Lithium secondary battery and method for producing same

Abstract

A lithium secondary battery 100 according to the present disclosure is provided with: a positive electrode 5 which contains MnO2; a negative electrode 6 which comprises a negative electrode active material that has adsorbed lithium ions; and an electrolyte solution which contains a mixed solvent of a cyclic carbonate and a chain carbonate, and with which the positive electrode 5 and the negative electrode 6 are impregnated. The negative electrode active material contains, for example, at least one substance that is selected from the group consisting of graphite and silicon. The cyclic carbonate comprises, for example, at least one substance that is selected from the group consisting of ethylene carbonate and vinylene carbonate. The chain carbonate comprises, for example, ethyl methyl carbonate.

Description

Lithium secondary battery and its manufacturing method

[0001] The present disclosure relates to a lithium secondary battery and a method for manufacturing the same.

[0002] The most well-known positive electrode active material for lithium secondary batteries is a cobalt-containing composite oxide. However, the estimated reserves of cobalt are not particularly large, raising concerns about their depletion in the near future. Positive electrode active materials that do not contain rare metals like cobalt include manganese dioxide and transition metal phosphates. Transition metal phosphates, such as lithium iron phosphate, are already in practical use. On the other hand, manganese dioxide has a capacity density significantly higher than transition metal phosphates and is inexpensive, making it a promising next-generation positive electrode active material.

[0003] Patent Document 1 describes a nonaqueous secondary battery that includes a positive electrode using manganese dioxide as an active material and a negative electrode using lithium or a lithium alloy as an active material, the positive electrode further containing LiMnO. The electrolyte used is a mixed solvent of propylene carbonate (PC) and dimethoxyethane (DME) in which lithium perchlorate is dissolved at a concentration of 1 mol / L.

[0004] Japanese Patent Application Laid-Open No. 63-114064 International Publication No. 2022 / 118545

[0005] The present disclosure provides a technique for improving the capacity density of a lithium secondary battery using MnO2 in the positive electrode.

[0006] The present disclosure provides a lithium secondary battery comprising: a positive electrode containing MnO; a negative electrode containing a negative electrode active material that occludes lithium ions; and an electrolyte solution impregnated in the positive electrode and the negative electrode, the electrolyte solution containing a mixed solvent of a cyclic carbonate and a chain carbonate.

[0007] According to the present disclosure, it is possible to improve the capacity density of a lithium secondary battery using MnO2 in the positive electrode.

[0008] FIG. 1 is a cross-sectional view of a lithium secondary battery according to an embodiment of the present disclosure. FIG. 2 is a process diagram illustrating a method for manufacturing a lithium secondary battery. FIG. 3A is a cross-sectional view illustrating an example of the positional relationship between a container, a Li metal foil, and an electrode group. FIG. 3B is a perspective view illustrating an example of the positional relationship between a container, a Li metal piece, and an electrode group. FIG. 4 is a graph showing initial charge / discharge curves of the lithium secondary batteries of Example 1 and Comparative Example 1. FIG. 5 is a graph showing initial charge / discharge curves of the lithium secondary batteries of Example 1 and Example 2. FIG. 6 is a graph showing the results of a charge / discharge cycle test of the lithium secondary battery of Example 1. FIG. 7A is a graph showing charge / discharge curves of the lithium secondary battery of Example 1 in charge / discharge test 2. FIG. 7B is a graph showing charge / discharge curves of the lithium secondary battery of Example 3 in charge / discharge test 2.

[0009] (Findings that form the basis of this disclosure) Because MnO2 does not contain lithium, lithium secondary batteries that use MnO2 as a positive electrode active material must be in a charged state upon completion of assembly. For example, the nonaqueous secondary battery of Patent Document 1 uses Li metal as a negative electrode active material.

[0010] As known to those skilled in the art, lithium secondary batteries using Li metal as the negative electrode active material have poor reversibility. When Li alloys, such as LiAl alloys, are used instead of Li metal, the cycle characteristics at shallow depths are good. However, repeated charging and discharging at deep depths results in the battery reaching the end of its life after a small number of charge-discharge cycles. Therefore, lithium secondary batteries using MnO2 as the positive electrode active material have only been put to practical use in applications requiring repeated charging and discharging at shallow depths, such as memory backup applications. Another problem is that Li alloys are hard and unsuitable for batteries with a wound structure.

