Lithium secondary battery and separator

Abstract

This lithium secondary battery comprises an electrode group and a non-aqueous electrolyte, wherein: the electrode group includes a positive electrode, a negative electrode, and a separator; the positive electrode and the negative electrode are wound with the separator interposed therebetween; in the negative electrode, lithium metal is precipitated during charging, and the lithium metal is dissolved in the non-aqueous electrolyte during discharge; the separator has a first surface facing the outside of the electrode group and a second surface facing the inside of the electrode group; at least one of the first surface and the second surface has a first region having first projecting portions in a first pattern and a second region having second projecting portions in a second pattern different from the first pattern; and the second region is disposed further toward the outer circumferential side of the electrode group than the first region.

Description

Lithium secondary battery and separator CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2023-122847, filed on July 27, 2023, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to lithium secondary batteries and separators.

[0003] Lithium ion batteries are known as high-capacity non-aqueous electrolyte secondary batteries. Lithium secondary batteries (lithium metal secondary batteries) are promising non-aqueous electrolyte secondary batteries with capacities even higher than lithium ion batteries. In lithium secondary batteries, lithium metal precipitates on the negative electrode during charging, and dissolves during discharging, releasing lithium ions into the non-aqueous electrolyte.

[0004] In lithium secondary batteries, lithium metal is deposited on the negative electrode during charging, so it is necessary to ensure a space between the separator and the electrode.

[0005] Patent Document 1 proposes a lithium secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte having lithium ion conductivity, wherein lithium metal is deposited on the negative electrode during charging and the lithium metal dissolves from the negative electrode during discharging; a spacer is provided between at least one of the positive electrode and the negative electrode and the separator; a first length of the separator in a first direction D1 is smaller than a second length of the separator in a second direction D2 intersecting with the first direction D1; and in a cross section of the spacer taken along the thickness direction of the separator and the first direction D1, at least one of an angle formed between the separator and the spacer on the spacer side and an angle formed between the spacer and an electrode in contact with the spacer is greater than 90°.

[0006] International Publication No. 2021 / 192645 Pamphlet

[0007] When an electrode assembly is constructed by winding a positive electrode and a negative electrode with a separator interposed therebetween, the height of the space for depositing lithium may be insufficient on the inner periphery of the electrode assembly, which can cause buckling of the electrodes due to excessive stress on the inner periphery.

[0008] One aspect of the present disclosure relates to a lithium secondary battery comprising an electrode group and a non-aqueous electrolyte, the electrode group comprising a positive electrode, a negative electrode, and a separator, the positive electrode and the negative electrode being wound with the separator interposed therebetween, lithium metal being precipitated in the negative electrode during charging and the lithium metal being dissolved in the non-aqueous electrolyte during discharging, the separator having a first surface facing the outside of the electrode group and a second surface facing the inside of the electrode group, at least one of the first surface and the second surface having a first region having first convex portions in a first pattern and a second region having second convex portions in a second pattern different from the first pattern, the second region being disposed closer to the outer periphery of the electrode group than the first region.

[0009] Another aspect of the present disclosure relates to a separator that is disposed between a positive electrode and a negative electrode and wound to form an electrode group, the separator having a first surface facing the outside of the electrode group and a second surface facing the inside of the electrode group, at least one of the first surface and the second surface having a first region having first convex portions in a first pattern and a second region having second convex portions in a second pattern different from the first pattern, the second region being disposed closer to the outer periphery of the electrode group than the first region.

[0010] Yet another aspect of the present disclosure relates to a separator having an elongated shape having a length D1 in a first direction and a length D2 (D1<D2) in a second direction intersecting the first direction, the separator having a first surface and a second surface opposite the first surface, at least one of the first surface and the second surface having a first region having first convex portions in a first pattern and a second region having second convex portions in a second pattern different from the first pattern, the second region being positioned farther from one end in the second direction than the first region.

[0011] According to the present disclosure, it is easy to ensure a space height for depositing lithium metal on the inner circumferential side of a wound electrode group of a lithium secondary battery. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0012] 1 is a longitudinal sectional view schematically showing an example of a lithium secondary battery according to an embodiment of the present disclosure; FIG. 2 is a sectional view schematically showing a part of the inner circumferential side of the lithium secondary battery shown in FIG. 1; FIG. 3 is a sectional view schematically showing a part of the outer circumferential side of the lithium secondary battery shown in FIG. 1; FIG. 4 is a top view showing an example of a first convex portion and a second convex portion; FIG. 5 is an enlarged view of a part of FIG. 3; FIG. 6 is a top view showing another example of the first convex portion and the second convex portion; FIG. 7 is a top view showing yet another example of the first convex portion and the second convex portion; FIG. 8 is a top view showing yet another example of the first convex portion and the second convex portion.

[0013] Hereinafter, embodiments according to the present disclosure will be described using examples, but the embodiments according to the present disclosure are not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and other materials may be applied as long as the invention according to the present disclosure can be implemented. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit.

[0014] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0015] (Lithium Secondary Battery) A lithium secondary battery according to an embodiment of the present disclosure includes a wound electrode group and a non-aqueous electrolyte. The electrode group includes, for example, a long (strip-shaped) positive electrode, a negative electrode, and a separator, and the positive electrode and negative electrode are wound with the separator interposed therebetween. Lithium metal precipitates in the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharge. Specifically, the negative electrode includes at least a negative electrode current collector, and lithium metal precipitates on the negative electrode current collector. The non-aqueous electrolyte has lithium ion conductivity. Lithium secondary batteries are also called lithium metal secondary batteries.

[0016] In a lithium secondary battery, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (or current from another perspective) at the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode of a lithium secondary battery differs from a negative electrode in which the movement of electrons at the negative electrode during charging and discharging is mainly due to the absorption and release of lithium ions by the negative electrode active material (e.g., graphite).

[0017] (Separator) The separator may have an elongated shape having a length D1 in a first direction parallel to the winding axis and a length D2 in a second direction intersecting the first direction (D1<D2). The first direction may be perpendicular to the second direction. The separator has a first surface facing the outside (outer periphery) of the electrode group and a second surface facing the inside (inner periphery) of the electrode group. One of the first surface and the second surface faces the positive electrode, and the other faces the negative electrode. At least one of the first surface and the second surface has a first region having first convex portions in a first pattern and a second region having second convex portions in a second pattern different from the first pattern. The second region is arranged closer to the outer periphery of the electrode group than the first region.

[0018] In other words, the separator has at least two regions, a first region and a second region, with the first region being disposed on the inner periphery of the electrode group and the second region being disposed on the outer periphery of the electrode group. The first region may be a region extending from the innermost edge of the first or second surface of the separator toward the outer periphery. The second region may be a region extending from the outermost edge of the first or second surface of the separator toward the inner periphery.

