Positive electrode active material layer, electrode, and solid-state battery

Abstract

Provided are: a positive electrode active material layer capable of forming an all-solid-state battery having excellent cycle characteristics; an electrode comprising said positive electrode active material layer; and a semi-solid-state or all-solid-state battery comprising said electrode.　The positive electrode active material layer contains a positive electrode active material (A), a solid electrolyte (B), and a binder (C), wherein the binder (C) contains a vinylidene fluoride copolymer which is a copolymer of vinylidene fluoride and hexafluoropropylene, or a copolymer of vinylidene fluoride and chlorotrifluoroethylene, and the vinylidene fluoride copolymer contains 83.0-97.0 mass% of a constituent unit derived from vinylidene fluoride on the basis of 100 mass% of the total constituent units.

Description

Positive electrode active material layer, electrode, and solid-state battery

[0001] The present invention relates to a positive electrode active material layer, an electrode, and a solid-state battery.

[0002] Conventional secondary batteries containing electrolytes have a high risk of ignition if an abnormality such as an internal short circuit occurs in the cell, causing the cell temperature to rise. Therefore, in order to improve cell safety, semi-solid or all-solid-state batteries in which most or all of the electrolyte is replaced with a solid electrolyte are being studied. Because semi-solid or all-solid-state batteries contain only a small amount or no electrolyte containing organic solvents, safety devices can be simplified and they are thought to be superior in terms of manufacturing cost and productivity.

[0003] The active material layer of an electrode in a semi-solid or all-solid-state battery is expected to contain a solid electrolyte in addition to the active material, conductive additive, and binder that constitute the electrode active material layer of a conventional secondary battery. However, batteries using electrodes containing a solid electrolyte have insufficient cycle characteristics and have problems with durability.

[0004] In response to this, for example, Patent Document 1 discloses a method of using a powder containing a specific element as a conductive additive for a positive electrode, and Patent Document 2 discloses a method of coating an active material and a conductive additive for a positive electrode.

[0005] JP 2013-084496 A JP 2018-055926 A

[0006] However, even the solid state batteries using the positive electrodes disclosed in Patent Documents 1 and 2 cannot be said to have sufficiently improved cycle characteristics, and a solid state battery with excellent cycle characteristics is desired.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a positive electrode active material layer capable of forming an all-solid-state battery having excellent cycle characteristics, an electrode including the positive electrode active material layer, and a semi-solid battery or all-solid-state battery including the electrode.

[0008] As a result of extensive research into solving the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by incorporating a specific vinylidene fluoride copolymer as a binder in a positive electrode active material layer containing a positive electrode active material and a solid electrolyte, and have thus completed the present invention.

[0009] An embodiment of the present invention relates to the following positive electrode active material layer, electrode, and semi-solid battery or all-solid battery, each containing a positive electrode active material (A), a solid electrolyte (B), and a binder (C).

[0010] [1] A cathode active material layer containing a cathode active material (A), a solid electrolyte (B), and a binder (C), wherein the binder (C) contains a vinylidene fluoride copolymer, the vinylidene fluoride copolymer being a copolymer of vinylidene fluoride and hexafluoropropylene or a copolymer of vinylidene fluoride and chlorotrifluoroethylene, and wherein, when all structural units of the vinylidene fluoride copolymer are taken as 100.0% by mass, the cathode active material layer contains 83.0% by mass or more and 97.0% by mass or less of the structural units derived from vinylidene fluoride.

[0011] [2] The positive electrode active material layer according to [1], wherein the vinylidene fluoride copolymer has an intrinsic viscosity of 1.5 dL / g or more. [3] The positive electrode active material layer according to [2], wherein the vinylidene fluoride copolymer has an intrinsic viscosity of 2.4 dL / g or less. [4] The positive electrode active material layer according to any one of [1] to [3], wherein the solid electrolyte (B) is an oxide-based solid electrolyte. [5] The positive electrode active material layer according to any one of [1] to [3], wherein the solid electrolyte (B) is an oxide-based solid electrolyte represented by the following formula (1): Li １＋ｘ＋ｙ Al ｘ Ti ２－ｘ Si ｙ P ３－ｙ O １２ ...(1) (in formula (1), x and y satisfy 0≦x≦1, 0≦y≦1). [6] The positive electrode active material layer according to [4], wherein the positive electrode active material (A) contains lithium metal phosphate, and the lithium metal phosphate is LiFePO ４ , LiMnPO ４ , LiCoPO ４ , and LiNiPO ４[7] A cathode active material layer according to any one of [1] to [5], selected from the group consisting of: [7] an electrode comprising a current collector and the cathode active material layer according to any one of [1] to [6]; [8] A semi-solid battery or an all-solid battery comprising the electrode according to [7].

