JP2002190321A - Nonaqueous electrolyte secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-security nonaqueous electrolyte secondary cell, using a porous solid state polymer electrolyte membrane. SOLUTION: The nonaqueous electrolyte secondary cell is provided with an anode, a cathode, and a porous polymer electrolyte membrane between the cathode and the anode, characterized by the fact that the porous proton- exchange membrane contains thermoplastic resin grains with its grain diameter of 0.015-5 μm, having the melting point of 80-150 deg.C lower than that of a macromolecule composing the porous polymer electrolyte membrane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery using a polymer electrolyte.

[0002]

2. Description of the Related Art With the rapid reduction in size and weight of electronic devices, there is an increasing demand for secondary batteries that are small, lightweight, have a high energy density, and can be repeatedly charged and discharged. In addition, due to environmental problems such as air pollution and an increase in carbon dioxide, early commercialization of electric vehicles is desired, and development of excellent secondary batteries having characteristics such as high efficiency, high output, high energy density, and light weight. Is required.

As a secondary battery satisfying these requirements, a secondary battery using a non-aqueous electrolyte has been put to practical use. This battery has several times the energy density of a battery using a conventional aqueous electrolyte solution. As an example, using a cobalt composite oxide, a nickel composite oxide or a spinel type lithium manganese oxide for the positive electrode, and storing and storing lithium in the negative electrode
Uses releasable carbon materials, metallic lithium, etc., and uses an organic electrolyte solution as the electrolyte.
Non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium batteries have been put to practical use.

In these non-aqueous electrolyte secondary batteries, the use of the following three types of separators has been studied.

[0005] The first is a polyolefin membrane such as polyethylene or polypropylene having micropores, which is currently used in commercially available lithium ion batteries and the like. In these separators, when the battery temperature rises abnormally, heat shrinks to close the micropores, which are the paths for ions, and stops the short-circuit current thereafter, thereby suppressing heat generation. Some have excellent functions, but due to the inclusion of organic electrolyte, care must be taken against liquid leakage, and there is a possibility that the organic electrolyte may burn due to abnormal heat generation of the battery, and from the viewpoint of safety as well. Special safety mechanisms such as safety valves and protection circuits were required.

Second, there is a so-called intrinsic solid polymer electrolyte in which a polymer such as polyethylene oxide and a lithium salt such as lithium perchlorate are mixed. Since these are completely solid, there is no fear of liquid leakage, but they have a drawback of low lithium ion conductivity at room temperature, and thus have not been put to practical use.

Third, in order to increase the lithium ion conductivity of the polymer electrolyte at room temperature, a mixture or gel system of a polymer electrolyte and an organic electrolyte is used, or a porous polymer electrolyte is used. Some measures have been taken, such as holding an organic electrolyte in the holes.

[0008] In particular, the porous polymer electrolyte membrane contains a lithium ion conductive non-aqueous electrolyte in the pores of the polymer membrane, and the polymer membrane itself swells or wets with the organic electrolyte to form lithium ion. It is a lithium ion conductive electrolyte membrane that can pass through, has high lithium ion conductivity at room temperature, and is suitable for batteries capable of high-rate discharge.

Furthermore, in a battery using a porous polymer electrolyte membrane, the electrolyte itself is rich in flexibility and can be made thinner. Further, the thinner the electrolyte, the higher the conductivity of the electrolyte. Then, after combining a positive electrode and a negative electrode with a porous polymer electrolyte membrane interposed therebetween, they are wound or laminated and used for a thin battery as a power generating element.

[0010]

The porous polymer electrolyte membrane contains a lithium ion conductive non-aqueous electrolyte in the pores of the polymer membrane, and the polymer membrane itself is capable of passing lithium ions. Although it is a conductive electrolyte membrane, the amount of organic electrolyte in the battery is small, so there is no possibility that the electrolyte leaks, and the safety is higher than a battery using a polyolefin membrane with fine pores and an organic electrolyte. Become.

However, since the porous polymer electrolyte membrane does not have a shutdown function unlike a polyolefin membrane having fine pores, there still remains a problem in terms of safety.

