Coated active material, positive electrode, and battery

Abstract

A coated active material 100 according to the present disclosure comprises: a positive electrode active material 110; and a coating layer 120 that coats at least a portion of the surface of the positive electrode active material 110. The coating layer 120 has a first layer 111 containing a first solid electrolyte, and a second layer 112 containing a second solid electrolyte and a first conductive assistant. The first layer 111 is located between the second layer 112 and the positive electrode active material 110. The first solid electrolyte contains Li, Ti, M1, and F, where M1 is at least one selected from the group consisting of Ca, Mg, Al, Y, Zr, and Nb. The second solid electrolyte has a composition different from that of the first solid electrolyte.

Description

Coated active material, positive electrode, and battery

[0001] The present disclosure relates to coated active materials, positive electrodes, and batteries.

[0002] In conventional batteries, electrolytes can be decomposed by active materials. When the electrolyte decomposes, a film of electrolyte decomposition products forms inside the electrodes. This leads to disadvantages such as an increase in the internal resistance of the battery and a decrease in the battery's cycle characteristics.

[0003] Coating an active material with an appropriate coating material can suppress electrolyte decomposition caused by the active material. For example, Patent Document 1 discloses coating a positive electrode active material with a solid electrolyte containing Li, Ti, M1, and F. M1 is at least one element selected from the group consisting of Ca, Mg, Al, Y, and Zr.

[0004] International Publication No. 2021 / 187391

[0005] There is room for improvement in the high-temperature storage characteristics of batteries using conventional active materials whose surfaces are coated with a coating material as disclosed in Patent Document 1. That is, there is a demand for improving the high-temperature storage characteristics of batteries using conventional techniques.

[0006] The coated active material of the present disclosure comprises: a cathode active material; and a coating layer that coats at least a portion of a surface of the cathode active material, wherein the coating layer has a first layer containing a first solid electrolyte and a second layer containing a second solid electrolyte and a first conductive additive, the first layer being located between the second layer and the cathode active material, the first solid electrolyte containing Li, Ti, M1, and F, where M1 is at least one selected from the group consisting of Ca, Mg, Al, Y, Zr, and Nb, and the second solid electrolyte having a different composition from the first solid electrolyte.

[0007] According to the present disclosure, it is possible to provide a coated active material that can improve the high-temperature storage characteristics of a battery.

[0008] Fig. 1 is a cross-sectional view showing a schematic configuration of a coated active material according to embodiment 1. Fig. 2 is a cross-sectional view showing a schematic configuration of a positive electrode according to embodiment 2. Fig. 3 is a cross-sectional view showing a schematic configuration of a battery according to embodiment 3.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0010] (Embodiment 1) Fig. 1 is a cross-sectional view showing a schematic configuration of a coated active material according to embodiment 1 of the present disclosure. The coated active material 100 includes a positive electrode active material 110 and a coating layer 120. The positive electrode active material 110 is, for example, in the form of particles. The coating layer 120 coats at least a portion of the surface of the positive electrode active material 110. The coating layer 120 includes a first layer 111 and a second layer 112. The first layer 111 is located between the second layer 112 and the positive electrode active material 110. The first layer 111 is a layer containing a first solid electrolyte. The second layer 112 is a layer containing a second solid electrolyte and a first conductive additive.

[0011] The first solid electrolyte is a solid electrolyte containing Li, Ti, M1, and F. M1 is at least one selected from the group consisting of Ca, Mg, Al, Y, Zr, and Nb. The second solid electrolyte is a solid electrolyte having a different composition from the first solid electrolyte.

[0012] The first solid electrolyte is a fluoride solid electrolyte and has excellent oxidation resistance due to the high electronegativity of fluorine. When the positive electrode active material 110 is coated with the coating layer 120 having the first layer 111 containing the first solid electrolyte, direct contact between the positive electrode active material 110 and the electrolyte is prevented. This reduces decomposition of the electrolyte by the positive electrode active material 110.

[0013] The coating layer 120 further includes a second layer 112 containing a second solid electrolyte having a different composition from the first solid electrolyte and a first conductive additive. The coated active material 100 including such a second layer 112 can suppress an increase in interfacial resistance of the positive electrode active material 110 in a high-temperature environment and improve durability against high-temperature storage. That is, the coated active material 100 according to the first embodiment can improve the high-temperature storage characteristics of the battery. In this way, the coated active material 100 according to the first embodiment can achieve the effect of improving the battery characteristics due to the first layer 111 of the coating layer 120, as well as the effect of improving the high-temperature storage characteristics of the battery due to the second layer 112.

[0014] As described above, the second layer 112 of the coating layer 120 contains the first conductive additive in addition to the second solid electrolyte. This improves the electronic conductivity of the coating layer 120, thereby reducing the possibility that the positive electrode active material 110 will become isolated in the positive electrode. As a result, the discharge capacity of a battery using the coated active material 100 can be improved.

[0015] In the present embodiment, the second layer 112 is, for example, a layer including the outermost surface of the coated active material 100. In other words, at least a portion of the outermost surface of the coated active material 100 is formed by the second layer 112. With such a configuration, the interface resistance between the coated active material 100 and other materials such as the solid electrolyte in the positive electrode is reduced, thereby further improving the discharge capacity of a battery using the coated active material 100 and reducing the resistance value of the battery.

[0016] In the present embodiment, the first layer 111 is in contact with, for example, the positive electrode active material 110. With such a configuration, contact between the positive electrode active material 110 and the electrolyte is more reliably reduced, and the effect of the coating layer 120 in improving the battery characteristics can be more effectively exerted.

[0017] In the coating layer 120, the first layer 111 and the second layer 112 may be provided in contact with each other as shown in Fig. 1 , or another layer may be further included. That is, the coating layer 120 may further include another layer provided between the first layer 111 and the second layer 112, for example. The other layer may include, for example, another solid electrolyte having a composition different from the first solid electrolyte and the second solid electrolyte.

