Recovery agent, energy storage device, recovery method, and method for manufacturing an energy storage device

Abstract

To further improve the charge and discharge characteristics of the energy storage device after recovery. [Solution] The recovery agent is a recovery agent that restores the capacity of an energy storage device that uses metal ions as carrier ions, and comprises a reduced aromatic hydrocarbon compound, a metal ion of the same type as the carrier ion, and a recovery agent solvent which is a compound having a tetrahydropyran skeleton.

Description

[Technical Field]

[0001] This disclosure relates to a recovery agent, an energy storage device, a recovery method, and a method for manufacturing an energy storage device. [Background technology]

[0002] Conventionally, a method has been proposed to restore the capacity of a non-aqueous electrolyte secondary battery that uses metal ions as carrier ions by adding a solution containing a reduced aromatic hydrocarbon compound and a metal ion of the same type as the carrier ion to the non-aqueous electrolyte secondary battery as a restorer (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-128795 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, while Patent Document 1 can restore the capacity of an energy storage device, there was a desire to further improve the charge and discharge characteristics of the energy storage device after restoration.

[0005] This disclosure was made to address these issues, with the primary objective being to further improve the charge and discharge characteristics of the energy storage device after recovery. [Means for solving the problem]

[0006] In order to achieve the above-mentioned objectives, the inventors conducted diligent research and discovered that using a compound having a tetrahydropyran skeleton as the solvent for the recovery agent further improves the charge and discharge characteristics of the energy storage device after recovery, thus completing the present invention.

[0007] In other words, the recovery agent of this disclosure is A recovery agent that restores the capacity of energy storage devices that use metal ions as carrier ions, The solution comprises a reduced aromatic hydrocarbon compound, a metal ion of the same type as the carrier ion, and a recovery agent solvent which is a compound having a tetrahydropyran skeleton.

[0008] Furthermore, the energy storage device of this disclosure is A power storage device that uses metal ions as carrier ions, It contains the recovery agent mentioned above.

[0009] Furthermore, the method for restoring this disclosure is: A recovery method for restoring the capacity of an energy storage device that uses metal ions as carrier ions, The method includes a recovery step in which the above-mentioned recovery agent is added to the energy storage device to restore the capacity of the energy storage device.

[0010] Furthermore, the method for manufacturing the energy storage device described herein is: A preparation process for preparing a capacity-degraded energy storage device using metal ions as carrier ions, A recovery step in which the above-mentioned recovery agent is added to the energy storage device to restore the capacity of the energy storage device, It includes. [Effects of the Invention]

[0011] This disclosure makes it possible to further improve the charge and discharge characteristics of the energy storage device after recovery. The reason for this effect is presumed to be as follows. For example, with conventional recovery agents, side reactions can occur during charging and discharging, such as the recovery agent solvent being co-inserted into the electrode along with carrier ions, which can reduce the charge and discharge efficiency of the energy storage device after recovery. Compounds having a tetrahydropyran skeleton are difficult to co-insert into the electrode due to their large molecular size, so it is presumed that using them as a recovery agent solvent can suppress side reactions, thereby further improving the charge and discharge characteristics of the energy storage device after recovery. [Brief explanation of the drawing]

[0012] [Figure 1] Explanatory drawing showing an outline of the configuration of the non-aqueous electrolyte secondary battery 20. [Figure 2] Explanatory drawing showing an example of a scheme of a recovery reaction. [Figure 3] Charge-discharge curves of the deteriorated battery and the recovered battery of Experimental Example 1. [Figure 4] Charge-discharge curves of the deteriorated battery and the recovered battery of Experimental Example 2.

Mode for Carrying Out the Invention

[0013] The recovery agent and the recovery method disclosed in this specification are a recovery agent and a recovery method for recovering the capacity of an electricity storage device having metal ions as carrier ions.

[0014] (Electricity storage device to be recovered) First, the electricity storage device to be recovered will be described. Examples of the electricity storage device include a hybrid capacitor, an electric double layer capacitor, an alkali metal secondary battery such as lithium or sodium, an alkali metal ion battery, an air battery, and the like. Examples of the metal ions as carrier ions include alkali metal ions such as Li, Na, K, and group 2 ions (alkaline earth metal ions) such as Mg, Ca, Sr, etc., among which lithium ions are preferred. Among these, as the electricity storage device, a lithium secondary battery, particularly a lithium ion secondary battery, is preferred. Here, the case where the electricity storage device is a lithium secondary battery will be mainly described. 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 that intervenes between the positive electrode and the negative electrode and conducts lithium ions. This electricity storage device may include a separator between the positive electrode and the negative electrode.

