Lithium ion secondary battery

Abstract

To provide a secondary battery with excellent cycle characteristic or a secondary battery with higher capacity.SOLUTION: A positive electrode active material layer includes a first graphene layer, a second graphene layer, and a positive electrode active material. The first graphene layer includes a first region covering the positive electrode active material, and the second graphene layer includes a second region covering the positive electrode active material and a third region overlapping the first region. The first region exists between the positive electrode active material and the third region and includes a surface formed of carbon six-membered rings that are arrayed. The positive electrode active material includes a fourth region with a layered rock salt type structure. A lithium layer of the layered rock salt type structure in the fourth region and a surface formed by the carbon six-membered ring in the second region are approximately perpendicular.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] 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. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a secondary battery, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a secondary battery, a power storage device, and a manufacturing method thereof.

[0002] In this specification, the term "secondary battery" or "power storage device" generally refers to elements and devices having a power storage function. [Background technology]

[0003] In recent years, there has been active development of various power storage devices, including lithium-ion secondary batteries, lithium-ion capacitors, and air batteries, as well as all-solid-state batteries. Demand for high-power, high-energy-density lithium-ion secondary batteries has rapidly expanded alongside the development of the semiconductor industry, and they are now essential to the modern information society as a rechargeable energy source, thanks to their high output and high energy density. These batteries are used in a variety of applications, including mobile phones, smartphones, tablets, and laptop computers, as well as portable music players, digital cameras, medical devices, and next-generation clean-energy vehicles, including hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs).

[0004] For example, lithium-ion batteries that use nickel cobalt manganese oxide (NCM) or lithium iron phosphate (LFP) as the positive electrode active material are now commercially available as large-scale secondary batteries for home use or secondary batteries for vehicles.

[0005] Meanwhile, graphene has attracted much attention in recent years due to its excellent conductivity, and large-scale production methods are being explored. As shown in Non-Patent Document 1, a compound obtained by reducing graphene oxide (GO) is sometimes called reduced GO (RGO), and its physical properties have attracted attention. For example, as shown in Non-Patent Document 2, there is research that characterizes the physical properties of GO using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and the like. Furthermore, Patent Document 1 describes the application of graphene to secondary batteries. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-030463 [Non-patent literature]

[0007] [Non-Patent Document 1] A. Bagri et al., “Structural evolution during the reduction of chemically derived graphene oxide”, NATURE CHEMISTRY, vol.2,2010,pp.581-587.

[0008] [Non-patent document 2] Burcu Saner et al., “Utilization of multiple graphene nanosheets in fuel cells: 2. The effect of oxidation process on the characteristics of graphene nanosheets”, Fuel, 90, 2011, pp. 2609-2616. Summary of the Invention [Problem to be solved by the invention]

[0009] Electric vehicles are vehicles that are driven solely by an electric motor, but there are also hybrid vehicles that are equipped with both an internal combustion engine and an electric motor. Multiple secondary batteries used in automobiles are grouped into a battery pack, and multiple sets of battery packs are installed underneath the automobile.

[0010] Furthermore, secondary batteries used in electric and hybrid vehicles deteriorate depending on the number of charges, depth of discharge, charging current, charging environment (temperature changes), etc. Deterioration also depends on how the user uses the battery, and factors such as the temperature during charging, frequency of quick charging, amount of charge by regenerative braking, and timing of charging by regenerative braking may also affect the deterioration.

[0011] Electric vehicles are prone to temperature changes depending on their operating conditions and environment, making temperature safety measures necessary. Among the components installed in electric vehicles, secondary batteries play the most important role as the power source for the vehicle. However, the problem with conventional secondary batteries is that the temperature range in which they can operate normally is narrow, which is the tolerance range for the environment in which the electric vehicle is used.

[0012] Secondary batteries have the problem of being prone to deterioration when internal chemical reactions occur at high temperatures. Also, in extremely cold regions where temperatures can drop to minus 50°C, the liquid components inside the secondary battery freeze, preventing discharge and causing the secondary battery to lose its function.

[0013] Even in areas other than extremely cold, when electric vehicles are used in low temperatures, lithium-ion secondary batteries experience a decrease in capacity, an increase in internal resistance, and a decrease in output voltage. Charging at low temperatures can also cause rapid deterioration due to the electrolytic deposition of lithium metal.

[0014] Therefore, one of the challenges is to improve the safety of secondary batteries.

[0015] Furthermore, in order to increase the driving distance of electric vehicles, it is necessary to increase the capacity of the battery pack, but increasing the capacity may result in longer charging times.

[0016] Rapid charging can be performed to increase the charging rate in order to shorten the charging time, but in the case of conventional lithium-ion secondary batteries, rapid charging has the disadvantage of accelerating the deterioration of the lithium-ion secondary battery, making it unusable and shortening the time to replace it.

[0017] Therefore, one of the objectives is to improve the secondary battery, reduce damage to the secondary battery and suppress deterioration even when the charge rate is increased, that is, to provide a secondary battery with good rate characteristics.

[0018] Another object of one embodiment of the present invention is to provide a secondary battery with favorable cycle characteristics, a secondary battery with higher capacity, or a novel power storage device.

[0019] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims. [Means for solving the problem]

[0020] One embodiment of the present invention provides a positive electrode active material layer including a first graphene layer, a second graphene layer, and a positive electrode active material. The first graphene layer has a first region covering the positive electrode active material. The second graphene layer has a second region covering the positive electrode active material and a third region overlapping with the first region. The first region is located between the positive electrode active material and the third region and has a plane formed of arranged hexagonal carbon rings. The positive electrode active material has a fourth region having a layered rock-salt structure and a crystal structure of lithium cobalt oxide represented by an R-3m space group, or a crystal structure of lithium nickel-cobalt-manganese oxide, or a crystal structure of a substance represented by LiMO2 (M is a metal element). The lithium layer with the layered rock-salt structure in the fourth region is approximately perpendicular to the plane formed by the hexagonal carbon rings in the second region. In the above configuration, it is preferable that the positive electrode active material further includes a third graphene layer, the third graphene layer having a surface composed of arranged six-membered carbon rings, and the positive electrode active material has a fifth region having a layered rock-salt structure and a crystal structure of lithium cobalt oxide expressed by the R-3m space group, or a crystal structure of lithium nickel-cobalt-manganese oxide, or a crystal structure of a substance expressed by LiMO2 (M is a metal element), and that the surface composed of the six-membered carbon rings of the third graphene layer and the (104) plane of the layered rock-salt structure of the fifth region are approximately parallel to each other.

[0021] Alternatively, one embodiment of the present invention is a positive electrode active material layer including a first graphene layer, a second graphene layer, and a positive electrode active material. The first graphene layer has a first region covering the positive electrode active material. The second graphene layer has a second region covering the positive electrode active material and a third region overlapping with the first region. The first region is located between the positive electrode active material and the third region and has a plane formed of arranged hexagonal carbon rings. The positive electrode active material has a fourth region having a layered rock-salt structure and a crystal structure of lithium cobalt oxide represented by the R-3m space group, or a crystal structure of lithium nickel-cobalt-manganese oxide, or a crystal structure of a substance represented by LiMO2 (M is a metal element). A (104) plane of the crystal structure of the fourth region is approximately parallel to the plane formed by the hexagonal carbon rings of the second region.

[0022] In the above-described structure, the fourth region preferably includes the surface of the positive electrode active material, or is preferably located within a range of less than 30 nm from the surface of the positive electrode active material.

[0023] In the above structure, the first graphene layer and the second graphene layer are preferably reduced graphene oxide layers. In the above structure, the positive electrode active material preferably contains lithium, nickel, cobalt, manganese, magnesium, oxygen, and fluorine.

[0024] Alternatively, one embodiment of the present invention includes an active material layer including a sheet-like carbon-containing compound and the carbon-containing compound having a first region located on the active material, a second region located on the active material, and a third region located on the active material, the second region being thicker than the first and third regions, the distance between the third region and the surface of the active material being longer than the distance between the second region and the surface of the active material, and the active material having a fourth region having a layered rock-salt structure, the plane of the layer having the layered rock-salt structure in the fourth region being approximately perpendicular to the surface of the second region. In the above configuration, the distance between the third region and the surface of the active material is preferably longer than the distance between the first region and the surface of the active material.

[0025] In the above structure, the fourth region is preferably a region including the surface of the active material, or is preferably located within a range of a distance of less than 30 nm from the surface of the active material.

[0026] In the above structure, the carbon-containing compound preferably includes graphene.

[0027] Another embodiment of the present invention includes the above-described positive electrode active material layer and a current collector, in which the positive electrode active material layer is a positive electrode provided over the current collector.

[0028] Another embodiment of the present invention is a secondary battery including a positive electrode having the above-described positive electrode active material layer, a negative electrode, and an electrolyte.

[0029] Another aspect of the present invention is a vehicle including the above-described secondary battery, an electric motor, and a control device, wherein the control device has a function of supplying electric power from the secondary battery to the electric motor.

[0030] In this specification, graphene has a carbon hexagonal lattice structure and includes single-layer graphene and multi-layer graphene having 2 to 100 layers. 2 A graphene compound refers to a single atomic layer of carbon molecular sheets with bonds. The term "multiple graphenes" refers to multi-layer graphene or multiple single-layer graphenes. Graphene is not limited to being composed solely of carbon; it may be partially bonded with oxygen, hydrogen, or functional groups, and can be called a graphene compound. Graphene compounds can have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds also have a planar shape. Graphene compounds enable surface contact with low contact resistance. Even thin graphene compounds can have very high conductivity, allowing efficient formation of conductive paths within the active material layer with a small amount. A spray-drying device can be used to form a graphene compound coating covering the entire surface of the active material. Here, graphene, multi-graphene, or RGO are particularly preferred. Here, RGO (reduced graphene oxide) refers to a compound obtained by reducing graphene oxide (GO), for example. [Effects of the Invention]

[0031] According to one embodiment of the present invention, a secondary battery with favorable rate characteristics can be provided, or a safer secondary battery can be provided, or a novel power storage device can be provided. [Brief explanation of the drawings]

[0032] [Figure 1] Fig. 1A is a perspective view showing an electrode according to one embodiment of the present invention, and Fig. 1B, Fig. 1C, Fig. 1D, and Fig. 1E are cross-sectional views showing electrodes according to one embodiment of the present invention. [Figure 2] 2A and 2B are cross-sectional views of an electrode and an active material layer, respectively, showing one embodiment of the present invention. [Figure 3] 3A, 3B, and 3C are diagrams showing examples of graphene. [Figure 4] 4A, 4B, 4C, and 4D are diagrams relating to graphene according to one embodiment of the present invention. [Figure 5] 5A, 5B, and 5C are cross-sectional views showing active material layers according to embodiments of the present invention. [Figure 6] 6A and 6B are diagrams showing an example of a crystal structure. [Figure 7] FIG. 7 is a cross-sectional view showing an active material layer according to one embodiment of the present invention. [Figure 8] 8A and 8B are cross-sectional views showing active material layers according to embodiments of the present invention. [Figure 9] FIG. 9 is a diagram showing an example of a crystal structure. [Figure 10] FIG. 10 is a diagram showing an example of a crystal structure. [Figure 11] FIG. 11 is a diagram showing an example of a flow showing one embodiment of the present invention. [Figure 12] FIG. 12 is an example of a cross-sectional view showing a process of one embodiment of the present invention. [Figure 13] FIG. 13 is a STEM photograph of active material particles showing one embodiment of the present invention. [Figure 14] FIG. 14A is a STEM photograph showing a comparative example, and FIG. 14B is a partially enlarged photograph thereof. [Figure 15] FIG. 15A shows the conditions of this embodiment, and FIG. 15B shows a comparative example. [Figure 16] FIG. 16 is a diagram showing the cycle characteristics of the secondary battery. [Figure 17]FIG. 17A is a perspective view of a secondary battery, FIG. 17B is a cross-sectional perspective view thereof, and FIG. 17C is a cross-sectional schematic view thereof during charging. [Figure 18] 18A is a perspective view of a secondary battery, FIG. 18B is a cross-sectional perspective view thereof, FIG. 18C is a perspective view of a battery pack including a plurality of secondary batteries, and FIG. 18D is a top view thereof. [Figure 19] 19A and 19B are diagrams illustrating an example of a secondary battery. [Figure 20] 20A and 20B are diagrams illustrating an example of a secondary battery. [Figure 21] 21A and 21B are diagrams illustrating an example of a secondary battery. [Figure 22] 22A, 22B, 22C, 22D, and 22E are perspective views showing electronic devices. [Figure 23] Figure 23A shows the charge rate characteristics, and Figure 23B shows the discharge rate characteristics. [Figure 24] 24A and 24B show the cycle characteristics. [Figure 25] 25A and 25B show the cycle characteristics. [Figure 26] 26A and 26B are SEM images. [Figure 27] 27A and 27B are SEM images. DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0034] (Embodiment 1) In this embodiment, an electrode for a secondary battery according to one embodiment of the present invention will be described.

[0035] FIG. 1A is a perspective view of an electrode 200. While FIG. 1A shows the electrode 200 in the form of a rectangular sheet, the shape of the electrode 200 is not limited to this and any shape can be appropriately selected. The electrode 200 is produced by applying an electrode paste to a current collector 201 and then drying the paste in a reducing atmosphere or under reduced pressure to form an active material layer 202. In FIG. 1A, the active material layer 202 is formed on only one side of the current collector 201, but the active material layer 202 may be formed on both sides of the current collector 201. Furthermore, the active material layer 202 does not need to be formed on the entire surface of the current collector 201; an uncoated region, such as a region for connecting to an electrode tab, can be appropriately provided.

[0036] The current collector 201 can be made of a material that is highly conductive and does not alloy with carrier ions such as lithium, such as metals such as stainless steel, gold, platinum, zinc, iron, copper, aluminum, and titanium, or alloys thereof. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can also be used. The current collector 201 can also be made of a metal element that reacts with silicon to form silicide. Examples of metal elements that react with silicon to form silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector 201 can be in any suitable shape, such as a sheet, mesh, punched metal, or expanded metal. The current collector 201 preferably has a thickness of 10 μm to 30 μm.

[0037] A titanium compound may be provided as a current collector by being laminated on the metal element shown above. Examples of titanium compounds include titanium nitride, titanium oxide, titanium nitride in which part of the nitrogen is substituted with oxygen, titanium oxide in which part of the oxygen is substituted with nitrogen, and titanium oxynitride (TiO x N yOne, or two or more selected from (0 < x < 2, 0 < y < 1) can be mixed or laminated and used. Among them, titanium nitride is particularly preferable because it has high conductivity and a high function of suppressing oxidation. By providing a titanium compound on the surface of the current collector, for example, the reaction between the material and the metal element in the active material layer formed on the current collector can be suppressed. When the active material layer contains a compound having oxygen, the oxidation reaction between the metal element and oxygen can be suppressed. For example, when aluminum is used as the current collector and the active material layer is formed using graphene oxide described later, there may be a concern about the oxidation reaction between the oxygen in graphene oxide and aluminum. In such a case, by providing a titanium compound on aluminum, the oxidation reaction between the current collector and graphene oxide can be suppressed.

