Positive electrode active material and method for producing same

Abstract

To provide a method for producing a positive electrode active material excellent in cycle characteristics.SOLUTION: A method for producing a positive electrode active material includes obtaining second particles containing a lithium transition metal composite oxide having a layered structure and a composition in which a ratio of a number of moles of nickel to a total number of moles of metals other than lithium is 0.7 or more and less than 1 by bringing the first particles containing the lithium transition metal composite oxide into contact with a solution containing sodium ions, obtaining a mixture by mixing the second particles with a boron compound, and heat-treating the mixture at a temperature of 100 °C or higher and 450 °C or lower.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material and a method for producing the same. [Background technology]

[0002] Lithium transition metal composite oxides, such as lithium cobalt oxide, lithium nickel oxide, and lithium nickel-cobalt manganese oxide, are used as positive electrode active materials in nonaqueous electrolyte secondary batteries. Lithium-nickel composite oxides, which use a higher proportion of nickel instead of cobalt, a scarce resource, have the advantage of high charge / discharge capacity per unit weight. However, lithium-nickel composite oxides are difficult to synthesize, and unreacted raw materials may remain as alkaline components. Residual alkaline components can cause thickening of the slurry during electrode fabrication and gas generation during charging. However, reducing the alkaline components by water washing or other methods may result in poor cycle performance.

[0003] In relation to the above, a technique has been proposed in which after washing with water, the material is brought into contact with an aqueous sulfate solution to cause sulfate to be present on the surface of the primary particles, thereby improving cycle characteristics (see, for example, Patent Document 1). Also, a technique has been proposed in which a lithium nickel oxide-based lithium transition metal composite oxide is washed with water, mixed with a boron compound, and heat-treated, thereby improving cycle characteristics (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-124086 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-088343 Summary of the Invention [Problem to be solved by the invention]

[0005] Positive electrode active materials for non-aqueous electrolyte secondary batteries are required to have further improved cycle characteristics. Therefore, an object of one aspect of the present disclosure is to provide a positive electrode active material with excellent cycle characteristics and a method for producing the same. [Means for solving the problem]

[0006] The first aspect is a method for producing a positive electrode active material, comprising: bringing first particles containing a lithium transition metal composite oxide having a layered structure, the lithium transition metal composite oxide having a composition in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.7 or more and less than 1, into contact with a solution containing sodium ions to obtain second particles containing the lithium transition metal composite oxide and elemental sodium; mixing the second particles with a boron compound to obtain a mixture; and heat-treating the mixture at a temperature of 100°C or more and 450°C or less.

[0007] The second aspect is a positive electrode active material that includes secondary particles formed by an aggregation of a plurality of primary particles that have a layered structure and include a lithium transition metal composite oxide in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium in the composition is 0.7 or more and less than 1, wherein a compound containing boron adheres to at least a portion of the surface of the primary particles and a compound containing sodium is present in at least a portion of the grain boundaries of the secondary particles, and the value obtained by dividing the standard deviation of the detected amount by the average detected amount of boron element in any three regions in the cross section of the secondary particles is less than 0.18.

[0008] A third aspect is a non-aqueous electrolyte secondary battery including the above-described positive electrode active material in a positive electrode. A fourth aspect is a non-aqueous electrolyte secondary battery including a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer including the above-described positive electrode active material and having a density of 2.8 g / cm. 3 More than 3.7g / cm 3 The following is an electrode for a non-aqueous electrolyte secondary battery. [Effects of the Invention]

[0009] According to one aspect of the present disclosure, it is possible to provide a positive electrode active material having excellent cycle characteristics and a method for producing the same. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 10 is a diagram showing an example of the distribution of boron elements in the lithium transition metal composite oxide of Example 5. [Figure 2] FIG. 10 is a diagram showing an example of the distribution of boron elements in the lithium transition metal composite oxide of Comparative Example 5. [Figure 3] FIG. 10 is a diagram showing an example of the distribution of boron elements in the lithium transition metal composite oxide of Comparative Example 6. [Figure 4] This is an equivalent circuit model for AC impedance measurement. DETAILED DESCRIPTION OF THE INVENTION

[0011] In this specification, the term "process" refers not only to an independent process, but also to a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Furthermore, the content of each component in a composition refers to the total amount of the multiple substances present in the composition, unless otherwise specified, when multiple substances corresponding to each component are present in the composition. Hereinafter, embodiments of the present invention will be described in detail. However, the embodiments described below are intended to exemplify the cathode active material and the manufacturing method thereof for embodying the technical concept of the present invention, and the present invention is not limited to the cathode active material and the manufacturing method thereof described below.

[0012] [Method of manufacturing positive electrode active material] The method for producing a positive electrode active material includes a washing step of contacting first particles containing a lithium transition metal composite oxide having a layered structure and in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium in the composition is 0.7 or more and less than 1 with a solution containing sodium ions to obtain second particles containing the lithium transition metal composite oxide and elemental sodium, a mixing step of mixing the second particles with a boron compound to obtain a mixture, and a heat treatment step of heat treating the mixture at a temperature of 100° C. or more and 450° C. If necessary, the method may also include other steps, such as a preparation step of preparing the first particles.

[0013] A positive electrode active material obtained by washing a lithium transition metal composite oxide having a nickel molar ratio of 0.7 or more but less than 1 with a sodium ion-containing aqueous solution and then heat-treating it together with a boron compound can achieve good cycle characteristics when used in a nonaqueous electrolyte secondary battery. This is thought to be because, for example, the presence of sodium at the grain boundaries of the secondary particles containing the lithium transition metal composite oxide that constitute the positive electrode active material allows boron to be uniformly distributed throughout the grain boundaries of the lithium transition metal composite oxide particles.

[0014] Preparation process In the preparation step, first particles containing a lithium transition metal composite oxide having a layered structure are prepared. The lithium transition metal composite oxide constituting the first particles contains nickel in its composition, and the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.7 or more and less than 1. The first particles may be prepared by appropriately selecting them from commercially available products, or may be prepared by the preparation method described below.

[0015] The method for preparing the first particles may include, for example, a precursor preparation step of preparing a precursor, and a synthesis step of synthesizing the first particles containing a lithium transition metal composite oxide from the precursor and a lithium compound. The first particles may be configured as secondary particles composed of a plurality of primary particles containing a lithium transition metal composite oxide.

[0016] In the precursor preparation step, a precursor containing a nickel-containing composite oxide is prepared. The precursor may be prepared by appropriately selecting from commercially available products, or by preparing a nickel-containing composite oxide having a desired composition by a conventional method. Examples of the precursor include nickel-containing composite oxides and composite oxides containing nickel and metals other than nickel (e.g., Co, Mn, Al, Ti, Nb, etc.).

[0017] Methods for obtaining a nickel-containing composite oxide having a desired composition include a method in which raw material compounds (hydroxides, carbonates, etc.) are mixed according to the target composition and decomposed into a nickel-containing composite oxide by heat treatment, as well as a coprecipitation method in which a solution in which the raw material compounds are dissolved is prepared, a precursor precipitate having the target composition is obtained by adjusting the temperature, adjusting the pH, adding a complexing agent, etc., and the nickel-containing composite oxide is obtained by heat treating the precursor precipitate. An example of a method for producing a nickel-containing composite oxide (hereinafter also simply referred to as a composite oxide) will be described below.

[0018] A method for obtaining a composite oxide by coprecipitation can include a seed generation step of adjusting the pH of a mixed solution containing metal ions in a desired composition ratio to obtain seed crystals, a crystallization step of growing the generated seed crystals to obtain a composite hydroxide having desired properties, and a step of heat-treating the obtained composite hydroxide to obtain a composite oxide. For details of the method for obtaining such a composite oxide, see, for example, JP 2003-292322 A and JP 2011-116580 A (US Patent Application Publication No. 2012 / 270107 A).

[0019] In the seed generation step, a liquid medium containing seed crystals is prepared by adjusting the pH of a mixed solution containing nickel ions at a desired composition ratio to, for example, 11 to 13. The seed crystals may contain, for example, a hydroxide containing nickel at a desired ratio. The mixed solution can be prepared by dissolving nickel salts in water at a desired ratio. Examples of nickel salts include sulfates, nitrates, and hydrochlorides. In addition to the nickel salt, the mixed solution may also contain other metal salts at a desired composition ratio as needed. The temperature in the seed generation step can be, for example, 40°C to 80°C. The atmosphere in the seed generation step can be a low-oxidizing atmosphere, and for example, the oxygen concentration may be maintained at 10% by volume or less.

