JP5966522B2 - Porous film and lithium ion secondary battery using the same - Google Patents

Description

  The present invention relates to a porous film and a lithium ion secondary battery using the same.

  The development of portable devices in recent years has been remarkable, and high energy batteries are the main driving force. In particular, lithium ion secondary batteries are expected as the mainstay of next-generation batteries. In the development of such a lithium ion secondary battery, improvement of components such as a positive electrode, a negative electrode, and an electrolytic solution is being promoted in order to achieve higher performance. In particular, various improvements have been made in order to suppress deterioration of characteristics due to decomposition of the electrolyte on the negative electrode surface.

For example, Patent Document 1 discloses that EC in initial charging is obtained by adding 1,3-propane sultone as an additive in an electrolytic solution using a cyclic carbonate such as ethylene carbonate (EC) as an electrolytic solution solvent. A technique is disclosed in which 1,3-propane sultone is reduced on the surface of the negative electrode carbon material before reductive decomposition, and the surface of the carbon material is coated with a passive film. Here, the passive film is a film formed so as to cover the surface of the negative electrode due to the decomposition of the additive added in the electrolytic solution, and has good lithium ion permeability and can be used in the negative electrode. It is a film | membrane which has the effect | action which suppresses decomposition reaction of electrolyte solution. Thereby, it is going to suppress degradation of electrolyte solution and deterioration of the negative electrode accompanying this.

Further, for example, in Patent Document 2 and Patent Document 3, by adding vinylene carbonate (VC) as an additive to propylene carbonate (PC), which is suitable for improving low temperature characteristics as an electrolyte solvent, during initial charging. Discloses a technique in which VC is reductively decomposed on the surface of the negative electrode carbon material before the PC reductive decomposition, and the surface of the carbon material is coated with a passive film. Thereby, it is going to suppress degradation of electrolyte solution and the accompanying deterioration of a negative electrode.

However, in the techniques disclosed in these Patent Documents 1 to 3, it is from the viewpoint of forming a passive film on the surface of the carbon material by adding an additive to the electrolytic solution. If there is a portion where the additive is not supplied, the passive film Is not formed. In particular, since a large battery has a large cell size, it takes time to circulate the electrolytic solution, and the influence of local deterioration of the electrolytic solution tends to be remarkable. Since the cell structure as a whole does not take into consideration the concentration distribution of the additive in the electrolytic solution, it is still insufficient as a countermeasure against the characteristic deterioration caused by the electrolytic solution.

JP 2000-3724 A JP-A-8-45545 JP 2001-167797 A

  In view of the above problems, an object of the present invention is to provide a porous film that suppresses deterioration of the composition of an electrolytic solution, and is intended to improve the cycle life of a lithium ion secondary battery.

  In order to achieve the above object, the inventors have reached the following invention described below.

  The porous film according to the present invention has a large number of pores, and an electrolyte solution diffusion hole larger than the pores exists on the membrane surface of the porous film. According to such a structure, the diffusion of the electrolyte solution is good at the electrode interface adjacent to the porous film as the separator. For this reason, the deteriorated electrolyte produced on the electrode surface can be easily diffused, and the cycle life of the lithium ion secondary battery can be improved.

  For example, when an additive is contained in the electrolytic solution, the supply of the additive in the electrolytic solution cannot catch up, and as a result, the reductive decomposition reaction of the electrolytic solution may proceed. Even in such a case, the additive solution needs to be sufficiently supplied not only in the vicinity of the electrode interface but also from the electrolyte solution present in the porous film. It is preferable that the diffusion hole is formed in a mortar shape.

  Furthermore, since the flow path of the electrolyte solution to be diffused is widened, the opening shape of the electrolyte solution diffusion hole is preferably elliptical. It is particularly preferable to take a mortar shape having a recess that has entered the porous film from the opening.

  Moreover, the composition deterioration of the electrolytic solution is particularly remarkable in the vicinity of the electrode terminal portion. Since the electrode itself also has an internal resistance, the state of the reductive decomposition reaction of the electrolytic solution varies depending on the distance from the terminal. That is, if an electrolyte solution diffusion hole is disposed near the electrode terminal where the decomposition reaction near the electrode terminal is active, the electrolyte solution can be effectively diffused, and additives necessary for the reaction can be supplied more efficiently.

  Therefore, in a lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolyte, and having the porous film between the positive electrode and the negative electrode, an electrode terminal and an electric terminal at one end of the positive electrode or the negative electrode. It is preferable that a large number of the electrolyte solution diffusion holes are unevenly distributed in the porous film facing one electrode connected to the electrode terminal in the vicinity thereof.

  Therefore, as a structure of the secondary battery, the electrolyte solution diffusion hole is preferably opposed to the negative electrode side.

