Positive electrode active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

A positive electrode active material comprising a lithium transition metal composite oxide containing 80 mol% or more of Ni relative to the total molar amount of metal elements other than Li. The lithium transition metal composite oxide comprises secondary particles formed by aggregation of primary particles, wherein at least one element A selected from Ca and Sr is present on the surfaces of the primary particles in an amount of 1 mol% or less, relative to the total molar amount of the metal elements other than Li, and at least one element B selected from Zr, Ti, Mn, Er, Pr, In, Sn, and Ba and S are present on the surfaces of the secondary particles.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material. Background Art

[0002] In non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, the positive electrode active material significantly influences battery performance, including power characteristics, capacity, cycle characteristics, and storage characteristics. Typically, lithium-transition metal composite oxides are used as positive electrode active materials. These lithium-transition metal composite oxides contain metal elements such as Ni, Co, Mn, and Al and are composed of secondary particles formed by aggregation of primary particles. Because the properties of positive electrode active materials vary significantly depending on their composition, particle shape, and other factors, extensive research has been conducted on various positive electrode active materials.

[0003] For example, Patent Document 1 discloses a method for manufacturing a positive electrode active material, wherein tungsten oxide and at least one selected from a sulfuric acid compound, a nitric acid compound, a boric acid compound, and a phosphoric acid compound are fixed to the surface of particles of a lithium transition metal composite oxide containing Ni, and the composite particles are heat-treated in an oxygen atmosphere.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2010-040383 Summary of the Invention

[0007] Lithium transition metal composite oxides with high Ni content are highly anticipated as positive electrode active materials that contribute to higher battery capacity. However, non-aqueous electrolyte secondary batteries using these positive electrode active materials have the problem of gas generation due to decomposition of the non-aqueous electrolyte during storage and charging. It should be noted that in the technology disclosed in Patent Document 1, increasing the Ni content in the positive electrode active material can lead to cation mixing, which can easily cause a decrease in initial capacity.

[0008] For a positive electrode active material for a non-aqueous electrolyte secondary battery in one embodiment of the present invention, it includes a lithium transition metal composite oxide, which contains 80 mol% or more of Ni relative to the total molar amount of metal elements other than Li, and the aforementioned lithium transition metal composite oxide includes secondary particles formed by aggregation of primary particles, and element A of at least one selected from Ca and Sr is present on the surface of the aforementioned primary particles in an amount of less than 1 mol% relative to the total molar amount of metal elements other than Li, and elements B and S of at least one selected from Zr, Ti, Mn, Er, Pr, In, Sn and Ba are present on the surface of the aforementioned secondary particles.

[0009] A nonaqueous electrolyte secondary battery according to one embodiment of the present invention includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and a nonaqueous electrolyte.

[0010] According to one embodiment of the present invention, in a non-aqueous electrolyte secondary battery using a positive electrode active material having a high Ni content, gas generation during charging and storage can be suppressed. By using a positive electrode active material as one embodiment of the present invention, for example, a non-aqueous electrolyte secondary battery with high capacity and excellent storage characteristics can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a cross-sectional view of a nonaqueous electrolyte secondary battery as an example of an embodiment.

[0012] Figure 2 This is a diagram schematically showing a cross section of a particle of a lithium-transition metal composite oxide constituting a positive electrode active material according to an example of an embodiment. DETAILED DESCRIPTION

[0013] As mentioned above, lithium transition metal composite oxides containing a large amount of Ni are useful positive electrode active materials that contribute to higher capacity and higher energy density of batteries. However, they have the problem of promoting the decomposition of non-aqueous electrolytes and increasing the amount of gas generated during battery charge storage.

[0014] The present inventors conducted in-depth research to solve this problem and found that in a lithium transition metal composite oxide (positive electrode active material) with a large amount of Ni, by making at least one selected from Ca and Sr (element A) present in a specified amount on the surface of the primary particles, and making at least one selected from Zr, Ti, Mn, Er, Pr, In, Sn and Ba (element B) and S present in specified amounts on the surface of the secondary particles, gas generation during battery charging and storage is specifically suppressed.

[0015] It is believed that for positive electrode active materials with a large amount of Ni, especially when the charge rate is high in a high temperature atmosphere, due to the activation of the particle surface, it is easy to cause the decomposition reaction of the non-aqueous electrolyte. Therefore, in a non-aqueous electrolyte secondary battery using a large amount of Ni positive electrode active material, the amount of gas generated during charging and storage becomes more. According to the positive electrode active material of the present invention, a stable protective layer is formed on the surface of the secondary particles of the composite oxide by the interaction of the coexistence of element A, element B and S, thereby improving the stability of the active material surface. It is thus believed that the decomposition reaction of the electrolyte on the surface of the active material is suppressed, and the amount of gas generated during charging and storage is greatly reduced.

[0016] It should be noted that, in the absence of element A, element B or S, a stable protective layer is not formed on the surface of the active material, and the effect of the present invention cannot be obtained. As described above, only when element A, element B and S coexist, the stability of the active material surface is specifically improved and gas generation is greatly suppressed. In addition, since there is an appropriate amount of addition of element A, element B and S, if the addition amount is not strictly controlled, not only can the effect of suppressing gas generation not be obtained, but also other battery performances will be reduced.

