Method for producing positive electrode active material for non-aqueous electrolyte secondary battery, positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

This method is characterized by including a mixing step of mixing a transition metal oxide containing Ni and lithium hydroxide to obtain a mixture and a firing step of firing the mixture, wherein the firing step includes: a first firing step of raising the temperature of the mixture from room temperature to a first temperature of 470-600°C, inclusive, and holding the mixture at the first temperature for 2-10 hours, inclusive; and a second firing step of, after the first firing step, raising the temperature of the mixture to a second temperature of 700-900°C, inclusive, at a raising rate of 5-15°C / min, inclusive, and holding the mixture at the second temperature for 1-10 hours, inclusive.

Description

Method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, positive electrode active material for a non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

[0001] This disclosure relates to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

[0002] Secondary batteries such as lithium-ion batteries are widely used in applications requiring high capacity, high durability, and high output, such as automotive and energy storage applications. The positive electrode active material, which is a major component of non-aqueous electrolyte secondary batteries, greatly affects these performance characteristics, and therefore, much research has been conducted on positive electrode active materials. For example, Patent Document 1 discloses a manufacturing method in which, during the firing of the positive electrode active material, it is heated at a temperature of 450°C or higher and 550°C or lower for a predetermined time, and then heated at a temperature of 650°C or higher and 800°C or lower for a predetermined time.

[0003] Japanese Patent Publication No. 2007-257985

[0004] There is a need for further improvement in the cycle characteristics of non-aqueous electrolyte secondary batteries. The manufacturing method described in Patent Document 1 is not sufficient to improve the cycle characteristics, and there is still room for improvement.

[0005] A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, comprises a mixing step of mixing a transition metal oxide containing Ni with lithium hydroxide to obtain a mixture, and a firing step of firing the mixture, wherein the firing step comprises a first firing step of raising the temperature of the mixture from room temperature to a first temperature of 470°C or higher and 600°C or lower, and holding it at the first temperature for 2 hours or more and 10 hours or less, and a second firing step of raising the temperature of the mixture to a second temperature of 700°C or higher and 900°C or lower at a heating rate of 5°C / min or higher and 15°C / min or less, and holding it at the second temperature for 1 hour or more and 10 hours or less.

[0006] According to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, a positive electrode active material capable of improving cycle characteristics can be obtained.

[0007] This is an axial cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. This figure schematically shows the structure and arrangement of primary particles inside secondary particles. This figure shows an example of a method for calculating the relative standard deviation.

[0008] The present disclosure provides a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: a crystallization step to obtain a Ni-containing transition metal hydroxide (hereinafter sometimes referred to as "Ni-containing hydroxide"); an oxidation step to obtain a Ni-containing transition metal oxide (hereinafter sometimes referred to as "Ni-containing oxide") by heat-treating the Ni-containing hydroxide; a mixing step to obtain a mixture by mixing the Ni-containing oxide with lithium hydroxide; a calcination step to calcine the mixture; a water washing step to wash the calcined product obtained in the calcination step with water; and a drying step to obtain a positive electrode active material (lithium transition metal composite oxide) by drying the wet powder obtained in the water washing step.

[0009] [Crystallization Process] In the crystallization process, for example, while stirring a solution of a metal salt containing Ni and an arbitrary metal element (Mn, Al, Co, etc.), an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to the alkaline side (for example, 8.5 or higher and 12.5 or lower), thereby precipitating (coprecipitation) a composite hydroxide (Ni-containing hydroxide) containing Ni and an arbitrary metal element. However, the method for producing the Ni-containing hydroxide is not limited to this.

[0010] [Oxidation Process] In the oxidation process, the Ni-containing hydroxide obtained in the crystallization process is heated and oxidized to obtain a Ni-containing oxide. The heating temperature in the oxidation process is, for example, 250°C to 700°C or lower, and may be 300°C or higher and 650°C or lower. The holding time at the maximum temperature is, for example, 1 hour or more and 15 hours or lower, and may be 2 hours or more and 12 hours or lower.

[0011] [Mixing Process] In the mixing process, the Ni-containing oxide obtained in the oxidation process and lithium hydroxide are mixed. Here, the ratio of the number of moles of lithium contained in lithium hydroxide to the total number of moles of metal elements contained in the Ni-containing oxide is preferably 0.9 or more and 1.2 or less, and more preferably 0.95 or more and 1.08 or less. By setting the ratio to 0.9 or more and 1.2 or less, the crystal structure of the finally obtained positive electrode active material is likely to be stabilized, and it is easy to achieve a higher capacity and improved cycle characteristics of the battery.

[0012] In the mixing process, in addition to the Ni-containing oxide and lithium hydroxide, a metal inorganic compound may be further added. Examples of the metal inorganic compound include compounds containing at least one of oxides, hydroxides, phosphates, sulfates, and chlorides containing at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Ca, Sr, Ba, B, Al, Ga, and In. Examples of the metal inorganic compound include, for example, TiO ４、 , ３ , ４ , ４ , ２ , Ti(OH) ４ , ZrSO ４ , ZrO ２ , VO, V ２ O ５ , Nb ２ O ５ , Nb(OH)​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​２ O ３ In ２ O ３ InCl ３ Examples include the above. The above metal-inorganic compounds may be used individually or in combination of two or more types. In addition, the above metal-inorganic compounds may be added before the oxidation step, in addition to the mixing step. That is, the oxidation step may be carried out with the Ni-containing hydroxide and the metal-inorganic compound already mixed.