  In addition, by disposing the electrolyte solution diffusion hole on the negative electrode side where the volume change is large and smoothing the diffusion of the electrolyte solution and the additive, deterioration as the negative electrode can be significantly suppressed. In particular, if the amount of electrolyte movement increases on the negative electrode side where the volume expansion is large, the amount of electrolyte supplied on the porous film side cannot temporarily catch up. If the electrolyte solution diffusion hole is arranged, the reaction of the negative electrode where the electrolyte enters and exits can be made uniform, and even when the secondary battery is used continuously for a long time, the electrolyte solution suddenly deteriorates and the safety decreases. Quality deterioration can be suppressed.

  According to the present invention, by having the electrolyte solution diffusion hole in the porous film, it is possible to suppress the deterioration of the composition of the electrolyte solution generated on the electrode surface, and the lithium ion secondary battery using this improves the cycle life. Can do.

It is a cross-sectional schematic diagram of the porous film which has the electrolyte solution diffusion hole in this embodiment on a film surface. It is a cross-sectional schematic diagram which shows the structure of a laminated body when the positive electrode in this embodiment, a porous film, and a negative electrode are laminated | stacked. It is a schematic sectional drawing of the lithium ion secondary battery using the porous film of this embodiment. It is an electron micrograph (10,000 times) of the porous film which has the electrolyte solution diffusion hole of Example 1 on a film | membrane surface.

  Hereinafter, preferred embodiments of the porous film 1 of the present invention will be described in detail. FIG. 1 is a schematic cross-sectional view of a porous film used in this embodiment. As shown in this figure, the porous film 1 of this embodiment has an electrolyte solution diffusion hole 2 on its main surface. For example, the porous film 1 has an average pore size of 0.05 μm to 0.3 μm mainly composed of polyethylene and a porosity of 20 to 70%, and on the main surface of the porous film 1, It is preferable to have the electrolyte solution diffusion hole 2 having a larger size than the pores existing in. In FIG. 1, a large number of pores in the porous film 1 are not shown. The electrolyte diffusion hole 2 makes it easy to replace the deteriorated electrolyte staying in the surrounding holes and the electrolyte without deterioration, so that the deterioration of the electrolyte can be prevented and the battery life can be extended. it can.

Although the raw material for the porous film 1 used in the present embodiment is not particularly limited to the above-described materials, thermoplastic trees can be preferably used. Further, an ultrahigh molecular weight polyolefin resin having a weight average molecular weight of 3 × 10 ⁵ or more, or an ultrahigh molecular weight polyolefin comprising at least 15% by weight of this ultrahigh 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.

  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).

In this embodiment, the ultrahigh molecular weight polyolefin resin has a weight average molecular weight in the range of 3 × 10 ^{5 to} 20 × 10 ⁶ , 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 in the present embodiment.

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 of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , and preferably 1 × 10 ^{4 to} 3 × 10 ⁵ . Examples of such a polyolefin resin include homopolymers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene, copolymers, and mixtures thereof. However, among these, high-density polyethylene resin is preferably used in the present embodiment.

  In this embodiment, when an ultrahigh molecular weight polyolefin resin composition is used, the composition needs to contain at least 15% by weight of the ultrahigh molecular weight polyolefin resin.

  In the production of the porous film according to the present 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 having a freezing point of −10 ° C. or lower. ~ 70 wt%, preferably 92-80 wt% and diffusion hole forming material 5-30 wt%, preferably 8-15 wt% are mixed in a uniform slurry, heated and stirred, An ultrahigh molecular weight polyolefin resin (composition) is dissolved in the solvent, and the resulting solution mixture is kneaded at a temperature in the range of 115 to 185 ° C. to prepare a kneaded product.

  The solvent is not particularly limited as long as it dissolves the ultrahigh molecular weight polyolefin resin (composition) well and has a freezing point of −10 ° C. or lower. Those having a freezing point in the range of -10 ° C to -45 ° C are preferably used. Preferable specific examples of such a solvent include, for example, aliphatic or cyclic hydrocarbons such as decane, decalin, and liquid paraffin, and mineral oil fractions corresponding to these freezing points. However, among these, a non-volatile solvent such as liquid paraffin is preferable, and in particular, a non-volatile solvent having a freezing point of −15 ° C. or lower and a kinematic viscosity at 40 ° C. of 65 cSt or lower is preferably used.

