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

Abstract

Provided is a positive electrode active material, a positive electrode, or a secondary battery having high charging load characteristics. This positive electrode active material contains a lithium composite oxide having a layered rock salt-type structure. The lithium composite oxide contains: lithium; a metal element containing at least one among cobalt and nickel; and potassium. The ratio of the substance amount of the potassium contained in the lithium composite oxide to the substance amount of the metal element contained in the lithium composite oxide is 0.0005-0.03. In the lithium composite oxide, the potassium is distributed so as to extend in a direction intersecting with the <001> direction of the layered rock salt-type structure.

Description

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

[0001] The present invention relates to a positive electrode active material, a positive electrode, a secondary battery, and a method for producing a positive electrode active material.

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

[0003] Patent No. 6809487

[0004] However, the positive electrode active material described in Patent Document 1 may have a reduced charge load characteristic depending on the distribution of potassium in the layered rock salt structure.

[0005] The present invention has been made in view of the above problems, and has an object to provide a positive electrode active material, a positive electrode, or a secondary battery that has excellent charging load characteristics.

[0006] A positive electrode active material according to one embodiment of the present invention includes a lithium composite oxide having a layered rock-salt structure, the lithium composite oxide including lithium, a metal element including at least one of cobalt and nickel, and potassium, wherein the ratio of the amount of substance of potassium included in the lithium composite oxide to the amount of substance of the metal element included in the lithium composite oxide is 0.0005 or more and 0.03 or less, and the potassium is distributed in the lithium composite oxide so as to extend in a direction intersecting the <001> direction of the layered rock-salt structure.

[0007] A positive electrode according to one aspect of the present invention includes the positive electrode active material.

[0008] A secondary battery according to one aspect of the present invention includes the positive electrode, a negative electrode, and an electrolyte.

[0009] A method for producing a positive electrode active material according to one embodiment of the present invention is a method for producing a positive electrode active material containing a lithium composite oxide having a layered rock salt structure, and includes a step of calcining a precursor containing lithium, at least one of cobalt and nickel, and potassium at a temperature of 200°C or higher and lower than 600°C to produce the lithium composite oxide.

[0010] According to the present invention, it is possible to provide a positive electrode active material, a positive electrode, or a secondary battery having excellent charging load characteristics.

[0011] FIG. 1 is a cross-sectional view showing an example of a secondary battery according to the first embodiment. FIG. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode body according to FIG. 1. FIG. 3A is a diagram showing a TEM observation image of a positive electrode active material according to an example of the first embodiment. FIG. 3B is a diagram showing an EDX mapping image of potassium in the region according to FIG. 3A. FIG. 4A is a diagram showing a TEM observation image of a positive electrode active material according to a comparative example of the first embodiment. FIG. 4B is a diagram showing an EDX mapping image of potassium in the region according to FIG. 4A. FIG. 5 is a cut-out view showing a different example of a secondary battery according to the first embodiment. FIG. 6 is a schematic diagram of a cross section taken along line VI-VI in FIG. 5.

[0012] The following describes an embodiment of the present invention. However, the present invention is not limited to this embodiment. Numerical values ​​include the range of rounding.

[0013] (Secondary battery) Fig. 1 is a cross-sectional view showing an example of a secondary battery according to the first embodiment. The secondary battery 1 shown in Fig. 1 is a laminated lithium-ion secondary battery. As shown in Fig. 1, the secondary battery 1 includes a battery element 20, an exterior member 30, and an adhesive 32.

[0014] The battery element 20 is provided inside an exterior member 30. As shown in FIG. 1 , 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 drawn from a positive electrode 210 (described later) to the outside of the exterior member 30. That is, the positive electrode lead 21 is a terminal that serves as a positive electrode of the secondary battery 1. In FIG. 1 , the positive electrode lead 21 is provided on an end surface of the electrode body 200. The negative electrode lead 22 is a terminal drawn from the inside of a negative electrode 220 (described later) to the outside of the exterior member 30. That is, the negative electrode lead 22 is a terminal that serves as a negative electrode of the secondary battery 1. In FIG. 1 , the negative electrode lead 22 is provided on an end surface of the electrode body 200. Details of the electrode body 200 will be described later.

[0015] The exterior member 30 is a case in which the battery element 20 is housed. The exterior member 30 includes two exterior sheets 30a and 30b. The exterior sheets 30a and 30b each include an insulating layer, a metal layer, and an outermost layer. In the example of FIG. 1 , the exterior sheet 30a has a recess 31. As a result, the battery element 20 is housed in the exterior member 30 by housing the battery element 20 in the recess 31 and bonding the peripheral edges of the exterior sheets 30a and 30b.

