Secondary-battery positive electrode and method for manufacturing same

Abstract

A secondary-battery positive electrode 5 according to the present disclosure comprises a positive electrode current collector 5a and a positive electrode active material layer 5b supported by the positive electrode current collector 5a. The positive electrode active material layer 5b contains active material particles and solid organic boronic acid. The organic boronic acid may contain a saturated hydrocarbon group in which at least one hydrogen atom may be substituted with a fluorine atom. The saturated hydrocarbon group may contain a cyclic, linear, or branched alkyl group.

Description

Positive electrode for secondary battery and method for producing same

[0001] The present disclosure relates to a positive electrode for a secondary battery and a method for producing the same.

[0002] As known to those skilled in the art, conventional battery electrolytes contain various components. For example, Patent Document 1 discloses that a non-aqueous electrolyte secondary battery containing vinylene carbonate as a non-aqueous solvent has good cycle characteristics.

[0003] Japanese Patent Application Laid-Open No. 2005-268230

[0004] One of the technical issues with non-aqueous electrolyte secondary batteries is the generation of gas during charge and discharge. The present disclosure provides a positive electrode for a secondary battery that can suppress gas generation and a decrease in discharge capacity during charge and discharge of the battery.

[0005] The present disclosure provides a positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains active material particles and a solid organoboronic acid.

[0006] The positive electrode for a secondary battery according to the present disclosure can suppress gas generation and a decrease in discharge capacity that occur during charge and discharge of the battery.

[0007] Fig. 1 is a cross-sectional view of a positive electrode for a secondary battery in Embodiment 1. Fig. 2 is a flowchart showing a manufacturing process of a positive electrode. Fig. 3 is a schematic cross-sectional view showing an example of a non-aqueous electrolyte secondary battery in Embodiment 2.

[0008] (Findings that Form the Basis of the Present Disclosure) One method for suppressing gas generation during charge and discharge is to add various additives to a non-aqueous electrolyte. However, even if such additives can suppress gas generation, they can sometimes accelerate the decrease in discharge capacity during charge and discharge. Suppressing gas generation does not necessarily mean that the decrease in discharge capacity can also be suppressed. The present inventors focused on suppressing the decrease in discharge capacity while taking advantage of the benefits of suppressing gas generation, and came up with the technology of the present disclosure.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0010] (Embodiment 1) Fig. 1 is a cross-sectional view of a positive electrode for a secondary battery in embodiment 1. The positive electrode for a secondary battery 5 includes a positive electrode current collector 5a and a positive electrode active material layer 5b. The positive electrode active material layer 5b is supported by the positive electrode current collector 5a. Hereinafter, the positive electrode for a secondary battery 5 may be simply referred to as "positive electrode 5."

[0011] The positive electrode active material layer 5b contains active material particles and a solid organic boronic acid. The organic boronic acid suppresses gas generation that accompanies charge and discharge of a secondary battery using the positive electrode 5.

[0012] Although the mechanism by which the solid organoboronic acid exerts the effect of suppressing gas generation is not entirely clear, the following mechanism is presumed. When a battery using the positive electrode 5 is operated, the organoboronic acid contained in the positive electrode 5 forms a coating containing boron on the surface of the active material particles. This can suppress decomposition of the nonaqueous solvent at the interface between the nonaqueous electrolyte and the positive electrode 5. The presence of the coating caused by the organoboronic acid can sometimes be confirmed by analyzing the components on the surface of the positive electrode 5 after charging and discharging the battery. A method for analyzing the components on the surface of the positive electrode 5 is, for example, ICP (Inductively Coupled Plasma) analysis.

[0013] According to this embodiment, a solid organoboronic acid is contained in the positive electrode 5. Therefore, the organoboronic acid can be contained in the positive electrode active material layer 5b at a higher content than when an electrolyte solution containing the organoboronic acid is subsequently impregnated into an electrode group. As a result, it is presumed that a coating containing boron is easily formed, and a high effect of suppressing gas generation can be obtained.

[0014] According to this embodiment, the decrease in discharge capacity due to charge and discharge can also be suppressed. According to this embodiment, the organic boronic acid can be contained in the positive electrode 5 even after the battery using the positive electrode 5 is assembled. This configuration can achieve a higher effect of suppressing side reactions at the negative electrode than when an electrode group is subsequently impregnated with an electrolyte solution containing an organic boronic acid. It is presumed that one reason for the effect of suppressing the decrease in discharge capacity is the result of suppressing such side reactions.

