Lithium-ion rechargeable battery

Abstract

To provide a lithium-ion secondary battery that can suppress the increase in resistance and improve storage capacity. [Solution] The lithium-ion secondary battery according to the present invention comprises a positive electrode, a negative electrode, and a separator. The positive electrode is composed of a positive electrode composite layer comprising a positive electrode active material, a conductive additive, and a binder, and a current collector foil. The positive electrode composite layer contains a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is lithium cobalt oxide (LiCoO2), and the second positive electrode active material is a phosphoric acid compound represented by the following formula (1). A coating layer made of inorganic filler is provided on the surface of the positive electrode composite layer and / or the separator. LiMn x M1 a Fe 1-x-a PO4 (however, 0

Description

Technical Field

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

Background Art

[0002] Lithium ion secondary batteries have the advantages of enabling small size, light weight, and large capacity due to their high energy density. For example, batteries using high-energy-density active materials, such as NCM mainly composed of nickel, cobalt and manganese, and layered compounds represented by lithium cobalt oxide (LCO), for the positive electrode are widely used ranging from small-scale applications such as wearable devices and power tools to large-scale applications including automobiles and industrial equipment. Among these, especially for large-scale applications, the consumption of lithium ion secondary batteries is large, and in recent years, demand for applications used in close proximity to people, such as automotive applications, has been increasing. In addition, in lithium ion secondary batteries, a separator using porous polyethylene (PE) is used as a separator.

[0003] Due to its high voltage, LCO may cause oxidation of PE separators during long-term storage. Accordingly, as a technique for suppressing oxidation of PE separators, it has been proposed to provide an inorganic coating layer such as alumina between the positive electrode and PE (see, for example, Patent Document 1).

[0004] Furthermore, for LCO, as the voltage becomes higher, more lithium ions are extracted, so structural changes are likely to occur. When the structure changes, cobalt elutes. Cobalt elution is particularly likely to occur when stored under high temperature. Accordingly, as disclosed in Patent Document 1, by providing an inorganic coating layer of alumina, the coating layer traps cobalt and suppresses cobalt elution.

Prior Art Literature

Patent Literature

[0005]

Patent Document 1

[0006] However, when the cobalt is trapped to some extent by the inorganic coating layer, the pores in the separator become clogged, leading to increased resistance and reduced storage capacity.

[0007] This invention has been made in view of the above circumstances, and aims to provide a lithium-ion secondary battery that can suppress the increase in resistance and improve storage capacity. [Means for solving the problem]

[0008] To address the above-mentioned problems, we conducted extensive research and discovered that surrounding LCO with a phosphate compound suppresses the deposition of cobalt from the dissolved LCO on the negative electrode surface, thereby preventing the induction of side reactions. This suppresses the increase in the internal resistance of the battery and improves its storage capacity, leading to the completion of the present invention.

[0009] The lithium-ion secondary battery according to the present invention is characterized in that, in first respect, it comprises a positive electrode, a negative electrode, and a separator, the positive electrode is composed of a positive electrode composite layer comprising a positive electrode active material, a conductive additive, and a binder, and a current collector foil, the positive electrode composite layer comprises a first positive electrode active material and a second positive electrode active material, the first positive electrode active material being lithium cobalt oxide (LiCoO2), the second positive electrode active material being a phosphoric acid compound represented by the following formula (1), and the surface of the positive electrode composite layer and / or the separator is provided with a coating layer made of an inorganic filler. LiMn x M1 a Fe 1-x-a PO4 (however, 0 <x≦0.9、0≦a<0.1、0<x+a<1) ···(1) In equation (1) above, M1 is at least one selected from sodium (Na), magnesium (Mg), scandium (Sc), yttrium (Y), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), aluminum (Al), titanium (Ti), chromium (Cr), lead (Pb), antimony (Sb), and boron (B).

[0010] Furthermore, the lithium-ion secondary battery according to the present invention is characterized in that, in addition to the first aspect, the mass ratio of the first positive electrode active material to the second positive electrode active material (first positive electrode active material: second positive electrode active material) is 95:5 to 90:10.

[0011] Furthermore, as a third aspect, the lithium-ion secondary battery according to the present invention is characterized in that, in addition to the first or second aspect, the particle size ratio of the first positive electrode active material to the second positive electrode active material (first positive electrode active material: second positive electrode active material) is 70:1 to 215:1.

