Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

This positive electrode active material contains a lithium transition metal composite oxide that contains 85% by mole or more of Ni relative to the total number of moles of metal elements excluding Li. The lithium transition metal composite oxide contains at least one metal element M1 that is selected from among Mg, Ti, Nb, Zr and V in an amount of 3% by mole or less relative to the total number of moles of metal elements excluding Li. In addition, the ratio of the concentration of the metal element M1 contained in the shell of each primary particle of the composite oxide to the concentration of the metal element M1 contained in the core is from 1.01 to 20; and at least one metal element M2 that is selected from among Ca and Sr is present on the surface of each primary particle in an amount of 1% by mole or less relative to the total number of moles of metal elements excluding Li.

Description

Technical Field

[0001] The present disclosure 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. [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-91626 [Patent Document 2] Special Publication No. 2010-535699 Summary of the Invention

[0006] A positive electrode active material with a high Ni content contributes to increasing the capacity of a battery, but has the problem of significantly reducing the capacity retention rate after storage of the battery. Note that the batteries using the positive electrode active materials of Patent Documents 1 and 2 still have room for improvement in terms of storage characteristics.

[0007] The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure comprises a lithium transition metal composite oxide containing 85 mol % or more of Ni relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide being secondary particles formed by aggregation of primary particles, and containing at least one metal element M1 selected from Mg, Ti, Nb, Zr, and V in an amount of 3 mol % or less relative to the total molar amount of metal elements excluding Li, and when a 10 nm thick region from the surface of the primary particles is defined as a shell and the inner region therebetween as a core, the ratio of the concentration of the metal element M1 contained in the shell to the concentration of the metal element M1 contained in the core is 1.01 or more and 20 or less, and at least one metal element M2 selected from Ca and Sr is present on the surface of the primary particles in an amount of 1 mol % or less relative to the total molar amount of metal elements excluding Li.

[0008] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0009] A nonaqueous electrolyte secondary battery using the positive electrode active material according to the present disclosure has excellent storage characteristics. The positive electrode active material according to the present disclosure has a high Ni content, which contributes to a high battery capacity and can sufficiently suppress a decrease in the capacity retention rate after storage of the battery. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] As mentioned above, lithium transition metal composite oxides with a high Ni content are useful positive electrode active materials that contribute to increasing the capacity and energy density of batteries. However, they have the trade-off of exhibiting a significant deterioration in capacity retention during storage tests.

[0012] As a result of intensive research to solve this problem, the inventors have found that by dissolving a predetermined amount of at least one metal element M1 selected from Mg, Ti, Nb, Zr, and V in a lithium transition metal composite oxide with a high Ni content, setting the ratio of the concentration of the metal element M1 contained in the shell to the concentration of the metal element M1 contained in the core of the primary particles to be 1.01 or more and 20 or less, and fixing a predetermined amount of at least one metal element M2 selected from Ca and Sr to the surface of the primary particles, the storage characteristics of the battery are greatly improved and a decrease in capacity retention rate in a storage test is specifically suppressed.

[0013] A coating layer containing metal element M2 is uniformly formed on the surface of the primary particles constituting the positive electrode active material according to the present disclosure. It is believed that this metal element M2 reacts and bonds with metal element M1 contained at a high concentration on the particle surface (shell), resulting in the uniform formation of a tightly bonded coating layer on the surface of the primary particles. This is thought to suppress oxygen desorption due to a side reaction between the active material and the electrolyte during storage tests, maintain a good crystalline structure of the active material, and improve storage characteristics.

[0014] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that it is initially anticipated that multiple embodiments and modifications described below may be selectively combined.

[0015] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can of the battery is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can made of a laminate sheet including a metal layer and a resin layer (laminated battery).The electrode assembly may also be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0016] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that houses the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has 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 cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0017] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0018] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. 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 direction and the width direction (short direction). 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 assembly 14 has 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.

[0019] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and 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, and the outer can 16 serves as the negative electrode terminal.

