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

Abstract

The present invention is characterized in that: a composite metal oxide includes at least a transition metal, and contains S and two or more elements selected from the group consisting of Li, Na, B, Ni, Mg, Al, Si, P, K, Ti, Mn, Fe, Co, Zr, Nb, Mo, Sn, W, Ca, Sr, and Bi; in secondary particles, the proportion of primary particles having an aspect ratio of at least 2 is at least 40%; a modified compound including a compound containing S is present on the surface of each of the primary particles; the content of S contained in the composite metal oxide satisfies 0.1 mol%≤S content≤3 mol%; and an S 2p spectrum by X-ray photoelectron spectroscopy has a first peak having a vertex at 170±2 eV and a second peak having a vertex at 167±2 eV.

Description

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

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

[0002] Secondary batteries such as lithium-ion batteries are widely used in applications requiring high capacity, high durability, and high output, such as automotive and energy storage applications. The positive electrode active material, which is a major component of non-aqueous electrolyte secondary batteries, greatly affects these performance characteristics, and therefore, much research has been conducted on positive electrode active materials. For example, Patent Document 1 discloses a positive electrode active material in which the proportion of primary particles with a large aspect ratio on the surface of the secondary particles is greater than the proportion of primary particles with a large aspect ratio in the center of the secondary particles.

[0003] Patent No. 6550598

[0004] In some cases, reducing the initial resistance of non-aqueous electrolyte secondary batteries is required from the standpoint of increasing power output. The technology described in Patent Document 1 does not consider the reduction of initial resistance, and there is still room for improvement.

[0005] A positive electrode active material for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, is a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a composite metal oxide, wherein the composite metal oxide contains at least a transition metal, and at least two elements selected from the group consisting of Li, Na, B, Ni, Mg, Al, Si, P, K, Ti, Mn, Fe, Co, Zr, Nb, Mo, Sn, W, Ca, Sr, and Bi, as well as S, and comprises secondary particles formed by the aggregation of primary particles, wherein the secondary particles have primary particles with an aspect ratio of 2 or more. The proportion of primary particles is 40% or more of the total number of primary particles, a modifying compound is present on the surface of the primary particles including the surface of the secondary particles, the modifying compound includes a compound containing sulfur, the sulfur content in the composite metal oxide satisfies 0.1 mol% ≤ S content ≤ 3 mol% relative to the total number of moles of transition metal elements in the composite metal oxide, and the S2p spectrum obtained by X-ray photoelectron spectroscopy has a first peak with a peak at 170 ± 2 eV and a second peak with a peak at 167 ± 2 eV.

[0006] Furthermore, a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, is characterized by comprising a positive electrode containing the positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0007] According to a positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, it is possible to provide a non-aqueous electrolyte secondary battery with reduced initial resistance.

[0008] This is an axial cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. This figure schematically shows the structure and arrangement of primary particles inside secondary particles. This is a figure of the S2p spectrum of the composite metal oxide of Example 1-1 obtained by X-ray photoelectron spectroscopy. This is a figure of the O1s spectrum of the composite metal oxide of Example 1-1 obtained by X-ray photoelectron spectroscopy. This is a figure of the S2p spectrum of the composite metal oxide of Comparative Example 1-2 obtained by X-ray photoelectron spectroscopy. This is a figure of the O1s spectrum of the composite metal oxide of Comparative Example 1-2 obtained by X-ray photoelectron spectroscopy.

[0009] The positive electrode active material may react with moisture during manufacturing and storage before being incorporated into a battery, forming an altered layer on its surface. Furthermore, after being incorporated into a battery, the positive electrode active material may react with the non-aqueous electrolyte, forming an altered layer on its surface. This altered layer inhibits the deintercalation and reintercalation of alkaline components between the positive electrode active material and the non-aqueous electrolyte, worsening the initial resistance.

[0010] Furthermore, in positive electrode active materials composed of secondary particles, the diffusivity of non-aqueous electrolytes into the secondary particles significantly affects the initial resistance. Therefore, to reduce the initial resistance, it is necessary to improve the diffusivity of non-aqueous electrolytes and Li into the secondary particles.

[0011] As a result of diligent research, the inventors have found that initial resistance can be specifically reduced by including a modified compound containing a sulfur-containing compound on the surface of primary particles, including the surface of secondary particles of a composite metal oxide, while ensuring that the spectrum obtained by X-ray photoelectron spectroscopy has a predetermined shape and that a predetermined proportion or more of primary particles have an aspect ratio of 2 or higher. This is presumed to be because the formation of an altered layer is suppressed by the modified compound, and the inclusion of a predetermined proportion or more of primary particles with an aspect ratio of 2 or higher facilitates the diffusion of non-aqueous electrolytes into the secondary particles, enabling the deintercalation and reintercalation of alkaline components throughout the secondary particles. However, if the above-mentioned modified compound is present and the spectrum obtained by X-ray photoelectron spectroscopy has a predetermined shape, but the inclusion of a predetermined proportion or more of primary particles with an aspect ratio of 2 or higher is not sufficient, the non-aqueous electrolyte does not diffuse sufficiently into the secondary particles, and the initial resistance cannot be reduced. Furthermore, if the material contains a predetermined proportion or more of primary particles with an aspect ratio of 2 or higher, but the above-mentioned modified compound is absent, or if the spectrum obtained by X-ray photoelectron spectroscopy does not have the predetermined shape, an altered layer is likely to form on the surface, making it impossible to reduce the initial resistance.

[0012] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery according to this disclosure will be described in detail. In the following, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 is given as an example of the non-aqueous electrolyte secondary battery, but the outer casing of the battery is not limited to a cylindrical outer casing. The non-aqueous electrolyte secondary battery according to this disclosure may be, for example, a prismatic battery with a prismatic outer casing, a coin-type battery with a coin-type outer casing, or a pouch-type battery with an outer casing made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body is not limited to a wound type, and may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator in between. Furthermore, the design of the non-aqueous electrolyte secondary battery according to this disclosure is not limited to the example design of the non-aqueous electrolyte secondary battery, and known designs of non-aqueous electrolyte secondary batteries may be applied.

[0013] Figure 1 is an axial cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte (not shown), and an outer casing 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. Hereafter, for convenience of explanation, the side of the battery with the sealing body 17 will be referred to as "upper," and the bottom side of the outer casing 16 will be referred to as "lower."

[0014] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all rectangular elongated bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape in the longitudinal direction. The separator 13 separates the positive electrode 11 and the negative electrode 12 from each other. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in both the longitudinal and transverse directions than the positive electrode 11. The two separators 13 are formed to be at least slightly larger in dimensions than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode body 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the transverse direction of the positive electrode 11 and the negative electrode 12 is the axial direction. In other words, the end faces in the short direction of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode body 14.

[0015] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0016] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0017] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side, and functions as a safety valve. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0018] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the non-aqueous electrolyte secondary battery 10, with particular emphasis on the positive electrode 11.

[0019] [Positive Electrode] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. Preferably, the positive electrode mixture layer is formed on both sides of the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The thickness of the positive electrode current collector is, for example, 10 μm or more and 30 μm or less.

[0020] The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, and a binder. The thickness of the positive electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the positive electrode current collector. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, conductive agent, etc., to the surface of the positive electrode current collector, drying the coating film, and then rolling it to form the positive electrode mixture layer on both sides of the positive electrode current collector.

[0021] Examples of conductive agents included in the positive electrode mixture layer include carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), graphene, and other carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0022] Examples of binders included in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyimide resins, acrylic resins, polyolefin resins, and polyacrylonitrile (PAN). These may be used individually or in combination of two or more types.

[0023] The positive electrode active material contained in the positive electrode mixture layer includes a composite metal oxide. The composite metal oxide contains at least a transition metal, and at least two elements selected from the group consisting of Li, Na, B, Ni, Mg, Al, Si, P, K, Ti, Mn, Fe, Co, Zr, Nb, Mo, Sn, W, Ca, Sr, and Bi, as well as S.

[0024] Composite metal oxides contain secondary particles formed by the aggregation of primary particles. The particle size of the primary particles is, for example, between 0.02 μm and 2 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM, e.g., JEOL JSM-7900F). The average particle diameter of the secondary particles is, for example, between 2 μm and 30 μm. Here, the average particle diameter refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of secondary particles can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell MT3000II) with water as the dispersion medium.

[0025] The composite metal oxide includes, for example, alkali metals. Examples of alkali metals include Li, Na, and K, with Li being preferred.

