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

Abstract

Provided is a positive electrode active material that is for a non-aqueous electrolyte secondary battery and that comprises a lithium transition metal composite oxide having a layered structure, said positive electrode active material being characterized in that: the lithium transition metal composite oxide is secondary particles formed by agglomeration of primary particles and is represented by the general formula LixNiaCobMncMdO2-y (in the formula, 0.95<x<1.10, 0.65≤a≤0.95, 0≤b≤0.15, 0≤c≤0.25, 0≤d≤0.10, 0≤y<0.05, a+b+c+d=1, and M is at least one element selected from the group consisting of W, Mo, Si, and Al); a phosphorus-containing compound exists on the surface or in the inside of the secondary particles; and the Gini coefficient of phosphorus (P) inside the secondary particles is not more than 0.6.

Description

Positive electrode active material for non-aqueous electrolyte secondary battery, non-aqueous electrolyte secondary battery, and method for manufacturing positive electrode active material for non-aqueous electrolyte secondary battery

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

[0002] In recent years, the applications of non-aqueous electrolyte secondary batteries have been expanding to power sources for electric vehicles and energy storage devices for utilizing natural energy. Since the positive electrode greatly affects battery characteristics including battery capacity, output characteristics, cycle characteristics, etc., many studies have been conducted on the positive electrode. For example, in Patent Document 1, a positive electrode active material having at least one additive selected from Li ３ BO ３ 、Li ３ PO ４ and Li ２ SO ４ is disclosed.

[0003] International Publication No. 2022 / 137583

[0004] In recent years, lithium transition metal composite oxides with a high Ni content have attracted attention as positive electrode active materials for realizing high battery capacity. However, when a lithium transition metal composite oxide with a high Ni content is used as a positive electrode active material, a large amount of highly reactive Ni ４＋ exists on the surface of the composite oxide particles in a state where the state of charge (SOC) is high, so side reactions between the lithium transition metal composite oxide and the non-aqueous electrolyte are likely to occur. And hydrogen fluoride generated by the side reaction reacts with the lithium transition metal composite oxide, and the product accumulates on the surface of the lithium transition metal composite oxide or the transition metal elutes from the lithium transition metal composite oxide. As a result, the reaction resistance of the battery may increase when charge and discharge are repeated.

[0005] A positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of the present disclosure, is a positive electrode active material for a non-aqueous electrolyte secondary battery containing a lithium transition metal composite oxide having a layered structure, wherein the lithium transition metal composite oxide is secondary particles formed by aggregation of primary particles, and has a general formula Li ｘ Ni ａ Co ｂ Mnｃ M ｄ O ２－ｙ (wherein the formula is expressed as 0.95≦x≦1.15, 0.65≦a≦0.95, 0≦b≦0.15, 0≦c≦0.25, 0≦d≦0.10, 0≦y<0.05, a+b+c+d=1, and M is at least one element selected from the group consisting of Zr, Nb, Ti, W, Mo, Si, and Al), and a phosphorus-containing compound is present on the surface and inside the secondary particle, and the Gini coefficient of phosphorus (P) inside the secondary particle is 0.6 or less.

[0006] One embodiment of the present disclosure is a non-aqueous electrolyte secondary battery, characterized by comprising a positive electrode containing the above-mentioned positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0007] A method for producing a positive electrode for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, comprises a mixing step of mixing a Ni metal compound containing at least Ni with a Li compound to obtain a mixture; a firing step of firing the mixture to obtain a fired product; a washing step of washing the fired product with water; and a dehydration step of dehydrating the product after the washing step to obtain a cake-like composition, further comprising a first adding step of adding a phosphorus-containing compound to at least one of the dispersion liquid into which the fired product in the washing step was added and the cake-like composition obtained in the dehydration step; and a second adding step of adding a boron-containing compound to at least one of the cake-like composition obtained in the dehydration step and the powder obtained by drying the cake-like composition, and then performing a heat treatment.

[0008] According to a positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, it is possible to secure the initial capacity while suppressing the increase in the battery's reaction resistance when it undergoes repeated charging and discharging.

[0009] This is a longitudinal cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment.

[0010] As described above, lithium transition metal composite oxides with a high Ni content are prone to side reactions with non-aqueous electrolytes, especially when the state of coolant (SOC) is high. Therefore, when such composite oxides are used as positive electrode active materials, hydrogen fluoride generated by side reactions reacts with the lithium transition metal composite oxide, causing products to accumulate on the surface of the lithium transition metal composite oxide or the transition metal to leach from the lithium transition metal composite oxide. As a result, the reaction resistance of the battery may increase after repeated charging and discharging.

[0011] As a result of our investigations, we have found that by placing a phosphorus-containing compound on the surface and inside the secondary particles of the lithium transition metal composite oxide used as the positive electrode active material, and by keeping the Gini coefficient of phosphorus (P) inside the secondary particles to 0.6 or less, it is possible to secure the initial capacity while suppressing the increase in the battery's reaction resistance when repeated charging and discharging. As will be described in detail later, the Gini coefficient is an index that indicates the degree of dispersion of the target element, and a smaller value means that it is uniformly dispersed inside the secondary particles. Furthermore, the phosphorus-containing compound preferentially reacts with hydrogen fluoride generated by side reactions. Therefore, by having the phosphorus-containing compound in an appropriate dispersion state, the lithium transition metal composite oxide can be protected from hydrogen fluoride. As a result, the reaction between the lithium transition metal composite oxide and hydrogen fluoride is suppressed, and the increase in the battery's reaction resistance when repeated charging and discharging can be suppressed.

[0012] In the following, 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 battery outer casing is not limited to a cylindrical shape. 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. In addition, the design of the non-aqueous electrolyte secondary battery according to this disclosure is not limited to the example non-aqueous electrolyte secondary battery design, and known non-aqueous electrolyte secondary battery designs may be applied.

[0013] Figure 1 is an axial cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. 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. In the following, for convenience of explanation, the side of the battery with the sealing body 17 will be referred to as the top, and the bottom side of the outer casing 16 as the bottom.

[0014] 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 than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal and transverse directions. The two separators 13 are formed to be at least slightly larger 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] [Positive Electrode] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. Preferably, the positive electrode mixture layer is formed on both sides of the positive electrode current collector. The positive electrode current collector 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 current collector is, for example, 10 μm or more and 30 μm or less.

