Secondary batteries

Abstract

To provide means that can suppress reduction of the discharge capacity of a battery in a secondary battery that uses a sulfide solid electrolyte and a positive electrode material made of a metal oxide in combination.SOLUTION: In a secondary battery having a power generation element formed by sequentially depositing a positive electrode containing a positive electrode material made of a metal oxide, a solid electrolyte containing a sulfide solid electrolyte, and a negative electrode containing a negative electrode active material, the amount of the sulfate radical in a surface of the metal oxide is controlled to be less than 1000 mass ppm.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a secondary battery.

Background Art

[0002] In recent years, in order to address global warming, reduction of carbon dioxide emissions has been urgently desired. In the automotive industry, expectations are focused on reducing carbon dioxide emissions by introducing electric vehicles (EVs) and hybrid electric vehicles (HEVs), and development of non-aqueous electrolyte secondary batteries such as secondary batteries for motor drive, which hold the key to the practical application of these vehicles, has been actively carried out.

[0003] As a secondary battery for motor drive, it is required to have extremely high output characteristics and high energy as compared with consumer lithium-ion secondary batteries used in mobile phones, notebook computers, etc. Therefore, lithium-ion secondary batteries having the highest theoretical energy among all practical batteries have attracted attention and are currently being rapidly developed.

[0004] Here, currently widely used lithium-ion secondary batteries use a flammable organic electrolyte as the electrolyte. In such liquid-based lithium-ion secondary batteries, safety measures against liquid leakage, short circuit, overcharge, etc. are required more strictly than other batteries.

[0005] Therefore, in recent years, research and development on all-solid-state batteries such as all-solid-state lithium-ion secondary batteries using oxide-based or sulfide-based solid electrolytes as the electrolyte have been actively carried out. A solid electrolyte is a material mainly composed of an ion conductor capable of ion conduction in a solid. Therefore, in all-solid-state lithium-ion secondary batteries, various problems caused by flammable organic electrolytes as in conventional liquid-based lithium-ion secondary batteries do not occur in principle. Also, generally, when using a high-potential and large-capacity positive electrode material and a large-capacity negative electrode material, a significant improvement in the output density and energy density of the battery can be achieved. All-solid-state lithium-ion secondary batteries using elemental sulfur (S) or sulfide-based materials as the positive electrode active material are promising candidates.

[0006] By the way, in an all-solid-state lithium-ion secondary battery using a sulfide solid electrolyte in combination with a positive electrode active material made of a metal oxide, there is a problem that a layer lacking lithium ions is generated due to the reaction between the surface of the metal oxide and the solid electrolyte at their interface, resulting in a decrease in the discharge capacity that can be extracted.

[0007] For the purpose of suppressing the occurrence of such problems, for example, Patent Document 1 discloses a technique of forming a coating layer made of a lithium ion conductive oxide such as LiNbO3 on the surface of a positive electrode active material made of a metal oxide and controlling the carbonate concentration in the positive electrode active material to a value within a predetermined range.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] According to the technique disclosed in Patent Document 1, a process of adding a coating layer is required, increasing the manufacturing cost. Also, it is difficult to form a coating layer on the entire surface of the particles constituting the positive electrode active material, and there is also a problem that the effect obtained by forming the coating layer is limited.

[0010] Therefore, an object of the present invention is to provide a means capable of suppressing a decrease in the discharge capacity of a secondary battery using a sulfide solid electrolyte in combination with a positive electrode active material made of a metal oxide.

Means for Solving the Problems

[0011] According to one embodiment of the present invention, a secondary battery is provided, comprising a power generation element in which a positive electrode containing a positive electrode active material made of a metal oxide, a solid electrolyte layer containing a sulfide solid electrolyte, and a negative electrode containing a negative electrode active material are stacked in this order, wherein the amount of sulfate ions present on the surface of the metal oxide is less than 1000 ppm by mass. [Effects of the Invention]

[0012] According to the present invention, in a secondary battery using a sulfide solid electrolyte in combination with a positive electrode active material made of a metal oxide, it is possible to suppress a decrease in the discharge capacity of the battery. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a perspective view showing the external appearance of a flat-stacked, all-solid-state lithium-ion secondary battery, which is one embodiment of the lithium-ion secondary battery according to the present invention. [Figure 2] Figure 2 is a cross-sectional view along the line 2-2 shown in Figure 1. [Figure 3] Figure 3 is a schematic cross-sectional view of a bipolar all-solid-state lithium-ion secondary battery, which is one embodiment of the lithium-ion secondary battery according to the present invention. [Figure 4] Figure 4 is a graph plotting the relationship between the amount of sulfate ions present on the surface of the positive electrode active material and the discharge capacity maintenance rate for Comparative Example 2 and Examples 3 to 6, which use an NMC composite oxide with a Ni ratio of 83% (0.83) as the positive electrode active material, as described in the Examples section below. [Modes for carrying out the invention]

[0014] 《Secondary battery》 One embodiment of the present invention is a secondary battery comprising a power generation element in which a positive electrode containing a positive electrode active material made of a metal oxide, a solid electrolyte layer containing a sulfide solid electrolyte, and a negative electrode containing a negative electrode active material are stacked in this order, wherein the amount of sulfate ions present on the surface of the metal oxide is less than 1000 ppm by mass.

[0015] The embodiments of the present invention described above will be explained below with reference to the drawings, but the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from the actual ratios.

