JP2992598B2 - Lithium ion battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator for improving the performance of a lithium ion battery and a lithium ion battery using the separator.

[0002]

2. Description of the Related Art A porous film made of polyethylene or polypropylene resin is used for a separator of a lithium ion battery. In these separators, except for the pores, the resin itself is used. Since the resin portion is hydrophobic and also an insulator, a polar organic solvent generally used as an electrolyte (electrolyte solution) of a lithium ion battery is used. And no affinity with lithium salts. Therefore, the passage for lithium ions to move is only the hole of the separator, and cannot move inside the polymer. In addition, the interface resistance is high due to poor adaptation at the contact interface with the electrode. Furthermore, these hydrophobic separators are poorly compatible with polar electrolytes and have low retention of electrolytes, so there is a risk of liquid leakage in the event of damage or the like.

[0003] When the internal temperature of the battery rises for some reason during use of the battery, the polyolefin-based resin separator increases the ionic conductivity of the electrolyte and may cause runaway. Therefore, it is desired to improve the disadvantages of such a hydrophobic separator having an insulator portion, reduce the internal resistance of the battery, and improve the safety and battery efficiency.

As an attempt to meet this demand, it has been studied to graft-polymerize a hydrophilic monomer onto a polypropylene resin porous membrane separator and use it in a lithium battery. Jo
urnalof Membrane Science 107 (1955) 155-164, JLGin
este, G. Pourcell. It is described that this method can increase the dissolution concentration of the electrolyte lithium salt in the electrolyte, improve the wettability with the electrode, and increase the number of charge / discharge cycles. Further, it has been studied to form a solid polymer electrolyte layer by enclosing a polymer electrolyte in pores of a polypropylene porous membrane. J. Electrochem. So
c. Vol. 142, No. 3, March 1995, KM Abraham, M. A
lamgir. This method avoids the risk of liquid leakage,
The electrical conductivity is reduced by one to two orders of magnitude.

As described above, if a polymer electrolyte material is used for a battery instead of a liquid electrolyte material, the problem of liquid leakage seen in a battery using a liquid electrolyte material can be solved, and a thinner film and a smaller size can be achieved. There are many benefits, such as being easier. As such a battery, a lithium battery using a lithium ion conductive polymer electrolyte and using lithium metal as a negative electrode active material is known. However, the solid electrolyte has an electric conductivity of 1 to ² S / cm of the liquid electrolyte,
0 ^over 7 there is a 10 ^over 8 S / cm and less problems. To solve this problem, using a polar organic solvent as a plasticizer in polymer electrolytes have been studied so-called gel electrolyte, its electrical conductivity even 10 ^{over 4} S / cm level at present. In addition, since the gel electrolyte has no mechanical strength, there is a danger that the electrodes may come into contact depending on the pressure.

[0006]

SUMMARY OF THE INVENTION The inventors of the present application
Improvements have been proposed for the electrolyte retention and ion permeability of the separator. That is, as a result of repeated studies focusing on the characteristics of such a polymer porous membrane and the characteristics of a polymer electrolyte, a polyvinylidene fluoride microporous membrane was swollen with a polar organic solvent, for example, as a separator membrane, and then impregnated. When the battery is constituted by using a separator in which the solvent is replaced with an electrolyte solution containing a lithium salt and using this as an electrolyte layer, at room temperature, ^10-2 S
/ Cm order electrical conductivity, and found that it can be applied to highly efficient and highly safe lithium ion batteries.

Although the mechanism for improving the conductivity is not known, since the polyvinylidene fluoride resin is a highly crystalline polymer, the porous film of this polymer does not dissolve in a polar organic solvent usually used in lithium batteries. . Therefore, in the battery, it functions as a separator which keeps the form as a membrane and prevents contact with the electrode. On the other hand, since polyvinylidene fluoride is a polar polymer, it swells to some extent in a polar organic solvent. That is, polyvinylidene fluoride swells in the polar organic solvent but does not dissolve. In addition, since polyvinylidene fluoride is a polymer having a very high dielectric constant, it has a function of promoting the dissociation of a lithium salt and increasing the concentration of lithium ions. In other words, it is presumed that conditions are created not only for the passage of ions in the pores, but also for the inside of swollen polyvinylidene fluoride which is a partition wall of the pores.

