WO2025215390A1 - Closed-type lithium-oxygen battery - Google Patents

Abstract

The present disclosure provides a means by which the charge/discharge efficiency of a closed-type lithium-oxygen battery can be improved. Provided is a battery for a closed-type lithium-oxygen battery, said battery having: electrodes in which an electrode active material layer containing lithium oxide, a catalyst, and a gel-forming polymer is disposed on the surface of a current collector; and an electrolyte layer in which an electrolyte solution is impregnated in a separator disposed adjacent to the electrodes, wherein the porosity x [%] of the electrode active material layer, the ratio y [%] of the volume of the gel-forming polymer to the volume of the electrode active material layer, and the liquid absorption ratio z [%] of the gel-forming polymer with respect to the electrolyte solution satisfy 0 < yz / x ≤ 8.7.

Description

Closed-type lithium-oxygen battery

The present invention relates to a closed-type lithium-oxygen battery.

In recent years, the widespread adoption of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being actively developed as on-board power sources for driving motors and other applications, which hold the key to the widespread adoption of these electric vehicles. Attention is being focused on non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, which are expected to offer high energy density and high output.

Lithium-oxygen batteries (lithium-oxygen secondary batteries) are known as a 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 an energy density significantly exceeding that of current lithium-ion secondary batteries. However, sufficient capacity characteristics have yet to be achieved, and further improvements are currently required.

Lithium-oxygen batteries are broadly divided into two types: those that use gaseous molecular oxygen ( _O2 ) as the oxygen source, consume _O2 during discharge, and generate _O2 during charge (lithium-air batteries), and those that do not consume or generate _O2 (narrowly defined lithium-oxygen batteries). Because the latter do not exchange _O2 with the atmosphere, they can be constructed as sealed cells and can be called "closed-type lithium-oxygen batteries."

Japanese Patent Laid-Open Publication No. 2015-159098 discloses the above-mentioned closed-type lithium-oxygen battery. This document attempts to achieve a high capacity of the electrode active material by mechanochemically pulverizing a raw material composition containing lithium oxide ( _Li2O ) as an electrode active material and a transition metal atom-containing oxide ( _Co3O4 , _etc. ) as a catalyst, while preventing atmospheric moisture, carbon dioxide, and the like from entering the cell as a closed-type battery.

The inventors' investigations have revealed that when closed-type lithium-oxygen batteries are fabricated using the technology disclosed in JP 2015-159098 A, it may not be possible to achieve sufficient charge/discharge efficiency (the ratio of discharge capacity to charge capacity).

The present invention therefore aims to provide a means for improving the charge/discharge efficiency of closed-type lithium-oxygen batteries.

One aspect of the present invention relates to a closed-type lithium-oxygen battery having an electrode in which an electrode active material layer containing lithium oxide, a catalyst, and a gel-forming polymer is disposed on the surface of a current collector, and an electrolyte layer in which an electrolytic solution is impregnated in a separator disposed adjacent to the electrode. The battery is characterized in that, when the porosity of the electrode active material layer is x [%], the volume ratio of the gel-forming polymer to the volume of the electrode active material layer is y [%], and the liquid absorption rate of the gel-forming polymer with respect to the electrolytic solution is z [%], the relationship of the following formula (1) is satisfied:
0<yz/x≦8.7 Formula (1)

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

One embodiment of the present invention is a battery for a closed-type lithium-oxygen battery, comprising: an electrode in which an electrode active material layer containing a lithium oxide, a catalyst, and a gel-forming polymer is disposed on the surface of a current collector; and an electrolyte layer in which an electrolytic solution is impregnated in a separator disposed adjacent to the electrode, wherein the porosity of the electrode active material layer is x [%], the ratio of the volume of the gel-forming polymer to the volume of the electrode active material layer is y [%], and the liquid absorption rate of the gel-forming polymer with respect to the electrolytic solution is z [%], the relationship of the following formula (1) is satisfied:
0<yz/x≦8.7 Formula (1)
In the course of searching for a means for improving the charge-discharge efficiency of a closed-type lithium-oxygen battery, the inventors discovered that incorporating a gel-forming polymer in addition to lithium oxide (Li ₂ O) and a catalyst into the electrode active material layer and controlling the above parameters (x, y, z) to have a predetermined relationship is effective in improving the charge-discharge efficiency.

