JP2010194726A - Porous composite film, method for producing the film, separator for battery, and nonaqueous electrolyte secondary battery using the separator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous composite film which can be used as a separator for a battery, which is high in reliability and can suppress a thermal runaway in the battery while maintaining discharge characteristics, the battery separator using the composite film, and a nonaqueous electrolyte secondary battery. <P>SOLUTION: The porous composite film includes a porous polyolefin film and a porous layer containing inorganic particles which are laminated on the polyolefin film. The porous layer is formed by the process in which a mixed solution (A) prepared by dissolving a polymeric material in the dispersion of the inorganic particles or a mixed solution (B) prepared by mixing the dispersion of the inorganic particles and a solution containing the polymeric material is fiberized by an electric field spinning method, and a porous film containing the inorganic particles which is formed by the obtained fibers is arranged on the porous polyolefin film. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a composite porous film in which a porous layer is laminated on a polyolefin-based porous film, a manufacturing method thereof, a battery separator using the composite porous film, and a non-aqueous electrolyte secondary solution using the same. Related to batteries.

  Lithium ion secondary batteries, which are non-aqueous electrolyte secondary batteries with high energy density, are widely used as the main power source with the reduction in weight and size of portable electronic devices such as mobile phones and notebook personal computers. Yes.

Lithium ion secondary batteries are typically represented by a positive electrode in which an active material layer containing a positive electrode active material such as a lithium compound typified by lithium cobaltate is formed on a metal current collector, and graphite. A negative electrode in which an active material layer containing a negative electrode active material such as a carbon material capable of occluding and releasing lithium is formed on a metal current collector, and an electrolyte such as a lithium salt such as LiPF ₆ is usually used as a protic non-aqueous solvent. And a separator made of a porous polymer membrane.

  Since the lithium ion secondary battery has a high energy density, a current flows at a time to generate a large amount of heat for abnormal use such as a short circuit or forced charge / discharge, and in the worst case, there is a risk of ignition. This is because when the temperature of a lithium ion secondary battery increases due to abnormal use, etc., a reaction between lithium and other battery constituents that has been suppressed near room temperature occurs, and the battery temperature further increases due to the reaction. It is understood that it occurs by state.

  As safety measures that can be contributed by a separator, attention is paid to a shutdown (SD) characteristic and a meltdown characteristic. Here, the shutdown characteristic is a characteristic in which, when an abnormal current (large current) flows and the battery generates heat, the pores of the separator are blocked and the current flow in the battery is cut off. The meltdown characteristic is a characteristic that a large hole opens in the separator when the temperature rises further than the temperature at which the shutdown characteristic appears. Therefore, in order to maintain higher safety, the film at a high temperature is lowered by lowering the shutdown temperature (temperature at which the shutdown characteristics are exhibited) and further increasing the meltdown temperature (temperature at which the meltdown characteristics are exhibited). It is effective to maintain insulation by suppressing and preventing shape deterioration (improving film shape maintenance characteristics at high temperatures).

  However, there is a trade-off between shutdown characteristics and meltdown characteristics (shape retention characteristics at high temperatures), and it has been very difficult to achieve a high level of both of these characteristics with a single-material film. . Therefore, in order to achieve both characteristics in a well-balanced manner, it is effective to combine appropriate materials and share the intended function among the respective materials.

  As an example of such a separator, a technique relating to a multilayer separator composed of a polyolefin layer having a shutdown function and a heat-resistant resin layer having a shape maintaining function at a high temperature has been proposed. For example, a technique related to a multilayer separator using aromatic polyamide (Patent Document 1) or polyimide (Patent Documents 2 and 3) as a material of a layer responsible for shape maintenance at high temperatures has been proposed. However, both of these have the problem that the heat-resistant resin layer may be electrochemically oxidized with charge and discharge.

  On the other hand, many techniques have been proposed for forming a porous layer containing inorganic particles on the surface of a polyolefin layer responsible for a shutdown function, for example, for the purpose of improving heat resistance and suppressing thermal runaway inside the battery. . For example, examples of the method for forming a porous layer containing inorganic particles include the following.

(1) A method of three-dimensionally crosslinking a binder containing a ceramic material (Patent Document 4)
(2) A method in which a heat-resistant nitrogen-containing aromatic polymer solution in which ceramic powder is dispersed is applied to the surface of a polyolefin separator and dried (Patent Document 5).
(3) A method in which inorganic particles are blended with a polyolefin-based material and manufactured through kneading, extrusion, stretching or extraction processes (Patent Document 6)

  However, in these techniques, the porous layer is directly formed on the separator surface and is firmly adhered to the separator. Therefore, when the separator contracts, the porous layer also contracts accordingly.

  Also, a thin film made of a material that dissolves in the non-aqueous electrolyte is formed on the surface of the polyolefin separator, and an inorganic oxide filler, a binder and a solvent are applied to the thin film surface and dried to form a porous layer. A method (Patent Document 7) has been proposed. In this technique, the porous layer and the separator can be separated by bringing the thin film into contact with the non-aqueous electrolyte after injecting the battery in the battery assembly process. Therefore, it can prevent that a porous layer shrinks with the shrinkage | contraction of a separator. However, with this technology, the pore structure for controlling the non-aqueous electrolyte retention function that plays the role of ionic conduction, for example, the pore size and porosity, and the dispersibility of the inorganic particles can be freely controlled to achieve a good It has the disadvantage that it is difficult to maintain the discharge characteristics.