[0017] Hereinafter, an embodiment of the positive electrode active material for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery using the positive electrode active material of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that it is initially envisioned that the multiple embodiments and modifications described below may be selectively combined.

[0018] The following example illustrates a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer can 16. However, the outer can of the battery is not limited to a cylindrical outer can. For example, a rectangular outer can (prismatic battery), a coin-shaped outer can (coin-shaped battery), or an outer can composed of a laminate sheet including a metal layer and a resin layer (laminated battery) may also be used. Furthermore, the electrode body may be a stacked type in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0019] Figure 1 FIG is a cross-sectional view of a non-aqueous electrolyte secondary battery 10 as an example of an embodiment. Figure 1 As shown, the nonaqueous electrolyte secondary battery 10 includes a wound electrode body 14, a nonaqueous electrolyte, and an outer can 16 for housing the electrode body 14 and the nonaqueous 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 spirally wound with the separator 13 interposed therebetween. The outer can 16 is a metal container with a bottom and a cylindrical shape, open on one side in the axial direction. The opening of the outer can 16 is closed by a sealing member 17. For ease of description, the battery's sealing member 17 side is considered the top, and the bottom side of the outer can 16 is considered the bottom.

[0020] The positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 are all strip-shaped elongated bodies, and are alternately stacked along the radial direction of the electrode body 14 by spiral winding. The negative electrode 12 is formed to be one circle larger than the positive electrode 11 to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction and the width direction (short side direction). The two separators 13 are formed to be at least one circle larger than the positive electrode 11, for example, and are arranged in a manner of clamping the positive electrode 11. The electrode body 14 has: a positive electrode lead 20 connected to the positive electrode 11 by welding, etc., and a negative electrode lead 21 connected to the negative electrode 12 by welding, etc.

[0021] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14 . Figure 1 In the example shown, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 toward the sealing body 17, while the negative electrode lead 21 extends through the outside of the insulating plate 19 toward the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of an internal terminal plate 23 of the sealing body 17 by welding or the like. The top plate of the sealing body 17, or lid 27, which is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, with the outer can 16 serving as the negative electrode terminal.

[0022] A gasket 28 is provided between the outer can 16 and the sealing member 17 to ensure the airtightness of the battery interior. The outer can 16 has a groove 22 formed inwardly, with a portion of its side surface protruding inwardly, to support the sealing member 17. The groove 22 is preferably formed in an annular shape along the circumference of the outer can 16, and its upper surface supports the sealing member 17. The sealing member 17 is secured to the upper portion of the outer can 16 by the groove 22 and the open end of the outer can 16, which is secured to the sealing member 17.

[0023] 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 cover 27 are stacked in order from the electrode body 14 side. The components constituting the sealing body 17 have, for example, a disc shape or a ring shape, and the components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat release, the lower valve body 24 is deformed and breaks in a manner that squeezes the upper valve body 26 toward the cover 27 side, thereby blocking the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure rises further, the upper valve body 26 breaks and the gas is discharged from the opening of the cover 27.

[0024] Hereinafter, the positive electrode 11 , the negative electrode 12 , the separator 13 , and the non-aqueous electrolyte, in particular, the positive electrode active material constituting the positive electrode 11 will be described in detail.

[0025] [positive electrode]

[0026] The positive electrode 11 includes a positive electrode core 30 and a positive electrode composite material layer 31 provided on the surface of the positive electrode core 30. The positive electrode core 30 can be made of a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a thin film having the metal disposed on the surface. The positive electrode composite material layer 31 preferably contains a positive electrode active material, a conductive material, and a binder, and is provided on both sides of the positive electrode core 30. The positive electrode 11 can be produced, for example, by coating a positive electrode composite material slurry containing a positive electrode active material, a conductive material, and a binder on the positive electrode core 30, drying the coating, and then compressing the coating to form the positive electrode composite material layers 31 on both sides of the positive electrode core 30.

[0027] Examples of the conductive material contained in the positive electrode composite material layer 31 include carbon materials such as carbon black, acetylene black, Ketjen black, graphite, and carbon nanotubes. Examples of the binder contained in the positive electrode composite material layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins can be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0028] Figure 2 This figure schematically shows a cross section of a particle of a lithium transition metal composite oxide 35 constituting an example of a positive electrode active material of an embodiment. The positive electrode active material of this embodiment includes a lithium transition metal composite oxide 35 (hereinafter referred to as "composite oxide 35"), and the lithium transition metal composite oxide 35 contains 80 mol% or more of Ni relative to the total molar amount of metal elements other than Li. The composite oxide 35 further preferably contains at least one selected from Co, Al, and Mn. In addition, as Figure 2 As shown, the composite oxide 35 includes secondary particles 37 formed by aggregation of primary particles 36 .

