Lithium ion secondary battery

Abstract

To provide a lithium ion secondary battery capable of improving safety while maintaining rate characteristics.SOLUTION: A lithium ion secondary battery according to the present invention includes a positive electrode including a positive electrode mixture layer containing a positive electrode active material, a conductive agent, and a binder, and a current collector foil, in which the positive electrode active material contains a first positive electrode active material that is a layered compound represented by a general formula shown in the following formula (1), and a second positive electrode active material that is a phosphate compound having an olivine structure represented by a general formula shown in the following formula (2), As the conductive agent, carbon nanotubes are contained at a ratio of more than 0.4% by weight and less than 1.0% by weight with respect to the total mass of the positive electrode mixture layer, the average fiber size of the carbon nanotubes is 4nm or more and 10nm or less, and the separators have a coating layer made of an inorganic material on at least one surface of a sheet serving as a base material of the separators. LiaNixCoyM11-x - yO2 (1) LiMnzM2bFe1 - z - bPO4 (2) SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries have the advantage of being small, lightweight, and capable of large capacity due to their high energy density. For example, batteries using high-energy-density active materials such as NCMs, which are primarily composed of nickel, cobalt, and manganese, or layered compounds such as lithium cobalt oxide (LCO), as their cathodes are widely used in a wide range of applications, from small-sized devices such as wearable devices and power tools to large-sized devices such as automobiles. The use of lithium-ion secondary batteries is particularly increasing for large-sized applications, and in recent years, safety has become important due to the growing demand for batteries used in close proximity to people, such as in automobiles.

[0003] Among battery safety issues, overcharging and external short circuits can be improved through system and module design. On the other hand, internal short circuits within cells cannot be prevented by module design alone, and countermeasures must be taken on the cell side. A known test for such countermeasures is the nail penetration test, which simulates an internal short circuit. In this test, a nail is inserted into the battery at a constant speed to forcibly short-circuit the positive and negative electrodes of the battery, enabling the battery's resistance to internal short circuits to be confirmed.

[0004] Lithium-ion secondary batteries use highly flammable organic solvents as electrolytes, and when thermally unstable layered compounds such as NCM are used as the positive electrode active material, there is a risk of fire in the event of an internal short circuit caused by puncture by an external metal piece or nail, etc., and therefore, improvements in safety are required.

[0005] On the other hand, in the structure of conventional lithium-ion secondary batteries, if the resistance of the electrodes is increased to improve safety in the event of an internal short circuit caused by nail penetration, the rate characteristics are sacrificed. Also, if the rate characteristics are increased, safety in the event of an internal short circuit caused by nail penetration tends to deteriorate, so achieving both performances at the same time has been an issue.

[0006] Patent Document 1 aims to suppress separation of the separator and electrodes during cycling and to improve the heat resistance of the separator, by disposing an inorganic particle layer, in which inorganic particles account for 80% or more by volume of the entire layer, on at least one surface of the separator's porous membrane, and by providing a porous resin layer, which is integrated with the inorganic particle layer, on the surface of the inorganic particle layer. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2020-64879 Summary of the Invention [Problem to be solved by the invention]

[0008] However, Patent Document 1 does not mention rate characteristics, and the separator according to Patent Document 1 may have an adverse effect on rate characteristics because the porous resin layer integrated with the inorganic particle layer inhibits the movement of lithium ions.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a lithium ion secondary battery that can improve safety while maintaining rate characteristics. [Means for solving the problem]

[0010] As a result of extensive research to solve the above problems, the inventors discovered that by using carbon nanotubes as the conductive agent for the positive electrode, satisfying certain conditions when mixed with the positive electrode active material, and combining it with a separator having an inorganic coating layer, it is possible to improve safety against nail penetration while maintaining rate characteristics, and thus completed the present invention.

[0011] The lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, and a separator. The positive electrode is composed of a positive electrode composite layer composed of a positive electrode active material, a conductive agent, and a binder, and a current collecting foil. The positive electrode active material includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is a layered compound represented by the general formula shown in Formula (1) below. The second positive electrode active material is a phosphate compound having an olivine structure represented by the general formula shown in Formula (2) below. The conductive agent includes carbon nanotubes in a proportion of more than 0.4 wt % and less than 1.0 wt % relative to the total weight of the positive electrode composite layer. The average fiber diameter of the carbon nanotubes is 4 nm or more and 10 nm or less. The separator has a coating layer made of an inorganic material on at least one surface of a sheet serving as a substrate of the separator. Li a Ni x Co y M1 1-x-y O2 (However, 0 <a≦1.2、0≦x≦0.9、0<y≦1、0<x+y≦1)···(1) LiMn z M2 b Fe 1-z-b PO4 (However, 0 <z≦0.9、0≦b≦0.1、0<z+b<1)···(2) M1 is one or more elements selected from Na, Mg, Sc, Y, Mn, Fe, Cu, Zn, Al, Ti, Cr, Pb, Sb and B, and M2 is one or more elements selected from Na, Mg, Sc, Y, Co, Ni, Cu, Zn, Al, Ti, Cr, Pb, Sb and B.

[0012] In the lithium ion secondary battery according to the present invention, it is more preferable that the carbon nanotubes have an average fiber length of more than 5 μm and not more than 30 μm.

[0013] In the lithium ion secondary battery according to the present invention, in the above invention, the total thickness of the coating layer is more preferably 2.0 μm or more.

[0014] In the lithium ion secondary battery according to the present invention, it is more preferable that the coating layer is formed on both sides of the substrate.