[0026] The composite metal oxide includes, for example, Ni. The Ni content relative to the total number of moles of transition metal elements in the composite metal oxide is, for example, 50 mol% ≤ Ni content ≤ 97 mol%. When the Ni content is within the above range, it becomes easier to achieve both high battery capacity and stabilization of the crystal structure. The Ni content is preferably 60 mol% ≤ Ni content ≤ 97 mol%, and more preferably 70 mol% ≤ Ni content ≤ 97 mol%. As the amount of Li extracted during charging increases, side reactions are more likely to occur between the positive electrode active material and the non-aqueous electrolyte, and the effects of this disclosure are significantly exhibited.

[0027] The composite metal oxide may contain Co. The Co content relative to the total number of moles of transition metal elements in the composite metal oxide is, for example, 0 mol% ≤ Co content ≤ 15 mol%. When the Co content is within the above range, safety can be improved while suppressing costs. Preferably, the Co content is 0 mol% ≤ Co content ≤ 12 mol%, and more preferably 0 mol% ≤ Co content ≤ 10 mol%.

[0028] The composite metal oxide contains, for example, Mn. The Mn content relative to the total number of moles of transition metal elements in the composite metal oxide is, for example, 0 mol% ≤ Mn content ≤ 50 mol%. When the Mn content is within the above range, it becomes easier to achieve both high battery capacity and stabilization of the crystal structure. Preferably, the Mn content is 0 mol% ≤ Mn content ≤ 40 mol%, and more preferably 0 mol% ≤ Mn content ≤ 30 mol%.

[0029] In composite metal oxides, it is preferable that the Co content and Mn content satisfy the condition 0 ≤ Co content / Mn content ≤ 2. In this case, it becomes easier to achieve both high battery capacity and safety.

[0030] The composite metal oxide contains sulfur (S). The S contained in the composite metal oxide is found in the S-containing compounds in the modified compounds present on the surface of the primary particles. Furthermore, some S may be present within the crystal structure. The S content in the composite metal oxide is 0.1 mol% ≤ S content ≤ 3 mol% relative to the total number of moles of transition metal elements in the composite metal oxide. When the S content is within this range, the surface state and crystal structure of the composite metal oxide are stabilized, and the initial resistance can be reduced. Preferably, the S content in the composite metal oxide is 0.1 mol% ≤ S content ≤ 2.7 mol%, and more preferably 0.1 mol% ≤ S content ≤ 2.5 mol%. In this case, the initial resistance can be further reduced.

[0031] The composite metal oxide may contain at least one of Ca and Sr. When the composite metal oxide contains at least one of Ca and Sr in addition to S, it is possible to improve, for example, the initial discharge capacity while reducing the initial resistance. At least one of Ca and Sr is included, for example, in the modified compound. That is, the modified compound may contain a compound containing S and a compound containing at least one of Ca and Sr. The compound containing S and the compound containing at least one of Ca and Sr may exist separately from each other or in a mixed state. Examples of compounds containing at least one of Ca and Sr are oxides, carbonates, and sulfates containing at least one of Ca and Sr.

[0032] The Ca and Sr content in the composite metal oxide may be, for example, 0 mol% < Ca content + Sr content ≤ 2 mol%, and 0.1 mol% ≤ Ca content + Sr content ≤ 2 mol%, relative to the total number of moles of transition metal elements in the composite metal oxide. It is also preferable that Ca content > Sr content. This can further reduce the initial resistance. The Ca and Sr content in the composite metal oxide can be confirmed, for example, by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0033] The composite metal oxide may contain M1 (M1 is at least one element selected from the group consisting of B, Mg, Al, Si, P, Ti, Fe, Zr, Nb, Mo, Sn, W, and Bi) in addition to alkali metals, Ni, Co, Mn, S, Ca, and Sr. The content of M1 relative to the total number of moles of transition metal elements in the composite metal oxide is, for example, 0 mol% ≤ M1 content ≤ 20 mol%. When the M1 content is within the above range, it becomes easier to achieve both high battery capacity and stabilization of the crystal structure. Preferably, the M1 content is 0 mol% ≤ M1 content ≤ 10 mol%.

[0034] Composite metal oxides include, for example, those with the general formula Li ａ Ni ｘ Co ｙ Mn ｚ M1 ｓ M2 ｔ S ｕ O ２－ｂ This is a composite oxide represented by the formula (wherein 0.8 ≤ a ≤ 1.2, 0.5 ≤ x ≤ 0.97, 0 ≤ y ≤ 0.15, 0 ≤ z ≤ 0.5, 0 ≤ s ≤ 0.20, 0 ≤ t ≤ 0.02, 0.001 ≤ u < 0.03, 0 ≤ b ≤ 0.05, x + y + z + w + s + t = 1, M1 is at least one element selected from the group consisting of B, Mg, Al, Si, P, Ti, Fe, Zr, Nb, Mo, Sn, W, and Bi, and M2 is at least one element of Ca and Sr). The proportion of metal elements contained in the composite metal oxide can be measured, for example, by an inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0035] In the composite metal oxide of this embodiment, in the secondary particles, the proportion of primary particles having an aspect ratio of 2 or more (hereinafter sometimes referred to as high aspect ratio primary particles) is 40% or more with respect to the total number of primary particles. Thereby, the diffusibility of the non-aqueous electrolyte into the secondary particles is improved, and the insertion and extraction of the alkali component are possible in the whole secondary particles. From the viewpoint of high capacity, the proportion of the high aspect ratio primary particles is preferably 40% or more and 85% or less, more preferably 45% or more and 75% or less with respect to the total number of primary particles.

[0036] The proportion of the high aspect ratio primary particles can be calculated as follows. (1) Expose the cross-section of the secondary particles. As a method for exposing the cross-section, for example, a method of embedding the secondary particles in a resin and processing them with a cross-section polisher (for example, IB19520CCP manufactured by JEOL Ltd.) to expose the cross-section of the secondary particles can be mentioned. (2) Using SEM (for example, JSM-7900F manufactured by JEOL Ltd.), take a reflected electron image of the cross-section of the exposed secondary particles. (3) Import the cross-sectional image obtained as described above into a computer, and calculate the aspect ratio of each primary particle using image analysis software (for example, ImageJ manufactured by the National Institutes of Health, USA). The aspect ratio of the primary particle is obtained by dividing the length of the long side of the primary particle by the length of the short side in the direction perpendicular to the longest diameter. (4) From the above measurement results, calculate the content of the high aspect ratio primary particles having an aspect ratio of 2 or more based on the following formula. (Proportion of high aspect ratio primary particles) = (Number of high aspect ratio primary particles) / (Total number of primary particles) × 100 (5) Perform the above measurement on 5 secondary particles contained in the same composite metal oxide, and use the average value as the proportion of the high aspect ratio primary particles.

[0037] The composite metal oxide may have a layered structure in which an alkali metal layer or an alkaline earth metal layer and a transition metal layer are alternately laminated. Examples of the layered structure of the composite metal oxide include a layered structure belonging to the space group R-3m, a layered structure belonging to the space group C2 / m, the space group P6 ３ mc belonging to, the space group P6 ３Examples include layered structures belonging to the / mmc space group. From the viewpoint of increasing capacity and crystalline structure stability, it is preferable that the composite metal oxide has a layered structure belonging to space group R-3m.

[0038] Figure 2 schematically shows the structure and arrangement of high-aspect-ratio primary particles 31 within secondary particles 30. Note that Figure 2 only shows a portion of the high-aspect-ratio primary particles 31 within secondary particles 30. As shown in Figure 2, the edge surfaces 32 of the alkali metal layer (similar to the alkaline earth metal layer) and the edge surfaces 33 of the transition metal layer are alternately stacked in the short-axis direction of the high-aspect-ratio primary particles 31. In other words, the edge surfaces 32 of the alkali metal layer and the edge surfaces 33 of the transition metal layer are oriented perpendicular to the short-axis direction of the high-aspect-ratio primary particles 31. Note that the term "edge surface" refers to the end of the layered structure (the surface where the stacking state is visible).

[0039] Here, in the secondary particles 30, among primary particles 31 with an aspect ratio of 2 or more, the proportion of primary particles 31 whose alkali metal layer edge surface 32 and transition metal layer edge surface 33 are oriented outward from the center of the secondary particle 30 is preferably 50% or more, and more preferably 60% or more, of the total number of primary particles 31 with an aspect ratio of 2 or more. By setting this proportion to 50% or more, the crystallinity of the composite metal oxide can be further improved and the initial resistance can be further reduced. There is no particular upper limit to the proportion of primary particles 31 whose alkali metal layer edge surface 32 and transition metal layer edge surface 33 are oriented outward from the center of the secondary particle 30; for example, it may be 100%. Note that primary particles 31 whose edge surfaces 32 and 33 are oriented outward from the center of the secondary particle 30 refer to primary particles 31 in which, when the circumscribed circle X of the secondary particle 30 is defined, the inclination angle of the edge surfaces 32 and 33 with respect to the radial direction of the circumscribed circle X is 30° or less, as shown in Figure 2.

