Positive electrode active material, lithium secondary battery, and method for manufacturing positive electrode active material

Abstract

The positive electrode active material contains Li x Mn y O2(1.012≥x≥0.683, and 0.91≤y≤1.0).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a positive electrode active material, a lithium secondary battery, and a method for manufacturing a positive electrode active material. BACKGROUND

[0002] As one of the positive electrode active materials of the lithium secondary battery, manganese dioxide having a tunnel structure or a layered structure is known. There are various crystal structures of the manganese dioxide, such as α-MnO2, β-MnO2, γ-MnO2, and the like. The manganese dioxide is expressed by a chemical formula of "MnO2", but is actually a compound of indefinite ratio.

[0003] The manganese dioxide does not contain Li, and thus a lithium secondary battery using the manganese dioxide as the positive electrode active material must be in a charged state at the end of assembly. That is, it is difficult to use graphite as the negative electrode active material, and it is necessary to use Li metal, LiAl alloy, or the like as the negative electrode active material.

[0004] As known by those skilled in the art, the lithium secondary battery using Li metal as the negative electrode active material has poor reversibility. In the case of using Li alloy such as LiAl alloy instead of Li metal, the cycle characteristics at a shallow depth are good. However, if the charge and discharge at a deep depth are repeated, the battery reaches the life in a small number of charge and discharge times. Therefore, the lithium secondary battery using the manganese dioxide as the positive electrode active material is practically used for applications in which the charge and discharge are repeated at a shallow depth, such as a memory backup application.

[0005] Patent Literature 1 discloses a scheme in which a product obtained by mixing and firing LiOH and MnO2 is used as the positive electrode active material. It is seen from the product that LiMnO2 is contained, but in fact, the valence number of Mn is not substantially changed from 4, and Li is not substantially doped in MnO2. That is, even if the raw material powders are mixed and fired, LiMnO2 is not generated, or even if it is generated, it is extremely small. Therefore, in the examples of Patent Literature 1, LiPb alloy is used as the negative electrode active material, and self-discharge is performed after the battery is assembled.

[0006] Non-Patent Literature 1 clarifies that the product obtained by mixing and firing LiOH and MnO2 is a composite of Li2MnO3 and MnO2.

[0007] Prior Art Documents

[0008] Patent Literature

[0009] Patent Literature 1: Japanese Patent Application Laid-Open No. 3-43968

[0010] Non-Patent Literature

[0011] Non-patent literature 1: Katsuhiko, Tsuneo, Kazuo, Ikio, Yuji, Toshihiko "Lithium-containing manganese dioxide as a positive active material for lithium secondary batteries" Electrochimica Acta, 57, No. 6, p533-538 (1989) SUMMARY

[0012] In view of the above, the present disclosure provides LiMnO2having lithium in advance.

[0013] The present disclosure provides a positive active material comprising

[0014] Li x Mn y O2(1.012≥x≥0.683, and 0.91≤y≤1.0).

[0015] According to the present disclosure, the positive active material comprises Li x Mn y O2having lithium in advance, and thus, a lithium secondary battery can be constructed using various negative active materials. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 A cross-sectional view of a lithium secondary battery according to an embodiment of the present disclosure.

[0017] Figure 2 A process diagram showing a manufacturing method of a positive active material.

[0018] Figure 3A A charge-discharge curve of the 1st cycle of a lithium secondary battery of Example 1.

[0019] Figure 3B A charge-discharge curve of the 2nd cycle of a lithium secondary battery of Example 1.

[0020] Figure 3C A graph showing the relationship between the discharge capacity density and the cycle number of discharge of a lithium secondary battery of Example 1.

[0021] Figure 4A A charge-discharge curve of the 1st cycle of a lithium secondary battery of Example 2.

[0022] Figure 4B A charge-discharge curve of the 2nd cycle of a lithium secondary battery of Example 2.

[0023] Figure 4C A graph showing the relationship between the discharge capacity density and the cycle number of discharge of a lithium secondary battery of Example 2.

[0024] Figure 5A A charge-discharge curve of the 1st cycle of a lithium secondary battery of Example 3.

[0025] Figure 5B Charge-discharge curve of the 2nd cycle of the lithium secondary battery of Example 3.

[0026] Figure 5C Graph showing the relationship between the discharge capacity density and the cycle number of discharge of the lithium secondary battery of Example 3.

[0027] Figure 6A Charge-discharge curve of the 1st cycle of the lithium secondary battery of Example 4.

