Lithium secondary battery

Abstract

Provided is a means capable of increasing discharge capacity in a lithium secondary battery in which manganese dioxide is used as a positive electrode active material and which contains a solid electrolyte. This lithium secondary battery has a power generation element comprising: a positive electrode which has a positive electrode active material containing manganese dioxide and a positive electrode active material layer containing a first solid electrolyte; a negative electrode which has a negative electrode active material layer containing a negative electrode active material; and a solid electrolyte layer which is located between the positive electrode and the negative electrode and contains a second solid electrolyte, wherein the average secondary particle diameter of the manganese dioxide is less than 2 μm, and the average secondary particle diameter of the manganese dioxide is less than 0.9 in relation to the average secondary particle diameter of the first solid electrolyte.

Description

Lithium secondary battery

[0001] The present invention relates to a lithium secondary battery.

[0002] In recent years, there has been active research and development into lithium secondary batteries that use oxide- or sulfide-based solid electrolytes. Solid electrolytes are materials primarily composed of ionic conductors that allow ionic conduction in a solid state. Therefore, lithium secondary batteries that use solid electrolytes do not, in principle, encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the battery's power density and energy density.

[0003] However, in lithium secondary batteries using a solid electrolyte, if the proportion of the positive electrode active material contained in the positive electrode active material layer is increased in order to improve the energy density, there may be a problem that the volume of the positive electrode active material increases, resulting in an increase in battery resistance. JP 2013-196968 A discloses a technology for reducing the battery resistance of the resulting battery by controlling the volume ratio and average particle size of the positive electrode active material and solid electrolyte in the positive electrode active material layer and hot-pressing the material for forming the positive electrode active material layer at a certain temperature or higher.

[0004] According to the investigations of the present inventors, it has been found that when a secondary battery using manganese dioxide as a positive electrode active material is produced using the technology described in JP 2013-196968 A, sufficient discharge capacity may not be obtained.

[0005] Therefore, an object of the present invention is to provide a means for increasing the discharge capacity of a lithium secondary battery that uses manganese dioxide as a positive electrode active material and contains a solid electrolyte.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that in a lithium secondary battery using manganese dioxide as a positive electrode active material, the above-mentioned problems can be solved by controlling the average secondary particle size of the manganese dioxide and controlling the particle size ratio to the solid electrolyte in the positive electrode active material, thereby completing the present invention.

[0007] One aspect of the present invention relates to a lithium secondary battery including a power generating element having a positive electrode having a positive electrode active material layer containing manganese dioxide as a positive electrode active material and containing a first solid electrolyte, a negative electrode having a negative electrode active material layer containing a negative electrode active material, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a second solid electrolyte, wherein the average secondary particle diameter of the manganese dioxide is less than 2 μm and the ratio of the average secondary particle diameter of the manganese dioxide to the average secondary particle diameter of the first solid electrolyte is less than 0.9.

[0008] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention.

[0009] One aspect of the present invention is a lithium secondary battery including a power generating element having a positive electrode having a positive electrode active material layer containing manganese dioxide as a positive electrode active material and containing a first solid electrolyte, a negative electrode having a negative electrode active material layer containing a negative electrode active material, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a second solid electrolyte, wherein the average secondary particle diameter of the manganese dioxide is less than 2 μm and the ratio of the average secondary particle diameter of the manganese dioxide to the average secondary particle diameter of the first solid electrolyte is less than 0.9. According to the present invention, in a lithium secondary battery including a solid electrolyte in which the positive electrode active material layer contains manganese dioxide as a positive electrode active material, the discharge capacity of the battery can be improved.

[0010] Hereinafter, 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 description of the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0011] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) all-solid-state lithium secondary battery (hereinafter also simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. The stacked-type structure allows the battery to be compact and have a high capacity. In this specification, the stacked-type lithium secondary battery (hereinafter also simply referred to as a "stacked-type battery") shown in FIG. 1 will be described in detail as an example.

[0012] As shown in FIG. 1 , the stacked battery 10 a of this embodiment has a structure in which a flat, approximately rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior material. Here, the power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are laminated. The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are disposed on both sides of a positive electrode current collector 11 ″. The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current collector 11 ′. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are laminated in this order so that one positive electrode active material layer 15 and the adjacent negative electrode active material layer 13 face each other with the solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one cell layer 19. Therefore, the stacked battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11' and the positive electrode current collector 11" respectively, and are configured to be sandwiched between the ends of the laminate film 29 and extended to the outside of the laminate film 29. A restraining pressure is applied to the stacked secondary battery 10a in the stacking direction of the power generation element 21 by a pressure member (not shown). Therefore, the volume of the power generation element 21 is kept constant.

[0013] The main components of the lithium secondary battery according to this embodiment will be described below.

