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. The lithium metal composite oxide contains a first metal element other than Li, and a second metal element other than Li and the first metal element. The first metal element is at least one element selected from the group consisting of Gd, Ce, Eu, Sm, and Yb.

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 part of the 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 of 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

Problems to be Solved by 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.

Means for Solving the Problems

[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, the lithium metal composite oxide including a first metal element other than Li, and a second metal element other than Li and the first metal element, the first metal element being at least one selected from the group consisting of Gd, Ce, Eu, Sm, and Yb, and 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, and an electrolyte, the positive electrode including the positive electrode active material for the secondary battery described above.

Effects of the Invention

[0009] According to the present disclosure, a secondary battery with a high energy density can be realized. The novel features of the present invention are described in the appended claims, but the present invention relates to both the configuration and the content, and will be better understood from the following detailed description in combination with the drawings and other objects and features of the present invention.

Brief Description of the Drawings

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

Modes for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present disclosure will be described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be exemplified, but other numerical values and materials may be applied as long as the effects of the present disclosure can be obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less". In the following description, when the lower limit and the upper limit of a numerical value regarding a specific physical property or condition are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be made as long as the lower limit is not more than the upper limit. When a plurality of materials are exemplified, one of them may be selected and used alone, or two or more of them may be used in combination.

[0012] Further, the present disclosure includes combinations of matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims. That is, as long as no technical contradiction occurs, matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims can be combined.

[0013] In the following description, the term "containing ~" or "including ~" is an expression that includes "containing (or including) ~", "substantially consisting of ~", and "consisting of ~".

[0014] Secondary batteries include at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries.

[0015] The positive electrode active material for a secondary battery according to the embodiments of this disclosure includes a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to space group Fm-3m. The crystal structure based on a rock salt structure belonging to space group Fm-3m is any crystal structure that can be attributed to space group Fm-3m. That is, this lithium metal composite oxide has a crystal structure similar to a rock salt structure belonging to space group Fm-3m. This lithium metal composite oxide includes a first metal element other than Li and a second metal element other than Li and the first metal element. The first metal element is at least one selected from the group consisting of Gd, Ce, Eu, Sm, and Yb.

[0016] The lithium metal composite oxides described above may have a crystal structure based on a rock salt structure, such as NaCl, in which oxygen atoms are arranged at the anion sites, and Li atoms and metal atoms other than Li (including primary and secondary metal elements) are irregularly arranged at the cation sites.

[0017] The inclusion of a first metal element increases the capacity of the lithium metal composite oxide having the above crystal structure. The reason for this is not clear, but it is presumed that one contributing factor is that the large ionic radius of the first metal element affects the ease of lithium ion movement. In the case of Gd and Ce, they are preferable because they allow for a higher average discharge potential.

[0018] In the above crystal structure, the cation sites may have vacancies in which Li atoms and metal atoms (atoms of the first and second metal elements) 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.

[0019] The lithium metal composite oxide may contain fluorine (F). Fluorine can substitute for oxygen atoms at the anion sites in the above crystal structure. Thereby, the state of Li excess is stabilized and a high capacity is obtained. Further, the average discharge potential increases due to the substitution of fluorine atoms. Note that the state of Li excess refers to a state where the number of Li atoms in the composite oxide is larger than the number of transition metal atoms.

[0020] In the above lithium metal composite oxide, the arrangement of Li at the cation sites is irregular and the bonding states of Li are various, so the width of the voltage distribution accompanying Li release is wide. For this reason, 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. Thereby, the available capacity further increases.

[0021] The second metal element may include a transition metal element (excluding the first metal element). Among them, the second metal element preferably contains Mn. The molar ratio of Mn in the lithium-containing composite oxide may be larger than the total molar ratio of the first metal element and the second metal element excluding Mn. That is, the lithium metal composite oxide 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 can be mentioned.

[0022] The lithium metal composite oxide is, for example, a composite oxide represented by the composition formula: Li a Mn b M1 c M2 d O 2-e F e is preferred. In the formula, M1 is at least one selected from the group consisting of Gd, Ce, Eu, Sm, and Yb. M2 is a metal element other than Li, Mn, Gd, Ce, Eu, Sm, and Yb. The above composition formula satisfies 0 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.15, 0 ≤ d ≤ 0.1, 0 ≤ e ≤ 0.75, 1.75 ≤ a + b + c + d ≤ 2.

[0023] From the viewpoint of increasing the average discharge potential, M1 is preferably at least one selected from the group consisting of Gd and Ce. From the viewpoint of easily obtaining the effect of adding M1, the ratio c of M1 may be 0.005 or more and 0.1 or less, or 0.005 or more and 0.075 or less, and preferably 0.025 or more and 0.075 or less.

[0024] In the above formula, M2 includes secondary metal elements, excluding the primary metal element and Mn. M2 may also include at least one selected from the group consisting of Ti, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Pr, Dy, and Er. The ratio d of M2 may be 0 or greater and 0.06 or less (or 0.04 or less).

