JP2016081621A - Separator and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery separator improved in permeability of an electrolyte in a separator.SOLUTION: In a separator used as a battery separator, a fibril branch end is formed on the surface thereof. Even if the separator is folded to be used when being stored in a lithium ion secondary battery including a positive electrode 1, a negative electrode 2, a separator 3 and a nonaqueous electrolyte, the fibril branch end on the surface has a larger free space than ordinary fibrils, so that a pore route can be secured largely on a film surface, and therefor soaking properties of an electrolyte is improved and the interface resistance of a cell can be also reduced.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a separator and a lithium ion secondary battery using the separator.

Lithium ion secondary batteries are mainly used in portable devices such as VTR cameras, notebook computers and mobile phones, taking advantage of the high energy density. In recent years, research and development of large-sized lithium ion secondary batteries for the purpose of electric vehicles and power storage have been actively conducted. In particular, in the automobile industry, in order to cope with environmental problems, development of electric vehicles using a motor as a power source and hybrid electric vehicles using both an internal combustion engine and a motor are underway. Has already been put to practical use.

In addition, lithium ion secondary batteries are classified into cylindrical shapes and rectangular shapes according to their shapes, but at present, cylindrical batteries are often used as power sources for electric vehicles. The inside of a typical cylindrical lithium ion secondary battery has a structure in which both the positive electrode plate and the negative electrode plate are coated with an active material on both sides of a metal foil, and a separator is sandwiched between these two electrodes so that they are not in direct contact with each other. It is said that.

A lithium ion secondary battery used for a power source of an electric vehicle is strongly required to have a high capacity and a high output, and a long life characteristic suitable for long-term use of the electric vehicle is also desired. However, in large batteries for electric vehicles, the movement of lithium ions does not follow the rapid flow of electrons that accompanies charging / discharging of a large current, resulting in a concentration gradient of lithium ions in the electrolyte in the separator or electrode. In some cases, the output may decrease. In order to solve this, a technique is disclosed in which the porosity of the separator separating the positive and negative electrodes is greater than or equal to the porosity of the positive electrode mixture and the negative electrode mixture (Patent Document 1). Moreover, the technique which suppresses the fall of a battery characteristic by making the ratio of the free electrolyte volume with respect to the space volume in a lithium ion secondary battery into an appropriate range is also disclosed (patent document 2).

  In particular, in the case of a cylindrical battery or a square battery, the separator is always bent when stored in a sealed container. According to the study by the present inventors, it is known that the fibrils present in the film pores are elongated at these bent portions, the porosity is decreased, and the internal resistance is increased. Here, the fibril is a bundle of polymer chains that are oriented (stretched). As described above, the change in the microstructure caused by the bending of the separator causes a decrease in the output of the battery. However, further improvement is considered necessary in this respect.

Japanese Patent Laid-Open No. 2001-223029 JP 2001-185223 A

  In view of the above, the present invention is intended to easily reduce the concentration gradient generated between the electrodes in the lithium ion secondary battery and improve the output characteristics of the battery.

  As a result of intensive studies to solve the above problems, the present inventors have found that it is effective to project the fibril free end on the surface of the resin sheet film in the separator according to the present invention.

The separator according to the present invention is a separator composed of a porous film having a fibril structure, and is characterized in that a fibril free end protrudes on the film surface of the porous film.

  Protruding the fibril end to the surface increases the free space on the surface, and even when bent, the fibril free end protrudes from the separator surface, allowing the distance between adjacent separators to be kept above a certain level. The free space thereby secures a diffusion path in the electrolyte, and no lithium ion concentration gradient occurs. Thereby, the output characteristics of the battery can be improved.

As for the porous film concerning this invention, it is preferable that a fibril free end has a branch part.

  More free ends can be increased by branching the fibril free end. The free end has more free space than continuously connected fibrils, so there is more free space around these branches than normal fibrils, resulting in less fibril overlap when bent, and lithium Ion movement is facilitated.

The fibril free end is preferably made of a resin having a carbonyl structure at least on the surface thereof.

  By having a carbonyl structure on the film surface, fibril free ends repel each other in the electrolyte solution, and contact between the free ends can be reduced.