Japanese Patent Laid-Open No. 10-6453 JP-A-10-302749 Japanese Patent Laid-Open No. 11-144697 JP 2006-310302 A JP 2000-30686 A Japanese Patent Laid-Open No. 2002-25531 Japanese Patent Laying-Open No. 2005-235508

  INDUSTRIAL APPLICABILITY The present invention can be used as a highly reliable battery separator that can suppress thermal runaway inside a battery when an internal short circuit due to contamination of foreign matters or abnormal heat generation due to misuse occurs while maintaining discharge characteristics. An object is to provide a composite porous film. Specifically, even when the polyolefin porous film contracts due to internal short circuit or abnormal heat generation, it can exist alone between the electrode and the separator without thermal contraction with the polyolefin porous film, An object of the present invention is to provide a composite porous film provided with a porous layer capable of preventing thermal runaway inside the battery. Furthermore, another object of the present invention is to provide a battery separator and a non-aqueous electrolyte secondary battery using such a composite porous film.

  As a result of intensive investigations in consideration of the above-described problems and problems, the present inventor has reached the present invention using the solution means described below.

  The composite porous film of the present invention is a composite porous film comprising a polyolefin-based porous film and a porous layer containing inorganic particles laminated on the polyolefin-based porous film, wherein the porous layer Is a mixed solution (A) prepared by dissolving a polymer material in the dispersion of inorganic particles, or a mixed solution (B) prepared by mixing the dispersion of inorganic particles and a solution containing a polymer material. ) By an electrospinning method, and an inorganic particle-containing porous film formed by the obtained fiber is disposed on the polyolefin-based porous film.

  The present invention also provides a battery separator using the composite porous film of the present invention.

  Furthermore, the present invention also provides a non-aqueous electrolyte secondary battery. A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution including a non-aqueous solvent and an electrolyte. An aqueous electrolyte secondary battery, wherein the separator is the battery separator of the present invention.

The present invention is a method for producing a composite porous film comprising a polyolefin-based porous film and a porous layer containing inorganic particles laminated on the polyolefin-based porous film,
(I) A mixed solution (A) prepared by dissolving a polymer material in a dispersion of the inorganic particles, or a mixed solution prepared by mixing the dispersion of inorganic particles and a solution containing a polymer material ( B), and a step of preparing
(II) fiberizing the mixed solution (A) or the mixed solution (B) by an electrospinning method, and producing an inorganic particle-containing porous membrane with the obtained fiber;
(III) disposing the inorganic particle-containing porous film on the polyolefin-based porous film to form the porous layer;
A method for producing a composite porous film is also provided.

  The composite porous film of the present invention has a configuration in which a porous layer containing inorganic particles is laminated on a polyolefin-based porous film. This porous layer containing inorganic particles is formed of fibers produced by electrospinning. According to the electrospinning method, the fiber diameter can be made extremely fine to nano size. Therefore, the porous layer of the composite porous film of the present invention can be formed by ultrafine fibers in which inorganic particles are efficiently filled. As a result, the composite porous film of the present invention has the control of the pore structure of the porous layer for controlling the non-aqueous electrolyte retention function that plays the role of ionic conduction, and the control of the inorganic particle density that is responsible for the heat resistance. Since both can be achieved, it can be used as a battery separator having excellent heat resistance while maintaining discharge characteristics. Furthermore, since the inorganic particles are uniformly distributed on the surface of the ultrafine fibers, the binding force between the fibers at a high temperature becomes strong, and an effect that the porous layer is hardly cracked even when exposed to a high temperature is obtained.

  Moreover, in the composite porous film of this invention, the inorganic particle containing porous film formed of the fiber obtained by the electrospinning method is arrange | positioned on the polyolefin-type porous film, and becomes a porous layer. Therefore, the respective layers (polyolefin-based porous film and porous layer) are not firmly bonded to each other. Therefore, the porous layer can exist independently without being affected by the thermal contraction of the polyolefin-based porous film due to internal short circuit or abnormal heat generation. Thereby, according to the composite porous film of this invention, the contact between positive and negative electrodes can be prevented, thermal runaway can be suppressed, and the nonaqueous electrolyte secondary battery excellent in safety can be provided.

  The battery separator of the present invention using the composite porous film of the present invention has excellent heat resistance and shape maintaining characteristics at high temperatures while maintaining excellent discharge characteristics. A non-aqueous electrolyte secondary battery excellent in performance can be provided.

  Moreover, since the non-aqueous electrolyte secondary battery of the present invention includes the battery separator having the above functions, it has excellent discharge characteristics and safety.

It is a partial cross section figure which shows the example of 1 structure of the non-aqueous electrolyte secondary battery of this invention. 4 is an electron micrograph of the surface of an inorganic particle-containing porous film produced in Example 2 and Example 4. FIG.

  Embodiments of the composite porous film of the present invention will be described with reference to the drawings. The following description does not limit the present invention.

  The composite porous film of the present invention is a composite porous film including a polyolefin-based porous film and a porous layer containing inorganic particles laminated on the polyolefin-based porous film.

  First, the polyolefin porous film used for the composite porous film of this Embodiment is demonstrated. In the present specification, the polyolefin-based porous film is a porous film formed using a material containing polyolefin as a main component (polyolefin content of 90 wt% or more).

  Polypropylene, polymethylpentene, polyethylene (including low-density polyethylene, linear low-density polyethylene, high-density polyethylene, and high-molecular-weight polyethylene) are used as materials for the polyolefin-based porous film used in the composite porous film of the present embodiment. And polyolefins such as polybutylene may be used alone, or a mixture of these resins may be used. In view of solvent resistance and oxidation-reduction resistance, a porous film made of polyolefin such as polyethylene and polypropylene is preferable. Among them, as a polyolefin-based porous film, the resin melts and heat-deforms when heated, so that the pores are blocked, and as a result, a so-called shutdown function can be imparted to the battery. A polyethylene porous film is particularly preferably used.

