Positive electrode active material and production method for positive electrode active material

Abstract

A positive electrode active material according to the present invention is characterized by including a lithium / transition metal composite oxide represented by the compositional formula LiNixM1yM2zO2 (0.7≤x≤1.0, 0≤y≤0.3, 0≤z≤0.1) and a surface compound that is present at the surface of the lithium / transition metal composite oxide and includes Li, Al, and sulfate ions. The positive electrode active material is also characterized by having a crystal lattice strain of 0.25%–0.35%. The positive electrode active material is also characterized in that the Al content of the surface compound is 0.01–2.0 mol% of the total molar amount of the positive electrode active material, and the sulfate ion content of the surface compound is at least 0.04 mol% of the total molar amount of the positive electrode active material.

Description

Positive electrode active material and method for producing the positive electrode active material

[0001] The present disclosure relates to a positive electrode active material and a method for manufacturing the positive electrode active material.

[0002] In recent years, the applications of non-aqueous electrolyte secondary batteries have expanded to include power sources for electric vehicles and power storage devices for utilizing natural energy. The positive electrode has a significant effect on battery characteristics, including battery capacity, output characteristics, and cycle characteristics, and therefore much research has been done on the positive electrode. For example, Patent Document 1 discloses a manufacturing method in which an Al-containing compound is added to a fired positive electrode active material and the material is fired again. According to the manufacturing method of Patent Document 1, LiAlO is applied to the surface of the positive electrode active material. ２ is formed, and the remaining Li present on the surface of the positive electrode active material can be removed.

[0003] Special table 2018-506141 publication

[0004] In non-aqueous electrolyte secondary batteries, improving cycle characteristics and durability are important issues. The technology described in Patent Document 1 cannot adequately address these issues, and there is still much room for improvement.

[0005] The positive electrode active material for a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure has the composition formula LiNi ｘ M1 ｙ M2 ｚ O ２ (M1 is at least one element selected from the group consisting of Co, Mn, and Al; M2 is at least one element selected from the group consisting of Ti, Nb, Si, Mo, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B; 0.7≦x≦1.0, 0≦y≦0.3, 0≦z≦0.1), and a surface compound present on the surface of the lithium transition metal composite oxide, the surface compound containing Li, Al, and sulfate ions; wherein the crystal lattice distortion of the positive electrode active material is 0.25% or more and 0.35% or less, the content of Al contained in the surface compound is 0.01 mol % or more and 2.0 mol % or less of the total molar amount of the positive electrode active material, and the content of sulfate ions contained in the surface compound is 0.04 mol % or more of the total molar amount of the positive electrode active material.

[0006] Furthermore, a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a mixing step of mixing a nickel-containing compound, a lithium-containing compound, aluminum hydroxide, and aluminum sulfate, which are positive electrode active material precursors, and a calcination step of calcining the mixture obtained in the mixing step, wherein the molar ratio of aluminum hydroxide to aluminum sulfate in the mixing step is 1:9 to 9:1.

[0007] The positive electrode active material according to one aspect of the present disclosure can improve cycle characteristics and battery durability.

[0008] 1 is a diagram schematically illustrating an axial cross section of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0009] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a nonaqueous electrolyte secondary battery according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that the scope of the present disclosure includes configurations obtained by selectively combining the respective components of the multiple embodiments and modifications described below.

[0010] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical, bottomed exterior body 16 is exemplified as a nonaqueous electrolyte secondary battery; however, the exterior body of the battery is not limited to a cylindrical exterior body. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic exterior body, a coin-type battery with a coin-type exterior body, or a pouch-type battery with an exterior body composed of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, but may also be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween. Furthermore, the design of the nonaqueous electrolyte secondary battery according to the present disclosure is not limited to the design of the exemplified nonaqueous electrolyte secondary battery, and known nonaqueous electrolyte secondary battery designs may also be applied.

[0011] FIG. 1 is an axial cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an exterior body 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 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 spirally wound with the separator 13 interposed therebetween. The exterior body 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the exterior body 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the sealing body 17 side of the battery will be referred to as the top, and the bottom side of the exterior body 16 will be referred to as the bottom.

