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

Abstract

A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure has a crystal structure belonging to space group R-3m, and is characterized in that the positive electrode active material is composed of secondary particles in which primary particles are aggregated, the secondary particles contain voids, and in a particle cross-section of the secondary particles, the particle cross-section of the secondary particles is larger than the particle cross-section of the secondary particles, the particle cross-section of the secondary particles is larger than the particle cross-section of the voids. The ratio of the area of the voids to the area of the particle cross-section is 15% or less, and the average aspect ratio of the voids is 2.1 or more. According to the positive electrode active material of the present disclosure, a high capacity of a non-aqueous electrolyte secondary battery can be achieved.

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. Background Art

[0002] In non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, the positive electrode active material significantly influences battery performance, including input / output characteristics, capacity, and durability. Consequently, extensive research has been conducted on positive electrode active materials. Lithium-transition metal composite oxides containing transition metal elements such as Ni, Mn, and Co are commonly used as positive electrode active materials. The type and amount of elements contained in lithium-transition metal composite oxides, as well as the crystal structure and particle structure of the composite oxide, significantly influence battery performance. Sometimes, even a slight change in their physical properties can prevent the target performance from being achieved.

[0003] For example, Patent Documents 1 and 2 focus on the particle structure of positive electrode active materials with the goal of improving battery performance, such as increasing capacity. Patent Document 1 discloses a positive electrode active material composed of two positive electrode active materials, wherein the second positive electrode active material is a particle having a hollow structure, and the voids present in the particle have a predetermined length and aspect ratio. Patent Document 2 discloses a positive electrode active material having a hollow structure with an aspect ratio of 1 or more and 1.33 or less, and containing a predetermined amount of voids in the center of the particle.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-120937

[0007] Patent Document 2: Japanese Patent Application Publication No. 2019-110136 Summary of the Invention

[0008] In recent years, non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have been used as power sources for driving vehicles, and further higher capacity is required. The positive electrode active materials of Patent Documents 1 and 2 still have much room for improvement from the perspective of higher capacity.

[0009] A positive electrode active material for a non-aqueous electrolyte secondary battery as one embodiment of the present invention is characterized in that it is a positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, and is composed of secondary particles formed by the aggregation of primary particles. The secondary particles contain voids, and in the particle cross-section of the secondary particles, the ratio of the area of ​​the voids to the area of ​​the particle cross-section is less than 15%, and the average aspect ratio of the voids is greater than 2.1.

[0010] As another embodiment of the present invention, a positive electrode active material for a non-aqueous electrolyte secondary battery is characterized in that it is a positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, and is composed of secondary particles formed by agglomeration of primary particles, and the secondary particles contain voids, and the voids include a first void having an aspect ratio of 6.0 or greater and a second void having an aspect ratio of less than 6.0, and in the particle cross section of the secondary particles, the average value of the area of ​​each void is 0.018 μm 2 Hereinafter, the ratio of the area of ​​the first voids to the area of ​​the particle cross section is 0.2% or more.

[0011] The nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode containing the above-mentioned positive electrode active material, a negative electrode, and a nonaqueous electrolyte.

[0012] According to the positive electrode active material according to the present disclosure, it is possible to increase the capacity of a non-aqueous electrolyte secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a longitudinal cross-sectional view of a nonaqueous electrolyte secondary battery as an example of an embodiment.

[0014] Figure 2 This is an image of a cross-section of particles of the positive electrode active material of Example 1.

[0015] Figure 3 This is an image of a cross-section of particles of the positive electrode active material of Example 2.

[0016] Figure 4 This is an image of a cross-section of particles of the positive electrode active material of Comparative Example 1. DETAILED DESCRIPTION

[0017] The present inventors have conducted extensive research on increasing the capacity of non-aqueous electrolyte secondary batteries and have discovered that, in a positive electrode active material having a crystal structure belonging to the space group R-3m, the introduction of voids with a high aspect ratio into the particles significantly increases the battery's charge and discharge capacity. Furthermore, even if voids are present in the positive electrode active material particles, if their aspect ratio is low, the capacity improvement effect is not achieved.

[0018] Some technologies for forming voids inside the particles of positive electrode active materials and controlling their aspect ratio have been proposed (for example, see Patent Documents 1 and 2). However, these technologies involve hollow particles with high porosity. When the positive electrode active material is used, there is a problem of reduced energy density of the battery. The inventors have successfully controlled the aspect ratio of the voids in the region with low porosity. According to the positive electrode active material disclosed in the present invention, the charge and discharge capacity of the non-aqueous electrolyte secondary battery can be greatly improved.

[0019] Hereinafter, an embodiment of 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 according to the present disclosure will be described in detail with reference to the accompanying drawings. Furthermore, technical configurations formed by selectively combining the various components of the various embodiments and variations described below are also included in the scope of the present disclosure.

