Positive electrode active material for secondary battery, and secondary battery

Abstract

A positive electrode active material for a secondary battery includes a lithium metal composite oxide having a crystal structure capable of belonging to space group Fm-3m, the lithium metal composite oxide including at least Li, Mn, Ti, and M, where M is a positive element different from Li, Mn, and Ti, and M is a negative element different from Li, Mn, and Ti. The crystallite size of the lithium metal composite oxide is in the range of 1 nm to 100 nm, or the half-value width of a diffraction peak attributed to the (200) plane in an X-ray diffraction (XRD) diagram of the lithium metal composite oxide using CuK [alpha] rays is in the range of 0.1 DEG to 1.8 DEG based on 2 [theta].

Description

[0001] Cross - reference to related applications

[0002] This disclosure claims the benefit of priority to Patent Application No. 2022 - 206532, filed with the Japan Patent Office on December 23, 2022. The entire contents of the above - mentioned patent application are incorporated herein by reference. Technical field

[0003] This disclosure relates to a positive electrode active material for a secondary battery and a secondary battery. Background art

[0004] Secondary batteries, particularly lithium - ion secondary batteries, have high output and high energy density, and are thus expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles. As a positive electrode active material for lithium - ion secondary batteries, a composite oxide of lithium and a transition metal (e.g., cobalt) has been used. By replacing a part of cobalt with nickel, high capacity can be achieved.

[0005] On the other hand, in recent years, due to the requirement for high energy density, Li - excess type lithium metal composite oxides based on the rock - salt structure Li 1+x Mn 1-x O2 have received attention.

[0006] In Patent Document 1, a positive electrode active material is disclosed, which contains a lithium transition metal composite oxide having a crystal structure belonging to the space group Fm - 3m and represented by the compositional formula Li 1+x Nb y Me z A p O2 (Me is a transition metal containing Fe and / or Mn, 0 < x < 1, 0 < y < 0.5, 0.25 ≤ z < 1, A is an element other than Nb and Me, 0 ≤ p ≤ 0.2, provided that Li 1+p Fe 1-q Nb q O2 with 0.15 < p ≤ 0.3 and 0 < q ≤ 0.3 is excluded.).

[0007] Prior art documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 6197029 Specification Summary of the invention

[0010] In Patent Document 1, high capacity can be achieved by controlling the composition (i.e., adding Nb). However, the effect of improving capacity is not sufficient, and there is still room for improvement.

[0011] In view of the above, one aspect of the present disclosure relates to a positive electrode active material for a secondary battery, which comprises a lithium metal composite oxide having a crystal structure that can be attributed to the space group Fm-3m, wherein the lithium metal composite oxide comprises at least Li, Mn, Ti and M, wherein M is a positive element (positive electroactive element) different from Li, Mn and Ti, and the crystallite size of the lithium metal composite oxide is in the range of 1 nm to 100 nm, or the half-value width of the diffraction peak attributed to the (200) plane in the X-ray diffraction (XRD) diagram of the lithium metal composite oxide using CuKα rays is in the range of 0.1° to 1.8° based on the 2θ standard.

[0012] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode includes the positive electrode active material for a secondary battery.

[0013] According to the present disclosure, a secondary battery with high energy density can be realized.

[0014] The novel features of the present invention are described in the claims, but both the configuration and content of the present invention, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a partially cutaway schematic perspective view of a secondary battery according to one embodiment of the present disclosure. DETAILED DESCRIPTION

[0016] The following examples are given to illustrate the embodiments of the present disclosure, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials are sometimes exemplified, but as long as the effects of the present disclosure can be obtained, other numerical values and materials can also be applied. In this specification, the description of "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be replaced by "above numerical value A and below numerical value B". In the following description, when the lower limit and upper limit of the numerical value for specific physical properties and conditions are exemplified, as long as the lower limit does not become above the upper limit, any one of the exemplified lower limits and any one of the exemplified upper limits can be arbitrarily combined. When multiple materials are exemplified, one of them can be selected and used alone, or two or more can be used in combination.