[0012] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all rectangular, elongated bodies that are spirally wound in the longitudinal direction and stacked alternately in the radial direction of the electrode assembly 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. Two separators 13 are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 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 assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the lateral direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the lateral end faces of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode assembly 14.

[0013] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the exterior body 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0014] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0015] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0016] Hereinafter, the positive electrode 11, the negative electrode 12, the separator 13, and the nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail with reference to FIG. 1, with the positive electrode 11 being particularly described.

[0017] [Positive Electrode] The positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 formed on the surface of the positive electrode core 30. The positive electrode mixture layer 31 is preferably formed on both sides of the positive electrode core 30. For the positive electrode core 30, a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface layer can be used. The thickness of the positive electrode core 30 is, for example, 10 μm or more and 30 μm or less.

[0018] The positive electrode mixture layer 31 includes, for example, a positive electrode active material, a conductive additive, and a binder. The positive electrode mixture layer 31 is provided on both sides of the positive electrode core 30 except for the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry including a positive electrode active material, a binder, a conductive additive, and the like to the surface of the positive electrode core 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0019] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black (CB) such as acetylene black (AB) and ketjen black, carbon nanotubes (CNT), graphene, graphite, and other carbon-based particles. These may be used alone or in combination of two or more.

[0020] Examples of the binder contained in the positive electrode mixture layer 31 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyimide-based resins, acrylic-based resins, polyolefin-based resins, polyacrylonitrile (PAN), etc. These may be used alone or in combination of two or more.

[0021] The positive electrode active material contained in the positive electrode mixture layer 31 has the composition formula LiNi ｘ M1 ｙ M2 ｚ O ２ (M1 is at least one element selected from the group consisting of Co, Mn, and Al; M2 is at least one element selected from the group consisting of Ti, Nb, Si, Mo, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B; 0.7≦x≦1.0, 0≦y≦0.3, 0≦z≦0.1) The content of the elements constituting the lithium transition metal composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0022] The Ni content in the lithium transition metal composite oxide is 70 mol% or more and 100 mol% or less relative to the total number of moles of metal elements excluding Li. This can improve battery capacity. The Ni content may be 75 mol% or more, or may be 80 mol% or more, preferably 85 mol% or more, and more preferably 90 mol% or more. Furthermore, from the viewpoint of structural stabilization, the Ni content is preferably 99 mol% or less, more preferably 95 mol% or less.

[0023] The lithium transition metal composite oxide preferably contains element M1 (M1 is at least one element selected from the group consisting of Co, Mn, and Al). When the lithium transition metal composite oxide contains Co, for example, it can improve the heat resistance of the battery. Furthermore, when the lithium transition metal composite oxide contains Mn or Al, it can stabilize the crystal structure, for example. Note that element M1 is an optional component. In other words, the lithium transition metal composite oxide does not need to contain element M1. The content of element M1 in the lithium transition metal composite oxide is 0 mol% or more and 30 mol% or less, preferably 5 mol% or more and 20 mol% or less, and more preferably 5 mol% or more and 15 mol% or less, relative to the total number of moles of metal elements excluding Li.

[0024] The lithium transition metal composite oxide may contain element M2 (M2 is at least one element selected from the group consisting of Ti, Nb, Si, Mo, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B). The element M2 is an optional component. In other words, the lithium transition metal composite oxide does not need to contain element M2. The content of element M2 in the lithium transition metal composite oxide is 0 mol% or more and 10 mol% or less, preferably 0 mol% or more and 7 mol% or less, and more preferably 0 mol% or more and 5 mol% or less, relative to the total number of moles of metal elements excluding Li.

[0025] The lithium transition metal composite oxide is, for example, a secondary particle formed by aggregation of primary particles. The particle size of the primary particles constituting the secondary particles of the lithium transition metal composite oxide is, for example, 0.02 μm or more and 2 μm or less. The particle size of the primary particles is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM). The average particle size of the secondary particles of the lithium transition metal composite oxide is, for example, 2 μm or more and 30 μm or less. Here, the average particle size refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the secondary particles of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as a dispersion medium.

