Positive electrode material, and positive electrode active material layer and lithium secondary battery using same

Abstract

The present invention provides a means capable of improving discharge capacity at a high rate in a lithium secondary battery. Provided is a positive electrode material comprising a first positive electrode active material that is not coated with a solid electrolyte, and a positive electrode active material composite that contains a second positive electrode active material and a first solid electrolyte that covers at least a portion of the surface of the second positive electrode active material.

Description

Positive electrode material, positive electrode active material layer using the same, and lithium secondary battery

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

[0002] In recent years, research and development into all-solid-state lithium secondary batteries using oxide- or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials primarily composed of ionic conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries, in principle, do not encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the battery's power density and energy density.

[0003] Conventionally, in all-solid-state lithium secondary batteries, a cathode active material layer is prepared by molding a mixture containing a cathode active material and a solid electrolyte using a method such as powder compaction. When a cathode active material layer is prepared using this method, oxidative decomposition of the solid electrolyte during charging increases the interfacial resistance between the cathode active material and the solid electrolyte, resulting in a decrease in the charge / discharge efficiency of the battery. To address this issue, for example, International Publication No. 2021 / 199618 (corresponding to U.S. Patent Application Publication No. 2023 / 0002244) discloses a coated cathode active material having a structure in which concave portions of the surface irregularities of the cathode active material are filled with a chlorine-containing solid electrolyte, leaving convex portions of the surface irregularities of the cathode active material exposed. According to the above document, this configuration makes it possible to reduce the interfacial resistance between the cathode active material and the solid electrolyte, thereby improving the charge / discharge efficiency of the battery.

[0004] However, according to the investigations of the present inventors, it has been found that even if a lithium secondary battery is fabricated using the technology described in the above document, sufficient capacity may not be obtained during high-rate discharge.

[0005] Therefore, an object of the present invention is to provide a means for improving the high-rate discharge capacity of a lithium secondary battery.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and in the process have found that the above-mentioned problems can be solved by forming a positive electrode active material layer using a mixture containing a positive electrode active material that is not coated with a solid electrolyte and a positive electrode active material that is coated with a solid electrolyte, thereby completing the present invention.

[0007] That is, a cathode material according to one embodiment of the present invention is characterized in that it includes a first cathode active material that is not coated with a solid electrolyte, and a cathode active material composite that includes a second cathode active material and the first solid electrolyte that coats at least a portion of the surface of the second cathode active material.

[0008] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) lithium secondary battery (stacked-type secondary battery) according to one embodiment of the present invention. FIG. 2 is a micrograph obtained by observing, using an SEM, a first cathode active material contained in the cathode material of Example 1. FIG. 3 is a micrograph obtained by observing, using an SEM, a cathode active material composite contained in the cathode material of Example 1. FIG. 4 is a micrograph obtained by observing, using an SEM, a cross section of the cathode active material layer produced in Example 1. FIG. 5 is a micrograph obtained by observing, using an SEM, a cross section of the cathode active material layer produced in Comparative Example 2. FIG. 6 is a micrograph obtained by observing, using an SEM, a cross section of the cathode active material layer produced in Comparative Example 4.

[0009] One aspect of the present invention is a cathode material including a first cathode active material that is not coated with a solid electrolyte, and a cathode active material composite including a second cathode active material and the first solid electrolyte that coats at least a portion of the surface of the second cathode active material. The cathode material according to this aspect can improve the high-rate discharge capacity of a lithium secondary battery.

[0010] Hereinafter, a cathode active material layer and a lithium secondary battery using the cathode material according to the present embodiment will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and duplicated explanations will be omitted. The dimensional ratios in the drawings are exaggerated for the sake of explanation and may differ from the actual ratios. The range "X to Y" means "X or more and Y or less."

[0011] Fig. 1 is a cross-sectional view schematically illustrating the overall structure of a stacked-type (internal parallel connection type) lithium secondary battery (hereinafter simply referred to as a "stacked-type secondary battery") according to one embodiment of the present invention. Fig. 1 shows a cross section of the stacked-type secondary battery during charging.

[0012] As shown in FIG. 1 , a power generating element 21 of a stacked secondary battery 10 a of this embodiment has a configuration in which a negative electrode in which negative electrode active material layers 13 containing lithium metal are disposed on both sides of a negative electrode current collector 11 ′, a solid electrolyte layer 17, and a positive electrode in which positive electrode active material layers 15 containing the positive electrode material according to one embodiment of the present invention are disposed on both sides of a positive electrode current collector 11 ″ are laminated. Specifically, the negative electrode, solid electrolyte layer, and positive electrode are laminated in this order such that one negative electrode active material layer 13 and an adjacent positive electrode active material layer 15 face each other with the solid electrolyte layer 17 interposed therebetween. As a result, adjacent negative electrode, solid electrolyte layer, and positive electrode constitute one unit cell layer 19. Therefore, the stacked secondary battery 10 a shown in FIG. 1 can also be said to have a configuration in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel.

[0013] A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29. The negative electrode current collector 25 and the positive electrode current collector 27 may be attached to the negative electrode current collector 11′ and the positive electrode current collector 11″ of the respective electrodes by ultrasonic welding, resistance welding, or the like, via a negative electrode terminal lead and a positive electrode terminal lead (not shown), respectively, as necessary.

[0014] In the above description, an embodiment of the lithium secondary battery according to one aspect of the present invention has been described using as an example a so-called lithium deposition type stacked (internal parallel connection type) all-solid-state lithium secondary battery in which lithium metal is deposited on the surface of the negative electrode current collector during charging. However, the type of lithium secondary battery to which the present invention can be applied is not particularly limited, and the present invention can also be applied to lithium secondary batteries that are not of the lithium deposition type and bipolar lithium secondary batteries.

[0015] The main components of a lithium secondary battery to which the positive electrode material according to this embodiment is applied will be described below.

[0016] [Current Collector] The current collector (negative electrode current collector, positive electrode current collector) has the function of mediating the movement of electrons from the electrode active material layer (negative electrode active material layer, positive electrode active material layer). There are no particular restrictions on the material that constitutes the current collector. Examples of materials that can be used for the current collector include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular restrictions on the thickness of the current collector, but an example is 10 to 100 μm.

