Lithium-ion rechargeable battery

Abstract

To provide a lithium-ion secondary battery that can suppress short circuits caused by metallic foreign objects. [Solution] The lithium-ion secondary battery according to the present invention comprises a positive electrode containing a positive electrode active material capable of intercepting and releasing lithium ions, a negative electrode containing a negative electrode active material capable of intercepting and releasing lithium ions, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolysis system, and when measuring the Li diffusion coefficient of the electrolyte in the separator in the thickness direction of the separator by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy, 7 The signal shape of the Li NMR spectrum shows a main peak and multiple shoulder peaks, or the separator has a comb-like structure composed of a highly crystalline support portion and an amorphous fibrous portion.

Description

[Technical Field]

[0001] This invention relates to a lithium-ion secondary battery. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, have become widely used due to their high energy density and other factors, and are incorporated as power sources for portable small devices such as mobile phones, digital cameras, and laptop computers. Furthermore, due to environmental concerns, the demand for rechargeable lithium-ion secondary batteries is increasing, and their applications are being developed primarily for electric vehicles, as well as for energy storage systems in homes and commercial facilities, and for industrial applications such as railways and ships. In particular, efforts to improve weight and volumetric energy density are becoming more active year by year.

[0003] Lithium-ion secondary batteries comprise a positive electrode containing a lithium transition metal oxide as an active material, a negative electrode containing carbon as an active material, a separator placed between the positive and negative electrodes, and an electrolyte containing an organic solvent in which a lithium salt is dissolved. From the perspective of energy density, thinning the separator is desirable. Thinning the separator reduces its volume and thus the amount of electrolyte it needs to hold, which is expected to lead to a lighter battery.

[0004] Lithium-ion battery electrodes may contain trace amounts of metallic foreign matter, such as iron, copper, zinc, tin, cobalt, nickel, and chromium, in amounts that do not pose a safety risk. These metallic foreign matter can enter the battery in solid or ionic form. If even a trace amount of metallic foreign matter is present in or on the positive electrode, the noble potential of the positive electrode may cause the foreign matter to dissolve and become metallic ions. Furthermore, if these metallic ions migrate to the negative electrode, the low potential of the negative electrode may cause metal to precipitate on the negative electrode. This deposition of metal on the negative electrode forms dendrites, and if these dendrites grow and reach the positive electrode, they can short-circuit the positive and negative electrodes. The aforementioned thinning of the separator increases the likelihood of such short circuits causing OCV (Open Circuit Voltage) failures, which reduce the output battery voltage, and even overheating and fire failures.

[0005] Patent Document 1 describes a configuration in which the positive and negative electrodes are handled in close contact with a coated separator in order to suppress defects caused by metallic foreign matter. It is stated that with such a structure, there is no gap between the electrodes and the separator, so metallic foreign matter introduced during assembly does not come into contact with the electrodes, and the formation of dendrites caused by metallic foreign matter becomes less likely.

[0006] Patent Document 2 describes a lithium-ion battery separator composed of a support layer and a separation function layer. Patent Document 2 states that the separation function layer has a smaller pore diameter and lower porosity than the support layer, and that this configuration, by making the separation function layer denser with a smaller pore diameter and lower porosity compared to the support layer, can suppress the movement of metallic foreign matter from the positive electrode side to the negative electrode side, thereby suppressing the deposition of metallic foreign matter on the negative electrode side, and thus ensuring battery performance and safety. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2013-127857 [Patent Document 2] Patent No. 6604479 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, in the configuration of Patent Document 1, although metal foreign matter introduced during assembly can be inactivated, metal foreign matter already introduced into the positive electrode during the source process cannot be removed. When metal foreign matter dissolves at the positive electrode, it moves to the negative electrode, where dendrites are formed. The formation of dendrites causes a short circuit between the positive and negative electrodes, leading to a decrease in the output battery voltage of the lithium-ion battery, or even overheating and ignition of the lithium-ion battery, resulting in a decrease in battery performance and safety. Furthermore, while Patent Document 2 describes a separator that can suppress the movement of metallic foreign matter from the positive electrode side to the negative electrode side, although the size of the pore diameter and the density of the porosity between the different layers depend to some extent on the electrophoresis during the dissolution and extraction of metallic foreign matter, there are aspects that cannot be explained by this alone, and it is unclear whether it can reliably suppress the movement of metallic foreign matter from the positive electrode side to the negative electrode side.

