JP7598705B2 - Electricity storage device and electrode - Google Patents

Description

This specification discloses an electricity storage device and an electrode.

Conventionally, as an electric storage device, a negative electrode active material including a layered structure having an organic framework layer containing an aromatic dicarboxylate anion and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in the carboxylic acid of the organic framework layer to form a framework has been proposed (see, for example, Patent Document 1). This layered structure is manufactured by a spray drying method in which a solution containing an aromatic dicarboxylate anion and an alkali metal cation is spray-dried. The spray drying method can obtain a layered structure with properties (shape, etc.) different from conventional methods. In addition, as a lithium ion capacitor, an electrolyte solution containing lithium bis(fluorosulfonyl)imide (LiFSI) and LiBF ₄ in a predetermined range, a positive electrode having a polyacene-based semiconductor material (PAS) as a positive electrode active material, and a negative electrode active material made of non-graphitizable carbon made of a phenolic resin raw material has been proposed (see, for example, Patent Document 2). It is said that this lithium ion capacitor can reduce the change in characteristics after being subjected to a high-temperature high-voltage environment. In addition, as an electrolyte solution for lithium ion batteries, an electrolyte solution containing LiFSI and water in a predetermined ratio has been proposed (see, for example, Patent Document 3). This electrolyte can reduce the ionic resistance more than conventional electrolytes. An electrolyte containing a carbonate having 3 to 6 carbon atoms, vinylene carbonate, lithium hexafluorophosphate, and 0.25 mol/L lithium bis(oxalatoborate) has been proposed (see, for example, Patent Document 4). When this electrolyte is used in a positive electrode having a positive electrode active material of lithium cobalt composite oxide and a negative electrode having a negative electrode active material of graphite, it is said that the cycle life can be improved in a high-temperature float test at 40°C.

Furthermore, a half cell has been proposed that uses a working electrode with graphite as an active material, a counter electrode of lithium metal, and an electrolyte solution containing 1M _LiPF6 as a supporting salt and 4-fluoroethylene carbonate (FEC) as an additive in a non-aqueous solvent containing ethylene carbonate and dimethyl carbonate interposed between the working electrode and the counter electrode (see, for example, Non-Patent Document 1). In this half cell, after 30 charge-discharge cycles at room temperature, it was stored at 55°C in a lithium ion desorbed state, and then returned to room temperature and subjected to 10 charge-discharge cycles. In this evaluation, it was found that adding 5% by mass of FEC can improve the capacity retention rate during high-temperature storage.

JP 2018-166060 A JP 2017-216310 A JP 2017-212153 A JP 2008-159588 A

J.Electrochem.Soc.162,(2015),A1683-A1692

However, in the electric storage device of Patent Document 1, for example, suppressing the deterioration of the charge/discharge characteristics after storage at high temperatures has not yet been fully considered. In addition, in the electric storage devices of Patent Documents 2 to 4 and Non-Patent Document 1, although it is said that the charge/discharge characteristics can be improved, the use of a layered structure as a negative electrode active material has not been considered. A layered structure having an organic framework layer and an alkali metal element layer has a high potential (0.7 V to 0.8 V based on Li) compared with the potential of graphite, which is a general negative electrode active material (0.1 V based on Li), and general substances cannot be used as they are, for example, when additives and supporting salts of general electrolytes are applied as they are, they do not show any effect. And, there are many combinations of additives, supporting salts, and non-aqueous solvents that can be applied to electrodes that use a layered structure having an organic framework layer and an alkali metal element layer as an electrode active material, and it is not easy to improve characteristics such as high-temperature storage stability. Thus, there has been a demand for further improving the charge/discharge characteristics of an electric storage device and an electrode that use a layered structure as a negative electrode active material after high-temperature storage.

The present disclosure has been made in consideration of these problems, and has as its main objective the provision of an electricity storage device and electrode that use a layered structure as an electrode active material and that can further improve the charge/discharge characteristics after high-temperature storage.

After extensive research to achieve the above-mentioned objective, the inventors discovered that by using a layered structure of an aromatic dicarboxylate metal salt as an electrode active material and forming a compound layer having specific properties on its surface, it is possible to further improve the charge/discharge characteristics after high-temperature storage, etc., and thus completed the disclosed invention.

That is, the electricity storage device of the present disclosure is
A positive electrode and
a negative electrode comprising, as a negative electrode active material, a layered structure including an organic framework layer containing an aromatic dicarboxylate anion and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in a carboxylic acid of the organic framework layer to form a framework, the photoelectron spectrum of a nitrogen 1s orbit being in the range of 396 eV to 403 eV, the photoelectron spectrum of a fluorine 1s orbit being in the range of 683 eV to 687 eV and 687 eV to 690 eV, and the photoelectron spectrum of a sulfur 1s orbit being in the range of 2475 eV to 2480 eV, when the surface is measured by hard X-ray photoelectron spectroscopy;
an ion conductive medium interposed between the positive electrode and the negative electrode and conducting carrier ions;
It is equipped with the following:

The electrode of the present disclosure comprises:
An electrode for use in an electricity storage device,
The electrode active material includes a layered structure including an organic framework layer containing an aromatic dicarboxylate anion and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in a carboxylic acid of the organic framework layer to form a framework, and when the surface is measured by hard X-ray photoelectron spectroscopy, the photoelectron spectrum of the nitrogen 1s orbit is in the range of 396 eV or more and 403 eV or less, the photoelectron spectrum of the fluorine 1s orbit is in the range of 683 eV or more and 687 eV or less and 687 eV or more and 690 eV or less, and the photoelectron spectrum of the sulfur 1s orbit is in the range of 2475 eV or more and 2480 eV or less.