[0028] Figure 6B Charge-discharge curve of the 2nd cycle of the lithium secondary battery of Example 4.

[0029] Figure 6C Graph showing the relationship between the discharge capacity density and the cycle number of discharge of the lithium secondary battery of Example 4.

[0030] Figure 7A Discharge curve of the 1st cycle of the lithium secondary battery of Reference Example 1.

[0031] Figure 7B Charge curve of the 1st cycle of the lithium secondary battery of Reference Example 1.

[0032] Figure 8 Graph showing the powder X-ray diffraction pattern of the positive electrode active material of Example 2 and Reference Example 1. DETAILED DESCRIPTION

[0033] (Insight underlying the present disclosure)

[0034] Generally, a secondary battery is also required to sufficiently tolerate deep charge-discharge at a deep depth. In one example, MnO2 is considered to be used for a positive electrode, and LiC6, which has previously absorbed and stored Li, is considered to be used for a negative electrode. However, LiC6 is complicated to produce, and it is not practical to use LiC6 for a negative electrode. Therefore, in order to use graphite, which has excellent reversibility, for a negative electrode, to exhibit good cycle characteristics at a deep depth, it is necessary to cause MnO2, which is a positive electrode active material, to contain a sufficient amount of Li, and to cause the valence number of Mn to be substantially 3.

[0035] Production of LiMnO2 was attempted by the existing method described in Patent Document 1, and Mn was not substantially reduced, and was substantially maintained at 4. The valence number of Mn is close to 4, and therefore, even if Li is contained, it is not practically possible to extract Li from a compound such as Li2MnO3.

[0036] Therefore, LiMnO2, which has lithium in advance, and which can be self-charged from the start, is sought.

[0037] (Summary of one embodiment of the present disclosure)

[0038] The positive electrode active material of the first aspect of the present disclosure contains Li x Mn y O2(1.012≥x≥0.683, and 0.91≤y≤1.0). The positive electrode active material contains Li x Mn y O2, and thus various negative electrode active materials can be used. Li x Mn y The lithium secondary battery using Li

[0039] In the second aspect of the present disclosure, for example, the positive electrode active material of the first aspect can satisfy the relationship 1.012≥x≥0.864. In this case, the capacity density of the lithium secondary battery is further increased.

[0040] In the third aspect of the present disclosure, for example, the positive electrode active material of the first or second aspect can have a valence number of Mn of 2.987 or more and 3.316 or less. The valence number of Mn is changed between 3 and 4, and thus the charge and discharge of the lithium secondary battery is smoothly performed.

[0041] In the fourth aspect of the present disclosure, for example, the positive electrode active material of any one of the first to third aspects can have a valence number of Mn of 2.987 or more and 3.136 or less.

[0042] The manufacturing method of the positive electrode active material of the fifth aspect of the present disclosure includes a step of bringing a lithium solution in which lithium metal is dissolved in an organic solvent into contact with MnO2.

[0043] The method of the present disclosure utilizes the strong reducing power of the organic solvent in which Li metal is dissolved. The valence number of Mn of MnO2 is reduced to 3 by the reducing power of the lithium solution, and Li ions are doped to MnO2 in the form of charge compensation. MnO2 is reduced to 3 by electrochemical reduction (discharge). An amount of 1 electron of Li is incorporated into MnO2.

[0044] In the sixth aspect of the present disclosure, for example, the manufacturing method of the positive electrode active material of the fifth aspect can include MnO2 containing at least one selected from the group consisting of γ-β-MnO2, ramsdellite, and β-MnO2. The ramsdellite, the γ-β-MnO2 which can be made from electrolytic manganese dioxide, and the β-MnO2 which can be made from the same electrolytic manganese dioxide all have a tunnel structure, are inexpensive, and are easily obtained.

[0045] In the seventh aspect of the present disclosure, for example, the manufacturing method of the positive electrode active material of the fifth or sixth aspect can include the aforementioned organic solvent containing at least one selected from the group consisting of glyme and chain carbonate.

[0046] In the 8th aspect of the present disclosure, in the method for producing a positive electrode active material of the 5th or 6th aspect, for example, the organic solvent can include at least one selected from the group consisting of triglyme, dimethoxyethane, and methyl ethyl carbonate.

[0047] In the 9th aspect of the present disclosure, in the method for producing a positive electrode active material of the 5th or 6th aspect, for example, the organic solvent can include a mixture of triglyme and methyl ethyl carbonate. If a mixture of triglyme and methyl ethyl carbonate is used, a lithium solution having a strong reducing power can be prepared, and doping of lithium to MnO2 can be performed rapidly.

