Secondary batteries

Abstract

A composite oxide which contains lithium and a transition metal, wherein: the crystal lattice of at least a part of the composite oxide is a hexagonal lattice; with n being an integer of not less than 1, an electron beam diffraction image thereof as acquired by having an electron beam enter from the direction of <2 -2 1> has a first group of diffraction spots and a second group of diffraction spots; the first group of diffraction spots comprises a (-3 1 8 / n) spot and a (1 2 2 / n) spot, which respectively appear in correspondence with the period of the unit lattice of the hexagonal lattice; and the second group of diffraction spots comprises 6 spots that appear with a 1 / 7 period between a (0 0 0) spot in the center and the (-3 1 8 / n) spot, and 6 spots that appear with a 1 / 7 period between the (0 0 0) spot in the center and the(1 2 2 / n) spot.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode active material for secondary batteries and to secondary batteries. [Background technology]

[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 due to their high output and high energy density. In recent years, lithium-rich lithium transition metal oxides have attracted attention in response to the demand for higher energy density (see Non-Patent Document 1).

[0003] Non-patent document 1 proposes a lithium-rich lithium transition metal oxide based on Li2MnO3. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] J Wu et al., J. Mater. Chem. A, 2020, 8, 18687-18697 [Overview of the project] [Problems that the invention aims to solve]

[0005] The lithium transition metal oxide described in Non-Patent Document 1 undergoes repeated charging and discharging, resulting in a breakdown of its crystal structure and a significant decrease in voltage and capacity. [Means for solving the problem]

[0006] In view of the above, one aspect of the present disclosure is a composite oxide containing lithium and a transition metal, wherein at least a part of the crystal lattice of the composite oxide is a hexagonal lattice, and when an electron beam is incident from the <2 -2n> direction where n is an integer of 1 or more, the electron diffraction image obtained has a first diffraction spot group and a second diffraction spot group. The first diffraction spot group has a {-3 1 8 / n} spot and a {1 2 2 / n} spot that appear corresponding to the period of the unit lattice of the hexagonal lattice respectively. The second diffraction spot group has six spots that appear at a 1 / 7 period between the central {0 0 0} spot and the {-3 1 8 / n} spot, and six spots that appear at a 1 / 7 period between the central {0 0 0} spot and the {1 2 2 / n} spot. The present disclosure relates to a composite oxide containing lithium and a transition metal.

[0007] Another aspect of the present disclosure is a composite oxide containing lithium and a transition metal, wherein at least a part of the crystal lattice of the composite oxide is a hexagonal lattice, and the electron diffraction image of the hexagonal lattice has a √7×√7 superlattice. The present disclosure relates to a composite oxide containing lithium and a transition metal.

[0008] Still another aspect of the present disclosure is a composite oxide containing lithium and a transition metal, wherein at least a part of the crystal lattice of the composite oxide is a hexagonal lattice, and the electron diffraction image of the hexagonal lattice includes a first diffraction spot group that appears corresponding to the period of the unit lattice of the hexagonal lattice, and a second diffraction spot group that appears at a ¼ period between the center of the electron diffraction image and any one of the first diffraction spot groups. The present disclosure relates to a composite oxide containing lithium and a transition metal.

[0009] Still another aspect of the present disclosure relates to a positive electrode active material for a secondary battery, which includes the above composite oxide containing lithium and a transition metal.

[0010] Still another aspect of the present disclosure relates to a secondary battery, which includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode includes the positive electrode active material for a secondary battery described above.

Advantages of the Invention

[0011] Provided is a composite oxide containing lithium and a transition metal, which suppresses a decrease in voltage and capacity of a secondary battery when used as a positive electrode active material for the secondary battery.

[0012] The novel features of the present invention are described in the appended claims, but the present invention will be better understood from the following detailed description in conjunction with the drawings, with reference to both the configuration and the content, as well as other objects and features of the present invention.

