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

Abstract

The present invention is characterized in that: an Ni-containing lithium transition metal oxide is included; the proportion of Ni in the lithium transition metal oxide is 70 mol% or higher with respect to the total number of moles of metal elements excluding Li; Al is dissolved in a solid state in the surface layer of primary particles of a lithium transition metal composite oxide; the lithium transition metal composite oxide has a crystal lattice strain of 0.1-0.25%; in a spectrum obtained by using a hard X-ray photoelectron spectroscopic method, a maximum peak is provided within a range from 1,557-1,559 eV in the range from 1,555-1,565 eV; and 0.04 mol% or more of sulfate ions with reference to the total molar quantity of the lithium transition metal composite oxide are contained.

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. Since the positive electrode significantly affects battery characteristics, including battery capacity, output characteristics, and cycle characteristics, much research has been conducted 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 resulting material is fired again, with the aim of improving cycle characteristics.

[0003] Japanese Patent Application Laid-Open No. 2022-000850

[0004] The positive electrode of a nonaqueous electrolyte secondary battery is required to have heat resistance. If the heat resistance of the positive electrode is low, the battery's heat generation initiation temperature will be low. If the temperature inside the battery rises due to overcharging, short circuit, or the like, the temperature rise may cause further heat-generating chemical reactions (exothermic reactions) to proceed inside the battery, further increasing the battery temperature. The technology described in Patent Document 1 is unable to sufficiently improve the heat resistance of the positive electrode, and there is still much room for improvement.

[0005] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a lithium transition metal oxide containing Ni, the lithium transition metal composite oxide being secondary particles formed by aggregation of primary particles, the proportion of Ni in the lithium transition metal oxide being 70 mol % or more relative to the total number of moles of metal elements excluding Li, Al being dissolved in a solid solution in the surface layers of the primary particles of the lithium transition metal composite oxide, a crystal lattice distortion of the lithium transition metal composite oxide being 0.1% or more and 0.25% or less, and having a maximum peak in a range of 1557 eV or more and 1559 eV or less within a range of 1555 eV or more and 1565 eV or less in a spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays, and the lithium transition metal composite oxide containing sulfate ions in an amount of 0.04 mol % or more of the total molar amount of the lithium transition metal composite oxide.

[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, and the maximum temperature during calcination in the calcination step is 750°C or higher.

[0007] According to the positive electrode active material for a nonaqueous electrolyte secondary battery according to one aspect of the present disclosure, the heat resistance of the battery can be improved.

[0008] 1 is a diagram showing a spectrum obtained by hard X-ray photoelectron spectroscopy of a positive electrode active material 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 mixture layer 31 contains a lithium transition metal composite oxide as a positive electrode active material. The lithium transition metal composite oxide is 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 a 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 referred to as 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.

[0022] 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.

[0023] The lithium transition metal composite oxide contains 70 mol % or more of Ni relative to the total number of moles of metal elements excluding Li. Furthermore, Al is solid-solved in the surface layer of the primary particles of the lithium transition metal composite oxide. Furthermore, LiAlO ２ There are almost no Li and Al containing compounds such as

[0024] 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 allows for improved battery capacity. Furthermore, the higher the Ni content, the lower the heat resistance of the lithium transition metal composite oxide tends to be, and therefore the effects of the present disclosure are more pronounced. 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.

[0025] 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, the heat resistance of the battery can be further improved. Furthermore, when the lithium transition metal composite oxide contains Mn or Al, for example, the crystal structure can be stabilized. 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.

[0026] 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.

[0027] The lithium transition metal composite oxide has, for example, 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 contents 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.

[0028] As described above, Al is dissolved in the surface layer of the primary particles of the lithium transition metal composite oxide. This suppresses side reactions with the non-aqueous electrolyte on the surface of the lithium transition metal composite oxide, improving heat resistance. Furthermore, it is preferable that Al is dissolved in the surface layer of the primary particles of the lithium transition metal composite oxide throughout the entire surface layer. This further improves heat resistance. Here, the surface layer refers to a region extending from the surface of the primary particles to a depth of 100 nm, preferably 70 nm. The state of Al dissolved in the surface layer of the primary particles can be observed by energy dispersive X-ray analysis (EDX).

[0029] The lithium transition metal composite oxide also contains sulfate ions. ４ When the lithium transition metal oxide is attached to the particle surface as a compound, SO ４ The term "lithium transition metal oxide" includes any form, such as when it is incorporated as a compound into secondary particles of the lithium transition metal oxide, or when it is contained within the crystal structure of the lithium transition metal oxide.