[0029] As mentioned above, composite oxide 35, which contains a high nickel content, is a useful positive electrode active material that contributes to higher battery capacity and energy density. However, it also suffers from the problem of high gas generation during battery charge and storage. In composite oxide 35, at least one element A selected from Ca and Sr is present on the surface of primary particles 36, and at least one element B selected from Zr, Ti, Mn, Er, Pr, In, Sn, and Ba, and S are present on the surface of secondary particles 37. When used as a positive electrode active material, these elements can significantly suppress gas generation during charge and storage.

[0030] The positive electrode active material of this embodiment has the composite oxide 35 as a main component. Here, the main component refers to the component with the largest mass proportion in the material constituting the positive electrode active material. In the positive electrode composite material layer 31, as a positive electrode active material, a composite oxide other than the composite oxide 35 may be contained within the scope that does not impair the purpose of the present invention, but the ratio of the composite oxide 35 is preferably 50% by mass or more, more preferably 80% by mass or more. In this embodiment, the positive electrode active material is described as being essentially composed only of the composite oxide 35. In addition, the positive electrode active material may also be composed of two or more composite oxides 35 with different compositions.

[0031] The composite oxide 35 preferably contains other metal elements in addition to Li, Ni, and the aforementioned elements A, B, and S. Examples of other metal elements include Co, Al, Mn, Nb, W, Fe, Zn, Er, K, Pr, Ca, Ba, Sc, Rb, Ga, In, Sn, and Sr. Among these, at least one selected from Co, Al, and Mn is preferably contained. The total amount of these other metal elements contained in the composite oxide 35 is preferably 20 mol% or less, more preferably 15 mol% or less, for example, 5 mol% or more and 20 mol% or less, relative to the total molar amount of the metal elements other than Li.

[0032] The Ni content of the composite oxide 35 is 80 mol% or more, preferably 85 mol% or more, and more preferably 90 mol% or more, relative to the total molar amount of metal elements other than Li. The upper limit of the Ni content is, for example, 95 mol%. If the Ni content is within this range, both high capacity / high energy density of the battery and good storage characteristics can be achieved. A suitable composite oxide 35 contains at least one selected from Co, Al, and Mn in an amount of 5 mol% or more and 20 mol% or less, relative to the total molar amount of metal elements other than Li. In this case, the structural stability of the composite oxide 35 is improved, contributing to the improvement of storage characteristics. The contents of Al and Mn are, for example, 1 mol% or more and 7 mol% or less, respectively.

[0033] The Co content of the composite oxide 35 may be less than 5 mol% relative to the total molar amount of metal elements other than Li, or the composite oxide 35 may contain substantially no Co. Co is rare and expensive, so by not using Co, the manufacturing cost of the battery can be reduced. It should be noted that the mole fractions of the elements contained in the composite oxide 35 are measured by inductively coupled plasma mass spectrometry (ICP-MS).

[0034] The composite oxide 35 preferably has a layered rock salt structure. Examples of the layered rock salt structure of the composite oxide 35 include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m.

[0035] As described above, the composite oxide 35 includes secondary particles 37 formed by aggregation of primary particles 36. The average particle size of the primary particles 36 is, for example, greater than 200 nm and less than 500 nm. The average particle size of the primary particles 36 is obtained by analyzing an SEM image of a particle cross section observed by a scanning electron microscope (SEM). For example, the positive electrode 11 is buried in a resin, a cross section is made using a cross-section polisher (CP), and the cross section is photographed using an SEM. 30 primary particles 36 are randomly selected from the SEM image and the grain boundaries are observed to determine the major axis (longest diameter) of each of the 30 primary particles 36, and the average value thereof is used as the average particle size.

[0036] The volume-based median diameter (hereinafter referred to as "D50") of the secondary particles 37 (composite oxide 35) is, for example, 1 μm to 30 μm, preferably 5 μm to 20 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle in the volume-based particle size distribution reaches 50%, also known as the median diameter. The particle size distribution of the secondary particles 37 can be measured using a laser diffraction particle size analyzer (e.g., MT3000II manufactured by MicrotracBEL Corp.) using water as the dispersion medium.

[0037] At least one element A selected from Ca and Sr is present on the surface of the primary particles 36 constituting the composite oxide 35 in an amount of 1 mol% or less relative to the total molar amount of metal elements other than Li. Element A is present on the surface of the secondary particles 37 and at the particle interfaces where the primary particles 36 are in contact with each other, and is present on the surfaces of all the primary particles 36 constituting the secondary particles 37 of the composite oxide 35. It is believed that element A is uniformly attached to the surface of the primary particles 36 in the form of a compound, forming a coating 36A containing element A on the surface of the primary particles 36. It should be noted that the element distribution in the particle cross-section of the composite oxide 35 can be confirmed by energy dispersive X-ray spectroscopy (TEM-EDX).

[0038] For example, element A is essentially present only on the surface of primary particles 36 and does not form a solid solution with Ni or the like. Even small amounts of element A contribute to suppressing gas generation through interaction with elements B and S. This effect becomes significant when element A is added at a concentration of 0.1 mol% or more relative to the total molar amount of metal elements excluding Li. On the other hand, when the content of element A exceeds 3 mol%, coating 36A containing element A becomes a resistive layer, reducing discharge capacity.

