Lithium secondary battery and method for manufacturing same

Abstract

Disclosed is a lithium secondary battery 10 comprising: a positive electrode 16; a negative electrode 13; and an electrolyte disposed between the positive electrode 16 and the negative electrode 13, wherein the positive electrode 16 includes a crystalline MnO2 and an electrochemical oxidation decomposition product of Li2MnO3. Lithium ions generated by the decomposition of Li2MnO3 during the initial charge of the lithium secondary battery 10 can be inserted into MnO2 during discharge.

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 its depletion in the near future. Examples of 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. Meanwhile, manganese dioxide (MnO2) has a capacity density significantly higher than transition metal phosphates and is inexpensive, making it a promising next-generation positive electrode active material.

[0003] Since MnO2 does not contain Li, when MnO2 is used as a positive electrode active material, it is necessary to use a negative electrode active material that already contains Li, or to have Li absorbed into the positive electrode or negative electrode in advance.

[0004] Patent Document 1 describes a non-aqueous secondary battery that includes a positive electrode using MnO2 as an active material and a negative electrode using lithium or a lithium alloy as an active material, and the positive electrode further contains Li2MnO3.

[0005] Patent Document 2 describes dissolving Li metal in an electrolyte solution to allow Li to be absorbed in the negative electrode in advance.

[0006] Japanese Patent Application Laid-Open No. 63-114064 International Publication No. 2024 / 024292

[0007] 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 are good at shallow depths. 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.

[0008] The method of electrochemically absorbing Li into the positive electrode or negative electrode tends to be costly and is not necessarily suitable for mass production of batteries.

[0009] An object of the present disclosure is to provide an inexpensive lithium secondary battery that uses MnO2 as a positive electrode active material and has good cycle characteristics.

[0010] The present disclosure provides a lithium secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode contains an electrochemical oxidative decomposition product of LiMnO and crystalline MnO.

[0011] According to the present disclosure, it is possible to provide a lithium secondary battery that uses MnO2 as a positive electrode active material and has good cycle characteristics at low cost.

[0012] 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 showing a method for manufacturing a lithium secondary battery. Fig. 3A is a graph showing charge / discharge curves for each cycle of the lithium secondary battery of Example 1. Fig. 3B is a graph showing the results of a charge / discharge test of the lithium secondary battery of Example 1. Fig. 4 is a graph showing X-ray diffraction patterns of the positive electrode mixtures of Samples 1 and 2.

[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] FIG. 1 is a cross-sectional view of a lithium secondary battery according to an embodiment of the present disclosure. The lithium secondary battery 10 of this embodiment includes a negative electrode 13, a positive electrode 16, a separator 17, and a container 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 disposed 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 disposed on the positive electrode current collector 14. A 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 electrode group including the negative electrode 13, the positive electrode 16, and the separator 17 is housed in a container 18. The negative electrode 13, the positive electrode 16, and the separator 17 are impregnated with an electrolyte.

[0015] The positive electrode current collector 14 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 14 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 14 as a conductive auxiliary material.

[0016] The positive electrode active material layer 15 contains an electrochemical oxidative decomposition product of LiMnO and crystalline MnO. The crystalline MnO serves as the positive electrode active material for the lithium secondary battery 10. LiMnO can be electrochemically oxidatively decomposed according to the following formula (a1). In other words, LiMnO serves as the lithium source for the lithium secondary battery 10. Some of the LiMnO may remain in the positive electrode active material layer 15 as a raw material residue. Despite the use of crystalline MnO, the lithium secondary battery 10 of this embodiment does not require the use of an active material containing lithium, such as lithium metal, in the negative electrode 13. Therefore, the lithium secondary battery 10 of this embodiment is inexpensive. There are no limitations on the material for the negative electrode 13, and highly reversible active materials, such as graphite, can be used for the negative electrode 13. Therefore, the lithium secondary battery 10 of this embodiment exhibits favorable cycle characteristics compared to lithium secondary batteries using lithium metal as the negative electrode active material.

