Positive electrode active material, positive electrode, and secondary battery

Abstract

Provided is a positive electrode active material which has low resistance and for which thermal stability is improved. The positive electrode active material includes positive electrode active material particles comprising a core part and a surface part provided on at least a part of the surface of the core part. The core part contains a first lithium nickel oxide represented by the compositional formula LiNi1-α-β-γCoαAlβMγO2, where: M is at least one transition metal element; and 0.85≤1-α-β-γ≤1 is satisfied. The surface part contains a second lithium nickel oxide represented by the compositional formula LiNi1-a-b-cXaYbCocO2, where: X is at least one element selected from Be, Al, and Ge, and Y is at least one element selected from Ti, Cr, V, and W; and 0.04<a<0.34, 0.02<b<0.24, and 1.0<a / b<4.5 are satisfied. The crystal structure of the surface part has no spinel structure.

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 battery that uses, as a positive electrode active material, a lithium nickel oxide that contains a large amount of Al and has a surface portion having a layered rock salt structure.

[0003] JP 2017-107827 A

[0004] However, the lithium nickel oxide described in Patent Document 1 may have a reduced ionic conductivity in the surface region and an increased resistance depending on the crystal structure of the surface region.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to provide a positive electrode active material, a positive electrode, or a secondary battery that has improved thermal stability and low resistance.

[0006] A positive electrode active material according to one aspect of the present invention is a positive electrode active material including a positive electrode active material particle having a core portion and a surface portion provided on at least a part of the surface of the core portion, wherein the core portion has a composition formula of LiNi １－α－β－γ Co α Al β M γ O ２ wherein M is at least one transition metal element, and the surface portion contains a first lithium nickel oxide satisfying the formula: LiNi １－ａ－ｂ－ｃ X ａ Y ｂ Co ｃ O ２ wherein X is at least one element selected from Be, Al, and Ge, and Y is at least one element selected from Ti, Cr, V, and W, and the crystal structure of the surface portion does not have a spinel 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] According to the present invention, it is possible to provide a positive electrode active material, a positive electrode, or a secondary battery that has improved thermal stability and low resistance.

[0010] 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. 3 is a schematic cross-sectional view showing positive electrode active material particles according to the first embodiment. FIG. 4 is a cutaway view showing another example of the secondary battery according to the first embodiment. FIG. 5 is a schematic cross-sectional view taken along line V-V in FIG. 4. FIG. 6 is a diagram showing the results of a thermal stability evaluation test according to Example 1. FIG. 7 is a diagram showing the results of a thermal stability evaluation test according to Comparative Example 1.

[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 1A. 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 1A. 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 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.

[0021] Fig. 3 is a schematic cross-sectional view showing a positive electrode active material particle according to the first embodiment. The positive electrode active material includes a positive electrode active material particle G1 shown in Fig. 3. The positive electrode active material particle G1 includes a core portion G2 and a surface portion G3.

[0022] The core portion G2 includes a first lithium nickel oxide. The first lithium nickel oxide has a composition formula of LiNi １－α－β－γ Co α Al β M γ O ２where M is at least one transition metal element and satisfies 0.85≦1−α−β−γ≦1. Here, M is, for example, at least one element selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Cu, Zn, Y, Zr, Nb, Mo, Hf, Ta, and W. This allows for increased capacity. Note that the values ​​of α, β, and γ are not particularly limited as long as 0.85≦1−α−β−γ≦1 is satisfied. For example, the first lithium nickel oxide may be free of at least one of Co, Al, and M.

[0023] The value of 1-α-β-γ can be identified or measured by EDS (Energy Dispersive X-ray Spectroscopy). More specifically, a line analysis is performed on a sample prepared by cross-sectionally processing the positive electrode active material layer 212, and the amounts of Ni, Co, Al, and M in the region occupied by the core portion G2 are quantified. Then, based on the quantification results, the ratio of the amount of substance of Ni to the total amount of substance of Ni, Co, Al, and M can be calculated as 1-α-β-γ.

