Composite material for battery, and battery

Abstract

The present disclosure provides a composite material for a battery, the composite material comprising: active material particles 12 that contain lithium oxide having an inverted fluorite structure; and a carbon coating layer 14 that contains a carbon material and covers the surfaces of the active material particles 12. The coverage of the active material particles 12 covered by the carbon coating layer 14 is, for example, 80% or more. The average thickness of the carbon coating layer 14 falls, for example, within the range of 20 nm to 400 nm.

Description

Composite materials for batteries and batteries

[0001] This disclosure relates to composite materials for batteries and batteries.

[0002] As described in Patent Document 1, it is known that the redox reaction between lithium oxide (Li2O) and lithium peroxide (Li2O2) can be applied to secondary batteries. However, one of the technical challenges has been the poor electronic conductivity of lithium oxide and lithium peroxide, which results in large overpotentials.

[0003] Patent Document 2 discloses that charging overvoltage can be reduced by solid-solving a transition metal in the crystal structure of lithium oxide.

[0004] Patent Document 3 discloses a composite oxide in which a transition metal element M and a typical element A are solid-dissolved in Li2O in a predetermined ratio.

[0005] Japanese Patent Publication No. 4554935, Japanese Patent Publication No. 6179944, International Publication No. 2019 / 163476

[0006] This disclosure provides a technology for improving the charge and discharge characteristics of batteries using lithium oxide.

[0007] This disclosure provides a composite material for batteries comprising: active material particles containing lithium oxide having an inverse fluorite-type structure; and a carbon coating layer containing a carbon material that covers the surface of the active material particles, wherein the coverage rate of the active material particles by the carbon coating layer is 80% or more.

[0008] According to this disclosure, the charge and discharge characteristics of batteries using lithium oxide can be improved.

[0009] Figure 1 is a cross-sectional view showing the schematic configuration of the composite material according to Embodiment 1. Figure 2 is a cross-sectional view showing the schematic configuration of the composite material according to a modified example. Figure 3 is a cross-sectional view showing the schematic configuration of the battery according to Embodiment 2. Figure 4A is a cross-sectional SEM image (30,000x magnification) of the positive electrode composite sheet of Embodiment 1. Figure 4B is a cross-sectional SEM image (30,000x magnification) of the positive electrode composite sheet of Embodiment 2. Figure 4C is a cross-sectional SEM image (30,000x magnification) of the positive electrode composite sheet of Comparative Example 1. Figure 4D is an SEM image and EDX elemental map of the positive electrode composite of Comparative Example 2. Figure 5 is a graph showing the relationship between the charge / discharge cycle and capacity retention rate of the test cells of Embodiment 1, Embodiment 2 and Comparative Example 1. Figure 6A is a cross-sectional SEM image and EDX elemental map of the positive electrode composite sheet of Embodiment 1. Figure 6B is a cross-sectional SEM image and EDX elemental map of the positive electrode composite sheet of Embodiment 2. Figure 7 is a graph showing the rate characteristics of the test cells of Embodiment 2 and Comparative Example 1.

[0010] The embodiments of this disclosure will be described below with reference to the drawings. This disclosure is not limited to the embodiments described below.

[0011] (Embodiment 1) Figure 1 is a cross-sectional view showing the schematic configuration of a composite material according to Embodiment 1. The composite material 10 comprises active material particles 12 and a carbon coating layer 14. The carbon coating layer 14 covers the surface of the active material particles 12. The active material particles 12 contain lithium oxide having an inverse fluorite-type structure. The composite material 10 is a material for batteries.

[0012] According to this embodiment, the carbon coating layer 14 imparts high electronic conductivity to the composite material 10. As a result, a battery using the composite material 10 can exhibit superior charge and discharge characteristics compared to a battery using active material particles without a carbon coating layer. "Charge and discharge characteristics" typically refer to cycle characteristics and rate characteristics.

[0013] The carbon coating layer 14 has the shape of a continuous film along the surface of the active material particles 12. The carbon coating layer 14 has a layered structure of carbon material particles. For example, as can be seen in Figure 4B, the carbon coating layer 14 has the shape of a film in which carbon material particles are layered and densely adhered to the surface of the active material particles 12.

[0014] The average particle size of the carbon material is, for example, in the range of 10 nm to 100 nm, and preferably in the range of 10 nm to 40 nm. It is presumed that a carbon coating layer 14 with a high coverage rate is easily formed using carbon material particles of this size. The average particle size of the carbon material can be measured by the same method as the method for calculating the average particle size of the active material particles 12, as will be described later.

[0015] Generally, as the proportion of conductive additive increases, the proportion of positive electrode active material decreases, leading to a reduction in discharge capacity. Therefore, in conventional lithium-ion secondary batteries, the amount of conductive additive tends to be limited. For example, when the positive electrode active material is lithium nickel cobalt manganese oxide, 3 parts by mass of conductive additive are used for 95 parts by mass of positive electrode active material.