[0038] An example of a method for providing a titanium compound on the current collector will be described. A titanium compound layer may be provided on the current collector using particles of the titanium compound. Alternatively, a titanium compound layer may be provided on the current collector by a thin film method such as the ALD method or the sputtering method. Alternatively, a titanium compound may be provided on the current collector using the sol-gel method.

[0039] FIG. 1A is a perspective view showing an example of the electrode 200, and the electrode 200 has a current collector 201 and an active material layer 202 on the current collector 201.

[0040] FIG. 1B is an enlarged view of the region surrounded by the square broken line in FIG. 1A and is a schematic view showing a longitudinal section of the active material layer 202.

[0041] FIG. 1C shows an example in which a titanium compound 201a is provided on the surface of the current collector 201 in addition to FIG. 1B.

[0042] The surface of current collector 201 may have an uneven surface, as shown in an example in FIG. 1D , etc. When the surface of current collector 201 has an uneven surface, the surface area is increased, and the contact area with active material layer 202 can be increased. The increased contact area with active material layer 202 can increase the conductivity of active material layer 202. Furthermore, when the surface of current collector 201 has an uneven surface, adhesion to active material layer 202 can be improved in some cases.

[0043] As shown in FIG. 1E, the active material layer 202 may be provided on both surfaces of the current collector 201, rather than on just one surface.

[0044] Figure 2A is an enlarged view of the view shown in Figure 1B. Figure 2B shows an enlarged view of the area enclosed by the dashed square in Figure 2A.

[0045] 2B, the active material layer 202 includes an active material 203 and a carbon-containing compound 207. The active material layer 202 also includes a binder (not shown). In the example shown in FIG. 2B, the active material 203 includes particles.

[0046] The active material 203 is a granular positive electrode active material composed of secondary particles having an average particle size and particle size distribution, which are obtained by, for example, mixing raw material compounds in a predetermined ratio, firing the mixture, pulverizing, granulating, and classifying the fired product by an appropriate means. While the active material 203 is shown as a sphere in Fig. 2B and other figures, the shape is not limited to this.

[0047] The active material 203 can be a material that can insert and extract lithium ions.

[0048] In addition, when the carrier ions are alkali metal ions other than lithium ions and alkaline earth metal ions, alkali metals (e.g., sodium, potassium, etc.) and alkaline earth metals (e.g., calcium, strontium, barium, beryllium, magnesium, etc.) may be used instead of lithium in the lithium compound and lithium-containing composite oxide as the positive electrode active material.

[0049] When the active material 203 is a positive electrode active material, for example, a lithium-containing composite oxide having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure can be used.

[0050] It is preferable to use a positive electrode active material having a layered crystal structure as the positive electrode active material of one aspect of the present invention.

[0051] Examples of the layered crystal structure include a layered rock salt-type crystal structure. Examples of the lithium-containing composite oxide having a layered rock salt-type crystal structure include LiM x O y (where x > 0 and y > 0, more specifically, for example, y = 2 and 0.8 < x < 1.2). Here, M is a metal element, preferably one or more selected from cobalt, manganese, nickel, and iron. Alternatively, M is, for example, two or more selected from cobalt, manganese, nickel, iron, aluminum, titanium, zirconium, lanthanum, copper, and zinc.

[0052] LiM x O y Examples of the lithium-containing composite oxide represented by include LiCoO2, LiNiO2, LiMnO2, etc. Also, examples of the NiCo-based represented by LiNi x Co 1-x O2 (0 < x < 1). Examples of the lithium-containing composite oxide represented by LiM x O y include the NiMn-based represented by LiNi x Mn 1-x O2 (0 < x < 1).

[0053] Also, examples of the lithium-containing composite oxide represented by LiMO2 include LiNi x Co y Mn zExamples include NiCoMn-based (also referred to as NCM) represented by O2(x>0, y>0, 0.8<x + y + z<1.2). Specifically, for example, it is preferable to satisfy 0.1x<y<8x and 0.1x<z<8x. As an example, x, y, and z preferably satisfy x:y:z = 1:1:1 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 5:2:3 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 8:1:1 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 6:2:2 or values in the vicinity thereof. Or as an example, x, y, and z preferably satisfy x:y:z = 1:4:1 or values in the vicinity thereof.

[0054] Examples of the lithium-containing composite oxide having a layered rock salt-type crystal structure include Li2MnO3, Li2MnO3-LiMeO2 (Me is Co, Ni, Mn), etc.

[0055] In the positive electrode active material having a layered crystal structure represented by the above lithium-containing composite oxide, a secondary battery with a high lithium content per volume and a high capacity per volume may be realized. In such a positive electrode active material, the amount of lithium desorbed per volume during charging is also large, and in order to perform stable charge and discharge, stabilization of the crystal structure after desorption is required. Also, the crystal structure may collapse during charge and discharge, which may inhibit high-speed charging and high-speed discharging.

[0056] The positive electrode active material according to one aspect of the present invention has high stability of the crystal structure during charge and discharge and is also suitable for applications of high-speed charging and high-speed discharging.

[0057] When the active material 203 is a negative electrode active material, a material capable of lithium dissolution / precipitation or lithium ion insertion / desorption can be used. For example, lithium metal, carbon-based materials, alloy-based materials, etc. can be mentioned.

[0058] Lithium metal has a low redox potential (-3.045 V vs. the standard hydrogen electrode) and a high specific capacity per weight and volume (3860 mAh / g and 2062 mAh / cm, respectively). 3 ) and is therefore preferable.

[0059] Examples of carbon-based materials include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0060] Examples of graphite include artificial graphite such as mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite, and natural graphite such as spheroidized natural graphite.

[0061] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), graphite exhibits a potential as low as that of metallic lithium (0.05 to 0.3 V vs. Li / Li + This allows lithium-ion batteries to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, small volume expansion, low cost, and higher safety compared to lithium metal.

[0062] As the negative electrode active material, an alloy-based material capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium can also be used. When the carrier ion is lithium ion, examples of the alloy-based material include materials containing at least one of Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Zn, Cd, In, and Ga. These elements have a large capacity relative to carbon, and silicon in particular has a dramatically high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Examples of alloy-based materials using such elements include Mg2Si, Mg2Ge, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn.

[0063] In addition, as the negative electrode active material, SiO, SnO, SnO2, titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), molybdenum oxide (MoO2), and other oxides can be used.

[0064] In addition, the negative electrode active material is a composite nitride of lithium and transition metals, Li3N-type 3-x M x N (M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 N3 is preferable because it exhibits a large charge / discharge capacity (900 mAh / g).

[0065] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, which is preferable because it can be combined with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material containing lithium ions is used as the positive electrode active material, the composite nitride of lithium and a transition metal can be used as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0066] In addition, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not undergo an alloying reaction with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 This phenomenon also occurs with sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N and Ge3N4, phosphides such as NiP2, FeP2 and CoP3, and fluorides such as FeF3 and BiF3. Note that the above fluorides have high potentials and may therefore be used as positive electrode active materials.

[0067] The active material layer of one embodiment of the present invention preferably contains a carbon-containing compound, which preferably functions as a conductive additive in the active material layer.

[0068] The carbon-containing compound of one embodiment of the present invention can impart electrical conductivity to the positive electrode active material in the positive electrode active material layer. The lithium insertion and desorption reactions in the positive electrode active material are, for example, oxidation-reduction reactions involving electron transfer. Therefore, imparting high electrical conductivity to the positive electrode active material by the carbon-containing compound of one embodiment of the present invention helps to achieve high-speed charging and discharging. Therefore, the use of the positive electrode active material layer of one embodiment of the present invention can realize high-speed charging and discharging of a secondary battery.

[0069] In the positive electrode active material layer of one embodiment of the present invention, a carbon-containing compound can impart high conductivity to the positive electrode active material and form an excellent conductive path in the active material layer, so that high output characteristics can be achieved even when the positive electrode active material layer is thick. For example, excellent output characteristics can be achieved even when the positive electrode active material layer is 40 μm or more, or even 55 μm or more. Increasing the thickness of the positive electrode active material layer can increase the capacity per volume of the secondary battery, and when the secondary battery is installed in an electric vehicle, the driving distance can be increased.

[0070] The carbon-containing compound preferably has a shape such as a sheet, a plate, or a plane. When the carbon-containing compound has a sheet or plate shape, the phrase "the predetermined plane and the carbon-containing compound are parallel" refers to, for example, that the thickness direction of the sheet or plate is in the same direction as the normal vector of the predetermined plane. Alternatively, the surface of the sheet or plate is parallel to the predetermined plane. The sheet-like shape of the carbon-containing compound enables surface contact with the active material with low contact resistance. Furthermore, covering a portion of the surface of the active material that is approximately perpendicular to the direction in which lithium insertion and desorption occurs may impart conductivity to the active material, thereby promoting lithium diffusion.

[0071] As used herein, "parallel" refers to a state in which two straight lines are arranged at an angle of -10° or more and 10° or less. This therefore includes cases in which the angle is -5° or more and 5° or less. "Substantially parallel" or "roughly parallel" refers to a state in which two straight lines are arranged at an angle of -30° or more and 30° or less. "Perpendicular" refers to a state in which two straight lines are arranged at an angle of 80° or more and 100° or less. This therefore includes cases in which the angle is 85° or more and 95° or less. "Substantially perpendicular" or "approximately perpendicular" refers to a state in which two straight lines are arranged at an angle of 60° or more and 120° or less.

[0072] Furthermore, when a sheet-like carbon-containing compound covers an active material, it may cover the entire active material, but it is preferable to cover only a portion of the active material. For example, when the active material is a particle, it is preferable to cover a portion of the particle. Furthermore, when a sheet-like carbon-containing compound has a curved shape, it is preferable that at least a portion of the sheet-like carbon-containing compound does not adhere tightly to the particle, and a space is provided between the carbon-containing compound and the particle through which lithium ions can enter and exit. For example, it is preferable to have a space between the carbon-containing compound and the particle through which an electrolyte solution can penetrate.

[0073] An example of the carbon-containing compound is graphene. Graphene has a shape such as a sheet, a plate, or a plane. Graphene preferably has a curved shape.

[0074] Alternatively, a graphene compound can be used as an example of the carbon-containing compound. The graphene compound is, for example, graphene having functional groups, characteristic groups, etc. The carbon-containing compound may be a graphene mixed sheet that contains graphene and is mixed with other materials and has a sheet-like shape. The carbon-containing compound may also be a sheet-like carbon-containing compound made of a plurality of graphenes.

[0075] Graphene or a graphene compound enables surface contact with low contact resistance. Furthermore, even when the graphene or graphene compound is thin, it may have very high conductivity, and a small amount of graphene or a graphene compound can efficiently form a conductive path within the active material layer. Therefore, using graphene or a graphene compound as a conductive additive is preferable because it can increase the contact area between the active material and the conductive additive. It is also preferable because it can sometimes reduce electrical resistance.

[0076] Graphene or a graphene compound has high flexibility and high mechanical strength. Therefore, by using an electrode containing an active material having graphene or a graphene compound formed on its surface in a battery, even if the active material expands and contracts due to repeated charge and discharge of the battery, the active material can be prevented from cleaving and cracking due to volume changes. Furthermore, the mechanical strength of the graphene or graphene compound can alleviate the pressure applied to the active material during electrode rolling in the electrode production process. This prevents the active material from cleaving and cracking, or from developing crystal defects, even when force is applied to the electrode.

[0077] Furthermore, there is a concern that the carbon-containing compound may decompose the electrolyte under high temperatures, high voltages, etc. The decomposition of the electrolyte may form a coating on the surface of the active material, which may increase the resistance of the electrode during charging and discharging. The decomposition of the electrolyte may also generate gas, which may create areas where contact between the active material and the electrolyte is difficult. Such decomposition reactions are thought to occur between the surface of the carbon-containing compound and the electrolyte. Graphene has a small surface area and can form an excellent conductive path even with a small amount added, so surface reactions can be suppressed.

[0078] Table 1 shows the specific surface areas of acetylene black (AB), vapor-grown carbon fiber (VGCF) (registered trademark), and graphene oxide (referred to as GO in the table) as examples of conductive additives, measured by the BET method. Graphene oxide was measured after its aqueous solution was dried by spray drying. It can be seen that the specific surface area of ​​acetylene black is significantly larger, while that of graphene oxide is smaller.

[0079] [Table 1]

[0080] 3A to 3C are diagrams showing examples of top views of graphene or graphene compounds in various shapes.

[0081] 3A is a diagram showing an example of the length 213 of one side of graphene 214. As shown in FIG. 3B, in a top view of graphene 214, the smallest circle that can contain graphene 214 may be created, and the diameter of the circle may be set as length 213 of one side. As shown in FIG. 3C, it is preferable that protrusion 212 is not included in length 213 of one side. Furthermore, length 213 of one side of graphene 214 is preferably 50 nm or more and 100 μm or less, more preferably 800 nm or more and 20 μm or less.

[0082] The graphene may be in the form of a single sheet of partially overlapping graphene. The overlapping graphene may have a region with a thickness of 0.33 nm to 50 μm, preferably 0.34 nm to 10 μm, and more preferably 0.34 nm to 50 nm.

[0083] As shown in FIG. 2B and other figures, a plurality of granular active materials 203 are covered with a plurality of sheet-like or flat-plate-like carbon-containing compounds 207. One carbon-containing compound 207 may be electrically connected to a plurality of granular active materials 203. A plurality of granular active materials 203 may also form an aggregate. The carbon-containing compound 207 may be arranged so as to surround the aggregate. A single carbon-containing compound 207 may also be electrically connected to a plurality of granular active materials 203 included in the aggregate.

[0084] When graphene is used as the carbon-containing compound 207, the graphene does not necessarily overlap with other graphene only on the surface of the active material layer 202, but a portion of the graphene is provided between the multiple active material layers 202. Furthermore, since graphene is an extremely thin film (sheet) composed of, for example, a single layer of carbon molecules or a laminate of these carbon molecules, it covers and contacts a portion of the surface of each active material particle 203, tracing the surface of the active material particle 203. Therefore, the portion that is not in contact with the active material particle 203 is bent, wrinkled, or stretched and taut between the multiple active material particles 203.

[0085] Here, a plurality of graphenes are bonded together to form a mesh-like graphene (hereinafter referred to as a graphene net). When the graphene net covers an active material, the graphene net can also function as a binder that binds the active material together. This allows the amount of binder to be reduced or eliminated, thereby improving the ratio of the active material to the electrode volume or weight. In other words, the capacity of the power storage device can be increased.

[0086] Graphene will be described with reference to FIG. 4. As shown in FIG. 4A, graphene 101 is, for example, a layer arrangement of carbon atoms and has six-membered rings made of carbon. As shown in FIG. 4, the six-membered carbon rings are arranged to form planes. Graphene 101 has π bonds between carbon atoms. In addition to the six-membered rings made of carbon, graphene 101 may also have five-membered rings made of carbon or seven or more-membered rings made of carbon. Here, in the vicinity of the multi-membered rings other than the six-membered rings, regions through which lithium ions can pass may be generated.

[0087] FIG. 4B shows how multiple graphenes 101 are stacked.

[0088] Furthermore, graphene having 2 to 100 layers (sometimes called multi-graphene) can be used as the graphene.

[0089] Alternatively, reduced graphene oxide (sometimes called RGO: Reduced Graphene Oxide) may be used as graphene.

