Positive electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

This positive electrode mixture layer is characterized by containing a positive electrode active material and a conductive auxiliary agent, the conductive auxiliary agent containing at least one selected from the group consisting of a particulate carbon material having an average particle diameter of 20 nm or less and a fibrous carbon material having an average fiber diameter of 20 nm or less, the positive electrode active material containing a lithium transition metal composite oxide and a modifying compound present on a surface of primary particles or at grain boundaries between the primary particles, the modifying compound containing a compound containing S, the content of S contained in the modifying compound satisfying 0.1 mol%≤content of S≤3 mol%, and the S2p spectrum obtained by X-ray photoelectron spectroscopy exhibiting a first peak with a vertex at 170±2 eV and a second peak with a vertex at 167±2 eV.

Description

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

[0001] This disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and 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, which is the main component of non-aqueous electrolyte secondary batteries, greatly affects these performance characteristics, and therefore, much research has been conducted on positive electrodes. For example, Patent Document 1 discloses a technique for suppressing side reactions that occur between the non-aqueous electrolyte and the positive electrode active material by coating the surface of the positive electrode active material with a sulfur compound.

[0003] Special Publication No. 2023-532367

[0004] In non-aqueous electrolyte secondary batteries, there is a need to suppress the increase in battery resistance when repeatedly charging and discharging, from the viewpoint of improving durability. The technology described in Patent Document 1 does not consider the increase in resistance when repeatedly charging and discharging, and there is still room for improvement.

[0005] A positive electrode for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, comprises a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector, wherein the positive electrode mixture layer includes a positive electrode active material, a conductive additive, and a binder, the conductive additive includes at least one selected from the group consisting of particulate carbon materials with an average particle size of 20 nm or less and fibrous carbon materials with an average fiber diameter of 20 nm or less, the positive electrode active material includes secondary particles formed by the aggregation of primary particles and comprises a lithium transition metal composite oxide containing at least Ni and a modifying compound present on the surface of the primary particles or at the grain boundaries between primary particles, the modifying compound includes a compound containing S, the S content in the modifying compound satisfies 0.1 mol% ≤ S content ≤ 3 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite 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 according to one aspect of this disclosure is characterized by comprising the positive electrode, the negative electrode, and the non-aqueous electrolyte.

[0007] According to a positive electrode for a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, it is possible to suppress the increase in battery resistance when charging and discharging is repeated.

[0008] This is an axial cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. This is a diagram of the S2p spectrum of the lithium composite oxide (Z) of Example 1-1 obtained by X-ray photoelectron spectroscopy. This is a diagram of the O1s spectrum of the lithium composite oxide (Z) of Example 1-1 obtained by X-ray photoelectron spectroscopy. This is a diagram of the S2p spectrum of the lithium composite oxide (Z) of Comparative Example 1-2 obtained by X-ray photoelectron spectroscopy. This is a diagram of the O1s spectrum of the lithium composite oxide (Z) of Comparative Example 1-2 obtained by X-ray photoelectron spectroscopy.

[0009] On the surface of the positive electrode active material, 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. As a result, with repeated charging and discharging, a modified layer may form on the surface of the positive electrode active material, or products generated by side reactions with the non-aqueous electrolyte may accumulate on the surface, potentially increasing resistance. Furthermore, with repeated charging and discharging, due to the expansion and contraction of the positive electrode active material, some positive electrode active material may not come into contact with the conductive additive contained in the positive electrode mixture layer, or cracks may develop in the positive electrode mixture layer, potentially increasing resistance.

[0010] As a result of diligent research, the inventors have found that by using a specially designed conductive additive with a unique microstructure, while simultaneously incorporating a modified compound containing sulfur on the surface of primary particles of a lithium transition metal composite oxide or at the grain boundaries between primary particles, the spectrum obtained by X-ray photoelectron spectroscopy has a predetermined shape. This is presumed to be because the modified compound suppresses the formation of a modified layer, and the conductive paths in the positive electrode mixture layer are maintained by the conductive additive with a unique microstructure. However, when the above-mentioned modified compound is present and the spectrum obtained by X-ray photoelectron spectroscopy has a predetermined shape, if a common conductive additive such as carbon black, which is widely used, is used, the formation of conductive paths becomes insufficient as charging and discharging is repeated, and the resistance increases. Furthermore, even when using a conductive additive with a unique microstructure, if the above-mentioned modified compound is absent or the spectrum obtained by X-ray photoelectron spectroscopy does not have a predetermined shape, a modified layer is more likely to form on the surface, and the resistance tends to increase.

[0011] 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.

[0012] 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."

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] [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.

[0019] The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive additive, 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 additive, etc., to the surface of the positive electrode current collector, drying the coating film, and then rolling it to form a positive electrode mixture layer on both sides of the positive electrode current collector.

[0020] The positive electrode mixture layer contains a positive electrode active material (hereinafter sometimes referred to as lithium composite oxide (Z)) comprising a lithium transition metal composite oxide and a modified compound. The positive electrode mixture layer may also contain positive electrode active materials other than lithium composite oxide (Z). In the positive electrode mixture layer, the content of lithium composite oxide (Z) is, for example, 90% by mass or more of the total mass of the positive electrode active material. Furthermore, the positive electrode mixture layer may contain substantially only lithium composite oxide (Z) as the positive electrode active material.

[0021] The lithium composite oxide (Z) contains, for example, Ni. The Ni content in the lithium transition metal composite oxide is preferably such that 50 mol% ≤ Ni content ≤ 97 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. If the Ni content is within the above range, a synergistic effect with the modified compound described later can further enhance both high 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%. Furthermore, as the amount of Li extracted during charging increases, side reactions between the positive electrode active material and the non-aqueous electrolyte become more likely, and the effects of this disclosure are significantly demonstrated.