[0012] According to the present invention, it is possible to provide a positive electrode active material layer capable of forming a semi-solid battery or an all-solid battery having excellent cycle characteristics, an electrode including the positive electrode active material layer, and a semi-solid battery or an all-solid battery including the electrode.

[0013] <Positive Electrode Active Material Layer> The positive electrode active material layer contains a positive electrode active material (A), a solid electrolyte (B), and a binder (C). The binder (C) contains a vinylidene fluoride copolymer. The vinylidene fluoride copolymer is a copolymer of vinylidene fluoride and hexafluoropropylene, or a copolymer of vinylidene fluoride and chlorotrifluoroethylene. The content of the vinylidene fluoride-derived structural unit in the copolymer is within a specific range, as described below. By containing the specific binder (C) in the positive electrode active material layer, a semi-solid battery or an all-solid battery having excellent cycle characteristics can be formed.

[0014] Essential and optional components contained in the positive electrode active material layer will be described below.

[0015] <Positive electrode active material (A)> The positive electrode active material (A) is not particularly limited, and for example, a conventionally known electrode active material for a positive electrode can be used. As the electrode active material for a positive electrode, a lithium-based electrode active material for a positive electrode containing at least lithium is preferred. As the electrode active material for a lithium-based positive electrode, for example, a material represented by the general formula Li ｘ M ｙ O ｚ (M is a transition metal element, x = 0.02 to 2.2, y = 1 to 2, z = 1.4 to 4). In the above general formula, M is at least one selected from the group consisting of Co, Mn, Ni, V, Fe, and Si, and may be at least one selected from the group consisting of Co, Ni, and Mn. Specific examples of such positive electrode active materials include LiCoO２ , LiMnO ２ , LiNiO ２ , LiVO ２ , LiNi １／３ Co １／３ Mn １／３ O ２ , LiMn ２ O ４ , Li(Ni ０．５ Mn １．５ ) O ４ , Li ２ FeSiO ４ , Li ２ MnSiO ４ etc.

[0016] In addition, the above general formula Li ｘ M ｙ O ｚ Other examples of the positive electrode active material (A) include lithium titanate (e.g., Li ４ Ti ５ O １２ ), lithium metal phosphate (LiFePO ４ , LiMnPO ４ , LiCoPO ４ , LiNiPO ４ ), transition metal oxide (V ２ O ５ , MoO ３ ), TiS ２ , LiCoN, Si, SiO ２ , Li ２ SiO ３ , Li ４ SiO ４ , and lithium-storing intermetallic compounds (e.g., Mg ２ Sn, Mg ２ Ge, Mg ２ Sb, Cu ３ Among these, lithium metal phosphate is preferred, and among the lithium metal phosphates, LiFePO ４ is preferred.

[0017] The shape of the positive electrode active material (A) is not particularly limited, and may be, for example, particulate or thin film, and may be particulate from the viewpoint of easy handling. When the positive electrode active material is particulate, the average particle size (D50) of the particles is, for example, preferably 1 nm or more and 100 μm or less, and more preferably 10 nm or more and 30 μm or less.

[0018] A coating layer containing a Li ion conductive oxide may be formed on the surface of the positive electrode active material (A) from the viewpoint of suppressing a reaction between the positive electrode active material and the solid electrolyte. Examples of the Li ion conductive oxide include LiNbO ３ , Li ４ Ti ５ O １２ , and Li ３ P.O. ４ The lower limit of the thickness of the coating layer is preferably 0.1 nm or more, and more preferably 1 nm or more. The upper limit of the thickness of the coating layer is preferably 100 nm or less, and more preferably 20 nm or less.

[0019] The content of the positive electrode active material (A) is not particularly limited, and is preferably 50% by mass or more and 98% by mass or less, more preferably 60% by mass or more and 95% by mass or less, and even more preferably 70% by mass or more and 90% by mass or less, relative to 100% by mass of the positive electrode active material layer.

[0020] <Solid Electrolyte (B)> As the solid electrolyte (B), a sulfide-based solid electrolyte, an oxide-based solid electrolyte, etc. can be used. Among these, it is preferable to use an oxide-based solid electrolyte from the viewpoint of the safety and stability of the electrolyte.

[0021] Examples of sulfide-based solid electrolytes include Li ２ S-SiS ２ , LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ S ５ , LiI-Li ２ O-Li ２ S-P ２ S ５ , LiI-Li２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ２ S-P ２ S ５ , Li ３ P.S. ４ etc.

