JP7769290B2 - Positive electrode material for electric device, and positive electrode for electric device and electric device using the same - Google Patents

Description

The present invention relates to a positive electrode material for an electric device, as well as a positive electrode for an electric device and an electric device using the same.

In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. The automotive industry is pinning its hopes on reducing carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and there has been active development of non-aqueous electrolyte secondary batteries, such as motor drive secondary batteries, which hold the key to commercializing these vehicles.

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, are attracting attention and are currently being rapidly developed.

Currently widely used lithium secondary batteries use a flammable organic electrolyte. These liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

In recent years, therefore, there has been active research and development into all-solid-state lithium secondary batteries that use oxide- or sulfide-based solid electrolytes. Solid electrolytes are materials primarily composed of ionic conductors that allow ions to be conducted in a solid state. For this reason, all-solid-state lithium secondary batteries do not, in principle, encounter the various problems associated with flammable organic electrolytes that occur with conventional liquid-based lithium secondary batteries.

A conventional technology has been known in which the surface of an electrode active material is coated with a sulfide solid electrolyte to suppress the formation of a high-resistance layer at the interface between the electrode active material and the sulfide solid electrolyte material, thereby increasing the output of the battery. However, when such electrode materials contain a conductive additive, the dispersibility of the conductive additive is insufficient, making it impossible to sufficiently reduce the internal resistance of the battery.

To solve these problems, Patent Document 1 proposes a technology for creating an electrode material by further blending composite active material particles, which are made by coating particles of electrode active material with a sulfide solid electrolyte, with a fibrous conductive additive and sulfide solid electrolyte particles with an average particle size smaller than that of the composite active material particles. Patent Document 1 claims that this configuration makes it possible to reduce the internal resistance of the battery.

Japanese Patent Application Laid-Open No. 2016-207418

The technology described in Patent Document 1 above aims to improve the output characteristics of a battery by reducing internal resistance primarily at the beginning of battery life (BOL; Beginning of Life). However, the inventors' investigations have revealed that when the technology described in Patent Document 1 is employed in a positive electrode material for an electrical device, the cycle durability of the electrical device to which the positive electrode material is applied may not be sufficiently ensured.

The present invention therefore aims to provide a means for further improving the cycle durability of electrical devices that use positive electrode materials containing a positive electrode active material capable of absorbing and releasing lithium ions, a conductive additive, and a solid electrolyte.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the above-mentioned problems could be solved by controlling the area profiles of the positive electrode active material and solid electrolyte in an image obtained by observing a cross section of a positive electrode active material layer obtained by molding a positive electrode material using a scanning electron microscope so that they satisfy a predetermined relationship, leading to the completion of the present invention.

One aspect of the present invention relates to a cathode material for an electric device, the cathode material comprising a cathode active material capable of absorbing and releasing lithium ions, a conductive additive, and a solid electrolyte. The cathode material is characterized in that, when a cross-section of a cathode active material layer obtained by molding the cathode material is observed using a scanning electron microscope, the area proportions of the cathode active material and the solid electrolyte in an observed image are defined as V _AM and V _SE , respectively, and the area proportions of the cathode active material and the solid electrolyte present at a distance from the conductive additive to an average particle diameter (D50) of the cathode active material in the observed image are defined as S _AM and S _SE , respectively.

The present invention makes it possible to further improve cycle durability in electrical devices that use a positive electrode material containing a positive electrode active material capable of absorbing and releasing lithium ions, a conductive additive, and a solid electrolyte.

1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery that is one embodiment of a positive electrode material according to the present invention. FIG. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. FIG. 2 is a diagram for explaining a method for calculating the SV ratio value of the positive electrode material according to the present invention.

The following describes the above-described embodiments of the present invention with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings are exaggerated for convenience of explanation and may differ from the actual proportions. The following describes the present invention using a stacked-type (internal parallel connection) all-solid-state lithium secondary battery, which is one form of secondary battery. As described above, the solid electrolyte that constitutes an all-solid-state lithium secondary battery is a material composed primarily of an ion conductor capable of ion conduction in a solid. Therefore, all-solid-state lithium secondary batteries have the advantage that, in principle, they do not encounter the various problems associated with flammable organic electrolytes, as occurs in conventional liquid-based lithium secondary batteries. Furthermore, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally has the advantage of significantly improving the battery's output density and energy density.

One embodiment of the present invention is a positive electrode material for an electric device, comprising a positive electrode active material capable of absorbing and releasing lithium ions, a conductive additive, and a solid electrolyte, wherein when a cross section of a positive electrode active material layer obtained by molding the positive electrode material is observed using a scanning electron microscope, the area proportions of the positive electrode active material and the solid electrolyte in an observed image are defined as V _AM and V _SE , respectively, and the area proportions of the positive electrode active material and the solid electrolyte present at a distance from the conductive additive to an average particle diameter (D50) of the positive electrode active material in the observed image are defined as S _AM and _S ...

This positive electrode material for an electric device is characterized by having an SV ratio represented by the formula: greater than 1. The positive electrode material for an electric device according to this embodiment can further improve the cycle durability of an electric device that uses a positive electrode material containing a positive electrode active material capable of absorbing and releasing lithium ions, a conductive additive, and a solid electrolyte.