[0026] The lithium transition metal composite oxide has, for example, a layered structure. Examples of the layered structure of the lithium transition metal composite oxide include a layered structure belonging to the space group R-3m and a layered structure belonging to the space group C2 / m. From the viewpoints of increasing capacity and stabilizing the crystal structure, the lithium transition metal composite oxide preferably has a layered structure belonging to the space group R-3m. The layered structure of the lithium transition metal composite oxide may include a transition metal layer and a Li layer.

[0027] On the particle surface of the lithium transition metal composite oxide, a surface compound containing Li, Al, and sulfate ions is present. The surface compound is, for example, LiAlO ２ and Al ２ (SO ４ ) ３ The surface compounds are present in the form of particles or layers on the surfaces of the primary particles of the lithium transition metal composite oxide. The surface compounds may be scattered so as to cover at least a portion of the surfaces of the primary particles, but are preferably present so as to cover substantially the entire surfaces of the secondary particles. The presence of the surface compounds can be confirmed, for example, by synchrotron XRD measurement, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), etc.

[0028] The content of Al contained in the surface compound is 0.01 mol % or more and 2.0 mol % or less, and preferably 0.1 mol % or more and 1.0 mol % or less, of the total molar amount of the positive electrode active material. When the Al content is within the above range, the particle surface of the positive electrode active material is less likely to deteriorate even after repeated cycles, and durability can be improved.

[0029] The Al content of the surface compound can be measured by adding 1 g of the positive electrode active material to 50 mL of hydrochloric acid, shaking the resulting sample solution at room temperature for 1 minute, filtering the stirred sample solution, collecting the filtrate, and measuring the amount of Al in the collected filtrate using an inductively coupled plasma atomic emission spectrometer (ICP-AES). Furthermore, when the lithium transition metal composite oxide does not contain Al, the Al content of the surface compound can also be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray analyzer (EDX).

[0030] The content of sulfate ions contained in the surface compound is 0.04 mol% or more, preferably 0.04 mol% or more and 2.0 mol% or less, and more preferably 0.05 mol% or more and 1.0 mol% or less, of the total molar amount of the positive electrode active material. When the content of sulfate ions is within the above range, the particle surface of the positive electrode active material is less likely to deteriorate even after repeated cycling, thereby improving durability. Furthermore, when the content of sulfate ions is within the above range, the surface of the lithium transition metal oxide is modified, resulting in specifically increased ionic conductivity, thereby reducing charge transfer resistance.

[0031] The content of sulfate ions contained in the surface compound can be measured by adding 1 g of the positive electrode active material to 50 mL of pure water, shaking the obtained sample solution at room temperature for 24 hours, filtering the stirred sample solution, collecting the filtrate, and measuring the amount of sulfate ions in the collected filtrate by ion chromatography.

[0032] Furthermore, the crystal lattice distortion of the positive electrode active material is 0.25% or more and 0.35% or less, preferably 0.26% or more and 0.34% or less. When the crystal lattice distortion of the positive electrode active material is 0.25% or more and 0.35% or less, the Al contained in the surface compound does not dissolve in the lithium transition metal composite oxide, and the Al can be present on the surface of the lithium transition metal composite oxide in the form of a surface compound. This makes the particle surface of the positive electrode active material less susceptible to deterioration even after repeated cycling, thereby improving durability. In other words, when the crystal lattice distortion of the positive electrode active material is less than 0.25% or more than 0.35%, the amount of Al present on the surface of the lithium transition metal composite oxide decreases, and durability cannot be sufficiently improved. Furthermore, when the crystal lattice distortion of the positive electrode active material is more than 0.35%, the crystal structure of the positive electrode active material becomes unstable, and battery capacity tends to decrease.

