Secondary batteries

Abstract

Provided is a positive electrode active material which achieves both high capacity and safety. This secondary battery has a positive electrode. The positive electrode includes a positive electrode active material. The positive electrode active material contains a lithium cobalt oxide containing magnesium, nickel, and aluminum. When the positive electrode is analyzed by powder X-ray diffraction using a CuKα1 ray source at a depth of charge of 0.8 or greater, diffraction peaks are present at 2θ=19.30±0.20° and 2θ=45.55±0.10° in the positive electrode active material. The positive electrode active material has a first region including a surface parallel to the (001) plane and a second region including a surface parallel to a plane intersecting the (001) plane. The concentration of nickel in the first region is higher than the concentration of nickel in the second region.

Description

Secondary battery, positive electrode active material, and method for producing positive electrode active material

[0001] The present invention relates to a product, a method, or a manufacturing method thereof. One embodiment of the present invention relates to a secondary battery, a cathode active material, and a manufacturing method of the cathode active material. Note that the present invention is not limited to the above fields, and relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, or an electronic device, or a manufacturing method thereof.

[0002] In recent years, demand for high-power, high-capacity lithium-ion secondary batteries (also referred to as lithium-ion batteries) has rapidly expanded, and they have become indispensable in modern society as a reusable energy source.

[0003] It is said that it is difficult to achieve both high capacity and safety in lithium-ion secondary batteries. For example, positive electrode active materials with a layered rock-salt crystal structure are expected to achieve high capacity because the lithium ion diffusion paths exist two-dimensionally within the crystal structure. However, positive electrode active materials with a layered rock-salt crystal structure are prone to thermal runaway if too many lithium ions are released during charging, which poses a safety issue.

[0004] As a positive electrode active material with a layered rock salt crystal structure, lithium cobalt oxide (LiCoO ２ ), or lithium nickel oxide (LiNiO ２ Lithium cobalt oxide has a layered rock salt crystal structure, and CoO ６ Because lithium ions can move two-dimensionally between the octahedral layers, lithium cobalt oxide also has good cycle characteristics. However, lithium cobalt oxide has a problem with phase changes that occur during charging and discharging. For example, if a certain amount of lithium ions are desorbed during charging, lithium cobalt oxide undergoes a phase change from hexagonal to monoclinic. Therefore, in order to maintain good cycle characteristics, the amount of lithium ions that can be desorbed from lithium cobalt oxide has been limited. To solve this problem, Patent Document 1 proposes a structure in which an additive element is added to lithium cobalt oxide.

[0005] Lithium nickel oxide also has a layered rock-salt crystal structure, so it is expected to have similar cycle characteristics to lithium cobalt oxide. Furthermore, nickel is cheaper than cobalt, and energy density can be improved in proportion to the nickel content, so lithium nickel oxide is being researched as a replacement for lithium cobalt oxide. However, lithium nickel oxide has issues with thermal stability and is less safe than lithium cobalt oxide, so it has not yet been put to practical use.

[0006] Furthermore, there is a problem caused by the change in the valence of nickel. Specifically, nickel is easily reduced to divalent during production, and since the ionic radius of nickel ions is close to that of lithium ions, nickel (divalent) substitutes for the site where lithium ions exist. This is called cation mixing. Due to cation mixing, the lithium content in lithium nickel oxide decreases, resulting in a low discharge capacity. Therefore, in order to improve the high energy density and cycle life, Patent Document 2 describes a method for producing LiCo obtained by a solid-phase method. ０．８ Ni ０．１ Mn ０．１ O ２ Furthermore, as shown in Non-Patent Document 1, LiNi １／２ Co １／２ 0 ２ Research is also being conducted on this.

[0007] Fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing and other processes, and their physical properties have been studied (see, for example, Non-Patent Document 2).

[0008] X-ray diffraction (XRD) is one of the techniques used to analyze the crystal structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) introduced in Non-Patent Document 3. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 4 can be referenced from the ICSD. For Rietveld analysis, the analysis program RIETAN-FP (Non-Patent Document 5) can be used. VESTA (Non-Patent Document 6) can be used as software for drawing crystal structures.

[0009] WO2020 / 026078 JP 2006-344509 A

[0010] Atsushi Ueda and Tsutomu Ohzuku, “Solid-State Redox Reactions of LiNi▲1 / 2▼Co▲1 / 2▼O▲2▼(R3-m) for 4 Volt “Secondary Lithium Cells” Journal of The Electrochemical Society, Vol. 141, No. 8, August 1994. W. E. Counts, R. Roy, and E. F. Osborn, “Fluoride Model Systems: II, The Binary Systems CaF▲2▼-BeF▲2▼, MgF▲2▼-BeF▲2▼, and LiF-MgF▲2▼”, Journal of the American Ceramic Society, 36 [1] 12-17 (1953). Belsky, A. et al. , “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Crystal. , (2002) B58 364-369. Akimoto, J. ; Gotoh, Y.; ; Oosawa, Y.; “Synthesis and structure refinement of LiCoO▲2▼ single crystals” Journal of Solid State Chemistry (1998) 141, p. 298-302. F. Izumi and K. Momma, Solid State Phenom. , 130, 15-20 (2007) K. Momma and F. Izumi, J. Appl. Cryst. (2011). 44, 1272-1276

[0011] Although it is possible to obtain a positive electrode active material according to the above-mentioned Patent Documents 1 and 2, there remains room for improvement in various aspects such as charge / discharge capacity, cycle characteristics, reliability, safety, and cost.

[0012] In view of the above, an object of one embodiment of the present invention is to provide a secondary battery, a positive electrode active material, and a manufacturing method thereof, which are stable at a high potential and / or a high temperature.Furthermore, an object of one embodiment of the present invention is to provide a secondary battery, a positive electrode active material, and a manufacturing method thereof, which are stable at a high potential and / or a high temperature.

[0013] Note that the description of the above-mentioned problems does not preclude the existence of other problems. Furthermore, problems other than the above-mentioned problems can be extracted from the description of the specification, drawings, and claims. One embodiment of the present invention does not necessarily solve all of the above-mentioned problems, but solves at least one of the above problems.

[0014] One aspect of the present invention is a secondary battery having a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material has a surface layer portion and an interior portion, the interior portion having at least cobalt and nickel, and the surface layer portion having at least cobalt and an additive element, wherein in the positive electrode active material, a ratio of nickel to the sum of cobalt and nickel, Ni / (Co+Ni), is greater than 0 and less than 0.05, and the additive element is one or more elements selected from magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium.

[0015] In one embodiment of the present invention, the positive electrode active material preferably has a single crystal structure.

[0016] In one aspect of the present invention, the positive electrode active material preferably has a crystallite size of 150 nm or more as calculated from the XRD pattern.

[0017] Another aspect of the present invention is a secondary battery including a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material includes lithium nickel cobalt oxide, magnesium, aluminum, and fluorine, and the positive electrode active material has an atomic ratio of nickel to the sum of the atomic ratio of cobalt and the atomic ratio of nickel, Ni / (Co+Ni), which is greater than 0.005 and less than 0.05, and when the positive electrode active material is analyzed by XRD, the crystallite size is 420 nm or greater and 530 nm or less.

[0018] Another aspect of the present invention is a secondary battery including a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material includes lithium nickel cobalt oxide, magnesium, aluminum, and fluorine, and the positive electrode active material has an atomic ratio of nickel to the sum of the atomic ratio of cobalt and the atomic ratio of nickel, Ni / (Co+Ni), which is greater than 0.005 and less than 0.05, and the positive electrode active material is CuKα １ When analyzed by powder X-ray diffraction using X-rays, the secondary battery has a crystallite size of 420 nm or more and 530 nm or less in a discharged state, and when the secondary battery uses lithium metal as a counter electrode and is charged at 25°C until a charge voltage reaches 4.6 V, the secondary battery has diffraction peaks at 2θ = 19.30 ± 0.20° and 2θ = 45.55 ± 0.10°.

[0019] In one aspect of the present invention, the ratio of the number of atoms of cobalt to the number of atoms of nickel in the positive electrode active material preferably satisfies Co:Ni=99:1 or a value close to that.

[0020] Another aspect of the present invention is a secondary battery having a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material includes lithium, cobalt, nickel, oxygen, and an additive element, and the positive electrode is used as the positive electrode of a test battery having a counter electrode composed of lithium metal, and a test is conducted at 25°C in which CCCV charging with an upper limit voltage of 4.6 V and CC discharging with a lower limit voltage of 2.5 V are repeated 50 times, and the discharge capacity value measured at the 50th cycle is 190 mAh / g or more.

[0021] Another aspect of the present invention is a secondary battery including a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material includes lithium, cobalt, nickel, oxygen, and an additive element. The positive electrode is used as the positive electrode of a test battery having a counter electrode composed of lithium metal, and a test is conducted at 25°C, in which a CCCV charge with an upper limit voltage of 4.6 V and a CC discharge with a lower limit voltage of 2.5 V are repeated 50 times, and the value of the discharge capacity measured at the 50th cycle satisfies 98% or more and less than 100% of the maximum value of the discharge capacity during the 50 cycles.

[0022] Another aspect of the present invention is a secondary battery having a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material includes lithium, cobalt, nickel, oxygen, and an additive element. The positive electrode is used as the positive electrode of a test battery having a counter electrode composed of lithium metal, and a test is conducted at 45°C, in which a CCCV charge with an upper limit voltage of 4.6 V and a CC discharge with a lower limit voltage of 2.5 V are repeated 50 times, and the discharge capacity value measured at the 50th cycle is 190 mAh / g or more.

[0023] Another aspect of the present invention is a secondary battery including a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material includes lithium, cobalt, nickel, oxygen, and an additive element. The positive electrode is used as the positive electrode of a test battery having a counter electrode composed of lithium metal, and a test is conducted at 45°C, in which a CCCV charge with an upper limit voltage of 4.6 V and a CC discharge with a lower limit voltage of 2.5 V are repeated 50 times, and the value of the discharge capacity measured at the 50th cycle satisfies 90% or more and less than 100% of the maximum value of the discharge capacity during the 50 cycles.

[0024] In one embodiment of the present invention, the additional element is preferably one or more elements selected from magnesium, aluminum, and fluorine.

[0025] Another aspect of the present invention is a secondary battery having a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material has lithium cobalt oxide containing magnesium, nickel, and aluminum, and the positive electrode active material is a secondary battery having a positive electrode active material and a negative electrode. １ The secondary battery has diffraction peaks at 2θ = 19.30 ± 0.20° and 2θ = 45.55 ± 0.10° when analyzed by powder X-ray diffraction according to a method described above, and the positive electrode active material has a first region including a surface parallel to the (00l) plane and a second region including a surface parallel to a plane other than the (00l) plane, and the nickel concentration in the first region is higher than the nickel concentration in the second region.

[0026] In one aspect of the present invention, the nickel concentration in the first region is preferably 0.7 atomic % or more and 2 atomic % or less, and the nickel concentration in the second region is preferably 0.3 atomic % or more and 1 atomic % or less.

[0027] Another aspect of the present invention is a secondary battery having a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material has lithium cobalt oxide containing magnesium, nickel, and aluminum, and a test battery using a positive electrode and lithium metal as a counter electrode, the test battery is charged to 4.6 V, and the positive electrode is irradiated with a CuKα radiation source. １ When analyzed by powder X-ray diffraction according to the method described above, the positive electrode active material has diffraction peaks at 2θ = 19.30 ± 0.20° and 2θ = 45.55 ± 0.10°, the positive electrode active material has a first region including a surface parallel to the (00l) plane and a second region including a surface parallel to a plane other than the (00l) plane, and the nickel concentration in the first region is higher than the nickel concentration in the second region.

[0028] In one aspect of the present invention, it is preferable that the nickel concentration in the first region is 0.7 atomic % or more and 2 atomic % or less, and the nickel concentration in the second region is 0.3 atomic % or more and 1 atomic % or less.

[0029] In one aspect of the present invention, the positive electrode active material preferably further contains fluorine.

[0030] In one embodiment of the present invention, the positive electrode preferably further includes a fibrous conductive material.

[0031] In one embodiment of the present invention, the negative electrode preferably contains graphite.

[0032] Another embodiment of the present invention is a positive electrode active material having a surface layer portion and an interior portion, the interior portion including at least cobalt and nickel, the surface layer portion including at least cobalt and an additional element, the atomic ratio of cobalt being higher than the atomic ratio of nickel, the additional element being present in a width of 2 nm to 30 nm, and the additional element being one or more selected from magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium.

[0033] Another aspect of the present invention is a method for producing a positive electrode active material, which comprises mixing an aqueous cobalt solution and an aqueous nickel solution to form a mixed solution, and reacting the mixed solution with an alkaline aqueous solution to coprecipitate a cobalt-nickel compound, wherein in the cobalt-nickel compound, the proportion of nickel in the sum of cobalt and nickel, Ni / (Co+Ni), is greater than 0 and less than 0.05.

[0034] Another aspect of the present invention is a method for producing a positive electrode active material, which includes mixing a cobalt aqueous solution and a nickel aqueous solution to form a mixed solution, reacting the mixed solution with an alkaline aqueous solution to form a cobalt-nickel compound, mixing the cobalt-nickel compound with a lithium compound, performing a first heat treatment to form a first composite oxide, mixing the first composite oxide with a compound having an additive element, and performing a second heat treatment, wherein in the cobalt-nickel compound, a ratio of nickel to the sum of cobalt and nickel, Ni / (Co+Ni), is greater than 0 and less than 0.05.

[0035] In one embodiment of the present invention, the additive element is preferably one or more elements selected from magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium.

[0036] Another aspect of the present invention is a method for producing a positive electrode active material, which includes forming a mixed solution in which a cobalt compound and a nickel compound are dissolved, reacting the mixed solution with an alkaline aqueous solution to form cobalt nickel hydroxide, mixing the cobalt nickel hydroxide with a lithium compound, and then performing a first heat treatment to form a first composite oxide. After crushing the first composite oxide, performing a second heat treatment to form a second composite oxide. After mixing the second composite oxide with a compound having an additive element, the method for producing a positive electrode active material includes performing a third heat treatment. In the cobalt nickel hydroxide, the atomic ratio of nickel to the sum of the atomic ratio of cobalt and the atomic ratio of nickel is greater than 0 and less than 0.05.

[0037] Another aspect of the present invention is a method for producing a positive electrode active material, comprising: forming a mixed solution in which a cobalt compound and a nickel compound are dissolved; reacting the mixed solution with an alkaline aqueous solution to form cobalt nickel hydroxide; mixing the cobalt nickel hydroxide with a lithium compound; performing a first heat treatment to form a first composite oxide; crushing the first composite oxide; performing a second heat treatment to form a second composite oxide; mixing the second composite oxide with a compound having a first additional element; performing a third heat treatment to form a third composite oxide; mixing the third composite oxide with a compound having a second additional element; and performing a fourth heat treatment;

[0038] In one embodiment of the present invention, it is preferable to react the mixed solution with an alkaline aqueous solution to obtain a suspension containing cobalt nickel hydroxide, to perform a first suction filtration of the suspension using water, and, after the first suction filtration, to perform a second suction filtration using an organic solvent to recover the cobalt nickel hydroxide.

[0039] In one aspect of the present invention, in the first composite oxide, the atomic ratio of lithium to the sum of the atomic ratios of cobalt and nickel is preferably 1.0 or more and 1.2 or less.

[0040] In one embodiment of the present invention, the additive element is preferably one or more elements selected from nickel, magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium.

[0041] In one aspect of the present invention, the first additional element or the second additional element is preferably one or more selected from nickel, magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium.

[0042] In one embodiment of the present invention, the temperature of the second heat treatment is preferably lower than the temperature of the first heat treatment.

[0043] In one embodiment of the present invention, the cobalt nickel hydroxide is preferably subjected to a drying step for 0.5 hours to 20 hours.

[0044] In one embodiment of the present invention, the cobalt nickel hydroxide is preferably subjected to a drying step for 12 hours or more and 20 hours or less.

[0045] According to one embodiment of the present invention, it is possible to provide a secondary battery, a cathode active material, and a method for manufacturing a cathode active material that are stable under high potential conditions and / or high temperature conditions. Also, according to one embodiment of the present invention, it is possible to provide a secondary battery, a cathode active material, and a method for manufacturing a cathode active material that are resistant to breakdown of a crystal structure even after repeated charge and discharge.

[0046] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc.

[0047] FIG. 1A shows a cathode active material, and FIGS. 1B and 1C show the distribution of additive elements. FIGS. 2A and 2B show an example of the distribution of additive elements contained in the cathode active material, and FIG. 2C is a diagram explaining the distribution of additive elements. FIG. 3 shows an example of a TEM image in which the crystal orientations are roughly consistent. FIGS. 4A and 4B show cathode active materials. FIGS. 5A to 5C show cathode active materials. FIG. 6 shows LiF and MgF ２FIG. 7 is a diagram illustrating a crystal structure of a positive electrode active material of one embodiment of the present invention. FIGS. 8A to 8C each illustrate a lattice constant calculated from XRD. FIG. 9 is a flowchart illustrating a manufacturing process for a positive electrode active material of one embodiment of the present invention. FIG. 10 is a flowchart illustrating a manufacturing process for a positive electrode active material of one embodiment of the present invention. FIGS. 11A to 11C are flowcharts illustrating a manufacturing process for a positive electrode active material of one embodiment of the present invention. FIG. 12 is a flowchart illustrating a manufacturing process for a positive electrode active material of one embodiment of the present invention. FIGS. 13A and 13B are flowcharts illustrating a manufacturing process for a positive electrode active material of one embodiment of the present invention. FIGS. 14A and 14B are cross-sectional views illustrating a positive electrode active material layer. FIGS. 15A and 15B are diagrams illustrating a solid electrolyte secondary battery. FIG. 16A is an exploded perspective view of a coin-type secondary battery, FIG. 16B is a perspective view of the coin-type secondary battery, and FIG. 16C is a cross-sectional perspective view thereof. FIG. 17A is a diagram illustrating an example of a cylindrical secondary battery. FIG. 17B is a diagram illustrating an example of a cylindrical secondary battery. FIG. 17C is a diagram showing an example of a plurality of cylindrical secondary batteries. FIG. 17D is a diagram showing an example of a power storage system having a plurality of cylindrical secondary batteries. FIGS. 18A and 18B are diagrams illustrating an example of a secondary battery, and FIG. 18C is a diagram showing the interior of a secondary battery. FIGS. 19A to 19C are diagrams illustrating an example of a secondary battery. FIGS. 20A and 20B are diagrams showing the appearance of a secondary battery. FIGS. 21A to 21C are diagrams illustrating a method of manufacturing a secondary battery. FIGS. 22A to 22D are diagrams illustrating an example of a transportation vehicle. FIG. 22E is a diagram illustrating an example of an artificial satellite. FIG. 23A is a diagram showing an electric bicycle, FIG. 23B is a diagram showing a secondary battery for the electric bicycle, and FIG. 23C is a diagram showing a scooter. FIGS. 24A to 24E are diagrams illustrating an example of an electronic device. FIG. 25A is a planar SEM image of a hydroxide, and FIG. 25B is a planar SEM image of an oxide. Figure 26A is a planar SEM image of the hydroxide, Figure 26B is a planar SEM image of the oxide, and Figure 26C is a cross-sectional SEM image of the oxide. Figure 27A is a planar SEM image of the hydroxide, Figure 27B is a planar SEM image of the oxide, and Figure 27C is a cross-sectional SEM image of the oxide. Figure 28A is a planar SEM image of the hydroxide, Figure 28B is a planar SEM image of the oxide, and Figure 28C is a cross-sectional SEM image of the oxide.FIG. 29A is a planar SEM image of the hydroxide, FIG. 29B is a planar SEM image of the oxide, and FIG. 29C is a cross-sectional SEM image of the oxide. FIG. 30A is a planar SEM image of the hydroxide, FIG. 30B is a planar SEM image of the oxide, and FIG. 30C is a cross-sectional SEM image of the oxide. FIG. 31 is a powder XRD pattern of the positive electrode active material. FIG. 32 is a charging XRD pattern of the positive electrode active material. FIG. 33 is an enlarged XRD pattern of a portion of FIG. 32. FIG. 34 is an enlarged XRD pattern of a portion of FIG. 32. FIGS. 35A and 35B are graphs showing the cycle characteristics of a secondary battery. FIGS. 36A and 36B are graphs showing the cycle characteristics of a secondary battery. FIGS. 37A and 37B are graphs showing the cycle characteristics of a secondary battery. FIGS. 38A and 38B are graphs showing the cycle characteristics of a secondary battery. FIGS. 39A and 39B are graphs showing the cycle characteristics of a secondary battery. 40A and 40B are graphs showing the cycle characteristics of a secondary battery. FIGS. 41A and 41B are graphs showing the cycle characteristics of a secondary battery. FIGS. 42A and 42B are graphs showing the cycle characteristics of a secondary battery. FIGS. 43A and 43B are graphs showing the cycle characteristics of a secondary battery. FIGS. 44A and 44B are graphs showing the results of STEM-EDX line analysis. FIGS. 45A and 45B are graphs showing the results of STEM-EDX line analysis. FIGS. 46A and 46B are graphs showing the results of STEM-EDX line analysis.

[0048] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0049] In this specification and the like, the positive electrode active material may be expressed as a composite oxide, a positive electrode material, a positive electrode material, a positive electrode material for lithium-ion secondary batteries, or the like. Furthermore, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably includes a compound. Furthermore, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably includes a composition. Furthermore, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably includes a composite.

[0050] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. In crystallography, space groups, crystal planes, and crystal directions are expressed by placing a superscript bar above the number. However, due to formatting constraints, in this specification, instead of placing a bar above the number, a minus sign (-) may be placed before the number. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. For ease of understanding the structure, trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice. Unless otherwise specified, the space group R-3m will also be expressed as a composite hexagonal lattice in this specification. Miller indices may also be expressed as (hkil) rather than (hkl). Here, i is −(h+k).

[0051] Furthermore, space groups are identified by XRD (X-ray diffraction), electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0052] Furthermore, if the anions have a structure in which three layers are stacked with a mutually offset, such as ABCABC, it is called a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. At the same time, since real crystals always have defects, the analysis results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM (transmission electron microscope) image, spots may appear at positions slightly different from the theoretical positions. For example, if the deviation between the theoretical position and orientation is 5° or less, or 2.5° or less, it can be said to have a cubic close-packed structure.

[0053] In this specification, the layered rock salt crystal structure refers to a crystal structure having a rock salt-type ion arrangement in which cations and anions are alternately arranged, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Strictly speaking, the layered rock salt crystal structure may have a distorted rock salt crystal lattice structure.

[0054] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0055] The theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the lithium ions that can be inserted and removed from the positive electrode active material are removed. For example, LiCoO ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 274mAh / g. ２ O ４ The theoretical capacity of LiCo is 148 mAh / g. （１−ｙ） Ni ｙ O ２ The theoretical capacity of the positive electrode active material expressed by (0<y<1) is set to 274 mAh / g.

[0056] The amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ MO ２ In this case, M is nickel and cobalt. In the case of a positive electrode active material in a lithium ion secondary battery, x = (theoretical capacity - charging capacity) / theoretical capacity. For example, LiMO ２ When a lithium ion secondary battery using as a positive electrode active material is charged at 219.2 mAh / g per weight of the positive electrode active material, Li ０．２ MO ２ Or we can say x = 0.2. ｘ MO ２In the above formula, "x" is small, for example, when 0.1<x≦0.24. The extent to which lithium has been released from the positive electrode active material relative to the theoretical capacity is sometimes referred to as the depth of charge. In this specification and elsewhere, the depth of charge corresponds to 1−x.

[0057] Li ｘ MO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above should be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data from a lithium-ion secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0058] When properly synthesized lithium cobalt nickel oxide is approximately stoichiometric before use in the positive electrode, Li ｘ MO ２ and x = 1. The lithium cobalt nickel oxide contained in the lithium ion secondary battery after discharge is also Li ｘ MO ２ It can be said that x = 1. The completion of discharge here refers to a state in which the voltage becomes 3.0 V or 2.5 V or less at a current of 100 mA / g or less per weight of the positive electrode active material.

[0059] In this specification and the like, the distribution of a certain element may refer to a region in which the element is continuously detected in a range that is not noise using a certain continuous analytical method.

[0060] Furthermore, in this specification and the like, when describing the characteristics of a positive electrode active material, it is not necessary for all of the positive electrode active materials in a secondary battery to have that characteristic. For example, when describing the characteristics of a coating of a positive electrode active material, for example, if 50% or more, preferably 70% or more, and more preferably 90% or more of the total number of three or more randomly selected positive electrode active materials have the characteristic of a coating (specifically, the surface on which the coating is formed is 50% or more, preferably 70% or more, and more preferably 90% or more of the positive electrode active material), it can be said that there is a sufficient effect of improving the characteristics of the positive electrode active material and a secondary battery containing it.

[0061] A short circuit in a lithium-ion secondary battery not only causes problems in the charging and / or discharging operations of the lithium-ion secondary battery, but can also lead to thermal runaway, heat generation, and fire. Short circuits can be classified into internal short circuits and external short circuits. In this specification, an internal short circuit in a lithium-ion secondary battery refers to contact between the positive electrode and the negative electrode inside the battery. An external short circuit in a lithium-ion secondary battery refers to contact between the positive electrode and the negative electrode outside the battery, which is assumed to occur due to misuse.