[0040] The proportion of primary particles 31 whose alkali metal layer edge surface 32 and transition metal layer edge surface 33 are oriented from the center outward of the secondary particle 30 is obtained from the orientation analysis of the edge surface using electron backscatter diffraction (EBSD) (e.g., Velocity, EDAX) under the following conditions. The proportion of primary particles 31 whose edge surfaces 32 and 33 are oriented from the center outward of the secondary particle 30 is measured for five secondary particles contained in the same composite metal oxide, and the average value is used. Acceleration voltage: 10kV WD: 15mm Sample tilt: 70° Orientation analysis: Inverse Pole Figure Map step: 0.05μm

[0041] In a composite metal oxide, the alkali metal contained in the composite metal oxide is Li, and the alkali metal layer may be a Li layer. In other words, the composite metal oxide may have a layered structure in which Li layers and transition metal layers are alternately stacked. Furthermore, the proportion of metal elements other than Li present in the Li layer of the layered structure is preferably 8 mol% or less relative to the total number of moles of metal elements other than Li in the composite metal oxide. If the proportion of metal elements other than Li in the Li layer exceeds 8 mol% relative to the total number of moles of metal elements other than Li in the composite metal oxide, the diffusivity of Li ions in the Li layer decreases, which may reduce the battery capacity. The metal elements other than Li present in the Li layer are mainly Ni, but other metal elements may also be included. The proportion of metal elements other than Li in the Li layer is, for example, 0.1 mol% or more and 8 mol% or less relative to the total number of moles of metal elements other than Li in the composite metal oxide.

[0042] The proportion of metal elements other than Li present in the layered Li layer can be obtained from the Rietveld analysis results of the X-ray diffraction pattern obtained by the following X-ray diffraction measurement of the composite metal oxide. For the Rietveld analysis of the X-ray diffraction pattern, for example, the Rietveld analysis software PDXL2 (Rigaku Corporation) can be used.

[0043] The X-ray diffraction pattern is obtained by powder X-ray diffraction using a powder X-ray diffractometer (manufactured by Rigaku Corporation, product name "SmartLab", source Cu-Kα) under the following conditions: Measurement range: 15-120° Scan rate: 4° / min Analysis range: 30-120° Background: B-spline profile Function: Split-type pseudo-Voigt function Constraints: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the respective Ni content)

[0044] Furthermore, the crystallite size s of the composite metal oxide, calculated by Scherrer's formula from the full width at half maximum of the diffraction peak of the (10⁴) plane in the X-ray diffraction pattern obtained by the above-mentioned X-ray diffraction, is preferably 300 Å ≤ s ≤ 700 Å, and more preferably 300 Å ≤ s ≤ 500 Å, from the viewpoint of reducing battery resistance and increasing battery capacity. Scherrer's formula is expressed as follows: In the formula below, s is the crystallite size, λ is the wavelength of the X-ray, B is the full width at half maximum of the diffraction peak of the (10⁴) plane, θ is the diffraction angle (rad), and K is Scherrer's constant. In this embodiment, K is set to 0.9. s = Kλ / Bcosθ

[0045] As described above, the composite metal oxide of this embodiment has a modified compound containing a sulfur-containing compound on the surface of the primary particles, including the surface of the secondary particles. The presence of the modified compound stabilizes the surface state and crystal structure of the composite metal oxide, thereby reducing the initial resistance.

[0046] The modifying compound may be uniformly dispersed throughout the composite metal oxide on the surface of the primary particles, or it may be present in only a portion of the composite metal oxide. For example, the modifying compound may be present partically or in layers on the surface of the primary particles. The surface of the primary particles of the composite metal oxide refers to the particle surface and its vicinity, for example, the near-surface region within 30 nm from the particle surface. The presence of the modifying compound on the surface of the primary particles of the composite metal oxide can be confirmed by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0047] The sulfur-containing compound in the modified compound is, for example, an inorganic sulfate containing Me (Me is at least one element included in the third period or later of alkali metals). Me is, for example, Na or K. The Me content in the composite metal oxide is preferably such that 0 mol% < Me content ≤ 6 mol% relative to the total number of moles of transition metal elements in the composite metal oxide. The lower limit of the Me content is, for example, 0.1 mol% relative to the total number of moles of transition metal elements in the composite metal oxide. Therefore, the Me content in the composite metal oxide may satisfy 0.1 mol% ≤ Me content ≤ 6 mol% relative to the total number of moles of transition metal elements in the composite metal oxide.

[0048] Here, inorganic sulfate refers to a sulfate that does not contain carbon. In this specification, sulfate refers to a sulfate in the broad sense that contains sulfur and oxygen. Inorganic sulfate is, for example, at least one compound selected from the group consisting of sulfites, disulfites, peroxodisulfates, and sulfates in the narrow sense, and more preferably at least one compound selected from the group consisting of sulfites, disulfites, peroxodisulfates. Sulfite refers to sulfite ions (SO4). ３ ２－ A salt consisting of ) and Me, for example, Na ２ SO ３ _K ２ SO ３ Examples include the disulfite ion (S). Disulfites are disulfite ions (S) ２ O ５ ２－ A salt consisting of ) and Me, for example, Na ２ SO ５ _K ２ SO ５ Examples include disulfate salts, which are composed of disulfate ions (S ２ O ７ ２－ A salt consisting of ) and Me, for example, Na ２ S ２ O ７ _K ２ S ２ O ７ Examples include the peroxodisulfate, which is a peroxodisulfate ion (S ２ O８ ２－ A salt consisting of ) and Me, for example, Na ２ S ２ O ８ _K ２ S ２ O ８ These are some examples. Sulfates in the narrow sense refer to sulfate ions (SO4). ４ ２－ It is a salt composed of ) and Me, and Na ２ SO ４ _K ２ SO ４ These are some examples.

[0049] The Me and S content in a composite metal oxide can be measured, for example, by an inductively coupled plasma atomic emission spectrometer (ICP-AES). While ICP-AES may sometimes quantify S originating from components in the composite hydroxide, if the S content is 0.1 mol% or higher, it can be said that the S quantified originates from the S-containing compound.

[0050] The composite metal oxide exhibits a first peak with a peak at 170 ± 2 eV and a second peak with a peak at 167 ± 2 eV in its S2p ​​spectrum obtained by X-ray photoelectron spectroscopy. The first peak is SO ４ ２－ The second peak is estimated to belong to SO ３ ２－ It is estimated that this is a peak attributable to SO. For example, in the method for producing the positive electrode active material described later, if a compound containing S is added during water washing, SO will be released from the positive electrode active material. ４ ２－ A first peak is obtained that is attributed to SO ３ ２－ A second peak attributable to the first peak cannot be obtained. Furthermore, for example, by adding a compound containing S to a cake-like composition or a powder obtained by drying the cake-like composition, as described later, a modified compound containing S can be made present on the surface of the primary particles of the composite metal oxide, thereby allowing both the first and second peaks to be obtained from the positive electrode active material.

[0051] The first and second peaks are obtained, for example, by separating the primary data of the S2p spectrum obtained by X-ray photoelectron spectroscopy into two peaks with peaks at 170±2 eV and 167±2 eV. After setting the peak positions of the two peaks to 170±2 eV and 167±2 eV respectively, peak fitting is performed by varying the peak width and peak height, so that each peak has a shape with a Gauss-Lorentz distribution.

[0052] Furthermore, in the O1s spectrum obtained by X-ray photoelectron spectroscopy, it is preferable that the peak of the third peak, which has a peak at 531±1 eV, is higher than the peak of the fourth peak, which has a peak at 529±1 eV. The third peak is presumed to be a peak attributed to oxygen contained in the compound present on the surface of the composite metal oxide, and the fourth peak is presumed to be a peak attributed to oxygen contained in the composite metal oxide. When the peak of the third peak is higher than the peak of the fourth peak, the protective effect of the modifying compound on the surface of the composite metal oxide is significantly exhibited, and the surface state and layered structure of the composite metal oxide can be stabilized. The third and fourth peaks can be confirmed in the primary data of the O1s spectrum obtained by X-ray photoelectron spectroscopy.

