Method for manufacturing molecular crystal and method for manufacturing solid electrolyte

Abstract

A molecular crystal is manufactured by a method that comprises: a step of mixing an organic molecule that includes at least one atom selected from the group consisting of a sulfur atom, an oxygen atom, a nitrogen atom, and a phosphorus atom with an alkali metal salt, while adding mechanical energy to obtain a mixture; and a step of heating the mixture.

Description

Method for producing molecular crystals and method for producing solid electrolytes

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims priority to Japanese Patent Application No. 2023-88980, filed on May 30, 2023, the entire contents of which are incorporated herein by reference. The present disclosure relates to a method for producing a molecular crystal and a method for producing a solid electrolyte.

[0002] Various types of energy storage devices have been put to practical use, including various secondary batteries such as nickel-metal hydride secondary batteries and lithium-ion secondary batteries, as well as electric double-layer capacitors. Among these, lithium-ion secondary batteries are used in a wide range of applications due to their high energy density and battery capacity. In recent years, sodium-ion secondary batteries, potassium-ion secondary batteries, and magnesium-ion secondary batteries have been attracting attention as post-lithium-ion secondary batteries that use elements that replace the rare metal lithium.

[0003] Lithium-ion secondary batteries, which are widely used as energy storage devices, have a negative electrode, a positive electrode, and an electrolyte, and are charged and discharged by transferring lithium ions between the electrodes via the electrolyte. Organic electrolyte solutions have traditionally been used primarily as electrolytes. In recent years, solid or gel electrolytes using inorganic or organic materials have been proposed as a way to eliminate concerns about electrolyte leakage and short circuits within batteries due to overcharge and overdischarge.

[0004] While organic solid electrolytes have the advantage of excellent adhesion to electrodes due to the moderate flexibility and high moldability of organic materials, they tend to have lower ionic conductivity than, for example, inorganic solid electrolytes. Therefore, in order to improve their practicality, various studies have been conducted to develop new materials for constituting organic solid electrolytes (see, for example, Non-Patent Document 1). Non-Patent Document 1 discloses the formation of a crystalline organic solid electrolyte from a soft solid crystal consisting of lithium chloride and isoquinoline.

[0005] Ionics, 2018, Vol. 24, pp. 343-349

[0006] Conventional organic solid electrolytes have the problem that they do not have sufficient ionic conductivity at room temperature, and furthermore, their ionic conductivity decreases significantly at temperatures below room temperature.

[0007] The present disclosure has been made in view of the above circumstances, and has as its object to provide a method for producing a molecular crystal that exhibits high ionic conductivity at room temperature and temperatures lower than room temperature, and as another object to provide a method for producing a solid electrolyte that exhibits high ionic conductivity at room temperature and temperatures lower than room temperature.

[0008] As a result of intensive research to solve the above problems, the present inventors have found that molecular crystals produced by a specific production method exhibit high ionic conductivity at room temperature and at temperatures lower than room temperature. According to the present disclosure, the following methods for producing molecular crystals and solid electrolytes are provided.

[0009] [Means 1] A method for producing a molecular crystal, comprising the steps of: mixing an organic molecule having at least one atom selected from the group consisting of a sulfur atom, an oxygen atom, a nitrogen atom, and a phosphorus atom with an alkali metal salt while applying mechanical energy to obtain a mixture; and heating the mixture.

[0010] [Means 2] The method for producing molecular crystals according to [Means 1], wherein the mixture is heated at a temperature equal to or higher than the melting point of the mixture. [Means 3] The method for producing molecular crystals according to [Means 1] or [Means 2], wherein the total number of carbon atoms per molecule of the organic molecules is 6 or less. [Means 4] The method for producing molecular crystals according to any of [Means 1] to [Means 3], wherein the organic molecules are at least one selected from the group consisting of sulfones, nitriles, and amides. [Means 5] The method for producing molecular crystals according to any of [Means 1] to [Means 4], wherein the alkali metal salt includes an imide-based alkali metal salt. [Means 6] A method for producing a solid electrolyte, wherein a solid electrolyte is produced using molecular crystals obtained by the production method according to any of [Means 1] to [Means 5].

