Lithium secondary battery

Abstract

To provide a means capable of increasing initial capacity in a lithium secondary battery using manganese dioxide as a positive electrode active material and using a solid electrolyte.SOLUTION: A lithium secondary battery includes a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material containing manganese dioxide, a negative electrode having a negative electrode active material layer containing a negative electrode active material containing metal lithium or a lithium-containing alloy, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, wherein a ratio of an average secondary particle size to an average primary particle size of the manganese dioxide is 150 or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[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 output density and energy density of the battery.

[0003] The electrode active material layer (positive electrode active material layer or negative electrode active material layer) of a lithium secondary battery using a solid electrolyte typically contains an electrode active material and a solid electrolyte. However, depending on the shape of the electrode active material, the formed electrode active material layer may be uneven or prone to voids, which may result in a decrease in battery performance, such as initial capacity, durability, and output characteristics. In response to this, Patent Document 1 proposes a technology in which, in an electrode having an electrode active material layer containing an electrode active material and a solid electrolyte, the particles of the electrode active material are secondary particles whose ratio of secondary particle size to primary particle size is 3 or less, or primary particles that are substantially unagglomerated. It is believed that the use of such an electrode active material can suppress the decrease in battery performance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-125921 Summary of the Invention [Problem to be solved by the invention]

[0005] Manganese dioxide is a high-capacity, low-cost positive electrode active material, and is widely used as a positive electrode material for batteries using liquid electrolytes (electrolytic solutions), such as alkaline manganese dry batteries. However, the inventors' studies have found that manganese dioxide has low crystallinity, making it difficult to set the ratio of secondary particle size to primary particle size to 3 or less, and therefore the deterioration of battery performance (particularly initial capacity) cannot be sufficiently suppressed.

[0006] Therefore, an object of the present invention is to provide a means for improving the initial capacity of a lithium secondary battery that uses manganese dioxide as a positive electrode active material and contains a solid electrolyte. [Means for solving the problem]

[0007] 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 that uses manganese dioxide as a positive electrode active material and includes a solid electrolyte, the above-mentioned problems can be solved by controlling the secondary particle size relative to the primary particle size of manganese dioxide within a predetermined range, thereby completing the present invention.

[0008] That is, one embodiment of the present invention is a lithium secondary battery comprising a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material including manganese dioxide, a negative electrode having a negative electrode active material layer containing a negative electrode active material including metallic lithium or a lithium-containing alloy, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, wherein the ratio of the average secondary particle size to the average primary particle size of the manganese dioxide is 150 or less. [Effects of the Invention]

[0009] According to the present invention, in a lithium secondary battery in which the positive electrode active material layer contains manganese dioxide as a positive electrode active material and which contains a solid electrolyte, the initial capacity of the battery can be improved. [Brief explanation of the drawings]

[0010] [Figure 1]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. [Figure 2] FIG. 2 is a graph showing the relationship between the degree of aggregation of the manganese dioxide used in each of the examples and comparative examples and the initial capacity of the obtained all-solid-state battery. DETAILED DESCRIPTION OF THE INVENTION

[0011] One aspect of the present invention is a lithium secondary battery comprising a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material including manganese dioxide, a negative electrode having a negative electrode active material layer containing a negative electrode active material including metallic lithium or a lithium-containing alloy, and a solid electrolyte layer interposed between the positive electrode and the negative electrode, wherein the ratio of the average secondary particle size to the average primary particle size of the manganese dioxide is not more than 150. According to the lithium secondary battery of this aspect, the positive electrode active material layer contains manganese dioxide as the positive electrode active material, and the initial capacity of the battery can be improved in a lithium secondary battery containing a solid electrolyte.

[0012] Hereinafter, embodiments of the positive electrode for a lithium secondary battery according to the present embodiment 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.

[0013] 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 made compact and have a high capacity. In this specification, the stacked-type lithium ion secondary battery (hereinafter also simply referred to as a "stacked-type battery") shown in FIG. 1 will be described in detail as an example.

[0014] As shown in FIG. 1, the stacked battery 10a 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 structured so as 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 battery 10a in the stacking direction of the power generating element 21 by a pressure member (not shown). Therefore, the volume of the power generating element 21 is kept constant.

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

[0016] [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.

[0017] 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.