[0020] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0021] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are layered. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for 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, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0022] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0023] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode composite layer provided on the surface of the positive electrode core. The positive electrode core can be 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 film with such a metal disposed on the surface. The positive electrode composite layer contains a positive electrode active material, a conductive material, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, a binder, etc., onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode composite layer on both sides of the positive electrode core.

[0024] Examples of conductive materials contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO).

[0025] The positive electrode active material includes a lithium transition metal composite oxide containing 85 mol% or more of Ni relative to the total molar amount of metal elements excluding Li. As described above, lithium transition metal composite oxides with a high Ni content can be used as useful positive electrode active materials that contribute to increasing the capacity and energy density of batteries, but they have the problem of significantly reducing the capacity retention rate after battery storage. In the lithium transition metal composite oxide that constitutes the positive electrode active material of this embodiment, metal element M1, such as Nb, is unevenly distributed on and near the particle surface, and metal element M2, such as Ca, is scattered on the particle surface, thereby significantly improving the storage characteristics of the battery.

[0026] The positive electrode active material is mainly composed of the lithium transition metal composite oxide. Here, "main component" refers to the component that accounts for the largest mass proportion of the materials constituting the positive electrode active material. The positive electrode mixture layer may contain a composite oxide other than the lithium transition metal composite oxide as the positive electrode active material, provided that the object of the present disclosure is not impaired. However, the proportion of the lithium transition metal composite oxide is preferably 50 mass% or more, more preferably 80 mass% or more, and may be substantially 100 mass%. The positive electrode active material may be composed of two or more types of composite oxides.

[0027] The lithium transition metal composite oxide preferably contains other metal elements in addition to Li, Ni, and the metal elements M1 and M2. Examples of other metal elements include Co, Al, Mn, B, Cr, Fe, Cu, Zn, Ga, In, Sn, Ta, and W. Of these, the composite oxide preferably contains at least one selected from Co, Al, and Mn. The content of metal elements other than Li, Ni, and the metal elements M1 and M2 contained in the lithium transition metal composite oxide is preferably 15 mol % or less, and more preferably 10 mol % or less, based on the total molar amount of metal elements excluding Li.

[0028] The lithium transition metal composite oxide preferably contains at least one selected from Al and Mn. In this case, for example, the Ni content is 90 mol% or more, the Al content is 7 mol% or less, and the Mn content is 5 mol% or less, relative to the total molar amount of metal elements excluding Li. Furthermore, an example of a suitable lithium transition metal composite oxide is a composite oxide that is substantially free of Co. Since Co is rare and expensive, not using Co can reduce the manufacturing cost of the battery. Co is expected to be mixed in as an impurity, but in that case, the Co content is less than 0.5 mol%.

[0029] The lithium transition metal composite oxide is a secondary particle formed by aggregation of primary particles. The average particle size of the primary particles is, for example, 200 nm or more and 500 nm or less. The average particle size of the primary particles is determined by analyzing an SEM image of the particle cross section observed with a scanning electron microscope (SEM). For example, the positive electrode 11 is embedded in a resin, a cross section is prepared by cross section polishing (CP) processing, and this cross section is photographed with an SEM. 30 primary particles are randomly selected from the SEM image, and the grain boundaries are observed. The major axis (longest diameter) of each of the 30 primary particles is determined, and the average value is taken as the average particle size.

[0030] The volume-based median diameter (hereinafter referred to as "D50") of the secondary particles (lithium transition metal composite oxide) is, for example, 1 μm or more and 30 μm or less, and preferably 5 μm or more and 20 μm or less. D50 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the secondary particles can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) using water as the dispersion medium.

[0031] The lithium transition metal composite oxide contains at least one metal element M1 selected from Mg, Ti, Nb, Zr, and V in an amount of 3 mol % or less relative to the total molar amount of metal elements excluding Li. If the content of metal element M1 exceeds 3 mol %, the energy density of the battery tends to decrease. Even a very small amount of metal element M1 contributes to improving storage characteristics compared to a battery without metal element M1, but the content of metal element M1 is, for example, 0.01 mol % or more.