[0019] The first and second convex portions function as spacers to form spaces between the separator and the electrodes. The lithium metal is accommodated in the spaces provided by the first and second convex portions, thereby suppressing expansion of the negative electrode during charging. The separator may be composed of a substrate layer and a spacer layer. In this case, the spacer layer constitutes the first and second convex portions.

[0020] The first convex portion and the second convex portion may be provided only on the surface of the first surface or the second surface that faces the negative electrode, or only on the surface that faces the positive electrode, or on both surfaces.

[0021] The first convex portions are formed in a first pattern, and the second convex portions are formed in a second pattern different from the first pattern. The pattern includes various styles of convex portions. At least one of the first convex portions and the second convex portions may be linear convex portions. The linear convex portions are formed along a linear pattern. At least one of the first convex portions and the second convex portions may be dot-shaped or island-shaped convex portions. The first pattern and the second pattern can be said to be different if the first convex portions and the second convex portions differ in at least one of their line width, diameter, height, density, pitch (the minimum distance between equivalent positions of the convex portions), shape (the path along which the convex portions are drawn), direction on the first surface or the second surface, etc.

[0022] By differentiating the first pattern of the first convex portions arranged on the inner periphery of the electrode assembly from the second pattern of the second convex portions arranged on the outer periphery of the electrode assembly, the size of the space formed on the inner periphery and outer periphery of the electrode assembly can be controlled. For example, if the ratio (R1) of the area occupied by the first convex portions in the first region is larger than the ratio (R2) of the area occupied by the second convex portions in the second region, the function of the first convex portions as spacers is relatively enhanced, thereby preventing a shortage of space formed on the inner periphery of the electrode assembly. R1 is the ratio (%) of the area where the first region and the first convex portions overlap with the area of ​​the first region when the first surface or the second surface is viewed in plan. R2 is the ratio (%) of the area where the second region and the second convex portions overlap with the area of ​​the second region when the first surface or the second surface is viewed in plan.

[0023] R1 and R2 may satisfy 1 < R1 / R2, 1.1 ≦ R1 / R2, or 1.2 ≦ R1 / R2. From the viewpoint of forming a necessary and sufficient space on the inner and outer circumferential sides, respectively, it is preferable that R1 / R2 ≦ 2.5 be satisfied. 1 < R1 / R2 ≦ 2.5, 1.1 ≦ R1 / R2 ≦ 2.5, or 1.2 ≦ R1 / R2 ≦ 2.5 may be satisfied. From the viewpoint of improving the function of the first convex portions as spacers while ensuring sufficient ion permeability of the separator, R1 is preferably 5% or more and 30% or less, and more preferably 10% or more and 25% or less.

[0024] Instead of satisfying R1 > R2, the air permeability P1 of the first region may be lower than the air permeability P2 of the second region. In this case, the function of the first convex portion as a spacer is enhanced compared to the second convex portion, thereby preventing a lack of space formed on the inner periphery of the electrode assembly. The unit of air permeability is "seconds / 100 mL," and the larger the air permeability value, the lower the air permeability. P1 and P2 may satisfy 1 < P1 / P2 or 1.05 ≦ P1 / P2. Furthermore, from the viewpoint of forming sufficient space on the inner and outer peripheries, respectively, it is preferable to satisfy P1 / P2 ≦ 1.5. The relationship 1 < P1 / P2 ≦ 1.5 or 1.05 ≦ P1 / P2 ≦ 1.5 may be satisfied. In addition to satisfying R1 > R2, the air permeability P1 of the first region may be lower than the air permeability P2 of the second region.

[0025] The air permeability (air resistance) is an index showing the time (seconds) required for a predetermined volume (100 mL) of air to permeate per unit area of ​​the separator when a predetermined pressure difference is applied between both sides of the separator. The air permeability was measured based on JIS P8117:2009, where the test area (permeable portion) of the separator is 6.42 cm². ２ The air permeability is measured by the Gurley tester method with an inner cylinder weight of 567 g. The air permeability may also be measured by the Oken tester method, and similar values ​​are obtained.

[0026] When the first convex portion and the second convex portion are each linear convex portion, the line width W1 of the first convex portion may be longer than the line width W2 of the second convex portion. This can relatively enhance the function of the first convex portion as a spacer. W1 and W2 may satisfy 1 < W1 / W2, 1.1 ≦ W1 / W2, or 1.2 ≦ W1 / W2. Furthermore, from the viewpoint of forming a necessary and sufficient space on the inner and outer circumferential sides, W1 / W2 ≦ 4.0 is preferable. 1 < W1 / W2 ≦ 4.0, 1.1 ≦ W1 / W2 ≦ 4.0, or 1.2 ≦ W1 / W2 ≦ 4.0 may be satisfied. Furthermore, from the viewpoint of enhancing the function of the first convex portion as a spacer while ensuring sufficient ion permeability of the separator, W1 is preferably 0.1 mm or more and 2 mm or less, more preferably 0.25 mm or more and 1 mm or less.

[0027] Here, the line width refers to the dimension in a direction perpendicular to the length direction of each of the linearly extending first and second convex portions and parallel to the main surface of the separator. The line width may be measured at any five or more locations on each of the first and second convex portions, and calculated as the arithmetic mean of the five or more measured values.

[0028] The height H1 of the first convex portion may be greater than the height H2 of the second convex portion. This can relatively enhance the function of the first convex portion as a spacer. H1 and H2 may satisfy 1 < H1 / H2, 1.1 ≦ H1 / H2, or 1.2 ≦ H1 / H2. Furthermore, from the viewpoint of forming a necessary and sufficient space on the inner and outer circumferential sides, respectively, it is preferable that H1 / H2 ≦ 2.5 be satisfied. 1 < H1 / H2 ≦ 2.5 may be satisfied, 1.1 ≦ H1 / H2 ≦ 2.5 may be satisfied, or 1.2 ≦ H1 / H2 ≦ 2.5 may be satisfied. Furthermore, from the viewpoint of enhancing the function of the first convex portion as a spacer, H1 is preferably 15 μm or more and 100 μm or less, and more preferably 30 μm or more and 70 μm or less.

[0029] Here, the heights of the first and second convex portions refer to the maximum dimensions of the first and second convex portions in a direction parallel to the thickness direction of the separator. H1 and H2 can be calculated by measuring at any five or more locations on each of the first and second convex portions and calculating the arithmetic mean of the five or more measurements. The heights of the convex portions can be measured by photographing a cross section of the separator parallel to the thickness direction using a scanning electron microscope (SEM).

[0030] From the viewpoint of increasing the ion permeability of the separator, the second region may be larger than the first region. For example, the length L1 of the first region in the second direction may be 25% or less of the total length L of the separator in the second direction. The first region may be, for example, a region extending from the innermost end of the first surface or the second surface of the separator toward the outer periphery, with a length L / 4 or less. This is because the space for lithium deposition is likely to be insufficient in such a region close to the innermost periphery of the electrode assembly.

