Method for producing positive electrode active material

Abstract

A secondary battery using lithium cobalt oxide as a positive electrode active material has a problem in that the battery capacity decreases due to repeated charging and discharging. To provide positive electrode active material particles with less deterioration.SOLUTION: The method includes a first step of arranging a container containing lithium oxide and fluoride in a heating furnace, and a second step of heating the inside of the heating furnace in an atmosphere containing oxygen, wherein the heating temperature in the second step is 750 °C or higher and 950 °C or lower. According to the above manufacturing method, fluorine is contained in the positive electrode active material particles, and fluorine improves the wettability of the surface of the positive electrode active material, so that homogenization and planarization can be achieved. In the positive electrode active material obtained in this manner, the crystal structure is less likely to collapse during repeated charging and discharging at a high voltage, and a secondary battery including the positive electrode active material having such characteristics has significantly improved cycle characteristics.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a secondary battery using a positive electrode active material and a method for producing the same.

[0002] One embodiment of the present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof.

[0003] In this specification, the term "electronic device" refers to any device having a power storage device, and includes electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like.

[0004] In this specification, the term "power storage device" refers to all elements and devices having a power storage function, including, for example, power storage devices such as lithium ion secondary batteries (also called secondary batteries), lithium ion capacitors, and electric double layer capacitors. [Background technology]

[0005] In recent years, there has been active development of various types of power storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries. Demand for high-power, high-energy-density lithium-ion secondary batteries, in particular, has rapidly expanded alongside the development of the semiconductor industry, and they are now essential to the modern information society as a rechargeable energy source, thanks to their high output and high energy density, and are used in a variety of applications, including mobile phones, smartphones, and laptop computers, as well as portable music players, digital cameras, medical devices, and next-generation clean-energy vehicles, such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs).

[0006] Therefore, improvements in the positive electrode active material have been investigated in order to improve the cycle characteristics and capacity of lithium ion secondary batteries (Patent Document 1).

[0007] Furthermore, the characteristics required of the power storage device include safety in various operating environments and improved long-term reliability.

[0008] On the other hand, fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing and the like, and their physical properties have been studied (Non-Patent Document 1). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2019-21456 [Non-patent literature]

[0010] [Non-Patent Document 1] WE Counts, R. Roy, and EF Osborn, “Fluoride Model Systems: II, The Binary Systems CaF2-BeF2, MgF2-BeF2, and LiF-MgF2”, Journal of the American Ceramic Society, 36 [1] 12-17 (1953). Summary of the Invention [Problem to be solved by the invention]

[0011] There is a demand for improvements in various aspects of lithium ion secondary batteries and positive electrode active materials used therein, such as capacity, cycle characteristics, charge / discharge characteristics, reliability, safety, and cost.

[0012] Furthermore, materials with a layered rock salt crystal structure, such as lithium cobalt oxide (LiCoO2), are known to have high discharge capacity and are excellent as positive electrode active materials for secondary batteries.

[0013] Secondary batteries using lithium cobalt oxide as the positive electrode active material have the problem that the battery capacity decreases due to repeated charging and discharging.

[0014] In view of the above, an object of one embodiment of the present invention is to provide positive electrode active material particles that are less susceptible to deterioration. Alternatively, an object of one embodiment of the present invention is to provide novel positive electrode active material particles. Alternatively, an object of one embodiment of the present invention is to provide a power storage device that is less susceptible to deterioration. Alternatively, an object of one embodiment of the present invention is to provide a highly safe power storage device. Alternatively, an object of one embodiment of the present invention is to provide a novel power storage device.

[0015] Another object of one embodiment of the present invention is to provide a novel substance, active material particles, a power storage device, or a manufacturing method thereof.

[0016] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims. [Means for solving the problem]

[0017] In order to solve at least one of the above problems, the present specification discloses a method for producing a positive electrode active material, which includes a first step of placing a container containing lithium oxide and fluoride in a heating furnace, and a second step of heating the inside of the heating furnace in an oxygen-containing atmosphere, wherein the heating temperature in the second step is 750°C or higher and 950°C or lower.

[0018] In the above, the heating temperature is preferably 775°C or higher and 925°C or lower, and more preferably 800°C or higher and 900°C or lower.

[0019] In the above method, it is preferable that the method further includes a step of covering the container before or during heating, and that the fluoride is lithium fluoride.

[0020] By using the above-mentioned manufacturing method, the positive electrode active material particles contain fluorine, which improves the wettability of the positive electrode active material surface, making it homogenous and flat. The positive electrode active material obtained in this way is resistant to collapse of its crystal structure during repeated high-voltage charge and discharge, and secondary batteries using such a positive electrode active material have significantly improved cycle characteristics.

[0021] In order to solve at least one of the above problems, another method for producing a positive electrode active material disclosed in the present specification includes a first step of subjecting a lithium source and a transition metal source to a first heating to produce lithium oxide; a second step of placing a container containing the lithium oxide and fluoride in a heating furnace; and a third step of performing a second heating in the heating furnace in an oxygen-containing atmosphere, wherein the second heating is performed at a temperature of 750°C or higher and 950°C or lower, and the first heating is performed at a temperature higher than that of the second heating.

[0022] By using the above-described manufacturing method, a layered rock salt crystal structure with few defects and distortions can be formed in the first heating, and then the surface of the positive electrode active material can be modified with fluoride or the like in the second heating.

[0023] It is preferable that the composite oxide containing lithium, a transition metal, and oxygen has a layered rock-salt type crystal structure with few defects and distortion. Therefore, it is preferable that the composite oxide contains few impurities. If the composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or distortion.

[0024] In order to prevent impurities from being mixed, it is preferable to modify the surface of the positive electrode active material by heating with a lid on after mixing with the fluoride. The timing of the lid can be any one of placing the lid on the container before heating and then placing it in the heating furnace, placing the lid on the container after placing it in the heating furnace, or placing the lid on during heating before the fluoride melts.

[0025] The above-described manufacturing method may include a step of increasing the oxygen concentration in the heating furnace before the second step, for example, by placing a lid on the container and then creating an oxygen atmosphere in the heating furnace.

[0026] Furthermore, as a means for solving at least one of the above problems, the present invention provides positive electrode active material particles that exhibit minimal deterioration by increasing the strength near the surface by controlling the surface roughness of the active material particles within a specific range. Preferably, the surface of the active material particles is smooth or glossy when observed in SEM photographs, with almost no cracks or protrusions. Fluorine is important for achieving a smooth surface, forming clean bonds on the surface. For example, positive electrode active material particles are produced by mixing lithium oxide and fluoride and heating the mixture. Furthermore, the presence of fluorine promotes solid-phase diffusion during heating, resulting in reduced crystal defects such as cracks, steps, and grain boundaries in the active material particles.

[0027] If pure LiCoO2 is exposed on the surface of the positive electrode active material particles, unevenness will occur, and cobalt or oxygen will be released during charging and discharging, causing the crystal structure to collapse and degradation. To prevent this exposed pure LiCoO2 from being exposed on the surface, it is preferable to uniformly cover the surface with a compound containing magnesium. Magnesium has the function of maintaining the crystal structure (layered rock salt crystal structure) even when Li is released during discharging. The presence of magnesium (or fluorine) near the surface of the positive electrode active material particles is also a feature.

[0028] Specifically, when particle surface irregularity information near the surface is quantified from measurement data in a cross section cut toward the particle center observed with a scanning transmission electron microscope (STEM), the positive electrode active material has a root mean square surface roughness (RMS) of less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

[0029] By adopting the above-described structure, cracks are less likely to occur when pressure is applied to a positive electrode containing a positive electrode active material during the production of a secondary battery, and the particle shape can be maintained. This reduces unnecessary cracks, thereby increasing the electrode density.

[0030] If the surface irregularities are larger and rougher than the above range, physical cracks or collapse of the crystal structure may occur, which may expose the pure LiCoO2 on the surface and accelerate degradation.

[0031] Furthermore, when preparing the positive electrode active material, the surface is modified by heating with a lid. This surface modification facilitates the formation of a positive electrode active material with the above-mentioned configuration, i.e., an RMS of less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. By covering the positive electrode active material with a lid, fluorine or fluoride does not diffuse outward during annealing, but is instead coated on the surface of other positive electrode active material particles, improving wettability with impurities and resulting in homogenization and flattening.

[0032] The lithium oxide is preferably a material having a layered rock salt crystal structure, such as a composite oxide represented by LiMO2. Examples of the element M include one or more selected from Co and Ni. Examples of the element M include one or more selected from Co and Ni, as well as one or more selected from Al and Mg.

[0033] By incorporating fluorine near the surface, it is possible to distribute not only fluorine but also magnesium, aluminum, or nickel in high concentrations near the surface. Annealing with a lid prevents fluorine from diffusing outward as a gas, while other elements such as aluminum diffuse into the solid. Fluorine improves the wettability of the positive electrode active material surface, making it homogenous or flat.

[0034] Another aspect of the present invention is a secondary battery having a positive electrode made of a positive electrode active material in which, when particle surface roughness information near the surface of a fluorine-containing lithium oxide particle is quantified from measurement data in a cross section cut toward the particle center as observed by STEM, at least a portion of the particle has a surface roughness of less than 3 nm.

[0035] In the above, the surface roughness is preferably an RMS value calculated from the standard deviation.

[0036] In the above, the positive electrode active material preferably has a surface roughness of at least 400 nm from the outer periphery of the particle.

[0037] Another embodiment of the present invention is a portable information terminal including the above secondary battery.

[0038] Another embodiment of the present invention is a vehicle including the above secondary battery. [Effects of the Invention]

[0039] According to one embodiment of the present invention, positive electrode active material particles that are less likely to deteriorate can be provided. Furthermore, according to one embodiment of the present invention, novel positive electrode active material particles can be provided. Furthermore, according to one embodiment of the present invention, a power storage device that is less likely to deteriorate can be provided. Furthermore, according to one embodiment of the present invention, a highly safe power storage device can be provided. Furthermore, according to one embodiment of the present invention, a novel power storage device can be provided.

[0040] According to one embodiment of the present invention, a novel substance, active material particles, a power storage device, or a manufacturing method thereof can be provided.

[0041] 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. [Brief explanation of the drawings]

[0042] [Figure 1] FIG. 1 is a phase diagram showing the relationship between the composition and temperature of lithium fluoride and magnesium fluoride. [Figure 2] FIG. 2 is a diagram illustrating the results of the DSC analysis. [Figure 3] FIG. 3 is a graph showing the weight loss rate when fluorides are heated. [Figure 4] FIG. 4 is a diagram showing an example of a flow illustrating one embodiment of the present invention. [Figure 5] FIG. 5 is an example of a cross-sectional view showing a process of one embodiment of the present invention. [Figure 6] FIG. 6A is a STEM photograph of active material particles showing one embodiment of the present invention, and FIG. 6B is a STEM photograph showing a comparative example. [Figure 7] FIG. 7A is a STEM photograph of active material particles showing one embodiment of the present invention, and FIG. 7B is a STEM photograph showing a comparative example. [Figure 8] FIG. 8A is a STEM photograph of an active material particle showing one embodiment of the present invention, FIG. 8B is an enlarged, cropped image, FIG. 8C is image data after binarization processing, FIG. 8D is image data with boundaries detected, FIG. 8E is a graph in which coordinate data was read from the extracted boundaries and plotted, and FIG. 8F is a graph in which only the roughness component has been extracted with the slope and waviness removed. [Figure 9] FIG. 9A is a STEM photograph of active material particles showing a comparative example, FIG. 9B is an enlarged cut-out image of a portion, and FIG. 9C is a graph in which only the roughness component has been extracted. [Figure 10] FIG. 10A shows condition 1, FIG. 10B shows condition 2, FIG. 10C shows condition 3, and FIG. 10D shows a comparative example. [Figure 11] FIG. 11 is a diagram showing the cycle characteristics of the secondary battery. [Figure 12] 12A is a STEM photograph of particles obtained under condition 1, FIG. 12B is a STEM photograph of particles obtained under condition 2, and FIG. 12C is a STEM photograph of particles obtained under condition 3. [Figure 13] Figure 13A is a graph showing the EDX results of particles obtained under condition 1, Figure 13B is a graph showing the EDX results of particles obtained under condition 2, and Figure 13C is a graph showing the EDX results of particles obtained under condition 3. [Figure 14] FIG. 14 is a diagram illustrating the crystal structure and magnetism of the positive electrode active material. [Figure 15] FIG. 15 is a diagram illustrating the crystal structure and magnetism of a conventional positive electrode active material. [Figure 16] 16A and 16B are cross-sectional views of an active material layer in which a graphene compound is used as the conductive material. [Figure 17] 17A and 17B are diagrams illustrating an example of a secondary battery. [Figure 18] 18A, 18B, 18C, 18D, and 18E are diagrams illustrating examples of secondary batteries. [Figure 19] FIG. 19A is a perspective view of a secondary battery, FIG. 19B is a cross-sectional perspective view thereof, and FIG. 19C is a cross-sectional schematic view thereof during charging. [Figure 20] 20A is a perspective view of a secondary battery, FIG. 20B is a cross-sectional perspective view thereof, FIG. 20C is a perspective view of a battery pack including a plurality of secondary batteries, and FIG. 20D is a top view thereof. [Figure 21] 21A and 21B are diagrams illustrating an example of a secondary battery. [Figure 22] 22A and 22B are diagrams illustrating a laminated secondary battery. [Figure 23] 23A, 23B, 23C, 23D, and 23E are perspective views showing electronic devices. [Figure 24] FIG. 24A is a STEM photograph of particles obtained in the comparative example, and FIG. 24B is a diagram showing the EDX results of the comparative example. [Figure 25] 25A and 25B are graphs showing the discharge characteristics of the positive electrode active material prepared in Example 1. [Figure 26] FIG. 26 is a graph showing the rate characteristics of the positive electrode active material prepared in Example 1. [Figure 27]27A and 27B are graphs showing the cycle characteristics of the positive electrode active material prepared in Example 1. [Figure 28] 28A and 28B are graphs showing the cycle characteristics of the positive electrode active material prepared in Example 1. [Figure 29] 29A and 29B are graphs showing the cycle characteristics of the positive electrode active material prepared in Example 1. [Figure 30] FIG. 30 is a graph showing the cycle characteristics of the positive electrode active material prepared in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0043] 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.

