Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery comprises: an electrode assembly (14) in which a positive electrode (11) and a negative electrode (12) are wound with a separator (13) therebetween; and a non-aqueous electrolyte. The separator (13) includes a first separator (50) disposed on one surface of the positive electrode (11) and a second separator (60) disposed on the other surface of the positive electrode (11). The first separator (50) has a first substrate layer (51) and a functional layer (52) disposed on a surface of the first substrate layer (51) facing the positive electrode (11). The functional layer (52) includes: a first heat-resistant layer (53) containing first inorganic particles; and resin particles (54) having an average particle size larger than the thickness of the first heat-resistant layer (53). The second separator (60) does not contain the resin particles (54).

Description

Nonaqueous electrolyte secondary battery

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

[0002] Non-aqueous electrolyte secondary batteries have been known that include an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween. The separator functions to retain the non-aqueous electrolyte while preventing contact between the positive electrode and the negative electrode. Patent Document 1 discloses a separator in which one surface of the separator is covered with a polymer adhesive layer.

[0003] Japanese Patent Application Laid-Open No. 2022-2216

[0004] As the capacity of non-aqueous electrolyte secondary batteries has increased in recent years, the volume change of electrode plates during charge and discharge has become larger. Accordingly, the electrode plates may expand during repeated charge and discharge. Furthermore, in order to improve output characteristics, etc., non-aqueous electrolyte secondary batteries are required to have reduced battery resistance. Conventional technologies, including those disclosed in Patent Document 1, still have significant room for improvement in terms of sufficiently reducing battery resistance while suppressing expansion of electrode plates.

[0005] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and a non-aqueous electrolyte, wherein the separator includes a first separator arranged on one surface side of the positive electrode and a second separator arranged on the other surface side of the positive electrode, the first separator having a first base material layer and a functional layer arranged on a surface of the first base material layer that faces the positive electrode, the functional layer including a first heat-resistant layer containing first inorganic particles and resin particles having an average particle size larger than the thickness of the first heat-resistant layer, and the second separator does not include resin particles.

[0006] According to the nonaqueous electrolyte secondary battery of one aspect of the present disclosure, it is possible to suppress the expansion of the electrode plates while sufficiently reducing the battery resistance.

[0007] 1 is an axial cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0008] 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.

[0009] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified as a nonaqueous electrolyte secondary battery, but the outer can of the battery is not limited to a cylindrical outer can. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery having a prismatic outer can, a coin battery having a coin-shaped outer can, or a pouch battery having an outer can made of a laminate sheet including a metal layer and a resin layer. 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 an electrode assembly 14, a nonaqueous electrolyte, and an outer can 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 outer can 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the nonaqueous electrolyte secondary battery 10 will be referred to as the "top" and the bottom side of the outer can 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] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0013] 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

[0014] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. 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. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0015] 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 outer can 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 outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0016] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 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 can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0017] 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.

[0018] Next, the positive electrode 11 and the negative electrode 12 that constitute the electrode body 14 will be described.

[0019] [Positive Electrode] As shown in FIG. 1 , the positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 32 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 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, and the like onto the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode mixture layer 32 on both sides of the positive electrode core 30.

[0020] 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.

[0021] 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.

[0022] [Negative Electrode] As shown in FIG. 1 , the negative electrode 12 has a negative electrode core 40 and a negative electrode mixture layer 42 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 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 core 40, drying the coating, and then compressing it to form the negative electrode mixture layer 42 on both sides of the negative electrode core 40.

[0023] The negative electrode mixture layer 42 preferably contains a carbon material and a silicon-containing material as negative electrode active materials. The inclusion of a silicon-containing material facilitates achieving a high capacity of the nonaqueous electrolyte secondary battery 10. Furthermore, silicon-containing materials undergo greater volumetric expansion and contraction than carbon materials during charge and discharge. Therefore, when the negative electrode mixture layer 42 contains a silicon-containing material, the electrode plate is more likely to elongate during charge and discharge. As will be described in more detail below, the separator 13 of the present disclosure suppresses elongation of the electrode plate during charge and discharge. Therefore, when the negative electrode mixture layer 42 contains a silicon-containing material, the effects of the present disclosure are more pronounced.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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).

