Positive electrode active material for secondary battery and secondary battery

Abstract

The following positive electrode active material for secondary batteries is used, comprising a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m. The lithium metal composite oxide comprises at least one element selected from the group consisting of Ca, Al, and Si. 1 The total content of Ca, Al and Si in lithium metal composite oxides, on a mass basis, is 10 to 1000 ppm relative to the total amount of lithium metal composite oxides.

Description

Technical Field

[0001] This invention relates to positive electrode active materials for secondary batteries and secondary batteries. Background Technology

[0002] Due to their high output and high energy density, secondary batteries, especially lithium-ion batteries, are expected to be used in small-scale consumer applications, energy storage devices, and as power sources for electric vehicles. As the positive electrode active material in lithium-ion secondary batteries, a composite oxide of lithium and a transition metal (such as cobalt) is used. High capacity is achieved by replacing a portion of the cobalt with nickel.

[0003] In recent years, based on the requirement of high energy density, Li with rock salt structure has been used... 1+x Mn 1-x O2-based Li-excess lithium metal composite oxides have attracted much attention.

[0004] Patent document 1 discloses a positive electrode active material, which comprises a crystal structure belonging to space group Fm-3m and with 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, excluding Li) 1+p Fe 1-q Nb q Lithium transition metal composite oxide (represented by O2 and 0.15 < p ≤ 0.3, 0 < q ≤ 0.3).

[0005] Existing technical documents

[0006] Patent Document 1: Japanese Patent No. 6197029 Specification Summary of the Invention

[0007] In Patent Document 1, high capacity can be obtained by controlling the composition (i.e., adding Nb). However, the capacity improvement is not sufficient and there is room for improvement.

[0008] In view of this, one aspect of the present invention relates to a positive electrode active material for secondary batteries, the positive electrode active material for secondary batteries comprising a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m, wherein the lithium metal composite oxide comprises at least one element selected from the group consisting of Ca, Al and Si. 1 .

[0009] Another aspect of the present invention relates to a secondary battery having a positive electrode, a negative electrode, an electrolyte, and a separator sandwiched between the positive electrode and the negative electrode, wherein the positive electrode comprises the aforementioned positive electrode active material for a secondary battery.

[0010] According to the present invention, a high-energy-density secondary battery can be realized. Attached Figure Description

[0011] Figure 1 A schematic perspective view showing a portion of a secondary battery according to one embodiment of the present invention after being cut open. Detailed Implementation

[0012] The positive electrode active material for secondary batteries according to embodiments of the present invention comprises a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m. That is, the lithium metal composite oxide has a crystal structure similar to that of rock salt belonging to space group Fm-3m. The lithium metal composite oxide contains at least one element selected from the group consisting of Ca, Al, and Si. 1 .

[0013] The lithium metal composite oxide contains Ca, Al, and Si (element A). 1 The total content of ( ) on a mass basis, relative to the total amount of lithium metal composite oxides, should be 10–1000 ppm. This is achieved by including trace amounts of element A. 1 This will increase capacity. Although the reason is not clear, it is believed that it is achieved by forming a layer of element A on at least a portion of the surface of the positive electrode active material. 1 One reason is that the dielectric layer composed of oxides causes changes in the electron orbital energy on the surface of the active material due to changes in the electric field distribution, thereby promoting electron tunneling (charge transfer reaction) that accompanies the movement of lithium ions between the electrolyte and the active material.

[0014] The aforementioned lithium metal composite oxide has a crystal structure based on rock salt, such as NaCl, which may have oxygen atoms at the anion sites and Li atoms and metal atoms other than Li (including element A) irregularly arranged at the cation sites. 1 The structure of ).

[0015] Lithium metal composite oxides are preferably composite oxides containing Mn as a transition metal element. The molar ratio of Mn in the lithium metal composite oxide can be greater than the total molar ratio of the transition metal elements other than Mn. That is, the lithium metal composite oxide can be a lithium metal composite oxide based on a composite oxide of Li and Mn. Examples of such Li and Mn composite oxides include Li... 1+x Mn 1-x O2.

