positive and negative electrode sheets are closely opposed to each other with a separator therebetween, and 1 mol of L per liter of a mixed solvent of ethylene carbonate, propylene carbonate and diethyl carbonate (mixing volume ratio is 3: 2: 5).
Using a solution obtained by dissolving iPF ₆ as an electrolyte,
This was impregnated between the positive and negative electrode sheets and the separator to produce a sealed AA lithium secondary battery.

Example 2 As a separator, a 20 μm-thick porous polypropylene sheet was joined to both sides of a 10 μm-thick porous polyethylene sheet by partial heat welding, and the airtightness was 300 Gurley ( Sec / 100 cc), and a sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that the battery used was a battery of 1 sec / 100 cc).

Example 3 As a separator, a 15-μm-thick porous polypropylene sheet was joined to only one side of a 10-μm-thick porous polyethylene sheet by partial thermal welding, and the air density was 200 Gurley. (Sec / 100 cc), and a sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that the porous polypropylene sheet layer was opposed to the negative electrode active material layer. .

Example 4 As a separator, a porous polyethylene sheet having a thickness of 3 μm was formed into a three-layer structure in which a porous polypropylene sheet having a thickness of 8 μm was joined to both sides by partial heat welding, and the air density was 1000 Gurley ( Sec / 100 cc), and a sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that the battery used was a battery of 1 sec / 100 cc).

Example 5 As the negative electrode active material, D002 was 0.336 nm and Lc was 5
0 nm, average outer diameter 10.2 μm, average length 30 μm
A sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that the graphite fiber was used.

Example 6 As the negative electrode active material, D002 was 0.336 nm and Lc was 5
0 nm, average outer diameter 10.2 μm, average length 50 μm
A sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that the graphite fiber was used.

Example 7 As a positive electrode active material, it passed through a 330 mesh Tyler sieve.
Fine-grained LiNiO _TwoSame as Example 1 except for using
Thus, a sealed AA type lithium secondary battery was produced.

Example 8 As a positive electrode active material, it was passed through a 330 mesh Tyler sieve.
Fine LiMnO _TwoSame as Example 1 except for using
Thus, a sealed AA type lithium secondary battery was produced.

Example 9 A sealed AA lithium secondary battery was prepared in the same manner as in Example 1 except that a mixed solvent of ethylene carbonate and ethyl methyl carbonate (mixing volume ratio was 1: 1) was used as an electrolytic solution. Produced.

Comparative Example 1 A sealed AA lithium secondary battery was produced in the same manner as in Example 1, except that a porous polyethylene sheet having a thickness of 25 μm was used as a separator.

Comparative Example 2 A sealed AA type lithium secondary battery was produced in the same manner as in Example 1 except that a porous polypropylene sheet having a thickness of 25 μm was used as a separator.

Comparative Example 3 A sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that flaky natural graphite having a D002 of 0.335 nm was used as the negative electrode active material.

Comparative Example 4 A sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that flaky artificial graphite having a D002 of 0.336 nm was used as the negative electrode active material.

Comparative Example 5 A sealed AA type lithium secondary battery was produced in the same manner as in Example 1 except that soft carbon (non-graphite) having D002 of 0.340 nm was used as the negative electrode active material.

Comparative Example 6 A sealed AA lithium secondary battery was produced in the same manner as in Example 1 except that hard carbon (non-graphite) having D002 of 0.345 nm was used as the negative electrode active material.

With respect to the sealed AA lithium secondary batteries of Examples 1 to 9 and Comparative Examples 1 to 6, the discharge capacity, self-discharge characteristics, shutdown temperature, and ignition temperature were measured by the following methods. . Measuring method of discharge capacity: Full charge state where discharge voltage becomes 4.2 V over 5 hours at charge current of 250 mA, then discharge at discharge current of 250 mA, discharge voltage of 3.0
The discharge time until the voltage drops to V is measured. The discharge capacity (mA · H / g) is calculated from the discharge time. Method for measuring self-discharge characteristics: A full charge state in which the discharge voltage reaches 4.2 V over a period of 5 hours at a charging current of 250 mA, allowed to stand naturally in that state, measured the discharge voltage after standing for two weeks, and discharged The voltage is used as the self-discharge characteristic. Measurement method of shutdown temperature: Charged current is 250 mA, and the discharge voltage is 4.2 V over 5 hours to make the battery fully charged, and then placed in an oven to increase the temperature in the oven at a rate of 5 ° C./min. At the same time, the electric resistance between the positive and negative electrodes of the battery, that is, the internal resistance of the battery is measured, and the temperature in the oven when the internal resistance becomes 100Ω is measured. This temperature is used as the shutdown temperature. Measuring method of ignition temperature: Increase the temperature in the oven in the same manner as in the measurement of the shutdown temperature, and measure the temperature in the oven when ignition of the battery is recognized by visual observation. This temperature is defined as the ignition temperature of the battery.

The results of the above measurements are shown in Table 1. In Comparative Example 1 using a separator consisting of only a porous polyethylene sheet, partial conduction between the positive and negative electrodes occurred due to poor puncture resistance to graphite fibers. For this reason, after leaving for 4.2 weeks by self-discharge from the initial 4.2 V during natural standing, it is 4.02 after leaving for two weeks.
V, or about 4.3%. In Comparative Example 2 using the separator composed of only the porous polypropylene sheet, the shutdown temperature is high, which is dangerous from the viewpoint of safe use of the battery. Comparative Examples 3 to 6, in which no graphite fiber was used as the negative electrode active material, were all inadequate in terms of discharge capacity. On the other hand, the batteries of Examples 1 to 9 have good results in all aspects.

[0039]

[Table 1]

[0040]

The nonaqueous electrolyte lithium secondary battery of the present invention has a problem that occurs when conductive graphite fibers are used as a negative electrode active material, that is, the separator breaks through the conductive graphite fibers and the positive electrode caused by the breakthrough. Since the problem of short circuit between the negative electrodes is improved, it is possible to industrially manufacture the conductive graphite fiber negative electrode active material at a high yield while utilizing well-known advantages such as a high discharge capacity and charge / discharge cycle characteristics. . Further, the battery of the present invention is excellent in safety in use.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H01M 10/40 H01M 10/40 A (56) References JP-A-8-83609 (JP, A) JP-A-8-250097 ( JP, A) JP-A-4-162370 (JP, A) JP-A-5-13088 (JP, A) JP-A-7-506699 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) H01M 2/14-2/18 H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (3)

(57) [Claims] At least one selected from the group consisting of a negative electrode using conductive graphite fibers as a negative electrode active material, a Li-Co-based composite oxide, a Li-Ni-based composite oxide, and a Li-Mn-based composite oxide. one to have a composite separator comprising a polyethylene layer and a polypropylene layer between the positive electrode to the positive electrode active material, the composite separator negative polypropylene layer
Electrolyte solvent is ethylene carbonate
-Carbonate, propylene carbonate and diethyl carbonate
A non-aqueous electrolyte lithium secondary battery, characterized mixed solvent der Rukoto Nate. 2. The conductive graphite fiber has an average outer diameter of 2 to 20 μm.
The non-aqueous electrolyte lithium secondary battery according to claim 1, wherein the average length is 10 to 200 μm. 3. The composite separator is porous;
The non-aqueous electrolyte lithium secondary battery according to claim 1 , wherein the diameter is 0.08 to 0.25 μm .