[0015] Organoboronic acid refers to a boronic acid that contains a boron-carbon covalent bond.

[0016] The organic boronic acid may include a saturated hydrocarbon group in which at least one hydrogen atom may be substituted with a fluorine atom. In other words, the organic boronic acid may be an alkylboronic acid. The number of carbon atoms in the saturated hydrocarbon group is not particularly limited and may be, for example, 1 to 6. This configuration sufficiently suppresses gas generation and a decrease in discharge capacity.

[0017] The saturated hydrocarbon group may include a cyclic, linear, or branched alkyl group. This configuration sufficiently suppresses the generation of gas and the decrease in discharge capacity.

[0018] The organic boronic acid may contain a compound represented by the following formula (1): In formula (1), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, and n is an integer from 0 to 6. This configuration sufficiently suppresses gas generation and a decrease in discharge capacity.

[0019]

[0020] In formula (1), n ​​is preferably an integer of 0 to 4, more preferably 1 to 4, and even more preferably 1 or 2. With this configuration, the effect of suppressing gas generation and the effect of suppressing a decrease in discharge capacity can be sufficiently obtained.

[0021] The organic boronic acid may contain a compound represented by the following formula (2): In formula (2), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom; m and n are each independently an integer from 0 to 6, and 0≦m+n≦6 is satisfied. This configuration satisfies the effects of suppressing gas generation and suppressing a decrease in discharge capacity.

[0022]

[0023] The organic boronic acid may contain at least one selected from the group consisting of a compound represented by formula (1) and a compound represented by formula (2).

[0024] The compound represented by formula (1) is an organic boronic acid having a linear alkyl group, such as methylboronic acid, ethylboronic acid, propylboronic acid, butylboronic acid, pentylboronic acid, and hexylboronic acid.

[0025] The compound represented by formula (2) is an organic boronic acid having a branched alkyl group. Examples of such organic boronic acids include isobutylboronic acid and isopropylboronic acid. Organic boronic acids having a branched alkyl group are also called isoalkylboronic acids.

[0026] Examples of boronic acids having a cycloalkyl group, which is a cyclic alkyl group, include cyclopentylboronic acid and cyclohexylboronic acid.

[0027] The organic boronic acids may be used alone or in combination of two or more, which can sufficiently suppress gas generation and decrease in discharge capacity.

[0028] The solid organoboronic acid may be present in a particulate state on the surfaces of the active material particles and between the active material particles, allowing the organoboronic acid to be contained in the positive electrode 5 even after the battery is assembled using the positive electrode 5.

[0029] The average particle size of the organic boronic acid particles is, for example, in the range of 0.1 μm to 10 μm, preferably 2 μm to 6 μm. By including such organic boronic acid particles in the positive electrode 5, it is possible to form a state in which the concentration of the organic boronic acid in the positive electrode active material layer 5b is minimally uneven. As a result, the effect of suppressing gas generation can be obtained throughout the positive electrode active material layer 5b.

[0030] The organoboronic acid particles may be nanoparticles, primary particles, secondary particles, or a mixture of primary and secondary particles.

[0031] The average particle size of particles such as active material particles and organic boronic acid particles can be calculated by the following method. A cross section of the positive electrode 5 is observed with a scanning electron microscope, and the area of ​​the particle in the microscopic image is calculated by image processing. The diameter of a circle having an area equal to the calculated area is regarded as the diameter of that particle. The diameters of an arbitrary number of particles (e.g., 10 particles) are calculated, and their average value is regarded as the average particle size. When selecting target particles, fine particles or coarse particles that deviate from the average size may be excluded. Using SEM-EDX, it is possible to examine the distribution of elements in the observation field, and the type of particle can be identified based on the distribution of elements.

[0032] The ratio of the mass of the organic boronic acid to the mass of the active material particles is, for example, in the range of 0.05% by mass or more and 2% by mass or less, preferably 0.5% by mass or more and 1.1% by mass or less. The organic boronic acid is a material that does not function as an active material. By adjusting the amount of the organic boronic acid within the above range, gas generation can be suppressed while also avoiding a significant decrease in the weight energy density of the battery. The mass of the active material particles and the mass of the organic boronic acid each refer to the mass per unit volume of the positive electrode active material layer 5b.