[0015] Furthermore, in the lithium ion secondary battery according to the present invention, when the larger of the tensile strengths in the TD direction and the MD direction of the substrate is defined as A and the smaller of the tensile strengths is defined as B, it is more preferable that the value of A / B is 1.0 or more and 5.0 or less. [Effects of the Invention]

[0016] According to the present invention, it is possible to improve safety while maintaining rate characteristics. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a perspective view illustrating the configuration of a lithium ion secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view taken along line AA shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following description. Furthermore, various modifications and improvements can be made to the present embodiments, and such modifications and improvements can also be included in the present invention. 1 and 2 show an example of the configuration of a laminated lithium ion secondary battery as an embodiment, but the shape of the lithium ion secondary battery of the present invention is not particularly limited and may be flat, cylindrical, prismatic, coin-shaped, etc. The exterior body of the lithium ion secondary battery is also not particularly limited and known materials such as laminate film, aluminum, aluminum alloy, and stainless steel can be used.

[0019] (Embodiment) Fig. 1 is a perspective view illustrating the configuration of a lithium ion secondary battery according to an embodiment of the present invention, and Fig. 2 is a cross-sectional view taken along line AA shown in Fig. 1.

[0020] The lithium ion secondary battery 1 is a laminated lithium ion battery having an exterior body 2 formed into a bag shape by stacking two laminate films with their heat-sealable resin layers facing each other and heat-sealing the outer periphery.

[0021] The exterior body 2 is formed into a bag shape by stacking two laminate films with their heat-sealable resin layers facing each other and heat-sealing the outer periphery. An electrode group 3 and a nonaqueous electrolyte are contained within the exterior body 2. The electrode group 3 is inserted through an opening in the exterior body 2, and the opening of the exterior body 2 is sealed by heat-sealing, thereby hermetically storing the electrode group 3 within the exterior body 2.

[0022] As the laminate film, a composite film in which a heat-sealable resin layer for heat sealing is provided on a metal layer is preferably used. The metal layer is not particularly limited as long as it prevents moisture from entering from the outside and improves the strength of the entire sheet. For example, aluminum foil, stainless steel foil, etc. can be used as the metal layer. The heat-sealable resin layer is not particularly limited, but polyethylene or polypropylene is preferably used from the viewpoint of the temperature range in which heat sealing is possible and the blocking property against the non-aqueous electrolyte. Here, to protect the metal layer, a protective layer may be provided on the surface opposite to the heat-sealable resin layer. The protective layer is not particularly limited, but nylon, PET, etc. are preferably used. Furthermore, to improve adhesion between the metal layer and the heat-sealable resin layer, an adhesive layer may be provided between them.

[0023] As shown in FIG. 2, the electrode group 3 has a structure in which a positive electrode 4, a negative electrode 5, and a separator 6 interposed between the positive electrode 4 and the negative electrode 5 are stacked as a set with the negative electrode 5 positioned as the outermost layer, and multiple such sets are stacked.

[0024] (positive electrode) The positive electrode 4 is composed of a positive electrode current collector 42 and a positive electrode mixture layer 41 formed on one or both sides of the positive electrode current collector 42. The positive electrode 4 is in the form of a plate.

[0025] Positive electrode mixture layer 41 contains a positive electrode active material, a conductive agent, and a binder.

[0026] <Cathode active material> The positive electrode active material includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is a layered compound represented by the following general formula (1). Li a Ni x Co y M1 1-x-y O2 (However, 0 <a≦1.2、0≦x≦0.9、0<y≦1、0<x+y≦1) ···(1) In the above formula (1), M1 is at least one selected from sodium (Na), magnesium (Mg), scandium (Sc), yttrium (Y), manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), aluminum (Al), titanium (Ti), chromium (Cr), lead (Pb), antimony (Sb), and boron (B). In the following description, the positive electrode active material represented by the above formula (1) is referred to as NCM.

[0027] Furthermore, for the purpose of improving cycle characteristics and thermal stability, the surface of the layered oxide may be coated with a film of an inorganic substance such as magnesium oxide, aluminum oxide, aluminum fluoride, niobium oxide, titanium oxide, or tungsten oxide, or an ion-conductive polymer such as polyethylene glycol, polyethylene oxide, or a derivative or salt thereof.

[0028] The second positive electrode active material is a phosphate compound represented by the following formula (2). LiMn z M2 b Fe 1-z-b PO4 (However, 0 <z≦0.9、0≦b≦0.1、0<z+b<1) ···(2) In the above formula (2), M2 is at least one selected from Na, Mg, Sc, Y, cobalt (Co), nickel (Ni), Cu, Zn, Al, Ti, Cr, Pb, Sb, and B. In the following description, the positive electrode active material represented by the above formula (2) is referred to as LMFP.

[0029] The phosphate compound represented by the above formula (2) has an olivine structure and is a material with a high energy density, with a charge / discharge potential similar to that of layered rock-salt oxides. When the active materials are mixed, they exhibit a high energy density in the same upper and lower voltage limits as when layered rock-salt oxides are used. The surface of the olivine compound may be coated with a carbon material to improve electrical conductivity. The weight ratio of the phosphate compound having an olivine structure to the total weight of the positive electrode active material is preferably 60% by weight or less, more preferably 25% by weight or less, from the viewpoint of energy density. Furthermore, the weight ratio is preferably 10% by weight or more, more preferably 20% by weight or more, from the viewpoint of sufficiently reducing the amount of heat generated during thermal decomposition.

[0030] From the viewpoint of energy density, the weight ratio of the positive electrode active material in positive electrode mixture layer 41 is preferably 90 wt % or more relative to the total weight of positive electrode mixture layer 41.

[0031] Whether or not the positive electrode active material contains NCM and LMFP can be confirmed by observation using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS), as well as X-ray diffraction (XRD).