The following describes the above-mentioned embodiments of the present invention with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings have been exaggerated for the sake of explanation and may differ from the actual proportions. In this specification, the range "X-Y" means "X or greater and Y or less." Furthermore, unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20-25°C) and a relative humidity of 40-50% RH.

1 is a cross-sectional view schematically illustrating a stacked (flat) closed-type lithium-oxygen battery (hereinafter 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 lithium-oxygen battery in which charge and discharge reactions proceed with the cell sealed (i.e., no exchange of molecular oxygen ( _O2 ) with the atmosphere occurs during the charge and discharge reactions). This "closed-type lithium-oxygen battery" is disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2015-159098 as well as in a document such as (Wang, J. et al., Reversible Conversion between Lithium Superoxide and Lithium Peroxide: A Closed "Lithium-Oxygen" Battery. Inorganics 2023, 11, 69.) Furthermore, although the present embodiment will be described taking as an example a case where the electrode according to the present embodiment is a positive electrode, the electrode can also be used as a negative electrode of a closed-type lithium-oxygen battery.

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 the actual charge/discharge reaction takes place, is sealed inside a laminate film 29. The power-generating element 21 is configured by stacking a positive electrode, in which a positive electrode active material layer 13 is disposed on both sides of a positive electrode current collector 11', an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode, in which a negative electrode active material layer 15 is disposed on both sides of a negative electrode current collector 12. Specifically, the positive electrode, electrolyte layer, and negative electrode are stacked 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 between them. The positive electrode, electrolyte layer, and negative electrode thus constitute a single cell layer 19. Therefore, the stacked lithium-oxygen battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel.

A positive electrode current collector 25 and a negative electrode current collector 27, which are electrically connected to the respective 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 edges of the laminate film 29 and extend outside the laminate film 29. If necessary, 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 each electrode by ultrasonic welding, resistance welding, or the like, via positive electrode terminal leads and negative electrode terminal leads (not shown).

The main components of the stacked lithium-oxygen battery according to this embodiment are described below.

[Current collector]
The current collector has a function of mediating the transfer 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.

[Electrode active material layer]
The electrode active material layer is disposed on the surface of the current collector and contains lithium oxide as an electrode active material, a catalyst, and a gel-forming polymer. In particular, the electrode active material layer is preferably a positive electrode active material layer from the viewpoint of more significant contribution to improving charge/discharge efficiency. In this case, the electrode active material (lithium oxide) functions as a positive electrode active material.

(lithium oxide)
Lithium oxide is a compound in which lithium and oxygen are covalently bonded, and there are several compounds depending on the atomic ratio. Specifically, the lithium oxide preferably contains one or more selected from the group consisting of Li ₂ O, LiO, Li ₂ O ₂ , and LiO _2. Among them, it is more preferable that the lithium oxide contains Li ₂ O, Li ₂ O ₂ , or LiO _2. In particular, it is particularly preferable that the lithium oxide contains Li ₂ O from the viewpoints of having a higher theoretical capacity and being chemically stable (less likely to decompose, and less reactivity with moisture and carbon dioxide in the air) compared to other lithium oxides.

In addition, when the electrode is a positive electrode, conventionally known materials can be used as the negative electrode active material constituting the negative electrode.Such negative electrode active materials include _, for example, carbon materials such as graphite, soft carbon, hard carbon, lithium-transition metal composite _oxides (for example, _Li4Ti5O12 ), metal materials (tin _, silicon), silicon-containing alloy-based negative electrode materials (for example _{, Si60Sn10Ti30} ), lithium alloy-based negative electrode materials (for example, lithium-tin alloy, lithium-silicon alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.).Preferably, from the viewpoint of capacity and output characteristics, silicon-containing alloy-based negative electrode materials, carbon materials, lithium-transition metal composite oxides, and lithium alloy-based negative electrode materials are preferably used as the negative electrode active material.

(catalyst)
A catalyst is a substance that has the function of promoting the above-mentioned bonding/dissociation reaction between lithium oxide and oxygen by reducing the activation energy of these reactions. Any conventionally known compound can be used as a catalyst as long as it can exhibit this function. As an example, the catalyst is preferably a compound containing a transition metal (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. Among these, 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., Co ₃ O ₄ ) is particularly preferred.