[0016] Figure 1 is a perspective view showing the external appearance of a flat-stacked all-solid-state lithium-ion secondary battery, which is one embodiment of the lithium-ion secondary battery according to the present invention. Figure 2 is a cross-sectional view along the line 2-2 shown in Figure 1. By using a stacked structure, the battery can be made compact and have a high capacity. In this specification, the flat-stacked non-bipolar lithium-ion secondary battery shown in Figures 1 and 2 (hereinafter also simply referred to as "stacked battery") will be used as an example to explain the details. However, in terms of the electrical connection configuration (electrode structure) inside the lithium-ion secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

[0017] As shown in Figure 1, the stacked battery 10a has a rectangular, flattened shape, with a negative electrode current collector plate 25 and a positive electrode current collector plate 27 extending from both sides for extracting power. The power generation element 21 is encased in the battery casing material (laminate film 29) of the stacked battery 10a, and its periphery is heat-sealed, so that the power generation element 21 is sealed with the negative electrode current collector plate 25 and the positive electrode current collector plate 27 extended to the outside.

[0018] Furthermore, the lithium-ion secondary battery according to this embodiment is not limited to a stacked, flat shape. In the case of a wound lithium-ion secondary battery, it may be cylindrical, or a cylindrical shape may be deformed into a rectangular, flat shape, and there are no particular limitations. In the case of the cylindrical shape described above, the outer material may be a laminate film or a conventional cylindrical can (metal can), and there are no particular limitations. Preferably, the power generation element is housed inside a laminate film containing aluminum. This embodiment can achieve weight reduction.

[0019] Furthermore, there are no particular restrictions on how the current collector plates (25, 27) shown in Figure 1 are removed. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be removed from the same side, or the negative electrode current collector plate 25 and the positive electrode current collector plate 27 may each be divided into multiple parts and removed from each side. In addition, in wound lithium-ion batteries, instead of tabs, for example, cylindrical cans (metal cans) may be used to form the terminals.

[0020] As shown in Figure 2, the stacked battery 10a of this embodiment has a structure in which a flattened, roughly rectangular power generation element 21, in which the charge-discharge reaction actually takes place, is sealed inside a laminate film 29, which is the battery's outer casing material. Here, the power generation element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which a positive electrode active material layer 15 containing positive electrode active material is arranged on both sides of a positive electrode current collector 11''. The negative electrode has a structure in which a negative electrode active material layer 13 containing negative electrode active material is arranged on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order, with one positive electrode active material layer 15 and an adjacent negative electrode active material layer 13 facing each other via a solid electrolyte layer 17. As a result, adjacent positive electrodes, solid electrolyte layers, and negative electrodes constitute a single cell layer 19. Therefore, the stacked battery 10a shown in Figure 1 can also be said to have a configuration in which multiple single cell layers 19 are stacked and electrically connected in parallel.

[0021] As shown in Figure 2, the outermost positive electrode current collectors located on both outermost layers of the power generation element 21 each have a positive electrode active material layer 15 on only one side, but active material layers may be provided on both sides. That is, instead of using a current collector specifically for the outermost layer with an active material layer on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. In addition, in some cases, the negative electrode active material layer 13 and the positive electrode active material layer 15 may be used as the negative electrode and positive electrode, respectively, without using current collectors (11',11”).

[0022] The negative electrode current collector 11' and the positive electrode current collector 11'' are each attached to a negative electrode current collector plate (tab) 25 and a positive electrode current collector plate (tab) 27, which are electrically connected to the respective electrodes (positive and negative electrodes), and are structured to be led out of the laminate film 29, which is the battery casing material, by being sandwiched between the edges of the laminate film 29. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be attached to the positive electrode current collector 11'' and the negative electrode current collector 11' of each electrode via positive electrode leads and negative electrode leads (not shown) as needed, by ultrasonic welding, resistance welding, or the like.

[0023] The main components of the lithium-ion secondary battery in this embodiment will be described below.

[0024] [Current collector] The current collector has the function of mediating the movement of electrons from the electrode active material layer. There are no particular restrictions on the materials that make up the current collector. For example, metals or conductive resins can be used as the constituent materials of the current collector.

[0025] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, clad materials of nickel and aluminum, or copper and aluminum may also be used. Alternatively, a foil in which aluminum is coated on a metal surface may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoint of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

[0026] Furthermore, examples of the latter conductive resins include resins to which conductive fillers are added as needed to non-conductive polymer materials.

[0027] Examples of non-conductive polymer materials include polyethylene (PE; high-density polyethylene (HDPE), low-density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamide-imide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), or polystyrene (PS). Such non-conductive polymer materials can have excellent potential resistance or solvent resistance.

[0028] Conductive fillers may be added to the above-mentioned conductive or non-conductive polymer materials as needed. In particular, if the resin that serves as the base material for the current collector consists solely of a non-conductive polymer, a conductive filler is necessarily required to impart conductivity to the resin.

[0029] The conductive filler can be any conductive material without particular limitations. For example, materials with excellent conductivity, potential resistance, or lithium ion shielding properties include metals and conductive carbon. There are no particular limitations on the metal, but it is preferable to include at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or an alloy or metal oxide containing these metals. There are no particular limitations on the conductive carbon. Preferably, it includes at least one selected from the group consisting of acetylene black, Vulcan®, Black Pearl®, carbon nanofiber, Ketjenblack®, carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene.

[0030] There are no particular restrictions on the amount of conductive filler added, as long as it is sufficient to impart sufficient conductivity to the current collector. Generally, it is 5 to 80% by mass relative to 100% of the total mass of the current collector.