[0029] Next, the separator 13 constituting the electrode assembly 14 will be described with reference to Fig. 2. Fig. 2 is an enlarged view of the electrode assembly 14 in Fig. 1, and is a diagram schematically illustrating the cross-sectional structure of the positive electrode 11, negative electrode 12, and separator 13 constituting the electrode assembly 14. For ease of explanation, Fig. 2 illustrates the positive electrode 11 and negative electrode 12 and the separator 13 with a gap therebetween.

[0030] 2 , the separator 13 includes a first separator 50 arranged on one surface side of the positive electrode 11 and a second separator 60 arranged on the other surface side of the positive electrode 11. In this embodiment, the first separator 50 is arranged on the inner surface side of the positive electrode 11, and the second separator 60 is arranged on the outer surface side of the positive electrode 11. That is, the first separator 50, the positive electrode 11, the second separator 60, and the negative electrode 12 are arranged in this order toward the radial outside of the electrode body 14.

[0031] The first separator 50 has a first base material layer 51 and a functional layer 52 disposed on the surface of the first base material layer 51 that faces the positive electrode 11. In this embodiment, the functional layer 52 is disposed only on the surface of the first base material layer 51 that faces the positive electrode 11, but the functional layer 52 may also be disposed on the surface of the first base material layer 51 that faces the negative electrode 12. In other words, the functional layer 52 may be disposed on both surfaces of the first base material layer 51.

[0032] The first substrate layer 51 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 first substrate layer 51 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 first substrate layer 51 may have a single-layer structure or a multi-layer structure.

[0033] The thickness of the first base layer 51 is preferably 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less.

[0034] From the viewpoint of ensuring the movement of lithium ions during charging and discharging of the battery, the porosity of the first substrate layer 51 is preferably, for example, 30% or more and 70% or less. The porosity of the first substrate layer 51 is measured by the following method. (1) Ten circular pieces with a diameter of 2 cm are punched out of the first substrate layer 51, and the thickness h and mass w of the center of each of the punched small pieces of the first substrate layer 51 are measured. (2) From the thickness h and mass w, the volume V and mass W of the 10 small pieces are determined, 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.

[0035] The functional layer 52 includes a first heat-resistant layer 53 containing first inorganic particles and resin particles 54 having an average particle size larger than the average thickness of the first heat-resistant layer 53. As a result, convex portions are formed on the surface of the functional layer 52 by the resin particles 54 protruding from the surface of the first heat-resistant layer 53. By providing the functional layer 52 on the surface of the first separator 50 facing the positive electrode 11, for example, the resin particles 54 are adhered to the positive electrode 11. As a result, even when charge and discharge are repeated, movement of the positive electrode 11 is suppressed, and elongation of the positive electrode 11 can be suppressed.

[0036] Furthermore, by providing the functional layer 52 on the surface of the first separator 50, an uneven structure is formed on the surface of the separator 13. This allows non-aqueous electrolyte to easily flow into the electrode assembly 14 through gaps in the uneven portion. As a result, even after repeated charge / discharge cycles, the amount of non-aqueous electrolyte inside the electrode assembly 14 is maintained, improving cycle characteristics. Furthermore, when an uneven structure is formed on the surface of the separator 13, internal stresses caused by expansion and contraction of the positive electrode 11 and the negative electrode 12 during charge / discharge tend to be alleviated. This makes it possible to suppress elongation of the positive electrode 11 and the negative electrode 12. In other words, a separator whose surface is covered with a polymer adhesive layer, as disclosed in Patent Document 1, cannot achieve the above-mentioned effect because an uneven shape cannot be formed on the separator surface.

[0037] Furthermore, since the movement of lithium ions is not hindered in the region where the resin particles 54 are not provided, it is possible to suppress an increase in battery resistance while suppressing elongation of the electrode plate by providing the functional layer 52 on the surface of the first separator 50. In other words, in the separator disclosed in Patent Document 1, the surface is covered with a polymer adhesive layer, which reduces the ionic conductivity of the separator and increases the battery resistance.

[0038] The first heat-resistant layer 53 is a layer containing first inorganic particles and a binder. The thickness of the first heat-resistant layer 53 is preferably smaller than the thickness of the first base layer 51, 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. The thickness of the first heat-resistant layer 53 can be measured, for example, from a cross-sectional image of the first separator 50 obtained with a scanning electron microscope (SEM, for example, SU8220 manufactured by Hitachi High-Technologies Corporation).