  The material that promotes the formation of diffusion holes larger in size than the pore film is an oligomer that is incompatible with the ultrahigh molecular weight polyolefin resin (composition) and is particularly soluble if it is soluble in a cleaning solvent described later. Although not limited, in particular, in the present embodiment, polyacrylonitrile having a weight average molecular weight of 1000 or less or a copolymer of polyacrylonitrile can also be used. Examples of copolymer monomers for polyacrylonitrile copolymers include vinyl acetate, methyl methacrylate, butyl methacrylate, methyl acrylate, butyl acrylate, itaconic acid, hydrogenated methyl acrylate, hydrogenated ethyl acrylate, acrylamide, and chloride. Vinyl, vinylidene fluoride, vinylidene chloride, or the like can be used. Further, acrylonitrile butadiene rubber, acrylonitrile butadiene styrene resin, acrylonitrile chlorinated polyethylene propylene diene styrene resin, acrylonitrile vinyl chloride resin, acrylonitrile methyl acrylate resin, acrylonitrile acrylate resin and the like can be used. Considering the entropy of mixing, the lower the weight average molecular weight, the more selectively gathers on the membrane surface, and the electrolyte solution diffusion holes are more likely to be formed.

  In the solution-like mixture of the ultrahigh molecular weight polyolefin resin (composition), the solvent and the diffusion hole forming material, when the ultrahigh molecular weight polyolefin resin (composition) exceeds 30% by weight, It is difficult to obtain sufficient entanglement of polymer chains because the solubility in the solvent is insufficient, and the ultrahigh molecular weight polyolefin resin is not stretched during kneading and is not unwound. Furthermore, as will be described later, when the kneaded product is cooled, it is molded into a sheet, the resin is crystallized, and then stretched to obtain a stretched film. On the other hand, when the ultrahigh molecular weight polyolefin resin (composition) is less than 5% by weight, the resulting porous film is inferior in strength.

  In this embodiment, when kneading a solution obtained by dissolving the above ultrahigh molecular weight polyolefin resin (composition) in a solvent, when kneading at a temperature exceeding 185 ° C., the viscosity of the solution is too low, A sufficient shearing force cannot be applied to the kneaded product, and on the other hand, when the kneading temperature is lower than 115 ° C., the mixture cannot be effectively kneaded, and thus in the kneading of the mixture, It is difficult to obtain sufficient entanglement of the polymer chains of the ultrahigh molecular weight polyolefin resin as described above.

  In this embodiment, in order to obtain sufficient entanglement of the polymer chains of such an ultrahigh molecular weight polyolefin resin, the solution mixture of the ultrahigh molecular weight polyolefin resin (composition), the solvent and the diffusion hole forming material is high. It is preferable to knead while applying a shearing force. When sufficient shearing force cannot be applied at the time of kneading, sufficient entanglement of the polymer chains of the ultrahigh molecular weight polyolefin resin cannot be obtained, and the diffusion hole forming material is not dispersed and has an appropriate size. There may be no electrolyte diffusion hole. Therefore, according to this embodiment, for kneading a solution-like mixture of an ultrahigh molecular weight polyolefin resin (composition) and a solvent, a kneader or a twin screw extruder that can give a strong shearing force to the mixture is usually used. Is preferably used.

  Next, according to this embodiment, the temperature below the freezing point of the solvent using the ultra-high-molecular-weight polyolefin resin (composition) thus obtained, the solvent-like kneaded material of the solvent and the diffusion hole forming material, preferably Is formed into a gel-like sheet while being cooled to a temperature in the range of −10 ° C. to −45 ° C., preferably in the range of −15 ° C. to −40 ° C. 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 is sandwiched between these metal plates, and the kneaded material may be pressed to be formed into a sheet.

  According to this embodiment, when the kneaded material is cooled and molded into a sheet, the kneaded material is rapidly cooled in order to finely crystallize the resin not only to the surface layer of the obtained sheet but also to the center of the sheet. Therefore, the cooling rate is preferably 50 ° C./min or more on average. In the solution state, that is, when the cooling rate at the time of molding from the kneaded product to the sheet is slow, the fibrils that are stretched and entangled return to the shape of the hair by forming kneading, forming a thick fiber. Therefore, it is difficult to form a porous membrane structure composed of uniform fibrils, and a porous membrane structure having large through holes is partially formed.

  The cooling rate is also closely related to the size of the electrolyte diffusion hole. As the sheet cools, the size of the polyacrylonitrile domain corresponding to the electrolyte diffusion hole portion begins to grow. Therefore, in order to control the size of the electrolyte diffusion hole, it is necessary to select an appropriate cooling rate, and it is necessary to balance with the rate at which the pore portion is crystallized.

