Recovery method, manufacturing method for power storage device, and recovery agent

Abstract

To provide a novel recovery method, a manufacturing method for a power storage device, and a recovery agent, which are capable of restoring the capacity of a power storage device containing an olivine-type active material.SOLUTION: This recovery method is a method for recovering the capacity of a power storage device that uses metal ions as carrier ions and contains an olivine-type compound as a positive electrode active material, and includes a recovery step of adding a recovery agent to the power storage device, the recovery agent including a reduced fluorene-based compound, metal ions of the same type as the carrier ions, and a recovery agent solvent, to recover the capacity of the power storage device.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a recovery method, a method for manufacturing an electricity storage device, and a recovery agent. [Background technology]

[0002] Conventionally, as a method for restoring the capacity of an electricity storage device containing an olivine-type positive electrode active material, a method has been considered in which a restoring agent containing a reduced aromatic hydrocarbon compound and metal ions of the same type as the carrier ions of the electricity storage device is injected into the electricity storage device (see, for example, Patent Documents 1 and 2). In an electricity storage device containing an olivine-type active material, the capacity may not be restored simply by injecting the restoring agent. However, Patent Document 1 states that capacity can be restored by injecting the restoring agent into the electricity storage device within a predetermined temperature range and maintaining a predetermined voltage by applying a constant voltage. Patent Document 2 states that capacity can be restored by using a restoring agent containing a cyclic diether compound. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-128795 [Patent Document 2] Japanese Patent Application Publication No. 2023-153526 Summary of the Invention [Problem to be solved by the invention]

[0004] However, Patent Document 1 requires the application of a constant voltage, which increases the number of steps required for the recovery process. Patent Document 2 also has the problem of limiting the types of solvents that can be used in the recovery agent. Therefore, a new recovery method capable of recovering the deteriorated capacity of an electricity storage device containing an olivine-type active material has been desired.

[0005] The present disclosure has been made to solve such problems, and a main object of the present disclosure is to provide a novel recovery method, a method for manufacturing an electricity storage device, and a recovery agent that can recover the capacity of an electricity storage device containing an olivine-type active material. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, the inventors discovered that the capacity of an electricity storage device containing an olivine-type active material can be restored by using a recovery agent containing a reduced fluorene-based compound, metal ions of the same type as carrier ions, and a recovery agent solvent, and thus completed the present invention.

[0007] That is, the recovery method of the present disclosure is as follows: A method for restoring the capacity of an electricity storage device that uses metal ions as carrier ions and contains an olivine-type compound as a positive electrode active material, comprising: a recovery step of adding a recovery agent to the electricity storage device, the recovery agent including a reduced fluorene-based compound, metal ions of the same type as the carrier ions, and a recovery agent solvent, to recover the capacity of the electricity storage device; It includes:

[0008] Further, the method for manufacturing an electricity storage device according to the present disclosure includes the steps of: a recovery step of recovering the capacity of the electricity storage device having metal ions as carrier ions and having a deteriorated capacity by the recovery method described above; It includes:

[0009] In addition, the recovery agent of the present disclosure is A recovery agent for recovering the capacity of an electricity storage device that uses metal ions as carrier ions and contains an olivine-type compound as a positive electrode active material, a reduced fluorene-based compound, metal ions of the same kind as the carrier ions, and a restoring agent solvent; It includes: [Effects of the Invention]

[0010] The present disclosure provides a novel recovery method, a method for manufacturing a power storage device, and a recovery agent capable of recovering the capacity of a power storage device containing an olivine-type active material. The reason for this effect is presumed to be as follows. For example, a recovery agent containing a reduced fluorene-based compound and metal ions acts directly on the positive electrode simply by being injected into the power storage device, causing a recovery reaction that supplies electrons and metal ions to the positive electrode. It is presumed that the reduced form of a fluorene-based compound has a lower overvoltage in the capacity recovery reaction and reacts under milder conditions than the reduced forms of other aromatic hydrocarbon compounds, and this is why the reduced form of a fluorene-based compound is suitable for recovering the capacity of a power storage device containing an olivine-type active material. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an explanatory diagram showing the outline of the configuration of a nonaqueous electrolyte secondary battery 20. [Figure 2] FIG. 1 is an explanatory diagram showing an example of a recovery reaction scheme. [Figure 3] Discharge curve for Experimental Example 1. [Figure 4] Discharge curve for Experimental Example 2. [Figure 5] Discharge curve of Experimental Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0012] The recovery method disclosed in this specification is a recovery method for recovering the capacity of an electricity storage device that uses metal ions as carrier ions and contains an olivine-type compound as a positive electrode active material, and includes a recovery step of recovering the capacity of the electricity storage device using a recovery agent.