[0011] According to the present disclosure, a molecular crystal exhibiting high ionic conductivity at room temperature and temperatures lower than room temperature can be obtained. Furthermore, by using the molecular crystal obtained by the manufacturing method of the present disclosure, a solid electrolyte exhibiting excellent ionic conductivity at room temperature and temperatures lower than room temperature can be obtained. By using such a solid electrolyte as an electrolyte for an electricity storage device such as a secondary battery or a capacitor, an electricity storage device can be obtained that combines ionic conductivity with the safety ensured by the solidification of the electrolyte.

[0012] FIG. 1 is a schematic diagram of a press die used to prepare the sample pellets.

[0013] Hereinafter, the method for producing a molecular crystal according to the present disclosure, and a solid electrolyte and an electricity storage device produced using the molecular crystal obtained by the production method will be described in detail.

[0014] <<Method for Producing Molecular Crystals>> Molecular crystals are solids in which structural units are regularly arranged in a crystal lattice. The method for producing molecular crystals of the present disclosure (hereinafter also referred to as "the present production method") produces molecular crystals using organic molecules having at least one atom selected from the group consisting of sulfur atoms, oxygen atoms, nitrogen atoms, and phosphorus atoms (hereinafter also referred to as "(A) organic molecules") and an alkali metal salt, and includes the following mechanical mixing step and heating step. Mechanical mixing step: A step of mixing the (A) organic molecules and the alkali metal salt while applying mechanical energy to obtain a mixture. Heating step: A step of heating the mixture obtained by the mechanical mixing step. Each step in the present production method will be described in detail below.

[0015] <Mechanical Mixing Step> In the mechanical mixing step, the (A) organic molecules and the alkali metal salt are mixed while applying mechanical energy to a substance containing the (A) organic molecules and the alkali metal salt by means of impact, shear, compression, friction, etc. (hereinafter also referred to as "mechanical mixing"), which preferably causes changes in the physicochemical properties of the (A) organic molecules, the alkali metal salt, or both.

[0016] (A) Organic Molecule The number of atoms of at least one kind selected from the group consisting of sulfur atoms, oxygen atoms, nitrogen atoms, and phosphorus atoms contained in the (A) organic molecule is not particularly limited. In the (A) organic molecule, the number of atoms of at least one kind selected from the group consisting of sulfur atoms, oxygen atoms, nitrogen atoms, and phosphorus atoms is preferably 1 to 8, and more preferably 2 to 6, per molecule. The (A) organic molecule preferably contains two or more, and more preferably 2 to 6, atoms of at least one kind selected from the group consisting of sulfur atoms, oxygen atoms, and nitrogen atoms.

[0017] The molecular weight of the (A) organic molecule is, for example, 300 or less, preferably 250 or less, and more preferably 200 or less. The lower limit of the molecular weight of the (A) organic molecule is, for example, 20 or more, preferably 40 or more, and more preferably 50 or more. From the viewpoint of promoting crystallization of the mixture of the (A) organic molecule and the alkali metal salt, the total number of carbon atoms per molecule of the (A) organic molecule is preferably 10 or less, more preferably 8 or less, and even more preferably 6 or less.

[0018] Specific examples of (A) organic molecules include organic molecules having a sulfur atom, such as sulfone, sulfide, thiol, thioester, thiocarbonate, sulfoxide, and sulfamide. Organic molecules having a nitrogen atom include nitrile, amine, and amide. Organic molecules having a phosphorus atom include phosphine, phosphine oxide, and phosphinimine. Organic molecules having an oxygen atom include organic molecules having an oxygen atom among the examples of organic molecules having a sulfur atom, organic molecules having an oxygen atom among the examples of organic molecules having a nitrogen atom, and organic molecules having an oxygen atom among the examples of organic molecules having a phosphorus atom, as well as ethers, esters, ketones, and carbonates.

[0019] In terms of being able to further increase the ionic conductivity of the molecular crystal, it is preferable that the (A) organic molecule be at least one selected from the group consisting of organic molecules having a sulfur atom and an oxygen atom, organic molecules having a nitrogen atom, and organic molecules having a phosphorus atom.

[0020] (Organic molecule having sulfur atom and oxygen atom) As the organic molecule having sulfur atom and oxygen atom, sulfone can be preferably used. Preferred examples of sulfone include those represented by the following formula (1): (In formula (1), R １ and R ２ are each independently an alkyl group or an alkoxy group, or R １ and R ２ and represents a cyclic structure formed by bonding together, and the total number of carbon atoms in formula (1) is 2 to 16.

