Positive electrode active material and secondary battery

Abstract

Provided are: a positive electrode active material which has excellent charge and discharge rate characteristics; and a secondary battery using the same. Provided is a positive electrode active material in which: the crystallite size calculated from an XRD pattern is 150 nm or more; the ratio of the number of nickel atoms to the sum of the number of transition metal atoms is larger in the inside than in a first surface layer part and in a second surface layer part; the ratio of the number of atoms of at least one element selected from cobalt and manganese to the sum of the number of transition metal atoms is larger in the second surface layer part than in the inside; and the concentration of at least one additive element is higher in the first surface layer part than in the inside and in the second surface layer part.

Description

Positive electrode active material and secondary battery

[0001] 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 power storage device including a secondary battery, a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, or a manufacturing method thereof.

[0002] 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.

[0003] In recent years, there has been active development of various types of energy storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. Demand for high-power, high-capacity lithium-ion secondary batteries has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0004] In particular, there is a high demand for secondary batteries with a large discharge capacity per weight and excellent cycle characteristics for mobile electronic devices, etc. To meet this demand, active materials for the positive electrodes of secondary batteries have been actively improved (for example, Patent Document 1 and Non-Patent Document 1).

[0005] JP 2020-068210 A

[0006] S. -W. Woo et al, “Improvement of electrochemical and thermal properties of Li[Ni▲0.8▼Co▲0.1▼Mn▲0.1▼]O▲2▼ “Positive electrode materials by multiple metal (Al, Mg) substance”, Electrochimica Acta 54 (2009) 3851-3856

[0007] Lithium ion secondary batteries still have room for improvement in various aspects, such as charge / discharge rate characteristics, discharge capacity, cycle characteristics, reliability, safety, and cost.

[0008] Therefore, there is a demand for positive electrode active materials that can improve issues such as charge / discharge rate characteristics, discharge capacity, cycle characteristics, reliability, safety, and cost when used in secondary batteries.

[0009] An object of one embodiment of the present invention is to provide a positive electrode active material or a composite oxide that can be used for a lithium-ion secondary battery and has excellent charge-discharge rate characteristics, or to provide a secondary battery that is safe or highly reliable.

[0010] Another object of one embodiment of the present invention is to provide a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof.

[0011] 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 of the specification, drawings, and claims.

[0012] One aspect of the present invention is a positive electrode active material having a transition metal M, oxygen, and an additive element, wherein the transition metal M is nickel, manganese, and cobalt, and the additive element is one or more selected from magnesium, aluminum, calcium, titanium, and zirconium; the positive electrode active material has a first surface layer portion, a second surface layer portion, and an interior portion, wherein the second surface layer portion is closer to the interior than the first surface layer portion, the interior portion has a higher ratio of nickel to the sum of the atoms of the transition metal M than the first surface layer portion and the second surface layer portion, the second surface layer portion has a higher ratio of the number of atoms of at least one element selected from cobalt and manganese to the sum of the atoms of the transition metal M than the interior, and the first surface layer portion has a higher concentration of at least one of the additive elements than the interior and the second surface layer portion.

[0013] Another aspect of the present invention is a secondary battery including a positive electrode having a positive electrode active material, and a negative electrode. The positive electrode active material includes a transition metal M, oxygen, and an additive element. The transition metal M is nickel, manganese, and cobalt, and the additive element is one or more selected from magnesium, aluminum, calcium, titanium, and zirconium. The positive electrode active material includes a first surface layer portion, a second surface layer portion, and an interior portion. The second surface layer portion is closer to the interior than the first surface layer portion. The interior portion has a higher ratio of nickel to the sum of the atoms of the transition metal M than the first surface layer portion and the second surface layer portion. The second surface layer portion has a higher ratio of the number of atoms of at least one element selected from cobalt and manganese to the sum of the atoms of the transition metal M than the interior. The first surface layer portion has a higher concentration of at least one additive element than the interior and the second surface layer portion.

[0014] In the above, the positive electrode active material preferably has a crystallite size of 150 nm or more as calculated from the XRD pattern.

[0015] Another aspect of the present invention is a cathode active material including a transition metal M and oxygen, wherein the transition metal M is nickel, manganese, and cobalt. The cathode active material has a crystallite size of 150 nm or more as calculated from an XRD pattern. A secondary battery using the cathode active material has a discharge capacity of 70 mAh / g or more at a CC (constant current) of 2000 mA / g after CC / CV (4.5 V, 100 mA / g, 10 mA / g cut) charging at 25°C.

[0016] Another embodiment of the present invention is a secondary battery including a positive electrode having a positive electrode active material and a negative electrode, wherein the positive electrode active material includes a transition metal M and oxygen, and the transition metal M is nickel, manganese, and cobalt. The positive electrode active material has a crystallite size of 150 nm or more as calculated from an XRD pattern. The secondary battery has a discharge capacity of 70 mAh / g or more at a CC (constant current) of 2000 mA / g after CC / CV (4.5 V, 100 mA / g, 10 mA / g cut) charging at 25°C.

[0017] According to one embodiment of the present invention, a positive electrode active material or a composite oxide that can be used in a lithium ion secondary battery and has excellent charge / discharge rate characteristics can be provided. Alternatively, a secondary battery with high safety or reliability can be provided.

[0018] According to one embodiment of the present invention, a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof can be provided.

[0019] 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.

[0020] FIGS. 1A to 1C are cross-sectional views of a positive electrode active material. FIG. 2 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 3A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 3B is an FFT pattern of a region of the rock salt-type crystal RS. FIG. 3C is an FFT pattern of a region of the layered rock salt-type crystal LRS. FIGS. 4A and 4B are cross-sectional views of a positive electrode active material. FIGS. 5A to 5C are cross-sectional views of a positive electrode active material. FIG. 6 is a diagram illustrating a method for manufacturing a positive electrode active material. FIG. 7 is a diagram illustrating a method for manufacturing a positive electrode active material. FIG. 8 is a diagram illustrating a method for manufacturing a positive electrode active material. FIG. 9 is a diagram illustrating a method for manufacturing a positive electrode active material. FIGS. 10A to 10D are cross-sectional views illustrating an example of a positive electrode of a secondary battery. FIG. 11A is an exploded perspective view of a coin-type secondary battery, FIG. 11B is a perspective view of the coin-type secondary battery, and FIG. 11C is a cross-sectional perspective view thereof. FIG. 12A shows an example of a cylindrical secondary battery. FIG. 12B illustrates an example of a cylindrical secondary battery. FIG. 12C illustrates an example of a plurality of cylindrical secondary batteries. FIG. 12D illustrates an example of a power storage system including a plurality of cylindrical secondary batteries. FIGS. 13A and 13B are diagrams illustrating an example of a secondary battery, and FIG. 13C is a diagram illustrating the interior of the secondary battery. FIGS. 14A to 14C are diagrams illustrating an example of a secondary battery. FIGS. 15A and 15B are diagrams illustrating the appearance of a secondary battery. FIGS. 16A to 16C are diagrams illustrating a manufacturing method of a secondary battery. FIG. 17A is a perspective view of a battery pack illustrating one embodiment of the present invention, FIG. 17B is a block diagram of the battery pack, and FIG. 17C is a block diagram of a vehicle including the battery pack. FIGS. 18A to 18D are diagrams illustrating an example of a transportation vehicle. FIG. 18E is a diagram illustrating an example of an artificial satellite. FIG. 19A is a diagram illustrating an electric bicycle, FIG. 19B is a diagram illustrating a secondary battery for the electric bicycle, and FIG. 19C is a diagram illustrating a scooter. Figures 20A to 20D are diagrams illustrating an example of an electronic device. Figures 21A to 21F are SEM images of the surface of a positive electrode active material. Figures 22A and 22B are graphs showing the discharge rate characteristics of a secondary battery. Figures 23A and 23B are graphs showing the charge rate characteristics of a secondary battery. Figures 24A to 24H are SEM images of the surface of a positive electrode active material.25A to 25C are cross-sectional SEM images of the positive electrode active material, and FIGS. 25D to 25F are graphs showing the results of EDX point analysis.

[0021] Hereinafter, examples of embodiments of the present invention will be described with reference to the drawings, etc. However, the present invention should not be construed as being limited to the following examples. The embodiments of the present invention can be modified within the scope of the spirit of the present invention.

[0022] In this specification and the like, the term "particle" is not limited to referring only to spherical particles (having a circular cross-sectional shape), and examples of the cross-sectional shape of individual particles include ellipsoids, rectangles, trapezoids, cones, squares with rounded corners, and asymmetric shapes, and further, individual particles may have an irregular shape.

[0023] Homogeneity refers to a state in which a certain element (e.g., A) is distributed with similar characteristics in a specific region of a solid composed of multiple elements (e.g., A, B, C). It is sufficient that the concentrations of elements in the specific regions are substantially the same. For example, it is sufficient that the difference in the detected amount of a certain element (e.g., the number of counts in STEM-EDX) between the specific regions is within 10%. Examples of specific regions include a surface layer, a surface, a convex portion, a concave portion, and an interior.

[0024] A cathode active material to which an additive element is added may be referred to as a composite oxide, a cathode material, a cathode ingredient, a cathode material for a secondary battery, or the like. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a compound. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composition. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composite.

[0025] Furthermore, when describing the characteristics of individual particles of the positive electrode active material in this specification, it is not necessary for all particles to have that characteristic. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected particles of the positive electrode active material have that characteristic, it can be said that there is a sufficient effect of improving the characteristics of the positive electrode active material and a secondary battery containing it.

[0026] As the charging voltage of a secondary battery increases, the potential of the positive electrode generally increases. The positive electrode active material of one embodiment of the present invention has a stable crystal structure even at high charging voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charging and discharging.

[0027] Furthermore, a short circuit in a secondary battery not only causes problems in the charging and / or discharging operations of the secondary battery, but may also lead to heat generation and fire. To achieve a safe secondary battery, it is preferable that the short circuit current be suppressed even at a high charging voltage. The positive electrode active material of one embodiment of the present invention suppresses the short circuit current even at a high charging voltage. Therefore, a secondary battery that achieves both high discharge capacity and safety can be obtained.

[0028] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in the secondary battery are described in their pre-degradation state. Note that a decrease in discharge capacity due to aging treatment (which may also be called burn-in treatment) during secondary battery manufacturing is not considered to be degradation. For example, a lithium-ion secondary cell or lithium secondary battery pack (hereinafter referred to as a lithium-ion secondary battery) that has a discharge capacity of 97% or more of its rated capacity can be considered to be in its pre-degradation state. For lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms not only to the above JIS standard but also to various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0029] Furthermore, the state of the materials in a secondary battery before deterioration is sometimes referred to as an initial product or initial state, and the state after deterioration (the state when the secondary battery has a discharge capacity of less than 97% of its rated capacity) is sometimes referred to as a product in use or in use state, or a used product or used state.

[0030] Embodiment 1 In this embodiment, a positive electrode active material 100 of one embodiment of the present invention will be described with reference to FIGS.

[0031] <Containing Elements> The positive electrode active material 100 contains lithium, a transition metal M, and oxygen. The transition metal M is one or more selected from nickel, manganese, and cobalt. It is preferable that the positive electrode active material 100 further contains an additive element. Alternatively, the positive electrode active material 100 may contain lithium nickel-manganese-cobalt oxide to which the additive element has been added.

[0032] A positive electrode active material for a lithium-ion secondary battery needs to contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted or extracted. The positive electrode active material 100 of one embodiment of the present invention contains nickel, manganese, and cobalt as the transition metal M responsible for the oxidation and reduction reaction.

[0033] A large proportion of nickel in the transition metals M contained in the positive electrode active material 100 is preferable because it is easier to increase the charge / discharge capacity even at a low charge voltage, compared to when cobalt accounts for the majority. Therefore, for example, nickel preferably accounts for 50% or more of the transition metals M, more preferably 60% or more, and even more preferably 75% or more.

[0034] The additive element contained in the positive electrode active material 100 is preferably one or more selected from magnesium, aluminum, calcium, titanium, zirconium, fluorine, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. The ratio of the additive element to the sum of the number of atoms of the transition metal M is preferably less than 25 atomic %, more preferably less than 10 atomic %, and even more preferably less than 5 atomic %.

[0035] As will be described later, these additive elements further stabilize the crystal structure of the positive electrode active material 100. In this specification and the like, the additive element has the same meaning as a mixture or a part of a raw material.

[0036] The additive elements do not necessarily have to include magnesium, aluminum, calcium, titanium, zirconium, fluorine, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium.

[0037] <Single Particles> The particles of the positive electrode active material 100 are preferably single crystals. When the positive electrode active material 100 is a single crystal particle, the single crystal particle may be referred to as a single particle. Alternatively, the positive electrode active material 100 preferably has a large crystallite size.