[0013] (Electricity storage device) First, the electricity storage device to be restored will be described. The electricity storage device uses metal ions as carrier ions. Examples of carrier ions include alkali metal ions such as Li, Na, and K, and Group 2 ions (alkaline earth metal ions) such as Mg, Ca, and Sr, among which lithium ions are preferred. Examples of electricity storage devices include hybrid capacitors, pseudo-electric double layer capacitors, lithium and sodium alkali metal secondary batteries, alkali metal ion batteries, and air batteries. Among these, lithium secondary batteries, particularly lithium ion secondary batteries, are preferred as electricity storage devices. Here, the electricity storage device will be mainly described as a lithium secondary battery. The electricity storage device may include, for example, a positive electrode having a positive electrode active material that occludes and releases lithium ions, a negative electrode having a negative electrode active material that occludes and releases lithium ions, and an ion-conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions. This electricity storage device may also include a separator between the positive electrode and the negative electrode.

[0014] The positive electrode contains an olivine-type compound as a positive electrode active material. The olivine-type compound may be, for example, lithium iron phosphate such as LiFePO4. The lithium iron phosphate may be doped with an additive element such as manganese. Lithium iron phosphate has the general formula Li x Fe 1-y M y PO4 (wherein M is one or more of Mn, Cr, Co, Cu, Ni, V, Mo, Y, Zn, Al, Ga, Mg, B, and Nb, and 0.05≦x≦1.2 and 0≦y≦0.8 are satisfied). The redox potential of the positive electrode active material may be 3.5 V or more, or 4.0 V or more versus Li metal.

[0015] The positive electrode may be formed, for example, by mixing a positive electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like positive electrode mixture, applying it to the surface of a current collector, drying it, and compressing it to increase electrode density as needed. The conductive material in the positive electrode may be, for example, one or a mixture of two or more of graphite, such as natural graphite (scale graphite, flake graphite) or artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, or metal (copper, nickel, aluminum, silver, gold, etc.). Among these, carbon black and acetylene black are preferred as the conductive material from the viewpoints of electronic conductivity and coatability. The binder serves to bind the active material particles and conductive material particles together. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber; thermoplastic resins such as polypropylene and polyethylene; ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR), either alone or in combination. Water-based binders, such as cellulose-based carboxymethyl cellulose (CMC), styrene butadiene copolymer (SBR), and aqueous dispersions of polyvinyl alcohol, can also be used. Examples of solvents that can be used to disperse the positive electrode active material, conductive material, and binder include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Alternatively, dispersants, thickeners, and the like can be added to water to form a slurry of the active material with a latex such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more kinds. Application methods include, for example, roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. Any of these can be used to obtain a desired thickness and shape.The weight of the positive electrode mixture is not particularly limited, but is, for example, 5 mg / cm. 2 May exceed 6 mg / cm 2 It may be more than 7 mg / cm 2 The weight of the positive electrode mixture may be, for example, 20 mg / cm 2 The following may also be used. Examples of current collectors include aluminum, titanium, stainless steel, nickel, iron, baked carbon, conductive polymers, conductive glass, and the like, as well as aluminum or copper whose surfaces have been treated with carbon, nickel, titanium, silver, or the like for the purpose of improving adhesion, conductivity, and oxidation resistance. The surfaces of these may also be subjected to oxidation treatment. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and formed fiber groups. The thickness of the current collector used is, for example, 1 to 500 μm.