[0022] The lithium composite oxide (Z) may further contain M1 (M1 is at least one element selected from the group consisting of B, Al, Si, Ti, Mn, Fe, Co, Zr, Nb, Mo, Sn, W, Ca, Sr, Na, K, Mg, S, P, and Bi). The amount of M1 contained in the lithium composite oxide (Z) satisfies, for example, 3 mol% ≤ amount of M1 ≤ 50 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. The ratio of the metal elements contained in the lithium composite oxide (Z) can be measured, for example, by an inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0023] The lithium composite oxide (Z) is, for example, represented by the general formula Li ａ Ni ｘ M1 ｙ O ２－ｂ (where 0.8 ≤ a ≤ 1.2, 0.50 ≤ x ≤ 0.97, 0.03 ≤ y ≤ 0.50, 0 ≤ b ≤ 0.05, x + y = 1, and M1 is at least one element selected from the group consisting of B, Al, Si, Ti, Mn, Fe, Co, Zr, Nb, Mo, Sn, W, Ca, Sr, Na, K, Mg, S, P, and Bi) and is a composite oxide. Also, the lithium transition metal composite oxide may be represented by the general formula Li ａ Ni ｘ Co ｙ Mn ｚ Al ｗ M2 ｖ O ２－ｂ (where 0.8 ≤ a ≤ 1.2, 0.50 ≤ x ≤ 0.97, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.50, 0 ≤ w ≤ 0.10, 0 ≤ v ≤ 0.10, 0 ≤ b ≤ 0.05, x + y + z + w + v = 1, and M2 is at least one element selected from the group consisting of B, Si, Ti, Fe, Zr, Nb, Mo, Sn, W, Ca, Sr, Na, K, Mg, S, P, and Bi) and is a composite oxide.

[0024] The lithium composite oxide (Z) may have a layered structure. Examples of layered structures for lithium transition metal composite oxides include a layered structure belonging to space group R-3m and a layered structure belonging to space group C2 / m. From the viewpoint of increasing battery capacity and ensuring crystal structure stability, it is preferable that the lithium composite oxide (Z) has a layered structure belonging to space group R-3m. The layered structure of the lithium composite oxide (Z) includes, for example, a Li layer and a transition metal layer. The charge and discharge reactions of the battery proceed as Li ions present in the Li layer reversibly move in and out.

[0025] In the layered structure of lithium composite oxide (Z), the proportion of metal elements other than Li present in the Li layer is preferably 8 mol% or less relative to the total number of moles of metal elements other than Li in the lithium transition metal composite oxide. If the proportion of metal elements other than Li in the Li layer exceeds 8 mol%, the diffusivity of Li ions in the Li layer decreases, which may reduce the battery capacity. The lower limit of the proportion of metal elements other than Li in the Li layer is, for example, 0.1 mol%, or it may be 1.0 mol%. The metal elements other than Li present in the Li layer are mainly Ni, but other metal elements may also be included.

[0026] 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 lithium composite oxide (Z). For example, the Rietveld analysis software PDXL2 (Rigaku Corporation) can be used for the Rietveld analysis of the X-ray diffraction pattern.

[0027] 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)

[0028] Furthermore, the crystallite size s of the lithium transition metal composite oxide, calculated by Scherrer's formula from the full width at half maximum of the (10⁴) plane diffraction peak of 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 (10⁴) plane diffraction peak, θ is the diffraction angle (rad), and K is Scherrer's constant. In this embodiment, K is set to 0.9. s = Kλ / Bcosθ

[0029] Lithium composite oxide (Z) contains 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). 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 (for example, Microtrac-Bell MT3000II) with water as the dispersion medium.

[0030] As described above, lithium composite oxide (Z) contains modifying compounds. These modifying compounds are present on the surface of the primary particles of the lithium transition metal composite oxide, or at the grain boundaries between primary particles. The presence of these modifying compounds makes side reactions between the lithium transition metal composite oxide and the non-aqueous electrolyte less likely to occur. As a result, even after repeated charging and discharging, the formation of an altered layer on the surface of the positive electrode active material and the deposition of products generated by side reactions with the non-aqueous electrolyte are suppressed, thereby suppressing the increase in resistance. Furthermore, the presence of these modifying compounds stabilizes the layered structure of the lithium transition metal composite oxide, improving the initial discharge capacity.

[0031] The modifying compound may be uniformly dispersed on the surface of primary particles or at grain boundaries between primary particles, or it may be partially present. For example, the modifying compound may exist particulately or in layers on the surface of primary particles or at grain boundaries between primary particles. Here, the surface of primary particles includes the surface of secondary particles of the lithium transition metal composite oxide. In other words, the surface of primary particles of a lithium transition metal composite oxide includes the surface of primary particles located inside the lithium transition metal composite oxide and the surface of primary particles exposed to the surface of the lithium transition metal composite oxide. Furthermore, the surface of primary particles means the particle surface of the primary particle 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 primary particles of a lithium transition metal composite oxide, or at grain boundaries between primary particles of a lithium transition metal composite oxide, can be confirmed by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0032] The modified compound includes a compound containing sulfur (S). The S content in the modified compound is preferably 0.1 mol% ≤ S content ≤ 3 mol%, more preferably 0.1 mol% ≤ S content ≤ 2.7 mol%, and more preferably 0.1 mol% ≤ S content ≤ 2.5 mol%, relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. By satisfying 0.1 mol% ≤ S content ≤ 3 mol%, side reactions between the lithium transition metal composite oxide and the non-aqueous electrolyte on the surface of the lithium transition metal composite oxide are less likely to occur. As a result, the increase in battery resistance when repeatedly charging and discharging can be suppressed. In addition, some of the S may be contained within the crystal structure of the lithium transition metal composite oxide.

[0033] Compounds containing S are, for example, inorganic sulfates containing Me (where Me is at least one element from the third period onward of the alkali metals). Me is, for example, Na or K.

[0034] The Me content in the modified compound is preferably such that 0 mol% < Me content ≤ 6 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. In this case, it is presumed that even after repeated charging and discharging, the formation of a modified layer on the surface of the positive electrode active material is further suppressed, and the increase in resistance is further suppressed. The lower limit of the Me content is, for example, 0.1 mol%. Therefore, the Me content in the modified compound may satisfy 0.1 mol% ≤ Me content ≤ 6 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0035] 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 O ８ 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.

[0036] The Me and S content in lithium composite oxide (Z) can be measured, for example, by 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 derived from the S-containing compound has been quantified.

[0037] The lithium composite oxide (Z) may contain at least one of Ca and Sr. When the lithium composite oxide (Z) contains at least one of Ca and Sr in addition to S, it leads to a reduction in initial resistance, for example, while further suppressing the increase in resistance. At least one of Ca and Sr is included, for example, in a modified compound. The modified compound includes, for example, 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. The compound containing at least one of Ca and Sr is, for example, an oxide, carbonate compound, or sulfate compound containing at least one of Ca and Sr.

