JP7472700B2 - Electrode active material, power storage device, and method for producing electrode active material - Google Patents

Description

This specification discloses an electrode active material, an electricity storage device, and a method for producing the electrode active material.

Conventionally, as an electricity storage device such as a lithium ion secondary battery, a layered structure has been proposed as a negative electrode active material, the layered structure having an organic framework layer containing an aromatic compound that is a dicarboxylate anion having an aromatic ring structure, and an alkali metal element layer in which an alkali metal element is coordinated to oxygen contained in the carboxylate anion to form a framework (see, for example, Patent Document 1). This layered structure is obtained by spray-drying a preparation solution in which an aromatic dicarboxylate anion and an alkali metal ion are dissolved, using a spray drying device,
The charge and discharge characteristics can be further improved.

JP 2018-166060 A

However, the problem with the power storage device of Patent Document 1 above is that when the open circuit potential is measured, there is a large high resistance region with large thermodynamic polarization. For this reason, there has been a demand for this electrode active material to have improved charge/discharge characteristics, such as by further increasing the low resistance region with small polarization.

The present disclosure has been made in consideration of these problems, and its main objective is to provide a new electrode active material, an electricity storage device, and a method for manufacturing an electrode active material that can further improve charge/discharge characteristics.

As a result of intensive research to achieve the above-mentioned objective, the inventors discovered that by creating a more suitable crystal structure, it is possible to produce a new electrode active material that can further improve the charge/discharge characteristics, and thus completed the present disclosure.

That is, the electrode active material disclosed in the present specification is
An electrode active material for use in an electricity storage device,
The layered structure includes an organic framework layer containing an aromatic dicarboxylate anion having a structure in which a plurality of aromatic rings are connected directly or via a carbon chain, 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 the layered structure is subjected to X-ray diffraction measurement, and the crystallite size D ₃₀₂ in the direction perpendicular to the 302 plane calculated using formula (1) is 25 nm or less.

The electricity storage device disclosed in the present specification is
A positive electrode and
A negative electrode containing the above-mentioned electrode active material;
an ion conductive medium interposed between the positive electrode and the negative electrode and conducting carrier ions;
It is equipped with the following:

The method for producing an electrode active material disclosed in the present specification includes the steps of:
A method for producing an electrode active material for use in an electricity storage device, comprising the steps of:
a pre-drying step of pre-drying an aromatic dicarboxylic acid having a structure in which a plurality of aromatic rings are connected directly or via a carbon chain;
a preparation step of preparing a solution in which alkali metal ions are added to the aromatic dicarboxylic acid dried in advance in a molar ratio of 1.9 to 2.1;
a step of spray-drying the prepared solution to produce a layered structure including an organic framework layer containing an aromatic dicarboxylate anion having a structure in which a plurality of aromatic rings are connected directly or via a carbon chain, 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;
It includes.

In the present disclosure, a novel electrode active material, a power storage device, and a method for manufacturing an electrode active material that can further improve the charge/discharge characteristics can be provided. The reason for obtaining such an effect is considered as follows. For example, if the crystallite size in the direction perpendicular to a specific crystal plane, particularly the crystallite size D ₃₀₂ in the direction perpendicular to the 302 plane, is 25 nm or less, the so-called alkali metal element layer becomes thinner, and it is presumed that the acceptability of the alkali metal ion, which is a carrier, can be further improved. Therefore, this electrode active material can expand the low resistance capacity region of the open circuit potential, and can further improve the charge/discharge characteristics.

FIG. 2 is an explanatory diagram of a compound structure having a layered structure of dilithium aromatic dicarboxylate. FIG. 2 is a schematic diagram showing an example of an electricity storage device 20. SEM photographs of Example 1 and Comparative Examples 1 and 2. XRD pattern of Example 1. XRD pattern of Comparative Example 1. XRD pattern of Comparative Example 2. Diffraction spectra of the 100 and 302 planes of each sample. 4 is a measurement result of the open circuit potential of Example 1. Measurement results of open circuit potential in Comparative Example 1. Measurement results of open circuit potential in Comparative Example 2. FIG. 1 is a graph showing the relationship between the crystallite size D and the proportion of a low resistance/capacitance region. Williamson-Hall plot versus strain ε _WH .