[0017] Li2MnO3-2e - →2Li + + MnO2 (amorphous) + O2 / 2 ... (a1)

[0018] As shown in formula (a1), electrochemical oxidative decomposition products of Li2MnO3 include lithium ions and MnO2. According to the manufacturing method of the present disclosure, which will be described later, lithium ions are generated by the decomposition of Li2MnO3 during the first charge of the lithium secondary battery 10, and these lithium ions are absorbed into the negative electrode 13. During the first discharge of the lithium secondary battery 10, lithium ions are released from the negative electrode 13 and inserted into the crystalline MnO2 according to formula (a2) below. This configuration enables charge and discharge from the second cycle onwards.

[0019] MnO2 (crystalline) + e - +Li + →LiMnO2...(a2)

[0020] The manganese oxide contained in the electrochemical oxidative decomposition product of LiMnO is amorphous, specifically, amorphous MnO. Although amorphous MnO does not exhibit the same capacity density as crystalline MnO, it can function as a positive electrode active material. Therefore, according to this embodiment, the capacity of amorphous MnO can be added to the capacity of crystalline MnO.

[0021] When Li2MnO3 remains as a raw material residue in the positive electrode 16, Li2MnO3 can also function as an active material. However, when the amount of Li2MnO3 remaining in the positive electrode 16 is small, it is difficult to confirm the change in the structure of Li2MnO3 due to charge and discharge by X-ray diffraction measurement.

[0022] The positive electrode 16 contains a mixture of Li2MnO3, an electrochemical oxidative decomposition product of Li2MnO3, and crystalline MnO2 as a positive electrode mixture. These components can be confirmed by X-ray diffraction measurement of the positive electrode mixture. That is, in the X-ray diffraction pattern of the positive electrode mixture measured using Cu-Kα radiation, peaks attributable to MnO2 exist in the diffraction angle 2θ ranges of 36° to 37° and 51° to 55°. Peaks attributable to Li2MnO3 exist in the diffraction angle 2θ ranges of 38° to 39° and 44° to 45°. This configuration allows for the provision of an inexpensive lithium secondary battery 10 that uses crystalline MnO2 as the positive electrode active material and has good cycle characteristics.

[0023] Li2MnO3 does not necessarily function as an active material. The fact that Li2MnO3 does not function as an active material can be confirmed by disassembling multiple lithium secondary batteries 10 manufactured under the same conditions and analyzing the positive electrodes 16. Specifically, one lithium secondary battery 10 in a charged state is disassembled to collect the positive electrode mixture, and X-ray diffraction measurement of the positive electrode mixture is performed. The positions of the peaks attributable to Li2MnO3 in the X-ray diffraction pattern are identified. Among the peaks attributable to Li2MnO3, a peak present in the diffraction angle 2θ range of 38° to 39° is defined as the first peak. A peak present in the diffraction angle 2θ range of 44° to 45° is defined as the second peak. In other words, the positions of the first and second peaks are identified. Next, another lithium secondary battery 10 in a discharged state is disassembled to collect the positive electrode mixture, and X-ray diffraction measurement of the positive electrode mixture is performed. The positions of the first and second peaks attributable to Li2MnO3 in the X-ray diffraction pattern are identified. The positions of the first peaks identified from the two X-ray diffraction patterns are consistent, and / or the positions of the second peaks identified from the two X-ray diffraction patterns are consistent. This is evidence that Li2MnO3 is not involved in charge / discharge. Note that if the difference in peak positions is within ±0.1° in terms of the diffraction angle 2θ, the peak positions can be considered to be consistent.

[0024] Crystalline MnO2 is a positive electrode active material. 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 "MnO2" in chemical formula, it is actually a non-stoichiometric compound.