[0038] Large primary particles suppress the formation of secondary particles due to aggregation and sintering of the primary particles. Furthermore, when the primary particle size is large, the crystallite size calculated from the half-width of the XRD diffraction pattern naturally also becomes large. Therefore, when the positive electrode active material 100 is a single particle, or when the crystallite size calculated from the XRD diffraction pattern is large, cracks that may occur between the primary particles are absent or few compared to a positive electrode active material formed by sintering a large number of primary particles. Therefore, it can be expected that cracks will be suppressed even if the volume of the positive electrode active material 100 changes due to charge and discharge.

[0039] For example, the crystallite size calculated from the half-width of the XRD diffraction pattern is preferably 150 nm or more, more preferably 180 nm or more, and even more preferably 200 nm or more.

[0040] However, when attempting to increase the size of the single crystal or the crystallite size, it may be necessary to heat for a long time or at a high temperature, or to add lithium in excess of the stoichiometric composition. However, long heating processes reduce productivity. Furthermore, heating at high temperatures may cause cation mixing of nickel ions and lithium ions. Furthermore, excess lithium may cause gelation of the binder when preparing the positive electrode slurry. To avoid these disadvantages, it is preferable to maintain the single crystal size and crystallite size at an appropriate level.

[0041] For example, the crystallite size calculated from the XRD diffraction pattern is preferably 1000 nm or less, and more preferably 800 nm or less. A positive electrode active material having a crystallite size calculated from the XRD diffraction pattern within the above range can be said to have a sufficiently large crystallite size and have characteristics similar to those of a single particle.

[0042] The XRD pattern used to calculate the full width at half maximum may be obtained from the positive electrode active material alone, or from the positive electrode containing the positive electrode active material, a current collector, a binder, a conductive material, and the like. However, in the positive electrode state, the positive electrode active material may be oriented due to the influence of pressure, etc., during the manufacturing process. If the positive electrode active material is strongly oriented, the crystallite size may not be accurately calculated. Therefore, it is more preferable to obtain the XRD pattern by a method that reduces the orientation, such as by peeling off the positive electrode active material layer from the positive electrode and removing some of the binder, etc., in the positive electrode active material layer using a solvent, etc., before loading the sample into a sample holder.

[0043] <<XRD>> The apparatus and conditions for XRD measurement when calculating the crystallite size are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １ Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0044] If the measurement sample is a powder, it can be set by placing it in a glass sample holder, or by sprinkling the sample on a greased silicone anti-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0045] The obtained XRD diffraction pattern can be analyzed using crystal structure analysis software (for example, TOPAS ver. 3) to calculate the crystallite size.

[0046] <Surface Layer Portion> Fig. 1A is a cross-sectional view of a positive electrode active material 100 in the form of a single particle. The positive electrode active material 100 preferably has a surface layer portion and an inner portion 100c. The surface layer portion preferably has a surface layer portion 100a and a surface layer portion 100b. The surface layer portion 100b is closer to the inner portion 100c than the surface layer portion 100a.

[0047] In this specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region within 200 nm from the surface toward the interior. The surface layer 100b of the positive electrode active material 100 refers to, for example, a region from the surface toward the interior that is greater than 200 nm and less than 1000 nm. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer is synonymous with the near-surface, near-surface region, or shell.

[0048] The region deeper than the surface layer of the positive electrode active material is referred to as an inner portion 100c, which is synonymous with an inner region or a core.

[0049] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer and the interior portion 100c. Therefore, the positive electrode active material 100 does not contain carbonates, hydroxyl groups, etc., chemically adsorbed after preparation. It also does not contain electrolytes, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material 100. The surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image, etc., is the boundary between the region where an electron beam combined image is observed and the region where it is not observed, and is the outermost region where bright spots originating from the atomic nuclei of metal elements with atomic numbers greater than that of lithium are observed. The surface in a cross-sectional STEM image, etc., may be determined in conjunction with the results of higher spatial resolution analyses, such as electron energy loss spectroscopy (EELS).

[0050] 1B and 1C are cross-sectional views of a positive electrode active material 100 having primary particles that are secondary particles with large crystallite sizes, and having a crystal grain boundary 101. Surface layer portions 100a and 100b may not be present around crystal grain boundary 101 as shown in FIG. 1B, or may be present as shown in FIG. 1C.

[0051] The crystal grain boundary 101 refers to, for example, a portion where primary particles of the positive electrode active material 100 are adhered to each other, a portion where the crystal orientation changes within the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. The crystal defect refers to a defect that can be observed in a cross-sectional TEM (transmission electron microscope), a cross-sectional STEM image, etc., that is, a structure in which other elements have entered between lattices, a cavity, etc. The crystal grain boundary 101 can be said to be one type of planar defect. The vicinity of the crystal grain boundary 101 refers to a region within 10 nm of the crystal grain boundary 101.

[0052] Grain boundaries are a type of planar defect. Therefore, like surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of added elements at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed, as will be described later.

[0053] Furthermore, when the concentration of the additive element is high at and near the grain boundary 101, even if cracks occur along the grain boundaries of the positive electrode active material 100 of one embodiment of the present invention, the concentration of the additive element is high near the surface where the cracks occur. Therefore, the crystal structure can be further stabilized even in the surface layer where the cracks occur.

[0054] <Crystalline Structure> The positive electrode active material 100 according to one embodiment of the present invention is in a discharged state, i.e., Li ｘ MO ２ When x=1 in the formula (where M is at least one of Ni, Co, and Mn), it is preferable that the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have a high discharge capacity, have two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion and desorption reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the interior 100c, which occupies the majority of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure.

[0055] On the other hand, the surface layer portion of the cathode active material 100 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100c, which is made up of octahedra of the transition metal M and oxygen, so that the layered structure formed by the octahedrons of the transition metal M and oxygen is not destroyed even when a large amount of lithium is released from the cathode active material 100 upon charging. Alternatively, the surface layer portion preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer portion, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion and inner portion 100c of the cathode active material 100, such as oxygen desorption, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100. In other words, "functioning as a barrier film" refers to, for example, the surface layer portion suppressing structural changes in the cathode active material 100 and suppressing oxidative decomposition of the electrolyte.

[0056] Therefore, the surface layer portion preferably has a different composition and crystal structure from the interior 100c. Furthermore, the surface layer portion preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior 100c. For example, at least a portion of the surface layer portion of the positive electrode active material 100 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0057] The surface layer is the region where lithium ions are first released during charging, and is the region where the lithium concentration is likely to be lower than that of the interior 100c. In addition, it can be said that the atoms on the surface of the positive electrode active material 100 in the surface layer are in a state where some of the bonds are broken. Therefore, the surface layer is likely to become unstable, and is the region where the deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer can be sufficiently stabilized, Li ｘ MO ２ Even when the value of x in the inner portion 100c is small, the layer structure of the inner portion 100c, which is made up of the transition metal M and oxygen octahedra, can be made less likely to break.Furthermore, the layer of the inner portion 100c, which is made up of the transition metal M and oxygen octahedra, can be prevented from shifting.

[0058] In order to provide the surface layer portion with a stable composition and crystal structure, the surface layer portion preferably contains an additive element, and more preferably contains a plurality of additive elements. Furthermore, it is preferable that the composition of the transition metal M be different between the surface layer portion and the inner portion 100c.

[0059] For example, it is preferable that the concentration peak of the additive element exists in the surface layer portion, and it is more preferable that the concentration peak of the additive element exists in the surface layer portion 100a closer to the surface.

[0060] Furthermore, the concentration of at least one of the transition metals M, cobalt and manganese, is preferably higher in the surface layer portion than in the interior 100c. Similarly, the concentration of nickel is preferably higher in the interior 100c than in the surface layer portion. Furthermore, at least one of cobalt and manganese preferably has a concentration gradient that increases toward the surface of the positive electrode active material 100. Similarly, nickel preferably has a concentration gradient that increases toward the interior of the positive electrode active material 100.

[0061] From the above, it is preferable that the surface layer portion 100b is a region that does not have a concentration peak of the additive element, but has higher concentrations of cobalt and manganese than the inner portion 100c.

[0062] Furthermore, the surface layer portion 100a preferably has a higher concentration of one or more selected from the additive elements than the surface layer portion 100b and the interior portion 100c. Furthermore, it is preferable that the one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. Furthermore, it is more preferable that the distribution of the additive elements in the positive electrode active material 100 differs depending on the additive element. For example, it is more preferable that the depth of the concentration peak from the surface differs depending on the additive element. The concentration peak here refers to the maximum concentration value in the surface layer portion 100a or within 200 nm from the surface.

[0063] For example, some of the additive elements, such as magnesium, fluorine, nickel, titanium, silicon, phosphorus, boron, and calcium, preferably have a concentration gradient that increases from the interior 100c toward the surface.

[0064] As will be described later, these additive elements further stabilize the crystal structure of the positive electrode active material 100. The additive elements may be contained in trace amounts in the transition metal M source, etc., in addition to those contained in the additive element source. Regardless of the material from which the additive elements are derived, the additive elements can contribute to the chemical stability of the positive electrode active material 100 as long as they have a preferred concentration and distribution.

[0065] For example, magnesium, which is one of the additive elements, can be present at an appropriate concentration in the lithium sites of the surface layer, making it easier to maintain the layered rock salt type crystal structure of the inner 100c. This is because magnesium present in the lithium sites is ２ It is thought that this is because they function as pillars supporting the layers.

[0066] At an appropriate concentration, magnesium does not adversely affect the intercalation and deintercalation of lithium during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect the intercalation and deintercalation of lithium. Furthermore, the effect on stabilizing the crystal structure may be reduced. In addition, unnecessary magnesium compounds (oxides, fluorides, etc.) that do not substitute for either the lithium site or the transition metal M site may segregate on the surface of the positive electrode active material and become resistance components in the secondary battery. Furthermore, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium entering the lithium site, reducing the amount of lithium contributing to charging and discharging.

[0067] Aluminum can also be present at the transition metal M site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Aluminum also has the effect of suppressing the elution of the surrounding transition metal M and improving continuous charging durability. Furthermore, because the Al—O bond is stronger than the transition metal M—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when used in secondary batteries. Furthermore, a positive electrode active material 100 can be obtained whose crystal structure is less likely to collapse even with repeated charging and discharging.

[0068] On the other hand, excessive aluminum may adversely affect the intercalation and deintercalation of lithium, so it is preferable that the total amount of aluminum contained in the positive electrode active material 100 is appropriate.

[0069] Furthermore, titanium oxide, which is one of the additive elements, is known to have superhydrophilicity. Therefore, by providing the cathode active material 100 with titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100 may have good wettability with highly polar solvents. When used in a secondary battery, this may improve the contact at the interface between the cathode active material 100 and a highly polar electrolyte, thereby suppressing an increase in internal resistance.

[0070] Furthermore, when phosphorus, which is one of the additive elements, is contained in the surface layer portion 100a, Li ｘ MO ２ When the value of x in the formula (100a) is kept small, it is possible to prevent short circuits, which is preferable. For example, it is preferable that the compound containing phosphorus and oxygen exists in the surface layer portion 100a. An example of the compound containing phosphorus and oxygen is lithium phosphate.

[0071] When the positive electrode active material 100 contains phosphorus, the hydrogen fluoride generated by decomposition of the electrolyte reacts with the phosphorus, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0072] The electrolyte is LiPF ６In the case where the electrolyte contains fluorine, there is a risk of hydrogen fluoride being generated by hydrolysis. Furthermore, hydrogen fluoride may also be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, and an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 (described later) may be suppressed. Furthermore, there is a risk of a decrease in adhesion due to gelation and / or insolubilization of PVDF.

[0073] When the positive electrode active material 100 contains phosphorus together with magnesium, Li ｘ MO ２ This is preferable because the stability is extremely high when x is small.

[0074] Furthermore, when the positive electrode active material 100 has cracks, the progression of the cracks can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the cracks on the surface, for example, in the embedded portion.

[0075] However, if the surface layer portion is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer portion to be occupied only by a structure in which magnesium oxide and an oxide of a divalent transition metal M are solid solutions. Therefore, the surface layer portion 100a must contain at least the transition metal M, and in a discharged state, it must also contain lithium, and it must have a path for the insertion and extraction of lithium.

[0076] In order to ensure sufficient paths for lithium insertion and desorption, it is preferable that the sum of the number of atoms of the transition metal M in the surface layer portion 100a is greater than the sum of the number of atoms of the additive element.

[0077] Furthermore, it is preferable that some of the added elements, particularly magnesium and aluminum, have a higher concentration in the surface layer than in the interior 100 c, but are also present randomly and dilutely in the interior 100 c. If magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100 c, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above.

[0078] Furthermore, it is preferable that the crystal structure continuously changes from the interior 100c toward the surface due to the concentration gradient of the added element as described above, or that the crystal orientation of the surface layer portion and the interior 100c generally coincide.

[0079] For example, it is preferable that the crystal structure continuously change from the interior 100c of the layered rock salt type toward the surface and surface layer portion having characteristics of the rock salt type or both the rock salt type and the layered rock salt type.Alternatively, it is preferable that the orientation of the interior 100c of the layered rock salt type is approximately the same as that of the rock salt type or the surface layer portion having characteristics of both the rock salt type and the layered rock salt type.