[0044] In this specification, "homogeneous" refers to a phenomenon in which a certain element (e.g., A) is distributed with similar characteristics in a specific region in a solid composed of multiple elements (e.g., A, B, C). Note that it is sufficient if the concentration of the element in each specific region is substantially the same. For example, it is sufficient if the difference in element concentration between each specific region is within 10%. Examples of specific regions include the surface, convex portions, concave portions, and interior.

[0045] In this specification, the layered rock-salt type crystal structure of a composite oxide containing lithium and a transition metal 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 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. Furthermore, strictly speaking, the layered rock-salt type crystal structure may have a distorted rock-salt type crystal lattice structure.

[0046] In this specification and the like, the rock salt type crystal structure refers to a structure in which cations and anions are arranged alternately, and it is also possible for there to be a deficiency of cations or anions.

[0047] In this specification, charging refers to the transfer of electrons from a positive electrode to a negative electrode in an external circuit. Regarding positive electrode active materials, charging refers to the removal of lithium ions. A positive electrode active material with a charge depth of 0.74 to 0.9, more specifically, 0.8 to 0.83, is considered to be a positive electrode active material charged at a high voltage. For example, a LiCoO2 positive electrode active material charged at 219.2 mAh / g is considered to be a positive electrode active material charged at a high voltage. Furthermore, a positive electrode active material charged at a constant current of 4.525 V to 4.65 V (for a lithium counter electrode) at 25°C, followed by constant voltage charging at 0.01 C or until the current drops to approximately 1 / 5 to 1 / 100 of the current value during constant current charging, is also considered to be a positive electrode active material charged at a high voltage.

[0048] Similarly, discharging refers to the transfer of electrons from the negative electrode to the positive electrode in an external circuit. For positive electrode active materials, discharging refers to the insertion of lithium ions. A fully discharged positive electrode active material is defined as a positive electrode active material with a charge depth of 0.06 or less, or a positive electrode active material that has been discharged from a high-voltage charged state to 90% or more of its charge capacity. For example, a LiCoO2 positive electrode active material with a charge capacity of 219.2 mAh / g is considered to be in a high-voltage charged state. A fully discharged positive electrode active material is defined as a positive electrode active material that has been discharged from this state to 197.3 mAh / g, or 90% of its charge capacity. Furthermore, a LiCoO2 positive electrode active material that has been discharged at a constant current until the battery voltage reaches 3 V or less (when using a lithium counter electrode) at 25°C is also defined as a fully discharged positive electrode active material.

[0049] Although this specification and the like may show examples of secondary batteries using a positive electrode and a positive electrode active material of one embodiment of the present invention, the secondary battery of one embodiment of the present invention is not limited to this example. Other materials, such as graphite and lithium titanate, may also be used for the negative electrode. The properties of the positive electrode and positive electrode active material of one embodiment of the present invention, such as their resistance to crystal structure collapse even after repeated charge and discharge and their excellent cycle characteristics, are not affected by the material of the negative electrode. Although examples of secondary batteries of one embodiment of the present invention using a lithium counter electrode and a relatively high charge voltage of 4.6 V are shown, they may also be charged and discharged at lower voltages. Charging and discharging at lower voltages is expected to result in even better cycle characteristics than those shown in this specification and the like.

[0050] (Embodiment 1) An example of a method for producing LiMO2 (where M is two or more metals including Co, and there is no particular limitation on the substitution position of the metal) will be described with reference to Figures 1 to 4. The following describes an example of a positive electrode active material containing Mg as a metal element other than Co contained in LiMO2.

[0051] This will be explained using the flow shown in Fig. 4. First, as the material for lithium oxide 901, a composite oxide containing lithium, a transition metal, and oxygen is used.

[0052] A composite oxide containing lithium, a transition metal, and oxygen can be synthesized by heating a lithium source and a transition metal source in an oxygen atmosphere. The transition metal source is preferably a metal that can form a layered rock-salt composite oxide belonging to the space group R-3m with lithium. For example, at least one of manganese, cobalt, and nickel can be used. That is, the transition metal source may be a cobalt source alone, a nickel source alone, a combination of a cobalt source and a manganese source, a combination of a cobalt source and a nickel source, or a combination of a cobalt source, a manganese source, and a nickel source. The heating temperature in this step is preferably higher than that in step S16, which will be described later. For example, the heating temperature can be 1000°C. This heating step is sometimes called calcination.

[0053] When using a pre-synthesized composite oxide containing lithium, transition metals, and oxygen, it is preferable to use one with few impurities. In this specification, the main components of the composite oxide containing lithium, transition metals, and oxygen, and the positive electrode active material, are lithium, cobalt, nickel, manganese, aluminum, and oxygen, and elements other than the main components are considered impurities. For example, when analyzed by glow discharge mass spectrometry, the total impurity concentration is preferably 10,000 ppmw (parts per million weight) or less, more preferably 5000 ppmw or less. In particular, the total impurity concentration of transition metals such as titanium and arsenic is preferably 3000 ppmw or less, more preferably 1500 ppmw or less.

[0054] For example, lithium cobalt oxide particles (product name: Cellseed C-10N) manufactured by Nippon Chemical Industry Co., Ltd. can be used as pre-synthesized lithium cobalt oxide. This lithium cobalt oxide has an average particle size (D50) of approximately 12 μm, and impurity analysis by glow discharge mass spectrometry (GD-MS) reveals that the magnesium and fluorine concentrations are 50 ppmw or less, the calcium, aluminum, and silicon concentrations are 100 ppmw or less, the nickel concentration is 150 ppmw or less, the sulfur concentration is 500 ppmw or less, the arsenic concentration is 1100 ppmw or less, and the concentrations of other elements other than lithium, cobalt, and oxygen are 150 ppmw or less.

[0055] The lithium oxide 901 in step S11 preferably has a layered rock-salt crystal structure with few defects and strain. Therefore, it is preferable that the composite oxide has few impurities. If a composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or strains.

[0056] Furthermore, fluoride 902 is prepared in step S12. Examples of fluorides that can be used include lithium fluoride (LiF), magnesium fluoride (MgF), aluminum fluoride (AlF), titanium fluoride (TiF), cobalt fluoride (CoF, CoF), nickel fluoride (NiF), zirconium fluoride (ZrF), vanadium fluoride (VF), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF), calcium fluoride (CaF), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF), cerium fluoride (CeF), lanthanum fluoride (LaF), and sodium aluminum hexafluoride (NaAlF). Fluoride 902 may be any fluoride that functions as a fluorine source. Therefore, instead of or as a part of the fluoride 902, for example, fluorine (F2), carbon fluoride, sulfur fluoride, oxygen fluoride (OF2, O2F2, O3F2, O4F2, O2F), etc. may be used and mixed in the atmosphere.

[0057] In this embodiment, lithium fluoride (LiF) is prepared as the fluoride 902. LiF is preferable because it has a cation common to LiCoO2. LiF is also preferable because it has a relatively low melting point of 848°C and is easily melted in the annealing step described below. In addition to LiF, a material containing magnesium, such as MgF2, may be used. When the fluoride 902 contains magnesium, magnesium can be distributed in a high concentration near the surface of the positive electrode active material.

[0058] The fluoride 902 may also be mixed with other element sources. For example, a titanium source, an aluminum source, a nickel source, a vanadium source, a manganese source, an iron source, a chromium source, a niobium source, a zinc source, a zirconium source, or the like may be mixed. For example, it is preferable to finely pulverize the hydroxides, fluorides, or the like of each element and mix them. The fine pulverization can be carried out, for example, by a wet method.

[0059] Moreover, either step S11 or step S12 may be performed first.

[0060] Next, in step S13, the mixture is mixed and pulverized. Mixing can be performed by either a dry or wet method, but a wet method is preferred because it allows for finer pulverization. When using a wet method, 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 more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, acetone is used.

[0061] For example, a ball mill, a bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. It is preferable to thoroughly perform this mixing and pulverizing process to finely pulverize the mixture 903.

[0062] The mixed and crushed materials are collected (step S14 in FIG. 4) to obtain a mixture 903 (step S15 in FIG. 4).

[0063] The mixture 903 preferably has a D50 of, for example, 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less.

[0064] Next, the mixture 903 is heated (step S16 in FIG. 4). This step is sometimes called annealing. The annealing generates LiMO2. Therefore, the conditions for performing step S16, such as the temperature, time, atmosphere, and weight of the mixture 903 to be annealed, are important. In this specification, annealing also refers to heating the mixture 903 or at least heating the heating furnace in which the mixture 903 is placed. In this specification, a heating furnace is a facility used to heat-treat (anneal) a substance or mixture, and has a heater, an atmosphere containing fluoride, and an inner wall that can withstand at least 600°C. The heating furnace may also be equipped with a pump that can reduce or increase the pressure inside the furnace. For example, pressure may be applied during the annealing in S16.

[0065] The annealing temperature in S16 must be equal to or higher than the temperature at which the reaction between the lithium oxide 901 and the fluoride 902 proceeds. The temperature at which the reaction proceeds here is a temperature at which mutual diffusion of elements contained in the lithium oxide 901 and the fluoride 902 occurs. Therefore, it may be lower than the melting temperature of these materials. For example, in the case of oxides, the melting temperature T m 0.757 times (Tanman temperature T d ) solid-state diffusion occurs. Therefore, for example, a temperature of 500°C or higher is sufficient.

[0066] However, the reaction proceeds more easily if the annealing temperature is equal to or higher than the temperature at which at least a portion of the mixture 903 melts. Therefore, the annealing temperature is preferably equal to or higher than the eutectic point of the fluoride 902. When the fluoride 902 contains LiF and MgF2, the eutectic point P of LiF and MgF2 is around 742°C (T1) as shown in Figure 1 (cited and added from Non-Patent Document 1, Figure 1471-A), so it is preferable to set the annealing temperature of S16 to 742°C or higher.

[0067] Here, differential scanning calorimetry (DSC) measurements of fluoride 902 and mixture 903 will be described with reference to FIG. 2. In FIG. 2, the vertical axis represents heat flow, and the horizontal axis represents temperature. Fluoride 902 in FIG. 2 is a mixture of LiF and MgF2. They were mixed so that the molar ratio of LiF:MgF2 was 1:3. Mixture 903 in FIG. 2 was made by mixing lithium cobalt oxide as lithium oxide 901 and LiF and MgF2 as fluoride 902. They were mixed so that the molar ratio of LiCoO2:LiF:MgF2 was 100:0.33:1.