[0016] In contrast, the electronic conductivity of lithium oxide used in the composite material 10 of this embodiment is lower than that of positive electrode active materials such as lithium nickel cobalt manganese oxide. Therefore, in this embodiment, the amount of conductive additive is adjusted so that a thick film of carbon material particles is formed in order to improve the electronic conductivity near the lithium oxide particles, which are the positive electrode active material. In this respect, the structure of the composite material 10 in this embodiment differs from the structure of conventional positive electrode composite materials.

[0017] The coverage rate of the active material particles 12 by the carbon coating layer 14 is, for example, 80% or more. Specifically, in the composite material powder 10, each of the multiple active material particles 12 is covered by the carbon coating layer 14. The average value of the coverage rate of the multiple active material particles 12 by the carbon coating layer 14 is 80% or more, preferably 90%. The higher the coverage rate, the better the conductive path to the active material particles 12, and thus the better the charge and discharge characteristics of the battery using the composite material 10.

[0018] There is no particular upper limit to the coverage rate, and it may be 100%.

[0019] The coverage rate can be calculated by observing a cross-section of a composite sheet made using the powder of the composite material 10 with a scanning electron microscope (SEM). For example, an SEM observation of the cross-section of the composite sheet is performed. The magnification for the SEM observation is between 10,000 and 50,000 times, for example, 30,000 times. An arbitrary number (for example, 100) of active material particles 12 are selected from the cross-sectional SEM image. The length of the outer edge of the active material particles 12 is calculated by image processing. The length of the contact interface between the active material particles 12 and the carbon coating layer 14 is calculated by image processing. The image resolution is, for example, 1280 pixels × 1026 pixels or higher. The coverage rate of the active material particles 12 is calculated as a percentage (%) by dividing the length of the contact interface by the length of the outer edge of the active material particles 12. The average value of the coverage rates calculated for each of the multiple active material particles 12 can be considered as the "coverage rate of the active material particles 12 by the carbon coating layer 14".

[0020] The average thickness of the carbon coating layer 14 is, for example, in the range of 20 nm to 400 nm. By appropriately adjusting the average thickness of the carbon coating layer 14, the effect of improving the charge and discharge characteristics of the battery is enhanced. It is desirable that the average thickness of the carbon coating layer 14 be in the range of 150 nm to 400 nm. The method for measuring the average thickness of the carbon coating layer 14 is as described in the examples below.

[0021] The carbon coating layer 14 contains carbon material. The main component of the carbon coating layer 14 is carbon material. "Main component" means the component that is present in the largest amount by mass. The fact that the main component of the carbon coating layer 14 is carbon material can be confirmed, for example, by energy-dispersive X-ray fluorescence analysis.

[0022] Carbon black is an example of a carbon material. Examples of carbon black include acetylene black and Ketjen black. At least one of these carbon materials is included in the carbon coating layer 14. Using carbon black makes it easy to form a carbon coating layer 14 of uniform thickness and imparts excellent electronic conductivity to the carbon coating layer 14.

[0023] The carbon coating layer 14 may contain a fibrous binder. Preferably, the fibrous binder contains a fluororesin that can be fibrillated (formed into fibers). The fibrous binder adheres to the surface of the active material particles 12 and intertwines with the carbon material. That is, the layered structure of the carbon coating layer 14 is formed by the carbon material being held in place by the fibrous binder which exists in a network-like structure. With this configuration, the carbon coating layer 14 can be made to adhere closely to the active material particles 12. It is considered advantageous for improving the charge and discharge characteristics of the battery using the composite material 10 by improving the adhesion between the carbon coating layer 14 and the active material particles 12.

[0024] Examples of fluororesins that can be fibrillated (formed into fibers) include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and polytetrafluoroethylene. At least one of these fluororesins is included in the carbon coating layer 14.

[0025] The fibrous binder can be produced, for example, by fibrillating polytetrafluoroethylene (PTFE) particles belonging to a fine powder that can be fibrillated (fibrousized). The average particle size of the PTFE particles is not particularly limited, and is, for example, in the range of 100 μm to 500 μm, preferably in the range of 100 μm to 400 μm. The average particle size of the PTFE particles can be measured by observing the PTFE particles with a scanning electron microscope (SEM). Specifically, the average value of the major axis of 100 arbitrarily selected particles can be considered as the average particle size. In addition to the fibrous binder, the carbon coating layer 14 may also contain particulate binders such as polyvinylidene fluoride (PVdF) that cannot be fibrillated.

[0026] Among the fluororesins mentioned above, polytetrafluoroethylene (PTFE) is suitable for the composite material 10. PTFE is suitable for forming the composite material 10 into a sheet by a dry process. The dry process is a method of forming the composite material 10 into a sheet without using a solvent.

[0027] The fibrous binder content in the carbon coating layer 14 is, for example, in the range of 0.1% by mass or more and 15% by mass or less, and may also be in the range of 1% by mass or more and 15% by mass or less, or in the range of 1% by mass or more and 12% by mass or less, or in the range of 5% by mass or more and 12% by mass or less. It is presumed that the adhesion between the carbon coating layer 14 and the active material particles 12 will be improved by appropriately adjusting the fibrous binder content. Furthermore, the carbon coating layer 14 can be adjusted to the intended thickness. The binder content in the carbon coating layer 14 can be measured by energy-dispersive X-ray fluorescence analysis (EDX).