[0048] In the 10th aspect of the present disclosure, in the method for producing a positive electrode active material of the 5th or 6th aspect, for example, the organic solvent can include a mixture of dimethoxyethane and methyl ethyl carbonate. If a mixture of dimethoxyethane and methyl ethyl carbonate is used, a lithium solution having a strong reducing power can be prepared, and doping of lithium to MnO2 can be performed rapidly.

[0049] In the 11th aspect of the present disclosure, in the method for producing a positive electrode active material of any one of the 5th to 10th aspects, for example, the lithium solution can further include an aromatic compound. A solution in which an aromatic compound is dissolved has a property of dissolving lithium as a cation with solvated electrons that release lithium.

[0050] In the 12th aspect of the present disclosure, in the method for producing a positive electrode active material of the 11th aspect, for example, the aromatic compound can include benzophenone.

[0051] The lithium secondary battery of the 13th aspect of the present disclosure includes a positive electrode including the positive electrode active material of any one of the 1st to 4th aspects. The positive electrode active material includes LiMnO2 that has lithium in advance, and thus various negative electrode active materials can be used. x Mn y O2, and thus various negative electrode active materials can be used.

[0052] In the 14th aspect of the present disclosure, in the lithium battery of the 13th aspect, for example, the positive electrode can include an aluminum positive electrode current collector. Aluminum and its alloys are inexpensive and easily thinned, and thus are suitable as a material for a positive electrode current collector.

[0053] In the 15th aspect of the present disclosure, in the lithium battery of the 13th or 14th aspect, the lithium battery can be in a discharged state at the end of assembly and can be self-charged. According to this configuration, a material having excellent cycle characteristics like graphite can be used as a negative electrode active material.

[0054] In the 16th aspect of the present disclosure, in the lithium battery of any one of the 13th to 15th aspects, for example, the lithium battery can further include a negative electrode including graphite. Graphite is not easily deteriorated even if repeated charging and discharging at a deep depth, and thus is particularly recommended.

[0055] In the 17th aspect of the present disclosure, the negative electrode can not have lithium at the end of assembly in the lithium battery of the 16th aspect. According to this configuration, a material having excellent cycle characteristics like graphite can be used as the negative electrode active material.

[0056] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0057] Figure 1 A cross-sectional view of a lithium secondary battery according to an embodiment of the present disclosure. The lithium secondary battery 10 according to the present embodiment includes a negative electrode 13, a positive electrode 16, a nonaqueous electrolyte 19, a separator 17, and a case 18. The negative electrode 13 includes a negative electrode current collector 11 and a negative electrode active material layer 12. The negative electrode active material layer 12 is provided on the negative electrode current collector 11. The positive electrode 16 includes a positive electrode current collector 14 and a positive electrode active material layer 15. The positive electrode active material layer 15 is provided on the positive electrode current collector 14. The separator 17 is disposed between the negative electrode 13 and the positive electrode 16. The negative electrode 13 and the positive electrode 16 face each other with the separator 17 interposed therebetween. The negative electrode 13, the positive electrode 16, the separator 17, and the nonaqueous electrolyte 19 are housed in the case 18.

[0058] The negative electrode active material layer 12 can include a negative electrode active material capable of occluding and releasing lithium ions. As the negative electrode active material capable of occluding and releasing lithium ions, graphite, silicon, silicon-containing oxide, zinc alloy, lithium metal, lithium alloy, and the like can be given. One kind selected from these negative electrode active materials can be used, or two or more kinds can be used in combination.

[0059] The negative electrode active material layer 12 can include graphite as the negative electrode active material. Graphite can be used alone as the negative electrode active material. Graphite is less likely to deteriorate even if repeated charging and discharging are performed with a deep depth, and thus is particularly recommended. Graphite alone as the negative electrode active material can be included in the negative electrode active material layer 12. Carbon materials other than graphite can be used as the negative electrode active material.

[0060] The negative electrode 13 can not have lithium at the end of assembly of the lithium secondary battery 10. In the present embodiment, the lithium secondary battery 10 is in a discharged state at the end of assembly, and can be self-charged from the start. That is, a material having excellent cycle characteristics like graphite can be used as the negative electrode active material. The "end of assembly" refers to any time during a period from the time when the manufacture of the lithium secondary battery 10 is completed to the time when an external power source is connected to the lithium secondary battery 10 and charging and discharging processing is performed.