[0015] The positive electrode contains a positive electrode active material. As the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, etc. can be used. Specifically, transition metal sulfides such as TiS2, TiS3, MoS3, FeS2, with the basic composition formula Li b , a MnO2 (0 < x < 1, etc., the same hereinafter), Li (1-x) Mn2O4, etc., lithium manganese composite oxides, with the basic composition formula Li (1-x) CoO2, etc., lithium cobalt composite oxides, with the basic composition formula Li (1-x) NiO2, etc., lithium nickel composite oxides, with the basic composition formula Li (1-x) Ni a Co b Mn c O2 (a + b + c = 1), etc., lithium nickel cobalt manganese composite oxides, with the basic composition formula LiV2O3, etc., lithium vanadium composite oxides, with the basic composition formula V2O5, etc., transition metal oxides can be used. Among these, lithium transition metal composite oxides, for example, LiCoO2, LiNiO2, LiMnO2, LiV2O3, etc. are preferable. Also, as the positive electrode active material, olivine-type compounds such as lithium iron phosphate can be used. Lithium iron phosphate may have the basic composition formula Li x Fe 1-y M y PO4 (where M is one or more of Mn, Cr, Co, Cu, Ni, V, Mo, Y, Zn, Al, Ga, Mg, B, Nb, and satisfies 0.05 ≦ x ≦ 1.2 and 0 ≦ y ≦ 0.8). The positive electrode active material preferably contains LiFePO4. Note that the "basic composition formula" means that other elements may be included. The positive electrode active material may have a redox potential of 3.5 V or more, 4.0 V or more, or 4.5 V or more based on the Li metal standard.

[0016] The positive electrode may be formed, for example, by mixing a positive electrode active material, a conductive material, and a binder, adding a suitable solvent to form a paste-like positive electrode composite, applying and drying it on the surface of a current collector, and compressing it as needed to increase the electrode density. In the positive electrode, the conductive material can be a mixture of one or more types of materials such as graphite (scaly graphite, flake graphite) or artificial graphite, acetylene black, carbon black, Ketjen black, carbon whiskers, needle coke, carbon fiber, or metals (copper, nickel, aluminum, silver, gold, etc.). Among these, carbon black and acetylene black are preferred as conductive materials from the viewpoint of electronic conductivity and coating properties. The binder serves to bind the active material particles and conductive material particles together. For example, fluororesins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR) can be used individually or as a mixture of two or more. Water-based binders such as cellulose-based carboxymethylcellulose (CMC), styrene-butadiene copolymer (SBR), and aqueous dispersions of polyvinyl alcohol can also be used. As a solvent for dispersing the positive electrode active material, conductive material, and binder, organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used. Alternatively, a dispersant and thickener may be added to water, and the active material may be slurryed with a latex such as SBR. As a thickening agent, polysaccharides such as carboxymethylcellulose and methylcellulose can be used alone or as a mixture of two or more. Coating methods include roller coating using applicator rolls, screen coating, doctor blade method, spin coating, and bar coating, and any thickness and shape can be achieved using any of these methods.Current collectors can be made from materials such as aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymers, and conductive glass. Additionally, aluminum and copper can be treated with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and oxidation resistance. These materials can also be oxidized. Current collector shapes include foil, film, sheet, net, punched or expanded materials, lath, porous materials, foams, and fiber clusters. For example, current collectors with a thickness of 1 to 500 μm are commonly used.

[0017] The negative electrode contains a negative electrode active material. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys, and tin compounds, carbonaceous materials capable of intercalating and releasing lithium ions, composite oxides containing multiple elements, and conductive polymers. Examples of carbonaceous materials include coke, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Of these, graphite, such as artificial graphite and natural graphite, is preferred because it has an operating potential close to that of metallic lithium, allows for charging and discharging at high operating voltages, suppresses self-discharge when a lithium salt is used as a supporting salt, and reduces 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 negative electrode active materials from a safety standpoint.