[0039] The first inorganic particles are composed of a metal compound whose main component is a metal other than an alkali metal or an alkaline earth metal. The metal compound constituting the first inorganic particles is preferably a metal compound having a lower oxidizing power than the lithium transition metal composite oxide used in the positive electrode active material. The metal compound contains at least one metal element selected from, for example, Al, Si, Ti, and Mn as the metal element other than an alkali metal or an alkaline earth metal.

[0040] Specific examples of the metal compound constituting the first inorganic particles include titanium oxide (titania), aluminum oxide (alumina), silicon oxide (silica), manganese oxide, aluminum hydroxide, boehmite, and Al ２ O ３ ・nH ２ Examples of the metal compound include aluminum oxide hydrates represented by the formula (I) and (II) where n is 1 to 3. The metal compound is, for example, at least one selected from metal oxides and metal hydroxides. Among these, the first inorganic particles include titanium oxide, aluminum oxide, aluminum hydroxide, boehmite, and Al ２ O ３ ・nH ２ It is preferable that the aluminum oxide composition contains at least one selected from aluminum oxide hydrates represented by the formula: O (n=1 to 3).

[0041] The volume-based average particle size (D50) of the first inorganic particles is, for example, 0.05 μm or more and 2 μm or less. The volume-based average particle size (D50) of the first inorganic particles 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 first inorganic particles can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II manufactured by Microtrac-Bell) using water as a dispersion medium.

[0042] The content of the first inorganic particles in the first heat-resistant layer 53 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 85% by mass or more and 99% by mass or less, relative to the total mass of the first heat-resistant layer 53.

[0043] The binder contained in the first heat-resistant layer 53 functions to bond the first inorganic particles to each other and to bond the first inorganic particles to the first base material layer 51. 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 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.

[0044] The resin particles 54 are made of a resin material and have an average particle size larger than the thickness of the first heat-resistant layer 53. The resin particles 54 preferably have adhesive properties to the positive electrode 11. When the non-aqueous electrolyte secondary battery 10 is assembled, the resin particles 54 may be adhered to the positive electrode 11. Furthermore, the resin particles 54 may exhibit adhesive properties to the positive electrode 11 when a non-aqueous electrolyte is held therein. Note that having adhesive properties means that when the first separator 50 (functional layer 52) and the positive electrode 11 are laminated and pressure-bonded together, the surfaces of the positive electrode 11 and the first separator 50 can be in contact with each other and cannot be separated.

[0045] The resin particles 54 are non-conductive particles, and are made of, for example, an acrylic resin made of an ethylenically unsaturated carboxylic acid alkyl ester such as methyl acrylate, butyl acrylate, ethyl acrylate, or 2-ethylhexyl acrylate, a resin made of a cyano group-containing ethylenically unsaturated monomer such as acrylonitrile, or a resin made of an ethylenically unsaturated carboxylic acid or a salt thereof such as acrylic acid, methacrylic acid, or maleic acid. These may be used alone or in combination of two or more types.

[0046] As described above, the volume-based average particle size (D50) of the resin particles 54 is larger than the thickness of the first heat-resistant layer 53. The volume-based average particle size (D50) of the resin particles 54 is preferably 2.0 μm or more and 10.0 μm or less, and more preferably 3.0 μm or more and 7.0 μm or less. The volume-based average particle size (D50) of the resin 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 referred to as the median diameter. The particle size distribution of the resin particles 54 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] In the functional layer 52, the mass ratio of the resin particles 54 to the total mass of the first heat-resistant layer 53 is preferably 0.5% by mass or more, and more preferably 1% by mass or more. In this case, the number of resin particles 54 arranged on the surface of the first separator 50 increases, thereby further suppressing the elongation of the electrode plate. Furthermore, the mass ratio of the resin particles 54 to the total mass of the first heat-resistant layer 53 is preferably 30% by mass or less, and more preferably 25% by mass or less. In this case, it is easy to ensure the ionic conductivity of the first separator 50, and an increase in the battery resistance of the nonaqueous electrolyte secondary battery 10 can be suppressed. Therefore, the mass ratio of the resin particles 54 to the total mass of the first heat-resistant layer 53 is preferably 0.5% by mass or more and 30% by mass or less, and more preferably 1% by mass or more and 25% by mass or less.