[0009] This invention has been made in view of the above circumstances, and aims to provide a lithium-ion secondary battery that can suppress short circuits caused by metallic foreign matter. [Means for solving the problem]

[0010] To solve the above problems, the lithium-ion secondary battery according to the present invention comprises a positive electrode containing a positive electrode active material capable of intercepting and releasing lithium ions, a negative electrode containing a negative electrode active material capable of intercepting and releasing lithium ions, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolysis system, and when measuring the Li diffusion coefficient of the electrolyte in the separator in the thickness direction of the separator by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy under the following measurement conditions, 7The signal shape of the Li NMR spectrum shows a main peak and a plurality of shoulder peaks, or the separator has a comb-like structure and is composed of a highly crystalline support part and an amorphous fiber part. Measurement conditions; Measurement set temperature: 25 °C Diffusion time: Δ 20 to 800 ms Magnetic field gradient pulse width δ: 0.6 to 1.2 ms Magnetic field gradient strength g: 0.20 to 14.99 T / m

Advantages of the Invention

[0011] According to the present invention, even when a trace amount of metal foreign matter is mixed in, a lithium ion secondary battery capable of suppressing a short circuit due to the metal foreign matter can be provided.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a perspective view for explaining 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 A-A shown in FIG. 1. [Figure 3] FIG. 3 is a diagram for explaining the mixing of metal foreign matter. [Figure 4] FIG. 4 is an enlarged view of an electrode into which metal foreign matter has been mixed. [Figure 5] FIG. 5 is a diagram showing a SEM image of a cross-section of a separator having a comb-like structure. [Figure 6] FIG. 6 is a diagram showing a SEM image of a cross-section of a separator having a mesh-like structure. [Figure 7] FIG. 7 is a diagram showing the signal shape of a PFG-NMR 7Li NMR spectrum.

Embodiments for Carrying Out the Invention

[0013] The embodiments of the present invention will be described below, but the present invention is not limited to the following description. Furthermore, various modifications or improvements can be made to these embodiments, and such modified or improved forms may also be included in the present invention. Although Figures 1 and 2 show an example configuration of a stacked lithium-ion secondary battery as an example of an embodiment, the shape of the lithium-ion secondary battery in the present invention is not particularly limited and may be flat, cylindrical, prismatic, or coin-shaped. Furthermore, the casing of the lithium-ion secondary battery is not particularly limited and known materials such as laminate film, aluminum, aluminum alloy, and stainless steel can be used.

[0014] (Embodiment) Figure 1 is a perspective view illustrating the configuration of a lithium-ion secondary battery according to one embodiment of the present invention. Figure 2 is a cross-sectional view taken along line AA shown in Figure 1.

[0015] The lithium-ion secondary battery 1 is a stacked lithium-ion battery comprising an outer casing 2 formed in a bag shape by overlapping two laminate films so that the heat-sealable resin layers face each other and heat-sealing the outer edges.

[0016] The outer casing 2 is formed into a bag shape by overlapping two laminate films so that the heat-sealable resin layers face each other, and then heat-sealing the outer edges. The electrode group 3 and the non-aqueous electrolyte are housed inside the outer casing 2. The electrode group 3 is inserted through the opening of the outer casing 2, and the opening of the outer casing 2 is heat-sealed to enclose the electrode group 3 in an airtight manner.

[0017] As the laminate film, a composite film having a metal layer with a heat-sealable resin layer is preferably used. The metal layer is not particularly limited as long as it prevents moisture from entering from the outside while improving the overall strength of the sheet. For example, aluminum foil, stainless steel foil, etc., can be used as the metal layer. Furthermore, while the heat-sealable resin layer is not particularly limited, polyethylene and polypropylene are preferably used from the viewpoint of a heat-sealable temperature range and barrier properties against non-aqueous electrolytes. Here, to protect the metal layer, a protective layer may be provided on the side opposite to the heat-fusible resin layer. The protective layer is not particularly limited, but nylon, PET, etc., are preferably used. In addition, an adhesive layer may be provided between the metal layer and the heat-fusible resin layer to improve their adhesion.