[0018] The negative electrode may be formed, for example, by tightly bonding a negative electrode active material and a current collector, or it may be formed by mixing a negative electrode active material, a conductive material, and a binder, adding a suitable solvent to form a paste-like negative electrode composite, which is then applied to the surface of the current collector, dried, and compressed as needed to increase the electrode density. The conductive material, binder, and solvent used for the negative electrode can be those exemplified for the positive electrode. The oxidation-reduction potential of the negative electrode active material may be 1.0V or less, 0.5V or less, or 0.3V or less based on Li metal. The basis weight of the negative electrode composite material is, for example, 3 mg / cm³. 2 It is acceptable to exceed this amount, 4 mg / cm³2 The above may also be used. The basis weight of the negative electrode composite material is, for example, 15 mg / cm³. 2 The following may also be used: For the negative electrode current collector, materials such as copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, and Al-Cd alloy can be used. In addition, for the purpose of improving adhesion, conductivity, and reduction resistance, materials such as copper with a surface treated with carbon, nickel, titanium, or silver can also be used. These materials can also be oxidized. The shape of the current collector can be the same as that of the positive electrode.

[0019] The ion-conducting medium may be a non-aqueous 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 individually or in combination. The concentration of the supporting salt is preferably 0.1 to 2.0 M, and more preferably 0.8 to 1.2 M. As the organic solvent, for example, an aprotic organic solvent can be used. Examples of such organic solvents include cyclic carbonates, linear carbonates, cyclic esters, cyclic ethers, and linear ethers. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of linear 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 linear ethers include dimethoxyethane and ethylene glycol dimethyl ether. These may be used individually or in combination. In addition, other non-aqueous electrolytes may be nitrile solvents such as acetonitrile and propylnitrile, ionic liquids, and gel electrolytes. Non-aqueous electrolytes 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.

[0020] The separator is not particularly limited as long as its composition can withstand the operating range of the energy storage device, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabric or polyphenylene sulfide nonwoven fabric, and thin microporous membranes of olefin resins such as polyethylene or polypropylene. These may be used individually or in combination.

[0021] The energy storage device may have an openable and closable injection port in the case that houses the positive electrode, negative electrode, and non-aqueous electrolyte. A recovery agent can be easily injected through the injection port.

[0022] The shape of the energy storage device is not particularly limited, but examples include coin-shaped, button-shaped, sheet-shaped, stacked, cylindrical, flat, and rectangular shapes. It may also be applied to larger devices used in electric vehicles, etc. Figure 1 is a schematic diagram showing an example of a non-aqueous electrolyte secondary battery 20, which is an example of an energy storage device. This non-aqueous electrolyte secondary battery 20 comprises a cup-shaped battery case 21, a positive electrode 22 having a positive electrode active material and provided at the bottom of the battery case 21, a negative electrode 23 having a negative electrode active material and provided at a position opposite the positive electrode 22 via a separator 24, a gasket 25 made 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 this non-aqueous electrolyte secondary battery 20, the space between the positive electrode 22 and the negative electrode 23 is filled with a non-aqueous electrolyte 27.

[0023] (Recovery item) Next, the recovery agent will be described. The recovery agent comprises a reduced aromatic hydrocarbon compound, a metal ion of the same type as the carrier ions in the energy storage device, and a recovery agent solvent which is a compound having a tetrahydropyran skeleton.

[0024] The recovery agent solvent is a compound having a tetrahydropyran skeleton (hereinafter also referred to as a tetrahydropyran compound). A tetrahydropyran compound has a saturated six-membered ring tetrahydropyran skeleton containing five carbon atoms and one oxygen atom. Tetrahydropyran compounds include tetrahydropyran and its derivatives. A tetrahydropyran derivative may have a substituent bonded to one or more of the six-membered ring carbons (carbons at positions 2 to 6) of tetrahydropyran. Examples of substituents include halogen atoms, alkyl groups, aryl groups, alkenyl groups, alkoxy groups, aryloxy groups, sulfonyl groups, amino groups, cyano groups, carbonyl groups, acyl groups, amide groups, and hydroxyl groups. Preferably, the substituent is an alkyl group having 1 to 3 carbon atoms, and a methyl group is preferred. Preferably, the substituent is bonded to the carbon at position 4 of tetrahydropyran. A tetrahydropyran compound may be, for example, tetrahydropyran (formula (A)) or 4-methyltetrahydropyran (formula (B)).