(Electrode active material)
The electrode active material disclosed in this specification is an electrode active material used in an electricity storage device. The electrode active material stores and outputs electric energy by absorbing and releasing alkali metal ions as carriers. Examples of the alkali metal ions as carriers include one or more of Li ions, Na ions, and K ions. The electrode active material is a layered structure including an organic framework layer and an alkali metal element layer. The organic framework layer has a structure including an aromatic dicarboxylate anion having a structure in which a plurality of aromatic rings are connected directly or via a carbon chain. The alkali metal element layer forms a skeleton by coordinating an alkali metal element to oxygen contained in a carboxylic acid of the organic framework layer. In this layered structure, the organic framework layer may have a structure in which aromatic rings are directly connected, or may have a structure in which aromatic rings are connected via a predetermined carbon chain. Examples of the predetermined carbon chain include alkenes and alkynes having 1 to 5 carbon atoms. The organic framework layer may have a structure in which one or more carboxy anions are bonded to an aromatic ring. 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. The metal ions contained in the non-aqueous electrolyte and absorbed and released by 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, it is not absorbed or released during charging and discharging.

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 preferably has an organic framework layer composed of an aromatic ring structure represented by the following formula (1). The organic framework layer preferably has a structure represented by formulas (2) to (4), and more preferably has a biphenyl structure represented by formula (2). The layered structure has a low resistance capacity region and a high resistance capacity region during charging and discharging at a single electrode (see Figures 8 to 10 described later). The layered structure is preferably provided with a structure represented by formula (5) in which four oxygen atoms of different dicarboxylate anions and an alkali metal element form a four-coordinated structure, which is structurally stable. However, in this formula (5), R has two or more aromatic ring structures, and when there are multiple aromatic rings, two or more of them may be the same, or one or more may be different. A is an alkali metal element. In addition, R may include an aromatic ring structure of the above formula (1) or formulas (2) to (4). This aromatic ring structure may have a substituent or a heteroatom in its structure. Specifically, instead of hydrogen, it 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, or the carbon of the aromatic ring may be replaced by nitrogen, sulfur, or oxygen. 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 absorption site (alkali metal ion absorption site) of the metal ion that is a carrier.

This electrode active material is one in which the crystallite size D ₃₀₂ perpendicular to the 302 plane calculated by performing X-ray diffraction measurement on the layered structure using formula (1) is 25 nm or less. For example, the X-ray diffraction measurement is performed by filling a sample into a capillary and measuring at the BL5S2 powder X-ray diffraction beamline under the conditions of light energy E = 20 keV (λ = 0.6195 Å), beam size 0.5 mm × 0.5 mm, exposure time 15 minutes, and 2θ = 0° to 30°. This crystallite size D ₃₀₂ is preferably smaller, more preferably 20 nm or less, and even more preferably 16 nm or less. This crystallite size D ₃₀₂ may be 10 nm or more in consideration of the difficulty of preparation.

When the layered structure of this electrode active material is subjected to the above-mentioned X-ray diffraction measurement, the crystallite size _D100 in the direction perpendicular to the 100 plane calculated using the Scherrer formula represented by the mathematical formula (1) is preferably 45 nm or less. This crystallite size _D302 is preferably smaller, more preferably 40 nm or less, and even more preferably 36 nm or less. If the crystallite size D in the direction perpendicular to a predetermined crystal plane is smaller, the acceptability of carrier ions can be further increased, which is preferable.

In addition, when the electrode active material is subjected to the above-mentioned X-ray diffraction measurement of the layered structure, the strain ε _WH ×1000 calculated using the Williamson-Hall formula represented by formula (2) is preferably 1.35 or more. The strain ε _WH ×1000 is obtained by plotting the βcosθ value against 2sinθ using all the diffraction peaks obtained by the X-ray diffraction measurement in the Williamson-Hall formula, and calculating the slope obtained by the least squares method. The value of this strain ε _WH ×1000 is a value reflecting the strain of the entire crystal grain, and a larger value indicates a larger strain of the entire crystal grain. The strain ε _WH ×1000 is preferably 1.5 or more, and more preferably 1.6 or more. In addition, the strain ε _WH ×1000 may be 2.0 or less in consideration of the retention of the crystals.