A lithium ion secondary battery comprising a positive electrode, a negative electrode, and the separator disposed between the positive and negative electrodes, and a non-aqueous electrolyte impregnated in the separator. By having a free end, the overlap of the fibrils at the bent portion that occurs during battery manufacture is suppressed, and a diffusion path for the electrolyte can be secured.

  There is provided a secondary battery in which a concentration gradient of lithium ions generated in a bent portion in the battery is reduced and output characteristics are improved.

It is a cross-sectional schematic diagram of the separator in a battery bending part. It is a schematic diagram of the wound electrical storage element accommodated in a lithium ion secondary battery. It is a cross-sectional schematic diagram of an electrical storage element. 2 is a scanning electron micrograph of the separator surface of Example 1. FIG.

  Hereinafter, the best mode for carrying out the present invention (hereinafter referred to as the present embodiment) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

  FIG. 1 shows a schematic cross-sectional view of the separator at the battery bent portion. The separator shown as the present embodiment is located at a position sandwiched between the positive electrode 1 and the negative electrode 2, and in the separator 18 composed of the porous film 3b having a fibril structure, the fibril free end is formed on the film surface of the porous film 3b. 3a protrudes. In FIG. 1, the fibril free end is not shown, but the layer where the fibril free end exists is shown as 3 a.

(Separator)
The material used for the porous film for producing the separator shown in the present embodiment is not particularly limited, but a thermoplastic resin is preferably used. Further, an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 5 × 10 ⁵ or more, or an ultrahigh molecular weight polyolefin comprising at least 15% by weight of this ultra high molecular weight polyolefin resin and a polyolefin resin having a weight average molecular weight of less than 5 × 10 ⁵ A resin composition is preferred.

  Hereinafter, in the present specification, for the sake of simplicity, the above ultrahigh molecular weight polyolefin resin and a composition containing the same are collectively referred to as an ultrahigh molecular weight polyolefin resin (composition).

The ultrahigh molecular weight polyolefin resin has a weight average molecular weight in the range of 5 × 10 ^{5 to} 60 × 10 ⁶ , and preferably in the range of 1 × 10 ⁶ to 15 × 10 ⁶ . Examples of such ultrahigh molecular weight polyolefin resins include homopolymers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene, copolymers, and mixtures thereof. . However, among these, ultra high molecular weight polyethylene resin is preferably used.

The ultra high molecular weight polyolefin resin composition may contain other polyolefin resins together with the ultra high molecular weight polyolefin resin. The polyolefin resin other than the ultrahigh molecular weight polyolefin resin has a weight average molecular weight in the range of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , preferably 1 × 10 ^{4 to} 3 ×, like the ultra high molecular weight polyolefin resin. It is in the range of 10 ⁵ .

  In the production of the separator of this embodiment, first, the ultrahigh molecular weight polyolefin resin (composition) is 5 to 30% by weight, preferably 8 to 20% by weight, and the solvent 95 to 70 having a freezing point of −10 ° C. or lower. % By weight, preferably 92 to 80% by weight, mixed in a uniform slurry, heated and stirred to dissolve the ultrahigh molecular weight polyolefin resin (composition) in the solvent, and the resulting solution The kneaded mixture is kneaded at a temperature in the range of 115 to 185 ° C. to prepare a kneaded product.

  As said solvent, while melt | dissolving the said ultra high molecular weight polyolefin resin (composition) well, a freezing point should just be -10 degrees C or less. Preferable specific examples of such a solvent include aliphatic or cyclic hydrocarbons such as decane, decalin, and liquid paraffin.

  Next, the temperature below the freezing point of the solvent using the solution-like kneaded product of the ultrahigh molecular weight polyolefin resin (composition) thus obtained and the solvent, preferably a temperature in the range of 25 ° C to -45 ° C. Preferably, the gel is formed into a gel sheet while cooling to a temperature in the range of 20 ° C. to −20 ° C., and the ultrahigh molecular weight polyolefin resin (and high density polyethylene resin) is crystallized. The range of 0.1 to 5.0 mm is usually appropriate for the thickness of the gel sheet.

