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 Cu and a transition metal element M1 that is other than Li and Cu. It is preferable that the lithium metal composite oxide is represented by composition formula LiaMnbCucA2 dO2-eFe (wherein A2 represents at least one element excluding Li, Mn, Cu, O and F; and 0 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.2, 0 ≤ d ≤ 0.2, 0 ≤ e ≤ 0.66 and 1.75 ≤ a + b + c + d ≤ 2 are satisfied).

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 of their 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 (e.g., 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, Li-excess type lithium metal composite oxides based on the rock salt structure Li 1+x Mn 1-x O2 have attracted attention.

[0004] Patent Document 1 discloses a positive electrode active material containing a lithium transition metal composite oxide having a crystal structure belonging to the space group Fm-3m and represented by the composition formula Li 1+x Nb y Me z A p O2 (Me is a transition metal containing Fe and / or Mn, 0 < x < 1, 0 < y < 0.5, 0.25 ≦ z < 1, A is an element other than Nb and Me, 0 ≦ p ≦ 0.2, provided that Li 1+p Fe 1-q Nb q O2 is excluded when 0.15 < p ≦ 0.3 and 0 < q ≦ 0.3).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

[0006] As described in Patent Document 1, lithium metal composite oxides based on rock salt structure Li 1+x Mn 1-x O2 tend to have a low average discharge voltage. Also, the distribution of the discharge voltage is wide, and there is a large amount of capacity that cannot be actually used. Therefore, the high capacity as expected has not been achieved, 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 includes Cu, and a transition metal element M other than Li and Cu 1 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, 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 a secondary battery.

[0009] According to the present disclosure, since the average discharge voltage of the lithium metal composite oxide having a rock salt structure can be increased, it becomes easy to realize a secondary battery with a high energy density.

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 includes Cu, and a transition metal element M other than Li and Cu 1 and. When the lithium metal composite oxide contains Cu, the average discharge voltage increases.

[0012] 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 (Cu and transition metal elements M) at the cation sites. 1 It may have a structure in which elements (including) are arranged irregularly.

[0013] Lithium metal composite oxides are composed of transition metal elements M 1 Preferably, it contains Mn. The molar ratio of Mn in the lithium metal composite oxide is the ratio of Mn to the transition metal elements excluding Mn. 1 The molar ratio of the total of the metals and Cu may be greater than the total molar ratio of the metals. In other words, 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, 1+x Mn 1-x O2 is one example.

[0014] In the above crystal structure, the cation sites contain Li atoms and transition metal elements M 1 It may have vacancies in which lithium atoms or metal atoms are not present. Here, having vacancies means that in the positive electrode active material extracted from a secondary battery immediately after manufacturing or after disassembly in a discharged state, 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 where lithium atoms or metal atoms can be placed. Having vacancies makes it easier for lithium ions to move through the vacancies and further improves the capacity.

[0015] Lithium metal composite oxides may contain fluorine (F). Fluorine can substitute for oxygen atoms at anion sites in the above crystal structure. This stabilizes the Li-rich state, resulting in high capacity. Furthermore, the substitution of fluorine atoms increases the average discharge potential. The Li-rich 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.

[0016] In the above lithium metal composite oxide, since the arrangement of Li in the cation site is irregular and the bonding states of Li are various, the width of the voltage distribution associated with 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, with the addition of Cu, the average discharge potential increases. In addition, due to the introduction of fluorine atoms, the voltage distribution associated with Li release moves to the higher potential side, making it easier to utilize the trailing part as capacity. As a result, the available capacity further increases.

[0017] Examples of the lithium metal composite oxide include those represented by the composition formula Li a Mn b Cu c A 2 d O 2-e F e (where 0 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.2, 0 ≤ d ≤ 0.2, 0 ≤ e ≤ 0.66, 1.75 ≤ a + b + c + d ≤ 2). Here, A 2 is at least one other element excluding Li, Mn, Cu, O, and F.

[0018] In the above composition formula, the x value represented by 2 - a - b - c - d (= x) represents the molar ratio of the vacancies present in 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 + d ≤ 1.98 is preferable, a + b + c + d ≤ 1.95 is more preferable, and a + b + c + d ≤ 1.9 is even more preferable. Also, the molar ratio of vacancies x is more preferably x ≤ 0.15 (a + b + c + d ≥ 1.85).

[0019] 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).

[0020] Alternatively, vacancies and their content ratio may be evaluated using a method that utilizes positron annihilation.

[0021] 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.

[0022] Lithium metal composite oxides contain elements other than Li, Mn, Cu, O, and F. 2 It may contain element A. 2 The composite material may include at least one selected from the group consisting of Ni, Fe, Co, Al, Sn, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, P, W, Ge, Si, Ga, La, Ce, Pr, Sm, Eu, Dy, and Er. Among lithium metal composite oxides, element A 2 Preferably, it contains at least one selected from the group consisting of Ni, Sn, W, Ge, Fe, Ta, P, Al, and Zn.

[0023] The above lithium metal composite oxides include, for example, lithium fluoride (LiF) and transition metal elements M. 1 Lithium oxides (e.g., lithium manganese oxide (LiMnO2) and copper oxide (CuO)) can be synthesized by mixing them 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 mixing them, 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.).

[0024] 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.

[0025] [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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] [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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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 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 contained 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.

[0037] 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.

[0038] 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.

[0039] [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. Also, as described later, a solid electrolyte may be used.

[0040] 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.

