Positive electrode active material for aqueous secondary battery, positive electrode for aqueous secondary battery using same, and aqueous secondary battery

Abstract

[Problem] To provide a means capable of improving discharging and charging efficiency of an aqueous secondary battery in which manganese dioxide is used as a positive electrode active material. [Solution] A substituted manganese dioxide, in which an oxygen element of manganese dioxide is substituted by one or more types of substitution elements, is used as a positive electrode active material of an aqueous secondary battery.

Description

Positive electrode active material for aqueous secondary battery, and aqueous secondary battery positive electrode and aqueous secondary battery using the same

[0001] The present invention relates to a positive electrode active material for aqueous secondary batteries, and a positive electrode for aqueous secondary batteries and aqueous secondary batteries using the same.

[0002] In recent years, the widespread use of various electric vehicles is expected to help solve environmental and energy problems. As on-board power sources for driving motors and other components that hold the key to the widespread use of these electric vehicles, extensive development has been conducted on aqueous secondary batteries, such as manganese oxide-zinc secondary batteries, which use manganese dioxide as the positive electrode active material and an electrolyte such as an alkaline aqueous solution.

[0003] Conventionally, lambda-type manganese dioxide (λ-MnO ２ In an alkaline battery, which is a primary battery using λ-MnO ２ A technique has been proposed in which a portion of the Mn constituting the cathode is substituted with one or more elements selected from the group consisting of Ni, Co, Ti, Fe, Mg, Zn, and Sn, and electrolytic manganese dioxide are used together as a cathode active material (Patent Document 1: JP 2007-123149 A). According to Patent Document 1, the use of such a cathode active material improves the stability of λ-MnO in an alkaline electrolyte. ２ It is said that this improves the stability of the crystal structure of the battery, and prevents the deterioration of high-load discharge characteristics after storage of the battery.

[0004] However, according to the investigations of the present inventors, the λ-MnO in which a part of Mn is substituted, as described in Patent Document 1, is used as a positive electrode active material for aqueous secondary batteries such as manganese zinc oxide secondary batteries. ２ It has been found that when using a battery, sufficient charge-discharge efficiency may not be obtained.

[0005] Therefore, an object of the present invention is to provide a means for improving the charge-discharge efficiency of an aqueous secondary battery that uses manganese dioxide as a positive electrode active material.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by using a substituted manganese dioxide, in which the oxygen element of manganese dioxide is substituted with a substituting element, as a positive electrode active material for an aqueous secondary battery, thereby completing the present invention.

[0007] That is, one aspect of the present invention is a positive electrode active material for an aqueous secondary battery, which contains a substituted manganese dioxide in which the oxygen element of manganese dioxide is substituted with one or more substituting elements.

[0008] 1 is a cross-sectional view schematically illustrating a stacked (flat) non-bipolar (internal parallel connection) manganese oxide zinc secondary battery according to one embodiment of the present invention.

[0009] In one aspect, the present invention provides a cathode active material for an aqueous secondary battery, which comprises a substituted manganese dioxide in which the oxygen element of manganese dioxide is substituted with one or more substituting elements. The cathode active material according to this aspect can improve the charge-discharge efficiency of an aqueous secondary battery using manganese dioxide as the cathode active material.

[0010] Hereinafter, the above-mentioned embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. In this specification, the range "X to Y" means "X or more and Y or less." Furthermore, unless otherwise specified, operations and measurements of physical properties, etc. are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0011] <<Aqueous Secondary Battery>> FIG. 1 is a cross-sectional view schematically illustrating a flat (stacked) non-bipolar (internal parallel connection) manganese zinc oxide secondary battery (hereinafter also simply referred to as a "stacked secondary battery"), which is one embodiment of the positive electrode active material for an aqueous secondary battery according to one aspect of the present invention.

[0012] 1 , the stacked secondary battery 10a of this embodiment has a structure in which a substantially rectangular power generating element 21, in which charge / discharge reactions actually proceed, is sealed inside a laminate film 29. Here, the power generating element 21 has a configuration in which a positive electrode in which a positive electrode active material layer 13 is disposed on both sides of a positive electrode current collector 11′, an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode in which a negative electrode active material layer 15 is disposed on both sides of a negative electrode current collector 12 are laminated. Specifically, the positive electrode, the electrolyte layer, and the negative electrode are laminated in this order, with one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 facing each other with the electrolyte layer 17 interposed therebetween.

