Positive electrode material and secondary battery using the same

Abstract

To provide means for further reducing the internal resistance of a secondary battery using a positive electrode active material including sulfur.SOLUTION: A positive electrode material includes: a conducting porous material; an alkali metal sulfide being a positive electrode active material; and a solid electrolyte. At least a portion of the alkali metal sulfide and at least a portion of the solid electrolyte are arranged in pores of the conducting porous material. The positive electrode material exhibits, in an X-ray diffraction measurement, a peak A derived from the conducting porous material in an area where 2θ is 20° or more and less than 25°and a peak B derived from the alkali metal sulfide and / or the solid electrolyte in an area where 2θ is 25 to 29°. A ratio (A / B) of intensity of the peak A with respect to intensity of the peak B exceeds 0.02.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode material and a secondary battery using the same. [Background technology]

[0002] In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. The automotive industry is hoping 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 secondary batteries for driving motors, which hold the key to putting these vehicles into practical use.

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

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

[0005] Therefore, in recent years, research and development of all-solid-state lithium secondary batteries using oxide- or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials primarily composed of ionic conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries, in principle, do not encounter the various problems associated with flammable organic electrolytes that are common 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 leads to significant improvements in the battery's power density and energy density. For example, elemental sulfur (S8) has an extremely high theoretical capacity of approximately 1670 mAh / g and is advantageously low cost and abundant in resources.

[0006] 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 performance.

[0007] To address this problem, Patent Document 1 proposes a technology for using a composite material containing a conductive agent and an alkali metal sulfide integrated on the surface of the conductive agent as a positive electrode material for an all-solid-state battery. Patent Document 1 states that by using a positive electrode material with this 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. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2012 / 102037 Summary of the Invention [Problem to be solved by the invention]

[0009] 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., that 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 charge and discharge. 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.

[0010] Therefore, an object of the present invention is to provide a means for further reducing the internal resistance of a secondary battery using a positive electrode active material containing sulfur. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by using a material that exhibits a specific X-ray diffraction spectrum in a positive electrode material having an alkali metal sulfide and a solid electrolyte contained in the pores of a conductive porous body, thereby completing the present invention.

[0012] One aspect of the present invention is a positive electrode material comprising a conductive porous body, an alkali metal sulfide as a positive electrode active material, and a solid electrolyte, wherein at least a portion of the alkali metal sulfide and at least a portion of the solid electrolyte are disposed in the pores of the conductive porous body, and wherein, in X-ray diffraction measurement, the positive electrode material exhibits a peak A derived from the conductive porous body in a 2θ region of 20° or more and less than 25°, and a peak B derived from the alkali metal sulfide and / or the solid electrolyte in a 2θ region of 25 to 29°, and wherein the ratio of the intensity of Peak A to the intensity of Peak B (A / B) exceeds 0.02. [Effects of the Invention]

[0013] According to the present invention, in a secondary battery using a positive electrode active material containing sulfur, the internal resistance of the secondary battery can be further reduced. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. [Figure 3] Figure 3(a) is a cross-sectional view of a cathode material according to the prior art, Figure 3(b) is a cross-sectional view of a cathode material according to one embodiment of the present invention, and Figure 3(c) is a cross-sectional view of a cathode material prepared by mechanical mixing. [Figure 4]4(a) is an XRD spectrum of the positive electrode material prepared in Example 4, and FIG. 4(b) is an XRD spectrum of the positive electrode material prepared in Comparative Example 2. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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 defined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios. Below, the present invention will be described using a stacked-type (internal parallel connection) all-solid-state lithium secondary battery, which is one type of secondary battery. As described above, the solid electrolyte constituting 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 various problems associated with flammable organic electrolytes, as in conventional liquid-based lithium secondary batteries. Furthermore, generally, the use of a high-potential, high-capacity positive electrode material and a high-capacity negative electrode material has the advantage of significantly improving the output density and energy density of the battery.

[0016] One aspect of the present invention is a cathode material comprising a conductive porous body, an alkali metal sulfide serving as a cathode active material, and a solid electrolyte, wherein at least a portion of the alkali metal sulfide and at least a portion of the solid electrolyte are disposed within the pores of the conductive porous body, and wherein the cathode material exhibits, in X-ray diffraction measurement, a peak A attributable to the conductive porous body in the 2θ range of 20° to less than 25° and a peak B attributable to the alkali metal sulfide and / or the solid electrolyte in the 2θ range of 25 to 29°, and wherein the ratio of the intensity of Peak A to the intensity of Peak B (A / B) exceeds 0.02. The cathode material according to this aspect can further reduce the internal resistance of secondary batteries.

[0017] FIG. 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1. The layered structure allows the battery to be compact and have a high capacity. In this specification, the flat-layered non-bipolar lithium secondary battery shown in FIGS. 1 and 2 (hereinafter also simply referred to as a "layered battery") will be used as an example for detailed explanation. However, in terms of the internal electrical connection configuration (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.

[0018] 1, the stacked battery 10a has a flat, rectangular shape, with a negative electrode current collector 25 and a positive electrode current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and the periphery is heat-sealed, with the negative electrode current collector 25 and positive electrode current collector 27 extending to the outside.

[0019] 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 into a rectangular, flat shape, and is not particularly limited. The cylindrical battery may use a laminate film or a conventional cylindrical can (metal can) as its exterior material, and is not particularly limited. Preferably, the power generating element is housed inside a laminate film containing aluminum. This configuration can achieve weight reduction.

[0020] There are also no particular limitations on how the current collectors (25, 27) shown in Fig. 1 are taken out. The negative current collector 25 and the positive current collector 27 may be taken 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 taken out from each side, and so on, and are not limited to what is shown in Fig. 1. Furthermore, in a wound-type lithium battery, terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

[0021] 2, the stacked battery 10a of this embodiment has a structure in which a flat, generally rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior material. Here, the power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are disposed on both sides of a positive electrode current collector 11". The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order such that one positive electrode active material layer 15 faces the adjacent negative electrode active material layer 13 with the solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, it can be said that the stacked battery 10a shown in FIG. 2 has a structure in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel.

