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

Abstract

The present disclosure provides a means capable of improving the energy density of a closed-type lithium-oxygen battery. 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 that is disposed on the surface of the current collector and contains a catalyst, lithium oxide, and an electrolyte solution containing a solvent and a lithium salt. In the unit volume of the positive electrode active material layer, if the molar amount of lithium ions constituting the lithium salt contained in the electrolytic solution is x[mol], and the molar amount of lithium ions contained in the lithium oxide is y[mol], the value of x / y is 0.010-0.035.

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 moisture and carbon dioxide from the atmosphere 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] According to the investigations of the present inventors, it has been found that when a closed-type lithium-oxygen battery is fabricated using the technology disclosed in JP 2015-159098 A, it may not be possible to achieve a sufficient energy density.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a means for improving the energy density of a closed-type lithium-oxygen battery.

[0008] One embodiment 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 an electrolyte solution containing a solvent and a lithium salt, a lithium oxide, and a catalyst, wherein, per unit volume of the positive electrode active material layer, when the molar amount of lithium ions constituting the lithium salt contained in the electrolyte solution is x [mol] and the molar amount of lithium ions contained in the lithium oxide is y [mol], the value of x / y is 0.010 to 0.035.

[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 embodiment 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 an electrolyte solution containing a solvent and a lithium salt, a lithium oxide, and a catalyst, wherein, per unit volume of the positive electrode active material layer, when the molar amount of lithium ions constituting the lithium salt contained in the electrolyte is x [mol] and the molar amount of lithium ions contained in the lithium oxide is y [mol], the value of x / y is 0.010 to 0.035. Use of a positive electrode having such a configuration can improve the energy density of the closed-type lithium-oxygen battery.

[0011] The above-mentioned embodiments of the present invention will be described below 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 ２ This "closed-type lithium-oxygen battery" is disclosed in the above-mentioned JP 2015-159098 A, 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.

[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 electrolyte solution including a solvent and a lithium salt, 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. The electrolyte solution contained in the electrolyte layer (which usually has a separator) also permeates the pores of the positive electrode active material layer 13 and the negative electrode active material layer 15 .

[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 an electrolyte solution containing a solvent and a lithium salt, lithium oxide, and a catalyst.

[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 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 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] (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 preferably contains a compound containing a transition metal (transition metal-containing compound). This transition metal-containing compound is preferably in the form of, for example, an oxide (including composite oxide), sulfide, halide, nitride, carbide, or the like containing the transition metal. In particular, from the viewpoint of excellent catalytic activity, it is preferable that the catalyst contains a transition metal-containing oxide. When the catalyst contains a transition metal-containing compound, there are no particular restrictions on the type of transition metal contained in the compound, and it may be an atom of 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.

[0021] The content of the catalyst in the lithium-containing composition 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%, still 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.

[0022] (Electrolyte) The electrolyte contains a non-aqueous solvent and a lithium salt, and functions as a carrier of lithium ions. The electrolyte is usually in the form of a lithium salt dissolved in a non-aqueous solvent.

[0023] 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).

[0024] 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).

[0025] The lithium salt is Li(CF ３ SO ２ ) ２ N(lithium bis(trifluoromethylsulfonyl)imide), Li(FSO ２ ) ２ N(lithium bis(fluorosulfonyl)imide; LiFSI), Li(C ２ F ５ SO ２ ) ２N (lithium bispentafluoroethanesulfonylimide; LiBETI), LiPF ６ , LiBF ４ , LiAsF ６ , LiTaF ６ , LiClO ４ and LiCF ３ SO ３ In particular, from the viewpoint of achieving excellent discharge capacity and energy density, the electrolyte solution preferably contains two or more types of lithium salts, more preferably two types of lithium salts, and particularly preferably contains one each of a lithium salt formed from a sulfonylimide compound and another lithium salt.

[0026] From the viewpoint of achieving excellent discharge capacity and energy density, the concentration of the lithium salt in the non-aqueous solvent is preferably 1.2 [mol / L] to 3.5 [mol / L], and more preferably 1.25 [mol / L] to 1.5 [mol / L].

[0027] The electrolyte solution may further contain a fluorine-containing carbonate such as a fluorine-containing cyclic carbonate or a fluorine-containing chain carbonate. This allows the battery to have excellent durability even when operated at high voltage. 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 chain 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 mass% of a fluorine-containing carbonate, particularly fluoroethylene carbonate, based on the total amount of the finally obtained electrolyte solution. This makes it possible to obtain the above-mentioned effects more significantly. When the electrolyte solution contains two or more types of fluorine-containing carbonates, it is preferable that the total amount thereof is within the above-mentioned range.

