Method for manufacturing a positive electrode for a secondary battery and a positive electrode active material for a secondary battery

Abstract

A positive electrode for secondary batteries according to the present invention comprises a positive electrode collector and a positive electrode mixture layer that is arranged on the positive electrode collector, and is characterized in that: the positive electrode mixture layer comprises a positive electrode active material that contains a lithium-containing composite oxide that is composed of secondary particles, each of which is formed of aggregated primary particles; a coating film that contains elemental Nb is formed on the surface of each primary particle of the lithium-containing composite oxide; and the Gini coefficient of elemental Nb as calculated from an element mapping image that is obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of a cross-section of the secondary particles is 0.5 or less.

Description

[Technical Field]

[0001] This invention relates to a positive electrode for secondary batteries and a technology for manufacturing a positive electrode active material for secondary batteries. [Background technology]

[0002] As a positive electrode active material for secondary batteries such as lithium-ion secondary batteries, for example, Patent Document 1 describes a material with the composition formula Li x Ni 1-y Co y-z M z O 2-a X b A positive electrode active material is disclosed, represented as (where M is at least Al), with a lattice constant of 2.81 to 2.91 Å in the a-axis and 13.7 to 14.4 Å in the c-axis, as measured by X-ray diffraction, and a ratio of the diffraction peak intensity of the (104) plane to the peak intensity of the (003) plane of 0.3 to 0.8. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 4197002 [Overview of the project]

[0004] This disclosure aims to provide a method for manufacturing a positive electrode for secondary batteries and a positive electrode active material for secondary batteries, which can increase the capacity of secondary batteries.

[0005] A positive electrode for a secondary battery according to one aspect of the present disclosure comprises a positive electrode current collector and a positive electrode composite material layer disposed on the positive electrode current collector, wherein the positive electrode composite material layer contains a positive electrode active material comprising a lithium-containing composite oxide composed of secondary particles formed by the aggregation of primary particles, a coating containing Nb elements is formed on the surface of the primary particles of the lithium-containing composite oxide, and the Gini coefficient of the Nb element calculated from an elemental mapping image obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of the cross-section of the secondary particles is 0.5 or less.

[0006] A method for producing a positive electrode active material for a secondary battery according to one aspect of the present disclosure is characterized by comprising: a first step of calcining a mixture containing a lithium compound and a metal element-containing compound other than lithium to obtain a lithium-containing composite oxide; and a second step of mixing the lithium-containing composite oxide obtained in the first step with a solution in which an Nb compound is dissolved, and calcining the resulting mixture.

[0007] According to one aspect of this disclosure, it is possible to provide a method for manufacturing a positive electrode for a secondary battery and a positive electrode active material for a secondary battery, which can increase the capacity of the secondary battery. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional view of a secondary battery, which is an example of an embodiment. [Figure 2] This is a schematic cross-sectional view of secondary particles of a lithium-containing composite oxide. [Figure 3] This is a diagram illustrating the method for calculating the Gini coefficient. [Modes for carrying out the invention]

[0009] An example of an embodiment will be described in detail below. The drawings referenced in the description of the embodiment are schematic representations, and the dimensional ratios of the components depicted in the drawings may differ from those of the actual objects.

[0010] Figure 1 is a schematic cross-sectional view of a secondary battery, which is an example of an embodiment. The secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, an electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, respectively, and a battery case 15 that houses the above components. The battery case 15 is composed of a bottomed cylindrical case body 16 and a sealing body 17 that closes the opening of the case body 16. In addition, other forms of electrode bodies may be used instead of the wound electrode body 14, such as a laminated electrode body in which the positive electrode and negative electrode are alternately stacked with a separator. Examples of battery cases 15 include metal cases such as cylindrical, rectangular, coin-shaped, and button-shaped cases, and resin cases formed by laminating resin sheets (so-called laminated type).

[0011] The electrolyte is, for example, a non-aqueous electrolyte comprising a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of lithium salts such as LiPF6 are used as the electrolyte salt. Note that the electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte using a gel-like polymer or the like.

[0012] The case body 16 is, for example, a metal container in the shape of a bottomed cylinder. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and its upper surface supports the sealing body 17.