[0053] The S2p and O1s spectra were obtained by X-ray photoelectron spectroscopy using an X-ray photoelectron spectrometer (PHI 5000 VersaProbe, ULVAC PHI, Inc.) under the following conditions: X-rays used: Monochrome Al-Kα (45W, 17kV) Analysis area: Approximately 200 μmφ

[0054] Next, an example of a method for producing a positive electrode active material according to this embodiment will be described. The method for producing a positive electrode active material includes, for example, a mixing step of mixing a metal compound containing Ni and a Li compound to obtain a mixture; a firing step of firing the mixture to obtain a fired product; a washing step of washing and dewatering the fired product to obtain a cake-like composition; and an addition step of adding a compound containing S to the cake-like composition or a powder obtained by drying the cake-like composition, and then performing a heat treatment.

[0055] Examples of metal compounds include metal hydroxides, metal oxides, and metal carbonate compounds. As an example of a metal compound, a metal oxide can be produced by separately adding a solution of a metal salt containing Ni, Co, Mn, etc., and an alkaline solution such as sodium hydroxide dropwise to a reaction vessel that is stirring a pH-adjusted solution, adjusting the pH to the alkaline side (for example, 8.5 or higher and 12.5 or lower), thereby precipitating (coprecipitation) a complex hydroxide, and then heat-treating the complex hydroxide.

[0056] Here, the aspect ratio of the final composite metal oxide can be adjusted by changing the conditions for precipitation of the composite hydroxide (concentration of the dropwise added solution, pH in the reaction vessel, temperature of the solution in the reaction vessel, complexing agent, etc.) and the conditions for heat treatment (heat treatment temperature, heat treatment time, heat treatment atmosphere, etc.). From the viewpoint of increasing the aspect ratio of the final composite metal oxide, it is preferable that the heat treatment temperature be 600°C or lower. For example, to increase the aspect ratio of the final composite metal oxide, the concentrations of the dropwise added metal salt solution and alkaline solution can be lowered, and the heat treatment can be performed within a specific temperature range.

[0057] Next, a mixture is obtained by mixing the obtained metal compound with the Li compound. Examples of Li compounds include Li ２ CO ３ LiOH, Li ２ O ２ Li ２ O, LiNO ３ LiNO ２ Li ２ SO ４ LiOH H ２ Examples include O, LiH, and LiF. The mixing ratio of the metal compound and the Li compound is preferably adjusted so that, for example, the molar ratio of the metal element excluding Li to Li is in the range of 1:0.98 to 1:1.09.

[0058] During mixing, at least one of the Ca compound and the Sr compound may be mixed. The Ca compound may be Ca(OH) ２ CaHPO ４ Ca(H ２ PO ４ ) ２ Ca３ (PO ４ ) ２ 、CaO、CaCO ３ 、CaSO ４ 、Ca(NO ３ ) ２、 CaCl ２ 、CaAlO ４ 等が挙げられる。Sr化合物としては、Sr(OH) ２ 、SrHPO ４ 、Sr(H ２ PO ４ ) ２ 、Sr ３ (PO ４ ) ２ 、SrO、SrCO ３ 、SrSO ４ 、Sr(NO ３ )<00​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​２ O ５ nH ２ O, Li ２ MoO ４ MoO ３ H ２ MoO ４ MoS ２ , SnO ２ WO ３ Li ２ WO ４ WS ２ Al ２ (WO ４ ) ３ , Bi(OH) ３ , Bi ２ O ３ These are some examples.

[0060] The mixture obtained in the mixing step is fired to obtain the fired product. The firing of the mixture is carried out, for example, in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream is set to 10 cm in the firing furnace. ３ The mixture is fired at a rate of 0.1 L / min or more and 4 L / min or more per kg of mixture. The first set temperature in the firing conditions is set to 450°C or lower. The holding time at the first set temperature is in the range of 0 hours or more and 8 hours or lower, and the heating rate below 450°C is in the range of 1.5°C / min or more and 6.0°C / min or less. The second set temperature is set to 450°C or more and 680°C or lower. The holding time at the second set temperature is in the range of 0 hours or more and 8 hours or lower, and the heating rate above 450°C or below 680°C is in the range of 1.0°C / min or more and 4.5°C / min or less. The maximum temperature that can be reached is in the range of 690°C or more and 900°C or lower. The heating rate from above 680°C to the maximum temperature can be, for example, 0.1°C / min or more and 3.5°C / min or less. Furthermore, the holding time at the highest temperature reached may be between one hour and ten hours. Also, this firing step may be a multi-stage firing, and multiple settings may be set for each temperature range, as long as they are within the range specified above. For example, by changing the first set temperature, the second set temperature, the respective heating rate to reach the highest temperature, and the respective holding time, the crystallinity of the composite metal oxide can be changed, and the proportion of metal elements other than Li present in the Li layer, the crystal size, etc., can be adjusted.

[0061] A cake-like composition is obtained by washing and dewatering the calcined material. The washing step is carried out, for example, using a 3 L reaction vessel, under conditions such as a solid-liquid ratio of 300 g / L or more and 2000 g / L or less, a washing time of 1 minute or more and 1 hour or less, and a stirring speed of 100 rpm or more. If the size of the reaction vessel is changed, the washing time and stirring speed may be changed. The water content of the cake-like composition obtained by dewatering is, for example, 10% or less, and may be 8% or less. The drying of the cake-like composition is carried out, for example, under conditions that include oxygen, nitrogen, air, etc., in a reduced pressure to a pressurized state. For example, a reduced pressure atmosphere, particularly a vacuum atmosphere, is preferred, and the drying is carried out under conditions such as a pressure of 1 kPa or less, a temperature of 120°C or more and 300°C or less, and a time of 1 hour or more and 10 hours or less.

[0062] A positive electrode active material (composite metal oxide) can be obtained by adding a sulfur-containing compound to a cake-like composition or a powder obtained by drying a cake-like composition, and then performing heat treatment. By adding the sulfur-containing compound after the calcination step, the sulfur-containing compound can be made present on the surface of the primary particles of the composite metal oxide.

[0063] The heat treatment of the cake-like composition after adding a compound containing sulfur, or the powder obtained by drying the cake-like composition, is carried out at a temperature of, for example, 150°C or higher and 500°C or lower. The atmosphere during the heat treatment may be a vacuum, an oxygen stream, or air. Also, during the heat treatment, for example, tungsten oxide (WO) may be used. ３ ), lithium tungstate (Li ２ WO ４ Li ４ WO ５ Li ６ W ２ O ９ ), boric acid (H ３ BO ３ ), lithium borate (Li ２ B ４ O ７ Li ３ BO ３ LiB ３ O ５ LiBO ２ ), lithium phosphate (Li ３－ｘ Hｘ PO ４ Additions such as (0 ≤ x ≤ 3) may also be added.

[0064] The sulfur-containing compound added in the addition step is, for example, an inorganic sulfate containing Me (Me is at least one element from the third period onward of the alkali metals). Me is, for example, Na or K.

[0065] The inorganic sulfate to be added is, for example, at least one compound selected from the group consisting of sulfites, disulfites, disulfates, peroxodisulfates, and sulfates in the narrow sense, and more preferably at least one compound selected from the group consisting of sulfites, disulfites, disulfates, and peroxodisulfates. Examples of inorganic sulfates to be added include Na ２ SO ３ _K ２ SO ３ Na ２ SO ５ _K ２ SO ５ Na ２ S ２ O ７ _K ２ S ２ O ７ Na ２ S ２ O ８ _K ２ S ２ O ８ Na ２ SO ４ _K ２ SO ４ These are some examples.

[0066] [Negative Electrode] The negative electrode 12 may, for example, have a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, or a metallic Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode current collector, and lithium metal may be deposited on the surface of the negative electrode current collector by charging. When the negative electrode 12 has a negative electrode mixture layer, it is preferable that the negative electrode mixture layer is formed on both sides of the negative electrode current collector. For the negative electrode current collector, a foil of a metal that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal arranged on the surface layer, can be used. The thickness of the negative electrode current collector is, for example, 5 μm or more and 30 μm or less. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode current collector. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., to the surface of a negative electrode current collector, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0067] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si and Sn, or lithium titanium composite oxides may be used as the negative electrode active material. Furthermore, materials with a carbon coating may also be used. For example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0068] Examples of binders included in the negative electrode mixture layer include styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0069] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0070] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0071] [Non-aqueous electrolytes] Non-aqueous electrolytes, for example, have lithium ion conductivity. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0072] A liquid electrolyte (electrolyte solution) includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0073] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0074] Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methylphenyl ether. Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0075] The electrolyte salt is preferably a lithium salt. A suitable lithium salt is LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO4). ２ F ２Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. Of these, LiPF is used from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ It is preferable to use the following. The concentration of the lithium salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 1.8 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0076] Non-aqueous electrolytes may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0077] Examples of unsaturated cyclic carbonate esters include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. Unsaturated cyclic carbonate esters may be used individually or in combination of two or more. Some hydrogen atoms in the unsaturated cyclic carbonate esters may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by the intermolecular condensation of multiple carboxylic acid molecules, but it is preferable that it be an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0078] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0079] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanate methylcyclohexane (BIMCH). Examples of sultone compounds include propanesultone and propensultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethylborate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethylphosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethylphosphite and tris(trimethylsilyl)phosphite.