Figure 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery, which is one embodiment of the positive electrode material of the present invention. Figure 2 is a cross-sectional view taken along line 2-2 in Figure 1. The layered structure allows the battery to be compact and have a high capacity. This specification will explain in detail the flat-layered, non-bipolar lithium secondary battery (hereinafter simply referred to as a "layered battery") shown in Figures 1 and 2 as an example. However, when viewed from the perspective of the internal electrical connection configuration (electrode structure) of the lithium secondary battery of this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

As shown in Figure 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 its periphery is heat-sealed, sealing the power generating element 21 with the negative electrode current collector 25 and positive electrode current collector 27 extending to the outside.

The lithium secondary battery according to this embodiment is not limited to a laminated, flat shape. A wound lithium secondary battery may be cylindrical, or may be a cylindrical battery modified to have a flat, rectangular 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.

Furthermore, there are no particular limitations on how the current collectors (25, 27) shown in Figure 1 are removed. The negative current collector 25 and the positive current collector 27 may be pulled out from the same side, or the negative current collector 25 and the positive current collector 27 may each be divided into multiple pieces and removed from each side; they are not limited to what is shown in Figure 1. Furthermore, in wound-type lithium batteries, terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

As shown in Figure 2, the stacked battery 10a of this embodiment has a structure in which a flat, approximately rectangular power generating element 21, where the actual charge/discharge reaction takes place, is sealed inside a laminate film 29, which is the battery exterior material. Here, the power generating element 21 has a structure 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 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 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 so that one positive electrode active material layer 15 faces the adjacent negative electrode active material layer 13 with the solid electrolyte layer 17 interposed between them. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute a single cell layer 19. Therefore, the stacked battery 10a shown in Figure 1 can be said to have a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel.

As shown in FIG. 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. In other words, 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. In some cases, the negative electrode active material layer 13 and the positive electrode active material layer 15 may be used as the negative electrode and positive electrode, respectively, without using current collectors (11', 11").

The negative electrode current collector 11' and the positive electrode current collector 11" are respectively fitted 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 edges of the laminate film 29, which is the battery outer casing, and extend to the outside of the laminate film 29. The positive electrode current collector 27 and the negative electrode current collector 25 may be attached to the positive electrode current collector 11" and the negative electrode current collector 11' of each electrode via a positive electrode lead and a negative electrode lead (not shown) by ultrasonic welding, resistance welding, or the like, as necessary.

The following describes the main components of a lithium secondary battery, which is an embodiment of the positive electrode material according to this embodiment.

[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.

Specific examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other materials that may be used include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Foils in which the metal surface is coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the standpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

The latter conductive resins include resins made from conductive polymer materials or non-conductive polymer materials to which conductive fillers have been added as needed.

Examples of non-conductive polymeric materials include polyethylene (PE; high-density polyethylene (HDPE), low-density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), and polystyrene (PS). Such non-conductive polymeric materials may have excellent potential resistance or solvent resistance.

Conductive fillers can be added to the above-mentioned conductive polymer materials or non-conductive polymer materials as needed. In particular, when the resin that forms the base material of the current collector consists solely of non-conductive polymers, a conductive filler is inevitably required to impart conductivity to the resin.

Any conductive filler can be used without particular restrictions, as long as it is a conductive material. Examples of materials with excellent conductivity, potential resistance, or lithium ion blocking properties include metals and conductive carbon. There are no particular restrictions on the metal, but it is preferable that it contains at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or an alloy or metal oxide containing such a metal. There are also no particular restrictions on the conductive carbon. It is preferable that it contains at least one material selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjen Black (registered trademark), carbon nanotubes, carbon nanohorns, carbon nanoballoons, and fullerenes.

There are no particular restrictions on the amount of conductive filler added, as long as it is an amount that can impart sufficient conductivity to the current collector. Generally, it is 5 to 80% by mass, based on 100% by mass of the total mass of the current collector.

The current collector may have a single layer structure made of a single material, or a laminate structure made of an appropriate combination of layers made of these materials. From the perspective of reducing the weight of the current collector, it is preferable for it to contain at least a conductive resin layer made of a conductive resin. Furthermore, from the perspective of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on part of the current collector. Furthermore, if the negative electrode active material layer and positive electrode active material layer described below are themselves conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate component from these electrode active material layers. In such a configuration, the negative electrode active material layer described below directly constitutes the negative electrode, and the positive electrode active material layer described below directly constitutes the positive electrode.