[0011] In addition, in Patent Document 1, a mixed solvent of propylene carbonate (PC) and dimethoxyethane (DME) is used as the solvent for the nonaqueous electrolyte. However, since the withstand voltage of this mixed solvent is low, the battery of Patent Document 1 can only utilize about half of the theoretical capacity of the positive electrode (MnO2).

[0012] It is conceivable to use lithium-occluded MnO2 as a positive electrode active material, as described in Patent Document 2. However, it is not easy to prepare such a material.

[0013] Therefore, there is a need for a technology to improve the capacity density of lithium secondary batteries using MnO2 as the positive electrode. In addition to improving the capacity density, lithium secondary batteries are also required to have practical reversibility. Specifically, lithium secondary batteries are required to exhibit good cycle characteristics even when charged and discharged at deep depths.

[0014] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0015] (Embodiment) FIG. 1 is a cross-sectional view of a lithium secondary battery according to an embodiment of the present disclosure. The lithium secondary battery 100 includes a container 1 and an electrode group 4. The electrode group 4 has a wound structure. The electrode group 4 is housed in the container 1. The electrode group 4 includes a positive electrode 5, a negative electrode 6, and a pair of separators 7. The electrode group 4 is impregnated with an electrolyte. The opening of the container 1 is closed with a sealing plate 2. The positive electrode 5 includes a positive electrode current collector 5a and a positive electrode active material layer 5b. One end of a positive electrode lead 5c is connected to the positive electrode 5. The other end of the positive electrode lead 5c is connected to the back surface of the sealing plate 2. An insulating gasket 3 is disposed around the periphery of the sealing plate 2. The negative electrode 6 includes a negative electrode current collector 6a and a negative electrode active material layer 6b. One end of a negative electrode lead 6c is connected to the negative electrode 6. The other end of the negative electrode lead 6c is connected to the bottom surface of the container 1. An insulating ring 8 is disposed on each of the upper and lower surfaces of the electrode group 4 .

[0016] In this embodiment, the container 1 has a negative polarity and the sealing plate 2 has a positive polarity. However, the container 1 may have a positive polarity and the sealing plate 2 may have a negative polarity.

[0017] The positive electrode current collector 5a can be a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable materials for the positive electrode current collector 5a because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, or the like may be used as the sheet or film. A carbon material such as carbon may be applied to the surface of the positive electrode current collector 5a as a conductive auxiliary material.

[0018] The positive electrode active material layer 5b contains MnO2 as a positive electrode active material. Specifically, the positive electrode active material layer 5b contains MnO2 when the assembly of the lithium secondary battery 100 is completed. The lithium secondary battery 100 is in a charged state when the assembly is completed. Therefore, the lithium secondary battery 100 can start discharging after assembly. "When the assembly is completed" means any point in time during the period from when the fabrication of the lithium secondary battery 100 is completed to when an external power source is connected to the lithium secondary battery 100 and a charge / discharge process is performed.

[0019] Electrolytic manganese dioxide can be used as MnO2. Electrolytic manganese dioxide is manganese dioxide produced by an electrolytic method. Electrolytic manganese dioxide is known as a material for primary batteries and is inexpensive. Electrolytic manganese dioxide may be heat-treated at temperatures between 150°C and 430°C. Although manganese dioxide is expressed as the chemical formula "MnO2," it is actually a non-stoichiometric compound.

[0020] The crystal structure of MnO2 is not particularly limited. Various crystal structures of MnO2, such as α-MnO2, β-MnO2, γ-MnO2, ε-MnO2, λ-MnO2, δ-MnO2, γ-β-MnO2, and R-MnO2 (ramsdellite-type manganese dioxide), can be used. MnO2 may contain multiple crystalline phases or may be a mixed crystal. MnO2 may contain at least one selected from the group consisting of γ-β-MnO2, ramsdellite, and β-MnO2. Ramsdellite, γ-β-MnO2, which can be prepared from electrolytic manganese dioxide, and β-MnO2, which can also be prepared from electrolytic manganese dioxide, all have a tunnel structure and are inexpensive and readily available. In particular, γ-β-MnO2 exhibits the highest discharge capacity density when used as an active material. In other words, Mn is easily reduced. Therefore, γ-β-MnO2 is also suitable as an active material for the lithium secondary battery 100. Note that γ-β-MnO2 represents a mixed crystal of γ-MnO2 and β-MnO2.