[0025] In the above compositional formula, the x value represented by 2-abcd(=x) represents the molar ratio of vacancies present at the cation sites. From the above compositional formula, the molar ratio x of vacancies is 0≦x≦0.25. Preferably, the molar ratio x of vacancies is x≧0.02, more preferably x≧0.05, and even more preferably x≧0.1. In other words, a+b+c+d≦1.98 is preferred, a+b+c+d≦1.95 is more preferred, and a+b+c+d≦1.9 is even more preferred. Furthermore, the molar ratio x of vacancies is x≦0.15 (a+b+c+d≧1.85).

[0026] 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-abcd 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).

[0027] Alternatively, vacancies and their content ratio may be evaluated using a method that utilizes positron annihilation.

[0028] 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 e of fluorine atoms in the compositional formula of the lithium metal composite oxide may be 0.1≦e≦0.58, 0.1≦e≦0.5, or 0.2≦e≦0.5.

[0029] The above lithium metal composite oxide can be synthesized, for example, by mixing lithium fluoride (LiF), an oxide of a first metal element (e.g., lithium manganate (LiMnO2) belonging to space group Fm-3m), and an oxide of a second metal element in an inert gas atmosphere such as Ar using a planetary ball mill. Li2O and Mn2O3 may also be used as raw materials. Furthermore, by adding lithium peroxide (Li2O2) to the above raw materials and performing the mixing process, a lithium metal composite oxide with vacancies can be synthesized. Instead of a planetary ball mill, a mixer capable of applying similar stirring and shearing 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.).

[0030] The secondary battery according to the embodiment of this disclosure comprises a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes the positive electrode active material for secondary batteries described above. The secondary battery according to the embodiment of this disclosure will be described in detail below.

[0031] [Positive electrode] The positive electrode comprises, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed, for example, by coating the surface of the positive electrode current collector with a positive electrode slurry, which is obtained by dispersing the positive electrode mixture 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.

[0032] The positive electrode mixture contains a positive electrode active material as an essential component and may include binders, conductive agents, etc., as optional components. Known materials can be used as binders and conductive agents.

[0033] The positive electrode active material contains the aforementioned lithium metal composite oxide having a crystalline 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.

[0034] 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.

[0035] The positive electrode active material may further contain other lithium metal composite oxides other than those described above. Other lithium metal composite 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 bExamples include O4, LiMePO4, and 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 includes 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.

[0036] As the positive electrode current collector, non-porous conductive substrates (such as metal foil) or porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0037] [Negative electrode] The negative electrode may, for example, have a negative electrode current collector and a negative electrode mixture layer supported on the negative electrode current collector. The negative electrode mixture layer can be formed, for example, by coating a negative electrode slurry, which is obtained by dispersing the negative electrode mixture in a dispersion medium, onto the surface of the negative electrode current collector and drying it. The dried coating may be rolled if necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector or on both surfaces. The negative electrode mixture layer may also be a negative electrode active material layer. Alternatively, lithium metal foil or lithium alloy foil may be attached to the negative electrode current collector.

[0038] The negative electrode mixture contains a negative electrode active material as an essential component and may include binders, conductive agents, etc., as optional components. Known materials can be used as binders and conductive agents.

[0039] The negative electrode active material includes materials that electrochemically intercalate and release lithium ions, lithium metals, and / or lithium alloys. Examples of materials that electrochemically intercalate and release 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.

[0040] As alloy materials containing silicon, for example, a silicon composite material can be used in which a lithium ion conductive phase and a silicon phase (e.g., silicon particles) are 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 a silicon phase dispersed in the silicate phase is preferred because it has high capacity and low irreversible capacity.

[0041] The silicate phase may include, 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, the lithium silicate phase is preferred because it has a small irreversible capacity and high initial charge-discharge efficiency.

[0042] 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 (0 < z < 2). z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2.

[0043] 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.

[0044] The carbon phase can 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.

[0045] The shape of the negative electrode current collector can be selected from shapes similar to those of the positive electrode current collector. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, and the like.

[0046] [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.

[0047] 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.

[0048] Other non-aqueous solvents include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts of fluorine-containing acids (e.g., LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (e.g., LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (e.g., LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0053] 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.

[0054] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0055] [Separator] It is desirable to have a separator between the positive and negative electrodes. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0056] A secondary battery may, for example, comprise a wound electrode group in which a positive electrode and a negative electrode are wound around a separator, or a laminated electrode group in which a positive electrode and a negative electrode are stacked around a separator. The secondary battery may take any form, such as cylindrical, prismatic, coin-type, button-type, or laminated type. In this disclosure, the type, shape, etc., of the secondary battery are not particularly limited.

[0057] Figure 1 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present disclosure. The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and a non-aqueous electrolyte (not shown). 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 of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, 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 electrolyte injection hole, which is sealed by a seal 8 after the electrolyte is injected.

[0058] 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.

