JP7662988B2 - 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, and 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. In the automotive industry, there is hope that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will help reduce carbon dioxide emissions, and there has been active development of non-aqueous electrolyte secondary batteries, such as motor drive secondary batteries, which hold the key to putting these into practical use.

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 developed at a rapid pace.

The lithium secondary batteries currently in widespread use use a flammable organic electrolyte as the electrolyte. These liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, etc. than other batteries.

In recent years, research and development of all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively conducted. A solid electrolyte is a material mainly composed of an ion conductor capable of ion conduction in a solid. Therefore, in principle, all-solid-state lithium secondary batteries do not cause various problems caused by flammable organic electrolytes as in conventional liquid-based lithium secondary batteries. In addition, in general, the use of a high-potential, large-capacity positive electrode material and a large-capacity negative electrode material can significantly improve the output density and energy density of a battery. For example, elemental sulfur (S ₈ ) has an extremely large theoretical capacity of about 1670 mAh/g, and has the advantages of being low cost and abundant in resources.

On the other hand, metallic lithium, which is a negative electrode active material that supplies lithium ions to the positive electrode, is known as a high-capacity negative electrode material that can be used in all-solid-state batteries. However, in all-solid-state batteries that use metallic lithium as the negative electrode active material and a sulfide solid electrolyte as the solid electrolyte, the metallic lithium and the sulfide solid electrolyte may react with each other, resulting in a deterioration in battery characteristics.

In order to address these problems, Patent Document 1 proposes a technology in which a composite material containing a conductive agent and an alkali metal sulfide integrated on the surface of the conductive agent is used as the positive electrode material of an all-solid-state battery. According to Patent Document 1, by using a positive electrode material having such a configuration, a positive electrode material and a lithium-ion battery are provided that have a high theoretical capacity and can also use a negative electrode active material that does not supply lithium ions to the positive electrode.

International Publication No. 2012/102037

Here, depending on the application of the secondary battery, it is required that sufficient capacity can be extracted when charging and discharging at a high charge and discharge rate (i.e., the so-called charge and discharge rate characteristics are sufficient). For example, a secondary battery with insufficient charge and discharge rate characteristics cannot utilize sufficient capacity in response to rapid charging and discharging. In order to improve the charge and discharge rate characteristics of a secondary battery, it is necessary to reduce the internal resistance of the battery.

The present invention aims to provide a means for further reducing the internal resistance of an electrical device that uses a positive electrode active material containing sulfur.

The present inventors conducted intensive research to solve the above problems. As a result, they discovered that the above problems can be solved by configuring a cathode material containing a conductive material having pores, a solid electrolyte, and a cathode active material containing sulfur so that the solid electrolyte and the cathode active material are arranged on the inner surfaces of the pores so as to be in contact with each other, and by appropriately controlling the profile of the pore volume of the conductive material, thereby completing the present invention.

One aspect of the present invention is a positive electrode material for an electric device, comprising a conductive material having pores, a solid electrolyte, and a positive electrode active material containing sulfur, in which at least a portion of the solid electrolyte and at least a portion of the positive electrode active material are disposed on the inner surfaces of the pores so as to be in contact with each other, and the percentage of the pore volume of pores having a pore diameter in the range of 1 to 4 nm to the pore volume of pores having a pore diameter in the range of 1 to 100 nm is 20% or less.

According to the present invention, in an electrical device using a positive electrode active material containing sulfur, the internal resistance of the electrical device can be further reduced.

FIG. 1 is a perspective view showing the appearance of a flat laminate 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. FIG. 3A is a cross-sectional schematic diagram of a positive electrode material in the prior art. FIG. 3B is a cross-sectional schematic diagram of a cathode material according to one embodiment of the present invention. FIG. 4 is a graph showing the pore size distribution in the range of 1 to 100 nm, measured according to the BJH method, for the carbons used in the comparative examples and examples described below.

The above-mentioned embodiment of the present invention will be described below with reference to the drawings, but the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. The dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios. The present invention will be described below using a stacked type (internal parallel connection type) all-solid-state lithium secondary battery, which is one form of secondary battery. As described above, the solid electrolyte constituting the all-solid-state lithium secondary battery is a material mainly composed of an ion conductor capable of ion conduction in a solid. For this reason, the all-solid-state lithium secondary battery has the advantage that, in principle, various problems caused by flammable organic electrolytes do not occur as in conventional liquid-based lithium secondary batteries. In addition, in general, the use of a high-potential, large-capacity positive electrode material and a large-capacity negative electrode material has the advantage that the output density and energy density of the battery can be significantly improved.

One aspect of the present invention is a positive electrode material for an electric device, comprising a conductive material having pores, a solid electrolyte, and a positive electrode active material containing sulfur, in which at least a portion of the solid electrolyte and at least a portion of the positive electrode active material are disposed on the inner surfaces of the pores so as to be in contact with each other, and in which the percentage of the pore volume of pores having a pore diameter in the range of 1 to 4 nm relative to the pore volume of pores having a pore diameter in the range of 1 to 100 nm is 20% or less. The positive electrode material for an electric device according to this aspect can further reduce the internal resistance of the electric device.

Figure 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. 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. In this specification, the flat-layered non-bipolar lithium secondary battery shown in Figures 1 and 2 (hereinafter also simply referred to as a "layered battery") will be described in detail as an example. However, when viewed from the perspective of the internal electrical connection form (electrode structure) of the lithium 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.