Brief Description of the Drawings

[0013] [Figure 1] It is a schematic perspective view of a part of a secondary battery according to an embodiment of the present disclosure, with a notch. [Figure 2] It is an example of an electron diffraction image of a composite oxide containing lithium and a transition metal according to an embodiment of the present disclosure. [Figure 3] It is a graph showing the relationship between the number of cycles and the discharge capacity in Evaluation 1 of the secondary battery fabricated in the Example. [Figure 4] It is a graph showing the relationship between the number of cycles and the average discharge voltage in Evaluation 1 of the secondary battery fabricated in the Example. [Figure 5] It is a graph showing the relationship between the number of cycles and the discharge capacity in Evaluation 2 of the secondary battery fabricated in the Example. [Figure 6] It is a graph showing the relationship between the number of cycles and the average voltage in Evaluation 2 of the secondary battery fabricated in the Example. [Figure 7] It is a graph showing the change in the relationship between the voltage and the discharge capacity accompanying charge and discharge of the secondary battery fabricated in the Example.

Mode for Carrying Out the Invention

[0014] The following describes, with examples, a composite oxide containing lithium and a transition metal, a positive electrode active material for a secondary battery, and embodiments of a secondary battery relating to this disclosure, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit is not greater than or equal to the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0015] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0016] A composite oxide comprising lithium and a transition metal according to the embodiments of this disclosure has a crystal lattice of at least a portion of the composite oxide that is, for example, a hexagonal lattice of space group R-3m, and the electron diffraction pattern obtained by incidenting an electron beam from the <2 -2 n> direction when n is an integer of 1 or more has a first diffraction spot group and a second diffraction spot group. n is not limited, but may be, for example, 1, 2, or 3.

[0017] <Complex Oxide A> Hereinafter, a composite oxide containing lithium and a transition metal from which an electron diffraction pattern having a first diffraction spot group and a second diffraction spot group can be obtained will also be referred to as "composite oxide A".

[0018] The first group of diffraction spots consists of {-3 1 8 / n} spots and {1 2 2 / n} spots, which appear corresponding to the period of the unit cell of the crystal lattice.

[0019] The second diffraction spot group has six spots that appear with a period of 1 / 7 between the central {0 0 0} spot and the {-3 1 8 / n} spot, and six spots that appear with a period of 1 / 7 between the central {0 0 0} spot and the {1 2 2 / n} spot.

[0020] When a second group of diffraction spots appears in addition to the first group of diffraction spots in the electron diffraction pattern, the crystal lattice corresponding to the measured portion of composite oxide A has a so-called √7×√7 superlattice (also called a "Maple-Leaf lattice"). The √7×√7 superlattice is formed based on a layered crystal structure in which layers of lithium and layers of transition metals are alternately stacked. The base layered crystal structure may be of the rock salt type.

[0021] In the region of the composite oxide having a √7×√7 superlattice, in the transition metal layer, a portion of the transition metal is substituted with lithium, resulting in a Li-rich (Li-excess) state compared to the base layered crystal structure. That is, composite oxide A has a layered crystal structure in which layers of lithium and transition metal are alternately stacked, and in this structure, the transition metal in the transition metal layer is partially substituted with lithium.

[0022] While crystal structures in a lithium-rich state are generally unstable, the durability of the crystal structure can be significantly improved by highly controlling the arrangement of lithium in the transition metal layer. When a first and second group of diffraction spots appear in the electron diffraction pattern, it can be said that the arrangement of lithium in the transition metal layer is highly controlled, and a certain degree of order has been formed.

[0023] It is sufficient to obtain an electron diffraction pattern having a first diffraction spot group and a second diffraction spot group in at least a portion of the composite oxide A, but it is desirable to obtain an electron diffraction pattern having a first diffraction spot group and a second diffraction spot group in a larger portion of the composite oxide A. For example, when obtaining an electron diffraction pattern by irradiating an electron beam from the <2 -2 1> direction at any multiple measurement points (e.g., 10 to 100 points) of a sample prepared from the powder of composite oxide A for electron diffraction, it is desirable to observe a first diffraction spot group and a second diffraction spot group at 30% or more, more than 50%, and especially more than 70% of the measurement points.

[0024] Because composite oxide A contains a √7×√7 superlattice in at least a portion of its crystal lattice, its crystal structure has improved durability, allowing for the electrochemical and reversible extraction of many lithium ions. In other words, secondary batteries containing composite oxide A as the positive electrode active material can absorb and release a large amount of lithium, resulting in high capacity. Moreover, due to the improved durability of the crystal structure of composite oxide A, secondary batteries are less prone to voltage and capacity degradation.

[0025] Composite oxide A can have an operating voltage in the range of 2V to 4.9V relative to the redox potential of metallic lithium, for example. That is, a secondary battery containing composite oxide A as the positive electrode active material can have a very wide potential window. Using composite oxide A makes it possible to realize a secondary battery with high voltage and high energy density.

