Positive electrode for closed-type lithium-oxygen batteries, and closed-type lithium-oxygen battery using same

Abstract

The present disclosure provides a means capable of improving peeling strength in a positive electrode for closed-type lithium-oxygen batteries that use polyvinylidene fluoride as a binder.　Disclosed is a positive electrode for closed-type lithium-oxygen batteries, the positive electrode comprising: a current collector; and a positive electrode active material layer that is disposed on a surface of the current collector and contains lithium oxide, a catalyst, and polyvinylidene fluoride that serves as a binder. The content of the polyvinylidene fluoride in the positive electrode active material layer is 20 mass% or less with respect to 100 mass% of the total solid content of the positive electrode active material layer at the time of complete battery discharge. In the Raman spectrum of the positive electrode active material layer, the ratio B / A of the intensity B of a peak top of a peak appearing in the range of 1110-1160 cm-1 to the intensity A of a peak top of a peak appearing in the range of 2940-2990 cm-1 is 3.8 or less.

Description

Positive electrode for closed-type lithium-oxygen battery and closed-type lithium-oxygen battery using the same

[0001] The present invention relates to a positive electrode for a closed-type lithium-oxygen battery and a closed-type lithium-oxygen battery using the same.

[0002] In recent years, the widespread use of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries have been actively developed as on-board power sources for driving motors and other applications, which are key to the widespread use of these electric vehicles. Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, have been attracting attention as they are expected to have high energy density and high output.

[0003] Lithium-oxygen batteries (lithium-oxygen secondary batteries) are known as one type of non-aqueous electrolyte secondary battery. These lithium-oxygen batteries have the highest theoretical energy density of all secondary batteries, including next-generation batteries, and are expected to offer battery performance with a high energy density that far exceeds that of current lithium-ion secondary batteries. However, sufficient capacity characteristics have yet to be achieved, and further improvements are currently required.

[0004] Lithium-oxygen batteries use gaseous molecular oxygen (O ２ ) is used, and O ２ When charging, ２ and a lithium-air battery that generates ２ The latter is a battery that does not consume or generate oxygen (lithium-oxygen battery in the narrow sense). ２ Since there is no exchange of electricity, the battery can be constructed as a sealed cell and can be called a "closed-type lithium-oxygen battery."

[0005] Japanese Patent Laid-Open Publication No. 2015-32515 discloses a transition metal-doped alkali metal oxide, which is a metal oxide containing alkali metal atoms such as lithium and transition metal atoms such as Co or Fe, and has a structure in which the transition metal atoms are dissolved in a solid solution within the crystalline structure of the alkali metal oxide. Japanese Patent Laid-Open Publication No. 2015-32515 discloses that, since the transition metal-doped alkali metal oxide allows an oxidation current to flow at a lower voltage than alkali metal oxides in which the transition metal is not dissolved, its use as a positive electrode material can result in a storage battery that can be charged at a lower voltage than a lithium-air battery.

[0006] However, according to the study by the present inventors, when a positive electrode for a closed-type lithium-oxygen battery is produced using the positive electrode material described in JP 2015-32515 A and polyvinylidene fluoride (PVdF) as a binder, the positive electrode of the Li(Ni-Mn-Co)O ２ It has been found that the peel strength of the positive electrode may be reduced in some cases compared to a positive electrode using a positive electrode active material such as a lithium transition metal composite oxide.

[0007] Therefore, an object of the present invention is to provide a means for improving the peel strength in a positive electrode for a closed-type lithium-oxygen battery that uses PVdF as a binder.

[0008] One aspect of the present invention is a cathode active material layer comprising a current collector and a cathode active material layer disposed on a surface of the current collector, the cathode active material layer including lithium oxide, a catalyst, and polyvinylidene fluoride as a binder, wherein the content of polyvinylidene fluoride in the cathode active material layer is 20 mass% or less relative to 100 mass% of a total solid content of the cathode active material layer when a battery is fully discharged, and a Raman spectrum of the cathode active material layer shows a peak at 2940 to 2990 cm －１ 1110 to 1160 cm for the peak top intensity A of the peak present in the range －１ The ratio B / A of the peak top intensities B of the peaks present in the range of 1.0 to 3.8 is a positive electrode for a closed-type lithium-oxygen battery.

[0009] 1 is a cross-sectional view schematically illustrating a stacked (flat) closed-type lithium oxygen battery according to one embodiment of the present invention.

[0010] One aspect of the present invention is a cathode active material layer comprising a current collector and a cathode active material layer disposed on a surface of the current collector, the cathode active material layer including a lithium oxide, a catalyst, and polyvinylidene fluoride (PVdF) as a binder, wherein the content of the polyvinylidene fluoride in the cathode active material layer is 20 mass% or less relative to 100 mass% of a total solid content of the cathode active material layer when the battery is fully discharged, and a Raman spectrum of the cathode active material layer shows a peak at 2940 to 2990 cm －１ 1110 to 1160 cm for the peak top intensity A of the peak present in the range －１ The positive electrode for a closed-type lithium-oxygen battery has a peak top intensity B of a peak present in the range of 0.01 to 0.02, and the ratio B / A of the peak top intensity B of a peak present in the range of 0.01 to 0.02 is 3.8 or less. By adopting such a configuration, the peel strength can be improved in a positive electrode for a closed-type lithium-oxygen battery using PVdF as a binder.