[0039] The content of element A should be controlled to 3 mol% or less, more preferably 0.7 mol% or less, and particularly preferably 0.5 mol% or less, relative to the total molar amount of metal elements other than Li. From the perspective of achieving both high battery capacity and good storage characteristics, the lower limit of the content of element A is preferably 0.15 mol%, more preferably 0.20 mol%, and particularly preferably 0.25 mol%. An example of a suitable content of element A is 0.20 mol% or more and 1 mol% or 0.25 mol% or more and 0.5 mol% or less.

[0040] At least one element B and S selected from the group consisting of Zr, Ti, Mn, Er, Pr, In, Sn, and Ba is present on the surface of the secondary particles 37 of the composite oxide 35. Like element A, elements B and S can be present on the entire surface of the primary particles 36, including the interior of the secondary particles 37. However, they are preferably present only on the surface of the secondary particles 37 and are substantially absent from the interior of the secondary particles 37. In this case, the amount of gas generated during charged storage can be effectively suppressed. It is believed that the elements B and S are uniformly attached to the surface of the secondary particles 37 in the form of a compound, forming a coating 37B containing the elements B and S on the surface of the secondary particles 37.

[0041] Element B, which does not form a solid solution with Ni or other elements, is present on the surface of secondary particles 37. Even a small amount of element B contributes to suppressing gas generation through interaction with elements A and S. This effect becomes significant when the element B content exceeds 0.02 mol% relative to the total molar amount of metal elements excluding Li. On the other hand, when the element B content exceeds 0.5 mol%, coating layer 37B containing element B becomes a resistive layer, reducing discharge capacity.

[0042] The content of element B is preferably 0.02 mol% or more and 0.5 mol% or less, more preferably 0.04 mol% or more, and particularly preferably 0.05 mol% or more relative to the total molar amount of metal elements other than Li. From the perspective of achieving both high battery capacity and good storage characteristics, the upper limit of the content of element B is more preferably 0.5 mol% or less, and particularly preferably 0.3 mol% or less. An example of a suitable content of element B is 0.02 mol% or more and 0.5 mol% or less, or 0.04 mol% or more and 0.3 mol% or less.

[0043] On the surface of the secondary particles 37, elements B and S are preferably present outside element A. That is, in the cross section of the composite oxide 35 particles, elements B and S, and element A are present in layers in this order from the particle surface side. On the surface of the secondary particles 37, for example, a coating layer 37B containing elements B and S is formed so as to cover the coating layer 36A containing element A. It should be noted that a portion of the elements B and S may also be directly attached to the surface of the secondary particles 37.

[0044] For the composite oxide 35, 1g of the composite oxide 35 is added to a mixed solution of 100mL of pure water, 1mL of a 35% by mass hydrochloric acid aqueous solution, 0.05mL of 46% by mass hydrofluoric acid, and 0.05mL of 64% by mass nitric acid. After stirring for 5 minutes, the mixed solution is filtered to obtain a filtrate. The ratio of the partial dissolution amount of element A in the filtrate obtained by ICP-MS to the total dissolution amount of element A obtained when all 1g of the composite oxide 35 is dissolved (partial dissolution amount / total dissolution amount) × 100) is preferably 60% or more, more preferably 65% ​​or more. The partial dissolution amount measured by this method represents the amount of element A present on and near the surface of the composite oxide 35 (the same applies to S and element B). When the dissolution amount ratio of element A meets this condition, gas generation during charge storage is easily suppressed compared to the case where it is not met.

[0045] For the total dissolution amount of composite oxide 35, 200 mg of composite oxide 35 was added to a mixed solution of 5 mL of 35 mass% hydrochloric acid, 2.5 mL of 46 mass% hydrofluoric acid, and 2.5 mL of 64 mass% nitric acid. Pure water was added to the mixed solution heated at about 90°C for 2 hours, and the volume was fixed to 100 mL. The total dissolution amount was calculated by ICP-MS.

[0046] For the composite oxide 35, 1 g of the composite oxide 35 is added to a mixed solution of 100 mL of pure water, 1 mL of a 35% by mass aqueous hydrochloric acid solution, 0.05 mL of a 46% by mass hydrofluoric acid solution, and 0.05 mL of a 64% by mass nitric acid solution. After stirring for 5 minutes, the mixed solution is filtered to obtain a filtrate. The ratio of the partial elution amount of S in the filtrate determined by ICP-MS to the total elution amount of S determined when all 1 g of the composite oxide 35 is dissolved (partial elution amount / total elution amount) × 100) is preferably 50% or more, more preferably 55% or more. When the S elution amount ratio satisfies this condition, gas generation during charge storage is more easily suppressed than when it is not satisfied.

[0047] For the composite oxide 35, 1 g of the composite oxide 35 is added to a mixed solution of 100 mL of pure water, 1 mL of a 35% by mass aqueous hydrochloric acid solution, 0.05 mL of a 46% by mass hydrofluoric acid solution, and 0.05 mL of a 64% by mass nitric acid solution. After stirring for 5 minutes, the mixed solution is filtered to obtain a filtrate. The ratio of the partial elution amount of element B in the filtrate determined by ICP-MS to the total elution amount of element B determined when all 1 g of the composite oxide 35 is dissolved (partial elution amount / total elution amount) × 100) is preferably 50% or more, more preferably 55% or more. When the elution amount ratio of element B satisfies this condition, gas generation during charged storage is more easily suppressed than when it is not satisfied.

