Solution for intercalation, intercalation method, method of manufacturing electrode, electrode, and electrochemical element

Abstract

To provide a solution, an intercalation method, a method of manufacturing an electrode, an electrode supporting metal ions, and an electrochemical device having an improved reducing power of radical anions.SOLUTION: A solution for intercalating metal ions into a solid material comprises radical anions of an aromatic compound and metal ions. The radical anions have an electron-donating substituent on an aromatic ring. An intercalation method comprises a step for contacting the solution with a solid material. A method for manufacturing an electrode includes steps for providing an initial electrode including a solid material and contacting the solution with the initial electrode. The electrode includes an aromatic compound having an electron-donating substituent on an aromatic ring. The electrochemical element includes the electrode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solution for intercalating metal ions into a solid material, an intercalation method, a method for producing an electrode, an electrode, and an electrochemical device. [Background technology]

[0002] Techniques for intercalating metal ions into the structure of a solid material using a solution are known. In the prior art disclosed in Patent Document 1, a solution containing a radical anion of an aromatic compound and a metal ion is brought into contact with a solid material, the solid material is reduced by the radical anion, and the metal ion is intercalated into the solid material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-204364 Summary of the Invention [Problem to be solved by the invention]

[0004] In the prior art, there is room for improvement in the amount of metal ions intercalated into solid materials, that is, the reducing power of radical anions.

[0005] The present invention has been made to solve this problem, and aims to provide a solution in which the reducing power of radical anions is improved, an intercalation method, a method for producing an electrode, an electrode carrying metal ions, and an electrochemical element. [Means for solving the problem]

[0006] A first aspect to achieve this object is a solution for intercalating metal ions into a solid material, comprising a radical anion of an aromatic compound and a metal ion, the radical anion having an electron-donating substituent on the aromatic ring.

[0007] In the second embodiment, in the first embodiment, the substituent has 10 or less carbon atoms, and the atom directly bonded to the aromatic ring is a carbon atom.

[0008] The third embodiment includes the metal ion of the first or second embodiment in the form of a simple substance of the metal.

[0009] In a fourth aspect, in any one of the first to third aspects, the compound has a maximum absorption wavelength in the range of 350-700 nm in the ultraviolet-visible absorption spectrum.

[0010] A fifth embodiment is an intercalation method comprising contacting a solid material with a solution of any of the first to fourth embodiments.

[0011] The sixth embodiment includes a state in which, after the operation in the fifth embodiment, the elemental metal of the metal ion and the solution coexist.

[0012] A seventh aspect is a method for producing an electrode, comprising the steps of providing an initial electrode comprising a solid material and contacting the initial electrode with a solution according to any one of the first to fourth aspects.

[0013] An eighth embodiment is an electrode comprising a solid material, the solid material comprising an aromatic compound having electron-donating substituents on the aromatic ring.

[0014] A ninth embodiment is an electrochemical device, comprising the electrode of the eighth embodiment. [Effects of the Invention]

[0015] The solution of the present invention contains a radical anion having an electron-donating substituent on the aromatic ring of an aromatic compound, which increases the electron density of the aromatic ring and the instability of the radical anion compared to a radical anion of an aromatic compound that does not have an electron-donating substituent on the aromatic ring. This increases the driving force for the reaction in which the radical anion returns to the original aromatic compound by intercalating a metal ion into the structure of the solid material, thereby improving the reducing power of the solution.

[0016] The intercalation method and electrode manufacturing method of the present invention include an operation of bringing a solution containing metal ions and radical anions having electron-donating substituents on the aromatic rings of an aromatic compound (a solution containing an arenide of metal ions) into contact with a solid material, and therefore, the amount of metal ions supported on the solid material can be increased.

