Separator for nonaqueous electrolyte secondary battery

Abstract

A separator (13) for a nonaqueous electrolyte secondary battery as an embodiment of the present disclosure is characterized in that resin particles (52) are present on at least one surface, and the resin particles (52) have a 20% compressive strength of 1-100 MPa after being immersed in a solvent comprising dimethyl carbonate at 25°C for 3 minutes.

Description

Separator for non-aqueous electrolyte secondary battery

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

[0002] In recent years, non-aqueous electrolyte secondary batteries have been widely used as high-power, high-energy density secondary batteries. These batteries include an electrode assembly having a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes, a non-aqueous electrolyte, and an exterior housing that houses these. The separator separates the positive electrode and the negative electrode while retaining the non-aqueous electrolyte. Patent Document 1 discloses a separator for non-aqueous electrolyte secondary batteries, in which a functional layer containing heat-resistant fine particles and resin particles is provided on the surface of a substrate layer made of a porous membrane in order to improve adhesion at low temperatures.

[0003] International Publication No. 2021 / 161842

[0004] As a result of investigations by the present inventors, it was found that when conventional separators provided with resin particles, including those disclosed in Patent Document 1, are used in non-aqueous electrolyte secondary batteries, the battery resistance increases and the charge / discharge cycle characteristics deteriorate.

[0005] A separator for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure is characterized in that resin particles are present on at least one surface thereof, and the resin particles have a 20% compressive strength of 1 MPa or more and 100 MPa or less after immersion in a solvent comprising dimethyl carbonate at 25°C for 3 minutes.

[0006] According to a separator for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, a non-aqueous electrolyte secondary battery can be provided that has reduced battery resistance and improved charge / discharge cycle characteristics.

[0007] It is a longitudinal cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment. It is a cross-sectional view of a separator according to an embodiment. It is a cross-sectional view of a separator according to another embodiment. It is a view for explaining a method of evaluating deformation of an electrode plate.

[0008] As described above, separators having resin particles disposed on their surfaces have been known. By disposing resin particles on the surface of the separator, an uneven structure is formed on the surface of the separator, and when the separator is incorporated into a non-aqueous electrolyte secondary battery, gaps are formed between the electrode plate and the separator. This reduces internal stress applied to the electrode plate when the electrode plate expands or contracts during charge and discharge, for example, and suppresses deformation of the electrode plate. Here, deformation of the electrode plate refers to bending of at least a portion of the electrode plate. However, when known separators having resin particles disposed on their surfaces are used in non-aqueous electrolyte secondary batteries, new problems arise: increased battery resistance and reduced charge / discharge cycle characteristics.

[0009] After extensive investigation, the inventors discovered that the increase in battery resistance and the deterioration of charge-discharge cycle performance are due to the deformation of resin particles when the separator is incorporated into a nonaqueous electrolyte secondary battery and during repeated charge-discharge cycles, resulting in clogging of the separator. Specifically, when the separator is compressed in the stacking direction during electrode assembly fabrication and when the separator is compressed in the stacking direction due to expansion of the electrode plates during charge-discharge cycles, the pressure load on the resin particles increases, causing the resin particles to compress and deform. The compressed and deformed resin particles then clog the pores of the separator, inhibiting the movement of Li ions, resulting in an increase in battery resistance and a deterioration of charge-discharge cycle performance.

[0010] Therefore, the inventors conducted further studies and found that by using a separator having resin particles on its surface that have a 20% compressive strength of 1 MPa or more and 100 MPa or less after immersion in a solvent made of dimethyl carbonate for 3 minutes at 25° C., deformation of the resin particles contained in the separator is suppressed, and an increase in battery resistance and a deterioration in charge-discharge cycle characteristics are suppressed. Note that, in this specification, the 20% compressive strength of the resin particles means the pressure required to compress the resin particles by 20% relative to their particle diameter, and can be measured by the method described below.

[0011] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0012] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical, bottomed exterior body 16 is exemplified as a nonaqueous electrolyte secondary battery; however, the exterior body of the battery is not limited to a cylindrical exterior body. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic exterior body, a coin-type battery with a coin-type exterior body, or a pouch-type battery with an exterior body composed of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, but may also be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween. Furthermore, the design of the nonaqueous electrolyte secondary battery according to the present disclosure is not limited to the design of the exemplified nonaqueous electrolyte secondary battery, and known nonaqueous electrolyte secondary battery designs may also be applied.

[0013] FIG. 1 is an axial cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an exterior body 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The exterior body 16 is a cylindrical metal container with a bottom and an opening on one axial side, and the opening of the exterior body 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the sealing body 17 side of the battery will be referred to as the top, and the bottom side of the exterior body 16 will be referred to as the bottom.

[0014] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all rectangular, elongated bodies that are spirally wound in the longitudinal direction and stacked alternately in the radial direction of the electrode assembly 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. Two separators 13 are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the lateral direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the lateral end faces of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode assembly 14.

[0015] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity) and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0016] The liquid electrolyte (electrolytic solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0017] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. Examples of the matrix polymer include a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.).

[0018] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the exterior body 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 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 internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0019] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0020] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 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 at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, 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 breaks, and gas is discharged from the opening of the cap 27.

[0021] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14, and in particular the separator 13, will be described in detail below.

[0022] [Positive Electrode] As shown in FIG. 1 , the positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 formed on the positive electrode core 30. The positive electrode core 30 can be made of a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. to the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0023] The positive electrode mixture layer 31 contains particulate lithium metal composite oxide as a positive electrode active material. The lithium metal composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal element constituting the lithium metal composite oxide is, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, Al, and Mn. Examples of suitable composite oxides include lithium metal composite oxides containing Ni, Co, and Mn, and lithium metal composite oxides containing Ni, Co, and Al.

[0024] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, and other carbon materials. Examples of the binder contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and the like. Furthermore, these resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like.