In the electricity storage device and electrode disclosed in this specification, the layered structure is used as the electrode active material, and the charge/discharge characteristics after high-temperature storage can be further improved. The reason for this effect is presumably that, for example, when a compound layer containing nitrogen, fluorine, or sulfur is formed as a coating on the surface of the electrode active material, which is a layered structure containing aromatic dicarboxylate anions, the interface is stabilized during initial lithium absorption and further decomposition of the ion conductive medium is suppressed, thereby improving, for example, the charge/discharge cycle characteristics at room temperature and the charge/discharge characteristics after high-temperature storage at 60°C and the like.

FIG. 2 is an explanatory diagram showing an example of the structure of a layered structure. FIG. 2 is an explanatory diagram showing an example of an electricity storage device 20. FIG. 2 is a graph showing the relationship between the capacity retention rate and the high-temperature storage time in Examples 1 to 3 and Comparative Example 1. FIG. 2 is a graph showing the relationship between the number of charge/discharge cycles and the capacity retention rate in Examples 1 to 3 and Comparative Example 1. Hard X-ray photoelectron spectroscopy spectra of Li in Examples 1 to 3 and Comparative Example 1. Hard X-ray photoelectron spectroscopy spectra of C in Examples 1 to 3 and Comparative Example 1. Hard X-ray photoelectron spectroscopy spectra of F in Examples 1 to 3 and Comparative Example 1. Hard X-ray photoelectron spectroscopy spectra of O in Examples 1 to 3 and Comparative Example 1. Hard X-ray photoelectron spectroscopy spectra of N and S in Examples 1 to 3.

(Electricity storage device)
The electric storage device of the present disclosure includes a positive electrode, a negative electrode, and an ion-conducting medium interposed between the positive electrode and the negative electrode and conducting carrier ions. The electric storage device may be, for example, an electric double layer capacitor, a hybrid capacitor, a pseudo electric double layer capacitor, or a lithium ion battery.

The positive electrode may be a known positive electrode used in a capacitor or a lithium ion capacitor. The positive electrode may contain, for example, a carbon material as a positive electrode active material. The carbon material is not particularly limited, but examples thereof include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. The activated carbon as a carbon material preferably has a specific surface area of 1000 m ² /g or more, more preferably 1500 m ² /g or more. When the specific surface area is 1000 m ² /g or more, the discharge capacity can be further increased. The specific surface area of this activated carbon is preferably 3000 m ² /g or less, more preferably 2000 ^{m 2} /g or less, from the viewpoint of ease of preparation. In addition, the positive electrode is considered to store electricity by adsorbing and desorbing at least one of anions and cations contained in the ion conductive medium, but it may also store electricity by inserting and desorbing at least one of anions and cations contained in the ion conductive medium.

Alternatively, the positive electrode may be a positive electrode used in a general lithium ion battery. In this case, the positive electrode active material may be a sulfide containing a transition metal element, or an oxide containing lithium and a transition metal element. Specifically, transition metal sulfides such as _TiS2 , _TiS3 , _MoS3 , and _FeS2 , lithium manganese composite oxides having a basic composition formula such as Li _{(1-x) MnO2} (0<x< ₁ , etc., the same applies below) and Li _{(1-x) Mn2O4} , lithium cobalt composite oxides having a basic composition formula such as Li _{(1-x) CoO2} , lithium nickel composite oxides having a basic composition formula such as Li _{(1-x) NiO2} , lithium nickel cobalt _manganese composite oxides having a basic composition formula such as Li( _{1 -x) NiaCobMncO2 (} a+b+c=1) and Li _{(1-x) NiaCobMncO4} (a+ _b + _c =2), and _lithium nickel cobalt manganese composite oxides having a basic composition formula such as _LiV2O Lithium vanadium composite oxides having a basic composition formula of _V2O5 , etc., and transition metal oxides having a basic composition formula of _V2O5, etc., can be used. The positive _electrode active material may be lithium iron phosphate, etc. Among these, lithium transition metal composite oxides, such as _LiCoO2 , _LiNiO2 , _LiMnO2 , and LiNi1 _/3Co1 / _3Mn1 / _{3O2 ,} are preferred. The term "basic composition formula" means that other elements may be included.

The positive electrode may be formed by, for example, mixing the above-mentioned positive electrode active material, conductive material, and 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 the electrode density as necessary. The conductive material is not particularly limited as long as it is an electronically conductive material that does not adversely affect the battery performance, and may be, for example, a mixture of one or more of graphite such as natural graphite (scale graphite, scale graphite) or artificial graphite, carbon materials such as acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, and carbon fibers, and metals (copper, nickel, aluminum, silver, gold, etc.). Among these, carbon black and acetylene black are preferred as the conductive material from the viewpoint of electronic conductivity and coatability. The positive electrode preferably contains the conductive material in the range of 5% by mass to 25% by mass of the entire positive electrode mixture, and may be 10% by mass or more, or may be 15% by mass or more. If it is 5% by mass or more, the electrode can have sufficient conductivity and the deterioration of the charge/discharge characteristics can be suppressed. Also, if it is 25% by mass or less, the amount of active material and binder is not too small, so the active material and binder can fully exert their functions.