[0016] The exterior sheets 30a, 30b are constructed by laminating an insulating layer, a metal layer, and an outermost layer in this order from the inside, i.e., the side where the battery element 20 is provided, and then bonding them together by lamination or other processing. The insulating layers of the exterior sheets 30a, 30b are made of resins such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, and polyolefin resins containing ethylene or propylene as monomers. This allows the exterior sheets 30a, 30b to reduce the moisture permeability of the secondary battery 1 and improve its airtightness. The metal layers of the exterior sheets 30a, 30b are metal plate or foil materials such as aluminum, stainless steel, nickel, and iron. The outermost layer may be made of any material, but is preferably made of the same resin as the insulating layer or a material with high resistance to tearing and punctures, such as nylon.

[0017] The adhesive 32 is a member for making the exterior member 30 airtight. The adhesive 32 is provided between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the adhesive 32 preferably has adhesion to 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 metal material, the adhesive 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. This allows the adhesive 32 to seal the gap between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the exterior member 30 airtight.

[0018] Fig. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode assembly in Fig. 1. More specifically, Fig. 2 is a cross-sectional view showing a portion of one layer of a positive electrode 210 and one layer of a negative electrode 220 of the electrode assembly 200. As shown in Fig. 2, the electrode assembly 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode assembly 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 interposed therebetween. The positive electrode 210 and the negative electrode 220 included in the electrode assembly 200 are layered members for the charge / discharge reaction of the secondary battery 1 according to the first embodiment.

[0019] The positive electrode 210 includes a positive electrode current collector 211 and a positive electrode active material layer 212. In the positive electrode 210, the positive electrode current collector 211 is laminated between the positive electrode active material layers 212.

[0020] The positive electrode current collector 211 is a conductive layer, and may be made of, for example, aluminum foil, stainless steel foil, etc. In the example of Fig. 1 , the positive electrode current collector 211 has a rectangular shape in plan view in the thickness direction, with protrusions on the positive electrode lead 21 side. The protrusions of the positive electrode current collector 211 are connected to the positive electrode lead 21.

[0021] The positive electrode active material layer 212 is a layer containing a positive electrode active material. The positive electrode active material layer 212 includes a 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, and may further include, for example, a dispersant.

[0022] The positive electrode active material includes a lithium composite oxide. The lithium composite oxide includes lithium, a metal element including at least one of cobalt and nickel, and potassium. In the first embodiment, the lithium composite oxide has a composition formula of Li １－ｘ K ｘ Ni １－ｙ Co ｙ O ２－ｚand satisfying the conditions 0.0005≦x≦0.03, 0≦y≦1, and −0.1≦z≦0.2. Here, x in this composition formula is the ratio of the amount of substance of potassium contained in the lithium composite oxide to the amount of substance of metal elements contained in the lithium composite oxide. By setting x to 0.0005 or more, it is possible to improve the charge / discharge load characteristics. By setting x to 0.03 or less, it is possible to suppress deterioration of the charge / discharge characteristics. Note that in the first embodiment, the positive electrode active material is particles made of lithium composite oxide, but is not limited thereto and may be, for example, a layer made of lithium composite oxide.

[0023] The values ​​of x and y can be measured by EDX (Energy Dispersive X-ray Spectroscopy). More specifically, a sample prepared by cross-sectionally processing the positive electrode active material or the positive electrode active material layer 212 is observed, and a rectangular region 150 nm or more inward from the surface of the lithium composite oxide, with sides of 50 nm to 200 nm, is selected, and the contents of Ni, Co, and K are measured by EDX. In the measurement, the contents of Ni, Co, and K are measured in five non-overlapping regions, and the molar ratio of the K content to the total Ni and Co content in each region is calculated. By calculating the arithmetic average of the molar ratios of the K content to the total Ni and Co content 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.

[0024] The value of z is calculated by the following method. First, for a sample prepared by cross-sectionally processing the positive electrode active material or the positive electrode active material layer 212, the Ni:Co composition ratio 1-y:y is measured using SEM (Scanning Electron Microscope)-EDX or ICP (Inductively Coupled Plasma). Then, the valence of Ni and Co is calculated using X-ray absorption spectroscopy (XAS) using synchrotron X-rays. Then, it is determined whether or not the formula (1) using the Ni:Co composition ratio 1-y:y is satisfied: 2.6≦(Ni valence)×(1-y)+(Co valence)×y≦3.2 (1) If formula (1) is satisfied, 2-z can be calculated using the following formula (2) with Li and K as monovalent ions and O as divalent anions. 2-z=[1×(1-x)+1×x+(Ni valence)×(1-y)+(Co valence)×y] / 2 (2) If formula (1) is not satisfied, it is assumed that there is a large loss of Li due to charging and discharging, etc., and z=0.