[0018] Furthermore, insulating tape may be used to fix the electrode group 3. The insulating tape comprises, for example, a base material and an adhesive layer. The base material may be made of a resin, for example, from the viewpoint of being readily available and low cost. The type of resin is not particularly limited as long as it has the desired elasticity, flexibility, and insulating properties. Examples of resins include polyimide, polyamide (such as aromatic polyamide), polyamide-imide, polyolefin (such as polypropylene (PP)), polyester (such as polyethylene naphthalate), polyphenylsulfone (PPS), and polyphenylene sulfide. These resins may be used individually or in combination. The adhesive layer contains an adhesive, and resin materials other than silicone can be used as the adhesive. Examples include acrylic resin, natural rubber, synthetic rubber (such as butyl rubber), epoxy resin, melamine resin, and phenolic resin. These may be used individually or in combination.

[0019] As shown in Figure 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 located in the outermost layer, and multiple such sets are stacked.

[0020] (positive electrode) The positive electrode 4 consists of a positive electrode current collector 42 and a positive electrode composite material layer 41 formed on both or one side of the positive electrode current collector 42. The positive electrode 4 is plate-shaped.

[0021] <Positive electrode composite layer> The positive electrode composite layer 41 contains, for example, a positive electrode active material, a conductive agent, and a binder. The positive electrode active material is not particularly limited, and known or commercially available materials can be used. For example, lithium-containing transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, LiFePO4, LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiCo 0.15 Ni 0.8 Al 0.05 O2, LiNi 0.5 Mn 1.5 O4 etc. can be used. Note that the positive electrode active material may be used by mixing a plurality of positive electrode active materials.

[0022] <Conductive agent> The conductive agent is not particularly limited, and known or commercially available materials can be used. For example, carbon blacks such as acetylene black and ketjen black, carbon nanotubes, carbon fibers, activated carbon, graphite, etc. can be used. These may be used alone or in combination of multiple types.

[0023] <Binder> The binder is not particularly limited, and known or commercially available materials can be used. For example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), acrylic resin, etc. can be used.

[0024] Note that the positive electrode 4 may further contain a dispersant. The dispersant is not necessarily required, but can be used when it is desired to disperse the conductive agent more. Typical dispersants include, for example, polyvinylpyrrolidone (PVP).

[0025] <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 foil made of aluminum, aluminum alloy, copper, copper alloy, nickel, or stainless steel, and porous metals such as porous aluminum. Among these candidates for the positive electrode current collector, aluminum or aluminum alloy is preferred because it has high electrical conductivity, excellent corrosion resistance in the electrolyte, and is a lightweight metal.

[0026] [Positive electrode manufacturing method] The positive electrode can be manufactured, for example, by the following method. First, the positive electrode active material, conductive agent, and binder mentioned above are dispersed in a solvent to prepare a positive electrode slurry. A thickening agent may be added to the positive electrode slurry. Subsequently, the positive electrode slurry is applied to one or both sides of the positive electrode current collector, dried to form a positive electrode composite layer, and then rolled to produce a plate-shaped positive electrode.

[0027] <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 solvents 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 carboxymethylcellulose (CMC) is used as the binder, it is preferable to use water as the solvent.

[0028] Furthermore, the positive electrode 4 has a positive electrode lead 43 extending from the positive electrode composite layer 41 by the positive electrode current collector 42. The positive electrode lead 43 extends from the right side, for example, in Figure 2. Each positive electrode lead 43 is bundled at its tip within the outer casing 2 and joined to each other by ultrasonic welding, resistance welding, or the like. The positive electrode terminal 7 has one end joined to the joint of the positive electrode lead 43, and the other end extends to the outside through the sealing portion of the outer casing 2.

[0029] (Negative electrode) The negative electrode 5 consists of a negative electrode current collector 52 and a negative electrode composite material layer 51 formed on one or both sides of the negative electrode current collector 52. Here, the negative electrode composite material layer 51 of the outermost negative electrode 5 is formed on the surface of the negative electrode current collector 52 facing the separator 6. In contrast, the negative electrode composite material layer 51 of the negative electrodes 5 located between the positive electrodes 4, excluding the outermost negative electrode 5, is formed on both sides of the negative electrode current collector 52.