[0030] SO attached to the particle surface or incorporated into secondary particles ４ Examples of the compound include Al ２ (SO ４ ) ３ , Ti(SO ４ ) ２ , MnSO ４ , Nb ２ (SO ４ ) ３, Si(SO ４ ) ２ , Mo(SO ４ ) ３ , Fe ２ (SO ４ ) ３ , BaSO ４ , CaSO ４ , CuSO ４ , MgSO ４ , SrSO ４ , ZnSO ４ etc.

[0031] The sulfate ion content of the lithium transition metal oxide is 0.04 mol% or more, preferably 0.05 mol% or more, and more preferably 0.06 mol% or more, of the total molar amount of the lithium transition metal composite oxide. By making the sulfate ion content of the lithium transition metal oxide 0.04 mol% or more of the total molar amount of the lithium transition metal composite oxide, the surface of the lithium transition metal oxide is modified, and heat resistance tends to be improved. Furthermore, by making the sulfate ion content of the lithium transition metal oxide 0.04 mol% or more of the total molar amount of the lithium transition metal composite oxide, the surface of the lithium transition metal oxide is modified, specifically increasing ionic conductivity and reducing charge transfer resistance. The upper limit of the sulfate ion content of the lithium transition metal oxide is, for example, 2.0 mol% of the total molar amount of the lithium transition metal composite oxide.

[0032] The content of sulfate ions contained in the lithium transition metal composite oxide can be determined by adding 1 g of the lithium transition metal composite oxide 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.

[0033] Furthermore, the crystal lattice distortion of the lithium transition metal composite oxide is 0.1% or more and 0.25% or less, and preferably 0.12% or more and 0.23% or less. When the crystal lattice distortion of the lithium transition metal composite oxide is 0.1% or more and 0.25% or less, Al is more likely to be dissolved in the surface layer of the primary particles of the lithium transition metal composite oxide, thereby improving heat resistance. When the crystal lattice distortion of the lithium transition metal composite oxide is 0.1% or more and 0.25% or less, the crystal structure of the lithium transition metal composite oxide is stabilized, which tends to improve battery capacity.

[0034] The crystal lattice distortion of the lithium transition metal composite oxide can be obtained from the results of Rietveld analysis of the X-ray diffraction pattern of the lithium transition metal composite oxide. Specifically, the X-ray diffraction pattern of the lithium transition metal composite oxide 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 crystal lattice distortion.

[0035] Furthermore, the lithium transition metal composite oxide has a maximum peak in the range of 1557 eV to 1559 eV in the range of 1555 eV to 1565 eV in the spectrum obtained by hard X-ray photoelectron spectroscopy (HAXPES) using 6.0 keV hard X-rays. Hard X-ray photoelectron spectroscopy can determine the bonding state of elements present near the surface of the particles.

[0036] 2 shows a spectrum of a lithium transition metal composite oxide according to this embodiment, obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays. Here, the peaks present in the range of 1557 eV or more and 1559 eV or less are peaks derived from elements such as Al that are solid-solubilized in the lithium transition metal composite oxide. Furthermore, the peaks present in the range of 1560 eV or more and 1562 eV or less are peaks derived from Al compounds (e.g., LiAlO) that adhere to the surface of the lithium transition metal composite oxide. ２ ) is a peak derived from

[0037] As described above, the lithium transition metal composite oxide of this embodiment has a maximum peak in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV. That is, the peak in the range of 1557 eV to 1559 eV is larger than the peak in the range of 1560 eV to 1562 eV. This means that Al is not fixed to the surface of the lithium transition metal composite oxide but is dissolved in the surface layer of the lithium transition metal composite oxide. This improves heat resistance. In other words, if the peak in the range of 1557 eV to 1559 eV is smaller than the peak in the range of 1560 eV to 1562 eV, Al is not sufficiently dissolved in the surface layer of the lithium transition metal composite oxide, and therefore heat resistance cannot be improved.

[0038] 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.

[0039] 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.

[0040] 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).

[0041] 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.

[0042] 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.

[0043] The aluminum contained in the aluminum hydroxide and aluminum sulfate added in the mixing step is dissolved in the surface layers of the primary particles of the lithium transition metal composite oxide through the firing step described below. The aluminum hydroxide and aluminum sulfate are preferably mixed and added so that the aluminum contained in the aluminum hydroxide and aluminum sulfate is 0.01 mol % or more and 2.0 mol % or less relative to the total molar amount of the lithium transition metal composite oxide.

