Non-aqueous electrolyte secondary batteries

Abstract

This non-aqueous electrolyte secondary battery comprises: an electrode assembly in which a positive electrode and a negative electrode are laminated via a separator; and a non-aqueous electrolyte. The non-aqueous electrolyte includes an SO2 bond-containing lithium salt and an isocyanate group-containing compound. With respect to the mass of the non-aqueous electrolyte, the concentration of the lithium salt is preferably 0.1-2.5 mass% and the concentration of the compound is preferably 0.1-8 mass%.

Description

[Technical Field]

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

[0002] It has been known that in non-aqueous electrolyte secondary batteries such as lithium-ion batteries, micro-short circuits can occur due to metallic foreign matter mixed into the electrode body. When metallic foreign matter is mixed into the electrode body, for example, the metallic foreign matter oxidizes and dissolves on the positive electrode side. The dissolved metal ions have a positive charge and move to the negative electrode side, where they are reduced and precipitated on the negative electrode surface, forming needle-shaped deposits called dendrites. The dendrites then grow and break through the separator, forming a conductive path between the positive and negative electrodes, causing a micro-short circuit.

[0003] Possible methods for preventing such short circuits include strengthening control over the amount of metallic foreign matter mixed in (preventive methods) and mitigating the effects of the mixed metallic foreign matter (neutralization methods). One proposed technology for neutralizing metallic foreign matter is to use a separator with a specific layered structure, control the air permeability in the thickness direction and surface direction of each layer, and further provide a negative electrode potential to some conductive layers, thereby performing low-rate, long-term minute charging (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-099275 Summary of the Invention

[0005] According to the method disclosed in Patent Document 1, it is possible to disperse and precipitate the mixed metal foreign matter, but the separator with controlled air permeability is expensive, and a mechanism for maintaining a negative electrode potential must be installed inside the battery to perform long-term micro-charging, which creates productivity issues. The purpose of this disclosure is to suppress the occurrence of micro-short circuits caused by mixed metal foreign matter in the electrode body without compromising battery productivity or battery performance such as output characteristics.

[0006] The nonaqueous electrolyte secondary battery according to the present disclosure is a nonaqueous electrolyte secondary battery including an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, and a nonaqueous electrolyte, the nonaqueous electrolyte including a lithium salt (A) containing an SO bond and a compound (B) containing an isocyanate group. In the nonaqueous electrolyte secondary battery according to the present disclosure, the composite layer of the negative electrode preferably has a porosity of 35% to 50%.

[0007] The nonaqueous electrolyte secondary battery according to the present disclosure can suppress the occurrence of micro-short circuits caused by metallic foreign matter without impairing battery productivity or battery performance such as output characteristics, etc. The nonaqueous electrolyte secondary battery according to the present disclosure can sufficiently mitigate the effects of metallic foreign matter even if the metallic foreign matter is mixed into the electrode body, thereby preventing the formation and growth of dendrites on the negative electrode surface. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view showing the appearance of a nonaqueous electrolyte secondary battery as an example of the embodiment. [Figure 2] FIG. 2 is a perspective view of an electrode assembly according to an embodiment. [Figure 3] FIG. 3 is a cross-sectional view of an electrode assembly according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] The inventors of the present invention conducted extensive research to prevent micro-short circuits caused by metallic impurities, and as a result, have succeeded in suppressing the growth of dendrites on the negative electrode surface by a simple method of adding two specific compounds to a non-aqueous electrolyte. It is believed that the two compounds added to the non-aqueous electrolyte (a lithium salt (A) containing an SO2 bond and a compound (B) containing an isocyanate group) promote the oxidative elution of metallic impurities at the positive electrode and inhibit the reductive deposition of the eluted metal ions at the negative electrode. By promoting the oxidative elution of metallic impurities, it is possible to elute all metallic impurities before the product is released to the market, for example, during the inspection process in the battery assembly process. In particular, it is believed that the function of the SO2 bond-containing lithium salt (A) promotes the oxidative elution of metallic impurities, and the function of the isocyanate compound (B) retards the deposition of eluted ions at the negative electrode.