Conventionally, a "solvent extraction method" has been used to make a polymer membrane porous. This solvent extraction method is the first
Is a method of obtaining a porous polymer by extracting a first solvent from a polymer solution obtained by dissolving a polymer in a solvent using a second solvent. By immersing in a second solvent that is insoluble and compatible with the first solvent, the first solvent is extracted, and the portion from which the first solvent has been removed becomes pores, and the porosity increases. A molecule is formed. In this solvent extraction method, a through hole having a circular opening can be formed in the polymer. However, this solvent extraction method has a problem that the process is complicated and takes a long time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a non-aqueous electrolyte secondary battery using a porous solid polymer electrolyte membrane, which does not leak electrolyte and has high safety. Is provided.

[0014]

A first aspect of the present invention is a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a porous polymer electrolyte membrane between the positive electrode and the negative electrode. The electrolyte membrane contains particles of a thermoplastic resin having a particle size of 0.015 to 5 μm, and the melting point of the thermoplastic resin is 80 to 170 ° C.
In addition, the melting point of the polymer constituting the porous polymer electrolyte membrane is lower.

According to the first aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery using a porous polymer electrolyte membrane which does not leak electrolyte, has excellent high-rate discharge characteristics, and is highly safe. be able to.

According to a second aspect of the present invention, in the non-aqueous electrolyte secondary battery, an organic solvent is removed from a paste containing a polymer constituting a porous polymer electrolyte membrane, particles of a thermoplastic resin, and an organic solvent. Characterized by using a porous polymer membrane obtained by the above method.

According to the second aspect of the present invention, a porous polymer membrane can be easily manufactured.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A nonaqueous electrolyte secondary battery according to the present invention comprises a positive electrode, a negative electrode, and a porous polymer electrolyte membrane between the positive electrode and the negative electrode. Has a particle size of 0.
015 to 5 μm thermoplastic resin particles, wherein the melting point of the thermoplastic resin is 80 to 170 ° C., and is lower than the melting point of the polymer constituting the porous polymer electrolyte membrane. .

According to the present invention, charging and discharging can be performed at a high rate by providing a porous polymer electrolyte membrane between the positive electrode and the negative electrode. Also, because the porous polymer electrolyte membrane contains particles of a thermoplastic resin, when the battery temperature abnormally rises,
In order to have a shutdown function that the thermoplastic resin dissolves, closes the pores of the porous polymer electrolyte membrane, ions do not move in the pores, stops the subsequent short-circuit current, and suppresses heat generation. The safety of the battery is dramatically improved.

The particle size of the thermoplastic resin contained in the porous polymer electrolyte membrane is 0.015 to 5 μm.
m. When the particle size is smaller than this range, sufficient ionic conductivity of the porous polymer electrolyte membrane cannot be secured, and when the particle size is larger than this range, the current distribution during charging and discharging becomes non-uniform.

As the shape of the particles of the thermoplastic resin, various shapes such as a sphere, a substantially sphere, a lump, and a fiber can be used, and it is particularly preferable to use a spherical particle.

The melting point of the thermoplastic resin is 80-17.
It must be in the range of 0 ° C. and lower than the melting point of the polymer constituting the porous polymer electrolyte membrane. When the melting point of the thermoplastic resin is 80 ° C or lower, the thermoplastic resin melts and shuts down even under normal battery use conditions, and when the melting point of the thermoplastic resin is 170 ° C or higher, Even if the battery becomes hot and becomes in a dangerous state, the thermoplastic resin does not melt, the shutdown function is not exhibited, and the safety of the battery cannot be ensured. Also, when the battery temperature rises, the thermoplastic resin must be melted before the porous polymer electrolyte membrane, so the melting point of the thermoplastic resin is the polymer constituting the porous polymer electrolyte membrane. Must be lower than the melting point.

The porous polymer membrane of the present invention can be prepared by a conventional solvent extraction method. In addition, the porous polymer membrane of the present invention first prepares a paste containing the polymer constituting the porous polymer electrolyte membrane, thermoplastic resin particles, and an organic solvent, and heats or decompresses the paste from this paste. The porous polymer electrolyte membrane is obtained by removing the organic solvent by such a method as described above, and is extremely simple and in a short time as compared with the conventional solvent extraction method.

