Secondary battery

Abstract

The present invention provides a means that is capable of improving the high-rate charge characteristics of a secondary battery. The present invention provides a secondary battery which comprises an electric power generation element that is provided with: a positive electrode which is obtained by arranging a positive electrode active material layer on the surface of a positive electrode collector; a negative electrode; and a solid electrolyte layer which is interposed between the positive electrode and the negative electrode, and contains a solid electrolyte. With respect to this secondary battery, the positive electrode active material layer is obtained by stacking a first layer, which is in contact with the solid electrolyte layer and contains a positive electrode active material, a solid electrolyte and a binder, and a second layer, which is in contact with the positive electrode collector and contains a positive electrode active material and a binder; the content of the solid electrolyte in the first layer is 1% by mass or more relative to 100% by mass of the total solid content in the first layer; and the content of the solid electrolyte in the second layer is 0% by mass, or more than 0% by mass but less than 1% by mass relative to 100% by mass of the total solid content in the second layer.

Description

secondary battery

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

[0002] In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. In the automotive industry, there is growing hope for a reduction in carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and non-aqueous electrolyte secondary batteries, such as secondary batteries for driving motors, which hold the key to their practical application, have been actively developed.

[0003] Secondary batteries for driving motors are required to have extremely high output characteristics and high energy compared to consumer lithium secondary batteries used in mobile phones, laptops, etc. Therefore, lithium secondary batteries, which have the highest theoretical energy of all practical batteries, have attracted attention and are currently being rapidly developed.

[0004] Currently widely used lithium secondary batteries use flammable organic electrolytes, and these liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

[0005] Therefore, in recent years, research and development on all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials composed primarily of ionic conductors that can conduct ions in a solid state. Therefore, all-solid-state lithium secondary batteries 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.

[0006] In a typical all-solid-state lithium secondary battery, the positive electrode has a configuration in which a positive electrode active material layer is disposed on the surface of a positive electrode current collector. The positive electrode active material layer contains, in addition to the positive electrode active material, a solid electrolyte for improving lithium ion conductivity in the positive electrode active material layer, and a binder for binding particles of the positive electrode active material and the solid electrolyte to each other and to the positive electrode current collector.

[0007] Here, WO 2020 / 241691 discloses an all-solid-state battery in which at least one layer selected from a positive electrode layer (positive electrode active material layer), a negative electrode layer (negative electrode active material layer), and a solid electrolyte layer contains a particulate first binder and a non-particulate second binder. According to WO 2020 / 241691, such a configuration is believed to provide an all-solid-state battery with good cycle characteristics.

[0008] However, according to the investigations of the present inventors, it has been found that when the technology described in International Publication No. 2020 / 241691 is adopted in the positive electrode active material layer of a secondary battery, sufficient rapid charging characteristics may not be obtained in the secondary battery to which the positive electrode active material layer is applied.

[0009] Therefore, an object of the present invention is to provide a means for improving the rapid charging characteristics of a secondary battery.

[0010] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by controlling the amount of solid electrolyte contained in the surface portion of the positive electrode active material layer that is in contact with the positive electrode current collector to a specific value or less, thereby completing the present invention.

[0011] That is, one aspect of the present invention relates to a secondary battery including a power generating element including a positive electrode having a positive electrode active material layer disposed on the surface of a positive electrode current collector, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte. In the secondary battery, the positive electrode active material layer is formed by stacking a first layer in contact with the solid electrolyte layer and containing a positive electrode active material, a solid electrolyte, and a binder, and a second layer in contact with the positive electrode current collector and containing a positive electrode active material and a binder. The solid electrolyte content in the first layer is 1% by mass or more relative to 100% by mass of the total solid content in the first layer, and the solid electrolyte content in the second layer is 0% by mass or more and less than 1% by mass relative to 100% by mass of the total solid content in the second layer.

[0012] Fig. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention. Fig. 2 is a cross-sectional view taken along line 2-2 shown in Fig. 1. Fig. 3 is a cross-sectional view schematically showing a positive electrode according to one embodiment of the present invention. Fig. 4 is a schematic diagram showing an example of a branched-chain fibrous binder.

[0013] One aspect of the present invention relates to a secondary battery including a power generating element including a positive electrode having a positive electrode active material layer disposed on the surface of a positive electrode current collector, a negative electrode, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte. In the secondary battery, the positive electrode active material layer is formed by stacking a first layer in contact with the solid electrolyte layer and containing a positive electrode active material, a solid electrolyte, and a binder, and a second layer in contact with the positive electrode current collector and containing a positive electrode active material and a binder. The solid electrolyte content in the first layer is 1% by mass or more relative to 100% by mass of the total solid content in the first layer, and the solid electrolyte content in the second layer is 0% by mass or more and less than 1% by mass relative to 100% by mass of the total solid content in the second layer. This aspect enables the secondary battery to have improved rapid charging characteristics.

[0014] The above-mentioned embodiments of the present invention will be described below with reference to the drawings. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. The present invention will be described below using as an example a flat stacked type (internal parallel connection type) all-solid-state lithium secondary battery, which is one type of secondary battery.

[0015] FIG. 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1 . The layered structure allows the battery to be compact and have a high capacity. This specification will be described in detail using the flat-layered, non-bipolar all-solid-state lithium secondary battery shown in FIGS. 1 and 2 (hereinafter simply referred to as a "layered battery"). However, in terms of the internal electrical connection configuration (electrode structure) of the secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

[0016] 1, the stacked battery 10a has a flat, rectangular shape, with a negative electrode current collector 25 and a positive electrode current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and the periphery is heat-sealed, with the negative electrode current collector 25 and the positive electrode current collector 27 extending to the outside.

[0017] The secondary battery according to this embodiment is not limited to a laminated, flat shape. A wound-type all-solid-state lithium secondary battery may be cylindrical, or may be a cylindrical battery modified into a rectangular, flat shape, and is not particularly limited. The cylindrical battery may use a laminate film or a conventional cylindrical can (metal can) as its exterior material, and is not particularly limited. Preferably, the power generating element is housed inside a laminate film containing aluminum. This configuration can achieve weight reduction.

