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

Abstract

The present disclosure provides a means by which the amount of oxygen gas generated in a closed-type lithium-oxygen battery can be reduced even when the battery is charged to near the theoretical capacity of lithium oxide, which is the positive electrode active material. Disclosed is a positive electrode for a closed-type lithium-oxygen battery, the positive electrode comprising a current collector and a positive electrode active material layer disposed on the surface of the current collector and containing a lithium oxide, a catalyst, and a binder, wherein at least a portion of the lithium oxide is coated with a branched polymer having a branched structure.

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] JP 2015-159098 A discloses the above-mentioned closed-type lithium-oxygen battery. This document describes a closed-type lithium-oxygen battery that prevents atmospheric moisture and carbon dioxide from entering the cell while retaining lithium oxide (Li ２ O) and a catalyst containing a transition metal atom (Co ３ O４ They are attempting to achieve a high capacity positive electrode active material by pulverizing a raw material composition containing the above-mentioned compounds by mechanochemical treatment.

[0006] Here, in the closed-type lithium-oxygen battery disclosed in JP 2015-159098 A, the battery is not charged up to the theoretical capacity (897 mAh / g) of the lithium oxide, which is the positive electrode active material, but the capacity during charging is controlled to 270 mAh / g, which is about 30% of the theoretical capacity.

[0007] On the other hand, according to the study by the present inventors, when charging is performed up to a level close to the theoretical capacity of lithium oxide, the lithium oxide, which is the positive electrode active material, reacts with oxygen (O ２ It was found that the lithium ions in the battery are oxidized to lithium ions, generating a large amount of oxygen gas. This oxygen gas would normally be reduced to lithium oxide during the discharge process, so the generation of a large amount of oxygen gas means that the battery capacity will irreversibly decrease.

[0008] Therefore, an object of the present invention is to provide a means for reducing the amount of oxygen gas generated in a closed-type lithium-oxygen battery even when the battery is charged to near the theoretical capacity of the lithium oxide positive electrode active material.

[0009] One aspect of the present invention is a positive electrode for a closed-type lithium-oxygen 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 a binder, wherein at least a portion of the lithium oxide is coated with a branched polymer having a branched structure.

[0010] 1 is a cross-sectional view schematically illustrating a stacked (flat) closed-type lithium oxygen battery according to one embodiment of the present invention.

[0011] One aspect of the invention is a positive electrode for a closed-type lithium-oxygen battery, comprising a current collector and a positive electrode active material layer disposed on the surface of the current collector, the positive electrode active material layer including lithium oxide, a catalyst, and a binder, wherein at least a portion of the lithium oxide is coated with a branched polymer having a branched structure. By using a positive electrode having such a configuration, it is possible to reduce the amount of oxygen gas generated even when the closed-type lithium-oxygen battery is charged to near the theoretical capacity of the lithium oxide, which is the positive electrode active material.

[0012] 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.

[0013] 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 ２ This "closed-type lithium-oxygen battery" is disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2015-159098, as well as in a document such as "Wang, J. et al., Reversible Conversion between Lithium Superoxide and Lithium Peroxide: A Closed "Lithium-Oxygen" Battery. Inorganics 2023, 11, 69."

[0014] 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.

[0015] 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.

[0016] The main components of the stacked lithium-oxygen battery according to this embodiment will be described below.

[0017] [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.

[0018] 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.

[0019] [Positive Electrode Active Material Layer] In this embodiment, the positive electrode active material layer includes a lithium oxide, a catalyst, and a binder. At least a portion of the lithium oxide is coated with a branched polymer having a branched structure.

[0020] (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 ２ It 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 ２It 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.

[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., Co ３ O ４ ) is particularly preferred.

[0022] The content of the catalyst in the positive electrode active material layer 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 when the battery is fully discharged.

[0023] (Branched polymer) A branched polymer is a polymer having a branched structure, and preferably a polymer having a branched carbon chain structure in the main chain of the polymer. As long as this definition is met, the specific structure of the branched polymer is not particularly limited.