[0011] Hereinafter, the above-mentioned embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. In this specification, the range "X to Y" means "X or more and Y or less." Furthermore, unless otherwise specified, operations and measurements of physical properties, etc. are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0012] FIG. 1 is a cross-sectional view showing a schematic representation of a stacked (flat) closed-type lithium-oxygen battery (hereinafter also simply referred to as a "stacked-type lithium-oxygen battery") according to one embodiment of the present invention. In this specification, the term "closed-type lithium-oxygen battery" refers to a battery in which charge and discharge reactions proceed in a sealed cell (i.e., the battery is free from the exchange of molecular oxygen (O ２This "closed-type lithium-oxygen battery" is disclosed in Japanese Patent Application Laid-Open No. 2015-159098 as well as in other documents (Wang, J. et al., "Reversible Conversion between Lithium Superoxide and Lithium Peroxide: A Closed "Lithium-Oxygen" Battery," Inorganics 2023, 11, 69.; Okuoka, S. et al., Scientific Reports, Online Edition: 2014 / 7 / 14, 4:5684, DOI: 10.1038 / srep05684, etc.).

[0013] As shown in FIG. 1 , the stacked lithium-oxygen battery 10a of this embodiment has a structure in which a substantially rectangular power-generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29. Here, the power-generating element 21 has a configuration in which a positive electrode, in which a positive electrode active material layer 13 is disposed on both sides of a positive electrode current collector 11′, an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode, in which a negative electrode active material layer 15 is disposed on both sides of a negative electrode current collector 12, are laminated. Specifically, the positive electrode, the electrolyte layer, and the negative electrode are laminated in this order, with one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 facing each other with the electrolyte layer 17 interposed therebetween. As a result, the positive electrode, the electrolyte layer, and the negative electrode constitute one unit cell layer 19. Therefore, the stacked lithium-oxygen battery 10a shown in FIG. 1 can also be said to have a configuration in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel. Although the positive electrode active material layer 13 is disposed on only one side of each of the outermost positive electrode current collectors located on both outermost layers of the power-generating element 21, active material layers may be disposed on both sides. That is, instead of using a current collector exclusively for the outermost layer with an active material layer disposed on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. Furthermore, by reversing the arrangement of the positive electrode and negative electrode from that shown in FIG. 1 , the outermost negative electrode current collectors may be located on both outermost layers of the power-generating element 21, and negative electrode active material layers may be disposed on one or both sides of the outermost negative electrode current collectors.

[0014] A positive electrode current collector 25 and a negative electrode current collector 27 that are electrically connected to the electrodes (positive and negative electrodes) are attached to the positive electrode current collector 11′ and the negative electrode current collector 12, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29. The positive electrode current collector 25 and the negative electrode current collector 27 may be attached to the positive electrode current collector 11′ and the negative electrode current collector 12 of the electrodes by ultrasonic welding, resistance welding, or the like, via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as needed.

[0015] The main components of the stacked lithium-oxygen battery according to this embodiment will be described below.

[0016] [Current Collector] The current collector has a function of mediating the movement of electrons from the positive electrode active material layer and the negative electrode active material layer described later. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

[0017] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. A foil having a metal surface coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering. Examples of conductive resins include resins containing a conductive filler added to a non-conductive polymer material.

[0018] [Positive Electrode Active Material Layer] In this embodiment, the positive electrode active material layer contains lithium oxide, a catalyst, and PVdF as a binder.

[0019] (Lithium oxide) Lithium oxide is a compound in which lithium is covalently bonded to oxygen, and there are several compounds depending on the atomic ratio. Specifically, lithium oxide is Li ２ O, LiO, Li ２ O ２ and LiO ２It is preferable that the lithium oxide contains one or more selected from the group consisting of Li ２ O, Li ２ O ２ or LiO ２ In particular, from the viewpoint of having a higher theoretical capacity and being chemically stable (hard to decompose and low in reactivity with moisture in the air and carbon dioxide) compared to other lithium oxides, it is more preferable to include Li ２ It is particularly preferred that the lithium oxide contains O. These lithium oxides have the functions of either releasing lithium ions (reacting in a direction that decreases the atomic ratio of lithium to oxygen) when the lithium-oxygen battery is charged, or absorbing lithium ions (reacting in a direction that increases the atomic ratio of lithium to oxygen) when the lithium-oxygen battery is discharged. From the above, it can be said that lithium oxides function as positive electrode active materials for lithium-oxygen batteries.

[0020] The content of lithium oxide in the positive electrode active material layer (the total amount when two or more types are included) is not particularly limited, but is preferably 5 to 50 mass %, more preferably 5 to 40 mass %, and even more preferably 10 to 30 mass %, relative to 100 mass % of the total solid content of the positive electrode active material layer when the battery is fully discharged.