  That is, in general, when a crystalline high molecular weight resin is crystallized, a lamellar crystal is formed, and the thickness of the lamellar crystal greatly depends on the crystallization temperature, and as the difference between the melting point and the crystallization temperature increases, the lamellar crystal The thickness of becomes smaller. According to this embodiment, the kneaded product of the ultrahigh molecular weight polyolefin resin (composition), the solvent and the diffusion hole forming material is heated to a temperature not higher than 115 to 185 ° C. and below the freezing point of the solvent used, ie −10. Since it is formed into a gel-like sheet while cooling to a temperature of ℃ or less, preferably while rapidly cooling, the thickness of the lamellar crystal is very small. Therefore, when the gel-like sheet is stretched to obtain a stretched film, As a result of the lamella crystal being divided into microcrystals, the fiber diameter is small and uniform fibrils, the curvature, which is the ratio of the penetration path to the film thickness, is very large, and the surface is larger than the pore diameter. It seems that a porous film having a microfibril structure in which electrolyte solution diffusion holes of a size exist can be obtained.

  Even if the kneaded product is rapidly cooled from the solution state and formed into a gel-like sheet and the resin is crystallized according to the present embodiment, the crystal structure of the resin in such a gel-like sheet is higher than the freezing point of the solvent used. When this gel-like sheet is stored at the same temperature as in the case where the cooling rate is slow, the fibrils that are stretched and entangled by kneading return to a hair-like shape, form thick fibers, and are fine. It is difficult to form a uniform porous structure, and large through holes are partially formed. Accordingly, if the kneaded product of the ultrahigh molecular weight polyolefin resin composition and the solvent is rapidly cooled, it is formed into a gel-like sheet, and thus the gel-like sheet having a crystal structure is immediately stretched or stored. It is desirable to store the obtained gel-like sheet at a temperature below the freezing point of the solvent.

  As described above, when the cooling rate at the time of forming the kneaded material into a sheet is slow or when the obtained sheet is stored at a temperature equal to or higher than the freezing point of the solvent, the above-described phenomenon occurs and is obtained. The porous film lacks the fineness and uniformity of the structure of the pore portion, and relatively large pores are formed, so that the strength, particularly the piercing strength, is inferior.

  According to this embodiment, when the melting point of the ultrahigh molecular weight polyolefin resin is M, the gel sheet is then at a temperature in the range of (M + 5) ° C. to (M−30) ° C., preferably from M ° C. (M-25) Biaxial stretching is performed at a temperature in the range of ° C. This biaxial stretching may be either sequential or simultaneous biaxial stretching, but preferably simultaneous biaxial stretching. In this embodiment, the stretch ratio of the gel sheet is 3 to 32 times in one direction, and 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.

  In the present embodiment, simultaneous biaxial stretching is performed in the TD direction (Transverse Direction) and the MD direction (Machine Direction), and by changing the speed ratio between the TD direction and the MD direction, electrolyte diffusion holes having different aspect ratios. Is formed. Since polyacrylonitrile and polyacrylonitrile copolymer domains existing on the film surface have a low molecular weight, the shape thereof is significantly affected by the film strain. Accordingly, it is possible to control the diffusion hole diameter at the drawing speed ratio.

  Next, the biaxially stretched film thus obtained is washed with an appropriate solvent to remove the solvent remaining in the film to form a porous film. Preferably, thereafter, heat shrinkage of the film is prevented. In order to do so, 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 sheet, for example, the sheet may be immersed in the solvent.

  The size of the formed diffusion hole is preferably 5 times or more, more preferably 10 times or more of the hole portion. Empirically, the hole diameter in the film thickness direction is preferably 5 times or more that of the pores, and more preferably 10 times or more, so that the electrolyte can be more effectively diffused.

  Considering the flow path of the electrolyte solution to be diffused, the shape of the electrolyte solution diffusion hole is more effective when it is circular or elliptical than square. The elliptical shape preferably has a major axis diameter / short axis diameter of the hole shape of, for example, 1.5 or more. The shape of the diffusion hole is preferably an ellipse in the surface direction and a mortar shape in the depth direction. In addition, when the same area is used, the electrolyte solution circulation holes are more effectively circulated in the elliptical shape than the plurality of circular electrolyte solution diffusion holes. This is presumably because the in-plane diffusion of the electrolyte is faster than that in the film thickness direction.

  Further, in the case of a porous film having no electrolyte solution diffusion hole on the membrane surface, a multilayer film formed by laminating a hole layer having a diffusion hole may be used. In that case, it is desirable that the thickness of the electrolyte solution diffusion layer (functional layer providing the electrolyte solution diffusion hole) is 0.1 to 1 μm. If the thickness of the electrolytic solution diffusion layer is 0.1 μm or more, it can be easily formed technically, and can be formed as a thin film that does not hinder the high capacity of the battery.

  The electrolyte solution diffusion layer is a porous material composed of at least one selected from the group of resin materials consisting of polyimide, polyetheretherketone, polyamide, polyamideimide, polyethersulfone, polyetherimide, polysulfone, and polyphenylene sulfide. It is preferable that it is a quality layer. If the electrolyte solution diffusion layer is a porous layer composed of at least one selected from the resin-based material group, it is easy to form voids functioning as electrolyte solution permeation and diffusion paths. Moreover, since the said resin system is excellent in mechanical strength and thermal stability, it can form the porous layer which cannot change easily within a battery.

