Positive electrode active material for secondary batteries and secondary batteries

Abstract

This positive electrode active material for secondary batteries contains a lithium metal composite oxide that has a crystal structure based on a rock salt structure belonging to the space group Fm-3m, and has a vacancy where a lithium atom and a metal atom are not placed in the crystal structure. For example, a composite oxide represented by composition formula LiaMnbMcO2-dFd (wherein M represents at least one metal element other than Li and Mn; and 0 < a ≤ 1.4, 0.4 ≤ b ≤ 0.95, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.66 and 1.75 ≤ a + b + c < 2 are satisfied) is able to be used as the lithium metal composite oxide.

Description

Technical Field

[0001] The present disclosure relates to secondary batteries, and particularly to improvements in positive electrodes used in secondary batteries.

Background Art

[0002] Secondary batteries, particularly lithium-ion secondary batteries, are expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles because of their high output and high energy density. As the positive electrode active material of lithium-ion secondary batteries, composite oxides of lithium and transition metals (for example, cobalt) are used. By replacing part of cobalt with nickel, it is possible to increase the capacity.

[0003] On the other hand, in recent years, due to the demand for high energy density, Li , , 1-q , p , y ,

[0005] , , , , , , , , q , 1+p , z , 1+x , , , , ,

[0006] ,

[0004] , Mn 1-x excess lithium metal composite oxides based on O2 have attracted attention.

[0004] Patent Document 1 discloses a positive electrode active material containing a lithium transition metal composite oxide having a crystal structure belonging to the space group Fm-3m and represented by the composition 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 is excluded when 0.15 < p ≦ 0.3 and 0 < q ≦ 0.3).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] In Patent Document 1, high capacity is enabled by controlling the composition (i.e., addition of Nb). However, the effect of improving the capacity is insufficient, and there is still room for improvement.

[0007] In view of the above, one aspect of the present disclosure 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 the space group Fm-3m, and in the crystal structure, having voids where lithium and metal atoms are not arranged.

[0008] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, an electrolytic solution, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode includes the positive electrode active material for the secondary battery described above.

[0009] According to the present disclosure, a secondary battery with a high energy density can be realized.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a schematic perspective view of a part of a secondary battery according to an embodiment of the present disclosure with a cutout.

Embodiments for Carrying Out the Invention

[0011] The positive electrode active material for a secondary battery according to an embodiment of the present disclosure includes a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to the space group Fm-3m. That is, this lithium metal composite oxide has a crystal structure similar to the rock salt structure belonging to the space group Fm-3m, and in the crystal structure, has voids where lithium and metal atoms are not arranged. Here, having voids means that in the positive electrode active material taken out by decomposing a secondary battery immediately after production or in a discharged state, voids not filled with Li atoms or metal atoms exist in the lithium metal composite oxide. The proportion of the voids can be 0.5% or more, preferably 1% or more, and more preferably 2% or more of the sites where lithium atoms or metal atoms can be arranged in the crystal structure.

[0012] The above lithium metal composite oxide has a crystal structure based on a rock salt structure represented by, for example, NaCl. Oxygen atoms are arranged at anion sites, and Li atoms and metal atoms other than Li are irregularly arranged at cation sites. However, some of the cation sites are not occupied by either Li atoms or metal atoms and are vacant. Through these vacancies, lithium ions can move more easily, and the capacity is improved.

[0013] Examples of the lithium metal composite oxide include those represented by the compositional 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, and 1.75 ≤ a + b + c < 2). M is at least one metal element other than Li and Mn. In the above compositional formula, the x value represented by 2 - a - b - c (= x) represents the molar ratio of the vacancies present at the cation sites. From the above compositional formula, the molar ratio of the vacancies x is 0 < x ≤ 0.25. The molar ratio of the vacancies x is preferably x ≥ 0.02, more preferably x ≥ 0.05, and even more preferably x ≥ 0.1. In other words, a + b + c ≤ 1.98 is preferable, a + b + c ≤ 1.95 is more preferable, and a + b + c ≤ 1.9 is even more preferable. Also, the molar ratio of the vacancies x is more preferably x ≤ 0.15 (a + b + c ≥ 1.85).

[0014] The vacancies and the content ratio thereof can be derived based on the crystal structure and composition of the lithium metal composite oxide. For example, in the case of a crystal structure similar to a rock salt structure belonging to the space group Fm-3m, the content ratio of the vacancies can be obtained by calculating x = 2 - a - b - c from the above compositional formula. The crystal structure of the lithium metal composite oxide is identified from the X-ray diffraction pattern measured using a powder X-ray diffractometer (for example, the desktop X-ray diffractometer MiniFlex manufactured by Rigaku Corporation, X-ray source: CuKα). The composition of the lithium metal composite oxide can be measured using an ICP emission spectroscopic analyzer (iCAP6300 manufactured by Thermo Fisher Scientific).