[0013] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the side of the electrode body 14. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their central portions, and the insulating member 25 is interposed between the peripheral portions of each. When the internal pressure of the secondary battery 10 rises due to heat generation caused by internal short circuit or the like, for example, the lower valve body 24 is deformed and broken so as to push up the upper valve body 26 toward the cap 27 side, and the current path between the lower valve body 24 and the upper valve body 26 is cut off. When the internal pressure further rises, the upper valve body 26 is broken, and gas is discharged from the opening of the cap 27.

[0014] In the secondary battery 10 shown in FIG. 1, the positive electrode lead 20 attached to the positive electrode 11 extends toward the sealing body 17 through the through hole of the insulating plate 18, and the negative electrode lead 21 attached to the negative electrode 12 extends to the bottom side of the case body 16 through the outside of the insulating plate 第十九 19 . The positive electrode lead 20 is connected to the lower surface of the filter 第二十三 23 which is the bottom plate of the sealing body 17 by welding or the like, and the cap 27 which is the top plate of the sealing body 17 electrically connected to the filter 第二十三 23 serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

[0015] The positive electrode 11, negative electrode 12, and separator 13 will be described in detail below.

[0016] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode composite layer provided on the positive electrode current collector. The positive electrode composite layer contains a positive electrode active material. Further, the positive electrode composite layer may contain a conductive material, a binder material, and the like. The positive electrode 11 is obtained, for example, by applying and drying a positive electrode composite slurry containing a positive electrode active material, a conductive material, a binder material, etc. on the positive electrode current collector to form a positive electrode composite layer on the positive electrode current collector and rolling the positive electrode composite layer. The positive electrode composite layer may be provided on one surface of the positive electrode current collector or on both surfaces of the positive electrode current collector.

[0017] For the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode such as aluminum or an aluminum alloy, a film having the metal disposed on the surface layer, etc. can be used. The positive electrode current collector has, for example, a thickness of about 10 μm to 100 μm.

[0018] The positive electrode active material contains a lithium-containing composite oxide composed of secondary particles in which primary particles are aggregated. The lithium-containing composite oxide is, in terms of achieving a high capacity of the battery, a composition formula: Li x Ni y M (1-y) O2 (where x and y satisfy 1 < x < 1.4 and 0.5 < y ≤ 1.0, and M contains at least one of Mn, Co, Al, and Fe) is preferably a lithium-containing composite oxide represented by. As long as M in the composition formula contains at least one of Mn, Co, Al, and Fe, it may contain other elements. Examples of other elements include Zr, B, Mg, Sc, Y, Ti, Cu, Zn, Cr, Pb, Sn, Na, K, Ba, Sr, Ca, Mo, Si, V, Hf, Ta, Nb, etc.

[0019] The composition of the lithium-containing composite oxide can be confirmed by, for example, performing Rietveld analysis on parameters obtained by inductively coupled plasma optical emission spectrometry and powder X-ray diffraction.

[0020] Figure 2 is a schematic cross-sectional view of secondary particles of a lithium-containing composite oxide. As shown in Figure 2, a coating 34 containing Nb is formed on the surface of primary particles 32 that constitute the secondary particles 30 of the lithium-containing composite oxide. The Nb element exists on the surface of the primary particles 32 in the form of compounds such as oxides. The Gini coefficient of the Nb element, calculated from the elemental mapping image obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of the cross-section of the secondary particles of the lithium-containing composite oxide, is 0.5 or less. The Gini coefficient of the Nb element is an indicator of the distribution of Nb elements present on the surface of the primary particles 32. The lower the value of the Gini coefficient of the Nb element, the less bias there is in the distribution of Nb elements on the surface of the primary particles 32, and the more uniformly dispersed they are across the entire surface of the primary particles 32. When the Gini coefficient of the Nb element is 0.5 or less, compared to cases where it exceeds the above range, the Nb element is dispersed across the entire surface of the primary particles 32, thus improving battery capacity due to the Nb addition effect. The lower limit of the Gini coefficient of the Nb element is not particularly limited, but 0.1 is sufficient. If the Gini coefficient for Nb is lower than 0.1, there may be too much Nb present on the surface of the primary particles, which can reduce battery capacity compared to the case where the Gini coefficient is in the range of 0.1 to 0.5.