[0014] [Current Collector] The current collector has a function of mediating the transfer of electrons from the electrode active material layer. 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.

[0015] 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 include at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector.

[0016] [Positive Electrode Active Material Layer] In the lithium secondary battery according to this embodiment, the positive electrode active material layer contains a positive electrode active material. The positive electrode active material contained in the positive electrode active material layer essentially contains manganese dioxide. The manganese dioxide has an average secondary particle diameter of less than 2 μm, and the average secondary particle diameter of the manganese dioxide is less than 0.9 times the average secondary particle diameter of the solid electrolyte (first solid electrolyte) in the positive electrode active material layer. By using such a positive electrode active material, the battery capacity of the lithium secondary battery can be improved. Although the mechanism behind the above effects is not fully understood, the following mechanism is presumed. Generally, the larger the contact area between the positive electrode active material and the solid electrolyte, the smaller the resistance in ion conduction and the improved battery capacity. In the positive electrode active material layer containing the solid electrolyte, manganese dioxide particles exist in the form of secondary particles formed by aggregation of primary particles, and the surfaces of the secondary particles have a relatively rough shape. Therefore, when manganese dioxide is used as a positive electrode active material, it is thought that, compared to when other positive electrode active materials, such as lithium-containing composite oxides, are used, sufficient contact points with the solid electrolyte are not ensured, resulting in reduced battery capacity. In the lithium secondary battery according to the present invention, by controlling the average secondary particle diameter of manganese dioxide to less than 2 μm, the outer surface area of ​​the secondary particles is ensured, and the contact area between the manganese dioxide as the positive electrode active material and the solid electrolyte contained in the positive electrode active material layer is increased. Therefore, it is presumed that resistance in ion conduction can be reduced. Furthermore, by appropriately adjusting the average secondary particle diameter of the solid electrolyte contained in the positive electrode active material layer and the average secondary particle diameter of manganese dioxide as the positive electrode active material, the contact area can be further increased. Furthermore, by controlling the average secondary particle diameter of manganese dioxide as the positive electrode active material to less than 2 μm, the migration path of lithium ions moving through the secondary particles of the positive electrode active material in the positive electrode active material layer is shortened. This is also thought to reduce resistance in ion conduction.

[0017] (Positive Electrode Active Material) The positive electrode active material contained in the positive electrode active material layer is manganese dioxide (MnO ２). The positive electrode active material may contain a positive electrode active material other than manganese dioxide, or the positive electrode active material may contain only manganese dioxide. In a preferred embodiment, the positive electrode active material layer does not contain a positive electrode active material other than manganese dioxide. That is, in this preferred embodiment, the positive electrode active material consists only of manganese dioxide. When the positive electrode active material consists only of manganese dioxide, the battery capacity of the lithium secondary battery according to the present invention is improved and the battery can be produced inexpensively, resulting in excellent economic efficiency.

[0018] Manganese dioxide as a positive electrode active material can form secondary particles. Secondary particles are aggregates of primary particles. Primary particles refer to the smallest unit of separable solid particles having interparticle boundaries. The average secondary particle diameter of manganese dioxide as a positive electrode active material is less than 2 μm, preferably less than 1.5 μm, more preferably less than 1 μm, and even more preferably less than 0.8 μm. If the average secondary particle diameter of manganese dioxide exceeds 2 μm, sufficient discharge capacity cannot be obtained. The lower limit is not particularly limited, but can 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 manganese dioxide as a positive electrode active material is, for example, 0.1 μm or more and less than 2 μm, preferably 0.3 μm or more and less than 1.5 μm, and more preferably 0.6 μm or more and less than 1 μm.

[0019] In this specification, the average secondary particle diameter of manganese dioxide is measured by a particle size distribution measuring device using a laser diffraction / scattering method, and is the 50% cumulative diameter (D ５０ ) is calculated as

[0020] There are no particular limitations on the method for obtaining manganese dioxide as a positive electrode active material. Manganese dioxide may be obtained by electrolysis, chemical synthesis, or other methods, but from the viewpoint of appropriately adjusting the secondary particle size, electrolytic manganese dioxide is preferred. In this specification, "electrolytic manganese dioxide" refers to manganese dioxide obtained by electrolysis. In this specification, "chemically synthesized manganese dioxide" refers to manganese dioxide obtained by chemical synthesis.

[0021] Electrolytic manganese dioxide can be appropriately produced by a conventionally known method. Specifically, for example, an electrolytic solution is prepared by using a mixture of an aqueous sulfuric acid solution and an aqueous manganese sulfate solution, and an electrolytic cell is prepared by appropriately selecting an anode and a cathode, and an electrolytic reaction is carried out, whereby electrolytic manganese dioxide is deposited and precipitated on the anode. In this case, the secondary particle diameter may be adjusted by changing the electrolytic current density or sulfuric acid concentration. For example, electrolytic manganese dioxide can be produced by the method described in JP 2017-179583 A. Electrolytic manganese dioxide may also be produced by an electrolytic reaction other than the above.

