Secondary battery and separator for secondary battery

Abstract

A 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. The separator includes a sheet-shaped first base material, a sheet-shaped second base material, and a spacer disposed between the first base material and the second base material. One of the first base material and the second base material is disposed on the positive electrode side, and the other is disposed on the negative electrode side.

Description

Secondary battery and separator for secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-29985, filed on February 29, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a secondary battery and a separator for a secondary battery.

[0003] Known high-capacity non-aqueous electrolyte secondary batteries include lithium ion batteries and lithium metal secondary batteries (hereinafter also referred to as "lithium secondary batteries"). In lithium secondary batteries, lithium metal precipitates on the negative electrode during charging, and the lithium metal dissolves during discharging and is released as lithium ions into the non-aqueous electrolyte.

[0004] In secondary batteries, the volume of the electrode group changes significantly due to the expansion and contraction of the electrodes during charging and discharging, which can lead to a deterioration in cycle performance. To address this issue, it is conceivable to reduce the volume change of the electrode group by placing a spacer between the separator substrate and the electrodes.

[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 is dissolved from the negative electrode during discharging; a spacer is provided between the separator and at least one of the positive electrode and the negative electrode; 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

[0007] There is room for further improvement in the cycle characteristics of secondary batteries equipped with spacers.

[0008] One aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the separator includes a first sheet-like substrate, a second sheet-like substrate, and a spacer disposed between the first substrate and the second substrate, wherein one of the first substrate and the second substrate is disposed on the positive electrode side, and the other of the first substrate and the second substrate is disposed on the negative electrode side.

[0009] Another aspect of the present disclosure relates to a separator for a secondary battery including a first substrate in a sheet form, a second substrate in a sheet form, and a spacer disposed between the first substrate and the second substrate.

[0010] According to the present disclosure, the cycle characteristics of a secondary battery can be improved. 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.

[0011] It is a longitudinal cross-sectional view showing an example of a secondary battery according to an embodiment of the present disclosure. It is a cross-sectional view showing a main part of the secondary battery of Fig. 1. It is a top view showing an example of a spacer. It is a top view showing another example of a spacer.

[0012] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "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. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0013] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The separator includes a first substrate sheet, a second substrate sheet, and a spacer disposed between the first substrate and the second substrate. One of the first substrate and the second substrate is disposed on the positive electrode side, and the other of the first substrate and the second substrate is disposed on the negative electrode side.

[0014] Hereinafter, the positive electrode, negative electrode, and separator are collectively referred to as an "electrode group." The shape of the electrode group is not particularly limited. The electrode group may be a wound electrode group in which a strip-shaped positive electrode and a strip-shaped negative electrode are spirally wound with a separator interposed therebetween, or may be a flat electrode group in which a wound electrode group is pressed in the radial direction. The electrode group may be configured by stacking a positive electrode and a negative electrode with a separator interposed therebetween, or may be configured by stacking a positive electrode and a negative electrode in a zigzag manner with a separator interposed therebetween.

[0015] Secondary batteries include lithium secondary batteries (lithium metal secondary batteries), lithium ion batteries, and the like. For example, the negative electrode of a lithium secondary battery expands due to the deposition of lithium metal during charging. The negative electrode of a lithium ion battery expands due to the occlusion of lithium ions during charging. Among secondary batteries, lithium secondary batteries have a large expansion rate of the negative electrode due to the deposition of Li during charging, resulting in a large change in the volume of the electrode group.

[0016] In secondary batteries, the electrode group undergoes large volume changes during charge and discharge, which can easily cause damage to the components of the electrode group, buckling of the electrodes, internal short circuits, and other problems, resulting in a deterioration in cycle performance. To address this issue, it is conceivable to form a separator by forming spacers on the main surfaces of the substrate and disposing the spacers between the substrate and the electrodes to suppress volume changes in the electrode group. The spacers serve to form a space to absorb the expansion of the electrodes.

[0017] However, when spacers are formed on the main surface of the substrate and disposed between the substrate and the electrode, the spacers are susceptible to the expansion and contraction of the electrode, and the adhesive strength between the substrate and the spacer decreases with repeated charge and discharge (expansion and contraction of the electrode). This reduces the spacer's ability to suppress volumetric changes in the electrode group (hereinafter also referred to as "spacer function"), which can lead to poor cycle performance. The above-mentioned deterioration in spacer function occurs due to spacer misalignment, wrinkles in the substrate due to spacer misalignment, or separation of the spacer from the main surface of the substrate. For example, in a lithium secondary battery in which the spacer is disposed on the negative electrode side and the substrate is disposed on the positive electrode side, Li may precipitate between the substrate and the spacer, causing the spacer to lift off the substrate, resulting in a deterioration in spacer function. In a lithium secondary battery in which the spacer is disposed on the positive electrode side and the substrate is disposed on the negative electrode side, Li deposition may cause a portion of the substrate (the portion exposed in the space formed by the spacer) to collapse toward the positive electrode. In this case, the spacer may shift and wrinkle the substrate, reducing the space (or the contact area between the spacer and the substrate), thereby reducing the spacer function.

[0018] In view of the above, the inventors have conducted extensive research and have newly discovered that by using two separator substrates and placing a spacer between the two substrates, it is possible to significantly suppress the deterioration of the spacer function and the resulting deterioration of cycle characteristics.

