Positive electrode active material for secondary battery and secondary battery

Abstract

The positive electrode active material for a secondary battery contains a lithium metal complex oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m, in which a vacancy having no lithium and metal atoms arranged therein is present. As the lithium metal complex oxide, for example, a complex oxide represented by the composition formula Li a Mn b M c O 2‑d F d (wherein M is at least one metal element other than Li and Mn, and satisfies 0 < a ≤ 1.4, 0.4 ≤ b ≤ 0.95, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.66, 1.75 ≤ a + b + c < 2).

Description

TECHNICAL FIELD

[0001] The present application relates to a secondary battery, and particularly to an improvement of a positive electrode for a secondary battery. BACKGROUND

[0002] Secondary batteries, particularly lithium ion secondary batteries, have high output and high energy density, and are therefore expected as power sources for small domestic uses, power storage devices, and electric vehicles. As a positive electrode active material of a lithium ion secondary battery, a lithium-transition metal (e.g., cobalt) composite oxide is used. A part of cobalt is replaced with nickel, and high capacity can be achieved.

[0003] On the other hand, in recent years, a lithium metal composite oxide of a lithium-excess type based on Li x Mn 1-x A lithium metal composite oxide of a lithium-excess type based on Li

[0004] Patent Literature 1 discloses a positive electrode active material including a lithium-transition metal composite oxide represented by a composition formula Li 1+x Nb y Me z A p O2 (Me is a transition metal including 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, wherein Li 1+p Fe 1-q Nb q O2 and 0.15 < p ≤ 0.3, 0 < q ≤ 0.3).

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent No. 6197029 SUMMARY

[0008] In Patent Literature 1, high capacity can be achieved by control of the composition, i.e., addition of Nb. However, the capacity improvement effect is not sufficient, and there is room for improvement.

[0009] In view of the above, one aspect of the present application relates to a positive electrode active material for a secondary battery, including a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to a space group Fm-3m, in which a vacancy having no lithium and metal atoms arranged therein is present.

[0010] Another aspect of the present application 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, the positive electrode including the above-described positive electrode active material for a secondary battery.

[0011] According to the present application, a secondary battery with high energy density can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is a simple perspective view of a part of a secondary battery according to an embodiment of the present application. DETAILED DESCRIPTION

[0013] The positive electrode active material for a secondary battery according to an embodiment of the present application includes a lithium metal complex oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m. That is, the lithium metal complex oxide has a crystal structure similar to a rock salt structure belonging to space group Fm-3m, and has vacancies in which lithium and metal atoms are not disposed. Here, having vacancies means that, in the positive electrode active material extracted from a secondary battery in a discharged state or immediately after manufacture, vacancies in which lithium atoms or metal atoms are not disposed exist in the lithium metal complex oxide. The proportion of vacancies can be 0.5% or more, preferably 1% or more, and more preferably 2% or more, of the points at which lithium atoms or metal atoms can be disposed in the crystal structure.

[0014] The above-described lithium metal complex oxide has a crystal structure based on a rock salt structure represented by, for example, NaCl, and has oxygen atoms disposed at anion sites and Li atoms and metal atoms other than Li irregularly disposed at cation sites. However, a part of the cation sites are not disposed with any Li atoms or metal atoms, and become vacancies. Lithium ions easily move through the vacancies, and the capacity is improved.

[0015] As the lithium metal complex oxide, for example, a lithium metal complex oxide represented by the composition formula Li a Mn b M c O 2-d F d(where 0 < a ≤ 1.4, 0.4 ≤ b ≤ 0.95, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.66, 1.75 ≤ a + b + c < 2) represents a substance. M is at least one metal element other than Li and Mn. In the above composition formula, the value of x represented by 2 - a - b - c (= x) represents the molar ratio of vacancies present at the cation site. According to the above composition formula, the molar ratio x of vacancies is 0 < x ≤ 0.25. The molar ratio of vacancies is preferably x ≥ 0.02, more preferably x ≥ 0.05, and further preferably x ≥ 0.1. In other words, it is preferable that a + b + c ≤ 1.98, more preferably a + b + c ≤ 1.95, and further preferably a + b + c ≤ 1.9. In addition, the molar ratio x of vacancies is more preferably x ≤ 0.15 (a + b + c ≥ 1.85).