[0016] The negative electrode may be formed by closely adhering a negative electrode active material to a current collector. Alternatively, the negative electrode may be formed by, for example, mixing a negative electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like negative electrode mixture, applying it to the surface of a current collector, drying it, and compressing it as needed to increase electrode density. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys, and tin compounds; carbonaceous materials capable of absorbing and releasing lithium ions; composite oxides containing multiple elements; and conductive polymers. Examples of carbonaceous materials include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Among these, graphites such as artificial graphite and natural graphite are preferred because they have an operating potential close to that of metallic lithium, allow for charging and discharging at high operating voltages, suppress self-discharge when using a lithium salt as a supporting electrolyte, and reduce irreversible capacity during charging. Examples of composite oxides include lithium-titanium composite oxide and lithium-vanadium composite oxide. Of these, carbonaceous materials are preferred as the negative electrode active material from the viewpoint of safety. The conductive material, binder, solvent, etc. used in the negative electrode can be the same as those exemplified for the positive electrode. The negative electrode active material may have an oxidation-reduction potential of 1.0 V or less, 0.5 V or less, or 0.3 V or less, relative to Li metal. The basis weight of the negative electrode composite is, for example, 3 mg / cm. 2 May exceed 4 mg / cm 2 The weight of the negative electrode composite may be, for example, 15 mg / cm 2 The negative electrode current collector may be made of copper, nickel, stainless steel, titanium, aluminum, baked carbon, conductive polymers, conductive glass, Al-Cd alloys, etc., or may be made of copper or the like whose surface has been treated with carbon, nickel, titanium, silver, etc., for the purpose of improving adhesion, conductivity, and reduction resistance. These surfaces can also be subjected to an oxidation treatment. The shape of the current collector can be the same as that of the positive electrode.

[0017] The ion-conducting medium may be a nonaqueous electrolyte containing a supporting salt and an organic solvent. Examples of supporting salts include inorganic salts such as LiPF6, LiClO4, LiAsF6, and LiBF4, and organic salts such as LiN(FSO2)2, LiN(CF3SO2)2, and LiN(C2F5SO2)2. These supporting salts may be used alone or in combination. The concentration of the supporting salt is preferably 0.1 to 2.0 M, more preferably 0.8 to 1.2 M. Examples of the organic solvent that can be used include aprotic organic solvents. Examples of such organic solvents include cyclic carbonates, chain carbonates, cyclic esters, cyclic ethers, and chain ethers. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of cyclic esters include gamma butyrolactone and gamma valerolactone. Examples of cyclic ethers include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of chain ethers include dimethoxyethane and ethylene glycol dimethyl ether. These may be used alone or in combination. Furthermore, other non-aqueous electrolytes may include nitrile solvents such as acetonitrile and propylnitrile, ionic liquids, and gel electrolytes. The non-aqueous electrolyte may contain additives such as film-forming agents and flame retardants. The ion-conducting medium may be a solid ion-conducting polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder.

[0018] The separator is not particularly limited as long as it has a composition that can withstand the range of use of non-aqueous electrolyte secondary batteries, and examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0019] The electricity storage device may have a case that houses the positive electrode, the negative electrode, and the non-aqueous electrolyte solution, and the case may have an openable / closable liquid inlet through which the recovery agent can be easily injected.

[0020] The shape of the electricity storage device is not particularly limited, and examples thereof include coin, button, sheet, laminate, cylindrical, flat, and rectangular shapes. The device may also be applied to large devices used in electric vehicles and the like. FIG. 1 is a schematic diagram showing an example of a nonaqueous electrolyte secondary battery 20, which is an example of an electricity storage device. The nonaqueous electrolyte secondary battery 20 includes a cup-shaped battery case 21, a positive electrode 22 having a positive electrode active material and disposed at the bottom of the battery case 21, a negative electrode 23 having a negative electrode active material and disposed opposite the positive electrode 22 with a separator 24 interposed therebetween, a gasket 25 formed of an insulating material, and a sealing plate 26 disposed at the opening of the battery case 21 and sealing the battery case 21 via the gasket 25. In the nonaqueous electrolyte secondary battery 20, a nonaqueous electrolyte 27 is filled in the space between the positive electrode 22 and the negative electrode 23.

[0021] (Recovery potion) Next, the recovery agent will be described. The recovery agent includes a reduced fluorene-based compound, metal ions of the same type as the carrier ions in the electricity storage device, and a recovery agent solvent. In the recovery agent, the reduced fluorene-based compound and the metal ions may be dissociated or associated.