[0022] 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. That is, instead of using a current collector exclusively for the outermost layer with an active material layer provided on only one side, a current collector having 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 the positive electrode, respectively, without using the current collectors (11', 11")

[0023] Negative electrode current collector 11′ and positive electrode current collector 11″ are respectively attached with negative electrode current collector (tab) 25 and positive electrode current collector (tab) 27 that are electrically connected to the respective electrodes (positive and negative electrodes), and are structured so as to be sandwiched between the ends of laminate film 29, which is the battery outer casing material, and extended to the outside of laminate film 29. Positive electrode current collector 27 and negative electrode current collector 25 may be attached to positive electrode current collector 11″ and negative electrode current collector 11′ of the respective electrodes by ultrasonic welding, resistance welding, or the like, via positive electrode leads and negative electrode leads (not shown) as necessary.

[0024] The main components of the lithium secondary battery according to this embodiment will be described below.

[0025] [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.

[0026] Specifically, examples of the metal include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Furthermore, foils in which aluminum is coated on a metal surface may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

[0027] Furthermore, examples of the resin having electrical conductivity include resins in which a conductive filler is added to a non-conductive polymer material.

[0028] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector includes at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the negative electrode active material layer and the positive electrode active material layer described later are electrically conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate member from these electrode active material layers. In such a configuration, the negative electrode active material layer described later constitutes the negative electrode, and the positive electrode active material layer described later constitutes the positive electrode.

[0029] [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 thereof 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 Nb2O5, Li4Ti5O 12 and the like. Furthermore, silicon-based negative electrode active materials and tin-based negative electrode active materials may also be used. Here, silicon and tin belong to the 14th group of elements, and are known to be negative electrode active materials that can significantly 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, SiO 2 disproportionated into two phases, an Si phase and a silicon oxide phase, is also used. xIt is also preferable to use silicon oxides such as (0.3≦x≦1.6). 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, examples of negative electrode active materials containing tin element (tin-based negative electrode active materials) include simple Sn, tin alloys (Cu—Sn alloy, Co—Sn alloy), amorphous tin oxide, tin silicon oxide, etc. Among these, examples of amorphous tin oxide include SnB 0.4 P 0.6 O 3.1 Examples of tin silicon oxides include SnSiO3. A metal containing lithium may also 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 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 contains metallic lithium, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably contains metallic lithium.

[0030] 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, its average particle diameter (D 50 ) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle diameter (D 50 The value of can be measured by a laser diffraction scattering method.

[0031] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably within the range of 40 to 99 mass %, and more preferably within the range of 50 to 90 mass %, for example.

[0032] The negative electrode active material layer preferably further contains a solid electrolyte. By including the solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes, and the sulfide solid electrolyte is preferred.

[0033] Examples of sulfide solid electrolytes include LiI-Li2S-SiS2, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, LiI-Li3PS4, LiI-LiBr-Li3PS4, Li3PS4, Li2S-P2S5-LiI, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z m S n (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In). The term "LiS-P2S5" refers to a sulfide solid electrolyte obtained using a raw material composition containing LiS and P2S5, and the same applies to other terms.

[0034] The sulfide solid electrolyte may have, for example, a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of sulfide solid electrolytes having a Li3PS4 skeleton include LiI-Li3PS4, LiI-LiBr-Li3PS4, and Li3PS4. Examples of sulfide solid electrolytes having a Li4P2S7 skeleton include Li-PS-based solid electrolytes known as LPS (for example, Li7P3S 11) can be mentioned. Also, as the sulfide solid electrolyte, for example, Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1), such as LGPS, may be used. Among them, it is preferably a sulfide solid electrolyte containing P element, and more preferably a material mainly composed of Li2S - P2S5. Furthermore, the sulfide solid electrolyte may contain halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte contains Li6PS5X (where X is Cl, Br or I, preferably Cl).

[0035] Also, when the sulfide solid electrolyte is of the Li2S - P2S5 system, the ratio of Li2S and P2S5 is preferably within the range of Li2S:P2S5 = 50:50 to 100:0 in terms of molar ratio, and more preferably Li2S:P2S5 = 70:30 to 80:20.

[0036] Also, the sulfide solid electrolyte may be a sulfide glass, a 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 (such as a ball mill) on the raw material composition. Also, the crystallized sulfide glass can be obtained, for example, by heat - treating the sulfide glass at a temperature above the crystallization temperature. Also, the ionic 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 ionic conductivity of the solid electrolyte can be measured by the alternating current impedance method.

[0037] As the oxide solid electrolyte, for example, compounds having a NASICON - type structure can be mentioned. As an example of a compound having a NASICON - type structure, a compound represented by the general formula Li 1+x Al x Ge 2-x (PO4)3 (0 ≤ x ≤ 2) (LAGP), general formula Li1+x Al x Ti 2-x (PO4)3 (0≦x≦2) (LATP) and the like. Another example of an oxide solid electrolyte is LiLaTiO (for example, Li 0.34 La 0.51 TiO3), LiPON (e.g., Li 2.9 PO 3.3 N 0.46 ), LiLaZrO (e.g., Li7La3Zr2O 12 ) etc.

[0038] The shape of the solid electrolyte may be, for example, a particulate shape such as a spherical shape or an oval spherical shape, or a thin film shape. When the solid electrolyte is particulate, the average particle diameter (D 50 ) 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. 50 ) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

[0039] The content of the solid electrolyte in the negative electrode active material layer is, for example, preferably in the range of 1 to 60 mass %, and more preferably in the range of 10 to 50 mass %.

[0040] 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.

[0041] Examples of conductive additives include, but are not limited to, metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium; alloys or metal oxides containing these metals; carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.); carbon nanotubes (CNTs); and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). Furthermore, particulate ceramic materials or resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive additives. Among these conductive additives, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon, more preferably at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon, and even more preferably at least one carbon. These conductive additives may be used alone or in combination.

[0042] 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.

[0043] When the conductive additive is particulate, its average particle size (primary particle size) is not particularly limited, but is preferably 0.01 to 10 μm from the viewpoint of the electrical properties of the battery. In this specification, the "particle size of the conductive additive" refers to the longest distance L between any two points on the contour line of the conductive additive. The value of the "average particle size of the conductive additive" is calculated as the average particle size 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).