[0031] It is desirable that the first and second protrusions are arranged on the separator in as uniform and dispersed a state as possible. This makes it possible to reduce the number of locations where lithium metal can be locally deposited while suppressing an increase in internal resistance, and to limit the amount of isolated lithium metal to as small an amount as possible. When a circular region having a diameter of any D1 / 3 (D1 is the length in the first direction (short side direction) of the strip-shaped separator) is set on the surface of the separator, it is desirable that a protrusion always exists in such a circular region.

[0032] At least one of the first pattern and the second pattern may be a geometric pattern. The convex portions of such a pattern are likely to be evenly arranged across the entire surface of the separator. The geometric pattern may be a network pattern. That is, at least one of the first convex portions and the second convex portions may be arranged across the entire surface of the separator along a network pattern. The network pattern may be an aggregate of polygons. An example of a network pattern includes a shape in which polygons are combined to share edges. Polygons include triangles, quadrilaterals, hexagons, etc. Different types of polygons may also be combined. It is desirable for the geometric pattern to have small anisotropy, such as an aggregate of regular polygons. The geometric pattern may be, for example, a honeycomb pattern (an aggregate of regular hexagons).

[0033] At least one of the first pattern and the second pattern may include a dotted pattern. At least one of the first pattern and the second pattern may be a pattern in which a linear pattern and a dotted pattern are combined. The planar shape of the dotted convex portions is not particularly limited and may be a circle (a perfect circle or an ellipse), a polygon (a triangle, a square, etc.), or the like.

[0034] Hereinafter, a separator having the above configuration will be referred to as "separator (A)". Furthermore, a lithium secondary battery having the above configuration will be referred to as "lithium secondary battery (B)". The separator (A) is used in the lithium secondary battery (B). Furthermore, hereinafter, an example will be described in which the separator is composed of a substrate layer and a spacer layer, and the spacer layer constitutes the first convex portion and the second convex portion.

[0035] The spacer layer may be formed, for example, by applying a dispersion liquid containing a spacer material to the portion of the surface of the base layer where the spacer layer is to be formed, and then drying the dispersion liquid. In this case, the spacer layer (first convex portion and second convex portion) can be bonded to the base layer with sufficient strength. The dispersion liquid is applied to the portion where the spacer layer is to be formed, and then dried. The dispersion liquid may be applied using a dispenser or a known printing method such as gravure printing, inkjet printing, or screen printing. The drying may also be performed by a known method such as drying by heating or natural drying.

[0036] The dispersion liquid containing the spacer material contains, for example, insulating particles, a binder resin, and a thickener. Therefore, the spacer layer (first convex portion and second convex portion) formed from the dispersion liquid contains insulating particles, a binder resin, and a thickener. The dispersion medium of the dispersion liquid is not particularly limited, but may be, for example, water, an organic solvent, or a mixture of water and an organic solvent. As the organic solvent, for example, N-methyl-2-pyrrolidone (NMP) may be used. Among these, the use of water is desirable because it reduces the environmental impact.

[0037] The shape of the insulating particles is not particularly limited, but may be spherical. However, spherical does not mean a strict perfect sphere, but rather a shape without sharp corners and with an aspect ratio (maximum diameter / maximum diameter in the direction perpendicular to the maximum diameter) in the range of, for example, 1 to 3. The median diameter (i.e., average particle diameter) in the volume-based particle size distribution of the insulating particles may be 1.0 μm to 10 μm. The median diameter is the particle diameter at which the cumulative volume is 50%. The median diameter of the insulating particles may be 1.0 μm to 2 μm. When the median diameter is within the above range, the insulating particles in the dispersion liquid tend to maintain a stable dispersed state, and the dispersed state is maintained even after application to the base layer, making it easy to form a spacer layer with a uniform morphology.

[0038] The median diameter in the volume-based particle size distribution of the particles can be measured, for example, by a laser diffraction / scattering particle size distribution measuring device (for example, Microtrac manufactured by Nikkiso Co., Ltd.) Alternatively, the cross section of the spacer layer may be observed with a transmission electron microscope (TEM), a TEM image may be taken, the area enclosed by the outlines of any 100 insulating particles may be calculated, the diameter of an equivalent circle (perfect circle) having the same area as the calculated area may be determined, and the average diameter of the 100 equivalent circles may be calculated.

[0039] The volume resistivity of the insulating particles is, for example, 1.0×10 ８ The volume resistivity of the insulating particles may be greater than 1.0×10. When the insulating particles have sufficient insulating properties, lithium metal is hardly deposited on the spacer layer, and deposition of lithium metal in the intended space is promoted. The volume resistivity of the insulating particles may be even greater, for example, 1.0×10 10 It may be Ω·cm or more.

[0040] The volume resistivity can be measured by a four-probe method. For example, the insulating particles are ２ The powder resistivity may be measured using a powder resistivity measuring device (for example, Loresta SP manufactured by Nitto Seiko Analytech Co., Ltd.).

[0041] Examples of insulating particles include inorganic particles such as metal oxides, metal hydroxides, metal nitrides, metal carbides, and metal sulfides. Examples of metal oxides include aluminum oxide (alumina, boehmite), magnesium oxide, titanium oxide (titania), zirconium oxide, and silicon oxide (silica). Examples of metal hydroxides include aluminum hydroxide. Examples of metal nitrides include silicon nitride, aluminum nitride, boron nitride, and titanium nitride. Examples of metal carbides include silicon carbide and boron carbide. Examples of metal sulfides include barium sulfate. Minerals such as aluminosilicates, layered silicates, barium titanate, and strontium titanate may also be used. Among these, alumina, silica, titania, and the like are preferably used.

[0042] The content of the insulating particles in the spacer layer is, for example, less than 80% by volume, and preferably 50 to 70% by volume. The content (volume ratio) of the insulating particles in the spacer layer is determined by observing the cross section of the spacer layer with a transmission electron microscope (TEM), taking a TEM image, and measuring the volume ratio of the insulating particles in the spacer layer by an arbitrary 10 μm ２ Alternatively, the volume fraction may be calculated by calculating the total area enclosed by the contours of the insulating particles in each field of view, and then calculating the ratio of the calculated total area to the area of ​​the field of view. In this case, it is desirable to calculate the volume fractions in three or more fields of view and calculate the average value thereof.

[0043] Examples of the binder resin include fluororesin, fluororubber, styrene-butadiene copolymer or hydrogenated product thereof, acrylonitrile-butadiene copolymer or hydrogenated product thereof, methacrylate ester-acrylate ester copolymer, styrene-acrylate ester copolymer, acrylonitrile-acrylate ester copolymer, ethylene propylene rubber, polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyimide, polyamide such as wholly aromatic polyamide (aramid), polyimide, polyamideimide, polyacrylonitrile, polyether, polyolefin, and alkyd resin.

[0044] The amount of binder resin may be, for example, 20 to 100 parts by volume, 20 to 80 parts by volume, 20 to 70 parts by volume, or 25 to 50 parts by volume per 100 parts by volume of insulating particles. Within this range, it is easy to increase the mechanical strength of the spacer layer and also to increase the binding strength between the base layer and the spacer layer.

