Enclosed lithium-oxygen battery

Abstract

The present application provides a means capable of improving the charge-discharge efficiency of a closed lithium-oxygen battery. A battery for a closed lithium-oxygen battery is disclosed, having: an electrode having an electrode active material layer containing a lithium oxide, a catalyst, and a gel-forming polymer disposed on the surface of a current collector; and an electrolyte layer formed by impregnating an electrolyte in a separator disposed adjacent to the electrode, the porosity x [%] of the electrode active material layer, the proportion y [%] of the volume of the gel-forming polymer in the volume of the electrode active material layer, and the liquid absorption rate z [%] of the gel-forming polymer with respect to the electrolyte satisfy 0 < yz / x ≤ 8.7.

Description

Technical Field

[0001] This invention relates to a closed-cell lithium-oxygen battery. Background Technology

[0002] In recent years, to address environmental and energy issues, the widespread adoption of electric vehicles is anticipated. As a key factor in the widespread adoption of these electric vehicles, the development of secondary batteries, such as those used for motor drives, has been actively pursued. Among these secondary batteries, non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, which promise high energy density and high output, have attracted significant attention.

[0003] Lithium-oxygen batteries (Li-Oxygen secondary batteries) are a type of non-aqueous electrolyte secondary battery. Among all secondary batteries, including next-generation batteries, Li-Oxygen batteries theoretically have the highest energy density and are expected to possess battery performance with energy densities significantly exceeding those of current lithium-ion secondary batteries. However, currently, their full capacity characteristics have not yet been achieved, requiring further improvements.

[0004] Lithium-oxygen batteries can be broadly categorized into two types: those that use molecular oxygen (O2) as an oxygen source, consuming O2 during discharge and generating O2 during charging (lithium-air batteries); and those that do not involve such O2 consumption or generation (lithium-oxygen batteries in the narrow sense). The latter does not involve exchange with atmospheric O2, and therefore can be constructed as a closed battery cell, which can be called a "closed-cell lithium-oxygen battery."

[0005] Previously, the aforementioned closed-type lithium-oxygen battery was disclosed in Japanese Patent Application Publication No. 2015-159098. In that document, as a closed-type battery, an attempt was made to prevent the mixing of atmospheric moisture, carbon dioxide, etc. into the battery cell, while micronizing a raw material composition containing lithium oxide (Li2O) as an electrode active material and oxides containing transition metal atoms (Co3O4, etc.) as a catalyst through mechanochemical processing, thereby achieving high capacity of the electrode active material. Summary of the Invention

[0006] The problem the invention aims to solve

[0007] According to the research of the inventors, it has been determined that when using the technology disclosed in Japanese Patent Application Publication No. 2015-159098 to manufacture a closed lithium-oxygen battery, it is sometimes impossible to achieve sufficient charge and discharge efficiency (the ratio of discharge capacity to charge capacity).

[0008] Therefore, the purpose of this invention is to provide a solution that can improve the charge and discharge efficiency of a closed lithium-oxygen battery.

[0009] Solution for solving the problem

[0010] One aspect of the present invention relates to a sealed lithium-oxygen battery, which comprises: an electrode, wherein 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, which is formed by impregnating a separator disposed adjacent to the electrode with an electrolytic solution. This battery is characterized in that when the porosity of the electrode active material layer is defined as x [%], the ratio of the volume of the gel-forming polymer to the volume of the electrode active material layer is defined as y [%], and the liquid absorption rate of the gel-forming polymer with respect to the electrolytic solution is defined as z [%], the relationship of the following formula (1) is satisfied:

[0011] 0<yz / x≤8.7 … Formula (1) Brief Description of the Drawings

[0012] Figure 1 is a cross-sectional view schematically showing a stacked (flat-type) sealed lithium-oxygen battery as one embodiment of the present invention. Detailed Description of the Embodiments

[0013] One aspect of the present invention is a battery for a sealed lithium-oxygen battery, which comprises: an electrode, wherein 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, which is formed by impregnating a separator disposed adjacent to the aforementioned electrode with an electrolytic solution. When the porosity of the aforementioned electrode active material layer is defined as x [%], the ratio of the volume of the aforementioned gel-forming polymer to the volume of the aforementioned electrode active material layer is defined as y [%], and the liquid absorption rate of the aforementioned gel-forming polymer with respect to the aforementioned electrolytic solution is defined as z [%], the relationship of the following formula (1) is satisfied:

[0014] 0<yz / x≤8.7 … Formula (1)

[0015] The present inventors have found that in the search for means to improve the charge-discharge efficiency of sealed lithium-oxygen batteries, making the electrode active material layer contain a gel-forming polymer in addition to lithium oxide (Li₂O) and a catalyst, and controlling the above parameters (x, y, z) to satisfy a specified relationship is useful for improving the charge-discharge efficiency.

[0016] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention should be determined based on the description of the claims, and is not limited to the following modes. It should be noted that the dimensional ratios in the drawings are exaggerated for convenience of description, and may differ from the actual ratios. In this specification, "X to Y" indicating a range means "X or more and Y or less". Unless otherwise specified, operations and measurements of physical properties and the like are carried out under the conditions of room temperature (20 to 25°C) and a relative humidity of 40 to 50%RH.