[0030] The negative electrode composite layer 51 includes 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 intercalating and releasing lithium. Examples of negative electrode active materials include pyrolysis carbons, pitch coke, needle coke, petroleum coke and other cokes, natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, glassy carbon, organic polymer compound sintered bodies (phenol resin, furan resin, etc., sintered and carbonized), carbon fibers, carbon black, activated carbon and other carbons, or metallic materials such as Al, Si, Sn and their alloys, SiO, SiO2, lithium titanate (Li4Ti50) 12 Examples include metal oxide materials such as )

[0031] <Conductive agent> The conductive agent is not particularly limited, and known or commercially available ones can be used. For example, the same conductive agent used for the positive electrode described above can be used. A configuration without a conductive agent is also possible.

[0032] <Binding agent> 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), fluororubber, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), core-shell binder, polyvinyl alcohol, or polyimide resins such as polyimide and polyamide-imide. The binder can be used individually, or as a mixture or copolymer of two or more of these substances.

[0033] <Negative electrode current collector> The negative electrode current collector 52 is not particularly limited, and known or commercially available ones can be used. For example, the negative electrode current collector 52 can be made of rolled foil made of copper, copper alloy, aluminum, aluminum alloy, or stainless steel, or porous metals such as porous aluminum. The negative electrode current collector is preferably made of copper or a copper alloy.

[0034] [Method for fabricating the negative electrode] The negative electrode can be manufactured, for example, by the following method. First, the aforementioned negative electrode active material, conductive agent, and binder are dispersed in a solvent to prepare a negative electrode slurry. A thickening agent may be further added to the negative electrode slurry. Subsequently, the negative electrode slurry is applied to one or both sides of the negative electrode current collector, dried to form a negative electrode composite layer, and then rolled to produce a plate-shaped negative electrode.

[0035] <Solvent> The solvent used in preparing the negative electrode slurry is not particularly limited, and known or commercially available solvents can be used. Examples of solvents 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 carboxymethylcellulose (CMC) is used as the binder, it is preferable to use water as the solvent. Furthermore, it is preferable to use polyvinylpyrrolidone (PVP) as the dispersant.

[0036] Furthermore, the negative electrode 5 has, for example, a negative electrode current collector 52 with a negative electrode lead 53 extending from the negative electrode composite layer 51. In Figure 2, for example, the negative electrode lead 53 extends from the left side. Each negative electrode lead 53 is bundled at its tip within the outer casing 2 and joined to one another. The negative electrode terminal 8 has one end joined to the joint of the negative electrode lead 53, and the other end extends to the outside through the sealing portion of the outer casing 2.

[0037] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent in which a lithium salt is dissolved. Lithium salts such as LiBF4, LiPF6, LiAsF6, LiCF3CFO, LiSO3CF3, LiCF2CF2SO3, LiClO4, LiN(COCF2CF3)2, and LiN(SO2CF3)2 can be used individually or in combination. The electrolyte concentration can be in the range of, for example, 0.3 mol / L to 3.0 mol / L.

[0038] (Separator) The separator 6 can use polyolefin resins such as polyethylene (PE) and polypropylene (PP) as the base layer. The electrolyte in the separator 6 in the thickness direction is measured by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy (PFG-NMR). 7The Li NMR spectrum shows a signal shape with multiple shoulder peaks or asymmetrical shoulder peaks. PFG-NMR is used to analyze these peaks when measuring the Li diffusion coefficient of the electrolyte contained in the separator.

[0039] 7 The signal of the Li NMR spectrum is measured under the following conditions, for example: Measurement setting temperature: 25℃ Diffusion time: Δ20~800ms Magnetic field gradient pulse width δ: 0.6~1.2ms Magnetic field gradient strength g: 0.20~14.99 T / m The measurement device used was a Bruker AVANCE III600.

[0040] The inventors found that when the signal has a shoulder peak shape as described above, it is highly likely that the signal has a multilayer structure in which layers with different Li diffusion coefficients are stacked. This multilayer structure may consist of multiple layers of resins of different materials as the base layer, or it may be a single base layer with an inorganic filler coated on at least one surface to form a multilayer structure. In the layers of the multilayer structure with a small Li diffusion coefficient in the thickness direction, the electrophoretic rate of the ionized metal becomes the rate-limiting factor in the electrophoretic rate of the separator 6, promoting diffusion in the planar direction and stagnating electrophoretic movement in the thickness direction.

