Positive electrode active material, positive electrode, and secondary battery

Abstract

According to the present invention, cycle characteristics are improved. This positive electrode active material comprises lithium nickel oxide and zirconium oxide. The lithium nickel oxide is represented by the composition formula LidNi1-a-b-cCoaAlbMcO2, where M is at least one element selected from among Na, Cu, W, Fe, and Zn, and 0≤a≤0.2, 0≤b≤0.2, 0≤c≤0.1, and 0.90≤d≤1.1 are satisfied.

Description

Positive electrode active material, positive electrode and secondary battery

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

[0002] Patent Document 1 discloses a positive electrode material containing a mixture of a positive electrode active material and lithium zirconate, which is an oxide containing zirconium.

[0003] Japanese Patent Application Laid-Open No. 2018-73562

[0004] However, the secondary battery disclosed in Patent Document 1 may have a reduced cycle characteristic.

[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, and a secondary battery that have excellent cycle characteristics.

[0006] A positive electrode active material according to one aspect of the present invention includes lithium nickel oxide and zirconium oxide, and the lithium nickel oxide has a composition formula of Li ｄ Ni １－ａ－ｂ－ｃ Co ａ Al ｂ M ｃ O ２ where M is at least one element selected from Na, Cu, W, Fe, and Zn, and satisfies 0≦a≦0.2, 0≦b≦0.2, 0≦c≦0.1, and 0.90≦d≦1.1.

[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 a positive electrode containing the positive electrode active material, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte.

[0009] According to the present invention, it is possible to provide a positive electrode active material, a positive electrode, and a secondary battery having excellent cycle characteristics.

[0010] Fig. 1 is a cross-sectional view showing an example of a secondary battery according to a first embodiment. Fig. 2 is an enlarged cross-sectional view showing a portion of a cross section of the electrode body according to Fig. 1. Fig. 3 is a schematic cross-sectional view showing a portion of a positive electrode active material particle according to the first embodiment. Fig. 4 is a cut-away view showing another example of a secondary battery according to the first embodiment. Fig. 5 is a schematic cross-sectional view taken along line V-V in Fig. 4.

[0011] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited to this embodiment.

[0012] (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.

[0013] 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.

[0014] 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.

[0015] 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 1A 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] The positive electrode active material layer 212 is a layer containing a positive electrode active material. The positive electrode active material contains lithium nickel oxide, zirconium oxide, and carbonate. This can improve the cycle retention rate while suppressing gas generation from the battery.

[0021] The lithium nickel oxide according to the first embodiment has a composition formula of Li ｄ Ni １－ａ－ｂ－ｃ Co ａ Al ｂ M ｃ O ２ Here, M is at least one element selected from Na, Cu, W, Fe, and Zn. The following conditions are satisfied: 0≦a≦0.2, 0≦b≦0.2, 0≦c≦0.1, and 0.90≦d≦1.1. That is, the lithium nickel oxide may not contain Co, Al, or M. The values ​​of M, a, b, and c of the lithium nickel oxide can be measured by ICP (Inductively Coupled Plasma) emission spectroscopy. The lithium nickel oxide preferably contains Co and Al. That is, in the above composition formula, it is preferable that 0<a≦0.2 and 0<b≦0.2 are satisfied, and it is more preferable that 0.01≦a≦0.2 and 0.01≦b≦0.2 are satisfied. In this case, the addition of Co and Al improves the chemical stability of the lithium nickel oxide.

[0022] The crystal system of zirconium oxide is monoclinic, tetragonal, or cubic. The crystal system of zirconium oxide is preferably monoclinic or tetragonal, which can further improve the cycle retention rate.

[0023] The crystal system of zirconium oxide can be identified by electron diffraction using a transmission electron microscope (TEM). More specifically, the crystal system of zirconium oxide can be identified by the following method. First, a sample containing a positive electrode active material is measured using a TEM. Next, a region containing Zr is identified using an energy dispersive X-ray spectroscopy (EDX) attached to the TEM. Then, the electron diffraction pattern of the identified region containing Zr can be obtained, thereby determining the crystal system of zirconium oxide. For example, if the diffraction pattern is determined to be in the space group P2 １ / c, it is said to be a monoclinic system and belongs to the space group P4 ２ When it belongs to / nmc, it can be said to be a tetragonal system, and when it belongs to Fm-3m, it can be said to be a cubic system.