[0020] 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 current collector. 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 current collector, drying the coating film, and then rolling it to form the positive electrode mixture layer on both sides of the positive electrode current collector.

[0021] Examples of conductive agents included in the positive electrode mixture layer include acetylene black (AB), carbon black (CB) such as Ketjenblack, carbon nanotubes (CNT), graphene, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types. The content of the conductive agent in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less, relative to the total mass of the positive electrode mixture layer.

[0022] 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. The binder content in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less, relative to the total mass of the positive electrode mixture layer.

[0023] The positive electrode composite layer contains a layered lithium transition metal composite oxide as the positive electrode active material. The lithium transition metal composite oxide is composed of secondary particles formed by the aggregation of primary particles. The particle size of the primary particles constituting the secondary particles of the lithium transition metal composite oxide is, for example, 0.02 μm or more and 2 μm or less. 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). The average particle diameter of the secondary particles of the lithium transition metal composite oxide is, for example, 2 μm or more and 30 μm or less. Here, the average particle diameter means the volume-based median diameter (D50). D50 means 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 the secondary particles of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (for example, Microtrac-Bell Co., Ltd., MT3000II) with water as the dispersion medium.

[0024] Examples of layered structures for 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 capacity and stabilizing the crystal structure, lithium transition metal composite oxides are preferably found to have a layered structure belonging to space group R-3m.

[0025] Lithium transition metal composite oxides have the general formula Li ｘ Ni ａ Co ｂ Mn ｃ M ｄ O ２－ｙ (The formula is expressed as follows: 0.95 < x < 1.10, 0.65 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.15, 0 ≤ c ≤ 0.25, 0 ≤ d ≤ 0.10, 0 ≤ y < 0.05, a + b + c + d = 1, and M is at least one element selected from the group consisting of Zr, Nb, Ti, W, Mo, Si, and Al). The content of the elements constituting the lithium transition metal composite oxide can be measured by inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0026] The Ni content in the lithium transition metal composite oxide is 65 mol% or more and 95 mol% or less, and Ni is an essential component. The higher the Ni content, the higher the capacity battery that can be obtained. Furthermore, from the viewpoint of achieving higher battery capacity, the Ni content in the lithium transition metal composite oxide is preferably 70 mol% or more and 95 mol% or less, more preferably 75 mol% or more and 95 mol% or less, and even more preferably 80 mol% or more and 95 mol% or less. Also, as described above, the higher the Ni content, the more likely side reactions between the lithium transition metal composite oxide and the non-aqueous electrolyte are to occur. Therefore, the higher the Ni content, the more pronounced the effects of this disclosure will be. Note that the charging capacity and discharge capacity can be increased by increasing the charging voltage regardless of the amount of Ni. However, as the voltage increases, side reactions such as oxidative decomposition of the non-aqueous electrolyte are more likely to occur, so the effects of this disclosure can be pronounced.

[0027] The Co content in lithium transition metal composite oxides is between 0 mol% and 15 mol%, and Co is an optional component. In other words, lithium transition metal composite oxides do not need to contain Co. By containing Co, lithium transition metal composite oxides can, for example, improve the heat resistance of batteries.

[0028] The Mn content in lithium transition metal composite oxides is between 0 mol% and 25 mol%, and Mn is an optional component. In other words, lithium transition metal composite oxides do not need to contain Mn. By containing Mn, lithium transition metal composite oxides can, for example, stabilize their crystal structure.

[0029] The content of M (where M is at least one element selected from the group consisting of Zr, Nb, Ti, W, Mo, Si, and Al) in lithium transition metal composite oxides is between 0 mol% and 10 mol%, and M is an optional component. In other words, lithium transition metal composite oxides do not need to contain M. By containing M, lithium transition metal composite oxides can, for example, stabilize their crystal structure. As a result, higher battery capacity can be achieved.

[0030] As described above, phosphorus-containing compounds are present at the primary particle grain boundaries of lithium transition metal composite oxides. Furthermore, phosphorus-containing compounds may be present not only at the primary particle grain boundaries but also on the surface of secondary particles. For example, the phosphorus-containing compounds exist particulately or in layers on the surface and within the secondary particles, and are not solid-dissolved within the primary particles. The phosphorus-containing compounds preferentially react with hydrogen fluoride generated by the side reaction between the lithium transition metal composite oxide and the non-aqueous electrolyte. Therefore, by presenting phosphorus-containing compounds at the primary particle grain boundaries, the lithium transition metal composite oxide can be protected from hydrogen fluoride. As a result, for example, the leaching of transition metals from the lithium transition metal composite oxide is suppressed, and the increase in the battery's reaction resistance during repeated charging and discharging can be suppressed. Additionally, by presenting phosphorus-containing compounds within the secondary particles, the lithium transition metal composite oxide can be protected from hydrogen fluoride even if repeated charging and discharging causes the secondary particles to crack and expose new cross-sections. The presence of phosphorus-containing compounds can be confirmed by synchrotron XRD measurements, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), etc.

[0031] Here, in the elemental concentration distribution of the cross-section of the lithium transition metal composite oxide using time-of-flight secondary ion mass spectrometry (TOF-SIMS), the Gini coefficient of phosphorus (P) inside the secondary particle is 0.6 or less. The Gini coefficient of phosphorus (P) inside the secondary particle is defined as the normalized intensity I of phosphorus (P) inside the secondary particle. Ｐ＿ＩＮ This value is twice the area enclosed by the diagonal and the Lorentz curve when the cumulative rate is expressed in order of intensity. The smaller the Gini coefficient of phosphorus (P) inside the secondary particle, the more uniformly the phosphorus-containing compound is dispersed inside the secondary particle (at the grain boundary of the primary particle). The minimum value of the Gini coefficient of phosphorus (P) inside the secondary particle is 0. In this specification, the Gini coefficient of phosphorus (P) inside the secondary particle refers to the value obtained by arbitrarily selecting 50 secondary particles, measuring the Gini coefficient of phosphorus (P) inside the secondary particle using the method described below, and taking the arithmetic mean of the measured values.