[0025] The lithium composite oxide has a layered rock-salt structure. In other words, at least a portion of the lithium composite oxide has a layered rock-salt structure. This can improve the volumetric energy density.

[0026] The crystal structure of the lithium composite oxide can be identified using a TEM (Transmission Electron Microscope) or a STEM (Scanning Transmission Electron Microscope). More specifically, the crystal structure of the lithium composite oxide can be identified by observing a sample prepared by cross-sectionally processing the positive electrode active material or the positive electrode active material layer 212, and performing crystal structure analysis using a TEM or STEM on a measurement point 150 nm or more inward from the surface of the lithium composite oxide. In this case, if a crystal structure belonging to the space group R-3m can be confirmed at at least one measurement point, the lithium composite oxide can be said to have a layered rock salt structure.

[0027] The distribution of potassium (K) in the lithium composite oxide will be described in detail below.

[0028] FIG. 3A is a diagram showing a TEM observation image of a cathode active material according to an example of the first embodiment. FIG. 3B is a diagram showing an EDX mapping image of potassium in the region according to FIG. 3A. FIG. 4A is a diagram showing a TEM observation image of a cathode active material according to a comparative example of the first embodiment. FIG. 4B is a diagram showing an EDX mapping image of potassium in the region according to FIG. 4A. The regions appearing in black or dark gray in FIGS. 3A and 4A are regions occupied by lithium composite oxide. The regions appearing in white in FIGS. 3B and 4B are regions where potassium is distributed. Here, the cathode active material according to FIGS. 3A and 3B is the cathode active material according to Example 1, which will be described later, and the cathode active material according to FIGS. 4A and 4B is the cathode active material according to Comparative Example 2, which will be described later.

[0029] 3B , in the lithium composite oxide according to the first embodiment, potassium is distributed so as to extend in a direction intersecting the <001> direction of the layered rock-salt structure. In the present disclosure, the <001> direction of the layered rock-salt structure refers to the direction normal to the crystal plane of the layered rock-salt structure that is equivalent to Miller indices <001>.

[0030] The distribution of potassium in the layered rock-salt structure can be measured by TEM-EDX or STEM-EDX. More specifically, a sample prepared by cross-sectionally processing the positive electrode active material or the positive electrode active material layer 212 is observed, and a rectangular region with sides of 50 nm to 200 nm, located at least 150 nm inward from the surface of the lithium composite oxide, is selected, and the <001> direction of the layered rock-salt structure is identified by analyzing the crystal orientation using TEM or STEM. Next, an EDX mapping image of potassium is obtained in that region, and the region in which potassium is distributed is identified.

[0031] Here, as shown in FIG. 3B , if the potassium distribution is linear in at least a portion of the region and extends in a direction intersecting the <001> direction of the layered rock-salt structure, it can be said that the potassium is distributed so as to extend in a direction intersecting the <001> direction of the layered rock-salt structure. Whether the potassium distribution is linear or not can be determined as follows: First, the potassium EDX mapping image is binarized. The threshold for binarization is the average value of the maximum pixel value and the minimum pixel value in the EDX mapping image. If the shape of the region with the larger pixel value has a length in one direction, it can be said that the potassium distribution is linear. On the other hand, if the potassium is dispersed in a dotted pattern as shown in FIG. 4B , the potassium distribution is not linear, and therefore it cannot be said that the potassium is distributed so as to extend in a direction intersecting the <001> direction of the layered rock-salt structure.

[0032] In the lithium composite oxide according to the first embodiment, lithium ions are inserted and extracted in a direction intersecting the <001> direction of the layered rock-salt structure. As shown in Fig. 3B, potassium, which has a larger ionic radius than Li ions, is arranged in a direction intersecting the <001> direction of the layered rock-salt structure, thereby increasing the inter-oxygen distance in the <001> direction. This suppresses the binding of lithium ions by oxygen, compared to the case in which potassium is dispersed and distributed as shown in Fig. 4B, thereby improving the diffusion rate of lithium ions and the charge / discharge load characteristics.

[0033] In the case where the positive electrode active material contains particles made of a lithium composite oxide, the particle size is preferably 10 μm or more. ５０ The positive electrode active material according to the first embodiment has high charge / discharge load characteristics even when the crystal particles are large, and thus can improve the charge capacity while suppressing a decrease in performance and a decrease in volumetric energy density due to an increase in surface area.

[0034] When the positive electrode active material contains particles made of a lithium composite oxide, the particle size may be 100 μm or less, which can improve the battery characteristics during high-speed charge and discharge.