[0017] [Positive Electrode Active Material Layer] The positive electrode active material layer essentially contains the positive electrode material according to the present embodiment. The positive electrode active material layer is typically disposed on the surface of a positive electrode current collector as shown in Fig. 1 . However, if the positive electrode active material layer itself has a certain degree of conductivity, the positive electrode active material layer itself may constitute a positive electrode without using a positive electrode current collector.

[0018] (Cathode Material) The cathode material according to this embodiment is characterized by including a first cathode active material that is not coated with a solid electrolyte, and a cathode active material composite including a second cathode active material and the first solid electrolyte that coats at least a portion of the surface of the second cathode active material. This configuration allows a cathode active material layer incorporating the cathode material to have both an electron conduction path formed by contact between the cathode active materials and a lithium ion conduction path via the solid electrolyte of the cathode active material composite. This is believed to improve the utilization efficiency of the cathode active material and the high-rate discharge capacity.

[0019] The types of the first and second positive electrode active materials are not particularly limited as long as they are materials capable of releasing lithium ions during charging and absorbing lithium ions during discharging. From the viewpoint of high capacity, the positive electrode active material preferably contains a lithium transition metal composite oxide. The lithium transition metal composite oxide is a compound belonging to the space group R3m and has a layered structure (layered rock salt structure) in which lithium atomic layers and transition metal atomic layers are alternately stacked. Therefore, the use of such a positive electrode active material can improve battery capacity.

[0020] Examples of lithium transition metal composite oxides include LiCoO ２ , LiNiO ２ , LiMnO ２ , Li(Ni-Mn-Co)O ２ , Li(Ni-Co-Al)O ２ and those in which a part of these transition metals has been substituted with other elements. More preferably, a composite oxide containing lithium and nickel is used, and even more preferably, Li(Ni—Mn—Co)O ２ and those in which part of these transition metals is replaced by other elements (hereinafter also referred to simply as "NMC composite oxides") or Li(Ni-Co-Al)O ２ Also, oxides in which part of these transition metals has been substituted with other elements (hereinafter simply referred to as "NCA composite oxides") are used, and NMC composite oxides are particularly preferred.

[0021] As described above, the NMC composite oxide and the NCA composite oxide also include composite oxides in which a portion of the transition metal element is replaced by another metal element. In this case, the other element may be Al, Sn, Nb, Ti, Zr, Mg, W, P, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, V, Cu, Ag, or Zn, preferably Al, Sn, Nb, Ti, Zr, Mg, W, P, V, Ca, Sr, or Cr, more preferably Al, Sn, Nb, Ti, Zr, Mg, P, or Cr, and even more preferably Al, Sn, Nb, Ti, Zr, or Mg from the viewpoint of improving cycle characteristics. However, the other metal element that can replace the transition metal element of the NCA composite oxide is other than Al.

[0022] The lithium transition metal composite oxide as the positive electrode active material has a high theoretical discharge capacity, and is therefore preferably a lithium transition metal composite oxide represented by the general formula (1): Li ｘ Ni ａ M ｂ N ｃ O ２(wherein x, a, b, c, and d satisfy 0.8≦x≦1.1, a+b+c=1, 0.33≦a≦0.95, 0.05≦b≦0.67, and 0≦c≦0.10. M is one or more elements selected from the group consisting of Mn and Co, and N is one or more elements selected from the group consisting of Al, Sn, Nb, Ti, Zr, and Mg). Here, x represents the atomic ratio of Li, a represents the atomic ratio of Ni, b represents the atomic ratio of M, and c represents the atomic ratio of N. The composition of each element can be measured, for example, by inductively coupled plasma (ICP) emission spectrometry. From the viewpoint of achieving a higher discharge capacity, it is preferable that in general formula (1), 0.80≦a≦0.95 (i.e., a high-nickel composite oxide).

[0023] Each of the first and second positive electrode active materials may contain an active material other than the lithium transition metal composite oxide. In some cases, two or more positive electrode active materials may be used in combination. However, in each of the first and second positive electrode active materials, the content of the lithium transition metal composite oxide relative to the total amount (100% by mass) of the positive electrode active material is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass. The first and second positive electrode active materials may be composed of the same type (composition) of positive electrode active material or different types (compositions). However, it is preferable that they be composed of the same type (composition) of positive electrode active material. It is more preferable that the first and second positive electrode active materials are both NMC composite oxides having the same composition.

[0024] The average particle diameters of the first and second positive electrode active materials are not particularly limited, but it is preferable that the average particle diameters of the first and second positive electrode active materials are different. This configuration reduces interparticle gaps in the positive electrode active material layer, increasing the number of contact points between the positive electrode active materials, thereby further improving high-rate discharge capacity. Furthermore, it is more preferable that the average particle diameter of the first positive electrode active material be larger than the average particle diameter of the second positive electrode active material. This configuration improves contact between particles of the positive electrode active material composite in the positive electrode active material layer, improving lithium ion conductivity and further improving high-rate discharge capacity. In this case, the ratio (d1 / d2) of the average particle diameter (d1) of the first positive electrode active material to the average particle diameter (d2) of the second positive electrode active material is preferably 1.1 to 5.0, more preferably 1.2 to 4.0, even more preferably 1.5 to 3.0, and particularly preferably 1.7 to 2.5. If the ratio (d1 / d2) is within the above range, electron conduction paths and lithium ion conduction paths are well formed, making it possible to further improve the discharge capacity at high rates.

[0025] The average particle diameter of the first positive electrode active material and the average particle diameter of the second positive electrode active material are each independently preferably 1 nm to 100 μm, more preferably 10 nm to 50 μm, even more preferably 100 nm to 20 μm, and particularly preferably 1 to 20 μm. In this specification, "particle diameter" refers to the maximum distance between any two points on the outline of a particle observed in several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). In addition, the value of "average particle diameter" refers to the arithmetic mean value of the "particle diameter".

[0026] In the positive electrode active material composite, the solid electrolyte used as the first solid electrolyte is not particularly limited, and a conventionally known solid electrolyte can be appropriately adopted. Examples of the solid electrolyte include sulfide-based solid electrolytes and oxide-based solid electrolytes. In this specification, the solid electrolyte refers to a material mainly composed of an ion conductor capable of ion conduction in a solid, and in particular, a material having a lithium ion conductivity of 1×10 at room temperature (25° C.). －５ S / cm or more, and this lithium ion conductivity is preferably 1×10 －４ The lithium ion conductivity is 2.5 S / cm or more. The lithium ion conductivity can be measured by an AC impedance method.

