Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery (10) comprises: an electrode body (14) that includes a positive electrode (20), a negative electrode (30), and a separator (40), the positive electrode (20) and the negative electrode (30) being wound with the separator (40) therebetween; and a nonaqueous electrolyte. The electrode body (14) has adhesive layers (50) respectively provided between the positive electrode (20) and the separator (40) and between the negative electrode (30) and the separator (40). The nonaqueous electrolyte contains 0.1-1.0 mass% of alicyclic diisocyanate in terms of the total mass of the electrolyte.

Description

Non-aqueous electrolyte secondary battery

[0001] This disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery equipped with a wound electrode body.

[0002] Conventionally, a wound electrode body is known that has adhesive layers provided between the positive electrode and the separator, and between the negative electrode and the separator, respectively, in order to suppress displacement of the positive and negative electrodes due to charging and discharging of the battery (see, for example, Patent Document 1). The adhesive layers maintain a constant distance between the electrodes and contribute to improving the cycle characteristics of the battery.

[0003] Japanese Patent Publication No. 2024-061427

[0004] Incidentally, achieving both good cycle characteristics and storage characteristics in non-aqueous electrolyte secondary batteries is an important challenge. However, our research has shown that when batteries using electrodes with an adhesive layer are stored in a high-temperature environment while charged, a large amount of gas is generated. It is thought that the decomposition of the non-aqueous electrolyte is accelerated by the moisture contained in the adhesive layer, resulting in increased gas generation and a decrease in the battery's storage characteristics.

[0005] The purpose of this disclosure is to provide a non-aqueous electrolyte secondary battery with excellent cycle characteristics and storage characteristics.

[0006] The non-aqueous electrolyte secondary battery according to this disclosure comprises an electrode body in which the positive electrode and the negative electrode are wound around the separator, and a non-aqueous electrolyte, wherein the electrode body has adhesive layers provided between the positive electrode and the separator, and between the negative electrode and the separator, and the non-aqueous electrolyte contains an alicyclic diisocyanate in an amount of 0.1% by mass or more and 1.0% by mass or less of the total mass of the electrolyte.

[0007] The non-aqueous electrolyte secondary battery relating to this disclosure has excellent cycle characteristics and storage characteristics.

[0008] This is a perspective view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. This is a cross-sectional view of an electrode body, which is an example of an embodiment. This is an enlarged view of a part of the cross-section of the electrode body.

[0009] As described above, our inventors' investigations have shown that when a battery using an electrode body with an adhesive layer is stored in a charged state in a high-temperature environment, the amount of gas generated increases, and the storage characteristics of the battery deteriorate. This deterioration in storage characteristics is thought to be mainly due to moisture contained in the adhesive layer. When water enters the battery, the non-aqueous electrolyte reacts with the water and decomposes, producing hydrofluoric acid. Hydrofluoric acid is thought to dissolve, for example, the metal components of the positive electrode active material, resulting in gas generation.

[0010] The inventors, through diligent research to solve the above problems, have found that by adding a specific amount of alicyclic diisocyanate to a non-aqueous electrolyte, the amount of gas generated is significantly reduced even when an electrode body having an adhesive layer is used. It is believed that the water contained in the adhesive layer is captured by the alicyclic diisocyanate, and the generation of hydrofluoric acid is effectively suppressed, resulting in improved storage characteristics. The non-aqueous electrolyte secondary battery according to this disclosure makes it possible to achieve both excellent cycle characteristics and storage characteristics. However, if the electrode body does not have an adhesive layer, storage characteristics are good even without adding alicyclic diisocyanate, but cycle characteristics are reduced.

[0011] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery according to this disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the components of the multiple embodiments and modified examples described below are included within the scope of this disclosure.