[0021] The molecule represented by the above formula (1) (hereinafter also referred to as "molecule (A-1)") has a sulfonyl group (-SO ２ -) in the above formula (1). １ and R ２ When R is an alkyl group or an alkoxy group, the alkyl group and the alkoxy group may be linear or branched. １ and R ２ The number of carbon atoms in each group of R １ and R ２ (A) From the viewpoint of inducing crystallization in a mixture of an organic molecule and an alkali metal salt, there is no particular limitation on the number of carbon atoms in R １ and R ２ Each of the groups preferably has 1 to 3 carbon atoms, more preferably 1 or 2.

[0022] R in the above formula (1) １ and R ２ But, R １ and R ２ When R represents a cyclic structure formed by bonding R to R, the cyclic structure may have a saturated ring skeleton or an unsaturated bond in the ring skeleton. １ and R ２ The cyclic structure formed by bonding R may be a structure in which an alkyl group (e.g., a methyl group or an ethyl group) is bonded to the ring skeleton. １ and R ２The number of carbon atoms in the cyclic structure formed by bonding of is preferably 2 to 8, more preferably 2 to 6, and even more preferably 3 to 5.

[0023] In order to improve the ionic conductivity of the present molecular crystal, R １ and R ２ is R １ and R ２ and more preferably, the cyclic structure has a saturated ring skeleton.

[0024] The total number of carbon atoms in the above formula (1) is 2 to 16. From the viewpoint of promoting crystallization of the mixture of the (A-1) molecule and the alkali metal salt, the total number of carbon atoms in the above formula (1) is preferably 2 to 8, more preferably 2 to 6, and even more preferably 2 to 5.

[0025] Specific examples of the (A-1) molecule include R １ and R ２ are each an alkyl group or an alkoxy group, examples of which include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, dioctyl sulfone, dimethyl sulfate, and diethyl sulfate. １ and R ２ But, R １ and R ２ Examples of the (A-1) molecule that represents a cyclic structure formed by bonding include tetrahydrothiophene 1,1-dioxide, 3-methylsulfolane, 3-sulfolene, 1,3-propane sultone, 1,4-butane sultone, 1,3,2-dioxathiolane 2,2-dioxide, etc. One type of molecule (A-1) may be used alone, or two or more types may be used in combination.

[0026] (Nitrogen-containing organic molecule) Nitriles can be preferably used as the nitrogen-containing organic molecule. Preferred examples of nitriles include those represented by the following formula (2): (In formula (2), R ３ represents a hydrocarbon chain, R ４ and R ５each independently represents a hydrogen atom, an alkyl group, or a cyano group, and the total number of carbon atoms in formula (2) is 16 or less.

[0027] The molecule represented by the above formula (2) (hereinafter also referred to as "molecule (A-2)") is a molecule having two or more cyano groups (-CN). ３ R may be a saturated hydrocarbon chain or may have an unsaturated bond. R is a suitable ionic conductive material for obtaining a molecular crystal having excellent ionic conductivity. ３ Preferably, R represents a saturated hydrocarbon chain. ３ has, for example, 1 to 10 carbon atoms, preferably 1 to 6 carbon atoms, and more preferably 2 to 4 carbon atoms.

[0028] R ４ or R ５ When R is an alkyl group, the alkyl group may be linear or branched. It is preferably linear. ４ and R ５ The number of carbon atoms in each group is preferably 0 to 3, and more preferably 0 or 1, from the viewpoint of causing crystallization by mixing (A) the organic molecule with the alkali metal salt.

[0029] The total number of carbon atoms in the above formula (2) may be 16 or less. From the viewpoint of promoting crystallization of the mixture of the (A-2) molecule and the alkali metal salt, the total number of carbon atoms in the above formula (2) is preferably 8 or less, and more preferably 6 or less.

[0030] Specific examples of the (A-2) molecule include succinonitrile, adiponitrile, 3-hexenedinitrile, 1,3,5-pentanetricarbonitrile, tert-butylmalononitrile, 1,2,2,3-propanetetracarbonitrile, etc. As the (A-2) molecule, one type may be used alone, or two or more types may be used in combination.

[0031] (Organic molecule having a nitrogen atom) As the organic molecule having a nitrogen atom, a compound having an amide group can be preferably used. The compound having an amide group is preferably a molecule having two or more amide groups in one molecule, in terms of high heat resistance. Specific examples of the compound having an amide group include malonamide, succinamide, glutaramide, adipamide, N,N,N',N'-tetramethylmalonamide, N,N,N',N'-tetraethylmalonamide, N,N,N',N'-tetramethylsuccinamide, and terephthalamide.