[0027] Examples of sulfide solid electrolytes include LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ２ S-P ２ S ５ , LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ , Li ２ S-P ２ S ５ - LiI, Li ２ S-P ２ S ５ -Li ２ O, Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ S-SiS ２ , Li ２ S-SiS ２ - LiI, Li ２ S-SiS ２ - LiBr, Li ２ S-SiS ２ -LiCl, Li ２S-SiS ２ -B ２ S ３ - LiI, Li ２ S-SiS ２ -P ２ S ５ - LiI, Li ２ S-B ２ S ３ , Li ２ S-P ２ S ５ -Z ｍ S ｎ (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li ２ S-GeS ２ , Li ２ S-SiS ２ -Li ３ P.O. ４ , Li ２ S-SiS ２ -Li ｘ MO ｙ (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In). ２ S-P ２ S ５ " is written by Li ２ S and P ２ S ５ The same applies to other descriptions.

[0028] The sulfide solid electrolyte is, for example, Li ３ P.S. ４ It may have a Li framework. ４ P ２ S ７ It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a skeleton include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li４ P ２ S ７ Examples of the sulfide solid electrolyte having a skeleton include a Li-P-S solid electrolyte called LPS (for example, Li ７ P ３ S １１ ) can be mentioned. In addition, examples of sulfide solid electrolytes include Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ (x satisfies 0<x<1), or the like may be used. Among them, a sulfide solid electrolyte containing a P element is preferable. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I), and an example thereof is Li. ６ P.S. ５ X (wherein X is Cl, Br or I, preferably Cl).

[0029] Examples of oxide solid electrolytes include compounds having a NASICON structure. An example of a compound having a NASICON structure is a compound represented by the general formula Li １＋ｘ Al ｘ Ge ２－ｘ (P.O. ４ ) ３ (0≦x≦2) (LAGP), a compound represented by the general formula Li １＋ｘ Al ｘ Ti ２－ｘ (P.O. ４ ) ３ (0≦x≦2) (LATP) and the like. Another example of the oxide solid electrolyte is LiLaTiO (for example, Li ０．３４ La ０．５１ TiO ３ ), LiPON (e.g., Li ２．９ P.O. ３．３ N ０．４６ ), LiLaZrO (e.g., Li ７ La ３ Zr ２ O １２ ) etc.

[0030] The method for producing the cathode active material composite is not particularly limited as long as it is a method capable of coating (immobilizing) the first solid electrolyte on the surface of the second cathode active material. As the coating (immobilization) method, a dry method is preferably used, in which mechanical energy is applied to the second cathode active material and the first solid electrolyte, causing particles to collide with each other to perform coating. The device used for coating (immobilization) is preferably one that can apply a large shear force to increase the frequency of particle collisions. An example of such a device is the Balance Gran BG-2L (manufactured by Freund Turbo Corporation).

[0031] The amount of the first solid electrolyte in the cathode active material composite is not particularly limited, but is preferably 1 to 20 parts by mass, and more preferably 2 to 15 parts by mass, relative to 100 parts by mass of the second cathode active material. When the amount of the first solid electrolyte is within the above range, the surface of the second cathode active material can be sufficiently coated, and the second cathode active material can easily come into contact with other cathode active materials in the cathode active material layer.

[0032] The positive electrode active material composite preferably has a shape closer to a sphere. More specifically, when observed with a scanning electron microscope (SEM), the positive electrode active material composite preferably has a circularity C defined by the following formula of 0.8 or more.

[0033]

[0034] In the above formula, π represents the ratio of the circumference of a circle to its diameter, and S represents the area of ​​the positive electrode active material composite (μm ２ ) and L represents the perimeter (μm) of the positive electrode active material composite.

[0035] By adopting such a configuration, the formability of the positive electrode active material layer can be improved when the positive electrode active material layer is prepared using a dry method as described in the Examples below. Furthermore, by adopting such a configuration, there is also the advantage that the density of the positive electrode active material layer can be improved. The circularity C is more preferably 0.85 or more, and more preferably 0.88 or more (upper limit: 1).

[0036] The bulk density of the positive electrode active material composite is not particularly limited, but is preferably 1.3 g / cm３ More than 2.0 g / cm ３ or less, more preferably 1.4 g / cm ３ 1.8g / cm or more ３ If the bulk density is within the above range, the formability of the positive electrode active material layer can be improved when the positive electrode active material layer is produced using a dry method as described in the examples below.

[0037] In the positive electrode material according to this embodiment, the contents of the first positive electrode active material and the positive electrode active material composite are not particularly limited, but the content of the first positive electrode active material is preferably more than 50% by mass, more preferably more than 50% by mass but not more than 95% by mass, even more preferably 60% by mass or more but not more than 95% by mass, particularly preferably 70% by mass or more but not more than 95% by mass, and most preferably 80% by mass or more but not more than 90% by mass, relative to the total mass of the first positive electrode active material and the positive electrode active material composite. When the first positive electrode active material is contained in a larger amount than the positive electrode active material composite, contact points between particles of the positive electrode active material in the positive electrode active material layer are more easily formed, and the discharge capacity at high rates can be further improved.

[0038] The positive electrode material according to this embodiment preferably contains a fibrous conductive additive in addition to the first positive electrode active material and the positive electrode active material composite. The conductive additive preferably has an electronic conductivity of 1 S / m or more, and more preferably 1×10 ２ S / m or more is more preferable, and 1×10 ４ It is more preferable that the density is 1×10 S / m or more. ５ The upper limit of the electronic conductivity of the conductive additive is not particularly limited, but is usually 1×10 ７S / m or less. By adopting such a configuration, an electron conduction path is formed between the positive electrode active materials, thereby improving the conductivity of the positive electrode active material layer. In this specification, the term "fibrous conductive additive" refers to a conductive additive having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less in an image observed using a scanning electron microscope (SEM). The type of fibrous conductive additive is not particularly limited as long as it has the above-mentioned shape, but examples include carbon fiber (specifically, vapor-grown carbon fiber (VGCF), polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, rayon-based carbon fiber, activated carbon fiber, etc.), graphene, and carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes). Among these, carbon fiber is preferred. Only one type of fibrous conductive additive may be used alone, or two or more types may be used in combination.