[0028] 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. ester, 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, 1,1-dimethyl-2-methylene ethylene carbonate, etc. These additives may be used alone or in combination of two or more. In addition, when an additive is used in the electrolytic solution, the amount used can be appropriately adjusted.

[0029] (Binder) In addition to the above-mentioned positive electrode material, the positive electrode active material layer preferably further comprises a binder as an additive component.The binder is not particularly limited, but may be, for example, the following materials: polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVdF) (including compounds in which hydrogen atoms are replaced by other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, thermoplastic polymers such as styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated product, styrene-isoprene-styrene block copolymer and its hydrogenated product, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene Fluorine resins such as ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber (VdF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VdF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VdF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VdF-PFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VdF-HFP-TFE ...hexafluoropropylene-tetrafluoroethylene fluororubber (VdF-HFP-TFE fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VdF-PFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VdF-HFP-TFE fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (Vd Examples of the fluororubber include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VdF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VdF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VdF-CTFE-based fluororubber), and epoxy resins.Among these, polyvinylidene fluoride (PVdF), polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are preferable.

[0030] The content of the binder that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is preferably 0.5 to 10 mass %, and more preferably 1 to 5 mass %, relative to 100 mass % of the solid content of the positive electrode active material layer when the battery is fully discharged.

[0031] (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.

[0032] 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.

[0033] The content of the conductive additive that can be contained in the positive electrode active material layer (the total amount when two or more kinds 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.

[0034] While there are no particular limitations on the form of the solids (lithium oxide, catalyst, 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, catalyst, and conductive additive are in the form of composite particles integrated by the binder. 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, catalyst, and conductive additive 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 the positive electrode active material, which can effectively contribute to further improving the capacity characteristics of the battery.

[0035] 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.

[0036] The porosity of the positive electrode active material layer is not particularly limited, but from the viewpoint of excellent discharge capacity and energy density, it is preferably 25% to 40% and more preferably 27% to 35%. Note that the porosity value of the positive electrode active material layer is a value obtained by using the method described in the Examples section below.

[0037] (Characteristics of the Positive Electrode Active Material Layer) As described above, the pores of the positive electrode active material layer of the closed-type lithium-oxygen battery positive electrode according to this embodiment are permeated with the electrolyte solution contained in the electrolyte layer (which typically includes a separator). Here, one of the characteristics of the positive electrode according to this embodiment is that, per unit volume of the positive electrode active material layer, the value of x / y (a dimensionless quantity) is 0.010 to 0.035, where x [mol] is the molar amount of lithium ions constituting the lithium salt contained in the electrolyte solution permeating the pores of the positive electrode active material layer, and y [mol] is the molar amount of lithium ions contained in the lithium oxide contained as the positive electrode active material. By using a positive electrode having such a configuration, the energy density of the closed-type lithium-oxygen battery can be improved. In particular, from the viewpoint of improving energy density, the value of x / y is preferably 0.011 to 0.020, and more preferably 0.0115 to 0.0150.

[0038] Here, the value of x can be calculated, for example, by calculating the pore volume per unit volume from the thickness and porosity of the positive electrode active material layer, and then calculating this value and the concentration of lithium salt in the electrolyte. The value of y can be calculated, for example, by calculating the solid content volume per unit volume from the thickness and porosity of the positive electrode active material layer, and then calculating this value and the lithium concentration in the positive electrode active material. The lithium concentration in the positive electrode active material can be calculated, for example, from the density and molecular weight of the positive electrode active material, and the abundance ratio of lithium atoms to positive electrode active material molecules.

[0039] The mechanism by which the above-described configuration improves energy density is not fully understood. However, unlike conventional lithium-ion secondary batteries, closed-type lithium-oxygen batteries are thought to have a structure in which the reaction proceeds in the positive electrode active material layer at the site where the positive electrode active material is in contact with both lithium ions and a catalyst. In relation to this, it is speculated that a value of x / y within a predetermined range creates a favorable reaction field for the reaction in the positive electrode active material layer, thereby improving energy density. More specifically, if the value of x / y is less than 0.010, the positive electrode active material layer may have a localized shortage of electrolyte (lithium salt), creating areas where the lithium ion insertion reaction into the active material is difficult to proceed. This may result in a decrease in discharge capacity and an increase in overvoltage, resulting in a decrease in energy density. On the other hand, if the value of x / y exceeds 0.035, the positive electrode active material layer may have a localized excess of electrolyte (lithium salt), creating areas where the lithium ion desorption reaction from the active material is difficult to proceed. This may result in a decrease in charge capacity and accelerated decomposition of the electrolyte, resulting in a decrease in energy density. It should be noted that this mechanism is merely based on speculation, and whether it is correct or not does not affect the technical scope of the present invention.

