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

Abstract

A positive electrode active material 32 according to an example of an embodiment of the present invention contains first composite oxides 33 and second composite oxides 34. The first composite oxides 33 are secondary particles formed as a result of primary particles 35 with a mean particle diameter of 0.3 μm or less being aggregated, and the second composite oxides 34 contain 《MK1》primary particles having a particle diameter of 0.5 μm or more《 / MK1》. D50a of the first composite oxides 33 is larger than D50b of the second composite oxides 34. The ratio of the mean cross-section area (Sb) of the second composite oxides 34 with respect to the mean cross-section area (Sa) of the primary particles 35 constituting the first composite oxides 33 satisfies the condition 80 ≤ (Sb / Sa) ≤ 600.

Description

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

[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.

[0002] The properties of a positive electrode active material, which is a main component of a non-aqueous electrolyte secondary battery, have a significant effect on performance such as battery capacity and charge / discharge cycle characteristics, and therefore various studies have been conducted on the positive electrode active material. For example, Patent Documents 1 and 2 disclose techniques using, as a positive electrode active material, a mixture of secondary particles formed by agglomeration of many primary particles and single particles formed by one or several (10 or less) primary particles.

[0003] JP 2022-63677 A JP 2019-021627 A

[0004] However, as a result of investigations by the present inventors, it was found that when single particles (also called non-aggregated particles) are used as the positive electrode active material, although the cycle characteristics are improved, the capacity is significantly reduced. In other words, simply using a mixture of appropriate secondary particles and single particles as the positive electrode active material, as in the techniques of Patent Documents 1 and 2, is not enough to achieve both high capacity and high durability (cycle characteristics).

[0005] The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure is a positive electrode active material comprising a first lithium-containing transition metal composite oxide and a second lithium-containing transition metal composite oxide, wherein the first lithium-containing transition metal composite oxide is a secondary particle formed by agglomeration of primary particles having an average particle size of 0.3 μm or less, the second lithium-containing transition metal composite oxide comprises primary particles having an average particle size of 0.5 μm or more, the volume-based median diameter (D50a) of the first lithium-containing transition metal composite oxide is larger than the volume-based median diameter (D50b) of the second lithium-containing transition metal composite oxide (D50a>D50b), and the ratio (Sb / Sa) of the average cross-sectional area (Sb) of the primary particles constituting the second lithium-containing transition metal composite oxide to the average cross-sectional area (Sa) of the primary particles constituting the first lithium-containing transition metal composite oxide is 80 to 600 (80≦(Sb / Sa)≦600).

[0006] The non-aqueous electrolyte secondary battery according to the present disclosure is characterized by comprising a positive electrode containing the above-described positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0007] The positive electrode active material according to the present disclosure can realize a nonaqueous electrolyte secondary battery with high capacity and excellent cycle characteristics.

[0008] 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention; 2 is a cross-sectional view of a particle of a positive electrode active material according to an embodiment of the present invention;

[0009] As described above, achieving both high capacity and high durability is an important challenge for non-aqueous electrolyte secondary batteries. As a result of extensive research aimed at solving this challenge, the inventors have found that the average cross-sectional area ratio (Sb / Sa) of the primary particles constituting the first composite oxide, which is a secondary particle formed by agglomeration of primary particles having an average particle size of 0.3 μm or less, and the second composite oxide, which contains primary particles of 0.5 μm or more, is an important factor in achieving both high capacity and high durability.

[0010] As described above, when the relationship between the average cross-sectional areas of the primary particles constituting the first and second composite oxides satisfies the condition 80≦(Sb / Sa)≦600, a nonaqueous electrolyte secondary battery with high capacity and excellent durability can be realized. This effect is specifically manifested only when the condition 80≦(Sb / Sa)≦600 is satisfied.

[0011] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that the present disclosure also includes embodiments that are formed by selectively combining multiple embodiments and modifications described below.

[0012] 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 rectangular outer can (prismatic battery) or 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.

[0013] FIG. 1 is a schematic diagram illustrating a longitudinal cross section 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 accommodates 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 with a bottom that is open on one axial side, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience 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.