[0032] <Conductive agent> The conductive agent is not particularly limited, and known or commercially available ones can be used. Examples of the conductive agent include carbon black such as acetylene black and ketjen black, carbon nanotubes, carbon fiber, activated carbon, graphite, and other conductive carbons. The conductive agent can be used alone or as a mixture of two or more of these.

[0033] <Binder> The binder is not particularly limited, and known or commercially available binders can be used. Examples include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), acrylic resin, polyvinyl alcohol (PVA), and carboxymethyl cellulose (CMC). The binder can be used as a single substance, or as a mixture or copolymer of two or more substances.

[0034] <Dispersant> A dispersant is not necessarily required, but may be added if it is desired to disperse the conductive agent more effectively. A typical dispersant is polyvinylpyrrolidone (PVP).

[0035] In the lithium-ion secondary battery 1, the positive electrode composite layer 41 preferably contains carbon nanotubes as a conductive agent in a proportion of more than 0.4 wt % and less than 1.0 wt % relative to the total mass of the positive electrode composite layer, and more preferably contains only carbon nanotubes as a conductive agent. If the amount of carbon nanotubes added is 0.4 wt % or less, the rate characteristics are significantly reduced, and if the amount of carbon nanotubes added is 1.0 wt % or more, fire is more likely to occur when a nail is penetrated. This is thought to be due to the increased current flowing from the electrode to the nail when the nail is penetrated.

[0036] The average fiber diameter of the carbon nanotubes is preferably 4 nm or more and 10 nm or less. If the average fiber diameter of the carbon nanotubes is less than 4 nm, the carbon nanotubes tend to tangle and form clumps, which tends to create areas of locally low resistance. If the average fiber diameter of the carbon nanotubes is greater than 10 nm, the number of added carbon nanotubes per weight decreases, resulting in areas where the conductive path is not sufficiently formed, resulting in a decrease in rate characteristics.

[0037] The presence or absence of carbon nanotubes and their average fiber diameter can be confirmed by observing the electrode surface with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fiber diameter can be determined by measuring the short-side length of the carbon nanotube fibers observed at a magnification of 20,000 or more. The average fiber diameter of the carbon nanotubes was determined by obtaining the fiber diameters of 100 or more carbon nanotubes using the above method and taking the number average.

[0038] The fiber length of the carbon nanotubes is preferably 5 μm or more and 30 μm or less. If the fiber length is less than 5 μm, the conductive paths between the particles may not be sufficiently formed, which may result in a decrease in rate characteristics. If the fiber length is longer than 30 μm, aggregation is likely to occur when the slurry is prepared, which may result in areas where the conductive paths are not sufficiently formed, resulting in a decrease in rate characteristics.

[0039] Although the detailed mechanism by which safety is improved while maintaining rate characteristics is not clear, it is believed that the following phenomenon occurs in lithium-ion secondary batteries.

[0040] When carbon black, which is widely used in general lithium-ion secondary batteries, is used as a conductive agent, a thicker layer of conductive agent is formed around the active material particles than when carbon nanotubes are used. This layer is formed by localized aggregation of the conductive agent and is thought to have high electronic conductivity. When a foreign object such as a nail pierces the electrode and causes an internal short circuit, a large current flows locally from the aggregation site, generating a large amount of heat.

[0041] On the other hand, carbon nanotubes are a conductive agent that can easily exist as a thin layer between active material particles, because they can sufficiently ensure rate characteristics even when used in reduced amounts compared to carbon black, etc. This layer is expected to have fewer localized agglomerations than a layer made of carbon black, and fewer areas with locally low resistance. Therefore, the current that flows when a nail penetrates the electrode will be smaller than when carbon black is used, and the amount of heat generated will also be smaller, which is thought to reduce the area where the active material thermally decomposes.

[0042] Furthermore, by using an olivine compound as part of the positive electrode active material and reducing the amount of heat generated during thermal decomposition of the positive electrode, the heat of decomposition of the active material can be kept to a moderate level.Even if a short circuit occurs when a foreign object such as a nail pierces the electrode, causing thermal decomposition of the positive electrode active material in an extremely small area, it is thought that the amount of heat generated can be kept to a level that does not cause thermal decomposition of the active material in the surrounding area.

[0043] From the above, it is speculated that by adding an appropriate amount of carbon nanotubes and further using an olivine compound as part of the positive electrode active material, it is possible to make the resistance distribution uniform and further reduce the amount of heat generated during thermal decomposition, thereby making it possible to maintain rate characteristics while keeping the current and heat generated by an internal short circuit to an appropriate level.

[0044] <Positive electrode current collector> The positive electrode current collector 42 is not particularly limited, and known or commercially available ones can be used. Examples of the positive electrode current collector 42 include rolled foils made of aluminum, aluminum alloys, copper, copper alloys, nickel, or stainless steel, and porous metals such as porous aluminum. Among these candidates for the positive electrode current collector, aluminum or aluminum alloys are preferred because they have high electrical conductivity, excellent corrosion resistance in the electrolyte, and are lightweight metals.

[0045] Whether the constituent materials in the electrode contain an active material capable of absorbing and desorbing lithium can be confirmed by cyclic voltammetry measurement. A coin cell is constructed by preparing the target electrode as the working electrode, using metallic lithium as the counter electrode, and sandwiching a separator between them. By performing several cycles of cyclic voltammetry measurement using an electrochemical test device, it is possible to confirm whether the electrode is capable of absorbing and desorbing lithium by confirming that the redox reaction peak occurs reversibly.

[0046] [Electrode manufacturing method] The positive electrode can be produced, for example, by the following method. First, the positive electrode active material, conductive agent, binder, and dispersant described above are dispersed in a solvent to prepare a positive electrode slurry. A thickener may be further added to the positive electrode slurry. Next, the positive electrode slurry is applied to one or both surfaces of a positive electrode current collector, and then dried to form a positive electrode composite layer. This is then rolled to produce a plate-shaped positive electrode.