[0020] In the crystallization step, the generated seed crystals are grown to obtain a precursor precipitate containing nickel with desired properties. The seed crystals can be grown, for example, by adding a mixed solution containing nickel ions and, if necessary, other metal ions to a liquid medium containing the seed crystals while maintaining the pH in the range of, for example, 7 to 12.5, preferably 7.5 to 12. The time period for adding the mixed solution is, for example, 1 hour to 24 hours, preferably 3 hours to 18 hours. The temperature in the crystallization step can be, for example, 40°C to 80°C. The atmosphere in the crystallization step is the same as that in the seed generation step.

[0021] The pH in the seed generation step and the crystallization step can be adjusted using an acidic aqueous solution such as an aqueous sulfuric acid solution or an aqueous nitric acid solution, or an alkaline aqueous solution such as an aqueous sodium hydroxide solution or an aqueous ammonia solution.

[0022] In the crystallization process, it is desirable to control the particle size of the precursor precipitate. The particle size of the precursor precipitate can be controlled by adjusting the temperature, pH, stirring speed, etc. of the reaction space. These conditions can be adjusted appropriately depending on actual conditions such as the shape of the vessel containing the reaction space, the starting materials, and the rate at which the starting materials are introduced into the reaction space. Furthermore, the particle size of the precursor precipitate can be controlled by the aging time after the onset of precipitation of the precursor precipitate, the stirring speed, etc. The conditions in this case may also be adjusted appropriately depending on actual conditions, since the particle growth rate, shape, etc. vary depending on the shape of the reaction vessel.

[0023] In the step of obtaining a composite oxide, the composite hydroxide-containing precursor precipitate obtained in the crystallization step is heat-treated to obtain the composite oxide. The heat treatment may be carried out by heating the composite hydroxide at a temperature of, for example, 500°C or lower, preferably 350°C or lower. The heat treatment temperature may be, for example, 100°C or higher, preferably 200°C or higher. The heat treatment time may be, for example, 0.5 to 48 hours, preferably 5 to 24 hours. The heat treatment atmosphere may be air or an oxygen-containing atmosphere. The heat treatment may be carried out using, for example, a box furnace, a rotary kiln furnace, a pusher furnace, a roller hearth kiln furnace, or the like.

[0024] The resulting composite oxide may contain other metal elements in addition to nickel. Examples of other metals include Co, Mn, Al, Ti, and Nb. At least one metal selected from the group consisting of these is preferred, and at least one metal selected from the group consisting of Co, Mn, and Al is preferred. When the composite oxide contains other metals, the aqueous solution used to obtain the precursor precipitate may contain the other metal ions in the desired composition. This allows the precursor precipitate to contain nickel and other metals, and the precursor precipitate can be heat-treated to obtain a composite oxide with the desired composition.

[0025] The average particle size of the composite oxide is, for example, 2 μm to 30 μm, preferably 3 μm to 25 μm. The average particle size of the composite oxide is a volume-average particle size, which is the value at which the volume integrated value from the small particle size side in the volume distribution obtained by a laser scattering method becomes 50%.

[0026] In the synthesis step of synthesizing first particles containing a lithium transition metal composite oxide, a mixture containing lithium obtained by mixing a composite oxide with a lithium compound is heat-treated at a temperature of 550° C. to 1000° C. The heat-treated product obtained has a layered structure and contains a lithium transition metal composite oxide containing nickel.

[0027] Examples of the lithium compound to be mixed with the composite oxide include lithium hydroxide, lithium carbonate, lithium oxide, etc. The particle size of the lithium compound to be mixed is, for example, 0.1 μm or more and 100 μm or less, preferably 2 μm or more and 20 μm or less, in terms of volume average particle size.

[0028] The ratio of the total number of moles of lithium to the total number of moles of metal elements constituting the composite oxide in the mixture may be, for example, 0.95 or more and 1.2 or less. The composite oxide and the lithium compound can be mixed using, for example, a high-speed shear mixer.

[0029] The mixture may further contain metal elements other than lithium and the metal elements constituting the composite oxide. Examples of the other metal elements include Al, Si, Zr, Ti, Mg, Ta, Nb, Mo, and W, and at least one selected from the group consisting of these is preferred. For example, when the mixture contains W, Nb, or the like as another metal element, the output characteristics are improved. For example, when the mixture contains Al, Zr, or the like, it is suitable for further improving the cycle characteristics. For example, when the mixture contains Ti, Si, or the like, it is suitable for further improving the cycle characteristics under high voltage. When the mixture contains other metal elements, a mixture can be obtained by mixing the other metal elements alone or metal compounds with the composite oxide and the lithium compound. Examples of metal compounds containing other metal elements include oxides, hydroxides, chlorides, nitrides, carbonates, sulfates, nitrates, acetates, oxalates, and the like.

[0030] When the mixture contains other metal elements, the ratio of the total number of moles of the metal elements constituting the composite oxide to the total number of moles of the other metal elements may be, for example, 1:0.0001 to 1:0.1, and preferably 1:0.0005 to 1:0.03, or 1:0.001 to 1:0.01.

[0031] The heat treatment temperature of the mixture is, for example, 550°C to 1000°C, preferably 600°C to 950°C, and more preferably 700°C to 950°C. The heat treatment of the mixture may be performed at a single temperature, but is preferably performed at multiple temperatures in terms of discharge capacity at high voltage. When performing heat treatment at multiple temperatures, it is desirable to, for example, hold the first temperature for a predetermined time, then further increase the temperature, and hold the second temperature for a predetermined time. The first temperature is, for example, 200°C to 600°C, preferably 400°C to 500°C, and the second temperature is, for example, 600°C to 900°C, preferably 650°C to 750°C. The heat treatment time is, for example, 0.5 to 48 hours. When performing heat treatment at multiple temperatures, each temperature can be 0.2 to 47 hours.

[0032] The heat treatment may be carried out in air or in an oxygen-containing atmosphere using, for example, a box furnace, a rotary kiln, a pusher furnace, a roller hearth kiln, or the like.

[0033] The lithium transition metal composite oxide obtained above has a composition in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.7 or more and less than 1, preferably 0.7 or more and 0.95 or less, more preferably 0.75 or more and 0.95 or less, and even more preferably 0.8 or more and 0.95 or less. The lithium transition metal composite oxide may contain cobalt. When the lithium transition metal composite oxide contains cobalt, the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium may be, for example, more than 0 and 0.3 or less, preferably 0.02 or more and 0.2 or less. The lithium transition metal composite oxide may contain manganese. When the lithium transition metal composite oxide contains manganese, the ratio of the number of moles of manganese to the total number of moles of metals other than lithium may be, for example, more than 0 and 0.3 or less, preferably more than 0 and 0.15 or less, and more preferably 0.01 or more and 0.15 or less. The lithium transition metal composite oxide may contain aluminum. When the lithium transition metal composite oxide contains aluminum, the ratio of the number of moles of aluminum to the total number of moles of metals other than lithium may be, for example, greater than 0 and less than 0.1, preferably greater than 0 and less than 0.05, and more preferably 0.01 or more and 0.04 or less. When the lithium transition metal composite oxide contains at least one of manganese and aluminum, the ratio of the total number of moles of manganese and aluminum to the total number of moles of metals other than lithium may be, for example, greater than 0 and less than 0.3, preferably greater than 0 and less than 0.25, and more preferably 0.01 or more and 0.15 or less.

[0034] The lithium transition metal composite oxide may have a composition represented by the following formula (1), for example. Li (1+p) Ni (1-x-y-z-w) Co x Mn y Al z Mw O2(1) In the formula, -0.05 ≤ p ≤ 0.2, 0 < x + y + z + w ≤ 0.3, 0 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.3, 0 ≤ z ≤ 0.1, and 0 ≤ w ≤ 0.03 are satisfied. M is at least one element selected from the group consisting of Zr, Ti, Mg, Ta, Nb, Mo, and W.