[0013] As a result, the positive electrode, electrolyte layer, and negative electrode constitute one cell layer 19. Therefore, the stacked secondary 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. Although the positive electrode active material layer 13 is disposed on only one side of each of the outermost positive electrode current collectors located on both outermost layers of the power-generating element 21, active material layers may be disposed on both sides. That is, instead of using a current collector exclusively for the outermost layer with an active material layer disposed on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. Furthermore, by reversing the arrangement of the positive electrode and negative electrode from FIG. 1 , the outermost negative electrode current collectors may be located on both outermost layers of the power-generating element 21, and negative electrode active material layers may be disposed on one or both sides of the outermost negative electrode current collectors.

[0014] A positive electrode current collector 25 and a negative electrode current collector 27 that are electrically connected to the electrodes (positive and negative electrodes) are attached to the positive electrode current collector 11′ and the negative electrode current collector 12, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29. The positive electrode current collector 25 and the negative electrode current collector 27 may be attached to the positive electrode current collector 11′ and the negative electrode current collector 12 of the electrodes by ultrasonic welding, resistance welding, or the like, via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as needed.

[0015] The main components of the manganese zinc oxide secondary battery according to this embodiment will be described below.

[0016] [Current Collector] The current collector has a function of mediating the movement of electrons from the positive electrode active material layer and the negative electrode active material layer described later. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

[0017] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. A foil having a metal surface coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering. Examples of conductive resins include resins containing a conductive filler added to a non-conductive polymer material.

[0018] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector includes at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, if the positive electrode active material layer and the negative electrode active material layer described later are electrically conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate member from these electrode active material layers. In such a configuration, the positive electrode active material layer described later constitutes the positive electrode, and the negative electrode active material layer described later constitutes the negative electrode.

[0019] [Positive Electrode Active Material Layer] In the manganese zinc oxide secondary battery according to this embodiment, the positive electrode active material layer contains a positive electrode active material, and the positive electrode active material contains the positive electrode active material according to one embodiment of the present invention.

[0020] <Positive Electrode Active Material> The positive electrode active material contains substituted manganese dioxide in which the oxygen element of manganese dioxide is substituted with one or more kinds of substitution elements.

[0021] The type of substituting element can be any element that can exhibit the effects of the present invention and does not adversely affect the performance or safety of the battery. For example, the substituting element can include an element having an electronegativity lower than that of oxygen. Examples of such elements include nitrogen, phosphorus, sulfur, selenium, chlorine, bromine, and iodine. On the other hand, the substituting element may include an element (e.g., fluorine) having an electronegativity higher than that of oxygen. The substituting element may be one type only, or two or more types. Among these, the substituting element preferably includes nitrogen or sulfur, and more preferably includes nitrogen.

[0022] It has been known that aqueous secondary batteries using manganese dioxide as a positive electrode active material have a high voltage during charging. This is due to the protons (H ＋ ) strongly binds to the oxygen that makes up manganese dioxide, and as a result, this bond is broken during charging, and protons (H ＋ This is thought to be due to the fact that an overvoltage is required to desorb the protons (H ＋ ) that can be desorbed during charging becomes low, resulting in the problem of insufficient charge-discharge efficiency.

[0023] On the other hand, as described above, if the substitution element contains an element having an electronegativity smaller than that of the oxygen element, the protons inserted during discharge (H ＋ ) and the substitution element are weaker than the bond with oxygen. As a result, protons (H ＋ ) is required to desorb a proton (H ＋ It is believed that this promotes the desorption of protons (H) during discharge, thereby improving the charge-discharge efficiency. ＋ The bond between the protons (H) and the substitution element is stronger than the bond with oxygen. As a result, the interatomic distance of this bond becomes shorter, creating gaps within the crystal structure of manganese dioxide, allowing protons (H＋ It is believed that the increase in the diffusion path of the cations promotes the charging reaction (i.e., improves the charge-discharge efficiency).

[0024] Manganese dioxide (MnO ２ The percentage of oxygen elements constituting the manganese dioxide that are substituted by the above-mentioned substituting elements can be determined appropriately in consideration of the desired performance. For example, relative to the total amount of oxygen elements constituting manganese dioxide (100 mol%), the substitution percentage of oxygen elements by the above-mentioned substituting elements is preferably 0.01 to 50 mol%, more preferably 0.02 to 30 mol%, even more preferably 0.03 to 20 mol%, particularly preferably 0.04 to 15 mol%, and most preferably 0.05 to 10 mol%. The value of this substitution percentage is measured by inductively coupled plasma analysis (ICP).