[0044] When the negative electrode active material layer contains a conductive additive, the content of the conductive additive in the negative electrode active material layer is not particularly limited, but is preferably 0 to 10 mass %, more preferably 2 to 8 mass %, and even more preferably 4 to 7 mass %, relative to the total mass of the negative electrode active material layer. Within such a range, a stronger electron conduction path can be formed in the negative electrode active material layer, which can effectively contribute to improving battery characteristics.

[0045] On the other hand, the binder is not particularly limited, but examples thereof include the following materials:

[0046] Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, 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 Examples of such fluororesins include ethylene-chlorotrifluoroethylene copolymer (ECTFE) and polyvinyl fluoride (PVF), vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubbers (VDF-HFP-based fluororubbers), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-HFP-TFE-based fluororubbers), vinylidene fluoride-pentafluoropropylene-based fluororubbers (VDF-PFP-based fluororubbers), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers), and epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

[0047] The thickness of the negative electrode active material layer varies depending on the intended configuration of the secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0048] [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.

[0049] The specific form of the solid electrolyte contained in the solid electrolyte layer is not particularly limited, and the solid electrolytes exemplified in the section on the negative electrode active material layer and their preferred forms can be similarly employed. In some cases, a solid electrolyte other than the above-mentioned solid electrolytes may be used in combination.

[0050] The solid electrolyte layer may further contain a binder in addition to the above-mentioned predetermined solid electrolyte. The examples and preferred forms described in the section on the negative electrode active material layer may also be adopted for the binder that can be contained in the solid electrolyte layer.

[0051] 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 800 μm or less, more preferably 700 μm or less, and even more preferably 600 μm or less. On the other hand, there is no particular restriction on the lower limit of the thickness of the solid electrolyte layer, but it is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more.

[0052] [Cathode active material layer] 1 and 2, the positive electrode active material layer contains a positive electrode material according to one embodiment of the present invention, which contains an alkali metal sulfide and a solid electrolyte in the pores of a conductive porous body.

[0053] (Cathode active material) The positive electrode material of this embodiment contains an alkali metal sulfide as a positive electrode active material, such as lithium sulfide (LiS), sodium sulfide (NaS), potassium sulfide (KS), rubidium sulfide (RbS), cesium sulfide (CsS), and francium sulfide (FrS). Of these, lithium sulfide or sodium sulfide is preferred, and lithium sulfide is more preferred.

[0054] The positive electrode material according to this embodiment may further contain, in addition to the alkali metal sulfide, a positive electrode active material other than the alkali metal sulfide (another positive electrode active material).

[0055] Other positive electrode active materials include, but are not limited to, sulfur-containing positive electrode active materials such as elemental sulfur (S) and particles or thin films of organic or inorganic sulfur compounds. These materials can utilize the sulfur redox reaction to release lithium ions during charging and absorb lithium ions during discharging. Examples of organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitriles, sulfur-modified polyisoprenes, rubeanic acid (dithiooxamide), and polycarbonates, as exemplified by the compounds described in International Publication No. 2010 / 044437. Inorganic sulfur compounds are preferred due to their excellent stability. Specific examples include metal sulfides such as TiS2, TiS3, TiS4, NiS, NiS2, CuS, FeS2, MoS2, MoS3, MnS, MnS2, CoS, and CoS2. The elemental sulfur (S) can be α-sulfur, β-sulfur, or γ-sulfur with an S8 structure.

[0056] Examples of sulfur-free positive electrode active materials include layered rock salt active materials such as LiCoO2, LiMnO2, LiNiO2, LiVO2, and Li(Ni-Mn-Co)O2; LiMn2O4; and LiNi 0.5 Mn 1.5 Examples of metal oxide active materials include spinel-type active materials such as LiFePO4 and LiMnPO4, olivine-type active materials such as LiFeSiO4 and LiMnSiO4, and Si-containing active materials such as LiFeSiO4 and LiMnSiO4. Examples of metal oxide active materials other than those mentioned above include LiTiO12 Examples include:

[0057] In some cases, two or more kinds 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 alkali metal sulfide in the total amount (100% by mass) of the positive electrode active material is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

[0058] From the viewpoint of reducing resistance, the positive electrode material of this embodiment preferably does not contain elemental sulfur. The content of elemental sulfur in 100% by mass of the total amount of the positive electrode active material is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, still more preferably 2% by mass or more, particularly preferably 1% by mass or less, and most preferably 0% by mass.

[0059] (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.

[0060] 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 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 because of its excellent ionic conductivity. In particular, it is preferable that the positive electrode active material is Li2S and the solid electrolyte contains lithium (the carrier ions contain lithium ions). By adopting such a configuration, the effects of the present invention can be more significantly obtained.

[0061] In still 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), a phosphorus atom and / or a boron atom. More preferably, the solid electrolyte is a sulfide solid electrolyte containing at least a lithium atom, a phosphorus atom, and a sulfur atom. Examples of such a sulfide solid electrolyte containing a lithium atom, a phosphorus atom, and a sulfur atom include, for example, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, LiI-Li3PS4, LiI-LiBr-Li3PS4, Li3PS4, Li2S-P2S5-LiI, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2-P2S5-LiI, Li2S-P2S5-Z m S n (where m and n are positive numbers, and Z is any one of Ge, Zn, and Ga), Li2S-SiS2-Li3PO4, etc. Examples of the sulfide solid electrolyte having a Li4P2S7 skeleton include, for example, a Li-P-S-based solid electrolyte called LPS (e.g., Li7P3S 11 ). Further, for example, Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1), such as LGPS, may be used. Among them, the sulfide solid electrolyte contained in the active material layer is more preferably a material mainly composed of Li2S-P2S5. Further, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte contains Li6PS5X (where X is Cl, Br or I, preferably Cl). Since these sulfide solid electrolytes have high ionic conductivity, they can also effectively contribute to the manifestation of the effects of the present invention.

[0062] (Conductive porous body) The positive electrode material according to this embodiment essentially contains a conductive porous body having pores. The specific form of the conductive porous body contained in the positive electrode material according to this embodiment is not particularly limited as long as it is a conductive material having pores, and any conventionally known material can be appropriately used. From the viewpoints of excellent conductivity and ease of processing, the conductive porous body is preferably a carbon material.