[0013] The metal-inorganic compound preferably contains at least one element selected from the group consisting of Nb, Ca, Sr, and Al. When the metal-inorganic compound contains Nb, cation mixing, in which transition metals such as Ni enter the sites for insertion and removal of Li ions during charging and discharging, is suppressed. As a result, the charge-discharge efficiency of the battery is improved, and the cycle characteristics are further enhanced. Furthermore, when the metal-inorganic compound contains at least one of Ca and Sr, a film containing at least one of Ca and Sr is formed on the particle surface of the positive electrode active material, further improving the cycle characteristics. In addition, when the metal-inorganic compound contains Al, the crystal structure of the positive electrode active material is more easily stabilized, making it easier to achieve higher battery capacity.

[0014] [Casturing Process] In the calcination process, the mixture obtained in the mixing process is placed in a calcination container and calcined to obtain a calcined product. Here, the calcination process is a two-stage calcination process that includes a first calcination process and a second calcination process, each with different heating temperatures. The calcination process is carried out, for example, under an oxygen stream. The flow rate of the oxygen stream during calcination is, for example, 20 mL / min or more per liter of calcination furnace or 0.3 L or more per 1 kg of mixture.

[0015] In the first firing step, the mixture is heated from room temperature to a first temperature of 470°C or higher and 600°C or lower, and held at this first temperature for 2 hours or more and 10 hours or less. By holding the mixture at a temperature of 470°C or higher, which is above the melting point of lithium hydroxide, the lithium hydroxide is sufficiently melted, and the synthesis reaction between the Ni-containing oxide and lithium hydroxide can proceed uniformly. As a result, the crystallite size distribution of the primary particles of the final positive electrode active material becomes sharper, and the major axis direction of the primary particles is more likely to be oriented from the center outward towards the periphery of the secondary particles. As a result, higher battery capacity and improved cycle characteristics can be achieved. In other words, if the first temperature is below 470°C, the lithium hydroxide will not melt sufficiently, and unevenness in the synthesis reaction is likely to occur. As a result, a decrease in battery capacity and cycle characteristics is likely to occur.

[0016] Furthermore, by maintaining a temperature below 600°C, it is possible to suppress the acceleration of crystal growth before the synthesis reaction in the first firing process, allowing the synthesis reaction to proceed more uniformly. As a result, it is possible to achieve higher battery capacity and improved cycle characteristics.

[0017] The first temperature, which is the highest temperature in the first firing process, may be between 470°C and 600°C, but is preferably between 470°C and 550°C. By setting the first temperature to between 470°C and 550°C, the lithium hydroxide can be sufficiently melted while suppressing the precipitation of the spinel phase in the crystal. As a result, higher battery capacity and improved cycle characteristics can be achieved. Note that the precipitation of the spinel phase is more pronounced when the Ni-containing oxide also contains Mn.

[0018] In the first firing process, the mixture is held at the first temperature for at least two hours and no more than ten hours. Holding it at the first temperature for at least two hours equalizes the temperature gradient within the firing container, allowing the synthesis reaction to proceed more uniformly. In other words, if the holding time at the first temperature is less than two hours, the temperature gradient within the firing container is not sufficiently eliminated due to endothermic reactions associated with the melting of lithium hydroxide, etc., and the synthesis reaction tends to proceed unevenly. Furthermore, holding it at the first temperature for ten hours or less can improve productivity, for example. Note that the holding time at the first temperature may be at least two hours and no more than eight hours, or at least three hours and no more than five hours.

[0019] The heating rate in the first firing process is not particularly limited, and may be, for example, 0.5°C / min or more and 15°C / min or less. Note that multiple heating rates may be set for each temperature range.

[0020] In the second firing process, the temperature is raised to a second temperature of 700°C or higher and 900°C or lower, and the material is held at this second temperature for 1 hour or more and 10 hours or less. By holding the material at this second temperature, crystal growth is promoted, and a positive electrode active material with a stable crystal structure can be obtained. The second temperature may be 700°C or higher and 850°C or 700°C or higher and 800°C or lower. In this case, a positive electrode active material with an even more stable crystal structure can be obtained. Furthermore, the holding time at the second temperature may be 2 hours or more and 7 hours or 2 hours or more and 5 hours or less. In this case, a positive electrode active material with an even more stable crystal structure can be obtained while improving productivity.

[0021] Here, the heating rate in the second firing process is between 5°C / min and 15°C / min. By keeping the heating rate in the second firing process within the above range, the crystallites of the primary particles tend to grow isotropically. As a result, cracking of the positive electrode active material becomes less likely when charging and discharging is repeated, and the cycle characteristics can be further improved. In other words, if the heating rate in the second firing process is less than 5°C / min, the crystallites tend to grow anisotropically, and the cycle characteristics cannot be sufficiently improved. Also, if the heating rate in the second firing process exceeds 15°C / min, excessive strain occurs in the crystal, making the primary particles more prone to cracking, which can lead to a decrease in battery capacity, for example.

[0022] The heating rate in the second firing process should be between 5°C / min and 15°C / min, but preferably between 5°C / min and 12°C / min, and more preferably between 6°C / min and 10°C / min. When the heating rate in the second firing process is between 5°C / min and 12°C / min, crystallites grow more isotropically, and the improvement in cycle characteristics is significant.

[0023] As in this embodiment, by including a first firing process and a second firing process, the actual temperature inside the firing furnace during the second firing process can be brought closer to the set temperature, and the variation in the actual temperature inside the firing furnace can be reduced. As a result, the crystallinity of the final cathode active material is improved, and the variation in the quality of the cathode active material can be further reduced.

