Battery

Abstract

Provided is a battery having excellent charge / discharge characteristics even in a low-temperature environment. The battery has a positive electrode, a negative electrode, an electrolytic solution, and a separator. The electrolytic solution and the separator are provided between the positive electrode and the negative electrode. The negative electrode includes a carbon material. The electrolytic solution contains a lithium salt, a potassium salt, a fluorinated cyclic carbonate, and a fluorinated chain-like carbonate. In the battery, the carbon material contains graphite, the anion of the lithium salt and the anion of the potassium salt are different from each other, the fluorinated cyclic carbonate includes fluoroethylene carbonate, and the fluorinated chain-like carbonate includes methyl trifluoropropionate.

Description

battery

[0001] One embodiment of the present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. One embodiment of the present invention relates to a power storage device including a battery (also referred to as a secondary battery), a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, a vehicle, or a manufacturing method thereof.

[0002] In this specification, the term "electronic device" refers to any device having a power storage device, and includes electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like.

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

[0004] In particular, there is a high demand for batteries for mobile electronic devices that have a large discharge capacity per unit weight and excellent cycle characteristics. To meet this demand, improvements have been made to the positive electrode active materials of battery positive electrodes. Research on the crystalline structure of positive electrode active materials has also been conducted (Non-Patent Documents 1 to 3).

[0005] X-ray diffraction (XRD) is one of the techniques used to analyze crystal structures. 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.

[0006] However, general lithium-ion batteries have problems with charging and discharging at low temperatures. In particular, they have a problem of being unable to obtain sufficient charge and discharge characteristics in low-temperature environments below 0°C. However, since it is desirable for batteries to exhibit stable performance regardless of the environment, development of battery components (such as positive electrode active materials, negative electrode active materials, and electrolytes) for use in lithium-ion batteries has been progressing in order to obtain sufficient charge and discharge characteristics even in low-temperature environments below 0°C (Patent Document 1).

[0007] As a method for improving the battery performance of a lithium ion battery, Patent Document 2 describes a conditioning method for reducing the reaction resistance in a negative electrode active material, in which sodium ions or potassium ions are inserted into graphite.

[0008] The phenomenon of potassium ions being inserted into graphite has been reported in detail in Non-Patent Document 6 and elsewhere.

[0009] In calculations related to graphite, it is important to take van der Waals forces into consideration. Corrections related to dispersion forces such as van der Waals forces in computational science have been reported in, for example, Non-Patent Document 7.

[0010] WO2023 / 73480 Pamphlet Japanese Patent Application Laid-Open No. 2005-302630

[0011] 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.17340−17348T.Motohashi,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 Electrochemical Society, 2002, 149 (12) A1604-A1609A. Belsky, et al. , “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Crystal. , (2002) B58 364-369. J. Akimoto, Y. Gotoh, Y. Oosawa, “Synthesis and structure refinement of LiCoO▲2▼ single crystals” Journal of Solid State Chemistry (1998) 141, p. 298-302. H. Onuma et al. , “Phase evolution of electrochemically potassium intercalated graphite.”Journal of Materials Chemistry A 9.18 (2021):11187-11200S. Grimme “Semiempirical GGA-type density functional constructed with a long-range dispersion correction.” Journal of Computational Chemistry 27.15 (2006): 1787-1799. ;

[0012] Patent Document 1 describes that a battery that can operate even in a low-temperature environment can be realized by using the nonaqueous solvent described in Patent Document 1. However, even with the battery described in Patent Document 1, the discharge capacity when discharged at a temperature of 0° C. or lower was not large at the time of filing the present invention, and further improvement is desired.

[0013] For example, Patent Document 1 discloses that the use of hard carbon facilitates charge and discharge in a low-temperature environment of 0° C. or less, but the reaction potential of hard carbon with lithium ions is higher than the reaction potential of graphite with lithium at 25° C. In other words, when hard carbon is used in the negative electrode of a battery, the operating voltage of a battery using hard carbon as the negative electrode active material is lower than that of a battery using graphite as the negative electrode active material.

[0014] Patent Document 2 describes a method for reducing the reaction resistance of a negative electrode containing graphite, but does not describe use in a low-temperature environment of 0° C. or below, nor does it describe an electrolyte or organic solvent suitable for charge / discharge in a low-temperature environment.

[0015] Therefore, an object of one embodiment of the present invention is to provide a battery using graphite as a negative electrode active material and having excellent battery characteristics even in a low-temperature environment (for example, 0° C. or lower, preferably −20° C. or lower, more preferably −30° C. or lower, more preferably −40° C. or lower). Note that the battery characteristics refer to charge capacity, discharge capacity, charge / discharge cycle characteristics, safety, reliability, and the like, and a battery is said to have excellent battery characteristics when one or more of these are excellent.

[0016] Another object of one embodiment of the present invention is to provide a battery with a high charging voltage, a battery with high safety or reliability, a battery with little deterioration, a battery with a long life, or a novel battery.

[0017] Another object is to provide a novel substance, active material, battery, or manufacturing method thereof.

[0018] 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.

[0019] One embodiment of the present invention is a battery including a positive electrode, a negative electrode, an electrolyte, and a separator, the separator being disposed between the positive electrode and the negative electrode; the negative electrode including a carbon material; the electrolyte including a lithium salt, a potassium salt, a fluorinated cyclic carbonate, and a fluorinated chain carbonate, wherein the anion of the lithium salt and the anion of the potassium salt are different from each other.

[0020] In the above, the carbon material is preferably graphite, and more preferably natural graphite.

[0021] In any one of the above, the lithium salt is LiPF ６ Preferably, the potassium salt comprises KFSI, the fluorinated cyclic carbonate comprises fluoroethylene carbonate, and the fluorinated chain carbonate comprises methyl trifluoropropionate.

[0022] In the above, the positive electrode is preferably lithium cobalt oxide containing magnesium, aluminum, and nickel, or the positive electrode is preferably lithium cobalt oxide containing magnesium, aluminum, nickel, and titanium.

[0023] According to one embodiment of the present invention, a battery can be provided that uses graphite as a negative electrode active material and has excellent battery characteristics even in a low-temperature environment (e.g., 0° C. or lower, preferably −20° C. or lower, more preferably −30° C. or lower, more preferably −40° C. or lower).

[0024] According to one embodiment of the present invention, a battery with a high charging voltage can be provided. Alternatively, a battery with high safety or reliability can be provided. Alternatively, a battery with little deterioration can be provided. Alternatively, a battery with a long life can be provided. Alternatively, a novel battery can be provided.

[0025] According to one embodiment of the present invention, a novel substance, an active material, a battery, or a manufacturing method thereof can be provided.

[0026] 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.

[0027] FIGS. 1A and 1B are diagrams illustrating an example of a cross section of an electrode. FIGS. 2A and 2B are diagrams illustrating a calculation model. FIGS. 3A and 3B are diagrams illustrating the calculation model. FIGS. 4A and 4B are diagrams illustrating the calculation model. FIGS. 5A and 5B are graphs illustrating calculation results. FIGS. 6A and 6B are graphs illustrating calculation results. FIG. 7 is a diagram illustrating the appearance of a secondary battery. FIGS. 8A to 8C are diagrams illustrating a method for fabricating a secondary battery. FIG. 9A is a cross-sectional view of a positive electrode active material, and FIG. 9B is a diagram illustrating the distribution of added elements in the positive electrode active material. FIG. 10 is a diagram illustrating the results of DSC analysis. FIG. 11 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 12A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 12B is an FFT pattern of the region of the rock salt-type crystal RS, and FIG. 12C is an FFT pattern of the region of the layered rock salt-type crystal LRS. FIG. 13 is a diagram illustrating the crystal structure of the positive electrode active material. FIG. 14 is a diagram illustrating the crystal structure of a conventional positive electrode active material. FIG. 15 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 16 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIGS. 17A and 17B are diagrams illustrating XRD patterns calculated from the crystal structure. FIG. 18 is a cross-sectional view of a positive electrode active material. FIGS. 19A to 19D are diagrams illustrating a method for producing a positive electrode active material. FIG. 20A is a diagram illustrating a cross-sectional schematic of a heating furnace, FIG. 20B is a diagram illustrating a top view of a lid, and FIG. 20C is a cross-sectional schematic diagram illustrating the heights of a container, a lid, and an object to be heated. FIG. 21 is a diagram illustrating a method for producing a positive electrode active material. FIGS. 22A to 22C are diagrams illustrating a method for producing a positive electrode active material. FIGS. 23A to 23H are diagrams illustrating an example of an electronic device. FIGS. 24A to 24D are diagrams illustrating an example of an electronic device. FIGS. 25A to 25C are diagrams illustrating an example of an electronic device. FIGS. 26A to 26C are diagrams illustrating an example of a vehicle. Figures 27A to 27D are diagrams illustrating an example of space equipment. Figures 28A and 28B are graphs illustrating the results of an example. Figures 29A and 29B are graphs illustrating the results of an example.

[0028] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0029] In the configuration of the invention described below, the same parts or parts having similar functions are denoted by the same reference numerals in different drawings, and repeated explanations thereof will be omitted. Furthermore, when referring to similar functions, the same hatching pattern may be used and no particular reference numeral may be assigned.

[0030] Furthermore, for ease of understanding, the position, size, range, etc. of each component shown in the drawings may not represent the actual position, size, range, etc. Therefore, the disclosed invention is not necessarily limited to the position, size, range, etc. disclosed in the drawings.

[0031] In this specification, the ordinal numbers such as "first" and "second" are used for convenience and do not limit the number of components or the order of the components (for example, the order of processes or the order of stacking). Furthermore, an ordinal number assigned to a component in one part of this specification may not match an ordinal number assigned to the same component in another part of this specification or in the claims.

[0032] (Embodiment 1) [Battery] A battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, an electrolyte, and a separator. The battery according to one embodiment of the present invention may also include an exterior body that covers at least a portion of the periphery of the positive electrode, the negative electrode, and the electrolyte.

[0033] In this embodiment, the description will focus on the battery configuration required to realize a battery that has excellent battery characteristics even in a low-temperature environment (for example, 0° C. or lower, preferably −20° C. or lower, more preferably −30° C. or lower, more preferably −40° C. or lower). Specifically, the description will focus on the negative electrode active material and the electrolyte contained in the negative electrode.

[0034] Fig. 1A is a schematic cross-sectional view illustrating the internal structure of a battery 10 according to one embodiment of the present invention, and Fig. 1B is an enlarged view of a region A indicated by a dashed line in Fig. 1A.

[0035] 1A , the positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 23. The negative electrode 30 has a negative electrode current collector 32 and a negative electrode active material layer 33. The positive electrode 20 and the negative electrode 30 are stacked with the separator 40 interposed between them, with the positive electrode active material layer 23 and the negative electrode active material layer 33 facing each other. The battery 10 also contains an electrolyte in the voids of the positive electrode active material layer 23, the negative electrode active material layer 33, and the separator 40.

[0036] [Negative electrode 1] The negative electrode 30 will be described with reference to Figure 1B. The negative electrode active material layer 33 is provided on the negative electrode current collector 32. The negative electrode active material layer 33 includes a negative electrode active material 34 and a binder 35. The negative electrode active material layer 33 also includes an electrolyte 45 in the voids therein. The negative electrode active material layer 33 may further include a conductive material 36.

[0037] For example, a metal foil can be used as the negative electrode current collector 32. The negative electrode active material layer 33 can be formed by applying a slurry onto the negative electrode current collector 32 and drying the slurry. The dried negative electrode active material layer 33 may be pressed.

[0038] The term "slurry" refers to a dispersion liquid used to form an active material layer on a current collector, and includes an active material, a binder, and a solvent, preferably further including a conductive material. A thickener may be added together with the binder. The slurry is sometimes called an electrode slurry or an active material slurry, and when forming a negative electrode active material layer, it is sometimes called a negative electrode slurry.

[0039] <Negative Electrode Active Material> A carbon material is preferably used as the negative electrode active material 34. As the carbon material, for example, graphite can be used.

[0040] 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 artificial graphite (spherical artificial graphite) having a spherical shape can be used as the artificial graphite. Examples of natural graphite include flake graphite and spherical natural graphite (spherical natural graphite).

[0041] When lithium ions are inserted between the crystal layers of graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V or more and 0.5 V or less vs. Li / Li ＋ ). This allows lithium-ion batteries using graphite to exhibit a high operating voltage. Graphite is also preferred because it has advantages such as a relatively large capacity per unit volume, a relatively small volume expansion, is inexpensive, and is safer than metallic lithium. Graphite is also preferred because it can exhibit a higher operating voltage when incorporated into the negative electrode of a lithium-ion battery compared to non-graphitizable carbon.

[0042] The negative electrode active material layer may contain, in addition to graphite, easily graphitizable carbon (soft carbon), hardly graphitizable carbon (hard carbon), carbon fiber (carbon nanotube, carbon nanofiber), graphene, carbon black, silicon (Si), tin (Sn), gallium (Ga), silicon monoxide (SiO), and lithium titanium oxide (Li ４ Ti ５ O １２ ) may have any one or more of.

[0043] The negative electrode current collector 32, binder 35, and conductive material 36 of the negative electrode 30 will be described later in [Negative electrode 2].

[0044] [Electrolyte] Examples of electrolytes that have good charge characteristics and / or discharge characteristics in a low-temperature environment and that can be used in the battery of one embodiment of the present invention will be described below.

[0045] The electrolyte described below is a liquid electrolyte in which a metal salt (e.g., lithium salt) is dissolved in an organic solvent, and is also referred to as an electrolyte solution or an organic electrolyte solution. The electrolyte solution used in a battery of one embodiment of the present invention contains a lithium salt and a metal M salt as metal salts. The molar concentration of the lithium salt contained in the electrolyte solution is preferably higher than the molar concentration of the metal M salt contained in the electrolyte solution. A battery having a lithium ion battery as a metal cation in this manner can be called a lithium ion battery. Alternatively, when the electrolyte solution used in a battery of one embodiment of the present invention contains two or more types of metal cations (e.g., when there is one or more types of metal M salt), the battery can be called a multi-cation battery or a multi-ion battery. When the electrolyte solution used in a battery of one embodiment of the present invention contains two types of metal cations (e.g., when there is one type of metal M salt), the battery can be called a dual-cation battery or a dual-ion battery.

[0046] Although the present specification describes a lithium-ion battery using lithium ions as the main carrier ions, the technical concept of the present invention is not limited to application to lithium-ion batteries. For example, a sodium-ion battery using sodium ions as the main carrier ions may have an electrolyte solution containing sodium salt and metal M salt.

[0047] <Lithium Salt> Examples of lithium salts include LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３, LiC(C ２ F ５ SO ２ ) ３ , LiN(FSO ２ ) ２ , 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 , lithium bis(oxalato)borate (LiBOB), or two or more of these in any combination and ratio.

[0048] <Metal M Salt> The metal M salt has a metal M as a cation. Elements that can be used as the metal M will be described below.

[0049] Here, it can be said that an element that can be used as the metal M is an element that can enter between the crystal layers of graphite and can widen the distance between the crystal layers of graphite more than when Li enters. Scientific calculations regarding elements that can be used as the metal M will be described below.

[0050] <<Calculation 1>> Using a model assuming bulk graphite, the formation energy of graphite intercalation compounds (GICs) for each intercalant and the spacing between graphite crystal layers were calculated by first-principles molecular dynamics calculations. Note that an intercalant refers to an element inserted between the graphite crystal layers.

[0051] Calculations were performed for inserting one intercalant between the crystal layers of graphite. In these calculations, the intercalants were lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), aluminum (Al), scandium (Sc), yttrium (Y), and lanthanum (La: La is used as a representative lanthanoid). The specific calculation conditions for the first-principles quantum molecular dynamics were as shown in Table 1. The models used in the calculations are shown in Figures 2A to 4B.

[0052]

[0053] The conditions for van der Waals force interactions in density functional theory (DFT-D2) calculations are as follows: IVDW: 1 VDW_RADIUS: 30 VDW_S6: 0.75 VDW_D: 20 VDW_C6: Values ​​corresponding to each element listed in Non-Patent Document 7 (Table 1) were used (however, the value 38.54 was used for Cs, Ba, and La) VDW_R0: Values ​​corresponding to each element listed in Non-Patent Document 7 (Table 1) were used (however, the values ​​1.771 was used for Cs, 1.738 for Ba, and 1.716 for La)

[0054] 2A and 2B are calculation models of graphite in which no intercalant is inserted between the crystal layers, Fig. 3A and 3B are calculation models of graphite in which an intercalant is inserted between the crystal layers, and Fig. 4A and 4B are calculation models in which the intercalant exists as a bulk metal. In each schematic diagram, the parallelepiped shown by the dashed line represents the lattice, which is the repeating unit of the calculation model.

[0055] The calculation model of Figures 2A and 2B shows 144 carbon atoms (C), with two layers of 72 carbon atoms each stacked. The calculation model of Figures 3A and 3B shows a structure in which one intercalant atom (metal M) is inserted between the layers in the calculation model of Figures 2A and 2B. The calculation model of Figures 4A and 4B shows a structure in which 128 atoms (metal) used as intercalants in the calculation model of Figures 3A and 3B are arranged.

[0056] The results of calculations performed under the above conditions are shown in Figures 5A, 5B, Tables 2 and 3. Figure 5A and Table 2 show the amount of change in energy ΔE due to the insertion of an intercalant between the crystal layers of graphite. Figure 5B and Table 3 show the amount of change in the c-axis length of the lattice ΔD due to the insertion of an intercalant between the crystal layers of graphite.

[0057]

[0058] The calculation method of ΔE shown in Fig. 5A and Table 2 will be described. The energy value obtained by performing a structural relaxation calculation (also called a structural optimization calculation) on the calculation model shown in Figs. 2A and 2B is defined as Eg, and the energy value obtained by performing a structural relaxation calculation on the calculation model shown in Figs. 3A and 3B is defined as E ＧＩＣ（Ｍ） The energy value obtained by performing structural relaxation calculations on the calculation models shown in FIGS. 4A and 4B is divided by 128 (the number of atoms) to obtain E ＭＥＴＡＬ（Ｍ） Then, ΔE = E ＧＩＣ（Ｍ） −(Eg + E ＭＥＴＡＬ（Ｍ） ) ΔE is calculated by the formula. For example, if the intercalant is lithium (Li), ΔE = E ＧＩＣ（Ｌｉ） −(Eg + E ＭＥＴＡＬ（Ｌｉ） In the structural relaxation calculations, the lattice size and atomic coordinates were optimized for each calculation model.

[0059] When ΔE is a negative value, the intercalant may enter between the crystalline layers of graphite. As shown in FIG. 5A and the calculation results in Table 2, elements that may enter between the crystalline layers of graphite are lithium, potassium, rubidium, cesium, strontium, and barium.

[0060]

[0061] The calculation method of ΔD shown in FIG. 5B and Table 3 will be described. The c-axis length of the lattice after structural relaxation calculation of the calculation model shown in FIGS. 2A and 2B is defined as D. Ｇ The c-axis length of the lattice after structural relaxation calculation of the calculation model shown in FIGS. 3A and 3B is D ＧＩＣ（Ｍ） When this is done, the change in the c-axis length of the lattice ΔD is ΔD Ｍ =D ＧＩＣ（Ｍ） For example, when the intercalant is lithium (Li), ΔD Ｌｉ =D ＧＩＣ（Ｌｉ） -Dg. Note that 1 Å = 10 −１０ m.

