Positive electrode active material, positive electrode, and battery

Abstract

This positive electrode active material includes Li, Mn, and O and can absorb and release lithium ions. The positive electrode active material includes a rock salt structure and an inverse fluorite structure, and the ratio of the amount of Mn atoms to the amount of Li atoms is greater than 0.1 but less than 0.6. Charge compensation for a reaction by which the positive electrode active material releases lithium ions and a reaction by which the positive electrode active material absorbs lithium ions can occur by oxidation / reduction of manganese and oxidation / reduction of oxygen.

Description

Positive electrode active material, positive electrode and battery

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

[0002] It has long been known that the redox reaction between lithium oxide (LiO) and lithium peroxide (LiO) can be applied to secondary batteries, as described in Patent Document 1. However, one of the technical challenges has been the poor electronic conductivity of lithium oxide and lithium peroxide, and the large overvoltage.

[0003] Patent Documents 2 and 3 disclose that charging overvoltage can be reduced by dissolving a transition metal in the crystal structure of lithium oxide.

[0004] Patent No. 4554935 Patent No. 6179944 Patent No. 6230149

[0005] J. Mater. Chem. A, 2018, 6, 13943-13951

[0006] The prior art has room for improvement in terms of discharge capacity.

[0007] The present disclosure provides a positive electrode active material that contains Li, Mn, and O and is capable of absorbing and desorbing lithium ions, the positive electrode active material having a rock salt structure and an anti-fluorite structure, and in which the ratio of the amount of substance of Mn atoms to the amount of substance of Li atoms is in the range of more than 0.1 and less than 0.6.

[0008] The positive electrode active material of the present disclosure can improve the discharge capacity of a battery.

[0009] FIG. 1 is a cross-sectional view showing a schematic configuration of a battery according to Embodiment 2. FIG. 2 is a graph showing the charge / discharge characteristics of the battery of Example 1. FIG. 3 is a graph showing the charge / discharge characteristics of the battery of Example 2. FIG. 4 is a graph showing the charge / discharge characteristics of the battery of Example 3. FIG. 5 is a graph showing the charge / discharge characteristics of the battery of Example 4. FIG. 6 is a graph showing the charge / discharge characteristics of the battery of Comparative Example 1. FIG. 7 is a graph showing the charge / discharge characteristics of the battery of Comparative Example 2. FIG. 8 is a graph showing the charge / discharge characteristics of the battery of Comparative Example 3. FIG. 9 shows X-ray diffraction patterns in a discharged state of the positive electrode composites of Examples and Comparative Examples. FIG. 10 is a graph showing the XANES spectrum of the Mn-K edge of the positive electrode composite of Example 1. FIG. 11 is a diagram showing the process of calculating the valence of Mn from the position of the absorption edge of the XANES spectrum. FIG. 12 is a graph showing a calibration curve for determining the valence of a sample from the energy of the K absorption edge. FIG. 13A is a graph showing the O-K edge XANES spectrum of the positive electrode composite in a charged or discharged state of Example 1. FIG. 13B is a graph showing the O-K edge XANES spectrum of the positive electrode composite in a charged or discharged state of Comparative Example 1. FIG. 13C is a graph showing the O-K edge XANES spectrum of the positive electrode composite in a charged or discharged state of Comparative Example 2. FIG. 13D is a graph showing the O-K edge XANES spectrum of the positive electrode composite in a charged or discharged state of Comparative Example 3. FIG. 14 is a graph showing the O-K edge differential spectra of the positive electrode composites of Example 1 and Comparative Examples 1 to 3.

[0010] (Findings that Form the Basis of the Present Disclosure) As described in Patent Document 2, mechanochemical treatment can be used to dissolve (do) transition metal atoms into the crystalline structure of lithium oxide. In mechanochemical treatment, for example, lithium oxide and α-iron oxide are mixed in a planetary ball mill. According to Patent Document 2, in some cases the iron is almost completely dissolved (Example 1), while in other cases the iron is only partially dissolved (Example 2). In the latter case, the product contains lithium oxide with dissolved iron and LiFeO2.

[0011] Patent Document 3 (Preparation Example 11) discloses that a solid powder obtained by mixing lithium oxide and manganese dioxide (MnO) in a planetary ball mill is a mixture of LiO and MnO. A battery using this mixture in the positive electrode (Example 7) was chargeable, but the discharge capacity (Figure 13) was very low.

[0012] Based on the above findings, the present inventors have conducted extensive research and have completed the positive electrode active material of the present disclosure.

[0013] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0014] (Embodiment 1) A cathode active material according to embodiment 1 includes Li, Mn, and O, and is capable of absorbing and releasing lithium ions. The cathode active material includes a rock salt structure and an inverse fluorite structure. In the cathode active material, the ratio of the amount of Mn atoms to the amount of Li atoms (Mn / Li) is greater than 0.1 and less than 0.6. When the ratio (Mn / Li) is within this range, the rock salt structure and the inverse fluorite structure can coexist, and the cathode active material can achieve a high discharge capacity. When the ratio (Mn / Li) is less than 0.1 or greater than 0.6, a structure in which the rock salt structure and the inverse fluorite structure coexist cannot be obtained, making it difficult to achieve a high discharge capacity.