[0031] The current collector may be a single-layer structure made of a single material, or it may be a laminated structure in which layers made of these materials are appropriately combined. From the viewpoint of reducing the weight of the current collector, it is preferable to include at least a conductive resin layer made of a conductive resin. Furthermore, from the viewpoint of blocking the movement of lithium ions between single cell layers, a metal layer may be provided on a part of the current collector. Moreover, if the negative electrode active material layer and positive electrode active material layer described later are conductive and can perform the current collecting function on their own, it is not necessary to use a current collector as a separate component from these electrode active material layers. In such a configuration, the negative electrode active material layer described later will directly constitute the negative electrode, and the positive electrode active material layer described later will directly constitute the positive electrode.

[0032] [Negative electrode (negative electrode active material layer)] In the secondary battery according to this embodiment, the negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of metal oxides include Nb2O5 and Li4Ti5O 12 These are some examples. Furthermore, silicon-based or tin-based negative electrode active materials may be used. Here, silicon and tin belong to Group 14 elements and are known to be negative electrode active materials that can greatly improve the capacity of non-aqueous electrolyte secondary batteries. These elements can intercalate and release a large number of charge carriers (such as lithium ions) per unit volume (mass), thus becoming high-capacity negative electrode active materials. Here, as the silicon-based negative electrode active material, it is preferable to use elemental Si. Similarly, SiO2 disproportionated into two phases, a Si phase and a silicon oxide phase, is also available. xIt is also preferable to use silicon oxides such as (0.3≦x≦1.6). In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, alloys containing silicon (silicon-containing alloy-based anode active materials) may be used. On the other hand, examples of anode active materials containing tin (tin-based anode active materials) include elemental Sn, tin alloys (Cu-Sn alloys, Co-Sn alloys), amorphous tin oxides, tin-silicon oxides, etc. Among these, SnB is an example of amorphous tin oxide. 0.4 P 0.6 O 3.1 Examples include the following. SnSiO3 is an example of a tin-silicon oxide. A lithium-containing metal may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples include metallic lithium and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li and at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, other negative electrode active materials may also be used. The negative electrode active material preferably contains metallic lithium, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and is particularly preferably metallic lithium.

[0033] The shape of the negative electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. If the negative electrode active material is in particulate form, its average particle size (D 50 The average particle size of the active material (D) is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle size of the active material (D) is used. 50 The value of ) can be measured by laser diffraction scattering.

[0034] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably in the range of 40 to 99% by mass, and more preferably in the range of 50 to 90% by mass.

[0035] The negative electrode active material layer preferably further contains a solid electrolyte. By including a solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes, but sulfide solid electrolytes are preferred.

[0036] Examples of the sulfide solid electrolyte include LiI-Li2S-SiS2, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, LiI-Li3PS4, LiI-LiBr-Li3PS 4、 Li3PS 4、 Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z m S n (where m and n are positive numbers, and Z is any one of Ge, Zn, and Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, and In), etc. The description of "Li2S-P2S5" means a sulfide solid electrolyte formed using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions.

[0037] The sulfide solid electrolyte may, for example, have a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of the sulfide solid electrolyte having a Li3PS4 skeleton include LiI-Li3PS4, LiI-LiBr-Li3PS 4、Examples include Li3PS4. Examples of sulfide solid electrolytes having a Li4P2S7 skeleton include, for example, a Li-P-S-based solid electrolyte called LPS (e.g., Li7P3S 11 ). Examples of sulfide solid electrolytes also include, for example, Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1), such as LGPS, may be used. Among them, the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing the P element, and more preferably a material mainly composed of Li2S-P2S5. Furthermore, the sulfide solid electrolyte may contain halogen (F, Cl, Br, I).

[0038] When the sulfide solid electrolyte is of the Li2S-P2S5 type, the ratio of Li2S and P2S5 is preferably within the range of Li2S:P2S5 = 50:50 to 100:0 in terms of molar ratio, and more preferably Li2S:P2S5 = 70:30 to 80:20.

[0039] The sulfide solid electrolyte may be a sulfide glass, a crystallized sulfide glass, or a crystalline material obtained by a solid-phase method. The sulfide glass can be obtained, for example, by performing mechanical milling (such as a ball mill) on a raw material composition. The crystallized sulfide glass can be obtained, for example, by heat-treating the sulfide glass at a temperature above the crystallization temperature. The ionic conductivity (e.g., Li ion conductivity) of the sulfide solid electrolyte at room temperature (25 °C) is preferably, for example, 1 × 10 -5 S / cm or more, and more preferably 1 × 10 -4 S / cm or more. The value of the ionic conductivity of the solid electrolyte can be measured by an alternating current impedance method.

[0040] Examples of oxide solid electrolytes include, for example, compounds having a NASICON-type structure. An example of a compound having a NASICON-type structure is the general formula Li 1+x Al x Ge2-x Compound represented by (PO4)3(0≦x≦2) (LAGP), general formula Li 1+x Al x Ti 2-x Examples include compounds represented by (PO4)3 (0≦x≦2), such as (LATP). Other examples of oxide solid electrolytes include LiLaTiO (for example, Li 0.34 La 0.51 TiO3), LiPON (for example, Li 2.9 PO 3.3 N 0.46 ), LiLaZrO (for example, Li7La3Zr2O) 12 Examples include:

[0041] Examples of solid electrolyte shapes include spherical, ellipsoidal, and other particle shapes, as well as thin film shapes. When the solid electrolyte is in the form of particles, its average particle size (D 50 The particle size (D) is not particularly limited, but is preferably 40 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. 50 The particle size is preferably 0.01 μm or larger, and more preferably 0.1 μm or larger.