[0035] The volume average particle diameter of the first particles containing the lithium transition metal composite oxide is, for example, 2 μm or more and 30 μm or less, preferably 3 μm or more and 25 μm or less.

[0036] Washing step In the washing step, the first particles containing the lithium transition metal composite oxide are brought into contact with a solution containing sodium ions (hereinafter also referred to as a washing solution) to obtain second particles containing the lithium transition metal composite oxide and sodium element. After contact with the washing solution, dehydration treatment, drying treatment, etc. may be carried out on the treated product as necessary. The washing step is, for example, a step of removing at least a part of the alkaline component of the unreacted raw material present in the first particles.

[0037] The solution containing sodium ions only needs to contain at least sodium ions and water. The solution containing sodium ions can be prepared, for example, by dissolving a sodium salt in a solvent. Examples of the sodium salt include sodium sulfate, sodium hydroxide, etc., and at least one selected from the group consisting of these is preferable, and it is more preferable to contain at least sodium sulfate. The solvent may, for example, contain at least water, and may contain a water-soluble organic solvent such as alcohol as necessary in addition to water. The content rate of sodium ions in the washing solution is, for example, 0.01 mol / L or more and 2.0 mol / L or less, preferably 0.05 mol / L or more and 2.0 mol / L or less, more preferably 0.1 mol / L or more and 1.5 mol / L or less, still more preferably 0.15 mol / L or more and 1.0 mol / L or less, and particularly preferably 0.15 mol / L or more and 0.6 mol / L or less.

[0038] The cleaning solution may contain metal ions other than sodium as needed. Examples of metal ions other than sodium include alkali metal ions such as lithium ions and potassium ions, and alkaline earth metal ions such as magnesium ions. When the cleaning solution contains metal ions other than sodium, the content of the metal ions other than sodium is, for example, 0.1 mol / L or less, and preferably less than 0.01 mol / L.

[0039] The temperature at which the first particles are contacted with the cleaning liquid is, for example, 5°C to 60°C, and preferably 10°C to 40°C. The contact time is, for example, 1 minute to 2 hours, and preferably 5 minutes to 30 minutes. The amount of the cleaning liquid used for contact is, for example, 0.25 to 10 times, and preferably 0.5 to 4 times the mass of the first particles.

[0040] The contact of the first particles with the cleaning liquid may be carried out by adding the first particles to the cleaning liquid to prepare a slurry. When the contact is carried out as a slurry, the solids concentration of the first particles in the slurry is, for example, 10% by mass or more and 80% by mass or less, preferably 20% by mass or more and 60% by mass or less. The contact may be carried out by passing the cleaning liquid through the first particles held on a filter, or by passing the cleaning liquid through a dehydrated cake obtained by washing the first particles with pure water or the like and then dehydrating them. When the cleaning liquid is passed through a dehydrated cake obtained by washing the first particles with pure water or the like and then dehydrating them, the total amount of the pure water and the cleaning liquid used is preferably 0.25 to 10 times, more preferably 0.5 to 4 times, the mass of the first particles. Note that a solution containing sodium ions (e.g., a sodium sulfate solution) has a higher solubility of residual alkali (e.g., lithium carbonate) than pure water, making it easier to remove residual alkali. Therefore, from the viewpoint of reducing damage to the lithium transition metal composite oxide, it is preferable to prioritize contact with the cleaning liquid. It is also preferable not to wash with pure water.

[0041] The second particles obtained in the washing step contain a sodium-containing compound in addition to the lithium transition metal composite oxide. The sodium-containing compound may be present, for example, at the grain boundaries of secondary particles composed of primary particles containing the lithium transition metal composite oxide. The content of the sodium-containing compound contained in the second particles is, for example, 100 ppm to 1400 ppm, preferably 150 ppm to 1300 ppm, more preferably 150 ppm to 1200 ppm, even more preferably 200 ppm to 1000 ppm, and particularly preferably 300 ppm to 1000 ppm, in terms of elemental sodium. When the content of the sodium-containing compound is within the above range, resistance components during charge and discharge are sufficiently reduced. The content of the sodium-containing compound in the second particles can be adjusted, for example, by the sodium ion concentration of the washing solution, the amount of water adhering to the dehydrated cake, etc.

[0042] The second particles obtained in the washing step may be dried. The drying process may be carried out by any method capable of removing at least a portion of the moisture adhering to the second particles, such as heat drying, air drying, or vacuum drying. The drying temperature in the case of heat drying may be any temperature at which the moisture contained in the second particles is sufficiently removed. The drying temperature is, for example, 80°C or higher and 300°C or lower, and preferably 100°C or higher and 250°C or lower. When the drying temperature is within the above range, the dissolution of lithium into the adhering water can be sufficiently suppressed. Furthermore, the collapse of the crystalline structure on the particle surface can be suppressed, and a decrease in charge / discharge capacity can be sufficiently suppressed. The drying time may be appropriately selected depending on the amount of moisture contained in the second particles. The drying time is, for example, 1 hour or higher and 10 hours or lower. The amount of moisture contained in the second particles after the drying process is, for example, 0.2 mass% or lower, and preferably 0.1 mass% or lower.

[0043] The degree of cleaning in the cleaning process can be confirmed by the lithium content, residual alkaline components, specific surface area, etc. of the second particles. Generally, a small specific surface area of ​​the second particles can sufficiently suppress particle cracking and the elution of lithium and elements constituting the composite oxide, which tends to further improve cycle characteristics. Furthermore, by ensuring a certain level of specific surface area, the residual alkaline components can be sufficiently reduced. A sodium salt aqueous solution such as sodium sulfate used as a cleaning solution has a higher solubility of lithium salts than pure water, a lithium salt aqueous solution, etc., and therefore the amount of solution required for alkali removal can be reduced. As a result, the specific surface area of ​​the second particles is reduced, and excessive lithium elution from the second particles can be suppressed.

[0044] The specific surface area of ​​the second particles obtained in the washing step is, for example, 0.5 m 2 / g or more 4m 2 / g or less, preferably 1.0m 2 / g or more 3.0m 2 / g or less, more preferably 1.0m 2 / g or more 1.6m 2 / g or less, more preferably 1.0m 2 / g or more 1.4m 2 The specific surface area can be measured by the BET method.

[0045] Mixing process In the mixing step, the second particles and the boron compound are mixed to obtain a mixture. The mixing of the second particles and the boron compound may be performed dry or wet. The mixing may be performed using, for example, a super mixer. In addition to the boron compound, other metal elements, alloys, or metal compounds may also be mixed in this mixing step. Examples of other metal elements include Al, Si, Zr, Ti, Mg, Ta, Nb, Mo, W, etc., and at least one selected from the group consisting of these elements is preferred.

[0046] The boron compound can be at least one selected from the group consisting of boron oxide, boron oxoacids, and boron oxoacid salts. Specific examples of boron compounds include lithium tetraborate (Li2B4O7), ammonium pentaborate (NH4B5O8), orthoboric acid (H3BO3; so-called ordinary boric acid), lithium metaborate (LiBO2), and boron oxide (BO3). At least one selected from the group consisting of these is preferred, and orthoboric acid is more preferred from the standpoint of cost.

[0047] The boron compound may be mixed with the second particles in a solid state, or may be mixed with the second particles as a solution of the boron compound. When a solid boron compound is used, the volume average particle size of the boron compound is, for example, 1 μm or more and 60 μm or less, and preferably 10 μm or more and 30 μm or less.

[0048] The content of the boron compound in the mixture is, for example, 0.1 mol% or more and 2 mol% or less, preferably 0.1 mol% or more and 1.5 mol% or less, and more preferably 0.1 mol% or more and 1.2 mol% or less, as the ratio of the number of moles of elemental boron to the total number of moles of metals other than lithium in the lithium transition metal composite oxide.

[0049] Heat Treatment Process In the heat treatment step, the mixture is heat-treated, for example, at a temperature of 100°C to 450°C to obtain a positive electrode active material. The heat treatment temperature may be 200°C to 400°C, preferably 220°C to 350°C, and more preferably 250°C to 350°C. By increasing the heat treatment temperature higher than the drying treatment temperature, the charge / discharge capacity may be further improved. The heat treatment atmosphere may be an oxygen-containing atmosphere or may be air. The heat treatment time is, for example, 1 hour to 20 hours, preferably 5 hours to 10 hours. The heat-treated product obtained in the heat treatment step may be subjected to crushing, classification, etc., as necessary.