[0040] [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 １２and 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. ｘ It 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 ３Examples 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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 configuration of the intended stacked battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0045] [Electrolyte Layer] The electrolyte layer contains an electrolytic solution (liquid electrolyte). The electrolyte layer preferably has a configuration in which a separator is impregnated with the electrolytic solution. The electrolytic solution contained in the electrolyte layer may have the same form as that described in the section on the positive electrode active material layer, including its preferred form, and therefore a detailed description thereof will be omitted here.

[0046] (Separator) When the electrolyte layer includes a separator, the separator has the function of retaining the electrolyte solution 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.

[0047] [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.

[0048] A closed-type lithium-oxygen battery using the positive electrode for a closed-type lithium-oxygen battery according to the present embodiment has improved energy density, and therefore is suitable for use as a power source for driving EVs and HEVs.

[0049] The above describes an embodiment of a positive electrode for a closed-type lithium-oxygen battery according to one aspect of the present invention. However, the present invention is not limited to the configurations described in the above-described embodiment, and can be modified as appropriate based on the claims.

[0050] The following embodiments are also included within the scope of the present invention: the positive electrode according to claim 1 having the features of claim 2; the positive electrode according to claim 1 or 2 having the features of claim 3; the positive electrode according to any one of claims 1 to 3 having the features of claim 4; the positive electrode according to any one of claims 1 to 4 having the features of claim 5; the positive electrode according to any one of claims 1 to 5 having the features of claim 6; the positive electrode according to any one of claims 1 to 6 having the features of claim 7; the positive electrode according to any one of claims 1 to 7 having the features of claim 8; the positive electrode according to claim 8 having the features of claim 9; and a closed-type lithium-oxygen battery comprising the positive electrode according to any one of claims 1 to 9.

[0051] 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.

[0052] <<Fabrication of Closed-Type Lithium-Oxygen Batteries>> Closed-type lithium-oxygen batteries in the form of coin cells were fabricated using the following method: The specifications of the positive electrode active material layer in each example and comparative example are shown in Table 1 below.

[0053] [Example 1] (Preparation of positive electrode material) Cobalt oxide (Co ３ O ４ 20 g of zirconia powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) was placed in a 70 mL planetary ball mill pot and crushed in the planetary ball mill (crushing conditions: 12 zirconia balls with 15 mm diameter were used at a rotation speed of 400 rpm for 1 hour).

[0054] In addition, lithium oxide (Li ２ 5 g of cobalt oxide (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 15 g of the cobalt oxide ground above were placed in a 70 mL planetary ball mill pot, and ground in the planetary ball mill (grinding conditions: using 40 g of zirconia balls with a diameter of 3 mm, at a rotation speed of 400 rpm for 50 hours).

[0055] Next, 2.5 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 an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVdF) (2.5 g of PVdF) as a binder were added to the mixture of lithium oxide and cobalt oxide mixed by the ball mill. The mixture was then kneaded using a planetary stirring mixer "Awatori Rentaro" (ARE-310, manufactured by Thinky Corporation) (kneading conditions: 2000 rpm for 4 minutes) to prepare a positive electrode active material slurry. In this example, the mixture ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF was 20:60:10:10.

[0056] (Preparation of Positive Electrode) The positive electrode active material slurry obtained above was uniformly applied onto aluminum foil placed on a smooth plate using a doctor blade (amount per unit area after drying: 5 mg / cm ２). Thereafter, it was dried for 30 minutes on a hot plate heated to 80°C, and then the obtained laminate was subjected to pressing using a roll press. Next, it was transferred to a vacuum dryer and dried under vacuum at 130°C for 8 hours, thereby producing a positive electrode of this example in which a positive electrode active material layer was formed on the surface of the aluminum foil. The thickness of the positive electrode active material layer was 22.6 [μm], and the porosity of the positive electrode active material layer was 32.6 [%]. In addition, the positive electrode active material (Li ２ The volume fraction of the positive electrode active material layer (x / y) was 22.4%. As a result, the value of x / y was calculated to be 0.0136. The thickness and porosity of the positive electrode active material layer were calculated using the following methods.