[0047] <Solvent> The solvent used in preparing the positive electrode slurry is not particularly limited, and known or commercially available solvents can be used. Examples of the solvent include N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and water. When polyvinylidene fluoride (PVDF) is used as the binder, it is preferable to use N-methyl-2-pyrrolidone (NMP) as the solvent. When styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), or carboxymethyl cellulose (CMC) is used as the binder, it is preferable to use water as the solvent. When polyvinylpyrrolidone (PVP) is used as the dispersant, it is preferable to use polyvinylpyrrolidone (PVP).

[0048] The positive electrode 4 also has a positive electrode lead 43, which is a positive electrode current collector 42 extending from the positive electrode mixture layer 41. The positive electrode lead 43 extends, for example, from the right side surface in FIG. 2 . The positive electrode leads 43 are bundled at their tip ends within the exterior body 2 and joined to each other by ultrasonic welding, resistance welding, or the like. One end of the positive electrode terminal 7 is joined to the joint of the positive electrode lead 43, and the other end extends to the outside through the sealing portion of the exterior body 2.

[0049] (Negative electrode) The negative electrode 5 is composed of a negative electrode current collector 52 and a negative electrode composite layer 51 formed on one or both sides of the negative electrode current collector 52. Here, the negative electrode composite layer 51 of the negative electrode 5 located in the outermost layer is formed on the surface of the negative electrode current collector 52 facing the separator 6. In contrast, the negative electrode composite layer 51 of the negative electrodes 5 located between the positive electrodes 4, excluding the negative electrode 5 located in the outermost layer, is formed on both sides of the negative electrode current collector 52.

[0050] Negative electrode mixture layer 51 contains a negative electrode active material, a conductive agent, and a binder. The negative electrode active material is not particularly limited as long as it is a material capable of absorbing and releasing lithium. Examples of the negative electrode active material include cokes such as pyrolytic carbons, pitch coke, needle coke, and petroleum coke; natural graphite; artificial graphite; mesocarbon microbeads (MCMB); hard carbon; soft carbon; glassy carbon; sintered organic polymer compounds (e.g., phenolic resins, furan resins, etc., sintered and carbonized); carbon fibers; carbon black; activated carbon; metal materials such as Al, Si, and Sn; and alloy materials. Preferred negative electrode active materials are cokes and carbons that have stable charge / discharge characteristics and are prone to forming a solid electrolyte interphase (SEI) coating on their surfaces. Graphite such as natural graphite or artificial graphite is more preferred.

[0051] <Conductive agent> The conductive agent is not particularly limited, and known or commercially available conductive agents can be used. For example, the conductive agent can be the same as that used in the positive electrode described above. Note that a configuration without a conductive agent is also possible.

[0052] <Binder> The binder is not particularly limited, and known or commercially available binders can be used. Examples include fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), fluororubbers, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), core-shell binders, polyvinyl alcohol, and polyimide resins such as polyimide and polyamideimide. The binder can be used as a single substance, or as a mixture or copolymer of two or more substances.

[0053] <Negative electrode current collector> The negative electrode current collector 52 is not particularly limited, and a known or commercially available one can be used. For example, rolled foil made of copper, a copper alloy, aluminum, an aluminum alloy, or stainless steel, or a porous metal such as porous aluminum can be used as the negative electrode current collector 52. The negative electrode current collector is preferably made of copper or a copper alloy.

[0054] Furthermore, the negative electrode 5 has, for example, a negative electrode current collector 52 having a negative electrode lead 53 extending from the negative electrode mixture layer 51. The negative electrode lead 53 extends, for example, from the left side surface in FIG. 2 . The negative electrode leads 53 are bundled at their tip ends within the exterior body 2 and joined to each other. One end of the negative electrode terminal 8 is joined to the joint of the negative electrode lead 53, and the other end extends to the outside through the sealing portion of the exterior body 2.

[0055] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent in which a lithium salt is dissolved. Examples of lithium salts include one or a mixture of two or more selected from the group consisting of LiBF, LiPF, Li(FSO)N, Li(CFSO)N, and LiPOF. The nonaqueous solvent is not particularly limited, but examples include one or a mixture of two or more selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl propionate, ethyl propionate, propyl propionate, methyl acetate, methyl formate, methyl butyrate, dioxolane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethoxyethane, γ-butyrolactone, acetonitrile, and benzonitrile. DMC, DEC, DPC, EMC, EC, and PC are particularly preferred. Among these, it is preferable to include EC, which is capable of forming a good coating on the negative electrode active material.

[0056] (separator) The separator 6 is not particularly limited, and known or commercially available separators can be used. Separator 6 can be, for example, a sheet made of a microporous membrane made of a polyolefin resin such as a polyethylene resin or a polypropylene resin, a polyimide resin, or a polytetrafluoroethylene (PTFE) resin. The microporous membrane or nonwoven fabric may have a single layer or a multilayer structure. Separator 6 has a coating layer made of an inorganic material formed on one or both sides of the microporous membrane to improve heat resistance and nonaqueous electrolyte replenishment.

[0057] The material of the coating layer on the separator 6 and whether or not there is a coating layer can be confirmed by SEM-EDS and XRD. In addition, the thickness of the coating layer on the separator can be confirmed by SEM-EDS after cross-sectioning the separator using a cross-section polisher, a razor, or the like.

