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 Ti and a metal element M1 that is other than Li and Ti. It is preferable that the metal element M1 additionally contains at least one element that is selected from the group consisting of Fe, Ge, Si and Ga.

Description

[Technical Field]

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

[0002] Secondary batteries, particularly lithium ion secondary batteries, have high output and high energy density, and thus are expected to be used as power supplies for small consumer applications, power storage devices, and electric vehicles. As a positive electrode active material for lithium ion secondary batteries, a composite oxide of lithium and a transition metal (e.g., cobalt) is used. Increasing the capacity can be achieved by replacing part of cobalt with nickel.

[0003] On the other hand, in recent years, in response to the demand for high energy density, rock salt-structured Li 1+x Mn 1-x O₂-based lithium-excess lithium metal composite oxides have attracted attention.

[0004] Patent Document 1 discloses that a lithium transition metal composite oxide having a crystal structure belonging to the space group Fm-3m and represented by the compositional formula Li 1+x Nb y Me z A p O₂ (wherein 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, excluding those which are Li 1+p Fe 1-q Nb q O₂ where 0.15<p≦0.3 and 0<q≦0.3) is contained as a positive electrode active material. [Prior Art Literature] [Patent Literature]

[0005] [Patent Document 1] Japanese Patent No. 6197029 Specification [Summary of the Invention]

[0006] In Patent Document 1, a high capacity is achieved by controlling the composition (i.e., adding Nb). However, the capacity improvement effect is insufficient, and there still remains 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, wherein the lithium metal composite oxide includes Ti, and a metal element M other than Li and Ti 1 and relates to a positive electrode active material for a secondary battery comprising the same.

[0008] Another aspect of the present disclosure relates to a secondary battery comprising 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 above positive electrode active material for a secondary battery.

[0009] According to the present disclosure, a secondary battery with high energy density can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] [Figure 1] FIG. 1 is a schematic perspective view with a part cut away of a secondary battery according to an embodiment of the present disclosure. MODES FOR CARRYING OUT THE INVENTION

[0011] 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. That is, 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 Ti, and a metal element M other than Li and Ti 1 and comprises the same.

[0012] The above lithium metal composite oxide has a crystal structure based on a rock salt structure, typified by NaCl for example, in which oxygen atoms are arranged at anion sites, and Li atoms and metal atoms other than Li (including Ti and the metal element M 1 ) can have a structure in which they are irregularly arranged.

[0013] The presence of Ti increases the capacity of the lithium metal composite oxide having the above crystal structure. The reason for this is not clear, but one possible reason is that in the lithium metal composite oxide, Ti has empty d orbitals. 4+ It is thought that this is because the rock salt structure, which has high symmetry, is more likely to be stable because it exists in this form. On the other hand, when electrons occupy part of the d orbital, a local structure with lower symmetry than the rock salt structure is formed, and the d orbital (for example, (t 2g ) 3 (e g ) 1 , (t 2g ) 6 (e g ) 1 In electron configurations such as those mentioned above (or when these electron configurations can be taken as perturbed states), the energy levels split, and electrons occupy lower-energy orbitals, which can lead to a more stable crystal structure with lower symmetry than the rock salt structure. However, Ti 4+ Because the d orbitals are empty, the symmetry of the rock salt structure is less likely to decrease, and the rock salt structure can be stabilized even after repeated charging and discharging.

[0014] Metal element M 1 Preferably, it contains at least one selected from the group consisting of Fe, Ge, Si, and Ga. In this case, the average discharge potential can be increased.

[0015] In the above crystal structure, the cation sites contain Li atoms and metal atoms (transition metal element M). 1 and metal element M 2 The lithium metal composite oxide may have vacancies in which lithium atoms are not present. 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 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.

[0016] The lithium metal composite oxide may contain fluorine (F). In the above crystal structure, fluorine can replace oxygen atoms at anion sites. This stabilizes the lithium-excess state and enables a high capacity to be obtained. Additionally, the substitution of fluorine atoms increases the average discharge potential. Note that the lithium-excess state refers to a state where the number of lithium atoms in the composite oxide is greater than the number of transition metal atoms.

[0017] In the above lithium metal composite oxide, the arrangement of Li at cation sites is irregular, and the bonding states of Li vary, so the voltage distribution accompanying Li release has a wide range. For this reason, it can be difficult to utilize the tail portion on the low potential side of the voltage distribution as capacity. However, the introduction of fluorine atoms shifts the voltage distribution accompanying Li release toward the high potential side, which makes it easier to utilize the tail portion as capacity. This further increases the available capacity.

[0018] The lithium metal composite oxide contains a transition metal element M 1 that preferably includes Mn. The molar ratio of Mn in the lithium-containing composite oxide may be greater than the total molar ratio of the metal element M excluding Mn and Ti. That is, 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 and Ti and the total molar ratio may be greater than that. That is, 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 O₂.