[0017] Figure 1 This is a schematic cross-sectional view of a stacked (flat) type closed lithium-oxygen battery (hereinafter also simply referred to as a "stacked lithium-oxygen battery") as an embodiment of the present invention. It should be noted that, in this specification, a "closed lithium-oxygen battery" refers to a lithium-oxygen battery in which the charge-discharge reaction is carried out in a sealed state (therefore, the exchange of molecular oxygen (O2) with the atmosphere does not occur during the charge-discharge reaction). Regarding this "closed lithium-oxygen battery," in addition to the aforementioned Japanese Patent Application Publication No. 2015-159098, it has also been disclosed in documents such as (Wang, J. et al., Reversible Conversion between Lithium Superoxide and Lithium Peroxide: A Closed "Lithium-Oxygen" Battery. Inorganics 2023, 11, 69.). Furthermore, in this embodiment, the case where the electrode of this method is a positive electrode is described as an example, but this electrode can also be used as the negative electrode of a closed lithium-oxygen battery.

[0018] like Figure 1 As shown, the stacked lithium-oxygen battery 10a of this embodiment has a structure in which a generally rectangular power generation element 21, which actually performs the charge-discharge reaction, is sealed inside a laminated film 29. Here, the power generation element 21 has the following structure: a positive electrode with positive active material layers 13 disposed on both sides of the positive current collector 11', an electrolyte layer 17 composed of a separator containing electrolyte, and a negative electrode with negative active material layers 15 disposed on both sides of the negative current collector 12, stacked together. Specifically, the positive electrode, electrolyte layer, and negative electrode are stacked sequentially with a positive active material layer 13 and an adjacent negative active material layer 15 facing each other across the electrolyte layer 17. Thus, the positive electrode, electrolyte layer, and negative electrode constitute a single cell layer 19. Therefore, Figure 1 The stacked lithium-oxygen battery 10a shown can be said to have a structure in which multiple single cell layers 19 are electrically connected in parallel.

[0019] The device has the following structure: a positive current collector plate 25 and a negative current collector plate 27, which are conductive to each electrode (positive and negative), are respectively mounted on the positive current collector 11' and the negative current collector 12, and are led out to the outside of the laminated film 29 by being clamped at the end of the laminated film 29. The positive current collector plate 25 and the negative current collector plate 27 can be mounted to the positive current collector 11' and the negative current collector 12 of each electrode via positive terminal leads and negative terminal leads (not shown), respectively, by ultrasonic welding and / or resistance welding, as needed.

[0020] The main components of the stacked lithium-oxygen battery of this embodiment will be described below.

[0021] [Current Collector]

[0022] The current collector mediates the movement of electrons from the positive and negative active material layers, which will be described later. There are no particular limitations on the materials used to construct the current collector. For example, metals or conductive resins can be used as materials for the current collector.

[0023] [Electrode active material layer]

[0024] An electrode active material layer is disposed on the surface of the current collector and comprises lithium oxide, a catalyst, and a gel-forming polymer as the electrode active material. In particular, from the viewpoint of contributing more significantly to the improvement of charge-discharge efficiency, the electrode active material layer is preferably a positive electrode active material layer. In this case, the electrode active material (lithium oxide) functions as the positive electrode active material.

[0025] (Lithium oxide)

[0026] Lithium oxides are compounds formed by the covalent bonding of lithium and oxygen, and several compounds exist depending on their atomic ratio. Specifically, lithium oxides preferably contain one or more of the following: Li₂O, LiO, Li₂O₂, and LiO₂. Li₂O, Li₂O₂, or LiO₂ is more preferred. In particular, from the viewpoint of higher theoretical capacity and chemical stability (less prone to decomposition and lower reactivity with moisture and carbon dioxide in the air) compared to other lithium oxides, the inclusion of Li₂O is especially preferred.

[0027] Furthermore, when the aforementioned electrode is the positive electrode, conventionally known materials can be used as the negative electrode active material constituting the negative electrode. Examples of such negative electrode active materials include, for instance, carbon materials such as graphite, soft carbon, and hard carbon, and lithium-transition metal composite oxides (e.g., Li₄Ti₅O₅). 12 Metallic materials (tin, silicon), silicon-containing alloy anode materials (e.g., Si), 60 Sn 10 Ti 30 Lithium alloy-based anode materials (such as lithium-tin alloys, lithium-silicon alloys, lithium-aluminum alloys, lithium-aluminum-manganese alloys, etc.) are preferred as anode active materials, based on capacity and output characteristics.