[0039] The length (average fiber length) of the fibrous conductive additive is preferably greater than the average particle diameter of the first positive electrode active material and the average particle diameter of the second positive electrode active material. This configuration increases the number of contact points between the fibrous conductive additive and the positive electrode active material in the positive electrode active material layer, thereby reducing cell resistance and further improving high-rate discharge capacity. The average fiber length of the fibrous conductive additive is preferably 10 μm or more, more preferably 10 to 100 μm, even more preferably 10 to 50 μm, particularly preferably 10 to 40 μm, and most preferably 15 to 40 μm. The average fiber diameter of the fibrous conductive additive is preferably 1 to 600 nm, more preferably 100 to 500 nm.

[0040] The content of the fibrous conductive additive in the positive electrode material according to this embodiment is not particularly limited, but is preferably 0.1 to 5 mass %, more preferably 0.2 to 4 mass %, and even more preferably 0.5 to 3 mass %, relative to 100 mass % of the positive electrode material.

[0041] The positive electrode material according to this embodiment may contain a particulate conductive additive (non-fibrous conductive additive) instead of or in addition to the fibrous conductive additive. The type of particulate conductive additive is not particularly limited, but examples include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals, and carbon such as carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). Furthermore, particulate ceramic materials or resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive additives. Only one type of particulate conductive additive may be used alone, or two or more types may be used in combination.

[0042] The content of the particulate conductive additive in the positive electrode material according to this embodiment is not particularly limited, but is preferably 0.1 to 5 mass %, more preferably 0.2 to 4 mass %, and even more preferably 0.5 to 3 mass %, relative to 100 mass % of the positive electrode material.

[0043] The cathode material according to this embodiment preferably includes a second solid electrolyte in addition to the first cathode active material and cathode active material composite described above. This configuration can further improve high-rate discharge capacity. The solid electrolyte used as the second solid electrolyte is not particularly limited, and the same solid electrolyte as used as the first solid electrolyte described above can be used. The average particle size of the second solid electrolyte is not particularly limited, but is preferably 0.01 to 40 μm, more preferably 0.1 to 20 μm, and even more preferably 1 to 10 μm.

[0044] The content of the second solid electrolyte in the positive electrode material according to this embodiment is not particularly limited, but is preferably 3 to 20 mass %, more preferably 5 to 15 mass %, relative to 100 mass % of the positive electrode material.

[0045] The positive electrode material according to this embodiment preferably includes a binder in addition to the first positive electrode active material and the positive electrode active material composite described above. This configuration can improve moldability when manufacturing the positive electrode active material layer. The type of binder is not particularly limited, but a fibrous binder that fibrillates when a shear force is applied can be preferably used. Preferred types of fibrous binders include polytetrafluoroethylene (PTFE), carboxymethyl cellulose, polyvinyl alcohol, and polyethylene, with polytetrafluoroethylene (PTFE) being more preferred. Only one type of fibrous binder may be used alone, or two or more types may be used in combination.

[0046] When the positive electrode active material layer contains a binder, a non-fibrous binder may be included instead of or in addition to the fibrous binder. The type of non-fibrous binder is not particularly limited, but styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), ethyl cellulose, and acrylic resin are preferred, and polyvinylidene fluoride (PVDF) is more preferred. Only one type of non-fibrous binder may be used alone, or two or more types may be used in combination. However, in the positive electrode active material layer, the mass ratio of the fibrous binder to the entire binder is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, particularly preferably 98% by mass or more, and most preferably 100% by mass.

[0047] The content of the binder (preferably a fibrous binder) in the positive electrode material according to this embodiment is not particularly limited, but is preferably 0.1 to 5 mass %, more preferably 0.2 to 4 mass %, and even more preferably 0.5 to 3 mass %, relative to 100 mass % of the positive electrode material.

[0048] The positive electrode active material layer is manufactured using the positive electrode material described above. The manufacturing method of the positive electrode active material layer is not particularly limited, but it can be manufactured by forming a dry positive electrode material that does not substantially contain liquid components such as solvents into a sheet. Manufacturing the positive electrode active material layer by such a method facilitates the formation of an electron conduction path due to contact between the positive electrode active materials and a lithium ion conduction path via the solid electrolyte of the positive electrode active material composite, as described above, which can further improve the discharge capacity at high rates.

[0049] In the positive electrode active material layer, it is preferable that the first positive electrode active material and the second positive electrode active material are in contact with each other. By adopting such a configuration, the utilization efficiency of the positive electrode active material can be improved, and the discharge capacity at high rates can be improved. Note that whether the first positive electrode active material and the second positive electrode active material are in contact with each other can be confirmed from an SEM observation image as described in the Examples below.

[0050] The density of the positive electrode active material layer is not particularly limited, but is preferably 3.40 g / cm ３ more preferably 3.40 g / cm ３ Exceeds 3.60 g / cm ３ If the density is within the above range, electron conduction paths and lithium ion conduction paths are well formed, and therefore the discharge capacity at high rates can be further improved.

[0051] The thickness of the positive electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 40 to 200 μm.

[0052] [Solid Electrolyte Layer] The solid electrolyte layer is interposed between the negative electrode and the positive electrode and contains a solid electrolyte (usually as a main component). The solid electrolyte contained in the solid electrolyte layer is not particularly limited, and the solid electrolytes described in the above-mentioned section on the positive electrode material can be similarly used.

[0053] The content of the solid electrolyte in the solid electrolyte layer is preferably 50 to 100% by mass, and more preferably 90 to 99% by mass.

[0054] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte. The binder that can be used in the solid electrolyte layer may be the same as that described in the section on the positive electrode material.

[0055] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 40 μm.

[0056] [Negative Electrode Active Material Layer] The negative electrode active material layer contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. A lithium-containing active material may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it contains lithium, and examples thereof include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. When metallic lithium or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery is preferably a so-called lithium deposition type, in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during charging, as shown in FIG. 1 . Such lithium deposition type lithium secondary batteries suffer from a problem of non-uniform reaction distribution in the active material layer, which can lead to the deposition and growth of lithium dendrites and the likelihood of short circuits. In the lithium secondary battery according to this embodiment, by using the above-described positive electrode material in the positive electrode active material layer, electron conduction paths and lithium ion conduction paths can be formed well, which has the advantage of making the reaction distribution in the active material layer uniform and suppressing the above-mentioned short circuit.

[0057] The thickness of the negative electrode active material layer (in the case of a lithium deposition type, the thickness at full charge) differs depending on the configuration of the intended lithium secondary battery, but is preferably within the range of, for example, 0.1 to 1000 μm.