[0032] The content of the metal element M1 is preferably 0.05 mol% or more, and more preferably 0.1 mol% or more, based on the total molar amount of metal elements excluding Li. The upper limit of the content of the metal element M1 is preferably 2.5 mol%, more preferably 2 mol%. Examples of suitable contents of the metal element M1 are 0.05 mol% or more and 2 mol% or less, 0.1 mol% or more and 2 mol% or less, 0.1 mol% or more and 1.5 mol% or less, or 0.1 mol% or more and 1 mol% or less.

[0033] The lithium transition metal composite oxide may contain one type of element as the metal element M1, or may contain two or more types of elements. The metal element M1 is solid-solved with other metal elements such as Li, Ni, and Al. The lithium transition metal composite oxide contains, for example, at least one element selected from Ti, Nb, and Zr as a solid solution. Among these, Nb and Zr are preferred, and Nb is more preferred.

[0034] In the lithium transition metal composite oxide, when the range of 10 nm in thickness from the surface of the primary particle is defined as the shell and the area inside as the core, the ratio of the concentration of the metal element M1 contained in the shell to the concentration of the metal element M1 contained in the core (R-M1) is 1.01 or more and 10 or less. In other words, the metal element M1 is dissolved at a higher concentration on or near the surface than inside the primary particle of the composite oxide. The concentration ratio of the metal element M1 (R-M1) can be determined by measuring the element distribution in the cross section of the primary particle using transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDX).

[0035] When the concentration ratio (R-M1) of the metal element M1 is greater than 1, it is possible to improve storage characteristics while ensuring a higher energy density compared to when it is 1 or less, but the concentration ratio (R-M1) is preferably 1.5 or more, more preferably 2 or more. From the viewpoint of charge transfer resistance, the upper limit of the concentration ratio (R-M1) is preferably 18 or less, more preferably 15 or less. Examples of suitable concentration ratios (R-M1) are 1.5 to 18, 1.5 to 15, or 2 to 15.

[0036] At least one metal element M2 selected from Ca and Sr is present on the surface of the primary particles of the lithium transition metal composite oxide in an amount of 1 mol % or less relative to the total molar amount of metal elements excluding Li. If the content of the metal element M2 exceeds 1 mol %, the energy density of the battery tends to decrease. Even a very small amount of the metal element M2 contributes to improving storage characteristics compared to a battery without the metal element M2, but the content of the metal element M2 is, for example, 0.01 mol % or more.

[0037] The metal element M2 is evenly distributed over the surface of each primary particle, including the surface and interior (interface between primary particles) of the secondary particles. In other words, the surface of the primary particles is covered with a coating layer of the metal element M2. This coating layer may contain elements other than the metal element M2, provided that the purpose of the present disclosure is not impaired. When the concentration ratio (R-M1) of the metal element M1 is 1.01 or more and 20 or less, the metal elements M1 and M2 easily react and bond, and the metal element M2 (coating layer) is firmly bonded to the surface of the primary particles. This is thought to suppress oxygen desorption due to side reactions and improve the storage characteristics of the battery.

[0038] The metal element M2 is not dissolved like the metal element M1 but is fixed on the surface of the primary particles. In other words, the metal element M2 is substantially absent inside the primary particles. The content of the metal element M2 is preferably 0.03 mol% or more, more preferably 0.05 mol% or more, relative to the total molar amount of the metal elements excluding Li. The upper limit value of the content of the metal element M2 is preferably 0.8 mol%. An example of a preferable content of the metal element M2 is 0.03 mol% or more and 0.8 mol% or less, 0.05 mol% or more and 1 mol% or less, or 0.05 mol% or more and 0.8 mol% or less.