The catalyst content in the electrode active material layer is not particularly limited, and although it depends on the type 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 especially preferably 250 to 350 mass%, relative to the total amount of lithium oxide (100 mass%) when the battery is fully discharged.

(Gel-forming polymer)
The gel-forming polymer is an ion-conductive polymer, and can form a structure together with the electrode active material to play a role in ion conduction. From the viewpoint of charge/discharge efficiency and output characteristics, the gel-forming polymer preferably contains at least one selected from the group consisting of polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene (PVdF-HEP), polymethyl methacrylate (PMMA), polyethylene glycol dimethacrylate, polyethylene glycol diacrylate, polypropylene glycol dimethacrylate, polypropylene glycol diacrylate, polymethyl methacrylate, and copolymers thereof. Furthermore, from the viewpoint of further improving charge/discharge efficiency and output characteristics, the gel-forming polymer is preferably selected from polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HEP), and polymethyl methacrylate (PMMA). Furthermore, in a more preferred embodiment, the gel-forming polymer includes one containing a structural unit derived from vinylidene fluoride in the main chain (e.g., polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HEP), etc.). In a particularly preferred embodiment, the gel-forming polymer includes polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HEP). PVdF-HFP has high swelling properties in electrolytes, which can improve the ionic conductivity of lithium ions and the like in the electrode active material layer. Furthermore, because PVdF-HFP is highly flexible, it can penetrate into the gaps between other components (electrode active materials, catalysts, conductive additives, etc.) contained in the electrode active material layer, improving their dispersibility. As a result, it is believed that electron conduction paths are well formed, enabling excellent charge/discharge efficiency.

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

The content of the gel-forming polymer in the electrode active material layer is not particularly limited, and although it depends on the type of lithium oxide and gel-forming polymer, it is preferably 10 to 100% by mass, more preferably 12 to 70% by mass, and even more preferably 15 to 60% by mass, relative to the total amount of the lithium oxide (100% by mass) when the battery is fully discharged.

(Binders other than gel-forming polymers)
The electrode active material layer may further contain a binder other than the gel-forming polymer. Examples of such binders include polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide. However, the lower the content of these binders, the better. Specifically, the content of the gel-forming polymer relative to the total amount (100% by mass) of the binder is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

(Conductive additive)
The electrode active material layer may further include a conductive additive. The conductive additive has a function of forming an electron conduction path (conductive passage) in the 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 may be reduced and the rate characteristics may be improved.

Conductive additives include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark), as well as 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 additive may be used alone, or two or more types may be used in combination.

The content of the conductive additive in the electrode active material layer is preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less, relative to 100% by mass of the total solids content of the electrode active material layer. At such an upper limit, aggregation of the conductive additives is suppressed, resulting in favorable formation of electron conduction paths, thereby further improving charge/discharge efficiency. There is no particular lower limit for the conductive additive content, but it is preferably greater than 0% by mass, and is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more. At such a lower limit, sufficient conductive additive is present to form electron conduction paths, thereby further improving charge/discharge efficiency.

While there are no particular restrictions on the form in which each component (lithium oxide, catalyst, gel-forming polymer, and optional conductive additive) in the electrode active material layer exists, it is preferable that these components be present so that the lithium oxide and catalyst involved in the battery reaction are in contact with each other. In particular, it is more preferable that the lithium oxide and catalyst are in the form of composite particles combined with a gel-forming polymer. Here, "in the form of composite particles" of the lithium oxide and catalyst means that when a charge/discharge reaction is carried out in a closed-type lithium-oxygen battery, the particles do not collapse and maintain their particle shape even if the lithium oxide expands and contracts during charge/discharge. This configuration allows the charge/discharge reaction to proceed more smoothly when lithium oxide is used as an electrode material, which can effectively contribute to further improving the capacity characteristics of the battery.