[0042] The solid electrolyte content in the negative electrode active material layer is preferably in the range of 1 to 60% by mass, and more preferably in the range of 10 to 50% by mass.

[0043] The negative electrode active material layer may further contain at least one of a conductive additive and a binder, in addition to the negative electrode active material and solid electrolyte described above.

[0044] Examples of conductive additives include, but are not limited to, metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals; carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNTs), and carbon black (specifically, acetylene black, Ketjenblack®, furnace black, channel black, thermal lamp black, etc.). Furthermore, a particulate ceramic material or resin material coated with the above-mentioned metal materials by plating or the like can also be used as a conductive additive. Among these conductive additives, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon; more preferably at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon; and even more preferably at least one carbon. These conductive additives may be used individually or in combination of two or more.

[0045] The conductive additive is preferably in the form of parts or fibers. If the conductive additive is in the form of parts, the shape of the particles is not particularly limited and may be any shape, such as powder, sphere, rod, needle, plate, columnar, irregular shape, flake, spindle, etc.

[0046] When the conductive additive is in particulate form, the average particle diameter (primary particle diameter) is not particularly limited, but from the viewpoint of the electrical characteristics of the battery, it is preferably 0.01 to 10 μm. In this specification, "particle diameter of the conductive additive" means the maximum distance L between any two points on the contour line of the conductive additive. The value of "average particle diameter of the conductive additive" shall be the value calculated as the average particle diameter of particles observed in several to tens of fields of view using observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0047] When the negative electrode active material layer contains a conductive additive, the content of the conductive additive in the negative electrode active material layer is not particularly limited, but is preferably 0 to 10% by mass, more preferably 2 to 8% by mass, and even more preferably 4 to 7% by mass, relative to the total mass of the negative electrode active material layer. Within this range, it is possible to form stronger electron conduction paths in the negative electrode active material layer, which can effectively contribute to improving battery characteristics.

[0048] On the other hand, while there are no particular limitations on the binder, examples include the following materials.

[0049] Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated products, styrene-isoprene-styrene block copolymer and its hydrogenated products, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), E Examples include fluororesins such as ethylene-chlorotrifluoroethylene copolymer (ECTFE) and polyvinyl fluoride (PVF), vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubbers (VDF-HFP-based fluororubbers), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-HFP-TFE-based fluororubbers), vinylidene fluoride-pentafluoropropylene-based fluororubbers (VDF-PFP-based fluororubbers), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers), epoxy resins, and the like. Among these, polyimide, styrene-butadiene rubber, carboxymethylcellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferred.

[0050] The thickness of the negative electrode active material layer varies depending on the configuration of the intended all-solid-state battery, but is preferably in the range of 0.1 to 1000 μm.

[0051] [Solid electrolyte layer] In the secondary battery according to this embodiment, the solid electrolyte layer is interposed between the positive electrode active material layer and the negative electrode active material layer described above, and is a layer that essentially contains a sulfide solid electrolyte. There are no particular restrictions on the specific form of the sulfide solid electrolyte contained in the solid electrolyte layer; the examples and preferred forms described in the section on the negative electrode active material layer may be used in the same way, and other solid electrolytes may be included as long as they essentially contain a sulfide solid electrolyte.

[0052] The solid electrolyte layer may further contain a binder in addition to the solid electrolyte described above. The binder that may be contained in the solid electrolyte layer may also be one of the examples and preferred forms described in the section on the negative electrode active material layer.

[0053] The thickness of the solid electrolyte layer varies depending on the intended configuration of the lithium-ion secondary battery, but from the viewpoint of improving the volumetric energy density of the battery, it is preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less. On the other hand, there is no particular limit on the lower limit of the thickness of the solid electrolyte layer, but it is preferably 10 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more.

[0054] [Cathode active material layer] In the secondary battery according to this embodiment, the positive electrode active material layer contains a positive electrode active material made of a metal oxide. The metal oxide that can function as the positive electrode active material is not particularly limited, but includes layered rock salt type active materials such as LiCoO2, LiMnO2, LiNiO2, LiVO2, Li(Ni-Mn-Co)O2, LiMn2O4, LiNi 0.5 Mn 1.5Examples include spinel-type active materials such as O4, olivine-type active materials such as LiFePO4 and LiMnPO4, and Si-containing active materials such as Li2FeSiO4 and Li2MnSiO4. Other metal oxides include, for example, Li4Ti5O 12 These include the following. Among these, composite oxides containing lithium and nickel are preferably used, and more preferably Li(Ni-Mn-Co)O2 and those in which some of these transition metals are substituted with other elements (hereinafter also simply referred to as "NMC composite oxides"). NMC composite oxides have a layered crystal structure in which lithium atomic layers and transition metal (Mn, Ni, and Co arranged in an orderly manner) atomic layers are alternately stacked with oxygen atomic layers in between. Each atom of the transition metal M contains one Li atom, and the amount of Li that can be extracted is twice that of spinel-based lithium manganese oxide, meaning that the supply capacity is twice as high and it can have a high capacity.

[0055] As mentioned above, NMC composite oxides also include composite oxides in which some of the transition metal elements are substituted by other metal elements. Examples of other elements in this case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, Zn, and the like. Preferably, the elements are Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, more preferably Ti, Zr, P, Al, Mg, Cr, and even more preferably Ti, Zr, Al, Mg, Cr from the viewpoint of improving cycle characteristics.