[0021] The positive electrode active material layer 5b may contain a positive electrode active material other than MnO2, or may contain only MnO2 as the positive electrode active material.

[0022] The positive electrode active material layer 5b may contain other materials such as a conductive additive and a binder.

[0023] The conductive additive is used to reduce the resistance of the positive electrode 5. 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.

[0024] The binder is used to improve the binding property of the material constituting the positive electrode 5. As the binder, 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 can be used.

[0025] The negative electrode current collector 6a may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. 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 such as carbon may be applied to the surface of the negative electrode current collector 6a as a conductive auxiliary material.

[0026] The negative electrode active material layer 6b contains a negative electrode active material that has absorbed lithium ions. Specifically, the negative electrode active material layer 6b contains lithium ions when the assembly of the lithium secondary battery 100 is completed. The lithium secondary battery 100 is in a charged state when the assembly is completed.

[0027] The negative electrode active material is a material capable of reversibly absorbing and releasing lithium ions. The negative electrode active material includes, for example, at least one selected from the group consisting of carbon materials and materials capable of forming an alloy with lithium. Examples of carbon materials include graphite. Examples of materials capable of forming an alloy with lithium include silicon, silicon-containing oxides, tin, zinc alloys, bismuth, and germanium. One or more of these negative electrode active materials may be used alone or in combination.

[0028] The negative electrode active material layer 6b may contain at least one selected from the group consisting of graphite and silicon as the negative electrode active material. Graphite may be the only negative electrode active material contained in the negative electrode active material layer 6b. Graphite is recommended because it is resistant to deterioration even when repeatedly charged and discharged at a deep depth. Carbon materials other than graphite may also be used as the negative electrode active material. Silicon exhibits a larger capacity than graphite and is therefore advantageous for increasing the capacity of the lithium secondary battery 100.

[0029] In this embodiment, the lithium secondary battery 100 is in a charged state upon completion of assembly and can begin discharging. When graphite is used as the negative electrode active material, the negative electrode 6 may contain LiC when the lithium secondary battery 100 is completed. When silicon is used as the negative electrode active material, the negative electrode 6 may contain LiC when the lithium secondary battery 100 is completed. 22 It may contain Si5.

[0030] The negative electrode active material layer 6b may contain a conductive additive, a binder, etc. As the conductive additive and the binder, the same materials that can be used for the positive electrode active material layer 5b can be used for the negative electrode active material layer 6b.

[0031] The electrolyte is a non-aqueous electrolyte impregnated into the positive electrode 5, the negative electrode 6, and the separator 7. The electrolyte may fill the internal space of the container 1. The electrolyte allows lithium ions to move between the positive electrode 5 and the negative electrode 6.

[0032] The electrolyte solution contains a non-aqueous solvent and a lithium salt.

[0033] A mixed solvent of a cyclic carbonate and a chain carbonate is used as the nonaqueous solvent. The mixed solvent of a cyclic carbonate and a chain carbonate has excellent voltage resistance. Therefore, by using a mixed solvent of a cyclic carbonate and a chain carbonate, it is possible to charge MnO2 to a high charging voltage. In other words, the capacity of the lithium secondary battery 100 can be increased by fully utilizing MnO2. Furthermore, since the negative electrode 6 contains a negative electrode active material that occludes lithium ions, the lithium secondary battery 100 exhibits good cycle characteristics even when charged and discharged at a deep depth.

[0034] The cyclic carbonate is typically a five-membered cyclic carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate, or vinylene carbonate. The cyclic carbonate may include at least one selected from the group consisting of ethylene carbonate and vinylene carbonate. By combining these cyclic carbonates with a chain carbonate, an electrolyte solution with high withstand voltage can be obtained.

[0035] At least one hydrogen atom of the cyclic carbonate may be substituted with a halogen atom. In other words, a halogen atom may be bonded to a carbon atom contained in the cyclic carbonate. Cyclic carbonates containing halogen atoms exhibit higher withstand voltage and are excellent in stability. Examples of cyclic carbonates containing halogen atoms include fluoroethylene carbonate and chloroethylene carbonate. The number of halogen atoms contained in the cyclic carbonate may be one or more.