[0014] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like 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 precipitation. That is, the negative electrode 12 is formed to be longer in the longitudinal direction and width direction (short direction) than the positive electrode 11. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and two separators 13 are arranged 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.

[0015] 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.

[0016] As described above, the outer can 16 is a cylindrical metal container with a bottom and an opening on one axial side. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure sealing of the battery interior and insulation between the outer can 16 and the sealing body 17. The outer can 16 has a grooved portion 22 that supports the sealing body 17, with part of the side surface protruding inward. 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.

[0017] 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 stacked. 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. If an abnormality occurs in the battery and the internal pressure increases, 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. If the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0018] 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.

[0019] [Positive Electrode] The positive electrode 11 includes a positive electrode core material 30 and a positive electrode mixture layer 31 formed on the surface of the positive electrode core material 30. The positive electrode core material 30 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 layer. An example of the positive electrode core material 30 is an aluminum or aluminum alloy foil having a thickness of 10 μm to 20 μm. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on both sides of the positive electrode core material 30. The thickness of the positive electrode mixture layer 31 is, for example, 30 μm to 100 μm on one side of the positive electrode core material 30. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. onto the positive electrode core material 30, drying the coating, and then compressing it to form a positive electrode mixture layer 31 on both sides of the positive electrode core material 30.

[0020] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes, carbon nanofibers, graphene, and other carbon materials. The content of the conductive agent is, for example, 0.01 to 10 parts by mass, and preferably 0.05 to 5 parts by mass, per 100 parts by mass of the positive electrode active material.

[0021] Examples of the binder contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, etc. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), etc. The content of the binder is, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of the positive electrode active material.

[0022] FIG. 2 is a diagram schematically illustrating a particle cross section of a positive electrode active material 32 according to an embodiment. As shown in FIG. 2 , the positive electrode active material 32 includes a first lithium-containing transition metal composite oxide 33 (hereinafter referred to as the "first composite oxide 33") and a second lithium-containing transition metal composite oxide 34 (hereinafter referred to as the "second composite oxide 34"). The positive electrode mixture layer 31 of this embodiment substantially contains only the first and second composite oxides 33, 34 as the positive electrode active material 32. The positive electrode mixture layer may also contain a third lithium-containing transition metal composite oxide, provided that the object of the present disclosure is not impaired. An example of the third composite oxide is a composite oxide that does not satisfy the particle size condition described below.

[0023] The first and second composite oxides 33, 34 are particles of composite oxides containing metal elements such as Ni, Co, Mn, and Al in addition to Li. The first and second composite oxides 33, 34 have, for example, a layered rock salt structure. Examples of the layered rock salt structure 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. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of increasing capacity and crystalline structure stability.

[0024] The first and second composite oxides 33 and 34 have the composition formula Li ｘ Ni ｙ M （１－ｙ） O ２ (0.95≦x≦1.40, 0.4≦y≦1.0, M is at least one element selected from the group consisting of Li, Mn, Co, Ca, Sr, Al, Ti, Zr, Fe, Nb, Ta, W, Mo, Si, Bi, B, P, V, Eu, La, and Sb) is preferred. In particular, it is preferred that the metal element M contains at least one element selected from the group consisting of Mn, Co, and Al.

[0025] From the viewpoint of increasing capacity, the first and second composite oxides 33, 34 preferably contain 70 mol% or more, and more preferably 80 mol% or more, of Ni relative to the total number of moles of metal elements excluding Li. Furthermore, as will be described in detail later, the effect of controlling the cross-sectional area ratio of the primary particles constituting each composite oxide is more pronounced when a composite oxide with a high Ni content is used. The Ni content may be 85 mol% or more, or even 90 mol% or more, relative to the total number of moles of metal elements excluding Li. The upper limit of the Ni content is, for example, 95 mol%.