[0035] The positive electrode binder contained in the positive electrode active material layer 212 may be any material, and may include, for example, one or more of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene-butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Examples of polymer compounds include polyvinylidene fluoride (PVdF) and polyimide.

[0036] The positive electrode conductive additive contained in the positive electrode active material layer 212 may be any material, and may include, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, and ketjen black. However, the positive electrode conductive additive contained in the positive electrode active material layer 212 is not limited to these materials as long as it is a conductive material, and may also be a metal material, a conductive polymer, or the like.

[0037] The negative electrode 220 includes 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 laminated between the negative electrode active material layers 222.

[0038] The negative electrode current collector 221 is a conductor, and for example, copper foil or the like can be used. In the example of Fig. 1 , the shape of the negative electrode current collector 221 is a rectangular sheet having protrusions on the negative electrode lead 22 side when viewed in a plan view in the thickness direction. The protrusions of the negative electrode current collector 221 are connected to the negative electrode lead 22.

[0039] The negative electrode active material layer 222 is a layer containing a negative electrode active material. The negative electrode active material layer 222 is not limited to being made of only a negative electrode active material, and may also contain, for example, a conductive additive and a binder.

[0040] The negative electrode active material refers to a reducing agent that can absorb and desorb charge carriers of the secondary battery 1 through charge and discharge reactions, such as a carbon material, a metal, a semimetal, a silicon alloy or compound, or a tin (Sn) alloy or compound.

[0041] The negative electrode active material containing silicon includes, for example, elemental silicon, silicon alloys, and silicon compounds. Examples of silicon alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of 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) as the second constituent element other than silicon. Examples of silicon compounds that can be used as the first negative electrode active material include silicon oxide (SiO ｘ Examples of the negative electrode active material include those containing oxygen (O) or carbon (C), such as silicon carbide (SiC), and may contain the second constituent element described above in addition to silicon. The negative electrode active material may also be doped with Li. ｘ In this case, it is preferable that Li is pre-doped by doping with Li in the process of producing the negative electrode 220. ｘ The negative electrode active material may be a composite of Si and other materials such as carbon, or a composite of a Si alloy and other materials such as carbon. In this case, the irreversible capacity can be reduced. In addition, it is preferable that the particle surfaces of the negative electrode active material are partially or entirely coated with carbon. This can improve the electronic conductivity of the particle surfaces of the negative electrode active material.

[0042] Examples of carbon materials that can be used as the negative electrode active material include MCMB (MesoCarbon MicroBeads), artificial graphite, natural graphite, non-graphitizable carbon, and graphitizable carbon. More specifically, examples of materials that can be used as the negative electrode active material include pyrolytic carbons, cokes, glassy carbon fiber, fired organic polymer compounds, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. Here, fired organic polymer compounds are produced by firing a polymer compound such as a phenolic resin or a furan resin at an appropriate temperature and carbonizing it.

[0043] The negative electrode active material is not limited to those listed above, and may contain other negative electrode active materials, such as metals, semimetal alloys or compounds, and tin (Sn) alloys or compounds, which can absorb and release lithium. Examples of metals and semimetals 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. Tin is more preferred because it has a high ability to absorb and release lithium and can achieve a high energy density.

[0044] Examples of tin alloys that can be used as the negative electrode active material include those containing at least one of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Examples of tin compounds that can be used as the negative electrode active material include those containing oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

[0045] The separator 230 is a film that insulates the positive electrode 210 from the negative electrode 220. The separator 230 is provided 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 come into direct contact with each other. In the example of Fig. 1 , the shape of the separator 230 is a rectangular sheet when viewed in plan in the thickness direction.

[0046] The separator 230 is preferably made of a material that is electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and is insulating. The separator 230 can be made of, for example, a polymer nonwoven fabric, a porous film, or a layer of glass or ceramic fibers. The separator 230 is more preferably made of a porous polyolefin film. This improves battery safety by preventing short circuits and providing a shutdown effect.

[0047] The electrolyte solution is impregnated into the separator 230. In the example of Fig. 1, the electrolyte solution fills the space inside the exterior member 30. The electrolyte solution is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0048] The electrolyte salt is, for example, lithium perchlorate (LiClO ４ ), lithium hexafluorophosphate (LiPF ６ ), lithium tetrafluoroborate (LiBF ４ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO ２ CF ３ ) ２ ), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO ２ C ２ F ５ ) ２ ), lithium hexafluoroarsenate (LiAsF ６ ) and other lithium salts.

[0049] Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0050] The electrolytic solution may further contain an additive such as a fluorinated carboxylic acid ester, a sulfonic acid ester, a sulfonic acid anhydride, or a carboxylic acid anhydride.