  Thus, the cooling to a temperature below the freezing point of the solvent using a solution-like kneaded product of the ultrahigh molecular weight polyolefin resin (composition) and the solvent is not particularly limited. Two metal plates may be cooled with dry ice, the kneaded material may be sandwiched between these metal plates, and the kneaded material may be pressed to form a sheet.

  Next, when the melting point of the ultrahigh molecular weight polyolefin resin is M, the gel-like sheet has a temperature in the range of (M + 5) ° C. to (M−30) ° C., preferably M ° C. to (M−25) ° C. Biaxial stretching at a temperature in the range. This biaxial stretching may be either sequential or simultaneous biaxial stretching, but preferably simultaneous biaxial stretching. In the present invention, the stretch ratio of the gel sheet is 3 to 32 times in one direction, the area stretch ratio is suitably in the range of 9 to 1024 times, preferably 3 to 20 times in one direction, The area stretch ratio is in the range of 9 to 400 times.

  Furthermore, the biaxially stretched film thus obtained is washed with an appropriate solvent to remove the solvent remaining in the sheet to obtain a separator, preferably after this, to prevent thermal shrinkage of this film Next, heat and heat set (heat setting). Examples of the solvent used for the desolvation treatment include volatile solvents such as hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, and ethers such as diethyl ether and dioxane. Is preferably used. These solvents are appropriately selected depending on the solvent used for preparing the solution of the ultrahigh molecular weight polyolefin resin (composition). In order to remove the solvent remaining in the separator, for example, the separator may be immersed in the solvent.

  The separator thus obtained is immersed in water and brought into contact with active oxygen species to etch the film surface. Examples of the active oxygen species to be used include superoxide and hydroxy radical. These can be generated by, for example, irradiating with ultraviolet rays while being in contact with ozone, or by ultrasonic treatment or electromagnetic wave treatment.

  In this case, ozone is generated by silent discharge in dry air or oxygen, irradiation with ultraviolet rays having a wavelength of 185 nm, electrolysis of dilute sulfuric acid at a low temperature, or heating of liquid oxygen according to a conventional method. be able to. As the ozone concentration, a range of 100 to 500 ppm is selected, but the etching rate increases as the concentration increases.

  In addition, the ultraviolet ray source irradiated at this time can be arbitrarily selected from those commonly used as an ultraviolet ray source, and is not particularly limited. Examples of such a radiation source include a low-pressure mercury lamp, a quartz mercury lamp, a hydrogen discharge tube, a xenon discharge tube, and a Lyman discharge tube.

  Next, hydroxy radicals are also generated during ultrasonic treatment in water. In this case, the ultrasonic wave can be oscillated using a vibrator such as a quartz vibrator, a barium titanate ceramic vibrator, a metal magnetostrictive vibrator, or a ferrite vibrator.

In the present etching method, by bringing the active oxygen species into contact with the separator, a fibril branch end by fibril cutting can be formed, and the surface thereof is modified to be hydrophilic. In this etching process, the fibril itself is cut, and a carbonyl group is generated on the surface. The generation of these hydrophilic groups can be confirmed by the appearance of an absorption peak near 1700 cm ⁻¹ derived from a carbonyl group in the ATR-IR spectrum.

  The fibril free end preferably has a carbonyl group on the surface, and the presence / absence and amount of the carbonyl group is determined by the element concentration of O (oxygen) / C (carbon) by X-ray photoelectron spectroscopy (XPS). It can confirm by a ratio, and it is preferable that O / C is 0.1-0.7 at this time.

  In addition, when an anionic functional group is added, the zeta potential decreases, but the zeta potential on the surface of the separator decreases due to contact with the active oxygen species, which also confirms the formation of a carbonyl group. . On the other hand, the fact that the surface of the separator has been modified can also be confirmed by reducing the contact angle of the surface with water.

  The separator thus obtained has a thickness in the range of 1 to 60 μm, preferably 5 to 45 μm, a porosity of 35 to 75%, preferably 50 to 70%, and an air permeability of 100 to 800 seconds. / 100 cc, preferably 100 to 500 seconds / 100 cc. The porosity may be measured by a mercury porosimeter, and the air permeability may be measured by a Gurley method in accordance with JIS P8117.