[0020] In the embodiment described below, a non-aqueous electrolyte secondary battery 10 is exemplified as a cylindrical battery in which a wound electrode body 14 is housed in an outer can 16 having a bottomed cylindrical shape. However, the outer can of the battery is not limited to a cylindrical outer can. Other embodiments of the non-aqueous electrolyte secondary battery disclosed herein include, for example, a square battery having a square outer can, a coin-shaped battery having a coin-shaped outer can, and a pouch-type battery having an outer can composed of a laminate sheet including a metal layer and a resin layer. In addition, the electrode body is not limited to a wound type, and may also be a stacked type electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator.

[0021] 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 non-aqueous electrolyte secondary battery 10 includes a wound electrode body 14, a non-aqueous electrolyte, and an outer can 16 for storing the electrode body 14 and the non-aqueous electrolyte. The non-aqueous electrolyte secondary battery 10 is, for example, a lithium-ion secondary battery. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a winding structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed therebetween. The outer can 16 is a metal container with a bottomed cylindrical shape that is open at one axial end, and the opening of the outer can 16 is blocked by a sealing body 17. In the following, for ease of explanation, the sealing body 17 side of the battery is set as the top, and the bottom side of the outer can 16 is set as the bottom.

[0022] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolyte) or a solid electrolyte.

[0023] Liquid electrolyte (electrolyte) contains non-aqueous solvent and electrolyte salt dissolved in non-aqueous solvent. Non-aqueous solvent, for example, uses esters, ethers, nitriles, amides, and mixed solvents of two or more thereof. As an example of non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents thereof, etc. can be cited. Non-aqueous solvent can also contain halogen substitutions (for example, fluoroethylene carbonate, etc.) obtained by replacing at least a portion of the hydrogen of these solvents with halogen atoms such as fluorine. Electrolyte salt, for example, uses lithium salts such as LiPF6.

[0024] As a solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As an inorganic solid electrolyte, materials known in all-solid lithium ion secondary batteries, etc. (such as oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte, for example, comprises a lithium salt and a matrix polymer or comprises a non-aqueous solvent, a lithium salt, and a matrix polymer. As a matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. As polymer materials, fluororesins, acrylic resins, polyether resins, etc. can be listed.

[0025] The positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 are all strip-shaped elongated bodies, which are alternately stacked in the radial direction of the electrode body 14 by being wound into a spiral. In order to prevent the precipitation of lithium, the negative electrode 12 is formed to be one circle larger than the positive electrode 11. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction and the width direction. The separator 13 is formed to be at least one circle larger than the positive electrode 11, for example, two sheets are arranged in a manner of sandwiching the positive electrode 11. The electrode body 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.

[0026] 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 cap 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.

[0027] 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 on its side, with a portion of the side protruding inward to support the sealing member 17. The groove 22 is preferably formed annularly 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 riveted to the sealing member 17.

[0028] 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 interposed between their respective peripheral portions. If the internal pressure of the battery rises due to abnormal heat, the lower valve body 24 is deformed and breaks in a manner that pushes the upper valve body 26 toward the cover 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and the gas is discharged from the opening of the cover 27.

[0029] Hereinafter, the positive electrode 11 , the negative electrode 12 , and the separator 13 constituting the electrode assembly 14 , particularly the positive electrode 11 , will be described in detail.

[0030] [positive electrode]

[0031] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer arranged on the positive electrode core. The positive electrode core can be made of a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, aluminum alloy, stainless steel, titanium, etc., and a film of the metal is arranged on the surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be manufactured, for example, by coating a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder on the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0032] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and Ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powders, and conductive whiskers. A conductive agent may be used alone or in combination. The conductive agent content is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0033] As the binder contained in the positive electrode mixture layer, fluororesins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, ethylene-propylene-butadiene copolymer and other olefin resins, polyacrylonitrile (PAN), polyimide, polyamide, ethylene-acrylic acid copolymer and the like can be exemplified. In addition, these resins can also be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO) and the like. The binder can be used alone or in combination with multiple types. Relative to the mass of the positive electrode mixture layer, the binder content is, for example, 0.1% by mass or more and 5% by mass or less.

[0034] The positive electrode active material has a layered rock salt structure belonging to the space group R-3m and is composed of secondary particles formed by the aggregation of primary particles. The secondary particles have voids. The voids include a first void with an aspect ratio of 6.0 or more and a second void with an aspect ratio of less than 6.0. The aspect ratio of the void is the ratio of the major diameter of the void to the minor diameter of the void (major diameter / minor diameter). In this specification, a void with an aspect ratio of 6.0 or more is defined as a "first void", and a void with an aspect ratio of less than 6.0 is defined as a "second void".