[0051] The battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to that shown in Fig. 1. Other examples will be described below, but the same components as those in Figs. 1 and 2 will be designated by reference numerals and will not be described again.

[0052] Fig. 5 is a cutaway view showing a different example of the secondary battery according to the first embodiment. Fig. 6 is a schematic view of a cross section taken along line VI-VI in Fig. 5. The secondary battery 1A shown in Figs. 5 and 6 differs from the example shown in Fig. 1 in that the electrode body 200 has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center.

[0053] The battery element 20A is provided inside the exterior member 30. As shown in FIG. 6 , 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 drawn from inside the battery element 20A to the outside of the exterior member 30, and the positive electrode lead 21A is provided near the center of the battery element 20A. The negative electrode lead 22A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the negative electrode lead 22A is provided near the center of the battery element 20A. The protective material 23 is a member that protects the outside of the battery element 20A. The protective material 23 is provided so as to be wrapped around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0054] 6 , the electrode assembly 200A is a laminate for the charge / discharge reaction of the secondary battery 1A according to the first embodiment. The electrode assembly 200A includes a positive electrode 210A including a positive electrode current collector 211A and a positive electrode active material layer 212A, a negative electrode 220A including a negative electrode current collector 221A and a negative electrode active material layer 222A, and a separator 230A. The electrode assembly 200A has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center, and is laminated in the following order from the outside, i.e., from the protective material 23 side: the negative electrode current collector 221A, the negative electrode active material layer 222A, the separator 230A, the positive electrode active material layer 212A, the positive electrode current collector 211A, the positive electrode active material layer 212A, the separator 230A, and the negative electrode active material layer 222A. In the electrode body 200A, no layers other than the negative electrode current collector 221A, separator 230A, and positive electrode current collector 211A are provided near the positive electrode lead 21A and the negative electrode lead 22A. With this structure, the positive electrode current collector 211A is connected to the positive electrode lead 21A, and the negative electrode current collector 221A is connected to the negative electrode lead 22A.

[0055] The electrolyte of the secondary battery according to the first embodiment may be an all-solid-state battery. That is, the electrolyte of the secondary battery according to the first embodiment may be a sintered body containing a solid electrolyte. In this case, too, the diffusion rate of lithium ions can be improved, and the charge / discharge load characteristics can be improved.

[0056] As described above, the positive electrode active material according to the first embodiment includes 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 potassium contained in the lithium composite oxide to the amount of metal elements contained in the lithium composite oxide is 0.0005 or more and 0.03 or less. In the lithium composite oxide, the potassium is distributed so as to extend in a direction intersecting the <001> direction of the layered rock-salt structure. This increases the diffusion rate of lithium ions and improves charge / discharge load characteristics.

[0057] In a preferred embodiment, the lithium composite oxide has a composition formula of Li １－ｘ K ｘ Ni １－ｙ Co ｙ O ２－ｚand satisfying 0.0005≦x≦0.03, 0≦y≦1, and −0.1≦z≦0.2. This ensures a sufficient amount of potassium, thereby further improving the charge and discharge load characteristics.

[0058] The positive electrode 210 according to the first embodiment contains the positive electrode active material according to the first embodiment, which can improve the charge / discharge load characteristics.

[0059] The secondary battery 1 according to the first embodiment includes the positive electrode 210 according to the first embodiment, the negative electrode 220, and an electrolyte, thereby improving the charge / discharge load characteristics.

[0060] (Method for manufacturing a positive electrode active material) Hereinafter, an example of the method for manufacturing a positive electrode active material according to the first embodiment will be described. The method for manufacturing a positive electrode active material according to the first embodiment includes a firing step.

[0061] The calcination step is a step of producing a lithium composite oxide by calcining a precursor containing lithium, at least one of cobalt and nickel, and potassium. The precursor is produced, for example, by mixing lithium hydroxide, cobalt oxide or nickel oxide, and potassium hydroxide. In the first embodiment, the precursor is calcined in a dry argon stream to form a molten salt, which is then cooled to obtain a calcined product. The calcined product is then pulverized, sieved by particle size, and washed with a polar solvent to remove precursor residue. This produces the positive electrode active material according to this embodiment.

[0062] Here, the precursor is fired at a temperature of 200° C. or higher but lower than 600° C. This prevents potassium from being dispersed in the crystal and makes it easier for potassium to be arranged in a direction intersecting the <001> direction of the layered rock salt structure, thereby improving the diffusion rate of lithium ions and the charge / discharge load characteristics.