[0033] The mass of the active material particles and the organic boronic acid present in a unit volume of the positive electrode active material layer 5b can be determined by the following method. The volume of the positive electrode active material layer 5b is calculated from the thickness and area of ​​the positive electrode active material layer 5b in a positive electrode 5 of any size. A positive electrode composite constituting the positive electrode active material layer 5b is sampled and washed with an organic solvent, and then its mass is calculated. Quantitative analysis of the elements contained in the positive electrode composite is performed by a chemical analysis method such as ICP analysis. By focusing on elements (e.g., transition metal elements) contained only in the active material particles, the content ratio of the active material particles in the positive electrode composite can be calculated. In other words, the mass of the active material particles present in a unit volume of the positive electrode active material layer 5b can be calculated. By focusing on elements (e.g., boron) contained only in the organic boronic acid, the content ratio of the organic boronic acid in the positive electrode composite can be calculated. In other words, the mass of the organic boronic acid present in a unit volume of the positive electrode active material layer 5b can be calculated.

[0034] The active material particles are particles of a positive electrode active material. The positive electrode active material can be a material capable of absorbing and releasing lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce the manufacturing cost of the battery and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0035] The positive electrode active material may contain lithium nickel oxide having a layered rock salt crystal structure. The proportion of Ni among metal elements other than Li contained in the lithium nickel oxide may be 50 atomic % or more. The lithium nickel oxide may also contain other transition metals. The lithium nickel oxide is useful for achieving a high operating voltage.

[0036] The lithium nickel oxide may be represented by the following composition formula (I): Element M1 is at least one selected from the group consisting of V, Co, and Mn. Element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. Composition formula (1) satisfies 0.9≦α≦1.10, −0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, and 0≦1−x1−x2≦0.5.

[0037] Li α Ni x1 M1 x2 M2 (1-x1-x2) O 2+β ...(I)

[0038] The positive electrode active material layer 5b may contain other materials such as a conductive additive and a binder.

[0039] The conductive additive is used to reduce the resistance of the positive electrode 5. Examples of the conductive additive include a carbon material and a conductive polymer compound. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene.

[0040] The binder is used to improve the binding property of the material constituting the positive electrode 5. As the binder, polymer materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide can be used.

[0041] The positive electrode current collector 5a can be a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable materials for the positive electrode current collector 5a because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the positive electrode current collector 5a as a conductive auxiliary material.

[0042] FIG. 2 is a flowchart showing the manufacturing process of the positive electrode 5 .

[0043] In step S1, a slurry containing active material particles is applied to a substrate to form a mixture layer. The slurry typically contains active material particles, a binder, a conductive additive, and a solvent. The solvent can be an organic solvent such as N-methyl-2-pyrrolidone, acetone, dimethylformamide, or tetrahydrofuran. The substrate can be the positive electrode current collector 5a or a transfer film such as a resin film. The slurry can be applied to the substrate by a coating method such as a doctor blade method.

[0044] In step S2, the mixture layer is dried. This removes the organic solvent from the mixture layer. Step S2 may be performed by heating the mixture layer to a temperature higher than room temperature (25°C). For example, the mixture layer may be dried at an ambient temperature of 60°C to 150°C.

[0045] After drying, in step S3, an organic boronic acid solution containing an organic boronic acid and a solvent is impregnated into the mixture layer. The method for impregnating the mixture layer with the organic boronic acid solution is not particularly limited. For example, the organic boronic acid solution can be impregnated into the mixture layer by dripping the organic boronic acid solution onto the mixture layer. By adjusting the amount dripped per unit area, the mass of the organic boronic acid relative to the mass of the active material particles can be adjusted to a desired value.

[0046] The organic boronic acid solution can be prepared by dissolving the organic boronic acid in a solvent. Examples of the solvent include organic solvents such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. The concentration of the organic boronic acid in the organic boronic acid solution can be adjusted taking into account the target content of the organic boronic acid in the positive electrode active material layer 5b and the amount of the organic boronic acid solution to be dripped onto the mixture layer per unit area. In one example, the concentration of the organic boronic acid in the organic boronic acid solution is in the range of 0.2% by mass to 4% by mass.

[0047] In step S4, the solvent is removed from the mixture layer. This causes solid organoboronic acid to precipitate on the surfaces of the active material particles and between the active material particles. The organoboronic acid may exist in a particulate state. Step S4 may be performed by heating the mixture layer to a temperature higher than room temperature (25°C). For example, the mixture layer may be dried at an ambient temperature of 60°C to 150°C.