[0028] (catalyst)

[0029] A catalyst is a substance that promotes the reaction of lithium oxides and oxygen by lowering the activation energy of the aforementioned binding / dissociation reaction. Any conventionally known compound can be used as a catalyst as long as it exhibits such a function. As an example, a catalyst is preferably a compound containing a transition metal (a transition metal-containing compound). This transition metal-containing compound is preferably, for example, in the form of oxides (including complex oxides), sulfides, halides, nitrides, carbides, etc., containing a transition metal. From the viewpoint of excellent catalytic activity, the catalyst preferably contains a transition metal oxide. When the catalyst contains a transition metal-containing compound, the type of transition metal contained in the compound is not particularly limited; it can be any metal atom classified as a transition metal, and one or more types can be used. From the viewpoint of catalytic activity, the transition metal is preferably at least one type belonging to groups 6 to 11 of the periodic table, more preferably one or more types selected from the group consisting of cobalt, manganese, iron, nickel, molybdenum, iridium, and rhodium, particularly preferably one or more types selected from the group consisting of cobalt, manganese, and iron, and most preferably cobalt. Examples of transition metal compounds include cobalt oxide, manganese oxide, iron oxide, nickel oxide, molybdenum oxide, iridium oxide, and rhodium oxide, which are transition metal oxides. Among these, cobalt oxide, manganese oxide, and iron oxide are preferred, and cobalt oxide (e.g., Co3O4) is particularly preferred.

[0030] The content of catalyst in the electrode active material layer is not particularly limited, depending on the type of lithium oxide and catalyst. When the battery is fully discharged, it is preferably 50 to 500% by mass relative to 100% of the total amount of lithium oxide, more preferably 100 to 450% by mass, further preferably 200 to 400% by mass, and particularly preferably 250 to 350% by mass.

[0031] (Gel-forming polymers)

[0032] Gel-forming polymers are ion-conducting polymers that form structures together with electrode active materials, enabling them to perform ion conduction. From the viewpoint of charge-discharge efficiency and output characteristics, gel-forming polymers preferably include 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). In a more preferred embodiment, the gel-forming polymer contains structural units derived from vinylidene fluoride (e.g., polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HEP), etc.) in its main chain. In a particularly preferred embodiment, the gel-forming polymer comprises polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HEP). PVdF-HEP has high swelling capacity in the electrolyte, thus improving the ionic conductivity of lithium ions, etc., in the electrode active material layer. Furthermore, due to its high flexibility, PVdF-HFP can penetrate the gaps between other components (electrode active materials, catalysts, conductive additives, etc.) contained in the electrode active material layer, thus improving their dispersion. As a result, it is believed that it can form good electron conduction pathways and achieve excellent charge and discharge efficiency.

[0033] There are no particular restrictions on whether PVdF-HFP is a copolymer of vinylidene fluoride and hexafluoropropylene. The lower limit of the proportion of hexafluoropropylene-derived structural units in PVdF-HFP relative to all structural units 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 in PVdF-HFP is preferably 20 mol% or less, more preferably 10 mol% or less, and even more preferably 8 mol% or less. By achieving the above-mentioned proportion of hexafluoropropylene-derived structural units, the swelling and flexibility to the electrolyte can be further improved. As a result, the ionic and electronic conductivity of the electrode active material layer is improved, enabling excellent charge and discharge efficiency.

[0034] The content of gel-forming polymer in the electrode active material layer is not particularly limited, and also depends on the type of lithium oxide and gel-forming polymer. When the battery is fully discharged, it is preferably 10 to 100% by mass relative to the total amount of the above-mentioned lithium oxide, more preferably 12 to 70% by mass, and even more preferably 15 to 60% by mass.

[0035] (Adhesives other than gel-forming polymers)

[0036] It should be noted that the electrode active material layer may further include a binder other than the aforementioned gel-forming polymer. Examples of such binders include polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide, with a lower content of these binders being more preferred. Specifically, the content of the gel-forming polymer in 100% by mass of the total binder is preferably 50% by mass or more, more preferably 80% by mass or more, further preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

[0037] (Conductive additive)

[0038] The electrode active material layer may further contain conductive additives. These conductive additives function to form electronic conduction pathways (conductive pathways) within the electrode active material layer. When such electronic conduction pathways are formed in the positive electrode active material layer, the battery's internal resistance decreases, and its rate performance is improved.

[0039] Examples of conductive additives include granular carbon materials such as acetylene black, carbon black, channel black, thermal cracking black, and Ketjen black (registered trademark), as well as fibrous carbon materials such as carbon nanotubes (single-layer and multi-layer carbon nanotubes), carbon nanofibers, vapor-grown carbon fibers, electric field-spun carbon fibers, polyacrylonitrile-based carbon fibers, and pitch-based carbon fibers. Conductive additives can be used alone or in combination with two or more.

[0040] The content of conductive additives 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 solid content of the electrode active material layer. At such an upper limit, electron conduction pathways are well formed by suppressing the aggregation of conductive additives, thus further improving charge-discharge efficiency. It should be noted that there is no particular limitation on the lower limit of the conductive additive content; it is more than 0% by mass, 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 additives are present for forming electron conduction pathways, thus further improving charge-discharge efficiency.