[0021] (Catalyst) A catalyst is a substance that has the function of promoting the bonding / dissociation reaction between the lithium oxide and oxygen by reducing the activation energy of these reactions. Any conventionally known compound can be used as a catalyst as long as it can exhibit this function. As an example, the catalyst is preferably a compound containing a transition metal (a transition metal-containing compound). This transition metal-containing compound is preferably in the form of, for example, an oxide (including composite oxides), sulfide, halide, nitride, carbide, or the like containing the transition metal. Among these, from the viewpoint of excellent catalytic activity, the catalyst preferably contains a transition metal-containing oxide. When the catalyst contains a transition metal-containing compound, there are no particular limitations on the type of transition metal contained in the compound, and the atom may be any metal classified as a transition metal, and one or more types may be used. Among these, from the viewpoint of catalytic activity, the transition metal is preferably at least one transition metal belonging to Groups 6 to 11 of the periodic table, more preferably one or more selected from the group consisting of cobalt, manganese, iron, nickel, molybdenum, iridium, and rhodium, particularly preferably one or more selected from the group consisting of cobalt, manganese, and iron, and most preferably cobalt. Examples of transition metal-containing compounds include transition metal-containing oxides such as cobalt oxide, manganese oxide, iron oxide, nickel oxide, molybdenum oxide, iridium oxide, and rhodium oxide. Among these, cobalt oxide, manganese oxide, and iron oxide are preferred, and cobalt oxide (e.g., tricobalt tetroxide (Co ３ O ４ )) is particularly preferred.

[0022] The content of the catalyst in the positive electrode active material layer (the total amount when two or more types are included) is not particularly limited, and although it depends on the types of lithium oxide and catalyst, it is preferably 50 to 500 mass %, more preferably 100 to 450 mass %, even more preferably 200 to 400 mass %, and particularly preferably 250 to 350 mass %, relative to 100 mass % of the total amount of the lithium oxide described above when the battery is fully discharged.

[0023] According to one embodiment, the content of the catalyst in the positive electrode active material layer (the total amount when two or more types are included) is, for example, 40 to 80 mass %, preferably 50 to 70 mass %, relative to 100 mass % of the total solids content of the positive electrode active material layer when the battery is fully discharged.

[0024] In the positive electrode active material layer, the lithium oxide and the catalyst may exist in the form of independent particles, or may exist as a composite of the lithium oxide and the catalyst (hereinafter also referred to as a "lithium oxide-catalyst composite"). Here, the "composite of the lithium oxide and the catalyst" refers to a state in which the lithium oxide particle and the catalyst particle are composited together, and the lithium oxide and the catalyst are mixed together within a single particle (the lithium oxide and the catalyst form a single particle). In this case, at least a portion of the transition metal atoms constituting the catalyst may be solid-solved in the lithium oxide. Forming such a composite can further promote the reaction between the lithium oxide and oxygen. This composite state can be achieved, for example, by employing a manufacturing method in which the lithium oxide and the catalyst are dry-kneaded using a ball mill or the like.

[0025] (Binder) The binder binds the components contained in the positive electrode active material layer and maintains the shape of the active material layer, thereby maintaining battery performance even when the lithium oxide shrinks and expands during charge and discharge. The positive electrode of this embodiment contains polyvinylidene fluoride (PVdF) as a binder. PVdF is a polymer containing units based on vinylidene fluoride (VdF) (hereinafter referred to as VdF units), and the content of VdF units is 90 mol% or more relative to all monomer units in the polymer. The PVdF may be a VdF homopolymer consisting only of VdF units, or may contain VdF units and units based on a monomer copolymerizable with VdF. However, a VdF homopolymer consisting only of VdF units is preferred. Examples of monomers copolymerizable with VdF include vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, hexafluoropropylene (HFP), (perfluoroalkyl)ethylene, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropeneethylene, and propylene.

[0026] The molecular weight of PVdF is not particularly limited, but from the viewpoint of binding property, the weight average molecular weight (polystyrene equivalent) measured by gel permeation chromatography is preferably 100,000 to 1,000,000, more preferably 300,000 to 900,000, and even more preferably 600,000 to 900,000.

[0027] In the positive electrode of this embodiment, the PVdF content in the positive electrode active material layer is 20% by mass or less, relative to 100% by mass of the total solids content of the positive electrode active material layer when the battery is fully discharged. If the PVdF content in the positive electrode active material layer exceeds 20% by mass, the electronic conductivity in the positive electrode active material layer decreases, resulting in a decrease in charge capacity. The PVdF content in the positive electrode active material layer is preferably 15% by mass or less, relative to 100% by mass of the total solids content of the positive electrode active material layer when the battery is fully discharged, and more preferably 10% by mass or less, from the viewpoint of suppressing overvoltage. The lower limit of the PVdF content in the positive electrode active material layer is not particularly limited, but is, for example, 2% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, from the viewpoint of ensuring adhesion.

[0028] In the positive electrode active material layer of the positive electrode according to this embodiment, at least a portion of the lithium oxide or the lithium oxide-catalyst composite is preferably coated with PVdF. In the positive electrode active material layer, the PVdF may coat only the lithium oxide, may coat only the lithium oxide-catalyst composite, may coat the lithium oxide together with other components (such as a catalyst or a conductive additive), or may coat the lithium oxide-catalyst composite together with a conductive additive or the like.

[0029] The coverage of the lithium oxide or lithium oxide-catalyst composite with PVdF is not particularly limited, but is preferably 50% or more. Here, "coverage" refers to the percentage of the surface area of ​​the lithium oxide or lithium oxide-catalyst composite particle that is covered with PVdF. From the perspective of more effectively achieving the effects of the present invention, the coverage is preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 90% or more. There is also no particular upper limit to the coverage, but it is typically 99% or less, preferably 95% or less. The "coverage" value can be calculated from an image obtained by observing the lithium oxide or lithium oxide-catalyst composite in the positive electrode active material using a scanning electron microscope (SEM).