[0048] The composite oxide 35 can be produced, for example, through the following steps: a first step of obtaining a composite oxide containing metal elements such as Ni and Al; a second step of mixing the composite oxide obtained in the first step, a compound containing element A, and a Li compound to obtain a mixture; a third step of calcining the mixture; and a fourth step of adding a compound containing element B and a compound containing S and performing a heat treatment. Note that, in the fourth step, a single compound containing elements B and S may also be added.

[0049] In the first step, for example, a solution containing a metal salt such as Ni or Al is stirred while an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to alkaline (e.g., 8.5 to 12.5), thereby precipitating (coprecipitating) a composite hydroxide containing metal elements such as Ni and Al. Thereafter, the composite hydroxide is calcined to synthesize a composite oxide containing metal elements such as Ni and Al. The calcination temperature is not particularly limited, and is, for example, 300° C. or higher and 600° C. or lower.

[0050] In the second step, the composite oxide obtained in the first step, the compound containing element A, and the lithium compound are mixed to obtain a mixture. Examples of the compound containing element A include Ca(OH)2, CaO, CaCO3, CaSO4, Ca(NO3)2, Sr(OH)2, Sr(OH)2·8H2O, SrO, SrCO3, SrSO4, and Sr(NO3)2. Furthermore, examples of the lithium compound include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF.

[0051] The mixing ratio of the composite oxide obtained in the first step and the Li compound is preferably such that the molar ratio of the metal element other than Li:Li is in the range of 1:0.98 to 1:1.1. In the second step, when the composite oxide obtained in the first step, the Li compound, and the compound containing element A are mixed, other metal raw materials may be added as needed. Other metal raw materials include oxides containing metal elements other than the metal elements constituting the composite oxide obtained in the first step.

[0052] In the third step, the mixture obtained in the second step is calcined in an oxygen atmosphere. Through this step, a coating layer 36A containing element A is formed on the surface of the primary particles 36. As an example of calcination conditions, the heating rate from 450°C to 680°C is set to 1.0°C / minute to 5.5°C / minute, and the maximum temperature is set to 700°C to 850°C. The heating rate from 680°C to the maximum temperature is, for example, 0.1°C / minute to 3.5°C / minute. The holding time at the maximum temperature can be 1 hour to 10 hours.

[0053] In the fourth step, for example, a compound containing elements B and S is mixed with the fired composite oxide, and the mixture is heat-treated. Through this step, a coating 37B containing elements B and S is formed on the surface of the secondary particles 37. The fired composite oxide obtained in the third step can be washed with water by a conventionally known method. After washing, the compound containing elements B and S can be added while the powder of the composite oxide is wet, and then heat-treated (dried). The compound containing elements B and S can be added in a powdered state or in a dissolved or dispersed state in water.

[0054] Examples of compounds containing elements B and S include zirconium sulfate, titanium sulfate, manganese sulfate, erbium sulfate, praseodymium sulfate, indium sulfate, tin sulfate, and barium sulfate. It should be noted that compounds containing element B and compounds containing S may be added separately. The heat treatment temperature is, for example, 150° C. to 300° C. in a vacuum atmosphere.

[0055] [negative electrode]

[0056] The negative electrode 12 includes a negative electrode core 40 and a negative electrode composite material layer 41 provided on the surface of the negative electrode core 40. The negative electrode core 40 may be made of a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a thin film having the metal disposed on the surface. The negative electrode composite material layer 41 preferably contains a negative electrode active material and a binder, and is provided on both sides of the negative electrode core 40. The negative electrode 12 can be produced, for example, by applying a negative electrode composite material slurry containing a negative electrode active material, a conductive material, and a binder to the surface of the negative electrode core 40, drying the coating, and then compressing the coating to form the negative electrode composite material layers 41 on both sides of the negative electrode core 40.

[0057] The negative electrode composite material layer 41 contains, for example, a carbon-based active material that reversibly absorbs and releases lithium ions as the negative electrode active material. Suitable carbon-based active materials include natural graphites such as flaky graphite, bulk graphite, and earthy graphite, as well as artificial graphites such as bulk artificial graphite (MAG) and graphitized mesocarbon microbeads (MCMB). Furthermore, a Si-based active material composed of at least one of Si and a Si-containing compound can be used as the negative electrode active material, or a combination of a carbon-based active material and a Si-based active material can be used.

[0058] As the conductive material contained in the negative electrode composite material layer 41, carbon materials such as carbon black, acetylene black, Ketjen black, and graphite can be used, as in the case of the positive electrode 11. Among the binding materials contained in the negative electrode composite material layer 41, fluororesins, PAN, polyimide, acrylic resins, polyolefins, etc. can also be used, as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferably used. In addition, for the negative electrode composite material layer, it is further preferred to include CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. Among them, SBR, CMC or its salt, PAA or its salt are suitably used in combination.