[0012] Furthermore, as a fourth aspect of the present invention, the lithium-ion secondary battery is characterized in that, in addition to the first to third aspects, a coating layer made of the inorganic filler is provided on both sides of the separator.

[0013] Furthermore, as a fifth aspect, the lithium-ion secondary battery according to the present invention is characterized in that, in addition to the first to fourth aspects, the average particle size of the first positive electrode active material is 6 μm or more and 12 μm or less.

[0014] Furthermore, as a sixth aspect of the present invention, the lithium-ion secondary battery is characterized in that, in addition to the first to fifth aspects, the average particle size of the second positive electrode active material is on the order of nanometers.

[0015] Furthermore, as a seventh aspect, the lithium-ion secondary battery according to the present invention is characterized in that, in addition to the first to sixth aspects, the voltage of the lithium-ion secondary battery is 4.40V or higher.

[0016] Furthermore, as an eighth aspect, the lithium ion secondary battery according to the present invention, in addition to the first to seventh aspects, includes a non-aqueous electrolyte, and a metal trapping agent is added to the non-aqueous electrolyte.

[0017] Furthermore, as a ninth aspect, the lithium ion secondary battery according to the present invention, in addition to the first to eighth aspects, is characterized in that the metal trapping agent traps cobalt. Effects of the Invention

[0018] According to the present invention, an increase in resistance can be suppressed, and the storage capacity can be improved. Brief Description of the Drawings

[0019] [Figure 1] Figure 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] Figure 2 is a cross-sectional view taken along line A-A shown in Figure 1. Mode for Carrying Out the Invention

[0020] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following description. Various modifications or improvements can be added to the present embodiment, and forms with such modifications or improvements can also be included in the present invention. Although FIGS. 1 and 2 show a configuration example of a stacked lithium ion secondary battery as an example of the embodiment, the shape of the lithium ion secondary battery in the present invention is not particularly limited, and may be flat, cylindrical, square, or coin-shaped. Also, the exterior body of the lithium ion secondary battery is not particularly limited either, and known materials such as laminate films, aluminum, aluminum alloys, and stainless steel can be used.

[0021] (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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] (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.

[0027] The positive electrode composite layer 41 includes a positive electrode active material, a conductive agent, and a binder.

[0028] <Cathode active material> The positive electrode active material includes a first positive electrode active material and a second positive electrode active material.

[0029] The first positive electrode active material can be lithium cobalt oxide (LiCoO2:LCO).

[0030] The second positive electrode active material is a phosphoric acid compound represented by the following formula (1). LiMn x M1 a Fe 1-x-a PO4 (however, 0 <x≦0.9、0≦a<0.1、0<x+a<1) ···(1) In equation (1) above, M1 is at least one selected from sodium (Na), magnesium (Mg), scandium (Sc), yttrium (Y), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), aluminum (Al), titanium (Ti), chromium (Cr), lead (Pb), antimony (Sb), and boron (B). In the following explanation, lithium iron manganese phosphate (LMFP) is used as the positive electrode active material represented by equation (1) above.

[0031] The phosphate compound represented by equation (1) above has an olivine structure and is a material with a charge / discharge potential close to that of layered rock salt oxides and a high energy density. When the respective active materials are mixed, they exhibit a high energy density in the same upper and lower voltage range as when layered rock salt oxides are used.

[0032] In the positive electrode active material, the phosphate compound (LMFP), which is the second positive electrode active material, surrounds the first positive electrode active material, LCO, thereby suppressing the deposition of dissolved cobalt on the negative electrode surface and inducing side reactions. In this case, the mixing ratio of LCO and LMFP is preferably 95:5 to 90:10 by mass ratio (LCO:LMFP), based on capacity density and charge / discharge curve shape. If the ratio of LMFP is lower than 5%, there will be insufficient LMFP surrounding the LCO, and the effect of suppressing the deposition of dissolved cobalt on the negative electrode surface and inducing side reactions may not be obtained. On the other hand, if the ratio of LMFP is higher than 10%, the proportion of LCO will decrease, which may reduce the storage capacity.