[0026] Composite oxide A can exhibit capacity within the operating voltage range through redox reactions between transition metal cations and oxygen anions. A portion of the capacity exhibited when composite oxide A intercalates or releases lithium is compensated for by the transition metal cations. Conversely, the remaining capacity that cannot be compensated for by the transition metal cations is compensated for by the oxygen anions.

[0027] Composite oxide A is, for example, one with the general formula (1): Li a M1 bO2 (where M1 is at least one selected from the group consisting of Cr, Mn, Fe, Co, and Ni, 0.14 ≦ a ≦ 1.2 (for example, 0.143 ≦ a ≦ 1.143) and 0.8 < b < 1). The range of a indicating the lithium amount increases or decreases when the composite oxide A occludes or releases lithium. Also, in the state where the depth of discharge (DOD) is 0% (the state of charge SOC = 100%), the larger the a value, the higher the capacity of the composite oxide A. From the perspective of increasing the capacity of the composite oxide A, in the state where DOD is 0%, the a value in the general formula (1) is preferably 0.56 ≦ a, more preferably 0.857 ≦ a or 1 ≦ a, and even more preferably 1.143 ≦ a.

[0028] Even for the composite oxide having the composition of the general formula (1), the crystal lattice does not necessarily include a √7×√7 superlattice. It is only established as the composite oxide A including a √7×√7 superlattice when the arrangement of lithium present in the transition metal layer is highly controlled and an order above a certain degree is formed.

[0029] The crystal lattice does not necessarily belong to R-3m. The crystal lattice can be slightly distorted from R-3m. For example, even if it seems almost unchanged on the surface, it is well known that the distorted crystal lattice strictly belongs to the space group C2 / m, and the lattice constants for the same plane can be about √3 times the lattice constant of R-3m in the a-axis direction and the same length in the b-axis (Inorg. Chem, 2012, 51, 6211 - 6220). In this case, in electron diffraction, the <2 -2 1> direction in R-3m becomes the <0 -1 3> direction, and the {-3 1 8} spot and {1 2 2} spot in R-3m correspond to the {5 -1 3} spot and {-4 -2 2} in C2 / m, and six spots appear at a 1 / 7 period between these spots and the central {0 0 0} spot.

[0030] The composite oxide A is, for example, of the general formula (1a): Li a Mn x M2 bO₂ (where M₂ is at least one selected from the group consisting of Cr, Fe, Co, and Ni, 0.14 ≦ a ≦ 1.2 (for example, 0.143 ≦ a ≦ 1.143), 0.8 < x + b < 1 and 0 < b < 1) may have a composition represented thereby. The composition of the general formula (1a) is a composition included in the general formula (1). In the case of the general formula (1a), M₁ contains Mn as an essential component. When M₁ is only Mn, the general formula (1) is, for example, Li 0.57 Mn 0.86 O₂, Li 1.14 Mn 0.86 O₂ and the like. This composite oxide A has a high capacity, but further increase in capacity can be expected by substituting a part of Mn with M₂. The range of b indicating the amount of M₂ may be 0 < b < 1, but 0.28 ≦ b ≦ 0.57 is more desirable. The desirable range of a is the same as that of the general formula (1).

[0031] <Method for producing composite oxide A> Hereinafter, an example of the method for producing the composite oxide A will be described. First, for example, Na a M₁ 0.86 O₂ (Na c M₁₃O₇) (where M₁ is at least one selected from the group consisting of Cr, Mn, Fe, Co, and Ni, 0.14 ≦ a ≦ 1.14, 0.5 ≦ c ≦ 4) is synthesized. For example, by mixing NaNO₃ and MnCO₃ at an atomic ratio of Na:Mn = 2:3 and firing in an oxidizing atmosphere (for example, in an oxygen stream or in air), Na₂Mn₃O₇ can be obtained. Considering the oxidation number of M₁, from the viewpoint of accelerating the reaction rate, it is desirable to fire in an oxygen stream so that the oxygen concentration is higher than that in air. The firing temperature may be, for example, 400 °C or higher, and 500 °C or higher or 550 °C or higher (for example, 600 °C) is more desirable. However, Na a M₁ 0.86 O₂ (for example, Na₂Mn₃O₇) will decompose if fired at too high a temperature. From the viewpoint of maintaining Na a M₁ 0.86 O₂ in a stable temperature range, the firing temperature is desirably 650 °C or lower. The firing time is appropriately selected so that the reaction proceeds sufficiently, but is, for example, 10 hours or more.