  The manufacturing method of the polyolefin-type porous film in this Embodiment is not specifically limited. A polyolefin-based porous film can be produced by using a dry method such as a dry stretching method, a wet method such as a phase separation method and a solvent extraction method, a method of rolling a nonwoven fabric made of a resin material, or the like. Such polyolefin-based porous films are commercially available from, for example, Asahi Kasei Corporation, Tonen Chemical Corporation, Celgard Corporation, Ube Industries, Ltd., and the like.

  When the composite porous film of the present embodiment is used as a battery separator, the pore size of the polyolefin-based porous membrane is preferably within the range of 0.01 to 5 μm, and preferably 0.01 to 0.5 μm. It is more preferable to be within the range. When the maximum pore diameter is less than 0.01 μm, the electrolyte solution tends to be insufficiently diffused, and the internal resistance of the battery may be increased. Further, when the maximum pore diameter exceeds 5 μm, for example, when used as a separator of a lithium ion secondary battery, it becomes difficult to suppress the generation of lithium dendriide (lithium needle crystals generated and generated during battery reaction) Short circuit may occur.

  When the composite porous film of the present embodiment is used as a battery separator, the porosity of the polyolefin-based porous film is preferably in the range of 10 to 95 vol%, more preferably in the range of 20 to 65 vol%, The range of ˜60 vol% is more preferable. If the porosity of the polyolefin-based porous film is too low, when the composite porous film is used as a battery separator, there are cases where the ion conductivity path is reduced and sufficient battery characteristics cannot be obtained. On the other hand, if the porosity of the polyolefin-based porous film is too high, the strength may be insufficient when the composite porous film is used as a battery separator. If the strength is insufficient, the thickness of the polyolefin-based porous film must be increased in order to obtain the required strength, which may increase the internal resistance of the battery.

  When the composite porous film of the present embodiment is used as a battery separator, a polyolefin-based porous film having an air permeability of preferably 1500 seconds / 100 cc or less, more preferably an air permeability of 1000 seconds / 100 cc or less is used. If the air permeability of the polyolefin-based porous film is too high, when the composite porous film is used as a battery separator, ionic conductivity may be lowered, and sufficient battery characteristics may not be obtained.

  The polyolefin porous film preferably has a puncture strength of 1 N or more. When the puncture strength is less than 1N, if the composite porous film of the present embodiment is used as a battery separator, the separator may break when an interfacial pressure is applied between the electrodes, causing an internal short circuit. It is. The puncture strength in this specification is a force necessary for a needle having a diameter of 1.0 mm and a radius of curvature of the tip of 0.5 mm to break through the separator at a speed of 2 cm / second.

  Next, the porous layer provided on the polyolefin-based porous film will be described.

  The porous layer provided in the composite porous film of the present embodiment is composed of an inorganic particle-containing porous film. This inorganic particle-containing porous membrane was prepared by mixing a mixed solution (A) prepared by dissolving a polymer material in a dispersion of inorganic particles, or a solution containing a dispersion of inorganic particles and a polymer material. The mixed solution (B) is made into a fiber by an electrospinning method, and is produced using a fiber containing inorganic particles obtained thereby. The polymer material contained in the mixed solution (A) and the mixed solution (B) serves as a matrix in the fiber containing inorganic particles. Further, the inorganic particle-containing porous membrane may be a thin film-shaped fiber produced by electrospinning, that is, a membrane composed only of the fiber, or a fiber produced by electrospinning. It may be a thin film sandwiched between sheet-like materials such as a Teflon (registered trademark) sheet.

  The inorganic particles contain as a main component at least one selected from the group consisting of electrically insulating metal oxides, non-metal oxides, metal nitrides, and metal carbides. The inorganic particles are preferably made of a material that does not swell or dissolve in a non-aqueous electrolyte solution that is a constituent material of a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. For example, alumina, silica, titanium dioxide, zirconium oxide or the like can be mentioned, and silica is preferably used in consideration of price and type. In this specification, the inorganic particles are mainly composed of at least one selected from the group consisting of metal oxides, non-metal oxides, metal nitrides, and metal carbides. It means containing 90 wt% or more.

  The average primary particle diameter of the inorganic particles used in the present invention is preferably 500 nm or less, and more preferably 100 nm or less. When the average particle diameter of the primary particles exceeds 500 nm, the fiber diameter increases when mixed with a polymer material to form a fiber, and the pore size of the porous layer composed of this fiber increases. The pore structure may be inhomogeneous. A particularly preferable range of the average primary particle diameter is 10 to 100 nm. In the present invention, the average primary particle diameter of the inorganic particles is an arithmetic average diameter obtained using a particle size distribution measured using a laser diffraction scattering method.

  The content of the inorganic particles is preferably 5 wt% to 80 wt%, more preferably 10 wt% to 70 wt%, based on the total weight of the porous layer containing the inorganic particles. When the content of the inorganic particles is less than 5 wt%, the effect of preventing a short circuit cannot be obtained sufficiently. On the other hand, if it exceeds 80 wt%, it becomes difficult to bind inorganic particles sufficiently during fiber formation, so that the film becomes brittle and handling properties may be deteriorated.

  The solvent (dispersion medium) in which the inorganic particles are dispersed is not particularly limited. However, when preparing a fiber containing inorganic particles, when using a mixed solution (A) prepared by dissolving a polymer material in a dispersion of inorganic particles, select a dispersion medium that dissolves the polymer material. There is a need to. When such a dispersion medium is used, since the polymer material can be directly dissolved in the dispersion medium, a solution having an arbitrary concentration can be easily prepared. In addition, when using a dispersion medium that does not dissolve the polymer material used for the preparation of the porous layer, separately prepare a solution in which the polymer material is dissolved in a solvent compatible with the dispersion medium. It is preferable to use a mixed solution (B) prepared by mixing each other.

