Positive electrode active material, positive electrode, secondary battery, and method for producing positive electrode active material

Abstract

Provided are a positive electrode active material, a positive electrode, or a secondary battery having high charge load characteristics. The positive electrode active material contains a lithium composite oxide having a layered rock salt structure. The lithium composite oxide includes lithium, a metal element including at least one of cobalt and nickel, and potassium. The ratio of the amount of substance of potassium contained in the lithium composite oxide to the amount of substance of the metal element contained in the lithium composite oxide is 0.0005-0.03 (inclusive). In the lithium composite oxide, potassium is distributed so as to extend in a direction intersecting the < 001 > direction of the layered rock salt structure.

Description

Technical Field

[0001] This invention relates to positive electrode active materials, positive electrodes, secondary batteries, and methods for manufacturing positive electrode active materials. Background Technology

[0002] Patent document 1 discloses a positive electrode active material comprising lithium nickel oxide having a layered rock salt structure.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent No. 6809487 Summary of the Invention

[0006] The technical problem that the invention aims to solve

[0007] However, the charging load characteristics of the positive electrode active material described in Patent Document 1 may be reduced due to the distribution of potassium in the layered rock salt type structure.

[0008] The present invention was made in view of the above-mentioned technical problems, and its purpose is to provide a positive electrode active material, a positive electrode or a secondary battery with high charging load characteristics.

[0009] Technical solutions for solving technical problems

[0010] One aspect of the present invention relates to a positive electrode active material comprising a lithium composite oxide having a layered rock salt-type structure, the lithium composite oxide comprising lithium, a metal element comprising at least one of cobalt and nickel, and potassium, wherein the ratio of the amount of potassium contained in the lithium composite oxide to the amount of the metal element contained in the lithium composite oxide is 0.0005 or more and 0.03 or less, and wherein potassium is distributed in the lithium composite oxide in a direction extending in a direction intersecting the <001> direction of the layered rock salt-type structure.

[0011] One embodiment of the present invention involves a positive electrode comprising the positive electrode active material.

[0012] One embodiment of the present invention relates to a secondary battery comprising the positive electrode, the negative electrode, and the electrolyte.

[0013] One aspect of the present invention relates to a method for manufacturing a positive electrode active material comprising a lithium composite oxide having a layered rock salt structure, wherein the method comprises a step of calcining a precursor comprising at least one of lithium, cobalt and nickel, and potassium at a temperature of 200°C or higher and less than 600°C to produce the lithium composite oxide.

[0014] Invention Effects

[0015] According to the present invention, a positive electrode active material, a positive electrode, or a secondary battery with high charge load characteristics can be provided. Attached Figure Description

[0016] Figure 1 This is a cross-sectional view showing an example of a secondary battery according to the first embodiment.

[0017] Figure 2 It is shown Figure 1 An enlarged cross-sectional view of a portion of the cross-section of the electrode involved.

[0018] Figure 3A This is a TEM image showing the positive electrode active material involved in the embodiment of the first embodiment.

[0019] Figure 3B It shows about Figure 3A A graph of the EDX mapping image of potassium in the region involved.

[0020] Figure 4A This is a TEM image showing the positive electrode active material involved in the comparative example of the first embodiment.

[0021] Figure 4B It shows about Figure 4A A graph of the EDX mapping image of potassium in the region involved.

[0022] Figure 5 This is a cutaway view showing different examples of the secondary battery involved in the first embodiment.

[0023] Figure 6 yes Figure 5 A schematic diagram of the cross section of line VI-VI. Detailed Implementation

[0024] The embodiments of the present invention will now be described. However, the present invention is not limited to these embodiments. Furthermore, numerical values ​​include rounding ranges.

[0025] (Secondary battery)

[0026] Figure 1 This is a cross-sectional view showing an example of a secondary battery according to the first embodiment. Figure 1 The secondary battery 1 shown is a laminated lithium-ion secondary battery. For example... Figure 1 As shown, the secondary battery 1 includes a battery element 20, an outer packaging component 30, and a sealing material 32.

[0027] The battery element 20 is located inside the outer packaging component 30. For example... Figure 1As shown, the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal that extends from the positive electrode 210 (described later) to the outside of the outer packaging component 30. That is, the positive electrode lead 21 is the terminal that serves as the positive electrode of the secondary battery 1. Figure 1 In this configuration, the positive electrode lead 21 is disposed on the end face of the electrode body 200. The negative electrode lead 22 is a terminal that extends from the interior of the negative electrode 220 (described later) to the exterior of the outer packaging component 30. That is, the negative electrode lead 22 is the terminal that serves as the negative electrode of the secondary battery 1. Figure 1 In this configuration, the negative electrode lead 22 is disposed on the end face of the electrode body 200. Details of the electrode body 200 are described below.

[0028] The outer packaging component 30 is the outer shell that houses the battery element 20. The outer packaging component 30 includes two outer packaging sheets 30a and 30b. The outer packaging sheets 30a and 30b have an insulating layer, a metal layer, and an outermost layer. Figure 1 In this example, a recess 31 is provided in the outer packaging sheet 30a. Thus, the battery element 20 is housed in the recess 31, and the battery element 20 is housed inside the outer packaging component 30 by bonding the periphery of the outer packaging sheets 30a and 30b.