[0033] The crystal lattice distortion of the positive electrode active material can be obtained from the results of Rietveld analysis of the X-ray diffraction pattern of the positive electrode active material. Specifically, the X-ray diffraction pattern of the positive electrode active material is measured using a powder X-ray diffractometer (manufactured by Bruker AXS, product name "D8ADVANCE"), and the obtained peak positions (2θ) and half-widths (β) of each peak are applied to a Williamson-Hall plot to calculate the distortion.

[0034] The positive electrode active material, which is one example of the embodiment, can be produced by the following method: Note that the production method described here is only an example, and the method for producing the positive electrode active material is not limited to this method.

[0035] The manufacturing process of the positive electrode active material includes a precursor preparation step of preparing a nickel-containing compound, which is a positive electrode active material precursor, a mixing step of mixing the nickel-containing compound, a lithium-containing compound, aluminum hydroxide, and aluminum sulfate, and a calcination step of calcining the mixture obtained in the mixing step.

[0036] In the precursor preparation step, a nickel-containing compound is prepared that contains, for example, 70 mol% to 100 mol% Ni, 0 mol% to 30 mol% M1 (M1 is at least one element selected from the group consisting of Co, Mn, and Al), and 0 mol% to 10 mol% M2 (M2 is at least one element selected from the group consisting of Ti, Nb, Si, Mo, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B).

[0037] The nickel-containing compound can be obtained, for example, by adding an alkaline solution such as sodium hydroxide dropwise to a stirred solution of a metal salt containing Ni and an arbitrary metal element (M1, M2, etc.) to adjust the pH to the alkaline side (e.g., 8.5 to 12.5) to precipitate (co-precipitate) a composite hydroxide containing Ni and the arbitrary metal element, and then heat-treating the composite hydroxide. The heat-treatment temperature is not particularly limited, but is, for example, in the range of 250°C to 600°C.

[0038] In the mixing step, a nickel-containing compound, a lithium-containing compound, aluminum hydroxide, and aluminum sulfate are mixed. The lithium-containing compound is, for example, Li ２ CO ３ , LiOH, Li ２ O ２ , Li ２ O, LiNO ３ , LiNO ２ , Li ２ SO ４ , LiOH·H ２ O, LiH, LiF, etc. In order to facilitate adjustment of the above-mentioned parameters to the above-mentioned ranges, the mixing ratio of the nickel-containing compound and the lithium-containing compound is preferably such that the molar ratio of the total amount of metal elements in the nickel-containing compound to Li falls within the range of 1:0.8 to 1:1.2, and particularly preferably 1:1.0 to 1:1.1.

[0039] The Al contained in the aluminum hydroxide and aluminum sulfate added in the mixing step is present on the surface of the lithium transition metal composite oxide as a surface compound after the firing step described below. Therefore, the aluminum hydroxide and aluminum sulfate are mixed and added so that the Al contained in the aluminum hydroxide and aluminum sulfate is 0.01 mol % or more and 2.0 mol % or less with respect to the total molar amount of the positive electrode active material.

[0040] In the mixing step, a compound containing M2 (M2 is at least one element selected from the group consisting of Ti, Nb, Si, Mo, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B) may be added. In this case, the amount of M2 added is mixed and added so as to be 0 mol % or more and 10 mol % or less.

[0041] Aluminum hydroxide has a lower melting point than aluminum sulfate. Therefore, the Al contained in aluminum hydroxide dissolves at low temperatures during the baking process described below and easily spreads throughout the positive electrode active material. In other words, if aluminum hydroxide alone is added without adding aluminum sulfate during the mixing process, Al is more likely to be dissolved in the lithium transition metal composite oxide, making it difficult for a surface compound containing Al to form on the surface of the lithium transition metal composite oxide. When the positive electrode active material is repeatedly charged and discharged, deterioration occurs preferentially from the particle surface, and if a sufficient amount of surface compound is not formed, cycle characteristics tend to deteriorate.