[0028] In this embodiment, the active material particles 12 are secondary particles. The carbon coating layer 14 coats the surface of the secondary active material particles 12. A composite material 10 having such a structure can also improve the charge and discharge characteristics of a battery. However, the active material particles 12 may be primary particles.

[0029] The active material particles 12 preferably contain lithium oxide in which transition metal M1 is dissolved. The lithium oxide in which transition metal M1 is dissolved has an inverse fluorite-type structure.

[0030] The inverse fluorite structure is a structure in which the positional relationship between cations and anions in the fluorite structure is reversed. When a transition metal M1 is dissolved in lithium oxide, the transition metal M1 occupies the lithium site, and oxygen atoms tetrahedral coordinate around the transition metal M1 to form an outer orbital complex. For example, in the case of cobalt, multiple unpaired electrons will exist in the 3d orbital, and these unpaired electrons confer electronic conductivity to the lithium oxide. Also, assuming that the valence of the transition metal M1 is +3, the valence of lithium is +1, so in order to maintain charge neutrality in the crystal, two lithium atoms other than the lithium atom substituted with the transition metal M1 are released from the crystal, forming two vacancies. These vacancies become conduction pathways for lithium ions, improving the ionic conductivity of lithium oxide. Lithium oxide in which a transition metal M1 is dissolved can be a substitutional solid solution.

[0031] As lithium is replaced by transition metal M1, the size of the crystal lattice changes. Since the ionic radius of the ions of transition metal M1 is larger than that of lithium ions, the substitution from lithium to transition metal M1 increases the size of the crystal lattice. On the other hand, vacancies decrease the size of the crystal lattice. Since the number of vacancies changes according to the valence of the substituted transition metal M1, the size of the crystal lattice changes according to the balance between the number of ions of transition metal M1 and the number of vacancies.

[0032] The transition metal M1 may be, for example, an element selected from the group of elements of Groups 3 to 11 in the 4th and 5th periods of the periodic table. Specifically, the transition metal M1 may contain at least one selected from the group consisting of Co, Cu, Ni, Mn, and Fe. The transition metal M1 may be Co. These elements are desirable from the viewpoints of suppressing an increase in the charging voltage of the battery using the composite material 10 and improving the discharge capacity of the battery.

[0033] Lithium oxide in which transition metal M1 is dissolved may have a composition represented by the following formulas (A) and (B) in the discharged state. According to quantum scientific calculations, when the valence of transition metal M1 is +3, preferably, α satisfies the relationship of 0.0327 ≦ α ≦ 0.1484. When the valence of transition metal M1 is +2, preferably, α satisfies the relationship of 0.0490 ≦ α ≦ 0.2224.

[0034] When the valence of transition metal M1 is +3: (Li (1-3α) M1 α )2O... (A) When the valence of transition metal M1 is +2: (Li (1-2α) M1 α )2O... (B)

[0035] The presence or absence of the inverse fluorite structure can be determined from the X-ray diffraction pattern of the composite material 10 measured using Cu-Kα rays. When the inverse fluorite structure is present, the X-ray diffraction pattern shows diffraction peaks attributed to the (111) plane of lithium oxide and diffraction peaks attributed to the (220) plane of lithium oxide. The diffraction peak attributed to the (111) plane of lithium oxide exists in the range of diffraction angle 2θ from 30° to 40°. The diffraction peak attributed to the (220) plane of lithium oxide exists in the range of diffraction angle 2θ from 52° to 62°.

[0036] The average particle size of the active material particles 12 is not particularly limited and is, for example, in the range of 100 nm or more and 500 nm or less. The average particle size of the active material particles 12 can be calculated by the following method. The cross-section of the structure containing the active material particles 12 is observed with a scanning electron microscope, and the area of a specific active material particle 12 in the microscope image is calculated by image processing. The diameter of a circle having an area equal to the calculated area is regarded as the diameter of the specific active material particle 12. The diameters of an arbitrary number (for example, 100) of active material particles 12 are calculated, and their average value is regarded as the average particle size of the active material particles 12. When selecting the target active material particles 12, fine particles deviating from the average size are excluded. The method for measuring the average particle size of the active material particles 12 is also applicable to the measurement of the average particle size of the carbon material particles.

[0037] The active material particles 12 may consist of Li and O, excluding inevitable impurities, or may consist of Li, M1, and O.

[0038] FIG. 2 is a cross-sectional view showing a schematic configuration of a composite material according to a modified example. In the composite material 10 shown in FIG. 1, a single secondary particle is coated with a carbon coating layer 14. In contrast, in the composite material 10a shown in FIG. 2, a plurality of secondary particles (active material particles 12) are bonded in a form sharing the carbon coating layer 14. The composite material 10a having such a structure can also have the effect of improving the charge and discharge characteristics of the battery.