The binder may include a water-soluble polymer. The water-soluble polymer may include at least carboxymethyl cellulose (CMC) and one or more of polyvinyl alcohol (PVA), styrene butadiene copolymer (SBR), and polyethylene oxide (PEO). The binder may be, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-containing resin such as fluororubber, or thermoplastic resin such as polypropylene or polyethylene, ethylene-propylene-diene-monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR), or the like. The positive electrode preferably contains the binder in the range of 5% by mass to 25% by mass of the entire positive electrode mixture, and may be 20% by mass or less, or 15% by mass or less. If it is 5% by mass or more, the electrode can have sufficient binding properties and deterioration of the charge and discharge characteristics can be suppressed. Furthermore, if the amount is 25% by mass or less, the active material and conductive material will not be too small, allowing the active material and conductive material to fully function.

As the solvent, water may be used, or an organic solvent such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, or tetrahydrofuran may be used. When a water-soluble polymer is used, it is preferable to use water as the solvent. As the current collector, in addition to copper, nickel, stainless steel, titanium, aluminum, baked carbon, conductive polymer, conductive glass, Al-Cd alloy, or the like, a material whose surface is treated with carbon, nickel, titanium, or silver for the purpose of improving adhesion, conductivity, and reduction resistance may also be used. Of these, it is more preferable that the current collector is made of aluminum metal. The shape of the current collector may be a foil, film, sheet, net, punched or expanded, lath, porous, foam, or fiber group formation. The thickness of the current collector is, for example, 1 to 500 μm.

The negative electrode contains a layered structure having an organic skeleton layer and an alkali metal element layer as a negative electrode active material. This negative electrode active material absorbs and releases metal ions that serve as carriers. The carrier ions are preferably alkali metal ions, such as Li ions, Na ions, and K ions, and Li ions are more preferred. This negative electrode active material contains a layered structure that includes an organic skeleton layer that contains an aromatic dicarboxylic acid anion, and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in the carboxylic acid of the organic skeleton layer to form a skeleton.

The layered structure may have an organic framework layer to which one or more aromatic ring structures are connected. The layered structure is preferably formed in layers by π-electron interactions of aromatic compounds and has a monoclinic crystal structure belonging to the space group P2 ₁ /c, which is structurally stable. The layered structure may have a structure represented by one or more of formulas (1) to (3). However, in the formulas (1) to (3), a is an integer of 1 to 5, and b is an integer of 0 to 3, and these aromatic compounds may have a substituent or a heteroatom in the structure. Specifically, the aromatic compound may have a halogen, a linear or cyclic 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, or a hydroxyl group as a substituent instead of the hydrogen of the aromatic compound, or may have a structure in which nitrogen, sulfur, or oxygen is introduced instead of the carbon of the aromatic compound. More specifically, the layered structure may be an aromatic compound represented by formula (4) or (5). In formulas (1) to (5), A is an alkali metal. In addition, it is preferable that the layered structure has a structure represented by the following formula (6) in which four oxygen atoms of different dicarboxylate anions and an alkali metal element form a tetracoordination, which is structurally stable. However, in this formula (6), R has one or two or more aromatic ring structures, and two or more of the multiple Rs may be the same, or one or more may be different. In addition, A is an alkali metal. In this way, it is preferable to have a structure in which the organic framework layer is bonded by an alkali metal element.

In this layered structure, when the organic framework layer has two or more aromatic ring structures, it may be, for example, an aromatic polycyclic compound in which two or more aromatic rings are bonded, such as biphenyl, or a condensed polycyclic compound in which two or more aromatic rings are condensed, such as naphthalene, anthracene, or pyrene. The aromatic ring may be a five-membered ring, a six-membered ring, or an eight-membered ring, and a six-membered ring is preferred. In addition, it is preferable that the aromatic ring is 2 to 5. When there are two or more aromatic rings, a layered structure is easily formed, and when there are five or less aromatic rings, the energy density can be further increased. The organic framework layer may have a structure in which one or more carboxy anions are bonded to an aromatic ring. It is preferable that the organic framework layer contains an aromatic compound in which one and the other of the dicarboxylate anions are bonded to diagonal positions of the aromatic ring structure. The diagonal position where the carboxylic acid is bonded may be the position farthest from the bonding position of one carboxylic acid to the bonding position of the other carboxylic acid. For example, if the aromatic ring structure is biphenyl, the 4,4' positions can be mentioned, and if it is naphthalene, the 2,6 positions can be mentioned.

In the alkali metal element layer, as shown in FIG. 1, for example, an alkali metal element is coordinated to oxygen contained in a carboxylate anion to form a skeleton. FIG. 1 is an explanatory diagram showing an example of the structure of a layered structure, taking dilithium 4,4'-biphenyldicarboxylate as a specific example. The alkali metal contained in the alkali metal element layer can be, for example, any one or more of Li, Na, K, etc., but Li is preferred. Note that the metal ions that are carriers of the power storage device and are absorbed and released in the layered structure by charging and discharging may be different from or the same as the alkali metal element contained in the alkali metal element layer, and can be, for example, any one or more of Li, Na, K, etc. In addition, since the alkali metal element contained in the alkali metal element layer forms the skeleton of the layered structure, it is presumed that it is not involved in ion migration accompanying charging and discharging, that is, is not absorbed and released during charging and discharging. The layered structure thus configured is structured, as shown in FIG. 1, by an organic skeleton layer and a Li layer (alkali metal element layer) present between the organic skeleton layer. In the energy storage mechanism, the organic framework layer of the layered structure functions as a redox (e ⁻ ) site, while the alkali metal element layer functions as an occlusion site for the metal ion (alkali metal ion occlusion site) which is a carrier. This layered structure is preferably one or more of, for example, an alkali metal salt of 2,6-naphthalenedicarboxylate and an alkali metal salt of 4,4′-biphenyldicarboxylate, and more preferably an alkali metal salt of 4,4′-biphenyldicarboxylate.