[0050] After the washing step, lithium-deficient regions may be formed near the surfaces of the second particles, and the lithium-deficient regions may inhibit the desorption and insertion of lithium ions. However, by mixing a boron compound with the second particles after the washing step and then subjecting them to heat treatment, it is believed that the lithium deficiency is compensated for, and the inhibition of the desorption and insertion of lithium ions is suppressed, thereby improving the charge / discharge characteristics and cycle characteristics.

[0051] [Cathode active material] The positive electrode active material has a layered structure and includes secondary particles formed by aggregating multiple primary particles containing a lithium transition metal composite oxide in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium in the composition is 0.7 or more and less than 1. A compound containing boron is attached to at least a portion of the surface of the primary particles. Furthermore, a compound containing sodium is present in at least a portion of the grain boundaries of the secondary particles. The coefficient of variation (CV), which is the value (σ1 / t1) obtained by dividing the standard deviation σ1 of the detected amount of boron element by the average value t1 of the detected amount for any three regions in the cross section of the secondary particles, is less than 0.18.

[0052] The positive electrode active material includes a lithium transition metal composite oxide and secondary particles formed by aggregation of multiple primary particles having a boron-containing compound attached to their surfaces. This improves the charge / discharge and cycle characteristics of a nonaqueous electrolyte secondary battery constructed using the positive electrode active material. Furthermore, the presence of a sodium-containing compound at the grain boundaries of the secondary particles is thought to allow the boron-containing compound to be uniformly distributed throughout the secondary particles, achieving favorable charge / discharge and cycle characteristics. The positive electrode active material can be efficiently manufactured using the above-described method for manufacturing a positive electrode active material.

[0053] The coefficient of variation of the detected amount of boron element in the cross section of the positive electrode active material particle is preferably less than 0.18, more preferably 0.15 or less, even more preferably 0.14 or less, and particularly preferably 0.13 or less. The lower limit of the coefficient of variation is, for example, 0.04 or more. A coefficient of variation of the detected amount of boron element equal to or less than a predetermined value is considered to indicate that the boron compound is uniformly distributed throughout the positive electrode active material particle.

[0054] The average value of the detected amount of boron element used to calculate the coefficient of variation is calculated by selecting three arbitrary regions in an arbitrary cross section of a secondary particle constituting the positive electrode active material, and calculating the arithmetic average of the detected amount in each region. The standard deviation of the detected amount of boron element is calculated from the obtained average value and the detected amount in each region. The detected amount of boron element in the cross section of the positive electrode active material particle may be measured using, for example, a secondary ion mass spectrometer (SIMS).

[0055] The region where elemental boron is detected can be selected from, for example, a region near the surface (surface region), a region near the center (center region), and a region intermediate between the surface region and the center region (middle region) in the cross section of the positive electrode active material particle. Alternatively, multiple regions may be selected from each of the surface region, the middle region, and the center region, and the arithmetic mean of the detected amounts in the multiple regions may be used as the detected amounts in the surface region, the middle region, and the center region.

[0056] The ratio of the moles of nickel to the total moles of metals other than lithium in the composition of the lithium transition metal composite oxide constituting the primary particles is 0.7 or more and less than 1, preferably 0.7 or more and 0.95 or less, more preferably 0.8 or more and 0.95 or less, and even more preferably 0.9 or more and 0.95 or less. A positive electrode active material containing a lithium transition metal composite oxide with a high mole ratio of nickel achieves better charge / discharge characteristics and cycle characteristics. The composition of the lithium transition metal composite oxide can be measured, for example, using an inductively coupled plasma optical emission spectrometer.

[0057] The lithium transition metal composite oxide may contain cobalt in its composition. When the lithium transition metal composite oxide contains cobalt, the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is, for example, greater than 0 and not more than 0.3, preferably not less than 0.02 and not more than 0.2. The lithium transition metal composite oxide may contain manganese. When the lithium transition metal composite oxide contains manganese, the ratio of the number of moles of manganese to the total number of moles of metals other than lithium is, for example, greater than 0 and not more than 0.3, preferably greater than 0 and not more than 0.15. The lithium transition metal composite oxide may contain aluminum. When the lithium transition metal composite oxide contains aluminum, the ratio of the number of moles of aluminum to the total number of moles of metals other than lithium is, for example, greater than 0 and not more than 0.1, preferably greater than 0 and not more than 0.05, more preferably not less than 0.01 and not more than 0.04.

[0058] When the lithium transition metal composite oxide contains nickel, cobalt, and at least one of manganese and aluminum, the content ratio Ni / Co / (Mn + Al) of nickel, cobalt, manganese, and aluminum can be, for example, 8 / 1 / 1, 8 / 1 / (0.5 + 0.5), etc. on a molar basis.

[0059] The lithium transition metal composite oxide may have a composition represented by, for example, the following formula (1). Li (1+p) Ni 1-x-y-z-w Co x Mn y Al z M w O2(1) In the formula, -0.05 ≦ p ≦ 0.2, 0 < x + y + z + w ≦ 0.3, 0 ≦ x ≦ 0.3, 0 ≦ y ≦ 0.3, 0 ≦ z ≦ 0.1, 0 ≦ w ≦ 0.03 are satisfied. M is at least one selected from the group consisting of Zr, Ti, Mg, Ta, Nb, Mo, and W.

[0060] In formula (1), from the viewpoint of output characteristics, p is preferably -0.02 or more, or 0.02 or more. Also, p is preferably 0.12 or less, or 0.06 or less. x is preferably 0 < x ≤ 0.3, more preferably 0.02 ≤ x ≤ 0.2. y is preferably 0 < y ≤ 0.3, more preferably 0 < y ≤ 0.15. z is preferably 0 < z ≤ 0.1, more preferably 0 < z ≤ 0.05, still more preferably 0.01 ≤ z ≤ 0.04.

[0061] On at least a part of the surface of the primary particles, a boron-containing compound is attached. Examples of the boron-containing compound include lithium metaborate (LiBO2) and the like. The boron-containing compound may form a complex with the lithium transition metal composite oxide. The content of the boron-containing compound in the positive electrode active material is, for example, 0.1 mol% or more and 2 mol% or less, preferably 0.1 mol% or more and 1.5 mol% or less, as the ratio of the number of moles of boron element to the total number of moles of metals other than lithium in the lithium transition metal composite oxide. The content of boron in the positive electrode active material can be measured, for example, by an inductively coupled plasma optical emission spectrometer.

[0062] In at least a part of the grain boundaries of the secondary particles, a sodium-containing compound is present. Examples of the sodium-containing compound include sodium sulfate (Na2SO4) and the like. The content of the sodium-containing compound in the positive electrode active material is, for example, 100 ppm or more and 1400 ppm or less in terms of sodium element, preferably 150 ppm or more and 1300 ppm or less, more preferably 150 ppm or more and 1200 ppm or less, still more preferably 200 ppm or more and 1000 ppm or less, particularly preferably 300 ppm or more and 1000 ppm or less. The content of sodium element in the positive electrode active material can be measured, for example, by an inductively coupled plasma optical emission spectrometer.

[0063] The specific surface area of the positive electrode active material is, for example, 0.2 m 2 / g or more and 3.0 m 2 / g or less, preferably 0.3 m 2 / g or more 2.0m 2 The specific surface area of ​​the positive electrode active material is measured by the BET method.

[0064] The positive electrode active material can be used in the positive electrode of a nonaqueous electrolyte secondary battery to form a nonaqueous electrolyte secondary battery that can achieve excellent cycle characteristics. The positive electrode active material can be included in a positive electrode active material layer disposed on a current collector to form a positive electrode. That is, the present invention encompasses an electrode for a nonaqueous electrolyte secondary battery that includes the positive electrode active material, and a nonaqueous electrolyte secondary battery that includes the electrode.