[0025] The substituted manganese dioxide may form secondary particles. Secondary particles refer to aggregates of primary particles. Primary particles refer to the smallest unit of separable solid particles having interparticle boundaries. The average secondary particle diameter of the substituted manganese dioxide as a positive electrode active material is, for example, less than 100 μm, preferably less than 20 μm, and more preferably less than 10 μm. The lower limit of the average secondary particle diameter of the substituted manganese dioxide is not particularly limited, but may be, for example, 0.1 μm or more, 0.3 μm or more, or 0.6 μm or more. That is, the average secondary particle diameter of the substituted manganese dioxide as a positive electrode active material is, for example, 0.1 μm or more and less than 100 μm, preferably 0.3 μm or more and less than 20 μm, and more preferably 0.6 μm or more and less than 10 μm. In this specification, the average secondary particle diameter of the substituted manganese dioxide as a positive electrode active material is measured using a particle size distribution analyzer using a laser diffraction / scattering method, and is expressed as a 50% cumulative diameter (D ５０ ) is the value calculated as

[0026] The average primary particle size of the substituted manganese dioxide as a positive electrode active material is not particularly limited, but the crystallite size estimated by Scherrer's formula from the 101 peak confirmed by XRD measurement is, for example, 0.005 μm to 1.5 μm. In addition, the ratio of the average primary particle size to the average secondary particle size of the substituted manganese dioxide (average secondary particle size / average primary particle size) is not particularly limited, but is, for example, 1 to 1,000.

[0027] Manganese dioxide (MnO), a raw material for substituted manganese dioxide, ２ ) has been widely used as a positive electrode active material for manganese dry batteries and the like, and its manufacturing method includes a liquid phase method such as an electrolytic method or a chemical synthesis method, but is not particularly limited here.

[0028] Electrolytic manganese dioxide can be appropriately produced by a conventionally known method, for example, by the method described in JP 2017-179583 A. Specifically, for example, a mixture of an aqueous sulfuric acid solution and an aqueous manganese sulfate solution is used as an electrolyte, and an electrolytic cell is prepared with appropriately selected anodes and cathodes, and an electrolytic reaction is allowed to proceed, resulting in electrolytic manganese dioxide being deposited and deposited on the anode.

[0029] Chemically synthesized manganese dioxide can be appropriately produced by a conventionally known method. Specifically, for example, a method of dissolving potassium permanganate in an aqueous hydrochloric acid solution and performing hydrothermal synthesis can be mentioned. The heat treatment (calcination) temperature during hydrothermal synthesis is, for example, 140 to 180°C, preferably 150 to 170°C. The heat treatment (calcination) time is, for example, 2 to 24 hours, preferably 6 to 18 hours. Alternatively, for example, manganese dioxide can be synthesized by mixing an aqueous potassium permanganate solution, an aqueous manganese sulfate solution, and an aqueous sodium hydroxide solution.

[0030] Alternatively, manganese dioxide obtained by electrolysis, chemical synthesis, or other methods may be appropriately pulverized to obtain a positive electrode active material having a desired particle size. For pulverization, for example, a roller mill, a jet mill, or the like may be used, but is not limited to these. Furthermore, manganese dioxide having a desired average secondary particle size may be produced by classification using an appropriate classification device, for example, a sieve, a classifier, or the like.

[0031] There is no particular limitation on the method for obtaining the substituted manganese dioxide as the positive electrode active material (the method for substituting the oxygen element with a substituting element). For example, when nitrogen is used as the substituting element, nitrogen gas (N ２ ) and ammonia gas (NH ３ A gas phase reaction can be employed in which manganese dioxide is heated in an atmosphere of fluorine (F) to promote the substitution. In this case, the substitution rate can be increased by extending the reaction time. In addition, fluorine (F) can be used instead of nitrogen gas. ２ ), chlorine (Cl ２ ), bromine (Br ２ ), iodine (I ２ Substitution with these elements is possible by performing a gas-phase reaction in the presence of gases such as HCl, ...