[0022] Chemically synthesized manganese dioxide can be produced by any conventional method. Specifically, for example, it can be synthesized by mixing an aqueous solution of potassium permanganate, an aqueous solution of manganese sulfate, and an aqueous solution of sodium hydroxide. Chemically synthesized manganese dioxide may also be produced by chemical reactions other than those described above.

[0023] Manganese dioxide obtained by electrolysis, chemical synthesis, or other methods may also be appropriately pulverized to produce manganese dioxide having a desired average secondary 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 also be produced by classification using an appropriate classification device, for example, a sieve, a classifier, or the like. Manganese dioxide having a desired average secondary particle size may also be produced by appropriately mixing two or more pulverized manganese dioxides having different average secondary particle sizes.

[0024] The positive electrode active material other than manganese dioxide contained in the positive electrode active material layer is not particularly limited, but may be LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ Among them, Li(Ni-Mn-Co)O ２ Also, those in which part of these transition metals has been replaced with other elements (hereinafter simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0025] In addition, a sulfur-based positive electrode active material may be used as a positive electrode active material other than manganese dioxide. Examples of the sulfur-based positive electrode active material include particles of an organic sulfur compound or an inorganic sulfur compound, and any material may be used as long as it utilizes the oxidation-reduction reaction of sulfur to release lithium ions during charging and absorb lithium ions during discharging. Examples of the organic sulfur compound include disulfide compounds, sulfur-modified polyacrylonitrile, sulfur-modified polyisoprene, rubeanic acid (dithiooxamide), polycarbon sulfide, etc., as typified by the compounds described in International Publication No. 2010 / 044437, but are not limited thereto. Examples of the inorganic sulfur compound include elemental sulfur (S), Li ２ S, S-carbon composite, TiS ２ , TiS ３ , TiS ４ , NiS, NiS ２ , CuS, FeS ２ , Li ２ S, MoS ２ , MoS ３ , MnS, MnS ２ , CoS, CoS ２ Examples include, but are not limited to, manganese dioxide. In addition to manganese dioxide, a positive electrode active material other than manganese dioxide may be used alone or in combination of two or more. In addition to manganese dioxide, a positive electrode active material other than the above may also be used.

[0026] Here, the proportion of the manganese dioxide content relative to the total mass of the positive electrode active material is preferably more than 50% by mass, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, even more preferably 95% by mass or more, particularly preferably 97% by mass or more, and most preferably 100% by mass.

[0027] The content of the positive electrode active material in the positive electrode active material layer is preferably 30 to 99 mass %, more preferably 40 to 85 mass %, and even more preferably 50 to 70 mass %, relative to the total mass of the positive electrode active material layer.

[0028] (Solid Electrolyte) In the lithium secondary battery according to this embodiment, the positive electrode active material layer contains a solid electrolyte. In this specification, the solid electrolyte contained in the positive electrode active material layer is referred to as the "first solid electrolyte," and the solid electrolyte contained in the solid electrolyte layer described below is referred to as the "second solid electrolyte." Therefore, the terms "first" and "second" are meaningless and are merely used to distinguish the locations where the solid electrolytes are present. The solid electrolyte (first solid electrolyte) contained in the positive electrode active material layer is not particularly limited as long as it has Li ion conductivity. Examples include halide solid electrolytes, sulfide solid electrolytes, hydride solid electrolytes, and oxide solid electrolytes. Among these, it is preferable to contain at least one of a halide solid electrolyte and a sulfide solid electrolyte, and it is particularly preferable to contain a halide solid electrolyte from the viewpoint of chemical stability in contact with manganese dioxide. In this specification, "sulfide solid electrolyte" refers to a solid electrolyte containing elemental sulfur, and "halide solid electrolyte" refers to a solid electrolyte containing elemental sulfur but not containing elemental sulfur.

[0029] The halide solid electrolyte includes Li ｐ M ｑ X ｒ(where M is a metal element, X is a halogen element, p = 1, 2, or 3, q ​​= 0 or 1, and r = an integer between 1 and 6. In this case, p, q, and r are appropriately selected so that the overall charge of the compound represented by the above composition formula is 0.) A solid electrolyte represented by the following composition formula is preferred. Examples of the metal element M include, but are not limited to, Al, Mg, Fe, Ga, Y, Zr, and In. Furthermore, the halogen element X is F, Cl, Br, or I, and X in the above composition formula may be composed of a single halogen element or multiple halogen elements.