As shown in FIG. 1, the stacked battery 10a has a flat rectangular shape, and a negative electrode current collector 25 and a positive electrode current collector 27 for extracting power are pulled out from both sides. 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, and the power generating element 21 is sealed with the negative electrode current collector 25 and the positive electrode current collector 27 pulled out 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 shape modified to have a rectangular flat shape, and is not particularly limited. The cylindrical shape may be a laminate film or a conventional cylindrical can (metal can) as the exterior material, and is not particularly limited. Preferably, the power generating element is housed inside a laminate film containing aluminum. This form can achieve weight reduction.

There are also no particular limitations on the way the current collectors (25, 27) shown in FIG. 1 are removed. The negative and positive current collectors 25 and 27 may be pulled out from the same side, or the negative and positive current collectors 25 and 27 may each be divided into multiple pieces and removed from each side; they are not limited to what is shown in FIG. 1. In addition, in a wound lithium battery, the 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 charge and discharge reactions actually proceed, 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 a positive electrode active material layer 15 containing a positive electrode active material is disposed on both sides of a positive electrode collector 11". The negative electrode has a structure in which a negative electrode active material layer 13 containing a negative electrode active material is disposed on both sides of a negative electrode 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 and the adjacent negative electrode active material layer 13 face each other through a solid electrolyte layer 17. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode form one single cell layer 19. Therefore, the stacked battery 10a shown in FIG. 1 can be said to have a structure in which a plurality of single cell layers 19 are stacked and electrically connected in parallel.

As shown in FIG. 2, the outermost negative electrode 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 collector dedicated to the outermost layer with an active material layer provided on only one side, a collector with active material layers on both sides may be used as the outermost layer collector as is. 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 the collectors (11', 11").

The negative electrode collector 11' and the positive electrode collector 11" are respectively attached with a negative electrode collector plate (tab) 25 and a positive electrode collector plate (tab) 27 that are electrically connected to each electrode (positive electrode and negative electrode), and are structured so as to be sandwiched between the ends of the laminate film 29, which is the battery exterior material, and are led out of the laminate film 29. The positive electrode collector plate 27 and the negative electrode collector plate 25 may be attached to the positive electrode collector 11" and the negative electrode collector 11' of each electrode by ultrasonic welding, resistance welding, or the like, respectively, via a positive electrode lead and a negative electrode lead (not shown) as necessary.

The main components of the lithium secondary battery according to this embodiment are described below.

[Current collector]
The current collector has a function of mediating the movement 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. In addition, clad materials of nickel and aluminum, clad materials of copper and aluminum, and the like may be used. Also, a foil in which a metal surface is coated with aluminum may be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

The latter conductive resins include conductive polymer materials and non-conductive polymer materials to which conductive fillers are added as necessary.

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), or polystyrene (PS). Such non-conductive polymeric materials may have excellent electric potential resistance or solvent resistance.

A conductive filler can be added to the conductive polymer material or non-conductive polymer material as necessary. In particular, when the resin that serves as the base material of the collector is made of only a non-conductive polymer, a conductive filler is necessarily required to impart conductivity to the resin.

The conductive filler can be used without any particular restriction as long as it is a material having electrical conductivity. For example, metals and conductive carbon can be mentioned as materials having excellent electrical conductivity, potential resistance, or lithium ion blocking properties. 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 these metals. There are also no particular restrictions on the conductive carbon. It is preferable that it contains at least one selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjen Black (registered trademark), carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene.

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

The current collector may be a single layer structure made of a single material, or may be 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 contains at least a conductive resin layer made of a resin having conductivity. In addition, from the viewpoint of blocking the movement of lithium ions between the single cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the negative electrode active material layer and the positive electrode active material layer described later are conductive and can perform a current collecting function by themselves, it is not necessary to use a current collector as a member separate from these electrode active material layers. In such a form, the negative electrode active material layer described later constitutes the negative electrode as it is, and the positive electrode active material layer described later constitutes the positive electrode as it is.

[Negative electrode (negative electrode active material layer)]
In the stacked battery according to the embodiment shown in FIG. 1 and FIG. 2, the negative electrode active material layer 13 includes a negative electrode active material. The type of the negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. Examples of the carbon material include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of the metal oxide include Nb ₂ O ₅ , Li ₄ Ti ₅ O ₁₂ , and the like. Furthermore, silicon-based negative electrode active materials and tin-based negative electrode active materials may be used. Here, silicon and tin belong to the 14th group of elements, and are known to be negative electrode active materials that can greatly improve the capacity of non-aqueous electrolyte secondary batteries. These simple substances can absorb and release a large number of charge carriers (lithium ions, etc.) per unit volume (mass), and therefore become high-capacity negative electrode active materials. Here, it is preferable to use Si simple substance as the silicon-based negative electrode active material. Similarly, it is also preferable to use silicon oxide such as SiO _x (0.3≦x≦1.6) disproportionated into two phases of Si phase and 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 oxides, tin silicon oxides, and the like. 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 a negative electrode active material is not particularly limited as long as it is an active material containing lithium, and examples thereof include metallic lithium as well as lithium-containing alloys. The lithium-containing alloy may be, for example, an alloy 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 the above may be used. The negative electrode active material preferably contains metallic lithium, a silicon-based negative electrode active material or a tin-based negative electrode active material, and particularly preferably contains metallic lithium.

The shape of the negative electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is particulate, the 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. In this specification, the average particle size (D ₅₀ ) of the active material can be measured by a laser diffraction scattering method.

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 %.

It is preferable that the negative electrode active material layer further contains a solid electrolyte. By containing 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 a sulfide solid electrolyte is preferable.

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 (wherein 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 (wherein x and y are positive numbers and M is P, Si, Ge, B, Al, Ga or In), etc. The description "Li ₂ S-P ₂ S ₅ " means a sulfide solid electrolyte obtained by using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other descriptions.

