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

Abstract

The present disclosure provides means with which it is possible to improve the discharge capacity of a closed-type lithium-oxygen battery. Disclosed is a positive electrode for a closed-type lithium-oxygen battery that has a positive electrode active material layer containing lithium oxide, a catalyst, a binder, and a carbon conductivity aid, the carbon conductivity aid having a BET specific surface area of 40-1000 m2 / g.

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] In the past, in Japanese Patent No. 6213998, the O ２This document discloses a lithium-oxygen battery (lithium-air battery) that generates a lithium-oxygen battery (lithium-air battery). This document attempts to achieve high capacity in lithium-air batteries, which are prone to problems caused by contamination with moisture and carbon dioxide from the atmosphere, by lowering the molar ratio (B / A) of alkali metal atoms (A) to transition metal atoms (B) in the electrode active material. This document also describes that it is preferable to add a conductive additive to an electrode material containing the above-mentioned electrode active material as an essential component in order to improve the conductivity of the electrode, and also describes electrodes formed using this electrode material.

[0006] As mentioned above, lithium-oxygen batteries have not yet achieved sufficient capacity characteristics, and further improvements are required.

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

[0008] One aspect of the present invention is a cathode active material layer including a lithium oxide, a catalyst, a binder, and a carbon conductive additive, wherein the carbon conductive additive has a BET specific surface area of ​​40 to 1000 m ２ / g.

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

[0010] One aspect of the present invention is a cathode active material layer including a lithium oxide, a catalyst, a binder, and a carbon conductive additive, wherein the carbon conductive additive has a BET specific surface area of ​​40 to 1000 m ２ / g. By using a positive electrode having such a configuration, the discharge capacity of a closed-type lithium-oxygen battery can be improved.

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

[0012] FIG. 1 is a cross-sectional view showing a schematic representation of a stacked (flat) closed-type lithium-oxygen battery (hereinafter also simply referred to as a "stacked-type lithium-oxygen battery") according to one embodiment of the present invention. In this specification, the term "closed-type lithium-oxygen battery" refers to a battery in which charge and discharge reactions proceed in a sealed cell (i.e., the battery is free from the exchange of molecular oxygen (O ２ This "closed-type lithium-oxygen battery" is disclosed in JP 2015-159098 A and 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] The positive electrode active material layer is a layer essentially containing lithium oxide, a catalyst, a binder, and a carbon conductive additive. The positive electrode active material layer is typically disposed on the surface of a positive electrode current collector as shown in Figure 1. However, if the positive electrode active material layer itself has a certain degree of conductivity, it may also constitute a positive electrode without using a positive electrode current collector.

[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 It is more preferable that the lithium oxide contains 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.

[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 substance can be used as a catalyst as long as it can exhibit this function. As an example, the catalyst preferably contains a transition metal or a compound containing a transition metal (a transition metal-containing compound). The transition metal-containing compound is preferably in the form of, for example, an oxide (including a composite oxide), sulfide, halide, nitride, carbide, or the like containing the transition metal. In particular, from the viewpoint of excellent catalytic activity, the catalyst preferably contains a transition metal or a transition metal-containing oxide, and more preferably contains a transition metal-containing oxide. When the catalyst contains a transition metal-containing compound (preferably, a transition metal-containing oxide), there are no particular limitations on the type of transition metal contained in the compound. The transition metal 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 the transition metal-containing compound 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 tricobalt tetroxide (Co ３ O４ ) is particularly preferred.

[0021] 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.

[0022] In the positive electrode active material layer, the lithium oxide and the catalyst may exist in the form of independent particles, or may exist as a composite of the lithium oxide and the catalyst (hereinafter also referred to as a "lithium oxide-catalyst composite"). Here, the "composite of the lithium oxide and the catalyst" refers to a state in which the lithium oxide particle and the catalyst particle are composited together, and the lithium oxide and the catalyst are mixed together within a single particle (the lithium oxide and the catalyst constitute a single particle). By forming such a composite, the reaction between the lithium oxide and oxygen can be further promoted. Note that such a composite state can be achieved, for example, by employing a manufacturing method in which the lithium oxide and the catalyst are dry-kneaded using a ball mill or the like.

[0023] (Binder) The binder binds the components contained in the positive electrode active material layer and maintains the shape of the active material layer even when the lithium oxide shrinks and expands during charging and discharging, thereby maintaining battery performance. The binder is not particularly limited, but examples thereof include the following materials: Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVdF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene Fluorine resins such as ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc., 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), 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.

[0024] 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 15 mass %, and more preferably 1 to 10 mass %, relative to 100 mass % of the solid content of the positive electrode active material layer when the battery is fully discharged.