[0019] That is, by disposing a spacer between the first substrate and the second substrate, the spacer is protected by the first substrate and the second substrate, making the spacer less susceptible to the effects of electrode expansion and contraction, suppressing a decrease in the adhesive strength between the substrate and the spacer and the resulting deterioration in spacer function. Therefore, the deterioration in cycle characteristics due to the aforementioned deterioration in spacer function is suppressed. For example, the occurrence of an internal short circuit in early cycles and the resulting decrease in capacity are suppressed. In particular, in lithium secondary batteries, the degree of expansion of the negative electrode due to Li deposition is large, so disposing a spacer between the first substrate and the second substrate to form a separator significantly suppresses the deterioration in spacer function.

[0020] From the viewpoints of facilitating the preparation of the electrode group and of facilitating the prevention of deterioration of the spacer function, it is preferable that the first substrate, the second substrate, and the spacer are integrated. From the same viewpoint, it is preferable that the first substrate and the second substrate each have an adhesion strength of 3 N / m or more between themselves and the spacer.

[0021] The adhesion strength between the first substrate and the spacer and the adhesion strength between the second substrate and the spacer are determined by disassembling an initial secondary battery, removing the separator from the electrode group, washing it with an organic solvent (dimethoxyethane), drying it, and then conducting a peel test using a universal testing machine.

[0022] When measuring the adhesion strength between the first substrate and the spacer, the second substrate is removed from the separator by cutting or the like to obtain a first sample in which the first substrate and the spacer are integrated, and a peel test is performed on the first sample.When measuring the adhesion strength between the second substrate and the spacer, the first substrate is removed from the separator by cutting or the like to obtain a second sample in which the second substrate and the spacer are integrated, and a peel test is performed on the second sample.

[0023] When a separator is produced by the method described below, either the first sample or the second sample may be a sample obtained by forming a spacer on either the first substrate or the second substrate by a coating method (printing method), or the other of the first sample and the second sample may be a sample obtained by heat-welding a spacer to the other of the first substrate or the second substrate.

[0024] The peel test can be carried out as follows: As the universal testing machine, for example, a universal testing machine AGS-X manufactured by Shimadzu Corporation can be used.

[0025] The first sample is cut to obtain a strip-shaped test piece (80 mm long, 15 mm wide). Double-sided tape is attached to the spacer side of the test piece and fixed to a fixture of a universal testing machine. The test piece is fixed so that the spacer side faces the fixture. One longitudinal end of the fixed test piece (first substrate) is fixed to the movable fixture of the universal testing machine. The peel test is performed at a peel angle of 90° and a peel rate of 20 mm / min. That is, the movable fixture is moved so as to peel the first substrate in a direction 90° relative to the fixture, and the first substrate of the test piece and the spacer are peeled at a rate of 20 mm / min. At this time, the tensile direction is always maintained at 90° relative to the fixing surface of the fixture fixing the test piece. The average tensile strength when the test piece is peeled over a range of 10 mm to 70 mm is determined as the adhesion strength (N / m) between the first substrate and the spacer. For the second sample, the adhesive strength between the second substrate and the spacer is determined in the same manner as for the first sample.

[0026] A separator in which the first substrate, the second substrate, and the spacer are integrated may be produced, for example, by forming a spacer on one main surface of the first substrate (or the second substrate) by a predetermined coating method (printing method) to obtain an intermediate, and then heat-welding an end of the spacer of the intermediate to one main surface of the second substrate (or the first substrate). In this case, the adhesive strength between the first substrate and the spacer and the adhesive strength between the second substrate and the spacer can both be increased to 3 N / m or more.

[0027] The first substrate and the second substrate may each comprise a porous sheet containing a polymer material. The melting point of the resin material contained in the spacer may be lower than the shutdown temperature of at least one of the porous sheets of the first substrate layer and the second substrate layer. In this case, in the event of abnormal heat generation due to a short circuit or the like, the resin material contained in the spacer melts, blocking the pores of the porous sheet and causing a shutdown, which is advantageous in terms of safety. Note that the shutdown temperature is the temperature at which the pores of the porous sheet melt and close, activating the battery's shutdown mechanism, in the event of abnormal heat generation due to a short circuit or the like in the battery. The blocking of the pores suppresses the movement of lithium ions between the positive and negative electrodes, cutting off the current and avoiding thermal runaway.

[0028] The secondary battery will be described in detail below.

[0029] (Lithium secondary battery) A lithium secondary battery includes a positive electrode, a negative electrode on which lithium metal precipitates during charging and dissolves in a non-aqueous electrolyte during discharge, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode includes at least a negative electrode current collector, and lithium metal precipitates on the negative electrode current collector during charging. The non-aqueous electrolyte has lithium ion conductivity.

[0030] 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).

[0031] (Separator) The separator includes a sheet-like first substrate, a sheet-like second substrate, and a spacer disposed between the first substrate and the second substrate. The spacer is disposed so as to be sandwiched between the first substrate and the second substrate. The first substrate and the second substrate may be the same or different in terms of the material constituting the substrate, the thickness of the substrate, etc. Below, the substrate (matters common to the first substrate and the second substrate) will be described in detail.

