Lithium-ion secondary battery

Abstract

To provide a lithium ion secondary battery which has flexibility and allows a good output characteristic to be achieved even in a high-output charge / discharge operation.SOLUTION: A lithium ion secondary battery according to an embodiment hereof comprises an electrode sheet 10 having: a current collector layer 14 integrated with an exterior packaging film 12; an electrode layer 16 containing active material particles 20 and a polymer solid electrolyte 22 filling gaps among the active material particles; and a separator layer 18 containing inorganic oxide particles 24 and a polymer solid electrolyte 22 filling gaps among the inorganic oxide particles. The polymer solid electrolyte 22 contains a lithium salt, an ionic liquid and a high-molecular weight polymer.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] FIELD OF THE INVENTION An embodiment of the present invention relates to a lithium-ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries using solid electrolytes instead of liquid electrolyte solutions are being actively developed. Attempts have been made to use solid electrolyte membranes instead of liquid electrolytes to obtain thin-film lithium-ion secondary batteries. Among solid electrolytes, polymer solid electrolytes offer greater flexibility in terms of shape and are considered suitable for use as solid electrolyte membranes in film-shaped batteries. However, most polymer solid electrolyte membranes do not provide effective ionic conductivity at room temperature. Therefore, attempts have been made to confine ionic liquids within polymer matrices (see Patent Documents 1 and 2) and to gel polymer electrolytes (see Patent Document 3).

[0003] On the other hand, among solid electrolytes, oxide-based inorganic solid electrolytes have high atmospheric stability and can be continuously coated at room temperature and normal pressure, so a thin separator layer of 10 μm or less can be formed by coating a thin film on a positive electrode active material or a negative electrode active material. Patent Document 4 discloses a method for realizing a thin film lithium-ion secondary battery by filling and joining the inorganic solid particle interfaces of the positive electrode active material, inorganic solid electrolyte, and negative electrode active material with a polymer solid electrolyte. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-149920 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-035250 [Patent Document 3] Japanese Patent Application Publication No. 2019-057425 [Patent Document 4] International Publication No. WO2018 / 180768 Brochure Summary of the Invention [Problem to be solved by the invention]

[0005] When attempting to obtain ionic conductivity comparable to that of a liquid electrolyte using a polymer solid electrolyte membrane obtained by the methods described in Patent Documents 1 to 3, the strength of the polymer solid electrolyte is reduced, impairing the membrane's isolation and self-supporting ability. Therefore, it cannot be used alone as a flexible thin film. In many cases, studies have been conducted to ensure self-supporting strength by impregnating a polymer solid electrolyte into an insulating film using a porous polymer film as a separator, as in conventional batteries. As a result, such thin-film lithium-ion secondary batteries have a structure in which a separator made of a polymer film impregnated with a polymer solid electrolyte is placed between a positive electrode coated with a positive electrode active material and a negative electrode coated with a negative electrode active material. This makes it difficult to realize a flexible film-shaped lithium-ion secondary battery.

[0006] Regarding the lithium ion secondary battery described in Patent Document 4, various oxide-based inorganic solid electrolytes with excellent ionic conductivity have been developed in recent years, but as mentioned above, there are no good polymer solid electrolytes that exhibit high ionic conductivity at room temperature. Until now, even in lithium ion secondary batteries in which such inorganic solid electrolyte particles are filled and joined with polymer solid electrolyte, the internal resistance of the battery increases due to the low ionic conductivity of the polymer solid electrolyte itself, and there has been a problem in that good output characteristics cannot be obtained during high-power charging and discharging.

[0007] In view of the above, an object of an embodiment of the present invention is to provide a lithium ion secondary battery that is flexible and can provide good output characteristics even during high-output charging and discharging. [Means for solving the problem]

[0008] A lithium-ion secondary battery according to an embodiment of the present invention comprises an electrode sheet having a current collector layer integrated with an exterior film, an electrode layer formed on the current collector layer and including active material particles and a polymer solid electrolyte filling the gaps between the active material particles, and a separator layer formed on the electrode layer and including inorganic oxide particles and the polymer solid electrolyte filling the gaps between the inorganic oxide particles. The polymer solid electrolyte includes a lithium salt, an ionic liquid, and a polymer.

[0009] In the lithium ion secondary battery, the ionic liquid preferably contains a cation component and a bis(fluorosulfonyl)imide anion as an anion component.

[0010] In the lithium ion secondary battery, the high molecular weight polymer preferably includes a crosslinked polymer obtained by crosslinking a reactive compound having a crosslinkable reactive group.

[0011] In the lithium ion secondary battery, the reactive compound preferably contains a tri- or higher functional polyether (meth)acrylate.

[0012] In the lithium ion secondary battery, the content of the lithium salt is preferably 2 mol / kg or more and 6 mol / kg or less with respect to the sum of the contents of the ionic liquid and the polymer.

[0013] In the lithium ion secondary battery, the total thickness of the electrode layer and the separator layer is preferably 0.02 mm or more and 0.3 mm or less. [Effects of the Invention]

[0014] The lithium ion secondary battery according to the embodiment of the present invention has flexibility and can obtain good output characteristics even during high-output charging and discharging. [Brief explanation of the drawings]

[0015] [Figure 1]1 is a cross-sectional schematic diagram of an electrode sheet for a lithium-ion secondary battery according to one embodiment; [Figure 2] Schematic cross-sectional view of a lithium-ion secondary battery according to an embodiment. [Figure 3] Plan view of the lithium ion secondary battery produced in the example [Figure 4] Cross section of line IV-IV in Figure 3 DETAILED DESCRIPTION OF THE INVENTION

[0016] A lithium-ion secondary battery according to an embodiment includes an electrode sheet, the electrode sheet including: a current collector layer integrated with an exterior film; an electrode layer formed on the current collector layer and including active material particles and a polymer solid electrolyte filling the gaps between the active material particles; and a separator layer formed on the electrode layer and including inorganic oxide particles and the polymer solid electrolyte filling the gaps between the inorganic oxide particles. The polymer solid electrolyte contains a lithium salt, an ionic liquid, and a polymer. Here, the electrode layer refers to either or both of the positive electrode and the negative electrode, and preferably both the positive electrode and the negative electrode are configured as described above.