[0080] In this specification, the layered rock-salt crystal structure of a composite oxide containing lithium and a transition metal M and belonging to the space group R-3m refers to a crystal structure having a rock-salt 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 vacancies of cations or anions may also be present. Strictly speaking, the layered rock-salt crystal structure may have a distorted rock-salt crystal lattice.

[0081] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0082] The fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0083] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction image corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the ideal rock salt type, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiMO ２ When comparing the electron diffraction patterns of LiMO ２ The bright spots on the (003) plane of MgO are observed at a distance about half the distance of the bright spots on the (111) plane of MgO. ２ In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0084] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0085] Layered rock salt crystals and the anions in rock salt crystals form a cubic close-packed structure (face-centered cubic lattice structure). Therefore, when layered rock salt crystals and rock salt crystals come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0086] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0087] However, since the space group of the layered rock salt type crystal is R-3m, which is different from the space group Fm-3m (space group of general rock salt type crystal) of the rock salt type crystal, the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal and the rock salt type crystal. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt type crystal and the rock salt type crystal are aligned, it may be said that the crystal orientations are approximately the same. In addition, having a three-dimensional structural similarity such that the crystal orientations are approximately the same, or having the same crystallographic orientation, is called topotaxis.

[0088] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron beam diffraction patterns, FFT patterns of TEM images, STEM images, etc. XRD (X-ray diffraction), electron beam diffraction, neutron beam diffraction, etc. can also be used as materials for the judgment.

[0089] 2 shows an example of a TEM image in which the orientations of the layered rock salt crystals LRS and RS are roughly the same. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0090] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in Figure 2) ＲＳ and L ＬＲＳ When the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0091] Furthermore, HAADF-STEM images provide contrast proportional to the atomic number, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium nickel-manganese-cobalt oxide belonging to the space group R-3m, the transition metals M, specifically manganese (atomic number 25), cobalt (atomic number 27), and nickel (atomic number 28), have high atomic numbers. Therefore, electron beams are strongly scattered at the positions of these atoms, and the arrangement of the transition metal M atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium nickel-manganese-cobalt oxide with a layered rock-salt crystal structure is observed perpendicular to the c-axis of the layered rock-salt crystal structure belonging to the space group R-3m, the arrangement of the transition metal M atoms is observed perpendicular to the c-axis as a bright line or an arrangement of highly bright dots, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium nickel-manganese-cobalt oxide contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0092] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0093] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0094] Figure 3A shows an example of an STEM image in which the orientations of the layered rock-salt crystal LRS and the rock-salt crystal RS roughly coincide. Figure 3B shows the FFT pattern of the region of the rock-salt crystal RS, and Figure 3C shows the FFT pattern of the region of the layered rock-salt crystal LRS. The left side of Figures 3B and 3C shows the composition, JCPDS card number, and the d-value and angle calculated from these. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction. While lithium cobalt oxide and cobalt oxide are shown here as examples, this is not a limitation of the present invention. For example, it is expected that the orientations of lithium nickel-manganese-cobalt oxide and nickel oxide, manganese oxide, and / or cobalt oxide will also roughly coincide.

[0095] The spot marked A in Figure 3B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 3C is derived from the 0003 reflection of the layered rock salt type. From Figures 3B and 3C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide. In other words, it can be seen that the line passing through AO in Figure 3B is roughly parallel to the line passing through AO in Figure 3C. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0096] In this way, in the FFT pattern and the electron beam diffraction pattern, when the orientations of the layered rock salt type crystal and the rock salt type crystal are roughly the same, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point being spot-like and not continuous with other reciprocal lattice points means high crystallinity.

[0097] Furthermore, as described above, when the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt crystal are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt crystal may be observed in a reciprocal lattice space different from the orientation of the 0003 reflection of the layered rock salt crystal. For example, the spot marked B in FIG. 3C is originating from the 1014 reflection of the layered rock salt crystal. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point originating from the 0003 reflection of the layered rock salt crystal (A in FIG. 3C), and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be the same. For example, equivalent reciprocal lattice points in each may be used.

[0098] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 3B originates from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 3B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to be the same. For example, equivalent reciprocal lattice points in each may be used.

[0099] When determining whether the crystal orientation is consistent, it is preferable to thin the sample so that the (0003) plane of the layered rock salt structure can be easily observed. Therefore, it is preferable to thin the sample using an FIB or the like so that the electron beam in the TEM or the like is incident on the [12-10] plane. ２It is known that layered rock salt type positive electrode active materials (where M is at least one of Ni, Co, and Mn) tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, by carefully observing the shape of the positive electrode active material using an SEM or the like, it is possible to slice it so that the (0003) plane can be easily observed using a TEM or the like.

[0100] <<XPS>> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), so the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be used to analyze the bonding state of the elements. The quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 1 atomic %, depending on the element.

[0101] In the cathode active material 100 according to one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer portion than in the interior portion 100c. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer portion is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer portion measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer portion 100a measured by XPS or the like is preferably higher than the magnesium concentration throughout the cathode active material 100. Furthermore, the aluminum concentration in at least a portion of the surface layer portion 100a is preferably higher than the aluminum concentration throughout the cathode active material 100.

[0102] As described above, the surface and surface layer of the positive electrode active material 100 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 100. The surface and surface layer also do not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 100. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0103] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0104] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be set to, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions: Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0105] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 and the transition metal M have a concentration gradient. It is more preferable that the depth from the surface of the concentration peak differs depending on the additive element in the positive electrode active material 100. The concentration gradients of the additive element and the transition metal M can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0106] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0107] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentrations of the additive element and the transition metal M in the surface layer portion, the interior 100c, and the vicinity of the grain boundary 101 of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the additive element. Furthermore, analysis that thins the sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0108] Therefore, when EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, the concentration of each added element in the surface layer portion is preferably higher than that in the interior portion 100 c. Also, the concentration of at least one of the transition metals M selected from cobalt and manganese in the surface layer portion is preferably higher than that in the interior portion 100 c. Similarly, the concentration of nickel in the interior portion 100 c is preferably higher than that in the surface layer portion.

[0109] For example, when EDX area analysis or EDX point analysis is performed on a positive electrode active material 100 having magnesium and / or aluminum as an added element, it is preferable that the magnesium and / or aluminum concentration in the surface layer portion is higher than the magnesium and / or aluminum concentration in the interior 100c.

[0110] Furthermore, while it is preferable that the surface of the positive electrode active material 100 is smooth and has few irregularities, this is not necessarily the case for the entire positive electrode active material 100. A composite oxide having an R-3m layered rock salt crystal structure is prone to slippage in planes parallel to the (001) plane, such as the plane where lithium is arranged. For example, as shown in FIG. 4A , when a (001) plane is present, slippage may occur parallel to the (001) plane, as indicated by the arrow in FIG. 4B , resulting in deformation, as a result of a process such as pressing.

[0111] In this case, the surface newly formed as a result of the slip and its surface layer 100a may be free of the added element or may contain the added element at a level below the detection limit. E-F in Fig. 4B are examples of the surface newly formed as a result of the slip and its surface layer 100a and surface layer 100b.

[0112] However, since slippage tends to occur parallel to the (001) plane, the newly formed surface and its surface layer 100a tend to have a (001) orientation. In this case, the diffusion path of lithium ions is not exposed and the surface is relatively stable, so there is almost no problem even if the added element is absent or below the detection limit.

[0113] As mentioned above, the composition is LiMO ２ In the composite oxide having a layered rock salt type crystal structure of R-3m, the transition metal M is arranged parallel to the (001) plane. ２ Among these, cobalt, which has the largest atomic number, has the highest brightness. Therefore, in a HAADF-STEM image, the arrangement of highly bright atoms can be considered to be the arrangement of the transition metal M. The repetition of this highly bright arrangement is synonymous with crystal fringes or lattice fringes.

[0114] The positive electrode active material 100 may have a coating film on at least a portion of the surface thereof. Figures 5A, 5B, and 5C show examples of the positive electrode active material 100 having a coating film 104.

[0115] The coating 104 is preferably formed, for example, by the deposition of decomposition products of the electrolyte solution during charge and discharge. Having a coating derived from the electrolyte solution on the surface of the positive electrode active material 100 is expected to improve charge and discharge cycle characteristics. This is due to reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing the elution of the transition metal M. The coating 104 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, a high-quality coating is easily obtained when LiBOB and / or SUN (suberonitrile) is used as part of the electrolyte. Therefore, a coating 104 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating and is therefore preferred. Furthermore, the coating 104 does not have to cover the entire positive electrode active material 100.

[0116] This embodiment can be used in combination with other embodiments.

[0117] Embodiment 2 In this embodiment, an example of a method for manufacturing a positive electrode active material 100 of one embodiment of the present invention will be described with reference to FIGS.

[0118] In order to prepare a positive electrode active material 100 having the distribution, composition, and / or crystal structure of the additive elements as described in the previous embodiment, the method of adding the additive elements is important.

[0119] Therefore, in the process of producing the positive electrode active material 100, it is preferable to first synthesize lithium nickel-manganese-cobalt oxide with a large crystallite size, and then mix in the additive element source and perform a heat treatment.

[0120] In order to synthesize lithium nickel-manganese-cobalt oxide with a large crystallite size, it is effective to repeat the step of adding a lithium source and heating multiple times.

[0121] Furthermore, it is difficult to increase the concentration of the additive element in the surface layer when synthesizing lithium nickel-manganese-cobalt oxide by mixing the additive element source simultaneously with the nickel source, manganese source, and cobalt source, or the lithium source. Furthermore, if the additive element source is simply mixed without heating after synthesizing lithium nickel-manganese-cobalt oxide, the additive element will simply adhere to the lithium nickel-manganese-cobalt oxide without dissolving in the lithium nickel-manganese-cobalt oxide. Without sufficient heating, it is also difficult to achieve a good distribution of the additive element. Therefore, it is preferable to mix the additive element source after synthesizing lithium nickel-manganese-cobalt oxide and then perform a heat treatment. This heat treatment after mixing the additive element source is sometimes called annealing.

[0122] <<Method 1 for Producing Positive Electrode Active Material>> Method 1 for producing a positive electrode active material 100 will be described with reference to FIGS. 6 and 7 .

[0123] 6, first, a transition metal M source, i.e., a nickel source (Ni source), a cobalt source (Co source), and a manganese source (Mn source) are prepared. These are preferably mixed in a ratio of nickel, cobalt, and manganese that allows a layered rock-salt crystal structure to be formed.

[0124] In particular, a positive electrode active material 100 containing a large amount of nickel as the transition metal M may be cheaper than a positive electrode active material containing a large amount of cobalt, and may also have an increased charge / discharge capacity per weight, which is preferable. For example, nickel preferably accounts for more than 50 atomic % of the transition metal M, more preferably 60 atomic % or more, and even more preferably 75 atomic % or more. However, if the proportion of nickel is too high, chemical stability and heat resistance may be reduced. Therefore, it is preferable that nickel accounts for 95 atomic % or less of the transition metal M.

[0125] Cobalt is preferred as the transition metal M, as it provides a high average discharge voltage and contributes to stabilizing the layered rock-salt structure, resulting in a highly reliable secondary battery. However, cobalt is more expensive and unstable than nickel and manganese, so if the proportion of cobalt is too high, the cost of manufacturing the secondary battery may increase. Therefore, for example, it is preferable that the cobalt content of the transition metal M be 2.5 atomic % or more and 34 atomic % or less.

[0126] The transition metal M does not necessarily have to contain cobalt.

[0127] The inclusion of manganese as the transition metal M is preferable because it improves heat resistance and chemical stability. However, if the proportion of manganese is too high, the discharge voltage and discharge capacity tend to decrease. Therefore, for example, it is preferable that the manganese content of the transition metal M is 2.5 atomic % or more and 34 atomic % or less.

[0128] The transition metal M does not necessarily have to contain manganese.

[0129] The transition metal M source is prepared as an aqueous solution containing the transition metal M. An aqueous solution of a nickel salt can be used as the nickel source. Examples of nickel salts that can be used include nickel sulfate, nickel chloride, nickel nitrate, and hydrates thereof. Organic acid salts of nickel, such as nickel acetate, and hydrates thereof can also be used. An aqueous solution of a nickel alkoxide or an organic nickel complex can also be used as the nickel source. In this specification and the like, organic acid salts refer to compounds of metals and organic acids, such as acetic acid, citric acid, oxalic acid, formic acid, and butyric acid.

[0130] Similarly, an aqueous solution of a cobalt salt can be used as the cobalt source. Examples of the cobalt salt include cobalt sulfate, cobalt chloride, cobalt nitrate, and hydrates thereof. Organic acid salts of cobalt, such as cobalt acetate, and hydrates thereof can also be used. An aqueous solution of a cobalt alkoxide or an organic cobalt complex can also be used as the cobalt source.

[0131] Similarly, an aqueous solution of a manganese salt can be used as the manganese source. Examples of manganese salts that can be used include manganese sulfate, manganese chloride, manganese nitrate, and hydrates thereof. Organic acid salts of manganese, such as manganese acetate, and hydrates thereof can also be used. An aqueous solution of a manganese alkoxide or an organic manganese complex can also be used as the manganese source.