[0062] The metal M is the amount of change ΔD in the c-axis length of the lattice when the metal M is inserted between the crystal layers of graphite. Ｍ The change in the c-axis length of the lattice when lithium is inserted between the crystal layers of graphite is ΔD Ｌｉ It is preferable that the element has a larger ΔD Ｌｉ ΔD greater than Ｍ When graphite has a region where metal M is inserted between layers, it is expected that the region where metal M is inserted will reduce the reaction resistance when lithium is inserted between the crystal layers of graphite.

[0063] Therefore, ΔD Ｌｉ ΔD greater than Ｍ When graphite has a region where metal M is intercalated between layers, it is believed that lithium can be intercalated between the crystalline layers of graphite in the region where metal M is intercalated, even in a low-temperature environment. In other words, the charge-discharge characteristics of the battery can be improved in a low-temperature environment.

[0064] As shown in FIG. 5B and the calculation results in Table 3, it is predicted that all of the elements calculated this time can widen the gap between the crystal layers of graphite more than when lithium is inserted.

[0065] <<Calculation 2>> Next, Calculation 2 was performed in which some of the calculation conditions were changed from Calculation 1.

[0066] As in Calculation 1, the calculation model used was the calculation model described with reference to FIGS. 2A to 4B.

[0067] The specific calculation conditions for the first-principles molecular dynamics calculation are shown in Table 4.

[0068]

[0069] The IVDW was set to 12 as a condition for van der Waals force interactions in the density functional theory (DFT-D3BJ) calculation.

[0070] The results of calculations performed under the same conditions as those shown in Calculation 1, except for the conditions related to the functionals, are shown in Figures 6A, 6B, Tables 5 and 6. Figure 6A and Table 5 show the amount of change in energy ΔE due to the insertion of an intercalant between the crystal layers of graphite. Also, Figure 6B and Table 6 show the amount of change in the c-axis length of the lattice ΔD due to the insertion of an intercalant between the crystal layers of graphite.

[0071]

[0072] Elements with a negative ΔE value have the potential to intercalate between the crystalline layers of graphite as intercalants. As shown in FIG. 6A and the calculation results in Table 5, elements that have the potential to intercalate between the crystalline layers of graphite are lithium, potassium, rubidium, cesium, calcium, strontium, barium, and lanthanum. Note that, as a result of Calculation 2, lanthanum has the potential to intercalate between the crystalline layers of graphite, and therefore, it is believed that not only lanthanum but also lanthanoids other than lanthanum are elements that have the potential to intercalate between the crystalline layers of graphite.

[0073]

[0074] As shown in FIG. 6B and the calculation results in Table 6, it is predicted that all of the elements calculated this time can widen the gap between the crystal layers of graphite more than when lithium is inserted.

[0075] Calculation 1 and Calculation 2 are calculations in which the correction conditions for the van der Waals forces are different. The calculation conditions for Calculation 1 are KC ８The calculation conditions for Calculation 2 are the conditions under which a calculated value (0.523 to 0.530 nm) close to the measured value (0.535 nm) of the interlayer distance of LiC ６ These are the conditions under which the calculated value (0.359 nm to 0.360 nm) of the interlayer distance is close to the measured value (0.372 nm). In this way, in calculations dealing with layered materials such as graphite, the calculation results may differ depending on the correction conditions for van der Waals forces, so calculations were performed under multiple correction conditions.

[0076] Based on the results of Calculation 1 and Calculation 2 shown above, elements that can be used as the metal M will be considered. In at least one of Calculation 1 and Calculation 2, an element that has a negative energy change when inserted between the crystal layers of graphite can be used as the metal M. In other words, the metal M can be any one or more of potassium, rubidium, cesium, calcium, strontium, barium, and lanthanum (or lanthanoids). Among these, it is more preferable to use either one or both of potassium and barium as the metal M because of its high safety.

[0077] As the anion of the metal M salt, for example, bis(fluorosulfonyl)imide (FSI − ), bis(trifluoromethanesulfonyl)imide (TFSI − ), P.F. ６ − , ClO ４ − , B.F. ４ − , and SCN − At least one anion among the above can be used.

[0078] When potassium is used as the metal M, potassium bis(fluorosulfonyl)imide (KFSI), potassium bis(trifluoromethanesulfonyl)imide (KTFSI), KPF ６ , KClO ４ , K.B.F. ４ At least one potassium salt of KFSI and KSCN can be used. KFSI is preferably used as the electrolyte to exhibit excellent battery characteristics in a low-temperature environment. The reason for this preference will be described later.

[0079] <Organic Solvent> Examples of organic solvents that can be used 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. Alternatively, the following organic solvents can be used as examples of organic solvents suitable for low-temperature environments.

[0080] <<Organic Solvent 1 Suitable for Low-Temperature Environments>> An example of an organic solvent suitable for low-temperature environments is one that contains EC, EMC, and DMC, and where the volume ratio of EC, EMC, and DMC is x:y:100-x-y (where 5≦x≦35 and 0<y<65) when the total amount of EC, EMC, and DMC is 100 vol%. More specifically, an organic solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 can be used. Note that the above volume ratio may be the volume ratio before mixing with the electrolytic solution, and the environment during mixing with the electrolytic solution may be room temperature (typically, 25°C).

[0081] EC is a cyclic carbonate with a high dielectric constant, which promotes the dissociation of lithium salts. However, EC has a high viscosity and a high freezing point (melting point) of 38°C, making it difficult to use EC alone as an organic solvent in low-temperature environments. Therefore, the organic solvent specifically described as one aspect of the present invention further includes EMC and DMC, rather than EC alone. EMC is a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −54°C. DMC is also a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −43°C. An electrolyte prepared using an organic solvent in which EC, EMC, and DMC having such physical properties are mixed in a volume ratio of x:y:100-x-y (where 5≦x≦35 and 0<y<65), assuming the total amount of these three organic solvents to be 100 vol%, is characterized by a freezing point of −40°C or lower.

[0082] A typical electrolyte used in lithium ion batteries freezes at about −20° C., making it difficult to fabricate a battery that can be charged and discharged at −40° C. The electrolyte described as an example in this embodiment has a freezing point of −40° C. or lower, making it possible to realize a lithium ion battery that can be charged and discharged even in a low-temperature environment of −40° C.

[0083] <<Organic Solvent 2 Suitable for Low-Temperature Environments>> Another example of an organic solvent suitable for low-temperature environments may include 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). Furthermore, the organic solvent preferably includes both a fluorinated cyclic carbonate and a fluorinated chain carbonate. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have electron-withdrawing substituents, and have lower solvation energies for lithium ions than organic compounds that do not have electron-withdrawing substituents. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable organic solvents.

[0084] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluorinated ethylene carbonate, fluoroethylene carbonate, FEC, and F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5. Since all of these fluorinated cyclic carbonates have electron-withdrawing substituents, the solvation energy of lithium ions is thought to be lower than that of EC.

[0085] The following structural formula (H10) is the structural formula of FEC: In FEC, the electron-withdrawing substituent is an F group.

[0086]

[0087] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The following structural formula (H22) is the structural formula of methyl 3,3,3-trifluoropropionate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0088]

[0089] An example of a fluorinated chain carbonate is trifluoromethyl 3,3,3-trifluoropropionate. The following structural formula (H23) is the structural formula of trifluoromethyl 3,3,3-trifluoropropionate. The electron-withdrawing substituent is CF ３ It is the base.

[0090]

[0091] An example of a fluorinated chain carbonate is trifluoromethyl propionate. The following structural formula (H24) is the structural formula of trifluoromethyl propionate. The electron-withdrawing substituent is CF ３ It is the base.

[0092]

[0093] An example of a fluorinated chain carbonate is methyl 2,2-difluoropropionate. The following structural formula (H25) is the structural formula of methyl 2,2-difluoropropionate. The electron-withdrawing substituent is CF ２ It is the base.

[0094]

[0095] The organic solvent contained in the electrolyte solution according to one embodiment of the present invention may include one or more fluorinated cyclic carbonates and one or more fluorinated chain carbonates. For example, the organic solvent described in this embodiment may include FEC and MTFP. The reason for this will be described below.

[0096] <<FEC and MTFP>> FEC is a cyclic carbonate with a high dielectric constant, and therefore, when used in an organic solvent, it promotes the dissociation of lithium salts. On the other hand, FEC has electron-withdrawing substituents, making it easier for desolvation with lithium ions to occur than ethylene carbonate (EC). Specifically, the solvation energy of lithium ions in FEC is lower than that of EC without electron-withdrawing substituents. Therefore, FEC easily releases lithium ions from the surfaces of the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery. Furthermore, FEC has a deep highest occupied molecular orbital (HOMO) level, making it less susceptible to oxidation and improving oxidation resistance. On the other hand, there are concerns about the high viscosity of FEC. Therefore, it is recommended to use a mixed organic solvent containing MTFP in addition to FEC in the electrolyte. MTFP is a type of chain carbonate, and can have the effect of reducing the viscosity of the electrolyte solution or maintaining the viscosity at room temperature (typically 25° C.) even at low temperatures (typically 0° C.). Furthermore, 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.

[0097] The organic solvent described above is free from granular dust or molecules other than the constituent molecules of the organic solvent (hereinafter simply referred to as "impurities"), including oxygen (O ２), water (H ２ The electrolyte preferably contains a low content of .O) or water, and is highly purified. It is also preferable that the reaction by-products produced during synthesis are suppressed through appropriate purification. Specifically, the impurity concentration of the electrolyte is 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. The concentration of water among the impurities can be detected by Karl Fischer titration.

[0098] 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.

[0099] 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 not observed.

[0100] FEC and MTFP having such physical properties can be mixed and used in a volume ratio of x:100-x (where 5≦x≦30, preferably 10≦x≦20), assuming the total content of these two organic solvents to be 100 vol%. The organic solvents can be mixed so that there is more MTFP than FEC. The volume ratio may be the volume ratio measured before mixing the organic solvents, and the outside air temperature when mixing the organic solvents may be room temperature (typically 25°C). An organic solvent mixed with FEC and MTFP is preferred because it exhibits a viscosity that allows operation as a lithium-ion battery and maintains an appropriate viscosity even in a low-temperature environment.

[0101] While FEC has been described as a representative example above, any of the organic compounds described as fluorinated cyclic carbonates has the effect of promoting the dissociation of lithium salts, has a low solvation energy and is prone to solvation with lithium ions, and has a high viscosity, making it difficult to use in low-temperature environments when used alone. Furthermore, although MTFP has been described as a representative example above, any of the organic compounds described as fluorinated chain carbonates has the effect of reducing or maintaining the viscosity of the electrolyte solution, which is an embodiment of the present invention. Therefore, if the organic solvent, which is an embodiment of the present invention, contains a fluorinated cyclic carbonate and a fluorinated chain carbonate, it is possible to satisfactorily insert and extract lithium ions into the negative electrode active material, and it is possible to provide a lithium-ion battery that can be charged and discharged over a wide temperature range, including low-temperature environments.

[0102] Therefore, the electrolyte solution of the battery of one embodiment of the present invention preferably contains a lithium salt, a metal M salt, and the above-mentioned "organic solvent 1 suitable for low-temperature environments" or "organic solvent 2 suitable for low-temperature environments."

[0103] A specific example of the electrolyte solution used in the battery according to one embodiment of the present invention is a lithium salt solution containing LiPF ６ The following shows an example of the composition of an electrolyte solution using potassium salt as the metal M salt and the above-mentioned "organic solvent 2 suitable for low-temperature environments" as the organic solvent.

[0104] By using a potassium salt as the metal M salt and inserting potassium ions between the crystalline layers of graphite, the gap between the crystalline layers of graphite can be widened more than when lithium ions are inserted. This makes it easier for lithium to enter between the crystalline layers of graphite, and therefore a battery in which the potassium ions exist between the crystalline layers of graphite has improved charge / discharge characteristics that involve the insertion and desorption of lithium ions in a low-temperature environment.

[0105] Here, in the battery according to one embodiment of the present invention, the electrolyte preferably contains a lithium salt and a metal M salt (here, a potassium salt) and uses the organic solvent suitable for low-temperature environments. In particular, it is particularly preferable to use a mixed solvent containing a fluorinated cyclic carbonate and a fluorinated chain carbonate, which has been described as the organic solvent 2 suitable for low-temperature environments.

[0106] A configuration for using an electrolytic solution containing a lithium salt and a metal M salt (here, a potassium salt) and an organic solvent suitable for the above-mentioned low-temperature environment will be described below.

[0107] FEC was used as the fluorinated cyclic carbonate, and MTFP was used as the fluorinated chain carbonate. LiPF was added to the mixed solvent to a concentration of 1 mol / L. ６ When potassium salt is further dissolved in the electrolyte, potassium hexafluorophosphate (KPF ６ ) is used, potassium salt is difficult to dissolve in the electrolyte, and the amount of potassium salt that can be added to the electrolyte is small, and 0.05 mol / L of KPF ６ However, some undissolved material was found.

[0108] Therefore, the electrolyte of the battery according to one embodiment of the present invention contains LiPF 5 as a lithium salt. ６ When using a potassium salt, the potassium salt contained in this electrolyte is KPF ６It is preferable that the potassium salt is not a mixed solvent containing FEC and MTFP. For example, potassium bis(fluorosulfonyl)imide (KFSI) can be used as the potassium salt for the electrolyte of the battery according to one embodiment of the present invention. This potassium salt is preferable because it can dissolve even an electrolyte in which a lithium salt is dissolved in a mixed solvent containing a fluorinated cyclic carbonate and a fluorinated chain carbonate, as described above as the organic solvent 2 suitable for low-temperature environments. More specifically, it is preferable to use a mixed solvent containing FEC and MTFP as the mixed solvent.

[0109] Alternatively, when LiFSI is used in the electrolyte of the battery according to one embodiment of the present invention, the potassium salt contained in the electrolyte is preferably not KFSI. In this case, for example, KPF ６ As the solvent, a mixed solvent containing a fluorinated cyclic carbonate and a fluorinated chain carbonate can be used. More specifically, as the mixed solvent, it is preferable to use a mixed solvent containing FEC and MTFP.

[0110] In other words, the above-described example of the electrolyte solution can be referred to as an electrolyte solution having a mixed solvent containing a fluorinated cyclic carbonate and a fluorinated chain carbonate, which contains multiple types of cations and multiple types of anions. Alternatively, the electrolyte solution can be referred to as an electrolyte solution having a mixed solvent containing two types of cations and two types of anions, which contains a fluorinated cyclic carbonate and a fluorinated chain carbonate. Note that the electrolyte solution for the battery of one embodiment of the present invention may be a Li ＋ and K. ＋ and as a cation, and PF ６ − and FSI − As an anion, the electrolyte of the battery of one embodiment of the present invention preferably contains a mixed solvent containing a fluorinated cyclic carbonate and a fluorinated chain carbonate. ＋ and K. ＋ and as a cation, and PF ６ − and FSI − and a mixed solvent containing FEC and MTFP as an anion.

[0111] In addition, PF６ − and FSI − When the anion is PF ６ − The molar concentration of FSI − A molar concentration higher than this is preferable because it can suppress an increase in the viscosity of the electrolyte solution.

[0112] [Negative Electrode 2] Here, items relating to the negative electrode that were not explained in [Negative Electrode 1] will be explained.

[0113] <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.

[0114] 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 carboxymethyl cellulose (CMC) such as carboxymethyl cellulose-sodium (CMC-Na), cellulose derivatives such as methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, and starch. Furthermore, it is even more preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0115] 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.

[0116] The binder may be used in combination with two or more of the above.

[0117] For example, a material with a particularly excellent viscosity adjusting effect may be used in combination with another material. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as a water-soluble polymer with a particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch may be used.

[0118] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials or other components when preparing electrode slurry. In this specification, cellulose and cellulose derivatives used as electrode binders also include their salts.

[0119] Water-soluble polymers stabilize viscosity by dissolving in water, allowing active materials and other materials combined as binders, such as styrene-butadiene rubber, to be stably dispersed in aqueous solutions. Furthermore, the presence of functional groups is expected to facilitate stable adsorption to the surface of active materials. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups such as hydroxyl or carboxyl groups, and the presence of these functional groups is expected to facilitate interactions between polymers, resulting in widespread coverage of the active material surface.

[0120] When the binder covers the surface of the active material or contacts the surface and forms a film, it is expected to function as a passive film and have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0121] <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.

[0122] The active material layers such as the positive electrode active material layer and the negative electrode active material layer preferably contain a conductive material.

[0123] 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.

[0124] 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.

[0125] The active material layer may also contain, as a conductive material, metal powder or metal fiber such as copper, nickel, aluminum, silver, or gold, or a conductive ceramic material.

[0126] The content of the conductive material relative to the total amount of the active material layer is preferably 0.1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0127] Unlike particulate conductive materials such as carbon black, which form point contacts with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the particulate active material and the graphene compound with a smaller amount than with 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 secondary battery.

[0128] 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. A secondary battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and stability, making it effective as an in-vehicle secondary battery.

[0129] <Current Collector> As the current collector, a material that has high conductivity and does not alloy with carrier ions such as lithium, such as metals such as stainless steel, gold, platinum, zinc, iron, copper, aluminum, and titanium, and alloys thereof, can be used. The current collector can be appropriately shaped, such as a sheet, mesh, punched metal, or expanded metal. It is preferable to use a current collector with a thickness of 10 μm or more and 30 μm or less.

[0130] The negative electrode current collector is preferably made of a material that does not alloy with carrier ions such as lithium.

[0131] [Positive electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer contains a positive electrode active material and may further contain at least one of a conductive material and a binder. The positive electrode current collector, conductive material, and binder may be those described in [Negative electrode 2].

[0132] The current collector can be a metal foil such as aluminum foil. The positive electrode can be formed by applying a slurry to the metal foil and drying it. Pressing may be performed after drying. The positive electrode is formed by forming an active material layer on the current collector.

[0133] The slurry is a material liquid used to form an active material layer on a current collector, and refers to a material containing an active material, a binder, and a solvent, preferably further mixed with a conductive material. The slurry is also called an electrode slurry or an active material slurry, and when forming a positive electrode active material layer, it is also called a positive electrode slurry.

[0134] <Positive Electrode Active Material> A positive electrode active material of one embodiment of the present invention will be described. As the positive electrode active material, lithium cobalt oxide and / or lithium nickel-cobalt-manganese oxide can be used.

[0135] As the lithium cobalt oxide, for example, lithium cobalt oxide to which magnesium is added is preferably used. Alternatively, lithium cobalt oxide to which magnesium, aluminum, nickel, and titanium are added is preferably used. Lithium cobalt oxide to which magnesium, aluminum, and nickel are added is preferably used. In other words, as the lithium cobalt oxide that can be used in the battery 10 of one embodiment of the present invention, "lithium cobalt oxide to which magnesium is added," "lithium cobalt oxide to which magnesium, aluminum, nickel, and titanium are added," or "lithium cobalt oxide to which magnesium, aluminum, and nickel are added" is preferably used. Note that lithium cobalt oxide that can be used in the battery 10 of one embodiment of the present invention will be described in detail as the positive electrode active material 100 in Embodiments 2 and 3.