[0063] The method for producing the positive electrode active material according to the present embodiment is merely an example and is not limited to the above-described method. For example, the method for producing the lithium composite oxide in the firing step is not limited to the molten salt crystallization method described above, and a crystal synthesis method that can be carried out at a temperature below 600°C, such as a flux synthesis method, a laser thin film deposition method, a sputtering thin film deposition method, a chemical vapor deposition method, an organic metal decomposition method, a solvothermal synthesis method, or a soft chemical synthesis method, may also be used.

[0064] As described above, the method for producing a positive electrode active material according to this embodiment is a method for producing a positive electrode active material containing a lithium composite oxide having a layered rock-salt structure, and includes a step of producing a lithium composite oxide by firing a precursor containing lithium, at least one of cobalt and nickel, and potassium at a temperature of 200° C. or higher and lower than 600° C. This prevents potassium from being dispersed in the crystal and makes it more likely to be arranged in a direction intersecting the <001> direction of the layered rock-salt structure, thereby increasing the diffusion rate of lithium ions and improving charge-discharge load characteristics.

[0065] Examples will be described below. Table 1 shows examples and comparative examples. Note that the present invention is not limited to these examples.

[0066] Example 1 The positive electrode active material according to Example 1 was prepared by the following method. Lithium hydroxide, cobalt oxide, and potassium hydroxide were mixed to prepare a precursor. In the calcination process, the prepared precursor was placed in a platinum crucible and calcined at 350°C for 3 hours in a dry argon stream, and then cooled to obtain a calcined product. The calcined product was pulverized, sieved based on particle size, and washed with a polar solvent to remove precursor residue. This yielded the positive electrode active material according to Example 1. The particle size of the positive electrode active material according to Example 1 was 10 μm.

[0067] In the battery according to Example 1, a positive electrode housed in an exterior cup and a negative electrode housed in an exterior can are stacked with a separator interposed therebetween. In the battery according to Example 1, the exterior can and the exterior cup are crimped together with a gasket interposed therebetween. The separator according to Example 1 is impregnated with an electrolyte solution.

[0068] The positive electrode according to Example 1 was prepared by the following method. First, 98 parts by mass of the positive electrode active material prepared above, 1.2 parts by mass of polyvinylidene fluoride as a positive electrode binder, and 0.8 parts by mass of Ketjen Black as a positive electrode conductive additive were mixed to prepare a positive electrode mixture. The prepared positive electrode mixture was then dispersed in N-methyl-2-pyrrolidone to prepare a paste-like positive electrode mixture slurry. The positive electrode mixture slurry was then applied to both sides of a 15 μm-thick aluminum foil as a positive electrode current collector layer using a coating device, and the positive electrode mixture slurry was then dried with hot air to form a positive electrode active material layer on the positive electrode current collector layer. The positive electrode according to Example 1 was then compression-molded using a hydraulic press to obtain the positive electrode active material layer.

[0069] The negative electrode according to Example 1 was fabricated by the following method. First, 95 parts by mass of graphite as the negative electrode active material and 5 parts by mass of polyvinylidene fluoride as the negative electrode binder were mixed to prepare a negative electrode mixture. The prepared negative electrode mixture was then dispersed in N-methyl-2-pyrrolidone to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a 12 μm-thick copper foil as a negative electrode current collector layer using a coating device, and the negative electrode mixture slurry was then dried with hot air to form a negative electrode active material layer on the negative electrode current collector layer. Thereafter, the negative electrode active material layer was compression-molded using a hydraulic press to obtain the negative electrode according to Example 1.

[0070] The negative electrode according to Example 1 was prepared by the following method: Lithium hexafluorophosphate was dissolved as an electrolyte salt in a solvent prepared by mixing ethylene carbonate and propylene carbonate in a volume ratio of 1:1. The content of the electrolyte salt relative to the solvent was 1 mol / cm. ３ It was prepared so that

[0071] The battery according to Example 1 was assembled in the following manner. First, the fabricated positive electrode was punched into a pellet having a diameter of 15 mm and placed inside an exterior cup. Then, the fabricated negative electrode was punched into a pellet having a diameter of 16 mm and placed inside an exterior can. Then, a porous polyolefin film was impregnated with the prepared electrolyte as a separator and laminated between the positive electrode and the negative electrode housed in the exterior can cup. Then, the exterior can and the exterior cup were crimped together via a gasket. In this manner, the battery according to Example 1 was fabricated.