[0032] (Organic Molecule Having a Phosphorus Atom) Examples of organic molecules having a phosphorus atom include ethylenebis(dimethylphosphine), ethylenebis(diphenylphosphine), methyl(diphenyl)phosphine oxide, triphenylphosphine oxide, methyl(diphenyl)phosphine imine, and triphenylphosphine imine.

[0033] From the viewpoint of ease of production of the molecular crystal, the organic molecule (A) is preferably at least one selected from the group consisting of sulfone, nitrile, and amide. Furthermore, the (A-1) molecule is more preferable in terms of superior ionic conductivity of the molecular crystal, and R １ and R ２ It is more preferable that the organic molecule (A) is a molecule having a cyclic structure formed by bonding of the following, and it is even more preferable that the cyclic structure has a saturated ring skeleton. Note that, as the organic molecule (A), one type may be used alone, or two or more types may be used in combination.

[0034] Alkali Metal Salt The alkali metal salt is not particularly limited as long as it generates alkali metal ions. Examples of the alkali metal salt include lithium salt, sodium salt, and potassium salt.

[0035] Specific examples of alkali metal salts include Li ２ CO ３ , LiBr, LiCl, LiI, LiSCN, LiBF ４ , LiAsF ６ , LiClO ４ , C.H. ３ COOLi, CF ３COOLi, LiCF ３ SO ３ , LiPF ６ , LiC(CF ３ SO ２ ) ３ , lithium bis(fluorosulfonyl)imide (Li ＋ (FSO ２ ) ２ N － ), lithium bis(trifluoromethanesulfonyl)imide (Li ＋ (CF ３ SO ２ ) ２ N － and salts of the anions of these lithium salts with alkali metals other than lithium (for example, sodium, potassium, etc.). Among these, lithium salts or sodium salts are preferred, and lithium salts are more preferred, since they have high ionic dissociation properties and can provide molecular crystals with higher ionic conductivity.

[0036] In terms of being able to further increase the ionic conductivity of the molecular crystal, the alkali metal salt used for producing the molecular crystal preferably contains an imide alkali metal salt. Among them, imide lithium salts are preferred in terms of high ionic dissociation, and among the imide lithium salts, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, or lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide are particularly preferred, lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide is more preferred, and lithium bis(fluorosulfonyl)imide is even more preferred.

[0037] The molecular weight of the alkali metal salt is, for example, 500 or less, preferably 400 or less, more preferably 350 or less, and even more preferably 300 or less. The lower limit of the molecular weight of the alkali metal salt is, for example, 20 or more, preferably 50 or more, more preferably 100 or more, and even more preferably 150 or more. One type of alkali metal salt may be used alone, or two or more types may be used in combination.

[0038] The melting point of the alkali metal salt is, for example, 60° C. or higher, preferably 70° C. or higher, and more preferably 80° C. or higher. There is no particular upper limit to the melting point of the alkali metal salt, but it may be, for example, 300° C. or lower, or 250° C. or lower.

[0039] When producing a molecular crystal by this production method, a substance other than the (A) organic molecule and the alkali metal salt (for example, a sulfur-containing organic molecule other than the (A) organic molecule) may be further used as a constituent unit in the crystal lattice, as long as the effects of the present disclosure are not impaired. However, in consideration of the ionic conductivity and ease of production of the molecular crystal, it is preferable to minimize the amount of the substance other than the (A) organic molecule and the alkali metal salt used. Specifically, the amount of the substance other than the (A) organic molecule and the alkali metal salt used is preferably 0.1 mol or less, more preferably 0.05 mol or less, per mol of the total amount of the (A) organic molecule and the alkali metal salt.

[0040] The compounding ratio of the (A) organic molecule to the alkali metal salt is not particularly limited and may be set depending on the types of the (A) organic molecule and the alkali metal salt, etc. The compounding ratio of the (A) organic molecule to the alkali metal salt may be, for example, 0.1 to 10 moles of the (A) organic molecule per mole of the alkali metal salt. The compounding ratio of the (A) organic molecule to the alkali metal salt is preferably 0.2 to 5 moles, more preferably 0.25 to 4 moles, and even more preferably 0.5 to 2 moles of the (A) organic molecule per mole of the alkali metal salt.