[0016] In the above crystal structure, vacancies may exist at the cation sites where neither Li atoms nor metal atoms are present. Here, "having vacancies" refers to the presence of vacancies not filled by Li atoms or metal atoms in the positive electrode active material obtained from the disassembly and removal of a freshly manufactured or discharged secondary battery. The proportion of vacancies can be 0.5% or more of the sites in the crystal structure where lithium atoms or metal atoms can be placed, preferably 1% or more, and more preferably 2% or more. By having vacancies, lithium ions can move more easily through the vacancies, and the capacity is further improved.

[0017] Lithium metal composite oxides can contain fluorine (F). In the crystal structure described above, fluorine can substitute for oxygen atoms at anion sites. This stabilizes the Li excess state, resulting in high capacity. Furthermore, the substitution of fluorine atoms increases the average discharge potential. It should be noted that the Li excess state refers to a state in which the number of Li atoms in the composite oxide exceeds the number of transition metal atoms.

[0018] In the aforementioned lithium metal composite oxides, the irregular arrangement of Li at the cation sites and the diverse binding states of Li result in a wide voltage distribution accompanying Li release. Consequently, the lower potential portion of this voltage distribution may be difficult to utilize as capacity. However, by introducing fluorine atoms, the voltage distribution accompanying Li release shifts towards the higher potential side, making the lower potential portion readily available as capacity. This further increases the usable capacity.

[0019] As lithium metal composite oxides, examples include those composed of the formula Li a Mn b M c O 2-d F d The composite oxide represented by (where 0 ≤ a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.66, 1.75 ≤ a + b + c ≤ 2) is defined here. M contains at least one metallic element other than Li and Mn, and at least element A. 1 .

[0020] In the above composition, the value of x, represented by 2-abc(=x), indicates the molar ratio of vacancies present at the cation sites. In the above composition, the molar ratio of vacancies x is 0 ≤ x ≤ 0.25. The molar ratio of vacancies x is preferably x ≥ 0.02, more preferably x ≥ 0.05, and even more preferably x ≥ 0.1. In other words, it is preferably a+b+c ≤ 1.98, more preferably a+b+c ≤ 1.95, and even more preferably a+b+c ≤ 1.9. Furthermore, the molar ratio of vacancies x is more preferably x ≤ 0.15 (a+b+c ≥ 1.85).

[0021] Vacancies and their proportions can be derived from the crystal structure and composition of lithium metal composite oxides. For example, in the case of a crystal structure similar to that of rock salt belonging to space group Fm-3m, the proportion of vacancies can be determined by calculating the composition of the lithium metal composite oxide and then calculating x = 2 - abc according to the composition formula. The crystal structure of the lithium metal composite oxide is determined by X-ray diffraction patterns obtained using a powder X-ray diffraction apparatus (e.g., the MiniFlex desktop X-ray diffraction apparatus manufactured by Rigaku Co., Ltd., X-ray source: CuKα). The composition of the lithium metal composite oxide can be determined using an ICP emission spectrometer (iCAP6300 manufactured by Thermo Fisher Scientific).

[0022] In addition, vacancies and their proportion can be evaluated using positron annihilation.

[0023] As shown in the above composition, a portion of the oxygen atoms at the anion sites can be replaced by fluorine atoms. This stabilizes the Li excess (a > 1) state, resulting in high capacity. Furthermore, as mentioned above, the average discharge potential increases, further enhancing the usable capacity. When a portion of the oxygen atoms are replaced by fluorine atoms, the substitution ratio d of the fluorine atoms in the composition of the lithium metal composite oxide can be 0.1 ≤ d ≤ 0.58, 0.1 ≤ d ≤ 0.5, or 0.2 ≤ d ≤ 0.5.

[0024] Lithium metal composite oxides except element A 1 In addition, it may contain a metallic element M other than Li and Mn. The lithium metal composite oxide may be a lithium metal composite oxide in which the metallic element M is selected from at least one of the following groups: Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Sm, Eu, Dy, and Er. Preferably, the lithium metal composite oxide contains at least one metallic element M selected from the group consisting of Ni, Sn, Mo, W, Ta, and Zn.