[0061] The negative electrode active material layer 12 can include a conductive aid, an ion conductor, a binder, and the like.

[0062] The conductive aid and the ion conductor are used to reduce the resistance of the negative electrode 13. As the conductive aid, carbon materials (carbon conductive aids) such as carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide; electrically conductive polymer compounds such as polyaniline, polypyrrole, and polythiophene can be used. As the ion conductor, gel electrolytes such as polymethyl methacrylate, polymethyl methacrylate, organic solid electrolytes such as polyethylene oxide, and inorganic solid electrolytes such as Li7La3Zr2O 12 and the like can be used.

[0063] The binder is used to improve the cohesiveness of the material constituting the negative electrode 13. As the binder, high molecular 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 can be used.

[0064] As the negative electrode current collector 11, a sheet or a film made of a metal material such as stainless steel, nickel, copper, and alloys thereof can be used. The sheet or the film can be porous or non-porous. As the sheet or the film, a metal foil, a metal mesh, and the like are used. A carbon material such as carbon can be coated on the surface of the negative electrode current collector 11 as an electrically conductive auxiliary material.

[0065] The positive electrode active material layer 15 contains Li x Mn y O2(1.012 ≥ x ≥ 0.683, and 0.91 ≤ y ≤ 1.0 as the positive electrode active material. In detail, the positive electrode active material has a composition of Li x Mn y O2(1.012 ≥ x ≥ 0.683, and 0.91 ≤ y ≤ 1.0 at the end of assembly of the lithium secondary battery 10. The positive electrode active material contains Li x Mn y O2, and thus various positive electrode active materials can be used. Li x Mn y O2 is used as the positive electrode active material, and graphite is used as the negative electrode active material, the lithium secondary battery 10 is in a discharged state at the end of assembly. Therefore, the lithium secondary battery 10 can start self-charging after assembly. The positive electrode active material layer 15 can contain a positive electrode active material other than Li x Mn y O2, or can contain only Li x Mn y O2 as the positive electrode active material.

[0066] Note that the manganese dioxide MnO zIn this case, the value of z can be 1.92 ≤ z ≤ 2.0 (see Oriental Carbon Research Report Vol. 20, No. 2, P. 133, Table 1, for example). This is converted to Mn y In the case of O2, the value of y becomes 1.82 / 2.0 ≤ y ≤ 2.0 / 2.0, that is, 0.91 ≤ y ≤ 1.0, and thus, Li x Mn y In O2, y satisfies 0.91 ≤ y ≤ 1.0. The value of y can be y = 1, for example.

[0067] Li x Mn y In O2, the relationship of 1.012 ≥ x ≥ 0.864 can be satisfied. In this case, the capacity density of the lithium secondary battery 10 is further increased.

[0068] Li x Mn y In O2, the valence of Mn is 2.987 or more and 3.316 or less, for example. The valence of Mn varies between 3 and 4, and thus, the charge and discharge of the lithium secondary battery 10 proceeds smoothly. The valence of Mn can be 2.987 or more and 3.136 or less.

[0069] The positive electrode active material layer 15 can include a conductive aid, an ion conductor, a binder, and the like. As the conductive aid, the ion conductor, and the binder, the same materials as those usable in the negative electrode active material layer 12 can be used for the positive electrode active material layer 15.

[0070] As the positive electrode current collector 14, a sheet or a film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof can be used. Aluminum and its alloys are inexpensive and easily thinned, and thus, are suitable as the material of the positive electrode current collector 14. The sheet or the film can be porous or non-porous. As the sheet or the film, a metal foil, a metal mesh, or the like can be used. A carbon material such as carbon can be applied to the surface of the positive electrode current collector 14 as a conductive auxiliary material.

[0071] The non-aqueous electrolyte 19 is, for example, an electrolyte solution impregnated in the negative electrode 13, the positive electrode 16, and the separator 17. The non-aqueous electrolyte 19 can fill the inside space of the case 18. Lithium ions can move between the negative electrode 13 and the positive electrode 16 in accordance with the movement of the non-aqueous electrolyte 19.

[0072] The non-aqueous electrolyte 19 includes a non-aqueous solvent and a lithium salt.

[0073] As the non-aqueous solvent, a cyclic carbonate, a chain carbonate, an ester, a cyclic ether, a chain ether, a nitrile, an amide, or the like can be used. One selected from these solvents can be used, or two or more can be used in combination.

[0074] As the lithium salt, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis-perfluoroethyisulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, lithium difluoro(oxalato)borate, and the like can be used. One selected from among these electrolyte salts can be used, or two or more can be used in combination.