[0068] As shown in Figure 2, an endothermic peak is observed around 735°C for fluoride 902. Also, an endothermic peak is observed around 830°C for mixture 903. Therefore, the annealing temperature is preferably 742°C or higher, more preferably 830°C or higher. Alternatively, it may be 800°C (T2 in Figure 1) or higher, which is between these temperatures.

[0069] The evaporation or sublimation of fluoride 902 will be explained using Figure 3. Figure 3 is a graph showing the weight loss rate of fluoride 902 mixed to give a molar ratio of LiF:MgF2 = 1:3 when heated in an uncovered container at 600°C, 700°C, 800°C and 900°C. All heating times were 10 hours in an oxygen atmosphere.

[0070] As shown in Figure 3, the weight loss after heating was 2% at 700°C, 8% at 800°C, and 26% at 900°C. This shows that the evaporation or sublimation of fluoride 902 begins to progress rapidly from around 800°C.

[0071] A higher annealing temperature is preferable because the reaction proceeds more easily, the annealing time can be shortened, and productivity is high.

[0072] However, the annealing temperature must be below the decomposition temperature of LiCoO2 (1130°C). Although the decomposition temperature of LiCoO2 is 1130°C, there is concern that a small amount of LiCoO2 may decompose at temperatures around that temperature. Therefore, the annealing temperature is preferably 1130°C or below, more preferably 1000°C or below, even more preferably 950°C (T4) or below, and even more preferably 900°C (T3) or below.

[0073] Therefore, the annealing temperature is preferably 850°C±100°C (750°C or higher and 950°C or lower) as shown in M1 in Figure 3, more preferably 850°C±75°C (775°C or higher and 925°C or lower) as shown in M2, and most preferably 850°C±50°C (800°C or higher and 900°C or lower) as shown in M3.

[0074] More specifically, the temperature is preferably 500°C or higher and 1130°C or lower, more preferably 500°C or higher and 1000°C or lower, even more preferably 500°C or higher and 950°C or lower, and even more preferably 500°C or higher and 900°C or lower. Also, the temperature 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, the temperature is preferably 800°C or higher and 1130°C or lower, more preferably 800°C or higher and 1000°C or lower, even more preferably 800°C or higher and 950°C or lower, and most preferably 800°C (T2) or higher and 900°C (T3) or lower (range L). Also, the temperature is preferably 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.

[0075] More specifically, by using LiF as the fluoride 902 and performing annealing S16 with a lid on, it is possible to produce a positive electrode active material 904 with good cycle characteristics, etc. Furthermore, it is believed that using LiF and MgF2 as the fluoride 902 promotes the reaction with LiCoO2, resulting in the production of LiMO2.

[0076] In addition, in this embodiment, LiF, which is a fluoride, is thought to function as a flux. Therefore, since the volume inside the heating furnace is larger than the volume of the container and is lighter than oxygen, it is expected that LiF will volatilize, and if the amount of LiF in the mixture 903 decreases, the generation of LiMO2 will be suppressed. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Furthermore, even if LiF is not used, Li and F on the surface of the lithium oxide 901 may react to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress the volatilization.

[0077] Therefore, by heating the mixture 903 in an atmosphere containing LiF, that is, by heating the mixture 903 in a furnace under conditions where the partial pressure of LiF is high, the volatilization of LiF in the mixture 903 is suppressed. By annealing with a fluoride (LiF or MgF) used to form the eutectic mixture and covering it, the annealing temperature can be lowered to below the decomposition temperature of LiCoO2 (1130°C), specifically, to between 742°C and 1000°C, allowing the formation of LiMO2 to proceed efficiently. As a result, a positive electrode active material with good properties can be produced, and the annealing time can also be shortened.

[0078] An example of the annealing method in S16 is shown in FIG.

[0079] The heating furnace 120 shown in FIG. 5 includes a furnace space 102, a hot plate 104, a heater 106, and an insulating material 108. Annealing is preferably performed with a lid 118 attached to the container 116. This configuration allows the space 119 defined by the container 116 and lid 118 to be filled with a fluoride-containing atmosphere. By maintaining the lid during annealing to maintain a constant or stable concentration of gasified fluoride in the space 119, fluorine and magnesium can be incorporated near the particle surfaces. Because the space 119 has a smaller volume than the furnace space 102, a small amount of fluoride volatilizes, creating a fluoride-containing atmosphere. In other words, the reaction system can be filled with a fluoride-containing atmosphere without significantly reducing the amount of fluoride contained in the mixture 903. Therefore, LiMO2 can be efficiently produced. Furthermore, the use of the lid 118 allows the mixture 903 to be annealed in a fluoride-containing atmosphere conveniently and inexpensively.

[0080] Here, the valence of Co (cobalt) in LiMO2 produced according to one embodiment of the present invention is preferably approximately trivalent. Cobalt can be divalent or trivalent. Therefore, to suppress the reduction of cobalt, the atmosphere in the heating furnace space 102 preferably contains oxygen, and more preferably the ratio of oxygen to nitrogen in the atmosphere in the heating furnace space 102 is equal to or higher than that of the air atmosphere, and even more preferably the oxygen concentration in the atmosphere in the heating furnace space 102 is equal to or higher than that of the air atmosphere. Therefore, it is necessary to introduce an oxygen-containing atmosphere into the heating furnace space. However, since cobalt atoms with nearby magnesium atoms may be more stable in a divalent state, not all cobalt atoms need to be trivalent.

[0081] Therefore, in one embodiment of the present invention, before heating, a step of creating an oxygen-containing atmosphere in the heating furnace space 102 and a step of placing the container 116 containing the mixture 903 in the heating furnace space 102 are performed. By performing these steps in this order, the mixture 903 can be annealed in an atmosphere containing oxygen and fluoride. Furthermore, it is preferable to seal the heating furnace space 102 during annealing to prevent gas from being transported to the outside. For example, it is preferable to perform annealing without flowing gas.

[0082] There are no particular limitations on the method for creating an oxygen-containing atmosphere in the heating furnace space 102, but examples include a method of evacuating the heating furnace space 102 and then introducing an oxygen-containing gas such as oxygen gas or dry air, or a method of infusing an oxygen-containing gas such as oxygen gas or dry air for a certain period of time. Among these, it is preferable to evacuate the heating furnace space 102 and then introduce oxygen gas (oxygen substitution). Note that the air in the heating furnace space 102 may be considered to be an oxygen-containing atmosphere.

[0083] When the lid 118 is placed on the container 116, an oxygen-containing atmosphere is created, and the container 116 is heated, an appropriate amount of oxygen enters the container 116 through the gap in the lid 118 placed on the container 116, and an appropriate amount of fluoride can be retained in the container 116.

[0084] Furthermore, there is a possibility that fluoride and the like adhering to the inner walls of the container 116 and the lid 118 may fly again due to heating and adhere to the mixture 903 .

[0085] There is no particular limitation on the process for heating the heating furnace 120. Heating may be performed using a heating mechanism provided in the heating furnace 120.

[0086] Furthermore, there are no particular limitations on how the mixture 903 is arranged when placed in the container 116, but it is preferable to arrange the mixture 903 so that the upper surface of the mixture 903 is flat with respect to the bottom surface of the container 116, in other words, so that the height of the upper surface of the mixture 903 is uniform, as shown in Figure 5.

[0087] The annealing in step S16 is preferably performed at an appropriate temperature and time. The appropriate temperature and time vary depending on conditions such as the size and composition of the lithium oxide 901 particles in step S11. If the particles are small, a lower temperature or shorter time may be more preferable than if the particles are large. After the annealing in step S16, a step of removing the lid is included.

[0088] For example, when the D50 of the particles in step S11 is about 12 μm, the annealing time is preferably, for example, 3 hours or more, and more preferably 10 hours or more.

[0089] On the other hand, when the D50 of the particles in step S11 is about 5 μm, the annealing time is preferably, for example, from 1 hour to 10 hours, and more preferably about 2 hours.

[0090] The temperature drop time after annealing is preferably, for example, 10 hours or more and 50 hours or less.

[0091] The annealed material is collected (step S17 in FIG. 4) to obtain positive electrode active material 904 (step S18 in FIG. 4).

[0092] Here, the difference in the particles obtained when annealing in S16 was performed with a lid and when annealing without a lid in the comparative example will be explained below.

[0093] Figure 6A is an example of a cross-sectional photograph taken by STEM of one of the positive electrode active material particles after annealing with a lid. Figure 6B is an example of a cross-sectional photograph taken by STEM of one of the positive electrode active material particles after annealing without a lid. In Figures 6A and 6B, the positive electrode active material particles are surrounded by a resin, and the STEM observation was performed after forming a protective film.

[0094] Furthermore, Fig. 7A is an enlarged view of a portion of the obtained photograph Fig. 6A. Fig. 7B is an enlarged view of a portion of Fig. 6B, which is a comparative example. It can be seen that the particle surface in Fig. 7A is smoother, more glossy, or more lustrous than that in Fig. 7B.

[0095] To clarify this difference in particle surface, the method and procedure for quantifying the unevenness of the particle surface are described below.

[0096] FIG. 8A is the same as FIG. 6A. FIG. 8B is an enlarged, cropped image of the area surrounded by the dotted line in FIG. 8A. In this embodiment, the area surrounded by the dotted line is trimmed as the area for determining particle roughness. The upper part of the image in FIG. 8B is the resin of the protective film formed for STEM observation, and the lower part is the positive electrode active material particle, with the interface representing the outermost shell of the particle surface.

[0097] To perform the noise processing shown in Figure 8B, Gaussian blurring (σ = 2) is performed, and then image processing software is used for binarization. The image after binarization is shown in Figure 8C. Further, interface extraction is performed using image processing software to obtain Figure 8D. Note that there are no particular limitations on the image processing software used for noise processing and interface extraction, but "ImageJ" can be used, for example. There are also no particular limitations on the spreadsheet software, but Microsoft Office Excel can be used, for example.

[0098] Use the magic hand tool to select the desired boundary line from the image data in Figure 8D, and extract the data into Excel. The extracted numerical data can be graphed in Excel to produce Figure 8E.

[0099] Using Excel's functions, correction was performed using the regression line (quadratic regression), and the roughness calculation parameters were obtained from the slope-corrected data. The slope-corrected data was converted to absolute values, averaged, and then the square root of the average was taken as the RMS, as shown in Figure 8F. This surface roughness is the RMS obtained by calculating the standard deviation. This surface roughness is also the surface roughness within at least 400 nm of the outer periphery of the positive electrode active material particles.

[0100] The particle surface of the positive electrode active material of this embodiment can be determined to have a roughness index (RMS) of 0.1 nm. After mixing with fluoride, the mixture is heated with a lid on to modify the surface, which makes it easy to obtain a positive electrode active material with an RMS of less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

[0101] Furthermore, when a similar RMS calculation is performed on the comparative example, Figure 9A is the same as Figure 6B, and Figure 9B is image data obtained by cropping the area surrounded by the dotted line in Figure 9A. Figure 9C can be obtained by graphing the numerical data using the same procedure as described above. Furthermore, the roughness (RMS), an index of roughness, of the particle surface of the comparative example can be calculated to be 3.3 nm.

[0102] (Embodiment 2) In this embodiment, an example of manufacturing a battery cell using LiMO2 manufactured by a manufacturing method of one embodiment of the present invention will be described. Note that since there are many common parts, the manufacturing method will be described with reference to Figure 4.

[0103] Lithium cobalt oxide is prepared as the lithium oxide 901. More specifically, Cellseed C-10N manufactured by Nippon Chemical Industry Co., Ltd. is prepared (step S11).

[0104] LiF and MgF2 are prepared as fluoride 902. The LiF and MgF2 are weighed out so that the molar ratio of LiF:MgF2 is 1:3, and acetone is added as a solvent, followed by wet mixing and pulverization. The ratio of LiF to lithium cobalt oxide is adjusted to 0.17 mol%. The ratio of MgF2 to lithium cobalt oxide is also adjusted to 0.5 mol%.

[0105] Lithium oxide 901 and fluoride 902 are mixed and recovered to obtain mixture 903 .