[0033] Furthermore, the average particle size of the LCO is preferably between 6 μm and 12 μm. If the average particle size is less than 6 μm, the specific surface area of ​​the active material increases, and the number of conductive paths relative to the specific surface area is insufficient, which may increase the reaction resistance. If the average particle size is greater than 12 μm, the diffusivity of lithium ions within the active material particles may decrease, and the specific surface area of ​​the active material decreases, reducing the reaction area, which may increase the reaction resistance. The average particle size is the median diameter (D50) value measured by the laser diffraction / scattering method described in JIS standard Z8825:2013. A laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) can be used for measurement. Alternatively, it is possible to obtain particle size distribution information and measure the average particle size by acquiring surface SEM images or cross-sectional SEM images and processing them with image analysis software.

[0034] The average particle size of LMFP is preferably smaller than the average particle size of LCO, from the perspective of LMFP covering LCO. The particle size ratio (LCO:LMFP) of the average particle size of LCO to the average particle size of LMFP is preferably 70:1 to 215:1, and the average particle size of LMFP is, for example, on the order of nanometers. Specifically, in order to uniformly cover an LCO of several μm with LMFP particles, the average particle size of LMFP is preferably 1 nm to 100 nm. The average particle size of LMFP can also be measured using the same measurement method as for LCO.

[0035] Whether or not LCO and LMFP are present as positive electrode active materials can be confirmed by observation using a scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).

[0036] <Conductive agent> The conductive agent is not particularly limited, and known or commercially available ones can be used. Examples of conductive agents include carbon black such as acetylene black and Ketjen black, carbon nanotubes, carbon fibers, activated carbon, and conductive carbon such as graphite. The conductive agent can be used individually or as a mixture of several of these.

[0037] <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) or polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), acrylic resin, polyvinyl alcohol (PVA), or carboxymethylcellulose (CMC). The binder can be used individually, or as a mixture or copolymer of two or more of these substances.

[0038] <Dispersant> Dispersants are not always necessary, but they can be added if you want to further disperse the conductive agent. Typical dispersants include polyvinylpyrrolidone (PVP), for example.

[0039] <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, 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.

[0040] [Electrode fabrication method] The positive electrode can be manufactured, for example, by the following method. First, the positive electrode active material, conductive agent, binder, and dispersant mentioned above are dispersed in a solvent to prepare a positive electrode slurry. A thickening agent may be further 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.

[0041] <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. Furthermore, it is preferable to use polyvinylpyrrolidone (PVP) as the dispersant.

[0042] 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.

[0043] (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.

[0044] The negative electrode composite 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 substance 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 (made by sintering and carbonizing phenolic resin, furan resin, etc.), carbon fibers, carbon black, activated carbon and other carbons, or metallic materials such as Al, Si, Sn, or alloy materials. As negative electrode active materials, cokes and carbons that have stable charge-discharge characteristics and readily form an SEI (Solid Electrolyte Interphase) film on their surface are preferred, with graphite such as natural graphite or artificial graphite being more preferred.

[0045] <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.

[0046] <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.

[0047] <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 or stainless steel, porous metal such as porous aluminum, etc. The negative electrode current collector is preferably made of copper or a copper alloy.

[0048] 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.

[0049] (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 more mixtures selected from the group consisting of LiBF4, LiPF6, Li(FSO2)2N, Li(CF3SO2)2N, and LiPO2F2. The non-aqueous solvent is not particularly limited, but examples include one or more mixed solvents 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. In particular, it is preferable to include EC that enables good film formation on the negative electrode active material. Furthermore, it is preferable that the non-aqueous electrolyte contains a nitrile-based metal trapping agent. The metal trapping agent traps cobalt and can suppress the deposition of cobalt from eluted LCO on the negative electrode surface, which can induce side reactions. Sucinonitrile, adiponitrile, etc., can be used as nitrile-based metal trapping agents. The amount of the metal trapping agent added is preferably 0.5 wt% to 5 wt% of the total mass of the non-aqueous electrolyte. If the amount added is less than 0.5 wt%, the effect of trapping metal ions will be reduced, and if the amount added is more than 5 wt%, an excessive film will form on the positive electrode, resulting in a significant decrease in output characteristics.

[0050] (Separator) The separator 6 is not particularly limited, and known or commercially available separators can be used. For example, the separator 6 can be a sheet made of a microporous membrane made of polyethylene resin, polyolefin resin such as polypropylene resin, polyimide resin, or polytetrafluoroethylene (PTFE) resin. The microporous membrane or nonwoven fabric may be a single layer or a multilayer structure.