  Since such a separator has a large number of fibril ends on the surface, the free volume on the surface of the membrane increases, so that, for example, the electrical resistance generated at the fibril bent portion generated when encapsulating in a sealed can is reduced, and the output A secondary battery having excellent characteristics can be provided.

Subsequently, the secondary battery using the above-described separator will be described by taking a lithium ion secondary battery as an example.
(Lithium ion secondary battery)
An electrode and a lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

  The lithium ion secondary battery mainly includes a power storage element 30, a case (not shown) for housing the power storage element 30 in a sealed state, and a pair of leads 60 and 62 connected to the power storage element 30. .

As shown in FIG. 3, the electricity storage element 30 is a wound body obtained by winding a pair of electrodes 10 and 20 that are arranged to face each other with the separator 18 interposed therebetween. In the positive electrode 10, a positive electrode active material layer 14 is provided on the positive electrode current collector 12, and in the negative electrode 20, a negative electrode active material layer 24 is provided on the negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. A non-aqueous electrolyte is contained inside the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. Leads 60 and 62 made of aluminum are connected to the ends of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the ends of the leads 60 and 62 extend to the outside of the case.

(Positive electrode current collector)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a binder, and a conductive auxiliary agent in an amount as necessary.

(Positive electrode active material)
Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions (for example, PF ₆ ⁻ ) of the lithium ions. The electrode is not particularly limited as long as it can be reversibly advanced, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and the general formula: LiNi _x Co _y Mn _z MaO ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Oxide, lithium vanadium compound (LiV ₂ O ₅ , LiVOPO ₄ ), olivine-type LiMPO ₄ (where M is one or more selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr) shows the element or VO), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1), Li excess based It includes complex metal oxides. In particular, LiVOPO ₄ and LiFePO ₄ excellent in thermal stability are more preferable.

(Positive electrode binder)
The binder binds the positive electrode active materials and the positive electrode current collector 12 together with the positive electrode active materials. The binder is not particularly limited as long as the above-described bonding is possible, and examples thereof include fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In addition to the above, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, or the like may be used as the binder. Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive assistant particles, it is not necessary to add the conductive assistant. As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) , Polyphosphazene, etc.) and a lithium salt such as LiClO ₄ , LiBF ₄ , LiPF ₆ , or an alkali metal salt mainly composed of lithium, and the like. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

Although content of the binder in the positive electrode active material layer 14 is not specifically limited, It is preferable that it is 1-10 mass% on the basis of the sum of the mass of a positive electrode active material, a conductive support agent, and a binder. By making content of a positive electrode active material and a binder into the said range, in the obtained positive electrode active material layer 14, the amount of binders is too small and the tendency which cannot form a strong positive electrode active material layer can be suppressed. In addition, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it is difficult to obtain a sufficient volume energy density can be suppressed.

(Positive electrode conductive aid)
The conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.

The content of the conductive additive in the positive electrode active material layer 14 is not particularly limited, but when added, it is preferably 0.5 to 5% by mass with respect to the mass of the positive electrode active material.

(Negative electrode current collector)
The negative electrode current collector 22 may be any conductive plate material, and for example, a metal thin plate (metal foil) of copper, nickel, stainless steel, or an alloy thereof can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a binder, and a conductive auxiliary agent in an amount as required.

(Negative electrode active material)
The negative electrode active material may be any compound that can occlude and release lithium ions, and known negative electrode active materials for batteries can be used. Examples of the negative electrode active material include carbon materials that can occlude and release lithium ions (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, and the like, aluminum, silicon Particles containing metals that can be combined with lithium, such as tin, amorphous compounds mainly composed of oxides such as silicon dioxide, tin dioxide, iron oxide, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), etc. Is mentioned. It is preferable to use graphite or silicon having a high capacity per unit mass and relatively stable.

(Negative conductive auxiliary)
The conductive auxiliary agent is not particularly limited, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as pyrolytic carbon such as carbon black, cokes, glassy carbons, organic polymer compound fired materials, carbon fibers, and activated carbon. Moreover, you may add negative electrode active material materials, such as non-graphitizable carbon, graphitizable carbon, and graphite, changing a shape.