[0041] In this case, the separator 6 has a comb-like structure and is composed of a highly crystalline support portion and an amorphous fibrous portion, or it forms a fine network structure in which fibrils are three-dimensionally linked to form a mesh. When the separator 6 is composed of highly crystalline lamellae (supporting parts) and amorphous fibrils (fibrous parts), such as a comb-like structure, the supporting parts act as barriers in the growth path of the metallic foreign matter, and the fibrils complicate the path and suppress growth in the thickness direction. To further suppress growth, it is preferable to coat the substrate layer with an inorganic filler or the like, which has the effect of complicating the growth path in the thickness direction between the positive and negative electrodes. Furthermore, if the separator 6 forms a fine network structure and the fibrils are connected three-dimensionally to form a uniform mesh, it is preferable to coat the substrate layer with an inorganic filler or the like.

[0042] Figure 3 is a diagram illustrating the inclusion of metallic foreign matter. Figure 4 is a magnified view of an electrode contaminated with metallic foreign matter. In a battery, metallic foreign matter 100 may be present. For example, metallic foreign matter 100 is made of copper and is present between the positive electrode 4 and the separator 6. Figure 4 shows a rectangular piece of copper metallic foreign matter.

[0043] As mentioned above, while metal foreign matter 100 can be inactivated if it is introduced during assembly, it cannot be removed if it has already been introduced into the positive electrode during the source process (e.g., the electrode manufacturing process). When metal foreign matter dissolves at the positive electrode, the dissolved metal foreign matter moves to the negative electrode, where dendrites are formed. The formation of dendrites causes a short circuit between the positive and negative electrodes, leading to problems such as a decrease in the output battery voltage of the lithium-ion battery, or the lithium-ion battery overheating and catching fire, thus reducing battery performance and safety.

[0044] The shape of the electrode elements is not particularly limited; the positive electrode, negative electrode, and separator may be wound together, stacked in a single-sheet configuration, or folded in a zigzag pattern.

[0045] (Method of manufacturing a non-aqueous electrolyte secondary battery) The following describes the manufacturing method of lithium-ion secondary batteries. Note that the numbers and characteristics are examples only. First, positive and negative electrodes, punched to a predetermined size, and separators, cut to a predetermined size, were alternately stacked. A separator was interposed between the positive and negative electrodes, and six positive electrodes and seven negative electrodes were alternately stacked to obtain a laminated element. This laminated element has a battery capacity of approximately 0.350 Ah. Terminal tabs were attached to the parts of the positive and negative electrodes of the laminated element that did not have a composite layer by ultrasonic welding or the like. The laminated element was sandwiched between aluminum laminate films so that the terminal tabs protruded to the outside, and three sides were laminated by heat welding. Electrolyte was injected through the one side that was left unsealed, and a lithium-ion secondary battery before initial charging was obtained by vacuum sealing. The amount of electrolyte was adjusted to approximately 11 g / Ah. The degree of reduced pressure (vacuum) during the sealing process is preferably around 1 hPa to 100 hPa. After obtaining the lithium-ion secondary battery with electrolyte injection, the separator, which has been impregnated with the electrolyte, is prone to wrinkling if left standing, so it should be quickly placed in a jig for pressurizing the battery. The pressurization at this time is 0.1 kgf / cm². 2 ~100 kgf / cm² 2 It is preferable to perform the charging under a moderate pressure. Afterwards, the battery was left to stand for about 3 hours to allow the electrolyte to permeate the constituent materials of the laminated element before starting the first charge. For the first charge, a sufficiently small current value was used, and for subsequent charges, a current value approximately five times larger than the first charge was used. The battery voltage was measured when the battery was charged to 50% of its rated capacity to check for short circuits caused by metallic foreign objects.

[0046] In the embodiment described above, in the lithium-ion secondary battery 1, the electrolyte in the separator in the thickness direction of the separator 6 7 The Li NMR spectrum shows a signal shape with multiple shoulder peaks or asymmetrical shoulder peaks. Furthermore, the separator 6 has a comb-like structure composed of highly crystalline support parts and amorphous fibrous parts, or it forms a fine network structure in which fibrils are three-dimensionally linked. According to this embodiment, the separator 6 having the above-described properties can stagnate the electrophoresis of metal (metal foreign matter) in the thickness direction, thereby suppressing short circuits caused by metal foreign matter. [Examples]

[0047] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by the following examples.