[0041] There is no particular limitation on the existing form of each constituent component (lithium oxide, catalyst, gel-forming polymer, and optional conductive auxiliary agent) in the electrode active material layer, and these constituent components are preferably present in a manner that the lithium oxide and the catalyst participating in the battery reaction are in contact with each other. In particular, the form of composite particles in which the lithium oxide and the catalyst are composited via the gel-forming polymer is more preferable. Here, the expression that the lithium oxide and the catalyst are in the "form of composite particles" refers to a state where when a closed lithium-oxygen battery is subjected to charge and discharge reactions, the particles do not collapse and maintain their particle shape even if the lithium oxide expands and contracts during charge and discharge. By adopting such a configuration, when lithium oxide is used as an electrode material, the charge and discharge reactions proceed more smoothly, which can effectively contribute to further improvement of the capacity characteristics of the battery.

[0042] [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]

[0043] As described above, the battery of the present embodiment is characterized in that when the porosity of the electrode active material layer is defined as x [%], the ratio of the volume of the gel-forming polymer to the volume of the electrode active material layer is defined as y [%], and the liquid absorption rate of the gel-forming polymer with respect to the electrolyte solution contained in the electrolyte layer is defined as z [%], Formula (1): 0 < yz / x ≤ 8.7 is satisfied. The present inventors have found that there is an appropriate range from the viewpoint of the charge and discharge efficiency of the battery among 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 rate (z) of the gel-forming polymer with respect to the electrolyte solution contained in the electrolyte layer, and this finding is quantified by the above relational expression.

[0044] If the yz / x value exceeds 8.7 (i.e., the volume proportion of the gel-forming polymer is too high or the liquid absorption rate is too large), sufficient charge-discharge efficiency cannot be achieved. This is believed to be because, when the lithium oxide, as the electrode active material, shrinks, the gel-forming polymer enters between the electrode active material and the catalyst, reducing their junction (reaction field) and making subsequent battery reactions difficult. Conversely, if the yz / x value is below 8.7, the gel-forming polymer can appropriately follow the shrinkage of the lithium oxide, and the contact between the electrode active material and the catalyst is adequately maintained throughout the charge-discharge reaction. Furthermore, by making the gel-forming polymer flexible, the movement of the electrode active material and catalyst within the active material layer can be suppressed, effectively preventing the shedding of these materials. Moreover, it is believed that due to these mechanisms, even after repeated charge-discharge cycles, the initial contact state between the electrode active material and the catalyst can be maintained, thus maintaining the initial capacity (i.e., improved charge-discharge efficiency). However, the above mechanism is merely a speculative one, and its correctness does not affect the scope of the present invention.

[0045] From the viewpoint of further improving charge and discharge efficiency, the value of yz / x is preferably satisfied by equation (2): 1.4 ≤ yz / x ≤ 8.7, more preferably by equation (3): 3.2 ≤ yz / x ≤ 5.7, and even more preferably by equation (4): 3.8 ≤ yz / x ≤ 4.8. If the value of yz / x is within these ranges, the gel-forming polymer exhibits better following properties relative to the shrinkage of lithium oxide, and the charge and discharge efficiency can be further improved. It should be noted that the values ​​of x, y, and z are calculated by the method described in the embodiments described later. In addition, in this specification, "improvement in charge and discharge efficiency" means that, in the case of a battery constituting this method, the initial charge and discharge efficiency is improved compared to the case of a battery constituting the same configuration except that it does not meet the requirements of a battery constituting this method (comparison battery). There is no particular limitation on the degree of improvement in the initial charge-discharge efficiency at this time. When the initial charge-discharge efficiency of the comparison battery is set to 100, the initial charge-discharge efficiency of the battery of this method is preferably 105 or higher, more preferably 119 or higher, even more preferably 125 or higher, and particularly preferably 127 or higher. It should be noted that the initial charge-discharge efficiency is measured using the method described in the Example section below.

[0046] The aforementioned 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, and the pressure conditions during electrode fabrication. From the viewpoint of further improving charge and 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.

[0047] Furthermore, the aforementioned y can be controlled by adjusting the content of gel-forming polymer in the electrode active material layer, etc. From the viewpoint of further improving charge and discharge efficiency, the proportion (y) of the volume of the gel-forming polymer in 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.

[0048] Furthermore, the aforementioned z can be controlled by changing the type of gel-forming polymer and the composition of the electrolyte. From the viewpoint of further improving charge-discharge efficiency, the liquid absorption rate (z) of the gel-forming polymer preferably satisfies 10 ≤ z ≤ 100, and more preferably satisfies 12 ≤ x ≤ 50.

[0049] There are no particular limitations on the thickness of the electrode active material layer, and prior knowledge regarding batteries can be appropriately referenced. For example, the thickness of the electrode active material layer is typically around 1–1000 μm, preferably 20–800 μm, more preferably 30–500 μm, and even more preferably 40–200 μm. A greater thickness of the electrode active material layer allows for better retention of electrode active material to achieve sufficient capacity (energy density). Conversely, a smaller thickness of the electrode active material layer results in improved discharge rate characteristics.

[0050] [Electrolyte layer]

[0051] The electrolyte layer contains an electrolyte (liquid electrolyte). The electrolyte layer preferably has a structure in which the electrolyte is impregnated in a separator.