[0036] The cyclic carbonate is represented by, for example, the following formula (1) or formula (2): 1 , R 2 , R 3 , and R 4 are each independently a hydrogen atom or a halogen atom. 5 and R 6 are each independently a hydrogen atom or a halogen atom. As shown in formula (1) or formula (2), the halogen atom may be bonded to the carbon atom at position 3 or 4 of the five-membered cyclic carbonate. The halogen atom may be F or Cl.

[0037]

[0038] The chain carbonate may be at least one selected from the group consisting of ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate. By combining these chain carbonates with cyclic carbonates, an electrolyte solution with high voltage resistance can be obtained. Among these, ethyl methyl carbonate is recommended.

[0039] 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. One selected from these lithium salts may be used, or two or more may be used in combination.

[0040] The separator 7 has lithium ion conductivity. The material of the separator 7 is not particularly limited as long as it allows lithium ions to pass through. The material of the separator 7 can be at least one selected from the group consisting of a gel electrolyte, an ion exchange resin membrane, a semipermeable membrane, and a porous membrane. Making the separator 7 from these materials can sufficiently ensure the safety of the lithium secondary battery 100. Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVdF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resins and porous membranes containing glass paper obtained by weaving glass fibers into nonwoven fabric.

[0041] The container 1 is made of a metal such as aluminum or stainless steel, and may have a cylindrical shape or a rectangular tube shape.

[0042] The electrode group 4 may be wound into a cylindrical shape or an oval shape.

[0043] There is no particular limitation on the shape of the lithium secondary battery 100. The lithium secondary battery 100 may have various shapes such as a cylindrical shape or a rectangular tubular shape.

[0044] Next, a method for manufacturing the lithium secondary battery 100 will be described.

[0045] In step S1, an electrode group 4 is produced. Specifically, after a positive electrode 5 and a negative electrode 6 are produced, the positive electrode 5, the negative electrode 6, and a pair of separators 7 are stacked and wound to form a spirally wound electrode group 4. In this way, the electrode group 4 is obtained.

[0046] The positive electrode 5 can be produced by applying a positive electrode mixture to one or both surfaces of a positive electrode current collector 5a, followed by drying and rolling. The positive electrode 5 has a thickness of, for example, 30 μm to 200 μm and is preferably highly flexible. The positive electrode mixture is obtained by kneading and uniformly dispersing materials such as a positive electrode active material, a binder, and a conductive additive using a dispersion medium.

[0047] The method for applying the positive electrode mixture to the positive electrode current collector 5a is not particularly limited. The positive electrode mixture in a slurry form can be applied to the positive electrode current collector 5a using a slit die coater, reverse roll coater, lip coater, blade coater, knife coater, gravure coater, dip coater, etc. The applied positive electrode mixture may be dried naturally or using a drying device.

[0048] After drying, the positive electrode 5 is rolled so that the positive electrode active material layer 5b has a predetermined thickness. The rolling may be performed multiple times using a roll press, or may be performed multiple times while changing the pressing pressure of the roll press. After rolling, a positive electrode lead 5c for extracting power is welded to the positive electrode current collector 5a.

[0049] The negative electrode 6 can be fabricated by applying a negative electrode composite to one or both sides of a negative electrode current collector 6a, drying, and rolling. After rolling, a negative electrode lead 6c for power extraction is welded to the negative electrode current collector 6a. The negative electrode 6 has a thickness of, for example, 30 μm to 210 μm and is preferably highly flexible. The negative electrode composite is obtained by kneading and uniformly dispersing materials such as a negative electrode active material, a binder, and a conductive additive using a dispersion medium.

[0050] As a method for producing the negative electrode 6, a dry method may be adopted instead of the above-mentioned wet method. Examples of dry film formation methods include evaporation, sputtering, and CVD (chemical vapor deposition). All of these methods can be performed in a vacuum. From the viewpoint of efficiently forming the negative electrode active material layer 6b, evaporation is preferable. As the evaporation method, either electron beam evaporation or resistance heating evaporation can be used. When depositing an oxide, nitride, or oxynitride, the oxide and / or nitride can be used as the evaporation material. Reactive evaporation may be performed by introducing oxygen gas, nitrogen gas, or ions or radicals containing elements constituting these gases between the negative electrode current collector 6a and the evaporation source while evaporating a negative electrode active material such as silicon or tin from the evaporation source.