This layered structure may be prepared by a spray drying method in which a solution containing an aromatic dicarboxylate anion and an alkali metal cation is spray-dried. The spray drying may be performed by a spray dryer. The spray drying conditions may be appropriately adjusted, for example, depending on the scale of the device and the amount of the electrode active material to be prepared. The concentration of the aromatic dicarboxylate anion in the preparation solution to be spray-dried is preferably 0.1 mol/L or more, more preferably 0.2 mol/L or more. In addition, the molar ratio B/A of the preparation solution, which is the molar number B (mol) of alkali metal cations to the molar number A (mol) of aromatic dicarboxylate anion, is preferably 2.2 or more. In this way, by making the alkali metal cations excessive, the resistance of the electrode for the electric storage device can be further reduced, which is preferable. This molar ratio B/A may be 2.5 or more. In addition, this molar ratio B/A may be 3.0 or less. The drying temperature is preferably in the range of, for example, 100°C or more and 250°C or less. At 100°C or higher, the solvent can be sufficiently removed, and at 250°C or lower, energy consumption can be further reduced, which is preferable. The drying temperature is more preferably 120°C or higher or 150°C or higher, and more preferably 220°C or lower. The amount of liquid supplied may be, for example, in the range of 0.1 L/h to 2 L/h, depending on the scale of production. The size of the nozzle for spraying the preparation solution may be, for example, in the range of 0.5 mm to 5 mm in diameter, depending on the scale of production.

This layered structure may have a hollow spherical structure formed by encapsulating a collection of flakes of the layered structure during preparation. The layered structure can have such a structure by preparation by spray drying. The hollow particles are obtained in the range of 0.1 μm to 10 μm. The thickness of the flakes in the hollow spherical structure or flake-like structure is, for example, 1 nm to 100 nm, preferably 1 nm to 20 nm. The maximum length of the flat part of the flake structure is 5 μm or less, and may be 2 μm or less. The negative electrode is oriented in a predetermined crystal plane because the hollow spherical structure is crushed and a flake-like layered structure is used. This negative electrode may have a peak intensity ratio P(300)/P(111) of 2.0 or more relative to the peak intensity of (111) when the electrode is subjected to X-ray diffraction measurement. This intensity ratio is more preferably 2.5 or more, and even more preferably 3.0 or more. This intensity ratio may be 5.0 or less. The negative electrode may be such that, when the electrode is subjected to X-ray diffraction measurement, the peak intensity ratio P(300)/P(011) of (300) to the peak intensity of (011) in the X-ray diffraction measurement is 2.0 or more. This intensity ratio is more preferably 2.5 or more, and even more preferably 3.0 or more. This intensity ratio may be 5.0 or less. The negative electrode may be such that, when the electrode is subjected to X-ray diffraction measurement, the peak intensity ratio P(100)/P(111) of (100) to the peak intensity of (111) in the X-ray diffraction measurement is 6.0 or more. This intensity ratio is more preferably 6.5 or more, and even more preferably 6.6 or more. This intensity ratio may be 10.0 or less. In addition, when the electrode is subjected to X-ray diffraction measurement, the negative electrode may have a peak intensity ratio P(100)/P(011) of 5.0 or more relative to the peak intensity of (011). This intensity ratio is more preferably 6.0 or more, and even more preferably 6.5 or more. This intensity ratio may be 10.0 or less. In addition, when the electrode is subjected to X-ray diffraction measurement, the negative electrode may have a peak intensity ratio P(100)/P(300) of 1.5 or more relative to the peak intensity of (300). This intensity ratio is more preferably 1.8 or more, and even more preferably 2.0 or more. This intensity ratio may be 5.0 or less. In this way, the negative electrode including the layered structure produced by the spray drying method has a specific orientation of small flakes of the active material present inside the electrode, and shows a tendency for the peak intensity corresponding to the n00 plane to increase. A negative electrode that satisfies this peak intensity ratio may in particular contain an alkali metal salt of 4,4'-biphenyldicarboxylic acid.

In this negative electrode, the photoelectron spectrum of the nitrogen 1s orbit measured by hard X-ray photoelectron spectroscopy on the surface is in the range of 396 eV to 403 eV. The photoelectron spectrum of the fluorine 1s orbit measured by hard X-ray photoelectron spectroscopy on the surface is in the range of 683 eV to 687 eV and 687 eV to 690 eV. Furthermore, the photoelectron spectrum of the sulfur 1s orbit measured by hard X-ray photoelectron spectroscopy on the surface is in the range of 2475 eV to 2480 eV. That is, this negative electrode may be one in which a compound layer (coating) containing nitrogen, fluorine and sulfur is formed in a film state or adsorbed state on the surface of the negative electrode active material. In this way, when a coating containing nitrogen, fluorine, sulfur, etc. is formed on the electrode active material of the layered structure, the interface is stabilized during initial lithium absorption of the active material, further decomposition of the electrolyte is suppressed, and high-temperature storage characteristics and charge/discharge cycle characteristics are improved. The compound layer may have a thickness of, for example, 10 nm or less, or 5 nm or less. The compound layer may have a thickness of 2 nm or more. When such a compound layer is formed on the surface of the layered structure, the charge-discharge cycle characteristics, including the capacity retention rate during the charge-discharge cycle, can be further improved. In addition, measurements using hard X-ray photoelectron spectroscopy can detect a depth of up to 20 nm from the surface.

