Positive electrode material and secondary battery using the same

Abstract

The purpose of the present invention is to provide means for further reducing the internal resistance of a secondary battery using a positive electrode active material containing sulfur. The present invention provides a positive electrode material including a porous carbon material, a sulfur-containing positive electrode active material, and a lithium halide material, the positive electrode material being characterized in that at least part of the positive electrode active material and at least part of the lithium halide are disposed within the pores of the porous carbon material.

Description

Positive electrode material and secondary battery using the same

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

[0002] In recent years, there has been a strong demand for a reduction in carbon dioxide emissions to combat global warming. In the automotive industry, there are high hopes for a reduction in carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and active development of non-aqueous electrolyte secondary batteries, such as secondary batteries for driving motors, is being carried out, which hold the key to the practical application of these vehicles.

[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 into all-solid-state lithium secondary batteries that use oxide-based or sulfide-based solid electrolytes has been actively conducted. A solid electrolyte is a material that is mainly composed of an ion conductor that can conduct ions in a solid state. Therefore, in principle, all-solid-state lithium secondary batteries do not encounter the various problems caused by flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, generally, the use of high-potential, large-capacity positive electrode materials and large-capacity negative electrode materials can significantly improve the output density and energy density of the 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.

[0006] On the other hand, elemental sulfur has high resistance, so it is difficult to ensure sufficient charge / discharge capacity when used as a positive electrode material at a practical current. To address this problem, JP 2010-95390 A proposes a technology for using a mesoporous carbon composite material containing at least mesoporous carbon and sulfur disposed in the mesopores of the mesoporous carbon as a positive electrode material for an all-solid-state battery. According to JP 2010-95390 A, by using a positive electrode material having such a configuration, the electron conductivity can be improved by finely dividing the sulfur and forming a composite with the mesoporous carbon, thereby improving the battery characteristics.

[0007] Depending on the application of the secondary battery, it is required that sufficient capacity can be extracted during charging and discharging at a high charge / discharge rate (i.e., sufficient charge / discharge rate characteristics). For example, a secondary battery with insufficient charge / discharge rate characteristics cannot utilize sufficient capacity in response to rapid charging and discharging. In order to improve the charge / discharge rate characteristics of a secondary battery, it is necessary to reduce the internal resistance of the battery. However, according to the study by the present inventors, it was found that it is difficult to sufficiently reduce the internal resistance even with the technology disclosed in JP 2010-95390 A.

[0008] 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.

[0009] 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 positive electrode material in which a lithium halide is contained in the pores of a porous carbon material together with a positive electrode active material containing sulfur, thereby completing the present invention.

[0010] One aspect of the present invention is a positive electrode material comprising a porous carbon material, a positive electrode active material containing sulfur, and a lithium halide, wherein at least a portion of the positive electrode active material and at least a portion of the lithium halide are disposed in pores of the porous carbon material.

[0011] FIG. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. 1. FIG. 3(a) is a cross-sectional schematic diagram of a conventional cathode material, and FIG. 3(b) is a cross-sectional schematic diagram of a cathode material according to one embodiment of the present invention. FIG. 4 is an XRD spectrum of the cathode material prepared in Example 1. FIG. 5 is an XRD spectrum of the cathode material prepared in Example 6. FIG. 6 is an XRD spectrum of the cathode material prepared in Comparative Example 1. FIG. 7 is an XRD spectrum of the cathode material prepared in Comparative Example 2.

[0012] 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 of not, in principle, encountering various problems associated with flammable organic electrolytes, as in conventional liquid-based lithium secondary batteries. Furthermore, the use of a high-potential, high-capacity positive electrode material and a high-capacity negative electrode material generally has the advantage of significantly improving the output density and energy density of the battery.

[0013] One aspect of the present invention is a cathode material comprising a porous carbon material, a cathode active material containing sulfur, and a lithium halide, wherein at least a portion of the cathode active material and at least a portion of the lithium halide are disposed in pores of the porous carbon material. The cathode material according to this aspect can further reduce the internal resistance of a secondary battery.

[0014] 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.

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

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

[0017] The 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.

[0018] There are also no particular limitations on the way 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. The present invention is not limited to what is shown in Fig. 1. In addition, in a wound-type lithium battery, terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

[0019] 2, the stacked battery 10a of this embodiment has a structure in which a flat, generally rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior material. Here, the power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are disposed on both sides of a positive electrode current collector 11". The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order such that one positive electrode active material layer 15 faces 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.

[0020] 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 layer current 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 the positive electrode, respectively, without using the current collectors (11′, 11″).

[0021] A negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the electrodes (positive and negative electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of a laminate film 29 that is a battery exterior material and led out of the laminate film 29. The positive electrode current collector 27 and the negative electrode current collector 25 may be attached to the positive electrode current collector 11″ and the negative electrode current collector 11′ of the electrodes by ultrasonic welding, resistance welding, or the like, respectively, via a positive electrode lead and a negative electrode lead (not shown) as necessary.

[0022] The main components of the lithium secondary battery according to this embodiment will be described below.

[0023] [Current Collector] The current collector has a function of mediating the transfer of electrons from the electrode active material layer (positive electrode active material layer or negative 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.

[0024] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. A foil having a metal surface coated with aluminum 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. Examples of conductive resins include resins containing a conductive filler added to a non-conductive polymer material.

[0025] 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.