[0024] Furthermore, in the firing process, it is preferable to fill the firing container with a mixture of 4 kg or more and less than 20 kg at a density of 0.5 kg / L or more and then perform the firing. In this case, a temperature gradient is easily generated within the firing container, so the effects of this disclosure are more pronounced. The amount of mixture to be filled may be 4.5 kg or more and 15 kg or less, or 5 kg or more and 10 kg or less. The filling density of the mixture may be 0.52 kg / L or more, or 0.55 kg or more. The upper limit of the filling density of the mixture is, for example, 1.5 kg / L.

[0025] [Water Washing Process] In the water washing process, the slurry obtained by mixing the calcined material obtained in the calcination process with water or an aqueous solution is stirred and washed with water. Before the water washing process, unreacted lithium hydroxide and other substances used during mixing may remain on the particle surface of the calcined material. By performing the water washing process, the unreacted lithium hydroxide and other substances remaining on the particle surface can be removed.

[0026] Washing is carried out by known methods. For example, the calcined material and water or an aqueous solution are placed in a reaction vessel equipped with a stirring device and stirred. In the washing process, the slurry produced in the washing process is separated into solid and liquid to obtain a cake-like wet powder. The method of solid-liquid separation is not particularly limited and is carried out by known methods. For example, a suction filter, centrifuge, or filter press can be used for solid-liquid separation.

[0027] [Drying Process] In the drying process, the wet powder obtained in the washing process is dried to obtain a positive electrode active material (lithium transition metal composite oxide). In the drying process, for example, from the viewpoint of suppressing deterioration of battery characteristics when used as a positive electrode active material, it is preferable to dry until the moisture content is 1.0% by mass or less. The drying conditions are, for example, drying at a temperature of 100°C or higher and 300°C or lower for 0.5 hours or more.

[0028] A non-aqueous electrolyte secondary battery to which the positive electrode active material produced by the above manufacturing method is applied can be obtained, for example, by housing an electrode body, in which electrodes (positive electrode, negative electrode) and a separator are stacked or wound together, in an outer casing such as an outer can or laminate together with a non-aqueous electrolyte.

[0029] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery according to this disclosure will be described in detail with reference to Figure 1. Figure 1 is an axial cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery, which is an example of an embodiment. Hereinafter, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 is given as an example of a non-aqueous electrolyte secondary battery, but the outer casing of the battery is not limited to a cylindrical outer casing. The non-aqueous electrolyte secondary battery according to this disclosure may be, for example, a prismatic battery with a prismatic outer casing, a coin-type battery with a coin-type outer casing, or a pouch-type battery with an outer casing made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body is not limited to a wound type, and may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator in between. Furthermore, the design of the non-aqueous electrolyte secondary battery according to this disclosure is not limited to the example design of the non-aqueous electrolyte secondary battery, and known designs of non-aqueous electrolyte secondary batteries may be applied.

[0030] As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte (not shown), and an outer casing 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. For the sake of explanation, the side of the battery with the sealing body 17 will be referred to as "upper," and the bottom side of the outer casing 16 will be referred to as "lower."

[0031] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all rectangular elongated bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape in the longitudinal direction. The separator 13 separates the positive electrode 11 and the negative electrode 12 from each other. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in both the longitudinal and transverse directions than the positive electrode 11. The two separators 13 are formed to be at least slightly larger in dimensions than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode body 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the transverse direction of the positive electrode 11 and the negative electrode 12 is the axial direction. In other words, the end faces in the short direction of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode body 14.

[0032] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0033] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0034] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side, and functions as a safety valve. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0035] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the non-aqueous electrolyte secondary battery 10, with particular emphasis on the positive electrode 11.

[0036] [Positive Electrode] The positive electrode 11 includes, for example, a positive electrode core and a positive electrode mixture layer formed on the surface of the positive electrode core. Preferably, the positive electrode mixture layer is formed on both sides of the positive electrode core. The positive electrode core can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The thickness of the positive electrode core is, for example, 10 μm or more and 30 μm or less.

[0037] The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, and a binder. The thickness of the positive electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the positive electrode core. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, conductive agent, etc., to the surface of the positive electrode core, drying the coating film, and then rolling it to form the positive electrode mixture layer on both sides of the positive electrode core.

[0038] Examples of conductive agents included in the positive electrode mixture layer include carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), graphene, and other carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0039] Examples of binders included in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyimide resins, acrylic resins, polyolefin resins, and polyacrylonitrile (PAN). These may be used individually or in combination of two or more types.

[0040] The positive electrode mixture layer contains a positive electrode active material (lithium transition metal composite oxide) produced by the above-described method. The positive electrode mixture layer may contain compounds other than the lithium transition metal composite oxide produced by the above-described method as the positive electrode active material, but the lithium transition metal composite oxide produced by the above-described method is the main component. Here, the main component means the component that accounts for the largest mass percentage of the positive electrode active material. The content of the lithium transition metal composite oxide produced by the above-described method is, for example, 90% by mass or more of the total mass of the positive electrode active material.

[0041] Lithium transition metal composite oxides contain secondary particles formed by the aggregation of primary particles. The particle size of the primary particles is, for example, between 0.02 μm and 2 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM, e.g., JEOL JSM-7900F). The average particle diameter of the secondary particles is, for example, between 2 μm and 30 μm. Here, the average particle diameter refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of secondary particles can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell MT3000II) with water as the dispersion medium.