[0044] Aluminum hydroxide has a lower melting point than aluminum sulfate. Therefore, the Al contained in aluminum hydroxide melts at low temperatures during the firing step described below and tends to spread throughout the lithium transition metal composite oxide. In other words, if aluminum hydroxide alone is added without adding aluminum sulfate during the mixing step, Al is dissolved deep into the lithium transition metal composite oxide, and the amount of Al dissolved in the surface layer of the lithium transition metal composite oxide decreases. As a result, the heat resistance cannot be sufficiently improved.

[0045] Furthermore, aluminum sulfate has a higher melting point than aluminum hydroxide, and therefore tends to be less likely to dissolve in the firing step described below. In other words, if aluminum sulfate alone is added without adding aluminum hydroxide in the mixing step, the aluminum sulfate will not dissolve sufficiently, making it difficult for Al to be dissolved in the surface layer of the lithium transition metal composite oxide. As a result, the heat resistance cannot be sufficiently improved.

[0046] As a result of investigations by the present inventors, it was found that by adjusting the molar ratio of aluminum hydroxide to aluminum sulfate to 1:9 to 9:1 in the mixing step, a sufficient amount of Al is dissolved in the surface layer of the lithium transition metal composite oxide after the firing step. Although the detailed mechanism is unclear, by mixing at the above ratio, aluminum hydroxide and aluminum sulfate form a eutectic point, and melting begins at a temperature range that is higher than that of aluminum hydroxide alone but lower than that of aluminum sulfate alone. As a result, in the firing step, Al does not dissolve deep into the lithium transition metal composite oxide, but is more likely to dissolve in the surface layer. As a result, heat resistance can be improved.

[0047] 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, a sufficient amount of Al is easily dissolved in the surface layer of the lithium transition metal composite oxide. As a result, the heat resistance can be further improved.

[0048] In the calcination step, the mixture obtained in the mixing step is calcined at a predetermined temperature for a predetermined time to obtain the positive electrode active material of this embodiment. The calcination temperature is 750°C or higher, preferably 750°C to 800°C, and more preferably 770°C to 790°C, from the viewpoint of dissolving Al throughout the surface layer of the lithium transition metal composite oxide and setting the crystal lattice strain of the lithium transition metal composite oxide to 0.1% to 0.25%. If the calcination temperature is lower than 750°C, it may be difficult to dissolve Al throughout the surface layer of the lithium transition metal composite oxide. Furthermore, if the calcination temperature is lower than 750°C, the crystal lattice strain of the positive electrode active material may exceed 0.25%. Furthermore, if the calcination temperature is higher than 800°C, the crystal lattice strain of the positive electrode active material may be less than 0.1%. The calcination temperature refers to the maximum temperature during calcination. The calcination time is preferably, for example, 1 hour to 24 hours. The firing is preferably carried out in an oxygen stream.

[0049] Fig. 2 shows, as an example of an embodiment, spectra obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays of lithium transition metal composite oxides prepared at firing temperatures of 755° C. and 785° C. For comparison, Fig. 2 also shows the spectrum of a lithium transition metal composite oxide to which no aluminum hydroxide or aluminum sulfate was added in the mixing step.

[0050] As shown in Figure 2, the lithium transition metal composite oxides prepared at firing temperatures of 755°C and 785°C have a maximum peak in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV. That is, the peak in the range of 1557 eV to 1559 eV is larger than the peak in the range of 1560 eV to 1562 eV. On the other hand, the sample to which aluminum hydroxide and aluminum sulfate were not added does not have a maximum peak in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV. This indicates that adding aluminum hydroxide and aluminum sulfate in a predetermined ratio and setting the firing temperature to 750°C or higher can form a solid solution of Al in the surface layer of the primary particles of the lithium transition metal composite oxide.

[0051] [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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] [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.

[0056] 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.

[0057] [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.

[0058] 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).

[0059] 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).

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] Examples of phenol compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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 way 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 lithium transition metal composite oxide. The molar ratio of aluminum hydroxide to aluminum sulfate was 9:1.

[0070] In the calcination step, the mixture was calcined in an oxygen stream at 775° C. for 15 hours to obtain a lithium transition metal composite oxide as a positive electrode active material.

[0071] The sulfate ion content of the lithium transition metal composite oxide was measured using the above-mentioned measurement method, and was found to be 0.06 mol% of the total molar amount of the lithium transition metal composite oxide. The crystal lattice distortion of the positive electrode active material was also measured using the above-mentioned measurement method, and was found to be 0.22%. Furthermore, the spectrum of the prepared lithium transition metal composite oxide obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays was measured, and it was confirmed that the maximum peak was in the range of 1557 eV to 1559 eV, within the range of 1555 eV to 1565 eV.