[0015] Alternatively, vacancies and their content ratio may be evaluated using a method that utilizes positron annihilation.

[0016] As shown in the above compositional formula, some of the oxygen atoms in the anion site may be substituted with fluorine atoms. This stabilizes the excess Li state. In addition, the introduction of fluorine atoms increases the average discharge potential. In the above lithium metal composite oxide, the arrangement of Li in the cation site is irregular and the bonding state of Li is varied, resulting in a wide voltage distribution associated with Li release. For this reason, it may be difficult to utilize the lower potential tail of the voltage distribution as capacitance. However, by introducing fluorine atoms, the voltage distribution associated with Li release shifts to the higher potential side, making it easier to utilize the tail as capacitance. This further increases the usable capacity.

[0017] When some oxygen atoms are replaced with fluorine atoms, the substitution ratio d of fluorine atoms in the composition formula of the lithium metal composite oxide may be 0.1 ≤ d ≤ 0.58, 0.1 ≤ d ≤ 0.5, or 0.2 ≤ d ≤ 0.5.

[0018] Lithium metal composite oxides may contain metal elements M other than Li and Mn. Lithium metal composite oxides may contain at least one metal element M 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. In particular, lithium metal composite oxides preferably contain at least one metal element M selected from the group consisting of Ni, Sn, Mo, W, Ta, and Zn.

[0019] The above lithium metal composite oxide can be synthesized, for example, by mixing lithium peroxide (Li2O2), lithium fluoride (LiF), and lithium manganese oxide (LiMnO2) in an inert gas atmosphere such as Ar using a planetary ball mill. LiO2 and Mn2O3 may also be used as raw materials. Alternatively, a mixer capable of applying similar stirring and shearing force to the powder may be used instead of a planetary ball mill, and the powder may be heated during the mixing process. The composition of the composite oxide can be adjusted to the desired range by changing, for example, the mixing ratio of LiF and LiMnO2 and the mixing conditions (rotation speed, processing time, processing temperature, etc.).

[0020] Next, a secondary battery according to the embodiments of this disclosure will be described in detail. The secondary battery comprises, for example, a positive electrode, a negative electrode, an electrolyte, and a separator as follows.

[0021] [Positive electrode] 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 is the positive electrode for secondary batteries described above. 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 containing a positive electrode active material, a binder, etc., is dispersed in a dispersion medium and drying it. The dried coating may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0022] The positive electrode mixture layer contains positive electrode active material as an essential component and may contain optional components such as binders, thickeners, conductive agents, positive electrode additives, etc. Known materials can be used as binders, thickeners, and conductive agents.

[0023] The positive electrode active material includes the aforementioned lithium metal composite oxide having a crystal structure similar to a rock salt structure belonging to space group Fm-3m. The composite oxide is, for example, a secondary particle formed by the aggregation of multiple primary particles. The particle size of the primary particles is generally 0.05 μm to 1 μm. The average particle size of the composite oxide is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. Here, the average particle size of the composite oxide refers to the median diameter (D50) at which the cumulative frequency in the volume-based particle size distribution reaches 50%, and is measured by a laser diffraction particle size distribution analyzer.

[0024] The content of the elements constituting the composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron beam microanalyzer (EPMA), or an energy-dispersive X-ray spectrometer (EDX), etc.

[0025] As the positive electrode active material, the above-mentioned lithium metal composite oxide having a crystal structure similar to the above-mentioned rock salt structure may be used in combination with other known lithium metal oxides. Other lithium metal oxides include, for example, Li a CoO2, Li a KiO2, 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 O 4、 LiMePO 4、Examples include lithium transition metal composite oxides such as Li2MePO4F. 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 one transition element (for example, 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 indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0026] The shape and thickness of the positive electrode current collector can be selected from the same shape and range as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0027] [Negative electrode] The negative electrode comprises, for example, 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 negative electrode active material, a binder, etc., is dispersed in a dispersion medium and drying it. The dried coating may be rolled if necessary. In other words, the negative electrode active material 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 of the negative electrode current collector or on both surfaces.

[0028] The negative electrode active material layer contains the negative electrode active material as an essential component and may contain optional components such as binders, conductive agents, and thickeners. Known materials can be used as binders, conductive agents, and thickeners.