The negative electrode may have a surface in which the photoelectron spectrum of lithium measured by hard X-ray photoelectron spectroscopy is in the range of 54 eV to 59 eV. The negative electrode may have a surface in which the photoelectron spectrum of carbon 1s orbital measured by hard X-ray photoelectron spectroscopy is in the range of 286 eV to 288 eV. The negative electrode may have a surface in which the photoelectron spectrum of phosphorus measured by hard X-ray photoelectron spectroscopy is not present. Even with such preliminary characteristics, the high-temperature storage characteristics and charge/discharge cycle characteristics can be improved.

In this negative electrode, the ratio of the number of atoms (element composition ratio) measured on the surface by hard X-ray photoelectron spectroscopy may be in the range of 20 at% to 30 at% for lithium. In addition, this element composition ratio may be in the range of 0.5 at% to 2 at% for nitrogen. Furthermore, this element composition ratio may be in the range of 2 at% to 4 at% for fluorine. Furthermore, this element composition ratio may be in the range of 1.5 at% to 3.5 at% for sulfur. And, this element composition ratio may be in the range of 0.5 at% or less for phosphorus. In this element composition ratio, for example, it is more preferable that nitrogen is 0.8 at% or more, and it may be 1.0 at% or more. Furthermore, it is more preferable that nitrogen is 1.8 at% or less, and it may be 1.66 at% or less. Furthermore, in this element composition ratio, it is more preferable that fluorine is 2.1 at% or more, and it may be 2.2 at% or more. In addition, fluorine is preferably 3.8 at% or less, and may be 3.0 at% or less. Furthermore, in this elemental composition ratio, sulfur is more preferably 1.6 at% or more, and may be 1.7 at% or more. Furthermore, sulfur is more preferably 3.2 at% or less, and may be 3.0 at% or less. When nitrogen, fluorine, and sulfur are contained in the compound layer in such ranges, high-temperature storage characteristics and charge/discharge cycle characteristics can be improved. In addition, in this elemental composition ratio, phosphorus is more preferably 0.2 at% or less, and even more preferably 0.1 at% or less. The elemental composition ratio of this coating is mainly composed of Li, C, O, N, F, and S.

The negative electrode may be a negative electrode composite material containing the layered structure as described above as an electrode active material, a binder, and a conductive material formed on a current collector. The conductive material, binder, solvent, and current collector used in the negative electrode may be, for example, those exemplified for the positive electrode, as appropriate. This negative electrode preferably contains a larger amount of negative electrode active material, and the negative electrode active material is preferably 65 mass% or more of the entire negative electrode composite, more preferably 70 mass% or more, and may be 75 mass% or more. The negative electrode active material may be in the range of 85 mass% or less or 75 mass% or less. When the negative electrode active material is contained in the range of 85 mass% or less, the amount of the conductive material and the binder is not too small, so that the functions of the conductive material and the binder can be fully exerted.

The ion conductive medium may be, for example, a non-aqueous electrolyte solution containing a supporting salt (supporting electrolyte) and an organic solvent. For example, when the carrier is lithium ions, the supporting salt is lithium bis(fluorosulfonyl)imide (LiFSI), which is an imide salt of lithium. The supporting salt preferably has a concentration of 0.1 mol/L or more and 5 mol/L or less in the non-aqueous electrolyte solution. When the concentration of the supporting salt is 0.1 mol/L or more, a sufficient current density can be obtained, and when the concentration is 5 mol/L or less, the electrolyte solution can be more stable. This concentration is more preferably 1.0 mol/L or more, even more preferably 1.1 mol/L or more, and may be 1.5 mol/L or more. Moreover, this concentration is more preferably 3.0 mol/L or less, and may be 2 mol/L or less. Moreover, a phosphorus-based, halogen-based, or other flame retardant may be added to the non-aqueous electrolyte solution. For example, an aprotic organic solvent can be used as the organic solvent. 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 ester carbonates 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. In addition, nitrile solvents such as acetonitrile and propylnitrile, ionic liquids, and gel electrolytes may also be used as non-aqueous electrolytes.

The electricity storage device may have a separator between the positive electrode and the negative electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the electricity storage device, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

The shape of the electric storage device is not particularly limited, but examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, and square type. The device may also be applied to large devices used in electric vehicles. FIG. 2 is a schematic diagram showing an example of an electric storage device 20. The electric storage device 20 includes 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 facing the positive electrode 22 via a separator 24, 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 electric storage device 20, an ion conductive medium 27 is filled in the space between the positive electrode 22 and the negative electrode 23. The negative electrode 23 has the layered structure of the aromatic dicarboxylic acid metal salt described above as the negative electrode active material.

In the above-described power storage device, the layered structure is used as the negative electrode active material, and the charge/discharge characteristics after high-temperature storage can be further improved. The reason for such an effect is presumed to be as follows. For example, in a layered structure having an organic framework layer and an alkali metal element layer, the charge/discharge potential is higher than that of graphite, which is a general negative electrode active material, so a coating similar to that of graphite was not formed. On the other hand, when a specific additive is added to a non-aqueous electrolyte, the components contained in the electrolyte can be decomposed and a compound layer can be formed even at a potential higher than that of graphite. However, it was difficult to improve the charge/discharge characteristics after high-temperature storage even with the compound layer. And, as a result of various studies, it has become clear that when a compound layer containing nitrogen, fluorine, or sulfur is formed as a coating on the surface of an electrode active material, which is a layered structure containing an aromatic dicarboxylate anion, the charge/discharge characteristics after high-temperature storage, such as at 60°C, are improved. The reason for this is presumably that, for example, when the electrode active material initially absorbs lithium, the decomposition of lithium bis(fluorosulfonyl)imide forms a coating on the electrode surface, stabilizing the interface and suppressing further side reactions with the electrolyte, etc.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and may be implemented in various forms as long as they fall within the technical scope of the present disclosure.