[0051] The material of the coating layer on the separator 6, and whether or not a coating layer exists, can be confirmed by SEM-EDS and XRD. Furthermore, the thickness of the coating layer on the separator can be confirmed by SEM-EDS after the separator's cross-section has been prepared using a cross-section polisher or razor.

[0052] In this embodiment, the positive electrode 4 or the separator 6 is provided with a coating layer made of an inorganic filler. The inorganic filler is not particularly limited, and for example, alumina, silica, barium sulfate, boehmite, etc. can be used alone or in combination of two or more, and the use of boehmite (AlOOH) is particularly preferred.

[0053] The thickness of the coating layer on each positive electrode or separator is preferably 1 μm to 4 μm per side. If the thickness of the coating layer is less than 1 μm, the safety function of the heat-resistant coating layer may not function, potentially reducing safety. If the thickness of the coating layer is greater than 4 μm, lithium ion electrophoresis may be inhibited during the battery reaction, potentially reducing output characteristics. For example, when the positive electrode 4 is coated with an inorganic filler, the positive electrode coated with the inorganic filler is produced by applying a coating agent containing inorganic filler powder and a binder to the surface of the positive electrode composite layer 41. Alternatively, the inorganic filler may be sprayed onto the positive electrode current collector 42 to form an inorganic filler layer on the surface of the positive electrode composite layer. When the separator 6 is coated with an inorganic filler, the inorganic filler is coated on one or both sides of the microporous film of the separator 6. The inorganic filler coating can improve heat resistance and the ability to replenish the non-aqueous electrolyte. If an inorganic filler coating layer is provided on the surface of the positive electrode composite layer 41 and on one of the separator 6, aramid may be coated on the other side (or the back surface of the coating layer). When forming a coating layer on only one side of the separator, it is preferable to form the coating layer on the side facing the positive electrode.

[0054] The average particle size of the inorganic filler used as the coating layer is preferably 1 nm to 2000 nm, and more preferably on the nano-order, from 10 nm to 500 nm. The average particle size of the inorganic filler can be determined, for example, by laser diffraction scattering.

[0055] 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.

[0056] It is preferable that such lithium-ion secondary batteries undergo pretreatment, such as initial charging and gas evacuation, before being used as batteries. For initial charging, for example, after injecting a non-aqueous electrolyte, constant current charging is performed at a small current value until the rated capacity reaches 10%. After such initial charging, gas generated inside the casing during the initial charging is evacuated. Gas evacuation is performed, for example, by placing the casing in a reduced pressure chamber with a part of the sealing portion open. Gas evacuation can be accelerated, for example, by pressing the casing from the outside during gas evacuation.

[0057] The presence of an active material capable of intercalating and deintercalating lithium in the electrode's constituent materials can be confirmed by cyclic voltammetry. By preparing the target electrode as the working electrode, using lithium metal as the counter electrode, and constructing a coin cell with separator 6 in between, and performing several cycles of cyclic voltammetry measurements using an electrochemical test apparatus, it is possible to confirm whether lithium is capable of intercalating and deintercalating lithium by observing the reversible occurrence of redox reaction peaks.

[0058] Furthermore, the capacity of the lithium-ion secondary battery 1 can be confirmed by charge-discharge testing. Using the upper and lower voltage limits specified by the manufacturer, charging is performed at 0.5 ItA, with CV switching at the upper voltage limit and a 0.05 ItA cutoff. Discharging is performed at 0.2 ItA, with a cutoff at the lower voltage limit. The battery capacity can then be confirmed from the discharge capacity.

[0059] The rate characteristics of lithium-ion secondary battery 1 can be confirmed by charge-discharge testing. Using the upper and lower voltage limits specified by the manufacturer, charging is performed at 0.5 ItA, with CV switching at the upper voltage limit and a 0.05 ItA cutoff. Discharging is performed at 0.2 ItA and 1 ItA, with a cutoff at the lower voltage limit. The discharge capacity at each discharge rate is obtained and confirmed by the ratio of the capacity at 1 ItA discharge to the capacity at 0.2 ItA discharge.