Moreover, polyvinylidene fluoride swells in the electrolytic solution, but does not dissolve in the electrolytic solution, so that the original electrical conductivity of the electrolytic solution can be maintained without increasing the viscosity of the electrolytic solution in the pores. In addition, the swollen polyvinylidene fluoride has good affinity with the electrodes, and has low electric resistance at the interface between both electrodes. An electrode of a lithium ion battery is generally one in which an active material is bound to copper or aluminum foil using polyvinylidene fluoride as a binder, and the presence of this polyvinylidene fluoride improves the affinity with a polyvinylidene fluoride separator. However, it is presumed that this is a reason for lowering the interface electric resistance. Furthermore, since the pores of the microporous membrane of polyvinylidene fluoride have high affinity with ordinary lithium-based electrolyte solutions,
It is also a great feature that the liquid retention property is good and liquid leakage hardly occurs.

When the internal temperature of the battery rises during use, the microporous polyvinylidene fluoride membrane absorbs and swells the organic electrolyte. For this reason, (1) the electrolyte solution in the micropores is reduced and the pore size is reduced (2) The swollen polyvinylidene fluoride becomes a gel electrolyte. As a result, the ionic conductivity decreases and the temperature inside the battery is prevented from rising. Furthermore, since the amount of the organic electrolyte in the battery is limited, polyvinylidene fluoride does not swell infinitely, that is, does not dissolve. (3) The organic solvent sucked into the polyvinylidene fluoride is retained in the gel by the osmotic pressure acting on the gel, and does not leak. (4) Since the gel adheres to the surface of the negative electrode, dendrites are less likely to be generated.

[0010] Porous membranes of polyvinylidene fluoride are known, and many prior arts relating to their production methods have been published. However, their use is as a separation membrane utilizing the chemical resistance and heat resistance of polyvinylidene fluoride. Describes a battery separator as a general use of a porous membrane, but does not show any effects and features when used as a separator of a lithium ion battery. (Japanese Patent Laid-Open No. 49-1265
72, 50-35265, 58-91731, 64-3
8448)

A dope in which a polymer is dissolved in a solvent is cast on a release material, and then immersed in water (non-solvent) containing dimethylformamide to form a porous film for a battery separator by a wet method. Are described in JP-A-2-27.
No. 6153 and Japanese Patent Application No. 7-46045. The morphology of the pores of the microporous membrane obtained by the wet method of the present application is described in the above-mentioned Japanese Patent Application No. Hei 7-46045 and can be referred to. However, in the present application, the electrolyte is prepared by swelling the separator. Consideration should be given to the fact that, because of the impregnation, the passage of ions inside the swollen resin body other than the pores that can be observed under magnification with an electron microscope or the like contributes to the conductivity.

[0012]

Example 1 Preparation of an electrolyte solution and a cell was carried out using a manipulator in a dry box filled with argon. 1) Preparation of positive electrode LiCoO ₂ as a positive electrode active material, graphite as a conductive agent,
To a powder prepared by mixing polyvinylidene fluoride as a binder, N-methyl-2-pyrrolidone as a solvent was added at 60% by weight of the powder and kneaded for 90 minutes to prepare a positive electrode slurry. The slurry was applied to a copper foil having a thickness of 20 μm by a coating apparatus, dried and solidified by a drier, and then passed through a press roller to obtain a uniform thickness (0.
2 mm) of the positive electrode was formed. Then, the dimensions were cut and the lead plate was welded to complete the positive electrode.