As carbon black, acetylene black, ketjen black and the like are particularly preferable, and ketjen black is particularly preferable. By including an electron conductive porous body, pores can be formed at the interface between the particles of the negative electrode active material and the binder, and the penetration of the non-aqueous electrolyte into the negative electrode active material layer 24 is facilitated by the pores. preferable.

(Negative electrode binder)
In addition to the material used for the positive electrode 10 described above, an acrylic resin such as PAA can also be used for the binder and the conductive assistant. In addition, the content of the binder and the conductive auxiliary agent is also adjusted as appropriate when the volume change of the negative electrode active material and the adhesion to the foil must be taken into account, and the same content as the content in the positive electrode 10 described above. Should be adopted. When adding, it is preferable that the addition amount of a binder is 2-20 mass% with respect to the mass of a negative electrode active material. It is preferable that the addition amount of a conductive support agent is 0.5-5 mass% with respect to the mass of a negative electrode active material.

With the components described above, the electrodes 10 and 20 can be produced by a commonly used method. For example, a paint containing an active material (a positive electrode active material or a negative electrode active material), a binder (a positive electrode binder or a negative electrode binder), a solvent, and a conductive additive (a positive electrode conductive additive or a negative electrode conductive aid) is placed on the current collector. It can manufacture by apply | coating and removing the solvent in the coating material apply | coated on the electrical power collector.

  As the solvent, for example, N-methyl-2-pyrrolidone, N, N-dimethylformamide, water and the like can be used.

There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

The method for removing the solvent in the paint applied on the current collectors 12 and 22 is not particularly limited, and the current collectors 12 and 22 applied with the paint are dried, for example, in an atmosphere of 80 ° C. to 150 ° C. Just do it.

  Then, the electrodes on which the active material layers 14 and 24 are formed in this way may then be pressed by a roll press device or the like as necessary. −

(Electrolytes)
The electrolyte is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte is not particularly limited, and for example, in the present embodiment, an electrolytic solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, so that the withstand voltage during charging is limited to a low level. As the electrolytic solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. It does not specifically limit as lithium salt, The lithium salt used as an electrolyte of a lithium ion secondary battery can be used. For example, examples of the lithium salt include inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiFSI, LiBOB, and Li ₂ B ₁₂ F ₁₂ , and organic compounds such as LiCF ₃ SO ₃ and (CF ₃ SO ₂ ) ₂ NLi. An acid anion salt or the like can be used.

Moreover, as the organic solvent, for example, aprotic high dielectric constant solvents such as ethylene carbonate and propylene carbonate, aprotic low viscosity such as acetic acid esters and propionic acid esters such as dimethyl carbonate and ethyl methyl carbonate, etc. A solvent is mentioned. It is desirable to use these aprotic high dielectric constant solvents and aprotic low viscosity solvents in combination at an appropriate mixing ratio. Furthermore, ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used. The counter anion is not particularly limited, and examples thereof include BF ₄ ⁻ , PF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ^{− and the} like. The ionic liquid can be used by mixing with the organic solvent described above.
The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 2.0 M from the viewpoint of electrical conductivity. The conductivity of the electrolyte at 25 ° C. is preferably 0.01 S / m or more, and is adjusted by the type of electrolyte salt or its concentration.

When the electrolyte is a solid electrolyte or gel electrolyte, poly (vinylidene fluoride) or the like can be contained as a polymer material.
Furthermore, you may add various additives in the electrolyte solution of this embodiment as needed. Examples of additives include vinylene carbonate and methyl vinylene carbonate for the purpose of improving cycle life, biphenyl and alkyl biphenyl for the purpose of preventing overcharge, various carbonate compounds for the purpose of deoxidation and dehydration, Carboxylic anhydride, various nitrogen-containing and sulfur-containing compounds can be mentioned.

  Between the positive electrode and the negative electrode produced as described above, the separator according to this embodiment described above is sandwiched between them, and after superposition, a winding core is used using a winding core. A wound product was produced. When the wound body 30 is enclosed in a case together with an electrolyte, a lithium ion secondary battery is completed.