[0030] In the positive electrode for a closed-type lithium-oxygen battery according to this embodiment, the Raman spectrum of the positive electrode active material layer shows a peak at 2940 to 2990 cm －１ 1110 to 1160 cm for the peak top intensity A of the peak present in the range －１ The ratio B / A of the peak top intensity B of the peak present in the range of 2940 to 2990 cm is 3.8 or less. By doing so, a positive electrode excellent in peel strength can be obtained. －１ The peaks in the range of 1110 to 1160 cm are due to PVdF. －１ The peaks present in the range are derived from the reaction product of PVdF and lithium oxide (decomposition product of PVdF). ２ Since PVdF is a Lewis base, it is highly reactive and is thought to react with PVdF as follows: (CH ２ -CF ２ ) ｎ +Li ２ O → (CH═CF) ｎ + LiOH + LiF In the Raman spectrum of the positive electrode active material layer, 2940 to 2990 cm －１ 1110 to 1160 cm for the peak top intensity A of the peak present in the range －１ When the ratio (peak top intensity ratio) B / A of the peak top intensity B of the peak present in the range is 3.8 or less, it can be said that the reaction between PVdF and lithium oxide is sufficiently suppressed and the amount of PVdF decomposition products is relatively small. Therefore, the binding property of the binder is maintained, resulting in a positive electrode with excellent binding property and high peel strength. The value of the peak top intensity ratio B / A is preferably 2.5 or less, more preferably 1.5 or less, and even more preferably 1.2 or less. Within the above range, the effects of the present invention can be obtained more significantly. The lower limit of the value of the peak top intensity ratio B / A is not particularly limited. The value of the peak top intensity ratio B / A is preferably as small as possible, and is preferably 0, but may be, for example, 0.1 or more. The value of the peak top intensity ratio B / A in the Raman spectrum of the positive electrode active material layer can be determined by the method described in the Examples.

[0031] The positive electrode active material layer may contain a binder (other binder) other than PVdF. Examples of binders other than PVdF include, but are not limited to, thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), and fluororesins, epoxy resins, and the like. However, the content of the other binders in the positive electrode active material layer (the total amount when two or more types are included) is less than the content of PVdF, and is, for example, less than 3 mass%, preferably less than 2 mass%, more preferably less than 1 mass%, and particularly preferably 0 mass%, relative to 100 mass% of the total solid content of the positive electrode active material layer when the battery is fully discharged.

[0032] (Conductive additive) The positive electrode active material layer preferably further contains a conductive additive as an additive component in addition to the above-described positive electrode material. The conductive additive has the function of forming an electron conduction path (conductive passage) in the positive electrode active material layer. When such an electron conduction path is formed in the positive electrode active material layer, the internal resistance of the battery can be reduced and the rate characteristics can be improved.

[0033] Examples of the conductive aid include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark), and fibrous carbon materials such as carbon nanotubes (single-walled carbon nanotubes, multi-walled carbon nanotubes, and multi-walled carbon nanotubes), carbon nanofibers, vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile-based carbon fibers, and pitch-based carbon fibers. One type of conductive aid may be used alone, or two or more types may be used in combination.

[0034] The content of the conductive additive that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 1.0 to 20 mass %, and more preferably 5 to 15 mass %, relative to 100 mass % of the total solid content of the positive electrode active material layer when the battery is fully discharged.

[0035] While there are no particular limitations on the form of the components (lithium oxide, catalyst, PVdF, and conductive additive) contained in the positive electrode active material layer, it is preferable that these components be present so that the lithium oxide and catalyst involved in the battery reaction are in contact with each other. Among these, it is more preferable that the lithium oxide and catalyst are in the form of composite particles coated together with PVdF (particularly, in the form of composite particles in which the lithium oxide-catalyst composite is coated with PVdF). Here, "in the form of composite particles" refers to a state in which, when a charge / discharge reaction is carried out in a closed-type lithium-oxygen battery, the particles containing the lithium oxide and the catalyst do not collapse and maintain their particle shape even if the lithium oxide expands and contracts during charge / discharge. By adopting such a configuration, when the lithium oxide is used as the positive electrode active material, the charge / discharge reaction proceeds more smoothly, which can effectively contribute to further improving the capacity characteristics of the battery.

[0036] The thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be appropriately referenced. For example, the thickness of the positive electrode active material layer is usually about 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, and even more preferably 40 to 200 μm. The thicker the positive electrode active material layer, the more positive electrode active material can be retained to achieve sufficient capacity (energy density). On the other hand, the thinner the positive electrode active material layer, the more the discharge rate characteristics can be improved.