2) Preparation of Negative Electrode 69% by weight of pitch coke as a carbonaceous material capable of doping and undoping lithium ions, 10% by weight of acetylene black as a conductive agent, and polyvinylidene fluoride 21 as a binder N-methyl-2-pyrrolidone as a solvent was added to a powder prepared by mixing the powders by weight at 140% by weight, and kneaded for 90 minutes to prepare a negative electrode slurry. This slurry is coated with a coating machine to a thickness of 10μ.
After being applied to an aluminum foil having a thickness of 0.2 m and solidified in a drying oven, a negative electrode having a uniform thickness (0.2 mm) was formed with a press roller. Thereafter, dimensions were cut and the lead plate was welded to complete the negative electrode.

3) Preparation of Polyvinylidene Fluoride Microporous Membrane 15 parts by weight of polyvinylidene fluoride were mixed with 85 parts by weight of N-methyl-2-pyrrolidone (NMP) as a solvent, 3 parts by weight of a nonionic activator as a film forming aid, and polyvinyl Pyrrolidone 2
The resulting mixture was mixed with parts by weight and stirred for 90 minutes to prepare a dope. The dope was cast on release paper, immersed in water containing 5 parts by weight of dimethylformamide to solidify, then washed with water and dried to obtain a fine powder of polyvinylidene fluoride having a thickness of 25 μm and a porosity of 60%. A porous membrane was made. Examples of the nonionic activator as the film forming aid include: ether type (eg, polyoxyethylene oleyl ether), alkylphenol type (eg, polyoxyethylene nonylphenyl ether, polyoxyethylene sesqui-2-ethylhexyl phosphate), ester type (eg, For example, polyoxyethylene monolaurate), sorbitan ester type (eg, sorbitan monostearate) and the like can be used.

4) Preparation of Electrolyte Layer Ethylene carbonate, dimethyl carbonate and diethyl carbonate are used in a polar organic solvent in a volume ratio of 2: 2:
The solvent mixed in 1 was used. As the electrolytic solution, a solution prepared by dissolving lithium hexafluorophosphate and lithium perchlorate in a concentration of 1 mol / 1 and 0.3 / 1, respectively, in this solvent was used. The polyvinylidene fluoride microporous membrane obtained by the wet method is immersed in the solvent for one day, and the solvent is sufficiently impregnated or permeated into the pores. The sufficiently substituted separator was used as an electrolyte layer (in a configuration in which a separator is sandwiched between a positive electrode and a negative electrode, the separator becomes an electrolyte layer connecting the two electrodes).

5) Measurement of electric conductivity The separator prepared above is sandwiched between a positive electrode and a negative electrode,
EG & G Measured on Princeton Applied Research M378 impedance system. Frequency 5Hz and 100
The electrical conductivity was determined using impedance spectroscopy between Hz.

[0017]

Comparative Example 1 As a porous membrane, Ce from Hoechst Celanease was used.
lgard (made of polypropylene and having a thickness of 25 μm and a porosity of 55)
%) Was used in the same manner as in Example 1 except that the comparison was made.

[0018]

The electrical conductivity of the separator at 25 ° C. in the above example was 7.5 × 10 ⁻³ S / cm, and Comparative Example 1 was used.
Was 3 × 10 ⁻⁴ S / cm. Further, in the case of Example 1, even if the film containing the electrolytic solution was dropped vertically, the solution did not drop and flow, and in the case of Comparative Example 1, the solution was easily dropped. By using a polyvinylidene fluoride microporous membrane as a separator for a lithium-ion battery as described above, a battery with good battery efficiency and a low risk of liquid leakage can be produced, and its electrolyte retention and ion permeability can be improved. The industrial value is extremely large.