[0060] In the embodiment described above, the lithium-ion secondary battery 1 includes LCO and LMFP as positive electrode active materials, and an inorganic filler is coated on the surface of the positive electrode composite layer 41 or the separator 6. As a result, the LMFP coats the LCO, suppressing the deposition of leached cobalt on the negative electrode surface, and the inorganic filler coating further traps the cobalt. Thus, even if cobalt leaches from the positive electrode active material, the inorganic filler coating layer traps the cobalt, thus suppressing clogging of the voids (pores) in the separator 6. According to this embodiment, the storage capacity can be improved by using LCO, the deposition of leached cobalt on the negative electrode surface can be suppressed, and the increase in resistance can be suppressed by trapping.

[0061] The lithium-ion secondary battery 1 according to this embodiment can be used at a high voltage of 4.40V or higher by having the above configuration. [Examples]

[0062] 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.

[0063] (Example 1) <Fabrication of the positive electrode> As the positive electrode active material, a mixture of 93.12 parts by mass of lithium cobalt oxide (LCO) and 2.88 parts by mass of lithium iron manganese phosphate (LMFP) was used. The ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 97:3 by mass. The ratio of particle sizes of LCO and LMFP at D50 was LCO:LMFP = 115:1 by particle size ratio. First, this mixture, along with 1.2 parts by mass of polyvinylidene fluoride (PVDF) as a binder, 2.6% by mass of carbon black as a conductive agent, and 0.2 parts by mass of polyvinylpyrrolidone (PVP) as a dispersant, was dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to both sides of an aluminum foil (thickness 12 μm), which served as the positive electrode current collector, at a coating rate of 154 g / m² per side. 2 The material was coated using a coating machine and dried at 80-130°C. Afterward, the positive electrode was fabricated by press processing until the electrode density reached 3.5 g / cc.

[0064] <Fabrication of the negative electrode> As the negative electrode active material, a mixture of 68.6 parts by mass of natural graphite and 29.4 parts by mass of artificial graphite was used. The ratio of natural graphite to artificial graphite in the negative electrode active material was natural graphite:artificial graphite = 7:3 by mass. First, this mixture, along with 1 part by mass each of styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) as binders, was dispersed in deionized water as a solvent to prepare a negative electrode slurry. Next, the obtained negative electrode slurry was applied to both sides of a copper foil (6 μm thick), which served as the negative electrode current collector, at a single-sided coating rate of 83 g / m². 2 The material was coated using a coating machine and dried at 80-110°C. Afterward, the negative electrode was fabricated by press processing until the electrode density reached 1.6 g / cc.

[0065] <Separator> As a separator, a 12 μm thick polyethylene (PE) resin microporous membrane was used, with a 2 μm thick coating layer of boehmite (AlOOH) on both sides. The average particle size of the boehmite coating layer was measured using a laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) and confirmed to be between 10 and 500 nm.

[0066] <Exterior> As the outer casing, a first laminate film having a rectangular shape and a recess for housing the electrode group and a flat portion surrounding the recess, and a second flat laminate film having a rectangular shape were prepared. Each laminate film has a structure in which a heat-sealable resin layer made of polyolefin, a metal layer made of aluminum foil, and a protective layer made of polyamide film are laminated in this order.

[0067] <Preparation of non-aqueous electrolyte> The non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.3 mol / L in a solvent made by mixing ethylene carbonate (EC), diethyl carbonate (DEC), ethyl propionate (EP), and propyl propionate (PP) in a volume ratio of 30:10:45:15 (=EC:DEC:EP:PP). In addition, as electrolyte additives, vinylene carbonate (VC) was added at a concentration of 2.25% by mass, allyl succinic anhydride (ASAh) at a concentration of 0.5% by mass, and adiponitrile (ADN) and succinonitrile (SN) at a concentration of 1.25% by mass each, relative to the total mass of the electrolyte.

[0068] <Assembly of prototype battery> An electrode group was fabricated by alternately stacking separators, five positive electrode plates, and six negative electrode plates, with separators interposed between the positive and negative electrode plates. During this process, the current collector leads extended from the sides of the electrode group. Negative electrode plates were positioned at the outermost layers in the stacking direction of the electrode group. Furthermore, separators were placed on both end faces of the electrode group, and insulating tape was applied to secure the electrode group.

[0069] Each positive lead and each negative lead were bundled at their tips and joined together by ultrasonic welding. An aluminum tab was attached to the joint of the positive leads as a positive terminal by ultrasonic welding. A copper tab was attached to the joint of the negative leads as a negative terminal by ultrasonic welding.