[0025] [ka]

[0026] Aromatic hydrocarbon compounds are not particularly limited, but are preferably compounds having a fluorene skeleton (hereinafter also referred to as fluorene compounds). Fluorene compounds have a fluorene skeleton in which two 6-membered rings are arranged so as to sandwich a 5-membered ring. Fluorene compounds include fluorene and its derivatives. Fluorene derivatives may have substituents on the aromatic ring or may contain heteroatoms within the aromatic ring. Examples of substituents include halogen atoms, alkyl groups, aryl groups, alkenyl groups, alkoxy groups, aryloxy groups, sulfonyl groups, amino groups, cyano groups, carbonyl groups, acyl groups, amide groups, hydroxyl groups, etc. Examples of heteroatoms include nitrogen, oxygen, sulfur, etc. Fluorene compounds may have a structure in which two hydrogens are bonded to the carbon at position 9 of the fluorene skeleton, as shown in formula (1) below, or they may have a structure in which two substituents are bonded to the carbon at position 9 of the fluorene skeleton, as shown in formula (2) below. Furthermore, the fluorene compound may be, for example, 9-methyl-9H-fluorene, in which one hydrogen atom and one substituent are bonded to the carbon atom at position 9 of the fluorene skeleton. The fluorene compound may be fluorene represented by formula (1) and without substituents or heteroatoms, or 9,9-dimethylfluorene represented by formula (2) and without substituents or heteroatoms. The aromatic hydrocarbon compound may be, for example, polyacene or polyphenyl as shown in Patent Document 1. The reduced aromatic hydrocarbon compound is, for example, the reduced state of the above-mentioned aromatic hydrocarbon compound (also called the reduced product), and may be, for example, an anion or a radical anion. The reduced aromatic hydrocarbon compound and the metal ion may be dissociated or associated.

[0027] [ka]

[0028] The metal ions may be of the same type as the carrier ions in the energy storage device, but it is preferable that they be one or more alkali metal ions such as lithium ions, sodium ions, and potassium ions.

[0029] The recovery agent may contain the aromatic hydrocarbon compounds described above, for example, one or more aromatic hydrocarbon compounds from formulas (1) and (2) above. Alternatively, the recovery agent may contain the aromatic hydrocarbon compound in a reduced state and a metal ion obtained by reacting the aromatic hydrocarbon compound with a metal, for example, one or more from the following formulas (3) and (4). When an aromatic hydrocarbon compound in which one hydrogen and one substituent are bonded to the carbon at position 9 of the fluorene skeleton is reacted with a metal, hydrogen is eliminated, similar to formula (3), and an anion of the aromatic hydrocarbon compound and a metal ion are produced.

[0030] [ka]

[0031] The recovery agent may be obtained by adding an aromatic hydrocarbon compound and a metal in a metallic state rather than an ionic state to a recovery agent solvent. For example, it may be obtained by reacting fluorene, 9,9-dimethylfluorene, or their derivatives with Li metal, as shown in formulas (5) and (6) below. The recovery agent may be obtained by reacting an aromatic hydrocarbon compound such as fluorene, 9,9-dimethylfluorene, or their derivatives with a metal such as Li metal in a recovery agent solvent. In this way, a recovery agent containing an aromatic hydrocarbon compound in a reduced state and a metal ion can be easily prepared. The recovery agent may also be prepared by adding a metal to a precursor obtained by adding an aromatic hydrocarbon compound to a recovery agent solvent. The recovery agent may be prepared under an inert atmosphere such as an argon atmosphere. The recovery agent may also be prepared under a low dew point environment such as below -20°C, below -40°C, or below -60°C. The recovery agent may also be prepared by stirring the recovery agent solvent, aromatic hydrocarbon compound, and metal, using a stirrer or the like.

[0032] [ka]

[0033] A solution composed of a reduced aromatic hydrocarbon compound, a metal ion of the same type as the carrier ions of the energy storage device, and a recovery solvent is also called a recovery stock solution. In the recovery stock solution, the concentrations of the reduced aromatic hydrocarbon compound and the metal ions may be 0.05 mol / L or higher, 0.1 mol / L or higher, or 0.5 mol / L or higher, respectively. Furthermore, this concentration may be less than or equal to the solubility, 5 mol / L or lower, or 2 mol / L or lower. Also, the number of moles (M) of the reduced aromatic hydrocarbon compound contained in the recovery stock solution may be... A (mol) and the number of moles of metal ions M B (mol) ratio M A / M B It is preferable to set it to 1 / 1, but it may also be 1.1 / 1.0 to 1.0 / 1.1 or 1.2 / 1.0 to 1.0 / 1.2.