[0106] Next, the mixture 903 is placed in a container, the lid is put on, and annealing is performed in an oxygen atmosphere in the heating furnace. The annealing temperature may vary depending on the weight of the mixture 903, but it is preferably set to 742°C or higher and 1000°C or lower. The "annealing temperature" is the temperature at which annealing is performed, and the "annealing time" is the time the annealing temperature is maintained. The temperature is increased at 200°C / h and decreased over 10 hours or more. It is also preferable not to actively supply gas during annealing to prevent gaseous fluoride from being transported to the outside. For example, it is preferable to perform annealing without flowing gas.

[0107] In this embodiment, the annealing is performed at a temperature of 850° C. for 60 hours in an oxygen atmosphere in the heating furnace.

[0108] After annealing, the cathode active material 904 can be obtained by recovery. If a smooth surface is obtained, the lid can be removed during heating to allow cooling. After cooling, the lid is removed, and the obtained cathode active material 904 is used to fabricate each cathode. A slurry of the cathode active material, AB, and PVDF mixed in a ratio of active material:AB:PVDF = 95:3:2 (weight ratio) is applied to a current collector. NMP is used as the solvent for the slurry.

[0109] After the slurry was applied to the current collector, the solvent was evaporated. Then, a pressure of 210 kN / m was applied, followed by a further pressure of 1467 kN / m. The positive electrode was obtained through these steps. The loading on the positive electrode was approximately 7 mg / cm. 2 The density of the positive electrode active material is set to be >3.8 g / cc.

[0110] Using the prepared positive electrode, a coin-type battery cell of the CR2032 type (diameter 20 mm, height 3.2 mm) is prepared.

[0111] Lithium metal is used as the counter electrode.

[0112] The electrolyte used in the electrolytic solution is 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7. 2 wt% vinylene carbonate (VC) is added to the electrolytic solution as an additive.

[0113] The separator is made of polypropylene with a thickness of 25 μm.

[0114] The positive electrode can and the negative electrode can are made of stainless steel (SUS).

[0115] Through the above steps, a secondary battery cell can be fabricated.

[0116] The following shows the experimental results obtained by comparing different annealing conditions.

[0117] Figure 10A shows condition 1, which is the same as the above-mentioned manufacturing method, and is the same as Figure 5, so the same symbols as Figure 5 are used. Furthermore, as condition 2, four lids are used as shown in Figure 10B. Furthermore, as condition 3, a triple layer is formed by using a container and a lid, as shown in Figure 10C. The lid and container are made of the same material, specifically a ceramic material. The lid is larger than the opening of the container and can be supported by its own weight. It is preferable to have as little gap as possible between the lid and the container, but there is a gap so that the inside of the container is not airtight with the lid. Conditions 1, 2, and 3 all have the same procedures and conditions except for the differences in the conditions shown in Figure 10.

[0118] The same annealing was performed under conditions 1, 2, and 3. SEM images of the particles obtained are shown in Figures 12A, 12B, and 12C. Condition 1 is shown in Figure 12A, Condition 2 in Figure 12B, and Condition 3 in Figure 12C. Under all conditions, with the lid, the surfaces of the active material particles were almost completely free of cracks or protrusions, and the surfaces appeared smooth or glossy when observed in SEM images. Fluorine is important for achieving flatness, i.e., a smooth, even surface. Using a lid to prevent fluorine from being released as a gas creates clean bonds on the particle surface. Furthermore, incorporating fluorine near the surface allows for high concentrations of not only fluorine but also magnesium to be distributed near the surface.

[0119] When a portion of the particles shown in Figure 12A under Condition 1 was measured by EDX, a magnesium peak was observed near the particle surface. The EDX results are shown in Figure 13A. The horizontal axis in Figure 13A represents the depth direction (distance). Since the cobalt detection position near the outermost shell of the particle can be determined, a magnesium peak can be observed near the particle surface. Figure 13B shows the EDX measurement results for a portion of the particles shown in Figure 12B. Figure 13C shows the EDX measurement results for a portion of the particles shown in Figure 12C. In all EDX results under both conditions, magnesium is clearly distributed on the surface of the positive electrode active material particles. The magnesium near the particle surface maintains its crystalline structure (layered rock-salt crystalline structure) even when Li is desorbed during discharge. While the EDX results show a magnesium peak only on the surface, magnesium is naturally present within the particles. Furthermore, the shift of the CoO2 layer can be reduced during repeated high-voltage charge / discharge. Furthermore, volume change during repeated charge / discharge can be reduced. Therefore, a secondary battery using a positive electrode active material having such characteristics has significantly improved cycle characteristics.

[0120] Figure 11 shows the cycle characteristics of the battery cells under conditions 1, 2, and 3. The cycle characteristics were evaluated at 25°C with charging as CCCV (0.5C, 4.6V, cut-off current 0.05C) and discharging as CC (0.5C, 2.5V). The results are shown in Figure 11.

[0121] FIG. 11 also shows the cycle characteristics of a comparative example battery cell fabricated without a lid, as shown in FIG. 10D, using the same fabrication procedure and conditions as Condition 1. FIG. 24A shows an SEM image of the particles of the comparative example. In the SEM image, the particle surface of the comparative example appears rough. Numerous fine protrusions are visible on the surface, which is significantly different from the results under Conditions 1, 2, and 3. FIG. 24B shows the EDX measurement results of a portion of the particles in FIG. 24A. The horizontal axis of FIG. 24B represents distance. No magnesium peaks are observed under the comparative example without a lid. In the comparative example where annealing without a lid was performed, fluorine was released from the inside of the particles to the outside, and almost no magnesium was present on the surface. Therefore, when charging and discharging were performed, the crystal structure collapsed due to distortion, resulting in a decrease in cycle characteristics, as shown in FIG. 11.

[0122] From the above, it can be confirmed that the annealing conditions with a lid (conditions 1, 2, and 3) exhibit good cycle characteristics compared to the comparative example of the annealing conditions without a lid.

[0123] (Embodiment 3) In this embodiment, an example of a structure of a positive electrode active material manufactured by a manufacturing method of one embodiment of the present invention will be described.

[0124] [Positive electrode active material structure] Materials with a layered rock-salt crystal structure, such as lithium cobalt oxide (LiCoO2), are known to have high discharge capacity and are excellent as positive electrode active materials for secondary batteries. An example of a material with a layered rock-salt crystal structure is a composite oxide represented by LiMO2. Examples of the element M include one or more selected from Co, Ni, and Mn. Examples of the element M include one or more selected from Co, Ni, and Mn, as well as one or more selected from Al and Mg.

[0125] It is known that the strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal.

[0126] In compounds containing nickel, distortion may occur due to the Jahn-Teller effect. Therefore, when LiNiO2 is charged and discharged at high voltages, there is a concern that the crystal structure may collapse due to distortion. In LiCoO2, the influence of the Jahn-Teller effect is suggested to be small, and LiCoO2 may have better durability against charge and discharge at high voltages, which is preferable.

[0127] The positive electrode active material will be described with reference to Figures 14 and 15. Figures 14 and 15 describe the case where cobalt is used as the transition metal contained in the positive electrode active material.

[0128] <Conventional positive electrode active materials> The positive electrode active material shown in Figure 15 is lithium cobalt oxide (LiCoO2) to which no halogen or magnesium is added. The crystal structure of the lithium cobalt oxide shown in Figure 15 changes depending on the depth of charge.

[0129] As shown in Figure 15, lithium cobalt oxide at a depth of charge of 0 (discharged state) has a region with a crystal structure of space group R-3m, with three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is six-coordinated with oxygen, and the layers are connected in a plane with edge sharing.

[0130] At a charge depth of 1, the crystal structure is of the space group P-3m1, with one CoO2 layer in the unit cell. Therefore, this crystal structure is sometimes called an O1-type crystal structure.

[0131] Furthermore, lithium cobalt oxide at a charge depth of approximately 0.88 has a crystal structure of the space group R-3m. This structure can be described as a structure in which a CoO2 structure such as P-3m1(O1) and a LiCoO2 structure such as R-3m(O3) are alternately stacked. Therefore, this crystal structure is sometimes referred to as an H1-3 crystal structure. In reality, the H1-3 crystal structure has twice the number of cobalt atoms per unit cell as other structures. However, in Figure 15 and other parts of this specification, for ease of comparison with other structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0132] As an example, the coordinates of cobalt and oxygen in the unit cell of the H1-3 crystal structure can be expressed as Co(0,0,0.42150±0.00016), O1(0,0,0.27671±0.00045), and O2(0,0,0.11535±0.00045). O1 and O2 are each an oxygen atom. Thus, the H1-3 crystal structure is expressed by a unit cell using one cobalt and two oxygen atoms. On the other hand, as described below, the O3' crystal structure of one embodiment of the present invention is preferably expressed by a unit cell using one cobalt and one oxygen atom. This indicates that the symmetry between cobalt and oxygen differs between the O3' crystal structure and the H1-3 structure, and that the O3' crystal structure exhibits smaller changes from the O3 structure than the H1-3 structure. The unit cell that is more preferably used to represent the crystal structure of the positive electrode active material may be selected, for example, so that the GOF (goodness of fit) value is smaller in Rietveld analysis of XRD.

[0133] When lithium cobalt oxide is repeatedly charged and discharged at a high voltage of 4.6 V or higher, based on the redox potential of lithium metal, or at a deep charge depth of 0.8 or higher, the crystal structure of the lithium cobalt oxide changes repeatedly (i.e., a non-equilibrium phase change) between the H1-3 crystal structure and the R-3m(O3) structure in the discharged state.

[0134] However, these two crystal structures have a large deviation in the CoO2 layers. As shown by the dotted line and arrow in Figure 15, in the H1-3 type crystal structure, the CoO2 layers are significantly deviated from the R-3m(O3) structure. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0135] Furthermore, the difference in volume is large: when compared per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the O3 crystal structure in the discharged state is more than 3.0%.

[0136] In addition, the H1-3 type crystal structure, which has continuous CoO2 layers such as P-3m1(O1), is likely to be unstable.

[0137] Therefore, repeated high-voltage charging and discharging causes the crystalline structure of lithium cobalt oxide to collapse, which leads to a deterioration in cycle characteristics. This is thought to be because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0138] <Positive Electrode Active Material of One Embodiment of the Present Invention> The positive electrode active material prepared according to one embodiment of the present invention can reduce the displacement of the CoO2 layer during repeated high-voltage charge and discharge. Furthermore, the volume change can be reduced. Therefore, the positive electrode active material can achieve excellent cycle characteristics. Furthermore, the compound can have a stable crystal structure in a high-voltage charged state. Therefore, the compound may be less likely to cause a short circuit when maintained in a high-voltage charged state. In such cases, safety is further improved, which is preferable.

[0139] In the positive electrode active material of one embodiment of the present invention, the change in crystal structure and the difference in volume per the same number of transition metal atoms between a fully discharged state and a high-voltage charged state are small.

[0140] 14 shows the crystal structures of a positive electrode active material 904 of one embodiment of the present invention before and after charge and discharge. The positive electrode active material 904 is a composite oxide containing lithium, cobalt as a transition metal, and oxygen. In addition to the above, the positive electrode active material 904 preferably contains magnesium as an additional element. Furthermore, the positive electrode active material 904 preferably contains a halogen such as fluorine or chlorine as an additional element.

[0141] The crystal structure at a charge depth of 0 (discharged state) in FIG. 14 is the same as that in FIG. 15, R-3m(O3). On the other hand, when the cathode active material 904 is fully charged, it has a crystal structure different from the H1-3 crystal structure. This structure is in the space group R-3m. Although it is not a spinel crystal structure, ions such as cobalt and magnesium occupy six oxygen coordination positions, and the cation arrangement has a symmetry similar to that of a spinel structure. The symmetry of the CoO2 layers in this structure is the same as that of an O3 type. Therefore, this structure is referred to herein as an O3' type crystal structure or a pseudo-spinel type crystal structure. Therefore, the O3' type crystal structure may also be referred to as a pseudo-spinel type crystal structure. In the diagram of the O3' type crystal structure shown in FIG. 14, lithium is omitted to explain the symmetry of the cobalt atoms and the oxygen atoms. However, in reality, lithium is present between the CoO2 layers at, for example, 20 atomic % or less relative to the cobalt. In both the O3-type and O3'-type crystal structures, magnesium is preferably present in a dilute form between the CoO2 layers, i.e., at the lithium sites, and halogens such as fluorine are preferably present randomly and dilutely at the oxygen sites.