[0017] Furthermore, the present disclosure includes combinations of matters described in two or more claims arbitrarily selected from the plurality of claims recited in the claims. In other words, matters described in two or more claims arbitrarily selected from the plurality of claims recited in the claims may be combined as long as no technical contradiction arises.

[0018] In the following description, the term “comprise (comprise) to (or, include (include) to)” includes expressions such as “comprise to (or, include to)”, “essentially consist of to”, and “constist of, composed of, formed of, made of, etc.”.

[0019] Secondary batteries include at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries, and all-solid batteries using solid electrolytes.

[0020] The positive electrode active material for a secondary battery according to an embodiment of the present disclosure includes a lithium metal composite oxide (hereinafter also referred to as "lithium metal composite oxide (Fm)") having a crystal structure that can be assigned to the space group Fm-3m. The lithium metal composite oxide (Fm) has, for example, a crystal structure based on a rock salt structure belonging to the space group Fm-3m, for example, a crystal structure similar to a rock salt structure represented by NaCl. In such a crystal structure, oxygen atoms are arranged at the anion site, and Li atoms and metal atoms other than Li can be irregularly arranged at the cation site.

[0021] The lithium metal composite oxide (Fm) contains at least Li, Mn, Ti, and M, where M is a positive element different from Li, Mn, and Ti.

[0022] In the lithium metal composite oxide (Fm), the main component of the metal element other than Li is preferably Mn. The number of atoms b occupied by Mn in the lithium metal composite oxide (Fm) may be the largest among the number of atoms occupied by metals other than Li in the lithium metal composite oxide (Fm). The number of atoms of Mn may be greater than the total number of atoms c of the metal elements excluding Li and Mn. The ratio (b / c) of the number of atoms b of Mn in the lithium metal composite oxide (Fm) to the number of atoms c of the metal elements other than Li and Mn is, for example, 1 or more and 15 or less, preferably 1 or more and 12 or less, may be greater than 1 and 12 or less, or may be 2 or more and 12 or less.

[0023] In addition, the lithium metal composite oxide (Fm) contains Ti. Ti has the effect of increasing the capacity of the lithium metal composite oxide (Fm) having the above-mentioned crystal structure. The reason for this is not clear, but one factor is considered to be that Ti can be used as a Ti in the lithium metal composite oxide with a d orbital empty. 4+In this case, the rock salt structure with high symmetry tends to become stable, and it is believed that the rock salt structure can be stabilized even after repeated charge and discharge. In the case where the lithium metal composite oxide (Fm) contains Ti, the ratio of the number of Mn atoms to the number of Ti atoms in the lithium metal composite oxide, Mn / Ti, may be 4 or more, may be 5 or more, may be 6 or more, and preferably 7 or more. In addition, Mn / Ti may be 70 or less, may be 30 or less, and preferably 15 or less. Mn / Ti may be 4 or more and 70 or less, may be 5 or more and 30 or less, may be 6 or more and 15 or less, and may be 7 or more and 15 or less.

[0024] The element M, which is a positive element, can be any element other than hydrogen and can be a metal element (including so-called semi-metal elements). In other words, the lithium metal composite oxide (Fm) is a lithium transition metal composite oxide containing at least four metals. By allowing Mn, Ti, and the element M to coexist, the charge and discharge efficiency is significantly improved. As a result, a higher energy density can be achieved.

[0025] As the element M, for example, at least one selected from Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al and Er can be used.

[0026] As the element M, for example, at least one selected from Ge, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al and Er can be used.

[0027] As the element M, for example, at least one selected from Ni, Co, Cu, Nb, Mo, Cr, Y, Zr, Ca, Mg, Ta, W and Al can be used. As the element M, for example, at least one selected from Fe, Ni, Co, Cu, Nb, Mo, Cr, Y, Zr, Ca, Mg, Ta, W and Al can be used.