[0024] The surface portion G3 is a layer provided on the surface of the core portion G2. Here, the surface portion G3 is not limited to a film-like layer that covers the entire surface of the core portion G2, but may be an island-like layer provided on a part of the surface of the core portion G2.

[0025] The surface portion G3 includes a second lithium nickel oxide. In the first embodiment, the surface portion G3 is made of a second lithium nickel oxide. The second lithium nickel oxide has a composition formula of Li ｄ Ni １－ａ－ｂ－ｃ X ａ Y ｂ Co ｃ O ２ Here, X is at least one element selected from beryllium (Be), aluminum (Al), and germanium (Ge), and Y is at least one element selected from titanium (Ti), chromium (Cr), vanadium (V), and tungsten (W).

[0026] The value of a satisfies the relationship 0.04<a<0.34. By making the value of a larger than 0.04, the thermal stability of the surface portion G3 can be improved. Furthermore, by making the value of a smaller than 0.3, the discharge capacity can be improved.

[0027] b satisfies the relationship 0.02<b<0.24. By making b greater than 0.02, a large amount of element Y, which has a smaller ionic radius than element X, is present, and the crystal structure of the surface portion G3 can be stably made into a layered rock salt structure, thereby reducing reaction resistance. Furthermore, by making b smaller than 0.2, the discharge capacity can be improved.

[0028] The ratio a / b of a to b satisfies 1.0<a / b<4.5. This allows the crystal structure of the surface portion G3 to be a stable layered rock salt structure, thereby reducing reaction resistance.

[0029] The value of c is not particularly limited as long as it satisfies the condition 0≦c<0.98. That is, the second lithium nickel oxide does not need to contain Co. In this case, the capacity can be increased and the manufacturing cost of the positive electrode active material can be reduced.

[0030] The presence or absence of the surface portion G3 can be determined by EDS (Energy Dispersive X-ray Spectroscopy). More specifically, the presence or absence of the surface portion G3 can be measured by observing a sample prepared by cross-sectionally processing the positive electrode active material layer 212 and performing elemental mapping of at least one of Ti, Cr, V, and W. Here, the region to be measured is a region including the surface of the unagglomerated positive electrode active material particles G1. If the mapping image shows a region in which the concentration of at least one of Ti, Cr, V, and W is clearly higher than that of the core portion G2, it can be said that the surface portion G3 is present in that region. In the following description, this region will be described as the region occupied by the surface portion G3.

[0031] Elements X and Y can be identified by EDS. More specifically, they can be measured by observing a sample prepared by cross-sectionally processing the positive electrode active material layer 212 and performing element mapping for Be, Al, Ge, Ti, Cr, V, and W. In this case, the element of Be, Al, or Ge present in the region occupied by surface portion G3 can be said to be element X, and the element of Ti, Cr, V, or W present in the region occupied by surface portion G3 can be said to be element Y.

[0032] The values ​​of a, b, and c can be measured by EDS. More specifically, a line analysis is performed on a sample prepared by cross-sectionally processing the positive electrode active material layer 212, and the amounts of Ni, Co, X, and Y in the region occupied by the surface portion G3 are quantified. Then, based on the quantification results, the proportions of the amounts of X, Y, and Co relative to the total amount of Ni, Co, X, and Y can be calculated as a, b, and c, respectively.

[0033] The surface portion G3 does not have a spinel structure of space group Fd-3m. In the first embodiment, the surface portion G3 has only a layered rock salt structure of space group R-3m. This improves ion conductivity, thereby reducing the interface resistance of the surface portion G3.