[0015] The inverse fluorite structure is presumed to be the crystalline structure of lithium oxide in which manganese is dissolved. The inverse fluorite structure is a structure in which the positional relationship between cations and anions in the fluorite structure is reversed. When manganese is dissolved in lithium oxide, manganese occupies the lithium site, and oxygen atoms are tetrahedral coordinated around the manganese, forming an outer orbital complex. As a result, multiple unpaired electrons exist in the 3d orbital, and these unpaired electrons impart electronic conductivity to the lithium oxide. Furthermore, assuming that the valence of manganese is +2, the valence of lithium is +1. Therefore, to maintain charge neutrality within the crystal, one lithium atom is released from the crystal in addition to the lithium atom substituted with manganese, forming a vacancy. This vacancy serves as a conduction path for lithium ions, improving the ionic conductivity of the lithium oxide. Lithium oxide in which manganese is dissolved can be a substitutional solid solution.

[0016] As lithium is substituted with manganese, the size of the crystal lattice changes. Because the ionic radius of manganese ions is larger than that of lithium ions, the substitution of lithium with manganese increases the size of the crystal lattice. On the other hand, vacancies decrease the size of the crystal lattice. Since the number of vacancies changes depending on the valence of the substituted manganese, the size of the crystal lattice changes depending on the balance between the number of manganese ions and the number of vacancies.

[0017] Although the atomic arrangement and composition ratio in the rock salt structure are not entirely clear, it is presumed that the rock salt structure is the crystalline structure of lithium manganese oxide for the following reasons. The lattice constant of manganese oxide (MnO) with a rock salt structure is 4.44 Å. The lattice constant of the rock salt structure contained in the positive electrode active material of the examples described below was approximately 4.1 Å. It is presumed that the lattice constant of the rock salt structure contained in the positive electrode active material of the examples is reduced from the lattice constant of MnO with a rock salt structure due to the small-sized lithium occupying the Mn sites of MnO.

[0018] The positive electrode active material may or may not contain a rock salt structure immediately after synthesis. In the latter case, the positive electrode active material may contain only an anti-fluorite structure immediately after synthesis. Although the reason is not entirely clear, a change from the anti-fluorite structure to the rock salt structure may occur after one initial charge / discharge cycle.

[0019] The presence or absence of a rock salt structure and an inverse fluorite structure can be determined from the X-ray diffraction pattern of the positive electrode active material measured using Cu-Kα radiation. When an inverse fluorite structure is present, the X-ray diffraction pattern shows a diffraction peak assigned to the (111) plane of lithium oxide and a diffraction peak assigned to the (220) plane of lithium oxide. The diffraction peak assigned to the (111) plane of lithium oxide exists in the diffraction angle 2θ range of 30° to 40°. The diffraction peak assigned to the (220) plane of lithium oxide exists in the diffraction angle 2θ range of 52° to 62°. When a rock salt structure is present, the X-ray diffraction pattern shows a peak assigned to the (200) plane and a peak assigned to the (220) plane.

[0020] The lattice constants of the antifluorite structure and the rock salt structure can be calculated from the X-ray diffraction pattern of the positive electrode active material. In this embodiment, the lattice constant a of the antifluorite structure satisfies 4.59 Å≦a≦4.70 Å. The lattice constant b of the rock salt structure satisfies 3.94 Å≦b≦4.20 Å.

[0021] Charge compensation for the reactions in which the positive electrode active material releases and absorbs lithium ions is carried out by the oxidation-reduction of manganese and the oxidation-reduction of oxygen. The involvement of both oxygen and manganese in the absorption and desorption of lithium ions is presumed to be one of the reasons why the positive electrode active material of the present disclosure can achieve a high discharge capacity. The oxidation-reduction of oxygen is presumed to be due to a redox reaction between lithium oxide and lithium peroxide. The oxidation-reduction of manganese refers to a change in the valence of manganese.

[0022] The proportion of charge compensation due to manganese oxidation-reduction to the total amount of charge compensation required for the reactions of the positive electrode active material to release lithium ions and the reactions of the positive electrode active material to absorb lithium ions is 5% to 70%. With this configuration, the positive electrode active material is likely to achieve a high discharge capacity. The remaining charge compensation is presumed to be due to oxygen oxidation-reduction.

[0023] In the oxygen (O)-K edge XANES spectrum of a positive electrode active material in a state in which lithium ions have been released, there is an absorption edge peak due to the 1s electron of an oxygen atom with an oxidation number of -1. When the oxygen in lithium oxide is reduced by charging to produce lithium peroxide, this absorption edge peak appears in the XANES spectrum.

[0024] The positive electrode active material of this embodiment can be produced, for example, by mechanochemical milling. First, a mixture is prepared by mixing lithium oxide powder and Mn raw material powder. At this time, the ratio of the amount of Mn atoms to the amount of Li atoms (Mn / Li) is determined so that it falls within a desired range. "Mn raw material" means a raw material containing Mn.

[0025] Examples of Mn raw materials include manganese oxide, lithium manganese oxide, and simple manganese. Examples of manganese oxide include MnO, Mn3O4, Mn2O3, and MnO2. Examples of lithium manganese oxide include LiMn2O4 and Li2MnO3. At least one material selected from these can be used as the Mn raw material.