[0072] [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.

[0073] [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.

[0074] [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).

[0075] [Evaluation of heat resistance] In an environment of 25°C, the battery was charged at 0.5 mA / cm until the battery voltage reached 4.2 V. ２ The battery was then disassembled, the positive electrode was removed, and 1 mg of the positive electrode mixture layer was scraped off and sealed in a pressure-resistant sealed container together with 1 μL of non-aqueous electrolyte to prepare a measurement sample. The measurement sample was heated from 25 ° C. to 550 ° C. at a heating rate of 10 ° C. / min using a differential scanning calorimeter (DSC), and the heat generation initiation temperature was measured. Note that a higher heat generation initiation temperature indicates higher heat resistance.

[0076] 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 sulfate ion content of the lithium transition metal composite oxide was measured using the above-described measurement method, and was found to be 0.10 mol% of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.23%. Furthermore, the spectrum of the prepared lithium transition metal composite oxide obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays was measured, and it was confirmed that the maximum peak was in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV.

[0077] 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 1:9 in the mixing step of preparing the positive electrode active material. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-described measurement method, and was found to be 0.22 mol% of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.23%. Furthermore, the spectrum of the prepared lithium transition metal composite oxide obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays was measured, and it was confirmed that the maximum peak was in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV.

[0078] 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 lithium transition metal composite oxide, and the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-described measurement method and found to be 0.05 mol % of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method and found to be 0.23%. Furthermore, the spectrum of the prepared lithium transition metal composite oxide obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays was measured and found to have a maximum peak in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV.

[0079] 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 lithium transition metal composite oxide, and the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-described measurement method and found to be 0.19 mol % of the total molar amount of the lithium transition metal composite oxide. The crystal lattice distortion of the positive electrode active material was also measured using the above-described measurement method and found to be 0.23%. Furthermore, the spectrum of the prepared lithium transition metal composite oxide obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays was measured, and it was confirmed that the maximum peak was in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV.

[0080] 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 sulfate ion content of the lithium transition metal composite oxide was measured using the above-mentioned measurement method, and was found to be 0.08 mol% of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-mentioned measurement method, and was found to be 0.21%. Furthermore, when the spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays of the produced lithium transition metal composite oxide was measured, it was confirmed that the maximum peak was in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV.

[0081] 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 785°C. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-mentioned measurement method, and was found to be 0.10 mol% of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-mentioned measurement method, and was found to be 0.21%. Furthermore, when the spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays of the prepared lithium transition metal composite oxide was measured, it was confirmed that the maximum peak was in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV.

[0082] 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 0:10, i.e., no aluminum hydroxide was added, during the mixing step of preparing the positive electrode active material. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-described measurement method, and was found to be 0.23 mol% of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.24%. Furthermore, when the spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays of the prepared lithium transition metal composite oxide was measured, it was found that within the range of 1555 eV to 1565 eV, there was no maximum peak in the range of 1557 eV to 1559 eV, but there was a maximum peak in the range of 1560 eV to 1562 eV. From this, it is presumed that in the prepared lithium transition metal composite oxide, Al is not dissolved in the surface layer of the lithium transition metal composite oxide, but is fixed to the surface of the lithium transition metal composite oxide.

[0083] Comparative 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 10:0, i.e., no aluminum sulfate was added, during the mixing step of preparing the positive electrode active material. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-described measurement method, and was found to be 0.01 mol% of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-described measurement method, and was found to be 0.23%. Furthermore, the spectrum of the prepared lithium transition metal composite oxide obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays was measured, and it was confirmed that the maximum peak was in the range of 1557 eV to 1559 eV within the range of 1555 eV to 1565 eV.

[0084] 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 aluminum hydroxide to aluminum sulfate was 7:3, and in the baking step of preparing the positive electrode active material, the baking temperature was 730°C. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-mentioned measurement method, and was 0.07 mol% of the total molar amount of the lithium transition metal composite oxide. Furthermore, the crystal lattice distortion of the positive electrode active material was measured using the above-mentioned measurement method, and was 0.27%. Furthermore, when the spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays of the prepared lithium transition metal composite oxide was measured, it was found that within the range of 1555 eV to 1565 eV, there was no maximum peak in the range of 1557 eV to 1559 eV, but there was a maximum peak in the range of 1560 eV to 1562 eV. From this, it is presumed that in the prepared lithium transition metal composite oxide, Al is not dissolved in the surface layer of the lithium transition metal composite oxide, but is fixed to the surface of the lithium transition metal composite oxide.