[Negative electrode (negative electrode active material layer)]
In the stacked 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. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of metal oxides include Nb ₂ O ₅ and Li ₄ Ti ₅ O _12. Silicon-based negative electrode active materials and tin-based negative electrode active materials may also be used. Silicon and tin belong to Group 14 elements and are known to be negative electrode active materials that can significantly improve the capacity of nonaqueous electrolyte secondary batteries. These elements can absorb and release a large number of charge carriers (such as lithium ions) per unit volume (mass), resulting in high-capacity negative electrode active materials. Here, it is preferable to use Si as the silicon-based negative electrode active material. Similarly, it is also preferable to use silicon oxides such as SiO _x (0.3≦x≦1.6) disproportionated into two phases, a Si phase and a silicon oxide phase. In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, an alloy containing silicon (silicon-containing alloy-based negative electrode active material) may be used. On the other hand, negative electrode active materials containing tin element (tin-based negative electrode active materials) include simple Sn, tin alloys (Cu—Sn alloys, Co—Sn alloys), amorphous tin oxide, tin silicon oxide, etc. Among these, an example of amorphous tin oxide is SnB _0.4 P _0.6 O _3.1 . An example of tin silicon oxide is SnSiO _3. Furthermore, a metal containing lithium may be used as the negative electrode active material. Such negative electrode active materials are not particularly limited as long as they are active materials containing lithium, and examples include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li and at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those mentioned above may also be used. The negative electrode active material preferably includes lithium metal, a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably includes lithium metal or a lithium-containing alloy. When the negative electrode active material includes lithium metal or a lithium-containing alloy, the secondary battery according to this embodiment may be a so-called lithium deposition type in which lithium metal or a lithium-containing alloy is deposited on the negative electrode current collector during charging. In this case, the layer of lithium metal or a lithium-containing alloy deposited on the negative electrode current collector during charging constitutes the negative electrode active material layer of the lithium secondary battery. Therefore, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer 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 or lithium-containing alloy may be present during full discharge.

The shape of the negative electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is particulate, its average particle diameter (D50) is preferably, for example, 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 (D50) of the active material can be measured by laser diffraction scattering.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably in the range of 40 to 99% by mass, and more preferably in the range of 50 to 90% by mass.

The negative electrode active material layer preferably further contains a solid electrolyte. By including a solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes, but a sulfide solid electrolyte is preferred.

Examples of the sulfide solid electrolyte include LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ S-P ₂ S ₅ , LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , Li ₃ PS ₄ , Li ₂ S-P ₂ S ₅ -LiI, 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, Li ₂ S-SiS ₂ -LiBr, Li ₂ S—SiS ₂ —LiCl, Li ₂ S—SiS ₂ —B ₂ S ₃ —LiI, Li ₂ S—SiS ₂ —P ₂ S ₅ —LiI, Li ₂ S—B ₂ S ₃ , Li ₂ S—P ₂ S ₅ —Z _m S _n (where m and n are positive numbers and Z is Ge, Zn or Ga), Li ₂ S—GeS ₂ , Li ₂ S—SiS ₂ —Li ₃ PO ₄ , Li ₂ S—SiS ₂ —Li _x MO _y (where x and y are positive numbers and M is P, Si, Ge, B, Al, Ga or In), etc. The term "Li ₂ S-P ₂ S ₅ " means a sulfide solid electrolyte obtained using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other terms.

The sulfide solid electrolyte may have, for example, a Li ₃ PS ₄ skeleton, a Li ₄ P ₂ S ₇ skeleton, or a Li ₄ P ₂ S ₆ skeleton. Examples of sulfide solid electrolytes having a Li ₃ PS ₄ skeleton include LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS ₄ , and Li ₃ PS ₄ . Examples of sulfide solid electrolytes having a Li ₄ P ₂ S ₇ skeleton include Li-P-S solid electrolytes known as LPS (for example, Li ₇ P ₃ S ₁₁ ). Examples of sulfide solid electrolytes that may be used include LGPS, which is represented by Li _(4-x) Ge _(1-x) P _x S ₄ (where x satisfies 0<x<1). In particular, 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 more preferably a material containing Li ₂ S—P ₂ S ₅ as a main component. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte contains Li ₆ PS ₅ X (where X is Cl, Br, or I, preferably Cl).

Furthermore, when the sulfide solid electrolyte is a Li ₂ S—P ₂ S ₅ system, the molar ratio of Li ₂ S and P ₂ S ₅ is preferably within the range of Li ₂ S:P ₂ S ₅ = 50:50 to 100:0, and particularly preferably Li ₂ S:P ₂ S ₅ = 70:30 to 80:20.

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 (ball mill, etc.) 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. The ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25°C) is preferably, for example, 1×10 ⁻⁵ S/cm or more, and more preferably 1×10 ⁻⁴ S/cm or more. The ionic conductivity value of the solid electrolyte can be measured by an AC impedance method.

Examples of oxide solid electrolytes include compounds having a NASICON structure. Examples of compounds having a NASICON structure include compounds (LAGP) represented by the general formula Li1 _{+ xAlxGe2 -x} ( _PO4 ) ₃ (0≦x≦2) and compounds (LATP) represented by the general formula Li1 _{+ xAlxTi2 -x} ( _PO4 ) _{3 (} 0≦ _{x ≦2). Other examples of oxide solid electrolytes include LiLaTiO (e.g., Li0.34La0.51TiO3), LiPON (e.g., Li2.9PO3.3N0.46 ) , and LiLaZrO (} e.g., _{Li7La3Zr2O12 )} .

The shape of the solid electrolyte can be, for example, a particulate shape such as a spherical or oval sphere, or a thin film shape. 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.

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

In addition to the above-mentioned negative electrode active material and solid electrolyte, the negative electrode active material layer may further contain at least one of a conductive additive and a binder.