[0026] Although the reason is not entirely clear, when a Mn raw material in which the formal valence of Mn is less than 4 is milled with lithium oxide, the discharge capacity of the battery tends to improve. Therefore, it is recommended to use a Mn raw material in which the formal valence of Mn is less than 4. The formal valence of Mn in the Mn raw material may be 3 or less. "Formal valence" means the valence determined from the composition formula.

[0027] The Mn raw material can be at least one selected from the group consisting of MnO, Mn3O4, and Mn2O3. The formal valence of Mn in these Mn raw materials is 2, 8 / 3, or 3. By using these Mn raw materials, the discharge capacity of the battery can be improved.

[0028] The synthesis of a positive electrode active material by mechanochemical milling is carried out using a device capable of exerting a mechanochemical effect, such as a ball mill or a bead mill. The atmosphere during synthesis is not particularly limited and may be air, an inert atmosphere, a dry atmosphere, or a dry inert atmosphere. To prevent the incorporation of inevitable impurities such as oxygen and water, it is desirable to synthesize the positive electrode active material in a dry inert atmosphere. For the inert atmosphere, for example, an inert gas such as nitrogen, argon, or helium is used.

[0029] The state of Mn solid solution in Li2O can be adjusted by adjusting conditions such as the rotation speed of the apparatus, treatment time, treatment temperature, particle size of the raw material, composition of the Mn raw material, and size of the grinding media.

[0030] (Embodiment 2) FIG. 1 is a cross-sectional view of a battery 100 according to Embodiment 2. The battery 100 includes a positive electrode 23, a negative electrode 26, a non-aqueous electrolyte 29, a separator 27, and an exterior casing 28. The positive electrode 23 includes a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 is disposed on the positive electrode current collector 21. The negative electrode 26 includes a negative electrode current collector 24 and a negative electrode active material layer 25. The negative electrode active material layer 25 is disposed on the negative electrode current collector 24. A separator 27 is disposed between the positive electrode 23 and the negative electrode 26. The positive electrode 23 and the negative electrode 26 face each other with the separator 27 interposed therebetween. The positive electrode 23, the negative electrode 26, the separator 27, and the non-aqueous electrolyte 29 are housed in the exterior casing 28. The battery 100 is typically a secondary battery.

[0031] The positive electrode active material layer 22 contains the positive electrode active material described in embodiment 1. With this configuration, the discharge capacity of the battery 100 is improved.

[0032] The positive electrode current collector 21 is a sheet or film made of a metal material such as aluminum, an aluminum alloy, stainless steel, titanium, or a titanium alloy. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material may be applied to the surface of the positive electrode current collector 21 as a conductive auxiliary material.

[0033] The positive electrode active material layer 22 may contain other materials such as a conductive additive, an ion conductor, and a binder.

[0034] The conductive additive and the ion conductor are used to reduce the resistance of the positive electrode 23. Examples of the conductive additive include a carbon material and a conductive polymer compound. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0035] Examples of ionic conductors include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate, organic solid electrolytes such as polyethylene oxide, and Li7La3Zr2O 12 At least one selected from these ion conductors can be used.

[0036] The binder is used to improve the binding properties of the materials constituting the negative electrode 26. Examples of binders include polymer materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one selected from these binders can be used.

[0037] The negative electrode current collector 24 is a sheet or film made of a metal material such as stainless steel, nickel, a nickel alloy, copper, or a copper alloy. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material may be applied to the surface of the negative electrode current collector 24 as a conductive auxiliary material.

[0038] The negative electrode active material layer 25 may contain a negative electrode active material capable of absorbing and desorbing lithium. Examples of negative electrode active materials capable of absorbing and desorbing lithium include lithium titanate, graphite, silicon, silicon-containing oxides, zinc alloys, lithium metal, and lithium alloys. At least one selected from these negative electrode active materials can be used. It is more preferable to use lithium metal as the negative electrode active material. The theoretical capacity of the positive electrode active material according to the present disclosure is much greater than that of other positive electrode active materials conventionally used. Therefore, lithium metal is suitable as the negative electrode active material.

[0039] The negative electrode active material layer 25 may contain other materials such as a conductive additive, an ion conductor, a binder, etc. Materials that can be used for the positive electrode active material layer 22 as the conductive additive, the ion conductor, and the binder can also be used for the negative electrode active material layer 25.

[0040] The non-aqueous electrolyte 29 may be impregnated into the positive electrode 23, the negative electrode 26, and the separator 27. The non-aqueous electrolyte 29 may fill the internal space of the exterior casing 28. The non-aqueous electrolyte 29 functions to allow lithium ions to travel between the positive electrode 23 and the negative electrode 26. The non-aqueous electrolyte 29 may include a non-aqueous electrolyte solution, a gel electrolyte, or an ionic liquid.

[0041] The non-aqueous electrolyte solution contains, for example, a non-aqueous solvent and a lithium salt.