[0018] Furthermore, there is no particular limitation on the way in which the current collectors (25, 27) shown in Fig. 1 are taken out. The negative electrode current collector 25 and the positive electrode current collector 27 may be taken out from the same side, or the negative electrode current collector 25 and the positive electrode current collector 27 may each be divided into a plurality of pieces and taken out from each side, and so on, and are not limited to what is shown in Fig. 1. Furthermore, in a wound-type all-solid-state lithium secondary battery, terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

[0019] 2, the stacked battery 10a of this embodiment has a structure in which a flat, generally 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 stacked. 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 stacked in this order such that one positive electrode active material layer 15 faces an adjacent negative electrode active material layer 13 with a solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, the stacked battery 10a shown in FIG. 2 can also be said to have a configuration in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel. Furthermore, a restraining pressure is applied to the stacked battery 10a in the stacking direction of the power generating element 21 by a restraining member (pressure member) (not shown). Therefore, the volume of the power generating element 21 is kept constant.

[0020] 2, the outermost negative electrode current collectors located on both outermost layers of the power generating element 21 each have a negative electrode active material layer 13 disposed on only one side, but active material layers may be provided on both sides. That is, instead of using a current collector exclusively for the outermost layer with an active material layer provided on only one side, a current collector with active material layers on both sides may be used as the outermost current collector as is.

[0021] Negative electrode current collector 11′ and positive electrode current collector 11″ are respectively attached with a negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the respective electrodes (positive and negative electrodes), and are structured so as to be sandwiched between the ends of laminate film 29, which is the battery outer casing material, and extended to the outside of laminate film 29. Positive electrode current collector 27 and negative electrode current collector 25 may be attached to positive electrode current collector 11″ and negative electrode current collector 11′ of the respective electrodes by ultrasonic welding, resistance welding, or the like, via positive electrode leads and negative electrode leads (not shown) as necessary.

[0022] FIG. 3 is a cross-sectional view schematically illustrating a positive electrode according to this embodiment. As shown in FIG. 3, the positive electrode 14 has a structure in which a positive electrode active material layer 15 is disposed on the surface of a positive electrode current collector 11″. Although not shown in FIG. 3, the exposed surface of the positive electrode active material layer 15 that is not in contact with the positive electrode current collector 11″ is in contact with the solid electrolyte layer 17 (see FIG. 2). The positive electrode active material layer 15 has a laminated structure in which a first layer 15a that is in contact with the solid electrolyte layer 17 and contains a positive electrode active material, a solid electrolyte, and a binder, and a second layer 15b that is in contact with the positive electrode current collector 11″ and contains a positive electrode active material and a binder, are stacked.

[0023] The main components of the secondary battery according to this embodiment will be described below.

[0024] [Current Collectors] The current collectors (negative electrode current collector 11′ and positive electrode current collector 11″) have 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 collectors. For example, metals and conductive resins can be used as the material constituting the current collectors.

[0025] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Foils in which aluminum is coated on the surface of a metal may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of active materials.

[0026] The latter conductive resin may be a conductive polymer material or a non-conductive polymer material to which a conductive filler is added as needed.

[0027] 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.

[0028] [Negative Electrode (Negative Electrode Active Material Layer)] In the laminated battery according to the embodiment shown in FIGS. 1 and 2 , the negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. Furthermore, a lithium-containing metal may be used as the 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, Sn, Mg, Au, Ag, and Zn. The negative electrode active material preferably contains metallic lithium or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably contains metallic lithium or a lithium-containing alloy. When metallic lithium or a lithium-containing alloy is used as the negative electrode active material, the secondary battery according to this embodiment may be a so-called lithium deposition type in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during charging. Therefore, in this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be present during full discharge.

[0029] 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 40 to 99 mass %, and more preferably within the range of 50 to 90 mass %, for example.

[0030] The negative electrode active material layer preferably further contains a solid electrolyte. By including the solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes, and sulfide solid electrolytes are preferred. In this specification, the term "solid electrolyte" refers to a material mainly composed of an ion conductor capable of ion conduction in a solid, and in particular, a material having a lithium ion conductivity of 1×10 at room temperature (25° C.). −５ S / cm or more, and this lithium ion conductivity is preferably 1×10 −４ The ionic conductivity is 100 S / cm or more. Here, the value of the ionic conductivity can be measured by an AC impedance method.

[0031] 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 -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, Li２ 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.

[0032] The sulfide solid electrolyte is, for example, Li ３ P.S. ４ It may have a Li framework. ４ P ２ 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 framework 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 (e.g., 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), etc. may be used. Among them, the sulfide solid electrolyte contained in the active material layer is preferably a sulfide solid electrolyte containing P element, and the sulfide solid electrolyte is preferably Li ２ S-P ２ S ５ It is more preferable that the sulfide solid electrolyte is a material mainly composed of Li. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte is a material mainly composed of Li. ６ P.S. ５ X, where X is Cl, Br or I, preferably Cl.

[0033] In addition, the sulfide solid electrolyte is Li ２ S-P ２ S ５ In the case of the system, Li ２ S and P ２ S ５ The ratio is the molar ratio of Li ２ S:P ２ S ５ It is preferable that the ratio is in the range of 50:50 to 100:0, and among these, Li ２ S:P ２ S ５ It is preferable that the ratio is 70:30 to 80:20.

[0034] The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid-phase method. Sulfide glass can be obtained, for example, by mechanical milling (such as a ball mill) a raw material composition. Crystallized sulfide glass can be obtained, for example, by heat-treating sulfide glass at a temperature equal to or higher than the crystallization temperature.