[0039] The composite material 10 shown in Figure 1 and the composite material 10a shown in Figure 2 can be manufactured by mixing active material particles 12 and a carbon material. If the carbon coating layer 14 contains a binder, the active material particles 12, the carbon material, and the binder are mixed together.

[0040] When the active material particles 12 are composed of lithium oxide in which a transition metal M1 is dissolved, the active material particles 12 are prepared in advance by a known method. Specifically, lithium oxide and a raw material containing the transition metal M1 are mixed and milled. Examples of raw materials for the transition metal M1 include oxides of the transition metal M1, complex oxides containing lithium and the transition metal M1, and elemental transition metal M1. At least one of these can be used.

[0041] The process for manufacturing the composite material 10 can also serve as the process for manufacturing an electrode mixture containing the composite material 10. That is, the carbon material and binder contained in the carbon coating layer 14 can serve as the conductive additive and binder, respectively, in the electrode mixture. For example, the active material particles 12, carbon material, and binder are mixed in a mortar in the mass ratio required for the target electrode mixture. If the amount of each material is too small, the force applied to the materials during mixing is too weak, or the mixing time is too short, the carbon coating layer will not be formed, as shown in the comparative example described later. According to the inventors' research, it is possible to form a carbon coating layer 14 of uniform thickness by mixing the active material particles 12, carbon material, and binder using a mortar and pestle for a sufficient amount of time.

[0042] (Embodiment 2) Figure 3 is a cross-sectional view of a battery 100 according to Embodiment 2. The battery 100 comprises a positive electrode 23, a negative electrode 26, a non-aqueous electrolyte 29, a separator 27, and an outer casing 28. The positive electrode 23 has a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 is disposed on the positive electrode current collector 21. The negative electrode 26 has a negative electrode current collector 24 and a negative electrode active material layer 25. The negative electrode active material layer 25 is disposed on the negative electrode current collector 24. A separator 27 is disposed between the positive electrode 23 and the negative electrode 26. The positive electrode 23 and the negative electrode 26 face each other via the separator 27. The positive electrode 23, the negative electrode 26, the separator 27, and the non-aqueous electrolyte 29 are housed in the outer casing 28. The battery 100 is typically a secondary battery.

[0043] The positive electrode active material layer 22 includes the composite material described in Embodiment 1. The battery 100 including the composite material described in Embodiment 1 can exhibit excellent charge and discharge characteristics.

[0044] The charge-discharge reaction equation for battery 100 is as follows: Reaction at positive electrode 23: Li2O2 + 2Li + +2e - ⇔2Li₂O Reaction at negative electrode 26: Li⇔Li + +e -

[0045] The positive electrode current collector 21 is a sheet or film made of a metallic material such as aluminum, aluminum alloy, stainless steel, titanium, or titanium alloy. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material may be coated on the surface of the positive electrode current collector 21 as a conductive auxiliary material.

[0046] The positive electrode active material layer 22 may contain other materials such as conductive additives, ion conductors, and binders.

[0047] The conductive additive and the ion conductor are used to reduce the resistance of the positive electrode 23. Examples of the conductive additive include carbon materials and conductive polymer compounds. Examples of the carbon materials include carbon black, graphite, carbon nanotubes, carbon nanofibers, graphene, fullerenes, and oxidized graphite. Examples of the conductive polymer compounds include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0048] Examples of the ion conductor include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate, organic solid electrolytes such as polyethylene oxide, and inorganic solid electrolytes such as Li7La3Zr2O 12 and the like. At least one selected from these ion conductors can be used.

[0049] The binder is used to improve the binding property of the materials constituting the positive electrode 23. Examples of the binder include polymer materials such as polyvinylidene fluoride, vinylidene fluoride - hexafluoropropylene copolymer, vinylidene fluoride - tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene - butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one selected from these binders can be used.

[0050] The conductive additive may be of the same type as the carbon material contained in the carbon coating layer 14 of the composite material 10. Similarly, the binder may be of the same type as the binder contained in the carbon coating layer 14 of the composite material 10.

[0051] [[ID=~14]]The negative electrode current collector 24 is, for example, a sheet or film made of a metal material such as stainless steel, nickel, nickel alloy, copper, or copper alloy. The sheet or film may be porous or non - porous. As the sheet or film, metal foil, metal mesh, etc. are used. A carbon material may be coated on the surface of the negative electrode current collector 24 as a conductive auxiliary material.

[0052] The negative electrode active material layer 25 may contain a negative electrode active material capable of intercalating and releasing lithium. Examples of negative electrode active materials capable of intercalating and releasing lithium include lithium titanate, graphite, silicon, silicon-containing oxides, zinc alloys, lithium metal, and lithium alloys. At least one of these negative electrode active materials can be used. It is more preferable to use lithium metal as the negative electrode active material.

[0053] The negative electrode active material layer 25 may contain other materials such as conductive additives, ion conductors, and binders. Materials usable in the positive electrode active material layer 22 can also be used in the negative electrode active material layer 25 as conductive additives, ion conductors, and binders.