[0058] In this embodiment, as described above, the separator 6 has a coating layer made of an inorganic material on at least one side of the substrate sheet. The substrate sheet is formed using a resin. From the viewpoint of ensuring safety against external short circuits and overcharging through the shutdown function, this sheet is preferably made of at least polyethylene (PE). The separator 9 may be made of PE alone, or may have a layer structure in which PE is combined with polypropylene (PP), polyimide, aramid, etc. from the viewpoint of heat resistance and oxidation resistance. For example, a separator having a three-layer structure of PP / PE / PP, in which PP is arranged on both sides of a substrate layer made of PE, may be used. Although the detailed mechanism of the safety improvement provided by the separator 6 is not clear, it is believed that the following phenomenon occurs.

[0059] Carbon nanotubes tend to be distributed thinly and uniformly around the active material particles, resulting in a uniform distribution of resistance. Therefore, when carbon nanotubes are selected as the conductive material for the positive electrode 4 and added in an amount sufficient to maintain rate characteristics, the area of ​​locally low resistance can be reduced. Furthermore, when an olivine compound is used as part of the positive electrode active material, the chain reaction of thermal decomposition caused by localized thermal decomposition of the active material is suppressed. Therefore, when a nail is penetrated, the heat generated during a short circuit is kept to a moderate temperature. If a separator 6 coated with an inorganic material is used, the shorted area will melt due to the generated heat, but the shrinkage of the separator will be minor, preventing the short from expanding and suppressing thermal runaway.

[0060] In contrast, even if carbon nanotubes are used within the amount range specified in this embodiment, if a separator without a coating layer made of an inorganic material is used, the heat generated by the short circuit will cause the separator to shrink, expanding the short circuit area, making it impossible to suppress the subsequent generation of heat, which is thought to lead to thermal runaway.

[0061] Furthermore, even when using an inorganic coated separator, if the electrode contains only the amount of carbon black (CB)-based conductive agent required to ensure the cell's rate characteristics, the acetylene black is unevenly distributed in the gaps between particles, creating areas of low resistance that act as paths, generating large currents locally. This increases the heat generated in the event of a short circuit, and the heated area of ​​the coated separator becomes large. Although the thermal shrinkage rate of the coated separator is small, the wide heating range increases the amount of shrinkage of the separator as a whole, making it impossible to prevent a short circuit and leading to thermal runaway.

[0062] Here, in the separator 6, from the viewpoint of improving internal short-circuit resistance, the base sheet preferably has an A / B value of 1.0 or more and 5.0 or less, where A is the larger tensile strength of the base material in the TD direction and B is the smaller tensile strength. Furthermore, the A / B value is more preferably 1.0 or more and 3.0 or less. Here, the MD direction is the direction in which the resin flows when forming the substrate, and the TD direction is the direction perpendicular to the direction in which the resin flows (MD direction).

[0063] In this case, if the A / B value is greater than 5.0, the separator will tear in one specific direction when a foreign object such as a nail pierces it, but the positive and negative electrodes will have circular or cross-shaped cuts due to the shape of the foreign object, and there will be many areas between the positive and negative electrodes where there is no separator.

[0064] Furthermore, if the A / B value is less than 1.0, the separator will be less likely to tear in one specific direction when pierced by a foreign object such as a nail, and will be more likely to tear in the same shape as the positive and negative electrodes, making it more likely that the separator will get caught between the positive and negative electrodes. When the A / B ratio is between 1.0 and 3.0, the separator is less likely to tear in one direction. This phenomenon reduces the number of short circuits inside the battery, which is thought to improve nail penetration resistance.

[0065] Here, the inorganic material forming the coating layer in the separator 6 is not particularly limited, and any material commonly used as a separator for a lithium ion secondary battery can be used. However, from the viewpoint of cost reduction, it is preferable to use any one of alumina, barium sulfate, and boehmite.

[0066] It is more preferable that the inorganic coating layer is formed on both sides of the substrate sheet. By forming inorganic coating layers on both sides of the substrate, it is possible to prevent the separator 6 from bending in a specific direction when it shrinks due to heat, and it is possible to more reliably suppress the expansion of short-circuit areas.

[0067] The total thickness of the inorganic coating layer per separator is preferably 2.0 μm or more, and more preferably 4.0 μm or more. When the inorganic coating layer has a total thickness of 2.0 μm or more, the thermal shrinkage rate of the coated separator can be significantly reduced, and when it is 4.0 μm or more, the thermal shrinkage rate of the coated separator can be reduced at higher temperatures, making it possible to suppress the expansion of short-circuit areas due to separator shrinkage. Furthermore, from the viewpoint of improving the energy density of the battery, the inorganic coating layer preferably has a thickness of 8.0 μm or less, and more preferably 6.0 μm or less.

[0068] The shape of the electrode element is not particularly limited, and the positive electrode, negative electrode, and separator may be wound, stacked in a sheet-by-sheet manner, or folded in a zigzag pattern.

[0069] Such lithium-ion secondary batteries are preferably subjected to pretreatments such as initial charging and gas evacuation before use. For example, initial charging involves injecting a nonaqueous electrolyte solution and then performing constant current charging at a minute current value down to 10% of the rated capacity. After such initial charging, gas generated within the exterior body during initial charging is vented. Gas evacuation is performed, for example, by placing the exterior body in a decompression chamber with a portion of the sealed portion open. Gas evacuation can be accelerated, for example, by applying pressure to the exterior body from the outside during gas evacuation.

[0070] The capacity of the lithium ion secondary battery 1 is preferably 50 Ah or less. When the capacity is 50 Ah or less, the time for a short circuit to occur in the electrode element when a nail penetrates it is shortened, making it possible to more reliably prevent ignition.