[0022] The oxide-based solid electrolyte includes an LLTO-based compound ((La, Li)TiO ３ ), Li ６ La ２ CaTa ２ O １２ , Li ６ La ２ ANb ２ O １２ (A: alkaline earth metal), Li ２ Nd ３ TeSbO １２ , Li ３ BO ２．５ N ０．５ , Li ９ SiAlO ８ , LAGP compound (Li １＋ｘ Al ｘ Ge ２－ｘ (PO4) ３ (0≦x≦1)), Li ２ O-Al ２ O ３ -TiO ２ -P ２ O ５ LATP-based compounds such as Li １＋ｘ Al ｘ Ti ２－ｘ (P.O. ４ ) ３ (0≦x≦1)), Li １＋ｘ Ti ２－ｘ Al ｘ Si ｙ (P.O. ４ ) ３－ｙ (0≦x≦1, 0≦y≦1), Li １＋ｙ Al ｘ M ２－ｘ (P.O. ４ ) ３(M is one or more elements selected from the group consisting of Ti, Ge, Sr, Sn, Zr, and Ca, and 0≦x≦1, 0≦y≦1), LiTi ｘ Zr ２－ｘ (P.O. ４ ) ３ (0≦x≦1), LISICON (Li ４－２ｘ Zn ｘ GeO ４ (0≦x≦1)), LIPON-based compounds (Li ３＋ｙ P.O. ４－ｘ N ｘ (0≦x≦1, 0≦y≦1)), NASICON-based compounds (LiTi ２ (P.O. ４ ) ３ etc.), garnet compounds (Li ７ La ３ Zr ２ O １２ , Li ７－ｘ La ３ Zr １－ｘ Nb ｘ O １２ (0≦x≦1, etc.).

[0023] Among oxide-based solid electrolytes, LATP-based compounds are preferred from the viewpoint of high lithium ion conductivity. １＋ｘ＋ｙ Al ｘ Ti ２－ｘ Si ｙ P ３－ｙ O １２ ...(1) (wherein x and y satisfy 0≦x≦1 and 0≦y≦1).

[0024] In addition, examples of preferred oxide-based solid electrolytes other than the LATP-based compounds include Li ７ La ３ Zr ２ O １２ (LLZO), Li ６．７５ La ３ Zr １．７５ Ta ０．２５ O １２ (LLZTO), Li ０．３３ La ０．５６ TiO ３(LLTO), Li １．６ Al ０．６ Ge １．４ (P.O. ４ ) ３ (LAGP) and the like.

[0025] The content of the solid electrolyte (B) is not particularly limited, and is preferably 1% by mass or more and 40% by mass or less, more preferably 2% by mass or more and 20% by mass or less, and even more preferably 5% by mass or more and 15% by mass or less, relative to 100% by mass of the positive electrode active material layer.

[0026] <Binder (C)> The binder (C) is a vinylidene fluoride copolymer. The vinylidene fluoride copolymer is a copolymer of vinylidene fluoride and hexafluoropropylene (hereinafter, sometimes referred to as "vinylidene fluoride copolymer (C-a)"), or a copolymer of vinylidene fluoride and chlorotrifluoroethylene (hereinafter, sometimes referred to as "vinylidene fluoride copolymer (C-b)"). The vinylidene fluoride copolymer (C-a) or the vinylidene fluoride copolymer (C-b) is used for the purpose of forming a semi-solid battery or an all-solid battery having excellent cycle characteristics. In the following, when there is no need to particularly distinguish between the vinylidene fluoride copolymer (C-a) and the vinylidene fluoride copolymer (C-b), they will be simply referred to as "vinylidene fluoride copolymer."

[0027] From the viewpoint of ensuring that the semi-solid battery or all-solid battery has excellent cycle characteristics, the content of constituent units derived from vinylidene fluoride (VDF) in the vinylidene fluoride copolymer is 83.0% by mass or more and 97.0% by mass or less, preferably 90.0% by mass or more and 96.0% by mass or less, and more preferably 93.0% by mass or more and 95.0% by mass or less, when the total amount of constituent units of the vinylidene fluoride copolymer is taken as 100.0% by mass.

[0028] (Vinylidene fluoride copolymer (C-a)) The vinylidene fluoride copolymer (C-a) may be a copolymer using only vinylidene fluoride (VDF) and hexafluoropropylene (HFP), or may be a copolymer of VDF and HFP with a monomer other than these monomers. As the other monomer, from the viewpoint of providing a semi-solid battery or an all-solid battery with excellent cycle characteristics, for example, a monomer having a hydrogen-bonding polar functional group is preferable, and a carboxyl group-containing monomer is more preferable. Examples of the carboxyl group-containing monomer copolymerizable with vinylidene fluoride include unsaturated monobasic acids such as acrylic acid, methacrylic acid, 2-carboxyethyl acrylate, and 2-carboxyethyl methacrylate; unsaturated dibasic acids such as maleic acid and citraconic acid; monoesters of unsaturated dibasic acids such as maleic acid monomethyl ester, maleic acid monoethyl ester, citraconic acid monomethyl ester, and citraconic acid monoethyl ester; acryloyloxyethyl succinic acid, acryloyloxypropyl succinic acid, methacryloyloxyethyl succinic acid, and methacryloyloxypropyl succinic acid.