[0054] The non-aqueous electrolyte 29 may be impregnated into the positive electrode 23, the negative electrode 26, and the separator 27. The non-aqueous electrolyte 29 may fill the internal space of the outer casing 28. Lithium ions can move between the positive electrode 23 and the negative electrode 26 through the action of the non-aqueous electrolyte 29. The non-aqueous electrolyte 29 may contain a non-aqueous electrolyte, a gel electrolyte, or an ionic liquid.

[0055] The non-aqueous electrolyte includes, for example, a non-aqueous solvent and a lithium salt.

[0056] Non-aqueous solvents that can be used include cyclic carbonate esters, linear carbonate esters, cyclic ethers, linear ethers, cyclic esters, linear esters, fluorinated solvents, and nitriles. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and butylene carbonate. Examples of linear carbonate esters include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of cyclic ethers include tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Examples of linear ethers include 1,2-dimethoxyethane and 1,2-diethoxyethane. Examples of cyclic esters include γ-butyrolactone. Examples of linear esters include methyl acetate. Examples of fluorinated solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethyl carbonate. Examples of nitriles include acetonitrile. At least one of these non-aqueous solvents can be used.

[0057] Examples of lithium salts include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. At least one of these lithium salts can be used.

[0058] Gel electrolytes can be materials obtained by impregnating a polymer material with a non-aqueous electrolyte. Examples of polymer materials include polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, and polymers having ethylene oxide bonds.

[0059] Examples of cations constituting ionic liquids include aliphatic quaternary cations, aliphatic cyclic ammonium compounds, and nitrogen-containing heterocyclic aromatic cations. Examples of aliphatic quaternary cations include tetraalkylammonium compounds and tetraalkylphosphonium compounds. Examples of aliphatic cyclic ammonium compounds include pyrrolidinium compounds, morpholinium compounds, imidazolinium compounds, tetrahydropyrimidinium compounds, piperadinium compounds, and piperidinium compounds. Examples of nitrogen-containing heterocyclic aromatic cations include pyridinium compounds and imidazolium compounds. An example of anion constituting ionic liquids is PF6. - BF4 - SbF6 - AsF6 - SO3CF3 - , N(SO2F)2 - , N(SO2CF3)2 - , N(SO2C2F5)2 - , N(SO2CF3)(SO2C4F9) - , C(SO2CF3)3 - These are some examples. The ionic liquid may also contain a lithium salt.

[0060] The separator 27 is an electrolyte layer having lithium ion conductivity. The material of the separator 27 is not particularly limited as long as the passage of lithium ions is permitted. The material of the separator 27 may be at least one selected from the group consisting of solid electrolytes, gel electrolytes, ion exchange resin membranes, semipermeable membranes, and porous membranes. If the separator 27 is made of these materials, the safety of the battery 100 can be sufficiently ensured. As for the solid electrolyte, sulfide solid electrolytes such as Li2S-P2S5 and Li7La3Zr2O 12 Examples of oxide solid electrolytes include (LLZ). Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVdF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resin and porous membranes made of glass paper obtained by weaving glass fibers into a nonwoven fabric.

[0061] The outer casing 28 is made of a material obtained by laminating a metal foil, such as aluminum foil, with a resin film, such as PET film. The outer casing 28 may also be a resin or metal container.

[0062] The shape of the battery 100 is not limited to a stacked type. Other shapes of the battery 100 include coin-type, cylindrical, prismatic, sheet-type, button-type, and flat-type.

[0063] Battery 100 may be a solid-state battery.

[0064] When the battery 100 is a solid-state battery, a solid electrolyte is used in place of the non-aqueous electrolyte 29 in the positive electrode 23, negative electrode 26, and separator 27. Examples of solid electrolytes include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, polymer solid electrolytes, and complex hydride solid electrolytes. At least one of these solid electrolytes can be used in each of the positive electrode 23, negative electrode 26, and separator 27.

[0065] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0066] (Technology 1) A composite material for batteries comprising: active material particles containing lithium oxide having an inverse fluorite-type structure; and a carbon coating layer containing a carbon material that covers the surface of the active material particles, wherein the coverage rate of the active material particles by the carbon coating layer is 80% or more.

[0067] According to this disclosure, the charge and discharge characteristics of batteries using lithium oxide can be improved.

[0068] (Technology 2) The composite material for batteries according to Technology 1, wherein the active material particles are secondary particles, and the carbon coating layer coats the surface of the secondary particles. A composite material having such a structure can also improve the charge and discharge characteristics of a battery.

[0069] (Technology 3) A composite material for batteries according to Technology 2, wherein a plurality of the secondary particles are bonded together in such a way that they share the carbon coating layer. A composite material having such a structure can also have the effect of improving the charge and discharge characteristics of a battery.

[0070] (Technology 4) A composite material for batteries according to any one of Technology 1 to 3, wherein the average thickness of the carbon coating layer is in the range of 20 nm to 400 nm. By appropriately adjusting the average thickness of the carbon coating layer, the effect of improving the charge and discharge characteristics of the battery is enhanced.