[0030] The composition formula of the halide solid electrolyte is specifically LiX, LiMX ４ , Li ２ MX ４ , Li ２ MX ６ , and Li ３ MX ６ Among them, from the viewpoint of Li ion conductivity, halide solid electrolytes are preferred. ３ MX ６ Preferably, M and X are the same as those of the above Li ｐ M ｑ X ｒ The definitions of M and X are the same as those in

[0031] Li ３ MX ６ Examples of the ３ AlF ６ , Li ３ AlCl ６ , Li ３ AlBr ６ , Li ３ All I ６ , Li ３ GaF ６ , Li ３ GaCl ６ , Li ３ GaBr ６ , Li ３ GaI ６ , Li ３ YF ６ , Li ３ YCl ６ , Li３ YBr ６ , Li ３ YI ６ , Li ３ InF ６ , Li ３ InCl ６ , Li ３ InBr ６ , and Li ３ InI ６ These can be produced by conventionally known methods, for example, by referring to Interdisciplinary Materials 2023;2:365-389. In one embodiment, the first solid electrolyte is Li ３ YCl ６ , Li ３ YBr ６ , and Li ３ YI ６ Preferably, the compound is at least one of Li ３ YCl ６ and / or Li ３ YBr ６ More preferably, Li ３ YBr ６ It is more preferable that:

[0032] Examples of sulfide solid electrolytes include LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ２ S-P ２ S ５ , LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ , Li ２ S-P ２ S ５ - LiI, Li ２ S-P ２ S５ -Li ２ O, Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ S-SiS ２ , Li ２ S-SiS ２ - LiI, Li ２ S-SiS ２ - LiBr, Li ２ S-SiS ２ -LiCl, Li ２ S-SiS ２ -B ２ S ３ - LiI, Li ２ S-SiS ２ -P ２ S ５ - LiI, Li ２ S-B ２ S ３ , Li ２ S-P ２ S ５ -Z ｍ S ｎ (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li ２ S-GeS ２ , Li ２ S-SiS ２ -Li ３ P.O. ４ , Li ２ S-SiS ２ -Li ｘ MO ｙ (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In). ２ S-P ２ S ５ " is written by Li ２ S and P ２ S ５ The same applies to other descriptions.

[0033] The sulfide solid electrolyte is, for example, Li ３ P.S. ４ It may have a Li framework. ４ P２ S ７ It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a skeleton include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li ４ P ２ S ７ Examples of the sulfide solid electrolyte having a skeleton include a Li-P-S solid electrolyte called LPS (for example, Li ７ P ３ S １１ ) can be mentioned. In addition, examples of sulfide solid electrolytes include Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ (x satisfies 0<x<1), or the like. Among them, the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing a P element, and the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing Li ２ S-P ２ S ５ It is more preferable that the sulfide solid electrolyte contains a halogen (F, Cl, Br, I). As the sulfide solid electrolyte containing a halogen, an argyrodite-type solid electrolyte (Li ６ P.S. ５ CX (wherein X is a halogen atom) is also a preferred material. The argyrodite-type solid electrolyte can be produced by a conventionally known method, for example, by referring to Solid State Ionics 318 (2018) 102-112.

[0034] The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid phase method.

[0035] The shape of the first solid electrolyte is a particle shape, and may be, for example, a spherical shape or an oval spherical shape. The average secondary particle diameter (D ５０ ) is not particularly limited, but is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 20 μm or less. The lower limit is not particularly limited, but is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1 μm or more. That is, the average secondary particle diameter of the first solid electrolyte is, for example, 0.01 μm or more and 500 μm or less, preferably 0.1 μm or more and 100 μm or less, more preferably 0.5 μm or more and 50 μm or less, and more preferably 1 μm or more and 20 μm or less. When the average secondary particle diameter of the first solid electrolyte is within the above range, it is advantageous in that the contact area between the first solid electrolyte and the positive electrode active material is more ensured.

[0036] In this specification, the average secondary particle diameter of the first solid electrolyte refers to the 50% cumulative diameter (D) based on the number of particles observed in several to several tens of fields of view when observed with a scanning electron microscope (SEM) (the maximum distance between any two points on the outline of the observed particles). ５０ ) will be adopted.

[0037] In addition, to obtain a first solid electrolyte having a desired average secondary particle size, the above material may be appropriately pulverized. For pulverization, for example, a roller mill, a jet mill, or the like may be used, but is not limited thereto. Furthermore, a first solid electrolyte having a desired average secondary particle size may be produced by classifying the material using an appropriate classification device, for example, a sieve, a classifier, or the like.

[0038] In this specification, the proportion of the content of the halide solid electrolyte relative to the total mass of the first solid electrolyte is preferably 50 mass% or more, more preferably 80 mass% or more, even more preferably 90 mass% or more, particularly preferably 95 mass% or more, and most preferably 100 mass%.