(Method for producing electrode active material)
The method for producing an electrode active material according to the present disclosure is a method for producing an electrode active material for the above-mentioned power storage device. This method may include a pre-drying step, a preparation step, and a production step. After the production step, the method may include a post-drying step in which the obtained layered structure is further dried. In this production method, the materials described in the above-mentioned electrode active material can be used.

In the pre-drying step, a process is performed to pre-dry aromatic dicarboxylic acid as a raw material, which has a structure in which multiple aromatic rings are connected directly or via a carbon chain. Examples of aromatic dicarboxylic acids include those having the structure of formula (1), and specific examples include those shown in formulas (2) to (4). Among these, biphenyl dicarboxylic acid having the structure shown in formula (2) is preferred. Pre-drying is more preferably performed under reduced pressure, for example. The reduced pressure condition is preferably, for example, minus 0.1 MPa or less, and more preferably minus 0.09 MPa or less. The drying temperature is preferably 80°C or more, more preferably 100°C or more, and even more preferably 120°C or more. This drying temperature is set to the decomposition temperature of aromatic dicarboxylic acid or less, and may be 250°C or less. The drying time is preferably, for example, in the range of 6 hours to 10 hours.

In the solution preparation step, a preparation solution is prepared by dissolving an aromatic dicarboxylic acid and an alkali metal ion. In this step, a solution is prepared by adding an alkali metal ion in a molar ratio of 1.9 to 2.1 to the aromatic dicarboxylic acid that has been dried in advance. The solvent of this preparation solution is not particularly limited, and may be an aqueous solvent or an organic solvent, but is preferably water. Examples of organic solvents include alcohols such as methanol and ethanol. In this step, it is preferable to prepare a preparation solution in which the total concentration of aromatic dicarboxylic acid anions is 0.1 mol/L or more, more preferably 0.2 mol/L or more. In this step, it is preferable to prepare a preparation solution in which the concentration of aromatic dicarboxylic acid anions is 5 mol/L or less. In such a concentration range, the next step is easier to carry out. In this step, it is preferable to prepare a preparation solution containing one or more alkali metal ions of lithium, sodium, and potassium. In this step, for example, it is preferable to obtain a prepared solution in which the molar ratio B/A, which is the number of moles B (mol) of alkali metal ions to the number of moles A (mol) of aromatic dicarboxylate anions, is in the range of 1.9 to 2.1, and it is more preferable to obtain a prepared solution in which B/A is in the range of 1.95 to 2.05. In this way, by making the alkali metal ions in a range close to the theoretical amount, it is possible to further suppress crystal growth in the thickness direction of the layered structure and further expand the low resistance capacity region of the open circuit potential of the electrode for the power storage device.

In the preparation step, the solution prepared in the preparation step is spray-dried to prepare a layered structure. In this step, the layered structure can be precipitated in a shorter time, which is preferable. Spray drying may be performed using a spray dryer. The spray drying conditions may be appropriately adjusted, for example, depending on the scale of the device and the amount of electrode active material to be prepared. In this step, the drying temperature may be, for example, room temperature or higher, for example, heating to 40°C or higher, but is preferably higher than the boiling point of the solvent, and is preferably in the range of 100°C to 330°C. At 100°C or higher, the solvent can be sufficiently removed, and at 330°C or lower, energy consumption can be further reduced, which is preferable. The drying temperature is more preferably 150°C or higher, more preferably 180°C or higher, and may be 200°C or higher.

In the post-drying step, the spray-dried layered structure is further dried. This step can be performed, for example, under the same conditions as the pre-drying step. The post-drying treatment may be performed under the same conditions as the pre-drying treatment, or under conditions different from those of the pre-drying treatment. In this manner, the electrode active material of the present disclosure can be produced.