[0032] Furthermore, substitution by a liquid phase reaction is also possible, not just a gas phase reaction. For example, urea (CO(NH) ２ ) ２ In the presence of a nitrogen source such as manganese sulfate (MnSO ４ Substituted manganese dioxide can also be obtained by hydrothermal synthesis, in which a metal salt of Mn(II), such as manganese dioxide (II), is oxidized with an oxidizing agent, such as sulfuric acid or ammonium peroxysulfate. In this case, the substitution ratio can be increased by extending the reaction time or increasing the amount of the nitrogen source.

[0033] Furthermore, when substitution is performed with an element (sulfur or phosphorus) that exhibits a simple substance with a melting point of 200° C. or less, it is possible to obtain substituted manganese dioxide substituted with this substitution element by melting the simple substance of the substitution element in an argon atmosphere and heating and holding it together with manganese dioxide at about 200° C. In this case, the substitution rate can be increased by extending the heating and holding time or by increasing the amount of the substitution element in the reaction system during heating.

[0034] The positive electrode active material may contain a positive electrode active material other than substituted manganese dioxide, may contain only substituted manganese dioxide and unsubstituted manganese dioxide, or may contain only substituted manganese dioxide. Here, the proportion of the substituted manganese dioxide relative to the total mass of the positive electrode active material is preferably greater than 50 mass%, more preferably 70 mass% or more, even more preferably 80 mass% or more, still more preferably 90 mass% or more, even more preferably 95 mass% or more, particularly preferably 97 mass% or more, and most preferably 100 mass%. In one embodiment, the positive electrode active material preferably does not contain any positive electrode active material other than substituted manganese dioxide and unsubstituted manganese dioxide, and more preferably does not contain any positive electrode active material other than substituted manganese dioxide. That is, the positive electrode active material preferably consists of only substituted manganese dioxide and unsubstituted manganese dioxide, and more preferably consists of only substituted manganese dioxide.

[0035] The content of the positive electrode active material in the positive electrode active material layer is preferably 30 to 99 mass %, more preferably 50 to 85 mass %, and even more preferably 60 to 80 mass %, relative to the total mass of the positive electrode active material layer.

[0036] <Additive Components> The positive electrode active material layer preferably further contains a conductive additive, a binder, or a metal (oxide) in addition to the above-described positive electrode active material.

[0037] (Conductive additive) The conductive additive has a function of forming an electron conduction path (conductive passage) in the positive electrode active material layer. When such an electron conduction path is formed in the positive electrode active material layer, the internal resistance of the battery can be reduced and the rate characteristics can be improved.

[0038] Examples of the conductive aid include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark), and fibrous carbon materials such as carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), carbon nanofibers, vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile-based carbon fibers, and pitch-based carbon fibers. One type of conductive aid may be used alone, or two or more types may be used in combination.

[0039] The content of the conductive additive that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 1 to 30 mass % and more preferably 5 to 20 mass % relative to 100 mass % of the total solid content of the positive electrode active material layer.

[0040] (Binder) The optional binder used in the positive electrode active material layer is not particularly limited, and examples thereof include the following materials: polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, thermoplastic polymers such as styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene Fluorine resins such as polytetrafluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluoroelastomers (VDF-HFP fluoroelastomers), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluoroelastomers (VDF-HFP-TFE fluoroelastomers), and vinylidene fluoride-pentafluoropropylene fluoroelastomers (VDF-PFP fluoroelastomers). vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers); and epoxy resins.Among these, polyvinylidene fluoride (PVDF), polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are preferred.

[0041] The content of the binder that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 0.5 to 20 mass % and more preferably 1 to 10 mass % with respect to the total solid content of the positive electrode active material layer.

[0042] (Metal (Oxide)) The metal (oxide) is a simple metal or its oxide, and is a substance that improves the discharge and charge efficiency when added to the positive electrode active material layer of the aqueous secondary battery according to this embodiment. There are no particular limitations on the type of such metal (oxide), but examples include bismuth, lead, tin, silver, copper, titanium, nickel, cobalt, iron, manganese, zinc, and oxides thereof. These metals (oxides) may be used alone or in combination of two or more. Among them, from the viewpoint of a high effect of improving the discharge and charge efficiency, it is preferable to use bismuth or its oxide (bismuth oxide), and it is particularly preferable to use bismuth oxide. When the positive electrode active material layer contains these metals (oxides), the cations of the metal enter between the layers or lattices of the manganese dioxide and function as pillars, thereby stabilizing the crystal structure of the manganese dioxide. In unsubstituted manganese dioxide, the pillars of this metal act as protons (H ＋ ) diffusion, but in the substituted manganese dioxide according to one embodiment of the present invention, the substitution element functions to inhibit the diffusion of protons (H ＋ ) diffusion path, which is thought to further improve the charge-discharge efficiency.