[0018] The positive electrode active material 110, the coating layer 120, the first solid electrolyte, the second solid electrolyte, and the first conductive additive will be described in more detail below.

[0019] The positive electrode active material 110 is, for example, a material that contains lithium and a transition metal and is capable of absorbing and releasing lithium.

[0020] Examples of the positive electrode active material 110 include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce battery manufacturing costs and increase the average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate. At least one selected from these positive electrode active materials can be used. In particular, lithium nickel cobalt manganese oxide (hereinafter referred to as "NCM") is a suitable positive electrode active material for combination with the coating layer 120 in embodiment 1. Therefore, the positive electrode active material 110 includes, for example, NCM.

[0021] The particles of the positive electrode active material 110 may be primary particles or secondary particles. The particles of the positive electrode active material 110 have an average particle size of, for example, 1 μm or more and 10 μm or less. The average particle size refers to the particle diameter (median diameter) when the cumulative volume in the volume-based particle size distribution is 50%. The volume-based particle size distribution is measured, for example, using a laser diffraction particle size distribution analyzer.

[0022] As described above, the first layer 111 of the coating layer 120 is a layer containing the first solid electrolyte. The first layer 111 may further contain other components in addition to the first solid electrolyte. For example, the first layer 111 may further contain a conductive additive in addition to the first solid electrolyte. The conductive additive here refers to a conductive additive used to reduce resistance in the positive electrode. Examples of the conductive additive include carbon materials and conductive polymer compounds. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compounds include polyaniline, polypyrrole, and polythiophene.

[0023] The first layer 111 of the coating layer 120 may consist essentially of the first solid electrolyte, or may consist solely of the first solid electrolyte. Here, "the first layer 111 consists essentially of the first solid electrolyte" means that the content of the first solid electrolyte in the first layer 111 is 90% by mass or more. As an example, the content may be 95% by mass or more.

[0024] As described above, the first solid electrolyte contained in the first layer 111 of the coating layer 120 contains Li, Ti, M1, and F. The first solid electrolyte may consist essentially of Li, Ti, M1, and F, or may consist only of Li, Ti, M1, and F. Here, "the first solid electrolyte consists essentially of Li, Ti, M1, and F" means that the molar ratio of the total amount of substance of Li, Ti, M1, and F to the total amount of substance of all elements constituting the first solid electrolyte is 90% or more. As an example, the molar ratio may be 95% or more.

[0025] In the first solid electrolyte, the ratio of the amount of substance of Li to the total amount of substance of Ti and M1 is, for example, 0.5 or more and 4.5 or less. When this ratio falls within this range, the first solid electrolyte has excellent lithium ion conductivity.

[0026] M1 may be at least one selected from the group consisting of Ca, Mg, and Al, in which case the first solid electrolyte has excellent lithium ion conductivity.

[0027] M1 may be Al, in which case the first solid electrolyte has excellent lithium ion conductivity.

[0028] The first solid electrolyte may have a composition represented by the following formula (1): Li6-(4-4x+m1x)b(Ti 1-x M1 x ) b F6 Formula (1) Here, in the above formula (1), 0<x<1 and 0<b≦2 are satisfied. Note that m1 is the valence of M1.

[0029] When the first solid electrolyte has the composition represented by the above formula (1), the first solid electrolyte has excellent lithium ion conductivity.

[0030] The first solid electrolyte may not contain sulfur. According to the above configuration, generation of hydrogen sulfide gas can be prevented, thereby realizing a battery with improved safety.

[0031] As described above, the second layer 112 of the coating layer 120 is a layer containing a second solid electrolyte and a first conductive additive. The first conductive additive refers to a conductive additive used to reduce resistance, similar to the conductive additive that can be used in the first layer 111. Examples of the first conductive additive include carbon materials and conductive polymer compounds. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymer compounds include polyaniline, polypyrrole, and polythiophene.

[0032] The first conductive additive contained in the second layer 112 of the coating layer 120 may be, for example, particulate. The first conductive additive may include, for example, acetylene black. As described below, the second layer 112 of the coating layer 120 is a thin layer, for example, with an average thickness of approximately 1 nm or more and 150 nm or less. By using a particulate conductive additive as the first conductive additive, even if the thickness of the second layer 112 is very thin, the second layer 112 containing the first conductive additive can be formed, and the effect of improving the battery characteristics provided by the second layer 112 can be more effectively exhibited. For example, by using a particulate conductive additive such as acetylene black as the first conductive additive, the electronic conductivity of the second layer is improved. As a result, the coated active material 100 can further improve the discharge capacity of the battery.

[0033] In the second layer 112, the ratio of the mass of the first conductive additive to the mass of the second solid electrolyte may be, for example, 0.01 mass% or more, or 0.1 mass% or more. In the second layer 112, the ratio of the mass of the first conductive additive to the mass of the second solid electrolyte may be, for example, 3 mass% or less, or 2 mass% or less. In the second layer 112, the ratio of the mass of the first conductive additive to the mass of the second solid electrolyte can be determined by energy dispersive X-ray spectroscopy in combination with a scanning transmission electron microscope (STEM-EDX).

[0034] The second solid electrolyte may contain, for example, a halide solid electrolyte. In this case, the second layer 112 has excellent lithium ion conductivity and can effectively reduce the interfacial resistance between the coated active material 100 and other materials, such as the solid electrolyte, in the positive electrode.

[0035] The second solid electrolyte contains, for example, Li, M2, Y, and X. M2 is at least one element selected from the group consisting of metal elements and semi-metal elements other than Li and Y. X is at least one element selected from the group consisting of F, Cl, Br, and I. In this case, the second layer 112 containing the second solid electrolyte has excellent lithium ion conductivity and can effectively reduce the interfacial resistance between the coated active material 100 and other materials, such as the solid electrolyte, in the positive electrode.

[0036] As used herein, "metalloid elements" include B, Si, Ge, As, Sb, and Te.