[0075] The separator 17 has lithium ion conductivity. The material of the separator 17 is not particularly limited as long as lithium ions are allowed to pass therethrough. The material of the separator 17 can be at least one selected from the group consisting of a solid electrolyte, a gel electrolyte, an ion exchange resin membrane such as a lithium cation exchange resin, a semipermeable membrane, and a porous membrane. If the separator 17 is made of these materials, the safety of the lithium secondary battery 10 can be sufficiently ensured. As the solid electrolyte, sulfide solid electrolytes such as Li2S-P2S5, oxide solid electrolytes such as Li7La3Zr2O 12 (LLZ), and the like can be given. As the gel electrolyte, a gel electrolyte containing a fluorine resin such as PVdF can be given. As the ion exchange resin membrane, a cation exchange membrane, an anion exchange membrane, and the like can be given. As the porous membrane, a porous membrane made of a polyolefin resin, a porous membrane formed of glass paper obtained by weaving glass fibers into a nonwoven fabric, and the like can be given.

[0076] The case 18 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 case 18 can be a resin-made or metal-made container.

[0077] The shape of the lithium secondary battery 10 is not particularly limited. Various shapes such as a coin type, a cylindrical type, a square type, a sheet type, a button cell type, a flat type, a stacked type, and the like can be adopted as the shape of the lithium secondary battery 10.

[0078] The lithium secondary battery 10 can be a full solid battery.

[0079] Next, a manufacturing method of the positive electrode active material will be described. Figure 2 A process chart of the manufacturing method of the positive electrode active material is shown.

[0080] In step S1, a lithium solution is prepared by dissolving lithium metal in an organic solvent. The lithium solution shows strong reducing power. As the organic solvent, at least one selected from the group consisting of glyme and chain carbonate can be used. These solvents can dissolve aromatic compounds such as benzophenone. If lithium metal is put in a solution prepared from these solvents and aromatic compounds such as benzophenone, the lithium metal donates peripheral electrons to the solution to become ions. The solution that has accepted the electrons is in a solvated state with the electrons, and becomes a state in which lithium metal is dissolved in the solution. Its potential is close to that of lithium metal, but its value is different from that of the combination of the aromatic compound and the solvent.

[0081] As the glyme, at least one selected from the group consisting of dimethoxyethane, diglyme, triglyme, tetraglyme, and polyethylene glycol dimethyl ether can be used. As the chain carbonate, at least one selected from the group consisting of ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate can be used.

[0082] Typically, the organic solvent contains at least one selected from the group consisting of triglyme, dimethoxyethane, and ethyl methyl carbonate.

[0083] The lithium solution can further contain an aromatic compound. A solution in which an aromatic compound is dissolved has the property of releasing solvated electrons of lithium, dissolving lithium as a cation. In other words, the aromatic compound has the property of accepting, as solvated electrons, the electrons released when lithium is dissolved in an organic solvent, and dissolving in the solvent of the lithium solution. Lithium having a molar mass equivalent to that of the aromatic compound can be dissolved in the organic solvent. For example, when the concentration of the aromatic compound in the lithium solution is 1 mol / liter, the concentration of lithium in the lithium solution is also approximately 1 mol / liter.

[0084] As the aromatic compound, at least one selected from the group consisting of benzophenone, biphenyl, phenanthrene, naphthalene, anthracene, o-terphenyl, benzophenanthrene, trans-stilbene, 2,2'-bipyridine, 4,4'-bipyridine, 3,3'-bipyridine, 2,3'-bipyridine, 2,4'-bipyridine, 3,4'-bipyridine, 1,10-phenanthroline, cis-stilbene, and fluorene can be used. The aromatic compound can typically be benzophenone. The impregnation time of Mn02in the lithium solution can be adjusted according to the kind and concentration of the aromatic compound.

[0085] The organic solvent can include a mixture of triethylene glycol dimethyl ether and methyl ethyl carbonate. If a mixture of triethylene glycol dimethyl ether and methyl ethyl carbonate is used, a lithium solution having a strong reducing power can be prepared, and thus, doping of lithium into Mn02can be performed rapidly. The mixing ratio of triethylene glycol dimethyl ether to methyl ethyl carbonate is not particularly limited, and can be adjusted within a range satisfying 0.1 < (triethylene glycol dimethyl ether / methyl ethyl carbonate) < 0.9 in terms of volume ratio. The organic solvent can include only triethylene glycol dimethyl ether and methyl ethyl carbonate as solvents.