[0043] The content of the metal (oxide) that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 0.5 to 20 mass % and more preferably 1 to 15 mass % with respect to the total solid content of the positive electrode active material layer.

[0044] The thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be appropriately referenced. For example, the thickness of the positive electrode active material layer is usually about 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, and even more preferably 40 to 200 μm. The thicker the positive electrode active material layer, the more positive electrode active material can be retained to achieve sufficient capacity (energy density). On the other hand, the thinner the positive electrode active material layer, the more the discharge rate characteristics can be improved.

[0045] [Negative Electrode Active Material Layer] In the manganese zinc oxide secondary battery according to this embodiment, the negative electrode active material layer contains a negative electrode active material.

[0046] (Negative electrode active material) The negative electrode active material of the manganese zinc oxide secondary battery contains zinc. The zinc may be in the form of zinc metal, a zinc compound, or a zinc alloy, as long as it has electrochemical activity suitable for the negative electrode. Preferred examples of the negative electrode active material include zinc oxide, zinc metal, and calcium zincate, with zinc metal or a zinc alloy being more preferred.

[0047] As the zinc alloy, a mercury- and lead-free zinc alloy known as a mercury-free zinc alloy can be used. For example, a zinc alloy containing 0.01 to 0.06 mass% indium, 0.005 to 0.02 mass% bismuth, and 0.0035 to 0.015 mass% aluminum is preferred from the perspective of its hydrogen gas generation suppression effect. In particular, indium and bismuth are advantageous in improving discharge performance. The use of a zinc alloy as a negative electrode active material has the advantage of slowing the rate of self-dissolution in alkaline electrolyte, thereby suppressing hydrogen gas generation and improving safety.

[0048] The negative electrode active material layer preferably further contains a thickener in addition to the above-described negative electrode active material. By including a thickener in the negative electrode active material layer, the negative electrode active material layer can be made into a gel. Examples of thickeners include polyvinyl alcohol, polyacrylate, carboxymethyl cellulose, alginic acid, etc., with polyacrylate being preferred due to its excellent chemical resistance to strong alkalis. Furthermore, the negative electrode active material layer may further contain the above-described conductive additive and binder, if necessary.

[0049] The shape of the negative electrode active material is not particularly limited, and may be powder or plate-like, but powder is preferred, which increases the surface area and enables it to withstand large current discharge. The average particle diameter of the negative electrode active material is preferably in the range of 50 to 1000 μm, more preferably 70 to 500 μm, and even more preferably 90 to 210 μm, in the case of a zinc alloy. Within this range, the large surface area makes it suitable for large current discharge, and it is easily mixed uniformly with the electrolyte and the thickener used as needed, making it easy to handle during battery assembly.

[0050] [Electrolyte Layer] In the manganese zinc oxide secondary battery according to this embodiment, the electrolyte layer is disposed adjacent to the electrode active material layer.

[0051] In the manganese oxide zinc secondary battery according to this embodiment, the electrolyte is preferably an aqueous solution containing an alkali metal hydroxide. Examples of alkali metal hydroxides include potassium hydroxide, sodium hydroxide, and lithium hydroxide, with potassium hydroxide being more preferred. Alternatively, an aqueous solution containing another hydroxide (such as ammonium hydroxide) may be used as the electrolyte. To suppress the self-dissolution of the zinc alloy, a zinc compound such as zinc oxide or zinc hydroxide may be added to the electrolyte. Typically, the electrolyte exists in a state where it has penetrated into the positive electrode active material layer and the negative electrode active material layer.

[0052] In order to prevent leakage of the electrolyte solution, the electrolyte solution may be gelled to form an electrolyte layer, or the separator may be impregnated with the electrolyte solution to form an electrolyte layer. As the gelling agent, a polymer that absorbs the solvent of the electrolyte solution and swells is preferably used, and examples of such polymers include polyethylene oxide, polyvinyl alcohol, polyacrylamide, and starch. Examples of the separator include a porous sheet separator made of a polymer or fiber that absorbs and retains the electrolyte solution, and a nonwoven fabric separator.

[0053] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 may be made of the same material or different materials.