[0021] The Gini coefficient of the Nb element is determined as follows. First, the positive electrode 11 is embedded in the resin, and a cross-section of the positive electrode composite layer is prepared by cross-section polishing (CP) or the like. Then, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is performed on the cross-section of secondary particles contained in the cross-section of the positive electrode composite layer. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a method in which an ion beam (primary ions) is irradiated onto the surface of a sample, and ions (secondary ions) that are emitted when some of the molecules constituting the surface are ionized are separated by mass using the difference in their time of flight (time of flight is proportional to the square root of the mass). Here, in order to obtain a profile in the thickness direction (depth direction) of the positive electrode composite layer, sputtered ions (O2 + ) and primary ions (Bi3 + The sample surface (secondary particle cross-section) is alternately irradiated with ) and the detected secondary ions (Nb ions: NbO 3- ) will be analyzed.

[0022] The measurement conditions for TOF-SIMS are as follows: Equipment: TOF-SIMS (TOF-SIMS5) (manufactured by ION-TOF) Sputter ion species: 16 O2 + Sputter ion acceleration energy: 1kV Sputtering area: 200 μm × 200 μm Probe ion species: 209 Bi3 + Probe ion acceleration energy: 30kV Measurement area: 50μm x 50μm Detected element: Negative Charge neutralization correction: None Number of scans: 200 Pixel count: 256 x 256 Cycle time (measurement time per pixel / scan): 75 μs Rear acceleration: 9.5kV Measurement vacuum degree: 1×10 -8 mbar The color image obtained by mapping a 50 μm × 50 μm measurement area is adjusted so that areas containing Nb appear brighter, and areas without Nb appear darker.

[0023] Figure 3 is a diagram illustrating the calculation method for the Gini coefficient. The Gini coefficient is an index that indicates the degree of homogeneity of each part of the subject being investigated, specifically how much of a certain quantity it possesses. In Figure 3, the horizontal axis represents the percentage (0-100%) of the total relative to the rank of each pixel, which is ordered by the intensity of the target quantity. The vertical axis represents the cumulative number of pixels up to each rank shown on the horizontal axis as a ratio (0-1) to the total. In Figure 3, the dotted lines represent the uniform distribution wiring, and the solid lines represent the Lorentz curve. The Lorentz curve is a graph obtained by rearranging each pixel in the mapping image obtained above in order of intensity and accumulating the number of pixels up to each rank. The Gini coefficient is then twice the area of ​​the region R closed by the uniform distribution wiring and the Lorentz curve (0-1).

[0024] The average particle size of the primary particles 32 in the lithium-containing composite oxide is preferably 10 μm or less in diameter, and more preferably 0.1 to 5 μm, in that it can further improve the capacity of the secondary battery.

[0025] The average particle diameter of primary particles 32 in a lithium-containing composite oxide is determined as follows. First, the positive electrode 11 is embedded in a resin, and a cross-section of the positive electrode composite layer is prepared by cross-section polishing (CP) or the like, exposing the cross-section of the secondary particles 30. Then, the cross-section of the secondary particles 30 is photographed using a scanning electron microscope (SEM), and 30 primary particles 32 are randomly selected from this cross-sectional SEM image. The grain boundaries of the selected 30 primary particles 32 are observed to determine the external shape of the primary particles 32, and the major axis (longest diameter) of each of the 30 primary particles 32 is measured. This major axis is taken as the particle diameter of the primary particles 32, and the average particle diameter of the primary particles 32 is determined.

[0026] The average particle size of the secondary particles 30 in the lithium-containing composite oxide is preferably 20 μm in diameter, and more preferably 3 to 15 μm, in that it can further improve the capacity of the secondary battery. The average particle size of the secondary particles 30 in the lithium-containing composite oxide is the volume-average particle size measured by laser diffraction, and is the median diameter at which the integrated volume value in the particle size distribution is 50%. The average particle size of the secondary particles 30 in the lithium-containing composite oxide can be measured by laser diffraction using, for example, Microtrac-Bell MT3000II.