[0029] (Vinylidene fluoride copolymer (C-b)) The vinylidene fluoride copolymer (C-b) may be a copolymer using only vinylidene fluoride (VDF) and chlorotrifluoroethylene (CTFE), or may be a copolymer of VDF and CTFE with a monomer other than these monomers. As the other monomer, from the viewpoint of providing a semi-solid battery or an all-solid battery with excellent cycle characteristics, for example, a monomer having a hydrogen-bonding polar functional group is preferable, and a carboxyl group-containing monomer is more preferable. Examples of the carboxyl group-containing monomer copolymerizable with vinylidene fluoride include unsaturated monobasic acids such as acrylic acid, methacrylic acid, 2-carboxyethyl acrylate, and 2-carboxyethyl methacrylate; unsaturated dibasic acids such as maleic acid and citraconic acid; monoesters of unsaturated dibasic acids such as maleic acid monomethyl ester, maleic acid monoethyl ester, citraconic acid monomethyl ester, and citraconic acid monoethyl ester; acryloyloxyethyl succinic acid, acryloyloxypropyl succinic acid, methacryloyloxyethyl succinic acid, and methacryloyloxypropyl succinic acid.

[0030] From the viewpoint of providing a semi-solid battery or an all-solid battery with excellent cycle characteristics, the lower limit of the intrinsic viscosity of the vinylidene fluoride copolymer is preferably 1.0 dL / g or more, more preferably 1.2 dL / g or more, and even more preferably 1.5 dL / g or more.

[0031] From the viewpoint of providing a semi-solid battery or an all-solid battery with excellent cycle characteristics, the upper limit of the intrinsic viscosity of the vinylidene fluoride copolymer is preferably 3.0 dL / g or less, more preferably 2.8 dL / g or less, and even more preferably 2.4 dL / g or less.

[0032] The content of the binder (C) is not particularly limited, and is preferably 1% by mass or more and 40% by mass or less, more preferably 2% by mass or more and 30% by mass or less, and even more preferably 3% by mass or more and 20% by mass or less, relative to 100% by mass of the positive electrode active material layer.

[0033] (Method for producing binder (C)) The method for producing the binder (C) is not particularly limited, and is usually carried out by a method such as suspension polymerization, emulsion polymerization, solution polymerization, etc. From the viewpoint of ease of post-treatment, etc., aqueous suspension polymerization and emulsion polymerization are preferred, and aqueous suspension polymerization is more preferred.

[0034] The aqueous suspension polymerization method is not particularly limited, and examples thereof include a method in which a monomer to be used for polymerization is polymerized in an aqueous medium in the presence of a suspending agent, a polymerization initiator, a chain transfer agent, etc.

[0035] The suspending agent is not particularly limited, and examples thereof include methyl cellulose, methoxylated methyl cellulose, propoxylated methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, polyvinyl alcohol, polyethylene oxide, gelatin, etc. The amount of the suspending agent used is not particularly limited, and is, for example, preferably 0.005 parts by mass or more and 1.0 parts by mass or less, and more preferably 0.01 parts by mass or more and 0.4 parts by mass or less, relative to 100 parts by mass of all monomers used in the polymerization.

[0036] The polymerization initiator is not particularly limited, and examples thereof include diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di-n-heptafluoropropyl peroxydicarbonate, isobutyryl peroxide, di(chlorofluoroacyl)peroxide, di(perfluoroacyl)peroxide, t-butyl peroxypivalate, etc. The amount of the polymerization initiator used is not particularly limited, and is, for example, preferably 0.05 parts by mass or more and 5 parts by mass or less, and more preferably 0.15 parts by mass or more and 2 parts by mass or less, relative to 100 parts by mass of all monomers used in the polymerization.

[0037] The chain transfer agent is not particularly limited, and examples thereof include ethyl acetate, propyl acetate, acetone, and diethyl carbonate.