[0136] As the lithium nickel-cobalt-manganese oxide, for example, lithium nickel-cobalt-manganese oxide having an atomic ratio of nickel:cobalt:manganese = 1:1:1, nickel:cobalt:manganese = 6:2:2, nickel:cobalt:manganese = 8:1:1, nickel:cobalt:manganese = 9:0.5:0.5, etc. Furthermore, as the lithium nickel-cobalt-manganese oxide, it is preferable to use lithium nickel-cobalt-manganese oxide to which one or more of aluminum, calcium, barium, strontium, and gallium have been added.

[0137] As a positive electrode active material that can be used in a low-temperature environment, it is preferable to use the lithium cobalt oxide described in Embodiment 2 or 3. The lithium cobalt oxide described in Embodiment 2 or 3 is a positive electrode active material that can be used up to a high charging voltage (hereinafter also referred to as "high charging voltage") and is a positive electrode active material that can insert and extract lithium ions at low temperatures.

[0138] Unless otherwise specified in this specification, the "charge voltage" is expressed based on the potential of lithium metal. In this specification, the "high charge voltage" refers to a charge voltage of, for example, 4.6 V or higher, preferably 4.65 V or higher, more preferably 4.7 V or higher, even more preferably 4.75 V or higher, and most preferably 4.8 V or higher. It is also possible to use two or more materials with different particle sizes and / or compositions as the positive electrode active material, as long as the material exhibits little deterioration during charging and discharging even at high charge voltages. In this specification, the term "different compositions" refers not only to cases where the composition of elements contained in the materials is different, but also to cases where the proportions of elements contained in the materials are different even if the composition of elements contained in the materials is the same.

[0139] As mentioned above, in this specification, a "high charge voltage" is defined as 4.6 V or higher based on the potential when the negative electrode is made of lithium metal, but when the potential when the negative electrode is made of graphite is used as the reference, a "high charge voltage" is defined as 4.5 V or higher. In short, in the case of a half cell using lithium metal as the negative electrode, a charge voltage of 4.6 V or higher is defined as a high charge voltage, and in the case of a full cell using graphite as the negative electrode, a charge voltage of 4.5 V or higher is defined as a high charge voltage.

[0140] <Second Positive Electrode Active Material> The positive electrode of one embodiment of the present invention can include a second positive electrode active material in addition to the lithium-containing positive electrode active material. The second positive electrode active material includes ions of a metal M and is capable of inserting and extracting the metal M during charging and discharging of the battery.

[0141] When potassium is used as the metal M, the second positive electrode active material may be, for example, K ｘ MnO ２ , K. ｘ CoO ２ , K. ｘ V ２ O ５ , K. ｘ CrO ２ , K. ｘ Fe ０．５ Mn ０．５ O ２ , K. ｘ Mn ０．８ Fe ０．２ O ２ , K. ｘ Co ０．５ Mn ０．５ O ２ , K. ｘ Ni ０．０５ Mn ０．９５ O ２ , K. ｘ Mn ０．７ Ni ０．３ O ２ , K. ｘ Mn ０．８ Fe ０．１ Ni ０．１ O ２ , and K ｘ Fe ０．１ Mn ０．８ Ti ０．１ O２ One or more of the following can be used. Note that x is 0 or more and 1 or less. Among them, K ｘ MnO ２ , K. ｘ CoO ２ , K. ｘ V ２ O ５ , K. ｘ CrO ２ It is preferable to use one or more of the following. In this example, the second positive electrode active material can be referred to as a positive electrode active material containing potassium. A positive electrode having a positive electrode active material containing potassium in addition to a positive electrode active material containing lithium can insert and extract lithium ions and potassium ions.

[0142] A positive electrode having a second positive electrode active material in addition to a lithium-containing positive electrode active material can be formed by applying a positive electrode slurry containing the lithium-containing positive electrode active material and the second positive electrode active material onto a metal foil and drying the slurry. Note that pressing may be performed after drying.

[0143] [Ion Insertion and Desorption] The insertion and desorption of ions (cations) will be described for a battery according to one embodiment of the present invention, in which the electrolyte contains a lithium salt and a metal M salt and graphite is used for the negative electrode. Note that the following description will be given of an example in which a potassium salt is used as the metal M salt.

[0144] In conventional lithium-ion batteries, lithium ions are inserted into or extracted from the negative electrode active material and the electrolyte solution, and the positive electrode active material and the electrolyte solution.

[0145] In one example of a battery according to one embodiment of the present invention, when the electrolyte contains a lithium salt and a metal M salt and the positive electrode contains only a positive electrode active material containing lithium, lithium ions and potassium ions are inserted or extracted from the negative electrode, and only lithium ions are inserted or extracted from the positive electrode.

[0146] Alternatively, in another example of the battery of one embodiment of the present invention, when the electrolyte contains a lithium salt and a metal M salt and the positive electrode contains a second positive electrode active material in addition to a positive electrode active material containing lithium, lithium ions and potassium ions are inserted and removed from the negative electrode, respectively, and lithium ions and potassium ions are inserted and removed from the positive electrode, respectively.

[0147] In this way, in the positive electrode and negative electrode of the battery according to one embodiment of the present invention, lithium ions and / or metal M ions are inserted into or extracted from the positive electrode or negative electrode in association with a charge reaction or a discharge reaction.

[0148] In addition, for example, in a low-temperature environment, if the metal M ions have a lower reaction resistance with the positive electrode or negative electrode than the lithium ions, it is considered that the metal M ions may contribute more to the charge / discharge reaction of the battery.

[0149] [Separator] A separator is disposed between the positive electrode and the negative electrode. The separator may be formed of, for example, polyimide, polyolefin, cellulose fiber, nonwoven fabric, glass fiber, ceramics, nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyurethane fiber, or the like. The separator is preferably processed into a bag shape and disposed so as to enclose either the positive electrode or the negative electrode.

[0150] The separator may have a multilayer structure. For example, an organic material film such as polypropylene, polyethylene, or polyimide may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture thereof. 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).

[0151] [Exterior Body] The exterior body of the battery can be formed of a metal material such as aluminum, stainless steel, or titanium. Alternatively, a film-like exterior body can be used. For example, a film-like exterior body can be a three-layer structure film in which a highly flexible metal thin film or metal foil such as aluminum, stainless steel, titanium, copper, or nickel is provided on a film made of a resin material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide-based resin or polyester-based resin is further provided on the metal thin film as the outer surface of the exterior body. Such a multilayer structure film can be called a laminate film. In this case, the laminate film may be referred to as an aluminum (aluminum) laminate film, a stainless steel laminate film, a titanium laminate film, a copper laminate film, a nickel laminate film, or the like, using the name of the material of the metal layer.

[0152] The material or thickness of the metal layer of the laminate film may affect the flexibility of the battery. For example, an aluminum laminate film having a polypropylene layer, an aluminum layer, and nylon is preferably used as an exterior body for a battery with excellent flexibility (flexibility). Here, the thickness of the aluminum layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. If the aluminum layer is thinner than 10 μm, there is a concern that pinholes in the aluminum layer may reduce the gas barrier properties, so the thickness of the aluminum layer is preferably 10 μm or more.

[0153] 7 and 8 are external views of a laminated secondary battery 500 as an example of the battery 10 of one embodiment of the present invention. Note that the shape of the battery 10 of one embodiment of the present invention is not limited to a laminated type and may be a cylindrical shape, a rectangular shape, a coin shape, or the like.

[0154] An example of a method for manufacturing a laminated secondary battery will be described with reference to FIGS. 8A to 8C . As shown in FIGS. 8A to 8C , the laminated secondary battery includes, as its constituent members, 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. As shown in FIG. 8A , the positive electrode 503 includes a positive electrode current collector 501 and a positive electrode active material layer 502, and the negative electrode 506 includes a negative electrode current collector 504 and a negative electrode active material layer 505. Note that if a flexible laminated secondary battery is mounted in an electronic device having at least a flexible portion, the secondary battery can also be bent in accordance with deformation of the electronic device.

[0155] First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked. FIG. 8B 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.

[0156] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are arranged on the outer casing 509 .

[0157] Next, as shown in Fig. 8C, 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.

[0158] 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.

[0159] By using the positive electrode active material described in Embodiment 2 or 3 for the positive electrode 503, the secondary battery 500 can have a high discharge capacity and excellent cycle characteristics.

[0160] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0161] Embodiment 2 In this embodiment, a positive electrode active material 100 that can be used for a battery 10 of one embodiment of the present invention will be described with reference to FIGS.

[0162] 9A is a cross-section of a positive electrode active material 100 that can be used for battery 10 of one embodiment of the present invention. Fig. 9B is a schematic diagram of an element concentration distribution when measurement is performed from the surface toward the inside in a cross-sectional analysis including the surface and a surface layer portion of the positive electrode active material 100. Measurement from the surface toward the inside is also referred to as measurement in the depth direction, and the X1-X2 and Y1-Y2 arrows in Fig. 9A are examples of the depth direction.

[0163] As shown in Fig. 9A, the positive electrode active material 100 has a surface layer 100a and an inner portion 100b. In Fig. 9A, the boundary between the surface layer 100a and the inner portion 100b is indicated by a dashed line. In the figure, (001) represents LiMO. ２ This shows the (001) plane of LiMO. ２ belongs to the space group R-3m.

[0164] In this specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region extending from the surface toward the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface toward the interior. Note that approximately perpendicular refers to an angle of 80 degrees or more and 100 degrees or less. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0165] The region deeper than the surface layer 100a of the positive electrode active material is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0166] Furthermore, when the positive electrode active material 100 has a layered rock salt crystal structure of space group R-3m, the surface layer portion 100a has an edge region and a basal region. Here, the edge region has a surface exposed in a direction intersecting with the (001) plane (also referred to as a surface other than the (001) orientation), and the edge region is a region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or substantially perpendicular from the surface to the interior. Here, "intersect" means that the angle between the perpendicular to the first plane (the (001) plane) and the normal to the second plane (the surface of the positive electrode active material 100) is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less, and even more preferably 50 degrees or more and 90 degrees or less.

[0167] The basal region has a surface parallel to the (001) plane (also referred to as a (001)-oriented surface), and the region extending from the surface to the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface to the interior is referred to as the basal region. Note that "parallel" here means that the angle formed between the perpendicular to the first surface (the (001) plane) and the normal to the second surface (the surface of the positive electrode active material 100) is 0 degrees or more and less than 10 degrees, preferably 0 degrees or more and 5 degrees or less, and more preferably 0 degrees or more and 2.5 degrees or less.

[0168] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer portion 100a and the inner portion 100b. ２ O ３ The positive electrode active material 100 does not include metal oxides having no lithium sites that can contribute to charging and discharging, such as lithium ions, carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material. The attached metal oxides refer to, for example, metal oxides whose crystal structure does not match that of the interior 100b.

[0169] Furthermore, the positive electrode active material 100 does not include the electrolyte, organic solvent, binder, conductive material, or compounds derived from these materials that are attached to the positive electrode active material 100 .

[0170] Although not shown, the positive electrode active material 100 may have a crystal grain boundary. The crystal grain boundary refers to, for example, a portion where particles of the positive electrode active material 100 are adhered to each other, a portion where the crystal orientation changes inside the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in a STEM (Scanning Transmission Electron Microscope) image becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. Furthermore, the crystal defect refers to a defect that can be observed in a cross-sectional TEM (Transmission Electron Microscope), a cross-sectional STEM image, etc., that is, a structure in which other atoms have entered between the lattices, a cavity, etc. The crystal grain boundary can be said to be one of the planar defects. Furthermore, the vicinity of the crystal grain boundary refers to a region within 10 nm from the crystal grain boundary.

[0171] <Containing Elements> The positive electrode active material 100 contains lithium and a transition metal M Ｔ , oxygen, and an additive element. Ｔ is at least one selected from cobalt, nickel, and manganese. Alternatively, the positive electrode active material 100 is a mixture of lithium and a transition metal M Ｔ The composite oxide (LiM Ｔ O ２ However, the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention may have the distribution or crystal structure of the additive element described later. Ｔ However, the atomic ratio is not limited to 1:1:2.

[0172] The positive electrode active material of a lithium ion secondary battery needs to contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and extracted. The positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention contains a transition metal M that is responsible for the oxidation and reduction reaction. ＴIt is preferable to use cobalt as the transition metal. When cobalt accounts for 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metals contained in the positive electrode active material 100, this is preferable because it has many advantages such as being relatively easy to synthesize and handle, and having excellent cycle characteristics.

[0173] In addition, the transition metal M of the positive electrode active material 100 Ｔ When cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, lithium nickel oxide (LiNiO ２ Compared to composite oxides in which nickel accounts for the majority of the transition metal, such as lithium nickel oxide, the stability is superior when a large amount of lithium is released by charging. This is thought to be because cobalt is less affected by distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. When octahedral-coordinated low-spin nickel (III), such as lithium nickel oxide, is used as the transition metal M, Ｔ Layered rock salt type composite oxides, which account for the majority of the total, are significantly affected by the Jahn-Teller effect, and the layers consisting of nickel and oxygen octahedra are prone to distortion. This increases the risk of the crystal structure collapsing during charge-discharge cycles. Nickel ions are also larger than cobalt ions, and are close in size to lithium ions. This is why nickel, as in lithium nickel oxide, is often used in combination with transition metals M Ｔ The layered rock salt type composite oxides, which account for the majority of the total, have the problem that cation mixing of nickel and lithium is likely to occur.

[0174] The additive elements contained in the positive electrode active material 100 are preferably one or more selected from magnesium, titanium, nickel, aluminum, fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium.

[0175] That is, the positive electrode active material 100 may include lithium cobalt oxide doped with magnesium, lithium cobalt oxide doped with magnesium and fluorine, lithium cobalt oxide doped with magnesium, nickel, and aluminum, lithium cobalt oxide doped with magnesium, nickel, aluminum, and fluorine, lithium cobalt oxide doped with magnesium and titanium, lithium cobalt oxide doped with magnesium, titanium, and fluorine, lithium cobalt oxide doped with magnesium, nickel, aluminum, and titanium, lithium cobalt oxide doped with magnesium, nickel, aluminum, titanium, and fluorine, and the like.

[0176] The additive element is preferably in a solid solution in the positive electrode active material 100. Therefore, for example, when performing line analysis by STEM-EDX, the depth at which the amount of the additive element detected increases is the depth at which the amount of the transition metal M Ｔ Preferably, the positive electrode active material 100 is located at a position deeper than the depth at which the amount of detected positive electrode active material 100 increases, that is, at an inner side of the positive electrode active material 100 .

[0177] In this specification and the like, the depth at which the amount of a certain element detected increases in STEM-EDX line analysis refers to the depth at which measurement values ​​that can be determined not to be noise in terms of intensity, spatial resolution, etc. are continuously obtained.

[0178] As will be described later, these additive elements further stabilize the crystal structure of the positive electrode active material 100. In this specification and the like, the additive element has the same meaning as a mixture or a part of a raw material.

[0179] The additive element does not necessarily have to include fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium.

[0180] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, etc., are further enhanced. The weight of manganese contained in the cathode active material 100 is preferably, for example, 600 ppm or less, and more preferably 100 ppm or less.

[0181] The positive electrode active material 100 has the above-described additive element in the surface layer portion 100a. It is more preferable that the positive electrode active material 100 has a plurality of additive elements. It is also preferable that the surface layer portion 100a has a higher concentration of one or more selected from the additive elements than the inner portion 100b. Alternatively, it is preferable that the surface layer portion 100a has a higher detected amount of one or more selected from the additive elements than the inner portion 100b. It is also preferable that the one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient.

[0182] Furthermore, it is more preferable that the distribution of the positive electrode active material 100 differs depending on the added element. For example, it is more preferable that the depth of the peak of the detected amount in the surface layer portion from the surface or a reference point in EDX-ray analysis described below differs depending on the added element. The peak of the detected amount here refers to the maximum value of the detected amount at a depth of 50 nm or less from the surface of the surface layer portion 100a. The detected amount refers to, for example, the count in EDX-ray analysis.

[0183] [Distribution] To achieve the above-described effects, it is preferable that the concentration of at least magnesium (Mg) and nickel (Ni) among the additive elements is higher in the surface layer 100a than in the interior 100b, as shown in Figures 9A and 9B. Alternatively, it is preferable that the detected amount in the surface layer 100a is greater than the detected amount in the interior 100b. Furthermore, it is preferable that the detected amount peak is located closer to the surface of the surface layer 100a. For example, it is preferable that the detected amount peak is located on the surface or within 3 nm from the reference point. It is also preferable that the distributions of magnesium and nickel overlap. The detected amount peaks of magnesium and nickel may be at the same depth, or the magnesium peak may be located closer to the interior, or the nickel peak may be located closer to the interior. The difference in depth between the detected amount peak of nickel and the detected amount peak of magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amount peak is narrow.

[0184] The overlapping region of the magnesium and nickel distributions is preferably located in the edge region of the surface layer 100a where lithium ions are inserted and removed. That is, the distribution of the additional element shown in the schematic diagram of Fig. 9B is preferably observed in the direction of the X1-X2 arrows in Fig. 9A. On the other hand, the overlapping region is not necessarily required in the basal region of the surface layer 100a.

[0185] Furthermore, the amount of nickel detected in the inner portion 100b is very small compared to the surface portion 100a, or may not be detected, or may be 1 atomic % or less.

[0186] Although not shown, it is preferable that the detectable amount of fluorine in the surface layer portion 100a is greater than the detectable amount inside, similar to magnesium or nickel. It is also preferable that the detectable amount peak in the surface layer portion 100a is closer to the surface. For example, it is preferable that the detectable amount peak is on the surface or within 3 nm from the reference point. Similarly, it is preferable that the detectable amount of silicon, phosphorus, boron and / or calcium is greater than the detectable amount inside the surface layer portion 100a. It is also preferable that the detectable amount peak is on the surface or within 3 nm from the reference point.

[0187] Furthermore, it is preferable that at least aluminum, among the added elements, has a peak of detected amount further inward than magnesium. As shown in FIG. 9B, the distributions of magnesium and aluminum may overlap, but there may be almost no overlapping region. 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 be present in a region of 5 nm to 30 nm from the surface or the reference point toward the interior.

[0188] The distribution of aluminum as described above can further stabilize the layered rock-salt crystal structure of the positive electrode active material 100. For example, it is expected that the change from the layered rock-salt crystal structure to the spinel crystal structure in the surface layer 100a of the positive electrode active material 100 can be suppressed. Note that the spinel crystal structure generated in the layered rock-salt crystal structure can be prevented by the presence of a transition metal M.Ｔ The defects, such as grain boundaries, in the positive electrode active material 100 may act as diffusion paths for lithium ions in the c-axis direction. Additional elements, such as aluminum, may be present near the defects. In other words, the presence of aluminum may facilitate the diffusion of lithium ions.

[0189] The reason why aluminum is more distributed to the interior than magnesium is thought to be because aluminum diffuses faster than magnesium. On the other hand, the amount of aluminum detected in the region closest to the surface is low, presumably because aluminum exists more stably in regions where magnesium and other elements are not present in solid solution at high concentrations than in regions where they are not.

[0190] 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.

[0191] Although not shown, it is preferable that manganese has a peak of detectable amount inside magnesium, similar to aluminum.