[0032] Normalized intensity I of phosphorus (P) inside secondary particles Ｐ＿ＩＮThis is obtained by measurement using a time-of-flight secondary ion mass spectrometer (TOF-SIMS5, IONTOF Corporation) under the following conditions: Primary ion: Bi ３＋ Ion voltage: 30 kV; Ion current: 0.03 pA @ 100 us; Observation range: 50 μm × 50 μm; Mass range: 60 us (~310 amu); Detection: 4 frames / scan, 150 scans

[0033] The image showing the concentration distribution of Ni and phosphorus (P) obtained from the above measurements is divided into 256 x 256 pixels, and the detection intensity of Ni and phosphorus (P) is calculated for each pixel. Furthermore, the ratio of the detection intensity of phosphorus (P) to the detection intensity of Ni is used to determine the normalized intensity of phosphorus (P). Ｐ It is calculated as follows.

[0034] The area from the surface of the secondary particle to 0.5 μm inward from the surface of the secondary particle recognized in the image above is defined as the surface of the secondary particle, and the area inside the surface of the secondary particle is defined as the interior of the secondary particle. Then, the pixels contained within this interior of the secondary particle are defined, and the corresponding I for each pixel is defined. Ｐ The set is I Ｐ＿ＩＮ This is the result. The sample used for cross-sectional observation may be a sample in which lithium transition metal composite oxide is embedded in resin or the like, or a positive electrode mixture layer containing lithium transition metal composite oxide.

[0035] The Gini coefficient of phosphorus (P) within the secondary particles is preferably 0.5 or less, and more preferably 0.4 or less. In this case, the phosphorus-containing compound is more uniformly dispersed within the secondary particles, thus better protecting the lithium transition metal composite oxide from hydrogen fluoride. As a result, even after repeated charging and discharging, the leaching of transition metals from the lithium transition metal composite oxide is suppressed, and the increase in the battery's reaction resistance can be suppressed.

[0036] The phosphorus-containing compound content is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, and even more preferably 0.01% by mass or more, based on the amount of phosphorus element of the lithium transition metal composite oxide. When the phosphorus-containing compound content is 0.001% by mass or more, based on the amount of phosphorus element of the lithium transition metal composite oxide, a sufficient amount of the phosphorus-containing compound can be present on the surface and inside the secondary particles. As a result, the phosphorus-containing compound protects the lithium transition metal composite oxide from hydrogen fluoride, and the increase in the reaction resistance of the battery can be suppressed.

[0037] The phosphorus-containing compound content is preferably 0.4% by mass or less, more preferably 0.2% by mass or less, and even more preferably 0.05% by mass or less, based on the amount of phosphorus element of the lithium transition metal composite oxide. If the phosphorus-containing compound content exceeds 0.4% by mass of the lithium transition metal composite oxide based on the amount of phosphorus element, the resistance of the lithium transition metal composite oxide may actually increase, making it difficult to improve the charge-discharge cycle characteristics. Therefore, the phosphorus-containing compound content is preferably 0.001% by mass or more and 0.4% by mass or less, more preferably 0.005% by mass or more and 0.2% by mass or less, and even more preferably 0.01% by mass or more and 0.05% by mass or less, based on the amount of phosphorus element of the lithium transition metal composite oxide. Furthermore, the phosphorus-containing compound content may be 0.001% by mass or more and 0.2% by mass or less, 0.001% by mass or more and 0.05% by mass or less, 0.005% by mass or more and 0.4% by mass or less, 0.005% by mass or more and 0.05% by mass or less, 0.01% by mass or more and 0.4% by mass or less, or 0.01% by mass or more and 0.2% by mass or less, based on the amount of phosphorus element. The phosphorus-containing compound content can be measured by inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0038] A phosphorus-containing compound can be any compound containing phosphorus (P), for example, a phosphate or phosphorus oxide containing at least one element selected from the group consisting of Li, Ca, and Sr. Specifically, a phosphorus-containing compound is Li ３ PO ４ Li ２ HPO ４ LiH ２ PO ４ LiFePO ４ Ca(H ２ PO ４ ) ２ Ca(H ２ PO ４ ) ２ ・H ２ O, CaHPO ４ CaHPO ４ (H ２ O), Ca ３ (PO ４ ) ２ , Sr(H ２ PO ４ ) ２ SrHPO ４ , Sr ３ (PO ４ ) ２ , P ４ O １０ , P ４ O ６ These are some examples. These compounds may exist individually or in combination of two or more types.

[0039] It is preferable that a boron-containing compound be present on at least one of the surface and interior of the secondary particles of the lithium transition metal composite oxide. The boron-containing compound may be present, for example, in particulate or layered form. The presence of the boron-containing compound stabilizes the crystal structure of the lithium transition metal composite oxide, enabling lower resistance and higher capacity of the battery.

[0040] Furthermore, when phosphorus-containing compounds and boron-containing compounds are present together, the phosphorus-containing compounds suppress the side reactions between the boron-containing compounds and non-aqueous electrolytes, thereby suppressing the elution of the boron-containing compounds. Therefore, by presenting phosphorus-containing compounds and boron-containing compounds together, the boron-containing compounds remain on the particle surface even after repeated charging and discharging. As a result, even after repeated charging and discharging, the battery capacity is maintained while further suppressing the increase in the battery's reaction resistance. The presence of boron-containing compounds can be confirmed in the same way as phosphorus-containing compounds by synchrotron XRD measurement, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), etc.

[0041] The content of the boron-containing compound is preferably 0.0015% by mass or more, more preferably 0.005% by mass or more, and even more preferably 0.01% by mass or more, based on the amount of boron element. When the content of the boron-containing compound is 0.0015% by mass or more, a sufficient amount of the boron-containing compound can be present on at least one of the surface and interior of the secondary particles. As a result, it is possible to achieve lower resistance and higher capacity in the battery.