[0056] NMC composite oxides are preferred because they have a high theoretical discharge capacity, and are based on the general formula (1):Li a Ni b Mn c Co d M xO2 (wherein, in the formula, a, b, c, d, and x satisfy 0.98 ≤ a ≤ 1.2, 0.6 ≤ b ≤ 0.9, 0 < c ≤ 0.4, 0 < d ≤ 0.4, 0 ≤ x ≤ 0.3, and b + c + d + x = 1. M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. Here, the problem to be solved by the present invention (the decrease in discharge capacity when a sulfide solid electrolyte and a positive electrode active material composed of a metal oxide are used in combination) is particularly prominent when the metal oxide as the positive electrode active material contains nickel at a high concentration, that is, in the case of a so-called high-nickel metal oxide. Therefore, the effects of the present invention are particularly significantly manifested when the metal oxide as the positive electrode active material is a high-nickel metal oxide. From this perspective, in the secondary battery according to this embodiment, it is particularly preferable that the metal oxide as the positive electrode active material is an NMC composite oxide satisfying 0.8 ≤ b ≤ 0.9 in the general formula (1) described above.

[0057] In the secondary battery according to this embodiment, as described above, the positive electrode active material layer essentially contains a positive electrode active material composed of a metal oxide, and it is also characterized in that the abundance of sulfate radicals on the surface of the positive electrode active material composed of the metal oxide is controlled to be less than 1000 mass ppm. By adopting such a configuration, it is possible to significantly suppress the decrease in discharge capacity when the positive electrode active material composed of a metal oxide is used in combination with a sulfide solid electrolyte. The mechanism by which the predetermined effects of the present application are achieved by adopting the configuration according to this embodiment has not been fully clarified. However, the inventors of the present invention presume that the problem of the decrease in discharge capacity when the abundance of sulfate radicals on the surface of the positive electrode active material is large is due to the progress of an insulator formation reaction between the positive electrode active material having sulfate radicals on the surface and the sulfide solid electrolyte, resulting in the formation of a layer through which carrier ions such as lithium ions cannot conduct. And by adopting the configuration according to this embodiment, the progress of the insulator formation reaction as described above is suppressed, and as a result, a conduction path for carrier ions is sufficiently ensured, and it is presumed that the decrease in discharge capacity is suppressed.

[0058] The amount of sulfate rhizomes present on the surface of the positive electrode active material is preferably 880 ppm by mass or less, more preferably 830 ppm by mass or less, even more preferably 670 ppm by mass or less, even more preferably 600 ppm by mass or less, particularly preferably 300 ppm by mass or less, and most preferably 200 ppm by mass or less. On the other hand, there is no particular limit on the lower limit of the amount of sulfate rhizomes present, but it is usually 10 ppm by mass or more.

[0059] Generally, metal oxides that can function as positive electrode active materials are produced by calcining the raw material, which is a metal salt. When sulfate is used as the raw material, sulfate ions (SO4) are present on the surface of the metal oxide obtained after the calcination process. 2- ) remain. In this specification, "sulfate ions" refers to sulfate ions present on the surface of the metal oxide after the calcination process. In this specification, the amount of sulfate ions present on the surface of the metal oxide is expressed as a mass percentage based on the total mass of the metal oxide, and has the unit "mass ppm". The specific measurement of this amount is performed by dissolving the metal oxide in hydrochloric acid and then measuring the amount of sulfur atoms by inductively coupled plasma atomic emission spectrometry (ICP). Next, the obtained amount of sulfur atoms is converted to the amount of sulfate ions. Then, the "amount of sulfate ions present on the surface of the metal oxide" is calculated as the mass percentage of the converted value obtained in this way relative to the original metal oxide.

[0060] There are no particular restrictions on specific methods for controlling the amount of sulfate rhizides present on the surface of the positive electrode active material within the above range, and conventionally known knowledge can be referenced as appropriate. For example, one method is to adjust the type and amount of salts (carbonates, sulfates, halides, etc.) used as raw materials when manufacturing metal oxides. Also, as disclosed in Japanese Patent Application Publication No. 2015-191848, for example, when the metal hydroxide, which is a precursor for the manufacture of metal oxides, contains a large amount of sulfate rhizides, a method can be used in which the amount of sulfate rhizides is reduced through ion exchange by washing with an aqueous carbonate solution, and then a calcination process is carried out.

[0061] In some cases, positive electrode active materials other than the metal oxides mentioned above may be used in combination. However, the proportion of metal oxides in the total amount of positive electrode active material (100% by mass) 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.

[0062] The shape of the positive electrode active material can be, for example, particulate (spherical, fibrous), thin film, etc. If the positive electrode active material is in particulate form, its average particle size (D 50 The average particle size of the active material (D) is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle size of the active material (D) is used. 50 The value of ) can be measured by laser diffraction scattering.

[0063] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 40 to 99% by mass, and more preferably in the range of 50 to 90% by mass. The positive electrode active material layer may further contain conductive additives and / or binders, and the specific forms and preferred forms thereof can be similar to those described in the section on the negative electrode active material layer above.

[0064] [Positive electrode current collector plate and negative electrode current collector plate] The materials constituting the current collector plates (25, 27) are not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Preferred materials for the current collector plates are metallic materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof. From the viewpoint of lightness, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material or different materials.

[0065] [Positive lead and negative lead] Furthermore, although not shown in the diagram, the current collectors (11, 12) and the current collector plates (25, 27) may be electrically connected via positive and negative leads. The materials used for the positive and negative leads can be the same as those used in known lithium-ion secondary batteries. It is preferable to cover the parts removed from the casing with heat-resistant, heat-shrinkable tubing or the like to prevent leakage current from contacting peripheral equipment or wiring and affecting the product (e.g., automotive parts, especially electronic equipment).