[0042] Lithium transition metal composite oxides contain Ni. The Ni content in the lithium transition metal composite oxide satisfies, for example, 50 mol% ≤ Ni content ≤ 95 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. If the Ni content is within the above range, it is possible to achieve both high capacity and stabilization of the crystal structure. The Ni content is preferably 70 mol% ≤ Ni content ≤ 95 mol%, and more preferably 80 mol% ≤ Ni content ≤ 95 mol%.

[0043] The lithium transition metal composite oxide further contains M1 (where M1 is at least one element selected from the group consisting of Co, Mn, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Ca, Sr, Ba, B, Al, Ga, and In). The amount of M1 contained in the lithium transition metal composite oxide satisfies, for example, 3 mol% ≤ M1 amount ≤ 50 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. It is preferable that M1 contains at least one element selected from the group consisting of Co, Mn, and Al.

[0044] Lithium transition metal composite oxides include, for example, those with the general formula Li ａ Ni ｘ M1 ｙ O ２－ｂ (In the formula, 0.9 ≤ a ≤ 1.2, 0.50 ≤ x ≤ 0.95, 0.03 ≤ y ≤ 0.50, 0 ≤ b ≤ 0.05, x + y = 1, and M1 is at least one element selected from the group consisting of Co, Mn, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Ca, Sr, Ba, B, Al, Ga, and In) is a composite oxide. In addition, lithium transition metal composite oxides are represented by the general formula Li ａ Ni ｘ Co ｙ Mn ｚ Al ｗ M2 ｖ O ２－ｂThe composite oxide may be represented by the formula (wherein 0.9 ≤ a ≤ 1.2, 0.50 ≤ x ≤ 0.95, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.20, 0 ≤ w ≤ 0.10, 0 ≤ v ≤ 0.10, 0 ≤ b ≤ 0.05, x + y + z + w + v = 1, and M2 is at least one element selected from the group consisting of at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Ca, Sr, Ba, B, Ga, and In). The proportion of metal elements contained in the lithium transition metal composite oxide can be measured, for example, by an inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0045] Lithium transition metal composite oxides have a layered structure. Examples of layered structures of lithium transition metal composite oxides include layered structures belonging to space group R-3m and layered structures belonging to space group C2 / m. From the viewpoint of increasing battery capacity and ensuring crystal structure stability, it is preferable for lithium transition metal composite oxides to have a layered structure belonging to space group R-3m. The layered structure of lithium transition metal composite oxides includes, for example, a lithium layer and a transition metal layer. The charge and discharge reactions of the battery proceed as Li ions present in the lithium layer reversibly move in and out.

[0046] In this embodiment, the lithium transition metal composite oxide has a primary particle ratio (minor axis / major axis) of less than 0.5, where the proportion of primary particles (hereinafter sometimes referred to as "primary particle X") is 30% or more of the total number of primary particles. This stabilizes the crystal structure, enabling higher battery capacity and improved cycle characteristics. The proportion of primary particle X can be adjusted by the firing process conditions in the above-described method for manufacturing the positive electrode active material. By setting the firing conditions in the first and second firing processes to the above-described conditions, the proportion of primary particle X can be set to 30% or more. In other words, for example, if the first temperature in the first firing process is less than 470°C or more than 600°C, the synthesis reaction may become heterogeneous, and the proportion of primary particle X may fall below 30%. The upper limit of the proportion of primary particle X is not particularly limited, for example, 70%.

[0047] The proportion of primary particles X whose ratio of minor axis to major axis (minor axis / major axis) is less than 0.5 can be calculated as follows: (1) Expose the cross-section of the secondary particles. One method for exposing the cross-section is to embed the secondary particles in resin and process them with a cross-section polisher (e.g., JEOL Ltd., IB19520CCP) to expose the cross-section of the secondary particles. (2) Use a SEM (e.g., JEOL Ltd., JSM-7900F) to take a backscattered electron image of the exposed cross-section of the secondary particles. (3) Import the cross-sectional images obtained above into a computer and use image analysis software (e.g., National Institutes of Health, ImageJ) to calculate the ratio of minor axis to major axis (minor axis / major axis) of each primary particle. (4) From the above measurement results, calculate the content of primary particles X whose ratio of minor axis to major axis (minor axis / major axis) is less than 0.5 based on the following formula. (Proportion of primary particles X) = (Number of primary particles X) / (Total number of primary particles) × 100 (5) Perform the above measurement on five secondary particles contained in the same composite metal oxide, and the average value shall be taken as the proportion of primary particles X.

[0048] In lithium transition metal composite oxides, it is preferable that the proportion of primary particles (hereinafter sometimes referred to as "primary particle Y") whose major axis direction is oriented from the center outward within the secondary particles is 30% or more of the total number of primary particles. In this case, cracking of secondary particles is less likely to occur when charging and discharging is repeated, and the cycle characteristics can be further improved. The proportion of primary particle Y can be adjusted by the conditions of the firing process in the above-described method for manufacturing the positive electrode active material, and by setting the firing conditions in the first and second firing processes to the above-described conditions, the proportion of primary particle Y can be set to 30% or more. In other words, for example, if the first temperature in the first firing process is less than 470°C or more than 600°C, the synthesis reaction may become non-uniform, and the proportion of primary particle Y may be less than 30%. There is no particular upper limit to the proportion of primary particle Y, for example, 70%.

[0049] Figure 2 schematically shows the structure and arrangement of primary particles 31 inside secondary particles 30. Note that Figure 2 only shows a portion of the primary particles 31 inside secondary particles 30. Here, a primary particle whose major axis direction is oriented from the center outward from the secondary particle 30 means a primary particle in which, when the circumscribed circle X of the secondary particle 30 is defined, the inclination angle of the major axis direction of the primary particle 31 with respect to the radial direction of the circumscribed circle X is 30° or less, as shown in Figure 2.