[0080] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0081] The present disclosure will be further illustrated below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0082] <Example 1-1> [Preparation of positive electrode active material] [Ni obtained by coprecipitation method ０．９０ Co ０．０５ Mn ０．０５ ] (OH) ２The composite hydroxide represented by was calcined at 400°C for 8 hours to obtain a metal oxide containing Ni, Co, and Mn. Next, the metal oxide and Ca(OH) were mixed so that the molar ratio of Ca to the total number of moles of Ni, Co, and Mn was 0.3 mol%, the molar ratio of Zr was 0.3 mol%, the molar ratio of Nb was 0.5 mol%, and the molar ratio of Sr was 0.1 mol%. ２ And, ZrO ２ And, Nb ２ O ５ And, Sr(OH) ２ The following are mixed, and further, the molar ratio of Li to the total number of moles of Ni, Co, Mn, Ca, Zr, Nb, and Sr is 105 mol%, so that lithium hydroxide monohydrate (LiOH·H) is formed as the Li compound. ２ O) was mixed to obtain a mixture (mixing step).

[0083] This mixture was heated under an oxygen stream with an oxygen concentration of 95% (flow rate of 3 L / min per 1 kg of mixture) at a heating rate of 4°C / min from room temperature to 400°C, then heated from 400°C to 650°C at a heating rate of 2°C / min. After that, the temperature was raised from 650°C to 730°C at a heating rate of 1°C / min and held for 5 hours to obtain a calcined product (calcination step). After crushing this calcined product, the calcined product was added to water in a 3 L reaction vessel so that the solid-liquid ratio was 500 g / L, and washed with water at a stirring speed of 300 rpm for 10 minutes, then dehydrated to obtain a cake-like composition (washing step). Furthermore, 0.1 mol% sodium sulfite (Na) relative to the total number of moles of Ni, Co, and Mn was added to this cake-like composition. ２ SO ３ The following was added: After that, heat treatment was performed for 2 hours in a vacuum atmosphere at a temperature of 150°C and a pressure of 10 Pa, followed by heat treatment at a temperature of 400°C and an oxygen atmosphere for 6 hours to obtain the positive electrode active material (composite metal oxide) of Example 1-1 (addition step).

[0084] Measurements of the obtained cathode active material using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy-dispersive X-ray spectroscopy (TEM-EDX) revealed that the S content, Ca content, and Sr content relative to the total number of moles of transition metal elements were 0.17 mol%, 0.3 mol%, and 0.1 mol%, respectively. TEM-EDX measurements also confirmed the presence of Ca, Sr, and inorganic sulfates on the primary particle surface of the composite metal oxide. Furthermore, as shown in Figure 3, the cathode active material of Example 1-1 exhibited both a first peak with a peak at 170±2 eV and a second peak with a peak at 167±2 eV in its S2p ​​spectrum obtained by X-ray photoelectron spectroscopy. Additionally, as shown in Figure 4, the cathode active material of Example 1-1 showed that the peak P3 of the third peak with a peak at 531±1 eV was higher than the peak P4 of the fourth peak with a peak at 529±1 eV in its O1s spectrum obtained by X-ray photoelectron spectroscopy.

[0085] Furthermore, using the cross-sectional SEM image of the secondary particles of the composite metal oxide, the proportion of high-aspect-ratio primary particles with an aspect ratio of 2 or higher relative to the total number of primary particles was calculated to be 57% using the method described above. In addition, X-ray diffraction measurements revealed that the proportion of metal elements other than Li present in the Li layer relative to the total number of moles of metal elements excluding Li in the composite metal oxide was 3.6 mol%. Furthermore, using electron backscatter diffraction (EBSD) of the cross-section of the positive electrode active material, orientation analysis of the edge plane was performed, and the proportion of primary particles whose edge planes in the Li layer and transition metal layer were oriented from the center outward was 61%. Finally, the crystallite size s of the composite metal oxide, calculated using Scherrer's formula from the full width at half maximum of the diffraction peak of the (10⁴) plane in the X-ray diffraction pattern, was 334 Å.

[0086] [Preparation of the positive electrode] The above-mentioned composite metal oxide was used as the positive electrode active material. The positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a solid content mass ratio of 100:3:2, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as the dispersion medium. The positive electrode mixture slurry was applied to both sides of a positive electrode core made of aluminum foil, and after the coating film was dried, the coating film was rolled using a roller and 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. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode core was exposed.

[0087] [Preparation of Non-Aqueous Electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF) was added to this mixed solvent. ６ A non-aqueous electrolyte was prepared by dissolving the substance to a concentration of 1.2 mol / liter.

[0088] [Preparation of Test Cell] A positive electrode lead was attached to the exposed portion of the positive electrode, and a negative electrode lead was attached to the Li metal foil serving as the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator. This electrode assembly was placed inside an outer casing made of aluminum laminate sheet, the non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to obtain the test cell.

[0089] [Evaluation of Initial Resistance] Under ambient temperature of 25°C, the test cell was charged with a constant current of 0.2C up to the charging termination potential (4.3V), and then charged with a constant voltage until the current was 0.01C at the charging termination potential. Subsequently, a constant current discharge was performed at a current of 0.2C until the cell voltage reached 2.5V. After that, constant current charging was performed again at a current of 0.2C up to the cell charging termination potential, and then constant voltage charging was performed until the current was 0.01C up to the charging termination potential. After that, the initial resistance value was taken by dividing the voltage drop when a current of 1C was applied for 5 seconds by the current value.

[0090] <Example 1-2> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) [Ni０．９０ Co ０．０４ Mn ０．０５ Al ０．０１ ] (OH) ２ The composite hydroxide represented by was calcined at 350°C for 8 hours to obtain a metal oxide containing Ni, Co, Mn, and Al. (2) In the mixing step, the metal oxide and Ca(OH) were mixed so that the molar ratio of Ca to the total number of moles of Ni, Co, Mn, and Al was 0.3 mol%, and the molar ratio of Ti was 0.1 mol%. ２ And, TiO ２ Furthermore, the Li compound was mixed with the total number of moles of Ni, Co, Mn, Al, Ca, and Ti such that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Al, Ca, and Ti was 103 mol%. (3) After the washing step, the dehydrated cake-like composition was vacuum-dried at 200°C to obtain a powder, to which in the addition step, 0.3 mol% sodium disulfite (Na) was added relative to the total number of moles of Ni, Co, Mn, and Al. ２ S ２ O ５ The following was added, and heat treatment was performed for 4 hours in an air atmosphere at a temperature of 200°C.

[0091] <Example 1-3> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the molar ratio of Al to the total number of moles of Ni, Co, and Mn was 0.3 mol%, the molar ratio of Ca was 0.1 mol%, and the molar ratio of Fe was 0.1 mol%, so that the metal oxide and Al(OH) ３ And Ca(OH) ２ And, Fe ２ O ３ Furthermore, a Li compound was mixed with the total number of moles of Ni, Co, Mn, Al, Ca, and Fe such that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Al, Ca, and Fe was 105 mol%. (2) In the addition step, 1.0 mol% of potassium disulfite (K) relative to the total number of moles of Ni, Co, and Mn was added to the cake-like composition. ２ S ２ O ５ The following was added, and heat treatment was performed for 2 hours under a vacuum atmosphere at a temperature of 200°C.

[0092] <Example 1-4> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) [Ni ０．８８ Co ０．０６ Mn ０．０３ Al ０．０３ ] (OH) ２ The composite hydroxide represented by was calcined at 550°C for 10 hours to obtain a metal oxide containing Ni, Co, Mn, and Al. (2) In the mixing step, the metal oxide and Nb were mixed such that the molar ratio of Nb to the total number of moles of Ni, Co, Mn, and Al was 0.3 mol%, the molar ratio of Ca was 0.5 mol%, and the molar ratio of Mo was 0.1 mol%. ２ O ５ And Ca(OH) ２ And, MoO ３ Furthermore, a Li compound was mixed with the total number of moles of Ni, Co, Mn, Al, Nb, Ca, and Mo such that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Al, Nb, Ca, and Mo was 107 mol%. (3) In the calcination step, the temperature was raised from room temperature to 400°C at a heating rate of 2°C / min, then raised from 400°C to 650°C at a heating rate of 2°C / min, and held at 650°C for 2 hours. After that, the temperature was raised from 650°C to 750°C at a heating rate of 1°C / min, and held for 5 hours to obtain the calcined product. (4) In the addition step, 0.1 mol% sodium sulfite (Na) relative to the total number of moles of Ni, Co, Mn, and Al was added to the cake-like composition. ２ SO ３ After adding ) and performing heat treatment for 2 hours in a vacuum atmosphere at a temperature of 200°C, 0.5 mol% boric acid (H ３ BO ３ The following were added, and the mixture was heat-treated for 10 hours in an oxygen atmosphere at a temperature of 400°C.