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 called LPS (for example, Li ₇ P ₃ S ₁₁ ). Examples of sulfide solid electrolytes include LGPS represented by Li _(4-x) Ge _(1-x) P _x S ₄ (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 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 (wherein 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 ratio of Li ₂ S and P ₂ S ₅ , in terms of molar ratio, is preferably within the range of Li ₂ S:P ₂ S ₅ =50:50 to 100:0, and in particular, 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. The sulfide glass can be obtained, for example, by performing mechanical milling (ball mill, etc.) on the raw material composition. The crystallized sulfide glass can be obtained, for example, by heat treating the sulfide glass at a temperature equal to or higher than the crystallization temperature. The ion conductivity (for example, 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 value of the ion conductivity 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 Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ (0≦x≦2), and compounds (LATP) represented by the general formula Li _1+x Al _x Ti _2-x (PO ₄ ) ₃ (0≦x≦2). Other examples of oxide solid electrolytes include LiLaTiO (e.g., Li _0.34 La _0.51 TiO ₃ ), LiPON (e.g., Li _2.9 PO _3.3 N _0.46 ), and LiLaZrO (e.g., Li ₇ La ₃ Zr ₂ O ₁₂ ).

The shape of the solid electrolyte may be, for example, a particulate shape such as a true sphere or an oval sphere, a thin film shape, etc. When the solid electrolyte is particulate, its average particle size ( _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 size ( _D50 ) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

The content of the solid electrolyte 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 %.

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

Examples of conductive assistants 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.). In addition, particulate ceramic materials and resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive assistants. Among these conductive assistants, 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 to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon, and even more preferably to include at least one carbon. These conductive assistants may be used alone or in combination of two or more.

The conductive assistant is preferably particulate or fibrous. When the conductive assistant is particulate, 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, spindle shape, etc.

When the conductive assistant 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 assistant" means the maximum distance L between any two points on the contour line of the conductive assistant. The value of the "average particle diameter of the conductive assistant" is calculated as the average particle diameter of particles observed in 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).

When the negative electrode active material layer contains a conductive assistant, the content of the conductive assistant 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, it is possible to form a stronger electron conduction path in the negative electrode active material layer, which can effectively contribute to improving the 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, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated product, styrene-isoprene-styrene block copolymer and its hydrogenated product, 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), 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-pentafluoropropylene fluororubber (VDF-PFP fluororubber), etc. Examples of the fluororubber 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), and epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

The thickness of the negative electrode active material layer varies depending on the configuration of the intended 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-mentioned 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 limitations 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 the same way. In some cases, solid electrolytes other than the above-mentioned solid electrolytes 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 may also be used for the binder that may be contained in the solid electrolyte layer.

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 limit 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.

[Positive electrode 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 contains a conductive material having pores, a solid electrolyte, and a positive electrode active material containing sulfur.

(Positive electrode active material containing sulfur)
The type of the positive electrode active material containing sulfur is not particularly limited, but may be, in addition to sulfur element (S), particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and may be any material capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur. Examples of the organic sulfur compound include disulfide compounds, sulfur-modified polyacrylonitriles, sulfur-modified polyisoprene, rubeanic acid (dithiooxamide), polycarbon sulfide, and the like, which are typified by compounds described in the pamphlet of International Publication No. 2010/044437. Among them, disulfide compounds, sulfur-modified polyacrylonitriles, and rubeanic acid are preferred, and sulfur-modified polyacrylonitriles are particularly preferred. As the disulfide compound, those having dithiobiurea derivatives, thiourea groups, thioisocyanates, or thioamide groups are more preferred. Here, the sulfur-modified polyacrylonitrile is a modified polyacrylonitrile containing sulfur atoms, which is obtained by mixing sulfur powder and polyacrylonitrile and heating under inert gas or reduced pressure. The estimated structure is, for example, as shown in Chem. Mater. 2011, 23, 5024-5028, in which polyacrylonitrile is ring-closed to form a polycyclic ring, and at least a part of S is bonded to C. The compound described in this document has strong peak signals at around 1330 cm ^-1 and 1560 cm ^-1 in the Raman spectrum, and further has peaks at around 307 cm ^-1 , 379 cm ^-1 , 472 cm ^-1 , and 929 cm ^-1 . On the other hand, inorganic sulfur compounds are preferred because of their excellent stability, and specific examples thereof include sulfur element (S), TiS ₂ , TiS ₃ , TiS ₄ , NiS, NiS ₂ , CuS, FeS ₂ , Li ₂ S, MoS ₂ , MoS ₃ , MnS, MnS ₂ , CoS, and CoS ₂ . Among them, S, S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , FeS ₂ and MoS ₂ are preferred, sulfur element (S), TiS ₂ and FeS ₂ are more preferred, and sulfur element (S) is particularly preferred from the viewpoint of high capacity. As the sulfur element (S), α-sulfur, β-sulfur or γ-sulfur having an S ₈ structure can be used.

The positive electrode material according to the present embodiment may further include a positive electrode active material that does not contain sulfur in addition to the positive electrode active material that contains sulfur. Examples of the positive electrode active material that does not contain sulfur include layered rock salt active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO 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 the above include Li ₄ Ti ₅ O ₁₂ .