[0142] In the O3'-type crystal structure, light elements such as lithium may occupy the oxygen tetracoordination site, and in this case too, the ion arrangement has a symmetry similar to that of the spinel type.

[0143] It can also be said that the O3' type crystal structure is similar to the CdCl2 type crystal structure, although it has random Li between the layers. This CdCl2 type-like crystal structure was observed when lithium nickel oxide was charged to a charge depth of 0.94 (Li 0.06 The crystal structure is similar to that of lithium cobaltate (NiO2), but it is known that pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt do not usually adopt this crystal structure.

[0144] In the positive electrode active material 904 of one embodiment of the present invention, when a large amount of lithium is released by charging at a high voltage, the change in the crystal structure is suppressed more than in conventional positive electrode active materials. For example, as shown by the dotted line in FIG. 14, there is almost no displacement of the CoO layer in these crystal structures.

[0145] More specifically, the cathode active material 904 of one embodiment of the present invention has high structural stability even at high charge voltages. For example, conventional cathode active materials adopt an H1-3 crystal structure at a charge voltage of approximately 4.6 V relative to the potential of lithium metal. However, the cathode active material 904 of one embodiment of the present invention can maintain an R-3m(O3) crystal structure even at a charge voltage of approximately 4.6 V. Even at higher charge voltages, for example, at a voltage of approximately 4.65 V to 4.7 V relative to the potential of lithium metal, the cathode active material 904 of one embodiment of the present invention can adopt an O3' crystal structure. When the charge voltage is further increased above 4.7 V, the H1-3 crystal structure may finally be observed in the cathode active material 904 of one embodiment of the present invention. Furthermore, even at lower charge voltages (for example, at a charge voltage of 4.5 V or higher but lower than 4.6 V relative to the potential of lithium metal), the cathode active material 904 of one embodiment of the present invention may adopt an O3' crystal structure.

[0146] Note that when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above-mentioned value by the potential of the graphite. The potential of graphite is approximately 0.05 V to 0.2 V relative to the potential of lithium metal. Therefore, even when the voltage of a secondary battery using graphite as the negative electrode active material is 4.3 V or higher and 4.5 V or lower, the positive electrode active material 904 of one embodiment of the present invention can maintain the R-3m(O3) crystal structure. Furthermore, even when the charge voltage is higher, for example, when the voltage of the secondary battery is higher than 4.5 V and lower than 4.6 V, the positive electrode active material 904 of one embodiment of the present invention can also have the O3'-type crystal structure. Furthermore, even when the charge voltage is lower, for example, when the voltage of the secondary battery is 4.2 V or higher and lower than 4.3 V, the positive electrode active material 904 of one embodiment of the present invention can sometimes have the O3'-type crystal structure.

[0147] Therefore, the crystal structure of the positive electrode active material 904 of one embodiment of the present invention is not easily destroyed even when repeatedly charged and discharged at a high voltage.

[0148] In addition, in the positive electrode active material 904, the difference in volume per unit cell between the O3 type crystal structure at a charge depth of 0 and the O3' type crystal structure at a charge depth of 0.88 is 2.5% or less, more specifically 2.2% or less.

[0149] In addition, the O3' type crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25.

[0150] An additive element, such as magnesium, randomly and dilutely present between CoO layers, i.e., at the lithium sites, has the effect of suppressing the misalignment of the CoO layers. Therefore, the presence of magnesium between CoO layers tends to result in an O3'-type crystal structure. Therefore, magnesium is preferably distributed throughout the particles of the positive electrode active material 904 of one embodiment of the present invention. Furthermore, to distribute magnesium throughout the particles, heat treatment is preferably performed during the manufacturing process of the positive electrode active material 904 of one embodiment of the present invention.

[0151] However, if the heat treatment temperature is too high, cation mixing occurs, increasing the possibility that added elements, such as magnesium, will enter the cobalt site. Magnesium present in the cobalt site is ineffective in maintaining the R-3m structure during high-voltage charging. Furthermore, if the heat treatment temperature is too high, there are concerns that adverse effects such as cobalt being reduced to a divalent state and lithium evaporating may occur.

[0152] Therefore, it is preferable to add a halogen compound such as a fluorine compound to the lithium cobalt oxide before the heat treatment to distribute magnesium throughout the particles. The addition of the halogen compound lowers the melting point of the lithium cobalt oxide. Lowering the melting point makes it easier to distribute magnesium throughout the particles at a temperature where cation mixing is unlikely to occur. Furthermore, the presence of a fluorine compound is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0153] However, if the magnesium concentration is increased above a desired value, the effect on stabilizing the crystal structure may be reduced. This is thought to be due to the incorporation of magnesium into the cobalt site in addition to the lithium site. The number of magnesium atoms in the positive electrode active material of one embodiment of the present invention is preferably 0.001 to 0.1 times the number of transition metal atoms, more preferably greater than 0.01 and less than 0.04 times, and even more preferably approximately 0.02 times. The magnesium concentration shown here may be, for example, a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using ICP-MS or the like, or may be based on the value of the raw material composition during the production process of the positive electrode active material.

[0154] Lithium cobalt oxide may contain, as an additive element, one or more metals other than cobalt, selected from nickel, aluminum, manganese, titanium, vanadium, and chromium, with nickel and aluminum being particularly preferred. Manganese, titanium, vanadium, and chromium may be stable and easily tetravalent, which may contribute significantly to structural stability. Addition of an additive element may result in a more stable crystal structure in the positive electrode active material of one embodiment of the present invention, for example, in a charged state at a high voltage. In the positive electrode active material of one embodiment of the present invention, the additive element is preferably added at a concentration that does not significantly alter the crystallinity of the lithium cobalt oxide. For example, the amount is preferably such that the Jahn-Teller effect, etc., described above, is not exhibited.

[0155] As shown in the legend in Figure 14, transition metals such as nickel and manganese and aluminum are preferably present at the cobalt site, but some may be present at the lithium site. Magnesium is also preferably present at the lithium site. Oxygen may be partially substituted with fluorine.

[0156] As the magnesium concentration in the positive electrode active material of one embodiment of the present invention increases, the capacity of the positive electrode active material may decrease. For example, this may be due to magnesium entering the lithium site, which may reduce the amount of lithium contributing to charge and discharge. Excessive magnesium may also produce magnesium compounds that do not contribute to charge and discharge. When the positive electrode active material of one embodiment of the present invention contains nickel as an additive element in addition to magnesium, the capacity per weight and per volume may be increased. When the positive electrode active material of one embodiment of the present invention contains aluminum as an additive element in addition to magnesium, the capacity per weight and per volume may be increased. When the positive electrode active material of one embodiment of the present invention contains nickel and aluminum as an additive element in addition to magnesium, the capacity per weight and per volume may be increased.

[0157] Hereinafter, the concentration of an element such as magnesium contained in a positive electrode active material of one embodiment of the present invention will be expressed in terms of the number of atoms.

[0158] The number of nickel atoms in the positive electrode active material of one embodiment of the present invention is preferably 10% or less of the number of cobalt atoms, more preferably 7.5% or less, even more preferably 0.05% to 4%, and particularly preferably 0.1% to 2%. The nickel concentration shown here may be a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0159] If the battery is charged at a high voltage for a long period of time, transition metals may leach out of the positive electrode active material into the electrolyte, causing the crystal structure to collapse. However, by including nickel in the above proportion, it may be possible to suppress the leach-out of transition metals from the positive electrode active material 904.

[0160] The number of aluminum atoms in the positive electrode active material of one embodiment of the present invention is preferably 0.05% to 4%, more preferably 0.1% to 2%, of the number of cobalt atoms. The aluminum concentration shown here may be a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0161] The positive electrode active material of one embodiment of the present invention preferably contains an additional element X, and phosphorus is preferably used as the additional element X. Furthermore, the positive electrode active material of one embodiment of the present invention more preferably contains a compound containing phosphorus and oxygen.

[0162] When the positive electrode active material of one embodiment of the present invention contains a compound containing an additional element X, a short circuit may be less likely to occur when the positive electrode active material is maintained in a charged state at a high voltage.

[0163] When the positive electrode active material of one embodiment of the present invention contains phosphorus as the additional element X, hydrogen fluoride generated by decomposition of the electrolyte solution may react with phosphorus, resulting in a decrease in the hydrogen fluoride concentration in the electrolyte solution.

[0164] When the electrolyte contains LiPF6, hydrogen fluoride may be generated by hydrolysis. Hydrogen fluoride may also be generated by the reaction of PVDF, which is used as a component of the positive electrode, with alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating may be suppressed. Furthermore, gelation and / or insolubilization of PVDF may be suppressed, which may reduce adhesion.

[0165] When the positive electrode active material of one embodiment of the present invention contains magnesium in addition to the additive element X, the stability in a high-voltage charged state is extremely high. When the additive element X is phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and even more preferably 0.7% to 4%. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS, or may be based on values ​​obtained by combining raw materials in the process of producing the positive electrode active material.

[0166] When the positive electrode active material has cracks, the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the cracks may inhibit the progression of the cracks.

[0167] As is clear from the oxygen atoms indicated by the arrows in Figure 14, the symmetry of the oxygen atoms is slightly different between the O3 and O3' crystal structures. Specifically, in the O3 crystal structure, the oxygen atoms are aligned along the (-1 0 2) plane indicated by the dotted line, whereas in the O3' crystal structure, the oxygen atoms are not strictly aligned along the (-1 0 2) plane. This is because, in the O3' crystal structure, as lithium decreases, tetravalent cobalt increases, increasing Jahn-Teller distortion and distorting the octahedral structure of CoO6. Another factor is that as lithium decreases, the repulsion between oxygen atoms in the CoO2 layer becomes stronger.

[0168] Magnesium is preferably distributed throughout the particles of the positive electrode active material 904 of one embodiment of the present invention, and in addition, the magnesium concentration in the surface layer is preferably higher than the average throughout the particles. For example, the magnesium concentration in the surface layer measured by XPS or the like is preferably higher than the average magnesium concentration throughout the particles measured by ICP-MS or the like.

[0169] When the positive electrode active material 904 of one embodiment of the present invention contains an element other than cobalt, such as one or more metals selected from nickel, aluminum, manganese, iron, and chromium, the concentration of the metal in the surface layer is preferably higher than the average concentration of the metal in the entire particle. For example, the concentration of the element other than cobalt in the surface layer measured by XPS or the like is preferably higher than the average concentration of the element in the entire particle measured by ICP-MS or the like.

[0170] The particle surface is essentially a crystal defect, and because lithium is released from the surface during charging, the lithium concentration is likely to be lower than in the interior. This makes the surface more unstable and prone to the collapse of the crystal structure. If the magnesium concentration in the surface layer is high, changes in the crystal structure can be more effectively suppressed. Furthermore, a high magnesium concentration in the surface layer is expected to improve corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte.

[0171] Furthermore, the concentration of halogen such as fluorine in the surface layer of the positive electrode active material 904 of one embodiment of the present invention is preferably higher than the average concentration in the entire particle. The presence of halogen in the surface layer, which is the region in contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid.

[0172] As described above, the surface portion of the positive electrode active material 904 of one embodiment of the present invention preferably has a different composition from the interior portion, that is, the concentration of additive elements, such as magnesium and fluorine, is higher than that of the interior portion. Furthermore, the composition preferably has a stable crystal structure at room temperature. Therefore, the surface portion may have a different crystal structure from the interior portion. For example, at least a part of the surface portion of the positive electrode active material 904 of one embodiment of the present invention may have a rock-salt crystal structure. Furthermore, when the surface portion and the interior portion have different crystal structures, the crystal orientations of the surface portion and the interior portion preferably roughly match.

[0173] The anions in layered rock salt crystals and rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in O3'-type crystals also have a cubic close-packed structure. When they contact, there is a crystal plane where the cubic close-packed structure formed by the anions is oriented in the same direction. However, the space group of layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space groups Fm-3m (the space group of general rock salt crystals) and Fd-3m (the space group of rock salt crystals with the simplest symmetry) of rock salt crystals. Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between layered rock salt crystals and O3'-type crystals and rock salt crystals. In this specification, when the cubic close-packed structure formed by the anions is oriented in the same direction in layered rock salt crystals, O3'-type crystals, and rock salt crystals, it may be said that the crystal orientations are approximately the same.