[0017] According to this embodiment, an electrode layer is formed on a current collector layer integrated with an exterior film, and a separator layer is further formed on the electrode layer using inorganic oxide particles and a polymer solid electrolyte. This makes it possible to reduce the total thickness of the electrode layer and separator layer and form a thin film structure in the form of a soft and flexible film, thereby obtaining a thin film lithium ion secondary battery.

[0018] Furthermore, the use of a polymer solid electrolyte containing a lithium salt, an ionic liquid, and a polymer allows for smooth migration of lithium ions in the polymer solid electrolyte, thereby providing a lithium ion secondary battery that maintains a high level of lithium ion conductivity, provides sufficient battery performance even at high charge / discharge rates, and is highly safe.

[0019] Furthermore, by filling and bonding the particle interfaces in the electrode layer with a polymer solid electrolyte, a lithium-ion secondary battery can be realized that has good output characteristics comparable to those of conventional batteries using a general liquid electrolyte, even during high-power charging and discharging.

[0020] [Electrode sheet] An electrode sheet for a lithium ion secondary battery according to one embodiment will be described with reference to FIG.

[0021] The electrode sheet 10 shown in FIG. 1 is a thin-film electrode sheet including a current collector layer 14 integrated with an exterior film 12, an electrode layer 16 formed on the current collector layer 14, and a separator layer 18 formed on the electrode layer 16.

[0022] In a lithium-ion secondary battery, the electrode sheet 10 may be used as a positive electrode sheet constituting a positive electrode, a negative electrode sheet constituting a negative electrode, or both a positive electrode sheet and a negative electrode sheet. In the case of a positive electrode sheet, the current collector layer 14 serves as a positive electrode current collector layer, and the electrode layer 16 serves as a positive electrode layer. In the case of a negative electrode sheet, the current collector layer 14 serves as a negative electrode current collector layer, and the electrode layer 16 serves as a negative electrode layer. The separator layer 18 may be provided on each of the positive electrode sheet and the negative electrode sheet, and the separator layers of both sheets may be joined together to form an integrated separator layer. Alternatively, a separator layer may be provided on either the positive electrode sheet or the negative electrode sheet, and the electrode layer of the other sheet may be joined to that separator layer, so that both sheets share a single separator layer.

[0023] Various materials having electronic conductivity can be used for the current collector layer 14. Examples of the positive electrode current collector layer include foils of aluminum, titanium, stainless steel, nickel, etc. Furthermore, for the purpose of improving adhesion, conductivity, and oxidation resistance, a material in which the surface of aluminum or the like is treated with carbon, nickel, titanium, silver, etc. may be used as the positive electrode current collector layer. Examples of the negative electrode current collector layer include foils of copper, stainless steel, nickel, aluminum, titanium, etc. Furthermore, for the purpose of improving adhesion, conductivity, and oxidation resistance, a material in which the surface of copper or the like is treated with carbon, nickel, titanium, silver, etc. may be used as the negative electrode current collector layer.

[0024] In this embodiment, a current collector layer 14 integrated with an exterior film 12 is used. Specifically, a laminate in which the exterior film 12 is bonded to one side of the current collector layer 14 may be used. The exterior film 12 is a film that constitutes the exterior of the lithium-ion secondary battery, and an electrically insulating polymer film is used. Examples of the exterior film 12 include polyester films such as polyethylene terephthalate (PET) film and polybutylene terephthalate (PBT) film, and polyamide films such as nylon 6 film and nylon 66 film.

[0025] The thickness of the current collector layer 14 is not particularly limited and may be, for example, 1 to 50 μm or 10 to 35 μm. The thickness of the exterior film 12 is not particularly limited and may be, for example, 5 to 50 μm or 10 to 20 μm. The thickness of the laminate of the exterior film 12 and the current collector layer 14 is not particularly limited and may be, for example, 6 to 100 μm or 20 to 50 μm.

[0026] The electrode layer 16 includes active material particles 20 and a polymer solid electrolyte 22 that fills the gaps between the active material particles 20. The electrode layer 16 may further include additives such as a conductive agent and a binder. The polymer solid electrolyte 22 is preferably provided so as to fill the gaps between the active material particles 20 throughout the entire electrode layer 16, from the surface of the current collector layer 14 to the interface with the separator layer 18.

[0027] When the electrode layer 16 is a positive electrode layer, the active material particles 20 are positive electrode active material particles. Positive electrode active material particles may be any material capable of inserting and extracting lithium ions, including metal oxides, composite oxides of lithium and transition metals, metal chalcogenides, and conductive polymer compounds. Examples of metal oxides include CuO, Cu2O, MnO2, MoO3, VO5, CrO3, MoO3, Fe2O3, Ni2O3, and CoO3. Examples of metal chalcogenides include TiS2, MoS2, and NbSe3. Examples of conductive polymer compounds include polyacene, polyparaphenylene, polypyrrole, and polyaniline.

[0028] As the positive electrode active material particles, composite oxides of lithium and transition metals are preferred because they are more likely to produce a high voltage. Examples of composite oxides of lithium and transition metals include LiCoO2, LiMnO2, LiMn2O4, LiNiO2, LiFePO4, and LiNi x Co (1-x) O2, LiMn a Ni b Co c (a+b+c=1), etc. Alternatively, a composite oxide of lithium and a transition metal doped with a small amount of an element such as fluorine, boron, aluminum, chromium, zirconium, molybdenum, or iron, or a lithium composite oxide particle surface treated with carbon, MgO, Al2O3, SiO2, or the like may be used as the positive electrode active material particles.