[0038] The Ca and Sr content in the modified compound satisfies, for example, the condition 0 mol% < Ca content + Sr content ≤ 2 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. The lower limit of the Ca content + Sr content is, for example, 0.1 mol%. It is also preferable that Ca content > Sr content. This can further suppress the increase in resistance. The Ca and Sr content in the modified compound can be confirmed, for example, by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0039] Lithium composite oxide (Z) 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 peak belongs 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. For example, as described later, by adding a compound containing S to a cake-like composition or a powder obtained by drying the cake-like composition, a modified compound containing S can be made present on the surface of the primary particles of the lithium transition metal composite oxide or at the grain boundaries between the primary particles of the lithium transition metal composite oxide, and both the first and second peaks can be obtained from the positive electrode active material.

[0040] 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.

[0041] 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 lithium composite oxide (Z), and the fourth peak is presumed to be a peak attributed to oxygen contained in the lithium transition metal composite 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 lithium transition metal composite oxide is significantly exhibited, and the surface state and layered structure of the lithium transition metal composite 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.

[0042] 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

[0043] 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 Ni compound containing at least Ni with 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.

[0044] Examples of Ni compounds include Ni-containing metal hydroxides, Ni-containing metal oxides, and Ni-containing metal carbonate compounds. While not particularly limited, for example, a solution of a metal salt containing Ni, Mn, Co, Al, etc., and an alkaline solution such as sodium hydroxide can be separately added dropwise to a reaction vessel where a pH-adjusted solution is being stirred, thereby adjusting the pH to the alkaline side (e.g., 8.5 to 12.5) to precipitate (coprecipitation) a composite hydroxide and obtain a Ni-containing metal hydroxide. Alternatively, a Ni-containing metal oxide can be produced by heat-treating the Ni-containing metal hydroxide. The heat treatment temperature is not particularly limited, but is, for example, in the range of 250°C to 600°C.

[0045] Next, a mixture is obtained by mixing at least a Ni compound and a 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.

[0046] 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 Ca ３ (PO ４ ) ２ CaO, CaCO ３ CaSO ４ Ca(NO ３ ) ２、 CaCl ２ CaAlO ４ Examples include Sr(OH) ２ SrHPO ４ , Sr(H ２ PO ４ ) ２ , Sr ３ (PO４ ) ２ 、SrO, SrCO ３ 、SrSO ４ 、Sr(NO ３ ) ２、 SrCl ２ 、SrAlO ４ 等。

[0047] Also, when mixing, an M2 compound containing M2 (M2 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) may be mixed. Examples of the M2 compound include H ３ BO ３ 、B ２ O ３ 、MgO, Al ２ O ３ 、Al(OH) ３ 、NaAl(OH) ４ 、AlPO ４ 、SiO, SiO ２ 、TiO ２ 、Ti(OH) ４ 、Fe(OH) ２ 、Fe ２ O ３ 、ZrO ２ 、ZrSiO ４ 、Zr(WO ４ ) ２ 、ZrCl ３ 、Nb ２ O[[ID=**61]] ５ 、Nb 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 等。 **Note**: There seems to be a formatting issue in the original text where the tag ` ２ ` and ` ５ ` might be incorrect in terms of the number of digits. I've translated it as is, but it's possible there was a mistake in the original input. Also, the text is a bit unclear in its chemical compound listings and relationships. If you have any further clarification or corrections regarding the original text, it would be helpful for a more accurate translation.

[0048] The mixture obtained in the mixing step is fired to obtain the fired product. The mixture is mixed, 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 less per 1 kg of mixture. In the firing conditions, the first set temperature 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 less, and the heating rate at 450°C or lower is in the range of more than 1.5°C / min and 6.0°C / min or less. The second set temperature is set to 450°C or higher 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 less, and the heating rate at 450°C or higher and 680°C or lower is in the range of more than 1.0°C / min and 4.5°C / min or less. The maximum temperature that can be reached is in the range of 690°C or higher 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.

[0049] 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.

[0050] A positive electrode active material (lithium composite oxide (Z)) 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 a sulfur-containing compound after the calcination step, the sulfur-containing compound can be made present on the surface of the primary particles of the lithium transition metal composite oxide or at the grain boundaries between the primary particles of the lithium transition metal composite oxide.

[0051] 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 H ｘ PO ４ Additions such as (0 ≤ x ≤ 3) may also be added.

[0052] 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.

[0053] 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.

[0054] The positive electrode mixture layer contains a binder and a conductive additive in addition to the positive electrode active material. Examples of binders include fluorine-containing polymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc. The binder content is, for example, 0.1 parts by mass or more and 2 parts by mass or less, preferably 0.5 parts by mass or more and 1.5 parts by mass or less, per 100 parts by mass of positive electrode active material.

[0055] The conductive additive includes at least one material selected from the group consisting of particulate carbon materials with an average particle size of 20 nm or less and fibrous carbon materials with an average fiber diameter of 20 nm or less. The positive electrode mixture layer may also contain carbon materials with an average particle size and / or average fiber diameter exceeding 20 nm as a conductive additive, but the main component shall be particulate carbon materials with an average particle size of 20 nm or less, or fibrous carbon materials with an average fiber diameter of 20 nm or less. The main component refers to the component that accounts for the largest mass percentage of the conductive additive.

[0056] The conductive additive may consist of at least one material selected from the group consisting of particulate carbon materials with an average particle size of 20 nm or less and fibrous carbon materials with an average fiber diameter of 20 nm or less. However, if the average particle size and average fiber diameter of the conductive additive contained in the positive electrode mixture layer exceed 20 nm, the formation of conductive paths will be insufficient, and resistance will tend to increase when charging and discharging is repeated.

[0057] The particulate carbon material may be Ketjenblack, Furnace Black, graphite, etc., but acetylene black (AB) is preferred. Therefore, an example of a suitable particulate carbon material is acetylene black with an average particle size of 20 nm or less.