[0071] (Technical 5) A composite material for a battery according to any one of Technical 1 to 4, wherein the carbon coating layer contains carbon black as the carbon material. Carbon black facilitates the formation of a carbon coating layer of uniform thickness and can impart excellent electronic conductivity to the carbon coating layer.

[0072] (Technical 6) A composite material for a battery according to any one of Technical 1 to 5, wherein the carbon coating layer includes a fibrous binder. With such a configuration, the carbon coating layer can be brought into close contact with the active material particles.

[0073] (Technical 7) A composite material for batteries according to any one of Technical 1 to 6, wherein the carbon coating layer contains polytetrafluoroethylene. PTFE is suitable for forming the composite material into a sheet by a dry method.

[0074] (Technical 8) A composite material for batteries according to Technical 6 or 7, wherein the binder content in the carbon coating layer is in the range of 0.1% by mass or more and 10% by mass or less. It is presumed that the adhesion between the carbon coating layer and the active material particles will be improved by appropriately adjusting the binder content.

[0075] (Technical 9) A composite material for a battery according to any one of Technical 1 to 8, wherein a transition metal is solid-dissolved in the lithium oxide. With such a configuration, the electronic conductivity of lithium oxide can be improved.

[0076] (Technical 10) A composite material for a battery according to Technical 9, wherein the transition metal comprises at least one selected from the group consisting of Co, Cu, Ni, Mn, and Fe. These elements are desirable from the viewpoint of suppressing an increase in the charging voltage of a battery using the composite material and from the viewpoint of improving the discharge capacity of the battery.

[0077] (Technical 11) A battery comprising electrodes containing a composite material for batteries as described in any one of Technical 1 to 10. A battery containing the composite material of this disclosure can exhibit excellent charge and discharge characteristics.

[0078] The following steps in the preparation of the composite material and the preparation of the test cell were performed under an argon atmosphere.

[0079] (Example 1) 1.00 g of Li2O and 0.81 g of LiCoO2 were placed in a zirconia container (45 mL) together with 45 g of zirconia balls (5 mm in diameter), and the mixture was milled in a planetary ball mill (Fritsch, PL-7 type) at 420 rpm for 100 hours. This dissolved Co into the crystalline structure of Li2O.

[0080] The fact that Li2O with solid solution of Co has an inverse fluorite-type structure was confirmed by powder X-ray diffraction measurements.

[0081] Using an agate mortar, Li2O with solid-solution Co, acetylene black (Denka Co., Ltd., Li-100, average particle size 35 nm), and PTFE were mixed in a mass ratio of 7:2:1. The amount of Li2O with solid-solution Co added was 0.35 g. The amount of acetylene black added was 0.1 g. The amount of PTFE added was 0.05 g. The agate mortar used had an inner diameter of 50 mm, an outer diameter of 60 mm, a depth of 18 mm, and weighed 135 g. The agate pestle weighed 38 g. These raw materials were mixed for 15 minutes while applying a large force (the force of an adult male's manual labor) to the pestle. This obtained the cathode composite material of Example 1.

[0082] The cathode composite material of Example 1 was rolled to produce a cathode composite material sheet of Example 1.

[0083] (Example 2) In Example 2, the amounts of Co-dissolved Li2O, acetylene black, and PTFE were doubled compared to Example 1. Specifically, the amount of Co-dissolved Li2O added was 0.7 g. The amount of acetylene black added was 0.2 g. The amount of PTFE added was 0.1 g. Except for changing the amount of raw materials added and changing the mixing time to 30 minutes, the cathode composite material and cathode composite material sheet of Example 2 were prepared in the same manner as in Example 1.

[0084] (Comparative Example 1) The cathode composite material and cathode composite material sheet of Comparative Example 1 were prepared in the same manner as in Example 1, except that the mixing time was changed to 10 minutes and the raw materials were mixed without applying great force to the pestle.

[0085] (Comparative Example 2) Li2O in which Co is dissolved and acetylene black were mixed in a mass ratio of 7:2. The mixture was placed in a WC container (45 mL) together with 45 g of WC balls (5 mm in diameter), and the mixture was milled in a planetary ball mill (Fritsch, PL-7 type) at 200 rpm for 10 hours. This yielded the positive electrode mixture of Comparative Example 2.

[0086] [Cross-sectional SEM-EDX analysis] The cross-sections of the cathode composite sheets of Example 1, Example 2, and Comparative Example 1 were processed using a cross-section polisher. SEM observation of the cross-sections was performed using a scanning electron microscope (JEOL, IT-800). The magnification for SEM observation was 30,000x. The results are shown in Figures 4A to 4C.

[0087] For the cathode composite material of Comparative Example 2, elemental analysis (carbon) was performed using SEM observation and an energy-dispersive X-ray fluorescence analyzer (EDX, JEOL, EX-64247ALSS). The magnification for SEM observation was 30,000x. The results are shown in Figure 4D.