[0058] [Positive current collector plate and negative current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive current collector plate 27 and the negative current collector plate 25 may be made of the same material or different materials.

[0059] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium ion secondary batteries can be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like so as to prevent contact with peripheral devices or wiring, etc., causing electrical leakage and affecting products (for example, automobile parts, particularly electronic devices, etc.).

[0060] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element as shown in FIG. 1 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable as the exterior material because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0061] Although one embodiment of the lithium secondary battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0062] The lithium secondary battery according to the present embodiment may or may not be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution).

[0063] The following embodiments are also included within the scope of the present invention: the cathode material according to claim 1 having the features of claim 2; the cathode material according to claim 1 or 2 having the features of claim 3; the cathode material according to any one of claims 1 to 3 having the features of claim 4; the cathode material according to any one of claims 1 to 4 having the features of claim 5; the cathode material according to any one of claims 1 to 5 having the features of claim 6; the cathode material according to claim 6 having the features of claim 7; the cathode material according to any one of claims 1 to 7 having the features of claim 8; the cathode material according to any one of claims 1 to 8 having the features of claim 9; the cathode active material layer according to claim 10, which employs the cathode material according to any one of claims 1 to 9; the cathode active material layer according to claim 10, which has the features of claim 11; and the lithium secondary battery according to claim 12, which employs the cathode active material layer according to claim 10 or 11.

[0064] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the operations were carried out in a glove box with a dew point of -60°C or less. Furthermore, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0065] <Examples of Preparation of Positive Electrode Material> [Example 1] (Preparation of Positive Electrode Active Material Composite) The second positive electrode active material, NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ , average particle size: 5 μm, circularity: 0.78, bulk density: 1.2 g / cm ３ 7.8 parts by mass of an argyrodite-type sulfide solid electrolyte (Li ６P.S. ５ The second positive electrode active material and the first solid electrolyte were placed in a Balance Gran BG-2L (manufactured by Freund Turbo Corporation) and processed for 30 minutes under conditions of a chopper (S-shaped) speed of 3,580 rpm and a scraper speed of 87 rpm, thereby obtaining a positive electrode active material composite in which the surface of the second positive electrode active material was coated with the first solid electrolyte.

[0066] (Preparation of Positive Electrode Material) The first positive electrode active material, NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ 79.2 g of the cathode active material composite (Li , average particle diameter: 5 μm), 8.8 g of the cathode active material composite, and argyrodite-type sulfide solid electrolyte (Li ) ６ P.S. ５ 10.2 g of the first positive electrode active material, the positive electrode active material composite, and the second solid electrolyte were weighed. The first positive electrode active material, the positive electrode active material composite, and the second solid electrolyte were placed in a 300 mL polypropylene (PP) container with an inner lid, and the container was placed in a low-frequency resonant acoustic mixer (LabRAM II, manufactured by RESODYN ACOUSTIC MIXERS, INC.) and treated for 10 minutes under conditions of an acceleration of 100 G and a frequency of 60 Hz (Mix I). Then, 1.0 g of carbon nanofiber (CNF) (average fiber diameter: 200 nm, average fiber length: 16 μm) as a fibrous conductive additive was placed in the container and treated for 10 minutes under conditions of an acceleration of 100 G and a frequency of 60 Hz in a low-frequency resonant acoustic mixer (Mix II). Then, 0.8 g of polytetrafluoroethylene (PTFE) as a binder was further added to the container, and the mixture was mixed in a low-frequency acoustic mixer at an acceleration of 100 G and a frequency of 60 Hz for 10 minutes (Mixing III). This resulted in a positive electrode material of this example having a composition of first positive electrode active material: second positive electrode active material: first solid electrolyte: second solid electrolyte: fibrous conductive additive: binder = 79.2:7.8:1.0:10.2:1.0:0.8 (mass ratio).

[0067] [Example 2] (Preparation of Positive Electrode Active Material Composite) A positive electrode active material composite was obtained in the same manner as in Example 1, except that 34.2 parts by mass of the second positive electrode active material and 1.0 part by mass of the first solid electrolyte were weighed out so that the total amount was approximately 900 g and placed in a Balance Gran BG-2L.

[0068] (Preparation of Positive Electrode Material) In the above (Mixture I), 52.8 g of the first positive electrode active material, 35.2 g of the positive electrode active material composite, and 10.2 g of the second solid electrolyte were weighed and placed in a 300 mL polypropylene (PP) container with an inner lid, and the same method as in Example 1 was used to obtain a positive electrode material of this example (first positive electrode active material: second positive electrode active material: first solid electrolyte: second solid electrolyte: fibrous conductive additive: binder = 52.8:34.2:1.0:10.2:1.0:0.8 (mass ratio)).

[0069] Example 3 (Preparation of Positive Electrode Active Material Composite) A positive electrode active material composite was obtained in the same manner as in Example 1.

[0070] (Preparation of Positive Electrode Material) In the above (Mix II), instead of the fibrous conductive additive CNF, a particulate conductive additive carbon black (CB) (manufactured by TIMCAL, SC65, BET specific surface area 62 m) was used. ２ A positive electrode material of this example (first positive electrode active material: second positive electrode active material: first solid electrolyte: second solid electrolyte: particulate conductive additive: binder = 79.2:7.8:1.0:10.2:1.0:0.8 (mass ratio)) was obtained in the same manner as in Example 1, except that a positive electrode active material containing 100% propylene glycol stearate and 100% propylene glycol stearate was used.

[0071] Example 4 (Preparation of Positive Electrode Active Material Composite) A positive electrode active material composite was obtained in the same manner as in Example 1, except that 25.4 parts by mass of the second positive electrode active material and 1.0 part by mass of the first solid electrolyte were weighed out so that the total amount was approximately 900 g and placed in a Balance Gran BG-2L.

[0072] (Preparation of Positive Electrode Material) In the above (Mixture I), 61.6 g of the first positive electrode active material, 26.4 g of the positive electrode active material composite, and 10.2 g of the second solid electrolyte were weighed and placed in a 300 mL polypropylene (PP) container with an inner lid, and the same method as in Example 1 was used to obtain a positive electrode material of this example (first positive electrode active material: second positive electrode active material: first solid electrolyte: second solid electrolyte: fibrous conductive additive: binder = 61.6:25.4:1.0:10.2:1.0:0.8 (mass ratio)).