[0024] The content of zirconium oxide is preferably 0.0001 or more and 0.01 or less, and more preferably 0.0002 or more and 0.005 or less, in terms of molar ratio relative to the content of nickel in the lithium nickel oxide in the positive electrode active material. The content of zirconium oxide is obtained by subjecting the positive electrode active material layer to X-ray Photoelectron Spectroscopy (XPS) analysis to quantitatively analyze the amount of zirconium atoms.

[0025] Carbonates consist of a cation and a carbonate ion (CO ３ ２－ ) The cation contains at least one element selected from K, Ca, Ba, Mg, Fe, Co, Na, Mn, Ni, Zn, Rb, Sr, Cs, Cd, and Ag. This causes the lithium carbonate to form a solid solution by heating during the production of the positive electrode active material layer 212, lowering the melting point of the lithium carbonate, thereby improving the bonding between the lithium nickel oxide particles, and further suppressing gas generation from the battery.

[0026] The content of carbonate, in terms of molar ratio relative to the content of nickel in the lithium nickel oxide in the positive electrode active material, is preferably 0.0002 or more and 0.02 or less, and more preferably 0.0004 or more and 0.01 or less. The content of carbonate can be obtained by quantitatively analyzing the amount of carbon atoms in carbonate ions by XPS analysis of the positive electrode active material layer.

[0027] The positive electrode active material layer 212 is not limited to the above-mentioned materials, and may further contain, for example, a positive electrode binder, a positive electrode conductive additive, and a dispersant.

[0028] The positive electrode binder may be any material, including at least one of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene-butadiene rubber, fluorine-containing rubber, and ethylene propylene diene. Examples of polymer compounds include polyvinylidene fluoride (PVdF) and polyimide.

[0029] The positive electrode conductive additive may be any material, including, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, and ketjen black. However, the positive electrode conductive additive is not limited to these materials, and may be a metal material, a conductive polymer, or the like, as long as it is a conductive material.

[0030] FIG. 3 is a schematic cross-sectional view showing a part of the positive electrode active material particle according to the first embodiment.

[0031] In the first embodiment, the positive electrode active material layer 212 is a layer of secondary particles formed by agglomeration of a plurality of lithium nickel oxide particles 212a (primary particles) in contact with one another, as shown in Fig. 3. The state of the lithium nickel oxide particles 212a in the positive electrode active material layer 212 can be evaluated by measuring a cross section of the positive electrode active material layer 212 with a scanning electron microscope (SEM) or by EDX (energy dispersive X-ray spectroscopy) mapping of Ni.

[0032] The average diameter of the lithium nickel oxide particles 212a is 0.1 μm or more and 3 μm or less. The average diameter of the lithium nickel oxide particles 212a in the positive electrode active material layer 212 can be evaluated by measuring a cross section of the positive electrode active material layer 212 using an SEM or EDX mapping for Ni. More specifically, the cross section of the positive electrode active material layer 212 is observed using an SEM, and the longest distance of the region surrounded by the surface or interface of 20 lithium nickel oxide particles is calculated for each particle in the obtained SEM observation image, and the arithmetic mean value of the longest distances is calculated, thereby calculating the average diameter of the lithium nickel oxide particles 212a.

[0033] The median diameter of secondary particles (D ５０ The median diameter of the secondary particles (D ５０ The median diameter (D) of the secondary particles can be calculated from a frequency distribution curve measured using a laser scattering particle size distribution analyzer (for example, a laser diffraction / scattering particle size distribution analyzer LA-960 manufactured by Horiba, Ltd.). ５０ A measurement sample for measuring the .sup.-electrode current collector 211 and the positive electrode active material layer 212 is prepared as follows. First, the positive electrode 210 including the positive electrode current collector 211 and the positive electrode active material layer 212 is immersed in water or N-methylpyrrolidone, and the positive electrode active material layer 212 is separated. Next, the separated positive electrode active material layer 212 is washed with water or N-methylpyrrolidone and dispersed in water or N-methylpyrrolidone to obtain a dispersion. Then, the dispersion is centrifuged to separate the secondary particles. Finally, the separated secondary particles are dispersed in a medium consisting of a mixture of water and N-methylpyrrolidone to obtain a measurement sample.