[0029] The amount of the positive electrode active material particles is not particularly limited, and for example, the mass per unit area of the positive electrode layer is 3 mg / cm. 2 More than 10mg / cm 2 The following may also be used.

[0030] When the electrode layer 16 is a negative electrode layer, the active material particles 20 are negative electrode active material particles. The negative electrode active material particles may be any material capable of inserting and extracting metallic lithium or lithium ions, and examples thereof include carbon materials, metallic materials, lithium transition metal nitrides, crystalline metal oxides, amorphous metal oxides, silicon compounds, and conductive polymers. Examples of carbon materials include natural graphite, artificial graphite, non-graphitizable carbon, and easily graphitizable carbon. Examples of metallic materials include metallic lithium, alloys, and tin compounds. Specific examples of negative electrode active material particles include Li4Ti5O 12 , NiSi5C6, etc. The negative electrode active material particles may be any one of the above listed materials or a combination of two or more of them. The amount of the negative electrode active material particles is not particularly limited, and for example, the mass per unit area of the negative electrode layer may be 1 mg / cm 2 More than 5mg / cm 2 The following may also be used.

[0031] The conductive agent used in the electrode layer 16 is not particularly limited and may, for example, be carbon black such as acetylene black or ketjen black, natural graphite (e.g., scaly graphite, flake graphite, or clay-like graphite), artificial graphite, carbon whiskers, carbon fiber, metal (e.g., copper, nickel, aluminum, silver, or gold) powder, metal fiber, or conductive ceramic material. These conductive agents may be used alone or in combination. The amount of conductive agent added is not particularly limited and may, for example, be 1% by mass or more and 20% by mass or less, or 1.5% by mass or more and 10% by mass or less, relative to the mass of the active material particles 20.

[0032] The binder used in the electrode layer 16 is not particularly limited, and examples thereof include polyvinylidene fluoride (PVDF), PVDF copolymer resin, fluorine-based resin, styrene-butadiene rubber (SBR), ethylene-propylene rubber (EPDM), styrene-acrylonitrile copolymer, etc. The amount of binder added is not particularly limited, and may be, for example, 1% by mass or more and 20% by mass or less, or 1.5% by mass or more and 10% by mass or less, relative to the mass of the active material particles 20.

[0033] The polymer solid electrolyte 22 filled between the active material particles 20 in the electrode layer 16 contains a lithium salt, an ionic liquid, and a polymer.

[0034] The lithium salt is dissolved in an ionic liquid, and lithium salts generally used as electrolytes for non-aqueous electrolytes can be used. Specific examples of the lithium salt include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2 (i.e., LiTFSI), LiN(FSO2)2 (i.e., LiFSI), and LiBC4O8. These lithium salts may be used alone or in combination of two or more. LiFSI and LiTFSI are preferred as the lithium salt.

[0035] Ionic liquids are solvents for transporting lithium ions, and dissolve lithium salts to form a non-aqueous electrolyte solution together with the lithium salts. Ionic liquids are composed of cationic and anionic components, are liquid at room temperature (25°C), are non-volatile, and have a relatively high decomposition temperature. By using an ionic liquid in the electrolyte solution that constitutes the polymer solid electrolyte, the heat resistance and safety of the electrolyte are superior compared to when a generally flammable organic solvent (e.g., cyclic carbonate, chain carbonate, etc.) is used. Furthermore, the polymer solid electrolyte according to this embodiment exhibits high performance even during high-power charging and discharging, resulting in a battery with high energy density and high voltage.

[0036] Examples of anionic components contained in ionic liquids include BF4 - , PF6 - , SbF6 - , NO3 - , CF3SO3 - , (FSO2)2N - (i.e., FSI anion), (CF3SO2)2N - (i.e., TFSI anion), (C2F5SO2)2N - , (CF3SO2)3C - , CF3CO2 - , C3F7CO2 -, CH3CO2 - , (CN)2N - These may be used alone or in combination of two or more.

[0037] As the anion component, it is preferable to use bis(fluorosulfonyl)imide anion (FSI anion). By including FSI anion as an anion component in an ionic liquid, battery performance during high-power charge / discharge can be further improved. When FSI anion is used as an anion component, an anion other than FSI anion may be used in combination. The method for preparing FSI anion is not particularly limited, but examples include a method in which fluorosulfonic acid is reacted with urea. Impurities can be confirmed by analysis using a plasma emission spectrometer (ICP).

[0038] The cationic component contained in the ionic liquid is not particularly limited, and examples thereof include cations that are compounds containing elements such as N, P, S, O, C, and Si, and have a chain structure or a cyclic structure such as a five-membered ring or a six-membered ring as a skeleton. Examples of cyclic structures such as five-membered rings and six-membered rings include heterocyclic structures such as a furan ring, thiophene ring, pyrrole ring, pyridine ring, oxazole ring, isoxazole ring, thiazole ring, isothiazole ring, furazan ring, imidazole ring, pyrazole ring, pyrazine ring, pyrimidine ring, pyridazine ring, pyrrolidine ring, piperidine ring, benzofuran ring, isobenzofuran ring, indole ring, isoindole ring, indolizine ring, and carbazole ring.

[0039] Among these cations, particularly preferred are chain or cyclic compounds containing a nitrogen element, which are industrially inexpensive and chemically and electrochemically stable. Examples of cations containing a nitrogen element include alkylammoniums such as triethylammonium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-methyl-1-propyl-pyrrolidinium, and methylpropylpiperidinium. These cations may be used alone or in combination of two or more.