[0026] [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 includes carbon materials, metal oxides, and metal active materials. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of metal oxides include Nb ２ O ５ , Li ４ Ti ５ O １２Examples include: silicon-based negative electrode active materials and tin-based negative electrode active materials. Silicon and tin belong to Group 14 elements and are known to be negative electrode active materials that can significantly improve the capacity of nonaqueous electrolyte secondary batteries. These elements can absorb and release a large number of charge carriers (such as lithium ions) per unit volume (mass), resulting in high-capacity negative electrode active materials. As the silicon-based negative electrode active material, it is preferable to use Si as the negative electrode active material. Similarly, it is also preferable to use silicon oxides such as SiOx (0.3≦x≦1.6) disproportionated into two phases: a Si phase and a silicon oxide phase. In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, silicon-containing alloys (silicon-containing alloy-based negative electrode active materials) may be used. On the other hand, examples of negative electrode active materials containing tin (tin-based negative electrode active materials) include simple Sn, tin alloys (Cu—Sn alloys, Co—Sn alloys), amorphous tin oxides, and tin silicon oxides. ０．４ P ０．６ O ３．１ Examples of tin silicon oxide include SnSiO ３ Examples include: anode active materials such as lithium-containing metals; and anode active materials such as lithium-containing metals may be used as the anode active material. The anode 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 anode active materials may be used in combination. Of course, anode active materials other than those described above may also be used. The anode active material preferably contains metallic lithium, a silicon-based anode active material, or a tin-based anode active material, and particularly preferably contains metallic lithium.

[0027] 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.

[0028] The negative electrode active material layer preferably further contains a solid electrolyte. By including the solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes, and sulfide solid electrolytes are preferred. In this specification, the solid electrolyte refers to a material mainly composed of an ion conductor capable of ion conduction in a solid, and in particular, a material having a lithium ion conductivity of 1×10 at room temperature (25° C.). −５ The ionic conductivity of a material is defined as a value of 0.05 S / cm or more. The ionic conductivity can be measured by an AC impedance method.

[0029] Examples of sulfide solid electrolytes include LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ２ S-P ２ S ５ , LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ , Li ２ S-P ２ S ５ - 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 -B２ S ３ - LiI, Li ２ S-SiS ２ -P ２ S ５ - LiI, Li ２ S-B ２ S ３ , Li ２ S-P ２ S ５ -Z ｍ S ｎ (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li ２ S-GeS ２ , Li ２ S-SiS ２ -Li ３ P.O. ４ , Li ２ S-SiS ２ -Li ｘ MO ｙ (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In). ２ S-P ２ S ５ " is written by Li ２ S and P ２ S ５ The same applies to other descriptions.

[0030] The sulfide solid electrolyte is, for example, Li ３ P.S. ４ It may have a Li framework. ４ P ２ S ７ It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a framework include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li ４ P２ S ７ Examples of the sulfide solid electrolyte having a skeleton include a Li-P-S solid electrolyte called LPS (e.g., Li ７ P ３ S １１ ) can be mentioned. In addition, examples of sulfide solid electrolytes include Li （４−ｘ） Ge （１−ｘ） P ｘ S ４ (x satisfies 0<x<1), etc. may be used. Among them, a sulfide solid electrolyte containing a P element is preferable, and Li ２ S-P ２ S ５ It is more preferable that the sulfide solid electrolyte is a material mainly composed of Li. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte is a material mainly composed of Li. ６ P.S. ５ X, where X is Cl, Br or I, preferably Cl.

[0031] In addition, the sulfide solid electrolyte is Li ２ S-P ２ S ５ In the case of the system, Li ２ S and P ２ S ５ The ratio of Li is the molar ratio. ２ S:P ２ S ５ It is preferable that the ratio is in the range of 50:50 to 100:0, and among these, Li ２ S:P ２ S ５ It is preferable that the ratio is 70:30 to 80:20.

[0032] 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 mechanical milling (ball mill, etc.) a 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 ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25°C) is, for example, 1 × 10 −５ S / cm or more, and preferably 1×10 −４ It is more preferable that the viscosity is 200 S / cm or more.

[0033] Examples of oxide solid electrolytes include compounds having a NASICON structure. An example of a compound having a NASICON structure is a compound represented by the general formula Li １＋ｘ Al ｘ Ge ２−ｘ (P.O. ４ ) ３ (0≦x≦2) (LAGP), a compound represented by the general formula Li １＋ｘ Al ｘ Ti ２−ｘ (P.O. ４ ) ３ (0≦x≦2) (LATP) and the like. Another example of the oxide solid electrolyte is LiLaTiO (for example, Li ０．３４ La ０．５１ TiO ３ ), LiPON (e.g., Li ２．９ P.O. ３．３ N ０．４６ ), LiLaZrO (e.g., Li ７ La ３ Zr ２ O １２ ) etc.

[0034] 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 %.

[0035] 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.

[0036] Examples of conductive additives include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals, carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNT), and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). Also, particulate ceramic materials or resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive additives. Among these conductive additives, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon, more preferably at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon, and even more preferably at least one carbon. These conductive additives may be used alone or in combination of two or more.

[0037] 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.

[0038] On the other hand, the binder is not particularly limited, and examples thereof include the following materials: polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, thermoplastic polymers such as styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychloro Fluorine resins such as trifluoroethylene (PCTFE), 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), 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 preferred.

[0039] 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.

[0040] [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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 1 and 2, the cathode active material layer includes a cathode material according to one embodiment of the present invention. The cathode material includes a porous carbon material, a cathode active material containing sulfur, and a lithium halide. At least a portion of the cathode active material and at least a portion of the lithium halide are disposed in the pores of the porous carbon material.

[0045] (Sulfur-containing cathode active material) The type of sulfur-containing cathode active material is not particularly limited, but includes elemental sulfur (S), as well as particles or thin films of organic sulfur compounds or inorganic sulfur compounds. Any substance can be used as long as it utilizes the oxidation-reduction reaction of sulfur 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), polycarbon sulfides, and the like, as typified by the compounds described in International Publication No. 2010 / 044437. Among these, disulfide compounds, sulfur-modified polyacrylonitriles, and rubeanic acid are preferred, and sulfur-modified polyacrylonitriles are particularly preferred. As disulfide compounds, dithiobiurea derivatives, those having a thiourea group, thioisocyanate, or thioamide group 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 the mixture under an inert gas or under reduced pressure. Its 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 a Raman spectrum of 1330 cm −１ and 1560 cm −１ There is a strong peak signal near 307 cm −１ , 379 cm −１ , 472 cm −１ , 929 cm −１ On the other hand, inorganic sulfur compounds are preferred because of their excellent stability. Specifically, elemental sulfur (S), S-carbon composite, TiS ２ , TiS ３ , TiS ４ , NiS, NiS ２ , CuS, FeS ２ , Li ２ S, MoS ２ , MoS ３ , MnS, MnS ２ , CoS, CoS２ Among them, elemental sulfur (S), TiS ２ , TiS ３ , TiS ４ , FeS ２ and MoS ２ are preferred, and elemental sulfur (S), TiS ２ and FeS ２ is more preferable, and elemental sulfur (S) is particularly preferable from the viewpoint of high capacity. ８ Alpha, beta, or gamma sulfur having the structure may be used.