[0088] For the cathode composite sheets of Examples 1 and 2, elemental analysis (fluorine) was performed by EDX in conjunction with cross-sectional SEM observation. The results are shown in Figures 6A and 6B.

[0089] [Calculation of Coverage] Coverage was calculated from cross-sectional SEM images at a magnification of 30,000x in multiple observation fields using the method described above. The resolution of the images used for image processing was 1280 pixels × 1026 pixels. The results are shown in Table 1.

[0090] [Calculation of Average Thickness of Carbon Coating Layer] 100 particles of the composite material were arbitrarily selected from multiple observation fields. The thickness of the carbon coating layer was measured at six arbitrary locations in the circumferential direction of each particle, and the average thickness was calculated. The "six arbitrary locations" were selected at approximately equal angular intervals in the circumferential direction of the particle. The obtained values ​​were considered to be the thickness of the carbon coating layer for the target particle. The average value of the carbon coating layer was calculated by summing the thicknesses of the carbon coating layer for each particle and dividing by the number of particles (100). The results are shown in Table 1.

[0091] [Measurement of Discharge Capacity] Using the positive electrode composite sheets of Example 1, Example 2, and Comparative Example 1, test cells were prepared using the following method. A sealed charge-discharge test cell (PAT-Cell-Press, manufactured by EL-CELL Corporation) was used as the test cell. An Al plunger was used for the lower current collector. A Cu plunger was used for the upper current collector. An FS-5P type separator was used.

[0092] A mixed solvent was prepared by mixing 1,3-dioxolane and dimethoxyethane in a 1:1 volume ratio. An electrolyte was prepared by dissolving LiFSI as a lithium salt and vinylethylene carbonate as an additive in the mixed solvent. The concentration of LiFSI in the electrolyte was 1 mol / liter. The concentration of vinylethylene carbonate in the electrolyte was 0.53 mol / liter.

[0093] A positive electrode composite sheet was placed between the lower current collector and the separator. The laminate of the lower current collector, positive electrode composite sheet, and separator was mounted in the cell container. Then, the electrolyte was added to the cell container. A lithium metal foil (0.3 mm thick) and the upper current collector were placed inside the cell container, and the cell container was sealed. In this way, test cells for Example 1, Example 2, and Comparative Example 1 were fabricated.

[0094] The test cell was placed in a measuring device (EL-CELL, PAT-Tester-i-16) and charged and discharged at a temperature of 25°C. Specifically, constant current charging was performed at a current of 0.06C up to 3.18V. Then, constant voltage charging was performed at a voltage of 3.18V until a current of 0.006C was reached. After a 20-minute pause, constant current discharge was performed at a current of 0.06C down to 1.8V. Then, constant voltage discharge was performed at a voltage of 1.8V until a current of 0.006C was reached. This charge-discharge cycle was considered as one charge-discharge cycle, and the charge-discharge cycle was repeated. A 20-minute pause was also provided between discharge and charge. The capacity retention rate after 20 cycles and the number of cycles required to reach an 80% capacity retention rate were measured. The results are shown in Table 1 and Figure 5.

[0095] The C-rate, which represents the current value, is a value based on the theoretical capacity of Li₂O. The "capacity retention rate" represents the ratio of the discharge capacity at any given number of cycles to the initial discharge capacity.

[0096]

[0097] Figure 4A is a cross-sectional SEM image (30,000x magnification) of the cathode composite sheet of Example 1. Figure 4B is a cross-sectional SEM image (30,000x magnification) of the cathode composite sheet of Example 2. Figure 4C is a cross-sectional SEM image (30,000x magnification) of the cathode composite sheet of Comparative Example 1. In Figures 4A to 4C, the white areas correspond to active material particles. The gray areas correspond to the conductive additive (acetylene black) and binder (PTFE).

[0098] As shown in Figures 4A and 4B, in Example 1 and Example 2, the surface of the active material particles was coated with a carbon coating layer. The average thickness of the carbon coating layer in Example 1 (Figure 4A) was 30 nm. The average thickness of the carbon coating layer in Example 2 (Figure 4B) was 200 nm.

[0099] In contrast, as shown in Figure 4C, in Comparative Example 1, acetylene black particles were attached to the surface of the active material particles, but no carbon coating layer was formed on the surface of the active material particles.

[0100] Figure 4D shows the SEM image and EDX elemental map of the cathode composite material of Comparative Example 2. The upper part of Figure 4D is the SEM image. The lower part of Figure 4D is the elemental map targeting carbon atoms. In the elemental map, the dark gray areas represent the presence of carbon atoms. As can be seen from Figure 4D, even when the Co-dissolved Li2O and acetylene black were milled using a ball mill, a carbon coating layer was not formed. In the cathode composite material of Comparative Example 2, only acetylene black particles were partially attached to the surface of the Li2O particles, and a carbon coating layer, such as one formed by the layering of carbon material, could not be observed.

[0101] In the SEM images from Figures 4A to 4D, the active material particles appear to be primary particles, but they are actually secondary particles.