[0035] Examples of oxide solid electrolytes include compounds having a NASICON structure. An example of a compound having a NASICON structure is a compound represented by the general formula Li １＋ｘ Al ｘ Ge ２−ｘ (P.O. ４ ) ３ (0≦x≦2) (LAGP), a compound represented by the general formula Li １＋ｘ Al ｘ Ti ２−ｘ (P.O. ４ ) ３ (0≦x≦2) (LATP) and the like. Another example of the oxide solid electrolyte is LiLaTiO (for example, Li ０．３４ La ０．５１ TiO ３ ), LiPON (e.g., Li ２．９ P.O. ３．３ N ０．４６ ), LiLaZrO (e.g., Li ７ La ３ Zr ２ O １２ ) etc.

[0036] Examples of the shape of the solid electrolyte include particulate shapes such as spherical and oval spheres, and thin films. When the solid electrolyte is particulate, its average particle diameter (D50) is not particularly limited, but is preferably 40 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. On the other hand, the average particle diameter (D50) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

[0037] The content of the solid electrolyte in the negative electrode active material layer is, for example, preferably in the range of 1 to 60 mass %, and more preferably in the range of 10 to 50 mass %.

[0038] The negative electrode active material layer may further contain at least one of a binder and a conductive additive in addition to the above-mentioned negative electrode active material and solid electrolyte.

[0039] The binder is not particularly limited, and examples thereof include thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene (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 (P Fluorine resins such as vinylidene fluoride (CTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc., vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride Examples of suitable fluororubbers 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), epoxy resins, and carboxymethyl cellulose. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferred.

[0040] The conductive additive is not particularly limited, and examples thereof 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. The electronic conductivity of the conductive additive is preferably 1 S / m or more, and more preferably 1×10 ２ S / m or more is more preferable, and 1×10 ４ It is more preferable that the density is 1×10 S / m or more. ５ The upper limit of the electronic conductivity of the conductive additive is not particularly limited, but is usually 1×10 ７ S / m or less.

[0041] 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.

[0042] When the conductive additive is particulate, the average particle diameter (primary particle diameter) is not particularly limited, but is preferably 0.01 to 10 μm from the viewpoint of the electrical properties of the battery. In this specification, the "particle diameter of the conductive additive" refers to the longest distance L between any two points on the contour line of the conductive additive. The value of the "average particle diameter of the conductive additive" is calculated as the average particle diameter of particles observed within several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0043] When the negative electrode active material layer contains a conductive additive, the content of the conductive additive in the negative electrode active material layer is not particularly limited, but is preferably 0 to 10 mass %, more preferably 2 to 8 mass %, and even more preferably 4 to 7 mass %, relative to the total mass of the negative electrode active material layer. Within such a range, a stronger electron conduction path can be formed in the negative electrode active material layer, which can effectively contribute to improving battery characteristics.

[0044] The thickness of the negative electrode active material layer varies depending on the intended configuration of the secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0045] [Solid Electrolyte Layer] In the stacked battery according to the embodiment shown in FIGS. 1 and 2 , the solid electrolyte layer 17 is interposed between the positive electrode active material layer and the negative electrode active material layer, and is a layer that essentially contains a solid electrolyte.

[0046] The specific form of the solid electrolyte contained in the solid electrolyte layer is not particularly limited, and the solid electrolytes exemplified in the section on the negative electrode active material layer and their preferred forms can be similarly employed. In some cases, a solid electrolyte other than the above-mentioned solid electrolytes may be used in combination.

[0047] The solid electrolyte layer may further contain a binder in addition to the above-mentioned solid electrolyte.

[0048] 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 600 μm or less, more preferably 500 μm or less, and even more preferably 400 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 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more.

[0049] 1 and 2 , the positive electrode active material layer 15 essentially contains a solid electrolyte and a binder. The positive electrode active material layer 15 has a configuration in which a first layer in contact with the solid electrolyte layer 17 and a second layer in contact with the negative electrode active material layer 13 are stacked.

[0050] (1) First Layer In the positive electrode active material layer, the first layer is disposed so as to be in contact with the solid electrolyte layer. The first layer essentially contains a positive electrode active material, a solid electrolyte, and a binder.

[0051] (Positive Electrode Active Material) The type of positive electrode active material contained in the first 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 , LiMnPO ４ 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 these, a composite oxide containing lithium and nickel is preferably used, and Li(Ni-Mn-Co)O is more preferably used.２ Also, those in which some of these transition metals have been replaced with other elements (hereinafter also simply referred to as "NMC composite oxides") are used. NMC composite oxides have a layered crystal structure in which lithium atomic layers and transition metal (Mn, Ni, and Co) atomic layers (orderly arranged) are alternately stacked with oxygen atomic layers interposed therebetween, and contain one Li atom per transition metal M atom, so that the amount of Li that can be extracted is twice that of spinel lithium manganese oxides, i.e., the supply capacity is doubled, and high capacity can be achieved.

[0052] As described above, the NMC composite oxide also includes composite oxides in which a portion of the transition metal element is substituted with another metal element. In this case, the other element may be Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, or Zn, preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, or Cr, more preferably Ti, Zr, P, Al, Mg, or Cr, and even more preferably Ti, Zr, Al, Mg, or Cr from the viewpoint of improving cycle characteristics.

[0053] Furthermore, in one preferred embodiment, a sulfur-based positive electrode active material is used. Examples of the sulfur-based positive electrode active material include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material can be used as long as it is capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur.

[0054] In some cases, two or more positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used.

[0055] The shape of the positive electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is particulate, its average particle size (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. ５０The value of can be measured by a laser diffraction scattering method.

[0056] The content of the positive electrode active material in the first layer is not particularly limited, but is preferably more than 50% by mass, more preferably in the range of more than 50% by mass to 95% by mass or less, and even more preferably in the range of 60% by mass to 90% by mass, relative to 100% by mass of the total solid content in the first layer.

[0057] (Solid Electrolyte) The type of solid electrolyte contained in the first layer is not particularly limited, but it is more preferable that the first layer contains a sulfide solid electrolyte. Specific and preferred forms of the solid electrolyte, such as the sulfide solid electrolyte, can be the same as those described in the section on the solid electrolyte layer above.