[0039] An example of a preferable lithium transition metal composite oxide is a composite oxide represented by the composition formula Li α Ni β Co x Al y Mn z M1 a M2 b O2 (where 0.9 ≦ α ≦ 1.2, 0.85 ≦ β ≦ 0.95, 0 ≦ x ≦ 0.05, 0 < y ≦ 0.07, 0 ≦ z ≦ 0.05, 0.0005 ≦ a ≦ 0.01, 0.0005 ≦ b ≦ 0.01). As shown by the composition formula, it is preferable to set the upper limit value of the Ni content to 0.95 mol% and add at least a predetermined amount of Al. In this case, the crystal structure is stabilized, contributing to the improvement of storage characteristics. The contents of the metal elements M1 and M2 may be substantially the same, or M1 > M2 or M1 < M2, but are preferably less than the contents of Ni, Co, Al, and Mn.

[0040] The lithium transition metal composite oxide of this embodiment can be synthesized by, for example, mixing a composite oxide containing Ni, Co, Al, and Mn, a compound containing a metal element M1 such as niobium hydroxide, a compound containing a metal element M2 such as calcium hydroxide, and a lithium compound, and then firing the mixture at a high temperature of 700°C to 850°C. The maximum temperature is maintained for 1 hour or more and 10 hours or less. Metal elements M1 such as Nb are easily dissolved in Ni and other metals, and therefore easily penetrate into the interior of the primary particles. On the other hand, metal elements M2 such as Ca are not easily dissolved in Ni and other metals, and therefore adhere to the surface of the primary particles. The concentration ratio (R-M1) of the metal element M1 tends to decrease with increasing the firing temperature and increase with decreasing the firing temperature. Therefore, the concentration ratio (R-M1) can be adjusted to the desired range by controlling the firing temperature.

[0041] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode composite layer provided on the surface of the negative electrode core. The negative electrode core can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material, a conductive material, a binder, etc. to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode composite layer on both sides of the negative electrode core.

[0042] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may also be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0043] As in the case of the positive electrode 11, the conductive material contained in the negative electrode mixture layer can be a carbon material such as carbon black, acetylene black, ketjen black, or graphite. As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). Furthermore, it is preferable that the negative electrode mixture layer further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use a combination of SBR with CMC or a salt thereof, or PAA or a salt thereof.

[0044] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefins, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as an aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 13.

[0045] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0046] Example 1 [Synthesis of positive electrode active material] A composite oxide containing Ni, Al, and Mn, niobium hydroxide, calcium hydroxide, and lithium hydroxide were mixed in a predetermined mass ratio, and the mixture was heated from room temperature to 650°C at a heating rate of 2.0°C / min in an oxygen stream, and then fired at a heating rate of 0.5°C / min from 650°C to 730°C to obtain a fired product. The fired product was washed with water and then dried to obtain a lithium transition metal composite oxide (cathode active material) containing the metal elements shown in Table 1. The contents of metal elements M1 and M2 shown in Table 1 are the amounts added externally relative to the total content of Ni, Co, Al, and Mn.

[0047] The obtained lithium transition metal composite oxide consisted of secondary particles with a D50 of 12 μm, formed by the aggregation of primary particles with an average particle size of 350 nm. TEM-EDX measurement of the element distribution in the cross section of the primary particles revealed that Nb was dissolved in the composite oxide, and the ratio of the Nb concentration in the shell (within a 10 nm thickness range from the particle surface) to the Nb concentration in the core (shell / core concentration ratio) was 9, confirming that Nb was unevenly distributed on the surface of the primary particles. Furthermore, the TEM-EDX measurement results confirmed that Ca was adhered almost uniformly to the surface of each primary particle, including the surface and interior of the secondary particles (a coating layer containing Ca was formed).

[0048] [Preparation of positive electrode] The lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a predetermined solid mass ratio, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to obtain a positive electrode.

[0049] [Preparation of negative electrode] A negative electrode composite slurry was prepared by mixing graphite, a dispersion of styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) at a predetermined solid content mass ratio, using water as a dispersion medium. Next, this negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, the coating was dried and compressed, and then cut to a predetermined electrode size to produce a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core.

[0050] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a predetermined volume ratio, and LiPF6 was added to the mixed solvent to obtain a non-aqueous electrolyte solution.