[0029] The negative electrode active material includes materials that electrochemically intercalate and release lithium ions, lithium metals, and / or lithium alloys. Examples of electrochemically intercalating and releasing lithium ions include carbon materials and alloying materials. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Among these, graphite is preferred due to its excellent charge-discharge stability and low irreversible capacity. Examples of alloying materials include those containing at least one metal capable of alloying with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide and tin oxide, which are formed by the bonding of these materials with oxygen, may also be used.

[0030] As alloy materials containing silicon, for example, a silicon composite material can be used, which consists of a lithium-ion conductive phase and silicon particles dispersed in the lithium-ion conductive phase. 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 of the silicon oxide phase (e.g., 95-100% by mass) may be silicon dioxide. Among these, a composite material composed of a silicate phase and silicon particles dispersed in the silicate phase is preferred because it has high capacity and low irreversible capacity.

[0031] The silicate phase may contain, for example, at least one element selected from the group consisting of Group 1 and Group 2 elements of the long-period periodic table. Examples of Group 1 and Group 2 elements of the long-period periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements that may be included include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), and titanium (Ti). Among these, a lithium-containing silicate phase (hereinafter also referred to as the lithium silicate phase) is preferred because it has a small irreversible capacity and high initial charge-discharge efficiency.

[0032] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may 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 has the formula: Li 2z SiO 2+z (where 0 < z < 2). z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2. Examples of elements other than Li, Si, and O that may be included in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), and the like.

[0033] The carbon phase may be composed of, for example, low-crystallinity amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or the like.

[0034] 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, net body, punching sheet, etc.) is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, and the like.

[0035] [Electrolyte solution] The electrolyte solution contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that dissociates into ions in the electrolyte solution. The solute may contain, for example, a lithium salt. Components of the electrolyte solution other than the solvent and the solute are additives. The electrolyte solution may contain various additives. The electrolyte solution is usually used in a liquid state, but may also be in a state where its fluidity is restricted by a gelling agent or the like.

[0036] The solvent used can be an aqueous or non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). The non-aqueous solvent may be used alone or in combination of two or more types.

[0037] Other non-aqueous solvents include cyclic ethers, linear ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0038] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0039] Examples of linear ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, ethylphenyl ether, butylphenyl ether, pentylphenyl 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, tetraethylene glycol dimethyl ether, and the like.

[0040] These solvents may be fluorinated solvents in which some of the hydrogen atoms are replaced by fluorine atoms. Fluoroethylene carbonate (FEC) may be used as the fluorinated solvent.

[0041] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts of fluorine-containing acids (such as LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0042] The lithium salt concentration in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0043] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0044] [Separator] A separator is interposed between the positive and negative electrodes. The separator has high ion permeability and possesses appropriate mechanical strength and insulating properties. Microporous thin films, woven fabrics, nonwoven fabrics, etc., can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred as the material of the separator.

[0045] One example of a secondary battery structure is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, and a non-aqueous electrolyte are housed in an outer casing. Alternatively, other forms of electrode groups may be used instead of the wound electrode group, such as a laminated electrode group in which the positive electrode and negative electrode are stacked with a separator. Secondary batteries may take any form, such as cylindrical, prismatic, coin-type, button-type, or laminated type.

[0046] Figure 1 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present disclosure.

[0047] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and a non-aqueous electrolyte. The electrode group 1 has a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them. The negative electrode current collector 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 gasket 7. The positive electrode current collector 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, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits 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 sealed by a seal 8 after injection.

[0048] The structure of the secondary battery may be cylindrical, coin-shaped, or button-shaped, and may have a metal battery case. It may also be a laminated battery, which has a battery case made of a laminate sheet that is a laminate of a barrier layer and a resin sheet. In this disclosure, the type, shape, etc. of the secondary battery are not particularly limited.

[0049] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0050] <Examples 1-8> [Fabrication of the positive electrode] Lithium fluoride (LiF), lithium peroxide (Li2O2), and lithium manganese oxide (LiMnO2) were mixed in a predetermined mass ratio. This mixed powder was placed in a planetary ball mill (Fritsch Premium-Line P7, rotation speed: 600 rpm, container: 45 mL, balls: φ3 mm Zr balls) and processed in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour operation followed by a 10-minute pause) to obtain lithium metal composite oxides with the compositions shown in Table 1.

[0051] The obtained lithium metal composite oxide, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 7:2:1, and a cathode composite slurry was prepared using N-methyl-2-pyrrolidone (NMP) as the dispersion medium. Next, the cathode composite slurry was applied to a cathode core made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to obtain the cathode.

[0052] [Preparation of electrolyte solution] An electrolyte was prepared by adding LiPF6 as a lithium salt to a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a predetermined volume ratio.