[0029] The outer packaging sheets 30a and 30b have the following structure: from the inside, i.e., the side where the battery element 20 is disposed, they are stacked in the order of an insulating layer, a metal layer, and an outermost layer, and are bonded together by lamination or the like. The insulating layer of the outer packaging sheets 30a and 30b is, for example, made of a resin containing polyethylene, polypropylene, modified polyethylene, modified polypropylene, ethylene, or propylene as monomers, such as a polyolefin resin. Therefore, the outer packaging sheets 30a and 30b can reduce the moisture permeability of the secondary battery 1 and improve airtightness. The metal layer of the outer packaging sheets 30a and 30b is a metal sheet or foil such as aluminum, stainless steel, nickel, or iron. The outermost layer can be any material; for example, it is preferably made of the same resin as the insulating layer, nylon, or other materials with high strength against breakage or puncture.

[0030] The sealing material 32 is a component used to make the outer packaging component 30 airtight. The sealing material 32 is disposed between the outer packaging component 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the sealing material 32 is preferably one that has a sealing effect on the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metallic material, the sealing material 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. Thus, the sealing material 32 can seal the gap between the outer packaging component 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the outer packaging component 30 airtight.

[0031] Figure 2 It is shown Figure 1An enlarged cross-sectional view of a portion of the cross-section of the electrode involved. More specifically, Figure 2 This is a cross-sectional view showing a portion of a positive electrode 210 and a negative electrode 220 in the electrode body 200. (See diagram below.) Figure 2 As shown, the electrode body 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode body 200 is a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 in between. The positive electrode 210 and the negative electrode 220 included in the electrode body 200 are layered components used for the charge and discharge reaction of the secondary battery 1 according to the first embodiment.

[0032] The positive electrode 210 has a positive current collector 211 and a positive active material layer 212. In the positive electrode 210, the positive current collector 211 is stacked between the positive active material layers 212.

[0033] The positive current collector 211 is a conductor layer, such as aluminum foil or stainless steel foil. Figure 1 In the example, when viewed from above in the thickness direction, the positive current collector 211 is a rectangular sheet with a protrusion on the side of the positive lead 21. The protrusion of the positive current collector 211 is connected to the positive lead 21.

[0034] The positive electrode active material layer 212 is a layer containing positive electrode active material. The positive electrode active material layer 212 includes the positive electrode active material, a positive electrode binder, and a positive electrode conductive additive. The positive electrode active material layer 212 is not limited to the materials listed above; for example, it may also contain a dispersant.

[0035] The positive electrode active material comprises a lithium composite oxide. The lithium composite oxide comprises lithium, a metal element comprising at least one of cobalt and nickel, and potassium. In a first embodiment, the lithium composite oxide has the following composition: Li 1-x K x Ni 1-y Co y O 2-z This means that the following conditions are met: 0.0005 ≤ x ≤ 0.03, 0 ≤ y ≤ 1, and -0.1 ≤ z ≤ 0.2. Here, x in the composition formula is the ratio of the amount of potassium contained in the lithium composite oxide to the amount of the metal element contained in the lithium composite oxide. By making x 0.0005 or more, the charge / discharge load characteristics can be improved. By making x 0.03 or less, the decrease in charge / discharge characteristics can be suppressed. Furthermore, in the first embodiment, the positive electrode active material is composed of particles made of lithium composite oxide, but it is not limited to this; for example, it may also be a layer made of lithium composite oxide.

[0036] The values ​​of x and y can be determined by EDX (Energy Dispersive X-ray Spectroscopy). More specifically, observing a sample prepared by cross-sectional processing of the positive electrode active material or the positive electrode active material layer 212, a rectangular region with a depth of 150 nm or more from the surface of the lithium composite oxide and a side length of 50 nm or more and 200 nm or less is selected, and the contents of Ni, Co, and K are determined by EDX. In the determination, the contents of Ni, Co, and K are measured in five non-overlapping regions, and the molar ratio of K content to the total contents of Ni and Co in each region is calculated. Thus, by calculating the arithmetic mean of the molar ratios of K content to the total contents of Ni and Co in each region, x can be calculated as the ratio of the amount of K to the total amount of Ni and Co, and y can be calculated as the ratio of the amount of Co to the total amount of Ni and Co.

[0037] The value of z is calculated using the following method. First, for samples prepared by cross-sectional processing of the positive electrode active material or the positive electrode active material layer 212, the composition ratio of Ni to Co, 1-y:y, is determined by SEM (Scanning Electron Microscope)-EDX or ICP (Inductively Coupled Plasma). Then, the valences of Ni and Co are calculated using X-ray absorption spectroscopy (XAS). Based on this, it is determined whether the composition ratio of Ni to Co, 1-y:y, is satisfied according to equation (1).

[0038] 2.6≤(valence of Ni)×(1-y)+(valence of Co)×y≤3.2 …(1)

[0039] Under the condition of satisfying equation (1), if Li and K are treated as monovalent ions and O is treated as divalent anion, 2-z can be obtained by the following equation (2).

