Separator for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

The purpose of the present disclosure is to provide a separator for nonaqueous electrolyte secondary batteries, said separator being capable of suppressing a decrease in the charge and discharge cycle characteristics, while being capable of suppressing slip drop of a heat-resistant porous film. A separator (13) for nonaqueous electrolyte secondary batteries according to one embodiment of the present disclosure comprises a porous base material (30) and a heat-resistant porous film (32) that is arranged on at least one surface of the porous base material (30) so as to face an electrode of a nonaqueous electrolyte secondary battery. The heat-resistant porous film (32) contains a filler and a binder; and at least a part of a facing part A of the heat-resistant porous film (32), said facing part A facing the edge portion of the electrode, has a higher binder content than a facing part B of the heat-resistant porous film (32), said facing part B facing the central portion of the electrode.

Description

[Technical Field]

[0001] The present disclosure relates to a separator for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] BACKGROUND ART In recent years, non-aqueous electrolyte secondary batteries having an electrode assembly in which a positive electrode and a negative electrode are arranged opposite each other with a separator interposed therebetween have been widely used as high-power, high-energy density secondary batteries.

[0003] For example, Patent Document 1 discloses a nonaqueous electrolyte secondary battery including an electrode assembly in which a positive electrode and a negative electrode are arranged opposite each other via a separator, the separator including a porous substrate and a heat-resistant porous membrane arranged on at least one surface of the porous substrate, and the heat-resistant porous membrane has a porosity of 55% or more. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-18600 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the heat-resistant porous membrane of the separator has a problem that it rubs against the electrode (especially the edge portion of the electrode) and part of it slides off the porous substrate. Furthermore, if the binder content is increased to prevent the heat-resistant porous membrane from sliding off, the binder clogs the pores of the porous substrate, which causes a problem of degrading the charge-discharge cycle characteristics.

[0006] Therefore, an object of the present disclosure is to provide a separator for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that can suppress the deterioration of charge / discharge cycle characteristics and the sliding off of a heat-resistant porous film. [Means for solving the problem]

[0007] A separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a porous substrate and a heat-resistant porous membrane disposed on at least one surface of the porous substrate and facing an electrode of the non-aqueous electrolyte secondary battery, the heat-resistant porous membrane containing a filler and a binder, and at least a part of a facing portion A of the heat-resistant porous membrane facing an edge portion of the electrode has a higher binder content than a facing portion B of the heat-resistant porous membrane facing a central portion of the electrode.

[0008] A non-aqueous electrolyte secondary battery according to an aspect of the present disclosure includes the electrode and the separator for a non-aqueous electrolyte secondary battery. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to suppress the deterioration of charge / discharge cycle characteristics and to suppress the slippage of the heat-resistant porous film. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view of a separator according to an embodiment of the present invention. [Figure 3] FIG. 3 is a schematic plan view of a separator according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0012] FIG. 1 is a schematic cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. The nonaqueous electrolyte secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing member 17 that closes the opening of the case body 16. Note that instead of the wound electrode assembly 14, a laminated electrode assembly formed by alternately stacking positive and negative electrodes with separators interposed therebetween may also be used. Examples of the battery case 15 include a cylindrical, prismatic, coin-shaped, or button-shaped metal case, and a pouch-shaped case formed by laminating a resin sheet.

[0013] Case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between case body 16 and sealing body 17 to ensure airtightness inside the battery. Case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports sealing body 17. Protruding portion 22 is preferably formed in an annular shape along the circumferential direction of case body 16, and supports sealing body 17 on its upper surface.

[0014] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to one another at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0015] 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in an insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside an insulating plate 19 and extends toward the bottom of the case body 16. The positive electrode lead 20 is connected to the underside of a filter 23, which is the bottom plate of the sealing body 17, by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

[0016] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector. The positive electrode current collector can be, for example, a foil of a metal such as aluminum that is stable within the potential range of the positive electrode, or a film having such a metal disposed on its surface. The positive electrode active material layer preferably contains a positive electrode active material and also contains a conductive material and a binder.

[0017] Examples of the positive electrode active material include lithium transition metal composite oxides, and specifically, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel manganese composite oxide, lithium nickel cobalt composite oxide, etc., and Al, Ti, Zr, Nb, B, W, Mg, Mo, etc. may be added to these lithium transition metal composite oxides.