[0045] The thickener may include, for example, at least one selected from the group consisting of carboxymethyl cellulose and carboxymethyl cellulose salts (hereinafter, at least one selected from the group consisting of carboxymethyl cellulose and carboxymethyl cellulose salts is also referred to as "CMC"). When a carboxymethyl cellulose salt is used as the CMC, a sodium salt, a lithium salt, a potassium salt, an ammonium salt, or the like may be used. Among these, it is preferable that the carboxymethyl cellulose salt includes a sodium salt.

[0046] To fully utilize the role of the CMC, it is desirable that the dispersion medium of the spacer material dispersion contains water, and the dispersion medium may be 50% by mass or more, or 70% by mass or more, 80% by mass or more, or 90% by mass or more of the dispersion medium may be water.

[0047] The amount of CMC may be, for example, 0.5 to 5 parts by volume or 1 to 3 parts by volume per 100 parts by volume of insulating particles. By using CMC in this range, a sufficient thickening effect of the CMC can be exhibited.

[0048] <Substrate Layer> A porous sheet having ion permeability and insulating properties is used for the substrate layer. Examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The material of the porous sheet is not particularly limited, but may be a polymer material. Examples of the polymer material include an olefin resin, a polyamide resin, and cellulose. Examples of the olefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The substrate layer may contain an additive, if necessary. Examples of the additive include an inorganic filler.

[0049] The thickness of the substrate layer is not particularly limited, but is, for example, 5 μm or more and 20 μm or less, and more preferably 10 μm or more and 20 μm or less.

[0050] The substrate layer may include a porous sheet and a composite material layer. The composite material layer may be formed on one or both main surfaces of the porous sheet. The composite material layer is a layer that allows lithium ions to permeate. The thickness of the composite material layer may be 5% to 50% of the total thickness of the separator.

[0051] The composite material layer includes a resin material and inorganic particles. The inorganic particles may include first particles and / or second particles. The first particles are particles of a lithium-containing phosphate. The first particles have the effect of suppressing heat generation in the battery under abnormal conditions. The second particles are particles other than the first particles.

[0052] When the composite material layer is formed on only one main surface of the porous sheet, it is desirable to provide the composite material layer on the main surface of the porous sheet facing the positive electrode. By disposing the composite material layer on the positive electrode side, deterioration of the porous sheet due to oxidation reactions can be suppressed. On the other hand, by disposing the composite material layer on the negative electrode side, deterioration of the porous sheet due to reduction reactions can be suppressed.

[0053] The phosphate constituting the first particles is lithium phosphate (Li ３ P.O. ４ ), dilithium hydrogen phosphate (Li ２ HPO ４ ), and lithium dihydrogen phosphate (LiH ２ P.O. ４ ) may be at least one selected from the group consisting of. Among these, lithium phosphate is preferred because it is highly effective in suppressing heat generation in the battery under abnormal conditions. The first particles may have a median diameter of 0.1 μm to 1.0 μm in a volume-based particle size distribution.

[0054] A preferred example of the second particles is particles made of an insulating inorganic compound that does not melt or decompose when the battery abnormally heats up. Examples of materials for the second particles include oxides, hydroxides, nitrides, carbides, sulfides, etc. Examples of oxides include aluminum oxide, boehmite, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, zinc oxide, etc. Examples of nitrides include silicon nitride, aluminum nitride, boron nitride, titanium nitride, etc. Examples of carbides include silicon carbide and boron carbide, etc. Examples of sulfides include barium sulfate, etc. Examples of hydroxides include aluminum hydroxide, etc. The median diameter in the volume-based particle size distribution of the second particles may be 0.2 to 2.0 μm.

[0055] The median diameter in the volume-based particle size distribution of the particles can be measured, for example, by a laser diffraction / scattering particle size distribution measuring device (for example, Microtrac manufactured by Nikkiso Co., Ltd.) Alternatively, the cross section of the base layer may be observed with a transmission electron microscope (TEM), a TEM image may be taken, the area surrounded by the outlines of any 100 first particles or second particles may be calculated, the diameter of an equivalent circle (perfect circle) having the same area as the calculated area may be determined, and the average diameter of the 100 equivalent circles may be calculated.

[0056] The resin material is preferably a polymeric material having higher heat resistance than the material of the porous sheet. Such a polymeric material preferably includes at least one selected from the group consisting of aromatic polyamides, aromatic polyimides, and aromatic polyamideimides. These are known to have high heat resistance. From the viewpoint of heat resistance, aramids, i.e., meta-aramids (meta-type wholly aromatic polyamides) and para-aramids (para-type wholly aromatic polyamides), are preferred.

[0057] The content of the inorganic particles in the composite material layer may be in the range of 50% by weight to 99% by weight (for example, in the range of 85% by weight to 99% by weight).

[0058] (Negative electrode) The negative electrode includes a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode upon charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode upon charging, becoming lithium metal, which is then deposited on the surface of the negative electrode. The lithium metal deposited on the surface of the negative electrode dissolves as lithium ions in the non-aqueous electrolyte upon discharging.

[0059] The negative electrode may include a lithium ion occlusion layer (a layer that develops capacity by occlusion and release of lithium ions by a negative electrode active material (such as graphite)) supported on a negative electrode current collector. In this case, the open circuit potential of the negative electrode when fully charged may be 70 mV or less relative to lithium metal (dissolution and deposition potential of lithium). If the open circuit potential of the negative electrode when fully charged is 70 mV or less relative to lithium metal, lithium metal is present on the surface of the lithium ion occlusion layer when fully charged. In other words, the negative electrode develops capacity by deposition and dissolution of lithium metal.

[0060] Here, "fully charged" refers to a state in which the battery is charged to a state of charge of, for example, 0.98 x C or more, where C is the rated capacity of the battery. The open circuit potential of the negative electrode at full charge can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell with lithium metal as the counter electrode. The nonaqueous electrolyte of the cell may have the same composition as the nonaqueous electrolyte in the disassembled battery.

[0061] The lithium ion occlusion layer is a layer of a negative electrode mixture containing a negative electrode active material. The negative electrode mixture may contain a binder, a thickener, a conductive agent, etc. in addition to the negative electrode active material.

[0062] Examples of the negative electrode active material include a carbonaceous material, a Si-containing material, and a Sn-containing material. The negative electrode may contain one or more negative electrode active materials. Examples of the carbonaceous material include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon).

[0063] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0064] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0065] The negative electrode current collector may be a conductive sheet, such as a foil or film.

[0066] The material of the negative electrode current collector (conductive sheet) may be any conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as a metal or alloy. The conductive material is preferably a material that does not react with lithium. More specifically, a material that does not form an alloy or an intermetallic compound with lithium is preferred. Examples of such conductive materials include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements, as well as graphite with a preferentially exposed basal plane. Examples of alloys include copper alloys and stainless steel (SUS). Among these, copper and / or copper alloys with high conductivity are preferred.