[0090] Graphene oxide (sometimes called graphene oxide) may have one or more of an oxygen-containing substituent, an oxygen-containing functional group, an oxygen-containing characteristic group, and oxygen. Examples of oxygen-containing functional groups include an epoxy group, a carbonyl group such as a carboxyl group, or a hydroxyl group. Figure 4C shows an example of graphene oxide.

[0091] Graphene oxide is easily dispersed in polar solvents due to its functional groups. Figure 4D shows an example of graphene oxide being dispersed in N-methyl-2-pyrrolidone (NMP). NMP is a compound with a five-membered ring structure and a polar solvent. The oxygen in NMP is electrically biased toward the negative (-) side, while the carbon double bonded to the oxygen is electrically biased toward the positive (+) side. Graphene oxide is added to a dilute solvent with this polarity. Because the oxygen in the functional groups of graphene oxide is negatively charged, different graphene oxides do not easily aggregate in polar solvents. However, they have a strong interaction with the polar solvent, NMP. Therefore, the functional groups, such as epoxy groups, present in graphene oxide interact with the polar solvent, inhibiting aggregation of graphene oxides. As a result, graphene oxide is uniformly dispersed in the dispersion medium.

[0092] Reduced graphene oxide may contain some oxygen or oxygen-containing substituents, functional groups, or characteristic groups, such as epoxy groups, carbonyl groups such as carboxyl groups, or hydroxyl groups.

[0093] The length of one side of the graphene oxide (also referred to as flake size) is 50 nm to 100 μm, preferably 800 nm to 20 μm. In particular, when the flake size is smaller than the average particle size of the active material particles 203, it becomes difficult for the graphene particles to come into surface contact with the active material particles 203 and to connect with each other, making it difficult to improve the electronic conductivity of the active material layer 202.

[0094] Graphene oxide has extremely high dispersibility in polar solvents due to its functional groups. Therefore, when graphene oxide and an active material are mixed in the process of forming the active material layer, they disperse well. After mixing, the graphene oxide is reduced to obtain reduced graphene. By forming the active material layer using graphene oxide, graphene can be dispersed uniformly within the active material layer 202. The solvent is volatilized and removed from the dispersion medium containing uniformly dispersed graphene oxide, and the graphene oxide is reduced to obtain graphene. The graphene remaining in the active material layer 202 partially overlaps with each other and is dispersed to such an extent that it is in surface contact with each other, thereby forming a three-dimensional conductive path. The reduction of graphene oxide may be performed, for example, by heat treatment or using a reducing agent.

[0095] Fig. 5A shows an example of active material 203 and carbon-containing compound 207 in a cross section of active material layer 202. In addition, Fig. 5B and Fig. 5C show enlarged views of the region indicated by the dashed square line in Fig. 5A.

[0096] 5B, consider the case where carbon-containing compound 207 is graphene. In FIG. 5B, the surface of active material 203 is covered with carbon-containing compound 207a, which is a first graphene, and carbon-containing compound 207b, which is a second graphene. Carbon-containing compound 207a overlaps with carbon-containing compound 207b and has a region sandwiched between active material 203 and carbon-containing compound 207b.

[0097] Here, in FIG. 5B, carbon-containing compound 207a and carbon-containing compound 207b may be considered together as a continuous carbon-containing compound. In such a case, for example, in FIG. 5B, in the continuous carbon-containing compound, region 221 is a region of carbon-containing compound 207a that does not overlap with carbon-containing compound 207b, region 222 is a region where carbon-containing compound 207a and carbon-containing compound 207b overlap, and region 223 is a region of carbon-containing compound 207b that does not overlap with carbon-containing compound 207a. In FIG. 5B, region 222 is thicker than regions 221 and 223. Also, in FIG. 5B, the distance between region 223 and the surface of active material 203 is longer than the distance between region 222 and active material 203. Also, the distance between region 223 and the surface of active material 203 is longer than the distance between region 221 and active material 203. The distance between region 223 and the surface of active material 203 may be, for example, the distance between the surface of region 223 that is closest to active material 203. The distance between region 222 and the surface of active material 203 may be, for example, the distance between the surface of region 222 that is closest to active material 203. The distance between region 221 and the surface of active material 203 may be, for example, the distance between the surface of region 221 that is closest to active material 203.

[0098] In a layered crystal structure, when a metal element that serves as a carrier ion, such as lithium, is arranged in layers, the metal element may tend to diffuse along the layers.

[0099] FIG. 6A shows the crystal structure of LiCoO2, which has a layered rock-salt structure. In FIG. 6A, lithium atoms are arranged in layers perpendicular to the c-axis (hereinafter referred to as lithium layers), and are alternately stacked with CoO2 layers containing cobalt and oxygen. Lithium atoms easily diffuse along the lithium layers. Therefore, by exposing the cross-sections of the lithium layers at the surface of the active material 203, lithium atoms can be easily inserted into and removed from the lithium layers. For example, when the active material 203 has a layered rock-salt structure, it is preferable that the cross-sections of the lithium layers be exposed at the surface of the active material 203.

[0100] For example, in an active material represented by LiMO2 (M is a metal element) and having a space group of R-3m, it is preferable that a plane including the c-axis is exposed. Alternatively, for example, it is preferable that a plane approximately perpendicular to the ab plane is exposed, for example, it is preferable that a plane forming an angle of 70° to 110° with the ab plane is exposed. Here, the ab plane is a plane including the a-axis direction and the b-axis direction within the plane. Furthermore, in an active material represented by LiMO2 (M is a metal element) and having a space group of R-3m, the surface energy of the (104) plane is stable. Therefore, for example, it is preferable that the (104) plane is exposed on the surface of the positive electrode active material.

[0101] Alternatively, the cross section of the lithium layer may not be exposed on the outermost surface of the active material 203. For example, a coating may be provided on the cross section of the lithium layer, and the carbon-containing compound 207 may be in contact with the coating. Alternatively, the cross section of the lithium layer may have a region with low crystallinity, and the carbon-containing compound 207 may be in contact with the region.

[0102] Figure 6B is a simplified diagram of the crystal structure of LiCoO2 for explanatory purposes. Lithium ions 884 present in the electrolyte receive electrons and enter the crystal from lithium layer 883. Lithium atoms 881 that have entered the crystal diffuse along lithium layer 883. For simplification, only cobalt atoms 882 in the CoO2 layer are shown in Figure 6B and Figure 7, which will be described later.

[0103] Fig. 7 is a schematic diagram showing an enlarged portion of Fig. 5B. Note that in Fig. 7, atoms such as carbon atoms are omitted from the carbon-containing compound.

[0104] In FIG. 7, carbon-containing compound 207a covers a portion of the region where the cross section of the lithium layer is exposed in active material 203. Carbon-containing compound 207a and carbon-containing compound 207b are stacked approximately parallel to the surface of active material 203. Both carbon-containing compound 207a and carbon-containing compound 207b preferably have a sheet-like shape, and stacking of sheet-like carbon-containing compounds can form a path for lithium to pass through. When lithium exists as lithium ions in the electrolyte, it is considered that the solvent solvates the lithium ions in at least some of the lithium ions. When lithium ions pass between carbon-containing compound 207a and carbon-containing compound 207b, the lithium ions may be desolvated. Lithium ions 884 pass between the stacked carbon-containing compounds 207a and 207b and are guided to the vicinity of the region where the cross section of the lithium layer is exposed on the surface of active material 203. Lithium ions 884 reach the region where the cross section of the lithium layer is exposed on the surface of the active material 203 , receive electrons, and lithium enters the active material 203 .

[0105] The active material 203 has a surface or a region near the surface that is covered with the carbon-containing compound 207a (hereinafter referred to as the first region). The first region of the active material 203 has a layered crystal structure. The carbon-containing compound 207a has a portion that overlaps with the first region of the active material 203 (hereinafter referred to as the first portion). The carbon-containing compound 207a is preferably in contact with the first region of the active material 203. Alternatively, the distance between the carbon-containing compound 207a and the first region of the active material 203 is preferably less than 30 nm, less than 15 nm, or less than 5 nm, for example. The first portion of the carbon-containing compound 207a overlaps with the carbon-containing compound 207b.

[0106] The first portion of the carbon-containing compound 207a is preferably parallel or approximately parallel to a plane including the c-axis direction of the crystal structure of the first region of the active material 203.

[0107] Alternatively, the first portion of the carbon-containing compound 207a is preferably parallel or approximately parallel to the (104) plane of the crystal structure of the first region of the active material 203.

[0108] Alternatively, the first portion of the carbon-containing compound 207a preferably forms an angle of 70° or more and 110° or less with respect to the ab plane of the crystal structure of the first region of the active material 203.

[0109] The vicinity of the surface of the active material 203 is, for example, a region that is less than 30 nm, less than 15 nm, or less than 5 nm away from the surface.

[0110] Furthermore, the portions of the graphene that are not in contact with the active material 203 may be bent, wrinkled, or stretched and taut between the multiple particles of active material 203. In Fig. 5C, the surfaces of active material 203a, which is the first active material, and active material 203b, which is the second active material, are covered with carbon-containing compound 207c, which is the third graphene. Carbon-containing compound 207c has a bent shape between active material 203a and active material 203b.

[0111] FIG. 8A shows an enlarged view of a portion of the cross section of an example of the active material layer 202. As shown in FIG. 8A, in the active material layer 202, secondary particles 208 may be formed from a plurality of active materials 203. Forming secondary particles 208 from a plurality of active materials 203 may increase the strength of the active material layer 202. The strength of the active material layer 202 refers, for example, to the strength of resistance to a peel test or the suppression of the active material from collapsing from the active material layer 202 after charge and discharge. Alternatively, forming secondary particles 208 from a plurality of active materials 203 may facilitate increasing the density of the active material layer 202. Increasing the density of the active material layer 202 can increase, for example, the energy density of a secondary battery. The secondary particles are, for example, aggregates formed by a plurality of active materials.

[0112] For example, as shown in FIG. 8B, the carbon-containing compound 207 preferably covers at least a portion of the secondary particles 208.

[0113] When the active material 203 is granular, the average particle size of the primary particles is, for example, 10 nm or more and 100 μm or less. When the average particle size of the primary particles is, for example, less than 5 μm, less than 3 μm, 1.5 μm or less, 900 nm or less, or 300 nm or less, the active material 203 preferably forms secondary particles in the active material layer.

[0114] The average particle size of the secondary particles is preferably 1 μm or more and 100 μm or less, and more preferably 3 μm or more and 30 μm or less.

[0115] Furthermore, when a first active material and a second active material having different average particle sizes are mixed in the active material layer, it is preferable that more secondary particles are formed in the active material with the smaller average particle size.

[0116] If the first active material and the second active material are different materials, the discharge curves, more specifically, the discharge voltage and the gradient of the discharge curve, may differ. In such cases, mixing the first active material and the second active material may cause an inflection point in the discharge curve of the secondary battery, making it easier to detect the capacity. Furthermore, mixing the first active material and the second active material may suppress deterioration of the secondary battery.

[0117] A positive electrode active material layer according to one embodiment of the present invention preferably includes, for example, lithium cobalt oxide having an average primary particle size of 5 μm or more and 100 μm or less, and lithium nickel cobalt manganese oxide having an average primary particle size of 10 nm or more and less than 5 μm, and the lithium nickel cobalt manganese oxide forms secondary particles having an average secondary particle size of 5 μm or more and 100 μm or less.

[0118] Furthermore, the active material layer 202 may have particulate carbon-containing compounds 207x (see FIG. 8B) in addition to the sheet-like carbon-containing compounds 207.

[0119] Furthermore, the active material layer 202 may have, in addition to the sheet-like carbon-containing compound 207, a fibrous carbon-containing compound 207y (see FIG. 8B).

[0120] Examples of particulate carbon-containing compounds that can be used include carbon materials such as carbon black (eg, AB) and graphite particles.

[0121] An example of a fibrous carbon-containing compound is VGCF (registered trademark). Alternatively, fibrous graphene, a compound in which graphene is rolled up into a carbon nanofiber, or the like can be used. Alternatively, a conductive polymer can be used. Alternatively, the carbon-containing compound may be in the form of a thread. It is preferable that the fibrous or thread-like carbon-containing compounds contact each other to form a conductive path. It is also preferable that the fibrous or thread-like carbon-containing compound and the sheet-like carbon-containing compound contact each other to form a conductive path. It is preferable that the conductive path formed by the carbon-containing compound is electrically connected to the active material 203.

[0122] Particulate carbon-containing compounds and fibrous carbon-containing compounds can easily enter tiny spaces. By combining a carbon-containing compound that can easily enter tiny spaces with a sheet-like carbon-containing compound that can impart conductivity across multiple particles, an excellent conductive path can be formed.

[0123] The surface resistivity of the graphene of one embodiment of the present invention is 1×10 4 It is preferably Ω / □ or less, and more preferably 50Ω / □ or less.

[0124] When the graphene has two or more layers, the interlayer distance between adjacent graphene layers is preferably 0.33 nm to 0.5 nm, more preferably 0.34 nm to 0.5 nm. The interlayer distance of graphene can be evaluated, for example, by observing the cross section with a transmission electron microscope (TEM). The interlayer distance can also be calculated (as the interplanar spacing) using, for example, X-ray diffraction (XRD).

[0125] The graphene of one embodiment of the present invention has an oxygen concentration of, for example, 20 atomic % or less, preferably 2 atomic % or more and 20 atomic % or less, more preferably 2 atomic % or more and 11 atomic % or less, and still more preferably 3 atomic % or more and 10 atomic % or less, relative to the entire graphene, as measured by XPS (X-ray photoelectron spectroscopy).

[0126] In addition, when graphene is measured by XPS, the spectrum of binding energy corresponding to C1s of carbon is analyzed by waveform separation, and the sp 2 The ratio of the peaks indicating sp 2 The proportion of sp is preferably 50% or more and 90% or less of the entire spectrum of C1s. 2 For example, by increasing the proportion of , the conductivity of graphene can be increased.

[0127] Note that the physical properties such as the lattice spacing, oxygen concentration, and electrical conductivity described above are merely examples, and the graphene compound of one embodiment of the present invention is not limited thereto.

[0128] Furthermore, the graphene oxide of one embodiment of the present invention may have lower electrical conductivity than graphene. For example, the surface resistivity of graphene oxide is measured to be 1×10 5 It may have a value greater than Ω / □.

[0129] The oxygen concentration of the graphene oxide of one embodiment of the present invention may exceed 30 atomic % relative to the entire graphene oxide, as measured by XPS (X-ray photoelectron spectroscopy). The conductivity of the graphene oxide can be increased by reducing it. The oxygen concentration of the graphene oxide of one embodiment of the present invention may be reduced to 20 atomic % or less by reducing it.

[0130] Note that the above description is an example, and the graphene of one embodiment of the present invention is not limited thereto.

[0131] In addition, the binder contained in the active material layer 202 may be a typical polyvinylidene fluoride (PVDF), as well as polyimide, polytetrafluoroethylene, polyvinyl chloride, ethylene propylene diene polymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, etc.

[0132] The amount of reduced graphene oxide contained in the positive electrode active material layer is 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%, based on the total amount of the active material, conductive additive, and binder. The binder may be contained in an amount of 1 wt% to 10 wt%, for example.