  The polymer material used for the preparation of the porous layer is not particularly limited as long as it is electrochemically and thermally stable. For example, polyamide, polyamideimide, polyimide, polysulfone, polyethersulfone, polyphenylsulfone, polyacrylonitrile, Examples thereof include celluloses such as fluorine-containing polymers and cellulose acetate. Here, the electrochemically stable polymer material means a chemical reaction or the like between other members in the device when the composite porous film of the present embodiment is incorporated as a member in a device. For example, when the composite porous film of the present embodiment is incorporated into a battery as a battery separator, a chemical change such as reaction with an electrolytic solution occurs. A polymeric material that does not wake up. In addition, the thermally stable polymer material is stable against the internal temperature of the battery at the time of charging / discharging, for example, when incorporated in a battery as a battery separator, and further, a polyolefin-based material when the battery is abnormally heated It is a material that can maintain its shape without melting even at a temperature at which it melts. In general, since the polyolefin-based material starts to melt at around 130 ° C., specifically, a polymer material having a melting temperature of 130 ° C. or higher is used.

  The porous layer constituting the composite porous film of the present invention is a mixed solution (A) prepared by dissolving a polymer material in a dispersion of inorganic particles, or a solution containing a dispersion of inorganic particles and a polymer material. The mixed solution (B) prepared by mixing the two is spun by an electrospinning method, and is produced from the obtained fiber. The electrospinning method is a method for producing fibers by spraying the solution by applying a high voltage to the polymer solution. The thickness of the fiber obtained depends on the applied voltage, the solution concentration, and the scattering distance of the solution during spraying. By supplying fibers continuously to the target using the electrospinning method, a three-dimensional porous film having a three-dimensional network formed by the fibers can be produced on the target.

  Furthermore, since the porous layer containing inorganic particles is an aggregate in which fibers are laminated, the pore size and porosity can be controlled by the fiber deposition thickness and fiber diameter. As a general rule, the pore size and the porosity are controlled by the thickness, the compression rate and the fiber diameter, but it is effective to adjust the fiber diameter in terms of increasing the porosity and reducing the film thickness. That is, when the pore size is reduced, the fiber is thinned. When the hole size is increased, the fiber is thickened.

  The fiber diameter of the fiber produced by the electrospinning method can be reduced to an average fiber diameter of 10 μm or less by controlling the viscosity of the mixed solution, and further to nanofibers (for example, up to about 0.05 μm). Is possible. Therefore, the pore size of the porous layer produced by depositing ultrafine fibers can be inevitably reduced. When the composite porous film of the present embodiment is used as a battery separator, the pore size of the porous layer may be 1 μm or less in consideration of the retention of non-aqueous electrolyte solution responsible for ion conduction and the internal resistance of the battery. desirable. For this reason, it is preferable that the average fiber diameter of the fiber which comprises a porous layer shall be 1 micrometer or less.

  In the composite porous film of the present embodiment, the inorganic particle-containing porous film is disposed on the polyolefin-based porous film to form a porous layer. Therefore, the respective layers (polyolefin-based porous film and porous layer) are not firmly bonded to each other. Therefore, the porous layer can exist independently without being affected by the thermal contraction of the polyolefin-based porous film due to internal short circuit or abnormal heat generation. Therefore, the composite porous film of the present embodiment can obtain excellent heat resistance and shape maintaining characteristics at high temperatures. For example, the area shrinkage rate when heat-treated at 350 ° C. for 2 hours is 5% or less. It is also possible.

As described above, the composite porous film of the present embodiment is
(I) A mixed solution (A) prepared by dissolving a polymer material in a dispersion of inorganic particles, or a mixed solution (B) prepared by mixing a dispersion of inorganic particles and a solution containing a polymer material And a step of preparing
(II) fiberizing the mixed solution (A) or the mixed solution (B) by an electrospinning method, and producing an inorganic particle-containing porous membrane with the obtained fibers;
(III) disposing the inorganic particle-containing porous film on a polyolefin-based porous film to form the porous layer;
Can be manufactured by a method including: Thus, in this embodiment, by separately producing an inorganic particle-containing porous film using fibers obtained by the electrospinning method, and arranging this inorganic particle-containing porous film on a polyolefin-based porous film, Manufactures composite porous films. In the composite porous film obtained by such a method, compared with the composite porous film obtained by the method of directly supplying the fiber obtained by the electrospinning method onto the polyolefin-based porous film to form a porous layer, The adhesion at the laminated interface is different. When the fiber spun on the polyolefin-based porous film is directly supplied, the spun fiber enters the polyolefin-based porous film, and the fiber is entangled with the porous structure of the polyolefin-based porous film near the interface, resulting in high adhesion. Thus, the polyolefin-based porous film and the porous layer are firmly bonded. On the other hand, when each layer is produced and laminated separately as in the present invention, the adhesiveness in the vicinity of the lamination interface is such that the laminated structure is maintained compared to the above method, and is not so strong. For this reason, when the polyolefin porous film shrinks due to internal short circuit or abnormal heat generation, the adhesion at the interface is released, and the porous layer containing inorganic particles is not affected by the shrinkage of the polyolefin porous film. The shape can be maintained.

  Next, the non-aqueous electrolyte secondary battery of the present invention will be described. A non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte containing a non-aqueous solvent and an electrolyte. . The separator used for this non-aqueous electrolyte secondary battery uses the above-mentioned composite porous film of the present invention as a battery separator. In addition, the battery separator using the composite porous film of the present invention is used in the state of being interposed between the positive electrode and the negative electrode, similarly to the known battery separator, and a non-aqueous electrolyte secondary battery is used. Can be assembled. The material of the positive electrode, the negative electrode, the battery case, the electrolyte, and the like and the arrangement structure of these constituent elements can be the same as those of a known non-aqueous electrolyte secondary battery.