[0042] The content of the boron-containing compound is preferably 1.0% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.2% by mass or less, based on the mass of the lithium transition metal composite oxide in terms of boron element. If the content of the boron-containing compound exceeds 1.0% by mass of the mass of the lithium transition metal composite oxide in terms of boron element, the resistance of the lithium transition metal composite oxide may actually increase. Therefore, the content of the boron-containing compound is preferably 0.0015% by mass or more and 1.0% by mass or less, more preferably 0.005% by mass or more and 0.5% by mass or less, and even more preferably 0.01% by mass or more and 0.2% by mass or less, based on the mass of the lithium transition metal composite oxide in terms of boron element. Furthermore, the content of the boron-containing compound may be 0.0015% by mass or more and 0.5% by mass or less, 0.0015% by mass or more and 0.2% by mass or less, 0.005% by mass or more and 1.0% by mass or less, 0.005% by mass or more and 0.2% by mass or less, 0.01% by mass or more and 1.0% by mass or less, or 0.01% by mass or more and 0.5% by mass or less, based on the mass of the lithium transition metal composite oxide in terms of boron element. The content of the boron-containing compound can be measured by inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0043] A boron-containing compound can be any compound containing boron (B), for example, boric acid (H ３ BO ３ ), boron oxide (B ２ O ３ ), lithium borate (Li ２ B ４ O ７ Li ３ BO ３ LiB ３ O ５ LiBO ２ These compounds may exist individually or in combination of two or more types.

[0044] The positive electrode mixture layer may contain positive electrode active materials other than lithium transition metal composite oxides (hereinafter sometimes referred to as lithium composite oxide (Z)) in which phosphorus-containing compounds are present on the surface and inside the secondary particles. In the positive electrode mixture layer, the content of lithium composite oxide (Z) is, for example, 90% by mass or more of the total mass of the positive electrode active materials. Alternatively, the positive electrode mixture layer may contain only lithium composite oxide (Z) as the positive electrode active material.

[0045] A positive electrode active material, which is an example of an embodiment, can be manufactured by the following method. Note that the manufacturing method described here is just one example, and the method for manufacturing the positive electrode active material is not limited to this method.

[0046] The manufacturing process for the positive electrode active material includes a mixing step of mixing a Ni metal compound containing at least Ni with a Li compound to obtain a mixture, a firing step of firing the mixture to obtain a fired product, a washing step of washing the fired product with water, and a dehydration step of dehydrating after the washing step to obtain a cake-like composition. Furthermore, the manufacturing process for the positive electrode active material also includes a first addition step of adding a phosphorus-containing compound at a predetermined timing.

[0047] In the mixing step, for example, a Ni metal compound containing 65 mol% to 95 mol% Ni, 0 mol% to 15 mol% Co, and 0 mol% to 25 mol% Mn is mixed with a Li compound to obtain a mixture.

[0048] Examples of Ni metal compounds include Ni-containing metal hydroxides, Ni-containing metal oxides, and Ni-containing metal carbonate compounds. Ni metal compounds can be obtained, for example, by stirring a solution of a metal salt containing Ni and an arbitrary metal element (Co, Mn, etc.), adding an alkaline solution such as sodium hydroxide 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 containing Ni and an arbitrary metal element, and then heat-treating the composite hydroxide. The heat treatment temperature is not particularly limited, but for example, it is in the range of 250°C or higher and 600°C or lower.

[0049] Examples of Li compounds include Li ２ CO ３, LiOH, Li ２ O ２ , Li ２ O, LiNO ３ , LiNO ２ , Li ２ SO ４ , LiOH·H ２ O, LiH, LiF, etc. may be mentioned. The mixing ratio of the Ni metal compound and the Li compound is preferably, for example, such that the molar ratio of the total amount of the metal elements in the Ni metal compound to Li is in the range of 1:0.8 or more and 1:1.2 or less, and more preferably in the range of 1:1.0 or more and 1:1.1 or less.

[0050] In the mixing step, a compound containing an element M (M is at least one element selected from the group consisting of Zr, Nb, Ti, W, Mo, Si, and Al) may be further mixed. The compound containing the element M is a compound containing at least one of a phosphate, a sulfate, an oxide, a hydroxide, a sulfide, and a chloride containing the element M. An example of the compound is ZrO ２ , ZrSiO ４ , Zr(SO ４ ), ２ , Zr(WO ４ ), ２ , ZrCl ３ , Nb ２ O ５ , Nb ２ O ５・ nH ２ O, TiO ２ , WO ３ , WS ２ , Al ２ (WO ４ ), ３ , MoO ２ , MoO ３ , MoS ２ , Al ２ O ３ , Al(OH) ３ , NaAl(OH) ４ , AlPO ４ etc. may be mentioned. These compounds may be used alone or in combination of two or more.

[0051] The firing process is a multi-stage firing process that includes, for example, a first firing process in which the product is fired at a temperature of 300°C or higher and 680°C or lower under an oxygen stream, and a second firing process in which the product obtained in the first firing process is fired at a temperature exceeding 680°C under an oxygen stream. In the first firing process, the temperature is raised to a first set temperature of 680°C or lower at a first heating rate of 0.2°C / min or higher and 4.5°C / min or lower. In the second firing process, the temperature is raised to a second set temperature of 900°C or lower at a rate of 0.5°C / min or higher and 3.5°C / min or lower. Note that the first and second heating rates may be set to multiple values ​​for each temperature range, as long as they are within the ranges specified above.

[0052] The holding time at the first set temperature in the first firing process is preferably 5 hours or less, and more preferably 3 hours or less. The holding time at the first set temperature is the time after the first set temperature is reached and maintained at that temperature; however, the holding time may be zero. The holding time at the second set temperature in the second firing process is preferably 1 hour or more and 10 hours or less, and more preferably 1 hour or more and 5 hours or less. The holding time at the second set temperature is the time after the second set temperature is reached and maintained at that temperature. The firing of the mixture is carried out, for example, in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream is set to 10 cm in the firing furnace. ３ The amount per unit area shall be 0.2 mL / min or more, 4 mL / min or less, or 0.3 L / min or more per 1 kg of the mixture.

[0053] In the washing step, the calcined product (lithium transition metal composite oxide) obtained in the calcination step is washed with water. Washing removes unreacted Li compounds added in the mixing step, as well as impurities other than Li compounds. The washing step is carried out, for example, using a 3 L reaction vessel, under conditions such as a solid-liquid ratio of 300 g / L or more and 2000 g / L or less, a washing time of 1 minute or more and 1 hour or less, and a stirring speed of 100 rpm or more. When the solid-liquid ratio is 300 g / L or more and 2000 g / L or less, the Gini coefficient of phosphorus (P) inside the secondary particles tends to be 0.6 or less when a phosphorus-containing compound, as described later, is added.