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 (CNT); and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). Conductive additives can also be used in which particulate ceramic or resin materials are coated with the above-mentioned metal materials by plating or other methods. Among these conductive additives, from the perspective of electrical stability, it is preferable to use at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon. It is more preferable to use at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon, and even more preferable to use at least one type of carbon. These conductive additives may be used alone or in combination.

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.

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 perspective of the electrical characteristics of the battery. In this specification, the "particle diameter of the conductive additive" refers to the 50% cumulative diameter (D50) measured using a laser diffraction/scattering particle size distribution analyzer. However, if measurement using this method is impossible or extremely difficult, the particle diameter is calculated using image recognition processing from an image of the cross section of the active material layer observed using a scanning electron microscope (SEM). In this case, to minimize the influence of statistical error, measurements are made using an image that contains the entire active material layer in the thickness direction.

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 these ranges, a stronger electron conduction path can be formed in the negative electrode active material layer, which can effectively contribute to improving battery characteristics.

On the other hand, binders are not particularly limited, but examples include the following materials:

Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced by other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, 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 (PCTFE), ethylene Fluorine resins such as ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), 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-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE ...hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylide Examples of suitable fluororubbers include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), as well as epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferred.

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

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

There are no particular restrictions on the specific form of the solid electrolyte contained in the solid electrolyte layer, and the solid electrolytes and their preferred forms exemplified in the section on the negative electrode active material layer can be used. In some cases, solid electrolytes other than those mentioned above may be used in combination.

The solid electrolyte layer may further contain a binder in addition to the above-mentioned specific solid electrolyte. The examples and preferred forms described in the section on the negative electrode active material layer can also be used for the binder that can be contained in the solid electrolyte layer.

The thickness of the solid electrolyte layer varies depending on the configuration of the desired lithium secondary battery, but from the perspective 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 lower limit on 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.

[Cathode active material layer]
1 and 2 , the positive electrode active material layer contains a positive electrode material for an electric device according to one embodiment of the present invention. The positive electrode material for an electric device includes a positive electrode active material capable of absorbing and releasing lithium ions, a conductive additive, and a solid electrolyte. The components of the positive electrode material according to one embodiment of the present invention will be described below.

(Positive electrode active material capable of absorbing and releasing lithium ions)
The positive electrode active material capable of absorbing and desorbing lithium ions is not particularly limited as long as it is a material that can release lithium ions during the charging process of the secondary battery and absorb lithium ions during the discharging process. An example of such a positive electrode active material is one that contains an M1 element and an O element, and the M1 element contains at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe, and P. Examples of such positive electrode active materials include layered rock salt active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , and Li(Ni—Mn—Co)O ₂ , spinel active materials such as LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ , olivine active materials such as LiFePO ₄ and LiMnPO ₄ , and Si-containing active materials such as Li ₂ FeSiO ₄ and Li ₂ MnSiO ₄ . Examples of oxide active materials other than those listed above include Li ₄ Ti ₅ O ₁₂ and LiVO ₂ . In some cases, two or more positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those listed above may also be used.

In a preferred embodiment, from the viewpoint of output characteristics, the positive electrode active material contains Li(Ni-Mn-Co) _O2 or an NMC composite oxide having a composition in which a portion of the transition metal in the NMC composite oxide is substituted with another element. Here, the content of the NMC composite oxide relative to the total amount (100 mass%) of the positive electrode active material is preferably 50 mass% or more, more preferably 70 mass% or more, even more preferably 80 mass% or more, even more preferably 90 mass% or more, particularly preferably 95 mass% or more, and most preferably 100 mass%.

The positive electrode active material is particulate. When the positive electrode active material is particulate, the particle shape is not particularly limited and may be any shape, such as powder, sphere, rod, needle, plate, column, irregular shape, scale, or spindle shape. From the perspective of output characteristics, the average particle diameter of the positive electrode active material is preferably 10 μm or less, more preferably 1 nm to 10 μm, even more preferably 10 nm to 9 μm, particularly preferably 100 nm to 8 μm, and particularly preferably 1 to 7 μm. In this specification, the "particle diameter of the positive electrode active material" refers to the 50% cumulative diameter (D50) measured using a laser diffraction/scattering particle size distribution analyzer. However, if measurement using this method is impossible or extremely difficult, the particle diameter is calculated using image recognition processing of an image of a cross section of the active material layer observed using a scanning electron microscope (SEM). In this case, measurement is performed using an image that contains the entire active material layer in the thickness direction to minimize the influence of statistical error.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 30 to 99% by mass, more preferably in the range of 40 to 90% by mass, and even more preferably in the range of 45 to 80% by mass.

(solid electrolyte)
The positive electrode material according to this embodiment essentially contains a solid electrolyte. There are no particular limitations on the specific form of the solid electrolyte contained in the positive electrode material according to this embodiment, and the solid electrolytes exemplified in the section on the negative electrode active material layer and their preferred forms may be similarly employed. In some cases, a solid electrolyte other than the above-mentioned solid electrolytes may be used in combination.