[0067] The thickness of the negative electrode current collector is not particularly limited and is, for example, 5 μm or more and 300 μm or less.

[0068] (Positive Electrode) The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer supported on the positive electrode current collector. The positive electrode composite layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode composite layer may be formed on only one side of the positive electrode current collector, or may be formed on both sides. The positive electrode is obtained, for example, by applying a positive electrode composite slurry including the positive electrode active material, the conductive material, and the binder to both sides of the positive electrode current collector, drying the coating, and then rolling.

[0069] The positive electrode active material is a material that absorbs and releases lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Among these, lithium-containing transition metal oxides are preferred because of their low production cost and high average discharge voltage.

[0070] The lithium contained in the lithium-containing transition metal oxide is released from the positive electrode as lithium ions during charging and precipitates as lithium metal on the negative electrode or negative electrode current collector. During discharging, the lithium metal dissolves from the negative electrode, releasing lithium ions, which are then absorbed into the composite oxide of the positive electrode. In other words, the lithium ions involved in charging and discharging are generally derived from the solute in the nonaqueous electrolyte and the positive electrode active material.

[0071] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Co, Ni, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, etc.

[0072] Among lithium-containing transition metal oxides, composite oxides containing Co, Ni, and / or Mn as transition metal elements, and optionally containing Al, and having a layered rock-salt crystal structure are preferred in terms of obtaining high capacity. In this case, in the lithium secondary battery, the molar ratio mLi / mM of the total amount of lithium contained in the positive electrode and negative electrode to the amount mM of metal M other than lithium contained in the positive electrode is set to, for example, 1.1 or less.

[0073] As the binder, conductive agent, etc., for example, those exemplified for the negative electrode can be used. The shape and thickness of the positive electrode current collector can be selected from the shape and range of the positive electrode current collector.

[0074] Examples of the material of the positive electrode current collector (conductive sheet) include metal materials containing Al, Ti, Fe, etc. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy, etc. The Fe alloy may be stainless steel (SUS).

[0075] The thickness of the positive electrode current collector is not particularly limited and is, for example, 5 μm or more and 300 μm or less.

[0076] (Non-aqueous electrolyte) The non-aqueous electrolyte having lithium ion conductivity may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives.

[0077] The gel electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0078] As the solid electrolyte, for example, a material known in all-solid-state lithium ion secondary batteries (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0079] The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent, which generates lithium ions and anions.

[0080] The anion is BF ４ － , ClO ４ － , P.F. ６ － , C.F. ３ SO ３ － , C.F. ３ CO ２ － , anions of imides, anions of oxalate complexes, etc. Examples of the anions of imides include N(SO ２ CF ３ ) ２ － , N(C ｍ F ２ｍ＋１ SO ２ ) ｘ (C ｎ F ２ｎ＋１ SO ２ ) y－ (m and n are each independently an integer of 0 or 1 or more, and x and y are each independently 0, 1, or 2, satisfying the relationship x+y=2). The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include bisoxalate borate anion and difluorooxalate borate anion (BF ２ (C ２ O ４ ) － ), P.F. ４ (C ２ O ４ ) － , P.F. ２ (C ２ O ４ ) ２ － The non-aqueous electrolyte may contain one of these anions alone or two or more of them.

[0081] From the viewpoint of suppressing the deposition of lithium metal in a dendritic form, the nonaqueous electrolyte preferably contains at least an anion of an oxalate complex, and more preferably contains an oxalate complex anion having fluorine. The interaction between the oxalate complex anion having fluorine and lithium facilitates the uniform deposition of lithium metal in the form of fine particles. This makes it easier to suppress local deposition of lithium metal. The oxalate complex anion having fluorine may be combined with another anion. The other anion may be PF ６ － and / or an anion of an imide.

[0082] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and halogen-substituted derivatives thereof. The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. Examples of halogen-substituted derivatives include fluorides.

[0083] Examples of esters include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and fluoroethylene carbonate (FEC). Examples of chain carbonate esters include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of chain carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate.

[0084] Examples of the ether include cyclic ethers and chain ethers. Examples of the cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.

[0085] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. The concentration of the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Furthermore, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0086] The non-aqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. The formation of a coating derived from the additive on the negative electrode makes it easier to suppress the formation of dendrites. Examples of such additives include vinylene carbonate, FEC, and vinyl ethyl carbonate (VEC).

[0087] The following description will be made with reference to the drawings. FIG. 1 is a longitudinal cross-sectional view schematically showing an example of a lithium secondary battery (B). The components described above can be applied to the components of the lithium secondary battery (B) described below. The components described below can be modified based on the above description. In the lithium secondary battery described below, components that are not essential for the lithium secondary battery according to the present disclosure may be omitted. In the following figures, the scale of the components has been changed to facilitate understanding.

[0088] (Embodiment 1) FIG. 1 is a longitudinal cross-sectional view schematically illustrating an example of a lithium secondary battery according to Embodiment 1. Note that FIG. 1 does not illustrate the spacer layers (first and second convex portions) and the spaces formed by the spacer layers. The cylindrical lithium secondary battery 10 shown in FIG. 1 includes a cylindrical battery case and a wound electrode group 14 and a nonaqueous electrolyte housed within the battery case. The battery case includes a case body 15, which is a cylindrical metal container with a bottom, and a sealing member 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing member 16. The gasket 27 ensures the hermeticity of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the direction of the winding axis.

[0089] Case body 15 has a step 21 formed, for example, by pressing a portion of the side wall of case body 15 from the outside. Step 21 may be formed in an annular shape along the circumferential direction of case body 15 on the side wall of case body 15. In this case, sealing body 16 is supported by the surface of step 21 on the opening side.

[0090] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These components are stacked in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 so that the cap 26 is located outside the case body 15 and the filter 22 is located inside the case body 15. The above-mentioned components constituting the sealing body 16 are, for example, disk-shaped or ring-shaped. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and an insulating member 24 is interposed between their respective peripheral edges. The filter 22 and the lower valve body 23 are connected to each other at their respective centers. The upper valve body 25 and the cap 26 are connected to each other at their respective centers. In other words, all components except the insulating member 24 are electrically connected to each other.

[0091] A vent hole (not shown) is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening (not shown) formed in the cap 26.

[0092] 2A is an enlarged view of a portion of the electrode group 14 where the first region of the separator is disposed on the inner periphery (e.g., a region two or three laps from the innermost periphery). Fig. 2B is an enlarged view of a portion of the electrode group 14 where the second region of the separator is disposed on the outer periphery (e.g., a region two or three laps from the outermost periphery). Figs. 2A and 2B include the portion near the positive electrode surrounded by region II in Fig. 1 and the portion near the negative electrode surrounded by region III in Fig. 1.