[0017] The electrode of the present invention includes an aromatic compound having an electron-donating substituent on the aromatic ring and a solid material, and the electrochemical device of the present invention includes the electrode, and can achieve a high energy density. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view of an electrochemical device according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic cross-sectional view of an electrochemical device 10 according to a first embodiment. The electrochemical device 10 is a device that converts chemical energy into electrical energy and vice versa. The electrochemical device 10 includes, in order, a negative electrode 11, a separator 14, and a positive electrode 15.

[0020] 1 shows an electrochemical element 10 having one set of anode 11, separator 14, and cathode 15, but the present invention is not limited to this and may include a plurality of sets of these. The electrochemical element 10 is not limited to one having a stacked structure, but may also have a wound structure in which the anode 11 and cathode 15 are stacked with the separator 14 interposed therebetween and wound.

[0021] Examples of the electrochemical element 10 include secondary batteries and electrochemical capacitors. Examples of secondary batteries include those in which the separator 14 is made of a solid electrolyte, those in which the separator 14 contains a polymer or gel electrolyte, those in which an electrolyte solution passes through the separator 14, and those in which a portion of the negative electrode 11 or positive electrode 15 is clay-like. Examples of secondary batteries include liquid-based batteries containing an electrolyte solution, as well as all-solid-state batteries, semi-solid batteries, quasi-solid batteries, and clay batteries. Examples of electrochemical capacitors include electric double layer capacitors, redox capacitors that utilize redox reactions of electrodes or redox reactions of ions in a non-aqueous electrolyte solution, and hybrid capacitors that combine electric double layers and redox reactions, or that combine them with secondary battery materials.

[0022] In the electrochemical device 10, metal ions move to carry charge between the positive electrode 15 and the negative electrode 11. Examples of metal ions include lithium ions, sodium ions, potassium ions, rubidium ions, cesium ions, and calcium ions. As the metal ions, ions of alkali metals are preferred, and lithium ions are particularly preferred.

[0023] Negative electrode 11 is formed by stacking current collector 12 and composite layer 13. Current collector 12 is a conductive member. Examples of materials for current collector 12 include metals selected from Ni, Ti, Fe, Cu, and Si, alloys containing two or more of these elements, and stainless steel.

[0024] The composite layer 13 contains a solid material capable of absorbing and releasing metal ions (charge carriers). Examples of the solid material include carbon-based materials such as porous carbon, natural graphite, artificial graphite, graphitizable carbon, non-graphitizable carbon, graphene, and carbon fiber, and Li4Ti5O 12 lithium transition metal oxides such as lithium transition metal nitrides, metallic lithium, lithium alloys such as Li-Al alloys, Li-Sn alloys, Li-Si alloys, Li-Mg alloys, and Li-Si alloys, In-Sb alloys, Si-Li alloys, Si, SiC, and compounds containing Si and O as constituent elements (hereinafter referred to as "SiO x" where x is a number between 0.5 and 1.5), and silicon-based materials are exemplified. Examples of solid materials include particles or powders of carbon-based materials, lithium alloys, silicon-based materials, etc., as well as mixtures or coatings of these materials.

[0025] SiO x Examples of the solid material include an oxide of Si and a material having a structure in which microcrystalline or amorphous Si is dispersed in an amorphous SiO2 matrix. The solid material is appropriately selected depending on the type of charge carrier and electrochemical device 10. It is particularly preferable that the solid material contains an element from Group 14 of the periodic table based on the IUPAC 1990 Recommendations. Examples of Group 14 elements include C, Si, Ge, Sn, and Pb.

[0026] The separator 14 is made of a porous material that is durable against the solid materials and electrolyte solution contained in the negative electrode 11 and the positive electrode 15, and that allows charge carriers to pass through but does not have electronic conductivity. Examples of the separator 14 include nonwoven fabrics and porous films made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0027] Positive electrode 15 is formed by stacking current collector 16 and composite layer 17. Current collector 16 is a conductive member. Examples of materials for current collector 16 include metals selected from Ni, Ti, Fe, and Al, alloys containing two or more of these elements, stainless steel, and carbon materials.