In the present invention, the following procedure can be adopted as a procedure for producing a battery provided with a porous polymer electrolyte membrane.

One method is to prepare a porous polymer membrane in advance, immerse the porous polymer membrane in an organic electrolyte, and hold the organic electrolyte in the pores of the porous polymer membrane. Then, the polymer of the porous polymer membrane is swollen or wetted with an organic electrolyte to form a porous polymer electrolyte membrane in which the polymer portion is also capable of conducting ions. The conductive polymer electrolyte membrane is interposed and laminated or wound, and housed in a battery case as a power generating element.

In another method, a porous polymer film is prepared in advance, laminated or wound with the porous polymer film sandwiched between a positive electrode and a negative electrode, and housed in a battery case as a power generating element. Then, an electrolytic solution is injected to convert the porous polymer membrane into a porous polymer electrolyte membrane.

In any of the above methods, the porous polymer film may be wound after being integrated with either the positive electrode or the negative electrode.

In these methods, a porous polymer electrolyte membrane may be contained in at least one of the positive electrode and the negative electrode, and the porous polymer electrolyte membrane is contained in both the inside of the positive electrode and the inside of the negative electrode. It is more preferable to include

The non-aqueous electrolyte secondary battery according to the present invention comprises a porous polymer electrolyte membrane containing thermoplastic resin particles, a positive electrode, and a negative electrode. And an electrode group in which an anode and a negative electrode are alternately stacked, or an electrode group in which an electrolyte membrane is interposed between a strip-shaped positive electrode plate and a strip-shaped negative electrode plate.

In the present invention, as the material of the thermoplastic resin contained in the porous polymer electrolyte membrane, polyethylene, polypropylene, polybutylene, polymethyl methacrylate, polyvinylidene chloride, polycarbonate, acrylonitrile-butadiene = styrene copolymer Merging (ABS
Resin) and the like, or a mixture thereof. The melting point of the thermoplastic resin is determined according to ASTM D21.
17.

The amount of the thermoplastic resin to be added to the porous polymer electrolyte membrane may be selected depending on the combination of the material of the porous polymer electrolyte used and the material of the thermoplastic resin. However, if the added amount is too small, the desired mechanical strength cannot be obtained, the effect of shutdown is small, and
If the addition amount is too large, the electric conductivity of the porous polymer electrolyte membrane decreases, and the battery characteristics deteriorate, so the addition amount needs to be limited to an appropriate range. Therefore, the addition amount of the thermoplastic resin is preferably 10 to 60 parts by weight based on 100 parts by weight of the porous polymer.

The porosity of the porous polymer electrolyte membrane is as follows:
To obtain high lithium ion conductivity at room temperature, 20
It is preferably about 90%.

As the polymer used for the porous polymer electrolyte membrane, polyvinylidene fluoride (PVdF), polyvinyl chloride (PVC), polyacrylonitrile (PAN),
Polyethylene oxide (PEO), polypropylene oxide, polymethyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinyl pyrrolidone, polyethylene imine, polybutadiene, polystyrene and polyisoprene, or a derivative thereof alone, Alternatively, they may be used as a mixture. Further, a polymer obtained by copolymerizing various monomers constituting the above polymer may be used.

Among these polymers, PVdF, PVd
When C and PAN were used, a battery showing particularly excellent characteristics was obtained. The reason is that PVd for organic electrolyte
This is because the swellability of F, PVC and PAN is higher than other polymers.

A mixed solution of an organic solvent and a lithium salt is used as the electrolytic solution contained in the solid polymer and the pores of the porous polymer electrolyte. Examples of the organic solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide,
A polar solvent such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolan, methyl acetate, or a mixture thereof may be used.

The lithium salts dissolved in the electrolyte include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ C
O ₂ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN
(SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ and L
Salts such as iN (COCF ₂ CF ₃ ) ₂ or mixtures thereof can be used.