[0048] In step S5, the mixture layer is pressed, thereby obtaining the positive electrode 5 of this embodiment.

[0049] According to the manufacturing method of this embodiment, it is not necessary to add an organic boronic acid to the slurry. Therefore, according to the manufacturing method of this embodiment, the slurry can be uniformly and smoothly applied to the substrate, and thus the positive electrode 5 of this embodiment can be efficiently manufactured. Note that adding an organic boronic acid to the slurry increases the viscosity of the slurry, making it difficult to apply the slurry to the substrate.

[0050] In steps S1 and S2, the positive electrode 5 can also be manufactured by a so-called dry method. In the dry method, active material particles, a binder, and a conductive additive are kneaded together. The resulting kneaded mixture is molded to obtain a mixture layer. The mixture layer may be formed directly on the positive electrode current collector 5a, or the mixture layer may be formed on a substrate such as a resin film and then transferred onto the positive electrode current collector 5a. A mixture layer may be disposed on each of both surfaces of the positive electrode current collector 5a.

[0051] (Embodiment 2) FIG. 3 is a schematic cross-sectional view showing an example of a nonaqueous electrolyte secondary battery according to Embodiment 2. The secondary battery 100 includes a container 1, an electrode group 4, and an electrolyte (not shown). The electrode group 4 has a wound structure. The electrode group 4 is housed in the container 1. The electrode group 4 includes a positive electrode 5, a negative electrode 6, and a pair of separators 7. The electrode group 4 is impregnated with an electrolyte. The opening of the container 1 is closed with a sealing plate 2. The positive electrode 5 includes a positive electrode current collector 5a and a positive electrode active material layer 5b. One end of a positive electrode lead 5c is connected to the positive electrode 5. The other end of the positive electrode lead 5c is connected to the back surface of the sealing plate 2. An insulating packing 3 is disposed around the periphery of the sealing plate 2. The negative electrode 6 includes a negative electrode current collector 6a and a negative electrode active material layer 6b. One end of a negative electrode lead 6c is connected to the negative electrode 6. The other end of the negative electrode lead 6c is connected to the bottom surface of the container 1. An insulating ring 8 is disposed on each of the upper and lower surfaces of the electrode group 4 .

[0052] By using the positive electrode 5 described in the first embodiment, gas generation and a decrease in discharge capacity that accompany charge and discharge of the secondary battery 100 can be suppressed.

[0053] The negative electrode current collector 6a may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector 6a as a conductive auxiliary material.

[0054] The negative electrode active material layer 6b includes a negative electrode active material. The negative electrode active material can be a material capable of absorbing and releasing lithium ions. The negative electrode active material includes, for example, at least one selected from the group consisting of carbon materials and materials capable of forming an alloy with lithium. Examples of carbon materials include graphite. Examples of materials capable of forming an alloy with lithium include silicon, silicon-containing oxides, tin, zinc alloys, bismuth, and germanium. One type selected from these negative electrode active materials may be used, or two or more types may be used in combination.

[0055] The negative electrode active material layer 6b may contain at least one selected from the group consisting of graphite and silicon as the negative electrode active material. The negative electrode active material layer 6b may contain only graphite as the negative electrode active material. Graphite is recommended because it is resistant to deterioration even when repeatedly charged and discharged at a deep depth. Carbon materials other than graphite may also be used as the negative electrode active material. Silicon has a larger capacity than graphite and is therefore advantageous for increasing the capacity of the secondary battery 100.

[0056] The negative electrode active material layer 6b may contain other materials such as a conductive additive, a binder, etc. Materials that can be used as the conductive additive and binder for the positive electrode active material layer 5b can also be used for the negative electrode active material layer 6b.

[0057] The electrolyte is a non-aqueous electrolyte impregnated into the positive electrode 5, the negative electrode 6, and the separator 7. The electrolyte may fill the internal space of the container 1. The electrolyte allows lithium ions to move between the positive electrode 5 and the negative electrode 6.

[0058] The electrolyte solution includes a non-aqueous solvent and a lithium salt.

[0059] Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, nitriles, amides, etc. One of these solvents may be used alone, or two or more of them may be used in combination.

[0060] Examples of lithium salts include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One selected from these lithium salts may be used, or two or more may be used in combination.