[0034] The crystal structure of the surface portion G3 can be measured by the following method. First, a field of view of 100 nm vertically and 100 nm horizontally, including the surface of the unagglomerated positive electrode active material particles G1, is observed using a TEM (Transmission Electron Microscope). Then, an EDS mapping image is obtained in that region, and the region occupied by the surface portion G3 is identified using the method described above. TEM diffraction images are then obtained at multiple measurement points within the region occupied by the surface portion G3. The multiple measurement points are at least two or more locations that are at least 2 nm apart from the core portion G2 in the thickness direction of the region occupied by the surface portion G3 and at least 10 nm apart from each other. If no spots belonging to the space group Fd-3m are observed at all of the multiple measurement points, it can be said that the surface portion G3 does not have a spinel structure.

[0035] The thickness of the surface portion G3 is 40 nm or less. This reduces reaction resistance and improves discharge capacity. The thickness of the surface portion G3 can be measured by EDS. More specifically, it can be measured by performing sample element mapping on a sample prepared by cross-sectionally processing the positive electrode active material layer 212. More specifically, in a mapping image of element Y, the length of the surface portion G3 in the thickness direction of the region occupied by the surface portion G3 is measured at multiple points, and the arithmetic average of these lengths can be measured as the thickness of the surface portion G3.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] As described above, the positive electrode active material according to the first embodiment is a positive electrode active material containing positive electrode active material particles G1 each having a core portion G2 and a surface portion G3 provided on at least a part of the surface of the core portion G2. The core portion G2 has a composition formula of LiNi １－α－β－γ Co α Al β M γ O ２ The surface portion G3 includes a first lithium nickel oxide having a composition formula of LiNi １－ａ－ｂ－ｃ X ａ Y ｂ Co ｃ O ２ wherein X is at least one element selected from Be, Al, and Ge, and Y is at least one element selected from Ti, Cr, V, and W, and the second lithium-nickel oxide satisfies the conditions 0.04<a<0.34, 0.02<b<0.24, and 1.0<a / b<4.5. The crystal structure of the surface portion G3 does not have a spinel structure. This allows for improved thermal stability and a low-resistance positive electrode active material to be provided.

[0057] In a preferred embodiment, the thickness of the surface portion G3 is less than 40 nm, which allows the positive electrode active material to have a lower resistance.

[0058] Furthermore, X is Al and Y is Ti, which makes it possible to provide a positive electrode active material with improved thermal stability and low resistance.

[0059] The positive electrode 210 according to the first embodiment includes the positive electrode active material according to the first embodiment, which makes it possible to provide a positive electrode 210 with improved thermal stability and low resistance.

[0060] 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. This makes it possible to provide a secondary battery 1 with improved thermal stability and low resistance.

[0061] (Method for manufacturing positive electrode mixture) Hereinafter, an example of the method for manufacturing the positive electrode active material according to the first embodiment will be described. The method for manufacturing the positive electrode active material according to the first embodiment includes a core portion forming step and a surface portion forming step.

[0062] In the core formation step, first lithium nickel oxide particles are obtained by the following method. First, an aqueous solution of sulfates of Ni, Al, and Co is prepared, and then poured into water such as deionized water and stirred. After stirring, the water in the solution is evaporated under vacuum and heated to dryness. Lithium hydroxide (LiOH) is then added, and the mixture is thoroughly crushed and mixed, followed by firing in an oxygen atmosphere. This yields first lithium nickel oxide particles.

[0063] In the surface portion forming step, the surface portion is formed on the outside of the first lithium nickel oxide particles (core portion) by the following method. The first lithium nickel oxide particles are placed in a solvent such as N-methyl-2-pyrrolidone (NMP) and stirred to disperse in the solvent. Then, a solution in which oxide fine particles of element X and oxide fine particles of element Y are dispersed is added dropwise and stirred. The solvent is then removed by suction filtration, and the solute is calcined in an oxygen atmosphere. The calcined product is then pulverized and sieved to obtain the positive electrode active material according to this embodiment.

[0064] Here, it is preferable that the median diameter of the oxide particles of element X and the oxide particles of element Y is 100 nm, which allows the thickness of the surface portion to be appropriate.