[0048] (Example 1) <Fabrication of the positive electrode plate> A positive electrode slurry was prepared by dispersing 90 parts by weight of lithium iron phosphate, the positive electrode active material, 5 parts by weight of PVdF, the binder, and 5 parts by weight of carbon black, the conductive agent, in NMP, the solvent. Next, this positive electrode slurry was applied to both sides of an aluminum foil (thickness: 20 μm) to be used as the positive electrode current collector. After drying the solvent and compressing the foil, the positive electrode plate, which is a rectangular positive electrode current collector with a positive electrode layer formed on it, was cut so that a strip-shaped positive electrode current collector lead extended from one side.

[0049] <Embedding of metallic foreign objects into the positive electrode plate> Copper was selected as the metal species to be used. Copper was chosen because it dissolves from a low oxidation potential of about 3.6-3.7V [Li / Li+] and precipitates from a high reduction potential of about 1.7-1.8V [Li / Li+], allowing for relatively quick acquisition of the effect of metal foreign matter on the battery from the initial charging and initialization stages. As a method for embedding metal foreign matter into the resulting positive electrode plate, the metal foreign matter was processed using a microscope and manipulator. A 5μm copper foil was processed into a 50×50μm square (equivalent to a volume with a diameter of approximately 30μm if made spherical). The processed copper foil was placed in the central part of the resulting positive electrode plate, and then compressed again while controlling the electrode density to increase to less than 0.05g / cc.

[0050] <Fabrication of the negative electrode plate> A negative electrode slurry was prepared by dispersing 98 parts by weight of graphite, the negative electrode active material, 1 part by weight of SBR, the binder, and 1 part by weight of CMC, the thickener, in deionized water, the solvent. Next, this negative electrode slurry was applied to both sides of a copper foil (thickness: 10 μm) to be used as the negative electrode current collector. After drying the solvent and compressing the foil, the negative electrode plate, which is a rectangular negative electrode current collector with the negative electrode layer formed on it, was cut so that a strip-shaped negative electrode current collector lead extended from one side.

[0051] <Fabrication of electrode groups> Next, an electrode group was fabricated by alternately stacking separators, six positive electrode plates, and seven negative electrode plates, with separators interposed between the positive and negative electrode plates. Polypropylene (PP) and polyethylene (PE) were used as the separators, and a 16 μm thick multilayer film (polypropylene / polyethylene / polypropylene) was formed by dry uniaxial stretching. The current collector leads were stacked so that they extended from the sides of the electrode group. Negative electrode plates were positioned at both ends of the electrode group in the stacking direction. Separators were then placed at both ends of the electrode group, and imide tape coated with acrylic adhesive was applied to four locations at the ends of the electrode group for fixing. Next, each current collector lead was bundled towards the center along the stacking direction of the electrode group, and joined to one side of one end of each terminal by ultrasonic welding. Imide tape coated with acrylic adhesive was applied to each ultrasonically welded joint to prevent damage to the laminate outer layer due to burrs. Next, a heat-sealable resin portion made of polyolefin was formed on the circumferential surface of the portion passing through the sealing portion of the positive and negative terminals.

[0052] <Fabrication of the exterior> Two rectangular laminate films were prepared, each having a recess for housing an electrode group and a flat portion surrounding the recess. Each laminate film has a structure in which a heat-sealable resin layer made of polyolefin, a metal layer made of aluminum foil, and protective layers made of nylon resin and polyester resin are laminated in that order.

[0053] <Battery assembly> Two laminate films were placed with their heat-sealable resin layers facing each other, and the laminate films were superimposed mirror-image-like so that the electrode group would be housed in two recessed areas. The electrode group was positioned so that the portion where the heat-sealable resin part of each terminal is formed passes between the periphery of the two laminate films, and a portion of each terminal is exposed to the outside. In this state, the heat-sealable resin layers at the periphery of the laminate films were heat-sealed together on three sides, including the side from which each terminal extends, to form a rectangular frame-shaped sealing portion. A heat sealing machine was used for the heat sealing. Next, a non-aqueous electrolyte was injected from the one side of the outer casing that was not heat-sealed. The non-aqueous electrolyte used was a mixed solvent in which EC, EMC, and DMC were mixed in a volume ratio of 2:5:3, with LiPF6 dissolved as the electrolyte at a concentration of 1.3 mol / L. Next, under reduced pressure, the remaining side of the outer casing (the final sealing section) was heat-sealed to manufacture a non-aqueous electrolyte secondary battery. The heat sealing was performed using a heat sealing machine.