[0018] As the conductive material, carbon powder such as carbon black, acetylene black, ketjen black, graphite, etc. may be used alone or in combination of two or more kinds.

[0019] Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more.

[0020] The negative electrode 12 includes, for example, a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector. The negative electrode current collector can be, for example, a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film having such a metal disposed on its surface. The negative electrode active material layer preferably contains a negative electrode active material and also contains a binder.

[0021] As the negative electrode active material, a carbon material capable of absorbing and releasing lithium ions can be used, and in addition to graphite, non-graphitizable carbon, graphitizable carbon, fibrous carbon, coke, carbon black, etc. Furthermore, as a non-carbon-based material, silicon, tin, and alloys and oxides mainly containing these can be used.

[0022] Examples of binders include fluorine-based resins, PAN, polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0023] FIG. 2 is a schematic cross-sectional view of a separator according to an embodiment. The separator 13 shown in FIG. 2 includes a porous substrate 30 and heat-resistant porous membranes 32 disposed on both sides of the porous substrate 30. Therefore, the heat-resistant porous membrane 32 disposed on one side of the porous substrate 30 faces (contacts) the positive electrode 11, and the heat-resistant porous membrane 32 disposed on the other side of the porous substrate 30 faces (contacts) the negative electrode 12. The separator 13 is designed to be larger in both width and length than the electrodes (positive electrode 11 and negative electrode 12) to prevent short circuits between the positive and negative electrodes. Therefore, when the electrodes and the separator 13 are stacked to form an electrode assembly 14, the separator 13 protrudes from the electrodes. The heat-resistant porous membrane 32 may be disposed on at least one surface of the porous substrate 30.

[0024] The porous substrate 30 is a porous sheet having ion permeability and insulating properties, and is made of, for example, a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The material of the porous substrate 30 is not particularly limited, but examples thereof include polyolefins such as polyethylene, polypropylene, and copolymers of polyethylene and α-olefin, acrylic resins, polystyrene, polyester, and cellulose. The porous substrate 30 may have a single-layer structure or a laminated structure. The thickness of the porous substrate 30 is not particularly limited, but is preferably in the range of, for example, 3 μm to 20 μm.

[0025] The porosity of the porous substrate 30 is preferably in the range of, for example, 30% to 70% in terms of lithium ion permeability, etc. The porosity of the porous substrate 30 is measured by the following method. (1) Punch out 10 circular pieces with a diameter of 2 cm from the substrate, and measure the thickness h and mass w of the center of each punched piece of substrate. (2) From the thickness h and mass w, the volume V and mass W of 10 small pieces are calculated, and the porosity ε is calculated using the following formula. Porosity ε(%)=((ρV-W) / (ρV))×100 ρ: Density of the material that makes up the base material

[0026] The average pore size of the porous substrate 30 is preferably in the range of 0.02 μm to 0.5 μm, and more preferably in the range of 0.03 μm to 0.3 μm. The average pore size of the porous substrate 30 is measured using a Perm Porometer (manufactured by Seika Sangyo Co., Ltd.) capable of measuring pore size by the bubble point method (JIS K3832, ASTM F316-86).

[0027] The heat-resistant porous film 32 contains a filler and a binder. When the separator 13 has the heat-resistant porous film 32, for example, the internal stress of the separator 13 that increases when the temperature rises is alleviated, thereby obtaining an effect of suppressing the thermal contraction of the separator 13. As a result, for example, it is possible to prevent the induction of short circuits between the positive and negative electrodes. The thickness of the heat-resistant porous film 32 is not particularly limited, but is preferably in the range of, for example, 1 μm to 10 μm.

[0028] The inclusion of a filler in the heat-resistant porous membrane 32 can, for example, impart a thermal shrinkage suppression effect to the heat-resistant porous membrane 32. The melting point or thermal softening point of the filler is preferably, for example, 150°C or higher, and more preferably 200°C or higher. Examples of the filler include metal oxide particles, metal nitride particles, metal fluoride particles, and metal carbide particles. Examples of metal oxide particles include aluminum oxide, titanium oxide, magnesium oxide, zirconium oxide, nickel oxide, silicon oxide, and manganese oxide. Examples of metal nitride particles include titanium nitride, boron nitride, aluminum nitride, magnesium nitride, and silicon nitride. Examples of metal fluoride particles include aluminum fluoride, lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, and barium fluoride. Examples of metal carbide particles include silicon carbide, boron carbide, titanium carbide, and tungsten carbide. The filler is zeolite (M 2 / n O·Al2O3·xSiO2·yH2O, where M is a metal element, x≧2, y≧0), talc (Mg3SiO4O 10 The inorganic filler may be a layered silicate such as tetrahydrofuran (Titanium dioxide) (OH)2), or a mineral such as barium titanate (BaTiO3) or strontium titanate (SrTiO3). These may be used alone or in combination of two or more.

