Separator for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

This separator (13) for a non-aqueous electrolyte secondary battery comprises a base layer (50) and a filler layer (52) disposed on the surface of the base layer (50), wherein the separator (13) is characterized in that the filler layer (52) is made of an inorganic material and includes porous particles (54) having pores with a pore diameter of 10-50 nm.

Description

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

[0001] The present disclosure relates to a separator for a non-aqueous electrolyte secondary battery and 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 not only retains the non-aqueous electrolyte but also isolates the positive and negative electrodes to prevent internal short circuits. Patent Document 1 discloses a separator for non-aqueous electrolyte secondary batteries in which a filler layer containing inorganic particles is disposed on the surface of a substrate layer made of a porous membrane.

[0003] Patent No. 4243323

[0004] However, as a result of investigations by the present inventors, it was found that in conventional filler layers including those described in Patent Document 1, the gaps between the inorganic particles are larger than the pore size of the base layer, and therefore the non-aqueous electrolyte may not be sufficiently retained on the surface of the separator. If the non-aqueous electrolyte cannot be sufficiently retained on the surface of the separator, there is a risk of deterioration in cycle characteristics when high-rate charge / discharge is repeated.

[0005] A separator for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a separator for a non-aqueous electrolyte secondary battery including a base material layer and a filler layer disposed on a surface of the base material layer, wherein the filler layer is made of an inorganic material and contains porous particles having pores with a pore size of 10 nm or more and 50 nm or less.

[0006] According to a separator for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, the non-aqueous electrolyte can be sufficiently retained on the surface of the separator, thereby improving cycle characteristics during repeated high-rate charge and discharge.

[0007] 1 is an axial cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention, and is a radial cross-sectional view of an electrode assembly according to an embodiment of the present invention, showing an enlarged view of the vicinity of a separator.

[0008] Hereinafter, with reference to the drawings, an example of an embodiment of a separator for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the same will be described in detail. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0009] 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.

[0010] FIG. 1 is an axial cross-sectional view of a 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 that is open 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 nonaqueous electrolyte secondary battery 10 will be referred to as the "top" and the bottom side of the exterior body 16 will be referred to as the "bottom."

[0011] 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.

[0012] As the non-aqueous solvent (organic solvent) of the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and two or more of these solvents can be mixed and used. When two or more solvents are mixed and used, it is preferable to use a mixed solvent containing a cyclic carbonate and a chain carbonate. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc. can be used as the chain carbonate. As the electrolyte salt of the non-aqueous electrolyte, LiPF ６ , LiBF ４ , LiCF ３ SO ３ The amount of electrolyte salt dissolved in the non-aqueous solvent can be, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0013] 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 through a through-hole in the center of 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 surface of the bottom of the exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0014] 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 upper 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.

[0015] 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 through a vent hole in the cap 27.

[0016] Next, the positive electrode 11 and the negative electrode 12 that constitute the electrode body 14 will be described.

[0017] [Positive Electrode] As shown in FIG. 1 , the positive electrode 11 has a positive electrode current collector 30 and a positive electrode mixture layer 32 formed on the positive electrode current collector 30. The positive electrode current collector 30 can be 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 its surface. The positive electrode mixture layer 32 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., onto the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode mixture layer 32 on both sides of the positive electrode current collector 30.

[0018] The positive electrode mixture layer 32 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.

[0019] Examples of the conductive agent contained in the positive electrode mixture layer 32 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 32 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.

[0020] [Negative Electrode] As shown in FIG. 1 , the negative electrode 12 has a negative electrode current collector 40 and a negative electrode mixture layer 42 formed on the negative electrode current collector 40. The negative electrode current collector 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 its surface. The negative electrode mixture layer 42 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 current collector 40, drying the coating, and then compressing it to form the negative electrode mixture layer 42 on both sides of the negative electrode current collector 40.

[0021] The negative electrode mixture layer 42 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 42 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.

[0022] 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 average particle size (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.

[0023] 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.

[0024] 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 42.

[0025] 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.

[0026] As with the positive electrode mixture layer 32, the binder contained in the negative electrode mixture layer 42 can be a fluorine-containing resin, PAN, polyimide, acrylic resin, polyolefin, or the like, but styrene butadiene rubber (SBR) is preferably used. The negative electrode mixture layer 42 preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use a combination of SBR with CMC or a salt thereof, PAA or a salt thereof, or the like. The negative electrode mixture layer 42 may also contain a conductive agent such as carbon nanotubes (CNT).