[0028] The composite layer 17 includes a solid material. The solid material is appropriately selected depending on the type of charge carrier and the electrochemical device 10. When the electrochemical device 10 is a lithium-ion secondary battery, the solid material may include an active material capable of reversibly absorbing and desorbing lithium ions. Examples of the active material include oxides containing lithium and transition metal elements as constituent metal elements (lithium transition metal oxides), such as lithium nickel oxide (e.g., LiNiO), lithium cobalt oxide (e.g., LiCoO), and lithium manganese oxide (e.g., LiMnO), as well as phosphates containing lithium and transition metal elements as constituent metal elements, such as lithium manganese phosphate (LiMnPO) and lithium iron phosphate (LiFePO). When the electrochemical device 10 is an electrochemical capacitor, examples of the active material include carbon-based materials such as porous carbon, natural graphite, artificial graphite, graphitizable carbon, non-graphitizable carbon, and carbon fiber.

[0029] In order to reduce the resistance of the composite layers 13 and 17, a conductive additive may be contained in the composite layers 13 and 17. Examples of the conductive additive include carbon black, acetylene black, ketjen black, carbon nanotubes, carbon fiber, Ni, Pt, and Ag.

[0030] The composite layers 13 and 17 may contain a binder or dispersant that binds the active material and conductive additive. Examples of binders include fluorinated resins, polyolefins, polyimides, polyvinylpyrrolidone, polyvinyl alcohol, cellulose ethers, and rubber-like polymers such as styrene-butadiene rubber. Examples of fluorinated resins include vinylidene fluoride polymers, polychlorotrifluoroethylene, polyvinyl fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers.

[0031] Intercalation is performed to pre-dope metal ions into the negative electrode 11 and lower the potential of the negative electrode 11. After pre-doping, the potential of the negative electrode 11 drops, so the electrochemical device 10 has a certain voltage before charging. When the electrochemical device 10 is charged, anions accumulate in the positive electrode 15, and metal ions are carried on the negative electrode 11. When the electrochemical device 10 is discharged, anions are released from the positive electrode 15, and metal ions are released from the negative electrode 11. Pre-doping the negative electrode 11 apparently increases the Coulombic efficiency of the negative electrode 11, so the energy density of the electrochemical device 10 can be increased.

[0032] The intercalation method includes contacting a solution with the solid material of the negative electrode 11. The solution contains a radical anion of an aromatic compound having an electron-donating substituent on the aromatic ring and a metal ion. The solution may also contain a simple metal ion (a metal formed when a metal ion accepts an electron).

[0033] Examples of metal elements constituting the metal ions (intercalates) and simple metals include lithium, sodium, potassium, rubidium, and cesium. These metal ion species may be used alone or in combination with other ions. The form of the metal can be appropriately selected as long as excessive heat generation during the radical anion-generating reaction is avoided, and it may be in the form of foil or particles. There are no limitations on the concentration of the metal ions in the solution, but a concentration of 0.5 mol / kg or more is preferred, and 1-10 mol / kg is more preferred.

[0034] Examples of aromatic rings that form the basic skeleton of radical anions include naphthalene, anthracene, phenanthrene, tetracene, pyrene, perylene, coronene, and hexaphene. A structure in which multiple aromatic rings form a conjugated system may also be used, such as biphenyl, p-terphenyl, m-terphenyl, o-terphenyl, 1-phenylnaphthalene, p-tolylnaphthalene, 1,1-binaphthyl, 1-(2-furyl)naphthalene, and positional isomers thereof. The aromatic ring may also be a heterocyclic compound containing atoms other than carbon, such as oxygen, nitrogen, or sulfur, in the ring.