[0086] The organic solvent can include a mixture of dimethoxyethane and methyl ethyl carbonate. If a mixture of dimethoxyethane and methyl ethyl carbonate is used, a lithium solution having a strong reducing power can be prepared, and thus, doping of lithium into Mn02can be performed rapidly. The mixing ratio of dimethoxyethane to methyl ethyl carbonate is not particularly limited, and can be adjusted within a range satisfying 0.1 < (dimethoxyethane / methyl ethyl carbonate) < 0.9 in terms of volume ratio. The organic solvent can include only dimethoxyethane and methyl ethyl carbonate as solvents.

[0087] In step S2, Mn02is brought into contact with the lithium solution. Typically, Mn02is impregnated with the lithium solution. Thereby, lithium is doped into Mn02having a layered structure or a tunnel structure, to produce Li x Mn y O2. Mn is reduced from a valence of 4 to a valence of 3. Mn02may be shaped into the shape of an electrode. That is, an electrode including Mn02as an active material can be impregnated with the lithium solution. In order to increase the reaction speed, the lithium solution can also be heated to a temperature higher than room temperature.

[0088] The crystal structure of Mn02is not particularly limited. Various crystal structures of Mn02such as α-Mn02, β-Mn02, γ-Mn02, ε-Mn02, λ-Mn02, δ-Mn02, γ-β-Mn02, R-Mn02(rhombohedral manganite type manganese dioxide), and the like can be used. Mn02may include a plurality of crystal phases, or can be a mixed crystal. Mn02may include at least one selected from the group consisting of γ-β-Mn02, rhombohedral manganite, and β-Mn02. γ-β-Mn02, which can be produced from rhombohedral manganite, electrolytic manganese dioxide, and β-Mn02, which can be produced from the same electrolytic manganese dioxide, all have a tunnel structure, are inexpensive, and are easily available. In particular, γ-β-Mn02shows the largest discharge capacity density when used as an active material of a primary battery. In other words, Mn is easily reduced. Note that γ-β-Mn02indicates a mixed crystal of γ-Mn02and β-Mn02.

[0089] After impregnating Mn02with the lithium solution, in step S3, waiting is performed until a prescribed time elapses. The "prescribed time" is a time until lithium is doped into Mn02to satisfy Lix Mn y O2(1.012≥x≥0.683 or 0.864) required time. The reduction speed of Mn varies depending on the strength of the reducing property of the lithium solution, the concentration of lithium in the lithium solution, the temperature of the lithium solution, the shape of MnO2 (whether it is a powder or a shaped body), and the like. Therefore, the optimum prescribed time can be investigated in advance by experiment.

[0090] After the prescribed time, in step S4, Li x Mn y O2is separated from the lithium solution. Li x Mn y O2is washed and dried. Thus, Li x Mn y O2(1.012≥x≥0.683, and 0.91≤y≤1.0) can be obtained as the positive electrode active material. The obtained Li x Mn y O2is used as the positive electrode active material, the lithium secondary battery 10 described with reference to Figure 1 can be manufactured. That is, Li x Mn y O2(1.012≥x≥0.683, and 0.91≤y≤1.0) is used as the positive electrode active material, and graphite is used as the negative electrode active material, the lithium secondary battery 10 can be manufactured. The lithium secondary battery 10 is in a discharged state at the end of assembly and is capable of self-charging from the start.

[0091] Example

[0092] (Example 1)

[0093] A slurry was prepared by mixing PVdF as a binder, γ-β-MnO2powder, and a solvent. The slurry was applied to an Al current collector to form an applied film. The applied film was dried and calendered to obtain an electrode having a size of 2 cm x 2 cm.

[0094] A mixed solution was prepared by mixing benzophenone and methyl ethyl carbonate. The concentration of benzophenone in the mixed solution was 1 mol / liter. A lithium solution was prepared by dissolving Li metal in the mixed solution until the saturation concentration. The concentration of lithium in the lithium solution was 1 mol / liter.

[0095] The electrode was immersed in the above-described lithium solution for 137.9 days. After 137.9 days, the electrode was taken out of the lithium solution, washed with methyl ethyl carbonate, and dried in a vacuum. Thus, the positive electrode of Example 1 was obtained.