[0025] [Negative Electrode] As shown in FIG. 1 , the negative electrode 12 has a negative electrode core 40 and a negative electrode mixture layer 41 formed on the negative electrode core 40. The negative electrode core 40 can be a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode mixture layer 41 contains a negative electrode active material, a binder, and, if necessary, a conductive agent. The negative electrode 12 can be produced by applying a negative electrode mixture slurry containing the negative electrode active material and the binder to the surface of the negative electrode core 40, drying the coating, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core 40.

[0026] The negative electrode mixture layer 41 preferably contains a carbon material and a silicon-containing material as the negative electrode active material. The inclusion of the silicon-containing material facilitates achieving a high capacity of the nonaqueous electrolyte secondary battery 10. For example, the negative electrode mixture layer 41 may use, as the negative electrode active material, a material containing at least one of an element that alloys with Li, such as Sn, and a material containing the element.

[0027] From the viewpoint of achieving high capacity, the content of the silicon-containing material is preferably 10% by mass or more, more preferably 12% by mass or more, and even more preferably 15% by mass or more of the total mass of the negative electrode active material. Generally, silicon-containing materials exhibit a larger volume change during charge and discharge than carbon materials. Therefore, when a silicon-containing material is included as the negative electrode active material, compressive stress is applied by the separator 13 during charge and discharge, making the resin particles 52 (see FIG. 2 ) more likely to be compressed and deformed. Therefore, when a silicon-containing material is included as the negative electrode active material, the effects of the present disclosure become more pronounced. The upper limit of the content of the silicon-containing material is, for example, 50% by mass of the total mass of the negative electrode active material.

[0028] The carbon material functioning as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, it is preferable to use at least artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, or a mixture thereof. The volume-based D50 of the carbon material is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less.

[0029] The silicon-containing material may be any material containing Si, and examples thereof include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. The D50 of the composite material is generally smaller than the D50 of graphite. The volume-based D50 of the composite material is, for example, 1 μm or more and 15 μm or less. One type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0030] A suitable silicon-containing material (composite material) is a composite particle including an ion-conducting phase, a Si phase dispersed in the ion-conducting phase, and a conductive layer covering the surface of the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, an amorphous carbon phase, a silicide phase, and a silicon oxide phase. The Si phase is formed by dispersing Si in the form of fine particles. The ion-conducting phase is a continuous phase composed of a collection of particles finer than the Si phase. The conductive layer is composed of a material with higher conductivity than the ion-conducting phase and forms a good conductive path in the negative electrode mixture layer 41.

[0031] An example of a suitable Si-containing composite material has a sea-island structure in which fine Si is dispersed almost uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The oxygen to silicon content (x) is, for example, 0.5≦x<2.0, preferably 0.8≦x≦1.5.

[0032] The binder contained in the negative electrode mixture layer 41 can be, as in the case of the positive electrode mixture layer 31, fluorine-containing resin, PAN, polyimide, acrylic resin, polyolefin, etc., but styrene butadiene rubber (SBR) is preferably used. The negative electrode mixture layer 41 also preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. Among these, it is preferable to use SBR in combination with CMC or a salt thereof, PAA or a salt thereof, etc. The negative electrode mixture layer 41 may also contain a conductive agent such as CNT.

[0033] [Separator] Fig. 2 is a diagram schematically illustrating a portion of a cross section of the separator 13. As shown in Fig. 2, the separator 13 includes, for example, a base layer 50, a heat-resistant layer 51 disposed on one surface of the base layer 50, and resin particles 52 disposed on the surface of the heat-resistant layer 51. Hereinafter, for convenience of explanation, the surface of the separator 13 on which the heat-resistant layer 51 is disposed will be referred to as a first surface 13A, and the surface opposite the first surface 13A will be referred to as a second surface 13B. In this embodiment, the first surface 13A faces the positive electrode 11, and the second surface 13B faces the negative electrode 12. Note that the second surface 13B of the separator 13 may face the positive electrode 11.

[0034] The porosity of the separator 13 is, for example, 30% or more and 70% or less. The porosity of the separator 13 is determined by the porosity of the base layer 50. The porosity of the separator 13 is measured by the following method. (1) The separator 13 is punched out into a circle with a diameter of 2 cm, and the thickness h and mass w of the center of the punched piece of the separator 13 are measured. (2) The volume V and mass W of the piece are calculated from the thickness h and mass w, and the porosity ε is calculated using the following formula: Porosity ε (%) = ((ρV - W) / (ρV)) × 100, where ρ is the density of the material constituting the separator 13.

[0035] The substrate layer 50 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. The material of the substrate layer 50 is not particularly limited, but examples include polyolefins such as polyethylene, polypropylene, and copolymers of polyethylene and α-olefins, acrylic resins, polystyrene, polyesters, cellulose, polyimides, polyphenylene sulfide, polyether ether ketones, and fluororesins. The substrate layer 50 may have a single-layer structure or a multi-layer structure. The thickness of the substrate layer 50 is preferably 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less.

[0036] The heat-resistant layer 51 contains, for example, inorganic particles. The thickness of the heat-resistant layer 51 is preferably smaller than the thickness of the base layer 50, for example, 0.5 μm or more and 5 μm or less. By providing the heat-resistant layer 51, the shape of the separator 13 is maintained when the battery abnormally heats up, and the occurrence of an internal short circuit can be suppressed. The heat-resistant layer 51 may be provided on both sides of the base layer 50, but is preferably provided on one side of the base layer 50 from the viewpoint of productivity, etc.