[0029] The BET specific surface area of the filler is not particularly limited, but for example, 2 / g~20m 2 / g is preferred, and 3m 2 / g~15m 2 The average particle size of the filler is not particularly limited, but is preferably, for example, 0.1 μm to 5 μm, and more preferably in the range of 0.2 μm to 1 μm.

[0030] The binder functions to bond the individual fillers together and the fillers to the porous substrate 30. The binder improves the peel strength between the porous substrate 30 and the heat-resistant porous membrane 32. Examples of binders include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, and polyvinyl alcohol (PVA). These may be used alone or in combination of two or more.

[0031] Fig. 3 is a schematic plan view of a separator according to an embodiment. The separator 13 shown in Fig. 3 is in a state prior to forming a wound electrode assembly 14. The wound electrode assembly 14 is obtained by placing a positive electrode 11 on one side of the separator 13 and a negative electrode 12 on the other side, and then winding them in the longitudinal direction.

[0032] The dashed line frame shown in Fig. 3 is the outer shape of an electrode (positive electrode or negative electrode) when the electrode is placed on the surface of the heat-resistant porous membrane 32 of the separator 13. Therefore, one long side of the dashed line frame shown in Fig. 3 is an opposing portion A1 of the heat-resistant porous membrane 32 that faces one edge portion in the lateral direction of the electrode (positive electrode or negative electrode) placed on the surface of the heat-resistant porous membrane 32 (i.e., one edge portion extending in the longitudinal direction), and the other long side is an opposing portion A1 of the heat-resistant porous membrane 32 that faces one edge portion in the lateral direction of the electrode (positive electrode or negative electrode) placed on the surface of the heat-resistant porous membrane 32. 3 is a facing portion A2 of the heat-resistant porous membrane 32 facing the other edge portion in the longitudinal direction of the electrode arranged on the surface of the heat-resistant porous membrane 32. One short side of the dashed-line frame shown in Fig. 3 is a facing portion B1 of the heat-resistant porous membrane 32 facing one edge portion in the longitudinal direction of the electrode arranged on the surface of the heat-resistant porous membrane 32 (i.e., one edge portion extending in the lateral direction), and the other short side is a facing portion B2 of the heat-resistant porous membrane 32 facing the other edge portion in the longitudinal direction of the electrode arranged on the surface of the heat-resistant porous membrane 32. In Fig. 3, symbol I indicates one edge portion in the lateral direction of the heat-resistant porous membrane 32, symbol II indicates the other edge portion in the lateral direction of the heat-resistant porous membrane 32, symbol III indicates one edge portion in the longitudinal direction of the heat-resistant porous membrane 32, and symbol IV indicates the other edge portion in the longitudinal direction of the heat-resistant porous membrane 32.

[0033] Here, at least a part of the opposing portions (A1, A2, B1, B2) of the heat-resistant porous membrane 32 has a higher binder content than the opposing portion of the heat-resistant porous membrane 32 opposing the central portion of the electrode. In other words, the opposing portion of the heat-resistant porous membrane 32 opposing the central portion of the electrode has a lower binder content than at least a part of the opposing portions (A1, A2, B1, B2) of the heat-resistant porous membrane 32. The central portion of the electrode refers to the center of the electrode in the longitudinal and lateral directions.

[0034] The heat-resistant porous membrane 32 slides off due to friction with the electrode, primarily due to friction with the edge of the electrode. Therefore, the opposing portions (A1, A2, B1, B2) of the heat-resistant porous membrane 32 that face the edge of the electrode are prone to slide off. However, in this embodiment, at least a portion of the opposing portions (A1, A2, B1, B2) of the heat-resistant porous membrane 32 that face the edge of the electrode has a higher binder content than the opposing portion of the heat-resistant porous membrane 32 that faces the center of the electrode, so the heat-resistant porous membrane 32 in the opposing portions has high adhesion. Therefore, even if friction with the edge of the electrode occurs, it is believed that the heat-resistant porous membrane 32 is prevented from sliding off. On the other hand, the portion of the heat-resistant porous membrane 32 facing the center portion of the electrode has a lower binder content than at least a portion of the facing portions (A1, A2, B1, B2) of the heat-resistant porous membrane 32, which prevents the binder from clogging the pores of the porous substrate 30. This is thought to prevent the movement of the non-aqueous electrolyte from being hindered during charge and discharge, thereby preventing a decrease in charge and discharge cycle characteristics.