In some cases, two or more types of positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used. However, the content of the positive electrode active material containing sulfur in the total amount of the positive electrode active material (100% by mass) is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

(Solid electrolyte)
The positive electrode material according to the present embodiment essentially contains a solid electrolyte. There is no particular limitation on the specific form of the solid electrolyte contained in the positive electrode material according to the present embodiment, and the solid electrolytes exemplified in the section on the negative electrode active material layer and their preferred forms can be similarly adopted. 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 is preferably a sulfide solid electrolyte. In another preferred embodiment, the solid electrolyte contained in the solid electrolyte layer contains an alkali metal atom. Here, examples of the alkali metal that can be contained in the solid electrolyte include Li, Na, and K, and Li is preferred in terms of excellent ion conductivity. In yet another preferred embodiment, the solid electrolyte contained in the positive electrode material contains an alkali metal atom (e.g., Li, Na, or K; preferably Li) and a phosphorus atom and/or a boron atom. 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, 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 any of Ge, Zn, and Ga), Li ₂ S-SiS ₂ -Li ₃ PO ₄ , and the like can be mentioned. In addition, as a sulfide solid electrolyte having a Li ₄ P ₂ S ₇ skeleton, for example, a Li-P-S solid electrolyte called LPS (for example, Li ₇ P ₃ S ₁₁ ) can be mentioned. In addition, for example, LGPS represented by Li _(4-x) Ge _(1-x) P _x S ₄ (x satisfies 0<x<1) and the like 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 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 (wherein X is Cl, Br or I, preferably Cl). These solid electrolytes have high ionic conductivity, and therefore can effectively contribute to the expression of the effects of the present invention.

(Conductive materials)
The positive electrode material according to the present embodiment essentially contains a conductive material having pores. The specific form of the conductive material contained in the positive electrode material according to the present embodiment is not particularly limited as long as it is a conductive material having pores and has a predetermined pore volume profile described later, and conventionally known materials can be appropriately adopted. From the viewpoints of excellent conductivity, ease of processing, and ease of designing a desired pore distribution, it is preferable that the conductive material having pores is a carbon material.

Examples of carbon materials having fine pores include activated carbon, Ketjen Black (registered trademark) (highly conductive carbon black), (oil) furnace black, channel black, acetylene black, thermal black, lamp black, and other carbon blacks, as well as carbon particles (carbon carriers) made of coke, natural graphite, artificial graphite, and the like. It is preferable that the main component of the carbon material is carbon. Here, "mainly made of carbon" means that the main component is carbon atoms, and is a concept that includes both carbon atoms alone and carbon atoms essentially. "Substantially made of carbon atoms" means that impurities of about 2 to 3 mass % or less can be mixed in.

In the positive electrode material according to the present embodiment, one of the characteristics is that the percentage of the pore volume of pores having a pore diameter in the range of 1 to 4 nm relative to the pore volume of pores having a pore diameter in the range of 1 to 100 nm (mesopores) is 20% or less. Qualitatively speaking, this characteristic means that the proportion of the pore volume occupied by pores having a relatively small pore diameter is small. Here, in this specification, the pore distribution (pore volume value) of the mesopores of the conductive material is obtained using the BJH method. The "BJH method" is an abbreviation for the "Barrett-Joyner-Halenda method" and is a method for determining the pore distribution of mesopores (pore diameter 1 to 100 nm). The total adsorption amount is expressed as the sum of the multilayer adsorption amount and the capillary condensation amount. Of these, capillary condensation is a phenomenon that usually occurs in extremely small pores, but in the mesopore region with a large pore diameter, it is considered that capillary condensation occurs in the gaps between molecules left after the multilayer adsorption layer is completed, and the pore distribution of the mesopores is analyzed from the thickness of the multimolecular adsorption layer at the time when capillary condensation occurs. Details of the BJH method are described in E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. chem. Soc., 73, 373 (1951). From the viewpoint of further exerting the effect of reducing internal resistance, the above percentage value is preferably 18% or less, more preferably 15% or less, even more preferably 12% or less, and particularly preferably 9%. On the other hand, there is no particular limit on the lower limit of the above percentage, but it is, for example, 3% or more.

The BET specific surface area of the conductive material having pores (preferably, carbon material) is preferably 200 m ² /g or more, more preferably 500 m ² /g or more, even more preferably 800 m ² /g or more, particularly preferably 1200 ^{m 2} /g or more, and most preferably 1500 m ² /g or more. The total pore volume of the conductive material having pores (preferably, carbon material) is preferably 1.0 mL / g or more, more preferably 1.3 mL / g or more, and even more preferably 1.5 mL / g or more. If the BET specific surface area and total pore volume of the conductive material are within such a range, it is possible to hold a sufficient amount of pores, and thus to hold a sufficient amount of positive electrode active material. The BET specific surface area and total pore volume of the conductive material can be measured by nitrogen adsorption and desorption measurement. The nitrogen adsorption and desorption measurements are performed using a BELSORP mini manufactured by Microtrac-Bell Corporation, using a multipoint method at a temperature of -196°C. The BET specific surface area is determined from the adsorption isotherm in the relative pressure range of 0.01 < P/P ₀ < 0.05. The pore volume is determined from the volume of adsorbed N ₂ at a relative pressure of 0.96.

The average pore diameter of the conductive material is not particularly limited, but is preferably 50 nm or less, and more preferably 30 nm or less. If the average pore diameter of the conductive material is within these ranges, electrons can be sufficiently supplied to the active material located away from the pore walls among the positive electrode active material containing sulfur arranged inside the pores. The average pore diameter of the conductive material can be calculated by nitrogen adsorption/desorption measurement, as in the case of determining the BET specific surface area and pore volume.

When the conductive material is particulate, the average particle size (primary particle size) is not particularly limited, but is preferably 0.05 to 50 μm, more preferably 0.1 to 20 μm, and even more preferably 0.5 to 10 μm. The definition of the "particle size of the conductive material" and the method of measuring the "average particle size of the conductive material" are the same as those described above for the conductive assistant.

As described above, the positive electrode material according to this embodiment includes a conductive material having pores, a solid electrolyte, and a positive electrode active material containing sulfur, and is characterized in that at least a portion of the solid electrolyte and at least a portion of the positive electrode active material are disposed on the inner surface of the pores of the conductive material so as to be in contact with each other.