In particular, the solid electrolyte contained in the positive electrode material according to this embodiment preferably contains a sulfide solid electrolyte from the viewpoint of excellent ionic conductivity. In another preferred embodiment, the solid electrolyte contained in the solid electrolyte layer contains alkali metal atoms. Here, examples of alkali metals that can be contained in the solid electrolyte include Li, Na, and K, with Li being preferred in terms of excellent ionic conductivity. In yet another preferred embodiment, the solid electrolyte contained in the positive electrode material contains alkali metal atoms (e.g., Li, Na, or K; preferably Li) and phosphorus and/or boron atoms. Examples of sulfide solid electrolytes containing such alkali metal atoms and phosphorus and/or boron atoms include LiI-Li ₂ S-P ₂ O ₅ , LiI-Li ₃ PO ₄ -P 2 S 5 , Li 2 S-P _{2 S 5} , LiI _{-Li 3 PS 4 , LiI-LiBr-Li 3 PS 4 , Li 3 PS 4 , Li 2 S-P 2 S 5 -LiI , 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 -B ₂ S ₃ -LiI, and 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-SiS ₂ -Li ₃ PO ₄ , etc. Examples of sulfide solid electrolytes having a Li ₄ P ₂ S ₇ skeleton include Li-P-S solid electrolytes known as LPS (e.g., Li ₇ P ₃ S ₁₁ ). Also, for example, LGPS, etc., represented by Li _(4-x) Ge _(1-x) P _x S ₄ (x satisfies 0<x<1), may be used. In particular, the sulfide solid electrolyte contained in the active material layer is preferably a sulfide solid electrolyte containing phosphorus atoms, and the sulfide solid electrolyte is more preferably a material containing Li ₂ S—P ₂ S ₅ as a main component. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte contains Li ₆ PS ₅ X (where X is Cl, Br, or I, preferably Cl). These solid electrolytes have high ionic conductivity and can therefore effectively contribute to the realization of the effects of the present invention.

The solid electrolyte contained in the positive electrode material according to this embodiment is particulate. When the solid electrolyte is particulate, the particle shape is not particularly limited and may be any shape, such as powder, sphere, rod, needle, plate, column, irregular shape, scale, or spindle shape. From the viewpoint of output characteristics, the average particle diameter of the solid electrolyte is preferably 5 μm or less, more preferably 1 nm to 5 μm, even more preferably 10 nm to 4.5 μm, particularly preferably 100 nm to 4 μm, and particularly preferably 1 to 3 μm. In this specification, the "particle diameter of the solid electrolyte" refers to the 50% cumulative diameter (D50) measured by a laser diffraction/scattering particle size distribution analyzer. However, if measurement using this method is impossible or extremely difficult, the particle diameter is calculated by image recognition processing from an image of a cross section of the active material layer observed using a scanning electron microscope (SEM). In this case, measurement is performed using an image that contains the entire active material layer in the thickness direction, to minimize the influence of statistical error.

The content of solid electrolyte in the positive electrode active material layer is not particularly limited, but is preferably in the range of 1 to 70 mass%, more preferably in the range of 10 to 60 mass%, and even more preferably in the range of 20 to 55 mass%.

(Conductive additive)
The positive electrode material according to this embodiment essentially contains a conductive additive in addition to the positive electrode active material and solid electrolyte described above. Specific and preferred forms of the conductive additive may be the same as those described above for the conductive additive as a constituent material of the negative electrode active material layer, and therefore detailed description thereof will be omitted here.

The content of the conductive additive in the positive electrode active material layer is not particularly limited, but is preferably 1 to 10 mass %, more preferably 2 to 8 mass %, and even more preferably 4 to 7 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.

The thickness of the positive electrode active material layer varies depending on the desired configuration of the all-solid-state battery, but is preferably within the range of 0.1 to 1000 μm, for example.

The positive electrode material for an electric device according to this embodiment is characterized in that, when a cross section of a positive electrode active material layer obtained by molding the positive electrode material is observed using a scanning electron microscope, the area profiles of the positive electrode active material and the solid electrolyte in an observation image are controlled to satisfy a predetermined relationship. Specifically, when the area proportions of the positive electrode active material and the solid electrolyte in an observation image of a cross section of a positive electrode active material layer obtained by molding the positive electrode material for an electric device according to this embodiment are defined as V _AM and V _SE , respectively, and the area proportions of the positive electrode active material and the solid electrolyte present at a distance from the conductive additive to the average particle diameter (D50) of the positive electrode active material in the observation image are defined as S _AM and S _SE , respectively, the value of the SV ratio, expressed by the following mathematical formula 1, is greater than 1:

The specific method for calculating the SV ratio value will be the method described in the Examples section below. Here, the method for calculating the SV ratio value will be briefly explained with reference to Figure 3.

First, a positive electrode active material layer is fabricated using the positive electrode material according to the present embodiment by the method described in the Examples section below. Next, a cross section of the positive electrode active material layer is observed using a scanning electron microscope (SEM) to obtain an observation image such as that shown in Figure 3(a).

Next, the presence of the conductive additive (acetylene black) is detected by mapping the C element in the observed image obtained above using energy dispersive X-ray analysis (EDX). Similarly, the presence of the positive electrode active material and the solid electrolyte is also detected. The area ratio of the positive electrode active material (V _AM ) and the area ratio of the solid electrolyte (V _SE ) in the observed image are then calculated (see FIG. 3( b)).