[0059] [Separator]

[0060] The separator 13 uses a porous sheet with ion permeability and insulation. As specific examples of the porous sheet, microporous films, woven fabrics, non-woven fabrics, etc. can be cited. As the material of the separator 13, polyolefins such as polyethylene, polypropylene, copolymers of ethylene and α-olefins, cellulose, etc. are suitable. The separator 13 can be a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, a heat-resistant layer composed of a resin with high heat resistance such as an aromatic polyamide resin, polyimide, or polyamide-imide can be formed on the surface of the separator 13.

[0061] [Non-aqueous electrolyte]

[0062] The non-aqueous electrolyte contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of them can be used. The non-aqueous solvent may also contain a halogenated product in which at least a part of the hydrogen of these solvents is substituted with a halogen atom such as fluorine. As the halogenated product, fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP), etc. can be cited.

[0063] As examples of the above esters, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, cyclic carboxylic acid esters such as γ-butyrolactone (GBL), γ-valerolactone (GVL), chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), etc. can be cited.

[0064] As examples of the above ethers, cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ethers, chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl 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, tetraethylene glycol dimethyl ether, etc. can be cited.

[0065] The electrolyte salt is preferably a lithium salt. As examples of the lithium salt, LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 10 , 10 , 2n+1 , x , 6-x , n (C n F<00,00003>) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10、LiCl、LiBr、LiI、lithium chloroborane、lower aliphatic carboxylic acid lithium、Li2B4O7、Li(B(C2O4)F2) and other borates、LiN(SO2CF3)2、LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2) {l, m are integers greater than or equal to 0} and other imide salts. Lithium salts may be used alone or in combination. Among them, LiPF6 is preferably used from the perspectives of ion conductivity and electrochemical stability. The concentration of the lithium salt per liter of non-aqueous solvent is 0.8 mol to 1.8 mol. Furthermore, vinylene carbonate, propane sultone-based additives, etc. may be added.

[0066] <Example>

[0067] Hereinafter, the present invention will be further described based on examples, but the present invention is not limited to these examples.

[0068] <Example 1>

[0069] [Synthesis of positive electrode active material]

[0070] A composite oxide containing Ni, Co, and Al (the molar ratio of Ni, Co, and Al is 92:4:4) obtained by a coprecipitation method, calcium hydroxide, and lithium hydroxide were mixed in a specified mass ratio. The mixture was heated from room temperature to 650°C at a heating rate of 2.0°C / minute in an oxygen gas stream, and then calcined from 650°C to 730°C at a heating rate of 0.5°C / minute to obtain a calcined product. After washing the calcined product with water, a specified amount of zirconium sulfate was added, and the product was dried at 180°C for 2 hours to obtain a lithium transition metal composite oxide (positive electrode active material) containing the elements shown in Table 1.

[0071] [Evaluation of the elution ratio of element A, element B, and S]

[0072] By the above method, the partial elution amount and the total elution amount of the elements A, B, and S in the obtained positive electrode active material were determined, and their ratio ((partial elution amount / total elution amount)×100) was determined.

[0073] [Production of positive electrode]

[0074] As the positive electrode active material, the above-mentioned lithium transition metal composite oxide is used. The positive electrode active material, acetylene black and polyvinylidene fluoride are mixed at a specified solid content mass ratio, and N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium to prepare a positive electrode composite material slurry. Next, the positive electrode composite material slurry is applied to the positive electrode core composed of aluminum foil, and the coating film is dried, compressed, and cut into the specified electrode size to obtain the positive electrode.

[0075] [Production of negative electrode]

[0076] A dispersion of graphite, styrene-butadiene rubber (SBR), and sodium carboxymethylcellulose (CMC-Na) are mixed at a specified solid content ratio by mass, using water as the dispersion medium to prepare a negative electrode composite material slurry. Next, the negative electrode composite material slurry is applied to both sides of a negative electrode core made of copper foil. The coating is dried, compressed, and then cut into the specified electrode size to produce a negative electrode with a negative electrode composite material layer formed on both sides of the negative electrode core.

[0077] [Preparation of non-aqueous electrolyte]

[0078] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed at a predetermined volume ratio. LiPF6 is added to the mixed solvent to obtain a non-aqueous electrolyte solution.

[0079] [Fabrication of Test Battery Cell (Non-aqueous Electrolyte Secondary Battery)]

[0080] The positive electrode attached to an aluminum positive lead and the negative electrode attached to a nickel negative lead were spirally wound with a polyethylene separator interposed therebetween to form a flat shape, creating a wound electrode assembly. This electrode assembly was housed in an outer casing made of an aluminum laminate sheet, and the non-aqueous electrolyte was injected. The opening of the outer casing was then sealed to create a test cell for evaluation.

[0081] [Evaluation of Gas Generation During Charge Storage]

[0082] The test cell, whose volume was measured by the Archimedean method, was initially charged (CCCV charging to a cell voltage of 4.2 V) at 25°C and then allowed to stand for 15 days at 60°C in this charged state. The volume of the test cell after charging and storage was measured by the Archimedean method, and the gas generation amount was calculated based on the difference from the volume before the initial charge. The gas generation amount is shown in Table 1 as a relative value, with the gas generation amount of the test cell of Comparative Example 1, described later, set as 100.