FIG. 3A is a schematic cross-sectional view of a cathode material 100' in the prior art. FIG. 3B is a schematic cross-sectional view of a cathode material 100 according to one embodiment of the present invention. In FIG. 3A and FIG. 3B, a carbon material (e.g., activated carbon) 110, which is a conductive material, has a large number of pores 110a. The inside of the pores 110a is filled with sulfur 120, which is a cathode active material. The cathode active material (sulfur) 120 is also disposed on the surface of the carbon material (activated carbon) 110. Here, in the cathode material 100' according to the prior art shown in FIG. 3A, a solid electrolyte (e.g., Li ₆ PS ₅ Cl, which is a sulfide solid electrolyte) 130 is disposed only on the surface of the carbon material (activated carbon) 110. In contrast, in the cathode material 100 according to one embodiment of the present invention shown in FIG. 3B, the solid electrolyte (for example, Li ₆ PS ₅ Cl, which is a sulfide solid electrolyte) 130 is disposed not only on the surface of the carbon material (porous carbon) 110 but also on the inner surface of the pores of the carbon material (porous carbon) 110. More specifically, a continuous phase consisting of the cathode active material (sulfur) 120 is filled inside the pores 110a, and the solid electrolyte 130 is disposed as a dispersed phase in the continuous phase. As a result, at least a part of the solid electrolyte disposed on the inner surface of the pores and at least a part of the cathode active material (sulfur) similarly disposed inside the pores are in contact with each other. In addition, the percentage of the pore volume of the pores having a pore diameter in the range of 1 to 4 nm to the pore volume of the pores having a pore diameter in the range of 1 to 100 nm among the pores of the conductive material is controlled to 20% or less. Here, it is possible to confirm whether the cathode active material or the solid electrolyte is disposed inside the pores of the conductive material using various conventionally known methods. For example, an elemental mapping of each material is performed using energy dispersive X-ray spectroscopy (EDX) on an image of a cross section of a conductive material observed by a transmission electron microscope (TEM), and the arrangement form of each material can be confirmed using the obtained elemental map and the count number of elements derived from each material relative to the count number of all elements as an index. For example, when a solid electrolyte contains phosphorus atoms and/or boron atoms, if there is no possibility that the phosphorus atoms and/or boron atoms are derived from other materials, it is possible to obtain the above elemental map for phosphorus and/or boron and confirm the arrangement form of the solid electrolyte from its distribution. It is also possible to confirm the arrangement form of the solid electrolyte from the value of the ratio of the count number of phosphorus and/or boron to the count number of all elements in EDX. In addition, when the arrangement form of the solid electrolyte is confirmed using the count number of elements derived only from the solid electrolyte in EDX as an index, if the value of the ratio of the count number of elements derived only from the solid electrolyte to the count number of all elements in EDX is 0.10 or more, it can be determined that the solid electrolyte is arranged inside the pores of the conductive material. The value of the ratio is preferably 0.15 or more, more preferably 0.20 or more, even more preferably 0.26 or more, and particularly preferably 0.35 or more. The larger this value is, the more the solid electrolyte is disposed on the inner surface of the pores of the conductive material. Therefore, when the value of the ratio is within these ranges, the effect of the present invention can be more significantly exhibited. There is no particular restriction on the preferred upper limit of the value of the ratio, but an example of a preferred upper limit is 0.50 or less, more preferably 0.45 or less.

According to the cathode material of the present embodiment having the above-mentioned configuration, in an electric device such as an all-solid-state lithium secondary battery using a cathode active material containing sulfur, it is possible to further reduce the internal resistance of the electric device. Although the mechanism by which such an excellent effect is achieved by using the configuration of the present embodiment is not completely clear, the following mechanism is presumed. That is, in order for the charge/discharge reaction to proceed in the cathode active material containing sulfur, it is necessary for the charge carriers such as electrons and lithium ions to smoothly enter and exit the surface of the cathode active material. Here, when the cathode active material (sulfur) 120 is held inside the pores 110a of a conductive material such as a carbon material 110 as in the cathode material 100' shown in FIG. 3A, the electrons can enter and exit smoothly to a certain extent through the conductive material on the surface of the cathode active material (sulfur) 120 located deep inside the pores 110a. However, since the solid electrolyte 130 is not arranged inside the pores 110a, the charge carriers such as lithium ions do not smoothly enter and exit the surface of the cathode active material (sulfur) 120 located deep inside the pores 110a. As a result, the charge/discharge reaction does not proceed sufficiently in the positive electrode active material (sulfur) 120 located deep inside the pores 110a, causing problems such as an increase in internal resistance and a decrease in charge/discharge rate characteristics.

In addition, when the percentage of the pore volume of pores with a pore diameter in the range of 1 to 4 nm to the pore volume of pores with a pore diameter in the range of 1 to 100 nm among the pores of the conductive material constituting the positive electrode material exceeds 20%, the proportion of pores with a relatively small pore diameter increases. Here, the sulfur-containing positive electrode active material has a small molecular size of less than about 1 nm, so it can penetrate into the inside of the pores of the conductive material (carbon material) and be supported. In contrast, the size of a single molecule of the solid electrolyte is large at about 2 nm, and it is possible that the size is further increased by solvation depending on the manufacturing method. In that case, the solid electrolyte cannot penetrate into the inside of pores with a pore diameter of about several nm. As a result, the charge/discharge reaction does not proceed sufficiently, and problems such as an increase in internal resistance and a decrease in charge/discharge rate characteristics occur.