[0071] Whether the constituent materials in the electrode contain an active material capable of absorbing and releasing lithium can be confirmed by cyclic voltammetry measurement. A coin cell is constructed by preparing the target electrode as the working electrode, using lithium metal as the counter electrode, and sandwiching a separator 6 between them. By performing several cycles of cyclic voltammetry measurement using an electrochemical test device, it can be confirmed that the redox reaction peak occurs reversibly, thereby confirming whether the electrode is capable of absorbing and releasing lithium.

[0072] The capacity of the lithium-ion secondary battery 1 can be confirmed by a charge / discharge test. Using the upper and lower limit voltages specified by the manufacturer, charging is performed at 0.5 ItA, CV switching at the upper limit voltage, and cutoff is performed at 0.05 ItA, and discharging is performed at 0.2 ItA and cutoff is performed at the lower limit voltage. This allows the battery capacity to be confirmed from the discharge capacity.

[0073] The rate characteristics of the lithium-ion secondary battery 1 can be confirmed by a charge / discharge test. Using the upper and lower limit voltages specified by the manufacturer, charging is performed at 0.5 ItA, CV switching at the upper limit voltage, and a cutoff of 0.05 ItA. Discharging is performed at 0.2 ItA and 1 ItA, and a cutoff at the lower limit voltage. The discharge capacity at each discharge rate is obtained, and the ratio of the capacity at 1 ItA discharge to the capacity at 0.2 ItA discharge can be confirmed.

[0074] In the present embodiment described above, the lithium ion secondary battery 1 contains carbon nanotubes as a conductive agent in a proportion of more than 0.4 wt % and less than 1.0 wt % relative to the total mass of the positive electrode composite layer, the separator 6 has a coating layer made of an inorganic material on at least one side of the sheet that serves as the base material of the separator, and the average fiber diameter of the carbon nanotubes is 4 nm or more and 10 nm or less, thereby making it possible to improve safety while maintaining rate characteristics. [Example]

[0075] The present invention will be described in more detail below by way of examples, but the present invention is not limited to the following examples in any way.

[0076] Example 1 <Preparation of positive electrode> LiNi as the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2 (commonly called NCM523) at 77.8 wt%, LiMn 0.7 Fe 0.3 A positive electrode active material slurry was prepared by mixing 19.5 wt% of PO4 (LMFP), 0.7 wt% of carbon nanotubes (CNTs) with an average fiber diameter of 8 nm and a length of 20 μm as a conductive agent, 2.0 wt% of polyvinylidene fluoride (PVDF) as a binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a viscosity adjusting solvent.

[0077] The obtained slurry was applied to both sides of a 20 μm thick aluminum foil without through holes as a current collector, dried, and then pressed with a roll press to obtain an electrode. The amount of the electrode mixture layer applied per side was 130 g / m. 2 The electrode density was adjusted to 3.1 g / cc. Thereafter, the positive electrode composite layer was cut so that the uncoated portion protruded from one side of the rectangular portion coated with the positive electrode composite layer in a rectangular shape as a positive electrode current collecting lead.

[0078] <Preparation of negative electrode> A negative electrode active material slurry was prepared by mixing 98.0 wt % graphite as a negative electrode active material, 1.0 wt % styrene butadiene rubber as a binder, 1.0 wt % carboxymethyl cellulose as a thickener, and an appropriate amount of ion-exchanged water as a viscosity adjusting solvent.

[0079] The obtained negative electrode active material slurry was applied to both sides of a 10 μm thick copper foil serving as a negative electrode current collector, and then dried to form a negative electrode composite layer. The amount of the negative electrode composite layer applied per side was 70 g / m. 2 Next, the positive electrode was subjected to press working to set the density of the negative electrode mixture layer to 1.45 g / cm. 3 Thereafter, the portion coated with the negative electrode composite layer was cut so that the uncoated portion protruded in a rectangular shape as a negative electrode current collecting lead from one side of the rectangle.

[0080] <Fabrication of electrode element> Next, electrode elements were fabricated by alternately stacking positive and negative electrodes with zigzag-like separators. The separators were made of a 12-μm-thick polyolefin substrate fabricated by wet-stretching, coated on both sides with alumina particles at a thickness of 2 μm per side (total thickness of 4 μm). The separators used had an A / B ratio of 2.2, where A is the larger tensile strength in the TD and MD directions and B is the smaller tensile strength. The positive and negative current collector leads were then bundled and ultrasonically welded to the positive and negative current collector tabs to fabricate the electrode elements.

[0081] The electrolyte was prepared by dissolving LiPF6 as a lithium salt at a rate of 1.3 mol / L in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:5:3, and adding vinylidene carbonate at a rate of 0.5 wt%.

[0082] <Fabrication of Non-Aqueous Electrolyte Secondary Battery> Two rectangular laminate films were prepared as exterior bodies, each having a structure in which a heat-sealed resin layer made of polyolefin, a metal layer made of aluminum foil, and a protective layer made of nylon resin and polyester resin were laminated in that order. A storage recess was formed in one of the laminate films, and the heat-sealed resin layers of the two laminate films were arranged opposite each other, and the laminate films were overlapped in a mirror-image manner so that the electrode element was stored in the storage recess. The electrode element was placed so that the portions of the heat-sealed resin portions of each terminal (tab) passed between the edges of the two laminate films, leaving a portion of each terminal exposed to the outside. In this state, the heat-sealed resin layers of the laminate films were heat-sealed to each other along three edges, including the edge from which each terminal extended. The nonaqueous electrolyte prepared above was then poured into the one edge of the exterior body that was not heat-sealed. Next, the remaining side of the exterior body was heat-sealed in a reduced pressure environment to produce a non-aqueous electrolyte secondary battery.