[0048] Furthermore, when the surface of the functional layer 52 is observed with a scanning electron microscope (SEM, for example, SU8220 manufactured by Hitachi High-Technologies Corporation), it is preferable that 40 to 300 resin particles 54 are detected in an area of ​​100 μm × 100 μm, and it is more preferable that 60 to 250 resin particles are detected. In this case, it is possible to sufficiently reduce the battery resistance while suppressing the elongation of the electrode plate.

[0049] A method for forming the functional layer 52 containing the first heat-resistant layer 53 and the resin particles 54 on the surface of the first base layer 51 includes preparing a dispersion liquid in which the first inorganic particles, the resin particles 54, and a binder are dispersed, and applying and drying the dispersion liquid to the surface of the first base layer 51. Examples of methods for applying the dispersion liquid to the surface of the first base layer 51 include gravure coating, spraying, die coating, roll coating, reverse roll coating, screen printing, and inkjet printing, and among these, gravure coating is preferred.

[0050] As shown in Fig. 2, the second separator 60 has a second base material layer 61 and a second heat-resistant layer 62 disposed on the surface of the second base material layer 61 facing the positive electrode 11. In this embodiment, the second heat-resistant layer 62 is disposed only on the surface of the second base material layer 61 facing the positive electrode 11, but it may also be disposed on the surface of the second base material layer 61 facing the negative electrode 12. In other words, the second heat-resistant layer 62 may be disposed on both surfaces of the second base material layer 61. By providing the second heat-resistant layer, the shape of the second separator 60 can be maintained even when the battery generates abnormal heat, and the occurrence of an internal short circuit can be suppressed.

[0051] Here, unlike the first separator 50, the second separator 60 does not contain resin particles 54. As a result of studies by the present inventors, it was found that when both the first separator 50 and the second separator 60 contain resin particles 54, the expansion of the electrode plates can be suppressed, but the battery resistance of the nonaqueous electrolyte secondary battery 10 tends to increase. This is presumably because, when separators 13 containing resin particles 54 are disposed on both sides of the positive electrode 11, the resin particles 54 inhibit the movement of lithium ions. Therefore, when only the first separator 50 contains resin particles 54 and the second separator 60 does not contain resin particles 54, the battery resistance can be sufficiently reduced while the expansion of the electrode plates is suppressed.

[0052] The second substrate layer 61 can be made of the same material as the first substrate layer 51. The second substrate layer 61 may have a single-layer structure or a multi-layer structure. The thickness, porosity, and other shapes of the second substrate layer 61 can be the same as those of the first substrate layer 51.

[0053] The second heat-resistant layer 62 is, for example, a layer containing second inorganic particles and a binder. The thickness of the second heat-resistant layer 62 is preferably smaller than the thickness of the second base layer 61, 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. The thickness of the first heat-resistant layer 53 can be measured, for example, from a cross-sectional image of the second separator 60 obtained with a scanning electron microscope (SEM, for example, SU8220 manufactured by Hitachi High-Technologies Corporation). The second inorganic particles can have the same form as the first inorganic particles.

[0054] A method for forming the second heat-resistant layer 62 containing the second inorganic particles on the surface of the second base layer 61 includes preparing a dispersion liquid in which the second inorganic particles and a binder are dispersed, applying the dispersion liquid to the surface of the second base layer 61, and drying the dispersion liquid. Examples of methods for applying the dispersion liquid to the surface of the second base layer 61 include gravure coating, spraying, die coating, roll coating, reverse roll coating, screen printing, and inkjet printing, and among these, gravure coating is preferred.

[0055] The second heat-resistant layer may also contain an aramid resin. Since an aramid resin has excellent heat resistance, when the second heat-resistant layer contains an aramid resin, the shape of the second separator 60 can be easily maintained even when the battery generates abnormal heat.

