Method for producing positive electrode active substance

Abstract

The present invention provides a method for producing a positive electrode active substance having good cycle characteristics. This method for producing a positive electrode active substance comprises: mixing lithium cobaltate having a median diameter (D50) of 10 µm or less with a first additional element source to form a first mixture; subjecting the first mixture to a first heating to form a first composite oxide; mixing the first composite oxide with a second additional element source to form a second mixture; subjecting the second mixture to second heating to form a second composite oxide; mixing the second composite oxide with a third additional element source to form a third mixture; and subjecting the third mixture to third heating. The first additional element source comprises a magnesium compound and a fluorine compound. The second additional element source comprises an aluminum compound and a nickel compound. The third additional element source comprises a titanium compound. The duration of the second heating and the duration of the third heating are each shorter than the duration of the first heating.

Description

Method for producing positive electrode active material

[0001] One embodiment of the present invention relates to a method for manufacturing a positive electrode active material. The present invention is not limited to the above fields, but also relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, a vehicle, and manufacturing methods thereof. The above-described semiconductor device, display device, light-emitting device, power storage device, lighting device, electronic device, and vehicle can use a secondary battery including the positive electrode active material of the present invention as a necessary power source. For example, the above-described electronic device includes a smartphone equipped with a secondary battery. The above-described power storage device includes a stationary power storage device.

[0002] Lithium-ion secondary batteries have a high volumetric energy density and are suitable as power sources for electronic devices. With the widespread use of portable electronic devices such as smartphones and tablet terminals, improvements in the safety and cycle characteristics of lithium-ion secondary batteries are desired. To meet these demands, improvements in the positive electrode active materials for lithium-ion secondary batteries have been actively pursued (e.g., Patent Documents 1 and 2).

[0003] Research on the crystalline structure of positive electrode active materials has also been conducted (Non-Patent Documents 1 to 3). X-ray diffraction (XRD) is one of the techniques used to analyze the crystalline structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) introduced in Non-Patent Document 4. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 5 can be referenced from the ICSD. For Rietveld analysis, the analysis program RIETAN-FP (Non-Patent Document 6) can be used. VESTA (Non-Patent Document 7) can be used as software for drawing crystal structures.

[0004] JP 2018-206747 A JP 2022-070247 A

[0005] Toyoki Okumura et.al.,”Correlation of lithium ion distribution and X−ray absorption near−edge structure in O3−and O2−lithium cobalt oxides from first−principle calculation”,Journal of Materials Chemistry,2012,22,p.17340−17348Motohashi,T.et.al.,”Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”,Physical Review B,80(16);165114Zhaohui Chen et.al.,“Staging Phase Transitions in Li▲x▼CoO▲2▼”,Journal of The Electro chemical Society,2002,149(12)A1604−A1609Belsky,A.et.al.,“New developments in the Inorganic Crystal Structure Database(ICSD):accessibility in support of materials research and design”,Acta Cryst.,(2002)B58 364−369.Akimoto,J.;Gotoh,Y.;Oosawa, Y. “Synthesis and structure refinement of LiCoO▲2▼ single crystals” Journal of Solid State Chemistry (1998) 141, p. 298-302. F. Izumi and K. Momma, “Three-Dimensional Visualization in Powder Diffraction”Solid State Phenom. 130, 15-20 (2007) K. Momma and F. Izumi, “VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data” J. Appl. Cryst. (2011). 44, 1272-1276;

[0006] Lithium-ion secondary batteries have room for improvement in various aspects, such as discharge capacity, cycle characteristics, reliability, safety, and cost. Therefore, an object of one embodiment of the present invention is to provide a manufacturing method of a positive electrode active material that exhibits good cycle characteristics. Another object of one embodiment of the present invention is to provide a manufacturing method that can reduce costs.

[0007] 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.

[0008] In order to solve the above-described problems, one aspect of the present invention is a method for producing a positive electrode active material, comprising mixing lithium cobalt oxide having a median diameter (D50) of 10 μm or less with a first additive element source to form a first mixture, performing a first heating on the first mixture to form a first composite oxide, mixing the first composite oxide with a second additive element source to form a second mixture, performing a second heating on the second mixture to form a second composite oxide, mixing the second composite oxide with a third additive element source to form a third mixture, and performing a third heating on the third mixture, wherein the first additive element source includes a magnesium compound and a fluorine compound, the second additive element source includes an aluminum compound and a nickel compound, and the third additive element source includes a titanium compound, and the second heating time and the third heating time are each shorter than the first heating time.

[0009] Another aspect of the present invention is a method for producing a positive electrode active material, comprising: mixing lithium cobalt oxide having a median diameter (D50) of 10 μm or less with a first additive element source having a median diameter (D50) of 100 μm or less to form a first mixture; performing a first heating process on the first mixture to form a first composite oxide; mixing the first composite oxide with a second additive element source having a median diameter (D50) of 100 μm or less to form a second mixture; performing a second heating process on the second mixture to form a second composite oxide; mixing the second composite oxide with a third additive element source to form a third mixture; and performing a third heating process on the third mixture.

[0010] Another aspect of the present invention is the method for producing a positive electrode active material, wherein the lithium cobalt oxide having a median diameter (D50) of 10 μm or less has a magnesium concentration of 40 wt ppm or less and a titanium concentration of 40 wt ppm or less.

[0011] According to one embodiment of the present invention, it is possible to manufacture positive electrode active material particles in which a decrease in discharge capacity during charge-discharge cycles is suppressed. Also, according to one embodiment of the present invention, it is possible to provide a manufacturing method that reduces costs.

[0012] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc.

[0013] FIG. 1 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 2A to 2C are diagrams illustrating a method for preparing a positive electrode active material. FIGS. 3A and 3B are diagrams illustrating a method for preparing a positive electrode active material. FIG. 4A is a cross-sectional view of a positive electrode active material, and FIGS. 4B and 4C are diagrams illustrating the distribution of added elements. FIG. 5 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 6A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 6B is an FFT pattern of the rock salt crystal RS region, and FIG. 6C is an FFT pattern of the layered rock salt crystal LRS region. FIG. 7 is a diagram illustrating the crystalline structure of a positive electrode active material. FIG. 8 is a diagram illustrating the crystalline structure of a conventional positive electrode active material. FIG. 9 is an XRD pattern. FIG. 10 is an XRD pattern. FIGS. 11A and 11B are XRD patterns. FIG. 12 is a cross-sectional view of a positive electrode active material. FIGS. 13A and 13B are diagrams illustrating a nail penetration test. FIG. 14 is a diagram showing the appearance of a secondary battery. FIGS. 15A to 15C are diagrams illustrating a method for manufacturing a secondary battery. FIGS. 16A to 16H are diagrams illustrating an example of an electronic device. FIGS. 17A to 17D are diagrams illustrating an example of an electronic device. FIGS. 18A to 18C are diagrams illustrating an example of an electronic device. FIGS. 19A to 19C are diagrams illustrating an example of a vehicle. FIGS. 20A and 20B are graphs showing the results of measuring discharge capacity by rate. FIG. 21 is a graph showing the results of measuring discharge capacity by rate. FIGS. 22A and 22B are graphs showing the results of charge / discharge cycle characteristics. FIGS. 23A and 23B are graphs showing the results of charge / discharge cycle characteristics. FIGS. 24A and 24B are graphs showing the results of charge / discharge cycle characteristics. FIG. 25 is a graph showing the results of charge / discharge curves. FIG. 26A is a graph showing the results of an impedance test, and FIG. 26B is a diagram showing the equivalent circuit used for analysis. FIG. 27A is a photograph before the nail penetration test, and FIG. 27B is a photograph after the nail penetration test.

[0014] Hereinafter, examples of embodiments of the present invention will be described with reference to the drawings, etc. However, the present invention should not be construed as being limited to the following examples. The embodiments of the present invention can be modified within the scope of the spirit of the present invention.

[0015] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. Trigonal crystals represented by the space group R-3m are generally sometimes expressed as a hexagonal composite hexagonal lattice to facilitate understanding of the structure. Miller indices may also be used in place of (hkl) or (hkil). Here, i is -(h+k). In this specification and the like, for the space group R-3m, unless otherwise specified, crystal planes and the like are expressed as a composite hexagonal lattice.

[0016] In this specification, etc., the space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification, etc., "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0017] In this specification, if the arrangement of anions is roughly close to cubic close packing, it can be considered cubic close packing. Cubic close packing of anions refers to a state in which a second layer of anions is arranged above the voids of the first layer of anions, and a third layer of anions is arranged directly above the voids of the second layer of anions, but not directly above the first layer of anions. Therefore, the anions do not necessarily have to be in a cubic lattice. Furthermore, since real crystals always have defects, analytical results do not necessarily conform to theory. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM (transmission electron microscope) image, spots may appear at positions slightly different from the theoretical positions. For example, a cubic close packing structure can be said to be present if the deviation between the theoretical position and orientation is 5 degrees or less, or 2.5 degrees or less.

[0018] In this specification etc., the term "particle" may be used to refer to the shape of an active material or conductive material, and includes a spherical shape (a circular cross-sectional shape). In this specification etc., the shape of an active material or conductive material is not limited to a spherical shape, and may include cross-sectional shapes such as an ellipse, rectangle, trapezoid, triangle, square with rounded corners, and asymmetric shapes, and individual particles may also have an irregular shape.

[0019] In this specification and the like, when simply referring to a positive electrode active material, the term may refer to a plurality of positive electrode active material particles or a single positive electrode active material particle, depending on the analytical method, etc. For example, in the case of descriptions relating to line analysis using a scanning transmission electron microscope-energy dispersive X-ray fluorescence detector (STEM; Scanning Transmission Electron Microscope-EDX: Energy Dispersive X-ray Spectroscopy), STEM-electron energy loss spectroscopy (STEM-EELS), and electron diffraction, the description refers to a single positive electrode active material particle unless otherwise specified. On the other hand, in the case of X-ray photoelectron spectroscopy (XPS), XRD, various mass analyses, and the like, unless otherwise specified, the description is of a plurality of positive electrode active material particles.

[0020] Furthermore, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments, it is not necessary for all particles in the secondary battery to have the characteristics. For example, as long as three or more randomly selected particles of the positive electrode active material have the characteristics, it can be said that the effects of improving the characteristics of the positive electrode active material and the secondary battery containing it are sufficient.

[0021] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in the secondary battery are described in their pre-degradation state. Note that a decrease in discharge capacity due to aging treatment during secondary battery manufacturing is not considered to be degradation. For example, a lithium-ion secondary battery having a discharge capacity of 97% or more of its rated capacity can be considered to be in its pre-degradation state. For lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0022] In this specification and the like, the state of the materials of a secondary battery before deterioration is sometimes referred to as an initial product or initial state, and the state after deterioration (the state when the secondary battery has a discharge capacity of less than 97% of the rated capacity) is sometimes referred to as a product in use or in use state, or a used product or used state.

[0023] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. ２ The theoretical capacity of LiNiO is 274 mAh / g per active material weight. ２ The theoretical capacity of LiMn is 275 mAh / g per active material weight. ２ O ４ The theoretical capacity of the active material is 148 mAh / g.

[0024] In this specification and the like, the amount of lithium remaining in the positive electrode active material that can be inserted and removed is determined by x in the composition formula, for example, Li ｘ CoO ２ In the case of a positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, LiCoO ２ When a secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２In the above formula, "x" is small, for example, when 0.1<x≦0.24. The extent to which lithium has been released from the positive electrode active material relative to the theoretical capacity is sometimes referred to as the depth of charge. In this specification and elsewhere, the depth of charge is defined as 1−x.

[0025] For example, if properly synthesized lithium cobalt oxide before use in the positive electrode approximately satisfies the stoichiometric ratio, LiCoO ２ and x = 1 (the depth of charge is 0 at this time). The lithium cobalt oxide contained in the secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The completion of discharge here refers to a state in which the voltage reaches 3.0 V or 2.5 V at a current of 100 mA / g or less, for example.

[0026] Li ｘ CoO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above should be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data on a secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0027] In this specification, the term "distribution of a certain element" refers to a region in which the element is continuously detected within a range that is not noise. The term "detection of a certain element continuously within a range that is not noise" includes detection of a certain element continuously within a range that is not noise when an analysis is performed multiple times.

[0028] In this specification, the term "supported amount" refers to the weight of the active material per unit surface area of ​​the current collector. The supported amount of the negative electrode active material can be adjusted according to the capacity of the positive electrode. In the case of double-sided coating, in which a slurry containing the active material is applied to both sides of the current collector, the supported amount is considered per side.

[0029] In this specification, even if a flame, spark, and / or smoke is observed in a nail penetration test, if the flame remains at the puncture site, i.e., the fire does not spread, and if the secondary battery does not experience thermal runaway, it is not considered to have ignited. For example, even if a flame, spark, and / or smoke is observed in a nail penetration test, if the flame remains at the puncture site, i.e., the fire does not spread, and if the secondary battery does not experience thermal runaway, it is not considered to have ignited. For example, if a secondary battery is subjected to a nail penetration test and the above-mentioned fire does not occur, it can be said to be a non-igniting secondary battery.

[0030] In this embodiment, a method for manufacturing a positive electrode active material according to one embodiment of the present invention will be described with reference to FIGS. 1 to 3. In the manufacturing method according to one embodiment of the present invention, lithium cobalt oxide (chemical formula: LiCoO) having a median diameter (D50) of less than 12 μm, specifically, a median diameter (D50) of 10 μm or less is used. ２ We focused on lithium cobalt oxide (LCO). A small median diameter (D50) means a large surface area. Furthermore, in the manufacturing method of one embodiment of the present invention, the heat treatment conditions were thoroughly considered to determine appropriate ranges, etc., in order to improve productivity. Furthermore, the types of additive elements and their appropriate concentration ranges were determined to obtain good cycle characteristics. A specific manufacturing method will now be described.

[0031] <<Production Method 1 of Positive Electrode Active Material>> A production method 1 of a positive electrode active material according to one embodiment of the present invention will be described.

[0032] <Step S11> In step S11 shown in FIG. 1, a lithium source (Li source in the figure) and a cobalt source (Co source in the figure) are prepared as starting materials. A lithium-containing compound is preferably used as the lithium source, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. The lithium source is preferably highly pure, such as a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher. The cobalt source is preferably a cobalt-containing compound, such as a cobalt oxide such as tricobalt tetroxide or cobalt hydroxide. The cobalt source is preferably highly pure, such as a material with a purity of 3N (99.9%) or higher, preferably 4N (99.995%) or higher. The use of highly pure materials allows for the control of impurities in the positive electrode active material. As a result, the decrease in discharge capacity during charge / discharge cycles can be suppressed. Furthermore, the reliability of the secondary battery is improved.

[0033] <Step S12> In step S12 shown in FIG. 1 , the lithium source and the cobalt source are mixed while being pulverized. This step produces a mixture (also referred to as a mixed material) containing the lithium source and the cobalt source. The pulverization and mixing method can be either a dry method or a wet method. The wet method is superior to the dry method in that it allows for finer pulverization. When using the 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 preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. The water content of dehydrated acetone is reduced to 10 ppm or less. By mixing the lithium source and the cobalt source with dehydrated acetone and pulverizing the mixture, the amount of impurities mixed into the mixture can be reduced.

[0034] A ball mill, a bead mill, or the like can be used as a means for grinding and mixing. A bead mill is a means using a stirring mechanism. A ball mill is a means for rotating a pot containing media. It is preferable to use aluminum oxide balls or zirconium oxide balls as the media. Typical zirconium oxide balls are made of a material that emits little impurities, so there is little contamination from the media. Furthermore, in order to suppress contamination from the media, it is preferable to set the peripheral speed of the ball mill pot to 100 mm / s or more and 2000 mm / s or less.

[0035] <Step S13> In step S13 shown in FIG. 1 , the mixture is heated. Heating is preferably performed at a temperature of 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably around 950°C. If the temperature is too low, the lithium source and the cobalt source may not be sufficiently melted. On the other hand, if the temperature is too high, lithium may sublimate from the lithium source and be reduced, and cobalt may be excessively reduced. For example, if cobalt is reduced from trivalent to divalent, oxygen defects may be induced. Such a high temperature is undesirable for the method of producing a positive electrode active material.

[0036] If the heating time is too short, lithium cobalt oxide may not be synthesized, which is undesirable, but if it is too long, productivity is thought to decrease. Therefore, the heating time is preferably 1 hour or more and 50 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0037] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate is preferably 200° C. / h.

[0038] For heating the composite oxide, the atmosphere preferably contains oxygen. Furthermore, it is preferable that the atmosphere has little water, and typically, the heating atmosphere is preferably dry air. For example, there is a method of continuously introducing dry air into the furnace. In this case, it is preferable that the flow rate of the dry air is 5 L / min or more and 20 L / min or less. The method of continuously introducing the atmospheric gas into the furnace and the gas flowing through the furnace as described above is called flow. In addition, as a method of preventing flow, a method can be used in which the furnace is depressurized and then filled with the atmospheric gas (this can also be called purging) to prevent the gas from entering or leaving the furnace. For example, it is preferable to depressurize the furnace until the differential pressure gauge reaches -970 hPa, and then fill the furnace with the atmospheric gas until it reaches 50 hPa. In the case of purging, dry air can also be used as the atmospheric gas.

[0039] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0040] A rotary kiln or a roller hearth kiln may be used as the furnace for heating. Heating in a rotary kiln can be performed while stirring, whether in a continuous or batch system.

[0041] The container for containing the mixture during heating is preferably a crucible or a setter (also called a sheath). The purity of the container is 3N (99.9%) or higher, preferably 4N (99.99%) or higher. Placing a lid on the container before heating can prevent unnecessary volatilization of the material. Aluminum oxide, zirconium oxide, or mullite-cordierite may also be used as the material for the crucible and setter.

[0042] Furthermore, it is preferable to use a container that has been used multiple times rather than a new one. In this specification, a new container refers to one that has undergone the process of adding lithium and a mixture and heating two or fewer times. A container that has been used multiple times refers to one that has undergone the process of adding a mixture and heating three or more times. This is because if a new container is used, there is a risk that some of the material will be absorbed and / or adhered to the container during heating. If some of the material is lost, there is a greater concern that a high-quality positive electrode active material will not be obtained. On the other hand, this risk is less with a container that has been used multiple times, and the decrease in discharge capacity during charge-discharge cycles may be suppressed.

[0043] After heating, the mixture may be crushed and sieved as necessary. When recovering the heated material, the mixture may be transferred from the container to a mortar and then recovered. The mortar is preferably an aluminum oxide mortar or a zirconium oxide mortar.

[0044] <Step S14> Through the above steps, lithium cobalt oxide can be synthesized as shown in step S14 in Fig. 1. It is preferable that the median diameter (D50) of the lithium cobalt oxide is 10 µm or less, because this suppresses a decrease in discharge capacity during charge-discharge cycles.

[0045] Although an example of producing lithium cobalt oxide by a solid phase method as in steps S11 to S14 has been shown, lithium cobalt oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0046] Alternatively, pre-synthesized lithium cobalt oxide may be used in step S14. The pre-synthesized lithium cobalt oxide has a median diameter (D50) of 10 μm or less. In this case, steps S11 to S13 can be omitted.

[0047] It is preferable that the pre-synthesized lithium cobalt oxide that can be used in step S14 has a concentration of elements other than the main component within a certain range. Note that the main component elements of lithium cobalt oxide refer to lithium, oxygen, and cobalt, and the elements other than the main component refer to elements other than lithium, oxygen, and cobalt. Elements that fall under the category of additive elements, which will be described later, can be considered elements other than the main component.

[0048] The concentrations of the main components and elements other than the main components will now be described. For example, when lithium cobalt oxide is analyzed using glow discharge mass spectrometry (GD-MS), the concentration of each element (unit: wt ppm) can be obtained. Tables 1 to 3 show the concentration of each element for four types of lithium cobalt oxide (materials Sm-1, Sm-2, Sm-3, and Sm-4). In the tables, "Matrix" refers to the main component, "Binder" refers to the auxiliary electrode, "Source" refers to the influence of components of the measurement device, "<" refers to a value below the detection limit, "≦" refers to the presence of interfering elements but below the numerical value, and "~" refers to the presence of variation or the presence of some interfering elements but a semi-quantitative value.

[0049]

[0050]

[0051]

[0052] From the above table, it is possible to read the concentration range of each element contained in the lithium cobalt oxide (material Sm-1, material Sm-2, material Sm-3, material Sm-4). For example, when material Sm-1 is used as the lithium cobalt oxide used in step S14, it can be read from the above table that the magnesium concentration is preferably 40 wt ppm or less and the titanium concentration is preferably 40 wt ppm or less.

[0053] Although not shown, after step S14, the lithium cobalt oxide may be heated (sometimes referred to as initial heating). This initial heating may further improve the distribution of the additive elements due to the effect of lithium being desorbed from a portion of the surface layer of the lithium cobalt oxide. Initial heating can also be expected to enhance the crystallinity of the internal crystalline structure of the positive electrode active material. However, after extensive research, the inventors have found that for lithium cobalt oxide with a median diameter (D50) of 10 μm or less, the distribution of the additive elements can be made appropriate even without this heating, and the decrease in discharge capacity during charge-discharge cycles is suppressed. Therefore, this heating is not performed in Production Method 1. This step is preferable because it improves productivity.

[0054] <Step S20> As shown in FIG. 1 , an additive element is added to lithium cobalt oxide in step S20. Specifically, a compound containing the additive element (referred to as the additive element source) is prepared. In the manufacturing method 1 for a positive electrode active material described in this embodiment, the additive element is added in multiple steps. Therefore, in the flow shown in FIG. 1 , the additive element prepared first is designated as A1, the additive element prepared second is designated as A2, and the additive element prepared third is designated as A3. The compound containing the additive element A1 (A1 source) is preferably a compound different from the compound containing the additive element A2 (A2 source), and further, the A2 source is preferably a compound different from the compound containing the additive element A3 (A3 source). Step S20 of adding the additive element A1 will be described in detail using FIG. 2A .