[0032] (Substrate) A porous sheet having ion permeability and insulating properties is used as the substrate. 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 may contain an additive, if necessary. Examples of the additive include an inorganic filler.

[0033] The thickness of the substrate 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.

[0034] The substrate may include a porous sheet and a composite material layer (heat-resistant 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 composite material layer contains inorganic particles. The composite material layer may contain a resin material as needed. The thickness of the composite material layer may be 5% to 50% of the total thickness of the substrate.

[0035] The composite material layer may be disposed on the side of the porous sheet facing the positive electrode, or on the side of the porous sheet facing the negative electrode. When the composite material layer is disposed on the positive electrode side, deterioration of the porous sheet due to oxidation reactions can be suppressed. When the composite material layer is disposed on the negative electrode side, deterioration of the porous sheet due to reduction reactions can be suppressed. A spacer may be disposed on the composite material layer. In this case, the effect of suppressing thermal shrinkage of the substrate is particularly enhanced.

[0036] The inorganic particles are preferably particles of an inorganic compound that is thermally stable and insulating, and thus unlikely to melt or decompose during abnormal heat generation due to a short circuit in the battery, etc. Examples of inorganic particle materials 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 inorganic particles may be 0.2 to 2.0 μm.

[0037] The median diameter in the volume-based particle size distribution of inorganic particles can be measured, for example, using a laser diffraction / scattering particle size distribution measuring device (for example, Microtrac manufactured by Nikkiso Co., Ltd.) Alternatively, the cross section of the substrate may be observed with a transmission electron microscope (TEM), a TEM image may be taken, the area surrounded by the outlines of any 100 inorganic 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.

[0038] Examples of resin materials contained in the composite material layer (heat-resistant layer) include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene, fluororubbers such as vinylidene fluoride-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer, styrene-butadiene copolymer or its hydrogenated product, acrylonitrile-butadiene copolymer or its hydrogenated product, methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, rubbers such as ethylene propylene rubber, ethyl cellulose, methyl cellulose, Examples of the resin include cellulose derivatives such as cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; vinyl resins such as polyvinyl alcohol and polyvinyl acetate; acrylic resins such as polymethyl methacrylate; polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyimide, polyamides such as wholly aromatic polyamide (aramid); polyamideimide, polyacrylonitrile, polyether, polyacrylic acid, polymethacrylic acid, polyester, polyolefin, silicone resin, urethane resin, melamine resin, urea resin, and epoxy resin.

[0039] The resin material contained in the composite material layer (heat-resistant layer) 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.

[0040] 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).

[0041] The composite material layer is formed, for example, by applying a coating liquid containing inorganic particles, a resin material, and a liquid component (dispersion medium) to a porous sheet and then drying the coating film. Examples of the liquid component include N-methyl-2-pyrrolidone.

[0042] (Spacer) In a lithium secondary battery, the main role of the spacer is to form a space for lithium metal to deposit in. By accommodating lithium metal in the space formed by the spacer, volume change of the electrode group is suppressed.

[0043] The spacer includes a conductive material and / or an insulating material. Among them, an insulating material is preferable. Since lithium metal is less likely to deposit on the surface of an insulating material, the effect of disposing the spacer can be easily obtained.

[0044] The spacer may contain a resin material (e.g., an insulating resin material) or may contain a resin material and particles. The proportion of the resin material in the spacer may be 10% by volume or more, 30% by volume or more, or 50% by volume or more, or may be 100% by volume or less, or 80% by volume or less.

[0045] Examples of resin materials contained in the spacer include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene, fluororubbers such as vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and ethylene-tetrafluoroethylene copolymer, rubbers such as styrene-butadiene copolymer or hydrogenated product, acrylonitrile-butadiene copolymer or hydrogenated product, methacrylic acid ester-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, and ethylene propylene rubber, and ethyl acrylate copolymer. Examples of the resin include cellulose derivatives such as cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose, vinyl resins such as polyvinyl alcohol and polyvinyl acetate, acrylic resins such as polymethyl methacrylate, polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyimide, polyamides such as wholly aromatic polyamide (aramid), polyamideimide, polyacrylonitrile, polyether, polyacrylic acid, polymethacrylic acid, polyester, polyolefin, silicone resin, urethane resin, melamine resin, urea resin, and epoxy resin.

[0046] Among the above resin materials, preferred materials that are impermeable to lithium ions include polyimide, polyvinylidene fluoride, and acrylonitrile-acrylic acid ester copolymers, and polyimide may also be used. A non-porous spacer of a certain height or greater made of these resin materials is a layer that is impermeable to lithium ions and has a non-porous structure that does not allow lithium ions to pass through. The placement of such a spacer is preferred from the viewpoint of suppressing an increase in the gas generation reaction rate during an internal short circuit.

[0047] The particles may be inorganic or organic. Examples of inorganic particles include insulating metal oxides, metal hydroxides, metal nitrides, metal carbides, and metal sulfides. Examples of metal oxides include aluminum oxide (alumina and 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, and titania are preferred.