[0053] [Preparation of test cells] A test cell was fabricated using the positive electrode described above and a negative electrode made of lithium metal foil. The positive electrode and the negative electrode were placed opposite each other with a separator in between to form an electrode body, and the electrode body was housed in a coin-shaped outer casing. After injecting electrolyte into the outer casing, the casing was sealed to obtain a coin-shaped secondary battery.

[0054] In Examples 1 to 8, the composition of the lithium metal composite oxide was changed as shown in Table 1, and test secondary batteries A1 to A8 were obtained, respectively.

[0055] <Comparative Example 1> In preparing the cathode, lithium manganese oxide (LiMnO2) was placed in a planetary ball mill (Fritsch Premium-Line P7, rotation speed: 600 rpm, container: 45 mL, balls: φ3 mm Zr balls) and processed in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour operation followed by a 10-minute pause).

[0056] Aside from this, a test secondary battery B1 was obtained in the same manner as in Example 1.

[0057] <Comparative Example 2> In preparing the cathode, lithium fluoride (LiF) and lithium manganese oxide (LiMnO2) were mixed in a predetermined mass ratio. This mixed powder was placed in a planetary ball mill (Fritsch Premium-Line P7, rotation speed: 600 rpm, container: 45 mL, balls: φ3 mm Zr balls) and processed in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour operation followed by a 10-minute pause) to obtain a lithium metal composite oxide. 1.15 Mn 0.85 O 1.7 F 0.3 I obtained it.

[0058] Aside from this, a test secondary battery B2 was obtained in the same manner as in Example 1.

[0059] [evaluation] (Initial discharge capacity) A secondary battery was charged at room temperature using a constant current of 0.05C until the battery voltage reached 4.95V. After a 20-minute pause, it was discharged at a constant current of 0.05C until the battery voltage reached 2.5V, and the discharge capacity was measured. The discharge capacity per unit mass of the positive electrode active material (lithium metal composite oxide) was determined and defined as the initial discharge capacity C0.

[0060] Table 1 shows the evaluation results of the initial discharge capacity C0, along with the composition of the lithium metal composite oxide used as the positive electrode active material in Examples 1 to 8 and Comparative Examples 1 and 2. a Mn b M c O 2-d F d The molar ratio of vacancies x (=2-abc) when expressed in this way is also shown.

[0061] In the lithium metal composite oxides used in Examples 1 to 8, vacancies exist at the cationic sites in the crystal structure where the molar ratio x of vacancies is in the range of 0.05 ≤ x ≤ 0.25. On the other hand, in the lithium metal composite oxides used in Comparative Examples 1 and 2, the molar ratio x of vacancies is theoretically 0 in the crystal structure, and there are substantially no vacancies at the cationic sites.

[0062] As shown in Table 1, the initial discharge capacity C0 of battery B1 in Comparative Example 1 is extremely small. On the other hand, in battery B2 in Comparative Example 2, the initial discharge capacity C0 was improved compared to battery B1 by introducing fluorine.

[0063] In Examples 1-8, batteries A1-A8 showed improved initial discharge capacity C0 compared to battery B2.

[0064] [Table 1] [Industrial applicability]

[0065] The secondary battery described herein provides a secondary battery with high capacity and excellent cycle characteristics. The secondary battery described herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of Symbols]

[0066] 1 electrode group 2 Positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing

Claims

1. Based on a rock salt structure belonging to space group Fm-3m, it contains a lithium metal composite oxide having vacancies in the cation sites of the crystal structure of the rock salt structure in which lithium and metal atoms are not arranged. The aforementioned lithium metal composite oxide has the compositional formula Li ａ Mn ｂ M ｃ O ２－ｄ F ｄ A positive electrode active material for a secondary battery, represented as follows: (where M is at least one metallic element other than Li and Mn, and satisfies 1.05 ≤ a ≤ 1.05, 0.70 ≤ b ≤ 0.80, 0 ≤ c ≤ 0.2, 0 < 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 the composition formula satisfies 0.1 ≤ d.

3. The positive electrode active material for a secondary battery according to claim 1, wherein the composition formula satisfies 0.2 ≤ d ≤ 0.

5.

4. The lithium metal composite oxide, wherein 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, is the positive electrode active material for a secondary battery according to any one of claims 1 to 3.

5. The positive electrode active material for a secondary battery according to claim 4, wherein the lithium metal composite oxide comprises at least one metal element M selected from the group consisting of Ni, Sn, Mo, W, Ta, and Zn.

6. The device comprises a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode. The positive electrode comprises a positive electrode active material for a secondary battery as described in any one of claims 1 to 5, in a secondary battery.

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