[0027] Next, the separator 13 that constitutes the electrode assembly 14 will be described with further reference to Fig. 2. Fig. 2 is a radial cross-sectional view of the electrode assembly 14, showing an enlarged view of the separator 13 and its vicinity.

[0028] [Separator] As shown in Fig. 2 , the separator 13 has a substrate layer 50 and a filler layer 52 disposed on the surface of the substrate layer 50 facing the positive electrode 11. In this embodiment, the filler layer 52 is disposed on only one side of the substrate layer 50, with the filler layer 52 facing the positive electrode 11 and the substrate layer 50 facing the negative electrode 12. Note that the form of the separator 13 is not limited to the example shown in Fig. 2 , and the filler layer 52 may be disposed on both sides of the substrate layer 50. By disposing the filler layer 52, 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.

[0029] The substrate layer 50 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. 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.

[0030] The thickness of the base 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.

[0031] The porosity of the substrate layer 50 is preferably, for example, 30% or more and 70% or less in order to ensure ionic conductivity during charging and discharging of the battery. The porosity of the substrate layer 50 is measured by the following method. (1) Ten circular pieces with a diameter of 2 cm are punched out of the substrate layer 50, and the thickness h and mass w of the center of each of the punched pieces of the substrate layer 50 are measured. (2) The volume V and mass W of the 10 pieces are determined 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 substrate.

[0032] The average pore diameter of the substrate layer 50 is, for example, 20 nm or more, preferably 30 nm or more, and more preferably 40 nm or more, from the viewpoint of ensuring ionic conductivity during charge and discharge of the battery. Furthermore, the average pore diameter of the substrate layer 50 is, for example, 100 nm or less, preferably 90 nm or less, and more preferably 80 nm or less, from the viewpoint of suppressing internal short circuits. Therefore, the average pore diameter of the substrate layer 50 is, for example, 20 nm or more and 100 nm or less, preferably 30 nm or more and 90 nm or less, and more preferably 40 nm or more and 80 nm or less. The average pore diameter of the substrate layer 50 can be measured by a nitrogen adsorption method (BJH method) using a gas adsorption measurement device (for example, BELSORP-MAX, manufactured by Microtrac-Bell Corporation).

[0033] The filler layer 52 includes inorganic particles 54 and a binder. The filler layer 52 is a layer formed by aggregating the inorganic particles 54, and is arranged over the entire surface of the base layer 50 facing the positive electrode 11. In the filler layer 52, gaps through which lithium ions pass are formed between the inorganic particles 54. In the example shown in FIG. 2 , the inorganic particles 54 are stacked in approximately two layers in the thickness direction of the separator 13, but this example is not limiting. The inorganic particles 54 may be present in only one layer in the thickness direction of the separator 13, or may be stacked in two or more layers.

[0034] The thickness of the filler layer 52 is preferably smaller than the thickness of the base layer 50, for example, 0.5 μm or more and 10.0 μm or less, and preferably 1.0 μm or more and 5.0 μm or less.

[0035] The content of the inorganic particles 54 in the filler layer 52 is, for example, 70% by mass or more and 99% by mass or less, preferably 80% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 99% by mass or less, relative to the total mass of the filler layer 52.

[0036] Here, the inorganic particles 54 include porous particles having a plurality of voids. The porous particles have at least pores with a pore size of 10 nm or more and 50 nm or less. As a result of studies by the present inventors, it has become clear that by disposing porous particles having pores with a pore size of 10 nm or more and 50 nm or less in the filler layer 52, the cycle characteristics during repeated high-rate charge and discharge are improved. Although the detailed mechanism is unclear, it is presumed that the nonaqueous electrolyte is retained in the voids of the porous particles, thereby allowing the nonaqueous electrolyte to be sufficiently retained on the surface of the separator 13. In other words, in a filler layer 52 in which only inorganic particles 54 without voids are disposed, the nonaqueous electrolyte cannot be sufficiently retained on the surface of the separator 13, resulting in a decrease in the cycle characteristics during repeated high-rate charge and discharge.