[0010] Furthermore, by controlling the porosity and pore size of the negative electrode composite layer within a specific range, the formation and growth of dendrites can be further suppressed, and the occurrence of micro-short circuits caused by metallic foreign matter can be more reliably prevented. Increasing the amount of isocyanate compound (B) added increases the effect of suppressing the reductive precipitation of metal ions, but this leads to a trade-off in that the input / output characteristics of the battery deteriorate. Similarly, increasing the amount of SO2 bond-containing lithium salt (A) added increases the effect of promoting oxidative elution and also contributes to improving the input / output characteristics, but this leads to a trade-off in that the amount of gas generated during high-temperature storage increases.

[0011] By adding an SO2 bond-containing lithium salt (A) and an isocyanate compound (B) to the non-aqueous electrolyte and controlling the porosity and pore size of the negative electrode composite layer within specific ranges, it is possible to prevent the occurrence of micro-short circuits caused by metallic foreign matter while suppressing the deterioration of input / output characteristics and the increase in the amount of gas generated during high-temperature storage.

[0012] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. It is anticipated from the beginning that multiple embodiments and modifications exemplified below may be selectively combined. Furthermore, in this specification, the expression "numerical value A to numerical value B" means "numerical value A or more and numerical value B or less," unless otherwise specified.

[0013] Fig. 1 is a perspective view showing the appearance of a nonaqueous electrolyte secondary battery 10 according to an embodiment, and Fig. 2 is a perspective view of an electrode assembly 11 constituting the nonaqueous electrolyte secondary battery 10. The nonaqueous electrolyte secondary battery 10 shown in Fig. 1 includes a bottomed, rectangular cylindrical outer can 14 as an outer casing, but the outer casing is not limited to this. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a cylindrical battery including a bottomed, cylindrical outer can, a coin-shaped battery including a coin-shaped outer can, or a laminate battery including an outer casing made of a laminate sheet including a metal layer and a resin layer.

[0014] As shown in FIGS. 1 and 2 , a nonaqueous electrolyte secondary battery 10 includes an electrode assembly 11, a nonaqueous electrolyte, a bottomed rectangular cylindrical outer can 14 that accommodates the electrode assembly 11 and the nonaqueous electrolyte solution, and a sealing plate 15 that closes the opening of the outer can 14. The nonaqueous electrolyte secondary battery 10 is a so-called prismatic battery. The electrode assembly 11 has a wound structure in which a positive electrode 20 and a negative electrode 30 are wound with a separator 40 interposed therebetween. The positive electrode 20, the negative electrode 30, and the separator 40 are all strip-shaped, elongated bodies, and the positive electrode 20 and the negative electrode 30 are stacked with the separator 40 interposed therebetween and wound around a winding axis. The electrode assembly may also be a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with separators interposed therebetween.

[0015] The nonaqueous electrolyte secondary battery 10 includes a positive electrode terminal 12 electrically connected to the positive electrode 20 via a positive electrode current collector 25, and a negative electrode terminal 13 electrically connected to the negative electrode 30 via a negative electrode current collector 35. In this embodiment, the sealing plate 15 has an elongated rectangular shape, with the positive electrode terminal 12 disposed at one longitudinal end of the sealing plate 15 and the negative electrode terminal 13 disposed at the other longitudinal end of the sealing plate 15. The positive electrode terminal 12 and the negative electrode terminal 13 are external connection terminals electrically connected to other nonaqueous electrolyte secondary batteries 10, various electronic devices, etc., and are attached to the sealing plate 15 via insulating members.

[0016] For ease of explanation, the height direction of the outer can 14 will be referred to as the "vertical direction" of the nonaqueous electrolyte secondary battery 10, the sealing plate 15 side will be referred to as the "top," and the bottom side of the outer can 14 will be referred to as the "bottom." Additionally, the direction along the longitudinal direction of the sealing plate 15 will be referred to as the "lateral direction" of the nonaqueous electrolyte secondary battery 10.