[0037] In this specification, the term "metal element" includes all elements included in Groups 1 to 12 of the periodic table excluding hydrogen, and all elements included in Groups 13 to 16 of the periodic table excluding B, Si, Ge, As, Sb, Te, C, N, P, O, S, and Se. In other words, metal elements are a group of elements that can become cations when forming an inorganic compound with a halogen element.

[0038] M2 may be at least one element selected from the group consisting of Group 3 to Group 13 elements. M2 may be at least one element selected from the group consisting of Sc, Ti, Zr, Hf, Nb, Ta, W, Zn, Al, Ga, and In. M2 is preferably at least one element selected from the group consisting of Ti, Hf, Zr, and In, and M2 may be Zr. In this case, the second layer 112 containing the second solid electrolyte has excellent lithium ion conductivity and can effectively reduce the interface resistance between the coated active material 100 and other materials, such as the solid electrolyte, in the positive electrode.

[0039] X may be Cl. In this case, second layer 112 including the second solid electrolyte has excellent lithium ion conductivity and can effectively reduce the interfacial resistance between coated active material 100 and other materials such as the solid electrolyte in the positive electrode.

[0040] The second solid electrolyte may have a composition represented by the following formula (2): d M2 e Y f X6 Formula (2) Here, M2 and X in the above formula (2) are as described above. Furthermore, in the above formula (2), 5.7<d+m2e+3f<6.3, d>0, e>0, and f≧0 are satisfied. Note that m2 is the valence of M2.

[0041] When the second solid electrolyte has a composition represented by the above formula (2), the second layer 112 containing the second solid electrolyte has excellent lithium ion conductivity and can effectively reduce the interfacial resistance between the coated active material 100 and other materials, such as the solid electrolyte, in the positive electrode.

[0042] The second solid electrolyte may not contain sulfur. According to the above configuration, generation of hydrogen sulfide gas can be prevented, thereby realizing a battery with improved safety.

[0043] The mass ratio of the second solid electrolyte to the mass of the positive electrode active material 110 is, for example, greater than 0 mass% and not more than 5 mass%. When the second solid electrolyte is contained in the coated active material 100 at the above mass ratio, the coated active material 100 can further improve the high-temperature storage characteristics of the battery. The mass ratio of the second solid electrolyte to the mass of the positive electrode active material 110 may be not more than 3 mass%, or may be not more than 2 mass%. In this case, the coated active material 100 can further improve the high-temperature storage characteristics of the battery. Furthermore, the mass ratio of the second solid electrolyte to the mass of the positive electrode active material 110 may be not less than 0.1 mass%, or may be not less than 0.5 mass%. In this case, the coated active material 100 can further improve the high-temperature storage characteristics of the battery.

[0044] The ratio of the mass of the second solid electrolyte to the mass of the positive electrode active material 110 can be determined by measuring the masses by, for example, inductively coupled plasma mass spectrometry (ICP-MS).

[0045] The average thickness of the first layer 111 is, for example, 1 nm or more and 150 nm or less. The average thickness of the first layer 111 is preferably 100 nm or less, and more preferably 50 nm or less. By appropriately adjusting the average thickness of the first layer 111, the effect of improving the high-temperature storage characteristics of the battery can be enhanced. The average thickness of the first layer 111 can be calculated from a scanning transmission electron microscope (STEM) image obtained by STEM. The average thickness can be the average value of thicknesses at any multiple points (e.g., five points).

[0046] The average thickness of the second layer 112 is, for example, 1 nm or more and 150 nm or less. The average thickness of the first layer 111 is preferably 100 nm or less. By appropriately adjusting the average thickness of the second layer 112, the effect of improving the high-temperature storage characteristics of the battery can be enhanced. The average thickness of the second layer 112, like the first layer 111, can be calculated from an STEM image obtained by STEM. The average thickness can be the average value of thicknesses at any multiple points (e.g., five points).

[0047] The coated active material 100 according to the first embodiment has an average particle size of, for example, 1 μm or more and 10 μm or less. The average particle size of the coated active material 100 refers to the particle diameter (median diameter) when the cumulative volume in the volume-based particle size distribution is 50%. The volume-based particle size distribution is measured, for example, using a laser diffraction particle size distribution measuring device. When the thickness of the coating layer 120 is on the order of nanometers, the average particle size of the coated active material 100 is approximately equal to the average particle size of the positive electrode active material 110.

[0048] The coated active material 100 according to the first embodiment can be produced, for example, by the following method.

[0049] First, the material of the first layer 111 is attached to the surface of the particles of the positive electrode active material 110. The material of the first layer 111 includes a first solid electrolyte. The material of the first layer 111 may also include a conductive additive.

[0050] The method for adhering the material of the first layer 111 to the surface of the positive electrode active material 110 is not particularly limited. For example, a mixture is obtained by mixing powder of the positive electrode active material 110 and powder of the material of the first layer 111 in an appropriate ratio. The mixture is then milled to impart mechanical energy to the mixture. A mixing device such as a ball mill can be used for the milling. To suppress oxidation of the material, the milling may be performed in a dry and inert atmosphere.

[0051] The first layer 111 may be manufactured by a dry particle compositing method. The treatment by the dry particle compositing method includes applying at least one mechanical energy selected from the group consisting of impact, compression, and shear to the positive electrode active material 110 and the material of the first layer 111.

[0052] Examples of devices that can impart mechanical energy to the mixture of the positive electrode active material 110 and the material of the first layer 111 include processing devices (particle composite devices) such as a ball mill, Mechanofusion (manufactured by Hosokawa Micron Corporation), and Nobilta (manufactured by Hosokawa Micron Corporation).

[0053] In either apparatus, the thickness of the first layer 111 can be controlled by adjusting conditions such as the rotation speed, processing time, and amount of material. Note that processing using the above apparatus is not essential. The first layer 111 may be produced by mixing the positive electrode active material 110 and the material for the first layer 111 using a mortar, a mixer, or the like. The material for the first layer 111 may be deposited on the surface of the positive electrode active material 110 by various methods such as a spray method, a spray-dry coating method, an electrodeposition method, an immersion method, or a mechanical mixing method using a disperser.