Further, as the positive electrode active material of the battery, inorganic
As the compound, the composition formula LixMO _TwoOr LiyM_Two
O_Four(Where M is a transition metal, 0 ≦ x ≦ 1, 0 ≦ y ≦
Complex oxide represented by 2), having tunnel-like vacancies
Oxide or layered metal chalcogenide
Can be. As a specific example, LiCoO_Two, LiN
iO_Two, LiMn_TwoO_Four, Li_TwoMn_TwoO_Four, MnO_Two, Fe
O_Two, V_TwoO_Five, V₆O₁₃, TiO_Two, TiS_TwoEtc.
You. Examples of the organic compound include polyaniline and the like.
And the like. In addition, inorganic compounds,
Regardless of the organic compound, using a mixture of the above various active materials
Is also good.

Further, as the negative electrode active material of the battery, a carbon material or graphite, which is a material capable of occluding and releasing lithium and / or lithium ions, may be used.
alloys of lithium with l, Si, Pb, Sn, Zn, Cd, etc., transition metal composite oxides such as LiFe ₂ O ₃ , WO ₂ , M
A transition metal oxide such as oO _{2, a} carbonaceous material such as graphite or carbon, a lithium nitride such as Li ₅ (Li ₃ N), a metal lithium foil, or a mixture thereof may be used.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to preferred embodiments.

Example 1 Lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, graphite as a negative electrode active material, modified polyvinylidene fluoride (PVdF) as a polymer material constituting a porous polymer electrolyte, and a porous material Low molecular weight polyethylene (P) is used as the thermoplastic resin contained in the polymer electrolyte membrane.
E), as an organic electrolyte, ethylene carbonate (E
C) and a mixed solvent of diethyl carbonate (DEC) (3
0: 70, to prepare a non-aqueous electrolyte secondary battery using a solution obtained by dissolving lithium hexafluorophosphate (LiPF ₆₎ of 1M in vol%).

The positive electrode plate is composed of 90 wt% of lithium cobalt oxide (LiCoO ₂ ) as an active material, 3 wt% of acetylene black as a conductive agent, 7 wt% of polyvinylidene fluoride (PVdF) as a binder, and n-solvent as a solvent. An appropriate amount of methyl-2-pyrrolidone (NMP) is mixed into a paste, and the paste is applied on an aluminum foil having a thickness of 20 μm, and dried at 150 ° C. to evaporate the NMP.
This operation was performed on both surfaces of the aluminum foil, followed by pressing. The thickness of the positive electrode plate after pressing is 170 μm, and the dimensions of the positive electrode plate are 47 mm in width,
The length was 235 mm.

The negative electrode plate is made of graphite 9 as an active material.
0 wt%, PVdF as a binder, and 10 wt% of PVdF as a binder are mixed with an appropriate amount of NMP as a solvent to form a paste. The paste is applied on a copper foil having a thickness of 14 μm, and dried at 150 ° C. to evaporate the NMP. After this operation was performed on both surfaces of the copper foil, it was produced by pressing.
The thickness of the negative electrode after pressing was 190 μm, and the dimensions of the negative electrode plate were 50 mm in width and 270 mm in length.

A porous polymer electrolyte membrane containing particles of a thermoplastic resin was prepared by the following method. First, 10 g of powder of modified polyvinylidene fluoride (PVdF) was added to 90 g of N2.
-Methylpyrrolidone (NMP)
7 g of low molecular weight polyethylene (PE) fine particles having an average particle size of 1 μm and a melting point of 120 ° C. were added and uniformly dispersed to obtain a paste.

The paste was wound while filling it between the positive electrode and the negative electrode to form a power generating element. The power generating element was dried under reduced pressure at 80 ° C. for 5 hours to remove NMP in the paste. At this time, holes were formed in the modified polyvinylidene fluoride (PVdF) film attached between the positive electrode and the negative electrode, which means that a porous polymer film was provided between the positive electrode and the negative electrode.

The power generating element provided with a porous polymer film between the positive electrode and the negative electrode is packed with an aluminum laminate film, 2 ml of an organic electrolyte is injected, and the porous polymer film is replaced with the porous polymer film. After forming the electrolyte membrane, the opening was heat-sealed and sealed, and the thickness was 3.8 mm, the width was 35 mm, and the length was 61 mm.
A thin battery having a nominal capacity of 700 mAh was produced, and this was designated as Battery A.