In contrast, as shown in FIG. 3B, in the case of the positive electrode material 100, in which the percentage of the pore volume of the pores having a pore diameter in the range of 1 to 4 nm relative to the pore volume of the pores having a pore diameter in the range of 1 to 100 nm is controlled to 20% or less, and the solid electrolyte 130 is held together with the positive electrode active material (sulfur) 120 on the inner surface of the pores 110a of the conductive material such as the carbon material 110, not only the inflow and outflow of electrons via the conductive material but also the inflow and outflow of charge carriers via the solid electrolyte 130 can proceed smoothly on the surface of the positive electrode active material (sulfur) 120 located deep inside the pores 110a. As a result, even around the positive electrode active material (sulfur) 120 located deep inside the pores 110a, a three-phase interface in which the positive electrode active material, the conductive material, and the solid electrolyte coexist can be sufficiently formed, and the charge and discharge reaction can proceed sufficiently. As a result, it is considered that the internal resistance of the battery is sufficiently reduced, and the charge and discharge rate characteristics are significantly improved.

The manufacturing method of the positive electrode material according to the present embodiment having the above-mentioned configuration is not particularly limited, but an example of the manufacturing method will be briefly described. As described in the Examples section below, first, a conductive material that satisfies the pore volume profile specified in the present application is prepared using a conventionally known method. Here, the method of obtaining such a conductive material will be described using an example in which the conductive material is a carbon material. First, a mold such as ceramics and a carbon raw material such as resin are mixed and fired in an inert atmosphere. Then, the mold is dissolved with acid to obtain a carbon material (conductive material) having a porous structure in which the shape of the mold is transferred. At this time, the pore size and pore volume of the obtained carbon material can be changed by appropriately adjusting the particle size of the mold and the compounding ratio of the carbon raw material.

Next, a solution is prepared by dissolving the solid electrolyte in an organic solvent, and the conductive material is dispersed therein to obtain a dispersion liquid. After that, the solvent is removed and then heat treatment is performed at a temperature of about 150 to 250°C for about 1 to 5 hours. As a result, a conductive material in which the pores of the conductive material are impregnated with the solid electrolyte is obtained. Next, the obtained conductive material is mixed with the positive electrode active material in a dry state, and further heat treatment is performed under the same conditions as above. As a result, the positive electrode active material melts and penetrates into the pores of the conductive material, and a composite material in which the positive electrode active material is arranged (filled) together with the solid electrolyte in the pores of the conductive material is obtained. The composite material obtained in this way may be used as it is as a positive electrode material, but it is preferable to further add a solid electrolyte to the composite material, mix it, and process it in an apparatus such as a ball mill as necessary to obtain a positive electrode material.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably within the range of 35 to 99 mass %, and more preferably within the range of 40 to 90 mass %. Note that this content value is calculated based on the mass of only the positive electrode active material, excluding the conductive material and solid electrolyte.

The positive electrode active material layer may further include a conductive assistant (one that does not hold the positive electrode active material or solid electrolyte inside the pores) and/or a binder, and the specific and preferred forms thereof may be the same as those described in the section on the negative electrode active material layer above. Similarly, the positive electrode active material layer preferably further includes a solid electrolyte, and more preferably includes a sulfide solid electrolyte. The specific and preferred forms of the solid electrolyte, such as the sulfide solid electrolyte, may be the same as those described in the section on the negative electrode active material layer above.

[Positive and negative current collector plates]
The material constituting the current collector plate (25, 27) is not particularly limited, and a known highly conductive material conventionally used as a current collector plate for a secondary battery can be used. As the material constituting the current collector plate, for example, a metal material such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof is preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. In addition, the same material may be used for the positive electrode current collector plate 27 and the negative electrode current collector plate 25, or different materials may be used.

[Positive and negative electrode leads]
Although not shown, the current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium secondary batteries may be similarly adopted. Note that the part taken out from the exterior is preferably covered with a heat-resistant insulating heat shrink tube or the like so as not to come into contact with peripheral devices or wiring, etc., causing leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[Battery exterior material]
As the battery exterior material, a known metal can case can be used, and a bag-shaped case using a laminate film 29 containing aluminum, which 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 preferable from the viewpoint of being excellent in high output and cooling performance and being suitable for use in batteries for large equipment for EVs and HEVs. In addition, since the group pressure applied from the outside to the power generating element can be easily adjusted, a laminate film containing aluminum is more preferable as the exterior body.

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

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

For example, the type of battery to which the lithium secondary battery according to the present embodiment can be applied includes a bipolar battery that 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.

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 is no particular limit to the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount is 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) 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 γ-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 the lithium salt include Li( _FSO2 ) _2N (lithium bis(fluorosulfonyl)imide; _LiFSI ), _Li ( _C2F5SO2 ) _2N , _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , and _LiCF3SO3 . Among _these , the lithium salt is preferably Li( _FSO2 ) _2N (LiFSI) from the viewpoints of battery output and charge/discharge cycle characteristics.

The liquid electrolyte (electrolyte) may further contain additives other than the above-mentioned components. Specific examples of such compounds include, for example, ethylene carbonate, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2-divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, and 1-ethyl-2-vinylethylene carbonate. Examples of the 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. The amount of additive used in the electrolyte can be adjusted as appropriate.

[Battery pack]
A battery pack is a device that consists of multiple batteries connected together. More specifically, it is a device that uses at least two batteries and is configured in series or parallel or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

A small, removable battery pack can be formed by connecting multiple batteries in series or in parallel. Then, multiple such small, removable battery packs can be connected in series or in 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. How many batteries are connected to form a battery pack, and how many stages of small battery packs are stacked 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 is to be installed.