[0048] The average particle size of the particles is not particularly limited, but may be 0.1 μm or more, 0.5 μm or more, or 10 μm or less, 5 μm or less, or 2 μm or less. The average particle size can be measured by the following method. First, a cross-section of the spacer in the thickness direction of the separator is photographed using an electron microscope to obtain an image of the cross-section. Next, the image is subjected to image processing such as binarization to identify the particle portion. Next, the diameter of a circle having the same area as the cross-section area of ​​each particle (equivalent circle diameter) is determined, and the arithmetic mean of the determined equivalent circle diameters can be used as the average particle size. The arithmetic mean can be determined, for example, from 100 or more particles. The average particle size of other particles contained in the electrode plate and separator can also be determined by a similar method.

[0049] When the spacer contains a resin material and particles, the particle content in the spacer is preferably 50% by volume or less, which makes it easier to ensure sufficient strength of the spacer.

[0050] The spacer includes one or more members (protrusions). The spacer may include a plurality of line-shaped members or a plurality of dot-shaped members. The line-shaped members may be arranged continuously. The line-shaped members may be arranged intermittently. That is, the line-shaped members may have defects in some parts. The line-shaped members may be straight or curved. The width of the line-shaped members may be 100 μm or more or 200 μm or more, and may be 2000 μm or less or 1000 μm or less.

[0051] The spacer preferably has a predetermined repeating pattern. That is, the spacer preferably includes one or more members (protrusions) arranged in a predetermined repeating pattern. The line-shaped members may be arranged in a striped pattern. The spacer may include a plurality of line-shaped members arranged parallel to each other along the length direction of the elongated first substrate and second substrate. The line-shaped members may also be arranged in a mesh-like pattern. The mesh-like pattern may be a collection of polygons. An example of a mesh-like pattern includes a shape in which polygons are combined to share sides. Polygons include triangles, quadrilaterals, hexagons, etc. Different types of polygons may also be combined. The mesh-like pattern may be a honeycomb pattern. Dot-shaped members may also be arranged in a predetermined repeating pattern.

[0052] When the separator is viewed from the normal direction of the main surface of the substrate, the ratio of the area S1 of the spacer disposed in the region where the positive electrode and the negative electrode face each other to the area S0 of the region where the positive electrode and the negative electrode face each other (S1 / S0×100) is preferably, for example, 5% or more and 20% or less. When the ratio of the area S1 is 5% or more, the spacer is likely to stably form a space. When the ratio of the area S1 is 20% or less, the internal resistance is likely to be reduced.

[0053] The height H (h in FIG. 2 ) of the spacer may be greater than the thickness T of the substrate. The ratio of the height H to the thickness T: H / T may be greater than 1, 1.5 or greater, 2 or greater, or 3 or greater. H / T may be 10 or less, 8 or less, 5 or less, or 4 or less. By making H / T 1.5 or greater, expansion of the electrode group can be particularly suppressed.

[0054] The height H can be measured by the following method. First, a cross section of the separator (substrate) in the thickness direction is photographed using an electron microscope to obtain an image of the cross section. Next, 20 arbitrary locations among the spacers are selected in the image, and the heights of the spacers at those locations are measured. Next, the heights measured at the 20 locations are arithmetically averaged, and the obtained average value is defined as the height H. The thickness T can also be measured using a similar procedure.

[0055] The spacers are formed, for example, by applying a coating liquid containing a spacer component and a liquid component to a predetermined location on the substrate and drying the coating. Examples of the liquid component include N-methyl-2-pyrrolidone. The coating may be performed using a dispenser or a known printing method such as gravure printing, inkjet printing, or screen printing. The drying may be performed by a known method such as heating or natural drying. The spacers may be formed, for example, on the surface of a porous sheet of the substrate or on the surface of a composite material layer of the substrate.

[0056] (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.

[0057] 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.

[0058] 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.

[0059] 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 material, and the like in addition to the negative electrode active material.

[0060] 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).

[0061] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0062] 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.

[0063] The negative electrode current collector may be a conductive sheet, such as a foil or film.

[0064] 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.

[0065] 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.

[0066] (Positive Electrode) The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector. The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode mixture 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 mixture 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.

[0067] 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.

[0068] 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.

[0069] 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 Ni, Co, 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.

[0070] Among lithium-containing transition metal oxides, composite oxides containing Ni, Co, and / or Mn as transition metal elements, and which may contain Al as an optional component, and which have 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.

[0071] As the binder, conductive material, 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.

[0072] 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).

[0073] 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.

[0074] (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.

[0075] 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.

[0076] 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.

[0077] The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent, which generates lithium ions and anions.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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).

[0085] (Lithium-ion battery) A lithium-ion battery includes a positive electrode, a negative electrode containing a negative electrode active material that absorbs and releases lithium ions, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The positive electrode and non-aqueous electrolyte may be the same as those exemplified for lithium secondary batteries. The positive electrode composite and positive electrode current collector contained in the positive electrode may be appropriately selected from those exemplified above. The non-aqueous solvent and lithium salt (anion) contained in the non-aqueous electrolyte may be appropriately selected from those exemplified above.

[0086] The negative electrode includes, for example, a negative electrode current collector and a negative electrode composite layer (the above-mentioned lithium ion occlusion layer) supported on a main surface of the negative electrode current collector. The negative electrode composite layer may be supported on one main surface of the negative electrode current collector, or may be supported on both main surfaces of the negative electrode current collector. The negative electrode composite and negative electrode current collector included in the negative electrode can be appropriately selected from those exemplified above.