[0040] The polymer contained in the polymer solid electrolyte 22 preferably includes a crosslinked polymer obtained by crosslinking a reactive compound having a crosslinkable reactive group. The crosslinked polymer improves the membrane strength and durability when the membrane is thinned, and also enhances the effect of trapping the ionic liquid in the polymer solid electrolyte 22.

[0041] Examples of crosslinkable reactive groups include double-bond functional groups such as (meth)acryloyl groups and allyl groups, and cyclic ether groups such as epoxy groups and oxetane groups. Examples of reactive compounds having such reactive groups include bifunctional or higher functional (meth)acrylate compounds and bifunctional or higher functional oxetane compounds. The polymer solid electrolyte 22 may contain an initiator for crosslinking the reactive groups. Here, the term "(meth)acryloyl group" refers to a concept that includes either or both of an acryloyl group and a methacryloyl group. Furthermore, the term "(meth)acrylate" refers to a concept that includes either or both of an acrylate and a methacrylate.

[0042] The difunctional or higher functional (meth)acrylate compound refers to an acrylate compound having two or more (meth)acryloyl groups in one molecule, such as tetrafunctional polyether acrylate, tetrafunctional polyether methacrylate, trifunctional polyether acrylate, trifunctional polyether methacrylate, bifunctional polyether acrylate, bifunctional polyether methacrylate, polyethylene glycol diacrylate, and polyethylene glycol dimethacrylate. These compounds may be used alone or in combination. Among these compounds, difunctional or higher functional polyether (meth)acrylates are preferred, and trifunctional or higher functional polyether (meth)acrylates are more preferred because they can further improve film strength and enhance durability when the film is thin. Furthermore, these polyether (meth)acrylates preferably have a polyethylene oxide skeleton.

[0043] The bifunctional or higher functional oxetane compound is a compound having two or more oxetane groups in one molecule, and examples thereof include a polymer obtained by polymerizing a monomer component containing a (meth)acrylate having an oxetane group. Specific examples include a copolymer of methyl methacrylate and oxetanyl methacrylate, and a copolymer of methyl methacrylate and (3-ethyloxetan-3-yl)methyl methacrylate.

[0044] In the polymer solid electrolyte 22, the content of the lithium salt is preferably 2 mol / kg or more and 6 mol / kg or less relative to the sum of the contents of the ionic liquid and the polymer. By increasing the content of lithium salt compared to that used in general lithium ion secondary batteries, the number of mobile lithium ions can be increased, thereby improving ionic conductivity. Furthermore, by keeping the lithium salt content at 6 mol / kg or less, an increase in viscosity of the nonaqueous electrolyte solution obtained by dissolving lithium salt in an ionic liquid can be suppressed, thereby suppressing a decrease in ionic conductivity. From the viewpoint of achieving excellent specific capacity at 1C discharge, the content of the lithium salt is preferably 5 mol / kg or less, more preferably 4 mol / kg or less, relative to the sum of the contents of the ionic liquid and the polymer.

[0045] In the polymer solid electrolyte 22, the content of the polymer is not particularly limited, but is preferably 25% by mass to 40% by mass, and more preferably 25% by mass to 35% by mass, of the total content of the ionic liquid and the polymer. This allows the polymer solid electrolyte 22 to maintain high lithium ion conductivity while maintaining high strength. The content of the ionic liquid is preferably 60% by mass to 75% by mass, and more preferably 65% by mass to 75% by mass, of the total content of the ionic liquid and the polymer.

[0046] The thickness of the electrode layer 16 is not particularly limited, but it is preferably 10 to 50 μm, and more preferably 15 to 30 μm for both the positive electrode layer and the negative electrode layer.

[0047] The separator layer 18 includes inorganic oxide particles 24 and a polymer solid electrolyte 22 that fills the gaps between the inorganic oxide particles 24. The separator layer 18 may further include additives such as a binder. The polymer solid electrolyte 22 is provided across the entire thickness of the separator layer 18. The separator layer 18 may have inorganic oxide particles 24 across the entire thickness, but preferably has a layer of polymer solid electrolyte 22 that is free of inorganic oxide particles 24, as shown in FIG. 1. That is, in the example shown in FIG. 1, the separator layer 18 includes an inorganic layer 26 that includes inorganic oxide particles 24 and is formed on the electrode layer 16, and a polymer solid electrolyte layer 28 that is free of inorganic oxide particles 24 and is formed on the inorganic layer 26.

[0048] As inorganic oxide particles 24, La, which has high lithium ion conductivity, 2 / 3-x Li 3x TiO3 (i.e., LLT), Li 1+x Al y Ti 2-y (PO4)3 (i.e., LATP), Li 1+x Al y Ge 2-y The inorganic oxide particles 24 may be inorganic solid electrolyte particles such as (PO4)3 (i.e., LAGP), or may be oxide particles that are not solid electrolytes, such as alumina or silica. The average particle size of the inorganic oxide particles 24 is not particularly limited, and the median diameter (D50) at which the cumulative particle volume distribution is 50% may be, for example, 0.1 μm to 10 μm, or 0.2 μm to 5 μm.

[0049] The binder used in the separator layer 18 is not particularly limited, and examples thereof include polyvinylidene fluoride (PVDF), PVDF copolymer resin, fluorine-based resin, styrene-butadiene rubber (SBR), ethylene-propylene rubber (EPDM), styrene-acrylonitrile copolymer, etc. The amount of binder added is not particularly limited, and may be, for example, 1% by mass or more and 20% by mass or less, or 1.5% by mass or more and 10% by mass or less, relative to the mass of the inorganic oxide particles 24.