[0093] The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The separator 13 has a base layer 50 and spacer layers 53A and 53B formed on the base layer 50. The spacer layer 53A, which is disposed on the inner circumferential side of the electrode group 14, constitutes a first convex portion. The spacer layer 53A (first convex portion) is formed in a first pattern. The spacer layer 53B, which is disposed on the outer circumferential side of the electrode group 14, constitutes a second convex portion. The spacer layer 53B (second convex portion) is formed in a second pattern. The first pattern and the second pattern are different, and the line width W1 of the first convex portion is longer than the line width W2 of the second convex portion. Furthermore, the ratio R1 of the area occupied by the spacer layer 53A (first convex portion) in the first region is greater than the ratio R2 of the area occupied by the spacer layer 53B (second convex portion) in the second region, so that the air permeability P1 of the first region is lower than the air permeability P2 of the second region.

[0094] The substrate layer 50 has a porous sheet 51 and a composite material layer 52. A spacer layer 53 is formed on the composite material layer 52. The positive electrode 11, the negative electrode 12, and the separator 50 are wound together so that the separator 13 is disposed between the positive electrode 11 and the negative electrode 12, thereby forming an electrode group 14.

[0095] The positive electrode 11 includes a positive electrode current collector 11a and a positive electrode composite layer 11b. The positive electrode current collector 11a is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. In FIG. 2, the negative electrode 12 is shown as a negative electrode (negative electrode current collector) on which no lithium metal is deposited. The negative electrode 12 is electrically connected to a case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20.

[0096] 2A and 2B , the separator 13 has a first main surface 50a facing the negative electrode 12 and a second main surface 50b facing the positive electrode 11. In the example shown in Figures 2A and 2B , the spacer layer 53 is formed on the first main surface 50a.

[0097] In the first embodiment, the composite material layer 52 is formed on one of the two main surfaces of the porous sheet 51, the main surface facing the negative electrode 12. However, the composite material layer 52 may be omitted, or may be formed on each of the two main surfaces of the porous sheet 51. The spacer layer 53 is formed on the composite material layer 52 and is in contact with the negative electrode 12. The spacer layer 53 forms a space 14s between the positive electrode 11 and the negative electrode 12 (between the negative electrode 12 and the separator 13). FIG. 2A shows the line width W1 and height H1 of the spacer layer 53A (first convex portion), and FIG. 2B shows the line width W2 and height H2 of the spacer layer 53B (second convex portion). In this embodiment, W1 > W2, and H1 and H2 are the same, but this is not limited thereto, and may be, for example, H1 > H2. The ratio R1 of the area occupied by the spacer layer 53A (first convex portion) in the first region is larger than the ratio R2 of the area occupied by the second convex portion in the second region.

[0098] During charging of the lithium secondary battery 10, lithium metal is deposited on the negative electrode 12. Because a space 14s exists between the positive electrode 11 and the negative electrode 12, the volume change of the electrode group 14 that accompanies the deposition of lithium metal is reduced, improving the cycle characteristics.

[0099] An example of the planar shape of the spacer layers 53A and 53B is shown in FIG. 3 , and an enlarged view of a portion of FIG. 3 (spacer layer 53B) is shown in FIG. 4 . In the example shown in FIG. 3 , the left end is the innermost end, and in the first region including this end, the spacer layer 53A forms linear first convex portions 53a with a long line width. On the other hand, the right end is the outermost end, and in the second region including this end, the spacer layer 53B forms linear second convex portions 53b with a short line width. The linear first convex portions 53a and second convex portions 53b are arranged in a mesh pattern. More specifically, in the first region, they are uniformly formed in a honeycomb pattern with a narrow pitch, and in the second region, they are uniformly formed in a honeycomb pattern with a wide pitch. The honeycomb pattern is a pattern in which multiple hexagons are arranged so that they share sides with each other. The regions where the linear first convex portions 53a and second convex portions 53b are not formed form spaces 14s. 4 shows the width W2 of the linear second protrusion 53b. The width W2 of the linear second protrusion 53b is the length (width) of the linear second protrusion 53b in a direction perpendicular to the direction in which the linear second protrusion 53b extends in a plan view.

[0100] 4, the linear second protrusions 53b have cutouts 54. The cutouts 54 promote the movement of ions in the non-aqueous electrolyte. In a pattern in which a plurality of polygons are arranged so as to share sides with each other, it is preferable to provide the cutouts 54.

[0101] 5 to 7 show other examples of the first convex portion 53a and the second convex portion 53b in plan view. In each example, the ratio R1 of the area occupied by the spacer layer 53A (first convex portion) in the first region is larger than the ratio R2 of the area occupied by the second convex portion in the second region.

[0102] 5 includes a plurality of linear first protrusions 53a arranged in a direction that intersects with each other, and the spacer layer 53B includes a plurality of linear second protrusions 53b arranged in a direction that intersects with each other. Gaps P exist between the plurality of first protrusions 53a and between the plurality of second protrusions 53b. Areas where no linear protrusions are formed form spaces 14s. The line width of the first protrusions 53a is long, and the line width of the second protrusions 53b is short.

[0103] 6 includes a plurality of first protrusions 53 a arranged in a stripe pattern and linearly extending in the second direction, and the spacer layer 53B includes a plurality of second protrusions 53 b arranged in a stripe pattern and linearly extending in the second direction. The number of first protrusions 53 a is smaller than the number of second protrusions 53 b, and the pitch between the first protrusions 53 a is narrower than the pitch between the second protrusions 53 b.

[0104] 7 includes linear first protrusions 53a arranged in a lattice (mesh) pattern, and the spacer layer 53B includes linear second protrusions 53b arranged in a lattice (mesh) pattern. The lattice size of the first protrusions 53a is smaller than the lattice size of the second protrusions 53b.

[0105] The combination of the first and second protrusions is not limited to the illustrated example. For example, the first protrusions may be formed in a honeycomb pattern, and the second protrusions may be formed in a pattern other than a honeycomb pattern.