[0032] Next, nitrate a M1 0.86 O2(Na c Replace the Na in M13O7 with Li. a M1 0.86 O2(Na c The method for substituting Na with Li in M13O7 is not particularly limited, but for example, a molten salt containing lithium as a cation and Na a M13O7 may be mixed with the molten salt in an excess of lithium and heated. The composition of the molten salt is not particularly limited, but may be a mixture of several types of lithium salts, for example, a mixture of a first lithium salt and a second lithium salt. The first lithium salt may be LiNO3, Li2SO4, Li2CO3, LiOH, Li2O, etc. The second lithium salt may be a lithium halide. More specifically, a mixture of LiNO3 and LiCl is mentioned, but is not particularly limited. The desirable reaction temperature differs depending on the composition of the molten salt used for ion exchange. A higher temperature is desirable in that ion exchange is promoted. On the other hand, if the reaction temperature is too high, the molten salt and the compound being ion-exchanged may be altered. A temperature slightly above the temperature at which the salt becomes molten is desirable. The heating time is appropriately selected so that the reaction proceeds sufficiently, but for example, it is 1 hour or more. Li produced when Na is replaced by Li a M1 0.86 O2(Li c The M13O7 can be washed with water and dried. While this example uses a molten salt containing Li, the method of ion exchange is not particularly limited. For example, reflux ion exchange or electrochemical ion exchange may be performed.

[0033] <Electron diffraction> Electron diffraction patterns are obtained, for example, by observing a transmission electron microscope image at an electron acceleration voltage of 200 kilovolts using JEOL Ltd.'s "JEM-ARM200F Dual-X," selecting the position for diffraction measurement, and acquiring the image using Gattan's "Orius." Measurement conditions may be changed as appropriate. Samples for electron diffraction are prepared by Ar-ion milling (Gattan's "PIPS 691") or focused ion beam (Thermo Fisher Scientific's "Helios G4").

[0034] 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.

[0035] [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 mixture layer can be formed, for example, by dispersing a positive electrode slurry containing a positive electrode mixture containing a positive electrode active material, a binder, etc., in a dispersion medium, applying it to the surface of the positive electrode current collector, and drying it. The dried coating may be rolled if necessary.

[0036] The positive electrode mixture layer contains a positive electrode active material as an essential component and may contain optional components such as binders, thickeners, and conductive agents. Known materials can be used as binders, thickeners, and conductive agents.

[0037] The positive electrode active material contains at least composite oxide A. Composite oxide A is, for example, secondary particles 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 composite oxide A is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. Here, the average particle size of composite oxide A 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. As the positive electrode active material, a mixture of composite oxide A and known materials may be used.

[0038] The composite oxide A may be combined with other composite oxides. For example, a solid solution of the composite oxide A and a composite oxide represented by LiMeO2 (hereinafter also referred to as "composite oxide B") may be used. Me is, for example, at least one selected from the group consisting of Ni, Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B, and preferably contains at least one selected from the group consisting of at least Ni, Co, and Mn. The overall composition of the solid solution is, for example, xLi a It is represented by Mn3O7·(1-x)LiMeO2. The range of x is not particularly limited as long as 0 < x, but a range of 0.5 < x ≦ 1 is preferred.

[0039] Similar to the composite oxide A, the composite oxide B is a layered composite oxide containing lithium and a transition metal. The composite oxide B is, for example, Li a Ni b M 1-b It may be a composite oxide represented by O2 (where 0 < a ≦ 1.2, 0.8 ≦ b ≦ 1, and M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B). Among them, M preferably contains at least one selected from the group consisting of Co, Mn, Al, and Fe. From the perspective of the stability of the crystal structure, M may contain Al. The value of a indicating the amount of lithium increases or decreases during charge and discharge.

[0040] The composition of the composite oxide A or the solid solution containing the same can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0041] For the positive electrode current collector, for example, a metal foil is used. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, and the like.

[0042] [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.

[0043] 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.

[0044] The negative electrode active material includes materials that electrochemically intercalate and release lithium ions, lithium metals, and / or lithium alloys. Examples of materials that electrochemically intercalate and release lithium ions include carbon materials and alloying materials. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). 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.