[Relationship between the porosity of the electrode active material layer, the volume ratio of the gel-forming polymer, and the liquid absorption rate of the gel-forming polymer]
As described above, the battery according to this embodiment is characterized in that, when the porosity of the electrode active material layer is x [%], the volume ratio of the gel-forming polymer to the volume of the electrode active material layer is y [%], and the liquid absorption rate of the gel-forming polymer with respect to the electrolyte solution contained in the electrolyte layer is z [%], the battery satisfies formula (1): 0 < yz/x ≦ 8.7. The inventors have found that there are appropriate ranges from the viewpoint of the charge/discharge efficiency of the battery among the porosity of the electrode active material layer (x), the volume ratio of the gel-forming polymer to the volume of the electrode active material layer (y), and the liquid absorption rate of the gel-forming polymer with respect to the electrolyte solution contained in the electrolyte layer (z), and have quantified this finding using the above relational formula.

If the yz/x value exceeds 8.7 (i.e., if the volume fraction of the gel-forming polymer is too high or the liquid absorption rate is too high), sufficient charge/discharge efficiency cannot be achieved. This is thought to be because, when the lithium oxide (electrode active material) shrinks, the gel-forming polymer penetrates between the electrode active material and the catalyst, reducing the contact point (reaction field) between them and hindering the subsequent battery reaction. In contrast, if the yz/x value is 8.7 or less, the gel-forming polymer can appropriately follow the contraction of the lithium oxide, maintaining sufficient contact between the electrode active material and the catalyst throughout the charge/discharge reaction. Furthermore, the flexibility of the gel-forming polymer suppresses the movement of the electrode active material and catalyst within the active material layer, effectively preventing their detachment. These mechanisms are thought to maintain the initial contact between the electrode active material and the catalyst even after repeated charge/discharge cycles, maintaining the initial capacity (i.e., improving charge/discharge efficiency). However, the above mechanism is merely speculative, and the accuracy of this mechanism does not affect the technical scope of the present invention.

From the perspective of further improving charge-discharge efficiency, the value of yz/x preferably satisfies formula (2): 1.4≦yz/x≦8.7, more preferably formula (3): 3.2≦yz/x≦5.7, and even more preferably formula (4): 3.8≦yz/x≦4.8. When the value of yz/x is within these ranges, the gel-forming polymer exhibits better compliance with the contraction of the lithium oxide, thereby further improving charge-discharge efficiency. The values of x, y, and z are calculated using the method described in the Examples below. In this specification, "improved charge-discharge efficiency" means that when a battery according to the present embodiment is constructed, the initial charge-discharge efficiency is improved compared to when a battery (comparative battery) having the same configuration except for not satisfying the specifications of the battery according to the present embodiment is constructed. There are no particular restrictions on the degree of improvement in the initial charge/discharge efficiency, but the initial charge/discharge efficiency of the battery according to this embodiment is preferably 105 or more, more preferably 119 or more, even more preferably 125 or more, and particularly preferably 127 or more, when the initial charge/discharge efficiency of the comparative battery is taken as 100. The initial charge/discharge efficiency is measured using the method described in the Examples section below.

The above x can be controlled by adjusting the particle size of the electrode active material, the amount of gel-forming polymer added, the thickness of the electrode active material layer, the pressing conditions during electrode fabrication, etc. From the perspective of further improving charge/discharge efficiency, the porosity (x) of the electrode active material layer preferably satisfies 25≦x≦70, more preferably 25≦x≦50, even more preferably 28≦x≦37, and particularly preferably 30≦x≦37.

Furthermore, the above y can be controlled by adjusting the content of the gel-forming polymer in the electrode active material layer. From the perspective of further improving charge/discharge efficiency, the volume ratio (y) of the gel-forming polymer to the volume of the electrode active material layer preferably satisfies 7.0≦y≦15.0, more preferably 9.0≦y≦13.0, and even more preferably 10.5≦y≦12.5.

Furthermore, the above z can be controlled by changing the type of gel-forming polymer and the composition of the electrolyte solution. From the perspective of further improving charge/discharge efficiency, the liquid absorption rate (z) of the gel-forming polymer preferably satisfies 10≦z≦100, and more preferably 12≦x≦50.

There are no particular restrictions on the thickness of the electrode active material layer, and conventionally known knowledge about batteries can be referenced as appropriate. For example, the thickness of the electrode active material layer is typically 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 electrode active material layer, the more electrode active material can be retained to achieve sufficient capacity (energy density). On the other hand, the thinner the electrode active material layer, the more improved the discharge rate characteristics can be.