[0132] In this embodiment, an aqueous solution in which nickel sulfate, cobalt sulfate, and manganese sulfate are dissolved in pure water is prepared as a source of the transition metal M. The atomic ratio of nickel, cobalt, and manganese is Ni:Co:Mn=8:1:1 or approximately this ratio. The aqueous solution is acidic.

[0133] <Step S13> A chelating agent may also be prepared, as shown in step S13 of FIG. 6 . Examples of chelating agents include glycine, oxine, 1-nitroso-2-naphthol 2-mercaptobenzothiazole, and EDTA (ethylenediaminetetraacetic acid). Multiple agents selected from glycine, oxine, 1-nitroso-2-naphthol, and 2-mercaptobenzothiazole may also be used. At least one of these agents is dissolved in pure water to form a chelating aqueous solution. A chelating agent is a complexing agent that forms a chelate compound and is preferable to general complexing agents. Of course, a complexing agent may be used instead of a chelating agent, and ammonia water may be used as the complexing agent. Using a chelating aqueous solution is preferable because it suppresses the generation of unnecessary crystal nuclei and promotes growth. Suppressing the generation of unnecessary nuclei suppresses the generation of fine particles, thereby producing a composite hydroxide with a good particle size distribution. Furthermore, using a chelating aqueous solution can slow the acid-base reaction, allowing the reaction to proceed gradually and resulting in nearly spherical secondary particles. Glycine has the effect of maintaining a constant pH value at a pH of 9 or more and 10 or less, and is therefore preferable when using a glycine aqueous solution as the chelate aqueous solution, as this makes it easier to control the pH of the reaction tank when obtaining the composite hydroxide 98.

[0134] <Step S14> Next, in step S14 of FIG. 6, a transition metal M source and a chelating agent are mixed to prepare an acid solution.

[0135] <Step S21> Next, in step S21 of Fig. 6, an alkaline solution is prepared. As the alkaline solution, for example, an aqueous solution containing sodium hydroxide, potassium hydroxide, lithium hydroxide, or ammonia can be used. An aqueous solution in which these are dissolved in pure water can be used. Alternatively, an aqueous solution in which multiple types selected from sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonia are dissolved in pure water may be used.

[0136] The pure water preferably used for the transition metal M source and the alkaline solution is water having a resistivity of 1 MΩ cm or more, more preferably 10 MΩ cm or more, and even more preferably 15 MΩ cm or more. Water satisfying this resistivity requirement has high purity and contains very few impurities.

[0137] <Step S22> As shown in step S22 of Fig. 6, it is preferable to prepare water in the reaction tank. This water may be an aqueous solution of a chelating agent, but pure water is more preferable. The use of pure water promotes nucleation, making it possible to produce composite hydroxides with small particle sizes. The water prepared in the reaction tank can be referred to as a filling liquid or an adjusting liquid for the reaction tank. When using an aqueous chelating agent solution, the description of step S13 can be taken into consideration.

[0138] 6, the acid solution and the alkaline solution are mixed and reacted with each other, which can be called a co-precipitation reaction, a neutralization reaction, or an acid-base reaction.

[0139] During the coprecipitation reaction in step S31, it is preferable to adjust the pH of the reaction system to 9.0 or more and 11.5 or less.

[0140] For example, when an alkaline solution is placed in a reaction tank and an acid solution is added dropwise to the reaction tank, it is advisable to maintain the pH of the aqueous solution in the reaction tank within the above-mentioned range. The same applies when an acid solution is placed in a reaction tank and an alkaline solution is added dropwise. When the solution in the reaction tank is 200 mL or more and 350 mL or less, it is preferable to set the drop rate of the acid or alkaline solution to 0.01 mL / min or less, as this makes it easier to control the pH conditions. The reaction tank has a reaction vessel or the like.

[0141] The aqueous solution may be stirred in the reaction tank using a stirring means. The stirring means may have a stirrer or stirring blades. Two to six stirring blades may be provided; for example, if four stirring blades are provided, they may be arranged in a cross shape when viewed from above. The rotation speed of the stirring means may be 800 rpm to 1200 rpm. Baffle plates may also be provided in the reaction tank to change the stirring direction and flow rate. The provision of baffle plates improves mixing efficiency, allowing the synthesis of composite hydroxide particles with more uniform size, etc.

[0142] The temperature of the reaction vessel is preferably adjusted to be 50° C. or higher and 90° C. or lower. The dropping of the alkaline solution or acid solution may be started after the reaction vessel has reached the appropriate temperature.

[0143] The reaction vessel may be filled with an inert atmosphere, such as nitrogen or argon. When a nitrogen atmosphere is used, nitrogen gas may be introduced at a flow rate of 0.5 L / min to 2 L / min.

[0144] The reactor may also be equipped with a reflux condenser, which allows nitrogen gas to be released from the reactor and water vapor to be returned to the reactor.

[0145] By the above coprecipitation reaction, a composite hydroxide 98 containing the transition metal M is precipitated.

[0146] <Step S32> To recover the composite hydroxide 98, filtration is preferably performed as shown in step S32 of Fig. 6. The filtration is preferably suction filtration. During filtration, the reaction product precipitated in the reaction tank may be washed with pure water, and then an organic solvent (e.g., acetone) may be used.

[0147] <Step S33> As shown in step S33 of Fig. 6, the composite hydroxide 98 after filtration may be dried. For example, it may be dried under vacuum at 60°C or higher and 200°C or lower for 0.5 hours or higher and 20 hours or lower. For example, it may be dried for 12 hours. In this manner, the composite hydroxide 98 can be obtained.

[0148] In this manner, a composite hydroxide 98 containing a transition metal M can be obtained. In this specification and the like, the composite hydroxide 98 refers to a hydroxide of multiple types of metals. The composite hydroxide 98 can be said to be a precursor of the positive electrode active material 100.

[0149] 7, a lithium source is prepared. At this time, since the step of adding the lithium source and heating is performed multiple times, an amount of lithium prepared in step S41 is less than the final amount. For example, when the sum of the numbers of atoms of nickel, cobalt, and manganese is 1, the ratio of lithium can be 0.5 to 0.9 (atomic ratio), and more preferably 0.7 (atomic ratio).

[0150] Examples of lithium sources that can be used include lithium hydroxide, lithium carbonate, and lithium nitrate. It is particularly preferable to use a material with a low melting point among lithium compounds, such as lithium hydroxide (melting point 462°C). Since positive electrode active materials with a high nickel content are more susceptible to cation mixing than lithium cobalt oxide and the like, heating in step S43 and other steps must be performed at low temperatures. Therefore, it is preferable to use a material with a low melting point.

[0151] Furthermore, the smaller the particle size of the lithium source, the more favorable the reaction will proceed. For example, a lithium source pulverized using a fluidized bed jet mill can be used. The particle size here refers to the median diameter.

[0152] <Step S42> Next, in step S42 of Fig. 7, the composite hydroxide 98 and the lithium source are mixed. Mixing can be performed by a dry method or a wet method. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. Furthermore, when using a ball mill, a bead mill, or the like, it is preferable to set the peripheral speed to 100 mm / sec or more and 2000 mm / sec or less in order to suppress contamination from the media or materials. The cobalt compound and the lithium compound may be pulverized simultaneously with mixing.

[0153] <Step S43> Next, the mixture of composite hydroxide 98 and the lithium source is heated. To distinguish from other heating steps, in Fig. 7, step S43 may be referred to as the first heating, step S53 as the second heating, and step S55 as the third heating.

[0154] The firing equipment for these heating processes may be a muffle furnace, a roller hearth kiln, a rotary kiln, or the like. The crucible, sheath, setter, and container used during heating are preferably made of materials that do not easily release impurities. For example, it is recommended to use a crucible made of aluminum oxide with a purity of 99.9%. For mass production, it is recommended to use a crucible made of, for example, mullite-cordierite (Al ２ O ３ SiO ２ It is recommended to use a sheath made of MgO.

[0155] The heating temperature in step S43 is preferably 400° C. to 750° C., more preferably 650° C. to 750° C. The heating time in step S43 is preferably 1 hour to 30 hours, more preferably 2 hours to 20 hours.

[0156] The heating is preferably carried out in an oxygen-containing atmosphere or a so-called dry air atmosphere containing oxygen with little water (for example, a dew point of -50°C or less, more preferably a dew point of -80°C or less).

[0157] Furthermore, it is preferable to include a crushing step after heating as step S44. Crushing can be performed, for example, in a mortar. Furthermore, classification can be performed using a sieve. By including the crushing step, the particle size and / or shape of the positive electrode active material 100 can be made more uniform. Through the above steps, a composite oxide is obtained (step S45).

[0158] <Step S51> Next, in step S51, a lithium source is prepared. The lithium source is prepared so that the final lithium amount, combined with step S41, is achieved. For example, if the sum of the numbers of atoms of nickel, cobalt, and manganese is 1 and the lithium is 0.7 (atomic ratio) in step S41, it is preferable to prepare, for example, 0.31 (atomic ratio) in step S51. Here, the final lithium amount when the sum of the numbers of atoms of nickel, cobalt, and manganese is 1 is 1.01 (atomic ratio), but this is not a limitation of one embodiment of the present invention. When the sum of the numbers of atoms of nickel, cobalt, and manganese is 1, the final lithium amount is preferably 0.95 to 1.25 (atomic ratio), and more preferably 1.00 to 1.10 (atomic ratio). The description of step S41 can be taken into consideration, except for the amount of the lithium source to be prepared.

[0159] 7 illustrates a method in which the lithium source is added in two separate steps, step S41 and step S51, and each step is heated, but this is not a limitation of one embodiment of the present invention. The lithium source may be added in three or more separate steps and each step may be heated.

[0160] <Step S52> Next, the composite oxide obtained in step S45 is mixed with the lithium source. The description of step S42 can be referred to for the mixing.

[0161] <Step S53> Next, the mixture of composite hydroxide 98 and lithium source is heated. The heating in step S53 is preferably performed at a sufficiently high temperature to increase the crystallite size of positive electrode active material 100, but the temperature range may differ depending on the composition of transition metal M.

[0162] When the proportion of nickel in the transition metal M is high, for example, 70% or more, the temperature is preferably 750°C or higher, more preferably 800°C or higher, and even more preferably 850°C or higher. On the other hand, if the temperature is too high, there is a risk that the transition metal M, such as nickel, may be reduced to a divalent state. Therefore, for example, the temperature is preferably 950°C or lower, more preferably 920°C or lower, and even more preferably 900°C or lower.

[0163] When the proportion of nickel in the transition metal M is 40% or more and less than 70%, for example, 900°C or higher is preferable, 950°C or higher is more preferable, and about 970°C is even more preferable. On the other hand, if the temperature is too high, the same disadvantages as those described above may occur, so 1020°C or lower is preferable, and 990°C or lower is even more preferable. For other heating conditions, the description of step S43 can be referred to.

[0164] It is also preferable to have a crushing step after heating as step S54. The description of step S44 can be referred to for the crushing step.

[0165] <Step S55> Furthermore, it is more preferable to perform heating in step S55. By performing this heating, residues of the lithium source and the like can be reduced. The heating temperature in step S55 is preferably 400°C or higher and 900°C or lower, more preferably 750°C or higher and 850°C or lower. Furthermore, the heating time in step S52 is preferably 1 hour or higher and 30 hours or lower, more preferably 2 hours or higher and 20 hours or lower. However, heating in step S55 does not have to be performed. For other heating conditions, the description of step S43 can be taken into consideration.

[0166] It is also preferable to have a crushing step after heating as step S56. The crushing step can be performed by referring to the description of step S44.

[0167] 7 illustrates a method in which the lithium source is mixed in step S51 and then heated twice in steps S53 and S55, but this is not a limitation of one embodiment of the present invention. Three or more heating steps may be performed.

[0168] Through the above steps, the positive electrode active material 100 can be produced.

[0169] 6 and 8 , a method 2 for manufacturing a positive electrode active material 100 will be described. The positive electrode active material 100 manufactured through the manufacturing method 2 contains an additive element. The following mainly describes steps different from the manufacturing method 1, and the description of the manufacturing method 1 can be referred to for the other steps.

[0170] <Step S71> In step S71 of FIG. 8, an additive element source is prepared.

[0171] The additive element may be, for example, one or more selected from magnesium, aluminum, calcium, titanium, zirconium, fluorine, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium.

[0172] As the magnesium source, for example, magnesium fluoride, magnesium hydroxide, magnesium carbonate, magnesium acetylacetonate (dihydrate), magnesium lactate, phthalocyanine magnesium (II), and other organic compounds containing magnesium can be used.

[0173] As the aluminum source, for example, aluminum hydroxide, aluminum fluoride, aluminum alkoxide, aluminum acetylacetonate, aluminum lactate, and other organic compounds containing aluminum can be used.

[0174] <Step S72> The additive element source and a composite oxide having a large crystallite size obtained by the same steps as in Preparation Method 1 are mixed.