[0055] 2A, an Al source to be added to lithium cobalt oxide is prepared. A lithium source may be prepared together with the Al source.

[0056] The additional element A1 can be one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0057] When magnesium is selected as the additive element, the A1 source can be called a magnesium source (Mg source in the figure). A magnesium compound can be used as the magnesium source, and representative examples include magnesium fluoride, magnesium oxide, magnesium hydroxide, and magnesium carbonate. A plurality of the above-mentioned magnesium sources may also be used.

[0058] When fluorine is selected as the additive element, the Al source can be called a fluorine source (F source in the figure). As the fluorine source, a fluorine compound can be used, for example, lithium fluoride (LiF), magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0059] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0060] The fluorine source is preferably a gas, and fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F), nitrogen trifluoride (NF ３ ) or the like may be used to mix the gas into the heated atmosphere. It is also preferable to use a plurality of the above-mentioned fluorine sources as the gas.

[0061] In the manufacturing method 1 described with reference to FIG. 2A, magnesium and fluorine are used as the additive element A1, and lithium fluoride (LiF) is prepared as the fluorine source and magnesium fluoride (MgF ２ ) is prepared. Magnesium fluoride is sometimes called the magnesium source.

[0062] <Step S22> Next, in step S22 shown in FIG. 2A, the magnesium source and the fluorine source are crushed and mixed. This step can be carried out under the crushing and mixing conditions selected from those described in step S12. Lithium fluoride and magnesium fluoride are mixed in a mixture of LiF:MgF ２ The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be too excessive, which may deteriorate the charge-discharge cycle characteristics. Therefore, the molar ratio of lithium fluoride and magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = 0.33 or its vicinity) is more preferable. In this specification, "or its vicinity" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0063] <Step S23> In step S23 shown in Fig. 2A, the pulverized and mixed materials are collected to obtain the Al source. If necessary, sieving may be performed during collection. The Al source shown in step S23 contains multiple starting materials and can be called a mixture.

[0064] The particle size of the mixture is preferably a median diameter (D50) of 100 μm or less, more preferably 1 μm or more and 60 μm or less. Even when a single material is used as the Al source, the median diameter (D50) is preferably 100 μm or less, more preferably 1 μm or more and 60 μm or less.

[0065] Such a finely powdered mixture (including the case where only one kind of additive element is included) is preferable because when it is mixed with lithium cobalt oxide in a later step, the mixture can be easily adhered uniformly to the surfaces of the lithium cobalt oxide particles, which facilitates uniform diffusion or distribution of the additive element in the surface layer portion of the positive electrode active material after heating.

[0066] 1, lithium cobalt oxide and an Al source are mixed. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the Al source is preferably Co:Mg=100:y (0.1≦y≦3), and more preferably Co:Mg=100:y (0.3≦y≦1).

[0067] The mixing in step S31 can be performed using a compounding process using mechanical energy. Compounding is a process in which mechanical energy (energy related to impact, compression, or shear) is applied to a powder layer using a rotor or the like, causing a mechanochemical reaction to form a new material. The rotor rotation speed in the compounding process is preferably 2000 rpm or more and 4000 rpm or less, and the processing time is preferably 5 minutes or more and 30 minutes or less. Since heat may be generated by rotation, the compounding treatment device may be equipped with a water cooling function. For lithium cobalt oxide with a median diameter (D50) of 10 μm or less, the compounding process can be performed favorably by setting the median diameter (D50) of the A1 source to 100 μm or less, particularly 1 μm or more and 60 μm or less.

[0068] A ball mill, a bead mill, or the like can be used for the mixing in step S31. It is preferable to use milder conditions than those in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or a shorter mixing time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing.

[0069] 1, the mixed materials are collected to obtain a mixture 901. When collecting the materials, sieving may be performed as necessary.

[0070] 1, the mixture 901 is heated under heating conditions selected from those described in step S13.

[0071] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds is preferably a temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 is preferably 650° C. or higher.

[0072] Of course, if the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 901, the reaction will proceed more easily. ２ When LiF and MgF ２ Since the eutectic point of LiF is around 742°C, it is preferable to set the heating temperature in step S33 to 742°C or higher. When the melting point of a fluoride such as lithium fluoride is lower than the melting point of the other additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source. For example, when LiF and MgF are used as additive element sources, ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, it is preferable that the heating temperature in step S33 be 742°C or higher.

[0073] Also, LiCoO ２ :LiF:MgF ２ The mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 showed an initial melting temperature T ｉｍ is 779 ° C, the melting peak temperature T ｐｍ is 815°C, and the melting end temperature T ｅｍ The heating temperature was 826° C. Therefore, it is preferable to set the heating temperature to 826° C. or higher.

[0074] The heating temperature is set to be lower than the melting point of lithium cobalt oxide (1130°C). At temperatures near the melting point, there is a concern that a small amount of lithium cobalt oxide may melt. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source, etc., within an appropriate range. If the temperature is too high, fluoride decreases due to sublimation. For example, the vapor pressure of lithium fluoride rises sharply from 900°C. Therefore, the heating temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, even more preferably 900°C or lower, and even more preferably 850°C or lower. When the sublimation of lithium fluoride is suppressed, the surface layer of the positive electrode active material can have appropriate concentrations of fluorine and lithium. Furthermore, if the surface layer contains sufficient lithium, a heterophase (MgTiO ３Another advantage is that it makes it harder for problems like these to occur.

[0075] In consideration of these, the heating temperature in step S33 is preferably 650°C or higher and 1130°C or lower, more preferably 650°C or higher and 1000°C or lower, even more preferably 650°C or higher and 950°C or lower, and even more preferably 650°C or higher and 900°C or lower. Also, it is preferably 742°C or higher and 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, even more preferably 742°C or higher and 950°C or lower, and even more preferably 742°C or higher and 900°C or lower. Also, it is preferably 826°C or higher and 1100°C or lower, preferably 826°C or higher and 1130°C or lower, more preferably 826°C or higher and 1000°C or lower, even more preferably 826°C or higher and 950°C or lower, and even more preferably 826°C or higher and 900°C or lower. The heating temperature in step S33 is preferably lower than that in step S13.

[0076] In manufacturing method 1, for example, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be set to, for example, 742°C or higher and 950°C or lower, improving productivity. However, since LiF has a lower specific gravity in a gaseous state than oxygen, LiF may sublimate upon heating, and if it sublimes, the amount of LiF in mixture 903 will decrease. This weakens its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the sublimation of LiF. Even if LiF is not used as the fluorine source, Li on the surface of the lithium cobalt oxide may react with F in the fluorine source to produce LiF, which may then sublimate. Therefore, even if a fluorine source other than LiF is used, it is still necessary to suppress sublimation.

[0077] Therefore, it is preferable to heat the mixture 901 in an atmosphere containing LiF by placing a lid on the container containing the mixture, for example, so that sublimation of LiF in the mixture 901 can be suppressed.

[0078] The furnace should be kept in an oxygen-containing atmosphere during heating. If there is a lack of oxygen, cobalt and other elements may be reduced, and the lithium cobaltate may no longer be able to maintain its layered rock salt crystal structure. For example, the pressure inside the furnace may be set to exceed atmospheric pressure.

[0079] In this step, it is preferable to heat the mixture 901 so that the particles do not stick together. If the particles of the mixture 901 stick together during heating, this may block the path for the added element (e.g., fluorine) to diffuse, which may worsen the distribution of the added element (e.g., magnesium and fluorine) in the surface layer. Furthermore, the sticking of the particles may reduce the contact area with oxygen.

[0080] When heating using a rotary kiln, it is preferable to heat by controlling the flow rate of the oxygen-containing atmosphere in the furnace. Furthermore, after the atmosphere is first purged and the oxygen atmosphere is introduced into the furnace, the atmosphere is not flowed. When heating using a roller hearth kiln, for example, the mixture 901 can be heated in an atmosphere containing LiF by placing a lid on a container containing the mixture 901.

[0081] A higher heating temperature is preferred because the reaction proceeds more easily, the heating time is shorter, and productivity is high. The heating time is preferably 2 hours or more, and more preferably 5 hours to 20 hours.

[0082] A supplementary note on heating conditions follows. The heating time varies depending on conditions such as the heating temperature, the particle size and composition of the lithium cobalt oxide in step S14, etc. When the median diameter (D50) of the lithium cobalt oxide is small, a lower temperature or a shorter time may be preferable than when the median diameter (D50) is large. When the median diameter (D50) of the lithium cobalt oxide in step S14 of FIG. 1 is about 7 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower, and more preferably 750° C. or higher and 950° C. or lower. The heating time is preferably, for example, 5 hours or higher and 20 hours or lower, and more preferably 8 hours or higher and 12 hours or lower.

[0083] 1, the heated material is recovered to obtain a composite oxide 902. In this step, after recovery, the material may be crushed as necessary.

[0084] <Step S40> In step S40 shown in FIG. 1, an A2 source is prepared. The additive element described in step S21 can be used as the additive element A2, and it is preferable to select an element different from the additive element A1 as the additive element A2. In manufacturing method 1, nickel and aluminum are used as the additive element A2. A nickel compound can be used as the nickel source, and typically nickel oxide, nickel hydroxide, etc. can be used. An aluminum compound can be used as the aluminum source, and typically aluminum oxide, aluminum hydroxide, etc. Step S40, in which the additive element A2 is added, will be described in detail using FIG. 2B.

[0085] <Steps S41 to S43> In step S41 of FIG. 2B , a nickel source (Ni source in the figure) and an aluminum source (Al source in the figure) are prepared. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of nickel atoms Ni in the A2 source is preferably Co:Ni = 100:z1 (0.2≦z1≦0.7), more preferably Co:Ni = 100:z1 (0.4≦z1≦0.6). The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of aluminum atoms Al in the A2 source is preferably Co:Al = 100:z2 (0.1≦z2≦0.5), more preferably Co:Al = 100:z2 (0.15≦z2≦0.3). Furthermore, in this step, the aluminum concentration is preferably lower than the nickel concentration.

[0086] Next, in step S42, the nickel source and the aluminum source are each pulverized. The pulverization conditions can refer to the conditions in step S22. Then, in step S43, the pulverized materials are mixed to obtain the A2 source.

[0087] The particle size of the A2 source is preferably a median diameter (D50) of 100 μm or less, more preferably 1 μm or more and 60 μm or less. Even when a single material is used as the A2 source, the median diameter (D50) is preferably 100 μm or less, more preferably 1 μm or more and 60 μm or less.

[0088] <Step S51> In step S51 shown in FIG. 1 , the composite oxide 902 and the A2 source are mixed. The description of step S31 can be referred to for the mixing conditions. Note that a composite treatment using mechanical energy can be applied to the mixing in step S51. For lithium cobalt oxide having a median diameter (D50) of 10 μm or less, the A2 source can be made to have a median diameter (D50) of 100 μm or less, particularly 1 μm or more and 60 μm or less, to suitably proceed with the composite treatment.

[0089] 1, the mixed materials are collected to obtain a mixture 903. When collecting the materials, sieving may be performed.

[0090] <Step S53> Next, in step S53 shown in FIG. 1, the mixture 903 is heated. For heating conditions, the description of step S33 can be referred to. Note that in this step as well, it is preferable to place a lid on the container that holds the mixture. Furthermore, a supplementary note will be made regarding the heating time in this step. A shorter heating time is preferable as it increases productivity. The heating time in this step can be shorter than that in step S33. Specifically, it is preferably one hour or more, and more preferably one hour or more and five hours or less.

[0091] 1, the heated material is recovered to obtain a composite oxide 904. In this step, after recovery, the material may be crushed as necessary.

[0092] <Step S60> In step S60 shown in Fig. 1, an A3 source is prepared. The additive element described in step S21 can be used as the additive element A3, and it is preferable to select an element different from the additive elements A1 and A2 as the additive element A3. In manufacturing method 1, titanium is used as the additive element A3. A titanium-containing compound (titanium compound) is used as the titanium source, and examples of titanium compounds include titanium oxide and lithium titanate. Examples of titanium oxide include TiO ２ There is a compound represented by the formula: Lithium titanate has Li ２ TiO ３ , LiTiO ２One or more compounds selected from the above-mentioned compounds can be used as the titanium source. Step S60 of adding the additional element A3 will be described in detail with reference to FIG. 2C.

[0093] 2C , a titanium source (Ti source in the figure) is prepared. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of titanium atoms Ti in the A3 source is preferably Co:Ti=100:z3 (0.01≦z3≦0.5), and more preferably Co:Ti=100:z3 (0.05≦z3≦0.25).

[0094] Next, in step S62, the titanium source is pulverized. The pulverization conditions can refer to the conditions in step S22. Then, in step S63, the pulverized material is recovered to obtain the A3 source.

[0095] The particle size of the A3 source is preferably a median diameter (D50) of 100 μm or less, more preferably 1 μm or more and 60 μm or less. Even when a single material is used as the A3 source, the median diameter (D50) is preferably 100 μm or less, more preferably 1 μm or more and 60 μm or less.

[0096] <Step S71> In step S71 shown in FIG. 1 , the composite oxide 904 and the A3 source are mixed. The description of step S31 can be referred to for the mixing conditions. Note that a composite treatment using mechanical energy can be applied to the mixing in step S51. For lithium cobalt oxide with a median diameter (D50) of 10 μm or less, the A2 source can be made to have a median diameter (D50) of 100 μm or less, particularly 1 μm or more and 60 μm or less, to suitably proceed with the composite treatment.

[0097] 1, the mixed materials are collected to obtain a mixture 905. When collecting the materials, sieving may be performed.

[0098] <Step S73> In step S73 shown in FIG. 1, the mixture 905 is heated. For heating conditions, the description of step S33 can be referred to. Note that in this step as well, it is preferable to place a lid on the container that holds the mixture. Furthermore, a supplementary note will be made regarding the heating time in this step. A shorter heating time is preferable because it increases productivity. The heating time in this step can be shorter than that in step S33. Specifically, it is preferably one hour or more, and more preferably one hour or more and five hours or less.

[0099] <Step S74> In step S74 shown in FIG. 1 , the heated material is recovered to obtain a positive electrode active material. Sieving may be performed during recovery. By the above-described manufacturing method 1, a positive electrode active material according to one embodiment of the present invention can be manufactured. Manufacturing method 1 is a highly productive method, and can provide a manufacturing method that reduces costs.

[0100] 3A and 3B , a method for producing a positive electrode active material 2, which is an embodiment of the present invention and differs from the method for producing 1, will be described. The method for producing 2 is different from the method for producing 1 mainly in the number of times that the additive element is added, and for other details, the description of the method for producing 1 can be referred to.

[0101] In manufacturing method 1, magnesium and fluorine were added as the additional element A1, and nickel and aluminum were added as the additional element A2 at different times, but magnesium, fluorine, nickel, and aluminum may also be added in the same step. A method of adding these in the same step as the A1+A2 source in step S20a of Figures 3A and 3B is shown. This manufacturing method can also be said to be highly productive because it can reduce the mixing and heating steps, and can provide a manufacturing method that reduces costs.

[0102] <<Production Method 3 for Cathode Active Material>> A description will now be given of Production Method 3 for a cathode active material, which is an embodiment of the present invention and differs from Production Methods 1 and 2. In Production Method 3 for a cathode active material, in step S11, an additive element source (A0 source) is prepared in addition to a lithium source and a cobalt source. In Production Method 2, magnesium, fluorine, nickel, and aluminum are added in the same step, but of these, a magnesium source and a fluorine source are prepared as the A0 source in the same step as the lithium source and cobalt source. This production method can also be said to be highly productive because it can reduce the mixing and heating steps, and can provide a production method that reduces costs.

[0103] Alternatively, a lithium cobalt oxide containing magnesium and fluorine may be used, in which the A0 source is added in the same step as the lithium source and the cobalt source, and in this case, steps S11 to S13 can be omitted.

[0104] <<Production Method 4 of Positive Electrode Active Material>> A description will be given of Production Method 4 of a positive electrode active material, which is one embodiment of the present invention and differs from Production Methods 1 to 3. Production Method 4 is a method in which an Al source is added, and then an aluminum source, a nickel source, a titanium source, and the like are added as an A2 source and an A3 source. This production method can also be said to be a highly productive method because it can reduce the mixing and heating steps, and can provide a production method that reduces costs.

[0105] A titanium compound is used as the A3 source, and the order of adding the A3 source after first adding at least the A1 source, or adding the A1 source and the A2 source before first adding the A3 source, is a preferred step in order to suppress a decrease in discharge capacity during charge-discharge cycles.

[0106] The manufacturing methods 1 to 4 can be combined as appropriate.

[0107] This embodiment mode can be used in combination with other embodiments.

[0108] Embodiment 2 In this embodiment, a positive electrode active material particle 100 of one embodiment of the present invention will be described with reference to FIGS.

[0109] 4A is a cross-section of a positive electrode active material particle 100 according to one embodiment of the present invention. The positive electrode active material particle 100 preferably has a surface layer portion 100a and an interior portion 100b. In FIG. 4A, the dashed line indicates an example of the boundary between the surface layer portion 100a and the interior portion 100b.

[0110] In this specification, the surface layer 100a can be referred to as a region within 20 nm in the depth direction from the particle surface, a region within 10 nm in the depth direction from the particle surface, a region within 5 nm in the depth direction from the particle surface, or a region within 3 nm in the depth direction from the particle surface. Note that the depth direction is a direction perpendicular or approximately perpendicular to the particle surface. Perpendicular or approximately perpendicular refers to a range of 80° to 100° relative to the surface. Surfaces formed by cracks and / or fissures may also be referred to as the particle surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0111] The region deeper than the surface layer 100a can be referred to as the inner portion 100b, which is synonymous with the inner region or the core.

[0112] The positive electrode active material particles 100 preferably contain lithium cobalt oxide as a main component and belong to the space group R-3m. The inner portion 100b preferably has a layered rock salt type crystal structure. The surface portion 100a preferably has a rock salt type crystal structure whose crystal orientation is identical or approximately identical to the layered rock salt type. Of course, the surface portion 100a must have a layered rock salt type crystal structure in part to enable the insertion and desorption of lithium ions. Therefore, it is preferable that the surface portion 100a have both a layered rock salt type crystal structure and a rock salt type crystal structure.

[0113] (001) in FIG. 4A indicates the (001) plane of lithium cobalt oxide. The surface layer portion 100a includes a region having a (001) plane (a (001)-oriented region, a region having a surface parallel to the (001) plane, called a basal region). The surface layer portion 100a also includes an edge region. The edge region is a region having a surface exposed in a direction intersecting the (001) plane, or a region other than a (001)-oriented region. In FIG. 4A, X1-X2 is a line along the cross section of the edge region, and Y1-Y2 is a line along the cross section of the basal region.

[0114] 4B and 4C are schematic diagrams showing the distribution of added elements in the edge region. In FIGS. 4B and 4C, the horizontal axis indicates the distance from the measurement point, and the vertical axis indicates the concentration of the element. By defining the particle surface on the horizontal axis, the distance from the particle surface can be determined. Schematic diagrams such as those in FIGS. 4B and 4C can be obtained from graphs of cross-sectional analysis results. The analysis direction of the cross-sectional analysis is from the surface of the particle to the interior, and analysis in this direction is called depth profile analysis. In other words, the distance on the horizontal axis can also be considered the depth of the particle. For cross-sectional analysis, for example, STEM-EDX line analysis can be used.

[0115] The positive electrode active material particles 100 do not contain any metal oxides, such as aluminum oxide, that do not have lithium sites that can contribute to charge and discharge, or carbonates or hydroxyl groups that are chemically adsorbed after the production of the positive electrode active material. Therefore, the surface of the positive electrode active material particles 100 refers to the surface of the particles that includes the surface layer portion 100a and the interior portion 100b. The attached metal oxide refers to, for example, a metal oxide whose crystal orientation does not match that of the interior portion 100b.

[0116] Furthermore, the cathode active material particles 100 do not include the electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom that are attached to the cathode active material particles 100. The attached electrolyte, organic solvent, binder, conductive material, or compounds derived therefrom can be removed by washing.

[0117] <Containing Elements> The positive electrode active material particles 100 contain lithium, a transition metal M, oxygen, and an additive element. The transition metal M is at least one selected from cobalt, nickel, and manganese. When cobalt is selected as the transition metal M, the positive electrode active material particles 100 contain lithium cobalt oxide (LiCoO ２ ) and an additive element. ２ A positive electrode active material having the lithium cobaltate and the additive element can be manufactured according to the first embodiment. However, the lithium cobaltate does not need to have a strict composition as shown in the chemical formula. In other words, the composition of the lithium cobaltate is not limited to Li:Co:O=1:1:2 (atomic ratio).

[0118] The positive electrode active material needs to contain a transition metal M that can be oxidized and reduced in order to maintain charge neutrality even when lithium ions are inserted and extracted. If the transition metals contained in the positive electrode active material particles 100 contain cobalt at 75 atomic % (sometimes referred to as atomic %) or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, this has many advantages, such as relatively easy synthesis and handling, and excellent charge-discharge cycle characteristics, making it preferable.

[0119] <Additive Element> The additive element contained in the positive electrode active material particles 100 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. Nickel is sometimes used as a main component of the positive electrode active material particles 100, and nickel may be further added to such positive electrode active material particles 100 as an additive element.

[0120] [Titanium] The presence of titanium in the surface layer portion 100a can promote the insertion and desorption of lithium ions into and from the positive electrode active material particles 100. This phenomenon will be described below.

[0121] Although titanium exists stably in the octahedral sites of hexavalent oxygen atoms in the oxide, titanium oxide and lithium titanate cannot form a stable layered rock salt type crystal structure. ２ The rutile crystal structure is the most stable, and lithium titanate, Li ４ Ti ５ O １２ has a spinel-type crystal structure. Therefore, when a small amount of titanium is dissolved in the surface layer 100a having a layered rock salt type or rock salt type crystal structure, defects occur in part of the crystal structure of the surface layer 100a. Lithium cobalt oxide is in a discharged state (a discharged state is referred to as Li ｘ CoO ２ It is known that the band gap is large and the resistance is high (this is also referred to as when x is 1 in the graph). Therefore, by introducing titanium defects into a part of the surface layer 100a, the band gap can be narrowed and the resistance can be reduced.