[0040] 2-z=[1×(1-x)+1×x+(valence of Ni)×(1-y)+(valence of Co)×y] / 2 …(2)

[0041] If equation (1) is not satisfied, it is considered that Li is significantly depleted during charging and discharging, and is set as z=0.

[0042] Lithium composite oxides possess a layered rock-salt structure. In other words, at least a portion of the lithium composite oxide exhibits a layered rock-salt structure. This allows for an increase in volumetric energy density.

[0043] The crystal structure of lithium composite oxides can be determined using TEM (Transmission Electron Microscope) or STEM (Scanning Transmission Electron Microscope). More specifically, by observing samples prepared by cross-sectional processing of the positive electrode active material or the positive electrode active material layer 212, and performing crystal structure analysis using TEM or STEM on measurement sites at least 150 nm inward from the surface of the lithium composite oxide, the crystal structure of the lithium composite oxide can be determined. If a crystal structure belonging to space group R-3m can be confirmed in at least one measurement site, it can be said that the lithium composite oxide has a layered rock-salt type structure.

[0044] The distribution of potassium (K) in lithium composite oxides is described in detail below.

[0045] Figure 3A This is a TEM image showing the positive electrode active material involved in the embodiment of the first embodiment. Figure 3B It shows about Figure 3A A graph of the EDX mapping image of potassium in the region involved. Figure 4A This is a TEM image showing the positive electrode active material involved in the comparative example of the first embodiment. Figure 4B It shows about Figure 4A A graph showing the EDX mapping image of potassium in the region involved. Figure 3A as well as Figure 4A The areas appearing black or dark gray are occupied by lithium composite oxides. Figure 3B as well as Figure 4B The white areas in the center represent areas where potassium is distributed. Here, Figure 3A as well as Figure 3B The positive electrode active material involved is the same as that involved in Example 1 described later. Figure 4A as well as Figure 4B The positive electrode active material involved is the positive electrode active material involved in Comparative Example 2, which will be described later.

[0046] like Figure 3B As shown, in the lithium composite oxide according to the first embodiment, potassium is distributed in a direction that intersects the <001> direction of the layered rock salt structure. In this disclosure, the <001> direction of the layered rock salt structure refers to the direction in which the Miller index is equivalent to <001> in the normal direction of the crystal plane of the layered rock salt structure.

[0047] The distribution of potassium in the layered rock-salt structure can be determined using TEM-EDX or STEM-EDX. More specifically, by observing a sample prepared by cross-section processing of the positive electrode active material or the positive electrode active material layer 212, a rectangular region at least 150 nm inward from the surface of the lithium composite oxide, with sides greater than 50 nm and less than 200 nm, is selected. The <001> orientation of the layered rock-salt structure is determined using TEM or STEM crystal orientation analysis. Then, an EDX mapping image of potassium is acquired in this region to determine the area of ​​potassium distribution.

[0048] Here, as Figure 3B As shown, in at least a portion of the region, potassium is distributed linearly, and if it extends in a direction intersecting the <001> direction of the layered rock salt structure, then potassium can be said to be distributed in a manner extending in a direction intersecting the <001> direction of the layered rock salt structure. The determination of whether potassium distribution is linear is performed using the following method. First, the EDX mapping image of potassium is binarized. The binarization threshold is set to the average of the maximum and minimum pixel values ​​in the EDX mapping image. Furthermore, if the shape of the region with large pixel values ​​has a length in one direction, then potassium distribution can be said to be linear. On the other hand, as... Figure 4B As shown, when potassium is dispersed in a point-like pattern, the distribution of potassium is not linear. Therefore, it cannot be said that potassium is distributed in a way that extends in a direction that intersects the <001> direction of the layered rock salt structure.

[0049] In the lithium composite oxide according to the first embodiment, lithium ion insertion and removal occur in a direction intersecting the <001> direction of the layered rock salt structure. For example... Figure 3B As shown, by placing potassium ions with larger ionic radii than Li ions in the direction intersecting the <001> direction with the layered rock salt structure, the inter-oxygen distance in the <001> direction increases. Therefore, compared with... Figure 4B Compared to the potassium dispersion shown, this method suppresses the binding of oxygen to lithium ions, increases the diffusion rate of lithium ions, and improves charge / discharge load characteristics.

[0050] When the positive electrode active material contains particles composed of lithium composite oxides, the particle size is preferably 10 μm or more. In this disclosure, particle size refers to the median diameter (D). 50 (Diameter). As for the positive electrode active material according to the first embodiment, even if the crystal particles are increased, the charge and discharge load characteristics are high. Therefore, it is possible to suppress the performance degradation and volumetric energy density reduction caused by the increase in surface area and to improve the charging capacity.

[0051] When the positive electrode active material contains particles composed of lithium composite oxides, the particle size can be less than 100 μm. In this case, the battery characteristics during high-speed charging and discharging can be improved.