[0175] Although this embodiment describes a preparation method in which an additive element is mixed in step S72, one embodiment of the present invention is not limited thereto. The additive element may be mixed in other steps. For example, the additive element may be mixed simultaneously with the lithium source in steps S42 and S52. Alternatively, the additive element may be mixed simultaneously with the transition metal M source in step S14.

[0176] <Step S73> Next, the mixture of the additive element source and the composite oxide is heated. The heating temperature in step S73 is preferably 400°C or higher and 900°C or lower, and more preferably 750°C or higher and 850°C or lower. The heating time in step S73 is preferably 0.5 hours or higher and 30 hours or lower, and more preferably 1 hour or higher and 10 hours or lower. For other heating conditions, the description of step S43 can be referred to.

[0177] It is also preferable to have a crushing step after heating as step S74. The crushing step can be carried out by referring to the description of step S44.

[0178] Through the above steps, the positive electrode active material 100 can be produced (step S75).

[0179] 6 and 9 , a method 3 for producing a positive electrode active material 100 will be described. The positive electrode active material 100 produced through the method 3 contains an additive element, and the ratio of the number of atoms of at least one of cobalt and manganese is greater in the surface layer portion than in the interior portion. The following mainly describes steps that differ from the method 2, and the description of the method 2 can be referred to for the other steps.

[0180] <Step S61> In step S61 of FIG. 9, at least one of a cobalt source and a manganese source is prepared.

[0181] As the cobalt source, for example, organic compounds containing cobalt, such as cobalt oxide, cobalt hydroxide, and cobalt alkoxide, can be used.

[0182] As the manganese source, for example, manganese oxide, manganese hydroxide, manganese alkoxide, and other organic compounds containing manganese can be used.

[0183] In step S61, a composite hydroxide may be prepared. For example, a composite hydroxide containing cobalt and manganese may be prepared as the cobalt source and the manganese source. Alternatively, a nickel-manganese-cobalt hydroxide, which has a lower proportion of nickel than the composite hydroxide prepared in FIG. 6, may be prepared.

[0184] <Step S62> At least one of the cobalt source and manganese source is mixed with a composite oxide having a large crystallite size obtained by a process similar to that of Preparation Method 1. The mixing method is not particularly limited. For example, when the cobalt source and / or the manganese source is an alkoxide, a sol-gel method can be used. Furthermore, when the cobalt source and / or the manganese source is a composite hydroxide, a mechanochemical method can be used.

[0185] <Step S63> Next, the mixture of the cobalt source and / or manganese source and the composite oxide is heated.

[0186] It is also preferable to have a crushing step after heating as step S64. The crushing step can be carried out by referring to the description of step S44.

[0187] After the composite oxide is produced by the above steps, an additive element is mixed in and heated in the same manner as in the positive electrode active material production method 2, whereby the positive electrode active material 100 can be produced.

[0188] 6 and 9 illustrate a method of adding at least one of a cobalt source and a manganese source before adding an additive element source, but this is not a limitation of the present invention. At least one of a cobalt source and a manganese source may be added after adding an additive element source, or may be added together with the additive element source. Furthermore, when preparing the composite hydroxide 98 shown in FIG. 6 , the composition of the transition metal M in the interior and the surface layer may be changed. In this case, the nickel ratio in the interior and the surface layer can be changed by, for example, changing from an acid solution containing a high nickel ratio in the transition metal M to an acid solution containing a low nickel ratio.

[0189] This embodiment can be used in combination with other embodiments.

[0190] Third Embodiment In this embodiment, each of the components constituting a lithium ion battery will be described.

[0191] [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 further include at least one of a conductive material and a binder. The positive electrode active material described in Embodiment 1 can be used.

[0192] FIG. 10A shows an example of a schematic cross-sectional view of a positive electrode.

[0193] The positive electrode current collector 21 can be, for example, a metal foil. The positive electrode can be formed by applying a slurry to a metal foil and drying it. Pressing may be performed after drying. The positive electrode is formed by forming an active material layer on the positive electrode current collector 21.

[0194] The slurry is a material liquid used to form an active material layer on the positive electrode current collector 21, and refers to a material containing an active material, a binder, and a solvent, preferably further mixed with a conductive material. The slurry is also called an electrode slurry or an active material slurry, and is sometimes called a positive electrode slurry when forming a positive electrode active material layer, and a negative electrode slurry when forming a negative electrode active material layer.

[0195] The positive electrode active material 100 has the function of absorbing and releasing lithium ions during charge and discharge. The positive electrode active material 100 used in one embodiment of the present invention can be a material that exhibits little deterioration during charge and discharge, even at high charge voltages. Unless otherwise specified, the charge voltage is expressed relative to the potential of lithium metal in this specification. Furthermore, in this specification, a high charge voltage is, for example, a charge voltage of 4.5 V or higher, preferably 4.55 V or higher, more preferably 4.6 V or higher, 4.65 V or higher, or 4.7 V or higher.

[0196] The positive electrode active material 100 used in one embodiment of the present invention can be any material that undergoes little deterioration due to charge and discharge even at a high charge voltage, and can be the material described in Embodiment 1 or 2. Note that the positive electrode active material 100 can be two or more materials with different particle sizes as long as the material undergoes little deterioration due to charge and discharge even at a high charge voltage.

[0197] The conductive material is also called a conductivity imparting agent or a conductivity aid, and a carbon material can be used. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. In this specification, the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0198] Specific examples of carbon materials that can be used as the conductive material include carbon black (furnace black, acetylene black, etc.).

[0199] Examples of positive electrodes are shown in Figures 10A to 10D.

[0200] FIG. 10A illustrates carbon black 43, which is an example of a conductive material, and an electrolyte 51 contained in the gaps located between the positive electrode active materials 100, and shows an example in which not only the positive electrode active material 100 but also a second positive electrode active material 110 is included.

[0201] A binder (resin) may be mixed to bond the positive electrode current collector 21, such as a metal foil, and the active material to form the positive electrode of the secondary battery. The binder is also called a binding agent. The binder is a polymer material, and if a large amount of the binder is added, the proportion of the active material in the positive electrode decreases, resulting in a smaller discharge capacity of the secondary battery. Therefore, it is preferable to mix the minimum amount of binder.

[0202] 10A shows an example in which the positive electrode active material 100 is illustrated as a sphere, but this is not particularly limited. For example, the cross-sectional shape of the positive electrode active material 100 may be an ellipse, a rectangle, a trapezoid, a triangle, a polygon with rounded corners, or an asymmetric shape. For example, FIG. 10B shows an example in which the positive electrode active material 100 has a polygonal shape with rounded corners.

[0203] 10B, graphene 42 is used as a carbon material used as a conductive material in the positive electrode. In FIG. 10B, a positive electrode active material layer including a positive electrode active material 100, graphene 42, and carbon black 43 is formed over a positive electrode current collector 21.

[0204] In the step of mixing graphene 42 and carbon black 43 to obtain electrode slurry, the weight of the carbon black to be mixed is preferably 1.5 to 20 times, more preferably 2 to 9.5 times, the weight of graphene.

[0205] Furthermore, when the mixture of graphene 42 and carbon black 43 is within the above range, the dispersion stability of carbon black 43 is excellent and agglomerations are less likely to occur during slurry preparation. Furthermore, when the mixture of graphene 42 and carbon black 43 is within the above range, a higher electrode density can be achieved than in a positive electrode using only carbon black 43 as the conductive material. Increasing the electrode density can increase the capacity per unit weight. Specifically, the density of the positive electrode active material layer measured gravimetrically can be 3.5 g / cc or more.

[0206] Although the electrode density is lower than that of a positive electrode using only graphene as a conductive material, by mixing the first carbon material (graphene) and the second carbon material (acetylene black) within the above range, it is possible to accommodate rapid charging, and therefore it is particularly effective when used as an in-vehicle secondary battery.

[0207] Fig. 10C illustrates an example of a positive electrode in which carbon fibers 44 are used instead of graphene. Fig. 10C illustrates an example different from Fig. 10B. The use of carbon fibers 44 can prevent aggregation of carbon black 43 and improve dispersibility.

[0208] In FIG. 10C, the regions not filled with the positive electrode active material 100, the carbon fibers 44, and the carbon black 43 indicate voids or binders.

[0209] Another example of a positive electrode is shown in Fig. 10D. Fig. 10C shows an example in which carbon fibers 44 are used in addition to graphene 42. Using both graphene 42 and carbon fibers 44 can prevent aggregation of carbon black such as carbon black 43 and further improve dispersibility.

[0210] In FIG. 10D , the regions that are not filled with the positive electrode active material 100 , the carbon fibers 44 , the graphene 42 , and the carbon black 43 indicate voids or binders.

[0211] A secondary battery can be produced by using any one of the positive electrodes shown in FIGS. 10A to 10D , placing a separator on the positive electrode, and placing the laminate obtained by placing the negative electrode on the separator in a container (such as an outer casing or a metal can) and filling the container with an electrolyte.

[0212] <Binder> As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0213] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0214] Alternatively, it is preferable to use, as the binder, materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0215] The binder may be used in combination with two or more of the above.

[0216] For example, a material with a particularly excellent viscosity adjusting effect may be used in combination with other materials. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as water-soluble polymers with a particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch may be used.

[0217] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials or other components when preparing electrode slurries. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0218] Water-soluble polymers stabilize viscosity by dissolving in water, allowing active materials and other materials combined as binders, such as styrene-butadiene rubber, to be stably dispersed in aqueous solutions. Furthermore, the presence of functional groups is expected to facilitate stable adsorption to the surface of active materials. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups such as hydroxyl or carboxyl groups, and the presence of these functional groups is expected to facilitate interactions between polymers, resulting in widespread coverage of the active material surface.

[0219] When the binder covers the surface of the active material or contacts the surface and forms a film, it is expected to function as a passive film and have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0220] <Conductive Material> The conductive material is also called a conductivity imparting agent or a conductivity aid, and is made of a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0221] The active material layers such as the positive electrode active material layer and the negative electrode active material layer preferably contain a conductive material.

[0222] As the conductive material, for example, one or more of carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fibers such as carbon nanofibers and carbon nanotubes, and graphene compounds can be used.

[0223] Examples of carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers and carbon nanotubes can also be used as carbon fibers. Carbon nanotubes can be produced by vapor phase growth, for example.

[0224] In this specification and the like, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, 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 to resemble carbon nanofibers.

[0225] The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0226] Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular active material and the graphene compound with a smaller amount than that of ordinary conductive materials. This allows the ratio of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the battery.

[0227] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, tend to fill microscopic spaces. Microscopic spaces refer to, for example, the spaces between multiple active materials. By combining a carbon-containing compound that easily fills microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased, resulting in the formation of an excellent conductive path. The battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and is stable, making it effective as an in-vehicle battery.

[0228] <Positive Electrode Current Collector> The positive electrode current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The positive electrode current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The positive electrode current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable that the positive electrode current collector have a thickness of 5 μm to 30 μm.

[0229] [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 further have a conductive material and a binder.

[0230] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material or a carbon material can be used.

[0231] In addition, the negative electrode active material can be an element capable of undergoing a charge-discharge reaction through alloying and dealloying reactions with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. 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, it is preferable to use silicon as the negative electrode active material. Alternatively, compounds containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements capable of undergoing charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0232] In this specification, "SiO" refers to, for example, silicon monoxide. ｘ Here, x preferably has a value of 1 or 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.

[0233] The carbon material may be graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, or the like.

[0234] 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 therefore 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.

[0235] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety compared to lithium metal.

[0236] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten oxide (WO ２ ), molybdenum oxide (MoO ２ ) and other oxides can be used.

[0237] In addition, as the negative electrode active material, a composite nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N ３ has a large discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0238] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the composite nitride of lithium and a transition metal be used as the negative electrode active material, even when a material containing lithium ions is used as the positive electrode active material, by first removing the lithium ions contained in the positive electrode active material.

[0239] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ , CuO, Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ This also occurs with fluorides such as

[0240] Another example of a negative electrode is one that does not have a negative electrode active material at the end of the battery fabrication. A negative electrode that does not have a negative electrode active material can be, for example, a negative electrode that has only a negative electrode current collector at the end of the battery fabrication, in which lithium ions released from the positive electrode active material upon charging the battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) battery, a negative electrode-less (anode-less) battery, or the like.

[0241] When a negative electrode without a negative electrode active material is used, a film for uniforming lithium deposition may be provided on the negative electrode current collector. For example, a solid electrolyte having lithium ion conductivity can be used as the film for uniforming lithium deposition. Examples of solid electrolytes that can be used include sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Among these, polymer-based solid electrolytes are suitable as films for uniforming lithium deposition because they are relatively easy to form uniformly on the negative electrode current collector. Furthermore, for example, a metal film that forms an alloy with lithium can be used as the film for uniforming lithium deposition. For example, a magnesium metal film can be used as the metal film that forms an alloy with lithium. Lithium and magnesium form a solid solution over a wide composition range, making them suitable as films for uniforming lithium deposition.