[0073] [Example 5] (Preparation of Positive Electrode Active Material Composite) NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ A positive electrode active material composite was obtained in the same manner as in Example 4, except that a granular material having a particle diameter of 4 μm was used.

[0074] (Preparation of Positive Electrode Material) In the above (Mixture I), NMC composite oxide (LiNi) ０．８ Mn ０．１ Co ０．１ O ２ A positive electrode material of this example (first positive electrode active material: second positive electrode active material: first solid electrolyte: second solid electrolyte: fibrous conductive additive: binder = 61.6:25.4:1.0:10.2:1.0:0.8 (mass ratio)) was obtained in the same manner as in Example 4, except that a first positive electrode active material: second positive electrode active material: first solid electrolyte: second solid electrolyte: fibrous conductive additive: binder = 61.6:25.4:1.0:10.2:1.0:0.8 (mass ratio) was used.

[0075] Example 6 (Preparation of Positive Electrode Active Material Composite) A positive electrode active material composite was obtained in the same manner as in Example 5, except that 34.2 parts by mass of the second positive electrode active material and 1.0 part by mass of the first solid electrolyte were weighed out so that the total amount was approximately 900 g and placed in a Balance Gran BG-2L.

[0076] (Preparation of Positive Electrode Material) In the above (Mixture I), 52.8 g of the first positive electrode active material, 35.2 g of the positive electrode active material composite, and 10.2 g of the second solid electrolyte were weighed and placed in a 300 mL polypropylene (PP) container with an inner lid, and the other components were subjected to the same procedure as in Example 5 to obtain a positive electrode material of this example (first positive electrode active material: second positive electrode active material: first solid electrolyte: second solid electrolyte: fibrous conductive additive: binder = 52.8:34.2:1.0:10.2:1.0:0.8 (mass ratio)).

[0077] Comparative Example 1 (Preparation of Positive Electrode Material) In the above (Mixture I), 87.0 g of the first positive electrode active material and 11.2 g of the second solid electrolyte were weighed and placed in a 300 mL polypropylene (PP) container with an inner lid, and the same procedure as in Example 1 was used to obtain a positive electrode material of this comparative example (first positive electrode active material:second solid electrolyte:fibrous conductive additive:binder=87.0:11.2:1.0:0.8 (mass ratio)).

[0078] Comparative Example 2 (Preparation of Positive Electrode Active Material Composite) A positive electrode active material composite was obtained in the same manner as in Example 1, except that 87.0 parts by mass of the second positive electrode active material and 1.4 parts by mass of the first solid electrolyte were weighed out so that the total amount was approximately 900 g and placed in a Balance Gran BG-2L.

[0079] (Preparation of Positive Electrode Material) In the above (Mixture I), 88.4 g of the positive electrode active material composite and 9.8 g of the second solid electrolyte were weighed and placed in a 300 mL polypropylene (PP) container with an inner lid, and the same procedure as in Example 1 was used to obtain a positive electrode material of this example (second positive electrode active material: first solid electrolyte: second solid electrolyte: fibrous conductive additive: binder = 87.0:1.4:9.8:1.0:0.8 (mass ratio)).

[0080] Comparative Example 3 (Preparation of Positive Electrode Active Material Composite) The second positive electrode active material, NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter: 5 μm) 87.0 parts by mass, and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５0.7 parts by mass of carbon black (CB) (manufactured by TIMCAL, SC65, BET specific surface area 62 m) as a particulate conductive additive. ２ 0.7 parts by mass of the second positive electrode active material, the first solid electrolyte, and the particulate conductive additive were weighed out to a total amount of approximately 900 g. The second positive electrode active material, the first solid electrolyte, and the particulate conductive additive were placed in a Balance Gran BG-2L (manufactured by Freund Turbo Corporation) and processed for 30 minutes under conditions of a chopper (S-shaped) of 3580 rpm and a scraper of 87 rpm, thereby obtaining a positive electrode active material composite in which the surface of the second positive electrode active material was coated with the first solid electrolyte and the particulate conductive additive.

[0081] (Preparation of Positive Electrode Material) 88.4 g of the positive electrode active material composite and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ 10.5 g of the first positive electrode active material, the positive electrode active material composite, and the second solid electrolyte were weighed. The first positive electrode active material, the positive electrode active material composite, and the second solid electrolyte were placed in a 300 mL polypropylene (PP) container with an inner lid, and the container was placed in a low-frequency resonant acoustic mixer (LabRAM II, manufactured by RESODYN ACOUSTIC MIXERS, INC.) and treated for 10 minutes under conditions of an acceleration of 100 G and a frequency of 60 Hz (Mix I). Then, 0.3 g of carbon nanofiber (CNF) (average fiber diameter: 200 nm, average fiber length: 16 μm) as a fibrous conductive additive was placed in the container, and the mixture was treated for 10 minutes under conditions of an acceleration of 100 G and a frequency of 60 Hz in a low-frequency resonant acoustic mixer (Mix II). Then, 0.8 g of polytetrafluoroethylene (PTFE) binder was added to the container and mixed for 10 minutes using a low-frequency acoustic mixer under conditions of an acceleration of 100 G and a frequency of 60 Hz (mixing III). This resulted in a positive electrode material of this comparative example having a composition of second positive electrode active material: first solid electrolyte: particulate conductive additive: second solid electrolyte: fibrous conductive additive: binder = 87.0: 0.7: 0.7: 10.5: 0.3: 0.8 (mass ratio).

[0082] Comparative Example 4 (Preparation of Positive Electrode Active Material Composite) The second positive electrode active material, NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２87.0 parts by mass of carbon black (CB) (manufactured by TIMCAL, SC65, BET specific surface area 62 m) as a particulate conductive additive. ２ The second positive electrode active material and the particulate conductive additive were placed in a Balance Gran BG-2L (manufactured by Freund Turbo Corporation) and processed for 30 minutes under conditions of a chopper (S-shaped) speed of 3,580 rpm and a scraper speed of 87 rpm, thereby obtaining a positive electrode active material composite in which the surface of the second positive electrode active material was coated with the particulate conductive additive.