[0017] The outer can 14 is a metal container in the shape of a rectangular cylinder with a bottom. An opening formed at the top end of the outer can 14 is closed, for example, by welding a sealing plate 15 to the edge of the opening. The sealing plate 15 is generally provided with a liquid injection portion 16 for injecting a non-aqueous electrolyte, a gas exhaust valve 17 for opening to exhaust gas in the event of a battery abnormality, and a current interruption mechanism (not shown). The outer can 14 and the sealing plate 15 are made of a metal material containing, for example, aluminum as a main component.

[0018] The electrode body 11 is a flat, wound electrode body including a flat portion and a pair of curved portions. The electrode body 11 is housed in the outer can 14 with the winding axis direction aligned with the lateral direction of the outer can 14 and the width direction of the electrode body 11, along which the pair of curved portions are aligned, aligned with the height direction of the battery. In this embodiment, a positive electrode side current collector formed by laminating the substrate exposed portion 23 of the positive electrode 20 at one axial end of the electrode body 11 and a negative electrode side current collector formed by laminating the substrate exposed portion 33 of the negative electrode 30 at the other axial end are formed, and each current collector is electrically connected to a terminal via a current collector. Note that an insulating electrode body holder (insulating sheet) may be disposed between the electrode body 11 and the inner surface of the outer can 14.

[0019] Hereinafter, the positive electrode 20, negative electrode 30, and separator 40 constituting the electrode assembly 11, particularly the negative electrode 30, will be described in detail with reference to Fig. 3. The nonaqueous electrolyte will also be described in detail. Fig. 3 shows a state in which a metallic foreign object 100 has been mixed between the positive electrode 20 and the separator 40.

[0020] [Positive electrode] As shown in FIG. 3 , the positive electrode 20 includes a positive electrode core 21 and a positive electrode composite layer 22 formed on the surface of the positive electrode core 21. The positive electrode core 21 can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 20, or a film having such a metal disposed on its surface. The positive electrode composite layer 22 contains a positive electrode active material, a conductive material, and a binder, and is preferably formed on both sides of the positive electrode core 21. In this embodiment, a core exposed portion 23, in which the core surface is exposed along the longitudinal direction, is formed at one end in the width direction of the positive electrode 20. The positive electrode 20 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, a binder, and the like onto the positive electrode core 21, drying the coating, and then compressing it to form the positive electrode composite layer 22 on both sides of the positive electrode core 21.

[0021] A lithium transition metal composite oxide is used as the positive electrode active material. Metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0022] Examples of the conductive material contained in the positive electrode mixture layer 22 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 22 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, and polyolefin resin. These resins may also be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or a salt thereof, or polyethylene oxide (PEO).

[0023] [Negative electrode] The negative electrode 30 includes a negative electrode core 31 and a negative electrode composite layer 32 formed on the surface of the negative electrode core 31. The negative electrode core 31 can be a foil of a metal, such as copper, that is stable within the potential range of the negative electrode 30, or a film with such a metal disposed on its surface. The negative electrode composite layer 32 contains a negative electrode active material and a binder, and is preferably formed on both sides of the negative electrode core 31. In this embodiment, a core exposed portion 33, in which the core surface is exposed along the longitudinal direction, is formed at one end in the width direction of the negative electrode 30. The positive electrode 20 and the negative electrode 30 are stacked via a separator 40 so that the core exposed portions 23, 33 are located on opposite sides of the electrode body 11 in the axial direction. The negative electrode 30 can be fabricated, for example, by applying a negative electrode composite slurry containing a negative electrode active material and a binder to the negative electrode core 31, drying the coating, and then compressing it to form a negative electrode composite layer 32 on both sides of the negative electrode core 31.

[0024] The negative electrode mixture layer 32 contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may also be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0025] Examples of Si-based active materials include compounds with a structure in which Si particles are dispersed in a silicon oxide phase (SiO), and compounds with a structure in which Si particles are dispersed in a lithium silicate phase. A preferred SiO has a sea-island structure in which fine Si particles are substantially uniformly dispersed in an amorphous silicon oxide matrix, and has the general formula SiO x (0.5 ≦ x ≦ 1.6). A preferred compound has a sea-island structure in which fine Si particles are substantially uniformly dispersed in a lithium silicate matrix represented by the general formula Li 2z SiO (2+z) (0 < z < 2).