(Electricity storage device)
The electric storage device disclosed in this specification includes a negative electrode containing the above-mentioned electrode active material, a positive electrode, and an ion conductive medium interposed between the positive electrode and the negative electrode and conducting carrier ions. This electric storage device may be, for example, an electric double layer capacitor, a hybrid capacitor, a pseudo electric double layer capacitor, a lithium ion secondary battery, or the like. The positive electrode may include a positive electrode active material that absorbs and releases carrier ions. The negative electrode may include the above-mentioned electrode active material that absorbs and releases alkali metal ions that are carriers. The ion conductive medium conducts carrier ions (either cations or anions). Here, an electric storage device in which the carrier of the negative electrode is lithium ions will be mainly described.

The negative electrode contains the above-mentioned electrode active material. The above-mentioned electrode active material has a potential of about 1.0 to 1.5 V based on lithium metal, so it is preferable to use it as the negative electrode active material. The negative electrode may be formed by mixing the above-mentioned electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like negative electrode mixture, applying it to the surface of a current collector, drying it, and compressing it to increase the electrode density as necessary. In this negative electrode, it is preferable that the above-mentioned electrode active material is contained as much as possible, and for example, it may be contained in the negative electrode mixture in a range of 60% by mass to 95% by mass. The conductive material may be, for example, a mixture of one or more of graphite such as natural graphite (scale graphite, scale graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.). Of these, carbon black and acetylene black are preferable as the conductive material from the viewpoint of electronic conductivity and coatability. The binder plays a role of connecting the active material particles and the conductive material particles, and can be, for example, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine rubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR), etc., used alone or as a mixture of two or more kinds. In addition, aqueous binders such as cellulose-based and aqueous dispersions of styrene butadiene rubber (SBR) can also be used. Examples of application methods include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc., and any thickness and shape can be obtained by using any of these. Examples of the current collector include aluminum, titanium, stainless steel, nickel, iron, baked carbon, conductive polymer, conductive glass, etc. The shape of the current collector can be a foil, a film, a sheet, a net, a punched or expanded one, a lath body, a porous body, a foam, a formed body of a fiber group, etc. The thickness of the current collector is, for example, 1 to 500 μm.

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 secondary 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 , _FeS2 , lithium manganese composite oxides having a basic composition formula such as Li _{(1-x) MnO2} (0≦x≦1, etc., the same applies below) or 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) or Li _{(1-x) NiaCobMncO4} (a+b+c=2), _{lithium vanadium} composite oxides having a basic composition formula such as _{LiV2O3 ,} lithium carbide composite oxides having a basic composition formula such as _LiV2O4 , lithium carbide ... _{2O5 ,} 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 , LiV2O3} , etc., are preferred. The "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, binder, solvent, and current collector used for the positive electrode may be, for example, any of those exemplified for the negative electrode.

In this power storage device, the ion conductive medium may be, for example, a non-aqueous electrolyte containing a supporting salt and an organic solvent. The supporting salt may contain a known lithium salt when the carrier is, for example, lithium ions. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li(CF ₃ SO ₂ ) ₂ N, and LiN(C ₂ F ₅ SO ₂ ) ₂ , among which LiPF ₆ and LiBF ₄ are preferred. The concentration of the supporting salt in the non-aqueous electrolyte is preferably 0.1 mol/L or more and 5 mol/L or less, and more preferably 0.5 mol/L or more and 2 mol/L or less. 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 can be more stabilized. In addition, a phosphorus-based or halogen-based flame retardant may be added to the non-aqueous electrolyte. For example, an aprotic organic solvent may 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 be used as the non-aqueous electrolyte.

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 electric storage device may also be applied to large devices used in electric vehicles and the like. 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 an 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 a layered structure having a crystallite size D ₃₀₂ perpendicular to the 302 surface of 25 nm or less as a negative electrode active material.