[0026] The first and second composite oxides 33 and 34 may have, for example, substantially the same composition, or may have different compositions as long as the composition satisfies the above-mentioned composition formula. The content of the elements constituting the composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0027] The first composite oxide 33 is a secondary particle formed by agglomeration of primary particles 35 having an average particle size of 0.3 μm or less. The second composite oxide 34 contains primary particles having a size of 0.5 μm or more and is also called a non-agglomerated particle or a single particle. The second composite oxide 34 is, for example, a single-crystal primary particle having no internal grain boundaries. The second composite oxide 34 may contain 10 or fewer primary particles, or 5 or fewer primary particles. The crystallinity of the composite oxide can be confirmed using a scanning ion microscope (SICM). The primary particles constituting one particle of the composite oxide are adhered to each other with enough strength to prevent separation even when a strong force is applied, such as during the preparation of a positive electrode mixture slurry.

[0028] The volumetric median diameter (D50a) of the first composite oxide 33 is larger than the volumetric median diameter (D50b) of the second composite oxide 34 (D50a>D50b). That is, in the first composite oxide 33, the particle size of the primary particles 35 constituting the first composite oxide 33 is smaller than the particle size of the primary particles constituting the second composite oxide 34, and the particle size of the entire particles is larger than the particle size of the second composite oxide 34. If this condition is not satisfied, it is impossible to achieve both high capacity and high durability.

[0029] D50a and D50b of the first and second composite oxides 33 and 34 mean particle sizes at which the cumulative frequency of smallest particle sizes reaches 50% in the volume-based particle size distribution. The particle size distribution of the first and second composite oxides 33 and 34 can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II manufactured by Microtrac-Bell Co., Ltd.) with water as the dispersion medium.

[0030] The ratio of D50a of the first composite oxide 33 to D50b of the second composite oxide 34 (D50a / D50b) should be greater than 1.0, but is preferably 1.5 to 10 (1.5≦(D50a / D50b)≦10). When the condition 1.5≦(D50a / D50b)≦10 is satisfied, it becomes easier to achieve both high capacity and high durability compared to when this condition is not satisfied. D50a / D50b is more preferably 2.0 to 7.0, and particularly preferably 2.5 to 4.0.

[0031] The D50a of the first composite oxide 33 is, for example, 10 μm to 30 μm, preferably 12 μm to 25 μm, and more preferably 15 μm to 20 μm. The D50b of the second composite oxide 34 is, for example, 0.5 μm to less than 10 μm, preferably 1 μm to 8 μm, and more preferably 3 μm to 7 μm. When the D50a and D50b of each composite oxide are within the above ranges and the D50a / D50b ratio satisfies the above condition, it becomes easy to achieve both high capacity and high durability.

[0032] The particle diameters of the primary particles constituting the first and second composite oxides 33 and 34 are determined by imaging the cross sections of the particles with a scanning electron microscope (SEM) and analyzing the SEM image. For example, the positive electrode or composite oxide is embedded in a resin, a cross section is prepared by cross-section polishing (CP) processing, and this cross section is imaged with an SEM. 50 primary particles are randomly selected from the SEM image, and the grain boundaries are observed. The area of ​​each of the 50 primary particles is determined, and the diameter of a circle corresponding to that area is calculated. The average diameter is determined as the average particle diameter.

[0033] As described above, the average particle size of the primary particles 35 constituting the first composite oxide 33 is 0.3 μm or less, more preferably 0.02 μm to 0.20 μm, and particularly preferably 0.05 μm to 0.15 μm. The particle size of the primary particles constituting the second composite oxide 34 is 0.5 μm or more; in other words, the second composite oxide 34 is a composite oxide particle containing primary particles with a particle size of 0.5 μm or more. The average particle size of the primary particles of the second composite oxide 34 is preferably 0.8 μm or more, more preferably 0.8 μm to 4.0 μm, and particularly preferably 1.0 μm to 3.0 μm.

[0034] The ratio (Sb / Sa) of the average cross-sectional area (Sb) of the primary particles constituting the second composite oxide 34 to the average cross-sectional area (Sa) of the primary particles 35 constituting the first composite oxide 33 is 80 to 600 (80≦(Sb / Sa)≦600). That is, the cross-sectional area of ​​each primary particle of the second composite oxide 34 is, on average, 80 to 600 times larger than the cross-sectional area of ​​each primary particle 35 of the first composite oxide 33. When this condition is satisfied, a nonaqueous electrolyte secondary battery with high capacity and excellent durability can be realized.