[0025] (Carbon Conductive Aid) The conductive aid has a function of forming an electron conduction path (conductive passage) in the positive electrode active material layer. In the positive electrode for a closed-type lithium-oxygen battery according to this embodiment, a conductive aid having a BET specific surface area of ​​40 to 1000 m is used as the conductive aid. ２ This battery is characterized by the use of a carbon conductive additive having a specific surface area of ​​40 to 400 m. This configuration makes it possible to improve the discharge capacity of a closed-type lithium-oxygen battery. From the viewpoint of further improving the discharge capacity, the BET specific surface area of ​​the carbon conductive additive is 40 to 400 m. ２ / g, and 100 to 250m ２ / g is more preferable. While the mechanism by which the above-described configuration achieves such an effect is not completely clear, the following mechanism is presumed. Because the positive electrode of a lithium-oxygen battery has lower conductivity than the positive electrode of a conventional lithium-ion secondary battery, a carbon conductive additive is used to ensure conductivity in the positive electrode active material layer. Furthermore, the reaction between lithium oxide (positive electrode active material) and oxygen mainly proceeds in the portion of the lithium oxide that is in contact with the catalyst. Therefore, it is desirable to arrange the carbon conductive additive so that sufficient electrons can be transferred to and from such a portion. However, compared with the positive electrode active materials used in conventional lithium-ion secondary batteries, lithium oxide contracts and expands significantly during charge-discharge reactions. Therefore, it is difficult to maintain the contact point between the lithium oxide and the carbon conductive additive (hereinafter simply referred to as "contact point"). The BET specific surface area of ​​the carbon conductive additive is 40 m. ２If the BET specific surface area of ​​the carbon conductive additive is less than 1000 m / g, the number of contact points between the lithium oxide, the catalyst, and the carbon conductive additive will be reduced, which may reduce the utilization efficiency of the lithium oxide. ２ If the BET specific surface area is larger than 40 to 1000 m / g, the proportion of the binder adhering to the surface of the carbon conductive additive increases, which may reduce the strength of the positive electrode active material layer. This may result in loss of contact when contraction or expansion occurs, and may also reduce the utilization efficiency of lithium oxide. In the positive electrode according to this embodiment, the BET specific surface area of ​​the carbon conductive additive is set to 40 to 1000 m ２ / g, a sufficient number of contacts can be maintained even with the contraction and expansion of the lithium oxide. It is presumed that this improves the utilization efficiency of the lithium oxide and ultimately improves the discharge capacity of closed-type lithium-oxygen batteries. Note that this mechanism is based solely on speculation, and its accuracy does not affect the technical scope of the present invention. In this specification, the BET specific surface area value is the value measured in accordance with the "Method for measuring the specific surface area of ​​powder (solid) by gas adsorption" described in JIS Z8830:2013 (ISO 9277:2010). When two or more materials are used as the carbon conductive additive, the weighted average of the BET specific surface areas of each material, weighted by mass ratio, is taken as the BET specific surface area of ​​the carbon conductive additive. The BET specific surface area of ​​the carbon conductive additive may change (the value of the BET specific surface area may increase) when subjected to kneading by a mechanochemical method using a ball mill or the like, but the value can be maintained as long as the additive is not subjected to such kneading.

[0026] Examples of carbon conductive additives include non-fibrous carbons such as acetylene black, carbon black, channel black, thermal black, Ketjen Black (registered trademark), graphene, and graphite; and fibrous carbons 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. In this specification, whether a carbon conductive additive is fibrous carbon is determined by the aspect ratio (fiber length / fiber diameter) of the carbon conductive additive observed using an observation tool such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Carbon conductive additives with an aspect ratio of greater than 3 are considered fibrous carbon.

[0027] The carbon conductive additive preferably contains fibrous carbon. When fibrous carbon is contained, the average aspect ratio of the fibrous carbon is preferably 50 or more, more preferably 100 or more, and even more preferably 150 or more. This configuration facilitates ensuring electrical conductivity in the positive electrode active material layer (particularly, electrical conductivity between lithium oxide particles and lithium oxide-catalyst composite particles). Furthermore, this configuration reduces the proportion of binder adhering to the surface of the carbon conductive additive, improving the strength of the positive electrode active material layer, thereby maintaining a sufficient number of contact points even with the contraction and expansion of lithium oxide. As a result, the utilization efficiency of lithium oxide is improved, and ultimately, the discharge capacity of closed-type lithium-oxygen batteries can be improved. In this specification, the average aspect ratio of fibrous carbon refers to the arithmetic mean value of the aspect ratios of fibrous carbon observed within several to several tens of fields of view using an observation tool such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0028] The carbon conductive aid preferably contains both non-fibrous carbon and fibrous carbon as described above. That is, according to a preferred embodiment of the present invention, the carbon conductive aid contains fibrous carbon and non-fibrous carbon. In this case, as described above, the average aspect ratio of the fibrous carbon is preferably 50 or more, more preferably 100 or more, and even more preferably 150 or more. The non-fibrous carbon adheres to the surface of lithium oxide particles, thereby contributing to an increase in contact points with the lithium oxide. Therefore, by using both fibrous carbon and non-fibrous carbon, it is possible to further improve the discharge capacity of a closed-type lithium-oxygen battery.