The electrode active material, the power storage device, and the method for producing the electrode active material described above can further improve the charge/discharge characteristics. The reason for obtaining such an effect is considered as follows. For example, if the crystallite size in the direction perpendicular to a specific crystal plane, particularly the crystallite size D ₃₀₂ in the direction perpendicular to the 302 plane, is 25 nm or less, the so-called alkali metal element layer becomes thinner, and it is presumed that the acceptability of the alkali metal ion, which is a carrier, can be further improved. Therefore, this electrode active material can expand the low resistance capacity region of the open circuit potential, and can further improve the charge/discharge characteristics.

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.

Below, we will explain a specific example of how to prepare an electrode active material.

(Synthesis of active material)
[Example 1]
4,4'-biphenyldicarboxylic acid (Bph(COOH) ₂ ) was pre-dried. 100 g of 4,4'-biphenyldicarboxylic acid was weighed and pre-dried at 120°C under reduced pressure of -0.09 MPa or less for 6 hours. Using this pre-dried sample, an aqueous solution in which 0.20 mol/L of 4,4'-biphenyldicarboxylic acid and 0.40 mol/L of lithium hydroxide were dissolved in ion-exchanged water was used as a raw material. This aqueous solution had a molar ratio B/A of 2.0, which is the molar number B (mol) of lithium hydroxide to the molar number A (mol) of 4,4'-biphenyldicarboxylic acid. The prepared 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 solution was sprayed at 0.4 L/hour, and the drying temperature was 200° C. to synthesize a layered structure of dilithium 4,4'-biphenyldicarboxylate. The obtained powder was dried at 120° C. for 8 hours under reduced pressure of 0.09 MPa or less. The obtained powder was used as the electrode active material of Example 1.

The obtained layered structure was observed at 1000 to 50000 times magnification using a scanning electron microscope (Quanta 200FEG manufactured by FEI Japan), and was found to be hollow particles with a particle size of 10 μm or less. Furthermore, when the particles with the exposed interior were viewed in a magnified view, the negative electrode active material had a hollow spherical structure formed by encapsulating a collection of flakes of the layered structure with a thickness of several nm. Furthermore, 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 irregularly moved from the outer periphery to the center). An electrode was produced using the flake-like layered structure obtained by crushing the hollow spherical structure.

[Comparative Example 1]
A powder obtained through the same process as in Example 1, except that an aqueous solution of pre-dried 0.20 mol/L 4,4'-biphenyldicarboxylic acid and 0.44 mol/L lithium hydroxide dissolved in ion-exchanged water was used as the raw material, was designated as Comparative Example 1. In Comparative Example 1, the molar ratio B/A was 2.2.

[Comparative Example 2]
A powder obtained through the same process as in Comparative Example 1, except that an aqueous solution of 0.20 mol/L of 4,4'-biphenyldicarboxylic acid and 0.44 mol/L of lithium hydroxide, which had not been dried in advance, dissolved in ion-exchanged water was used as the raw material, was designated Comparative Example 2. In Comparative Example 1, the molar ratio B/A was 2.2.

(Preparation of 4,4'-biphenyldicarboxylate dilithium electrode)
80 parts by mass of 4,4'-biphenyldicarboxylate dilithium prepared by the above method, 20 parts by mass of carbon black (Tokai Carbon, TB5500) as a particulate carbon conductive material, 3 parts by mass of carboxymethylcellulose (Daicel, CMC-1120) as a water-soluble polymer, 4 parts by mass of polyethylene oxide (PEO, molecular weight 2 million), and 3 parts by mass of styrene butadiene copolymer (JSR, TRD2001) were mixed in a ratio of 10 parts by mass, 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 current collector so that the negative electrode active material per unit area was 3 mg/cm ² , and the coated sheet was prepared by vacuum heating and drying at 120 ° C. Then, the coated sheet was subjected to pressure pressing and punched out to an area of 10 cm ² to prepare an electrode.

(X-ray diffraction measurement: XRD)
The electrode composite material of the coated portion was peeled off from the electrode prepared above, and filled into a capillary having a diameter of 0.5 mm. Measurement was performed at the Aichi Synchrotron Light Center BL5S2 (powder diffraction) beamline under the conditions of E = 20 keV (λ = 0.6195 Å), exposure time 15 minutes, and 2θ = 0° to 30°.