[0070] An electrode group with welded positive and negative terminals is placed in a recess in the first laminate film. A flat second laminate film is then placed over the flat edge surrounding the recess in the first laminate film so that parts of the positive and negative terminals extend outwards. Three of the four overlapping sides of the first and second laminate films are heat-sealed together, and the electrode group is housed within an outer casing made of the first and second laminate films.

[0071] Next, 6.1 g of non-aqueous electrolyte was injected through the remaining side of the outer casing. Then, the side was heat-sealed under reduced pressure of 1 hPa to 100 hPa to obtain the prototype battery after the injection of the non-aqueous electrolyte. The theoretical capacity of this prototype battery is 2.0 Ah.

[0072] <Chemical conversion process> After the non-aqueous electrolyte was injected, the prototype battery was left to stand for 12 hours to allow the non-aqueous electrolyte to permeate the constituent materials of the electrode group. Next, the positive and negative terminals of the prototype battery were connected to a power supply (charge / discharge device), and the prototype battery was initially charged to 10% of its rated capacity at a current of 0.25 ItA. Then, the prototype battery was placed in a reduced pressure chamber with a portion of the sealing part of the casing open to exhaust any gas generated inside the prototype battery. Subsequently, the sealing part of the casing was sealed again, and the prototype battery was charged a second time at a current of 0.5 ItA to 10-100% of its rated capacity. After that, the prototype battery was left to stand in a high-temperature (35°C) environment for 17 hours, then constant current charging was performed at a current of 0.5 ItA up to 4.40V, followed by constant voltage charging to 0.1 ItA. Finally, it was discharged to 2.5V at a current of 0.2 ItA, and the rated capacity was measured to obtain the prototype battery according to Example 1.

[0073] <Weight ratio of LCO to LMFP> It was checked whether LCO and LMFP coexist by surface SEM images or cross-sectional SEM images. If their presence was confirmed, the positive electrode active material was obtained from the electrode to determine the weight ratio, and elemental analysis was performed by ICP. The weight ratio was calculated from Co, and Mn or Fe based on the ICP analysis results.

[0074] <Particle size ratio of LCO to LMFP> Surface SEM images or cross-sectional SEM images were acquired, and information on particle size distribution was obtained by performing image processing with image analysis software, so as to measure the average particle size of LCO and the average particle size of LMFP.

[0075] <Confirmation of existence of boehmite> Whether boehmite is present on the surface side of the positive electrode mixture layer was visually checked by using cellophane tape or the like to see if there is any adhesion to the cellophane tape. Whether boehmite is present on the separator side was confirmed by performing component analysis through FT-IR or XRD analysis.

[0076] <Evaluation of whether the separator is oxidized> After continuous charging at 45°C for 600 hours, or full-charge storage at 85°C for 3 days, the prototype battery was disassembled, and the separator was visually observed to confirm the presence or absence of oxidation. The presence or absence of oxidation was determined based on the presence or absence of black spots that appear due to oxidation, and a sample with no black spots at all was determined as "absent".

[0077] <Measurement of IR degradation rate, degradation rate (residual), and degradation rate (recovered)> The cell was fully charged (CCCV-4.45V-1C 0.1C cutoff), discharged at 0.2C, and the pre-storage capacity was obtained. After fully charging the cell again, the IR (Internal Resistance) was measured and the pre-storage IR was obtained. Then, the fully charged prototype battery was stored in a 60°C constant temperature bath for 10 days. After being removed from storage and sufficiently cooled in a 25°C environment, the IR was measured and the post-storage IR was obtained. It was also discharged at 0.2C to obtain the remaining capacity. After fully charging the prototype battery, it was discharged again at 0.2C to obtain the recovered capacity. Each degradation rate (%) was determined as follows. IR degradation rate = (IR after storage / IR before storage) × 100 Degradation rate (remaining) = (remaining volume / volume before storage) × 100 Degradation rate (recovery) = (recovery capacity / storage capacity) × 100 Furthermore, a smaller IR degradation rate (%) is considered to indicate better performance, while a larger degradation rate (residual) (%) and degradation rate (recovery) (%) are considered to indicate better performance.

[0078] <Overall Rating> A comprehensive evaluation was conducted based on the results of separator oxidation, IR degradation rate, degradation rate (remaining), and degradation rate (recovery). In the overall evaluation, if all conditions were met—separator oxidation was "none," IR degradation rate was less than 300%, degradation rate (remaining) was 60% or higher, and degradation rate (recovery) was 80% or higher—it was marked "○." If even one condition was not met, it was marked "×."