[0052] The positive electrode binder contained in the positive electrode active material layer 212 can be any material, such as any one or more of synthetic rubber and polymer compounds. Examples of synthetic rubbers include styrene-butadiene rubber, fluorinated rubber, and ethylene propylene diene monomer (EPDM) rubber. Examples of polymer compounds include polyvinylidene fluoride (PVdF) and polyimide.

[0053] The positive electrode active material layer 212 can contain any material as a positive electrode conductive additive, such as carbon. Examples of carbon include graphite, carbon black, acetylene black, and Ketjen black. However, the positive electrode conductive additive in the positive electrode active material layer 212 is not limited to these materials as long as it is a conductive material; it can also be a metallic material, a conductive polymer, etc.

[0054] The negative electrode 220 has a negative electrode current collector 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector 221 is stacked between the negative electrode active material layers 222.

[0055] The negative current collector 221 is a conductor, such as copper foil. Figure 1 In the example, when viewed from above in the thickness direction, the negative current collector 221 is a rectangular sheet with a protrusion on the side of the negative lead 22. The protrusion of the negative current collector 221 is connected to the negative lead 22.

[0056] The negative electrode active material layer 222 is a layer containing negative electrode active material. The negative electrode active material layer 222 is not limited to being composed solely of negative electrode active material; for example, it may also contain conductive additives and binders.

[0057] Negative electrode active materials refer to reducing agents such as carbon materials, metals, half-metals, silicon alloys or compounds, and tin (Sn) alloys or compounds that can absorb the charge carriers that have been released from the secondary battery 1 through charge-discharge reactions.

[0058] Anode active materials containing silicon include, for example, silicon monomers, silicon alloys, and silicon compounds. Examples of silicon alloys that can be used as anode active materials include alloys containing at least one of the following as a second constituent element other than silicon: tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr). Furthermore, examples of silicon compounds that can be used as first anode active materials include silicon oxide (SiO₂). xCompounds containing oxygen (O) or carbon (C), such as silicon carbide (SiC), may also contain the aforementioned second constituent element in addition to silicon. Furthermore, the negative electrode active material can also be doped with Li. When the negative electrode active material is SiO... x In this case, it is preferable to pre-dope Li by doping Li during the fabrication process of the negative electrode 220. This reduces the SiO₂ content as the active material of the negative electrode. x The irreversible capacity is reduced. Furthermore, the negative electrode active material can also be a composite of Si and other materials such as carbon, or a composite of Si alloys and other materials such as carbon. In this case, the irreversible capacity can be reduced. Additionally, the particle surface of the negative electrode active material is preferably partially or completely covered by carbon. This improves the electronic conductivity of the particle surface of the negative electrode active material.

[0059] Examples of carbon materials that can be used as negative electrode active materials include MCMB (MesoCarbon MicroBeads), artificial graphite, natural graphite, difficult-to-graphitize carbon, and easily-graphitize carbon. More specifically, materials that can be used as negative electrode active materials include pyrolytic carbon, coke, glassy carbon fibers, sintered organic polymer compounds, activated carbon, and carbon black. Coke includes pitch coke, needle coke, and petroleum coke. Here, sintered organic polymer compounds are produced by sintering and carbonizing polymers such as phenolic resin and furan resin at appropriate temperatures.

[0060] Furthermore, the negative electrode active material is not limited to the substances listed above, and may also include other negative electrode active materials, such as alloys or compounds of metals or half-metals, alloys or compounds of tin (Sn), and other materials capable of lithium insertion and extraction. Examples of metals and half-metals that can be used as negative electrode active materials include tin (Sn), lead (Pb), aluminum (Al), indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, germanium, tin, and lead are preferred. Moreover, tin has a high capacity for lithium insertion and extraction, resulting in high energy density, and is therefore more preferred.

[0061] Examples of tin alloys that can be used as negative electrode active materials include alloys comprising at least one of the following groups as a second constituent element other than tin: nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. Furthermore, examples of tin compounds that can be used as negative electrode active materials include compounds containing oxygen or carbon, which may also contain the aforementioned second constituent element in addition to tin.

[0062] The separator 230 is a membrane that insulates the positive electrode 210 and the negative electrode 220. The separator 230 is disposed between the main surface of the positive electrode 210 and the main surface of the negative electrode 220, so that the positive electrode 210 and the negative electrode 220 do not directly contact each other. Figure 1 In the example, when viewed from above in the thickness direction, the diaphragm 230 is a rectangular sheet.

[0063] The material of the separator 230 is preferably electrically stable, chemically stable to the positive electrode active material, the negative electrode active material, and the electrolyte, and also has insulating properties. The separator 230 can be, for example, a layer composed of polymer nonwoven fabric, porous membrane, glass, or ceramic fibers. More preferably, the material of the separator 230 comprises a porous polyolefin membrane. This improves battery safety through short-circuit protection and disconnection effects.

[0064] The electrolyte is impregnated in the diaphragm 230. Figure 1 In this example, the electrolyte fills the space within the outer packaging component 30. The electrolyte is a non-aqueous electrolyte containing an electrolyte salt and a solvent for dissolving the electrolyte salt.

[0065] Electrolyte salts include lithium salts such as lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO2C2F5)2), and lithium hexafluoroarsenate (LiAsF6).