[0035] For example, if the cross-sectional area of ​​the primary particles 35 of the first composite oxide 33 is increased to make Sb / Sa less than 80 without changing the cross-sectional area of ​​the primary particles of the second composite oxide 34, it is believed that the cycle characteristics will deteriorate due to the generation of new surfaces caused by particle cracking during charge and discharge. From the viewpoint of cycle characteristics, a smaller average cross-sectional area (Sa) of the primary particles 35 is preferable, but if Sa becomes too small, a tendency for a decrease in capacity is observed.

[0036] Although it varies somewhat depending on the composition of the composite oxide, a suitable range of Sb / Sa is 100 to 500, more preferably 150 to 400, and particularly preferably 200 to 350. The average cross-sectional area (Sa) of the primary particles 35 of the first composite oxide 33 is, for example, 0.001 μm ２ ~0.050μm ２ and preferably 0.005 μm ２ ~0.025 μm ２ The average cross-sectional area (Sb) of the primary particles of the second composite oxide 34 is, for example, 0.50 μm ２ ~5.0 μm ２and preferably 1.0 μm ２ ~4.0 μm ２ is.

[0037] The mixing ratio of the first and second composite oxides 33, 34 is not particularly limited, and the content of the first composite oxide 33 may be, for example, 10% to 90% of the total mass of the positive electrode active material 32. Preferably, however, the content of the first composite oxide 33 is equal to or greater than the content of the second composite oxide 34. The first composite oxide 33 is preferably contained in an amount of 50% to 90% of the total mass of the positive electrode active material 32, and more preferably in an amount of 60% to 85%. If the content of the first composite oxide 33 is within this range, it becomes easy to achieve both high capacity and high durability. In this embodiment, since the positive electrode active material 32 essentially contains only the first and second composite oxides 33, 34, the content of the second composite oxide 34 is preferably 10% to 50% of the total mass of the positive electrode active material 32, and more preferably 15% to 40%.

[0038] The first and second composite oxides 33 and 34 can be synthesized by the method described in the Examples below. The first composite oxide 33 can be synthesized, for example, by lowering the firing temperature compared to the synthesis of the second composite oxide 34. Furthermore, when synthesizing the first composite oxide 33, an element that suppresses crystal growth may be added. The particle size and cross-sectional area of ​​the primary particles vary depending on the type of added element. For example, in a lithium-containing transition metal composite oxide containing 80 mol % Ni and 20 mol % Mn relative to the metal elements excluding Li, adding 0.1 to 1 mol % of Ca, Sr, Al, Ti, Zr, Fe, Nb, Ta, W, Mo, Si, Bi, B, P, V, Eu, La, Sb, or the like tends to result in a smaller cross-sectional area of ​​the primary particles compared to when these elements are not added.

[0039] The mixing ratio of the first and second complex oxides 33, 34 may vary in the thickness direction of the positive electrode mixture layer 31. As an example, when the positive electrode mixture layer 31 is divided into two equal parts in the thickness direction and defined as a first region and a second region in that order from the surface side of the positive electrode 11, the mixing ratio may be varied between the first region and the second region. In this embodiment, the mixing ratio of the first and second complex oxides 33, 34 is substantially constant throughout the positive electrode mixture layer 31.

[0040] [Negative Electrode] The negative electrode 12 includes a negative electrode core material 40 and a negative electrode mixture layer 41 formed on the surface of the negative electrode core material 40. The negative electrode core material 40 can be a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. An example of the negative electrode core material 40 is a copper or copper alloy foil having a thickness of 5 μm to 15 μm. The negative electrode mixture layer 41 contains a negative electrode active material and a binder, and is preferably formed on both sides of the negative electrode core material 40. The thickness of the negative electrode mixture layer 41 is, for example, 30 μm to 150 μm on one side of the negative electrode core material 40. The negative electrode 12 can be produced by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc. to the negative electrode core material 40, drying the coating, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core material 40.