[0012] In the following, a pouch-type battery comprising an outer casing 11 composed of laminate sheets 11a and 11b is given as an example, but the outer casing is not limited thereto. Another example of an embodiment is a non-aqueous electrolyte secondary battery, which may be, for example, a cylindrical battery with a cylindrical outer casing, a rectangular battery with a rectangular outer casing, or a coin-type battery with a coin-shaped outer casing. However, as will be described in detail later, the configuration of the present disclosure is suitable for batteries with flattened electrode bodies, and in particular for pouch-type batteries in which the outer casing is composed of a flexible sheet.

[0013] Figure 1 is a perspective view showing the external appearance of a non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises an outer casing 11 composed of two laminate sheets 11a and 11b. The non-aqueous electrolyte secondary battery 10 also comprises an electrode body 14 housed in the outer casing 11 and a non-aqueous electrolyte. The outer casing 11 has a substantially rectangular shape in plan view and includes a housing portion 12 in which the electrode body 14 and the non-aqueous electrolyte are housed, and a sealing portion 13 formed around the housing portion 12. The sealing portion 13 is formed, for example, by heat sealing the peripheral edges of the laminate sheets 11a and 11b together.

[0014] The laminate sheets 11a and 11b are flexible sheets composed of a film including a metal layer and a resin layer. An example of the metal layer is an aluminum layer or an aluminum alloy layer. The resin layer is, for example, made of polyolefin. The housing portion 12 can be provided by forming a recess capable of housing the electrode body 14 in at least one of the laminate sheets 11a and 11b. In the example shown in Figure 1, the recess is formed only in the laminate sheet 11a. The sealing portion 13 is formed by joining the peripheral edges of the laminate sheets 11a and 11b. In the example shown in Figure 1, the sealing portion 13 is formed in a rectangular frame shape in plan view with approximately the same width, surrounding the housing portion 12.

[0015] The non-aqueous electrolyte secondary battery 10 further includes a positive electrode lead 15 and a negative electrode lead 16 extending from the end of the outer casing 11. The positive electrode lead 15 and the negative electrode lead 16 are thin, plate-shaped conductive members. In the housing 12, the positive electrode lead 15 is connected to the positive electrode 20 of the electrode body 14, and the negative electrode lead 16 is connected to the negative electrode 30 of the electrode body 14. In the example shown in Figure 1, the positive electrode lead 15 and the negative electrode lead 16 are drawn out from the same short side of the outer casing 11, which is roughly rectangular in plan view, at a distance from each other in the short direction of the outer casing 11, and do not come into contact with each other.

[0016] Figure 2 is a cross-sectional view of the electrode body 14 cut perpendicular to the winding axis direction. As shown in Figure 2, the electrode body 14 includes a positive electrode 20, a negative electrode 30, and a separator 40, and has a winding structure in which the positive electrode 20 and the negative electrode 30 are wound via the separator 40. The positive electrode 20, the negative electrode 30, and the separator 40 are all strip-shaped elongated bodies, and are alternately laminated in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 30 is formed with a dimension slightly larger than that of the positive electrode 20 in order to prevent the precipitation of lithium. That is, the negative electrode 30 is formed longer than the positive electrode 20 in the longitudinal direction and the width direction (short side direction). The separator 40 is formed with a dimension at least slightly larger than that of the positive electrode 20, and two sheets are arranged so as to sandwich the positive electrode 20 or the negative electrode 30.

[0017] The electrode body 14 is formed in a flat shape. The electrode body 14 has a flat portion and a pair of curved portions, and is formed by pressing a cylindrical winding structure in the radial direction. The flat portion is a portion where the outer peripheral surface of the electrode body 14 is flat. The curved portion is a portion curved so as to protrude outward in the radial direction of the electrode body 14, and is formed at both ends of the flat portion. In a cross-sectional view of the electrode body 14, the flat portion has a substantially rectangular shape, and the curved portion has a substantially semicircular shape.

[0018] In the positive electrode 20 and the negative electrode 30 constituting the electrode body 14, when the volume changes during charge and discharge, non-uniform stress is likely to act, and for example, displacement of the positive electrode 20 and the negative electrode 30 is more likely to occur compared to a cylindrical electrode body. Further, in the case of a pouch-type battery, since the exterior body 11 is composed of flexible laminate sheets 11a and 11b, the force restraining the movement of the electrode body 14 is weaker compared to a rectangular battery, and non-uniform stress is likely to act on the electrode plate. For this reason, it is preferable to adhere the positive electrode 20 and the negative electrode 30 to the surface of the separator 40 by an adhesive layer 50 (see Figure 3).