[0063] Examples of carbon materials 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; carbon particles (carbon supports) made from coke, natural graphite, artificial graphite, and the like. Alternatively, a ceramic or other mold may be mixed with a carbon raw material such as a resin, fired under an inert atmosphere, and then the mold may be dissolved with acid to synthesize a carbon material having a porous structure in which the shape of the mold has been transferred. This carbon material may then be used. In this case, the pore size and pore volume of the resulting carbon material can be altered by appropriately adjusting the particle size of the mold and the blending ratio of the carbon raw materials.

[0064] It is preferable that the carbon material is mainly composed of carbon. Here, "mainly composed of carbon" means that the material contains carbon atoms as the main component, and is a concept that includes both "consisting only of carbon atoms" and "consisting essentially of carbon atoms." "Consisting essentially of carbon atoms" means that the inclusion of impurities of about 2 to 3 mass % or less is acceptable.

[0065] In the positive electrode material according to this embodiment, the conductive porous body preferably has pores with a pore diameter in the range of 1 to 100 nm. Furthermore, the percentage of the pore volume of pores with a pore diameter in the range of 1 to 4 nm relative to the pore volume of pores with a pore diameter in the range of 1 to 100 nm is preferably 20% or less. This allows the positive electrode active material and / or solid electrolyte to be easily retained deep inside the pores. Furthermore, even inside the pores, not only can electrons enter and exit the surface of the positive electrode active material via the conductive porous body, but also charge carriers can enter and exit the surface of the positive electrode active material via the solid electrolyte smoothly. As a result, even around the positive electrode active material located deep inside the pores, a three-phase interface where the positive electrode active material, the conductive porous body, and the solid electrolyte coexist is sufficiently formed, allowing the charge-discharge reaction to proceed smoothly. This is believed to further reduce the internal resistance of the battery. Herein, the pore distribution of the conductive porous body is determined using the BJH method. From the viewpoint of further exerting the effect of reducing internal resistance, the above percentage is more preferably 18% or less, even more preferably 15% or less, even more preferably 12% or less, and particularly preferably 9% or less. On the other hand, there is no particular restriction on the lower limit of the above percentage, but it is, for example, 3% or more.

[0066] The BET specific surface area of ​​the conductive porous body (preferably a carbon material) is 200 m 2 / g or more is preferable, and 500m 2 / g or more is more preferable, and 800m 2 / g or more is more preferable, and 1200m 2 / g or more is particularly preferred, and 1500m 2It is most preferable that it is / g or more. Further, the total pore volume of the conductive porous body (preferably a 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 the total pore volume of the conductive porous body are within such ranges, a sufficient amount of pores can be retained, and thus a sufficient amount of the positive electrode active material can be retained. Note that the values of the BET specific surface area and the total pore volume of the conductive porous body can be measured by nitrogen adsorption / desorption measurement. This nitrogen adsorption / desorption measurement is performed using BELSORP mini manufactured by MicrotracBEL Corporation at a temperature of -196°C by the multi-point method. The BET specific surface area is determined from the adsorption isotherm in the relative pressure range of 0.01 < P / P0 < 0.05. Also, the total pore volume is determined from the volume of adsorbed N2 at a relative pressure of 0.96.

[0067] The average pore diameter of the conductive porous body is not particularly limited, but is preferably 50 nm or less, and particularly preferably 30 nm or less. If the average pore diameter of the conductive porous body is within these ranges, electrons can be sufficiently supplied to the active material existing at a position far from the pore wall among the positive electrode active materials containing sulfur disposed inside the pores. Note that the value of the average pore diameter of the conductive porous body can be calculated by nitrogen adsorption / desorption measurement in the same manner as when obtaining the values of the BET specific surface area and the total pore volume.

[0068] When the conductive porous body is in the form of particles, the average particle diameter (primary particle diameter) 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. Note that the definition of the "particle diameter of the conductive porous body" and the measurement method of the "average particle diameter of the conductive porous body" are the same as those described above for the conductive aid.

[0069] As described above, the positive electrode material according to this embodiment includes a conductive porous body, a solid electrolyte, and an alkali metal sulfide as a positive electrode active material, and at least a portion of the solid electrolyte and at least a portion of the positive electrode active material are disposed in the pores of the conductive porous body.

[0070] FIG. 3(a) is a cross-sectional schematic diagram of a cathode material 100′ in Patent Document 1, a prior art. FIG. 3(b) is a cross-sectional schematic diagram of a cathode material 100 according to one embodiment of the present invention. In FIGS. 3(a) and 3(b), a conductive porous body (e.g., a carbon material) 110 has numerous pores 110a. In the method described in Patent Document 1, elemental sulfur 140, which is a raw material for alkali metal sulfide 120, is thermally impregnated into the pores 110a of the conductive porous body 110. The elemental sulfur 140 is then chemically reacted to form the alkali metal sulfide 120 on the surface of the conductive porous body 110. A solid electrolyte 130 is then added by mechanical mixing to obtain a cathode material 100′ having the alkali metal sulfide 120 and the solid electrolyte 130 on the surface of the conductive porous body 110. Alternatively, as described in JP 2013-80637 A, a compound serving as a raw material for the solid electrolyte 130 can be added and reacted to obtain a positive electrode material 100′ in which the alkali metal sulfide 120 and the solid electrolyte 130 are combined with the conductive porous body 110.

[0071] However, in the above method, the reaction of converting elemental sulfur 140 into alkali metal sulfide 120 mainly proceeds on the surface (region outside the pores) of the conductive porous body 110, and the elemental sulfur 140 impregnated inside the pores 110a of the conductive porous body 110 remains unreacted. Because elemental sulfur 140 is insulating, using the prior art positive electrode material 100' results in high electrode resistance.

[0072] Furthermore, in the above-described method, since the solid electrolyte 130 or its raw materials are added later, it is difficult to introduce the solid electrolyte 130 into the pores 110a of the conductive porous body 110. Therefore, in the positive electrode material 100′ shown in FIG. 3(a), a reaction region is formed on the surface of the conductive porous body 110, where the alkali metal sulfide 120 (positive electrode active material), the solid electrolyte 130, and the conductive porous body 110 coexist, and an electrode reaction proceeds near this region. However, the solid electrolyte 130 does not fill the pores 110a, and the charge / discharge reaction does not proceed sufficiently. This is thought to result in high electrode resistance.