[0057] <Method for measuring porosity of positive electrode active material layer> (1) The mass per unit area of ​​the positive electrode active material layer was measured. Then, the mass of each material per unit area of ​​the positive electrode active material layer was calculated from the compounding ratio of the materials; (2) The thickness [A] of the positive electrode active material layer was measured using a micrometer; (3) The thickness [B] of the positive electrode active material layer when the porosity was 0% was calculated using the mass of each material and the density of each material calculated in (1); (4) The pore volume of the positive electrode active material layer was calculated from the difference (A-B) between the measured thickness and the calculated thickness of the positive electrode active material layer, and the pore volume per m of the positive electrode active material layer was calculated. ３ The pore volume per unit area was determined, and the percentage of the obtained value was taken as the porosity [%].

[0058] (Fabrication of Closed-Type Lithium-Oxygen Battery (Coin Cell)) The positive electrode and lithium counter electrode fabricated above were placed opposite each other, with two separators (polyolefin, thickness of each separator: 20 μm) placed between them. Next, the stack of the positive electrode, separator, and lithium counter electrode was placed on the bottom side of a coin cell (CR2032, material: stainless steel (SUS316)). Furthermore, a gasket was attached to maintain insulation between the electrodes, the electrolyte was injected using a syringe, a spring and spacers were stacked, and the upper sides of the coin cells were overlapped and crimped to seal, thereby fabricating the closed-type lithium-oxygen battery (coin cell) of this example. The electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF ), a lithium salt, in an organic solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of EC:DEC = 3:7 (volume ratio). ６ ) dissolved at a concentration of 1.25 mol / L was used.

[0059] Example 2: LiPF in electrolyte ６ A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 1 described above, except that the concentration of Li was changed to 1.5 mol / L. The thickness of the positive electrode active material layer was 23.3 μm, and the porosity of the positive electrode active material layer was 34.8%. The percentage of the positive electrode active material (Li ２ The volume fraction of SiO2 was 21.7%. As a result, the value of x / y was calculated to be 0.0180.

[0060] Example 3: LiPF in electrolyte ６ A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 1 described above, except that the concentration of Li was changed to 2.0 mol / L. The thickness of the positive electrode active material layer was 22.2 μm, and the porosity of the positive electrode active material layer was 31.6%. The percentage of the positive electrode active material (Li ２ The volume ratio of SiO2 was 22.7%. As a result, the value of x / y was calculated to be 0.0208.

[0061] [Example 4] The lithium salt contained in the electrolyte was 1.0 mol / L LiPF ６ A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 1, except that the cathode active material layer was changed to a mixture of 0.25 mol / L LiFSI (lithium bis(fluorosulfonyl)imide) and 0.25 mol / L LiFSI. The thickness of the cathode active material layer was 21.6 μm, and the porosity of the cathode active material layer was 29.5%. The cathode active material (LiFSI) in the cathode active material layer was 0.25 mol / L. ２ The volume fraction of SiO2 was 23.4%. As a result, the value of x / y was calculated to be 0.0118.

[0062] [Example 5] The lithium salt contained in the electrolyte was 1.0 mol / L LiPF ６ A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 1, except that the cathode active material layer was changed to a mixture of 0.5 mol / L LiFSI and 0.5 mol / L LiFSI. The thickness of the cathode active material layer was 21.1 μm, and the porosity of the cathode active material layer was 28.0%. The cathode active material (LiFSI) in the cathode active material layer was 0.5 mol / L. ２ The volume fraction of SiO2 was 23.9%. As a result, the value of x / y was calculated to be 0.0131.

[0063] [Example 6] A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 5, except that the number of presses when pressing the positive electrode active material layer using a roll press was changed. The thickness of the positive electrode active material layer was 21.8 [μm], and the porosity of the positive electrode active material layer was 30.2 [%]. In addition, the percentage of the positive electrode active material (Li ２ The volume fraction of SiO2 was 23.2%. As a result, the value of x / y was calculated to be 0.0146.

[0064] [Example 7] The lithium salt contained in the electrolyte was 1.0 mol / L LiPF ６A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 1, except that the cathode active material layer was changed to a mixture of 1.0 mol / L LiFSI and 1.0 mol / L LiFSI. The thickness of the cathode active material layer was 21.9 μm, and the porosity of the cathode active material layer was 30.7%. The cathode active material (LiFSI) in the cathode active material layer was 0.01 mol / L. ２ The volume ratio of SiO2 was 23.0%. As a result, the value of x / y was calculated to be 0.0199.