[0041] The method for mixing the (A) organic molecules and the alkali metal salt while applying mechanical energy is not particularly limited. In the mechanical mixing step, the (A) organic molecules and the alkali metal salt can be mixed using various devices, such as a ball mill, a bead mill, a blender, a homogenizer, a stamp mill, a homomixer, or a disperser-type mixer. Furthermore, when producing molecular crystals on a small scale, the (A) organic molecules and the alkali metal salt can also be mechanically mixed using a mortar and pestle. The (A) organic molecules and the alkali metal salt can be mechanically mixed in a dry or wet manner. Furthermore, the (A) organic molecules and the alkali metal salt can be mechanically mixed at room temperature or at a low temperature. For example, when the melting point of the (A) organic molecules is lower than room temperature, the (A) organic molecules in a liquid state can be mechanically mixed with the alkali metal salt. Alternatively, the (A) organic molecules in a solid state can be mechanically mixed with the alkali metal salt at a temperature lower than the melting point of the (A) organic molecules.

[0042] The mechanical mixing of the (A) organic molecules and the alkali metal salt may be carried out while heating. In this case, in order to suppress volatilization or sublimation of the (A) organic molecules, the mechanical mixing step is preferably carried out at a temperature lower than the heating temperature in the heating step carried out after the mechanical mixing step. Specifically, the mechanical mixing step is carried out, for example, at 50°C or lower, preferably 40°C or lower, and more preferably 35°C or lower. The time for which mechanical mixing is carried out varies depending on the amounts and types of the (A) organic molecules and alkali metal salt used, but may be, for example, 1 to 60 minutes, or 1 to 30 minutes.

[0043] The mixture of (A) organic molecules and alkali metal salt obtained by mechanical mixing (hereinafter also simply referred to as "mixture") preferably has a melting point of 40°C or higher, more preferably 50°C or higher, and even more preferably 55°C or higher. Furthermore, the melting point of the mixture is preferably 110°C or lower, more preferably 100°C or lower, and even more preferably 95°C or lower, in order to suppress decomposition of molecular crystals accompanying volatilization or sublimation of (A) organic molecules in the heating step after the mechanical mixing step. In this specification, the melting point of the mixture of (A) organic molecules and alkali metal salt is a value obtained by differential scanning calorimetry (DSC). Details of the measurement method follow the method described in the Examples below.

[0044] <Heating Step> In the heating step, after the mechanical mixing step, the mixture obtained by the mechanical mixing step is heated. The method for heating the mixture is not particularly limited. The heating temperature of the mixture can be appropriately set depending on the types of (A) organic molecules and alkali metal salt used as raw materials. From the viewpoint of making the (A) organic molecules and alkali metal salt in the mixture more uniform, it is preferable that in the heating step of this production method, the mixture is heated at a temperature equal to or higher than the melting point of the mixture of the (A) organic substance and the alkali metal salt obtained by mechanical mixing. Setting the heating temperature equal to or higher than the melting point of the mixture melts the mixture, thereby promoting uniform mixing of the (A) organic molecules and the alkali metal salt.

[0045] The temperature at which the mixture is heated is preferably higher than the melting point of the mixture, from the viewpoint of homogenizing the (A) organic molecules and the alkali metal salt in the mixture by melting the mixture. Furthermore, from the viewpoint of promoting homogenization of the (A) organic molecules and the alkali metal salt in the mixture, the heating temperature of the mixture is more preferably 5°C or higher, and even more preferably 7°C or higher, than the melting point of the mixture (unit: °C). With regard to the upper limit of the heating temperature of the mixture, from the viewpoint of suppressing volatilization or sublimation of the components contained in the mixture, when the melting point of the mixture is expressed as Mp (unit: °C), it is preferably (Mp + 20)°C or lower, and more preferably (Mp + 15)°C or lower. The heating time is not particularly limited as long as it can homogenize the (A) organic molecules and the alkali metal salt in the mixture. The heating time is, for example, 1 minute to 3 hours, preferably 15 minutes to 2 hours, and more preferably 30 minutes to 2 hours. Furthermore, the heat treatment is usually carried out under normal pressure, but may also be carried out under increased pressure or reduced pressure.

[0046] When the mixture is heated, it is preferable to heat the mixture while stirring it to further homogenize the (A) organic molecules and the alkali metal salt. The stirring method is not particularly limited, and examples thereof include a magnetic stirrer, a stirring rod, a stirrer with stirring blades, and external circulation stirring. Stirring may also be performed using a mechanical operation that can generate a larger shear force, such as a homomixer, a disperser-type mixer, or a homogenizer.