  In the present embodiment, a cylindrical lithium ion secondary battery 1 as shown in FIG. 1 will be described as an example of the non-aqueous electrolyte secondary battery of the present invention. In FIG. 1, some hatching is omitted for the purpose of making the drawing easier to see.

  As shown in FIG. 1, in the lithium ion secondary battery 1, the positive electrode 2, the negative electrode 3, and the separator 4 disposed between the positive electrode 2 and the negative electrode 3 are integrally wound in a spiral shape, The battery case 5 with a bottom is accommodated. A positive electrode lead (not shown) connected to the positive electrode 2 is electrically connected to the battery case 5 via a lower insulating sleeve (not shown). In the figure, reference numeral 5a denotes a positive terminal portion. The negative electrode tab 7 electrically connected to the negative electrode 3 is electrically connected to the negative electrode terminal portion 6 a through the cavity portion 8 a of the upper insulating sleeve 8. The battery is filled with a non-aqueous electrolyte (not shown). The battery case 5 is sealed by a lid body 6 including a negative electrode terminal portion 6a and a packing 9 that closes a gap between the lid body 6 and the battery case 5, so that a non-aqueous electrolyte cannot leak out of the battery. ing. In addition, the separator 4 is impregnated with an electrolytic solution. As a result, the ion carrier is moved between the positive electrode 2 and the negative electrode 3 sandwiching the separator 4, and can be discharged and charged as a secondary battery. Become. In this example, two separators 4 are bonded together to form a bag shape, and the negative electrode 3 is inserted therein and wound together with the positive electrode 2. However, the separator 4 need not necessarily be formed in a bag shape because the separator 4 may be disposed between the positive electrode 2 and the negative electrode 3 adjacent to each other in the state after winding.

  The positive electrode 2 is formed of an active material that occludes and releases lithium ions, a binder, and a current collector. The positive electrode 2 can be produced, for example, by preparing a paste by mixing the active material in a solvent in which a binder is dissolved, applying the paste onto a current collector, and drying the paste. You may press further after drying.

As a positive electrode active material, the well-known compound used as a positive electrode active material of a lithium ion secondary battery can be used. Specifically, lithium-containing transition metal oxides such as LiCoO ₂ , LiMnO ₂ , and LiNiO _2, or lithium-containing transition metal oxides in which a part of the transition metal is substituted with another transition metal, titanium disulfide, Examples include chalcogen compounds such as molybdenum disulfide.

  As the binder, a known resin can be used as the binder constituting the positive electrode 2. For example, a fluororesin such as polyvinylidene fluoride (PVDF), hexafluoropropylene and polytetrafluoroethylene, a hydrocarbon resin such as styrene butadiene rubber and ethylene propylene terpolymer, or a mixture thereof can be used. Moreover, you may add electroconductive powder, such as carbon black, as a conductive support agent.

  As the current collector of the positive electrode 2, a metal excellent in oxidation resistance is used. For example, aluminum processed into a foil shape or a mesh shape is preferably used.

  The negative electrode 3 is formed of a carbon-based active material or a lithium-containing alloy, a binder, and a current collector. The negative electrode 3 can also be produced by the same method as the positive electrode 2. Further, the same binder as the binder used in the positive electrode 2 can be used.

  Examples of the carbon-based active material include artificial graphite, natural graphite, fired bodies such as coke and pitch, and those obtained by sintering phenol resin, polyimide, cellulose, and the like. Examples of the lithium-containing metal include Al, Sn, and Si alloys.

  As the current collector of the negative electrode 3, a metal excellent in reduction stability is used. For example, copper processed into a foil shape or a mesh shape is preferably used.

The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte. Specifically, an electrolytic solution in which a lithium salt (electrolyte) is dissolved in a non-aqueous solvent, a gel electrolytic solution containing the electrolytic solution, a solid electrolyte in which a lithium salt is dissolved and decomposed into a polymer such as polyethylene oxide, lithium ions, etc. The well-known electrolyte solution used for a secondary battery is mentioned. Specific examples of the lithium salt used as the electrolyte include lithium borotetrafluoride (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), and lithium trifluorosulfonate (LiCF ₃ SO). ₃ ) etc. can be used. Examples of the non-aqueous solvent include propylene carbonate (PC), ethylene carbonate (EC), methyl ethyl carbonate (MEC), 1,2-dimethoxyethane (DME), and γ-butyrolactone (γ-BL). Or a mixed solvent thereof can be used.

  For the separator 4, the above-described composite porous film of the present invention is used. Therefore, a separator having excellent heat resistance and shape maintaining characteristics at high temperatures while maintaining excellent discharge characteristics is obtained.

  In this lithium ion secondary battery, since the composite porous film of the present invention is used as a separator, excellent discharge characteristics and safety can be realized.

  In the present embodiment, a cylindrical non-aqueous electrolyte secondary battery has been described as an example of the non-aqueous electrolyte secondary battery of the present invention. However, other configurations such as a cylindrical or laminated type are described. The configuration of the present invention can be applied even to a non-aqueous electrolyte secondary battery.

  Next, the composite porous film, battery separator and non-aqueous electrolyte secondary battery of the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples. Absent.

  First, the measurement method of the physical property and battery characteristic of the composite porous film produced by each Example and comparative example which are mentioned later is demonstrated.