[0028] As the element M, for example, at least one selected from Ni, Co, Nb, Cr, Zr, Ca and Al can be used, and as the element M, for example, at least one selected from Fe, Ni, Co, Nb, Cr, Zr, Ca and Al can be used.

[0029] However, in order to achieve improved charge and discharge efficiency and high energy density, the crystals of the lithium metal composite oxide (Fm) need to satisfy at least one of the following conditions (A) and (B). Both conditions (A) and (B) may also be satisfied.

[0030] Condition (A): The crystallite size of the lithium metal composite oxide (Fm) is in the range of 1 nm to 100 nm.

[0031] Condition (B): In the X-ray diffraction (XRD) pattern of the lithium metal composite oxide (Fm) using CuKα radiation, the half-value width of the diffraction peak attributed to the (200) plane is in the range of 0.1° to 1.8° based on 2θ.

[0032] When condition (A) is satisfied, the crystallite size of the lithium metal composite oxide (Fm) is preferably 80 nm or less, more preferably 60 nm or less. The crystallite size of the lithium metal composite oxide (Fm) is preferably 4 nm or more, more preferably 5 nm or more or 6 nm or more. The upper and lower limits of the above-mentioned crystallite size can be arbitrarily combined. The crystallite size of the lithium metal composite oxide (Fm) can be 4 nm or more and 80 nm or less, 5 nm or more and 60 nm or less, or 6 nm or more and 60 nm or less.

[0033] Generally, lithium-metal composite oxides have large crystallite sizes, and their capacity is generally improved when their crystal structure is fully developed. On the other hand, it is believed that small lithium-metal composite oxides (Fm) with crystallite sizes of 100 nm or less significantly enhance the effect of promoting the release and absorption of lithium ions due to the increase in the area of the grain boundaries, thereby improving the capacity.

[0034] The crystallite size is calculated based on the Scherrer equation from the half-value width of the diffraction peak attributed to the (200) plane in the X-ray diffraction (XRD) pattern of the lithium metal composite oxide (Fm) using CuKα radiation. In the X-ray diffraction (XRD) pattern of the lithium metal composite oxide (Fm) using CuKα radiation, the half-value width of the diffraction peak attributed to the (200) plane can be, for example, in the range of 0.1° to 1.8° on a 2θ basis.

[0035] When condition (B) is satisfied, in the X-ray diffraction (XRD) pattern of the lithium metal composite oxide (Fm) using CuKα radiation, the half-value width of the diffraction peak attributable to the (200) plane may be 1.0° or less, 0.8° or less, or 0.5° or less, based on 2θ. The half-value width of the diffraction peak may be 0.2° or more, or 0.3° or more, based on 2θ. The upper and lower limits of the half-value widths may be arbitrarily combined.

[0036] The lithium metal composite oxide (Fm) may be a lithium-excess type lithium metal composite oxide. The lithium metal composite oxide (Fm) may be based on a composite oxide of Li and Mn. The basic composite oxide of Li and Mn may be a Li-excess type lithium metal composite oxide. 1+x Mn 1-x O2 or Li 1+x Mn 1-x-y M y It is believed that a crystal structure similar to a rock salt structure belonging to the space group Fm-3m with excess Li has a greater effect of improving capacity by reducing the crystallite size to less than 100 nm, compared with a crystal structure clearly belonging to the space group Fm-3m.

[0037] The lithium metal composite oxide (Fm) may contain fluorine (F). Fluorine can replace oxygen atoms at anion sites in the above-mentioned crystal structure. As a result, the state of excess Li is stabilized, and a higher capacity can be obtained. In addition, the average discharge potential increases by the replacement of fluorine atoms. Furthermore, the state of excess Li refers to a state in which the number of atoms of Li in the lithium metal composite oxide (Fm) is greater than the total number of atoms of metal elements other than Li.

[0038] In lithium metal composite oxides (Fm), the configuration of Li at the cation site is irregular, and the binding state of Li is various, so the voltage distribution associated with the release of Li is wide. Therefore, it becomes difficult to utilize the lower part of the voltage distribution on the low potential side as capacity. However, by introducing fluorine atoms, the voltage distribution associated with the release of Li moves to the high potential side, making it easy to utilize the lower part as capacity. As a result, the usable capacity is further increased.