[0046] The positive electrode material according to this embodiment may further include a positive electrode active material that does not contain sulfur (another positive electrode active material) in addition to the sulfur-containing positive electrode active material. Examples of the other positive electrode active material include LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ Examples include:

[0047] 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 sulfur-containing positive electrode active material in 100% by mass of the total amount 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.

[0048] (Lithium Halide) The positive electrode material according to the present embodiment contains lithium halide (LiX, where X is a halogen atom). Examples of lithium halide include lithium fluoride, lithium chloride, lithium bromide, and lithium iodide. In the positive electrode material according to the present embodiment, lithium chloride or lithium iodide is preferred because of its excellent ionic conductivity, and lithium iodide is more preferred. Two or more types of lithium halides may be used in combination.

[0049] (Porous Carbon Material) The positive electrode material according to the present embodiment essentially contains a carbon material having pores (porous carbon material). The specific form of the porous carbon material contained in the positive electrode material according to the present embodiment is not particularly limited as long as it is a carbon material having pores, and conventionally known materials can be appropriately adopted.

[0050] Examples of porous 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 of 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 porous carbon material can be varied by appropriately adjusting the particle size of the mold and the compounding ratio of the carbon raw materials.

[0051] It is preferable that the porous 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 material can tolerate impurities of about 2 to 3 mass % or less.

[0052] The BET specific surface area of ​​the porous carbon material is not particularly limited, but is preferably 200 m ２ / g or more, and ２ / g or more is more preferable, and 800m ２ / g or more, and more preferably 1200m ２ / g or more is particularly preferred, and 1500m ２ / g or more is most preferable. The total pore volume of the porous 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 porous carbon material are within these ranges, a sufficient amount of pores can be retained, and therefore a sufficient amount of positive electrode active material can be retained. The BET specific surface area and total pore volume of the porous carbon material can be measured by nitrogen adsorption / desorption measurement. This nitrogen adsorption / desorption measurement is performed using a BELSORP mini manufactured by Microtrac-Bell Corporation, at a temperature of -196°C using a multi-point method. 0.01<P / P ０ The BET specific surface area is determined from the adsorption isotherm in the range of relative pressure <0.05. The total pore volume is calculated from the adsorption N at a relative pressure of 0.96. ２ Calculate from the volume.

[0053] The average pore diameter of the porous carbon material is not particularly limited, but is preferably 100 nm or less, more preferably 50 nm or less, and particularly preferably 30 nm or less. When the average pore diameter of the porous carbon material is within these ranges, the contact area with the positive electrode active material or lithium halide can be increased, thereby increasing the reaction area. Furthermore, sufficient electrons can be supplied to the active material located far from the pore walls among the sulfur-containing positive electrode active material disposed inside the pores. As a result, the effects of the present invention can be more significantly achieved. The average pore diameter of the porous carbon material can be calculated by nitrogen adsorption / desorption measurement, similar to the case of determining the BET specific surface area and total pore volume. In this specification, the pore distribution of the porous carbon material obtained using the BJH method is used. The lower limit of the average pore diameter of the porous carbon material is not particularly limited, but is, for example, 1 nm or more.

[0054] In the positive electrode material according to this embodiment, the porous carbon material 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 lithium halide to be easily retained deep inside the pores. Furthermore, even inside the pores, not only the movement of electrons through the porous carbon material but also the movement of charge carriers through the lithium halide can proceed smoothly on the surface of the positive electrode active material. 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 porous carbon material, and the lithium halide coexist is sufficiently formed, allowing the charge-discharge reaction to proceed sufficiently. As a result, it is believed that the internal resistance of the battery is further reduced. From the viewpoint of further exerting the effect of reducing the internal resistance, the above percentage is more preferably 18% or less, even more preferably 15% or less, still 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.

[0055] The average particle diameter (primary particle diameter) of the porous carbon material 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. In this specification, the "particle diameter of the porous carbon material" refers to the longest distance L between any two points on the contour line of a particle of the porous carbon material. The value of the "average particle diameter of the porous carbon material" is calculated as the arithmetic mean value of particle diameters 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).

[0056] As described above, the positive electrode material according to this embodiment includes a porous carbon material, a positive electrode active material containing sulfur, and a lithium halide, and at least a portion of the positive electrode active material and at least a portion of the lithium halide are disposed in the pores of the porous carbon material.

[0057] Fig. 3(a) is a cross-sectional view of a conventional cathode material 100'. Fig. 3(b) is a cross-sectional view of a cathode material 100 according to one embodiment of the present invention. In Fig. 3(a) and Fig. 3(b), a porous carbon material 110 has a large number of pores 110a, and the pores 110a are filled with a cathode active material 120 containing sulfur.

[0058] However, when an electrode is fabricated using the cathode material 100′ shown in FIG. 3A, the reaction resistance of the electrode reaction is high. This is because the cathode active material 120 containing sulfur has low conductivity, and in addition, during the electrode reaction, the cathode active material 120 generates Li, which has low ionic conductivity, near the interface 140 with the solid electrolyte 130. ２ This is thought to be because S is generated, which increases the resistance to the diffusion of lithium ions in the positive electrode material.