For example, in the above-mentioned embodiment, the electrode is described as an electricity storage device, but is not limited thereto, and may be an electrode for an electricity storage device. This electrode includes a layered structure having an organic framework layer containing an aromatic dicarboxylic acid anion and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in the carboxylic acid of the organic framework layer to form a framework, as an electrode active material, and when the surface is measured by hard X-ray photoelectron spectroscopy, the photoelectron spectrum of the nitrogen 1s orbit is in the range of 396 eV to 403 eV, the photoelectron spectrum of the fluorine 1s orbit is in the range of 683 eV to 687 eV and 687 eV to 690 eV, and the photoelectron spectrum of the sulfur 1s orbit is in the range of 2475 eV to 2480 eV. If this electrode is used in an electricity storage device, the same effect as the above-mentioned electricity storage device can be achieved.

In the above embodiment, the layered structure is described as being produced by a spray drying method, but the present invention is not limited to this, and may be synthesized, for example, by a solution mixing method. In the solution mixing method, a layered structure can be obtained by heating and stirring an aromatic dicarboxylic acid and an alkali hydroxide in a solvent. The above-mentioned compound layer may be formed on the surface of such a layered structure.

Below, we will explain a specific example of how the electricity storage device disclosed herein was fabricated. As an example, we will explain an example in which a layered structure was synthesized, an electrode and an ion-conducting medium were fabricated, and an electricity storage device was evaluated.

(Negative Electrode Active Material: Synthesis of Layered Structure of Dilithium 4,4'-Biphenyldicarboxylate)
A layered structure was prepared by a spray drying method. For the synthesis of dilithium 4,4'-biphenyldicarboxylate, 4,4'-biphenyldicarboxylic acid and lithium hydroxide monohydrate (LiOH.H ₂ O) were used as starting materials. Lithium hydroxide was added to water and stirred to obtain an aqueous solution of 0.44 mol/L. Then, an aqueous solution was prepared so that the molar ratio B/A, which is the number of moles B (mol) of lithium hydroxide to the number of moles A (mol) of 4,4'-biphenyldicarboxylic acid, was 2.2, that is, the amount of 4,4'-biphenyldicarboxylic acid was 0.20 mol/L. The aqueous solution was spray-dried using a spray dryer (Mini Spray Dryer B-290, manufactured by Nippon Buchi Co., Ltd.) to precipitate dilithium 4,4'-biphenyldicarboxylate. The nozzle diameter of the spray dryer used was 1.4 mm, the amount of solution sprayed was 0.4 L/hour, and the drying temperature was 150 ° C., and 4,4'-dilithium biphenyldicarboxylate was synthesized. The obtained layered structure was observed at 1000 to 50,000 times magnification using a scanning electron microscope (Quanta 200FEG manufactured by FEI Japan Co., Ltd.), and the particle size was found to be hollow particles of 10 μm or less. In addition, when the particles with the exposed inside were viewed in an enlarged view, the negative electrode active material had a hollow spherical structure formed by encapsulating a collection of flakes of the layered structure having a thickness of several nm. In addition, the negative electrode active material had a structure in which the flakes of the layered structure irregularly moved from the center to the outer periphery (synonymous with a structure in which the flakes moved irregularly from the outer periphery to the center). An electrode was produced using a flake-like layered structure obtained by crushing this hollow spherical structure.

(Negative electrode: Preparation of 4,4'-biphenyldicarboxylate dilithium electrode)
The 4,4'-biphenyldicarboxylate dilithium prepared by the above method was mixed at 73% by mass, 18% by mass of carbon black (Tokai Carbon, TB5500) as a particulate carbon conductive material, 2.7% by mass of carboxymethylcellulose (Daicel, CMC-1120) as a water-soluble polymer, 3.6% by mass of polyethylene oxide (PEO, molecular weight 2 million), and 2.7% by mass of styrene-butadiene copolymer (JSR, TRD2001), and an appropriate amount of water was added as a dispersion medium and dispersed to form a slurry. This slurry was uniformly applied to a 10 μm thick copper foil collector so that the negative electrode active material per unit area was 2.5 mg/cm ² , and the coated sheet was prepared by vacuum heating and drying at 120 ° C. The coated sheet was then subjected to pressure pressing to prepare a rectangular electrode of 10 cm ² .

(Preparation of Positive Electrode)
Activated carbon (Cataler, EXC-11G) as a positive electrode active material was 90.0% by mass, Denka Black (Denka) as a particulate carbon conductive material was 4.0% by mass, carboxymethyl cellulose (Daicel, CMC-2200) as a water-soluble polymer was 1.0% by mass, and styrene butadiene copolymer (JSR, TRD102A) was 5.0% by mass. The mixture was mixed in a ratio of 1.0% by mass, and an appropriate amount of water was added as a dispersion medium to form a slurry-like composite material. This slurry was uniformly applied to a 10 μm thick aluminum foil current collector so that the positive electrode active material per unit volume was 4.0 mg / cm ² , and the mixture was vacuum dried at 120 ° C. to prepare a coated sheet. The coated sheet was then subjected to a pressure press treatment to prepare a rectangular electrode of 10 cm ² .