[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.

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

Preferred non-aqueous solvents are those that readily dissolve lithium salts, such as 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 replaced with fluorine atoms; ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate. fluorine-containing cyclic carbonates in which some of the hydrogen atoms of these cyclic carbonates have been replaced 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 gamma-butyrolactone (GBL).

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

Examples of lithium salts include Li( _FSO2 ) _2N (lithium bis(fluorosulfonyl)imide; _LiFSI ), _Li ( _C2F5SO2 ) _2N , _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , _{and LiCF3SO3} .

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

It is also preferable that the electrolyte solution further contains 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. Furthermore, these fluorine-containing carbonates form a protective film on the surface of the positive electrode active material, thereby improving the voltage resistance of the positive electrode active material.

In this case, preferred fluorine-containing carbonates include fluorine-containing cyclic carbonates such as fluoroethylene carbonate (FEC), difluoroethylene carbonate, and 4-fluoropropylene carbonate; and fluorine-containing 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 fluorine-containing carbonate, particularly fluoroethylene carbonate, based on the total amount of the finally obtained electrolyte solution. This makes it possible to achieve 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 range.

The electrolyte may further contain additives other than the components mentioned above. 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 additives 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, the amount used can be adjusted as appropriate.

(separator)
The separator constituting the electrolyte layer has the function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and also functions as a partition wall between the positive electrode and the negative electrode.

Examples of the separator's form include a porous sheet separator made of polymer or fiber that absorbs and retains the electrolyte, and a nonwoven fabric separator.

As a porous sheet separator made of a polymer or fiber, for example, a microporous material (microporous membrane) can be used. Specific forms of the porous sheet made of polymer or fiber include microporous (microporous membrane) separators made of polyolefins such as polyethylene (PE) and polypropylene (PP); laminates of multiple layers of these (for example, a laminate with a three-layer structure of PP/PE/PP); hydrocarbon resins such as polyimide, aramid, and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP); and glass fibers.

Nonwoven fabric separators may be made from conventional materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide, and aramid, either alone or in combination.

The thickness of the separator should be the same as that of the electrolyte layer, preferably 5 to 200 μm, and particularly preferably 10 to 100 μm.

Furthermore, a separator with a heat-resistant insulating layer laminated on a porous substrate (separator with heat-resistant insulating layer) can be used. The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. A highly heat-resistant separator with a melting point or thermal softening point of 150°C or higher, preferably 200°C or higher, is used. The presence of a heat-resistant insulating layer alleviates the internal stress of the separator that increases with temperature rise, thereby suppressing thermal shrinkage. As a result, short circuits between battery electrodes can be prevented, resulting in a battery configuration that is less susceptible to performance degradation due to temperature rise. Furthermore, the presence of a heat-resistant insulating layer improves the mechanical strength of the separator with heat-resistant insulating layer, making it less likely to rupture. Furthermore, the heat-shrinkage suppression effect and high mechanical strength make the separator less likely to curl during the battery manufacturing process.

[Positive electrode current collector plate and negative electrode current collector plate]
The material constituting the current collector plates (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 plates. 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.

As described above, the closed-type lithium-oxygen battery according to this embodiment has excellent charge/discharge efficiency. Therefore, the closed-type lithium-oxygen battery according to this embodiment is suitable for use as a power source for driving EVs and HEVs.

The above describes a closed-type lithium-oxygen battery according to one embodiment of the present invention, but the present invention is not limited to the configuration described in the above embodiment and can be modified as appropriate based on the claims.

The following embodiments are also within the scope of the present invention: the battery according to claim 1 having the characteristics of claim 2; the battery according to claim 1 or 2 having the characteristics of claim 3; the battery according to any one of claims 1 to 3 having the characteristics of claim 4; the battery according to any one of claims 1 to 4 having the characteristics of claim 5; the battery according to any one of claims 1 to 5 having the characteristics of claim 6; the battery according to claim 6 having the characteristics of claim 7; the battery according to any one of claims 1 to 7 having the characteristics of claim 8; the battery according to any one of claims 1 to 8 having the characteristics of claim 9; the battery according to any one of claims 1 to 9 having the characteristics of claim 10; and the battery according to any one of claims 1 to 10 having the characteristics of claim 11.