[0093] <Example 1-5> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) [Ni ０．９２ Co ０．０３ Mn ０．０４ Al ０．０１ ] (OH) ２The composite hydroxide represented by was calcined at 400°C for 8 hours to obtain a metal oxide containing Ni, Co, Mn, and Al. (2) In the mixing step, the metal oxide and Mg(OH) were mixed so that the molar ratio of Mg to the total number of moles of Ni, Co, Mn, and Al was 0.1 mol%, the molar ratio of Ti was 0.3 mol%, and the molar ratio of Sr was 0.3 mol%. ２ And, TiO ２ And, SrCO ３ Furthermore, a Li compound was mixed with the total number of moles of Ni, Co, Mn, Al, Mg, Ti, and Sr such that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Al, Mg, Ti, and Sr was 101 mol%. (3) In the calcination step, the temperature was raised from room temperature to 350°C at a heating rate of 6°C / min, and then held at 350°C for 2 hours. After that, the temperature was raised from 350°C to 680°C at a heating rate of 2°C / min, and then raised from 680°C to 700°C at a heating rate of 1°C / min, and held for 5 hours to obtain the calcined product. (4) In the addition step, 0.3 mol% of Na relative to the total number of moles of Ni, Co, Mn, and Al was added to the cake-like composition. ２ SO ４ The substance was added, and heat treatment was performed for 4 hours under a vacuum atmosphere at a temperature of 300°C.

[0094] <Comparative Example 1-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) The composite hydroxide was calcined at 650°C for 10 hours to obtain a metal oxide containing Ni, Co, and Mn. (2) In the mixing step, the metal oxide and Ca(OH) were mixed so that the molar ratio of Ca to the total number of moles of Ni, Co, and Mn was 0.3 mol%, and the molar ratio of Zr was 0.3 mol%. ２ And, ZrO ２ Furthermore, a Li compound was mixed with the total number of moles of Ni, Co, Mn, Ca, and Zr such that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Ca, and Zr was 103 mol%. (3) After the washing step, the dehydrated cake-like composition was vacuum-dried at 200°C to obtain a powder, to which in the addition step, 0.3 mol% sodium disulfite (Na) was added relative to the total number of moles of Ni, Co, and Mn. ２ S ２ O ５ The following was added, and heat treatment was performed for 10 hours in an air atmosphere at a temperature of 300°C.

[0095] <Comparative Example 1-2> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the molar ratio of Ca to the total number of moles of Ni, Co, and Mn was 0.3 mol%, the molar ratio of Zr was 0.3 mol%, and the molar ratio of Al was 0.3 mol%, so that the metal oxide and Ca(OH) ２ And, ZrO ２ And, Al ２ O ３ Furthermore, a Li compound was mixed with the total number of moles of Ni, Co, Mn, Ca, Zr, and Al such that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Ca, Zr, and Al was 105 mol%. (2) In the calcination step, the temperature was raised from room temperature to 730°C at a heating rate of 1°C / min, and then held for 5 hours to obtain the calcined product. (3) After the water washing step, the dehydrated cake-like composition was subjected to vacuum drying only at 200°C.

[0096] When the positive electrode active materials of Comparative Examples 1-2 were measured by TEM-EDX, it was confirmed that Ca was present in the primary particles of the composite metal oxide, but no sulfur-containing compounds were present. Furthermore, although no sulfur-containing compounds were added, 0.07 mol% sulfur was quantitatively determined from the components contained in the composite hydroxide. In addition, as shown in Figure 5, in the S2p spectrum of the positive electrode active materials of Comparative Examples 1-2, only the first peak with a peak at 170 ± 2 eV was observed, and the second peak with a peak at 167 ± 2 eV was not observed. Furthermore, as shown in Figure 6, in the O1s spectrum of the positive electrode active materials of Comparative Examples 1-2, the peak P3 of the third peak with a peak at 531 ± 1 eV was lower than the peak P4 of the fourth peak with a peak at 529 ± 1 eV.

[0097] <Comparative Example 1-3> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) [Ni ０．９０ Co ０．０３ Mn ０．０５ Al ０．０２ ] (OH) ２The composite hydroxide represented by was calcined at 600°C for 10 hours to obtain a metal oxide containing Ni, Co, Mn, and Al. (2) In the mixing step, the metal oxide and TiO were mixed such that the molar ratio of Ti to the total number of moles of Ni, Co, Mn, and Al was 0.1 mol%, and the molar ratio of Zr was 0.3 mol%. ２ And, ZrO ２ Furthermore, the Li compound was mixed with the total number of moles of Ni, Co, Mn, Al, and Zr such that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Al, and Zr was 103 mol%. (3) After the washing step, the dehydrated cake-like composition was vacuum-dried at 200°C to obtain a powder, to which in the addition step, 0.3 mol% sodium ascorbate (C) was added relative to the total number of moles of Ni, Co, Mn, and Al. ６ H ７ O ６ Na was added, and heat treatment was performed at 300°C in an oxygen atmosphere for 5 hours.

[0098] <Comparative Example 1-4> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) [Ni ０．９０ Co ０．０２ Mn ０．０８ ] (OH) ２ (1) The composite hydroxide represented by was calcined at 300°C for 10 hours to obtain a metal oxide containing Ni, Co, and Mn. (2) In the mixing step, the metal oxide and the Li compound were mixed so that the molar ratio of Li to the total number of moles of Ni, Co, and Mn was 103 mol%. (3) In the addition step, 0.1 mol% of sodium dodecyl sulfate (NaC) relative to the total number of moles of Ni, Co, and Mn was added to the cake-like composition. １２ H ２５ SO ４ The following was added, and heat treatment was performed for 4 hours under a vacuum atmosphere at a temperature of 150°C.

[0099] Table 1 shows the conditions for preparing the positive electrode active materials for Examples 1-1 to 5 and Comparative Examples 1-1 to 4. Table 2 shows the evaluation results of the test cells for Examples 1-1 to 5 and Comparative Examples 1-1 to 4. In Table 2, the initial resistance of each test cell is expressed relatively, with the initial resistance of the test cell in Comparative Example 1-1 set to 100; a smaller value indicates lower resistance. Table 2 also shows the following information along with the evaluation results. (1) The content of S, Me, Ca, and Sr contained in the composite metal oxide relative to the total number of moles of transition metal elements in the composite metal oxide, calculated from the measurement results of ICP-AES and TEM-EDX. (2) The presence or absence of S-containing compounds and Ca or Sr on the surface of the primary particles of the composite metal oxide. (3) The proportion of metal elements other than Li present in the Li layer of the layered structure. (4) The presence or absence of the first and second peaks in the S2p spectrum by X-ray photoelectron spectroscopy, and whether the peak P3 of the third peak in the O1s spectrum by X-ray photoelectron spectroscopy is higher than the peak P4 of the fourth peak. (5) The proportion of high aspect ratio primary particles with an aspect ratio of 2 or more relative to the total number of primary particles. (6) The proportion of primary particles whose edge faces are oriented from the center of the secondary particles toward the outer periphery. (7) The crystallite size (s) calculated by Scherrer's formula from the full width at half maximum of the diffraction peak of the (104) plane of the X-ray diffraction pattern.

[0100]

[0101]

[0102] As shown in Table 2, the initial resistance of the test cell in the example is reduced compared to the initial resistance of the test cell in the comparative example. Therefore, it can be said that the initial resistance can be specifically reduced by including a modified compound containing a sulfur-containing compound on the surface of the primary particles, and by including a predetermined proportion or more of primary particles whose spectra obtained by X-ray photoelectron spectroscopy have a predetermined shape and an aspect ratio of 2 or more.