[0042] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, cyclic esters, chain esters, fluorine-containing solvents, and nitriles. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, and butylene carbonate. Examples of chain carbonates include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. Examples of cyclic ethers include tetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Examples of chain ethers include 1,2-dimethoxyethane and 1,2-diethoxyethane. Examples of cyclic esters include γ-butyrolactone. Examples of chain esters include methyl acetate. Examples of fluorine-containing solvents include fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethyl carbonate. Examples of nitriles include acetonitrile. At least one selected from these non-aqueous solvents can be used.

[0043] Examples of lithium salts include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. At least one selected from these lithium salts can be used.

[0044] The gel electrolyte can be a material obtained by impregnating a polymer material with a non-aqueous electrolyte solution, such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, or a polymer having an ethylene oxide bond.

[0045] Examples of cations constituting ionic liquids include aliphatic chain quaternary cations, aliphatic cyclic ammonium, and nitrogen-containing heterocyclic aromatic cations. Examples of aliphatic chain quaternary cations include tetraalkylammonium and tetraalkylphosphonium. Examples of aliphatic cyclic ammonium include pyrrolidiniums, morpholiniums, imidazoliniums, tetrahydropyrimidiniums, piperaziniums, and piperidiniums. Examples of nitrogen-containing heterocyclic aromatic cations include pyridiniums and imidazoliums. Examples of anions constituting ionic liquids include PF6 - , BF4 - , SbF6 - , AsF6 - , SO3CF3 - , N(SO2F)2 - , N(SO2CF3)2 - , N(SO2C2F5)2 - , N(SO2CF3)(SO2C4F9) - , C(SO2CF3)3 - The ionic liquid may contain a lithium salt.

[0046] The separator 27 is an electrolyte layer having lithium ion conductivity. There are no particular limitations on the material of the separator 27 as long as it allows the passage of lithium ions. The material of the separator 27 can be at least one selected from the group consisting of solid electrolytes, gel electrolytes, ion exchange resin membranes, semipermeable membranes, and porous membranes. If the separator 27 is made of these materials, the safety of the battery 100 can be sufficiently ensured. Examples of solid electrolytes include sulfide solid electrolytes such as Li2S-P2S5, Li7La3Zr2O 12Examples of the electrolyte include oxide solid electrolytes such as LLZ. Examples of the gel electrolyte include gel electrolytes containing fluororesins such as PVdF. Examples of the ion exchange resin membrane include cation exchange membranes and anion exchange membranes. Examples of the porous membrane include porous membranes made of polyolefin resins and porous membranes made of glass paper obtained by weaving glass fibers into nonwoven fabric.

[0047] The exterior 28 is made of a material obtained by laminating a metal foil such as an aluminum foil with a resin film such as a PET film, for example. The exterior 28 may also be a resin or metal container.

[0048] The shape of the battery 100 is not limited to the laminated type. Other shapes of the battery 100 include a coin type, a cylindrical type, a square type, a sheet type, a button type, and a flat type.

[0049] The battery 100 may be a solid-state battery.

[0050] When the battery 100 is a solid-state battery, a solid electrolyte is used in the positive electrode 23, the negative electrode 26, and the separator 27 instead of the nonaqueous electrolyte 29. Examples of the solid electrolyte include a halide solid electrolyte, a sulfide solid electrolyte, an oxide solid electrolyte, a polymer solid electrolyte, and a complex hydride solid electrolyte. At least one selected from these solid electrolytes can be used in each of the positive electrode 23, the negative electrode 26, and the separator 27.

[0051] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0052] (Technology 1) A positive electrode active material containing Li, Mn, and O and capable of absorbing and releasing lithium ions, the positive electrode active material having a rock salt structure and an inverse fluorite structure, wherein the ratio of the amount of substance of Mn atoms to the amount of substance of Li atoms is in the range of more than 0.1 and less than 0.6.

[0053] The positive electrode active material of the present disclosure can improve the discharge capacity of a battery.

[0054] (Technology 2) The positive electrode active material according to Technology 1, wherein the lattice constant a of the antifluorite structure satisfies 4.59 Å≦a≦4.70 Å, and the lattice constant b of the rock salt structure satisfies 3.94 Å≦b≦4.20 Å.

[0055] (Technology 3) The cathode active material according to Technology 1 or 2, wherein charge compensation for the reaction in which the cathode active material releases lithium ions and the reaction in which the cathode active material absorbs lithium ions is performed by oxidation-reduction of manganese and oxidation-reduction of oxygen. It is presumed that the involvement of both oxygen and manganese in the absorption and desorption of lithium ions is one of the reasons why the cathode active material of the present disclosure can achieve a high discharge capacity.

[0056] (Technology 4) The cathode active material according to Technology 3, wherein the ratio of charge compensation due to oxidation-reduction of manganese to the total amount of charge compensation required for the reaction of the cathode active material to release lithium ions and the reaction of the cathode active material to absorb lithium ions is 5% or more and 70% or less. With this configuration, the cathode active material is likely to achieve a high discharge capacity. The remaining charge compensation is presumed to be due to oxidation-reduction of oxygen.

[0057] (Technology 5) The positive electrode active material according to any one of Technologies 1 to 4, wherein an absorption edge peak resulting from a 1s electron of an oxygen atom having an oxidation number of −1 is present in a XANES spectrum at the O-K edge of the positive electrode active material in a state in which lithium ions have been released. When lithium oxide is reduced to produce lithium peroxide, this absorption edge peak appears in the XANES spectrum.