[0062] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in lithium-ion secondary batteries are described in their pre-degradation state. A decrease in discharge capacity due to aging and burn-in treatments during the manufacturing stage of lithium-ion secondary batteries is not considered to be degradation. For example, a lithium-ion secondary battery consisting of a single cell or a battery pack can be said to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. For lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0063] In this specification, a lithium ion secondary battery refers to a battery that uses lithium ions as carrier ions, but the carrier ions of the present invention are not limited to lithium ions. For example, alkali metal ions or alkaline earth metal ions can be used as carrier ions of the present invention, and specifically, sodium ions can be used. In this case, the present invention can be understood by replacing lithium ions with sodium ions. Furthermore, when there is no limitation on the carrier ions, the battery may be referred to as a secondary battery.

[0064] In this specification and the like, the active material may be referred to as an active material particle, but the shape may vary and is not limited to a particle shape. For example, the shape of the active material (active material particle) may be, in addition to a circle, an ellipse, a rectangle, a trapezoid, a triangle, a square with rounded corners, or an asymmetric shape in one cross section.

[0065] In this specification, the term "smooth surface of an active material" refers to a cross-section of the active material having a surface roughness of at least 10 nm or less, when surface irregularity information is quantified from measurement data. In this specification, the cross-section is, for example, a cross-section obtained when observing with a scanning transmission electron microscope (STEM) image.

[0066] In this specification etc., the term "A and / or B" may be used, but this is an example of a description that includes only A, only B, or both A and B.

[0067] In this specification, etc., secondary particles refer to particles formed by aggregation of primary particles. Also, in this specification, etc., primary particles refer to particles that do not have grain boundaries on their appearance. Also, in this specification, etc., single particles are particles that do not have grain boundaries on their appearance and have a single crystal.

[0068] Embodiment 1 In this embodiment, a positive electrode active material 100 of one embodiment of the present invention will be described with reference to FIGS.

[0069] 1A shows a cross-sectional view of a positive electrode active material 100 according to one embodiment of the present invention. As shown in FIG. 1A, the positive electrode active material 100 has a surface layer 100a and an interior 100c. In FIG. 1A, the boundary between the surface layer 100a and the interior 100c is indicated by a dashed line.

[0070] <Single Particle> The positive electrode active material 100 preferably has high crystallinity, and more preferably is a single crystal. That is, the positive electrode active material 100 preferably has a single particle. When the positive electrode active material 100 according to one embodiment of the present invention is a single particle, cracks are less likely to occur even when a volume change occurs in the positive electrode active material 100 due to charge and discharge, which is preferable. Furthermore, when the positive electrode active material 100 is a single particle, a secondary battery using the positive electrode active material 100 is thought to be less likely to ignite, thereby improving safety.

[0071] <Crystallite Size> The crystallite size can be determined, for example, by the following Scherrer formula.

[0072]

[0073] In addition, all diffraction peaks detected in the 2θ range of 15° to 90° can be used to calculate the crystallite size. After determining the crystallite size of each diffraction peak, correction can be applied, and the average crystallite size can be calculated. For correction, LiCoO ２ The literature value (ICSD coll.code.172909) can be used.

[0074] To increase the crystallite size, excess lithium can be added and heated. However, excess lithium can cause gelation of the binder during electrode fabrication, such as the positive electrode. To avoid this drawback, it is advisable to set an upper limit on the crystallite size. For example, by setting the crystallite size calculated from the XRD diffraction pattern to 600 nm or less, preferably 500 nm or less, it is possible to avoid the above drawbacks.

[0075] For example, the crystallite size of the positive electrode active material 100 calculated from the half-width of the XRD diffraction pattern is preferably 250 nm or more, and more preferably 420 nm or more. The range of the crystallite size can be determined by any combination of the above paragraph and this paragraph.

[0076] The XRD diffraction pattern for calculating the half-width is preferably obtained from the positive electrode active material alone, but may also be obtained from the positive electrode including the positive electrode active material, a current collector, a binder, a conductive material, etc. However, in the positive electrode state, the positive electrode active material may be oriented due to the influence of pressure, etc., during the manufacturing process. If the orientation is strong, accurate calculation of the crystallites may not be possible. Therefore, it is more preferable to obtain the pattern by removing the positive electrode active material layer from the positive electrode, removing some of the binder, etc., in the positive electrode active material layer using a solvent, etc., and then filling the sample into a sample holder.

[0077] <XRD> The measurement conditions for the above XRD will be explained. The device and conditions for XRD measurement are not particularly limited as long as the device is properly adjusted and calibrated using a standard sample. For example, the measurement can be performed using the following device and conditions. XRD device: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): Set to 0.01° Counting time: 1 second / step Sample stage rotation: 15 rpm The standard sample used for adjustment and calibration can be, for example, NIST (National Institute of Standards and Technology) standard aluminum oxide sintered plate SRM 1976.

[0078] If the measurement sample is a powder such as a positive electrode active material, it is set by placing it on a glass sample holder, or by sprinkling the sample on a greased silicone non-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode is attached to the substrate with double-sided tape, and the positive electrode active material layer on the positive electrode is set to match the measurement surface required by the device.

[0079] The characteristic X-rays may be monochromated using a filter or by using XRD data analysis software after obtaining an XRD diffraction pattern. For example, CuKα ２ Excluding the peak due to the line, CuKα １ It is possible to extract only the line peaks. The software can also be used to remove background noise.

[0080] The obtained XRD diffraction pattern can be analyzed using crystal structure analysis software (such as TOPAS) to calculate the crystallites.

[0081] <Surface Layer Portion> In this specification and the like, the surface layer portion 100a of the positive electrode active material 100 refers to, for example, a region within 200 nm from the surface toward the inside, preferably within 100 nm, more preferably within 50 nm, and even more preferably within 20 nm. The surface layer portion is synonymous with the surface vicinity or the surface vicinity region.

[0082] <Surface> Because the positive electrode active material 100 is a composite oxide capable of inserting and desorbing carrier ions, typically lithium ions, it does not contain carbonates, hydroxyl groups, or the like that are chemically adsorbed after production. It also does not contain electrolytes, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material 100. Therefore, the surface of the positive electrode active material 100 refers to the surface of a composite oxide capable of inserting and desorbing carrier ions, typically lithium ions, and the above-mentioned members that cannot be called composite oxides do not constitute the surface of the positive electrode active material 100. Furthermore, surfaces that occur in the positive electrode active material 100 due to cracks can also be considered the surface.

[0083] <Interior> The region deeper than the surface layer portion 100a of the positive electrode active material 100 is called the interior 100c. The interior 100c is synonymous with the interior region or core.

[0084] <Containing Elements> The positive electrode active material 100 needs to contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when carrier ions, typically lithium ions, are inserted and removed. The positive electrode active material 100 of one embodiment of the present invention may contain a plurality of transition metals, and it is preferable to use nickel in addition to cobalt. A composite oxide containing both cobalt and nickel as transition metals may be called lithium nickel cobalt oxide or lithium cobalt nickel oxide, and has the composition formula LiCo １−ｙ Ni ｙ O ２ It can be expressed as follows. １−ｙ Ni ｙ O ２ has a layered rock-salt crystal structure and belongs to the space group R-3m. Because the positive electrode active material 100 according to one embodiment of the present invention contains nickel, deviation of the layered structure of the layered rock-salt crystal structure is suppressed, or the crystal structure is stabilized.

[0085] The positive electrode active material 100 of one embodiment of the present invention preferably uses cobalt as a main component of a transition metal. In this specification and the like, the main component of a transition metal refers to a transition metal having the highest atomic ratio among a plurality of transition metals contained in the positive electrode active material 100. Specifically, LiCo １−ｙ Ni ｙ O ２In the formula, y is preferably more than 0 and less than 0.1, more preferably more than 0 and 0.05 or less, and even more preferably more than 0 and 0.01 or less. When y has such a value, the effects of the additive elements described below become significant.

[0086] LiCo １−ｙ Ni ｙ O ２ In this case, when y is 0.01, for example, when expressed as the atomic ratio of the transition metals in the positive electrode active material 100, it can be expressed as Co:Ni = 99:1. Therefore, in the positive electrode active material 100, it is preferable that Co:Ni = 99:1 or its vicinity is satisfied. Furthermore, when y is 0.05, it can be expressed as Co:Ni = 95:5, and in the positive electrode active material 100, it is preferable that Co:Ni = 95:5 or its vicinity is satisfied. Note that the above-mentioned vicinity used in the atomic ratio ratio includes 0.9 times or more and 1.1 times or less of that value. When such an atomic ratio is satisfied, the effect of the added element described below becomes significant.

[0087] In the positive electrode active material 100 of one embodiment of the present invention, it can be said that the atomic ratio of nickel to the sum of the atomic ratio of cobalt and the atomic ratio of nickel (referred to as Ni / (Co+Ni)) is preferably greater than 0 and less than 0.1. It is more preferable that Ni / (Co+Ni) is greater than 0 and 0.05 or less. It is even more preferable that Ni / (Co+Ni) is greater than 0 and 0.01 or less. It is also preferable that Ni / (Co+Ni) is greater than 0.005 and less than 0.1. It is even more preferable that Ni / (Co+Ni) is greater than 0.005 and 0.05 or less. It is even more preferable that Ni / (Co+Ni) is greater than 0.005 and 0.01 or less. When such atomic ratios are satisfied, the effects of the additive elements described below become significant.

[0088] In the positive electrode active material 100 of one embodiment of the present invention, when line analysis is performed using scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDX), nickel is preferably located in the interior 100c of the positive electrode active material 100. To locate nickel in the interior 100c in this manner, a coprecipitation method or the like is preferably used.

[0089] The EDX measurement device may be attached to a scanning electron microscope (SEM), but it is preferable to use an EDX measurement device attached to an STEM for line analysis of the positive electrode active material 100 .

[0090] In the EDX method, some elements cannot be detected depending on the measurement device, measurement conditions, and the element to be measured. When the content is greater than 0 and less than 1 atomic % (sometimes referred to as at%), the element may not be detected. In this case, the element can be analyzed in combination with other analytical methods, such as elemental analysis of the entire particle using ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry) detects impurity elements, so detection is on the order of ppm wt. For example, Ni detected at 140 ppm wt can be converted to 0.014 wt%. Furthermore, when the composition of lithium cobalt oxide is LiCoO ２ When the Ni content can be expressed as 0.006 atomic %, the Ni content of 140 ppm by weight in the lithium cobalt oxide can be converted to 0.006 atomic %.

[0091] In the positive electrode active material 100 of one embodiment of the present invention, lithium, which is a carrier ion, may be in excess or deficient. Therefore, the atomic ratio of lithium in the positive electrode active material 100 is not limited in any way. In other words, the composition of the positive electrode active material 100, where M is a transition metal such as cobalt and / or nickel, is not strictly limited to Li:M:O=1:1:2. However, according to the examples described later, it is clear that the atomic ratio of lithium to the atomic ratio of cobalt is preferably 1.0 or more and 1.09 or less.

[0092] Furthermore, it is more preferable that the positive electrode active material 100 of one embodiment of the present invention has a unique crystal structure during charging. The unique crystal structure will be described later. A secondary battery using such a positive electrode active material 100 is less susceptible to deterioration after charging and discharging, and is safer because it is less likely to catch fire.

[0093] <Additive Element> The positive electrode active material 100 of one embodiment of the present invention preferably contains an additive element. In other words, the positive electrode active material 100 more preferably contains lithium, cobalt, nickel, oxygen, and an additive element. In other words, lithium cobalt nickel oxide (LiCo １−ｙ Ni ｙ O ２ The additive element may be one or more selected from the group consisting of magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium.

[0094] If the additive element is too little, it will not be able to sufficiently chemically stabilize the positive electrode active material 100, but if it is too much, there is a concern that it may have an adverse effect on the discharge capacity, etc. Therefore, for example, the positive electrode active material 100 containing the additive element A may be １−ｙ−ｚ Ni ｙ O ２ A ｚ In this case, z is preferably greater than 0 and equal to or less than 0.3. It is more preferable that z is greater than 0 and equal to or less than 0.1, and even more preferably greater than 0 and equal to or less than 0.05.

[0095] The additive element is preferably in a solid solution in the positive electrode active material 100. Alternatively, the additive element is preferably substituted for any of the transition metal, oxygen, and lithium sites that constitute the positive electrode active material 100. When STEM-EDX line analysis is performed on the positive electrode active material 100, the additive element present in such a state is determined to be located inside the positive electrode active material 100. In other words, when STEM-EDX line analysis is performed on the positive electrode active material 100, the position where the count number of the additive element begins to increase is preferably deeper than the position where the count number of the transition metal, such as cobalt or nickel, begins to increase.

[0096] When STEM-EDX linear analysis is performed on the positive electrode active material 100, the distribution of magnesium among the added elements is preferably such that the amount detected in the surface layer 100a is greater than the amount detected in the interior 100c. Furthermore, it is preferable that the peak of the detected amount is in a region of the surface layer 100a closer to the surface. In this specification and the like, the "peak of the detected amount" refers to the maximum value of the intensity on the vertical axis in the EDX linear analysis chart. Furthermore, in the above chart, the vertical axis may be represented as the number of counts, and the "peak of the detected amount" may refer to the maximum value of the count number in the EDX linear analysis chart.

[0097] When STEM-EDX line analysis is performed on the positive electrode active material 100, the distribution of fluorine, one of the added elements, is preferably such that the amount detected in the surface layer 100a is greater than the amount detected in the interior 100c. Furthermore, it is preferable that the peak of the detected amount of fluorine is in a region of the surface layer 100a closer to the surface. The fluorine distribution may overlap with the magnesium distribution. The terms "distribution" and "overlap" include the positions of the peaks of the detected amount coinciding with each other. Furthermore, the fluorine distribution does not have to overlap with the magnesium distribution.

[0098] Of the added elements, the distribution of aluminum preferably has a peak of detected amount inside compared to magnesium when STEM-EDX line analysis is performed on the positive electrode active material 100. The reason why aluminum is distributed inside compared to magnesium is thought to be because aluminum diffuses more easily than magnesium. On the other hand, the reason why the detected amount of aluminum is low in the region closest to the surface is presumed to be because aluminum is more stable in regions where magnesium and the like are not dissolved at high concentrations than in regions where they are not.

[0099] The aluminum distribution may overlap with the magnesium distribution, or the aluminum distribution and the magnesium distribution may barely overlap. The peak of the detected amount of aluminum may be present in the surface layer 100a, or may be present deeper than the surface layer 100a. For example, it is preferable that the detected amount peak be in a region of 5 nm to 30 nm from the surface toward the interior.

[0100] As described above, the additive element does not need to have the same concentration gradient or distribution throughout the entire region of the surface layer portion 100 a of the positive electrode active material 100 .

[0101] The above-described additive elements can further stabilize the crystalline structure of the positive electrode active material 100 during charging, as will be described later. Of course, as long as the crystalline structure of the positive electrode active material 100 can be further stabilized during charging, the additive elements do not necessarily have to be included. In other words, the additive elements do not necessarily have to be magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, or gallium.

[0102] Nickel may be used as an additive element in the cathode active material 100 according to one embodiment of the present invention. The effects of nickel include suppressing the misalignment of the layered structure or stabilizing the crystal structure, as described above. To achieve these effects, nickel may be present in either the interior 100c or the surface layer 100a. The nickel concentration may be different between the interior 100c and the surface layer 100a. A higher nickel concentration in the interior 100c than in the surface layer 100a is preferred because the above-described effects are efficiently manifested in the interior 100c, which accounts for the majority of the cathode active material 100. A higher nickel concentration in the surface layer 100a than in the interior 100c is preferred because the above-described effects are efficiently manifested in the surface layer 100a, where degradation of the cathode active material 100 begins. One method for increasing the nickel concentration in the surface layer 100a is to add nickel as an additive element after forming a composite oxide.

[0103] With regard to the additive element other than nickel, the atomic ratio of the additive element is preferably less than 30 at %, more preferably less than 10 at %, and even more preferably less than 5 at %, with respect to the atomic ratio of the transition metal (the sum of the atomic ratios when there are multiple transition metals) contained in the positive electrode active material 100. As described above, it is desirable for the additive element to be located in the surface layer portion 100 a, and therefore it can be said that it is sufficient if at least the surface layer portion 100 a satisfies the atomic ratio of the additive element other than nickel described above.

[0104] The atomic ratio of the added elements can be determined by, for example, XPS (X-ray photoelectron spectroscopy) analysis or EPMA (electron probe microanalysis) in addition to EDX line analysis.

[0105] As described above, the presence of the additive element stabilizes the crystalline structure of the positive electrode active material, particularly during high-voltage charging. Furthermore, it is believed that the crystalline structure can be stabilized even when exposed to a high-temperature environment, for example, a 45°C environment, and even when charged at a high voltage. Secondary batteries using this positive electrode active material are less likely to catch fire, making them preferable.

[0106] <Crystalline Surface> In the cathode active material 100 shown in FIG. 1A, the inner portion 100c is a composite oxide having a layered rock-salt crystal structure, and both the surface portion 100a and the inner portion 100c have a surface parallel to the (001) plane. In this specification, the (001) plane and the (003) plane may be collectively referred to as the (001) plane. In this specification, the (001) plane may be referred to as the C-plane, basal plane, or the like. In addition, it can be said that the diffusion path of lithium ions in the cathode active material 100 exists along the basal plane. In this specification, the surface on which the diffusion path of lithium ions is exposed, i.e., the surface on which lithium is inserted and extracted, specifically, a surface other than the (001) plane, may be referred to as an edge plane.

[0107] <Differences in Distribution of Additional Element Depending on Crystal Plane> The distribution of the additional element does not necessarily have to be the same in all regions of the surface layer portion 100a of the positive electrode active material 100. For example, the distribution of the additional element may differ depending on the crystal plane. The regions marked A-B in FIG. 1A have surfaces parallel to the edge planes, and the regions marked C-D in FIG. 1A have surfaces parallel to the basal planes. A case will be described in which the distribution of the additional element differs for each of these regions. For example, in the region having a surface parallel to the edge plane in FIG. 1B, the concentration of the additional element increases toward the surface. Furthermore, in the region having a surface parallel to the basal plane in FIG. 1C, even if the additional element is the same, the concentration increases toward the boundary between the surface layer portion 100a and the interior 100c. In this way, even if the additional element is the same, the distribution may differ depending on the crystal plane.

[0108] This is thought to be due to the difference in the stability of the crystal plane. In the layered rock salt type crystal structure of R-3m, the cations are arranged parallel to the (001) plane. This crystal structure is arranged parallel to the (001) plane. ２ It can be said that the structure is composed of alternately stacked layers (M is cobalt and / or nickel) and lithium layers. Therefore, the diffusion path of lithium ions also exists on a plane parallel to the (001) plane. Furthermore, the main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane. MO ２The layer is relatively stable and no lithium ions are inserted or extracted, so the basal plane and the surface layer portion having the basal plane are stable.

[0109] On the other hand, the edge surfaces and the surface layer portion 100a having the edge surfaces expose the diffusion paths of lithium ions. Therefore, the edge surfaces and the surface layer portion 100a having the edge surfaces are important regions for maintaining the diffusion paths of lithium ions, and at the same time, they are prone to become unstable because lithium ions are first desorbed from them. Reinforcing such edge surfaces and the surface layer portion 100a having the edge surfaces with an additive element such as magnesium is important for maintaining the crystal structure of the entire positive electrode active material 100.

[0110] The additive element may also be introduced from the edge surface and the surface layer portion 100a having the edge surface by utilizing the diffusion path of lithium ions. To efficiently add the additive element to the edge surface and the surface layer portion 100a having the edge surface, the additive element may be added after the composite oxide is formed.

[0111] For example, magnesium is easily added to the edge surface and the surface layer portion 100a having the edge surface, and therefore, in the positive electrode active material 100, the concentration of magnesium may be higher on the edge surface shown in FIG. 1B than on the basal surface shown in FIG. 1C.

[0112] As will be described in the embodiment below, since the positive electrode active material 100 is obtained via a coprecipitation method, the distribution of the additive elements as shown in FIGS. 1B and 1C may be achieved by adding an additive element to the hydroxide, which is the precursor of the composite oxide. This is because a diffusion path for lithium ions may also be formed in the hydroxide. In this specification and the like, the hydroxide in the stage before the composite oxide is produced is called a precursor.

[0113] In order to maintain the crystalline structure of the entire positive electrode active material 100, it is preferable that the additive element is located in the surface layer portion 100a, where deterioration is likely to begin. Therefore, regardless of the timing at which the additive element is added, it is sufficient that the additive element is located in the surface layer portion 100a so as to stabilize the crystalline structure.

[0114] Next, an example of the profile of each added element when EDX ray analysis was performed in A-B attached to Fig. 1A is shown in Fig. 2A. Also, an example of the profile of each added element when EDX ray analysis was performed in C-D attached to Fig. 1A is shown in Fig. 2B. In this way, the added element does not need to have the same concentration gradient or distribution throughout the entire surface layer portion 100a of the positive electrode active material 100.

[0115] 2A and 2B, it is preferable that the detected amount of at least magnesium among the additive elements is greater in the surface layer portion 100a than in the interior portion 100c, i.e., the maximum count number of magnesium is detected in the surface layer portion 100a. Furthermore, it is preferable that magnesium has a narrow peak of detected amount in a region closer to the surface of the surface layer portion 100a. For example, it is preferable that the peak of detected amount is on the surface or within 3 nm from the reference point.

[0116] Although not shown, it is preferable that the detectable amount of fluorine in the surface layer portion 100a is greater than the detectable amount inside, as with magnesium. It is also preferable that the detectable amount peak in the surface layer portion 100a is closer to the surface. For example, it is preferable that the detectable amount peak is on the surface or within 3 nm from the reference point. Similarly, it is preferable that the detectable amount of titanium, silicon, phosphorus, boron, and / or calcium is greater than the detectable amount inside the surface layer portion 100a. It is also preferable that the detectable amount peak is on the surface or within 3 nm from the reference point.

[0117] Furthermore, it is preferable that at least aluminum, among the additive elements, has a peak of detection amount deeper than magnesium. As shown in Figures 2A and 2B, the distributions of magnesium and aluminum may partially overlap, or, as not shown, the distributions of magnesium and aluminum may barely overlap. The peak of detection amount of aluminum, i.e., the maximum count number, may be present in the surface layer 100a or may be deeper than the surface layer 100a. However, it is preferable that aluminum is distributed more deeply in the positive electrode active material 100 than magnesium. For example, it is preferable that the peak is present in a region of 5 nm to 30 nm from the surface or the reference point toward the interior. Furthermore, as shown in Figures 2A and 2B, it is preferable that the maximum count number of magnesium is greater than the maximum count number of aluminum.

[0118] The reason why aluminum is more concentrated in the interior than magnesium, i.e., in the deeper regions, is thought to be because aluminum has a faster diffusion rate than magnesium. On the other hand, the reason why the maximum count number of aluminum in the region closest to the surface is small is presumably because aluminum exists more stably in regions where magnesium and other elements are not present in solid solution at high concentrations than in regions where they are not.

[0119] More specifically, in the region of the layered rock salt type of space group R-3m or the cubic rock salt type where magnesium is dissolved at a high concentration, layered rock salt type LiAlO ２ Compared to the case of cobalt, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. ＋ is Mg ２＋ The valence change due to substitution to Co ３＋ From Co ２＋ However, since Al can only be trivalent, it is thought that it is difficult for it to coexist with magnesium in a rock salt or layered rock salt structure.

[0120] Although not shown, it is preferable that manganese has a peak of detectable amount inside magnesium, similar to aluminum.

[0121] In addition, the distribution of the additive element may not be a normal distribution. For example, as shown in FIG. 2C, the distribution of aluminum is Ａｌ When the sample is divided by the maximum amount of aluminum detected, the length of the skirt may differ between the surface side and the inner side. Ａｌ ) 1 / 5 height (1 / 5 Max Ａｌ ) is divided in half by a perpendicular line drawn from the maximum value to the horizontal axis, the peak width W ｓ The inner peak width W ｃ may be large.

[0122] <Approximate Matching> Due to the concentration gradient of the added element as described above, for example, the interior 100c may have a layered rock salt crystal structure, while the surface and surface layer 100a may have a rock salt crystal structure or a crystal structure having characteristics of both the rock salt and layered rock salt structures. In this case, it is preferable that the crystal structure continuously changes from the interior 100c toward the surface layer 100a. Alternatively, it is preferable that the crystal orientations of the surface layer 100a and the interior 100c are approximately the same.

[0123] 3 shows an example of a TEM image in which the orientation of the layered rock-salt crystals LRS in the interior 100c and the orientation of the rock-salt crystals RS in the surface layer 100a are roughly the same. In a high-resolution TEM image, contrast originating from the crystal plane is obtained. Due to the diffraction and interference of the electron beam, for example, when an electron beam is incident perpendicular to the c-axis of the layered rock-salt composite hexagonal lattice, a repetition of bright bands (bright strips) and dark bands (dark strips) is obtained, which are the contrast originating from the (0003) plane. Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in FIG. 3) are not aligned with each other. ＲＳ and L ＬＲＳ When the angle between the dark lines is 5° or less, or 2.5° or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5° or less, or 2.5° or less, it can be determined that the crystal orientations are roughly aligned.