  As mentioned above, although the example of the separator of this embodiment, its manufacturing method, and the lithium ion secondary battery using a separator was demonstrated, this invention is not limited to the said embodiment. The separator etc. of this invention can be suitably set according to a use etc.

  For example, the separator of the present embodiment can be used for electrochemical elements other than lithium ion secondary batteries. Electrochemical elements include secondary batteries other than lithium ion secondary batteries such as metal lithium secondary batteries (those using the active material of the present invention as the cathode and metal lithium as the anode), and electric batteries such as lithium capacitors. Examples include chemical capacitors. These electrochemical elements can be used for power sources such as self-propelled micromachines and IC cards, and distributed power sources arranged on or in a printed circuit board.

Example 1
(Preparation of separator)
A 15% by weight ultra-high molecular weight polyethylene resin (melting point: 134 ° C.) having a weight average molecular weight of 1 million and 85% by weight of liquid paraffin are uniformly mixed in a slurry, charged in a small kneader, and heated at a temperature of 160 ° C. for about 50 minutes. Then, the mixture was heated, dissolved, and kneaded to obtain a kneaded product of an ultrahigh molecular weight polyethylene resin and a solvent. Thereafter, the kneaded product was molded into a sheet having a thickness of 0.5 mm while rapidly cooling to 15 ° C. to crystallize the ultrahigh molecular weight polyethylene resin.

Next, the sheet was simultaneously biaxially stretched 4 × 4 times in length and width at a temperature of about 115 ° C. and then immersed in methylene chloride to remove the solvent. The separator thus obtained was further heat set at 120 ° C. for 10 seconds to obtain a porous film having a thickness of 25 μm and a porosity of 40%. The porosity at this time was calculated using a mercury porosimeter (Autoscan 33, manufactured by Yuasa Ionics). The thickness of the separator was determined by observing the cross section of the separator with a scanning electron microscope (SEM).

Further, only half of the obtained separator was masked with polyfluorinated ethylene, accommodated in a quartz test tube (inner diameter 150 mm, length 300 mm), 300 ml of distilled water was added, and the wavelength from the low-pressure mercury lamps on both sides during ozone aeration Irradiation with ultraviolet rays of 254 nm was performed. At this time, the distance between the quartz test tube and the low-pressure mercury lamp was 3 mm. Thus, after performing the etching process for 120 minutes, the separator was taken out, washed with water, the masking was peeled off, and the separator of Example 1 was obtained.

(Production of lithium ion secondary battery)
(Preparation of negative electrode)
Artificial graphite was used as the negative electrode active material, styrene-butadiene rubber and carboxymethyl cellulose were used as the binder, and water was used as a solvent to prepare a paint. This paint was applied to a copper foil (thickness: 15 μm) as a current collector by a doctor blade method, dried at 80 ° C., and then rolled to form a positive electrode active material layer on the surface of the copper foil. The copper foil was provided with a portion to which no paint was applied in order to connect the external lead terminal. As the external lead terminal, a nickel foil wound with polypropylene grafted with maleic anhydride was prepared for the purpose of improving the sealing performance with the exterior body. The nickel foil and the copper foil after the coating material was applied and dried were ultrasonically welded to produce a negative electrode.

(Preparation of positive electrode)
LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride as a binder, carbon black and graphite as conductive aids, and N-methyl-2-pyrrolidone as a solvent were mixed to prepare a paint. This paint was applied to an aluminum foil (thickness 20 μm) as a current collector by a doctor blade method, dried at 100 ° C., and then rolled to form a positive electrode active material layer on the surface of the aluminum foil. The aluminum foil was provided with a portion to which no paint was applied in order to connect the external lead terminal. As an external lead terminal, for the purpose of improving the sealing property with the exterior body, an aluminum foil wound with polypropylene grafted with maleic anhydride was prepared. The aluminum foil and the aluminum foil after the coating material was applied and dried were ultrasonically welded to produce a positive electrode.
(Production of evaluation lithium-ion secondary battery)
The positive electrode prepared as described above and the negative electrode are stacked with the separator prepared as described above interposed therebetween, and the stacked body is wound using a winding machine using a winding core. What was made was produced. The wound body was sealed in an aluminum cylindrical case together with the electrolytic solution, so that the separator was impregnated with the electrolytic solution, and the lithium ion secondary battery for evaluation of Example 1 was completed.
As the electrolytic solution, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, and LiPF ₆ was dissolved as a supporting salt so as to have a concentration of 1.0 mol / L.