[0037] The pore size of the porous particles may be 10 nm or more, preferably 15 nm or more, and more preferably 20 nm or more. If the pore size of the porous particles is less than 10 nm, the amount of non-aqueous electrolyte retained in the pores will be reduced. Furthermore, the pore size of the porous particles may be 50 nm or less, preferably 45 nm or less, and more preferably 40 nm or less. If the pore size of the porous particles exceeds 50 nm, the non-aqueous electrolyte may not remain in the pores, and the non-aqueous electrolyte may not be sufficiently retained. Therefore, the pore size of the porous particles is preferably 15 nm or more and 45 nm or less, and more preferably 20 nm or more and 40 nm or less. The pore size of the porous particles can be measured by the nitrogen adsorption method (BJH method) using a gas adsorption measurement device (for example, Microtrac-Bell Corporation, product name: BELSORP-MAX).

[0038] The porous particles may have pores with diameters of less than 10 nm or more than 50 nm, in addition to pores with diameters of 10 nm or more and 50 nm or less. In this case, in the pore size distribution of the porous particles, the volume of pores with diameters of 10 nm or more and 50 nm or less is preferably 50% or more, more preferably 75% or more, of the total pore volume.

[0039] The inorganic particles 54 may be composed of only porous particles, or may be a mixture of porous particles and particles other than porous particles. The content of porous particles in the inorganic particles 54 is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more, relative to the total mass of the inorganic particles 54. Note that particles other than porous particles contained in the inorganic particles 54 refer to particles that do not have pores with a pore diameter of 10 nm or more and 50 nm or less.

[0040] The material of the porous particles is, for example, silica (SiO ２ ), alumina (Al ２ O ３ ), titania (TiO ２ ), magnesia (MgO), and zeolite. These may be used alone or in combination of two or more.

[0041] The volume-based average particle size (D50) of the porous particles (inorganic particles 54) is, for example, 0.05 μm or more and 2 μm or less. The volume-based average particle size (D50) of the porous particles (inorganic particles 54) refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of the porous 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.

[0042] The binder contained in the filler layer 52 functions to bond the inorganic particles 54 serving as fillers to each other and to bond the fillers to the base material layer 50. The binder 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.

[0043] In addition to the inorganic particles 54, the filler layer 52 may contain resin particles having an average particle size larger than that of the inorganic particles 54. The resin particles are preferably arranged so as to protrude from the inorganic particle layer formed by the inorganic particles 54, forming convex portions on the surface of the separator 13. When the filler layer 52 contains the resin particles, an uneven structure is formed on the surface of the separator 13, and when the separator 13 is incorporated into the non-aqueous electrolyte secondary battery 10, a gap is generated between the positive electrode 11 and the separator 13. This reduces internal stress applied to the positive electrode 11 when expansion and contraction of the positive electrode 11 or the negative electrode 12 occurs during charge and discharge, for example, and deformation of the positive electrode 11 can be suppressed.

[0044] Examples of the material for the resin particles include acrylic resins made from ethylenically unsaturated carboxylic acid alkyl esters such as methyl acrylate, butyl acrylate, ethyl acrylate, and 2-ethylhexyl acrylate; resins made from cyano group-containing ethylenically unsaturated monomers such as acrylonitrile; and resins made from ethylenically unsaturated carboxylic acids and salts thereof such as acrylic acid, methacrylic acid, and maleic acid.

[0045] The resin particles have adhesive properties with the positive electrode 11, and in the nonaqueous electrolyte secondary battery 10, the resin particles are preferably adhered to the positive electrode 11. By the resin particles adhering to the positive electrode 11, movement of the positive electrode 11 due to charge and discharge is suppressed, and peeling of the positive electrode mixture layer 32 can be suppressed. The resin particles may exhibit adhesive properties to the positive electrode 11, for example, when a nonaqueous electrolyte is held therein.

[0046] The volume-based average particle size (D50) of the resin particles is, for example, 2.0 μm or more and 20.0 μm or less, and preferably 2.0 μm or more and 10.0 μm or less. The volume-based average particle size (D50) of the resin particles, like the volume-based average particle size (D50) of the inorganic particles 54, refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the resin 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.

[0047] When the surface of the filler layer 52 is observed with a scanning electron microscope (SEM, for example, SU8220 manufactured by Hitachi High-Technologies Corporation), preferably 10 to 150 resin particles are detected in an area of ​​100 μm × 100 μm, and more preferably 40 to 150 resin particles are detected. This forms an appropriate uneven structure on the surface of the separator 13, which can alleviate internal stress caused by expansion and contraction of the positive electrode 11 and the negative electrode 12 when the nonaqueous electrolyte secondary battery 10 is charged and discharged. As a result, deformation of the positive electrode 11 can be suppressed.