[0035] In a positive electrode active material as an example of an embodiment, in a particle cross section of a secondary particle, the ratio of the area of ​​the voids to the area of ​​the particle cross section (sometimes referred to as "void ratio") is 15% or less, and the average aspect ratio of the voids is 2.1 or more. In a positive electrode active material as another example of an embodiment, in a particle cross section of a secondary particle, the average area of ​​each void is 0.018 μm 2 Hereinafter, the ratio of the area of ​​the first void to the area of ​​the particle cross section is 0.2% or more. That is, the positive electrode active material is not a hollow particle having a large void formed in the center of the particle, but rather a particle with a low porosity having a plurality of elongated voids formed in a wide range throughout the particle.

[0036] Figure 2 and Figure 3 : is a particle cross-sectional image of a positive electrode active material prepared in an embodiment described later. For comparison, a particle cross-sectional image of a positive electrode active material prepared in a comparative example described later is shown in FIG. Figure 4 . (a) of each figure is a scanning electron microscope (SEM) image of a particle cross section, and (b) of each figure is a binary image in which the void portion is made white and the portion where no void exists is made black through image processing. Compared with the conventional positive electrode active materials, the positive electrode active material of this embodiment has a plurality of fine line-shaped or strip-shaped voids formed everywhere in the entire particle. It is believed that the positive electrode active material of this embodiment has such voids, which allows the electrolyte to penetrate smoothly into the interior of the particle, greatly contributing to the high capacity of the battery.

[0037] The positive electrode active material is mainly composed of a composite oxide (hereinafter referred to as "Li composite oxide") having the above-mentioned voids formed inside the secondary particles. Here, the so-called main component refers to the component with the highest mass ratio among the components of the positive electrode active material. In the mixture layer of the positive electrode 11, composite oxides other than Li composite oxides can also be used as the positive electrode active material, but the content of Li composite oxide is preferably 50% by mass or more, and can also be substantially 100% by mass.

[0038] Li composite oxide is a composite oxide having a crystal structure belonging to space group R-3m and containing at least one selected from Ni, Co, Mn, and Al. From the viewpoint of both high capacity and material cost reduction, the Li composite oxide preferably contains at least Ni, and more preferably contains Ni and Mn. With respect to the total molar number of metal elements other than Li, the Ni content is preferably 35 mol% or more and 85 mol% or less, more preferably 40 mol% or more and 75 mol% or less, and may also be 40 mol% or more and 60 mol% or less.

[0039] For example, Li composite oxide is represented by the composition formula (Li 1-x Na x ) a M b X 1-b O c The composite oxide represented by the formula, wherein M is at least one selected from Ni, Co, Mn, and Al, X is at least one selected from transition metal elements and typical elements (elements of Group 1, Group 2, and Group 13 to Group 18 of the periodic table) other than Li, Na, Ni, Co, Mn, and Al, 0≤x≤0.2, 0.90≤a≤1.15, 0<b≤1, and c is a value that satisfies electrical neutrality. As described above, M preferably contains at least Ni and Mn. Furthermore, the composition of the positive electrode active material can be measured using an ICP emission spectrometer (e.g., iCAP 6300 manufactured by Thermo Fisher Scientific).

[0040] In the composition formula (Li 1-x Na x ) a M b X 1-b O c In the present invention, the total molar ratio (a) of Li and Na is preferably 0.80 or more and 1.15 or less (0.80≤a≤1.15), and may be less than 1.00. The molar ratio (x) of Na is preferably 0.2 or less (0≤x≤0.2), more preferably 0.15 or less, and particularly preferably 0.13 or less. The Li composite oxide can be produced by ion-exchanging Na in the Na composite oxide with Li, but the total amount of Na can be replaced by Li, and the molar ratio (x) of Na can be substantially 0. The Li composite oxide can contain a predetermined amount of Na, and the molar ratio (x) of Na can be, for example, 0.002≤x≤0.02.

[0041] In the composition formula (Li 1-x Na x ) a M b X 1-b O cThe molar ratio (c) of O is a value that satisfies electroneutrality. In other words, it is a value that satisfies the valence of O in the positive electrode active material. The molar ratio (c) of O is, for example, 1.80 or more and 2.15 or less (1.80 ≤ e ≤ 2.15). Furthermore, when the molar ratio (c) of O is less than 2.0, the layered rock salt structure is oxygen-deficient.

[0042] For example, Li composite oxide is represented by the composition formula (Li 1-x Na x ) a Ni 1-d-e Mn d X e O c As described above, the molar ratio (1-de) of Ni is preferably 0.95 or less, more preferably 0.85 or less. In addition, the molar ratio (1-de) of Ni is preferably 0.30 or more, more preferably 0.40 or more. An example of a preferred range of the molar ratio (1-de) of Ni is 0.35≤1-de≤0.95, 0.40≤1-de≤0.85, or 0.40≤1-de≤0.60. In this case, it is possible to achieve both high capacity and material cost reduction at a higher level.