[0022] Fluorene-based compounds have a fluorene skeleton with two six-membered rings sandwiching a five-membered ring. Fluorene-based compounds include fluorene and its derivatives. Fluorene derivatives may have a substituent on the aromatic ring or may contain a heteroatom in the aromatic ring. Examples of the substituent include a halogen atom, an alkyl group, an aryl group, an alkenyl group, an alkoxy group, an aryloxy group, a sulfonyl group, an amino group, a cyano group, a carbonyl group, an acyl group, an amide group, and a hydroxyl group. Examples of the heteroatom include nitrogen, oxygen, and sulfur. Fluorene-based compounds may have a structure in which two hydrogen atoms are bonded to the carbon atom at the 9th position of the fluorene skeleton, as shown in the following formula (1), or may have a structure in which two substituents are bonded to the carbon atom at the 9th position of the fluorene skeleton, as shown in the following formula (2). The fluorene-based compound may be, for example, 9-methyl-9H-fluorene, in which one hydrogen atom and one substituent are bonded to the carbon atom at the 9th position of the fluorene skeleton. The fluorene-based compound may be fluorene represented by formula (1) and containing no substituents or heteroatoms, or 9,9-dimethylfluorene represented by formula (2) and containing no substituents or heteroatoms. The reduced fluorene-based compound is, for example, the above-mentioned fluorene-based compound in a reduced state (also referred to as a reduced form), and is, for example, an anion or a radical anion.

[0023] [ka]

[0024] The metal ions in the recovery agent may be of the same type as the carrier ions in the electricity storage device, but are preferably one or more alkali metal ions such as lithium ions, sodium ions, and potassium ions.

[0025] The restoring agent may be one obtained by reacting a fluorene compound with a metal to contain a reduced fluorene compound and a metal ion, and may be one obtained by, for example, one or more of the following formulas (3) and (4): When a metal is reacted with a fluorene compound in which one hydrogen and one substituent are bonded to the carbon at the 9-position of the fluorene skeleton, hydrogen is eliminated to generate an anion of the fluorene compound and a metal ion, as in formula (3).

[0026] [ka]

[0027] In the recovery agent, the recovery agent solvent may be an organic solvent. The recovery agent solvent is preferably an ether compound, and may be a cyclic ether or a chain ether. The recovery agent solvent may contain one or more solvents selected from the group consisting of tetrahydrofuran (THF), dimethoxyethane (DME), diethoxyethane (DEE), dioxolane (DOL), and dioxane (DOX).

[0028] The recovery agent may be obtained by adding a fluorene-based compound and a metal in a metallic state, not an ion state, to a recovery agent solvent. For example, as shown in the following formulas (5) and (6), fluorene, 9,9-dimethylfluorene, or a derivative thereof may be reacted with Li metal in a DME solvent. Alternatively, fluorene, 9,9-dimethylfluorene, or a derivative thereof may be reacted with Li metal in a THF solvent or a DOL solvent, as shown in formulas (5) and (6). This facilitates the preparation of a recovery agent containing a reduced fluorene-based compound and metal ions. The recovery agent may be prepared by adding a metal to a precursor obtained by adding a fluorene-based compound to a recovery agent solvent. The recovery agent may be prepared in an inert atmosphere, such as an argon atmosphere. The recovery agent may be prepared in a low dew point environment, such as a dew point of −20°C or lower, −40°C or lower, or −60°C or lower. The restoring agent may be prepared by stirring a restoring agent solvent, a fluorene-based compound, and a metal, and a stirrer or the like may be used in this process.

[0029] [ka]

[0030] An allenide solution composed of a reduced fluorene-based compound, metal ions of the same type as the carrier ions in the power storage device, and a recovery agent solvent is also referred to as a recovery agent stock solution. In the recovery agent stock solution, the concentrations of the reduced fluorene-based compound and the metal ions may each be 0.05 mol / L or more, 0.1 mol / L or more, or 0.5 mol / L or more. Furthermore, this concentration may be equal to or less than the solubility, 5 mol / L or less, or 2 mol / L or less. Furthermore, the number of moles M of the reduced fluorene-based compound contained in the recovery agent stock solution is A (mol) and the number of moles of metal ions, M B (mol) and its ratio M A / M B is preferably 1 / 1, but may be 1.1 / 1.0 to 1.0 / 1.1, or may be 1.2 / 1.0 to 1.0 / 1.2.