[0083] [Evaluation of initial capacity]

[0084] The test cell was charged at a constant current of 0.3 C to a battery voltage of 4.2 V under a temperature environment of 25° C., and then discharged at a constant current of 0.2 C to a battery voltage of 2.5 V. The discharge capacity at this time is shown in Table 1 as the initial capacity.

[0085] <Example 2>

[0086] A test cell was prepared in the same manner as in Example 1 except that the amount of calcium hydroxide added was changed during the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0087] <Example 3>

[0088] A test cell was prepared in the same manner as in Example 1 except that titanium sulfate was added instead of zirconium sulfate in the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0089] <Example 4>

[0090] In the synthesis of the positive electrode active material, a composite oxide containing Ni and Al (the molar ratio of Ni to Al was 94:6) was used instead of the composite oxide containing Ni, Co, and Al. A test cell was prepared in the same manner as in Example 1, and the gas generation amount was evaluated.

[0091] <Example 5>

[0092] A test cell was prepared in the same manner as in Example 1, except that a composite oxide containing Ni and Mn (Ni:Mn molar ratio of 94:6) was used in place of the composite oxide containing Ni, Co, and Al in the synthesis of the positive electrode active material. Gas generation and other evaluations were performed. The evaluation results are shown in Table 2. Gas generation is a relative value, with the gas generation of the test cell of Comparative Example 5, described later, set as 100.

[0093] <Example 6>

[0094] A test cell was prepared in the same manner as in Example 1, except that strontium hydroxide was added instead of calcium hydroxide during the synthesis of the positive electrode active material, and the gas generation amount and other evaluations were performed. The evaluation results are shown in Table 2. The gas generation amount is a relative value, with the gas generation amount of the test cell of Comparative Example 5 described later being set as 100.

[0095] <Example 7>

[0096] A test cell was prepared in the same manner as in Example 1, except that a composite oxide containing Ni, Co, and Al (with a molar ratio of Ni, Co, and Al of 83:14:3) was used in place of the composite oxide containing Ni, Co, and Al in the synthesis of the positive electrode active material. The gas generation amount and other parameters were evaluated. The evaluation results are shown in Table 3. The gas generation amount is a relative value, with the gas generation amount of the test cell of Comparative Example 7, described later, being set as 100.

[0097] <Comparative Example 1>

[0098] A test cell was prepared in the same manner as in Example 1 except that zirconium sulfate was not added during the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0099] <Comparative Example 2>

[0100] A test cell was prepared in the same manner as in Comparative Example 1 except that the amount of calcium hydroxide added was changed during the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0101] <Comparative Example 3>

[0102] A test cell was prepared in the same manner as in Example 1 except that calcium hydroxide and zirconium sulfate were not added during the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0103] <Comparative Example 4>

[0104] In the synthesis of the positive electrode active material, a composite oxide containing Ni and Al (Ni:Al molar ratio of 94:6) was used instead of the composite oxide containing Ni, Co, and Al. A test cell was prepared in the same manner as in Comparative Example 1, and the gas generation amount and the like were evaluated.

[0105] <Comparative Example 5>

[0106] In the synthesis of the positive electrode active material, a composite oxide containing Ni and Mn (the molar ratio of Ni and Mn was 94:6) was used instead of the composite oxide containing Ni, Co and Al. A test battery cell was prepared in the same manner as in Comparative Example 1, and the gas generation amount was evaluated.

[0107] <Comparative Example 6>

[0108] A test cell was prepared in the same manner as in Comparative Example 1 except that strontium hydroxide was added instead of calcium hydroxide during the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0109] <Comparative Example 7>

[0110] A test cell was prepared in the same manner as in Example 7 except that zirconium sulfate was not added during the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0111] <Comparative Example 8>

[0112] A test cell was prepared in the same manner as in Comparative Example 1 except that zirconium oxide was added instead of zirconium sulfate during the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0113] <Comparative Example 9>

[0114] A test cell was prepared in the same manner as in Comparative Example 1 except that lithium sulfate was added instead of zirconium sulfate in the synthesis of the positive electrode active material, and the amount of gas generated and the like were evaluated.

[0115] [Table 1]

[0116]

[0117] [Table 2]

[0118]

[0119] [Table 3]

[0120]

[0121] As shown in Tables 1 to 3, the test battery cells of the embodiments have less gas generation during charge storage than the test battery cells of the comparative examples, and the storage characteristics are excellent. According to the results shown in Tables 1 to 3, when using the positive electrode active material in which element A, element B and S are not present on the surface of the primary particles of the lithium transition metal composite oxide (Comparative Example 3), the positive electrode active material in which element B and S are not present on the surface of the secondary particles (Comparative Examples 1 to 7, 9), and the positive electrode active material in which only S is absent (Comparative Example 8), the gas generation during charge storage increases. That is, through the interaction of element A, element B and S, the stability of the active material surface is improved, and the generation of gas during charge storage is specifically suppressed.