[0043] In the composition formula (Li 1-x Na x ) a Ni 1-d-e Mn d X e O c In the embodiment of the present invention, the molar ratio (d) of Mn is preferably 0.75 or less, more preferably 0.65 or less. In addition, the molar ratio (d) of Mn is preferably 0.05 or more, more preferably 0.15 or more. An example of a preferred range of the molar ratio (b) of Mn is 0.20≤d≤0.75, 0.25≤d≤0.65, or 0.40≤d≤0.60. In this case, it is possible to achieve both high capacity and material cost reduction at a higher level.

[0044] In the composition formula (Li 1-x Na x ) a Ni 1-d-e Mn d X e O cIn the embodiment, X can be at least one element selected from Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, Co, and Al. When a small amount of X is added, the improvement effect of the charge and discharge capacity becomes more significant. The molar ratio (e) of X is preferably 0.1 or less (0≤e≤0.1), more preferably 0.05 or less (0≤e≤0.05). X is preferably at least one element selected from Al, Co, and Zr, and particularly preferably Al or Co.

[0045] An example of a volume-based median diameter (D50) of a Li composite oxide is 1 μm or more and 30 μm or less, or 3 μm or more and 20 μm or less. The D50 of a composite oxide is the particle size at which the volume cumulative value in the particle size distribution measured by a laser diffraction scattering method becomes 50%. The BET specific surface area of ​​a Li composite oxide is, for example, 0.1 m 2 / g or more and 10m 2 / g or less, or 0.5m 2 / g or more and 6m 2 The BET specific surface area of ​​the composite oxide is measured according to the BET method (nitrogen adsorption method) described in JIS R 1626. When D50 and the BET specific surface area are within this range, it is easy to increase the capacity.

[0046] Regarding Li composite oxides, the ratio of the area of ​​voids in the cross-section of the secondary particles to the area of ​​the particle cross-section (porosity) is preferably 15% or less, more preferably 13% or less, and particularly preferably 11% or less. In this case, the durability of the secondary particles can be ensured while achieving high capacity. Furthermore, the porosity is preferably 5% or more, more preferably 6% or more, and particularly preferably 7% or more. In other words, an example of a preferred range for the porosity is 5% or more and 15% or less.

[0047] The average value of the aspect ratio of the voids is preferably 2.1 or more, more preferably 2.3 or more, and particularly preferably 2.5 or more. In this case, the electrolyte easily penetrates into the interior of the particles through the voids, and the improvement effect of the charge-discharge capacity becomes significant. In addition, the upper limit of the average aspect ratio is not particularly limited, but is 6.0 as an example. That is, an example of a preferred range of the average aspect ratio is 2.1 or more and 6.0% or less.

[0048] Regarding the Li composite oxide, the number of voids in the secondary particle cross section is preferably 300 or more, more preferably 400 or more, and particularly preferably 450 or more. It is preferred that each void is small, and a plurality of such small voids are formed. The average area of ​​each void in the secondary particle cross section is preferably 0.018 μm 2Below, more preferably 0.016 μm 2 Below, particularly preferably 0.014 μm 2 the following.

[0049] The lower limit of the average value of the area of ​​each void is not particularly limited, but is 0.001 μm as an example. 2 The average value of the area of ​​each void is, for example, 0.001 μm 2 Above and 0.018μm 2 Below, 0.002μm 2 Above and 0.016μm 2 Below, or 0.003μm 2 Above and 0.014μm 2 When the average value of the area of ​​each void is within this range and the porosity is within the above range, the effect of improving the charge-discharge capacity becomes significant.

[0050] The voids include a first void having an aspect ratio of 6.0 or greater and a second void having an aspect ratio of less than 6.0, but the ratio of the area of ​​the first void to the area of ​​the cross-section of the secondary particle is preferably 0.2% or greater. More preferably, it is 0.5% or greater, and particularly preferably 1.0% or greater. By increasing the proportion of the first void in the voids, the improvement effect of the charge-discharge capacity becomes significant. The ratio of the area of ​​the first void to the area of ​​the cross-section of the secondary particle is, for example, 0.5% or greater and 3.0% or less, or 1.0% or greater and 2.0% or less.

[0051] The ratio of the area of ​​voids with an aspect ratio of 5.0 or greater to the area of ​​the secondary particle cross section, and the ratio of the area of ​​voids with an aspect ratio of 7.0 or greater to the area of ​​the secondary particle cross section are, for example, 0.2% to 4.0% and 0.05% to 1.5%, respectively. Furthermore, Li composite oxides may also contain voids with an aspect ratio of 10 or greater. Furthermore, conventional positive electrode active materials do not contain voids with an aspect ratio of 10 or greater, and generally do not contain voids with an aspect ratio of 6 or greater.

[0052] In the lithium composite oxide, the ratio of the number of primary voids to the number of voids in the cross section of the secondary particle is preferably 2% or greater, more preferably 3% or greater, and particularly preferably 5% or greater. In this case, the improvement in charge and discharge capacity is significant. The ratio of the number of primary voids to the number of voids in the cross section of the secondary particle is, for example, 2% or greater and 15% or less, 3% or greater and 13% or less, or 5% or greater and 10% or less.