[0047] After heating a mixture containing (A) an organic molecule and an alkali metal salt, the heated mixture can be crystallized by lowering the temperature of the heated mixture, thereby obtaining the desired molecular crystal. The temperature of the heated mixture can be lowered by gradually cooling the mixture (e.g., slowly lowering the temperature at room temperature over 1 to 48 hours). Alternatively, the heated mixture can be rapidly cooled (e.g., placing the heated mixture in a thermostatic bath at a temperature lower than room temperature (e.g., 15°C or lower) for a short period of time). Among these methods, gradually lowering the temperature of the mixture is preferred from the viewpoint of favorable crystallization of the heated mixture. The fact that the product obtained by this production method is a molecular crystal containing (A) an organic molecule and an alkali metal salt can be detected by powder X-ray diffraction measurement of the product. Details of the measurement method follow those described in the Examples below.

[0048] Here, when molecular crystals are produced by applying only the mechanical mixing step out of the mechanical mixing step and the heating step, it is possible to suppress the volatilization and sublimation of the (A) organic molecules during production, but the (A) organic molecules and the alkali metal salt cannot be mixed uniformly, and the ionic conductivity of the resulting molecular crystals may be insufficient.Furthermore, when molecular crystals are produced by applying only the heating step out of the mechanical mixing step and the heating step, the (A) organic molecules are likely to volatilize and sublimate due to heating, making it difficult to obtain molecular crystals of the desired composition.

[0049] In contrast, in the present production method including the above-mentioned mechanical mixing step and heating step, the (A) organic molecules and the alkali metal salt are mechanically mixed in advance, and then the mixture containing the (A) organic molecules and the alkali metal salt is heated. This makes it possible to uniformly mix the (A) organic molecules and the alkali metal salt while suppressing volatilization and sublimation of the (A) organic molecules, and as a result, it is believed that a molecular crystal with excellent ionic conductivity can be obtained.

[0050] In addition, according to the present production method, which includes a mechanical mixing step and a heating step, it is believed that the mechanical mixing performed before the heat treatment during the production of molecular crystals causes intermolecular interactions between the alkali metal ions derived from the alkali metal salt and the (A) organic molecules. Therefore, in the present production method, in which the heat treatment is performed after the mechanical mixing, it is believed that sublimation and volatilization of the (A) organic molecules are suppressed during the heat treatment, thereby suppressing the variation in the composition of the molecular crystals. Furthermore, it is believed that the suppression of the variation in the composition of the molecular crystals leads to the formation of uniform ion conduction paths, enabling the production of molecular crystals that exhibit excellent ion conductivity.

[0051] <Solid Electrolyte> The molecular crystal obtained by the above-described production method is a crystalline organic material containing (A) organic molecules and an alkali metal salt as structural units of the crystal. The molecular crystal has an ion conduction path due to the regular arrangement of structural units containing (A) organic molecules and an alkali metal salt, thereby exhibiting excellent ion conductivity. Therefore, the molecular crystal obtained by the production method is suitable as a material for a solid electrolyte. In other words, by using the molecular crystal obtained by the production method, a solid electrolyte exhibiting excellent ion conductivity can be obtained.

[0052] The solid electrolyte of the present disclosure can be obtained by molding an electrode material containing molecular crystals obtained by the above-described manufacturing method into a desired shape. The molding method is not particularly limited, and known methods such as extrusion molding, injection molding, pressure molding, slip casting, mold cast molding, and tape casting can be used. Among these, obtaining a molded body by pressure molding is preferred because it can minimize the porosity of the resulting solid electrolyte and facilitates the formation of ion conduction paths. The shape of the molded body is not particularly limited and can be appropriately set depending on the shape of the applied electricity storage device. The shape of the molded body is, for example, rectangular or circular.

[0053] The solid electrolyte of the present disclosure may further contain components other than the molecular crystal of the present disclosure, as long as the effects of the present disclosure are not impaired. Examples of other components include molecular crystals different from the molecular crystal of the present disclosure, polymer solid electrolytes, polymer gel electrolytes, inorganic electrolytes (e.g., ceramic electrolytes, glass electrolytes, etc.), binders, etc.