[0124] In addition to TEM images, images reflecting the crystal structure can also be obtained using HAADF-STEM images, ABF-STEM images, and the like.

[0125] In HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium cobalt nickel oxide (R-3m) belonging to the space group R-3m, the atomic numbers of cobalt (atomic number 27) and nickel (atomic number 28) are large, so the electron beam is strongly scattered at the positions of the cobalt and nickel atoms, and the arrangement of the cobalt and nickel atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobalt nickel oxide with a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt and nickel atoms is observed perpendicular to the c-axis as a bright line or an arrangement of highly bright dots, while the arrangement of the lithium and oxygen atoms is observed as a dark line or a low-brightness region. When lithium cobalt nickel oxide contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements, it is also observed as a dark line or a low-brightness region.

[0126] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 or less or 2.5 or less, it can be determined that the atomic arrangements are roughly the same, i.e., the crystal orientations are roughly the same. Similarly, when the angle between the dark lines is 5 or less or 2.5 or less, it can also be determined that the crystal orientations are roughly the same.

[0127] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0128] Furthermore, it can be determined by electron beam diffraction, TEM images, cross-sectional STEM images, etc. that the surface layer 100a etc. has both characteristics of the layered rock salt type and the rock salt type crystal structure.

[0129] In cross-sectional STEM images, etc., when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis, layers observed with high brightness and layers observed with low brightness are observed alternately. In the case of the rock-salt type, there is no distinction in the cation sites, so such characteristics are not observed. In the case of a crystal structure that has the characteristics of both the rock-salt type and the layered rock-salt type, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, etc., and furthermore, metals with atomic numbers higher than that of lithium are present in part of the low-brightness layers, i.e., the lithium layers.

[0130] Layered rock salt crystals and the anions of rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there is a crystal plane where the orientation of the cubic close-packed structure formed by the anions is aligned. In this specification, when the orientation of the cubic close-packed structure formed by the anions is aligned in a layered rock salt crystal and a rock salt crystal, it may be said that the crystal orientations are approximately the same. In addition, having a three-dimensional structural similarity such that the crystal orientations are approximately the same, or having the same crystallographic orientation, is called topotaxis.

[0131] <Smooth surface> The surface of the positive electrode active material 100 is preferably smooth and has few irregularities. A smooth surface of the positive electrode active material 100 refers to a state in which there are few irregularities, the positive electrode active material 100 is rounded overall, and the corners are also rounded. Furthermore, a state in which there is little foreign matter adhering to the surface is called smooth. Foreign matter is thought to be a cause of irregularities, so it is preferable that no foreign matter adheres to the surface.

[0132] <Secondary Particles> The cathode active material 100 according to one embodiment of the present invention may be a secondary particle as long as the crystallites are large. FIGS. 4A and 4B show cross-sectional views of the cathode active material 100, which is a secondary particle and has primary particles with large crystallites. Also, FIGS. 4A and 4B show grain boundaries 101. The surface layer 100a may not be present in the vicinity of the grain boundaries 101 as shown in FIG. 4A , or may be present in the vicinity of or along the grain boundaries 101 as shown in FIG. 4B .

[0133] Large crystallites in the positive electrode active material 100 refer to large crystallites calculated from the half-width of the XRD diffraction pattern, which is equivalent to large primary particles. Large primary particles eliminate the need for a large number of primary particles to form a sintered body and become secondary particles. The positive electrode active material 100 of one embodiment of the present invention contains secondary particles with large crystallites, but these have few or no grain boundaries that can occur between primary particles. Furthermore, although cracks can occur due to grain boundaries, the positive electrode active material 100 of one embodiment of the present invention is expected to suppress the occurrence of cracks even when the volume of the positive electrode active material 100 changes during charge and discharge. Thus, secondary particles with large crystallites can achieve the same effect as the single particles described above.

[0134] 4A and 4B corresponds to, for example, an interface between adhered primary particles, a surface where the crystal orientation changes inside the positive electrode active material 100, i.e., a surface where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a surface containing many crystal defects, a surface where the crystal structure is disordered, etc. Crystal defects include defects that can be observed in a cross-sectional TEM (transmission electron microscope), a cross-sectional STEM image, etc., i.e., a structure in which other elements have entered between lattices, cavities, etc. The vicinity of the grain boundary 101 refers to at least a region of the primary particles within 10 nm from the grain boundary 101.

[0135] The crystal structure of the surface layer 100 a, the grain boundaries 101 , etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0136] The grain boundary 101 is one type of planar defect. Therefore, the grain boundary 101, like the surface layer 100a, is likely to become unstable and change in the crystal structure is likely to occur. Therefore, if the concentration of the added element at and near the grain boundary 101 is high, the change in the crystal structure can be more effectively suppressed, as will be described later. Therefore, it is preferable that the added element is located in the surface layer 100a as shown in FIG. 4B.

[0137] When the concentration of the additive element is high at the grain boundary 101 and in its vicinity, even if cracks occur in the positive electrode active material 100 of one embodiment of the present invention due to the grain boundary 101, the concentration of the additive element is high on the surface newly formed by the cracks or in its vicinity. Thus, the additive element can stabilize the crystal structure even in the surface portion formed by the cracks.

[0138] For example, the concentration of the additive element, typically the magnesium concentration, at and near the grain boundary 101 of the positive electrode active material 100 is preferably higher than that in other regions of the interior 100c. Also, the nickel concentration at and near the grain boundary 101 is preferably higher than that in other regions of the interior 100c.

[0139] When fluorine is used as the additive element, it is preferable that the fluorine concentration in and around the grain boundary 101 is higher than that in other regions of the interior 100 c, thereby improving the corrosion resistance to hydrofluoric acid even after cracks have occurred in the positive electrode active material.

[0140] <Coating> The positive electrode active material 100 may have a coating on at least a portion of its surface. Fig. 5A shows an example in which a coating 104 is provided on the positive electrode active material 100 shown in Fig. 1A. Figs. 5B and 5C show examples in which a coating 104 is provided on the positive electrode active material 100 shown in Fig. 4A and Fig. 4B, respectively.

[0141] The coating 104 is preferably formed, for example, by the deposition of decomposition products of the electrolyte solution during charge and discharge. Having a coating derived from the electrolyte solution on the surface of the positive electrode active material 100 is expected to improve charge and discharge cycle characteristics. This is due to reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing cobalt elution. The coating 104 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, a high-quality coating is easily obtained when LiBOB and / or SUN (suberonitrile) is used as part of the electrolyte. Therefore, a coating 104 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating and is therefore preferred. Furthermore, the coating 104 does not have to cover the entire positive electrode active material 100.

[0142] <Crystalline Structure> The positive electrode active material 100 of one embodiment of the present invention has a unique crystalline structure. The crystalline structure will be described in comparison with conventional lithium cobalt oxide. In describing the crystalline structure, the amount of desorbed lithium ions is represented by x, and the positive electrode active material 100 is represented by Li ｘ Co （１−ｙ） Ni ｙ O ２ The following description will be made with a focus on the value x. Note that the amount of desorption x is different from the amount of added lithium.

[0143] <Li ｘ Co （１−ｙ） Ni ｙ O ２ When x is 1 in the graph, the crystal structure of the positive electrode active material 100 according to one embodiment of the present invention is shown in FIG. ｘ Co （１−ｙ） Ni ｙ O ２ In the above formula, it is preferable that the positive electrode active material 100 has a layered rock-salt type crystal structure when x = 1. In particular, it is preferable that the inner portion 100c, which occupies the majority of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure belonging to the space group R-3m.

[0144] Figure 7 shows the layered rock salt type crystal structure with R-3m O3. In Figure 7, O3 is added under the space group, and this crystal structure has lithium occupying octahedral sites, and a layer consisting of a transition metal M (M is cobalt and / or nickel) and an octahedron of oxygen (hereinafter referred to as MO) in the unit cell. ２ Since there are three layers of MO3, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer refers to a structure in which an octahedral structure in which oxygen is six-coordinated to a transition metal M is continuous on a plane in an edge-sharing state. In addition, although Fig. 7 shows that all lithium sites are occupied by lithium ions, as mentioned above, ions of an added element, such as magnesium ions, may be located at the lithium sites.

[0145] The surface layer portion 100 a of the positive electrode active material 100 of one embodiment of the present invention is such that the MO in the inner portion 100 c remains even when lithium is released from the positive electrode active material 100 due to charging. ２The surface layer portion 100a preferably functions as a barrier film for the positive electrode active material 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the positive electrode active material 100, preferably reinforces the positive electrode active material 100. The term "reinforcement" used here refers to the ability to prevent oxygen desorption and / or MO ２ This means suppressing structural changes in the surface layer portion 100a and the interior portion 100c of the positive electrode active material 100, such as shifting of the layered structure, and / or suppressing decomposition of the organic electrolyte solution or the like on the surface of the positive electrode active material 100. Magnesium can suppress oxygen desorption from the surroundings, so the above reinforcement can be achieved by including at least magnesium as an additive element.

[0146] For example, the surface layer portion 100a may have a different crystal structure from the interior portion 100c. Furthermore, if the surface layer portion 100a has a crystal structure that is more stable at room temperature (25°C) than the interior portion 100c, the above-described reinforcement effect can be achieved, which is preferable. For example, at least a portion of the surface layer portion 100a of the positive electrode active material 100 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0147] The surface layer 100a is the region where lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 100c. In addition, the atoms on the surface of the particles of the positive electrode active material 100 in the surface layer 100a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and can be said to be a region where deterioration of the crystal structure is likely to begin. For example, in the surface layer 100a, MO ２ If the crystalline structure of the layered structure consisting of layers is displaced, the influence thereof is transmitted to the inner portion 100c, and the crystalline structure of the layered structure is also displaced in the inner portion 100c, which is thought to lead to deterioration of the crystalline structure of the entire positive electrode active material 100. On the other hand, if the surface layer portion 100a can be sufficiently stabilized, Li ｘ Co （１−ｙ） Ni ｙ O ２ Even when x in the internal 100c is small,２ Furthermore, the layered structure of the inner 100c can be made less fragile. ２ The misalignment of the layers can be suppressed.

[0148] Furthermore, as described above, the distribution of the additive element on the (001) plane of the positive electrode active material 100 may be different from that on the planes other than the (001) plane. ２ Since the layer is relatively stable, the surface of the positive electrode active material 100 is more stable if it is a (001) plane. Furthermore, surfaces other than the (001) plane exist on the surface of the positive electrode active material 100, and lithium ion diffusion paths are exposed on surfaces other than the (001) plane. This is thought to be because the additive element is less likely to distribute on the stable (001) plane than on surfaces other than the (001) plane, and is more likely to diffuse and distribute using the above diffusion paths on surfaces other than the (001) plane. While the main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane, surfaces other than the (001) plane expose the main diffusion path of lithium ions and are important regions for maintaining the lithium ion diffusion path. Furthermore, surfaces other than the (001) plane are prone to instability because they are the regions from which lithium ions are first desorbed. Therefore, in order to maintain the overall crystal structure of the positive electrode active material 100, it is preferable to reinforce surfaces other than the (001) plane.

[0149] Therefore, in the case of magnesium, the distribution in the (001) plane and the surface layer portion 100a having this plane preferably has a half-width of 5 nm to 150 nm, more preferably 10 nm to 100 nm, and even more preferably 20 nm to 80 nm. Furthermore, the distribution of magnesium in planes other than the (001) plane and the surface layer portion 100a having this plane preferably has a half-width of more than 150 nm to 280 nm, more preferably more than 180 nm to 250 nm, and even more preferably 200 nm to 230 nm. When the half-width is used as the distribution width, the distribution width in the magnesium profile in the (001) plane and the surface layer portion 100a having this plane is preferably 10 nm to 300 nm. Furthermore, the distribution width of magnesium in planes other than the (001) plane and the surface layer portion 100a having this plane is preferably more than 300 nm to 500 nm. Since magnesium may increase the resistance value of the surface layer 100a, it is preferable that magnesium be distributed in a narrow width as described above.

[0150] In a manufacturing method in which heating is performed after mixing the additive element, which will be described in a later embodiment, the additive element may spread mainly via the diffusion path of lithium ions. Therefore, in order to achieve a preferred range of distribution of the additive element in a plane other than the (001) plane and in the surface layer portion 100a having such a plane, a method in which the additive element is mixed after manufacturing a composite oxide containing lithium cobalt nickel oxide or with a hydroxide that is a precursor of the composite oxide is preferred. However, magnesium has a large ionic radius and is likely to remain in the surface layer portion 100a regardless of the process in which it is added, so it is preferred.

[0151] [Magnesium] Since the ionic radius of magnesium is close to that of lithium ion, magnesium ions can easily enter the lithium site in the layered rock salt crystal structure. When magnesium is present at an appropriate concentration in the lithium site of the surface layer 100a, the crystal structure of the interior 100c can be easily maintained. This is because magnesium present in the lithium site is easily absorbed by MO ２ It is presumed that this is because it functions as a pillar supporting the layers.ｘ Co （１−ｙ） Ni ｙ O ２ Even when the value of x in the graph is small, oxygen desorption from the periphery of magnesium can be suppressed, thereby suppressing the thermal decomposition reaction. Furthermore, if the magnesium concentration in the surface layer portion 100a is high, it can be expected that the corrosion resistance against hydrofluoric acid generated by the decomposition of the organic electrolyte solution or the like will be improved.

[0152] [Fluorine] Fluorine is an anion and may substitute for some of the oxygen, which is also an anion. That is, fluorine may substitute for some of the oxygen in the surface layer portion 100a at an appropriate concentration. Furthermore, the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. That is, in the absence of fluorine, cobalt ions change from trivalent to tetravalent upon lithium desorption. On the other hand, in the presence of fluorine, cobalt ions change from divalent to trivalent upon lithium desorption. The redox potential of cobalt ions differs between the two, and the presence of fluorine reduces the energy required for lithium desorption from the positive electrode active material 100. Therefore, the insertion and desorption of lithium ions near the fluorine is more likely to occur smoothly. Therefore, fluorine is preferably present on the surface or surface layer portion 100a of the positive electrode active material 100. When the fluorine-containing positive electrode active material 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved.

[0153] Furthermore, the presence of fluorine on the surface or surface layer that contacts the electrolyte, or the adsorption or adhesion of fluoride to the surface, can suppress excessive reaction between the cathode active material 100 and the electrolyte. Furthermore, the presence of fluorine on the surface or surface layer that contacts the electrolyte can be expected to improve corrosion resistance against hydrofluoric acid produced by decomposition of the organic electrolyte, etc. The above-mentioned adsorption includes chemical adsorption and physical adsorption. Chemical adsorption is the formation of a chemical bond through a chemical reaction between fluorine and the surface of the cathode active material 100, while physical adsorption is adsorption due to intermolecular forces (van der Waals forces) acting between fluorine and the surface of the cathode active material 100.

[0154] Furthermore, the melting points of fluorine compounds (sometimes called fluorides), including lithium fluoride, which are considered as fluorine sources, may be lower than the melting points of other additive element sources. That is, fluorine compounds may function as fluxes (also called fluxing agents) that lower the melting points of other additive element sources. Fluorine compounds such as LiF and MgF ２ As shown in FIG. 6 (quoted and added from FIG. 5 of Non-Patent Document 2), ２ Since the eutectic point P of the alloy is around 742°C, it is preferable to set the heating temperature at or near 742°C in the heating step after mixing the additive elements.

[0155] Nickel has a lower oxidation-reduction potential than cobalt, and therefore, it can be said that nickel easily releases lithium during charging, for example. Therefore, a positive electrode active material 100 having a higher atomic ratio of nickel is expected to have a faster charge / discharge speed.

[0156] In addition, the order of ionization tendency is greatest for magnesium, aluminum, cobalt, and nickel. Therefore, nickel is thought to be less likely to dissolve into the electrolyte than the other elements listed above during charging. Therefore, nickel is highly effective in stabilizing the crystalline structure of the surface layer portion in a charged state, and it is desirable for nickel to be present in both the interior portion 100c and the surface layer portion 100a.

[0157] [Aluminum] Aluminum can exist in the cobalt site of the layered rock salt crystal structure. Aluminum is a trivalent typical element and its valence does not change, so lithium around the aluminum is difficult to move even during charging and discharging. Therefore, MO ２ Therefore, even if the positive electrode active material 100 is subjected to a force that causes it to expand and contract in the c-axis direction due to the insertion and extraction of lithium ions, that is, even if the force that causes it to expand and contract in the c-axis direction due to a change in the depth of charge or the charging rate, deterioration of the positive electrode active material 100 can be suppressed.

[0158] Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charge durability. Furthermore, since the Al—O bond is stronger than the MO bond, specifically the CoO bond, it can suppress oxygen desorption from the aluminum's periphery. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0159] [Synergistic Effect of Multiple Elements] Furthermore, when magnesium and nickel coexist in the surface layer portion 100a, nickel may be more stable near the magnesium. ｘ Co （１−ｙ） Ni ｙ O ２ Even when x is small, the coexistence of magnesium and nickel in the surface layer portion 100a can suppress the elution of magnesium, which can contribute to the stabilization of the surface layer portion 100a.

[0160] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0161] However, if the surface layer 100a is occupied only by a compound of the added element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by a structure in which MgO is solid-solved. Therefore, the surface layer 100a must contain at least cobalt, and in a discharged state, it must also contain lithium, and must have a path for lithium insertion and extraction. In order to ensure sufficient paths for lithium insertion and extraction, it is preferable that the surface layer 100a have a higher concentration of cobalt than magnesium. It is also acceptable for the surface layer 100a to have a higher concentration of nickel than magnesium.

[0162] Furthermore, magnesium, which is one of the additive elements, is preferably present at a higher concentration in the surface layer 100 a than in the interior 100 c, but is also preferably present randomly and dilutely in the interior 100 c. If magnesium is present at an appropriate concentration at the lithium sites in the interior 100 c, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above.

[0163] Furthermore, it is preferable that aluminum, which is one of the additive elements, be present at a higher concentration in the surface layer 100 a than in the interior 100 c, but it is also preferable that aluminum be present randomly and dilutely in the interior 100 c. If aluminum is present at an appropriate concentration at the lithium sites in the interior 100 c, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above.

[0164] When nickel is present in the inner portion 100c, MO ２ Furthermore, when nickel is present in the surface layer portion 100a, the MO ２ The shifting of the layered structure can be suppressed.

[0165] <Li ｘ Co （１−ｙ） Ni ｙ O ２ When x is small in the positive electrode active material 100 of one embodiment of the present invention has the above-described distribution of the additional element and / or the crystal structure, ｘ Co （１−ｙ） Ni ｙ O ２ The crystal structure when x is small, i.e., when the battery is charged at a high voltage, differs from that of conventional lithium cobalt oxide. Here, "small x" refers to, for example, 0.1<x≦0.24. Furthermore, "high voltage in the charged state" refers to 4.5 V or higher, 4.6 V or higher, preferably 4.7 V or higher, and more preferably 4.8 V or higher.

[0166] First, conventional lithium cobalt oxide will be described. It is known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x = 0.5 or so and belongs to the monoclinic space group P2 / m. This structure has CoO in the unit cell. ２There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0167] Furthermore, conventional lithium cobalt oxide when x = 0 has a crystal structure of the trigonal space group P-3m1, and also contains CoO in the unit cell. ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0168] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２ It can also be said that the structure of and the structure of are alternately stacked. Therefore, this crystal structure is sometimes called an H1-3 type crystal structure. Note that, since actual lithium insertion / desorption does not necessarily occur uniformly within the positive electrode active material, the change in the crystal structure is not strictly related to the amount of lithium desorption, and the value of the amount of lithium desorption may be the timing when the crystal change begins.

[0169] In conventional lithium cobalt oxide, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure changes (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0170] These two crystal structures are CoO ２ The layer misalignment is large. In the H1-3 type crystal structure, CoO ２ The layers are significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure. Furthermore, the two crystal structures can undergo irreversible structural changes.

[0171] Furthermore, the difference in volume between these two crystal structures is large: per equivalent number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0172] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type.２ A structure with continuous layers is likely to be unstable.

[0173] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0174] Next, a positive electrode active material 100 of one embodiment of the present invention will be described. In the positive electrode active material 100 of one embodiment of the present invention shown in FIG. 7, Li ｘ Co （１−ｙ） Ni ｙ O ２ The change in the crystal structure when x is small, for example, when x is about 0.2, differs from that of conventional lithium cobalt oxide. Figure 7 shows a crystal structure belonging to the trigonal space group R-3m as a positive electrode active material 100 of one embodiment of the present invention when x is about 0.2. ２ The layer symmetry is the same as that of O3. Therefore, this crystal structure will be called an O3'-type crystal structure. In Figure 7, this crystal structure is labeled R-3m O3'. Furthermore, since the XRD pattern of this crystal structure may resemble a spinel structure, this crystal structure may also be called a pseudo-spinel structure.

[0175] As shown by the dotted line in FIG. 7, in the positive electrode active material 100 according to one embodiment of the present invention, the R-3m O3 in the discharged state and the O3′-type crystal structure are different from each other. ２ That is, in the positive electrode active material 100 of one embodiment of the present invention, the MoO ２ The layer misalignment is small. Furthermore, in the positive electrode active material 100 of one embodiment of the present invention, the change in volume per transition metal atom can be small. Therefore, the positive electrode active material 100 of one embodiment of the present invention is less likely to misalign, i.e., less likely to collapse, even when repeatedly charged and discharged such that x is about 0.2, specifically, 0.24 or less, and thus the site where lithium can stably exist is maintained, thereby achieving excellent cycle characteristics.

[0176] The positive electrode active material 100 of one embodiment of the present invention can stably utilize more lithium than conventional lithium cobalt oxide, and therefore has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be manufactured.

[0177] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ Co （１−ｙ） Ni ｙ O ２ When x is 0.24 or less, the positive electrode active material 100 according to one embodiment of the present invention can have a more stable crystal structure than conventional lithium cobalt oxide. ｘ Co （１−ｙ） Ni ｙ O ２ Even when the value of x in the positive electrode active material 100 is kept at 0.24 or less, oxygen is unlikely to be released, a thermal decomposition reaction can be suppressed, and a lithium-ion secondary battery using the positive electrode active material 100 is not likely to catch fire even when subjected to a nail penetration test. In other words, a secondary battery using the positive electrode active material 100 of one embodiment of the present invention is preferable because it has improved safety.

[0178] In this specification, "not igniting" in a nail penetration test means that no flame is observed outside the exterior body or that thermal runaway of the secondary battery does not occur. In other words, even if sparks and / or smoke are observed, the fire does not spread, which is equivalent to not igniting.

[0179] The O3'-type crystal structure of the positive electrode active material 100 can have typical cobalt and oxygen coordinates in a unit cell within the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell in the O3'-type crystal structure is 2.797×10 for the a-axis. −１０ ≦a≦2.837×10 −１０ (m) is preferred, and 2.807 × 10 −１０ ≦a≦2.827×10 −１０ (m) is more preferable, and typically a=2.817×10 −１０ (m). The c-axis is 13.681 × 10 −１０ ≦c≦13.881×10−１０ (m) is preferred, and 13.751 × 10 −１０ ≦c≦13.811×10 −１０ (m) is more preferable, and typically c=13.781×10 −１０ (m).

[0180] Also Li ｘ Co （１−ｙ） Ni ｙ O ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ Co （１−ｙ） Ni ｙ O ２ The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when the battery is charged at a constant current (CC) voltage of 4.6 V or higher relative to the potential of lithium metal in an environment of 25° C., followed by constant voltage (CV) charging at a voltage of 4.6 V or higher, which is the upper limit voltage for CC charging (this is referred to as CCCV charging), the H1-3 crystal structure begins to appear in conventional lithium cobalt oxide. On the other hand, the positive electrode active material 100 of one embodiment of the present invention is preferable because it can maintain a crystal structure with the symmetry of R-3m O3 even when CCCV charging is performed at a high charging voltage, for example, a voltage of 4.6 V or higher in an environment of 25° C.

[0181] In this specification, unless otherwise specified, the charging voltage is expressed based on the potential of lithium metal. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery and the potential of the positive electrode differ. For example, when focusing on the potential of the positive electrode, charging to 4.5 V when the counter electrode is graphite is roughly equivalent to charging to 4.6 V when the counter electrode is lithium.

[0182] In addition, in the O3'-type crystal structure of Figure 7, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. Lithium may exist disproportionately at some lithium sites. The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0183] As described above, the positive electrode active material 100 according to one embodiment of the present invention is Li ｘ Co （１−ｙ） Ni ｙ O ２It is preferable that the crystal structure of the lithium cobalt oxide changes uniquely depending on the change in x in the lithium cobalt oxide. ｘ Co （１−ｙ） Ni ｙ O ２ The change in x in the equation is equal to the change in the state of charge, and when x=0.2, the state of charge corresponds to 1−0.2=0.8.