[0041] Other non-aqueous solvents include cyclic ethers, linear ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0048] The electrolyte may be a solid electrolyte. Ionic conductive inorganic solid electrolytes can be used as solid electrolytes. Examples of inorganic solid electrolytes include sulfides, hydrides, and oxides. Hydrides generally include solid electrolytes called complex hydrides. The crystalline state of the solid electrolyte is not particularly limited and may be either crystalline or amorphous.

[0049] Examples of sulfides (sulfide-based solid electrolytes) include those containing Li2S and one or more sulfides containing at least one element selected from the group consisting of Group 13, Group 14, and Group 15 elements of the periodic table. Specific examples of sulfides include Li2S-SiS2, Li2S-P2S5, Li2S-GeS2, Li2S-B2S3, Li2S-Ga2S3, Li2S-Al2S3, Li2S-GeS2-P2S5, Li2S-Al2S3-P2S5, Li2S-P2S3, Li2S-P2S3-P2S5, LiI-Li2S-P2S5, LiI-Li2S-SiS2, LiI-Li2S-B2S3, and LiI-Li2S-P2O5. Other examples such as LiI-Li3PO4-P2S5 may also be used.

[0050] Examples of hydrides (hydride-based solid electrolytes) include lithium borohydride complex hydrides. Examples of complex hydrides include LiBH4-LiI complex hydrides, LiBH4-LiNH2 complex hydrides, LiBH4-P2S5, and LiBH4-P2I4.

[0051] Examples of oxides (oxide-based solid electrolytes) include LiPON, Li3PO4, Li2SiO2, Li2SiO4, Li 0.5 La 0.5 TiO3, Li 1.3 Al 0.3 Ti 0.7 (PO4)3, La 0.51 Li 0.34 TiO 0.74 Li 1.5 Al 0.5 Ge 1.5 Examples include (PO4)3.

[0052] These solid electrolytes may be used individually, or two or more may be used in combination as needed.

[0053] [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.

[0054] 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.

[0055] Figure 1 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present disclosure.

[0056] 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.

[0057] 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.

[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 1-3> [Fabrication of the positive electrode] Lithium fluoride (LiF), lithium manganese oxide (LiMnO2), and copper oxide (CuO) 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.

[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 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.

[0061] In Examples 1 to 3, lithium metal composite oxides X1 to X3 with different compositions were synthesized in this manner, and cathodes were obtained using each of the lithium metal composite oxides X1 to X3.

[0062] [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.

[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 A3 were fabricated using positive electrodes each employing lithium metal composite oxides X1 to X3 as the positive electrode active material. Secondary batteries A1 to A3 correspond to Examples 1 to 3.

[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 dominant 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 a predetermined composition.

[0066] <Comparative Example 2> In preparing the positive electrode, lithium fluoride (LiF), lithium peroxide (Li2O2), and lithium manganate (LiMnO2) were mixed in a predetermined mass ratio. This mixed powder was placed in a dominant 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 Y2 having a predetermined composition.

[0067] Using the obtained lithium metal composite oxides Y1 and Y2, positive electrodes were fabricated in the same manner as in Example 1 to obtain test secondary batteries B1 and B2.

[0068] For lithium metal composite oxides X1-X3, Y1, and Y2, 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.

[0069] [evaluation] (Average discharge voltage) A secondary battery was charged at room temperature with a constant current of 0.05C until the battery voltage reached 4.95V. After a 20-minute pause, it was discharged at a constant current of 0.2C until the battery voltage reached 2.5V, and the discharge capacity was measured. The time-averaged battery voltage during constant-current discharge was calculated from the time change in the battery voltage and defined as the average discharge voltage V0.

[0070] Table 1 shows the evaluation results of the average discharge voltage V0, along with the composition of the lithium metal composite oxide used as the positive electrode active material in each battery.

[0071] As shown in Table 1, in batteries A1 to A3 of Examples 1 to 3, the average discharge voltage increased compared to batteries B1 and B2 of Comparative Examples 1 and 2, which did not contain Cu, by including Cu in the lithium metal composite oxide.

[0072] [Table 1] [Industrial applicability]

[0073] The secondary battery described herein has a high capacity and is useful as the main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0074] 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 aforementioned lithium metal composite oxide has the compositional formula Li ａ Mn ｂ Cu ｃ A ２ ｄ O ２－ｅ F ｅ (However, A ２ (where is at least one element other than Li, Mn, Cu, O, and F), 1 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.2 (excluding the range 0 < c < 0.1), 0 ≤ d ≤ 0.2, 0.1 ≤ e ≤ 0.58, and, 1. A positive electrode active material for secondary batteries that satisfies the conditions 1.75 ≤ a + b + c + d ≤ 2.

2. The molar ratio of Mn in the lithium metal composite oxide is greater than the total molar ratio of the transition metal element contained in the element A ２ and Cu, and the positive electrode active material for a secondary battery according to claim 1.

3. 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 as described in claim 1 or 2.

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

5. A positive electrode active material for a secondary battery according to any one of claims 1 to 4, wherein the composition formula satisfies 1.85 ≤ a + b + c + d ≤ 1.

98.

6. In the lithium metal composite oxide, element A ２ The positive electrode active material for a secondary battery according to any one of claims 1 to 5, wherein the positive electrode active material comprises at least one selected from the group consisting of Ni, Fe, Co, Al, Sn, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, P, W, Ge, Si, Ga, La, Ce, Pr, Sm, Eu, Dy, and Er.

7. In the lithium metal composite oxide, element A ２ The positive electrode active material for a secondary battery according to claim 6, wherein the positive electrode active material comprises at least one selected from the group consisting of Ni, Sn, W, Ge, Fe, Ta, P, Al, 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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