Positive electrode active material for secondary batteries and secondary batteries

Abstract

The present invention uses a positive electrode active material for secondary batteries, said positive electrode active material containing a lithium metal composite oxide that has a crystal structure based on a rock salt structure belonging to the space group Fm-3m. The lithium metal composite oxide contains at least one element A1 that is selected from the group consisting of Ca, Al and Si; and the total content of Ca, Al and Si contained in the lithium metal composite oxide is from 10 ppm to 1,000 ppm on a mass basis relative to the total amount of the lithium metal composite oxide.

Description

Technical Field

[0001] The present disclosure relates to a positive electrode active material for a secondary battery and a secondary battery.

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 they have high output and high energy density. As the positive electrode active material of a lithium-ion secondary battery, a composite oxide of lithium and a transition metal (for example, cobalt) is used. By replacing a 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, a Li-excess type lithium metal composite oxide based on a rock salt structure Li 1+x Mn 1-x O2 has 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., adding 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 includes a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to the space group Fm-3m, and the lithium metal composite oxide is at least one element A selected from the group consisting of Ca, Al, and Si 1 relates to a positive electrode active material for a secondary battery.

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

[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.

Modes 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. This lithium metal composite oxide is at least one element A selected from the group consisting of Ca, Al, and Si 1 contains.

[0012] The total content of Ca, Al, and Si (element A 1 ) contained in the lithium metal composite oxide may be 10 to 1000 ppm based on the mass with respect to the total amount of the lithium metal composite oxide. Element A 1The presence of trace amounts of element A improves capacity. The reason for this is unclear, but at least a portion of the surface of the positive electrode active material contains element A. 1 One possible reason is that the formation of a dielectric layer composed of oxides causes a change in the electric field distribution, which alters the electron orbital energy on the surface of the active material, thereby promoting electron tunneling (charge transfer reaction) associated with lithium ion movement between the electrolyte and the active material.

[0013] The lithium metal composite oxides described above, such as NaCl, have a crystal structure based on a rock salt structure, with oxygen atoms positioned at the anion sites and Li atoms and other metal atoms (element A) at the cation sites. 1 It may have a structure in which elements (including) are arranged irregularly.

[0014] Lithium metal composite oxides are preferably those containing Mn as a transition metal element. The molar ratio of Mn in the lithium metal composite oxide may be greater than the molar ratio of the total transition metal elements excluding Mn. In other words, lithium metal composite oxides may be based on a composite oxide of Li and Mn. As such a composite oxide of Li and Mn, Li 1+x Mn 1-x O2 is one example.

[0015] In the above crystal structure, the cation sites may have vacancies in which Li atoms and metal atoms are not disposed. Here, having vacancies means that in the positive electrode active material immediately after manufacturing or after disassembling a discharged secondary battery, there are vacancies in the lithium metal composite oxide that are not filled with Li atoms or metal atoms. The proportion of vacancies may be 0.5% or more, preferably 1% or more, and more preferably 2% or more of the sites in the crystal structure in which lithium atoms or metal atoms can be disposed. Having vacancies makes it easier for lithium ions to move through the vacancies and further improves the capacity.

[0016] The lithium metal composite oxide may contain fluorine (F). In the above crystal structure, fluorine can substitute for oxygen atoms at anion sites. As a result, the Li-excess state is stabilized and a high capacity is obtained. In addition, the substitution of fluorine atoms increases the average discharge potential. The Li-excess state refers to a state where the number of Li atoms in the composite oxide is greater than the number of transition metal atoms.

[0017] In the above lithium metal composite oxide, the arrangement of Li at the cation site is irregular and the bonding states of Li are various, so the width of the voltage distribution accompanying Li release is wide. Therefore, it may be difficult to utilize the trailing part on the low potential side of the voltage distribution as capacity. However, due to the introduction of fluorine atoms, the voltage distribution accompanying Li release moves to the high potential side, making it easier to utilize the trailing part as capacity. As a result, the available capacity further increases.

[0018] Examples of the lithium metal composite oxide include those represented by the composition formula Li a Mn b M c O 2-d F d (where 0 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.66, 1.75 ≤ a + b + c ≤ 2 are satisfied). Here, M contains at least one metal element other than Li and Mn, and contains at least element A 1 .

[0019] In the above composition formula, the x value represented by 2 - a - b - c (= x) represents the molar ratio of the vacancies present at the cation site. From the above composition formula, 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, 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 vacancies x is more preferably x ≤ 0.15 (a + b + c ≥ 1.85).