[0037] [Method for Manufacturing Positive Electrode] A positive electrode for a closed-type lithium-oxygen battery according to one embodiment of the present invention can be manufactured by any manufacturing method that allows a positive electrode active material layer disposed on the surface of a current collector to contain lithium oxide, a catalyst, and PVdF as a binder, the PVdF content in the positive electrode active material layer to be within a predetermined range, and that achieves a predetermined peak intensity ratio in the Raman spectrum of the positive electrode active material layer. One example of such a manufacturing method includes: preparing a coated positive electrode material by mixing lithium oxide, a catalyst, and PVdF in an amount of 60% by mass or less relative to the total input; mixing the coated positive electrode material with the remaining PVdF to prepare a positive electrode mixture; and disposing the positive electrode mixture on the surface of a current collector to form a positive electrode active material layer. This manufacturing method allows for efficient production of the positive electrode for a closed-type lithium-oxygen battery according to this embodiment. According to this production method, a coated cathode material is obtained in which lithium oxide and a catalyst are pre-coated with PVdF (pre-added PVdF) in an amount of 60 mass% or less relative to the total amount of PVdF added, thereby allowing the surface of the lithium oxide to be coated with PVdF. When obtaining a coated cathode material, it is preferable to first dry-mix the lithium oxide and the catalyst using a mechanical mixing method such as a mechanochemical method to form a lithium oxide-catalyst composite. When obtaining a coated cathode material by mixing a lithium oxide-catalyst composite with PVdF, it is preferable to add PVdF dissolved or dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) and mix the materials together. After mixing, some or all of the solvent may be volatilized away, and it is preferable to volatilize away all of the solvent.

[0038] The coated cathode material obtained as described above is then mixed with the remaining PVdF (later-added PVdF) to obtain a cathode mixture, which is then placed on the surface of a current collector to form a cathode active material layer. By employing such a method, it is possible to suppress decomposition of PVdF and improve the binding strength of the entire cathode active material layer.

[0039] Lithium oxide is a Lewis base and can react with PVdF by donating an electron pair. Therefore, in an electrode using PVdF as a binder, when lithium oxide is used as the positive electrode active material, decomposition of PVdF may occur due to reaction with lithium oxide. As a result, the peel strength of the electrode may be reduced compared to when a positive electrode active material such as a lithium transition metal composite oxide is used. However, according to the above method, in the coated positive electrode material, the surface of the lithium oxide is coated with the first-added PVdF. Therefore, even if a portion of the first-added PVdF comes into contact with the surface of the lithium oxide and decomposes, the later-added PVdF does not come into contact with the lithium oxide, thereby suppressing the reaction between lithium and the later-added PVdF. Therefore, a higher proportion of PVdF does not decompose, resulting in a positive electrode with high peel strength.

[0040] The amount of pre-added PVdF is preferably more than 0% by mass and not more than 60% by mass, more preferably 10 to 60% by mass, and even more preferably 10 to 30% by mass, relative to the total amount of PVdF added. When the amount is within this range, the positive electrode of this embodiment can be easily obtained. Furthermore, the peel strength can be further improved.

[0041] To dispose the positive electrode mixture on the surface of the current collector, typically, a positive electrode mixture in the form of a solution or slurry containing a solvent (positive electrode active material slurry) is applied to the surface of the current collector, and the solvent is then volatilized away. In the positive electrode active material layer obtained in this manner, its components are in the form of composite particles. Furthermore, the later-added PVdF is preferably added in a dissolved or dispersed state in a solvent such as N-methyl-2-pyrrolidone (NMP). Furthermore, it is preferable to add and mix a conductive additive to the coated positive electrode material together with, or before or after, the later-added PVdF (solution). Adding the conductive additive at this stage enables sufficient formation of an electron conduction path in the positive electrode active material layer.

[0042] [Negative Electrode Active Material Layer] (Negative Electrode Active Material) The negative electrode active material layer contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of metal oxides include Nb ２ O ５ , Li ４ Ti ５ O １２ and the like. Furthermore, silicon-based negative electrode active materials and tin-based negative electrode active materials may be used. Here, silicon and tin belong to Group 14 elements and are known to be negative electrode active materials that can greatly improve the capacity of secondary batteries. These simple substances can absorb and release a large number of charge carriers (lithium ions, etc.) per unit volume (mass), and therefore become high-capacity negative electrode active materials. Here, it is preferable to use Si simple substance as the silicon-based negative electrode active material. Similarly, SiO disproportionated into two phases, a Si phase and a silicon oxide phase, is also used. ｘIt is also preferable to use silicon oxides such as (0.3≦x≦1.6). In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, an alloy containing silicon (silicon-containing alloy-based negative electrode active material) may be used. On the other hand, examples of negative electrode active materials containing tin (tin-based negative electrode active materials) include simple Sn, tin alloys (Cu—Sn alloys, Co—Sn alloys), amorphous tin oxides, tin silicon oxides, etc. Among these, examples of amorphous tin oxides include SnB ０．４ P ０．６ O ３．１ Examples of tin silicon oxide include SnSiO ３ Examples include: a lithium-containing metal; and a lithium-containing alloy may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include, but are not limited to, alloys of Li with at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those described above may also be used. The negative electrode active material preferably includes lithium metal, a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably includes lithium metal or a lithium-containing alloy. When the negative electrode active material includes lithium metal or a lithium-containing alloy, the lithium-oxygen battery according to this embodiment may be a so-called lithium deposition type battery in which lithium metal or a lithium-containing alloy is deposited on the negative electrode current collector during charging. In this case, a layer of lithium metal or a lithium-containing alloy deposited on the negative electrode current collector during charging serves as the negative electrode active material layer of the lithium-oxygen battery according to this embodiment. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer need not be present during full discharge, but in some cases, a certain amount of the negative electrode active material layer made of lithium metal or a lithium-containing alloy may be present during full discharge.