[0037] The inorganic particles contained in the heat-resistant layer 51 are composed of insulating inorganic compounds that are resistant to melting and decomposition during abnormal battery heat generation. Examples of inorganic particles include metal oxide particles, metal nitride particles, metal fluoride particles, and metal carbide particles. The volume-based particle size (D50) of the inorganic particles is, for example, 0.05 μm or more and 2 μm or less. The volume-based particle size (D50) of the inorganic particles refers to the particle size at which the cumulative frequency of the smallest particle size in a volume-based particle size distribution is 50% from the smallest particle size, also known as the median diameter. The particle size distribution of the inorganic particles can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrac-Bell) using water as a dispersion medium. The volume-based particle size (D50) of the inorganic particles is a value measured before the separator 13 is incorporated into the nonaqueous electrolyte secondary battery 10.

[0038] 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. Inorganic particles include zeolite (M ２／ｎ O.Al ２ O ３ xSiO ２ ・yH ２ O, M is a metal element, n is the valence of M, x≧2, y≧0), porous aluminosilicates such as talc (Mg３ Si ４ O １０ (OH) ２ ), layered silicates such as barium titanate (BaTiO ３ ), strontium titanate (SrTiO ３ These may be used alone or in combination of two or more.

[0039] The content of inorganic particles in the heat-resistant layer 51 is, for example, 45% by mass or more and 95% by mass or less, preferably 50% by mass or more and 95% by mass or less, and more preferably 55% by mass or more and 95% by mass or less, relative to the total mass of the heat-resistant layer 51.

[0040] The heat-resistant layer 51 may contain a binder. The binder contained in the heat-resistant layer 51 is preferably a polymer material, and examples thereof include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), polyimide-based resins, polyamide-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.

[0041] Next, the resin particles 52 present on at least one surface of the separator 13 will be described in detail.

[0042] First, the properties of the resin particles 52 will be described. The resin particles 52 have a 20% compressive strength (hereinafter sometimes simply referred to as 20% compressive strength) of 1 MPa or more, preferably 10 MPa or more, and more preferably 20 MPa or more after immersion in a solvent made of dimethyl carbonate at 25°C for 3 minutes. By setting the 20% compressive strength of the resin particles 52 to 1 MPa or more, compressive deformation of the resin particles 52 is suppressed. As a result, clogging of the separator 13 is suppressed, and an increase in the battery resistance and a deterioration in the charge-discharge cycle characteristics of the non-aqueous electrolyte secondary battery 10 are suppressed. Note that the 20% compressive strength after immersion in a solvent made of dimethyl carbonate at 25°C for 3 minutes is used as an indicator of the strength of the resin particles 52 because the inventors have found a very good correlation between the 20% compressive strength of the resin particles 52 in this state and the ease of compressive deformation of the resin particles 52 when incorporated into a battery.

[0043] Furthermore, the resin particles 52 have a 20% compressive strength of 100 MPa or less. If the 20% compressive strength of the resin particles 52 exceeds 100 MPa, the deformation of the resin particles 52 becomes too small, resulting in increased stress on the electrode plate from the resin particles 52 during charging and discharging, and thus increased internal stress on the electrode plate. As a result, the electrode plate is deformed, which is likely to increase battery resistance and deteriorate charge / discharge cycle characteristics. Furthermore, the 20% compressive strength of the resin particles 52 is preferably 60 MPa or less, and more preferably 40 MPa or less. In this case, the stress on the electrode plate from the resin particles 52 during charging and discharging can be reduced, and deformation of the electrode plate can be sufficiently suppressed. In other words, if the 20% compressive strength of the resin particles 52 exceeds 40 MPa, the stress on the electrode plate from the resin particles 52 during charging and discharging tends to be increased. Therefore, the 20% compressive strength of the resin particles 52 is 1 MPa or more and 100 MPa or less, preferably 10 MPa or more and 60 MPa or less, and more preferably 20 MPa or more and 40 MPa or less. Note that the resin particles 52 may contain resin particles 52 having a 20% compressive strength of less than 1 MPa or more than 100 MPa, but the resin particles 52 are mainly composed of resin particles 52 having a 20% compressive strength of 1 MPa or more and 100 MPa or less. In this specification, the term "main component" refers to the component that accounts for the largest proportion by mass of the resin particles 52.

[0044] The 20% compression strength of the resin particles 52 is obtained by measurement under the following measurement conditions using a microcompression tester ("MCT-W201" manufactured by Shimadzu Corporation). Specifically, the deformation amount and load of each sample particle are measured when a load is applied to each sample particle at the following loading rate. The compressive fracture strength is then calculated by substituting the load (N) when the sample particle is compressed 20% of its particle diameter before deformation (particle diameter measured by a CCD camera) and the particle diameter of the sample particle before deformation into the following formula. The 20% compression strength of the resin particles 52 is a value measured after immersing the separator 13 in a solvent made of dimethyl carbonate at 25°C for 3 minutes before being incorporated into the nonaqueous electrolyte secondary battery 10. 20% compression strength (MPa) = load (N) / {π × (particle diameter (mm)) ２}

[0045] <Compression strength measurement conditions> Test temperature: room temperature (25°C) Upper pressure indenter: flat indenter with a diameter of 50 μm (material: diamond) Lower pressure plate: SKS flat plate Measurement mode: compression test Test load: minimum 10 mN, maximum 50 mN Load rate: minimum 0.178 mN / sec, maximum 0.221 mN / sec Full scale displacement: 20 μm