[0058] Generally, carbon blacks such as acetylene black are composed of the smallest particles called domains or particles, primary aggregates of these smallest particles, and secondary aggregates of primary aggregates. Here, primary aggregates are generally called structures or aggregates, and secondary aggregates are generally called agglomerates, etc. In this disclosure, the average particle size refers to the average particle size of the smallest particles mentioned above. The average particle size of particulate carbon materials is determined by image analysis using a transmission electron microscope (TEM). The average particle size of particulate carbon materials is determined by arbitrarily selecting 100 particulate carbon materials, measuring the major axis of the smallest particles, and taking the arithmetic mean of the measured values. The lower limit of the average particle size of particulate carbon materials is not particularly limited, but one example is 1 nm.

[0059] Furthermore, the aspect ratio (ratio of the major axis to the minor axis) of the particulate carbon material is, for example, 1.2 times or more and 2.5 times or less, or 1.3 times or more and 2 times or less. Note that the aspect ratio of the particulate carbon material refers to the aspect ratio of the primary aggregates mentioned above.

[0060] The fibrous carbon material may be carbon nanofiber (CNF) or the like, but is preferably carbon nanotube (CNT). Carbon nanotube is a conductive carbon fiber with an outer diameter of several tens of nanometers or less, and has an extremely large aspect ratio (ratio of fiber length to fiber diameter). The average aspect ratio of carbon nanotube is, for example, 20 times or more, preferably 50 times or more. With carbon nanotube with a high aspect ratio, contact with the active material and core material becomes linear contact rather than point contact. Therefore, a good conductive path can be formed with a small amount of addition.

[0061] The average fiber diameter of the carbon nanotubes used in the positive electrode 11 is 20 nm or less, preferably 15 nm or less, and more preferably 10 nm or less. The fiber diameter refers to the length in the direction perpendicular to the fiber length. An average fiber diameter of 20 nm or less can suppress the increase in resistance during repeated charging and discharging. The lower limit of the average fiber diameter of the carbon nanotubes is not particularly limited, but one example is 1 nm. The average fiber diameter of the carbon nanotubes is determined by image analysis using TEM. The average fiber diameter of the carbon nanotubes is determined by arbitrarily selecting 100 carbon nanotubes, measuring their fiber diameters, and taking the arithmetic mean of the measured values.

[0062] The average fiber length of carbon nanotubes is, for example, 0.5 μm or more, and may be 1 μm or more. Fiber length refers to the length of a carbon nanotube when stretched in a straight line. An average fiber length of 0.5 μm or more can further suppress the increase in resistance during repeated charging and discharging. There is no particular upper limit to the average fiber length of carbon nanotubes, but one example is 100 μm. The average fiber length of carbon nanotubes is determined by image analysis using a scanning electron microscope (SEM). The average fiber length of carbon nanotubes is determined by arbitrarily selecting 100 carbon nanotubes, measuring their lengths, and taking the arithmetic mean of the measured values. Note that carbon nanotubes present in the positive electrode mixture layer may exist as bundles of multiple carbon nanotubes. In that case, the length of a single carbon nanotube within the bundle is used to calculate the average fiber length.

[0063] Carbon nanotubes may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), and a combination of both may be used. Single-walled carbon nanotubes have a structure in which one layer of graphite sheet is formed into a tube, while multi-walled carbon nanotubes have a structure in which multiple layers of graphite sheet are formed into a tube. An example of a multi-walled carbon nanotube is a double-walled carbon nanotube having a two-layer structure.

[0064] The optimal BET specific surface area for carbon nanotubes varies slightly depending on the type of carbon nanotube, but one example is 200 m². ２ It is 250 m or more per g, and more preferably 250 m ２ It is 1 / g or more. The upper limit of the BET specific surface area is not particularly limited, but one example is 2000 m ２ The value is / g. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0065] Carbon nanotubes are supplied to the manufacturing process of the positive electrode 11 in the form of a conductive additive dispersion, in which carbon nanotubes are dispersed in a liquid containing a dispersant and an aprotic polar solvent (dispersion medium), and are added to the positive electrode mixture slurry. In the dispersion, the dispersant is dissolved in the polar solvent, and the carbon nanotubes are dispersed in the polar solvent by the action of the dispersant. The solid content concentration (carbon nanotubes and dispersant) of the dispersion is, for example, 0.1% by mass or more and 20% by mass or less, preferably 0.2% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less, from the viewpoint of balancing the dispersibility of carbon nanotubes and productivity.

[0066] The content of the conductive additive in the positive electrode mixture layer is preferably 1 part by mass or less, and more preferably 0.8 parts by mass or less, per 100 parts by mass of positive electrode active material. In this case, the amount of positive electrode active material contained in the positive electrode mixture layer can be secured, making it easier to achieve high capacity batteries. Furthermore, by using a carbon material with an average particle size and average fiber diameter of 20 nm or less as the conductive additive, good conductive paths can be formed in the positive electrode mixture layer even if the content of the conductive additive is 1 part by mass or less per 100 parts by mass of positive electrode active material. The lower limit of the content of the conductive additive is, for example, 0.1 parts by mass per 100 parts by mass of positive electrode active material. Therefore, the content of the conductive additive may be 0.1 parts by mass or more and 1 part by mass or less, or 0.1 parts by mass or more and 0.8 parts by mass or less, per 100 parts by mass of positive electrode active material. The amount of particulate carbon material contained in the conductive additive may be, for example, 0.1 parts by mass or more and 1 part by mass or 0.3 parts by mass or more and 1 part by mass or less per 100 parts by mass of positive electrode active material. The amount of fibrous carbon material contained in the conductive additive may be, for example, 0.001 parts by mass or more and 1 part by mass or 0.001 parts by mass or more and 0.5 parts by mass or less per 100 parts by mass of positive electrode active material.

[0067] Furthermore, it is preferable that the conductive additive includes particulate carbon material with an average particle size of 20 nm or less, and fibrous carbon material with an average fiber diameter of 20 nm or less. In this case, it becomes easier to form good conductive paths in the positive electrode mixture layer, and the increase in resistance is further suppressed even when charging and discharging is repeated.

[0068] When using particulate carbon material with an average particle size of 20 nm or less and fibrous carbon material with an average fiber diameter of 20 nm or less in combination as conductive additives, the content of particulate carbon material may be greater than the content of fibrous carbon material. In this case, it becomes easier to form good conductive paths in the positive electrode mixture layer. In the positive electrode mixture layer, the mass ratio of the content of particulate carbon material to the content of fibrous carbon material is, for example, 2 or more and 30 or less.