[0019] Hereinafter, referring to Figure 3, the positive electrode 20, the negative electrode 30, the separator 40, the adhesive layer 50, and the non-aqueous electrolyte will be described in detail, particularly the adhesive layer 50 and the non-aqueous electrolyte. Figure 3 is an enlarged view of a part of the cross-section of the electrode body 14 shown in Figure 2.

[0020] As shown in Figure 3, the electrode body 14 has adhesive layers 50 provided between the positive electrode 20 and the separator 40, and between the negative electrode 30 and the separator 40. That is, the positive electrode 20 and the separator 40 are bonded to each other by the adhesive layer 50. Similarly, the negative electrode 30 and the separator 40 are bonded to each other by the adhesive layer 50. The adhesive layer 50 maintains a constant distance between the electrodes and contributes to improving the battery's cycle characteristics. As described above, the effect of the adhesive layer 50 is particularly significant in the flattened electrode body 14, and it is also useful from the viewpoint of suppressing electrode plate deformation.

[0021] The adhesive layer 50 only needs to be interposed between the surfaces of the positive electrode 20 and the negative electrode 30 and the surface of the separator 40. It may also be formed on the surfaces of the positive electrode 20 and the negative electrode 30, but in this embodiment, from the viewpoint of improving the productivity of the electrode body 14, it is formed on the surface of the separator 40. Preferably, the adhesive layer 50 is formed on both sides of the separator 40 with the same thickness. The adhesive layer 50 is formed, for example, by applying a paint containing an adhesive component to the surface of the separator 40 and drying the paint film. Alternatively, a sheet constituting the adhesive layer 50 may be attached to the surface of the separator 40.

[0022] [Positive Electrode] The positive electrode 20 comprises a positive electrode core 21 and a positive electrode mixture layer 22 provided on the positive electrode core 21. The positive electrode core 21 can be made of a metal foil that is stable in the potential range of the positive electrode 20, such as aluminum, aluminum alloy, stainless steel, or titanium, or a film with the metal arranged on its surface. The thickness of the positive electrode core 21 is, for example, 10 μm or more and 30 μm or less. The positive electrode mixture layer 22 contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core 21. The average thickness of the positive electrode mixture layer 22 is, for example, 60 μm or more and 120 μm or less on one side of the positive electrode core 21. The positive electrode 20 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto a positive electrode core 21, drying the coating, and then compressing it to form a positive electrode mixture layer 22 on both sides of the positive electrode core 21.

[0023] As the positive electrode active material, a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn is used. Examples of the metal elements contained in the composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, Bi, etc. Among them, it is preferable to contain at least one selected from Ni, Co, and Mn. The lithium transition metal composite oxide has, for example, a layered rock salt structure. The lithium transition metal composite oxide may be used alone or in combination of multiple types. The content of the positive electrode active material is, for example, 90% by mass or more and 99.8% by mass or less based on the mass of the positive electrode mixture layer 22.

[0024] Examples of the conductive agent contained in the positive electrode mixture layer 22 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powders, conductive whiskers, etc. The conductive agent may be used alone or in combination of multiple types. The content of the conductive agent is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less based on the mass of the positive electrode mixture layer 22.

[0025] Examples of the binder contained in the positive electrode mixture layer 22 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer, and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. Further, these resins may be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), etc. The binder may be used alone or in combination of multiple types. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less based on the mass of the positive electrode mixture layer 22.