[0051] [Preparation of test cell (non-aqueous electrolyte secondary battery)] The positive electrode with an aluminum positive electrode lead attached and the negative electrode with a nickel negative electrode lead attached were spirally wound with a polyethylene separator interposed therebetween and flattened to prepare a wound electrode assembly. This electrode assembly was housed in an exterior body made of aluminum laminate, and after the nonaqueous electrolyte solution was poured into it, the opening of the exterior body was sealed to prepare a test cell for evaluation.

[0052] [Preservation test] The fabricated test cell was subjected to one charge / discharge cycle at a current of 0.5 It in a temperature environment of 25°C with a charge cut-off voltage of 4.2 V and a discharge cut-off voltage of 2.5 V to measure the discharge capacity, and then charged to 4.2 V and left to stand in a high-temperature environment of 50°C for 45 days. After storage, the test cell was discharged to 2.5 V at a current of 0.5 It in a temperature environment of 25°C, and the discharge capacity was measured.

[0053] The capacity retention rate after the storage test was calculated using the following formula, and the results are shown in Table 1.

[0054] Capacity retention rate = (discharge capacity after storage test / discharge capacity before storage test) x 100 <Example 2> Test cells were prepared in the same manner as in Example 1, except that in the synthesis of the positive electrode active material, strontium hydroxide was added in place of calcium hydroxide so as to obtain the metal element contents shown in Table 1, and a storage test was carried out.

[0055] Example 3 Test cells were prepared in the same manner as in Example 1, except that in the synthesis of the positive electrode active material, a composite oxide containing Ni, Co, and Al was added instead of the composite oxide containing Ni, Al, and Mn, and zirconium oxide was added instead of niobium hydroxide, so as to achieve the contents of the metal elements shown in Table 1, and a storage test was performed.

[0056] Example 4 Test cells were prepared in the same manner as in Example 1, except that in the synthesis of the positive electrode active material, a composite oxide containing Ni and Al was added in place of the composite oxide containing Ni, Al, and Mn so as to achieve the contents of metal elements shown in Table 1, and a storage test was performed.

[0057] <Example 5> Test cells were prepared and subjected to a storage test in the same manner as in Example 4, except that in the synthesis of the positive electrode active material, titanium oxide was added in addition to niobium hydroxide so as to achieve the metal element contents shown in Table 1. The amounts of Nb and Ti added were set to a molar ratio of 1:1.

[0058] <Comparative Example 1> A test cell was prepared in the same manner as in Example 1, except that niobium hydroxide and calcium hydroxide were not added in the synthesis of the positive electrode active material, and a storage test was carried out.

[0059] <Comparative Example 2> A test cell was prepared in the same manner as in Example 1, except that calcium hydroxide was not added in the synthesis of the positive electrode active material, and a storage test was carried out.

[0060] <Comparative Example 3> A test cell was prepared in the same manner as in Example 1, except that niobium hydroxide was not added in the synthesis of the positive electrode active material, and a storage test was carried out.

[0061] <Comparative Example 4> A test cell was prepared in the same manner as in Example 1, except that in synthesizing the positive electrode active material, the maximum temperature reached during firing was set to 780°C so that the shell / core concentration ratio was less than 1.01, and a storage test was carried out.

[0062] [Table 1]

[0063] As shown in Table 1, all of the test cells of the examples have higher capacity retention rates after the storage test and superior storage characteristics compared to the test cells of the comparative examples. The results shown in Table 1 show that the capacity retention rate after storage is significantly lower than that of the test cell of Example 1 when Ca (metal element M2) is not fixed to the surface of the primary particles of the lithium transition metal composite oxide (Comparative Example 2), when Nb (metal element M1) is not solid-dissolved in the composite oxide (Comparative Example 3), when both metal elements M1 and M2 are not added (Comparative Example 1), and when the shell / core concentration ratio of metal element M1 is 0.9 (Comparative Example 4).