[0025] The aforementioned lithium metal composite oxides can be processed by a planetary ball mill in an inert gas atmosphere such as Ar to produce, for example, lithium fluoride (LiF), lithium manganese oxide (LiMnO2), and elemental A. 1The composite oxide is synthesized by mixing the oxides. Li₂O and Mn₂O₃ can be used as raw materials. Alternatively, lithium peroxide (Li₂O₂) can be added to the above raw materials for mixing to synthesize a lithium metal composite oxide with vacancies. Instead of a planetary ball mill, a mixer capable of imparting the same stirring shear force to the powder can be used, and the powder can be heated during the mixing process. The composition of the composite oxide can be adjusted to the target range by changing, for example, the mixing ratio of LiF and LiMnO₂, and the mixing conditions (rotation speed, processing time, processing temperature, etc.).

[0026] The lithium metal composite oxides synthesized by the above method sometimes contain Ca derived from the Li raw material used in the synthesis. Additionally, they sometimes contain Al or Si derived from the material constituting the processing container during the mixing process. If the Ca, Al, and / or Si contained in the composite oxide are in trace amounts, as described above, this helps to improve the capacity. A specified amount of CaO, Al₂O₃, and SiO₂ can be added to LiF and LiMnO₂ for mixing.

[0027] Next, a detailed description of the secondary battery according to embodiments of the present invention will be provided. The secondary battery includes, for example, a positive electrode, a negative electrode, an electrolyte, and a separator, as shown below.

[0028] [positive electrode]

[0029] The positive electrode includes a positive current collector and a positive electrode additive layer formed on the surface of the positive current collector, which contains a positive electrode active material. For example, the positive electrode additive layer can be formed by coating a positive electrode slurry containing a positive electrode active material, a binder, etc., dispersed in a dispersion medium onto the surface of the positive current collector and then drying it. The dried coating can be calendered as needed. The positive electrode additive layer can be formed on one side or both sides of the positive current collector.

[0030] The positive electrode mixture layer contains the positive electrode active material as an essential component, and can include binders, thickeners, conductive agents, positive electrode additives, etc., as an optional component. Commonly known materials can be used as binders, thickeners, and conductive agents.

[0031] As the positive electrode active material, it comprises the aforementioned lithium metal composite oxide having a crystal structure similar to that of rock salt 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 particle size (D50) that accumulates to 50% of the frequency in the volumetric particle size distribution, measured using a laser diffraction-type particle size distribution measuring device.

[0032] It should be noted that the content of elements constituting the composite oxide can be determined by inductively coupled plasma emission spectrometry (ICP-AES), electron beam microanalyzer (EPMA), or energy dispersive X-ray analyzer (EDX).

[0033] As a positive electrode active material, other known lithium metal oxides, not belonging to the aforementioned lithium metal composite oxides, can be mixed with the lithium metal composite oxide having a crystal structure similar to that of rock salt described above. Examples of other lithium metal oxides include, for instance, 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 Lithium transition metal composite oxides such as O4, LiMePO4, and Li2MePO4F. Here, M is selected from at least one element chosen 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 (e.g., at least one element 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. It should be noted that the value of 'a', representing the molar ratio of lithium, is increased or decreased through charging and discharging.

[0034] The shape and thickness of the positive current collector are selected from those of the negative current collector. Examples of materials for the positive current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0035] [negative electrode]

[0036] The negative electrode includes, for example, a negative current collector and a layer of negative active material formed on the surface of the negative current collector. For example, a layer of negative active material can be formed by coating a negative electrode slurry containing a negative electrode agent, binder, etc., dispersed in a dispersion medium onto the surface of the negative current collector and then drying it. The dried coating can be calendered as needed. That is, the negative active material can be an agent layer. Alternatively, lithium metal foil or lithium alloy foil can be attached to the negative current collector. The layer of negative active material can be formed on one side or both sides of the negative current collector.

[0037] The negative electrode active material layer contains the negative electrode active material as an essential component, and can include binders, conductive agents, thickeners, etc. as optional components. Known materials can be used as binders, conductive agents, and thickeners.