[0018] [Cathode active material layer] (Cathode active material) 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 contains manganese dioxide having a ratio of the average secondary particle size to the average primary particle size (average secondary particle size / average primary particle size) of 150 or less. With this configuration, the initial capacity can be improved in a lithium ion secondary battery using manganese dioxide as the positive electrode active material and a solid electrolyte.

[0019] Manganese dioxide (MnO2) is widely used as a positive electrode active material in batteries that use liquid electrolytes (electrolytic solutions), such as alkaline manganese batteries. However, it has been found that when manganese dioxide is used as a positive electrode active material in lithium secondary batteries that contain solid electrolytes, the battery capacity, especially the initial capacity, decreases compared to batteries that use liquid electrolytes (liquid-type batteries).

[0020] Manganese dioxide is often produced by a liquid-phase method such as electrolysis, and has lower crystallinity than positive electrode active materials such as lithium-transition metal composite compounds, which are mainly produced by solid-state sintering. Furthermore, manganese dioxide particles exist in the positive electrode active material layer containing a solid electrolyte in the form of secondary particles formed by aggregation of primary particles. In this case, in the positive electrode active material layer containing a solid electrolyte, the contact points of the manganese dioxide particles with the solid electrolyte are limited to the outer surface regions of the secondary particles. This is thought to result in insufficient contact points between the manganese dioxide positive electrode active material and the solid electrolyte, resulting in a decrease in battery capacity.

[0021] In contrast, the lithium secondary battery of this embodiment uses manganese dioxide as the positive electrode active material, whose ratio of the average secondary particle size to the average primary particle size (average secondary particle size / average primary particle size, also referred to as agglomeration degree) is 150 or less. In the positive electrode active material layer, lithium ions migrate from the solid electrolyte to the positive electrode active material and diffuse within the particles of the positive electrode active material. The diffusion of lithium ions within the secondary particles of the positive electrode active material can proceed through intragranular diffusion of the primary particles and grain boundary diffusion between the primary particles. However, when the primary particles aggregate to form secondary particles, the contact points between the manganese dioxide particles and the solid electrolyte are limited to the outer surface regions of the secondary particles, and the primary particles present inside the secondary particles cannot contact the solid electrolyte. This results in a higher resistance to grain boundary diffusion compared to liquid-phase batteries. In the lithium secondary battery of this embodiment, controlling the agglomeration degree of manganese dioxide to a predetermined value reduces the frequency of grain boundaries, thereby reducing the distance and resistance of grain boundary diffusion of lithium ions. As a result, the discharge capacity of the battery can be improved. Furthermore, the utilization rate of manganese dioxide can be improved.

[0022] In the case of liquid-based batteries, the resistance to grain boundary diffusion is low because the liquid electrolyte (electrolytic solution) fills the spaces between the primary particles. Therefore, the degree of aggregation of manganese dioxide has little effect on the discharge capacity of the battery. In other words, the present invention can be said to solve the problems specific to lithium secondary batteries containing solid electrolytes.

[0023] The degree of aggregation of manganese dioxide, which is the positive electrode active material, is not particularly limited as long as it is 150 or less. If the degree of aggregation of manganese dioxide exceeds 150, the diffusion resistance of lithium ions increases, making it difficult to obtain sufficient initial capacity. The degree of aggregation of manganese dioxide is preferably 140 or less, more preferably 130 or less, even more preferably 120 or less, even more preferably 110 or less, even more preferably 100 or less, even more preferably 90 or less, even more preferably 80 or less, even more preferably 70 or less, even more preferably 60 or less, even more preferably 50 or less, and particularly preferably 42 or less. Within the above ranges, the effects of the present invention can be more significantly obtained. The lower limit of the degree of aggregation of manganese dioxide is not particularly limited, but is, for example, 1 or more, preferably 3 or more, more preferably more than 3, even more preferably 10 or more, and even more preferably 20 or more. In particular, a degree of aggregation of manganese dioxide of 10 or more is preferred because it allows for stable production.

[0024] Here, secondary particles are aggregates of primary particles. Primary particles are the smallest unit of separable solid particles with boundaries between particles. Primary particles can be independent crystalline particles.