[0026] [Negative Electrode] The negative electrode 30 comprises a negative electrode core 31 and a negative electrode mixture layer 32 disposed on the negative electrode core 31. The negative electrode core 31 can be made of a metal foil that is stable in the potential range of the negative electrode 30, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with the metal disposed on its surface. The negative electrode mixture layer 32 contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core 31. The negative electrode 30 can be manufactured, for example, by applying a negative electrode slurry containing a negative electrode active material and a binder onto the negative electrode core 31, drying the coating, and then compressing it to form the negative electrode mixture layer 32 on both sides of the negative electrode core 31.

[0027] The negative electrode active material is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally, carbon materials such as graphite are used. Alternatively, elements that alloy with Li, such as Si and Sn, or materials containing these elements may be used as the negative electrode active material. Among these, silicon-containing materials, including Si, are preferred. Furthermore, lithium titanate, which has a higher charge-discharge potential than carbon materials, can also be used as the negative electrode active material. The negative electrode active material may be used alone or in combination of multiple types.

[0028] The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. In particular, it is preferable to use artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and earthy graphite, or mixtures thereof. Examples of silicon-containing materials that function as the negative electrode active material include silicon alloys, silicon compounds, and composite materials containing Si. A preferred silicon-containing material is a composite particle containing an ionic conducting phase and a Si phase dispersed in the ionic conducting phase.

[0029] The binder contained in the negative electrode mixture layer 32 may be fluororesin, olefin resin, PAN, polyimide, polyamide, acrylic resin, etc., as in the case of the positive electrode 20, but polyvinyl acetate, styrene-butadiene rubber (SBR), etc. may also be used. Among these, the use of SBR is preferred. One type of binder may be used alone, or multiple types may be used in combination. Furthermore, the negative electrode mixture layer 32 preferably contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. These function as thickeners in the negative electrode slurry. The binder content is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less of the mass of the negative electrode mixture layer 32. The negative electrode mixture layer 32 may also contain conductive agents such as CNTs.

[0030] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 40. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 40 include polyethylene, polyolefins such as polypropylene, and cellulose. Among these, a separator 40 made of polyolefin is preferred. The separator 40 may have a single-layer structure or a multi-layer structure. The thickness of the separator 40 is, for example, 5 μm to 30 μm, and more preferably 5 μm to 20 μm.

[0031] A heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 40. Alternatively, a filler layer containing an inorganic filler may be formed on the surface of the separator 40. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the separator 40 and drying the coating film. In this embodiment, an adhesive layer 50 is further formed on the surface of the porous sheet constituting the separator 40.

[0032] [Adhesive Layer] The adhesive layer 50 is a layer containing an adhesive that adheres to the surfaces of the positive electrode 20 and the negative electrode 30, and contributes to improving cycle characteristics by suppressing misalignment between the positive electrode 20 and the negative electrode 30 and maintaining a constant distance between the electrodes. The thickness of the adhesive layer 50 is, for example, 0.5 μm or more and 10 μm or less, preferably 0.5 μm or more and 5 μm or less. The adhesive layer 50 constitutes the outermost surface of the separator 40 that is in contact with the positive electrode 20 and the negative electrode 30, and it is preferable that it is formed over a wide area of ​​the surface of the porous sheet that constitutes the separator 40. The adhesive layer 50 is formed over the entire surface of the porous sheet, for example, but may be formed in a predetermined pattern such as stripes, dots, or a grid in plan view.

[0033] The adhesive constituting the adhesive layer 50 is made of a material that has adhesive strength to the surfaces of the positive electrode 20, the negative electrode 30, and the separator 40, and does not dissolve in the non-aqueous electrolyte. The adhesive can be the same resin as the binder of the positive electrode mixture layer 22, for example, fluororesins such as PTFE and PVDF, olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer and ethylene-propylene-butadiene copolymer, or acrylic resins such as PAN, polyimide, polyamide, and ethylene-acrylic acid copolymer. PVDF is preferred among these. The adhesive may be used alone or in combination of multiple types. The adhesive layer 50 may also contain materials other than the adhesive, such as inorganic fillers.