[0058] The content of the solid electrolyte in the first layer is essentially 1% by mass or more, based on 100% by mass of the total solid content in the first layer. If the content of the solid electrolyte in the first layer is less than 1% by mass, the ionic conductivity of the first layer will be low, which may increase the internal resistance of the battery and prevent sufficient rapid charging characteristics from being obtained. The content of the solid electrolyte in the first layer is preferably 1% by mass or more and 70% by mass or less, more preferably 5% by mass or more and 50% by mass or less, and even more preferably 10% by mass or more and 30% by mass or less. If the content of the solid electrolyte in the first layer is within the above range, the ionic conductivity and energy density of the first layer can both be achieved.

[0059] (Binder) The type of binder contained in the first layer is not particularly limited, and the binder described in the above-mentioned section on the negative electrode active material layer can be similarly employed. In particular, the binder contained in the first layer preferably contains a fibrous binder. More specifically, when a cross section of the first layer is observed using a scanning electron microscope (SEM), the area ratio of the fibrous binder to the total area of ​​the binder in an observation image is preferably greater than 50%, more preferably 80% to 100%, even more preferably 90% to 100%, particularly preferably 95% to 100%, and most preferably 100%. When the area ratio of the fibrous binder in the first layer is within the above range, the contact area between particles of the positive electrode active material, the solid electrolyte, and the optionally contained conductive additive is increased, thereby reducing the internal resistance of the battery and further improving the rapid charging characteristics. In this specification, the term "cross section of a layer" refers to a cross section of the layer cut in the stacking direction of the secondary battery (i.e., a cross section of the layer cut in the thickness direction). In addition, in this specification, the term "fibrous binder" refers to a binder mainly composed of fibers having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less in an observation image when a cross section of a positive electrode active material layer is observed using a scanning electron microscope (SEM). The aspect ratio is calculated by dividing the maximum Feret diameter by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between two parallel lines when the outline of the binder is sandwiched between the lines, and the minimum Feret diameter is the minimum distance between the lines when the outline of the binder is sandwiched between two parallel lines. When a binder is "mainly composed" of the above fibers, it means that the area ratio of the above fiber portion to the total area of ​​the binder in the SEM observation image is 50% or more. A single fibrous binder may include a portion other than the fiber having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less (a portion having an aspect ratio of less than 10 or a portion having a minimum Feret diameter of more than 0.2 μm).However, the area ratio of the non-fiber portion to the total area of ​​the fibrous binder in the SEM observation image must be less than 50%, preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less (the lower limit is 0%). Fibrous binders include not only those composed of only one fiber, but also those having a configuration in which two or more fibers are connected to each other. Specific shapes of binders having a configuration in which two or more fibers are connected to each other include branched chain, radial, and mesh shapes, as well as combinations of these.

[0060] Here, we will explain how to determine the maximum and minimum Feret diameters for a binder having a structure in which two or more fibers are connected to each other. FIG. 4 is a schematic diagram showing an example of a branched fibrous binder. The binder 30 shown in FIG. 4 has a structure in which fiber X, fiber Y, and fiber Z are connected to each other. Each dashed line represents a line connecting the center of the fiber width (half width), and points A, B, and C represent the ends of each dashed line. Note that the ends of each dashed line coincide with the ends of the fibers. Point D represents the intersection of the three dashed lines. In other words, the binder 30 shown in FIG. 4 can also be said to have a shape in which fiber X from point A to point D, fiber Y from point B to point D, and fiber Z from point C to point D are bonded at point D. The maximum Feret diameter of fiber X in the binder 30 shown in FIG. 4 is defined as the distance from point A to point D. Similarly, the maximum Feret diameter of fiber Y is the distance from point B to point D, and the maximum Feret diameter of fiber Z is the distance from point C to point D. Furthermore, the minimum Feret diameter of fiber X is the minimum distance between two parallel lines when the outline of the binder (fiber) from point A to point D is sandwiched between the lines. The same applies to the minimum Feret diameters of fiber Y and fiber Z. In binder 30 shown in FIG. 4 , fiber Y and fiber Z have an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less, while fiber X has an aspect ratio of less than 10. However, since the area of ​​fiber X occupies less than 50% of the total area of ​​binder 30, the binder shown in FIG. 4 can be said to be a fibrous binder.

[0061] The type of fibrous binder is not particularly limited as long as it has the above-mentioned shape in the positive electrode active material layer, but a binder that fibrillates when shear force is applied can be suitably used. Examples of such fibrillizable binders include polytetrafluoroethylene (PTFE), carboxymethyl cellulose, polyethylene oxide, polyvinyl alcohol, and polyethylene, with polytetrafluoroethylene (PTFE) being more preferred. The use of such binders further improves both the binding properties and the contact between the particles, thereby further improving the rapid charging characteristics. One type of fibrous binder may be used alone, or two or more types may be used in combination. In this specification, the compound name of the binder may refer not only to the compound indicated by the compound name, but also to forms in which the terminal or part of the side chain is substituted (modified) with other substituents. In cases where the terminals or part of the side chains are substituted (modified) with other substituents, the proportion of structural units whose terminals or side chains have been substituted (modified) with other substituents in 100 mol % of all structural units is preferably 10 mol % or less, and more preferably 5 mol % or less.

[0062] (Conductive Aid) The first layer may contain a conductive aid as needed. The type of conductive aid is not particularly limited, and those described in the section on the negative electrode active material layer above may be similarly employed. Among them, the conductive aid contained in the first layer preferably contains fibrous carbon. In this specification, "fibrous carbon" refers to conductive carbon having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less in an image observed using a scanning electron microscope (SEM). As described above, the electronic conductivity of the conductive carbon is preferably 1 S / m or more, and 1×10 ２ S / m or more is more preferable, and 1×10 ４ It is more preferable that the density is 1×10 S / m or more. ５ The upper limit of the electronic conductivity of the conductive carbon is not particularly limited, but is usually 1×10 ７ S / m or less.

[0063] The type of fibrous carbon is not particularly limited as long as it has the above-mentioned shape, and examples thereof include carbon fiber (carbon nanofiber), graphene, and carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes). Of these, carbon fiber (carbon nanofiber) is preferred. One type of fibrous carbon may be used alone, or two or more types may be used in combination.