[0039] Lithium metal composite oxide (Fm) can be represented by the composition formula Li a Mn b Ti c M d O e F f In this case, 1≤a≤1.4, 0.3≤b≤0.9, 0<c≤0.5, 0<d≤0.25, 0≤f≤0.7, and 1.7≤e+f≤2 can be satisfied. Alternatively, 1≤a≤1.4, 0.5≤b≤0.9, 0<c≤0.4, 0≤f≤0.67, and 1.7≤e+f≤2.2 can be satisfied. e+f is usually less than 2, can be less than 1.94, can be less than 1.9, or can be less than 1.8.

[0040] As shown in the above composition formula, some of the oxygen atoms at the anion sites can be replaced by fluorine atoms. This stabilizes the state of excess Li (a>1), enabling high capacity. Furthermore, as mentioned above, the average discharge potential rises, further increasing the usable capacity.

[0041] Lithium metal composite oxide (Fm) can be obtained by calcining a mixture of raw materials containing the elements that constitute the lithium metal composite oxide (Fm). Calcination promotes the growth of crystals similar to a rock salt structure belonging to the space group Fm-3m, resulting in a lithium metal composite oxide with large crystallite size. Subsequent pulverization can control the crystallite size to within the range of 1 nm to 100 nm.

[0042] As raw materials of elements constituting lithium metal composite oxides, Mn oxides, Ti oxides, oxides of element M, lithium oxides, lithium salts, lithium manganate (LiMnO2), lithium titanate, Li2O, TiO2, Mn2O3, etc. As a fluorine source, lithium fluoride (LiF) can be used.

[0043] Next, the secondary battery according to the embodiment of the present disclosure will be described in detail. The secondary battery includes, for example, the following positive electrode, negative electrode, electrolyte, and separator.

[0044] [positive electrode]

[0045] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode used in the secondary battery described above is used as the positive electrode. The positive electrode mixture layer can be formed, for example, by applying a positive electrode slurry onto the surface of the positive electrode current collector and drying it. The positive electrode slurry is obtained by dispersing a positive electrode mixture containing a positive electrode active material, a binder, etc. in a dispersion medium. The dried coating can also be rolled as needed. The positive electrode mixture layer can be formed on one surface of the positive electrode current collector or on both surfaces thereof.

[0046] The positive electrode mixture layer contains a positive electrode active material as an essential component and may contain a binder, a thickener, a conductive agent, a positive electrode additive, etc. as optional components. As the binder, the thickener, and the conductive agent, known materials can be used.

[0047] As a positive electrode active material, the above-mentioned lithium metal composite oxide (Fm) having a crystal structure similar to a rock salt structure belonging to the space group Fm-3m is included. The lithium metal composite oxide (Fm) is, for example, a secondary particle formed by the aggregation of multiple primary particles. The particle size of the primary particles is generally 0.01μm to 1μm. The average particle size of the lithium metal composite oxide (Fm) is, for example, 1μm to 50μm, preferably 2μm to 25μm. Here, the average particle size of the lithium metal composite oxide (Fm) means the median diameter (D50) at which the cumulative frequency in the volume-based particle size distribution becomes 50%, which is measured using a laser diffraction particle size distribution measuring device. The D10 diameter (the diameter at which the cumulative frequency in the volume-based particle size distribution becomes 10%) of the lithium metal composite oxide (Fm) can be 0.6μm or less.

[0048] The BET surface area of the composite oxide is preferably 0.01 m 2 / g~15m 2 / g range.