[0034] In the first embodiment, as shown in FIG. 3 , the zirconium oxide 212b covers at least a portion of the surface of the lithium nickel oxide particles 212a. In other words, the zirconium oxide 212b is present on the surface or interface of the lithium nickel oxide particles 212a. This improves the conductivity between the lithium nickel oxide particles 212a. The distribution of the zirconium oxide 212b can be observed by EDX mapping of Zr in the cross section of the positive electrode active material layer 212. Here, if the region occupied by Zr, i.e., the region where the zirconium oxide 212b is distributed, is in contact with the surface or interface of the lithium nickel oxide particles 212a, it can be said that the zirconium oxide 212b covers at least a portion of the surface of the lithium nickel oxide particles 212a.

[0035] In the first embodiment, as shown in FIG. 3 , the carbonate 212c covers at least a portion of the surface of the lithium nickel oxide particles 212a. In other words, the carbonate 212c is present on the surface or interface of the lithium nickel oxide particles 212a. This improves the bonding between the lithium nickel oxide particles 212a. Here, the carbonate 212c is a mixture with lithium carbonate due to the substitution of some of the cations with lithium. The distribution of the carbonate 212c can be observed, for example, by comparing EDX mapping of O, Ni, and Zr on the cross section of the positive electrode active material layer 212. Specifically, the region where the carbonate 212c is distributed can be defined as the region where O is present, excluding the region where Ni contained in the positive electrode active material is present and the region where Zr contained in zirconium oxide is present. Here, if the region where the carbonate 212c is distributed is in contact with the surface or interface of the lithium nickel oxide particles 212a, it can be said that the carbonate 212c covers at least a portion of the surface of the lithium nickel oxide particles 212a.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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 negative electrode production process. ｘThe 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] The separator 230 is a film that insulates the positive electrode 210 from the negative electrode 220. The separator 230 is provided between the positive electrode 210 and 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 a plan view in the thickness direction.

[0045] 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.

[0046] 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.

[0047] 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 F５ ) ２ ), lithium hexafluoroarsenate (LiAsF ６ ) and other lithium salts.

[0048] 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.

[0049] 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.

[0050] 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 using the drawings, but the same components as those in Figs. 1 and 2 will be designated by reference numerals and will not be described again.

[0051] Fig. 4 is a cutaway view showing a different example of the secondary battery according to the first embodiment. Fig. 5 is a schematic cross-sectional view taken along line VV in Fig. 4. The secondary battery 1A shown in Figs. 4 and 5 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.

[0052] The battery element 20A is provided inside the exterior member 30. As shown in FIG. 5 , 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.

[0053] 5 , 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.

[0054] As described above, the positive electrode active material according to the first embodiment includes lithium nickel oxide and zirconium oxide 212b. The lithium nickel oxide has a composition formula of Li ｄ Ni １－ａ－ｂ－ｃ Co ａ Al ｂ M ｃ O ２where M is at least one element selected from Na, Cu, W, Fe, and Zn, and satisfies 0≦a≦0.2, 0≦b≦0.2, 0≦c≦0.1, and 0.90≦d≦1.1. This makes it possible to improve the cycle retention rate.

[0055] In a desirable embodiment, the zirconium oxide 212b covers at least a part of the surface of the lithium nickel oxide, thereby improving the conductivity between the lithium nickel oxide particles 212a.

[0056] In a preferred embodiment, the lithium nickel oxide is in the form of particles. The average diameter of the lithium nickel oxide is 0.1 μm or more and 3 μm or less. This reduces the electrical resistance of the positive electrode active material layer 212A and improves the cycle characteristics.

[0057] In a preferred embodiment, the crystal system of zirconium oxide is a monoclinic system or a tetragonal system, which can further improve cycle characteristics.

[0058] In a preferred embodiment, the molar ratio of the zirconium oxide content to the nickel content in the lithium nickel oxide is 0.0001 or more and 0.01 or less, which can reduce the electrical resistance of the positive electrode active material layer 212A and improve the cycle characteristics.

[0059] In a preferred embodiment, the coating material further contains carbonate 212c, which can suppress gas generation.

[0060] In a preferred embodiment, the carbonate 212c covers at least a portion of the surface of the lithium nickel oxide, thereby improving the binding between the lithium nickel oxide particles 212a.

[0061] In a preferred embodiment, the molar ratio of the content of carbonate 212c to the content of nickel in the lithium nickel oxide is 0.0002 or more and 0.02 or less, which can reduce the electrical resistance of the positive electrode active material layer 212A and improve the cycle characteristics.