[0174] The general alignment of the crystal orientations of the two regions can be determined from TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, and ABF-STEM (annular bright-field scanning transmission electron microscope) images. X-ray diffraction (XRD), electron diffraction, and neutron diffraction can also be used for this determination. If the crystal orientations are generally aligned, the difference in the orientation of the alternating linear array of cations and anions can be observed in TEM images, etc., to be 5 degrees or less, more preferably 2.5 degrees or less. Light elements such as oxygen and fluorine may not be clearly visible in TEM images, but in such cases, the alignment of the orientations can be determined from the arrangement of metal elements.

[0175] However, if the surface layer is only MgO or only a solid solution of MgO and CoO(II), it becomes difficult to insert and extract lithium. Therefore, the surface layer must contain at least cobalt, and in the discharged state, it must also contain lithium, providing a path for lithium insertion and extraction. It is also preferable that the concentration of cobalt is higher than that of magnesium.

[0176] The additional element X is preferably located in the surface layer portion of the particles of the positive electrode active material 904 of one embodiment of the present invention. For example, the positive electrode active material 904 of one embodiment of the present invention may be covered with a coating containing the additional element X.

[0177] ≪Grain boundary≫ The additional element X contained in the positive electrode active material 904 of one embodiment of the present invention may be present randomly and dilutely inside the positive electrode active material 904, but it is more preferable that a part of the additional element X segregates at the grain boundaries.

[0178] In other words, the concentration of the additional element X in and around the grain boundary of the positive electrode active material 904 of one embodiment of the present invention is preferably higher than that in other regions inside the grain boundary.

[0179] Like particle surfaces, grain boundaries are also planar defects. This makes them prone to instability and facilitates changes in the crystal structure. Therefore, if the concentration of the added element X at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0180] Furthermore, when the concentration of the additional element X is high at and near the grain boundary, even if cracks occur along the grain boundary of particles of the positive electrode active material 904 of one embodiment of the present invention, the concentration of the additional element X becomes high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the cracks occur.

[0181] In this specification and the like, the vicinity of the grain boundary refers to the region up to about 10 nm from the grain boundary.

[0182] <Particle size> If the particle size of the positive electrode active material 904 of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector occur. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with an electrolyte solution occur. Therefore, the average particle size (D50: also referred to as median diameter) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less.

[0183] <Analysis method> Whether a certain positive electrode active material exhibits the O3'-type crystal structure when charged at a high voltage can be determined by analyzing the positive electrode charged at a high voltage using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. XRD is particularly preferred because it can analyze the symmetry of transition metals such as cobalt 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 crystallite size, and it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery.

[0184] As described above, the positive electrode active material 904 of one embodiment of the present invention is characterized by minimal change in crystal structure between a high-voltage charged state and a discharged state. Materials in which a crystal structure that exhibits a significant change between a high-voltage charged state and a discharged state occupies 50 wt% or more of a chargeable material are undesirable because they cannot withstand high-voltage charge and discharge. It should be noted that the desired crystal structure may not be achieved simply by adding an additive element. For example, even if both materials share the common feature of being lithium cobalt oxide containing magnesium and fluorine, there are cases in which the O3'-type crystal structure occupies 60 wt% or more of a chargeable material in a high-voltage charged state, and cases in which the H1-3-type crystal structure occupies 50 wt% or more of a chargeable material in a high-voltage charged state. Furthermore, at a certain voltage, the O3'-type crystal structure may be nearly 100 wt%, and further increasing the voltage may result in the H1-3-type crystal structure. Therefore, crystal structure analysis, such as XRD, is required to determine whether a material is the positive electrode active material 904 of one embodiment of the present invention.

[0185] However, when positive electrode active materials are charged or discharged at high voltage, their crystal structure may change when exposed to air. For example, they may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is recommended that all samples be handled in an inert atmosphere such as an argon atmosphere.

[0186] (Fourth embodiment) In this embodiment, an example of a secondary battery of one embodiment of the present invention will be described with reference to FIGS.

[0187] <Secondary battery configuration example 1> The following description will be given taking as an example a secondary battery in which a positive electrode, a negative electrode, and an electrolyte are enclosed in an exterior body.

[0188] [Positive electrode] 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 is formed by the method described in the above embodiment. Hereinafter, a cross-sectional structure example in which a graphene compound is used as a conductive material in the active material layer 200 will be described as an example.

[0189] 16A shows a vertical cross-sectional view of active material layer 200. Active material layer 200 includes granular positive electrode active material 101, graphene compound 201 as a conductive material, and a binder (not shown).

[0190] In this specification and the like, graphene compounds 201 include graphene, multilayer graphene, multigraphene, graphene oxide (GO), multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide (RGO), reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up into a shape similar to carbon nanofibers. Graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength.

[0191] 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.

[0192] In this specification, 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. It may also be called a carbon sheet. Although a single sheet of reduced graphene oxide can function, multiple sheets may also be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is between 2 atomic % and 15 atomic %. By achieving these 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 between the G band and 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.

[0193] In a longitudinal cross section of the active material layer 200, as shown in FIG. 16B , sheet-like graphene compounds 201 are dispersed approximately uniformly within the active material layer 200. In FIG. 16B , the graphene compounds 201 are schematically represented by thick lines, but in reality, they are thin films having a thickness corresponding to a single layer or multiple layers of carbon molecules. The plurality of graphene compounds 201 are formed so as to partially cover the plurality of granular positive electrode active material 101 or to be attached to the surfaces of the plurality of granular positive electrode active material 101, and are therefore in surface contact with each other. This increases the contact area between the active material and the conductive material.

[0194] Here, a plurality of graphene compounds are bonded together to form a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net). When an active material is covered with a graphene net, the graphene net can also function as a binder that binds the active materials together. Therefore, the amount of binder can be reduced or no binder can be used, thereby improving the ratio of the active material to the electrode volume or weight. In other words, the capacity of the secondary battery can be increased.

[0195] Here, it is preferable to use graphene oxide as the graphene compound 201, mix it with an active material to form a layer that will become the active material layer 200, and then reduce it. That is, the completed active material layer preferably has reduced graphene oxide. By using graphene oxide, which has extremely high dispersibility in a polar solvent, to form the graphene compound 201, it is possible to substantially uniformly disperse the graphene compound 201 inside the active material layer 200. Since the solvent is volatilized and removed from the dispersion medium containing the uniformly dispersed graphene oxide and the graphene oxide is reduced, the graphene compound 201 remaining in the active material layer 200 partially overlaps and is dispersed to such an extent that it is in surface contact with each other, thereby forming a three-dimensional conductive path. Note that the reduction of the graphene oxide may be performed, for example, by heat treatment or using a reducing agent.

[0196] Therefore, unlike a granular conductive material such as acetylene black that makes point contact with the active material, the graphene compound 201 enables surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular positive electrode active material 101 and the graphene compound 201 with a smaller amount than that of a typical conductive material. This allows the proportion of the positive electrode active material 101 in the active material layer 200 to be increased, thereby increasing the discharge capacity of the secondary battery.

[0197] Furthermore, by using a spray dryer in advance, a graphene compound, which is a conductive material, can be formed as a coating that covers the entire surface of the active material, and further a conductive path can be formed between the active material particles by the graphene compound.

[0198] [Negative electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material, and may also have a conductive material and a binder.

[0199] [Negative electrode active material] As the negative electrode active material, for example, an alloy-based material or a carbon-based material can be used.

[0200] The negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, materials containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium can be used. These elements have a larger capacity than carbon, and silicon, in particular, has a high theoretical capacity of 4200 mAh / g. For this reason, silicon is preferred as the negative electrode active material. Compounds containing these elements can also be used. Examples include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes called alloy-based materials.

[0201] 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, and more preferably 0.3 or more and 1.2 or less.

[0202] 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.

[0203] 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.

[0204] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), graphite exhibits a low potential similar to that of metallic lithium (0.05 V to 0.3 V vs. Li / Li + ) This allows lithium-ion secondary batteries to exhibit high operating voltages. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, a relatively small volume expansion, low cost, and a higher level of safety compared to lithium metal.

[0205] In addition, titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), molybdenum oxide (MoO2), and other oxides can be used.

[0206] In addition, the negative electrode active material is a composite nitride of lithium and transition metals, Li3N-type 3-x M x N (M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 N3 has a large charge / discharge capacity (900mAh / g, 1890mAh / cm 3 ) and is preferred.

[0207] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, and therefore it can be preferably combined with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material containing lithium ions is used as the positive electrode active material, the composite nitride of lithium and a transition metal can be used as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0208] In addition, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 It also occurs with sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N and Ge3N4, phosphides such as NiP2, FeP2 and CoP3, and fluorides such as FeF3 and BiF3.

[0209] 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.

[0210] [Negative electrode current collector] The negative electrode current collector may be made of the same material as the positive electrode current collector, but it is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0211] [Electrolyte] The electrolytic solution contains a solvent and an electrolyte. The solvent for the electrolytic solution is preferably an aprotic organic solvent, such as 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, or sultone, or any combination and ratio of two or more of these.

[0212] 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 or catching fire even if the internal temperature rises due to an internal short circuit or overcharging of the secondary battery. Ionic liquids are composed of cations and anions, including organic cations and anions. 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.

[0213] Examples of the electrolyte to be dissolved in the solvent include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B 10 Cl 10 , Li2B 12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, etc. can be used alone or in any combination and ratio of two or more of these.

[0214] The electrolyte used in the secondary battery 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, the weight ratio of impurities to the electrolyte is preferably 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0215] The electrolyte may also contain additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalato)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive may be, for example, 0.1 wt % to 5 wt % of the total solvent.

[0216] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0217] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0218] 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.

[0219] 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, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0220] In addition, instead of an electrolyte solution, a solid electrolyte containing inorganic materials such as sulfides or oxides, or a solid electrolyte containing polymeric materials such as polyethylene oxide (PEO) can be used. When a solid electrolyte is used, the installation of a separator and spacer becomes unnecessary. Furthermore, since the entire battery can be solidified, there is no risk of leakage, dramatically improving safety.

[0221] [Separator] The secondary battery preferably has a separator. Examples of the separator include paper, nonwoven fabric, glass fiber, ceramics, and synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably envelope-shaped and disposed so as to encase either the positive electrode or the negative electrode.

[0222] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials that can be used include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials that can be used include PVDF and polytetrafluoroethylene. Examples of polyamide materials that can be used include nylon and aramid (meta-aramid, para-aramid).

[0223] 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 materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0224] 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.

[0225] 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 capacity per volume of the secondary battery can be increased.

[0226] [Exterior body] The exterior body of the secondary battery can be made of a metal material such as aluminum or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body.

[0227] <Configuration example 2 of secondary battery> As an example of the configuration of a secondary battery, the configuration of a secondary battery using a solid electrolyte layer will be described below. In this specification, the term "solid-state battery" refers not only to secondary batteries using only a solid electrolyte, but also to batteries using a polymer gel electrolyte, a trace amount of electrolytic solution, or a combination of these.

[0228] As shown in FIG. 17A, a secondary battery 400 according to one embodiment of the present invention includes a positive electrode 410, a solid electrolyte layer 420, and a negative electrode 430.

[0229] 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 material and a binder.

[0230] 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.

[0231] The negative electrode 430 has a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 has a negative electrode active material 431 and a solid electrolyte 421. The negative electrode active material layer 434 may also have a conductive material and a binder. When metallic lithium is used for the negative electrode 430, the negative electrode 430 may not have a solid electrolyte 421, as shown in FIG. 17B. Using metallic lithium for the negative electrode 430 is preferable because it can improve the energy density of the secondary battery 400.

[0232] As the solid electrolyte 421 of 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.

[0233] Sulfide-based solid electrolytes include thiosilicon-based (Li 10 GeP2S 12 , Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glasses (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·38SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), sulfide crystallized glasses (Li7P3S 11 , Li 3.25 P 0.95 Sulfide-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.