[0035] Examples of electron-donating substituents bonded to aromatic rings include alkyl groups such as methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl, i-butyl, and tert-butyl, alkenyl groups, alkynyl groups, methoxy groups, ethoxy groups, propoxy groups, and butoxy groups. Examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl groups. The electron-donating substituents may be the same or different, and may be bonded to each other to form a ring structure. Examples of such substituents include derivatives of 1,2,3,4-tetrahydroanthracene and acenaphthene. Additional substituents may be bonded to the substituent as long as they do not adversely affect the generation and stability of radical anions.

[0036] The substituent preferably has 10 or fewer carbon atoms, such as an alkyl group, where the atom directly bonded to the carbon atom of the aromatic ring is a carbon atom. By ensuring that the carbon chain length of the substituent is within the appropriate range, the viscosity caused by intermolecular forces does not become too high, making handling easier. At the same time, because it has a certain degree of volatility, residual aromatic compounds can be easily removed from the solid material after the intercalation reaction by vacuum drying or other methods. Furthermore, substituents with a structure in which a carbon atom is directly bonded to the aromatic ring have an appropriate electron-donating strength to the aromatic ring and can generate highly stable radical anions in solvents.

[0037] The aromatic compound may be a compound in which a plurality of aromatic rings are bonded to each other by a partial structure having electron-donating properties to the aromatic ring, such as a methylene chain or an ether bond. From these aromatic compounds, an appropriate compound is selected in consideration of handling safety, chemical properties, and reactivity. For example, when intercalation is performed at 25°C, compounds that are liquid at room temperature are preferred from the viewpoint of handling. In this case, 1-methylnaphthalene, 2-methylnaphthalene, 1-ethylnaphthalene, 2-ethylnaphthalene, 1-propylnaphthalene, 1-isopropylnaphthalene, 2-propylnaphthalene, 2-isopropylnaphthalene, 1-butylnaphthalene, 1-isobutylnaphthalene, 1-tert-butylnaphthalene, 2-butylnaphthalene, and 2-tert-butylnaphthalene are preferred. 1-Methylnaphthalene and 2-methylnaphthalene are particularly preferred.

[0038] On the other hand, taking into consideration that electron donating properties to aromatic rings act additively, from such a viewpoint, the aromatic compound is preferably a dialkylnaphthalene represented by 1,3-dimethylnaphthalene, 1,4-dimethylnaphthalene, 1,5-dimethylnaphthalene, 1,6-dimethylnaphthalene, 1,7-dimethylnaphthalene, 1,8-dimethylnaphthalene, 2,3-dimethylnaphthalene, 2,6-dimethylnaphthalene, or 2,7-dimethylnaphthalene.

[0039] The radical anion and metal ion can be used in a dissolved state in tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, cyclic ethers such as crown ethers, or chain ethers such as glymes. Among these solvents, cyclic ethers are preferred over chain ethers, and among cyclic ethers, 2-methyltetrahydrofuran and 2,5-dimethyltetrahydrofuran are more preferred because they are less likely to cause co-intercalation of the solvent.

[0040] The ether solvent may be used alone, or a mixture of ether solvents with other organic solvents may be used to obtain the appropriate dielectric constant, viscosity, or vapor pressure. The ether solvent may be dehydrated in advance, or may be dehydrated by distillation, contact with a desiccant such as zeolite, or contact with the metal element of the intercalate ion as a pre-processing step before the intercalation reaction. The organic solvent may contain a radical scavenger, such as dibutylhydroxytoluene (BHT), as an antioxidant and stabilizer, as long as it does not significantly adversely affect the intercalation reaction.

[0041] The solution preferably has a maximum absorption wavelength in the range of 350-700 nm in the ultraviolet-visible absorption spectrum measured at an optical path length of 10 mm and a temperature of 25°C ± 2°C. This is because when the reducing power of the radical anions in the solution is high, the maximum absorption wavelength tends to be in the range of 350-700 nm.