[0133] The density of the active material layer is, for example, preferably 30% or more, more preferably 50% or more, and even more preferably 70% or more of the density of the material used as the active material. When the active material layer of one embodiment of the present invention uses LiFePO4 as the active material, the density of the active material layer is preferably 1.1 g / cm 3 , more preferably 1.8 g / cm 3 More preferably, 2.6 g / cm 3 That's all.

[0134] Graphene is formed, for example, by performing a reduction treatment on graphene oxide, which has an oxygen to carbon atomic ratio of 0.405 or more.

[0135] Graphene oxide, which has an oxygen to carbon atomic ratio of 0.405 or more, can be produced using an oxidation method called the Hummers method.

[0136] The Hummers method involves adding a sulfuric acid solution of potassium permanganate, hydrogen peroxide, or other chemicals to graphite powder to induce an oxidation reaction, producing a dispersion containing graphite oxide. Oxidation of the carbon in graphite bonds functional groups such as epoxy groups, carbonyl groups, carboxyl groups, and hydroxyl groups to graphite oxide. This results in a longer interlayer distance between multiple graphene layers than in graphite, facilitating separation and flaking. Next, ultrasonic vibrations are applied to the dispersion containing graphite oxide to cleave the graphite oxide with a long interlayer distance, separating the graphene oxide and producing a dispersion containing graphene oxide. Finally, powdered graphene oxide can be obtained by removing the solvent from the dispersion containing graphene oxide.

[0137] Here, graphene oxide with an oxygen to carbon atomic ratio of 0.405 or more can be produced by appropriately adjusting the amount of an oxidizing agent such as potassium permanganate. That is, the degree of oxidation of graphene oxide (oxygen to carbon atomic ratio) can be increased by increasing the amount of oxidizing agent relative to graphite powder. Therefore, the amount of oxidizing agent relative to the graphite powder used as raw material can be determined according to the amount of graphene oxide to be produced.

[0138] Note that the method for preparing graphene oxide is not limited to the Hummers method using a sulfuric acid solution of potassium permanganate, and for example, a Hummers method using nitric acid, potassium chlorate, sodium nitrate, or the like, or a method for preparing graphene oxide other than the Hummers method may be used as appropriate.

[0139] Furthermore, graphite oxide may be flaked by applying ultrasonic vibration, or by irradiating it with microwaves, radio waves, or thermal plasma, or by applying physical stress.

[0140] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0141] (Embodiment 2) In this embodiment, graphene contained in an electrode included in a secondary battery of one embodiment of the present invention will be described.

[0142] Graphene is a carbon material with a crystalline structure in which a hexagonal carbon skeleton is extended into a plane. Graphene is an atomic plane of a graphite crystal, and has astonishing electrical, mechanical, and chemical properties. It has attracted attention for its potential applications in a variety of fields, including high-mobility field-effect transistors, highly sensitive sensors, highly efficient solar cells, and next-generation transparent conductive films.

[0143] In this specification, graphene includes single-layer graphene and multi-layer graphene having 2 to 100 layers. Single-layer graphene refers to a sheet of carbon molecules having one atomic layer with π bonds. Graphene oxide refers to a compound obtained by oxidizing the graphene. When graphene is formed by reducing graphene oxide, not all of the oxygen contained in the graphene oxide is released, and some oxygen remains in the graphene. When graphene contains oxygen, the proportion of oxygen is 2 to 20 atomic % of the entire graphene, preferably 3 to 15 atomic %, as measured by XPS.

[0144] Here, when the graphene is multilayer graphene, the graphene is obtained by reducing graphene oxide, and thus the interlayer distance of the graphene is 0.34 nm to 0.5 nm, preferably 0.38 nm to 0.42 nm, and more preferably 0.39 nm to 0.41 nm. While the interlayer distance of single-layer graphene in ordinary graphite is 0.34 nm, the graphene used in the secondary battery according to one embodiment of the present invention has a longer interlayer distance, which facilitates the movement of carrier ions between layers of the multilayer graphene.

[0145] In the electrode for a secondary battery according to one embodiment of the present invention, graphene is dispersed in an active material layer so as to overlap with and contact a plurality of active material particles. In other words, a network for electron conduction by graphene is formed in the active material layer. This maintains the bonding between the plurality of active material particles, resulting in the formation of an active material layer with high electron conductivity.

[0146] An active material layer containing graphene as a conductive additive can be produced by the following method. First, graphene is dispersed in a dispersion medium (also called a solvent), and then an active material is added and kneaded to produce a mixture. A binder is added to this mixture and kneaded to produce an electrode paste. Finally, the electrode paste is applied to a current collector, and the dispersion medium is volatilized to produce an active material layer containing graphene as a conductive additive.

[0147] Graphene oxide is a polar substance having functional groups such as epoxy groups, carbonyl groups, carboxyl groups, and hydroxyl groups. It is thought that the functional groups such as epoxy groups possessed by graphene oxide interact with polar solvents, inhibiting the aggregation of graphene oxide particles, resulting in uniform dispersion of graphene oxide in the dispersion medium.

[0148] From the above, in order to use graphene as a conductive additive and to construct a network with high electronic conductivity in the active material layer, it is very effective to use highly dispersible graphene oxide in the dispersion medium when preparing the electrode paste. The dispersibility of graphene oxide in the dispersion medium is thought to depend on the amount of oxygen-containing functional groups such as epoxy groups (in other words, the degree of oxidation of graphene oxide).

[0149] Therefore, one embodiment of the present invention is graphene oxide used as a raw material of a conductive additive for an electrode for a secondary battery, in which the atomic ratio of oxygen to carbon is 0.405 or more.

[0150] Here, the atomic ratio of oxygen to carbon is an index showing the degree of oxidation, and is the ratio of the weight of carbon and oxygen among the constituent elements of graphene oxide, based on carbon. The weight of the elements constituting graphene oxide can be measured, for example, by X-ray photoelectron spectroscopy (XPS).

[0151] The atomic ratio of oxygen to carbon in graphene oxide of 0.405 or more means that graphene oxide has high dispersibility in polar solvents, and is a polar substance with sufficient functional groups such as epoxy groups, carbonyl groups, carboxyl groups, and hydroxyl groups bonded to it.

[0152] Therefore, by dispersing and kneading graphene oxide, which has an atomic ratio of oxygen to carbon of 0.405 or more, in a dispersion medium together with an active material and a binder, applying the mixture to a current collector, and heating it, it is possible to form an electrode for a secondary battery that contains graphene that is highly dispersible and has an electron-conducting network.

[0153] The graphene oxide preferably has a side length of 50 nm or more and 100 μm or less, and more preferably 800 nm or more and 20 μm or less.

[0154] Another embodiment of the present invention is an electrode for a secondary battery including, on a current collector, an active material layer including a plurality of granular active material particles, a conductive additive including a plurality of graphenes, and a binder, in which the graphenes are larger than the average particle size of the granular active material particles, the graphenes are dispersed in the active material layer to be in surface-to-surface contact with one or more adjacent graphene particles, and the graphenes are in surface-to-surface contact with each other so as to wrap around part of a surface of the granular active material particles.

[0155] Another embodiment of the present invention is an electrode for a secondary battery including an active material layer over a current collector, the active material layer including a plurality of granular active materials, a conductive additive including a plurality of graphenes, and a binder, in which, with respect to a bonding state of carbon included in the active material layer, a proportion of C═C bonds is 35% or more and a proportion of C═C bonds is 5% or more and 20% or less.

[0156] Another embodiment of the present invention is a method for manufacturing an electrode for a secondary battery, comprising: dispersing graphene oxide having an oxygen to carbon atomic ratio of 0.405 or more in a dispersion medium; adding an active material to the dispersion medium in which the graphene oxide is dispersed and kneading the mixture; adding a binder to the mixture and kneading the mixture to prepare an electrode paste; applying the electrode paste to a current collector; and reducing the graphene oxide after or simultaneously with volatilizing the dispersion medium contained in the applied electrode paste, thereby forming an active material layer containing graphene on the current collector.

[0157] When graphene contains oxygen, the oxygen content is 2 atomic % to 20 atomic % of the entire graphene, preferably 3 atomic % to 15 atomic % as measured by XPS. The lower the oxygen content, the higher the conductivity of graphene, resulting in the formation of a network with high electron conductivity. Furthermore, the higher the oxygen content, the more gaps that serve as ion passages can be formed in graphene.

[0158] The amount of reduced graphene oxide contained in the positive electrode active material layer is in the range of 0.5 wt % to 10 wt %, preferably more than 1 wt % to 5 wt %.

[0159] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0160] (Embodiment 3) In this embodiment, an example of a structure of a positive electrode active material manufactured by a manufacturing method of one embodiment of the present invention will be described.

[0161] The positive electrode active material of one embodiment of the present invention contains fluorine. Fluorine can improve the wettability of the surface of the positive electrode active material and homogenize it. The resulting positive electrode active material is resistant to breakdown of its crystal structure during repeated high-voltage charge and discharge. A secondary battery using such a positive electrode active material exhibits significantly improved cycle characteristics.

[0162] In addition, the positive electrode active material of one embodiment of the present invention provides positive electrode active material particles that are less susceptible to deterioration by increasing the strength near the surface by controlling the surface roughness of the active material particles to a specific range. For example, the positive electrode active material particles are prepared by mixing lithium oxide and fluoride and heating the mixture.

[0163] If pure LiCoO2 is exposed on the surface of the positive electrode active material particles, unevenness will occur, and cobalt and oxygen will be released during charging and discharging, causing the crystal structure to collapse and degradation. To prevent this exposed pure LiCoO2 from being exposed on the surface, it is preferable to uniformly cover the surface with a compound containing magnesium. Magnesium has the function of maintaining the crystal structure (layered rock salt crystal structure) even when Li is released during discharging. The presence of magnesium (or fluorine) near the surface of the positive electrode active material particles is also a feature.

[0164] By adopting the above-described structure, cracks are less likely to occur when pressure is applied to a positive electrode containing a positive electrode active material during the production of a secondary battery, and the particle shape can be maintained. This reduces excess cracks, thereby increasing the electrode density.

[0165] If the surface irregularities are larger and rougher than the above range, physical cracks and collapse of the crystal structure may occur, which may expose the pure LiCoO2 on the surface and accelerate degradation.

[0166] The lithium oxide is preferably a material having a layered rock salt crystal structure, such as a composite oxide represented by LiMO2. Examples of the element M include one or more selected from Co and Ni. Examples of the element M include one or more selected from Co and Ni, as well as one or more selected from Al and Mg.

[0167] By incorporating fluorine near the surface, it is possible to distribute not only fluorine but also magnesium, aluminum, and nickel in high concentrations near the surface. Annealing with a lid prevents fluorine from diffusing outward as a gas, while other materials such as aluminum diffuse into the solid. Fluorine improves the wettability of the positive electrode active material surface, making it homogenous.

[0168] It is preferable that the composite oxide containing lithium, a transition metal, and oxygen has a layered rock-salt type crystal structure with few defects and distortion. Therefore, it is preferable that the composite oxide contains few impurities. If the composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or distortion.

[0169] In order to prevent impurities from being mixed, it is preferable to modify the surface of the positive electrode active material by heating with a lid on after mixing with the fluoride. The timing of the lid can be any one of placing the lid on the container before heating and then placing it in the heating furnace, placing the lid on the container after placing it in the heating furnace, or placing the lid on during heating before the fluoride melts.

[0170] [Positive electrode active material structure] Materials with a layered rock-salt crystal structure, such as lithium cobalt oxide (LiCoO2), are known to have high discharge capacity and are excellent as positive electrode active materials for secondary batteries. An example of a material with a layered rock-salt crystal structure is a composite oxide represented by LiMO2. Examples of element M include one or more selected from Co and Ni. Examples of element M include one or more selected from Co and Ni, as well as one or more selected from Al and Mg.

[0171] It is known that the strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal.

[0172] In compounds containing nickel, distortion may occur due to the Jahn-Teller effect. Therefore, when LiNiO2 is charged and discharged at high voltages, there is a concern that the crystal structure may collapse due to distortion. In LiCoO2, the influence of the Jahn-Teller effect is suggested to be small, and LiCoO2 may have better durability against charge and discharge at high voltages, which is preferable.

[0173] The positive electrode active material will be described with reference to Figures 9 and 10. Figures 9 and 10 describe the case where cobalt is used as the transition metal contained in the positive electrode active material.

[0174] The positive electrode active material manufactured according to one embodiment of the present invention can reduce the displacement of the CoO2 layer during repeated high-voltage charge and discharge. Furthermore, the volume change can be reduced. Therefore, the positive electrode active material according to one embodiment of the present invention can achieve excellent cycle characteristics. Furthermore, the positive electrode active material according to one embodiment of the present invention can have a stable crystal structure in a high-voltage charged state. Therefore, the positive electrode active material according to one embodiment of the present invention may be less susceptible to short circuits when maintained in a high-voltage charged state. In such cases, safety is further improved, which is preferable.

[0175] In the positive electrode active material of one embodiment of the present invention, the change in crystal structure and the difference in volume per the same number of transition metal atoms between a fully discharged state and a high-voltage charged state are small.

[0176] FIG. 9 shows the crystal structure of a positive electrode active material according to one embodiment of the present invention before and after charge and discharge. The positive electrode active material according to one embodiment of the present invention is a composite oxide containing lithium, cobalt, and oxygen. In addition to the above, it preferably contains magnesium. It also preferably contains a halogen such as fluorine or chlorine. It also preferably contains aluminum and nickel.

[0177] The crystal structure at a charge depth of 0 (discharged state) in Figure 9 is the same as that in Figure 10, R-3m (O3). On the other hand, the positive electrode active material shown in Figure 10 has a crystal structure different from the H1-3 crystal structure (space group R-3m) when fully charged. This structure is in the space group R-3m and is not a spinel crystal structure. However, ions such as cobalt and magnesium occupy six oxygen coordination positions, and the cation arrangement has a symmetry similar to that of a spinel structure. Furthermore, the symmetry of the CoO2 layer in this structure is the same as that of an O3 structure. Therefore, this structure is referred to herein as an O3' crystal structure or a pseudospinel crystal structure. Note that, in the diagram of the O3' crystal structure shown in Figure 9, lithium can be present at any lithium site with a probability of approximately 20%, but this is not limited to this. Lithium may also be present at only a specific portion of the lithium sites. In both the O3-type and O3'-type crystal structures, magnesium is preferably present in a dilute form between the CoO2 layers, i.e., at the lithium sites, and halogens such as fluorine are preferably present randomly and dilutely at the oxygen sites.

[0178] In the O3'-type crystal structure, light elements such as lithium may occupy the oxygen tetracoordination site, and in this case too, the arrangement of ions has a symmetry similar to that of the spinel type.

[0179] The O3'-type crystal structure can also be said to be a crystal structure similar to the CdCl2-type crystal structure, although it has random Li between the layers. This CdCl2-type-like crystal structure was observed when lithium nickel oxide was charged to a charge depth of 0.94 (Li 0.06 The crystal structure is similar to that of lithium cobaltate (NiO2), but it is known that pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt do not usually adopt this crystal structure.