The present invention will be explained in more detail below using examples. However, the technical scope of the present invention is not limited to the following examples.

<<Creating a closed-type lithium-oxygen battery>>
[Measurement method]
In the following examples and comparative examples, the values of x, y and z were measured using the following measurement methods.

(Measurement of Porosity (x) of Positive Electrode Active Material Layer)
The porosity (x) of the positive electrode active material layer was measured as follows:
(1) The mass per unit area of the positive electrode active material layer was measured, and 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) Using the mass of each material determined in (1) and the density of each material, the thickness [B] of the positive electrode active material layer when the porosity is 0% was calculated.
(4) The pore volume of the positive electrode active material layer was calculated from the difference (A-B) between the measured thickness of the positive electrode active material layer and the calculated thickness of the positive electrode active material layer, and the pore volume per 1 ^m3 of the positive electrode active material layer was determined. The percentage of the obtained value was defined as the porosity x [%].

(Measurement of the volume ratio (y) of the gel-forming polymer to the volume of the positive electrode active material layer)
Regarding the volume ratio (y) of the gel-forming polymer to the volume of the positive electrode active material layer, the volume ratio y [%] of the gel-forming polymer to the volume of the positive electrode active material layer obtained was calculated based on the following formula:
Volume of gel-forming polymer = (mass of gel-forming polymer) / (density of gel-forming polymer)
The volume ratio of the gel-forming polymer (y)=(volume of the gel-forming polymer)/[total volume of the positive electrode active material layer×(1−x/100)].

(Measurement of the absorption rate (z) of the gel-forming polymer for the electrolyte)
The liquid absorption rate (z) of the gel-forming polymer with respect to the electrolyte was calculated by measuring the weight of the gel-forming polymer before and after immersion in the electrolyte, using the following formula:
Liquid absorption rate (%)=[(weight of gel-forming polymer after immersion in electrolyte solution−weight of gel-forming polymer before immersion in electrolyte solution)/weight of gel-forming polymer before immersion in electrolyte solution]×100
In this case, the same electrolyte as that used in the production of the battery was used.

[Example 1]
(Preparation of positive electrode material)
Lithium oxide (Li ₂ O, manufactured by Kojundo Chemical Laboratory Co., Ltd.) as the positive electrode active material (lithium oxide) and cobalt oxide (Co ₃ O ₄ , manufactured by Kojundo Chemical Laboratory Co., Ltd.) as the catalyst were placed in a separate 70 mL planetary ball mill pot and each was ground using a planetary ball mill (grinding conditions: 40 g of 3 mm diameter zirconia balls and 15 g of 15 mm diameter zirconia balls at a rotation speed of 400 rpm for 1 hour).

Next, 5 g of crushed lithium oxide and 15 g of cobalt oxide were placed in a 70 mL planetary ball mill pot and mixed using a planetary ball mill (mixing conditions: 40 g of 3 mm diameter zirconia balls and 15 15 mm diameter zirconia balls, rotating at 400 rpm for 1 hour).

Meanwhile, the gel-forming polymer PVdF-HFP (Kyner Flex 2501, manufactured by Arkema; proportion of hexafluoropropylene-derived structural units: 6.9 mol%; liquid absorption rate (z) = 13%]) and an appropriate amount of N-methyl-2-pyrrolidone (NMP, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a solvent were kneaded (kneading conditions: 2000 rpm for 5 minutes) using a planetary stirring mixer/kneader "Awatori Rentaro" (ARE-310, manufactured by Thinky Corporation) to dissolve the PVdF-HFP in NMP. The resulting NMP solution of PVdF-HFP was mixed with the ball mill mixture of lithium oxide and cobalt oxide and the conductive additive acetylene black (AB) (Li-400, manufactured by Denka Co., Ltd., average primary particle size: 48 nm, aspect ratio: 1), and further mixed using the mixer/kneader (kneading conditions: 2000 rpm for 5 minutes). The resulting mixture was transferred to a metal tray and placed on a hot plate heated to 120°C to volatilize the NMP. The remaining powder was then collected and mixed by hand in an agate mortar for 5 minutes to prepare the positive electrode material of Example 1. In this example, the mixture ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF-HFP was 20:60:10:10.