[0054] At this point, a phosphorus-containing compound may be added while stirring the dispersion containing the calcined material (lithium transition metal composite oxide) (first addition step). By adding the phosphorus-containing compound to the water during washing, the phosphorus-containing compound is uniformly dispersed inside the secondary particles (grain boundaries of the primary particles), and the Gini coefficient of phosphorus (P) inside the secondary particles becomes 0.6 or less.

[0055] In the first addition step, a phosphorus-containing compound may be added by dropwise adding an aqueous phosphoric acid solution to the dispersion containing the calcined product (lithium transition metal composite oxide). When an aqueous phosphoric acid solution is added dropwise, for example, the lithium on the surface of the primary particles reacts with the phosphoric acid, and Li is added to at least the primary particle interface. ３ PO ４ A phosphate containing Li is formed.

[0056] In the first addition step, it is preferable to add an aqueous solution containing a phosphorus-containing compound in an amount of 0.001% by mass or more and 0.5% by mass or less, relative to the lithium transition metal composite oxide, and more preferably an aqueous solution containing a phosphorus-containing compound in an amount of 0.005% by mass or more and 0.3% by mass or less, relative to the lithium transition metal composite oxide. By adding an aqueous solution containing a phosphorus-containing compound in an amount of 0.001% by mass or more and 0.5% by mass or less, relative to the lithium transition metal composite oxide, the mass of the phosphorus-containing compound contained in the positive electrode active material can be reduced to 0.001% by mass or more and 0.4% by mass or less, relative to the phosphorus-containing compound. Furthermore, when using an aqueous phosphoric acid solution, it is preferable to add an aqueous solution containing a phosphorus-containing compound in an amount of 0.1% by mass or more and 85% by mass or less, more preferably an aqueous solution containing a phosphorus-containing compound in an amount of 0.3% by mass or more and 50% by mass or less, and even more preferably an aqueous solution containing a phosphorus-containing compound in an amount of 0.5% by mass or more and 10% by mass or less, relative to the lithium transition metal composite oxide. In this case, an appropriate amount of phosphorus-containing compound can be present at the grain boundaries of at least the primary particles of the lithium transition metal composite oxide, and a significant effect in suppressing side reactions with the electrolyte is achieved.

[0057] Furthermore, in the addition process, the amount of phosphorus-containing compound added per minute is preferably 0.0001% by mass or more and 0.05% by mass or less, in terms of phosphorus element, relative to the total mass of the lithium transition metal composite oxide. By setting the amount of phosphorus-containing compound added per minute within the above range, the phosphorus-containing compound becomes more uniformly dispersed inside the secondary particles (at the grain boundaries of the primary particles).

[0058] In the dehydration step, the cake-like composition is obtained by dehydration. The method of dehydration is not particularly limited and can be carried out by, for example, a filter press. A phosphorus-containing compound may be added to the cake-like composition obtained in the dehydration step (first addition step). By adding a phosphorus-containing compound to the cake-like composition, the phosphorus-containing compound is uniformly dispersed inside the secondary particles (grain boundaries of primary particles), and the Gini coefficient of phosphorus (P) inside the secondary particles becomes 0.6 or less. Drying of the cake-like composition is carried out, for example, under conditions that include oxygen, nitrogen, air, etc., under reduced pressure to increased pressure. For example, a reduced pressure atmosphere, particularly a vacuum atmosphere, is preferred, and the drying is carried out under conditions such as a pressure of 1 kPa or less, a temperature of 120°C or higher and 300°C or lower, and a time of 1 hour or more and 10 hours or less.

[0059] Furthermore, the manufacturing process for the positive electrode active material may include a second addition step in which a boron-containing compound is added to at least one of the cake-like composition obtained in the dehydration step and the powder obtained by drying the cake-like composition, and then heat-treated. By performing the second addition step, the boron-containing compound can be present on at least one of the surface and interior of the secondary particles of the lithium transition metal composite oxide.

[0060] The boron-containing compound added in the additive step is, for example, boric acid (H ３ BO ３ ), boron oxide (B ２ O ３ ), lithium borate (Li ２ B ４ O ７ Li ３ BO ３ LiB ３ O ５ LiBO ２These compounds may be used individually or in combination of two or more.

[0061] The heat treatment of the cake-like composition after the addition of the boron-containing compound, or the powder obtained by drying the cake-like composition, is carried out at a temperature of, for example, 150°C or higher and 500°C or lower. The atmosphere during the heat treatment may be a vacuum, an oxygen stream, or air.

[0062] [Negative Electrode] The negative electrode 12 may, for example, have a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, or a metallic Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode current collector, and lithium metal may be deposited on the surface of the negative electrode current collector 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 current collector. For the negative electrode current collector, 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 current collector 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 current collector. 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 a negative electrode current collector, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0063] 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.

[0064] 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.

[0065] [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.

[0066] 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.

[0067] [Non-aqueous electrolytes] Non-aqueous electrolytes, for example, have lithium ion conductivity. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0068] 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).

[0069] 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).

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0075] 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.

[0076] 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.

[0077] The present disclosure will be further illustrated below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0078] <Example 1> [Preparation of positive electrode active material] [Ni obtained by coprecipitation method ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ ] (OH) ２The composite hydroxide represented by was calcined at 400°C for 8 hours to obtain a Ni-containing metal oxide containing Ni, Co, Mn, and Al. Next, lithium hydroxide monohydrate (LiOH·H) was prepared so that the molar ratio of Li to the total number of moles of Ni, Co, Mn, and Al was 103 mol%. ２ O) was mixed to obtain a mixture (mixing step). This mixture was heated under an oxygen stream with an oxygen concentration of 95% (flow rate of 3 L / min per 1 kg of mixture) at a heating rate of 4°C / min from room temperature to 400°C, and then heated from 400°C to 650°C at a heating rate of 2°C / min. After that, the temperature was heated from 650°C to 850°C at a heating rate of 1°C / min, and then held for 10 hours to obtain a calcined product (calcination step).