Comparative Example 1 Using the same positive electrode, negative electrode, and organic electrolyte as in Example 1, a polyolefin separator having fine pores was sandwiched between the positive electrode and the negative electrode, and wound to form a power generating element. After packing with a laminate film and injecting 2 ml of the organic electrolyte, the opening was heat-sealed and sealed to produce a thin battery with a nominal capacity of 700 mAh,
This was designated as Battery B.

Comparative Example 2 The same as Example 1 except that TiO ₂ fine particles having an average particle diameter of 0.01 μm were used instead of the low molecular weight polyethylene (PE) fine particles contained in the porous polymer membrane. Thus, a thin battery having a nominal capacity of 700 mAh was produced, and this was designated as Battery C.

First, a liquid leakage test from a battery was performed.
Five batteries A, B, and C were prepared, and a part of an aluminum laminate film as an exterior body of each battery was intentionally cracked, and the entire battery was pressed with a force of 500 kgf.
After holding for 1 hour, the state of liquid leakage from each battery was observed.

As a result, in the battery B of Comparative Example 1, leakage of the electrolyte from the crack was observed in all five batteries. On the other hand, Example 1 using a porous polymer electrolyte membrane
In the battery A of Comparative Example 2 and the battery C of Comparative Example 2, no leakage of the electrolytic solution was observed in all the batteries. The reason is,
This is because in the batteries A and C, the amount of the organic electrolyte in the batteries is smaller than that in the battery B.

Next, a short-circuit test was performed. Battery A,
Five batteries B and C were prepared, and each battery was charged at 25 ° C. with a constant current of 1 CmA (700 mA) up to 4.4 V, followed by a constant voltage of 4.4 V, and a constant current and a constant voltage total of 3 V.
After charging for an hour, the positive terminal and the negative terminal were short-circuited, and the state of the battery at that time was observed.

As a result, in the battery C of Comparative Example 2,
Gas ejection occurred in all five batteries, and the surface temperature of the battery was 300
The temperature rose to above ℃. On the other hand, in the battery A of Example 1 and the battery B of Comparative Example 1, the surface temperatures of all the batteries are 90 to 90.
Although the temperature reached 100 ° C., no abnormal decrease such as ejection of gas from the battery or opening of the laminate film case was observed. The reason is that the low molecular weight polyethylene (PE) fine particles in the porous polymer electrolyte membrane in the battery A and the polyolefin separator having micropores in the battery B exhibited the shutdown function.

[0052]

As described above, in the nonaqueous electrolyte secondary battery according to the present invention, since the porous polymer electrolyte membrane contains a thermoplastic resin, there is no liquid leakage from the battery. Due to the shutdown function of the thermoplastic resin, a battery excellent in safety can be obtained. In addition, since the porous polymer electrolyte membrane has high ionic conductivity, it can be charged and discharged at a high rate, and a battery having a large discharge capacity can be obtained.

Further, the porous polymer membrane of the present invention is prepared by first preparing a paste containing a polymer constituting the porous polymer electrolyte membrane, particles of a thermoplastic resin, and an organic solvent. It is obtained by removing the organic solvent by a method such as heating or depressurization, and can obtain a porous polymer electrolyte membrane extremely simply and in a short time as compared with the conventional solvent extraction method.

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a porous polymer electrolyte membrane between the positive electrode and the negative electrode,
The porous polymer electrolyte membrane contains particles of a thermoplastic resin having a particle size of 0.015 to 5 μm, the melting point of the thermoplastic resin is 80 to 170 ° C., and the porous polymer electrolyte membrane constitutes the porous polymer electrolyte membrane. A non-aqueous electrolyte secondary battery characterized by having a melting point lower than the melting point of the polymer. 2. Use of a porous polymer membrane obtained by removing an organic solvent from a paste containing a polymer constituting a porous polymer electrolyte membrane, particles of a thermoplastic resin, and an organic solvent. The non-aqueous electrolyte secondary battery according to claim 1.