[0073] In contrast, in the cathode material 100 according to one embodiment of the present invention shown in FIG. 3(b), the alkali metal sulfide 120 and solid electrolyte 130 serving as cathode active materials are disposed not only on the surface of the conductive porous body 110 but also on the inner surface of the pores 110a of the conductive porous body 110. On the surface of the alkali metal sulfide 120 located deep within the pores 110a, not only electrons but also charge carriers can smoothly enter and exit through the conductive porous body 110, but also through the solid electrolyte 130. As a result, even around the alkali metal sulfide 120 located deep within the pores 110a, a three-phase interface where the alkali metal sulfide 120, the conductive porous body 110, and the solid electrolyte 130 coexist is sufficiently formed, allowing charge and discharge reactions to proceed smoothly. As a result, the alkali metal sulfide present inside the pores 110a can also be utilized as an active material in the electrode reaction, which is believed to significantly reduce the internal resistance of the battery.

[0074] Here, whether or not a positive electrode active material or a solid electrolyte is disposed inside the pores of a conductive porous body can be confirmed using various conventionally known techniques. For example, elemental mapping of each material can be performed using energy dispersive X-ray spectroscopy (EDX) on a cross-sectional image of a conductive porous body observed with a transmission electron microscope (TEM). The arrangement of each material can be confirmed using the obtained elemental map and the counts of elements from each material relative to the counts of all elements as indicators. For example, if the solid electrolyte contains phosphorus atoms and / or boron atoms, and the phosphorus and / or boron atoms are unlikely to be derived from other materials, the above-mentioned elemental map for phosphorus and / or boron can be obtained, and the arrangement of the solid electrolyte can be confirmed from their distribution. The arrangement of the solid electrolyte can also be confirmed from the ratio of the counts of phosphorus and / or boron to the counts of all elements in EDX.

[0075] Furthermore, the positive electrode material according to this embodiment is characterized in that, in an X-ray diffraction measurement of the positive electrode material using CuKα radiation, the positive electrode material exhibits a peak A derived from the conductive porous body in a 2θ region of 20° or more and less than 25°, and a peak B derived from the alkali metal sulfide and / or solid electrolyte in a 2θ region of 25 to 29°, and the ratio of the intensity of the peak A to the intensity of the peak B (A / B) exceeds 0.02.

[0076] Here, Fig. 4(a) is a graph showing a spectrum obtained by performing X-ray diffraction measurement using CuKα radiation on the powder of the positive electrode material prepared in Example 4 described below. As shown in Fig. 4(a), the X-ray diffraction spectrum of the positive electrode material according to this embodiment first shows peak A, which is derived from the conductive porous body, in the 2θ region of 20° or more and less than 25°. It also shows peak B, which is derived from the alkali metal sulfide and / or solid electrolyte, in the 25 to 29° region. The ratio of the intensity of peak A to the intensity of peak B (A / B) exceeds 0.02.

[0077] The inventors' studies have revealed that constructing an all-solid-state lithium secondary battery using such a positive electrode material can further reduce the internal resistance of the secondary battery. Although the mechanism is not fully understood, the following mechanism is presumed. Specifically, the positive electrode material according to this embodiment can be produced, for example, by dissolving an alkali metal sulfide and a solid electrolyte, which are positive electrode active materials, in a solvent to obtain a solution, as described below, and then adding a conductive porous body to the solution to simultaneously impregnate the alkali metal sulfide and the solid electrolyte into the conductive porous body. In this case, the alkali metal sulfide and the solid electrolyte are uniformly introduced into the surface and pores of the conductive porous body, preventing excessive coverage of the surface of the conductive porous body. Therefore, it is believed that the wide-angle peak A derived from the conductive porous body is clearly observed. It is believed that the uniform introduction of the alkali metal sulfide and the solid electrolyte into the surface and pores of the conductive porous body increases the reaction area, thereby reducing resistance.

[0078] On the other hand, a positive electrode material can also be prepared by mechanically mixing the alkali metal sulfide 120, the solid electrolyte 130, and the conductive porous body 110. However, in Comparative Example 2, in which the positive electrode material was prepared by mechanical mixing, the A / B ratio was 0.02 or less, as shown in FIG. 4(b). In the positive electrode material 100″ prepared by mechanical mixing, the alkali metal sulfide 120 and the solid electrolyte 130 are not introduced into the pores 110a of the conductive porous body 110, as shown in FIG. 3(c), but are present in large quantities on the surface of the conductive porous body 110. This is thought to be why the intensity of the wide-angle peak A originating from the conductive porous body in the XRD spectrum is relatively small. Furthermore, in the case of mechanical mixing, the solid electrolyte is not introduced into the pores, which is thought to result in an insufficient ion conduction path and insufficient reduction of resistance. Furthermore, it is thought that the mechanical mixing may cause the solid electrolyte 130 to decay or denature, resulting in the generation of decay products or denature products 131, which may increase resistance.

[0079] In addition, even in the positive electrode materials according to conventional techniques such as Patent Document 1, alkali metal sulfide and solid electrolyte tend to be present in large amounts on the surface of the conductive porous body 110. Therefore, the intensity of the wide-angle peak A originating from the conductive porous body in the XRD spectrum becomes relatively small, and the above-mentioned specific A / B value is not achieved.

[0080] The A / B ratio is preferably 0.03 or more, more preferably 0.10 or more, and even more preferably 0.20 or more. Within this range, the effects of the present invention can be more pronounced. Meanwhile, the upper limit of the A / B ratio is not particularly limited, but is, for example, 100 or less, preferably 10 or less, and even more preferably 1 or less. The A / B ratio can be controlled, for example, by adjusting the ratio of the alkali metal sulfide, solid electrolyte, and conductive porous body in the production process in which the alkali metal sulfide, which is the positive electrode active material, and the solid electrolyte are simultaneously impregnated into the conductive porous body, as described below. Here, the intensities of Peak A and Peak B refer to the peak heights minus the baseline. Furthermore, when two or more peaks having the same attribution are observed in the specified region, the height of the highest peak is taken as the peak intensity.

[0081] The positive electrode material of this embodiment preferably does not have a peak derived from elemental sulfur in the X-ray diffraction spectrum.