[0049] The content of the elements constituting the composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0050] As the positive electrode active material, other known lithium metal oxides other than the above-mentioned lithium metal composite oxides may be mixed with the above-mentioned lithium metal composite oxide having a crystal structure similar to the rock salt structure. Examples of other lithium metal oxides include Li a CoO2、Li a NiO2、Li a MnO2、Li a Co b Ni 1-b O2、Li a Co b M 1-b O c 、Li a Ni 1-b M b O c 、Li a Mn2O4、Li a Mn 2-b M bLithium transition metal composite oxides such as O4, LiMePO4, and Li2MePO4F. Here, M is at least one selected from Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element (for example, at least one selected from Mn, Fe, Co, and Ni). Here, 0 ≤ a ≤ 1.2, 0 ≤ b ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Furthermore, the value a, which represents the molar ratio of lithium, increases or decreases with charge and discharge.

[0051] The shape and thickness of the positive electrode current collector can be selected based on the shape and range of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0052] [negative electrode]

[0053] The negative electrode, for example, includes a negative electrode current collector and may include a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry onto the surface of the negative electrode current collector and drying it. The negative electrode slurry is obtained by dispersing a negative electrode mixture containing a negative electrode active material, a binder, etc. in a dispersion medium. The dried coating may also be rolled as needed. In other words, the negative electrode active material layer may be a mixture layer. Alternatively, lithium metal foil or lithium alloy foil may be attached to the negative electrode current collector. The negative electrode active material layer may be formed on one surface or on both surfaces of the negative electrode current collector.

[0054] The negative electrode active material layer contains the negative electrode active material as an essential component and may contain a binder, a conductive agent, a thickener, etc. as optional components. As the binder, the conductive agent, and the thickener, known materials can be used.

[0055] The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions, lithium metal, and / or a lithium alloy. As a material that electrochemically absorbs and releases lithium ions, a carbon material, an alloy material, etc. can be used. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and difficultly graphitized carbon (hard carbon). Among them, graphite having excellent charge and discharge stability and low irreversible capacity is preferred. As alloy materials, materials containing at least one metal that can form an alloy with lithium can be listed, and examples include silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide, tin oxide, and the like formed by combining these with oxygen can also be used.

[0056] As an alloy-based material containing silicon, for example, a silicon composite material in which a lithium-ion conductive phase and silicon particles are dispersed in the lithium-ion conductive phase can be used. As the lithium-ion conductive phase, for example, a silicon oxide phase, a silicate phase, and / or a carbon phase can be used. The main component (e.g., 95 to 100% by mass) of the silicon oxide phase can be silicon dioxide. Among them, a composite material composed of a silicate phase and silicon particles dispersed in the silicate phase has a high capacity and a small irreversible capacity, and is preferred from this point of view.

[0057] The silicate phase can contain, for example, at least one selected from Group 1 elements and Group 2 elements of the long-period type periodic table. As the Group 1 element and Group 2 element of the long-period type periodic table, for example, lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. can be used. As other elements, aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), titanium (Ti), etc. can be included. Among them, from the aspect of small irreversible capacity and high initial charge-discharge efficiency, a lithium-containing silicate phase (hereinafter also referred to as a lithium silicate phase) is preferred.

[0058] The lithium silicate phase only needs to be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and other elements can also be included. The atomic ratio O / Si of O to Si in the lithium silicate phase is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio Li / Si of Li to Si in the lithium silicate phase is, for example, greater than 0 and less than 4. The lithium silicate phase can have a composition represented by the formula Li 2z SiO 2+z (0 < z < 2). z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2. As elements other than Li, Si, and O that can be included in the lithium silicate phase, for example, iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), etc. can be cited.

[0059] The carbon phase can be composed of, for example, low-crystalline amorphous carbon (i.e., non-crystalline carbon). The amorphous carbon can be, for example, hard carbon, soft carbon, or other amorphous carbon.

[0060] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh body, a net body, a punched sheet, etc.) can be used. As the material of the negative electrode current collector, stainless steel, nickel, nickel alloy, copper, copper alloy, etc. can be exemplified.

[0061] [Electrolyte]

[0062] The electrolyte may be a liquid electrolyte (electrolyte), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolyte solution comprising a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolyte solution is, for example, not less than 0.5 mol / L and not more than 2 mol / L. The electrolyte solution may also contain known additives.