[0031] The recovery agent may further contain an electrolyte solvent. Examples of the electrolyte solvent include the organic solvents described above that are used in the electrolyte solution of the electricity storage device. The recovery agent may further contain a supporting salt. Examples of the supporting salt include the supporting salts described above that are contained in the non-aqueous electrolyte solution of the electricity storage device. The organic solvent and the supporting salt are preferably the same as the organic solvent and the supporting salt contained in the electrolyte solution of the electricity storage device to be recovered. The recovery agent may contain an electrolyte solution in which a supporting salt is dissolved in an electrolyte solvent. In this case, the electrolyte solution is preferably the same as the non-aqueous electrolyte solution of the electricity storage device to be recovered. When the recovery agent contains an electrolyte solution or an electrolyte solvent, the content of the electrolyte solution or the electrolyte solvent may be 10% by volume or more, 20% by volume or more, or 30% by volume or more. The content of the electrolyte solution or the electrolyte solvent may be 70% by volume or less, 60% by volume or less, or 50% by volume or less.

[0032] The redox potential of the recovery agent may be higher than that of the negative electrode and lower than that of the positive electrode. The redox potential of the recovery agent may be, for example, 0.7 V or higher, 0.8 V or higher, or 1.0 V or higher relative to Li metal. The redox potential of the recovery agent may be, for example, 2.5 V or lower, 2.0 V or lower, 1.5 V or lower, or 1.2 V or lower relative to Li metal.

[0033] (Recovery process) In the recovery process, the recovery agent described above is added to the above-mentioned electricity storage device to recover the capacity of the electricity storage device. The electricity storage device to be recovered may be an electricity storage device with degraded capacity (also referred to as a degraded battery). The electricity storage device with degraded capacity may be, for example, an electricity storage device in a state where its capacity has degraded relative to the rated capacity of the electricity storage device. The electricity storage device with degraded capacity may be an unused product or a used product. Even an unused product may experience capacity degradation due to long-term storage, etc.

[0034] In the recovery step, the recovery agent described above is added to the electricity storage device. When adding the recovery agent, the electricity storage device may be opened and the recovery agent added to seal the opening, or the recovery agent may be added to the electricity storage device by injection or the like to seal the perforations. The recovery agent may be added in an inert atmosphere such as an argon atmosphere. The recovery agent may be added so as to be in contact with at least the positive electrode, or may be mixed with the non-aqueous electrolyte of the electricity storage device. The amount of recovery agent can be determined appropriately depending on the configuration and degree of deterioration of the electricity storage device. The amount of recovery agent may be, for example, 1% to 100% of the volume of the non-aqueous electrolyte contained in the electricity storage device, 10% to 75% of the volume, or 25% to 50% of the volume of the non-aqueous electrolyte contained in the electricity storage device.

[0035] In the recovery step, the recovery agent may be added to the electricity storage device and maintained in an open circuit state for a period of time, for example, from 1 hour to 48 hours, from 6 hours to 36 hours, or from 12 hours to 24 hours.

[0036] In the recovery step, a recovery agent is added to the electricity storage device, and a predetermined voltage lower than the full charge voltage may be maintained by applying a constant voltage. The predetermined voltage is preferably lower than the full charge voltage, and is preferably a voltage such that the potential of the positive electrode is higher than the potential of the recovery agent. The full charge voltage may be the upper charge voltage limit set for the electricity storage device. The predetermined voltage may be determined appropriately depending on the configuration of the electricity storage device, and may be, for example, 3.0 V or higher but lower than 4.1 V, or 3.5 V or higher and 4.0 V or lower. The application of the constant voltage may be continued before or during the addition of the recovery agent, or may be started after the addition of the recovery agent while the voltage drop of the electricity storage device due to the supply of metal ions to the positive electrode is negligibly small (for example, within 5 minutes, preferably within 3 minutes, and more preferably within 1 minute). The constant voltage application time may be determined appropriately depending on the configuration of the electricity storage device, the degree of deterioration, the amount of recovery agent, and the like. The constant voltage application time may be, for example, 1 hour or more and 48 hours or less, 6 hours or more and 36 hours or less, or 12 hours or more and 24 hours or less.