[0122] Cobalt is also present in the oxide. ３＋ is the most stable, while titanium is Ti ４＋ Therefore, the charge on the lithium ion around titanium is relatively lower than that around cobalt. ４＋ Defects may be induced in the cation sites near the titanium, and such defects reduce the diffusion resistance of cations, particularly lithium ions. Therefore, the diffusion resistance of lithium ions is reduced around the titanium. Therefore, the presence of titanium in the surface layer portion 100a can reduce the diffusion resistance of lithium ions at the interface between the electrolyte and the positive electrode active material particles 100.

[0123] If the titanium content is too low, the above-mentioned effects are not fully exhibited. On the other hand, if the titanium content is too high, it may form a different phase from other added elements such as magnesium (for example, Mg in an ilmenite-type crystal structure). ２ TiO ４ Furthermore, magnesium may be taken away by the formation of a different phase, which may reduce the magnesium concentration in the surface layer portion 100a. ＋ When the battery is charged at a high voltage exceeding 100 V, it is preferable for the surface layer 100a to contain magnesium in order to suppress phase change. Therefore, the loss of magnesium due to the formation of a different phase is a major disadvantage. Furthermore, if there are too many defects due to titanium, there is a concern that oxygen may be more likely to be desorbed from the surface. Therefore, it is preferable that titanium is present in the surface layer 100a together with magnesium, fluorine, etc., or that titanium is present in the surface or near-surface region of the surface layer 100a at a concentration lower than that of magnesium.

[0124] Specifically, the number of Ti atoms in the entire positive electrode active material particle 100 is preferably 0.0001 to 0.005 times (0.01% to 0.5%) the number of Co atoms, and more preferably 0.0005 to 0.0025 times (0.05% to 0.25%). The amount of titanium in the entire positive electrode active material particle 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material particle 100 using, for example, inductively coupled plasma-mass spectrometry (ICP-MS), or may be based on the value of the blending of raw materials in the manufacturing process of the positive electrode active material particle 100. Furthermore, the titanium concentration determined by GD-MS is preferably 300 wt ppm to 600 wt ppm, and the Mg / Ti ratio determined by GD-MS preferably satisfies 2 to 7.

[0125] The titanium concentration in the surface layer portion 100a determined by XPS measurement is preferably 0.5 atomic% or more and 2.5 atomic% or less. Furthermore, the Co / Ti ratio in the surface layer portion 100a determined by XPS measurement preferably satisfies 3 or more and 8 or less. Titanium is preferably present at a higher concentration in the surface layer portion 100a than in the interior portion 100b. The high concentration of titanium present in the surface layer portion 100a can reduce the diffusion resistance of lithium ions at the interface between the electrolyte and the positive electrode active material particles 100. Furthermore, the Mg / Ti ratio in the surface layer portion 100a determined by XPS measurement preferably satisfies 2 or more and 7 or less, and titanium is preferably present at a lower concentration than magnesium.

[0126] [Magnesium] The presence of magnesium in the surface layer 100a helps to maintain the layered rock-salt crystal structure in the interior 100b. This phenomenon will be described below.

[0127] Magnesium ions are divalent cations, and since magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock salt crystal structure, they tend to enter the lithium site. ２ Because they function as pillars supporting the layers, the layered rock salt crystal structure is easily maintained.

[0128] In addition, the presence of magnesium ｘ CoO ２ When x in the formula (100a) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, if the magnesium concentration in the surface layer portion 100a is high, it can be expected that the corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte will be improved. In order to achieve the effect of corrosion resistance to hydrofluoric acid, it is preferable that the magnesium concentration in the surface is high.

[0129] If the magnesium content is too low, the above-mentioned effects will not be fully achieved. On the other hand, if the magnesium content is too high, the capacity will decrease. Therefore, in the entire positive electrode active material particle 100, the number of magnesium atoms is preferably 0.002 to 0.06 times the number of cobalt atoms, more preferably 0.005 to 0.03 times, and even more preferably approximately 0.01 times. The amount of magnesium in the entire positive electrode active material particle 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material particle 100 using, for example, ICP-MS, or may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material particle 100. Furthermore, the magnesium concentration determined by GD-MS is preferably 800 wt ppm to 2000 wt ppm.

[0130] The magnesium concentration in the surface layer portion 100a determined by XPS measurement is preferably 3 atomic % or more and 10 atomic % or less. Furthermore, the Co / Mg ratio in the surface layer portion 100a determined by XPS measurement is preferably 2 or more and 4 or less. It is preferable that magnesium is present at a higher concentration in the surface layer portion 100a than in the interior portion 100b.

[0131] [Nickel] In lithium cobalt oxide, which has a layered rock-salt crystal structure, nickel can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, it has a lower redox potential than cobalt, making it easier to release lithium and electrons during charging. This is expected to result in faster charge and discharge speeds.

[0132] Furthermore, when nickel is present at the lithium site, the deviation of the layer structure consisting of the octahedrons of cobalt and oxygen can be suppressed. This is because nickel present at the lithium site also acts as a CoO ２ This is thought to be because nickel functions as a pillar supporting the layers. Furthermore, the presence of nickel at the lithium sites is expected to make the crystal structure more stable when the material is charged at high temperatures, for example, above 45°C.

[0133] Furthermore, the order of ionization tendency is lowest in magnesium, followed by aluminum, cobalt, and nickel (Mg>Al>Co>Ni). Therefore, nickel is less likely to dissolve into the electrolyte than the other elements listed above during charging. Therefore, nickel is considered to be highly effective in stabilizing the crystal structure of the surface layer 100a in a charged state.

[0134] Furthermore, nickel is Ni ２＋ , Ni ３＋ , Ni ４＋ Of which Ni ２＋ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. Therefore, nickel is thought to have the effect of suppressing the phase change from the layered rock salt type to the spinel-type crystal structure.

[0135] On the other hand, excessive nickel is undesirable because it increases the influence of distortion due to the Jahn-Teller effect. Furthermore, excessive nickel may adversely affect lithium insertion and desorption. Therefore, it is preferable that the amount of nickel contained in the entire positive electrode active material particle 100 is appropriate. Specifically, the number of nickel atoms in the entire positive electrode active material particle 100 is preferably more than 0% but not more than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. The amount of nickel in the entire positive electrode active material particle 100 referred to here may be, for example, a value obtained by elemental analysis of the entire positive electrode active material using ICP-MS or the like, or may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material. The nickel concentration determined by GD-MS is preferably 3000 wt ppm to 5000 wt ppm, and the Mg / Ni ratio determined by GD-MS is preferably 0.1 to 0.8.

[0136] The nickel concentration in the surface layer 100a determined by XPS measurement is preferably 0.8 atomic % or more and 1.8 atomic % or less. Furthermore, the Co / Ni ratio in the surface layer 100a determined by XPS measurement is preferably 9 or more and 12 or less.

[0137] [Aluminum] Aluminum can exist at the cobalt site in a layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is unlikely to move during charging and discharging. Therefore, the aluminum and the lithium around it function as pillars, suppressing changes in the crystal structure. Therefore, even if the positive electrode active material particle 100 is subjected to a force that causes it to expand and contract in the c-axis direction due to the insertion and desorption of lithium ions, i.e., even if a force that causes it to expand and contract in the c-axis direction due to a change in the charge depth or charge rate is applied, deterioration of the positive electrode active material particle 100 can be suppressed.

[0138] Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charge durability. Furthermore, because the Al-O bond is stronger than the Co-O bond, it can suppress the desorption of oxygen from the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum can improve the safety of secondary batteries.

[0139] On the other hand, excessive aluminum may adversely affect lithium insertion and desorption. Therefore, it is preferable that the cathode active material particles 100 as a whole contain an appropriate amount of aluminum. For example, the number of aluminum atoms contained in the cathode active material particles 100 as a whole is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount of aluminum contained in the cathode active material particles 100 as a whole may be, for example, a value obtained by performing elemental analysis of the entire cathode active material particles 100 using ICP-MS or the like, or may be based on the value of the raw material composition during the manufacturing process of the cathode active material particles 100. The aluminum concentration determined by GD-MS is preferably 1200 wt ppm to 1800 wt ppm, and the Mg / Al ratio determined by GD-MS is preferably 0.5 to 1.2.

[0140] [Fluorine] Fluorine is a monovalent anion, and when part of the oxygen in the surface layer portion 100a is substituted with fluorine, the lithium desorption energy decreases. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. In other words, when fluorine is not present, cobalt ions change from trivalent to tetravalent upon lithium desorption. On the other hand, when fluorine is present, cobalt ions change from divalent to trivalent upon lithium desorption. The redox potential of cobalt ions differs between the two. Therefore, when part of the oxygen in the surface layer portion 100a of the positive electrode active material particle 100 is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when the positive electrode active material particle 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved.

[0141] Furthermore, the presence of fluorine on the surface, which is the portion in contact with the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the cathode active material particles 100 and the electrolyte. Furthermore, corrosion resistance to hydrofluoric acid can be effectively improved. The fluorine concentration determined by GD-MS is preferably 2800 wt ppm or more and 3500 wt ppm or less, and the Mg / F ratio determined by GD-MS is preferably 0.2 or more and 1 or less.

[0142] The fluorine concentration in the surface layer portion 100a determined by XPS measurement is preferably 5 atomic % or more and 10 atomic % or less. Furthermore, the Co / F ratio in the surface layer portion 100a determined by XPS measurement is preferably 1 or more and 3 or less. Furthermore, the Mg / F ratio in the surface layer portion 100a determined by XPS measurement is preferably 0.8 or more and 1.2 or less.

[0143] [Other additive elements] When phosphorus is contained in the surface layer portion 100a, Li ｘ CoO ２ When x in the formula (I) is kept small, short-circuiting between the positive electrode and the negative electrode can be suppressed, which is preferable. For example, it is preferable that the formula (I) is present in the surface layer portion 100a as a compound containing phosphorus and oxygen.

[0144] When the positive electrode active material particles 100 contain phosphorus, the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or the electrolyte, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable. ６ If the electrolyte contains fluorine, it may react with water to generate hydrogen fluoride. Furthermore, hydrogen fluoride may also be generated by the reaction of polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, with an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 may be suppressed. Furthermore, it may be possible to suppress a decrease in adhesion due to gelation and / or insolubilization of PVDF.

[0145] Furthermore, when the positive electrode active material particle 100 has a crack, the progression of the crack can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material particle with the crack on the surface, for example, in the embedded portion.

[0146] [Synergistic Effect of Multiple Added Elements] When the surface layer portion 100a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0147] For the above reasons, it is preferable that magnesium be added to the positive electrode active material in a process before nickel is added. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide regardless of the process of addition, whereas nickel can diffuse widely into the interior of lithium cobalt oxide in the absence of magnesium. Therefore, if nickel is added before magnesium, there is a concern that nickel will diffuse into the interior of the lithium cobalt oxide and not remain in the desired amount in the surface layer.

[0148] Furthermore, when the surface layer 100a contains magnesium, nickel, and titanium, the presence of nickel results in the formation of MgTiO with an ilmenite-type crystal structure. ３ The formation of heterophases such as MgTiO is suppressed. The ilmenite crystal structure has hexagonal close-packed anions, which differs from the rock salt and layered rock salt crystal structures in that the anions are cubic close-packed. Therefore, a certain amount of activation energy is required for the phase transition. NiO(II) is a compound with low chemical activity, so MgTiO ３ It has the effect of suppressing the production of

[0149] For the above reasons, if titanium is added to the positive electrode active material in a manufacturing process subsequent to the process for adding magnesium and nickel, the effect of nickel in suppressing the formation of heterogeneous phases can be more pronounced.

[0150] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0151] However, if the surface layer 100a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by MgO or a structure in which MgO and MO(II) are solid-solved. Therefore, the surface layer 100a must contain at least a transition metal M such as cobalt, and also contain lithium in the discharged state, so that it has a path for the insertion and extraction of lithium.

[0152] In order to ensure sufficient paths for lithium insertion and desorption, the surface layer 100a preferably has a higher cobalt concentration than magnesium. For example, when the surface of the positive electrode active material is measured by XPS, the ratio of the number of magnesium atoms Mg to the number of cobalt atoms Co (Mg / Co) is preferably 0.62 or less. The surface layer 100a preferably has a higher cobalt concentration than nickel. The surface layer 100a preferably has a higher cobalt concentration than aluminum. The surface layer 100a preferably has a higher cobalt concentration than fluorine.

[0153] Furthermore, since an excessive amount of nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium is higher than that of nickel in the surface layer portion 100 a. For example, when measured from the surface of the positive electrode active material particle 100 by XPS, the number of nickel atoms is preferably 1 / 6 or less of the number of magnesium atoms.

[0154] Furthermore, although it is preferable that some of the added elements, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, they are also preferably present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it is possible to suppress the deviation of the layered structure consisting of cobalt and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing magnesium elution can be expected, as described above.

[0155] The additive element indicates the detected amount of characteristic X-rays (sometimes simply referred to as the detected amount) that can be determined not to be noise in the STEM-EDX line analysis from the viewpoints of intensity, spatial resolution, etc. Furthermore, the state in which the detected amount is obtained continuously is called the distribution of the additive element. Furthermore, it is preferable that the distribution has a maximum value of the detected amount, that is, a peak of the detected amount. It is also possible that the distribution has multiple such peaks.

[0156] In the case where the additive element is magnesium, fluorine, nickel, or titanium, the peak of the detectable amount is preferably located in the surface layer portion 100a. In other words, the peak of the detectable amount of each of magnesium, fluorine, nickel, and titanium is preferably located 20 nm or less from the surface. The additive element may have multiple detectable amount peaks, but it is preferable that each of magnesium, fluorine, nickel, and titanium has the maximum value of the peak (maximum peak) in the surface layer portion 100a or 20 nm or less from the surface.

[0157] [Distribution] Among the additive elements, desirable distributions of aluminum, magnesium, and titanium are illustrated in Fig. 4B. Among the additive elements, desirable distributions of nickel, aluminum, magnesium, and titanium are illustrated in Fig. 4C. The distance on the horizontal axis in Fig. 4B and Fig. 4C corresponds to X1 to X2 of the positive electrode active material particle 100 illustrated in Fig. 4A, and may be referred to as the distance from the measurement start point.

[0158] 4B and 4C, magnesium is preferably distributed so that the concentration in the surface layer 100a is higher than the concentration in the interior 100b. Titanium is preferably distributed so that the concentration in the surface layer 100a is higher than the concentration in the interior 100b. The above-mentioned concentration in the surface layer 100a being higher than the concentration in the interior 100b means that the detected amount in the surface layer 100a is greater than the detected amount in the interior 100b.

[0159] As shown in Figures 4B and 4C, when the position where the magnesium distribution begins (referred to as the rising position) overlaps with the position where the titanium distribution begins, and when the position where the magnesium distribution ends (referred to as the falling position) overlaps with the position where the titanium distribution begins, the distributions are said to overlap. In other words, it is preferable for the distributions of magnesium and titanium to overlap. Furthermore, when there is a position where the titanium distribution begins and ... magnesium distribution begins and a position where the titanium distribution begins and a position where the titanium distribution begins and a position where the titanium distribution begins and a position where the titanium distribution ends are both offset, the configuration having an overlapping region also includes a state in which the overlapping region is present.

[0160] Furthermore, as shown in Figures 4B and 4C, the magnesium peak, typically the maximum peak, preferably exists in the surface layer 100a, and preferably exists in a region closer to the surface within the surface layer 100a. For example, the magnesium concentration peak preferably exists on the surface or within 3 nm from the reference point. Furthermore, the titanium peak, typically the maximum peak, preferably exists in the surface layer 100a, and preferably exists in a region closer to the surface within the surface layer 100a, and preferably exists in a region closer to the surface within the surface layer 100a. The magnesium peak position and the titanium peak position may overlap. Furthermore, if the magnesium peak position and the titanium peak position are different, the difference in peak position should be within 3 nm, preferably within 1 nm. When the full width at half maximum of the distribution based on the peak position can be calculated, it is preferable that the full width at half maximum of the titanium distribution be narrower than the full width at half maximum of the magnesium distribution.

[0161] As shown in FIG. 4C , the nickel distribution preferably overlaps with the magnesium distribution and the titanium distribution. Alternatively, the nickel distribution preferably has an overlapping region with the magnesium distribution and the titanium distribution. The nickel peak position is preferably located in the surface layer 100a, and more preferably in a region of the surface layer 100a closer to the surface. For example, the nickel concentration peak position is preferably located on the surface or within 3 nm from the reference point. Furthermore, if the full width at half maximum (FWHM) based on the peak position can be determined, it is preferable that the full width at half maximum (FWHM) of the nickel distribution be narrower than that of the magnesium distribution.

[0162] 4B and 4C, the distribution of magnesium, nickel, and titanium is preferably present in the edge region of the surface layer portion 100a. On the other hand, the above distribution is not necessarily required in the basal region of the surface layer portion 100a.

[0163] Although it has been described that the amount of magnesium detected in the interior 100b is small compared to the surface layer 100a, it is preferable that magnesium be present in a dilute state in the interior 100b. The amount of titanium detected in the interior 100b may be very small compared to the surface layer 100a, may not be detected, or may be 1 atomic % or less. Furthermore, the amount of nickel detected in the interior 100b may be very small compared to the surface layer 100a, may not be detected, or may be 1 atomic % or less.

[0164] Although not shown, it is preferable that the amount of fluorine detected in the surface layer 100a is greater than the amount of fluorine detected in the interior 100b, as with magnesium. It is also preferable that the peak of the detection amount is located closer to the surface of the surface layer 100a. For example, it is preferable that the peak of the detection amount is located on the surface or within 3 nm from the reference point. Similarly, it is preferable that the amount of silicon, phosphorus, boron, and / or calcium detected in the surface layer 100a is greater than the amount of fluorine detected in the interior 100b. It is also preferable that the peak of the detection amount is located closer to the surface of the surface layer 100a. For example, it is preferable that the peak of the detection amount is located on the surface or within 3 nm from the reference point.

[0165] Furthermore, it is preferable that the peak position of the detected amount of aluminum is located inside the distribution of magnesium or titanium, as shown in Figures 4B and 4C. In other words, it is preferable that at least aluminum, among the added elements, has a peak of detected amount inside magnesium or titanium. The distribution of magnesium or titanium may have an overlapping region with the distribution of aluminum, but there may be almost no overlapping region as shown in Figures 4B and 4C. The peak of detected amount of aluminum may be present in the surface layer portion 100a, or may be deeper than the surface layer portion 100a. For example, it is preferable that the peak position is located on the surface, or in a region of 5 nm to 30 nm from the reference point toward the interior.

[0166] The reason why aluminum is distributed deeper than magnesium or titanium is thought to be because the diffusion rate of aluminum is faster than that of magnesium, etc. On the other hand, the reason why the amount of aluminum detected in the region closest to the surface is low is presumably because aluminum exists more stably in regions where magnesium, etc. are not present than in regions where they are dissolved at high concentrations.

[0167] More specifically, in the region of the layered rock salt type of space group R-3m or the cubic rock salt type where magnesium is dissolved at a high concentration, layered rock salt type LiAlO ２ Compared to the case of cobalt, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. ＋ is Mg ２＋ The valence change due to substitution to Co ３＋ From Co ２＋ However, since Al can only be trivalent, it is thought that it is difficult for it to exist stably near magnesium in rock salt or layered rock salt structures.

[0168] The distribution of the additive elements in the basal region may differ from the distributions shown in FIGS. 4B and 4C . For example, the basal region and the surface layer portion 100a having the basal region may have a lower detectable amount of one or more selected from the additive elements compared to the edge region and the surface layer portion 100a having the basal region. Specifically, the detectable amount of one or more of magnesium, nickel, and titanium may be lower. Alternatively, the basal region and the surface layer portion 100a having the basal region may have no detectable amount of one or more selected from the additive elements, or the detectable amount may be 1 atomic % or less. Specifically, the detectable amount of nickel may be no detectable amount of nickel, or 1 atomic % or less. In particular, with an analysis method that detects characteristic X-rays, such as EDX, the Kβ of cobalt and the Kα of nickel are close in energy, making it difficult to detect trace amounts of nickel in a material in which cobalt is the main element. Alternatively, the basal region and the surface layer portion 100a having the basal region may have a peak of detectable amount of one or more selected from the additive elements that is shallower from the surface compared to the edge region and the surface layer portion 100a having the edge region. Specifically, the peaks of the detected amounts of magnesium and aluminum may be shallower than the edge region and the surface layer portion 100a having the edge region.

[0169] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ It can be said that the structure is one in which the CoO layer and the lithium layer are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (001) plane. ２ Since the layer is relatively stable, it is more stable for the surface of the positive electrode active material particle 100 to have a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0170] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface other than the (001) orientation and the surface layer portion 100a are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface other than the (001) orientation and the surface layer portion 100a is extremely important for maintaining the crystal structure of the entire positive electrode active material particle 100. Therefore, in the positive electrode active material particle 100, it is preferable that the distribution of the added element on the surface other than the (001) orientation and the surface layer portion 100a thereof is, for example, a distribution as shown in FIG. 4B or FIG. 4C .

[0171] As explained in the first embodiment, high-purity LiCoO ２ In a manufacturing method in which the additive element is mixed and heated after the preparation of the silicon nitride film, the additive element spreads mainly through the diffusion path of lithium ions, and therefore the distribution of the additive element in the surface other than the (001) orientation and in the surface layer portion 100a thereof can be easily controlled to a preferred range.

[0172] The additive element does not necessarily have to contain one selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0173] For example, positive electrode active material particles 100 that are substantially free of manganese have the advantages of being relatively easy to synthesize and handle, and having excellent cycle characteristics. Therefore, the weight of manganese contained in positive electrode active material particles 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0174] <Grain Boundary> In addition to the distribution described above, it is more preferable that at least a portion of the additive element contained in the positive electrode active material particle 100 is unevenly distributed at and near the grain boundary. In this specification, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, specifically, the element being present at a higher concentration than in other regions. This is synonymous with segregation, precipitation, non-uniformity, bias, or the presence of a mixture of high-concentration and low-concentration regions.