[0184] <Particle Diameter> In a particle size distribution curve in which the horizontal axis is cumulative percent, the particle diameter at the 10% point on the horizontal axis is called the 10% diameter or D10, the particle diameter at the 50% point on the horizontal axis is called the 50% diameter or D50, and the particle diameter at the 90% point on the horizontal axis is called the 90% diameter or D90, and D50 is sometimes called the median diameter. D50 is often used to represent particle diameter. If the particle diameter of the positive electrode active material 100 of one embodiment of the present invention is too large, problems such as difficulty in lithium diffusion and excessive roughness of the surface of the active material layer when applied to a current collector may occur. On the other hand, if the particle diameter is too small, problems such as excessive reaction with the electrolyte may occur. Therefore, in the positive electrode active material 100, D50 is preferably 1 μm or more and 100 μm or less, more preferably 15 μm or more and 40 μm or less, and even more preferably 15 μm or more and 35 μm or less.

[0185] Furthermore, using a mixture of particles with different particle sizes in the positive electrode can increase the electrode density, which is preferable because it allows for a secondary battery with high energy density. A positive electrode active material 100 with a relatively small particle size is expected to have high charge / discharge rate characteristics. A secondary battery using a positive electrode active material 100 with a relatively large particle size is expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0186] <Analysis Method> Whether a certain positive electrode active material has an O3'-type crystal structure during discharge can be determined by Li ｘ Co （１−ｙ） Ni ｙ O ２ This can be determined by analyzing a positive electrode having a positive electrode active material in which x is small using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. For example, x can be set to 0.2.

[0187] However, since a positive electrode active material with a small x may undergo a change in crystal structure when exposed to air, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0188] In particular, XRD is preferable in that it can analyze the symmetry of the transition metal contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallites, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD, powder X-ray diffraction can obtain diffraction peaks that reflect the crystalline structure of the interior 100c of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

[0189] <Charging Method> Charging for determining whether a certain positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention can be performed by fabricating a coin-shaped cell (CR2032 type, diameter 20 mm, height 3.2 mm) using lithium metal as a counter electrode, for example.

[0190] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0191] As described above, lithium metal can be used for the counter electrode, but materials other than lithium metal may also be used. When a material other than lithium metal is used, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode.

[0192] The lithium salt contained in the electrolyte solution is 1 mol / L lithium hexafluorophosphate (LiPF ６ The electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7. The electrolyte may contain 2 wt % of vinylene carbonate (VC) as an additive.

[0193] The separator may be a 25 μm thick porous polypropylene film.

[0194] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0195] The coin cell fabricated under the above conditions is charged at a desired voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). For example, when charging by CCCV, the CC charging current can be 20 mA / g or more and 100 mA / g or less per weight of positive electrode active material. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less per weight of positive electrode active material. To observe the phase change of the positive electrode active material, charging at such a low current value is desirable. The XRD measurement temperature should be 25°C. After charging in this manner, the coin cell can be disassembled in an argon-atmosphere glove box and the positive electrode removed to obtain a positive electrode active material with a desired charge capacity, i.e., a desired depth of charge. When performing various subsequent analyses, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed in a sealed container sealed in an argon atmosphere. It is also preferable to remove the positive electrode promptly after charging is complete and subject it to analysis. Specifically, it is preferable to do so within one hour after the completion of charging, and more preferably within 30 minutes.

[0196] Furthermore, when analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions for charge. For example, charging may be performed by CC charging up to a given voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V) at a current value of 20 mA / g or more and 100 mA / g or less per weight of positive electrode active material, followed by CV charging until the current value reaches 2 mA / g or more and 10 mA / g or less per weight of positive electrode active material, and then CC discharging at a current value of 20 mA / g or more and 100 mA / g or less per weight of positive electrode active material until the voltage reaches 2.5 V.

[0197] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, CC discharge can be performed at a current value of 20 mA / g or more and 100 mA / g or less per weight of the positive electrode active material until the voltage reaches 2.5 V, for example.

[0198] <XRD> The apparatus and conditions for XRD measurement are not particularly limited as long as appropriate adjustments and calibrations are made. For example, the XRD conditions described above can be used. In this specification and the like, when the 2θ value of a certain diffraction peak is mentioned, it refers to the 2θ value at which the apex of the diffraction peak appears in the XRD pattern after fitting a calculation model. The crystal structure analysis software used for fitting is not particularly limited, but for example, TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker) can be used. When the measurement sample is a powder, it is called powder X-ray diffraction. The radiation source is CuKα １ can be used.

[0199] If the positive electrode active material 100 has an O3'-type crystal structure, diffraction peaks appear at 2θ = 19.30 ± 0.20° (19.10° or more and 19.50° or less) and 2θ = 45.55 ± 0.10° (45.45° or more and 45.65° or less). Note that the above-described 2θ values ​​were obtained by estimating the crystal structure from the XRD pattern of the positive electrode active material 100 of one embodiment of the present invention and fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker).

[0200] Li ｘ Co （１−ｙ） Ni ｙ O ２ When x is small, for example, when x is 0.24 or less, diffraction peaks appear at least at 2θ = 19.30 ± 0.20° and 2θ = 45.55 ± 0.10°, which can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0201] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0202] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ Co （１−ｙ） Ni ｙ O ２When x in the formula is small, the particles have an O3'-type crystal structure, but not all of the particles need to have an O3'-type crystal structure. They may contain other crystal structures, or some of the particles may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure preferably accounts for 50% or more, more preferably 60% or more, and even more preferably 66% or more.

[0203] Similarly, when Rietveld analysis is carried out, it is preferable that the H1-3 type and O1 type crystal structures are 40% or less.

[0204] The sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width, for example, the full width at half maximum, is narrow. The half-width varies depending on the XRD measurement conditions and the value of 2θ, even for peaks arising from the same crystalline phase.

[0205] As described above, it is preferable that the positive electrode active material 100 according to one embodiment of the present invention be small in influence of the Jahn-Teller effect. In the positive electrode active material, the range of the nickel proportion and lattice constant in which the influence of the Jahn-Teller effect is estimated to be small will be considered using XRD analysis.

[0206] 8 shows the results of calculating the a-axis and c-axis lattice constants using XRD for a positive electrode active material 100 of one embodiment of the present invention having a layered rock-salt crystal structure and containing cobalt and nickel. FIG. 8A shows the a-axis result, and FIG. 8B shows the c-axis result. Note that the XRD patterns used for these calculations are for powder after synthesis of the positive electrode active material, but before incorporation into a positive electrode. The nickel concentration on the horizontal axis is equal to the atomic ratio of nickel when the sum of the atomic ratios of cobalt and nickel is taken as 100%.

[0207] FIG. 8C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active materials whose lattice constant results are shown in FIGS. 8A and 8B.

[0208] 8C shows that the a-axis / c-axis tends to change significantly when the nickel concentration increases from 5 at% to 7.5 at%. FIG. 8A shows that the a-axis distortion increases at a nickel concentration of 7.5 at%. This distortion may be due to the Jahn-Teller distortion of trivalent nickel. It is suggested that an excellent positive electrode active material with a small Jahn-Teller distortion can be obtained when the nickel concentration is less than 7.5 at%.

[0209] Such a composite oxide with a small Jahn-Teller distortion is preferably present in the interior 100c of the positive electrode active material 100. In other words, the above-described range of nickel concentration does not necessarily apply to the surface layer 100a of the positive electrode active material 100. In other words, the concentration in the surface layer 100a may be higher than the above-described concentration.

[0210] As a result of considering a preferable range of the lattice constant from the above, it was found that in the positive electrode active material 100 of one embodiment of the present invention, the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge / discharge is performed or in a discharged state, which can be estimated from the XRD pattern, has an a-axis lattice constant of 2.814 × 10 −１０ m is greater than 2.817 x 10 −１０ m and the lattice constant of the c-axis is 14.05 × 10 −１０ m or larger, 14.07 x 10 −１０ It has been found that the value is preferably smaller than m. The state in which no charge and discharge are performed may be, for example, the state of powder before the positive electrode of the secondary battery is produced.

[0211] In the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, it is preferable that the value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) is greater than 0.20000 and smaller than 0.20049.

[0212] When XRD analysis is performed on the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, a first peak may be observed at 2θ of not less than 18.50° and not more than 19.30°, and a second peak may be observed at 2θ of not less than 38.00° and not more than 38.80°.

[0213] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), and therefore it is possible to quantitatively analyze the concentration of each element in a region that is about half the depth of the surface layer 100a of the positive electrode active material 100. Furthermore, narrow scan analysis can be used to analyze the bonding state of the elements.

[0214] In the cathode active material 100 according to one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer portion 100a than in the interior portion 100c. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer portion 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration of at least a portion of the surface layer portion 100a measured by XPS or the like is preferably higher than the magnesium concentration throughout the cathode active material 100. Furthermore, the nickel concentration of at least a portion of the surface layer portion 100a is preferably higher than the nickel concentration throughout the cathode active material 100. Furthermore, the aluminum concentration of at least a portion of the surface layer portion 100a is preferably higher than the aluminum concentration throughout the cathode active material 100. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 100 a is higher than the fluorine concentration in the entire positive electrode active material 100 .

[0215] The concentration of the added element may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances chemisorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 to 1.5. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 to 0.06.

[0216] Similarly, in order to ensure sufficient paths for lithium insertion and desorption, the positive electrode active material 100 preferably has higher concentrations of lithium and cobalt than the respective additive elements in the surface layer portion 100 a. This means that the concentrations of lithium and cobalt in the surface layer portion 100 a are preferably higher than the concentrations of one or more additive elements selected from the additive elements contained in the surface layer portion 100 a, as measured by XPS or the like.

[0217] Furthermore, when XPS analysis was performed on the positive electrode active material 100 of one embodiment of the present invention, the atomic ratio of magnesium to cobalt was preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times. Furthermore, the atomic ratio of aluminum to cobalt was preferably 0.12 or less, more preferably 0.09 or less. The above-described ranges indicate that each additive element is widely distributed at a preferred concentration in the surface layer 100a of the positive electrode active material 100.

[0218] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be set to, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions. Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα rays (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0219] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0220] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is a value different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0221] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. More preferably, the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy, electron probe microanalysis (EPMA), or the like.

[0222] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0223] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100c, and near the grain boundary 101 of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0224] Therefore, when EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, the concentration of each additional element, particularly the additional element, in the surface layer portion 100a is preferably higher than that in the interior portion 100c.

[0225] Furthermore, when EDX ray analysis, EDX areal analysis, or EDX point analysis is performed on the positive electrode active material 100, the ratio of the atomic ratio of magnesium Mg to the atomic ratio of cobalt Co (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.4 or less. The ratio of the atomic ratio of aluminum Al to the atomic ratio of cobalt Co (Al / Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.45 or less.

[0226] From the results of EDX-ray analysis, the surface of positive electrode active material 100 can be estimated, for example, as follows: For an element that is uniformly present in interior 100c of positive electrode active material 100, such as oxygen or cobalt, the point where the amount detected in interior 100c is half that of interior 100c is used as the surface of positive electrode active material 100.

[0227] Since the positive electrode active material 100 is a composite oxide, the surface can be estimated using the detected amount of oxygen. Specifically, first, the average oxygen concentration O ａｖｅ At this time, oxygen O, which is thought to be due to chemical adsorption or background, is found in the area that can be clearly determined to be outside the surface. ｂｇ If detected, O ｂｇ The average oxygen concentration O ａｖｅ This average value O ａｖｅ Half the value of O ａｖｅ The measurement point showing the measurement value closest to / 2 can be estimated to be the surface of the positive electrode active material 100.

[0228] Similarly, the surface of the positive electrode active material 100 can be estimated using the detected amount of cobalt. Alternatively, the sum of the detected amounts of multiple transition metals can be used for similar estimation. The detected amounts of transition metals, including cobalt, are suitable for estimating the surface because they are less susceptible to the influence of chemical adsorption.

[0229] This embodiment mode can be used in combination with other embodiment modes or examples.

[0230] Embodiment 2 In this embodiment, a method for manufacturing a positive electrode active material 100 according to one embodiment of the present invention will be described with reference to flowcharts shown in FIGS.

[0231] <Manufacturing Method 1> The manufacturing methods described in Figures 9 and 10 include a coprecipitation method, making them suitable for mass production. Coprecipitation is a method in which a poorly soluble salt precipitates from an aqueous solution containing two or more metal ions when the ion concentration becomes supersaturated. Compared to mixing solid materials, the coprecipitation method achieves a higher uniformity of the metal salt mixture in the precipitate, making it easier to position nickel in the interior 100c. Note that Figure 10 is a flow diagram that omits some of the steps in Figure 9 and is an example of a highly productive process.

[0232] <Cobalt Source> As shown in FIGS. 9 and 10 , a cobalt source 81 (referred to as a Co source in the drawings) is prepared. The cobalt source 81 is one of the starting materials for the positive electrode active material. The cobalt source 81 is a compound containing cobalt (referred to as a cobalt compound). Examples of the cobalt compound include cobalt sulfate, cobalt chloride, cobalt nitrate, and hydrates thereof. Cobalt alkoxides or organic cobalt complexes may also be used as the cobalt compound. Furthermore, organic acids of cobalt, such as cobalt acetate, or hydrates thereof may also be used as the cobalt compound. In this specification and the like, organic acids include citric acid, oxalic acid, formic acid, butyric acid, and the like, in addition to acetic acid.

[0233] When a solution is used as the cobalt source 81, an aqueous solution containing the above-mentioned cobalt compound (hereinafter referred to as an aqueous cobalt solution) is prepared.

[0234] <Nickel Source> As shown in FIGS. 9 and 10 , a nickel source 82 (referred to as Ni source in the drawings) is prepared. The nickel source 82 is one of the starting materials for the positive electrode active material. A compound containing nickel (referred to as nickel compound) is used as the nickel source 82. The nickel compound may be, for example, nickel sulfate, nickel chloride, or nickel nitrate, or a hydrate thereof. Nickel alkoxides or organic nickel complexes may also be used as the nickel compound. Furthermore, organic nickel acids such as nickel acetate, or a hydrate thereof may also be used as the nickel compound.

[0235] When a solution is used as the nickel source 82, an aqueous solution containing the above nickel compound (referred to as an aqueous nickel solution) is prepared.

[0236] When lithium cobalt oxide is obtained as the positive electrode active material 100, the proportion of nickel should be smaller than the proportion of cobalt. For example, LiCo １−ｙ Ni ｙ O ２ The cobalt source and the nickel source may be prepared so that y in the above formula is greater than 0 and less than 0.1, preferably greater than 0 and 0.05 or less, and preferably greater than 0 and 0.01 or less.

[0237] In other words, the atomic ratio of nickel to the sum of the atomic ratio of cobalt and the atomic ratio of nickel is preferably greater than 0 and less than 0.5, preferably greater than 0 and less than 0.1, preferably greater than 0 and less than 0.05, preferably greater than 0 and less than 0.01, and the cobalt source and nickel source may be prepared so as to satisfy this relationship. The atomic ratio of nickel to the sum of the atomic ratio of cobalt and the atomic ratio of nickel may be expressed as Ni / (Ni+Co), which may also be referred to as the proportion of nickel to the sum of cobalt and nickel. Note that the atomic ratio of cobalt to nickel when prepared as a starting material as in this paragraph does not have to satisfy the same atomic ratio in the positive electrode active material 100.

[0238] Furthermore, an excess amount of lithium may be mixed into the positive electrode active material 100, and the lithium ratio is not limited in any way in the present invention. That is, when a transition metal such as cobalt or nickel is defined as M, the composition is not strictly limited to Li:M:O=1:1:2.

[0239] The positive electrode active material of the present invention may contain manganese, but more preferably does not contain manganese. A positive electrode active material that does not contain manganese substantially has the great advantage of being relatively easy to synthesize and handle, and having excellent cycle characteristics. "Substantially free" can be considered to mean that the content of manganese in the positive electrode active material is small. Specifically, the weight of manganese in the positive electrode active material is 600 ppm or less, more preferably 100 ppm or less.

[0240] <Chelating Agent> A chelating agent 83 is prepared as shown in Fig. 9. The chelating agent 83 may be omitted, and Fig. 10 shows a flow diagram in which the chelating agent 83 is not prepared.

[0241] The chelating agent 83 is an aqueous solution containing glycine, oxine, 1-nitroso-2-naphthol, 2-mercaptobenzothiazole, or EDTA (ethylenediaminetetraacetic acid), and is referred to as an aqueous chelating solution. Note that multiple types of glycine, oxine, 1-nitroso-2-naphthol, or 2-mercaptobenzothiazole may be used. Water, preferably pure water, is used as the solvent.

[0242] The chelating agent described above is preferable to a general complexing agent in that it is a complexing agent that forms a chelate compound. Of course, a general complexing agent such as aqueous ammonia may be used instead of the chelating agent.

[0243] The use of a chelating agent is preferable because it can suppress the generation of unnecessary crystal nuclei and promote crystal growth. Suppressing the generation of unnecessary nuclei suppresses the generation of fine particles, resulting in the production of hydroxides (sometimes called precursors) with a good particle size distribution. Furthermore, the use of a chelating agent can slow the acid-base reaction, allowing the reaction to proceed gradually, resulting in the production of hydroxides that are nearly spherical.

[0244] The use of an aqueous glycine solution is preferable because it makes it easier to control the pH of the solution in the reaction vessel during the coprecipitation reaction in step S31 in Figures 9 and 10. Furthermore, the glycine concentration of the aqueous glycine solution is preferably 0.05 mol / L or more and 0.5 mol / L or less, and more preferably 0.1 mol / L or more and 0.2 mol / L or less.

[0245] <Pure Water> The pure water used in the chelate aqueous solution is water having a resistivity of 1 MΩ cm or more, more preferably 10 MΩ cm or more, and even more preferably 15 MΩ cm or more. Water satisfying the above resistivity range has high purity and contains very few impurities, and is therefore preferred for use in acid-base reactions.

[0246] <Step S14> Next, step S14 shown in Figures 9 and 10 will be described. In step S14, a cobalt source 81 and a nickel source 82 are mixed. By mixing in this step, a mixed solution 91 of a cobalt compound and a nickel compound is obtained. The cobalt compound and the nickel compound are dissolved in the mixed solution. The water used in this step may be the above-mentioned pure water. The mixed solution 91 is an acidic solution and can be called an acid solution.

[0247] <Alkaline Aqueous Solution> Next, the alkaline aqueous solution 84 shown in FIGS. 9 and 10 will be described.

[0248] The alkaline aqueous solution 84 may be, for example, an aqueous solution containing sodium hydroxide, potassium hydroxide, lithium hydroxide, or ammonia, but is not limited to these aqueous solutions as long as it functions as a pH adjuster. For example, it may be an aqueous solution in which two or more species selected from sodium hydroxide, potassium hydroxide, and lithium hydroxide are dissolved in water. The water used may be pure water.

[0249] <Chelating Agent (Filling Liquid)> A chelating agent 85 is prepared as shown in Fig. 9. The chelating agent 85 may be made of the same material as the chelating agent 83 described above. The chelating agent 85 may be omitted; for example, Fig. 10 shows a flow diagram in which the chelating agent 85 is not prepared. In the case of Fig. 10, however, it is recommended that water 86, preferably pure water, be placed in the reaction vessel as the filling liquid.

[0250] A supplementary note about the chelating agent 85 shown in FIG. 9 . The chelating agent 85 may be placed in a reaction vessel used in step S31, which will be described later, and may be referred to as a charging solution or an adjusting solution. In other words, the chelating agent 85 refers to an aqueous solution in the initial state of the reaction. Here, too, the use of the chelating agent 85 can suppress the generation of unnecessary crystal nuclei and promote their growth, as described above. Suppressing the generation of unnecessary nuclei suppresses the generation of fine particles, thereby producing hydroxides with a good particle size distribution. Alternatively, the acid-base reaction can be delayed, and the gradual progress of the reaction can produce hydroxides that are close to spherical.

[0251] <Step S31> Step S31 shown in FIGS. 9 and 10 will be described. In step S31, the mixed solution 91 and the alkaline aqueous solution 84 are mixed in a reaction vessel. The mixing in step S31 causes the mixed solution 91 and the alkaline aqueous solution 84 to react with each other, producing a hydroxide 95 as a precursor. The chemical reaction in step S31 can be referred to as a neutralization reaction, an acid-base reaction, or a coprecipitation reaction. The coprecipitation reaction precipitates a composite hydroxide 95 (simply referred to as hydroxide 95) having cobalt and nickel as the transition metals M. The hydroxide 95 can be referred to as a precursor of the positive electrode active material 100. The hydroxide 95 can also be referred to as a compound of cobalt and nickel, and is therefore sometimes referred to as a cobalt-nickel compound.

[0252] In step S31, the solution in the reaction vessel is preferably stirred using a stirring means. The stirring means may have a stirrer or stirring blades. When stirring the solution, the stirring may be performed at a rotation speed of 500 rpm to 1500 rpm, preferably 800 rpm to 1200 rpm. Two to six stirring blades may be provided. For example, when using four stirring blades, they may be arranged in a cross shape when viewed from above.

[0253] The coprecipitation reaction in step S31 allows nickel and cobalt to be uniformly mixed, i.e., hydroxide 95 in which nickel is present in the interior 100c can be obtained. The positive electrode active material 100 formed through the hydroxide 95 can enjoy the effects of nickel.

[0254] However, when nickel is difficult to dissolve in cobalt, or when the material undergoes heat treatment, etc., as described below, nickel may be unevenly distributed in the surface layer portion 100a rather than in the interior portion 100c of the positive electrode active material 100. Even when nickel is present in the surface layer portion 100a, the positive electrode active material 100 can still enjoy the effects of nickel. For example, nickel can suppress defects in the positive electrode active material. In this specification and the like, uneven distribution refers to the uneven or uneven distribution of elements such as nickel. The uneven distribution may also be referred to as segregation or precipitation.

[0255] <Reaction Conditions> When reacting the mixed solution 91 with the alkaline aqueous solution 84 according to the coprecipitation reaction, the pH of the solution in the reaction vessel is adjusted to 9 or more and 13 or less, preferably 9.8 or more and 12.5 or less. This range is preferable because it allows for a large particle size of the hydroxide 95. Outside this range, productivity may decrease, and the obtained hydroxide 95 may be more likely to contain impurities.

[0256] When the mixed solution 91 is placed in a reaction vessel and the alkaline aqueous solution 84 is added dropwise to the reaction vessel, it is advisable to maintain the pH of the solution in the reaction vessel within the range of the above conditions. Also, when the alkaline aqueous solution 84 is placed in a reaction vessel and the mixed solution 91 is added dropwise to the reaction vessel, it is advisable to maintain the pH of the solution in the reaction vessel within the range of the above conditions.

[0257] The liquid feed rate (also referred to as the drip rate) of the mixed liquid 91 or the alkaline aqueous solution 84 is preferably 0.01 mL / min to 1 mL / min, and more preferably 0.05 mL / min to 0.5 mL / min, when the volume of the solution in the reaction vessel is 200 mL to 350 mL. The alkaline aqueous solution 84 may be dripped so that the pH of the solution in the reaction vessel remains constant. A pump is provided in the tank that stores the mixed liquid 91 or the alkaline aqueous solution 84, etc., and the drip rate can be controlled by the pump. The pump can also control the amount of dripping. The drip rate may be changed in multiple stages, for example, the drip rate may be gradually increased.

[0258] The temperature of the solution in the reaction vessel is adjusted to be 50° C. or higher and 90° C. or lower. After checking the temperature of the solution, it is advisable to start the dropwise addition. The above range is preferable because it allows the particle size of the hydroxide 95 obtained to be large.

[0259] The reaction vessel may be filled with an inert atmosphere. For example, when a nitrogen atmosphere is used, nitrogen gas may be introduced at a flow rate of 0.5 L / min to 1.2 L / min. Furthermore, nitrogen gas may be introduced by bubbling into the liquid in the reaction vessel.

[0260] The reaction vessel may also be equipped with a reflux condenser, which allows nitrogen gas to escape from the reaction vessel and water to return to the reaction vessel.

[0261] After the above reaction, a precipitate 92 is obtained as a reaction product in the reaction vessel. The precipitate 92 contains cobalt nickel hydroxide.

[0262] <Steps S32 and S33> The filtration in step S32 and the drying process in step S33 shown in FIG. 9 will now be described. The precipitate 92 contains impurities other than the hydroxide 95. Therefore, in order to recover the hydroxide 95, the filtration in step S32 is preferably performed. The filtration can be performed by suction filtration or vacuum filtration. Centrifugation may also be used instead of filtration. When suction filtration is used, it is preferable to wash the reaction product precipitated in the reaction vessel with water (e.g., pure water) and then with an organic solvent with a low boiling point (e.g., acetone). It is also preferable to perform suction filtration multiple times. Note that, as shown in FIG. 10, step S32 does not necessarily have to be performed.