[0192] However, the added element does not necessarily have to have the same concentration gradient or distribution throughout the entire surface layer portion 100 a of the positive electrode active material 100 .

[0193] The (001)-oriented surface of the positive electrode active material 100 may have a different distribution of additive elements than the other surfaces. For example, the (001)-oriented surface and its surface layer 100a may have a lower detectable amount of one or more additive elements compared to surfaces other than the (001)-oriented surface. Specifically, the detectable amount of one or more of magnesium and nickel may be low. Alternatively, the (001)-oriented surface and its surface layer 100a may have no detectable amount of one or more additive elements, or the detectable amount may be 1 atomic % or less. Specifically, the detectable amount of nickel may be no detectable amount, or 1 atomic % or less. In particular, with analytical methods that detect 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 materials in which cobalt is the primary element. Alternatively, the (001)-oriented surface and its surface layer 100a may have a peak detectable amount of one or more additive elements shallower from the surface than surfaces other than the (001)-oriented surface. Specifically, the (001) oriented surface and its surface layer 100a may have peak positions of detected amounts of magnesium and aluminum that are shallower than those of surfaces other than the (001) oriented surface.

[0194] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. Ｔ O ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and the diffusion path of lithium ions is also parallel to the (001) plane.

[0195] M Ｔ O ２ Since the layer is relatively stable, it is more stable if the surface of the positive electrode active material 100 has a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0196] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface and the surface layer portion 100a other than the (001) orientation 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 and the surface layer portion 100a other than the (001) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0197] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, it is important that the distribution of the additive elements in the surface other than the (001) orientation and in the surface layer 100a thereof is, for example, as shown in FIG. 9B . Among the additive elements, nickel is particularly preferably detected in the surface other than the (001) orientation and in the surface layer 100a thereof. On the other hand, the concentration of the additive element in the (001)-oriented surface and in the surface layer 100a thereof may be low or absent, as described above.

[0198] For example, the magnesium distribution in the (001)-oriented surface and its surface layer 100a preferably has a half-width of 10 nm to 200 nm, more preferably 50 nm to 150 nm, and even more preferably 80 nm to 120 nm. The magnesium distribution in the non-(001)-oriented surface and its surface layer 100a preferably has a half-width of more than 200 nm to 500 nm, more preferably more than 200 nm to 300 nm, and even more preferably 230 nm to 270 nm.

[0199] Furthermore, the nickel distribution in the surface that is not (001) oriented and in the surface layer 100a thereof preferably has a half-width of 30 nm or more and 150 nm or less, more preferably 50 nm or more and 130 nm or less, and even more preferably 70 nm or more and 110 nm or less.

[0200] High purity LiCoO, which will be described in a later embodiment ２In the fabrication method of fabricating the silicon nitride film, the additive element is mixed in and heated after fabrication, and the additive element spreads mainly through the diffusion path of lithium ions, so that 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.

[0201] [Magnesium] Magnesium is divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by M Ｔ O ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２ When x in the formula (1) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100. Furthermore, a high magnesium concentration in the surface layer portion 100a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0202] At an appropriate concentration, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying cobalt sites in addition to lithium sites. Furthermore, excess magnesium compounds (e.g., oxides and fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become resistance components in secondary batteries. Furthermore, increasing the magnesium concentration of the positive electrode active material may decrease the discharge capacity of the positive electrode active material. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0203] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, 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 about 0.01 times. The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0204] [Nickel] Nickel is LiM Ｔ O ２ In the layered rock salt type crystal structure of , it can exist in either the cobalt site or the lithium site. When it exists in the cobalt site, it has a lower redox potential compared to cobalt, so it can be said that it is easier to release lithium and electrons during charging, for example. Therefore, it is expected that the charge and discharge speed will be faster. Therefore, even with the same charging voltage, the transition metal M Ｔ However, a larger charge / discharge capacity is obtained with nickel than with cobalt.

[0205] Furthermore, when nickel is present at the lithium site, the shift in the layer structure consisting of octahedra of cobalt and oxygen can be suppressed. Also, the change in volume caused by charging and discharging is suppressed. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel present at the lithium site also acts as a Ｔ O ２ This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0206] In addition, the distance between the cations and anions of nickel oxide (NiO) is closer to that of LiCoO than that of rock salt MgO and rock salt CoO. ２ The average distance between the cations and anions is close to that of LiCoO ２ The orientation is likely to match.

[0207] In addition, the order of ionization tendency is lowest for magnesium, aluminum, cobalt, and nickel (Mg>Al>Co>Ni). Therefore, nickel is thought to be less likely to dissolve into the electrolyte than the other elements listed above during charging. Therefore, it is thought to be highly effective in stabilizing the crystalline structure of the surface layer in the charged state.

[0208] 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.

[0209] On the other hand, an excess of nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and may also adversely affect the insertion and extraction of lithium.

[0210] Therefore, it is preferable that the entire positive electrode active material 100 contains an appropriate amount of nickel. For example, the number of nickel atoms contained in the positive electrode active material 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%. Alternatively, more than 0% but not more than 4% is preferable. Alternatively, more than 0% but not more than 2% is preferable. Alternatively, 0.05% to 7.5% is preferable. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 7.5% is preferable. Alternatively, 0.1% to 4% is preferable. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0211] [Aluminum] Aluminum can also be present 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 less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Therefore, as described below, even if the positive electrode active material 100 experiences 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 changes in the charge depth or charge rate occurs, deterioration of the positive electrode active material 100 can be suppressed.

[0212] 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's surroundings. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0213] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0214] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 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 contained in the entire positive electrode active material 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0215] [Fluorine] Fluorine is a monovalent anion, and when a portion 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. That is, in the absence of fluorine, 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 a portion of the oxygen in the surface layer portion 100a of the positive electrode active material 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 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 100a, which has a surface that contacts the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the positive electrode active material 100 and the electrolyte. Furthermore, corrosion resistance to hydrofluoric acid can be effectively improved.

[0216] Furthermore, 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. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, it is preferable to set the heating temperature to 742°C or higher in the heating step after mixing the additive element.

[0217] Here, the differential scanning calorimetry (DSC) measurement of the fluorides and mixtures will be explained with reference to Fig. 10. The fluorides in Fig. 10 are LiF and MgF ２ It is a mixture of LiF:MgF ２ The mixture in FIG. 10 contains lithium cobalt oxide as the lithium oxide, and LiF and MgF as the fluorides. ２ It is a mixture of LiCoO ２ :LiF:MgF ２ The components were mixed so that the molar ratio was 100:0.33:1.

[0218] As shown in Fig. 10, an endothermic peak is observed in the vicinity of 735°C for the fluoride. Also, an endothermic peak is observed in the vicinity of 830°C for the mixture. Therefore, the heating temperature after mixing the additive element is preferably 742°C or higher, more preferably 830°C or higher. Alternatively, a temperature of 800°C or higher, which is between these temperatures, may be used.

[0219] [Titanium] The positive electrode active material 100 may contain titanium. The presence of titanium as an additive element in the surface layer portion 100a is expected to have the effect of promoting the insertion and desorption of lithium ions into the positive electrode active material 100. On the other hand, if the titanium concentration is too high or if the region containing only titanium is too large, the layered rock-salt crystal structure of the positive electrode active material 100 may be distorted. Therefore, it is preferable that other additive elements, such as nickel and magnesium, are also present in the surface layer portion 100a. It is expected that the distortion of the crystal structure is alleviated by other additive elements such as nickel and magnesium.

[0220] When the positive electrode active material 100 contains titanium, it is preferable that the distributions of magnesium and titanium overlap. The peaks of the detected amounts of magnesium and titanium may be at the same depth, or the magnesium peak may be closer to the center, or the titanium peak may be closer to the center. The difference in depth between the peak of detected amount of titanium and the peak of detected amount of magnesium is preferably within 3 nm, and more preferably within 1 nm. Furthermore, it is preferable that the half-width of the detected amount is narrow.

[0221] In other words, it is preferable that the distributions of magnesium, nickel, and titanium overlap. The difference in depth between the peaks of the detected amount of titanium, nickel, and magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amount is narrow.

[0222] The above-mentioned region where the distributions of magnesium and nickel overlap, the region where the distributions of magnesium and titanium overlap, or the region where the distributions of magnesium, nickel, and titanium overlap is preferably located in the edge region of the surface layer portion 100 a where lithium ions are inserted and desorbed. On the other hand, the above-mentioned overlapping region is not necessarily required in the basal region of the surface layer portion 100 a.

[0223] [Other additive elements] When phosphorus is contained in the surface layer portion 100a, Li ｘ CoO ２ When the value of x in the graphite layer 100 is kept small, it is possible to prevent short circuits, which is preferable. For example, it is preferable that the graphite layer 100 is present in the surface layer 100a as a compound containing phosphorus and oxygen.

[0224] When the positive electrode active material 100 contains phosphorus, the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or electrolyte, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0225] The electrolyte is LiPF ６ In the case where the electrolyte contains the above-mentioned compound, hydrogen fluoride may be generated by hydrolysis. Furthermore, hydrogen fluoride may be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, and an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating portion 104 may be suppressed. Furthermore, a decrease in adhesion due to gelation and / or insolubilization of PVDF may be suppressed.

[0226] When the positive electrode active material 100 contains phosphorus together with magnesium, Li ｘ CoO ２This is preferable because stability is extremely high when x is small in the positive electrode active material 100. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Alternatively, 1% to 10% is preferable. Alternatively, 1% to 8% is preferable. Alternatively, 2% to 20% is preferable. Alternatively, 2% to 8% is preferable. Alternatively, 3% to 20% is preferable. Alternatively, 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and more preferably 0.7% to 4%. Alternatively, 0.1% to 5% is preferable. Alternatively, 0.1% to 4% is preferable. Alternatively, 0.5% to 10% is preferable. Alternatively, 0.5% to 4% is preferable. Alternatively, 0.7% to 10% is preferable. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire cathode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on values ​​of the composition of raw materials in the process of producing the cathode active material 100.

[0227] Furthermore, when the positive electrode active material 100 has cracks, the progression of the cracks can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the cracks on the surface, for example, in the embedded portion.

[0228] [Synergistic Effect of Multiple Added Elements] Furthermore, 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 M Ｔ O 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.

[0229] For the same reason, when adding an additive element to lithium cobalt oxide in the manufacturing process, it is preferable to add magnesium in a step before adding nickel. Alternatively, it is preferable to add magnesium and nickel in the same step. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide regardless of the step in which it is added, 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 surface layer in a desired amount.

[0230] Furthermore, having additive elements with different distributions in combination is preferable because it can stabilize the crystal structure in a wider region. For example, if the positive electrode active material 100 has both magnesium and nickel distributed in a region closer to the surface of the surface layer 100a and aluminum distributed in a deeper region, it can stabilize the crystal structure in a wider region than if it had only one of these elements. In this way, when the positive electrode active material 100 has additive elements with different distributions in combination, aluminum is not essential to the surface because surface stabilization can be sufficiently achieved by magnesium, nickel, etc. Rather, it is preferable for aluminum to be widely distributed in a deeper region. For example, it is preferable for aluminum to be continuously detected in a region from 1 nm to 25 nm in the depth direction from the surface. A wide distribution in a region from 0 nm to 100 nm from the surface, preferably a region from 0.5 nm to 50 nm from the surface, is preferable because it can stabilize the crystal structure in a wider region.

[0231] 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.

[0232] However, if the surface layer 100a is occupied only by a compound of the additional element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. ＴIt is not preferable that the surface layer 100a is occupied only by a structure in which O(II) is solid-solved. Therefore, the surface layer 100a is made of at least a transition metal M such as cobalt. Ｔ In a discharged state, the material must also contain lithium and have a path for lithium insertion and desorption.

[0233] In order to ensure sufficient paths for lithium insertion and desorption, the surface layer portion 100a preferably has a higher cobalt concentration than magnesium. For example, when measured from the surface of the positive electrode active material 100 using XPS (X-ray Photoelectron Spectroscopy), the ratio Mg / Co of the number of magnesium atoms Mg to the number of cobalt atoms Co is preferably 0.62 or less. The surface layer portion 100a preferably has a higher cobalt concentration than nickel. The surface layer portion 100a preferably has a higher cobalt concentration than aluminum. The surface layer portion 100a preferably has a higher cobalt concentration than fluorine.

[0234] Furthermore, since an excessive amount of nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium in the surface layer 100 a is higher than that of nickel. For example, when measured from the surface of the positive electrode active material 100 by XPS, the number of nickel atoms is preferably 1 / 6 or less of the number of magnesium atoms.

[0235] 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.

[0236] It is preferable that the crystal structure continuously changes from the interior 100b toward the surface due to the concentration gradient of the added element as described above, or that the crystal orientation of the surface layer 100a and the interior 100b roughly coincide.

[0237] For example, 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, which has a rock salt type or both a rock salt type and a layered rock salt type crystal structure. Alternatively, it is preferable that the crystal orientation of the surface layer 100a, which has the characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are approximately the same.

[0238] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a structure in which the lattice of the rock-salt crystal is distorted.

[0239] The rock salt crystal structure refers to a cubic crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately, and may contain cation or anion defects.

[0240] The fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0241] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is taken as the origin (000), the bright spot closest to the central spot is, for example, the (111) plane in the ideal rock salt type, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２ The distance between the bright spots on the (003) plane of LiCoO is observed to be about half the distance between the bright spots on the (111) plane of MgO. ２ In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0242] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0243] Layered rock salt crystals and anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that anions in the O3'-type and monoclinic O1(15) crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure composed of anions is aligned.

[0244] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0245] However, the space group of the layered rock salt crystal and the O3'-type crystal is R-3m, which is different from the space group Fm-3m (the space group of a general rock salt crystal) of the rock salt crystal, and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt crystal and the O3'-type crystal and the rock salt crystal. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt crystal, the O3'-type and the rock salt crystal 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.

[0246] 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) images, ABF-STEM (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 (X-ray diffraction), neutron diffraction, etc. can also be used as materials for determination.

[0247] 11 shows an example of a TEM image in which the orientations of the layered rock salt crystals LRS and RS are roughly the same. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0248] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in FIG. 11) 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.

[0249] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers being observed 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 position of the cobalt atoms, 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 atoms 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.

[0250] 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.

[0251] 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.

[0252] Figure 12A shows an example of an STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS are roughly the same. Figure 12B shows the FFT pattern of the region of the rock salt crystal RS, and Figure 12C shows the FFT pattern of the region of the layered rock salt crystal LRS. The left side of Figures 12B and 12C shows the composition, JCPDS card number, and d value and angle calculated from the JCPDS card data. The right side shows the measured values. The spot marked with O is the zeroth order diffraction.

[0253] The spot marked A in Figure 12B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 12C is derived from the 0003 reflection of the layered rock salt type. From Figures 12B and 12C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide. In other words, it can be seen that the line passing through AO in Figure 12B and the line passing through AO in Figure 12C are roughly parallel. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0254] In this way, in the FFT pattern and the electron beam diffraction pattern, when the orientations of the layered rock salt type crystal and the rock salt type crystal are roughly the same, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point being spot-like and not continuous with other reciprocal lattice points means high crystallinity.

[0255] Furthermore, as described above, when the orientation of the cubic 11-1 reflection and the orientation of the layered rock salt 0003 reflection are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the layered rock salt 0003 reflection may be observed in a reciprocal lattice space different from the orientation of the layered rock salt 0003 reflection. For example, the spot marked B in Figure 12C is originating from the layered rock salt 10-14 reflection. This spot may be observed at an angle of 52 degrees or more and 56 degrees or less (i.e., ∠AOB is 52 degrees or more and 56 degrees or less) from the orientation of the reciprocal lattice point originating from the layered rock salt 0003 reflection (A in Figure 12C), and at a location with a d value of 0.19 nm or more and 0.21 nm or less. Note that this index is merely an example and does not necessarily have to be identical. For example, a reciprocal lattice point equivalent to 0003 and 10-14 may also be used.

[0256] 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. 12B originates from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54 degrees or more and 56 degrees or less (i.e., ∠AOB is 54 degrees or more and 56 degrees or less) from the orientation of the reflection (A in FIG. 12B) 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.

[0257] It is known that layered rock salt type positive electrode active materials, including 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, first select positive electrode active material particles in which a crystal plane expected to be the (0003) plane is observed using a SEM or the like, 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 layered rock salt type so that the (0003) plane can be easily observed.

[0258] <Crystal structure> <Liｘ M Ｔ O ２ When x is 1 in the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, ｘ M Ｔ O ２ In the case where x = 1 in the formula, the layered rock salt type composite oxide has a layered rock salt type crystal structure belonging to the space group R-3m. The layered rock salt type composite oxide has a high discharge capacity, has two-dimensional lithium ion diffusion paths, is suitable for lithium ion insertion / extraction reactions, and is excellent as a positive electrode active material for secondary batteries. Therefore, it is particularly preferable that the inner part 100b, which occupies most of the volume of the positive electrode active material 100, has a layered rock salt type crystal structure. Figure 13 shows the layered rock salt type crystal structure labeled R-3m O3. R-3m O3 has a lattice constant a = 2.81610 (× 10 −１０ m), b=2.81610(×10 −１０ m), c=14.05360(×10 −１０ m), α = 90.0000 (degrees), β = 90.0000 (degrees), γ = 120.0000 (degrees), and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0,0,0), Co(0,0,0.5), and O(0,0,0.23951) (Non-Patent Document 5).

[0259] On the other hand, the surface layer portion 100 a of the positive electrode active material 100 that can be used for the battery 10 of one embodiment of the present invention is such that even if lithium is removed from the positive electrode active material 100 by charging, the transition metal M Ｔ It is preferable that the surface layer 100a has a function of reinforcing the layer structure consisting of an octahedron of oxygen and oxygen so that it is not broken. Alternatively, it is preferable that the surface layer 100a functions as a barrier film for the positive electrode active material 100. Alternatively, it is preferable that the surface layer 100a, which is the outer periphery of the positive electrode active material 100, reinforces the positive electrode active material 100. The reinforcement here means that the surface layer 100a functions as a barrier film for the positive electrode active material 100, and / or the outer periphery of the positive electrode active material 100. Ｔ This means suppressing structural changes in the surface layer portion 100 a and the interior portion 100 b of the positive electrode active material 100, such as a shift in the layer structure formed of an octahedron of oxygen and oxygen, and / or suppressing oxidative decomposition of the electrolyte on the surface of the positive electrode active material 100.

[0260] Therefore, the surface layer portion 100a preferably has a different crystal structure from the interior portion 100b. Furthermore, the surface layer portion 100a preferably has a composition and a crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0261] The surface layer 100a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 100b. In addition, it can be said that the atoms on the surface of the particles of the positive electrode active material 100 in the surface layer 100a are in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and is a region where deterioration of the crystal structure is likely to begin. For example, in the surface layer 100a, a transition metal M Ｔ If the crystal structure of the layered structure consisting of octahedra of Li and oxygen is displaced, the influence of this will be transmitted to the inner portion 100b, causing the crystal structure of the layered structure to be displaced in the inner portion 100b as well, which is thought to lead to deterioration of the crystal structure of the entire positive electrode active material 100. On the other hand, if the surface layer portion 100a can be sufficiently stabilized, ｘ M Ｔ O ２ Even when x in the inner portion 100b is small, for example, even when x is 0.24 or less, the transition metal M Ｔ Furthermore, the layer structure consisting of the octahedrons of the transition metal M in the inner portion 100b can be made hard to break. Ｔ The displacement of the layer consisting of the octahedron of oxygen can be suppressed.