[0054] <Voltage Measurement> To check for a short circuit, the output battery voltage of the constructed battery was measured. The reference voltage was 3.3000V, and a voltage below this value was considered to indicate a short circuit.

[0055] Table 1 shows the battery configuration and test results in Example 1. Figure 5 shows an SEM image of a cross-section of a separator having a comb-like structure. As shown in Figure 5, the comb-like structure is composed of highly crystalline support parts and amorphous fibrous parts arranged in a comb-like pattern. [Table 1]

[0056] (Example 2) In Example 2, the separator was fabricated in the same manner as in Example 1, except that it was changed to a single layer film with a thickness of 16 μm, made from polypropylene (PP) using a dry uniaxial stretching method. The battery configuration and test results in Example 2 are shown in Table 1.

[0057] (Comparative Example 1) Comparative Example 1 was prepared in the same manner as in Example 1, except that the separator was changed to a single layer film with a thickness of 16 μm, formed by wet biaxial stretching using polypropylene (PE) as the material. The battery configuration and test results for Comparative Example 1 are shown in Table 1. Figure 6 shows an SEM image of a separator cross-section having a mesh-like structure. As shown in Figure 6, the mesh-like structure is a fine network structure in which fibrils are connected three-dimensionally to form a mesh.

[0058] <Presence or absence of short circuit due to separator> Cells that were short-circuited were identified as "present" based on the measured battery voltage, and cells that were not short-circuited were identified as "absent." Comparative Example 1 was short-circuited, while Examples 1 and 2 were not. Figure 7 shows PFG-NMR 7 This figure shows the signal shape of the LiNMR spectrum. In Figure 7, Comparative Example 1 has peak P1, Example 1 has peak P2, and Example 2 has peak P3. As shown in Figure 7, Example 1 has a shoulder peak (peak P2) in its signal, and it is presumed that the difference in diffusivity of each layer of the multilayer film suppresses the growth of metallic foreign matter in the thickness direction. Specifically, it is presumed that the peak (peak P2) of Example 1 is formed by the overlapping of the Li ion peak in the outer layer (left side) and the Li ion peak in the inner layer (right side). Although the peak (peak P3) of Example 2 does not have a shoulder peak, it has a comb-like structure. Furthermore, it is presumed that Example 1 and Example 2 are separated into a support part for the highly crystalline region and a fibrous part for the amorphous region by the comb-like structure, and that the support part acts as a wall on the path through which metallic foreign matter grows, suppressing its growth.

[0059] The lithium-ion secondary battery of the present invention can be applied to a variety of applications requiring rapid charging and discharging, such as power tools, drones, robots, electric motorcycles, and many other applications. [Explanation of symbols]

[0060] 1. Lithium-ion rechargeable battery 2. Exterior 3 electrode groups 4 Positive electrode 5 Negative electrode 6 Separators 7 Positive terminal 8 Negative terminal 41. Positive electrode composite layer 42 Positive electrode current collector 51 Negative electrode layer 52 Negative electrode current collector

Claims

[Claim 1] A positive electrode containing a positive electrode active material capable of intercepting and releasing lithium ions, A negative electrode containing a negative electrode active material capable of intercalating and releasing lithium ions, A separator is interposed between the positive electrode and the negative electrode, and non-aqueous electrolysis is performed. Equipped with, When measuring the Li diffusion coefficient of the electrolyte in the separator in the thickness direction of the separator by pulsed magnetic field gradient nuclear magnetic resonance spectroscopy under the following measurement conditions, ７ The signal shape of the Li NMR spectrum shows a main peak and multiple shoulder peaks, or the separator has a comb-like structure and is composed of a highly crystalline support portion and an amorphous fibrous portion. A lithium-ion secondary battery characterized by the following features. Measurement conditions; Measurement setting temperature: 25℃ Diffusion time: Δ20-800 ms Magnetic field gradient pulse width δ: 0.6–1.2 ms Magnetic field gradient strength g: 0.20–14.99 T / m

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