[0065] [Example 8] A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 1, except that the lithium salt contained in the electrolyte was changed to 3.0 mol / L LiFSI. The thickness of the positive electrode active material layer was 22.0 [μm], and the porosity of the positive electrode active material layer was 30.8 [%]. The percentage of the positive electrode active material (Li ２ The volume ratio of SiO2 was 23.0%. As a result, the value of x / y was calculated to be 0.0300.

[0066] Comparative Example 1 The lithium salt contained in the electrolyte was 1.0 mol / L LiPF ６ A coin cell (closed-type lithium-oxygen battery) of this example was fabricated using the same method as in Example 1 described above, except that the thickness of the positive electrode active material layer was 21.9 μm, and the porosity of the positive electrode active material layer was 30.5%. In addition, the positive electrode active material (Li ２ The volume fraction of SiO2 was 23.1%. As a result, the value of x / y was calculated to be 0.0099.

[0067] [Comparative Example 2] A coin cell (closed-type lithium-oxygen battery) of this example was produced using the same method as in Example 1 described above, except that the lithium salt contained in the electrolyte was changed to 4.0 mol / L LiFSI (lithium bis(fluorosulfonyl)imide). The thickness of the positive electrode active material layer was 22.0 [μm], and the porosity of the positive electrode active material layer was 31.0 [%]. In addition, the percentage of the positive electrode active material (Li ２The volume ratio of SiO2 was 23.0%. As a result, the value of x / y was calculated to be 0.0402.

[0068]

[0069] Evaluation of Closed-Type Lithium-Oxygen Batteries (Measurement of Gas Generation Amount During Charging) The coin cells (closed-type lithium-oxygen batteries) prepared in the above Examples and Comparative Examples were subjected to the following charge-discharge test (initial charge-discharge) in a thermostatic chamber set at 298 K (25°C), and the discharge capacity [mAh / g] and average discharge voltage [V] were measured. These values ​​were then multiplied to calculate the weight energy density [mWh / g] of the coin cell. The results are shown in Table 2 below.

[0070] (Charge / Discharge Test Conditions) Charging / Discharging Tester: TOSCAT-3000, Model TYS-30TU10 (manufactured by Toyo Systems Co., Ltd.) Charging / Discharging Conditions: [Charging process] 0.02 C (current density 19.7 mA / g), upper limit voltage 4.6 V (CC) [Discharging process] 0.02 C (current density 19.7 mA / g), lower limit voltage 1.8 V (CC) Rest time between charging and discharging processes: 3 minutes.

[0071]

[0072] The results shown in Tables 1 and 2 show that, according to the present invention, the energy density of a closed-type lithium-oxygen battery can be improved by controlling the relationship between the molar amount (x [mol]) of lithium ions constituting the lithium salt contained in the electrolyte and the molar amount (y [mol]) of lithium ions contained in the lithium oxide within a predetermined range per unit volume of the positive electrode active material layer of the closed-type lithium-oxygen battery.

[0073] 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

A current collector; a positive electrode active material layer disposed on a surface of the current collector, the positive electrode active material layer including an electrolyte solution containing a solvent and a lithium salt, a lithium oxide, and a catalyst; and a positive electrode for a closed-type lithium-oxygen battery, wherein, in a unit volume of the positive electrode active material layer, a molar amount of lithium ions constituting the lithium salt contained in the electrolytic solution is defined as x [mol], and a molar amount of lithium ions contained in the lithium oxide is defined as y [mol], and a value of x / y is 0.010 to 0.

035.

2. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein the concentration of lithium ions constituting the lithium salt contained in the electrolyte is 1.2 [mol / L] to 3.5 [mol / L].

3. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein the electrolyte solution contains two or more kinds of lithium salts.

4. The positive electrode for a closed-type lithium-oxygen battery according to claim 3, wherein the value of x / y is 0.011 to 0.

020. The lithium salt contained in the electrolyte solution is Li(CF ３ SO ２ ) ２ N, Li(FSO ２ ) ２ N, Li(C ２ F ５ SO ２ ) ２ N, LiPF ６ , LiBF ４ , LiAsF ６ , LiTaF ６ , LiClO ４ and LiCF ３ SO ３ 4. The positive electrode for a closed-type lithium-oxygen battery according to claim 3, comprising one or more selected from the group consisting of:

3. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein the positive electrode active material layer has a porosity of 25% to 40%. 　 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:

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

9. The positive electrode for a closed-type lithium-oxygen battery according to claim 8, wherein the transition metal contained in the transition metal-containing oxide comprises one or more selected from the group consisting of cobalt, manganese, iron, nickel, molybdenum, iridium, and rhodium. 　 A closed-type lithium-oxygen battery comprising the positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2.

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