[0026] For the binder contained in the negative electrode composite material layer 32, similar to the case of the positive electrode 20, a fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc. can be used, but it is preferable to use styrene-butadiene rubber (SBR). Further, the negative electrode composite material layer 32 preferably further contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. Among them, it is preferable to use SBR in combination with CMC or its salt, and PAA or its salt.

[0027] The volume-based median diameter (hereinafter referred to as "D50") of the carbon-based active material is, for example, 5 μm to 30 μm, preferably 10 μm to 25 μm. The negative electrode composite material layer 32 may contain two or more types of carbon-based active materials having different D50s. D50 means the particle diameter at which the cumulative frequency becomes 50% from the smaller particle diameter in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of the graphite particles can be measured using a laser diffraction type particle size distribution measuring device (for example, MT3000II manufactured by Microtrac Bell Co., Ltd.) with water as the dispersion medium.

[0028] The BET specific surface area of the carbon-based active material is, for example, 3 m 2 / g to 8 m 2 / g, preferably 4 m 2 / g to 5 m 2 / g. When the BET specific surface area of the carbon-based active material is within this range, for example, a nonaqueous electrolyte solution can easily penetrate into negative electrode mixture layer 32, resulting in good output characteristics. The BET specific surface area of the carbon-based active material is measured by the BET method using a conventionally known specific surface area measuring device (for example, Macsorb (registered trademark) HM model-1201 manufactured by Mountech Co., Ltd.).

[0029] The porosity of the negative electrode mixture layer 32 is preferably 25% to 50%, more preferably 30% to 50%, and particularly preferably 35% to 50%. If the porosity of the negative electrode mixture layer 32 is within this range, it is possible to prevent the occurrence of micro-short circuits caused by metallic foreign matter 100 while suppressing a decrease in input / output characteristics and an increase in the amount of gas generated during high-temperature storage. Note that if the porosity exceeds 50%, the negative electrode mixture layer 32 becomes more likely to fall off from the surface of the negative electrode core 31. The porosity of the negative electrode mixture layer 32 is calculated by the formula: porosity (%) = mixture layer density / true density. The true density of the negative electrode mixture layer 32 can be measured using a pycnometer.

[0030] It is preferable that the negative electrode mixture layer 32 satisfies the above porosity and has a median pore diameter of 1.90 μm or less as measured by mercury intrusion porosimetry. The median pore diameter refers to the pore diameter at which the cumulative frequency in the pore distribution is 50% from the smallest pore diameter. If the porosity of the negative electrode mixture layer 32 is approximately the same, a smaller median pore diameter and a larger number of pores is considered to facilitate the penetration of metal ions generated by the oxidative dissolution of the foreign metal matter 100 into the negative electrode 30 and the progression of precipitation therein, thereby suppressing the formation and growth of dendrites.

[0031] The median pore size of the negative electrode composite layer 32 is preferably 1.85 μm or less, more preferably 1.80 μm or less, and particularly preferably 1.75 μm or less. The lower limit of the median pore size is not particularly limited, but an example is 1.3 μm. The median pore size of the negative electrode composite layer 32 is measured by mercury intrusion porosimetry using a conventionally known pore size distribution measuring device (e.g., AutoPore IV 9500 manufactured by Micromeritic).

[0032] The porosity and median pore diameter of the negative electrode mixture layer 32 can be controlled within the above-described preferred ranges by appropriately changing the types of components of the negative electrode mixture layer 32, the compounding ratio of the components, the degree of compression of the negative electrode mixture layer 32, etc. For example, physical properties such as the D50, BET specific surface area, and compressive strength of the carbon-based active material affect the porosity and pore diameter of the negative electrode mixture layer 32. Generally, when an active material having a particle size distribution with good packing properties is used and the negative electrode mixture layer 32 is strongly compressed, the porosity and median pore diameter of the negative electrode mixture layer 32 tend to become smaller.

[0033] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 40. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 40 include polyethylene, polypropylene, polyolefins such as copolymers of ethylene and α-olefins, and cellulose. The separator 40 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 40.