(Example 2)
Example 2 was the same as Example 1 except that the etching time was 60 minutes.

(Example 3)
Example 3 was the same as Example 1 except that the etching time was 180 minutes.

(Comparative Example 1)
Comparative Example 1 was the same as Example 1 except that no etching process was performed.

(Comparative Example 2)
Comparative Example 2 was the same as Example 1 except that the etching time was 20 minutes.

  The material used, the characteristics of the produced separator and the characteristics of the secondary battery using the separator were evaluated as follows.

(Confirmation of fibril branch end by SEM)
A separator cut to an appropriate size was fixed to a sample stage with a conductive double-sided tape, and an osmium plasma coating having a thickness of about 10 nm was applied to prepare a sample for speculum. Next, the surface structure of the separator was observed using a scanning electron microscope (SEM) to confirm the fibril branch end. The surface photograph of the separator produced in Example 1 is shown in FIG. As is apparent from the surface photograph, it can be seen that a fibril free end formed by cutting the fibril exists on the separator surface.

(Measurement of contact angle with water)
The contact angle of the separator with respect to water was measured with an automatic contact angle meter (CA-Z type) manufactured by Kyowa Interface Science Co., Ltd.

(XPS measurement)
Using X-ray photoelectron spectroscopy (XPS) (ESCA-5500MC manufactured by PERKIN ELMER PHI), surface element composition analysis of the sample was performed, and an element concentration ratio of O (oxygen) / C (carbon) was calculated. As the measurement conditions, the excitation source used was Al-Kα ray, the output was 14 kV, 150 W, the monochromator was used, the analysis area was 0.8 mm × 3.5 mm, and the extraction angle was 65 degrees.

(Cell initial impedance)
The initial impedance of the battery was measured with a Solartron 1260 chemical impedance analyzer.

(Discharge test)
The lithium ion secondary battery was subjected to constant current and constant voltage charging under conditions of a maximum voltage of 4.2 V, a current density of 0.068 mA / cm ² , and a final current density of 0.034 mA / cm ² . After that, the capacity when discharged at the final voltage of 2.75 V and the current density of 0.068, 0.341, and 1.361 mA / cm ² is 0.1 C, 0.5 C, and 2.0 C, respectively. Asked. The capacity is described as a relative value with the discharge capacity of 0.1 C of Example 1 being 100. Further, the discharge capacity after a 500 cycle charge / discharge test at a discharge rate of 0.5 C was also measured.
(Measurement of immersion time of electrolyte)
The characteristics of the separator described so far and the characteristics of the secondary battery using the separator have all been evaluated using a wound body having a diameter of 16 mm. Regarding the evaluation of the immersion time measurement of the electrolyte, Two types of wound bodies having different diameters were prepared. The diameter of the wound body was 16 mm and 24 mm, and was adjusted by changing the diameter of the shaft core. The end of the produced wound body was immersed in the electrolytic solution, and the time until the weight change disappeared was defined as the permeation time. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ in a nonaqueous solvent of EC (ethylene carbonate) / DEC (diethyl carbonate) = 30/70 (weight ratio) to a concentration of 1 mol / cm ³ was used.
Table 1 shows the characteristics of the separator described above and the results of the characteristics of the secondary battery using the separator. As a result, the batteries of Examples 1 to 3 were clearly improved in battery characteristics as compared with the comparative example.

Claims (4)

  The separator which consists of a porous film which has a fibril structure, The fibril free end protrudes on the film | membrane surface of the said porous film, The separator characterized by the above-mentioned. The separator according to claim 1, wherein the fibril free end has a branching portion. The separator according to claim 1 or 2, wherein the fibril free end is made of a resin having a carbonyl structure at least on the surface thereof. A lithium ion battery comprising: a positive electrode, a negative electrode, and a separator according to any one of claims 1 to 3 disposed between the positive and negative electrodes; and a nonaqueous electrolytic solution impregnated in the separator. Next battery.

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