[0020] The vacancies and their proportion 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 space group Fm-3m, the composition of the lithium metal composite oxide can be determined, and the vacancy proportion can be calculated by calculating x = 2 - abc from the empirical 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 (e.g., Rigaku Corporation's MiniFlex desktop X-ray diffractometer, X-ray source: CuKα). The composition of the lithium metal composite oxide can be measured using an ICP emission spectrometer (Thermo Fisher Scientific's iCAP6300).

[0021] Alternatively, vacancies and their content ratio may be evaluated using a method that utilizes positron annihilation.

[0022] 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 state of excess Li (a>1) and allows for high capacity. Furthermore, as mentioned above, the average discharge potential increases, further increasing the usable capacity. When some of the oxygen atoms are substituted with fluorine atoms, the substitution ratio d of fluorine atoms in the compositional 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.

[0023] Lithium metal composite oxides are composed of element A 1 In addition to Li and Mn, it may also contain other metal elements M. The lithium metal composite oxide 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, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Sm, Eu, Dy, and Er. In particular, it is preferable that the lithium metal composite oxide contains at least one metal element M selected from the group consisting of Ni, Sn, Mo, W, Ta, and Zn.

[0024] The above lithium metal composite oxides include, for example, lithium fluoride (LiF), lithium manganese oxide (LiMnO2), and element A 1 The oxides can be synthesized by mixing them in an inert gas atmosphere such as Ar using a planetary ball mill. Li2O and Mn2O3 may be used as raw materials. In addition, lithium metal composite oxides with vacancies can be synthesized by adding lithium peroxide (Li2O2) to the above raw materials and mixing them. Instead of a planetary ball mill, a mixer capable of applying a similar stirring shear force to the powder may be used, 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.).

[0025] The lithium metal composite oxide synthesized by the above method may contain Ca derived from the Li raw material used in the synthesis. It may also contain Al or Si derived from the materials constituting the processing container during the mixing process. These Ca, Al, and / or Si present in the composite oxide, in trace amounts, contribute to increased capacity, as described above. Alternatively, predetermined amounts of CaO, Al2O3, and SiO2 may be added to LiF and LiMnO2, and the mixing process may be carried out.

[0026] 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.

[0027] [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.

[0028] 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.

[0029] 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.

[0030] The content of the elements constituting the composite oxide can be measured using inductively coupled plasma atomic emission spectrometer (ICP-AES), electron beam microanalyzer (EPMA), or energy dispersive X-ray spectrometer (EDX), etc.

[0031] 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 than the above-mentioned lithium metal composite oxide. 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.

[0032] 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.

[0033] [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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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 may have a composition represented by the formula: Li 2z SiO 2+z (0 < z < 2). z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2. 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.

[0039] The carbon phase may be composed of, for example, low-crystalline amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or the like.

[0040] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh body, a net body, a 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.

[0041] [Electrolyte] The electrolyte contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that dissociates into ions in the electrolyte. The solute may contain, for example, a lithium salt. Components of the electrolyte other than the solvent and the solute are additives. The electrolyte may contain various additives. The electrolyte 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.

[0042] 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.

[0043] Other non-aqueous solvents include cyclic ethers, linear ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0050] [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.

[0051] 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.

[0052] Figure 1 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present disclosure.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] <Examples 1 and 2> [Fabrication of the positive electrode] Lithium fluoride (LiF), lithium manganese (LiMnO2), calcium oxide (CaO), aluminum oxide (Al2O3), and silicon oxide (SiO2) 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: φ5 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 having a predetermined composition.

[0057] 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.

[0058] [Preparation of electrolytes] A non-aqueous 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 predetermined volume ratios.

[0059] [Preparation of test cells] A test cell was fabricated using the above-mentioned positive electrode and a negative counter electrode made of lithium metal foil. The positive electrode and negative counter 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 the electrolyte into the outer casing, the casing was sealed to obtain a coin-shaped test secondary battery.

[0060] In Examples 1 and 2, lithium metal composite oxides X1 and X2 with different compositions were synthesized, and secondary batteries A1 and A2 were fabricated, respectively, using lithium metal composite oxide X1 as the positive electrode active material. Secondary battery A1 corresponds to Example 1, and secondary battery A2 corresponds to Example 2.