[0263] The filtered product may be further dried in step S33. For example, the drying is performed at 60°C to 90°C for 0.5 to 20 hours, preferably 12 to 20 hours. Drying is preferably performed in an oxygen-poor atmosphere. For example, when drying is performed under vacuum, a bell-jar-type vacuum apparatus having a container (referred to as a bell jar) capable of evacuating the interior and a vacuum pump connected to the bell jar can be used. Alternatively, when drying is performed in a vacuum atmosphere, a vacuum drying furnace may be used, which has a vacuum pump connected to the drying furnace. The vacuum pumps of the bell-jar-type vacuum apparatus and vacuum drying furnace may include a dry pump, turbomolecular pump, oil-sealed rotary pump, cryopump, or mechanical booster pump. The vacuum atmosphere in the bell-jar-type vacuum apparatus and vacuum drying furnace includes an atmosphere reduced in pressure so that the differential pressure gauge of each apparatus indicates a pressure of -0.1 MPa or more but less than -0.08 MPa. When heating in a nitrogen atmosphere, a nitrogen-containing gas may be flowed into the container of the bell-jar-type vacuum apparatus and vacuum drying furnace. In this way, hydroxide 95 can be obtained. Thorough drying is preferable because it reduces impurities, such as moisture or hydroxyl groups, from the obtained hydroxide 95. Note that step S33 does not necessarily have to be performed, as shown in FIG.

[0264] Heating may be performed instead of or in addition to the drying in step S33. The heating temperature is preferably 700° C. or higher and lower than 1200° C., more preferably 800° C. or higher and lower than 1100° C., and even more preferably 900° C. or higher and lower than 1000° C. The heating time can be, for example, 1 hour or higher and 100 hours or lower, and preferably 2 hours or higher and 20 hours or lower.

[0265] The hydroxide 95 obtained through this process is Co １−ｙ Ni ｙ (OH) ２ The hydroxide 95 may be a single particle or a secondary particle, but preferably has a large crystallite.

[0266] <Lithium Source> A lithium compound is prepared as the lithium source 88 (referred to as Li source in the drawings) shown in FIGS.

[0267] As the lithium compound, lithium hydroxide, lithium carbonate, lithium oxide, or lithium nitrate is prepared. The atomic ratio of lithium to the sum of the atomic ratios of transition metals such as cobalt and nickel (referred to as Li / (Co+Ni)) is 0.9 or more and 1.2 or less, preferably 1.0 or more and 1.09 or less. The lithium compound is weighed so as to satisfy the above range. Using hydroxide 95 with reduced moisture or hydroxyl groups is preferable, as Li / Co+Ni becomes an appropriate value.

[0268] The lithium compound may be pulverized in advance. For example, the mortar may be pulverized for 5 to 15 minutes. The mortar is preferably made of a material that does not easily release impurities, and specifically, a mortar made of aluminum oxide (hereinafter referred to as alumina) with a purity of 90% or more, preferably 99% or more, may be used. A wet pulverization method using a ball mill or the like may also be used. In the wet pulverization method, acetone or dehydrated acetone may be used as the solvent. The pulverized lithium compound may also be sieved. Furthermore, the lithium compound may be pulverized using a pulverization and classification device.

[0269] 9 and 10, step S51 will be described. In step S51, hydroxide 95 and lithium source 88 are mixed. Thereafter, a mixed mixture 96 is obtained. A mortar, a rotation / revolution stirring device, or the like may be used as a means for mixing hydroxide 95 and lithium source 88.

[0270] When mixing and pulverizing the hydroxide 95 and the lithium source 88 simultaneously, it is preferable to use a ball mill or a bead mill as the media. Alumina balls or zirconia balls can be used for the ball mill or the bead mill. In the ball mill or the bead mill, centrifugal force is applied to the media, which enables microparticulation. However, if there is a concern about contamination from the media, it is preferable to use the zirconia balls and set the peripheral speed to 100 mm / sec or more and 2000 mm / sec or less.

[0271] When mixing and pulverization are performed simultaneously, the pulverization method that can be used includes a dry pulverization method and a wet pulverization method. The dry pulverization method involves pulverization in an inert gas or air, and can pulverize to a particle size of 3.5 μm or less, preferably 3 μm or less. The wet pulverization method involves pulverization in a liquid, and can pulverize to a particle size of 1 μm or less. In other words, if you want to reduce the particle size, you should use the wet pulverization method.

[0272] In this way, a mixture 96 is obtained.

[0273] <Step S54> Next, step S54 shown in Figures 9 and 10 will be described. In step S54, the mixture 96 is heated. Step S54 may be referred to as main baking. The heating step may be performed multiple times, or pre-baking at a lower temperature may be performed before the main baking.

[0274] In step S54, the heating temperature is preferably 700° C. or higher and lower than 1200° C., more preferably 800° C. or higher and lower than 1100° C., and even more preferably 900° C. or higher and lower than 1000° C. When producing the oxide 98 through this heat treatment, heating is performed at a temperature at which at least the hydroxide 95 and the lithium source 88 diffuse into each other.

[0275] The heating time in step S54 can be, for example, 1 hour or more and 100 hours or less, and is preferably 2 hours or more and 20 hours or less.

[0276] The atmosphere in the processing chamber in step S54 preferably contains oxygen. Examples of the oxygen-containing atmosphere include an oxygen atmosphere, a dry air atmosphere, an air atmosphere, and an atmosphere in which oxygen is mixed with other gases (e.g., one or more selected from nitrogen and noble gases). Examples of noble gases include argon. Alternatively, the atmosphere may be a mixture of nitrogen, a noble gas, or two or more selected from nitrogen and noble gases.

[0277] The atmosphere in the processing chamber in step S54 preferably has a low moisture content. The dew point of the atmosphere is preferably, for example, −50° C. or less, and more preferably −80° C. or less. Dry air can be suitably used in the temperature increasing step and the temperature maintaining step. In addition, the CH ４, CO, CO ２ , and H ２ By setting the impurity concentrations of the above to 5 ppb (parts per billion) or less, it may be possible to suppress impurities that may be mixed into the material.

[0278] One method involves continuously introducing gas into the processing chamber in step S54. This method can also be considered as flowing gas into the processing chamber. In this case, the gas flow rate may be, for example, 0.1 L / min to 0.7 L / min per 1 L of processing chamber volume. When the processing chamber volume is 40 L, a flow rate of 10 L / min or thereabouts is preferable. The gas may be, for example, oxygen gas, dry air, nitrogen gas, noble gas, or a mixture of two or more selected from these gases.

[0279] After the atmosphere in the processing chamber in step S54 is replaced with a desired gas, a method may be used to prevent the gas from entering or leaving the processing chamber. For example, the atmosphere in the processing chamber may be replaced with a gas containing oxygen to prevent the gas from entering or leaving the processing chamber. Alternatively, the processing chamber may be depressurized before introducing the gas. Specifically, for example, the processing chamber may be depressurized until a differential pressure gauge indicates -970 hPa, and then the gas may be introduced until the pressure reaches 50 hPa.

[0280] It is preferable that the crucible, sheath, setter, or container used during heating be made of a material that does not easily release impurities. For example, it is recommended to use a crucible made of alumina with a purity of 99.9%. For mass production, mullite-cordierite (Al ２ O ３ , SiO ２ , MgO) sheaths are preferably used.

[0281] Furthermore, when recovering the material after heating, it is preferable to transfer it from the crucible to a mortar and then recover it, since this prevents impurities from being mixed into the material. The mortar is also preferably made of a material that does not easily release impurities, and specifically, a mortar made of alumina or zirconia with a purity of 90% or more, preferably 99% or more, may be used.

[0282] <Step S55> Step S55 shown in Fig. 9 will now be described. In step S55, a crushing step is carried out. For example, classification may be carried out using a sieve with a mesh size of 40 µm or more and 60 µm or less. However, as shown in Fig. 10, oxide 98 can be obtained without carrying out the crushing step of step S55.

[0283] 9 and 10 will be described. The oxide 98 is formed through at least the heating in step S54, and may be referred to as a composite oxide. The oxide 98 can also be used as the positive electrode active material 100.

[0284] <Step S56: Heat Treatment> Step S56 shown in FIG. 9 will now be described. Heating is more preferable to obtain oxide 98 without defects. This heating may be called initial heating to distinguish it from other heating treatments. The heat treatment of step S56 is expected to have the effects of reducing defects, increasing the crystallinity of the internal layered rock-salt crystal structure, or smoothing the surface. While all of these effects can be considered to be on oxide 98, because the shape of the positive electrode active material 100 may reflect the shape of the oxide 98, the effects of reducing defects, increasing the crystallinity of the internal layered rock-salt crystal structure, or smoothing the surface can also be expected for positive electrode active material 100.

[0285] The heating conditions in step S56 can be selected from the heating conditions described in step S54. The heating temperature in step S56 is preferably lower than the heating temperature in step S54.

[0286] The atmosphere in the processing chamber in step S56 can be selected from the atmospheres described in step S54. The atmosphere in the processing chamber in step S56 preferably contains oxygen.

[0287] The heat treatment in step S56 causes lithium to be released from a portion of the surface layer of the oxide 98, which further improves the distribution of the additive elements, which will be described later. More specifically, it is believed that the heat treatment makes it easier to vary the distribution depending on the additive element through the following mechanism. First, lithium is released from a portion of the surface layer of the oxide 98 through the heat treatment in step S56. Next, the oxide 98, which is deficient in lithium from the surface layer, is mixed with an additive element source, such as a nickel source, an aluminum source, or a magnesium source, as will be described later, and heated. Of the additive elements, magnesium is a divalent typical element, and nickel is a transition metal, but is easily converted into a divalent ion. Therefore, Mg is released from a portion of the surface layer of the oxide 98. ２＋ and Ni ２＋ and Co reduced by lithium deficiency. ２＋ However, since this phase is formed only in a part of the surface layer of the oxide 98, it may not be clearly observed in an electron microscope image such as an STEM or an electron beam diffraction pattern.

[0288] If lithium is released from a part of the surface layer of the oxide 98 by the heat treatment in step S56, a lithium source may be added to the oxide 98 before step S56.

[0289] However, the heat treatment in step S56 does not necessarily have to be performed.

[0290] 9 and 10 will be described. The additive element source 89 is preferably a compound containing one or more elements selected from magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium.

[0291] When the additive element is magnesium, the additive element source 89 can be referred to as a magnesium source. A compound containing magnesium is used as the magnesium source. Examples of the magnesium-containing compound that can be used include magnesium fluoride, magnesium oxide, magnesium hydroxide, and magnesium carbonate. A plurality of the above-mentioned magnesium sources may also be used.

[0292] When the additive element is fluorine, the additive element source 89 can be referred to as a fluorine source. A fluorine-containing compound is used as the fluorine source. Examples of fluorine-containing compounds that can be used include lithium fluoride, magnesium fluoride, aluminum fluoride, titanium fluoride, cobalt fluoride, nickel fluoride, zirconium fluoride, vanadium fluoride, manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride, calcium fluoride, sodium fluoride, potassium fluoride, barium fluoride, cerium fluoride, lanthanum fluoride, and sodium aluminum hexafluoride. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating step described below.

[0293] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as both a fluorine source and a lithium source.

[0294] The fluorine source may be a gas, such as fluorine, carbon fluoride, sulfur fluoride, or oxygen fluoride, which may be mixed into the atmosphere during the heating step described below. A plurality of the above-mentioned fluorine sources may also be used.

[0295] When preparing the additive element source 89, two or more types of additive elements can be used as shown in Figures 11A to 11C. Figure 11A illustrates an example in which two types of additive elements, an Mg source and an F source, Figure 11B illustrates an example in which three types of additive elements, an Mg source, an F source, and an A source, are used, and Figure 11C illustrates an example in which four types of additive elements, an Mg source, an F source, an Ni source, and an Al source, are used. In all of Figures 11A to 11C, a milling step and / or a mixing step is performed in step S22 to obtain the additive element source 89.

[0296] When lithium fluoride and magnesium fluoride are used together as the additive element source 89, the molar ratio of lithium fluoride to magnesium fluoride is LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x:1 (x = 0.33 or its vicinity) is more preferable. Note that "x = 0.33 or its vicinity" refers to a value that is greater than 0.9 times and less than 1.1 times the value.

[0297] When two or more kinds of additive element sources 89 are used, it is preferable to mix the additive element sources 89 together first, as in step S22 of Figures 11A to 11C. There are two methods for mixing, one in which the raw materials are mixed while being pulverized, and the other in which the raw materials are mixed without being pulverized. When two or more kinds of additive element sources 89 are mixed first, it is preferable to mix them while being pulverized. This is because the particle size of the additive element sources 89 can be made uniform and the particle size can be further reduced.

[0298] Furthermore, when recovering the additive element source 89 after mixing, etc., it may be classified using a sieve with an opening diameter of 250 μm to 350 μm, so that the particle size can be made uniform.

[0299] Methods of mixing while pulverizing include dry pulverization and wet pulverization. Wet pulverization is preferred because it can produce smaller particle sizes than dry pulverization. When wet pulverization is performed, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is preferable to use dehydrated acetone with a purity of 99.5% or more as the solvent. Using dehydrated acetone with the above purity can reduce impurities that may be mixed in.

[0300] In the method of mixing while pulverizing, media such as a ball mill or a bead mill can be used. Alumina balls or zirconia balls can be used as media for the ball mill and the bead mill, respectively. In the ball mill and the bead mill, centrifugal force is applied to the media, which enables microparticulation. However, if contamination from the media or the like is a concern, it is preferable to use the above-mentioned zirconia balls and set the peripheral speed to 100 mm / s or more and 2000 mm / s or less.

[0301] The additive element source 89 can be introduced into the oxide 98 by a solid-phase method, a liquid-phase method such as a sol-gel method, a sputtering method, a vapor deposition method, a CVD (chemical vapor deposition) method, a PLD (pulsed laser deposition) method, etc. In this embodiment, a case where the solid-phase method is used will be described.

[0302] 9 and 10, an additive element source 89 and an oxide 98 are mixed. Then, a mixture 99 is formed. The mixing conditions in step S71 can be selected from the mixing conditions in step S51. The mixing conditions in step S71 should be set to a rotation speed of 100 rpm or more and 200 rpm or less so that the oxide 98 does not disintegrate.

[0303] <Step S72> Step S72 shown in Figures 9 and 10 will be described below. In step S72, the mixture 99 is heated.

[0304] Here, a supplementary note about the heating temperature will be provided. The heating in step S72 must be performed at a temperature at which the reaction between the oxide 98 and the additive element source 89 proceeds or higher. The temperature at which the reaction proceeds may be any temperature at which interdiffusion between the oxide 98 and the additive element source 89 occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, and the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) and therefore, the heating temperature in step S72 should be 500° C. or higher.

[0305] Of course, the reaction is easily promoted at a temperature equal to or higher than the temperature at which the oxide 98 and the additive element source 89 are partially melted, which is preferable. For example, LiF and MgF are used as the additive element source 89. ２ In this case, the heating in step S72 is preferably performed at 700° C. or higher. ２ Since the eutectic point of is around 742°C, it is preferable to heat in step S72 to 742°C or higher.

[0306] A high heating temperature is preferable because it facilitates the reaction, shortens the heating time, and increases productivity.

[0307] However, the heating temperature must be lower than the decomposition temperature of the oxide 98. That is, the heating temperature in step S72 should be lower than the heating temperature in step S52. The heating temperature in step S72 should also be lower than the heating temperature in step S54. At temperatures near the decomposition temperature, there is a concern that the oxide 98 may decompose, albeit to a small extent. The melting point of lithium cobalt oxide is 1130°C, and at temperatures just below that, around 1000°C, lithium evaporation or cation mixing of lithium and cobalt becomes more likely to occur. Therefore, the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower.

[0308] In consideration of these, the heating temperature of step S72 is preferably 500°C or higher and lower than 1130°C, more preferably 700°C or higher and 1000°C or lower, even more preferably 700°C or higher and 950°C or lower, and even more preferably 700°C or higher and 900°C or lower. Also, it is preferably 742°C or higher and 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, even more preferably 742°C or higher and 950°C or lower, and even more preferably 742°C or higher and 900°C or lower. Also, it is preferably 800°C or higher and 1130°C or lower, or 830°C or higher and 1130°C or lower, more preferably 830°C or higher and 1000°C or lower, even more preferably 830°C or higher and 950°C or lower, and even more preferably 830°C or higher and 900°C or lower.

[0309] Furthermore, when the mixture 99 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like in the heating environment within an appropriate range.

[0310] In this manufacturing method, the fluorine source LiF may function as a flux. This function allows the heating temperature in step S72 to be lower than the decomposition temperature of oxide 98, for example, to a temperature between 742°C and 950°C, and also allows the additive element source to spread uniformly in the surface layer. As a result, a positive electrode active material 100 having the additive element in the surface layer can be manufactured.

[0311] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF may sublime when heated, and if it sublimes, the amount of LiF in the mixture 99 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the sublimation of LiF. Note that even if LiF is not used as the fluorine source, Li on the surface of the oxide 98 may react with F in a fluorine source other than LiF, resulting in the generation of LiF, which may then sublimate. Therefore, even if a fluoride having a higher melting point than LiF is used as a fluorine source other than LiF, it is still necessary to suppress sublimation.

[0312] To suppress sublimation, there is a method of heating the mixture 99 in an atmosphere containing F. This method involves making the atmosphere in the heating furnace that heats the mixture 99 into a state where the partial pressure of LiF is high. Another method is to place a lid on the reaction vessel that contains the mixture 99. By such a method or the like, it is possible to suppress the sublimation of LiF in the mixture 99, i.e., the decrease in LiF.

[0313] The heating in step S72 can be performed using a roller hearth kiln. The roller hearth kiln allows the mixture 99 to be heated while being moved within the kiln with a lid placed on the container containing the mixture 99. By placing a lid, the mixture 99 can be heated in an atmosphere containing LiF, and the sublimation, i.e., reduction, of LiF in the mixture 99 can be suppressed.

[0314] Alternatively, the heating in step S72 can be performed using a rotary kiln. It is preferable that the atmosphere in the rotary kiln contains oxygen, and heating is performed while controlling the flow rate of oxygen. To suppress the sublimation, i.e., reduction, of LiF in the mixture 99, it is preferable to reduce the flow rate of oxygen. One way to reduce the flow rate of oxygen is to first introduce oxygen into the kiln and hold it there for a certain period of time, and then not introduce oxygen thereafter.

[0315] It is believed that such a process can produce a positive electrode active material 100 with a smooth surface and minimal irregularities.

[0316] <Step S73> Step S73 shown in Figure 9 will now be described. In step S73, a crushing step is carried out. For example, classification may be carried out using a sieve with a mesh size of 40 µm or more and 60 µm or less. This can prevent particles from adhering to each other. However, as shown in Figure 10, positive electrode active material 100 can be obtained without carrying out the crushing step of step S73.

[0317] The positive electrode active material 100 can be manufactured in the manner described above. The positive electrode active material 100 can reflect the shape of the hydroxide 95, which is the precursor. Furthermore, by following the manufacturing method described above, lithium cobalt oxide can be obtained that also contains nickel inside. Nickel that is not dissolved may diffuse to the surface layer of the lithium cobalt oxide. Furthermore, lithium cobalt oxide can be obtained in which the additive element remains in the surface layer. It is preferable that the additive element is dissolved in the lithium cobalt oxide in the surface layer.

[0318] Furthermore, lithium cobalt oxide is preferable because it contains few impurities. However, when sulfide is used as the starting material, sulfur may be detected in the lithium cobalt oxide. The sulfur concentration can be measured by performing elemental analysis of the entire particles of the positive electrode active material using GD-MS, ICP-MS, etc.

[0319] <Manufacturing Method 2> In the above-mentioned Manufacturing Method 1, a method of introducing the additive element source 89 into the oxide 98 in one go was exemplified, but it may also be introduced into the oxide 98 in two or more separate steps. Therefore, in Manufacturing Method 2, a method of preparing a first additive element A1 source 89a and a second additive element A2 source 89b and dividing the introduction into two separate steps will be described with reference to Figure 12. Note that steps up to step S55 in Figure 12 are the same as step S55 described in Manufacturing Method 1, and in Manufacturing Method 2, oxide 98a can be obtained via step S55.

[0320] <First additive element A1 source> As shown in Fig. 12, a first additive element A1 source 89a is prepared. It is preferable to use a compound containing an additive element selected from the additive element sources described above, and for example, magnesium and / or fluorine can be used as the additive element. Specifically, it is preferable to prepare a first additive element A1 source 89a containing a Mg source and a F source as shown in Fig. 11A.

[0321] 12, steps S71a to S73a are performed. Note that steps S71a to S73a are similar to steps S71 to S73 described in manufacturing method 1, and therefore a description thereof will be omitted. In manufacturing method 2, oxide 98b can be obtained through step S73a.

[0322] <Second Additive Element A2 Source> Next, as shown in FIG. 12, a second additive element A2 source 89b is prepared. It is preferable to use a compound containing an additive element selected from the additive element sources described above. Furthermore, it is preferable to use an additive element different from that of the first additive element A1 source 89a for the second additive element A2 source 89b. Aluminum and / or nickel can be used as such an additive element. Specifically, the second additive element A2 source 89b may be prepared from an Al source and a Ni source as shown in FIG. 13A or 13B. While FIG. 13A shows that the Al source and Ni source are simultaneously milled and / or mixed in step S42, FIG. 13B differs in that the Al source and Ni source are separately milled and / or mixed in step S42. In this manner, the second additive element A2 source 89b can be obtained.

[0323] 12, steps S71b to S73b are performed. Note that steps S71b to S73b are similar to steps S71a to S73b described above, and therefore will not be described again. In manufacturing method 2, positive electrode active material 100 can be obtained through step S73b.

[0324] This embodiment mode can be used in combination with other embodiment modes or examples.

[0325] Embodiment 3 In this embodiment, an example of a secondary battery of one embodiment of the present invention will be described with reference to FIGS.

[0326] <Configuration Example 1 of Secondary Battery> Hereinafter, a secondary battery in which a positive electrode, a negative electrode, and an electrolyte are housed in an exterior body will be described as an example.

[0327] 14A shows an example of a cross-sectional view of a positive electrode 503 used in a secondary battery. The positive electrode 503 has a positive electrode active material layer 502 on a positive electrode current collector 501. The positive electrode active material layer 502 contains a positive electrode active material 100, a positive electrode active material 562, a conductive material 553, a conductive material 554, and an electrolyte solution 530. The positive electrode active material layer 502 also contains a binder (not shown). The secondary battery may have a configuration including either the conductive material 553 or the conductive material 554.

[0328] The D50 of the positive electrode active material 100 is preferably 1 μm or more and 100 μm or less, more preferably 15 μm or more and 40 μm or less, and even more preferably 15 μm or more and 35 μm or less. To increase the packing density, a positive electrode active material 562 having a different D50 may be added. The D50 of the positive electrode active material 562 is preferably 1 / 10 to 1 / 6 of the D50 of the positive electrode active material 100. When particle size distribution measurement is performed on an active material containing a mixture of the positive electrode active material 100 and the positive electrode active material 562, two peaks with different maximum values ​​are observed. Of course, two or more peaks may be observed. It is possible to increase the packing density without the positive electrode active material 562.

[0329] In FIG. 14A, the boundary between the surface layer and the interior is indicated by a dotted line, but the boundary is not necessarily as clear as in FIG. 14A.

[0330] The active material of the positive electrode active material 100 may be the same as or different from the active material of the positive electrode active material 562. The same active material includes the same main raw material of the active material, and may differ in the presence or absence of an additive element, etc. The different active material includes the active material that is different in the main raw material of the active material.

[0331] Again, it is preferable that the positive electrode active material 100 and the positive electrode active material 562 contain an additive element. The additive element may be unevenly distributed or thinly distributed inside.

[0332] The additive element may be present in the surface layer portion. The concentration of the additive element in the surface layer portion may be different from the concentration of the additive element in the interior, and it is preferable that the concentration of the additive element in the surface layer portion is higher than the concentration in the interior. This is sometimes referred to as the additive element being unevenly distributed in the surface layer portion.

[0333] Although the positive electrode active material 100 and the positive electrode active material 562 are sometimes called positive electrode active material particles, the shape of the positive electrode active material can be various shapes other than particulate. Unlike Fig. 14A, Fig. 14B shows a positive electrode 503 having a positive electrode active material in a shape other than particulate. Fig. 14B is the same as Fig. 14A except for the shape of the positive electrode active material, and therefore a description thereof will be omitted.

[0334] 14A and 14B are shown as primary particles, they may be secondary particles. Furthermore, the positive electrode active material 100 and the positive electrode active material 562 are preferably single particles.

[0335] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may include a conductive material and a binder. The positive electrode active material may be a positive electrode active material manufactured by the manufacturing method described in the above embodiment. For example, a positive electrode active material having a relatively small D50 and a positive electrode active material having a relatively large D50 may be mixed and used.

[0336] The positive electrode active material of one embodiment of the present invention may be mixed with another positive electrode active material.

[0337] Other examples of the positive electrode active material include composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. For example, LiFePO ４ , LiFeO ２ , LiNiO ２ , LiMn ２ O ４ , V ２ O ５ , Cr ２ O ５ , MnO ２ and the like compounds.

[0338] Other positive electrode active materials include LiMn ２ O ４ Lithium-containing materials having a spinel-type crystal structure containing manganese, such as lithium nickel oxide (LiNiO ２ or LiNi １−ｘ M ｘ O ２It is preferable to mix (0<x<1) (M=Co, Al, etc.) This configuration can improve the characteristics of the secondary battery.