[0027] The amount of Nb present in the positive electrode active material is preferably 0.1% or less, and more preferably 0.5% or less, relative to the combined amount of Ni and Mn in the lithium-containing composite oxide, in order to further improve the capacity of the secondary battery. Here, in the process of coating the surface of the primary particles 32 of the lithium-containing composite oxide with a film 34 containing Nb, some of the Nb may become solid-dissolved in the lithium-containing composite oxide. That is, some of the Nb may be incorporated into the crystal structure of the lithium-containing composite oxide and contained inside the particles. In this case, if Nb is solid-dissolved in the primary particles 32 of the lithium-containing composite oxide, the amount of Nb present in the positive electrode composite layer includes not only the amount of Nb present on the surface of the primary particles 32 of the lithium-containing composite oxide, but also the amount of Nb that is solid-dissolved in the primary particles 32 of the lithium-containing composite oxide.

[0028] An example of a method for producing the positive electrode active material of this embodiment is described below.

[0029] The method for producing a positive electrode active material according to this embodiment comprises a first step of calcining a mixture containing a lithium compound and a compound containing a metal element other than lithium to obtain a lithium-containing composite oxide, and a second step of mixing the lithium-containing composite oxide obtained in the first step with a solution in which an Nb compound is dissolved, and calcining the resulting mixture.

[0030] Examples of lithium compounds in the first step include lithium carbonate and lithium hydroxide. Examples of metal element-containing compounds other than lithium include hydroxides and oxides containing transition metals such as Ni and Mn. Preferably, the metal element-containing compounds other than lithium are composite hydroxides and composite oxides containing Ni and Mn. Note that composite hydroxides containing Ni and Mn can be obtained, for example, by known coprecipitation methods.

[0031] The firing conditions for the mixture in the first step are preferably, for example, firing in air at a firing temperature in the range of 600°C to 1100°C for a firing time of 5 hours to 24 hours.

[0032] The solution in which the Nb compound is dissolved in the second step is, for example, an alcohol solution in which niobethoxide is dissolved.

[0033] The firing conditions for the mixture in the second step are preferably, for example, firing in air at a firing temperature in the range of 250°C to 650°C for a firing time of 1 hour to 10 hours.

[0034] As described above, by adding a solution containing dissolved Nb compounds and firing the lithium-containing composite oxide after its preparation, the Nb element can be dispersed across the entire surface of the primary particles of the lithium-containing composite oxide. This makes it easy to control the Gini coefficient of the Nb element to 0.5 or less. The coverage of the Nb element can be controlled, for example, by adjusting the firing temperature and firing time in the second step. Furthermore, the particle size of the lithium-containing composite oxide can be controlled by adjusting the firing temperature and firing time in the first step.

[0035] The conductive materials included in the positive electrode composite layer include, for example, carbon black (CB), acetylene black (AB), Ketjenblack, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0036] Examples of binders included in the positive electrode composite layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), cellulose derivatives such as polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, carboxymethylcellulose (CMC) or its salts, and polyethylene oxide (PEO). These may be used individually or in combination of two or more types.

[0037] [Negative electrode] The negative electrode 12 comprises a negative electrode current collector, such as a metal foil, and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode current collector can be a metal foil that is stable in the negative electrode potential range, such as copper, or a film with the metal arranged on its surface. The negative electrode composite layer includes, for example, a negative electrode active material and a binder.

[0038] The negative electrode 12 is obtained, for example, by applying and drying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., onto the negative electrode current collector to form a negative electrode mixture layer on the negative electrode current collector, and then rolling the negative electrode mixture layer. The negative electrode mixture layer may be provided on one side of the negative electrode current collector or on both sides of the negative electrode current collector.

[0039] The negative electrode active material can be any material capable of intercalating and releasing lithium ions, specifically lithium alloys such as metallic lithium, lithium-aluminum alloy, lithium-lead alloy, lithium-silicon alloy, and lithium-tin alloy; carbon materials such as graphite, coke, and calcined organic materials; and metal oxides such as SnO2, SnO, and TiO2. These can be used individually or in combination of two or more.