[0046] The resin particles 52 preferably have a volumetric particle size (D50) of 1.0 μm or more, more preferably 3.0 μm or more, and even more preferably 5.0 μm or more. When the resin particles 52 have a volumetric particle size (D50) of 1.0 μm or more, it becomes easier to form an uneven structure on the surface of the separator 13. As a result, when the electrode plate expands and contracts during charge and discharge, the internal stress applied to the electrode plate is further alleviated, and deformation of the electrode plate can be further suppressed. Furthermore, the resin particles 52 preferably have a volumetric particle size (D50) of 20.0 μm or less, more preferably 15.0 μm or less, and even more preferably 10.0 μm or less. When the resin particles 52 have a volumetric particle size (D50) of 20.0 μm or less, clogging of the separator 13 is suppressed, and an increase in battery resistance and a deterioration in charge and discharge cycle characteristics are further suppressed. Therefore, the volumetric particle size (D50) of the resin particles 52 is preferably 1.0 μm or more and 20.0 μm or less, more preferably 3.0 μm or more and 15.0 μm or less, and even more preferably 5.0 μm or more and 10.0 μm or less. The volumetric particle size (D50) of the resin particles 52, like the volumetric particle size (D50) of inorganic particles, refers to the particle size at which the cumulative frequency in the volumetric particle size distribution is 50% from the smallest particle size, and is also referred to as the median diameter. The particle size distribution of the resin particles 52 can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrac-Bell) using water as a dispersion medium. The volumetric particle size (D50) of the resin particles 52 is a value measured before the separator 13 is incorporated into the nonaqueous electrolyte secondary battery 10.

[0047] The glass transition temperature of the resin particles 52 is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 140°C or higher. In this case, it becomes easy to increase the 20% compressive strength of the resin particles 52, and the effects of the present disclosure are significantly exhibited. In other words, if the glass transition temperature of the resin particles 52 is less than 100°C, it becomes difficult to increase the 20% compressive strength of the resin particles 52 to 1 MPa or higher. The upper limit of the glass transition temperature of the resin particles 52 is, for example, 250°C.

[0048] The glass transition temperature of the resin particles 52 can be determined using a differential scanning calorimetry (DSC) measuring device (e.g., an EXSTAR DSC6220 manufactured by SII NanoTechnology Inc.). Specifically, the resin particles 52 are weighed into an aluminum pan, and a differential scanning calorimetry (DSC) curve is obtained by measuring the resin particles 52 under the conditions specified in JIS Z 8703 using an empty aluminum pan as a reference, within a measurement temperature range of −100°C to 200°C, at a heating rate of 10°C / min. The DSC curve is then heated to a temperature of 0.05 mW / min / mg or greater during this heating process. The glass transition temperature (°C) can then be determined from the intersection of the baseline immediately before the endothermic peak of the DSC curve at which the differential signal (DDSC) is 0.05 mW / min / mg or greater and the tangent to the DSC curve at the first inflection point that appears after the endothermic peak.

[0049] The 20% compressive strength, volume-based particle size (D50), and glass transition temperature of the resin particles 52 can be adjusted by the type and amount of metal hydroxide used in preparing the resin particles 52, as well as the preparation method and preparation conditions of the resin particles 52. Details of the metal hydroxide will be described below.

[0050] Here, examples of the monomer units contained in the resin particles 52 include aromatic monovinyl monomer units, crosslinkable monomer units, and (meth)acrylic acid alkyl ester monomer units. In this specification, when a polymer "contains a monomer unit," it means that "a polymer obtained using that monomer contains a structural unit derived from the monomer." In this specification, "(meth)acrylic" means acrylic and / or methacrylic.

[0051] Examples of aromatic monovinyl monomers capable of forming aromatic monovinyl monomer units include, but are not limited to, styrene, α-methylstyrene, butoxystyrene, vinylnaphthalene, etc. Among these, styrene is preferred. These aromatic monovinyl monomers may be used alone or in combination of two or more at any ratio.

[0052] Examples of crosslinkable monomers capable of forming crosslinkable monomer units include polyfunctional monomers having two or more polymerization reactive groups in the monomer. Examples of such polyfunctional monomers include (meth)acrylic acid allyl ester monomers such as allyl methacrylate; aromatic divinyl monomers such as divinylbenzene and divinylnaphthalene; di(meth)acrylic acid ester monomers such as diethylene glycol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, and 1,3-butylene glycol diacrylate; tri(meth)acrylic acid ester monomers such as trimethylolpropane trimethacrylate and trimethylolpropane triacrylate; and ethylenically unsaturated monomers containing an epoxy group such as allyl glycidyl ether and glycidyl methacrylate. Here, divinylbenzene is preferred as the aromatic divinyl monomer, ethylene glycol dimethacrylate is preferred as the di(meth)acrylic acid ester monomer, trimethylolpropane trimethacrylate is preferred as the tri(meth)acrylic acid ester monomer, and glycidyl methacrylate is preferred as the ethylenically unsaturated monomer containing an epoxy group. These crosslinkable monomers may be used alone or in combination of two or more in any ratio.

[0053] Here, the crosslinkable monomer unit is preferably at least one type of crosslinkable monomer unit selected from the group consisting of an aromatic divinyl monomer unit, a di(meth)acrylic acid ester monomer unit, a tri(meth)acrylic acid ester monomer unit, and an ethylenically unsaturated monomer unit containing an epoxy group. That is, the resin particles 52 are preferably composed of particles containing resin particles 52 containing at least one crosslinkable monomer unit selected from the group consisting of an aromatic divinyl monomer, a di(meth)acrylic acid ester monomer unit, a tri(meth)acrylic acid ester monomer unit, and an ethylenically unsaturated monomer unit containing an epoxy group, more preferably particles containing resin particles 52 containing at least one crosslinkable monomer unit selected from the group consisting of an aromatic divinyl monomer unit, a di(meth)acrylic acid ester monomer unit, and an ethylenically unsaturated monomer unit containing an epoxy group, even more preferably particles containing both an aromatic divinyl monomer unit or a di(meth)acrylic acid ester monomer unit and an ethylenically unsaturated monomer unit containing an epoxy group, and particularly preferably particles containing both a di(meth)acrylic acid ester monomer unit and an ethylenically unsaturated monomer unit containing an epoxy group.