[0061] The separator 7 has lithium ion conductivity. The material of the separator 7 is not particularly limited as long as it allows lithium ions to pass through. The material of the separator 7 can be at least one selected from the group consisting of a gel electrolyte, an ion exchange resin membrane, a semipermeable membrane, and a porous membrane. Making the separator 7 from these materials can sufficiently ensure the safety of the nonaqueous electrolyte secondary battery 100. Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVdF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resins and porous membranes containing glass paper obtained by weaving glass fibers into nonwoven fabric.

[0062] The container 1 is made of a metal such as aluminum or stainless steel, and may have a cylindrical shape or a rectangular tube shape.

[0063] The electrode group 4 may be wound into a cylindrical shape or an oval shape.

[0064] The shape of the secondary battery 100 is not limited to a cylindrical shape, and various shapes such as a coin shape, a square shape, a sheet shape, a button shape, a flat shape, and a laminated shape can be adopted as the shape of the secondary battery 100.

[0065] The technology of the present disclosure can be applied to various types of secondary batteries, such as sodium secondary batteries and magnesium secondary batteries, in addition to lithium secondary batteries.

[0066] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0067] (Technology 1) A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains active material particles and a solid organoboronic acid.

[0068] The positive electrode for a secondary battery according to the present disclosure can suppress gas generation and a decrease in discharge capacity that occur during charge and discharge of the battery.

[0069] (Technology 2) The positive electrode for a secondary battery according to Technology 1, wherein the organic boronic acid contains a saturated hydrocarbon group in which at least one hydrogen atom may be substituted with a fluorine atom. With this configuration, the effects of suppressing gas generation and suppressing a decrease in discharge capacity can be sufficiently obtained.

[0070] (Technology 3) The positive electrode for a secondary battery according to Technology 2, wherein the saturated hydrocarbon group includes a cyclic, linear, or branched alkyl group. With this configuration, the effects of suppressing gas generation and suppressing a decrease in discharge capacity can be sufficiently obtained.

[0071] (Technology 4) The positive electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the organic boronic acid includes a compound represented by the following formula (1), wherein in formula (1), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, and n is an integer of 0 to 6. With this configuration, the effect of suppressing gas generation and the effect of suppressing a decrease in discharge capacity can be sufficiently obtained.

[0072] (Technology 5) The positive electrode for a secondary battery according to Technology 4, wherein in the formula (1), n ​​is 1 to 4. With such a configuration, the effect of suppressing gas generation and the effect of suppressing a decrease in discharge capacity can be sufficiently obtained.

[0073] (Technology 6) The positive electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the organic boronic acid includes a compound represented by the following formula (2), wherein in formula (2), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, R is a methyl group or an ethyl group, and m and n are each independently an integer of 0 to 6, and 0≦m+n≦6 is satisfied. With this configuration, the effect of suppressing gas generation and the effect of suppressing a decrease in discharge capacity can be sufficiently obtained.

[0074] (Technology 7) The positive electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the organic boronic acid includes at least one selected from the group consisting of ethylboronic acid, propylboronic acid, butylboronic acid, pentylboronic acid, hexylboronic acid, isobutylboronic acid, and isopropylboronic acid. With this configuration, the effect of suppressing gas generation and the effect of suppressing a decrease in discharge capacity can be sufficiently obtained.

[0075] (Technology 8) The positive electrode for a secondary battery according to any one of Technologies 1 to 7, wherein the ratio of the mass of the organic boronic acid to the mass of the active material particles is in the range of 0.05 mass% to 2 mass%. By adjusting the amount of the organic boronic acid to the above range, gas generation can be suppressed while also avoiding a significant decrease in the weight energy density of the battery.

[0076] (Technology 9) The positive electrode for a secondary battery according to any one of Technologies 1 to 8, wherein the organic boronic acid is present in a particulate state on the surfaces of the active material particles and between the active material particles. With this configuration, the organic boronic acid can be contained in the positive electrode even after assembly of a battery using the positive electrode.

[0077] (Technology 10) A method for manufacturing a positive electrode for a secondary battery, comprising: forming a mixture layer containing active material particles on a substrate; impregnating the mixture layer with an organoboronic acid solution containing an organoboronic acid and a solvent; and removing the solvent from the mixture layer.

[0078] According to the manufacturing method of the present disclosure, the positive electrode for a secondary battery of the present disclosure can be manufactured efficiently.