[0041] The negative electrode composite layer 41 generally contains, as the negative electrode active material, a carbon material that reversibly absorbs and releases lithium ions. Suitable examples of the carbon material include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). Alternatively, the negative electrode active material may be an active material containing at least one of an element that alloys with Li, such as Si or Sn, and a material containing such an element. A suitable example of the active material is SiO ２ The negative electrode active material is a Si-containing material in which fine Si phases are dispersed in an ion-conducting phase such as a silicon phase, a silicate phase such as lithium silicate, or an amorphous carbon phase. Graphite and a Si-containing material may be used in combination as the negative electrode active material.

[0042] The binder contained in the negative electrode mixture layer 41 can be a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, polyolefin, or styrene-butadiene rubber (SBR). The negative electrode mixture layer 41 may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, or polyvinyl alcohol (PVA). The binder content is, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of the negative electrode active material. A conductive agent such as carbon black or carbon nanotubes may also be added to the negative electrode mixture layer 41.

[0043] [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 and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0044] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0045] The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0046] Example 1 Synthesis of First Composite Oxide First, a composite hydroxide containing Ni and Mn in a molar ratio of 8:2 (Ni ０．８ Mn ０．２ (OH) ２ The composite hydroxide obtained was heated in air at 700°C for 2 hours to obtain the composite oxide (Ni ０．８ Mn ０．２ O ２ The resulting composite oxide (Ni ０．８ Mn ０．２ O ２ ) with 0.002 moles of WO per mole of Ni and Mn combined. ３ and further, 1.11 moles of lithium hydroxide monohydrate (LiOH·H ２O) were mixed, and the resulting mixture was fired. Specifically, the mixture was fired under an oxygen stream with an oxygen concentration of 95% (flow rate of 5 L / min per 1 kg of mixture) at a temperature increase rate of 2.0°C / min from room temperature to 650°C, and then at a temperature increase rate of 1°C / min from 650°C to 820°C. A first composite oxide was obtained. This composite oxide is a secondary particle formed by the aggregation of a large number of primary particles, and its composition, D50, and average cross-sectional area (Sa) of the primary particles are as follows: D50: 16.2 μm Sa: 0.022 μm ２

[0047] [Synthesis of Second Composite Oxide] First, a composite hydroxide containing Ni and Mn in a molar ratio of 8:2 (Ni ０．８ Mn ０．２ (OH) ２ The composite hydroxide obtained was heated in air at 700°C for 2 hours to obtain the composite oxide (Ni ０．８ Mn ０．２ O ２ ) was converted into 1.11 moles of lithium hydroxide monohydrate (LiOH·H ２ O) were mixed, and the resulting mixture was fired. Specifically, the mixture was fired under an oxygen stream with an oxygen concentration of 95% (flow rate of 5 L / min per 1 kg of mixture) at a temperature increase rate of 2.0°C / min from room temperature to 650°C, and then at a temperature increase rate of 1°C / min from 650°C to 870°C. A second composite oxide was obtained. This composite oxide is a particle (non-agglomerated particle) containing one or 10 or less primary particles, and its composition, D50, and average cross-sectional area (Sb) of the primary particles are as follows: D50: 5.43 μm Sb: 2.02 μm ２

[0048] [Preparation of Positive Electrode] A mixture of a first composite oxide and a second composite oxide in a mass ratio of 70:30 was used as the positive electrode active material. This positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a mass ratio of 98:1:1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode core material made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode core material.

[0049] [Fabrication of Negative Electrode] As the negative electrode active material, graphite powder and SiO ｘ A mixture of the negative electrode active material and a Si-containing material represented by the formula (I) was used, in a mass ratio of 95:5. This negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and a dispersion of styrene-butadiene rubber (SBR) were mixed in a solids mass ratio of 98:1:1, and a negative electrode mixture slurry was prepared using water as a dispersion medium. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode core material made of copper foil, and the coating was dried and compressed. The negative electrode core was then cut to a predetermined electrode size, resulting in a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core material.