[0050] The polymer solid electrolyte 22 contained in the separator layer 18 is the same as the polymer solid electrolyte 22 contained in the electrode layer 16. Preferably, the polymer solid electrolyte 22 in the electrode layer 16 and the polymer solid electrolyte 22 in the separator layer 18 are integrally formed. Here, "integrally formed" means that they are formed by simultaneous curing from a single polymer solid electrolyte solution.

[0051] The thickness of the separator layer 18 is not particularly limited, but is preferably 3 to 150 μm, more preferably 3 to 40 μm, in the electrode sheet 10 before being manufactured into a battery. The thicknesses of the inorganic layer 26 and the polymer solid electrolyte layer 28 that constitute the separator layer 18 are also not particularly limited, but is preferably 2 to 20 μm, more preferably 2 to 10 μm, in the electrode sheet 10 before being manufactured into a battery, in the inorganic layer 26, and is preferably 1 to 130 μm, more preferably 1 to 30 μm, in the polymer solid electrolyte layer 28.

[0052] A method for manufacturing an electrode sheet 10 according to one embodiment may include, for example, (S1) a step of forming an active material layer on a current collector layer 14 integrated with an exterior film 12, (S2) a step of forming an inorganic oxide layer on the active material layer, and (S3) a step of filling the active material layer and the inorganic oxide layer with a polymer solid electrolyte solution and curing the solution.

[0053] In step (S1), active material particles 20 are mixed with additives such as a conductive agent and a binder in a dispersion medium to prepare a slurry-like active material coating. The active material coating is then applied to the current collector layer 14, and the dispersion medium is then dried and removed. Examples of the dispersion medium include organic solvents such as N-methyl-2-pyrrolidone and water. To make the active material coating slurry, a water-soluble polymer such as carboxymethyl cellulose (CMC) may be added as a viscosity adjuster. The coating method is not particularly limited, and examples include application using a well-known coating machine, as well as spray printing and screen printing.

[0054] In step (S2), inorganic oxide particles 24 are mixed with additives such as a binder in a dispersion medium to prepare a slurry of inorganic oxide paint, which is then applied to the active material layer, and the dispersion medium is then dried and removed. Examples of the dispersion medium that can be used include organic solvents such as N-methyl-2-pyrrolidone and water. The application method is not particularly limited, and examples include spray printing and screen printing.

[0055] In step (S3), a polymer solid electrolyte solution is applied to the surface of the inorganic oxide layer and allowed to penetrate into the active material layer and the inorganic oxide layer. The polymer solid electrolyte solution preferably contains, in addition to a lithium salt and an ionic liquid, a reactive compound that forms a crosslinked polymer as a polymer by a crosslinking reaction, and an initiator for the crosslinking reaction. The polymer solid electrolyte solution may also contain an organic solvent as a diluent. The method for applying the polymer solid electrolyte solution is not particularly limited as long as the polymer solid electrolyte solution applied from the surface of the inorganic oxide layer can penetrate the entire active material layer and the inorganic oxide layer, and known coating methods can be used. After applying the polymer solid electrolyte solution, the diluent solvent is volatilized, followed by heat treatment to crosslink and harden the solution, thereby obtaining a polymer solid electrolyte. In this case, it is preferable that the surface of the polymer solid electrolyte has tackiness, which can improve interfacial adhesion when bonding separator layers 18 together or the separator layer 18 and the electrode layer 16 together.

[0056] [Lithium-ion secondary battery] A lithium ion secondary battery according to one embodiment is a film-shaped battery including the electrode sheet 10 shown in Fig. 1. A lithium ion secondary battery 30 shown in Fig. 2 is formed by using the electrode sheet 10 as a positive electrode sheet 10A and a negative electrode sheet 10B, and bonding these positive electrode sheet 10A and negative electrode sheet 10B together.

[0057] 2, lithium-ion secondary battery 30 includes first exterior film 12A, positive electrode current collector layer 14A, positive electrode layer 16A, separator layer 32, negative electrode layer 16B, negative electrode current collector layer 14B, and second exterior film 12B laminated in this order. Separator layer 32 separating the positive electrode and negative electrode includes first inorganic layer 26A in contact with positive electrode layer 16A, second inorganic layer 26B in contact with negative electrode layer 16B, and polymer solid electrolyte layer 28A sandwiched between first inorganic layer 26A and second inorganic layer 26B.

[0058] The positive electrode sheet 10A has a first exterior film 12A, a positive electrode current collector layer 14A, a positive electrode layer 16A, a first inorganic layer 26A, and a portion of a polymer solid electrolyte layer 28A, and the details of each of these components are as described for the electrode sheet 10. That is, the positive electrode current collector layer 14A is integrated with the first exterior film 12A. The positive electrode layer 16A includes positive electrode active material particles 20A and a polymer solid electrolyte 22A that fills the gaps between the positive electrode active material particles 20A. The first inorganic layer 26A and a portion of the polymer solid electrolyte layer 28A form the separator layer 18, and the separator layer 18 includes inorganic oxide particles 24A and a polymer solid electrolyte 22A that fills the gaps between the inorganic oxide particles 24A.

[0059] The negative electrode sheet 10B includes a second exterior film 12B, a negative electrode current collector layer 14B, a negative electrode layer 16B, a second inorganic layer 26B, and a portion of a polymer solid electrolyte layer 28A. The details of each of these components are as described for the electrode sheet 10. That is, the negative electrode current collector layer 14B is integrated with the second exterior film 12B. The negative electrode layer 16B includes negative electrode active material particles 20B and a polymer solid electrolyte 22B that fills the gaps between the negative electrode active material particles 20B. The second inorganic layer 26B and a portion of the polymer solid electrolyte layer 28A form the separator layer 18, which includes inorganic oxide particles 24B and a polymer solid electrolyte 22B that fills the gaps between the inorganic oxide particles 24B. The polymer solid electrolyte 22A of the positive electrode sheet 10A and the polymer solid electrolyte 22B of the negative electrode sheet 10B are preferably made of the same material.