[0085] Comparative 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 0.001 mol % of the total molar amount of the lithium transition metal composite oxide, and the molar ratio of aluminum hydroxide to aluminum sulfate was 7:3. The sulfate ion content of the lithium transition metal composite oxide was measured using the above-described measurement method, and was found to be 0.00 mol % of the total molar amount of the lithium transition metal composite oxide. 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%. Furthermore, when the spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays of the prepared lithium transition metal composite oxide was measured, it was found that within the range of 1555 eV to 1565 eV, there was no maximum peak in the range of 1557 eV to 1559 eV, but there was a maximum peak in the range of 1560 eV to 1562 eV. From this, it is presumed that in the prepared lithium transition metal composite oxide, Al is not dissolved in the surface layer of the lithium transition metal composite oxide, but is fixed to the surface of the lithium transition metal composite oxide.

[0086] Table 1 shows the heat generation initiation temperatures of the test cells of the examples and comparative examples.

[0087]

[0088] As shown in Table 1, the test cells of the examples exhibited high heat generation initiation temperatures. From this, it can be said that heat resistance can be improved by dissolving Al in the surface layers of the primary particles of the lithium transition metal composite oxide, setting the crystal lattice distortion to 0.1% or more and 0.25% or less, having a maximum peak in the range of 1557 eV or more and 1559 eV or less within the range of 1555 eV or more and 1565 eV or less in a spectrum obtained by hard X-ray photoelectron spectroscopy, and containing sulfate ions in an amount of 0.04 mol % or more of the total molar amount of the lithium transition metal composite oxide.

[0089] On the other hand, the test cells of Comparative Examples 1 and 2, in which only either aluminum hydroxide or aluminum sulfate was added, Comparative Example 3, in which the firing temperature was less than 750°C, and Comparative Example 4, in which the amounts of aluminum hydroxide and aluminum sulfate added were extremely small, were unable to form a sufficient solid solution of Al in the surface layer of the lithium transition metal composite oxide, and therefore had low heat generation initiation temperatures.

[0090] The present disclosure is further described by the following embodiments. Configuration 1: A positive electrode active material comprising a lithium transition metal oxide containing Ni, the lithium transition metal composite oxide being secondary particles formed by aggregation of primary particles, the proportion of Ni in the lithium transition metal oxide being 70 mol % or more relative to the total number of moles of metal elements excluding Li, Al being dissolved in a surface layer of the primary particles of the lithium transition metal composite oxide, the crystal lattice distortion of the lithium transition metal composite oxide being 0.1% or more and 0.25% or less, and the spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays having a maximum peak in the range of 1557 eV or more and 1559 eV or less within the range of 1555 eV or more and 1565 eV or less, and the lithium transition metal composite oxide containing sulfate ions in an amount of 0.04 mol % or more of the total molar amount of the lithium transition metal composite oxide. Aspect 2: The cathode active material according to Aspect 1, wherein the content of sulfate ions contained in the lithium transition metal composite oxide is 0.04 mol % or more and 2.0 mol % or less of the total molar amount of the lithium transition metal composite oxide. 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 a cathode active material precursor; 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, and the calcination temperature in the calcination step is 750°C or more. Aspect 4: The method for producing a cathode active material according to Aspect 3, wherein the calcination temperature in the calcination step is 750°C or more and 800°C or less.

[0091] 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 comprising a lithium transition metal oxide containing Ni, wherein the lithium transition metal composite oxide is secondary particles formed by aggregation of primary particles, a ratio of Ni in the lithium transition metal oxide is 70 mol % or more relative to the total number of moles of metal elements excluding Li, Al is dissolved in a surface layer of the primary particles of the lithium transition metal composite oxide, a crystal lattice distortion of the lithium transition metal composite oxide is 0.1% or more and 0.25% or less, and a spectrum obtained by hard X-ray photoelectron spectroscopy using 6.0 keV hard X-rays has a maximum peak in the range of 1557 eV or more and 1559 eV or less within the range of 1555 eV or more and 1565 eV or less, and the lithium transition metal composite oxide contains sulfate ions in an amount of 0.04 mol % or more of the total molar amount of the lithium transition metal composite oxide.

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

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 calcination step of calcining the mixture obtained in the mixing step, wherein in the mixing step, the molar ratio of aluminum hydroxide to aluminum sulfate is 1:9 to 9:1, and the calcination temperature in the calcination step is 750°C or higher.

4. The method for producing a positive electrode active material according to claim 3, wherein the firing temperature in the firing step is 750°C or higher and 800°C or lower.

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