[vehicle]
A battery or a battery pack formed by combining a plurality of batteries can be mounted on a vehicle. In the present invention, a battery with excellent long-term reliability and long life can be configured, so that a plug-in hybrid electric vehicle with a long EV driving distance or an electric vehicle with a long driving distance per charge can be configured by mounting such a battery. For example, a battery or a battery pack formed by combining a plurality of batteries can be used in a hybrid vehicle, a fuel cell vehicle, or an electric vehicle (including four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles, etc.), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles) to make the vehicle long-life and highly reliable. However, the use is not limited to automobiles, and the battery pack can be applied to various power sources of other vehicles, such as moving bodies such as trains, and can also be used as a mounted power source for an uninterruptible power supply device, for example.

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.

Example of test cell preparation
[Comparative Example 1]
(Preparation of solid electrolyte-impregnated carbon)
In a glove box in an argon atmosphere with a dew point of -68°C or less, 0.500g of sulfide solid electrolyte (Ampcera, Li ₆ PS ₅ Cl) was added to 100mL of ultra-dehydrated ethanol (FUJIFILM Wako Pure Chemical Industries, Ltd.), and the solution was stirred until it became transparent to dissolve the solid electrolyte in ethanol. 1.00g of carbon (Kansai Thermochemical Industries, Ltd., activated carbon, MSC-30) was added to the obtained solid electrolyte ethanol solution, and the carbon was thoroughly dispersed in the solution by stirring. The container containing this carbon dispersion was connected to a vacuum device, and the container was reduced in pressure to 1 Pa or less by an oil rotary pump while stirring the carbon dispersion in the container with a magnetic stirrer. Since ethanol, which is a solvent, volatilizes under reduced pressure, the ethanol was removed over time, and the carbon impregnated with the solid electrolyte remained in the container. In this way, the ethanol was removed under reduced pressure, and then the solid electrolyte was heated to 180°C under reduced pressure and heat-treated for 3 hours to prepare a solid electrolyte-impregnated carbon. The carbon used in this comparative example was prepared according to E.P. Fig. 4 shows a graph of the pore size distribution in the range of 1 to 100 nm, measured according to the BJH method described in Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 73, 373 (1951). The horizontal axis of Fig. 4 is the pore size Dp [nm] of the carbon, and the vertical axis is the pore volume Vp [cm ³ /g]. The percentage of the pore volume of pores with a pore size in the range of 1 to 4 nm to the pore volume of pores with a pore size in the range of 1 to 100 nm was calculated to be 79%.

(Preparation of sulfur/solid electrolyte/carbon composites by hot impregnation with sulfur)
In a glove box in an argon atmosphere with a dew point of -68°C or less, 2.50 g of sulfur (manufactured by Aldrich) was added to 0.750 g of the solid electrolyte-impregnated carbon prepared above and mixed thoroughly in an agate mortar, and the mixed powder was then placed in a sealed pressure-resistant autoclave and heated at 170°C for 3 hours to melt the sulfur and impregnate the solid electrolyte-impregnated carbon with the sulfur. In this way, a sulfur/solid electrolyte/carbon composite material was prepared.

(Preparation of sulfur-containing positive electrode material)
In a glove box in an argon atmosphere with a dew point of -68°C or less, 40 g of 5 mm diameter zirconia balls, 0.130 g of the sulfur/solid electrolyte-impregnated carbon prepared above, and 0.070 g of solid electrolyte (Li6PS5Cl, manufactured by Ampcera) were placed in a 45 mL capacity zirconia container, and processed at 370 rpm for 6 hours using a planetary ball mill (Premium line P-7, manufactured by Fritsch) to obtain a powder of a sulfur-containing positive electrode material. The composition of the sulfur-containing positive electrode material was sulfur:solid electrolyte:carbon = 50:40:10.

The amount of solid electrolyte contained within the pores of the conductive material (carbon) constituting the powder of sulfur-containing positive electrode material obtained above was quantified by EDX mapping, and it was confirmed that the solid electrolyte was disposed within the pores of the conductive material (carbon) together with the sulfur, which is the positive electrode active material.

(Preparation of test cells (all-solid-state lithium secondary batteries))
The battery was fabricated in a glove box in an argon atmosphere with a dew point of −68° C. or lower.

A SUS cylindrical convex punch (10 mm diameter) was inserted into one side of a cylindrical tube jig (inner diameter 10 mm, outer diameter 23 mm, height 20 mm) manufactured by Macor, and 80 mg of a sulfide solid electrolyte (Li ₆ PS ₅ Cl manufactured by Ampcera) was placed from the top of the cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte, and a hydraulic press was used to press the solid electrolyte at a pressure of 75 MPa for 3 minutes to form a solid electrolyte layer with a diameter of 10 mm and a thickness of about 0.6 mm in the cylindrical tube jig. Next, the cylindrical convex punch inserted from the top was once removed, and 7.5 mg of the sulfur-containing positive electrode mixture prepared above was placed on one side of the solid electrolyte layer in the cylindrical tube, and a cylindrical convex punch (also serving as a positive electrode current collector) was inserted from the top again, and pressed at a pressure of 300 MPa for 3 minutes to form a positive electrode active material layer with a diameter of 10 mm and a thickness of about 0.06 mm on one side of the solid electrolyte layer. Next, the lower cylindrical convex punch (also serving as a negative electrode current collector) was removed, and a lithium foil (manufactured by Nilaco Corporation, thickness 0.20 mm) punched to a diameter of 8 mm and an indium foil (manufactured by Nilaco Corporation, thickness 0.30 mm) punched to a diameter of 9 mm were stacked as negative electrodes, and the indium foil was inserted from the bottom of the cylindrical tube jig so that it was located on the side of the solid electrolyte layer, and the cylindrical convex punch was inserted again, and pressed at a pressure of 75 MPa for 3 minutes to form a lithium-indium negative electrode.