[0024] Examples of branched polymers include those containing one or more structures selected from the group consisting of polypropylene oxide (PPO), polyacrylonitrile (PAN), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), polymethyl methacrylate (PMMA), polymethyl acrylate (PMA), poly(vinyl ethyl ether), polystyrene (PS), and styrene-butadiene rubber (SBR). Also preferred are one or more polymers selected from the above group. These branched polymers may be further substituted with a substituent such as a halogen atom, an alkyl group, an alkenyl group, an alkynyl group, an alkoxy group, an acyl group, an alkoxycarbonyl group, a nitro group, an amino group, a sulfo group, an alkylsulfo group, a carboxy group, an alkylcarboxy group, or a hydroxy group.

[0025] In particular, from the viewpoint of excellent suppression of gas generation, it is more preferable that the branched polymer has a structural unit derived from hexafluoropropylene in the main chain (for example, vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HEP)). In a particularly preferred embodiment, the branched polymer includes polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HEP). In addition, PVdF-HFP has high swelling property in the electrolyte solution, which can improve the ionic conductivity of lithium ions and the like in the positive electrode active material layer. Furthermore, PVdF-HFP has high flexibility, so it can penetrate into the gaps of other components (positive electrode active material, catalyst, conductive additive, etc.) contained in the positive electrode active material layer, improving the dispersibility of these components. As a result, an electron conduction path is well formed, and excellent charge / discharge efficiency can be exhibited.

[0026] PVdF-HFP is not particularly limited as long as it is a copolymer of vinylidene fluoride and hexafluoropropylene. The lower limit of the proportion of hexafluoropropylene-derived structural units contained in PVdF-HFP is preferably 4 mol% or more, more preferably 6 mol% or more, and even more preferably 6.5 mol% or more, relative to the total structural units of PVdF-HFP. Furthermore, the lower limit of the proportion of hexafluoropropylene-derived structural units contained in PVdF-HFP is preferably 20 mol% or less, more preferably 10 mol% or less, and even more preferably 8 mol% or less. By having the proportion of hexafluoropropylene-derived structural units within the above range, swelling in an electrolyte solution and flexibility can be further improved. As a result, the effect of suppressing gas generation is excellent, the ionic conductivity and electronic conductivity in the electrode active material layer are improved, and excellent charge / discharge efficiency can be demonstrated.

[0027] In the positive electrode according to the present embodiment, the average molecular weight of the branched polymer is not particularly limited, and conventionally known knowledge can be appropriately referred to. For example, the weight average molecular weight (in terms of polystyrene) measured by gel permeation chromatography is 1,000 to 1,000,000, preferably 5,000 to 50,000, and more preferably 10,000 to 30,000.

[0028] The content of the branched polymer in the positive electrode active material layer is not particularly limited and depends on the types of lithium oxide, catalyst, and branched polymer. From the viewpoint of achieving a high effect of suppressing the amount of gas generation, the content of the branched polymer in the positive electrode active material layer (the total amount when two or more types are included) is preferably 0.5 to 10.0 mass%, more preferably 1.0 to 8.0 mass%, and even more preferably 2.0 to 6.0 mass%, relative to 100 mass% of the solids content of the positive electrode active material layer when the battery is fully discharged.

[0029] As described above, in the positive electrode active material layer of the positive electrode according to this embodiment, at least a portion of the lithium oxide is coated with a branched polymer having a branched structure. In the positive electrode active material layer, the branched polymer may coat only the lithium oxide, or may coat the lithium oxide together with other components (such as a catalyst or a conductive additive).

[0030] The coverage of the lithium oxide with the branched polymer is not particularly limited, but is preferably 50% or more. Here, "coverage" refers to the percentage of the surface area of ​​the lithium oxide particle that is covered with the branched polymer. From the viewpoint 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. The upper limit of the coverage is not particularly limited, but is usually 99% or less, preferably 95% or less. The "coverage" value can be calculated from an image of the lithium oxide in the positive electrode active material observed using a scanning electron microscope (SEM).