[Film properties]
<Thickness of polyolefin-based porous film and inorganic particle-containing porous film>
Measurement was performed with a 1/10000 mm thickness gauge.

<Porosity of polyolefin-based porous film>
Using each value of area S (cm ² ), average thickness t (cm), weight W (g), and density d (g / cm ³ ) of the resin constituting the polyolefin porous film, the polyolefin porous film. The following formula was used.
Porosity (vol%) = (1−W / (S × t × d)) × 100

<Air permeability of polyolefin porous film>
It calculated | required based on JISP8117.

<Puncture strength of polyolefin porous film>
It measured using the compression tester "KES-G5" by Kato Tech Co., Ltd. The maximum load was read from the load displacement curve obtained by the measurement and used as the puncture strength. A needle having a diameter of 1.0 mm and a radius of curvature of the tip of 0.5 mm was used. The puncture speed was 2 cm / second.

<Observation of inorganic particle-containing porous membrane by electron microscope>
The surface of the inorganic particle-containing porous film was observed using VE-7800 manufactured by KEYENCE Corporation.

<Heat resistance test>
A film to be tested was cut into a size of 35 mm × 35 mm, placed on an aluminum cup and placed in an oven at 350 ° C. for 2 hours, and then the shape was observed to obtain the area shrinkage. The area shrinkage ratio was calculated by (film area before heat treatment−film area after heat treatment) × 100 / (film area before heat treatment). The film area after the heat treatment was calculated by image analysis after taking a photograph.

[Battery characteristics]
<Trickle charge test>
A secondary battery charge / discharge test apparatus “BST2005W” manufactured by Nagano Co., Ltd. was used. Install the battery in a constant temperature bath at 60 ° C, perform constant current constant voltage trickle charging at 4 mA, 4.25 V for 7 days, perform 4 mA, 2.75 V termination constant current discharge, and discharge to the charge capacity of the battery The oxidation resistance was evaluated as a percentage (%) of the capacity. Here, trickle charging is a charging method in which a minute current is continuously supplied in order to compensate for the spontaneous discharge of the secondary battery and maintain a fully charged state. When the lithium ion secondary battery is in a fully charged state, the separator is exposed to a strong oxidizing atmosphere. Therefore, when the oxidation resistance of the separator is poor, the separator is oxidized and deteriorated, and the discharge capacity is reduced. Therefore, the charge / discharge efficiency (100 × (discharge capacity) / (charge capacity) (%)) obtained using the result of the trickle charge test can be used as an index of the oxidation resistance of the separator.

  Next, the composite porous film and lithium ion secondary battery produced as Examples 1-4 and Comparative Examples 1-2 are demonstrated in detail.

[Example 1]
<Composite porous film>
A dimethylacetamide sol containing 20 wt% of silica (SiO ₂ ) particles having an average primary particle diameter of 10 to 20 nm was purchased from Nissan Chemical Co., Ltd. On the other hand, a solution in which polyacrylonitrile ([—CH ₂ CHCN—] _n ) purchased from Aldrich as a polymer material was dissolved in N, N-dimethylformamide (hereinafter referred to as DMF) was prepared. Dimethylacetamide sol and the above solution were blended so that SiO ₂ was 50 wt% with respect to the polymer weight to prepare a mixed solution (mixed solution (B)) having a solid content concentration of 15 wt%.

  Using this mixed solution, an inorganic particle-containing porous membrane was produced by electrospinning. A nanofiber electrospinning unit manufactured by Kato Tech Co., Ltd. was used for the production of the porous membrane. Manufacturing conditions are: liquid feeding speed: 0.01 to 0.05 cm / min, target speed: 12 m / min, applied voltage: 15.0 kV, distance between needle tip and target: 13 cm, temperature: 22 to 24 ° C., humidity: 20 to 50%. Furthermore, an aluminum foil was prepared, wound around a target drum, and fibers containing inorganic particles were supplied onto the aluminum foil to form a thin film. The thin-film fibers were peeled off from the aluminum foil, and this was sandwiched between Teflon (registered trademark) sheets and laminated with a heat roll heated to 120 ° C. to prepare an inorganic particle-containing porous film having a thickness of 10 μm.

  The inorganic particle-containing porous membrane produced as described above was placed on a polyethylene porous film having a thickness of 16 μm, a porosity of 60 vol%, an air permeability of 92 seconds / 100 cc, and a puncture strength of 4.1 N. did.

  The composite porous film produced as described above was cut into a size of 35 mm × 35 mm and used as a battery separator.

<Positive electrode>
89 parts by weight of lithium cobalt oxide (LiCoO ₂ ), 5 parts by weight of acetylene black, 6 parts by weight of PVDF, and 90 parts by weight of NMP (N-methylpyrrolidone) were mixed to obtain a positive electrode mixture slurry. This positive electrode mixture slurry was passed through a 70-mesh net to remove solids having a large particle size, and then uniformly applied to one side of a positive electrode current collector made of an aluminum foil having a thickness of 20 μm. The coating film was dried and then pressed to obtain a positive electrode. In this example, the application area (W1 × W2) of the positive electrode was 27 × 27 (mm ² ). Note that the positive electrode was provided with a current collecting portion to which no active material was applied, and the positive electrode ear portion was welded to a tab (aluminum).

<Negative electrode>
Graphite mesocarbon microbeads (MCMB, manufactured by Osaka Gas Chemical Co., Ltd., d (002) = 0.34 nm) powder 95 parts by weight, PVDF 5 parts by weight, NMP 110 parts by weight were mixed to obtain a negative electrode mixture slurry. This negative electrode mixture slurry was applied to one side of a negative electrode current collector made of a copper foil having a thickness of 18 μm. The coating film was dried and then pressed to obtain a negative electrode. In this example, the negative electrode coating area (W1 × W2) was 29 × 29 (mm ² ). The negative electrode was provided with a current collecting part to which no active material was applied, and the negative electrode ear was welded to a tab (nickel).