[0087] The substrate and spacer included in the separator can be the same as those used in lithium secondary batteries. The substrate may include a porous sheet containing a polymer material. The substrate may further include a composite material layer containing a resin material and inorganic particles. The spacer may include a resin material.

[0088] In the case of a lithium-ion battery, for example, the spacer may be disposed at a location where stress is likely to increase when the negative electrode expands. The spacer may be disposed at a folded portion of an electrode assembly in which positive and negative electrodes are stacked in a zigzag pattern, at the innermost periphery of a wound electrode assembly, or at a location with a small radius of curvature of a flat electrode assembly.

[0089] FIG. 1 is a longitudinal sectional view schematically illustrating an example of a secondary battery according to an embodiment of the present disclosure.

[0090] 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 (not shown) 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 body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16. The gasket 27 ensures the airtightness 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 winding axis direction.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 2 is an enlarged view of a portion of the electrode group 14. FIG. 2 includes a portion near the positive electrode surrounded by region II in FIG. 1 and a portion near the negative electrode surrounded by region III in FIG.

[0095] The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 50. The positive electrode 11, the negative electrode 12, and the separator 50 (substrates 60, 70) are all strip-shaped. The electrode group 14 is formed by winding the positive electrode 11, the negative electrode 12, and the separator 50 such that the separator is disposed between the positive electrode 11 and the negative electrode 12.

[0096] The positive electrode 11 includes a positive electrode current collector and positive electrode mixture layers formed on both sides of the positive electrode current collector. The positive electrode current collector 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.

[0097] The separator 50 includes a sheet-like first substrate 60, a sheet-like second substrate 70, and a spacer 80 disposed between the first substrate 60 and the second substrate 70. The first substrate 60 is disposed on the negative electrode 12 side, and the second substrate 70 is disposed on the positive electrode 11 side. The first substrate 60 includes a porous sheet 61 and a composite material layer 62 (heat-resistant layer). The second substrate 70 includes a porous sheet 71 and a composite material layer 72 (heat-resistant layer).

[0098] The composite material layers 62, 72 are formed on the main surfaces of the porous sheets 61, 71 on the spacer 80 side. The spacer 80 forms a space 14s between the positive electrode 11 and the negative electrode 12 (between the first substrate 60 and the second substrate 70). Figure 2 shows the height h of the spacer 80.

[0099] In the illustrated example, the first substrate 60 is positioned on the negative electrode 12 side, and the second substrate 70 is positioned on the positive electrode 11 side, but the first substrate 60 may also be positioned on the positive electrode 11 side, and the second substrate 70 may also be positioned on the negative electrode 12 side.

[0100] In the illustrated example, the composite material layer 62 is disposed on the main surface of the porous sheet 61 facing the spacer 80, but may be disposed on the main surface of the porous sheet 61 facing the negative electrode 12. The composite material layer 72 is disposed on the main surface of the porous sheet 71 facing the spacer 80, but may be disposed on the main surface of the porous sheet 71 facing the positive electrode 11.

[0101] 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.

[0102] 3 and 4 show examples of the planar shape of the spacer 80. Figures 3 and 4 are top views of an intermediate 90 in which the spacer 80 is formed on one main surface (the main surface on the second substrate 70 side) of the first substrate 60, and are views of the spacer 80 as viewed from the normal direction of the main surface of the first substrate 60. In the plan view, the spacer 80 is disposed on one main surface (the main surface on the second substrate 70 side) of the first substrate 60, and the spacer 80 is formed of linear members 80a. A space 14s is formed in an area where the spacer 80 is not disposed.

[0103] The spacer 80 in Fig. 3 includes linear members 80a arranged in a honeycomb pattern. The spacer 80 in Fig. 4 includes a plurality of linear members 80a arranged in a stripe pattern. The linear members 80a are arranged parallel to one another along the length direction (winding direction) of the strip-shaped first substrate 60. The linear members 80a are arranged spaced apart from one another.

[0104] In Fig. 4, six linear members 80a are arranged, but the number is not limited to this. While the linear members 80a in Figs. 3 and 4 are formed continuously, they may be formed intermittently by providing defects in some of them. The defects may be provided randomly or in a regular repeating pattern. The arrangement pattern of the spacers is not limited to the honeycomb pattern in Fig. 3 or the stripe pattern in Fig. 4. The spacers may be arranged in a dot pattern, for example.

[0105] The spacers 80 (linear members 80 a) located on both sides of the positive electrode 11 may be arranged to overlap each other entirely or partially, or may be arranged not to overlap each other. The same applies to the spacers 80 (linear members 80 a) located on both sides of the negative electrode 12.

[0106] 1 is a cylindrical lithium secondary battery equipped with a wound electrode group, but the secondary battery according to the present disclosure is not limited to this. The shape of the secondary battery can be appropriately selected from various shapes such as cylindrical, coin, prismatic, sheet, and flat depending on the application. The shape of the electrode group is also not particularly limited, and may be a stacked type.