[0039] The content of the first solid electrolyte is, for example, preferably in the range of 1 to 60 mass %, more preferably in the range of 10 to 55 mass %, and even more preferably in the range of 25 to 50 mass %, relative to the total mass of the positive electrode active material layer.

[0040] The average secondary particle diameter of the manganese dioxide is less than 0.9 relative to the average secondary particle diameter of the first solid electrolyte (the ratio of the average secondary particle diameter of the manganese dioxide to the average secondary particle diameter of the first solid electrolyte (average secondary particle diameter of manganese dioxide / average secondary particle diameter of the first solid electrolyte) is less than 0.9). The ratio of the average secondary particle diameter of the manganese dioxide to the average secondary particle diameter of the first solid electrolyte is, for example, less than 0.3, preferably less than 0.1, and more preferably less than 0.08. The lower limit of this ratio is not particularly limited, but is, for example, 0.01 or more, preferably 0.03 or more, and more preferably 0.05 or more. That is, the average secondary particle diameter of the manganese dioxide is, for example, 0.01 or more but less than 0.3, preferably 0.03 or more but less than 0.1, and more preferably 0.05 or more but less than 0.08 relative to the average secondary particle diameter of the first solid electrolyte. When the ratio of the average secondary particle size of manganese dioxide to the average secondary particle size of the first solid electrolyte is within the above range, the contact area between the manganese dioxide as the positive electrode active material and the first solid electrolyte increases, thereby further improving the capacity of the battery.

[0041] The positive electrode active material layer may further contain at least one of a conductive additive and a binder in addition to the above-mentioned positive electrode active material and solid electrolyte.

[0042] Examples of conductive additives include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals, carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNT), and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). Also, particulate ceramic materials or resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive additives. Among these conductive additives, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon, more preferably at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon, and even more preferably at least one carbon. These conductive additives may be used alone or in combination of two or more.

[0043] The conductive additive is preferably in the form of particles or fibers. When the conductive additive is in the form of particles, the shape of the particles is not particularly limited and may be any shape such as powder, sphere, rod, needle, plate, column, irregular shape, scale, or spindle shape.

[0044] When the conductive additive is in a particulate form, the average particle size (primary particle size) is not particularly limited, but is preferably 0.01 to 10 μm from the viewpoint of the electrical characteristics of the battery.

[0045] When the positive electrode active material layer contains a conductive additive, the content of the conductive additive in the positive electrode active material layer is not particularly limited, but is preferably 0 to 10 mass %, more preferably 1 to 5 mass %, relative to the total mass of the positive electrode active material layer. Within such a range, a stronger electron conduction path can be formed in the positive electrode active material layer, which can effectively contribute to improving battery characteristics.

[0046] On the other hand, the binder is not particularly limited, but examples thereof include the following materials:

[0047] Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), carboxymethyl cellulose, polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene (PTFE), polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene Fluorine resins such as polytetrafluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc., vinylidene fluoride-hexafluoropropylene fluoroelastomers (VDF-HFP fluoroelastomers), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluoroelastomers (VDF-HFP-TFE fluoroelastomers), vinylidene fluoride-pentafluoropropylene fluoroelastomers (VDF-PFP fluoroelastomers), etc. Examples of the binder include 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, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferred. The binder content is not particularly limited.

[0048] The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within a range of 0.1 to 1000 μm, more preferably within a range of 0.5 to 100 μm, and even more preferably within a range of 1 to 50 μm.

[0049] [Negative Electrode (Negative Electrode Active Material Layer)] In the lithium secondary battery according to the above embodiment, the negative electrode active material layer 13 contains a negative electrode active material. When the positive electrode active material further contains a lithium-containing positive electrode active material in addition to manganese dioxide, the negative electrode active material is not particularly limited, and may include, but is not limited to, a carbon material, a metal oxide, and a metal active material. On the other hand, when the positive electrode active material does not contain a lithium-containing positive electrode active material, the negative electrode active material is a lithium-containing negative electrode active material. A lithium-containing metal may be used as the lithium-containing negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include metallic lithium and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those specifically listed above may also be used. From the viewpoint of improving the battery capacity, the negative electrode active material preferably contains metallic lithium. Alternatively, the negative electrode active material may consist solely of metallic lithium.

[0050] The negative electrode active material may be in the form of particles (spherical, fibrous), thin film, etc. When the negative electrode active material is in the form of particles, the average particle diameter is preferably in the range of, for example, 1 nm to 100 μm.

[0051] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably in the range of 40 to 100 mass %. The negative electrode active material layer may further contain a solid electrolyte, a conductive additive, and / or a binder, and specific and preferred forms thereof may be the same as those described in the section on the positive electrode active material layer above.