[0042] Furthermore, aluminum sulfate has a higher melting point than aluminum hydroxide, and therefore tends to be less soluble in the calcination step described below. In other words, if aluminum sulfate alone is added without adding aluminum hydroxide in the mixing step, the aluminum sulfate does not dissolve sufficiently, making it difficult for surface compounds to form on the entire surface of the lithium transition metal composite oxide. As a result, when charging and discharging are repeated, deterioration tends to proceed preferentially from areas of the particle surface of the positive electrode active material where no surface compounds have formed, resulting in poor cycle performance.

[0043] As a result of the inventors' investigations, it was found that by adjusting the molar ratio of aluminum hydroxide to aluminum sulfate to 1:9 to 9:1 during the mixing step, surface compounds can be formed over the entire surface of the lithium transition metal composite oxide. Although the detailed mechanism is unclear, mixing at this ratio allows aluminum hydroxide and aluminum sulfate to form a eutectic point, and melting begins at a temperature range higher than that of aluminum hydroxide alone but lower than that of aluminum sulfate alone. This prevents Al from being dissolved in the lithium transition metal composite oxide during the firing step, and makes it easier for it to spread over the entire surface of the lithium transition metal composite oxide as a surface compound. As a result, the particle surfaces of the positive electrode active material are less susceptible to deterioration even after repeated cycles, improving durability.

[0044] In the mixing step, the molar ratio of aluminum hydroxide to aluminum sulfate may be 1:9 to 9:1, preferably 3:7 to 9:1, and more preferably 5:5 to 9:1. In this case, it becomes easier to uniformly form a surface compound over the entire surface of the lithium transition metal composite oxide. As a result, the particle surfaces of the positive electrode active material are less susceptible to deterioration even after repeated cycles, and durability can be further improved.

[0045] In the calcination step, the mixture obtained in the mixing step is calcined at a predetermined temperature and time to obtain the positive electrode active material of this embodiment. The calcination temperature is preferably 720°C or higher and 770°C or lower, and more preferably 740°C or higher and 760°C or lower, from the viewpoint of facilitating the formation of surface compounds on the entire surface of the lithium transition metal composite oxide and maintaining a crystal lattice distortion of the positive electrode active material of 0.25% or higher and 0.35% or lower. If the calcination temperature is lower than 720°C, the crystal lattice distortion of the positive electrode active material may exceed 0.35%. Furthermore, if the calcination temperature is higher than 770°C, a portion of the Al may dissolve in the lithium transition metal composite oxide, resulting in a crystal lattice distortion of the positive electrode active material of less than 0.25%. The calcination temperature refers to the maximum temperature during calcination. The calcination time is preferably, for example, 1 hour or higher and 24 hours or lower. Furthermore, calcination is preferably performed in an oxygen stream.

[0046] [Negative Electrode] The negative electrode 12 may have, for example, a negative electrode core 40 and a negative electrode mixture layer 41 formed on the surface of the negative electrode core 40, or a metal Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have the negative electrode core 40, and lithium metal may be deposited on the surface of the negative electrode core 40 upon charging. When the negative electrode 12 has the negative electrode mixture layer 41, the negative electrode mixture layer 41 is preferably formed on both sides of the negative electrode core 40. The negative electrode core 40 may be made of a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a film having such a metal disposed on the surface layer. The thickness of the negative electrode core 40 is, for example, 5 μm or more and 30 μm or less.

[0047] The negative electrode mixture layer 41 contains, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer 41 is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode core 40. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and the like to the surface of the negative electrode core 40, drying the coating, and then rolling it to form the negative electrode mixture layer 41 on both sides of the negative electrode core.

[0048] The negative electrode active material contained in the negative electrode mixture layer 41 is not particularly limited as long as it can reversibly absorb and release lithium ions, and generally, a carbon material such as graphite is used. The graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, or artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads. Furthermore, as the negative electrode active material, metals that alloy with Li, such as Si and Sn, metal compounds containing Si, Sn, and lithium-titanium composite oxides, may also be used. Furthermore, these may be provided with a carbon coating. For example, SiO ｘ (0.5≦x≦1.6) or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase represented by (0<y<2) may be used in combination with graphite.