[0058] (Technology 6) A positive electrode comprising the positive electrode active material according to any one of Technologies 1 to 5. By using the positive electrode active material of the present disclosure, the discharge capacity of the battery can be improved.

[0059] (Technology 7) A battery including the positive electrode according to Technology 6. By using the positive electrode active material of the present disclosure, the discharge capacity of the battery can be improved.

[0060] (Technology 8) A method for producing a cathode active material containing Li, Mn, and O, the method comprising milling a Mn raw material in which the formal valence of Mn is less than tetravalent with lithium oxide, wherein the ratio of the substance amount of Mn atoms contained in the cathode active material to the substance amount of Li atoms contained in the cathode active material is greater than 0.1 and less than 0.6. Although the reason is not necessarily clear, when a Mn raw material in which the formal valence of Mn is less than tetravalent with lithium oxide is milled, the discharge capacity of a battery tends to improve.

[0061] (Technology 9) The method for producing a positive electrode active material according to Technology 8, wherein the formal valence of Mn in the Mn raw material is 2, 8 / 3, or 3. With this configuration, the discharge capacity of the battery can be improved.

[0062] (Technology 10) The method for producing a positive electrode active material according to Technology 8 or 9, wherein the Mn raw material contains at least one selected from the group consisting of MnO, Mn3O4, and Mn2O3. By using these Mn raw materials, the discharge capacity of the battery can be improved.

[0063] Hereinafter, each operation in the preparation of the positive electrode active material and each operation in the preparation of the battery were performed in a dry atmosphere or an argon atmosphere. A dry atmosphere is an air atmosphere with a dew point of −40° C. or less. Some of the examples and comparative examples include cases in which the same operation was performed multiple times to obtain a sufficient amount of sample.

[0064] [Preparation of Positive Electrode Active Material] (Example 1) 1.10 g of Li2O and 0.87 g of MnO were placed in a planetary ball mill (manufactured by Fritsch GmbH, P-7 type, 45 mL container) together with 45 g of zirconia balls (diameter 5 mm), and the mixture was milled at 420 rpm for 140 hours. This produced the positive electrode active material of Example 1.

[0065] Example 2 A positive electrode active material of Example 2 was prepared in the same manner as in Example 1, except that 0.80 g of Li2O and 1.23 g of Mn3O4 were used.

[0066] Example 3 A positive electrode active material of Example 3 was prepared in the same manner as in Example 1, except that 0.45 g of Li2O and 0.59 g of Mn2O3 were used.

[0067] Example 4 A positive electrode active material of Example 4 was prepared in the same manner as in Example 1, except that 0.70 g of Li2O and 1.66 g of MnO were used.

[0068] Comparative Example 1 A positive electrode active material of Comparative Example 1 was prepared in the same manner as in Example 1, except that 1.00 g of Li2O and 0.49 g of MnO2 were used.

[0069] Comparative Example 2 A positive electrode active material of Comparative Example 2 was prepared in the same manner as in Example 1, except that 1.40 g of Li2O and 0.55 g of MnO were used.

[0070] Comparative Example 3 A positive electrode active material of Comparative Example 3 was prepared in the same manner as in Example 1, except that 0.50 g of Li2O and 1.48 g of MnO were used.

[0071] [Battery Fabrication] Coin batteries conforming to the CR2016 standard were fabricated using the positive electrode active materials of the Examples and Comparative Examples. The positive electrode consisted of a positive electrode composite containing the positive electrode active material, acetylene black, and polytetrafluoroethylene in a mass ratio of 7:2:1. The negative electrode consisted of a 0.3 mm thick lithium metal foil. The separator consisted of a three-layer separator consisting of nonwoven fabric, polyolefin resin film, and nonwoven fabric.

[0072] The electrolyte solution used in the batteries of the Examples and Comparative Examples was prepared as follows: Ethylene carbonate and diethyl carbonate were mixed in a volume ratio of 1:1. LiPF was dissolved in the resulting mixed solvent at a concentration of 1 mol / L to prepare the electrolyte solution.

[0073] [Charge / Discharge Test] Charge / discharge tests were performed on the batteries of the examples and comparative examples using the following method. The upper limit of charge capacity was set to 900 mAh / g, and constant current charging was performed at a current value of 50 mA / g up to 3.4 V. Subsequently, constant voltage charging was performed at a voltage of 3.4 V until a current value of 5 mA / g was reached. After a 20-minute pause, the upper limit of discharge capacity was set to 600 mAh / g, and constant current discharging was performed at a current value of 50 mA / g up to 1.8 V or 1.5 V. Subsequently, constant voltage discharging was performed at a voltage of 1.8 V or 1.5 V until a current value of 5 mA / g was reached.

[0074] 2 to 8 are graphs showing the charge-discharge characteristics of the batteries of Examples 1 to 4 and Comparative Examples 1 to 3, respectively.

[0075] The unit of capacity "mAh / g" represents the capacity per 1 g of Li 2 O. The unit of current value "mA / g" represents the current value per 1 g of Li 2 O.