[0064] The content of fibrous carbon in the total amount (100% by mass) of the conductive additive contained in the first layer is preferably greater than 50% by mass, more preferably 80% by mass or more and 100% by mass or less, even more preferably 90% by mass or more and 100% by mass or less, particularly preferably 95% by mass or more and 100% by mass or less, and most preferably 100%.

[0065] The thickness of the first layer varies depending on the configuration of the intended secondary battery, but is preferably 50 μm or more and 120 μm or less, and more preferably 60 μm or more and 100 μm or less. When the thickness of the first layer is within the above range, sufficient energy density can be ensured.

[0066] The manufacturing method of the first layer is not particularly limited, and known methods can be used as appropriate. For the first layer containing a fibrous binder, a mixture containing a positive electrode active material, a solid electrolyte, a fibrillizable binder, and an optional conductive additive (preferably fibrous carbon) is subjected to shear force by an appropriate method to fibrillate the binder. The fibrillated mixture is then formed into a sheet to obtain the first layer. The mixture is preferably a powder mixture substantially free of liquid components. The liquid component content in the mixture is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, even more preferably 0.5% by mass or less, particularly preferably 0.1% by mass or less, and most preferably 0% by mass, relative to 100% by mass of the mixture.

[0067] (2) Second Layer In the positive electrode active material layer, the second layer is disposed so as to be in contact with the positive electrode current collector. The second layer essentially contains a positive electrode active material and a binder.

[0068] (Positive electrode active material) The positive electrode active material contained in the second layer may be the same as or different from the positive electrode active material contained in the first layer, but is preferably the same as or different from the positive electrode active material contained in the second layer.

[0069] The content of the positive electrode active material in the second layer is not particularly limited, but is preferably more than 50% by mass, more preferably more than 50% by mass to 95% by mass or less, and even more preferably 60% by mass to 90% by mass, relative to 100% by mass of the total solid content in the second layer. When the content of the positive electrode active material in the second layer is within the above range, the energy density can be maintained.

[0070] (Binder) The type of binder contained in the second layer is not particularly limited, and the binder described in the above-mentioned section on the negative electrode active material layer can be similarly adopted. Among them, the binder contained in the second layer preferably contains a non-fibrous binder. More specifically, when the cross section of the second layer is observed using a scanning electron microscope (SEM), the area ratio of the non-fibrous binder to the total area of ​​the binder in the observation image is preferably greater than 50%, more preferably 80% to 100% or less, even more preferably 90% to 100% or less, particularly preferably 95% to 100% or less, and most preferably 100% (that is, the area ratio of the fibrous binder is preferably less than 50%, more preferably 0% to 20% or less, even more preferably 0% to 10% or less, particularly preferably 0% to 5% or less, and most preferably 0%). When the area ratio of the non-fibrous binder in the second layer is within the above range, the adhesion between the second layer and the positive electrode current collector is improved, and the second layer can be prevented from peeling off from the positive electrode current collector. As a result, the internal resistance of the battery is kept low, and the rapid charging characteristics can be further improved. In this specification, the term "non-fibrous binder" refers to a binder other than the above-mentioned "fibrous binder." That is, in this specification, the term "non-fibrous binder" refers to a binder having an aspect ratio of less than 10 or a minimum Feret diameter of greater than 0.2 μm in an image obtained by observing the cross section of the positive electrode active material layer using a scanning electron microscope (SEM). The determination of whether or not a binder is a "non-fibrous binder" and the calculation of the "area ratio of the non-fibrous binder" are performed by the method described in the Examples below.

[0071] The type of non-fibrous binder is not particularly limited as long as it has the above-mentioned shape in the positive electrode active material layer. However, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), ethyl cellulose, and acrylic resin are preferred, and polyvinylidene fluoride (PVDF) is more preferred. By using such a binder, the second layer and the positive electrode current collector can be firmly adhered to each other, thereby further improving the rapid charging characteristics. Only one type of non-fibrous binder may be used alone, or two or more types may be used in combination. As described above, the binder may include a form in which the terminal or part of the side chain is substituted (modified) with another substituent.

[0072] (Conductive Aid) The second layer may contain a conductive aid as needed. The type of conductive aid is not particularly limited, and those described in the section on the negative electrode active material layer above may be similarly employed. In particular, the conductive aid contained in the second layer preferably contains fibrous carbon. The type of conductive aid is not particularly limited, and those described in the section on the first layer above may be similarly employed. The conductive aid (preferably fibrous carbon) contained in the first layer and the conductive aid (preferably fibrous carbon) contained in the second layer may be the same or different, but are preferably the same.

[0073] The content of fibrous carbon in the total amount (100% by mass) of the conductive additive contained in the second layer is preferably greater than 50% by mass, more preferably 80% by mass or more and 100% by mass or less, even more preferably 90% by mass or more and 100% by mass or less, particularly preferably 95% by mass or more and 100% by mass or less, and most preferably 100% by mass.

[0074] (Solid Electrolyte) In the present invention, the second layer is characterized by not containing a solid electrolyte, or even if it does contain a solid electrolyte, the amount is extremely small. More specifically, the content of the solid electrolyte in the second layer is 0% by mass or more than 0% by mass and less than 1% by mass, relative to 100% by mass of the total solid content in the second layer. If the content of the solid electrolyte in the second layer is 1% by mass or more, rapid charging characteristics may be insufficient. This is thought to be because, when the potential of the positive electrode current collector increases during charging, the solid electrolyte in contact with the positive electrode current collector in the second layer deteriorates due to oxidation, resulting in an increase in the internal resistance of the battery. For these reasons, the content of the solid electrolyte in the second layer is preferably 0% by mass or more than 0% by mass and 0.5% by mass or less, relative to 100% by mass of the total solid content in the second layer, more preferably 0% by mass or more than 0% by mass and 0.1% by mass or less, and particularly preferably 0% by mass.