[0107] (Additional Notes) The above embodiments disclose the following technologies. (Technology 1) A 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, wherein the separator includes a sheet-like first substrate, a sheet-like second substrate, and a spacer disposed between the first substrate and the second substrate, wherein one of the first substrate and the second substrate is disposed on the positive electrode side, and the other of the first substrate and the second substrate is disposed on the negative electrode side. (Technology 2) The secondary battery according to Technology 1, wherein the first substrate, the second substrate, and the spacer are integrated. (Technology 3) The secondary battery according to Technology 1 or 2, wherein the first substrate and the second substrate each have an adhesion strength of 3 N / m or more between themselves and the spacer. (Technology 4) The secondary battery according to any one of Technology 1 to 3, wherein the spacer includes an insulating material. (Technology 5) The secondary battery according to any one of Technologies 1 to 4, wherein the spacer has a non-porous structure that is impermeable to lithium ions. (Technology 6) The secondary battery according to any one of Technologies 1 to 5, wherein the spacer includes a resin material. (Technology 7) The secondary battery according to Technology 6, wherein the first substrate and the second substrate each include a porous sheet including a polymer material, and wherein the melting point of the resin material included in the spacer is lower than the shutdown temperature of the porous sheet of at least one of the first substrate layer and the second substrate layer. (Technology 8) The secondary battery according to any one of Technologies 1 to 7, wherein the spacer includes one or more members arranged in a predetermined repeating pattern. (Technology 9) The secondary battery according to any one of Technologies 1 to 8, wherein the first substrate and the second substrate each have an elongated shape, and wherein the spacer includes a plurality of linear members arranged parallel to each other along the length direction of the first substrate and the second substrate. (Technology 10) The secondary battery according to any one of Technologies 1 to 9, wherein lithium metal precipitates in the negative electrode during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharging. (Technology 11) A separator for a secondary battery, comprising: a first substrate in a sheet shape, a second substrate in a sheet shape, and a spacer disposed between the first substrate and the second substrate.(Technology 12) The secondary battery separator according to Technology 11, wherein the first substrate, the second substrate, and the spacer are integrated. (Technology 13) The secondary battery separator according to Technology 11 or 12, wherein the first substrate and the second substrate each have an adhesion strength of 3 N / m or more between themselves and the spacer. (Technology 14) The secondary battery separator according to any one of Technology 11 to 13, wherein the spacer contains an insulating material. (Technology 15) The secondary battery separator according to any one of Technology 11 to 14, wherein the spacer has a non-porous structure that is impermeable to lithium ions. (Technology 16) The secondary battery separator according to any one of Technology 11 to 15, wherein the spacer contains a resin material. (Technology 17) The secondary battery separator according to Technology 16, wherein the first substrate and the second substrate each comprise a porous sheet containing a polymer material, and the melting point of the resin material contained in the spacer is lower than the shutdown temperature of the porous sheet of at least one of the first substrate layer and the second substrate layer. (Technology 18) The secondary battery separator according to any one of Technology 11 to 17, wherein the spacer includes one or more members arranged in a predetermined repeating pattern. (Technology 19) The secondary battery separator according to any one of Technology 11 to 18, wherein the first substrate and the second substrate each have an elongated shape, and the spacer includes a plurality of linear members arranged parallel to each other along the length direction of the first substrate and the second substrate.

[0108] [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.

[0109] Secondary Batteries A1-A2 (Preparation of Positive Electrode) A positive electrode active material, acetylene black (AB; conductive material), polyvinylidene fluoride (PVdF; binder), and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were mixed to prepare a positive electrode mixture slurry. The positive electrode active material was 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 was 1.0) and having a layered structure. The mass ratio of NCA:AB:PVdF in the positive electrode mixture slurry was 95:2.5:2.5. The positive electrode mixture slurry was applied to both sides of a strip of Al foil (positive electrode current collector), and the coating was dried and rolled to form a positive electrode mixture layer. The positive electrode current collector having the positive electrode mixture layers formed on both sides was cut to a predetermined size to obtain a positive electrode.

[0110] (Preparation of Negative Electrode Current Collector) A strip of electrolytic copper foil (thickness: 12 μm) was prepared as a negative electrode current collector.

[0111] (Preparation of Substrate) A polyethylene microporous thin film (thickness 10 μm, porosity 50%) was prepared. One main surface of the microporous thin film was coated with a coating liquid containing paraphenylene terephthalamide, an aromatic polyamide, as the resin material and alumina as inorganic particles. The coating liquid was an N-methyl-2-pyrrolidone solution containing 5.8% by mass of dissolved calcium chloride, and the concentration was adjusted to 2 wt% of aromatic polyamide and 4 wt% of alumina. The substrate with the coating film formed was left for 1 hour in an atmosphere at 25°C and 70% relative humidity to precipitate the aromatic polyamide. Next, NMP and calcium chloride in the coating film were removed by rinsing with water. The coating film was dried at 60°C for 5 minutes to form a composite material layer (heat-resistant layer). In this way, a substrate comprising a microporous thin film and a composite material layer was obtained. The thickness of the substrate, including the microporous thin film and the composite material layer, was 15 μm.