[0175] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material particles 100 is higher than that in the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that in the interior 100b. It is also preferable that the nickel concentration at and near the grain boundaries is higher than that in the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that in the interior 100b. It is also preferable that the titanium concentration at and near the grain boundaries is higher than that in the interior 100b.

[0176] Grain boundaries are a type of planar defect. Therefore, like particle surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of added elements at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0177] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the positive electrode active material particles 100, subsequent heating or the like increases the magnesium concentration and fluorine concentration near the cracked surface. Therefore, even after cracks have occurred, the corrosion resistance to hydrofluoric acid of the positive electrode active material can be improved. Furthermore, even after cracks have occurred, side reactions between the electrolyte and the positive electrode active material can be suppressed.

[0178] <Particle size> If the particle size of the positive electrode active material particles 100 is too large, problems arise such as difficulty in diffusing lithium, and the surface of the active material layer becomes too rough when coated on a current collector. On the other hand, if the particle size is too small, problems arise such as excessive reaction with the electrolyte solution.

[0179] The particle size of the positive electrode active material particles 100 can be measured, for example, by a laser diffraction particle size distribution analyzer. The particle size of the positive electrode active material measured by the laser diffraction particle size distribution analyzer has a median diameter (D50) of 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. Alternatively, 1 μm or more and 40 μm or less is preferred. Alternatively, 1 μm or more and 30 μm or less is preferred. Alternatively, 2 μm or more and 100 μm or less is preferred. Alternatively, 2 μm or more and 30 μm or less is preferred. Alternatively, 5 μm or more and 100 μm or less is preferred. Alternatively, 5 μm or more and 40 μm or less is preferred.

[0180] The median diameter (D50) is the particle diameter at which the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. The particle diameter at which the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 10% is called the median diameter (D10), and the particle diameter at which the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 90% is called the median diameter (D90).

[0181] Furthermore, when particles of different particle sizes are mixed and used in the positive electrode, the electrode density can be increased, and a secondary battery with high energy density can be obtained, which is preferable. Positive electrode active material particles 100 with a relatively small particle size are expected to have high charge / discharge rate characteristics. Positive electrode active material particles 100 with a relatively large particle size are expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0182] The interior 100b preferably has a low density of defects, including dislocations. Dislocations in the interior 100b can be observed, for example, by TEM. If the density of defects, including dislocations, is sufficiently low, they may not be observed within a specific 1 μm square of the observation sample. Note that dislocations are a type of crystal defect and are different from vacancy defects.

[0183] The positive electrode active material preferably has a large crystallite size measured by XRD. In other words, the inner portion 100b preferably has high crystallinity. The larger the crystallite size, the greater the amount of Li, as will be described later. ｘ CoO ２When x is small, the O3'-type crystal structure is easily maintained and the contraction of the c-axis length is easily suppressed. Furthermore, it is believed that the fewer defects, including dislocations, observed by TEM, the larger the crystallite size measured by XRD.

[0184] The crystallite size is calculated using, for example, a Bruker D8 ADVANCE with a CuKα X-ray source. １ The diffraction pattern obtained using a LYNXEYE XE-T detector with a 2θ angle of 15° to 90° increments of 0.005 can be used, along with ICSD Coll. Code 172909 as the literature value for lithium cobalt oxide. Analysis can be performed using DIFFRAC. TOPAS ver. 6 crystal structure analysis software, and it is preferable to use the L Vol -IB value as the crystallite size. Note that if the calculated Preferred Orientation is less than 0.8, the sample may be too strongly oriented and therefore not suitable for determining the crystallite size.

[0185] XRD measurements for calculating the crystallite size are preferably performed on the positive electrode active material alone, but may also be performed on the positive electrode, which includes the positive electrode active material, a current collector, a binder, a conductive material, and the like. However, in the positive electrode state, the positive electrode active material may be oriented due to pressure and other factors during the manufacturing process. Strong orientation can prevent accurate calculation of the crystallite size. Therefore, it is more preferable to obtain the sample by removing the positive electrode active material layer from the positive electrode, removing some of the binder and other materials in the positive electrode active material layer using a solvent, and then loading the sample into a sample holder. Alternatively, for powder samples, grease can be applied to a silicon non-reflective plate and the sample attached to it.

[0186] <Crystalline Structure> Due to the distribution of the added elements as described above, it is preferable that the crystalline structure continuously changes from the interior 100b toward the surface, or that the crystal orientation of the surface layer 100a and the interior 100b roughly coincide.

[0187] It is preferable that the crystal structure continuously changes from the interior 100b of the layered rock salt type toward the surface and surface layer 100a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure. Alternatively, it is preferable that the crystal orientation of the surface layer 100a having a rock salt type crystal structure or both a rock salt type crystal structure and a layered rock salt type crystal structure and the interior 100b of the layered rock salt type generally coincide.

[0188] In this specification, a layered rock-salt crystal structure belonging to the space group R-3m refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which cobalt and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a distorted rock-salt crystal lattice structure, which may result in a lower symmetry than the rock-salt crystal structure.

[0189] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0190] The presence of both the layered rock salt type crystal structure and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0191] In the rock salt crystal structure, there is no distinction in the cation sites, but in the layered rock salt crystal structure, there are two types of cation sites, one of which is mostly occupied by lithium and the other by cobalt. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same in both the rock salt crystal structure and the layered rock salt crystal structure. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in an ideal rock salt crystal structure, and, for example, the (003) plane in a layered rock salt crystal structure. For example, MgO, which has a rock salt crystal structure, and LiCoO, which has a layered rock salt crystal structure, ２When comparing the electron diffraction patterns of LiCoO ２ The distance between the bright spots on the (003) plane of MgO is observed to be about half the distance between the bright spots on the (111) plane of MgO. ２ When the two phases are present, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type crystal structure and the layered rock salt type crystal structure have strong brightness, while bright spots occurring only in the layered rock salt type crystal structure have weak brightness.

[0192] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. Rock-salt crystal structures do not exhibit this characteristic because there is no distinction between cation sites. When a material has both a rock-salt crystal structure and a layered rock-salt crystal structure, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers greater than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0193] The layered rock salt crystal structure and the anions in the rock salt crystal structure have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in the O3'-type crystal structure and the monoclinic O1(15)-type crystal structure, which will be described later, also have a cubic close-packed structure. Therefore, when the layered rock salt crystal structure and the rock salt crystal structure come into contact, there are crystal planes where the cubic close-packed structures formed by the anions are aligned.

[0194] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. The layered rock salt crystal structure is in the space group R-3m and is a rhombohedral structure, but is generally expressed as a compound hexagonal lattice to make the structure easier to understand, and the (0001) plane of the layered rock salt crystal structure has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt crystal structure. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0195] However, the space group of the layered rock salt type crystal structure and the O3' type crystal structure is R-3m, which is different from the space group Fm-3m of the rock salt type crystal structure (the space group of a general rock salt type crystal structure). Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal structure and the O3' type crystal structure and the rock salt type crystal structure. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt type crystal structure, the O3' type crystal structure, and the rock salt type crystal structure are aligned, it may be said that the crystal orientations are approximately the same. Furthermore, having a three-dimensional structural similarity such that the crystal orientations are approximately the same, or having the same crystallographic orientation, is called topotaxis.

[0196] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM, high-angle scattering annular dark-field scanning transmission electron microscope) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope, annular bright-field scanning transmission electron microscope) images, electron diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and FFT patterns of STEM images, etc. Furthermore, XRD, neutron diffraction, etc. can also be used as materials for determination.

[0197] 5 shows an example of a TEM image in which the orientation of the layered rock salt-type crystal structure LRS and the rock salt-type crystal structure RS roughly coincides. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0198] For example, in high-resolution TEM images, contrast originating from crystal planes is observed. When an electron beam is incident perpendicularly to the c-axis of a composite hexagonal lattice of a layered rock-salt crystal structure, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in Figure 5) ＲＳ and L ＬＲＳ When the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0199] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the cobalt atom positions, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate having a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of highly bright dots, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0200] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0201] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0202] Figure 6A shows an example of an STEM image in which the orientations of the layered rock-salt-type crystal structure LRS and the rock-salt-type crystal structure RS are roughly the same. Figure 6B shows the FFT pattern of the region of the rock-salt-type crystal structure RS, and Figure 6C shows the FFT pattern of the region of the layered rock-salt-type crystal structure LRS. The left side of Figures 6B and 6C shows the composition, JCPDS card number, and the d value and angle calculated from the JCPDS card number. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction.

[0203] The spot marked A in Figure 6B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 6C is derived from the 0003 reflection of the layered rock salt type. From Figures 6B and 6C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type crystal structure roughly coincide. In other words, it can be seen that the line passing through AO in Figure 6B is roughly parallel to the line passing through AO in Figure 6C. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0204] In this way, in the FFT pattern and the electron diffraction pattern, when the orientations of the layered rock salt type crystal structure and the rock salt type crystal structure roughly coincide, the <0003> orientation of the layered rock salt type crystal structure and the <11-1> orientation of the rock salt type crystal structure may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point that is spot-like and not continuous with other reciprocal lattice points indicates high crystallinity.

[0205] Furthermore, as described above, when the orientation of the 11-1 reflection of a cubic crystal and the orientation of the 0003 reflection of a layered rock-salt crystal structure are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock-salt crystal structure may be observed in a reciprocal lattice space different from the orientation of the 0003 reflection of the layered rock-salt crystal structure. For example, the spot marked B in FIG. 6C originates from the 1014 reflection of the layered rock-salt crystal structure. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point (A in FIG. 6C) originating from the 0003 reflection of the layered rock-salt crystal structure, and at a location with a d value of 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be identical. For example, reciprocal lattice points equivalent to 0003 and 1014 may also be used.

[0206] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 6B is originating from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 6B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to match this. For example, reciprocal lattice points equivalent to the 11-1 and 200 reflections of the cubic crystal may also be used.

[0207] It is known that positive electrode active materials with a layered rock salt crystal structure, such as lithium cobalt oxide, tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, when observing the (0003) plane using a TEM or the like, it is preferable to first select positive electrode active material particles whose shape is observed using a SEM or the like to reveal a crystal plane expected to be the (0003) plane, and then thin-section the positive electrode active material particles using a FIB (Focused Ion Beam) or the like so that the (0003) plane can be observed in a TEM or the like with an electron beam incident in the [12-10] direction. When determining whether the crystal orientation is consistent, it is preferable to thin-section the particles so that the (0003) plane of the layered rock salt crystal structure can be easily observed.

[0208] <Change in Crystal Structure> Using Figs. 7 to 11, ｘ CoO ２ The change in the crystal structure accompanying the change in x in the formula (I) will be described. Specifically, the description will be made by comparing a conventional positive electrode active material shown in FIG. 8 with a positive electrode active material according to one embodiment of the present invention shown in FIG. 7. Note that the conventional positive electrode active material is lithium cobalt oxide that does not particularly contain any additional elements.

[0209] <Li ｘ CoO ２ When x is 1 in the positive electrode active material of one embodiment of the present invention shown in FIG. ｘ CoO ２ In the case where x=1 in the graph, the positive electrode active material has a layered rock salt type crystal structure belonging to the space group R-3m. ｘ CoO ２ In the case where x=1, the crystal structure has a layered rock salt type belonging to the space group R-3m. ｘ CoO ２ The layered rock salt type crystal structure when x = 1 in the formula is designated as R-3m O3. R-3m O3 has a lattice constant of a = 2.81610 × 10 −１０ (m), b=2.81610×10 −１０ (m), c=14.05360×10 −１０ (m), α = 90.0000, β = 90.0000, γ = 120.0000, and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0,0,0), Co(0,0,0.5), O(0,0,0.23951) (Non-Patent Document 5). While Figures 7 and 8 show lithium present at all lithium sites with equal probability, this is not a limitation. Lithium may be present disproportionately at some of the lithium sites. The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0210] A positive electrode active material having a layered rock salt crystal structure has a high discharge capacity, has two-dimensional lithium ion diffusion paths, and is suitable for lithium ion insertion / extraction reactions. Therefore, it is particularly preferable that the inner portion 100b, which occupies most of the volume of the positive electrode active material, has a layered rock salt crystal structure.

[0211] <Li ｘ CoO ２ The positive electrode active material of one embodiment of the present invention shown in FIG. 7 contains the above-described additional element, and therefore, ｘ CoO ２ The crystal structure when x is small differs from that of the conventional positive electrode active material shown in Fig. 8. Here, "small x" means that 0.1<x≦0.24.

[0212] First, it is known that when x = 0.5, the conventional lithium cobalt oxide shown in Figure 8 has a crystal structure that has high lithium symmetry and belongs to the monoclinic space group P2 / m. The conventional positive electrode active material is lithium cobalt oxide that does not contain any added elements, and changes in its crystal structure are described in Non-Patent Documents 1 to 3, etc. This structure has CoO in the unit cell. ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0213] The conventional lithium cobalt oxide shown in FIG. 8 has a crystal structure of the trigonal space group P-3m1 when x = 0, and also contains CoO in the unit cell. ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0214] The conventional lithium cobalt oxide shown in Figure 8 has a crystal structure of the space group R-3m when x = 0.12. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２It can also be said that this crystal structure is a structure in which the structure of and the structure of are stacked alternately. Therefore, this crystal structure is sometimes referred to as an H1-3 crystal structure. In a unit cell of an H1-3 crystal structure, the number of cobalt atoms per unit cell is twice that of other structures. However, in Figure 8 and other parts of this specification, to facilitate comparison with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half that of the unit cell. Furthermore, actual lithium insertion / extraction does not necessarily occur uniformly within the positive electrode active material, and the lithium concentration may be uneven, so experimentally, an H1-3 crystal structure is observed from approximately x = 0.25.

[0215] As an example, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in the unit cell of the H1-3 type 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), where O1 and O2 are oxygen atoms.

[0216] The unit cell that should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, the unit cell that results in the smallest goodness of fit (GOF) value is adopted.

[0217] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, the conventional lithium cobalt oxide shown in Figure 8 repeatedly changes its crystal structure (i.e., undergoes a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state. However, these two crystal structures are easily separated by the CoO ２ The layer displacement is large. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0218] Furthermore, there is a large difference in volume between these two crystal structures. The specific volume difference will be described later.

[0219] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0220] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0221] On the other hand, in the positive electrode active material of one embodiment of the present invention shown in FIG. ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material of one embodiment of the present invention is ｘ CoO ２ When x is 0.24 or less, a more stable crystal structure than that of conventional positive electrode active materials can be obtained, which is preferable as it further improves the safety of the secondary battery.

[0222] In the positive electrode active material of one embodiment of the present invention shown in FIG. ｘ CoO ２ The graph shows the crystal structures when x is 1, approximately 0.2, and approximately 0.15. When x = 1, the positive electrode active material particles 100 have the same R-3m O3 crystal structure as conventional lithium cobalt oxide. However, when x is 0.24 or less, for example, approximately 0.2 or 0.15, at which point conventional lithium cobalt oxide has an H1-3 type crystal structure, the positive electrode active material particles 100 have a different crystal structure.

[0223] In the positive electrode active material of one embodiment of the present invention shown in Figure 7, lithium cobalt oxide has a crystal structure belonging to the trigonal space group R-3m when x = approximately 0.2. This has the same symmetry as the CoO2 layer O3. Therefore, this crystal structure is referred to as an O3'-type crystal structure. This crystal structure is shown in Figure 7 with the notation R-3m O3'.

[0224] In the O3'-type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. Furthermore, the lattice constant of the unit cell is preferably 2.797≦a≦2.837 (Å), more preferably 2.807≦a≦2.827 (Å), typically a=2.817 (Å). The c-axis is preferably 13.681≦c≦13.881 (Å), more preferably 13.751≦c≦13.811 (Å), typically c=13.781 (Å).

[0225] 7, lithium cobalt oxide may have a crystal structure belonging to the monoclinic space group P2 / m when x is about 0.15. This is because the unit cell contains CoO ２ There is one layer. In addition, the lithium present in the positive electrode active material particle 100 at this time is about 15 atomic % in the discharged state. Therefore, this crystal structure is called a monoclinic O1(15) type crystal structure. This crystal structure is shown in Figure 7 with the P2 / m monoclinic O1(15) symbol.

[0226] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２The lattice constants of the unit cell are a = 4.880 ± 0.05 Å, b = 2.817 ± 0.05 Å, c = 4.839 ± 0.05 Å, α = 90°, β = 109.6 ± 0.1°, and γ = 90°.

[0227] This crystal structure can also show the lattice constant in the space group R-3m if some error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constants of the unit cell are a = 2.817 ± 0.02 Å and c = 13.68 ± 0.1 Å.

[0228] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy hexacoordinated oxygen sites, although light elements such as lithium and magnesium may occupy tetracoordinated oxygen sites.

[0229] As shown by the dotted line in FIG. 7, the R-3m O3 in the discharged state, the O3'-type crystal structure, and the monoclinic O1(15)-type crystal structure show a difference in the CoO ２ There is almost no layer misalignment.

[0230] The table below shows the difference in volume per cobalt atom between discharged R-3m O3, the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, the H1-3-type, and the trigonal O1-type. Regarding the lattice constants of each crystal structure used in the calculation, literature values ​​can be referenced for discharged R-3m O3 and trigonal O1 (ICSD coll.code.172909 and 88721). For H1-3, Non-Patent Document 3 can be referenced. The O3'-type crystal structure and the monoclinic O1(15)-type crystal structure can be calculated from experimental values ​​obtained by XRD.

[0231]

[0232] Using the volume per cobalt atom in the table above, the difference in volume between the H1-3 crystal structure and the R-3mO3 crystal structure in a discharged state can be calculated when comparing the volumes per cobalt atom. As shown in the volume change rate (%) in the table above, when the denominator is the R-3mO3 crystal structure in a discharged state, the difference in volume is 3.9%. This means that the difference in volume exceeds 3.5%, which is extremely large.

[0233] Using the volume per cobalt atom in the table above, the difference in volume between R-3m O3 in a discharged state and the O3'-type crystal structure can be calculated by comparing the volumes per the same number of cobalt atoms. As shown in the volume change rate (%) in the table above, when the denominator is the R-3m O3-type crystal structure in a discharged state, the difference in volume is 1.8%. In other words, the difference in volume is 2.5% or less, more specifically, 2.2% or less, which is very small.

[0234] Using the volume per cobalt atom in the table above, the difference in volume between R-3m O3 in a discharged state and the monoclinic O1(15) crystal structure can be calculated by comparing the volumes per cobalt atom. As shown in the volume change rate (%) in the table above, when the denominator is the R-3m O3 crystal structure in a discharged state, the difference in volume is 2.5%. In other words, the difference in volume is 3.3% or less, more specifically, 3.0% or less, which is very small.

[0235] As described above, in the positive electrode active material of one embodiment of the present invention, Li ｘ CoO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the positive electrode active material suppresses the decrease in charge / discharge capacity during charge / discharge cycles. Furthermore, because it can stably utilize more lithium than conventional positive electrode active materials, the positive electrode active material has a large discharge capacity per weight and per volume. Therefore, by using this positive electrode active material, secondary batteries with high discharge capacity per weight and per volume can be fabricated.

[0236] Note that the positive electrode active material of one embodiment of the present invention is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, the O3' type crystal structure may be present, and it is estimated that even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is present. ｘ CoO ２ It has been confirmed that when x is greater than 0.1 and less than 0.2, typically when x is 0.15 or more and less than 0.17, it may have a monoclinic O1(15) type crystal structure. However, the crystal structure is Li ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0237] Therefore, the positive electrode active material of one embodiment of the present invention is Li ｘ CoO ２ When x in the positive electrode active material is greater than 0.1 and less than or equal to 0.24, the positive electrode active material may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, in the positive electrode active material of one embodiment of the present invention, not all of the particles in the interior 100b may have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or a portion may be amorphous.

[0238] Also Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ CoO ２In other words, a state where x is small can be referred to as a state where the material is charged at a high charging voltage. In other words, the positive electrode active material of one embodiment of the present invention is preferable because it can maintain a crystal structure with R-3m O3 symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25°C. For example, when constant-current charging and constant-voltage charging (CCCV charging) are performed at a voltage of 4.6 V or higher relative to the potential of lithium metal in a 25°C environment, a H1-3 crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be referred to as a high charging voltage. Unless otherwise specified, charging voltages are expressed relative to the potential of lithium metal.

[0239] In addition, 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 about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0240] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x is small, whether the positive electrode active material of one embodiment of the present invention has an O3′-type and / or monoclinic O1(15)-type crystal structure can be determined by Li ｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0241] XRD is particularly preferred in that 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 as is.

[0242] When analyzing the crystallite size by powder XRD, it is preferable to measure the crystallite size while excluding the influence of orientation due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the sample.

[0243] Furthermore, even in the positive electrode active material of one embodiment of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9 V, an H1-3 type or trigonal O1 type crystal structure may be formed. However, a positive electrode active material with a small x may undergo a change in crystal structure when exposed to air. For example, the O3' type and monoclinic O1(15) type crystal structures may change to an H1-3 type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0244] <<Coin Cell to be Charging>> Charging to determine whether a certain composite oxide is a positive electrode active material of one embodiment of the present invention can be performed by fabricating a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a lithium counter electrode and charging it. It is also possible to fabricate a coin cell using a positive electrode removed from a secondary battery. In other words, the positive electrode is prepared by coating a slurry containing a positive electrode active material, a conductive material, and a binder on an aluminum foil positive electrode current collector.

[0245] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0246] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte can be a mixed organic solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in an EC:DEC=3:7 (volume ratio), in which vinylene carbonate (VC) is mixed at 2 wt % with respect to the mixed organic solvent.

[0247] The separator may be a 25 μm thick porous polypropylene film.