[0180] Layered rock salt crystals and the anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in pseudospinel crystals also have a cubic close-packed structure. When these crystals contact, there is a crystal plane where the cubic close-packed structure formed by the anions is aligned. However, the space group of layered rock salt crystals and the O3'-type crystal structure is R-3m, which is different from the space groups of rock salt crystals, Fm-3m (the space group of general rock salt crystals) and Fd-3m (the space group of rock salt crystals with the simplest symmetry). Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between layered rock salt crystals and the O3'-type crystal structure and the rock salt crystal. In this specification, when the cubic close-packed structure formed by the anions is aligned in layered rock salt crystals, the O3'-type crystal structure, and the rock salt crystal, the crystal orientation may be said to be approximately aligned.

[0181] In the positive electrode active material according to one embodiment of the present invention, when a large amount of lithium is released during charging at a high voltage, the change in the crystal structure is suppressed more than in the comparative example described below. For example, as shown by the dotted line in Figure 9, there is almost no displacement of the CoO2 layers in these crystal structures.

[0182] More specifically, the cathode active material of one embodiment of the present invention exhibits high structural stability even at high charge voltages. For example, in a comparative example, the H1-3 crystal structure is formed at a voltage of approximately 4.6 V relative to the potential of lithium metal. However, the cathode active material of one embodiment of the present invention can maintain the R-3m(O3) crystal structure even at a charge voltage of approximately 4.6 V. Even at higher charge voltages, for example, at voltages of approximately 4.65 V to 4.7 V relative to the potential of lithium metal, the cathode active material of one embodiment of the present invention can adopt the O3' crystal structure. When the charge voltage is further increased above 4.7 V, the H1-3 crystal structure may finally be observed in the cathode active material of one embodiment of the present invention. Furthermore, at lower charge voltages (for example, even when the charge voltage is 4.5 V or more but less than 4.6 V relative to the potential of lithium metal), the cathode active material of one embodiment of the present invention may adopt the O3' crystal structure. Note that when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is approximately 0.05 V to 0.2 V relative to the potential of lithium metal. Therefore, even when the voltage of a secondary battery using graphite as the negative electrode active material is 4.3 V or higher and 4.5 V or lower, the positive electrode active material of one embodiment of the present invention can maintain the R-3m(O3) crystal structure. Furthermore, even when the charge voltage is higher, for example, when the voltage of the secondary battery is higher than 4.5 V and lower than 4.6 V, the positive electrode active material of one embodiment of the present invention can also adopt the O3'-type crystal structure. Furthermore, even when the charge voltage is lower, for example, when the voltage of the secondary battery is 4.2 V or higher but lower than 4.3 V, the positive electrode active material of one embodiment of the present invention may adopt the O3'-type crystal structure.

[0183] Therefore, in the positive electrode active material of one embodiment of the present invention, the crystal structure is not easily destroyed even when charge and discharge are repeated at a high voltage.

[0184] In addition, in a positive electrode active material according to one embodiment of the present invention, the difference in volume per unit cell between the O3-type crystal structure at a state of charge of 0 and the O3'-type crystal structure at a state of charge of 0.8 is 2.5% or less, more specifically 2.2% or less. Note that the O3'-type crystal structure can have the coordinates of cobalt and oxygen in the unit cell expressed as Co(0,0,0.5), O(0,0,x), where 0.20≦x≦0.25.

[0185] Magnesium, which exists randomly and dilutely between the CoO2 layers, i.e., at the lithium sites, has the effect of suppressing the displacement of the CoO2 layers when charged at high voltage. Therefore, when magnesium exists between the CoO2 layers, the O3'-type crystal structure is easily formed.

[0186] However, if the heat treatment temperature is too high, cation mixing occurs, increasing the possibility that magnesium will enter the cobalt site. Magnesium present in the cobalt site is ineffective in maintaining the R-3m structure during high-voltage charging. Furthermore, if the heat treatment temperature is too high, there are concerns that adverse effects such as cobalt being reduced to a divalent state and lithium evaporating may occur.

[0187] Therefore, it is preferable to add a halogen compound such as a fluorine compound to the lithium cobalt oxide before the heat treatment to distribute magnesium throughout the particles. The addition of the halogen compound lowers the melting point of the lithium cobalt oxide. Lowering the melting point makes it easier to distribute magnesium throughout the particles at a temperature where cation mixing is unlikely to occur. Furthermore, the presence of a fluorine compound is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0188] Note that increasing the magnesium concentration above a desired value may reduce the effect on stabilizing the crystal structure. This is thought to be because magnesium occupies not only the lithium site but also the cobalt site. The number of magnesium atoms in the positive electrode active material prepared according to one embodiment of the present invention is preferably 0.001 to 0.1 times the number of cobalt atoms, more preferably greater than 0.01 and less than 0.04, and even more preferably approximately 0.02. The magnesium concentration shown here may be, for example, a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using ICP-MS or the like, or may be based on the value of the raw material composition during the preparation of the positive electrode active material.

[0189] The number of nickel atoms in the positive electrode active material of one embodiment of the present invention is preferably 7.5% or less of the number of cobalt atoms, preferably 0.05% to 4%, and more preferably 0.1% to 2%. The nickel concentration shown here may be a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0190] <Particle size> If the particle size of the positive electrode active material of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector arise. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with the electrolyte occur. Therefore, the average particle size (D50: also referred to as median diameter) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less.

[0191] <Analysis method> Whether a certain positive electrode active material exhibits a pseudospinel crystal structure when charged at a high voltage can be determined by analyzing the positive electrode charged at a high voltage using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. XRD is particularly preferred because it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, and it can obtain sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery.

[0192] As mentioned above, positive electrode active materials are characterized by minimal change in their crystal structure between the high-voltage charged and discharged states. Materials with a crystal structure that exhibits significant changes between the high-voltage charged and discharged states (50 wt% or more) are undesirable because they cannot withstand high-voltage charging and discharging. It is important to note that simply adding impurity elements may not result in the desired crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine has the same characteristics, when charged at high voltage, the pseudo-spinel crystal structure may be 60 wt% or more, or the H1-3 crystal structure may be 50 wt% or more. Furthermore, at a certain voltage, the pseudo-spinel crystal structure may be nearly 100 wt%, and further increasing the voltage may result in the H1-3 crystal structure. Therefore, it is preferable to analyze the crystal structure of positive electrode active materials using XRD or other methods. By combining these methods with XRD or other methods, more detailed analysis can be performed.

[0193] However, when positive electrode active materials are charged or discharged at high voltage, their crystal structure may change when exposed to air. For example, they may change from a pseudospinel crystal structure to an H1-3 crystal structure. Therefore, it is recommended that all samples be handled in an inert atmosphere, such as an argon-containing atmosphere.

[0194] <Comparative Example> The positive electrode active material shown in Fig. 10 is lithium cobalt oxide (LiCoO2) to which no halogen or magnesium is added using a manufacturing method described below. The crystal structure of the lithium cobalt oxide shown in Fig. 10 changes depending on the depth of charge.

[0195] As shown in Figure 10, lithium cobalt oxide at a depth of charge of 0 (discharged state) has a region with a crystal structure of space group R-3m, with three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is six-coordinated with oxygen, and the layers are connected in a plane with edge sharing.

[0196] At a charge depth of 1, the crystal structure is of the space group P-3m1, with one CoO2 layer in the unit cell. Therefore, this crystal structure is sometimes called an O1-type crystal structure.

[0197] Furthermore, lithium cobalt oxide at a charge depth of approximately 0.8 has a crystal structure of the space group R-3m. This structure can be described as a structure in which a CoO2 structure such as P-3m1(O1) and a LiCoO2 structure such as R-3m(O3) are alternately stacked. For this reason, this crystal structure is sometimes referred to as an H1-3 crystal structure. In reality, the H1-3 crystal structure has twice the number of cobalt atoms per unit cell as other structures. However, in Figure 10 and other parts of this specification, for ease of comparison with other structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0198] As an example, the coordinates of cobalt and oxygen in the unit cell of the H1-3 crystal structure can be expressed as Co(0,0,0.42150±0.00016), O1(0,0,0.27671±0.00045), and O2(0,0,0.11535±0.00045). O1 and O2 are each an oxygen atom. Thus, the H1-3 crystal structure is expressed by a unit cell using one cobalt and two oxygen atoms. On the other hand, the O3' crystal structure of one embodiment of the present invention is preferably expressed by a unit cell using one cobalt and one oxygen atom. This indicates that the symmetry between cobalt and oxygen differs between the O3' crystal structure and the H1-3 structure, and that the O3' crystal structure exhibits smaller changes from the O3 structure than the H1-3 structure. The unit cell that is more preferably used to represent the crystal structure of the positive electrode active material may be selected, for example, so that the GOF (good of fitness) value is smaller in Rietveld analysis of XRD.

[0199] When lithium cobalt oxide is repeatedly charged and discharged at a high voltage of 4.6 V or higher, based on the redox potential of lithium metal, or at a deep charge depth of 0.8 or higher, the crystal structure of the lithium cobalt oxide changes repeatedly (i.e., a non-equilibrium phase change) between the H1-3 crystal structure and the R-3m(O3) structure in the discharged state.

[0200] However, these two crystal structures have a large deviation in the CoO2 layers. As shown by the dotted lines and arrows in Figure 10, in the H1-3 type crystal structure, the CoO2 layers are significantly deviated from the R-3m(O3) structure. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0201] Furthermore, the difference in volume is large: when compared per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the O3 crystal structure in the discharged state is more than 3.0%.

[0202] In addition, the continuous CoO2 layer structure, such as P-3m1(O1), which is contained in the H1-3 type crystal structure, is likely to be unstable.

[0203] Therefore, repeated high-voltage charging and discharging causes the crystalline structure of lithium cobalt oxide to collapse, which leads to a deterioration in cycle characteristics. This is thought to be because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0204] <Surface roughness and specific surface area> The positive electrode active material of one embodiment of the present invention preferably has a smooth surface with little unevenness. A smooth surface with little unevenness is one factor indicating good distribution of additives near the surface.

[0205] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or cross-sectional TEM image of the positive electrode active material, the specific surface area of ​​the positive electrode active material, or the like.

[0206] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material as shown below.

[0207] First, the cathode active material is processed using FIB or the like to expose its cross section. At this time, it is preferable to cover the cathode active material with a protective film, protective agent, or the like. Next, an SEM image of the interface between the protective film or the like and the cathode active material is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ=2) is performed, followed by binarization. The interface is then extracted using image processing software. The interface line between the protective film or the like and the cathode active material is selected using a magic hand tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from the regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction. The root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness within at least 400 nm of the outer periphery of the cathode active material particles.

[0208] On the particle surfaces of the positive electrode active material of this embodiment, the roughness (RMS: root mean square surface roughness), which is an index of roughness, is preferably less than 3 nm, more preferably less than 1 nm, and even more preferably less than 0.5 nm.

[0209] The image processing software used for noise processing, boundary extraction, etc. is not particularly limited, but for example, "ImageJ" can be used. Similarly, the spreadsheet software is not particularly limited, but for example, Microsoft Office Excel can be used.

[0210] For example, the actual specific surface area A measured by the gas adsorption method using the constant volume method R and the ideal specific surface area A i The smoothness of the surface of the positive electrode active material can also be quantified from the ratio of

[0211] Ideal specific surface area A i is calculated assuming that all particles have the same diameter as D50, the same weight, and an ideal spherical shape.

[0212] The median diameter D50 can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. The specific surface area can be measured by a specific surface area measuring device using a gas adsorption method based on a constant volume method, for example.

[0213] The positive electrode active material of one embodiment of the present invention has an ideal specific surface area A calculated from the median diameter D50. i and the actual specific surface area A R Ratio A R / A i is preferably 2 or less.

[0214] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0215] (Fourth embodiment) An example of a method for producing LiMO2 (where M represents two or more metals including Co, and the substitution position of the metal is not particularly limited) will be described with reference to FIG. 11. The following describes an example of a positive electrode active material containing Mg as a metal element other than Co in LiMO2. Note that the positive electrode active material of one embodiment of the present invention has a crystal structure of a lithium composite oxide represented by LiMO2, but its composition is not limited to Li:M:O=1:1:2.

[0216] First, as the material of the lithium oxide 901, a composite oxide containing lithium, a transition metal, and oxygen is used.

[0217] When using a pre-synthesized composite oxide containing lithium, transition metals, and oxygen, it is preferable to use one with few impurities. In this specification, the main components of the composite oxide containing lithium, transition metals, and oxygen, and the positive electrode active material, are lithium, cobalt, nickel, manganese, aluminum, and oxygen, and elements other than the main components are considered impurities. For example, when analyzed by glow discharge mass spectrometry, the total impurity concentration is preferably 10,000 wt ppm or less, more preferably 5000 wt ppm or less. In particular, the total impurity concentration of transition metals such as titanium and arsenic is preferably 3000 wt ppm or less, more preferably 1500 wt ppm or less.

[0218] For example, lithium cobalt oxide particles (product name: Cellseed C-10N) manufactured by Nippon Chemical Industry Co., Ltd. can be used as pre-synthesized lithium cobalt oxide. This lithium cobalt oxide has an average particle diameter (D50) of approximately 12 μm, and impurity analysis by glow discharge mass spectrometry (GD-MS) reveals that the magnesium and fluorine concentrations are 50 wt ppm or less, the calcium, aluminum, and silicon concentrations are 100 wt ppm or less, the nickel concentration is 150 wt ppm or less, the sulfur concentration is 500 wt ppm or less, the arsenic concentration is 1100 wt ppm or less, and the concentrations of other elements other than lithium, cobalt, and oxygen are 150 wt ppm or less.

[0219] The lithium oxide 901 in step S11 preferably has a layered rock-salt crystal structure with few defects and strain. Therefore, it is preferable that the composite oxide has few impurities. If a composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or strains.

[0220] Further, in step S12, a fluoride 902 is prepared. In this embodiment, lithium fluoride (LiF) is prepared as the fluoride 902. LiF is preferable because it has a cation common to LiCoO. LiF is also preferable because it has a relatively low melting point of 848° C. and is easily melted in an annealing step described later. In addition to LiF, MgF may also be used. The fluoride that can be used in one embodiment of the present invention is not limited to LiF or MgF.

[0221] Moreover, either step S11 or step S12 may be performed first.

[0222] Next, in step S13, the mixture is mixed and pulverized. Mixing can be performed by either a dry or wet method, but a wet method is preferred because it allows for finer pulverization. When using a wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, acetone is used.

[0223] For example, a ball mill, a bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. It is preferable to thoroughly perform this mixing and pulverizing process to finely pulverize the mixture 903.

[0224] The mixed and crushed materials are collected (step S14 in FIG. 11) to obtain a mixture 903 (step S15 in FIG. 11).

[0225] The mixture 903 preferably has a D50 of, for example, 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less.

[0226] Next, the mixture 903 is heated (step S16 in FIG. 11). This step is sometimes called annealing. LiMO2 is produced by annealing. Therefore, the conditions for performing step S16, such as the temperature, time, atmosphere, and weight of the mixture 903 to be annealed, are important. In this specification, annealing also refers to heating the mixture 903 or at least heating a heating furnace containing the mixture 903. In this specification, a heating furnace is a facility used to heat-treat (anneal) a substance or mixture, and has a heater, an atmosphere containing fluoride, and an inner wall that can withstand at least 600°C. The heating furnace may also be equipped with a pump that can reduce or increase the pressure inside the furnace. For example, pressure may be applied during the annealing step S16.