[0066] Solvents include, for example, lactone solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate solvents such as ethylene carbonate, propylene carbonate, butyl carbonate, vinylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate; ether solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile solvents such as acetonitrile; sulfolane solvents; phosphoric acid solvents; phosphate solvents; pyrrolidone solvents; and other non-aqueous solvents.

[0067] As additives, electrolytes may also contain fluorinated carboxylic acid esters, sulfonates, sulfonic anhydrides, carboxylic anhydrides, and other additives.

[0068] The battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to... Figure 1 The secondary battery shown is illustrated below. Other examples will be explained below, but for those related to... Figure 1 as well as Figure 2 The same structural labels are used in the same figures, and the descriptions are omitted.

[0069] Figure 5This is a cutaway view showing different examples of the secondary battery involved in the first embodiment. Figure 6 yes Figure 5 A schematic diagram of the cross section of line VI-VI. Figure 5 as well as Figure 6 The secondary battery 1A shown is structurally similar to the one where the electrode body 200A is wound around the positive electrode lead 21A and the negative electrode lead 22A. Figure 1 The examples involved are different.

[0070] Battery element 20A is located inside the outer packaging component 30. For example... Figure 6 As shown, the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal extending from the inside of the battery element 20A to the outside of the outer packaging component 30, and is located near the center of the battery element 20A. The negative electrode lead 22A is a terminal extending from the inside of the battery element 20A to the outside of the outer packaging component 30, and is located near the center of the battery element 20A. The protective material 23 is a component that protects the exterior of the battery element 20A. The protective material 23 is provided in a manner that it is wound around the electrode body 200A. The protective material 23 is, for example, an insulating strip.

[0071] exist Figure 6 In this example, electrode body 200A is a laminate used for the charge-discharge reaction of the secondary battery 1A according to the first embodiment. Electrode body 200A includes: a positive electrode 210A, having a positive current collector 211A and a positive active material layer 212A; a negative electrode 220A, having a negative current collector 221A and a negative active material layer 222A; and a separator 230A. Electrode body 200A is a structure wound around the positive electrode lead 21A and the negative electrode lead 22A as the center, and is laminated from the outside, i.e., from the side of the protective material 23, in the following order: negative current collector 221A, negative active material layer 222A, separator 230A, positive active material layer 212A, positive current collector 211A, positive active material layer 212A, separator 230A, and negative active material layer 222A. The electrode body 200A does not have any layers other than the negative current collector 221A, the separator 230A, and the positive current collector 211A near the positive lead 21A and the negative lead 22A. With this structure, the positive current collector 211A is connected to the positive lead 21A, and the negative current collector 221A is connected to the negative lead 22A.

[0072] Furthermore, the secondary battery according to the first embodiment can also be an all-solid-state battery. That is, the electrolyte of the secondary battery according to the first embodiment can also be a sintered body containing a solid electrolyte. In this case, the diffusion rate of lithium ions can be increased, and the charge / discharge load characteristics can also be improved.

[0073] As explained above, the positive electrode active material according to the first embodiment comprises a lithium composite oxide having a layered rock salt-type structure. The lithium composite oxide comprises lithium, a metal element including at least one of cobalt and nickel, and potassium. The ratio of the amount of potassium in the lithium composite oxide to the amount of the metal element in the lithium composite oxide is 0.0005 or more and 0.03 or less. In the lithium composite oxide, potassium is distributed in a direction extending in a direction intersecting the <001> direction of the layered rock salt-type structure. This improves the diffusion rate of lithium ions and enhances the charge / discharge load characteristics.

[0074] As a preferred approach, the composition of the lithium composite oxide is derived from Li 1-x K x Ni 1-y Co y O 2-z This means that 0.0005≤x≤0.03, 0≤y≤1, and -0.1≤z≤0.2 are satisfied. Therefore, the amount of potassium becomes sufficient, thus further improving the charge / discharge load characteristics.

[0075] The positive electrode 210 according to the first embodiment includes the positive electrode active material according to the first embodiment. As a result, the charge and discharge load characteristics can be improved.

[0076] The secondary battery 1 according to the first embodiment includes a positive electrode 210, a negative electrode 220, and an electrolyte as described in the first embodiment. This improves the charge / discharge load characteristics.

[0077] (Method for manufacturing positive electrode active material)

[0078] Hereinafter, an example of a method for manufacturing the positive electrode active material according to the first embodiment will be described. The method for manufacturing the positive electrode active material according to the first embodiment includes a calcination process.

[0079] The calcination process is a process of producing lithium composite oxides by calcining a precursor containing at least one of lithium, cobalt, and nickel, as well as potassium. The precursor is, for example, prepared by mixing lithium hydroxide, cobalt oxide or nickel oxide, and potassium hydroxide. In the first embodiment, the precursor is calcined in a dry argon gas stream, and after temporarily becoming a molten salt, a calcined product is obtained by cooling. The obtained calcined product is then pulverized, sieved according to particle size, and washed with a polar solvent to remove precursor residues. Thus, the positive electrode active material according to this embodiment is obtained.