[0025] 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 crystal phases or may be a mixed crystal. MnO2 may contain at least one selected from the group consisting of γ-β-MnO2, γ-MnO2, and β-MnO2. 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 produced from electrolytic manganese dioxide, and β-MnO2, which can also be produced from electrolytic manganese dioxide, all have a tunnel structure, are inexpensive, and are 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 suitable as an active material for the lithium secondary battery 10. Note that γ-β-MnO2 refers to a mixed crystal of γ-MnO2 and β-MnO2.

[0026] The positive electrode active material layer 15 may contain a positive electrode active material other than MnO2, or may contain only MnO2 as the positive electrode active material.

[0027] The positive electrode active material layer 15 may contain other materials such as a conductive additive, a binder, and an ion conductor.

[0028] The conductive additive is used to reduce the resistance of the positive electrode 16. 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.

[0029] The binder is used to improve the binding property of the material that constitutes the positive electrode 16. Examples of the binder that can be used 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.

[0030] The ion conductor is used to reduce the resistance of the positive electrode 16. Examples of the ion conductor include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate, organic solid electrolytes such as polyethylene oxide, and LiLaZrO. 12 and inorganic solid electrolytes such as those listed below.

[0031] The negative electrode current collector 11 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 11 as a conductive auxiliary material.

[0032] The negative electrode active material layer 12 contains a negative electrode active material.

[0033] 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.

[0034] The negative electrode active material layer 12 may contain at least one selected from the group consisting of graphite and silicon as the negative electrode active material. The negative electrode active material layer 12 may contain only graphite as the negative electrode active material. 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 has a larger capacity than graphite and is therefore advantageous for increasing the capacity of the lithium secondary battery 10.

[0035] The negative electrode active material layer 12 may contain other materials such as a conductive additive, a binder, an ion conductor, etc. The same materials that can be used for the positive electrode active material layer 15 can be used for the negative electrode active material layer 12 as the conductive additive, binder, and ion conductor.

[0036] The electrolyte is disposed between the positive electrode 16 and the negative electrode 13. Specifically, the electrolyte is an electrolytic solution impregnated into the negative electrode 13, the positive electrode 16, and the separator 17. The electrolytic solution may fill the internal space of the container 18. The electrolytic solution allows lithium ions to move between the positive electrode 16 and the negative electrode 13.

[0037] The electrolyte solution contains a non-aqueous solvent and a lithium salt.

[0038] 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 the mixed solvent of a cyclic carbonate and a chain carbonate, MnO2 can be charged to a high charging voltage. In other words, the capacity of the lithium secondary battery 10 can be increased by fully utilizing MnO2.

[0039] Furthermore, if the non-aqueous solvent has a high withstand voltage, it is permissible to apply a high voltage to the positive electrode 16 to decompose Li2MnO3.

[0040] 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 such a cyclic carbonate with a chain carbonate, an electrolyte solution having a high withstand voltage can be obtained.

[0041] 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.

[0042] The cyclic carbonate ester 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 ester. The halogen atom may be F or Cl.

[0043]

[0044] The chain carbonate ester may be at least one selected from the group consisting of ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate. By combining these chain carbonate esters with cyclic carbonate esters, an electrolyte solution with high voltage resistance can be obtained. Among these, ethyl methyl carbonate is recommended.

[0045] 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.

[0046] The separator 17 has lithium ion conductivity. There are no particular limitations on the material of the separator 17 as long as it allows the passage of lithium ions. The material of the separator 17 can be at least one selected from the group consisting of solid electrolytes, gel electrolytes, ion exchange resin membranes such as lithium cation exchange resins, semipermeable membranes, and porous membranes. If the separator 17 is made of these materials, the safety of the lithium secondary battery 10 can be sufficiently ensured. Examples of solid electrolytes include sulfide solid electrolytes such as Li2S-P2S5, Li7La3Zr2O 12 Examples 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 container 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 container 18 may be a resin or metal container.