[0065] [Nonaqueous electrolyte secondary battery electrode] The electrode for a non-aqueous electrolyte secondary battery includes a current collector and a positive electrode active material layer disposed on the current collector and including the above-described positive electrode active material or a positive electrode active material produced by the above-described production method. A non-aqueous electrolyte secondary battery including such an electrode can achieve excellent cycle characteristics.

[0066] The density of the positive electrode active material layer is, for example, 2.6 g / cm 3 More than 3.9g / cm 3 It may be less than 2.8 g / cm 3 More than 3.8g / cm 3 or less, more preferably 3.1 g / cm 3 More than 3.7g / cm 3 More preferably, 3.2 g / cm or less 3 More than 3.6g / cm 3 The density of the active material layer is calculated by dividing the mass of the active material layer by the volume of the active material layer. The density of the active material layer can be adjusted by applying pressure to an electrode composition described below after applying it to a current collector.

[0067] Examples of materials for the current collector include aluminum, nickel, and stainless steel. The positive electrode active material layer can be formed by applying an electrode composition obtained by mixing the above-mentioned positive electrode active material, a conductive material, a binder, and the like together with a solvent onto the current collector, followed by drying and pressure treatments. Examples of conductive materials include natural graphite, artificial graphite, and acetylene black. Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, and polyamide acrylic resin. Examples of solvents include N-methyl-2-pyrrolidone (NMP).

[0068] [Non-aqueous electrolyte secondary battery] The non-aqueous electrolyte secondary battery includes the above-mentioned electrodes for a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery is configured to include, in addition to the electrodes for a non-aqueous electrolyte secondary battery, a negative electrode for a non-aqueous secondary battery, a non-aqueous electrolyte, a separator, etc. For the negative electrode, non-aqueous electrolyte, separator, etc. of the non-aqueous electrolyte secondary battery, those for non-aqueous electrolyte secondary batteries described in, for example, JP 2002-075367 A, JP 2011-146390 A, JP 2006-12433 A (the disclosures of which are incorporated herein by reference in their entirety) can be appropriately used. [Example]

[0069] Examples of the present invention are described below, but the present invention is not limited to these examples. The volume-average particle size was determined by the volume distribution obtained by laser scattering, where the cumulative volume from the small particle size side was 50%. Specifically, the volume-average particle size was measured using a laser diffraction particle size distribution analyzer (MALVERN Inst. MASTERSIZER 2000). The specific surface area was measured using a BET specific surface area analyzer (Macsorb, manufactured by Mountec Co., Ltd.) by a gas adsorption method (single-point method) using nitrogen gas. The alkaline content was measured by adding the positive electrode active material to pure water, titrating the dissolved lithium with sulfuric acid, and measuring up to the second neutralization point. The amount of alkaline content neutralized by sulfuric acid was defined as the lithium hydroxide (LiOH) content. The composition was measured using an inductively coupled plasma atomic emission spectrometry (ICP-AES; manufactured by PerkinElmer). The amount of sulfuric acid was measured using an ICP-AES (manufactured by Hitachi). The sodium content (Na content) was measured using an atomic absorption spectrometry (AAS; manufactured by Hitachi).

[0070] [Example 1] Precursor preparation process By coprecipitation method, the volume average particle size of the secondary particles is 20 μm, and (Ni 0.95 Co 0.05 Composite oxide particles having a composition represented by the formula:

[0071] Synthesis process The obtained composite oxide particles were mixed with lithium hydroxide and aluminum hydroxide in a molar ratio of Li:(Ni+Co):Al=1.10:0.97:0.03 to obtain a raw material mixture. The obtained raw material mixture was heat-treated in the atmosphere. The heat treatment was carried out at a first temperature of 450°C for 3 hours and at a second temperature of 680°C for 4 hours. After the heat treatment, a dispersion treatment was carried out to determine the composition of the lithium transition metal composite oxide. 1.03 Ni 0.92 Co 0.05 Al 0.03 We obtained the first particle, which is O2.

[0072] Cleaning process The obtained first particles were added to a sodium sulfate aqueous solution prepared so that the sodium ion concentration was 0.469 mol / L to prepare a slurry with a solids concentration of 45% by mass. The solids concentration was calculated by dividing the mass of the first particles by the mass of the first particles plus the mass of the washing solution. The slurry was stirred for 30 minutes, then dehydrated in a funnel and separated as a cake. The separated cake was dried at 150°C for 10 hours to obtain second particles as washed particles.

[0073] Mixing process Orthoboric acid was added in an amount such that the boron content was 1 mol % relative to the total number of moles of metals other than lithium in the lithium transition metal composite oxide contained in the obtained second particles, and the mixture was mixed and stirred to obtain a mixture.

[0074] Heat Treatment Process The resulting mixture was heat-treated in air at 250° C. for 10 hours to obtain the target positive electrode active material E1 containing a lithium transition metal composite oxide.

[0075] [Example 2] A positive electrode active material E2 was obtained in the same manner as in Example 1, except that the concentration of the aqueous sodium sulfate solution in the washing step was adjusted so that the sodium ion concentration was 0.156 mol / L.

[0076] [Example 3] A positive electrode active material E3 was obtained in the same manner as in Example 1, except that the aqueous sodium sulfate solution in the washing step was changed to an aqueous sodium hydroxide solution.

[0077] [Comparative Example 1] A positive electrode active material C1 was obtained in the same manner as in Example 1, except that pure water was used instead of the aqueous sodium sulfate solution in the washing step and the solid content concentration of the slurry was set to 42 mass %.

[0078] Comparative Example 2 A positive electrode active material C2 was obtained in the same manner as in Example 1, except that in the washing step, a lithium sulfate aqueous solution prepared so as to have a lithium ion concentration of 0.469 mol / L was used instead of the sodium sulfate aqueous solution, and the solid content concentration of the slurry was set to 32 mass %.

[0079] [Cycle characteristics evaluation 1] The cycle characteristics of the positive electrode active materials obtained in Examples 1 to 3 and Comparative Examples 1 and 2 were evaluated as follows.

[0080] Preparation of the positive electrode 96.5 parts by mass of the positive electrode active material, 65 parts by mass of SUPER-C (manufactured by TIMICAL Corporation), and 2 parts by mass of PVDF (polyvinylidene fluoride) were dispersed and dissolved in NMP (N-methyl-2-pyrrolidone) to prepare a positive electrode slurry. The obtained positive electrode slurry was applied to an aluminum foil current collector plate, dried, and then pressed with a roll press until the density of the positive electrode active material layer reached 3.5 g / cm. 3 Compression molded to a size of 15cm 2 The positive electrode was obtained by cutting the sheet into pieces.

[0081] Preparation of non-aqueous electrolyte A mixed solvent was prepared by mixing EC (ethylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) in a volume ratio of 3:4:3. Lithium hexafluorophosphate (LiPF6) was dissolved in the mixed solvent to a concentration of 1 mol / L to prepare a nonaqueous electrolyte solution.

[0082] Assembling the evaluation battery A lead electrode was attached to the positive electrode current collector, and then vacuum dried at 120°C. A separator made of porous polyethylene was placed on the positive electrode in a dry box and stored in a bag-shaped laminate pack. After storage, the positive electrode was vacuum dried at 60°C to remove moisture adsorbed to each component. After vacuum drying, the positive electrode current collector covered with the separator and a Li foil adhered to a SUS plate in an argon box were placed opposite each other and inserted into the laminate pack. The aforementioned nonaqueous electrolyte was injected into the laminate pack and sealed, yielding a laminate-type nonaqueous electrolyte secondary battery for evaluation.

[0083] aging The obtained evaluation battery was subjected to one charge-discharge cycle consisting of a constant-voltage / constant-current charge at a charge voltage of 4.25 V (counter electrode Li) and a charge current of 0.2 C (1 C ≡ a current at which discharge is completed in 1 hour), and a constant-current discharge at a discharge voltage of 2.75 V (counter electrode Li) and a discharge current of 0.2 C.

[0084] Capacity retention rate measurement After aging, one cycle consisted of a constant-voltage / constant-current charge at a charge voltage of 4.25 V (Li counter electrode) and a charge current of 0.3 C, followed by a constant-current discharge at a discharge voltage of 2.75 V (Li counter electrode) and a discharge current of 0.3 C. The discharge capacity after each cycle was measured at a constant temperature of 45°C. The ratio of the discharge capacity Ed(n) after n cycles to the discharge capacity Ed(1) after 1 cycle (≡Ed(n) / Ed(1)) was defined as the capacity retention rate Rs(n) after n cycles. The number of cycles was n = 30. The evaluation results are shown in Table 1.