[0079] Example 1 An organic boronic acid solution was prepared by dissolving propylboronic acid in dimethyl carbonate. The concentration of propylboronic acid in the organic boronic acid solution was 2% by mass.

[0080] LiNi 0.8 Mn 0.2A positive electrode slurry was prepared by mixing and stirring positive electrode active material particles having a composition of O, acetylene black (AB), polyvinylidene fluoride (PVDF), and N-methyl-pyrrolidone (NMP). The mass ratio of the positive electrode active material particles, AB, and PVDF was positive electrode active material:AB:PVDF=96:2:2. The positive electrode slurry was applied to the surface of aluminum foil to form a coating film.

[0081] After drying the coating film, an organic boronic acid solution was dropped onto the coating film and allowed to soak in. The amount of the organic boronic acid solution dropped was adjusted so that the ratio of propylboronic acid to the mass of the positive electrode active material was 0.3 mass%. The coating film was then dried and rolled. This produced a positive electrode of Example 1.

[0082] Example 2 A positive electrode of Example 2 was produced in the same manner as in Example 1, except that the amount of the organic boronic acid solution added was adjusted so that the ratio of the mass of propylboronic acid to the mass of the positive electrode active material was 0.7 mass %.

[0083] Example 3 A positive electrode of Example 3 was produced in the same manner as in Example 1, except that the amount of the organic boronic acid solution dropped was adjusted so that the ratio of the mass of propylboronic acid to the mass of the positive electrode active material was 1.1 mass %.

[0084] Comparative Example 1 A positive electrode of Comparative Example 1 was produced in the same manner as in Example 1, except that the organic boronic acid solution was not dropped onto the coating film.

[0085] Reference Example 1 As a positive electrode of Reference Example 1, the same one as that of Comparative Example 1 was prepared.

[0086] [Preparation of Evaluation Cells for Examples and Comparative Examples] Ethylene carbonate (EC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of EC:FEC:DMC:EMC = 10:10:75:5 to prepare a non-aqueous solvent. LiPF was dissolved in the obtained non-aqueous solvent to a concentration of 1.3 mol / L to obtain a non-aqueous electrolyte solution.

[0087] A laminate half cell was fabricated using a positive electrode, a Li metal foil (2 cm×2 cm, 200 μm thick) as a counter electrode, a separator (Celgard, #2320), and a non-aqueous electrolyte.

[0088] In this manner, evaluation cells for Examples 1 to 3 and Comparative Example 1 were obtained.

[0089] [Preparation of Evaluation Cell for Reference Example] Propylboronic acid was dissolved at a concentration of 0.2 mass% in the nonaqueous electrolyte used to prepare the evaluation cells for Examples and Comparative Examples. The evaluation cell for Reference Example 1 was obtained by the same method as in Examples and Comparative Examples, except that a nonaqueous electrolyte containing propylboronic acid was used. The concentration of 0.2 mass% was converted to a mass ratio of 0.6 mass% relative to the positive electrode active material.

[0090] [Measurement of Amount of Gas Generated During Charge and Discharge, Discharge Capacity, and Resistance] The evaluation cells of Examples 1 to 3, Comparative Example 1, and Reference Example 1 were charged and discharged according to the following procedure, and the amount of gas generated during charge and discharge and the discharge capacity were measured. Charging and discharging were performed in an ambient atmosphere at 55°C. The evaluation cells were charged to 4.3 V (fully charged state) and then discharged at a constant current to 2.5 V (fully discharged state). The current value at which the discharge time was 1 hour was defined as "1 C."

[0091] (First cycle) Constant current charging was performed at a current value of 0.2 C until the voltage reached 4.3 V, and constant voltage charging was performed at a voltage of 4.3 V until the current value reached 0.02 C. After a rest period of 20 minutes, constant current discharging was performed at a current value of 0.2 C until the voltage reached 2.5 V. After the rest period of 20 minutes, the initial volume of the evaluation cell was measured by the Archimedes method in an ambient atmosphere at 20°C.

[0092] (2nd to 25th cycles) Constant current charging was performed at a current value of 0.3 C until the voltage reached 4.3 V, followed by constant voltage charging at a voltage of 4.3 V until the current value reached 0.02 C. Subsequently, constant current discharging was performed at a current value of 0.5 C until the voltage reached 2.5 V. A 20-minute rest period was provided between charging and discharging. This charge / discharge process was repeated 24 times, for a total of 25 charge / discharge cycles. The volume of the evaluation cell was then measured using the Archimedes method under an ambient atmosphere at 20°C. The amount of gas generated at the 25th cycle was calculated by subtracting the initial volume from the volume of the evaluation cell at the 25th cycle. The results are shown in Table 1.