[0048] There are no particular limitations on the shape of the lithium secondary battery 10. The lithium secondary battery 10 may be in a variety of shapes, such as a coin shape, a cylindrical shape, a square shape, a sheet shape, a button shape, a flat shape, or a laminated shape.

[0049] The lithium secondary battery 10 may be a solid-state battery.

[0050] Next, a method for manufacturing the lithium secondary battery 10 will be described.

[0051] In step S1, an electrode assembly is produced including a negative electrode 13, a positive electrode 16, and a separator 17. More specifically, the process of step S1 includes a step of producing a negative electrode 13, a step of producing a positive electrode 16, and a step of stacking the negative electrode 13, the positive electrode 16, and the separator 17. In this way, an electrode assembly is obtained.

[0052] The positive electrode 16 can be fabricated by applying a positive electrode mixture to one or both surfaces of the positive electrode current collector 14, followed by drying and rolling. The positive electrode mixture is obtained by kneading and uniformly dispersing materials such as LiMnO, crystalline MnO, a binder, and a conductive additive. The resulting positive electrode 16 contains a mixture of LiMnO and crystalline MnO.

[0053] In the mixture of LiMnO and crystalline MnO, the ratio (M2 / M1) of the amount of substance M2 of LiMnO to the amount of substance M1 of MnO is, for example, 2 / 3 or more. By adjusting the ratio (M2 / M1) within this range, the capacity density of the lithium secondary battery 10 can be improved. The upper limit of the ratio (M2 / M1) is, for example, 1.

[0054] There are no particular limitations on the method for applying the positive electrode mixture to the positive electrode current collector 14. A slit die coater, reverse roll coater, lip coater, blade coater, knife coater, gravure coater, dip coater, or the like can be used to apply the positive electrode mixture in a slurry form to the positive electrode current collector 14. The applied positive electrode mixture may be dried naturally or may be dried using a drying device.

[0055] After drying, the positive electrode 16 may be rolled so that the positive electrode active material layer 15 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 for extracting power is welded to the positive electrode current collector 14.

[0056] The negative electrode 13 can be fabricated by applying a negative electrode mixture to one or both sides of the negative electrode current collector 11, drying, and rolling. After rolling, a negative electrode lead for power extraction is welded to the negative electrode current collector 11. The negative electrode mixture is obtained by kneading and uniformly dispersing materials such as a negative electrode active material, a binder, and a conductive additive. The negative electrode active material may be a material that does not contain removable Li. Lithium ions generated by the decomposition of Li2MnO3 may be absorbed into the negative electrode active material. This configuration allows the use of an active material with excellent reversibility, such as graphite, for the negative electrode 13. "Removable Li" refers to Li atoms that can be reversibly inserted and removed.

[0057] A dry method may be used instead of a wet method to form the negative electrode 13. Examples of dry film formation methods include evaporation, sputtering, and CVD (chemical vapor deposition).

[0058] In step S2, the electrode group and the electrolyte are placed in the container 18. The electrolyte is impregnated into the electrode group.

[0059] In step S3, Li2MnO3 contained in the positive electrode 16 is electrochemically decomposed. Specifically, a voltage is applied to the positive electrode 16 to decompose Li2MnO3. As described with reference to formula (a1), the decomposition of Li2MnO3 generates lithium ions and oxygen gas. This supplies lithium ions involved in charge and discharge to the electrode assembly. According to the manufacturing method of this embodiment, even though crystalline MnO2 is used, it is not necessary to use an active material that already contains lithium, such as lithium metal, in the negative electrode 13. Because lithium ions can be supplied by decomposing Li2MnO3 contained in the positive electrode 16, the manufacturing method of this embodiment allows for inexpensive production of the lithium secondary battery 10.

[0060] The lithium ions generated by the decomposition of LiMnO can be absorbed into the negative electrode active material, thus preventing the deposition of lithium metal on the surface of the electrode assembly. For this purpose, it is desirable that the negative electrode active material be one that does not contain desorbable Li, such as graphite or silicon.