[0082] The mass ratio of alkali metal sulfide to solid electrolyte in the positive electrode material of this embodiment is not particularly limited, but is, for example, alkali metal sulfide:solid electrolyte = 1:99 to 99:1. The mass ratio is preferably alkali metal sulfide:solid electrolyte = 50:50 to 1:99, more preferably 1:1 to 1:20. Within the above range, a sufficient amount of positive electrode active material can be secured, resulting in a high-performance battery. This is also preferable because it ensures the ionic conductivity of the positive electrode material and ensures the formation of a sufficient ionic conduction network within the pores. Furthermore, this is preferable because decomposition of the solid electrolyte due to an excessive amount of solid electrolyte is unlikely to occur.

[0083] The positive electrode material of this embodiment preferably has a pore filling rate of 50% or more, preferably 80% or more, and more preferably 100% or more, defined as the ratio of the volume of the positive electrode active material and solid electrolyte to the total pore volume of the conductive porous body. The effects of the present invention can be more pronounced within the above range. The upper limit of the pore filling rate is not particularly limited, but is, for example, 200% or less, preferably 170% or less. The pore filling rate can be determined by the method described in the Examples.

[0084] According to a preferred embodiment of the present invention, the mass ratio of alkali metal sulfide to solid electrolyte is alkali metal sulfide:solid electrolyte=1:1 to 1:5, and the pore filling rate is 100% or more. This embodiment is preferable because a sufficient ion conduction network is formed even within the pores. Furthermore, it is preferable because decomposition of the solid electrolyte due to an excessive amount of solid electrolyte is unlikely to occur. It is also preferable from the viewpoint of energy density.

[0085] In the positive electrode material of this embodiment, the mass ratio of the alkali metal sulfide to the conductive porous body is not particularly limited, but for example, the ratio (alkali metal sulfide / conductive porous body) is 0.5 to 5, and preferably 1 to 4. Within the above range, the effects of the present invention can be more significantly obtained.

[0086] Although there are no particular limitations on the manufacturing method of the cathode material according to this embodiment having the above-described configuration, it is preferable that the manufacturing method includes dissolving an alkali metal sulfide and a solid electrolyte in a solvent capable of dissolving both of them and impregnating a conductive porous body with the resulting solution. This allows the alkali metal sulfide and the solid electrolyte to be impregnated into the conductive porous body in a homogeneous mixture. Therefore, the alkali metal sulfide and the solid electrolyte are arranged in a homogeneous mixture on the surface and within the pores of the conductive porous body, thereby increasing the reaction area. An example of such a manufacturing method will be briefly described below.

[0087] First, a solution is prepared by dissolving an alkali metal sulfide and a solid electrolyte in a solvent capable of dissolving both of them.

[0088] In this specification, "a certain solid can be dissolved in a certain solvent" means that the solubility of the certain solid in the solvent is 0.1 g / 100 g solvent or more at normal pressure and 25° C. "Able to dissolve both a metal sulfide and a solid electrolyte" means that the solubilities of the metal sulfide and the solid electrolyte when dissolved separately in a solvent are both within the above ranges.

[0089] Examples of such solvents include alcohols such as methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, glycerin, capryl alcohol, lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, oleyl alcohol, and linolyl alcohol; ethers such as diethyl ether, tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane; etc. Alcohols are preferred because they have high solubility for alkali metal sulfides and solid electrolytes and are less likely to decompose.

[0090] The water content of the solvent is preferably 0.2% by mass or less. A water content of 0.2% by mass or less can prevent the water in the solvent from reacting with the alkali metal sulfide or solid electrolyte and decomposing. More preferably, the water content in the solvent is 0.1% by mass or less, even more preferably 0.05% by mass or less, even more preferably 0.02% by mass or less, even more preferably 0.01% by mass or less, even more preferably 0.005% by mass or less, particularly preferably 0.002% by mass or less, and most preferably 0.001% by mass or less. The water content in the solvent can be measured, for example, by Karl Fischer coulometric titration.

[0091] The route for obtaining a solvent with a low water content is not particularly limited. If a product with a low water content is commercially available, that product may be purchased and used, or a commercially available product with a relatively high water content may be purchased and then the water content of the product may be reduced. The method for reducing the water content of the solvent is also not particularly limited, and conventionally known knowledge may be referred to as appropriate. Examples include heat drying, drying under reduced pressure, drying with a desiccant (e.g., silica gel or sodium sulfate), and distillation.

[0092] Next, a conductive porous body is dispersed in the resulting solution to obtain a dispersion. The solvent is then removed from the dispersion while stirring, preferably under reduced pressure. Thereafter, the dispersion is heat-treated, preferably under reduced pressure, at a temperature of, for example, 60 to 250°C, preferably about 150 to 180°C, for, for example, 1 to 120 hours, preferably 1 to 10 hours. This allows for the production of a composite cathode material in which the cathode active material is disposed (filled) together with the solid electrolyte inside the pores of the conductive porous body.

[0093] The step of preparing a solution in which an alkali metal sulfide and a solid electrolyte are dissolved in a solvent capable of dissolving both of them, and the step of dispersing a conductive porous body in the solution to obtain a dispersion liquid are preferably carried out in an inert gas atmosphere with a controlled dew point.

[0094] The step of preparing a solution in which an alkali metal sulfide and a solid electrolyte are dissolved in a solvent capable of dissolving both of them, the step of dispersing a conductive porous body in the solution to obtain a dispersion, and the step of removing the solvent are preferably carried out at a temperature of 70° C. or less, and more preferably at a temperature of 20 to 50° C. This effectively suppresses decomposition of the solid electrolyte and reduces resistance.

[0095] 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 35 to 99 mass %, more preferably in the range of 40 to 90 mass %, for example. Note that this content value is calculated based on the mass of only the positive electrode active material, excluding the conductive porous body and solid electrolyte.

[0096] The positive electrode active material layer may further include a conductive additive (one that does not hold the positive electrode active material or solid electrolyte inside the pores) and / or a binder, and specific and preferred forms thereof may be similarly adopted 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 particularly preferably includes a sulfide solid electrolyte. Specific and preferred forms of the solid electrolyte, such as the sulfide solid electrolyte, may also be similarly adopted as those described in the section on the negative electrode active material layer above.

[0097] The thickness of the positive electrode active material layer varies depending on the intended configuration of the secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0098] [Positive and negative current collector plates] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plates. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive current collector plate 27 and the negative current collector plate 25 may be made of the same material or different materials.

[0099] [Positive and negative leads] Although not shown, the current collector and the current collecting plate may be electrically connected via a positive electrode lead and 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 leads. 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., automobile parts, particularly electronic devices).

[0100] [Battery exterior materials] As the battery exterior material, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element as shown in 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 to these. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0101] The stacked battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability, and is therefore suitable for use as a driving power source for EVs and HEVs.

[0102] 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.

[0103] For example, the type of battery to which the lithium secondary battery according to the present embodiment is applied includes a bipolar battery including a bipolar electrode having a positive electrode active material layer electrically coupled to one surface of a current collector and a negative electrode active material layer electrically coupled to the opposite surface of the current collector.

[0104] 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 is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution). [Example]

[0105] The present invention will be described in more detail below with reference to examples, although the technical scope of the present invention is not limited to the following examples.

[0106] <<Example of test cell production>> [Example 1] (Preparation of positive electrode material) In a glove box with an argon atmosphere at a dew point below -68°C, 0.167 g of lithium sulfide (LiS), a positive electrode active material, and 1.667 g of sulfide solid electrolyte (LiPSCl, manufactured by Ampcera) were added to 40 mL of ultra-dehydrated ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., water content 0.001% by mass or less) and stirred until no solid particles were visible, dissolving the positive electrode active material and solid electrolyte in the ethanol. To the resulting solution, 0.167 g of conductive porous carbon powder (Knobel® P(3)010, manufactured by Toyo Tanso Co., Ltd.) was added and stirred thoroughly to thoroughly disperse the porous carbon powder in the solution. The container containing the porous carbon powder dispersion was connected to a vacuum device, and the container was reduced in pressure to 1 Pa or less using an oil rotary pump while stirring the dispersion in the container with a magnetic stirrer. Since the solvent ethanol volatilizes under reduced pressure, the ethanol is removed over time, leaving the conductive porous body impregnated with the positive electrode active material and solid electrolyte in the container. After removing the ethanol under reduced pressure, the container was heated to 180°C under reduced pressure and heat-treated for 3 hours to prepare the positive electrode material.

[0107] The positive electrode material obtained above was subjected to powder X-ray diffraction measurement. The measurement was performed using a SmartLab manufactured by Rigaku Corporation under the following conditions: Scan speed (2θ / θ): 2° / min Step width (2θ / θ): 0.02° Source: CuKα:λ=1.5418Å.

[0108] In the XRD spectrum of the positive electrode material obtained in this example, a wide-angle peak A derived from carbon was observed in the 2θ region of 20° or more and less than 25°, and a peak B derived from Li2S was observed in the 2θ region of 20 to 25°, and the intensity ratio (A / B) of these peaks was 0.25.

[0109] The pore filling rate of the positive electrode material obtained above was also determined. The pore filling rate is the ratio of the volume occupied by the positive electrode active material and solid electrolyte to the total pore volume of the conductive porous body, and was estimated from nitrogen adsorption / desorption measurements and the material loading ratio. Nitrogen adsorption / desorption measurements were performed using a Microtrack-Bell BELSORP mini at a temperature of -196°C using the multipoint method. The total pore volume was calculated from the volume of N2 adsorbed at a relative pressure of 0.96. The results are shown in Table 1.

[0110] (Preparation of positive electrode mixture) In a glove box in an argon atmosphere with a dew point of −68° C. or lower, 40 g of 5 mm diameter zirconia balls, 0.130 g of the positive electrode material prepared above, and 0.070 g of a solid electrolyte (Li6PS5Cl, manufactured by Ampcera) were placed in a 45 mL zirconia container, and the mixture was 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 positive electrode mixture.

[0111] (Production of test cells (all-solid-state lithium secondary batteries)) The battery was fabricated in a glove box with an argon atmosphere at a dew point of −68° C. or lower.

[0112] A 10 mm diameter stainless steel cylindrical convex punch was inserted into one side of a McCor cylindrical tube jig (10 mm inner diameter, 23 mm outer diameter, 20 mm height). 80 mg of sulfide solid electrolyte (Li6PS5Cl, manufactured by Ampcera) was placed into the cylindrical tube jig from the top. Another SUS cylindrical convex punch was then inserted to sandwich the solid electrolyte. The tube was then pressed for 3 minutes at 75 MPa using a hydraulic press to form a 10 mm diameter, approximately 0.6 mm thick solid electrolyte layer in the cylindrical tube jig. The cylindrical convex punch inserted from the top was then temporarily removed, and 7.5 mg of the prepared cathode mixture was placed on one side of the solid electrolyte layer inside the cylindrical tube. A new cylindrical convex punch (also serving as a cathode current collector) was inserted from the top and pressed for 3 minutes at 300 MPa to form a 10 mm diameter, approximately 0.06 mm thick cathode active material layer on one side of the solid electrolyte layer. Next, the lower cylindrical convex punch (which also served as the negative electrode current collector) was removed, and a lithium foil (manufactured by Nilaco Corporation, thickness 0.20 mm) punched to a diameter of 8 mm was stacked on top of an indium foil (manufactured by Nilaco Corporation, thickness 0.30 mm) punched to a diameter of 9 mm as the negative electrode.The cylindrical tube jig was inserted from the bottom so that the indium foil was positioned on the solid electrolyte layer side, and the cylindrical convex punch was inserted again.Then, the lithium-indium negative electrode was formed by pressing at a pressure of 75 MPa for 3 minutes.

[0113] In this manner, 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.

[0114] [Example 2] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the positive electrode material, the amounts of lithium sulfide, sulfide solid electrolyte, and porous carbon powder were changed to 0.174 g, 1.739 g, and 0.087 g, respectively.

[0115] In the XRD spectrum of the positive electrode material prepared in Example 2, the ratio of the intensity of peak A to the intensity of peak B (A / B) was 0.22.

[0116] [Example 3] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1 above, except that in the preparation of the positive electrode material, the amounts of lithium sulfide, sulfide solid electrolyte, and porous carbon powder were changed to 0.667 g, 1.000 g, and 0.333 g, respectively.

[0117] In the XRD spectrum of the positive electrode material prepared in Example 3, the ratio of the intensity of peak A to the intensity of peak B (A / B) was 0.33.

[0118] [Example 4] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1, except that in the preparation of the positive electrode material, the amounts of lithium sulfide, sulfide solid electrolyte, and porous carbon powder were changed to 0.727 g, 1.091 g, and 0.182 g, respectively.

[0119] 4(a) shows the results of measuring the XRD spectrum of the positive electrode material prepared in Example 4. The ratio of the intensity of Peak A to the intensity of Peak B (A / B) was 0.20.

[0120] [Example 5] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1 above, except that in the preparation of the positive electrode material, the amount of lithium sulfide was changed to 1.000 g, the amount of sulfide solid electrolyte was changed to 0.500 g, and the amount of porous carbon powder was changed to 0.500 g.

[0121] In the XRD spectrum of the positive electrode material prepared in Example 5, the ratio of the intensity of peak A to the intensity of peak B (A / B) was 0.15.

[0122] [Example 6] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1 above, except that in the preparation of the positive electrode material, the amounts of lithium sulfide, sulfide solid electrolyte, and porous carbon powder were changed to 1.143 g, 0.571 g, and 0.286 g, respectively.

[0123] In the XRD spectrum of the positive electrode material prepared in Example 6, the ratio of the intensity of peak A to the intensity of peak B (A / B) was 0.03.

[0124] [Comparative Example 1] In a glove box with an argon atmosphere and a dew point of -68°C or less, 6.104 g of sulfur (manufactured by Aldrich) and 1.896 g of carbon (Kansai Coke and Chemicals Co., Ltd., activated carbon, P(3)010) were thoroughly mixed in an agate mortar and then mixed for 15 minutes in a planetary ball mill. The mixed powder was placed in a sealed pressure-resistant autoclave and heated at 170°C for 3 hours to melt the sulfur and impregnate the carbon. This yielded a sulfur / carbon composite.

[0125] To the obtained sulfur / carbon composite material, lithium triethylborohydride was added and reacted according to Example 1 of JP 2013-80637 A, to obtain a lithium sulfide / carbon composite. Diphosphorus pentasulfide was added to the obtained lithium sulfide / carbon composite and reacted to obtain a positive electrode material.

[0126] In the XRD spectrum of the positive electrode material prepared in Comparative Example 1, the ratio of the intensity of Peak A to the intensity of Peak B (A / B) was 0.01.

[0127] Comparative Example 2 In a glove box with an argon atmosphere and a dew point of -68°C or lower, 40 g of 5 mm diameter zirconia balls, 0.667 g of lithium sulfide, 1.000 g of solid electrolyte (Li6PS5Cl manufactured by Ampcera), and 0.333 g of porous carbon powder (Knobel® P(3)010 manufactured by Toyo Tanso Co., Ltd.) were placed in a 45 ml zirconia container and processed at 370 rpm for 6 hours in a planetary ball mill to obtain a positive electrode material consisting of a lithium sulfide / solid electrolyte / carbon mixed powder. An all-solid-state lithium secondary battery was fabricated using the same method as in Example 1, except for the above.

[0128] 4(b) shows the results of measuring the XRD spectrum of the positive electrode material prepared in Comparative Example 2. The ratio of the intensity of Peak A to the intensity of Peak B (A / B) was 0.02.

[0129] <Test cell evaluation example> The internal resistance of the test cells prepared in the above comparative examples and examples was measured by the following method. All of the following measurements were carried out using a charge / discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a constant temperature bath set at 25°C.

[0130] (internal resistance measurement) The measurement device used was a high-performance electrochemical measurement system, product name "SP-200" manufactured by Bio-Logic SAS (France), and impedance was measured as the electrochemical characteristics under measurement conditions of a frequency range of 7 MHz to 0.1 Hz, temperature conditions of room temperature, and applied voltage of 50 mV. The test cell was placed in a thermostatic chamber, and evaluation was carried out after the cell temperature became constant. The results are shown in Table 1 below.

[0131] [Table 1]

[0132] The results shown in Table 1 show that, according to the present invention, the internal resistance can be further reduced in an all-solid-state lithium secondary battery using an alkali metal sulfide as the positive electrode active material. [Explanation of symbols]

[0133] 10a stacked battery, 11' negative electrode current collector, 11” positive electrode current collector, 13 negative electrode active material layer, 15 positive electrode active material layer, 17 solid electrolyte layer, 19 cell layer, 21 power generation elements, 25 negative current collector plate, 27 positive current collector plate, 29 Laminating film, 100, 100', 100" cathode material, 110 Conductive porous body, 110a pores, 120 Alkali metal sulfide (positive electrode active material), 130 solid electrolyte, 131 Decay products or modifications of solid electrolytes; 140 Elemental sulfur (inert sulfur).

Claims

1. A positive electrode material comprising a carbon material that is a conductive porous body, lithium sulfide that is a positive electrode active material, and a solid electrolyte, at least a portion of the lithium sulfide and at least a portion of the solid electrolyte are disposed in the pores of the conductive porous body; In X-ray diffraction measurement using CuKα radiation, the positive electrode material exhibits a peak A derived from the conductive porous body in a region where 2θ is equal to or greater than 20° and less than 25°, and a peak B derived from the lithium sulfide and / or the solid electrolyte in a region where 2θ is 25 to 29°, the ratio of the intensity of Peak A to the intensity of Peak B (A / B) is greater than 0.02; A positive electrode material, characterized in that a mass ratio of the lithium sulfide to the solid electrolyte is lithium sulfide:solid electrolyte=50:50 to 1:

99.

2. The positive electrode material described in claim 1, wherein the solid electrolyte contains lithium.

3. 3. The cathode material according to claim 1, wherein the solid electrolyte comprises at least P, Li, and S.

4. 4. The positive electrode material according to claim 1, wherein a mass ratio of the lithium sulfide to the solid electrolyte is lithium sulfide:solid electrolyte=1:1 to 1:

20.

5. A secondary battery comprising the positive electrode material according to any one of claims 1 to 4.

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