[0050] The proportion of primary particles Y is obtained from the results of orientation analysis using electron beam backscatter diffraction (EBSD) (e.g., Velocity, EDAX) on the cross-section of secondary particles under the following conditions. The proportion of primary particles Y is measured for five secondary particles contained in the same lithium transition metal composite oxide, and the average value is used. Acceleration voltage: 10 kV, WD: 15 mm, Sample tilt: 70°, Orientation analysis: Inverse Pole, Figure Map, step: 0.05 μm

[0051] In lithium transition metal composite oxides, it is preferable that the relative standard deviation of the crystallite size of the primary particles is less than 0.003. In this case, cracking of secondary particles is less likely to occur when charging and discharging is repeated, and the cycle characteristics can be further improved. The relative standard deviation of the crystallite size can be adjusted by the conditions of the firing process in the above-described method for manufacturing the positive electrode active material, and by setting the firing conditions in the first and second firing processes to the above-described conditions, the relative standard deviation of the crystallite size can be made less than 0.003.

[0052] Here, the relative standard deviation of crystallite size (and the parameters described later) is calculated by dividing the positive electrode active material inside the firing container 40 used in the firing process into 27 sections in total: (1), (2), and (3) in the X direction, (A), (B), and (C) in the Y direction, and (a), (b), and (c) in the Z direction which is perpendicular to the X and Y directions, as shown in Figure 3, and measuring the crystallite size of the primary particles in each section. Alternatively, it may be calculated by extracting 50 arbitrary secondary particles from the positive electrode mixture layer and measuring the crystallite size of the primary particles contained in each secondary particle.

[0053] The crystallite size of the primary particles can be calculated by determining the full width at half maximum (FWHM) of the peaks attributed to each phase in the X-ray diffraction pattern obtained by the X-ray diffraction measurement described below, and then using Scherrer's equation. If there are multiple peaks attributed to each phase, the FWHM of the peak with the highest intensity is determined, and Scherrer's equation is applied to it. Scherrer's equation is expressed as follows: In the equation below, s is the crystallite size, λ is the wavelength of the X-ray, B is the FWHM of the diffraction peak, θ is the diffraction angle (rad), and K is Scherrer's constant. In this embodiment, K is set to 0.9. s = Kλ / Bcosθ

[0054] The X-ray diffraction pattern is obtained by powder X-ray diffraction using a powder X-ray diffractometer (manufactured by Rigaku Corporation, product name "RINT-TTR", source Cu-Kα) under the following conditions: Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-spline profile Function: Split-type pseudo-Voigt function Constraints: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the respective Ni content) ICSD No.: 98-009-4814

[0055] Furthermore, it is preferable that the lithium transition metal composite oxide has a relative standard deviation of less than 0.005 in the ratio of the number of moles of lithium to the total number of moles of transition metals. In this case, it becomes easier to achieve higher battery capacity. The relative standard deviation of the ratio of moles of lithium can be adjusted by the conditions of the firing process in the above-described method for manufacturing the positive electrode active material. By setting the firing conditions in the first and second firing processes to the above-described conditions, the relative standard deviation of the ratio of moles of lithium can be made less than 0.005. The ratio of lithium and transition metal contained in the lithium transition metal composite oxide can be measured by inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0056] Furthermore, it is preferable that the relative standard deviation of the proportion of lithium elements present in the lithium layer of the lithium transition metal composite oxide is less than 0.002. In this case, it becomes easier to achieve higher battery capacity. The relative standard deviation of the proportion of lithium elements present in the lithium layer can be adjusted by the conditions of the firing process in the above-described method for manufacturing the positive electrode active material. By setting the firing conditions in the first and second firing processes to the above-described conditions, the relative standard deviation of the proportion of lithium elements present in the lithium layer can be made less than 0.002. The proportion of lithium elements present in the lithium layer is obtained from the Rietveld analysis results of the X-ray diffraction pattern obtained by the above-described X-ray diffraction measurement. For example, the Rietveld analysis software PDXL2 (Rigaku Corporation) can be used for the Rietveld analysis of the X-ray diffraction pattern.

[0057] [Negative Electrode] The negative electrode 12 may, for example, have a negative electrode core and a negative electrode mixture layer formed on the surface of the negative electrode core, or a metallic Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode core, and lithium metal may be deposited on the surface of the negative electrode core by charging. When the negative electrode 12 has a negative electrode mixture layer, it is preferable that the negative electrode mixture layer is formed on both sides of the negative electrode core. For the negative electrode core, a foil of a metal that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal arranged on the surface layer, can be used. The thickness of the negative electrode core is, for example, 5 μm or more and 30 μm or less. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode core. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., to the surface of the negative electrode core, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0058] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si and Sn, or lithium titanium composite oxides may be used as the negative electrode active material. Furthermore, materials with a carbon coating may also be used. For example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0059] Examples of binders included in the negative electrode mixture layer include styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0060] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0061] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0062] [Non-aqueous electrolytes] Non-aqueous electrolytes are ionic conductive (for example, lithium ion conductive). Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0063] A liquid electrolyte (electrolyte solution) includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, 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 halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0064] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0065] Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methylphenyl ether. Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0066] The electrolyte salt is preferably a lithium salt. A suitable lithium salt is LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO4). ２ F ２Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. Of these, LiPF is used from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ It is preferable to use the following. The concentration of the lithium salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 1.8 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0067] Non-aqueous electrolytes may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0068] Examples of unsaturated cyclic carbonate esters include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. Unsaturated cyclic carbonate esters may be used individually or in combination of two or more. Some hydrogen atoms in the unsaturated cyclic carbonate esters may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by the intermolecular condensation of multiple carboxylic acid molecules, but it is preferable that it be an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0069] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0070] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanate methylcyclohexane (BIMCH). Examples of sultone compounds include propanesultone and propensultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethylborate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethylphosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethylphosphite and tris(trimethylsilyl)phosphite.