[0227] The annealing temperature of S16 is preferably equal to or higher than the temperature at which the mixture 903 melts. The annealing temperature must be equal to or lower than the decomposition temperature of LiCoO2 (1130°C). Although the decomposition temperature of LiCoO2 is 1130°C, there is concern that a small amount of LiCoO2 may decompose at temperatures around that temperature. Therefore, the annealing temperature is preferably equal to or lower than 1130°C, and more preferably equal to or lower than 1000°C.

[0228] By using LiF as the fluoride 902 and annealing S16 with a lid on, it is possible to produce a positive electrode active material 904 with good cycle characteristics, etc. Furthermore, when LiF and MgF2 are used as the fluoride 902, the eutectic point of LiF and MgF2 is around 742°C, so if the annealing temperature of S16 is set to 742°C or higher, it is thought that the reaction with LiCoO2 is promoted and LiMO2 is produced.

[0229] Furthermore, LiF, MgF2, and LiCoO2 exhibit endothermic peaks in differential scanning calorimetry (DSC) at around 820° C. Therefore, the annealing temperature is preferably 742° C. or higher, and more preferably 820° C. or higher.

[0230] Therefore, the annealing temperature is preferably 742° C. or higher and 1130° C. or lower, and more preferably 742° C. or higher and 1000° C. or lower. Also, the annealing temperature is preferably 820° C. or higher and 1130° C. or lower, and more preferably 820° C. or higher and 1000° C. or lower.

[0231] In this embodiment, LiF, a fluoride, is thought to function as a flux. Therefore, since the volume inside the heating furnace is larger than the volume of the container and is lighter than oxygen, it is expected that LiF will volatilize and the amount of LiF in the mixture 903 will decrease, suppressing the generation of LiMO2. Therefore, it is necessary to heat the mixture while suppressing the volatilization of LiF.

[0232] Therefore, by heating the mixture 903 in an atmosphere containing LiF, that is, by heating the mixture 903 in a heating furnace under a high LiF partial pressure, the volatilization of LiF in the mixture 903 is suppressed. By annealing with a lid using a fluoride (LiF or MgF) that forms a eutectic mixture, the annealing temperature can be lowered to below the decomposition temperature of LiCoO2 (1130°C), specifically, to between 742°C and 1000°C, allowing the formation of LiMO2 to proceed efficiently. As a result, a positive electrode active material with good characteristics can be produced, and the annealing time can also be shortened.

[0233] An example of the annealing method in S16 is shown in FIG.

[0234] The heating furnace 120 shown in FIG. 12 includes a furnace space 103, a hot plate 104, a heater 106, and an insulating material 108. Annealing is preferably performed with a lid 118 attached to the container 116. This configuration allows the space 119 defined by the container 116 and lid 118 to be filled with a fluoride-containing atmosphere. By maintaining the lid during annealing to maintain a constant or stable concentration of gasified fluoride in the space 119, fluorine and magnesium can be incorporated near the particle surfaces. Because the space 119 has a smaller volume than the furnace space 103, a small amount of fluoride volatilizes, creating a fluoride-containing atmosphere. In other words, the reaction system can be filled with a fluoride-containing atmosphere without significantly reducing the amount of fluoride contained in the mixture 903. Therefore, LiMO2 can be efficiently produced. Furthermore, the use of the lid 118 allows the mixture 903 to be annealed in a fluoride-containing atmosphere conveniently and inexpensively.

[0235] Furthermore, there is a possibility that fluoride and the like adhering to the inner walls of the container 116 and the lid 118 may fly again due to heating and adhere to the mixture 903 .

[0236] Here, the valence of Co (cobalt) in LiMO2 produced according to one embodiment of the present invention is preferably trivalent. Cobalt can be divalent or trivalent. Therefore, in order to suppress the reduction of cobalt, the atmosphere in the heating furnace space 103 preferably contains oxygen, more preferably the ratio of oxygen to nitrogen in the atmosphere in the heating furnace space 103 is equal to or higher than that of the air atmosphere, and even more preferably the oxygen concentration in the atmosphere in the heating furnace space 103 is equal to or higher than that of the air atmosphere. Therefore, it is necessary to introduce an oxygen-containing atmosphere into the heating furnace space.

[0237] Therefore, in one embodiment of the present invention, before heating, a step of creating an oxygen-containing atmosphere in the heating furnace space 103 and a step of placing the container 116 containing the mixture 903 in the heating furnace space 103 are performed. By performing these steps in this order, the mixture 903 can be annealed in an atmosphere containing oxygen and fluoride. Furthermore, it is preferable to seal the heating furnace space 103 during annealing to prevent gas from being transported to the outside. For example, it is preferable not to flow gas during annealing.

[0238] There are no particular limitations on the method for creating an oxygen-containing atmosphere in the heating furnace space 103, but examples include a method of evacuating the heating furnace space 103 and then introducing an oxygen-containing gas such as oxygen gas or dry air, or a method of injecting an oxygen-containing gas such as oxygen gas or dry air for a certain period of time. Among these, it is preferable to evacuate the heating furnace space 103 and then introduce oxygen gas (oxygen substitution). Note that the air in the heating furnace space 103 may be considered to be an oxygen-containing atmosphere.

[0239] There is no particular limitation on the process for heating the heating furnace 120. Heating may be performed using a heating mechanism provided in the heating furnace 120.

[0240] Furthermore, there are no particular limitations on how the mixture 903 may be arranged when placed in the container 116, but it is preferable to arrange the mixture 903 so that the top surface of the mixture 903 is flat with respect to the bottom surface of the container 116, in other words, so that the height of the top surface of the mixture 903 is uniform, as shown in FIG. 12.

[0241] The annealing in step S16 is preferably performed at an appropriate temperature and time. The appropriate temperature and time vary depending on conditions such as the size and composition of the lithium oxide 901 particles in step S11. If the particles are small, a lower temperature or shorter time may be more preferable than if the particles are large. After the annealing in step S16, a step of removing the lid is included.

[0242] For example, when the average particle size (D50) of the particles in step S11 is about 12 μm, the annealing time is preferably, for example, 3 hours or more, and more preferably 10 hours or more.

[0243] On the other hand, when the average particle size (D50) of the particles in step S11 is about 5 μm, the annealing time is preferably, for example, from 1 hour to 10 hours, and more preferably about 2 hours.

[0244] The temperature drop time after annealing is preferably, for example, 10 hours or more and 50 hours or less.

[0245] The annealed material is collected (Step S17 in FIG. 11) to obtain positive electrode active material 904 (Step S18 in FIG. 11).

[0246] Here, the difference in the particles obtained when annealing in S16 was performed with a lid and when annealing without a lid in the comparative example will be explained below.

[0247] FIG. 13 is an example of a cross-sectional photograph taken by SEM of one of the positive electrode active material particles annealed with the lid on.

[0248] 14B is an enlarged view of a portion of the comparative example shown in FIG. 14A. It can be seen that the particle surface in FIG. 13 is smoother than those in FIGS. 14A and 14B.

[0249] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0250] (Embodiment 5) In this embodiment, an example of manufacturing a battery cell using LiMO manufactured by a manufacturing method of one embodiment of the present invention will be described. Note that since there are many common parts, the manufacturing method will be described with reference to FIG.

[0251] Lithium cobalt oxide is prepared as the lithium oxide 901. More specifically, Cellseed C-10N manufactured by Nippon Chemical Industry Co., Ltd. is prepared (step S11).

[0252] LiF and MgF2 are prepared as fluoride 902 (step S12). LiF and MgF2 are weighed out so that the molar ratio of LiF:MgF2 is 1:3, and acetone is added as a solvent, followed by wet mixing and pulverization. The ratio of LiF to lithium cobalt oxide is adjusted to 0.17 mol%. The ratio of MgF2 to lithium cobalt oxide is also adjusted to 0.5 mol%.

[0253] Lithium oxide 901 and fluoride 902 are mixed (step S13) and collected (step S14), to obtain mixture 903 (step S15).

[0254] Next, the mixture 903 is placed in a container, which is then covered, and annealed in an oxygen atmosphere in the heating furnace (step S16). The annealing temperature may vary depending on the weight of the mixture 903, but is preferably set to 742°C or higher and 1000°C or lower. The annealing temperature is the temperature at which annealing is performed, and the "annealing time" is the time the annealing temperature is maintained. The temperature is increased at 200°C / h and decreased over 10 hours or longer. It is also preferable to seal the space 103 inside the heating furnace during annealing to prevent gas from being transported to the outside. For example, it is preferable to perform annealing without gas flow.

[0255] In this embodiment, the annealing is performed at a temperature of 850° C. for 60 hours in an oxygen atmosphere in the heating furnace.

[0256] After annealing, the cathode active material 904 can be obtained by recovery (step S18). If a smooth surface is obtained, the lid can be removed during heating to allow cooling. After cooling, the lid is removed, and the obtained cathode active material 904 is used to fabricate each cathode. A current collector is used that is coated with a slurry in which the cathode active material, AB, and PVDF are mixed in a ratio of active material:AB:PVDF = 95:3:2 (weight ratio). NMP is used as the solvent for the slurry.

[0257] After the slurry is applied to the current collector, the solvent is evaporated, and then the current collector is pressed. Through these steps, a positive electrode can be obtained.

[0258] Using the prepared positive electrode, a coin-type battery cell of the CR2032 type (diameter 20 mm, height 3.2 mm) is prepared.

[0259] Lithium metal is used as the counter electrode.

[0260] The electrolyte used in the electrolytic solution is 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7. 2 wt% vinylene carbonate (VC) is added to the electrolytic solution.

[0261] The separator is made of polypropylene with a thickness of 25 μm.

[0262] The positive electrode can and the negative electrode can are made of stainless steel (SUS).

[0263] Through the above steps, a secondary battery cell can be fabricated.

[0264] The following shows the experimental results obtained by comparing different annealing conditions.

[0265] Figure 15A shows the same conditions as the above-mentioned manufacturing method, and is the same as Figure 12, so the same symbols as Figure 12 are used. The lid and container are made of the same material, specifically, a ceramic material. The lid is larger than the opening of the container and can be supported by its own weight. It is preferable to have as little gap as possible between the lid and the container, but there is a gap so that the inside of the container is not sealed by the lid.

[0266] The cycle characteristics of the battery cell are shown in Figure 16. The cycle characteristics were evaluated at 25°C with charging as CCCV (0.5C, 4.6V, cut-off current 0.05C) and discharging as CC (0.5C, 2.5V). The results are shown in Figure 16.

[0267] FIG. 16 also shows the cycle characteristics of a comparative example battery cell fabricated without a lid, as shown in FIG. 15B, using the same fabrication procedures and conditions as in this embodiment.

[0268] From the above, it can be confirmed that the annealing conditions with a lid show better cycle characteristics than the comparative example with an annealing condition without a lid.

[0269] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0270] (Sixth embodiment) In this embodiment, an example of the shape of a secondary battery including a positive electrode active material manufactured by the manufacturing method described in the previous embodiment will be described. The description in the previous embodiment can be referred to for the material used in the secondary battery described in this embodiment.

[0271] [Coin-type secondary battery] First, an example of a coin-type secondary battery will be described. Fig. 17A is an external view of a coin-type (single-layer flat) secondary battery, and Fig. 17B is a cross-sectional view thereof.

[0272] In a coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector.

[0273] It is to be noted that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each only need to have an active material layer formed on one side.

[0274] Positive electrode can 301 and 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, it is preferable to coat them with nickel, aluminum, or the like. Positive electrode can 301 is electrically connected to positive electrode 304, and negative electrode can 302 is electrically connected to negative electrode 307.

[0275] These negative electrode 307, positive electrode 304, and separator 310 are impregnated with an electrolyte, and as shown in FIG. 17B, the positive electrode can 301 is placed downward, and the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order, and the positive electrode can 301 and negative electrode can 302 are crimped together via gasket 303, to produce coin-type secondary battery 300.

[0276] By using the positive electrode active material particles described in the above embodiment for the positive electrode 304, the coin-type secondary battery 300 can be made less susceptible to deterioration and highly safe.

[0277] [Separator] The secondary battery preferably has a separator. Examples of the separator include fibers containing cellulose, such as paper, nonwoven fabrics, glass fibers, ceramics, and synthetic fibers made of nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably formed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0278] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials that can be used include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials that can be used include PVDF and polytetrafluoroethylene. Examples of polyamide materials that can be used include nylon and aramid (meta-aramid, para-aramid).

[0279] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0280] 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.

[0281] 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.

[0282] Here, we will explain the current flow during charging of a secondary battery using Figure 17C. When a lithium-based secondary battery is considered as a closed circuit, the movement of lithium ions and the flow of current are in the same direction. In lithium-based secondary batteries, the anode (positive electrode) and cathode (negative electrode) are interchanged during charging and discharging, and the oxidation and reduction reactions are alternated. Therefore, the electrode with the higher reaction potential is called the positive electrode, and the electrode with the lower reaction potential is called the negative electrode. Therefore, in this specification, whether during charging, discharging, when a reverse pulse current is applied, or when a charging current is applied, the positive electrode will be called the "positive electrode" or "+ electrode," and the negative electrode will be called the "negative electrode" or "- electrode." Using the terms anode (positive electrode) and cathode (negative electrode), which are related to oxidation and reduction reactions, could lead to confusion because their roles are reversed during charging and discharging. Therefore, the terms anode (positive electrode) and cathode (negative electrode) will not be used in this specification. If the terms anode and cathode are used, it should be clearly stated whether they are used during charging or discharging, and whether they correspond to the positive or negative pole.

[0283] 17C is connected to a charger to charge the secondary battery 300. As the charging of the secondary battery 300 progresses, the potential difference between the electrodes increases.

[0284] [Cylindrical secondary battery] An example of a cylindrical secondary battery will be described with reference to Figures 18A to 18D. As shown in Figure 18A, a cylindrical secondary battery 600 has a positive electrode cap (battery lid) 601 on the top surface, and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0285] FIG. 18B is a schematic diagram showing the cross section of a cylindrical secondary battery. Inside a hollow cylindrical battery can 602, a battery element is provided, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a center pin. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal that is corrosion-resistant to the electrolyte, such as nickel, aluminum, or titanium, or an alloy of these metals or alloys of these metals with other metals (e.g., stainless steel). Furthermore, a coating of nickel, aluminum, or the like is preferable to prevent corrosion by the electrolyte. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. A nonaqueous electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The non-aqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0286] Because the positive and negative electrodes used in cylindrical storage batteries are wound, it is preferable to form active materials on both sides of the current collector. A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be made of a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 612 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature rises, and the increased resistance limits the amount of current to prevent abnormal heat generation. Barium titanate (BaTiO3) based semiconductor ceramics or the like can be used for the PTC element.

[0287] 18C , a module 615 may be configured by sandwiching a plurality of secondary batteries 600 between conductive plates 613 and 614. The plurality of secondary batteries 600 may be connected in parallel, in series, or in parallel and then in series. By configuring a module 615 having a plurality of secondary batteries 600, a large amount of power can be extracted.

[0288] FIG. 18D is a top view of module 615. For clarity, conductive plate 613 is shown with dotted lines. As shown in FIG. 18D, module 615 may have conductive wires 616 that electrically connect multiple secondary batteries 600. A conductive plate can be superimposed on the conductive wires 616. Furthermore, a temperature control device 617 may be provided between multiple secondary batteries 600. When a secondary battery 600 becomes overheated, it can be cooled by the temperature control device 617, and when a secondary battery 600 becomes too cold, it can be heated by the temperature control device 617. This makes it less likely that the performance of module 615 will be affected by the outside temperature.