[0066] [Battery casing material] As the battery casing material, known metal can cases can be used, or, as shown in Figures 1 and 2, a bag-shaped case made of aluminum-containing laminate film 29 that can cover the power generation elements can be used. For example, a three-layer laminate film made by laminating PP, aluminum, and nylon in that order can be used, but there are no limitations to these. Laminate film is preferable from the viewpoint of high output and excellent cooling performance, and can be suitably used for batteries in large equipment for EVs and HEVs. Furthermore, since the group pressure applied to the power generation elements from the outside can be easily adjusted, an aluminum-containing laminate film is more preferable for the casing.

[0067] The stacked battery according to this embodiment has a configuration in which multiple single cell layers are connected in parallel, resulting in high capacity and excellent cycle durability. Therefore, the stacked battery according to this embodiment is suitable for use as a power source for EVs and HEVs.

[0068] Although one embodiment of a lithium-ion secondary battery has been described above, the present invention is not limited to the configuration described in the above-described embodiment, and can be modified as appropriate based on the claims.

[0069] For example, one type of battery to which the lithium-ion secondary battery according to the present invention is applied is a bipolar battery, which includes a bipolar electrode having a positive electrode active material layer electrically coupled to one side of a current collector and a negative electrode active material layer electrically coupled to the opposite side of the current collector.

[0070] Figure 3 is a schematic cross-sectional view of a bipolar lithium-ion secondary battery (hereinafter also simply referred to as "bipolar secondary battery"), which is one embodiment of the lithium-ion secondary battery according to the present invention. The bipolar secondary battery 10b shown in Figure 3 has a structure in which a substantially rectangular power generation element 21, in which the charge and discharge reaction actually proceeds, is sealed inside a laminate film 29 which is the battery casing.

[0071] As shown in Figure 3, the power generation element 21 of the bipolar secondary battery 10b in this embodiment has a plurality of bipolar electrodes 23, each having a positive electrode active material layer 15 electrically coupled to one side of a current collector 11 and a negative electrode active material layer 13 electrically coupled to the opposite side of the current collector 11. Each bipolar electrode 23 is stacked via a solid electrolyte layer 17 to form the power generation element 21. The solid electrolyte layer 17 has a structure in which a solid electrolyte is formed in layers. As shown in Figure 3, the solid electrolyte layer 17 is sandwiched between the positive electrode active material layer 15 of one bipolar electrode 23 and the negative electrode active material layer 13 of another bipolar electrode 23 adjacent to the first bipolar electrode 23.

[0072] The adjacent positive electrode active material layer 15, solid electrolyte layer 17, and negative electrode active material layer 13 constitute a single cell layer 19. Therefore, it can be said that the bipolar secondary battery 10b has a structure in which single cell layers 19 are stacked. The outermost current collector 11a on the positive electrode side, located in the outermost layer of the power generation element 21, has the positive electrode active material layer 15 formed on only one side. The outermost current collector 11b on the negative electrode side, located in the outermost layer of the power generation element 21, has the negative electrode active material layer 13 formed on only one side.

[0073] Furthermore, in the bipolar secondary battery 10b shown in Figure 3, the positive electrode current collector plate (positive electrode tab) 25 is positioned adjacent to the outermost current collector 11a on the positive electrode side, and this is extended and leads out from the laminate film 29 which is the battery casing. On the other hand, the negative electrode current collector plate (negative electrode tab) 27 is positioned adjacent to the outermost current collector 11b on the negative electrode side, and similarly this is extended and leads out from the laminate film 29.

[0074] The number of times the single cell layers 19 are stacked is adjusted according to the desired voltage. In the case of the bipolar secondary battery 10b, the number of times the single cell layers 19 are stacked may be reduced if sufficient output can be secured even with the battery thickness made as thin as possible. In the case of the bipolar secondary battery 10b, in order to prevent external shocks and environmental degradation during use, it is preferable to have a structure in which the power generation element 21 is sealed under reduced pressure in the laminate film 29 which is the battery casing, and the positive electrode current collector plate 27 and the negative electrode current collector plate 25 are brought out to the outside of the laminate film 29.

[0075] Furthermore, the secondary battery according to this embodiment does not have to be all-solid type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte). There are no particular restrictions on the amount of liquid electrolyte (electrolyte) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolyte) does not occur.

[0076] The liquid electrolytes (electrolytes) that can be used have a form in which a lithium salt is dissolved in an organic solvent. Examples of organic solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyl dioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). In particular, the organic solvent is preferably a linear carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

[0077] Examples of lithium salts include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3. Among these, Li(FSO2)2N(LiFSI) is preferred from the viewpoint of battery output and charge / discharge cycle characteristics.

[0078] The liquid electrolyte (electrolyte) may further contain additives other than the components mentioned above. Specific examples of such compounds include, for example, ethylene carbonate, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, and 1-ethyl-2-vinylethylene Examples include ethylene carbonate, vinylvinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acrylicoxymethyl ethylene carbonate, methacrylicoxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used individually or in combination of two or more. The amount of additive used in the electrolyte can be adjusted as appropriate.

[0079] [Battery pack] A battery pack is a device made by connecting multiple batteries together. More specifically, it uses at least two batteries connected in series, parallel, or both. By connecting them in series or parallel, it becomes possible to freely adjust the capacity and voltage.