[0053] The method for measuring the porosity and the aspect ratio of the voids in the cross section of the secondary particle of the Li composite oxide is as follows.

[0054] (1) The presence and number of voids are determined by analyzing the SEM image of the secondary particle cross section. In order to expose the cross section of the secondary particle, a focused ion beam (FIB) processing device is used to obtain a cross section of the central portion (diameter) of the positive electrode active material. Then, the particle cross section (secondary particle cross section) is observed using SEM. In this specification, the observation magnification of the SEM when observing the particle cross section of the Li composite oxide is set to 20,000 times.

[0055] (2) For the extraction of gap areas, use image analysis software such as Image J or Avizo-Materials Science. Define low-brightness areas as gaps based on the brightness threshold of the captured image and convert it into a binary image (see Figures 2 to 4 (b)) The continuous white pixels are marked as a void. The void ratio is calculated by observing the cross section and the ... 2 The aspect ratio of the voids was determined by taking the maximum Feret's diameter of each void as the major axis length and the minimum Feret's diameter as the minor axis length, and the result was calculated as major axis length / minor axis length.

[0056] Li composite oxide is produced, for example, through the following steps: (1) a step of mixing and calcining a sodium raw material and a nickel raw material to synthesize a sodium composite oxide; and (2) a step of reacting the sodium composite oxide with a lithium compound to ion-exchange the Na in the sodium composite oxide into Li. In step (1), it is preferred to also add a manganese raw material, and a raw material containing element X may also be added. In the synthesis step of the Li composite oxide, the synthesis conditions are controlled so that a plurality of small voids with a high aspect ratio are introduced. In controlling the voids, for example, the calcination conditions in step (1) are important.

[0057] The sodium raw material is at least one selected from metallic sodium and sodium compounds. The sodium compound is not particularly limited as long as it contains Na. Examples thereof include acetates such as CH3COONa and CH3COONa·3H2O, nitrates such as NaNO3, sulfates such as Na2SO4, carbonates such as Na2CO3, bicarbonates such as NaHCO3, hydroxides such as NaOH, and oxides such as Na2O and Na2O2. Among these, Na2CO3, NaHCO3, NaOH, and NaNO3 are preferred.

[0058] The nickel raw material is at least one selected from metallic nickel and nickel compounds. The nickel compound is not particularly limited as long as it contains nickel, and examples thereof include oxides such as NiO, hydroxides such as NiOH, Ni(OH)2, and NiOOH, nitrates such as NiNO3, carbonates such as NiCO3 and Ni4CO3(OH)6(H2O)4, and sulfates such as NiSO4. Among these, Ni(OH)2 is preferred.

[0059] The manganese raw material uses at least one selected from metallic manganese and manganese compounds. As the manganese compound, as long as it contains Mn, there is no particular limitation. For example, oxides such as MnO, Mn2O3, Mn3O4, MnO2, hydroxides such as Mn(OH)2, MnOOH, carbonates such as MnCO3, nitrates such as Mn(NO3)2, sulfates such as MnSO4, etc. can be cited. Among them, Mn(OH)2 is preferred.

[0060] The raw material containing element X uses at least one selected from element X and compounds of element X. As the compound containing element X, as long as it contains X, there is no particular limitation, and oxides, hydroxides, carbonates, nitrates, sulfates, etc. can be cited. Furthermore, as the raw material for the sodium composite oxide, a compound containing Ni and Mn, a compound containing Ni and X, a compound containing Mn and X, or a compound containing Ni, Mn, and X can be used.

[0061] The mixing ratio of the raw materials for the sodium composite oxide can be appropriately set. For example, it is preferably set so as to have a chemical composition represented by the composition formula Na a [[ID=!0]]Ni 1-d-e Mn d X e O c (where 0.95 ≤ a ≤ 1.05, 0.05 < d ≤ 0.65, 0 ≤ e ≤ 0.05, 0.4 ≤ 1 - d - e < 0.95, and c is a value that satisfies electroneutrality). In addition, the method of mixing the raw materials is not particularly limited as long as it can uniformly mix the raw materials, and mixing using a known mixer such as a stirring mixer can be exemplified.

[0062] The mixture of the above raw materials is fired using a firing furnace in the atmosphere or in an oxygen stream. The firing temperature is preferably 700 °C or higher and 900 °C or lower, more preferably 750 °C or higher and 850 °C or lower. The heating rate is preferably slow, for example, 0.3 °C / minute or higher and 3.0 °C / minute or lower, or 0.5 °C / minute or higher and 2.0 °C / minute or lower. The firing time is preferably 20 hours or longer when the firing temperature is 750 °C or higher and 850 °C or lower. Here, the so-called firing time means the time from when the temperature of the firing furnace reaches the firing temperature until the firing ends and cooling starts. The fired product is rapidly cooled in the atmosphere, for example, by taking it out of the firing furnace.