[0054] Although not shown, the current collectors (11', 12) and the current collector plates (25, 27) may be electrically connected via positive and negative electrode leads. Materials used in known aqueous secondary batteries may be used as the constituent materials for the positive and negative electrode leads. It is preferable that the portion removed from the exterior be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting products (e.g., automotive parts, particularly electronic devices).

[0055] [Battery Exterior Body] As the battery exterior body, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element, as shown in FIG. 1, can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of its high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminate laminate is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside and allows for easy adjustment of the electrolyte layer thickness to the desired value.

[0056] Although a manganese oxide zinc secondary battery has been described above as an example of an embodiment of an aqueous secondary battery, the present invention is not limited to the configurations described in the above-described embodiment and can be modified as appropriate based on the claims. For example, the type of aqueous secondary battery according to this embodiment may be any battery in which manganese dioxide functions as a positive electrode active material, such as a manganese hydrogen battery or a redox flow battery.

[0057] The following items are also included within the scope of the present invention: Item 1: A positive electrode active material for an aqueous secondary battery, comprising a substituted manganese dioxide in which the oxygen element of manganese dioxide is substituted with one or more substitution elements;Item 2: The positive electrode active material for an aqueous secondary battery according to Item 1, wherein the substituting element comprises an element having an electronegativity smaller than that of oxygen; Item 3: The positive electrode active material for an aqueous secondary battery according to Item 1, wherein the substituting element comprises an element having an electronegativity larger than that of oxygen; Item 4: The positive electrode active material for an aqueous secondary battery according to Item 1 or 2, wherein the substituting element comprises nitrogen or sulfur; Item 5: The positive electrode active material for an aqueous secondary battery according to any one of Items 1 to 4, wherein a substitution ratio of the oxygen element with the substituting element is 0.01 to 50 mol %; Item 6: The positive electrode active material for an aqueous secondary battery according to Item 5, wherein a substitution ratio of the oxygen element with the substituting element is 0.05 to 1 mol %; Item 7: A positive electrode for an aqueous secondary battery having a positive electrode active material layer, wherein the positive electrode active material layer comprises the positive electrode active material for an aqueous secondary battery according to any one of Items 1 to 6; Item 8: The positive electrode for an aqueous secondary battery according to Item 7, wherein the positive electrode active material layer further contains one or more additives selected from the group consisting of bismuth, lead, tin, silver, copper, titanium, nickel, cobalt, iron, manganese, zinc, and oxides thereof; Item 9: The positive electrode for an aqueous secondary battery according to Item 8, wherein the additive contains bismuth oxide; Item 10: The positive electrode for an aqueous secondary battery according to any one of Items 7 to 9, wherein the positive electrode active material layer further contains a conductive additive and / or a binder; Item 11: The positive electrode for an aqueous secondary battery according to Item 10, wherein the positive electrode active material layer contains acetylene black as the conductive additive and polytetrafluoroethylene as the binder; Item 12: An aqueous secondary battery comprising the positive electrode for an aqueous secondary battery according to any one of Items 7 to 11, an electrolyte layer containing an electrolytic solution, and a negative electrode containing a negative electrode active material; Item 13: The aqueous secondary battery according to Item 12, wherein the negative electrode active material contains zinc; Item 14: The aqueous secondary battery according to Item 13, wherein the negative electrode active material comprises zinc metal, a zinc compound, or a zinc alloy; Item 15: The aqueous secondary battery according to any one of Items 12 to 14, wherein the electrolyte is an aqueous solution containing an alkali metal hydroxide; Item 16: The aqueous secondary battery according to Item 15, wherein the alkali metal hydroxide comprises potassium hydroxide;

[0058] The present invention will be explained in more detail below using examples and comparative examples, but it should be understood that the present invention is not limited to the following examples.

[0059] <<Fabrication of Aqueous Secondary Battery>> [Example 1] (Preparation of Positive Electrode Active Material) Manganese dioxide (MnO ２ Manganese dioxide (manufactured by Tosoh Corporation) was prepared. This manganese dioxide was placed in a furnace purged with nitrogen and baked at 200°C for 1 hour. This resulted in the preparation of a positive electrode active material made of substituted manganese dioxide in which 0.06 mol% of the oxygen element constituting the manganese dioxide was substituted with nitrogen (confirmed by inductively coupled plasma analysis (ICP)).