[0048] Next, the pore size distribution of the separator 13 will be described. The pore size distribution of the separator 13 exhibits at least two peaks. Specifically, a peak (first peak) derived from the pores within the porous particles and a peak (second peak) derived from the pores (gaps) between the porous particles (inorganic particles 54) are shown. The first peak, for example, appears in a pore size range of 10 nm to 50 nm, and the second peak, for example, appears in a pore size range of 100 nm to 300 nm. The pore size distribution of the separator 13 can be measured by a nitrogen adsorption method (BJH method) using a gas adsorption measurement device (e.g., Quadrasorb evo manufactured by Anton Paar). The distribution can be shown in a graph with the pore size on the horizontal axis and the cumulative volume percentage (relative to 100% total pore volume) on the vertical axis.

[0049] As a result of investigations by the present inventors, it was found that there is a correlation between the magnitude of the first peak and the second peak and the cycle characteristics during repeated high-rate charge and discharge. Specifically, when the maximum value in the pore size distribution of the separator 13 in the range of pore size from 10 nm to 50 nm is defined as A and the maximum value in the range of pore size from 100 nm to 300 nm is defined as B, it was found that the cycle characteristics during repeated high-rate charge and discharge can be significantly improved by setting the ratio of A to B (A / B) to be 0.3 or more.

[0050] Although the detailed mechanism is unclear, it is presumed that by setting (A / B) to 0.3 or more, the ratio of pores between the porous particles (inorganic particles 54) can be reduced while ensuring the amount of nonaqueous electrolyte in the porous particles, thereby preventing excessive supply of nonaqueous electrolyte from the separator 13 to the positive electrode 11. In other words, when (A / B) is less than 0.3, i.e., when the second peak is larger than the first peak, excessive nonaqueous electrolyte may be supplied to the positive electrode 11 through the pores between the porous particles (inorganic particles 54). As a result, improvement in cycle characteristics may be insufficient when high-rate charge / discharge is repeated.

[0051] The above (A / B) is preferably 0.35 or more, more preferably 0.40 or more. By increasing (A / B), the cycle characteristics when high-rate charge / discharge is repeated can be further improved. The upper limit of (A / B) is not particularly limited, but is, for example, 2.0.

[0052] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0053] Example 1 [Fabrication of Positive Electrode] As a positive electrode active material, LiNi ０．８８ Co ０．０９ Al ０．０３ O ２ An 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. Then, a positive electrode current collector exposed portion in which the positive electrode mixture layer was not present and the positive electrode current collector surface was exposed was provided at one end of the approximately central portion in the longitudinal direction of the positive electrode, and an aluminum positive electrode lead was welded to the positive electrode current collector exposed portion.

[0054] [Preparation of Negative Electrode] 95 parts by mass of graphite, 5 parts by mass of silicon 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. Then, a negative electrode exposed portion in which the negative electrode mixture layer was not present and the surface of the negative electrode current collector 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.

[0055] [Preparation of Separator] Porous silica particles (porous particles) having an average particle size (D50) of 0.7 μm and an average pore size of 25 nm as inorganic particles and an acrylic acid ester-based binder emulsion were mixed in a solid content mass ratio of 100:3, and then an appropriate amount of water was added so that the solid content concentration was 10 mass % to prepare a first dispersion.

[0056] A 12 μm thick polyolefin porous substrate was used as the substrate layer. The first dispersion was applied to one surface of the porous substrate as the substrate layer. The coating was then dried in an oven at 50° C. for 4 hours to prepare a 3 μm thick filler layer.

[0057] The pore size distribution of the produced separator was measured by the nitrogen adsorption method (BJH method) using a gas adsorption measurement device (Quadrasorb evo manufactured by Anton Paar). The ratio (A / B) of the maximum value A in the pore size range of 10 nm or more and 50 nm or less to the maximum value B in the pore size range of 100 nm or more and 300 nm or less was 0.61.

[0058] [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.