[0079] Table 1 shows the battery configuration and test results in Example 1. [Table 1]

[0080] (Example 2) In Example 2, the same configuration and experimental method as in Example 1 were used, except that the positive electrode active material was changed to 91.2 parts by mass of LCO and 4.8 parts by mass of LMFP. In Example 2, the ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 95:5. The configuration of the battery and the test results in Example 2 are shown in Table 1.

[0081] (Example 3) In Example 3, the same configuration and experimental method as in Example 1 were used, except that the positive electrode active material was changed to 86.4 parts by mass of LCO and 9.6 parts by mass of LMFP. In Example 3, the ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 90:10. The configuration of the battery and the test results in Example 3 are shown in Table 1.

[0082] (Example 4) In Example 4, the same configuration and experimental method as in Example 1 were used, except that the positive electrode active material was changed to 81.6 parts by mass of LCO and 14.4 parts by mass of LMFP. In Example 4, the ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 85:15. The configuration of the battery and the test results in Example 4 are shown in Table 1.

[0083] (Example 5) In Example 5, the same configuration and experimental method as in Example 1 were used, except that the positive electrode active material was changed to 76.8 parts by mass of LCO and 19.2 parts by mass of LMFP. In Example 5, the ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 80:20. The configuration of the battery and the test results in Example 5 are shown in Table 1.

[0084] (Example 6) In Example 6, the same configuration and experimental method as in Example 1 were used, except that the separator had a coating layer made of boehmite (AlOOH) with a thickness of 2 μm on one side (the positive electrode side). The configuration of the battery in Example 6 and the test results are shown in Table 1.

[0085] (Example 7) In Example 7, the same configuration and experimental method as in Example 1 were used, except that a coating layer made of boehmite (AlOOH) was provided on the surface of the positive electrode composite layer, and the separator was a microporous membrane made of PE resin without a coating layer. The configuration of the battery in Example 7 and the test results are shown in Table 1. The average particle size of the boehmite coating layer was measured using a laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) and confirmed to be between 10 and 500 nm.

[0086] (Example 8) In Example 8, the same configuration and experimental method as in Example 1 were used, except that a coating layer made of boehmite (AlOOH) was provided on the surface of the positive electrode composite layer and on both sides of the separator. The configuration of the battery in Example 8 and the test results are shown in Table 1. The average particle size of the boehmite coating layer was measured using a laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) and confirmed to be between 10 and 500 nm.

[0087] (Example 9) In Example 9, the same configuration and experimental method as in Example 1 were used, except that a coating layer made of boehmite (AlOOH) was provided on the surface of the positive electrode composite layer, and a 4 μm thick coating layer made of aramid was provided on one side (positive electrode side) of the separator. The configuration of the battery in Example 9 and the test results are shown in Table 1. The average particle size of the aramid coating layer was measured using a laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) and confirmed to be between 10 and 500 nm.

[0088] (Example 10) In Example 10, the same configuration and experimental method as in Example 1 were used, except that the particle size ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 70:1. The configuration of the battery and the test results in Example 10 are shown in Table 1.

[0089] (Example 11) In Example 11, the same configuration and experimental method as in Example 1 were used, except that the particle size ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 85:1. The configuration of the battery and the test results in Example 11 are shown in Table 1.

[0090] (Example 12) In Example 12, the same configuration and experimental method as in Example 1 were used, except that the particle size ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 170:1. The configuration of the battery and the test results in Example 12 are shown in Table 1.

[0091] (Example 13) In Example 13, the same configuration and experimental method as in Example 1 were used, except that the particle size ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 215:1. The configuration of the battery and the test results in Example 13 are shown in Table 1.

[0092] (Comparative Example 1) In Comparative Example 1, the same configuration and experimental method as in Example 1 were used, except that the positive electrode active material consisted only of 96.0 parts by mass of LCO. The configuration of the battery and the test results in Comparative Example 1 are shown in Table 1.

[0093] (Comparative Example 2) In Comparative Example 2, the same configuration and experimental method as in Comparative Example 1 were used, except that a 4 μm thick coating layer made of aramid was applied to one side (positive electrode side) of the separator. The configuration of the battery in Comparative Example 2 and the test results are shown in Table 1. The average particle size of the aramid coating layer was measured using a laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation) and confirmed to be between 10 and 500 nm.