[0103] <Example 2-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows and the charge termination potential for evaluating the initial resistance was set to 4.4V. (1) [Ni ０．８０ Mn ０．２０ ] (OH)２ The composite hydroxide represented by was calcined at 350°C for 10 hours to obtain a metal oxide containing Ni and Mn. (2) In the mixing step, the metal oxide and Ca(OH) were mixed so that the molar ratio of Ca to the total number of moles of Ni and Mn was 1.0 mol%, the molar ratio of Sr was 0.1 mol%, and the molar ratio of Nb was 0.1 mol%. ２ And, Sr(OH) ２ And, Nb ２ O ５ Furthermore, the Li compound was mixed with the total number of moles of Ni, Mn, Ca, Sr, and Nb so that the molar ratio of Li to the total number of moles of Ni, Mn, Ca, Sr, and Nb was 105 mol%. (3) In the calcination step, the temperature was raised from room temperature to 300°C at a heating rate of 5°C / min, and then raised from 300°C to 650°C at a heating rate of 3°C / min. After that, the temperature was raised from 650°C to 750°C at a heating rate of 1.5°C / min and held for 5 hours to obtain the calcined product (calcination step). After crushing this calcined product, the calcined product was added to water in a 3 L reaction vessel so that the solid-liquid ratio was 1000 g / L, and washed with water at a stirring speed of 300 rpm for 10 minutes, and then dehydrated to obtain a cake-like composition (washing step). (4) After the washing step, the dehydrated cake-like composition is vacuum-dried at 150°C to obtain a powder, to which in the addition step, 0.1 mol% sodium disulfite (Na) is added relative to the total number of moles of Ni and Mn. ２ S ２ O ５ The following was added, and the mixture was heat-treated at 300°C in an oxygen atmosphere for 8 hours.

[0104] <Example 2-2> A test cell was prepared and evaluated in the same manner as in Example 2-1, except that the positive electrode active material was prepared as follows: (1) [Ni ０．８０ Mn ０．２０ ] (OH) ２ The composite hydroxide represented by was calcined at 600°C for 10 hours to obtain a metal oxide containing Ni and Mn. (2) In the mixing step, the metal oxide and Ca(OH) were mixed so that the molar ratio of Ca to the total number of moles of Ni and Mn was 0.2 mol%, the molar ratio of Sr was 0.1 mol%, and the molar ratio of Si was 0.1 mol%. ２ And, Sr(OH) ２ And, SiO ２Furthermore, the Li compound was mixed with the total number of moles of Ni, Mn, Ca, Sr, and Si such that the molar ratio of Li to Si was 105 mol%. (3) In the addition step, 0.1 mol% of sodium sulfite (Na) relative to the total number of moles of Ni and Mn was added to the cake-like composition. ２ SO ３ The following steps were performed: adding the substance and heat-treating it in a vacuum atmosphere at 200°C for 2 hours, followed by heat-treating it in an oxygen atmosphere at 300°C for 8 hours.

[0105] <Example 2-3> A test cell was prepared and evaluated in the same manner as in Example 2-1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the molar ratio of Ca to the total number of moles of Ni and Mn was 0.5 mol%, the molar ratio of Sr was 0.1 mol%, the molar ratio of Nb was 0.3 mol%, and the molar ratio of Ti was 0.1 mol%, so that the metal oxide and Ca(OH) ２ And, Sr(OH) ２ And, Nb ２ O ５ And, TiO ２ Furthermore, a Li compound was mixed with the total number of moles of Ni, Mn, Ca, Sr, Nb, and Ti such that the molar ratio of Li to the total number of moles of Ni was 105 mol%. (3) After the washing step, the dehydrated cake-like composition was vacuum-dried at 150°C to obtain a powder, to which in the addition step, 0.1 mol% sodium disulfite (Na) was added relative to the total number of moles of Ni and Mn. ２ S ２ O ５ The following was added, and the mixture was heat-treated at 400°C in an oxygen atmosphere for 8 hours.

[0106] <Comparative Example 2-1> A test cell was prepared and evaluated in the same manner as in Example 2-1, except that the positive electrode active material was prepared as follows: (1) [Ni ０．８０ Mn ０．２０ ] (OH) ２(1) The composite hydroxide represented by was calcined at 650°C for 10 hours to obtain a metal oxide containing Ni and Mn. (2) In the mixing step, the metal oxide and the Li compound were mixed so that the molar ratio of Li to the total number of moles of Ni and Mn was 105 mol%. (3) After the washing step, the dehydrated cake-like composition was vacuum-dried at 150°C to obtain a powder, to which 0.3 mol% sodium disulfite (Na) relative to the total number of moles of Ni, Co and Mn was added in the addition step. ２ S ２ O ５ The following was added, and heat treatment was performed for 2 hours in an oxygen atmosphere at a temperature of 300°C.

[0107] Table 3 shows the conditions for preparing the positive electrode active materials for Examples 2-1 to 2-3 and Comparative Example 2-1. Table 4 shows the evaluation results of the test cells for Examples 2-1 to 2-3 and Comparative Example 2-1. In Table 4, the initial resistance of each test cell is expressed relatively, with the initial resistance of the test cell in Comparative Example 2-1 set to 100; a smaller value indicates lower resistance. In addition to the evaluation results, Table 4 also shows the following information, similar to Table 2. (1) The content of S, Me, Ca, and Sr contained in the composite metal oxide relative to the total number of moles of transition metal elements in the composite metal oxide, calculated from the measurement results of ICP-AES and TEM-EDX. (2) The presence or absence of S-containing compounds and Ca or Sr on the surface of the primary particles of the composite metal oxide. (3) The proportion of metal elements other than Li present in the Li layer of the layered structure. (4) The presence or absence of the first and second peaks in the S2p spectrum by X-ray photoelectron spectroscopy, and whether the peak P3 of the third peak in the O1s spectrum by X-ray photoelectron spectroscopy is higher than the peak P4 of the fourth peak. (5) The proportion of high aspect ratio primary particles with an aspect ratio of 2 or more relative to the total number of primary particles. (6) The proportion of primary particles whose edge faces are oriented from the center of the secondary particles toward the outer periphery. (7) The crystallite size (s) calculated by Scherrer's formula from the full width at half maximum of the diffraction peak of the (104) plane of the X-ray diffraction pattern.

[0108]

[0109]

[0110] As shown in Table 4, the initial resistance of the test cell in the example is lower than that of the test cell in the comparative example.

[0111] <Example 3-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows and the charging termination potential for evaluating the initial resistance was set to 4.4V. (1) [Ni ０．６０ Co ０．２０ Mn ０．２０ ] (OH) ２ The composite hydroxide represented by was calcined at 400°C for 10 hours to obtain a metal oxide containing Ni, Co, and Mn. (2) In the mixing step, the metal oxide and CaCO were mixed such that the molar ratio of Ca to the total number of moles of Ni, Co, and Mn was 0.3 mol%, the molar ratio of Zr was 0.3 mol%, and the molar ratio of W was 0.1 mol%. ３ And, ZrO ２ And, WO ３ Furthermore, the Li compound was mixed so that the molar ratio of Li to the total number of moles of Ni, Co, Mn, Ca, Zr, and W was 105 mol%. (3) In the calcination step, the temperature was raised from room temperature to 300°C at a heating rate of 5°C / min, then from 300°C to 650°C at a heating rate of 3°C / min, and held at 650°C for 2 hours. After that, the temperature was raised from 650°C to 800°C at a heating rate of 1.5°C / min and held for 8 hours to obtain the calcined product. (4) In the washing step, the obtained calcined product was pulverized, and using a 3 L reaction vessel, the calcined product was added to water so that the solid-liquid ratio was 1300 g / L, washed with water at a stirring speed of 300 rpm for 10 minutes, and then dehydrated to obtain a cake-like composition. (5) After the washing step, the dehydrated cake-like composition is vacuum-dried at 150°C to obtain a powder, to which in the addition step, 0.3 mol% sodium disulfite (Na) is added relative to the total number of moles of Ni, Co, and Mn. ２ S ２ O ５ The following was added, and heat treatment was performed for 2 hours in an air atmosphere at a temperature of 500°C.

[0112] <Comparative Example 3-1> A test cell was prepared and evaluated in the same manner as in Example 3-1, except that the positive electrode active material was prepared as follows: (1) In the mixing step, the metal oxide and the Li compound were mixed so that the molar ratio of Li to the total number of moles of Ni, Co, and Mn was 105 mol%. (2) After the water washing step, the dehydrated cake-like composition was subjected to vacuum drying only at 150°C.