[0248] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0249] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). The charging method is not particularly limited as long as charging can be performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be terminated at a current of 2 mA / g or more and 10 mA / g or less per weight of positive electrode active material. To observe the phase change of the positive electrode active material, it is desirable to charge at such a small current value. On the other hand, if the current does not reach 2 mA / g or more and 10 mA / g or less even after long-term CV charging, it is considered that the current is being consumed not for charging the positive electrode active material but for decomposing the electrolyte. Therefore, CV charging may be terminated after a sufficient time has elapsed since the start of charging. In this case, a sufficient time can be, for example, 1.5 hours or more and 3 hours or less. The temperature is set to 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain a positive electrode active material with a desired charge capacity. When various analyses are performed thereafter, it is preferable to seal the cell in an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to quickly remove the positive electrode and subject it to analysis after charging is complete. Specifically, it is preferable to do so within one hour after charging is complete, and more preferably within 30 minutes.

[0250] <XRD> The XRD device and conditions are not particularly limited. The software for analyzing the crystal structure is also not particularly limited. It is preferable to use a highly accurate device, conditions, and corresponding software, as this allows for more accurate crystal structure analysis. For example, the following device, conditions, and analysis software can be used for measurement and analysis. XRD device: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm The obtained XRD pattern was analyzed using analysis software such as DIFFRAC.EVA to separate the background and CuKα ２ The peaks of the lines can be removed.

[0251] CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model １ Ideal powder XRD patterns based on the line are shown in Figures 9, 10, 11A, and 11B. ｘ CoO ２ LiCoO where x=1 ２ Also shown are ideal XRD patterns calculated from the crystal structure of O3 and trigonal O1 with x = 0. Figures 11A and 11B show the XRD patterns of the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, and the H1-3-type crystal structure, with Figure 11A showing an enlarged view of the region where 2θ is between 18° and 21°, and Figure 11B showing an enlarged view of the region where 2θ is between 42° and 46°. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from the Inorganic Crystal Structure Database (ICSD) (see Non-Patent Document 4). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The crystal structures of the O3' type and monoclinic O1(15) type crystal structure patterns were estimated from the XRD pattern of the positive electrode active material, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same way as the others.

[0252] As shown in FIGS. 9, 11A and 11B, in the O3' type crystal structure, diffraction peaks appear in the 2θ range of 19.13° or more and less than 19.37°, and in the 2θ range of 45.37° or more and less than 45.57°.

[0253] In addition, in the monoclinic O1(15) type crystal structure, diffraction peaks appear in the 2θ range of 19.37° or more and 19.57° or less, and in the 2θ range of 45.57° or more and 45.67° or less.

[0254] However, as shown in Figures 10, 11A and 11B, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of peaks at 19.13° or greater and less than 19.37° and / or 19.37° or greater and less than 19.57°, and at 45.37° or greater and less than 45.57° and / or 45.57° or greater and less than 45.67° when x is small can be said to be a characteristic of the positive electrode active material of one embodiment of the present invention.

[0255] This can be said to be because, in the positive electrode active material of one embodiment of the present invention, the positions at which XRD diffraction peaks appear are close between the crystal structures where x = 1 and where x ≦ 0.24. More specifically, the difference in 2θ between the main diffraction peaks of the crystal structures where x = 1 and where x ≦ 0.24 appears at a 2θ angle of 42° to 46° is 0.7° or less, more preferably 0.5° or less.

[0256] When Rietveld analysis is performed on the XRD pattern of the positive electrode active material of one embodiment of the present invention, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. When the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0257] Similarly, when Rietveld analysis is performed, the H1-3 type and O1 type crystal structures are preferably 50% or less, or preferably 34% or less, or more preferably substantially not observed.

[0258] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0259] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the full width at half maximum is narrow. For example, a narrow full width at half maximum is preferable. Even for peaks arising from the same crystalline phase, the full width at half maximum varies depending on the XRD measurement conditions and the value of 2θ. Under the above-mentioned measurement conditions, for peaks observed between 2θ = 43° and 46°, the full width at half maximum is preferably, for example, 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to sufficient stabilization of the crystal structure after charging.

[0260] Furthermore, the crystallite size of the O3′-type and monoclinic O1(15) crystal structures of the positive electrode active material particles 100 of one embodiment of the present invention is approximately equal to that of LiCoO in a discharged state. ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘCoO ２ When x in the formula is small, the peaks of the crystal structure of the O3' type and / or monoclinic O1(15) can be clearly observed. ２ In this case, even if some of the crystal structure resembles the O3' type and / or monoclinic O1(15) crystal structure, the crystallite size will be small and the peaks will be broad and small. The crystallite size can be determined from the full width at half maximum of the XRD peak.

[0261] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), so the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be used to analyze the bonding state of the elements. The quantitative accuracy of XPS is often about ±1 atomic %, with a lower limit of about 1 atomic %, depending on the element.

[0262] Note that the surface and surface layer portion 100a of the positive electrode active material particle 100 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the production of the positive electrode active material particle 100. Furthermore, the surface of the positive electrode active material particle 100 also does not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material particle 100. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0263] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0264] The concentration of the added element may also be compared with the cobalt concentration. Using the ratio to cobalt is preferable because it allows comparison while reducing the influence of carbonates and other substances chemisorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 or more and 1.5 or less. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0265] Similarly, in order to ensure sufficient lithium insertion / extraction paths, the positive electrode active material particles 100 preferably have higher concentrations of lithium and cobalt in the surface layer portion 100a than the respective additive elements. This means that the concentrations of lithium and cobalt in the surface layer portion 100a are preferably higher than the concentrations of one or more additive elements selected from the additive elements contained in the surface layer portion 100a as measured by XPS or the like. For example, the concentration of cobalt in at least a portion of the surface layer portion 100a as measured by XPS or the like is preferably higher than the concentration of magnesium in at least a portion of the surface layer portion 100a as measured by XPS or the like. Similarly, the concentration of lithium is preferably higher than the concentration of magnesium. Furthermore, the concentration of cobalt is preferably higher than the concentration of nickel. Similarly, the concentration of lithium is preferably higher than the concentration of nickel. Furthermore, the concentration of cobalt is preferably higher than the concentration of aluminum. Similarly, the concentration of lithium is preferably higher than the concentration of aluminum. Furthermore, the concentration of cobalt is preferably higher than the concentration of fluorine. Similarly, the concentration of lithium is preferably higher than the concentration of fluorine.

[0266] Furthermore, it is more preferable that aluminum be widely distributed in a deep region, for example, on the surface or in a region having a depth of 5 nm to 50 nm from the reference point. Therefore, although aluminum is detected in an analysis of the entire cathode active material particle 100 using ICP-MS, GD-MS, or the like, it is more preferable that the concentration of aluminum is not detected by XPS or the like, or is 1 atomic % or less.

[0267] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions. Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 2 to 5 nm (take-off angle 15° to 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0268] Furthermore, when the positive electrode active material of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0269] Furthermore, when the positive electrode active material of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0270] Furthermore, in at least a portion of the surface layer portion 100a measured by XPS, it is preferable that a bond between magnesium and fluorine is present, and it is more preferable that a bond between magnesium and fluorine and oxygen (O-Mg-F bond) is present rather than a bond between magnesium and fluorine (Mg-F bond). The bond between magnesium and fluorine in the surface layer portion 100a is one factor indicating that the flux effect of the fluoride is fully exerted, the surfaces of the magnesium and lithium cobalt oxide are melted, and the magnesium is fully segregated.

[0271] The excitation X-rays used in XPS measurements are monochromated Al, and the detection area can be 100 μmφ. The take-off angle (the angle between the tilt of the sample stage and the detection direction of the detector) can be 45° or 15°. The detection depth is approximately 4 to 5 nm when the take-off angle is 45°, and approximately 2 nm when the take-off angle is 15°.

[0272] The presence of an O-Mg-F bond in the XPS spectrum of Mg1s, which is one of the magnesium bonding states, means that when the XPS spectrum is waveform separated, the bond energy of the O-Mg-F bond is different from that of the MgO bond and the MgF bond. ２ The fitting is performed assuming that the bond is in the middle of the bond.

[0273] It is preferable that O-Mg-F bonds are confirmed at both take-off angles of 45° and 15°. Furthermore, if there is a difference in the peak values ​​of O-Mg-F bonds between take-off angles of 45° and 15°, it can be said that the proportion of O-Mg-F bonds present in the XPS detection region is different. For example, when the take-off angle of 15° is larger than 45°, it can be said that O-Mg-F bonds are mainly present in a very shallow region up to 2 nm from the surface.

[0274] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material particles 100 have a concentration gradient. More preferably, the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material particles 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material particle 100 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, electron probe microanalysis (EPMA), or the like.

[0275] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0276] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material particle 100. Furthermore, EDX ray analysis can analyze the distribution of the added element and its maximum concentration. Furthermore, analysis that thins the sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0277] Since the positive electrode active material particles 100 are compounds containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present is defined as the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multi-layer film of carbon, metal, oxide, resin, etc.

[0278] In STEM-EDX analysis, the graph of the detected amount of characteristic X-rays of the element does not change sharply in principle or due to measurement errors, and it may be difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX analysis, the transition metal M is considered to be the average value M of the detected amount inside. ＡＶＥ and the average background value M ＢＧ The point where the oxygen concentration is 50% of the sum of the two, or the oxygen concentration is the average value of the internal detection amount O ＡＶＥ and the average background value O ＢＧ The point where the sum of the interior and background is 50% is taken as the reference point on the surface. Note that if the 50% point of the sum of the interior and background is different for the transition metal M and oxygen, it is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen adhering to the surface. Therefore, the average value M of the detected amount inside the transition metal M is used as the reference point. ＡＶＥ and the average background value M ＢＧ In the case of a positive electrode active material having a plurality of transition metals M, the M of the element with the largest count in the inner portion 100b can be used. ＡＶＥ and M ＢＧThe reference point can be determined using the following formula:

[0279] The average value M of the background of the transition metal M ＢＧ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the vicinity where the detected amount of transition metal M starts to increase. ＡＶＥ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably more than 50 nm, from the region where the counts of the transition metal M and oxygen are saturated and stable, for example, the region where the detected amount of the transition metal M starts to increase. ＢＧ and the average value of the amount of oxygen detected inside O ＡＶＥ can also be found in the same way.

[0280] Furthermore, the surface of the positive electrode active material particle 100 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between an area where an image derived from the crystalline structure of the positive electrode active material is observed and an area where it is not observed, and is the outermost area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are observed. The surface in an STEM image or the like may be determined in conjunction with an analysis with higher spatial resolution.

[0281] Furthermore, a peak in STEM-EDX-ray analysis refers to the maximum value (the apex of a convex shape) of characteristic X-rays that appears in a graph of the characteristic X-ray intensity for each element, and when multiple peaks are confirmed, refers to the maximum value of the characteristic X-rays. Note that noise in STEM-ED X-ray analysis may be a measured value of a full width at half maximum that is less than the spatial resolution (R), for example, less than R / 2.

[0282] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured over six scans can be used as the detected amount of each element. The number of scans is not limited to six, and more scans can be performed, and the average can be used as the detected amount of each element.

[0283] A sample to be subjected to STEM-EDX ray analysis is prepared, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material. For example, carbon can be vapor-deposited using a carbon coating unit of an ion sputtering apparatus (MC1000 manufactured by Hitachi High-Technologies).

[0284] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). Pickup is performed using an MPS (microprobing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0285] In order to increase the spatial resolution in STEM-EDX ray analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. The beam diameter in STEM-EDX ray analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. In order to reduce the beam diameter, it is preferable to perform aberration correction. Specifically, an aberration correction mechanism (Cs-collector) is arranged in the optical system.

[0286] STEM-EDX ray analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W (Dual EDS) EDX detector. During EDX ray analysis, the acceleration voltage of the STEM device is set to 200 kV, and the emission current is set to 6 μA or more and 10 μA or less, and a portion of the thinned sample with minimal depth and unevenness is measured. The magnification is, for example, about 150,000 times.

[0287] <Coating Portion> A coating portion may be attached to at least a portion of the surface of the positive electrode active material particle 100. Fig. 12 shows an example of a positive electrode active material particle 100 to which a coating portion 104 is attached.

[0288] The coating portion 104 is preferably formed by the accumulation of decomposition products of the electrolyte and the organic electrolytic solution during charging and discharging. ｘ CoO ２When repeated charging is performed such that x in the formula is 0.24 or less, the presence of a coating portion derived from the electrolyte on the surface of the positive electrode active material particle 100 is expected to improve the charge-discharge cycle characteristics. This is due to reasons such as suppressing an increase in impedance on the positive electrode active material surface or suppressing cobalt elution. The coating portion 104 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, when LiBOB and / or SUN (suberonitrile) are used as the electrolyte, a high-quality coating portion is easily obtained. Therefore, a coating portion 104 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating portion and is therefore preferred. Furthermore, the coating portion 104 does not have to cover the entire positive electrode active material particle 100. For example, it is sufficient for the coating portion 104 to cover 50% or more of the surface of the positive electrode active material particle 100, with 70% or more being more preferable and 90% or more being even more preferable.

[0289] <<Resistance DCIR (Direct Current Internal Resistance)>> In the above <<Charging Method>>, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) is prepared with a lithium counter electrode and charged as a charging method for determining whether a certain composite oxide is a positive electrode active material of one embodiment of the present invention. Using a coin cell prepared in a similar manner, it is possible to measure the low resistance DCIR, which is one of the characteristics of the positive electrode active material of one embodiment of the present invention.

[0290] A coin cell prepared under the same conditions as in the above charging method is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.60 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging can be performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be completed at 2 mA / g or more and 10 mA / g or less.

[0291] The charged coin cell is placed in a state where neither charging nor discharging is performed. This is called a rest period, and the rest period is preferably 5 minutes or more and 30 minutes or less. Then, the resistance (DCIR) is measured. The resistance (DCIR) is measured by discharging for 1 second at a current of 2 mA / g or more and 10 mA / g or less, typically 6 mA / g, per weight of the positive electrode active material. The resistance (DCIR) can be calculated by dividing the difference between the open circuit voltage before discharge and the voltage during discharge by the current (e.g., 6 mA / g).

[0292] When the open circuit voltage before discharge (after charge) is 4.50 V or more and 4.60 V or less, the resistance DCIR of a coin cell having the positive electrode active material of one embodiment of the present invention is preferably 40 Ω or less, and more preferably 25 Ω or less. Note that a series of measurements of the resistance DCIR may be performed in the same temperature environment as the charge-discharge cycle test or the rate-specific discharge capacity test, for example, in a 25°C environment or a 45°C environment.

[0293] <<Powder Resistivity Measurement>> The positive electrode active material particles 100 according to one embodiment of the present invention have a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge. The positive electrode active material particles 100 having the above-described excellent properties are characterized by the fact that, in the above-described <<XRD>>, Li ｘ CoO ２ It has been explained that when x in the formula (I) is small, the particle has an O3'-type and / or monoclinic O1(15)-type crystal structure. Furthermore, in the above section <<EDX>>, a preferred distribution of the added elements when the positive electrode active material particle 100 is subjected to STEM-EDX analysis has been explained. Furthermore, the positive electrode active material particle 100 according to one embodiment of the present invention is also characterized by the volume resistivity of the powder.

[0294] A feature of the positive electrode active material particles 100 according to one embodiment of the present invention is that the volume resistivity of the powder of the positive electrode active material particles 100 is 1.0×10 ８ Ω・cm or more 1.0×10 １０ It is preferable that the resistance is Ω cm or less, and 5.0 × 10 ８ Ω・cm or more 1.5×10 ９ It is more preferable that the resistivity is Ω·cm or less.

[0295] The positive electrode active material particles 100 having the above volume resistivity have a stable crystal structure even at high voltages, and can be used as an indicator that the surface layer portion 100a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been well formed.

[0296] A method for measuring the volume resistivity of the powder of the positive electrode active material particles 100 according to one embodiment of the present invention will be described.

[0297] The volume resistivity of a powder is preferably measured using an instrument with a resistance measurement terminal and a mechanism for applying pressure to the powder to be measured. The resistance measurement terminal preferably has four terminals (also referred to as four-point probes). For example, the MCP-PD51 manufactured by Mitsubishi Chemical Analytech Co., Ltd. can be used as a measurement device having a resistance measurement terminal and a mechanism for applying pressure to the powder (sample) to be measured. The resistance measurement device can be the Loresta-GP low resistance meter or the Hiresta-GP high resistance meter. The Loresta-GP can be used to measure low resistance samples, and the Hiresta-GP can be used to measure high resistance samples. The measurement environment is preferably a stable environment such as a dry room. A dry room environment, for example, with a temperature of 25°C and a dew point of -40°C or lower, is preferable. When performing measurements in a humid environment, the electrical resistance may decrease due to the influence of moisture in the air, potentially preventing the actual physical property values ​​from being obtained.

[0298] Measurement of the volume resistivity of powder using the measuring device shown above will be described. First, a powder sample is set in the measuring unit. The measuring unit is structured so that the powder sample and a terminal for resistance measurement are in contact with each other and can apply pressure to the powder sample. The measuring unit also has a structure for measuring the volume of the powder sample. Specifically, the measuring unit has a cylindrical space in which the powder sample is set. The structure for measuring the volume of the powder sample described above can measure the volume occupied by the powder at that time by measuring the height of the powder set in the space.

[0299] In measuring the volume resistivity of a powder, the electrical resistance of the powder and the volume of the powder are measured while pressure is applied to the powder. The pressure applied to the powder can be measured under a variety of conditions. For example, the electrical resistance and volume of the powder can be measured under pressure conditions of 16 MPa, 25 MPa, 38 MPa, 51 MPa, and 64 MPa. The volume resistivity of the powder can be calculated from the measured electrical resistance and volume of the powder.

[0300] When the above-described measurement is performed, the volume resistivity of the powder of the positive electrode active material particles 100 according to one embodiment of the present invention is 1.0 × 10 when measured under a pressure of 64 MPa. ８ Ω・cm or more 1.0×10 １０ When the capacitance is 5.0×10 Ω cm or less, favorable cycle characteristics are exhibited in a charge-discharge cycle test under high charge voltage conditions. ８ Ω・cm or more 1.5×10 ９ When the electrical resistance is Ω·cm or less, more preferable cycle characteristics are exhibited in a charge / discharge cycle test under high voltage conditions.

[0301] Unless otherwise specified in the present specification, the volume resistivity measured as above is the volume resistivity of the powder.

[0302] <Nail Penetration Test> Next, the nail penetration test will be described. In the nail penetration test, a secondary battery is fully charged and a nail having a predetermined diameter selected from 2 mm to 20 mm is inserted into the secondary battery at a predetermined speed. Full charge refers to a state in which the charge rate, expressed as State Of Charge (hereinafter referred to as SOC), is 100%. In this embodiment, the nail penetration test device will first be described.

[0303] FIG. 13A shows a side view of a nail penetration test device 1000. The nail penetration test device 1000 includes a stage 1001, a drive unit 1002, a nail 1003, a voltage measuring device 1015, a temperature measuring device 1016, and a control unit 1018. The drive unit 1002 has a drive mechanism 1012 that moves the nail 1003 in the direction of the arrow in the figure, and the drive mechanism 1012 operates to cause the nail 1003 to penetrate a secondary battery 1004 installed on the stage 1001. At this time, the secondary battery 1004 is in a fully charged state, and this operation is called a nail penetration operation. Note that the dashed line shown in FIG. 13A indicates a recess in the stage 1001 that is provided to accommodate the nail 1003 that has penetrated the secondary battery 1004 during the nail penetration operation.

[0304] The voltage measuring device 1015 transmits information about the voltage of the secondary battery during the nail penetration operation to the control unit 1018. Specifically, the amount of voltage change and the like are transmitted to the control unit 1018. Furthermore, the temperature measuring device 1016 transmits information about the temperature during the nail penetration operation to the control unit 1018. When controlling the operating conditions of the nail 1003, the control unit 1018 can transmit a control signal to the drive unit 1002.

[0305] FIG. 13B is a perspective view illustrating the vicinity of the upper portion of the stage 1001 of the nail penetration test device 1000. The secondary battery 1004 placed on the stage 1001 is electrically connected to wires 1005a and 1005b. The wires 1005a and 1005b belong to a voltage measuring device 1015, and the wires 1005a and 1005b are electrically connected to the positive and negative electrode tabs of the secondary battery 1004, respectively, to measure the voltage of the secondary battery 1004. The voltage of the secondary battery 1004 is simply referred to as voltage, voltage value between the positive and negative electrodes, battery voltage, cell voltage, or open-circuit voltage. When a temperature sensor is used as the temperature measuring device 1016, the temperature sensor is disposed so as to contact the surface of the exterior body of the secondary battery 1004.

[0306] 13B shows an example in which a first temperature sensor 1006a and a second temperature sensor 1006b are arranged on the secondary battery 1004, and a third temperature sensor 1006c is further provided on the nail 1003 shown in FIG. 13A, but one or three or more temperature sensors may be arranged on the secondary battery 1004. In the secondary battery 1004, the first temperature sensor 1006a is arranged on the side where the wiring 1005a and the wiring 1005b are not arranged, and the second temperature sensor 1006b is arranged on the side where the wiring 1005a and the wiring 1005b are arranged. Arranging two or more temperature sensors in this manner is preferable because the other temperature sensors can be used even if one temperature sensor becomes unusable due to expansion of the exterior body or the like.

[0307] Furthermore, there is a welded region on the side where the wiring 1005a and the wiring 1005b are arranged, but the exterior body is folded back on the side where the wiring 1005a and the wiring 1005b are not arranged, so there is no adhesive region. Therefore, even if the exterior body expands, the expansion is suppressed on the side where the wiring 1005a and the wiring 1005b are not arranged, and the second temperature sensor 1006b is more unlikely to peel off than the first temperature sensor 1006a, which is preferable.

[0308] The dashed ellipse in FIG. 13B represents the area where the nail 1003 penetrates the secondary battery 1004 during the nail penetration operation. The first temperature sensor 1006a and the second temperature sensor 1006b provided on the secondary battery 1004 are preferably provided equidistant from the area where the nail 1003 penetrates. Typically, the first temperature sensor 1006a and the second temperature sensor 1006b are provided within 5 cm, preferably within 2 cm, of the area where the nail 1003 penetrates. By providing the sensors in this manner, it is possible to grasp temperature changes in the area where the nail 1003 penetrates and its vicinity, which is preferable. Here, "vicinity" refers to an area within 1 cm of the penetration area. When two or more temperature sensors are provided, it is preferable to start the nail penetration operation after confirming that the temperature difference between the temperature sensors is within ±5°C, preferably within ±2°C.

[0309] To prevent a secondary battery from igniting or from experiencing thermal runaway in a nail penetration test, it is considered effective to suppress the temperature rise of the secondary battery and to ensure that the negative electrode, positive electrode, and / or electrolyte have stable properties at high temperatures. An example of a configuration that suppresses the temperature rise is a structure of the positive electrode active material that slows the rate of current flow. Specifically, the rate of current flow can be slowed by a configuration in which an additive element such as magnesium is segregated in the surface layer of the positive electrode active material.

[0310] <Characteristics of Secondary Battery in Nail Penetration Test> When AC impedance measurement is performed on a secondary battery in a fully charged state, it is preferable that the AC impedance value at a frequency of 1 kHz satisfy an internal resistance of 100 mΩ or less, preferably less than 90 mΩ. Even with such low internal resistance, a highly safe secondary battery can be provided by, for example, segregating additive elements such as magnesium in the surface layer. When fully charged, the secondary battery's charge capacity can be 2000 mAh or more, preferably 2400 mAh or more. Even a high capacity of 2400 mAh or more is preferable because it is difficult to burn. Because a fully charged secondary battery may discharge over time or its resistance may increase due to decomposition of the electrolyte, AC impedance measurement should be performed within 24 hours, preferably within 12 hours, and more preferably within 6 hours after being fully charged.

[0311] This embodiment can be used in combination with other embodiments.

[0312] Embodiment 3 In this embodiment, an example of a secondary battery of one embodiment of the present invention will be described with reference to FIGS.

[0313] <Configuration Example of Secondary Battery> Hereinafter, a secondary battery shown in FIG. 14 will be described as an example, in which a positive electrode, a negative electrode, and an electrolyte are enclosed in an exterior body.

[0314] [Positive Electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and may include a conductive material (synonymous with a conductive additive) and a binder. The positive electrode active material is prepared by the preparation method described in the above embodiment.

[0315] The positive electrode active material described in the above embodiment may be mixed with another positive electrode active material.

[0316] Other examples of the positive electrode active material include composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. For example, LiFePO ４ , LiFeO ２ , LiNiO ２ , LiMn ２ O ４ , V ２ O ５ , Cr ２ O ５ , MnO ２ and the like compounds.

[0317] Other positive electrode active materials include LiMn ２ O ４ Lithium-containing materials having a spinel-type crystal structure containing manganese, such as lithium nickel oxide (LiNiO ２ or LiNi １−ｘ M ｘ O ２ It is preferable to mix (0<x<1) (M=Co, Al, etc.) This configuration can improve the characteristics of the secondary battery.

[0318] <Conductive Material> The conductive material is also called a conductivity imparting agent or a conductivity aid, and is made of a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0319] The active material layers such as the positive electrode active material layer and the negative electrode active material layer preferably contain a conductive material.

[0320] As the conductive material, for example, one or more of carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fiber such as carbon nanofiber and carbon nanotube, and graphene compound can be used.

[0321] Examples of carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers and carbon nanotubes can also be used as carbon fibers. Carbon nanotubes can be produced by vapor phase growth, for example.

[0322] In this specification and the like, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0323] In this specification and the like, graphene oxide refers to a material that contains carbon and oxygen, has a sheet shape, and has a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

[0324] In this specification and the like, reduced graphene oxide refers to a material containing carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by six-membered carbon rings. A single sheet of reduced graphene oxide can function, but multiple sheets may be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is 2 atomic % or more and 15 atomic % or less. By achieving such carbon and oxygen concentrations, reduced graphene oxide can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0325] Graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds may also have a sheet-like shape. Graphene compounds may have curved surfaces, enabling surface contact with low contact resistance. Even when thin, they may have very high conductivity, allowing a small amount of material to efficiently form a conductive path within an active material layer. Therefore, using a graphene compound as a conductive material can increase the contact area between the active material and the conductive material. It is preferable that the graphene compound covers 80% or more of the active material. It is preferable that the graphene compound clings to at least a portion of the active material particles. It is also preferable that the graphene compound overlaps at least a portion of the active material particles. It is also preferable that the shape of the graphene compound matches at least a portion of the shape of the active material particles. The shape of the active material particles refers, for example, to the unevenness of a single active material particle or the unevenness formed by multiple active material particles. It is also preferable that the graphene compound surrounds at least a portion of the active material particles. The graphene compound may also have holes.

[0326] When active material particles having a small particle size, for example, 1 μm or less, are used, the specific surface area of ​​the active material particles is large, and more conductive paths connecting the active material particles are required. In such cases, it is preferable to use a graphene compound that can efficiently form conductive paths even in a small amount.

[0327] Because of the properties described above, graphene compounds are particularly effective as conductive materials for secondary batteries that require rapid charging and rapid discharging. For example, rapid charging and rapid discharging may be required for secondary batteries for two-wheeled or four-wheeled vehicles, secondary batteries for drones, etc. Rapid charging characteristics may also be required for mobile electronic devices, etc. Rapid charging and discharging refers to, for example, charging and discharging at 200 mA / g, 400 mA / g, or 1000 mA / g or more per weight of the positive electrode active material.

[0328] The plurality of graphenes or graphene compounds are formed so as to partially cover the plurality of granular positive electrode active material particles or to be attached to the surfaces of the plurality of granular positive electrode active material particles, and are therefore preferably in surface contact with each other.

[0329] Here, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphenes or graphene compounds together. When an active material is covered with a graphene net, the graphene net can also function as a binder that bonds the active materials together. Therefore, the amount of binder can be reduced or no binder can be used, thereby improving the ratio of active material to the electrode volume and electrode weight. In other words, the discharge capacity of a secondary battery can be increased.

[0330] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO ２ , SiO ｘ(x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particles preferably have a median diameter (D50) of 1 μm or less, more preferably 100 nm or less.

[0331] The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0332] Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular active material and the graphene compound with a smaller amount than that of ordinary conductive materials. This allows the ratio of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the battery.

[0333] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, tend to fill microscopic spaces. Microscopic spaces refer to, for example, the spaces between multiple active materials. By combining a carbon-containing compound that easily fills microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased, resulting in the formation of an excellent conductive path. The battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and is stable, making it effective as an in-vehicle battery.

[0334] [Binder] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0335] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include one or more of cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, and starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0336] Alternatively, it is preferable to use, as the binder, materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0337] The binder may be used in combination with two or more of the above.

[0338] [Current Collector] The current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable to use a current collector with a thickness of 5 μm to 30 μm.

[0339] [Negative Electrode] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also include a conductive material and a binder.

[0340] [Negative Electrode Active Material] As the negative electrode active material, for example, an alloy-based material and / or a carbon-based material can be used.

[0341] As the negative electrode active material, an element capable of undergoing a charge-discharge reaction by alloying / de-alloying reaction with lithium can be used. For example, a material containing one or more elements selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger charge-discharge capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, a compound containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements capable of undergoing charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0342] In this specification and the like, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO ｘHere, x preferably has a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. Alternatively, x is preferably 0.2 or more and 1.2 or less. Alternatively, x is preferably 0.3 or more and 1.5 or less.

[0343] Examples of carbonaceous materials that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0344] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0345] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of lithium metal (0.05 V or more and 0.30 V or less vs. Li / Li ＋ This allows lithium-ion secondary batteries to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high charge / discharge capacity per unit volume, relatively little volume expansion, low cost, and greater safety than lithium metal.

[0346] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO ２ ) and other oxides can be used.

[0347] In addition, as the negative electrode active material, a nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large charge / discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0348] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, is used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the material can be combined with a material such as the above. Even when a material containing lithium ions is used as the positive electrode active material, it is possible to use a nitride of lithium and a transition metal as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0349] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ , CuO, Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ Fluorides such as:

[0350] The conductive material and binder that can be contained in the negative electrode active material layer can be the same as the conductive material and binder that can be contained in the positive electrode active material layer.

[0351] [Negative electrode current collector] The negative electrode current collector can be made of the same material as the positive electrode current collector. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0352] [Electrolyte] One form of electrolyte can be an electrolytic solution having a solvent and an electrolyte dissolved in the solvent. The electrolytic solution contains a solvent and a lithium salt. The solvent for the electrolytic solution is preferably an aprotic organic solvent, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone, or any combination and ratio of two or more of these.

[0353] When ethylene carbonate (EC) and diethyl carbonate (DEC) are contained as the solvent of the electrolytic solution, when the total content of ethylene carbonate and diethyl carbonate is 100 vol%, the volume ratio of ethylene carbonate to diethyl carbonate can be x:100-x (where 20≦x≦40). More specifically, a mixed organic solvent containing EC and DEC in a volume ratio of EC:DEC=30:70 can be used.

[0354] When the solvent of the electrolytic solution contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), when the total content of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 100 vol%, the volume ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate can be x:y:100-x-y (where 5≦x≦35 and 0<y<65). More specifically, a mixed organic solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 can be used.

[0355] As the solvent for the electrolyte, a mixed organic solvent containing a fluorinated cyclic carbonate (sometimes referred to as a fluorinated cyclic carbonate) or a fluorinated chain carbonate (sometimes referred to as a fluorinated chain carbonate) can be used. Furthermore, it is preferable that the mixed organic solvent contains both a fluorinated cyclic carbonate and a fluorinated chain carbonate. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have electron-withdrawing substituents, which are preferable because they lower the solvation energy of lithium ions. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable for the electrolyte, and a mixed organic solvent containing these is also suitable.

[0356] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC has isomers such as cis-4,5 and trans-4,5. Since all fluorinated cyclic carbonates have electron-withdrawing substituents, they are considered to have low solvation energy for lithium ions. The electron-withdrawing substituent in FEC is an F group.

[0357] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0358] FEC is a cyclic carbonate with a high dielectric constant, and when used in an organic solvent, it promotes the dissociation of lithium salts. On the other hand, because FEC has electron-withdrawing substituents, it is more likely to desolvate with lithium ions than ethylene carbonate (EC). Specifically, the solvation energy of lithium ions in FEC is lower than that of EC without electron-withdrawing substituents. Therefore, it is easier to separate lithium ions from the surfaces of the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery. Furthermore, because FEC has a deep highest occupied molecular orbital (HOMO), it is less susceptible to oxidation and has improved oxidation resistance. On the other hand, there are concerns about the high viscosity of FEC. Therefore, it is recommended to use a mixed organic solvent containing MTFP in addition to FEC alone in the electrolyte. MTFP is a type of chain carbonate, and can have the effect of reducing the viscosity of the electrolyte solution or maintaining the viscosity at room temperature (typically 25° C.) even at low temperatures (typically 0° C.). Furthermore, although MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, it may form a solvate with lithium ions when used in the electrolyte solution.

[0359] A compound containing phosphorus, fluorine, carbon, oxygen, and nitrogen, or a mixed organic solvent containing such a compound, can be used as the solvent for the electrolyte. In this case, it is preferable that phosphorus has a double bond with oxygen and a bond with fluorine. The phosphorus-fluorine bond reduces the polarity of the molecule compared to a compound containing a phosphorus-hydrogen bond, facilitating desolvation of lithium ions. Nitrile groups also have high stability against oxidation and reduction. Furthermore, in the case of linear compounds, having different substituents at both ends of the linear chain can reduce viscosity at low temperatures rather than having the same substituent at both ends. Therefore, it is preferable to use a chain compound containing a phosphorus-oxygen double bond and a phosphorus-fluorine bond, and further having a nitrile group at one end, for the electrolyte. Using such a compound as the solvent for the electrolyte can be expected to improve safety or low-temperature characteristics.

[0360] The organic solvent described above is preferably highly purified, with a low content of particulate dust or molecules other than the constituent molecules of the organic solvent (hereinafter simply referred to as "impurities," including oxygen (O), water (HO), or moisture). It is also preferable that the reaction by-products produced during synthesis are suppressed through appropriate purification. Specifically, the impurities in the electrolyte are 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. The concentration of moisture among the impurities can be detected by Karl Fischer titration.

[0361] Furthermore, it is preferable that the above-mentioned organic solvent has almost no peaks due to impurities that can be confirmed by NMR measurement or the like. "Almost no peaks can be confirmed" means that the ratio of the integrated area of ​​the peak due to the impurity to the integrated area of ​​the peak due to the main component (simply referred to as "integral ratio") is 0.005 or less, preferably 0.002 or less. The device used for NMR measurement is not particularly limited, but for example, Bruker's "AVANCE III 400" can be used. Furthermore, among the five peaks of acetonitrile derived from acetonitrile-d3 used as a solvent in 1H-NMR measurement, the central peak can be located at 1.94 ppm.

[0362] For example, in the case of MTFP, it is known that when 1H-NMR is measured using acetonitrile-d3 solvent, four peaks appear at δ between 3.29 ppm and 3.43 ppm. However, if other peaks appear in this vicinity, for example, if a peak appears at δ between 3.24 ppm and 3.29 ppm, the peak is considered to be derived from impurities. Therefore, if the ratio (integral ratio) of the peak area between 3.24 ppm and 3.29 ppm to the peak area between 3.29 ppm and 3.43 ppm is 0.005 or less, preferably 0.002 or less, it can be said that peaks due to impurities are almost impossible to confirm.

[0363] When the total content of the mixed organic solvent containing FEC and MTFP having such physical properties is taken as 100 vol%, they are preferably mixed and used in a volume ratio of x:100-x (where 5≦x≦30, preferably 10≦x≦20). In other words, it is preferable to mix MTFP in a larger amount than FEC in the mixed organic solvent.

[0364] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from exploding and / or catching fire even if the internal temperature of the secondary battery rises due to an internal short circuit, overcharging, or the like. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0365] [Lithium Salt] The lithium salt (also called an electrolyte) to be dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used alone or in any combination and ratio of two or more of these. The lithium salt is preferably used in an amount of 0.5 mol / L or more and 3.0 mol / L or less relative to the solvent. ６ , LiBF ４ The use of such improves the safety of lithium ion secondary batteries.

[0366] The above-mentioned electrolyte is preferably a highly purified electrolyte with a low content of granular dust or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is 1 wt % or less, preferably 0.1 wt % or less, and more preferably 0.01 wt % or less.

[0367] [Additives] The electrolyte may contain additives. The additives can suppress reactive decomposition of the electrolyte that may occur on the positive electrode surface or the negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. Examples of additives that can be used include vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), and lithium bis(oxalate)borate (LiBOB). LiBOB is particularly preferred because it easily forms a good coating. VC or FEC is preferred because it can form a good coating on the negative electrode during battery aging or charging in the early stages of use, thereby improving cycle characteristics.

[0368] The additive may be one or more dinitrile compounds, such as succinonitrile, glutaronitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE).

[0369] Fluorobenzene may also be added to the organic solvent. The concentration of the additive is preferably, for example, 0.1 wt% to 5 wt% of the total electrolyte. PS or EGBE is preferred because it forms a good coating on the positive electrode during charge and discharge, improving cycle characteristics. FB is preferred because it improves the wettability of the organic solvent to the positive and negative electrodes. Dinitrile compounds are preferred because their nitrile groups orient to the positive and negative electrodes, inhibiting oxidative decomposition of the organic solvent and improving high-voltage resistance. Furthermore, dinitrile compounds are preferred because they can prevent copper dissolution during overdischarge when a copper-containing current collector is used on the negative electrode. Considering the use of secondary batteries at high voltages, adding a nitrile compound is preferred.

[0370] [Gel Electrolyte] A polymer gel in which a polymer is swollen with an electrolytic solution may be used as the gel electrolyte. By using a polymer gel electrolyte, a semi-solid electrolyte layer can be provided, and safety against leakage and the like can be improved. In addition, it is possible to reduce the thickness and weight of the secondary battery.

[0371] 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.

[0372] Examples of polymers that can be used include polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0373] [Solid Electrolyte] Instead of an electrolytic solution, a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a solid electrolyte containing a polymer material such as a PEO (polyethylene oxide) can be used. When a solid electrolyte is used, the installation of a separator and / or spacer is unnecessary. Furthermore, since the entire battery can be solidified, there is no risk of leakage, and safety is dramatically improved.

[0374] [Separator] The secondary battery preferably has a separator. Examples of separators that can be used include paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into an envelope shape and disposed so as to encase either the positive electrode or the negative electrode.

[0375] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0376] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging 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.

[0377] 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.

[0378] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the discharge capacity per volume of the secondary battery can be increased.

[0379] [Exterior Body] The exterior body of the secondary battery can be made of, for example, a metal material such as aluminum and / or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure film in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body.

[0380] <Laminated Secondary Battery and Fabrication Method> An example of an external view of a laminated secondary battery 500 is shown in FIGS. 14 and 15. FIGS. 14 and 15 show a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511. The positive electrode 503 has a positive electrode current collector 501 and a positive electrode active material layer 502, and the negative electrode 506 has a negative electrode current collector 504 and a negative electrode active material layer 505. If the laminated secondary battery has a flexible configuration, when it is mounted in an electronic device having at least a flexible portion, the secondary battery can also be bent to match the deformation of the electronic device. An example of a method for fabricating the laminated secondary battery will be described with reference to FIGS. 15A to 15C.

[0381] First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked. FIG. 15B shows the stacked negative electrode 506, the separator 507, and the positive electrode 503. Here, 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.

[0382] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are arranged on the outer casing 509 .

[0383] Next, as shown in Fig. 15C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding or the like may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0384] Next, an electrolyte solution (not shown) is introduced into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of the electrolyte solution is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, the laminated secondary battery 500 can be produced.

[0385] By using the positive electrode active material described in the above embodiment for the positive electrode 503, the secondary battery 500 can have a high discharge capacity and excellent cycle characteristics.

[0386] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0387] Embodiment 4 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted on an electronic device will be described with reference to FIGS. 16A to 18C.

[0388] 16A to 16G show examples in which the secondary battery having the positive electrode active material described in the above embodiment is mounted in an electronic device. Examples of electronic devices to which the secondary battery is applied include television devices (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0389] Furthermore, a secondary battery having a flexible shape can be incorporated along the curved surfaces of the inner or outer walls of houses, buildings, etc., and the interior or exterior of automobiles.

[0390] 16A illustrates an example of a mobile phone. The mobile phone 7400 includes a display portion 7402 built into a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 includes a secondary battery 7407. By using the secondary battery of one embodiment of the present invention as the secondary battery 7407, a lightweight mobile phone with a long lifetime can be provided.

[0391] Figure 16B shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed by an external force and bent as a whole, the secondary battery 7407 provided inside is also bent. Figure 16C shows the state of the bent secondary battery 7407 at that time. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a bent state. The secondary battery 7407 has lead electrodes electrically connected to the current collectors.

[0392] FIG. 16D shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display portion 7102, operation buttons 7103, and a secondary battery 7104. FIG. 16E shows a bent secondary battery 7104. When the secondary battery 7104 is worn on a user's arm in a bent state, the housing deforms, causing a change in the curvature of part or the entire secondary battery 7104. Note that the degree of curvature at any point on the curve, expressed as the radius of the corresponding circle, is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or the entire main surface of the housing or the secondary battery 7104 changes when the radius of curvature is in the range of 40 mm to 150 mm. High reliability can be maintained when the radius of curvature of the main surface of the secondary battery 7104 is in the range of 40 mm to 150 mm. By using the secondary battery of one embodiment of the present invention as the secondary battery 7104, a lightweight and long-life portable display device can be provided.

[0393] 16F shows an example of a wristwatch-type portable information terminal 7200. The portable information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, operation buttons 7205, an input / output terminal 7206, and the like.

[0394] The portable information terminal 7200 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0395] The display surface of the display portion 7202 is curved, and a display can be performed along the curved display surface. The display portion 7202 also includes a touch sensor, and can be operated by touching the screen with a finger or a stylus. For example, an application can be started by touching an icon 7207 displayed on the display portion 7202.

[0396] The operation button 7205 can have 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 7205 can be freely set by an operating system incorporated in the portable information terminal 7200.

[0397] The portable information terminal 7200 is also capable of performing standardized short-range wireless communication. For example, hands-free conversation is possible by communicating with a wirelessly enabled headset.

[0398] The portable information terminal 7200 also includes an input / output terminal 7206, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal 7206. Note that charging may be performed by wireless power supply without using the input / output terminal 7206.

[0399] The display portion 7202 of the portable information terminal 7200 includes the secondary battery of one embodiment of the present invention. By using the secondary battery of one embodiment of the present invention, a lightweight portable information terminal with a long life can be provided. For example, the secondary battery 7104 shown in FIG. 16E can be incorporated into the housing 7201 in a curved state or into the band 7203 in a bendable state.

[0400] The portable information terminal 7200 preferably has a sensor. For example, a fingerprint sensor, a pulse sensor, a human body sensor such as a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted as the sensor.

[0401] 16G illustrates an example of an armband-type display device. The display device 7300 includes a display portion 7304 and the secondary battery of one embodiment of the present invention. The display device 7300 can also be provided with a touch sensor in the display portion 7304 and can also function as a portable information terminal.

[0402] The display surface of the display portion 7304 is curved, and display can be performed along the curved display surface. The display state of the display device 7300 can be changed by short-range wireless communication according to a communication standard.

[0403] The display device 7300 also includes an input / output terminal, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal. Note that charging may be performed by wireless power supply without using the input / output terminal.

[0404] By using the secondary battery of one embodiment of the present invention as the secondary battery included in the display device 7300, a lightweight display device with a long lifetime can be provided.