[0079] To this calcined material, a 2 L reaction vessel was used to add the calcined material to water so that the solid-liquid ratio was 1250 g / L, and the mixture was washed with water at a stirring speed of 200 rpm for 10 minutes (washing step). At this time, H as a phosphorus-containing compound ３ PO ４ A phosphoric acid aqueous solution containing 0.05% by mass relative to the total mass of the calcined product was added dropwise (addition step). Note that the amount of H per minute ３ PO ４ The amount added was 0.005% by mass relative to the total mass of the calcined product. Then, dehydration was performed using a filter press to obtain a cake-like composition (dehydration step). Next, 0.5% by mass of boric acid relative to the mass of the lithium transition metal composite oxide was added to the obtained cake-like composition. After this, heat treatment was performed for 2 hours under a vacuum atmosphere at a temperature of 300°C to obtain the positive electrode active material (second addition step).

[0080] When the fabricated positive electrode active material was measured by TOF-SIMS, the presence of phosphorus-containing compounds and boron-containing compounds on the surface and inside the secondary particles was confirmed. Furthermore, the Gini coefficient of phosphorus (P) inside the secondary particles was calculated to be 0.3 using the method described above. In addition, when the content of phosphorus-containing compounds (calculated as phosphorus element) and boron-containing compounds (calculated as boron element) was measured by ICP-AES, the results were 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0081] [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 mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode current collector made of aluminum foil, the coating was dried and compressed, and then the positive electrode current collector was cut to a predetermined electrode size to obtain a positive electrode in which positive electrode mixture layers were arranged on both sides of the positive electrode current collector. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode current collector was exposed.

[0082] [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.

[0083] [Preparation of Test Cell] A positive electrode lead was attached to the exposed portion of the positive electrode, and a negative electrode lead was attached to the lithium metal foil serving as the negative electrode. A wound electrode body was then constructed by spirally winding the positive and negative electrodes together via a polyolefin separator. This electrode body was placed inside an outer casing made of aluminum laminate sheet, and after injecting the non-aqueous electrolyte, the opening of the outer casing was sealed to obtain a test cell.

[0084] [Evaluation of Initial Discharge Capacity] The test cell was charged to 4.4V (vsLi) with a constant current of 0.2C at a temperature of 25°C, and then charged to 0.02C with a constant voltage of 4.4V. After that, it was discharged to 2.5V with a constant current of 0.2C, and the initial discharge capacity was measured.

[0085] [Evaluation of reaction resistance after cycling] The test cell was charged to 4.4V (vsLi) with a constant current of 0.2C at a temperature of 25°C, and then charged at a constant voltage of 4.4V until the current value was equivalent to 0.01C. After a rest period of 1 hour, it was discharged to 2.5V with a constant current of 0.2C, and this was considered one cycle, which was repeated for 30 cycles. Then, the test cell after 30 cycles was charged at a constant voltage of 4.4V (vsLi) with a current value equivalent to 0.01C using a Solartron 1255B (manufactured by Solartron Corporation) at a temperature of 25°C, and the AC impedance was measured with an applied voltage of 10mV in the measurement frequency range of 0.01 to 200kHz, and the reaction resistance value was determined from the Nyquist plot (arc of approximately 1Hz to 0.1Hz).

[0086] <Example 2> In the second addition step for preparing the positive electrode active material, boric acid as a boron-containing compound was added to the dried powder of the cake-like composition, and the same heat treatment was performed. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds and boron-containing compounds were present on the surface and inside the secondary particles. Furthermore, the Gini coefficient of phosphorus (P) inside the secondary particles was calculated to be 0.3 using the method described above. In addition, when the content of phosphorus-containing compounds and boron-containing compounds, calculated in terms of phosphorus elements and boron elements, was measured by ICP-AES, it was found to be 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0087] <Example 3> In the first addition step of preparing the positive electrode active material, H as a phosphorus-containing compound ３ PO ４A test cell was prepared and evaluated in the same manner as in Example 1, except that a phosphoric acid aqueous solution containing 0.01% by mass relative to the total mass of the calcined material was added dropwise. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds and boron-containing compounds were present on the surface and inside the secondary particles. Furthermore, the Gini coefficient of phosphorus (P) inside the secondary particles was calculated to be 0.5 using the method described above. In addition, the phosphorus-containing compounds and boron-containing compounds were converted to phosphorus element equivalents by ICP-AES. When the content of the boron-containing compounds was measured, it was found to be 0.003% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0088] <Example 4> In the mixing step of preparing the positive electrode active material, [Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ ] (OH) ２ Instead of [Ni ０．９２ Co ０．０３ Mn ０．０５ ] (OH) ２ A test cell was prepared and evaluated in the same manner as in Example 1, except for the use of [specific material]. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds and boron-containing compounds were present on the surface and inside the secondary particles. Furthermore, the Gini coefficient of phosphorus (P) inside the secondary particles was calculated to be 0.3 using the method described above. In addition, when the content of phosphorus-containing compounds (calculated as phosphorus element) and boron-containing compounds (calculated as boron element) was measured by ICP-AES, it was found to be 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0089] <Example 5> In the mixing step of preparing the positive electrode active material, [Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ ] (OH) ２ Instead of [Ni ０．８２ Co ０．０８ Mn ０．０５ Al ０．０５ ] (OH) ２Using the same method as in Example 1, a test cell was prepared and evaluated in the same manner as in Example 1, except that in the second addition step, boric acid as a boron-containing compound was added not only to the cake-like composition but also to the powder obtained by drying the cake-like composition, and the same heat treatment was performed. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds and boron-containing compounds were present on the surface and inside the secondary particles. Furthermore, when the Gini coefficient of phosphorus (P) inside the secondary particles was calculated using the method described above, it was found to be 0.3. In addition, when the content of phosphorus-containing compounds and boron-containing compounds, calculated in terms of phosphorus elements and boron elements, was measured by ICP-AES, it was found to be 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0090] <Example 6> In the first addition step of preparing the positive electrode active material, H as a phosphorus-containing compound ３ PO ４ A test cell was prepared and evaluated in the same manner as in Example 5, except that a phosphoric acid aqueous solution containing 0.5% by mass relative to the total mass of the calcined material was added dropwise. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds and boron-containing compounds were present on the surface and inside the secondary particles. Furthermore, the Gini coefficient of phosphorus (P) inside the secondary particles was calculated to be 0.3 using the method described above. In addition, when the content of phosphorus-containing compounds (calculated as phosphorus element) and boron-containing compounds (calculated as boron element) was measured by ICP-AES, it was found to be 0.2% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0091] <Example 7> In the mixing step of preparing the positive electrode active material, [Ni ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ ] (OH) ２ Instead of [Ni ０．９０ Co ０．０２ Mn ０．０３ Al ０．０５ ] (OH) ２A test cell was prepared and evaluated in the same manner as in Example 1, except that the second addition step was omitted and the cake-like composition was dried under a vacuum atmosphere at 180°C for 2 hours. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds were present on the surface and inside the secondary particles. Furthermore, the Gini coefficient of phosphorus (P) inside the secondary particles was calculated to be 0.3 using the method described above. In addition, when the content of the phosphorus-containing compound, converted to phosphorus element, was measured by ICP-AES, it was found to be 0.05% by mass of the lithium transition metal composite oxide.