[0050] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 20:5:75 (25°C). LiPF was added to this mixed solvent to a concentration of 1.4 mol / L. ６ was dissolved to prepare a non-aqueous electrolyte solution.

[0051] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] Leads were attached to the positive electrode and the negative electrode, respectively, and the positive electrode and the negative electrode were spirally wound with a separator interposed therebetween to obtain a wound electrode assembly. The electrode assembly was housed in a cylindrical outer can with a bottom, and the negative electrode lead was welded to the inner bottom surface of the outer can, and the positive electrode lead was welded to the internal terminal plate of the sealing body. The non-aqueous electrolyte solution was then poured into the outer can, and the opening edge of the outer can was crimped and fixed to the sealing body to produce a cylindrical non-aqueous electrolyte secondary battery.

[0052] Example 2 In the synthesis of the first composite oxide, WO ３ By setting the amount of addition to 0.0025 moles per mole of the total of Ni and Mn, the average cross-sectional area (Sa) of the primary particles constituting the secondary particles is 0.016 μm ２ Except for this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1. In this case, the cross-sectional area ratio (Sb / Sa) of the primary particles constituting each composite oxide was 126.3.

[0053] Example 3 In the synthesis of the first composite oxide, WO ３ By setting the amount of addition to 0.005 moles per mole of Ni and Mn in total, the average cross-sectional area (Sa) of the primary particles constituting the secondary particles is 0.009 μm ２ Except for this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1. In this case, the cross-sectional area ratio (Sb / Sa) of the primary particles constituting each composite oxide was 224.4.

[0054] Example 4 In the synthesis of the first composite oxide, WO ３ The amount of Nb added is 0.005 moles per mole of Ni and Mn combined, and 0.0025 moles of Nb is added per mole of Ni and Mn combined, so that the average cross-sectional area (Sa) of the primary particles constituting the secondary particles is 0.006 μm ２ Except for this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1. In this case, the cross-sectional area ratio (Sb / Sa) of the primary particles constituting each composite oxide was 336.7.

[0055] Example 5: A second composite oxide having the composition formula LiNi ８８ Co ９ Mn ３ O ２ The average cross-sectional area (Sb) of the primary particles constituting the secondary particles is 3.00 μm. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 3, except that a composite oxide having the formula:

[0049] In this case, the cross-sectional area ratio (Sb / Sa) of the primary particles constituting each composite oxide was 333.3.

[0056] Example 6: A second composite oxide having the composition formula LiNi ８８ Co９ Mn ３ O ２ The average cross-sectional area (Sb) of the primary particles constituting the secondary particles is 3.00 μm. ２ A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 4, except that a composite oxide having the formula:

[0049] In this case, the cross-sectional area ratio (Sb / Sa) of the primary particles constituting each composite oxide was 500.0.

[0057] Comparative Example 1 In the synthesis of the first composite oxide, WO ３ The sample was fired from room temperature to 650°C at a temperature increase rate of 2.0°C / min without adding any additives, and then fired from 650°C to 840°C at a temperature increase rate of 1°C / min. ２ Except for this, a positive electrode active material and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 1. In this case, the cross-sectional area ratio (Sb / Sa) of the primary particles constituting each composite oxide was 29.3.

[0058] Comparative Example 2 In the synthesis of the first composite oxide, WO ３ The sample was fired from room temperature to 650°C at a temperature increase rate of 2.0°C / min without adding any additives, and then fired from 650°C to 820°C at a temperature increase rate of 1°C / min. ２ Except for this, a positive electrode active material and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 1. In this case, the cross-sectional area ratio (Sb / Sa) of the primary particles constituting each composite oxide was 74.8.

[0059] <Comparative Example 3> A positive electrode active material having a D50 of 11.9 μm and an average cross-sectional area (Sa) of the primary particles constituting the secondary particles of 0.063 μm ２ A positive electrode active material and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that the first composite oxide represented by the formula (I) was used alone.