[0062] In a more preferred embodiment, the cation of the carbonate 212c contains at least one element selected from K, Ca, Ba, Mg, Fe, Co, Na, Mn, Ni, Zn, Rb, Sr, Cs, Cd, and Ag, which can further suppress gas generation.

[0063] The positive electrode 210 according to the first embodiment includes the positive electrode active material according to the first embodiment, which can improve the cycle retention rate.

[0064] The secondary battery 1 according to the first embodiment includes a positive electrode 210 including the positive electrode active material according to the first embodiment, a negative electrode 220, a separator 230 between the positive electrode 210 and the negative electrode 220, and an electrolyte solution, thereby improving the cycle retention rate.

[0065] (Method for manufacturing a cathode active material) An example of a method for manufacturing a cathode active material according to the first embodiment will be described below. Note that the method for manufacturing a cathode active material according to this embodiment is merely an example and is not limited to the following. The method for manufacturing a cathode active material according to the first embodiment includes a lithium nickel oxide preparation step and a coating step.

[0066] In the lithium nickel oxide production process, lithium nickel oxide Li is produced by the following method. ｄ Ni １－ａ－ｂ－ｃ Co ａ Al ｂ M ｃ O ２ (M is at least one element selected from Na, Cu, W, Fe, and Zn, and 0≦a≦0.2, 0≦b≦0.2, 0≦c≦0.1, 0.90≦d≦1.1) is prepared. First, an aqueous solution of sulfates of Ni, Al, and Co is prepared. Then, the sulfate solution is added to an alkaline aqueous solution and stirred to precipitate a coprecipitate. Here, the alkaline aqueous solution can be an aqueous solution of an ammonia solution, a hydroxide of metal M, or the like. Then, the coprecipitate is washed and dried, and mixed with lithium hydroxide to prepare a precursor. Then, the precursor is fired to obtain lithium nickel oxide.

[0067] In the coating process, the zirconium oxide and carbonate are coated on the lithium nickel oxide by the following method. First, the zirconium alkoxide and carbonate are added to a solvent and stirred to prepare a solution. The solvent into which the zirconium alkoxide and carbonate are added is, for example, isopropyl alcohol (IPA), but any liquid capable of dissolving the zirconium alkoxide and carbonate is acceptable. The prepared lithium nickel oxide is then added to the solution containing the zirconium alkoxide and carbonate, followed by further stirring. The precursor is then heated and dried to remove the solvent, and the resulting precursor is fired in a high-oxygen atmosphere to obtain a sintered body. The sintered body is then pulverized to obtain a positive electrode active material coated with the zirconium oxide and carbonate.

[0068] Zirconium alkoxides have the general formula Zr(OR) ４ where R is an alkyl group. Specific examples include zirconium n-propoxide and zirconium n-butoxide.

[0069] In the coating step, the amount of zirconium alkoxide added to the solution is preferably 0.0001 or more and 0.01 or less in molar ratio to the nickel content in the lithium nickel oxide.

[0070] In the coating step, the amount of carbonate added to the solution is preferably 0.0002 or more and 0.02 or less in molar ratio with respect to the nickel content in the lithium nickel oxide.

[0071] In the coating step, the firing temperature is preferably 600° C. or higher and 800° C. or lower, which allows the crystal system of zirconium oxide to be monoclinic or tetragonal, thereby improving the cycle retention rate.

[0072] Through the above steps, the positive electrode active material according to the first embodiment can be produced.

[0073] Examples will be described below. Table 1 shows examples and comparative examples. Note that the present invention is not limited to these examples. In Table 1, the amount of gas generated is a relative value to Example 1, and represents the volume ratio of the gas generated from the battery to Example 1.

[0074]

[0075] Example 1 A positive electrode active material according to Example 1 was prepared by the following method.

[0076] As a lithium nickel oxide production process, the lithium nickel oxide according to Example 1 was produced by the following method.

[0077] First, nickel (II) sulfate heptahydrate, aluminum sulfate, and cobalt sulfate were dissolved in distilled water to obtain an aqueous solution of sulfates of Ni, Al, and Co. The aqueous solution of sulfates of Ni, Al, and Co was prepared so that the molar ratio of Ni, Al, and Co was 10:1:1 and the sum of the molar concentrations of Ni, Al, and Co ions was 1.5 mol / L.