[0085] [Table 1]

[0086] As shown in Table 1, the test batteries constructed using the positive electrode active materials E1 to E3 of Examples 1 to 3, which had been washed with a sodium salt aqueous solution, mixed with a boron compound, and heat-treated, had excellent cycle characteristics. The positive electrode active material C1 of Comparative Example 1 had a large specific surface area of ​​the second particles. This is thought to be due to, for example, washing with pure water. The test battery constructed using the positive electrode active material C1 also had a low capacity retention rate and poor battery performance. The positive electrode active material C2 of Comparative Example 2 had a smaller specific surface area of ​​the second particles than Comparative Example 1. This is thought to be due, for example, to washing with a lithium sulfate aqueous solution. The test battery constructed using the positive electrode active material C2 also had a low capacity retention rate and poor battery performance.

[0087] [Example 4] Precursor preparation process By coprecipitation method, the volume average particle size is 18 μm, and (Ni 0.85 Co 0.15 Composite oxide particles having a composition represented by the formula:

[0088] Synthesis process The primary particles were obtained in the same manner as in Example 1, except that the mixing ratio in the synthesis step was changed to Li:(Ni+Co):Al=1.10:0.96:0.04 and the second temperature was changed to 745°C.

[0089] Cleaning process Second particles were obtained in the same manner as in Example 1, except that the sodium sulfate aqueous solution used in the washing step was prepared so that the sodium ion concentration was 0.313 mol / L and the solid content concentration was 40 mass %.

[0090] Mixing process Positive electrode active material E4 was obtained in the same manner as in Example 1, except that the amount of boric acid added in the mixing process was 0.3 mol% in terms of elemental boron relative to the total number of moles of metals other than lithium in the lithium transition metal composite oxide contained in the second particles.

[0091] Comparative Example 3 A positive electrode active material C3 was obtained in the same manner as in Example 4, except that pure water was used instead of the aqueous sodium sulfate solution in the washing step and the solid content concentration was set to 45 mass %.

[0092] Comparative Example 4 A positive electrode active material C4 was obtained in the same manner as in Example 4, except that in the washing step, an aqueous lithium sulfate solution prepared so as to have a lithium ion concentration of 0.313 mol / L was used instead of the aqueous sodium sulfate solution.

[0093] The positive electrode active materials obtained in Example 4 and Comparative Examples 3 and 4 were evaluated in the same manner as in Cycle Performance Evaluation 1. The evaluation results are shown in Table 2.

[0094] [Table 2]

[0095] As shown in Table 2, the evaluation batteries constructed with the positive electrode active materials C3 and C4 of Comparative Examples 3 and 4 had lower capacity retention rates and inferior battery performance compared to the evaluation battery constructed with the positive electrode active material E4 of Example 4 that had been washed with a sodium salt aqueous solution.

[0096] [Example 5] Precursor preparation process By coprecipitation method, the volume average particle size was 22 μm, and (Ni 0.88 Co 0.09 Mn 0.03 Composite oxide particles having a composition represented by the formula:

[0097] Synthesis process The primary particles were obtained in the same manner as in Example 1, except that the mixing ratio in the synthesis step was changed to Li:(Ni+Co+Mn):Al=1.12:0.98:0.02 and the second temperature was changed to 730°C.

[0098] Cleaning process As in Example 1, the primary particles were washed with an aqueous sodium sulfate solution prepared so that the sodium ion concentration was 0.469 mol / L, to obtain secondary particles.

[0099] Mixing process A mixture was obtained in the same manner as in Example 1, except that the amount of boric acid added in the mixing step was 0.5 mol% in terms of elemental boron relative to the total number of moles of metals other than lithium in the lithium transition metal composite oxide contained in the second particles, and tungsten oxide was added so as to be 0.3 mol% relative to the total number of moles of metals other than lithium in the lithium transition metal composite oxide contained in the second particles.

[0100] Heat Treatment Process The mixture was heat-treated in the same manner as in Example 1 to obtain a positive electrode active material E5.

[0101] Comparative Example 5 A positive electrode active material C5 was obtained in the same manner as in Example 5, except that pure water was used instead of the aqueous sodium sulfate solution in the washing step and the solid content concentration of the slurry was set to 37 mass %.

[0102] Comparative Example 6 A positive electrode active material C6 was obtained in the same manner as in Example 5, except that in the washing step, a lithium sulfate aqueous solution prepared so as to have a lithium ion concentration of 0.469 mol / L was used instead of the sodium sulfate aqueous solution, and the solid content concentration of the slurry was set to 28 mass %.

[0103] [Cycle characteristics evaluation 2] The cycle characteristics of the positive electrode active materials obtained in Example 5 and Comparative Examples 5 and 6 were evaluated as follows.

[0104] Preparation of positive electrode active material The volume average particle size is 4.5 μm, and Li 1.03 Ni 0.835 Co 0.14 Al 0.025 A positive electrode active material A was prepared, which contained a lithium transition metal composite oxide having a composition represented by O2 and was obtained through a washing process using a sodium sulfate aqueous solution. Positive electrode active materials E5, C5, and C6 obtained in Example 5 and Comparative Examples 5 to 6 were mixed with positive electrode active material A at a weight ratio of 7:3 to prepare mixed positive electrode active materials for evaluation.

[0105] Preparation of the positive electrode 92 parts by mass of the mixed positive electrode active material obtained above, 3 parts by mass of acetylene black, and 5 parts by mass of PVDF (polyvinylidene fluoride) were dispersed and dissolved in NMP (N-methyl-2-pyrrolidone) to prepare a positive electrode slurry. The obtained positive electrode slurry was applied to a current collector made of aluminum foil, dried, and then pressed with a roll press until the density of the positive electrode active material layer became 3.3 g / cm. 3 Compression molded to a size of 15cm 2 The positive electrode was obtained by cutting the sheet into pieces.

[0106] Preparation of the negative electrode A negative electrode slurry was prepared by dispersing 97.5 parts by mass of artificial graphite, 1.5 parts by mass of CMC (carboxymethyl cellulose), and 1.0 part by mass of SBR (styrene butadiene rubber) in water. The resulting negative electrode slurry was applied to a copper foil, dried, and then compression-molded to obtain a negative electrode.

[0107] Preparation of non-aqueous electrolyte A mixed solvent was prepared by mixing EC (ethylene carbonate) and EMC (ethyl methyl carbonate) at a volume ratio of 3:7. Lithium hexafluorophosphate (LiPF6) was dissolved in the mixed solvent to a concentration of 1 mol / L, thereby preparing a non-aqueous electrolyte solution.

[0108] Assembling the evaluation battery The evaluation battery was assembled in the same manner as in cycle performance evaluation 1. Specifically, lead electrodes were attached to the current collectors of the positive and negative electrodes, respectively, and then vacuum dried at 120°C. Next, the separator was placed between the positive and negative electrodes, and the resulting assembly was housed in a bag-shaped laminate pack. After storage, the assembly was vacuum dried at 60°C to remove moisture adsorbed to each component. After vacuum drying, a nonaqueous electrolyte was injected into the laminate pack and sealed, yielding a laminate-type nonaqueous electrolyte secondary battery as the evaluation battery.

[0109] aging The obtained evaluation battery was subjected to one charge-discharge cycle consisting of a constant-voltage / constant-current charge at a charge voltage of 4.2 V (counter electrode C) and a charge current of 0.1 C, and a constant-current discharge at a discharge voltage of 2.75 V (counter electrode C) and a discharge current of 0.2 C. Thereafter, the charge current was changed to 0.2 C, and the charge-discharge cycle was repeated twice to allow the nonaqueous electrolyte to become familiar with the positive and negative electrodes.

[0110] The discharge capacity retention rate was measured in the same manner as in Cycle Characteristics 1, except that the charge voltage was 4.2 V (counter electrode C), the discharge voltage was 2.75 V (counter electrode C), and the number of cycles was n = 100. The evaluation results are shown in Table 3.