[0069] [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.

[0070] 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.

[0071] 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.

[0072] [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.

[0073] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of 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.

[0074] [Non-aqueous electrolytes] Non-aqueous electrolytes, for example, have lithium ion conductivity. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0075] 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).

[0076] 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).

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0082] 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.

[0083] 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.

[0084] 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.

[0085] <Example 1-1> [Preparation of positive electrode active material] [Ni obtained by coprecipitation method ０．９０ Co ０．０５ Mn ０．０５ ] (OH) ２A Ni-containing composite hydroxide represented by was calcined at 400°C for 8 hours to obtain a Ni-containing composite oxide containing Ni, Co, and Mn. Next, the above Ni-containing 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).

[0086] 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 of Example 1-1 (addition step).

[0087] 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 sulfur (S), calcium (Ca), and syrup (Sr) content relative to the total moles of transition metal elements were 0.17 mol%, 0.3 mol%, and 0.1 mol%, respectively. Furthermore, TEM-EDX measurements confirmed the presence of Ca, Sr, and inorganic sulfates on the primary particle surface of the cathode active material.

[0088] Furthermore, the crystallite size s of the lithium transition metal composite 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. 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 molar amount of metal elements excluding Li in the lithium transition metal composite oxide was 3.6 mol%.

[0089] Furthermore, as shown in Figure 2, the positive electrode active material of Example 1-1 was confirmed to have 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. In addition, as shown in Figure 3, the positive electrode 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.

[0090] [Preparation of Acetylene Black Dispersion] Acetylene black (AB-1), a particulate carbon material with an average particle size (average major diameter) of 14 nm and an average aspect ratio of 1.3 times, was used as a conductive additive, and polyvinylpyrrolidone (PVP) was used as a dispersant. The conductive additive and the dispersant were mixed in a mass ratio of 10:1, and this mixture was added to N-methyl-2-pyrrolidone (NMP). The mixture was kneaded using a ball mill to obtain a conductive additive dispersion (AB-1 dispersion) containing approximately 10% by mass of the conductive additive.

[0091] [Preparation of Carbon Nanotube Dispersion] Carbon nanotubes (CNT-1), a fibrous carbon material with an average fiber diameter of 18 nm, an average fiber length of 1 μm, and an average aspect ratio of 125 times, were used as the conductive additive, and polyvinylpyrrolidone (PVP) was used as the dispersant. A mixture of the above conductive additive and the above dispersant in a mass ratio of 3:1 was added to N-methyl-2-pyrrolidone (NMP), and kneaded using a ball mill to obtain a conductive additive dispersion (CNT-1 dispersion) containing approximately 3% by mass of the conductive additive.

[0092] [Preparation of the positive electrode] The above-mentioned lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black dispersion (AB-1 dispersion), carbon nanotube dispersion (CNT-1 dispersion), and polyvinylidene fluoride (PVDF) were mixed in a solid content mass ratio of 100:0.5:0.2:1, excluding the dispersant, 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.

[0093] [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.

[0094] [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.

[0095] [Evaluation of Resistance Increase Rate] Under an 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 value was 0.01C at the charging termination potential. Subsequently, a constant current discharge was performed at a current value of 0.2C until the cell voltage reached 2.5V. After that, constant current charging was performed again at a current value of 0.2C up to the cell charging termination potential, and then constant voltage charging was performed until the current value 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.

[0096] Subsequently, the cell was charged with a constant current of 0.2C until it reached the charging termination potential. Then, at the charging termination potential, it was charged with a constant voltage until the current was reduced to 0.01C. This constant current discharge at 0.2C was repeated 30 times until the cell voltage reached 2.5V. After that, constant current charging at 0.2C was performed again until it reached the cell charging termination potential, and then constant voltage charging was performed until it reached the charging termination potential until the current was reduced to 0.01C. The resistance after 30 cycles was then calculated by dividing the voltage drop when a current of 1C was applied for 5 seconds by the current value. The resistance increase rate was then calculated using the following formula: Resistance increase rate = Resistance after 30 cycles / Initial resistance

[0097] <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 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%, with the Ni-containing composite 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(3) A CNT-2 dispersion was used as a conductive additive for the positive electrode, instead of the AB-1 dispersion and the CNT-1 dispersion, and the amount of CNT-2 added was 0.3 parts by mass per 100 parts by mass of positive electrode active material. The method for preparing the CNT-2 dispersion is as follows.

[0098] [Preparation of Carbon Nanotube Dispersion] Carbon nanotubes (CNT-2), a fibrous carbon material with an average fiber diameter of 10 nm, an average fiber length of 1 μm, and an average aspect ratio of 69 times, were used as the conductive additive, and polyvinylpyrrolidone (PVP) was used as the dispersant. A mixture of the above conductive additive and the above dispersant was mixed in a mass ratio of 3:1 and added to N-methyl-2-pyrrolidone (NMP). The mixture was kneaded using a ball mill to obtain a conductive additive dispersion (CNT-2 dispersion) containing approximately 3% by mass of the conductive additive.

[0099] <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 ０．８８ Co ０．０６ Mn ０．０３ Al ０．０３ ] (OH) ２ The Ni-containing composite hydroxide represented by was calcined at 550°C for 10 hours to obtain a Ni-containing composite oxide containing Ni, Co, Mn, and Al. (2) In the mixing step, the Ni-containing composite 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 ３Furthermore, 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 ) was added and heat treatment was carried out for 10 hours in an oxygen atmosphere at a temperature of 400°C. (5) As a conductive additive for the positive electrode, a CNT-2 dispersion was used instead of the AB-1 dispersion and the CNT-1 dispersion, and the amount of solid content of CNT-2 added was 0.5 parts by mass per 100 parts by mass of positive electrode active material.

[0100] <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 ０．９２ Co ０．０３ Mn ０．０４ Al ０．０１ ] (OH) ２ (2) In the mixing step, the Ni-containing composite hydroxide represented by was calcined at 400°C for 8 hours to obtain a Ni-containing composite oxide containing Ni, Co, Mn, and Al. ２ And, TiO ２ And, SrCO ３Furthermore, 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 following was added and heat treatment was performed for 4 hours under a vacuum atmosphere at a temperature of 300°C. (5) As a conductive additive for the positive electrode, AB-2 dispersion was used instead of AB-1 dispersion and CNT-1 dispersion, and the amount of solid content of AB-2 added was 0.8 parts by mass per 100 parts by mass of positive electrode active material. The method for preparing AB-2 dispersion is as follows.