[0038] The negative electrode active material includes materials that electrochemically absorb and release lithium ions, lithium metal, and / or lithium alloys. Carbon materials and alloy materials are used as materials for electrochemically absorbing and releasing lithium ions. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Graphite, which exhibits excellent charge-discharge stability and low irreversible capacity, is preferred. Alloy materials include those containing at least one metal capable of forming an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide or tin oxide, which are combined with oxygen, can also be used.

[0039] As a silicon-containing alloy material, a silicon composite material, such as a lithium-ion conductive phase and silicon particles dispersed in the lithium-ion conductive phase, can be used. As the lithium-ion conductive phase, silicon oxide phase, silicate phase, and / or carbon phase can be used, for example. The main component of the silicon oxide phase (e.g., 95-100% by mass) can be silicon dioxide. From the perspective of high capacity and low irreversible capacity, a composite material composed of a silicate phase and silicon particles dispersed in the silicate phase is preferred.

[0040] The silicate phase may contain at least one element selected from the group consisting of elements from Group 1 and Group 2 of the long-period periodic table. Examples of elements from Group 1 and Group 2 of the long-period periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements may include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), and titanium (Ti). Among these, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred due to its small irreversible capacity and high initial charge / discharge efficiency.

[0041] The lithium silicate phase can be any oxide phase containing lithium (Li), silicon (Si), and oxygen (O), but may also contain other elements. The atomic ratio 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 of Li to Si in the lithium silicate phase is, for example, greater than 0 and less than 4. The lithium silicate phase may have the formula: Li 2z SiO 2+z The composition represented by (0 < z < 2). z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2. Elements other than Li, Si and O that can be included in the lithium silicate phase include, for example, iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), etc.

[0042] The carbon phase can be composed of, for example, amorphous carbon with low crystallinity (i.e., non-crystalline carbon). Amorphous carbon can be, for example, hard carbon, soft carbon, or other types of carbon.

[0043] As the negative current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as a mesh, net, or perforated sheet) can be used. Examples of materials that can be used as the negative current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0044] [Electrolytes]

[0045] An electrolyte comprises a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that undergoes ion dissociation in the electrolyte. For example, the solute may contain a lithium salt. The components of an electrolyte other than the solvent and solute are additives. Various additives may be included in an electrolyte. Electrolytes are typically used directly in a liquid state, but they can also be used in a state where their flowability is restricted by gelling agents or other similar methods.

[0046] The solvent can be an aqueous or non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates 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 chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). A single non-aqueous solvent can be used, or two or more can be used in combination.

[0047] Other examples of non-aqueous solvents include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0048] Examples of cyclic ethers include 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-eucalyptol, crown ethers, etc.

[0049] Examples of 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, tetraethylene glycol dimethyl ether, etc.

[0050] These solvents can be fluorinated solvents in which some of the hydrogen atoms are replaced by fluorine atoms. Fluorinated ethylene carbonate (FEC) can be used as a fluorinated solvent.

[0051] As lithium salts, lithium salts containing chloric acid, such as LiClO4, LiAlCl4, and LiB2, can be used. 10 Cl 10 Lithium salts include those containing fluorine acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts containing fluorinated imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), and lithium halides (LiCl, LiBr, LiI, etc.). Lithium salts can be used alone or in combination of two or more.

[0052] The concentration of lithium salt in the electrolyte can be above 1 mol / L and below 2 mol / L, or above 1 mol / L and below 1.5 mol / L. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and moderate viscosity can be obtained. However, the lithium salt concentration is not limited by the above conditions.

[0053] Electrolytes may contain other known additives. Examples of such additives include 1,3-propanesulfonyl lactone, methyl benzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0054] [Separator]

[0055] A separator is sandwiched between the positive and negative electrodes. The separator has high ion permeability and possesses moderate mechanical strength and insulation. Microporous membranes, woven fabrics, non-woven fabrics, etc., can be used as the separator. Polyolefins such as polypropylene and polyethylene are preferred as the material for the separator.

[0056] As an example of a secondary battery structure, one can exemplify a structure in which an electrode assembly consisting of a positive and negative electrode sandwiched by a separator is wound together, with the non-aqueous electrolyte housed in an outer casing. Alternatively, other forms of electrode assemblies, such as a stacked electrode assembly consisting of a positive and negative electrode sandwiched by a separator, can be used instead of the wound electrode assembly. Secondary batteries can take any shape, such as cylindrical, square, coin-shaped, button-shaped, or laminated.