[0076]

[0077] The Mn / Li molar ratio is the ratio of the amount of Mn atoms contained in the positive electrode active material to the amount of Li atoms contained in the positive electrode active material, and is a value calculated from the amount of LiO used and the amount of Mn raw material used.

[0078] As shown in Table 1, the batteries of Examples 1 to 4 had high discharge capacities, while the batteries of Comparative Examples 1 to 3 had low discharge capacities. There was a correlation between the Mn / Li molar ratio and discharge capacity. The batteries of Comparative Examples 1 and 2 had low Mn / Li molar ratios and did not exhibit sufficient discharge capacity. The battery of Comparative Example 3 had a high Mn / Li molar ratio and did not exhibit sufficient discharge capacity. According to the results shown in Table 1, the lower limit of the preferred range of the Mn / Li molar ratio is thought to be between the value of Example 1 and the value of Comparative Example 2. The upper limit of the preferred range of the Mn / Li molar ratio is thought to be between the value of Example 4 and the value of Comparative Example 3. That is, in order to improve the discharge capacity of a battery, it is necessary for the Mn / Li molar ratio to satisfy the condition 0.1<Mn / Li<0.6.

[0079] In the examples and comparative examples, four types of Mn raw materials were used: MnO, Mn3O4, Mn2O3, and MnO2. The formal valences of Mn in these Mn raw materials were 2, 8 / 3, 3, and 4, respectively. The batteries of Examples 1 to 3, which used MnO, Mn3O4, and Mn2O3 as the Mn raw materials, all exhibited high discharge capacities. Although the Mn / Li molar ratio of Comparative Example 1, which used MnO2 as the Mn raw material, was approximately the same as the Mn / Li molar ratio of Comparative Example 2, which used MnO as the Mn raw material, the discharge capacity of the battery of Comparative Example 1 was lower than that of Comparative Example 2. From these results, it is inferred that when the formal valence of Mn in the Mn raw material is less than 4, a positive electrode active material exhibiting a high discharge capacity tends to be obtained.

[0080] After the charge-discharge test, the batteries of the examples and comparative examples were disassembled, and the positive electrode composite was removed, washed with a diethyl carbonate solution, and vacuum dried. This resulted in a discharged positive electrode composite. Various measurements were performed using the discharged positive electrode composite.

[0081] [X-ray Diffraction Measurement] Powder X-ray diffraction measurement was performed on the positive electrode composites of the Examples and Comparative Examples. For the X-ray diffraction measurement, a powder X-ray diffractometer (MiniFlex600, manufactured by Rigaku Corporation) was used. The measurement conditions are as follows.

[0082] X-ray source: Cu Ka line Detector: HyPix400MF Step size: 0.005 deg Scan speed: 0.4 deg / min Scan range: 10-80 deg

[0083] FIG. 9 shows X-ray diffraction patterns of the positive electrode composites of the examples and comparative examples in a discharged state. The "PTFE" peak is the peak of polytetrafluoroethylene, which is a binder. The "AB" peak is the peak of acetylene black, which is a conductive additive. In the X-ray diffraction pattern of a known lithium manganese oxide having a rock salt structure (see Non-Patent Document 1), the peak assigned to the (200) plane and the peak assigned to the (220) plane appear at diffraction angles 2θ of 43° to 46° and 62° to 67°, respectively. Therefore, in this example as well, if peaks appeared at these positions, it was determined that the sample had a rock salt structure.

[0084] The X-ray diffraction patterns of the positive electrode composites of Comparative Examples 1 and 2 had a diffraction peak attributable to the (111) plane of LiO and a diffraction peak attributable to the (220) plane of LiO, but did not have a peak attributable to the (220) plane of the rock salt structure. In other words, the positive electrode composites of Comparative Examples 1 and 2 did not have a rock salt structure.

[0085] The X-ray diffraction pattern of the positive electrode composite of Comparative Example 3 showed that a large amount of the raw material MnO remained. The lattice constant of the MnO with a rock salt structure calculated from the X-ray diffraction pattern of the positive electrode composite of Comparative Example 3 was 4.44 Å. This value was significantly different from the lattice constants of the rock salt structures in Examples 1 to 4. In the X-ray diffraction pattern of MnO with a rock salt structure, the peaks assigned to the (200) plane and the (220) plane appear at diffraction angles 2θ of 40° to 41° and 58° to 59°, respectively.

[0086] In contrast, the X-ray diffraction patterns of the positive electrode composites of Examples 1 to 4 had a diffraction peak assigned to the (111) plane of LiO, a diffraction peak assigned to the (220) plane of LiO, a peak assigned to the (200) plane of the rock salt structure, and a peak assigned to the (220) plane of the rock salt structure. In other words, the positive electrode active materials contained in the positive electrode composites of Examples 1 to 4 had both a rock salt structure and an anti-fluorite structure. The lattice constants of the rock salt structure and the anti-fluorite structure in Examples 1 to 4 were as follows:

[0087] Example 1: 4.12 Å (rock salt structure), 4.65 Å (inverse fluorite structure) Example 2: 4.01 Å (rock salt structure), 4.63 Å (inverse fluorite structure) Example 3: 4.11 Å (rock salt structure), 4.67 Å (inverse fluorite structure) Example 4: 4.14 Å (rock salt structure), 4.61 Å (inverse fluorite structure)