[0101] [Preparation of Acetylene Black Dispersion] Acetylene black (AB-2), a particulate carbon material with an average particle size (average major diameter) of 17 nm and an average aspect ratio of 1.5 times, was used as a conductive additive, and polyvinylpyrrolidone (PVP) was used as a dispersant. The above conductive additive and dispersant were mixed in a mass ratio of 10:1 and added to N-methyl-2-pyrrolidone (NMP). By kneading the mixture using a ball mill, a conductive additive dispersion (AB-2 dispersion) containing approximately 10% by mass of the conductive additive was obtained.

[0102] <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) [Ni ０．９０ Co ０．０３ Mn ０．０５ Al ０．０２ ] (OH) ２ (2) In the mixing step, the Ni-containing composite hydroxide represented by was calcined at 600°C for 10 hours to obtain a Ni-containing composite oxide containing Ni, Co, Mn, and Al.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 (4) Na was added and heat treatment was performed at 300°C in an oxygen atmosphere for 5 hours as a conductive additive for the positive electrode, with CNT-3 dispersion used instead of AB-1 dispersion and CNT-1 dispersion, and the amount of CNT-3 added was 0.3 parts by mass per 100 parts by mass of positive electrode active material. The method for preparing the CNT-3 dispersion is as follows.

[0103] [Preparation of Carbon Nanotube Dispersion] Carbon nanotubes (CNT-3), a fibrous carbon material with an average fiber diameter of 15 nm, an average fiber length of 1 μm, and an average aspect ratio of 10⁴ times, were used as the conductive additive, and polyvinylpyrrolidone (PVP) was used as the dispersant. A mixture of the conductive additive and the dispersant in a mass ratio of 3:1 was added to N-methyl-2-pyrrolidone (NMP), and the mixture was kneaded using a ball mill to obtain a conductive additive dispersion (CNT-3 dispersion) containing approximately 3% by mass of the conductive additive.

[0104] <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%, with the Ni-containing composite oxide and Ca(OH) ２ And, ZrO ２ And, Al ２ O ３Furthermore, 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) 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 a calcined product. (3) After the water washing step, the dehydrated cake-like composition was subjected to vacuum drying only at 200°C. (4) As a conductive additive for the positive electrode, AB-3 dispersion was used instead of AB-1 dispersion and CNT-1 dispersion, and the amount of AB-3 added was 0.5 parts by mass per 100 parts by mass of positive electrode active material. The method for preparing AB-3 dispersion is as follows.

[0105] [Preparation of Acetylene Black Dispersion] Acetylene black (AB-3), a particulate carbon material with an average particle size (average major diameter) of 25 nm and an average aspect ratio of 2, was used as the conductive additive, and polyvinylpyrrolidone (PVP) was used as the dispersant. The conductive additive and the dispersant were mixed in a mass ratio of 10:1, and this mixture was added to N-methyl-2-pyrrolidone (NMP). The mixture was kneaded using a ball mill to obtain a conductive additive dispersion (AB-3 dispersion) containing approximately 10% by mass of the conductive additive.

[0106] 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% of sulfur was quantitatively determined, originating from components contained in the Ni-containing composite hydroxide. In addition, when X-ray diffraction measurements were performed, the proportion of metal elements other than Li in the Li layer relative to the total molar amount of metal elements excluding Li in the composite metal oxide was 2.2 mol%. Moreover, as shown in Figure 4, in the S2p spectrum obtained by X-ray photoelectron spectroscopy for 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 5, in the O1s spectra obtained by X-ray photoelectron spectroscopy, the peak P3 of the third peak, which has a peak at 531±1 eV, was lower than the peak P4 of the fourth peak, which has a peak at 529±1 eV.

[0107] <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) The Ni-containing composite hydroxide was calcined at 650°C for 10 hours to obtain a Ni-containing composite oxide containing Ni, Co, and Mn. (2) In the mixing step, the Ni-containing composite oxide and Ca(OH) were mixed such 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 (4) The following was added and heat treatment was performed for 10 hours in an air atmosphere at a temperature of 300°C as a conductive additive for the positive electrode: (4) Instead of the AB-1 dispersion and CNT-1 dispersion, the AB-3 dispersion was used, and the amount of AB-3 added was 1.5 parts by mass per 100 parts by mass of positive electrode active material.

[0108] <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 (2) In the mixing step, the Ni-containing composite hydroxide represented by was calcined at 300°C for 10 hours to obtain a Ni-containing composite oxide containing Ni, Co, and Mn. ２ O) was mixed with (3) In the addition step, 0.1 mol% sodium dodecyl sulfate (NaC) relative to the total number of moles of Ni, Co, and Mn was added to the cake composition. １２ H ２５ SO ４(4) The following was added and heat treatment was performed for 4 hours in a vacuum atmosphere at a temperature of 150°C. (4) As a conductive additive for the positive electrode, AB-2 dispersion was used instead of AB-1 dispersion and CNT-1 dispersion, and the amount of solid content of AB-2 added was 0.8 parts by mass per 100 parts by mass of positive electrode active material.

[0109] Table 1 shows the conditions for preparing the positive electrode active materials for Examples 1-1 to 4 and Comparative Examples 1-1 to 4. Table 2 shows the evaluation results of the test cells for Examples 1-1 to 4 and Comparative Examples 1-1 to 4. In Table 2, the resistance increase rate for each test cell is expressed relatively, with the resistance increase rate of the test cell for Comparative Example 1-1 set to 100. Table 2 also shows the following information along with the evaluation results. (1) S content, Me content, Ca content, and Sr content relative to the total number of moles of transition metal elements, calculated from the measurement results of ICP-AES and TEM-EDX. (2) Presence or absence of S-containing compounds and Ca or Sr on the surface or grain boundaries of the primary particles of the composite metal oxide. (3) Proportion of metal elements other than Li present in the Li layer of the layered structure. (4) 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) Crystallite size (s) calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks of the (104) plane of the X-ray diffraction pattern. (6) Average particle size and average fiber diameter of the conductive additive.