[0057] Figure 1 A schematic perspective view showing a portion of a square secondary battery according to one embodiment of the present invention after being cut open.

[0058] The battery comprises a square-shaped battery casing 4, and an electrode assembly 1 and a non-aqueous electrolyte housed within the battery casing 4. The electrode assembly 1 has a strip-shaped negative electrode, a strip-shaped positive electrode, and a separator between them. The negative current collector of the negative electrode is electrically connected to the negative terminal 6 located on the sealing plate 5 via a negative lead 3. The negative terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive current collector of the positive electrode is electrically connected to the inner surface of the sealing plate 5 via a positive lead 2. That is, the positive electrode is electrically connected to the battery casing 4, which also serves as the positive terminal. The periphery of the sealing plate 5 is fitted into the open end of the battery casing 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is sealed by a sealing plug 8 after electrolyte injection.

[0059] It should be noted that the structure of the secondary battery can be cylindrical, coin-shaped, button-shaped, etc., with a metal battery casing, or it can be a laminated battery with a laminated battery casing consisting of a barrier layer and a resin sheet. In this invention, there are no particular limitations on the type or shape of the secondary battery.

[0060] The present invention will be specifically described below based on embodiments and comparative examples, but the present invention is not limited to the following embodiments.

[0061] <Examples 1 and 2>

[0062] [The production of the positive electrode]

[0063] Lithium fluoride (LiF), lithium manganese oxide (LiMnO2), calcium oxide (CaO), aluminum oxide (Al2O3), and silicon oxide (SiO2) were mixed according to a specified mass ratio. The mixed powder was fed into a planetary ball mill (Fritsch Premium-Line P7, speed: 600 rpm, container: 45 mL, balls: φ5 mm Zr balls) and treated at room temperature in an Ar atmosphere for 35 hours (with a cycle of 1 hour operation followed by a 10-minute stop, repeated 35 times) to obtain a lithium metal composite oxide with a specified composition.

[0064] The obtained lithium metal composite oxide, acetylene black, and polyvinylidene fluoride were mixed at a solid content mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was coated onto a positive electrode core made of aluminum foil. After drying and compressing the coating, it was cut into specified electrode sizes to obtain the positive electrode.

[0065] [Preparation of Electrolytes]

[0066] A non-aqueous electrolyte is prepared by adding LiPF6 as a lithium salt in a mixed solvent in a specified volume ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0067] [Preparation of the experimental battery]

[0068] A test battery was fabricated using the aforementioned positive electrode and a negative electrode made of lithium metal foil. The positive and negative electrodes were aligned face-to-face using a separator to form an electrode body, which was then housed in a coin-shaped outer container. After injecting electrolyte into the outer container, it was sealed to obtain a coin-shaped test secondary battery.

[0069] In Examples 1 and 2, lithium metal composite oxides X1 and X2 with different compositions were synthesized, and secondary batteries A1 using lithium metal composite oxide X1 as the positive electrode active material and A2 using lithium metal composite oxide X2 as the positive electrode active material were fabricated, respectively. Secondary battery A1 corresponds to Example 1, and secondary battery A2 corresponds to Example 2.

[0070] The compositions of lithium metal composite oxides X1 and X2 were determined by ICP-spectroscopy. The results indicate that the composition of lithium metal composite oxide X1 is approximately Li... 1.29 Mn 0.71 O 1.42 F 0.58 It contains 40 ppm Ca, 30 ppm Al, and 100 ppm Si by mass. Similarly, the composition of lithium metal composite oxide X2 is roughly evaluated as Li 1.29Mn 0.71 O 1.42 F 0.58 It contains 100 ppm Ca, 60 ppm Al, and 250 ppm Si, respectively, based on mass.

[0071] <Comparative Examples 1 and 2>

[0072] In the fabrication of the positive electrode, lithium fluoride (LiF), lithium manganese oxide (LiMnO2), calcium oxide (CaO), aluminum oxide (Al2O3), and silicon oxide (SiO2) were mixed in a specified mass ratio. This mixed powder was fed into a planetary ball mill in the same manner as in Example 1, and treated at room temperature in an Ar atmosphere to obtain a lithium metal composite oxide with a specified composition. Using the obtained lithium metal composite oxide, a positive electrode was fabricated in the same manner as in Example 1 to obtain a test secondary battery.