[0035] The facing portion of the heat-resistant porous membrane 32 having a higher binder content than the facing portion of the heat-resistant porous membrane 32 facing the central portion of the electrode may be at least one of facing portions A1, A2, B1, and B2, or at least one of a part of facing portion A1, a part of facing portion A2, a part of facing portion B1, and a part of facing portion B2.

[0036] In the case of a wound-type electrode body, since a single electrode sheet is usually cut along the longitudinal direction to create multiple strip-shaped electrodes, burrs are likely to occur on the short-side edge portions of the electrodes (i.e., edge portions extending in the longitudinal direction). Therefore, in the case of a wound-type electrode body, it is preferable that the opposing portions A1 and A2 of the heat-resistant porous membrane 32 opposing the short-side edge portions of the electrode have a higher binder content than the opposing portion of the heat-resistant porous membrane opposing the center portion of the electrode.

[0037] In the case of a laminated electrode body, since multiple electrodes are usually produced by punching one electrode sheet into a predetermined shape (rectangle, circle, etc.), burrs are likely to occur all over the edge portions of the electrodes. Therefore, in the case of a laminated electrode body, it is preferable that all of the opposing portions of the heat-resistant porous membrane 32 facing the edge portions of the electrodes have a higher binder content than the opposing portion of the heat-resistant porous membrane 32 facing the center portion of the electrode.

[0038] The above is just one example, and in any electrode body, it is sufficient that at least a portion of the opposing portion of the heat-resistant porous membrane 32 facing the edge portion of the electrode has a higher binder content than the opposing portion of the heat-resistant porous membrane 32 facing the central portion of the electrode.

[0039] The binder content of the heat-resistant porous membrane 32 may increase stepwise or continuously from the portion of the heat-resistant porous membrane 32 facing the center portion of the electrode toward the portion of the heat-resistant porous membrane 32 facing the edge portion of the electrode. The binder content of the heat-resistant porous membrane 32 may increase or decrease stepwise or continuously from the portion of the heat-resistant porous membrane 32 facing the edge portion of the electrode toward the edge portion of the heat-resistant porous membrane 32, or may remain the same.

[0040] The binder content in at least a portion of the heat-resistant porous membrane 32 facing the edge portion of the electrode is preferably in the range of 5% to 15% by mass, for example, in order to effectively prevent the heat-resistant porous membrane from slipping off, etc. The binder content in the heat-resistant porous membrane 32 facing the center portion of the electrode is preferably in the range of 1% to 10% by mass, for example, in order to effectively prevent a decrease in the charge-discharge cycle of the nonaqueous electrolyte secondary battery, etc.

[0041] An example of a method for producing the separator 13 will be described. A first slurry containing a filler, a binder, etc. is prepared. A second slurry containing the same filler, binder, etc. as the first slurry but with a higher binder content than the first slurry is prepared. The second slurry is then applied to the surface of the porous substrate, for example, from edge portion III to edge portion IV in the longitudinal direction, over a width including edge portion I and opposing portion A1 as shown in FIG. 3, and the second slurry is also applied to the surface of the porous substrate over a width including edge portion II and opposing portion A2 as shown in FIG. 3, over a width including edge portion III to edge portion IV in the longitudinal direction. The first slurry is then applied to the surface of the porous substrate between the application spaces for the second slurry. The first and second slurries may be applied simultaneously or separately. After application, the porous substrate is dried for a predetermined period of time to obtain a separator having a heat-resistant porous film formed on the surface of the porous substrate.

[0042] The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt. The nonaqueous electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte using a gel-like polymer or the like. Examples of the electrolyte salt include lithium salts such as LiFSI, LiTFSI, LiBF4, and LiPF6. Examples of the solvent include esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl acetate (MA), and methyl propionate (MP), ethers, nitriles, amides, and mixtures of two or more of these. The nonaqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0043] Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylates such as methyl fluoropropionate (FMP).