[0289] By using the positive electrode active material manufactured by the manufacturing method described in the above embodiment for the positive electrode 604, the cylindrical secondary battery 600 can be one that is less susceptible to deterioration and has high safety.

[0290] [Example of secondary battery structure] Another structural example of the power storage device will be described with reference to FIGS.

[0291] 19A shows the structure of wound body 950. Winding body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. Winding body 950 is a wound body in which negative electrode 931 and positive electrode 932 are stacked on top of each other with separator 933 sandwiched therebetween, and the stacked sheet is wound. Note that multiple stacks of negative electrode 931, positive electrode 932, and separator 933 may be stacked.

[0292] A secondary battery 913 shown in Fig. 19B has a wound body 950 provided with terminals 951 and 952 inside a housing 930. The wound body 950 is impregnated with an electrolyte inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that in Fig. 19B, for convenience, the housing 930 is shown separated, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (such as aluminum) or a resin material.

[0293] [Laminated secondary battery] Next, an example of a laminated secondary battery will be described with reference to FIGS. 20A and 20B.

[0294] Fig. 20A shows an example of an external view of a laminated secondary battery 500. Fig. 20B shows another example of an external view of a laminated secondary battery 500.

[0295] 20A and 20B include 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.

[0296] The laminated secondary battery 500 has a plurality of wound or strip-shaped positive electrodes 503, separators 507, and negative electrodes 506.

[0297] The wound body has a negative electrode 506, a positive electrode 503, and a separator 507. Similar to the wound body described in Fig. 19A, the wound body is formed by stacking the negative electrode 506 and the positive electrode 503 on top of each other with the separator 507 sandwiched therebetween, and winding the laminated sheet.

[0298] A secondary battery may be provided in which a plurality of rectangular positive electrodes 503, separators 507, and negative electrodes 506 are arranged in a space formed by a film that serves as exterior body 509.

[0299] A method for producing a secondary battery having a plurality of rectangular positive electrodes 503, separators 507, and negative electrodes 506 will be described below.

[0300] First, the negative electrode 506, the separator 507, and the positive electrode 503 are laminated. In this embodiment, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0301] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are placed on the exterior body 509 .

[0302] The outer casing 509 may be a three-layer laminate film having a highly flexible thin metal film made of aluminum, stainless steel, copper, nickel, etc. on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and further having an insulating synthetic resin film made of polyamide resin, polyester resin, etc. on the thin metal film as the outer surface of the outer casing.

[0303] The exterior body 509 is folded to sandwich the laminated layer. Then, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding may be used for the joining. During this joining, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0304] 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 secondary battery 500, which is a laminated secondary battery, can be produced.

[0305] By using the positive electrode active material particles described in the above embodiment for the positive electrode 503, the secondary battery 500 can be one that is less susceptible to deterioration and has high safety.

[0306] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0307] (Embodiment 7) In this embodiment, the configuration of a solid-state secondary battery will be described. In this specification, the term "solid-state battery" refers not only to secondary batteries that use only a solid electrolyte, but also to batteries that use a polymer gel electrolyte, a small amount of electrolytic solution, or a combination of these.

[0308] As shown in FIG. 21A, a secondary battery 400, which is a solid-state battery according to one embodiment of the present invention, includes a positive electrode 410, a solid electrolyte layer 420, and a negative electrode 430. FIG. 21A illustrates a case where a solid electrolyte is used. When a solid electrolyte is used, the installation of a separator or spacer is unnecessary. Furthermore, since the entire battery can be solidified, there is no risk of leakage, and safety is dramatically improved.

[0309] The positive electrode 410 includes a positive electrode current collector 413 and a positive electrode active material layer 414. The positive electrode active material layer 414 includes a positive electrode active material 411 and a solid electrolyte 421. The positive electrode active material 411 can be the positive electrode active material 904 described in the previous embodiment. The positive electrode active material layer 414 may include a conductive agent and a binder. The conductive agent can be a carbon material such as carbon black (e.g., acetylene black (AB)), graphite particles, carbon nanotubes (CNT), or fullerene. Metal powders or metal fibers of copper, nickel, aluminum, silver, or gold, or conductive ceramic materials can also be used. A graphene compound may also be used as the conductive agent. Graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds have a planar shape. Graphene compounds enable surface contact with low contact resistance. Furthermore, even when thin, graphene compounds can exhibit very high conductivity, allowing for efficient formation of conductive paths within the active material layer with a small amount. Therefore, using a graphene compound as a conductive additive is preferable because it can increase the contact area between the active material and the conductive additive. It is also preferable because it can reduce electrical resistance. Examples of graphene compounds include graphene, multilayer graphene, multi-graphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, and graphene quantum dots. Reduced graphene oxide is also known as reduced graphene oxide (hereinafter, RGO). Here, RGO refers to a compound obtained by reducing graphene oxide (GO). When using active material particles with a small particle size, such as active material particles with a diameter of 1 μm or less, the specific surface area of ​​the active material particles is large, requiring more conductive paths connecting the active material particles. In such cases, it is particularly preferable to use a graphene compound that can efficiently form conductive paths even in a small amount.In this specification, graphene oxide refers to a material containing carbon and oxygen, having a sheet-like shape, and having functional groups, particularly epoxy groups, carboxy groups, or hydroxy groups. Furthermore, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphene compounds together. When an active material is coated with a graphene net, the graphene net can also function as a binder that binds the active material together. Therefore, the amount of binder can be reduced or eliminated, thereby improving the ratio of active material to the electrode volume or weight. In other words, the capacity of a secondary battery can be increased.

[0310] Solid electrolyte layer 420 has solid electrolyte 421. Solid electrolyte layer 420 is located between positive electrode 410 and negative electrode 430, and is a region that has neither positive electrode active material 411 nor negative electrode active material 431.

[0311] The negative electrode 430 includes a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 includes a negative electrode active material 431 and a solid electrolyte 421. The negative electrode active material layer 434 may also include a conductive agent and a binder. When metallic lithium is used for the negative electrode 430, the negative electrode 430 may not include the solid electrolyte 421, as shown in FIG. 21B. Using metallic lithium for the negative electrode 430 is preferable because it can improve the energy density of the secondary battery 400. Note that in FIGS. 21A and 21B, the solid electrolyte 421, the positive electrode active material 411, and the negative electrode active material 431 are shown as spherical particles, which is an ideal particle shape. However, since they actually have various shapes, they are illustrated schematically for convenience.

[0312] The solid electrolyte 421 included in the solid electrolyte layer 420 and the material used for the solid electrolyte layer 420 may be, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halide-based solid electrolyte, or the like.

[0313] Sulfide-based solid electrolytes include thiosilicon-based (Li 10 GeP2S12 , Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glasses (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·38SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), sulfide crystallized glasses (Li7P3S 11 , Li 3.25 P 0.95 Sulfide-based solid electrolytes have the advantages of being highly conductive, being able to be synthesized at low temperatures, and being relatively soft, which makes it easy to maintain conductive paths even after charging and discharging.

[0314] Oxide-based solid electrolytes include materials with a perovskite crystal structure (La 2 / 3-x Li 3x TiO3, etc.), materials with NASICON-type crystal structure (Li 1-X Al X Ti 2-X (PO4)3, etc.), materials with garnet-type crystal structure (Li7La3Zr2O 12 etc.), materials with LISICON-type crystal structure (Li 14 ZnGeO 16 etc.), LLZO(Li7La3Zr2O 12 ), oxide glass (Li3PO4-Li4SiO4, 50Li4SiO4·50Li3BO3, etc.), oxide glass-ceramics (Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0315] In this specification and the like, the NASICON-type crystal structure refers to a compound represented by M2(XO4)3 (M: transition metal, X: S, P, As, Mo, W, etc.), which has a structure in which MO6 octahedra and XO4 tetrahedra are arranged three-dimensionally, sharing vertices.

[0316] Halide-based solid electrolytes include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, LiI, etc. Composite materials in which these halide-based solid electrolytes are filled into the pores of porous alumina or porous silica can also be used as solid electrolytes.

[0317] Also, different solid electrolytes may be mixed and used.

[0318] The electrolytes may also be mixed and used.

[0319] The electrolyte to be mixed with the solid electrolyte is preferably a highly purified electrolyte with a low content of granular dust and 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.

[0320] In addition, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalato)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolyte solution mixed with the solid electrolyte. The concentration of the added material may be, for example, 0.1 wt % to 5 wt % of the total solvent.

[0321] Furthermore, a polymer gel electrolyte obtained by swelling a polymer with an electrolytic solution may be used as a material to be mixed with the solid electrolyte.

[0322] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0323] 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.

[0324] 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.

[0325] Furthermore, the present invention can be implemented in appropriate combination with other embodiments.

[0326] (Embodiment 8) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted in an electronic device or a mobile object will be described.

[0327] 22A to 22E show examples of mounting the secondary battery described in the previous embodiment in an electronic device. Examples of electronic devices that use a bendable secondary battery include television devices (also called televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0328] Furthermore, the secondary battery can be applied to a mobile object, typically an automobile. Examples of the automobile include next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs), and the secondary battery can be applied as one of the power sources mounted on the automobile. The mobile object is not limited to an automobile. Examples of the mobile object include trains, monorails, ships, aircraft (helicopters, unmanned aerial vehicles (drones), airplanes, and rockets), electric bicycles, and electric motorcycles, and the secondary battery of one embodiment of the present invention can be applied to these mobile objects.

[0329] Furthermore, the secondary battery of this embodiment may be applied to a ground-mounted charging device installed in a house or a charging station installed in a commercial facility.

[0330] 22A shows an example of a mobile phone. Mobile phone 2100 includes a display unit 2102 built into housing 2101, as well as operation buttons 2103, an external connection port 2104, a speaker 2105, and a microphone 2106. Mobile phone 2100 also includes a secondary battery 2107.

[0331] 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.

[0332] The operation button 2103 can be provided with various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system built into the mobile phone 2100.

[0333] 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.

[0334] The mobile phone 2100 also has an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Charging may also be performed by wireless power supply without using the external connection port 2104.

[0335] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, or other human body sensor, a touch sensor, a pressure sensor, or an acceleration sensor.

[0336] FIG. 22B shows an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. The secondary battery according to one embodiment of the present invention is highly safe and can be used safely for a long period of time, making it suitable as a secondary battery to be installed in the unmanned aerial vehicle 2300.

[0337] As shown in FIG. 22C, a secondary battery 2602 including a plurality of secondary batteries 2601 of one embodiment of the present invention may be mounted in a hybrid vehicle (HV), an electric vehicle (EV), a plug-in hybrid vehicle (PHV), or other electronic devices.

[0338] FIG. 22D shows an example of a vehicle equipped with a secondary battery 2602. The vehicle 2603 is an electric vehicle that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid vehicle that can appropriately select and use an electric motor or an engine as a power source for traveling. The vehicle 2603 using an electric motor has multiple ECUs (Electronic Control Units), and the ECUs perform engine control and the like. The ECUs include microcomputers. The ECUs are connected to a Controller Area Network (CAN) provided in the electric vehicle. CAN is one of the serial communication standards used for in-vehicle LANs. By using the secondary battery of one embodiment of the present invention, the secondary battery can function as a power source for the ECU, thereby realizing a vehicle that is highly safe and has a long cruising range.

[0339] The secondary battery can not only drive an electric motor (not shown) but also supply power to light-emitting devices such as headlights, room lights, etc. The secondary battery can also supply power to display devices and semiconductor devices such as a speedometer, a tachometer, and a navigation system that the vehicle 2603 has.

[0340] The vehicle 2603 can charge the secondary battery of the secondary battery 2602 by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like.

[0341] FIG. 22E shows a state in which a vehicle 2603 is being charged via a cable from a ground-mounted charging device 2604. Charging may be performed using a predetermined charging method, connector specifications, or the like, such as CHAdeMO (registered trademark) or Combo. For example, plug-in technology can be used to charge a secondary battery 2602 mounted on the vehicle 2603 using external power supply. Charging can be performed by converting AC power to DC power via a conversion device such as an AC-DC converter. The charging device 2604 may be installed in a home as shown in FIG. 22E, or may be a charging station installed in a commercial facility.

[0342] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between vehicles. Furthermore, a solar cell can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0343] 22E includes a power storage system 2612 including a secondary battery which is one embodiment of the present invention, and a solar panel 2610. The power storage system 2612 is electrically connected to the solar panel 2610 via wiring 2611 or the like. The power storage system 2612 may be electrically connected to a ground-mounted charging device 2604. The power obtained by the solar panel 2610 can be charged to the power storage system 2612. The power stored in the power storage system 2612 can be charged to a secondary battery 2602 included in a vehicle 2603 via the charging device 2604.

[0344] The power stored in the power storage system 2612 can also be supplied to other electronic devices in the house. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the power storage system 2612 of one embodiment of the present invention can be used as an uninterruptible power supply, enabling the use of electronic devices.

[0345] This embodiment mode can be implemented in appropriate combination with other embodiment modes. [Example]

[0346] In this example, a secondary battery including reduced graphene oxide as a conductive agent and a secondary battery including acetylene black were manufactured, and their characteristics were evaluated.

[0347] <Preparation of secondary battery> For evaluation, a coin-type secondary battery of the CR2032 type (diameter 20 mm, height 3.2 mm) was fabricated.

[0348] The positive electrode active material of the secondary battery was nickel-cobalt-manganese lithium oxide (manufactured by MTI) with an atomic ratio of Ni:Co:Mn = 5:2:3. This is sometimes referred to as NCM523.

[0349] Graphene oxide (GO) or acetylene black (AB) was used as the conductive agent. Graphene oxide was reduced in a later process. Graphene oxide was mixed at 3 wt% of the total amount of the active material, graphene oxide, and binder.

[0350] PVDF was used as the binder.

[0351] The positive electrode active material, conductive agent, and binder were mixed to prepare a slurry. NMP was used as the solvent. The slurry was applied to a current collector, which was an aluminum foil with a carbon undercoat, and then dried.

[0352] Next, chemical and thermal reduction were performed on the sample using graphene oxide as the conductive agent.

[0353] L-ascorbic acid was used as the reducing agent for chemical reduction. The solvent was water:NMP = 1:9 (volume ratio), and a solution containing 0.078 mol / L L-ascorbic acid and 0.074 mol / L lithium hydroxide was prepared. A current collector coated with a positive electrode active material layer was immersed in the ascorbic acid solution and reacted at 60°C for 1 hour.

[0354] Next, thermal reduction was carried out at a heating temperature of 170°C for 10 hours.

[0355] After the reduction treatment, the mixture was pressed at a linear pressure of 210 kN / m and then at a linear pressure of 1467 kN / m to form a positive electrode.

[0356] The counter electrode was made of lithium metal.

[0357] The electrolyte used was 1 mol / L lithium hexafluorophosphate (LiPF6), which 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.

[0358] The separator was made of polypropylene with a thickness of 25 μm.

[0359] The positive electrode can and the negative electrode can were made of stainless steel (SUS).

[0360] <Battery characteristics> Next, a charge / discharge test was carried out on the prepared sample.