[0112] (Formation of spacers on the main surface of the substrate) A coating liquid containing polyvinylidene fluoride and alumina particles (inorganic filler) was applied to the main surface of the substrate facing the microporous thin film, and the coating was dried to form spacers on the main surface of the substrate facing the microporous thin film.

[0113] The spacers were formed in the pattern shown in Fig. 3 (honeycomb pattern) or the pattern shown in Fig. 4 (striped pattern). The linear members constituting the spacers had a width of 0.5 mm and a height of 30 µm. The spacing (pitch) between the linear members in the width direction of the substrate was 5 mm. In this way, an intermediate body having the substrate and spacers (intermediate body 90 in Fig. 3 or 4) was obtained.

[0114] (Preparation of Separator: Integration of Intermediate and Substrate) The same substrate as above was separately prepared. The ends of the spacer (linear member) of the intermediate obtained above were heat-welded to the main surface of the substrate on the microporous thin film side. In this way, the spacer was disposed between the two substrates to obtain a separator. The adhesion strength between the two substrates and the spacer, determined by the method described above, was 3 N / m or more.

[0115] (Preparation of non-aqueous electrolyte) 1,2-dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (CHF ２ (CF ２ OCH ２ )CF ３ An ether-based mixed solvent containing lithium bis(sulfonylimide) (LiFSI) at 1 mol / L and LiBF at a volume ratio of 1:2 was prepared. ２ (C ２ O ４ ) was dissolved in the ethanol to a concentration of 0.1 mol / L to prepare a liquid non-aqueous electrolyte (ether-based electrolyte solution).

[0116] (Assembly of Secondary Battery) In an inert gas atmosphere, a positive electrode and a negative electrode current collector were spirally wound with a separator interposed therebetween to prepare an electrode assembly. In this way, a wound electrode assembly having the structure shown in Fig. 2 was obtained. At this time, a separator was disposed between the positive electrode and the negative electrode so that one of the two substrates was disposed on the negative electrode side and the other of the two substrates was disposed on the positive electrode side.

[0117] The electrode group was housed in a cylindrical case body with a bottom, and a non-aqueous electrolyte was poured into it. A sealing member was placed over the opening of the case body via a gasket, sealing the electrode group and non-aqueous electrolyte inside the battery case. In this way, a lithium secondary battery having the structure shown in FIG. 1 was completed.

[0118] Secondary Battery B1 Secondary battery B1 was fabricated in the same manner as secondary battery A1, except that the separator was formed by overlapping two substrates without using a spacer.

[0119] Secondary Battery B2: A different substrate from the one described above was used as one of the two substrates. The different substrate was a porous sheet with a high porosity and thickness. That is, the different substrate was constructed by forming the same composite material layer as described above on one main surface of a polypropylene porous sheet (porosity 66%, thickness 50 μm). Except for the above, secondary battery B2 was fabricated in the same manner as secondary battery B1.

[0120] Secondary Battery B3: The intermediate was used as a separator. The separator was arranged so that the main surface of the substrate on which the spacer was formed faced the negative electrode. Except for the above, secondary battery B3 was fabricated in the same manner as secondary battery A2.

[0121] Secondary Battery B4: The intermediate was used as a separator. The separator was arranged so that the main surface of the substrate on which the spacer was formed faced the positive electrode. Except for the above, secondary battery B4 was fabricated in the same manner as secondary battery A2.

[0122] The secondary batteries obtained above were evaluated as follows.

[0123] [Evaluation 1: Number of cycles at which abnormality occurred] (Charge-discharge cycle test) Each of the obtained batteries was subjected to a charge-discharge cycle test. In the charge-discharge cycle test, the battery was charged in a thermostatic chamber at 25° C. under the following conditions, then rested for 20 minutes, and discharged under the following conditions.

[0124] (Charging) The battery was charged at a constant current of 10 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 4.1 V. Thereafter, the battery was charged at a constant voltage of 4.1 V until the current value per unit area of ​​the electrode reached 1 mA.

[0125] (Discharge) The battery was discharged at a constant current of 10 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 3.0 V.

[0126] The above charge and discharge constitute one cycle, and charge and discharge were repeated. When the charge capacity at the mth cycle was 1% or more higher than the charge capacity at the previous (m-1)th cycle, it was determined that abnormal charging had occurred due to the occurrence of a small internal short circuit, and the charge and discharge test was terminated. The number of cycles at this time, m, was calculated as the number of cycles at which abnormal charging had occurred. If abnormal charging had not occurred up to 300 cycles, it was determined that there was "no abnormality."

[0127] [Evaluation 2: Swelling ratio of electrode group] Charging was performed under the same conditions as above to obtain a secondary battery in a charged state after the initial charge. The distance D0 between the negative electrode and the positive electrode was determined using a CT image of the cross section of this secondary battery (electrode group). The distance D0 was determined by measuring the distance between the positive and negative electrodes at four arbitrarily selected points and calculating the average value.

[0128] Three cycles of charge and discharge were performed under the same conditions as above, and then charging was performed to obtain a secondary battery in a charged state after three cycles of charge and discharge. The distance D1 between the negative electrode and the positive electrode was determined using the same method as above. The swelling ratio of the electrode group was calculated by (D1 / D0) x 100.

[0129] The evaluation results are shown in Table 1. In Table 1, A1 and A2 are examples, and B1 to B4 are comparative examples.