[0049] Examples of the binder contained in the negative electrode mixture layer 41 include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0050] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0051] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0052] [Non-aqueous electrolyte] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0053] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0054] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates 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 chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0055] Examples of the 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 methyl phenyl ether. and chain ethers such as ethyl phenyl ether, butyl phenyl ether, pentyl phenyl 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.

[0056] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include 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 , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ Among these, LiPF is preferred from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ The concentration of the lithium salt may be, for example, 4 mol or less, or 3 mol or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less, per 1 L of the non-aqueous solvent.

[0057] The non-aqueous electrolyte may contain an additive such as an unsaturated carbonate ester, an acid anhydride, a phenol compound, a benzene compound, a nitrile compound, an isocyanate compound, a sultone compound, a sulfate compound, a borate ester compound, a phosphate ester compound, or a phosphite ester compound.

[0058] Examples of unsaturated cyclic carbonates 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. One type of unsaturated cyclic carbonate may be used alone, or two or more types may be used in combination. Some of the hydrogen atoms in the unsaturated cyclic carbonate may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by intermolecular condensation of multiple carboxylic acid molecules, but is preferably an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0059] Examples of phenol compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0060] 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 bisisocyanatomethylcyclohexane (BIMCH). Examples of sultone compounds include propane sultone and propene sultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethyl borate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethyl phosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethyl phosphite and tris(trimethylsilyl)phosphite.

[0061] 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, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, 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 the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0062] Hereinafter, the present disclosure will be further described with reference to examples and comparative examples, but the present disclosure is not limited to the following examples. Example 1 [Preparation of Positive Electrode Active Material] In the precursor preparation step, [Ni ０．９０ Co ０．０５ Mn ０．０５ ](OH) ２was calcined at 500° C. for 10 hours to obtain a nickel-containing compound, which is a precursor of a positive electrode active material containing Ni, Co, and Mn.

[0063] In the mixing step, the nickel-containing compound, lithium hydroxide (LiOH), and powdered aluminum hydroxide (Al(OH) ３ ) and powdered aluminum sulfate (Al ２ (SO ４ ) ３ The aluminum hydroxide and aluminum sulfate were mixed together in such a manner that the molar ratio of Al contained in the aluminum hydroxide and aluminum sulfate was 0.5 mol % of the total molar amount of the positive electrode active material. The molar ratio of aluminum hydroxide to aluminum sulfate was 1:9.

[0064] In the firing step, the mixture was fired in an oxygen stream at 750° C. for 15 hours to obtain a positive electrode active material.

[0065] The Al and sulfate ion contents of the surface compound were measured by the above-mentioned measurement method and found to be 0.5 mol % and 0.21 mol %, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured by the above-mentioned measurement method and found to be 0.26%.

[0066] [Preparation of Positive Electrode] The positive electrode active material, acetylene black (AB), and polyvinylidene fluoride were mixed in a mass ratio of 86:10:4, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode having a positive electrode mixture layer disposed on both sides of the positive electrode core.

[0067] [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 hexafluorophosphate (LiPF ６ ) was dissolved in the solution to a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0068] [Preparation of Test Cell] A lithium metal foil was used as the negative electrode, and the positive and negative electrodes were arranged facing each other with a separator interposed therebetween to prepare an electrode assembly. This electrode assembly and the nonaqueous electrolyte solution were placed in a coin-shaped outer can, and the opening of the outer can was sealed with a gasket and a sealing member to prepare a test cell (nonaqueous electrolyte secondary battery).

[0069] [Evaluation of Cycle Characteristics (Capacity Retention)] The prepared test cells were charged to 4.2 V at a constant current of 0.2 C in a temperature environment of 25°C, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 C. Thereafter, the cells were discharged at a constant current of 0.2 C until the voltage reached 2.5 V. This constitutes one cycle, and the discharge capacity at 1.0 C after 30 cycles was measured. The capacity retention was then calculated using the following formula: Capacity retention [%] = (Discharge capacity after 30 cycles / Discharge capacity at first cycle) × 100

[0070] Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3 in the mixing step of preparing the positive electrode active material. The contents of Al and sulfate ions contained in the surface compound were measured using the above-described measurement method, and were found to be 0.5 mol % and 0.09 mol %, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.27%.