[0064] In other words, adding predetermined amounts of metal elements M1 and M2 to the composite oxide and distributing metal element M1 unevenly on the surface of the composite oxide primary particles specifically improves the storage characteristics of the battery. It is believed that the reaction and bonding of metal elements M1 and M2 results in the uniform formation of a tightly bonded coating layer containing metal element M2 on the surface of the primary particles. This suppresses oxygen desorption due to side reactions between the active material and electrolyte during storage, and maintains the favorable crystalline structure of the active material, thereby improving storage characteristics.

[0065] <Comparative Example 5> A test cell was prepared in the same manner as in Example 1, except that the maximum temperature reached during firing was set to 700°C so that the shell / core concentration ratio exceeded 20 during synthesis of the positive electrode active material, and a storage test was performed. The same test cell was charged at a current of 0.5 It in a temperature environment of 25°C to a charge cut-off voltage of 4.2 V, and then AC impedance measurements were performed in the range of 10 mHz to 100 kHz, and a Cole-Cole plot was created. The charge transfer resistance of the positive electrode composite layer was calculated from the size of the approximate semicircle appearing in the obtained Cole-Cole plot. For Examples 1 and 2, the charge transfer resistance of the positive electrode composite layer was calculated in the same manner, and the evaluation results are shown in Table 2.

[0066] [Table 2]

[0067] The test cell of Comparative Example 5 had a capacity retention rate after the storage test equivalent to that of the test cells of the Examples. However, as shown in Table 2, the charge transfer resistance of the positive electrode mixture layer was significantly higher than those of Examples 1 and 2, making it difficult to put the configuration of Comparative Example 5 into practical use. In other words, only when the Shell / Core concentration ratio of the metal element M1 in the positive electrode active material satisfies the condition of 1.01 or more and 20 or less, can the storage characteristics be improved without impairing other battery performance such as charge transfer resistance.

[0068] In this example, Nb, Zr, and Ti were used as the metal element M1, but similar effects of improving storage characteristics can also be obtained when Mg and V are used. [Explanation of symbols]

[0069] 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 16 outer can 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Grooved part 23 Internal terminal board 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket

Claims

1. a lithium transition metal composite oxide containing 90 mol % or more of Ni relative to the total molar amount of metal elements excluding Li; The lithium transition metal composite oxide is a secondary particle formed by aggregation of primary particles, At least one metal element M1 selected from Mg, Ti, Nb, Zr, and V is contained in an amount of 0.01 mol % or more and 3 mol % or less with respect to the total molar amount of metal elements excluding Li, When a region of 10 nm in thickness from the surface of the primary particle is defined as a shell and a region inside the shell is defined as a core, the ratio of the concentration of the metal element M1 contained in the shell to the concentration of the metal element M1 contained in the core is 1.01 or more and 20 or less, at least one metal element M2 selected from Ca and Sr is present on the surface of the primary particles in an amount of 0.01 mol % or more and 1 mol % or less with respect to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide contains at least one selected from Al and Mn, and when Al is contained, the Al content is 5 mol % or more and 7 mol % or less, relative to the total molar amount of metal elements excluding Li, and when Mn is contained, the Mn content is 3 mol % or more and 5 mol % or less.

2. a lithium transition metal composite oxide containing 85 mol% or more of Ni relative to the total molar amount of metal elements excluding Li; The lithium transition metal composite oxide is a secondary particle formed by aggregation of primary particles, At least one metal element M1 selected from Mg, Ti, Nb, Zr, and V is contained in an amount of 0.01 mol % or more and 3 mol % or less with respect to the total molar amount of metal elements excluding Li, When a region of 10 nm in thickness from the surface of the primary particle is defined as a shell and a region inside the shell is defined as a core, the ratio of the concentration of the metal element M1 contained in the shell to the concentration of the metal element M1 contained in the core is 1.01 or more and 20 or less, at least one metal element M2 selected from Ca and Sr is present on the surface of the primary particles in an amount of 0.01 mol % or more and 1 mol % or less with respect to the total molar amount of metal elements excluding Li, The lithium transition metal composite oxide is a positive electrode active material for a non-aqueous electrolyte secondary battery, which is substantially free of Co.

3. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material according to claim 1 or 2, a negative electrode, and a non-aqueous electrolyte.

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