[0063] By extending the firing time in step (1), a plurality of fine, linear voids with a high and small aspect ratio can be introduced into the particles of the finally obtained Li composite oxide. As described above, the porosity is preferably 15% or less, the aspect ratio of the voids is preferably 2.5 or more, and the proportion of the first voids is preferably 1.0% or more. However, in order to control these within this range, the firing time is preferably set to 20 hours or more and 40 hours or 22 hours or more and 28 hours or less at a firing temperature of 750°C or more and 850°C or less. The sodium composite oxide can be obtained, for example, by rapidly cooling the fired product in the atmosphere and then pulverizing it using a known method.

[0064] In step (2), Na in the sodium composite oxide is ion-exchanged with Li. In step (2), for example, the total amount of Na is ion-exchanged with Li, but the ion exchange may be performed so that a predetermined amount of Na remains. As a suitable ion exchange method, a method of adding a molten salt bed of a lithium salt to the sodium composite oxide and heating it can be cited. The lithium salt is, for example, at least one selected from lithium nitrate, lithium sulfate, lithium chloride, lithium carbonate, lithium hydroxide, lithium iodide, and lithium bromide.

[0065] The heating temperature in the ion exchange process is preferably 200°C or more and 400°C or less, more preferably 250°C or more and 350°C or less. If the heating temperature exceeds 400°C, the reaction proceeds rapidly, and there is a possibility of an uneven reaction. On the other hand, if the heating temperature is less than 200°C, the reaction does not proceed fully, and Na tends to remain excessively. The heating treatment time is set to 3 hours or more and 10 hours or less, for example, after the temperature is increased at a rate of 3.0°C / min or more and 8.0°C / min or less to reach the maximum treatment temperature. After the heat treatment, the product is cooled to obtain a Li composite oxide. The product can also be washed with water, ethanol, methanol, or the like. Drying after washing is carried out in the atmosphere or in a vacuum.

[0066] [negative electrode]

[0067] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer arranged on the negative electrode core. The negative electrode core can be made of a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, nickel alloy, or a film having the metal arranged on the surface. The negative electrode mixture 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 coating a negative electrode mixture slurry containing a negative electrode active material and a binder on the negative electrode core, drying the coating, and then compressing the coating to form negative electrode mixture layers on both sides of the negative electrode core. Furthermore, the negative electrode mixture layer may also contain a conductive agent such as CNT.

[0068] Negative electrode active materials generally use carbon materials that can reversibly absorb and release lithium ions. Alternatively, elements such as Si and Sn that alloy with lithium, or materials containing these elements, may be used as negative electrode active materials. Silicon-containing materials containing Si are preferred. Furthermore, materials such as lithium titanate, which have a higher charge and discharge potential relative to metallic lithium than carbon materials, may also be used as negative electrode active materials. Negative electrode active materials may be used singly or in combination.

[0069] The carbon material that functions as the negative electrode active material is, for example, at least one selected from natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, artificial graphite such as blocky artificial graphite (MAG) and graphitized mesocarbon microbeads (MCMB), natural graphite such as flaky graphite, blocky graphite, and earthy graphite, or a mixture thereof, is preferably used. Silicon-containing materials that function as negative electrode active materials include, for example, silicon alloys, silicon compounds, and composite materials containing Si. Very suitable silicon-containing materials are composite particles comprising an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

[0070] The binder contained in the negative electrode mixture layer can also use fluororesins, olefin resins, PAN, polyimide, polyamide, acrylic resins, etc. in the same way as in the case of the positive electrode 11, but polyvinyl acetate, styrene-butadiene rubber (SBR), etc. can also be used. Among them, SBR is preferably used. The binder can be used alone or in combination. In addition, the negative electrode mixture layer preferably contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. They act as thickeners in the negative electrode mixture slurry. The content of the binder relative to the mass of the negative electrode mixture layer is, for example, not less than 0.1% by mass and not more than 5% by mass.

[0071] [Diaphragm]

[0072] The separator 13 is a porous sheet having ion permeability and insulation. Specific examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, and the like. The separator 13 is preferably made of polyolefins such as polyethylene and polypropylene, or cellulose. The separator 13 may be a single-layer structure or a multi-layer structure. Alternatively, a heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0073] 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 the inorganic filler include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphate compounds. 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.

[0074] Example

[0075] The present disclosure is further illustrated below by way of examples, but the present disclosure is not limited by these examples.

[0076] <Example 1>

[0077] [Preparation of positive electrode active material]

[0078] A hydroxide containing Ni and Mn at a molar ratio of 1:1 was mixed with sodium carbonate in a molar ratio of Ni:Mn:Na = 0.5:0.5:1.05. The resulting mixture was heated at a heating rate of 1°C / minute, calcined in air at 800°C for 24 hours, and then rapidly cooled in air to produce a Na-containing composite oxide. A Li-containing molten salt was prepared by mixing lithium nitrate and lithium chloride in a molar ratio of 88:12. The Li-containing molten salt and the Na-containing composite oxide were mixed and heated at a heating rate of 5°C / minute and then heated in air at 280°C for 5 hours. The product was then cooled at a cooling rate of 2°C / minute, washed with a sufficient amount of water, and then heated in a vacuum at 160°C for 4 hours to produce a Li composite oxide in which Na was ion-exchanged with Li.