[0083] (Preparation of Positive Electrode Material) 88.0 g of the positive electrode active material composite and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ 11.2 g of the second solid electrolyte (C1, average particle size: 0.8 μm) was weighed out. The above positive electrode active material composite and the second solid electrolyte were placed in a 300 mL polypropylene (PP) container with an inner lid, and the container was placed in a low-frequency resonant acoustic mixer (LabRAM II, manufactured by RESODYN ACOUSTIC MIXERS, INC.) and treated for 10 minutes under conditions of an acceleration of 100 G and a frequency of 60 Hz (Mix I). Then, 0.8 g of polytetrafluoroethylene (PTFE) as a binder was further added to the container and treated for 10 minutes under conditions of an acceleration of 100 G and a frequency of 60 Hz in the low-frequency resonant acoustic mixer (Mix III). This resulted in a positive electrode material of this comparative example having a composition of second positive electrode active material: particulate conductive additive: second solid electrolyte: binder = 87.0: 1.0: 11.2: 0.8 (mass ratio).

[0084] <Example of Preparation of Evaluation Cell> (Preparation of Laminate of Solid Electrolyte Layer and Negative Electrode Current Collector) An argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５95 parts by mass of styrene-butadiene rubber (SBR) and 5 parts by mass of styrene-butadiene rubber (SBR) as a binder were dispersed in 1,3,5-trimethylbenzene as a solvent to a solids content of 47% by mass. The binder was previously adjusted to a concentration of 6% by mass using 1,3,5-trimethylbenzene. The resulting dispersion was stirred at 1000 rpm for 3 minutes using a planetary mixer to prepare a slurry. The resulting slurry was applied to a negative electrode current collector (SUS430, thickness 10 μm) so that the film thickness after drying would be 40 μm. The solvent was then dried on a hot plate at 80°C for 30 minutes to obtain a laminate of a solid electrolyte layer and a negative electrode current collector.

[0085] (Preparation of Positive Electrode Active Material Layer) The positive electrode material was formed into a sheet using a hand roller and stretched to a thickness of 200 μm, and then passed through a tabletop roll press to prepare a positive electrode active material layer with a thickness of 150 μm.

[0086] (Fabrication of Cell Exterior Body) A positive electrode current collecting portion to which a positive electrode current collecting foil (made of aluminum, thickness 20 μm, size 44 mm × 31 mm) and a tab (made of aluminum) were previously welded, a negative electrode current collecting portion to which a negative electrode current collecting foil (made of SUS430, thickness 10 μm, size 44 mm × 31 mm) and a tab lead (made of nickel) were previously welded, and a Kapton film (thickness 75 μm, size 48 mm × 34 mm (partially punched into a 21 mm × 21 mm square)) for insulating the current collecting foil was sandwiched between two laminate films (thickness 160 μm, size 60 mm × 56 mm), and the tab portion was welded and fixed to produce an exterior body.

[0087] (Preparation of Lithium Secondary Battery) Using a 25 mm × 25 mm (corner R = 1 mm) punching machine manufactured by Nogami Giken Co., Ltd., the laminate of the solid electrolyte layer and the negative electrode current collector prepared above was punched into a 25 mm × 25 mm square. Using a 20 mm × 20 mm (corner R = 1 mm) punching machine manufactured by Nogami Giken Co., Ltd., the positive electrode active material layer prepared above was punched into a 20 mm × 20 mm square. Also, using a 25 × 25 mm (corner R = 1 mm) punching machine manufactured by Nogami Giken Co., Ltd., a carbon aluminum foil (SHX-PT-T20-MS manufactured by Shohoku Laminate Industry Co., Ltd.: a 20 μm thick aluminum foil surface with a 1 μm thick carbon coating layer) serving as a positive electrode current collector was punched into a 25 mm × 25 mm square. A positive electrode active material layer was placed on a positive electrode current collector, and a laminate of a solid electrolyte layer and a negative electrode current collector was then laminated on top of this so that the solid electrolyte layer and the positive electrode active material layer were adjacent to each other to form a power generation element. The power generation element was sandwiched between SUS304 foil (50 μm thick, 30 × 30 mm), placed in a 35 mm × 35 mm laminate film bag, and sealed with a vacuum sealer. The above was subjected to isostatic pressing (CIP) at 25 °C and 700 MPa for 1 minute.

[0088] (Arrangement of power generating element in cell exterior body) The four sides of the laminated bag containing the power generating element (solid electrolyte layer / positive electrode active material layer / carbon aluminum foil joint body) subjected to CIP as described above were cut with scissors to remove the power generating element. The Kapton film of the cell exterior body was punched out into a 21 mm x 21 mm square, and a 20 mm x 20 mm square positive electrode active material layer was placed in the punched area. This was vacuum sealed (10 －３ Pa) to obtain a cell for evaluation, which was a lithium deposition type lithium secondary battery.

[0089] <Evaluation of Positive Electrode Active Material Composite> [Circularity (a)] The positive electrode active material composite was observed with a scanning electron microscope (SEM) at a magnification of 10,000 times. The area (projected area of ​​particles) (μm ２) and perimeter (length of the outline) (μm). The circularity of the positive electrode active material composite was then calculated using the following formula. In this manner, the circularity of 10 positive electrode active material composites was determined, and their arithmetic mean value was calculated. FIG. 2 shows a micrograph of the first positive electrode active material contained in the positive electrode material of Example 1, observed using a scanning electron microscope (SEM). FIG. 3 shows a micrograph of the positive electrode active material composite contained in the positive electrode material of Example 1, observed using a scanning electron microscope (SEM).

[0090]

[0091] In the above formula, π represents the ratio of the circumference of a circle to its diameter, and S represents the area of ​​the positive electrode active material composite (μm ２ ) and L represents the perimeter (μm) of the positive electrode active material composite.

[0092] [Bulk Density (b)] The positive electrode active material composite was weighed and gently placed in a measuring cylinder, and the volume of the positive electrode active material composite was measured. Then, the bulk density (g / cm ３ ) = volume (cm ３ The bulk density was calculated from the following equation: (weight) / (mass (g)).

[0093] <Evaluation of Positive Electrode Active Material Layer> [Sheet Formability (c)] The positive electrode material was formed into a sheet using a hand roller. When the positive electrode material could be stretched to a thickness of 100 μm, the sheet was evaluated as having good formability, and when the sheet could not be stretched to a thickness of 100 μm, the sheet was evaluated as having poor formability.