[0054] The content of the crosslinkable monomer units in the resin particles 52 is preferably 2% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more, and is preferably 99% by mass or less, more preferably 75% by mass or less, and even more preferably 50% by mass or less, where the amount of all repeating units in the resin particles 52 is taken as 100% by mass. If the content of the crosslinkable monomer units is 2% by mass or more, an increase in battery resistance can be suppressed. On the other hand, if the content of the crosslinkable monomer units is 99% by mass or less, a deterioration in charge-discharge cycle characteristics can be suppressed.

[0055] Examples of (meth)acrylic acid alkyl ester monomers that can form the (meth)acrylic acid alkyl ester monomer units include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, butyl acrylates such as n-butyl acrylate and t-butyl acrylate, octyl acrylates such as pentyl acrylate, hexyl acrylate, heptyl acrylate and 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate, etc. Examples of the (meth)acrylic acid alkyl ester include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, butyl methacrylates such as n-butyl methacrylate and t-butyl methacrylate, octyl methacrylates such as pentyl methacrylate, hexyl methacrylate, heptyl methacrylate and 2-ethylhexyl methacrylate, and alkyl methacrylates such as nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate and stearyl methacrylate. Of these, n-butyl acrylate, 2-ethylhexyl acrylate and methyl methacrylate are preferred. These (meth)acrylic acid alkyl ester monomers may be used alone or in combination of two or more in any ratio.

[0056] Next, a method for preparing the resin particles 52 will be described. The resin particles 52 can be prepared by polymerizing a monomer composition containing the above-mentioned monomers in an aqueous solvent such as water. Here, the proportion of each monomer in the monomer composition is usually the same as the proportion of each monomer unit in the polymer that constitutes the resin particles 52.

[0057] The polymerization method is not particularly limited, and any of the following methods can be used: suspension polymerization, emulsion polymerization aggregation, pulverization, dissolution suspension, seed polymerization, etc. Among these, suspension polymerization is preferred because it allows for high productivity in producing the resin particles 52. Furthermore, any of the following reactions can be used as the polymerization reaction: radical polymerization, living radical polymerization, etc.

[0058] The monomer composition used in preparing particles containing polymer A may contain other additives such as a chain transfer agent, a polymerization regulator, a polymerization reaction retarder, a reactive fluidizing agent, a filler, a flame retardant, an antioxidant, and a colorant in any amount.

[0059] Here, a method for preparing the resin particles 52 by suspension polymerization will be described as an example, but the method for preparing the resin particles 52 is not limited to this.

[0060] (1) Preparation of Monomer Composition First, the monomers constituting the desired resin particles 52 and other compounding ingredients added as needed are mixed to prepare a monomer composition.

[0061] (2) Formation of droplets Next, the monomer composition is dispersed in water, a polymerization initiator is added, and then droplets of the monomer composition are formed. Here, the droplets can be formed, for example, by shearing and stirring the water containing the monomer composition using a disperser such as an emulsifying disperser.

[0062] Examples of the polymerization initiator include oil-soluble polymerization initiators such as t-butylperoxy-2-ethylhexanoate, azobisisobutyronitrile, etc. The polymerization initiator may be added after the monomer composition is dispersed in water and before droplets are formed, or may be added to the monomer composition before it is dispersed in water.

[0063] From the viewpoint of stabilizing the formed droplets of the monomer composition in water, it is preferable to form droplets of the monomer composition by adding a dispersion stabilizer to water. In this case, examples of the dispersion stabilizer that can be used include metal hydroxides such as magnesium hydroxide, calcium phosphate, and sodium dodecylbenzenesulfonate. Here, the dispersion stabilizer may be added in the form of a colloidal dispersion in which the dispersion stabilizer is dispersed in water.

[0064] As shown in FIG. 2 , resin particles 52 are present on the surface of the heat-resistant layer 51. In the example shown in FIG. 2 , the resin particles 52 are attached to the surface of the heat-resistant layer 51, and the entire resin particles 52 are present in a state exposed from the heat-resistant layer 51. The presence of the resin particles 52 on the surface of the heat-resistant layer 51 forms an uneven structure on the surface of the separator 13. This creates gaps between the positive electrode 11 and the separator 13 when the separator 13 is incorporated into a nonaqueous electrolyte secondary battery 10. As a result, when the electrode plate expands and contracts during charge and discharge, the internal stress applied to the electrode plate is alleviated, and deformation of the electrode plate is suppressed. The resin particles 52 are dispersed, for example, throughout the entire surface of the heat-resistant layer 51. Note that the resin particles 52 may be present on the second surface 13B instead of or in addition to the first surface 13A. Furthermore, a portion of the resin particles 52 may not be directly attached to the surface of the heat-resistant layer 51, but may be attached to the surface of the resin particles 52.

[0065] When viewed from the surface of the first surface 13A, the ratio of the area of ​​the region where the resin particles 52 are exposed to the entire area of ​​the first surface 13A is preferably 2% or more, more preferably 5% or more, and even more preferably 10% or more. In this case, it is possible to form an appropriately sized uneven structure on the first surface 13A of the separator 13. Furthermore, the ratio of the area of ​​the region where the resin particles 52 are exposed to the entire area of ​​the first surface 13A is preferably 30% or less, more preferably 25% or less, and even more preferably 20% or less. In this case, smooth movement of Li ions within the separator 13 is possible, and an increase in battery resistance is suppressed. Therefore, an example of a suitable range for the ratio of the area of ​​the region where the resin particles 52 are exposed to the entire area of ​​the first surface 13A is 2% or more and 30% or less, more preferably 5% or more and 25% or less, and even more preferably 10% or more and 20% or less. The ratio of the area of ​​the region where the resin particles 52 are exposed to the entire area of ​​the first surface 13A can be calculated using a laser microscope (for example, VK-X3000 manufactured by Keyence Corporation).