[0075] When the second layer contains a solid electrolyte, the type of solid electrolyte is not particularly limited, but it is more preferable that the second layer contains a sulfide solid electrolyte. Specific and preferred forms of solid electrolytes such as sulfide solid electrolytes can be similar to those described in the solid electrolyte layer section above. The solid electrolyte contained in the first layer and the solid electrolyte contained in the second layer may be the same or different, but are preferably the same.

[0076] The thickness of the second layer is preferably 0.1 μm or more and less than 5 μm, more preferably 1 μm or more and 4 μm or less. When the thickness of the second layer is 0.1 μm or more, deterioration of the solid electrolyte is effectively suppressed, and an increase in the internal resistance of the battery can be prevented. When the thickness of the second layer is less than 5 μm, the energy density can be maintained.

[0077] The method for producing the second layer is not particularly limited, and known methods can be used as appropriate. For example, a slurry is prepared by adding a suitable dispersion medium to a positive electrode active material, a binder, an optional conductive additive (preferably fibrous carbon), and an optional solid electrolyte. The slurry is applied to a support (e.g., a positive electrode current collector), and the dispersion medium is removed to obtain the second layer. For another example, the second layer can be obtained by kneading a mixture containing a positive electrode active material, a binder, an optional conductive additive (preferably fibrous carbon), and an optional solid electrolyte, and then forming the mixture into a sheet. In this case, the mixture preferably does not substantially contain a liquid component. The preferred content of the liquid component in the mixture is the same as that described above for the mixture used to produce the first layer.

[0078] In the secondary battery according to this embodiment, at least one of the first layer and the second layer preferably contains a conductive additive made of fibrous carbon, and both the first layer and the second layer preferably contain a conductive additive made of fibrous carbon. By containing fibrous carbon as a conductive additive in the first layer and / or the second layer, the internal resistance of the battery can be kept low, and the rapid charging characteristics can be further improved.

[0079] In the secondary battery according to this embodiment, it is particularly preferable that the binder in the first layer contains a fibrous binder, and the area ratio of the fibrous binder to the area of ​​the binder in an image observed when a cross section of the first layer is observed using a scanning electron microscope is greater than 50%; the binder in the second layer contains a non-fibrous binder, and the area ratio of the non-fibrous binder to the area of ​​the binder in an image observed when a cross section of the second layer is observed using a scanning electron microscope is greater than 50%; and both the first layer and the second layer further contain a conductive additive made of fibrous carbon, the fibrous binder containing polytetrafluoroethylene, and the non-fibrous binder containing polyvinylidene fluoride. As shown in Example 1 described below, having the above configuration exhibits particularly excellent rapid charging characteristics.

[0080] In the secondary battery according to this embodiment, the positive electrode active material layer may include one or more other layers between the first layer and the second layer, but it is preferable that the first layer and the second layer are disposed adjacent to each other.

[0081] [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.

[0082] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead. Materials used in known lithium secondary batteries may be used as the constituent materials of the positive electrode and the negative electrode lead. It is preferable that the portion removed from the exterior be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, which may cause a current leak and affect the product (e.g., automotive parts, particularly electronic devices).

[0083] [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 can be used, as shown in Figures 1 and 2 . 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 for the exterior body because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0084] 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.

[0085] Although one embodiment of the secondary battery of the present invention 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.

[0086] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0087] <Example of Preparation of Evaluation Cell> [Example 1] (Preparation of Positive Electrode Active Material Layer) (1) Preparation of First Layer As a constituent material of the first layer, NMC composite oxide (LiNi) was used as a positive electrode active material. ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D50): 1 μm) and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ The following materials were prepared: carbon nanofiber (CNF) (VGCF (registered trademark), aspect ratio: 60, average fiber diameter: approximately 150 nm, average fiber length: approximately 9 μm, manufactured by Showa Denko K.K.) as a conductive additive; and polytetrafluoroethylene (PTFE) as a fibrillizable binder. In a glove box with an argon atmosphere and a dew point of −68°C or lower, the positive electrode active material, solid electrolyte, conductive additive, and binder were weighed out to a mass ratio of 79:16:3:2 and kneaded in an agate mortar. After confirming that the binder had fibrillated, the resulting mixture was formed into a sheet using a hand roller and then punched into a circle with a diameter of 19 mm to obtain a first layer with a thickness of 80 μm.

[0088] (2) Preparation of the second layer As a constituent material of the second layer, NMC composite oxide (LiNi) ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D50): 1 μm), carbon nanofiber (CNF) as a conductive additive (VGCF (registered trademark), manufactured by Showa Denko K.K., aspect ratio: 60, average fiber diameter: approximately 150 nm, average fiber length: approximately 9 μm), and polyvinylidene fluoride (PVDF) as a binder were prepared. In a glove box with an argon atmosphere and a dew point of −68°C or lower, the positive electrode active material, conductive additive, and binder were weighed out to a mass ratio of 79:19:2 and kneaded in an agate mortar. The resulting mixture was formed into a sheet using a hand roller and then punched into a circle with a diameter of 19 mm to obtain a second layer with a thickness of 3 μm.

[0089] (Preparation of Solid Electrolyte Layer) In a glove box with an argon atmosphere having a dew point of −68° C. or less, an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ A solid electrolyte slurry was prepared by mixing 95 parts by mass of styrene-butadiene rubber (SBR) as a binder with 5 parts by mass of mesitylene as a solvent. The obtained solid electrolyte slurry was applied to the surface of a stainless steel foil as a support using an applicator, dried, and then punched out into a circle with a diameter of 25 mm to obtain a solid electrolyte layer with a thickness of 40 μm.

[0090] (Preparation of Negative Electrode Intermediate Layer) Silver nanoparticles and carbon black nanoparticles were weighed and mixed in a mass ratio of 1:3. Five parts by mass of the resulting mixture was added to a binder solution (0.5 parts by mass of styrene-butadiene rubber (SBR) as a binder dissolved in mesitylene as a solvent) and mixed to prepare a negative electrode intermediate layer slurry. The resulting negative electrode intermediate layer slurry was applied to the surface of a stainless steel foil negative electrode current collector using an applicator, dried, and then punched into a circle with a diameter of 21 mm to obtain a negative electrode intermediate layer with a thickness of 10 μm.