(Preparation of ionically conductive medium)
An ion-conducting medium was prepared by adding lithium bis(fluorosulfonyl)imide (LiFSI) as a supporting electrolyte to a non-aqueous solvent in a volume ratio of 30:40:30, so as to have a concentration of 1.1 mol/L.

(Adjustment of negative electrode: pre-doping)
A bipolar cell was prepared by sandwiching a separator impregnated with an ion-conducting medium between the negative electrode described above as a working electrode and a lithium metal foil as a counter electrode. The capacity of the negative electrode was confirmed by charging and discharging the bipolar cell at a temperature of 20° C., a voltage range of 0.5 to 1.5 V (vs. Li/Li ⁺ ), and a current value of 0.5 mA (equivalent to C/10), and the negative electrode was made to absorb lithium equivalent to SOC 75%.

[Electricity storage device of Example 1]
An asymmetric capacitor was prepared by sandwiching a separator impregnated with the ion conductive medium between the above-mentioned positive electrode and a pre-doped negative electrode. This was designated as Example 1.

[Electricity storage devices of Examples 2 and 3]
An electricity storage device of Example 2 was prepared in the same manner as in Example 1, except that the concentration of LiFSI in the ion conductive medium was set to 1.5 mol/L. An electricity storage device of Example 3 was prepared in the same manner as in Example 1, except that the concentration of LiFSI in the ion conductive medium was set to 2.0 mol/L.

[Electricity storage device of Comparative Example 1]
An electricity storage device of Comparative Example 1 was prepared in the same manner as in Example 1, except that the ion conductive medium contained 1.1 mol/L of LiPF ₆ as a supporting salt.

(Evaluation of Electricity Storage Device)
A charge-discharge cycle test was performed using the prepared capacitor. A conditioning treatment was performed by repeating five cycles of charging and discharging in a temperature environment of 20° C. under a voltage range of 1.5 to 3.1 V (vs. Li/Li ⁺ ) and a current value of 1.5 mA (corresponding to 1 C).

(High temperature storage test)
A high-temperature storage test was performed on the prepared capacitor. The test environment temperature was set to 60°C, and the prepared capacitor was charged to 3.1V at a Li reference potential with a current value of 1.5mA (equivalent to 1C), and then left in a test temperature environment (60°C) without current flow. After a predetermined high-temperature storage time had elapsed, the test environment temperature was set to 20°C, the voltage range at the Li reference potential was set to 1.5V to 3.1V, and a constant current charge/discharge test was performed at a current value of 1.5mA (equivalent to 1C). From the obtained discharge curve, the capacity, capacity retention rate, and resistance after high-temperature storage were calculated. The capacity retention rate (%) in the high-temperature storage test was calculated from the formula _Qp / _Q0 x 100 using the initial capacity _Q0 (mAh/g) and the capacity _Qp (mAh/g) after high-temperature storage. The resistance was calculated by dividing the voltage change 1 second after the start of charging by the current value. The high-temperature storage times for determining the capacity after high-temperature storage were 50 hours, 100 hours, 200 hours, 300 hours, and 400 hours.

(Charge-discharge cycle test)
Using the produced capacitor, charging and discharging was repeated 1000 cycles at a temperature environment of 20°C, with a voltage range of 1.5 to 3.1 V (vs. Li/Li ⁺ ) and a current value of 10 mA (corresponding to 10 C), and the initial discharge capacity _Q1 and the discharge capacity _Q1000 after 1000 cycles were calculated. Then, the capacity retention rate represented by the formula _Q1000 × 100/ _Q1 was calculated.

(Analysis of the negative electrode)
The negative electrode subjected to the conditioning treatment was subjected to hard X-ray photoelectron spectroscopy to detect the coating components formed on the negative electrode active material. The negative electrode was removed by disassembling the power storage device after the conditioning treatment, and the negative electrode was washed with dimethyl carbonate, washed with anhydrous acetonitrile, and dried three times. The analysis conditions were as follows: the light source was SPring-8 BL16XU (Sunbeam), the measurement method was HAXPES, the incident X-ray energy was 7.947 keV, the take off angle was 85°, and the X-ray irradiation area was 52 × 380 μm. The ranges of binding energy (eV) at which the peaks of the Li 1s orbital, C 1s orbital, N 1s orbital, O 1s orbital, F 1s orbital, P 1s orbital, and S 1s orbital obtained by the measurement appeared are summarized in Table 1.

(Results and Discussion)
FIG. 3 is a relationship diagram of the capacity retention rate versus high-temperature storage time in Examples 1 to 3 and Comparative Example 1. FIG. 4 is a relationship diagram of the number of 20° C. charge/discharge cycles versus the capacity retention rate in Examples 1 to 3 and Comparative Example 1. FIG. 5 is a hard X-ray photoelectron spectroscopy spectrum of the Li 1s orbital in the negative electrodes of Examples 1 to 3 and Comparative Example 1. FIG. 6 is a hard X-ray photoelectron spectroscopy spectrum of the C 1s orbital in the negative electrodes of Examples 1 to 3 and Comparative Example 1. FIG. 7 is a hard X-ray photoelectron spectroscopy spectrum of the F 1s orbital in the negative electrodes of Examples 1 to 3 and Comparative Example 1. FIG. 8 is a hard X-ray photoelectron spectroscopy spectrum of the O 1s orbital in the negative electrodes of Examples 1 to 3 and Comparative Example 1. FIG. 9 is a hard X-ray photoelectron spectroscopy spectrum of the N and S 1s orbitals in the negative electrodes of Examples 1 to 3. Table 2 shows the composition of the electrolyte, the element composition ratio (at %) determined by hard X-ray photoelectron spectroscopy, the capacity retention rate after high-temperature storage at 60° C., and the capacity retention rate (%) after 1000 cycles at 20° C.