[0262] Furthermore, the interior 100b of the positive electrode active material 100 preferably has a low density of defects, including dislocations. Furthermore, the positive electrode active material 100 preferably has a large crystallite size as measured by XRD. In other words, the interior 100b preferably has high crystallinity. Furthermore, the surface of the positive electrode active material 100 preferably has a smooth surface. These characteristics are important factors supporting the reliability of the positive electrode active material 100 when used in a secondary battery. If the reliability of the positive electrode active material is high, the upper limit of the charging voltage of the secondary battery can be increased, resulting in a secondary battery with a high charge / discharge capacity.

[0263] Dislocations in the interior 100b can be observed, for example, using a 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.

[0264] The larger the crystallite size, the greater the amount of Li ｘ CoO ２ When x is small, the O3' type crystal structure is easily maintained, and contraction of the c-axis length is easily suppressed.

[0265] It is believed that the fewer defects, including dislocations, observed by TEM, the larger the crystallite size measured by XRD.

[0266] When calculating the crystallite size, the XRD diffraction pattern is preferably obtained from the positive electrode active material alone. However, it may also be obtained from the positive electrode, which includes a current collector, binder, conductive material, etc., in addition to the positive electrode active material. However, in the positive electrode state, due to the influence of pressure, etc., during the manufacturing process, the particles of the positive electrode active material may be oriented so that the crystal planes of the particles of the positive electrode active material are aligned in one direction. Strong orientation may prevent accurate calculation of the crystallite size. Therefore, it is more preferable to obtain the XRD diffraction pattern by removing the positive electrode active material layer from the positive electrode, removing some of the binder, etc., in the positive electrode active material layer using a solvent, etc., and then loading the sample into a sample holder. Another method involves applying grease to a silicon non-reflective plate and then attaching the powder sample of the positive electrode active material, etc., to the silicon non-reflective plate.

[0267] The crystallite size can be calculated using, for example, a Bruker D8 ADVANCE, with CuKα X-rays, 2θ between 15 degrees and 90 degrees, increment 0.005, and a LYNXEYE XE-T detector, and the diffraction pattern obtained using ICSD Coll. Code. 172909 as the literature value for lithium cobalt oxide. Analysis can be performed using DIFFRAC. TOPAS ver. 6 as crystal structure analysis software, and can be set, for example, as follows: Emission Profile: CuKa5. lam Background: Chebychev polynomial, 5th Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2Th angular range: 2.9 FDS angle: 0.3 Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5 Secondary Sollers: 2.5 Corrections Specimen displacement: Refine LP Factor: 0

[0268] It is preferable to use the value of LVol-IB, which is the crystallite size calculated by the above method, as the crystallite size. Note that if the calculated preferred orientation is less than 0.8, the orientation of the sample may be too strong and it may not be suitable for determining the crystallite size.

[0269] <Li ｘ M Ｔ O ２ The positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention has the above-described distribution and / or crystal structure of the additive element in a discharged state, and therefore, ｘ M Ｔ O ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0270] 13 to 17, Li ｘ M Ｔ O ２ The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention.

[0271] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 14. The conventional positive electrode active material shown in FIG. 14 is lithium cobalt oxide (LiCoO ２ In particular, changes in the crystal structure of lithium cobalt oxide that does not contain any added elements are described in Non-Patent Documents 1 to 3, etc.

[0272] In Figure 14, R-3m O3 is added to Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0273] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0274] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO 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.

[0275] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２ It can also be said that the structure of and the structure of are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 crystal structure. Note that, since the actual insertion and desorption of lithium does not necessarily occur uniformly within the positive electrode active material and the lithium concentration may become uneven, the H1-3 crystal structure is experimentally observed from about x = 0.25. In fact, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in Figure 14 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 the unit cell.

[0276] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in a unit cell can be expressed as Co (0,0,0.42150±0.00016), O1 (0,0,0.27671±0.00045), and O2 (0,0,0.11535±0.00045). O1 and O2 are each oxygen atoms. Which unit cell 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, it is sufficient to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0277] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0278] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 14, in the H1-3 type crystal structure, CoO ２The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0279] Furthermore, the difference in volume between these two crystal structures is large, so that, when compared per equal number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure exceeds 3.5%, typically 3.9% or more.

[0280] 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.

[0281] 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.

[0282] On the other hand, in the positive electrode active material 100 that can be used for the battery 10 of one embodiment of the present invention shown in FIG. 13, Li ｘ M Ｔ O 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. Ｔ O ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 100 that can be used in the battery 10 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 can achieve excellent cycle characteristics. Furthermore, the positive electrode active material 100 that can be used in the battery 10 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. ｘ M Ｔ O ２ When x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ MＴ O ２ When the value of x in the formula (1) is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0283] Li ｘ M Ｔ O ２ The crystal structure of the inner portion 100b of the positive electrode active material 100 when x is 1, approximately 0.2, and approximately 0.15 is shown in FIG. 13. The inner portion 100b occupies the majority of the volume of the positive electrode active material 100 and is the portion that contributes greatly to charge and discharge. Ｔ O ２ The most problematic areas are layer misalignment and volume changes.

[0284] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0285] However, the positive electrode active material 100 has a different crystal structure from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.15, which results in an H1-3 type crystal structure.

[0286] The positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention when x is about 0.2 has a crystal structure that belongs to the trigonal space group R-3m. ２ The symmetry of the layers is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 13 with the notation R-3m O3'.

[0287] 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. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１０ m), and 2.807≦a≦2.827(×10 −１０ m) is more preferable, and typically a=2.817(×10 −１０ The c-axis is 13.681 ≤ c ≤ 13.881 (× 10 −１０ m), and 13.751≦c≦13.811 (×10 −１０m) is more preferable, and typically c=13.781 (×10 −１０ m).

[0288] The positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention when x is about 0.15 may have a crystal structure that belongs to the monoclinic space group P2 / m. ２ There is one layer. In addition, the lithium present in the positive electrode active material 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 13 with the P2 / m monoclinic O1(15) attached.

[0289] 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 Ｏ２ The lattice constant of the unit cell is a = 4.880 ± 0.05 (× 10 −１０ m), b=2.817±0.05(×10 −１０ m), c=4.839±0.05(×10 −１０ m), α = 90 degrees, β = 109.6 ± 0.1 degrees, γ = 90 degrees.

[0290] 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 constant of the unit cell is a = 2.817 ± 0.02 (× 10 −１０ m), c=13.68±0.1(×10 −１０ m).

[0291] 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.

[0292] As shown by the dotted line in FIG. 13, the CoO ２ There is almost no layer misalignment.

[0293] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0294] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the monoclinic O1(15) type crystal structure is 3.3% or less, more specifically 3.0% or less, typically 2.5%.

[0295] Table 7 shows the difference in volume per cobalt atom between discharged R-3m O3, O3', monoclinic O1(15), H1-3, and trigonal O1. For the lattice constants of the crystal structures of discharged R-3m O3 and trigonal O1 used in the calculations of Table 7, ICSD coll. code. 172909 and 88721 can be referenced. For discharged R-3m O3 and trigonal O1, literature values ​​can be referenced (ICSD coll. code. 172909 and 88721). For H1-3, Non-Patent Document 3 can be referenced. For O3' and monoclinic O1(15), calculations can be made from experimental XRD values.

[0296]

[0297] In this way, in the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, Li ｘ M Ｔ O ２When 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 100 is resistant to collapse of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the positive electrode active material 100 suppresses the decrease in charge / discharge capacity during charge / discharge cycles. Furthermore, because more lithium can be stably utilized than conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0298] The positive electrode active material 100 is Li ｘ M Ｔ O 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 presumed that even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is present. ｘ M Ｔ O 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, the crystal structure may be monoclinic O1(15) type. However, the crystal structure is Li ｘ M Ｔ O 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.

[0299] Therefore, the positive electrode active material 100 is Li ｘ M Ｔ O ２ When x is greater than 0.1 and equal to or less than 0.24, the positive electrode active material 100 may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, not all of the particles in the interior 100b of the positive electrode active material 100 have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or some may be amorphous.

[0300] Also Li ｘ M Ｔ O２ To make the x in the cell small, it is generally necessary to charge the cell at a high charging voltage. ｘ M Ｔ O ２ The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage.

[0301] Therefore, in other words, the positive electrode active material 100 that can be used for the battery 10 of one embodiment of the present invention is preferable because it can maintain a crystal structure having the symmetry of R-3m O even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C.

[0302] Even with the positive electrode active material 100, an H1-3 crystal structure may be observed only when the charge voltage is further increased. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, even when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or higher and lower than 4.6 V at 25° C., the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention may have an O3′ crystal structure. Similarly, when the positive electrode active material 100 is charged at a voltage of 4.65 V or higher and 4.7 V or lower at 25° C., the positive electrode active material 100 may have a monoclinic O1(15) crystal structure.

[0303] 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.

[0304] In addition, in O3' and monoclinic O1(15) in FIG. 13, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, for example, in monoclinic O1(Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0305] The crystal structure of O3' and monoclinic O1(15) type has random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt are usually CdCl ２ It is known that it does not have a typical crystal structure.

[0306] <Grain Boundary> In addition to the above distribution, at least a part of the additive element contained in the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention is preferably unevenly distributed in and near the grain boundary.

[0307] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of areas with high concentration and areas with low concentration.

[0308] For example, the magnesium concentration at and near the grain boundaries of the positive electrode active material 100 is preferably higher than that in other regions of the interior 100b. The fluorine concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 100b. The nickel concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 100b. The aluminum concentration at and near the grain boundaries is also preferably higher than that in other regions of the interior 100b.

[0309] 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.

[0310] 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 100 that can be used in the battery 10 of one embodiment of the present invention, the magnesium concentration and fluorine concentration are high near the surface where the cracks occur. Therefore, even after the cracks occur, the corrosion resistance to hydrofluoric acid of the positive electrode active material can be improved. Furthermore, even after the cracks occur, a side reaction between the electrolyte and the positive electrode active material can be suppressed.

[0311] <Particle size> If the particle size of the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector may occur. On the other hand, if the particle size is too small, problems such as excessive reaction with the electrolyte solution may occur.

[0312] The particle size of the positive electrode active material 100 can be measured, for example, by a laser diffraction particle size distribution analyzer. The particle size of the positive electrode active material 100 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 preferable. Alternatively, 1 μm or more and 30 μm or less is preferable. Alternatively, 2 μm or more and 100 μm or less is preferable. Alternatively, 2 μm or more and 30 μm or less is preferable. Alternatively, 5 μm or more and 100 μm or less is preferable. Alternatively, 5 μm or more and 40 μm or less is preferable.

[0313] Furthermore, using a mixture of particles with different particle sizes in the positive electrode can increase the electrode density, which is preferable because it allows for a secondary battery with high energy density. Positive electrode active material 100 with a relatively small particle size is expected to have high charge / discharge rate characteristics. Positive electrode active material 100 with a relatively large particle size is expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0314] <Analysis method> A certain positive electrode active material is ｘ M Ｔ O ２When x in the formula (I) is small, whether the positive electrode active material 100 can be used in the battery 10 of one embodiment of the present invention having an O3′-type and / or monoclinic O1(15)-type crystal structure can be determined by Li ｘ M Ｔ O ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0315] In particular, XRD is preferable 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, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies the majority of the volume of the positive electrode active material 100.

[0316] When analyzing the crystallite size by powder XRD, it is preferable to measure the size while excluding the influence of the orientation of the positive electrode active material particles 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 size.

[0317] As described above, the positive electrode active material 100 that can be used in the battery 10 according to one embodiment of the present invention is Li ｘ M Ｔ O ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50% or more of the crystal structure are not preferable because they cannot withstand repeated high-voltage charging and discharging.

[0318] It should also be noted that simply adding an additive element may not result in an O3' or monoclinic O1(15) crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additive element, Li ｘ M Ｔ O ２In some cases, x is 0.24 or less and the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and in other cases the H1-3 type crystal structure accounts for 50% or more.

[0319] Furthermore, even in the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be formed when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether the positive electrode active material 100 can be used in the battery 10 of one embodiment of the present invention, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0320] However, when a positive electrode active material with a small x is exposed to air, its crystal structure may change. For example, the crystal structure may change from O3'-type or monoclinic O1(15)-type to H1-3-type. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0321] Furthermore, whether or not the distribution of the additive elements contained in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), EPMA (Electron Probe Micro Analyzer), or the like.

[0322] The crystal structure of the surface layer 100 a and the grain boundaries can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0323] Charging can be performed to determine whether a certain composite oxide is the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention by fabricating a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) using the composite oxide for a positive electrode and lithium metal for a counter electrode. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0324] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0325] 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.

[0326] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0327] The separator may be a 25 μm thick porous polypropylene film.

[0328] The positive electrode can and the negative electrode can may be made of stainless steel.

[0329] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V). 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 2 mA / g or more and 10 mA / g or less. 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.

[0330] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions. For example, charging may be performed by constant current charging at a current value of 20 mA / g or more and 100 mA / g or less up to an arbitrary voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 2 mA / g or more and 10 mA / g or less, and discharging by constant current discharging at a current value of 20 mA / g or more and 100 mA / g or less with an end voltage of 2.5 V.

[0331] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, the discharge can be constant current discharge at a current value of 20 mA / g or more and 100 mA / g or less, with an end voltage of 2.5 V, for example.

[0332] <XRD> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray: CuKα １ Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5 degrees Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15 degrees to 90 degrees Step width (2θ): 0.01 degree setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0333] If the measurement sample is a powder, it can be set by placing it in a glass sample holder, or by sprinkling the sample on a greased silicone anti-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0334] 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 15, 16, 17A, and 17B. 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 17A and 17B 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 17A showing an enlarged view of the region where 2θ is in the range of 18 degrees to 21 degrees, and Figure 17B showing an enlarged view of the region where 2θ is in the range of 42 degrees to 46 degrees. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from the crystal structure information obtained from ICSD (see Non-Patent Document 4). The 2θ range was from 15 to 75 degrees, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10.−１０ m 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.

[0335] As shown in Figures 15, 17A and 17B, in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.25 ± 0.12 degrees (19.13 degrees or more and less than 19.37 degrees) and 2θ = 45.47 ± 0.10 degrees (45.37 degrees or more and less than 45.57 degrees).

[0336] In addition, in the monoclinic O1(15) type crystal structure, diffraction peaks appear at 2θ = 19.47 ± 0.10 degrees (19.37 degrees or more and 19.57 degrees or less) and 2θ = 45.62 ± 0.05 degrees (45.57 degrees or more and 45.67 degrees or less).

[0337] However, as shown in Figures 16, 17A and 17B, 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 degrees or more and less than 19.37 degrees and / or 19.37 degrees or more and less than 19.57 degrees and at least 45.37 degrees or more and less than 45.57 degrees and / or at least 45.57 degrees or more and less than 45.67 degrees when x is small can be said to be a characteristic of the positive electrode active material 100 that can be used for the battery 10 of one embodiment of the present invention.

[0338] This means that in the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, the positions at which XRD diffraction peaks appear are close to each other in the crystal structures where x = 1 and x ≦ 0.24. More specifically, among the main diffraction peaks of the crystal structures where x = 1 and x ≦ 0.24, the difference in 2θ between the peaks that appear at 2θ of 42 degrees or more and 46 degrees or less is 0.7 degrees or less, more preferably 0.5 degrees or less.

[0339] Note that the positive electrode active material 100 that can be used for the battery 10 of one embodiment of the present invention is Li ｘ CoO ２ When x in the is small, it may have an O3'-type and / or monoclinic O1 (15)-type crystal structure, but not all of the particles may have an O3'-type and / or monoclinic O1 (15)-type crystal structure. It may contain other crystal structures, or it may be partially amorphous. However, when Rietveld analysis is performed on the XRD pattern, it is preferable that 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. If 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, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0340] 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.

[0341] 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.

[0342] 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 half-width is narrow. For example, a narrow full width at half maximum is preferable. Even for peaks arising from the same crystalline phase, the half-width varies depending on the XRD measurement conditions and the value of 2θ. Under the above-mentioned measurement conditions, for peaks observed at 2θ = 43 degrees or more and 46 degrees or less, the full width at half maximum is preferably, for example, 0.2 degrees or less, more preferably 0.15 degrees or less, and even more preferably 0.12 degrees 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 the stabilization of the crystal structure after sufficient charging.

[0343] The crystallite size of the O3'-type and monoclinic O1(15) crystal structures of the positive electrode active material 100 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 ｘ CoO ２ When x in the formula is small, the peaks of the O3' type and / or monoclinic O1(15) crystal structure can be clearly observed. ２ In this case, even if a part of the crystal structure resembles the O3' type and / or monoclinic O1(15) crystal structure, the crystallite size will be small and the peak will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0344] As described above, the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention preferably has a small influence of the Jahn-Teller effect. As long as the influence of the Jahn-Teller effect is small, the positive electrode active material 100 may contain a transition metal such as nickel or manganese as an additive element in addition to cobalt.

[0345] For example, when the nickel concentration is 5 atomic % and 7.5 atomic %, the value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) tends to change significantly, and when the nickel concentration is 7.5 atomic %, the change in the lattice constant of the a-axis becomes large. This change may be due to the Jahn-Teller distortion of trivalent nickel. Therefore, the transition metal M contained in the positive electrode active material 100 Ｔ Of these, nickel is preferably less than 7.5 atomic %.

[0346] Furthermore, when the manganese concentration is 5 atomic % or more, the behavior of the change in lattice constant is different, suggesting that it does not follow Vegard's law. Ｔ Of these, manganese is preferably 4 atomic % or less.

[0347] The above ranges of nickel concentration and manganese concentration do not necessarily apply to the surface layer 100a, that is, the concentrations in the surface layer 100a may be higher than the above ranges.

[0348] As a result of considering a preferable range of the lattice constant from the above, it was found that in the positive electrode active material of one embodiment of the present invention, the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge and discharge are performed or in a discharged state, which can be estimated from the XRD pattern, has an a-axis lattice constant of 2.814 × 10 −１０ m is greater than 2.817 x 10 −１０ m and the lattice constant of the c-axis is 14.05 × 10 −１０ m or larger, 14.07 x 10 −１０ It has been found that the value is preferably smaller than m. The state in which no charge and discharge are performed may be, for example, the state of powder before the positive electrode of the secondary battery is produced.

[0349] Alternatively, in the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, the a-axis / c-axis ratio is preferably greater than 0.20000 and smaller than 0.20049.

[0350] Alternatively, when XRD analysis is performed on the layered rock salt type crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, a first peak may be observed at 2θ of not less than 18.50 degrees and not more than 19.30 degrees, and a second peak may be observed at 2θ of not less than 38.00 degrees and not more than 38.80 degrees.

[0351] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as X-rays, 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 elements. The quantitative accuracy of XPS is often about ±1 atomic %, with a lower limit of about 1 atomic %, depending on the element.

[0352] In the cathode active material 100 that can be used in the battery 10 of one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer portion 100a than in the interior portion 100b. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer portion 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer portion 100a measured by XPS or the like is preferably higher than the average magnesium concentration throughout the cathode active material 100. Furthermore, the titanium concentration in at least a portion of the surface layer portion 100a is preferably higher than the average titanium concentration throughout the cathode active material 100. It is also preferable that the nickel concentration in at least a portion of the surface layer portion 100a is higher than the average nickel concentration in the entire positive electrode active material 100. It is also preferable that the aluminum concentration in at least a portion of the surface layer portion 100a is higher than the average aluminum concentration in the entire positive electrode active material 100. It is also preferable that the fluorine concentration in at least a portion of the surface layer portion 100a is higher than the average fluorine concentration in the entire positive electrode active material 100.

[0353] Note that the surface and surface layer portion 100a of the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 100. Furthermore, the surface of the positive electrode active material 100 also does not contain an electrolyte, a binder, a conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 100. Therefore, when quantifying elements contained in the positive electrode active material, correction 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 correction may be made to exclude C—F bonds derived from the binder.

[0354] 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 or the like to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. At this time, lithium may dissolve in the solvent or the like 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.

[0355] The concentration of the added element may be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances chemisorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 to 1.5. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 to 0.06.

[0356] Similarly, in order to ensure sufficient lithium insertion / extraction paths, the positive electrode active material 100 preferably has a higher concentration of lithium and cobalt in the surface layer portion 100a than the concentrations of 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.

[0357] Furthermore, it is more preferable that aluminum be widely distributed in a deep region, for example, the surface or a region at 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 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.

[0358] Furthermore, when XPS analysis was performed on the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, the number of magnesium atoms relative to the number of cobalt atoms was preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times. The number of nickel atoms relative to the number of cobalt atoms was preferably 0.15 times, more preferably 0.03 to 0.13 times. The number of aluminum atoms relative to the number of cobalt atoms was preferably 0.12 times, more preferably 0.09 times. The number of fluorine atoms relative to the number of cobalt atoms was preferably 0.3 to 0.9 times, more preferably 0.1 to 1.1 times. The above ranges indicate that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100 but are widely distributed in the surface layer 100a of the positive electrode active material 100 at preferred concentrations.

[0359] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. The take-off angle can be set to, for example, 45 degrees. Measurement can be performed, for example, using the following equipment and conditions. Measurement equipment: PHI Quantera II X-rays: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45 degrees) Measurement spectrum: wide scan, narrow scan for each detected element

[0360] Furthermore, when XPS analysis is performed on the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, 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).

[0361] Furthermore, when XPS analysis was performed on the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, the peak representing the bond energy between magnesium and another element was preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is a value different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0362] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. More preferably, the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), EPMA, or the like.

[0363] 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.

[0364] 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 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0365] The positive electrode active material 100 is a compound containing a transition metal capable of inserting and extracting lithium and oxygen, and therefore contains a transition metal M that is oxidized and reduced as lithium is inserted and extracted. Ｔ The interface between a region where metals (e.g., Co, Ni, Mn, Fe, etc.) and oxygen are present and a region where they are 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. As the protective film, a single-layer film or a multilayer film of carbon, metal, oxide, resin, etc. may be used.

[0366] Therefore, when referring to the depth direction in STEM-EDX ray analysis, etc., the transition metal M Ｔ The detected amount of characteristic X-rays is Ｔ The average value M of the detected amount of characteristic X-rays ＴＡＶＥ and the background transition metal M Ｔ The average value M of the detected amount of characteristic X-rays ＴＢＧ The point where the detected amount of oxygen characteristic X-rays is 50% of the sum of the detected amount of oxygen characteristic X-rays and the average value O ＡＶＥ and the average value O of the detected amount of characteristic X-rays of oxygen in the background ＢＧ The reference point is the point where the sum of the transition metal M Ｔ The amount of detected characteristic X-rays is Ｔ The average detected amount of characteristic X-rays of the transition metal M ＴWhen the point where the detected amount of characteristic X-rays of oxygen is 50% of the sum of the average values ​​of the detected amount of characteristic X-rays of internal oxygen and the average value of the detected amount of characteristic X-rays of background oxygen is different from the point where the detected amount of characteristic X-rays of oxygen is 50% of the sum of the average values ​​of the detected amount of characteristic X-rays of internal oxygen and the average values ​​of the detected amount of characteristic X-rays of background oxygen, it is considered that this is due to the influence of metal oxides, carbonates, etc. containing oxygen attached to the surface. Ｔ The amount of detected characteristic X-rays is Ｔ The average value M of the detected amount of characteristic X-rays ＴＡＶＥ and the background transition metal M Ｔ The average value M of the detected amount of characteristic X-rays ＴＢＧ The 50% point of the sum of the transition metal M Ｔ In the case of a positive electrode active material having a plurality of elements, the element M having the largest amount of characteristic X-rays detected inside is ＴＡＶＥ and M ＴＢＧ The reference point can be determined using the following formula:

[0367] The background transition metal M Ｔ The average value M of the detected amount of characteristic X-rays ＴＢＧ is, for example, a transition metal M Ｔ The amount of the characteristic X-rays detected can be determined by averaging the area of ​​2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the area where the amount of the characteristic X-rays detected begins to increase. Ｔ The average value M of the detected amount of characteristic X-rays ＴＡＶＥ is the transition metal M Ｔ and the region where the amount of detected characteristic X-rays of oxygen is saturated and stable, for example, the transition metal M Ｔ The average value of the detected amount of characteristic X-rays of oxygen in the background can be obtained by averaging the 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 detected amount of characteristic X-rays of oxygen starts to increase. ＢＧ and the average value of the detected amount of characteristic X-rays of oxygen inside ＡＶＥ can also be found in the same way.

[0368] Furthermore, the surface of the positive electrode active material 100 in a cross-sectional STEM 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 an image is not observed, and is the outermost area of ​​an 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 confirmed.

[0369] Furthermore, the spatial resolution of STEM-EDX is approximately 1 nm. Therefore, the position where the detected amount of characteristic X-rays of the additive element is maximum may be shifted by approximately 1 nm. For example, even if the detected amount of characteristic X-rays of an additive element such as magnesium is maximum at a position outside the surface determined above, the difference between the maximum value and the surface can be considered an error if it is less than 1 nm.

[0370] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated values ​​measured over six scans can be used to graph the characteristic X-rays of each element. The number of scans is not limited to six, and more scans can be performed, and the average can be used to graph the characteristic X-rays of each element.

[0371] The STEM-EDX analysis can be performed, 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 an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0372] 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.

[0373] 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 EDX detector. During EDX ray analysis, the emission current of the STEM device 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, approximately 150,000 times. The conditions for EDX ray analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and 6 or more frames.

[0374] When EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, it is preferable that the concentration of each added element, particularly an added element such as magnesium, in the surface layer portion 100a is higher than that in the interior 100b.

[0375] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, it is preferable that the magnesium concentration in the surface layer 100a is higher than that in the interior 100b. Furthermore, when EDX analysis is performed, the peak of the magnesium concentration in the surface layer 100a is preferably present on the surface of the cathode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, it is preferable that the magnesium concentration decays to 60% or less of the peak at a depth of 1 nm from the peak. Furthermore, it is preferable that the magnesium concentration decays to 30% or less of the peak at a depth of 2 nm from the peak. Note that the concentration peak (also referred to as peak top) here refers to the maximum concentration value.

[0376] Furthermore, when EDX-ray analysis is performed, the maximum value of the magnesium concentration in the surface layer 100a (detected amount of magnesium / sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) is preferably 0.5 atomic % or more and 10 atomic % or less, and more preferably 1 atomic % or more and 5 atomic % or less.

[0377] Furthermore, when EDX-ray analysis is performed, the maximum value of the titanium concentration (detected amount of titanium / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.2 atomic % or more and 5 atomic % or less, and more preferably 0.5 atomic % or more and 2 atomic % or less.

[0378] Furthermore, when EDX-ray analysis is performed, the maximum value of the nickel concentration (detected amount of nickel / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.2 atomic % or more and 5 atomic % or less, and more preferably 0.5 atomic % or more and 3 atomic % or less.

[0379] In the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably has a region overlapping with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration and the peak of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0380] Furthermore, when EDX-ray analysis is performed, the fluorine concentration peak of the surface layer 100a is preferably present on the surface of the positive electrode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, if the magnesium concentration peak is slightly more inward than the fluorine concentration peak, resistance to hydrofluoric acid is increased, which is more preferable. For example, the magnesium concentration peak is more preferably at least 0.5 nm more inward than the fluorine concentration peak, and even more preferably at least 1.5 nm more inward.

[0381] Furthermore, in the positive electrode active material 100 containing nickel as an additive element, the nickel concentration peak in the surface layer 100a is preferably present at a depth of 3 nm from the surface or reference point toward the center of the positive electrode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, in the positive electrode active material 100 containing magnesium and nickel, the nickel distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the nickel concentration peak and the magnesium concentration peak is preferably within 3 nm, more preferably within 1 nm.

[0382] In the cathode active material 100 containing titanium as an additive element, the titanium concentration peak in the surface layer 100a preferably exists at a depth of 3 nm from the surface or reference point toward the center of the cathode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. In the cathode active material 100 containing magnesium and titanium, the titanium distribution preferably has an overlapping region with the magnesium distribution. For example, the difference in depth between the titanium concentration peak and the magnesium concentration peak is preferably within 3 nm, more preferably within 1 nm.

[0383] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, it is preferable that the peak of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100 a when EDX-ray analysis is performed. For example, the peak of the aluminum concentration is preferably present on the surface of the positive electrode active material 100 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0384] When the positive electrode active material 100 contains magnesium, titanium, nickel, and aluminum as described above, a battery using the positive electrode active material 100 can achieve both "high cycle characteristics" that can suppress deterioration in discharge capacity due to repeated high-voltage charging (for example, charging with an upper limit of 4.6 V) and discharging, and "high low-temperature characteristics" that can obtain a large discharge capacity at low temperatures (for example, 0°C, -20°C, -40°C).

[0385] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) (Al / Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) (Ni / Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) (F / Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0386] Furthermore, when the positive electrode active material 100 is subjected to linear or area analysis, the ratio of the number of atoms of the additional element A to the cobalt Co in the vicinity of the grain boundaries (A / Co) is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less.

[0387] For example, when the additive element is magnesium, when linear or area analysis is performed on the positive electrode active material 100, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) near the grain boundaries is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less. Furthermore, when the ratio is within the above range at multiple locations, for example, three or more locations, on the positive electrode active material 100, it can be said that this indicates that the additive element is not attached to a narrow area on the surface of the positive electrode active material 100, but is widely distributed at a preferred concentration in the surface layer portion 100a of the positive electrode active material 100.

[0388] <EPMA> EPMA can also quantify elements. Area analysis can analyze the distribution of each element.

[0389] When EPMA surface analysis is performed on a cross section of the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, it is preferable that one or more selected from the additive elements have a concentration gradient, as in the EDX analysis results. It is also more preferable that the depth from the surface of the concentration peak differs depending on the additive element. The preferred range of the concentration peak for each additive element is also the same as in the EDX analysis.

[0390] However, EPMA analyzes a region from the surface to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from the measurement results obtained using other analytical methods. For example, when the surface of the positive electrode active material 100 is analyzed using EPMA, the concentration of each added element present in the surface layer 100 a may be lower than the result obtained using XPS.

[0391] <Raman Spectroscopy> As described above, the positive electrode active material 100 usable in the battery 10 of one embodiment of the present invention preferably has at least a portion of the surface layer 100a having a rock salt crystal structure. Therefore, when the positive electrode active material 100 and a positive electrode including the positive electrode active material 100 are analyzed by Raman spectroscopy, it is preferable to observe not only the layered rock salt crystal structure but also cubic crystal structures such as rock salt crystal structure. In the STEM image and the electron microbeam diffraction pattern described below, unless cobalt is substituted at lithium positions with a certain frequency in the depth direction during observation, or cobalt is present at the oxygen tetracoordination positions, they cannot be detected as bright spots in the STEM image and the electron microbeam diffraction pattern. On the other hand, because Raman spectroscopy is an analysis that captures the vibrational modes of bonds such as Co—O, peaks of the wavenumbers of the corresponding vibrational modes may be observed even when the amount of the corresponding Co—O bond is small. Furthermore, Raman spectroscopy can be performed on a surface layer having an area of ​​several μm ２ Since it is possible to measure a range of about 1 μm in depth, it is possible to capture with high sensitivity the state that exists only on the particle surface.

[0392] For example, when the laser wavelength is 532 nm, layered rock salt LiCoO ２ So, 470 cm −１ ~490cm −１ , 580 cm −１ ~600cm −１ Peak at (vibration mode: E ｇ , A １ｇ ) is observed. On the other hand, cubic CoO ｘ (0<x<1) (rock salt type Co １−ｙ O (0<y<1) or spinel type Co ３ O ４ ) is 665 cm −１ ~685cm −１ Peak at (Vibration mode: A １ｇ ) is observed.

[0393] Therefore, the integrated intensity of each peak was calculated at 470 cm −１ ~490cm −１ I1,580cm −１ ~600cm −１ I2, 665 cm −１ ~685cm −１When I3 is taken as I3, the value of I3 / I2 is preferably 1% or more and 10% or less, and more preferably 3% or more and 9% or less.

[0394] If a cubic crystal structure such as a rock salt type is observed within the above range, it can be said that the surface layer 100a of the positive electrode active material 100 has a rock salt type crystal structure within a preferred range.

[0395] <<Electron Diffraction Pattern>> As with Raman spectroscopy, it is preferable that the characteristics of the rock salt-type crystal structure are observed in the electron diffraction pattern as well as the layered rock salt crystal structure. However, in the STEM image and the electron diffraction pattern, taking into account the above-mentioned difference in sensitivity, it is preferable that the characteristics of the rock salt-type crystal structure are not too strong in the surface layer 100a, particularly in the outermost surface (for example, 1 nm deep from the surface). This is because, rather than the outermost surface being covered with a rock salt-type crystal structure, it is preferable that an additive element such as magnesium is present in the lithium layer while maintaining the layered rock salt-type crystal structure, which can ensure a lithium diffusion path and has a stronger function of stabilizing the crystal structure.

[0396] Therefore, for example, when a micro-electron beam diffraction pattern is obtained from a region having a depth of 1 nm or less from the surface and a micro-electron beam diffraction pattern from a region having a depth of 3 nm to 10 nm, it is preferable that the difference in lattice constant calculated from these patterns is small.

[0397] For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is 0.1 (×10 −１０ m) or less, and the c-axis is preferably 1.0 (×10 −１０ It is preferable that the a-axis is 0.05 (×10 −１０ m) or less, and the c-axis is preferably 0.6 (×10 −１０ It is more preferable that the a-axis is 0.04 (×10 −１０ m) or less, and the c-axis is more preferably 0.3 (×10 −１０ It is more preferable that the thickness is 1 / 2 m or less.

[0398] <Surface Roughness and Specific Surface Area> The positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention preferably has a smooth surface with few irregularities. A smooth surface with few irregularities indicates that the effect of a flux, which will be described later, is fully exerted, and the surfaces of the additive element source and lithium cobalt oxide are melted. Therefore, this is one factor indicating that the distribution of the additive element in the surface layer portion 100 a is good.

[0399] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or cross-sectional TEM image of the positive electrode active material 100, the specific surface area of ​​the positive electrode active material 100, or the like.

[0400] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as follows.

[0401] First, the cathode active material 100 is processed using an FIB or the like to expose a cross section. At this time, it is preferable to cover the cathode active material 100 with a protective film, protective agent, or the like. Next, an SEM image of the interface between the protective film or the like and the cathode active material 100 is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ = 2) is performed, followed by binarization. Interface extraction is then performed using image processing software. The interface line between the protective film or the like and the cathode active material 100 is selected using an automatic selection tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness at least within 400 nm of the outer periphery of the particle of the cathode active material.

[0402] The particle surfaces of the positive electrode active material 100 of this embodiment preferably have a root mean square (RMS) surface roughness, which is an index of roughness, of less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

[0403] For example, the actual specific surface area S measured by the gas adsorption method using the constant volume method Ｒ and the ideal specific surface area S ｉ The smoothness of the surface of the positive electrode active material 100 can also be quantified from the ratio of

[0404] Ideal specific surface area S ｉ is calculated assuming that all particles have the same diameter as D50, the same weight, and an ideal spherical shape.

[0405] The median diameter D50 can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. The specific surface area can be measured by a specific surface area measuring device using a gas adsorption method based on a constant volume method, for example.

[0406] The positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention has an ideal specific surface area S calculated from the median diameter D50. ｉ and the actual specific surface area S Ｒ The ratio S Ｒ / S ｉ is preferably 2.1 or less.

[0407] Alternatively, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

[0408] First, a surface SEM image of the positive electrode active material 100 is obtained. At this time, a conductive coating may be applied as a pretreatment for observation. The observation surface is preferably perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area are the same.

[0409] Next, image processing software (for example, "ImageJ") is used to convert the SEM image to, for example, 8 bits to obtain an image (called a grayscale image). The grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be expressed in 2 to the power of 8 = 256 gradations. Dark areas have lower gradations, and bright areas have higher gradations. The luminance change can be quantified in relation to the number of gradations. This numerical value is called a grayscale value. By obtaining the grayscale value, it is possible to evaluate the unevenness of the positive electrode active material as a numerical value.

[0410] Furthermore, it is possible to display the brightness change of the target area as a histogram. A histogram is a three-dimensional representation of the gradation distribution in the target area, and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0411] When evaluating positive electrode active material 100 that can be used in battery 10 of one embodiment of the present invention, the difference between the maximum and minimum grayscale values ​​is preferably 120 or less, more preferably 115 or less, and even more preferably 70 to 115. The standard deviation of the grayscale values ​​is preferably 11 or less, more preferably 8 or less, and even more preferably 4 to 8.

[0412] <Particle Size Distribution Analysis Using Cross-Sectional SEM Image of Positive Electrode> The particle size distribution of the positive electrode active material 100 can also be calculated from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

[0413] First, an analysis region is cut out from the acquired cross-sectional SEM image. A range having a sufficient area for image analysis can be cut out, for example, a range of 50 μm or more × 100 μm or more, but this is not limited thereto. Depending on factors such as the size of the positive electrode active material, a smaller or larger area may be cut out.

[0414] The cross-sectional SEM image may be cut out using a function of image processing software. For example, ImageJ may be used as the image processing software, and the image may be cut out using its crop function.

[0415] Next, the cut-out first image is binarized using image processing software, and particle analysis is performed.

[0416] ImageJ, for example, can be used as the image processing software. The binarization process will be described below. A first image displayed on a 256-value grayscale is used as a frequency graph excluding black (value 0) and white (value 255), and the low-value side (HWHM_L) and high-value side (HWHM_H) are determined as the half-width at half maximum (HWHM) of the maximum peak in the frequency graph. Next, a minimum value a in the range twice the width of HWHM_L on the low-value side from the value that is the peak top (maximum frequency) of the maximum peak, and a maximum value b in the range twice the width of HWHM_H on the high-value side are determined.