[0088] [Measurement of X-ray absorption near edge structure (Mn-K edge)] For Examples 1 to 4, X-ray absorption near edge structure (XANES) was measured using the discharged positive electrode composite and the charged positive electrode composite. The charged positive electrode composite was prepared separately from the discharged positive electrode composite. That is, the batteries of Examples 1 to 4 were fabricated by the method described above. Thereafter, the batteries of Examples 1 to 4 were subjected to constant current and constant voltage charging by the method described above. The charged batteries of Examples 1 to 4 were disassembled, and the positive electrode composite was removed, washed with a diethyl carbonate solution, and vacuum dried. This resulted in a charged positive electrode composite.

[0089] For the positive electrode composites of Examples 1 to 4 in the charged or discharged state, the Mn K-edge X-ray absorption near edge structure (XANES) was measured. The measurements were carried out at BL16B2 of the SPring-8 synchrotron radiation facility. The measurement method used was a transmission method. Energy calibration was performed with the Cu pre-edge at 12.7190 deg.

[0090] [X-ray absorption near edge structure measurement (O-K edge)] The O K-edge X-ray absorption near edge structure (XANES) was measured for the positive electrode composites in the charged or discharged state of Examples 1 to 4. The measurements were carried out at Ritsumeikan University SR Center BL-11. The measurement method used was the partial fluorescence yield method. Energy calibration was performed with the pre-edge of amorphous iron oxide (a-Fe2O3) set to 529.4 eV.

[0091] 10 is a graph showing the XANES (X-ray absorption near edge structure) spectrum of the Mn-K edge of the positive electrode composite of Example 1. As shown in FIG. 10, an energy difference occurred between the spectrum in the discharged state and the spectrum in the charged state.

[0092] 11 is a diagram showing the process of calculating the Mn valence from the position of the absorption edge in the XANES spectrum. As shown in FIG. 11, for Example 1, the vertical axis (normalized X-ray absorption) was selected to have a value in the range of 0.2 to 1.2, and a rectangle was defined with an area equal to the integral value (area of ​​the gray part) obtained by integration. The energy corresponding to one side of the rectangle on the high-energy side was regarded as the energy of the K absorption edge (Edge).

[0093] 12 is a graph showing a calibration curve for determining the valence of a sample from the energy (E) of the K-absorption edge. The positive electrode active materials prepared in Examples 1 to 3 and Comparative Example 1 before charge / discharge were used as standard samples for creating the calibration curve. The precise Mn valence of the standard samples was determined by redox titration, and then the standard samples were subjected to XANES measurement to identify the energy (E) of the K-absorption edge, and the calibration curve shown in FIG. 12 was created.

[0094] [Determination of valence by redox titration] The Mn valence of the standard sample was determined by redox titration. 250 mg of ammonium iron(II) sulfate hexahydrate (Fe(SO4)2(NH4)2.6H2O) was placed in a beaker and dissolved in 50 mL of pure water, 10 mL of 95% sulfuric acid, and 9.4 mL of 85% phosphoric acid. This resulted in an ammonium iron(II) sulfate hexahydrate solution. 25 mg of a sample was added to the ammonium iron(II) sulfate hexahydrate solution and dissolved. The resulting solution was titrated with 5 mmol / L potassium permanganate solution. The average valence m of Mn was calculated from the following formula (1) using the amount of ammonium iron (II) sulfate hexahydrate added, the molecular weight of ammonium iron (II) sulfate hexahydrate, the amount of sample, the Mn content in the sample, the atomic weight of Mn, the concentration of the potassium permanganate solution, the concentration factor of the potassium permanganate solution, and the titration amount of the potassium permanganate solution.

[0095]

[0096] m: average valence of Mn W Fe : Amount of Fe(SO4)2(NH4)2.6H2O (mg) MW Fe : Molecular weight of Fe(SO4)2(NH4)2.6H2O (392 g / mol) C Mn AW: Mn content in the sample (mass%, based on the ICP quantification results) Mn W: atomic weight of Mn (54.9 g / mol) SP : Amount of sample (mg) C KMnO4 F: Concentration of KMnO4 solution (5 mmol / L) KMn4 : Concentration factor of KMnO4 solution (1.000) V KMn4 : Titration volume of KMnO4 solution (mL)

[0097] Using the calibration curve of Figure 12, the valence of Mn in the positive electrode active material in the charged state and the valence of Mn in the positive electrode active material in the discharged state were determined for Examples 1 to 4 and Comparative Examples 1 to 3. From the valence change from the charged state to the discharged state, the discharge capacity due to this valence change was calculated. From the discharge capacity, the contribution rate of charge compensation due to the valence change of Mn was calculated. The results are shown in Table 2.