[0025] In this specification, the average secondary particle diameter of manganese dioxide as a positive electrode active material is measured by a particle size distribution measuring device using a laser diffraction / scattering method, and is the 50% cumulative diameter (D 50 ) is the value calculated as

[0026] The average primary particle size of manganese dioxide as a positive electrode active material is the average crystallite size estimated by Scherrer's formula from the 110 peak confirmed by XRD measurement.

[0027] The average secondary particle size of manganese dioxide is not particularly limited as long as it achieves the above-mentioned predetermined degree of aggregation, but is, for example, 8 μm or less, preferably 5 μm or less, more preferably 3 μm or less, even more preferably 2.5 μm or less, even more preferably 2 μm or less, even more preferably 1.5 μm or less, and particularly preferably 1 μm or less. Within the above range, contact with the solid electrolyte in the positive electrode active material layer can be further improved, resulting in a further improvement in initial capacity. Furthermore, the lower limit of the average secondary particle size of manganese dioxide is, for example, 0.01 μm or more, preferably 0.05 μm or more, more preferably 0.1 μm or more, even more preferably 0.2 μm or more, even more preferably 0.3 μm or more, and even more preferably 0.5 μm or more.

[0028] The average primary particle size of manganese dioxide is not particularly limited as long as it achieves the above-mentioned predetermined degree of aggregation, but is, for example, 0.001 to 0.1 μm, preferably 0.005 to 0.05 μm, and more preferably 0.01 to 0.02 μm. Within the above range, the effects of the present invention can be more significantly achieved.

[0029] The crystal structure of manganese dioxide (MnO2) is not particularly limited, and any crystal structure can be used, such as α-type (hollandite type), β-type (rutile type), γ-type, δ-type, λ-type (spinel type), and ramsdellite type.

[0030] There are no particular limitations on the method for obtaining manganese dioxide having the above-mentioned predetermined degree of aggregation. The degree of aggregation of manganese dioxide can be adjusted by appropriately selecting the method and conditions for producing manganese dioxide, the method and conditions for pulverization, etc.

[0031] Manganese dioxide may be obtained by electrolysis, chemical synthesis, or other methods, but from the viewpoint of more easily obtaining manganese dioxide having a predetermined specific surface area, manganese dioxide obtained by electrolysis or chemical synthesis is preferred, and chemical synthesis is more preferred. In this specification, "electrolytic manganese dioxide" means manganese dioxide obtained by electrolysis. In this specification, "chemically synthesized manganese dioxide" means manganese dioxide obtained by chemical synthesis.

[0032] Electrolytic manganese dioxide can be produced by any conventional method. For example, it can be produced 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 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 degree of aggregation of electrolytic manganese dioxide can be changed by changing the electrolysis conditions, such as the temperature, electrolysis current density, and sulfuric acid concentration.

[0033] Chemically synthesized manganese dioxide can be appropriately produced by a conventionally known method. Specific examples include a method in which potassium permanganate is dissolved in an aqueous hydrochloric acid solution and hydrothermal synthesis is performed. The heat treatment (calcination) temperature during hydrothermal synthesis is, for example, 140 to 180°C, and preferably 150 to 170°C. The heat treatment (calcination) time is, for example, 2 to 24 hours, and preferably 6 to 18 hours. Alternatively, it can be synthesized by mixing an aqueous potassium permanganate solution, an aqueous manganese sulfate solution, and an aqueous sodium hydroxide solution. Chemically synthesized manganese dioxide may also be produced by chemical reactions other than those described above.

[0034] Manganese dioxide obtained by electrolysis, chemical synthesis, or other methods may also be pulverized appropriately to obtain 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.

[0035] 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.

[0036] The positive electrode active material other than manganese dioxide contained in the positive electrode active material layer is not particularly limited, but may be a layered rock salt type active material such as LiCoO2, LiMnO2, LiNiO2, LiVO2, or Li(Ni-Mn-Co)O2, LiMn2O4, LiNi 0.5 Mn 1.5 Examples of oxide active materials include spinel-type active materials such as LiFePO4 and LiMnPO4, olivine-type active materials such as LiFeSiO4 and LiMnSiO4, and Si-containing active materials such as LiFeSiO4 and LiMnSiO4. 12 For example, Li(Ni-Mn-Co)O2 and those in which part of the transition metals in these oxides is replaced with other elements (hereinafter also simply referred to as "NMC composite oxides") can be used.