[0019]

SUMMARY OF THE INVENTION Continuing with the subsequent studies by the present inventors, a dope is coated on a mold release material, and immediately before solidification, an ultra-thin reinforcing material such as a nonwoven fabric is laminated or embedded to form a polyfluoride. It has become possible to provide a reinforcing effect on the mechanical properties of the microporous membrane of vinylidene resin. That is, the outline of the invention to be added in the present application is that a polyvinylidene fluoride resin is dissolved in a solvent, and a fibrous base material as a reinforcing material is cast in the dope on the release material in a state where the dope is cast on the release material. A porous membrane of polyvinylidene fluoride resin having electrolyte retention and ion permeability, formed by a wet film forming method of solidifying or laminating in a non-solvent after being impregnated and immersed in a non-solvent, and integrated. And a lithium ion battery using the separator membrane. In the above description, the present invention relates to a method in which a fibrous base material as a reinforcing material is laminated and bonded to form a composite fibrous base material, and then used for impregnation and immersion in a dope on a release material.

A preferred range of the main conditions for reinforcing the fiber base material is as follows. 1) Dope preparation Polyvinylidene fluoride resin: Same as the basic invention. Resin concentration in dope: 5-30%. Solvent: N-methyl-2-pyrrolidone (NMP) and others,
Dimethyl formaldehyde (DMF), dimethyl sulfoxide (DMSO). Surfactant: polyoxyethylene sesqui 2-ethylhexyl phosphate, other ether type, ester type, sorbita ester type nonionic surfactant. Other compounding agents: polyvinylpyrrolidone (molecular weight: 10,0
00-1,200,000), polyethylene glycol, water-insoluble partially crosslinked polyethylene glycol. Dope viscosity: 3,000-30,000 cps / 20 ° C.

2) Fibrous base material: A nonwoven fabric formed by mixing or mixing polyethylene, polypropylene, or a fiber material having polypropylene as a core material and polyethylene as a sheath material in a wet or dry manner and forming a low melting point portion as a binder into a sheet. Available, weight: 5-40.0 g /
m ² , thickness: 10-70 μm, tensile strength: vertical
0-2.0Kg / 15mm width, air permeability: 0.0-100
sec / 100cc (Gurley, JIS-P-8117)
And a hydrophilic fiber is applied so that the constituent fibers of the fiber base material have a surface tension comparable to the surface tension of the dope solvent (NMP surface tension: 41 dynes / cm, 25 ° C.). Thin nonwoven fabrics are preferred.

Further, the reinforcing fiber base material may be a nonwoven fabric having a perpendicular arrangement in a weft direction in addition to a normal nonwoven fabric in which the directions of constituent fibers are randomly arranged. Further, a plurality of nonwoven fabrics may be laminated to form a composite reinforcing material. You may use it.

[0023] 3) separator membrane conditional doped with weight: 5.0-40.0 (dry) g / m 2 membrane thickness: 25 over 75μm film Weight: 15.0-65.0g / m ² Air permeability: 5-500sec / 100cc

[0024]

Example 2 The nonwoven fabric used in Example 2 was obtained by wet-mixing a mixed fiber using polyethylene and polypropylene (weight: 11 ± 3 g / m ² , thickness: 0.030−).
0.032 mm, air permeability: 0.0 ± 1 sec / 100c
c (JIS-P-8117), surface tension of fiber: 38 dynes / cm or more.

The film forming conditions are the same as in the first embodiment except that the fiber reinforced base material is laminated and embedded in the dope cast on the release material. That is, 15 parts by weight of polyvinylidene fluoride were mixed with 85 parts by weight of N-methyl-2-pyrrolidone (NMP) as a solvent, 3 parts by weight of a nonionic activator as a film-forming aid, and 2 parts by weight of polyvinylpyrrolidone, followed by stirring for 90 minutes. To make a dope. After casting the dope on release paper, the fiber base material is embedded in the dope, immersed in water containing 5 parts by weight of dimethylformamide to solidify, and then washed with water and dried to obtain a fiber. A reinforced microporous membrane of polyvinylidene fluoride was prepared.