[0034] The recovery agent may further contain an electrolyte solvent. Examples of the electrolyte solvent include the organic solvents mentioned above that are used in the electrolytes of energy storage devices. The electrolyte solvent is a different solvent from the recovery agent solvent. The recovery agent may further contain a supporting salt. Examples of the supporting salt include the supporting salts mentioned above that are contained in the non-aqueous electrolyte of energy storage devices. It is preferable that the organic solvent and supporting salt are the same as those contained in the electrolyte of the energy storage device to be recovered. The recovery agent may also contain an electrolyte in which the supporting salt is dissolved in the electrolyte solvent, in which case it is preferable that the electrolyte is the same as the non-aqueous electrolyte of the energy storage device to be recovered. When the recovery agent contains an electrolyte or electrolyte solvent, the content of the electrolyte or electrolyte solvent may be 10% by volume or more, 20% by volume or more, or 30% by volume or more. In addition, the content of the electrolyte or electrolyte solvent may be 70% by volume or less, 60% by volume or less, or 50% by volume or less.

[0035] The recovery agent may have an oxidation-reduction potential higher than the oxidation-reduction potential of the negative electrode and lower than the oxidation-reduction potential of the positive electrode. The oxidation-reduction potential of the recovery agent may be, for example, 0.7V or higher, 0.8V or higher, or 1.0V or higher based on the Li metal. Alternatively, the oxidation-reduction potential of the recovery agent may be, for example, 2.5V or lower, 2.0V or lower, 1.5V or lower, or 1.2V or lower based on the Li metal.

[0036] (Energy storage device) Next, the energy storage device will be described. This energy storage device is configured in the same way as the energy storage device to be restored described above, except that it contains a recovery agent. The recovery agent is preferably added to the ion conducting medium. The amount of the recovery agent may be, for example, 1% to 100%, 10% to 75%, or 25% to 50% of the volume of the ion conducting medium (without the recovery agent).

[0037] (Recovery method and method for manufacturing an energy storage device) Next, a recovery method and a method for manufacturing an energy storage device will be described. This recovery method and method for manufacturing an energy storage device may include a preparation step of preparing an energy storage device with degraded capacity, and a recovery step of adding a recovery agent to the energy storage device to restore its capacity. By performing the recovery step, the capacity of the energy storage device with degraded capacity can be restored, thereby enabling the manufacture of an energy storage device with restored capacity using the energy storage device with degraded capacity.

[0038] In the preparation step, a degraded energy storage device is prepared. Examples of a degraded energy storage device include the energy storage devices to be restored as described above. A degraded energy storage device may be, for example, an energy storage device whose capacity has degraded relative to its rated capacity. A degraded energy storage device may be unused or used. Even unused devices may degrade in capacity due to long-term storage, etc.

[0039] In the recovery process, the recovery agent described above is added to the energy storage device whose capacity has deteriorated. When adding the recovery agent, the energy storage device may be opened, the recovery agent added, and the opening sealed, or the recovery agent may be added to the energy storage device by injection or other means to seal the perforation. When adding the recovery agent, it may be added under an inert atmosphere such as an argon atmosphere. The recovery agent should be added so as to be in contact with at least the positive and negative electrodes, but it may also be mixed with the non-aqueous electrolyte of the energy storage device. The amount of recovery agent can be appropriately determined according to the configuration of the energy storage device and the degree of deterioration. For example, the amount of recovery agent may be 1% to 100%, 10% to 75%, or 25% to 50% of the volume of non-aqueous electrolyte contained in the energy storage device.

[0040] In the recovery process, the recovery agent may be added to the energy storage device and kept in an open-circuit state. The holding time may be, for example, 1 hour to 48 hours, 6 hours to 36 hours, or 12 hours to 24 hours.

[0041] In the recovery process, a recovery agent is added to the energy 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 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 limit charge voltage set for the energy storage device. The predetermined voltage may be determined appropriately depending on the configuration of the energy storage device, etc., but for example, it may be 3.0V or more and less than 4.1V, or 3.5V or more and 4.0V or less. Constant voltage application may be continued before adding the recovery agent or during the addition of the recovery agent, or it may be started after adding the recovery agent while the voltage drop of the energy 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, more preferably within 1 minute). The constant voltage application time can be determined appropriately depending on the configuration of the energy storage device, the degree of degradation, the amount of recovery agent, etc. 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.

[0042] In the recovery process, prior to adding the recovery agent to the energy storage device, the voltage of the energy storage device may be adjusted to the predetermined voltage described above. In this case, for example, the voltage of the energy storage device may be adjusted by charging with constant current charging (CC charging) or constant current constant voltage charging (CCCV charging). This voltage adjustment is not required, but it is preferable that the voltage of the energy storage device be the predetermined voltage described above.