[0037] In addition, sulfur-based positive electrode active materials other than manganese dioxide may also be used. Examples of sulfur-based positive electrode active materials include particles of organic sulfur compounds or inorganic sulfur compounds, as long as they utilize the sulfur redox reaction to release lithium ions during charging and absorb lithium ions during discharging. Examples of organic sulfur compounds include, but are not limited to, disulfide compounds, sulfur-modified polyacrylonitriles, sulfur-modified polyisoprenes, rubeanic acid (dithiooxamide), and polycarbon sulfides, as exemplified by the compounds described in International Publication No. 2010 / 044437. Examples of inorganic sulfur compounds include, but are not limited to, elemental sulfur (S), Li2S, S-carbon composites, TiS2, TiS3, TiS4, NiS, NiS2, CuS, FeS2, Li2S, MoS2, MoS3, MnS, MnS2, CoS, and CoS2.

[0038] Here, the ratio of the manganese dioxide content 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 90% by mass or more, still more preferably 95% by mass or more, even more preferably 97% by mass or more, particularly preferably 99% by mass or more, and most preferably 100% by mass.

[0039] Preferably, the proportion of the content of manganese dioxide having the above-mentioned predetermined degree of aggregation relative to the total mass of the positive electrode active material is preferably more than 50 mass%, more preferably 70 mass% or more, even more preferably 90 mass% or more, still more preferably 95 mass% or more, even more preferably 97 mass% or more, particularly preferably 99 mass% or more, and most preferably 100 mass%.

[0040] 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.

[0041] (solid electrolyte) The positive electrode active material layer preferably 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 order of "first" and "second" itself is meaningless, and these terms 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 because these materials are softer than manganese dioxide and can further improve contact with manganese dioxide. It is particularly preferable to contain a halide solid electrolyte because they have excellent chemical stability when in contact with manganese dioxide and can achieve a higher discharge capacity. In this specification, the term "sulfide solid electrolyte" refers to a solid electrolyte containing elemental sulfur, and the term "halide solid electrolyte" refers to a solid electrolyte containing a halogen element that does not contain elemental sulfur.

[0042] As a halide solid electrolyte, Li p M q X r (wherein M is a metal element, X is a halogen element, p=1, 2, or 3, q=0 or 1, and r=an integer of 1 to 6. In this case, p, q, and r are appropriately selected so that the charge of the entire compound represented by the above composition formula is 0.) A solid electrolyte represented by the following composition formula is preferred.

[0043] The metal element M may include, but is not limited to, at least one selected from the group consisting of Al, Mg, Fe, Ga, Y, Zr, and In. The halogen element X may include at least one selected from the group consisting of F, Cl, Br, and I, and X in the above composition formula may be composed of a single halogen element or multiple halogen elements.

[0044] Specific examples of the composition formula of the halide solid electrolyte include, but are not limited to, at least one selected from the group consisting of LiX, LiMX4, Li2MX4, Li2MX6, and Li3MX6. Among these, the halide solid electrolyte is preferably Li3MX6, since this provides a higher performance lithium secondary battery. M and X are the above-mentioned Li p M q X r The definitions of M and X are the same as those in the above. With this configuration, the effects of the present invention can be more significantly achieved.

[0045] Examples of LiMX6 include, but are not limited to, LiAlF6, LiAlCl6, LiAlBr6, LiAlI6, LiGaF6, LiGaCl6, LiGaBr6, LiGaI6, LiYF6, LiYCl6, LiYBr6, LiYI6, LiInF6, LiInCl6, LiInBr6, and LiInI6. These can be appropriately produced by conventionally known methods, for example, by appropriately referring to Interdisciplinary Materials 2023;2:365-389. In one embodiment, the first solid electrolyte is preferably at least one of LiYCl6, LiYBr6, and LiYI6, more preferably LiYCl6 and / or LiYBr6, and even more preferably LiYBr6.

[0046] Examples of sulfide solid electrolytes include LiI-Li2S-SiS2, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, LiI-Li3PS4, LiI-LiBr-Li3PS4, Li3PS4, Li2S-P2S5-LiI, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z m Sn (where m and n are positive numbers, and Z is any one of Ge, Zn, and Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, and In), and the like. The description of "Li2S-P2S5" means a sulfide solid electrolyte formed using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions.