[0078] Then, an aqueous ammonium solution with a concentration of 1.5 mol / L was charged into a 2-L reaction vessel, the reaction vessel was heated to 50°C with a heater, and the aqueous sulfate solution was added while stirring with a paddle-type stirring blade. The aqueous sulfate solution was added at a rate of 5.0 g / min over 24 hours. The aqueous sulfate solution was added so as to maintain the pH at 11.0. This resulted in the precipitation of a coprecipitate.

[0079] The resulting coprecipitate was washed by repeated pressure filtration and dispersion in distilled water to remove impurities. The washing was completed when the electrical conductivity of the filtrate became less than 20 mS / m. The washed coprecipitate was dried by heating at 100°C for 10 hours.

[0080] The resulting coprecipitate and lithium hydroxide were mixed so that the molar ratio of Li to the total amount of Ni, Al, and Co was 1.04 to obtain a precursor, which was then pre-baked at 500°C for 5 hours in an oxygen atmosphere and then post-baked at 730°C for 16 hours to obtain lithium nickel oxide particles.

[0081] Thereafter, in the coating step, the lithium nickel oxide particles were coated with zirconium oxide and carbonate by the following method.

[0082] 0.250 g (0.76 mmol) of zirconium (IV) n-propoxide and potassium carbonate (K ２ CO ３ 0.250 g (1.81 mmol) of the lithium nickel oxide particles was added to 62.50 g of isopropyl alcohol (IPA) as a solvent and stirred for 10 minutes to prepare a solution. After stirring, 60 g of the lithium nickel oxide particles (nickel content: 0.79 mol) prepared above was added to the solution and stirred at room temperature for 30 minutes.

[0083] After stirring, the mixture was heated and dried at 100°C for 3 hours to remove the solvent, yielding a powder of a precursor of the positive electrode active material. The powder of the precursor of the positive electrode active material was compressed into pellets. The precursor of the positive electrode active material was then heated at a temperature increase rate of 10°C / min in an atmosphere with an oxygen concentration of 97% by volume or more, fired at 680°C for 20 hours, and cooled to 30°C at a temperature decrease rate of 5°C / min to obtain a sintered body. The sintered body was crushed in a mortar and then sieved with a 90 μm mesh to remove coarse particles, yielding a powder of the positive electrode active material according to Example 1.

[0084] Thereafter, 95% by mass of the prepared powder of the positive electrode active material, 3% by mass of carbon black as a positive electrode conductive assistant, and 2% by mass of polyvinylidene fluoride as a positive electrode binder were mixed and dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry positive electrode mixture.

[0085] Then, this slurry-like positive electrode mixture was uniformly applied to a strip-shaped aluminum foil (positive electrode current collector) having a thickness of 15 μm to form a coating film. Next, this coating film was dried with hot air and then compression-molded using a roll press to form a positive electrode sheet having a positive electrode active material layer. The prepared positive electrode sheet was punched to a diameter of 16.5 mm and vacuum-dried at 120 ° C. for 10 hours using a vacuum dryer to obtain a positive electrode according to Example 1.

[0086] The negative electrode according to Example 1 was fabricated by laminating a negative electrode active material layer on a negative electrode current collector layer. The negative electrode active material layer according to Example 1 was fabricated by punching a 0.24 mm thick lithium (Li) foil into a disk with a diameter of 17 mm. The negative electrode current collector layer according to Example 1 was a 200 μm thick stainless steel (SUS) plate.

[0087] The separator according to Example 1 was prepared by punching a polyolefin sheet having a thickness of 15 μm into a disk having a diameter of 17.5 mm.

[0088] The electrolyte solution according to Example 1 was prepared by dissolving lithium hexafluorophosphate (LiPF ) as a solute in a solvent containing 30% by mass of ethylene carbonate (EC) and 70% by mass of ethyl methyl carbonate (EMC). ６ ) was dissolved to a concentration of 1 mol / L.