[0060] In the lithium-ion secondary battery 30, the total thickness of the electrode layers and separator layers is preferably 0.02 mm or more and 0.3 mm or less. A total thickness of 0.02 mm or more facilitates manufacturing. A total thickness of 0.3 mm or less improves the flexibility of the lithium-ion secondary battery 30, suppresses damage to the internal structure when bending the battery, and increases the capacity retention rate in a bending test. The total thickness is more preferably 0.2 mm or less, and even more preferably 0.1 mm or less. Here, the total thickness of the electrode layers and separator layers refers to the total thickness of all electrode layers and separator layers included in the lithium-ion secondary battery 30. In the example of FIG. 2, it is the total thickness of the positive electrode layer 16A, separator layer 32, and negative electrode layer 16B.

[0061] The thickness of the lithium ion secondary battery 30 is preferably 0.5 mm or less, and more preferably 0.3 mm or less. There are no particular limitations on the thickness of each layer of the lithium ion secondary battery 30, and it is as explained for the electrode sheet 10. There are no particular limitations on the thickness of the separator layer 32 in the lithium ion secondary battery, but it is preferably 6 to 200 μm, and more preferably 6 to 140 μm.

[0062] A method for manufacturing a lithium ion secondary battery 30 according to one embodiment may include, for example, (S11) a step of preparing a positive electrode sheet 10A, (S12) a step of preparing a negative electrode sheet 10B, and (S13) a step of bonding the positive electrode sheet 10A and the negative electrode sheet 10B together.

[0063] In steps (S11) and (S12), the positive electrode sheet 10A and the negative electrode sheet 10B are produced according to the method for producing the electrode sheet 10 described above.

[0064] In step (S13), the separator layers of the positive electrode sheet 10A and the negative electrode sheet 10B are overlapped with each other to bond the positive electrode sheet 10A and the negative electrode sheet 10B together, thereby joining the separator layers of both sheets together to form an integrated separator layer 32.

[0065] Instead of providing separator layers on both the positive electrode sheet 10A and the negative electrode sheet 10B, a separator layer may be provided on either the positive electrode sheet or the negative electrode sheet, and the electrode layer of the other sheet may be bonded to the separator layer so that both sheets share a single separator layer. That is, one of the positive electrode sheet and the negative electrode sheet may have an electrode layer and a separator layer laminated on a current collector layer integrated with an exterior film, and the other sheet may have an electrode layer laminated on a current collector layer integrated with an exterior film. The lithium-ion secondary battery according to this embodiment may then be produced by bonding the two sheets together so that the separator layer of one sheet overlaps the electrode layer of the other sheet.

[0066] In this embodiment, when bonding the positive electrode sheet and the negative electrode sheet together, the tackiness of the surface of the separator layer and / or electrode layer containing the polymer solid electrolyte can be improved by including an ionic liquid in the polymer solid electrolyte. Therefore, when bonding the separator layers of the positive electrode sheet and the negative electrode sheet together, or when bonding the separator layer of one of the positive electrode sheet and the negative electrode sheet to the electrode layer of the other sheet, the integration effect at the bonding interface between the two can be improved, and as a result, contact resistance can be reduced.

[0067] According to the present embodiment described above, a thin film lithium ion secondary battery can be obtained that is safe, flexible, and has good output characteristics even during high-power charging and discharging. Also, thin film lithium ion secondary batteries with large areas or various shapes can be obtained.

[0068] The lithium ion secondary battery according to this embodiment is useful not only as a power source for mobile devices, but also as a medium- or large-sized lithium ion secondary battery installed in wearable devices, power tools, electric bicycles, electric wheelchairs, robots, electric vehicles, emergency power sources, and large-capacity stationary power sources. [Example]

[0069] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0070] Example 1 [Fabrication of exterior material and current collector] An adhesive was applied to the entire surface of one side of a 35 μm-thick metal foil (aluminum foil) constituting the current collector layer, and a biaxially oriented polyethylene terephthalate (PET) film made of heat-resistant resin was attached to the exterior film. The PET film had a thickness of 15 μm. Next, an adhesive was applied to the other side of the aluminum foil except for the exposed metal portion, and a heat-sealable film (polypropylene film) made of thermoplastic resin was attached. The heat-sealable film was then cut along the periphery of the exposed metal portion to produce an exterior material / current collector with an exposed metal portion. The heat-sealable film had a thickness of 40 μm. The exposed metal portion consisted of an electrode formation portion measuring 35 mm x 35 mm and a terminal formation portion measuring 35 mm x 5 mm.

[0071] Fig. 3 is a plan view of a lithium-ion secondary battery fabricated in this example, and Fig. 4 is a cross-sectional view taken along line IV-IV of Fig. 3. As shown in the figure, an exterior material / current collector 40 has an aluminum foil 44 forming a current collector layer integrated with a PET film 42 that serves as an exterior film. A heat-sealable resin layer 46 made of a heat-sealable film is provided on the surface of the aluminum foil 44 opposite the PET film 42, and an electrode-forming portion 48 and a terminal-forming portion 50 are provided as exposed metal portions where the heat-sealable resin layer 46 has been removed.

[0072] [Preparation of positive electrode active material layer] LiNi as positive electrode active material particles 1 / 3 Mn 1 / 3 Co 1 / 3A positive electrode active material coating with a solids content of 60% by mass was obtained by mixing 92 g of O2, 2 g of acetylene black (Li-400, manufactured by Denka Co., Ltd.) as a conductive agent, 4 g of synthetic graphite (KS6, manufactured by Timcal Co., Ltd.), 4 g of polyvinylidene fluoride (PVDF) (manufactured by Kureha Co., Ltd.) as a binder, and 67 g of N-methyl-2-pyrrolidone as a dispersion medium in a planetary mixer. The positive electrode active material coating was then print-applied to the electrode-forming portion of the exterior material / current collector using a coating machine, and then dried under reduced pressure at 130°C to form a positive electrode active material layer.