[0019] Examples of the lithium metal composite oxide include those represented by the composition formula Li a Mn b Ti c M 2 d O 2-e F e (wherein 0 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0 < c ≤ 0.15, 0 ≤ d ≤ 0.1, 0 ≤ e ≤ 0.66, 1.75 ≤ a+b+c+d ≤ 2 is satisfied). Here, M 2 is the aforementioned metal element M 1This is obtained by removing Mn. The Ti ratio c is more preferably 0.025 ≤ c ≤ 0.1, and even more preferably 0.025 ≤ c ≤ 0.075.

[0020] 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).

[0021] 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).

[0022] Alternatively, vacancies and their content ratio may be evaluated using a method that utilizes positron annihilation.

[0023] 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.

[0024] Lithium metal composite oxides are composed of elements other than Li, Mn, and Ti, such as M 2 It may also contain the metal element M. 2 It may include at least one selected from the group consisting of Fe, Ge, Si, Ga, Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Sm, Eu, Dy, and Er. Among these, the metallic element M 2 Therefore, it is preferable to include at least one selected from the group consisting of Fe, Ge, Si, and Ga, as this allows for a higher average discharge potential.

[0025] The above lithium metal composite oxides include, for example, lithium fluoride (LiF), element M 1 Oxides of (for example, lithium manganate (LiMnO2) belonging to space group Fm-3m), and element M 2 The oxides can be synthesized by mixing them in an inert gas atmosphere such as Ar using a planetary ball mill. Li2O and Mn2O3 may be used as raw materials. In addition, lithium metal composite oxides with vacancies can be synthesized by adding lithium peroxide (Li2O2) to the above raw materials and mixing them. Instead of a planetary ball mill, a mixer capable of applying a similar stirring shear force to the powder may be used, and the powder may be heated during the mixing process. The composition of the composite oxide can be adjusted to the desired range by changing, for example, the mixing ratio of LiF and LiMnO2 and the mixing conditions (rotation speed, processing time, processing temperature, etc.).

[0026] Next, a secondary battery according to the embodiments of this disclosure will be described in detail. The secondary battery comprises, for example, a positive electrode, a negative electrode, an electrolyte, and a separator as follows.

[0027] [Positive electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode used is the positive electrode for secondary batteries described above. The positive electrode mixture layer can be formed, for example, by coating the surface of the positive electrode current collector with a positive electrode slurry in which a positive electrode mixture containing a positive electrode active material, a binder, etc., is dispersed in a dispersion medium and drying it. The dried coating may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0028] The positive electrode mixture layer contains positive electrode active material as an essential component and may contain optional components such as binders, thickeners, conductive agents, positive electrode additives, etc. Known materials can be used as binders, thickeners, and conductive agents.

[0029] The positive electrode active material includes the aforementioned lithium metal composite oxide having a crystal structure similar to a rock salt structure belonging to space group Fm-3m. The composite oxide is, for example, a secondary particle formed by the aggregation of multiple primary particles. The particle size of the primary particles is generally 0.05 μm to 1 μm. The average particle size of the composite oxide is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. Here, the average particle size of the composite oxide refers to the median diameter (D50) at which the cumulative frequency in the volume-based particle size distribution reaches 50%, and is measured by a laser diffraction particle size distribution analyzer.

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

[0031] As the positive electrode active material, the above-mentioned lithium metal composite oxide having a crystal structure similar to the above-mentioned rock salt structure may be used in combination with other known lithium metal oxides other than the above-mentioned lithium metal composite oxide. Other lithium metal oxides include, for example, Li a CoO2, Li a KiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c Li a Ni 1-b M b O c Li a Mn2O4, Li a Mn 2-b M b O 4、 LiMePO 4、 Examples include lithium transition metal composite oxides such as Li2MePO4F. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least one transition element (for example, at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. Note that the value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0032] The shape and thickness of the positive electrode current collector can be selected from the same shape and range as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0033] [Negative electrode] The negative electrode comprises, for example, a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by coating the surface of the negative electrode current collector with a negative electrode slurry in which a negative electrode mixture containing negative electrode active material, a binder, etc., is dispersed in a dispersion medium and drying it. The dried coating may be rolled if necessary. In other words, the negative electrode active material may be a mixture layer. Alternatively, lithium metal foil or lithium alloy foil may be attached to the negative electrode current collector. The negative electrode active material layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0034] The negative electrode active material layer contains the negative electrode active material as an essential component and may contain optional components such as binders, conductive agents, and thickeners. Known materials can be used as binders, conductive agents, and thickeners.