[0242] Furthermore, when a negative electrode that does not have a negative electrode active material is used, a negative electrode current collector having projections and recesses can be used. When a negative electrode current collector having projections and recesses is used, the recesses of the negative electrode current collector become cavities into which lithium contained in the negative electrode current collector can be easily deposited, and therefore, when lithium is deposited, it is possible to prevent it from forming a dendritic shape.

[0243] 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.

[0244] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper, etc. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0245] [Electrolyte] The electrolyte contains a solvent and an electrolyte. The solvent for the electrolyte is preferably an aprotic organic solvent, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone, or any combination and ratio of two or more of these.

[0246] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from exploding and / or catching fire even if the internal temperature of the secondary battery rises due to an internal short circuit, overcharging, or the like. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0247] The electrolyte to be dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ These lithium salts may be used alone or in any combination and ratio of two or more thereof.

[0248] 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.

[0249] In addition, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), dinitrile compounds such as succinonitrile and adiponitrile, fluorobenzene, and ethyleneglycolbis(propionitrile)ether may be added to the electrolyte. The concentration of each additive may be, for example, 0.1 wt% to 5 wt% of the total solvent. VC and LiBOB are particularly preferred because they easily form a good coating. The additives may form a coating that adheres to the active material surface during the aging process of the secondary battery. Therefore, in secondary batteries that have undergone even a small amount of charging and discharging, at least some of the additives may not be detected in the electrolyte.

[0250] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0251] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0252] 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.

[0253] 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.

[0254] In addition, instead of an electrolytic solution, a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a solid electrolyte containing a polymer material such as a PEO (polyethylene oxide) can be used. When a solid electrolyte is used, the installation of a separator and / or spacer is unnecessary. Furthermore, since the entire battery can be solidified, the risk of leakage is eliminated, dramatically improving safety.

[0255] [Separator] When the electrolyte contains an electrolytic solution, a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0256] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0257] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide-based materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0258] 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.

[0259] 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.

[0260] [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 film 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.

[0261] Embodiment Mode 4 In this embodiment mode, examples of shapes of a secondary battery having a positive electrode manufactured by the manufacturing method described in the previous embodiment mode will be described.

[0262] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 11A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 11B is an external view, and Fig. 11C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0263] 11A is a schematic diagram showing the overlapping of components (upper and lower positions and positional relationships) for ease of understanding, and therefore, FIGS. 11A and 11B are not completely identical corresponding views.

[0264] In Fig. 11A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. These are sealed with a negative electrode can 302 and a positive electrode can 301 by a gasket. Note that the gasket for sealing is not shown in Fig. 11A. The spacer 322 and the washer 312 are used to protect the inside or to fix the position inside the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 322 and the washer 312 are made of stainless steel or an insulating material.

[0265] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0266] FIG. 11B is a perspective view of the completed coin-type secondary battery.

[0267] In the coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector. The negative electrode 307 is not limited to a laminated structure, and may be formed of lithium metal foil or a lithium-aluminum alloy foil.

[0268] Note 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.

[0269] The positive electrode can 301 and the negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, etc., they are preferably coated with nickel, aluminum, or the like. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0270] These negative electrode 307, positive electrode 304, and separator 310 are immersed in an electrolyte solution, and as shown in FIG. 11C , the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downwards, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303, thereby producing a coin-type secondary battery 300.

[0271] By having the above-described configuration, the coin-type secondary battery 300 can have a high discharge capacity and excellent cycle characteristics.

[0272] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 12A. As shown in Fig. 12A, a cylindrical secondary battery 616 has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0273] 12B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in FIG. 12B 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.

[0274] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal, such as nickel, aluminum, or titanium, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel), which is corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat the battery can 602 with nickel, aluminum, or the like to prevent corrosion by the electrolyte. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. A nonaqueous electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The nonaqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0275] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector.

[0276] By using the positive electrode active material 100 obtained in Embodiments 1 and 2 or the like for the positive electrode 604, a cylindrical secondary battery 616 having a high capacity, a high discharge capacity, and excellent cycle characteristics can be obtained.

[0277] 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 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature increases, and the increased resistance limits the amount of current to prevent abnormal heat generation. The PTC element is made of barium titanate (BaTiO ３ )-based semiconductor ceramics, etc. can be used.

[0278] 12C shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616. The positive electrodes of the respective secondary batteries are in contact with and electrically connected to conductors 624 separated by insulators 625. The conductors 624 are electrically connected to a control circuit 620 via wiring 623. The negative electrodes of the respective secondary batteries are electrically connected to the control circuit 620 via wiring 626. The control circuit 620 may be a charge / discharge control circuit that performs charging and discharging, or a protection circuit that prevents overcharging and / or overdischarging.

[0279] 12D shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of secondary batteries 616 may be connected in parallel, in series, or in parallel and then further connected in series. By configuring the power storage system 615 to have a plurality of secondary batteries 616, it is possible to extract a large amount of power.

[0280] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0281] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When the secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside air temperature.

[0282] 12D , the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of secondary batteries 616 via a conductive plate 614.

[0283] [Another Example of Secondary Battery Structure] An example of the structure of a secondary battery will be described with reference to FIGS. 13 and 14. FIG.

[0284] The secondary battery 913 shown in FIG. 13A has a wound body 950 provided with terminals 951 and 952 inside a housing 930. The wound body 950 is immersed in an electrolyte inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that in FIG. 13A , for convenience, the housing 930 is shown separated, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a resin material.

[0285] 13B, the housing 930 shown in Fig. 13A may be formed of a plurality of materials. For example, the secondary battery 913 shown in Fig. 13B has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0286] The housing 930a can be made of an insulating material such as organic resin. In particular, by using a material such as organic resin on the surface on which the antenna is formed, it is possible to suppress shielding of the electric field by the secondary battery 913. Note that if the shielding of the electric field by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0287] 13C shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0288] Alternatively, a secondary battery 913 may be provided that has a wound body 950a as shown in Fig. 14. The wound body 950a shown in Fig. 14A has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0289] By using the positive electrode active material 100 obtained in Embodiment 1, 2, or the like for the positive electrode 932, the secondary battery 913 can have a large capacity, a high discharge capacity, and excellent cycle characteristics.

[0290] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0291] 14B , the negative electrode 931 is electrically connected to a terminal 951 by ultrasonic bonding, welding, or crimping. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952 by ultrasonic bonding, welding, or crimping. The terminal 952 is electrically connected to a terminal 911b.

[0292] 14C , the wound body 950 a and the electrolyte are covered by the housing 930 to form the secondary battery 913. It is preferable to provide the housing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of the housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0293] As shown in Fig. 14B, the secondary battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a can result in a secondary battery 913 with a larger discharge capacity. For other elements of the secondary battery 913 shown in Figs. 14A and 14B, the descriptions of the secondary battery 913 shown in Figs. 13A to 13C can be referred to.

[0294] 15A and 15B show examples of external views of a laminated secondary battery, which includes 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.

[0295] 16A shows an external view of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as a tab region). The negative electrode 506 has a negative electrode current collector 504, and a negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., a tab region. Note that the area or shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 16A .

[0296] <Method of Manufacturing Laminated Secondary Battery> An example of a method of manufacturing the laminated secondary battery whose external view is shown in FIG. 15A will be described with reference to FIGS. 16B and 16C.

[0297] First, the negative electrode 506, separator 507, and positive electrode 503 are stacked. FIG. 16B shows the stacked negative electrode 506, separator 507, and positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. This can also be called a laminate consisting of a negative electrode, a separator, and a positive electrode. Next, the tab regions of the positive electrode 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.

[0298] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are arranged on the outer casing 509 .

[0299] Next, as shown in Fig. 16C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0300] 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.

[0301] By using the positive electrode active material 100 obtained in Embodiments 1 and 2 or the like for the positive electrode 503, the secondary battery 500 can have a high capacity, a high discharge capacity, and excellent cycle characteristics.

[0302] Embodiment 5 In this embodiment, an example of a vehicle including a secondary battery of one embodiment of the present invention will be described.

[0303] The secondary battery can be applied to a typical vehicle, such as an automobile. Examples of the automobile include next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHEVs or PHVs). The secondary battery can be used as one of the power sources mounted on the automobile. The vehicle is not limited to an automobile. Examples of the vehicle include trains, monorails, ships, submersibles (deep-sea exploration vessels, unmanned submersibles), aircraft (helicopters, unmanned aerial vehicles (drones), airplanes, rockets, and artificial satellites), electric bicycles, and electric motorcycles. The secondary battery of one embodiment of the present invention can be applied to these vehicles.

[0304] 17C shows an example in which the secondary battery of the present invention is applied to an electric vehicle (EV). The electric vehicle is equipped with first batteries 1301a and 1301b as main driving secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0305] The internal structure of the first battery 1301a may be a wound type shown in Fig. 13C or 14A or a stacked type shown in Fig. 15A or 15B. The first battery 1301a may use the all-solid-state battery of Embodiment 6. Use of the all-solid-state battery of Embodiment 6 for the first battery 1301a allows for a high capacity, improved safety, and reductions in size and weight.

[0306] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple secondary batteries, it is possible to extract large amounts of power. The multiple secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of secondary batteries is also called a battery pack.

[0307] In addition, in a secondary battery for vehicle use, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a in order to cut off power from multiple secondary batteries.

[0308] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DCDC circuit 1306. When a rear motor 1317 is provided for the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0309] In addition, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0310] Next, the first battery 1301a will be described with reference to FIG. 17A.

[0311] FIG. 17A shows an example in which nine prismatic secondary batteries 1300 are combined into one battery pack 1415. Furthermore, nine prismatic secondary batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also called a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (such as from the road surface), it is preferable to fix multiple secondary batteries using the fixing portions 1413 and 1414 or a battery housing box. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0312] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit portion 1320. A charge control circuit or a battery control system including a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor (BTOS).

[0313] It is preferable to use a metal oxide that functions as an oxide semiconductor. For example, a metal oxide such as In-M2-Zn oxide (wherein the element M2 is one or more selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, etc.) may be used as the metal oxide. In particular, the In-M-Zn oxide that can be used as the metal oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In—Ga oxide or an In—Zn oxide may be used as the metal oxide. The CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis oriented in a specific direction. The specific direction is the thickness direction of the CAAC-OS film, the normal direction to the surface on which the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. The crystalline regions are regions in which the atomic arrangement is periodic. When the atomic arrangement is considered as a lattice arrangement, the crystalline regions are also regions in which the lattice arrangement is aligned.

[0314] Note that "CAC-OS" has a mosaic structure in which a material is separated into a first region and a second region, and the first region is distributed throughout the film (hereinafter, also referred to as a cloud structure). That is, CAC-OS is a composite metal oxide having a structure in which the first region and the second region are mixed. However, it may be difficult to observe a clear boundary between the first region and the second region.

[0315] For example, in the case of CAC-OS in an In—Ga—Zn oxide, EDX mapping obtained using energy dispersive X-ray spectroscopy (EDX) can confirm that the CAC-OS has a structure in which a region containing In as a main component (first region) and a region containing Ga as a main component (second region) are unevenly distributed and mixed.

[0316] When a CAC-OS is used in a transistor, the conductivity due to the first region and the insulating property due to the second region act complementarily, thereby providing the CAC-OS with a switching function (a function of turning on / off). That is, a CAC-OS has a conductive function in a part of the material and an insulating function in a part of the material, and functions as a semiconductor as a whole. By separating the conductive function and the insulating function, both functions can be maximized. Therefore, by using a CAC-OS in a transistor, a high on-current (I ｏｎ ), high field-effect mobility (μ), and good switching behavior can be achieved.

[0317] Oxide semiconductors have a variety of structures, each of which has different characteristics. The oxide semiconductor of one embodiment of the present invention may include two or more of an amorphous oxide semiconductor, a polycrystalline oxide semiconductor, an a-like OS, a CAC-OS, an nc-OS, and a CAAC-OS.

[0318] Furthermore, because the control circuit unit 1320 can be used in high-temperature environments, it is preferable to use a transistor using an oxide semiconductor. To simplify the process, the control circuit unit 1320 may be formed using a unipolar transistor. Transistors using an oxide semiconductor for the semiconductor layer have a wider operating ambient temperature range than single-crystal Si, from −40° C. to 150° C., and their characteristics change less when the secondary battery is heated than single-crystal Si. The off-current of a transistor using an oxide semiconductor is below the lower limit of measurement regardless of temperature, even at 150° C., whereas the off-current characteristics of single-crystal Si transistors are highly temperature-dependent. For example, at 150° C., the off-current of a single-crystal Si transistor increases, and the current on / off ratio is not sufficiently large. The control circuit unit 1320 can improve safety. Furthermore, a synergistic effect on safety can be achieved by combining the positive electrode active material 100 obtained in Embodiments 1 and 2 with a secondary battery using the positive electrode. The secondary battery and control circuit unit 1320 using the positive electrode active material 100 obtained in the first or second embodiment or the like for the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0319] The control circuit unit 1320, which uses a memory circuit including transistors using oxide semiconductors, can also function as an automatic control device for a secondary battery to address 10 causes of instability, such as micro-short circuits. The functions for addressing the 10 causes of instability include overcharging prevention, overcurrent prevention, overheating control during charging, cell balancing in the battery pack, over-discharging prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of degradation, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits. The control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for a secondary battery can be miniaturized.