[0102] Figure 5 is a graph showing the relationship between charge / discharge cycles and capacity retention rate for the test cells of Example 1, Example 2, and Comparative Example 1. As shown in Table 1 and Figure 5, the discharge capacity of the test cells of Example 1 and Example 2 after 20 cycles was significantly higher than that of the test cell of Comparative Example 1 after 20 cycles. The number of cycles required for the test cells of Example 1 and Example 2 to reach a capacity retention rate of 80% was significantly higher than the number of cycles required for the test cell of Comparative Example 1 to reach a capacity retention rate of 80%. In other words, the test cells of Example 1 and Example 2 showed excellent cycle characteristics.

[0103] The results of the test cell in Example 2 were superior to those of the test cell in Example 1. From these results, it can be inferred that a moderately large average thickness of the carbon coating layer is advantageous for improving the battery's cycle characteristics.

[0104] Figure 6A shows a cross-sectional SEM image and an EDX elemental map of the cathode composite sheet of Example 1. Figure 6B shows a cross-sectional SEM image and an EDX elemental map of the cathode composite sheet of Example 2. The upper images in Figures 6A and 6B are SEM images. The lower images in Figures 6A and 6B are elemental maps targeting fluorine atoms. As shown in Figures 6A and 6B, fluorine atoms originating from PTFE were also present in the portion corresponding to the carbon coating layer.

[0105] [Measurement of Discharge Rate Characteristics] Test cells for Example 2 and Comparative Example 1 were prepared using the method described above. Each test cell was set in a measuring device (PAT-Tester-i-16, manufactured by EL-CELL Corporation) and charged and discharged at a temperature of 25°C. Specifically, constant current charging was performed at a current value of 0.06C up to 3.18V. Then, constant voltage charging was performed at a voltage of 3.18V until a current value of 0.006C was reached. After a 20-minute pause, constant current discharge was performed at a current value of 0.06C down to 1.8V. After recharging the test cell under the same charging conditions, charging and discharging were performed by increasing the discharge current value in the order of 0.11C, 0.22C, 0.33C, 0.56C, 0.67C, 1.00C, and 1.34C, and the discharge capacity at each current value was measured. A 20-minute pause was also provided between discharge and charging. The results are shown in Table 2 and Figure 7.

[0106]

[0107] Figure 7 is a graph showing the rate characteristics of the test cells of Example 2 and Comparative Example 1. The horizontal axis of Figure 7 represents the current value in Table 2. The vertical axis of Figure 7 represents the capacity retention rate in Table 2. The capacity retention rate of the test cell of Comparative Example 1 represents the ratio of the discharge capacity at an arbitrary current value to the discharge capacity at 0.06C. The capacity retention rate of the test cell of Example 2 represents the ratio of the discharge capacity at an arbitrary current value to the discharge capacity at 0.06C. When the current value was 0.06C, 0.11C, or 0.22C, the discharge capacity of the test cell of Example 2 was equal to the discharge capacity of the test cell of Comparative Example 1. As the current value increased, the discharge capacity of both the test cell of Example 2 and the test cell of Comparative Example 1 decreased. With increasing current, the discharge capacity of the test cell of Comparative Example 1 decreased rapidly. In contrast, the decrease in the discharge capacity of the test cell of Example 2 was gradual. In other words, the test cell of Example 2 had superior discharge rate characteristics compared to the test cell of Comparative Example 1. Based on these results, it can be said that the composite material of Example 2 is suitable for improving the rate characteristics of a battery.

[0108] The technology disclosed herein is useful for batteries such as lithium-ion batteries.

Claims

1. A composite material for batteries comprising: active material particles containing lithium oxide having an inverse fluorite-type structure; and a carbon coating layer containing a carbon material that covers the surface of the active material particles, wherein the coverage rate of the active material particles by the carbon coating layer is 80% or more.

2. The composite material for a battery according to claim 1, wherein the active material particles are secondary particles, and the carbon coating layer coats the surface of the secondary particles.

3. The composite material for a battery according to claim 2, wherein a plurality of the secondary particles are bonded together in such a way that they share the carbon coating layer.

4. The composite material for a battery according to claim 1, wherein the average thickness of the carbon coating layer is in the range of 20 nm to 400 nm.

5. The composite material for a battery according to claim 1, wherein the carbon coating layer contains carbon black as the carbon material.

6. The composite material for a battery according to claim 1, wherein the carbon coating layer comprises a fibrous binder.

7. The composite material for a battery according to claim 1, wherein the carbon coating layer contains polytetrafluoroethylene.

8. The composite material for batteries according to claim 6, wherein the content of the binder in the carbon coating layer is in the range of 0.1% by mass or more and 10% by mass or less.

9. The composite material for a battery according to claim 1, wherein a transition metal is solid-dissolved in the lithium oxide.

10. The composite material for a battery according to claim 9, wherein the transition metal comprises at least one selected from the group consisting of Co, Cu, Ni, Mn, and Fe.

11. A battery comprising electrodes containing the composite material for batteries described in claim 1.

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