[0092] <Example 8> In the addition step for preparing the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 7, except that the phosphoric acid aqueous solution was not added to the dispersion during the water washing step, but was added to the cake-like composition obtained in the dehydration step. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds were present on the surface and inside the secondary particles. Furthermore, when the Gini coefficient of phosphorus (P) inside the secondary particles was calculated using the method described above, it was found to be 0.5. In addition, when the content of the phosphorus-containing compound, converted to phosphorus element, was measured by ICP-AES, it was found to be 0.05% by mass relative to the mass of the lithium transition metal composite oxide.

[0093] <Comparative Example 1> A test cell was prepared and evaluated in the same manner as in Example 1, except that the first and second addition steps were not performed in the preparation of the positive electrode active material.

[0094] <Comparative Example 2> In the first addition step of preparing the positive electrode active material, H as a phosphorus-containing compound ３ PO ４A test cell was prepared and evaluated in the same manner as in Example 2, except that a phosphoric acid aqueous solution containing 1.0% by mass relative to the total mass of the calcined material was added dropwise. When the prepared positive electrode active material was measured by TOF-SIMS, it was confirmed that phosphorus-containing compounds and boron-containing compounds were present on the surface and inside the secondary particles. Furthermore, the Gini coefficient of phosphorus (P) inside the secondary particles was calculated to be 0.8 using the method described above. In addition, when the content of phosphorus-containing compounds (calculated as phosphorus element) and boron-containing compounds (calculated as boron element) was measured by ICP-AES, it was found to be 0.3% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0095] <Comparative Example 3> In the mixing step for the preparation of the positive electrode active material, Li as a phosphorus-containing compound is added so that it is 0.05% by mass relative to the mass of the lithium transition metal composite oxide. ３ PO ４ A test cell was prepared and evaluated in the same manner as in Example 2, except that the phosphorus-containing compound was not added during the washing process after mixing the materials. The Gini coefficient of phosphorus (P) inside the secondary particles was calculated using the method described above and was found to be 0.8. Furthermore, the content of the phosphorus-containing compound, converted to phosphorus element equivalent, and the boron-containing compound, converted to boron element equivalent, were measured by ICP-AES and found to be 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0096] <Comparative Example 4> A test cell was prepared and evaluated in the same manner as in Example 2, except that in the water washing process for preparing the positive electrode active material, an aqueous phosphoric acid solution was added dropwise to the water before adding the calcined material, but no aqueous phosphoric acid solution was added dropwise after adding the calcined material. The Gini coefficient of phosphorus (P) inside the secondary particles was calculated using the method described above and was found to be 0.8. Furthermore, the content of phosphorus-containing compounds converted to phosphorus element and boron-containing compounds converted to boron element were measured by ICP-AES and were found to be 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0097] <Comparative Example 5> In the preparation of the positive electrode active material, no phosphorus-containing compound was added during the water washing, and the powder after drying was given a phosphorus-containing compound of Li such that it was 0.05% by mass relative to the mass of the lithium transition metal composite oxide. ３ PO ４ A test cell was prepared and evaluated in the same manner as in Example 2, except for the addition of [substance name]. The Gini coefficient of phosphorus (P) inside the secondary particles was calculated using the method described above and was found to be 0.7. Furthermore, the content of the phosphorus-containing compound, converted to phosphorus element equivalent, and the boron-containing compound, converted to boron element equivalent, were measured by ICP-AES and found to be 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0098] <Comparative Example 6> When preparing the cathode active material, no phosphorus-containing compound was added during the water washing process. When preparing the cathode mixture slurry in the cathode preparation process, the amount of phosphorus-containing compound Li was added such that it was 0.05% by mass relative to the mass of the lithium transition metal composite oxide. ３ PO ４ A test cell was prepared and evaluated in the same manner as in Example 2, except for the addition of [substance name]. The Gini coefficient of phosphorus (P) inside the secondary particles was calculated using the method described above and was found to be 0.8. Furthermore, the content of phosphorus-containing compounds converted to phosphorus element equivalent and boron-containing compounds converted to boron element equivalent were measured by ICP-AES and were found to be 0.02% by mass and 0.1% by mass, respectively, relative to the mass of the lithium transition metal composite oxide.

[0099] <Comparative Example 7> A test cell was prepared and evaluated in the same manner as in Example 2, except that the first addition step was not performed in the preparation of the positive electrode active material. The content of the boron-containing compound, converted to boron elemental value, was measured by ICP-AES and was found to be 0.1% by mass relative to the mass of the lithium transition metal composite oxide.

[0100] Table 1 shows the initial discharge capacity and reaction resistance after cycling of the test cells for the examples and comparative examples. Note that the initial discharge capacity and reaction resistance shown in Table 1 are expressed relatively, with the initial discharge capacity and reaction resistance of the test cell for Comparative Example 1 set to 100. A higher initial discharge capacity value indicates higher capacity, while a lower reaction resistance value indicates lower resistance.

[0101]

[0102] As shown in Table 1, the test cells of the examples in which the Gini coefficient of phosphorus (P) is 0.6 or less show a decrease in reaction resistance after cycling while maintaining initial capacity compared to the test cells of the comparative examples. This is presumed to be because the reaction between lithium transition metal composite oxide and hydrogen fluoride was suppressed by having the phosphorus-containing compound in an appropriate dispersion state. Furthermore, the test cells of Examples 1 to 6, which contain a boron-containing compound, show an increase in initial capacity compared to the test cells of Examples 7 and 8, which do not contain a boron-containing compound. Therefore, it can be said that by including a boron-containing compound and keeping the Gini coefficient of phosphorus (P) at 0.6 or less, it is possible to achieve high capacity while suppressing the increase in the reaction resistance of the battery after cycling.