[0071] Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that the molar ratio of aluminum hydroxide to aluminum sulfate was 9:1 in the mixing step of preparing the positive electrode active material. The contents of Al and sulfate ions contained in the surface compound were measured using the above-described measurement method, and were found to be 0.5 mol % and 0.06 mol %, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.27%.

[0072] Example 4 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the mixing step of preparing the positive electrode active material, the molar ratio of Al contained in aluminum hydroxide and aluminum sulfate was 0.1 mol % of the total molar amount of the positive electrode active material, and the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3. The contents of Al and sulfate ions contained in the surface compound were measured using the above-mentioned measurement method, and were found to be 0.1 mol % and 0.04 mol %, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-mentioned measurement method, and was found to be 0.27%.

[0073] Example 5 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the mixing step of preparing the positive electrode active material, the molar ratio of Al contained in aluminum hydroxide and aluminum sulfate was 1.0 mol % of the total molar amount of the positive electrode active material, and the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3. The contents of Al and sulfate ions contained in the surface compound were measured using the above-mentioned measurement method, and were found to be 1.0 mol % and 0.19 mol %, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-mentioned measurement method, and was found to be 0.25%.

[0074] Example 6 In the precursor preparation step for preparing a positive electrode active material, [Ni ０．９０ Co ０．０５ Al ０．０５ ](OH) ２ A composite hydroxide represented by the formula (I) was calcined at 500°C for 8 hours to obtain a nickel-containing compound, which is a positive electrode active material precursor containing Ni, Co, and Al. A test cell was produced and evaluated in the same manner as in Example 1, except that in the mixing step of producing the positive electrode active material, the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3. The contents of Al and sulfate ions contained in the surface compound were measured using the above-mentioned measurement method, and were found to be 0.5 mol% and 0.08 mol%, respectively, of the total molar amount of the positive electrode active material. The crystal lattice distortion of the positive electrode active material was also measured using the above-mentioned measurement method, and was found to be 0.25%.

[0075] Example 7 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the mixing step of preparing the positive electrode active material, the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3, and in the baking step of preparing the positive electrode active material, the baking temperature was 735°C. The contents of Al and sulfate ions contained in the surface compound were measured using the above-mentioned measurement method, and were found to be 0.5 mol% and 0.09 mol%, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-mentioned measurement method, and was found to be 0.28%.

[0076] Comparative Example 1 A test cell was prepared and evaluated in the same manner as in Example 1, except that the molar ratio of aluminum hydroxide to aluminum sulfate was 10:0, i.e., no aluminum sulfate was added, in the mixing step of preparing the positive electrode active material. The contents of Al and sulfate ions contained in the surface compound were measured using the above-described measurement method, and were found to be 0.5 mol % and 0.01 mol %, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.29%.

[0077] Comparative Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the mixing step of preparing the positive electrode active material, the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3, and in the baking step of preparing the positive electrode active material, the baking temperature was 790°C. The contents of Al and sulfate ions contained in the surface compound were measured using the above-mentioned measurement method, and were found to be 0.5 mol% and 0.07 mol%, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-mentioned measurement method, and was found to be 0.18%.

[0078] Comparative Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the mixing step of preparing the positive electrode active material, the molar ratio of Al contained in aluminum hydroxide and aluminum sulfate was 0.001 mol % of the total molar amount of the positive electrode active material, and the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3. The contents of Al and sulfate ions contained in the surface compound were measured using the above-described measurement method, and were found to be 0.001 mol % and 0.00 mol %, respectively, of the total molar amount of the positive electrode active material. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.28%.

[0079] Table 1 shows the capacity retention rates of the test cells of the examples and comparative examples.