[0096] A lithium secondary battery of Example 1 was produced using the positive electrode of Example 1, Li metal as the negative electrode, and an electrolyte. The electrolyte was prepared by dissolving LiPF6 at a concentration of 1 mol / liter in an organic solvent containing ethylene carbonate and methyl ethyl carbonate at a volume ratio of 1:1. A charge-discharge test was performed on the resulting lithium secondary battery. The test was started from charging. The charge-discharge current was 0.05 mA, the charge termination voltage was 4.3 V, and the discharge termination voltage was 1.5 V. The results are shown in Figure 3A , Figure 3B and Figure 3C .

[0097] Figure 3A is a charge-discharge curve of the first cycle of the lithium secondary battery of Example 1. Figure 3B is a charge-discharge curve of the second cycle of the lithium secondary battery of Example 1. Figure 3C is a graph showing the relationship between the discharge capacity density and the cycle number of discharge of the lithium secondary battery of Example 1. Figure 3A and Figure 3B The horizontal axis indicates the capacity density (mAh / g) of the lithium secondary battery, and the vertical axis indicates the voltage (V). Figure 3C The horizontal axis indicates the number of discharges (cycle number), and the vertical axis indicates the discharge capacity density (mAh / g).

[0098] As shown in Figure 3A , the lithium secondary battery of Example 1 was in a discharged state immediately after assembly and was able to charge from the first cycle. This indicates that a sufficient amount of lithium was doped into the MnO2 to form LiMnO2.

[0099] As shown in Figure 3A and Figure 3B , the discharge capacity density of the first cycle was approximately the same as that of the second cycle. As shown in Figure 3C , even if the charge-discharge of 10 cycles was repeated, the discharge capacity density was substantially unchanged.

[0100] The amount of substance of Li doped in the MnO2 during the reduction treatment and the valence number of Mn were calculated from the charge capacity density (309.5 mAh / g) of the first cycle. As a result, the amount of substance of the doped Li was 1.012 moles with respect to 1 mole of MnO2. The valence number of Mn after the reduction treatment was 2.987.

[0101] (Example 2)

[0102] A mixed solution was prepared by adding benzophenone at a concentration of 1 mol / liter in a mixed solvent containing triethylene glycol dimethyl ether and methyl ethyl carbonate at a volume ratio of 1:1. A lithium solution was prepared by dissolving Li metal in the mixed solution. The concentration of lithium in the lithium solution was 1 mol / liter.

[0103] The same electrode as in Example 1 was immersed in the above lithium solution for 1.8 days. Thereafter, a lithium secondary battery of Example 2 was fabricated in the same manner as in Example 1, and a charge-discharge test was performed. The results are shown in Figure 4A , Figure 4B and Figure 4C . Figure 4A , Figure 4B and Figure 4C are graphs showing the test results for the same items as Figure 3A , Figure 3B and Figure 3C .

[0104] As shown in Figure 4A , the lithium secondary battery of Example 2 was in a discharged state immediately after assembly and was able to charge from the first cycle. As shown in Figure 4C , even if the charge-discharge of 10 cycles was repeated, the discharge capacity density was substantially unchanged.

[0105] The amount of Li doped in MnO2 in the reduction treatment and the valence number of Mn were calculated from the charge capacity density (276 mAh / g) of the first cycle. As a result, the amount of doped Li was 0.905 mole with respect to 1 mole of MnO2. The valence number of Mn after the reduction treatment was 3.094. The time of the reduction treatment in Example 2 was 1.8 days, which was substantially shorter than the time of the reduction treatment in Example 1.

[0106] (Example 3)

[0107] The immersion time of the electrode in the lithium solution was changed to 2.8 days, and otherwise, a lithium secondary battery of Example 3 was fabricated in the same manner as in Example 2, and a charge-discharge test was performed. The results are shown in Figure 5A , Figure 5B and Figure 5C . Figure 5A , Figure 5B and Figure 5C are graphs showing the test results for the same items as Figure 3A , Figure 3B and Figure 3C .

[0108] As shown in Figure 5A , the lithium secondary battery of Example 3 was in a discharged state immediately after assembly and was able to charge from the first cycle. As shown in Figure 5C , even if the charge-discharge of 10 cycles was repeated, the discharge capacity density was substantially unchanged.

[0109] The amount of substance of the doped Li in the MnO2 and the valence number of Mn in the reduction treatment were calculated from the charge capacity density of the 1st cycle (276 mAh / g). As a result, the amount of substance of the doped Li was 0.864 mole with respect to 1 mole of MnO2. The valence number of Mn after the reduction treatment was 3.136. The time of the reduction treatment in Example 3 was 2.8 days, which was substantially shorter than the time of the reduction treatment in Example 1.