[0059] In contrast, the cathode material 100 of the present invention has the lithium halide 150 inside the pores 110a of the porous carbon material 110. This allows for the establishment of an ion conduction path in the cathode active material 120 that is favorable for the movement of lithium ions, and allows the lithium ions to be efficiently introduced into the pores 110a of the porous carbon material 110 via the lithium halide 150. At the surface of the cathode active material 120 located deep within the pores 110a, not only can electrons move in and out via the porous carbon material 110, but also lithium ions can move in and out smoothly from the solid electrolyte 130 via the lithium halide 150. Therefore, in the cathode material 100 of the present invention, a reaction region in which the cathode active material 120, the lithium halide 150, and the porous carbon material 110 coexist is formed not only on the surface of the porous carbon material 110 but also inside the pores 110a, allowing the electrode reaction to proceed sufficiently. As a result, the positive electrode active material 120 present inside the pores 110a can also be used as an active material in the electrode reaction, and it is believed that the internal resistance of the battery is sufficiently reduced.

[0060] Furthermore, the charge / discharge reaction of a battery using a general cathode material containing sulfur is expressed as shown in the following formula (1), whereas the charge / discharge reaction of a battery using the cathode material of the present invention is expressed as shown in the following formula (2). That is, since more electrons participate in the reaction in formula (2), electron exchange is promoted, and the reaction of sulfur is promoted, which is thought to enable the battery reaction to proceed more efficiently.

[0061]

[0062] Here, whether or not the positive electrode active material or lithium halide is arranged inside the pores of the porous carbon material can be confirmed by various conventionally known methods. For example, an image of a cross section of the porous carbon material observed with a transmission electron microscope (TEM) is subjected to element mapping of each material using energy dispersive X-ray spectroscopy (EDX), and the arrangement of each material can be confirmed using the obtained element map and the count number of the element from each material relative to the count number of all elements as indicators.

[0063] In the positive electrode material according to this embodiment, it is preferable that the lithium halide is present in a larger amount inside the pores of the porous carbon material than outside the pores (i.e., on the surfaces of the particles of the porous carbon material). This can more advantageously establish a conduction path for lithium ions in the positive electrode material, thereby further reducing the internal resistance of the battery.

[0064] Furthermore, in the positive electrode material according to the present embodiment, in an X-ray diffraction measurement using CuKα radiation of the positive electrode material, a crystalline peak that is not attributable to any of the porous carbon material, the sulfur-containing positive electrode active material, and the lithium halide is preferably present between the two most intense peaks attributable to the lithium halide. That is, it is preferable that a new peak that is not attributable to any of the raw materials (starting materials) is present. Although the details are unknown, it is believed that the crystalline structure that gives this new peak favors ion migration and contributes to reducing resistance.

[0065] In the positive electrode material of the present invention, the content of lithium halide relative to the total mass of the positive electrode material is, for example, 4% by mass or more, and preferably 5% by mass or more but less than 50% by mass. When the content of lithium halide is within the above range, a sufficient resistance reduction effect can be easily obtained. More preferably, the content of lithium halide is 5 to 30% by mass, even more preferably 10 to 30% by mass, and even more preferably 20 to 30% by mass. When two or more types of lithium halides are used in combination, it is preferable that the total amount is within the above range.

[0066] The mass ratio of the sulfur-containing cathode active material to the lithium halide in the cathode material of this embodiment is not particularly limited, but may be, for example, cathode active material:lithium halide = 100:1 to 100:10000. The mass ratio is preferably cathode active material:lithium halide = 100:1 to 100:100, more preferably 100:1 to 100:50, and even more preferably 100:5 to 100:45. Within the above range, a sufficient amount of cathode active material can be secured, resulting in a high-performance battery. Furthermore, this is preferable because it ensures the ionic conductivity of the cathode material and allows a sufficient ionic conduction network to be formed within the pores.

[0067] In the positive electrode material of this embodiment, the mass ratio of the sulfur-containing positive electrode active material to the porous carbon material is not particularly limited, but for example, the ratio (sulfur-containing positive electrode active material / porous carbon material) is 0.5 to 10, and preferably 2 to 8. Within the above range, the effects of the present invention can be more significantly obtained.

[0068] In addition, although not particularly limited, it is preferable that the pores of the porous carbon material are not filled with a solid electrolyte in the positive electrode material of this embodiment. Solid electrolytes are expensive and highly reactive with moisture. Therefore, handling a solid electrolyte requires an environment that is not exposed to the atmosphere, such as a low dew point (dew point < -60°C), and a glove box or super dry room must be used. The positive electrode material of this embodiment can establish an ion conduction path favorable for lithium ion migration even without using a solid electrolyte by disposing a lithium halide together with a positive electrode active material inside the pores of the porous carbon material. Furthermore, not using a solid electrolyte allows for inexpensive preparation in an atmospheric environment, simplifying the equipment and surrounding environment. In the positive electrode material of this embodiment, the content of the solid electrolyte among the components filling the pores of the porous carbon material is preferably 3% by mass or less, more preferably 2% by mass or less, even more preferably 1% by mass or less, and most preferably 0% by mass.

[0069] In the positive electrode material of this embodiment, it is preferable that only the sulfur-containing positive electrode active material and the lithium halide are filled into the pores of the porous carbon material. By doing so, the effects of the present invention can be more significantly achieved. In the positive electrode material of this embodiment, the total content of the sulfur-containing positive electrode active material and the lithium halide among the components filled into the pores of the porous carbon material is preferably 95 mass%, more preferably 99 mass% or more, and most preferably 100 mass%.

[0070] The method for producing the cathode material according to the present embodiment having the above-described configuration is not particularly limited, but the following (Production Method 1) or (Production Method 2) can be preferably used. In particular, (Production Method 1) is more effective in reducing the reaction resistance.

[0071] (Production Method 1) In Production Method 1, first, lithium halide is introduced into the pores of a porous carbon material by solution impregnation. Next, the resulting porous carbon material having lithium halide in the pores is further impregnated with a sulfur-containing cathode active material by thermal impregnation to place the lithium halide in the pores of the porous carbon material. Specifically, first, a solution is prepared by dissolving lithium halide in a solvent, and then the porous carbon material is dispersed therein to obtain a dispersion. Examples of solvents include water; 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; and ethers such as diethyl ether, tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Due to the high solubility of lithium halide, water or alcohols are preferred as the solvent, with alcohols being more preferred. Thereafter, after removing the solvent, the mixture is subjected to a heat treatment at a temperature of about 150 to 250°C for about 1 to 5 hours. This results in a porous carbon material in which the pores are impregnated with lithium halide. Next, the porous carbon material in which the pores are impregnated with lithium halide is dry-mixed with a positive electrode active material, and further subjected to a heat treatment under the same conditions as above. This causes the positive electrode active material to melt and penetrate into the pores of the porous carbon material, and a positive electrode material can be obtained as a composite material in which the positive electrode active material is disposed (filled) together with lithium halide in the pores of the porous carbon material.

[0072] (Manufacturing Method 2) In Manufacturing Method 2, first, a sulfur-containing cathode active material is introduced into the pores of a porous carbon material by thermal impregnation. Next, the resulting porous carbon material having the cathode active material inside the pores is further impregnated with a lithium halide solution to dispose the cathode active material inside the pores of the porous carbon material. Specifically, first, the porous carbon material is dry-mixed with the cathode active material and then heat-treated at a temperature of approximately 150 to 250°C for approximately 1 to 5 hours. This results in a porous carbon material in which the cathode active material is impregnated inside the pores. Next, a solution is prepared by dissolving lithium halide in a solvent, and the porous carbon material in which the cathode active material is impregnated inside the pores is dispersed therein to obtain a dispersion. Specific examples of the solvent are the same as above. Then, the solvent is removed and the mixture is heat-treated under the same conditions as above. This results in a cathode material as a composite material in which the cathode active material and lithium halide are disposed (filled) inside the pores of the porous carbon material.

[0073] Since the pore size of the porous carbon material is nanoscale, simply mechanically mixing the lithium halide and the porous carbon material in the solid state does not allow the lithium halide to be placed inside the pores of the porous carbon material. In this case, the lithium halide only adheres to the surface of the particles of the porous carbon material. Similarly, mechanically mixing the lithium halide and the porous carbon material thermally impregnated with the positive electrode active material in the solid state does not allow the lithium halide to be introduced inside the pores of the porous carbon material. Furthermore, the positive electrode active material cannot be introduced into the pores of the porous carbon material by mechanical mixing alone in the solid state.

[0074] 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 %. Note that this content value is calculated based on the mass of only the positive electrode active material, excluding the porous carbon material and lithium halide from the positive electrode material.

[0075] 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.

[0076] 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.

[0077] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material or different materials.

[0078] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead. Materials used in known lithium secondary batteries may be used as the constituent materials of the positive electrode and the negative electrode lead. It is preferable that the portion removed from the exterior be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, which may cause a current leak and affect the product (e.g., automotive parts, particularly electronic devices).

[0079] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element, as shown in FIGS. 1 and 2 , can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating polypropylene (PP), aluminum, and nylon in this order, but is not limited thereto. From the viewpoint of achieving high output and excellent cooling performance and being suitable for use in batteries for large equipment such as EVs and HEVs, a laminate film is desirable as the exterior material. Furthermore, an aluminum-containing laminate film is more preferable as the exterior material because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0080] The stacked battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability, and is therefore suitable for use as a power source for driving EVs and HEVs.

[0081] 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.

[0082] 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.

[0083] Furthermore, the secondary battery according to the present 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).

[0084] The present invention encompasses the following aspects and configurations: 1. A cathode material comprising a porous carbon material, a cathode active material containing sulfur, and lithium halide, characterized in that at least a portion of the cathode active material and at least a portion of the lithium halide are disposed in the pores of the porous carbon material; 2. The cathode material according to 1. above, in which, in X-ray diffraction measurement, a crystalline peak not attributable to the porous carbon material, the cathode active material, or the lithium halide is present between the two most intense peaks among those attributable to the lithium halide; 3. The cathode material according to 1. or 2. above, in which the content of the lithium halide is 5% by mass or more and less than 50% by mass; 4. The cathode material according to any of 1. to 3. above, in which the lithium halide is present more inside the pores of the porous carbon material than outside the pores; 5. The cathode material according to any of 1. to 4. above, in which the average pore diameter of the porous carbon material is 100 nm or less; 6. 6. The positive electrode material according to any one of 1. to 5., wherein 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 among the pores possessed by the porous carbon material is 20% or less; 7. A secondary battery comprising the positive electrode material according to any one of 1. to 6.

[0085] 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.

[0086] Example 1: Preparation of a Positive Electrode Material In a glove box with an argon atmosphere and a dew point of −68° C. or less, 1.0 g of lithium iodide (LiI, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) was added to 33 mL of ultra-dehydrated ethanol (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and stirred until the solution became clear, dissolving the LiI in the ethanol. 0.50 g of porous carbon material powder (Knobel® P(3)010, manufactured by Toyo Tanso Co., Ltd.) was added to the resulting solution and stirred thoroughly to thoroughly disperse the porous carbon material in the solution. The container containing this dispersion was connected to a vacuum device, and the container was evacuated to a pressure of 1 Pa or less using an oil-sealed rotary pump while stirring the dispersion in the container with a magnetic stirrer. Because the solvent ethanol volatilizes under reduced pressure, the ethanol was removed over time, leaving the porous carbon material impregnated with LiI in the container. After the ethanol was removed under reduced pressure in this manner, the mixture was heated to 180° C. under reduced pressure and subjected to heat treatment for 3 hours to prepare a LiI-impregnated porous carbon material.

[0087] Before the above treatment, the average pore diameter of the porous carbon material used in this example was measured by nitrogen adsorption / desorption measurement. This nitrogen adsorption / desorption measurement was performed using a BELSORP mini manufactured by Microtrac-Bell Corporation at a temperature of -196°C using a multipoint method. 0.01<P / P ０ The BET specific surface area was calculated by the multipoint method from the adsorption isotherm in the relative pressure range of <0.05. The total pore volume was calculated by the adsorption N at a relative pressure of 0.96. ２ The pore size distribution was measured according to the BJH method. The average pore size d was calculated from the BET specific surface area S and the total pore volume V, assuming cylindrical pores, by d = 4V / S. As a result, the average pore size of the porous carbon material used in this example was 10 nm.

[0088] In addition, the pore size distribution in the pore size range of 1 to 100 nm was measured according to the BJH method, and the percentage of the pore volume of pores in the pore size range of 1 to 4 nm to the pore volume of pores in the pore size range of 1 to 100 nm was calculated to be 9%.

[0089] Next, in a glove box under an argon atmosphere with a dew point of −68° C. or lower, 0.563 g of the LiI-impregnated porous carbon material prepared above was added to 0.938 g of sulfur (manufactured by Aldrich Chemical Co.) and thoroughly mixed 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 LiI-impregnated porous carbon material. In this way, a porous carbon material in which sulfur and LiI were filled in the pores was prepared as a positive electrode material.

[0090] (TEM-EDX Measurement) It was confirmed by TEM-EDX that sulfur and lithium iodide were located inside the pores of the porous carbon material for the positive electrode material prepared in this example.

[0091] Specifically, powder particles of the positive electrode material were exfoliated to a thickness of approximately 100 nm using a plasma-focused ion beam processing device (Helios G4 PFIB CXe, manufactured by Thermo Scientific, accelerating voltage: 30 kV). The exfoliated specimen was transferred to a TEM (JEOL JEM-F200 multifunctional analytical transmission electron microscope, accelerating voltage: 200 kV) without exposure to the atmosphere to confirm the microstructure. Elemental map data of the area corresponding to the interior of the particles was obtained using an EDX (energy dispersive X-ray spectrometer, JEOL Dual SDD, accelerating voltage: 200 kV) attached to the TEM (EDX mapping characteristic X-ray measurement energy band: 0-5 keV). The obtained elemental map data confirmed that sulfur and lithium iodide were located inside the pores of the porous carbon material. In this example, iodine or sulfur was used as a labeling element to determine whether lithium iodide or a positive electrode active material was filled inside the pores of the porous carbon material. It was also confirmed that more lithium iodide was located inside the pores of the porous carbon material than outside the pores. Lithium iodide can be detected using either iodine or lithium as a labeling element.

[0092] Powder X-ray diffraction measurement was performed on the positive electrode material obtained above. 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°, radiation source: CuKα: λ=1.5418 Å.

[0093] The XRD spectrum of the positive electrode material obtained in this example is shown in Figure 4. For comparison, the XRD spectrum of a sample in which only elemental sulfur was thermally impregnated into a porous carbon material using the same method as above, and the XRD spectrum of LiI are also shown. In the positive electrode material obtained in this example, a new peak (indicated by an arrow) that could not be attributed to any of the starting materials was observed between the two most intense peaks indicated by a ▼ among the peaks attributed to LiI.

[0094] (Preparation of Positive Electrode Mixture) In a glove box with an argon atmosphere having a dew point of −68° C. or less, 40 g of zirconia balls with a diameter of 5 mm, 0.762 g of the positive electrode material prepared above, and a solid electrolyte (Li, manufactured by Ampcera Corporation) were mixed. ６ P.S. ５ The resulting mixture and 0.238 g of sulfur, lithium iodide, solid electrolyte, and porous carbon material were placed in a 45 mL zirconia container and processed for 6 hours at 370 rpm in a planetary ball mill (Fritsch GmbH, Premium Line P-7), to obtain a powder of a positive electrode mixture. The resulting positive electrode mixture had a composition of sulfur:lithium iodide:solid electrolyte:porous carbon material=48:19:24:10 (mass ratio).

[0095] (Fabrication of Test Cell (All-Solid-State Lithium Secondary Battery)) Fabrication of the battery was carried out in a glove box in an argon atmosphere with a dew point of −68° C. or lower.

[0096] A cylindrical convex punch (10 mm diameter) made of stainless steel was inserted into one side of a cylindrical tube jig (inner diameter 10 mm, outer diameter 23 mm, height 20 mm) made by Macor, and a sulfide solid electrolyte (Li, manufactured by Apcera) was inserted from the top of the cylindrical tube jig. ６ P.S. ５80 mg of SUS304 (C1) was placed inside the cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the sulfide solid electrolyte, and the tube was pressed for 3 minutes at a pressure of 75 MPa using a hydraulic press to form a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm in the cylindrical tube jig. Next, the cylindrical convex punch inserted from above was temporarily removed, and 7.5 mg of the positive electrode mixture prepared above was placed on one side of the solid electrolyte layer inside the cylindrical tube. A cylindrical convex punch (also serving as a positive electrode current collector) was inserted again from above, and the tube was pressed for 3 minutes at a pressure of 300 MPa to form a positive electrode active material layer with a diameter of 10 mm and a thickness of approximately 0.06 mm 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 and an indium foil (manufactured by Nilaco Corporation, thickness 0.30 mm) punched to a diameter of 9 mm were stacked as negative electrodes. 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. The mixture was pressed at a pressure of 75 MPa for 3 minutes to form a lithium-indium negative electrode.

[0097] 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.

[0098] [Example 2] (Preparation of Positive Electrode Material) In a glove box filled with an argon atmosphere having a dew point of −68° C. or lower, 2.5 g of sulfur (manufactured by Aldrich Chemical Co., Ltd.) as a positive electrode active material and 0.5 g of a powder of a porous carbon material (manufactured by Toyo Tanso Co., Ltd., Knobel (registered trademark) P(3)010) were thoroughly mixed 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 porous carbon material with the sulfur. A sulfur-impregnated porous carbon material was thus prepared.

[0099] In a glove box with an argon atmosphere and a dew point of −68°C or less, 1.0 g of lithium iodide (LiI, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) was added to 33 mL of ultra-dehydrated ethanol (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and stirred until the solution became clear, dissolving the LiI in the ethanol. 3.0 g of the sulfur-impregnated porous carbon material prepared above was added to the resulting solution and stirred thoroughly to thoroughly disperse the sulfur-impregnated porous carbon material in the solution. The container containing this dispersion was connected to a vacuum device, and while stirring the dispersion in the container with a magnetic stirrer, the container was reduced in pressure to 1 Pa or less using an oil rotary pump. Because the solvent ethanol volatilizes under reduced pressure, the ethanol was removed over time, leaving a porous carbon material with sulfur and LiI filled in the pores 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 a positive electrode material.

[0100] Other than the above, an all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1. In Example 2, the composition of the positive electrode mixture was sulfur:lithium iodide:solid electrolyte:porous carbon material=48:19:24:10 (mass ratio).

[0101] [Example 3] In the preparation of the positive electrode material in Example 1, the amount of lithium iodide used in the preparation of the LiI-impregnated porous carbon material was changed from 1.0 g to 0.5 g. 2.5 g of sulfur was then impregnated into 1.0 g of the obtained LiI-impregnated porous carbon material. Except for the above, an all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1. In Example 3, the composition of the positive electrode mixture was sulfur:lithium iodide:solid electrolyte:porous carbon material = 55:11:24:11 (mass ratio).

[0102] Example 4 In the preparation of the positive electrode material in Example 1, the amount of lithium iodide used in the preparation of the LiI-impregnated porous carbon material was changed from 1.0 g to 0.15 g. 2.5 g of sulfur was then impregnated into 0.65 g of the obtained LiI-impregnated porous carbon material. Except for the above, an all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1. In Example 4, the composition of the positive electrode mixture was sulfur:lithium iodide:solid electrolyte:porous carbon material=60:4:24:12 (mass ratio).

[0103] [Example 5] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1, except that ultrapure water was used instead of ultra-dehydrated ethanol in preparing the positive electrode material. In Example 5, the composition of the positive electrode mixture was sulfur:lithium iodide:solid electrolyte:porous carbon material=48:19:24:10 (mass ratio).

[0104] In the positive electrode materials prepared in Examples 2 to 5, the same TEM-EDX measurements as above confirmed that sulfur and lithium iodide were located inside the pores of the porous carbon material. It was also confirmed that more lithium iodide was located inside the pores of the porous carbon material than outside the pores. Furthermore, in the positive electrode materials prepared in Examples 2 to 5, a new peak was observed between the two most intense peaks attributed to LiI in the XRD spectrum, as described above. In Examples 3 and 4, it was confirmed that the intensity of the new peak decreased as the lithium iodide content decreased.

[0105] [Example 6] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1, except that lithium iodide was changed to lithium chloride (LiCl) in the preparation of the positive electrode material. In Example 6, the composition of the obtained positive electrode mixture was sulfur:lithium chloride:solid electrolyte:porous carbon material = 48:19:24:10 (mass ratio).

[0106] In the positive electrode material prepared in Example 6, the same TEM-EDX measurement as above confirmed that sulfur and lithium chloride were located inside the pores of the porous carbon material, and that more lithium chloride was located inside the pores of the porous carbon material than outside the pores.

[0107] The XRD spectrum of the positive electrode material obtained in Example 6 was measured in the same manner as described above. The results are shown in FIG. 5. For comparison, the XRD spectrum of a sample in which only elemental sulfur was thermally impregnated into a porous carbon material and the XRD spectrum of LiCl are also shown. In the positive electrode material obtained in Example 6, a new peak (indicated by an arrow) that could not be attributed to any of the starting materials was observed between the two most intense peaks indicated by ▼ among the peaks attributed to LiCl.

[0108] [Example 7] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1, except that lithium iodide was changed to lithium bromide (LiBr) in the preparation of the positive electrode material. In Example 7, the composition of the obtained positive electrode mixture was sulfur:lithium bromide:solid electrolyte:porous carbon material = 48:19:24:10 (mass ratio).

[0109] In the positive electrode material prepared in Example 7, the same TEM-EDX measurement as above confirmed that sulfur and lithium bromide were located inside the pores of the porous carbon material. It was also confirmed that more lithium bromide was located inside the pores of the porous carbon material than outside the pores. Furthermore, the XRD spectrum of the positive electrode material obtained in Example 7 was measured in the same manner as above. In the XRD spectrum of the positive electrode material obtained in Example 7, a new peak that could not be attributed to any of the starting materials was observed between the two most intense peaks attributed to LiBr.

[0110] [Example 8] An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1, except that lithium iodide was changed to lithium fluoride (LiF) in the preparation of the positive electrode material. In Example 8, the composition of the obtained positive electrode mixture was sulfur:lithium fluoride:solid electrolyte:porous carbon material = 48:19:24:10 (mass ratio).

[0111] In the positive electrode material prepared in Example 8, the same TEM-EDX measurement as above confirmed that sulfur and lithium fluoride were arranged inside the pores of the porous carbon material. It was also confirmed that more lithium fluoride was arranged inside the pores of the porous carbon material than outside the pores. Furthermore, the XRD spectrum of the positive electrode material obtained in Example 8 was measured in the same manner as above. In the XRD spectrum of the positive electrode material obtained in Example 8, a new peak that could not be attributed to any of the starting materials was observed between the two most intense peaks attributed to LiF.

[0112] Comparative Example 1: In a glove box with an argon atmosphere having a dew point of −68° C. or less, 40 g of 5 mm diameter zirconia balls, 2.5 g of sulfur (manufactured by Aldrich Chemicals), 0.5 g of porous carbon material powder (manufactured by Toyo Tanso Co., Ltd., Knobel® P(3)010), and 1.0 g of lithium iodide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., LiI) were placed in a 45 ml zirconia container and milled at 370 rpm for 6 hours in a planetary ball mill to obtain a sulfur / LiI / porous carbon material mixed powder cathode material. An all-solid-state lithium secondary battery was fabricated using the same method as in Example 1, except for the above. In Comparative Example 1, the resulting cathode mixture had a composition of sulfur:lithium iodide:solid electrolyte:porous carbon material = 48:19:24:10 (mass ratio).

[0113] When the positive electrode material prepared in Comparative Example 1 was subjected to TEM-EDX measurement in the same manner as above, it was found that lithium iodide was present outside the pores of the porous carbon material and was not located inside the pores.

[0114] The XRD spectrum of the positive electrode material obtained in Comparative Example 1 was measured in the same manner as described above. The results are shown in FIG. 6. For comparison, the XRD spectrum of elemental sulfur and the XRD spectrum of the positive electrode material of Example 1 are also shown. In the positive electrode material of Comparative Example 1, peaks derived from starting materials such as elemental sulfur and LiI were observed, but no new peaks were observed between the two most intense peaks attributed to LiI.

[0115] Comparative Example 2: In a glove box with an argon atmosphere having a dew point of −68° C. or less, 40 g of 5 mm diameter zirconia balls, 2.5 g of sulfur (manufactured by Aldrich Chemical), 0.5 g of porous carbon material powder (manufactured by Toyo Tanso Co., Ltd., Knobel® P(3)010), and 1.0 g of lithium chloride (LiCl) were placed in a 45 ml zirconia container and milled at 370 rpm for 6 hours in a planetary ball mill to obtain a cathode material composed of a sulfur / LiCl / porous carbon material mixed powder. An all-solid-state lithium secondary battery was fabricated using the same method as in Example 1, except for the above. In Comparative Example 2, the resulting cathode mixture had a mass ratio of sulfur:lithium chloride:solid electrolyte:porous carbon material of 48:19:24:10.

[0116] When the positive electrode material prepared in Comparative Example 2 was subjected to TEM-EDX measurement in the same manner as above, it was found that lithium chloride was present outside the pores of the porous carbon material and was not located inside the pores.

[0117] The XRD spectrum of the positive electrode material obtained in Comparative Example 2 was measured in the same manner as described above. The results are shown in FIG. 7. For comparison, the XRD spectrum of elemental sulfur and the XRD spectrum of the positive electrode material of Example 6 are also shown. In the positive electrode material of Comparative Example 2, peaks derived from starting materials such as elemental sulfur and LiCl were observed, but no new peaks were observed between the two most intense peaks attributed to LiCl.

[0118] Comparative Example 3 (Preparation of Cathode Material) In a glove box with an argon atmosphere having a dew point of −68° C. or lower, 2.5 g of sulfur (manufactured by Aldrich Chemical Co., Ltd.) as a cathode active material and 0.5 g of porous carbon material powder (manufactured by Toyo Tanso Co., Ltd., Knobel® P(3)010) were thoroughly mixed in an agate mortar. 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 porous carbon material with sulfur. This resulted in a sulfur-impregnated porous carbon material, which was used as a cathode material. An all-solid-state lithium secondary battery was fabricated using the same method as in Example 1, except for the above. In Comparative Example 3, the composition of the resulting cathode mixture was sulfur:solid electrolyte:porous carbon material = 50:24:10 (mass ratio).

[0119] Comparative Example 4 In the preparation of the positive electrode material of Example 1, lithium iodide was replaced with a sulfide solid electrolyte (manufactured by Ampcera, Li ６ P.S. ５ An all-solid-state lithium secondary battery was fabricated in the same manner as in Example 1, except that the composition of the sulfur, solid electrolyte, and porous carbon material was changed from sulfur to sulfur, solid electrolyte, and porous carbon material in a glove box with an argon atmosphere having a dew point of −68° C. or less, and that the operation was changed to air. In Comparative Example 4, the composition of the obtained positive electrode mixture was sulfur:solid electrolyte:porous carbon material=48:43:10 (mass ratio).

[0120] <<Test Cell Evaluation Example>> The resistance values ​​of the test cells prepared in the above Comparative Examples and Examples were 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.

[0121] (Resistance Measurement) A high-performance electrochemical measurement system "SP-200" manufactured by Bio-Logic SAS (France) was used as the measurement device, 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. Note that the resistance of the battery of Comparative Example 4 was too high to be measured.

[0122]

[0123] The results shown in Table 1 show that the positive electrode material of the present invention can further reduce the internal resistance of a secondary battery using a positive electrode active material containing sulfur.

[0124] This application is based on Japanese Patent Application No. 2022-058716, filed on March 31, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0125] 10a laminated 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 single 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 porous carbon material, 110a pores, 120 sulfur-containing positive electrode active material, 130 solid electrolyte, 140 region near interface, 150 lithium halide.

Claims

1. A porous carbon material, a positive electrode active material containing sulfur, and a lithium halide, at least a portion of the positive electrode active material and at least a portion of the lithium halide are disposed in pores of the porous carbon material; A positive electrode material, characterized in that the lithium halide disposed in the pores of the porous carbon material has lithium ion conductivity.

2. 2. The positive electrode material according to claim 1, wherein a lithium ion conduction path is formed in the pores of the porous carbon material via the lithium halide.

3. 3. The positive electrode material according to claim 1, wherein, in an X-ray diffraction measurement, a crystal peak not attributable to any of the porous carbon material, the positive electrode active material, and the lithium halide is present between the two most intense peaks among the peaks attributable to the lithium halide.

4. The positive electrode material according to claim 1 or 2, wherein the content of the lithium halide is 5% by mass or more and less than 50% by mass.

5. The positive electrode material according to claim 1 or 2, wherein the content of the lithium halide is 10 to 30 mass %.

6. 3. The positive electrode material according to claim 1, wherein the lithium halide is present in a greater amount inside the pores of the porous carbon material than outside the pores.

7. 3. The positive electrode material according to claim 1, wherein the lithium halide is lithium iodide or lithium chloride.

8. 3. The positive electrode material of claim 1, wherein the lithium halide is lithium iodide.

9. The positive electrode material according to claim 1 or 2, wherein the porous carbon material has an average pore size of 100 nm or less.

10. 3. The positive electrode material according to claim 1, wherein 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.

11. The positive electrode material according to claim 1 or 2, wherein the positive electrode material is obtained by introducing the lithium halide into the inside of the pores of the porous carbon material by solution impregnation, removing the solvent, and then performing a heat treatment at a temperature of 150 to 250° C.

12. A secondary battery comprising the positive electrode material according to claim 1 or 2.