(Preparation of Positive Electrode)
NMP was added to the positive electrode material prepared above to adjust the solid content to 25% by mass, and the viscosity was adjusted to prepare a positive electrode slurry. The positive electrode slurry was uniformly applied to aluminum foil placed on a smooth plate using a doctor blade so that the final thickness of the positive electrode active material layer was 200 μm. The resulting laminate was then dried for 30 minutes on a hot plate heated to 80°C, and then pressed using a roll press. The laminate was then transferred to a vacuum dryer and dried under vacuum at 130°C for 8 hours to prepare a positive electrode of this example in which a positive electrode active material layer was formed on the surface of the aluminum foil. The porosity (x) of the positive electrode active material layer was 28%. The volume ratio (y) of the gel-forming polymer (PVdF-HFP) to the volume of the positive electrode active material layer was 12.2. As a result, the value of yz/x was calculated to be 12.2 × 13/28 = 5.7.

(Fabrication of closed-type lithium-oxygen batteries (coin cells))
The positive electrode and lithium counter electrode prepared above were placed opposite each other, and a separator (polyolefin, thickness: 20 μm) was placed between them. Next, the laminate of the positive electrode, separator, and lithium counter electrode was placed in a coin cell (CR2032, material: stainless steel (SUS316)), and the following electrolyte solution was injected using a syringe and sealed to prepare a closed-type lithium-oxygen battery (coin cell) of this example. The electrolyte used was an organic solvent mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of EC:DEC = 3:7 (volume ratio), in which lithium hexafluorophosphate (LiPF ₆ ), a lithium salt, was dissolved at a concentration of 1 mol/L.

[Examples 2 to 4]
The closed-type lithium-oxygen batteries (coin cells) of these Examples were produced using the same method as in Example 1 described above, except that the conditions for the pressing treatment were changed so that the porosity (x [%]) of the positive electrode active material layer and the volume ratio (y [%]) of the gel-forming polymer to the volume of the positive electrode active material layer were the values shown in Table 1 below.

[Example 5]
The closed-type lithium-oxygen battery (coin cell) of this example was produced using the same method as in Example 1 described above, except that the amounts of the positive electrode active material (lithium oxide) and the catalyst (cobalt oxide) used were changed to the values shown in Table 1 below, and the conditions of the pressing treatment were changed so that the porosity (x [%]) of the positive electrode active material layer and the volume ratio (y [%]) of the gel-forming polymer to the volume of the positive electrode active material layer were the values shown in Table 1 below.

[Example 6]
A closed-type lithium-oxygen battery (coin cell) of this example was produced using the same method as in Example 1 described above, except that PVdF-HFP (Kyner Flex 2501) was replaced with PVdF-HFP (Kyner Flex 2851, manufactured by Arkema; proportion of the number of constitutional units derived from hexafluoropropylene: 2.4 mol %; liquid absorption rate (z)=45[%]) as the gel-forming polymer, and the conditions of the pressing treatment were changed so that the porosity (x[%]) of the positive electrode active material layer and the volume ratio (y[%]) of the gel-forming polymer to the volume of the positive electrode active material layer were the values shown in Table 1 below.

[Example 7]
A closed-type lithium-oxygen battery (coin cell) of this example was produced using the same method as in Example 5 described above, except that PVDF (Kureha KF Polymer W#9700, manufactured by Kureha; liquid absorption rate (z)=95[%]) was used as the gel-forming polymer instead of PVdF-HFP (Kyner Flex 2501), and the conditions for the press treatment were changed so that the porosity (x[%]) of the positive electrode active material layer and the volume ratio (y[%]) of the gel-forming polymer to the volume of the positive electrode active material layer were the values shown in Table 1 below.

[Comparative Example 1]
A closed-type lithium-oxygen battery (coin cell) of this comparative example was produced using the same method as in Example 1 described above, except that the amounts of the positive electrode active material (lithium oxide), catalyst (cobalt oxide), and gel-forming polymer (PVdF-HFP) used were changed to the values shown in Table 1 below, and the conditions of the pressing process were changed so that the porosity (x [%]) of the positive electrode active material layer and the volume ratio (y [%]) of the gel-forming polymer to the volume of the positive electrode active material layer were the values shown in Table 1 below.