[0034] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt. Examples of the non-aqueous solvent that can be used include 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 a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0035] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate, chain carbonates 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 chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate. Among these, it is preferable to use at least one selected from EC, EMC, and DMC, and it is particularly preferable to use a mixed solvent of EC, EMC, and DMC.

[0036] Examples of the ethers include 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, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, and the like. Examples of suitable ethers include chain ethers such as ethyl 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, and tetraethylene glycol dimethyl ether.

[0037] The non-aqueous electrolyte further contains a lithium salt (A) containing an SO bond and an isocyanate compound (B) containing an isocyanate group. As described above, adding the SO bond-containing lithium salt (A) and the isocyanate compound (B) to the non-aqueous electrolyte promotes oxidative elution of the foreign metal matter 100 at the positive electrode 20 and suppresses reductive precipitation of the eluted metal ions on the surface of the negative electrode 30. This suppresses the formation and growth of dendrites on the surface of the negative electrode 30, preventing the occurrence of micro-short circuits caused by the foreign metal matter 100. The SO bond-containing lithium salt (A) and the isocyanate compound (B) are dissolved in the non-aqueous solvent.

[0038] Examples of the SO bond-containing lithium salt (A) include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium fluorosulfonate, lithium trifluoromethanesulfonate, etc. Among these, compounds containing fluorine are preferred, and LiFSI is particularly preferred.

[0039] The concentration of the SO2 bond-containing lithium salt (A) is preferably 0.1 to 2.5 mass%, more preferably 0.5 to 2.5 mass%, and particularly preferably 1.0 to 2.5 mass%, relative to the mass of the non-aqueous electrolyte. If the content of the SO2 bond-containing lithium salt (A) is within this range, it is possible to prevent the occurrence of micro-short circuits caused by metallic foreign matter 100, while suppressing a decrease in input / output characteristics and an increase in the amount of gas generated during high-temperature storage.

[0040] The isocyanate compound (B) is preferably a compound containing two or more isocyanate groups in the molecule. Specific examples include hexamethylene diisocyanate (HDI), dicyclohexylmethane diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, 1,3-bisisocyanatomethylcyclohexane, diisocyanatomethane, 1,3-diisocyanatopropane, 1,4-diisocyanatobutane, 1,5-diisocyanatopentane, 1,6-diisocyanatohexane, 1,7-diisocyanatoheptane, 1,8-diisocyanatooctane, 1,12-diisocyanatododecane, 1,3-diisocyanato-2-fluoropropane, 1,4-diisocyanato-2-butene, 1,4-diisocyanato-2-fluorobutane, 1,4-diisocyanato-2,3-difluorobutane, 1,5-diisocyanato diisocyanato-2-pentene, 1,5-diisocyanato-2-methylpentane, 1,6-diisocyanato-2-hexene, 1,6-diisocyanato-3-hexene, 1,6-diisocyanato-3-fluorohexane, 1,6-diisocyanato-3,4-difluorohexane, 1,3-bis(isocyanatomethyl)cyclohexane, carbonyl diisocyanate, 1,4-diisocyanatobutane-1,4-dione, 1,5-diisocyanatopentane-1,5-dione, diisocyanatobenzene, xylene diisocyanate, ethyl diisocyanatobenzene, trimethyl diisocyanatobenzene, diisocyanatonaphthalene, diisocyanatobiphenyl, 2,2-bis(isocyanatophenyl)hexafluoropropane, and the like. Among these, at least one selected from HDI, dicyclohexylmethane diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, and 1,3-bisisocyanatomethylcyclohexane is preferred, and HDI is particularly preferred.

[0041] The concentration of the isocyanate compound (B) is preferably 0.1% by mass to 8% by mass, more preferably 0.5% by mass to 8% by mass, and particularly preferably 1.5% by mass to 8% by mass with respect to the mass of the non-aqueous electrolyte. If the content of the isocyanate compound (B) is within this range, it is possible to prevent the occurrence of minute short circuits caused by metal foreign matter 100 while suppressing the deterioration of input / output characteristics and the increase in the amount of gas generated during storage at high temperatures.

[0042] In addition to the SO2 bond-containing lithium salt (A), the non-aqueous electrolyte preferably contains other lithium salts as electrolyte salts. Specific examples of the other lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), and the like. Among them, LiPF6 is preferable.