[0044] Next, an example will be described. [Example]

[0045] <Example> [Separator fabrication] Titania (TiO2) with a particle size of 0.5 μm, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:0.5:4.5 in an aqueous solvent using a mixer to prepare a first slurry for a heat-resistant porous membrane with a solid content of 30%. Titania (TiO2) with a particle size of 0.5 μm, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:0.5:9.5 in an aqueous solvent using a mixer to prepare a second slurry for a heat-resistant porous membrane with a solid content of 30%.

[0046] Using a stripe coater, the prepared first and second slurries were applied to both sides of a polyethylene porous substrate. Specifically, the second slurry was applied to one surface of the porous substrate from edge portion III to edge portion IV in the longitudinal direction, with a width including edge portion I to opposing portion A1 shown in FIG. 3, and to one surface of the porous substrate from edge portion III to edge portion IV in the longitudinal direction, with a width including edge portion II to opposing portion A2 shown in FIG. 3. Simultaneously with the application of the second slurry, the first slurry was applied to the surface of the porous substrate between the application spaces of the second slurry. After application, the substrate was dried for a predetermined time. The first and second slurries were similarly applied to the other surface of the porous substrate and dried. In this way, a separator having a heat-resistant porous film formed on both sides of the porous substrate was obtained.

[0047] [Preparation of positive electrode] In N-methyl-2-pyrrolidone (NMP) solvent, LiNi 0.8 Co 0.15 Al 0.05A positive electrode active material represented by O2, acetylene black (AB), and polyvinylidene fluoride (PVDF) with an average molecular weight of 1.1 million were mixed in a mass ratio of 98:1:1 using a mixer to prepare a positive electrode composite slurry with a solid content of 70%. The positive electrode composite slurry was applied to both sides of aluminum foil, dried, and then rolled using a roller. In this way, a positive electrode was obtained in which a positive electrode active material layer was formed on both sides of the positive electrode current collector. This positive electrode was cut into strips of a predetermined width and used as the positive electrode of the example.

[0048] [Preparation of negative electrode] 95 parts by mass of graphite powder, 5 parts by mass of Si oxide, 1 part by mass of carboxymethyl cellulose (CMC), and an appropriate amount of water were mixed, and 1.2 parts by mass of styrene butadiene rubber (SBR) and an appropriate amount of water were added to this mixture and mixed to prepare a negative electrode composite slurry. The negative electrode composite slurry was applied to both sides of copper foil, dried, and then rolled using a roller. In this way, a negative electrode having a negative electrode active material layer formed on both sides of the negative electrode current collector was obtained. This negative electrode was cut into strips of a predetermined width and used as the negative electrode of the example.

[0049] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by adding 5 parts by mass of vinylene carbonate (VC) to 100 parts by mass of a mixed solvent prepared by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:3, and dissolving 1 mol / L of LiPF6.

[0050] [Fabrication of non-aqueous electrolyte secondary battery] (1) A positive electrode lead was attached to the positive electrode current collector, and a negative electrode lead was attached to the negative electrode current collector. The separator and the positive electrode were then aligned so that the heat-resistant porous film formed from the second slurry on one side of the separator faced the edge of the positive electrode in the short direction. The separator and the negative electrode were also aligned so that the heat-resistant porous film formed from the second slurry on the other side of the separator faced the edge of the negative electrode in the short direction. The separator was then placed between the positive electrode and the negative electrode. These were then wound to produce a wound electrode assembly. (2) Insulating plates were placed above and below the electrode body, the negative electrode lead was welded to the case body, and the positive electrode lead was welded to the sealing member, and the electrode body was housed within the case body. (3) After the non-aqueous electrolyte was poured into the case body, the open end of the case body was sealed with a sealing member via a gasket, completing a non-aqueous electrolyte secondary battery.

[0051] <Comparative Example 1> In producing the separator, a first slurry was applied to one surface of the porous substrate from edge portion III to edge portion IV in the longitudinal direction, with a width including edge portion I to opposing portion A1 shown in FIG. 3, and from edge portion II to opposing portion A2 shown in FIG. 3, with a width including edge portion III to edge portion IV in the longitudinal direction. Simultaneously with the application of the first slurry, a second slurry was applied to the surface of the porous substrate between the application spaces of the first slurry and dried for a predetermined time. The first and second slurries were also applied to the other surface of the porous substrate in the same manner and dried. Otherwise, the separator was produced in the same manner as in the example.