[0054] Next, the second layer 112 is formed by attaching a material for the second layer 112 to the surface of the particles on which the first layer 111 is formed on the surface of the positive electrode active material 110. The material for the second layer 112 includes a second solid electrolyte and a first conductive additive.

[0055] The method of adhering the material of the second layer 112 to the surface of the particles on which the first layer 111 is formed on the surface of the positive electrode active material 110 can be the same as the method exemplified above for forming the first layer 111.

[0056] (Embodiment 2) A positive electrode according to embodiment 2 includes the coated active material according to embodiment 1. The coated active material according to embodiment 1 can improve the high-temperature storage characteristics of a battery, as described in embodiment 1. Therefore, the positive electrode according to embodiment 2 can improve the high-temperature storage characteristics of a battery.

[0057] Furthermore, the coated active material according to embodiment 1 can also improve the discharge capacity of the battery, as described in embodiment 1. Therefore, the positive electrode according to embodiment 2 can also improve the discharge capacity of the battery.

[0058] 2 is a cross-sectional view showing a schematic configuration of a positive electrode 200 according to embodiment 2. The positive electrode 200 includes, for example, a positive electrode current collector 210 and a positive electrode active material layer 220 supported on the positive electrode current collector 210. The positive electrode active material layer 220 includes the coated active material 100 according to embodiment 1.

[0059] The positive electrode active material layer 220 may contain only the coated active material 100 according to the first embodiment as the positive electrode active material, or may further contain a positive electrode active material different from the coated active material 100 according to the first embodiment.

[0060] The positive electrode active material layer 220 includes, for example, a solid electrolyte. Hereinafter, the solid electrolyte included in the positive electrode active material layer 220 will be referred to as a third solid electrolyte.

[0061] As shown in FIG. 2 , the positive electrode active material layer 220 is composed of a coated active material 100 and a material phase 221 composed of materials other than the coated active material 100. The material phase 221 includes, for example, a third solid electrolyte. In addition to the third solid electrolyte, the material phase 221 may further include a second conductive additive or the like. The positive electrode active material layer 220 has, for example, a sea-island structure in which the coated active material 100 is regarded as an island and the region of the material phase 221 including the third solid electrolyte is regarded as a sea. In the coated active material 100, the second layer 112 of the coating layer 120 includes a first conductive additive. This effectively reduces the interfacial resistance between the coated active material 100 and other materials, such as the third solid electrolyte.

[0062] Examples of the third solid electrolyte include halide solid electrolytes, sulfide solid electrolytes, polymer solid electrolytes, and complex hydride solid electrolytes. Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-SiS2, Li2S-B2S3, Li2S-GeS2, and Li 3.25 Ge 0.25 P 0.75 S4, Li 10 GeP2S 12Examples of the polymer solid electrolyte include a compound of a polymer compound having an ethylene oxide structure and a lithium salt. The lithium salt may be, for example, at least one selected from the group consisting of LiPF6, LiBF4, LiSbF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), and LiC(SO2CF3)3. Examples of the complex hydride solid electrolyte include LiBH4-LiI and LiBH4-P2S5. The halide solid electrolyte may be a material represented by the following formula (3):

[0063] Li α M3 β X' γ In formula (3), α, β, and γ each independently represent a value greater than 0, M3 represents at least one element selected from the group consisting of metal elements and metalloid elements other than Li, and X′ represents at least one element selected from the group consisting of F, Cl, Br, and I.

[0064] The third solid electrolyte may be at least one selected from the group consisting of a sulfide solid electrolyte and a halide solid electrolyte, in which case the positive electrode 200 has excellent lithium ion conductivity.

[0065] The third solid electrolyte may be a halide solid electrolyte, in which case the positive electrode 200 has excellent lithium ion conductivity.

[0066] When the third solid electrolyte is a halide solid electrolyte, the third solid electrolyte may contain Cl.

[0067] The third solid electrolyte may not contain sulfur. According to the above configuration, generation of hydrogen sulfide gas can be prevented, thereby realizing a battery with improved safety.

[0068] The positive electrode active material layer 220 may contain other materials such as a conductive additive and a binder as needed. That is, the material phase 221 may contain other materials such as a conductive additive and a binder in addition to the third solid electrolyte. Hereinafter, the conductive additive contained in the material phase 221 will be referred to as a second conductive additive.

[0069] The second conductive additive is used to reduce the resistance of the positive electrode 200. Examples of the second conductive additive include a carbon material and a conductive polymer compound. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0070] The binder is used to improve the binding properties of the materials that make up the positive electrode 200. Examples of binders 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.

[0071] When the material phase 221 contains a third solid electrolyte and a second conductive additive, the first conductive additive contained in the second layer 112 of the coating layer 120 of the coated active material 100 and the second conductive additive contained in the material phase 221 may differ from each other in at least one property selected from the group consisting of material and shape. According to this configuration, a conductive additive suitable for the second layer 112 of the coating layer 120 of the coated active material 100 can be selected as the first conductive additive, and a conductive additive suitable for the material phase 221 can be selected as the second conductive additive. This makes it possible to effectively improve battery characteristics such as discharge capacity.

[0072] When the material phase 221 includes a third solid electrolyte and a second conductive additive, for example, the first conductive additive contained in the second layer 112 of the coating layer 120 of the coated active material 100 may be particulate, and the second conductive additive contained in the material phase 221 may be fibrous. The particulate first conductive additive can further improve the discharge capacity of the battery, as described in the first embodiment. The use of a fibrous conductive additive as the second conductive additive in the material phase 221 can ensure sufficient electronic conductivity of the material phase 221 while reducing the proportion of the conductive additive in the material phase 221. This improves the discharge capacity of the battery. In this way, a configuration in which the first conductive additive is particulate and the second conductive additive is fibrous can effectively improve the discharge capacity of the battery. For example, acetylene black may be used as the particulate conductive additive. For example, carbon nanofibers may be used as the fibrous conductive additive.