[0043] <Example> Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0044] <Example 1> [Production of positive electrode] As the positive electrode active material, the general formula LiNi 1 / 3 Co 1 / 3 Mn 1 / 3A lithium transition metal composite oxide represented by O2 was used. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 90.3:7:2.7, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil, leaving only the area where the positive electrode lead was connected. The coating was then dried and compressed, and cut to the specified electrode size to obtain a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core.

[0045] [Preparation of negative electrode] As a negative electrode active material, D50 is 1.85 μm, BET specific surface area is 4.5 m 2 / g of graphite was used. The negative electrode active material, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a solids mass ratio of 99:0.6:0.4, and water was used as the dispersion medium to prepare a negative electrode composite slurry. Next, the negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, leaving only the area where the negative electrode lead was connected. The coating was dried, compressed with a predetermined force, and then cut to the specified electrode size to obtain a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core. The porosity of the negative electrode composite layer was 31.8%, and the median pore diameter measured by mercury porosimetry was 1.85 μm.

[0046] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 25:35:40 (25°C, 1 atmosphere). LiFSI was added to the mixed solvent to a concentration of 2.5 mass% relative to the mass of the non-aqueous electrolyte, and hexamethylene diisocyanate (HDI) was added to a concentration of 8 mass%. LiPF6 was also added to a concentration of 1.15M to obtain a non-aqueous electrolyte.

[0047] [Test cell construction] Test cells for testing micro-short circuits due to the inclusion of metallic foreign matter and test cells for performance evaluation of output characteristics and gas generation amount were fabricated using the following methods.

[0048] <Micro-short circuit test cell> An aluminum lead was ultrasonically welded to a positive electrode cut to the specified shape and dimensions, and a nickel lead was ultrasonically welded to a negative electrode cut to the specified shape and dimensions. The positive electrode was a square 38 mm long and 27 mm wide, with a base on which composite layers were formed on both sides and an exposed core portion (12 mm long and 10 mm wide) protruding from one end of the base in the vertical direction. The negative electrode was a square 41 mm long and 30 mm wide, with a base on which composite layers were formed on both sides and an exposed core portion (10 mm long and 10 mm wide) protruding from one end of the base in the vertical direction.

[0049] Next, the positive and negative electrodes with welded leads were stacked via a multilayer separator including a polypropylene layer and a polyethylene layer to produce a laminated electrode assembly. At this time, spherical metal (copper) particles with a diameter of 30 μm were mixed between the positive electrode and the separator. The fabricated electrode assembly and the nonaqueous electrolyte were housed in an exterior case made of an aluminum laminate sheet, and the opening was sealed to obtain a micro-short circuit test cell (100 mm long, 45 mm wide).

[0050] <Performance evaluation cell> An aluminum lead was ultrasonically welded to a positive electrode cut to a predetermined shape and dimensions, and a nickel lead was ultrasonically welded to a negative electrode cut to a predetermined shape and dimensions. The positive electrode was a strip-shaped, long, strip with a width of 50 mm and a length of 230 mm, with a core exposed portion formed at one end in the longitudinal direction. The positive electrode lead was welded to one side of the core exposed portion at a position 7 mm from one end in the longitudinal direction of the positive electrode. The negative electrode was a strip-shaped, long, strip with a width of 52 mm and a length of 330 mm, with core exposed portions formed at both ends in the longitudinal direction. The negative electrode lead was welded to one side of the core exposed portion at a position 18 mm from one end in the longitudinal direction of the negative electrode.

[0051] Next, the positive and negative electrodes with welded leads were spirally wound with a multilayer separator including a polypropylene layer and a polyethylene layer interposed therebetween, and then pressed at a predetermined pressure to produce a flat wound electrode assembly. The positive and negative electrodes were wound so that the leads of the positive and negative electrodes were positioned on the outside of the winding. The produced electrode assembly and the nonaqueous electrolyte solution were housed in an exterior case made of an aluminum laminate sheet, and the opening was sealed to obtain a performance evaluation cell.