[0052] (Electrolyte)

[0053] The electrolyte functions as a carrier of lithium ions. The electrolyte is in a form where the lithium salt is dissolved in a non-aqueous solvent. Preferably, the electrolyte is prepared by further adding a fluorocarbonate to a solution obtained by dissolving the lithium salt in a non-aqueous solvent.

[0054] As a non-aqueous solvent, non-aqueous solvents that readily dissolve lithium salts are preferred, such as chain carbonates like dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and methyl ethyl carbonate (MEC); fluorinated chain carbonates in which a portion of the hydrogen atoms of these chain carbonates are replaced by fluorine atoms; cyclic carbonates like ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC); fluorinated cyclic carbonates in which a portion of the hydrogen atoms of these cyclic carbonates are replaced by fluorine atoms; methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL).

[0055] From the viewpoint of further improving fast charging and output characteristics, the non-aqueous solvent preferably includes a chain carbonate, and more preferably includes at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0056] Examples of lithium salts include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3.

[0057] The concentration of lithium salt in the above-mentioned non-aqueous solvent is preferably 0.1~3.0 mol / L, more preferably 0.8~2.2 mol / L.

[0058] Furthermore, the electrolyte preferably also contains fluorinated carbonates such as fluorinated cyclic carbonates and fluorinated chain carbonates. This ensures excellent durability even when the battery is operated at high voltages. Additionally, these fluorinated carbonates form a protective film on the surface of the positive electrode active material, improving its voltage resistance.

[0059] At this time, fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), difluoroethylene carbonate, and 4-fluoropropylene carbonate, and fluorinated chain carbonates such as ethyl trifluoromethyl carbonate, 2,2,2-trifluoroethylmethyl carbonate, and bis(2,2,2-trifluoroethyl) carbonate are preferred as fluorinated carbonates. The content of fluorinated carbonates is not particularly limited. In a preferred embodiment, the total amount of electrolyte relative to the final electrolyte contains 0.5 to 10% by mass of fluorinated carbonates, particularly fluoroethylene carbonate. This allows for a more significant attainment of the aforementioned effects. It should be noted that when the electrolyte contains two or more fluorinated carbonates, their total amount is preferably within the above range.

[0060] The electrolyte may further contain additives other than those mentioned above. Specific examples of such compounds include, for instance, 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, 1-ethyl-2-vinyl ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, ethoxymethyl ethylene carbonate, allyl ethyl ethylene carbonate, acryloyl ethyl ethylene carbonate, methacryl ethyl ethylene carbonate, ethynyl ethylene carbonate, propyryneyl ethylene carbonate, ethynyl ethyl ethylene carbonate, propyryneyl ethylene carbonate, methylene ethylene carbonate, 1,1-dimethyl-2-methylene ethylene carbonate, etc. These additives can be used alone or in combination of two or more. Furthermore, the amount of additive used in the electrolyte can be adjusted appropriately.

[0061] (Separator)

[0062] The separators that make up the electrolyte layer have the functions of retaining the electrolyte to ensure lithium-ion conductivity between the positive and negative electrodes, and also serve as a partition between the positive and negative electrodes.

[0063] Examples of separators include porous sheets made of polymers or fibers that absorb and retain the electrolyte, and nonwoven fabric separators.

[0064] As a separator for a porous sheet formed of polymers and / or fibers, a microporous material (microporous membrane) can be used, for example. Specific forms of porous sheets formed of such polymers and / or fibers include, for example, polyolefins such as polyethylene (PE) and polypropylene (PP); laminates formed by stacking multiple of them (e.g., a laminate forming a 3-layer structure of PP / PE / PP), microporous materials (microporous membranes) formed of hydrocarbon resins such as polyimide, aramid, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and glass fibers.

[0065] As a nonwoven fabric separator, it can be made of cotton, rayon, cellulose acetate, nylon, polyester; polyolefins such as PP and PE; and previously known nonwoven fabrics such as polyimide and aramid, either alone or in combination.

[0066] The thickness of the separator only needs to be the same as that of the electrolyte layer, preferably 5~200μm, and particularly preferably 10~100μm.

[0067] Furthermore, as a separator, a separator with a heat-resistant insulating layer laminated on a porous matrix (a separator with a heat-resistant insulating layer) can be used. The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. The separator with the heat-resistant insulating layer uses a separator with high heat resistance, having a melting point or thermal softening point of 150°C or higher, preferably 200°C or higher. By having a heat-resistant insulating layer, the internal stress of the separator, which increases with temperature rise, is mitigated, thus achieving a heat shrinkage suppression effect. As a result, it is possible to prevent short circuits between the electrodes of the battery, thus creating a battery structure that is less prone to performance degradation due to temperature rise. In addition, by having a heat-resistant insulating layer, the mechanical strength of the separator with the heat-resistant insulating layer is improved, making it less likely for the separator to rupture. Furthermore, due to the heat shrinkage suppression effect and the high mechanical strength, the separator is less likely to curl during the battery manufacturing process.