[0073] The ratio of the first conductive additive in the second layer 112 of the coated active material 100 and the ratio of the second conductive additive in the material phase 221 may be different from each other. According to this configuration, the first conductive additive can be contained in a ratio suitable for the second layer 112 of the coating layer 120 of the coated active material 100, and the second conductive additive can be contained in a ratio suitable for the material phase 221. This makes it possible to effectively improve battery characteristics such as high-temperature storage characteristics and discharge capacity. Furthermore, in this case, even if the second solid electrolyte constituting the second layer 112 and the third solid electrolyte constituting the material phase 221 have the same composition, the interface between the coated active material 100 and the material phase 221 (i.e., the interface between the islands and the sea in a sea-island structure) can be identified by the difference in the ratio of the conductive additive.

[0074] Whether the ratio of the first conductive additive in the second layer 112 and the ratio of the second conductive additive in the material phase 221 are different from each other can be confirmed by elemental mapping of a cross section including the interface between the coated active material 100 and the material phase 221 and its vicinity. The elemental map can be obtained by energy dispersive X-ray spectroscopy (STEM-EDX) in combination with a scanning transmission electron microscope.

[0075] The positive electrode current collector 210 is a sheet or film made of a metal material such as aluminum, an aluminum alloy, stainless steel, titanium, or a titanium alloy. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material may be applied to the surface of the positive electrode current collector 210 as a conductive auxiliary material.

[0076] The positive electrode 200 according to the second embodiment can be produced, for example, by mixing the material constituting the positive electrode active material layer 220 with a solvent to prepare a positive electrode slurry, applying the positive electrode slurry onto the positive electrode current collector 210 to form a coating film, and then drying the coating film.

[0077] (Embodiment 3) A battery according to Embodiment 3 includes the positive electrode according to Embodiment 2, a negative electrode, and a solid electrolyte layer. The solid electrolyte layer is disposed between the positive electrode and the negative electrode. As described in Embodiment 2, the positive electrode according to Embodiment 2 can improve the high-temperature storage characteristics of the battery. Therefore, the battery according to Embodiment 3 can improve the high-temperature storage characteristics.

[0078] Furthermore, the positive electrode according to embodiment 2 can also improve the discharge capacity of the battery, as described in embodiment 2. Therefore, the battery according to embodiment 3 can also improve the discharge capacity.

[0079] 3 is a cross-sectional view showing a schematic configuration of a battery 300 according to Embodiment 3. The battery 300 includes a positive electrode 310, a negative electrode 330, and a solid electrolyte layer 320 disposed between the positive electrode 310 and the negative electrode 330. The positive electrode 310 is the positive electrode 200 according to Embodiment 2. With this configuration, the battery 300 can have improved high-temperature storage characteristics and discharge capacity.

[0080] The battery 300 according to the third embodiment may be an all-solid-state battery.

[0081] The negative electrode 330 includes a negative electrode active material. The negative electrode active material is a material capable of absorbing and desorbing lithium. Examples of negative electrode active materials capable of absorbing and desorbing lithium include lithium titanate, graphite, silicon, silicon-containing oxides, zinc alloys, lithium metal, and lithium alloys. At least one selected from these negative electrode active materials can be used.

[0082] The negative electrode 330 may contain other materials such as a conductive additive, a binder, etc. Materials that can be used for the positive electrode 310 as the conductive additive and the binder can also be used for the negative electrode 330.

[0083] The solid electrolyte layer 320 includes a solid electrolyte. Examples of the solid electrolyte that can be used include a halide solid electrolyte, a sulfide solid electrolyte, an oxide solid electrolyte, a polymer solid electrolyte, and a complex hydride solid electrolyte. The solid electrolyte layer 320 may be formed of a plurality of solid electrolytes having different compositions. The solid electrolyte layer 320 may also be a stack of a plurality of solid electrolyte films.

[0084] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0085] (Technology 1) A coated active material comprising: a cathode active material; and a coating layer coating at least a portion of a surface of the cathode active material, wherein the coating layer has a first layer containing a first solid electrolyte and a second layer containing a second solid electrolyte and a first conductive additive, the first layer being located between the second layer and the cathode active material, the first solid electrolyte containing Li, Ti, M1, and F, where M1 is at least one selected from the group consisting of Ca, Mg, Al, Y, Zr, and Nb, and the second solid electrolyte having a different composition from the first solid electrolyte.

[0086] With this configuration, the high-temperature storage characteristics of the battery can be improved.

[0087] (Technology 2) The coated active material according to Technology 1, wherein the second layer is a layer including the outermost surface of the coated active material.

[0088] With this configuration, the high-temperature storage characteristics of the battery can be effectively improved.

[0089] (Technology 3) The coated active material according to Technology 1 or 2, wherein the first layer is in contact with the positive electrode active material.

[0090] With this configuration, the high-temperature storage characteristics of the battery can be effectively improved.

[0091] (Technology 4) The coated active material according to any one of Technologies 1 to 3, wherein in the first solid electrolyte, a ratio of the amount of substance of Li to the total amount of substance of Ti and the M1 is 0.5 or more and 4.5 or less.

[0092] This provides the first solid electrolyte with excellent lithium ion conductivity.

[0093] (Technology 5) The coated active material according to any one of Technologies 1 to 4, wherein M1 is at least one selected from the group consisting of Ca, Mg, and Al.

[0094] This provides the first solid electrolyte with excellent lithium ion conductivity.

[0095] (Technology 6) The coated active material according to Technology 5, wherein M1 is Al.

[0096] This provides the first solid electrolyte with excellent lithium ion conductivity.

[0097] (Technology 7) The first solid electrolyte has a composition represented by the following formula (1): Li6-(4-4x+m1x)b(Ti 1-x M1 x ) b F6 Formula (1) wherein, in the formula (1), 0<x<1 and 0<b≦2 are satisfied, and m1 is the valence of M1.