[0051] In step S2, a Li metal foil is wrapped around the electrode group 4. When the negative electrode active material is graphite, the amount of Li in the Li metal foil is adjusted to satisfy, for example, a molar ratio of lithium:carbon=1:6. When the negative electrode active material is silicon, the amount of Li in the Li metal foil is adjusted to satisfy, for example, a molar ratio of lithium:silicon=22:5.

[0052] In step S3, the electrode group 4 is placed in the container 1. Specifically, the electrode group 4 and the Li metal foil are arranged inside the container 1 so that the Li metal foil is electrically connected to the negative electrode 6 of the electrode group 4 and is located between the electrode group 4 and the inner surface of the container 1. With this configuration, lithium ions can be supplied to the negative electrode 6 and absorbed by injecting an electrolyte solution.

[0053] 3A is a cross-sectional view showing an example of the positional relationship between the container 1, the Li metal foil 9, and the electrode group 4. In the example shown in FIG. 3A, the container 1 is a cylindrical container. By placing the electrode group 4 wrapped with the Li metal foil 9 in the container 1, the Li metal foil 9 can be disposed between the electrode group 4 and the inner surface of the container 1. The electrode group 4, the Li metal foil 9, and the container 1 are arranged in this order in the radial direction of the container 1. The Li metal foil 9 is in contact with the container 1 and is electrically connected to the negative electrode 6 via the container 1. With this configuration, lithium ions can be supplied to and absorbed into the negative electrode 6 by injecting an electrolyte solution.

[0054] The lithium ion supply source is not limited to the Li metal foil 9 shown in FIG. 3A . FIG. 3B is a perspective view showing an example of the positional relationship between the container 1, the Li metal piece 19, and the electrode group 4. In the example shown in FIG. 3B , the lithium ion supply source is at least one Li metal piece 19. The container 1 is a rectangular cylindrical container. The Li metal piece 19 is disposed at at least one of the four corners of the container 1. This allows the Li metal to be disposed between the electrode group 4 and the inner surface of the container 1. The Li metal piece 19 is in contact with the container 1 and electrically connected to the negative electrode 6 via the container 1. With this configuration, lithium ions can be supplied to and absorbed in the negative electrode 6 by injecting an electrolyte solution. When the rectangular cylindrical container 1 is combined with the cylindrical electrode group 4, dead spaces are generated at the four corners. According to this embodiment, the dead spaces at the four corners of the rectangular cylindrical container 1 can be effectively utilized. Note that the four corners of the rectangular cylindrical container 1 may be curved.

[0055] When Li metal pieces 19 are used as the lithium ion supply source, the above-mentioned step S2 is not necessary. In step S3, after the electrode group 4 is placed in the container 1, the Li metal pieces 19 may be placed inside the container 1, or the Li metal pieces 19 may be placed in the container 1 together with the electrode group 4. Alternatively, the Li metal pieces 19 may be placed in the container 1 first, and then the electrode group 4 may be placed in the container 1.

[0056] The Li metal pieces 19 may be placed at each of the four corners of the container 1 .

[0057] 3A, the step of placing the electrode group 4 in the container 1 also serves as a step of disposing Li metal between the electrode group 4 and the inner surface of the container 1. According to the example shown in Fig. 3B, the step of placing the electrode group 4 in the container 1 may also serve as a step of disposing Li metal between the electrode group 4 and the inner surface of the container 1, or these steps may be separate steps.

[0058] In step S4, an electrolyte solution is poured into the container 1. Because the negative electrode 6 is electrically connected to the Li metal foil 9 or the Li metal piece 19, pouring the electrolyte solution causes the Li metal foil 9 or the Li metal piece 19 to dissolve, supplying lithium ions to the electrolyte solution. The lithium ions move to the negative electrode 6 through the electrolyte solution and are absorbed into the negative electrode 6. With this configuration, lithium ions can be easily absorbed into the negative electrode 6.

[0059] In step S5, the container 1 is sealed.