[0063] The gel electrolyte comprises a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that gels by absorbing the non-aqueous solvent. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0064] As the solid electrolyte, for example, a material known in all-solid-state lithium-ion secondary batteries and the like (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halide-based solid electrolyte, etc.) is used.

[0065] For example, a liquid nonaqueous electrolyte can be prepared by dissolving a salt in a nonaqueous solvent. The salt is an electrolyte salt that undergoes ion dissociation in the electrolyte, and may include, for example, a lithium salt. The electrolyte may contain various additives. While the electrolyte is typically used in a liquid state, it may also be in a state where its fluidity is restricted, such as with a gelling agent.

[0066] As the non-aqueous solvent, for example, cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates etc. can be used. As cyclic carbonates, propylene carbonate (PC), ethylene carbonate (EC), vinylene carbonate (VC) etc. can be listed. As chain carbonates, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) etc. can be listed. In addition, as cyclic carboxylates, gamma-butyrolactone (GBL), gamma-valerolactone (GVL) etc. can be listed. As chain carboxylates, methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP) etc. can be listed. The non-aqueous solvent can be used alone or in combination of two or more.

[0067] Examples of the nonaqueous solvent include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0068] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxolane, Alkane, 1,4-diol Alkane, 1,3,5-tri Alkanes, furans, 2-methylfuran, 1,8-cineole, crown ethers, etc.

[0069] Examples of the chain ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0070] These solvents may be fluorinated solvents in which a portion of hydrogen atoms are substituted with fluorine atoms. Fluorinated ethylene carbonate (FEC) can be used as the fluorinated solvent.

[0071] As lithium salts, for example, lithium salts containing chloric acid (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorinated acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorinated acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.). The lithium salts may be used alone or in combination of two or more.

[0072] The concentration of the lithium salt in the electrolyte can be from 1 mol / L to 2 mol / L, or from 1 mol / L to 1.5 mol / L. By controlling the lithium salt concentration within this range, an electrolyte with excellent ion conductivity and moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above range.

[0073] The electrolyte may contain other known additives. Examples of the additives include 1,3-propane sultone, toluenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0074] [Diaphragm]

[0075] The separator is located between the positive and negative electrodes. It has high ion permeability and possesses appropriate mechanical strength and insulation properties. Microporous films, woven fabrics, and nonwoven fabrics can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred materials for separators.

[0076] As an example of the structure of a secondary battery, an electrode group and a non-aqueous electrolyte are housed in an outer body, wherein the electrode group is formed by winding a positive electrode and a negative electrode with a separator. Alternatively, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator can be used instead of a wound electrode group. The secondary battery can be, for example, cylindrical, square, coin-shaped, button-shaped, laminated, or any other form.

[0077] Figure 1 This is a schematic perspective view of a partially cutaway rectangular secondary battery according to one embodiment of the present disclosure.

[0078] The battery comprises a square-bottomed battery case 4, an electrode group 1 housed in the battery case 4, and a non-aqueous electrolyte (not shown). The electrode group 1 comprises a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator therebetween. The negative electrode collector of the negative electrode is electrically connected to a negative terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode collector of the positive electrode is electrically connected to the back of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4 which also serves as the positive terminal. The periphery of the sealing plate 5 is fitted into the open end of the battery case 4, and the fitting portion is laser welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is blocked by a sealing plug 8 after injection.

[0079] Furthermore, the secondary battery structure may be cylindrical, coin-shaped, button-shaped, or the like, with a metal battery case, or a laminated battery with a battery case made of a laminate sheet serving as a laminate of a barrier layer and a resin sheet. In this disclosure, the type and shape of the secondary battery are not particularly limited.

[0080] Hereinafter, the present disclosure will be specifically described based on Examples and Comparative Examples, but the present disclosure is not limited to the following Examples.