[0047] The sulfide solid electrolyte may, for example, have a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of the sulfide solid electrolyte having a Li3PS4 skeleton include LiI-Li3PS4, LiI-LiBr-Li3PS4, and Li3PS4. Further, examples of the sulfide solid electrolyte having a Li4P2S7 skeleton include a Li-P-S-based solid electrolyte called LPS (for example, Li7P3S 11 ). Also, as the sulfide solid electrolyte, for example, Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1), such as LGPS, may be used. Among them, the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing a P element, and more preferably a material mainly composed of Li2S-P2S5. Further, the sulfide solid electrolyte preferably contains a halogen (F, Cl, Br, I). Examples of the sulfide solid electrolyte containing a halogen include an argyrodite-type solid electrolyte (Li6PS5CX, where X is a halogen atom), and this is also a material that can be preferably used. The argyrodite-type solid electrolyte can be produced by a conventionally known method, and for example, it can be produced by referring to Solid State Ionics 318 (2018) 102-112.

[0048] The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid phase method.

[0049] The shape of the first solid electrolyte may be, for example, a particulate shape, such as a spherical shape or an oval spherical shape. 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 secure.

[0050] In this specification, the average secondary particle diameter of a solid electrolyte is defined as 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 contour line of the observed particles). 50 ) will be adopted.

[0051] The solid electrolyte may be synthesized using a known method, or a commercially available product may be used. Furthermore, these materials may be appropriately pulverized to obtain a solid electrolyte having a desired average secondary particle size. For pulverization, for example, a roller mill, a jet mill, or the like may be used, but this is not limited thereto. Furthermore, a solid electrolyte having a desired average secondary particle size may be produced by classifying the solid electrolyte using an appropriate classification device, for example, a sieve, a classifier, or the like.

[0052] In one embodiment, the content of the halide solid electrolyte relative to the total mass of the first solid electrolyte is preferably more than 50 mass%, more preferably 80 mass% or more, even more preferably 90 mass% or more, particularly preferably 95 mass% or more, and most preferably 100 mass%.

[0053] 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.

[0054] In one embodiment, the ratio of the average secondary particle size of manganese dioxide to the average secondary particle size of the first solid electrolyte (average secondary particle size of manganese dioxide / average secondary particle size of first solid electrolyte) is not particularly limited, but is, for example, less than 0.9, preferably less than 0.3, and more preferably 0.2 or less. Within this range, the contact area between manganese dioxide and the first solid electrolyte becomes larger, thereby further improving the battery capacity. The lower limit of the ratio of the average secondary particle size of manganese dioxide to the average secondary particle size of the first solid electrolyte (average secondary particle size of manganese dioxide / average secondary particle size of first solid electrolyte) is also not particularly limited, but is, for example, 0.01 or more, preferably 0.03 or more, and more preferably 0.05 or more. Within this range, the contact area between manganese dioxide and the first solid electrolyte becomes larger, thereby further improving the battery capacity. That is, the ratio of the average secondary particle size of manganese dioxide to the average secondary particle size of the first solid electrolyte is, for example, 0.01 or more and less than 0.9, preferably 0.03 or more and less than 0.1, and more preferably 0.05 or more and 0.2 or less.

[0055] In one embodiment, the mass ratio of the positive electrode active material to the total mass of the positive electrode active material and the first solid electrolyte contained in the positive electrode active material layer is, for example, 30 to 90 mass%, preferably 40 to 80 mass%, more preferably 50 to 70 mass%, and even more preferably 50 to 60 mass%. Within this range, the effects of the present invention can be obtained more significantly.

[0056] 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.

[0057] Examples of conductive additives include, but are not limited to, 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 (CNTs); and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). Furthermore, 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.

[0058] 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.

[0059] When the conductive assistant is in 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 properties of the battery.

[0060] 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 more than 0 mass % to 10 mass % or less, more preferably 1 to 5 mass %, relative to the total mass of the positive electrode active material layer. Within this range, a stronger electron conduction path can be formed in the positive electrode active material layer, which can effectively contribute to improving battery characteristics.

[0061] On the other hand, the binder is not particularly limited, but examples thereof include the following materials:

[0062] 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, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene block copolymer and its hydrogenated products, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (P Examples of suitable fluororesins include fluoroelastomers such as vinylidene fluoride-hexafluoropropylene copolymer (VDF-HFP), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF); vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene copolymer (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene copolymer (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene copolymer (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene copolymer (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene copolymer (VDF-CTFE-based fluororubber); and epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable. There are no particular restrictions on the content of the binder.