[0060] Through the above steps, the lithium secondary battery 100 shown in FIG. 1 is obtained.

[0061] After the assembly is completed, the electrode group 4 may be discharged and charged. In one example, the end-of-charge voltage during charging is 4.2 V or higher. Since the electrolyte used in this embodiment has excellent voltage resistance, it is possible to charge the lithium secondary battery 100 up to an end-of-charge voltage of 4.2 V or higher. According to this embodiment, the capacity density of MnO2 can be fully exhibited.

[0062] Even if Li metal is not completely absorbed in the negative electrode 6 after the electrolyte solution is poured into the container 1, discharging the lithium secondary battery 100 causes the Li metal to dissolve in the electrolyte solution and lithium ions to be supplied to the positive electrode 5. When the lithium secondary battery 100 is subsequently charged, the lithium ions are absorbed in the active material, such as graphite, in the negative electrode 6.

[0063] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0064] (Technology 1) A lithium secondary battery comprising: a positive electrode containing MnO2; a negative electrode containing a negative electrode active material that occludes lithium ions; and an electrolyte solution impregnated in the positive electrode and the negative electrode, the electrolyte solution containing a mixed solvent of a cyclic carbonate and a chain carbonate.

[0065] This configuration allows MnO2 to be charged to a high charging voltage. In other words, the capacity of the lithium secondary battery can be increased by fully utilizing MnO2. Furthermore, since the negative electrode contains a negative electrode active material that occludes lithium ions, the lithium secondary battery exhibits good cycle characteristics even when charged and discharged at high depths.

[0066] (Technology 2) The lithium secondary battery according to Technology 1, wherein the negative electrode active material comprises at least one selected from the group consisting of graphite and silicon. Graphite is recommended because it is resistant to deterioration even when repeatedly charged and discharged at a deep depth. Silicon exhibits a larger capacity than graphite and is therefore advantageous for increasing the capacity of the lithium secondary battery.

[0067] (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the cyclic carbonate comprises at least one selected from the group consisting of ethylene carbonate and vinylene carbonate. By combining these cyclic carbonates with chain carbonates, an electrolyte solution with high withstand voltage can be obtained.

[0068] (Technology 4) The lithium secondary battery according to Technology 1 or 2, wherein a halogen atom is bonded to a carbon atom contained in the cyclic carbonate. Cyclic carbonates having halogen atoms exhibit higher withstand voltage and are excellent in stability.

[0069] (Technology 5) The lithium secondary battery according to any one of Technologies 1 to 4, wherein the chain carbonate includes ethyl methyl carbonate. By combining ethyl methyl carbonate with a cyclic carbonate, an electrolyte solution with high withstand voltage can be obtained.

[0070] (Technology 6) The lithium secondary battery according to any one of Technologies 1 to 5, wherein the MnO2 contains electrolytic manganese dioxide. Electrolytic manganese dioxide is known as a material for primary batteries and is inexpensive.

[0071] (Technology 7) A method for manufacturing a lithium secondary battery, comprising: arranging an electrode group and Li metal inside a container such that Li metal is electrically connected to a negative electrode of the electrode group having a positive electrode containing MnO2 and a negative electrode containing a negative electrode active material, and the Li metal is located between the electrode group and an inner surface of the container; and injecting an electrolyte solution containing a mixed solvent of a cyclic carbonate and a chain carbonate into the container.

[0072] According to this configuration, by injecting the electrolyte, lithium ions can be supplied to the negative electrode and absorbed therein.

[0073] (Technology 8) The method for manufacturing a lithium secondary battery according to Technology 7, wherein the container is a cylindrical container, and the electrode group around which the Li metal is wound is placed in the container, thereby disposing the electrode group and the Li metal inside the container. With this configuration, by injecting an electrolyte solution, lithium ions can be supplied to the negative electrode and absorbed.

[0074] (Technology 9) The method for manufacturing a lithium secondary battery according to Technology 7, wherein the container is a rectangular cylindrical container and the Li metal is disposed in at least one of the four corners of the container. With this configuration, lithium ions can be supplied to the negative electrode and absorbed by injecting an electrolyte. In addition, the dead space at the four corners of the rectangular cylindrical container can be effectively utilized.