(SEM Observation)
The surface of the electrode thus prepared was observed under an SEM at a magnification of 1,000 to 50,000 using a scanning electron microscope (Quanta 200FEG manufactured by FEI Japan).

(Fabrication of Electricity Storage Device)
A non-aqueous electrolyte solution was prepared by adding lithium hexafluorophosphate to a volume of 1 mol/L in a non-aqueous solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:40:30. The electrode prepared above was used as a working electrode, and a lithium metal (thickness 300 μm) as a counter electrode was opposed to the separator (manufactured by Toray Tonen) impregnated with the non-aqueous electrolyte solution through the separator to form a bipolar evaluation cell. In a temperature environment of 20° C., the battery was reduced to 0.5 V (Li reference potential) at 0.5 mA, which corresponds to a 10-hour discharge or charge, and then oxidized to 0.5 mA and 1.5 V.

(Open circuit potential measurement)
Using the bipolar evaluation cell prepared above, a current of 0.5 mA was applied for 30 minutes in the potential range of 0.5 V to 1.5 V. This was an electric capacity corresponding to 0.1 electrons for a two-electron reaction corresponding to the theoretical capacity. After that, the cell was left standing until the change in potential was 4.5 mV per hour or until 100 hours, and the potential after leaving the cell standing was taken as the open circuit potential.

(Results and discussion)
Table 1 summarizes the molar ratios B/A of Example 1, Comparative Examples 1, and 2, the presence or absence of pre-drying of the aromatic dicarboxylic acid, the crystallite size perpendicular to the 302 and 100 planes according to the Scherrer formula, the strain ε _WH ×1000 according to the Williamson-Hall formula, and the proportion (-) of the low resistance and capacitance region. Table 1 also lists the crystallite size predicted from the crystal structure of Non-Patent Document 1 (Communs. Chem. 1, 65(2018)) and the predicted value of the open circuit voltage of Non-Patent Document 2 (Chem. Mater. 2020, 32, 3396-3404). Figure 3 is an SEM photograph of Example 1, Comparative Examples 1, and 2. Figure 4 is an XRD pattern of Example 1. Figure 5 is an XRD pattern of Comparative Example 1. Figure 6 is an XRD pattern of Comparative Example 2. Figure 7 is a diffraction spectrum of the 100 and 302 planes of each sample. Fig. 8 shows the results of measurement of the open circuit potential in Example 1. Fig. 9 shows the results of measurement of the open circuit potential in Comparative Example 1. Fig. 10 shows the results of measurement of the open circuit potential in Comparative Example 2. Fig. 11 shows the relationship between the crystallite size D and the proportion of the low resistance capacitance region. Fig. 12 shows the relationship between the Williamson-Hall plot and the strain ε _WH .

As shown in FIG. 3, in Comparative Example 2, the area of the flat surface of the particles on the flat plate was larger, and the layered structure was thick. On the other hand, in Comparative Example 1, in which the raw material aromatic dicarboxylic acid was pre-dried, it was found that fine layered structure particles with a smaller area of the flat surface were obtained. And, in Example 1, in which the raw material Li was not excessively added and the theoretical value was used, and the pre-dried, it was found that layered structure particles with a thinner thickness were obtained. This was expected to be related to the fact that crystals tend to grow in the thickness direction when excessive Li is included. As shown in FIGS. 4 to 7, it was found that the diffraction peak of the 100 plane and the diffraction peak of the 302 plane were smaller in the order of Comparative Example 2, 1, and Example 1. In addition, the half-width β of this diffraction peak, the Bragg angle (rad), the wavelength λ (0.61992 Å), the constant K (0.9), etc. were used to calculate the crystallite size in the direction perpendicular to the (hkl) plane using the Scherrer formula shown in Equation (1). As shown in Table 1, it was found that the crystallite sizes D ₃₀₂ and D ₁₀₀ decreased in the order of Comparative Example 2, 1, and Example 1.