[0405] An example in which the secondary battery having good cycle characteristics described in the above embodiment is mounted on an electronic device will be described with reference to FIGS. 16H to 18C. FIG.

[0406] By using a secondary battery of one embodiment of the present invention as a secondary battery in daily electronic devices, products that are lightweight and have a long life can be provided. For example, daily electronic devices include electric toothbrushes, electric shavers, and electric beauty devices. For secondary batteries in these products, a stick-shaped secondary battery that is easy for users to hold, small, lightweight, and has a large discharge capacity is desired.

[0407] FIG. 16H is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In FIG. 16H, the electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 that includes a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit that prevents overcharging and / or over-discharging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 16H has external terminals so that it can be connected to a charging device. Because the secondary battery 7504 is the tip portion when held, it is desirable that the total length be short and the weight be light. The secondary battery of one embodiment of the present invention has a high discharge capacity and good cycle characteristics, making it possible to provide a compact and lightweight electronic cigarette 7500 that can be used for a long period of time.

[0408] 17A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0409] For example, the secondary battery of one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 17A . The eyeglasses-type device 4000 includes a frame 4000 a and a display unit 4000 b. Mounting the secondary battery on the temples of the curved frame 4000 a makes it possible to provide the eyeglasses-type device 4000 with a lightweight design, a good weight balance, and a long continuous use time. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to a smaller housing can be realized.

[0410] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b and / or the earphone unit 4001c. By including the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to a miniaturized housing can be realized.

[0411] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. By providing the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0412] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. By providing the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0413] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted inside the belt portion 4006a. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0414] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided on the display portion 4005a or the belt portion 4005b. By providing the secondary battery of one embodiment of the present invention, a structure that can accommodate space saving due to miniaturization of the housing can be realized.

[0415] The display unit 4005a can display not only the time but also various other information such as incoming emails and phone calls.

[0416] Furthermore, since the wristwatch-type device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0417] FIG. 17B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0418] 17C shows a side view of the display portion 4005a. FIG. 17C shows a state in which a secondary battery 913 is built inside the display portion 4005a. The secondary battery 913 is the secondary battery described in Embodiment 4. The secondary battery 913 is provided at a position overlapping with the display portion 4005a, and is small and lightweight.

[0419] 17D shows an example of a wireless earphone. Here, the wireless earphone is shown having a pair of main bodies 4100a and 4100b, but this does not necessarily have to be a pair.

[0420] The main bodies 4100a and 4100b each include a driver unit 4101, an antenna 4102, and a secondary battery 4103. They may also include a display portion 4104. They also preferably include a substrate on which a circuit such as a wireless IC is mounted, a charging terminal, and the like. They may also include a microphone.

[0421] The case 4110 has a secondary battery 4111. It is preferable that the case 4110 also has a board on which circuits such as a wireless IC and a charge control IC are mounted, and a charging terminal. It may also have a display unit, buttons, and the like.

[0422] The main units 4100a and 4100b can wirelessly communicate with other electronic devices such as smartphones. This allows sound data and the like sent from other electronic devices to be played back on the main units 4100a and 4100b. If the main units 4100a and 4100b have microphones, the sound picked up by the microphones can be sent to the other electronic device, and the sound data after processing by the electronic device can be sent back to the main units 4100a and 4100b for playback. This allows the devices to be used as, for example, translation devices.

[0423] Furthermore, the secondary battery 4103 included in the main body 4100a can be charged from the secondary battery 4111 included in the case 4110. The coin-type secondary battery, the cylindrical secondary battery, or the like described in the above embodiments can be used as the secondary battery 4111 and the secondary battery 4103. A secondary battery using the positive electrode active material obtained in Embodiment 1 for its positive electrode has high energy density. By using the secondary battery 4103 and the secondary battery 4111 as the secondary battery, a structure that can accommodate space saving due to miniaturization of the wireless earphone can be realized.

[0424] 18A shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0425] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 to determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or an electronic component therein. By using the secondary battery 6306 according to one embodiment of the present invention in the cleaning robot 6300, the cleaning robot 6300 can be a highly reliable electronic device with a long operating time.

[0426] Fig. 18B shows an example of a robot. The robot 6400 shown in Fig. 18B includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0427] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0428] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0429] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0430] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component inside the robot 6400. By using the secondary battery according to one embodiment of the present invention in the robot 6400, the robot 6400 can be a highly reliable electronic device with a long operating time.

[0431] Fig. 18C shows an example of an aircraft 6500. The aircraft 6500 shown in Fig. 18C includes a propeller 6501, a camera 6502, a secondary battery 6503, and the like, and has the function of flying autonomously.

[0432] For example, image data captured by the camera 6502 is stored in the electronic component 6504. The electronic component 6504 can analyze the image data and detect the presence or absence of an obstacle when moving. Furthermore, the electronic component 6504 can estimate the remaining battery charge from a change in the storage capacity of the secondary battery 6503. The flying object 6500 includes the secondary battery 6503 according to one embodiment of the present invention therein. By using the secondary battery according to one embodiment of the present invention in the flying object 6500, the flying object 6500 can be an electronic device with a long operating time and high reliability.

[0433] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0434] Embodiment 5 In this embodiment, an example in which a secondary battery including the positive electrode active material of one embodiment of the present invention is mounted on a vehicle will be described.

[0435] By installing a secondary battery in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HV), electric vehicles (EV), or plug-in hybrid vehicles (PHV) can be realized.

[0436] FIG. 19 illustrates an example of a vehicle using a secondary battery according to one embodiment of the present invention. An automobile 8400 illustrated in FIG. 19A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. By using one embodiment of the present invention, a vehicle with a long cruising distance can be realized. Furthermore, the automobile 8400 includes a secondary battery. For example, secondary battery modules can be arranged on the floor of the interior of the vehicle. The secondary battery not only drives the electric motor 8406 but also supplies power to light-emitting devices such as a headlight 8401 and a room light (not shown).

[0437] The secondary battery can also supply power to display devices such as a speedometer and a tachometer included in the automobile 8400. The secondary battery can also supply power to semiconductor devices such as a navigation system included in the automobile 8400.

[0438] The automobile 8500 shown in FIG. 19B can charge the secondary battery of the automobile 8500 by receiving power from an external charging facility using a plug-in method and / or a wireless power supply method. FIG. 19B shows a state in which a ground-mounted charging device 8021 charges a secondary battery 8024 mounted on the automobile 8500 via a cable 8022. The charging method and connector specifications may be determined as appropriate using a predetermined method such as CHAdeMO (registered trademark) or Combo. The charging device 8021 may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge the secondary battery 8024 mounted on the automobile 8500 using external power supply. Charging can be performed by converting AC power to DC power using a conversion device such as an AC-DC converter.

[0439] Although not shown, a power receiving device can be mounted on a vehicle and can be charged by receiving power contactlessly from a ground-based power transmitting device. In the case of this contactless power supply method, by incorporating a power transmitting device into the road and / or 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, and the secondary battery can be charged while the vehicle is stopped and / or moving. For such contactless power supply, an electromagnetic induction method and / or a magnetic field resonance method can be used.

[0440] 19C shows an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 19C includes a secondary battery 8602, a side mirror 8601, and a turn signal light 8603. The secondary battery 8602 can supply electricity to the turn signal light 8603.

[0441] 19C is capable of storing a secondary battery 8602 in under-seat storage 8604. Even if under-seat storage 8604 is small, secondary battery 8602 can be stored in under-seat storage 8604. Secondary battery 8602 is removable, and when charging, secondary battery 8602 can be carried indoors, charged, and stored before riding.

[0442] According to one aspect of the present invention, the cycle characteristics of the secondary battery are improved, and the discharge capacity of the secondary battery can be increased. Therefore, the secondary battery itself can be made smaller and lighter. If the secondary battery itself can be made smaller and lighter, it will contribute to reducing the weight of the vehicle, thereby improving the cruising range. Furthermore, the secondary battery installed in the vehicle can also be used as a power supply source for purposes other than the vehicle. In this case, for example, it is possible to avoid using a commercial power source during peak power demand. Avoiding the use of a commercial power source during peak power demand can contribute to energy conservation and reduction of carbon dioxide emissions. Furthermore, if the cycle characteristics are good, the secondary battery can be used for a long period of time, and the amount of rare metals used, such as cobalt, can be reduced.

[0443] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0444] In this example, a positive electrode active material containing at least titanium as an additive element was produced, and the characteristics of the positive electrode active material were analyzed.

[0445] <Preparation of Positive Electrode Active Material> The conditions for producing the positive electrode active material of this example will be described with reference to the steps shown in FIGS. 1 to 2C.

[0446] <Sample 1A> LiCoO in step S14 of FIG. ２ As the lithium cobalt oxide, commercially available lithium cobalt oxide (Cellseed C-5H, manufactured by Nippon Chemical Industry Co., Ltd.) containing no particular additive elements was prepared. It is preferable to use lithium cobalt oxide in this step that has sufficiently low concentrations of magnesium and titanium, which are additive elements described below, i.e., a low impurity concentration, since this facilitates control of the distribution of the additive elements described below. Furthermore, it is preferable that the lithium cobalt oxide in this step have a median diameter (D50) of 10 μm or less, typically 7.0 μm. The median diameter (D50) can be measured using a laser diffraction particle size analyzer SALD-2200. Heating of the lithium cobalt oxide in this step was not performed.

[0447] 1, magnesium and fluorine were added as the additive element A1. First, as shown in step S21 of FIG. 2A, magnesium fluoride (MgF ２ ), lithium fluoride (LiF) was prepared as a fluorine source. Magnesium fluoride can also be called a fluorine source. LiF:MgF ２ LiF and MgF were mixed in a molar ratio of 1:3. ２ Next, LiF and MgF were dissolved in dehydrated acetone. ２The mixture was mixed at a rotation speed of 500 rpm for 20 hours (step S22). A ball mill was used for mixing, and zirconium oxide balls with a diameter of 1 mm were used as media. After mixing, the dehydrated acetone was removed using a drying oven, and the mixture was sieved using a 300 μm mesh sieve and a 53 μm mesh sieve to obtain an additive element source (A1 source) (step S23). By sieving through the 53 μm sieve, the median diameter (D50) of the A1 source became 100 μm or less, which is suitable for using Picoline, which will be described later.

[0448] Next, in step S31, MgF contained in the Al source ２ The Al source and lithium cobalt oxide were mixed using a composite treatment to obtain a mixture 901 (step S32). A Picoline (manufactured by Hosokawa Micron Co., Ltd.) equipped with a Nobilta rotor was used as the composite treatment device, and stirring was performed at a rotation speed of 3000 rpm for 10 minutes. Cooling water was used during the composite treatment to suppress heat generation.

[0449] Next, in step S33, the mixture 901 was heated. The heating conditions were 850°C and 10 hours. During heating, the mixture 903 was placed in a sagger and a lid was placed on it. The sagger was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Co., Ltd.) and heated at the above-mentioned heating temperature. Oxygen was flowed in the furnace at 10 L / min (O ２ Flow). The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace reached 5 Pa, creating a positive pressure inside the furnace. After heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C. In this way, a composite oxide 902 containing Mg and F was obtained (step S34).

[0450] Next, nickel and aluminum were added as the additive element A2 according to steps S40 to S54. First, as shown in steps S41 and S43 of FIG. 2B, nickel hydroxide (Ni(OH)) that had been subjected to a pulverization process (step S42) was used as the nickel source. ２ ) was prepared as an aluminum source, and aluminum hydroxide (Al(OH)) that had been subjected to a crushing process (step S42) was used.３ ) was prepared. The grinding process was a wet method using dehydrated acetone. After grinding, the dehydrated acetone was removed using a drying oven, and the resulting material was sieved using a 300 μm sieve and a 53 μm sieve. By sieving through the 53 μm sieve, the particle sizes of the A2 source were reduced to 100 μm or less, which is suitable for using Picoline, which will be described later.

[0451] The nickel hydroxide and aluminum hydroxide were weighed so that the number of nickel atoms in the nickel hydroxide was 0.50 atomic % relative to the number of cobalt atoms in the lithium cobalt oxide, and the number of aluminum atoms in the aluminum hydroxide was 0.25 atomic % relative to the number of cobalt atoms in the lithium cobalt oxide, and these were mixed with the composite oxide 902 using a composite treatment to obtain a mixture 903 (step S52). A Picoline (manufactured by Hosokawa Micron) equipped with a Nobilta rotor was used as the apparatus for the composite treatment, and stirring was performed for 10 minutes at a rotation speed of 3000 rpm. Cooling water was used to suppress heat generation during the composite treatment.

[0452] Next, in step S53, the mixture 903 was heated. The heating conditions were 850°C and 2 hours. By going through the mixing step using the composite treatment, this step S53 could be completed in a shorter time than step S33, improving productivity. During heating, the mixture 903 was placed in a sagger and a lid was placed on it. The sagger was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Company Ltd.) and heated at the above heating temperature. Oxygen was flowed through the furnace at 10 L / min (O ２ Flow). The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace reached 5 Pa, creating a positive pressure inside the furnace. After heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C. In this way, a composite oxide 904 containing Mg, F, Al, and Ni was obtained (step S54).

[0453] Next, titanium was added as the additional element A3 in accordance with steps S60 to S74. As shown in steps S61 to S63 of FIG. 2C, lithium titanate (Li) that had been subjected to a pulverization process (step S62) was used as the titanium source. ２ TiO ３Using zirconium oxide balls as a medium, lithium titanate was placed in dehydrated acetone and stirred at a rotation speed of 400 rpm for 12 hours. The mixture was then sieved through a sieve with 300 μm openings to obtain source A3 (step S63).

[0454] Next, in step S71, titanium was weighed out as the A3 source so that the atomic number of titanium was 0.1 atomic % relative to the cobalt of the lithium cobalt oxide, and this was mixed with the composite oxide 904 by a composite treatment to obtain a mixture 905 (step S72). A Picoline (manufactured by Hosokawa Micron) equipped with a Nobilta rotor was used as the composite treatment device, and stirring was performed for 10 minutes at a rotation speed of 3000 rpm. Cooling water was used to suppress heat generation during the composite treatment.

[0455] Next, in step S73, the mixture 905 was heated. The heating conditions were 850°C and 2 hours. This step S73 could be completed in a shorter time than step S33, improving productivity. During heating, the mixture 903 was placed in a sagger and a lid was placed on it. The sagger was placed in a roller hearth kiln simulator furnace (manufactured by Noritake Company Ltd.) and heated at the above heating temperature. Oxygen was flowed through the furnace at 10 L / min (O ２ The flow rate, specifically the opening width of the exhaust port, was adjusted so that the differential pressure gauge of the furnace reached 5 Pa, creating a positive pressure inside the furnace. After heating, the furnace was cooled at a rate of 200°C / hour, and the oxygen flow was not stopped until the temperature reached 200°C.

[0456] The composite oxide containing Mg, F, Al, Ni, and Ti thus obtained was used as the positive electrode active material of Example 1.

[0457] <GD-MS Measurement Results> GD-MS measurements were performed on the positive electrode active material of Example 1. The results are shown in the table below.

[0458]

[0459] From the table above, the titanium concentration determined by GD-MS was 460 wt ppm, which satisfied the requirement of 300 wt ppm to 600 wt ppm. From the table above, the magnesium concentration determined by GD-MS was 1500 wt ppm, which satisfied the requirement of 800 wt ppm to 2000 wt ppm. Titanium has a lower concentration than magnesium, and the Mg / Ti ratio determined by GD-MS was 3.3, which satisfied the requirement of 2 to 7.

[0460] From the above table, the nickel concentration determined by GD-MS was 4000 wt ppm, which satisfied the requirement of 3000 wt ppm or more and 5000 wt ppm or less. The aluminum concentration determined by GD-MS was 1600 wt ppm, which satisfied the requirement of 1200 wt ppm or more and 1800 wt ppm or less.

[0461] <XPS Measurement Results> XPS measurement was performed on the positive electrode active material of Example 1. The results are shown in the table below.

[0462]

[0463] From the above table, the titanium concentration determined by XPS was 1.9 atomic%, which satisfied the requirement of 0.5 atomic% or more and 2.5 atomic% or less. Furthermore, from the above table, the Co / Ti ratio determined by XPS was 6.1, which satisfied the requirement of 3 or more and 8 or less. Furthermore, from the above table, the Mg / Ti ratio determined by XPS was 4.4, which satisfied the requirement of 2 or more and 7 or less.

[0464] From the above table, the magnesium concentration determined by XPS was 8.4 atomic%, which satisfied the requirement of 3 atomic% or more and 10 atomic% or less. Furthermore, from the above table, the Co / Mg ratio determined by XPS was 1.4, which satisfied the requirement of 2 or more and 4 or less.

[0465] From the above table, the nickel concentration determined by XPS was 1.1 atomic%, which satisfied the requirement of 0.8 atomic% or more and 1.8 atomic% or less. Furthermore, from the above table, the Co / Ni ratio determined by XPS was 11.0, which satisfied the requirement of 9 or more and 12 or less. Nickel is more likely to dissolve in lithium cobalt oxide than magnesium, so there is a possibility that nickel diffused throughout the positive electrode active material.

[0466] As can be seen from the above table, the aluminum concentration determined by XPS was below the lower limit of detection. Considering the measurement range in the depth direction of XPS, it is believed that aluminum is present in the interior 100b.

[0467] <Preparation of Positive Electrode A> A positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were prepared. PVDF was dissolved in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, the positive electrode active material, AB, and PVDF were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. After applying the slurry to the positive electrode current collector, the solvent was evaporated to form a positive electrode active material layer.

[0468] Thereafter, in order to increase the density of the positive electrode active material layer, a pressing process was performed using a roll press under a linear pressure of 210 kN / m, with the upper and lower rolls of the roll press both set at 120°C.

[0469] By this method, a positive electrode A was produced. In the positive electrode A, the amount of the positive electrode active material supported per area of ​​the positive electrode was about 7 mg / cm. ２ It was.

[0470] <Preparation of Positive Electrode B> Positive electrode B was prepared, which differed from positive electrode A in slurry ratio, pressing conditions, and loading amount. The slurry ratio of positive electrode B was positive electrode active material:AB:PVDF=96:2:2 (weight ratio), and the pressing conditions were a linear pressure of 210 kN / m followed by a linear pressure of 1467 kN / m. The other conditions were the same as positive electrode A. In positive electrode B, the loading amount of positive electrode active material per area of ​​the positive electrode was approximately 14.5 mg / cm. ２ It was.

[0471] <Assembly of Coin Cells> In this example, coin cells A and B (CR2032 type, diameter 20 mm, height 3.2 mm) equipped with the above-mentioned positive electrodes A and B and having a lithium metal counter electrode were assembled in a glove box under an argon atmosphere. Stainless steel (SUS) was used for the positive electrode cans and negative electrode cans of coin cells A and B, respectively. Polypropylene was used for the separators of coin cells A and B. The electrolyte of coin cells A and B was a mixture of an organic solvent with an EC:DEC ratio of 30:70 (volume ratio) and LiPF ６ 1.0 mol / l solution (LiPF ６ _EC+DEC) was prepared, and VC was added as an additive, and LiPF ６ _EC + DEC was used with 2 wt% added. When exposed to the test, dendrites may form on the lithium metal, making it difficult to continue the test, but the additive VC can suppress dendrites, making long-term testing possible.

[0472] First, a positive electrode punched to fit the size of a coin cell was immersed in electrolyte to prepare a positive electrode impregnated with electrolyte. The electrolyte-impregnated positive electrode was placed on a positive electrode can. The positive electrode current collector was in contact with the positive electrode can. Next, a separator punched to fit the size of the coin cell was immersed in electrolyte to prepare a separator impregnated with electrolyte. The separator impregnated with electrolyte was placed on the positive electrode, and the electrolyte was poured into the separator. A gasket was then placed on the separator, and lithium metal was placed on the gasket. The gasket was preferably made of an insulating material containing a fluorine compound and had a ring shape. The gasket ensured airtightness inside the coin cell. A spacer was then placed on top of the lithium metal, and a washer was placed on top of the spacer. The spacer was then placed to prevent contact between the lithium metal and the washer. The negative electrode can was then placed on top, and the negative electrode can and positive electrode can were crimped together. In this way, coin cells A and B used in the tests of this example were completed.

[0473] <Charge Capacity Measurement 1 by Rate> The following charge / discharge cycle was used to measure the charge capacity by rate for coin cell A. First, the temperature of the thermostatic chamber in which coin cell A was placed was set to 25° C., and cycles under the following charge / discharge conditions were repeated in order. 1st and 2nd cycles: Charging conditions: CCCV charging, 0.2C rate, 4.6V, 0.02C cutoff. Discharging conditions: CC discharge, 0.2C rate, 2.5V cutoff (fixed conditions, omitted for subsequent cycles). 3rd cycle: Charging conditions: CC charging, 0.2C rate, 4.6V cutoff. 4th cycle: Charging conditions: CC charging, 0.5C rate, 4.6V cutoff. 5th cycle: Charging conditions: CC charging, 1C rate, 4.6V cutoff. 6th cycle: Charging conditions: CC charging, 2C rate, 4.6V cutoff. 7th cycle: Charging conditions: CC charging, 3C rate, 4.6V cutoff. 8th cycle: Charging conditions: CC charging, 4C rate, 4.6V cutoff. 9th cycle: Charging conditions: CC charging, 5C rate, 4.6V cutoff. 10th cycle Charging conditions: CC charging, 0.2 C rate, 4.6 V cutoff. In this test, the current value corresponding to 1 C was 200 mA / g per weight of positive electrode active material. The 4.6 V etc. in the charging condition is called the upper limit voltage, and is maintained at the upper limit voltage during the CV charging period. The 2.5 V in the discharging condition is called the lower limit voltage. A 10-minute rest period was provided between the charging condition and the next discharging condition.