[0081] <Example (Series A)>

[0082] [Production of positive electrode]

[0083] Manganese oxide (Mn2O3), lithium carbonate (Li2CO3), titanium oxide (TiO2), and an oxide of element M (Fe2O3, CoO, ZrO2, Cr2O3, Al2O3, CaO, NiO, Nb2O5) were mixed in a predetermined mass ratio to obtain a lithium metal composite oxide having the composition shown in Table 1. The mixture was calcined at 950°C for 10 hours to obtain a lithium metal composite oxide.

[0084] The sintered lithium metal composite oxide was analyzed by ICP emission spectrometry to identify its composition. Furthermore, the oxygen content was evaluated using an oxygen / nitrogen analyzer (HORIBA, EMGA-920). The results showed that the oxygen content of the sintered lithium metal composite oxide was lower than that of the charged material. Furthermore, in the table, m refers to a fluctuation from a theoretical value and can be a positive or negative value.

[0085] The sintered lithium metal composite oxide was pulverized in a planetary ball mill under various conditions. The conditions for change were the rotation speed and the processing time. As an example, the lithium metal composite oxide of battery A1 was described. The sintered lithium metal composite oxide was put into a planetary ball mill (Fritsch Premium-Line P7, rotation speed: 300 rpm, container: 45 mL, balls: Zr balls) were placed in an Ar atmosphere and treated at room temperature for 12 hours (12 cycles of operation for 1 hour followed by a 10-minute rest period were performed), thereby obtaining a lithium metal composite oxide having a specified composition.

[0086] The obtained lithium metal composite oxide was measured and analyzed by X-ray diffraction using a powder X-ray diffraction apparatus using CuKα radiation. The number and peak positions of the XRD peaks confirmed the formation of a lithium metal composite oxide (Fm) having a rock salt-type crystal structure belonging to the space group Fm-3m. In addition, the half-value width of the diffraction peak attributable to the (200) plane of the lithium metal composite oxide (Fm) was determined. The crystallite size was calculated based on the half-value width using the Scherrer equation.

[0087] The resulting lithium metal composite oxide (Fm), acetylene black, and polyvinylidene fluoride were mixed at a solids mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode slurry. The slurry was then applied to a positive electrode current collector made of aluminum foil. The coated film was dried, compressed, and then cut into the desired electrode size to obtain a positive electrode.

[0088] [Preparation of electrolyte]

[0089] A non-aqueous electrolyte was prepared by adding LiPF 6 as a lithium salt to a mixed solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a predetermined volume ratio.

[0090] [Preparation of test unit]

[0091] A test cell was fabricated using the aforementioned positive electrode and a negative counter electrode composed of lithium metal foil. The positive and negative counter electrodes were placed facing each other with a separator between them to form an electrode assembly, which was then housed in a coin-shaped outer can. After injecting electrolyte into the outer can, the can was sealed to produce a coin-shaped test secondary battery (Series A).

[0092] <Comparative Example (Series B)>

[0093] In the production of the positive electrode, manganese oxide (Mn2O3), lithium carbonate (Li2CO3), titanium oxide (TiO2) and oxides of element M (Fe2O3, CoO, ZrO2, NiO, Nb2O5) are mixed in a prescribed mass ratio to obtain a lithium metal composite oxide having the composition shown in Table 2. The mixture is calcined at 950°C for 10 hours to obtain a lithium metal composite oxide. The sintered lithium metal composite oxide is pulverized using a mortar or a planetary ball mill under various conditions. Except for using the obtained lithium metal composite oxide, a coin-shaped secondary battery for testing (B series) is obtained in the same manner as in Example (A series).