[0059] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] A wound electrode assembly was fabricated by spirally winding the positive and negative electrodes with a separator interposed therebetween. At this time, the filler layer of the separator faced the positive electrode. Insulating plates were placed above and below the electrode assembly, and the electrode assembly was housed in an exterior case. The negative electrode tab was welded to the bottom of the cylindrical exterior case with a bottom, and the positive electrode tab was welded to a sealing member. After injecting a non-aqueous electrolyte into the exterior case, the opening of the exterior case was sealed with a sealing member via a gasket, completing the fabrication of a non-aqueous electrolyte secondary battery.

[0060] [High-Rate Charge-Discharge Cycle Test] The fabricated nonaqueous electrolyte secondary battery was charged to 4.2 V at a constant current of 0.7 C in a temperature environment of 25°C, and then charged at a constant voltage of 4.2 V until the current value reached 0.01 C. After a 1-hour rest, the battery was discharged to 2.5 V at a constant current of 0.5 C. This constituted one cycle, and the discharge capacity at 0.5 C after 100 cycles was measured. The capacity retention rate during high-rate charging was calculated using the following formula: Capacity retention rate [%] = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) × 100

[0061] Example 2 In the preparation of a separator, inorganic particles were mixed in a solid content mass ratio of 50:50:3, including porous silica particles (porous particles) having an average particle size (D50) of 0.7 μm and an average pore size of 25 nm, boehmite particles having an average particle size (D50) of 0.7 μm, and an acrylic acid ester-based binder emulsion. A second dispersion was then prepared by adding an appropriate amount of water to adjust the solid content to 10 mass %. A nonaqueous electrolyte secondary battery was prepared and evaluated in the same manner as in Example 1, except that the second dispersion was applied to one surface of a porous substrate serving as a substrate layer to form a filler layer.

[0062] The pore size distribution of the produced separator was measured by a nitrogen adsorption method (BJH method) using a gas adsorption measurement device (Quadrasorb evo manufactured by Anton Paar). The ratio (A / B) of the maximum value A in the pore size range of 10 nm or more and 50 nm or less to the maximum value B in the pore size range of 100 nm or more and 300 nm or less was 0.37.

[0063] Example 3 In preparing a separator, porous silica particles (porous particles) having an average particle size (D50) of 0.7 μm and an average pore size of 25 nm as inorganic particles, acrylic resin particles having an average particle size (D50) of 3.0 μm as resin particles, and an acrylic acid ester-based binder emulsion were mixed in a solid content mass ratio of 100:2:5, and then an appropriate amount of water was added to prepare a third dispersion so that the solid content concentration was 10 mass%. Then, a nonaqueous electrolyte secondary battery was prepared and evaluated in the same manner as in Example 1, except that the third dispersion was applied to one surface of a porous substrate serving as a substrate layer to prepare a filler layer.

[0064] The pore size distribution of the produced separator was measured by the nitrogen adsorption method (BJH method) using a gas adsorption measurement device (Quadrasorb evo manufactured by Anton Paar). The ratio (A / B) of the maximum value A in the pore size range of 10 nm or more and 50 nm or less to the maximum value B in the pore size range of 100 nm or more and 300 nm or less was 0.55.

[0065] Comparative Example 1 In preparing a separator, boehmite particles having an average particle size (D50) of 0.7 μm as inorganic particles and an acrylic acid ester-based binder emulsion were mixed in a solid content mass ratio of 100:3, and then an appropriate amount of water was added to obtain a solid content concentration of 10 mass% to prepare a fourth dispersion. A nonaqueous electrolyte secondary battery was prepared and evaluated in the same manner as in Example 1, except that the fourth dispersion was applied to one surface of a porous substrate serving as a substrate layer to prepare a filler layer. In other words, the filler layer of the separator of Comparative Example 1 did not contain porous particles.

[0066] The pore size distribution of the produced separator was measured by a nitrogen adsorption method (BJH method) using a gas adsorption measurement device (Quadrasorb ev, manufactured by Anton Paar). The ratio (A / B) of the maximum value A in the pore size range of 10 nm or more and 50 nm or less to the maximum value B in the pore size range of 100 nm or more and 300 nm or less was 0.21.

[0067] Comparative Example 2: In preparing a separator, boehmite particles having an average particle size (D50) of 0.7 μm as inorganic particles, acrylic resin particles having an average particle size (D50) of 3.0 μm as resin particles, and an acrylic acid ester-based binder emulsion were mixed in a solid content mass ratio of 100:2:5, and then an appropriate amount of water was added to obtain a solid content concentration of 10 mass% to prepare a fifth dispersion. A nonaqueous electrolyte secondary battery was prepared and evaluated in the same manner as in Example 1, except that the fifth dispersion was applied to one surface of a porous substrate serving as a base layer to prepare a filler layer. In other words, the filler layer of the separator of Comparative Example 2 did not contain porous particles.