[0093] (26th to 50th cycles) Next, the charge-discharge cycle was repeated 25 more times using the same method as the charge-discharge cycles from the 2nd to 25th cycles. The volume of the evaluation cell was then measured by Archimedes' method in an ambient atmosphere at 20°C. The amount of gas generated at the 50th cycle was calculated by subtracting the initial volume from the volume of the evaluation cell at the 50th cycle. The results are shown in Table 1.

[0094] The capacity retention rate at 50 cycles was calculated using the following formula (3). A high capacity retention rate indicates that the decrease in discharge capacity due to charge and discharge was suppressed. The results are shown in Table 1.

[0095] (Capacity maintenance rate) = 100 x C 50 / C1 (3) C1: Discharge capacity at the first cycle C 50 : Discharge capacity at 50th cycle

[0096] The resistance ratio at 50 cycles was calculated using the following formula (4). The results are shown in Table 1.

[0097] (Resistance ratio at 50 cycles) = R 50 / R1 (4) R1: Internal resistance of the cell at one cycle R 50 : Internal resistance of the cell at 50 cycles

[0098]

[0099] In Table 1, the items "Gas generation amount (%) at 25 cycles" and "Gas generation amount (%) at 50 cycles" represent values ​​relative to the values ​​in Comparative Example 1.

[0100] As shown in Table 1, the amount of gas generated in the evaluation cell of Example 1 was lower than the amount of gas generated in Comparative Example 1. The amounts of gas generated in the evaluation cells of Examples 2 and 3 were significantly lower than the amount of gas generated in Comparative Example 1. This result indicates that gas generation was suppressed by the organoboronic acid contained in the positive electrode of the evaluation cell.

[0101] The capacity retention rates at 50 cycles for the evaluation cells of Examples 1 to 3 all exceeded the capacity retention rate for the evaluation cell of Comparative Example 1. The positive electrodes of Examples 1 to 3 were able to not only suppress gas generation but also suppress a decrease in discharge capacity due to charge and discharge.

[0102] The resistance ratios of the evaluation cells of Examples 1 to 3 at 50 cycles were all lower than the resistance ratio of the evaluation cell of Comparative Example 1 at 50 cycles. The resistance values ​​of the evaluation cells of Examples 1 to 3 after 50 cycles were lower than the resistance value of the evaluation cell of Comparative Example 1 after 50 cycles.

[0103] The amount of gas generated in the evaluation cell of Reference Example 1 was lower than the amount of gas generated in Comparative Example 1. Meanwhile, the capacity retention rate at 50 cycles in Reference Example 1 was lower than the capacity retention rate of the evaluation cell of Comparative Example 1. The resistance ratio at 50 cycles in Reference Example 1 was higher than that of Comparative Example 1. In contrast, the evaluation cells of Examples 1 to 3, which were manufactured using positive electrodes containing an organic boronic acid in advance, were able to suppress not only the amount of gas generated but also the decrease in discharge capacity and the increase in resistance value.

[0104] Examples 11 to 17 Organoboronic acid solutions were prepared by dissolving various organoboronic acids shown in Table 2 in dimethyl carbonate. The concentration of each organoboronic acid in the organoboronic acid solution was 2% by mass.

[0105] The positive electrodes of Examples 11 to 17 were prepared by the same method as in Example 1, while adjusting the amount of the organic boronic acid solution dropped so that the ratio of the mass of the organic boronic acid to the mass of the positive electrode active material was the value shown in Table 2. The positive electrode of Comparative Example 1 was prepared under the same conditions as the positive electrode of Comparative Example 1 shown in Table 1. The positive electrode of Example 12 was prepared under the same conditions as the positive electrode of Example 1 shown in Table 1.

[0106] [Preparation of Evaluation Cells] Evaluation cells of Examples 11 to 17 were prepared in the same manner as in Example 1 using the positive electrodes of Examples 11 to 17.

[0107] [Measurement of gas generation amount and discharge capacity during charge and discharge] The evaluation cells of Examples 11 to 17 were charged and discharged in the same manner as described above, and the amount of gas generated during charge and discharge was measured. The results are shown in Table 2.