[0080] Here, the precursor is sintered at a temperature above 200°C and below 600°C. This suppresses potassium dispersion in the crystals and facilitates its placement in the direction intersecting the <001> direction of the layered rock salt structure, thereby increasing the lithium-ion diffusion rate and improving charge / discharge load characteristics.

[0081] Furthermore, the method for manufacturing the positive electrode active material described in this embodiment is merely one example and is not limited to the manufacturing method described above. For example, the method for producing lithium composite oxides in the calcination process is not limited to the molten salt crystallization method described above, and can also use crystallization synthesis methods that can be carried out at temperatures below 600°C, such as flux synthesis, laser thin film deposition, sputtering thin film deposition, chemical vapor deposition, organometallic decomposition, solvothermal synthesis, and soft chemical synthesis.

[0082] As explained above, the method for manufacturing the positive electrode active material according to this embodiment is a method for manufacturing a positive electrode active material comprising a lithium composite oxide having a layered rock salt structure. It includes a step of calcining a precursor comprising at least one of lithium, cobalt, and nickel, and potassium, at a temperature of 200°C or higher and less than 600°C to produce the lithium composite oxide. This suppresses potassium dispersion in the crystals and facilitates its arrangement in the direction intersecting the <001> direction of the layered rock salt structure, thereby increasing the lithium-ion diffusion rate and improving charge / discharge load characteristics.

[0083] (Example)

[0084] The embodiments will be described below. Table 1 is a table showing the embodiments and comparative examples. However, the present invention is not limited to these embodiments.

[0085] Table 1

[0086] (Example 1)

[0087] The positive electrode active material involved in Example 1 was prepared by the following method. A precursor was prepared by mixing lithium hydroxide, cobalt oxide, and potassium hydroxide. As a calcination step, the prepared precursor was added to a platinum crucible and calcined at 350°C for 3 hours in a dry argon gas stream. After cooling, a calcined product was obtained. The calcined product was pulverized, sieved according to particle size, and washed with a polar solvent to remove precursor residues. Thus, the positive electrode active material involved in Example 1 was obtained. The particle size of the positive electrode active material involved in Example 1 was 10 μm.

[0088] In the battery of Example 1, the positive electrode housed inside the outer packaging cup and the negative electrode housed inside the outer packaging can are stacked together with a separator in between. In the battery of Example 1, the outer packaging can and the outer packaging cup are riveted together with gaskets in between. Furthermore, the separator of Example 1 is impregnated with electrolyte.

[0089] The positive electrode involved in Example 1 was manufactured by the following method. First, 98 parts by weight of the positive electrode active material prepared above, 1.2 parts by weight of polyvinylidene fluoride as a positive electrode binder, and 0.8 parts by weight of Ketjenblack as a positive electrode conductive additive were mixed to prepare a positive electrode mixture. Then, the prepared positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a paste-like positive electrode mixture slurry. Then, the positive electrode mixture slurry was coated on both sides of an aluminum foil with a thickness of 15 μm, which serves as the positive electrode current collector layer, using a coating apparatus. The positive electrode mixture slurry was then dried with hot air to form a positive electrode active material layer in the positive electrode current collector layer. Subsequently, the positive electrode active material layer was compressed and molded using a hydraulic press to obtain the positive electrode involved in Example 1.

[0090] The negative electrode involved in Example 1 was manufactured by the following method. First, 95 parts by weight of graphite as the negative electrode active material and 5 parts by weight of polyvinylidene fluoride as the negative electrode binder were mixed to prepare a negative electrode mixture. Then, the prepared negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry was coated on both sides of a 12 μm thick copper foil serving as the negative electrode current collector layer using a coating apparatus. Then, the negative electrode mixture slurry was hot-air dried to form a negative electrode active material layer on the negative electrode current collector layer. Subsequently, the negative electrode active material layer was compressed and molded using a hydraulic press to obtain the negative electrode involved in Example 1.

[0091] The negative electrode involved in Example 1 was prepared by the following method. Lithium hexafluorophosphate, as the electrolyte salt, was dissolved in a liquid in which ethylene carbonate and propylene carbonate, as solvents, were mixed at a volume ratio of 1:1. Here, the electrolyte salt content relative to the solvent was 1 mol / cm³. 3 It is prepared in the manner described above.

[0092] The battery described in Example 1 was assembled using the following method. First, the prepared positive electrode was punched into granules with a diameter of 15 mm and housed inside an outer packaging cup. Then, the prepared negative electrode was punched into granules with a diameter of 16 mm and housed inside an outer packaging can. Next, the prepared electrolyte was impregnated in a porous polyolefin membrane serving as a separator, and this porous polyolefin membrane was laminated between the positive electrode housed in the outer packaging cup and the negative electrode housed in the outer packaging can. Afterward, the outer packaging can and the outer packaging cup were riveted together with gaskets in between. Thus, the battery described in Example 1 was manufactured.