[0061] To sufficiently promote the decomposition reaction of Li2MnO3, it is desirable to apply a moderately high voltage to the positive electrode 16. From this perspective, it is desirable to use an electrolyte with excellent voltage resistance. When the voltage applied to the positive electrode 16 to electrochemically decompose the Li2MnO3 contained in the positive electrode 16 is a first voltage, the first voltage is higher than the upper limit charging voltage of the lithium secondary battery 10 when used as a product. This configuration can suppress decomposition of the electrolyte and improve the cycle characteristics of the lithium secondary battery 10. Because the process of electrochemically decomposing Li2MnO3 corresponds to only one charging cycle, applying a relatively high voltage to the positive electrode 16 is unlikely to significantly decompose the electrolyte. In one example, the first voltage is a DC voltage of +5 V relative to the voltage of the negative electrode 13. The upper limit charging voltage is a DC voltage of +4.3 V relative to the voltage of the negative electrode 13.

[0062] As explained with reference to formula (a1), oxygen gas is generated when Li2MnO3 decomposes. Therefore, it is desirable to perform step S3 before sealing the container. In this embodiment, after the electrode group is placed in the container 18, Li2MnO3 is electrochemically decomposed, and the oxygen gas generated by the decomposition of Li2MnO3 is released to the outside of the container 18. This prevents oxygen gas from accumulating inside the container 18.

[0063] If the container is provided with a gas vent valve, it may be possible to carry out step S3 after sealing the container.

[0064] After Li2MnO3 is decomposed, the container 18 is sealed in step S4, thereby obtaining the lithium secondary battery 10 of this embodiment.

[0065] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0066] (Technology 1) A lithium secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode contains an electrochemical oxidative decomposition product of Li2MnO3 and crystalline MnO2.

[0067] According to the present disclosure, it is possible to provide a lithium secondary battery that uses MnO2 as a positive electrode active material and has good cycle characteristics at low cost.

[0068] (Technology 2) The lithium secondary battery according to Technology 1, wherein the MnO2 includes at least one selected from the group consisting of γ-β-MnO2, γ-MnO2, and β-MnO2. These MnO2 are suitable as active materials for lithium secondary batteries.

[0069] (Technology 3) The lithium secondary battery according to Technology 1 or 2, wherein the positive electrode further contains Li2MnO3, and lithium ions generated by decomposition of Li2MnO3 during the initial charge of the lithium secondary battery are inserted into the MnO2 during discharge. With this configuration, charge and discharge from the second cycle onwards becomes possible.

[0070] (Technology 4) The lithium secondary battery according to any one of Technologies 1 to 3, wherein the electrochemical oxidative decomposition product contains amorphous manganese oxide. With this configuration, the capacity of amorphous MnO2 can be added to the capacity of crystalline MnO2.

[0071] (Technology 5) A lithium secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode contains a mixture of Li2MnO3, an electrochemical oxidative decomposition product of Li2MnO3, and crystalline MnO2 as a positive electrode mixture; and in an X-ray diffraction pattern of the positive electrode mixture measured using Cu-Kα radiation, peaks attributable to the MnO2 exist in the diffraction angle 2θ ranges of 36° to 37° and 51° to 55°, and peaks attributable to the Li2MnO3 exist in the diffraction angle 2θ ranges of 38° to 39° and 44° to 45°.

[0072] According to this configuration, a lithium secondary battery having good cycle characteristics can be provided at low cost while using crystalline MnO2 as the positive electrode active material.

[0073] (Technology 6) The lithium secondary battery according to Technology 5, wherein, among the peaks attributable to LiMnO, the peak present in a diffraction angle 2θ range of 38° to 39° is defined as a first peak, and the peak present in a diffraction angle 2θ range of 44° to 45° is defined as a second peak, and the position of the first peak in the X-ray diffraction pattern of the positive electrode mix when the lithium secondary battery is in a charged state coincides with the position of the first peak in the X-ray diffraction pattern of the positive electrode mix when the lithium secondary battery is in a discharged state, and / or the position of the second peak in the X-ray diffraction pattern of the positive electrode mix when the lithium secondary battery is in a charged state coincides with the position of the second peak in the X-ray diffraction pattern of the positive electrode mix when the lithium secondary battery is in a discharged state. X-ray diffraction measurement can confirm the presence of LiMnO.