[0066] Examples of methods for causing the resin particles 52 to be present on the surface of the heat-resistant layer 51 include gravure coating, spraying, bar coating, die coating, knife coating, roll coating, reverse roll coating, screen printing, inkjet printing, lamination, and electrophotography, among which gravure coating is preferred. Alternatively, the resin particles 52 may be caused to be present on the surface of the heat-resistant layer 51 by preparing a liquid composition in which the resin particles 52 are dispersed in a dispersion medium, applying the liquid composition to the surface of the heat-resistant layer 51, and drying the coating.

[0067] In the embodiment shown in FIG. 2 , the resin particles 52 are present on the surface of the heat-resistant layer 51, but the arrangement of the resin particles 52 is not limited thereto. For example, as shown in FIG. 3 , a portion of the resin particles 52 may be present inside the heat-resistant layer 51. By making the particle size of the resin particles 52 larger than the thickness of the heat-resistant layer 51, the resin particles 52 protrude from the heat-resistant layer 51. This allows the resin particles 52 to be present on the surface of the separator 13, forming an uneven structure on the separator surface. Although it is preferable that all of the resin particles 52 protrude from the heat-resistant layer 51, at least some of the resin particles 52 may not protrude from the heat-resistant layer 51 but may be embedded in the heat-resistant layer 51. The heat-resistant layer 51 containing the resin particles 52 can be produced, for example, by applying a slurry composition containing inorganic particles, resin particles 52, a binder, and a dispersion medium to the surface of the substrate layer 50 and drying the coating.

[0068] 2 and 3, the heat-resistant layer 51 is disposed on one surface of the base layer 50, but the heat-resistant layer 51 does not have to be disposed. That is, the resin particles 52 may be attached directly to the surface of the base layer 50. Even in this case, an uneven structure caused by the resin particles 52 is formed on the surface of the separator 13, and the effects of the present disclosure can be exerted.

[0069] The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0070] Example 1 [Fabrication of Positive Electrode] As a positive electrode active material, LiNi ０．８８ Co ０．０９ Al ０．０３ O２ Aluminum-containing lithium nickel cobalt oxide represented by the formula (I) was used. 100 parts by mass of the positive electrode active material, 1 part by mass of acetylene black (AB), and 0.9 parts by mass of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a strip-shaped positive electrode current collector made of aluminum foil with a thickness of 15 μm, dried, rolled, and cut to a predetermined electrode plate size to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector. A positive electrode exposed portion in which the positive electrode mixture layer was not present and the current collector surface was exposed was provided in approximately the center of the positive electrode in the longitudinal direction, and an aluminum positive electrode lead was welded to the positive electrode exposed portion.

[0071] [Preparation of Negative Electrode] 95 parts by mass of graphite, 5 parts by mass of Si oxide (SiO), 1 part by mass of sodium carboxymethyl cellulose (CMC-Na), and 1 part by mass of styrene butadiene rubber (SBR) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a strip-shaped negative electrode current collector made of copper foil with a thickness of 8 μm, dried, rolled, and cut to a predetermined electrode plate size to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector. A negative electrode exposed portion in which the negative electrode mixture layer was not present and the current collector surface was exposed was provided at the inner end of the winding of the negative electrode, and a nickel negative electrode lead was welded to the negative electrode exposed portion.

[0072] [Preparation of Resin Particles] <Preparation of Monomer Composition> A monomer composition was prepared by mixing 85 parts of styrene as an aromatic monovinyl monomer, 5 parts of glycidyl methacrylate as a crosslinkable monomer, and 10 parts of ethylene glycol dimethacrylate.

[0073] <Preparation of Metal Hydroxide> A colloidal dispersion containing magnesium hydroxide as the metal hydroxide was prepared by gradually adding, with stirring, an aqueous solution prepared by dissolving 7.0 parts of sodium hydroxide in 50 parts of ion-exchanged water to an aqueous solution prepared by dissolving 10.0 parts of magnesium chloride in 200 parts of ion-exchanged water.

[0074] <Suspension Polymerization Method> Resin particles were prepared by suspension polymerization method. Specifically, the monomer composition obtained as described above was added to the colloidal dispersion containing the magnesium hydroxide, and after further stirring, 3.0 parts of t-butylperoxy-2-ethylhexanoate ("Perbutyl O" manufactured by NOF Corporation) was added as a polymerization initiator to obtain a mixed solution. The obtained mixed solution was subjected to high-shear stirring at 12,000 rpm for 1 minute using an in-line emulsifying disperser ("Cavitron" manufactured by Pacific Machinery Works, Ltd.) to form droplets of the monomer composition in the colloidal dispersion containing magnesium hydroxide.

[0075] The colloidal dispersion containing magnesium hydroxide in which droplets of the monomer composition were formed was placed in a reactor, and the temperature was raised to 90° C., whereupon a polymerization reaction was carried out for 5 hours. The resulting dispersion was purified by subjecting it to a reduced pressure treatment at 90° C. for 2 hours using an evaporator, to obtain an aqueous dispersion containing resin particles.

[0076] Further, while stirring the aqueous dispersion containing the resin particles, sulfuric acid was added dropwise at room temperature (25°C) and acid washing was performed until the pH became 6.5 or less. Next, filtration and separation were performed, and 500 parts of ion-exchanged water was added to the obtained solid matter to re-slurry it, and the water washing treatment (washing, filtration, and dehydration) was repeated 10 times. Then, filtration and separation were performed, and the obtained solid matter was placed in a container of a dryer and dried at 40°C for 48 hours to obtain dried resin particles (first resin particles).

[0077] [Preparation of separator] A porous polyethylene substrate having a thickness of 12 μm was used as the substrate layer. Alumina (α-Al ２ O ３ ) particles, an acrylic acid ester-based binder emulsion, and carboxymethyl cellulose as a thickener were mixed in a solid content mass ratio of 100:3:1.5, and then an appropriate amount of water was added to make the solid content concentration 40 mass% to prepare a dispersion. This dispersion was applied to the entire surface of the porous substrate as the substrate layer using a gravure coater set at a conveying speed of 1.5 m / min and a drying oven temperature of 40°C, and α-Al ２ O ３ A heat-resistant layer having a thickness of 3 μm containing the above was prepared.