[0060] For each battery of the Examples and Comparative Examples, the initial charge capacity, initial discharge capacity, and capacity retention rate after cycle testing were evaluated by the following methods, and the evaluation results, along with the physical properties of the positive electrode active material, are shown in Table 1. In Table 1, Examples 1 to 6 are designated A1 to A6, and Comparative Examples 1 to 3 are designated B1 to B3.

[0061] [Evaluation of Charge / Discharge Capacity] The batteries to be evaluated were charged at a constant current of 0.2 C at a temperature of 25°C until the battery voltage reached 4.5 V, and then charged at a constant voltage of 0.02 C at 4.5 V. The batteries were then discharged at a constant current of 0.2 C until the battery voltage reached 2.5 V. The charge and discharge capacities per gram of positive electrode active material were calculated from the charge / discharge curves obtained at this time. The evaluation results shown in Table 1 are relative values, with the value for the battery of Comparative Example 1 (B1) set to 100.

[0062] [Evaluation of Cycle Characteristics (Capacity Retention Rate)] The above charge / discharge cycle was repeated 50 times, and the capacity retention rate was calculated by the following formula: Capacity retention rate = (Discharge capacity at 50th cycle / Discharge capacity at 1st cycle) x 100

[0063]

[0064] As shown in Table 1, the batteries of Examples 1 to 6 all have a higher capacity retention rate after the cycle test and are excellent in cycle characteristics compared to the batteries of Comparative Examples 1 to 3. Furthermore, the batteries of the Examples have excellent cycle characteristics while ensuring high capacity, achieving both high capacity and high durability.

[0065] In contrast, the battery of Comparative Example 3, which used only the first composite oxide (secondary particles) as the positive electrode active material, had a high charge / discharge capacity but a lower capacity retention rate after the cycle test compared to the batteries of Examples. Similarly, a decrease in cycle characteristics was confirmed for the batteries of Comparative Examples 1 and 2, which used a positive electrode active material in which the average cross-sectional area ratio (Sb / Sa) of the primary particles was less than 80.

[0066] REFERENCE SIGNS LIST 10 Non-aqueous 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 electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode core material, 31 Positive electrode mixture layer, 32 Positive electrode active material, 33 First composite oxide, 34 Second composite oxide, 35 Primary particles, 40 Negative electrode core body, 41 Negative electrode mixture layer

Claims

1. A positive electrode active material comprising a first lithium-containing transition metal composite oxide and a second lithium-containing transition metal composite oxide, the first lithium-containing transition metal composite oxide is a secondary particle formed by agglomeration of primary particles having an average particle size of 0.3 μm or less, The second lithium-containing transition metal composite oxide contains primary particles having a size of 0.5 μm or more, The volume-based median diameter (D50a) of the first lithium-containing transition metal composite oxide is larger than the volume-based median diameter (D50b) of the second lithium-containing transition metal composite oxide (D50a>D50b); a ratio (Sb / Sa) of an average cross-sectional area (Sb) of the primary particles constituting the second lithium-containing transition metal composite oxide to an average cross-sectional area (Sa) of the primary particles constituting the first lithium-containing transition metal composite oxide is 80 to 600 (80≦(Sb / Sa)≦600).

2. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein a ratio (D50a / D50b) of a volume-based median diameter (D50a) of the first lithium-containing transition metal composite oxide to a volume-based median diameter (D50b) of the second lithium-containing transition metal composite oxide is 1.5 to 10 (1.5≦(D50a / D50b)≦10).

3. The first and second lithium-containing transition metal composite oxides have a composition formula Li ｘ Ni ｙ M （１－ｙ） O ２ 2. The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, which is a composite oxide represented by the formula: (0.95≦x≦1.40, 0.4≦y≦1.0, M is at least one selected from the group consisting of Li, Mn, Co, Ca, Sr, Al, Ti, Zr, Fe, Nb, Ta, W, Mo, Si, Bi, B, P, V, Eu, La, and Sb).

4. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the first lithium-containing transition metal composite oxide is contained in an amount of 50% to 90% based on the total mass of the positive electrode active material.

5. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 4; A negative electrode; A non-aqueous electrolyte; A non-aqueous electrolyte secondary battery comprising: