Positive electrode active material for secondary batteries, and secondary battery

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

Positive electrode active material for secondary battery and secondary battery

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

[0002] Secondary batteries, especially lithium-ion secondary batteries, have high power output and high energy density, and are expected to be used in small consumer applications, power storage devices, and electric vehicles. The cathode active material of lithium-ion secondary batteries is a composite oxide of lithium and a transition metal (e.g., cobalt). Replacing part of the cobalt with nickel can increase the battery capacity.

[0003] On the other hand, in recent years, in response to the demand for high energy density, the rock salt structure Li １＋ｘ Mn １－ｘ O ２ Li-excess lithium metal composite oxides based on the above have been attracting attention.

[0004] Patent Document 1 describes a compound having a crystal structure belonging to the space group Fm-3m and having the composition formula Li １＋ｘ Nb ｙ Me ｚ A ｐ O ２ (Me is a transition metal including Fe and / or Mn, 0<x<1, 0<y<0.5, 0.25≦z<1, A is an element other than Nb and Me, 0≦p≦0.2, provided that Li １＋ｐ Fe １－ｑ Nb ｑ O ２ and excluding those in which 0.15<p≦0.3 and 0<q≦0.3), a positive electrode active material containing a lithium transition metal composite oxide represented by the formula (2) is disclosed.

[0005] Patent No. 6197029 specification

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

[0007] In view of the above, one aspect of the present disclosure relates to a positive electrode active material for a secondary battery, comprising 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 comprising a first metal element other than Li and a second metal element other than Li and the first metal element, and the first metal element being at least one selected from the group consisting of Gd, Ce, Eu, Sm, and Yb.

[0008] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the above-described positive electrode active material for a secondary battery.

[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 set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away;

[0011] The following describes embodiments of the present disclosure using 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 are obtained. In this specification, the term "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 lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0012] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0013] In the following description, the terms "contain" or "comprise" encompass "contain (or include)," "consist essentially of," and "consist of."

[0014] The secondary battery includes at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries.

[0015] A 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. The crystal structure based on a rock salt structure belonging to the space group Fm-3m may be any crystal structure that can be assigned to the space group Fm-3m. In other words, this lithium metal composite oxide has a crystal structure similar to a rock salt structure belonging to the 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 oxide may have a crystal structure based on a rock salt structure, such as that represented by NaCl, in which oxygen atoms are arranged at the anion sites and Li atoms and metal atoms other than Li (including the first metal element and the second metal element) are irregularly arranged at the cation sites.

[0017] The inclusion of the first metal element increases the capacity of the lithium metal composite oxide having the above-mentioned crystal structure. Although the reason for this is unclear, it is presumed that one factor is that the large ionic radius of the first metal element affects the ease of lithium ion migration. Gd and Ce are preferred in that they can increase the average discharge potential.

[0018] In the above crystal structure, the cation sites may have vacancies where Li atoms and metal atoms (atoms of the first metal element and the second metal element) are not arranged. Here, "having vacancies" refers to the presence of vacancies not filled with Li atoms or metal atoms in the lithium metal composite oxide in the positive electrode active material immediately after production or taken out by disassembling a secondary battery in a discharged state. 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 where lithium atoms or metal atoms can be arranged. The presence of vacancies facilitates the movement of lithium ions through the vacancies, further improving capacity.

[0019] The lithium metal composite oxide may contain fluorine (F). Fluorine can substitute oxygen atoms at the anion site in the crystal structure. This stabilizes the Li-excess state, resulting in high capacity. Furthermore, the substitution of fluorine atoms increases the average discharge potential. The Li-excess state refers to a state in which the number of Li atoms in the composite oxide is greater than the number of transition metal atoms.

[0020] In the lithium metal composite oxide, the arrangement of Li at the cation site is irregular and the bonding state of Li is varied, resulting in a wide voltage distribution associated with Li release. This can make it difficult to utilize the low-potential tail of the voltage distribution as capacitance. However, the introduction of fluorine atoms shifts the voltage distribution associated with Li release to the high-potential side, making it easier to utilize the tail as capacitance. This further increases the available capacity.

[0021] The second metal element may contain 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 greater than the total molar ratio of the first metal element and the second metal element excluding Mn. In other words, the lithium metal composite oxide may be based on a composite oxide of Li and Mn. Examples of such a composite oxide of Li and Mn include Li, １＋ｘ Mn １－ｘ O ２ Examples include:

[0022] The lithium metal composite oxide has, for example, the composition formula: Li ａ Mn ｂ M1 ｃ M2 ｄ O ２－ｅ F ｅ In the formula, M1 is at least one element 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 composition formula satisfies the following conditions: 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 effects of adding M1, the ratio c of M1 may be 0.005 or more and 0.1 or less, or may be 0.005 or more and 0.075 or less, and is preferably 0.025 or more and 0.075 or less.

[0024] In the above formula, M2 includes a second metal element excluding the first metal element and Mn. M2 may include at least one element 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 more and 0.06 or less (or 0.04 or less).

[0025] In the above composition formula, the x value represented by 2-a-b-c-d (= x) represents the molar ratio of vacancies present in the cation sites. According to the above composition formula, the molar ratio x of vacancies is 0≦x≦0.25. The molar ratio x of vacancies is preferably 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 more preferably x≦0.15 (a+b+c+d≧1.85).

[0026] The vacancies and the vacancy content ratio can be derived based on the crystal structure and composition of the lithium metal composite oxide. For example, in the case of a crystal structure similar to a rock salt structure belonging to the space group Fm-3m, the vacancy content ratio can be determined by determining the composition of the lithium metal composite oxide and calculating x = 2-a-b-c-d from the composition 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., a desktop X-ray diffractometer MiniFlex manufactured by Rigaku Corporation, X-ray source: CuKα). The composition of the lithium metal composite oxide can be measured using an ICP optical emission spectrometer (iCAP6300 manufactured by Thermo Fisher Scientific).

[0027] Alternatively, vacancies and the vacancy content may be evaluated by a method utilizing positron annihilation.

[0028] As shown in the composition formula, some of the oxygen atoms in the anion site may be substituted with fluorine atoms. This stabilizes the Li-excess state (a>1) and results in high capacity. Furthermore, as described above, the average discharge potential increases, further increasing the available capacity. When some of the oxygen atoms are substituted with fluorine atoms, the substitution ratio e of fluorine atoms in the composition 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 lithium metal composite oxide may be, for example, lithium fluoride (LiF), an oxide of a first metal element (for example, lithium manganate (LiMnO) belonging to the space group Fm-3m), or the like. ２ )) and an oxide of the second metal element can be synthesized by mixing them in an inert gas atmosphere such as Ar using a planetary ball mill. ２ O and Mn ２ O ３ In addition to the above raw materials, lithium peroxide (Li ２ O ２) and mixing, a lithium metal composite oxide having vacancies can be synthesized. Instead of the 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 may be, for example, LiF and LiMnO ２ The mixing ratio and mixing conditions (rotation speed, processing time, processing temperature, etc.) can be adjusted to fall within the desired range.

[0030] The secondary battery according to the embodiment of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains the above-described positive electrode active material for a secondary battery. The secondary battery according to the embodiment of the present 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 applying a positive electrode slurry in which the positive electrode mixture is dispersed in a dispersion medium to the surface of the positive electrode current collector and drying it. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector.

[0032] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, etc. Known materials can be used as the binder and the conductive agent.

[0033] The positive electrode active material includes the above-mentioned lithium metal composite oxide having a crystal structure similar to a rock salt structure belonging to the space group Fm-3m. The composite oxide is, for example, a secondary particle formed by aggregation of a plurality of 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 a volume-based particle size distribution is 50%, and is measured using a laser diffraction particle size distribution analyzer.

[0034] The content of elements constituting the composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0035] The positive electrode active material may further contain other lithium metal composite oxides other than the above-mentioned lithium metal composite oxides. Examples of other lithium metal composite oxides include 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 Examples of suitable lithium-ion batteries include Li2O4, LiMePO4, and Li2MePO4F. Here, M is at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element (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. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0036] The positive electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the positive electrode current collector include stainless steel, aluminum, an aluminum alloy, and titanium.

[0037] [Negative Electrode] The negative electrode may have, for example, a negative electrode current collector and may include a negative electrode mixture layer supported on the negative electrode current collector. The negative electrode mixture layer may be formed, for example, by applying a negative electrode slurry in which the negative electrode mixture is dispersed in a dispersion medium to the surface of the negative electrode current collector and drying it. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector. The negative electrode mixture layer may be a negative electrode active material layer. Alternatively, a lithium metal foil or a 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 contain optional components such as a binder, a conductive agent, etc. Known materials can be used as the binder and the conductive agent.

[0039] The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions, lithium metal, and / or a lithium alloy. Examples of materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxides and tin oxides formed by bonding these with oxygen may also be used.

[0040] Examples of alloy-based materials containing silicon include a lithium ion conductive phase and a silicon composite material in which a silicon phase (e.g., silicon particles) is dispersed in the lithium ion conductive phase. Examples of the lithium ion conductive phase include a silicon oxide phase, a silicate phase, and / or a carbon phase. The main component of the silicon oxide phase (e.g., 95 to 100% by mass) can be silicon dioxide. Among these, composite materials composed of a silicate phase and a silicon phase dispersed in the silicate phase are preferred because of their high capacity and low irreversible capacity.

[0041] The silicate phase may contain, for example, at least one element selected from the group consisting of Group 1 elements and Group 2 elements of the long periodic table. Examples of Group 1 elements and Group 2 elements of the long 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, the lithium silicate phase is preferred due to its 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 It may have a composition expressed as (0<z<2). It is preferable that z satisfies the relationship 0<z<1, and more preferably z=1 / 2.

[0043] Examples of elements other than Li, Si, and O that can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0044] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other.

[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, and copper alloy.

[0046] [Electrolyte] The electrolyte includes a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that ionically dissociates in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and the solute are additives. The electrolyte may include 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 may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that can be used 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). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0048] Other examples of 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 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, and tetraethylene glycol dimethyl ether.