[0043] The shape of the negative electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is particulate, the average particle diameter is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm.

[0044] The content of the negative electrode active material in the negative electrode active material layer is, for example, 60% by mass or more and less than 100% by mass, preferably 80% by mass or more and 99.5% by mass or less, more preferably more than 95% by mass and 99.0% by mass or less, and even more preferably 97% by mass or more and 98.5% by mass or less, relative to 100% by mass of the total solid content. When the content of the negative electrode active material is within the above range, both battery capacity and output characteristics can be achieved.

[0045] Furthermore, the negative electrode active material layer may further contain other additives such as a conductive aid and a binder, as described above for the positive electrode active material layer, if necessary.

[0046] The thickness of the negative electrode active material layer (in the case of a lithium deposition type lithium oxygen battery, the thickness when fully charged) differs depending on the intended configuration of the stacked battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0047] [Electrolyte Layer] The electrolyte layer contains an electrolytic solution (liquid electrolyte) and preferably has a configuration in which a separator is impregnated with the electrolytic solution.

[0048] (Electrolyte) The electrolyte functions as a carrier of lithium ions. The electrolyte has a form in which a lithium salt is dissolved in a non-aqueous solvent. Preferably, the electrolyte is obtained by further adding a fluorine-containing carbonate to the non-aqueous solvent in which the lithium salt is dissolved.

[0049] The non-aqueous solvent is preferably one that can easily dissolve lithium salts, and examples thereof include chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and methyl ethyl carbonate (MEC); fluorine-containing chain carbonates in which some of the hydrogen atoms of these chain carbonates have been substituted with fluorine atoms; ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (MC). fluorine-containing cyclic carbonates in which some of the hydrogen atoms of these cyclic carbonates have been substituted with fluorine atoms; methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL).

[0050] In particular, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the non-aqueous solvent preferably contains a chain carbonate, and more preferably contains at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0051] The lithium salt is Li(FSO ２ ) ２ N(lithium bis(fluorosulfonyl)imide; LiFSI), Li(C ２ F ５ SO ２ ) ２ N, LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiCF ３ SO ３ etc.

[0052] The concentration of the lithium salt in the non-aqueous solvent is preferably 0.1 to 3.0 mol / L, and more preferably 0.8 to 2.2 mol / L.

[0053] In addition, it is preferable that the electrolyte solution further contains a fluorine-containing carbonate such as a fluorine-containing cyclic carbonate or a fluorine-containing linear carbonate. This allows the battery to have excellent durability even when operated at high voltage. Furthermore, these fluorine-containing carbonates form a protective film on the surface of the positive electrode active material, thereby enhancing the voltage resistance of the positive electrode active material. In this case, preferred fluorine-containing carbonates include fluorine-containing cyclic carbonates such as fluoroethylene carbonate (FEC), difluoroethylene carbonate, and 4-fluoropropylene carbonate; and fluorine-containing linear carbonates such as ethyl trifluoromethyl carbonate, 2,2,2-trifluoroethyl methyl carbonate, and bis(2,2,2-trifluoroethyl)carbonate. The content of the fluorine-containing carbonate is not particularly limited. In a preferred embodiment, the electrolyte solution contains 0.5 to 10% by mass of a fluorine-containing carbonate, particularly fluoroethylene carbonate, based on the total amount of the finally obtained electrolyte solution. This allows the above-mentioned effects to be obtained more significantly. When the electrolytic solution contains two or more kinds of fluorine-containing carbonates, the total amount thereof is preferably within the above range.

[0054] The electrolyte may further contain additives other than the above-mentioned components. Specific examples of such compounds include vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate, 1-methyl-1-vinyl ethylene carbonate, 1-methyl-2-vinyl ethylene carbonate, 1-ethyl-1-vinyl ethylene carbonate, and 1-ethyl-2-vinyl ethylene carbonate. Examples of the additive include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte solution, the amount used can be adjusted as appropriate.

[0055] (Separator) The separator constituting the electrolyte layer has the function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and the function of acting as a partition wall between the positive electrode and the negative electrode. Examples of the separator include a porous sheet separator made of a polymer or fiber that absorbs and retains the electrolyte solution, and a nonwoven fabric separator. The thickness of the separator may be the same as that of the electrolyte layer, and is preferably 5 to 200 μm, and particularly preferably 10 to 100 μm.

[0056] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for lithium ion secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 may be made of the same material or different materials.

[0057] Since the positive electrode for a closed-type lithium-oxygen battery according to the present embodiment has excellent peel strength, a closed-type lithium-oxygen battery using the same can exhibit excellent battery performance. Therefore, a closed-type lithium-oxygen battery using the positive electrode according to the present embodiment can be suitably used as a driving power source for EVs and HEVs.

[0058] The above describes one embodiment of the positive electrode (closed-type lithium-oxygen battery) according to one aspect of the present invention. However, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0059] The following embodiments are also included within the scope of the present invention: a positive electrode according to claim 1 having the features of claim 2; a positive electrode according to claim 1 or 2 having the features of claim 3; a positive electrode according to any one of claims 1 to 3 having the features of claim 4; a positive electrode according to any one of claims 1 to 4 having the features of claim 5; a closed-type lithium-oxygen battery comprising a positive electrode according to any one of claims 1 to 5; and a method for producing a positive electrode according to any one of claims 1 to 5 having the features of claim 7.