[0110] (Example 4)

[0111] A treatment solution was prepared by adding benzophenone at a concentration of 1 mol / liter in a mixed solvent of dimethoxyethane and methyl ethyl carbonate at a volume ratio of 1:1. A lithium solution was prepared by dissolving Li metal in the treatment solution. The concentration of lithium in the lithium solution was 1 mol / liter.

[0112] The same electrode as in Example 1 was immersed in the above lithium solution for 1.8 days. Thereafter, a lithium secondary battery of Example 4 was fabricated in the same manner as in Example 1, and a charge-discharge test was performed. The results are shown in Figure 6A 、 Figure 6B and Figure 6C . Figure 6A 、 Figure 6B and Figure 6C are graphs showing the results of tests of the same items as Figure 3A 、 Figure 3B and Figure 3C .

[0113] As shown in Figure 6A , the lithium secondary battery of Example 4 was in a discharged state immediately after assembly and was able to charge from the 1st cycle. As shown in Figure 6C , even if the charge-discharge of 10 cycles was repeated, the discharge capacity density was substantially unchanged.

[0114] The amount of substance of the doped Li in the MnO2 and the valence number of Mn in the reduction treatment were calculated from the charge capacity density of the 1st cycle (210.54 mAh / g). As a result, the amount of substance of the doped Li was 0.683 mole with respect to 1 mole of MnO2. The valence number of Mn after the reduction treatment was 3.316. The time of the reduction treatment in Example 4 was 1.8 days, which was substantially shorter than the time of the reduction treatment in Example 1.

[0115] (Reference Example 1)

[0116] A slurry was prepared by mixing PVdF as a binder, γ-β-MnO2 powder, and a solvent. The slurry was coated onto an Al current collector to form a coating film. The coating film was dried and calendered to obtain an electrode with dimensions of 2 cm × 2 cm. This electrode was used as the positive electrode. Otherwise, a lithium secondary battery similar to that of Reference Example 1 was fabricated using the same method as in Example 1, and charge-discharge tests were performed. However, the tests started from discharge. The results are shown below. Figure 7A and Figure 7B .

[0117] Figure 7A The discharge curve of the first cycle of the lithium secondary battery in Reference Example 1 is shown. Figure 7B The charging curve of the lithium secondary battery in Reference Example 1 for the first cycle is shown. From the discharge capacity density of the first cycle, the mass of Li doped in the positive electrode and the valence of Mn during the first cycle discharge are calculated. The results show that the mass of doped Li relative to 1 mole of MnO2 is 0.929 moles. The valence of Mn is 3.071.

[0118] (Powder X-ray diffraction measurement)

[0119] Powder X-ray diffraction measurements were performed on the positive electrode active material of Example 2 and the positive electrode active material of Reference Example 1. The results are shown below. Figure 8 . Figure 8 The relationship between the results shown and the sample is as described below.

[0120] DIS… After the initial discharge of Example 1

[0121] DIS-CHA… After the initial charge as shown in Example 1

[0122] DIS-CHA-DIS… After the second cycle of discharge in Example 1

[0123] Li doping… Charge and discharge after Example 2 (after Li doping)

[0124] Blank…polyethylene bag used during the measurement

[0125] Depend on Figure 8 The results show that the "DIS", "DIS-CHA-DIS", and "Li-doped" samples have the same structure.

[0126] Industrial availability

[0127] The technology disclosed herein can be used in lithium secondary batteries.

[0128] Explanation of reference numerals in the attached figures

[0129] 10 Lithium secondary batteries

[0130] 11 Negative current collector

[0131] 12 negative active material layer

[0132] 13 negative electrode

[0133] 14 positive current collector

[0134] 15 positive active material layer

[0135] 16 positive electrode

[0136] 17 separator

[0137] 18 case

[0138] 19 nonaqueous electrolyte

Claims

1. A method for producing a positive electrode active material, comprising the step of bringing a lithium solution in which lithium metal is dissolved in contact with Mn02, the lithium solution further containing benzophenone, the organic solvent being a mixture of triglyme dimethyl ether and methyl ethyl carbonate, or a mixture of dimethoxyethane and methyl ethyl carbonate.

2. The method for manufacturing a positive electrode active material according to claim 1, wherein Mn02 comprises at least one selected from the group consisting of γ-β-Mn02, ramsdellite, and β-Mn02. Mn02 comprises at least one selected from the group consisting of γ-β-Mn02, ramsdellite, and β-Mn02.

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