[0037] In the recovery step, prior to adding the recovery agent to the electricity storage device, the voltage of the electricity storage device may be adjusted to the above-mentioned predetermined voltage. In this case, the voltage of the electricity storage device may be adjusted by, for example, constant current charging (CC charging) or constant current constant voltage charging (CCCV charging). This voltage adjustment is not necessary, but it is preferable that the voltage of the electricity storage device be the above-mentioned predetermined voltage.

[0038] When a recovery agent is injected in the recovery step, a recovery reaction such as that shown in FIG. 2 is thought to occur. FIG. 2 is an explanatory diagram showing an example of the recovery reaction scheme, and is an explanatory diagram showing the scheme when the positive electrode active material is an olivine-type compound such as lithium iron phosphate. In FIG. 2, metal ions are + , the reduced form of fluorene-based compound Flu - olivine-type compounds, Li n-yAs shown in Figure 2, the recovery agent is a compound containing a reduced form of a fluorene-based compound and metal ions, which functions as a reducing agent that donates electrons. When applied to a deteriorated positive electrode, the reducing agent donates electrons and metal ions to the positive electrode, restoring the capacity.

[0039] The recovery agent and recovery method described above can recover the capacity of an electricity storage device containing an olivine-type active material. The reason for this effect is presumed to be as follows. For example, a recovery agent containing a reduced fluorene-based compound and metal ions can directly act on the positive electrode simply by being injected into the electricity storage device, causing a recovery reaction that supplies electrons and metal ions to the positive electrode. The reduced form of a fluorene-based compound exhibits a lower overvoltage in the capacity recovery reaction involving the reduction of the positive electrode than the reduced forms of other aromatic hydrocarbon compounds. Therefore, even if the potential of the positive electrode decreases as the reaction progresses, the potential difference required for the reaction with the recovery agent is thought to be sufficiently maintained. Furthermore, the reduced form of a fluorene-based compound is thought to react under mild conditions. Therefore, the capacity of an electricity storage device containing an olivine-type active material can be suitably recovered. This can also be expected to enable suitable capacity recovery without the application of a constant voltage, thereby reducing the effort and cost required for applying a constant voltage.

[0040] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure.

[0041] For example, in the above-described embodiment, a method for restoring an electricity storage device has been described. However, this method for restoring an electricity storage device can be used to manufacture a new nonaqueous electrolyte secondary battery with restored capacity using an electricity storage device with degraded capacity. Therefore, this restoration method may be considered a method for manufacturing an electricity storage device. This manufacturing method may manufacture an electricity storage device by performing the restoration step in the above-described method for restoring an electricity storage device. The method for manufacturing an electricity storage device may include a preparation step of preparing a capacity-degraded electricity storage device that uses metal ions as carrier ions, and a restoration step of restoring the capacity of the electricity storage device using the above-described method for restoring an electricity storage device. The electricity storage device prepared in the preparation step may be the same as the degraded battery described in the above-described embodiment.

[0042] The present disclosure may be as set forth in the following [1] to [8]. [1] A method for restoring the capacity of an electricity storage device that uses metal ions as carrier ions and contains an olivine-type compound as a positive electrode active material, the method comprising: a restoring step of adding to the electricity storage device a restoring agent that contains a reduced fluorene-based compound, metal ions of the same type as the carrier ions, and a restoring agent solvent, thereby restoring the capacity of the electricity storage device. [2] The recovery method according to [1], wherein the recovery step uses a recovery agent containing at least one of a reduced form of a compound of formula (1) and a reduced form of a compound of formula (2) as the reduced fluorene-based compound. [ka] [3] The recovery method according to [1] or [2], wherein in the recovery step, the recovery agent is added to the electricity storage device and the device is maintained in an open circuit state. [4] The recovery method according to any one of [1] to [3], wherein the recovery step uses a recovery agent containing an ether compound as the recovery agent solvent. [5] The recovery method according to any one of [1] to [4], wherein the recovery step uses a recovery agent containing a recovery agent stock solution including the reduced fluorene-based compound, metal ions of the same type as the carrier ions, and the recovery agent solvent, and an electrolyte solvent used in the electricity storage device. [6] The recovery method according to any one of [1] to [5], wherein the recovery step recovers the capacity of the electricity storage device containing lithium iron phosphate as the olivine type compound. [7] A method for manufacturing an electricity storage device, comprising: a recovery step of recovering the capacity of the electricity storage device, the capacity of which has deteriorated and which uses metal ions as carrier ions, by the recovery method according to any one of [1] to [6]. [8] A recovery agent for recovering the capacity of an electrical storage device that uses a metal ion as a carrier ion and contains an olivine-type compound as a positive electrode active material, the recovery agent comprising a reduced fluorene-based compound, a metal ion of the same type as the carrier ion, and a recovery agent solvent. [Example]