[0071] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0072] The present disclosure will be further explained below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples. <Example 1> [Preparation of positive electrode active material] Ni by coprecipitation method ０．９０ Co ０．０５ Mn ０．０５ (OH) ２A Ni-containing hydroxide represented by was obtained (crystallization step). The obtained Ni-containing hydroxide was heated at 400°C for 6 hours under an oxygen stream to obtain Ni as a transition metal oxide (Ni-containing oxide). ０．９０ Co ０．０５ Mn ０．０５ O ２ This was obtained (oxidation step). Subsequently, the transition metal oxide was mixed with lithium hydroxide (LiOH) so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the above transition metal oxide (Li / Me ratio) was 1.03. Furthermore, Nb as a metal inorganic compound was added so that the molar ratio of Nb to the total number of moles of metal elements (Me) contained in the above transition metal oxide was 0.25 mol%. ２ O ５ The ingredients were mixed to obtain a mixture (mixing step).

[0073] The obtained 6 kg mixture was packed into a firing vessel to a density of 0.55 kg / L. The mixture was then heated under an oxygen stream at a heating rate of 2.5°C / min from room temperature to 470°C (first temperature) and held for 3 hours (first firing step). Subsequently, the temperature was raised at a heating rate of 6°C / min to 755°C (second temperature) and held for 2 hours (second firing step).

[0074] Subsequently, the calcined product obtained in the second calcination step was added to water, and the mixture was washed with water at a stirring speed of 300 rpm for 10 minutes. The mixture was then dewatered using a filter press to obtain wet powder (washing step). The obtained wet powder was then dried under a vacuum atmosphere at 180°C for 2 hours to obtain the positive electrode active material (lithium transition metal composite oxide) of Example 1 (drying step).

[0075] [Preparation of the positive electrode] The above positive electrode active material, acetylene black (AB), and polyvinylidene fluoride were mixed in a mass ratio of 86:10:4, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode in which positive electrode slurry layers were arranged on both sides of the positive electrode core.

[0076] [Preparation of Non-Aqueous Electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF) was added to this mixed solvent. ６ A non-aqueous electrolyte was prepared by dissolving the substance to a concentration of 1.2 mol / liter.

[0077] [Preparation of Test Cell] A lithium metal foil was used as the negative electrode, and the positive and negative electrodes were arranged facing each other via a separator to form an electrode body. This electrode body and the non-aqueous electrolyte were placed in a coin-shaped outer casing, and the opening of the outer casing was sealed with a gasket and a sealing body to produce a test cell (non-aqueous electrolyte secondary battery).

[0078] [Evaluation of Capacity Retention Rate] Under an ambient temperature of 25°C, the cell was charged with a constant current at 0.3C to 4.2V, and then charged with a constant voltage at 4.2V until the voltage dropped to 0.01C. Afterward, it was discharged with a constant current at 0.2C to 2.5V, and the initial discharge capacity was measured. This charge-discharge cycle was considered one cycle, and 30 cycles were performed. The capacity retention rate of the test cell in the charge-discharge cycle was calculated using the following formula: Capacity Retention Rate [%] = (Discharge Capacity at Cycle 30 / Initial Discharge Capacity) × 100

[0079] <Example 2> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 600°C (first temperature) under an oxygen stream and held for 3 hours.

[0080] <Example 3> A test cell was fabricated and evaluated in the same manner as in Example 1, except that the positive electrode active material was fabricated as follows. (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed such that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 550 °C (the first temperature) under an oxygen stream and held for 4 hours. (3) In the second firing step, the temperature was raised to 770 °C (the second temperature) at a rate of 10 °C / min and held for 2 hours.

[0081] <Example 4> A test cell was fabricated and evaluated in the same manner as in Example 1, except that the positive electrode active material was fabricated as follows. (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed such that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 550 °C (the first temperature) under an oxygen stream and held for 3 hours. (3) In the second firing step, the temperature was raised to 755 °C (the second temperature) at a rate of 6 °C / min and held for 3 hours.

[0082] <Example 5> A test cell was fabricated and evaluated in the same manner as in Example 1, except that the positive electrode active material was fabricated as follows. (1) A composite hydroxide represented by ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ (OH) ２ was heated at 400 °C for 6 hours to obtain a transition metal oxide of Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ O ２(2) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.07. (3) In the mixing step, Nb was mixed so that the molar ratio of Nb to the total number of moles of metal elements (Me) contained in the transition metal oxide was 0.45 mol%. ２ O ５ (4) In the first firing process, the mixture was heated from room temperature to 550°C (first temperature) under an oxygen stream and held for 3 hours. (5) In the second firing process, the mixture was heated to 755°C (second temperature) at a heating rate of 6°C / min and held for 3 hours.

[0083] <Example 6> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ (OH) ２ The composite hydroxide represented by was heated at 400°C for 6 hours, and Ni as a transition metal oxide ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ O ２ (2) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.04. (3) In the first firing step, the mixture was heated from room temperature to 550°C (first temperature) under an oxygen stream and held for 3 hours. (4) In the second firing step, the mixture was heated to 755°C (second temperature) at a heating rate of 6°C / min and held for 4 hours.