[0068] The pore size distribution of the produced separator was measured by the nitrogen adsorption method (BJH method) using a gas adsorption measurement device (Quadrasorb evo manufactured by Anton Paar). The ratio (A / B) of the maximum value A in the pore size range of 10 nm or more and 50 nm or less to the maximum value B in the pore size range of 100 nm or more and 300 nm or less was 0.21.

[0069] Table 1 shows the capacity retention rates of the nonaqueous electrolyte secondary batteries of Examples and Comparative Examples in a high-rate charge-discharge cycle test. A higher capacity retention rate in the high-rate charge-discharge cycle test indicates improved cycle characteristics after repeated high-rate charge-discharge. Table 1 also shows the (A / B) ratio calculated from the pore size distribution of each separator.

[0070]

[0071] As shown in Table 1, the nonaqueous electrolyte secondary batteries of Examples had improved capacity retention rates compared to the nonaqueous electrolyte secondary batteries of Comparative Examples. This is presumably because the nonaqueous electrolyte was sufficiently retained on the surface of the separator due to the presence of porous particles having pores with a pore size of 10 nm or more and 50 nm or less in the filler layer.

[0072] The present disclosure is further described by the following embodiments. Aspect 1: A separator for a non-aqueous electrolyte secondary battery, comprising a substrate layer and a filler layer disposed on a surface of the substrate layer, wherein the filler layer is made of an inorganic material and contains porous particles having pores with a pore size of 10 nm or more and 50 nm or less. Aspect 2: The separator for a non-aqueous electrolyte secondary battery according to Aspect 1, wherein, in a pore size distribution of the separator, when the maximum value in the pore size range of 10 nm or more and 50 nm or less is defined as A and the maximum value in the pore size range of 100 nm or more and 300 nm or less is defined as B, the ratio of A to B (A / B) is 0.3 or more. Aspect 3: The separator for a non-aqueous electrolyte secondary battery according to Aspect 1 or 2, wherein the porous particles contain at least one selected from the group consisting of silica, alumina, titania, magnesia, and zeolite. Configuration 4: The separator for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the filler layer further contains resin particles. Configuration 5: The separator for a non-aqueous electrolyte secondary battery according to Configuration 4, wherein the resin particles have a volume-based average particle size of 2.0 μm or more and 20.0 μm or less. Configuration 6: The separator for a non-aqueous electrolyte secondary battery according to Configuration 4 or 5, wherein, when the surface of the filler layer is observed with a scanning electron microscope, 10 to 150 of the resin particles are detected in an area of ​​100 μm x 100 μm. Configuration 7: A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and the separator according to any one of Configurations 1 to 6.

[0073] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 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, 32 Positive electrode mixture layer, 40 Negative electrode core, 42 Negative electrode mixture layer, 50 Base layer, 52 Filler layer, 54 Inorganic particles (porous particles)

Claims

1. A separator for a non-aqueous electrolyte secondary battery, comprising: a base layer; and a filler layer disposed on a surface of the base layer, wherein the filler layer is made of an inorganic material and contains porous particles having pores with a pore size of 10 nm or more and 50 nm or less.

2. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein, in the pore size distribution of the separator, when the maximum value in the pore size range of 10 nm or more and 50 nm or less is defined as A and the maximum value in the pore size range of 100 nm or more and 300 nm or less is defined as B, the ratio of A to B (A / B) is 0.3 or more.

3. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the porous particles contain at least one material selected from the group consisting of silica, alumina, titania, magnesia, and zeolite.

4. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the filler layer further contains resin particles.

5. The separator for a non-aqueous electrolyte secondary battery according to claim 4, wherein the resin particles have a volume-based average particle size of 2.0 μm or more and 20.0 μm or less.

6. The separator for a non-aqueous electrolyte secondary battery according to claim 4, wherein, when the surface of said filler layer is observed with a scanning electron microscope, 10 to 150 of said resin particles are detected in an area of ​​100 μm x 100 μm.

7. A non-aqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode; a non-aqueous electrolyte; and the separator according to any one of claims 1 to 6.

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