[0060] (Preparation of Positive Electrode) Next, a powder composition consisting of 75.0 parts by mass of the positive electrode active material (substituted manganese dioxide) prepared above and 16.7 parts by mass of acetylene black (AB) (manufactured by Denka Co., Ltd., Denka Black (registered trademark); average particle size (primary particle size): 0.023 μm) as a conductive additive was mixed at 2000 rpm for 1 minute using a planetary stirring type mixing kneader "Awatori Rentaro" (ARE-310, manufactured by Thinky Corporation). Next, a PTFE aqueous dispersion (60% dispersion) containing polytetrafluoroethylene (PTFE) (8.3 parts by mass) as a binder was added to the powder composition, and the mixture was mixed at 2000 rpm for 1 minute using the same device. Thereafter, the obtained powder composition was kneaded manually for 5 minutes, rolled using a roll press, and dried at 60 ° C. for 3 minutes to obtain a positive electrode active material layer. This positive electrode active material layer was then cut into a 2 cm square and attached to a positive electrode current collector made of nickel foil by pressing, thereby producing the positive electrode of this example.

[0061] (Preparation of aqueous secondary battery) Nickel hydroxide (Ni(OH) ２ A negative electrode active material slurry containing powder of the above compound and a binder was applied to the negative electrode and dried to form a negative electrode active material layer, thereby producing a negative electrode.

[0062] A vinylon separator was placed between the positive electrode active material layer of the positive electrode and the negative electrode active material layer of the negative electrode, facing each other, to obtain a laminate (power generating element). Nickel tabs were attached to each of the positive electrode current collector and the negative electrode current collector, and the power generating element was placed inside an exterior body made of an aluminum laminate film so that these tabs were exposed to the outside. An electrolyte solution was then injected, and the laminate film was vacuum-sealed to produce the aqueous secondary battery of this example. An 8 mol / L potassium hydroxide aqueous solution was used as the electrolyte solution.

[0063] [Example 2] (Preparation of Positive Electrode) A powder composition consisting of 62.0 parts by mass of a positive electrode active material (substituted manganese dioxide) prepared using the same method as in Example 1 described above and 16.7 parts by mass of acetylene black (Denka Black (registered trademark), manufactured by Denka Co., Ltd.; average particle size (primary particle size): 0.023 μm) as a conductive additive was mixed for 1 minute at 2000 rpm using a planetary stirring type mixing kneader "Awatori Rentaro" (ARE-310, manufactured by Thinky Corporation). Next, 13.0 parts by mass of bismuth oxide as an additive was added, and the mixture was mixed for 1 minute at 2000 rpm using the same device. Furthermore, a PTFE aqueous dispersion (60% dispersion) containing polytetrafluoroethylene (PTFE) (8.3 parts by mass) as a binder was added to the powder composition, and the mixture was mixed for 1 minute at 2000 rpm using the same device. The resulting powder composition was then kneaded manually for 5 minutes, rolled using a roll press, and dried for 3 minutes at 60° C. to obtain a positive electrode active material layer. This positive electrode active material layer was then cut into a 2 cm square and attached to a positive electrode current collector made of nickel foil by pressing, thereby producing the positive electrode of this example.

[0064] The aqueous secondary battery of this example was fabricated in the same manner as in Example 1, except that the positive electrode fabricated above was used.

[0065] [Example 3] (Preparation of Positive Electrode Active Material) Manganese dioxide (MnO ２ This manganese dioxide was mixed with sulfur (manufactured by Aldrich) (MnO ２The mixture was mixed with 10% by mass of sulfur dioxide (10% by mass relative to the mass of the manganese dioxide) and evaporated. The mixture was then heated at 150°C for 2 hours under an argon atmosphere, and finally washed with toluene to remove unreacted sulfur. This resulted in the preparation of a positive electrode active material made of substituted manganese dioxide in which 10 mol% of the oxygen element constituting the manganese dioxide had been substituted with sulfur (confirmed by inductively coupled plasma analysis (ICP)).

[0066] The aqueous secondary battery of this example was fabricated in the same manner as in Example 1 above, except that the positive electrode active material prepared above was used.

[0067] Example 4 A water-based secondary battery of this example was fabricated in the same manner as in Example 2, except that the positive electrode active material prepared in Example 3 was used.

[0068] Comparative Example 1 A water-based secondary battery of this comparative example was fabricated using the same method as in Example 1, except that manganese dioxide prepared as a raw material was used as the positive electrode active material instead of the substituted manganese dioxide.