[0098] This provides the first solid electrolyte with excellent lithium ion conductivity.

[0099] (Technology 8) The coated active material according to any one of Technologies 1 to 7, wherein the first conductive additive is in a particulate form.

[0100] With this configuration, the discharge capacity of the battery can be improved.

[0101] (Technology 9) The coated active material according to any one of Technologies 1 to 8, wherein the second solid electrolyte contains a halide solid electrolyte.

[0102] With this configuration, the second layer of the coating layer has excellent lithium ion conductivity and can effectively reduce the interface resistance between the coated active material and other materials such as the solid electrolyte in the positive electrode.

[0103] (Technology 10) The coated active material according to Technology 9, wherein the second solid electrolyte is a halide solid electrolyte.

[0104] With this configuration, the second layer of the coating layer has better lithium ion conductivity and can effectively reduce the interface resistance between the coated active material and other materials such as the solid electrolyte in the positive electrode.

[0105] (Technology 11) The coated active material according to any one of technologies 1 to 10, wherein the second solid electrolyte contains Li, M2, Y, and X, wherein M2 is at least one element selected from the group consisting of metal elements and semi-metal elements other than Li and Y, and X is at least one element selected from the group consisting of F, Cl, Br, and I.

[0106] With this configuration, the second layer of the coating layer has excellent lithium ion conductivity and can effectively reduce the interface resistance between the coated active material and other materials such as the solid electrolyte in the positive electrode.

[0107] (Technology 12) The coated active material according to Technology 11, wherein M2 is at least one element selected from the group consisting of Group 3 elements to Group 13 elements.

[0108] With this configuration, the second layer of the coating layer has better lithium ion conductivity and can effectively reduce the interface resistance between the coated active material and other materials such as the solid electrolyte in the positive electrode.

[0109] (Technology 13) The coated active material according to Technology 12, wherein M2 is at least one selected from the group consisting of Sc, Ti, Zr, Hf, Nb, Ta, W, Zn, Al, Ga, and In.

[0110] With this configuration, the second layer of the coating layer has better lithium ion conductivity and can effectively reduce the interface resistance between the coated active material and other materials such as the solid electrolyte in the positive electrode.

[0111] (Technology 14) The second solid electrolyte has a composition represented by the following formula (2): Li d M2 e Y f X6 Formula (2) wherein, in the formula (2), 5.7<d+m2e+3f<6.3, d>0, e>0, and f≧0 are satisfied, and the m2 is a valence of the M2.

[0112] With this configuration, the second layer of the coating layer has better lithium ion conductivity and can effectively reduce the interface resistance between the coated active material and other materials such as the solid electrolyte in the positive electrode.

[0113] (Technology 15) The coated active material according to any one of Techniques 11 to 14, wherein X is Cl.

[0114] With this configuration, the second layer of the coating layer has better lithium ion conductivity and can effectively reduce the interface resistance between the coated active material and other materials such as the solid electrolyte in the positive electrode.

[0115] (Technology 16) The coated active material according to any one of Technologies 1 to 15, wherein a ratio of the mass of the second solid electrolyte to the mass of the positive electrode active material is greater than 0 mass % and is 5 mass % or less.

[0116] With this configuration, the high-temperature storage characteristics of the battery can be effectively improved.

[0117] (Technology 17) The coated active material according to any one of Technologies 1 to 16, wherein the first layer has an average thickness of 1 nm or more and 150 nm or less.

[0118] With this configuration, the high-temperature storage characteristics of the battery can be further improved.

[0119] (Technology 18) The coated active material according to any one of Technologies 1 to 17, wherein the second layer has an average thickness of 1 nm or more and 150 nm or less.

[0120] With this configuration, the high-temperature storage characteristics of the battery can be further improved.

[0121] (Technology 19) A positive electrode comprising the coated active material according to any one of technologies 1 to 18.

[0122] With this configuration, the high-temperature storage characteristics of the battery can be improved.

[0123] (Technology 20) The positive electrode according to Technology 19, wherein the positive electrode comprises a positive electrode active material layer including the coated active material and a material phase excluding the coated active material, the material phase including a third solid electrolyte and a second conductive additive, and the first conductive additive and the second conductive additive differ from each other in at least one selected from the group consisting of material and shape.

[0124] With this configuration, the discharge capacity of the battery can be improved.

[0125] (Technology 21) The positive electrode according to Technology 20, wherein the first conductive additive is particulate and the second conductive additive is fibrous.

[0126] With this configuration, the discharge capacity of the battery can be further improved.

[0127] (Technology 22) The positive electrode according to Technology 20 or 21, wherein a ratio of the first conductive additive in the second layer of the coated active material and a ratio of the second conductive additive in the material phase are different from each other.

[0128] With this configuration, battery characteristics such as high-temperature storage characteristics and discharge capacity can be effectively improved.

[0129] (Technology 23) A battery comprising: the positive electrode according to any one of Technologies 19 to 22; a negative electrode; and a solid electrolyte layer disposed between the positive electrode and the negative electrode.

[0130] With this configuration, the high-temperature storage characteristics of the battery can be improved.

[0131] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0132] Example 1 Preparation of First Solid Electrolyte In an argon atmosphere having a dew point of -60°C or less, raw material powders LiF, TiF, and AlF were weighed in a molar ratio of LiF:TiF:AlF = 2.7:0.3:0.7. These were pulverized and mixed in a mortar to obtain a mixture. Thereafter, the mixture was milled using a φ5 mm zirconia ball and a planetary ball mill (Fritsch, P-7 model) at 500 rpm for 12 hours. As a result, Li 2.7 Ti 0.3 Al 0.7 A powdery first solid electrolyte having a composition of LiF was obtained. 2.7 Ti 0.3 Al 0.7 F6 is written as "LTAF".