[0417] Next, binarization is performed so that values ​​less than a are white, values ​​greater than a and less than b are black, and values ​​greater than b are white. Specifically, the threshold function of ImageJ is used to perform binarization as Threshold (a, b). After that, random bright spots thought to be caused by the conductive material are removed using the Gray Morphology (radius = 3, operator = open, type = circle) and Gray Morphology (radius = 1, operator = close, type = circle) conditions, and a second image can be obtained.

[0418] Next, using the second image, the particle size (projected area) was determined to be 0.5 μm using the Analyze Particles function of ImageJ. ２ 700 μm or more ２ The following particles are detected, and the area S of each particle is obtained. Next, the diameter r of each particle is calculated based on the area S of each particle (Equation 1).

[0419]

[0420] In this way, the particle size distribution of each particle can be calculated from the cross-sectional SEM image. Performing the above analysis is called performing particle size distribution analysis using a cross-sectional SEM image of the positive electrode.

[0421] A coating portion may be attached to at least a portion of the surface of the positive electrode active material 100. Fig. 18 shows an example of a positive electrode active material 100 to which a coating portion 104 is attached.

[0422] The coating portion 104 is preferably formed by the accumulation of decomposition products of the electrolyte and the organic electrolyte solution during charging and discharging. ｘ CoO ２ When repeated charging and discharging are performed such that x in the formula is 0.24 or less, it is expected that the charge-discharge cycle characteristics will be improved by having a coating portion derived from the electrolyte on the surface of the positive electrode active material 100. This is due to reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing cobalt elution. The coating 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 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 100, with 70% or more being more preferable and 90% or more being even more preferable.

[0423] <<Powder Resistivity Measurement>> The positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention has 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 100 having the above-described excellent properties is characterized in that, in the above-described <<XRD>>, Li ｘ CoO ２ It has been explained that when x in the formula (I) is small, the positive electrode active material 100 has an O3'-type and / or monoclinic O1(15)-type crystal structure. Furthermore, in the above section <<EDX>>, a preferred distribution of the additive elements when the positive electrode active material 100 is subjected to STEM-EDX analysis has been explained. Furthermore, the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention is also characterized by the volume resistivity of the powder.

[0424] The positive electrode active material 100 that can be used for the battery 10 of one embodiment of the present invention is characterized in that the volume resistivity of the powder of the positive electrode active material 100 is 1.0×10 ８ Ω・cm or more 1.0×10 １０It 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.

[0425] The positive electrode active material 100 having the above volume resistivity has a stable crystal structure even at high voltages. Therefore, the volume resistivity of the powder of the positive electrode active material 100 being within the above range can be used as an indicator that the surface layer portion 100 a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been successfully formed.

[0426] A method for measuring the volume resistivity of powder of the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention will be described.

[0427] The powder volume resistivity measuring device preferably has an instrument part with terminals for resistance measurement and a mechanism for applying pressure to the powder to be measured. The resistance measurement terminals preferably have 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 measuring device having terminals for resistance measurement and a mechanism for applying pressure to the powder (sample) to be measured. The resistance measuring device can be the Loresta-GP low resistance measuring device or the Hiresta-GP high resistance measuring device. 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, is preferably a temperature environment of 25°C and a dew point environment of -40°C or lower. 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.

[0428] 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.

[0429] 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.

[0430] When the above-described measurement is performed, the volume resistivity of the powder of the positive electrode active material 100 that can be used in the battery 10 of 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.

[0431] Unless otherwise specified in the present specification, the volume resistivity measured as above is the volume resistivity of the powder.

[0432] <Ion Chromatography> A method for measuring the powder of the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention by ion chromatography will be described. The ion chromatography measurement involves a pretreatment step of dissolving the powder of the positive electrode active material 100 in acid to obtain a solution for measurement, and a measurement step of measuring the solution.

[0433] The ion chromatography apparatus and conditions are not particularly limited. For example, the measurement can be performed using the apparatus and conditions described below. As the ion chromatography apparatus, for example, a Dionex ICS-2100 ion chromatography system manufactured by Thermo Fisher Scientific can be used.

[0434] An example of pretreatment for ion chromatography will be described. 250 mg of powder of the positive electrode active material 100 and 0.05 Mol / L of H ２ SO ４ 2 ml of the aqueous solution is prepared, placed in a glass container with a lid, and mixed to obtain a first mixed solution. It is recommended that ultrasonic waves be applied for approximately 1 hour during the mixing process. The container is then left to stand at room temperature for 12 hours or more. Then, 1 ml of the filtrate obtained by filtering the first mixed solution is mixed with 9 ml of pure water to obtain a second mixed solution. In this manner, the powder of the positive electrode active material 100 can be pretreated.

[0435] Next, the second mixed solution obtained by the pretreatment is subjected to ion chromatography, which may be used to perform anion analysis and cation analysis.

[0436] An example of anion analysis conditions is shown below. Anion analysis can be performed at 35°C using a Dionex IonPac AG20 (2 x 50 mm) or Dionex IonPac AS20 (2 x 250 mm) column. A KOH aqueous solution is used as the eluent, and the flow rate is preferably 0.44 ml / min. It is preferable to perform gradient measurement so that the concentration of the KOH aqueous solution gradually increases. An electrical conductivity detector is used as the detector, and a mixed anion standard solution manufactured by Kanto Chemical Co., Ltd. can be used to create a calibration curve.

[0437] An example of the conditions for cation analysis is shown below. Cation analysis can be performed at 30°C using a Dionex IonPac CG16 (3 x 50 mm) or Dionex IonPac CS16 (3 x 250 mm) column. The eluent should be an aqueous methanesulfonic acid (MSA) solution, and the flow rate should be 0.36 ml / min. It is recommended that the concentration of the MSA solution be kept constant and that isocratic measurement be performed. An electrical conductivity detector is used as the detector, and a cation mixed standard solution manufactured by Kanto Chemical Co., Ltd. can be used to create a calibration curve.

[0438] The ion chromatography measurement described above allows quantitative measurement of anions such as fluorine (F) and chlorine (Cl), and cations such as lithium (Li), magnesium (Mg), cobalt (Co), and nickel (Ni).

[0439] In ion chromatography measurement of a powder of the positive electrode active material 100 that can be used in the battery 10 of one embodiment of the present invention, the weight of fluorine relative to the weight of the powder is preferably 100 ppm to 1000 ppm, more preferably 100 ppm to 200 ppm.

[0440] This embodiment can be used in combination with other embodiments.

[0441] Embodiment 3 In this embodiment, an example of a method for manufacturing a positive electrode active material 100 that can be used for a battery 10 of one embodiment of the present invention will be described.

[0442] In order to prepare the positive electrode active material 100 having the distribution, composition, and / or crystalline structure of the additive elements as described in the previous embodiment, the method of adding the additive elements is important. At the same time, it is also important that the crystallinity of the inner portion 100b is good.

[0443] Therefore, in the process of producing the positive electrode active material 100, it is preferable to first synthesize lithium cobalt oxide, and then mix in the additive element source and perform a heat treatment.

[0444] In the method of synthesizing lithium cobalt oxide containing an additive element by mixing an additive element source simultaneously with a cobalt source and a lithium source, it is difficult to increase the concentration of the additive element in the surface layer portion 100a. Furthermore, if the additive element source is simply mixed without heating after synthesizing lithium cobalt oxide, the additive element will simply adhere to the lithium cobalt oxide without dissolving in the lithium cobalt oxide. Without sufficient heating, it is difficult to achieve a good distribution of the additive element. Therefore, it is preferable to mix the additive element source after synthesizing lithium cobalt oxide and then perform a heat treatment. This heat treatment after mixing the additive element source is sometimes called annealing.

[0445] However, if the annealing temperature is too high, cation mixing occurs, increasing the possibility that an added element, such as magnesium, enters the cobalt site. ｘ CoO ２ When the value of x in the matrix is ​​small, the layered rock salt crystal structure of R-3m cannot be maintained. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent and the evaporation of lithium.

[0446] Therefore, it is preferable to mix a material that functions as a flux with the additive element source. If the melting point is lower than that of lithium cobalt oxide, the material can function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux lowers the melting point of the additive element source and lithium cobalt oxide. Lowering the melting point makes it easier to distribute the additive element well at a temperature where cation mixing is unlikely to occur.

[0447] <Example 1 of Method for Manufacturing Positive Electrode Active Material> An example of a method for manufacturing a positive electrode active material that can be used as one embodiment of the present invention (Example 1 of Method for Manufacturing Positive Electrode Active Material) will be described with reference to FIGS. 19A to 19D .

[0448] First, in step S10, lithium cobalt oxide is prepared as a starting material. The starting lithium cobalt oxide may have a particle size (median diameter (D50)) of 10 μm or less (preferably 8 μm or less). The lithium cobalt oxide having a median diameter (D50) of 10 μm or less may be a known or commonly used (in short, commercially available) lithium cobalt oxide, or may be prepared through steps S11 to S14 shown in FIG. 19B . A representative example of commercially available lithium cobalt oxide having a median diameter (D50) of 10 μm or less is lithium cobalt oxide (product name "CellSeed C-5H") manufactured by Nippon Chemical Industry Co., Ltd. CellSeed C-5H has a median diameter (D50) of approximately 7 μm. A preparation method for obtaining lithium cobalt oxide having a median diameter (D50) of 10 μm or less through steps S11 to S14 is described below.

[0449] <Step S11> In step S11 shown in FIG. 19B, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0450] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0451] As the cobalt source, it is preferable to use a compound containing cobalt, such as tricobalt tetroxide or cobalt hydroxide. The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased, and the reliability of the secondary battery is improved.

[0452] <Step S12> Next, in step S12 shown in FIG. 19B , the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed in a dry or wet manner. Wet pulverization and mixing allows for smaller pulverization, which is preferable for obtaining lithium cobalt oxide with a median diameter (D50) of 10 μm or less as a starting material. When performing 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). However, 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. It is preferable to mix the lithium source and the transition metal source in dehydrated acetone with a purity of 99.5% or higher, with a water content of 10 ppm or less, and then pulverize and mix the resulting mixture. By using dehydrated acetone with the above purity, it is possible to reduce impurities that may be mixed in.

[0453] <Step S13> Next, in step S13 shown in FIG. 19B , the mixed material is heated. The heating temperature is preferably 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably approximately 950°C (1000°C or lower). If the temperature is too low, the decomposition and melting of the lithium source and transition metal source may be insufficient. On the other hand, if the temperature is too high, lithium may evaporate from the lithium source and / or cobalt may be excessively reduced, resulting in defects. For example, cobalt may change from trivalent to divalent, causing oxygen defects.

[0454] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. Therefore, the heating time should be 1 hour or more and 100 hours or less, preferably 2 hours or more and 20 hours or less, and more preferably 2 hours or more and 10 hours or less.

[0455] 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 should be 200° C. / h.

[0456] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２ The impurity concentrations of the above should be 5 ppb (parts per billion) or less.

[0457] The heating atmosphere is preferably an atmosphere containing oxygen. For example, dry air may be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method in which oxygen is continuously introduced into the reaction chamber and flows through the reaction chamber is called "flow."

[0458] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing the oxygen to flow may be used. For example, a method of reducing the pressure of the reaction chamber and then filling it with oxygen to prevent the oxygen from entering or leaving the reaction chamber may be used, which is called purging. For example, the reaction chamber may be reduced in pressure to -970 hPa and then filled with oxygen to 50 hPa.

[0459] 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.

[0460] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0461] The container for containing the object to be heated during heating is preferably an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). An aluminum oxide crucible is a material that is almost free of impurities. In this embodiment, a setter made of aluminum oxide with a purity of 99.9% is used. Note that it is preferable to place a lid on the crucible or setter before heating, as this prevents the material from volatilizing.

[0462] After the heating is completed, the mixture may be crushed and sieved as necessary.

[0463] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized. ２ ) is an oxide containing multiple metal elements in its structure, and therefore can be called a composite oxide. In this specification and the like, the term "composite oxide" refers to an oxide containing multiple metal elements in its structure. After step S13, a crushing step and a classification step are carried out to adjust the particle size distribution, and then lithium cobalt oxide (LiCoO ２ ) may be obtained.

[0464] Although the example of producing the composite oxide by the solid phase method in steps S11 to S14 has been shown, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0465] Through steps S11 to S14, lithium cobalt oxide can be obtained as a starting material for obtaining a positive electrode active material that can be used in lithium ion batteries and has excellent charge-discharge characteristics even in low-temperature environments. Specifically, lithium cobalt oxide having a median diameter (D50) of 10 μm or less can be obtained as the starting lithium cobalt oxide.

[0466] 19A, the lithium cobalt oxide starting material is heated. The heating in step S15 is sometimes referred to as initial heating in this specification, etc., because it is the first heating of the lithium cobalt oxide. Alternatively, because it is heating performed before step S31 described below, it is sometimes referred to as preheating or pretreatment.

[0467] The initial heating causes lithium compounds and the like unintentionally remaining on the surface of the lithium cobalt oxide to be desorbed. It is also expected to have the effect of enhancing the internal crystallinity. Although impurities may be present in the lithium source and / or cobalt source prepared in step S11, etc., the initial heating can reduce the impurities from the lithium cobalt oxide starting material. The effect of enhancing the internal crystallinity is, for example, the effect of alleviating distortion, misalignment, and the like resulting from differential shrinkage of the lithium cobalt oxide prepared in step S14.

[0468] Furthermore, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. Initial heating also has the effect of mitigating cracks, crystal defects, and the like that the lithium cobalt oxide has. In this specification and elsewhere, a "smooth" surface refers to a surface with few irregularities, a rounded overall surface, and rounded corners. Alternatively, a state in which there is little foreign matter attached to the surface is also referred to as "smooth." Foreign matter is considered to be a cause of irregularities, so it is preferable not to allow it to adhere to the surface.

[0469] In this initial heating, it is not necessary to separately prepare a material that functions as a lithium source, an additive element source, or a flux.

[0470] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. An appropriate heating time range can be selected, for example, from the heating conditions described in step S13. The heating temperature in step S15 is preferably lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in step S15 is preferably shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating is preferably performed at a temperature of 700°C or higher and 1000°C or lower (more preferably, 800°C or higher and 900°C or lower) for 1 hour or higher and 20 hours or lower (more preferably, 1 hour or higher and 5 hours or lower).

[0471] The heating in step S13 can cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference can induce a shrinkage difference. It is also thought that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and this energy is sometimes called strain energy. It is thought that the internal stress is removed by the initial heating in step S15; in other words, the strain energy is homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in the lithium cobalt oxide is relaxed. This smooths the surface of the lithium cobalt oxide. Alternatively, it can be said that the surface is improved. In other words, by going through step S15, the shrinkage difference that occurred in the lithium cobalt oxide is alleviated, resulting in a smooth surface for the composite oxide.

[0472] Furthermore, the shrinkage difference may cause microscopic misalignment, such as crystal misalignment, in the lithium cobalt oxide. To reduce this misalignment, it is preferable to perform step S15. By performing step S15, it is possible to equalize the misalignment of the composite oxide (alleviate the misalignment of crystals, etc., that has occurred in the composite oxide, or align the crystal grains). As a result, the surface of the composite oxide becomes smooth.

[0473] As described above, in step S10, pre-synthesized lithium cobalt oxide having a median diameter (D50) of 12 μm or less, preferably 10 μm or less, and more preferably 8 μm or less may be used. In this case, steps S11 to S13 can be omitted. It is useful to perform step S15 on pre-synthesized lithium cobalt oxide, and this is a preferred step because it allows lithium cobalt oxide with a smooth surface to be obtained.

[0474] Note that step S15 is not an essential configuration in one aspect of the present invention, and therefore an aspect in which step S15 is omitted is also included in one aspect of the present invention.

[0475] The positive electrode active material of one embodiment of the present invention preferably contains an additional element A. A method for adding the additional element A in the following steps will be described.

[0476] <Step S20> Next, details of step S20 for preparing the additional element A as the A source will be described with reference to FIGS. 19C and 19D.

[0477] <Step S21> Step S20 shown in FIG. 19C includes steps S21 to S23. In step S21, an additive element A is prepared. Specific examples of the additive element A include one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium. FIG. 19C illustrates a case where a magnesium source (Mg source) and a fluorine source (F source) are prepared. In step S21, a lithium source may be separately prepared in addition to the additive element A.

[0478] When magnesium is selected as the additional element A, the source of the additional element A can be called a magnesium source. As the magnesium source, magnesium fluoride (MgF ２ ), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) ２ ), or magnesium carbonate (MgCO ３ ) etc. A plurality of magnesium sources may be used.

[0479] When fluorine is selected as the additional element A, the source of the additional element A can be called a fluorine source. Examples of the fluorine source include lithium fluoride (LiF) and 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.

[0480] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can also be used as a lithium source. Other lithium sources that can be used in step S21 include lithium carbonate.

[0481] The fluorine source may also be a gas, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O２ F) or the like may be used and mixed into the atmosphere in the heating step described below. A plurality of fluorine sources may be used.

[0482] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and the magnesium source. ２ ) is prepared. In addition, when the melting point of a fluorine compound (sometimes called a fluoride) such as lithium fluoride is lower than the melting point of the other additive element source, the fluorine compound can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source. ２ When LiF and MgF ２ The eutectic point of LiF is around 742°C. Therefore, LiF and MgF are used as fluorides. ２ When a mixed fluoride having the formula (I) is used as the source of the additive element, it is preferable to set the heating temperature to 742° C. or higher in the heating step after mixing the additive element.

[0483] Lithium fluoride and magnesium fluoride are also known as LiF:MgF ２ When the ratio of lithium fluoride to magnesium fluoride is about 65:35, the effect of lowering the melting point is maximized. In addition, if the ratio of lithium fluoride is too high, there is a concern that the lithium will be excessive, which may deteriorate the cycle characteristics. Therefore, the molar ratio of lithium fluoride to 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 and the like, "near a certain value" means a value that is greater than 0.9 times and smaller than 1.1 times that value, unless otherwise specified.

[0484] 19C, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0485] 19C, the pulverized and mixed materials are collected to obtain a source of the additional element A. The source of the additional element A shown in step S23 includes a plurality of starting materials, and can also be called a mixture.

[0486] The particle size of the mixture is preferably such that the median diameter (D50) is 100 nm or more and 10 μm or less, more preferably 300 nm or more and 5 μm or less. Even when a single material is used as the source of the additive element A, the median diameter (D50) is preferably 100 nm or more and 10 μm or less, more preferably 300 nm or more and 5 μm or less.

[0487] The mixture (including the case where only one type of additive element is contained) pulverized in step S22 is likely to be uniformly adhered to the surface of the lithium cobalt oxide when mixed with the lithium cobalt oxide in a later step. If the mixture is uniformly adhered to the surface of the lithium cobalt oxide, the additive element can be easily distributed or diffused uniformly in the surface layer portion 100a of the composite oxide after heating, which is preferable.

[0488] <Step S21> A process different from that shown in Fig. 19C will be described with reference to Fig. 19D. Step S20 shown in Fig. 19D includes steps S21 to S23.

[0489] In step S21 shown in Fig. 19D, four types of additive element A sources to be added to lithium cobalt oxide are prepared. That is, Fig. 19D differs from Fig. 19C in the types of additive element A sources. In addition to the additive element A sources, a lithium source may be separately prepared.

[0490] As sources of four types of additive element A, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described in FIG. 19C . Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0491] <Step S22> and <Step S23> Next, step S22 and step S23 shown in FIG. 19D are the same as step S22 and step S23 described with reference to FIG. 19C.