[0339] In addition, as another positive electrode active material, the composition formula Li ａ Mn ｂ M ｃ O ｄ A lithium-manganese composite oxide that can be expressed by the formula (1) can be used. Here, element M is preferably a metal element selected from among lithium and manganese, or silicon or phosphorus, and more preferably nickel. Furthermore, when measuring the entire lithium-manganese composite oxide particle, it is preferable that 0<a / (b+c)<2, c>0, and 0.26≦(b+c) / d<0.5 (where a, b, c, and d are excluding 0) are satisfied during discharge. The composition of metals, silicon, phosphorus, etc. in the entire lithium-manganese composite oxide particle can be measured, for example, using an inductively coupled plasma mass spectrometer (ICP-MS). The oxygen composition in the entire lithium-manganese composite oxide particle can be measured, for example, using energy dispersive X-ray spectroscopy (EDX). Furthermore, the composition can be determined by valence evaluation using fusion gas analysis and XAFS (X-ray absorption fine structure) analysis in combination with ICP-MS analysis. The lithium manganese composite oxide refers to an oxide containing at least lithium and manganese, and may contain one or more elements selected from the group consisting of chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, phosphorus, and the like.

[0340] [Conductive Material] The conductive material functions to assist the current path between the active material and the current collector, or the current path between multiple active materials. To fulfill this function, the conductive material preferably has a lower resistance than the active material. Due to its role, the conductive material is also called a conductive auxiliary or a conductive agent.

[0341] The conductive material is typically a carbon material or a metal material. The conductive material is particulate, and examples of such particulate conductive materials include carbon black (furnace black, acetylene black, graphite, etc.). Many carbon blacks have a particle size smaller than that of the positive electrode active material. The conductive material is fibrous, and examples of such fibrous conductive materials include carbon nanotubes (CNT) and VGCF (registered trademark). The conductive material can be in sheet form, and an example of a sheet-like conductive material is multilayer graphene. Sheet-like conductive materials may appear thread-like in the cross section of the positive electrode.

[0342] Particulate conductive materials can penetrate into gaps in the positive electrode active material and are prone to aggregation. Therefore, particulate conductive materials can assist in the conductive paths between nearby positive electrode active materials. Fibrous conductive materials have bent regions, but are larger than the positive electrode active materials. Therefore, fibrous conductive materials can assist in the conductive paths between distant positive electrode active materials in addition to adjacent positive electrode active materials. In this way, it is recommended to mix two or more shapes of conductive materials.

[0343] When multilayer graphene is used as the sheet-like conductive material and carbon black is used as the particulate conductive material, the weight of the carbon black in a mixed slurry state is preferably 1.5 to 20 times, and more preferably 2 to 9.5 times, the weight of the multilayer graphene.

[0344] When the mixing ratio of multilayer graphene to carbon black is within the above range, the carbon black does not aggregate and is easily dispersed. Furthermore, when the mixing ratio of multilayer graphene to carbon black is within the above range, the electrode density can be made higher than when only carbon black is used as the conductive material. By increasing the electrode density, the capacity per unit weight can be increased.

[0345] Furthermore, by setting the mixing ratio of multilayer graphene and carbon black within the above range, rapid charging can be achieved.

[0346] In this specification, graphene includes multi-layer graphene and multi-graphene. In other words, graphene refers to a substance that contains carbon, has a shape such as a plate or sheet, and has a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings is sometimes called a carbon sheet. Furthermore, graphene compounds include graphene oxide, multi-layer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multi-layer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, and the like. In other words, graphene compounds may have functional groups. Furthermore, graphene or graphene compounds preferably have a curved shape. Furthermore, graphene or graphene compounds may be rolled, and rolled graphene is sometimes called a carbon nanofiber.

[0347] In this specification and the like, graphene oxide refers to a material that contains carbon and oxygen, has a sheet shape, and has a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

[0348] In this specification and the like, reduced graphene oxide refers to a material containing carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by six-membered carbon rings. A single sheet of reduced graphene oxide can function, but multiple sheets may be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is 2 atomic % or more and 15 atomic % or less. By achieving such carbon and oxygen concentrations, reduced graphene oxide can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0349] Fluorine-containing graphene may be used as the graphene compound. The fluorine in the graphene compound is preferably adsorbed on the surface. Fluorine-containing graphene can be produced by contacting graphene with a fluorine compound (called fluorination treatment). Fluorination treatment involves the addition of fluorine (F ２) or a fluorine compound. As the fluorine compound, hydrogen fluoride, halogen fluoride (ClF ３ , IF ５ etc.), gaseous fluorides (BF ３ , N.F. ３ , P.F. ５ , SiF ４ , SF ６ etc.), metal fluorides (LiF, NiF ２ , AlF ３ , MgF ２ For the fluorination treatment, it is preferable to use a gaseous fluoride, and the gaseous fluoride may be diluted with an inert gas. The temperature for the fluorination treatment is preferably room temperature, but is preferably 0°C or higher and 250°C or lower, which includes room temperature. When the fluorination treatment is performed at 0°C or higher, fluorine can be adsorbed on the surface of graphene.

[0350] Graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds may also have a sheet-like shape. Graphene compounds may have curved surfaces, enabling surface contact with low contact resistance. Even when thin, they may have very high conductivity, allowing a small amount of material to efficiently form a conductive path within an active material layer. Therefore, using a graphene compound as a conductive material can increase the contact area between the active material and the conductive material. It is preferable that the graphene compound covers 80% or more of the active material. It is preferable that the graphene compound clings to at least a portion of the active material particles. It is also preferable that the graphene compound overlaps at least a portion of the active material particles. It is also preferable that the shape of the graphene compound matches at least a portion of the shape of the active material particles. The shape of the active material particles refers, for example, to the unevenness of a single active material particle or the unevenness formed by multiple active material particles. It is also preferable that the graphene compound surrounds at least a portion of the active material particles. The graphene compound may also have holes.

[0351] When active material particles having a small particle size, for example, 1 μm or less, are used, the specific surface area of ​​the active material particles is large, and more conductive paths connecting the active material particles are required. In such cases, it is preferable to use a graphene compound that can efficiently form conductive paths even in a small amount.

[0352] Because of the properties described above, graphene compounds are particularly effective as conductive materials for secondary batteries that require rapid charging and rapid discharging. For example, rapid charging and rapid discharging may be required for secondary batteries for two-wheeled or four-wheeled vehicles, secondary batteries for drones, etc. Rapid charging characteristics may also be required for mobile electronic devices, etc. Rapid charging and discharging refers to, for example, charging and discharging at 200 mA / g, 400 mA / g, or 1000 mA / g or more per weight of the positive electrode active material.

[0353] In the active material layer, the sheet-like graphene or graphene compound may be uniformly dispersed. The plurality of graphene or graphene compounds are formed so as to partially cover the plurality of active materials or to be attached to the surfaces of the plurality of granular active materials, and are in surface contact with each other.

[0354] Here, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphenes or graphene compounds together. When an active material is covered with a graphene net, the graphene net can also function as a binder that bonds the active materials together. Therefore, the amount of binder can be reduced or no binder can be used, thereby improving the ratio of active material to the electrode volume and electrode weight. In other words, the discharge capacity of a secondary battery can be increased.

[0355] Here, it is preferable to use graphene oxide as the graphene or graphene compound, mix it with an active material to form an active material layer, and then reduce it. That is, the completed active material layer preferably contains reduced graphene oxide. By using graphene oxide, which has extremely high dispersibility in polar solvents, to form graphene or graphene compounds, it is possible to achieve a substantially uniform dispersion of the graphene or graphene compound within the active material layer. The solvent is volatilized and removed from the dispersion medium containing the uniformly dispersed graphene oxide, and the graphene oxide is reduced. Therefore, the graphene or graphene compound remaining in the active material layer partially overlaps and is dispersed to the extent that it is in surface contact with each other, thereby forming a three-dimensional conductive path. The reduction of graphene oxide may be performed, for example, by heat treatment or using a reducing agent.

[0356] Therefore, unlike granular conductive materials such as acetylene black that make point contact with an active material, graphene or a graphene compound enables surface contact with low contact resistance, and therefore, the electrical conductivity between a smaller amount of active material and graphene or a graphene compound than with a normal conductive material can be improved.

[0357] Furthermore, by using a spray dryer in advance, the entire surface of the active material can be covered with a graphene compound, which is a conductive material, to form a coating portion, and further, conductive paths can be formed between the active material particles by the graphene compound.

[0358] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO ２ , SiO ｘ (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particle diameter is measured using D50, and D50 is preferably 1 μm or less, and more preferably 100 nm or less.

[0359] In addition to graphene, acetylene black (abbreviated as AB) can also be used as a conductive material. Fluorine-containing acetylene black may also be used. The fluorine in fluorine-containing acetylene black is preferably adsorbed on the surface. Fluorine-containing acetylene black can also be produced by contacting acetylene black with a fluorine compound (called fluorination treatment). The fluorination treatment described for graphene can also be applied to acetylene black.

[0360] In addition to graphene and acetylene black, carbon fiber materials (also referred to as carbon nanotubes or CNTs) can be used as conductive materials. Fluorine-containing carbon nanotubes may also be used. The fluorine in the fluorine-containing carbon nanotubes is preferably adsorbed on the surface. Fluorine-containing carbon nanotubes can also be produced by contacting carbon nanotubes with a fluorine compound (called fluorination treatment). The fluorination treatment described for graphene can also be applied to carbon nanotubes.

[0361] [Binder] The binder is necessary to firmly bond the powdered active material without covering the surface of the active material. Furthermore, the binder must be adhesive to the current collector. In other words, the binder should contain a material that exhibits binding properties. Furthermore, in consideration of the expansion of the active material, the binder should be sufficiently flexible and able to adapt to changes in the state of the active material. The binder must also be compatible with the electrolyte. Furthermore, because extremely strong oxidation and reduction reactions occur in secondary batteries, a binder that does not deteriorate in these reactions or has low reactivity is desired.

[0362] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0363] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include one or more of cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, and starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0364] Alternatively, it is preferable to use, as the binder, materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0365] The binder may be used in combination with two or more of the above.

[0366] For example, a material with a particularly excellent viscosity adjusting effect may be used in combination with another material. For example, while rubber materials have excellent adhesive strength and / or elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as water-soluble polymers with a particularly excellent viscosity adjusting effect, the aforementioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, and starch may be used.

[0367] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium and ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with the active material and other components when preparing electrode slurry. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0368] Water-soluble polymers stabilize viscosity by dissolving in water, and can stably disperse active materials and other materials combined as binders, such as styrene-butadiene rubber, in aqueous solutions. Furthermore, the presence of functional groups is expected to facilitate stable adsorption to the surface of active materials. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups, such as hydroxyl and carboxyl groups, and the presence of these functional groups is expected to facilitate interactions between polymers, resulting in widespread coverage of the active material surface.

[0369] When the binder covering or contacting the surface of the active material forms a film, it is expected to function as a passive film and have the effect of suppressing decomposition of the electrolyte. Here, the passive film is a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0370] [Positive Electrode Current Collector] The positive electrode current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The positive electrode current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable that the current collector have a thickness of 5 μm to 30 μm.

[0371] [Negative Electrode] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also include a conductive material and a binder.

[0372] [Negative Electrode Active Material] As the negative electrode active material, for example, an alloy-based material and / or a carbon-based material can be used.

[0373] As the negative electrode active material, an element capable of undergoing a charge-discharge reaction by alloying / de-alloying reaction with lithium can be used. For example, a material containing one or more elements selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger charge-discharge capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g per active material weight. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, a compound containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0374] In this specification and the like, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO ｘ Here, x preferably has a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. Alternatively, x is preferably 0.2 or more and 1.2 or less. Alternatively, x is preferably 0.3 or more and 1.5 or less.

[0375] Examples of carbonaceous materials that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0376] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0377] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a low potential similar to that of lithium metal (0.05 V to 0.3 V vs. Li / Li ＋This allows lithium-ion secondary batteries to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high discharge capacity per unit volume, relatively little volume expansion, low cost, and greater safety than lithium metal.

[0378] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO ２ ) and other oxides can be used.

[0379] In addition, as the negative electrode active material, a nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large charge / discharge capacity (900 mAh / g, 1890 mAh / cm per active material weight). ３ ) and is preferred.

[0380] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the material can be combined with a material such as the above. Even when a material containing lithium ions is used as the positive electrode active material, it is possible to use a nitride of lithium and a transition metal as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0381] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ This also occurs with fluorine compounds such as

[0382] The conductive material and binder that can be contained in the negative electrode active material layer can be the same as the conductive material and binder that can be contained in the positive electrode active material layer.

[0383] [Negative electrode current collector] The negative electrode current collector can be made of the same material as the positive electrode current collector. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0384] [Electrolyte] The electrolyte contains a solvent and a lithium salt. The solvent of the electrolyte is preferably an aprotic organic solvent, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone, or any combination and ratio of two or more of these.

[0385] Furthermore, a mixed organic solvent containing a fluorinated cyclic carbonate (sometimes referred to as a fluorinated cyclic carbonate) or a fluorinated chain carbonate (sometimes referred to as a fluorinated chain carbonate) can be used as the electrolyte. Furthermore, it is preferable that the mixed organic solvent contains both a fluorinated cyclic carbonate and a fluorinated chain carbonate. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have electron-withdrawing substituents, which are preferable because they lower the solvation energy of lithium ions. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable for the electrolyte, and a mixed organic solvent containing these is also suitable.

[0386] As the fluorinated cyclic carbonate, fluorinated ethylene carbonate, for example, fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), or tetrafluoroethylene carbonate (F4EC), can be used. DFEC has isomers such as cis-4,5 and trans-4,5. Since all fluorinated cyclic carbonates have electron-withdrawing substituents, the solvation energy of lithium ions is thought to be low. In FEC, the electron-withdrawing substituent is an F group.

[0387] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0388] FEC is a cyclic carbonate with a high dielectric constant, and when used in an organic solvent, it promotes the dissociation of lithium salts. Specifically, the solvation energy of lithium ions in FEC is smaller than that of ethylene carbonate (EC), which does not have electron-withdrawing substituents. Therefore, desolvation with lithium ions is facilitated, and lithium ions are easily released from the surface of the positive electrode active material, thereby reducing the internal resistance of the secondary battery. Furthermore, FEC is thought to have a deep highest occupied molecular orbital (HOMO) level. A deep HOMO level makes it less susceptible to oxidation and improves oxidation resistance. On the other hand, there are concerns about the high viscosity of FEC. Therefore, it is recommended to use a mixed organic solvent containing MTFP in addition to FEC in the electrolyte. MTFP is a type of chain carbonate, and can have the effect of reducing the viscosity of the electrolyte solution or maintaining the viscosity at room temperature (typically 25° C.) even at low temperatures (typically 0° C.). Furthermore, MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, but may form a solvate with lithium ions when used in the electrolyte solution.

[0389] It is also preferable to use an electrochemically stable material for the electrolyte. In particular, since the positive electrode active material 100 of one embodiment of the present invention is prevented from deteriorating in its crystalline structure even when charged and discharged at a high voltage, it is preferable to combine it with an electrolyte that is chemically stable even at a high potential. For example, it is preferable to use an electrolyte that does not produce a large peak at 5.0 V or less when measured by LSV (linear sweep voltammetry). More specifically, a mixture of AB:PVdF = 1:1 was applied to a carbon-coated aluminum foil (1.130 cm) for the working electrode. ２) was used, Li metal was used as the counter electrode, a polypropylene separator was used, and the voltage scanning rate was 1.0 mV·s −１ When LSV measurement was performed at a temperature of 25°C, the current density was 1.0 mA cm at any voltage of 5.0 V or less. −２ It is preferable to use an electrolyte such as the following:

[0390] Examples of electrolytes that can achieve a current density in the above range in LSV measurement include a mixture of EC and MTFP in an EC:MTFP ratio of 2:8 (volume ratio), a mixture of FEC and MP in an FEC:MP ratio of 2:8 (volume ratio), a mixture of EC and MP in an EC:MP ratio of 2:8 (volume ratio), a mixture of FEC and MTFP in an FEC:MTFP ratio of 2:8 (volume ratio), a mixture of EC, EMC, and DMC in an EC:EMC:DMC ratio of 3:3.5:3.5 (volume ratio), a mixture of EC, EMC, and MP in an EC:EMC:MP ratio of 2:2:4 (volume ratio), and a mixture of EC and MTFP in an EC:MTFP ratio of 2:8 (volume ratio).

[0391] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from exploding and / or catching fire even if the internal temperature of the secondary battery rises due to an internal short circuit, overcharging, or the like. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0392] [Lithium Salt] The lithium salt (also called an electrolyte) to be dissolved in the solvent is, for example, LiPF ６ , LiClO４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used in an amount of 0.5 mol / L or more and 3.0 mol / L or less relative to the solvent. ６ , LiBF ４ The use of such improves the safety of lithium ion secondary batteries.

[0393] The above-mentioned electrolyte is preferably a highly purified electrolyte with a low content of granular dust or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, it is preferable that the weight ratio of impurities to the electrolyte be 1 wt % or less, preferably 0.1 wt % or less, and more preferably 0.01 wt % or less.

[0394] [Additives] Additives such as vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolyte solution. The concentration of the added material may be, for example, 0.1 wt % to 5 wt % of the total solvent. VC or LiBOB is particularly preferred because it is easy to form a good coating portion.

[0395] [Gel Electrolyte] A polymer gel in which a polymer is swollen with an electrolytic solution may be used as the gel electrolyte. By using a polymer gel electrolyte, a semi-solid electrolyte layer can be provided, and safety against leakage and the like can be improved. In addition, it is possible to reduce the thickness and weight of the secondary battery.

[0396] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel.

[0397] Examples of polymers that can be used include polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0398] [Separator] The secondary battery preferably has a separator. Examples of separators that can be used include those made of paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into an envelope shape and disposed so as to encase either the positive electrode or the negative electrode.

[0399] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0400] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide-based materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0401] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0402] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the discharge capacity per volume of the secondary battery can be increased.

[0403] [Exterior Body] The exterior body of the secondary battery can be made of, for example, a metal material such as aluminum and / or a resin material. A film-like exterior body can also be used. Examples of films that can be used include a three-layer film in which a highly flexible metal thin film made of aluminum, stainless steel, copper, nickel, or the like is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film made of polyamide resin, polyester resin, or the like is further provided on the metal thin film as the outer surface of the exterior body. A multilayer film containing aluminum is sometimes referred to as an aluminum laminate film.

[0404] <Secondary Battery Configuration Example 2> [Solid Electrolyte] Instead of an electrolytic solution, a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a solid electrolyte containing a polymer material such as a PEO (polyethylene oxide) can be used. When a solid electrolyte is used, the installation of a separator and / or spacer is unnecessary. Furthermore, since the entire battery can be solidified, there is no risk of leakage, and safety is dramatically improved.

[0405] A secondary battery using a solid electrolyte (also called a solid-state secondary battery) is expected to be chemically stable even at a high potential compared to a secondary battery having a liquid electrolyte. Therefore, an all-solid-state secondary battery using the positive electrode active material obtained in the above embodiment is expected to have good charge / discharge characteristics even when the charge voltage is 4.8 V or higher, for example, 5.0 V.

[0406] The configuration of a solid secondary battery will be described below as an example of the configuration of a secondary battery.

[0407] As shown in FIG. 15A , a secondary battery 400 of one embodiment of the present invention includes a positive electrode 410 , a solid electrolyte layer 420 , and a negative electrode 430 .

[0408] The positive electrode 410 includes a positive electrode current collector 413 and a positive electrode active material layer 414. The positive electrode active material layer 414 includes a positive electrode active material 411 and a solid electrolyte 421. The positive electrode active material 411 is formed using the positive electrode active material formed by the method described in the above embodiment. The positive electrode active material layer 414 may include a conductive agent and a binder.

[0409] Solid electrolyte layer 420 has solid electrolyte 421. Solid electrolyte layer 420 is located between positive electrode 410 and negative electrode 430, and is a region that has neither positive electrode active material 411 nor negative electrode active material 431.

[0410] The negative electrode 430 includes a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 includes a negative electrode active material 431 and a solid electrolyte 421. The negative electrode active material layer 434 may also include a conductive agent and a binder. When metallic lithium is used for the negative electrode 430, the negative electrode 430 may not include the solid electrolyte 421, as shown in FIG. 15B . Using metallic lithium for the negative electrode 430 is preferable because it can improve the energy density of the secondary battery 400.

[0411] As the solid electrolyte 421 included in the solid electrolyte layer 420, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halide-based solid electrolyte, or the like can be used.

[0412] The sulfide-based solid electrolytes include thiolithium-based (Li １０ GeP ２ S １２ , Li ３．２５ Ge ０．２５ P ０．７５ S ４ etc.), sulfide glass (70Li ２ S・30P ２ S ５ , 30Li ２ S・26B ２ S ３ ・44LiI, 63Li ２ S・36SiS ２ ・1Li ３ P.O. ４ , 57Li ２ S・38SiS ２ ・5Li ４ SiO ４ , 50Li ２ S・50GeS ２ etc.), sulfide crystallized glass (Li ７ P ３ S １１ , Li ３．２５ P ０．９５ S ４Sulfide-based solid electrolytes have the advantages of being highly conductive, being able to be synthesized at low temperatures, and being relatively soft, which makes it easy to maintain conductive paths even after charging and discharging.

[0413] The oxide-based solid electrolyte includes a material having a perovskite crystal structure (La ２／３−ｘ Li ３ｘ TiO ３ etc.), materials having a NASICON type crystal structure (Li １＋Ｘ Al Ｘ Ti ２−Ｘ (P.O. ４ ) ３ etc.), materials having a garnet-type crystal structure (Li ７ La ３ Zr ２ O １２ etc.), materials having a LISICON type crystal structure (Li １４ ZnGe ４ O １６ etc.), LLZO (Li ７ La ３ Zr ２ O １２ ), oxide glass (Li ３ P.O. ４ -Li ４ SiO ４ , 50Li ４ SiO ４ ・50Li ３ BO ３ etc.), oxide crystallized glass (Li １．０７ Al ０．６９ Ti １．４６ (P.O. ４ ) ３ , Li １．５ Al ０．５ Ge １．５ (P.O. ４ ) ３ Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0414] Halide-based solid electrolytes include LiAlCl ４ , Li ３ InBr ６, LiF, LiCl, LiBr, LiI, etc. Furthermore, composite materials in which these halide-based solid electrolytes are filled into the pores of porous aluminum oxide and / or porous silica can also be used as solid electrolytes.

[0415] Also, different solid electrolytes may be mixed and used.

[0416] Among them, Li having a NASICON type crystal structure １＋ｘ Al ｘ Ti ２−ｘ (P.O. ４ ) ３ (0<x<1) (hereinafter, LATP) is preferable because it contains aluminum and titanium, elements that may be contained in the positive electrode active material used in the secondary battery 400 of one embodiment of the present invention, and therefore a synergistic effect can be expected in improving cycle characteristics. In addition, productivity can be expected to be improved by reducing the number of steps. Note that in this specification and the like, the NASICON-type crystal structure refers to a structure having a NASICON-type crystal structure, which is a structure having a NASICON-type crystal structure, and which is ... ２ (XO ４ ) ３ (M: transition metal, X: S, P, As, Mo, W, etc.), ６ Octahedron and XO ４ It refers to a structure in which tetrahedrons are arranged three-dimensionally with their vertices shared.

[0417] This embodiment mode can be implemented in appropriate combination with other embodiment modes or examples.

[0418] Embodiment Mode 4 In this embodiment mode, an example of a secondary battery having a positive electrode manufactured by the manufacturing method described in the previous embodiment mode will be described.

[0419] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 16A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 16B is an external view, and Fig. 16C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0420] 16A is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships) for ease of understanding, and therefore, FIGS. 16A and 16B are not completely identical corresponding views.

[0421] In Fig. 16A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. These are sealed with a negative electrode can 302 and a positive electrode can 301 by a gasket. Note that the gasket for sealing is not shown in Fig. 16A. The spacer 322 and the washer 312 are used to protect the inside or to fix the position inside the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 322 and the washer 312 are made of stainless steel or an insulating material.

[0422] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0423] FIG. 16B is a perspective view of the completed coin-type secondary battery.

[0424] In the coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector. The negative electrode 307 is not limited to a laminated structure, and may be formed of lithium metal foil or a lithium-aluminum alloy foil.

[0425] It is to be noted that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each only need to have an active material layer formed on one side.

[0426] The positive electrode can 301 and the negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, etc., they are preferably coated with nickel, aluminum, or the like. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0427] These negative electrode 307, positive electrode 304, and separator 310 are immersed in an electrolyte solution, and as shown in FIG. 16C , the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downwards, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303, thereby producing a coin-type secondary battery 300.

[0428] By having the above-described configuration, the coin-type secondary battery 300 can have a high discharge capacity and excellent cycle characteristics.

[0429] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 17A. As shown in Fig. 17A, a cylindrical secondary battery 616 has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0430] 17B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in FIG. 17B has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0431] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal, such as nickel, aluminum, or titanium, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel), which is corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat the battery can 602 with nickel, aluminum, or the like to prevent corrosion by the electrolyte. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. A nonaqueous electrolyte (not shown) is injected inside the battery can 602, in which the battery element is provided. The nonaqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0432] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector.

[0433] By using the positive electrode active material 100 of one embodiment of the present invention for the positive electrode 604, the cylindrical secondary battery 616 can have a large capacity, a large discharge capacity, and excellent cycle characteristics.