[0110]

[0111]

[0112] As shown in Table 2, the resistance increase rate of the test cell in the example is lower than that of the test cell in the comparative example. Therefore, it can be said that the resistance increase rate 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 conductive additive with a microstructure whose spectrum obtained by X-ray photoelectron spectroscopy has a predetermined shape.

[0113] <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 (2) In the mixing step, the Ni-containing composite hydroxide represented by was calcined at 650°C for 10 hours to obtain a Ni-containing composite oxide containing Ni and Mn. ２ O) was mixed with (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, 0.3 mol% sodium disulfite (Na) was added relative to the total number of moles of Ni, Co, and Mn. ２ S ２ O ５ (4) The following was added and heat treatment was performed for 2 hours in an oxygen atmosphere at a temperature of 300°C. (4) As a conductive additive for the positive electrode, AB-2 dispersion was used instead of AB-1 dispersion and CNT-1 dispersion, and the amount of solid content of AB-2 added was 1 part by mass per 100 parts by mass of positive electrode active material.

[0114] <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(1) A Ni-containing composite hydroxide represented by was calcined at 350°C for 10 hours to obtain a Ni-containing composite oxide containing Ni and Mn. (2) In the mixing step, 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%, and further, the Li compound was mixed 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 then held for 5 hours to obtain the calcined product. (4) In the washing step, the obtained calcined material was crushed, and then, using a 3 L reaction vessel, the calcined material was added to water so that the solid-liquid ratio was 1000 g / L, and washed with water at a stirring speed of 300 rpm for 10 minutes, after which it was dehydrated to obtain a cake-like composition. (5) In the addition step, 0.1 mol% sodium disulfite (Na) relative to the total number of moles of Ni and Mn was added to the powder obtained by vacuum drying the cake-like composition dehydrated after the washing step at 150°C. ２ S ２ O ５ (6) The following was added and heat treatment was performed for 8 hours in an oxygen atmosphere at a temperature of 300°C as a conductive additive for the positive electrode: Instead of the AB-2 dispersion, the AB-1 dispersion and the CNT-3 dispersion were used, with the solid content amounts of AB-2 and CNT-3 being 0.5 and 0.03 parts by mass, respectively, per 100 parts by mass of the positive electrode active material.

[0115] <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) [Ni ０．８０ Mn ０．２０ ] (OH) ２ (2) In the mixing step, the Ni-containing composite hydroxide represented by was calcined at 600°C for 10 hours to obtain a Ni-containing composite oxide containing Ni and Mn. ２ And, Sr(OH) ２And, SiO ２ Furthermore, 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 (4) As a conductive additive for the positive electrode, only the CNT-1 dispersion was used instead of the AB-1 dispersion and the CNT-1 dispersion, and the amount of CNT-1 solids added was 0.5 parts by mass per 100 parts by mass of positive electrode active material.

[0116] <Example 2-4> 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 (2) In the mixing step, the Ni-containing composite hydroxide represented by was calcined at 350°C for 10 hours to obtain a Ni-containing composite oxide containing Ni and Mn. 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(4) Instead of the AB-2 dispersion, the AB-2 dispersion and CNT-2 dispersion were used as conductive additives for the positive electrode. The amount of solid content added to AB-2 was 0.8 parts by mass per 100 parts by mass of positive electrode active material, and the amount of solid content added to CNT-2 was 0.03 parts by mass per 100 parts by mass of positive electrode active material.

[0117] <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) In the mixing step, the molar ratio of Fe to the total number of moles of Ni and Mn was 1.0 mol%, and the molar ratio of Sr was 0.3 mol%, so that Ni-containing metal oxide and Fe ２ O ３ And, Sr(OH) ２ Furthermore, the Li compound was mixed with the total number of moles of Ni, Mn, Fe, and Sr so that the molar ratio of Li to the total number of moles of Ni, Mn, Fe, and Sr was 105 mol%. (2) After the water washing step, the dehydrated cake-like composition was subjected to vacuum drying only at 150°C. (3) As a conductive additive for the positive electrode, the amount of AB-2 added was 1 part by mass per 100 parts by mass of positive electrode active material.

[0118] Table 3 shows the conditions for preparing the positive electrode active materials for Examples 2-1 to 2-4 and Comparative Example 2-1. Table 4 shows the evaluation results of the test cells for Examples 2-1 to 2-4 and Comparative Example 2-1. In Table 4, the resistance increase rate for each test cell is expressed relatively, with the resistance increase rate of the test cell for Comparative Example 2-1 set to 100. In addition to the evaluation results, Table 4 also shows the following information, similar to Table 2. (1) S content, Me content, Ca content, and Sr content relative to the total number of moles of transition metal elements, calculated from the measurement results of ICP-AES and TEM-EDX. (2) Presence or absence of S-containing compounds and Ca or Sr on the surface or grain boundaries of the primary particles of the composite metal oxide. (3) Proportion of metal elements other than Li present in the Li layer of the layered structure. (4) 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) Crystallite size (s) calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks of the (104) plane of the X-ray diffraction pattern. (6) Average particle size and average fiber diameter of the conductive additive.

[0119]

[0120]

[0121] As shown in Table 4, the resistance increase rate of the test cell in the example is lower than that of the test cell in the comparative example.

[0122] <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 (2) In the mixing step, the Ni-containing composite hydroxide represented by was calcined at 400°C for 10 hours to obtain a Ni-containing composite oxide containing Ni, Co, and Mn. ３And, 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 (6) The following was added and heat treatment was performed for 2 hours in an air atmosphere at a temperature of 500°C as a conductive additive for the positive electrode. Instead of the AB-1 dispersion and the CNT-1 dispersion, only the CNT-1 dispersion was used, and the amount of CNT-1 added was 0.8 parts by mass per 100 parts by mass of positive electrode active material.

[0123] <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, Ni-containing composite oxide and lithium hydroxide monohydrate (LiOH·H) were mixed so that the molar ratio of Li to the total number of moles of Ni, Co, and Mn was 105 mol%. ２ (O) was mixed with (2) After the water washing step, the dehydrated cake-like composition was vacuum dried at 150°C. (3) As a conductive additive for the positive electrode, AB-2 dispersion was used instead of CNT-1 dispersion, and the amount of AB-2 added was 0.8 parts by mass per 100 parts by mass of positive electrode active material.