[0060] The present invention will be described in more detail below with reference to examples, although the technical scope of the present invention is not limited to the following examples.

[0061] Fabrication of Closed-Type Lithium-Oxygen Battery Example 1 Preparation of Lithium Oxide-Catalyst Composite Lithium oxide (Li) was used as the positive electrode active material (lithium oxide). ２O, manufactured by Kojundo Chemical Laboratory Co., Ltd.) and cobalt oxide (tricobalt tetroxide (Co ３ O ４ ), manufactured by Kojundo Chemical Laboratory Co., Ltd.) were placed in separate 70 mL pots for a planetary ball mill, and each was ground in a planetary ball mill (grinding conditions: treatment for 1 hour at 400 rpm using 40 g of 3 mmφ zirconia balls and 15 g of 15 mmφ zirconia balls).

[0062] Next, 5 g of the crushed lithium oxide and 15 g of cobalt oxide were placed in a 70 mL planetary ball mill pot and mixed in a planetary ball mill (mixing conditions: 40 g of 3 mmφ zirconia balls and 15 15 mmφ zirconia balls were used, and the mixture was treated at 400 rpm for 50 hours), thereby obtaining a lithium oxide-catalyst composite.

[0063] (Coating of Lithium Oxide-Catalyst Composite with Binder) 0.1 g of polyvinylidene fluoride (PVdF, manufactured by Sigma-Aldrich Co. LLC, weight-average molecular weight 880,000) powder as a binder and 11 g of N-methyl-2-pyrrolidone (NMP, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a solvent were kneaded (kneading conditions: 2000 rpm for 5 minutes) using a planetary stirring mixer / kneader "Awatori Rentaro" (ARE-310, manufactured by Thinky Corporation) to completely dissolve the PVdF in the NMP. 4 g of the lithium oxide-catalyst composite obtained above was added to the NMP solution of PVdF thus obtained, and further kneading was performed using the mixer / kneader (kneading conditions: 2000 rpm for 5 minutes) to coat the surfaces of the lithium oxide-catalyst composite particles with PVdF (pre-added PVdF). The resulting mixture was transferred to a metal tray and placed on a hot plate heated to 120° C. to volatilize the NMP. The remaining powder was then collected and mixed by hand in an agate mortar for 5 minutes to obtain a coated positive electrode material.

[0064] (Preparation of Positive Electrode Active Material Slurry) 0.82 g of the coated positive electrode material obtained above was mixed with 0.1 g of acetylene black (AB) (Li-400, manufactured by Denka Co., Ltd., average primary particle size: 48 nm, aspect ratio: 1) as a conductive additive, and NMP was added to the mixture so that the solids concentration was 25% by mass. The mixture was further kneaded using the mixing and kneading device (kneading conditions: 2000 rpm for 2 minutes) to obtain a slurry. Thereafter, 1.14 g of a separately prepared NMP solution containing 7% by mass of PVdF (later-added PVdF) was added to the slurry, and the mixture was further kneaded using the mixing and kneading device (kneading conditions: 2000 rpm for 2 minutes) to obtain a positive electrode active material slurry. The ratio (mass ratio) of lithium oxide: cobalt oxide: acetylene black: PVdF in the positive electrode active material slurry was 20:60:10:10. The proportion of the first-added PVdF relative to the total amount of PVdF was 20 mass %, and the proportion of the last-added PVdF was 80 mass %.

[0065] (Preparation of Positive Electrode) The positive electrode active material slurry prepared above was uniformly applied to the aluminum foil placed on a smooth plate using a doctor blade so that the thickness of the positive electrode active material layer was 200 μm. After that, it was dried on a hot plate heated to 80 ° C for 30 minutes, and then transferred to a vacuum dryer and dried at 130 ° C for 8 hours under vacuum, thereby preparing a positive electrode of this example in which a positive electrode active material layer was formed on the surface of the aluminum foil.

[0066] [Example 2] In the above (coating of lithium oxide-catalyst composite with binder), the amount of PVdF powder was changed from 0.1 g to 0.2 g. In the above (preparation of positive electrode active material slurry), the amount of coated positive electrode material was changed from 0.82 g to 0.84 g. Furthermore, the amount of NMP solution containing 7% by mass of PVdF was changed from 1.14 g to 0.86 g. A positive electrode active material slurry was obtained in the same manner as in Example 1, except for the above. The ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF in the positive electrode active material slurry was 20:60:10:10. The proportion of the first-added PVdF relative to the total amount of PVdF was 40% by mass, and the proportion of the later-added PVdF was 60% by mass. A positive electrode of this example was prepared in the same manner as in the above (preparation of positive electrode), except for using the positive electrode active material slurry obtained in this manner.

[0067] [Example 3] In the above (coating of lithium oxide-catalyst composite with binder), the amount of PVdF powder was changed from 0.1 g to 0.3 g. In the above (preparation of positive electrode active material slurry), the amount of coated positive electrode material was changed from 0.82 g to 0.86 g. Furthermore, 1.14 g of an NMP solution containing 7% by mass of PVdF was changed to 0.57 g. A positive electrode active material slurry was obtained in the same manner as in Example 1, except for the above. The ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF in the positive electrode active material slurry was 20:60:10:10. The proportion of the first-added PVdF relative to the total amount of PVdF was 60% by mass, and the proportion of the later-added PVdF was 40% by mass. A positive electrode of this example was prepared in the same manner as in the above (preparation of positive electrode), except for using the positive electrode active material slurry obtained in this manner.