[0079] The obtained Li composite oxide was subjected to composition analysis using an ICP emission spectrometer (iCAP 6300 manufactured by Thermo Fisher Scientific). The composition was Li 0.932 Na 0.01 Ni 0.5 Mn 0.5 O 0.185 . The results of observing the Li composite oxide with SEM confirmed that the Li composite oxide is a secondary particle formed by the aggregation of primary particles. In addition, a particle cross-section of the Li composite oxide was made and the particle cross-section was observed with SEM. As a result, a plurality of elongated voids were confirmed inside the particle. In addition, the ratio of voids, aspect ratio, etc. were calculated using the above method, and their values ​​are shown in Table 1 (the same applies to other embodiments and comparative examples). Furthermore, 949 voids were confirmed in the observed secondary particle cross-section.

[0080] [Production of positive electrode]

[0081] The above-mentioned lithium composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a solid content mass ratio of 92:5:3, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode mixture slurry. This positive electrode slurry was applied to a positive electrode core composed of aluminum foil. After the coating film was dried, it was rolled using a calendering roller to obtain a positive electrode with a positive electrode mixture layer formed on the positive electrode core.

[0082] [Preparation of non-aqueous electrolyte]

[0083] A non-aqueous electrolyte solution was prepared by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent obtained by mixing fluoroethylene carbonate (FEC) and methyl propionate (FMP) in a volume ratio of 1:3 to obtain a mixed solvent.

[0084] [Preparation of test cell]

[0085] Lithium metal foil was used as the negative electrode. The positive and negative electrodes were arranged facing each other with a separator interposed therebetween to form an electrode assembly. This electrode assembly and the non-aqueous electrolyte were placed in a coin-shaped outer can. The opening of the outer can was sealed with a gasket and a sealing member to produce a test cell (non-aqueous electrolyte secondary battery).

[0086] <Example 2>

[0087] The Na-containing composite oxide synthesized by the same method as in Example 1 was mixed with lithium hydroxide, and the mixture thus obtained was heated at a heating rate of 5°C / min, heated at a temperature of 280°C in air for 5 hours, and then cooled at a cooling rate of 2°C / min to obtain a Li composite oxide (positive electrode active material). Except for this, the positive electrode active material and the test cell were prepared in the same manner as in Example 1. The composition of the obtained Li composite oxide was analyzed using an ICP emission spectrometer (iCAP 6300 manufactured by Thermo Fisher Scientific). As a result, the composition was Li 0.843 Na 0.12 Ni 0.5 Mn 0.5 O 0.193 Furthermore, 493 voids were confirmed in the observed cross section of the secondary particles of the positive electrode active material.

[0088] <Comparative Example 1>

[0089] A hydroxide containing Ni and Mn at a molar ratio of 1:1 was mixed with lithium hydroxide at a molar ratio of Ni:Mn:Li = 0.5:0.5:1.05. The resulting mixture was heated at a heating rate of 5°C / minute, calcined in air at 900°C for 10 hours, and then cooled at a cooling rate of 10°C / minute to produce a lithium-transition metal composite oxide. A test cell was fabricated in the same manner as in Example 1, except that this composite oxide was used as the positive electrode active material. Furthermore, 269 voids were observed in the cross-section of the secondary particles of the observed positive electrode active material.

[0090] [Evaluation of discharge capacity]

[0091] Each test cell of the Examples and Comparative Examples was charged at 25°C at a constant current of 0.2C until the battery voltage reached 4.5V. It was then charged at a constant voltage of 4.5V until the current reached 0.02C. After a 20-minute rest, the cells were discharged at a constant current of 0.2C until the battery voltage reached 2.5V, and the discharge capacity was determined. The discharge capacity measurement results are shown in Table 1, along with the composition of the positive electrode active material.

[0092] Table 1

[0093]

[0094] As shown in Table 1, the test cells of the embodiments all have higher capacities than the test cells of the comparative examples. Figure 2 and Figure 3 is a particle cross-sectional image of the positive electrode active material of Examples 1 and 2. Figure 4 These are particle cross-sectional images of the positive electrode active material of Comparative Example 1. These particle cross-sectional images clearly show that the positive electrode active material of Example 1 contains numerous voids with a higher aspect ratio than the positive electrode active material of Comparative Example 1. The voids in the positive electrode active material of Example 1 are individually small and formed in stripes throughout the particle. Using a positive electrode active material with such voids incorporated into the particles can significantly improve the charge and discharge capacity of a battery.