[0038] The amount of monomers used in the polymerization, expressed as a mass ratio of total monomers to water, is typically 1:1 to 1:10, preferably 1:2 to 1:5. The polymerization conditions, such as polymerization temperature and polymerization time, used in suspension polymerization are not particularly limited, and known polymerization conditions may be employed. The polymerization temperature T is appropriately selected depending on the 10-hour half-life temperature T10 of the polymerization initiator, and is typically selected within the range of T10 - 25°C ≦ T ≦ T10 + 25°C. For example, the T10 values ​​of t-butyl peroxypivalate and diisopropyl peroxydicarbonate are 54.6°C and 40.5°C, respectively (see NOF Corporation product catalog). Therefore, in polymerizations using t-butyl peroxypivalate and diisopropyl peroxydicarbonate as polymerization initiators, the polymerization temperature T is appropriately selected within the ranges of 29.6°C ≦ T ≦ 79.6°C and 15.5°C ≦ T ≦ 65.5°C, respectively. The polymerization time is not particularly limited, but is preferably 1 to 24 hours in consideration of productivity and the like.

[0039] <Other Components> The positive electrode active material layer may contain components (hereinafter also referred to as "other components") other than the above-described positive electrode active material (A), solid electrolyte (B), and binder (C) as long as the effects of the present invention are not impaired. As the other components, any known additives can be used, for example, a conductive additive, an insulating inorganic filler such as alumina, magnesia, or silica, an insulating organic filler such as polytetrafluoroethylene, polyimide, or polyacrylonitrile, a plasticizer such as ethylene carbonate, propylene carbonate, or ethyl methyl carbonate, LiPF ６ , Li salts such as LiFSI and LiTFSI, dispersants, flame retardants, antifoaming agents, etc.

[0040] Examples of the conductive additive (D) include carbon black, carbon nanotubes, etc. These may be used alone or in combination of two or more.

[0041] The content of the conductive additive (D) is not particularly limited, but is preferably 0.05% by mass or more and 20% by mass or less, more preferably 0.1% by mass or more and 15% by mass or less, and even more preferably 0.5% by mass or more and 10% by mass or less, relative to 100% by mass of the positive electrode active material layer.

[0042] <Electrode> The electrode refers to a positive electrode. The electrode includes a current collector and the above-described positive electrode active material layer. By including the above-described positive electrode active material layer, the electrode can form a semi-solid battery or an all-solid battery having excellent cycle characteristics.

[0043] (Current Collector) The current collector is a terminal for extracting electricity. The material of the current collector is not particularly limited, and metal foil or metal mesh of aluminum, copper, iron, stainless steel, steel, nickel, titanium, etc. can be used. Alternatively, the current collector may be formed by applying the above metal foil or metal mesh to the surface of another medium.

[0044] The bulk density of the electrode is not particularly limited, and is, for example, 1.5 g / cm ３ 5.0g / cm or more ３ The weight of the electrode is not particularly limited, and is, for example, 20 g / m ２ More than 1000g / m ２ It is preferable that:

[0045] <Electrode Manufacturing Method> Examples of the electrode manufacturing method include a method including a step of mixing the above-described positive electrode active material (A), the solid electrolyte (B), the binder (C), and, if necessary, other components and the non-aqueous solvent (S) to prepare a positive electrode slurry, and a step of applying the obtained positive electrode slurry to a current collector and then drying the applied slurry. That is, in the above embodiment, a positive electrode active material layer is produced and an electrode is also produced through a step of drying the positive electrode slurry.

[0046] Examples of the non-aqueous solvent (S) include N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP"), dimethylformamide, N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N,N-dimethyl sulfoxide, hexamethylphosphoamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate, acetone, cyclohexanone, methyl ethyl ketone, and tetrahydrofuran. These may be used alone or in combination of two or more.

[0047] The coating method is not particularly limited, and for example, a doctor blade method, a reverse roll method, a comma bar method, a gravure method, an air knife method, a die coating method, a dip coating method, etc. can be used.

[0048] The drying temperature is, for example, preferably 80°C or higher and 300°C or lower, more preferably 90°C or higher and 200°C or lower, and even more preferably 100°C or higher and 180°C or lower. The drying time is, for example, preferably 10 seconds or higher and 300 minutes or lower, and more preferably 1 minute or higher and 200 minutes or lower. Drying may be performed multiple times at different temperatures. Pressure may be applied during drying.

[0049] All-solid-state battery includes the electrode of the above-described embodiment. As the all-solid-state battery, conventionally known components other than the positive electrode, such as the negative electrode and separator, can be used.

[0050] <Semi-solid battery> A semi-solid battery includes the electrode of the above-described embodiment. For the semi-solid battery, conventionally known components other than the positive electrode, such as a negative electrode and a separator, can be used. The amount of electrolyte contained in the semi-solid battery is preferably 1 to 95%, more preferably 5 to 90%, and even more preferably 10 to 80%, when the volume of the electrolyte contained in a conventional secondary battery is taken as 100%.

[0051] (Cycle Characteristics) The cycle characteristics of the solid state battery are such that, when constant current charging and discharging is performed under conditions of an ambient temperature of 25°C, a charge / discharge rate of 0.5C, and a voltage range of 2.5V to 4.0V, the discharge capacity after 50 cycles is preferably 140 mAh / g or more, more preferably 145 mAh / g or more, and even more preferably 150 mAh / g or more.