[0042] In the intercalation method, the solution can be brought into contact with the solid material by applying or spraying the solution to the solid material placed on the negative electrode 11, or by immersing the solid material in the solution. Any contact between the solution and the solid material is sufficient; the solid material does not necessarily have to be brought into contact with a pre-prepared solution. For example, a solvent and an aromatic compound having electron-donating groups can be mixed first, followed by the addition of a metal element serving as a metal ion source to generate metal ions, after which the solid material can be immersed in the solution. Alternatively, the solid material can be immersed in the solvent first, followed by the addition of the aromatic compound having electron-donating groups and the metal element serving as a metal ion source. These materials can be added in any order, either all at once or gradually as the reaction progresses. It is particularly preferable to gradually add the metal element depending on the reaction state in which radical anions are generated.

[0043] When preparing the solution, the molar ratio of aromatic compound to metal element does not necessarily have to be 1:1. Considering that the metal element or solution is deactivated by moisture in the environment, and that using an excess of metal element generates a dianion, further lowering the equilibrium potential, it is preferable to have more metal element than aromatic compound. Note that the radical anion and dianion may be mixed. The radical anion may be prepared by mixing multiple aromatic compounds, and may also contain a neutral aromatic compound that has no charge.

[0044] In the intercalation method, after the solution is brought into contact with the solid material, the solid material, the solution, and the metal element may coexist. This is because even when metal ions move from the solution into the solid material and the radical anions return to the aromatic compound as neutral molecules, radical anions and metal ions are generated by reaction with the metal element, and the concentration of the radial anions, which are the reactants, is maintained, thereby preventing a decrease in the rate of the intercalation reaction.

[0045] The solution may be removed from the solid material that has been brought into contact with the solution by methods such as filtration, centrifugation, washing with an organic solvent, and drying under reduced pressure. These methods may be used alone or in combination. In particular, when the negative electrode 11 is an electrode for a secondary battery, a method is preferred in which the negative electrode is washed by immersing it in an organic solvent multiple times and then removing the organic solvent by evaporation. The solution and the organic solvent used for washing can be recovered, filtered, impurities removed, purified, and dehydrated as necessary, and then reused in the solution.

[0046] After contacting the solid material with the solution and then removing the solution, the composite layer 13 of the negative electrode 11 contains aromatic compounds having electron-donating substituents on the aromatic ring. The aromatic compounds can be detected by immersing the composite layer 13 in a solvent, dissolving the substances attached to the solid material in the solvent, and then ionizing them using a gas chromatograph mass spectrometer (GC-MS), separating them according to the mass of the ions. If the GC-MS confirms the presence of trace amounts or more of aromatic compounds having electron-donating substituents on the aromatic ring, it is determined that the negative electrode 11 contains aromatic compounds.

[0047] When an aromatic compound having an electron-donating substituent on the aromatic ring is contained in the negative electrode 11, the aromatic compound is eluted from the negative electrode 11, and when a potential exceeding the oxidation potential of the aromatic compound is applied to the electrochemical device 10, the aromatic compound undergoes an electrochemical oxidation reaction, which is expected to have the effect of suppressing an increase in voltage. Furthermore, although it depends on the structure and material composition of the electrochemical device 10, when elemental metals are deposited within the electrochemical device 10, the deposited elemental metals are consumed in a reaction that forms a radical anion of the aromatic compound, which is expected to have the effect of suppressing the deposition of elemental metals. [Example]

[0048] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0049] (Preparation of initial electrodes) A negative electrode (hereinafter referred to as "negative electrode A") was prepared by applying a slurry prepared so that the solid composition was graphite: acetylene black: styrene butadiene rubber: carboxymethyl cellulose = 97:1:1:1 (volume ratio) onto copper foil. Similarly, a negative electrode (hereinafter referred to as "negative electrode B") was prepared by applying a slurry prepared so that the solid composition was graphite: polyvinylidene fluoride = 93:7 (weight ratio) onto copper foil. A negative electrode (hereinafter referred to as "negative electrode C") was prepared by applying a slurry prepared so that the solid composition was hard carbon: graphite: carbon black: styrene butadiene rubber: carboxymethyl cellulose = 60:30:3:5:2 (weight ratio) onto copper foil. Using a hand punch, negative electrodes A, B, and C were each punched into a circle with a diameter of 15.95 mm, and then vacuum-dried at 110 ° C for 12 hours.