[0320] Furthermore, a "micro-short" refers to a tiny short circuit inside a secondary battery, which is not so small that the positive and negative electrodes of the secondary battery are short-circuited and render it unable to be charged or discharged, but rather refers to a phenomenon in which a small short-circuited part allows a small amount of short-circuit current to flow. Even if the short-circuit occurs in a relatively short period of time and in a small location, a large voltage change occurs, and this abnormal voltage value may affect subsequent estimations.

[0321] One of the causes of micro-short circuits is said to be that multiple charge and discharge cycles cause uneven distribution of the positive electrode active material, resulting in localized current concentration in parts of the positive electrode and negative electrode, causing parts of the separator to stop functioning, or the generation of by-products due to side reactions, resulting in micro-short circuits.

[0322] In addition to detecting micro-shorts, the control circuit 1320 can also be said to detect the terminal voltage of the secondary battery and manage the charge / discharge state of the secondary battery. For example, to prevent overcharging, it can turn off both the output transistor and the cutoff switch of the charging circuit almost simultaneously.

[0323] Next, an example of a block diagram of the battery pack 1415 shown in FIG. 17A is shown in FIG. 17B.

[0324] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the secondary battery used and limits the upper limit of the current from the outside or the upper limit of the output current to the outside. The range between the lower limit and the upper limit of the secondary battery's voltage is within the recommended voltage range, and when the secondary battery falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and / or overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. Furthermore, a PTC element may be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0325] The switch portion 1324 can be configured by combining n-channel transistors or p-channel transistors. The switch portion 1324 is not limited to a switch having a Si transistor using single crystal silicon. For example, the switch portion 1324 may be formed using a power transistor having Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), or GaOx (gallium oxide; x is a real number greater than 0). Furthermore, memory elements using OS transistors can be freely arranged by stacking them on circuits using Si transistors, and therefore integration can be easily achieved. Furthermore, OS transistors can be manufactured using the same manufacturing equipment as Si transistors, and therefore can be manufactured at low cost. That is, the control circuit portion 1320 using OS transistors can be stacked on the switch portion 1324 and integrated into a single chip. The volume occupied by the control circuit section 1320 can be reduced, which allows for miniaturization.

[0326] The first batteries 1301a and 1301b mainly supply power to on-board equipment in the 42V system (high voltage system), while the second battery 1311 supplies power to on-board equipment in the 14V system (low voltage system). Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Lead-acid batteries have the disadvantage of being more self-discharged than lithium-ion batteries and being prone to deterioration due to a phenomenon called sulfation. Using a lithium-ion battery as the second battery 1311 offers the advantage of being maintenance-free. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b have remaining capacity, the second battery 1311 is charged to maintain a full charge state by supplying power from the first battery to the second battery.

[0327] In this embodiment, an example in which lithium ion batteries are used for both the first battery 1301a and the second battery 1311 is shown. A lead-acid battery, an all-solid-state battery, or an electric double layer capacitor may be used for the second battery 1311. For example, the all-solid-state battery of Embodiment 6 may be used. By using the all-solid-state battery of Embodiment 6 for the second battery 1311, high capacity can be achieved, and reductions in size and weight can be achieved.

[0328] Furthermore, regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 via the motor controller 1303 or the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b be capable of rapid charging.

[0329] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions in accordance with the charging characteristics of the secondary battery used, and can perform rapid charging.

[0330] Although not shown, when an external charger is connected, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a and 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the functions of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a and 1301b via the control circuit unit 1320 to prevent overcharging. The control circuit unit 1320 may also be provided in the connection cable or the charger's connection cable. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU includes a microcomputer. The ECU uses a CPU or a GPU.

[0331] External chargers installed at charging stations and the like include 100V-200V outlets, or three-phase 200V and 50kW. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0332] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0333] Furthermore, by using graphene as a conductive material, a secondary battery with significantly improved electrical characteristics can be realized, as a synergistic effect of suppressing capacity decline and maintaining high capacity even when the electrode layer is thickened and the amount of graphene supported is increased. This is particularly effective for secondary batteries used in vehicles, and it is possible to provide a vehicle with a long driving range, specifically a driving distance of 500 km or more per charge, without increasing the ratio of the weight of the secondary battery to the total weight of the vehicle.

[0334] In particular, the secondary battery of the present embodiment described above can increase the operating voltage of the secondary battery by using the positive electrode active material 100 described in Embodiments 1, 2, etc., and can increase the usable capacity as the charging voltage increases. Furthermore, by using the positive electrode active material 100 described in Embodiments 1, 2, etc., in the positive electrode, a secondary battery for a vehicle having excellent cycle characteristics can be provided.

[0335] Next, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0336] 12D, 14C, and 17A can be installed in a vehicle to realize next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Furthermore, the secondary battery can also be installed in agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, artificial satellites, space probes, planetary probes, or spacecraft. The secondary battery of one embodiment of the present invention can be a high-capacity secondary battery. Therefore, the secondary battery of one embodiment of the present invention is suitable for miniaturization and weight reduction and can be suitably used in transportation vehicles.

[0337] 18A to 18D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 shown in FIG. 18A is an electric automobile that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a secondary battery is installed in a vehicle, an example of the secondary battery described in Embodiment 4 is installed in one or more locations. The automobile 2001 shown in FIG. 18A includes a battery pack 2200, which includes a secondary battery module to which multiple secondary batteries are connected. It is preferable that the automobile further includes a charge control device electrically connected to the secondary battery module.

[0338] Furthermore, the automobile 2001 can charge its secondary battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging may be performed using a predetermined charging method or connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The charging device may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge the power storage device installed in the automobile 2001 using external power supply. Charging can be performed by converting AC power to DC power via a conversion device, such as an AC-DC converter.

[0339] Furthermore, although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0340] 18B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The secondary battery module of the transport vehicle 2002 is, for example, a four-cell unit of secondary batteries with a nominal voltage of 3.0 V to 5.0 V, with 48 cells connected in series for a maximum voltage of 170 V. Apart from the number of secondary batteries constituting the secondary battery module of the battery pack 2201, the transport vehicle 2002 has the same functions as those shown in FIG. 18A, and therefore a description thereof will be omitted.

[0341] FIG. 18C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The secondary battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, which is obtained by connecting in series one hundred or more secondary batteries with a nominal voltage of 3.0 V or more and 5.0 V or less. Therefore, a secondary battery with minimal variation in characteristics is required. By using a secondary battery in which the positive electrode active material 100 described in embodiments 1 and 2 is used as the positive electrode, a secondary battery with stable battery characteristics can be manufactured, enabling mass production at low cost from the standpoint of yield. Furthermore, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2202, the battery pack 2202 has the same functions as those shown in FIG. 20A, and therefore a description thereof will be omitted.

[0342] Fig. 18D shows, as an example, an aircraft 2004 having an engine that burns fuel. The aircraft 2004 shown in Fig. 18D has wheels for takeoff and landing, and can therefore be considered a type of transport vehicle, and has a battery pack 2203 that includes a secondary battery module formed by connecting multiple secondary batteries, and the secondary battery module and a charge control device.

[0343] The secondary battery module of the aircraft 2004 has, for example, eight 4 V secondary batteries connected in series to produce a maximum voltage of 32 V. Other than the number of secondary batteries constituting the secondary battery module of the battery pack 2203, the secondary battery module has the same functions as those shown in Fig. 18A, and therefore a description thereof will be omitted.

[0344] 18E illustrates, as an example, a satellite 2005 equipped with a secondary battery 2204. Because the satellite 2005 is used in space at extremely low temperatures, it is preferable that the satellite 2005 be equipped with the secondary battery 2204, which is one embodiment of the present invention and has excellent low-temperature resistance. It is more preferable that the secondary battery 2204 be mounted inside the satellite 2005 while being covered with a heat-insulating member.

[0345] Embodiment 6 In this embodiment, an example in which a lithium-ion battery according to one embodiment of the present invention is mounted on a motorcycle or a bicycle will be described as an example in which a secondary battery is mounted on a vehicle.

[0346] 19A illustrates an example of an electric bicycle using the power storage device of one embodiment of the present invention. The power storage device of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 19A. The power storage device of one embodiment of the present invention includes, for example, a plurality of storage batteries and a protection circuit.

[0347] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state detached from the bicycle in FIG. 19B . The power storage device 8702 includes a plurality of built-in storage batteries 8701 included in the power storage device of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit 8703. The power storage device 8702 also includes a control circuit 8704 that can control charging or detect an abnormality of the secondary battery, an example of which is shown in Embodiment 7. The control circuit 8704 is electrically connected to the positive electrode and the negative electrode of the storage battery 8701. A synergistic effect in terms of safety can be obtained by combining the positive electrode active material 100 obtained in Embodiments 1, 2, and the like with a secondary battery whose positive electrode is used. The secondary battery and the control circuit 8704 using the positive electrode active material 100 obtained in Embodiments 1 and 2 for the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0348] 19C illustrates an example of a two-wheeled vehicle using the power storage device of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 19C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. The power storage device 8602 includes a plurality of secondary batteries each using the positive electrode active material 100 obtained in Embodiments 1 and 2 for a positive electrode. This power storage device 8602 can have a high capacity and contribute to miniaturization.

[0349] 19C can store a power storage device 8602 in an under-seat storage compartment 8604. The power storage device 8602 can be stored in the under-seat storage compartment 8604 even if the under-seat storage compartment 8604 is small.

[0350] Embodiment 7 In this embodiment, an example of mounting a secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a secondary battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0351] 20A shows an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 also includes a secondary battery 2107. By including the secondary battery 2107 using the positive electrode active material 100 described in Embodiments 1 and 2, etc. as a positive electrode, a high capacity can be achieved, and a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0352] 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.

[0353] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0354] 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.

[0355] The mobile phone 2100 also includes an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0356] Furthermore, the mobile phone 2100 preferably has a sensor. As the sensor, for example, a fingerprint sensor, a pulse sensor, a body temperature sensor or other human body sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted.

[0357] FIG. 20B illustrates an unmanned aerial vehicle 2300 having a plurality of rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 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. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2 for its positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Thus, the secondary battery is suitable as a secondary battery to be mounted on the unmanned aerial vehicle 2300.

[0358] Fig. 20C shows an example of a robot. The robot 6400 shown in Fig. 20C includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0359] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0360] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0361] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0362] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1, 2, and the like for a positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery is suitable as the secondary battery 6409 to be mounted on the robot 6400.

[0363] 20D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0364] The cleaning robot 6300 can analyze an image captured by the camera 6303 and determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1, 2, and the like for its positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery 6306 is suitable for use in the cleaning robot 6300.

[0365] In this example, a positive electrode active material 100 with a large crystallite size was produced, and its characteristics were evaluated.

[0366] <Preparation of Positive Electrode Active Material> Samples prepared in this example will be described with reference to the preparation method shown in FIGS.

[0367] 6, nickel(II) sulfate was prepared as the nickel source, cobalt(II) sulfate as the cobalt source, and manganese(II) sulfate as the manganese source. In step S13, glycine was prepared as a chelating agent. These transition metal M sources were weighed out to 2 mol / L and Ni:Co:Mn=8:1:1 (atomic ratio), and glycine was weighed out to 0.200 mol / L. Pure water was added to these sources and dissolved them (step S14), producing an acid solution.

[0368] A 5 mol / L aqueous solution of sodium hydroxide was used as the alkaline solution.

[0369] Pure water was used as the charging liquid. Nitrogen was bubbled into the charging liquid at a nitrogen flow rate of 1 L / min. Note that water or aqueous solution that is initially placed in the reaction vessel, such as pure water here, may be referred to as the charging liquid. The charging liquid may also be referred to as the adjustment liquid. The charging liquid and adjustment liquid refer to water or aqueous solution before the reaction, that is, water or aqueous solution in the initial state.

[0370] The acid solution was mixed into the charging solution at a rate of 0.10 mL / min while stirring at 1000 rpm. The alkaline solution was added dropwise as needed to maintain the charging solution at pH 11.0. The temperature of the charging solution was maintained at 50°C. A baffle plate was installed in the reaction vessel to change the direction and flow rate of stirring. An OptiMax (Mettler-Toledo) was used for these coprecipitation reactions.

[0371] The precipitate formed by the above coprecipitation reaction was filtered with pure water and acetone, and dried in a vacuum drying oven at 200° C. for 12 hours to obtain a composite hydroxide.

[0372] In step S41 of Fig. 7, lithium hydroxide was prepared as a lithium source. The lithium hydroxide was pulverized using a fluidized bed jet mill. In step S42, the composite hydroxide obtained above was mixed with the lithium source. The mixing ratio was set to 0.7 (atomic ratio) when the sum of the numbers of atoms of nickel, cobalt, and manganese was 1.