[0059] <Examples 1-17> [Fabrication of the positive electrode] Lithium fluoride (LiF), lithium peroxide (Li2O2), lithium manganese oxide (LiMnO2), and an oxide of a first metal element were mixed in a predetermined mass ratio. The oxide of the first metal element used was Gd2O3, CeO2, Eu2O3, Sm2O3, or Yb2O3. The 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.

[0060] 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 positive electrode slurry was prepared using N-methyl-2-pyrrolidone (NMP) as the dispersion medium. Next, the positive electrode slurry was applied to aluminum foil, which served as the positive electrode current collector. After drying and compressing the coating, the film was cut to a predetermined size to obtain the positive electrode.

[0061] In this way, lithium metal composite oxides X1 to X17 shown in Tables 1 and 2 were synthesized as positive electrode active materials, and positive electrodes using lithium metal composite oxides X1 to X17 were obtained.

[0062] [Electrolyte preparation] 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.

[0063] [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.

[0064] Secondary batteries A1 to A17 were fabricated using positive electrodes made from lithium metal composite oxides X1 to X17, respectively. Secondary batteries A1 to A17 correspond to Examples 1 to 17.

[0065] <Comparative Example 1> In preparing the positive electrode, 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 in the same manner as in Example 1 and treated at room temperature in an Ar atmosphere to obtain a lithium metal composite oxide Y1 having the composition shown in Table 1 as the positive electrode active material (Comparative Example 1).

[0066] Using the obtained lithium metal composite oxide Y1, a positive electrode was fabricated in the same manner as in Example 1 to obtain a test secondary battery B1.

[0067] For lithium metal composite oxides X1-X17 and Y1, the X-ray diffraction patterns of the composite oxides were measured and analyzed using a powder X-ray diffractometer. From 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.

[0068] [evaluation] The initial discharge capacity was measured for each of the secondary batteries A1-A17 and B1.

[0069] (Initial discharge capacity) The secondary battery was charged with a constant current of 0.1C at room temperature until the voltage reached 4.95V, and then charged with a constant voltage of 4.95V until the current reached 0.01C. After a 20-minute pause, it was discharged with a constant current of 0.1C until the voltage reached 2.5V. The discharge capacity at this time was measured. The discharge capacity per unit mass of the positive electrode active material (lithium metal composite oxide) was determined and used as the initial discharge capacity.

[0070] (Average discharge voltage) In measuring the initial discharge capacity of secondary batteries A1 and B1, the time average of the battery voltage was determined from the time change of the battery voltage during constant current discharge, and this was defined as the average discharge voltage.

[0071] Table 1 shows the evaluation results for initial discharge capacity and average discharge voltage for secondary batteries A1 and B1. Table 2 shows the evaluation results for initial discharge capacity for secondary batteries A2 to A17.

[0072] [Table 1]

[0073] [Table 2]

[0074] As shown in Table 1, in battery A1, the initial discharge capacity was improved compared to battery B1, which does not contain Gd, by including Gd in the lithium metal composite oxide. In battery A1, the average discharge voltage was higher than in battery B1.

[0075] As shown in Table 2, the initial discharge capacity was improved in batteries A2 to A5 by including Gd in the lithium metal composite oxide.

[0076] As shown in Table 2, in batteries A6 to A17, the initial discharge capacity was improved compared to battery B1, which did not contain Ce, Eu, Sm, and Yb, by including these elements in the lithium metal composite oxide. [Industrial applicability]

[0077] The positive electrode active material for secondary batteries according to this disclosure is suitably used in secondary batteries where high capacity is required. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of Symbols]

[0078] 1: Electrode group, 2: Positive lead, 3: Negative lead, 4: Battery case, 5: Sealing plate, 6: Negative terminal, 7: Gasket, 8: Sealing plug

Claims

1. It contains a lithium metal composite oxide having a crystalline structure based on a rock salt structure belonging to space group Fm-3m, The lithium metal composite oxide is represented by the formula: Li a Mn b M1 c M2 d O 2-e Fe e, During the ceremony, M1 is at least one selected from the group consisting of Gd, Ce, Eu, Sm, and Yb. M2 is a metallic element other than Li, Mn, Gd, Ce, Eu, Sm, and Yb. Satisfying 0 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.15, 0 ≤ d ≤ 0.1, 0 ≤ e ≤ 0.75, and 1.75 ≤ a + b + c + d ≤ 2, Positive electrode active material for secondary batteries.

2. The lithium metal composite oxide has vacancies at the cation sites in the crystal structure, as described in claim 1, for a positive electrode active material for a secondary battery.

3. The lithium metal composite oxide comprises fluorine, as described in claim 1 or 2, for a positive electrode active material for a secondary battery.

4. The positive electrode active material for a secondary battery according to claim 1 or 2, wherein M2 is at least one selected from the group consisting of Ti, Ni, Co, Sn, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Pr, Dy, and Er.

5. It comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises the positive electrode active material for a secondary battery described in claim 1 or 2, in a secondary battery.

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