[0052] The thickness of the negative electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0053] [Solid Electrolyte Layer] In the lithium secondary battery according to the above embodiment, the solid electrolyte layer is a layer interposed between the above-described positive electrode active material layer and negative electrode active material layer, and essentially contains a solid electrolyte. The specific form of the solid electrolyte (second solid electrolyte) contained in the solid electrolyte layer is not particularly limited, and the exemplary and preferred forms described in the section on the positive electrode active material layer may be similarly adopted. However, from the viewpoint of excellent ionic conductivity and further improvement of battery performance, the second solid electrolyte preferably contains a sulfide solid electrolyte, and more preferably contains an argyrodite-type solid electrolyte.

[0054] The solid electrolyte layer may further contain a binder in addition to the second solid electrolyte described above. The examples and preferred embodiments described in the section on the positive electrode active material layer may also be used for the binder that can be contained in the solid electrolyte layer.

[0055] The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but from the viewpoint of improving the volumetric energy density of the battery, it is preferably 1000 μm or less, more preferably 800 μm or less, and even more preferably 700 μm or less. On the other hand, there is no particular restriction on the lower limit of the thickness of the solid electrolyte layer, but it is preferably 10 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more.

[0056] [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 27 and the negative electrode current collector plate 25 may be made of the same material or different materials.

[0057] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium secondary batteries can be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like so as to prevent contact with peripheral devices or wiring, etc., causing electrical leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[0058] [Battery Exterior Material] As the battery exterior material, 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 achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable as the exterior material because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0059] The stacked battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability, and is therefore suitable for use as a power source for driving EVs and HEVs.

[0060] Although one embodiment of a lithium secondary battery has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0061] For example, the type of battery to which the lithium secondary battery according to the present invention is applied includes a bipolar battery including a bipolar electrode having a positive electrode active material layer electrically coupled to one surface of a current collector and a negative electrode active material layer electrically coupled to the opposite surface of the current collector.

[0062] Furthermore, the lithium secondary battery according to the present embodiment may or may not be an all-solid-state type. That is, the solid electrolyte layer may or may not contain a conventionally known liquid electrolyte (electrolytic solution). When the solid electrolyte layer contains a liquid electrolyte (electrolytic solution), there is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount be such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolytic solution) does not occur.

[0063] The following items are also included within the scope of the present invention: Item 1: A lithium secondary battery comprising a power generating element including: a positive electrode containing manganese dioxide as a positive electrode active material and having a positive electrode active material layer containing a first solid electrolyte; a negative electrode having a negative electrode active material layer containing a negative electrode active material; and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a second solid electrolyte, wherein the average secondary particle diameter of the manganese dioxide is less than 2 μm, and the ratio of the average secondary particle diameter of the manganese dioxide to the average secondary particle diameter of the first solid electrolyte is less than 0.9 μm; Item 2: The lithium secondary battery according to Item 1, wherein the average secondary particle diameter of the manganese dioxide is less than 1.5 μm (preferably less than 1 μm, more preferably less than 0.8 μm); Item 3: The lithium secondary battery according to Item 1 or 2, wherein the average secondary particle diameter of the manganese dioxide is less than 0.3 (preferably less than 0.1, more preferably less than 0.08) to the average secondary particle diameter of the first solid electrolyte; Item 4: The lithium secondary battery according to any one of Items 1 to 3, wherein the first solid electrolyte is at least one of a halide solid electrolyte and a sulfide solid electrolyte (preferably a halide solid electrolyte); Item 5: The halide solid electrolyte is Li ３ MX ６ Item 6: The lithium secondary battery according to Item 4, wherein the halide solid electrolyte is a solid electrolyte represented by the composition: (M is a metal element, and X is a halogen element); ３ YCl ６ , Li ３ YBr ６ , and Li ３ YI ６ At least one of (preferably Li ３ YCl ６ and / or Li ３ YBr ６ , more preferably Li ３ YBr ６Item 7: The lithium secondary battery according to any one of Items 1 to 6, wherein the manganese dioxide has an average secondary particle diameter of 0.1 μm or more (preferably 0.3 μm or more, more preferably 0.6 μm or more); Item 8: The lithium secondary battery according to any one of Items 1 to 7, wherein the first solid electrolyte has an average secondary particle diameter of 0.01 μm or more and 500 μm or less (preferably 0.1 μm or more and 100 μm or less, more preferably 0.5 μm or more and 50 μm or less, and more preferably 1 μm or more and 20 μm or less); Item 9: The lithium secondary battery according to any one of Items 1 to 8, wherein the manganese dioxide comprises electrolytic manganese dioxide; Item 10: The lithium secondary battery according to any one of Items 1 to 9, wherein the second solid active material is an argyrodite-type solid electrolyte; Item 11: The lithium secondary battery according to any one of Items 1 to 10, wherein the negative electrode active material comprises metallic lithium.