[0063] In one embodiment, the total content of the positive electrode active material, the first solid electrolyte, the conductive additive, and the binder relative to the total mass of the positive electrode active material layer is preferably 90 mass% or more, more preferably 95 mass% or more, even more preferably 98 mass% or more, particularly preferably 99 mass% or more, and most preferably 100 mass%.

[0064] In one embodiment, the total content of the positive electrode active material, the first solid electrolyte, and the conductive additive relative to the total mass of the positive electrode active material layer is preferably 90 mass% or more, more preferably 95 mass% or more, even more preferably 98 mass% or more, particularly preferably 99 mass% or more, and most preferably 100 mass%.

[0065] The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is preferably within the range of 1 to 1000 μm, more preferably within the range of 10 to 100 μm, and even more preferably within the range of 10 to 50 μm.

[0066] [Negative electrode (negative electrode active material layer)] The negative electrode active material layer contains a negative electrode active material. In the lithium secondary battery according to this embodiment, the negative electrode active material essentially contains metallic lithium (Li) or a lithium-containing alloy. The type of the negative electrode active material is not particularly limited, and examples of the lithium-containing alloy include an alloy of lithium and at least one material that can be alloyed with lithium. Examples of materials that can be alloyed with lithium include Si, Au, In, Ge, Sn, Pb, Al, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Cd, Hg, Ga, Tl, C, N, Sb, Bi, O, S, Se, Te, and Cl. Among these, from the viewpoint of constructing a battery with excellent capacity and energy density, the material that can be alloyed with lithium preferably contains at least one element selected from the group consisting of Si, Au, In, Ge, Sn, Pb, Al, and Zn, and more preferably contains Si, Au, or In. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those mentioned above may also be used as long as they essentially contain metallic lithium or a lithium-containing alloy.

[0067] In a preferred embodiment, the negative electrode active material contains metallic lithium. This configuration can further improve the battery capacity. In a preferred embodiment, the negative electrode active material consists solely of metallic lithium.

[0068] The shape of the negative electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc., but thin film is preferred. When the negative electrode active material is particulate, its average particle diameter (D 50 ) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle diameter (D 50 The value of can be measured by a laser diffraction scattering method.

[0069] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably within the range of, for example, 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.

[0070] 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.

[0071] [Solid electrolyte layer] In the lithium secondary battery according to the above embodiment, the solid electrolyte layer is a layer interposed between the positive electrode active material layer and the negative electrode active material layer and containing 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 achieving excellent ion conductivity and further improving battery performance, the second solid electrolyte preferably contains a sulfide solid electrolyte, and more preferably contains an argyrodite-type solid electrolyte.

[0072] 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.

[0073] 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.

[0074] [Positive and negative current collector plates] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plates. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive current collector plate 27 and the negative current collector plate 25 may be made of the same material or different materials.

[0075] [Positive and negative leads] Although not shown, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via positive and negative electrode leads. Materials used in known lithium ion secondary batteries may be used as the constituent materials of the positive and negative electrode leads. The parts removed from the exterior are preferably 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., automobile parts, particularly electronic devices).

[0076] [Battery exterior materials] As the battery exterior material, a known metal can case can be used, or 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 to these. 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 devices 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.

[0077] 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 driving power source for EVs and HEVs.

[0078] 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.

[0079] 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.

[0080] Furthermore, the lithium secondary battery according to this 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.