[0043] When a recovery agent is injected during the recovery process, a recovery reaction like the one shown in Figure 2 is expected to occur. Figure 2 is an explanatory diagram showing an example of the recovery reaction scheme, illustrating the scheme when the positive electrode active material is an olivine-type compound such as lithium iron phosphate and the aromatic hydrocarbon compound is a fluorene-type compound. In Figure 2, the metal ions are Li + , the reduced form of fluorene compounds is Flu - , olivine-type compounds Li n-y This will be explained using MePO4 as an example. As shown in Figure 2, in the recovery agent, a compound containing a reduced form of a fluorene compound and metal ions functions as an electron-donating reducing agent. By acting on a degraded positive electrode, electrons and metal ions are donated from the reducing agent to the positive electrode, thereby restoring its capacity.

[0044] The energy storage device after the recovery process preferably has a charge / discharge efficiency of 95% or higher. This charge / discharge efficiency is preferably 97% or higher, and more preferably 99% or higher.

[0045] In the embodiments described in detail above, the charge and discharge characteristics of the energy storage device after recovery can be further improved. The reason for this effect is presumed to be as follows. For example, with conventional recovery agents, side reactions can occur during charge and discharge, such as the recovery agent solvent being co-inserted into the electrode along with carrier ions, which can reduce the charge and discharge efficiency of the energy storage device after recovery. Compounds having a tetrahydropyran skeleton are difficult to co-insert into the electrode due to their large molecular size, so it is presumed that using them as a recovery agent solvent can suppress side reactions, thereby further improving the charge and discharge characteristics of the energy storage device after recovery. Co-insertion of solvents and the resulting side reactions are often problematic in graphite anodes, so the application of this disclosure is particularly significant in energy storage devices containing graphite as the anode active material. Compounds having a tetrahydropyran skeleton are considered difficult to insert into graphite anodes.

[0046] Furthermore, a recovery agent containing a reduced aromatic hydrocarbon compound and metal ions directly acts on the positive electrode simply by being injected into a non-aqueous electrolyte secondary battery, generating a recovery reaction that supplies electrons and metal ions to the positive electrode, thus easily restoring the capacity of the energy storage device. In particular, it is presumed that capacity recovery is more favorably achieved when the aromatic hydrocarbon compound is a fluorene-based compound. Since the reduced form of the fluorene-based compound has favorable reducing power, side reactions such as reductive decomposition of the positive electrode by the reduced aromatic hydrocarbon compound and oxidation of the recovery agent are less likely to occur, and it is presumed that the charge and discharge characteristics of the energy storage device after recovery can be further improved. In addition, since the reduced form of the fluorene-based compound has a lower overpotential in the capacity recovery reaction involving reduction of the positive electrode compared to the reduced form of other aromatic hydrocarbon compounds, it is presumed that even if the potential of the positive electrode decreases as the reaction progresses, a sufficient potential difference necessary for the reaction with the recovery agent is maintained, and capacity recovery is more favorably achieved. Furthermore, it is expected that capacity recovery can be favorably achieved without applying a constant voltage, thus reducing the effort and cost of applying a constant voltage. Furthermore, since the reduced forms of fluorene compounds react under milder conditions compared to the reduced forms of other aromatic hydrocarbon compounds, it is presumed that they can suitably restore capacity even in energy storage devices containing olivine-type cathode active materials.

[0047] It goes without saying that this disclosure is not limited in any way to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of this disclosure.

[0048] This disclosure may also be as described in [1] to

[10] below. [1] A recovery agent for restoring the capacity of an energy storage device that uses metal ions as carrier ions, comprising a reduced aromatic hydrocarbon compound, a metal ion of the same type as the carrier ion, and a recovery agent solvent which is a compound having a tetrahydropyran skeleton. [2] The recovery agent according to [1], wherein the recovery agent solvent is 4-methyltetrahydropyran. [3] The recovery agent according to [1] or [2], wherein the aromatic hydrocarbon compound is a compound having a fluorene skeleton. [4] A restorative agent according to any one of [1] to [3], which restores the capacity of the energy storage device containing graphite as a negative electrode active material. [5] A restorative agent according to any one of [1] to [4], which restores the capacity of the energy storage device comprising lithium iron phosphate as a positive electrode active material. [6] An energy storage device that uses metal ions as carrier ions, comprising a recovery agent described in any one of [1] to [5]. [7] A recovery method for restoring the capacity of an energy storage device that uses metal ions as carrier ions, comprising a recovery step of adding a recovery agent described in any one of [1] to [5] into the energy storage device to restore the capacity of the energy storage device. [8] The recovery method according to [7], wherein in the recovery step, the recovery agent is added to the energy storage device and held in an open circuit state. [9] The recovery method according to [7] or [8], wherein the energy storage device after the recovery step has a charge / discharge efficiency of 95% or more.