[0075] (Technology 10) The method for producing a lithium secondary battery according to any one of Techniques 7 to 9, wherein the Li metal is electrically connected to the negative electrode via the container. With this configuration, lithium ions can be easily absorbed into the negative electrode 6.

[0076] (Technology 11) The method for producing a lithium secondary battery according to any one of Technologies 7 to 10, further comprising discharging and charging the electrode group, wherein an end-of-charge voltage during charging is 4.2 V or more. With this configuration, the capacity density of MnO2 can be fully exhibited.

[0077] Example 1 A positive electrode composite was prepared by mixing a binder, a conductive additive, γ-β-MnO2 powder, and a solvent. The positive electrode composite was applied to an Al current collector to form a coating film. The coating film was dried and rolled to obtain a positive electrode. PVdF was used as the binder. Acetylene black was used as the conductive additive. N-methylpyrrolidone was used as the solvent. The ratio of the binder, conductive additive, and γ-β-MnO2 powder was 10:5:85 by mass.

[0078] A binder, graphite powder, and a solvent were mixed to prepare a negative electrode mixture. The negative electrode mixture was applied to a Cu current collector to form a coating film. The coating film was dried and rolled to obtain a negative electrode.

[0079] A positive electrode lead and a negative electrode lead were welded to the positive electrode and the negative electrode, respectively. Then, an electrode group having a wound structure was fabricated using the positive electrode and the negative electrode. A microporous polyolefin film was used as a separator.

[0080] Next, a Li metal foil was wrapped around the electrode group, and the amount of Li in the Li metal foil was adjusted to satisfy a lithium:carbon molar ratio of 1:6.

[0081] Next, the electrode group was placed in a cylindrical container so that the Li metal foil was electrically connected to the negative electrode through the container. An electrolyte solution was then poured into the container. The electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate and methyl ethyl carbonate in a 1:1 volume ratio. A sealing plate was attached to the container, and a lithium secondary battery with the structure shown in Figure 1 was obtained.

[0082] Example 2 A lithium secondary battery of Example 2 was fabricated in the same manner as in Example 1, except that the electrolyte solution was prepared using a mixed solvent containing vinylene carbonate (VC) and methyl ethyl carbonate (MEC) in a volume ratio of 1:1.

[0083] Example 3 A lithium secondary battery of Example 3 was fabricated in the same manner as in Example 1, except that the electrolyte solution was prepared using a mixed solvent containing fluoroethylene carbonate (FEC) and methyl ethyl carbonate (MEC) in a volume ratio of 1:1.

[0084] Comparative Example 1 A lithium secondary battery of Comparative Example 1 was fabricated in the same manner as in Example 1, except that the electrolyte solution was prepared using a mixed solvent containing propylene carbonate (PC) and dimethoxyethane (DME) in a volume ratio of 1:1.

[0085] (Charge / Discharge Test 1) Charge / discharge tests were conducted on the lithium secondary batteries of Example 1, Example 2, and Comparative Example 1. First, the lithium secondary batteries were subjected to a discharge treatment, and then the initial charge capacity and initial discharge capacity were measured. In Examples 1 and 2, the charge / discharge current was 0.025 C, the end-of-charge voltage was 4.2 V, and the end-of-discharge voltage was 2.0 V. In Comparative Example 1, the charge / discharge current was 0.025 C, the end-of-charge voltage was 3.6 V, and the end-of-discharge voltage was 2.0 V.

[0086] FIG. 4 is a graph showing the initial charge / discharge curves of the lithium secondary batteries of Example 1 and Comparative Example 1. The horizontal axis represents capacity density (mAh / g). The vertical axis represents voltage (V). As shown in FIG. 4, the lithium secondary battery of Example 1 exhibited a capacity density of approximately 270 mAh / g. In Example 1, no significant decomposition of the electrolyte was observed up to a charging voltage of 4.2 V. In contrast, the capacity density of the lithium secondary battery of Comparative Example 1 was approximately 150 mAh / g. In Comparative Example 1, significant decomposition of the electrolyte occurred, making it impossible to charge at voltages higher than 3.6 V.