[0074] (Technology 7) The lithium secondary battery according to any one of Techniques 1 to 6, wherein the electrolyte is a liquid electrolyte containing a non-aqueous solvent, and the non-aqueous solvent contains a cyclic carbonate ester and a chain carbonate ester. A mixed solvent of a cyclic carbonate ester and a chain carbonate ester has excellent voltage resistance. When the non-aqueous solvent has excellent voltage resistance, it is permissible to apply a high voltage to the positive electrode to decompose LiMnO.

[0075] (Technology 8) The lithium secondary battery according to any one of Technologies 1 to 7, wherein the negative electrode contains 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.

[0076] (Technology 9) A method for manufacturing a lithium secondary battery, comprising: preparing a positive electrode containing a mixture of Li2MnO3 and crystalline MnO2; and electrochemically decomposing the Li2MnO3 contained in the positive electrode to generate lithium ions.

[0077] According to the manufacturing method of the present disclosure, lithium ions can be supplied in an amount sufficient to decompose the Li2MnO3 contained in the positive electrode, making it possible to manufacture lithium secondary batteries at low cost.

[0078] (Technology 10) The method for producing a lithium secondary battery according to Technology 9 further comprises preparing a negative electrode including a negative electrode active material that does not contain desorbable Li, and the lithium ions generated by decomposition of the Li2MnO3 are absorbed into the negative electrode active material. With this configuration, deposition of lithium metal on the surface of the electrode group is prevented.

[0079] (Technology 11) The method for producing a lithium secondary battery according to Technology 9 or 10, wherein in the mixture, the ratio of the amount of substance of the Li2MnO3 to the amount of substance of the MnO2 is 2 / 3 or more. By adjusting the ratio of the amount of substances to be in such a range, the capacity density of the lithium secondary battery can be improved.

[0080] (Technology 12) The method for manufacturing a lithium secondary battery according to any one of Techniques 9 to 11, further comprising placing an electrode group including the positive electrode in a container and sealing the container, wherein after placing the electrode group in the container, the Li2MnO3 is electrochemically decomposed, oxygen gas generated by the decomposition of the Li2MnO3 is released to the outside of the container, and after the Li2MnO3 is decomposed, the container is sealed. With this configuration, it is possible to prevent oxygen gas from accumulating inside the container.

[0081] (Technology 13) The method for manufacturing a lithium secondary battery according to any one of Techniques 9 to 12, wherein the Li2MnO3 contained in the positive electrode is electrochemically decomposed by applying a first voltage to the positive electrode, the first voltage being higher than an upper limit charging voltage of the lithium secondary battery when it is used as a product. With this configuration, decomposition of the electrolyte can be suppressed, thereby improving the cycle characteristics of the lithium secondary battery.

[0082] (Example 1) Li2O powder and γ-β-MnO2 powder were mixed in a substance weight ratio of 1:1, and then the mixture was fired in air at 250°C for 12 hours, thereby obtaining powdered Li2MnO3.

[0083] A positive electrode mixture was prepared by mixing a binder, a conductive additive, γ-β-MnO2 powder, Li2MnO3 powder, and a solvent. The positive electrode mixture was applied to an Al current collector to form a coating film. The coating film was dried and rolled to obtain a 2 cm x 2 cm 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, Li2MnO3 powder, and γ-β-MnO2 powder was 5:3:39.56:48.76 by mass. The ratio of Li2MnO3 to γ-β-MnO2 was Li2MnO3:γ-β-MnO2 = 43:57 by mass.

[0084] 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.