[0080]

[0081] As shown in Table 1, the test cells of the examples exhibit high capacity retention rates. This indicates that durability can be improved by setting the crystal lattice distortion of the positive electrode active material to 0.25% or more and 0.35% or less, setting the content of Al contained in the surface compound to 0.01 mol % or more and 2.0 mol % or less of the total molar amount of the positive electrode active material, and setting the content of sulfate ions contained in the surface compound to 0.04 mol % or more of the total molar amount of the positive electrode active material.

[0082] On the other hand, the test cells of Comparative Example 1, in which the content of sulfate ions contained in the surface compound was less than 0.04 mol % of the total molar amount of the positive electrode active material; Comparative Example 2, in which the crystal lattice distortion of the positive electrode active material was less than 0.25%; and Comparative Example 3, in which the content of Al contained in the surface compound was less than 0.01 mol % of the total molar amount of the positive electrode active material, had smaller capacity retention rates and insufficient durability compared to the test cells of the Examples.

[0083] The present disclosure is further described by the following embodiments. Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery, having a composition formula of LiNi ｘ M1 ｙ M2 ｚ O ２(M1 is at least one element selected from the group consisting of Co, Mn, and Al; M2 is at least one element selected from the group consisting of Ti, Nb, Si, Mo, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B; 0.7≦x≦1.0, 0≦y≦0.3, 0≦z≦0.1), and a surface compound present on a surface of the lithium transition metal composite oxide, the surface compound containing Li, Al, and sulfate ions, wherein the crystal lattice distortion of the positive electrode active material is 0.25% or more and 0.35% or less, the content of Al contained in the surface compound is 0.01 mol % or more and 2.0 mol % or less of a total molar amount of the positive electrode active material, and the content of sulfate ions contained in the surface compound is 0.04 mol % or more of a total molar amount of the positive electrode active material. Aspect 2: The cathode active material according to Aspect 1, wherein the content of sulfate ions contained in the surface compound is 0.04 mol % or more and 2.0 mol % or less of the total molar amount of the cathode active material. Aspect 3: A method for producing a cathode active material for a non-aqueous electrolyte secondary battery, comprising: a mixing step of mixing a nickel-containing compound, a lithium-containing compound, aluminum hydroxide, and aluminum sulfate as cathode active material precursors; and a calcination step of calcining the mixture obtained in the mixing step, wherein the molar ratio of aluminum hydroxide to aluminum sulfate in the mixing step is 1:9 to 9:1. Aspect 4: The method for producing a cathode active material according to Aspect 3, wherein the maximum temperature during the calcination step is 720°C or more and 770°C or less.

[0084] REFERENCE SIGNS LIST 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 exterior body, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode core, 31 positive electrode mixture layer, 40 negative electrode core, 41 negative electrode mixture layer.

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, having the composition formula LiNi ｘ M1 ｙ M2 ｚ O ２ 1. A positive electrode active material comprising: a lithium transition metal composite oxide represented by the formula (X, Y, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B; 0.7≦x≦1.0, 0≦y≦0.3, 0≦z≦0.1); and a surface compound present on a surface of the lithium transition metal composite oxide, the surface compound containing Li, Al, and sulfate ions; wherein the crystal lattice distortion of the positive electrode active material is 0.25% or more and 0.35% or less; the content of Al contained in the surface compound is 0.01 mol % or more and 2.0 mol % or less of a total molar amount of the positive electrode active material; and the content of sulfate ions contained in the surface compound is 0.04 mol % or more of a total molar amount of the positive electrode active material.

2. The positive electrode active material according to claim 1, wherein the content of sulfate ions contained in the surface compound is 0.04 mol % or more and 2.0 mol % or less of the total molar amount of the positive electrode active material.

3. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: a mixing step of mixing a nickel-containing compound, a lithium-containing compound, aluminum hydroxide, and aluminum sulfate, which are positive electrode active material precursors; and a firing step of firing the mixture obtained in the mixing step, wherein the molar ratio of aluminum hydroxide to aluminum sulfate in the mixing step is 1:9 to 9:

1.

4. The method for producing a positive electrode active material according to claim 3, wherein the firing temperature in the firing step is 720°C or higher and 770°C or lower.

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