[0089] The battery according to Example 1 was fabricated by the following method. First, the fabricated negative electrode was placed in a stainless steel cup-shaped anode cup. The negative electrode current collector was placed so that it was in contact with the anode cup. The fabricated separator was then laminated on the negative electrode active material layer. Then, 150 μL of the fabricated electrolyte was impregnated into the separator, so that it also penetrated into the gap between the negative electrode and the anode cup. The fabricated positive electrode was then laminated on the separator so that the positive electrode active material layer was in contact with the separator. An aluminum plate was then laminated on the positive electrode current collector layer of the positive electrode, and a stainless steel cup-shaped cathode cup was laminated on the aluminum plate. A gasket made of an insulating material was then provided between the periphery of the anode cup and the periphery of the cathode cup. The anode cup and the cathode cup were then sealed using a crimping machine to form an exterior housing, thereby fabricating the battery according to Example 1.

[0090] <Analysis of the positive electrode active material> The lithium nickel oxide particles prepared were subjected to ICP emission spectroscopy, and the composition formula was determined to be Li １．０４ Ni ０．８４ Co ０．０８ Al ０．０８ O ２In addition, a cross section of the produced positive electrode active material layer was observed by SEM and EDX, and it was confirmed that the positive electrode active material layer contained a plurality of lithium nickel oxide particles, that zirconium oxide and carbonate were distributed at the interface or part of the surface of the lithium nickel oxide particles, and that the zirconium oxide and carbonate coated at least a part of the surface of the lithium nickel oxide.

[0091] The cross section of the prepared positive electrode was measured with an SEM to measure the average diameter of the lithium nickel oxide particles. In measuring the average diameter, the longest distance of the region surrounded by the surface or interface of 20 lithium nickel oxide particles in the SEM observation image was calculated for each particle, and the arithmetic mean of the longest distances was calculated to calculate the average diameter of the lithium nickel oxide particles. The average diameter of the lithium nickel oxide particles in Example 1 was 0.5 μm.

[0092] <<Initial Charge / Discharge Test>> An initial charge / discharge test was performed on the prepared battery. The initial charge / discharge test was performed in a thermostatic chamber at 25°C. In the initial charge / discharge test, the prepared battery was charged at a constant current and constant voltage at a current of 0.1 C up to an upper limit voltage of 4.25 V and a lower limit current of 0.005 C, then rested for 10 minutes, and discharged at a current of 0.1 C down to a lower limit voltage of 2.0 V to confirm that charging and discharging were possible.

[0093] <Charge-Discharge Cycle Test> A charge-discharge cycle test was conducted on the prepared battery. In the charge-discharge cycle test, charge-discharge cycles were performed in a thermostatic chamber at 60°C. In the charge-discharge cycle test, the prepared battery was rested for 3 hours in advance, then subjected to constant current / constant voltage charging at a current of 1.0 C to an upper limit voltage of 4.25 V and a lower limit current of 0.01 C, rested for 1 minute, and then discharged at a current of 5.0 C to a lower limit voltage of 2.5 V, and rested for 5 minutes. In the charge-discharge cycle test, the discharge capacity of the battery after 100 cycles of the above charge-discharge cycle was measured, and the 100-cycle retention rate was calculated as a ratio of the discharge capacity according to the following formula (1): Cycle retention rate (%) = (discharge capacity after 100 cycles) / (discharge capacity after 1 cycle) × 100 (1)

[0094] <Gas Generation Amount Evaluation Test> A gas generation amount evaluation test was conducted on the prepared batteries. In the gas generation amount evaluation test, float charging was performed in a thermostatic chamber at 60°C. In the gas generation amount evaluation test, the prepared batteries were rested for 3 hours in advance, and then float charged at a current of 1.0 C with an upper limit voltage of 4.25 V. A current was continuously applied for 100 hours to maintain 4.25 V, and then the batteries were discharged at 1.0 C at 25°C to a lower limit voltage of 2.0 V. After discharge, the volume of gas generated from the batteries was measured by the Archimedes method, and the amount of gas generated was calculated.

[0095] (Example 2) In Example 2, as shown in Table 1, calcium carbonate (CaCO ３ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 39, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68,

[0096] (Example 3) In Example 3, as shown in Table 1, barium carbonate (BaCO ３ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 39, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68,

[0097] (Example 4) In Example 4, as shown in Table 1, sodium carbonate (Na ２ CO ３ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 39, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68,

[0098] (Example 5) In Example 5, as shown in Table 1, magnesium carbonate (MgCO ３ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 39, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68,

[0099] (Example 6) In Example 6, as shown in Table 1, iron carbonate (FeCO ３ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 39, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68,

[0100] (Example 7) In Example 7, as shown in Table 1, silver carbonate (Ag ２ CO ３ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 39, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68,

[0101] (Example 8) In Example 8, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the firing rate in the coating process was set to 100°C / min and the firing temperature was set to 750°C.