[0068] [Positive current collector and negative current collector]

[0069] The materials used to construct the current collectors (25, 27) are not particularly limited, and well-known highly conductive materials conventionally used in current collectors for lithium-ion secondary batteries can be used. Preferred materials for the current collectors include, for example, metallic materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and their alloys. From the viewpoints of lightweight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. It should be noted that the negative current collector 25 and the positive current collector 27 can be made of the same material or different materials.

[0070] As described above, the closed-cell lithium-oxygen battery of this method exhibits excellent charge and discharge efficiency. Therefore, the closed-cell lithium-oxygen battery of this method is suitable for use as a power source for EVs and HEVs.

[0071] The above describes one aspect of the closed-cell lithium-oxygen battery of the present invention. However, the present invention is not limited to the configuration described in the above embodiments and can be appropriately modified based on the claims.

[0072] It should be noted that the following embodiments are also included within the scope of the present invention: the battery of claim 1 having the features of claim 2; the battery of claim 1 or 2 having the features of claim 3; the battery of any one of claims 1 to 3 having the features of claim 4; the battery of any one of claims 1 to 4 having the features of claim 5; the battery of any one of claims 1 to 5 having the features of claim 6; the battery of claim 6 having the features of claim 7; the battery of any one of claims 1 to 7 having the features of claim 8; the battery of any one of claims 1 to 8 having the features of claim 9; the battery of any one of claims 1 to 9 having the features of claim 10; and the battery of any one of claims 1 to 10 having the features of claim 11.

[0073] Example

[0074] The present invention will now be described in more detail through embodiments. However, the scope of the present invention is not limited to the following embodiments.

[0075] Fabrication of a Closed-Type Lithium-Oxygen Battery

[0076] [Determination Method]

[0077] In the following examples and comparative examples, the values ​​of x, y, and z were measured using the following measurement methods.

[0078] (Determination of porosity (x) of the positive electrode active material layer)

[0079] The porosity (x) of the positive electrode active material layer is measured as follows:

[0080] (1) Measure the mass of each unit area of ​​the positive electrode active material layer. Then, determine the mass of each material per unit area of ​​the positive electrode active material layer from the mixing ratio of the materials.

[0081] (2) Use a micrometer to measure the thickness of the positive electrode active material layer [A].

[0082] (3) Using the mass and density of each material obtained in (1), calculate the thickness of the positive electrode active material layer when the porosity is 0% [B].

[0083] (4) Calculate the pore volume of the positive electrode active material layer from the difference (AB) between the measured thickness and the calculated thickness of the positive electrode active material layer, and determine the pore volume per 1m³. 3 The pore volume of the positive electrode active material layer is used as the percentage of the obtained value as porosity x [%).

[0084] (Determination of the proportion (y) of the volume of gel-forming polymer in the volume of the positive electrode active material layer)

[0085] Regarding the proportion (y) of the volume of the gel-forming polymer in the volume of the positive electrode active material layer, the proportion y[%] of the volume of the gel-forming polymer in the volume of the positive electrode active material layer is calculated based on the following formula:

[0086] Volume of gel-forming polymer = (mass of gel-forming polymer) / (density of gel-forming polymer)

[0087] The proportion of the volume of gel-forming polymer (y) = (volume of gel-forming polymer) / [total volume of positive electrode active material layer × (1-x / 100)].

[0088] (Determination of the electrolyte uptake rate (z) of gel-forming polymers)

[0089] Regarding the absorbance (z) of the gel-forming polymer relative to the electrolyte, the weight of the gel-forming polymer before and after impregnation in the electrolyte was measured and calculated using the following formula:

[0090] Liquid absorption rate (%) = [(Weight of gel-forming polymer after electrolyte impregnation - Weight of gel-forming polymer before electrolyte impregnation) / Weight of gel-forming polymer before electrolyte impregnation] × 100

[0091] At this point, the same electrolyte used in the manufacture of the battery is used as the electrolyte.

[0092] [Example 1]

[0093] (Preparation of cathode materials)

[0094] Lithium oxide (Li2O, manufactured by High Purity Chemical Research Institute Co., Ltd.) as the positive electrode active material (lithium oxide) and cobalt oxide (Co3O4, manufactured by High Purity Chemical Research Institute Co., Ltd.) as the catalyst were placed in a separate 70mL jar for a planetary ball mill and pulverized separately using a planetary ball mill (pulverization conditions: 40g of 3mmφ zirconia balls and 15 15mmφ zirconia balls were used at a speed of 400rpm for 1 hour).

[0095] Next, 5g of pulverized lithium oxide and 15g of cobalt oxide were placed in a 70mL jar for a planetary ball mill and mixed in a planetary ball mill (mixing conditions: using 40g of 3mmφ zirconia balls and 15 15mmφ zirconia balls, processing at 400rpm for 1 hour).