[0122] <Examples 8 to 10>

[0123] In the synthesis of the positive electrode active material, the amount of calcium hydroxide added was changed so that the amount of element A was as shown in Table 4. A test cell was prepared in the same manner as in Example 1, and the gas generation amount and the like were evaluated. The gas generation amount is a relative value with the gas generation amount of the test cell of Comparative Example 1 being set to 100.

[0124] <Examples 11 to 14>

[0125] In the synthesis of the positive electrode active material, the amount of zirconium sulfate added was changed so that the amounts of elements B and S were as shown in Table 4. A test cell was prepared in the same manner as in Example 1, and the gas generation amount and other evaluations were performed. The gas generation amount is a relative value with the gas generation amount of the test cell of Comparative Example 1 being set to 100.

[0126] [Table 4]

[0127]

[0128] As shown in Table 4, when a specified amount of element A is included, both initial capacity and good storage characteristics can be achieved. However, if the amount of element A is too small, the effect of suppressing gas generation is small. If the amount of element A is too large, although there is an effect of suppressing gas generation, the initial capacity of the battery tends to decrease (Examples 8 to 10). In addition, if the amount of element B is too large, similar to the case of excessive addition of element A, although there is an effect of suppressing gas generation, the initial capacity of the battery tends to decrease (Example 14). In other words, in order to suppress gas generation during charge storage without compromising other battery performance, it is important to control the amounts of element A, element B, and S to appropriate levels.

[0129] Description of Reference Numerals

[0130] 10Non-aqueous electrolyte secondary battery

[0131] 11 positive electrode

[0132] 12 negative electrode

[0133] 13 dividers

[0134] 14-electrode body

[0135] 16 outer cans

[0136] 17 Sealing body

[0137] 18, 19 insulation board

[0138] 20 positive lead

[0139] 21 negative lead

[0140] 22 groove part

[0141] 23 internal terminal board

[0142] 24 lower valve body

[0143] 25 Insulation components

[0144] 26 upper valve body

[0145] 27 covers

[0146] 28 gaskets

[0147] 30 positive electrode core

[0148] 31 positive electrode composite material layer

[0149] 35 Lithium transition metal composite oxide (composite oxide)

[0150] 36 primary particles

[0151] 36A, 37B coating

[0152] 37 secondary particles

[0153] 40 negative electrode core

[0154] 41 negative electrode composite material layer

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide contains 80 mol % or more of Ni relative to the total molar amount of metal elements other than Li, The lithium transition metal composite oxide includes secondary particles formed by aggregation of primary particles. At least one element A selected from Ca and Sr is present on the surface of the primary particles in an amount of 0.1 mol % or more and 0.5 mol % or less relative to the total molar amount of metal elements excluding Li, At least one element selected from Zr and Ti, B and S, are present on the surface of the secondary particles, The element B is present in an amount of 0.02 mol% or more and 0.5 mol% or less relative to the total molar amount of the metal elements excluding Li.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein The lithium transition metal composite oxide contains at least one selected from Co, Al, and Mn.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein 1 g of the lithium transition metal composite oxide is added to a mixed solution of 100 mL of pure water, 1 mL of a 35% by mass hydrochloric acid aqueous solution, 0.05 mL of a 46% by mass hydrofluoric acid, and 0.05 mL of a 64% by mass nitric acid. After stirring for 5 minutes, the mixed solution is filtered to obtain a filtrate. The ratio of the partial dissolution amount of S and element B in the filtrate determined by inductively coupled plasma mass spectrometry to the total dissolution amount of S and element B determined when all 1 g of the lithium transition metal composite oxide is dissolved is respectively greater than 50%.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein 1 g of the lithium transition metal composite oxide is added to a mixed solution of 100 mL of pure water, 1 mL of a 35% by mass hydrochloric acid aqueous solution, 0.05 mL of a 46% by mass hydrofluoric acid, and 0.05 mL of a 64% by mass nitric acid. After stirring for 5 minutes, the mixed solution is filtered to obtain a filtrate. The ratio of the partial dissolution amount of element A in the filtrate determined by inductively coupled plasma mass spectrometry to the total dissolution amount of element A determined when all 1 g of the lithium transition metal composite oxide is dissolved is greater than 60%.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 3, wherein 1 g of the lithium transition metal composite oxide is added to a mixed solution of 100 mL of pure water, 1 mL of a 35% by mass hydrochloric acid aqueous solution, 0.05 mL of a 46% by mass hydrofluoric acid, and 0.05 mL of a 64% by mass nitric acid. After stirring for 5 minutes, the mixed solution is filtered to obtain a filtrate. The ratio of the partial dissolution amount of element A in the filtrate determined by inductively coupled plasma mass spectrometry to the total dissolution amount of element A determined when all 1 g of the lithium transition metal composite oxide is dissolved is greater than 60%. 6 . A non-aqueous electrolyte secondary battery comprising: a positive electrode containing the positive electrode active material according to claim 1 , a negative electrode, and a non-aqueous electrolyte.

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