[0072] <Analysis of Cathode Active Material> The prepared cathode active material was subjected to composition analysis using EDX. In the analysis, a rectangular region of 500 nm length and 500 nm width, which was located 150 nm or more inward from the surface of the particle of the cathode active material in the cathode active material layer, was selected using SEM-EDX (S-4800, Hitachi High-Technologies), and the Co and K contents were measured. In the measurement, the Co and K contents were measured in five non-overlapping regions, and the molar ratio of the K content to the Co content in each region was calculated. The arithmetic mean of the molar ratio of the K content to the Co content in each region was calculated, thereby calculating x as the ratio of the amount of K substance to the amount of Co substance. As a result of the above measurement, x = 0.03 in Example 1. Here, since Example 1 does not contain nickel, y = 1. Also, in Example 1, z = 0.

[0073] The crystal structure of the prepared positive electrode active material was analyzed using a TEM (JEM-F200, JEOL Ltd.). The analysis was performed using a JEM-F200 microscope at an acceleration voltage of 200 kV, at a measurement point 150 nm or more inward from the surface of the positive electrode active material particle in the positive electrode active material layer, to measure the crystal structure of the lithium composite oxide. As a result of the above measurement, a crystal structure belonging to the space group R-3m was confirmed at the measurement point in Example 1. Therefore, it was determined that the positive electrode active material according to Example 1 has a layered rock salt structure.

[0074] The potassium distribution in the layered rock-salt structure of the prepared positive electrode active material was measured by TEM-EDX. The measurement was performed using a TEM (JEM-F200, JEOL) and an attached EDX analyzer (Noran System 7, Thermo Fisher Scientific) at an accelerating voltage of 200 kV. A rectangular region measuring 500 nm in length and 500 nm in width, located at least 150 nm inward from the surface of the lithium composite oxide, was selected, and the <001> direction of the layered rock-salt structure was identified by analyzing the crystal orientation using the TEM. Next, an EDX mapping image of potassium was obtained in this region, and the region in which potassium was distributed was identified. As a result of the above measurement, in Example 1, as shown in FIG. 3B, the potassium distribution was linear, extending in a direction intersecting the <001> direction of the layered rock-salt structure. Therefore, it was determined that in Example 1, potassium was distributed in a direction intersecting the <001> direction of the layered rock-salt structure. In Table 1, when potassium is distributed so as to extend in a direction intersecting the <001> direction of the layered rock salt structure, the K distribution state is defined as "A."

[0075] <Charge Capacity Measurement Test> A charge test was performed on the prepared battery under the following conditions, and the electrical capacity per unit mass of the positive electrode active material was calculated as the charge capacity. Charging method: CC / CV charging, Charging rate: 0.05 C, Upper limit voltage: 4.3 V

[0076] <<Initial Efficiency Measurement Test>> A charge / discharge test was performed on the prepared battery under the following conditions. The electric capacity per unit mass of the positive electrode active material during charging was defined as the charge capacity, and the electric capacity per unit mass of the positive electrode active material during subsequent discharge was defined as the discharge capacity. The ratio of the discharge capacity to the charge capacity was calculated as the initial efficiency. Charging method: CC / CV charging, Charge rate: 0.05 C, Upper limit voltage: 4.3 V, Discharging method: CC discharge, Discharge rate: 0.05 C, Lower limit voltage: 3 V

[0077] <<Discharge Load Measurement Test>> Two discharge tests with different discharge rates were performed on the prepared batteries under the following conditions, and the ratio of the electrical capacity per unit mass of the positive electrode active material when discharged at a discharge rate of 0.2 C to the electrical capacity per unit mass of the positive electrode active material when discharged at a discharge rate of 2 C was calculated as the discharge load. Discharge method: CC discharge Discharge rate: 2 C, 0.2 C Lower limit voltage: 3 V

[0078] <Charge Load Measurement Test> Two charge tests with different charge rates were performed on the prepared batteries under the following conditions, and the ratio of the electric capacity per unit mass of the positive electrode active material during charging at a charge rate of 0.2 C to the electric capacity per unit mass of the positive electrode active material during charging at a charge rate of 2 C was calculated as the charge load. Charging method: CC discharge; Charge rates: 2 C, 0.2 C; Upper limit voltage: 3 V

[0079] In Example 2, a positive electrode active material and a battery were produced, analyzed, and tested in the same manner as in Example 1. Analysis of the positive electrode active material revealed that the composition, crystal structure, K distribution, and particle size of the positive electrode active material according to Example 2 were the same as those of Example 1.

[0080] In Example 3, a positive electrode active material was prepared so that x was 0.005 and the particle size was 20 μm, and other than this, a positive electrode active material and a battery were prepared, analyzed, and tested in the same manner as in Example 1. As a result of analyzing the positive electrode active material, the crystal structure and K distribution state of the positive electrode active material according to Example 3 were similar to those of Example 1.