[0095] In the positive electrode active material of Example 1, the ratios of the area of ​​voids with an aspect ratio of 5 or greater and the area of ​​voids with an aspect ratio of 7 or greater to the cross-sectional area of ​​the secondary particles were 1.9% and 0.9%, respectively. Furthermore, in the positive electrode active material of Example 2, the ratios of the area of ​​voids with an aspect ratio of 5 or greater and the area of ​​voids with an aspect ratio of 7 or greater to the cross-sectional area of ​​the secondary particles were 1.8% and 0.7%, respectively. Meanwhile, in the positive electrode active material of Comparative Example 1, the ratios of the area of ​​voids with an aspect ratio of 5 or greater and the area of ​​voids with an aspect ratio of 7 or greater to the cross-sectional area of ​​the secondary particles were 0.2% and 0%, respectively.

[0096] The present disclosure is further illustrated by the following embodiments.

[0097] Technical composition 1: A positive electrode active material for a non-aqueous electrolyte secondary battery, which is a positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, and is composed of secondary particles formed by the aggregation of primary particles, and the secondary particles contain voids. In the particle cross-section of the secondary particles, the ratio of the area of ​​the voids to the area of ​​the particle cross-section is less than 15%, and the average aspect ratio of the voids is greater than 2.1.

[0098] Technical Configuration 2: A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a crystal structure belonging to the space group R-3m, the positive electrode active material being composed of secondary particles formed by agglomeration of primary particles, the secondary particles containing voids, the voids comprising a first void having an aspect ratio of 6.0 or greater and a second void having an aspect ratio of less than 6.0, wherein the average area of ​​each void in a particle cross section of the secondary particles is 0.018 μm 2 Hereinafter, the ratio of the area of ​​the first voids to the area of ​​the cross section of the particle is 0.2% or more.

[0099] Technical Configuration 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Technical Configuration 2, wherein in the particle cross section of the secondary particle, the ratio of the number of the first voids to the number of the voids is 2% or more and 15% or less.

[0100] Technical Configuration 4: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Technical Configurations 1 to 3, wherein the average aspect ratio of the voids in the particle cross section of the secondary particles is 2.1 or more and 6.0 or less.

[0101] Technical Configuration 5: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Technical Configurations 1 to 4, wherein the number of the voids in the particle cross section of the secondary particle is 300 or more.

[0102] Technical Configuration 6: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Technical Configurations 1 to 5, wherein the positive electrode active material has a composition formula (Li 1-x Na x ) a M b X 1-b O c The composite oxide composition represented by the formula, wherein M is at least one selected from Ni, Co, Mn, and Al, X is at least one selected from transition metal elements and typical elements other than Li, Na, Ni, Co, Mn, and Al, 0≤x≤0.2, 0.90≤a≤1.15, 0<b≤1, and c is a value that satisfies electrical neutrality.

[0103] Technical Configuration 7: A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode comprises the positive electrode active material according to any one of Technical Configurations 1 to 6.

[0104] Description of Reference Numerals

[0105] 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 plates; 20: Positive electrode lead; 21: Negative electrode lead; 22: Slotted portion; 23: Internal terminal plate; 24: Lower valve body; 25: Insulating member; 26: Upper valve body; 27: Cover; 28: Gasket.

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, the positive electrode active material having a crystal structure belonging to the space group R-3m, It is composed of secondary particles formed by the aggregation of primary particles. The secondary particles contain voids, In the particle cross section of the secondary particle, a ratio of the area of ​​the voids to the area of ​​the particle cross section is 15% or less, and an average value of the aspect ratio of the voids is 2.1 or more.

2. A positive electrode active material for a non-aqueous electrolyte secondary battery, the positive electrode active material having a crystal structure belonging to the space group R-3m, It is composed of secondary particles formed by the aggregation of primary particles. The secondary particles contain voids, The voids include a first void having an aspect ratio of 6.0 or greater and a second void having an aspect ratio of less than 6.0, In the particle cross section of the secondary particle, the average area of ​​each of the voids is 0.018 μm 2 Hereinafter, the ratio of the area of ​​the first voids to the area of ​​the cross section of the particle is 0.2% or more.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, In a particle cross section of the secondary particle, a ratio of the number of the first voids to the number of the voids is 2% or more and 15% or less.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, In the particle cross section of the secondary particle, the average value of the aspect ratio of the voids is 2.1 or more and 6.0 or less.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, In the particle cross section of the secondary particle, the number of the voids is 300 or more.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, Composition formula (Li 1-x Na x ) a M b X 1-b O c The composite oxide composition represented by In the formula, M is at least one selected from Ni, Co, Mn, and Al, X is at least one selected from transition metal elements and typical elements other than Li, Na, Ni, Co, Mn, and Al, 0≤x≤0.2, 0.90≤a≤1.15, 0<b≤1, and c is a value that satisfies electrical neutrality. 7 . A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode comprises the positive electrode active material according to claim 1 .

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