[0113] Table 5 shows the conditions for preparing the positive electrode active materials for Example 3-1 and Comparative Example 3-1. Table 6 shows the evaluation results of the test cells for Example 3-1 and Comparative Example 3-1. In Table 6, the initial resistance of each test cell is expressed relatively, with the initial resistance of the test cell in Comparative Example 3-1 set to 100; a smaller value indicates lower resistance. In addition to the evaluation results, Table 6 also shows the following information, similar to Tables 2 and 4. (1) The content of S, Me, Ca, and Sr contained in the composite metal oxide relative to the total number of moles of transition metal elements in the composite metal oxide, calculated from the measurement results of ICP-AES and TEM-EDX. (2) The presence or absence of S-containing compounds and Ca or Sr on the surface of the primary particles of the composite metal oxide. (3) The proportion of metal elements other than Li present in the Li layer of the layered structure. (4) The presence or absence of the first and second peaks in the S2p spectrum by X-ray photoelectron spectroscopy, and whether the peak P3 of the third peak in the O1s spectrum by X-ray photoelectron spectroscopy is higher than the peak P4 of the fourth peak. (5) The proportion of high aspect ratio primary particles with an aspect ratio of 2 or more relative to the total number of primary particles. (6) The proportion of primary particles whose edge faces are oriented from the center of the secondary particles toward the outer periphery. (7) The crystallite size (s) calculated by Scherrer's formula from the full width at half maximum of the diffraction peak of the (104) plane of the X-ray diffraction pattern.

[0114]

[0115]

[0116] As shown in Table 6, the initial resistance of the test cell in the example is lower than that of the test cell in the comparative example.

[0117] This disclosure is further illustrated by the following embodiments. Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a composite metal oxide, wherein the composite metal oxide contains at least a transition metal, at least two elements selected from the group consisting of Li, Na, B, Ni, Mg, Al, Si, P, K, Ti, Mn, Fe, Co, Zr, Nb, Mo, Sn, W, Ca, Sr, and Bi, and S, and comprises secondary particles formed by the aggregation of primary particles, wherein the proportion of primary particles with an aspect ratio of 2 or more in the secondary particles is 4 in relation to the total number of primary particles. A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the content of sulfur is 0% or more, a modifying compound is present on the surface of the primary particles including the surface of the secondary particles, the modifying compound includes a compound containing sulfur, the sulfur content in the composite metal oxide satisfies 0.1 mol% ≤ S content ≤ 3 mol% with respect to the total number of moles of transition metal elements in the composite metal oxide, and the S2p spectrum obtained by X-ray photoelectron spectroscopy has a first peak with a peak at 170 ± 2 eV and a second peak with a peak at 167 ± 2 eV. Configuration 2: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the compound containing sulfur is an inorganic sulfate containing Me (Me is at least one element included in the third period or later of alkali metals). Configuration 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 2, wherein the Me content in the composite metal oxide satisfies 0 mol% < Me content ≤ 6 mol% with respect to the total number of moles of transition metal elements in the composite metal oxide. Configuration 4: A positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein, in the O1s spectrum obtained by X-ray photoelectron spectroscopy, the peak having a peak at 531 ± 1 eV is higher than the peak having a peak at 529 ± 1 eV. Configuration 5: A positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the modified compound comprises a compound containing at least one of Ca and Sr. Configuration 6: A positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 5, wherein the sum of the Ca content and Sr content contained in the composite metal oxide satisfies 0 mol% < Ca content + Sr content ≤ 2 mol% with respect to the total number of moles of transition metal elements in the composite metal oxide.Configuration 7: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 5 or 6, wherein the amount of Ca contained in the composite metal oxide and the amount of Sr contained in the composite metal oxide satisfy Ca content > Sr content. Configuration 8: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein the composite metal oxide contains Mn, and in the composite metal oxide, the amount of Co relative to the total number of moles of transition metal elements and the amount of Mn relative to the total number of moles of transition metal elements satisfy 0 ≤ Co content / Mn content ≤ 2. Configuration 9: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein the composite metal oxide contains Ni, and in the composite metal oxide, the amount of Ni relative to the total number of moles of transition metal elements satisfies 50 mol% ≤ Ni content ≤ 97 mol%. Configuration 10: The composite metal oxide contains an alkali metal or an alkaline earth metal, and the composite metal oxide has a layered structure in which alkali metal layers or alkaline earth metal layers and transition metal layers are alternately stacked, as described in any one of Configurations 1 to 9. Configuration 11: The composite metal oxide is a positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 10, wherein, among the primary particles with an aspect ratio of 2 or more, the proportion of primary particles whose edge surfaces are oriented from the center of the alkali metal layer or the alkaline earth metal layer, and whose edge surfaces are oriented from the center of the secondary particles toward the outer periphery, is 50% or more of the total number of primary particles with an aspect ratio of 2 or more. Configuration 12: The alkali metal is Li, and the alkali metal layer is a Li layer, as described in Configuration 10 or 11. Configuration 13: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 12, wherein the proportion of metal elements other than Li present in the Li layer is 8 mol% or less relative to the total number of moles of metal elements excluding Li in the composite metal oxide. Configuration 14: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 12 or 13, wherein the crystallite size s of the composite metal oxide, calculated by Scherrer's formula from the full width at half maximum of the diffraction peak of the (104) plane of the X-ray diffraction pattern obtained by X-ray diffraction, satisfies 300 Å ≤ s ≤ 700 Å.Configuration 15: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for non-aqueous electrolyte secondary batteries described in any one of Configurations 1 to 14, a negative electrode, and a non-aqueous electrolyte.

[0118] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Secondary particles, 31 Primary particles, 32, 33 Edge surface

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery containing a composite metal oxide, wherein the composite metal oxide contains at least a transition metal, at least two elements selected from the group consisting of Li, Na, B, Ni, Mg, Al, Si, P, K, Ti, Mn, Fe, Co, Zr, Nb, Mo, Sn, W, Ca, Sr, and Bi, and S, and contains secondary particles formed by the aggregation of primary particles, wherein the proportion of primary particles with an aspect ratio of 2 or more in the secondary particles is 40% or more of the total number of primary particles, a modifying compound is present on the surface of the primary particles including the surface of the secondary particles, the modifying compound includes a compound containing S, and the S content contained in the composite metal oxide satisfies 0.1 mol% ≤ S content ≤ 3 mol% with respect to the total number of moles of transition metal elements in the composite metal oxide. A positive electrode active material for a non-aqueous electrolyte secondary battery, having a first peak with a peak at 170 ± 2 eV and a second peak with a peak at 167 ± 2 eV in the S2p spectrum obtained by X-ray photoelectron spectroscopy.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the compound containing S is an inorganic sulfate containing Me (Me is at least one element included in the third period or later of alkali metals).

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the Me content in the composite metal oxide satisfies 0 mol% < Me content ≤ 6 mol% with respect to the total number of moles of transition metal elements in the composite metal oxide.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein, in the O1s spectrum obtained by X-ray photoelectron spectroscopy, the peak having a peak at 531±1 eV is higher than the peak having a peak at 529±1 eV.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the modified compound comprises a compound containing at least one of Ca and Sr.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 5, wherein the sum of the Ca content and the Sr content contained in the composite metal oxide satisfies 0 mol% < Ca content + Sr content ≤ 2 mol% with respect to the total number of moles of transition metal elements in the composite metal oxide.

7. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 5, wherein the amount of Ca contained in the composite metal oxide and the amount of Sr contained in the composite metal oxide satisfy the condition Ca content > Sr content.

8. The composite metal oxide contains Mn, and in the composite metal oxide, the ratio of the Co content to the total number of moles of transition metal elements and the ratio of the Mn content to the total number of moles of transition metal elements satisfy 0 ≤ Co content / Mn content ≤ 2, as described in claim 1.

9. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the composite metal oxide contains Ni, and in the composite metal oxide, the content of Ni relative to the total number of moles of transition metal elements satisfies 50 mol% ≤ Ni content ≤ 97 mol%.

10. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the composite metal oxide contains an alkali metal or an alkaline earth metal, and the composite metal oxide has a layered structure in which alkali metal layers or alkaline earth metal layers and transition metal layers are alternately stacked.

11. The composite metal oxide is a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 10, wherein, among the primary particles having an aspect ratio of 2 or more, the proportion of primary particles whose edge surfaces are oriented from the center of the secondary particles toward the outer periphery is 50% or more of the total number of primary particles having an aspect ratio of 2 or more.

12. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 10, wherein the alkali metal is Li, and the alkali metal layer is a Li layer.

13. The proportion of metal elements other than Li present in the Li layer is 8 mol% or less relative to the total number of moles of metal elements other than Li in the composite metal oxide, as described in claim 12.

14. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 12, wherein the crystallite size s of the composite metal oxide, calculated by Scherrer's formula from the full width at half maximum of the diffraction peak of the (10⁴) plane of the X-ray diffraction pattern obtained by X-ray diffraction, satisfies 300 Å ≤ s ≤ 700 Å.

15. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery described in any one of claims 1 to 14, a negative electrode, and a non-aqueous electrolyte.

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