[0093] Analysis of Positive Electrode Active Materials

[0094] Compositional analysis of the prepared positive electrode active material was performed using EDX. In the analysis, a SEM-EDX (S-4800, Hitachi High-Technologies) was used. A rectangular region, 500 nm x 500 nm, was selected, extending at least 150 nm inward from the surface of the positive electrode active material particles in the positive electrode active material layer, to determine the contents of Co and K. The contents of Co and K were measured in five non-overlapping regions, and the molar ratio of K content to Co content in each region was calculated. Thus, x was calculated as the arithmetic mean of the molar ratios of K content to Co content in each region, representing the amount of K to the amount of Co. The results of the above measurements showed that x = 0.03 in Example 1. Here, in Example 1, since nickel was not included, y = 1. Furthermore, in Example 1, z = 0.

[0095] The crystal structure of the prepared positive electrode active material was analyzed using TEM. In the analysis, a TEM (JEM-F200, Japan Electron) was used at an accelerating voltage of 200 kV to determine the crystal structure of a region extending more than 150 nm inward from the surface of the positive electrode active material particles in the positive electrode active material layer. The results showed that, in Example 1, a crystal structure belonging to space group R-3m was confirmed in this region. Therefore, it was determined that the positive electrode active material involved in Example 1 has a layered rock salt type structure.

[0096] For the prepared positive electrode active material, the distribution of potassium in the layered rock-salt structure was determined using TEM-EDX. In the determination, a TEM (JEM-F200, Nippon Electron) and its associated EDX analyzer (Noran system 7, ThermoFisher Scientific) were used at an accelerating voltage of 200 kV. A rectangular region, 500 nm x 500 nm, extending at least 150 nm inward from the surface of the lithium composite oxide, was selected. The <001> orientation of the layered rock-salt structure was determined by resolving the crystal orientation using the TEM. Subsequently, an EDX mapping image of potassium was acquired in this region to determine the potassium distribution area. The results of the above determinations, as in Example 1, show that... Figure 3BAs shown, potassium is distributed linearly and extends in a direction intersecting the <001> direction of the layered rock salt structure. Therefore, in Example 1, it is determined that potassium is distributed in a manner that extends in a direction intersecting the <001> direction of the layered rock salt structure. In Table 1, when potassium is distributed in a manner that extends in a direction intersecting the <001> direction of the layered rock salt structure, the K distribution state is "A".

[0097] Charging Capacity Measurement Test

[0098] Under the following conditions, a charging test was conducted on the manufactured battery, and the capacity per unit mass of the positive electrode active material was calculated as the charging capacity.

[0099] Charging method: CC / CV charging

[0100] Charging rate: 0.05C

[0101] Upper voltage limit: 4.3V

[0102] Initial Efficiency Measurement Test

[0103] Under the following conditions, charge and discharge tests were conducted on the manufactured battery. The capacity per unit mass of the positive electrode active material during charging was defined as the charging capacity, and the capacity per unit mass of the positive electrode active material during subsequent discharging was defined as the discharging capacity. The ratio of the discharging capacity to the charging capacity was calculated as the initial efficiency.

[0104] Charging method: CC / CV charging

[0105] Charging rate: 0.05C

[0106] Upper voltage limit: 4.3V

[0107] Discharge method: CC discharge

[0108] Discharge rate: 0.05C

[0109] Lower limit voltage: 3V

[0110] Discharge Load Measurement Test

[0111] Two discharge tests with different discharge rates were conducted on the battery under the following conditions. The ratio of the capacity per unit mass of the positive electrode active material during discharge at a discharge rate of 0.2C to the capacity per unit mass of the positive electrode active material during discharge at a discharge rate of 2C was calculated as the discharge load.

[0112] Discharge method: CC discharge

[0113] Discharge rate: 2C, 0.2C

[0114] Lower limit voltage: 3V

[0115] Charging Load Measurement Test

[0116] Two charging tests with different charging rates were conducted on the battery under the following conditions. The ratio of the capacity per unit mass of the positive electrode active material during charging at a charging rate of 0.2C to the capacity per unit mass of the positive electrode active material during charging at a charging rate of 2C was calculated as the charging load.

[0117] Charging method: CC discharge

[0118] Charging rate: 2C, 0.2C

[0119] Upper voltage limit: 3V

[0120] (Example 2)

[0121] In Example 2, the positive electrode active material and battery were fabricated in the same manner as in Example 1, and analysis and experiments were conducted. The analysis results of the positive electrode active material showed that the composition, crystal structure, K distribution state, and particle size of the positive electrode active material involved in Example 2 were the same as those in Example 1.

[0122] (Example 3)

[0123] In Example 3, except that the positive electrode active material was fabricated with x = 0.005 and a particle size of 20 μm, the positive electrode active material and battery were fabricated in the same manner as in Example 1, and analysis and experiments were performed. The analysis results of the positive electrode active material showed that the crystal structure and K distribution of the positive electrode active material involved in Example 3 were the same as those in Example 1.

[0124] (Example 4)

[0125] In Example 4, except that the positive electrode active material was fabricated with x = 0.0005 and a particle size of 50 μm, the positive electrode active material and battery were fabricated in the same manner as in Example 1, and analysis and experiments were performed. The analysis results of the positive electrode active material showed that the crystal structure and K distribution of the positive electrode active material involved in Example 4 were the same as those in Example 1.