[0103] This disclosure is further illustrated by the following embodiments. Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide having a layered structure, wherein the lithium transition metal composite oxide is a secondary particle formed by the aggregation of primary particles, and the general formula is Li ｘ Ni ａ Co ｂ Mn ｃ M ｄ O ２－ｙA positive electrode active material for a non-aqueous electrolyte secondary battery, represented by the formula (wherein 0.95≦x≦1.15, 0.65≦a≦0.95, 0≦b≦0.15, 0≦c≦0.25, 0<d≦0.1, 0≦y<0.05, a+b+c+d=1, M is at least one element selected from the group consisting of Zr, Nb, Ti, W, Mo, Si, and Al), wherein a phosphorus-containing compound is present on the surface and inside the secondary particles, and the Gini coefficient of phosphorus (P) inside the secondary particles is 0.6 or less. Configuration 2: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein a boron-containing compound is present on at least one of the surface and inside the secondary particles. Configuration 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the content of the phosphorus-containing compound is 0.001% by mass or more and 0.4% by mass or less of the mass of the lithium transition metal composite oxide, in terms of phosphorus element. Configuration 4: The phosphorus-containing compound comprises at least one of a phosphate salt containing at least one element selected from the group consisting of Li, Ca, and Sr, and phosphorus oxide, as described in any one of Configurations 1 to 3, as a positive electrode active material for a non-aqueous electrolyte secondary battery. Configuration 5: The boron-containing compound content is 0.0015% by mass or more and 1.0% by mass or less of the mass of the lithium transition metal composite oxide, as described in Configuration 2, as an amount of boron element, as a positive electrode active material for a non-aqueous electrolyte secondary battery. Configuration 6: The boron-containing compound comprises at least one of boric acid and lithium borate, as described in Configuration 2 or 5, as described in Configuration 7: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery described in any one of Configurations 1 to 6, a negative electrode, and a non-aqueous electrolyte.Configuration 8: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: a mixing step of mixing a Ni metal compound containing at least Ni with a Li compound to obtain a mixture; a firing step of firing the mixture to obtain a fired product; a washing step of washing the fired product with water; and a dehydration step of dehydrating after the washing step to obtain a cake-like composition, further comprising: a first adding step of adding a phosphorus-containing compound to at least one of the dispersion in which the fired product was added in the washing step and the cake-like composition obtained in the dehydration step; and a second adding step of adding a boron-containing compound to at least one of the cake-like composition obtained in the dehydration step and the powder obtained by drying the cake-like composition, and then performing a heat treatment. Configuration 9: The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 8, wherein in the first adding step, an aqueous phosphoric acid solution containing the phosphorus-containing compound in an amount of 0.0005% by mass or more and 2% by mass or less with respect to the total mass of the lithium transition metal composite oxide is added. Configuration 10: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 8 or 9, wherein in the addition step, the amount of the phosphorus-containing compound added per minute is 0.0001% by mass or more and 0.05% by mass or less in terms of phosphorus element relative to the total mass of the lithium transition metal composite oxide. Configuration 11: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 9, wherein the concentration of the phosphoric acid aqueous solution is 0.1% by mass or more and 85% by mass or less.

[0104] 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.

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide having a layered structure, wherein the lithium transition metal composite oxide is a secondary particle formed by the aggregation of primary particles, and the general formula is Li ｘ Ni ａ Co ｂ Mn ｃ M ｄ O ２－ｙ A positive electrode active material for a non-aqueous electrolyte secondary battery, represented by the formula (wherein 0.95 ≤ x ≤ 1.15, 0.65 ≤ a ≤ 0.95, 0 ≤ b ≤ 0.15, 0 ≤ c ≤ 0.25, 0 < d ≤ 0.1, 0 ≤ y < 0.05, a + b + c + d = 1, and M is at least one element selected from the group consisting of Zr, Nb, Ti, W, Mo, Si, and Al), wherein a phosphorus-containing compound exists on the surface and inside the secondary particles, and the Gini coefficient of phosphorus (P) inside the secondary particles is 0.6 or less.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein a boron-containing compound is present on at least one of the surface and interior of the secondary particles.

3. The content of the phosphorus-containing compound is 0.001% by mass or more and 0.4% by mass or less of the mass of the lithium transition metal composite oxide, as described in claim 1.

4. The phosphorus-containing compound comprises at least one of a phosphate salt containing at least one element selected from the group consisting of Li, Ca, and Sr, and phosphorus oxide, as a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the content of the boron-containing compound is 0.0015% by mass or more and 1.0% by mass or less of the mass of the lithium transition metal composite oxide, in terms of boron element.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the boron-containing compound comprises at least one of boric acid and lithium borate.

7. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery described in any one of claims 1 to 6, a negative electrode, and a non-aqueous electrolyte.

8. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: a mixing step of mixing a Ni metal compound containing at least Ni with a Li compound to obtain a mixture; a firing step of firing the mixture to obtain a fired product; a washing step of washing the fired product with water; and a dehydration step of dehydrating the product after the washing step to obtain a cake-like composition, further comprising: a first adding step of adding a phosphorus-containing compound to at least one of the dispersion in which the fired product was added in the washing step and the cake-like composition obtained in the dehydration step; and a second adding step of adding a boron-containing compound to at least one of the cake-like composition obtained in the dehydration step and the powder obtained by drying the cake-like composition, and then performing a heat treatment.

9. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 8, wherein in the first addition step, an aqueous phosphoric acid solution containing the phosphorus-containing compound is added in an amount of 0.001% by mass or more and 0.5% by mass or less on an amount equivalent to phosphorus, relative to the total mass of the lithium transition metal composite oxide.

10. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 8, wherein in the first addition step, the amount of the phosphorus-containing compound added per minute is 0.0001% by mass or more and 0.05% by mass or less in terms of phosphorus element relative to the total mass of the lithium transition metal composite oxide.

11. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 9, wherein the concentration of the phosphoric acid aqueous solution is 0.1% by mass or more and 85% by mass or less.

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