[0029] The content of the carbon conductive additive in the positive electrode active material layer (the total amount when two or more types are included) 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. Note that the positive electrode active material layer may contain a conductive additive other than the carbon conductive additive (for example, a conductive additive made of a metal), but preferably does not contain one.

[0030] There is no particular limitation on the form of the solid contents (lithium oxide, catalyst, binder, and carbon conductive additive) contained in the positive electrode active material layer. From the viewpoint of further promoting the reaction of lithium oxide by the catalyst, the positive electrode active material layer preferably contains the above-mentioned composite of lithium oxide and catalyst. In addition, when the positive electrode active material layer contains such a composite, the surface area x [m ２ ] and the surface area y [m ２] more preferably satisfies 5≦x / y≦120, and even more preferably satisfies 5≦x / y≦50. By adopting such a configuration, the conductivity in the positive electrode active material layer is more easily ensured. Furthermore, by adopting such a configuration, the proportion of binder adhering to the surface of the carbon conductive additive can be reduced, the strength of the positive electrode active material layer can be improved, and a sufficient number of contacts can be maintained even with the contraction and expansion of lithium oxide. As a result, the utilization efficiency of lithium oxide is improved, and ultimately the discharge capacity of the closed-type lithium-oxygen battery can be further improved. In this specification, the surface area x [m ２ ] is the BET specific surface area [m ２ / g] by the mass [g] of the carbon conductive additive. Similarly, in this specification, the surface area y [m ２ ] is the BET specific surface area [m ２ / g] by the mass of the complex [g]. ２ The value of

[0045] can be controlled by adjusting the conditions when the lithium oxide and the catalyst are kneaded in a dry manner using a ball mill or the like.

[0031] 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.

[0032] There is no particular restriction on the porosity of the positive electrode active material layer, but from the viewpoint of excellent discharge capacity and energy density, it is preferably 25 to 40%, and more preferably 27 to 35%.

[0033] [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. 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.

[0034] The shape of the negative electrode active material may be, for example, particulate (non-fibrous, 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.

[0035] 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.

[0036] 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.

[0037] The thickness of the negative electrode active material layer (in the case of a lithium deposition type lithium oxygen battery, the thickness when fully charged) differs depending on the intended configuration of the stacked battery, but is preferably within the range of, for example, 0.1 to 1000 μm.

[0038] [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.

[0039] 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.

[0040] The non-aqueous solvent is preferably one that easily dissolves lithium salts, for example, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC) and other chain carbonates; ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and other cyclic carbonates; 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).

[0041] 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).

[0042] 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 ) ２ N (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.

[0043] From the viewpoint of achieving excellent discharge capacity and energy density, the concentration of the lithium salt in the non-aqueous solvent is, for example, 1 mol / L to 3.5 mol / L, preferably 1.2 mol / L to 3.5 mol / L, and more preferably 1.25 mol / L to 1.5 mol / L.

[0044] The electrolyte solution may further contain 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. 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 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 be within the above-mentioned range.

[0045] 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.

[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 an improved discharge capacity, and therefore the 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.

[0049] The positive electrode for a closed-type lithium-oxygen battery according to one embodiment of the present invention and the closed-type lithium-oxygen battery using the same have been described above. However, the present invention is not limited to the configurations described in the above-described embodiments, 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 claim 7 having the features of claim 8; and the closed-type lithium-oxygen battery according to claim 9, comprising the positive electrode according to any one of claims 1 to 8.

[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 Battery>> 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. The BET specific surface area described below was calculated by measuring nitrogen gas as the adsorbed gas by the static capacitance method in accordance with "Method for measuring the specific surface area of ​​powder (solid) by gas adsorption" described in JIS Z8830:2013 (ISO 9277:2010), and analyzing the result by the multipoint method. In addition, the BET specific surface area [m ２ / g] by the mass [g] of the carbon conductive additive contained in the positive electrode active material layer, and the resulting value is the surface area x [m ２ The BET specific surface area [m ２ / g] by the mass [g] of the lithium oxide-catalyst composite contained in the positive electrode active material layer, and the resulting value is the surface area y [m ２ ].