[0016] The vacancies and the contained proportion of vacancies can be derived from the crystal structure and the composition of the lithium metal complex oxide. For example, in the case of a crystal structure similar to the rock salt structure belonging to the space group Fm-3m, the contained proportion of vacancies is found by calculating x = 2 - a - b - c according to the above composition formula. The crystal structure of the lithium metal complex oxide is identified from the X-ray diffraction pattern measured using a powder X-ray diffractometer (for example, a table-top X-ray diffractometer MiniFlex manufactured by Rigaku Corporation, X-ray source: CuKα). The composition of the lithium metal complex oxide can be measured using an ICP emission spectrometer (iCAP6300 manufactured by Thermo Fisher Scientific).

[0017] In addition, the vacancies and the contained proportion of vacancies can also be evaluated by a method utilizing positron annihilation.

[0018] As shown in the above composition formula, a part of the oxygen atoms at the anion site can also be substituted with fluorine atoms. Thereby, the state of Li excess is stabilized. In addition, due to the introduction of fluorine atoms, the average discharge potential rises. In the above lithium metal complex oxide, due to the irregular arrangement of Li at the cation site, the binding state of Li is various, and thus the width of the voltage distribution accompanying Li release is wide. Therefore, the lower portion of the voltage distribution on the low potential side becomes difficult to use as capacity. However, since the voltage distribution accompanying Li release moves to the high potential side due to the introduction of fluorine atoms, it becomes easy to use the lower portion as capacity. Thereby, the available capacity further increases.

[0019] In the case where a part of the oxygen atoms is substituted with fluorine atoms, the substitution ratio d of fluorine atoms in the composition formula of the lithium metal complex oxide can be 0.1 ≤ d ≤ 0.58, or can be 0.1 ≤ d ≤ 0.5 or 0.2 ≤ d ≤ 0.5.

[0020] The lithium metal complex oxide can include a metal element M other than Li and Mn. The lithium metal complex oxide can include at least one selected from the group consisting of 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, Sm, Eu, Dy, and Er as the metal element M. Among them, the lithium metal complex oxide preferably includes at least one selected from the group consisting of Ni, Sn, Mo, W, Ta, and Zn as the metal element M.

[0021] The above-described lithium metal complex oxide can be synthesized, for example, by mixing treatment of lithium peroxide (Li2O2), lithium fluoride (LiF), and lithium manganate (LiMnO2) using a planetary ball mill in a non-reactive gas atmosphere such as Ar. Raw materials can use Li2O and Mn2O3. In addition, instead of the planetary ball mill, a mixing machine capable of applying the same stirring shear force to the powder can also be used, and the powder can also be heated during the mixing treatment. The composition of the complex oxide and the like can be adjusted to the target range, for example, by changing the mixing ratio of LiF to LiMnO2, the mixing conditions (rotation speed, treatment time, treatment temperature, and the like).

[0022] Next, a secondary battery related to the embodiment of the present application is described in detail. The secondary battery has, for example, the following positive electrode, negative electrode, electrolyte, and separator.

[0023] [Positive electrode]

[0024] The positive electrode has a positive electrode current collector, and a positive electrode mixture layer including a positive electrode active material formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed, for example, by coating the surface of the positive electrode current collector with a positive electrode slurry in which a positive electrode mixture including a positive electrode active material, a binder, and the like is dispersed in a dispersion medium, and drying it. The dried coating film can also be calendered as needed. The positive electrode mixture layer can be formed on one surface of the positive electrode current collector, or on both surfaces.

[0025] The positive electrode mixture layer can include a positive electrode active material as an essential component, and can include a binder, a thickener, a conductive agent, a positive electrode additive, and the like as arbitrary components. As the binder, thickener, and conductive agent, publicly known materials can be used.

[0026] As the positive electrode active material, the above-described lithium metal complex oxide having a crystal structure similar to the rock salt structure belonging to the space group Fm-3m is included. The complex oxide is, for example, a secondary particle in which a plurality of primary particles are aggregated. The particle diameter of the primary particle is generally 0.05 μm to 1 μm. The average particle diameter of the complex oxide is, for example, 3 μm to 30 μm, and preferably 5 μm to 25 μm. Here, the average particle diameter of the complex oxide refers to the median particle diameter (D50) at which the cumulative frequency is 50% in the particle size distribution on a volume basis, and is measured by a laser diffraction type particle size distribution measuring device.

[0027] Note that the content of each element constituting the complex oxide can be measured by an inductively coupled plasma emission spectrometry device (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray analysis device (EDX), or the like.