[0052] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0053] [Examples 1 to 5 and Comparative Examples 1 to 6] <Materials Used> In the examples and comparative examples, the following A1 was used as the positive electrode active material (A). A1: LiFePO ４

[0054] In the examples and comparative examples, the following B1 was used as the solid electrolyte (B): B1: Li １＋ｘ＋ｙ Al ｘ Ti ２-ｘ Si ｙ P ３-y O １２ (LATP) (manufactured by Ohara Corporation, "LICGC ＴＭ PW-01 (particle size 1μm)")

[0055] In the examples and comparative examples, the following binders C1 to C10 were prepared and used as binders (C). The intrinsic viscosity of each prepared binder was measured according to the following method.

[0056] (Measurement of intrinsic viscosity) 80 mg of each binder was dissolved in 20 ml of N,N-dimethylformamide to prepare a binder-containing solution. １ was measured using an Ubbelohde viscometer in a thermostatic bath at 30° C. Similarly, the viscosity η of N,N-dimethylformamide was measured using an Ubbelohde viscometer in a thermostatic bath at 30° C. ０ Then, the intrinsic viscosity η was calculated based on the following formula: Intrinsic viscosity η = (1 / C) ln(η １ / η ０ In the above formula, C is the concentration per unit of the binder-containing solution, which is 0.4 g / dl in this case.

[0057] (Preparation Example of Binder C1 (VDF / HFP)) 290 parts by mass of ion-exchanged water, 0.15 parts by mass of methyl cellulose, 57.9 parts by mass of vinylidene fluoride (VDF), 7.8 parts by mass of hexafluoropropylene (HFP), and 0.9 parts by mass of diisopropyl peroxydicarbonate (IPP) were charged into a 2 L autoclave and polymerized at 45°C. 34.3 parts by mass of VDF was aftercharged to obtain a copolymer. The obtained copolymer was heat-treated at 95°C for 60 minutes, dehydrated, washed with water, and further dried at 80°C for 20 hours to obtain Binder C1. Binder C1 contained 94.3% by mass of VDF and 5.7% by mass of HFP based on all structural units. The intrinsic viscosity (η) of Binder C1 was 1.5 dL / g.

[0058] (Preparation Example of Binder C2 (VDF / HFP)) A 2L autoclave was charged with 253.0 parts by mass of ion-exchanged water, 0.05 parts by mass of methyl cellulose, 90.0 parts by mass of vinylidene fluoride (VDF), 10.0 parts by mass of hexafluoropropylene (HFP), 0.4 parts by mass of IPP, and 1.0 parts by mass of ethyl acetate, and polymerized at 45°C. The resulting copolymer was heat-treated at 95°C for 60 minutes, dehydrated, washed with water, and further dried at 80°C for 20 hours to obtain Binder C2. Binder C2 contained 93.0% by mass of VDF and 7.0% by mass of HFP based on all structural units. The intrinsic viscosity (η) of Binder C2 was 1.9 dL / g.

[0059] (Preparation Example of Binder C3 (VDF / HFP)) VDF and HFP were polymerized in the same manner as for Binder C2, except that 256.4 parts by mass of ion-exchanged water, 0.05 parts by mass of methyl cellulose, 82.0 parts by mass of VDF, 18.0 parts by mass of HFP, 0.4 parts by mass of IPP, and 0.9 parts by mass of ethyl acetate were used, to obtain Binder C3. Binder C3 contained 85.0% by mass of VDF and 15.0% by mass of HFP based on all structural units. The intrinsic viscosity (η) of Binder C3 was 1.8 dL / g.

[0060] (Preparation Example of Binder C4 (VDF / CTFE)) VDF and CTFE were polymerized in the same manner as in the preparation example of binder C2, except that 256.6 parts by mass of ion-exchanged water, 0.05 parts by mass of methyl cellulose, 96.0 parts by mass of VDF, 4.0 parts by mass of chlorotrifluoroethylene (CTFE) instead of HFP, 0.6 parts by mass of IPP, and 0.59 parts by mass of ethyl acetate were used, to obtain binder C4. Binder C4 contained 96.0% by mass of VDF and 4.0% by mass of CTFE based on the total structural units. The intrinsic viscosity (η) of binder C4 was 2.1 dL / g.