[0045] For example, metal foil is used as the negative electrode current collector. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0046] [Electrolyte] An electrolyte, for example, 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 a gelling agent, and solid electrolytes may also be used.

[0047] The solvent used can be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. A single non-aqueous solvent may be used, or two or more may be used in combination.

[0048] 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.

[0049] The concentration of lithium salt in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. [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.

[0050] One example of a secondary battery structure is one in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, and an electrolyte are housed in an outer casing. 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 can take any form, such as cylindrical, prismatic, coin-type, button-type, or laminated type.

[0051] Figure 1 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present disclosure. The battery comprises a bottomed rectangular battery case 4, an electrode group 1 and an electrolyte (not shown) housed within the battery case 4. The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them. The negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting portion is laser welded. The sealing plate 5 has an electrolyte injection hole, which is sealed by a seal 8 after the electrolyte is injected.

[0052] 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.

[0053] <Example 1> [Fabrication of the positive electrode] NaNO3 and MnCO3 were mixed in an atomic ratio of Na:Mn=2:3 and calcined in an oxygen stream at 600°C for 20 hours to obtain Na2Mn3O7.

[0054] Next, lithium nitrate and lithium chloride were mixed in a molar ratio of LiNO3:LiCl=88:12 to prepare a molten salt. The molten salt and Na2Mn3O7 were mixed in an atomic ratio of Li:Na=15:1, and the resulting mixture was heated at 280°C for 4 hours to replace the Na in Na2Mn3O7 with Li. 0.57 Mn 0.86 O2 (Li2Mn3O7) was produced.

[0055] Next, Li 0.57 Mn 0.86 O2(Li2Mn3O7) was washed with water, dried in air at 80°C for 12 hours, and then dried again in a vacuum at 105°C for 12 hours to obtain the dry composite oxide A(Li 0.57 Mn 0.86O2(Li2Mn3O7) was obtained.

[0056] The obtained Li 0.57 Mn 0.86 A sample for electron diffraction measurement was prepared from O2(Li2Mn3O7), and an electron beam was incident on the crystal lattice of composite oxide A from the <2 -2 1> direction to obtain an electron diffraction pattern. The electron diffraction pattern is shown in Figure 2.

[0057] In the electron diffraction pattern, the spot located at the center with the highest brightness is the {0 0 0} spot, which is included in the first diffraction spot group. Several other spots from the first diffraction spot group with high brightness can be observed at a predetermined distance from the {0 0 0} spot. The {-3 1 8} spot and the {1 2 2} spot can also be observed as spots from the first diffraction spot group.

[0058] On the straight line connecting the {0 0 0} spot and the {-3 1 8} spot, low-luminance spots included in the 6 second spot groups are observed at positions that divide the distance between the {0 0 0} spot and the {-3 1 8} spot into approximately 7 equal parts. Similarly, low-luminance spots included in the 6 second spot groups are observed at positions that divide the distance between the {0 0 0} spot and the {1 2 2} spot into approximately 7 equal parts.

[0059] The obtained Li 0.57 Mn 0.86 A positive electrode slurry was prepared by mixing O2 (Li2Mn3O7), acetylene black, and polyvinylidene fluoride in a mass ratio of 7:2:1, and using N-methyl-2-pyrrolidone (NMP) as the dispersion medium. Next, the positive electrode slurry was applied to a positive electrode current collector made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to obtain the positive electrode.

[0060] [Preparation of electrolytes] An 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.

[0061] [Preparation of test cells] A test cell was fabricated using the positive electrode described above and a negative electrode made of lithium metal foil. The positive and negative electrodes 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.

[0062] [Rating 1] The secondary battery was charged at room temperature using a constant current of xC as described below until the battery voltage reached 4.7V. At 4.7V, constant voltage charging was performed until the current decreased to 0.5 × xC. After a 20-minute pause, the battery was discharged using a constant current of xC until the battery voltage reached 2V. After another 20-minute pause, the same measurements were performed again. Here, charging and discharging were performed with x=0.01 for cycles 1-3, x=0.05 for cycles 4-6, x=0.1 for cycles 7-9, x=0.1 for cycles 10-12, x=0.1 for cycles 13-27, x=0.01 for cycles 28-29, x=0.01 for cycles 30-49, and x=0.01 for cycle 50. The discharge capacity during constant current discharge was determined, and the time average of the battery voltage, calculated from the time change of the battery voltage, was defined as the average discharge voltage. Note that 0.1C in this measurement corresponds to 18.4mA / g.