[0102] (Example 9) In Example 9, as shown in Table 1, calcium carbonate (CaCO ３ A battery was fabricated and measured in the same manner as in Example 1, except that the firing rate in the coating step was 100°C / min and the firing temperature was 750°C.

[0103] (Example 10) In Example 10, as shown in Table 1, barium carbonate (BaCO ３ A battery was fabricated and measured in the same manner as in Example 1, except that the firing rate in the coating step was 100°C / min and the firing temperature was 750°C.

[0104] (Example 11) In Example 11, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the firing rate in the coating process was set to 50°C / min and the firing temperature was set to 900°C.

[0105] Comparative Example 1 In Comparative Example 1, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that in the coating step, aluminum alkoxide was used instead of zirconium alkoxide, so that aluminum oxide was coated on lithium nickel oxide instead of zirconium oxide, and the firing temperature was set to 700°C.

[0106] Comparative Example 2 In Comparative Example 2, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the step of coating with lithium nickel oxide was not carried out.

[0107] (Example 12) In Example 12, as shown in Table 1, lithium carbonate (Li ２ CO ３ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,

[0108] Example 13 In Example 13, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that no carbonate was used in the coating step.

[0109] As shown in Table 1, in Examples 1 to 13, by using a positive electrode active material containing zirconium oxide, it was possible to improve the cycle retention rate compared to Comparative Examples 1 and 2, in which the positive electrode active material did not contain zirconium oxide.

[0110] As shown in Table 1, in Examples 1 to 10, 12, and 13, the zirconium oxide had a monoclinic or tetragonal crystal system, which enabled the cycle characteristics to be improved more than in Example 11, in which the zirconium oxide had a cubic crystal system.

[0111] As shown in Table 1, in Examples 1 to 12, by using a positive electrode active material containing a carbonate, the amount of gas generated could be further suppressed compared to Example 13 and Comparative Example 2, in which the positive electrode active material did not contain a carbonate.

[0112] As shown in Table 1, in Examples 1 to 11, by using a carbonate other than lithium carbonate, the amount of gas generated could be further reduced compared to Example 12, in which the carbonate was lithium carbonate.

[0113] 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.

[0114] The present invention can also take the following forms: (1) A lithium nickel oxide and a zirconium oxide, wherein the lithium nickel oxide has a composition formula of Li ｄ Ni １－ａ－ｂ－ｃ Co ａ Al ｂ M ｃ O ２(2) The positive electrode active material according to (1), wherein M is at least one element selected from Na, Cu, W, Fe, and Zn, and the following relationships are satisfied: 0≦a≦0.2, 0≦b≦0.2, 0≦c≦0.1, and 0.90≦d≦1.1. (2) The positive electrode active material according to (1), wherein the zirconium oxide covers at least a portion of the surface of the lithium nickel oxide. (3) The positive electrode active material according to (1) or (2), wherein the lithium nickel oxide is in the form of particles, and the lithium nickel oxide has an average diameter of 0.1 μm or more and 3 μm or less. (4) The positive electrode active material according to any one of (1) to (3), wherein the crystal system of the zirconium oxide is a monoclinic system or a tetragonal system. (5) The positive electrode active material according to any one of (1) to (4), wherein the molar ratio of the zirconium oxide content to the nickel content in the lithium nickel oxide is 0.0001 or more and 0.01 or less. (6) The cathode active material according to any one of (1) to (5), further comprising a carbonate. (7) The cathode active material according to (6), wherein the carbonate coats at least a portion of the surface of the lithium nickel oxide. (8) The cathode active material according to (6) or (7), wherein the molar ratio of the content of the carbonate to the content of nickel in the lithium nickel oxide is 0.0002 or more and 0.02 or less. (9) The cathode active material according to any one of (6) to (8), wherein the cation of the carbonate contains at least one element selected from K, Ca, Ba, Mg, Fe, Co, Na, Mn, Ni, Zn, Rb, Sr, Cs, Cd, and Ag. (10) A cathode comprising the cathode active material according to any one of (1) to (9). (11) A secondary battery comprising: a positive electrode containing the positive electrode active material according to any one of (1) to (9); a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte solution.

[0115] 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