Then, for all of the conductive additives detected above, the regions of the positive electrode active material and solid electrolyte present at a distance from the conductive additive to the average particle diameter (D50) of the positive electrode active material are identified, and S _AM (positive electrode active material) and S _SE (solid electrolyte) are calculated as the area ratios of each identified region to the observed image (see FIGS. 3( c) and 3(d)). Then, the SV ratio is calculated from the values of V _AM , V _SE , S _AM , and S _SE calculated in this way according to Equation 1.

The positive electrode material for an electrical device according to this embodiment is characterized in that the SV ratio calculated as described above is greater than 1. Furthermore, the larger the SV ratio, the greater the effect of improving the cycle durability of the resulting electrical device. From this perspective, the SV ratio is preferably 1.1 or greater, more preferably 1.3 or greater, and even more preferably 1.5 or greater. There is no particular upper limit to the SV ratio, but it is usually 2.0 or less.

Although the mechanism by which an SV ratio greater than 1 significantly improves the cycle durability of an electric device such as a lithium secondary battery is not completely clear, the following mechanism is presumed. That is, the denominator on the right side of Equation 1 (V _AM /(V _AM +V _SE )) can be considered to represent the volume fraction (feed ratio) of the positive electrode active material in the total of the positive electrode active material and solid electrolyte contained in the positive electrode material. On the other hand, the numerator on the right side of Equation 1 (S _AM /(S _AM +S _SE )) can be considered to represent the volume fraction (abundance ratio) of the positive electrode active material present around the conductive additive in the positive electrode active material layer. From this, it can be said that an SV ratio greater than 1 means that the numerator of Equation 1 is greater than the denominator of Equation 1, i.e., that the positive electrode active material is selectively present around the conductive additive (i.e., the solid electrolyte is present at a lower ratio) compared to the ratio of the positive electrode active material estimated from the volume fraction (feed ratio) of the positive electrode active material to the total positive electrode active material and solid electrolyte contained in the positive electrode material. When the positive electrode active material is present around the conductive additive (i.e., no solid electrolyte is present around the conductive additive), a conductive path from the conductive additive to the positive electrode active material is sufficiently secured while preventing the conduction of electrons from the conductive additive to the solid electrolyte, which would cause deterioration of battery performance. In other words, since solid electrolytes undergo irreversible decomposition reactions (electrochemical degradation) when in contact with electrons, it is believed that suppressing electron conduction from the conductive additive to the solid electrolyte by setting the SV ratio greater than 1 prevents degradation due to decomposition of the solid electrolyte and improves battery performance, such as cycle durability. In order to control the SV ratio, techniques such as adjusting the blending ratio of each component in the positive electrode material or the blending order when preparing the positive electrode material can be adopted.

[Positive electrode current collector plate and negative electrode current collector plate]
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.

[Positive electrode lead and negative electrode lead]
Although not shown, the current collector and the current collecting 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 negative electrode lead. It is preferable that the portion removed from the outer casing be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting the product (e.g., automotive parts, particularly electronic devices).

[Battery exterior material]
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 Figures 1 and 2 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of its high output and excellent cooling performance, making it suitable for use in batteries for large 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.

The stacked battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the stacked battery according to this embodiment is suitable for use as a power source for driving EVs and HEVs.

The above describes one embodiment of a lithium secondary battery, but the present invention is not limited to the configuration described in the above embodiment and can be modified as appropriate based on the claims.

For example, the lithium secondary battery according to this embodiment can be used in a bipolar battery, which includes a bipolar electrode having a positive electrode active material layer electrically bonded to one surface of a current collector and a negative electrode active material layer electrically bonded to the opposite surface of the current collector.

Furthermore, the secondary battery according to this embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There are no particular restrictions 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.

The liquid electrolyte (electrolyte solution) that can be used has a form in which a lithium salt is dissolved in an organic solvent. Examples of organic solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and gamma-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the organic solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Examples of lithium salts include Li( _FSO2 ) _2N (lithium bis( _{fluorosulfonyl)imide; LiFSI), Li(C2F5SO2 ) 2N} , _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , _{and LiCF3SO3 .} Of these, the lithium salt is preferably Li( _FSO2 ) _2N (LiFSI) from the viewpoints of battery output and charge/discharge cycle characteristics.

The liquid electrolyte (electrolytic solution) may further contain additives other than the components described above. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate, 1-methyl-1-vinyl ethylene carbonate, 1-methyl-2-vinyl ethylene carbonate, 1-ethyl-1-vinyl ethylene carbonate, and 1-ethyl-2-vinyl ethylene carbonate. Examples of additives include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte, the amount used can be adjusted as appropriate.

[Battery assembly]
A battery pack is made up of multiple batteries connected together. Specifically, it is made up of at least two batteries connected in series, parallel, or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

Multiple batteries can be connected in series or parallel to form a small, detachable battery pack. Then, multiple such small, detachable battery packs can be connected in series or parallel to form a large-capacity, high-output battery pack (battery module, battery pack, etc.) suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric output density. The number of batteries connected to form a battery pack, and the number of stacked small battery packs to form a large-capacity battery pack, can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which it will be installed.