[0028] As the positive electrode active material, other known lithium metal oxides can also be mixedly used in the above-described lithium metal complex oxide having a crystal structure similar to the above-described rock salt structure. As the other lithium metal oxides, for example, 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 b O4, LiMePO4, Li2MePO4F, and the like lithium transition metal complex oxides. Here, M is at least one selected from the group consisting of 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, contains at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0 ≤ a ≤ 1.2, 0 ≤ b ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Note that the value of a, which represents the molar ratio of lithium, is increased or decreased by charging and discharging.

[0029] The shape and thickness of the positive electrode current collector can be selected from those based on the shape and range of the negative electrode current collector, respectively. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, and the like can be exemplified.

[0030] [Negative electrode]

[0031] The negative electrode, for example, has a negative electrode current collector and 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 coating the surface of the negative electrode current collector with a negative electrode slurry in which a negative electrode mixture containing a negative electrode active material, a binder, and the like is dispersed in a dispersion medium, and drying the same. The dried coating film can be calendered as needed. That is, the negative electrode active material can be a mixture layer. Alternatively, a lithium metal foil or a lithium alloy foil can be attached to the negative electrode current collector. The negative electrode active material layer can be formed on one surface of the negative electrode current collector or on both surfaces.

[0032] The negative electrode active material layer can contain a negative electrode active material as an essential component, and can contain a binder, a conductive agent, a thickening agent, and the like as arbitrary components. As the binder, the conductive agent, and the thickening agent, publicly known materials can be used.

[0033] The negative electrode active material contains a material that electrochemically occludes and releases lithium ions, lithium metal, and / or a lithium alloy. As the material that electrochemically occludes and releases lithium ions, a carbon material, an alloy-based material, and the like are used. As the carbon material, for example, graphite, easily graphitizable carbon (soft carbon), and difficultly graphitizable carbon (hard carbon), and the like can be exemplified. Among them, graphite is preferred because of its excellent stability in charge and discharge and small irreversible capacity. As the alloy-based material, a material containing at least one metal that can form an alloy with lithium can be exemplified, and silicon, tin, a silicon alloy, a tin alloy, a silicon compound, and the like can be exemplified. Silicon oxide or tin oxide, and the like in which oxygen is combined with them can also be used.

[0034] As the 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, and the like can be used. The main component (for example, 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 is preferred from the viewpoint of high capacity and small irreversible capacity.

[0035] The silicate phase can contain at least one selected from the group consisting of a long-period-type Periodic Table Group 1 element and a long-period-type Periodic Table Group 2 element. As the long-period-type Periodic Table Group 1 element and the long-period-type Periodic Table Group 2 element, for example, lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and the like can be used. As other elements, aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), titanium (Ti), and the like can be contained. Among them, a silicate phase containing lithium (hereinafter referred to as a lithium silicate phase) is preferred because of small irreversible capacity and high initial charge and discharge efficiency.

[0036] The lithium silicate phase can be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and can also contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si 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 of 0 < z < 1, more preferably z = 1 / 2. As the elements other than Li, Si, and O that can be contained in the lithium silicate phase, for example, iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), and the like can be listed.

[0037] The carbon phase can be composed of, for example, amorphous carbon (i.e., non-crystalline carbon) having low crystallinity. The amorphous carbon can be, for example, hard carbon, soft carbon, or the like.

[0038] As the negative electrode current collector, a non-porous conductive substrate (metal foil or the like), a porous conductive substrate (mesh body, grid body, punched sheet, or the like) is used. As the material of the negative electrode current collector, stainless steel, nickel, nickel alloy, copper, copper alloy, or the like can be exemplified.

[0039] [Electrolyte solution]

[0040] The electrolyte solution contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that undergoes ion dissociation in the electrolyte solution. The solute can contain, for example, a lithium salt. The components of the electrolyte solution other than the solvent and the solute are additives. The electrolyte solution can contain various additives. The electrolyte solution is usually used directly in a liquid state, but can also be in a state in which the flowability is restricted using a gelling agent or the like.

[0041] The solvent uses an aqueous solvent or a non-aqueous solvent. As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylate, a chain carboxylate, or the like is used. As the cyclic carbonate, propylene carbonate (PC), ethylene carbonate (EC), vinylene carbonate (VC), or the like can be listed. As the chain carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or the like can be listed. In addition, as the cyclic carboxylate, γ-butyrolactone (GBL), γ-valerolactone (GVL), or the like can be listed. As the chain carboxylate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), or the like can be listed. The non-aqueous solvent can be used alone as one kind, or two or more kinds can be used in combination.