[0098]

[0099] As shown in Table 2, the contribution of Mn valence change to the discharge reaction of the batteries of Examples 1 to 4 and Comparative Example 1 was 10% to 66%. This result indicates that the discharge reaction occurred due to charge compensation other than Mn valence change. A possible charge compensation reaction other than charge compensation due to Mn valence change is an oxygen redox reaction. That is, in the batteries of Examples 1 to 4 and Comparative Example 1, charge compensation is presumed to have occurred due to oxygen valence change. However, because the Mn raw material used in Comparative Example 1 had a tetravalent valence, the discharge capacity of Comparative Example 1 was extremely low. The battery of Comparative Example 1 is difficult to function as a practical battery. On the other hand, the contribution of Mn valence change to the discharge reaction of the batteries of Comparative Examples 2 and 3 was 95% or more. It is presumed that an oxygen redox reaction did not occur in the batteries of Comparative Examples 2 and 3. From these results, it is considered that the ratio of charge compensation by manganese oxidation-reduction to the total amount of charge compensation required for the charge and discharge reactions is generally in the range of 5% to 70% in the batteries using the positive electrode active materials of Examples 1 to 4. Taking the discharge capacity into consideration, the ratio of charge compensation by manganese oxidation-reduction is preferably in the range of 5% to 50%, more preferably in the range of 10% to 40%.

[0100] Based on the above results, O-K edge XANES measurements were performed to examine the presence or absence of an oxygen redox reaction. Figures 13A to 13D are graphs showing O-K edge XANES spectra of the positive electrode composites of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 in a charged or discharged state, respectively. The positive electrode composites in a charged or discharged state were the same as those used in the Mn-K edge XANES measurements. Figure 14 is a graph showing the O-K edge difference spectra of the positive electrode composites of Example 1 and Comparative Examples 1 to 3. The difference spectrum represents the difference between the O-K edge XANES spectrum of the positive electrode composite in a charged state and the O-K edge XANES spectrum of the positive electrode composite in a discharged state.

[0101] As shown in FIGS. 13A and 14, in the XANES spectrum of the O-K edge of the positive electrode composite of Example 1, a peak of divalent oxygen in LiO appears at 534 eV, and a peak of monovalent oxygen dimer [O] 2-It is known that a peak appears at 530 eV. In the XANES spectrum of the O-K edge of the positive electrode composite of Example 1, a peak appears around 530 eV in the charged state and disappears in the discharged state. The peak around 534 eV is stronger in the discharged state than in the charged state. In other words, in the charged state in which lithium ions are released, oxygen in Li2O is reduced to monovalent oxygen dimer [O2] 2- In other words, it is thought that an absorption edge peak due to the 1s electron of an oxygen atom with an oxidation number of -1 appeared. In a discharged state in which lithium ions are absorbed, a monovalent oxygen dimer [O2] 2- It is believed that Li2O was generated by oxidation of Li. This result indicates that a reduction reaction of oxygen occurred during charging, and an oxidation reaction of oxygen occurred during discharging.

[0102] On the other hand, as shown in FIGS. 13B, 13C, 13D, and 14, in Comparative Examples 1 to 3, the monovalent oxygen dimer [O] at around 530 eV was generated during charging. 2- This indicates that the reduction reaction of oxygen hardly occurred in the positive electrode mixtures of Comparative Examples 1 to 3.

[0103] The technology of the present disclosure is useful for batteries such as lithium secondary batteries.

Claims

1. A positive electrode active material containing Li, Mn, and O and capable of absorbing and releasing lithium ions, the positive electrode active material having a rock salt structure and an inverse fluorite structure, wherein the ratio of the amount of Mn atoms to the amount of Li atoms is greater than 0.1 and less than 0.

6.

2. The positive electrode active material according to claim 1, wherein the lattice constant a of the antifluorite structure satisfies 4.59 Å≦a≦4.70 Å, and the lattice constant b of the rock salt structure satisfies 3.94 Å≦b≦4.20 Å.

3. The positive electrode active material according to claim 1, wherein charge compensation for the reaction of the positive electrode active material releasing lithium ions and the reaction of the positive electrode active material absorbing lithium ions is performed by oxidation-reduction of manganese and oxidation-reduction of oxygen.

4. The positive electrode active material according to claim 3, wherein a ratio of charge compensation due to oxidation-reduction of manganese to a total amount of charge compensation required for the reaction of the positive electrode active material releasing lithium ions and the reaction of the positive electrode active material absorbing lithium ions is 5% or more and 70% or less.

5. The positive electrode active material according to claim 1, wherein an absorption edge peak resulting from a 1s electron of an oxygen atom having an oxidation number of −1 is present in the XANES spectrum of the O-K edge of the positive electrode active material in a state in which lithium ions have been released.

6. A positive electrode comprising the positive electrode active material according to claim 1.

7. A battery comprising the positive electrode according to claim 6.

8. A method for producing a positive electrode active material containing Li, Mn, and O, comprising milling a Mn raw material in which the formal valence of Mn is less than tetravalent and lithium oxide, wherein the ratio of the amount of Mn atoms contained in the positive electrode active material to the amount of Li atoms contained in the positive electrode active material is greater than 0.1 and less than 0.

6.

9. The method for producing a positive electrode active material according to claim 8, wherein the formal valence of Mn in the Mn raw material is 2, 8 / 3, or 3.

10. The method for producing a positive electrode active material according to claim 8, wherein the Mn raw material includes at least one selected from the group consisting of MnO, Mn3O4, and Mn2O3.

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