[0081] In Example 4, a positive electrode active material was prepared so that x was 0.0005 and the particle size was 50 μm, and other than this, a positive electrode active material and a battery were prepared, analyzed, and tested in the same manner as in Example 1. As a result of analyzing the positive electrode active material, the crystal structure and K distribution state of the positive electrode active material according to Example 4 were the same as those of Example 1.

[0082] Comparative Example 1 The positive electrode active material according to Comparative Example 1 was prepared by the following method. Lithium carbonate and cobalt oxide were mixed to prepare a precursor. In the calcination step, the prepared precursor was placed in an alumina crucible and calcined at 850°C for 3 hours in a dry air stream, and then cooled to obtain a calcined product. The calcined product was pulverized, sieved based on particle size, and washed with a polar solvent to remove precursor residue. This resulted in the positive electrode active material according to Comparative Example 1. The particle size of the positive electrode active material according to Comparative Example 1 was 20 μm.

[0083] Thereafter, a battery was fabricated and analyzed and tested in the same manner as in Example 1. Analysis of the positive electrode active material revealed that the crystalline structure of the positive electrode active material according to Comparative Example 1 was the same as that of Example 1. On the other hand, measurement of the potassium distribution in the layered rock salt structure by TEM-EDX revealed that potassium was not distributed.

[0084] Comparative Example 2 In Comparative Example 2, a cathode active material and a battery were prepared in the same manner as in Comparative Example 1, except that potassium hydroxide was further used as a precursor raw material so that x was 0.001. Analysis revealed that the crystalline structure of the cathode active material according to Example 4 was the same as that of Example 1. Meanwhile, as a result of measuring the potassium distribution in the layered rock-salt structure using TEM-EDX, in Comparative Example 2, as shown in FIGS. 4A and 4B, potassium was dispersed in a dotted pattern along the surface of the particles of the cathode active material, and the potassium distribution was not linear. Therefore, in Example 1, it was determined that potassium was not distributed in a direction intersecting the <001> direction of the layered rock-salt structure. In Table 1, when potassium was dispersed in a dotted pattern and the potassium distribution was not linear, the potassium distribution state was determined to be "B."

[0085] As shown in Table 1, in Examples 1 to 3, the K distribution state was "A," and thus the charge load and discharge load were improved without a significant decrease in the charge capacity and initial efficiency, compared to Comparative Example 1, which did not contain K, and Comparative Example 2, which had a K distribution state of "B." Therefore, it can be seen that by setting the K distribution state to "A," it is possible to improve the charge and discharge load characteristics while suppressing a decrease in the charge and discharge characteristics.

[0086] As shown in Table 1, in Example 4, by setting the K distribution state to "A" and the particle size to 50 μm, the charge capacity was improved without significantly deteriorating battery characteristics such as initial capacity, charge load, and charge load, compared to Comparative Example 1, which does not contain K and has a particle size of 20 μm, and Comparative Example 2, which has the K distribution state to "B" and a particle size of 20 μm. Therefore, it can be seen that the charge capacity can be improved by increasing the particle size. It can also be seen that by setting the K distribution state to "A," it is possible to suppress a significant deterioration in charge / discharge load characteristics caused by increasing the particle size.

[0087] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit thereof, and equivalents thereof are also included in the present invention.

[0088] REFERENCE SIGNS LIST 1, 1A Secondary battery 20, 20A Battery element 21, 21A Positive electrode lead 22, 22A Negative electrode lead 23 Protective material 30 Exterior member 30a, 30b Exterior sheet 31 Recess 32 Adhesive 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 comprising lithium, a metal element comprising at least one of cobalt and nickel, and potassium, a ratio of the substance amount of potassium contained in the lithium composite oxide to the substance amount of the metal element contained in the lithium composite oxide being 0.0005 or more and 0.03 or less, and in the lithium composite oxide, potassium is distributed so as to extend in a direction intersecting with the <001> direction of the layered rock-salt structure.

2. The lithium composite oxide has a composition formula of Li １－ｘ K ｘ Ni １－ｙ Co ｙ O ２－ｚ 2. The positive electrode active material according to claim 1, wherein x is an integer of 0 to 1, and y is an integer of 0 to 1.

3. A positive electrode comprising the positive electrode active material according to claim 1 or 2.

4. A secondary battery comprising the positive electrode according to claim 3, a negative electrode, and an electrolyte.

5. A method for producing a positive electrode active material containing a lithium composite oxide having a layered rock salt structure, comprising the step of calcining a precursor containing lithium, at least one of cobalt and nickel, and potassium at a temperature of 200°C or higher and lower than 600°C to produce the lithium composite oxide.

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