[0085] A positive electrode lead and a negative electrode lead were welded to the positive electrode and the negative electrode, respectively. Then, the positive electrode, the separator, and the negative electrode were stacked to obtain a plate-shaped electrode assembly. A microporous polyolefin film was used as the separator.

[0086] The electrode group was placed in a container made of laminated film, and an electrolyte solution was poured into the container. The electrolyte solution was prepared by dissolving LiPF at a concentration of 1 mol / L in an organic solvent containing ethylene carbonate and methyl ethyl carbonate in a volume ratio of 1:1.

[0087] With the container open, a voltage was applied between the positive and negative electrodes to perform the initial charge and discharge. The initial charge was performed at a constant current of 0.3 mA. The end-of-charge voltage was 5 V. The initial discharge was performed at a constant current of 0.3 mA. The end-of-discharge voltage was 2 V.

[0088] After the first charge and discharge, the container was sealed, thereby obtaining the battery of Example 1.

[0089] [Charge / Discharge Test] The lithium secondary battery of Example 1 was repeatedly charged and discharged 50 times, including the initial charge and discharge. Charging and discharging were performed at a constant current of 0.3 mA. The end-of-charge voltage was 4.3 V. The end-of-discharge voltage was 2 V.

[0090] Fig. 3A is a graph showing charge / discharge curves for each cycle of the lithium secondary battery of Example 1. The horizontal axis represents capacity density (mAh / g). The vertical axis represents voltage (V). The charge / discharge curves are for the first, second, fifth, tenth, and twentieth cycles. Fig. 3B is a graph showing the results of a charge / discharge 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 capacity density (mAh / g).

[0091] The first charge shown in the graph in Figure 3A was for decomposing LiMnO. From the second cycle onward, the battery was able to be charged and discharged smoothly. The ratio of discharge capacity to charge capacity during the first charge and discharge was 98%.

[0092] As shown in FIG. 3B, the lithium secondary battery of Example 1 maintained a high capacity density even after 50 charge / discharge cycles.

[0093] Example 2: LiOH.HO powder and γ-β-MnO powder were mixed in a ratio of 1.9:1, and the mixture was then heat-treated in air at 250°C for 12 hours using a rotary kiln. This resulted in powdered LiMnO. Lithium secondary batteries of Examples 2-1 and 2-2 were then fabricated using the same method as in Example 1.

[0094] The lithium secondary battery of Example 2-1 was initially charged in the same manner as in Example 1. The lithium secondary battery of Example 2-1 in a charged state was disassembled to collect the positive electrode mixture of Sample 1.

[0095] The lithium secondary battery of Example 2-2 was subjected to an initial charge and an initial discharge in the same manner as in Example 1. The lithium secondary battery of Example 2-2 in a discharged state was disassembled to collect a positive electrode mixture of Sample 2.

[0096] [X-ray Diffraction Measurement] Powder X-ray diffraction measurement was performed on the positive electrode mixtures of Samples 1 and 2. For the X-ray diffraction measurement, a powder X-ray diffractometer (MiniFlex600, manufactured by Rigaku Corporation) was used. The measurement conditions are as follows.

[0097] X-ray source: Cu-Kα ray Detector: HyPix400MF Step size: 0.005 deg Scan speed: 0.4 deg / min Scan range: 10-80 deg

[0098] FIG. 4 is a graph showing the X-ray diffraction patterns of the positive electrode mixtures of Sample 1 and Sample 2. Peaks A1, A2, and A3, which exist within the diffraction angle 2θ ranges of 36° to 37°, 40° to 42°, and 51° to 55°, are peaks derived from γ-β-MnO2. The shift in the peak derived from γ-β-MnO2 is due to the insertion and desorption of lithium ions into γ-β-MnO2, which has a tunnel structure. Peaks B1 and B2, which exist within the diffraction angle 2θ ranges of 38° to 39° and 44° to 45°, are peaks derived from Li2MnO3. As can be seen from the X-ray diffraction patterns shown in FIG. 4, γ-β-MnO2 and Li2MnO3 were detected in both the charged and discharged states. In other words, a portion of Li2MnO3 remained in the positive electrode without decomposition.