As shown in Figures 3 and 4, the high-temperature storage characteristics and cycle characteristics were improved in Examples 1 to 3 compared to Comparative Example 1. In addition, as shown in Figures 5 to 9 and Tables 1 to 2, the relationship between the high-temperature storage characteristics and cycle characteristics and the coating composition on the negative electrode was inferred as follows. In the photoelectron spectrum of each element, when a peak exists within the range of binding energy shown in Examples 1 to 3 in Table 1, it was inferred that the high-temperature storage characteristics and cycle characteristics are improved. Specifically, in Comparative Example 1, no peaks of N or S are observed, but in Examples 1 to 3, it was found that a coating was formed on the negative electrode in which the photoelectron spectrum of the 1s orbit of nitrogen exists in the range of 396 eV to 403 eV and the photoelectron spectrum of the 1s orbit of sulfur exists in the range of 2475 eV to 2480 eV. In addition, in Examples 1 to 3, it was found that a coating was formed on the negative electrode in which the photoelectron spectrum of the 1s orbit of C exists in the range of 286 eV to 288 eV and the photoelectron spectrum of the 1s orbit of F exists in the range of 683 eV to 687 eV. In addition, as shown in FIG. 7A, the photoelectron spectrum of F-SO ₂ was in the range of 687 eV to 689 eV, which is particularly noticeable in Example 3. Furthermore, in Examples 1 to 3, the photoelectron spectrum of P was not obtained. That is, in Examples 1 to 3, it was found that by including LiFSI as a supporting salt, the additives and components of the electrolyte decompose during initial charging and discharging, and a coating containing N, F, S, etc. is formed on the negative electrode active material. And, when such a coating is formed, it is presumed that the interface is stabilized during the initial lithium absorption of the negative electrode active material, further decomposition of the electrolyte is suppressed, and high-temperature storage characteristics and charge-discharge cycle characteristics are improved.

As shown in Table 2, it was found that when the atomic ratio in the coating is in the range of 0.5 at% to 2 at% for nitrogen, 2 at% to 4 at% for fluorine, and 1.5 at% to 3.5 at% for sulfur, the high-temperature storage characteristics and charge-discharge cycle characteristics are improved. The compound layer containing N, F, S, etc. formed on the negative electrode active material had a thickness of 10 nm. It was presumed that at this thickness, the surface compound layer would not provide resistance to the absorption and release of Li.

It goes without saying that the present disclosure is in no way limited to the above examples, and may be implemented in a variety of ways as long as they fall within the technical scope of the present disclosure.

This disclosure is applicable to the battery industry.

20: power storage device, 21: battery case, 22: positive electrode, 23: negative electrode, 24: separator, 25: gasket, 26: sealing plate, 27: ion conductive medium.

Claims (7)

A positive electrode and
a negative electrode comprising, as a negative electrode active material, a layered structure including an organic framework layer containing an aromatic dicarboxylate anion and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in a carboxylic acid of the organic framework layer to form a framework, the photoelectron spectrum of a nitrogen 1s orbit being in the range of 396 eV to 403 eV, the photoelectron spectrum of a fluorine 1s orbit being in the range of 683 eV to 687 eV and 687 eV to 690 eV, and the photoelectron spectrum of a sulfur 1s orbit being in the range of 2475 eV to 2480 eV, when the surface is measured by hard X-ray photoelectron spectroscopy;
an ion conductive medium interposed between the positive electrode and the negative electrode and conducting carrier ions;
The negative electrode of the power storage device comprises, as the negative electrode active material, the layered structure having a structure represented by one or more of formulas (1) to (3).

The negative electrode has a surface with a ratio of the number of atoms of nitrogen measured by hard X-ray photoelectron spectroscopy of 0.5 at
2. The power storage device according to claim 1, wherein the atomic percentage of fluorine is in the range of 2 at % to 4 at %, and the atomic percentage of sulfur is in the range of 1.5 at % to 3.5 at %. The electricity storage device according to claim 1 or 2, wherein the negative electrode does not have a phosphorus photoelectron spectrum. The electric storage device according to any one of claims 1 to 3, wherein the negative electrode has a compound layer having a thickness of 10 nm or less that contains nitrogen, fluorine, and sulfur formed in a film state or in an adsorbed state on the surface of the negative electrode active material. The electric storage device according to any one of claims 1 to 4, wherein the ion conductive medium contains lithium bis(fluorosulfonyl)imide in the range of 1.0 mol/L or more and 2 mol/L or less. 6. The power storage device according to claim 1, wherein the positive electrode contains activated carbon having a specific surface area of 1000 m ² /g or more as a positive electrode active material. An electrode for use in an electricity storage device,
the layered structure includes, as an electrode active material, an organic framework layer containing an aromatic dicarboxylate anion and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in a carboxylic acid of the organic framework layer to form a framework, and when the surface is measured by hard X-ray photoelectron spectroscopy, the photoelectron spectrum of the nitrogen 1s orbit is in the range of 396 eV or more and 403 eV or less, the photoelectron spectrum of the fluorine 1s orbit is in the range of 683 eV or more and 687 eV or less and 687 eV or more and 690 eV or less, and the photoelectron spectrum of the sulfur 1s orbit is in the range of 2475 eV or more and 2480 eV or less,
The electrode comprises, as the electrode active material, the layered structure having a structure represented by one or more of formulas (1) to (3).

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