[0034] [Non-aqueous electrolyte] The non-aqueous electrolyte is ionic conductive and, for example, comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. It is preferable to use a lithium salt as the electrolyte salt. The non-aqueous electrolyte further contains an alicyclic diisocyanate in an amount of 0.1% to 1.0% by mass of the total mass of the electrolyte. The adhesive layer 50 of the electrode body 14 contributes to improving the cycle characteristics but is a factor that reduces the storage characteristics. However, by adding an alicyclic diisocyanate to the non-aqueous electrolyte, the reduction in storage characteristics is effectively suppressed.

[0035] Non-aqueous solvents can include, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0036] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0037] Examples of the above ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, etc., and chain ethers such as 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0038] As an example of the lithium salt, LiBF １０ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , Li(P(C ２ O ４ )F ４ ), LiPF ６－ｘ (CnF ２ｎ＋１ ) ｘ (1 < x < 6, n is 1 or 2), LiB １０ Cl １０ , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylic acid, Li[B(C ２ O ４ ) ２ , Li ２ B４ O ７ , Li(B(C ２ O ４ ) F ２ ) and other borates, bisfluorosulfonylimid lithium (LiN(FSO ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ), LiN(C １ F ２ｌ＋１ SO ２ ) (C ｍ F ２ｍ＋１ SO ２ Examples include imide salts such as {l and m are integers greater than or equal to 0}. Among them, LiPF ６ It is preferable.

[0039] The concentration of the lithium salt is, for example, 0.5 mol or more and 3 mol or less per liter of non-aqueous solvent, preferably 0.8 mol or more and 1.5 mol or less. One type of lithium salt may be used alone, or multiple types may be used in combination.

[0040] As described above, alicyclic diisocyanates play an important role in achieving both good cycle characteristics and storage characteristics. In a non-aqueous electrolyte secondary battery 10, it is thought that the decomposition of the non-aqueous electrolyte is accelerated by the moisture contained in the adhesive layer 50, leading to a decrease in storage characteristics. For example, LiPF in the non-aqueous electrolyte ６When this reacts with water, hydrofluoric acid is produced, which dissolves the metal components of the positive electrode active material, generating gas. Our investigations have shown that alicyclic diisocyanates effectively suppress gas generation compared to other diisocyanates such as linear diisocyanates and aromatic diisocyanates. Alicyclic diisocyanates have a high affinity for the electrode plate and are thought to efficiently capture water in the vicinity of the electrode plate.

[0041] The alicyclic diisocyanate is preferably at least one compound selected from those containing a six-membered ring and having 8 to 15 carbon atoms. Examples of suitable alicyclic diisocyanates include at least one selected from isophorone diisocyanate (IPDI), dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexylene diisocyanate, and cyclohexane-1,2- or 1,3-diylbis(methylene)diisocyanate (1,2- or 1,3-bis(isocyanatomethyl)cyclohexane). Among these, 1,3-bis(isocyanatomethyl)cyclohexane is preferred.

[0042] As described above, the content of alicyclic diisocyanate is 0.1% by mass or more and 1.0% by mass or less relative to the total mass of the non-aqueous electrolyte. If the content of alicyclic diisocyanate is 0.1% by mass or more, gas generation during high-temperature storage of the battery can be effectively suppressed. On the other hand, even if an amount of alicyclic diisocyanate exceeding 1.0% by mass is added, the gas generation suppression effect does not increase compared to the case where the content is 1.0% by mass, and problems such as increased resistance can be expected, so it is preferable to set the upper limit of the alicyclic diisocyanate content to 1.0% by mass. The content of alicyclic diisocyanate is more preferably 0.1% by mass or more and 0.5% by mass or less relative to the total mass of the non-aqueous electrolyte, and particularly preferably 0.2% by mass or more and 0.5% by mass or less. In this case, the storage characteristics of the battery can be suppressed more efficiently and effectively.

[0043] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0044] <Example 1> [Preparation of the positive electrode] Lithium cobalt oxide was used as the positive electrode active material. The positive electrode active material, carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97.6:1.2:1.2, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode in which positive electrode mixture layers were formed on both sides of the positive electrode core. Aluminum leads were welded to the exposed parts of the positive electrode core.