[0065] The method for manufacturing a positive electrode active material according to the present embodiment is merely an example and is not limited to the above-described method. For example, in the surface portion forming step, metal nanoparticles or alloy nanoparticles containing element X or element Y may be used instead of the oxide fine particles of element X and the oxide fine particles of element Y.

[0066] Examples will be described below. Table 1 shows examples and comparative examples. Note that the present invention is not limited to these examples.

[0067]

[0068] Example 1 A positive electrode according to Example 1 was produced by the following method.

[0069] The positive electrode active material according to Example 1 was prepared by the following method.

[0070] For the core formation process, first lithium nickel oxide particles were prepared by the following method. Nickel (II) nitrate hexahydrate (Sigma-Aldrich), cobalt (II) nitrate (Sigma-Aldrich), and aluminum nitrate nonahydrate (Sigma-Aldrich) were added to deionized water and stirred to obtain an aqueous solution containing Ni ions, Co ions, and Al ions in a molar ratio of 0.85:0.12:0.03. The obtained aqueous solution was heated to 60°C in a vacuum to evaporate the water, and then heated to 110°C and held for 15 hours to dry the solution, obtaining a nitrate mixture. Lithium hydroxide (Kanto Chemical) in an amount equal to the total molar amount of the transition metals in the nitrates was added to the obtained nitrate mixture, and the mixture was thoroughly crushed and mixed in an agate mortar to obtain a core precursor. The obtained core precursor was calcined at 680°C for 12 hours in an oxygen atmosphere to obtain first lithium nickel oxide particles. The composition formula of the first lithium nickel oxide particles was LiNi ０．８５ Co ０．１２ Al ０．０３ O ２ It was.

[0071] In the surface portion forming step, a surface portion was formed on the first lithium nickel oxide particles (core portion) by the following method. The first lithium nickel oxide particles obtained above were dispersed in NMP to prepare a solution of first lithium nickel oxide particles. Furthermore, aluminum oxide microparticles (median diameter 5 nm, US Research Nanomaterials) were dispersed in NMP to a concentration of 5 mass%. Titanium oxide microparticles (median diameter 18 nm, US Research Nanomaterials) were dispersed in NMP to a concentration of 5 mass%. Here, the aluminum oxide microparticles and titanium oxide microparticles were prepared. Then, while stirring the solution of the first lithium nickel oxide particles at 200 rpm, the aluminum oxide microparticle solution and the titanium oxide microparticle solution were each added dropwise so that the aluminum oxide concentration and the titanium oxide concentration were both 0.8 mass% and mixed for 60 minutes. Subsequently, the solvent was separated and removed by suction filtration using a pump to obtain a precursor of a positive electrode active material. The obtained precursor of the positive electrode active material was heated to 760°C at 5°C / min in an oxygen atmosphere, held at 760°C for 2 hours, and cooled to room temperature at 5°C / min to obtain a fired product. The fired product was pulverized in an agate mortar and sieved through a 100 µm mesh to obtain the positive electrode active material according to Example 1.

[0072] Thereafter, 95.5% by mass of the positive electrode active material according to Example 1, 2.5% by mass of carbon black as a positive electrode conductive assistant, and 2.0% by mass of polyvinylidene fluoride as a positive electrode binder were mixed together, NMP was added so that the ratio of the solute to the solution was 70% by mass, and the mixture was kneaded for 3 minutes at 2000 rpm with a stirring defoamer (Thinky) to prepare a slurry positive electrode mixture.

[0073] The resulting slurry of positive electrode mixture was then uniformly applied to a 15 μm thick strip of aluminum foil (positive electrode current collector) to form a coating film, which was then dried to remove the NMP, forming a positive electrode sheet having a positive electrode active material layer. The resulting positive electrode sheet was punched into a diameter of 15 mm using a punch (Nogami Giken Co., Ltd.), thereby obtaining a positive electrode according to Example 1.

[0074] The negative electrode active material layer according to Example 1 was prepared by punching a metallic lithium (Li) foil (Honjo Metals) having a thickness of 1 mm into a disk having a diameter of 15.5 mm.