[0474] As a result, FIG. 20A shows a graph in which, with the vertical axis representing the charge capacity per weight of the positive electrode active material, the third cycle (charge rate / discharge rate=0.2 C / 0.2 C, and so on), the fourth cycle (0.5 C / 0.2 C), the fifth cycle (1 C / 0.2 C), the sixth cycle (2 C / 0.2 C), the seventh cycle (3 C / 0.2 C), the eighth cycle (4 C / 0.2 C), and the ninth cycle (5 C / 0.2 C) are shown in order from the left.

[0475] <Measurement of discharge capacity by rate 2> The discharge capacity by rate was measured using the following charge / discharge cycle for coin cell A. First, the temperature of the thermostatic chamber in which coin cell A was placed was set to 25°C, and cycles under the following charge / discharge conditions were repeated in order. 1st to 3rd cycles: Charge conditions: CCCV charge, 0.2C rate, 4.6V, 0.02C cutoff (fixed conditions, omitted for subsequent cycles) Discharge conditions: CC discharge, 0.2C rate, 2.5V cutoff 4th cycle: Discharge conditions: CC discharge, 0.5C rate, 2.5V cutoff 5th cycle: Discharge conditions: CC discharge, 1C rate, 2.5V cutoff 6th cycle: Discharge conditions: CC discharge, 2C rate, 2.5V cutoff 7th cycle: Discharge conditions: CC discharge, 3C rate, 2.5V cutoff 8th cycle: Discharge conditions: CC discharge, 4C rate, 2.5V cutoff 9th cycle: Discharge conditions: CC discharge, 5C rate, 2.5V cutoff 10th cycle: Discharge conditions: CC discharge, 0.2C rate, 2.5V cutoff In this test, the current value corresponding to 1 C was 200 mA / g of positive electrode active material weight. The 4.6 V or other voltage limit for charging is called the upper voltage limit, and the upper voltage limit is maintained during CV charging. The 2.5 V or lower voltage limit for discharging was called the lower voltage limit. A 10-minute rest period was provided between each charging condition and the next discharging condition.

[0476] As a result, the vertical axis represents the discharge capacity per weight of the positive electrode active material, and the graph shows, from the left, the third cycle (charge rate / discharge rate = 0.2 C / 0.2 C), the fourth cycle (0.2 C / 0.5 C), the fifth cycle (0.2 C / 1 C), the sixth cycle (0.2 C / 2 C), the seventh cycle (0.2 C / 3 C), the eighth cycle (0.2 C / 4 C), and the ninth cycle (0.2 C / 5 C) in that order, as shown in FIG. 20B .

[0477] <Measurement 3 of Discharge Capacity by Rate> The discharge capacity by rate was measured using the following charge / discharge cycle for coin cell B. First, the temperature of the thermostatic chamber in which coin cell B was placed was set to 25° C., and cycles under the following charge / discharge conditions were repeated in order. First cycle: Charge conditions: CCCV charge, 0.2 C rate, 4.6 V, 0.05 C cutoff. Discharge conditions: CC discharge, 0.2 C rate, 3.0 V cutoff. Second cycle: Charge conditions: CCCV charge, 0.5 C rate, 4.6 V, 0.05 C cutoff. Discharge conditions: CC discharge, 0.1 C rate, 3.0 V cutoff. Third cycle: Charge conditions: CCCV charge, 0.5 C rate, 4.6 V, 0.05 C cutoff. Discharge conditions: CC discharge, 1.0 C rate, 3.0 V cutoff. Fourth cycle: Charge conditions: CCCV charge, 0.5 C rate, 4.6 V, 0.05 C cutoff. Discharge conditions: CC discharge, 2.0 C rate, 3.0 V cutoff. In this test, the current value corresponding to 1 C was 200 mA / g of positive electrode active material weight. The charging condition of 4.6 V is called the upper limit voltage, and is maintained at the upper limit voltage during CV charging. The discharging condition of 3.0 V is called the lower limit voltage. A 10-minute rest period was provided between the charging condition and the next discharging condition.

[0478] As a result, the vertical axis represents the discharge capacity per weight of the positive electrode active material, and the graph in this order from the left shows the first cycle (0.2 C / 0.2 C), the second cycle (0.5 C / 0.1 C), the third cycle (0.5 C / 1.0 C), and the fourth cycle (0.5 C / 2.0 C).

[0479] This experiment revealed that the positive electrode active material according to one embodiment of the present invention has sufficient rate-specific discharge capacity and rate-specific charge capacity.

[0480] <Charge-discharge cycle test> A charge-discharge cycle test was performed using coin cells A and B. Coin cell A was prepared under the same conditions as the charge-discharge cycle test. Coin cell B used in rate-specific discharge capacity measurement 2 was used in the charge-discharge cycle test.

[0481] The conditions for the charge-discharge cycle test for coin cell A are described below. Coin cell A was placed in a thermostatic chamber maintained at 25°C or 45°C, and 50 cycles were performed under the following charge-discharge conditions. No aging was performed prior to this test. Condition 1: Charge: CCCV charge, 0.5C rate, 4.60V, 0.02C cutoff. Discharge: CC discharge, 0.5C rate, 2.5V cutoff. In this test, the current value corresponding to 1C was 200mA / g of positive electrode active material weight. Under each condition, a 10-minute rest period was allowed between the end of charge and the next discharge. If the cutoff current of the charge condition was not reached, time cutoff may be performed. In this test, time cutoff was performed when the cutoff current was not reached for more than 3 hours. The charge conditions of 4.60V, 4.65V, and 4.70V are called upper voltage limits, and the CV charge period was maintained at these upper voltage limits. The discharge condition of 2.5 V is called the lower limit voltage.

[0482] <Charge Capacity and Discharge Capacity> In the charge / discharge cycle test, current is measured using a charge / discharge meter, and this is taken as the charge capacity and discharge capacity. In measuring charge and discharge, the current flowing through the secondary battery is measured using the four-terminal method. During charging, electrons flow from the positive electrode terminal through the charge / discharge meter to the negative electrode terminal, so the charge current flows from the negative electrode terminal through the charge / discharge meter to the positive electrode terminal. During discharge, electrons flow from the negative electrode terminal through the charge / discharge meter to the positive electrode terminal, so the discharge current flows from the positive electrode terminal through the charge / discharge meter to the negative electrode terminal. The charge current and discharge current are measured using an ammeter included in the charge / discharge meter. The integrated amount of charge flowing during one charge cycle corresponds to the charge capacity. The integrated amount of charge flowing during one discharge cycle corresponds to the discharge capacity. For example, the integrated amount of discharge current flowing during the first discharge cycle can be referred to as the first cycle discharge capacity, and the integrated amount of discharge current flowing during the 50th discharge cycle can be referred to as the 50th cycle discharge capacity. The discharge capacity is converted into a value per weight of the positive electrode active material, and a higher discharge capacity is preferable as a battery characteristic. The charge capacity is also converted into a value per weight of the positive electrode active material.

[0483] <Discharge Capacity Retention Rate> The discharge capacity retention rate (%) at the nth cycle was calculated as (discharge capacity at the nth cycle / maximum value of discharge capacity during the 1st to nth cycles) × 100, where n is a natural number excluding zero, and in this example, n = 50. A higher discharge capacity retention rate at the end of 50 cycles is more desirable as a battery characteristic because it suppresses the decrease in battery capacity after repeated charge and discharge.

[0484] As a result of the test at 25°C under Condition 1, FIG. 22A shows a graph with the vertical axis representing the discharge capacity (mAh / g) and the horizontal axis representing the number of cycles, and FIG. 22B shows a graph with the horizontal axis representing the discharge capacity retention rate (%) and the horizontal axis representing the number of cycles.

[0485] As a result of the test at 45°C under Condition 1, FIG. 23A shows a graph with the vertical axis representing the discharge capacity (mAh / g) and the horizontal axis representing the number of cycles, and FIG. 23B shows a graph with the horizontal axis representing the discharge capacity retention rate (%) and the horizontal axis representing the number of cycles.

[0486] At 25°C under Condition 1, the maximum discharge capacity was 216 mAh / g at the first cycle, and the discharge capacity remained between 200 mAh / g and 220 mAh / g up to the 50th cycle. The discharge capacity retention rate was also 96% at the 50th cycle.

[0487] At 45°C under Condition 1, the maximum discharge capacity was 219 mAh / g at the first cycle, and the discharge capacity remained at 190 mAh / g or more and 220 mAh / g or less up to the 50th cycle. The discharge capacity retention rate was also 90% at the 50th cycle.

[0488] This experiment revealed that the positive electrode active material of one embodiment of the present invention exhibited a discharge capacity of 200 mAh / g or more and 220 mAh / g or less when charged to 4.6 V at 25° C., and a discharge capacity retention rate at the 50th cycle of 96% or more. Furthermore, the positive electrode active material of one embodiment of the present invention exhibited a discharge capacity of 190 mAh / g or more and 220 mAh / g or less when charged to 4.6 V at 45° C., and a discharge capacity retention rate at the 50th cycle of 90% or more. It was found that the positive electrode active material of one embodiment of the present invention sufficiently suppressed the decrease in discharge capacity.

[0489] Next, the conditions for the charge-discharge cycle test for coin cell B will be described. Coin cell B was placed in a thermostatic chamber maintained at 45°C, and 50 cycles were performed under the following charge-discharge conditions. No aging was performed prior to this test. Condition 2: Charge: CCCV charge, 0.5C rate, 4.60V, 0.05C cutoff. Discharge: CC discharge, 1C rate, 3.0V cutoff. In this charge-discharge cycle test, the current value corresponding to 1C was 200mA / g per weight of positive electrode active material. Under these conditions, a 10-minute rest period was provided after the end of charge before the next discharge. If the cutoff current of the charge condition is not reached, time cutoff may be performed. In this test, time cutoff was performed when the cutoff current was not reached for more than 3 hours. The charge condition of 4.60V is called the upper limit voltage, and the CV charge period is maintained at this upper limit voltage. The discharge condition of 3.0V is called the lower limit voltage.

[0490] As a result of the test at 45°C under Condition 2, FIG. 24A shows a graph with the vertical axis representing the discharge capacity (mAh / g) and the horizontal axis representing the number of cycles, and FIG. 24B shows a graph with the horizontal axis representing the discharge capacity retention rate (%) and the horizontal axis representing the number of cycles.

[0491] At 45°C under condition 2, the maximum discharge capacity was 220 mAh / g at the first cycle, and the discharge capacity remained at 170 mAh / g or more and 225 mAh / g or less up to the 50th cycle. The discharge capacity retention rate was also 78% at the 50th cycle.

[0492] This experiment revealed that the positive electrode active material of one embodiment of the present invention exhibited a discharge capacity of 170 mAh / g or more and 225 mAh / g or less when charged to 4.6 V at 45°C, and a discharge capacity retention rate of 75% or more at the 50th cycle. Although the battery characteristics appear to be lower than those of Condition 1, this is thought to be due to the continuous use of Coin Cell B from the rate-specific discharge capacity measurement through the charge-discharge cycle test. Even taking Condition 2 into consideration, it was found that the decrease in discharge capacity was sufficiently suppressed by the positive electrode active material of one embodiment of the present invention.

[0493] <AC Impedance Test> A secondary battery using the positive electrode active material of one embodiment of the present invention was subjected to an AC impedance test in a fully charged state. A method for manufacturing the secondary battery used in the test is described below.

[0494] <Preparation of Positive Electrode C> Positive electrodes C and D were prepared, each having a different loading amount from that of positive electrode A. In positive electrodes C and D, the loading amount of the positive electrode active material per area of ​​the positive electrode was approximately 21 mg / cm ２ The other conditions were the same as those for positive electrode A.

[0495] <Preparation of Negative Electrodes C and D> Carbon materials were prepared as negative electrode active materials C and D. Spheroidized natural graphite was used as negative electrode active material C, and artificial graphite X was used as negative electrode active material D. The spherical natural graphite had a median diameter (D50) of 20.9 μm and a specific surface area of ​​1.01 m. ２ Formula BT SLC1520T (manufactured by Superior Graphite) with a pore size of 1.57 m / g was used. Negative electrode active material D was artificial graphite X with a median diameter (D50) of 10.1 μm and a specific surface area of ​​1.57 m / g. SBR was prepared as a binder, and CMC was prepared as a thickener. Carbon fiber (VGCF (registered trademark) manufactured by Showa Denko K.K.) was prepared as a conductive material. Next, a slurry was prepared by mixing spherical natural graphite:VGCF:CMC:SBR in a weight ratio of 97:1:1:1. Water was used as the solvent for the slurry. The slurry was applied to a copper negative electrode current collector, and the solvent was then evaporated. The thickness of the copper negative electrode current collector was 18 μm.

[0496] Thereafter, in order to increase the density of the negative electrode active material layer, after the solvent was evaporated, the negative electrode active material layer was pressed using a roll press. The pressing conditions were a linear pressure of 28 kN / m, which was lower than that of the positive electrode. After pressing, the thickness of the negative electrode active material layer was 100 μm or more and 124 μm or less.

[0497] Through the above steps, a negative electrode was obtained.

[0498] <Electrolyte> Lithium hexafluorophosphate (LiPF) was dissolved at 1 mol / L in a mixed organic solvent containing ethylene carbonate (abbreviated as EC, flash point 143°C, boiling point 238°C, melting point 38°C) and diethyl carbonate (abbreviated as DEC, flash point 25°C, boiling point 127°C, melting point -43°C) in a volume ratio of EC:DEC = 30:70. ６ An electrolyte solution containing the above-mentioned ammonium hydroxide was prepared. No additives were used.

[0499] <Separator> A polypropylene film having a thickness of 25 μm was used as the separator.

[0500] <Exterior Body> An aluminum laminate film was used as the exterior body. The aluminum laminate film had, from the inside of the exterior body, a first polypropylene (thickness 22.5 μm), a second polypropylene (thickness 22.5 μm), an aluminum foil (thickness 40 μm), an adhesive layer (thickness 3 μm), and nylon (thickness 25 μm).

[0501] <Preparation of Test Secondary Batteries: Full Cells C and D> A lithium ion secondary battery (full cell C) was prepared using the positive electrode C prepared above, the negative electrode C prepared above, the electrolyte solution, a separator, and an exterior body. A lithium ion secondary battery (full cell D) was prepared using the positive electrode C prepared above, the negative electrode D prepared above, the electrolyte solution, a separator, and an exterior body. After a predetermined amount of electrolyte solution was injected into full cells C and D, the cells were sealed by reducing the pressure to −60 kPa using a vacuum sealer capable of reducing pressure.

[0502] The outer dimensions of full cells C and D were 76 mm x 89 mm. The area of ​​the positive electrode was 20.5 cm ２ The area of ​​the negative electrode is 23.8 cm ２ In the secondary battery, the area of ​​the negative electrode is preferably larger than the area of ​​the positive electrode, preferably 1.1 times or more and 1.3 times or less. Fifteen positive electrodes with positive electrode active material layers coated on both sides were prepared. Fourteen negative electrodes with negative electrode active material layers coated on both sides and two negative electrodes with negative electrode active material layers coated on one side were prepared. The positive and negative electrodes were stacked so as to face each other. A separator was placed between the positive and negative electrodes. The negative electrode with negative electrode active material layers coated on one side was positioned on the outermost side.

[0503] In the full cells C and D, the amounts of the positive electrode active material and the negative electrode active material were determined so that the positive electrode capacity and the negative electrode capacity were the same. Specifically, the amount of the positive electrode active material was 20 mg / cm. ２ 22mg / cm or more ２ Hereinafter, the amount of negative electrode active material carried is 13 mg / cm ２ 15mg / cm or more ２ The adjustments were made with the following aims in mind: When active material layers are formed on both sides of the current collector, the amounts of positive electrode active material carried and negative electrode active material carried are shown as values ​​per active material layer on one side of the current collector.

[0504] In full cells C and D, the positive and negative electrode capacity ratio (%) was set to 80% or more and 90% or less, preferably 83% or more and 85% or less. The positive and negative electrode capacity ratio (%) was calculated by calculating the ratio of the negative electrode capacity to the positive electrode capacity, assuming that the capacity is the product of the amount of support, the charge capacity of the active material (when charged at a charge voltage of 4.5 V (counter electrode graphite), a capacity of 200 mAh / g per weight of positive electrode active material and 300 mAh / g per weight of negative electrode active material used), and the area. The total capacity of full cells C and D was set to 2.5 Ah.

[0505] The table below summarizes the conditions for producing the test secondary batteries.

[0506]

[0507] <Aging> First, full cells C and D were aged. The aging method is shown in the table below. Aging is sometimes called initial charge / discharge or conditioning. In the table below, 1C = 200mA / g (per weight of positive electrode active material). Step A4 in the table below is called degassing and resealing. The environmental temperatures in the table below allow for an error of ±5°C.

[0508]

[0509] <Charge Capacity Measurement> Figure 25 shows the charge / discharge curves corresponding to steps A7 and A8 for full cell C. The discharge capacity was excellent, at 200 mAh / g or more. After the aging treatment, full cell C was fully charged according to step A7 in Table 6, and the charge capacity was measured. A charge / discharge measurement system (TOSCAT-3000) manufactured by Toyo Systems Co., Ltd. was used as the charge / discharge measuring device for measuring the charge capacity. The charge capacity (total capacity) of full cell C in the fully charged state was close to 2.5 Ah, and no abnormalities in the charge capacity were observed.

[0510] <AC Impedance Measurement> AC impedance measurement (AC impedance measurement) was performed on full cell C in the fully charged state. Specifically, AC impedance measurement was started on full cell C within 2 hours of reaching the fully charged state. The device used for AC impedance measurement was a Biologic VMP3 (multi-channel potentiostat / galvanostat). The measurement conditions were an amplitude voltage of 10 mV and a frequency sweep ranging from 200 kHz to 10 mHz, after which the impedance value corresponding to 1 kHz was extracted. 1001.6 Hz was used as the frequency closest to 1 kHz. Note that 10 points per digit were measured during the sweep. The measurement environment temperature was 25°C.

[0511] Fig. 26A shows a Nyquist diagram as a measurement result of the AC impedance measurement. Fig. 26B shows an equivalent circuit used for analyzing the measurement data of the AC impedance measurement. Based on the Nyquist diagram of Fig. 26A, fitting calculations were performed using the equivalent circuit of Fig. 26B. For data analysis including fitting calculations, Zfit, a measurement and analysis software from EC-Lab manufactured by Biologic, was used.

[0512] In FIG. 26B , it is assumed that Rl corresponds to the resistance due to the battery leads and wiring, L corresponds to the inductance due to the battery leads and wiring, RS corresponds to the resistance of the electrolyte and the electrical resistance of the electrode, RA corresponds to the film resistance on the positive electrode surface, the film resistance on the negative electrode surface, and the charge transfer resistance at the negative electrode surface (resistance associated with solvation and desolvation of Li ions and deintercalation of Li ions into the active material), RC corresponds to the charge transfer resistance at the positive electrode surface (resistance associated with solvation and desolvation of Li ions and deintercalation of Li ions into the active material), and CPE (constant phase element) corresponds to the capacitance component at the porous electrode, and are arranged in the equivalent circuit.

[0513] The analysis result of the AC impedance measurement showed that the impedance value of Full Cell C at a frequency of 1 kHz was 45 mΩ.

[0514] <Nail Penetration Test> Steps A7 and A8 were repeated three times for full cell D. Then, following step A7, full cell D was fully charged and subjected to a nail penetration test. Figure 27A shows a photograph of full cell D before nail penetration. The nail penetration test was performed at room temperature, specifically, at 25°C. For the nail penetration test, a tester such as that shown in Figures 13A and 13B was used, specifically, an Advanced Safety Tester manufactured by Espec Corporation.

[0515] A nail having a diameter of 3 mm was used as the nail 1003 in Fig. 13A. The nail penetration speed was 5 mm / s. The nail penetration depth was 10 mm. Other than these, the nail penetration test was performed in accordance with the description of SAE J2464 "Safety and Abuse Testing for Electric and Hybrid Vehicle Energy Storage Systems."

[0516] 27B shows a photograph of full cell D immediately after the nail was pierced. As can be seen from Fig. 27B, full cell D did not even emit smoke, and it was determined that it did not catch fire. It was found that the safety of a secondary battery using the positive electrode active material according to one embodiment of the present invention is improved.

[0517] 100: Positive electrode active material particles, 100a: Surface layer portion, 100b: Interior portion, 104: Coating portion, 901: Mixture, 902: Composite oxide, 903: Mixture, 904: Composite oxide, 905: Mixture

Claims

mixing lithium cobalt oxide having a median diameter (D50) of 10 μm or less with a first additive element source to form a first mixture; subjecting the first mixture to a first heating to form a first composite oxide; mixing the first composite oxide with a second additive element source to form a second mixture; subjecting the second mixture to a second heating to form a second composite oxide; mixing the second composite oxide with a third additive element source to form a third mixture; A method for producing a positive electrode active material by subjecting the third mixture to a third heating, comprising: the first additive element source comprises a magnesium compound and a fluorine compound; the second additive element source comprises an aluminum compound and a nickel compound; the third additive element source comprises a titanium compound; a time period for the second heating and a time period for the third heating are each shorter than a time period for the first heating; A method for producing a positive electrode active material. 　 mixing lithium cobalt oxide having a median diameter (D50) of 10 μm or less with a first additive element source having a median diameter (D50) of 100 μm or less to form a first mixture; performing a first heating process on the first mixture to form a first composite oxide; mixing the first composite oxide with a second additive element source having a median diameter (D50) of 100 μm or less to form a second mixture; subjecting the second mixture to a second heating to form a second composite oxide; mixing the second composite oxide with a third additive element source to form a third mixture; A method for producing a positive electrode active material by subjecting the third mixture to a third heating, comprising: the first additive element source comprises a magnesium compound and a fluorine compound; the second additive element source comprises an aluminum compound and a nickel compound; the third additive element source comprises a titanium compound; a time period for the second heating and a time period for the third heating are each shorter than a time period for the first heating; A method for producing a positive electrode active material. 　 In claim 1 or 2, the lithium cobalt oxide having a median diameter (D50) of 10 μm or less has a magnesium concentration of 40 wt ppm or less and a titanium concentration of 40 wt ppm or less. A method for producing a positive electrode active material.

Images