[0083] <Rate characteristic test> Using the fabricated battery, a charge-discharge test was carried out in a thermostatic chamber at 25° C. After initial charging and activation, the battery was subjected to a discharge rate characteristic test. For the initial charge, one cycle was performed with a constant current / constant voltage charge of 0.1 ItA, a voltage of 4.2 V, and a cutoff current of 0.05 ItA, followed by a constant current / constant voltage discharge of 0.1 ItA and a cutoff voltage of 2.7 V. For activation, four cycles were performed with a constant current / constant voltage charge of 0.2 ItA, a voltage of 4.2 V, and a cutoff current of 0.05 ItA, followed by a constant current / constant voltage discharge of 0.2 ItA and a cutoff voltage of 2.7 V. For the discharge rate characteristic test, charging was performed at a constant current and constant voltage of 0.5 ItA, 4.2 V, and a cutoff current of 0.05 C, while discharging was performed at a constant current of 0.2 ItA and 2.0 ItA with a cutoff voltage of 2.7 V. In each test, a rest period of 15 minutes was set after charging and discharging.

[0084] <Nail penetration test> The fabricated non-aqueous electrolyte secondary battery (cell) was previously subjected to constant current-constant voltage charging (cutoff current: 0.05C) with an upper limit voltage of 4.2V and a current value of 0.5C. A nail (made of iron, 5mm diameter) was inserted into the center of the cell at a nail penetration rate of 10mm / s until it penetrated the cell, and the maximum surface temperature of the cell after nail penetration (hereinafter referred to as "surface temperature") was measured. In addition, the appearance of the cell was checked one hour after nail penetration to confirm the presence or absence of cracking in areas other than the nail penetration area.

[0085] <Overall rating> An overall evaluation was made based on the results of the discharge rate characteristics and nail penetration test. The overall evaluation was made based on the combination of the results of each test, as shown in Table 1 below, and was ranked in order of best to worst as follows: ◎, ◯, △, ×. [Table 1]

[0086] The configuration of the battery in Example 1 and the test results are shown in Table 2. [Table 2]

[0087] Example 2 In Example 2, except that the amount of carbon nanotubes added to the positive electrode was 0.5 wt %, the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Example 2 are shown in Table 2.

[0088] Example 3 In Example 3, except that the amount of carbon nanotubes added to the positive electrode was 0.8 wt %, the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Example 3 are shown in Table 2.

[0089] Example 4 In Example 4, except that the positive electrode active material was LiCoO2 (LCO), the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Example 4 are shown in Table 2.

[0090] Example 5 In Example 5, except that the fiber length of the carbon nanotubes was 2 μm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 5 and the test results are shown in Table 2.

[0091] Example 6 In Example 6, except that the fiber length of the carbon nanotubes was 5 μm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 6 and the test results are shown in Table 2.

[0092] Example 7 In Example 7, except that the fiber length of the carbon nanotubes was 30 μm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 7 and the test results are shown in Table 2.

[0093] Example 8 In Example 8, except that the fiber length of the carbon nanotubes was 50 μm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 8 and the test results are shown in Table 2.

[0094] Example 9 In Example 9, except that the diameter of the carbon nanotubes was 4 nm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 9 and the test results are shown in Table 2.

[0095] Example 10 In Example 10, except that the diameter of the carbon nanotubes was 5 nm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 10 and the test results are shown in Table 2.

[0096] Example 11 In Example 11, except that the diameter of the carbon nanotubes was 6 nm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 11 and the test results are shown in Table 2.

[0097] Example 12 In Example 12, except that the diameter of the carbon nanotubes was 10 nm, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Example 12 and the test results are shown in Table 2.

[0098] Example 13 In Example 13, except that the thickness of one side of the coating layer was 3.0 μm and the total thickness was 6 μm, the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Example 13 are shown in Table 2.

[0099] Example 14 In Example 14, the same configuration and experimental method were used as in Example 1, except that the thickness of one side of the coating layer was 1.0 μm and the total thickness was 2 μm. The configuration and test results of the battery in Example 14 are shown in Table 2.

[0100] Example 15 In Example 15, except that the thickness of one side of the coating layer was 3.0 μm and the total thickness was 6 μm, the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Example 15 are shown in Table 2.

[0101] Example 16 In Example 16, the same configuration and experimental method were used as in Example 1, except that the thickness of one side of the coating layer was 5.0 μm and the total thickness was 10 μm. The configuration and test results of the battery in Example 16 are shown in Table 2.

[0102] Example 17 In Example 17, the same configuration and experimental method were used as in Example 1, except that the A / B value of the separator was 0.5. The configuration and test results of the battery in Example 17 are shown in Table 2.

[0103] Example 18 In Example 18, the same configuration and experimental method were used as in Example 1, except that the A / B value of the separator was 1.1. The configuration and test results of the battery in Example 18 are shown in Table 2.

[0104] Example 19 In Example 19, the same configuration and experimental method were used as in Example 1, except that the A / B value of the separator was 5.0. The configuration and test results of the battery in Example 19 are shown in Table 2.

[0105] (Comparative Example 1) In Comparative Example 1, except that the amount of carbon nanotubes added to the positive electrode was 0.4 wt %, the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Comparative Example 1 are shown in Table 2.

[0106] (Comparative Example 2) In Comparative Example 2, the same configuration and experimental method as in Example 1 were used, except that the amount of carbon nanotubes added to the positive electrode was 1.0 wt % and the separator was coated with alumina. The configuration and test results of the battery in Comparative Example 2 are shown in Table 2.

[0107] (Comparative Example 3) In Comparative Example 3, carbon black (CB) and graphite were used as conductive agents for the positive electrode, the amount of carbon black added was 3.0 wt %, the amount of graphite added was 2.0 wt %, and the separator was coated with alumina, except that the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Comparative Example 3 are shown in Table 2.