[0053] [Example 1] (Preparation of lithium oxide-catalyst composite) Tricobalt tetroxide (Co ３ O ４ 20 g of lithium ion-exchange powder (LiO, manufactured by Kojundo Chemical Laboratory Co., Ltd.) was placed in a 70 mL planetary ball mill pot and subjected to planetary ball mill pulverization (pulverization conditions: 12 zirconia balls with a diameter of 15 mm were used at a rotation speed of 400 rpm for 1 hour). ２ 5 g of the cobalt tetroxide (manufactured by Kojundo Chemical Laboratory Co., Ltd.) and 15 g of the ground tricobalt tetroxide were placed in a 70 mL planetary ball mill pot, and the pot was subjected to planetary ball milling (milling conditions: using 40 g of 3 mm diameter zirconia balls at a rotation speed of 400 rpm for 50 hours) to produce a lithium oxide-catalyst composite (BET specific surface area: 1.03 m ２ / g).

[0054] (Preparation of Positive Electrode Active Material Slurry) 1.6 g of the lithium oxide-catalyst composite prepared above and carbon nanotubes (CNT-1, BET specific surface area: 200 m), which are fibrous carbon, as a carbon conductive assistant were mixed. ２ / g, average aspect ratio: 909 (average fiber diameter: 11 nm, average fiber length: 10 μm), bulk density: 0.03 to 0.15 g / cm ３ ) and an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVdF) (0.2 g of PVdF) as a binder were kneaded (kneading conditions: 2000 rpm for 4 minutes) using a planetary stirring mixer / kneader "Awatori Rentaro" (ARE-310, manufactured by Thinky Corporation) to prepare a positive electrode active material slurry. The ratio (mass ratio) of lithium oxide:tricobalt tetroxide:carbon conductive additive:PVdF in the positive electrode active material slurry was 20:60:10:10.

[0055] (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 (basis weight 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 in vacuum at 130°C for 8 hours, thereby producing a positive electrode for a closed-type lithium-oxygen battery of this example in which a positive electrode active material layer was formed on the surface of the aluminum foil.

[0056] (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 mol / L was used.

[0057] [Example 2] In the above (preparation of positive electrode active material slurry), as the carbon conductive additive, fibrous carbon CNT-1 was replaced with fibrous carbon carbon nanotubes (CNT-2, BET specific surface area: 42 m ２ 0.2 g of a cathode active material slurry having a fiber diameter of 300 nm and a fiber length of 20 μm was used. A cathode active material slurry and a cathode were prepared in the same manner as in Example 1, except for the above. The cathode was used to prepare a closed-type lithium-oxygen battery (coin cell) of this example.

[0058] Example 3 In the above (preparation of positive electrode active material slurry), instead of fibrous carbon CNT-1, non-fibrous carbon graphene (graphene-1, manufactured by Sigma-Aldrich, graphene nanoplatelets, BET specific surface area: 500 m) was used as the carbon conductive additive. ２ / g, particle size: less than 2 μm, bulk density: 0.2 to 0.4 g / cm ３ Except for this, a positive electrode active material slurry and a positive electrode were prepared in the same manner as in Example 1, and the positive electrode was used to prepare a closed-type lithium-oxygen battery (coin cell) of this example.

[0059] Example 4 In the above (preparation of positive electrode active material slurry), as the carbon conductive additive, graphene (graphene-2, BET specific surface area: 951 m), which is non-fibrous carbon, was used instead of CNT-1, which is fibrous carbon. ２ / g, particle size: 12.1 μm, bulk density: 0.0015 to 0.0030 g / cm ３ Except for this, a positive electrode active material slurry and a positive electrode were prepared in the same manner as in Example 1, and the positive electrode was used to prepare a closed-type lithium-oxygen battery (coin cell) of this example.

[0060] [Example 5] In the above (preparation of positive electrode active material slurry), 0.1 g of carbon nanotube (CNT-1) which is fibrous carbon and 0.1 g of acetylene black (AB, manufactured by Denka Co., Ltd., Li-400, BET specific surface area: 39 m) which is non-fibrous carbon were used as the carbon conductive assistant. ２ / g, bulk density: 0.15 g / cm ３ The BET specific surface area of ​​the carbon conductive additive (a mixture of CNT-1 and AB) was 119.5 m ２ Other than this, the positive electrode active material slurry and the positive electrode were prepared in the same manner as in Example 1, and the positive electrode was used to prepare a closed-type lithium-oxygen battery (coin cell) of this example.