[0099] The technology of the present disclosure is useful for lithium secondary batteries.

Claims

1. A lithium secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode contains an electrochemical oxidative decomposition product of Li2MnO3 and crystalline MnO2.

2. The lithium secondary battery according to claim 1, wherein the MnO2 includes at least one selected from the group consisting of γ-β-MnO2, γ-MnO2, and β-MnO2.

3. The lithium secondary battery according to claim 1, wherein the positive electrode further contains Li2MnO3, and lithium ions generated by decomposition of Li2MnO3 during initial charging of the lithium secondary battery are inserted into the MnO2 during discharge.

4. The lithium secondary battery according to claim 1, wherein the electrochemical oxidative decomposition product comprises amorphous manganese oxide.

5. A lithium secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode contains a mixture of Li2MnO3, an electrochemical oxidative decomposition product of Li2MnO3, and crystalline MnO2 as a positive electrode mixture, and wherein, in an X-ray diffraction pattern of the positive electrode mixture measured using Cu-Kα radiation, peaks attributable to the MnO2 exist in the diffraction angle 2θ ranges of 36° to 37° and 51° to 55°, and peaks attributable to the Li2MnO3 exist in the diffraction angle 2θ ranges of 38° to 39° and 44° to 45°.

6. The lithium secondary battery according to claim 5, wherein, when the peak originating from Li2MnO3 that exists in a diffraction angle 2θ range of 38° to 39° is defined as a first peak and the peak that exists in a diffraction angle 2θ range of 44° to 45° is defined as a second peak, the position of the first peak in the X-ray diffraction pattern of the positive electrode mixture when the lithium secondary battery is in a charged state coincides with the position of the first peak in the X-ray diffraction pattern of the positive electrode mixture when the lithium secondary battery is in a discharged state, and / or the position of the second peak in the X-ray diffraction pattern of the positive electrode mixture when the lithium secondary battery is in a charged state coincides with the position of the second peak in the X-ray diffraction pattern of the positive electrode mixture when the lithium secondary battery is in a discharged state.

7. The lithium secondary battery according to claim 1, wherein the electrolyte is a liquid electrolyte containing a non-aqueous solvent, and the non-aqueous solvent contains a cyclic carbonate ester and a chain carbonate ester.

8. The lithium secondary battery according to claim 1, wherein the negative electrode contains at least one selected from the group consisting of graphite and silicon.

9. A method for manufacturing a lithium secondary battery, comprising: preparing a positive electrode containing a mixture of Li2MnO3 and crystalline MnO2; and electrochemically decomposing the Li2MnO3 contained in the positive electrode to generate lithium ions.

10. The method for producing a lithium secondary battery according to claim 9, further comprising preparing a negative electrode containing a negative electrode active material that does not contain desorbable Li, and the lithium ions produced by decomposition of the Li2MnO3 are absorbed into the negative electrode active material.

11. The method for producing a lithium secondary battery according to claim 9, wherein in the mixture, the ratio of the amount of substance of the Li2MnO3 to the amount of substance of the MnO2 is 2 / 3 or more.

12. The method for producing a lithium secondary battery according to claim 9, further comprising: placing an electrode group including the positive electrode in a container; and sealing the container; after placing the electrode group in the container, electrochemically decomposing the Li2MnO3, releasing oxygen gas produced by the decomposition of the Li2MnO3 to the outside of the container; and sealing the container after the Li2MnO3 has been decomposed.

13. The method for producing a lithium secondary battery according to claim 9, wherein the Li2MnO3 contained in the positive electrode is electrochemically decomposed by applying a first voltage to the positive electrode, and the first voltage is a voltage higher than an upper limit charging voltage when the lithium secondary battery is used as a product.

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