[0073] [Preparation of negative electrode active material layer] Li4Ti5O as negative electrode active material particles 12 A negative electrode active material coating with a solids content of 40% by mass was obtained by mixing 92 g of the above, 5 g of acetylene black (Li-400, manufactured by Denka Company Limited) as a conductive agent, 1.5 g of CMC (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as a thickener, 1.5 g (solids content equivalent) of SBR (manufactured by JSR Corporation) as a binder, and 75 g of pure water as a dispersion medium in a planetary mixer. The negative electrode active material coating was applied by print coating to the electrode formation portion of the exterior material / current collector using a coating machine, and then dried under reduced pressure at 130°C to form a negative electrode active material layer.

[0074] [Creation of inorganic oxide layer] LAGP(Li) as inorganic oxide particles 1.5 Al 0.5 Ge 1.5 P3O 12 97 g of cellulose acetate (C100, D50 = 1.2 μm), 3 g of polyvinylidene fluoride (PVDF) (manufactured by Kureha Corporation) as a binder, and 40 g of N-methyl-2-pyrrolidone as a dispersion medium were mixed in a planetary mixer to obtain an inorganic oxide paint with a solid content of 60 mass %. The paint was applied to the surfaces of the positive electrode active material layer and the negative electrode active material layer using a spray printer so that the thickness after drying would be 8 μm, and then dried under reduced pressure at 130°C to obtain a positive electrode sheet and a negative electrode sheet each having an inorganic oxide layer on their surfaces.

[0075] [Preparation of polymer solid electrolyte solution] A solution of a polymer solid electrolyte was prepared by mixing 56 parts by weight of lithium salt lithium bis(fluorosulfonyl)imide (LiFSI) (Kishida Chemical Co., Ltd., lithium battery grade (LBG)), 70 parts by weight of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImFSI) (Dai-ichi Kogyo Seiyaku Co., Ltd., Elexel IL-110) as an ionic liquid electrolyte solvent, 30 parts by weight of tetrafunctional polyether acrylate (Dai-ichi Kogyo Seiyaku Co., Ltd., Elexel TA-210) as a reactive compound to become a polymer, 0.9 parts by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) (Wako Pure Chemical Industries, Ltd., V-65), and 144 parts by weight of dimethoxyethane (Kishida Chemical Co., Ltd., 1,2-Dimethoxyethane) (DME) as a dilution solvent. The lithium salt content was adjusted to 3.0 mol / kg relative to the sum of the contents of the ionic liquid and the polymer.

[0076] [Fabrication of lithium-ion secondary batteries] The polymer solid electrolyte solution was applied in an amount of 200 g / m onto the inorganic oxide layer of the positive electrode sheet on which the inorganic oxide layer was provided. 2 After coating, dimethoxyethane was distilled off at room temperature under vacuum conditions, and then the polymer solid electrolyte was cured by heat treatment at 80°C for 12 hours or more under vacuum conditions to prepare a positive electrode sheet with a separator layer. In addition, the polymer solid electrolyte was cured in the same manner for the negative electrode sheet provided with the inorganic oxide layer to prepare a negative electrode sheet with a separator layer.

[0077] The resulting positive and negative electrode sheets were laminated together, with their separator layers overlapping, and then heat-sealed with the heat-sealable resin layer of the current collector. Specifically, as shown in FIGS. 3 and 4, the positive electrode sheet 10A and the negative electrode sheet 10B were laminated together so that their terminal-forming portions 50, 50 were exposed and not overlapping, and the separator layer 18A of the positive electrode sheet 10A and the separator layer 18B of the negative electrode sheet 10B were overlapped to form an integrated separator layer 32. In this example, when the separator layers 18A and 18B containing the cured polymer solid electrolyte were laminated together, the tackiness of their surfaces ensured good adhesion at the bonding interface, resulting in seamless, integrated bonding of the separator layers 18A and 18B. The overlapping portions of the heat-sealable resin layers 46 of the positive electrode sheet 10A and the negative electrode sheet 10B were then heated to fuse them together. This resulted in a positive electrode area of 10.2 cm. 2 ], and the negative electrode area is 9.0 [cm 2 A film-shaped lithium ion secondary battery was obtained.

[0078] For the obtained lithium ion secondary battery of Example 1, the thickness of each of the positive electrode layer 16A, the negative electrode layer 16B, and the separator layer 32, as well as the total thickness thereof, are shown in Table 1 below, and the 1C discharge characteristics and R10 bending test were evaluated using the evaluation methods described below.

[0079] <Examples 2 to 6, Comparative Example 2> For Examples 2 to 6 and Comparative Example 2, lithium ion secondary batteries were fabricated in the same manner as in Example 1, except that the conditions were changed to those shown in Table 1 below, and the 1C discharge characteristics and R10 bending test were evaluated.

[0080] Specifically, alumina (Al2O3, D50=0.5 μm) was used as the inorganic oxide particles instead of LAGP in Examples 2 to 6. In Examples 3 and 4, the thicknesses of the positive electrode layer 16A, the negative electrode layer 16B, and the separator layer 32 were changed as shown in Table 1, and in Examples 5 and 6, the lithium salt concentration in the polymer solid electrolyte was changed as shown in Table 1.

[0081] On the other hand, in Comparative Example 2, the polymer solid electrolyte solution was composed of 56 parts by mass of LiFSI, 100 parts by mass of tetrafunctional polyether acrylate, 3 parts by mass of an azo-based initiator, and 144 parts by mass of dimethoxyethane, and did not contain any ionic liquid.