[0040] The binder may be, for example, fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 11, or it may be styrene-butadiene rubber (SBR), CMC or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc.

[0041] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulating properties can be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include olefin resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it may be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with a material such as aramid resin or ceramic coated on its surface may be used. [Examples]

[0042] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0043] <Examples> [Fabrication of positive electrode active material] Lithium hydroxide and a Ni and Mn-containing composite hydroxide were mixed so that the molar ratio of Li to the total amount of Ni and Mn was 1.07:1. This mixture was calcined in air at 900°C for 10 hours and then pulverized to obtain a lithium-containing composite oxide. Next, 400 μl of an ethanol solution containing niobethoxide was added to the obtained lithium-containing composite oxide so that it constituted 0.25% of the total amount of Ni and Mn in the oxide, and mixed. This mixture was calcined in air at 300°C for 3 hours and then pulverized to obtain a lithium-containing composite oxide in which a film containing Nb was formed on the surface of the primary particles. This was used as the positive electrode active material. Furthermore, analysis by inductively coupled plasma (ICP) emission spectroscopy revealed that the amount of Nb present in the positive electrode active material was 0.25 mol% relative to the total amount of Ni and Mn in the lithium-containing composite oxide.

[0044] [Fabrication of the positive electrode] A positive electrode slurry was prepared by mixing 95 parts by mass of positive electrode active material, 5 parts by mass of acetylene black (AB) as a conductive material, and 3 parts by mass of polyvinylidene fluoride as a binder, and then adding an appropriate amount of N-methyl-2-pyrrolidone (NMP). Next, this positive electrode slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and after the coating film was dried, it was rolled using a rolling roller. In this way, a positive electrode was produced in which positive electrode slurry layers were formed on both sides of the positive electrode current collector.

[0045] From the obtained cathode, a cross-section of the cathode composite layer was prepared by the cross-section polishing (CP) process described above. Then, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed on the secondary particle cross-section under the conditions described above, and the Gini coefficient of the Nb element was calculated from the elemental mapping image obtained from this analysis to be 0.33.

[0046] Furthermore, the cross-section of the positive electrode composite layer was observed using SEM, and the particle size of the primary particles of the positive electrode active material was measured under the aforementioned conditions. As a result, the average particle size of the primary particles of the positive electrode active material was 0.5 μm.

[0047] [Preparation of non-aqueous electrolytes] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 20:75:5. LiPF6 was dissolved in this mixed solvent to a concentration of 1.3 mol / L to prepare a non-aqueous electrolyte.

[0048] [Manufacturing of secondary batteries] Electrode leads were attached to the metallic lithium used as the positive and negative electrodes. An electrode body was then fabricated with a separator interposed between the positive and negative electrodes. This electrode body was then housed in an aluminum laminate film, the non-aqueous electrolyte was injected, and the film was sealed. This constituted the secondary battery of the example.

[0049] <Comparative Example 1> [Fabrication of positive electrode active material] Lithium hydroxide and a Ni and Mn-containing composite hydroxide were mixed so that the molar ratio of Li to the total amount of Ni and Mn was 1.07:1. This mixture was calcined in air at 900°C for 10 hours and then pulverized to obtain a lithium-containing composite oxide. Next, the obtained lithium-containing composite oxide and niobium oxide (N) were mixed. b A lithium-containing composite oxide was obtained by mixing 2O5 powder with a lithium-containing composite oxide, firing the mixture in air at 600°C for 3 hours, and then grinding it to form a coating containing Nb on the surface of the primary particles. This was used as the positive electrode active material.

[0050] Furthermore, analysis by inductively coupled plasma (ICP) emission spectroscopy revealed that the amount of Nb present in the positive electrode active material was 0.25 mol% relative to the combined amount of Ni and Mn in the lithium-containing composite oxide.

[0051] A positive electrode was fabricated using this positive electrode active material in the same manner as in the example. A cross-section of the positive electrode composite layer was prepared from the obtained positive electrode by the cross-section polishing (CP) process described above. Then, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was performed on the secondary particle cross-section under the conditions described above, and the Gini coefficient of the Nb element was calculated from the elemental mapping image obtained from the analysis, and it was found to be 0.6.