[0124] 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 resistance increase rate for each test cell is expressed relatively, with the resistance increase rate of the test cell for Comparative Example 3-1 set to 100. In addition to the evaluation results, Table 6 also shows the following information, similar to Tables 2 and 4. (1) S content, Me content, Ca content, and Sr content relative to the total number of moles of transition metal elements, calculated from the measurement results of ICP-AES and TEM-EDX. (2) Presence or absence of S-containing compounds and Ca or Sr on the surface or grain boundaries of the primary particles of the composite metal oxide. (3) Proportion of metal elements other than Li present in the Li layer of the layered structure. (4) 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) Crystallite size (s) calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks of the (104) plane of the X-ray diffraction pattern. (6) Average particle size and average fiber diameter of the conductive additive.

[0125]

[0126]

[0127] As shown in Table 6, the resistance increase rate of the test cell in the example is lower than that of the test cell in the comparative example.

[0128] This disclosure is further illustrated by the following embodiments. Configuration 1: A positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive additive, and a binder, the conductive additive comprises at least one selected from the group consisting of particulate carbon materials with an average particle size of 20 nm or less and fibrous carbon materials with an average fiber diameter of 20 nm or less, the positive electrode active material comprises a lithium transition metal composite oxide containing at least Ni and secondary particles formed by the aggregation of primary particles, and a modifying compound present on the surface of the primary particles or at the grain boundaries between the primary particles, the modifying compound comprising a compound containing S, the S content in the modifying compound satisfying 0.1 mol% ≤ S content ≤ 3 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, and having a first peak with a peak at 170 ± 2 eV and a second peak with a peak at 167 ± 2 eV in an S2p spectrum measured by X-ray photoelectron spectroscopy, the positive electrode for a non-aqueous electrolyte secondary battery. Configuration 2: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 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). Configuration 3: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 2, wherein the Me content in the modified compound satisfies 0 mol% < Me content ≤ 6 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. Configuration 4: The positive electrode 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 with a peak at 531 ± 1 eV is higher than the peak with a peak at 529 ± 1 eV. Configuration 5: The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the modified compound includes a compound containing at least one of Ca and Sr. Configuration 6: The positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 5, wherein the sum of the Ca content and Sr content contained in the modified compound satisfies 0 mol% < Ca content + Sr content ≤ 2 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.Configuration 7: A positive electrode for a non-aqueous electrolyte secondary battery according to Configuration 5 or 6, wherein the Ca content and Sr content in the modified compound satisfy Ca content > Sr content. Configuration 8: A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein the Ni content in the lithium transition metal composite oxide satisfies 50 mol% ≤ Ni content ≤ 97 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. Configuration 9: A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein the lithium transition metal composite oxide has a layered structure, and the crystallite size s of the lithium transition metal composite 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 10: The lithium transition metal composite oxide has a layered structure, and the proportion of metal elements other than Li present in the Li layer of the layered structure is 8 mol% or less relative to the total number of moles of metal elements other than Li in the lithium transition metal composite oxide, as described in any one of Configurations 1 to 9. Configuration 11: The conductive additive comprises particulate carbon material with an average particle size of 20 nm or less, and fibrous carbon material with an average fiber diameter of 20 nm or less, as described in any one of Configurations 1 to 10. Configuration 12: The mass ratio of the content of particulate carbon material to the content of fibrous carbon material is 2 or more and 30 or less, as described in any one of Configurations 1 to 11. Configuration 13: The particulate carbon material is acetylene black with an average particle size of 20 nm or less, as described in any one of Configurations 1 to 12. Configuration 14: The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 13, wherein the fibrous carbon material is a carbon nanotube with an average fiber diameter of 20 nm or less. Configuration 15: The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 14, wherein the fibrous carbon material has an average aspect ratio of 50 or more. Configuration 16: The positive electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 15, wherein the content of the conductive additive is 1 part by mass or less per 100 parts by mass of the positive electrode active material.Configuration 17: A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery as described in any one of Configurations 1 to 16, a negative electrode, and a non-aqueous electrolyte.

[0129] 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

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive additive, and a binder, the conductive additive comprises at least one selected from the group consisting of particulate carbon materials with an average particle size of 20 nm or less and fibrous carbon materials with an average fiber diameter of 20 nm or less, the positive electrode active material comprises a lithium transition metal composite oxide containing at least Ni and secondary particles formed by the aggregation of primary particles, and a modifying compound present on the surface of the primary particles or at the grain boundaries between the primary particles, the modifying compound comprises a compound containing S, and the S content in the modifying compound satisfies 0.1 mol% ≤ S content ≤ 3 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, A positive electrode for a non-aqueous electrolyte secondary battery, exhibiting 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.

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

4. The positive electrode 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 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 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 modified compound satisfies 0 mol% < Ca content + Sr content ≤ 2 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

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

8. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the Ni content in the lithium transition metal composite oxide satisfies 50 mol% ≤ Ni content ≤ 97 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

9. The lithium transition metal composite oxide has a layered structure, and the crystallite size s of the lithium transition metal composite oxide, calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks of the (10⁴) plane of the X-ray diffraction pattern obtained by X-ray diffraction, satisfies 300 Å ≤ s ≤ 700 Å, the positive electrode for a non-aqueous electrolyte secondary battery according to claim 1.

10. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide has a layered structure, and the proportion of metal elements other than Li present in the Li layer of the layered structure is 8 mol% or less with respect to the total number of moles of metal elements other than Li in the lithium transition metal composite oxide.

11. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the conductive additive comprises a particulate carbon material with an average particle size of 20 nm or less and a fibrous carbon material with an average fiber diameter of 20 nm or less.

12. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the mass ratio of the content of particulate carbon material to the content of fibrous carbon material is 2 or more and 30 or less.

13. The particulate carbon material is acetylene black with an average particle size of 20 nm or less, as described in claim 1, for a positive electrode for a non-aqueous electrolyte secondary battery.

14. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the fibrous carbon material is a carbon nanotube having an average fiber diameter of 20 nm or less.

15. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the fibrous carbon material has an average aspect ratio of 50 or more.

16. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the conductive additive is 1 part by mass or less per 100 parts by mass of the positive electrode active material.

17. A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 16, a negative electrode, and a non-aqueous electrolyte.

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