[0052] [Micro-short circuit test] The micro-short circuit test cells were charged at 25°C and adjusted to a state of charge (SOC) of 8%. They were then left standing at 25°C for 12 hours, and the voltage drop from the start of the standing period was monitored to determine whether a micro-short circuit had occurred. This test was performed on 10 test cells, and the number of cells in which a micro-short circuit had occurred was counted to determine the micro-short circuit occurrence rate.

[0053] [Evaluation of output characteristics] The performance evaluation cell was charged in a temperature environment of 25°C and adjusted to a state of charge of 50% SOC. After that, it was discharged for 10 seconds at current values of 1 to 36 It at 25°C, and the output resistance (DCIR) was calculated from the voltage drop 10 seconds after the start of discharge.

[0054] [Evaluation of gas generation rate] The performance evaluation cell was charged in a temperature environment of 25°C and adjusted to a state of charge of 50% SOC. It was then left to stand at 75°C for 16 hours, and the amount of gas generated was calculated using the Archimedes method.

[0055] <Example 2> Test cells were produced in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, the LiFSI concentration was changed to 1.0 mass % and the HDI concentration was changed to 1.5 mass %, and evaluations were carried out.

[0056] Example 3 A test cell was produced in the same manner as in Example 2, except that in producing the negative electrode, the specified pressure during compression was changed, and the porosity of the negative electrode composite layer was set to 36.4% and the median pore diameter was set to 1.75 μm, and various evaluations were performed.

[0057] Example 4 A test cell was produced in the same manner as in Example 2, except that in producing the negative electrode, the specified pressure during compression was changed, and the porosity of the negative electrode composite layer was set to 40.9% and the median pore diameter was set to 1.65 μm, and each evaluation was performed.

[0058] <Comparative Example 1> Test cells were prepared in the same manner as in Example 2, except that LiFSI was not added in the preparation of the non-aqueous electrolyte solution, and various evaluations were carried out.

[0059] <Comparative Example 2> Test cells were prepared in the same manner as in Example 2, except that HDI was not added in the preparation of the non-aqueous electrolyte solution, and various evaluations were carried out.

[0060] <Comparative Example 3> Test cells were prepared in the same manner as in Example 3, except that LiFSI and HDI were not added in preparing the non-aqueous electrolyte solution, and various evaluations were carried out.

[0061] [Table 1]

[0062] As shown in Table 1, the test cells of the examples all had a lower incidence of micro-short circuits than the test cells of the comparative examples, significantly reducing the occurrence of short circuits caused by metallic foreign matter. In particular, the test cells of Examples 1, 3, and 4 had a 0% incidence of short circuits, more reliably preventing the occurrence of short circuits. Furthermore, the test cells of Examples 3 and 4, which had a higher porosity in the negative electrode composite layer than the test cells of Examples 1 and 2, had excellent output characteristics, generated less gas during high-temperature storage, and had a low incidence of short circuits. [Explanation of symbols]

[0063] 10 Nonaqueous electrolyte secondary battery 11 Electrode body 12 Positive terminal 13 Negative terminal 14 Outer can 15 Sealing plate 16 Injection section 17 Gas exhaust valve 20 positive electrode 21 Positive electrode core 22 Positive electrode mixture layer 23,33 Exposed core part 25 Positive electrode current collector 30 negative electrode 31 Negative electrode core 32 Negative electrode composite layer 35 Negative electrode current collector 40 Separator 100 Metallic foreign bodies

Claims

[Claim 1] A non-aqueous electrolyte secondary battery including an electrode assembly formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, and a non-aqueous electrolyte, The non-aqueous electrolyte is SO ２ a lithium salt (A) containing a bond and a compound (B) containing an isocyanate group, the negative electrode includes a negative electrode core and a negative electrode mixture layer formed on a surface of the negative electrode core, the porosity of the negative electrode mixture layer is 35% to 50%, the negative electrode mixture layer has a median pore diameter measured by mercury porosimetry of 1.75 μm or less, a concentration of the lithium salt (A) of 0.1% by mass to 2.5% by mass and a concentration of the compound (B) of 0.1% by mass to 8% by mass relative to the mass of the nonaqueous electrolyte;

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