[0234] Oxide-based solid electrolytes include materials with a perovskite crystal structure (La 2 / 3-x Li 3x TiO3, etc.), materials with NASICON-type crystal structure (Li 1+X Al X Ti 2-X (PO4)3, etc.), materials with garnet-type crystal structure (Li7La3Zr2O 12 etc.), materials with LISICON-type crystal structure (Li 14 ZnGeO 16etc.), oxide glass (Li3PO4-Li4SiO4, 50Li4SiO4·50Li3BO3 etc.), oxide glass-ceramics (Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0235] Halide-based solid electrolytes include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, LiI, etc. Composite materials in which these halide-based solid electrolytes are filled into the pores of porous aluminum oxide or porous silica can also be used as solid electrolytes.

[0236] Also, different solid electrolytes may be mixed and used.

[0237] Among them, Li with NASICON type crystal structure 1+x Al x Ti 2-x (PO4)3 (0≦x≦1) (hereinafter, LATP) 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 is therefore expected to have a synergistic effect in improving cycle characteristics, making it preferable. It is also expected to improve productivity by reducing the number of steps. In this specification and elsewhere, the NASICON-type crystal structure refers to a compound represented by M2(XO4)3 (M: transition metal, X: S, P, As, Mo, W, etc.), which has a structure in which MO6 octahedra and XO4 tetrahedra are arranged three-dimensionally with vertices shared.

[0238] [Shapes of exterior body and secondary battery] The exterior of the secondary battery 400 of one embodiment of the present invention can be made of various materials and in various shapes, but preferably has a function of applying pressure to the positive electrode, the solid electrolyte layer, and the negative electrode.

[0239] For example, Figures 18A, 18B, and 18C are examples of cells for evaluating materials for all-solid-state batteries.

[0240] 18A is a cross-sectional schematic diagram of the evaluation cell, which has a lower member 761, an upper member 762, and a fixing screw and wing nut 764 that fix them in place, and electrode plate 753 is pressed to fix the evaluation material by rotating a holding screw 763. An insulator 766 is provided between lower member 761 and upper member 762, both of which are made of stainless steel. An O-ring 765 is also provided between upper member 762 and holding screw 763 to provide a tight seal.

[0241] The evaluation material is placed on electrode plate 751, surrounded by insulating tube 752, and pressed from above by electrode plate 753. An enlarged perspective view of the evaluation material and its surroundings is shown in Figure 18B.

[0242] The evaluation material is an example of a laminate of a positive electrode 750a, a solid electrolyte layer 750b, and a negative electrode 750c, and its cross-sectional view is shown in Figure 18C. Note that the same reference numerals are used for the same parts in Figures 18A, 18B, and 18C.

[0243] The electrode plate 751 and lower member 761, which are electrically connected to the positive electrode 750a, can be said to correspond to a positive electrode terminal. The electrode plate 753 and upper member 762, which are electrically connected to the negative electrode 750c, can be said to correspond to a negative electrode terminal. Electrical resistance and the like can be measured by applying pressure to the evaluation material via the electrode plate 751 and the electrode plate 753.

[0244] The secondary battery of one embodiment of the present invention preferably uses an airtight package for its exterior. For example, a ceramic package or a resin package can be used. Furthermore, the exterior is preferably sealed in a sealed atmosphere, such as in a glove box, while blocking external air.

[0245] 18D shows a perspective view of a secondary battery of one embodiment of the present invention, which has an exterior body and shape different from those shown in Fig. 18A, Fig. 18B, and Fig. 18C. The secondary battery in Fig. 18D has external electrodes 771 and 772 and is sealed in an exterior body having multiple package members.

[0246] An example of a cross section taken along the dashed line in Figure 18D is shown in Figure 18E. A laminate including positive electrode 750a, solid electrolyte layer 750b, and negative electrode 750c is enclosed and sealed within package member 770a, which is a flat plate with electrode layer 773a provided thereon, frame-shaped package member 770b, and package member 770c, which is a flat plate with electrode layer 773b provided thereon. Package members 770a, 770b, and 770c can be made of an insulating material, such as a resin material or ceramic.

[0247] The external electrode 771 is electrically connected to the positive electrode 750a via the electrode layer 773a and functions as a positive electrode terminal, while the external electrode 772 is electrically connected to the negative electrode 750c via the electrode layer 773b and functions as a negative electrode terminal.

[0248] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0249] (Embodiment 5) In this embodiment, an example of the shape of a secondary battery including a positive electrode active material manufactured by the manufacturing method described in the previous embodiment will be described. The description of the previous embodiment can be referred to for the material used in the secondary battery described in this embodiment.

[0250] [Coin-type secondary battery] First, an example of a coin-type secondary battery will be described. Fig. 19A is an external view of a coin-type (single-layer flat) secondary battery, and Fig. 19B is a cross-sectional view thereof.

[0251] In a 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.

[0252] 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.

[0253] Positive electrode can 301 and negative electrode can 302 can be made of metals such as nickel, aluminum, or titanium that are corrosion-resistant to the electrolyte, or alloys of these metals or alloys of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, it is preferable to coat them with nickel, aluminum, or the like. Positive electrode can 301 is electrically connected to positive electrode 304, and negative electrode can 302 is electrically connected to negative electrode 307.

[0254] These negative electrode 307, positive electrode 304, and separator 310 are impregnated with an electrolyte, and as shown in FIG. 9B, the positive electrode can 301 is placed downward, and the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order, and the positive electrode can 301 and negative electrode can 302 are crimped together via gasket 303, to produce coin-type secondary battery 300.

[0255] By using the positive electrode active material particles described in the above embodiment for the positive electrode 304, the coin-type secondary battery 300 can be made less susceptible to deterioration and highly safe.

[0256] Here, we will explain the current flow during charging of a secondary battery using Figure 19C. When a lithium-based secondary battery is considered as a closed circuit, the movement of lithium ions and the flow of current are in the same direction. In lithium-based secondary batteries, the anode (positive electrode) and cathode (negative electrode) are interchanged during charging and discharging, and the oxidation and reduction reactions are alternated. Therefore, the electrode with the higher reaction potential is called the positive electrode, and the electrode with the lower reaction potential is called the negative electrode. Therefore, in this specification, the positive electrode will be called the "positive electrode" or "+ electrode (plus electrode)," and the negative electrode will be called the "negative electrode" or "- electrode (minus electrode)," regardless of whether the battery is being charged or discharged, whether a reverse pulse current is being applied, or whether a charging current is being applied. Using the terms anode (positive electrode) and cathode (negative electrode), which are related to oxidation and reduction reactions, may lead to confusion because their roles are reversed during charging and discharging. Therefore, the terms anode (positive electrode) and cathode (negative electrode) will not be used in this specification. If the terms anode and cathode are used, it should be clearly stated whether they are used during charging or discharging, and whether they correspond to the positive or negative pole.

[0257] 19C is connected to a charger to charge the secondary battery 300. As the charging of the secondary battery 300 progresses, the potential difference between the electrodes increases.

[0258] [Cylindrical secondary battery] An example of a cylindrical secondary battery will be described with reference to Figures 20A to 20D. As shown in Figure 20B, a cylindrical secondary battery 600 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.

[0259] FIG. 20B is a schematic diagram showing the cross section of a cylindrical secondary battery. Inside a hollow cylindrical battery can 602, a battery element is provided, 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 center pin. 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 alloys of these metals with other metals (e.g., stainless steel), which are corrosion-resistant to the electrolyte. Furthermore, a coating of nickel, aluminum, or the like is preferable 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 poured into the battery can 602, in which the battery element is provided. The non-aqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0260] Because the positive and negative electrodes used in cylindrical storage batteries are wound, it is preferable to form active materials on both sides of the current collector. A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be made of a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 612 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature rises, and the increased resistance limits the amount of current to prevent abnormal heat generation. The PTC element can be made of barium titanate (BaTiO3)-based semiconductor ceramics or the like.

[0261] 20C , a module 615 may be configured by sandwiching a plurality of secondary batteries 600 between conductive plates 613 and 614. The plurality of secondary batteries 600 may be connected in parallel, in series, or in series after being connected in parallel. By configuring a module 615 having a plurality of secondary batteries 600, a large amount of power can be extracted.

[0262] FIG. 20D is a top view of module 615. For clarity, conductive plate 613 is shown with dotted lines. As shown in FIG. 20D, module 615 may have conductive wires 616 that electrically connect multiple secondary batteries 600. A conductive plate can be superimposed on the conductive wires 616. Furthermore, a temperature control device 617 may be provided between multiple secondary batteries 600. When a secondary battery 600 overheats, it can be cooled by the temperature control device 617, and when a secondary battery 600 is too cold, it can be heated by the temperature control device 617. This makes it less likely that the performance of module 615 will be affected by the outside temperature.

[0263] By using the positive electrode active material manufactured by the manufacturing method described in the above embodiment for the positive electrode 604, the cylindrical secondary battery 600 can be one that is less susceptible to deterioration and has high safety.

[0264] [Example of secondary battery structure] Another structural example of the secondary battery will be described with reference to FIGS.

[0265] 21A shows the structure of wound body 950. Winding body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. Winding body 950 is a wound body in which negative electrode 931 and positive electrode 932 are stacked on top of each other with separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple stacks of negative electrode 931, positive electrode 932, and separator 933 may be stacked.

[0266] A secondary battery 913 shown in FIG. 21B has a wound body 950 in which terminals 951 and 952 are provided inside a housing 930. The wound body 950 is impregnated with 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. 21B, 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 (such as aluminum) or a resin material.

[0267] [Laminated secondary battery] Next, an example of a laminated secondary battery will be described with reference to FIGS. 22A and 22B.

[0268] Fig. 22A shows an example of an external view of a laminated secondary battery 500. Fig. 22B shows another example of an external view of a laminated secondary battery 500.

[0269] 22A and 22B show 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.

[0270] The laminated secondary battery 500 has a plurality of wound or strip-shaped positive electrodes 503, separators 507, and negative electrodes 506.

[0271] The wound body has a negative electrode 506, a positive electrode 503, and a separator 507. The wound body is formed by stacking the negative electrode 506 and the positive electrode 503 with the separator 507 sandwiched between them, in the same manner as the wound body described in Fig. 21A, and winding the laminated sheet.

[0272] A secondary battery may be provided in which a plurality of rectangular positive electrodes 503, separators 507, and negative electrodes 506 are arranged in a space formed by a film that serves as exterior body 509.

[0273] A method for producing a secondary battery having a plurality of rectangular positive electrodes 503, separators 507, and negative electrodes 506 will be described below.

[0274] First, the negative electrode 506, the separator 507, and the positive electrode 503 are laminated. In this embodiment, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0275] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are placed on the exterior body 509 .

[0276] The outer casing 509 may be a three-layer laminate film having a highly flexible thin metal film made of aluminum, stainless steel, copper, nickel, etc. on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and further having an insulating synthetic resin film made of polyamide resin, polyester resin, etc. on the thin metal film as the outer surface of the outer casing.

[0277] The exterior body 509 is folded to sandwich the laminated layer. Then, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding may be used for the joining. During this joining, 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.

[0278] 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.

[0279] By using the positive electrode active material particles described in the above embodiment for the positive electrode 503, the secondary battery 500 can be one that is less susceptible to deterioration and has high safety.

[0280] This embodiment mode can be freely combined with other embodiment modes.

[0281] (Sixth embodiment) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted in an electronic device or a mobile object will be described.

[0282] 23A to 23E show examples of electronic devices incorporating the secondary battery described in some of the above embodiments. Examples of electronic devices that use the bendable secondary battery include television sets (also called televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0283] Furthermore, the secondary battery can be applied to a mobile object, typically an automobile. Examples of the automobile include next-generation clean energy automobiles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs), and the secondary battery can be applied as one of the power sources mounted on the automobile. The mobile object is not limited to an automobile. Examples of the mobile object include trains, monorails, ships, aircraft (helicopters, unmanned aerial vehicles (drones), airplanes, and rockets), electric bicycles, and electric motorcycles, and the secondary battery of one embodiment of the present invention can be applied to these mobile objects.

[0284] The secondary battery of this embodiment may also be applied to a ground-mounted charging device installed in a home or a charging station installed in a commercial facility.

[0285] 23A shows an example of a mobile phone. Mobile phone 2100 includes a display unit 2102 built into housing 2101, as well as operation buttons 2103, an external connection port 2104, a speaker 2105, and a microphone 2106. Mobile phone 2100 also includes a secondary battery 2107.

[0286] 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.

[0287] 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 built into the mobile phone 2100.

[0288] 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.

[0289] The mobile phone 2100 also has an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Charging may also be performed by wireless power supply without using the external connection port 2104.