[0061] [Example 6] In the above (preparation of positive electrode active material slurry), 0.1 g of carbon nanotubes (CNT-2) which are fibrous carbon and 0.1 g of acetylene black (AB, manufactured by Denka Co., Ltd., Li-400, BET specific surface area: 39 m) which are non-fibrous carbon were used as the carbon conductive assistant. ２ / g, bulk density: 0.15 g / cm ３ The BET specific surface area of ​​the carbon conductive additive (a mixture of CNT-2 and AB) was 40.5 m ２ Other than this, the positive electrode active material slurry and the positive electrode were prepared in the same manner as in Example 1, and the positive electrode was used to prepare a closed-type lithium-oxygen battery (coin cell) of this example.

[0062] Comparative Example 1 In the above (preparation of positive electrode active material slurry), as the carbon conductive assistant, instead of the fibrous carbon CNT-1, a fibrous carbon carbon nanotube (CNT-3, VGCF, manufactured by Showa Denko K.K., BET specific surface area: 13 m) was used. ２ / g, average aspect ratio: 40 (average fiber diameter: 150 nm, average fiber length: 6 μm), bulk density: 0.04 g / cm ３ Except for this, a positive electrode active material slurry and a positive electrode were prepared in the same manner as in Example 1, and the positive electrode was used to prepare a closed-type lithium-oxygen battery (coin cell) of this comparative example.

[0063] Comparative Example 2 In the above (preparation of positive electrode active material slurry), 0.2 g of non-fibrous carbon acetylene black (AB) was used as the carbon conductive additive instead of the fibrous carbon CNT-1. Except for this, a positive electrode active material slurry and a positive electrode were prepared in the same manner as in Example 1, and a closed-type lithium-oxygen battery (coin cell) of this comparative example was fabricated using this positive electrode.

[0064] <Evaluation of Closed-Type Lithium-Oxygen Batteries (Measurement of Initial Discharge Capacity)> 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] (discharge capacity per gram of lithium oxide contained in the positive electrode active material layer) was measured. The results are shown in Table 1 below.

[0065] (Charge / Discharge Test Conditions) Charging / Discharging Tester: TOSCAT-3000, Model TYS-30TU10 (manufactured by Toyo Systems Co., Ltd.) Charging / Discharging Conditions: [Charging process] 0.02C, Upper limit voltage: 4.6V (CC) [Discharging process] 0.02C, Lower limit voltage: 1.8V (CC) Rest time between charging and discharging processes: 3 minutes.

[0066]

[0067] From the results shown in Table 1, according to the present invention, it is clear that the BET specific surface area is 40 to 1000 m ２ It can be seen that the discharge capacity of a closed-type lithium-oxygen battery can be improved by incorporating a carbon conductive additive having a carbon content of 0.1 wt. / g into the positive electrode active material layer.

[0068] In addition, when Examples 1 to 6 are compared, the BET specific surface area of ​​the carbon conductive additive is 40 to 400 m ２ / g, the discharge capacity is further improved. Comparison between Example 1 and Example 5, and comparison between Example 2 and Example 6 also reveals that the discharge capacity is further improved when the carbon conductive assistant contains fibrous carbon and non-fibrous carbon.

[0069] 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 cathode active material layer containing lithium oxide, a catalyst, a binder, and a carbon conductive additive, wherein the BET specific surface area of ​​the carbon conductive additive is 40 to 1000 m ２ / g.

2. The BET specific surface area of ​​the carbon conductive additive is 40 to 400 m ２ 2. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein the positive electrode has a SiO2 content of 0.1 wt % or less.

3. The positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, wherein the carbon conductive additive contains fibrous carbon, and the average aspect ratio of the fibrous carbon is 50 or more.

4. The positive electrode for a closed-type lithium-oxygen battery according to claim 1 or 2, wherein the carbon conductive additive comprises fibrous carbon and non-fibrous carbon.

5. The positive electrode active material layer contains a composite of the lithium oxide and the catalyst, and the surface area x [m ２ ] and the surface area y [m ２ 3. The positive electrode for a closed-type lithium-oxygen battery according to claim 1, wherein x / y satisfies 5≦x / y≦120.

6. 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:

7. 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.

8. The positive electrode for a closed-type lithium-oxygen battery according to claim 7, wherein the transition metal contained in the transition metal-containing oxide includes one or more selected from the group consisting of cobalt, manganese, iron, nickel, molybdenum, iridium, and rhodium.

9. 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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