[0068] Comparative Example 1 In Example 1, the coating of the lithium oxide-catalyst composite with a binder was not performed. In the preparation of the positive electrode active material slurry, 0.82 g of the coated positive electrode material was replaced with 0.8 g of the lithium oxide-catalyst composite, and 1.14 g of an NMP solution containing 7% by mass of PVdF was replaced with 1.43 g. A positive electrode active material slurry was obtained in the same manner as in Example 1, except for the above. The ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF in the positive electrode active material slurry was 20:60:10:10. The proportion of the first-added PVdF relative to the total amount of PVdF was 0% by mass, and the proportion of the later-added PVdF was 100% by mass. A positive electrode of this comparative example was prepared in the same manner as in the preparation of the positive electrode, except for the use of the positive electrode active material slurry obtained in this manner.

[0069] <<Measurement of Raman Spectrum of Positive Electrode Active Material Layer>> Microscopic Raman measurement was performed on the positive electrodes prepared in each Example and Comparative Example to obtain the Raman spectrum of the positive electrode active material layer. The measurement conditions were as follows: Apparatus: Renishaw Microscopic Raman Spectrometer inVia Qontor 532 / 633 Condition: Electrode Surface Measurement Number of Accumulations: 1 Laser Intensity: 1% Laser Wavelength: 532 nm Irradiation Time: 20 seconds Objective Lens Magnification: 50x Measurement Range: 100 to 3500 cm －１ .

[0070] In the positive electrodes of the examples and comparative examples, the Raman spectra obtained were －１ Peak A was observed around 1130 cm. This peak A is thought to be derived from PVdF. －１ Peak B was observed near the peak A. This peak B is thought to be derived from a decomposition product of PVdF. The background was subtracted from these peaks in the usual manner, and the peak top heights were determined to be the peak top intensities. The ratio of the peak top intensity B of peak B to the peak top intensity A of peak A (peak intensity ratio) B / A was then calculated. The results are shown in Table 1 below.

[0071] <<Measurement of Peel Strength>> The positive electrodes prepared in each Example and Comparative Example were cut to 2 cm × 10 cm, and the positive electrode active material layer portion of the positive electrode was attached to the base of a tensile strength tester (manufactured by Imada Co., Ltd., model: ZTA-5N) with double-sided tape (manufactured by Nitto Denko Corporation, 23 mm width), and the current collector side was attached to the tensile strength tester. The tensile strength was measured at a pulling rate of 100 mm / min, and this was taken as the peel strength. The results are shown in Table 1 below.

[0072]

[0073] The results shown in Table 1 indicate that the positive electrodes of Examples 1 to 3, which have a predetermined peak intensity ratio B / A in the Raman spectrum of the positive electrode active material layer, have high peel strength. In particular, the positive electrode of Example 1, which has a peak intensity ratio B / A of 1.2 or less, exhibits even better peel strength. On the other hand, the positive electrode of Comparative Example 1, which does not exhibit the predetermined peak intensity ratio B / A, did not achieve sufficient peel strength.

[0074] This application is based on International Patent Application PCT / JP2024 / 014217, filed on April 8, 2024, the disclosure of which is incorporated by reference in its entirety.

[0075] 10a: laminated lithium-oxygen battery; 11': positive electrode current collector; 12: negative electrode current collector; 13: positive electrode active material layer; 15: negative electrode active material layer; 17: electrolyte layer; 19: single cell layer; 21: power generating element; 25: positive electrode current collector (positive electrode tab); 27: negative electrode current collector (negative electrode tab); 29: laminate film.

Claims

1. A battery comprising: a current collector; and a positive electrode active material layer disposed on a surface of the current collector, the positive electrode active material layer including lithium oxide, a catalyst, and polyvinylidene fluoride as a binder, wherein the content of polyvinylidene fluoride in the positive electrode active material layer is 20 mass% or less relative to 100 mass% of the total solid content of the positive electrode active material layer when the battery is fully discharged; and a Raman spectrum of the positive electrode active material layer shows a peak at 2940 to 2990 cm －１ 1110 to 1160 cm for the peak top intensity A of the peak present in the range －１ a ratio B / A of the peak top intensities B of the peaks present in the range of 3.8 or less.

2. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein the content of polyvinylidene fluoride in the positive electrode active material layer is 10 mass % or less relative to 100 mass % of the total solids content of the positive electrode active material layer when the battery is fully discharged.

3. The positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, wherein the B / A ratio is 1.2 or less.

4. The lithium oxide is Li ２ O, LiO, Li ２ O ２ and LiO ２ 3. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, comprising one or more selected from the group consisting of:

5. The positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, wherein the catalyst contains a transition metal-containing oxide.

6. A closed-type lithium-oxygen battery comprising the positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2.

7. A method for producing a positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, comprising: preparing a coated positive electrode material by mixing the lithium oxide, the catalyst, and the polyvinylidene fluoride in an amount of 60 mass% or less based on the total amount input; preparing a positive electrode mixture by mixing the coated positive electrode material and the remainder of the polyvinylidene fluoride; and disposing the positive electrode mixture on the surface of the current collector to form the positive electrode active material layer.

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