[0133] [Preparation of Second Solid Electrolyte] In an argon atmosphere having a dew point of -60°C or less, raw material powders LiCl, ZrCl4, and YCl3 were weighed in a molar ratio of LiCl:ZrCl4:YCl3 = 2.5:0.5:0.5. These were pulverized and mixed in a mortar to obtain a mixture. Thereafter, the mixture was milled using a φ5 mm zirconia ball and a planetary ball mill (manufactured by Fritsch, Model P-7) at 500 rpm for 12 hours. As a result, Li 2.5 Zr 0.5 Y 0.5 A powdery second solid electrolyte having a composition of LiCl was obtained. 2.5 Zr 0.5 Y0.5 Cl6 is abbreviated as "LZYC".

[0134] [Preparation of Coated Cathode Active Material] An NCM powder (average particle size 5 μm) was prepared as the cathode active material. A first solid electrolyte was attached to the surface of the NCM particles to form a first coating layer. The first coating layer was formed by compressive shear treatment using a particle compositer (NOB-MINI, manufactured by Hosokawa Micron Corporation). Specifically, the NCM and the first solid electrolyte were mixed in a mass ratio of 100:3, and the mixture was treated under the conditions of a rotation speed of 6000 rpm and a treatment time of 50 min. This resulted in a cathode active material (a cathode active material with a first layer) having a first coating layer formed on its surface.

[0135] A mixture of a second solid electrolyte and a first conductive additive was attached to the surface of particles of the positive electrode active material with the first layer to form a second layer of the coating layer. Carbon nanofibers (manufactured by Showa Denko K.K.) were used as the first conductive additive. The second solid electrolyte and the first conductive additive were mixed such that the mass ratio of the second solid electrolyte to the first conductive additive was 100:0.5. The second layer of the coating layer was formed by a compressive shear treatment using a particle compositer (NOB-MINI, manufactured by Hosokawa Micron Corporation). Specifically, the positive electrode active material with the first layer and the mixture of the second solid electrolyte and the first conductive additive were processed at a rotation speed of 6000 rpm for a processing time of 50 minutes so that the mass ratio of the NCM to the mixture of the second solid electrolyte and the first conductive additive was NCM:(mixture of the second solid electrolyte and the first conductive additive) = 100:0.5. As a result, the second layer of the coating layer was formed, and the coated active material of Example 1 was obtained.

[0136] [Preparation of Positive Electrode Composite] The coated active material, third solid electrolyte, and second conductive additive were mixed in an agate mortar to prepare a positive electrode composite. The mass ratio of the coated active material, third solid electrolyte, and second conductive additive was coated active material: third solid electrolyte: second conductive additive = 64:34:2. LZYC was used as the third solid electrolyte. Carbon nanofiber (manufactured by Showa Denko K.K.) was used as the second conductive additive. Hereinafter, carbon fiber will be referred to as CNF.

[0137] [Preparation of Negative Electrode Mixture] A mixture of LiYBrCl (hereinafter referred to as "LYBC") as a solid electrolyte and LiTiO as a negative electrode active material was mixed in a dry argon atmosphere. 12 The negative electrode mixture was prepared by mixing the negative electrode active material (average particle size 2.5 μm) and the conductive additive in an agate mortar. The mass ratio of the negative electrode active material, solid electrolyte, and conductive additive was 64:34:2. CNF (manufactured by Showa Denko K.K.) was used as the conductive additive.

[0138] [Battery Fabrication] 63.3 mg of a positive electrode composite, 50.9 mg of LZYC, 60.0 mg of LYBC, and 82.3 mg of a negative electrode composite were stacked in this order in an insulating outer cylinder having an inner diameter of 9.4 mm. The positive electrode composite, solid electrolyte, and negative electrode composite were press-molded at a pressure of 720 MPa. This produced a laminate having a positive electrode, an electrolyte layer, and a negative electrode. Next, stainless steel current collectors were placed on the top and bottom of the laminate, and current collector leads were attached to the current collectors. Finally, the insulating outer cylinder was sealed using an insulating ferrule to isolate the interior of the insulating outer cylinder from the outside atmosphere.

[0139] [Battery Evaluation] <Measurement of Initial Discharge Capacity> The initial discharge capacity of the battery of Example 1 was measured by the following method. After constant current charging at a current of 0.01 C until the voltage reached 2.75 V, constant current discharging was performed at a current of 0.01 C until the voltage reached 0.9 V. The discharge capacity measured at this time was considered to be the initial discharge capacity. The rest time between charge and discharge was 60 minutes. Charging and discharging were performed at a temperature condition of 25°C (ambient temperature). The results are shown in Table 1.

[0140] <High-Temperature Storage Test> A high-temperature storage test was conducted on the battery of Example 1 as follows. After continuous charging at 100°C (ambient temperature) for 500 hours, charging was stopped and the battery was stored in an environment at 100°C for 100 hours. The temperature was then returned to 25°C and charge / discharge measurements were conducted. The discharge capacity measured at this time was regarded as the recovered discharge capacity. The ratio of the recovered discharge capacity to the initial discharge capacity was defined as the recovery rate. The results are shown in Table 1.

[0141] Example 2 In the preparation of the coated active material, when forming the second layer, the mass ratio of the second solid electrolyte to the first conductive additive was changed to 100:1. Except for this change, the coated active material and battery of Example 2 were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0142] Example 3 In the preparation of the coated active material, acetylene black (manufactured by Denka Co.) was used as the first conductive additive instead of CNF when forming the second layer. Except for this change, the coated active material and battery of Example 3 were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1. Hereinafter, acetylene black will be referred to as AB.