[0052] In the fabrication of the non-aqueous electrolyte secondary battery, the separator and the positive electrode were aligned so that the heat-resistant porous film formed from the first slurry on one side of the separator faced the edge portion of the positive electrode in the lateral direction, and the separator and the negative electrode were aligned so that the heat-resistant porous film formed from the first slurry on the other side of the separator faced the edge portion of the negative electrode in the lateral direction, and the separator was disposed between the positive electrode and the negative electrode. Otherwise, the non-aqueous electrolyte secondary battery was fabricated in the same manner as in the examples.

[0053] <Comparative Example 2> A separator was produced in the same manner as in Example 1, except that the first slurry was applied to both sides of the porous substrate in the production of the separator. A nonaqueous electrolyte secondary battery was produced using the produced separator in the same manner as in Example 1.

[0054] <Comparative Example 3> A separator was produced in the same manner as in Example 1, except that the second slurry was applied to both sides of the porous substrate in the production of the separator. A nonaqueous electrolyte secondary battery was produced using the produced separator in the same manner as in Example 1.

[0055] [Charge / discharge cycle characteristics] The nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples were subjected to constant current charging at a current of 0.3 It up to 4.2 V, followed by constant voltage charging at 4.2 V down to a current of 0.05 It. They were then subjected to constant current discharging at a current of 0.5 It down to 2.5 V. This charge / discharge cycle was repeated 100 times, and the capacity retention rate was determined. The results are summarized in Table 1. Capacity retention rate (%) = (100th cycle discharge capacity / 1st cycle discharge capacity) x 100

[0056] After the 100 charge-discharge cycles, the nonaqueous electrolyte secondary batteries were disassembled, the separators were removed, and the presence or absence of the heat-resistant porous membrane was visually checked. The number of batteries checked was 100. The results of whether or not the heat-resistant porous membrane had slipped off are summarized in Table 1.

[0057] [Table 1]

[0058] As can be seen from the results in Table 1, in Example 1, the deterioration of charge-discharge cycle characteristics is suppressed, and the slippage of the heat-resistant porous membrane is also suppressed.This is presumably because at least a part of the facing part of the heat-resistant porous membrane facing the edge part of the electrode has a higher binder content than the facing part of the heat-resistant porous membrane facing the center part of the electrode, which improves the adhesion of the heat-resistant porous membrane and prevents the heat-resistant porous membrane from slipping off due to friction with the edge part of the electrode.In addition, the facing part of the heat-resistant porous membrane facing the center part of the electrode has a lower binder content than at least a part of the facing part of the heat-resistant porous membrane facing the edge part of the electrode, which prevents the pores of the porous substrate from being blocked by the binder, which prevents the movement of non-aqueous electrolyte from being hindered during charge-discharge, which is presumably why the deterioration of charge-discharge cycle characteristics is suppressed. [Explanation of symbols]

[0059] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 battery case, 16 case body, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 protruding portion, 23 filter, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 porous substrate, 32 heat-resistant porous membrane.

Claims

1. a porous substrate; and a heat-resistant porous membrane disposed on at least one surface of the porous substrate and facing an electrode of a non-aqueous electrolyte secondary battery; The heat-resistant porous membrane contains a filler and a binder, a separator for a non-aqueous electrolyte secondary battery, wherein at least a part of an opposing portion A of the heat-resistant porous membrane facing an edge portion in a lateral direction of the electrode and at least a part of an opposing portion B of the heat-resistant porous membrane facing an edge portion in a longitudinal direction of the electrode have a higher binder content than an opposing portion C of the heat-resistant porous membrane facing a central portion of the electrode.

2. the content of the binder relative to the mass of all components contained in at least a portion of the opposing portion A of the heat-resistant porous membrane and the content of the binder relative to the mass of all components contained in at least a portion of the opposing portion B of the heat-resistant porous membrane are each 5% by mass to 15% by mass, 2. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the binder contained in the opposing portion C of the heat-resistant porous membrane is 1% by mass to 10% by mass.

3. 3. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the heat-resistant porous membrane is disposed on both sides of the porous substrate.

4. A non-aqueous electrolyte secondary battery comprising the electrode and the separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3.

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