[10] A method for manufacturing an energy storage device, comprising: a preparation step of preparing an energy storage device with degraded capacity using metal ions as carrier ions; and a recovery step of adding a recovery agent described in any one of [1] to [5] into the energy storage device to restore the capacity of the energy storage device. [Examples]

[0049] The following describes examples of lithium-ion battery recovery using the recovery agent of this disclosure. Experimental Example 1 corresponds to the embodiment, and Experimental Example 2 corresponds to the comparative example.

[0050] [Experimental Example 1] (Basic battery configuration) A positive electrode was prepared by coating aluminum foil with a positive electrode mixture consisting of 92% by mass of LiFePO4, 5% by mass of acetylene black, and 3% by mass of polyvinylidene fluoride. A negative electrode was prepared by coating copper foil with a negative electrode mixture consisting of 98% by mass of graphite, 1% by mass of carboxymethylcellulose, and 1% by mass of styrene-butadiene rubber. The electrolyte 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 single-layer polyethylene microporous membrane was used as the separator. The battery contained 0.7 mL of the electrolyte.

[0051] (Making degraded batteries) First, a positive electrode with a simulated reduced capacity was fabricated using the following procedure. A separator soaked in electrolyte was placed between the positive and negative electrodes, and the cell was sealed in a laminate film to create a laminate cell (battery before degradation). The fabricated laminate cell was charged with constant current (CC) at 2.0 mA to 3.65 V at 25°C, discharged with CC at 2.0 mA to 2.5 V, charged with constant current-constant voltage (CCCV) at 2.0 mA for 2 hours to 3.65 V, and finally discharged with CC at 1.0 mA to 2.5 V. The discharge capacity of the second CC discharge was defined as the battery capacity. Subsequently, the cell was charged to a capacity equivalent to 10% of its electrical capacity (SOC = 10%), thereby extracting lithium from the positive electrode and obtaining a positive electrode with a simulated reduced capacity (also referred to as a degraded positive electrode). Subsequent charge-discharge and open-circuit potential measurements were performed at 25°C. The cell was then disassembled, and the degraded positive electrode was removed. A laminate cell was fabricated in the same manner as the pre-degradation battery, except that the obtained degraded positive electrode was used as the positive electrode. This was then used as the degraded battery. The discharge capacity of the capacity-degraded battery was measured in the same manner as the pre-degradation battery.

[0052] (Preparation of recovery agent) Under an inert atmosphere, fluorene was dissolved in 4-methyltetrahydropyran (MTHP) solvent to a concentration of 1.0 mol / L. Then, 1.0 mol / L of lithium metal was added and stirred to prepare the recovery agent stock solution. The reaction between fluorene and lithium metal was presumed to be represented by the above formula (5). The recovery agent stock solution was an orange solution. Then, an equal volume of electrolyte was added to the recovery agent stock solution and stirred to prepare the recovery agent. The recovery agent was pale yellow. The electrolyte used was the same composition as the electrolyte used in the aforementioned battery. That is, a mixed solvent containing 30 vol% EC, 40 vol% DMC, and 30 vol% EMC was used, in which LiPF6 was dissolved to a concentration of 1.0 mol / L.

[0053] (Recovery of degraded batteries) A degraded battery was charged to 3.65V at 1.0mA using CCCV charging, and the voltage was adjusted to 3.65V. Then, a portion of the degraded battery was opened under an argon atmosphere, 0.5mL of a recovery agent was injected using a pipette, and the opening was sealed. Only the open-circuit potential was measured for 25 hours. The recovered battery was designated as the recovered battery.

[0054] (Evaluation of regenerative batteries) For the recovered batteries, the discharge capacity was measured in the same way as for the degraded batteries.

[0055] [Experimental Example 2] The preparation of the recovery agent was carried out in the same manner as in Experimental Example 1, except that the solvent was changed from MTHP to dimethoxyethane (DME).