[0434] A positive electrode terminal (positive electrode current collecting lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collecting lead) 607 is connected to the negative electrode 606. The positive electrode terminal 603 can be made of a metal material such as aluminum. The negative electrode terminal 607 can be made of a metal material such as copper. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases when the temperature rises, and the increase in resistance limits the amount of current to prevent abnormal heat generation. ３ )-based semiconductor ceramics, etc. can be used.

[0435] 17C shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616. The positive electrodes of each secondary battery are in contact with and electrically connected to conductors 624 separated by insulators 625. The conductors 624 are electrically connected to a control circuit 620 via wiring 623. The negative electrodes of each secondary battery are electrically connected to the control circuit 620 via wiring 626. The control circuit 620 may be a charge / discharge control circuit that performs charging and discharging, or a protection circuit that prevents overcharging and / or overdischarging.

[0436] 17D shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of secondary batteries 616 may be connected in parallel, in series, or in parallel and then further connected in series. By configuring the power storage system 615 to have a plurality of secondary batteries 616, it is possible to extract a large amount of power.

[0437] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0438] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When the secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside air temperature.

[0439] 17D , the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of secondary batteries 616 via a conductive plate 614.

[0440] [Another Example of Secondary Battery Structure] Another example of the secondary battery structure will be described with reference to FIGS. 18 and 19. FIG.

[0441] A secondary battery 913 shown in FIG. 18A has a wound body 950 provided with terminals 951 and 952 inside a housing 930. The wound body 950 is immersed in an electrolyte inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that in FIG. 18A , for convenience, the housing 930 is shown separated, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a laminate of a metal material and a resin material.

[0442] 18B, the housing 930 shown in Fig. 18A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 18B has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0443] The housing 930a can be made of a metal material (for example, aluminum) or a laminate of a metal material and a resin material. In particular, by using a material such as organic resin on the surface on which the antenna is formed, it is possible to suppress shielding of the electric field by the secondary battery 913. Note that if the shielding of the electric field by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of a metal material (for example, aluminum) or a laminate of a metal material and a resin material.

[0444] 18C shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0445] 19A may be a secondary battery 913 having a wound body 950a as shown in Fig. 19A. The wound body 950a shown in Fig. 19A has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0446] By using the positive electrode active material 100 of one embodiment of the present invention for the positive electrode 932, the secondary battery 913 can have a large capacity, a high discharge capacity, and excellent cycle characteristics.

[0447] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0448] 19B , the negative electrode 931 is electrically connected to a terminal 951 by ultrasonic bonding, welding, or crimping. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952 by ultrasonic bonding, welding, or crimping. The terminal 952 is electrically connected to a terminal 911b.

[0449] 19C , the wound body 950 a and the electrolyte are covered by the housing 930 to form the secondary battery 913. It is preferable to provide the housing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the internal pressure inside the housing 930 reaches a predetermined value to prevent the battery from exploding.

[0450] As shown in Fig. 19B, the secondary battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a can result in a secondary battery 913 with a larger discharge capacity. For other elements of the secondary battery 913 shown in Figs. 19B and 19C, refer to the description of the secondary battery 913 shown in Figs. 18A and 18B.

[0451] 20A and 20B show examples of external views of a laminated secondary battery. As shown in Fig. 20A and 20B, a laminated secondary battery 500 has a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0452] 20A shows an external view of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as a tab region). The negative electrode 506 has a negative electrode current collector 504, and a negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., a tab region. Note that the area or shape of the tab regions of the positive electrode 503 and the negative electrode 506 are not limited to the example shown in FIG. 20A .

[0453] <Method of Manufacturing Laminated Secondary Battery> An example of a method of manufacturing the laminated secondary battery 500 whose external view is shown in FIG. 20A will be described with reference to FIGS. 21A to 21C.

[0454] First, as shown in FIG. 21A , a positive electrode 503 and a negative electrode 506 are prepared. The positive electrode 503 has a positive electrode active material layer 502 and a tab 501. The tab 501 is part of the positive electrode current collector. The negative electrode 506 has a negative electrode active material layer 505 and a tab 504. The tab 504 is part of the negative electrode current collector. Next, the negative electrode 506, a separator 507, and a positive electrode 503 are laminated. FIG. 21B shows the laminated negative electrode 506, separator 507, and positive electrode 503. Here, an example is shown in which five pairs of negative electrodes 506 and four pairs of positive electrodes 503 are used. This can also be called a laminate consisting of the negative electrode 506, the separator 507, and the positive electrode 503. Next, the tab regions of the positive electrode 503 are joined together, and a positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode 503. The joining may be performed by, for example, ultrasonic welding, etc. Similarly, the tab regions of the negative electrodes 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode 506.

[0455] Next, as shown in FIG. 21C, the above-described laminate is placed on an exterior body 509.

[0456] Next, as shown in Fig. 21C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding or the like may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0457] Next, the electrolyte solution is introduced into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of the electrolyte solution is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, the laminated secondary battery 500 can be produced.

[0458] By using the positive electrode active material 100 of one embodiment of the present invention for the positive electrode 503, the secondary battery 500 can have a high capacity, a high discharge capacity, and excellent cycle characteristics.

[0459] This embodiment mode can be implemented in appropriate combination with other embodiment modes or examples.

[0460] Embodiment 5 In this embodiment, an example of a vehicle including a secondary battery according to one embodiment of the present invention will be described. The secondary battery according to one embodiment of the present invention is preferable because it is less likely to catch fire.

[0461] When a secondary battery according to one embodiment of the present invention is installed in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs) can be realized. Furthermore, the secondary battery can also be installed in agricultural machinery, motorized bicycles including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, artificial satellites, space probes, planetary probes, or spacecraft. The secondary battery according to one embodiment of the present invention can be a high-capacity secondary battery. Therefore, the secondary battery according to one embodiment of the present invention is suitable for miniaturization and weight reduction and can be suitably used in transportation vehicles.

[0462] 22A to 22D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 illustrated in FIG. 22A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a secondary battery is installed in a vehicle, an example of the secondary battery described in Embodiment 4 is installed in one or more locations. The automobile 2001 illustrated in FIG. 22A includes a battery pack 2200, which includes a secondary battery module to which multiple secondary batteries are connected. It is preferable that the automobile further include a charge control device electrically connected to the secondary battery module.

[0463] Furthermore, automobile 2001 can charge its secondary battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging may be performed using a predetermined charging method or connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The charging facility may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge an electric storage device mounted on automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device, such as an AC-DC converter.

[0464] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0465] Figure 22B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. Transport vehicle 2002 has a battery pack 2201, which has a secondary battery module in which multiple secondary batteries are connected. The secondary battery module of transport vehicle 2002 is, for example, a four-cell unit of secondary batteries with a nominal voltage of 3.0 V to 5.0 V, with 48 cells connected in series for a maximum voltage of 170 V. Apart from the number of secondary batteries constituting the secondary battery module of battery pack 2201, the secondary battery module has the same functions as those shown in Figure 22A, and therefore a description thereof will be omitted.

[0466] FIG. 22C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The transport vehicle 2003 has a battery pack 2202, which has a secondary battery module in which multiple secondary batteries are connected. The secondary battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, in which one hundred or more secondary batteries with a nominal voltage of 3.0 V or more and 5.0 V or less are connected in series. Therefore, a secondary battery with little variation in characteristics is required. By using a secondary battery in which the positive electrode active material 100 described in Embodiments 1 and 2 is used as a positive electrode, a secondary battery with stable battery characteristics can be manufactured, and mass production at low cost from the standpoint of yield is possible.

[0467] Fig. 22D shows, as an example, an aircraft 2004 having an engine that burns fuel. Since the aircraft 2004 shown in Fig. 22D has wheels for takeoff and landing, it can also be said to be part of a transportation vehicle, and has a battery pack 2203 that includes a secondary battery module formed by connecting multiple secondary batteries and the secondary battery module and a charge control device.

[0468] The secondary battery module of the aircraft 2004 has, for example, eight 4 V secondary batteries connected in series, with a maximum voltage of 32 V. Other than the number of secondary batteries constituting the secondary battery module of the battery pack 2203, the secondary battery module has the same functions as those in Fig. 22A, and therefore a description thereof will be omitted.

[0469] 22E illustrates, as an example, a satellite 2005 equipped with a secondary battery 2204. Because the satellite 2005 is used in space at extremely low temperatures, it is preferable that the satellite 2005 be equipped with the secondary battery 2204, which is one embodiment of the present invention and has excellent low-temperature resistance. It is more preferable that the secondary battery 2204 be mounted inside the satellite 2005 while being covered with a heat-insulating member.

[0470] This embodiment mode can be implemented in appropriate combination with other embodiment modes or examples.

[0471] Embodiment 6 In this embodiment, an example in which a lithium-ion battery according to one embodiment of the present invention is mounted on a motorcycle or a bicycle will be described as an example in which a secondary battery is mounted on a vehicle.

[0472] 23A illustrates an example of an electric bicycle using the power storage device of one embodiment of the present invention. The power storage device of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 23A. The power storage device of one embodiment of the present invention includes, for example, a plurality of storage batteries and a protection circuit.

[0473] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state detached from the bicycle in FIG. 23B . The power storage device 8702 includes a plurality of built-in storage batteries 8701, which are included in the power storage device of one embodiment of the present invention, and a display unit 8703 can display the remaining battery charge and other information. The power storage device 8702 also includes a control circuit 8704 that can control charging or detect an abnormality of the secondary battery. The control circuit 8704 is electrically connected to the positive electrode and negative electrode of the storage battery 8701. Furthermore, a synergistic effect in terms of safety can be obtained by combining the power storage device 8702 with a secondary battery using the positive electrode active material 100 of one embodiment of the present invention for its positive electrode. The secondary battery using the positive electrode active material 100 of one embodiment of the present invention for its positive electrode and the control circuit 8704 can significantly contribute to eliminating accidents, such as fires, caused by secondary batteries.

[0474] 23C illustrates an example of a two-wheeled vehicle using the power storage device of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 23C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. The power storage device 8602, which includes a plurality of secondary batteries each using the positive electrode active material 100 of one embodiment of the present invention for its positive electrode, can have a high capacity and contribute to miniaturization.

[0475] 23C can store a power storage device 8602 in an under-seat storage compartment 8604. The power storage device 8602 can be stored in the under-seat storage compartment 8604 even if the under-seat storage compartment 8604 is small.

[0476] This embodiment mode can be implemented in appropriate combination with other embodiment modes or examples.

[0477] (Embodiment 7) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted in an electronic device will be described. The secondary battery according to one embodiment of the present invention is preferable because it is less likely to ignite. Examples of electronic devices in which a secondary battery is mounted include television devices (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0478] 24A shows an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, an operation button 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 includes a secondary battery 2107. When the secondary battery 2107 includes the positive electrode active material 100 described in Embodiments 1 and 2, the secondary battery 2107 can have a high capacity, and a space-saving configuration can be realized by miniaturizing the housing 2101.

[0479] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0480] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0481] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0482] The mobile phone 2100 also includes an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0483] Furthermore, the mobile phone 2100 preferably has a sensor. As the sensor, for example, a fingerprint sensor, a pulse sensor, a body temperature sensor or other human body sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted.

[0484] FIG. 24B illustrates an unmanned aerial vehicle 2300 having a plurality of rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 of one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. A secondary battery using the positive electrode active material 100 of one embodiment of the present invention for its positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Thus, the secondary battery is suitable as a secondary battery to be installed in the unmanned aerial vehicle 2300.

[0485] Fig. 24C shows an example of a robot. A robot 6400 shown in Fig. 24C includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0486] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0487] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0488] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0489] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. A secondary battery using the positive electrode active material 100 according to one embodiment of the present invention for its positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery is suitable as the secondary battery 6409 to be mounted on the robot 6400.

[0490] FIG. 24D illustrates an example of a portable fan. The portable fan 6200 illustrated in FIG. 24D includes a secondary battery 6209 of one embodiment of the present invention, an operation button 6205, a fan 6202, an external connection port 6204, and the like. The secondary battery 6209 is housed in a housing 6201. The portable fan 6200 can operate a motor using power supplied from the secondary battery 6209 to rotate the fan 6202. The secondary battery 6209 can be charged through the external connection port 6204. Although an example of a cylindrical secondary battery is illustrated as the secondary battery 6209, the shape of the secondary battery 6209 is not particularly limited. A secondary battery using the positive electrode active material 100 of one embodiment of the present invention has a stable crystal structure and is therefore unlikely to ignite. Therefore, the secondary battery 6209 is suitable as the secondary battery 6306 to be mounted in the portable fan 6200.

[0491] 24E shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck the dust 6310 from a suction port provided on the bottom surface of the housing 6301.

[0492] The cleaning robot 6300 can analyze an image captured by the camera 6303 and determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. A secondary battery using the positive electrode active material 100 according to one embodiment of the present invention for its positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery 6306 is suitable for use in the cleaning robot 6300.

[0493] This embodiment mode can be implemented in appropriate combination with other embodiment modes or examples.

[0494] In this example, samples of the positive electrode active material were prepared according to the above embodiment and measurements were carried out. The steps for preparing the samples are described in detail below.

[0495] <Preparation of Samples> <Sample 1> In this example, samples were prepared by adjusting the ratio of Co and Ni. First, the preparation process of Sample 1, which does not use Ni, will be described. Cobalt (II) sulfate heptahydrate (CoSO 4 ) was used as the cobalt source 81 shown in FIG. 12 of the above embodiment. ４ ・7H ２ 0) was prepared, and first glycine was prepared as a chelating agent 83, and these were mixed as in step S14 to obtain a mixed solution 91 in which cobalt sulfate was dissolved. In the mixed solution 91, the combined concentration of cobalt sulfate and nickel sulfate was set to 2 mol / L, and the concentration of the first glycine was set to 0.1 mol / L.

[0496] Next, sodium hydroxide dissolved in pure water (aqueous sodium hydroxide solution) was prepared as the alkaline aqueous solution 84 shown in Fig. 12. The concentration of sodium hydroxide was adjusted to 5 mol / L.

[0497] 12, an aqueous solution containing a second glycine was prepared as the chelating agent 85. The concentration of the second glycine in the aqueous solution containing the second glycine was adjusted to 0.1 mol / L. The aqueous solution containing the second glycine is referred to as the charging solution.

[0498] <Coprecipitation Conditions> An OptiMax (Mettler Toledo) coprecipitation apparatus was used. The reaction vessel of the coprecipitation apparatus was filled with the charging solution, stirred with a stirrer at a rotation speed of 1000 rpm, and an aqueous sodium hydroxide solution was appropriately added dropwise to maintain the liquid temperature at 50°C and the pH at 10.8. The mixed solution 91 was added dropwise to the reaction vessel at a rate of 0.10 mL / min. A tube for adding the mixed solution 91 was inserted into the charging solution. In this case, the dropping process cannot be observed, so the dropping process can be referred to as "liquid delivery." Nitrogen was also supplied from the top of the reaction vessel at a rate of 1 L / min. The coprecipitation reaction proceeded in the reaction vessel. After the dropping was completed, the liquid temperature was maintained at 25°C.

[0499] According to step S32 shown in Fig. 12, the suspension produced by the coprecipitation reaction was subjected to suction filtration with pure water, and then suction filtration with acetone to obtain a precipitate. Thereafter, according to step 33 shown in Fig. 12, the precipitate was dried in a vacuum drying oven at 80°C for 12 hours to obtain hydroxide 95 (Sample 1). It is believed that by thoroughly drying the hydroxide 95 as in Sample 1, it is possible to obtain hydroxide 95 from which impurities have been removed. The hydroxide 95 is cobalt hydroxide. The hydroxide 95 is sometimes referred to as a precursor.

[0500] Lithium hydroxide was prepared as the lithium source 88 in Figure 12. The lithium hydroxide was crushed in a fluidized bed jet mill at 10,000 rpm or 60,000 rpm for 45 minutes or 90 minutes. In step S42, the molar ratio of lithium hydroxide to the precursor (referred to as Li / Co or Li / (Ni+Co)) was adjusted to 1.03. Note that Li is set to 1 when calculating the theoretical capacity and depth of charge.

[0501] Next, according to steps S51 and S54 of FIG. 12 , the precursor and lithium hydroxide were mixed three times for 1.5 minutes using a rotation / revolution mixer at 2000 rpm to obtain mixture 96. Mixture 96 was then heated at 1000°C for 10 hours. A muffle furnace was used as the heating furnace, and oxygen was passed through at a flow rate of 5 L / min. The mixture was then cooled to room temperature to obtain oxide 98a. Note that oxide 98a is called a composite oxide, and can function as a positive electrode active material even at this stage. Furthermore, the atomic ratio of the positive electrode active material obtained through these processes may not be equal to the adjusted molar ratio. Step S56 of FIG. 12 was not performed.

[0502] As the first additive element A1 source 89a in FIG. 12, a compound containing an additive element was prepared. Specifically, LiF:MgF ２ The first additive element A1 source 89a was prepared by mixing LiF and MgF adjusted to the above molar ratio. ２ The above was mixed in dehydrated acetone using zirconia balls with a diameter of 1 mm, and then sieved through a sieve with openings of 300 μm. ２ The mixture having the above formula was mixed three times for 1.5 minutes at 2000 rpm using a planetary centrifugal mixer to obtain mixture 99a.

[0503] The mixture 99a was heated according to step S72a of Fig. 12. The heating was carried out at 900°C for 20 hours. A muffle furnace was used as the heating furnace, and oxygen was flowed at a flow rate of 5 L / min. The mixture was then cooled to room temperature, and an oxide 98b was obtained.

[0504] A compound containing an additive element was prepared as the second additive element A2 source 89b in FIG. 12. Specifically, aluminum hydroxide (Al(OH) ３ ), nickel hydroxide (Ni(OH) ２These were mixed in dehydrated acetone and then sieved through a 300 μm mesh sieve. Aluminum hydroxide was weighed out so that it was 0.5 mol % of the transition metal M (M = Co + Ni) and nickel hydroxide was weighed out so that it was 0.5 mol % of the transition metal M.

[0505] According to step S71b in FIG. 12, oxide 98b and Al(OH) ３ and Ni(OH) ２ The mixture was mixed three times for 1.5 minutes at 2000 rpm using a planetary centrifugal mixer, and then mixed three times for 1.5 minutes at 2000 rpm using a planetary centrifugal mixer to obtain mixture 99b.

[0506] Mixture 99b was heated according to step S72b in FIG. 12 . Heating was performed at 850° C. for 10 hours. A muffle furnace was used as the heating furnace, and oxygen was flowed at a flow rate of 5 L / min. The mixture was then cooled to room temperature, yielding cathode active material 100. Note that in this example, the crushing step S73 in FIG. 12 was not performed. The cathode active material produced through the above steps was designated Sample 1.

[0507] <Sample 2> Nickel (II) sulfate hexahydrate (NiSO ) was used as the nickel source 82 together with the cobalt source 81 shown in FIG. ４ ・6H ２ A positive electrode active material was prepared in the same manner as Sample 1 except that the molar ratio of cobalt sulfate to nickel sulfate in the mixed solution 91 was 99:1 and the pH of the coprecipitation conditions was 11. This positive electrode active material was designated Sample 2. For example, the positive electrode active material of one embodiment of the present invention was prepared by mixing LiCo （１−ｙ） Ni ｙ O ２ In Sample 2, the molar ratio of the raw materials was adjusted so that y was 0.01. However, the atomic ratio of the positive electrode active material obtained through the coprecipitation reaction, suction filtration, mixing of the lithium compound, and / or heating process may not be equal to the adjusted molar ratio.

[0508] <Sample 3> In addition to the cobalt source 81 shown in FIG. 12, nickel sulfate (NiSO 4 ) was used as the nickel source 82. ４) was prepared, the molar ratio of cobalt sulfate to nickel sulfate in the mixed solution 91 was set to 95:5, and the pH of the coprecipitation conditions was set to 11. Except for this, a positive electrode active material was prepared in the same manner as in Sample 1. （１−ｙ） Ni ｙ O ２ In Sample 3, the molar ratio of the raw materials was adjusted so that y was 0.05.

[0509] <Sample 4> In addition to the cobalt source 81 shown in FIG. 12, nickel sulfate (NiSO 4 ) was used as the nickel source 82. ４ The molar ratio of cobalt sulfate to nickel sulfate in the mixed solution 91 was adjusted to 90:10, and the pH of the coprecipitation conditions was adjusted to 11. The positive electrode active material was prepared in the same manner as in Sample 1, and designated Sample 4. That is, LiCo （１−ｙ） Ni ｙ O ２ In Sample 4, the molar ratio of the raw materials was adjusted so that y was 0.10.

[0510] <Sample 5> Nickel sulfate (NiSO ) was used as the nickel source 82 together with the cobalt source 81 shown in FIG. ４ The molar ratio of cobalt sulfate to nickel sulfate in the mixed solution 91 was set to 80:20, and the pH of the coprecipitation conditions was set to 11. The positive electrode active material was prepared in the same manner as in Sample 1, and designated Sample 5. （１−ｙ） Ni ｙ O ２ In Sample 5, the molar ratio of the raw materials was adjusted so that y was 0.20.

[0511] <Sample 6> Nickel sulfate (NiSO ) was used as the nickel source 82 together with the cobalt source 81 shown in FIG. ４ ) was prepared, the molar ratio of cobalt sulfate to nickel sulfate in the mixed solution 91 was set to 68:32, and the pH of the coprecipitation conditions was set to 11. Except for this, a positive electrode active material was prepared in the same manner as in Sample 1, and was designated Sample 6. That is, LiCo （１−ｙ） Ni ｙ O ２ In Sample 6, the molar ratio of the raw materials was adjusted so that y was 0.32.

[0512] For Samples 1 to 6, the amount of nickel (molar ratio) relative to the sum of nickel and cobalt in the raw materials is shown in Table 1. The molar ratio can also be converted to atomic %. For example, when the composition formula of the obtained positive electrode active material is LiMO ２ When calculated assuming that M is Co and Ni, the Co content is 24.8 atomic % and the Ni content is 0.25 atomic % in Sample 2. When the additive elements (Mg, Al, Ni, and F) of the obtained positive electrode active material are included, the Co content is 24.5 atomic % and the Ni content is 0.4 atomic % in Sample 2, for example.

[0513]

[0514] <SEM Image 1 of Precursor (Hydroxide) and Oxide> SEM images were obtained of the precursor (hydroxide) and oxide corresponding to Samples 1 to 6. The oxides used for observing the SEM images were prepared by crushing the lithium source 88 at 10,000 rpm.

[0515] Fig. 25A shows a planar SEM image of the precursor of Sample 1, i.e., the oxide 95 in Fig. 11. Fig. 25B shows a planar SEM image of Sample 1 before the addition of the additive element source, i.e., the oxide 98a in Fig. 11. The SEM images were taken using an S4800 manufactured by Hitachi High-Technologies Corporation, and the accelerating voltage was fixed at 5 kV.

[0516] Similarly, Fig. 26A shows a planar SEM image of the precursor of Sample 2, Fig. 26B shows a planar SEM image of the oxide corresponding to Sample 2 before the addition of the additive element source, and Fig. 26C shows a cross-sectional SEM image of the oxide corresponding to Sample 2 before the addition of the additive element source.

[0517] Similarly, a planar SEM image of the precursor of Sample 3 is shown in FIG. 27A, a planar SEM image of the oxide corresponding to Sample 3 and prior to the addition of the additive element source is shown in FIG. 27B, and a cross-sectional SEM image is shown in FIG. 27C.

[0518] Similarly, a planar SEM image of the precursor of Sample 4 is shown in FIG. 28A, a planar SEM image of the oxide corresponding to Sample 4 before the addition of the additive element source is shown in FIG. 28B, and a cross-sectional SEM image is shown in FIG. 28C.

[0519] Similarly, a planar SEM image of the precursor of Sample 5 is shown in FIG. 29A, a planar SEM image of the oxide corresponding to Sample 5 and prior to the addition of the additive element source is shown in FIG. 29B, and a cross-sectional SEM image is shown in FIG. 29C.

[0520] Similarly, a planar SEM image of the precursor of Sample 6 is shown in FIG. 30A, a planar SEM image of the oxide corresponding to Sample 6 before the addition of the additive element source is shown in FIG. 30B, and a cross-sectional SEM image is shown in FIG. 30C.

[0521] 25A to 30C, in all samples, the oxides mixed with the lithium source and heated in step S54 had smoother surfaces than the precursors. In addition, in samples 2 and 3 containing nickel, deposits were sometimes observed as indicated by the arrows in FIGS. 26B and 27B, but no deposits were observed in the oxide corresponding to sample 1, which does not contain nickel.

[0522] <EDX Point Analysis Measurement> Point analysis was performed on Samples 2 to 6 before the addition of the additive element source, i.e., the oxides corresponding to oxide 98a in Fig. 11, using a cross-sectional scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) method. The EDX measurement device used was an energy dispersive X-ray analyzer Ultim Max 170 manufactured by Oxford Instruments, with an acceleration voltage of 3 kV to 30 kV, and a Si drift detector for X-ray detection. In EDX elemental analysis, the detection depth is several nm to several μm depending on the acceleration voltage, and the energy resolution is 130 eV to 140 eV.