The properties of the obtained film are as follows. Weight: 19.0 g / m ² Thickness: 41 μm Air permeability: 18-19 sec / 100 cc Tensile strength: Vertical; 5.14 kg / 50 mm width / 200 mm / min Horizontal; 1.53 kg / 50 mm width / 200 mm / min

Measurement of Electric Conductivity The result obtained by the above measurement method is 7.5 × 10 ⁻³ S / c.
m, 25 ° C., corresponding to the result of Example 1.

[0028]

Example 3 A fiber reinforced film was prepared under the same conditions as in Example 2 except that the amount of the polyvinylidene fluoride resin was changed to 12.0 g / m ² (dry), and other nonwoven fabrics, mold release materials and processing film forming conditions were the same. I got The properties of the obtained film are as follows. Weight: 23.0 g / m ² Thickness: 53 μm Air permeability: 12-13 sec / 100 cc Tensile strength: Vertical; 5.33 kg / 50 mm width / 200 mm / min Horizontal; 1.54 kg / 50 mm width / 200 mm / min FIG. 1 shows a cross-sectional photograph of the microporous film obtained in Example 3. The membrane of this example has a structure in which polyvinylidene fluoride layers are formed on the front and back of the reinforcing material, and the reinforcing material layer is embedded.

Findings obtained from Examples 2, 3 and other test examples. If the surface tension of the fiber of the reinforcing fiber base is 35 dynes / cm or more (preferably 38 dynes / cm or more), the dope of polyvinylidene fluoride on the release material has good affinity with the fiber base, and Can be integrated by embedding and immersion in If the surface tension of the fiber is less than 35 dynes / cm, the fibers can be integrated in a laminated or bonded state. Doping amount: 8.0 g / m ² , non-woven fabric weight: 11.0
With a combination of g / m ² and thickness: 8.0 μm, a microporous polyvinylidene fluoride film is formed substantially uniformly on the front and back. When the doping amount is 10.0 g / m ² (dry) or more, the thickness of the front and back polyvinylidene fluoride microporous membranes becomes large, the cross-sectional structure becomes nearly symmetrical, and the distribution of the pore diameter becomes asymmetric. The symmetrical structure of the membrane can also be selected by changing the weight and thickness of the fiber reinforcement.

[0030]

The mechanical strength of the obtained microporous membrane is as follows:
It responds to the strength required in industrial battery assembly work and greatly contributes to practical application.

[0031]

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a fiber-reinforced microporous membrane. 1 is a front resin layer, 2 is a back resin layer, 3 is a reinforcing fiber base, and 4 is a cavity. This is the side where the back resin layer was in contact with the release material.

──────────────────────────────────────────────────続 き Continued on the front page (72) Natsuichiro Kamibayashi, Inventor 6-29-1, Tarui, Sennan-shi, Osaka Toyo Cross Co., Ltd. (72) Akira Miura 6-29-1, Tarui, Sennan-shi, Osaka, Japan Toyo Cross Co., Ltd. (72) Inventor Shizuo Maeda 6-29-1, Tarui, Sennan-shi, Osaka Toyo Cross Co., Ltd. (72) Inventor Shiro 6-29-1, Tarui, Sennan-shi, Osaka Toyo Cross Co., Ltd. (56) References JP-A-8-250127 (JP, A) JP-A-8-236097 (JP, A) US Pat. No. 5,296,318 (US, A) (58) Fields investigated (Int. Cl. ⁶ , DB Name) H01M 2/16

Claims (1)

(57) [Claims] 1. A polyvinylidene fluoride resin is dissolved in a solvent, and a fibrous base material as a reinforcing material is impregnated and immersed in the dope in a state where the dope is cast on a release material. Lithium-ion battery separator made of a microporous polyvinylidene fluoride resin having electrolyte retention and ion permeability, formed by a wet film forming method of solidifying in a non-solvent.