[0050] Example 1 In an argon-atmosphere glove box, a solution of the aromatic compound 1-methylnaphthalene dissolved in the solvent 2-methyltetrahydrofuran was prepared in a glass container with a synthetic resin screw cap, so that the concentration of the aromatic compound was 0.5 mol / L. Metallic lithium was then added in an amount three times the number of moles of 1-methylnaphthalene, and the solution was stirred. As a result, the solution turned deep purple, indicating that radical anions had been generated.

[0051] Twenty-four hours after the addition of the entire amount of metallic lithium, one negative electrode A was immersed in 1.5 mL of solution in a container containing unreacted metallic lithium. After immersing negative electrode A in the solution in the presence of metallic lithium for 4 days, negative electrode A was removed from the container. After washing negative electrode A with 2-methyltetrahydrofuran, the 2-methyltetrahydrofuran was evaporated, and lithium ions were intercalated to obtain the negative electrode of Example 1.

[0052] Example 2 The negative electrode of Example 2 was obtained in the same manner as in Example 1, except that negative electrode C was used instead of negative electrode A and negative electrode C was immersed in the solution for 6 days.

[0053] Example 3 The negative electrode of Example 3 was obtained in the same manner as in Example 1, except that 1.5 mL of the solution in Example 1 was removed from the container and transferred to another container, and negative electrode A was immersed in the solution for 1 hour in a state in which unreacted metallic lithium was not present.

[0054] Example 4 The negative electrode of Example 4 was obtained in the same manner as in Example 3, except that negative electrode A was immersed in the solution for 5 minutes.

[0055] (Comparative Example 1) The negative electrode of Comparative Example 1 was obtained in the same manner as in Example 3, except that 1-phenylnaphthalene, which does not have an electron-donating substituent, was used instead of the aromatic compound 1-methylnaphthalene.

[0056] (Comparative Example 2) The negative electrode of Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that negative electrode B was used instead of negative electrode A.

[0057] (Measurement of maximum absorption wavelength) The maximum absorption wavelength of the ultraviolet-visible absorption spectrum of the solution in Example 1-4 was measured using an ultraviolet-visible spectrophotometer (Evolution 201, manufactured by Thermo Fisher Scientific K.K.) under conditions of an optical path length of 10 mm and a temperature of 25° C.±2° C. As a result, the maximum absorption wavelength of the solution in Example 1-4 was 415 nm.

[0058] (Analysis of aromatic compounds contained in the negative electrode) GC-MS confirmed that the negative electrodes in Examples 1-4 contained trace amounts of aromatic compounds.

[0059] (Cell preparation) We prepared a case and cap, a separator (polypropylene, 20 μm thick), lithium metal foil (15 mm diameter), a gasket, a spacer, and a spring washer for a CR2032 coin cell, as well as a commercially available battery-grade electrolyte solution containing 1 mol / L of lithium hexafluorophosphate and 1 wt% vinylene carbonate in a 1:1:1 (volume ratio) mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0060] In a glove box under an argon atmosphere, the case, gasket, lithium metal foil, separator, negative electrode in Example 1, spacer, washer, and cap were stacked in this order while injecting the electrolyte solution, to obtain the coin-type cell in Example 1. Similarly, the negative electrodes in Examples 2-4 and Comparative Examples 1 and 2 were used instead of the negative electrode in Example 1 to obtain the cells in Examples 2-4 and Comparative Examples 1 and 2.

[0061] (Measurement of negative electrode potential) In a thermostatic chamber at 25°C, the voltage immediately after connecting the cell to the charge / discharge device was measured as the negative electrode potential (V vs. Li / Li + ) was decided.