[0361] FIG. 23A shows the charge rate characteristics at 0°C. FIG. 23B shows the discharge rate characteristics at 0°C. Charge was CC (0.2C, 1C, 2C, 5C, or 10C, end voltage 4.3V), and discharge was CC (0.2C, 1C, 2C, 5C, or 10C, end voltage 2.0V). In this example, 1C was 170mA / g.

[0362] The sample using reduced graphene oxide as a conductive agent (labeled RGO in the figure) showed better high-rate performance than the sample using acetylene black (labeled AB in the figure). [Example]

[0363] For evaluation, a coin-type secondary battery of the CR2032 type (diameter 20 mm, height 3.2 mm) was fabricated.

[0364] The positive electrode active material of the secondary battery was nickel-cobalt-manganese lithium oxide (manufactured by MTI) with an atomic ratio of Ni:Co:Mn=5:2:3.

[0365] Graphene oxide (GO) or acetylene black (AB) was used as the conductive agent. Graphene oxide was reduced in a later process. Graphene oxide was mixed at 1 wt% or 3 wt% of the total amount of the active material, graphene oxide, and binder.

[0366] PVDF was used as the binder, and the binder was mixed at 2 wt% of the total amount of the active material, graphene oxide, and binder.

[0367] The conditions for preparing the slurry and reducing the graphene oxide were the same as those in Example 1.

[0368] After the reduction treatment, the mixture was pressed at a linear pressure of 210 kN / m and then at a linear pressure of 1467 kN / m to form a positive electrode. The amount of the support on the positive electrode was approximately 7 mg / cm. 2 It was.

[0369] The counter electrode, electrolyte, separator, positive electrode can, and negative electrode can shown in Example 1 were used.

[0370] The cycle characteristics of the fabricated secondary battery were evaluated. The measurement temperature was 60°C. Charging was performed by CC charging at 1C with a cut-off voltage of 4.3V, followed by CV charging at 4.3V with a cut-off condition of 0.1C for 1.5 hours. CC discharging was performed at 1C with a cut-off voltage of 2.5V.

[0371] Figure 24A shows the measurement results for a sample using 3 wt% graphene oxide as a conductive agent (labeled RGO:3%), and Figure 24B shows the measurement results for a sample using 3 wt% acetylene black as a conductive agent (labeled AB:3%). Figure 25A shows the measurement results for a sample using 1 wt% graphene oxide as a conductive agent (labeled RGO:1%), and Figure 25B shows the measurement results for a sample using 1 wt% acetylene black as a conductive agent (labeled AB:1%).

[0372] The sample using reduced graphene oxide as a conductive agent showed less variation in properties than the sample using acetylene black, and the rapid capacity loss with cycling was suppressed. [Example]

[0373] In this example, a laminate type secondary battery was fabricated and its volume change was evaluated.

[0374] <Preparation of negative electrode> Specific surface area is 1.5m 2 / g of MCMB graphite was mixed with a conductive agent, CMC-Na (sodium carboxymethylcellulose), and SBR (styrene butadiene rubber) in a weight ratio of graphite:conductive agent:CMC-Na:SBR = 96:1:1:2, and a slurry was prepared using water as the solvent.

[0375] The degree of polymerization of the CMC-Na used was 600 to 800, and the viscosity of the aqueous solution when used as a 1 wt% solution was in the range of 300 mPa·s to 500 mPa·s. The conductive agent was vapor-grown carbon fiber VGCF (registered trademark)-H (Showa Denko K.K., fiber diameter 150 nm, specific surface area 13 m). 2 / g) was used.

[0376] The prepared slurry was applied to a current collector and dried to form a negative electrode active material layer on the current collector. The current collector was made of copper foil with a thickness of 18 μm. The negative electrode active material layer was formed on one side of the current collector.

[0377] <Preparation of positive electrode> Next, a positive electrode was fabricated. Nickel-cobalt-manganese lithium oxide (manufactured by MTI) with an atomic ratio of Ni:Co:Mn=5:2:3 was used as the positive electrode active material for the secondary battery.

[0378] Graphene oxide (GO) or acetylene black (AB) was used as the conductive agent. Graphene oxide was reduced in a later process. Graphene oxide was mixed at 3 wt% of the total amount of the active material, graphene oxide, and binder.

[0379] PVDF was used as the binder, and the binder was mixed at 2 wt% of the total amount of the active material, graphene oxide, and binder.

[0380] The conditions for preparing the slurry and reducing the graphene oxide were the same as those in Example 1.

[0381] After the reduction treatment, the cathode was prepared by applying a linear pressure of 120 kN / m. The amount of the support on the cathode was approximately 10 mg / cm. 2 It was.

[0382] <Preparation of secondary battery> Using the positive electrode and negative electrode prepared above, a secondary battery was prepared using a film as an exterior body.

[0383] The separator was made of polypropylene with a thickness of 25 μm.

[0384] The positive electrode, separator, and negative electrode were stacked in this order. The positive electrode active material provided on one side of the current collector was arranged to face the negative electrode active material with the separator sandwiched therebetween.

[0385] Leads were attached to the positive and negative electrodes, respectively.

[0386] The laminate, which was made by stacking the positive electrode, the negative electrode, and the separator, was sandwiched between an exterior body folded in half, and the laminate was positioned so that one end of the lead was exposed to the outside of the exterior body. Next, one side of the exterior body was left open, and the other sides were sealed.

[0387] The film used for the exterior body was a film in which a polypropylene layer, an acid-modified polypropylene layer, an aluminum layer, and a nylon layer were laminated in this order. The film had a thickness of approximately 110 μm. The film for the exterior body was folded so that the nylon layer was on the surface that would be placed on the outside of the exterior body, and the polypropylene layer was on the surface that would be placed on the inside. The thickness of the aluminum layer was approximately 40 μm, the thickness of the nylon layer was approximately 25 μm, and the total thickness of the polypropylene layer and the acid-modified polypropylene layer was approximately 45 μm.

[0388] Next, under an argon gas atmosphere, an electrolyte was injected from the side that was left as an open portion.

[0389] The electrolyte used was 1 mol / L lithium hexafluorophosphate (LiPF6), a mixture of propylene carbonate (PC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of PC:EMC:DMC = 3:3.5:3.5.

[0390] Next, one side of the exterior body that had been left as an open portion was sealed in a reduced pressure atmosphere.

[0391] Using the above steps, six secondary batteries using reduced graphene oxide as a conductive agent (hereinafter referred to as cells RGO-C1, RGO-C2, RGO-C3, RGO-C4, RGO-C5, and RGO-C6) and five secondary batteries using acetylene black (hereinafter referred to as cells AB-C1, AB-C2, AB-C3, AB-C4, and AB-C5) were fabricated.

[0392] Gas may be generated inside the secondary battery during aging, retention tests, electrical property evaluation, etc., causing the exterior body of the secondary battery to swell and change in volume. In this example, the change in volume was evaluated by the following method.

[0393] When an object is immersed in liquid, it experiences a buoyant force equal to the weight of the volume of liquid it displaces. Here, the buoyant force is F [N] and the density of the liquid is d [kg / m 3 ], the volume of the part of the object immersed in the liquid is V [m 3 ], and gravitational acceleration is g [m / s 2 ], then the following formula (1) holds.

[0394]

number

[0395] When gravity and buoyancy are balanced, the following equation (2) holds: Weight m [g] is the weight of the liquid displaced by immersing the object.

[0396]

number

[0397] From equation (2), the volume V of the part of the object that is immersed in liquid can be expressed by the following equation (3) using the weight m and density d, and can be calculated from the weight of the liquid displaced by immersing the object.

[0398]

number

[0399] <Weight measurement 1> The fabricated secondary battery was placed in a tank of water, the lead electrode portion was clamped with a clip, and the battery was immersed in water. The clip was hanging from a structure independent of the weighing scale. The weight increase caused by immersing the secondary battery in water was measured. The results are shown in the "weight 1" column in Table 2.

[0400] [Table 2]

[0401] <Aging> Next, the secondary battery was subjected to aging.

[0402] First, the secondary battery was sandwiched between two plates and charged at CC (0.01C, capacity 15mAh / g). CC stands for constant current. The capacity of the secondary battery was calculated per weight of the positive electrode active material. The C rate was calculated by adjusting 1C to the charge / discharge cycle conditions.

[0403] Next, charging was carried out at CC (0.1C, capacity 120mAh / g).

[0404] <Weight measurement 2> The two plates were then removed, and the secondary battery was placed in a tank of water to measure the weight increase caused by immersing the secondary battery in water. The results are shown in the "weight 2" column in Table 2 above.

[0405] <Keep> Then, the mixture was kept at 40°C for 24 hours.

[0406] <Weight measurement 3> The secondary battery was then placed in a water tank and immersed in water, and the weight increase due to immersion of the secondary battery in water was measured. The results are shown in the "weight 3" column in Table 2 above.

[0407] As shown in Table 3, the secondary battery using reduced graphene oxide as the conductive agent in the positive electrode was lighter in weight than the secondary battery using acetylene black. This suggests that using reduced graphene oxide as the conductive agent in the positive electrode suppressed gas generation during aging and when maintained at 40°C, thereby suppressing swelling of the secondary battery's exterior body. [Example]

[0408] In this example, SEM observation of the electrodes fabricated in Example 2 was performed.

[0409] <Disassembly of the battery> The secondary battery that had been charged and discharged until its discharge capacity rapidly decreased was disassembled, and the positive electrode was taken out. The disassembly was carried out under an argon atmosphere. After disassembly, it was washed with DMC and the solvent was volatilized.

[0410] <SEM observation> Next, the positive electrode taken out by disassembling the secondary battery was placed in a container held under an argon atmosphere, carried into the transfer chamber, depressurized by a pump to discharge the atmospheric atmosphere, and then the container was emptied to expose the positive electrode to the atmospheric atmosphere without observation. The observation was carried out using a scanning electron microscope (SEM). The SEM used was SU8030 manufactured by Hitachi High-Technologies Corporation. The acceleration voltage was set to 5 kV.

[0411] [[ID=(16)]]Fig. 26 shows SEM images of the positive electrode using 3 wt% of graphene as the conductive agent. Fig. 26A is the positive electrode with no decrease in discharge capacity, and Fig. 26B is the positive electrode with a slightly decreased discharge capacity. Graphene compound 2700 is shown in Figs. 26A and 26B.

[0412] Fig. 27 shows SEM images of the positive electrode using 3 wt% of acetylene black as the conductive agent. Fig. 27A is the positive electrode with no decrease in discharge capacity, and Fig. 27B is the positive electrode with a rapidly decreased discharge capacity.

[0413] As described above, when the positive electrode after charge and discharge cycles was observed, graphene compounds were observed. In the electrode using acetylene black, substances considered to be decomposition products of the electrolyte were seen, and the decrease in capacity may be due to the decomposition products. On the other hand, the electrode using graphene oxide compound had fewer substances considered to be decomposition products compared to the electrode using acetylene black. Therefore, it is considered that the decrease in capacity accompanying charge and discharge cycles was suppressed.

Explanation of symbols

[0414] 101: graphene, 103: space inside heating furnace, 104: hot plate, 106: heater part, 108: heat insulating material, 116: container, 118: lid, 119: space, 120: heating furnace, 200: electrode, 201: current collector, 201a: titanium compound, 202: active material layer, 203: active material, 203a: active material, 203b: active material, 207: carbon-containing compound, 207a: carbon-containing compound, 207b: carbon-containing compound, 207c: carbon-containing compound, 207x: carbon-containing compound, 207y: carbon-containing compound, 208: secondary particle, 212: protrusion, 214: graphene, 221: region, 222 : Region, 223: Region, 300: Secondary battery, 301: Positive electrode can, 302: Negative electrode can, 303: Gasket, 304: Positive electrode, 305: Positive electrode current collector, 306: Positive electrode active material layer, 307: Negative electrode, 308: Negative electrode current collector, 309: Negative electrode active material layer, 310: Separator, 400: Secondary battery, 410: Positive electrode, 411: Positive electrode active material, 413: Positive electrode current collector, 414: Positive electrode active material layer, 420: Solid electrolyte layer, 421: Solid electrolyte, 430: Negative electrode, 431: Negative electrode active material, 433: Negative electrode current collector, 434: Negative electrode active material layer, 500: Secondary battery, 503: Positive electrode, 506: Negative electrode, 507: Separator , 509: exterior body, 510: positive electrode lead electrode, 511: negative electrode lead electrode, 600: secondary battery, 601: positive electrode cap, 602: battery can, 603: positive electrode terminal, 604: positive electrode, 605: separator, 606: negative electrode, 607: negative electrode terminal, 608: insulating plate, 609: insulating plate, 611: PTC element, 612: safety valve mechanism, 613: conductive plate, 614: conductive plate, 615: module, 616: conducting wire, 617: temperature control device, 881: lithium atom, 882: cobalt atom, 883: lithium layer, 884: lithium ion, 901: lithium oxide, 902: fluoride, 90 3: mixture, 904: positive electrode active material, 913: secondary battery, 930: housing, 931: negative electrode, 932: positive electrode, 933: separator, 950: wound body, 951: terminal, 952: terminal, 2100: mobile phone, 2101: housing, 2102: display unit, 2103: operation button, 2104: external connection port, 2105: speaker, 2106: microphone, 2107: secondary battery, 2300: unmanned aerial vehicle, 2301: secondary battery, 2302: rotor, 2303: camera, 2601: secondary battery, 2602: secondary battery, 2603: vehicle, 2604: charging device, 2610: solar panel,2611: Wiring, 2612: Energy storage system, 2700: Graphene compound,

Claims

1. a first graphene layer, a second graphene layer, and first positive electrode active material particles; the first positive electrode active material particles contain magnesium, aluminum, nickel, fluorine, and lithium cobalt oxide represented by the R-3m space group; the first graphene layer and the second graphene layer each have a structure in which sheet-like carbon-containing compounds are stacked, the first positive electrode active material particles have a region on a surface of the first positive electrode active material particle that is covered with the first graphene layer, the second graphene layer has a portion overlapping the region; a lithium ion secondary battery having lithium between the first graphene layer and the second graphene layer;

2. a first graphene layer, a second graphene layer, and first positive electrode active material particles; the first positive electrode active material particles contain magnesium, aluminum, nickel, fluorine, and lithium cobalt oxide represented by the R-3m space group; the first graphene layer and the second graphene layer each have a structure in which sheet-like carbon-containing compounds are stacked, the first positive electrode active material particles have a region on a surface of the first positive electrode active material particle that is covered with the first graphene layer, the second graphene layer has a portion overlapping the region; The lithium ion secondary battery has a passage for lithium in the overlapping portion.

3. In claim 1, the lithium present between the first graphene layer and the second graphene layer is contained in the first positive electrode active material particles.

4. In any one of claims 1 to 3, the first graphene layer is spaced from the first positive electrode active material particles by a distance of less than 5 nm.

5. In any one of claims 1 to 4, The lithium ion secondary battery, wherein the portion is parallel or approximately parallel to the (104) plane of the crystal structure of the region.

6. In any one of claims 1 to 5, a lithium ion secondary battery, wherein the portion forms an angle of 70° or more and 110° or less with respect to the ab plane of the crystal structure of the region.

7. In any one of claims 1 to 6, further comprising second positive electrode active material particles and a third graphene layer, the first positive electrode active material particles and the second positive electrode active material particles are covered with the third graphene layer.

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