  In the formation of the electrolyte diffusion layer, a slurry is prepared by mixing the resin material, a soluble solvent of these resins, and a diffusion hole forming material such as polyacrylonitrile or a copolymer of polyacrylonitrile. It can form by apply | coating to the surface of this. Examples of the solvent include a polar amide solvent or a polar urea solvent. Examples of the polar amide solvent include N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone and the like. Examples of the polar urea solvent include N, N, N ′, N′-tetramethylurea and the like. Among these, N-methyl-2-pyrrolidone is preferable in terms of chemical stability.

  Examples of the method for forming the electrolyte solution diffusion layer include coating methods such as knife, blade, bar, gravure, and die. Coating of bars, knives and the like is simple, but industrially, die coating having a structure in which the solution does not come into contact with outside air is preferable.

In addition, inorganic particles may be mixed in order to improve the mechanical strength and thermal stability of the electrolyte solution diffusion layer. Specific examples of the constituent material of the inorganic particles include inorganic oxides such as iron oxide, Al ₂ O ₃ (alumina), SiO ₂ (silica), TiO ₂ , BaTiO ₃ , ZrO ₂ ; aluminum nitride, silicon nitride, etc. Inorganic nitrides of the above; poorly soluble ionic crystals such as calcium fluoride, barium fluoride and barium sulfate; covalent bonds such as silicon and diamond; clays such as montmorillonite; Here, the inorganic oxide may be boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, or other mineral resource-derived substances or artificial products thereof.

(Lithium ion secondary battery)
Subsequently, a secondary battery using the above-described porous film will be described by taking a lithium ion secondary battery as an example.

  FIG. 3 is a schematic cross-sectional view of an example of the lithium ion secondary battery 100 using the porous film 18 described above.

  The lithium ion secondary battery 100 is mainly impregnated in the laminate 30, the case 50 that accommodates the laminate 30 in a sealed state, the pair of leads 60 and 62 connected to the laminate 30, and the laminate 30. It has an electrolyte.

  The laminated body 30 is configured such that a pair of the positive electrode 10 and the negative electrode 20 are opposed to each other with the porous film 18 interposed therebetween. The positive electrode 10 is obtained by providing a positive electrode active material layer 14 on a plate-like (film-like) positive electrode current collector 12. The negative electrode 20 is obtained by providing a negative electrode active material layer 24 on a plate-like (film-like) 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 porous film 18. Leads 62 and 60 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 10 and the negative electrode 20 are collectively referred to as electrodes 10 and 20, and the positive electrode current collector 12 and the negative electrode current collector 22 are collectively referred to as current collectors 12 and 22, and the positive electrode active material layer 14 and the negative electrode The active material layers 24 are collectively referred to as active material layers 14 and 24.

(Positive electrode)
First, description will be made in order from the positive electrode.
The positive electrode current collector 12 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

The positive electrode active material layer 14 includes an active material according to the present embodiment, a binder, and a conductive material in an amount as necessary.
As the positive electrode active material, for example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), layered lithium manganate (LiMnO ₂ ), or LiMn _x Ni _y Co, which is a composite oxide containing a plurality of transition metals. Layered compounds such as _z O ₂ (x, y and z satisfy the formula x + y + z = 1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ x <1), and one or more kinds of these compounds Substituted transition metal element, lithium manganate (Li _{1 + x} Mn _2−x O ₄ (where x is a number from 0 to 0.33)), Li _{1 + x} Mn _2−xy M _y O ₄ (however, , M includes at least one metal selected from the group consisting of Ni, Co, Cr, Cu, Fe, Al, and Mg, x represents a number from 0 to 0.33, and y is from 0 to 1.0. Number and x and y satisfy the formula of 2-xy> 0), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , LiMn _2−x M _x O ₂ (where M is Co, Ni, Fe, Cr, Including at least one metal selected from the group consisting of Zn and Ta, x represents a number of 0.01 to 0.1, Li ₂ Mn ₃ MO ₈ (where M is Fe, Co, Ni, At least one metal selected from the group consisting of Cu and Zn), copper-lithium oxide (Li ₂ CuO ₂ ), iron-lithium oxide (LiFe ₃ O ₄ ), LiFePO ₄ , LiV ₃ O ₈ , Examples include vanadium oxides such as V ₂ O ₅ and Cu ₂ V ₂ O ₇ , disulfide compounds, and Fe ₂ (MoO ₄ ) ₃ .

  (Conductive aid) Examples of the conductive aid include metals such as nickel, aluminum, copper, and silver, and conductive carbon materials. Examples of the conductive carbon material include carbon blacks such as acetylene black and ketjen black, and carbon fibers such as graphite and carbon nanofibers. As the conductive assistant, carbon black is particularly preferable. In addition, it is not necessary to contain a conductive support agent.