[0061] (Preparation Example of Binder C5 (VDF / HFP / MMM)) VDF, HFP, and MMM were polymerized in the same manner as in the Preparation Example of Binder C2, except that 259 parts by mass of ion-exchanged water, 0.15 parts by mass of methyl cellulose, 90.0 parts by mass of VDF, 10.0 parts by mass of HFP, 0.5 parts by mass of monomethyl maleate (MMM), and 0.5 parts by mass of IPP were used, to obtain binder C5. The amount of each monomer in binder C5 was determined by calculating the VDF / HFP ratio by 19F-NMR, calculating the VDF / MMM ratio by the method described below, and then calculating so that the total of VDF, HFP, and MMM was 100 mol%. The VDF / MMM ratio in the polymer (the molar ratio of the amount of structural units derived from vinylidene fluoride to the amount of structural units derived from monomethyl maleate) was calculated based on the calculation method using IR spectra and a calibration curve disclosed in WO 2009 / 084483. Binder C5 contained 92.5% by mass of VDF, 7.0% by mass of HFP, and 0.5% by mass of MMM relative to all structural units. The intrinsic viscosity (η) of binder C5 was 2.6 dL / g.

[0062] (Preparation Example of Binder C6 (PVDF)) VDF was polymerized in the same manner as in the preparation example of Binder C2, except that 231.8 parts by mass of ion-exchanged water, 0.05 parts by mass of methyl cellulose, 100 parts by mass of VDF, 0.7 parts by mass of IPP, and 0.7 parts by mass of ethyl acetate were used, to obtain Binder C6, a VDF homopolymer. Binder C6 contained 100.0% by mass of VDF based on all structural units. The intrinsic viscosity (η) of Binder C6 was 2.1 dL / g.

[0063] (Preparation Example of Binder C7 (PVDF)) VDF was polymerized in the same manner as in the preparation example of Binder C2, except that 213.8 parts by mass of ion-exchanged water, 0.05 parts by mass of methyl cellulose, 100 parts by mass of VDF, 0.3 parts by mass of IPP, and 0.4 parts by mass of ethyl acetate were used, to obtain Binder C7, a VDF homopolymer. Binder C7 contained 100.0% by mass of VDF based on all structural units. The intrinsic viscosity (η) of Binder C7 was 3.1 dL / g.

[0064] (Preparation Example of Binder C8 (VDF / HFP / MMM)) VDF, HFP, and MMM were polymerized in the same manner as in the Preparation Example of Binder C2, except that 240.2 parts by mass of ion-exchanged water, 0.2 parts by mass of methyl cellulose, 96.7 parts by mass of VDF, 3.0 parts by mass of HFP, 0.3 parts by mass of MMM, and 0.47 parts by mass of IPP were used, to obtain Binder C8. Binder C8 contained 97.4% by mass of VDF, 2.3% by mass of HFP, and 0.3% by mass of MMM based on all structural units. The intrinsic viscosity (η) of Binder C8 was 3.1 dL / g.

[0065] (Preparation Example of Binder C9 (VDF / TFE)) VDF and TFE were polymerized with reference to Japanese Patent No. 5949915 to obtain binder C9. Binder C9 contained 78.0% by mass of VDF and 22.0% by mass of TFE based on all structural units. The intrinsic viscosity (η) of binder C9 was 2.4 dL / g.

[0066] (Preparation Example of Binder C10 (VDF / TFE)) VDF and TFE were polymerized with reference to Japanese Patent No. 5949915 to obtain binder C10. Binder C10 contained 86.8 mass% of VDF and 13.2 mass% of TFE based on all structural units. The intrinsic viscosity (η) of binder C9 was 2.3 dL / g.

[0067] (Preparation Example of Binder C11 (VDF / HFP)) VDF and HFP were polymerized in the same manner as in the preparation example of Binder C2, except that 258.0 parts by mass of ion-exchanged water, 0.05 parts by mass of methyl cellulose, 75 parts by mass of VDF, 25 parts by mass of HFP, and 0.7 parts by mass of IPP were used, to obtain Binder C11. Binder C11 contained 81.2% by mass of VDF and 18.8% by mass of HFP based on all structural units. The intrinsic viscosity (η) of Binder C11 was 1.6 dL / g.

[0068] Detailed information on the obtained binders C1 to C11 is shown in Table 1.

[0069]

[0070] In the examples and comparative examples, the following D1 was used as the conductive additive (D): D1: carbon nanotube (CNT) dispersion

[0071] In the examples and comparative examples, the following S1 was used as the non-aqueous solvent (S): S1: N-methyl-2-pyrrolidone (NMP)

[0072] (Preparation of Positive Electrode Slurry) Each binder (C) was dispersed in N-methyl-2-pyrrolidone (NMP) at room temperature so that the concentration of the binder (C) was adjusted to a desired concentration. The solution temperature was then raised to 50°C to dissolve each binder (C) (hereinafter referred to as the "binder solution"). The positive electrode active material (A), solid electrolyte (B), conductive additive (D), and each of the binder solutions were mixed so that the ratio of positive electrode active material (A) / solid electrolyte (B) / conductive additive (D) / binder (C) was 90 / 10 / 2 / 10 (mass%) to obtain each positive electrode slurry. The solid content of the positive electrode slurry was adjusted using NMP.