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

[Example 1]
An all-solid-state lithium secondary battery was produced in the same manner as in Comparative Example 1 described above, except that in the preparation of the solid electrolyte-impregnated carbon, porous carbon powder (Knobel (registered trademark) P(3)010, manufactured by Toyo Tanso Co., Ltd.) was used as the carbon instead of MSC-30. Note that, in this example as well, it was confirmed that the solid electrolyte was disposed inside the pores of the conductive material (carbon) together with sulfur, which was the positive electrode active material.

The pore size distribution of the carbon used in this example, measured according to the BJH method described in E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 73, 373 (1951), is shown in FIG. 4. The percentage of the pore volume of pores with a pore size of 1 to 4 nm relative to the pore volume of pores with a pore size of 1 to 100 nm was calculated to be 9%.

[Example 2]
An all-solid-state lithium secondary battery was produced in the same manner as in Comparative Example 1, except that in the preparation of the solid electrolyte-impregnated carbon, carbon black (Ketjenblack (registered trademark) EC600JD, manufactured by Lion Specialty Chemicals Co., Ltd.) was used as the carbon instead of MSC-30. Note that, in this example as well, it was confirmed that the solid electrolyte was disposed inside the pores of the conductive material (carbon) together with sulfur, which was the positive electrode active material.

The pore size distribution of the carbon used in this example, measured according to the BJH method described in E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 73, 373 (1951), is shown in FIG. 4. The percentage of the pore volume of pores with a pore size of 1 to 4 nm relative to the pore volume of pores with a pore size of 1 to 100 nm was calculated to be 20%.

<Test cell evaluation example>
The internal resistance of the test cells prepared in each of the comparative examples and examples was measured by the following method. All of the following measurements were performed using a charge/discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a constant temperature bath set at 25°C.

(Internal resistance measurement)
A test cell was placed in a thermostatic chamber, and after the cell temperature became constant, a constant current discharge was performed at a current density of 0.2 mA/cm ² to a cell voltage of 0.5 V as cell conditioning, followed by a constant current constant voltage charge of 2.5 V at the same current density with a cutoff current of 0.01 mA/cm ^2. The capacity value (mAh/g) per mass of the positive electrode active material was calculated from the charge/discharge capacity value obtained after repeating this conditioning charge/discharge cycle 10 times and the mass of the positive electrode active material contained in the positive electrode. Next, a constant current discharge was performed at a current density of 0.2 mA/cm ² to a capacity of 50% (SOC50%) with respect to the capacity value calculated in this way of 100%. After a 30-minute pause, discharge was performed for 10 seconds at a discharge rate of 1C, and the direct current resistance (DCR) was calculated according to Ohm's law from the voltage drop and current value at that time, and was taken as the internal resistance value of the test cell. The results are shown in Table 1 below. The internal resistance values (DCR) shown in Table 1 are relative values when the value in Comparative Example 1 is set to 1.

From the results shown in Table 1, it can be seen that, according to the present invention, the internal resistance can be further reduced in an all-solid-state lithium secondary battery using a cathode active material containing sulfur. In addition, when the internal resistance values of the cathodes in Examples 1 and 2 were compared, the internal resistance value of Example 1, which has a smaller value of the above percentage (Vp1-4nm/Vp@1-100nm), was reduced to approximately 80% of that of Example 2. For this reason, in the present invention, it is considered that the smaller the value of the above percentage (Vp1-4nm/Vp@1-100nm) is, the more preferable it is from the standpoint of reducing internal resistance.

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,
27 positive electrode current collector,
29 Laminate film,
100, 100' positive electrode material,
110 Carbon materials (activated carbon, porous carbon),
110a pore,
120 positive electrode active material (sulfur),
130 Solid electrolyte.

Claims (12)

The positive electrode active material includes a conductive material having fine pores, a solid electrolyte, and a positive electrode active material including sulfur,
at least a portion of the solid electrolyte and at least a portion of the positive electrode active material are disposed on inner surfaces of the pores so as to be in contact with each other;
a continuous phase made of the positive electrode active material is filled inside the pores, and the solid electrolyte is disposed as a dispersed phase in the continuous phase; and
A positive electrode material for an electric device, wherein, among the pores, a pore volume of pores having a pore diameter in the range of 1 to 4 nm accounts for 20% or less of a pore volume of pores having a pore diameter in the range of 1 to 100 nm. The positive electrode material for an electrical device according to claim 1, wherein the percentage is 9% or less. 3. The positive electrode material for an electric device according to claim 1 , wherein in an observation image of a cross section of the conductive material by TEM-EDX, a ratio of the count number of elements derived only from the solid electrolyte to the count number of all elements is 0.10 or more. The positive electrode material for an electric device according to any one of claims 1 to 3 , wherein the conductive material is a carbon material. The positive electrode material for an electric device according to any one of claims 1 to 4 , wherein the conductive material has a total pore volume of 1.0 mL/g or more. The positive electrode material for an electric device according to claim 1 , wherein the conductive material has an average pore size of 50 nm or less. The positive electrode material for an electric device according to any one of claims 1 to 6 , wherein the solid electrolyte is a sulfide solid electrolyte. 8. The positive electrode material for an electric device according to claim 1 , wherein the solid electrolyte contains an alkali metal atom, and a phosphorus atom and/or a boron atom. 9. The positive electrode material for an electrical device according to claim 8 , wherein the alkali metal atom is a lithium atom. A positive electrode for an electric device, comprising the positive electrode material for an electric device according to any one of claims 1 to 9 . An electric device comprising the positive electrode for an electric device according to claim 10 . The electrical device of claim 11 , which is an all-solid-state lithium secondary battery.