[0106] (Additional Notes) The above embodiments disclose the following techniques: (Technology 1) A lithium secondary battery comprising an electrode group and a non-aqueous electrolyte, wherein the electrode group comprises a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are wound with the separator interposed therebetween, wherein lithium metal precipitates on the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging, wherein the separator has a first surface facing the outside of the electrode group and a second surface facing the inside of the electrode group, wherein at least one of the first surface and the second surface has a first region having first convex portions in a first pattern and a second region having second convex portions in a second pattern different from the first pattern, and wherein the second region is disposed closer to the outer periphery of the electrode group than the first region. (Technology 2) The lithium secondary battery according to Technology 1, wherein the proportion of the area occupied by the first convex portions in the first region is greater than the proportion of the area occupied by the second convex portions in the second region. (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the air permeability of the first region is lower than the air permeability of the second region. (Technology 4) The lithium secondary battery according to any one of Technology 1 to 3, wherein the first convex portions and the second convex portions are each linear convex portions, and wherein the line width of the first convex portions is greater than the line width of the second convex portions. (Technology 5) The lithium secondary battery according to any one of Technology 1 to 4, wherein the height of the first convex portions is greater than the height of the second convex portions. (Technology 6) The lithium secondary battery according to any one of Technologies 1 to 5, wherein the separator has an elongated shape having a length D1 in a first direction parallel to a winding axis and a length D2 (D1<D2) in a second direction intersecting the first direction, and the length of the first region in the second direction is 25% or less of the entire length of the separator in the second direction. (Technology 7) The lithium secondary battery according to any one of Technologies 1 to 6, wherein at least one of the first pattern and the second pattern is a geometric pattern.(Technology 8) A separator disposed between a positive electrode and a negative electrode and wound to form an electrode group, the separator having a first surface facing the outside of the electrode group and a second surface facing the inside of the electrode group, at least one of the first surface and the second surface having a first region having first convex portions in a first pattern and a second region having second convex portions in a second pattern different from the first pattern, the second region being disposed closer to the outer periphery of the electrode group than the first region. (Technology 9) The separator according to Technology 8, wherein the proportion of the area occupied by the first convex portions in the first region is larger than the proportion of the area occupied by the second convex portions in the second region. (Technology 10) The separator according to Technology 8 or 9, wherein the air permeability of the first region is lower than the air permeability of the second region. (Technology 11) The separator according to any one of Technologies 8 to 10, wherein the first convex portions and the second convex portions are linear convex portions, and the line width of the first convex portions is longer than the line width of the second convex portions. (Technology 12) The separator according to any one of Technologies 8 to 11, wherein the height of the first convex portions is longer than the height of the second convex portions. (Technology 13) The separator according to any one of Technologies 8 to 12, wherein the separator has an elongated shape having a length D1 in a first direction and a length D2 in a second direction intersecting the first direction (D1<D2), and wherein the length of the first region in the second direction is 25% or less of the entire length of the separator in the second direction. (Technology 14) The separator according to any one of Technologies 8 to 13, wherein at least one of the first pattern and the second pattern is a geometric pattern. (Technology 15) A separator having an elongated shape with a length D1 in a first direction and a length D2 (D1<D2) in a second direction intersecting the first direction, the separator having a first surface and a second surface opposite to the first surface, at least one of the first surface and the second surface having a first region with first convex portions in a first pattern and a second region with second convex portions in a second pattern different from the first pattern, the second region being positioned farther from one end in the second direction than the first region.

[0107] [Examples] The lithium secondary battery according to the present disclosure will be described in more detail below based on examples and comparative examples, although the present disclosure is not limited to the following examples.

[0108] Example 1 (1) Preparation of Positive Electrode A rock-salt lithium-containing transition metal oxide (NCA: positive electrode active material) containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al is 1.0) and having a layered structure, acetylene black (AB: conductive material), and polyvinylidene fluoride (PVdF: binder) were mixed in a mass ratio of NCA:AB:PVdF = 95:2.5:2.5, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode composite slurry. The resulting positive electrode composite slurry was applied to both sides of a strip-shaped Al foil (positive electrode current collector), dried, and the coating of the positive electrode composite was rolled using a roller. Finally, the resulting laminate of the positive electrode current collector and the positive electrode mixture was cut to a predetermined electrode size, to obtain a positive electrode having a positive electrode mixture layer on both sides of the positive electrode current collector.

[0109] (2) Preparation of Negative Electrode A strip of electrolytic copper foil (thickness: 15 μm) was prepared as a negative electrode current collector.

[0110] (3) Separator (3-1) Base Layer (Porous Sheet) A microporous thin film made of polyethylene and having a thickness of 20 μm was prepared.

[0111] (3-2) Formation of spacer layer on the main surface of the base material layer Insulating particles (median diameter 3 μm, volume resistivity 10 １４ A dispersion of the spacer material was prepared by mixing 50 parts by volume of SiO 2 (Ω·cm), 50 parts by volume of polyvinylidene fluoride (PVdF) as a binder resin, and N-methyl-2-pyrrolidone (NMP) as a dispersion medium.

[0112] Next, a dispersion of the spacer material was discharged onto the microporous thin film using a dispenser in a honeycomb pattern, and the coating was vacuum dried to form first and second convex portions. Specifically, one surface (first main surface) of the microporous thin film was divided into a first region and a second region, and the first convex portions were uniformly formed in the first honeycomb pattern in the first region, and the second convex portions were uniformly formed in the second honeycomb pattern in the second region.

[0113] The first region was a region extending from the innermost end of the first surface of the base material layer toward the outer periphery with a length of L / 4, where L is the total length of the base material layer in the second direction. That is, the length L1 of the first region in the second direction was 25% of the total length L of the base material layer (i.e., the separator) in the second direction.

[0114] The line width and height of the first convex portion and the second convex portion were the same, W1=W2=0.5 mm, and H1=H2=50 μm.

[0115] In the first pattern, the distance (pitch) between a pair of opposing sides of the regular hexagon was 5 mm, and in the second pattern, the distance (pitch) between a pair of opposing sides of the regular hexagon was 10 mm. As a result, the area ratio R1 of the first region occupied by the first convex portions was 19%, and the area ratio R2 of the second region occupied by the second convex portions was 9.7%. The air permeability P1 of the first region was 370 s / 100 mL, and the air permeability P2 of the second region was 345 s / 100 mL.

[0116] (4) Preparation of non-aqueous electrolyte Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:DMC=30:70, and LiPF was added to the resulting mixed solvent. ６ 1 mol / L, LiBF ２ (C ２ O ４ ) were dissolved in a concentration of 0.1 mol / L to prepare a liquid non-aqueous electrolyte.

[0117] (5) Battery Assembly An electrode assembly was prepared by spirally winding the positive electrode and negative electrode current collector with the separator interposed therebetween in an inert gas atmosphere. The separator was positioned so that the spacer layer faced the negative electrode. The electrode assembly was housed in a bag-shaped exterior case made of a laminate sheet with an Al layer. After injecting a non-aqueous electrolyte, the exterior case was sealed to complete the lithium secondary battery A1.

[0118] Example 2: Battery A2 was fabricated in the same manner as Battery A1, except that in the first pattern, the spacing (pitch) between a pair of opposing sides of the regular hexagon was changed to 10 mm and the line width W1 of the first convex portion was changed to 1.0 mm. In the second pattern, R1 was 19% and R2 was 9.7%. P1 was 373 seconds / 100 mL and P2 was 322 seconds / 100 mL.

[0119] Example 3: In the first pattern, the spacing (pitch) between a pair of opposing sides of the regular hexagon was changed to 2.25 mm, and the line width W1 of the first convex portion was changed to 0.25 mm. The second pattern was also changed to a striped pattern. Specifically, multiple linear second convex portions extending in the second direction were formed in the second region. The spacing (pitch) between the multiple linear second convex portions in the first direction was 5 mm, and the line width W2 was 0.5 mm. Battery A3 was fabricated in the same manner as Battery A1, except for the above. R1 was 21% and R2 was 10%. P1 was 381 seconds / 100 mL, and P2 was 345 seconds / 100 mL.