[0075] The separator according to Example 1 was prepared by punching a 25 μm thick porous polypropylene film into a disk shape with a diameter of 19.4 mm.

[0076] The electrolyte solution according to Example 1 was prepared by mixing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 3:6:1, and adding lithium hexafluorophosphate (LiPF ) as a solute to the solvent. ６ ) was dissolved to a concentration of 1 mol / L.

[0077] The battery according to Example 1 was fabricated by stacking the above-prepared positive electrode, separator, and negative electrode in a coin cell case (diameter 20 mm, thickness 16 mm) and sealing it in. Here, 100 μL of the prepared electrolyte solution was added to the battery according to Example 1, and the battery was allowed to stand at room temperature for 12 hours to allow the electrolyte to be impregnated.

[0078] <Analysis of Cathode Active Material> The composition of the surface layer of the prepared cathode active material was analyzed using EDS. In the analysis, a TEM-EDS (Noran System 7, Hitachi High-Tech) was used to observe a 100 nm x 80 nm field of view in the cross section of the cathode active material layer, including the surface of unagglomerated cathode active material particles. In the analysis, the region in the mapping image of element Y, i.e., Ti, where the Ti concentration was clearly higher than that of the core portion was identified as the region occupied by the surface portion. Line analysis was then performed in the thickness direction of the surface portion in this field of view with a step width of 0.8 nm to quantify Ni, Co, Al, and Ti in the region where the surface portion existed. Based on the quantification results, the ratios of the amounts of Al, Ti, and Co to the total amount of Ni, Co, Al, and Ti were calculated as a, b, and c, respectively. The results of the above measurements were as follows: a = 0.19, b = 0.09, and c = 0 in Example 1. Therefore, in Example 1, a / b=2.1.

[0079] The crystal structure of the surface of the prepared positive electrode active material was analyzed using a TEM. In the analysis, a TEM (JEM-F200, JEOL Ltd.) was used at an acceleration voltage of 200 kV to observe a 100 nm x 100 nm field of view in the cross section of the positive electrode active material layer, including the surface of non-aggregated positive electrode active material particles. An EDS mapping image was then obtained in this region, and the region occupied by the surface was identified using the method described above. TEM diffraction images were then obtained at three measurement points within the region occupied by the surface. The three measurement points were selected to be at least 2 nm apart from the surface of the core portion in the thickness direction of the region occupied by the surface portion, and at least 10 nm apart from each other. If only spots belonging to the space group R-3m were observed at all three measurement points, and no spots belonging to the space group Fd-3m were observed, it was determined that the crystal structure of the surface portion had a layered rock salt structure and not a spinel structure. On the other hand, if a spot belonging to the space group R-3m was observed at at least one of the three measurement points and a spot belonging to the space group Fd-3m was observed at at least one of the three measurement points, it was determined that the crystalline structure of the surface portion had a layered rock-salt structure and a spinel structure. As a result of the above measurements, the crystalline structure of the surface portion of Example 1 had a layered rock-salt structure and did not have a spinel structure.

[0080] The thickness of the surface portion of the prepared positive electrode active material was measured using EDS. In the measurement, a TEM-EDS (Noran system 7, Hitachi High-Tech) was used to observe a 100 nm x 100 nm field of view in the cross section of the positive electrode active material layer, including the surface of non-aggregated positive electrode active material particles. In the analysis, the region in the mapping image of element Y, i.e., Ti, where the Ti concentration was clearly higher than that of the core portion was identified as the region occupied by the surface portion, and the length in the thickness direction of the region occupied by the surface portion was measured at multiple locations, and the arithmetic average of these lengths was measured as the thickness of the surface portion. As a result of the above measurement, the thickness of the surface portion according to Example 1 was 6 nm.