[0096] On the other hand, PVdF-HFP (Kyner Flex 2501, manufactured by Arkema, with a structural unit ratio derived from hexafluoropropylene of 6.9 mol%; liquid absorption rate (z) = 13%), which is a gel-forming polymer, and an appropriate amount of N-methyl-2-pyrrolidone (NMP, manufactured by Fujifilm and Kojun Pharmaceutical Co., Ltd.) as a solvent were mixed in a planetary stirring mixing apparatus "Defoaming Rentarō" (ARE-310, manufactured by THINKY Co., Ltd.) (mixing conditions: 2000 rpm for 5 minutes), and PVdF-HFP was dissolved in NMP. The resulting PVdF-HFP NMP solution was mixed with the lithium oxide and cobalt oxide mixture obtained by ball milling, and acetylene black (AB) (Li-400, manufactured by Denka Co., Ltd., average primary particle size: 48 nm, aspect ratio: 1) as a conductive additive. The mixture was further kneaded using the aforementioned mixing apparatus (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 allow the NMP to evaporate. The remaining powder was then recovered and manually mixed for 5 minutes using an agate mortar to prepare the cathode material of Example 1. It should be noted that in the cathode material of this example, the mixing ratio (mass ratio) of lithium oxide:cobalt oxide:acetylene black:PVdF-HFP is 20:60:10:10.

[0097] (The production of the positive electrode)

[0098] NMP was added to the prepared cathode material at a solid content concentration of 25% by mass to adjust the viscosity and prepare a cathode slurry. The cathode slurry was uniformly coated onto an aluminum foil placed on a smoothing disc using a doctor blade to achieve a final cathode active material layer thickness of 200 μm. It was then dried on a hot plate heated to 80°C for 30 minutes, followed by pressure treatment of the resulting laminate using a roller press. Next, it was transferred to a vacuum dryer and dried under vacuum at 130°C for 8 hours, thereby producing the cathode of this embodiment with a cathode active material layer formed on the surface of the aluminum foil. It should be noted that the porosity (x) of the cathode active material layer is 28%. Furthermore, the volume ratio (y) of the gel-forming polymer (PVdF-HFP) in the cathode active material layer is 12.2. As a result, the value of yz / x is calculated to be 12.2 × 13 / 28 = 5.7.

[0099] (Fabrication of a closed-cell lithium-oxygen battery (coin cell))

[0100] The positive electrode and the counter electrode lithium electrode prepared above are positioned opposite each other, with a separator (polyolefin, thickness: 20 μm) placed between them. Next, the stack of the positive electrode, separator, and counter electrode lithium electrode is placed into a coin cell (CR2032, material: stainless steel (SUS316)), and the electrolyte described below is injected using a syringe. The cell is then sealed to fabricate the closed lithium-oxygen battery (coin cell) of this embodiment. It should be noted that the electrolyte described above is prepared by dissolving lithium hexafluorophosphate (LiPF6) as a lithium salt in an organic solvent prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of EC:DEC=3:7 (volume ratio) at a concentration of 1 mol / L.

[0101] [Examples 2-4]

[0102] The conditions for the pressurization treatment were varied such that the porosity (x[%)) of the positive electrode active material layer and the proportion of the volume of the gel-forming polymer in the volume of the positive electrode active material layer (y[%)) were the values ​​shown in Table 1 below. Otherwise, the closed lithium-oxygen batteries (coin cell cells) of these embodiments were manufactured using the same method as in Example 1 above.

[0103] [Example 5]

[0104] The amounts of positive electrode active material (lithium oxide) and catalyst (cobalt oxide) were changed to the values ​​shown in Table 1 below, and the conditions for pressurization treatment were changed in such a way that the porosity (x [%)) of the positive electrode active material layer and the proportion of the volume of the gel-forming polymer in the volume of the positive electrode active material layer (y [%)) were changed to the values ​​shown in Table 1 below. Otherwise, the closed lithium-oxygen battery (coin cell) of this embodiment was manufactured using the same method as in Example 1 above.

[0105] [Example 6]

[0106] As the gel-forming polymer, PVdF-HFP (Kyner Flex 2851, manufactured by Arkema, with a structural unit ratio derived from hexafluoropropylene of 2.4 mol%; liquid absorption rate (z) = 45%) was used instead of PVdF-HFP (Kyner Flex 2501). The conditions for pressurization were changed so that the porosity (x%) of the positive electrode active material layer and the proportion of the volume of the gel-forming polymer in the volume of the positive electrode active material layer (y%) were the values ​​shown in Table 1 below. Otherwise, the closed lithium-oxygen battery (coin cell) of this embodiment was fabricated using the same method as in Example 1 above.

[0107] [Example 7]

[0108] As the gel-forming polymer, PVDF (KUREHA KF Polymer W#9700, manufactured by Kureha; liquid absorption rate (z) = 95%) was used instead of PVdF-HFP (Kyner Flex 2501), and the conditions for pressurization were changed so that the porosity (x%) of the positive electrode active material layer and the proportion of the volume of the gel-forming polymer in the volume of the positive electrode active material layer (y%) were the values ​​shown in Table 1 below. Otherwise, the closed lithium-oxygen battery (coin cell) of this embodiment was manufactured using the same method as in Example 5 above.