8 to 10 and Table 1, it was found that the region in which the open circuit potential exhibits low resistance capacity during charging and discharging is expanded in the order of Comparative Example 2, 1, and Example 1. As shown in FIG. 11, a linear relationship is obtained between Example 1, Comparative Example 1, and 2 in the relationship between the open circuit potential and each crystallite size, and it was inferred that, for example, the crystallite size D ₃₀₂ is preferably 25 nm or less, more preferably 20 nm or less, and even more preferably 16 nm or less. It was also inferred that, for the crystallite size D ₁₀₀ , it is preferably 45 nm or less, more preferably 40 nm or less, and even more preferably 36 nm or less.

In addition, using the half-width β of this diffraction peak, the Bragg angle (rad), the wavelength λ (0.61992 Å), the constant K (0.9), etc., and using the Williamson-Hall equation shown in formula (2), it was found that the distortion ε _WH ×1000 is preferably 1.35 or more, and more preferably 1.6 or more. As shown in FIG. 12 and Table 1, the (100) plane shows a crystallite in the a-axis direction, and the (302) plane corresponds to _the Li ion supply surface during charging and discharging. Therefore, it was inferred that when the size of each crystallite decreases or the defects in the crystal structure increase, the Li acceptance surface increases, and the low resistance capacity region increases. In this embodiment, the aromatic dicarboxylic acid is pre-dried, and the lithium amount is made equivalent and spray-dried. However, the present invention is not limited to this manufacturing method. It is important that the crystallite size _D302 and the crystallite size _D100 are equal to or less than a predetermined value. It is presumed that if the crystallite size D is equal to or less than a predetermined value, the low resistance capacitance region can be expanded.

It goes without saying that the present disclosure is in no way limited to the above-mentioned embodiments, and may be implemented in various forms 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 (10)

An electrode active material for use in an electricity storage device,
A layered structure comprising an organic framework layer containing an aromatic dicarboxylate anion having a structure in which a plurality of aromatic rings are connected directly or via a carbon chain, 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 layered structure being subjected to X-ray diffraction measurement, and having a crystallite size _D302 in a direction perpendicular to a 302 plane calculated using formula (1) of 25 nm or less.
Electrode active material.
2. The electrode active material of claim 1, wherein the crystallite size _D302 is less than or equal to 20 nm. 3. The electrode active material according to claim 1, wherein the layered structure is subjected to X-ray diffraction measurement and has a crystallite size D ₁₀₀ in a direction perpendicular to a 100 plane calculated using formula (1) of 45 nm or less. 4. The electrode active material according to claim 3, wherein the crystallite size _D100 is 40 nm or less. 5. The electrode active material according to claim 1, wherein the layered structure is subjected to X-ray diffraction measurement and has a strain ε _WH ×1000 calculated using formula (2) of 1.35 or more.
The electrode active material according to claim 5 , wherein the strain ε _WH ×1000 is 1.6 or more. The electrode active material according to any one of claims 1 to 6, wherein the layered structure has a structure represented by chemical formula (1).

A positive electrode and
A negative electrode having an electrode mixture containing the electrode active material according to any one of claims 1 to 7;
an ion conductive medium interposed between the positive electrode and the negative electrode and conducting alkali metal ions;
A power storage device comprising: A method for producing an electrode active material for use in an electricity storage device, comprising the steps of:
a pre-drying step of pre-drying an aromatic dicarboxylic acid having a structure in which a plurality of aromatic rings are connected directly or via a carbon chain;
a preparation step of preparing a solution in which alkali metal ions are added to the aromatic dicarboxylic acid dried in advance in a molar ratio of 1.9 to 2.1;
a step of spray-drying the prepared solution to produce a layered structure including an organic framework layer containing an aromatic dicarboxylate anion having a structure in which a plurality of aromatic rings are connected directly or via a carbon chain, 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;
A manufacturing method comprising: The method according to claim 9, wherein in the pre-drying step, the aromatic dicarboxylic acid is dried under reduced pressure at 100°C or higher.