[0081] <<Initial Charge / Discharge Test (Discharge Capacity Measurement Test)>> An initial charge / discharge test was performed on the fabricated battery under the following conditions to confirm that charging and discharging were possible. In the test, the amount of electricity per 1 g of positive electrode active material removed during discharge was measured as the discharge capacity. Charge / discharge test equipment: TOSCAT-3100 (Toyo Systems) Charging method: CC Charging rate: 0.1 C Upper limit voltage: 4.25 V Discharging method: CC Discharge rate: 0.1 C Lower limit voltage: 2.5 V

[0082] <<Thermal Stability Evaluation Test>> The battery after the initial charge / discharge was charged under the following conditions. Charging method: CC, Charge rate: 0.05C, Upper limit voltage: 4.4V. The battery was then disassembled, the positive electrode removed, washed with dimethyl carbonate, and vacuum dried at 100°C for 2 hours, after which the positive electrode active material layer was removed. 1 mg of the removed positive electrode active material layer was placed in a brass container (Rigaku), 1 mg of dimethyl carbonate was added, and the brass lid was crimped and sealed at a pressure of 6 MPa to prepare a measurement sample. The measurement sample was placed in a DSC (Differential Scanning Calorimetry) device (7000X, Hitachi High-Tech Corporation), heated to 350°C at 10°C / min, and the heat flux of the measurement sample during heat generation was measured. In the measurement, the heat flux at 80°C was defined as 0 W / g, and the temperature at which the heat flux first reached 1 W / g was defined as the heat generation initiation temperature.

[0083] <<Reaction Resistance Measurement Test>> After the initial charge / discharge, the battery was charged under the following conditions: Charging method: CCCV, Charge rate: 0.2, Cut current: 0.05 C, Upper limit voltage: 4.25 V. Then, AC impedance Z was measured using an electrochemical measurement device (Solartron, Solartron Analytical) in the frequency range from 1 MHz to 100 MHz with a potential amplitude of 10 mV. In the measurement, the reaction resistance was calculated by subtracting the real part Z' of AC impedance Z at 1000 Hz from the real part Z' of AC impedance Z at 1 Hz.

[0084] (Example 2) In Example 2, a battery was fabricated in the same manner as Example 1, and analyzed and tested, except that in the surface portion formation process, the concentration of the aluminum oxide microparticle solution was 1.6 mass % and the concentration of the titanium oxide microparticles was 1.6 mass %.

[0085] In Example 2, a = 0.26, b = 0.18, c = 0, and a / b = 1.4. The crystal structure of the surface portion in Example 2 had a layered rock salt structure and did not have a spinel structure. The thickness of the surface portion in Example 2 was 6 nm.

[0086] Comparative Example 1 In Comparative Example 1, except that the surface portion forming step was not performed, a battery was fabricated and analyzed and tested in the same manner as in Example 1. That is, in Comparative Example 1, the positive electrode active material particles did not have a surface portion.

[0087] Comparative Example 2 In Comparative Example 2, a battery was fabricated in the same manner as in Example 1, except that only an aluminum oxide microparticle solution was added in the surface portion forming step, and no titanium oxide microparticle solution was added. That is, in Comparative Example 2, the second lithium nickel oxide did not contain Ti, which is element Y. Furthermore, in Comparative Example 2, measurements were performed in the same manner as in Example 1, except that the thickness of the surface portion was measured by defining the region where the concentration of Al was clearly high in the mapping image of element X, i.e., Al, as the region occupied by the surface portion.

[0088] In Comparative Example 2, a = 0.2 and c = 0. The crystal structure of the surface portion in Comparative Example 2 had a layered rock salt structure and a spinel structure. The thickness of the surface portion in Comparative Example 2 was 25 nm.

[0089] (Comparative Example 3) In Comparative Example 3, a battery was fabricated in the same manner as in Example 1, and analyzed and tested, except that in the surface portion formation process, the concentration of the aluminum oxide microparticle solution was set to 0.2 mass % and the concentration of the titanium oxide microparticles was set to 0.2 mass %.