[0073] In Comparative Examples 1 and 2, lithium metal composite oxides Y1 and Y2 with different compositions were synthesized, and secondary batteries B1 and B2, respectively, using lithium metal composite oxide Y1 as the positive electrode active material and Y2 as the positive electrode active material, were fabricated. Secondary battery B1 corresponds to Comparative Example 1, and secondary battery B2 corresponds to Comparative Example 2.

[0074] The compositions of lithium metal composite oxides Y1 and Y2 were determined by ICP-spectroscopy. The results indicate that the composition of lithium metal composite oxide Y1 is approximately Li... 1.28 Mn 0.62 Al 0.1 O 1.7 F 0.3 It contains 40 ppm Ca, 34300 ppm Al, and 200 ppm Si by mass. Similarly, the composition of lithium metal composite oxide X2 is roughly evaluated as Li 1.25 Mn 0.65 Si 0.1 O 1.7 F 0.3 It contains 30 ppm Ca, 30 ppm Al, and 35,400 ppm Si, respectively, based on mass.

[0075] Regarding lithium metal composite oxides X1, X2, Y1, and Y2, after measuring and analyzing the X-ray diffraction patterns of the composite oxides using a powder X-ray diffraction device, the number and position of the XRD peaks confirmed that the composite oxides have a crystal structure based on the rock salt type belonging to the space group Fm-3m.

[0076] [evaluate]

[0077] (Initial discharge capacity)

[0078] The secondary battery was charged at a constant current of 0.05C to a battery voltage of 4.95V at room temperature. Then, after a 20-minute pause, it was discharged at a constant current of 0.2C to a battery voltage of 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 set as the initial discharge capacity C0.

[0079] Table 1 shows the evaluation results of the initial discharge capacity C0, along with the composition and Ca, Al, and Si contents of the lithium metal composite oxides used as positive electrode active materials in each battery.

[0080] As shown in Table 1, in batteries A1 and A2 of Examples 1 and 2, the initial discharge capacity was improved by including Ca, Al, and Si in trace amounts in the range of 10 to 1000 ppm. In batteries B1 and B2 of Comparative Examples 1 and 2, the initial discharge capacity decreased because Ca, Al, and Si were included in large quantities exceeding 1000 ppm.

[0081] [Table 1]

[0082]

[0083] Industrial availability

[0084] The secondary battery according to the present invention provides a high-capacity battery with excellent cycle characteristics. The secondary battery of the present invention is effectively used as a main power source for mobile communication devices, portable electronic devices, etc.

[0085] Explanation of reference numerals in the attached figures

[0086] 1 Electrode assembly

[0087] 2 Positive lead

[0088] 3 Negative lead

[0089] 4. Battery casing

[0090] 5 Sealing board

[0091] 6 Negative extremes

[0092] 7 gaskets

[0093] 8. Sealing plug

Claims

1. A positive electrode active material for secondary batteries, comprising a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m. The lithium metal composite oxide contains element A composed of Ca, Al and Si. 1 , The total content of Ca, Al, and Si in the lithium metal composite oxide, on a mass basis, is 10~1000 ppm relative to the total amount of the lithium metal composite oxide. The lithium metal composite oxide is composed of the formula Li a Mn b M c O 2-d F d In this formula, M is at least one metallic element other than Li and Mn, and contains at least element A. 1 The following conditions must be met: 0 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.2, 0 ≤ d ≤ 0.66, and 1.75 ≤ a + b + c ≤ 2.

2. The positive electrode active material for secondary batteries according to claim 1, wherein, The lithium metal composite oxide has vacancies at the cation sites in the crystal structure.

3. The positive electrode active material for secondary batteries according to claim 1, wherein, The lithium metal composite oxide contains fluorine.

4. The positive electrode active material for secondary batteries according to claim 1, wherein, In the lithium metal composite 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, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Sm, Eu, Dy, and Er.

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

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

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