[vehicle]
A battery or a battery pack formed by combining a plurality of such batteries can be mounted on a vehicle. Since the present invention can provide a long-life battery with excellent long-term reliability, the use of such a battery can be used to construct plug-in hybrid electric vehicles with long EV driving distances and electric vehicles with long driving distances per charge. For example, a battery or a battery pack formed by combining a plurality of such batteries can be used in hybrid vehicles, fuel cell vehicles, and electric vehicles (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles) to produce a vehicle with a long life and high reliability. However, the use of the battery is not limited to automobiles, and it can also be applied to various power sources for other vehicles, such as trains, and can also be used as an on-board power source for uninterruptible power supplies and the like.

The present invention will be described in more detail below using examples. However, the technical scope of the present invention is not limited to the following examples.

[Preparation of evaluation cell]
Example 1
(Preparation of Positive Electrode)
First, _{LiNi0.8Mn0.1Co0.1O2} (average particle size ( _D50 ) = _{10 μm} ) as the positive electrode active material, acetylene black as the conductive additive, sulfide solid electrolyte (LPS ( _Li2S - _{P2S5 )} , average particle size (D50) = 5 μm), and binder (styrene-butadiene rubber (SBR)) were weighed out to a mass ratio of 50:30:20:2. Next, all components except the sulfide solid electrolyte were mixed in an agate mortar in a glove box, and then further mixed and stirred in a planetary ball mill. Thereafter, the sulfide solid electrolyte and an appropriate amount of a slurry viscosity adjusting solvent (methylene chloride) were further added to the obtained mixed powder, and the mixture was mixed and stirred in a planetary ball mill to prepare a positive electrode active material slurry.

The positive electrode active material slurry prepared above was then applied to the surface of aluminum foil serving as a positive electrode current collector, dried, and pressed to form a positive electrode active material layer (50 μm thick), thereby producing a positive electrode. The cross section of the positive electrode active material layer of the positive electrode thus produced was observed using a scanning electron microscope (SEM), and the image shown in Figure 3(a) was obtained. The SV ratio was then calculated using the following method.

Specifically, the presence of the conductive additive (acetylene black) was detected by mapping the C element in the observation image shown in Figure 3(a) using energy dispersive X-ray analysis (EDX). Similarly, the presence of the positive electrode active material and the solid electrolyte was also detected. The area ratio of the positive electrode active material (V _AM ) and the area ratio of the solid electrolyte (V _SE ) in the observation image were then calculated (see Figure 3(b)).

Thereafter, for all of the conductive additives detected above, the regions of the positive electrode active material and solid electrolyte present at a distance from the conductive additive to the average particle diameter (D50) of the positive electrode active material were identified, and S _AM (positive electrode active material) and S _SE (solid electrolyte) were calculated as the area ratio of each identified region to the observed image (see FIGS. 3(c) and 3(d)). The SV ratio was then calculated from the values of V _AM , V _SE , S _AM , and S _SE calculated in this manner according to Equation 1. As a result, the SV ratio value of the positive electrode material according to this example was 1.5.

(Preparation of solid electrolyte layer)
A solid electrolyte slurry was prepared by adding 2 parts by mass of styrene-butadiene rubber (SBR) to 100 parts by mass of the same sulfide solid electrolyte (LPS (Li ₂ S—P ₂ S ₅ )) as above, and adding methylene chloride as a solvent. The solid electrolyte slurry prepared above was then applied to the surface of a stainless steel foil support and dried to obtain a solid electrolyte layer (thickness 30 μm) as a free-standing membrane.

(Preparation of evaluation cell)
The solid electrolyte layer prepared above was transferred to the positive electrode active material layer side of the positive electrode prepared above by cold isostatic pressing (CIP) so that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer. Finally, a stainless steel foil was laminated as a negative electrode current collector on the exposed surface of the solid electrolyte layer to prepare an evaluation cell (lithium deposition-type all-solid-state lithium secondary battery) without a negative electrode active material layer.

Example 2
The evaluation cell of this example was produced by the same S method as in Example 1 described above, except that the type and blending ratio of each component in the positive electrode material or the blending order during preparation of the positive electrode material were adjusted so that the SV ratio of the positive electrode material would be 1.3 (specifically, the type of binder was changed to one with lower binding properties (low-molecular-weight SBR)).

Example 3
The evaluation cell of this example was produced in the same manner as in Example 1 described above, except that the type and blending ratio of each component in the positive electrode material or the blending order during preparation of the positive electrode material were adjusted so that the SV ratio of the positive electrode material would be 1.1 (specifically, all components were added at the same time and mixed and stirred).

<Comparative Example 1>
The evaluation cell of this comparative example was produced in the same manner as in Example 3 described above, except that the type and blending ratio of each component in the positive electrode material or the blending order during preparation of the positive electrode material were adjusted so that the SV ratio of the positive electrode material was 1.0 (specifically, a positive electrode active material having an average particle diameter (D50) of 15 μm was used).