[0126] (Comparative Example 1)

[0127] The positive electrode active material involved in Comparative Example 1 was prepared by the following method: A precursor was prepared by mixing lithium carbonate and cobalt oxide. As a calcination step, the prepared precursor was added to an alumina crucible and calcined at 850°C for 3 hours in a dry air stream. After calcination, the calcined material was cooled to obtain a calcined product. The obtained calcined product was pulverized, sieved according to particle size, and washed with a polar solvent to remove the precursor residue. Thus, the positive electrode active material involved in Comparative Example 1 was obtained. Here, the particle size of the positive electrode active material involved in Comparative Example 1 was 20 μm.

[0128] Subsequently, a battery was fabricated in the same manner as in Example 1, and analysis and testing were performed. Analysis of the positive electrode active material showed that the crystal structure of the positive electrode active material involved in Comparative Example 1 was the same as that in Example 1. On the other hand, TEM-EDX determination of the potassium distribution in the layered rock-salt structure showed no potassium distribution.

[0129] (Comparative Example 2)

[0130] In Comparative Example 2, the positive electrode active material was prepared in the same manner as in Comparative Example 1, except that potassium hydroxide was used as a precursor with x = 0.001. Analysis and testing were then performed. The analytical results showed that the crystal structure of the positive electrode active material in Example 4 was the same as that in Example 1. On the other hand, the distribution of potassium in the layered rock-salt structure was determined using TEM-EDX. The results showed that in Comparative Example 2, as... Figure 4A as well as Figure 4B As shown, potassium is dispersed in a dotted pattern along the surface of the positive electrode active material particles, and the distribution of potassium is not linear. Therefore, in Comparative Example 2, it was determined that potassium was not distributed in a manner that extends in a direction intersecting the <001> direction of the layered rock salt type structure. In Table 1, when potassium is dispersed in a dotted pattern and the distribution of potassium is not linear, the K distribution state is "B".

[0131] As shown in Table 1, in Examples 1 to 3, by setting the K distribution state to "A", compared with Comparative Example 1 (which does not contain K) and Comparative Example 2 (which has a K distribution state of "B"), the charging capacity and initial efficiency are not significantly reduced, and the charging load and discharging load are improved. Therefore, it can be seen that by setting the K distribution state to "A", the charging and discharging load characteristics can be improved while suppressing the reduction of charging and discharging characteristics.

[0132] As shown in Table 1, in Example 4, by using a K distribution state of "A" and a particle size of 50 μm, compared to Comparative Example 1 (which does not contain K and has a particle size of 20 μm) and Comparative Example 2 (which uses a K distribution state of "B" and has a particle size of 20 μm), the battery characteristics such as initial capacity, charging load, and charging load were not significantly reduced, thus improving the charging capacity. Therefore, it can be seen that increasing the particle size can improve the charging capacity. Furthermore, it can be seen that by using a K distribution state of "A", the significant reduction in charge / discharge load characteristics due to increasing the particle size can be suppressed.

[0133] The above embodiments are provided for easy understanding of the present invention and are not intended to limit the interpretation of the embodiments of the present invention. The present invention can be modified / improved without departing from its spirit, and equivalents are also included in the present invention.

[0134] Explanation of reference numerals in the attached figures

[0135] 1, 1A: Secondary battery; 20, 20A: Battery element; 21, 21A: Positive electrode lead; 22, 22A: Negative electrode lead; 23: Protective material; 30: Outer packaging component; 30a, 30b: Outer packaging sheet; 31: Indentation; 32: Sealing material; 200, 200A: Electrode body; 210, 210A: Positive electrode; 211, 211A: Positive electrode current collector; 212, 212A: Positive electrode active material layer; 220, 220A: Negative electrode; 221, 221A: Negative electrode current collector; 222, 222A: Negative electrode active material layer; 230, 230A: Separator.

Claims

1. A positive electrode active material comprising a lithium composite oxide having a layered rock salt structure. The lithium composite oxide comprises lithium, a metal element comprising at least one of cobalt and nickel, and potassium. The ratio of the amount of potassium contained in the lithium composite oxide to the amount of the metal element contained in the lithium composite oxide is 0.0005 or more and 0.03 or less. In the lithium composite oxide, potassium is distributed in a direction that intersects the <001> direction of the layered rock salt structure.

2. The positive electrode active material according to claim 1, wherein, The composition of the lithium composite oxide is Li 1-x K x Ni 1-y Co y O 2-z This means that 0.0005≤x≤0.03, 0≤y≤1, and -0.1≤z≤0.2 are satisfied.

3. A positive electrode comprising the positive electrode active material as described in claim 1 or 2.

4. A secondary battery comprising the positive electrode, negative electrode, and electrolyte as described in claim 3.

5. A method for manufacturing a positive electrode active material, comprising a lithium composite oxide having a layered rock salt structure. The method for manufacturing the positive electrode active material includes a step of calcining a precursor at a temperature above 200°C and below 600°C to produce the lithium composite oxide, wherein the precursor comprises at least one of lithium, cobalt and nickel, and potassium.

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