[0081] The following items are also included within the scope of the present invention: Item 1: A positive electrode having a positive electrode active material layer containing a positive electrode active material including manganese dioxide; a negative electrode having a negative electrode active material layer containing a negative electrode active material including metallic lithium; a solid electrolyte layer interposed between the positive electrode and the negative electrode; a power generating element having a lithium secondary battery, wherein the ratio of the average secondary particle size to the average primary particle size of the manganese dioxide is 150 or less; Item 2: The lithium secondary battery according to Item 1, wherein the ratio of the average secondary particle size to the average primary particle size of the manganese dioxide is 130 or less (preferably 100 or less, more preferably 80 or less, even more preferably 50 or less, and still more preferably 42 or less); Item 3: The lithium secondary battery according to Item 1 or 2, wherein the manganese dioxide has an average secondary particle size of 5 μm or less (preferably 3 μm or less, more preferably 2.5 μm or less, even more preferably 2 μm or less, even more preferably 1.5 μm or less, and even more preferably 1 μm or less); Item 4: The lithium secondary battery according to any one of Items 1 to 3, wherein the positive electrode active material layer further contains a halide solid electrolyte; Item 5: The positive electrode active material layer further contains a halide solid electrolyte, and the halide solid electrolyte is Li p M q X r (wherein M is a metal element, X is a halogen element, p=1, 2, or 3, q=0 or 1, and r=an integer of 1 to 6, and p, q, and r are appropriately selected so that the charge of the entire compound represented by the above composition formula is 0); Item 6: The lithium secondary battery according to any one of Items 1 to 5, wherein the positive electrode active material layer further contains a halide solid electrolyte, and the halide solid electrolyte has a composition of Li3MX6 (M is a metal element, and X is a halogen element); Item 7: The lithium secondary battery according to any one of Items 1 to 6, wherein the positive electrode active material layer further contains a halide solid electrolyte, and the halide solid electrolyte is selected from the group consisting of Li3AlF6, Li3AlCl6, Li3AlBr6, Li3AlI6, Li3GaF6, Li3GaCl6, Li3GaBr6, Li3GaI6, Li3YF6, Li3YCl6, Li3YBr6, Li3YI6, Li3InF6, Li3InCl6, Li3InBr6, and Li3InI6; Item 8: The lithium secondary battery according to any one of Items 1 to 7, wherein the positive electrode active material layer does not contain any positive electrode active material other than manganese dioxide. [Example]

[0082] 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.

[0083] [Example 1] (Fabrication of all-solid-state batteries) The fabrication of the all-solid-state battery was carried out in a glove box with a dew point of -68°C or less. The tools and equipment used in the glove box were thoroughly dried beforehand.

[0084] (Preparation of positive electrode material) In a glove box with an argon atmosphere having a dew point of −68°C or lower, manganese dioxide (average primary particle diameter: 0.0150 μm, average secondary particle diameter: 0.63 μm, degree of aggregation: 42) as a positive electrode active material, Li3YBr6 (average secondary particle diameter: 10 μm) as a halide solid electrolyte, and acetylene black (manufactured by Denka Co., Ltd.) as a conductive additive were weighed out in a mass ratio of manganese dioxide:halide solid electrolyte:acetylene black = 54:44:2, and then mixed in an agate mortar to obtain a positive electrode material.

[0085] (Preparation of evaluation cells) 100 mg of sulfide solid electrolyte (Li6PS5Cl) was weighed, placed in a McCol tube, clamped between hard Cr-plated SLD pins, and pressed at 400 MPa at room temperature for 1 minute to obtain a solid electrolyte layer. The cathode material prepared above was weighed and placed on one side of the solid electrolyte layer to a thickness of 0.02 mm after pressing, and pressed at 200 MPa at room temperature for 1 minute to obtain a cathode active material layer. A 0.1 mm thick In foil (9 mm diameter) and a 0.1 mm thick Li foil (5 mm diameter) were placed on the other side of the solid electrolyte layer, in that order, and pressed at 100 MPa at room temperature for 1 second to produce an evaluation cell for this example. The confining pressure during cell assembly was 80 MPa.

[0086] (Preparation of liquid battery) Manganese dioxide (average primary particle size: 0.0150 μm, average secondary particle size: 0.63 μm, degree of aggregation: 42) as a positive electrode active material, acetylene black as a conductive additive, and polytetrafluoroethylene (PTFE) as a binder were weighed out in a mass ratio of manganese dioxide:acetylene black:PTFE = 80:16:4, and then mixed in an agate mortar to obtain a sheet-like positive electrode active material layer. The positive electrode active material layer was then punched out with a 5 mm diameter belt punch and pressed at room temperature against a 10 mm diameter aluminum mesh as a current collector at a pressure of 200 MPa to obtain a positive electrode.

[0087] Using the above positive electrode, a 2032-type coin cell was fabricated for evaluation. The electrolyte used was a 3:7 volume mixture of ethylene carbonate and diethyl carbonate, with LiPF6 dissolved at a concentration of 1M. A 0.6 mm thick Li foil was used as the negative electrode.