[0108] The capacity retention rate at 25 cycles was calculated using the following formula (5). The results are shown in Table 2.

[0109] (Capacity maintenance rate) = 100 x C 25 / C1 (5) C1: Discharge capacity at the first cycle C 25 : Discharge capacity at 25th cycle

[0110]

[0111] In Table 2, the item "Gas generation amount (%) at 25 cycles" indicates a relative value to that of Comparative Example 1.

[0112] As shown in Table 2, at the 25th cycle, the gas generation rates of the evaluation cells of Examples 11 to 17 were significantly lower than the gas generation rate of the evaluation cell of Comparative Example 1. In other words, the effect of suppressing gas generation was obtained regardless of the type of organoboronic acid.

[0113] The capacity retention rates at 25 cycles for the evaluation cells of Examples 11 to 17 all exceeded the capacity retention rate of the evaluation cell of Comparative Example 1. In other words, regardless of the type of organoboronic acid, the decrease in discharge capacity associated with charge and discharge was suppressed.

[0114] [ICP Analysis] Evaluation cells for Example 1 and Reference Example 1 were prepared using the method described above. The evaluation cells for Example 1 and Reference Example 1 were subjected to two cycles of charge and discharge using the method described above. The evaluation cells were then disassembled, and the post-charge and discharge positive electrodes were removed. The post-charge and discharge positive electrodes were immersed in a container filled with dimethyl carbonate for 30 seconds or more and stirred to remove the nonaqueous electrolyte adhering to the surface of the positive electrode. The boron atomic concentration in the positive electrode was then measured using an ICP analyzer (iCAP7400 Duo, manufactured by Thermo Fisher Scientific).

[0115] The boron atom concentration in the positive electrode taken out of the evaluation cell of Example 1 was 0.025% by mass. This result indicates that a portion of the organoboronic acid in the positive electrode was consumed in the formation of the boron-containing coating. The boron atom concentration in the positive electrode taken out of the evaluation cell of Reference Example 1 was 0.005% by mass, which corresponds to 20% of the value in Example 1. It is presumed that in Example 1, the formation of a larger amount of the boron-containing coating on the surface of the positive electrode not only suppressed the amount of gas generation but also suppressed the decrease in discharge capacity.

[0116] The technology of the present disclosure is useful for, for example, lithium ion secondary batteries.

Claims

1. A positive electrode for a secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains active material particles and a solid organoboronic acid.

2. The positive electrode for a secondary battery according to claim 1, wherein the organic boronic acid contains a saturated hydrocarbon group in which at least one hydrogen atom may be substituted with a fluorine atom.

3. The positive electrode for a secondary battery according to claim 2, wherein the saturated hydrocarbon group includes a cyclic, linear, or branched alkyl group.

4. The organic boronic acid includes a compound represented by the following formula (1):

2. The positive electrode for a secondary battery according to claim 1 , wherein, in formula (1), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, and n is an integer of 0 to 6.

5. The positive electrode for a secondary battery according to claim 4, wherein in said formula (1), n ​​is 1 to 4.

6. The organic boronic acid includes a compound represented by the following formula (2): wherein, in formula (2), X is a methyl group in which at least one hydrogen atom may be substituted with a fluorine atom, R is a methyl group or an ethyl group, m and n are each independently an integer of 0 to 6, and 0≦m+n≦6 is satisfied. The positive electrode for a secondary battery according to claim 1 .

7. The positive electrode for a secondary battery according to claim 1, wherein the organic boronic acid comprises at least one selected from the group consisting of ethylboronic acid, propylboronic acid, butylboronic acid, pentylboronic acid, hexylboronic acid, isobutylboronic acid, and isopropylboronic acid.

8. The positive electrode for a secondary battery according to claim 1, wherein the ratio of the mass of the organic boronic acid to the mass of the active material particles is in the range of 0.05 mass % or more and 2 mass % or less.

9. The positive electrode for a secondary battery according to claim 1, wherein the organic boronic acid is present in the form of particles on the surfaces of the active material particles and between the active material particles.

10. A method for producing a positive electrode for a secondary battery, comprising: forming a mixture layer containing active material particles on a substrate; impregnating the mixture layer with an organoboronic acid solution containing an organoboronic acid and a solvent; and removing the solvent from the mixture layer.

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