[0523] The results of the EDX point analysis measurements are shown in Tables 2 to 6. The measurement locations are also shown in Figures 26C, 27C, 28C, 29C, and 30C.

[0524]

[0525]

[0526]

[0527]

[0528]

[0529] As shown in Table 2, nickel was below the detection limit at all measurement points in Sample 2 before the addition of the additive element source. This was thought to be due to the fact that nickel was more difficult to precipitate than cobalt when synthesizing hydroxide by the coprecipitation method.

[0530] As shown in Table 3, nickel was detected at every measurement point in Sample 3 before the addition of the additive element sources. Ni / (Ni+Co) was 0.033 or more and 0.046 or less.

[0531] As shown in Table 4, nickel was detected at every measurement point in Sample 4 before the addition of the additive element sources. Ni / (Ni+Co) was 0.085 or more and 0.100 or less.

[0532] As shown in Table 5, nickel was detected at all measurement points in Sample 5 before the addition of the additive element sources. Ni / (Ni+Co) was 0.185 or more and 0.202 or less.

[0533] As shown in Table 6, nickel was detected at all measurement points in Sample 6 before the addition of the additive element sources. Ni / (Ni+Co) was 0.282 or more and 0.310 or less.

[0534] <XRD Measurement> The powder XRD measurement results of the positive electrode active materials of Samples 1 to 6 are shown in FIG. 31. For comparison, O3 (LiCoO ２ ) and the crystal structure of O3 (LiNiO ２ The XRD pattern corresponding to the crystal structure of the sample is also shown. The vertical axis of the XRD measurement results represents intensity (arb. unit) and the horizontal axis represents 2θ. The XRD measurement conditions were as follows: XRD device: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １ Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 75° Step width (2θ): 0.005° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0535] The positive electrode active materials corresponding to Samples 1 to 6 were all O3 (LiCoO ２The diffraction pattern was similar to that of the crystal structure of O3 (LiNiO). As the Ni / (Ni+Co) ratio increased, the peaks tended to shift to lower angles. ２ ) crystal structure.

[0536] The crystalline structure of the positive electrode active material was analyzed using Diffrac.TOPAS for the XRD pattern shown in Figure 31. The space group was R-3m for all of the samples. The crystallite size, a-axis, and c-axis lattice constants are shown in Table 7.

[0537]

[0538] As shown in Table 7, no significant difference was observed in the lattice constants. In Samples 4 to 6, the crystallite size tended to decrease as Ni / (Ni+Co) increased.

[0539] In Samples 2 to 6, which contained nickel as the raw material, the crystallite size was 250 nm or more and 600 nm or less. In Samples 2 and 3, which contained nickel as the raw material with a Ni / (Co+Ni) ratio of more than 0.005 and less than 0.1, the crystallite size was 420 nm or more and 550 nm or less.

[0540] <Preparation of Positive Electrodes and Test Batteries for XRD Testing> Samples 1 to 6 were prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder. PVDF was prepared by dissolving it in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, the positive electrode active material, AB, and PVDF were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After applying the slurry to the positive electrode current collector, the solvent was evaporated. No pressing process was performed.

[0541] By the above steps, positive electrodes including Samples 1 to 6 were obtained.

[0542] A test battery (referred to as a half cell) was fabricated using the above positive electrode and lithium metal as a counter electrode. The test battery was a coin-type cell (CR2032 type, diameter 20 mm, height 3.2 mm).

[0543] The electrolyte for the half-cell consisted of a mixed organic solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 30:70, to which lithium hexafluorophosphate (LiPF) was added at a concentration of 1 mol / L. ６ An organic electrolyte solution was prepared by dissolving VC in the organic electrolyte solution at a concentration of 2 wt %.

[0544] A porous polypropylene film having a thickness of 25 μm was used as the separator of the half cell.

[0545] Coin-shaped half cells containing Samples 1 to 6 were fabricated using positive and negative electrode cans made of stainless steel (SUS316).

[0546] Half cells containing Samples 1 to 6 were subjected to one charge-discharge cycle for aging, followed by charging for XRD measurement. Specifically, for aging (first charge-discharge cycle), charging was performed using CCCV (0.2 C, 4.5 V, 0.02 C cut) and discharging using CC (0.2 C, 3.0 V cut). Charging for XRD measurement (second charge-discharge cycle) was performed using CCCV (0.2 C, 4.6 V, 0.02 C cut). The depth of charge for charging for XRD measurement was 0.8 or more for Sample 2. In this example, 1 C = 200 mA / g (per weight of positive electrode active material). Table 8 shows the charge-discharge capacities for Samples 1 to 5.

[0547]

[0548] As shown in Table 8, for Samples 2 to 5, which contained nickel as a raw material, the irreversible capacity tended to increase as the Ni / (Ni + Co) ratio increased. This was thought to be due to cation mixing. However, the depth of charge, including the irreversible capacity, was approximately 80% for all samples during the second charge for XRD measurement.

[0549] Immediately after charging was completed, the charged half-cell was disassembled in a glove box under an argon atmosphere, the positive electrode was removed, and the electrolyte was removed by washing with DMC (dimethyl carbonate). The removed positive electrode was attached to a flat substrate with double-sided tape and sealed in an airtight sample holder under an argon atmosphere. The positive electrode active material layer was set to match the measurement surface required by the device. XRD measurement was performed at room temperature. The XRD measurement conditions were as follows: XRD device: D8 ADVANCE manufactured by Bruker AXS; X-ray: CuKα; Output: 40 kV, 40 mA; Divergence angle: Div. Slit, 0.5°; Detector: LynxEye; Scan method: 2θ / θ continuous scan; Measurement range (2θ): 15° to 75°; Step width (2θ): 0.01° setting; Counting time: 1 second / step; Sample stage rotation: 15 rpm

[0550] Diffraction patterns of the positive electrodes having the positive electrode active materials of Samples 1 to 6 at 4.6 charge times are shown in Figure 32. For comparison, the diffraction patterns of H1-3 and O3' are also shown. Figures 33 and 34 are enlarged diffraction patterns of a portion of Figure 32. The apex of a diffraction pattern is called a diffraction peak.

[0551] As shown in Figures 32 to 34, diffraction peaks were detected at 2θ = 19.29° and 2θ = 45.52° in Sample 1, and at 2θ = 19.31° and 2θ = 45.53° in Sample 2. These diffraction peaks satisfied the diffraction peaks that appear when the O3'-type crystal structure described in the above embodiment is present, namely, 2θ = 19.30 ± 0.20° (19.10° or more and 19.50° or less) and 2θ = 45.55 ± 0.10° (45.45° or more and less than 45.65°). Therefore, it was found that Sample 1 and Sample 2 have an O3'-type crystal structure. Furthermore, diffraction peaks were detected at 2θ = 19.05° and 2θ = 45.32° in Sample 1. These were diffraction peaks of O3, and it was found that Sample 1 also has an O3-type crystal structure. No H1-3 diffraction peaks were observed in Sample 1 or Sample 2.

[0552] 32 to 34, Rietveld analysis revealed that the abundance ratio of the O3 type crystal structure was 17 wt % and the abundance ratio of the O3' type crystal structure was 83 wt % in Sample 1. Furthermore, the abundance ratio of the O3 type crystal structure was 6 wt % and the abundance ratio of the O3' type crystal structure was 94 wt % in Sample 2. Samples 1 and 2, which have the O3' type crystal structure, are preferable because they exhibited high discharge capacity retention rates even under high voltage conditions in charge-discharge cycle tests.

[0553] On the other hand, diffraction peaks of H1-3 were detected in Samples 4 and 5. Also, diffraction peaks of O3 were detected in Sample 6. This is because the value of Ni / (Ni+Co) increased and the crystal structure became LiNiO ２ This was thought to be due to the fact that it was approaching

[0554] <Preparation of Positive Electrodes for Charge-Discharge Cycle Test and Test Batteries> Batteries for charge-discharge cycle test were prepared in the same manner as the positive electrodes for XRD test and test batteries, except that a press treatment was performed using a roll press machine to increase the density of the positive electrode active material layer on the positive electrode current collector. The press treatment was performed under a linear pressure of 210 kN / m. The upper and lower rolls of the roll press machine were both set to 120°C. In Samples 1 to 6, the loading amount was 7 mg / cm. ２ In this specification and the like, the amount carried is the sum of the amounts of the positive electrode active material, the conductive material, and the binder per area of ​​the current collector.

[0555] Similarly, batteries for charge-discharge cycle tests were also prepared for the samples before the addition of the additive elements, that is, for the oxides corresponding to oxide 98a in FIG.

[0556] A charge / discharge cycle test of the half cell was performed using a charge / discharge measuring system (TOSCAT-3100) manufactured by Toyo Systems Co., Ltd. The charge / discharge cycle test using the half cell allows the performance of the positive electrode alone to be understood.

[0557] The rate of charge-discharge cycle test conditions will now be described. The rate during discharge is called the discharge rate, which is the relative ratio of the current during discharge to the battery capacity and is expressed in units of C. For a battery with a rated capacity of X (Ah), the current equivalent to 1 C is X (A). When a battery is discharged at a current of 2X (A), it is said to have been discharged at 2C, and when a battery is discharged at a current of X / 2 (A), it is said to have been discharged at 0.5C. The rate during charging is called the charge rate. Similarly to the charge rate, when a battery is charged at a current of 2X (A), it is said to have been charged at 2C, and when a battery is charged at a current of X / 2 (A), it is said to have been charged at 0.5C. The charge rate and discharge rate are collectively referred to as the charge-discharge rate.

[0558] In the charge-discharge cycle test, the above charge and discharge were counted as one cycle, and the cycle count was repeated 50 times. The value calculated by (discharge capacity at the 50th cycle / maximum discharge capacity during the 50th cycle) × 100 was taken as the discharge capacity retention rate (capacity retention) (%) at the 50th cycle. That is, when a test was conducted in which 50 charge-discharge cycles were repeated and the discharge capacity was measured for each cycle, the ratio of the discharge capacity measured at the 50th cycle to the maximum discharge capacity during the 50th cycle (referred to as the maximum discharge capacity) was calculated. A higher discharge capacity retention rate is desirable as a battery characteristic because it suppresses the capacity decrease of the battery after repeated charge and discharge. Note that the above number of cycles is an example.

[0559] In the charge-discharge cycle test, current is measured. Specifically, in charge and discharge measurements, the battery voltage and current flowing through the battery are preferably measured using a four-terminal method. During charging, electrons flow from the positive electrode terminal through a charge-discharge meter to the negative electrode terminal, so the charging current flows from the negative electrode terminal through the charge-discharge meter to the positive electrode terminal. During discharging, electrons flow from the negative electrode terminal through the charge-discharge meter to the positive electrode terminal, so the discharging current flows from the positive electrode terminal through the charge-discharge meter to the negative electrode terminal. The charging current and discharging current are measured using an ammeter included in the charge-discharge meter, and the integrated amounts of current flowing in one charge and one discharge correspond to the charge capacity and discharge capacity, respectively. For example, the integrated amount of discharge current flowing in the first discharge cycle can be referred to as the first-cycle discharge capacity, and the integrated amount of discharge current flowing in the 50th discharge cycle can be referred to as the 50th-cycle discharge capacity.

[0560] The above-described charge-discharge cycle test was carried out on Samples 1 to 6 at environmental temperatures of 25° C. and 45° C. (hereinafter, the environmental temperature will be omitted). The environmental temperature is the temperature of the thermostatic bath in which each sample was placed.

[0561] Each sample placed in a thermostatic chamber was charged at a constant current at a rate of 0.5 C (in this example, 1 C = 200 mA / g (per weight of positive electrode active material)) until the upper limit voltage reached 4.6 V, and then charged at a constant voltage until the current amount reached 0.05 C. Constant voltage charging after constant current charging is referred to as CCCV charging. Discharge was performed at a constant current at a rate of 0.5 C until the lower limit voltage reached 2.5 V. A rest period was provided between charge and discharge, and in this example, the rest period was 10 minutes.

[0562] 35A shows a graph of the change in discharge capacity versus the number of cycles when Sample 1 was heated to 25° C. FIG. 35B shows a graph of the change in discharge capacity versus the number of cycles when Sample 1 was heated to 45° C.

[0563] Similarly, Figure 36A shows a graph of the change in discharge capacity versus cycle number for Sample 2 and the corresponding half cell having the oxide before the addition of the additive element at 25°C, and Figure 36B shows a graph of the change in discharge capacity versus cycle number for Sample 2 at 45°C.

[0564] FIG. 37A shows a graph of the change in discharge capacity versus cycle number for Sample 3 and the corresponding half-cell containing the oxide before the addition of the additive element at 25°C, and FIG. 37B shows a graph of the change in discharge capacity versus cycle number for Sample 3 at 45°C.

[0565] FIG. 38A shows a graph of the change in discharge capacity versus cycle number for Sample 4 and the corresponding half-cell containing the oxide before the addition of the additive element at 25°C, and FIG. 38B shows a graph of the change in discharge capacity versus cycle number for Sample 4 at 45°C.

[0566] FIG. 39A shows a graph of the change in discharge capacity versus cycle number for Sample 5 and the corresponding half-cell containing the oxide before the addition of the additive element at 25°C, and FIG. 39B shows a graph of the change in discharge capacity versus cycle number for Sample 5 at 45°C.

[0567] FIG. 40A shows a graph of the change in discharge capacity versus cycle number for Sample 6 and the corresponding half-cell containing the oxide before the addition of the additive element at 25°C, and FIG. 40B shows a graph of the change in discharge capacity versus cycle number for Sample 6 at 45°C.

[0568] As shown in Figures 35A to 40B, in Samples 1 to 6, Samples 1 to 6 containing the additive elements had better charge-discharge cycle characteristics than the oxides before the addition of the additive elements corresponding to each sample. In particular, Samples 1 and 2 showed very good charge-discharge capacity cycle characteristics. Among them, Sample 2 containing nickel as a raw material showed extremely good charge-discharge cycle characteristics. In Sample 2, which had an appropriate nickel concentration as a raw material, the above-mentioned XRD test results showed that the proportion of O3'-type crystal structures was high, and it is thought that the effects of the additive elements such as magnesium were efficiently expressed.

[0569] For example, Sample 1 had a discharge capacity of 196.4 mAh / g per weight of positive electrode active material at 25° C. and a discharge capacity of 199.4 mAh / g per weight of positive electrode active material at 45° C. after 50 cycles. Sample 2 had a discharge capacity of 203.8 mAh / g per weight of positive electrode active material at 25° C. and a discharge capacity of 195.5 mAh / g per weight of positive electrode active material at 45° C. after 50 cycles. All of these values ​​were 170 mAh / g or more per weight of positive electrode active material, more specifically, 190 mAh / g or more per weight of positive electrode active material.

[0570] On the other hand, in Samples 4 to 6, which had a high concentration of nickel as the raw material, the effect of the added elements such as magnesium was lower than in Samples 1 and 2.

[0571] For example, even at 45°C, Sample 4, which had the best charge-discharge cycle characteristics among Samples 4 to 6, had a discharge capacity of 150.4 mAh / g per weight of positive electrode active material after 50 cycles, and more specifically, a discharge capacity of 160 mAh / g or more per weight of positive electrode active material.

[0572] 41A shows the discharge capacity retention rates of Samples 1 to 6 at 25° C. FIG. 41B shows the discharge capacity retention rates of Samples 1 to 6 at 45° C.

[0573] 41A , the discharge capacity retention rate after 50 cycles at 25° C. was 96.3% for Sample 1 and 98.2% for Sample 2. That is, both Sample 1 and Sample 2 had a discharge capacity retention rate of 90% or more and less than 100%, more specifically, 95% or more and less than 100%. Furthermore, in Sample 2, in which the Ni / (Co+Ni) ratio in the raw material was more than 0.005 and less than 0.1, the discharge capacity retention rate was 98% or more and less than 100%.

[0574] On the other hand, even for Sample 4, which had the best charge / discharge cycle characteristics among Samples 3 to 6, the discharge capacity retention rate after 50 cycles was 63.4%, which was below 65%.

[0575] 41B , the discharge capacity retention rate after 50 cycles at 45° C. was 91.6% for Sample 1 and 90.1% for Sample 2. That is, for both Sample 1 and Sample 2, the retention rate was 85% or more and less than 100%, more specifically, 90% or more and less than 100%.

[0576] On the other hand, even for Sample 4, which had the best charge / discharge cycle characteristics among Samples 3 to 6, the discharge capacity retention rate after 50 cycles was 74.6%, which was below 75%.

[0577] 42A shows the average discharge voltages of Samples 1, 2, 4 to 6 at 25° C. FIG. 42B shows the average discharge voltages of Samples 1, 2, 4 to 6 at 45° C.

[0578] 42A, the average discharge voltage after 50 cycles at 25° C. was 4.02 V for Sample 1 and 4.08 V for Sample 2. In other words, both Sample 1 and Sample 2 were 4.00 V or higher.

[0579] On the other hand, sample 5, which had the best charge-discharge cycle characteristics among samples 4 to 6, had an average discharge voltage of 3.72 V after 50 cycles, which was 3.8 V or less.

[0580] 42B, the average discharge voltage after 50 cycles at 45° C. was 4.02 V for Sample 1 and 4.02 V for Sample 2. In other words, both Sample 1 and Sample 2 were 4.00 V or higher.

[0581] On the other hand, sample 5, which had the best charge-discharge cycle characteristics among samples 4 to 6, had an average discharge voltage of 3.76 V after 50 cycles, which was 3.8 V or less.

[0582] 43A shows the discharge energy densities of Samples 1, 2, 4 to 6 at 25° C. FIG. 43B shows the discharge energy densities of Samples 1, 2, 4 to 6 at 45° C.

[0583] 43A, the discharge energy density after 50 cycles at 25° C. was 789 mWh for Sample 1 and 831 mWh for Sample 2. In other words, both Sample 1 and Sample 2 had discharge energy densities of 750 mWh or more.

[0584] On the other hand, Sample 4, which had the best charge / discharge cycle characteristics among Samples 4 to 6, had a discharge energy density of 449 mWh after 50 cycles, which was less than 500 mWh.

[0585] 43B, the discharge energy density after 50 cycles at 45° C. was 801 mWh for Sample 1 and 786 mWh for Sample 2. In other words, both Sample 1 and Sample 2 had discharge energy densities of 750 mWh or more.

[0586] On the other hand, Samples 4 and 5, which had the best charge-discharge cycle characteristics among Samples 4 to 6, both had discharge energy densities of 538 mWh after 50 cycles, which was below 550 mWh.

[0587] <STEM-EDX> Next, the positive electrode active materials of Samples 1 and 2, which showed good results in the charge-discharge cycle test, and the positive electrode active material of Sample 4 were subjected to line analysis by STEM-EDX. A Hitachi High-Tech HD-2700 was used as the STEM device, and the acceleration voltage was set to 200 kV. An Ametec Octane T Ultra W (detector area: 100 mm) was used as the EDX detector. ２ The EDX software used was TEAM manufactured by Ametec.

[0588] As a pretreatment for analysis, the samples were thinned using the FIB method. Gallium etching was used to thin the samples. A carbon film was formed as a protective film on the samples. Two types of samples were prepared: Sample 1 Basal, in which a region with a surface parallel to the basal plane was processed, and Sample 1 Edge, in which a region with a surface (edge ​​plane) parallel to the plane intersecting the basal plane was processed. Similarly, two types of Sample 2 and Sample 4 were prepared: Sample 2 Basal and Sample 4 Basal, and Sample 2 Edge and Sample 4 Edge.

[0589] FIG. 44A shows the profile of STEM-EDX ray analysis for Sample 1 Edge, and FIG. 44B shows the profile of STEM-EDX ray analysis for Sample 1 Basal. FIGS. 44A and 44B show the calculated content of each element (cobalt, magnesium, aluminum, and nickel; other elements are not shown) from the profile of detection intensity (counts) by STEM-EDX. The content of each element was calculated when the total amount of carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, calcium, titanium, manganese, cobalt, nickel, copper, and gallium was taken as 100 atomic %. The horizontal axis represents distance [nm], and the vertical axis represents the content [atomic %] of that element. Furthermore, from the profile in FIG. 44A , the reference point of the distance shown on the horizontal axis was set to the position where it is half the average value of the oxygen content. Specifically, the average oxygen concentration O was calculated from the region where the amount of oxygen detected inside the active material particle is stable (region at a distance of 20 nm or more). ａｖｅ Calculate the average value O ａｖｅ The value of 1 / 2 of this was taken as half the oxygen content. The half-value of the oxygen content was at a position at a distance of approximately 20 nm on the horizontal axis. Since the reference point can be said to correspond to the surface of the active material, a position deeper than the reference point was taken as the interior of the particle. The reference point was also determined in the same way for the profile in Figure 44B. To make it easier to compare with Figure 44A, the reference point in Figure 44B was also set at a position at a distance of approximately 20 nm on the horizontal axis.

[0590] As shown in Figure 44A, Mg has the highest concentration peak near the surface (within a depth range of 3 nm or less from the surface), and the maximum concentration content corresponding to the peak is approximately 7.8 atomic %. Furthermore, Al has a concentration peak that overlaps with Mg, and the maximum concentration content corresponding to the peak is approximately 3.5 atomic %. "Overlapping" of concentration peaks means that the difference between the peaks is less than 3 nm.

[0591] As shown in Figure 44B, Mg had the highest concentration peak near the surface (within a depth of approximately 1 nm from the surface), with a maximum value of approximately 4.3 atomic %. Al had a concentration peak that overlapped with Mg, with a maximum value of approximately 2.5 atomic %. "Overlapping" of the concentration peaks means that the difference between the peaks is less than 3 nm.

[0592] FIG. 45A shows the STEM-EDX ray analysis profile of Sample 2 Edge, and FIG. 45B shows the STEM-EDX ray analysis profile of Sample 2 Basal. The content of each element (cobalt, magnesium, aluminum, and nickel; other elements are not shown) was calculated from the STEM-EDX detection intensity (count) profile. The content of each element was calculated when the total amount of carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, calcium, titanium, manganese, cobalt, nickel, copper, and gallium was set to 100 atomic %. In each case, the horizontal axis represents the analysis distance [nm], and the vertical axis represents the content [atomic %] of that element. Furthermore, in FIGS. 45A and 45B, a reference point was determined in the same way as in FIG. 44A. To facilitate comparison with FIG. 44A and other figures, the reference point was set at a distance of approximately 20 nm on the horizontal axis in FIGS. 45A and 45B.

[0593] 45A, the highest Mg concentration peak was near the surface (within a depth of 3 nm from the surface), with a maximum value of approximately 4.2 atomic %.Al peaks were located deeper than the Mg peak (within a depth of 25 nm from the surface) and were present over a wide range (within a depth of approximately 60 nm from the surface), with a maximum concentration corresponding to the peak being approximately 1.4 atomic %.

[0594] In Sample 2 Edge, the nickel content in the surface layer was 2 atomic % or less, and the nickel content in the interior was 0.7 atomic %. As an example of the interior of Sample 2, the average nickel content (atomic %) from the surface to a depth of 100 nm to 150 nm was calculated and used as the nickel content in the interior.

[0595] As shown in Figure 45B, no peaks of Mg and Al contents were confirmed. In Sample 2 Basal, the nickel content in the surface layer was 1 atomic % or less, and the nickel content in the interior was 0.3 atomic %. The nickel content in the interior was determined in the same manner as Sample 2 Edge.

[0596] FIG. 46A shows the STEM-EDX ray analysis profile of Sample 4 Edge, and FIG. 46B shows the STEM-EDX ray analysis profile of Sample 4 Basal. The content of each element (cobalt, magnesium, aluminum, and nickel; other elements are not shown) was calculated from the STEM-EDX detection intensity (count) profile. The content of each element was calculated when the total amount of carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, calcium, titanium, manganese, cobalt, nickel, copper, and gallium was taken as 100 atomic %. In each case, the horizontal axis represents the analysis distance [nm], and the vertical axis represents the content [atomic %] of that element. Furthermore, in FIGS. 46A and 46B, a reference point was determined in the same manner as in FIG. 44A. To facilitate comparison with FIG. 44A and other figures, the reference point was set at a distance of approximately 20 nm on the horizontal axis in FIGS. 46A and 46B.

[0597] As shown in Figure 46A, Mg had the highest concentration peak near the surface (within a depth of 3 nm from the surface), and the maximum concentration corresponding to the peak was approximately 1.0 atomic %. Furthermore, Al had a peak at a deeper position than Mg (within a depth of 5 nm from the surface) and was present over a wide range (within a depth of approximately 10 nm from the surface), and the maximum concentration corresponding to the peak was approximately 0.9 atomic %. Furthermore, nickel in the interior was 3.5 atomic %...

Claims

[Claim 1] A secondary battery having a positive electrode, The positive electrode has a positive electrode active material, The positive electrode active material has a surface layer and an interior, The interior contains at least cobalt and nickel, The surface layer comprises at least cobalt and an additive element, The atomic ratio of cobalt is higher than that of nickel. The aforementioned additive element is present in a width of 2 nm to 30 nm. The aforementioned additive element is one or more selected from magnesium, fluorine, calcium, aluminum, silicon, vanadium, copper, and gallium. Secondary battery.