[0088] [Example 2] An evaluation cell for this example was produced using the same method as in Example 1 described above (Production of an all-solid-state battery), except that the manganese dioxide was changed to manganese dioxide having an average primary particle diameter of 0.0135 μm, an average secondary particle diameter of 1.99 μm, and an aggregation degree of 147.

[0089] Furthermore, in the above (Production of a liquid battery), an evaluation cell for a liquid battery was obtained in the same manner as above, except that the manganese dioxide was changed to manganese dioxide having an average primary particle diameter of 0.0135 μm, an average secondary particle diameter of 1.99 μm, and an aggregation degree of 147.

[0090] [Comparative Example 1] An evaluation cell for this comparative example was produced using the same method as in Example 1 described above (Production of an all-solid-state battery), except that the manganese dioxide was changed to manganese dioxide having an average primary particle diameter of 0.0157 μm, an average secondary particle diameter of 5.08 μm, and an aggregation degree of 324.

[0091] Furthermore, in the above (Preparation of a liquid battery), an evaluation cell for a liquid battery was obtained in the same manner as above, except that the manganese dioxide was changed to manganese dioxide having an average primary particle diameter of 0.0157 μm, an average secondary particle diameter of 5.08 μm, and an aggregation degree of 324.

[0092] <Measurement of initial discharge capacity> For the evaluation cells of the all-solid-state battery and the liquid-based battery fabricated above, a test was conducted to confirm the initial discharge capacity (initial capacity) using the constant current intermittent titration method. The measurement temperature was 25°C, and the evaluation equipment used was a charge / discharge evaluation device, TOSCAT-3100, manufactured by Toyo Systems Co., Ltd. The evaluation of the all-solid-state battery was carried out by repeatedly applying a current of 0.01C for 10 minutes followed by a 2-hour rest. The evaluation of the liquid-based battery was carried out by repeatedly applying a current of 0.01C for 10 minutes followed by a 30-minute rest.

[0093] The initial discharge capacity of each evaluation cell was calculated by determining the cell voltage after rest obtained by the constant current intermittent titration method as 2V [vs. Li / Li + The capacity when the manganese dioxide reached the initial capacity was recorded. The results are shown in Table 1 below and Figure 2. Figure 2 is a graph showing the relationship between the degree of aggregation of the manganese dioxide used in each Example and Comparative Example and the initial capacity of the resulting all-solid-state battery. In Figure 2, the initial capacity of the all-solid-state battery obtained in each Example and Comparative Example is expressed as a ratio to the initial capacity of a liquid-based battery produced using the same positive electrode active material.

[0094] [Table 1]

[0095] As shown in Table 1 and FIG. 2, the all-solid-state batteries of Examples 1 and 2, which used manganese dioxide as the positive electrode active material, in which the ratio of the average secondary particle size to the average primary particle size was 150 or less, achieved excellent initial capacity. Furthermore, there was little or no decrease in initial capacity compared to liquid electrolyte batteries, confirming that lithium batteries using solid electrolytes can achieve good ionic conductivity similar to that of liquid electrolyte batteries. On the other hand, when the ratio of the average secondary particle size to the average primary particle size of manganese dioxide exceeded 150, as in Comparative Example 1, the lithium secondary battery using a solid electrolyte failed to achieve sufficient initial capacity. [Explanation of symbols]

[0096] 10a stacked 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 cell layer, 21 power generation elements, 25 negative current collector plate, 27 positive current collector plate, 29 Laminating film.

Claims

1. a positive electrode having a positive electrode active material layer containing a positive electrode active material including manganese dioxide; a negative electrode having a negative electrode active material layer containing a negative electrode active material including metallic lithium or a lithium-containing alloy; a solid electrolyte layer interposed between the positive electrode and the negative electrode; a power generating element having A lithium secondary battery, wherein the ratio of the average secondary particle size to the average primary particle size of the manganese dioxide is 150 or less.

2. 2. The lithium secondary battery according to claim 1, wherein the ratio of the average secondary particle size to the average primary particle size of the manganese dioxide is 130 or less.

3. 3. The lithium secondary battery according to claim 1, wherein the manganese dioxide has an average secondary particle size of 5 μm or less.

4. The lithium secondary battery according to claim 1 , wherein the positive electrode active material layer further contains 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).

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