[0130]

[0131] In secondary batteries A1 and A2, the volume change of the electrode group was small, no abnormalities were observed after 300 cycles, the occurrence of internal short circuits was suppressed, and cycle characteristics were improved. In secondary batteries A1 and A2, in which the negative electrode swells significantly due to Li deposition during charging, the deterioration of the spacer function was suppressed, and the volume change of the electrode group during charging and discharging was small.

[0132] In secondary batteries B1 and B2, the volume change of the electrode group was large because no spacer was placed, and abnormalities occurred early in the cycle. In secondary battery B2, a different substrate with high porosity and thickness was used, which somewhat absorbed the swelling of the negative electrode compared to secondary battery B1, and the volume change of the electrode group was slightly smaller, but the significant effect of using a spacer was not obtained. In addition, the strength of the different substrate was low, so abnormalities occurred early in the cycle.

[0133] In secondary batteries B3 and B4, the spacer function deteriorated during charge and discharge, the volume of the electrode group changed significantly, and an abnormality (internal short circuit) occurred early in the cycle. In secondary battery B3, the spacer was placed on the negative electrode side and the substrate was placed on the positive electrode side. In this secondary battery B3, Li precipitated between the substrate and the spacer, causing the spacer to lift off the substrate, which is thought to have deteriorated the spacer function.

[0134] In secondary battery B4, the spacer was placed on the positive electrode side and the substrate was placed on the negative electrode side. In this secondary battery B4, Li deposition caused a part of the substrate (the part exposed in the space formed by the spacer) to collapse toward the positive electrode, which in turn caused the spacer to shift and wrinkle the substrate, reducing the space (or the contact area between the spacer and the substrate). This is thought to have reduced the spacer function.

[0135] The secondary battery separator of the present disclosure can be used in lithium ion batteries, lithium secondary batteries, and the like.

[0136] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0137] 10: Lithium secondary battery, 11: Positive electrode, 12: Negative electrode, 14: Electrode group, 14s: Space, 15: Case body, 16: Sealing body, 23: Lower valve body, 25: Upper valve body, 60: First substrate, 70: Second substrate, 61, 71: Porous sheets, 62, 72: Composite material layers, 80: Spacer, 80a: Line-shaped member, 90: Intermediate body

Claims

1. A 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, wherein the separator includes a first sheet-like substrate, a second sheet-like substrate, and a spacer disposed between the first substrate and the second substrate, wherein one of the first substrate and the second substrate is disposed on the positive electrode side, and the other of the first substrate and the second substrate is disposed on the negative electrode side.

2. The secondary battery according to claim 1, wherein the first substrate, the second substrate, and the spacer are integrated.

3. The secondary battery according to claim 2, wherein the first substrate and the second substrate each have an adhesion strength of 3 N / m or more between themselves and the spacer.

4. The secondary battery according to any one of claims 1 to 3, wherein the spacer includes an insulating material.

5. The secondary battery according to any one of claims 1 to 3, wherein the spacer has a non-porous structure that is impermeable to lithium ions.

6. The secondary battery according to any one of claims 1 to 3, wherein the spacer contains a resin material.

7. The secondary battery according to claim 6, wherein the first substrate and the second substrate each comprise a porous sheet containing a polymer material, and the melting point of the resin material contained in the spacer is lower than the shutdown temperature of the porous sheet of at least one of the first substrate layer and the second substrate layer.

8. The secondary battery according to any one of claims 1 to 3, wherein the spacer comprises one or more members arranged in a predetermined repeating pattern.

9. A secondary battery according to any one of claims 1 to 3, wherein the first substrate and the second substrate are each elongated, and the spacer includes a plurality of linear members arranged parallel to one another along the length of the first substrate and the second substrate.

10. The secondary battery according to any one of claims 1 to 3, wherein lithium metal is precipitated on the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharging.

11. A separator for a secondary battery, comprising: a sheet-like first substrate; a sheet-like second substrate; and a spacer disposed between the first substrate and the second substrate.

12. The secondary battery separator according to claim 11, wherein the first substrate, the second substrate, and the spacer are integrated.

13. The secondary battery separator according to claim 12, wherein the first substrate and the second substrate each have an adhesive strength of 3 N / m or more between themselves and the spacer.

14. The secondary battery separator according to any one of claims 11 to 13, wherein the spacer includes an insulating material.

15. The secondary battery separator according to any one of claims 11 to 13, wherein the spacer has a non-porous structure that is impermeable to lithium ions.

16. The secondary battery separator according to any one of claims 11 to 13, wherein the spacer contains a resin material.

17. A separator for a secondary battery according to claim 16, wherein the first substrate and the second substrate each comprise a porous sheet containing a polymer material, and the melting point of the resin material contained in the spacer is lower than the shutdown temperature of the porous sheet of at least one of the first substrate layer and the second substrate layer.

18. The secondary battery separator according to any one of claims 11 to 13, wherein the spacer comprises one or more members arranged in a predetermined repeating pattern.

19. A secondary battery separator according to any one of claims 11 to 13, wherein the first substrate and the second substrate are each elongated, and the spacer includes a plurality of linear members arranged parallel to one another along the length of the first substrate and the second substrate.

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