[0042] As the non-aqueous solvent, in addition to the above, a cyclic ether, a chain ether, an acetonitrile or the like nitrile, a dimethylformamide or the like amide, or the like can be listed.

[0043] As examples of cyclic ethers, 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, and the like can be given.

[0044] As examples of chain ethers, 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, amyl 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-dimethoxy methane, 1,1-diethoxy ethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and the like can be given.

[0045] These solvents can be fluorinated solvents in which a part of hydrogen atoms is replaced with fluorine atoms. As the fluorinated solvents, fluoroethylene carbonate (FEC) can be used.

[0046] As the lithium salt, for example, lithium salts of 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.), and the like can be used. The lithium salt can be used alone or in combination of two or more.

[0047] The concentration of the lithium salt in the electrolyte solution can be 1 mol / liter or more and 2 mol / liter or less, and can be 1 mol / liter or more and 1.5 mol / liter or less. By controlling the lithium salt concentration in the above range, an electrolyte solution having excellent ion conductivity and having moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0048] The electrolyte solution can contain other publicly known additives. As the additives, 1,3-propanolide, methyl benzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, and the like can be given.

[0049] [Separator]

[0050] A separator is interposed between the positive electrode and the negative electrode. The separator has a high ion permeability, a moderate mechanical strength, and a moderate insulating property. As the separator, a microporous film, a woven fabric, a nonwoven fabric, or the like can be used. As the material of the separator, a polyolefin such as polypropylene or polyethylene is preferable.

[0051] As an example of the structure of the secondary battery, a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and a nonaqueous electrolyte are accommodated in an exterior body can be given. Alternatively, instead of the wound-type electrode group, another type of electrode group such as a stacked-type electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween can be used. The secondary battery can be of any form such as a cylindrical type, a square type, a coin type, a button type, a laminate type, or the like.

[0052] Figure 1 is a schematic perspective view in which a part of a square secondary battery according to an embodiment of the present application is cut away.

[0053] The battery has a square battery case 4 with a bottom, and an electrode group 1 and a nonaqueous electrolyte accommodated in the battery case 4. The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin-made gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4 serving as a positive electrode terminal. The edge of the sealing plate 5 is fitted to the open end portion of the battery case 4, and the fitted portion is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is plugged with a sealing plug 8 after the injection of the electrolyte.

[0054] Note that the structure of the secondary battery can be a cylindrical, coin, or button type having a battery case made of metal, or a laminate type having a battery case made of a laminate of a barrier layer and a resin sheet. In the present application, the type, shape, and the like of the secondary battery are not particularly limited.

[0055] Hereinafter, the present application will be specifically described on the basis of examples and comparative examples, but the present application is not limited to the following examples.

[0056] Examples 1 to 8

[0057] [Production of Positive Electrode]

[0058] Lithium fluoride (LiF), lithium peroxide (Li2O2), and lithium manganate (LiMnO2) were mixed at a prescribed mass ratio. The mixed powder was put into a planetary ball mill (Premium-Line P7 manufactured by Fritsch, rotation speed: 600 rpm, container: 45 mL, balls: Zr balls of φ 3 mm) and treated in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour of operation and 10 minutes of stoppage) to obtain a lithium metal complex oxide having the composition shown in Table 1.

[0059] The obtained lithium metal complex oxide, acetylene black, and polyvinylidene fluoride were mixed at a solid component mass ratio of 7:2:1, and a positive electrode composite material slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode composite material slurry was applied to a positive electrode core composed of an aluminum foil, and the coated film was dried and compressed, and then cut to a prescribed electrode size to obtain a positive electrode.

[0060] [Preparation of electrolyte]

[0061] An electrolyte was prepared by adding LiPF6 as a lithium salt to a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) mixed at a prescribed volume ratio.

[0062] [Manufacture of test cell]

[0063] A test cell was manufactured using the above-described positive electrode and a negative electrode composed of a lithium metal foil. The above-described positive electrode and negative electrode were arranged opposite each other with a separator interposed therebetween to form an electrode body, and the electrode body was housed in a coin-shaped outer can. After the electrolyte was injected into the outer can, the outer can was sealed to obtain a coin-shaped secondary cell.

[0064] In Examples 1 to 8, the composition of the lithium metal complex oxide was changed as shown in Table 1, and secondary cells Al to A8 for testing were obtained, respectively.