[0031] In the positive electrode for a closed-type lithium-oxygen battery according to this embodiment, the lithium oxide contained in the positive electrode active material layer is coated with a branched polymer, thereby sufficiently reducing the amount of gas generation even when the closed-type lithium-oxygen battery is charged to the theoretical capacity of the lithium oxide. As a result, irreversible reduction in battery capacity can be prevented. Although the mechanism behind this effect is not fully understood, the following mechanism is speculated. Specifically, by coating the lithium oxide with a branched polymer having polarity within its molecules, gases such as oxygen gas generated during battery charging are trapped in the gap between the lithium oxide and the branched polymer and are less likely to be released to the outside of the system. Therefore, according to Le Chatelier's principle, the equilibrium on the surface of the lithium oxide constantly shifts toward a direction that makes oxygen gas generation less likely. As a result, the amount of gas generation can be suppressed. In contrast, even if the lithium oxide is coated with a linear polymer without a branched structure, the gas trapping effect described above is hardly exhibited and the effect of suppressing gas generation cannot be sufficiently achieved. However, the above mechanism is merely speculative, and the correctness of this mechanism does not affect the technical scope of the present invention.

[0032] (Binder) The binder used in the positive electrode active material layer is not particularly limited, but preferably contains a polymer other than a branched polymer (a linear polymer having a linear structure), and is preferably a linear polymer. Examples of such linear polymers include polyvinylidene fluoride (PVdF), polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), and polyvinyl chloride (PVC). Among them, the linear polymer preferably contains polyvinylidene fluoride (PVdF).

[0033] The content of the binder in the positive electrode active material layer (the total amount when two or more types are included) is preferably 0.5 to 10.0 mass%, more preferably 1.0 to 8.0 mass%, and even more preferably 2.0 to 6.0 mass%, relative to 100 mass% of the solids content of the positive electrode active material layer when the battery is fully discharged. Furthermore, the content of the branched polymer relative to 100 mass% of the total of the binder and the branched polymer in the positive electrode active material layer is preferably 10 to 70 mass%, more preferably 30 to 70 mass%, and even more preferably 35 to 50 mass%. When this content is 10 mass% or more, the coverage can be sufficiently increased, and the effect of suppressing gas generation by capturing gas can be fully exerted. Furthermore, when this content is 70 mass% or less, sufficient electron conduction paths can be secured, and an increase in gas generation due to current concentration in the positive electrode active material layer can be prevented.

[0034] (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.

[0035] 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 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.

[0036] 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.

[0037] While there are no particular limitations on the form of the components (lithium oxide, catalyst, branched polymer, binder, 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. In particular, it is more preferable that the lithium oxide and catalyst are in the form of composite particles coated with a branched polymer. Here, "in the form of composite particles" means that when a charge / discharge reaction is carried out in a closed-type lithium-oxygen battery, the particles containing the lithium oxide and catalyst maintain their particle shape without collapsing, even if the lithium oxide expands and contracts during charge / discharge. This configuration allows the charge / discharge reaction to proceed more smoothly when the lithium oxide is used as a positive electrode active material, which can effectively contribute to further improving the capacity characteristics of the battery.

[0038] 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.

[0039] [Method for Manufacturing Positive Electrode] The positive electrode for a closed-type lithium-oxygen battery according to one embodiment of the present invention can be manufactured by any manufacturing method capable of realizing a state in which a positive electrode active material layer disposed on the surface of a current collector contains lithium oxide, a catalyst, and a binder, and at least a portion of the lithium oxide is coated with a branched polymer. One example of such a manufacturing method includes: preparing a composite electrode material by mixing lithium oxide, a catalyst, and a branched polymer; mixing the composite electrode material and a binder to prepare an electrode mixture; and disposing the electrode mixture on the surface of a current collector to form a positive electrode active material layer. This manufacturing method produces a composite electrode material in which lithium oxide and the catalyst are pre-coated with a branched polymer, thereby ensuring reliable coating of the surface of the lithium oxide with the branched polymer. Note that, when preparing the composite electrode 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 mixed powder. Furthermore, when a composite electrode material is obtained by mixing a lithium oxide, a catalyst, and a branched polymer, it is preferable to add the branched polymer in a state of being dissolved or dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) and mix it in. After mixing, some or all of the solvent may be removed by evaporation, and it is preferable to remove all of the solvent by evaporation.