[0090] In Comparative Example 3, a = 0.04, b = 0.02, c = 0, and a / b = 2.0. The crystalline structure of the surface portion in Comparative Example 3 consisted only of a layered rock salt structure and did not contain a spinel structure. The thickness of the surface portion in Comparative Example 3 was 2 nm.

[0091] (Comparative Example 4) In Comparative Example 4, a battery was fabricated in the same manner as in Example 1, and analyzed and tested, except that in the surface portion formation process, the concentration of the aluminum oxide microparticle solution was 2.4 mass % and the concentration of the titanium oxide microparticles was 2.4 mass %.

[0092] In Comparative Example 4, a = 0.34, b = 0.24, c = 0, and a / b = 1.4. The crystalline structure of the surface portion in Comparative Example 4 had a layered rock salt structure and a spinel structure. The thickness of the surface portion in Comparative Example 4 was 40 nm.

[0093] (Comparative Example 5) In Comparative Example 5, a battery was fabricated in the same manner as in Example 1, and analyzed and tested, except that in the surface portion formation process, the concentration of the aluminum oxide microparticle solution was set to 0.8 mass % and the concentration of the titanium oxide microparticles was set to 0.2 mass %.

[0094] In Comparative Example 5, a = 0.18, b = 0.04, c = 0, and a / b = 4.5. The crystalline structure of the surface portion in Comparative Example 5 had a layered rock salt structure and a spinel structure. The thickness of the surface portion in Comparative Example 5 was 5 nm.

[0095] (Comparative Example 6) In Comparative Example 6, a battery was fabricated in the same manner as in Example 1, and analyzed and tested, except that in the surface portion formation process, the concentration of the aluminum oxide microparticle solution was set to 0.8 mass % and the concentration of the titanium oxide microparticles was set to 0.16 mass %.

[0096] In Comparative Example 6, a = 0.11, b = 0.11, c = 0, and a / b = 1.0. The crystal structure of the surface portion in Comparative Example 6 had a layered rock salt structure and a spinel structure. The thickness of the surface portion in Comparative Example 5 was 6 nm.

[0097] FIG. 6 shows the results of a thermal stability evaluation test for Example 1. FIG. 7 shows the results of a thermal stability evaluation test for Comparative Example 1. As shown in Table 1, FIGS. 6 and 7, Examples 1 and 2 had a surface portion containing a second lithium nickel oxide, and thus had a higher heat generation initiation temperature than Comparative Example 1, which did not have a surface portion. Here, heat generation when the measurement sample was heated is thought to be due to alteration, such as a phase change, occurring in the positive electrode active material due to heat or a reaction with the solvent in contact. Therefore, it is thought that providing a surface portion containing a second lithium nickel oxide can improve thermal stability.

[0098] As shown in Table 1, in Examples 1 and 2, the second lithium nickel oxide contained element Y, and thus the crystalline structure of the surface portion had a layered rock salt structure and no spinel structure, while in Comparative Example 2, in which the surface portion did not contain element Y, the crystalline structure of the surface portion had a layered rock salt structure and a spinel structure. Furthermore, in Comparative Example 2, the second lithium nickel oxide contained element X but not element Y, and thus the heat generation initiation temperature was higher than in Comparative Example 1, which did not have the surface portion, but the reaction resistance increased. On the other hand, in Examples 1 and 2, the surface portion further contained element Y, and thus the reaction resistance was reduced without lowering the heat generation initiation temperature compared to Comparative Example 2, in which the second lithium nickel oxide did not contain element Y. It is believed that the further inclusion of element Y in the second lithium nickel oxide stabilizes the layered rock salt structure of the surface portion, thereby improving thermal stability and reducing reaction resistance.

[0099] As shown in Table 1, in Examples 1 and 2, by satisfying a > 0.04 and b > 0.02, the heat generation initiation temperature was higher than in Comparative Example 3, in which a ≦ 0.04 and b ≦ 0.02. This suggests that by satisfying a > 0.04 and b > 0.02, thermal stability can be improved.