[0078] Next, the first resin particles, an acrylic acid ester-based binder emulsion, and carboxymethyl cellulose as a thickener were mixed in a solids mass ratio of 100:10:10, and then an appropriate amount of water was added to prepare a dispersion so that the solids concentration was 10% by mass. This dispersion was applied to the entire surface of the heat-resistant layer using a gravure coater set at a conveying speed of 4 m / min and a drying oven temperature of 85 °C, thereby adhering the first resin particles to the surface of the heat-resistant layer. After immersion of the first resin particles in a solvent composed of dimethyl carbonate at 25 °C for 3 minutes, the 20% compressive strength was 20.5 MPa, the volume-based particle size (D50) was 5.0 μm, and the glass transition temperature was 124 °C. Furthermore, when the first surface of the separator was observed from the surface side using a laser microscope (Keyence Corporation VK-X3000), the ratio of the area of ​​the region where the first resin particles were exposed to the entire area of ​​the first surface was 10%.

[0079] [Preparation of non-aqueous electrolyte] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 3:7, and lithium hexafluorophosphate (LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.5 mol / L of ammonium hydroxide in water.

[0080] [Fabrication of a Non-Aqueous Electrolyte Secondary Battery] A wound electrode assembly was fabricated by spirally winding the positive and negative electrodes with a separator interposed therebetween, with the heat-resistant layer of the separator facing the positive electrode. Insulating plates were placed on the top and bottom of the electrode assembly, respectively, and the electrode assembly was housed in an outer can. The negative electrode lead was welded to the bottom of the cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member. After pouring the electrolyte into the outer can, the opening of the outer can was sealed with a sealing member via a gasket, completing the fabrication of a non-aqueous electrolyte secondary battery.

[0081] [Evaluation of Battery Resistance] The test cell was subjected to constant current charging at 0.2 C at a temperature of 25° C. until the battery voltage reached 4.2 V, and then constant voltage charging at 4.2 V until the current value reached 0.02 C. Subsequently, constant current discharging was performed at 1.0 C for 10 seconds, and the battery resistance was determined by dividing the voltage drop by the current value.

[0082] [Evaluation of Charge-Discharge Cycle Characteristics] The test cell was charged at a constant current of 0.2 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 C. Thereafter, the test cell was discharged at a constant current of 1 C until the battery voltage reached 2.5 V, and the discharge capacity at this time was taken as the initial discharge capacity. This charge-discharge cycle was counted as one cycle, and 200 cycles were repeated. The initial discharge capacity and the discharge capacity at the 200th cycle were determined, and the capacity retention rate was calculated using the following formula. A higher capacity retention rate indicates better charge-discharge cycle characteristics. Capacity retention rate (%) = Discharge capacity at 200th cycle / Initial discharge capacity × 100

[0083] [Evaluation of Plate Deformation] The test cell was charged at a constant current of 0.2 C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C. Subsequently, the battery was discharged at a constant current of 0.2 C until the battery voltage reached 2.5 V. This charge-discharge cycle was repeated 200 times, with a 20-minute rest period between each cycle. After 200 cycles, the nonaqueous electrolyte secondary battery was charged at a constant current of 0.2 C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C, resulting in a charged state. The nonaqueous electrolyte secondary battery in this charged state was observed for a cross section near the center of the winding of the electrode assembly using an X-ray CT scanner (Shimadzu Corporation, SMX-225CT FPD HR). Then, as shown in FIG. 4, if deformation (bending) of the electrode plate (at least one of the positive electrode and the negative electrode) occurred, the angle θ of plate deformation was confirmed. The degree of electrode plate deformation was classified into A, B, and C based on the following evaluation criteria: A: angle θ is 170° or more B: angle θ is more than 150° and less than 170° C: angle θ is 150° or less When deformation (bending) of the electrode plate (at least one of the positive electrode and negative electrode) was confirmed such that the angle θ is 150° or less, it was determined that electrode plate deformation was present, and the electrode plate deformation was evaluated.

[0084] Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that, instead of the first resin particles, 70 parts of styrene as an aromatic monovinyl monomer, and 5 parts of glycidyl methacrylate and 25 parts of ethylene glycol dimethacrylate as crosslinkable monomers were mixed to prepare a monomer composition and obtain resin particles (second resin particles). The second resin particles had a 20% compressive strength of 24.9 MPa after immersion in a solvent consisting of dimethyl carbonate at 25°C for 3 minutes, a volume-based particle size (D50) of 5.0 μm, and a glass transition temperature of 164°C. Furthermore, when the first surface of the separator was observed from the surface side using a laser microscope (Keyence Corporation VK-X3000), the ratio of the area of ​​the region where the second resin particles were exposed to the entire area of ​​the first surface was 10%.

[0085] Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that, instead of the first resin particles, 45 parts of styrene as an aromatic monovinyl monomer, and 5 parts of glycidyl methacrylate and 50 parts of ethylene glycol dimethacrylate as crosslinkable monomers were mixed to prepare a monomer composition and obtain resin particles (third resin particles). The third resin particles had a 20% compressive strength of 40.1 MPa after immersion in a solvent consisting of dimethyl carbonate at 25°C for 3 minutes, a volume-based particle size (D50) of 5.0 μm, and a glass transition temperature above the upper limit of the measurement temperature range (>200°C). Furthermore, when the first surface of the separator was observed from the surface side using a laser microscope (Keyence Corporation VK-X3000), the ratio of the area of ​​the region where the third resin particles were exposed to the entire area of ​​the first surface was 10%.

[0086] Comparative Example 1 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the separator, resin particles were not attached to the surface of the heat-resistant layer.