  (Binder) As the binder, polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene chloride (CTFE) copolymer [PVDF-CTFE)] Fluorine such as vinylidene fluoride-hexafluoropropylene fluororubber, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene fluororubber [PVDF-TFE-HFP)], vinylidene fluoride-tetrafluoroethylene-perfluoroalkyl vinyl ether fluororubber, etc. Polymers such as those are preferred. As the vinylidene fluoride-based polymer, those in which vinylidene fluoride is 50% by weight or more, particularly 70% by weight or more are preferable, and in particular, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene (HFP), A copolymer of vinylidene fluoride and ethylene trifluoride [PVDF-CTFE]] is preferred.

(Negative electrode)
Next, the negative electrode will be described.
The negative electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

  As the negative electrode active material, for example, lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers, can be occluded and released. One kind or a mixture of two or more kinds of carbon-based materials is used. In addition, elements such as Si, Sn, Ge, Bi, Sb and In and alloys thereof, lithium-containing nitrides, oxides and other compounds that can be charged and discharged at a low voltage close to lithium metal, or lithium metals and lithium / aluminum alloys Can also be used as a negative electrode active material. Then, a negative electrode mixture obtained by appropriately adding a conductive additive (carbon material such as carbon black) or a binder such as PVDF to these negative electrode active materials is finished into a molded body using the current collector as a core material. A negative electrode mixture layer can be formed on the current collector surface.

(Electrolytes)
Although not shown, the electrolyte is contained in the laminate 30. The electrolyte is not particularly limited, and, for example, in the present embodiment, an electrolyte 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, and the withstand voltage during charging is limited to a low level. As the electrolyte solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. For example, as the non-aqueous solvent, methyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite, 1,2-dimethoxyethane, 1,3 -The organic solvent which consists only of 1 type, such as dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, diethyl ether, or these 2 or more types of mixed solvents is mentioned.

Examples of the lithium _{salt, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN (CF 3 SO 2 ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] and the like.

  The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / L, and more preferably 0.9 to 1.25 mol / L.

  Furthermore, non-aqueous electrolytes contain esters with double bonds, such as vinylene carbonate, sulfur-containing organic compounds, such as propane sultone, and fluorine-containing compounds, such as fluorobenzene, for the purpose of improving battery cycle life and load characteristics. It is preferable to add an additive such as an aromatic compound.

  In the present embodiment, the electrolyte may be a gel electrolyte obtained by adding a gelling agent in addition to liquid. Further, instead of the electrolyte solution, a solid electrolyte (a solid polymer electrolyte or an electrolyte made of an ion conductive inorganic material) may be contained.

  The case 50 seals the laminate 30 and the electrolyte solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 3, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). preferable.

  The leads 60 and 62 are made of a conductive material such as aluminum.

  The material of the case is not particularly limited, and examples thereof include a cylindrical steel can (such as a rectangular tube or a cylinder) and an aluminum can that are used in a conventionally known lithium ion secondary battery. In addition, a laminate film obtained by vapor-depositing a metal on a resin film can be used for the exterior body.

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

  For example, the porous film of the present invention can be used for electrochemical devices 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
Slurry of ultra high molecular weight polyethylene resin having a weight average molecular weight of 2 million (melting point 134 ° C.) 15% by weight, liquid paraffin (kinematic viscosity -15 ° C., kinematic viscosity 59 cSt at 40 ° C.) 83.5% by weight and polyacrylonitrile 1.5% by weight The mixture was uniformly mixed into a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, 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 porous film 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%.

(Example 2)
Uniform slurry of 15% by weight of ultra-high molecular weight polyethylene resin (melting point 134 ° C) with a weight average molecular weight of 2 million, 83% by weight of liquid paraffin (kinematic viscosity -15 ° C, kinematic viscosity 59 cst at 40 ° C) and 2% by weight of polyacrylonitrile. This was mixed, charged into a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, 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 porous film 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%.

(Example 3)
Slurry of 15% by weight of ultra high molecular weight polyethylene resin (melting point 134 ° C.) with a weight average molecular weight of 2 million, 83.5% by weight of liquid paraffin (kinematic viscosity of 59 cst at a freezing point of −15 ° C. and 40 ° C.) and 1.5% by weight of polyacrylonitrile. The mixture was uniformly mixed into a small kneader, heated at a temperature of 160 ° C. for about 50 minutes, 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 at a temperature of about 115 ° C. in a length of 4 × 4.4 times, and then immersed in methylene chloride to remove the solvent. The porous film 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%.

Example 4
The porous film obtained in Example 1 was coated with a polyamideimide (PAI) porous layer. The procedure is shown below.