[0091] (Preparation of Evaluation Cell) The second layer and the first layer prepared above were sequentially stacked on an aluminum foil (circular, 19 mm diameter) serving as a positive electrode current collector. The solid electrolyte layer formed on the stainless steel foil surface prepared above was transferred onto the first layer by cold isostatic pressing (CIP) so that the exposed surface of the solid electrolyte layer faced the first layer. After peeling off the stainless steel foil adjacent to the solid electrolyte layer, the negative electrode intermediate layer formed on the stainless steel foil (negative electrode current collector) prepared above was placed on the transferred solid electrolyte layer so that the exposed surface of the negative electrode intermediate layer faced the solid electrolyte layer, and the resulting mixture was pressed by cold isostatic pressing (CIP) to obtain an evaluation cell (lithium deposition-type all-solid-state lithium secondary battery).

[0092] [Example 2] An evaluation cell for this example was produced in the same manner as in Example 1, except that in "(2) Production of the second layer", styrene-butadiene rubber (SBR) was used as the binder instead of polyvinylidene fluoride (PVDF).

[0093] [Example 3] An evaluation cell for this example was produced in the same manner as in Example 2, except that in "(2) Production of the second layer", acetylene black (Denka Black (registered trademark) HS-100, manufactured by Denka Company Ltd., average primary particle diameter: 36 nm, aspect ratio: less than 10) was used as the conductive additive instead of carbon nanofiber (CNF).

[0094] [Comparative Example 1] (Preparation of Positive Electrode Active Material Layer) As a constituent material of the positive electrode active material layer, NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D50): 1 μm) and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５The following materials were prepared: acetylene black (D50: 0.2 μm), a conductive additive (Denka Black (registered trademark) HS-100, manufactured by Denka Co., Ltd., average primary particle diameter: 36 nm, aspect ratio: less than 10), and polytetrafluoroethylene (PTFE; fibrillizable) and polyvinylidene fluoride (PVDF) as binders. In a glove box with an argon atmosphere and a dew point of −68°C or lower, the positive electrode active material, solid electrolyte, conductive additive, and binder were weighed out to a mass ratio of 79:16:3:2 (the mass ratio of PTFE to PVDF was 1:1), and kneaded in an agate mortar. After confirming that the PTFE had fibrillated, the resulting mixture was formed into a sheet using a hand roller and then punched into a circle with a diameter of 19 mm, obtaining a positive electrode active material layer with a thickness of 83 μm.

[0095] An evaluation cell for this comparative example was produced in the same manner as in Example 1, except that the positive electrode active material layer obtained above was used.

[0096] Comparative Example 2 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 1, except that in the above "(Preparation of Positive Electrode Active Material Layer)", polytetrafluoroethylene (PTFE) was not used as the binder, but only polyvinylidene fluoride (PVDF) was used; and the positive electrode active material, solid electrolyte, conductive additive, and binder were weighed out to a mass ratio of 79:16:3:2, and kneaded in an agate mortar.

[0097] Comparative Example 3 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 1, except that in the above "(Preparation of Positive Electrode Active Material Layer)", carbon nanofiber (CNF) (VGCF (registered trademark), manufactured by Showa Denko K.K., aspect ratio: 60, average fiber diameter: approximately 150 nm, average fiber length: approximately 9 μm) was used as the conductive additive instead of acetylene black; polytetrafluoroethylene (PTFE) was used alone as the binder without polyvinylidene fluoride (PVDF); and the positive electrode active material, solid electrolyte, conductive additive, and binder were weighed out to a mass ratio of 79:16:3:2 and kneaded in an agate mortar.

[0098] Comparative Example 4 (Preparation of Positive Electrode Active Material Layer) (1) Preparation of First Layer As a constituent material of the first layer, NMC composite oxide (LiNi) was used as a positive electrode active material. ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D50): 1 μm) and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ The following materials were prepared: carbon nanofiber (CNF) (VGCF (registered trademark), aspect ratio: 60, average fiber diameter: approximately 150 nm, average fiber length: approximately 9 μm, manufactured by Showa Denko K.K.) as a conductive additive; and polytetrafluoroethylene (PTFE) as a fibrillizable binder. In a glove box with an argon atmosphere and a dew point of −68°C or lower, the positive electrode active material, solid electrolyte, conductive additive, and binder were weighed out to a mass ratio of 79:16:3:2 and kneaded in an agate mortar. After confirming that the binder had fibrillated, the resulting mixture was formed into a sheet using a hand roller and then punched into a circle with a diameter of 19 mm to obtain a first layer with a thickness of 80 μm.

[0099] (2) Preparation of the second layer As a constituent material of the second layer, NMC composite oxide (LiNi) ０．８ Mn ０．１ Co ０．１ O ２ The positive electrode active material, solid electrolyte, conductive additive, and binder were weighed in a mass ratio of 79:16:3:2 in a glove box containing an argon atmosphere with a dew point of −68°C or lower and kneaded in an agate mortar. The resulting mixture was formed into a sheet using a hand roller and then punched into a circle with a diameter of 19 mm to obtain a second layer with a thickness of 3 μm.

[0100] An evaluation cell for this comparative example was produced in the same manner as in Example 1, except that the positive electrode active material layer obtained above was used.

[0101] Comparative Example 5 An evaluation cell for this comparative example was produced in the same manner as in Comparative Example 1, except that in the above "(Preparation of Positive Electrode Active Material Layer)", carbon nanofiber (CNF) (VGCF (registered trademark), manufactured by Showa Denko K.K., aspect ratio: 60, average fiber diameter: approximately 150 nm, average fiber length: approximately 9 μm) was used as the conductive additive instead of acetylene black.

[0102] [Comparative Example 6] An evaluation cell for this comparative example was prepared in the same manner as in Example 1, except that in "(2) Preparation of the second layer", no positive electrode active material was used as the constituent material of the second layer.