[0073] (Preparation of Positive Electrodes) Each of the obtained positive electrode slurries was applied to an Al foil (thickness: 15 μm) and then dried at 120° C. Each of the obtained preliminary positive electrodes was pressed and further subjected to heat treatment at 120° C. for 3 hours. As a result, the electrode bulk density became 2.3 g / cm ３ , basis weight 200g / m ２ Each positive electrode was obtained.

[0074] (Preparation of Test Cells) Each of the obtained positive electrodes was punched out to a diameter of 14 mm using a hand punch, and a 2016 size coin cell was prepared using metallic Li as the counter electrode and a PE single-layer separator (thickness: 20 μm) as the separator. The electrolyte was a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC / EMC = 3 / 7 (volume ratio)) with 1.2 M LiPF ６ A solution containing 1% by mass of vinylene carbonate (VC) was used.

[0075] <Evaluation> The charge / discharge characteristics of each of the obtained test cells were measured according to the following method.

[0076] (Cycle Characteristics) Each test cell was subjected to CCCV charge / CC discharge under the conditions of an ambient temperature of 25° C., a charge / discharge rate of 0.5 C, a voltage range of 2.5 V to 4.0 V, and a CCCV charge cut-off current value of 0.05 C, and the discharge capacity after 50 cycles was measured. The results are shown in Table 2.

[0077]

[0078] From Table 2, in the positive electrode active material layers of Comparative Examples 1 and 2, which used a VDF homopolymer as the binder (C), the maximum discharge capacity after 50 cycles was only 123.7 mAh / g. Furthermore, in the positive electrode active material layers of Comparative Examples 4 and 5, which used a copolymer of VDF and TFE as the binder (C), the discharge capacity after 50 cycles was all less than 140 mAh / g. Similarly, in the positive electrode active material layer of Comparative Example 3, which used a copolymer of VDF and HFP as the binder (C), but with a VDF-derived structural unit content of more than 97.0 mass%, the discharge capacity after 50 cycles was less than 140 mAh / g. Furthermore, in the positive electrode active material layer of Comparative Example 6, which used a copolymer of VDF and HFP as the binder (C), but with a VDF-derived structural unit content of less than 83 mass%, the discharge capacity after 50 cycles was less than 140 mAh / g. On the other hand, in the case of a positive electrode active material layer using a copolymer of VDF and HFP, and a copolymer of VDF and CTFE, in which the VDF-derived structural unit is 83.0 mass % or more and 97.0 mass % or less, the discharge capacity after 50 cycles is 140 mAh / g or more, as shown in Examples 1 to 5, and it is found that the positive electrode active material layer has excellent cycle characteristics.

Claims

1. A positive electrode active material layer containing a positive electrode active material (A), a solid electrolyte (B), and a binder (C), The binder (C) contains a vinylidene fluoride copolymer, The vinylidene fluoride copolymer is a copolymer of vinylidene fluoride and hexafluoropropylene or a copolymer of vinylidene fluoride and chlorotrifluoroethylene, The vinylidene fluoride copolymer contains 83.0% by mass or more and 97.0% by mass or less of the structural units derived from vinylidene fluoride, when the total structural units of the vinylidene fluoride copolymer is taken as 100.0% by mass, The vinylidene fluoride copolymer has an intrinsic viscosity of 1.5 dL / g or more.

2. 2. The positive electrode active material layer according to claim 1, wherein the vinylidene fluoride copolymer has an intrinsic viscosity of 2.4 dL / g or less.

3. The positive electrode active material layer according to claim 1 or 2, wherein the solid electrolyte (B) is an oxide-based solid electrolyte.

4. The solid electrolyte (B) is represented by the following formula (1): Li １＋ｘ＋ｙ Al ｘ Ti ２－ｘ Si ｙ P ３－ｙ O １２ ・・・(1) (In formula (1), x and y satisfy 0≦x≦1 and 0≦y≦1.) The positive electrode active material layer according to claim 3 , comprising a material represented by the formula:

5. The positive electrode active material (A) contains lithium metal phosphate, The lithium metal phosphate is LiFePO ４ , LiMnPO ４ , LiCoPO ４ , and LiNiPO ４ The positive electrode active material layer according to claim 1 or 2, selected from the group consisting of:

6. An electrode comprising a current collector and the positive electrode active material layer according to claim 1 or 2.

7. A semi-solid or all-solid battery comprising the electrode according to claim 6.