[0111] [Table 3]

[0112] [Evaluation of boron element distribution] The distribution of boron element inside the particles was evaluated for the positive electrode active materials obtained in Example 5 and Comparative Examples 5 and 6. Specifically, positive electrodes were prepared in the same manner as above, and the obtained positive electrodes were processed under vacuum conditions using an ion milling device IM400PLUS (manufactured by Hitachi Corporation) to obtain cross-sectional samples of the positive electrode active material particles. An Ar beam was used for processing, and the processing time was 1 hour. The detected amount of each element in the cross section of the positive electrode active material particle in the obtained cross-sectional sample was measured using a double-focusing sector magnetic field mass spectrometer (NanoSIMS 50L; manufactured by Cameca Corporation). The primary ion species was Cs + The primary acceleration voltage was set to 8 kV, and -8 kV was applied to the sample stage. + By irradiating the secondary ions, BO 2- A signal of (mass number 42.97) was measured. A cross-sectional image was created based on the intensity of the measured signal. 2- An example of an image showing the distribution of BO in the positive electrode active material particles obtained in Comparative Example 5 is shown in FIG. 2- An example of an image showing the distribution of BO in the positive electrode active material particles obtained in Comparative Example 6 is shown in FIG. 2- An example of an image showing the distribution of each is shown in FIG.

[0113] Using image analysis software (OpenMIMS ImageJ Plugin), the obtained image data was used to identify the BO in the area near the particle surface (surface area), the area in the middle layer of the particle (middle layer area), and the area near the center of the particle (center area). 2- The amount of detected ions was calculated by analysis. The analysis was carried out for three positive electrode active material particles for each Example and Comparative Example, selecting two regions from each of the surface, middle layer, and center. Each region was selected on a line crossing the particle cross section so as to be approximately symmetrical with respect to the center of the particle, and the area of ​​each region was approximately 1.5 × 10 -11 m 2 The two central regions were selected as one continuous region. 2-The amount of BO detected was calculated as the arithmetic average for each of the two regions. From the detected amounts, the amount of BO detected in the three regions of the surface, middle layer, and center of the particles of the positive electrode active material was calculated. 2- The arithmetic mean value t1, standard deviation σ1, and coefficient of variation (CV; σ1 / t1) of the detected amount of boron were calculated. In this case, the standard deviation σ1 was calculated using the STDEV.P function in Excel. For this coefficient of variation CV, the arithmetic mean results of three particles for each example and comparative example are shown in Table 4, and these results were evaluated as the distribution of boron element.

[0114] [Table 4]

[0115] As shown in Tables 3 and 4, the positive electrode active materials obtained in Comparative Examples 5 and 6 had lower capacity retention rates and inferior battery performance compared to Example 5, which was washed with a sodium salt aqueous solution. Furthermore, compared to Example 5, the boron-containing compound was not uniformly distributed throughout the particles. This is thought to be due to, for example, the absence of sodium at the grain boundaries of the positive electrode active material particles.

[0116] [Example 6] Precursor preparation process By coprecipitation method, the volume average particle size is 4 μm, and (Ni 0.885 Co 0.115 )O3 composite oxide particles were obtained.

[0117] Synthesis process The same procedure as in Example 1 was carried out to obtain primary particles, except that the mixing ratio in the synthesis step was changed to Li:(Ni+Co):Al=1.10:0.97:0.03 and the second temperature was changed to 690°C.

[0118] Cleaning process Second particles were obtained in the same manner as in Example 1, except that the aqueous sodium sulfate solution used in the washing step was adjusted to have a sodium ion concentration of 0.156 mol / L.

[0119] Mixing process Positive electrode active material E6 was obtained in the same manner as in Example 1, except that the amount of boric acid added in the mixing process was 0.1 mol% in terms of elemental boron relative to the total number of moles of metals other than lithium in the lithium transition metal composite oxide contained in the second particles.

[0120] Comparative Example 7 A positive electrode active material C7 was obtained in the same manner as in Example 6, except that in the washing step, a lithium sulfate aqueous solution prepared so as to have a lithium ion concentration of 0.156 mol / L was used instead of the sodium sulfate aqueous solution, and the solid content concentration of the slurry was set to 30 mass %.

[0121] The cycle characteristics of the positive electrode active materials obtained in Example 6 and Comparative Example 7 were evaluated as follows.

[0122] The volume average particle size is 22 μm, and Li 1.03 Ni 0.86 Co 0.09 Mn 0.03 Al 0.02 A positive electrode active material B containing a lithium transition metal composite oxide having a composition represented by O2 and obtained through a washing process using a sodium sulfate aqueous solution was prepared. The positive electrode active materials E6 and C7 obtained in Example 6 and Comparative Example 7 were mixed with the positive electrode active material B in a weight ratio of 3:7 to obtain a mixed positive electrode active material for evaluation. The cycle performance was evaluated in the same manner as in Evaluation 2. The evaluation results are shown in Table 5.

[0123] [Table 5]

[0124] As shown in Table 5, the positive electrode active material obtained in Comparative Example 7 had a lower capacity retention rate than Example 6, and was inferior in battery performance.

[0125] [Drag increase rate evaluation] For the positive electrodes containing the positive electrode active materials obtained by the same manufacturing methods as in Example 5 and Comparative Examples 5 and 6, the resistance increase rate was evaluated as follows.

[0126] [Example 7] Preparation of positive electrode active material A positive electrode active material for evaluation was prepared in the same manner as in Example 5.

[0127] Preparation of the positive electrode 92 parts by mass of the positive electrode active material obtained above, 3 parts by mass of acetylene black, and 5 parts by mass of PVDF (polyvinylidene fluoride) were dispersed and dissolved in NMP (N-methyl-2-pyrrolidone) to prepare a positive electrode slurry. The obtained positive electrode slurry was applied to a current collector made of aluminum foil, dried, and then pressed with a roll press until the density of the positive electrode active material layer became 2.8 g / cm. 3 Compression molded to a size of 15cm 2 The positive electrode of Example 7 was obtained by cutting the positive electrode active material layer so that the thickness of the positive electrode active material layer was measured with a micrometer and the density of the positive electrode active material layer was calculated by dividing the mass of the positive electrode active material layer by the volume of the positive electrode active material layer, which was calculated by measuring the thickness of the positive electrode active material layer with a micrometer.

[0128] Assembling the evaluation battery A battery for evaluation was obtained in the same manner as in cycle performance evaluation 2, except that the positive electrode obtained above was used.

[0129] [Example 8] The density of the positive electrode active material layer is 3.3 g / cm 3 A positive electrode of Example 8 was obtained in the same manner as in Example 7, except that it was compression-molded so that the positive electrode had a thickness of 100 μm. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this positive electrode was used.

[0130] [Example 9] The density of the positive electrode active material layer is 3.5 g / cm 3 A positive electrode of Example 9 was obtained in the same manner as in Example 7, except that it was compression-molded so that the positive electrode had a thickness of 100 μm. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this positive electrode was used.

[0131] [Example 10] The density of the positive electrode active material layer is 3.7 g / cm 3A positive electrode of Example 10 was obtained in the same manner as in Example 7, except that it was compression-molded so that the positive electrode had a thickness of 100 μm. A battery for evaluation was then obtained in the same manner as in Example 7, except that this positive electrode was used.

[0132] [Comparative Example 8] A positive electrode was obtained in the same manner as in Example 7, except that the positive electrode active material obtained by the manufacturing method of Comparative Example 5 was used as the positive electrode active material. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this was used.

[0133] Comparative Example 9 The density of the positive electrode active material layer is 3.3 g / cm 3 A positive electrode of Comparative Example 9 was obtained in the same manner as in Comparative Example 8, except that it was compression molded so that the positive electrode had a thickness of 100 μm. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this positive electrode was used.

[0134] [Comparative Example 10] The density of the positive electrode active material layer is 3.5 g / cm 3 A positive electrode of Comparative Example 10 was obtained in the same manner as in Comparative Example 8, except that it was compression molded so that the positive electrode had a thickness of 100 μm. A battery for evaluation was then obtained in the same manner as in Example 7, except that this positive electrode was used.