[0143] Example 4 In the preparation of the coated active material, when forming the second layer, AB (manufactured by Denka Corporation) was used as the first conductive additive instead of CNF, and the mass ratio of the second solid electrolyte to the first conductive additive was changed to 100:1. Except for these changes, the coated active material and battery of Example 4 were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0144] (Comparative Example 1) In the preparation of the coated active material, the second layer was not formed. That is, only the first layer was formed on the surface of the NCM to prepare the coated active material of Comparative Example 1. Except for this change, the coated active material and battery of Comparative Example 1 were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0145] Reference Example 1 In the preparation of the coated active material, only the second solid electrolyte was used to form the second layer. That is, the second layer was formed without containing a conductive additive and comprised only the second solid electrolyte. Except for this change, the coated active material and battery of Reference Example 1 were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0146]

[0147] As shown in Table 1, the initial discharge capacities of the batteries of Examples 1 to 4 were greater than the initial discharge capacity of the battery of Comparative Example 1.

[0148] As shown in Table 1, the recovery rates of the recovered discharge capacity after the high-temperature storage test relative to the initial discharge capacity were good in the batteries of Examples 1 to 4, and exceeded the recovery rate of the battery of Comparative Example 1. In other words, the batteries of Examples 1 to 4 were excellent in durability against storage in a charged state at high temperatures, and had improved high-temperature storage characteristics compared to the battery of Comparative Example 1.

[0149] Furthermore, as shown in Table 1, the battery of Reference Example 1, in which the coating layer of the coated active material had a second layer but the second layer did not contain a conductive additive, had an initial discharge capacity higher than those of the batteries of Examples 1 to 3 and comparable to that of the battery of Example 4. However, the recovery rate of the battery of Reference Example 1 was lower than that of the batteries of Examples 1 to 4. In other words, the batteries of Examples 1 to 4 had improved high-temperature storage characteristics compared to the battery of Reference Example 1.

[0150] As described above, a coated active material coated with a coating layer having a first layer and a second layer containing a solid electrolyte and a conductive additive can improve high-temperature storage characteristics and initial discharge capacity compared to a coated active material coated with a coating layer consisting only of the first layer. Furthermore, a coated active material coated with a coating layer having a first layer and a second layer containing a solid electrolyte and a conductive additive can improve high-temperature storage characteristics compared to a coated active material coated with a coating layer in which the second layer does not contain a conductive additive.

[0151] The technology of the present disclosure is useful for, for example, lithium ion secondary batteries.

Claims

1. A coated active material comprising: a positive electrode active material; and a coating layer covering at least a portion of a surface of the positive electrode active material, wherein the coating layer has a first layer containing a first solid electrolyte and a second layer containing a second solid electrolyte and a first conductive additive, the first layer being located between the second layer and the positive electrode active material, the first solid electrolyte containing Li, Ti, M1, and F, where M1 is at least one selected from the group consisting of Ca, Mg, Al, Y, Zr, and Nb, and the second solid electrolyte having a different composition from the first solid electrolyte.

2. The coated active material according to claim 1, wherein the second layer is a layer including the outermost surface of the coated active material.

3. The coated active material according to claim 1, wherein the first layer is in contact with the positive electrode active material.

4. The coated active material according to claim 1, wherein in the first solid electrolyte, the ratio of the amount of substance of Li to the total amount of substance of Ti and M1 is 0.5 or more and 4.5 or less.

5. The coated active material according to claim 1, wherein M1 is at least one selected from the group consisting of Ca, Mg, and Al.

6. The coated active material according to claim 5, wherein M1 is Al.

7. The first solid electrolyte has a composition represented by the following formula (1): Li6-(4-4x+m1x)b(Ti 1-x M1 x ) b F6 Formula (1) wherein, in said formula (1), 0<x<1 and 0<b≦2 are satisfied, and said m1 is the valence of said M1.

8. The coated active material according to claim 1, wherein the first conductive additive is in particulate form.

9. The coated active material according to claim 1, wherein the second solid electrolyte contains a halide solid electrolyte.

10. The coated active material according to claim 9, wherein the second solid electrolyte is a halide solid electrolyte.

11. The coated active material according to claim 1, wherein the second solid electrolyte contains Li, M2, Y, and X, wherein M2 is at least one element selected from the group consisting of metal elements and semi-metal elements other than Li and Y, and X is at least one element selected from the group consisting of F, Cl, Br, and I.

12. The coated active material according to claim 11, wherein M2 is at least one element selected from the group consisting of elements of Groups 3 to 13.

13. The coated active material according to claim 12, wherein M2 is at least one selected from the group consisting of Sc, Ti, Zr, Hf, Nb, Ta, W, Zn, Al, Ga, and In.

14. The second solid electrolyte has a composition represented by the following formula (2): Li d M2 e Y f X6 Formula (2) wherein, in said formula (2), the conditions 5.7<d+m2e+3f<6.3, d>0, e>0, and f≧0 are satisfied, and said m2 is the valence of said M2. The coated active material according to claim 11 .

15. The coated active material according to claim 11, wherein X is Cl.

16. The coated active material according to claim 1, wherein the ratio of the mass of the second solid electrolyte to the mass of the positive electrode active material is greater than 0 mass % and is 5 mass % or less.

17. The coated active material according to claim 1, wherein the average thickness of the first layer is 1 nm or more and 150 nm or less.

18. The coated active material according to claim 1, wherein the average thickness of the second layer is 1 nm or more and 150 nm or less.

19. A positive electrode comprising the coated active material according to any one of claims 1 to 18.

20. The positive electrode according to claim 19, wherein the positive electrode comprises a positive electrode active material layer consisting of the coated active material and a material phase excluding the coated active material, the material phase including a third solid electrolyte and a second conductive additive, and the first conductive additive and the second conductive additive differ from each other in at least one property selected from the group consisting of material and shape.

21. The positive electrode according to claim 20, wherein the first conductive additive is in particulate form and the second conductive additive is in fibrous form.

22. The positive electrode according to claim 20, wherein the proportion of the first conductive additive in the second layer of the coated active material is different from the proportion of the second conductive additive in the material phase.

23. A battery comprising: the positive electrode according to claim 19; a negative electrode; and a solid electrolyte layer disposed between the positive electrode and the negative electrode.

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