[0056] [Experimental Results] Figure 3 shows the charge-discharge curves of the degraded and recovered batteries in Experimental Example 1. Figure 4 shows the charge-discharge curves of the degraded and recovered batteries in Experimental Example 2. In Figures 3 and 4, the charge-discharge curves shown are the discharge curve of the second CC discharge and the charge curve of the CCCV charge immediately preceding it. From Figures 3 and 4, it was found that Experimental Example 1 has recovery performance equal to or better than that of Experimental Example 2.

[0057] Table 1 shows the charge capacity, discharge capacity, and charge / discharge efficiency of the recovered batteries in Experimental Example 1 and Experimental Example 2. The charge capacity and discharge capacity were obtained from the charge / discharge curves shown in Figures 3 and 4, and the charge / discharge efficiency was defined as the ratio of the discharge capacity to the charge capacity. In Experimental Example 1, which used a recoverant containing MTHP, the charge / discharge efficiency of the recovered battery was 100%, whereas in Experimental Example 2, which used a recoverant containing DME, the charge / discharge efficiency of the recovered battery was 91%. This indicates that using a recoverant containing MTHP can improve the charge / discharge efficiency of the recovered battery.

[0058] Furthermore, regarding fluorene, as shown in equation (5) above, it is known that it reacts with lithium in an ether-based solvent to generate a radical anion, after which the hydrogen at position 9 spontaneously detaches, changing back into an anion (Reference 1: CAN Journal CHEM 38 (1960) 2450-2456). In Experimental Examples 1 and 2, it was inferred that the anion thus generated reacted with the positive electrode inside the battery, transferring lithium ions and electrons to the positive electrode while changing itself into an electrochemically stable chemical species inside the battery, thus carrying out the chemical reaction.

[0059] Two reasons were hypothesized for the deterioration of charge / discharge efficiency due to the injection of the recovery agent. One is the oxidative decomposition of the recovery agent solvent (ether, etc.) on the positive electrode, and regarding oxidative decomposition, the oxidative decomposition of tetrahydropyran was 5.3V vs. Li / Li + Based on what happens in this case (Reference 2: Patent No. 5306749), it was inferred that oxidative decomposition does not occur within the normal range of use of lithium-ion batteries. Another possibility is co-insertion of lithium ions and the recovery agent solvent into the negative electrode graphite, etc. However, in Experimental Example 1, it was inferred that the large molecular size of methyltetrahydropyran suppressed co-insertion, thereby improving the charge and discharge efficiency of the recovery battery.

[0060] It goes without saying that this disclosure is not limited in any way to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of this disclosure.

[0061] [Table 1] [Industrial applicability]

[0062] This disclosure is applicable to the field of energy storage devices. [Explanation of Symbols]

[0063] 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 recovery agent that restores the capacity of energy storage devices that use metal ions as carrier ions, The solvent comprises a reduced aromatic hydrocarbon compound, a metal ion of the same type as the carrier ion, and a recovery agent solvent which is a compound having a tetrahydropyran skeleton. A recovery item.

2. The aforementioned recovery agent solvent is 4-methyltetrahydropyran. The recovery agent according to claim 1.

3. The aforementioned aromatic hydrocarbon compound is a compound having a fluorene skeleton. The recovery agent according to claim 1 or 2.

4. To restore the capacity of the energy storage device containing graphite as the negative electrode active material, The recovery agent according to claim 1 or 2.

5. To restore the capacity of the energy storage device containing lithium iron phosphate as the positive electrode active material, The recovery agent according to claim 1 or 2.

6. A power storage device that uses metal ions as carrier ions, A restorative agent comprising the one described in claim 1 or 2, Energy storage device.

7. A recovery method for restoring the capacity of an energy storage device that uses metal ions as carrier ions, A recovery method comprising a recovery step of adding the recovery agent according to claim 1 or 2 to the energy storage device to restore the capacity of the energy storage device.

8. In the recovery step, the recovery agent is added to the energy storage device and kept in an open circuit state. The recovery method according to claim 7.

9. The energy storage device after the recovery process has a charge / discharge efficiency of 95% or more. The recovery method according to claim 7.

10. A preparation process for preparing a capacity-degraded energy storage device using metal ions as carrier ions, A recovery step of adding the recovery agent according to claim 1 or 2 into the energy storage device to restore the capacity of the energy storage device, A method for manufacturing an energy storage device, including [the specified element].

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