[0082] <Comparative Example 1> A polyolefin monolayer separator (Celgard Corporation; thickness 25 μm, porosity 55%, Gurley air permeability 200 sec) was sandwiched between the positive electrode sheet obtained in "Preparation of positive electrode active material layer" and the negative electrode sheet obtained in "Preparation of negative electrode active material layer" in Example 1, and laminated. A positive electrode terminal and a negative electrode terminal were ultrasonically welded to each positive and negative electrode. This laminate was placed in an aluminum laminate packaging material and heat-sealed, leaving an opening for injection. A positive electrode area of 10.2 cm was obtained. 2 ], negative electrode area 9.0[cm 2 Next, an electrolyte solution prepared by dissolving LiPF6 in a mixed solvent of ethylene carbonate and diethyl carbonate was poured into the battery, and the opening was heat-sealed to obtain a lithium-ion secondary battery of Comparative Example 1. The obtained lithium-ion secondary battery was evaluated for 1C discharge characteristics and an R10 bending test.

[0083] <Comparative Example 3> A lithium ion secondary battery was produced in the same manner as in Example 1, except that the inorganic oxide layer was not provided, and the 1C discharge characteristics and R10 bending test were evaluated.

[0084] [Charge and discharge test of lithium-ion secondary batteries] As a capacity confirmation test, the cell was charged at a constant current (CC) of 0.1C, followed by CC discharge at a current of 0.1C to determine the 0.1C discharge capacity. Both charging and discharging were performed for approximately 10 hours. The voltage range for charging and discharging was set to 1.7V to 2.8V. The 0.1C current is 0.1 times the 1C current that can discharge the cell capacity in 1 hour.

[0085] In a 1C discharge characteristic test, the cells were charged at a current of 0.1C, followed by CC discharge at a current of 1C. The specific capacity and capacity retention were calculated. The charge / discharge voltage range was set to 1.7V to 2.8V. Here, the specific capacity [mAh / g] indicates the charge / discharge capacity per mass [g] of the positive electrode active material particles contained in the cell. The capacity retention [%] was calculated from the ratio of the 1C discharge capacity to the 0.1C discharge capacity obtained in the capacity confirmation test.

[0086] For the R10 bending test, the test battery was wrapped around a rod with a curvature radius of approximately 10 mm and bent. The battery was then charged at a current of 0.1 C and then discharged at a current of 0.1 C. The 0.1 C discharge capacity was determined. The charge / discharge voltage range was set to 1.7 V to 2.8 V. The capacity retention rate [%] for the R10 bending test was calculated as the ratio of the 0.1 C discharge capacity obtained in the R10 bending test to the 0.1 C discharge capacity obtained in the capacity confirmation test.

[0087] [Table 1]

[0088] The results are shown in Table 1. Comparative Example 1, which is a lithium ion secondary battery using a general-purpose liquid electrolyte, had excellent 1C discharge characteristics, but was poor in flexibility, so the battery could not be wrapped around a rod in the R10 bending test, and therefore, the R10 bending test evaluation could not be performed. Therefore, it is marked with "X" in Table 1.

[0089] In contrast, in Examples 1 to 6, the capacity retention rate at a 1C charge / discharge rate was almost the same as that of Comparative Example 1, and the discharge characteristics were excellent. Furthermore, the capacity retention rate in the R10 bending test was high, and the batteries were soft and flexible. As for the separator layer, not only in Example 1, in which the solid electrolyte LAGP was used as the inorganic oxide particles, but also in Example 2, in which alumina was used, the specific capacity and capacity retention rate in the 1C discharge characteristics were equivalent, and excellent discharge characteristics were obtained. In Example 4, the capacity retention rate in the R10 bending test was slightly lower than in the other Examples. From this, it can be said that a thinner battery is preferable from the viewpoint of flexibility.

[0090] On the other hand, in Comparative Example 2, since the polymer solid electrolyte did not contain an ionic liquid, sufficient 1C discharge characteristics were not obtained. Therefore, the R10 bending test was not performed, and this is indicated as "X" in Table 1. In Comparative Example 3, since the separator layer made of inorganic oxide particles and polymer solid electrolyte was not included, the capacity retention rate in the R10 bending test was reduced.

[0091] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

Claims

1. a current collector layer integrated with an exterior film; an electrode layer formed on the current collector layer, the electrode layer including active material particles and a polymer solid electrolyte filling gaps between the active material particles; a separator layer formed on the electrode layer, the separator layer including inorganic oxide particles and the polymer solid electrolyte filling gaps between the inorganic oxide particles; an electrode sheet having the polymer solid electrolyte in the electrode layer and the polymer solid electrolyte in the separator layer are integrally formed, the polymer solid electrolyte includes a lithium salt, an ionic liquid, and a polymer; The ionic liquid contains a cation component and a bis(fluorosulfonyl)imide anion as an anion component, the polymer includes a crosslinked polymer obtained by crosslinking a reactive compound having a crosslinkable reactive group, the reactive compound being at least one selected from the group consisting of a difunctional or higher functional (meth)acrylate compound and a difunctional or higher functional oxetane compound; The content of the ionic liquid is 60% by mass or more and 75% by mass or less with respect to the sum of the contents of the ionic liquid and the polymer. Lithium-ion secondary battery.

2. The lithium ion secondary battery according to claim 1 , wherein the reactive compound comprises a tri- or higher functional polyether (meth)acrylate.

3. 3. The lithium ion secondary battery according to claim 1, wherein the content of the lithium salt is 2 mol / kg or more and 6 mol / kg or less with respect to the sum of the contents of the ionic liquid and the polymer.

4. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the total thickness of the electrode layer and the separator layer is 0.02 mm or more and 0.3 mm or less.

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