[0084] <Example 7> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ (OH) ２ The composite hydroxide represented by was heated at 400°C for 6 hours, and Ni as a transition metal oxide０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ O ２ (2) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (3) In the mixing step, Nb was mixed so that the molar ratio of Nb to the total number of moles of metal elements (Me) contained in the transition metal oxide was 0.45 mol%. ２ O ５ (4) In the first firing process, the mixture was heated from room temperature to 550°C (first temperature) under an oxygen stream and held for 5 hours. (5) In the second firing process, the mixture was heated to 755°C (second temperature) at a heating rate of 6°C / min and held for 5 hours.

[0085] <Comparative Example 1> A test cell was prepared and evaluated in the same manner as in Example 1, except that the first firing step was omitted, and in the second firing step, the temperature was raised from room temperature to 755°C (second temperature) at a heating rate of 6°C / min and held for 2 hours.

[0086] <Comparative Example 2> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 550°C (first temperature) under an oxygen stream and held for 1 hour. (3) In the second firing step, the temperature was raised to 755°C (second temperature) at a heating rate of 6°C / min and held for 3 hours.

[0087] <Comparative Example 3> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ (OH) ２ The composite hydroxide represented by was heated at 400°C for 6 hours, and Ni as a transition metal oxide０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ O ２ (2) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (3) In the first firing step, the mixture was heated from room temperature to 550°C (first temperature) under an oxygen stream and held for 2 hours. (4) In the second firing step, the mixture was heated to 755°C (second temperature) at a heating rate of 1°C / min and held for 2 hours.

[0088] <Comparative Example 4> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 550°C (first temperature) under an oxygen stream and held for 3 hours. (3) In the second firing step, the temperature was raised to 755°C (second temperature) at a heating rate of 1°C / min and held for 2 hours.

[0089] <Comparative Example 5> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ (OH) ２ The composite hydroxide represented by was heated at 400°C for 6 hours, and Ni as a transition metal oxide ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ O ２(2) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (3) In the first firing step, the mixture was heated from room temperature to 550°C (first temperature) under an oxygen stream and held for 3 hours. (4) In the second firing step, the mixture was heated to 755°C (second temperature) at a heating rate of 2°C / min and held for 2 hours.

[0090] <Comparative Example 6> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 450°C (first temperature) under an oxygen stream and held for 3 hours.

[0091] <Comparative Example 7> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 300°C (first temperature) under an oxygen stream and held for 3 hours.

[0092] <Comparative Example 8> A test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the above transition metal oxide and lithium hydroxide (LiOH) were mixed so that the ratio of the number of moles of Li to the total number of moles of metal elements (Me) contained in the transition metal oxide (Li / Me ratio) was 1.05. (2) In the first firing step, the mixture was heated from room temperature to 700°C (first temperature) under an oxygen stream and held for 3 hours.

[0093] Table 1 shows the conditions for preparing the positive electrode active materials for Examples 1-7 and Comparative Examples 1-8, and Table 2 shows the measurement results of the capacity retention rate of the test cells for Examples 1-7 and Comparative Examples 1-8. Table 2 also shows the crystallite size of the primary particles, the relative standard deviation of the crystallite size, the relative standard deviation of the ratio of moles of lithium to the total number of moles of transition metals, and the relative standard deviation of the proportion of lithium elements present in the lithium layer for the positive electrode active materials of Examples 1-7 and Comparative Examples 1-8. The relative standard deviation of each parameter was calculated by dividing the inside of the firing container into 27 sections and measuring the values ​​in each region, as described above. Table 2 also shows, for the positive electrode active materials of Examples 1 and 4, and Comparative Examples 1 and 6, the proportion of primary particles with a ratio of minor axis to major axis (minor axis / major axis) of less than 0.5, measured using the method described above, and the proportion of primary particles whose major axis direction is oriented from the center of the secondary particle towards the outer circumference.

[0094]

[0095]

[0096] As shown in Table 2, the capacity retention rates of the test cells in the examples were all improved compared to the capacity retention rates of the test cells in the comparative examples. On the other hand, the test cell of Comparative Example 2, in which the holding time in the first firing process was less than 2 hours, did not show sufficient improvement in capacity retention even after two-stage firing. This is presumed to be because the short holding time created a temperature gradient inside the firing container, causing the synthesis reaction to proceed unevenly. Similarly, the test cells of Comparative Examples 3 to 5, in which the heating rate in the second firing process was less than 5°C / min, also did not show sufficient improvement in capacity retention. This is presumed to be because the slow heating rate made it easier for crystallites to grow anisotropically, resulting in particle cracking and other damage when charging and discharging were repeated. Furthermore, the test cells of Comparative Examples 6 and 7, in which the heating temperature in the first firing process was less than 470°C, also did not show sufficient improvement in capacity retention. This is presumed to be because the lithium hydroxide did not melt sufficiently, causing the synthesis reaction to proceed unevenly. Finally, the test cell of Comparative Example 8, in which the heating temperature in the first firing process exceeded 600°C, also did not show sufficient improvement in capacity retention. This is presumed to be because crystal growth was promoted before the synthesis reaction, causing the synthesis reaction to proceed unevenly.