[Comparative Example 2]
A closed-type lithium-oxygen battery (coin cell) of this comparative example was produced using the same method as in Example 5 described above, except that PVDF (Kureha KF Polymer W#7200, manufactured by Kureha; liquid absorption rate (z) = 120 [%]) was used as the gel-forming polymer instead of PVdF-HFP (Kyner Flex 2501), and the conditions for the press treatment were changed so that the porosity (x [%]) of the positive electrode active material layer and the volume ratio (y [%]) of the gel-forming polymer to the volume of the positive electrode active material layer were the values shown in Table 1 below.

<Coin cell evaluation (measurement of initial charge/discharge efficiency)>
The closed-type lithium-oxygen batteries (coin cells) of the Examples and Comparative Examples prepared above were subjected to the following charge-discharge test (initial charge-discharge) in a thermostatic chamber set at 300 K (27°C), and the charge capacity and discharge capacity were measured. The ratio of the discharge capacity to the charge capacity was calculated to define the initial charge-discharge efficiency. The results are shown in Table 1 below. The initial charge-discharge efficiency values shown in Table 1 are relative values, with the measured value for Comparative Example 2 set to 100.

(Charge/discharge test conditions)
Charge/discharge tester:
TOSCAT-3000, model TYS-30TU10 (manufactured by Toyo Systems Co., Ltd.)
Charge/discharge conditions:
[Charging process] 0.02 C (current density 18 mA / g), 1.8 V → 4.6 V (CCCV; 0.01 C cutoff)
[Discharge process] 0.02C (current density 18mA/g), 4.6V → 1.8V (CC)
Rest time between charging and discharging processes: 30 minutes.

The results shown in Table 1 show that, according to the present invention, the charge/discharge efficiency of a closed-type lithium-oxygen battery can be improved by incorporating a gel-forming polymer in addition to lithium oxide (Li ₂ O) and a catalyst into the electrode active material layer and by controlling the above parameters (x, y, z) to have a predetermined relationship.

10a: stacked 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 cell layer,
21 power generating element,
25 Positive electrode current collector (positive electrode tab),
27 negative electrode current collector plate (negative electrode tab),
29 Laminating film.

Claims (11)

The battery comprises an electrode in which an electrode active material layer containing lithium oxide, a catalyst, and a gel-forming polymer is disposed on the surface of a current collector, and an electrolyte layer in which a separator disposed adjacent to the electrode is impregnated with an electrolytic solution,
A closed-type lithium-oxygen battery, wherein the relationship of the following formula (1) is satisfied when the porosity of the electrode active material layer is x [%], the volume ratio of the gel-forming polymer to the volume of the electrode active material layer is y [%], and the liquid absorption rate of the gel-forming polymer with respect to the electrolyte is z [%]:
0<yz/x≦8.7 Formula (1) The closed-type lithium-oxygen battery according to claim 1, further satisfying the following formula (2):
1.4≦yz/x≦8.7...Formula (2) The closed-type lithium-oxygen battery according to claim 2, further satisfying the following formula (3):
3.2≦yz/x≦5.7...Formula (3) The closed-type lithium-oxygen battery of claim 1 or 2, wherein y satisfies 7.0≦y≦15.0. The closed-type lithium-oxygen battery according to claim 1 or 2, wherein x satisfies 25≦x≦50. The closed-type lithium-oxygen battery described in claim 1 or 2, wherein the gel-forming polymer contains structural units derived from vinylidene fluoride in its main chain. The closed-type lithium-oxygen battery described in claim 6, wherein the gel-forming polymer includes vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP). 3. The closed-type lithium-oxygen battery according to claim 1, wherein the lithium oxide comprises one or more selected from the group consisting _{of Li2O} , LiO, _Li2O2 , and _LiO2 . The closed-type lithium-oxygen battery according to claim 1 or 2, wherein the catalyst comprises a transition metal-containing oxide. The closed-type lithium-oxygen battery of claim 9, 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. The closed-type lithium-oxygen battery according to claim 1 or 2, wherein the electrode active material layer is a positive electrode active material layer.