[0051] These solvents may be fluorinated solvents in which some of the hydrogen atoms are substituted with fluorine atoms, such as fluoroethylene carbonate (FEC).

[0052] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF ２ O ２ , LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(CF4F9SO2), LiN(CF5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0053] The concentration of the lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or may be 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having 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, such as 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0055] [Separator] It is desirable to have a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0056] The secondary battery may include, for example, a wound electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, or a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween. The secondary battery may be in any form, such as a cylindrical type, a prismatic type, a coin type, a button type, or a laminate type. In the present disclosure, the type, shape, etc. of the secondary battery are not particularly limited.

[0057] FIG. 1 is a schematic perspective view, partially cut away, of a prismatic secondary battery according to an embodiment of the present disclosure. The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. 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 is laser-welded. The sealing plate 5 has an electrolyte injection hole, which is closed with a seal plug 8 after injection.

[0058] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0059] Examples 1 to 17 [Preparation of Positive Electrode] Lithium fluoride (LiF), lithium peroxide (Li ２ O ２ ), lithium manganese oxide (LiMnO ２ ) and an oxide of the first metal element were mixed in a predetermined mass ratio. ２ O ３ , CeO ２ , Eu ２ O ３ , Sm ２ O ３ , or Yb ２ O ３ The mixed powder was charged into a planetary ball mill (Premium-Line P7 manufactured by Fritsch, rotation speed: 600 rpm, container: 45 mL, ball: Zr ball with a diameter of 5 mm) and treated in an Ar atmosphere at room temperature for 35 hours (35 cycles of operation for 1 hour followed by a 10-minute break), thereby obtaining 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 N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to an aluminum foil positive electrode current collector, and the coating was dried, compressed, and then cut to a predetermined size to obtain a positive electrode.

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

[0062] [Preparation of Electrolyte] 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) at a predetermined volume ratio.

[0063] [Preparation of Test Cell] A test cell was prepared using the above-described positive electrode and a negative electrode counter electrode made of lithium metal foil. The above-described positive electrode and negative electrode counter electrode were arranged opposite each other with a separator interposed therebetween to form an electrode assembly, and the electrode assembly was housed in a coin-shaped outer can. After injecting an electrolyte into the outer can, the outer can was sealed to obtain a coin-shaped test secondary battery.

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

[0065] Comparative Example 1 In the preparation of a positive electrode, lithium fluoride (LiF) and lithium manganese oxide (LiMnO ２ ) were mixed in a predetermined mass ratio. The mixed powder was charged into a planetary ball mill and treated in an Ar atmosphere at room temperature in the same manner as in Example 1, to obtain lithium metal composite oxide Y1 having the composition shown in Table 1 as a positive electrode active material (Comparative Example 1).

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

[0067] For each of the lithium metal composite oxides X1 to 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 positions of XRD peaks, it was confirmed that the composite oxides had a crystal structure based on a rock salt type belonging to the space group Fm-3m.

[0068] [Evaluation] The initial discharge capacity of each of the secondary batteries A1 to A17 and B1 was measured.

[0069] (Initial Discharge Capacity) The secondary battery was subjected to constant current charging at a current of 0.1 C in a room temperature environment until the voltage reached 4.95 V, and then constant voltage charging at a voltage of 4.95 V until the current reached 0.01 C. After a 20-minute break, constant current discharging was performed at a current of 0.1 C until the voltage reached 2.5 V. The discharge capacity at this time was measured. The discharge capacity per 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 calculated from the time change during constant current discharge, and this was taken as the average discharge voltage.

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

[0072]

[0073]

[0074] As shown in Table 1, the initial discharge capacity of Battery A1 was improved by incorporating Gd into the lithium metal composite oxide compared to Battery B1, which did not contain Gd. Battery A1 also had a higher average discharge voltage than Battery B1.

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

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

[0077] The positive electrode active material for a secondary battery according to the present disclosure is suitable for use in secondary batteries requiring high capacity. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and variations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to include all modifications and variations that do not depart from the true spirit and scope of the present invention.

[0078] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug

Claims

1. comprising a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to the space group Fm-3m wherein 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 selected from the group consisting of Gd, Ce, Eu, Sm, and Yb, and is a positive electrode active material for a secondary battery.

2. The positive electrode active material for a secondary battery according to claim 1, wherein the second metal element contains Mn.

3. The positive electrode active material for a secondary battery according to claim 1 or 2, wherein the lithium metal composite oxide has pores in the cation sites in the crystal structure.

4. The positive electrode active material for a secondary battery according to claim 1 or 2, wherein the lithium metal composite oxide contains fluorine.

5. The lithium metal composite oxide has the formula: Li ａ Mn ｂ M1 ｃ M2 ｄ O ２－ｅ F ｅ and is represented by wherein 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; 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; The positive electrode active material for a secondary battery according to claim 1 or 2.

6. The positive electrode active material for a secondary battery according to claim 5, 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.

7. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte; wherein the positive electrode contains the positive electrode active material for a secondary battery according to claim 1 or 2.