<Production of battery>
A battery was produced using the composite porous film, positive electrode and negative electrode produced as described above. The composite porous film was used as a battery separator. An electrode laminate was produced by alternately laminating the positive electrode and the negative electrode via the composite porous film. After this electrode laminate was charged into an aluminum laminate package, lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate / diethyl carbonate (weight ratio: 1/1) at a concentration of 1.0 mol / L in the package. The electrolyte solution was injected, then the package was sealed, and a laminate seal type lithium ion secondary battery was assembled. The lithium ion secondary battery of Example 1 was subjected to initial charge / discharge twice at a rate of 0.2 CmA at a constant temperature of 25 ° C., and then a trickle charge test was performed. The test results are as shown in Table 1.

[Example 2]
A composite porous film, a positive electrode, a negative electrode, and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the thickness of the inorganic particle-containing porous film was about 25 μm. For the obtained lithium ion secondary battery of Example 2, a trickle charge test was conducted in the same manner as in Example 1. The results are as shown in Table 1. In this example, the surface of the inorganic particle-containing porous film was also observed with an electron microscope. Furthermore, a heat resistance test was also conducted on the inorganic particle-containing porous membrane of this example.

[Example 3]
A dimethylacetamide sol containing 20 wt% of silica (SiO ₂ ) particles having an average primary particle diameter of 10 to 20 nm was purchased from Nissan Chemical Co., Ltd. On the other hand, a solution was prepared by dissolving polyacrylonitrile purchased from Aldrich as a polymer material in DMF. Dimethylacetamide sol and the above solution were blended so that SiO ₂ was 30 wt% with respect to the polymer weight to prepare a mixed solution (mixed solution (B)) having a solid content concentration of 15 wt%.

  Using this mixed solution, an inorganic particle-containing porous membrane was produced by electrospinning. An inorganic particle-containing porous membrane was produced in the same manner as in Example 1 except that the thickness was 15 μm. This inorganic particle-containing porous film was disposed on a polyethylene porous film having a thickness of 16 μm, a porosity of 60 vol%, an air permeability of 92 seconds / 100 cc, and a puncture strength of 4.1 N to form a porous layer.

  The composite porous film produced as described above was cut into a size of 35 mm × 35 mm and used as a battery separator. In the same manner as in Example 1, a positive electrode, a negative electrode, and a lithium ion secondary battery were produced. For the lithium ion secondary battery of Example 3 obtained, a trickle charge test was performed in the same manner as in Example 1. The results are as shown in Table 1.

[Example 4]
A composite porous film, a positive electrode, a negative electrode, and a lithium ion secondary battery were produced in the same manner as in Example 3 except that the thickness of the inorganic particle-containing porous film was about 25 μm. For the obtained lithium ion secondary battery of Example 4, a trickle charge test was conducted in the same manner as in Example 1. The results are as shown in Table 1. In this example, the surface of the inorganic particle-containing porous film was also observed with an electron microscope. Furthermore, a heat resistance test was also conducted on the inorganic particle-containing porous membrane of this example.

[Comparative Example 1]
A polyethylene porous film having a thickness of 16 μm, a porosity of 60 vol%, an air permeability of 92 seconds / 100 cc, and a puncture strength of 4.1 N was prepared and cut into a size of 35 mm × 35 mm to obtain a battery separator. A positive electrode, a negative electrode, and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the battery separator was different. For the lithium ion secondary battery of Comparative Example 1 obtained, a trickle charge test was conducted in the same manner as in Example 1. The results are as shown in Table 1. Moreover, the heat resistance test was also done about the polyethylene porous film used as a battery separator in this comparative example.

[Comparative Example 2]
<Composite porous film>
The polyacrylonitrile used in Example 1 purchased from Aldrich was dissolved in DMF to prepare a solution with a solid content concentration of 12.5 wt%. A porous film was produced by electrospinning using this solution. For the production of the porous membrane, a nanofiber electrospinning unit manufactured by Kato Tech Co., Ltd. was used. Manufacturing conditions are: liquid feeding speed: 0.01 to 0.05 cm / min, target speed: 12 m / min, applied voltage: 15.0 kV, distance between needle tip and target: 13 cm, temperature: 22 to 24 ° C., humidity: 20 to 50%. Furthermore, an aluminum foil was prepared, wound around a target drum, and fibers obtained by electrospinning were supplied onto the aluminum foil to form a thin film. The thin fiber was peeled off from the aluminum foil, and was sandwiched between Teflon (registered trademark) sheets and laminated with a heat roll heated to 120 ° C. to prepare a porous film having a thickness of 15 μm.

  The porous film produced as described above was disposed on a polyethylene porous film having a thickness of 16 μm, a porosity of 60 vol%, an air permeability of 92 seconds / 100 cc, and a puncture strength of 4.1 N to form a porous layer.

  The composite porous film produced as described above was cut into a size of 35 mm × 35 mm and used as a battery separator.

  A positive electrode, a negative electrode, and a lithium ion secondary battery were produced in the same manner as in Example 1 except that the battery separator was different. For the lithium ion secondary battery of Comparative Example 2 obtained, a trickle charge test was conducted in the same manner as in Example 1. The results are as shown in Table 1. In addition, a heat resistance test was also conducted on the porous film made of only polyacrylonitrile of this comparative example.

  Below, each evaluation test result (a trickle charge test, a heat test, and an electron microscope observation) is demonstrated.