[0043] Hereinafter, examples in which the method for recovering an electricity storage device according to the present disclosure was carried out will be described as examples, with Experimental Examples 1 to 3 all corresponding to working examples.

[0044] [Experimental Example 1] (Battery configuration) The positive electrode mixture was a mixture of 92% by mass of LiFePO4, 5% by mass of acetylene black, and 3% by mass of polyvinylidene fluoride. The positive electrode mixture was coated on aluminum foil. The negative electrode mixture was a mixture of 98% by mass of graphite, 1% by mass of carboxymethyl cellulose, and 1% by mass of styrene-butadiene rubber. The negative electrode mixture was coated on copper foil. An electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing 30% by volume of ethylene carbonate (EC), 40% by volume of dimethyl carbonate (DMC), and 30% by volume of ethyl methyl carbonate (EMC). A polyethylene monolayer microporous membrane was used as the separator. Each battery contained approximately 1 mL of electrolyte.

[0045] (Method for manufacturing and measuring capacity degradation cells) First, a simulated positive electrode with reduced capacity was prepared using the following procedure. The positive and negative electrodes were placed opposite each other with a separator impregnated with electrolyte interposed between them and sealed in a laminate film to prepare a laminate cell. The prepared laminate cell was charged at 2.0 mA constant current (CC) to 3.65 V at 25 °C, discharged at 2.0 mA constant current to 2.5 V, charged at 2.0 mA constant current / constant voltage (CCCV) to 3.65 V for 2 hours at 2.0 mA, and finally discharged at 1.0 mA constant current to 2.5 V. The discharge capacity of the second CC discharge was recorded as the battery capacity.

[0046] The cell was then charged to a capacity equivalent to 10% of the cell's electrical capacity (SOC = 10%), extracting Li from the positive electrode to obtain a simulated positive electrode with reduced capacity (also referred to as a degraded positive electrode). Further charge / discharge and open-circuit potential measurements were performed at 25°C. The cell was then disassembled, and the degraded positive electrode was removed. The degraded positive electrode and negative electrode were placed opposite each other with a separator impregnated with electrolyte interposed between them, and then sealed in a laminate film to produce a laminate cell, which was used as a degraded battery. The discharge capacity of the capacity-degraded battery was measured in the same way as for the battery before degradation.

[0047] (How to prepare recovery agent) Under an inert atmosphere, fluorene was dissolved in dimethoxyethane (DME) solvent to a concentration of 1.0 mol / L. Then, 1.0 mol / L of lithium metal was added and stirred to obtain an orange solution (restoring agent stock solution). The reaction between fluorene and lithium metal was presumably expressed by the above formula (5). An equal volume of electrolyte was added to this restoring agent stock solution and stirred to obtain a light yellow solution (restoring agent). The electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent (electrolyte solvent) containing 30 vol% EC, 40 vol% DMC, and 30 vol% EMC.

[0048] (Recovering degraded batteries) The degraded battery was CCCV charged at 1.0 mA up to 3.65 V, the voltage was adjusted to 3.65 V, part of the degraded battery was opened under an argon atmosphere, 0.5 mL of recovery agent was injected using a pipette, the opened part was sealed, and only the open circuit potential was measured for 25 hours.

[0049] (Recovery Battery Evaluation) The discharge capacity of the recovered battery was measured in the same manner as for the deteriorated battery.

[0050] [Experimental Example 2] The same procedure as in Experimental Example 1 was carried out except that the solvent for the recovery agent was changed from DME to tetrahydrofuran (THF).