[0052] Furthermore, the cross-section of the positive electrode composite layer was observed using SEM, and the particle size of the primary particles of the positive electrode active material was measured under the aforementioned conditions. As a result, the average particle size of the primary particles of the positive electrode active material was 1.0 μm.

[0053] Then, a secondary battery was fabricated using the positive electrode of Comparative Example 1, in the same manner as in the Example.

[0054] <Comparative Example 2> [Fabrication of positive electrode active material] Lithium hydroxide and a Ni and Mn-containing composite hydroxide were mixed so that the molar ratio of Li to the total amount of Ni and Mn was 1.07:1. This mixture was calcined in air at 900°C for 10 hours and then pulverized to obtain a lithium-containing composite oxide. A secondary battery was prepared in the same manner as in the example, except that this was used as the positive electrode active material in Comparative Example 2.

[0055] [Measurement of initial battery capacity] The secondary batteries of the examples and each comparative example were charged with a constant current of 0.1C at a temperature of 25°C until the cell voltage reached 4.7V, and then charged with a constant voltage of 4.7V until the current value reached 0.05C. Next, they were discharged with a constant current of 0.1C until the cell voltage reached 2.5V. The discharge capacity at this time was defined as the capacity of the secondary battery.

[0056] The capacity of the secondary battery in the example was 226.45 mAh / g, the capacity of the secondary battery in Comparative Example 1 was 222.82 mAh / g, and the capacity of the secondary battery in Comparative Example 2 was 218.1 mAh / g. From these results, it can be said that the capacity of a secondary battery can be improved by using a positive electrode active material that includes a lithium-containing composite oxide with a film containing Nb elements formed on the surface of the primary particles, and by using a positive electrode in which the Gini coefficient of the Nb element calculated from the elemental mapping image obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of the cross-section of the secondary particles is 0.5 or less. [Explanation of Symbols]

[0057] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Filter, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Secondary particles, 32 Primary particles, 34 Coating.

Claims

1. It comprises a positive electrode current collector and a positive electrode composite material layer disposed on the positive electrode current collector. The positive electrode composite layer has a positive electrode active material containing a lithium-containing composite oxide, which is composed of secondary particles formed by the aggregation of primary particles. A coating containing Nb is formed on the surface of the primary particles of the lithium-containing composite oxide. The Gini coefficient of the Nb element, calculated from the elemental mapping image obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of the cross-section of the aforementioned secondary particles, is between 0.1 and 0.

5. The lithium-containing composite oxide is represented by the compositional formula: Li x Ni y M (1-y) O 2 (wherein x and y satisfy 1 < x < 1.4 and 0.5 < y ≤ 1.0, and M contains Mn), The amount of Nb element present in the positive electrode active material is 0.01 to 0.1% of the total amount of Ni and Mn in the lithium-containing composite oxide, in a positive electrode for a secondary battery.

2. The positive electrode for a secondary battery according to claim 1, wherein the average particle diameter of the primary particles is 0.1 to 10 μm.

3. The positive electrode for a secondary battery according to claim 1 or 2, wherein the average particle diameter of the secondary particles is 3 to 20 μm.

4. A first step involves calcining a mixture containing a lithium compound and a compound containing a metal element other than lithium to obtain a lithium-containing composite oxide; and a second step involves mixing the lithium-containing composite oxide obtained in the first step with a solution in which an Nb compound is dissolved, and calcining the resulting mixture to obtain a lithium-containing composite oxide composed of secondary particles in which primary particles have aggregated. (i) A coating containing Nb is formed on the surface of the primary particle, (ii) The Gini coefficient of the Nb element calculated from the elemental mapping image obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) of the cross-section of the secondary particle is 0.1 or more and 0.5 or less. (iii) The empirical formula is represented as: Li x Ni y M (1-y) O 2 (wherein x and y satisfy 1 < x < 1.4 and 0.5 < y ≤ 1.0, and M contains Mn), A method for producing a positive electrode active material for a secondary battery, comprising: (iv) a second step of producing the lithium-containing composite oxide in which the amount of Nb element is 0.01 to 0.1% of the total amount of Ni and Mn.

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