[0063] In the above charge-discharge cycle, the discharge capacity at the 35th cycle was approximately 160 mAh / g, and the average discharge voltage at the 35th cycle was 3.14 V. The discharge capacity and average discharge voltage for each cycle are shown in Figures 3 and 4.

[0064] [Rating 2] The prototype battery was first charged to 4.8V at a constant current of 0.1C, and then left to rest for 20 minutes. Afterward, it was discharged to 2.0V at the same constant current of 0.1C as during charging, and then left to rest for 20 minutes. The same charging and discharging conditions were repeated thereafter. Note that 0.1C in this measurement corresponds to 18.3mA / g.

[0065] The discharge capacity and average discharge voltage at cycle 35 were approximately 242 mAh / g and 3.11 V, respectively. The discharge capacity and average discharge voltage for each cycle are shown in Figures 5 and 6. Figure 7 shows a graph illustrating the change in the relationship between voltage and discharge capacity during charging and discharging.

[0066] In Non-Patent Document 1, Li2MnO3 is used as the positive electrode active material, and constant current charging is performed at 20mA / g up to 4.8V, followed by constant current discharging at the same current value down to 2.0V. When this charge-discharge cycle is repeated, the discharge capacity at 35 cycles is approximately 80mAh / g, and the average discharge voltage at 35 cycles has decreased to 2.8V. Although the charge-discharge conditions differ between the above example and Non-Patent Document 1, the discharge capacity at 35 cycles in the above example was measured at a constant current value close to that of Non-Patent Document 1. Furthermore, in all cycles measured under the condition x=0.1C in the above example, the above example maintains a higher capacity than Non-Patent Document 1. [Industrial applicability]

[0067] The lithium-transition metal composite oxide relating to this disclosure is useful as a positive electrode active material for the main power supply of mobile communication devices, portable electronic devices, etc., because it has high capacity and excellent durability.

[0068] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of symbols]

[0069] 1: Electrode group, 2: Positive lead, 3: Negative lead, 4: Battery case, 5: Sealing plate, 6: Negative terminal, 7: Gasket, 8: Sealing plug

Claims

1. A positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, It is equipped with, The positive electrode comprises a composite oxide containing lithium and a transition metal, The aforementioned composite oxide is General formula (1): Li a Mn b O 2 (0.14 ≤ a ≤ 1.2 and 0.8 < b < 1) or General formula (2): Li a Mn x M2 b O 2 (where M2 is at least one selected from the group consisting of Cr, Fe, Co and Ni, 0.14 ≤ a ≤ 1.2, 0.8 < x + b < 1 and 0 < b < 0.57), At least a portion of the crystal lattice of the composite oxide is a hexagonal lattice, and the electron diffraction pattern obtained by incidenting an electron beam from the direction <2 - 2n> with n as an integer of 1 or more has a first diffraction spot group and a second diffraction spot group. The first group of diffraction spots each has {-3 1 8 / n} spots and {1 2 2 / n} spots that appear corresponding to the period of the unit cell of the hexagonal lattice, The second group of diffraction spots is, Six spots appear between the central {0 0 0} spot and the aforementioned {-3 1 8 / n} spot at a 1 / 7 period, It has six spots that appear at a 1 / 7 period between the central {0 0 0} spot and the {1 2 2 / n} spot, A secondary battery in which the composite oxide has an operating voltage in the range of 2V to 4.9V relative to the redox potential of metallic lithium, and is charged and discharged within the range of the operating voltage so as to exhibit capacity through the redox reaction of oxygen anions in addition to the cations of the transition metal.

2. The secondary battery according to claim 1, wherein the composite oxide is charged and discharged so as to have an operating voltage in the range of 4.7 V or more with respect to the oxidation-reduction potential of metallic lithium.

3. The secondary battery according to claim 1, wherein the electron diffraction pattern of the hexagonal lattice has a √7 × √7 superlattice.

4. The secondary battery according to claim 1, wherein the electron diffraction pattern of the hexagonal lattice includes a first group of diffraction spots that appear corresponding to the period of the unit cell of the hexagonal lattice, and a second group of diffraction spots that appear with a period of 1 / 7 between the center of the electron diffraction pattern and any of the first group of diffraction spots.

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