[0045] [Fabrication of the negative electrode] Graphite was used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR) dispersion were mixed in a solid content mass ratio of 97.6:1.2:1.2, and water was used as the dispersion medium to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to a negative electrode core made of copper foil, the coating was dried and compressed, and then the negative electrode core was cut to a predetermined electrode size to obtain a negative electrode in which negative electrode mixture layers were formed on both sides of the negative electrode core. Nickel leads were welded to the exposed parts of the negative electrode core.

[0046] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 30:70 (25°C) to form a solvent, to which LiPF is added. ６ The solution was dissolved at a concentration of 1.2 mol / L, and 1,3-bis(isocyanatomethyl)cyclohexane was added to prepare a non-aqueous electrolyte solution so that the alicyclic diisocyanate content was 0.2% by mass.

[0047] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] As a separator, a microporous polyethylene film with a thickness of 16 μm was used, with adhesive layers made of polyvinylidene fluoride (PVDF) formed on both sides. The adhesive layers were formed by applying a PVDF solution, prepared by dissolving PVDF in NMP, to the surface of the microporous polyethylene film and drying the coating. The thickness of each adhesive layer was 2 μm. The positive electrode and the negative electrode were wound in a spiral shape via the separator having the adhesive layers, and the wound body was pressed into a flat shape to obtain an electrode body. The electrode body and the non-aqueous electrolyte were housed in an outer casing made of a laminate sheet containing an aluminum metal layer in an inert atmosphere, and the peripheral edge of the outer casing was heat-sealed to form a sealed portion, thereby fabricating a non-aqueous electrolyte secondary battery, which is a pouch-type battery with dimensions of 69 mm in length, 56 mm in width, 4.9 mm in thickness, and a rated capacity of 3150 mAh.

[0048] <Example 2> A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that the content of 1,3-bis(isocyanatomethyl)cyclohexane was changed to 0.4% by mass.

[0049] <Comparative Example 1> A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that a separator without an adhesive layer was used in the preparation of the electrode body, and alicyclic diisocyanate was not added in the preparation of the non-aqueous electrolyte.

[0050] <Comparative Example 2> A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that a separator without an adhesive layer was used in the preparation of the electrode body.

[0051] <Comparative Example 3> A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that an alicyclic diisocyanate was not added in the preparation of the non-aqueous electrolyte.

[0052] The cycle characteristics and high-temperature storage characteristics of each battery in the examples and comparative examples were evaluated using the method described below, and the evaluation results are shown in Table 1.

[0053] [Evaluation of Cycle Characteristics] Under a temperature environment of 23°C, each battery was charged with a constant current of 0.7C until the battery voltage reached 4.45V, and then charged again with a constant voltage of 4.45V to 1 / 50C. After that, it was discharged with a constant current of 0.5C until the battery voltage reached 3.00V. This charge-discharge cycle was repeated, and the number of cycles at which the discharge capacity fell to 80% or less of the initial discharge capacity of the first cycle was determined.

[0054] [Evaluation of Storage Characteristics] Each battery was charged at a constant current of 0.7C at a temperature of 23°C until the battery voltage reached 4.45V, and then charged again at a constant voltage of 4.45V to 1 / 50C. Afterward, the charged batteries were left (stored) at a temperature of 80°C for 24 hours, and the expansion rate was calculated from the thickness of the batteries before and after the storage test. A lower expansion rate indicates less gas generation and superior storage characteristics. Expansion rate = [(Thickness after storage - Thickness before storage) / Thickness before storage] × 100 (%)

[0055]

[0056] As shown in Table 1, the battery in the example has a higher number of cycles at which the discharge capacity falls below 80% of the initial discharge capacity compared to the battery in the comparative example. Furthermore, the swelling rate of the battery in the example is about the same as or smaller than that of the batteries in comparative examples 1 and 2. In other words, the battery in the example can achieve both good cycle characteristics and storage characteristics.