[0094] [evaluate]

[0095] (Initial discharge capacity)

[0096] The secondary battery was charged at a constant current of 0.1C under normal temperature until the battery voltage reached 4.95V, and the CC charge capacity was obtained. Then, constant voltage charging was further performed at a constant voltage of 4.95V until the current reached 0.01C, and the CC-CV charge capacity was obtained. Then, after a rest period of 20 minutes, constant current discharge was performed at a constant current of 0.1C or 0.05C until the battery voltage reached 2.5V, and the 0.1C discharge capacity and 0.05C discharge capacity were measured. The charge capacity and discharge capacity per unit mass of the positive electrode active material (lithium metal composite oxide), as well as the charge and discharge efficiency representing the ratio obtained by dividing the 0.05C discharge capacity by the CC-CV charge capacity, are shown in Tables 1 and 2.

[0097] In Tables 1 and 2, the full width at half maximum (FWHM) of the XRD diffraction peak attributed to the (200) plane and the crystallite size (nm) are shown for Series A of the Examples and Series B of the Comparative Examples. The values in parentheses represent the deviation of the minimum digit (last digit). For example, 1.74 (3) means that it is within the range of 1.71 to 1.77.

[0098] Table 1

[0099]

[0100] Table 2

[0101]

[0102] As can be understood from Tables 1 and 2, when the lithium metal composite oxide contains Mn, Ti, and M and satisfies Condition A or Condition B, the charge and discharge efficiency is significantly improved.

[0103] Industrial applicability

[0104] The secondary battery cathode active material disclosed herein can provide a secondary battery with high energy density. The secondary battery disclosed herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0105] The present invention has been described with reference to the presently preferred embodiments, but such disclosure is not to be construed as limiting. A person skilled in the art will readily appreciate various modifications and variations upon reading the foregoing disclosure. Therefore, the claims should be construed as encompassing all modifications and variations that do not depart from the true spirit and scope of the present invention.

[0106] Description of Reference Numerals

[0107] 1: Electrode group; 2: Positive lead; 3: Negative lead; 4: Battery case; 5: Sealing plate; 6: Negative terminal; 7: Gasket; 8: Sealing plug.

Claims

1. A positive electrode active material for a secondary battery, A lithium metal composite oxide having a crystal structure attributable to the space group Fm-3m is included, The lithium metal composite oxide contains at least Li, Mn, Ti and M, The M is a positive element different from Li, Mn and Ti, The crystallite size of the lithium metal composite oxide is in the range of 1 nm to 100 nm, or the half-value width of the diffraction peak attributed to the (200) plane in the X-ray diffraction (XRD) pattern of the lithium metal composite oxide using CuKα rays is in the range of 0.1° to 1.8° based on the 2θ standard.

2. The positive electrode active material for a secondary battery according to claim 1, The M comprises at least one selected from Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al and Er.

3. The positive electrode active material for a secondary battery according to claim 1, The M comprises at least one selected from Ge, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al and Er.

4. The positive electrode active material for a secondary battery according to any one of claims 1 to 3, The lithium metal composite oxide has a composition formula of Li a Mn b Ti c M d O e F f Indicates that 1≤a≤1.4, 0.3≤b≤0.9, 0<c≤0.5, 0<d≤0.25, 0≤f≤0.7, 1.7≤e+f≤2.2 is satisfied.

5. The positive electrode active material for a secondary battery according to any one of claims 1 to 3, The crystallite size of the lithium metal composite oxide is 80 nm or less.

6. The positive electrode active material for a secondary battery according to any one of claims 1 to 3, The crystallite size of the lithium metal composite oxide is 60 nm or less.

7. The positive electrode active material for a secondary battery according to any one of claims 1 to 3, The ratio b / c of the number b of Mn atoms to the number c of metal atoms other than Li and Mn in the lithium metal composite oxide is 1 or more and 15 or less.

8. The positive electrode active material for a secondary battery according to any one of claims 1 to 3, The lithium metal composite oxide contains fluorine.

9. The positive electrode active material for a secondary battery according to any one of claims 1 to 3, The lithium metal composite oxide is a lithium-excess type lithium metal composite oxide.

10. A secondary battery, It includes a positive electrode, a negative electrode, an electrolyte, and a separator between the positive electrode and the negative electrode. The positive electrode comprises the positive electrode active material for a secondary battery according to any one of claims 1 to 3.

Images