[0056] As described above, the separator 13 of this embodiment includes the first separator 50 and the second separator 60. The first separator 50 has a functional layer 52 containing resin particles 54 on the surface facing the positive electrode 11. This suppresses movement of the positive electrode 11 and suppresses elongation of the electrode plate, even during repeated charge and discharge. Furthermore, the second separator 60 does not contain resin particles 54. This allows smooth movement of lithium ions, thereby sufficiently reducing the battery resistance of the nonaqueous electrolyte secondary battery 10. In other words, the nonaqueous electrolyte secondary battery 10 including the separator 13 of this embodiment can suppress elongation of the electrode plate while sufficiently reducing the battery resistance. Furthermore, because it is not necessary to use resin particles 54 in the second separator 60, the amount of resin particles 54 used can be reduced, thereby reducing production costs.

[0057] The above-described embodiment can be modified as appropriate without departing from the scope of the present disclosure. For example, in the above-described embodiment, the first separator 50 is disposed on the inner winding surface side of the positive electrode 11 and the second separator 60 is disposed on the outer winding surface side of the positive electrode 11. However, the second separator 60 may be disposed on the inner winding surface side of the positive electrode 11 and the first separator 50 may be disposed on the outer winding surface side of the positive electrode 11. Note that the surface pressure of the electrode assembly 14 tends to increase toward the inner winding surface. Therefore, when the first separator 50 is disposed on the inner winding surface side of the positive electrode 11 and the second separator 60 is disposed on the outer winding surface side of the positive electrode 11, elongation of the electrode plate can be further suppressed.

[0058] The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0059] Example 1 [Fabrication of Positive Electrode] As a positive electrode active material, LiNi ０．８８ Co ０．０９ Al ０．０３ O ２An aluminum-containing lithium nickel cobalt oxide represented by the formula (I) was used. The above positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a solids mass ratio of 100:1:0.9, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a strip-shaped positive electrode core 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 core. Then, a positive electrode core exposed portion in which no positive electrode mixture layer was present and the positive electrode core surface was exposed was provided in the longitudinal middle portion of the positive electrode. In addition, an aluminum positive electrode lead was welded to the positive electrode core exposed portion.

[0060] [Preparation of Negative Electrode] Graphite, silicon oxide (SiO), sodium carboxymethyl cellulose (CMC-Na), and styrene butadiene rubber (SBR) were mixed in a solids mass ratio of 88:12:1:1, 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 negative electrode core made of copper foil with a thickness of 19 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 core. Then, a negative electrode core exposed portion was provided at the inner end of the negative electrode winding, where the negative electrode mixture layer was not present and the negative electrode core surface was exposed. In addition, a nickel negative electrode lead was welded to the negative electrode core exposed portion.

[0061] [Preparation of First Separator] Boehmite particles as inorganic particles having an average particle size (D50) of 0.7 μm, acrylic resin particles as resin particles having a D50 of 3.0 μm, and an acrylic acid ester-based binder emulsion were mixed in a solid content mass ratio of 100:5:3, and then an appropriate amount of water was added to make the solid content concentration 10 mass % to prepare a first dispersion.

[0062] A polyethylene porous substrate having a thickness of 11 μm was used as the substrate layer. The first dispersion was applied to one side of the porous substrate as the substrate layer. The coating was then dried by heating in an oven at 50 ° C for 4 hours, and a functional layer was produced in which resin particles protruded from the surface of a 2.5 μm thick first heat-resistant layer formed by a binder. When the surface of the functional layer was observed with a scanning electron microscope (SU8220 manufactured by Hitachi High-Tech Corporation), 130 resin particles were detected in an area of ​​100 μm × 100 μm.

[0063] [Preparation of second separator] Boehmite particles as inorganic particles having an average particle size (D50) of 0.7 μm 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 make the solid content concentration 10 mass % to prepare a second dispersion.

[0064] A polyethylene porous substrate having a thickness of 11 μm was used as the substrate layer. The second dispersion was applied to one side of the porous substrate as the substrate layer. The coating was then dried in an oven at 50° C. for 4 hours to produce a second heat-resistant layer having a thickness of 2.5 μm and formed from the binder.

[0065] [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.

[0066] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] A wound electrode assembly was fabricated by spirally winding a positive electrode and a negative electrode with a separator interposed therebetween. The first separator was positioned on the inner surface of the positive electrode, and the second separator was positioned on the outer surface of the positive electrode. Insulating plates were placed on the top and bottom of the electrode assembly, 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 a non-aqueous 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.