[0109] [Comparative Example 1]

[0110] The amounts of positive electrode active material (lithium oxide), catalyst (cobalt oxide), and gel-forming polymer (PVdF-HFP) were changed to the values ​​shown in Table 1 below, and the conditions for pressurization were changed in such a way that the porosity (x [%)) of the positive electrode active material layer and the proportion of the volume of the gel-forming polymer in the volume of the positive electrode active material layer (y [%)) were the values ​​shown in Table 1 below. Otherwise, the closed lithium-oxygen battery (coin cell) of this comparative example was manufactured using the same method as in Example 1 above.

[0111] [Comparative Example 2]

[0112] As the gel-forming polymer, PVDF (Kureha KF Polymer W#7200, manufactured by Kureha; liquid absorption rate (z) = 120%) was used instead of PVdF-HFP (Kyner Flex 2501), and the pressure treatment conditions were changed so that the porosity (x%) of the positive electrode active material layer and the proportion of the volume of the gel-forming polymer in the volume of the positive electrode active material layer (y%) were the values ​​shown in Table 1 below. Otherwise, the closed lithium-oxygen battery (coin cell) of this comparative example was manufactured using the same method as in Example 5 above.

[0113] Evaluation of Coin Cells (Determination of Initial Charge / Discharge Efficiency)

[0114] For the enclosed lithium-oxygen batteries (coin cell cells) of the above-described embodiments and comparative examples, the following charge-discharge tests (initial charge-discharge) were conducted in a constant temperature bath set to 300K (27°C), and the charging capacity and discharging capacity were measured respectively. Then, the ratio of the discharging capacity to the charging capacity was calculated as the initial charge-discharge efficiency. The results are shown in Table 1 below. It should be noted that the initial charge-discharge efficiency values ​​shown in Table 1 are relative values ​​when the measured value of Comparative Example 2 is set to 100.

[0115] (Charge and discharge test conditions)

[0116] Charge and discharge test machine:

[0117] TOSCAT-3000, model TYS-30TU10 (manufactured by Toyosystem Corporation)

[0118] Charge and discharge conditions:

[0119] [Charging process] 0.02C (current density 18mA / g), 1.8V→4.6V (CCCV; 0.01C cutoff)

[0120] [Discharge process] 0.02C (current density 18mA / g), 4.6V→1.8V (CC)

[0121] The pause time between the charging and discharging processes is 30 minutes.

[0122] [Table 1]

[0123]

[0124] As shown in Table 1, according to the present invention, by including a gel-forming polymer in addition to lithium oxide (Li2O) and a catalyst in the electrode active material layer, and by controlling the above parameters (x, y, z) to a predetermined relationship, the charge and discharge efficiency of a closed lithium-oxygen battery can be improved.

[0125] Explanation of reference numerals in the attached figures

[0126] 10A stacked lithium-oxygen battery

[0127] 11' Positive current collector,

[0128] 12 Negative current collector,

[0129] 13. Positive electrode active material layer

[0130] 15. Negative electrode active material layer

[0131] 17 Electrolyte layer

[0132] 19 Single-cell layer

[0133] 21. Power generation components

[0134] 25 Positive current collector (positive electrode tab)

[0135] 27. Negative current collector (negative electrode tab)

[0136] 29-layer laminated film.

Claims

1. A closed lithium-oxygen battery, comprising: an electrode, wherein an electrode active material layer containing a lithium oxide, a catalyst and a gel-forming polymer is disposed on a surface of a current collector; and an electrolyte layer formed by impregnating a separator disposed adjacent to the electrode with an electrolytic solution, where when the porosity of the electrode active material layer is defined as x[%], the ratio of the volume of the gel-forming polymer to the volume of the electrode active material layer is defined as y[%], and the liquid absorption rate of the gel-forming polymer relative to the electrolytic solution is defined as z[%], the relationship of the following formula (1) is satisfied: 0<yz / x≤8.7 … Formula (1).

2. The closed lithium-oxygen battery according to claim 1, which further satisfies the following formula (2): 1.4≤yz / x≤8.7 … Formula (2).

3. The closed lithium-oxygen battery according to claim 2, which further satisfies the following formula (3): 3.2≤yz / x≤5.7 … Formula (3).

4. The closed-type lithium-oxygen battery according to claim 1 or 2, wherein, y satisfies 7.0≤y≤15.

0.

5. The closed-type lithium-oxygen battery according to claim 1 or 2, wherein, x satisfies 25≤x≤50.

6. The closed-cell lithium-oxygen battery according to claim 1 or 2, wherein, The gel-forming polymer contains a structural unit derived from vinylidene fluoride in its main chain.

7. The closed-cell lithium-oxygen battery according to claim 6, wherein, The gel-forming polymer comprises vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP).

8. The closed-type lithium-oxygen battery according to claim 1 or 2, wherein, The lithium oxide includes one or two or more selected from the group consisting of Li₂O, LiO, Li₂O₂ and LiO₂.

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

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

11. The closed-cell lithium-oxygen battery according to claim 1 or 2, wherein, The electrode active material layer is a positive electrode active material layer.

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