[0080] Multiple batteries can be connected in series or parallel to form a small, removable battery pack. Furthermore, multiple of these removable battery packs can be connected in series or parallel to form a large-capacity, high-output battery pack (battery module, battery pack, etc.) suitable for vehicle power supplies and auxiliary power sources requiring high volumetric energy density and high volumetric power density. The number of batteries used to create a battery pack, and the number of layers of small battery packs stacked to create a large-capacity battery pack, should be determined according to the battery capacity and output of the vehicle (electric vehicle) in which it will be installed.

[0081] [vehicle] A battery or a battery pack consisting of multiple batteries can be mounted in a vehicle. The present invention allows for the construction of a long-life battery with excellent long-term reliability. Therefore, mounting such a battery can result in a plug-in hybrid electric vehicle with a long EV driving range or an electric vehicle with a long driving range on a single charge. This is because using a battery or a battery pack consisting of multiple batteries in vehicles such as hybrid vehicles, fuel cell vehicles, or electric vehicles (including four-wheeled vehicles (passenger cars, trucks, buses and other commercial vehicles, light vehicles, etc.), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles) results in a long-life and highly reliable vehicle. However, its application is not limited to automobiles; for example, it can be applied to various power sources for other vehicles, such as trains, and can also be used as an onboard power source for uninterruptible power supplies. [Examples]

[0082] 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.

[0083] Examples of test cell preparations [Comparative Example 1] A lithium-ion conductive sulfide solid electrolyte, LPS (Li2S-P2S5 (mixing ratio 80:20 (mol%))), was prepared. A lithium-containing metal oxide (composition = LiNi) was also prepared as the positive electrode active material. 0.6 Mn 0.2 Co0.2 O2) and acetylene black, a conductive additive, were prepared. The positive electrode active material was LiNi 0.6 Mn 0.2 Co 0.2 The amount of sulfate ions present on the surface of O2 was measured using the following method and found to be 2000 ppm by mass relative to the total amount of positive electrode active material.

[0084] (Method for measuring the amount of sulfate ions present on the surface of positive electrode active material (metal oxide)) The positive electrode active material (metal oxide) was dissolved in hydrochloric acid, and then measured using inductively coupled plasma atomic emission spectrometry (ICP) to determine the amount of sulfur atoms. Next, the obtained amount of sulfur atoms was converted to the amount of sulfate rhizates. Then, the "amount of sulfate rhizates present on the surface of the metal oxide" was calculated as the mass ratio of this converted value to the original metal oxide.

[0085] 50 parts by mass of the positive electrode active material, 30 parts by mass of the conductive additive, and 20 parts by mass of the sulfide solid electrolyte prepared above were mixed in an agate mortar and then further mixed and stirred using a planetary ball mill. Next, 2 parts by mass of styrene-butadiene rubber (SBR), which is a binder, was added to the resulting mixed powder, and an appropriate amount of xylene was added as a solvent, and the mixture was prepared to prepare a positive electrode active material slurry.

[0086] The positive electrode active material slurry prepared above was coated onto one surface of an aluminum foil (20 μm thick), which serves as the positive electrode current collector, and the solvent was evaporated to form a positive electrode active material layer (50 μm thick).

[0087] Next, the sulfide solid electrolyte prepared above and the binder (SBR) were mixed in a mass ratio of 95:5, an appropriate amount of xylene was added as a solvent, and the mixture was combined to prepare a solid electrolyte slurry.

[0088] The solid electrolyte slurry prepared above was coated onto the exposed surface of the positive electrode active material layer prepared above, and the solvent was evaporated to form a solid electrolyte layer (80 μm thick). The resulting laminate consisting of the positive electrode current collector / positive electrode active material layer / solid electrolyte layer was then cut out so that the coated area of ​​the positive electrode active material layer was 2.5 cm × 2.0 cm.

[0089] Subsequently, a negative electrode of the same dimensions was attached to the exposed surface of the solid electrolyte layer of the laminate cut out as described above, and pressed together with the solid electrolyte layer by applying pressure. The negative electrode used was a laminate consisting of metallic lithium (Li) foil (100 μm thick) and nickel (Ni) foil (20 μm thick), with the Li foil positioned on the solid electrolyte layer side.

[0090] Finally, aluminum (Al) and nickel (Ni) tab leads were welded to the aluminum (Al) foil, which constitutes the positive electrode current collector, and to the Ni foil, which constitutes the negative electrode, respectively. The resulting cell was then sealed inside an outer casing made of aluminum laminate film to produce a test cell for this comparative example.

[0091] [Comparative Example 2] As the positive electrode active material, lithium-containing metal oxide (composition = LiNi 0.83 Mn 0.07 Co 0.1 The test cell for this comparative example was prepared using the same method as in Comparative Example 1 described above, except that O2 (amount of sulfate ions on the surface = 1000 ppm by mass) was used.

[0092] [Example 1] As the positive electrode active material, lithium-containing metal oxide (composition = LiNi 0.5 Mn 0.3 Co 0.2 The test cell for this example was prepared using the same method as in Comparative Example 1 described above, except that O2 (amount of sulfate ions on the surface = 300 ppm by mass) was used.

[0093] [Example 2] As the positive electrode active material, lithium-containing metal oxide (composition = LiNi0.6 Mn 0.2 Co 0.2 The test cell for this example was prepared using the same method as in Comparative Example 1 described above, except that O2 (amount of sulfate ions on the surface = 872 mass ppm) was used.