[0065] <Comparative Example 1>

[0066] In the manufacture of the positive electrode, lithium manganate (LiMnO2) was put into a planetary ball mill (Premium-Line P7 manufactured by Fritsch, rotation speed: 600 rpm, container: 45 mL, balls: Zr balls of φ 3 mm) and treated in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour of operation and 10 minutes of stoppage).

[0067] Other than this, the same as in Example 1 was performed, and a secondary cell Bl for testing was obtained.

[0068] <Comparative Example 2>

[0069] In the production of the positive electrode, lithium fluoride (LiF) and lithium manganate (LiMnO2) were mixed at a prescribed mass ratio. The mixed powder was put into a planetary ball mill (Premium-Line P7 manufactured by Fritsch, rotation speed: 600 rpm, container: 45 mL, balls: Zr balls of φ 3 mm) and treated in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour of operation and 10 minutes of stoppage) to obtain Li 1.15 Mn 0.85 O 1.7 F 0.3 as a lithium metal complex oxide.

[0070] In addition thereto, the same was carried out as in Example 1 to obtain a secondary battery B2 for testing.

[0071] [Assessment]

[0072] (initial discharge capacity)

[0073] The secondary battery was subjected to constant current charging at a constant current of 0.05 C until the battery voltage was 4.95 V in a normal temperature environment. Then, the charging was stopped for 20 minutes, and the battery was subjected to constant current discharging at a constant current of 0.05 C until the battery voltage was 2.5 V, and the discharge capacity was measured. The discharge capacity per unit mass of the positive electrode active material (lithium metal complex oxide) was calculated as the initial discharge capacity Co.

[0074] In Table 1, the assessment results of the initial discharge capacity Coare shown together with the composition of the lithium metal complex oxide used as the positive electrode active material in Examples 1 to 8 and Comparative Examples 1 and 2. In Table 1, the composition of the lithium metal complex oxide used in each example is also shown as the molar ratio x (= 2 - a - b - c) of the vacancies at the time of the synthesis. a Mn b M c O 2-d F d of the vacancies at the time of the synthesis.

[0075] In the crystal structure of the lithium metal complex oxide used in Examples 1 to 8, vacancies exist at the cation sites in the range of 0.05 ≤ x ≤ 0.25 of the molar ratio x of the vacancies. On the other hand, in the crystal structure of the lithium metal complex oxide used in Comparative Examples 1 and 2, the molar ratio x of the vacancies is theoretically 0, and substantially no vacancies exist at the cation sites.

[0076] As shown in Table 1, the initial discharge capacity Coof the battery Bl of Comparative Example 1 is extremely small. On the other hand, the initial discharge capacity Coof the battery B2 of Comparative Example 2 is improved by the introduction of fluorine as compared with the battery Bl.

[0077] The initial discharge capacity Coof the batteries Al to A8 of Examples 1 to 8 is also improved as compared with the battery B2.

[0078] [Table 1]

[0079]

[0080] Industrial applicability

[0081] According to the secondary battery relating to the present application, a secondary battery having high capacity and excellent cycle characteristics can be provided. The secondary battery relating to the present application is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0082] Explanation of reference numerals

[0083] 1 electrode group

[0084] 2 positive electrode lead

[0085] 3 negative electrode lead

[0086] 4 battery case

[0087] 5 sealing plate

[0088] 6 negative electrode terminal

[0089] 7 gasket

[0090] 8 sealing plug

Claims

1. A positive electrode active material for a secondary battery, comprising a lithium metal complex oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m, in the crystal structure, having a vacancy in which lithium and metal atoms are not disposed, wherein The lithium metal complex oxide is represented by a composition formula Li a Mn b M c O 2-d F d wherein M is at least one metal element other than Li and Mn, and satisfies 0 < a ≤ 1.4, 0.4 ≤ b ≤ 0.95, 0 ≤ c ≤ 0.2, 0.2 ≤ d ≤ 0.66, and 1.85 ≤ a + b + c ≤ 1.

98.

2. The positive electrode active material for a secondary battery according to claim 1, wherein in the lithium metal complex oxide, the metal element M comprises at least one selected from the group consisting of 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, Sm, Eu, Dy, and Er.

3. The positive electrode active material for a secondary battery according to claim 2, wherein in the lithium metal complex oxide, the metal element M comprises at least one selected from the group consisting of Ni, Sn, Mo, W, Ta, and Zn.

4. A secondary battery provided with: a positive electrode, a negative electrode, an electrolyte, and a separator interposed 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.

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