[0135] [Comparative Example 11] The density of the positive electrode active material layer is 3.7 g / cm 3 A positive electrode of Comparative Example 11 was obtained in the same manner as in Comparative Example 8, except that it was compression-molded so that the positive electrode had a thickness of 100 μm. A battery for evaluation was then obtained in the same manner as in Example 7, except that this positive electrode was used.

[0136] [Comparative Example 12] A positive electrode was obtained in the same manner as in Example 7, except that the positive electrode active material obtained by the manufacturing method of Comparative Example 6 was used as the positive electrode active material. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this was used.

[0137] [Comparative Example 13] The density of the positive electrode active material layer is 3.3 g / cm 3A positive electrode of Comparative Example 13 was obtained in the same manner as in Comparative Example 12, except that it was compression-molded so that the positive electrode had a thickness of 100 μm. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this positive electrode was used.

[0138] [Comparative Example 14] The density of the positive electrode active material layer is 3.5 g / cm 3 A positive electrode of Comparative Example 14 was obtained in the same manner as in Comparative Example 12, except that it was compression-molded so that the positive electrode had a thickness of 100 μm. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this positive electrode was used.

[0139] [Comparative Example 15] The density of the positive electrode active material layer is 3.7 g / cm 3 A positive electrode of Comparative Example 15 was obtained in the same manner as in Comparative Example 12, except that it was compression-molded so that the positive electrode had a thickness of 100 μm. Then, a battery for evaluation was obtained in the same manner as in Example 7, except that this positive electrode was used.

[0140] aging The obtained evaluation battery was subjected to one charge-discharge cycle consisting of a constant-voltage / constant-current charge at a charge voltage of 4.2 V (counter electrode C) and a charge current of 0.1 C, and a constant-current discharge at a discharge voltage of 2.75 V (counter electrode C) and a discharge current of 0.2 C. Thereafter, the charge current was changed to 0.2 C and the battery was subjected to two charge-discharge cycles to allow the nonaqueous electrolyte to become familiar with the positive and negative electrodes.

[0141] AC impedance measurement After aging, the batteries were charged to 100% SOC by constant-voltage, constant-current charging at a charging voltage of 4.2 V and a charging current of 0.2 C. Resistance measurements were performed using an impedance measuring device (1470E or 1455A, both manufactured by Solartron) using the AC impedance method in the range of 1 MHz to 0.1 Hz, and a Nyquist plot was obtained. After the resistance measurements, the batteries were discharged at a constant current of 2.75 V and a discharge current of 0.2 C. The test batteries were then subjected to 200 charge-discharge cycles at a constant temperature of 45°C, consisting of a constant-voltage, constant-current charge at a charging voltage of 4.2 V and a charging current of 1 C, followed by a constant-current discharge at a discharge voltage of 2.75 V and a discharge current of 1 C. After the 200 charge-discharge cycles, the batteries were charged to 100% SOC by constant-voltage, constant-current charging at a charging voltage of 4.2 V and a charging current of 0.2 C. Resistance measurements were similarly performed using the impedance measuring device, and a Nyquist plot was obtained.

[0142] Calculation of resistance increase rate Based on the obtained Nyquist plot, an equivalent circuit model (Figure 4) was constructed and fitting calculations were performed. The higher peak frequency of the arc component of the impedance obtained by the measurement was taken as the resistance derived from the negative electrode, and the lower peak frequency was taken as the resistance derived from the positive electrode, R. The resistance derived from the positive electrode before cycling was taken as R(p) and the resistance derived from the positive electrode after cycling was taken as R(a). The resistance increase rate was calculated as R(a) / R(p) × 100(%). Next, as in Comparative Example 8 for Example 7, the resistance increase rate calculated for the Examples or Comparative Examples cleaned with a cleaning solution was calculated by dividing the resistance increase rate for a Comparative Example prepared with the same electrode plate density and cleaned with pure water. The evaluation results for Examples 7 to 10, which were cleaned with a sodium sulfate aqueous solution, and Comparative Examples 12 to 15, which were cleaned with a lithium sulfate aqueous solution, are shown in Table 6.

[0143] [Table 6]

[0144] As shown in Table 6, the positive electrodes of the examples containing the positive electrode active material obtained through the washing process using an aqueous solution containing sodium ions had low relative resistance increase rates and suppressed deterioration in output after cycling, regardless of the plate density. These results confirmed that the characteristics of nonaqueous electrolyte secondary batteries containing the positive electrode active material obtained through the washing process using an aqueous solution containing sodium ions were improved not only in capacity retention but also in resistance.

Claims

1. contacting first particles having a layered structure with a solution containing sodium ions to obtain second particles containing the lithium transition metal composite oxide and sodium element; mixing the second particles with a boron compound to obtain a mixture; heat treating the mixture at a temperature of 100°C or higher and 450°C or lower; wherein the solution is heated to a temperature of 5°C or higher and 60°C or lower.

2. The method according to claim 1 , wherein the content of sodium element contained in the second particles is 100 ppm or more and 1400 ppm or less.

3. 3. The method according to claim 1, wherein the mixture contains the boron compound in an amount such that the ratio of the number of moles of boron element to the total number of moles of metals other than lithium in the lithium transition metal composite oxide is 0.1 mol % or more and 2 mol % or less.

4. 4. The method according to claim 1, wherein the lithium transition metal composite oxide contains cobalt, and the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is 0.3 or less.

5. 5. The method according to claim 1, wherein the lithium transition metal composite oxide contains at least one of manganese and aluminum, and the ratio of the number of moles of manganese and aluminum to the total number of moles of metals other than lithium is 0.3 or less.

6. The method according to any one of claims 1 to 5, wherein the lithium transition metal composite oxide has a composition represented by the following formula (1): Li （１＋ｐ） Ni （１－ｘ－ｙ－ｚ－ｗ） Co ｘ Mn ｙ Al ｚ M ｗ O ２ (1) (-0.05≦p≦0.2, 0<x+y+z+w≦0.3, 0≦x≦0.3, 0≦y≦0.3, 0≦z≦0.1, 0≦w≦0.03, and M is at least one element selected from the group consisting of Zr, Ti, Mg, Ta, Nb, Mo, and W).

7. The secondary particles are formed by aggregating a plurality of primary particles containing a lithium transition metal composite oxide having a layered structure, a boron-containing compound adheres to at least a portion of the surface of the primary particles; a compound containing sodium is present in at least a portion of the grain boundaries of the secondary particles; the value obtained by dividing the standard deviation of the detected amount by the average value of the detected amount of boron element in any three regions in the cross section of the secondary particle is less than 0.18; The sodium element content is 100 ppm or more and 1400 ppm or less, A positive electrode active material having an LiOH content of 0.57% by mass or less.

8. 8. The positive electrode active material according to claim 7, wherein the ratio of the number of moles of elemental boron to the total number of moles of metals other than lithium is 0.1 mol % or more and 2 mol % or less.

9. 9. The positive electrode active material according to claim 7, wherein the lithium transition metal composite oxide contains cobalt, and the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is 0.3 or less.

10. 10. The positive electrode active material according to claim 7, wherein the lithium transition metal composite oxide contains at least one of manganese and aluminum, and the ratio of the number of moles of manganese and aluminum to the total number of moles of metals other than lithium is 0.3 or less.

11. 11. The positive electrode active material according to claim 7, wherein the lithium transition metal composite oxide has a composition represented by the following formula (1): Li （１＋ｐ） Ni （１－ｘ－ｙ－ｚ－ｗ） Co ｘ Mn ｙ Al ｚ M ｗ O ２ (1) (-0.05≦p≦0.2, 0<x+y+z+w≦0.3, 0≦x≦0.3, 0≦y≦0.3, 0≦z≦0.1, 0≦w≦0.03, and M is at least one selected from the group consisting of Zr, Ti, Mg, Ta, Nb, Mo, and W).

12. A non-aqueous electrolyte secondary battery containing the positive electrode active material according to any one of claims 7 to 11 in a positive electrode.

13. a current collector and a positive electrode active material layer disposed on the current collector; The positive electrode active material layer contains the positive electrode active material according to any one of claims 7 to 12, Density is 2.8 g / cm ３ 3.7g / cm or more ３ An electrode for a non-aqueous electrolyte secondary battery, which is as follows:

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