Hybrid mixer containing 50% by weight of polyamideimide solution (HPC-5000-30 manufactured by Hitachi Chemical Co., Ltd.), 15% by weight of polyethylene glycol (PEG 4000 manufactured by Daiichi Kogyo Seiyaku), 5% by weight of polyacrylonitrile, and 30% by weight of N-methyl-2-pyrrolidone. For 1 h. Thereafter, a 4 μm heat-resistant resin solution layer was formed on the porous film obtained in Example 1 using a blade coater. When the liquid layer was immediately washed with water (temperature: 40 ° C.) in a water bath, polyamideimide (PAI) resin was precipitated, and a yellowish white film was formed on the porous film obtained in Example 1. The obtained porous film on which the heat-resistant layer was formed was immersed in a hot water bath for 5 minutes, and then immersed in ion-exchanged water and washed. After washing with ion exchanged water for 12 hours or more, the wet membrane was taken out of the water, and the water was blown off with air. Then, the whole was put into a heat oven and the membrane was dried while reducing the pressure at 60 ° C., and the polyethylene porous film and the polyamideimide porous film having a total thickness of 0.5 μm formed only on one surface thereof Got.

(Example 5)
Polyamideimide solution (HPC-5000-30, manufactured by Hitachi Chemical Co., Ltd.) 50% by weight, polyethylene glycol (PEG 4000, manufactured by Daiichi Kogyo Seiyaku), polyacrylonitrile 7.5% by weight, N-methyl-2-pyrrolidone 27.5% by weight % Was mixed for 1 h with a hybrid mixer. Thereafter, a 4 μm heat-resistant resin solution layer was formed on the porous film obtained in Example 1 using a blade coater. When the liquid layer was immediately washed in a water bath (temperature: 40 ° C.), a polyamideimide resin was precipitated, and a yellowish white film was formed on the porous film obtained in Example 1. The obtained porous film on which the heat-resistant layer was formed was immersed in a hot water bath for 5 minutes, and then immersed in ion-exchanged water and washed. After washing with ion exchanged water for 12 hours or more, the wet membrane was taken out of the water, and the water was blown off with air. Then, the whole was put into a heat oven and the membrane was dried while reducing the pressure at 60 ° C., and the polyethylene porous film and the polyamideimide porous film having a total thickness of 0.5 μm formed only on one surface thereof Got.

(Comparative Example 1)
An ultra-high molecular weight polyethylene resin having a weight average molecular weight of 2 million (melting point: 134 ° C.) 15% by weight and liquid paraffin (solidifying point: −15 ° C., kinematic viscosity 59 cst at 40 ° C.) 85% by weight are uniformly mixed in a slurry state. The mixture was charged in a small kneader, heated at 160 ° C. for about 50 minutes, 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 porous film 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%.

(Comparative Example 2)
A polyamideimide solution (HPC-5000-30 manufactured by Hitachi Chemical Co., Ltd.) 50% by weight and N-methyl-2-pyrrolidone 250% by weight were mixed with a hybrid mixer for 1 hour. Thereafter, a 4 μm heat-resistant resin solution layer was formed on the porous film obtained in Example 1 using a blade coater. When the liquid layer was immediately washed in a water bath (temperature: 40 ° C.), a polyamideimide resin was precipitated, and a yellowish white film was formed on the porous film obtained in Example 1. The obtained porous film on which the heat-resistant layer was formed was immersed in a hot water bath for 5 minutes, and then immersed in ion-exchanged water and washed. After washing with ion exchanged water for 12 hours or more, the wet membrane was taken out of the water, and the water was blown off with air. Then, the whole was put into a heat oven and the membrane was dried while reducing the pressure at 60 ° C., and the polyethylene porous film and the polyamideimide porous film having a total thickness of 0.5 μm formed only on one surface thereof Got.

(Battery production)
"Positive electrode"
LiMn ₂ O ₄ as a positive electrode active material, carbon black as a conductive auxiliary agent, and (PVDF-CTFE) as a binder were prepared. These were mixed so that the positive electrode active material: conducting aid: binder = 90: 5: 5 by weight ratio. The obtained mixture and NMP (N-methyl-2-pyrrolidone) solvent were mixed at a weight ratio of 1: 1.3 and dispersed at room temperature to prepare a cathode slurry. The obtained cathode slurry was formed into a coating film by the doctor blade method and dried to prepare a cathode.

"Negative electrode"
An easily graphitizable carbon material was prepared as a negative electrode active material, carbon black was prepared as a conductive additive, and (PVDF-CTFE) was prepared as a binder. These were mixed so that the negative electrode active material: conducting aid: binder = 90: 5: 5 by weight ratio. The obtained mixture and NMP (N-methyl-2-pyrrolidone) solvent were mixed at a weight ratio of 1: 1 and dispersed at room temperature to prepare an anode slurry. The obtained anode slurry was formed into a coating film by a doctor blade method and dried to prepare an anode.