[0097] This disclosure is further illustrated by the following embodiments. Configuration 1: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a mixing step of mixing a transition metal oxide containing Ni with lithium hydroxide to obtain a mixture, and a firing step of firing the mixture, wherein the firing step comprises a first firing step of raising the temperature of the mixture from room temperature to a first temperature of 470°C or higher and 600°C or lower, and holding it at the first temperature for 2 hours or more and 10 hours or less, and a second firing step of raising the temperature of the mixture to a second temperature of 700°C or higher and 900°C or lower at a heating rate of 5°C / min or higher and 15°C / min or less, and holding it at the second temperature for 1 hour or more and 10 hours or less, the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery. Configuration 2: The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein in the mixing step, the ratio of the number of moles of lithium contained in the lithium hydroxide to the total number of moles of metal elements contained in the transition metal oxide is 0.9 or more and 1.2 or less. Configuration 3: The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein in the mixing step, the ratio of the number of moles of lithium contained in the lithium hydroxide to the total number of moles of metal elements contained in the transition metal oxide is 0.95 or more and 1.08 or less. Configuration 4: The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein in the mixing step, a metal-inorganic compound is further added. Configuration 5: The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 4, wherein the metal-inorganic compound contains at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Ca, Sr, Ba, B, Al, Ga, and In. Configuration 6: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 4 or 5, wherein the metal-inorganic compound comprises at least one element selected from the group consisting of Nb, Ca, Sr, and Al. Configuration 7: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the first temperature is 470°C or higher and 550°C or lower. Configuration 8: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein in the first firing step, the material is held at the first temperature for 3 hours or more and 5 hours or less.Configuration 9: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein in the second firing step, the temperature is raised to the second temperature at a heating rate of 6°C / min or more and 10°C / min or less. Configuration 10: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 9, wherein the second temperature is 700°C or more and 800°C or less. Configuration 11: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 10, wherein in the second firing step, the mixture is held at the second temperature for 2 hours or more and 5 hours or less. Configuration 12: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 11, wherein in the firing step, 4 kg or more and less than 20 kg of the mixture is filled into a firing container at a density of 0.5 kg / L or more and firing is performed. Configuration 13: A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising secondary particles formed by the aggregation of primary particles, wherein the proportion of primary particles in which the ratio of the minor axis to the major axis (minor axis / major axis) in the primary particles is less than 0.5 is 30% or more of the total number of primary particles. Configuration 14: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 13, wherein, within the secondary particles, the proportion of primary particles in which the major axis direction of the primary particles is oriented from the center of the secondary particle toward the outer circumference is 30% or more of the total number of primary particles. Configuration 15: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 13 or 14, wherein the relative standard deviation of the crystallite size of the positive electrode active material is less than 0.003. Configuration 16: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 13 to 15, having a layered structure, wherein the relative standard deviation of the proportion of lithium elements present in the lithium layer is less than 0.002. Configuration 17: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for non-aqueous electrolyte secondary batteries described in any one of Configurations 13 to 16, a negative electrode, and a non-aqueous electrolyte.

[0098] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Secondary particles, 31 Primary particles

Claims

1. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: a mixing step of mixing a transition metal oxide containing Ni with lithium hydroxide to obtain a mixture; and a firing step of firing the mixture, wherein the firing step comprises: a first firing step of raising the temperature of the mixture from room temperature to a first temperature of 470°C or higher and 600°C or lower, and holding it at the first temperature for 2 hours or more and 10 hours or less; and a second firing step of raising the temperature of the mixture to a second temperature of 700°C or higher and 900°C or lower at a heating rate of 5°C / min or higher and 15°C / min or lower, and holding it at the second temperature for 1 hour or more and 10 hours or less.

2. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein in the mixing step, the ratio of the number of moles of lithium contained in the lithium hydroxide to the total number of moles of metal elements contained in the transition metal oxide is 0.9 or more and 1.2 or less.

3. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein in the mixing step, the ratio of the number of moles of lithium contained in the lithium hydroxide to the total number of moles of metal elements contained in the transition metal oxide is 0.95 or more and 1.08 or less.

4. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein a metal inorganic compound is further added in the mixing step.

5. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, wherein the metal-inorganic compound comprises at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mg, Ca, Sr, Ba, B, Al, Ga, and In.

6. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, wherein the metal-inorganic compound comprises at least one element selected from the group consisting of Nb, Ca, Sr, and Al.

7. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the first temperature is 470°C or higher and 550°C or lower.

8. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein in the first firing step, the material is held at the first temperature for 3 hours or more and 5 hours or less.

9. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein in the second firing step, the temperature is raised to the second temperature at a heating rate of 6°C / min or more and 10°C / min or less.

10. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the second temperature is 700°C or higher and 800°C or lower.

11. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein in the second firing step, the material is held at the second temperature for two hours or more and five hours or less.

12. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein in the firing step, 4 kg or more and less than 20 kg of the mixture is filled into a firing container at a density of 0.5 kg / L or more and fired.

13. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising secondary particles formed by the aggregation of primary particles, wherein the proportion of primary particles in which the ratio of the minor axis to the major axis (minor axis / major axis) is less than 0.5 is 30% or more of the total number of primary particles.

14. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 13, wherein, within the secondary particles, the proportion of primary particles whose major axis direction is oriented from the center of the secondary particle toward the outer circumference is 30% or more of the total number of primary particles.

15. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 13, wherein the relative standard deviation of the crystallite size of the positive electrode active material is less than 0.

003.

16. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 13, having a layered structure, wherein the relative standard deviation of the proportion of lithium elements present in the lithium layer is less than 0.

002.

17. A non-aqueous electrolyte secondary battery comprising a positive electrode containing a positive electrode active material for a non-aqueous electrolyte secondary battery as described in any one of claims 13 to 16, a negative electrode, and a non-aqueous electrolyte.

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