[0040] The composite electrode material obtained as described above is then mixed with a binder to obtain an electrode mixture, which is then placed on the surface of a current collector to form a positive electrode active material layer. By employing such a method, it is possible to improve the overall adhesiveness of the positive electrode active material layer. To place the composite electrode material on the surface of a current collector, the composite electrode material is typically applied to the surface of the current collector in the form of a solution or slurry containing a solvent, and the solvent is then volatilized off. In the positive electrode active material layer obtained in this manner, its components are in the form of composite particles. The binder added at this time 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 electrode mixture together with the binder (solution), or before or after this. Adding the conductive additive at this stage makes it possible to sufficiently form an electron conduction path in the positive electrode active material layer.

[0041] [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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] The thickness of the negative electrode active material layer (in the case of a lithium deposition type secondary 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.

[0046] [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.

[0047] (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.

[0048] 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).

[0049] 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).

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] (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.

[0055] [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.

[0056] A closed-type lithium-oxygen battery using the positive electrode for a closed-type lithium-oxygen battery according to the present embodiment generates a sufficiently reduced amount of gas even when charged to the theoretical capacity of the lithium oxide, preventing irreversible reduction in battery capacity. Therefore, a closed-type lithium-oxygen battery using the positive electrode according to the present embodiment is suitable for use as a power source for driving EVs and HEVs.

[0057] 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.

[0058] 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 claim 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 positive electrode according to claim 5 having the features of claim 6; a positive electrode according to any one of claims 1 to 6 having the features of claim 7; a positive electrode according to any one of claims 1 to 7 having the features of claim 8; a positive electrode according to any one of claims 1 to 8 having the features of claim 9; a positive electrode according to claim 9 having the features of claim 10; a closed-type lithium-oxygen battery comprising the positive electrode according to any one of claims 1 to 10; and a method for producing a positive electrode according to any one of claims 1 to 10 having the features of claim 12.

[0059] 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.

[0060] <<Fabrication of Closed-Type Lithium-Oxygen Battery>> [Example 1] (Preparation of Positive Electrode Mixture) Lithium oxide (Li) was used as the positive electrode active material (lithium oxide). ２ O, manufactured by Kojundo Chemical Laboratory Co., Ltd.) and cobalt oxide (Co ３ O ４ , manufactured by Kojundo Chemical Laboratory Co., Ltd.) and another 70 mL pot for a planetary ball mill were placed in each pot and crushed in a planetary ball mill (crushing conditions: treatment for 1 hour at 400 rpm using 40 g of 3 mmφ zirconia balls and 15 g of 15 mmφ zirconia balls).

[0061] 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 using a planetary ball mill (mixing conditions: treatment for 1 hour at a rotation speed of 400 rpm using 40 g of 3 mmφ zirconia balls and 15 15 mmφ zirconia balls).

[0062] Separately, 0.0125 g of vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP, manufactured by Sigma-Aldrich Co. LLC) as a branched polymer 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 dissolve PVdF-HFP in NMP. The mixture of lithium oxide and cobalt oxide mixed in the ball mill described above was added to the NMP solution of PVdF-HFP obtained in this manner, and further kneading was performed using the mixer / kneader (kneading conditions: 2000 rpm for 5 minutes) to coat the surfaces of the lithium oxide particles with PVdF-HFP. The resulting mixture was transferred to a metal tray and placed on a hot plate heated to 120°C to volatilize the NMP, yielding a composite electrode material. The remaining composite electrode material was then collected and mixed by hand in an agate mortar for 5 minutes. An NMP solution of polyvinylidene fluoride (PVdF), a linear polymer, as a binder and acetylene black (AB) (Li-400, manufactured by Denka Co., Ltd., average primary particle size: 48 nm, aspect ratio: 1) as a conductive additive were then added, and the mixture was further mixed using the mixer / kneader (kneading conditions: 2000 rpm for 5 minutes) to prepare a positive electrode mixture. In the positive electrode mixture of this example, the mixing ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF-HFP:PVdF was 20:60:10:2:8 (the content of the branched polymer relative to the total of the binder and the branched polymer (100 mass%) was 20 mass%).