[0492] <Step S31> Next, in step S31 shown in FIG. 19A , the lithium cobalt oxide that has undergone step S15 (initial heating) is mixed with an additive element A source (Mg source). Here, the ratio of the number of cobalt atoms Co in the lithium cobalt oxide that has undergone step S15 to the number of magnesium atoms Mg in the additive element A is preferably Co:Mg = 100:y (0.1≦y≦6), and more preferably Co:Mg = 100:y (0.3≦y≦3). Note that adding the additive element A to the lithium cobalt oxide that has undergone initial heating allows the additive element A to be added evenly. For this reason, it is preferable to add the additive element A after initial heating (step S15), rather than adding the additive element A and then performing initial heating (step S15).

[0493] Furthermore, when nickel is selected as the additional element A, it is preferable to perform the mixing in step S31 so that the number of nickel atoms in the nickel source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. Furthermore, when aluminum is selected as the additional element A, it is preferable to perform the mixing in step S31 so that the number of aluminum atoms in the aluminum source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0494] The mixing in step S31 is preferably performed under milder conditions than the pulverization and mixing in step S12 so as not to destroy the shape of the lithium cobalt oxide. For example, it is preferable to perform the mixing under conditions of a lower rotation speed or a shorter time than in step S12. Also, dry mixing is preferable. For example, a particle compositer, a ball mill, a bead mill, etc. can be used for mixing.

[0495] Known particle compositing devices include Mechanofusion (registered trademark) and Nobilta (registered trademark) manufactured by Hosokawa Micron Corporation. Mechanofusion has a fixed blade inside a cylindrical container, and the rotation of the cylindrical container applies mechanical energy to the powder, thereby enabling mixing. Nobilta has a rotating blade inside a cylindrical container, and the rotation of the blade applies mechanical energy to the powder, thereby enabling mixing. In this embodiment, mixing is performed using Nobilta at 3000 rpm for 10 minutes.

[0496] <Step S32> Next, in step S32 of Fig. 19A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0497] <Step S33> Next, in step S33 shown in FIG. 19A, the mixture 903 is heated. The heating temperature in step S33 is preferably 800°C or higher and 1100°C or lower, more preferably 800°C or higher and 950°C or lower, and even more preferably 850°C or higher and 900°C or lower. The heating time in step S33 may be 1 hour or higher and 100 hours or lower, and preferably 1 hour or higher and 10 hours or lower. The lower limit of the heating temperature in step S33 needs to be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element A source proceeds. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element A source occurs, and may be lower than the melting temperature of these materials. For example, taking an oxide as an example, the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs, so the heating temperature in step S33 may be 500° C. or higher.

[0498] The reaction is more likely to proceed when the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903. For example, LiF and MgF are used as the source of the additive element A. ２ As described above, when ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0499] Also, LiCoO ２ :LiF:MgF ２ As described above, the mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830°C in differential scanning calorimetry (DSC measurement). Therefore, the lower limit of the heating temperature is more preferably 830°C or higher.

[0500] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0501] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower.

[0502] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0503] In the manufacturing method described in this embodiment, some materials, for example, LiF as a fluorine source, may function as a flux, which allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, for example, 742° C. to 950° C., and allows additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0504] Incidentally, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF may volatilize or sublime when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. In this case, the function as a flux will be weakened. Therefore, it is preferable to heat the mixture while suppressing the volatilization or sublimation of LiF.

[0505] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization or sublimation of LiF in the mixture 903.

[0506] Furthermore, the heating in this step is preferably performed so as not to cause adhesion between particles of the mixture 903. If the particles of the mixture 903 adhere to each other during heating, the contact area with oxygen in the atmosphere will decrease, and the route along which the additive elements (e.g., fluorine) diffuse will be blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0507] Furthermore, when the additive element (e.g., fluorine) is uniformly distributed in the surface layer portion, a smooth cathode active material with few irregularities can be obtained. Therefore, in this process, in order to maintain or further smooth the surface by heating in step S15, it is preferable that the particles of mixture 903 do not adhere to each other.

[0508] An example in which step S33 is performed in a heating furnace is shown in FIG. 20A.

[0509] The heating furnace 220 shown in FIG. 20A has a heating furnace space 202, a hot plate 204, a pressure gauge 221, a heater unit 206, and a heat insulating material 208. A container 216 and a lid 218 are shown as setters for accommodating the object to be heated. It is preferable to heat the container 216 with the lid 218 attached. FIG. 20B shows a top view of the lid 218, and FIG. 20C shows a schematic cross-sectional view of the container 216 and the lid 218. Simply placing the lid 218 on the container 216 creates a sealed space, but because it is not completely sealed, the inside of the container does not become abnormally high pressure, ensuring safety. Because a source of additive element A (typically fluoride) is added to the container 216 in advance, a fluoride-containing atmosphere can be created within the space 219 defined by the container 216 and the lid 218. During heating, by keeping the concentration of gasified fluoride in the space 219 constant or preventing it from decreasing by covering the space 219, it is possible to contain additive elements A, such as fluorine and magnesium, near the particle surfaces of the mixture 903. Because the volume of the space 219 is smaller than that of the heating furnace space 202, a small amount of fluoride volatilizes, thereby creating an atmosphere containing fluoride. In other words, the atmosphere of the reaction system can be made to contain fluoride without significantly reducing the amount of fluoride contained in the mixture 903. Furthermore, by using the lid 218, the mixture 903 can be heated in an atmosphere containing fluoride simply and inexpensively.

[0510] Furthermore, before heating in the heating furnace space 202, a step of creating an oxygen-containing atmosphere in the heating furnace space 202 and a step of placing the container 216 containing the mixture 903 in the heating furnace space 202 are performed. By performing these steps in this order, the mixture 903 can be heated in an atmosphere containing oxygen and fluoride. For example, heating can be performed while a gas is flowing (flow). The gas can be introduced from the bottom of the heating furnace space 202 and exhausted to the top. Furthermore, the heating furnace space 202 can be sealed during heating to create a closed space to prevent the gas from being transported to the outside (purging).

[0511] There are no particular limitations on the method for creating an oxygen-containing atmosphere in the heating furnace space 202, but examples include a method of evacuating the heating furnace space 202 and then introducing an oxygen-containing gas such as oxygen gas or dry air, or a method of infusing an oxygen-containing gas such as oxygen gas or dry air for a certain period of time. Among these, it is preferable to evacuate the heating furnace space 202 and then introduce oxygen gas (oxygen substitution). Note that the air in the heating furnace space 202 may be considered to be an oxygen-containing atmosphere.

[0512] Furthermore, a source of the additive element A (typically a fluoride) that has been impregnated into the inner walls of the container 216 and the lid 218 can be re-emitted by heating and attached to the mixture 903 .

[0513] There is no particular limitation on the process for heating the heating furnace 220. Heating may be performed using a heating mechanism provided in the heating furnace 220.

[0514] The conditions for placing the mixture 903 in the container 216 will be described with reference to FIG. 20C . As shown in FIG. 20C , it is preferable to place the mixture 903 so that the top surface of the mixture 903 is flat relative to the bottom surface of the container 216, in other words, so that the height H of the top surface of the mixture 903 is uniform. The height H of the top surface of the mixture 903 is preferably 4.0 mm or less, and more preferably 2.0 mm or less. By setting the height H of the top surface of the mixture 903 to the above conditions, oxygen in the atmosphere can reach the mixture 903 near the bottom surface of the container 216. On the other hand, if the height H of the top surface of the mixture 903 is higher than 4.0 mm, the amount of oxygen reaching the mixture 903 near the bottom surface of the container 216 will be insufficient, resulting in reduced battery characteristics when the cathode active material subjected to this process is used in a battery. Furthermore, if the height H of the top surface of the mixture 903 is too low, the amount of mixture 903 that can be placed in the container 216 will be reduced, resulting in reduced productivity. Therefore, the height H of the upper surface of the mixture 903 is preferably 0.5 mm or more, or 1.0 mm or more. To summarize the above, the height H of the upper surface of the mixture 903 is preferably 0.5 mm or more and 4.0 mm or less, and more preferably 1.0 mm or more and 2.0 mm or less.

[0515] The heating in step S33 is preferably performed while controlling the pressure inside the furnace with the pressure gauge 221. The inside of the furnace is preferably kept at atmospheric pressure or a pressurized state. For example, it is thought that when exposed to a pressurized state, the surface of lithium cobalt oxide is more likely to melt. Therefore, LiF and MgF ２ The surface of the lithium cobalt oxide heated together can be melted by applying pressure.

[0516] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluorine compounds resulting from the fluorine source or the like within an appropriate range. The partial pressure can also be controlled by heating the container used in this step with a lid on. As described above, the lid can prevent the material from volatilizing or sublimating.

[0517] In the manufacturing method described in this embodiment, a material for the source of the additive element A (typically a fluoride), for example, LiF as a fluorine source, may function as a flux. This function allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, for example, to a temperature between 742°C and 950°C, and allows the additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0518] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF will volatilize or sublime when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Note that 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, producing LiF, which may then volatilize. Therefore, even if a fluorine compound with a higher melting point than LiF is used, it is still necessary to suppress volatilization.

[0519] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of LiF in the mixture 903. In order to suppress the volatilization of LiF, it is also preferable to place a lid on the setter container. Since the setter container and the lid are exposed to high temperatures, if they are made of materials with different thermal expansion coefficients, there is a risk that the gap between the setter container and the lid will become large. Therefore, it is preferable that the setter container and the lid are made of the same material.

[0520] 19A , the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the recovered positive electrode active material 100. Through the above steps, it is possible to produce the positive electrode active material 100 (composite oxide) having a median diameter (D50) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less).

[0521] When lithium cobalt oxide to which magnesium, aluminum, nickel, and titanium are added is used as the positive electrode active material 100, a process of adding titanium can be performed after step S34.

[0522] The titanium addition process may involve, for example, mixing the composite oxide recovered in step S34 (lithium cobalt oxide to which magnesium, aluminum, and nickel have been added) with a titanium source and heating the mixture. Examples of titanium sources that can be used include lithium titanate, titanium oxide, and titanium hydroxide. For details of the heating process, see, for example, the description of step S33.

[0523] 21 and 22 , another example of a method for producing a positive electrode active material that can be used as one embodiment of the present invention (Example 2 of method for producing a positive electrode active material) will be described. Example 2 of method for producing a positive electrode active material differs from Example 1 of method for producing a positive electrode active material described above in the number of times that an additive element is added and the mixing method, but the other descriptions in Example 1 of method for producing a positive electrode active material can be applied.

[0524] 21, steps S10 and S15 are performed in the same manner as in FIG. 19A to prepare lithium cobalt oxide that has undergone initial heating. Note that step S15 is not an essential configuration in one aspect of the present invention, and therefore an aspect in which step S15 is omitted is also included in one aspect of the present invention.

[0525] <Step S20a> Next, as shown in step S20a, a first additive element A1 source (A1 source) is prepared. Details of step S20a will be described with reference to FIG.

[0526] <Step S21> In step S21 shown in Fig. 22A, a first additive element A1 source (A1 source) is prepared. The A1 source can be selected from the additive elements A described in step S21 shown in Fig. 19C and used. For example, the additive element A1 can be one or more selected from magnesium, fluorine, and calcium. Fig. 22A illustrates an example in which a magnesium source (Mg source) and a fluorine source (F source) are used as the additive element A1.

[0527] Steps S21 to S23 shown in Fig. 22A can be fabricated under the same conditions as steps S21 to S23 shown in Fig. 19C. As a result, an additional element A1 source (A1 source) can be obtained in step S23.

[0528] Steps S31 to S33 shown in FIG. 21 can be performed under the same conditions as steps S31 to S33 shown in FIG. 19A.

[0529] <Step S34a> Next, the material heated in step S33 is recovered to obtain lithium cobalt oxide containing the additional element A1. Here, this is also referred to as the second composite oxide to distinguish it from the lithium cobalt oxide (first composite oxide) that has been subjected to step S15.

[0530] <Step S40> In step S40 shown in Fig. 21, a second additive element A2 source (A2 source) is prepared. Step S40 will be described with reference to Figs. 22B and 22C.

[0531] <Step S41> In step S40 shown in Figure 22B, a second additive element A2 source (A2 source) is prepared. The A2 source can be selected from the additive elements A described in step S20 shown in Figure 19C and used. For example, the additive element A2 can be any one or more selected from nickel, titanium, boron, zirconium, and aluminum. Figure 22B illustrates an example in which a nickel source and an aluminum source are used as the additive element A2.

[0532] Steps S41 to S43 shown in Fig. 22B can be performed under the same conditions as steps S21 to S23 shown in Fig. 19C. As a result, an additional element A2 source (A2 source) can be obtained in step S43.

[0533] Steps S41 to S43 shown in Figure 22C are a modified example of Figure 22B. In step S41 shown in Figure 22C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are independently pulverized. As a result, in step S43, a plurality of second additional element A2 sources (A2 sources) are prepared. Thus, step S40 in Figure 22C differs from step S40 in Figure 22B in that the additional element sources are independently pulverized in step S42a.

[0534] <Steps S51 to S53> Next, steps S51 to S53 shown in FIG. 21 can be performed under the same conditions as steps S31 to S34 shown in FIG. 19A. The conditions for step S53, which is related to the heating step, are preferably the same as or lower than the heating temperature of step S33 shown in FIG. 21. The heating time is preferably shorter than that of step S33. Specifically, the heating temperature is preferably 800°C or higher and 950°C or lower, more preferably 820°C or higher and 870°C or lower, and even more preferably 850°C±10°C. The heating time is preferably 0.5 hours or higher and 8 hours or lower, and more preferably 1 hour or higher and 5 hours or lower.

[0535] When nickel is selected as the additional element A2, it is preferable to perform the mixing in step S51 so that the number of nickel atoms in the nickel source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15. When aluminum is selected as the additional element A2, it is preferable to perform the mixing in step S51 so that the number of aluminum atoms in the aluminum source is 0.05% or more and 4% or less of the number of cobalt atoms in the lithium cobalt oxide that has undergone step S15.

[0536] <Step S54> Next, in step S54 shown in FIG. 21 , the heated material is recovered and crushed as necessary to obtain a positive electrode active material 100. Through the above steps, a positive electrode active material 100 (composite oxide) having a median diameter (D50) of 12 μm or less (preferably 10.5 μm or less, more preferably 8 μm or less) can be produced. Alternatively, a positive electrode active material 100 applicable to lithium-ion batteries having excellent charge / discharge characteristics even in low-temperature environments can be produced. The positive electrode active material 100 contains the additive element A1 and the additive element A2.

[0537] In the above-described Example 2 of the manufacturing method, the additive element into the lithium cobalt oxide is introduced separately as a first additive element A1 and a second additive element A2, as shown in Figures 21 and 22. By introducing the additive elements separately, the distribution of each additive element in the depth direction can be changed.

[0538] This embodiment can be used in combination with other embodiments.

[0539] 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. 23A to 25C.

[0540] 23A to 23G 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, sound reproduction devices, and large game machines such as pachinko machines.

[0541] 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., or the interior or exterior of automobiles.

[0542] 23A 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.

[0543] Figure 23B 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 23C 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.

[0544] FIG. 23D 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. 23E 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.

[0545] 23F 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.

[0546] 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.

[0547] The display surface of the display portion 7202 is curved, and 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.

[0548] 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.

[0549] 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.

[0550] 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 wirelessly without using the input / output terminal 7206.

[0551] 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. 23E can be incorporated into the housing 7201 in a curved state or into the band 7203 in a bendable state.

[0552] 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.

[0553] 23G 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.

[0554] 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.

[0555] 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 wirelessly without using the input / output terminal.

[0556] 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.

[0557] 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. 23H to 25C. FIG.

[0558] 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.

[0559] FIG. 23H is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In FIG. 23H, 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. 23H 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.

[0560] 24A 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.

[0561] 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. 24A . 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. The inclusion of the secondary battery of one embodiment of the present invention allows for a configuration that can accommodate space savings associated with a smaller housing.

[0562] 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.

[0563] 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.

[0564] 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.

[0565] 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.

[0566] 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 configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0567] The display unit 4005a can display not only the time but also various other information such as incoming emails and phone calls.

[0568] 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.

[0569] FIG. 24B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0570] 24C shows a side view of the display portion 4005a. The display portion 4005a includes a secondary battery 913. The secondary battery 913 is the same as the secondary battery described in the previous embodiment. The secondary battery 913 is provided at a position overlapping with the display portion 4005a, and is small and lightweight.

[0571] 24D 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.

[0572] 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.

[0573] 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.

[0574] 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 main units 4100a and 4100b to be used as, for example, a translation device.

[0575] 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 100 described in Embodiment 2 or 3 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 wireless earphones can be realized.

[0576] 25A 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 provided on the bottom surface.

[0577] 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 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.

[0578] Fig. 25B shows an example of a robot. The robot 6400 shown in Fig. 25B 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.

[0579] 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.

[0580] 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.

[0581] 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.

[0582] 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.

[0583] Fig. 25C shows an example of an aircraft 6500. The aircraft 6500 shown in Fig. 25C includes a propeller 6501, a camera 6502, a secondary battery 6503, and the like, and has the function of autonomous flight.

[0584] 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.

[0585] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0586] 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.

[0587] When a secondary battery is installed in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), or plug-in hybrid vehicles (PHVs) can be realized.

[0588] FIG. 26 illustrates an example of a vehicle using a secondary battery according to one embodiment of the present invention. An automobile 8400 illustrated in FIG. 26A 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).

[0589] 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.

[0590] The automobile 8500 shown in FIG. 26B can charge its secondary battery by receiving power from an external charging facility using a plug-in system and / or a wireless power supply system. FIG. 26B 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 system, 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.

[0591] 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.

[0592] 26C is an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 26C 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.

[0593] 26C 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 then stored before riding.

[0594] 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.

[0595] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0596] Embodiment 6 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted on space equipment will be described.

[0597] 27A shows an artificial satellite 6800 as an example of space equipment. The artificial satellite 6800 has a body 6801, a solar panel 6802, an antenna 6803, and a secondary battery 6805. The solar panel is sometimes called a solar cell module.

[0598] When sunlight is irradiated onto the solar panel 6802, the power required for the operation of the satellite 6800 is generated. However, for example, in a situation where sunlight is not irradiated onto the solar panel or in a situation where the amount of...

Claims

The battery has a positive electrode, a negative electrode, an electrolyte, and a separator, The separator is disposed between the positive electrode and the negative electrode, The negative electrode comprises a carbon material, The electrolyte solution includes a lithium salt, a potassium salt, a fluorinated cyclic carbonate, and a fluorinated chain carbonate, a battery in which the anion of the lithium salt and the anion of the potassium salt are different. 　 In claim 1, The battery, wherein the carbon material comprises graphite. 　 In claim 2, The battery, wherein the graphite comprises natural graphite. 　 In any one of claims 1 to 3, The lithium salt is LiPF ６ having The potassium salt has KFSI, The fluorinated cyclic carbonate comprises fluoroethylene carbonate; The battery, wherein the fluorinated chain carbonate has methyl trifluoropropionate. 　 In claim 4, The battery, wherein the positive electrode is a lithium cobalt oxide having magnesium, aluminum, and nickel.

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