[0103] <Observation of Binder Shape> The positive electrode active material layer was removed from the evaluation cell prepared above, and the cross section of the first layer was observed using a scanning electron microscope (SEM). The maximum and minimum Feret diameters of each binder in the SEM image were determined. The maximum Feret diameter was divided by the minimum Feret diameter to calculate the aspect ratio. The proportion of the area of ​​the fibrous portion having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less to the total area of ​​each binder was determined to be greater than 50%, thereby determining whether each binder was a fibrous binder or a non-fibrous binder. This operation was performed on 10 SEM images (each including 10 or more binders), and the proportion of the area of ​​the fibrous binder to the area of ​​the binder in the observed image was calculated as a percentage. The results are shown in Table 1 below.

[0104] <Evaluation of rapid charging characteristics> A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector of the evaluation cell prepared above, respectively, and three charge / discharge cycles were performed under the following charge / discharge test conditions: During this test, a restraining pressure of 3 MPa was applied in the stacking direction of the evaluation cell using a pressure member.

[0105] (Charge / discharge test conditions) 1) Charging / discharging conditions [Voltage range] 3.0 to 4.3 V [Charging process] CCCV (0.01 C cutoff) [Discharging process] CC [Charge / discharge rate] 0.1 C or 2 C (30-minute rest after each charging / discharging) 2) Evaluation temperature: 298 K (25°C).

[0106] The evaluation cells were charged in a constant current / constant voltage (CCCV) mode in a thermostatic chamber set at the evaluation temperature. The charging process (to deposit lithium metal on the negative electrode current collector) was performed in a constant current / constant voltage (CCCV) mode, with the charge / discharge rate set at 3.0 V to 4.3 V (0.01 C cutoff). The discharging process (to dissolve lithium metal on the negative electrode current collector) was then performed in a constant current (CC) mode, with the charge / discharge rate set at 4.3 V to 3.0 V. Here, 1 C refers to the current value at which the battery is fully charged (100% charged) after one hour of charging. Three cycles of charging / discharging at 0.1 C and 2 C were performed. The ratio of the third-cycle charge capacity (2 C) to the third-cycle charge capacity (0.1 C) was calculated to evaluate the rapid charge characteristics. The results are shown in Table 1 below.

[0107]

[0108] The results shown in Table 1 demonstrate that the present invention can improve rapid charging characteristics. The reason for the poor rapid charging characteristics of Comparative Example 4 is believed to be that if the second layer contains a large amount of solid electrolyte, the solid electrolyte in contact with the positive electrode current collector in the second layer will degrade due to oxidation when the potential of the positive electrode current collector increases during charging, resulting in an increase in the internal resistance of the battery. The reason for the poor rapid charging characteristics of Comparative Example 6 is believed to be that if the second layer does not contain a positive electrode active material, the potential of the second layer will increase during charging, causing the solid electrolyte in contact with the second layer in the first layer to degrade due to oxidation, resulting in an increase in the internal resistance of the battery.

[0109] A comparison between Example 2 and Example 3 showed that the rapid charging characteristics were further improved by including fibrous carbon as a conductive additive in both the first layer and the second layer. Also, a comparison between Example 1 and Example 2 showed that the rapid charging characteristics were further improved by including polytetrafluoroethylene as a fibrous binder in the first layer and polyvinylidene fluoride as a non-fibrous binder in the second layer.

[0110] 10a, 100 Stacked battery, 11' Negative electrode current collector, 11" Positive electrode current collector, 13 Negative electrode active material layer, 14 Positive electrode, 15 Positive electrode active material layer, 15a First layer, 15b Second 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, 30 Fibrous binder.

Claims

a positive electrode having a positive electrode active material layer disposed on the surface of a positive electrode current collector; a negative electrode; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; A secondary battery having a power generating element including: the positive electrode active material layer is formed by stacking a first layer in contact with the solid electrolyte layer and containing a positive electrode active material, a solid electrolyte, and a binder, and a second layer in contact with the positive electrode current collector and containing a positive electrode active material and a binder; a content of the solid electrolyte in the first layer is 1% by mass or more relative to 100% by mass of the total solid content in the first layer; A secondary battery, wherein the content of the solid electrolyte in the second layer is 0% by mass or more than 0% by mass and less than 1% by mass, relative to 100% by mass of the total solid content in the second layer. 　 In the first layer, the binder contains a fibrous binder; 2. The secondary battery according to claim 1, wherein an area ratio of the fibrous binder to a total area of ​​the binder in an image obtained by observing a cross section of the first layer using a scanning electron microscope is greater than 50%.

3. The secondary battery according to claim 2, wherein the fibrous binder contains at least one selected from the group consisting of polytetrafluoroethylene, carboxymethyl cellulose, polyethylene oxide, polyvinyl alcohol, and polyethylene. 　 In the second layer, the binder comprises a non-fibrous binder; 4. The secondary battery according to claim 2, wherein the area ratio of the non-fibrous binder to the total area of ​​the binder in an image obtained by observing a cross section of the second layer using a scanning electron microscope is greater than 50%.

5. The secondary battery according to claim 4, wherein the non-fibrous binder includes at least one selected from the group consisting of styrene-butadiene rubber, polyvinylidene fluoride, ethyl cellulose, and acrylic resin.

3. The secondary battery according to claim 1, wherein at least one of the first layer and the second layer further contains a conductive additive made of fibrous carbon.

5. The secondary battery according to claim 4, wherein both the first layer and the second layer further contain a conductive additive made of fibrous carbon, the fibrous binder includes polytetrafluoroethylene, and the non-fibrous binder includes polyvinylidene fluoride. 　 The secondary battery according to claim 1 , wherein the content of the positive electrode active material in the second layer exceeds 50% by mass relative to 100% by mass of the total solid content in the second layer.

3. The secondary battery according to claim 1, wherein the second layer has a thickness of 0.1 μm or more and less than 5 μm.

3. The secondary battery according to claim 1, wherein the first layer has a thickness of 50 μm or more and 120 μm or less.