[0290] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0291] FIG. 23B shows an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 according to 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. The secondary battery according to one embodiment of the present invention is highly safe and can be used safely for a long period of time, making it suitable as a secondary battery to be installed in the unmanned aerial vehicle 2300.

[0292] As shown in FIG. 23C, a secondary battery 2602 including a plurality of secondary batteries 2601 of one embodiment of the present invention may be mounted in a hybrid electric vehicle (HEV), an electric vehicle (EV), a plug-in hybrid electric vehicle (PHEV), or other electronic devices.

[0293] FIG. 23D shows an example of a vehicle equipped with a secondary battery 2602. The vehicle 2603 is an electric vehicle that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid vehicle that can appropriately select and use an electric motor or an engine as a power source for traveling. The vehicle 2603 that uses an electric motor has multiple ECUs (Electronic Control Units), and the ECUs perform engine control and the like. The ECUs include a microcomputer. The ECUs are connected to a Controller Area Network (CAN) provided in the electric vehicle. CAN is one of the serial communication standards used for in-vehicle LANs. By using the secondary battery of one embodiment of the present invention, the vehicle can function as a power source for the ECU, thereby achieving a highly safe vehicle with a long cruising range.

[0294] The secondary battery can not only drive an electric motor (not shown) but also supply power to light-emitting devices such as headlights or room lights, etc. The secondary battery can also supply power to display devices and semiconductor devices such as a speedometer, a tachometer, and a navigation system included in the vehicle 2603.

[0295] The vehicle 2603 can charge the secondary battery of the secondary battery 2602 by receiving power supply from an external charging facility by a plug-in system or a contactless power supply system.

[0296] FIG. 23E shows a state in which a vehicle 2603 is being charged via a cable from a ground-mounted charging device 2604. Charging may be performed using a predetermined charging method or connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. For example, plug-in technology can be used to charge a secondary battery 2602 mounted on the vehicle 2603 using external power supply. Charging can be performed by converting AC power to DC power via a converter such as an AC-DC converter. The charging device 2604 may be installed in a home, as shown in FIG. 23E, or may be a charging station installed in a commercial facility.

[0297] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground 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 vehicles. Furthermore, a solar cell 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.

[0298] 23E includes a power storage system 2612 including a secondary battery which is one embodiment of the present invention, and a solar panel 2610. The power storage system 2612 is electrically connected to the solar panel 2610 via wiring 2611 or the like. The power storage system 2612 may be electrically connected to a ground-mounted charging device 2604. The power obtained by the solar panel 2610 can be charged to the power storage system 2612. The power stored in the power storage system 2612 can be charged to a secondary battery 2602 included in a vehicle 2603 via the charging device 2604.

[0299] The power stored in the power storage system 2612 can also be supplied to other electronic devices in the house. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the power storage system 2612 of one embodiment of the present invention can be used as an uninterruptible power supply, enabling the use of electronic devices.

[0300] This embodiment mode can be implemented in appropriate combination with other embodiment modes. [Example]

[0301] In this example, the characteristics of the positive electrode active material produced by the production method of the first embodiment were evaluated.

[0302] The sample produced in this example will be described with reference to the production method shown in FIG.

[0303] C-10N (manufactured by Nippon Chemical Industry Co., Ltd.) was used as the lithium oxide 901. Lithium fluoride and magnesium fluoride were used as the fluoride 902. Furthermore, aluminum hydroxide was mixed as the aluminum source and nickel hydroxide as the nickel source into the fluoride 902. The mixture was such that, when the number of cobalt atoms contained in the lithium oxide 901 was taken as 100, the number of lithium fluoride molecules was 0.33, the number of magnesium fluoride molecules was 1, the number of aluminum hydroxide molecules was 0.5, and the number of nickel hydroxide molecules was 0.5.

[0304] 4, lithium oxide 901 and fluoride 902 containing an aluminum source and a nickel source were mixed to prepare mixture 903. Mixture 903 was placed in an alumina container, which was then covered and placed in a muffle furnace.

[0305] Then, mixture 903 was heated in the same manner as in step S16. The heating conditions were 900° C., 20 hours, and an oxygen atmosphere. The positive electrode active material thus produced was designated as Sample 1.

[0306] Furthermore, lithium cobalt oxide (C-10N) to which no fluoride 902 or the like was added and which was not heated was used as Sample 2 (Comparative Example).

[0307] Furthermore, as a comparative example, nickel-cobalt-manganese lithium oxide (NCM523) manufactured by MTI, in which the ratio of nickel, cobalt, and manganese was Ni:Co:Mn=5:2:3, was used as a positive electrode active material without adding fluoride 902 or the like and without heating. This was designated as Sample 3.

[0308] The preparation conditions for Samples 1, 2 and 3 are shown in Table 1.

[0309] [Table 1]

[0310] Secondary batteries were fabricated using the positive electrode active materials of Samples 1, 2, and 3. First, the positive electrode active materials of Samples 1 to 3, AB, and PVDF were mixed in a weight ratio of positive electrode active material:AB:PVDF=95:3:2 to prepare a slurry, which was then applied to an aluminum current collector. NMP was used as the solvent for the slurry.

[0311] After the slurry was applied to the current collector, the solvent was evaporated. After that, a pressure of 210 kN / m was applied, and then a pressure of 1467 kN / m was applied. A positive electrode was obtained through these steps. The loading amount of the positive electrode was approximately 7 mg / cm. 2 The density of the positive electrode active material layer in which the positive electrode active material, AB, and PVDF were mixed was 3.987 g / cc for Sample 1 and 3.415 g / cc for Sample 3.

[0312] Using the prepared positive electrode, a coin-type battery cell of the CR2032 type (diameter 20 mm, height 3.2 mm) was fabricated.

[0313] The counter electrode was made of lithium metal.

[0314] The electrolyte used in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7, to which 2 wt% vinylene carbonate (VC) was added as an additive.

[0315] The separator was made of polypropylene with a thickness of 25 μm.

[0316] The positive electrode can and the negative electrode can were made of stainless steel (SUS).

[0317] The discharge characteristics of secondary batteries using Samples 1 and 3 are shown in Figures 25A and 25B. Figure 25A shows the discharge capacity per weight, and Figure 25B shows the discharge capacity per volume of the positive electrode active material layer. Measurements were taken at 25°C. Charge was CC / CV (0.5C, 4.6V, 0.05C cut), and discharge was CC (0.5C, 2.5V cut), with a 10-minute rest period before the next charge. In this example, 1C was 200mA / g.

[0318] The discharge capacity of sample 1 was 215.8 mAh / g by weight and 860.5 mAh / cm by volume. 3 The discharge capacity of Sample 3 was 200.7 mAh / g by weight and 685.2 mAh / cm by volume. 3 It was.

[0319] The energy density per volume of Sample 1 was about 1.3 times that of Sample 3. As described above, the positive electrode active material of one embodiment of the present invention was shown to have high energy density. For example, by using the positive electrode active material of one embodiment of the present invention as a battery for an electric vehicle (EV), the number of batteries used in the EV battery (the number of batteries connected in series or in parallel) can be reduced.

[0320] Next, the rate characteristics were evaluated for the secondary batteries using Sample 1 and Sample 2. The rate characteristics at 0.2C, 0.5C, 1C, 2C, 3C, 4C, and 5C are shown in FIG.

[0321] Discharge rate 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 1C is X (A). When discharging at a current of 2X (A), it is said to have been discharged at 2C, and when discharging at a current of X / 5 (A), it is said to have been discharged at 0.2C. The same is true for charge rate; when charging at a current of 2X (A), it is said to have been charged at 2C, and when charging at a current of X / 5 (A), it is said to have been charged at 0.2C.

[0322] Constant current charging, for example, refers to a method of charging at a constant charge rate. Constant voltage charging, for example, refers to a method of charging at a constant voltage once the upper voltage limit is reached. Constant current discharging, for example, refers to a method of discharging at a constant discharge rate.

[0323] The charging voltage for Sample 1 was set to 4.60 V. Sample 2 cannot withstand high-voltage charging, so a charging voltage of 4.2 V was used to ensure stable operation. Measurements were taken at 25°C. Charging was performed using CC / CV (0.2 C, 4.20 V or 4.60 V, 0.02 C cut), and discharging was performed using CC (0.2 C, 0.5 C, 1 C, 2 C, 3 C, 4 C, or 5 C, 2.5 V), with a 10-minute rest period before the next charge.

[0324] Table 2 shows the percentage of each rate when 0.2C is 100%.

[0325] [Table 2]

[0326] As shown in Fig. 26 and Table 2, Sample 1 exhibited extremely good rate characteristics compared to Sample 2. In addition, the decrease in discharge capacity at high rates was small.

[0327] The amount of the positive electrode active material layer is approximately 8 mg / cm 2 27 to 29 show the cycle characteristics of secondary batteries using Samples 1, 2, and 3, which were prepared in the same manner as above except that the density of the positive electrode active material layer was set to 3.8 g / cc or more. Measurements were made at 25°C for Fig. 27A, 45°C for Fig. 27B, 50°C for Fig. 28A, 55°C for Fig. 28B, 65°C for Fig. 29A, and 85°C for Fig. 29B. The other charge / discharge conditions were the same as those for measuring the discharge capacity.

[0328] Table 2 also shows the capacity retention rate after 50 cycles for Sample 1 at each measurement temperature.

[0329] [Table 3]

[0330] 27A to 28A , Sample 1, which is a positive electrode active material of one embodiment of the present invention, showed little deterioration at 25° C. to 50° C. and exhibited extremely good high-temperature characteristics compared to Sample 2, which is lithium cobalt oxide without addition or heating, etc. The high-temperature characteristics were comparable to those of Sample 3, which is NCM523.

[0331] 28B to 29B, at 55° C., 65° C., and 85° C., the cycle characteristics of Sample 3 were superior to those of Sample 1. However, the characteristics were superior to those of Sample 2.

[0332] As described above, Sample 1, which is a positive electrode active material of one embodiment of the present invention, exhibited excellent characteristics in a cycle test up to 45° C., which is required for a secondary battery mainly to be installed in mobile electronic devices.

[0333] Next, laminated secondary batteries were fabricated using the positive electrode active materials of Sample 1 and Sample 2 (Comparative Example) with artificial graphite as the negative electrode, and their cycle characteristics were evaluated. The evaluation results are shown in Figure 30. Note that Figure 30 shows the actual measured values ​​and extrapolated values ​​for "Sample 1 with additive," "Sample 1 without additive," and "Sample 2 (Comparative Example)." Note that the extrapolated values ​​were calculated by linear approximation based on the actual measured values ​​for "Sample 1 with additive" up to 282 charge / discharge cycles, and are indicated by the dashed line in Figure 30.

[0334] The electrolyte used in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolyte was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of EC:DEC = 3:7. A secondary battery with additive was prepared by adding 1 wt% LiBOB to the electrolyte, and a secondary battery without additive was prepared by adding no LiBOB.

[0335] The cycle characteristics were measured at 45° C. The charge was CCCV (0.5 C, 4.5 V, cut-off current 0.2 C) and the discharge was CC (0.5 C, 3.0 V).

[0336] As shown in FIG. 30, Sample 1 containing the additive exhibited extremely good cycle characteristics even at 45°C.

[0337] From the above results, it is expected that the use of a positive electrode active material according to one embodiment of the present invention will enable the achievement of the target secondary battery shown in Table 3 below.

[0338] [Table 4] [Explanation of symbols]

[0339] 101: positive electrode active material, 102: space inside the heating furnace, 104: hot plate, 106: heater part, 108: heat insulating material, 116: container, 118: lid, 119: space, 120: heating furnace, 901: lithium oxide, 902: fluoride, 904: positive electrode active material

Claims

1. A first step of placing a container containing lithium oxide and fluoride in a heating furnace; a second step of heating the inside of the heating furnace in an atmosphere containing oxygen, and covering the container before or during the heating; the lithium oxide contains cobalt; the fluoride includes lithium fluoride and magnesium fluoride; the heating temperature in the second step is 750°C or higher and 950°C or lower; The method for producing a positive electrode active material further comprises covering the container with the lid so that the concentration of the fluoride in the space defined by the container and the lid remains constant or does not decrease.

2. In claim 1, The method for producing a positive electrode active material, wherein no gas is allowed to flow during the heating.

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