[0069] Comparative Example 2 A water-based secondary battery of this comparative example was fabricated using the same method as in Example 2, except that manganese dioxide prepared as a raw material was used as the positive electrode active material instead of the substituted manganese dioxide.

[0070] Evaluation of Aqueous Secondary Batteries The battery performance (charge-discharge efficiency) of the aqueous secondary batteries prepared above was evaluated using the following method. The experiment was carried out in a thermostatic chamber at 298 K (25° C.).

[0071] (Charge / Discharge Test Conditions) Charging / Discharging Tester: TOSCAT-3000, Model TYS-30TU10 (manufactured by Toyo Systems Co., Ltd.) Discharge / Charge Conditions: [Discharge process] 0.2 C (current density 12 mA / g), 0.6 V → -1.5 V (CC) [Charge process] 0.2 C (current density 12 mA / g), -1.5 V → 0.6 V (CC).

[0072] The initial process for the aqueous secondary battery fabricated above was the discharge process. First, the initial discharge and charge processes were performed under the above conditions to eliminate the influence of irreversible capacity. Next, discharge and charge processes were performed under the same conditions as above, and the capacity (discharge capacity and charge capacity) during each process was measured. The percentage of the charge capacity relative to the discharge capacity was calculated to determine the discharge and charge efficiency. The results are shown in Table 1 below. Note that the discharge and charge efficiency values ​​shown in Table 1 are relative values, with the value for Comparative Example 2 set to 100.

[0073]

[0074] The results shown in Table 1 demonstrate that the present invention can improve the charge-discharge efficiency of aqueous secondary batteries using manganese dioxide as a positive electrode active material by substituting a portion of the oxygen element in manganese dioxide with a substituting element such as nitrogen or sulfur. It also demonstrates that the effect of improving charge-discharge efficiency can be enhanced by further adding an additive such as bismuth oxide to a system using the positive electrode active material of the present invention.

[0075] Although nickel hydroxide was used as the negative electrode active material in the above experimental examples, the reactions at the positive electrode during charging and discharging are the same in aqueous secondary batteries using other negative electrode active materials. Therefore, it is believed that the effects of the present invention demonstrated above will also be achieved in aqueous secondary batteries using negative electrode active materials other than nickel hydroxide (for example, those containing zinc).

[0076] This application claims priority to Japanese Patent Application No. 2024-052853, filed on March 28, 2024, the contents of which are incorporated herein by reference in their entirety.

[0077] 10a Stacked secondary battery, 11' Positive electrode current collector 12 Negative electrode current collector 13 Positive electrode active material layer, 15 Negative electrode active material layer, 17 Electrolyte layer, 19 Single cell layer, 21 Power generating element, 25 Positive electrode current collector (positive electrode tab), 27 Negative electrode current collector (negative electrode tab), 29 Laminate film.

Claims

1. A positive electrode active material for aqueous secondary batteries, comprising substituted manganese dioxide in which the oxygen element of manganese dioxide is substituted with one or more substitution elements.

2. The positive electrode active material for an aqueous secondary battery according to claim 1, wherein the substituting element includes an element having an electronegativity smaller than that of oxygen.

3. The positive electrode active material for aqueous secondary batteries according to claim 1 or 2, wherein the substituting element includes a nitrogen element or a sulfur element.

4. The positive electrode active material for aqueous secondary batteries according to claim 1 or 2, wherein the substitution ratio of the oxygen element by the substitution element is 0.01 to 50 mol %.

5. A positive electrode for an aqueous secondary battery having a positive electrode active material layer, wherein the positive electrode active material layer contains the positive electrode active material for an aqueous secondary battery according to claim 1 or 2.

6. The positive electrode for an aqueous secondary battery according to claim 5, wherein the positive electrode active material layer further contains one or more additives selected from the group consisting of bismuth, lead, tin, silver, copper, titanium, nickel, cobalt, iron, manganese, zinc, and oxides thereof.

7. The positive electrode for an aqueous secondary battery according to claim 6, wherein the additive comprises bismuth oxide.

8. An aqueous secondary battery comprising the positive electrode for aqueous secondary batteries according to claim 5, an electrolyte layer containing an electrolytic solution, and a negative electrode containing a negative electrode active material.

9. The aqueous secondary battery of claim 8, wherein the negative electrode active material comprises zinc.

10. The aqueous secondary battery according to claim 8, wherein the electrolyte is an aqueous solution containing an alkali metal hydroxide.

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