[0067] [Evaluation of elongation rate of electrode plate (positive electrode)] The fabricated nonaqueous electrolyte secondary battery was subjected to X-ray CT imaging using an inspeXio SMX-255CT FPD HR manufactured by Shimadzu Corporation. The nonaqueous electrolyte secondary battery was then charged to 4.2 V at a constant current of 0.3 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 one-hour rest, the battery was discharged to 2.5 V at a constant current of 0.3 C. This constituted one cycle, and 100 cycles were performed.

[0068] Thereafter, X-ray CT images of the nonaqueous electrolyte secondary batteries after cycling were taken using an inspeXio SMX-255CT FPD HR manufactured by Shimadzu Corporation. Then, in the X-ray CT images after cycling, the width A1 of the positive electrode at the Xth turn from the innermost periphery, where the width of the positive electrode was maximum, was measured. Also, in the X-ray CT images before cycling, the width A2 of the positive electrode at the Xth turn from the innermost periphery was measured. Then, the elongation of the electrode plate was evaluated using the following formula: Elongation of electrode plate [%] = A1 / A2 × 100

[0069] [Evaluation of Battery Resistance] Battery resistance at a 30% state of charge (SOC 30%) was determined by AC impedance measurement in a 25°C environment. Specifically, AC impedance was measured for the nonaqueous electrolyte secondary battery using a Solartron 1255B (manufactured by Solartron) at an applied voltage of 10 mV and a measurement frequency range of 0.01 Hz to 100 kHz. A Nyquist diagram was plotted from the measurement data, and the battery resistance was determined from the size of the arc between 10 Hz and 0.1 Hz.

[0070] Example 2 A test cell was produced and evaluated in the same manner as in Example 1, except that in the production of a nonaqueous electrolyte secondary battery, the second separator was placed on the inner surface side of the wound positive electrode and the first separator was placed on the outer surface side of the wound positive electrode.

[0071] Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the first separator, boehmite particles as inorganic particles having an average particle size (D50) of 0.7 μm, acrylic resin particles as resin particles having a D50 of 3.0 μm, and an acrylic acid ester-based binder emulsion were mixed in a solids mass ratio of 100:8:3. When the surface of the functional layer was observed with a scanning electron microscope (SU8220 manufactured by Hitachi High-Technologies Corporation), 200 resin particles were detected in an area of ​​100 μm × 100 μm.

[0072] Example 4 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the first separator, boehmite particles as inorganic particles having an average particle size (D50) of 0.7 μm, acrylic resin particles as resin particles having a D50 of 3.0 μm, and an acrylic acid ester-based binder emulsion were mixed in a solids mass ratio of 100:3:3. When the surface of the functional layer was observed with a scanning electron microscope (SU8220 manufactured by Hitachi High-Technologies Corporation), 70 resin particles were detected in an area of ​​100 μm × 100 μm.

[0073] Example 5 A test cell was produced and evaluated in the same manner as in Example 1, except that in the production of the second separator, a solution of an aramid resin was applied to one surface of a porous substrate serving as a substrate layer and then dried to form a second heat-resistant layer containing an aramid resin.

[0074] Comparative Example 1 A test cell was fabricated and evaluated in the same manner as in Example 1, except that a first separator was used instead of a second separator in the fabrication of a nonaqueous electrolyte secondary battery. That is, in the nonaqueous electrolyte secondary battery of Comparative Example 1, first separators containing resin particles were disposed on both sides of the positive electrode.

[0075] Comparative Example 2 A test cell was fabricated and evaluated in the same manner as in Example 1, except that a second separator was used instead of the first separator in the fabrication of a nonaqueous electrolyte secondary battery. That is, in the nonaqueous electrolyte secondary battery of Comparative Example 2, second separators containing no resin particles were disposed on both sides of the positive electrode.

[0076] The evaluation results of the elongation percentage of the electrode plates and the battery resistance of the nonaqueous electrolyte secondary batteries of Examples and Comparative Examples are shown in Table 1. The battery resistance is a relative value when the battery resistance of the nonaqueous electrolyte secondary battery of Comparative Example 1 is set to 100.