[0051] [Experimental Example 3] The same procedure as in Experimental Example 1 was carried out except that the solvent of the recovery agent was changed from DME to 1,3-dioxolane (DOL).

[0052] [Results and Discussion] The discharge curves for Experimental Examples 1, 2, and 3 are shown in Figures 3, 4, and 5, respectively. In all of Experimental Examples 1, 2, and 3, the battery capacity recovered without the application of a constant voltage. It is known that fluorene reacts with lithium in an ethereal solvent to generate a radical anion, which then spontaneously loses the hydrogen at the 9-position and transforms into an anion (Reference 1: CAN Journal CHEM 38 (1960) 2450-2456). In Experimental Examples 1, 2, and 3, it is presumed that the anion thus generated reacted with the positive electrode inside the battery, transferring lithium ions and electrons to the positive electrode while transforming itself into an electrochemically stable chemical species inside the battery, thereby causing the chemical reaction to proceed.

[0053] In the aforementioned Patent Document 1, it was necessary to maintain the positive electrode at a high potential by applying a constant voltage. It was inferred that this constant voltage application was necessary because battery capacity recovery by the recovery agent proceeds using the energy difference between the positive electrode reaction and the recovery agent reaction as the driving force. In contrast, in Experimental Examples 1 to 3 of the present disclosure, the battery voltage after injection of the recovery agent was 3.3 V, and the reaction proceeded under milder conditions than in the aforementioned Patent Document 1. Furthermore, while the aforementioned Patent Document 2 uses a cyclic diether compound as a solvent to facilitate capacity recovery, in Experimental Examples 1 to 3 of the present disclosure, the recovery reaction proceeded regardless of the type of solvent. This is inferred to be due to the difference in reactivity between the fluorene of the present disclosure and the naphthalene of Patent Document 2 due to the different solvation structures. Furthermore, when comparing the discharge curves before and after recovery for Experimental Examples 1 to 3, they were similar. From this, it was inferred that the injection of the recovery agent also has little effect on the internal resistance of the battery. From the above, it was expected that the recovery agent of the present disclosure is a recovery agent that can be applied to various battery systems. [Industrial Applicability]

[0054] The present invention can be used in the technical field of electricity storage devices. [Explanation of symbols]

[0055] 20 non-aqueous electrolyte secondary battery, 21 battery case, 22 positive electrode, 23 negative electrode, 24 separator, 25 gasket, 26 sealing plate, 27 non-aqueous electrolyte.

Claims

1. A method for restoring the capacity of an electricity storage device that uses metal ions as carrier ions and contains an olivine-type compound as a positive electrode active material, comprising: a recovery step of adding a recovery agent to the electricity storage device, the recovery agent including a reduced fluorene-based compound, metal ions of the same type as the carrier ions, and a recovery agent solvent, to recover the capacity of the electricity storage device; Recovery methods including.

2. The recovery method according to claim 1, wherein the recovery step uses a recovery agent containing one or more of the reduced form of the compound of formula (1) and the reduced form of the compound of formula (2) as the reduced fluorene-based compound. 【Chemical 1】

3. The recovery method according to claim 1 or 2, wherein the recovery step comprises adding the recovery agent to the electricity storage device and maintaining the device in an open circuit state.

4. The recovery method according to claim 1 or 2, wherein the recovery step uses a recovery agent containing an ether compound as the recovery agent solvent.

5. 3. The recovery method according to claim 1, wherein the recovery step uses a recovery agent containing a recovery agent stock solution including the reduced fluorene-based compound, metal ions of the same type as the carrier ions, and the recovery agent solvent, and an electrolyte solvent used in the electricity storage device.

6. The recovery method according to claim 1 or 2, wherein the recovery step recovers the capacity of the electricity storage device containing lithium iron phosphate as the olivine type compound.

7. a recovery step of recovering the capacity of the electricity storage device having metal ions as carrier ions and having a deteriorated capacity by the recovery method according to claim 1 or 2; A method for manufacturing an electricity storage device, comprising:

8. A recovery agent for recovering the capacity of an electricity storage device that uses metal ions as carrier ions and contains an olivine-type compound as a positive electrode active material, A restoring agent comprising a reduced fluorene-based compound, metal ions of the same type as the carrier ions, and a restoring agent solvent.

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