[0373] In step S43, the mixture of composite hydroxide and lithium source was heated. For heating, an aluminum oxide crucible was used, and the mixture was heated in an oxygen atmosphere in a muffle furnace at 700°C for 10 hours. The oxygen flow rate was 5 L / min, and the temperature was increased at 100°C / hour. Thereafter, the mixture was cooled to room temperature and crushed (step S44), and a composite oxide was obtained (step S44).

[0374] In step S51, lithium hydroxide similar to that in step S41 was prepared. In step S52, the composite oxide obtained above was mixed with a lithium source. The mixing ratio was such that the lithium was 0.31 (atomic ratio) when the sum of the numbers of atoms of nickel, cobalt, and manganese was 1. In other words, the total lithium mixed in steps S42 and S52 was 1.01 (atomic ratio) when the sum of the numbers of atoms of nickel, cobalt, and manganese was 1.

[0375] In step S53, the mixture of the composite oxide and the lithium source was heated. The procedure was the same as in step S43, except that the heating temperature was 850° C. Thereafter, the mixture was allowed to cool to room temperature and crushed (step S54), thereby obtaining a positive electrode active material. This was designated as Sample 1.

[0376] Sample 2 was prepared in the same manner as Sample 1 except that the heating in step S53 was set to 875°C.

[0377] Sample 3 was prepared in the same manner as Sample 1 except that the heating in step S53 was set to 900°C.

[0378] As a comparative example, a positive electrode active material was prepared by mixing the lithium source only once. Specifically, in step S41, the lithium was mixed so that the ratio (atomic ratio) of nickel, cobalt, and manganese was 1.01 when the sum of the atomic numbers of nickel, cobalt, and manganese was 1, and no mixing was performed in step S52. Sample 4 was prepared in the same manner as Sample 1 except for the above.

[0379] Sample 4 was further heated at 800° C. for 10 hours to prepare a positive electrode active material, which was designated as Sample 5.

[0380] <Samples 6 to 9> In step S13, glycine was weighed out so that the concentration in the acid solution was 0.100 mol / L. A 0.100 M glycine aqueous solution was used as the charging solution. The acid solution was mixed with the charging solution at a rate of 0.0443 mL / min. After the coprecipitation reaction, the solution temperature was controlled to 25°C, and the mixture was filtered with pure water and then with acetone.

[0381] Furthermore, Samples 6, 7, 8, and 9 were further heated at 800°C for 10 hours after step S54 (step S55) and crushed (step S56). Other conditions were the same as in step S43. The other conditions were the same as in Samples 1 to 4, and the positive electrode active materials were designated Samples 6 to 9.

[0382] <Sample 21> Sample 21 was prepared in the same manner as Sample 8, except that after the heating and cooling in step S55, heating was again carried out at 800° C. for 2 hours.

[0383] The preparation conditions for Samples 1 to 9 and Sample 21 are shown in Table 1.

[0384]

[0385] <SEM> SEM images of Sample 1 are shown in FIG. 21A, Sample 2 in FIG. 21B, Sample 3 in FIG. 21C, Sample 6 in FIG. 21D, Sample 7 in FIG. 21E, and Sample 8 in FIG. 21F. All of these positive electrode active materials were confirmed to have large primary particles. Furthermore, in Samples 1 to 3, which were not heated in S55, deposits presumed to be residues of a lithium source or the like were observed on the surface of the positive electrode active material. On the other hand, in Samples 6 to 8, which were heated in S55, smooth surfaces with almost no residue were observed.

[0386] <Crystallite Size> The crystallite sizes of Samples 1 to 9 were calculated by XRD analysis. The XRD device and calculation method were as described in Embodiment 1. The crystallite sizes are also shown in Table 1.

[0387] As shown in Table 1, Samples 1 to 3 and Samples 6 to 8, in which the lithium source was added in multiple batches, had larger crystallite sizes than Samples 4, 5, and 9, in which the lithium source was added in a single batch. The crystallite sizes of the samples in which the lithium source was added in a single batch were less than 140 nm, whereas the crystallite sizes of the samples in which the lithium source was added in multiple batches were 140 nm or more, more specifically 150 nm or more.

[0388] <Half-cell charge / discharge rate characteristics> Half-cells were assembled using the positive electrode active materials of Samples 7 to 9, and the charge / discharge rate characteristics were evaluated. The performance of the positive electrode alone was determined by evaluating the cycle characteristics of the half-cells.

[0389] The conditions for the half-cell are explained below. First, the above-mentioned positive electrode active material was prepared. Acetylene black (AB) was prepared as a conductive material. Polyvinylidene fluoride (PVDF) was prepared as a binder. These were mixed in a weight ratio of positive electrode active material:AB:PVDF=95:3:2 to prepare a slurry. The slurry was then applied to an aluminum current collector. NMP was used as the solvent for the slurry.

[0390] After the slurry was applied to the current collector, the solvent was evaporated. A positive electrode was obtained by the above steps. The amount of active material carried on the positive electrode was approximately 7 mg / cm.２ It was decided.

[0391] The electrolyte solution used was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, to which 2 wt% vinylene carbonate (VC) was added as an additive. The electrolyte contained 1 mol / L of lithium hexafluorophosphate (LiPF ６ ) was used. Polypropylene was used as the separator.

[0392] Metallic lithium was prepared as the counter electrode, and a coin-shaped half cell equipped with the above positive electrode and other components was formed.

[0393] <Discharge Rate Characteristics> Discharge rate characteristics were measured using the above half cell.

[0394] Figure 22A shows the discharge capacities at 0.5C, 10C, and 20C measured at 25°C, and Figure 22B shows the same discharge capacities measured at 65°C. In all cases, charging was performed under CC / CV (constant current / constant voltage) conditions (0.5C, 4.5V, 0.05C cut), and discharging was performed under CC (constant current) conditions (0.5C, 10C, or 20C, 2.5V cut). Note that 1C was defined as 200mA / g.

[0395] As shown in FIGS. 22A and 22B , secondary batteries using positive electrode active materials with large crystallite sizes, such as Samples 7 and 8, exhibited higher discharge capacities at high discharge rates exceeding 10C compared to positive electrode active materials with small crystallite sizes, such as Sample 9. For example, the discharge capacities at 25°C and 10C were 70 mAh / g or higher, more specifically, Sample 8 was 85 mAh / g, and Sample 7 was 98 mAh / g. The discharge capacities at 65°C and 10C were 150 mAh / g or higher, more specifically, Sample 7 was 158 mAh / g, and Sample 8 was 168 mAh / g. The discharge capacities at 65°C and 20C were 100 mAh / g or higher, more specifically, Sample 8 was 111 mAh / g, and Sample 7 was 124 mAh / g.

[0396] <Charge Rate Characteristics> Next, a half cell was similarly prepared using Sample 21, and the charge rate characteristics were evaluated.

[0397] Figure 23A shows the discharge capacities at 0.1C, 0.5C, 1C, and 5C measured at 25°C, and Figure 23B shows the same discharge capacities measured at 65°C. In all cases, charging was performed at CC / CV (0.1C, 0.5C, 1C, or 5C, 4.5V, 0.05C cut), and discharging was performed at CC (0.5C, 2.5V cut). The horizontal axis shows the charge and discharge rates as C-rate. Tests were conducted twice for each of the charge rates of 0.1C, 0.5C, 1C, and 5C.

[0398] As shown in Figures 23A and 23B, there was no significant change in discharge capacity when the charge rate was changed from 0.1C to 0.5C, indicating that sufficient charge and discharge were possible even at high charge rates. For example, the discharge capacity at 25°C and 5 / 0.5C (charge / discharge) was 150mAh / g or more, more specifically, 170mAh / g in both cycles. Furthermore, the discharge capacity at 65°C and 5 / 0.5C was 170mAh / g or more, more specifically, 180mAh / g in the first 5 / 0.5 cycle and 184mAh / g in the second cycle.

[0399] The above results demonstrate that the positive electrode active material of one embodiment of the present invention, which has a large crystallite size, has excellent charge / discharge rate characteristics.

[0400] In this example, a positive electrode active material 100 was produced in which the crystallite size of the primary particles was large and the surface layer portion 100a contained an additive element.

[0401] <Preparation of Positive Electrode Active Material> Samples prepared in this example will be described with reference to the preparation method shown in FIGS.

[0402] First, a composite oxide was prepared through steps S11 to S56 in the same manner as in Example 1 (step S57). Next, in step S71 of Fig. 8, aluminum hydroxide was prepared as an aluminum source for the additive element. In step S72, the composite oxide was mixed with the aluminum source. The mixing ratio was such that the sum of the numbers of atoms of nickel, cobalt, and manganese was 1, and the aluminum was 0.01 (atomic ratio).

[0403] In step S73, the mixture of the composite oxide and the aluminum source was heated. The procedure was the same as in step S43, except that the heating temperature was 800°C and the heating time was 2 hours. The mixture was then cooled to room temperature and crushed (step S74), yielding a positive electrode active material. This was designated sample 32.

[0404] Sample 33 was prepared in the same manner as Sample 32, except that magnesium was used as the additive element, magnesium carbonate was prepared as the magnesium source, and the mixing ratio was such that the sum of the numbers of atoms of nickel, cobalt, and manganese was 1 and the magnesium was 0.01 (atomic number ratio).

[0405] Sample 34 was prepared in the same manner as Samples 32 and 33, except that the additive elements were aluminum and magnesium, with the aluminum being 0.005 (atomic ratio) and the magnesium being 0.005 (atomic ratio).

[0406] Sample 31 was prepared in the same manner as Sample 32 except that no additional elements were added.

[0407] The preparation conditions for Samples 31 to 34 are shown in Table 2.

[0408]

[0409] <Surface SEM> A surface SEM image of sample 31 is shown in Fig. 24A, a surface SEM image of sample 32 is shown in Fig. 24B, a surface SEM image of sample 33 is shown in Fig. 24C, and a surface SEM image of sample 34 is shown in Fig. 24D. An enlarged image of the square portion in Fig. 24A is shown in Fig. 24E, an enlarged image of the square portion in Fig. 24B is shown in Fig. 24F, an enlarged image of the square portion in Fig. 24C is shown in Fig. 24G, and an enlarged image of the square portion in Fig. 24D is shown in Fig. 24H.

[0410] <Cross-Sectional SEM-EDX> Next, a cross-sectional SEM image of sample 32 is shown in FIG. 25A, a cross-sectional SEM image of sample 33 is shown in FIG. 25B, and a cross-sectional SEM image of sample 34 is shown in FIG. 25C. EDX point analysis was performed on the locations indicated by (1) to (4) in FIG. 25A, and the measured aluminum concentrations are shown in FIG. 25D. Similarly, the magnesium concentrations at the locations indicated by (1) to (4) in FIG. 25B are shown in FIG. 25E. The aluminum and magnesium concentrations at the locations indicated by (1) to (4) in FIG. 25C are shown in FIG. 25F. In all cases, the aluminum and magnesium concentrations decreased from the surface layer 100a toward the interior.

[0411] As shown in FIGS. 24A to 25F, Samples 32 to 34 were positive electrode active materials with large crystallite sizes and higher concentrations of the added elements in the surface layer 100a than in the interior.

[0412] 100: positive electrode active material, 100a: surface layer, 100b: surface layer, 100c: interior, 101: grain boundary, 104: coating

Claims

1. A positive electrode active material having a transition metal M, oxygen, and an additive element, The transition metal M is nickel, manganese, and cobalt. The aforementioned additive element is one or more selected from magnesium, aluminum, calcium, titanium, and zirconium. The positive electrode active material has a first surface layer, a second surface layer, and an interior. The second surface layer is closer to the interior than the first surface layer. In the interior, the ratio of nickel to the sum of the number of transition metal M atoms is larger than in the first and second surface layers. The ratio of the number of atoms of at least one element selected from cobalt and manganese to the sum of the number of atoms of the transition metal M is greater in the second surface layer than in the interior. A positive electrode active material wherein the first surface layer has a higher concentration of at least one of the additive elements than the interior and the second surface layer.

2. In claim 1, The positive electrode active material is a positive electrode active material whose crystallite size, calculated from the XRD pattern, is 150 nm or larger.

3. A secondary battery having a positive electrode with a positive electrode active material and a negative electrode, The positive electrode active material comprises a transition metal M, oxygen, and an additive element. The transition metal M is nickel, manganese, and cobalt. The aforementioned additive element is one or more selected from magnesium, aluminum, calcium, titanium, and zirconium. The positive electrode active material has a first surface layer, a second surface layer, and an interior. The second surface layer is closer to the interior than the first surface layer. In the interior, the ratio of nickel to the sum of the number of transition metal M atoms is larger than in the first and second surface layers. The ratio of the number of atoms of at least one element selected from cobalt and manganese to the sum of the number of atoms of the transition metal M is greater in the second surface layer than in the interior. A secondary battery wherein the first surface layer has a higher concentration of at least one of the additive elements than the interior and the second surface layer.

4. In claim 3, The aforementioned positive electrode active material is a secondary battery in which the crystallite size of the positive electrode active material calculated from the XRD pattern is 150 nm or larger.