[0035] The negative electrode active material includes materials that electrochemically intercalate and release lithium ions, lithium metals, and / or lithium alloys. Examples of electrochemically intercalating and releasing lithium ions include carbon materials and alloying materials. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Among these, graphite is preferred due to its excellent charge-discharge stability and low irreversible capacity. Examples of alloying materials include those containing at least one metal capable of alloying with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide and tin oxide, which are formed by the bonding of these materials with oxygen, may also be used.

[0036] As alloy materials containing silicon, for example, a silicon composite material can be used, which consists of a lithium-ion conductive phase and silicon particles dispersed in the lithium-ion conductive phase. As the lithium-ion conductive phase, for example, a silicon oxide phase, a silicate phase, and / or a carbon phase can be used. The main component of the silicon oxide phase (e.g., 95-100% by mass) may be silicon dioxide. Among these, a composite material composed of a silicate phase and silicon particles dispersed in the silicate phase is preferred because it has high capacity and low irreversible capacity.

[0037] The silicate phase may, for example, contain at least one selected from the group consisting of Group 1 elements and Group 2 elements of the long-form periodic table. Examples of Group 1 elements and Group 2 elements of the long-form periodic table that can be used include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements such as aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), and titanium (Ti) may also be included. Among these, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferable because of its low irreversible capacity and high initial charge-discharge efficiency.

[0038] The lithium silicate phase only needs to be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si, O / Si, in the lithium silicate phase is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si, Li / Si, in the lithium silicate phase 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) It may have a composition represented by . z preferably satisfies the relationship 0<z<1, and z=1 / 2 is more preferable. 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), and aluminum (Al).

[0039] The carbon phase may, for example, be composed of amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other types of carbon.

[0040] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) and porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0041] [Electrolyte] An electrolyte comprises a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that undergoes ionic dissociation in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and solute are additives. Various additives may be included in the electrolyte. Electrolytes are usually used in liquid form, but they may also be in a state where their fluidity is restricted by gelling agents or other means.

[0042] The solvent used can be an aqueous or non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). The non-aqueous solvent may be used alone or in combination of two or more types.

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

[0044] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0045] Examples of linear ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and the like.

[0046] These solvents may be fluorinated solvents in which some of the hydrogen atoms are replaced by fluorine atoms. Fluoroethylene carbonate (FEC) may be used as the fluorinated solvent.

[0047] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10Lithium salts of fluorine-containing acids (such as LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0048] The lithium salt concentration in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

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

[0050] [Separator] A separator is interposed between the positive and negative electrodes. The separator has high ion permeability and possesses appropriate mechanical strength and insulating properties. Microporous thin films, woven fabrics, nonwoven fabrics, etc., can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred as the material of the separator.

[0051] One example of a secondary battery structure is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, and a non-aqueous electrolyte are housed in an outer casing. Alternatively, other forms of electrode groups may be used instead of the wound electrode group, such as a laminated electrode group in which the positive electrode and negative electrode are stacked with a separator. Secondary batteries may take any form, such as cylindrical, prismatic, coin-type, button-type, or laminated type.

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

[0053] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and a non-aqueous electrolyte. The electrode group 1 has a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting portion is laser-welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is sealed by a seal 8 after injection.

[0054] The structure of the secondary battery may be cylindrical, coin-shaped, or button-shaped, and may have a metal battery case. It may also be a laminated battery, which has a battery case made of a laminate sheet that is a laminate of a barrier layer and a resin sheet. In this disclosure, the type, shape, etc. of the secondary battery are not particularly limited.

[0055] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0056] <Examples 1-7> [Fabrication of the positive electrode] Lithium fluoride (LiF), lithium peroxide (Li2O2), lithium manganese oxide (LiMnO2), and titanium dioxide (TiO2) were mixed in a predetermined mass ratio. This mixed powder was placed in a planetary ball mill (Fritsch Premium-Line P7, rotation speed: 600 rpm, container: 45 mL, balls: φ5 mm Zr balls) and processed in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour operation followed by a 10-minute pause) to obtain a lithium metal composite oxide having a predetermined composition.

[0057] The obtained lithium metal composite oxide, acetylene black, and polyvinylidene fluoride were mixed in a solid content mass ratio of 7:2:1, and a cathode composite slurry was prepared using N-methyl-2-pyrrolidone (NMP) as the dispersion medium. Next, the cathode composite slurry was applied to a cathode core made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to obtain the cathode.

[0058] In Examples 1 to 7, lithium metal composite oxides X1 to X7 with different compositions were synthesized in this manner, and cathodes were obtained using each of the lithium metal composite oxides X1 to X7.

[0059] In Examples 3 and 4, iron oxide (Fe2O3) was further added to the above raw materials and mixed in a predetermined mass ratio to obtain lithium metal composite oxides X3 and X4, respectively.