[0064] The present invention will be described in more detail below with reference to examples, although the technical scope of the present invention is not limited to the following examples.

[0065] <Example of Preparation of Evaluation Cell> The manganese dioxides used in this example, each having a different average secondary particle size, were prepared by appropriately changing the electrolysis conditions and pulverization conditions when preparing the manganese dioxide by electrolysis. The first solid electrolyte used in this example was Li prepared by appropriately referring to Nano Energy 77, 2020, 105097 and Interdisciplinary Materials 2023; 2: 365-389. ３ YBr ６ The average secondary particle diameters of the manganese dioxide and the first solid electrolyte used in this example were measured as follows.

[0066] [Measurement of Average Secondary Particle Diameter of Manganese Dioxide] The average secondary particle diameter of manganese dioxide particles was measured using a particle size distribution measuring device that uses a laser diffraction / scattering method before preparing the positive electrode material, and was calculated based on the volume as the 50% cumulative diameter (D ５０ ) was adopted.

[0067] Measurement device: HORIBA LA-960 Measurement solvent: water [Measurement of average secondary particle diameter of solid electrolyte] The average secondary particle diameter of the particles of the solid electrolyte is the 50% cumulative diameter (D) of the particle diameters of the particles observed in several to several tens of fields of view when observed with a scanning electron microscope (SEM) before the preparation of the positive electrode material (the maximum distance among the distances between any two points on the outline of the observed particles). ５０ ) was adopted.

[0068] Measurement device: JEOL JSM-6610LV Example 1 Preparation of positive electrode material In a glove box in an argon atmosphere with a dew point of −68° C. or less, 54 parts by mass of manganese dioxide (average secondary particle diameter: 0.64 μm) as a positive electrode active material and 54 parts by mass of a halide solid electrolyte (Li ３ YBr ６ 44 parts by mass of cellulose acylate (average secondary particle diameter: 10 μm) and 2 parts by mass of carbon black HS-100 (manufactured by Denka Co., Ltd.) as a conductive additive were kneaded in an agate mortar for 30 minutes.

[0069] (Preparation of Test Cell) The preparation of the test cell was carried out in a glove box in an argon atmosphere with a dew point of −68° C. or lower.

[0070] A cylindrical convex punch (10 mm diameter) made of stainless steel was inserted into one side of a cylindrical tube jig (tube inner diameter 10 mm, outer diameter 23 mm, height 20 mm) made by Macol, and an argyrodite-type solid electrolyte (Li ６ P.S. ５Cl) was inserted. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte, and a hydraulic press was used to press the solid electrolyte at a pressure of 400 MPa (Φ10 equivalent) for 1 minute, thereby forming a solid electrolyte layer with a diameter of 10 mm and a thickness of 650 μm in the cylindrical tube jig. Next, the cylindrical convex punch inserted from above was temporarily removed, and the positive electrode material prepared above was placed on one side of the solid electrolyte layer in the cylindrical tube. A cylindrical convex punch (also serving as a positive electrode current collector) was inserted again from above, and pressed at a pressure of 400 MPa (Φ10 equivalent) for 1 minute, thereby forming a positive electrode active material layer with a diameter of 10 mm and a thickness of 20 μm on one side of the solid electrolyte layer. Next, the lower cylindrical convex punch (which also served as the negative electrode current collector) was removed, and a 0.1 mm thick In foil (manufactured by Honjo Metals Co., Ltd., diameter 9 mm) and a 0.1 mm thick Li foil (manufactured by Nilaco Corporation, diameter 5 mm) were inserted as negative electrodes from the bottom of the cylindrical tube jig so that the In foil was positioned on the solid electrolyte layer side, and the cylindrical convex punch was inserted again, followed by pressing at a pressure of 100 MPa for 1 second.

[0071] In this manner, a test cell (all-solid-state lithium secondary battery) was produced in which the negative electrode current collector (punch), lithium-indium negative electrode, solid electrolyte layer, positive electrode active material layer, and positive electrode current collector (punch) were stacked in this order.

[0072] Example 2 An evaluation cell for this example was produced in the same manner as in Example 1, except that the average secondary particle diameter of manganese dioxide was changed from 0.64 μm to 0.90 μm.

[0073] Example 3 An evaluation cell for this example was produced in the same manner as in Example 1, except that the average secondary particle diameter of manganese dioxide was changed from 0.64 μm to 1.3 μm.

[0074] Comparative Example 1 An evaluation cell for this example was produced in the same manner as in Example 1, except that the average secondary particle diameter of manganese dioxide was changed from 0.64 μm to 3.3 μm.