<Comparative Example 2>
The evaluation cell of this comparative example was produced in the same manner as in Example 1 described above, except that the type and blending ratio of each component in the positive electrode material or the blending order during preparation of the positive electrode material were adjusted so that the SV ratio of the positive electrode material would be 0.5 (specifically, a sulfide solid electrolyte having an average particle size (D50) of 7 μm was used, and the components other than the positive electrode active material were mixed and stirred in advance, and then the positive electrode active material and a slurry viscosity adjusting solvent were added, and further mixing and stirring were performed).

<Comparative Example 3>
The evaluation cell of this comparative example was produced in the same manner as in Comparative Example 2 described above, except that the type and blending ratio of each component in the positive electrode material or the blending order during preparation of the positive electrode material were adjusted so that the SV ratio of the positive electrode material would be 0.2 (specifically, a sulfide solid electrolyte having an average particle size (D50) of 5 μm was used, and the type of binder was changed to one with higher binding properties (high molecular weight SBR)).

[Evaluation of evaluation cells (measurement of charge/discharge efficiency)]
A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector, respectively, of the evaluation cells produced in the above Examples and Comparative Examples, and 50 cycles of charge and discharge were carried out under the following charge and discharge test conditions.

(Charge/discharge test conditions)
1) Charge/discharge conditions [Voltage range] 3.0 to 4.3 V
[Charging process] CCCV
[Discharge process] CC
[Charge/discharge rate] 0.2C
(After charging and discharging, rest for 30 minutes each)
2) Evaluation temperature: 298K (25°C).

The evaluation cell was charged in a constant current/constant voltage (CCCV) mode at 0.2 C from 3.0 V to 4.3 V in a thermostatic chamber set at the evaluation temperature using a charge/discharge tester. During the charge process (lithium metal deposition on the negative electrode current collector), the cell was charged at 0.2 C from 3.0 V to 4.3 V. During the discharge process (lithium metal dissolution on the negative electrode current collector), the cell was discharged at 0.2 C from 4.3 V to 3.0 V in CCCV mode. Here, 1 C refers to the current value at which the battery is fully charged (100% charged) after one hour of charging. During the charge/discharge process, the charge capacity (battery capacity at charge) and discharge capacity (battery capacity at discharge) of the evaluation cell were measured. The initial charge/discharge efficiency was calculated as the ratio of the battery capacity at discharge to the battery capacity at charge in the first cycle. The discharge capacity retention rate was also measured as the percentage of the discharge capacity at the 50th cycle relative to the discharge capacity at the first cycle. The values of each parameter used to calculate the SV ratio in the above-mentioned examples and comparative examples are shown in Table 1 below. Note that the values shown in Table 1 have been normalized to take into account the different magnifications of the cross-sectional images used to calculate each parameter. The measurement results of the discharge capacity retention rate are shown in Table 2 below.

As can be seen from the results shown in Table 2, in Examples 1 to 3, where the SV ratio of the positive electrode material was greater than 1, the discharge capacity retention rate and initial charge/discharge efficiency were significantly improved compared to Comparative Examples 1 to 3, where the SV ratio was 1.0 or less. Furthermore, in Examples 1 and 2, where the SV ratio was 1.3 or greater, the battery performance was particularly improved, and in Example 1, where the SV ratio was 1.5, the battery performance was significantly improved.

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 generating element,
25 negative electrode current collector plate,
27 positive electrode current collector plate,
29 Laminating film.

Claims (11)

A positive electrode material for an electric device, comprising: a positive electrode active material capable of absorbing and releasing lithium ions; a conductive additive; and a solid electrolyte,
When a cross section of a positive electrode active material layer obtained by molding a positive electrode material is observed using a scanning electron microscope, the area proportions of the positive electrode active material and the solid electrolyte in an observed image are defined as V _AM and V _SE , respectively, and the area proportions of the positive electrode active material and the solid electrolyte present at a distance from the conductive additive to the average particle diameter (D50) of the positive electrode active material in the observed image are defined as S _AM and S _SE , respectively, and the following formula 1 is obtained:

A positive electrode material for an electric device, characterized in that the SV ratio represented by the following formula is greater than 1. The positive electrode material for an electrical device according to claim 1, wherein the SV ratio is 1.3 or greater. 3. The positive electrode material for an electric device according to claim 1, wherein the positive electrode active material comprises Li(Ni-Mn-Co) _O or an NMC composite oxide having a composition in which a portion of the transition metal in the NMC composite oxide is substituted with another element. The positive electrode material for an electrical device according to any one of claims 1 to 3, wherein the average particle diameter (D50) of the positive electrode active material is 10 μm or less. The positive electrode material for an electrical device according to any one of claims 1 to 4, wherein the solid electrolyte has an average particle diameter (D50) of 5 μm or less. The positive electrode material for an electrical device according to any one of claims 1 to 5, wherein the solid electrolyte comprises a sulfide solid electrolyte. The positive electrode material for an electrical device according to claim 6, wherein the solid electrolyte contains alkali metal atoms and phosphorus atoms and/or boron atoms. The positive electrode material for an electrical device according to claim 7, wherein the alkali metal atoms are lithium atoms. A positive electrode for an electrical device comprising the positive electrode material for an electrical device according to any one of claims 1 to 8. An electrical device comprising the positive electrode for an electrical device according to claim 9. The electrical device described in claim 10 is an all-solid-state lithium secondary battery.