[0094] (Comparative Example 3) In Comparative Example 3, the same configuration and experimental method as in Example 2 were used, except that the particle size ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 16:1, and the separator was a microporous membrane made of PE resin without a coating layer. The configuration and test results of the battery in Comparative Example 3 are shown in Table 1.

[0095] (Comparative Example 4) In Comparative Example 4, the positive electrode active material was changed to 57.6 parts by mass of LCO and 38.4 parts by mass of LMFP, and the same configuration and experimental method as in Example 1 was used, except that the particle size ratio was LCO:LMFP = 16:1. In Comparative Example 4, the ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 60:40. The configuration of the battery and the test results in Comparative Example 4 are shown in Table 1.

[0096] (Comparative Example 5) In Comparative Example 5, the positive electrode active material was changed to 95.04 parts by mass of LCO and 0.96 parts by mass of LMFP, and the same configuration and experimental method as in Example 1 was used, except that the particle size ratio was LCO:LMFP = 16:1. In Comparative Example 5, the ratio of LCO to LMFP in the positive electrode active material was LCO:LMFP = 99:1. The configuration of the battery and the test results in Comparative Example 5 are shown in Table 1.

[0097] Table 1 shows that by including LCO and LMFP as the positive electrode active material and providing a coating layer made of boehmite on either the surface of the positive electrode composite layer or the separator, oxidation of the separator did not occur, and compared to Comparative Examples 1 to 5, good IR degradation rates, degradation rates (residual), and degradation rates (recovery) were observed. Therefore, it can be said that Examples 1 to 13 can suppress the increase in resistance and improve storage capacity. In particular, Examples 2, 3, 6 to 13, in which the ratio of LCO to LMFP was in the range of 95:5 to 90:10, showed relatively low IR degradation rates compared to Examples 1, 4, and 5.

[0098] The lithium-ion secondary battery of the present invention can be applied to a variety of applications requiring rapid charging and discharging and high capacity, such as power tools, drones, robots, and electric motorcycles. [Explanation of Symbols]

[0099] 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

1. Equipped with a positive electrode, a negative electrode, and a separator, The positive electrode is composed of a positive electrode composite layer comprising a positive electrode active material, a conductive additive, and a binder, and a current collector foil. The positive electrode composite layer comprises a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is lithium cobalt oxide (LiCoO2). ２ ) and The second positive electrode active material is a phosphoric acid compound represented by the following formula (1): The surface of the positive electrode composite layer and / or the separator is provided with a coating layer made of an inorganic filler. A lithium-ion secondary battery characterized by the following features. LiMn ｘ M1 ａ Fe １－ｘ－ａ PO ４ (However, 0 < x ≤ 0.9, 0 ≤ a < 0.1, 0 < x + a < 1) ... (1) In the above formula (1), M1 is at least one selected from sodium (Na), magnesium (Mg), scandium (Sc), yttrium (Y), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), aluminum (Al), titanium (Ti), chromium (Cr), lead (Pb), antimony (Sb), and boron (B).

2. The mass ratio of the first positive electrode active material to the second positive electrode active material (first positive electrode active material: second positive electrode active material) is 95:5 to 90:

10. The lithium-ion secondary battery according to feature 1.

3. The particle size ratio of the first positive electrode active material to the second positive electrode active material (first positive electrode active material: second positive electrode active material) is 70:1 to 215:

1. The lithium-ion secondary battery according to feature 1.

4. A coating layer consisting of the inorganic filler is provided on both sides of the separator. The lithium-ion secondary battery according to feature 1.

5. The average particle size of the first positive electrode active material is 6 μm or more and 12 μm or less. The lithium-ion secondary battery according to feature 1.

6. The average particle size of the second positive electrode active material is on the order of nanometers. The lithium-ion secondary battery according to feature 1.

7. The voltage of the lithium-ion secondary battery is 4.40V or higher. The lithium-ion secondary battery according to feature 1.

8. Contains a non-aqueous electrolyte, A metal trapping agent is added to the non-aqueous electrolyte. The lithium-ion secondary battery according to feature 1.

9. The lithium-ion secondary battery according to claim 8, characterized in that the metal trapping agent traps cobalt.

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