"Electrolyte"
An electrolyte was prepared by dissolving LiPF ₆ in a non-aqueous solvent with EC (ethylene carbonate) / DEC (diethyl carbonate) = 30/70 (weight ratio) to a concentration of 1 mol / cm ³ .

"battery"
The above-mentioned positive electrode (diameter 14 mm) and negative electrode (diameter 15 mm) are laminated via the porous film (diameter 16 mm) produced in the example, sealed in a container together with the electrolyte, and a button battery having a capacity of 4.2 mAh ( 2032 type).

  The materials used, the characteristics of the formed porous film, and the characteristics of the secondary battery using the same were evaluated as follows.

(Weight average molecular weight)
Using a gel permeation chromatograph (manufactured by Waters, GPC-150C), o-dichlorobenzene was used as a solvent, and Shodex-80M (manufactured by Showa Denko) was used as a column at a temperature of 135 ° C. Data processing was performed using a TRC data processing system. The molecular weight was calculated based on polystyrene.

(Thickness)
It is measured with a 1/10000 mm thickness gauge and from a 10,000 times scanning electron micrograph of the cross section of the porous film.

(Porosity)
Using a mercury porosimeter (Autoscan 33, manufactured by Yuasa Ionics Co., Ltd.), the pore volume (ml / g) was determined, and the density of the resin composition was 0.95 (g / ml). Calculated.
(Formula 1)
Porosity (%) = (pore volume) / (1 / density + pore volume) × 100

(Air permeability)
It is measured by a method according to JIS P8117.

(Surface observation of porous film: shape observation of electrolyte diffusion hole)
A porous film 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. Using the following ultra-high resolution scanning electron microscope apparatus (UHRSEM: Hitachi ultra-high resolution scanning electron microscope S-900), under the conditions of an acceleration voltage of 1.0 kV and an imaging speed of 40 seconds / frame, The surface structure of the porous film was observed at a predetermined magnification. In FIG. 4, the photograph (500 times) which image | photographed the main surface of the porous film obtained in Example 1 with the scanning electron microscope is shown. The opening area of the electrolyte diffusion hole was calculated by image analysis based on this photograph. Whether or not the electrolyte solution diffusion hole has a mortar shape or the like can be determined by checking a cross section perpendicular to the main surface of the porous film with a scanning electron microscope.

(Discharge test and cycle life)
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 . Then, 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 (unit:%) with the discharge capacity of 0.1 C of Example 1 as 100. Further, the discharge capacity after a 500 cycle charge / discharge test at discharge rates of 0.1 C, 0.5 C, and 2.0 C was also measured to evaluate the cycle life. After 2000 cycles, the cell was opened and the state of the negative electrode was visually confirmed. Table 1 shows the conditions and results.

In addition, when the porous film of an Example was observed with the scanning electron microscope, all the things of an Example were electrolyte solution of the hole diameter 10 times or more of the hole diameter to a fine hole in the state as shown in FIG. Diffusion holes were confirmed. On the other hand, the comparative example did not have an electrolyte solution diffusion hole.

  From the results of Table 1, it was found that the example was a lithium ion secondary battery excellent in cycle life due to suppression of composition deterioration as compared with the comparative example.

Claims (5)

A separator for a lithium ion secondary battery comprising a porous film made of an ultrahigh molecular weight polyolefin resin and having a large number of pores, and having an average pore diameter of 0.05 μm to 0.3 μm on the membrane surface of the porous film There are micropores and electrolyte diffusion holes larger than 10 times the average aperture diameter of the micropores,
The lithium ion secondary battery separator, wherein an area occupied by the electrolyte solution diffusion hole with respect to the membrane surface of the porous film is 5.1% or more and 12.2% or less. The separator for a lithium ion secondary battery according to claim 1, wherein the electrolyte solution diffusion hole is formed in a mortar shape. The separator for a lithium ion secondary battery according to claim 1 or 2, wherein the opening shape of the electrolyte solution diffusion hole is elliptical. A separator for a lithium ion secondary battery according to any one of claims 1 to 3, comprising a positive electrode, a negative electrode, and an electrolyte, and between the positive electrode and the negative electrode,
The positive electrode and the negative electrode are electrically connected to an electrode terminal at one end thereof,
A lithium ion secondary battery in which a large number of electrolyte solution diffusion holes are unevenly distributed in the lithium ion secondary battery separator facing each other in the vicinity of the electrode terminal of at least one of the positive electrode and the negative electrode. 5. The lithium ion secondary battery according to claim 4, wherein the opening of the electrolyte solution diffusion hole is opposed to the negative electrode.