[0077]

[0078] As shown in Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 to 5 were able to suppress plate elongation while sufficiently reducing battery resistance. On the other hand, the nonaqueous electrolyte secondary battery of Comparative Example 1, in which a first separator containing resin particles was disposed on both sides of the positive electrode, was able to suppress plate elongation, but the battery resistance increased. This is presumably because the resin particles inhibit the movement of lithium ions when separators containing resin particles are disposed on both sides of the positive electrode. Furthermore, the nonaqueous electrolyte secondary battery of Comparative Example 2, in which a second separator not containing resin particles was disposed on both sides of the positive electrode, had a reduced battery resistance but an increased plate elongation.

[0079] Furthermore, the nonaqueous electrolyte secondary battery of Example 1, in which a first separator containing resin particles is disposed on the inner surface side of the wound positive electrode, exhibits more suppressed elongation of the electrode plate than the nonaqueous electrolyte secondary battery of Example 2, in which a first separator containing resin particles is disposed on the outer surface side of the wound positive electrode. Therefore, it can be said that when a first separator containing resin particles is disposed on the inner surface side of the wound positive electrode, it is possible to further suppress the elongation of the electrode plate while sufficiently reducing the battery resistance.

[0080] The present disclosure is further described by the following embodiments. Aspect 1: A non-aqueous electrolyte secondary battery including an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and a non-aqueous electrolyte, wherein the separator includes a first separator disposed on one surface side of the positive electrode and a second separator disposed on the other surface side of the positive electrode, the first separator having a first base layer and a functional layer disposed on a surface of the first base layer facing the positive electrode, the functional layer including a first heat-resistant layer containing first inorganic particles and resin particles having an average particle size larger than a thickness of the first heat-resistant layer, and the second separator does not include the resin particles. Aspect 2: The non-aqueous electrolyte secondary battery according to Aspect 1, wherein the second separator has a second base layer and a second heat-resistant layer disposed on a surface of the second base layer facing the positive electrode, the second heat-resistant layer including at least one of second inorganic particles and an aramid resin. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the first separator is disposed on the inner surface side of the positive electrode and the second separator is disposed on the outer surface side of the positive electrode.Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein, when the surface of the functional layer is observed with a scanning electron microscope, 40 to 300 resin particles are detected in an area of ​​100 μm × 100 μm.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the resin particles have an average particle size of 2.0 μm to 10.0 μm.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the resin particles have an average particle size of 3.0 μm to 7.0 μm. The nonaqueous electrolyte secondary battery of any one of Configurations 1 to 6, wherein the ratio of the mass of the resin particles to the total mass of the first heat-resistant layer in the functional layer is 0.5 mass % or more and 30 mass % or less. The nonaqueous electrolyte secondary battery of any one of Configurations 1 to 7, wherein the resin particles are non-conductive particles.

[0081] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 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 First separator, 51 First base layer, 52 Functional layer, 53 First heat-resistant layer, 54 Resin particles, 60 Second separator, 61 Second base layer, 62 Second heat-resistant layer

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween; and a non-aqueous electrolyte, wherein the separator includes a first separator arranged on one surface side of the positive electrode and a second separator arranged on the other surface side of the positive electrode, the first separator having a first base material layer and a functional layer arranged on a surface of the first base material layer facing the positive electrode, the functional layer including a first heat-resistant layer containing first inorganic particles and resin particles having an average particle size larger than the thickness of the first heat-resistant layer, and the second separator does not include the resin particles.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the second separator has a second base material layer and a second heat-resistant layer disposed on a surface of the second base material layer facing the positive electrode, and the second heat-resistant layer contains at least one of second inorganic particles and aramid resin.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first separator is disposed on the inner surface side of the wound positive electrode, and the second separator is disposed on the outer surface side of the wound positive electrode.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein when the surface of the functional layer is observed with a scanning electron microscope, 40 to 300 of the resin particles are detected in an area of ​​100 μm×100 μm.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the resin particles have an average particle size of 2.0 μm or more and 10.0 μm or less.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the resin particles have an average particle size of 3.0 μm or more and 7.0 μm or less.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the functional layer, the ratio of the mass of the resin particles to the total mass of the first heat-resistant layer is 0.5 mass % or more and 30 mass % or less.

8. The non-aqueous electrolyte secondary battery according to claim 1, wherein the resin particles are non-conductive particles.

Images