[0120] Example 4: In both the first and second patterns, the spacing (pitch) between a pair of opposing sides of the regular hexagon was changed to 2.25 mm, and the line width W1 of the first convex portion and the line width W2 of the second convex portion were changed to 0.25 mm. Furthermore, two 0.25 mm-wide notches were provided in each honeycomb in the first convex portion (see FIG. 4), and two 0.5 mm-wide notches were provided in each honeycomb in the second convex portion. Battery A4 was fabricated in the same manner as Battery A1, except for the above. R1 was 16.5%, and R2 was 14.5%. P1 was 370 seconds / 100 mL, and P2 was 345 seconds / 100 mL.

[0121] Example 5 Battery A5 was fabricated in the same manner as Battery A1, except that the height of the first convex portion in the first pattern was changed to 60 μm. R1 was 19%, R2 was 9.7%, P1 was 370 seconds / 100 mL, and P2 was 345 seconds / 100 mL.

[0122] Comparative Example 1 Battery B1 was fabricated in the same manner as Battery A1, except that first convex portions were formed in the first region in the same pattern as the second pattern. R1 = R2 = 9.7%, and P1 = P2 = 345 seconds / 100 mL.

[0123] Comparative Example 2 Battery B2 was fabricated in the same manner as Battery A1, except that the patterns of the first and second regions were reversed. R1 was 9.7%, R2 was 19%, P1 was 345 seconds / 100 mL, and P2 was 370 seconds / 100 mL.

[0124] [Evaluation 1] For each battery obtained, the height of the space between the electrode and the base material layer (the distance between the negative electrode and the base material layer) was measured using X-rays for the three innermost turns around which the first region of the separator was wound, and the average value for the three turns was calculated. The height of the space between the electrode and the base material layer was also measured for the three outermost turns around which the second region of the separator was wound, and the average value for the three turns was calculated. The results are shown in Table 1. The values ​​in Table 1 are relative values, with the space height for Battery A1 of Example 1 set to 100.

[0125] [Evaluation 2] Each of the obtained batteries was subjected to a charge / discharge test under the following conditions in a thermostatic chamber at 25° C. The rest time between charge and discharge was 20 minutes.

[0126] (Charging) The battery was charged at a constant current of 2.15 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 4.1 V, and then charged at a constant voltage of 4.1 V until the current value per unit area of ​​the electrode reached 0.54 mA.

[0127] (Discharge) Constant current discharge was carried out at a current of 2.15 mA per unit area of ​​the electrode until the battery voltage reached 3.75 V.

[0128] The above charge and discharge cycles were repeated, and the number of cycles until voltage fluctuation due to buckling of the electrodes was detected was determined.

[0129]

[0130] The results in Table 1 show that in the batteries A1 to A5 of the examples, a space of sufficient height for lithium deposition was formed, enabling long-term charge-discharge cycles, whereas in the batteries B1 and B2 of the comparative examples, the space was too short, making the electrodes more susceptible to buckling.

[0131] The lithium secondary battery of the present disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet devices, electric vehicles including hybrids and plug-in hybrids, and home storage batteries combined with solar cells. While the present invention has been described with reference to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and alterations will undoubtedly become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to include all modifications and alterations without departing from the true spirit and scope of the present invention.

[0132] REFERENCE SIGNS LIST 10 Lithium secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode group 14s Space 50 Base material layer 50A Base material layer 50B Spacer layer 51 Porous sheet 52 Composite material layer 53A, 53B Spacer layer 53a First convex portion 53b Second convex portion

Claims

1. A lithium secondary battery comprising: an electrode group; and a non-aqueous electrolyte; the electrode group comprises a positive electrode, a negative electrode, and a separator; the positive electrode and the negative electrode are wound with the separator interposed between them; lithium metal precipitates in the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharging; the separator has a first surface facing the outside of the electrode group and a second surface facing the inside of the electrode group; at least one of the first surface and the second surface has a first region having first convex portions in a first pattern and a second region having second convex portions in a second pattern different from the first pattern; and the second region is disposed on the outer periphery of the electrode group relative to the first region.

2. The lithium secondary battery according to claim 1, wherein the proportion of the area of ​​said first region occupied by said first convex portion is greater than the proportion of the area of ​​said second region occupied by said second convex portion.

3. The lithium secondary battery according to claim 1, wherein the air permeability of said first region is lower than the air permeability of said second region.

4. The lithium secondary battery according to claim 1, wherein the first convex portion and the second convex portion are each linear convex portions, and the line width of the first convex portion is greater than the line width of the second convex portion.

5. The lithium secondary battery according to claim 1, wherein the height of the first convex portion is greater than the height of the second convex portion.

6. A lithium secondary battery according to any one of claims 1 to 5, wherein the separator has an elongated shape having a length D1 in a first direction parallel to the winding axis and a length D2 in a second direction intersecting the first direction (D1<D2), and the length of the first region in the second direction is 25% or less of the entire length of the separator in the second direction.

7. The lithium secondary battery according to any one of claims 1 to 5, wherein at least one of the first pattern and the second pattern is a geometric pattern.

8. A separator disposed between a positive electrode and a negative electrode and wound to form an electrode group, the separator having a first surface facing the outside of the electrode group and a second surface facing the inside of the electrode group, at least one of the first surface and the second surface having a first region having a first convex portion of a first pattern and a second region having a second convex portion of a second pattern different from the first pattern, the second region being disposed closer to the outer periphery of the electrode group than the first region.

9. The separator according to claim 8, wherein a proportion of the area of ​​said first region occupied by said first convex portions is greater than a proportion of the area of ​​said second region occupied by said second convex portions.

10. The separator according to claim 8, wherein the air permeability of said first region is lower than the air permeability of said second region.

11. The separator according to claim 8, wherein the first convex portion and the second convex portion are each a linear convex portion, and the line width of the first convex portion is greater than the line width of the second convex portion.

12. The separator according to claim 8, wherein the height of the first protrusion is greater than the height of the second protrusion.

13. A separator according to any one of claims 8 to 12, wherein the separator has an elongated shape having a length D1 in a first direction parallel to the winding axis and a length D2 in a second direction intersecting the first direction (D1<D2), and the length of the first region in the second direction is 25% or less of the entire length of the separator in the second direction.

14. The separator according to any one of claims 8 to 12, wherein at least one of the first pattern and the second pattern is a geometric pattern.

15. A separator having an elongated shape having a length D1 in a first direction and a length D2 (D1<D2) in a second direction intersecting the first direction, the separator having a first surface and a second surface opposite the first surface, at least one of the first surface and the second surface having a first region having a first convex portion of a first pattern and a second region having a second convex portion of a second pattern different from the first pattern, the second region being positioned farther from one end in the second direction than the first region.

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