[0094] [Example 3] As the positive electrode active material, lithium-containing metal oxide (composition = LiNi 0.83 Mn 0.07 Co 0.1 The test cell for this example was prepared using the same method as in Comparative Example 1 described above, except that O2 (amount of sulfate ions on the surface = 196 ppm by mass) was used.

[0095] [Example 4] As the positive electrode active material, lithium-containing metal oxide (composition = LiNi 0.83 Mn 0.07 Co 0.1 The test cell for this example was prepared using the same method as in Comparative Example 1 described above, except that O2 (amount of sulfate ions on the surface = 670 ppm by mass) was used.

[0096] [Example 5] As the positive electrode active material, lithium-containing metal oxide (composition = LiNi 0.83 Mn 0.07 Co 0.1 The test cell for this example was prepared using the same method as in Comparative Example 1 described above, except that O2 (amount of sulfate ions on the surface = 830 ppm by mass) was used.

[0097] [Example 6] As the positive electrode active material, lithium-containing metal oxide (composition = LiNi 0.83 Mn 0.07 Co 0.1 The test cell for this example was prepared using the same method as in Comparative Example 1 described above, except that O2 (amount of sulfate ions on the surface = 598 ppm by mass) was used.

[0098] 《Example of evaluation of test cells》 For the test cells prepared in each of the comparative examples and examples described above, the discharge capacity per unit mass of the positive electrode active material was measured using the following method.

[0099] First, the test cell was sandwiched between two 5mm thick stainless steel plates, and the clamping pressure was set to 1000 kgf / cm². 2 To achieve this, pressure was applied using a flat plate press machine with a hydraulic jack.

[0100] After 1 hour of pressurization, the test cell was placed inside a constant temperature bath set to 25°C, connected to a charge / discharge device, and a charge / discharge test was performed to measure the charge / discharge capacity. During the charging process, a current equivalent to 0.05C was applied, and CC-CV charging was performed with an upper voltage limit of 4.2V. This charging process was terminated when the current value decreased to a value equivalent to 0.01C, or when 40 hours had elapsed since the start of charging. After the charging process was completed, the cell was left for 1 hour before the discharge process was performed. During the discharge process, CC discharge was performed with a current value equivalent to 0.05C and a lower voltage limit of 2.5V. The capacity (discharge capacity) was measured during the discharge process and normalized by the mass of the positive electrode active material used in each test cell to calculate the discharge capacity per unit mass of the active material. Furthermore, the percentage of the discharge capacity per unit mass of the active material calculated in this way relative to the theoretical discharge capacity (discharge capacity maintenance rate) was calculated and used as an evaluation index for the discharge characteristics. The results are shown in the table below. Furthermore, for Comparative Example 2 and Examples 3 to 6, which used an NMC composite oxide with a Ni ratio of 83% (0.83) as the positive electrode active material, Figure 4 shows a graph plotting the relationship between the amount of sulfate present on the surface of the positive electrode active material and the discharge capacity maintenance rate.

[0101] [Table 1]

[0102] The results shown in Table 1 indicate that, when comparing series with the same positive electrode active material composition, the discharge capacity maintenance rate improves when the amount of sulfate ions on the surface of the positive electrode active material is less than 1000 ppm by mass compared to cases where it is greater.

[0103] In particular, when the metal oxide constituting the positive electrode active material is an NMC composite oxide with a Ni ratio of 60% (0.6) or higher, it can be seen that the discharge capacity retention rate is dramatically improved by keeping the amount of sulfate ions present on the surface of the positive electrode active material below 1000 ppm by mass. In particular, as shown in Figure 4, when so-called high-nickel NMC811, which has an extremely high Ni ratio of 80% (0.8), is used as the positive electrode active material, the effect of applying the present invention on improving the discharge capacity retention rate is extremely excellent. [Explanation of symbols]

[0104] 10,10A stacked battery, 10b bipolar battery, 11 Current collector, 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 layers, 21 Power generation elements, 25 Negative electrode current collector plate, 27 Positive electrode current collector plate, 29. Laminating film.

Claims

1. The following general formula (1): Li ａ Ni ｂ Co ｃ Mn ｄ M ｘ O ２ (1) In the equation, a, b, c, d, and x satisfy 0.98 ≤ a ≤ 1.2, 0.6 ≤ b ≤ 0.9, 0 < c ≤ 0.4, 0 < d ≤ 0.4, 0 ≤ x ≤ 0.3, and b + c + d + x = 1. M is at least one element selected from Ti, Zr, W, P, Al, Mg, V, Ca, Sr, and Cr. A positive electrode containing a positive electrode active material made of a composite oxide having a composition represented by, A solid electrolyte layer containing a sulfide solid electrolyte, A negative electrode containing a negative electrode active material, It has power generation elements stacked in this order, A secondary battery wherein the amount of sulfate ions present on the surface of the composite oxide is 196 ppm by mass or more and 880 ppm by mass or less.

2. The secondary battery according to claim 1, wherein the positive electrode active material has a spherical shape.

3. The secondary battery according to claim 1 or 2, wherein in the general formula (1) above, 0.8 ≤ b ≤ 0.

9.

4. The secondary battery according to any one of claims 1 to 3, wherein the amount of sulfate ions present on the surface of the composite oxide is 830 ppm by mass or less.

5. The secondary battery according to claim 4, wherein the amount of sulfate ions present on the surface of the composite oxide is 600 ppm by mass or less.

6. The secondary battery according to claim 5, wherein the amount of sulfate ions present on the surface of the composite oxide is 200 ppm by mass or less.

7. A secondary battery according to any one of claims 1 to 6, which is an all-solid-state lithium-ion secondary battery.

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