[Evaluation result 1: Trickle charge test]
For the laminated seal type lithium ion secondary batteries obtained in Examples 1 to 4 and Comparative Examples 1 and 2, the initial discharge capacity is the second discharge capacity of the initial charge / discharge, and the second charge / discharge of the initial charge / discharge is performed. The efficiency was defined as initial charge / discharge efficiency. The results are shown in Table 1. The trickle charge test results are also shown in Table 1.

  From the results shown in Table 1, normal composite charging / discharging is performed without hindering ion movement inside the battery by using a composite porous membrane film in which a porous layer containing inorganic particles is laminated on a polyethylene porous film as a separator. Thus, it has been clarified that a decrease in discharge capacity due to deterioration of the separator is suppressed even during long-term charging under heating conditions, and a lithium ion secondary battery excellent in long-term reliability can be obtained. Moreover, even if the result of Examples 1-4 is compared with the comparative example 2 which laminated | stacked the polyacrylonitrile porous layer which does not contain an inorganic particle on a polyethylene porous film, the discharge efficiency of the same grade is obtained. It was also confirmed that the discharge efficiency was not significantly reduced by the presence of inorganic particles. On the other hand, in the comparative example 1 which used only the polyethylene porous film as a separator, the charge / discharge efficiency of the trickle charge test was as low as about 10.3%. From these results, it was found that when the composite porous film of the present invention having a porous layer formed of fibers containing inorganic particles was used as a battery separator, good discharge characteristics could be realized.

[Evaluation result 2: heat resistance test]
When the composite porous films of Example 2 and Example 4 were put into an oven at 350 ° C. for 2 hours, the area shrinkage rates of Example 2 and Example 4 were both 4.4%. Furthermore, when the shape change after heat processing was observed, the composite porous film of Example 2 and Example 4 did not produce the crack in a plane, but was maintaining the same shape as before heat processing substantially. On the other hand, Comparative Example 1 using only the polyethylene porous film could not maintain the shape before the heat treatment at all and was deformed into a lump, so that the area shrinkage rate was 100%. On the other hand, in the comparative example 2 of the porous layer not containing inorganic particles, the film was carbonized, and cracks were generated in the surface along with the shrinkage, and the film became brittle. The area shrinkage rate of Comparative Example 2 was 26.5%. From this result, it was found that the porous layer formed of fibers containing inorganic particles has excellent heat resistance and excellent shape maintaining characteristics at high temperatures.

[Evaluation Result 3: Electron Microscopic Observation of Porous Layer Prepared by Electrospinning Method]
FIG. 2 shows a state in which the surface of the inorganic particle-containing porous film produced in Example 2 and Example 4 was observed with an electron microscope. As shown in the electron micrograph of FIG. 2, fibers produced by electrospinning using a mixed solution containing inorganic particles and a polymer material were extremely fine with an average fiber diameter of 1 μm or less. Furthermore, from the 100,000 times surface photograph of Example 2, it was confirmed that the inorganic particles were uniformly dispersed in the fiber of Example 2.

  The composite porous film of the present invention has shape maintaining characteristics at high temperatures and is excellent in oxidation resistance, and therefore can be applied to battery separators, capacitors and the like. In addition, the battery separator of the present invention can be used as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery.

1 Nonaqueous electrolyte secondary battery 2 Positive electrode 3 Negative electrode 4 Separator (composite porous film)
5 Battery Case 5a Positive Terminal 6 Lid 6a Negative Terminal 7 Negative Tab 8 Upper Insulating Sleeve 8a Cavity 9 Packing

Claims (8)

A composite porous film comprising a polyolefin-based porous film and a porous layer containing inorganic particles laminated on the polyolefin-based porous film,
The porous layer was prepared by mixing a mixed solution (A) prepared by dissolving a polymer material in the inorganic particle dispersion, or a mixture containing the inorganic particle dispersion and a polymer material. A composite porous film formed by fiberizing the mixed solution (B) by an electrospinning method and disposing an inorganic particle-containing porous film formed by the obtained fiber on the polyolefin-based porous film. The average primary particle diameter of the inorganic particles is 10 nm to 500 nm, the content of the inorganic particles in the porous layer is 10 wt% to 70 wt%, and
The composite porous film according to claim 1, wherein an average fiber diameter of the fibers containing the inorganic particles is 0.05 μm to 10 μm.   3. The composite porous film according to claim 1, wherein the inorganic particles are mainly composed of at least one selected from the group consisting of metal oxides, non-metal oxides, metal nitrides, and metal carbides.   The polymer material is at least one selected from the group consisting of polyamide, polyamideimide, polyimide, polysulfone, polyethersulfone, polyphenylsulfone, polyacrylonitrile, a fluorine-containing polymer, and celluloses. The composite porous film according to any one of?   The composite porous film according to any one of claims 1 to 4, wherein an area shrinkage ratio when heat-treated at 350 ° C for 2 hours is 5% or less.   The battery separator using the composite porous film of any one of Claims 1-5. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution containing a non-aqueous solvent and an electrolyte,
A non-aqueous electrolyte secondary battery, wherein the separator is the battery separator according to claim 6. A method for producing a composite porous film comprising a polyolefin porous film and a porous layer containing inorganic particles laminated on the polyolefin porous film,
(I) A mixed solution (A) prepared by dissolving a polymer material in a dispersion of the inorganic particles, or a mixed solution prepared by mixing the dispersion of inorganic particles and a solution containing a polymer material ( B), and a step of preparing
(II) fiberizing the mixed solution (A) or the mixed solution (B) by an electrospinning method, and producing an inorganic particle-containing porous membrane with the obtained fiber;
(III) disposing the inorganic particle-containing porous film on the polyolefin-based porous film to form the porous layer;
A method for producing a composite porous film.