[0063] (Preparation of Positive Electrode) NMP was added to the positive electrode mixture prepared above so that the solid content concentration was 25% by mass, and the viscosity was adjusted to prepare a positive electrode slurry. The positive electrode slurry was uniformly applied to an 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. Then, the positive electrode was dried for 30 minutes on a hot plate heated to 80 ° C., and then transferred to a vacuum dryer and dried at 130 ° C. for 8 hours under vacuum to prepare a positive electrode having a positive electrode active material layer formed on the surface of the aluminum foil.

[0064] (Fabrication of a Closed-Type Lithium-Oxygen Battery (Laminated Cell)) The positive electrode obtained above was placed in a 12.4 cm ２ On the other hand, a laminate of lithium metal foil (thickness 200 μm) and a negative electrode current collector (copper foil) was cut into a rectangle of 13.02 cm, which is approximately similar to the positive electrode. ２ The laminate was cut into a rectangular shape. An aluminum terminal was welded to the positive electrode current collector (aluminum foil), and a nickel terminal was welded to the negative electrode current collector (copper foil). Next, a separator (Celgard® 3501, manufactured by Celgard, made of polypropylene (PP), 25 μm thick) was inserted into the electrode active material layer side of the positive and negative electrodes to form a laminate. This laminate was sandwiched between a heat-sealed aluminum laminate film (150 μm thick) that served as an exterior body, and 280 μL of electrolyte was injected. The interior of the exterior body was then depressurized to a vacuum using a vacuum sealer. The pressure was released once and returned to atmospheric pressure, and then the pressure was reduced again to 99.7% and sealed to produce a laminate cell (closed-type lithium-oxygen battery) having a power generation element in which the positive electrode and negative electrode were stacked so that they faced each other via the separator. The electrolyte solution is prepared by dissolving lithium hexafluorophosphate (LiPF ), a lithium salt, in an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed in a volume ratio of 3:7. ６ ) dissolved at a concentration of 1 mol / L was used.

[0065] Example 2 A laminate cell (closed-type lithium-oxygen battery) of this example was produced using the same method as in Example 1 described above, except that the mixing ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF-HFP:PVdF in the positive electrode mixture (positive electrode slurry) was 20:60:10:4:6 (the content of the branched polymer relative to the total of 100 mass% of the binder and the branched polymer was 40 mass%).

[0066] Example 3 A laminate cell (closed-type lithium-oxygen battery) of this example was produced using the same method as in Example 1 described above, except that the mixing ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF-HFP:PVdF in the positive electrode mixture (positive electrode slurry) was 20:60:10:6:4 (the content of the branched polymer relative to the total of 100 mass% of the binder and the branched polymer was 60 mass%).

[0067] Comparative Example 1 A laminate cell (closed-type lithium-oxygen battery) of this comparative example was produced using the same method as in Example 1 described above, except that the branched polymer (PVdF-HFP) was not added when preparing the positive electrode mixture (positive electrode slurry), and the mixing ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF-HFP:PVdF was 20:60:10:0:10.

[0068] Comparative Example 2 A laminate cell (closed-type lithium-oxygen battery) of this comparative example was produced using the same method as in Example 1 described above, except that a linear polymer (PVdF) was used instead of the branched polymer (PVdF-HFP) when preparing the positive electrode mixture (positive electrode slurry).