[0094] [Cracks in the Positive Electrode Active Material Layer (d)] The free-standing film of the positive electrode active material layer obtained in the above (Preparation of the Positive Electrode Active Material Layer) was punched out to a diameter of 14 mm and sandwiched between two sheets of stainless steel foil (50 μm thick). This laminate was placed in a laminate film bag and sealed with a vacuum sealer. The sealed laminate was sandwiched between an aluminum plate and a rubber plate, placed in a resin bag, and sealed with a vacuum sealer to obtain a test laminate. The test laminate was subjected to isostatic pressing (CIP) at 25°C, 700 MPa, and 1 minute. The pressed laminate was disassembled to remove the positive electrode active material layer. The cross section in the stacking direction was exposed by ion milling, and the cross section was observed with a scanning electron microscope (SEM) at 10,000x magnification. The percentage of positive electrode active material particles with cracks among 100 positive electrode active material particles in the observed image was calculated. A ratio of less than 1% was evaluated as ◯ (good), a ratio of 1% or more but less than 20% was evaluated as △ (passable), and a ratio of 20% or more was evaluated as × (poor). Note that Figures 4 to 6 show micrographs of the cross sections of the positive electrode active material layers produced in Example 1, Comparative Example 2, and Comparative Example 4, observed using a scanning electron microscope (SEM).

[0095] [Distance (e) between positive electrode active materials] In the SEM observation image obtained in the above [Cracks in positive electrode active material], the distance between positive electrode active material particles (the shortest distance between two particles) was measured at 100 locations between the particles, and the arithmetic average value was calculated. The obtained value was evaluated as ◯ (good) when it was less than 0.1 μm, Δ (fair) when it was 0.1 μm or more and less than 1 μm, and × (poor) when it was 1 μm or more. Note that, as shown in Table 1, in the positive electrode active material layers prepared using the positive electrode materials of Examples 1 to 5, it was confirmed that the distance between the positive electrode active materials was less than 0.1 μm, and that the first positive electrode active material and the second positive electrode active material were in contact with each other.

[0096] [Density (f) of Positive Electrode Active Material Layer] The density of the positive electrode active material layer of the CIP obtained in [Cracks in the Positive Electrode Active Material Layer] above was calculated using the following formula.

[0097]

[0098] <Charge / Discharge Test> [Discharge Capacity (g)] A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector, respectively, of the above evaluation cell (restrained under a restraining pressure of 3 MPa), and the following evaluation was performed at 25°C.

[0099] First, aging treatment was performed at charge rates of 0.01C and 0.5C (CC mode) (upper limit voltage 4.3V). Next, the voltage range was set to 3.0 to 4.3V, and the initial charge / discharge capacity was measured at a charge / discharge rate of 0.1C (CC mode). A 30-minute rest period was provided between the charge and discharge treatments (same below). Next, the charge / discharge capacity under high-rate conditions was measured under the same charge / discharge conditions as above, except that the charge / discharge rate was set to 2.0C (CC mode). The ratio (percentage) of the discharge capacity at 2.0C to the discharge capacity at 0.1C was then calculated.

[0100] [Lithium Deposition State (h)] After the charge-discharge test for [Discharge Capacity] described above, a further charging process was performed, and the appearance of the negative electrode surface of the evaluation cell was observed to confirm the state of lithium deposition. The sum of the areas of convex portions due to localized lithium deposition was calculated, and the ratio (percentage) of the total area of ​​convex portions to the area of ​​the negative electrode current collector was calculated. A ratio of less than 1% was evaluated as ◯ (good), a ratio of 1% to less than 5% was evaluated as △ (passable), and a ratio of 5% or more was evaluated as × (poor).

[0101] [Minor Short Circuit (i)] The ratio (percentage) of the initial discharge capacity to the initial charge capacity in the [Discharge Capacity] was calculated. A ratio of 85% or more was evaluated as ◯ (no minor short circuit) and a ratio of less than 85% was evaluated as × (minor short circuit).

[0102] The results are shown in Table 1 below.

[0103]

[0104] As shown in Table 1, the present invention can improve the high-rate discharge capacity of a lithium secondary battery. A comparison between Examples 4 and 5 reveals that Example 5, in which the average particle diameter of the first positive electrode active material is different from the average particle diameter of the second positive electrode active material (the average particle diameter of the first positive electrode active material is larger than the average particle diameter of the second positive electrode active material), further improves the high-rate discharge capacity. A comparison between Examples 1 and 3 also reveals that the inclusion of a fibrous conductive additive in the positive electrode material further improves the high-rate discharge capacity.

[0105] 10a: laminated battery, 11': negative electrode current collector, 11'': positive electrode current collector, 13: negative electrode active material layer, 15: positive electrode active material layer, 17: solid electrolyte layer, 19: single cell layer, 21: power generating element, 25: negative electrode current collector, 27: positive electrode current collector, 29: laminate film.

Claims

a first positive electrode active material that is not covered with a solid electrolyte; a cathode active material composite including a second cathode active material and a first solid electrolyte that coats at least a portion of a surface of the second cathode active material; a positive electrode material comprising: 　 The positive electrode material according to claim 1 , wherein the first positive electrode active material and the second positive electrode active material have different average particle sizes. 　 The cathode material according to claim 2 , wherein an average particle size of the first cathode active material is larger than an average particle size of the second cathode active material.

3. The positive electrode material according to claim 1, wherein a content of the first positive electrode active material is more than 50 mass% with respect to a total mass of the first positive electrode active material and the positive electrode active material composite.

3. The positive electrode material according to claim 1, wherein the positive electrode active material composite has a circularity C of 0.8 or more as determined by the following formula when observed with a scanning electron microscope (SEM): In the above formula, π represents the ratio of the circumference of a circle to its diameter, and S represents the area of ​​the positive electrode active material composite (μm ２ ) and L represents the perimeter (μm) of the positive electrode active material composite. 　 The positive electrode material according to claim 1 or 2, further comprising a fibrous conductive additive. 　 The positive electrode material according to claim 6 , wherein the length of the fibrous conductive additive is greater than the average particle diameter of the first positive electrode active material and the average particle diameter of the second positive electrode active material. 　 The cathode material of claim 1 or 2, further comprising a second solid electrolyte. 　 The positive electrode material according to claim 1 or 2, further comprising a binder. 　 A positive electrode active material layer comprising the positive electrode material according to claim 1 or 2. 　 The positive electrode active material layer according to claim 10 , wherein the first positive electrode active material and the second positive electrode active material are in contact with each other. 　 A positive electrode having the positive electrode active material layer according to claim 10; a negative electrode having a negative electrode current collector and on which lithium metal is deposited during charging; a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; A lithium secondary battery comprising a power generating element having the above structure.

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