[0060] In Example 5, silicon oxide (SiO2) was further added to the above raw materials and mixed in a predetermined mass ratio to obtain lithium metal composite oxide X5.

[0061] In Examples 6 and 7, germanium oxide (GeO2) was further added to the above raw materials and mixed in a predetermined mass ratio to obtain lithium metal composite oxides X6 and X7, respectively.

[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 A7 were fabricated using positive electrodes each employing lithium metal composite oxides X1 to X7 as the positive electrode active material. Secondary batteries A1 to A7 correspond to Examples 1 to 7.

[0065] <Comparative Examples 1 and 2> 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 (Comparative Example 1).

[0066] In Comparative Example 2, iron oxide (Fe2O3) was further added to the above raw materials and mixed in a predetermined mass ratio to obtain lithium metal composite oxide Y2.

[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] <Comparative Examples 3 and 4> In preparing the positive electrode, lithium peroxide (Li2O2), lithium fluoride (LiF), lithium manganese oxide (LiMnO2), and silicon oxide (SiO2) 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 Y3 having a predetermined composition (Comparative Example 3).

[0069] In Comparative Example 4, the above raw materials were mixed in a predetermined mass ratio with germanium oxide (GeO2) instead of silicon oxide (SiO2) to obtain lithium metal composite oxide Y4.

[0070] Using the obtained lithium metal composite oxides Y3 and Y4, positive electrodes were fabricated in the same manner as in Example 1 to obtain test secondary batteries B3 and B4.

[0071] For lithium metal composite oxides X1-X7 and Y1-Y4, 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.

[0072] [evaluation] (Initial discharge capacity) A secondary battery was charged at a constant current of 0.05C to a battery voltage of 4.95V under normal temperature conditions. After a 20-minute pause, it was discharged at a constant current of 0.2C to a battery voltage of 2.5V, and the discharge capacity was measured. The discharge capacity per unit mass of the positive electrode active material (lithium metal composite oxide) was determined and defined as the initial discharge capacity C0.

[0073] (Average discharge voltage) In the measurement of the initial discharge capacity described above, 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 V0.

[0074] Table 1 shows the evaluation results for initial discharge capacity C0 and average discharge voltage V0, along with the composition of the lithium metal composite oxide used as the positive electrode active material in each battery.

[0075] As shown in Table 1, in batteries A1 to A7 of Examples 1 to 7, the initial discharge capacity was improved compared to batteries B1 to B4 of Comparative Examples 1 to 4, which did not contain Ti, by including Ti in the lithium metal composite oxide.

[0076] In batteries A3 to A7, adding Fe, Si, and Ge to the lithium metal composite oxide in addition to Ti slightly reduces the initial discharge capacity, but tends to increase the average discharge voltage. On the other hand, as shown in batteries B2 to B4, when the lithium metal composite oxide does not contain Ti and only contains Fe, Si, or Ge individually, no effect of increasing the average discharge voltage is observed.

[0077] [Table 1] [Industrial applicability]

[0078] The secondary battery described herein provides a secondary battery with high capacity and excellent cycle characteristics. The secondary battery described herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of Symbols]

[0079] 1 electrode group 2 Positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing

Claims

1. A crystal structure based on a rock salt structure belonging to space group Fm-3m, comprising a lithium metal composite oxide having vacancies at the cation sites in the crystal structure of the rock salt structure belonging to space group Fm-3m, The lithium metal composite oxide has a number of Li atoms greater than the number of transition metal atoms, and its compositional formula is Li ａ Mn ｂ Ti ｃ M ２ ｄ O ２－ｅ F ｅ (However, M ２ (where is at least one metallic element other than Mn, Ti, and Li), 1 < a ≤ 1.35, 0.4 ≤ b ≤ 0.9, 0.025 ≤ c ≤ 0.1, 0 ≤ d ≤ 0.1, 0 ≤ e ≤ 0.66, and, Satisfying 1.85 ≤ a + b + c + d ≤ 1.98, The metal element M ２ comprises at least one selected from the group consisting of Fe, Ge, Si and Ga, which is a positive electrode active material for a secondary battery.

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

3. The molar ratio of Mn in the lithium metal composite oxide is the ratio of the metal elements M excluding Mn. ２ A positive electrode active material for a secondary battery according to claim 1 or 2, wherein the molar ratio of the active material is greater than the total molar ratio of Ti.

4. In the lithium metal composite oxide, the metal element M ２ The positive electrode active material for a secondary battery according to claim 1, further comprising at least one selected from the group consisting of Ni, Co, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Sm, Eu, Dy, and Er.

5. 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 4, in a secondary battery.

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