[0057] In contrast, the battery of Comparative Example 2, which has an electrode body with an adhesive layer and an alicyclic diisocyanate added to the non-aqueous electrolyte, shows a slight improvement in storage characteristics compared to the battery of Comparative Example 1, which does not contain either an adhesive layer or an alicyclic diisocyanate, but no improvement in cycle characteristics is observed. In particular, since the above cycle test is a high-voltage cycle, the volume change of the electrode body (battery) is large, and in the absence of an adhesive layer, adding an alicyclic diisocyanate to the electrolyte does not improve the cycle characteristics.

[0058] Furthermore, the battery of Comparative Example 3, which has an electrode body with an adhesive layer and does not contain alicyclic diisocyanate, shows a slight improvement in cycle characteristics compared to the battery of Comparative Example 1, but its storage characteristics are significantly worse. This is thought to be because a large amount of gas is generated during high-temperature storage due to the influence of moisture contained in the adhesive layer.

[0059] As described above, by adding a specific amount of alicyclic diisocyanate to a non-aqueous electrolyte, gas generation is significantly reduced even when an electrode body with an adhesive layer is used. Water contained in the adhesive layer is captured by the alicyclic diisocyanate, effectively suppressing the generation of hydrofluoric acid. As a result, high-temperature storage characteristics are expected to be greatly improved. In addition, the distance between the positive and negative electrodes is stably maintained over a long period of time by the adhesive layer, resulting in excellent cycle characteristics.

[0060] The configuration of the non-aqueous electrolyte secondary battery described herein exhibits the above effects even when applied to batteries used with low charging voltages, but is particularly effective in batteries with high charging voltages, for example. An example of a suitable charging voltage is 4.45V or higher.

[0061] The present disclosure is further illustrated by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, and a non-aqueous electrolyte, wherein the electrode body has adhesive layers provided between the positive electrode and the separator, and between the negative electrode and the separator, and the non-aqueous electrolyte contains an alicyclic diisocyanate in an amount of 0.1% by mass or more and 1.0% by mass or less of the total mass of the electrolyte. Configuration 2: The non-aqueous electrolyte secondary battery according to Configuration 1, wherein the alicyclic diisocyanate includes a six-membered ring and is selected from at least one compound having 8 to 15 carbon atoms. Configuration 3: The non-aqueous electrolyte secondary battery according to Configuration 2, wherein the alicyclic diisocyanate is 1,3-bis(isocyanatomethyl)cyclohexane. Configuration 4: A non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the content of the alicyclic diisocyanate is 0.1% by mass or more and 0.5% by mass or less of the total mass of the non-aqueous electrolyte. Configuration 5: A non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the electrode body is formed in a flattened shape.

[0062] 10 Non-aqueous electrolyte secondary battery, 11 Outer casing, 11a, 11b Laminate sheets, 12 Housing section, 13 Sealing section, 14 Electrode body, 15 Positive electrode lead, 16 Negative electrode lead, 20 Positive electrode, 21 Positive electrode core, 22 Positive electrode mixture layer, 30 Negative electrode, 31 Negative electrode core, 32 Negative electrode mixture layer, 40 Separator, 50 Adhesive layer

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, and a non-aqueous electrolyte, wherein the electrode body has adhesive layers provided between the positive electrode and the separator, and between the negative electrode and the separator, and the non-aqueous electrolyte contains an alicyclic diisocyanate in an amount of 0.1% by mass or more and 1.0% by mass or less of the total mass of the electrolyte.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the alicyclic diisocyanate is at least one selected from compounds comprising a six-membered ring and having 8 to 15 carbon atoms.

3. The non-aqueous electrolyte secondary battery according to claim 2, wherein the alicyclic diisocyanate is 1,3-bis(isocyanatomethyl)cyclohexane.

4. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the content of the alicyclic diisocyanate is 0.1% by mass or more and 0.5% by mass or less of the total mass of the non-aqueous electrolyte.

5. The electrode body is formed in a flattened shape, as described in any one of claims 1 to 3, for the non-aqueous electrolyte secondary battery.

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