[0087] Comparative Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the resin particles, instead of the first resin particles, 68 parts of styrene as an aromatic monovinyl monomer, 26.5 parts of n-butyl acrylate as a (meth)acrylic acid alkyl ester monomer, and 5 parts of glycidyl methacrylate and 0.5 parts of ethylene glycol dimethacrylate as crosslinkable monomers were mixed to prepare a monomer composition and obtain resin particles (fourth resin particles). Note that the fourth resin particles had a 20% compressive strength of 0.0 MPa (below the device sensitivity) after immersion in a solvent consisting of dimethyl carbonate at 25°C for 3 minutes, a volume-based particle size (D50) of 5.0 µm, and a glass transition temperature of 61°C. Furthermore, when the first surface of the separator was observed from the surface side using a laser microscope (VK-X3000 manufactured by Keyence Corporation), the ratio of the area of ​​the region where the fourth resin particles were exposed to the entire area of ​​the first surface was 10%.

[0088] The evaluation results of the test cells of the examples and comparative examples are shown in Table 1. Table 1 also shows the 20% compressive strength, volume-based particle size (D50), and glass transition temperature of the resin particles after immersion in a solvent made of dimethyl carbonate at 25°C for 3 minutes. The battery resistance and capacity retention rate shown in Table 1 are shown relative to the battery resistance and capacity retention rate of the test cell of Comparative Example 1, which are set at 100. A larger value for battery resistance indicates a lower resistance, and a larger value for capacity retention rate indicates better charge-discharge cycle characteristics.

[0089]

[0090] As shown in Table 1, the test cells of the examples had improved battery resistance and capacity retention compared to the test cells of the comparative examples. In other words, by providing resin particles on the separator surface with a 20% compressive strength of 1 MPa or more after immersion in a dimethyl carbonate solvent at 25°C for 3 minutes, battery resistance was reduced, resulting in a nonaqueous electrolyte secondary battery with improved charge-discharge cycle characteristics. Furthermore, the test cell of Comparative Example 1, in which no resin particles were present on the separator surface, experienced plate deformation during charge and discharge. This is presumably due to the lack of a gap between the separator and the electrode plate, which increases internal stress when the electrode plate expands. Furthermore, the test cell of Comparative Example 2, in which resin particles on the separator surface with a 20% compressive strength of less than 1 MPa after immersion in a dimethyl carbonate solvent at 25°C for 3 minutes were present, did not experience plate deformation, but did experience increased battery resistance and a worsening capacity retention. This is presumably due to the compressive deformation of the resin particles as the electrode plate expanded, resulting in separator clogging.

[0091] The present disclosure is further described by the following embodiments. Aspect 1: A separator for a non-aqueous electrolyte secondary battery, wherein resin particles are present on at least one surface of the separator, and the resin particles have a 20% compressive strength of 1 MPa or more and 100 MPa or less after immersion in a solvent comprising dimethyl carbonate at 25° C. for 3 minutes. Aspect 2: The separator for a non-aqueous electrolyte secondary battery according to Aspect 1, wherein the resin particles have a 20% compressive strength of 10 MPa or more and 60 MPa or less after immersion in a solvent comprising dimethyl carbonate at 25° C. for 3 minutes. Aspect 3: The separator for a non-aqueous electrolyte secondary battery according to Aspect 1 or 2, wherein the resin particles have a 20% compressive strength of 20 MPa or more and 40 MPa or less after immersion in a solvent comprising dimethyl carbonate at 25° C. for 3 minutes. Configuration 4: The separator for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the separator has a base layer and a heat-resistant layer disposed on one surface of the base layer, the heat-resistant layer containing inorganic particles, and the resin particles are present in at least one of the interior of the heat-resistant layer, the surface of the heat-resistant layer, and the other surface of the base layer.Configuration 5: The separator for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the resin particles have a volume-based particle size (D50) of 1.0 μm or more and 20.0 μm or less.Configuration 6: The separator for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the resin particles have a glass transition temperature of 100° C. or more. Configuration 7: The separator for a nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein, in a surface view of the separator, the surface on which the resin particles are provided has an area in which the resin particles are exposed, and the area of ​​the region in which the resin particles are exposed has a ratio of 2% to 30% of the entire surface area of ​​the separator.

[0092] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 13A First surface, 13B Second surface, 14 Electrode body, 16 Exterior body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core, 31 Positive electrode mixture layer, 40 Negative electrode core, 41 Negative electrode mixture layer, 50 Base layer, 51 Heat-resistant layer, 52 Resin particles

Claims

1. A separator for a non-aqueous electrolyte secondary battery, comprising resin particles present on at least one surface of the separator, and the resin particles have a 20% compressive strength of 1 MPa or more and 100 MPa or less after immersion in a solvent comprising dimethyl carbonate at 25°C for 3 minutes.

2. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the resin particles have a 20% compressive strength of 10 MPa or more and 60 MPa or less after immersion in a solvent comprising dimethyl carbonate at 25°C for 3 minutes.

3. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the resin particles have a 20% compressive strength of 20 MPa or more and 40 MPa or less after immersion in a solvent comprising dimethyl carbonate at 25°C for 3 minutes.

4. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the separator comprises a base layer and a heat-resistant layer disposed on one surface of the base layer, the heat-resistant layer comprises inorganic particles, and the resin particles are present in at least one of the interior of the heat-resistant layer, the surface of the heat-resistant layer, and the other surface of the base layer.

5. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the resin particles have a volume-based particle size (D50) of 1.0 μm or more and 20.0 μm or less.

6. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the glass transition temperature of the resin particles is 100°C or higher.

7. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein, in a surface view of the surface of the separator on which the resin particles are provided, the ratio of the area of ​​the region where the resin particles are exposed to the area of ​​the entire surface of the separator is 2% or more and 30% or less.

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