[0087] 5 is a graph showing the initial charge / discharge curves of the lithium secondary batteries of Examples 1 and 2. Even when the cyclic carbonate in the mixed solvent was changed from ethylene carbonate (Example 1) to vinylene carbonate (Example 2), charging at a voltage of 4.2 V was possible, and a high capacity density of 250 mAh / g or more was achieved.

[0088] (Charge-Discharge Cycle Test) The lithium secondary battery of Example 1 was repeatedly charged and discharged 10 times. The charge-discharge conditions were as described in the charge-discharge test section. The results are shown in FIG.

[0089] 6 is a graph showing the results of a charge-discharge cycle test of the lithium secondary battery of Example 1. The horizontal axis represents the number of cycles, which is the number of charge-discharge cycles. The vertical axis represents the discharge capacity density (mAh / g). As shown in FIG. 6, the lithium secondary battery of Example 1 maintained a discharge capacity density of 200 mAh / g or more even after 10 cycles of charge-discharge at a deep charge depth.

[0090] (Charge / Discharge Test 2) A charge / discharge test was conducted on the lithium secondary batteries of Examples 1 and 3. First, the lithium secondary batteries were subjected to a discharge treatment, and then a first charge / discharge treatment was conducted. Then, a second charge / discharge treatment was conducted, and the charge capacity and discharge capacity were measured. In the charge / discharge test 2, the charge / discharge current was 0.015 C, the end-of-charge voltage was 4.2 V, and the end-of-discharge voltage was 2.0 V.

[0091] FIG. 7A is a graph showing the charge / discharge curves of the lithium secondary battery of Example 1 in charge / discharge test 2. FIG. 7B is a graph showing the charge / discharge curves of the lithium secondary battery of Example 3 in charge / discharge test 2. As shown in FIGS. 7A and 7B , in charge / discharge test 2, the lithium secondary battery of Example 1 exhibited a capacity density of approximately 180 mAh / g. The lithium secondary battery of Example 3 exhibited a capacity density of approximately 160 mAh / g. The current value in charge / discharge test 2 was approximately 0.6 times the current value in charge / discharge test 1. Even under these conditions, the lithium secondary batteries of Examples 1 and 3 exhibited a capacity density greater than the capacity density (approximately 150 mAh / g) of the lithium secondary battery of Comparative Example 1 in charge / discharge test 1. Furthermore, the lithium secondary battery of Example 3 had a capacity density comparable to that of the lithium secondary battery of Example 1.

[0092] The technology of the present disclosure is useful for lithium secondary batteries.

Claims

1. MnO 2 A positive electrode comprising: a negative electrode including a negative electrode active material that absorbs lithium ions; an electrolyte solution impregnated in the positive electrode and the negative electrode and including a mixed solvent of a cyclic carbonate and a chain carbonate; Equipped with At least one of the following requirements (i) and (ii) is satisfied: (i) the cyclic carbonate comprises at least one selected from the group consisting of ethylene carbonate and vinylene carbonate; (ii) the chain carbonate includes ethyl methyl carbonate; Lithium secondary battery.

2. The negative electrode active material includes at least one selected from the group consisting of graphite and silicon. The lithium secondary battery according to claim 1 .

3. A halogen atom is bonded to a carbon atom contained in the cyclic carbonate. The lithium secondary battery according to claim 1 .

4. The MnO 2 contains electrolytic manganese dioxide; The lithium secondary battery according to claim 1 .

5. MnO 2 and a negative electrode including a negative electrode active material, the electrode group and the Li metal being electrically connected to the negative electrode, and the Li metal being located between the electrode group and an inner surface of the container; and Injecting an electrolyte solution containing a mixed solvent of a cyclic carbonate and a chain carbonate into the container; A method for producing a lithium secondary battery, comprising:

6. The container is a cylindrical container, placing the electrode group with the Li metal wound thereon into the container, thereby disposing the electrode group and the Li metal inside the container; The method for producing the lithium secondary battery according to claim 5 .

7. The container is a rectangular cylindrical container, disposing the Li metal in at least one of the four corners of the container; The method for producing the lithium secondary battery according to claim 5 .

8. The Li metal is electrically connected to the negative electrode through the container. The method for producing the lithium secondary battery according to claim 5 .

9. Further comprising discharging and charging the electrode group; The end-of-charge voltage during charging is 4.2 V or higher. The method for producing the lithium secondary battery according to claim 5 .