[0075] <Measurement of discharge capacity> A confining pressure of 80 MPa was applied to the evaluation cell prepared above in the stacking direction using a pressure member, and the cell was placed in a thermostatic chamber set at 25°C, where a constant current of 0.05 C was applied at a voltage of 2 V (vs. Li / Li＋ ) and then discharged to a current value of 0.01C at 2V (vs. Li / Li ＋ The discharge capacity was measured. The results are shown in Table 1 below.

[0076]

[0077] From the results in Table 1, it was confirmed that the discharge capacities of the evaluation cells of Examples 1 to 3 were larger than the discharge capacity of the evaluation cell of Comparative Example 1, in which the average secondary particle diameter of manganese dioxide as the positive electrode active material was 2 μm or more.

[0078] On the other hand, in a so-called liquid-type lithium battery, in which the electrolyte is liquid, the measurement results of the discharge capacity when manganese dioxide is used as the positive electrode active material are shown as a reference example in the following Table 2. The method for producing the liquid-type lithium battery and the conditions for measuring the discharge capacity are as follows.

[0079] <Example of Preparation of Liquid-Based Evaluation Cell> (Preparation of Positive Electrode for Liquid-Based Battery) Manganese dioxide (the same manganese dioxide as in Examples 1 to 3 and Comparative Example 1 above was used), acetylene black (average particle size: 35 nm), and PTFE were weighed out to a mass ratio of 80:16:4, and then mixed in an agate mortar to obtain a sheet-like positive electrode material. The positive electrode material was then punched out with a 5 mm diameter belt punch, and pressed against an aluminum mesh with a diameter of 10 mm at room temperature at a pressure of 200 MPa to form a positive electrode active material layer.

[0080] (Preparation of evaluation cell for liquid battery) Evaluation was carried out using a 2032 type coin cell. The electrolyte was 1M LiPF ６ (ethylene carbonate:diethyl carbonate=3:7 (volume ratio) solution), and a Li foil with a thickness of 0.6 mm was used as the negative electrode.

[0081] <Measurement of discharge capacity of liquid battery> A confining pressure of 80 MPa was applied to the evaluation cell prepared above in the stacking direction using a pressure member, and the cell was placed in a thermostatic chamber set at 25°C, where the voltage was 2 V (vs. Li / Li) at a constant current of 0.05 C. ＋ ) and then discharged to a current value of 0.01C at 2V (vs. Li / Li ＋) and the discharge capacity was measured.

[0082]

[0083] The results in Table 2 confirm that the discharge capacity of liquid-type lithium batteries varies little with the average secondary particle size of manganese dioxide. These results indicate that in batteries using a liquid electrolyte, the external surface area of ​​the secondary particles of the positive electrode active material has little effect on the contact between the electrolyte and the positive electrode active material. On the other hand, in batteries using a solid electrolyte, the discharge capacity of the battery can be improved by appropriately controlling the secondary particle size of the positive electrode active material and the particle size ratio between the positive electrode active material and the solid electrolyte.

[0084] 10a: laminated battery, 11': negative electrode current collector, 11'': positive electrode current collector, 13: negative electrode active material layer, 15: positive electrode active material layer, 17: solid electrolyte layer, 19: single cell layer, 21: power generating element, 25: negative electrode current collector, 27: positive electrode current collector, 29: laminate film.

Claims

1. A lithium secondary battery comprising a power generating element including: a positive electrode containing manganese dioxide as a positive electrode active material and having a positive electrode active material layer containing a first solid electrolyte; a negative electrode having a negative electrode active material layer containing a negative electrode active material; and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a second solid electrolyte, wherein the average secondary particle diameter of the manganese dioxide is less than 2 μm and the ratio of the average secondary particle diameter of the manganese dioxide to the average secondary particle diameter of the first solid electrolyte is less than 0.

9.

2. The lithium secondary battery according to claim 1, wherein the manganese dioxide has an average secondary particle size of less than 1 μm.

3. The lithium secondary battery according to claim 1 or 2, wherein the average secondary particle size of the manganese dioxide is less than 0.1 times the average secondary particle size of the first solid electrolyte.

4. The lithium secondary battery according to claim 1, wherein the first solid electrolyte is a halide solid electrolyte.

5. The halide solid electrolyte is Li ３ MX ６ 5. The lithium secondary battery according to claim 4, having a composition of (M is a metal element, and X is a halogen element).

6. The lithium secondary battery according to claim 1, wherein the manganese dioxide has an average secondary particle size of 0.6 μm or more.

7. The lithium secondary battery according to claim 1, wherein the manganese dioxide comprises electrolytic manganese dioxide.

8. The lithium secondary battery according to claim 1, wherein the negative electrode active material includes metallic lithium.

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