[0100] As shown in Table 1, in Examples 1 and 2, by setting a < 0.34 and b < 0.24, the crystalline structure of the surface portion was a layered rock salt structure and not a spinel structure, while in Comparative Example 4, by setting a ≧ 0.3 and b ≧ 0.2, the crystalline structure of the surface portion was a layered rock salt structure and a spinel structure. Furthermore, in Examples 1 and 2, by setting a < 0.34 and b < 0.24, the reaction resistance was reduced and the discharge capacity was increased compared to Comparative Example 4, by setting a ≧ 0.3 and b ≧ 0.2. This suggests that by setting a < 0.34 and b < 0.24, the layered rock salt structure of the surface portion is stabilized, the reaction resistance can be suppressed, and the discharge capacity can be improved.

[0101] As shown in Table 1, in Examples 1 and 2, by setting the thickness of the surface portion to less than 40 nm, the reaction resistance was smaller than in Comparative Example 4, in which the thickness of the surface portion was 40 nm or more. This shows that the reaction resistance can be suppressed by setting the thickness of the surface portion to less than 40 nm.

[0102] As shown in Table 1, in Examples 1 and 2, by setting a / b within the range of 1.0<a / b<4.5, the crystalline structure of the surface portion had a layered rock salt structure and no spinel structure, whereas in Comparative Example 5, where a / b≧4.5, and Comparative Example 6, where a / b≦1, the crystalline structure of the surface portion had a layered rock salt structure and a spinel structure. Furthermore, in Examples 1 and 2, by setting a / b within the range of 1.0<a / b<4.5, the reaction resistance was reduced compared to Comparative Example 5, where a / b≧4.5, and Comparative Example 6, where a / b≦1. It is therefore believed that by setting a / b within the range of 1.0<a / b<4.5, the layered rock salt structure of the surface portion can be stabilized, and reaction resistance can be suppressed.

[0103] 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.

[0104] The present invention may take the following forms: (1) A positive electrode active material including positive electrode active material particles having a core portion and a surface portion provided on at least a part of the surface of the core portion, wherein the core portion has a composition formula of LiNi １－α－β－γ Coα Al β M γ O ２ wherein M is at least one transition metal element, and the surface portion contains a first lithium nickel oxide having a composition formula of LiNi １－ａ－ｂ－ｃ X ａ Y ｂ Co ｃ O ２ (2) A positive electrode active material comprising a second lithium nickel oxide represented by the formula (I) where X is at least one element selected from Be, Al, and Ge, and Y is at least one element selected from Ti, Cr, V, and W, and the crystal structure of the surface portion does not have a spinel structure. (3) The positive electrode active material according to (1), wherein the thickness of the surface portion is less than 40 nm. (4) The positive electrode active material according to (1) or (2), wherein X is Al, and Y is Ti. (5) A positive electrode comprising the positive electrode active material according to any one of (1) to (3). (6) A secondary battery comprising the positive electrode according to (4), a negative electrode, and an electrolyte.

[0105] 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 including a positive electrode active material particle having a core portion and a surface portion provided on at least a part of the surface of the core portion, wherein the core portion has a composition formula of LiNi １－α－β－γ Co α A β M γ O ２ The surface portion includes a first lithium nickel oxide represented by the formula LiNi, M being at least one transition metal element, and satisfying 0.85≦1-α-β-γ≦1. １－ａ－ｂ－ｃ X ａ Y ｂ Co ｃ O ２ wherein X is at least one element selected from Be, Al, and Ge, and Y is at least one element selected from Ti, Cr, V, and W, and wherein 0.04<a<0.34, 0.02<b<0.24, and 1.0<a / b<4.5 are satisfied, and a crystal structure of the surface portion does not have a spinel structure.

2. The positive electrode active material according to claim 1, wherein the thickness of the surface portion is less than 40 nm.

3. The positive electrode active material according to claim 1 or 2, wherein X is Al, and Y is Ti.

4. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 3.

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

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