[0108] Comparative Example 4 In Comparative Example 4, carbon black (CB) and graphite were used as the conductive agent for the positive electrode, and the same configuration and experimental method were used as in Example 1, except that the carbon black content was 0.4 wt % and the graphite content was 0.3 wt %. The configuration and test results of the battery in Comparative Example 4 are shown in Table 2.

[0109] (Comparative Example 5) In Comparative Example 5, the carbon nanotubes had a diameter of 12 nm, and a separator with a coating layer (4 μm thick) on only one side was used, but the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Comparative Example 5 are shown in Table 2.

[0110] (Comparative Example 6) In Comparative Example 6, except that a separator without a coating layer was used, the same configuration and experimental method were used as in Example 1. The configuration of the battery in Comparative Example 6 and the test results are shown in Table 2.

[0111] (Comparative Example 7) In Comparative Example 7, the same configuration and experimental method were used as in Example 1, except that the A / B value of the separator was 9.9. The configuration and test results of the battery in Comparative Example 7 are shown in Table 2.

[0112] (Comparative Example 8) In Comparative Example 8, except that lithium cobalt oxide (LCO) was used as the positive electrode active material, the same configuration and experimental method were used as in Comparative Example 3. The configuration and test results of the battery in Comparative Example 8 are shown in Table 2.

[0113] (Comparative Example 9) In Comparative Example 9, except that lithium cobalt oxide (LCO) was used as the positive electrode active material, the same configuration and experimental method were used as in Example 1. The configuration and test results of the battery in Comparative Example 9 are shown in Table 2.

[0114] Table 2 shows the electrode parameters, electrochemical properties, and nail penetration test results for Examples and Comparative Examples. Examples 1 to 11 and 13 to 19 did not ignite or emit smoke when penetrated with a nail, and also exhibited high discharge rate characteristics exceeding 90%. Furthermore, Example 12, in which the diameter of the carbon nanotubes was 10 nm, emitted smoke when penetrated with a nail, but exhibited a high discharge rate characteristic of 85%. On the other hand, in Comparative Example 1, in which 0.4 wt% of carbon nanotubes was added to the conductive agent, the discharge rate characteristics were below 90%. This is presumably because the amount of conductive agent added was too small to form a sufficient conductive path in the electrode. In Comparative Example 2, which used 1.0 wt% carbon nanotubes as the conductive agent, and Comparative Examples 3 and 4, which used carbon black and graphite as the conductive agent and a separator coated on both sides with alumina, fires were confirmed when the nail was pierced. This is presumably because when the nail came into contact with the positive electrode, the nail came into contact with the area where the conductive agent had agglomerated, causing a large current to flow locally. In Comparative Examples 5 and 6, which used separators without a coating layer, and Comparative Example 7, in which A / B was greater than 5, fires were confirmed when the nail was inserted. This is thought to be because the separator was unable to prevent the expansion of the short-circuit area when the nail was inserted. In Comparative Examples 8 and 9, which did not use an olivine compound as the positive electrode active material, fire was confirmed when the nail was penetrated. This is thought to be due to a chain reaction of thermal decomposition caused by the heat of decomposition of the positive electrode active material due to Joule heat generated locally when the nail was penetrated.

[0115] The lithium ion secondary battery of the present invention can be used in a variety of applications that require rapid charging and discharging, such as power tools, drones, robots, and electric motorcycles. [Explanation of symbols]

[0116] 1. Lithium-ion secondary battery 2. Exterior body 3 electrode groups 4 Positive electrode 5 Negative electrode 6 Separator 7 Positive terminal 8 Negative terminal 41 Positive electrode mixture layer 42 Positive electrode current collector 51 negative electrode layer 52 Negative electrode current collector

Claims

1. It has a positive electrode, a negative electrode, and a separator, the positive electrode is composed of a positive electrode mixture layer made of a positive electrode active material, a conductive agent, and a binder, and a current collecting foil; the positive electrode active material includes a first positive electrode active material and a second positive electrode active material, The first positive electrode active material is a layered compound represented by the following general formula (1): The second positive electrode active material is a phosphate compound having an olivine structure represented by the following general formula (2): The conductive agent contains carbon nanotubes in an amount of more than 0.4 wt % and less than 1.0 wt % relative to the total mass of the positive electrode mixture layer, the average fiber diameter of the carbon nanotubes is 4 nm or more and 10 nm or less; The separator has a coating layer made of an inorganic material on at least one surface of a sheet serving as a base material of the separator. A lithium-ion secondary battery characterized by: Li ａ Ni ｘ Co ｙ M1 １－ｘ－ｙ O ２ (where 0<a≦1.2, 0≦x≦0.9, 0<y≦1, 0<x+y≦1) ... (1) LiMn ｚ M2 ｂ Fe １－z－ｂ P.O. ４ (where 0<z≦0.9, 0≦b≦0.1, 0<z+b<1) ... (2) M1 is one or more elements selected from Na, Mg, Sc, Y, Mn, Fe, Cu, Zn, Al, Ti, Cr, Pb, Sb, and B; M2 is one or more elements selected from Na, Mg, Sc, Y, Co, Ni, Cu, Zn, Al, Ti, Cr, Pb, Sb, and B.

2. The average fiber length of the carbon nanotubes is 5 μm or more and 30 μm or less.

2. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is a lithium ion secondary battery.

3. The total thickness of the coating layer is 2.0 μm or more.

2. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is a lithium ion secondary battery.

4. The coating layer is coated on both sides of the substrate.

2. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is a lithium ion secondary battery.

5. When the larger tensile strength of the substrate in the TD direction and the smaller tensile strength in the MD direction is designated as A and B, the value of A / B is 1.0 or more and 5.0 or less.

2. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is a lithium ion secondary battery.

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