[0069] Comparative Example 3 A laminate cell (closed-type lithium-oxygen battery) of this comparative example was produced using the same method as in Example 2 described above, except that a linear polymer (PVdF) was used instead of the branched polymer (PVdF-HFP) when preparing the positive electrode mixture (positive electrode slurry).

[0070] Comparative Example 4 A laminate cell (closed-type lithium-oxygen battery) of this comparative example was produced using the same method as in Example 3 described above, except that a linear polymer (PVdF) was used instead of the branched polymer (PVdF-HFP) when preparing the positive electrode mixture (positive electrode slurry).

[0071] Evaluation of Closed-Type Lithium-Oxygen Batteries (Measurement of Gas Generation Amount During Charging) The following charging test (initial charging) was conducted on the laminated cells (closed-type lithium-oxygen batteries) prepared in the above Examples and Comparative Examples. The following charging test was conducted under conditions in which the battery was charged to the theoretical capacity of lithium oxide (897 mAh / g). The experiment was also conducted in a thermostatic chamber at 300 K (27°C).

[0072] (Charge test conditions) Charge / discharge tester: TOSCAT-3000, model TYS-30TU10 (manufactured by Toyo Systems Co., Ltd.) Charging conditions: [Charge process] 0.02 C (current density 18 mA / g), 1.8 V → 4.6 V (CCCV; 0.01 C cutoff) Here, the volume of the laminated cell before and after the above-mentioned charge test was measured using the Archimedes method. Then, the value obtained by subtracting the volume before the charge test from the volume after the charge test was used as the volume of the positive electrode active material (Li ２ The amount of gas generated per unit mass of the positive electrode active material (cc / g) was calculated by dividing the calculated amount by the mass of the positive electrode active material (0). The results are shown in Table 1 below. The gas generation amounts shown in Table 1 are relative values ​​when the value for Comparative Example 4 is set to 1.

[0073]

[0074] The results shown in Table 1 show that, according to the present invention, by using a configuration in which lithium oxide as a positive electrode active material is coated with a branched polymer, the amount of gas generated can be sufficiently reduced even when a closed-type lithium-oxygen battery is charged up to the theoretical capacity of the lithium oxide. In particular, a comparison of Examples 2 and 3 with Example 1 shows that a significant effect in reducing the amount of gas generated can be obtained when the content of the branched polymer is 30 to 70 mass% relative to 100 mass% of the total of the binder and branched polymer.

[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 positive electrode for a closed-type lithium-oxygen 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 a binder, wherein at least a portion of the lithium oxide is coated with a branched polymer having a branched structure.

2. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein the branched polymer comprises one or more structures selected from the group consisting of polypropylene oxide, polyacrylonitrile, vinylidene fluoride-hexafluoropropylene copolymer, polymethyl methacrylate, polymethyl acrylate, poly(vinyl ethyl ether), polystyrene, and styrene-butadiene rubber.

3. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein the branched polymer has a structural unit derived from hexafluoropropylene in the main chain.

4. The positive electrode for a closed-type lithium-oxygen battery according to claim 3, wherein the branched polymer comprises a vinylidene fluoride-hexafluoropropylene copolymer.

5. The positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, wherein the binder contains a linear polymer having a linear structure.

6. The positive electrode for a closed-type lithium-oxygen battery according to claim 5, wherein the binder comprises polyvinylidene fluoride.

7. The positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, wherein in the positive electrode active material layer, the content of the branched polymer is 30 to 70 mass % relative to 100 mass % in total of the binder and the branched polymer.

8. 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:

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

10. The positive electrode for a closed-type lithium-oxygen battery according to claim 9, wherein the transition metal contained in the transition metal-containing oxide includes one or more transition metals selected from the group consisting of cobalt, manganese, iron, nickel, molybdenum, iridium, and rhodium.

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

12. A method for producing a positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, comprising: mixing the lithium oxide, the catalyst, and the branched polymer to prepare a composite electrode material; mixing the composite electrode material and the binder to prepare an electrode mixture; and disposing the electrode mixture on the surface of the current collector to form the positive electrode active material layer.

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