[0061] The composition of each of the lithium metal composite oxides X1 and X2 was identified by ICP emission spectrometry. As a result, the composition of lithium metal composite oxide X1 was approximately Li 1.29 Mn 0.71 O 1.42 F 0.58 It was evaluated as containing 40 ppm Ca, 30 ppm Al, and 100 ppm Si by mass. Similarly, the composition of lithium metal composite oxide X2 was approximately Li 1.29 Mn 0.71 O 1.42 F 0.58It was evaluated as containing 100 ppm Ca, 60 ppm Al, and 250 ppm Si by mass.

[0062] <Comparative Examples 1 and 2> In preparing the positive electrode, lithium fluoride (LiF), lithium manganese oxide (LiMnO2), calcium oxide (CaO), aluminum oxide (Al2O3), and silicon oxide (SiO2) were mixed in a predetermined mass ratio. This mixed powder was placed in 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 having a predetermined composition. Using the obtained lithium metal composite oxide, a positive electrode was prepared in the same manner as in Example 1 to obtain a test secondary battery.

[0063] In Comparative Examples 1 and 2, lithium metal composite oxides Y1 and Y2 with different compositions were synthesized, and secondary batteries B1 and B2 were fabricated, respectively, using lithium metal composite oxide Y1 as the positive electrode active material. Secondary battery B1 corresponds to Comparative Example 1, and secondary battery B2 corresponds to Comparative Example 2.

[0064] The composition of lithium metal composite oxides Y1 and Y2 was identified by ICP emission spectrometry. The results showed that the composition of lithium metal composite oxide Y1 was approximately Li 1.28 Mn 0.62 Al 0.1 O 1.7 F 0.3 It was evaluated as containing 40 ppm Ca, 34,300 ppm Al, and 200 ppm Si by mass. Similarly, the composition of lithium metal composite oxide X2 was approximately Li 1.25 Mn 0.65 Si 0.1 O 1.7 F 0.3 It was evaluated as containing 30 ppm Ca, 30 ppm Al, and 35,400 ppm Si by mass.

[0065] For lithium metal composite oxides X1, X2, Y1, and Y2, the X-ray diffraction patterns of the composite oxides were measured and analyzed using a powder X-ray diffractometer. Based on the number and position of the XRD peaks, it was confirmed that the composite oxides have a crystal structure based on a rock salt type belonging to the space group Fm-3m.

[0066] [evaluation] (Initial discharge capacity) A secondary battery was charged at a constant current of 0.05C to a battery voltage of 4.95V under normal temperature conditions. 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 defined as the initial discharge capacity C0.

[0067] 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 each battery, and the Ca, Al, and Si content.

[0068] As shown in Table 1, in batteries A1 and A2 of Example 1 and Example 2, the initial discharge capacity was improved by including trace amounts of Ca, Al, and Si in a total range of 10 to 1000 ppm. In batteries B1 and B2 of Comparative Example 1 and Comparative Example 2, the initial discharge capacity was decreased by including large amounts of Ca, Al, and Si exceeding 1000 ppm in total.

[0069] [Table 1] [Industrial applicability]

[0070] 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]

[0071] 1 electrode group 2 Positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing

Claims

1. It has a rock salt crystal structure belonging to space group Fm-3m, or it is based on the rock salt structure and contains a lithium metal composite oxide having vacancies at the cation sites in the rock salt crystal structure. The lithium metal composite oxide contains Ca, Al, and Si as elements A １ Included as, The lithium metal composite oxide is represented by the compositional formula Li a Mn b M c O 2-d F d (where M is at least one metallic element other than Li and Mn, and contains at least element A 1, satisfying 0 < a ≤ 1.35 (except in the range of 0 < a < 1.29), 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.2, 0 ≤ d ≤ 0.66, and 1.75 ≤ a + b + c ≤ 2), A positive electrode active material for a secondary battery, wherein the total content of Ca, Al, and Si in the lithium metal composite oxide is 10 to 1000 ppm by mass relative to the total amount of the lithium metal composite oxide.

2. The positive electrode active material for a secondary battery according to claim 1, wherein the total content of Ca, Al, and Si in the lithium metal composite oxide is 170 ppm or more and 410 ppm or less by mass relative to the total amount of the lithium metal composite oxide.

3. The positive electrode active material for a secondary battery according to claim 1 or 2, wherein the Ca content in the lithium metal composite oxide is greater than the Al content in the lithium metal composite oxide and less than the Si content in the lithium metal composite oxide.

4. The lithium metal composite oxide has vacancies at the cation sites in the crystal structure, wherein the positive electrode active material for a secondary battery is according to any one of claims 1 to 3.

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

6. 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, 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 5.

7. The positive electrode active material for a secondary battery according to claim 6, 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.

8. 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 7, in a secondary battery.

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