[0062] (Measurement of the amount of metal ions intercalated into the negative electrode) The cell was connected to a charge / discharge device in a thermostatic chamber at 25°C, and the capacity (mAh) was measured when the cell was discharged at a current of 0.5 mA down to a voltage of 1.5 V. The value obtained by dividing the capacity by the mass (g) of the active material of the negative electrode was taken as the amount of metal ions intercalated into the negative electrode (mAh / g, hereinafter referred to as "doping depth").

[0063] The negative electrode potentials and doping depths measured using the cells in Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Table 1.

[0064] [Table 1]

[0065] According to Table 1, it was confirmed that the negative electrodes in Examples 1-4, which used 1-methylnaphthalene having an electron-donating substituent, could have a lower potential than the negative electrodes in Comparative Examples 1 and 2, which used 1-phenylnaphthalene having no electron-donating substituent. It was also confirmed that the amount of metal ions doped into the negative electrodes in Examples 1-4 was greater than the amount of metal ions doped into the negative electrodes in Comparative Examples 1 and 2.

[0066] It was confirmed that the solutions in Examples 1-4, which used aromatic compounds having electron-donating substituents, could dope with a larger amount of metal ions, i.e., had higher reducing power, than the solutions in Comparative Examples 1 and 2, which used aromatic compounds without electron-donating substituents. It is presumed that the radical anions in Examples 1-4 had a higher electron density in the aromatic ring than radical anions without electron-donating substituents on the aromatic ring, which increased the instability of the radical anions and increased the driving force for the reaction in which the metal ions are inserted into the negative electrode and the radical anions return to their original aromatic compound, resulting in an improved reducing power of the solutions.

[0067] The cells in Examples 1-4 have a larger amount of metal ions supported on the negative electrode than the cells in Comparative Examples 1 and 2. Therefore, even if the irreversible capacity of the negative electrode is large when charged by a conventional electrochemical method, the intercalation of metal ions into the negative electrode increases the apparent Coulombic efficiency of the negative electrode and enables the positive electrode to discharge to a potential lower than the voltage at the start of discharge. Therefore, the energy density of the cells in Examples 1-4 can be increased.

[0068] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0069] In the embodiment, electrochemical element 10 has been described as including negative electrode 11 in which composite layer 13 is provided on one surface of current collector 12, and positive electrode 15 in which composite layer 17 is provided on one surface of current collector 16, but this is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to an electrochemical element including electrode layers (so-called bipolar electrodes) in which composite layer 13 and composite layer 17 are provided on both surfaces of current collector 12. If bipolar electrodes and separators 14 are alternately stacked and housed in a case (not shown), an electrochemical element with a so-called bipolar structure can be obtained. [Explanation of symbols]

[0070] 10 Electrochemical elements 11 negative electrode (electrode)

Claims

1. 1. A solution for intercalating metal ions into a solid material, comprising: a radical anion of an aromatic compound and the metal ion; The radical anion has an electron-donating substituent on the aromatic ring.

2. 2. The solution according to claim 1, wherein the substituent has 10 or less carbon atoms, and the atom directly bonded to the aromatic ring is a carbon atom.

3. The solution according to claim 1 or 2, which contains a simple metal of the metal ion.

4. 3. The solution according to claim 1, which has an absorption maximum wavelength in the range of 350 to 700 nm in the ultraviolet-visible absorption spectrum.

5. An intercalation method comprising contacting a solid material with the solution according to claim 1 or 2.

6. The intercalation method according to claim 5, wherein after the operation, a state in which the metal ions in the form of simple metals and the solution coexist is included.

7. A method for producing an electrode, comprising the steps of preparing an initial electrode comprising a solid material and contacting the initial electrode with the solution according to claim 1 or 2.

8. An electrode comprising a solid material, An electrode comprising an aromatic compound having an electron-donating substituent on the aromatic ring.

9. An electrochemical device comprising the electrode according to claim 8.

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