[0054] The content of the molecular crystal of the present disclosure in the solid electrolyte is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on the total amount of the solid electrolyte, particularly from the viewpoint of ensuring ionic conductivity in a low-temperature environment.

[0055] The solid electrolyte obtained as described above exhibits high ionic conductivity at room temperature and temperatures lower than room temperature. Specifically, the ionic conductivity of a solid electrolyte having a thickness of about 200 μm (specifically, 200±20 μm) measured at 25° C. using an AC impedance method is 1×10 －６ From the viewpoint of obtaining an electricity storage device with excellent performance, the ionic conductivity under the same conditions is preferably 2×10 S / cm or more. －６ S / cm or more is more preferable, and 5×10 －６ It is more preferable that the viscosity is 25 S / cm or more.

[0056] In addition, the ionic conductivity of a solid electrolyte having a thickness of about 200 μm (specifically, 200±20 μm) measured at −20° C. using an AC impedance method is 2×10 －８ S / cm or more, and 5×10 －８ S / cm or more is more preferable, and 1×10 －７ The ionic conductivity is more preferably 25 S / cm or more. Details of the method for measuring the ionic conductivity follow the method described in the examples below.

[0057] The solid electrolyte of the present disclosure exhibits high ionic conductivity not only in a room temperature environment but also in a low-temperature environment of 0° C. or less. Therefore, by using the solid electrolyte of the present disclosure as the electrolyte of an electricity storage device, an electricity storage device with high ionic conductivity can be obtained.

[0058] <Electricity Storage Device> The electricity storage device of the present disclosure (hereinafter also referred to as "the device") includes the solid electrolyte of the present disclosure. Examples of the device include a secondary battery, a capacitor, etc. When the device is a secondary battery, one embodiment is an all-solid-state battery, and a lithium-ion secondary battery is preferred because of its excellent ionic conductivity.

[0059] An all-solid-state lithium-ion secondary battery, which is one embodiment of the device, will now be described. A lithium-ion secondary battery is a laminate comprising electrodes consisting of a positive electrode and a negative electrode, and a solid electrolyte, with the solid electrolyte being disposed between the positive electrode and the negative electrode so that the solid electrolyte is in contact with the electrode. The materials constituting the positive electrode and the negative electrode are not particularly limited, and can be appropriately selected from materials known as electrode materials for lithium-ion secondary batteries. For example, a metal foil such as aluminum or stainless steel can be used as the positive electrode current collector. A metal foil such as copper foil or lithium foil can be used as the negative electrode current collector.

[0060] In the lithium-ion secondary battery of the present disclosure, the solid electrolyte is formed using (A) a molecular crystal containing an organic molecule and an alkali metal salt. The thickness of the solid electrolyte is not particularly limited and can be appropriately set depending on the application of the secondary battery, etc. The thickness of the solid electrolyte is, for example, 5 to 500 μm.

[0061] The method for producing a lithium ion secondary battery is not particularly limited, and known methods can be appropriately adopted depending on the battery structure, etc. For example, a laminate including a positive electrode, a solid electrolyte, and a negative electrode may be produced by annealing a molded body formed using molecular crystals and sandwiching the resulting solid electrolyte between a positive electrode and a negative electrode. Alternatively, a laminate including a positive electrode, a solid electrolyte, and a negative electrode may be produced by placing an electrolyte material including molecular crystals between a positive electrode and a negative electrode in a container and then annealing the container. A laminate including a positive electrode, a solid electrolyte, and a negative electrode is usually housed in a case and used as a secondary battery.

[0062] The device is not limited to the above-described configuration in which the carrier for ion conduction is lithium ions, but may be a secondary battery in which other ions, such as sodium ions, are used as carriers. The device may also be a capacitor. One form of the capacitor includes a positive electrode, a negative electrode, and a solid electrolyte, with the solid electrolyte disposed between the positive electrode and the negative electrode so that the solid electrolyte is in contact with the electrodes.

[0063] The power storage device including the solid electrolyte can be used for a variety of purposes, specifically as a power source for various mobile devices such as mobile phones, personal computers, smartphones, game consoles, and wearable devices; various moving objects such as electric vehicles, hybrid vehicles, robots, and drones; and various electric and electronic devices such as digital cameras, video cameras, music players, power tools, and home appliances.

[0064] The present disclosure will be specifically described below based on examples. However, the present disclosure is not limited to these examples. In the following, "parts" and "%" mean "parts by mass" and "% by mass", respectively, unless otherwise specified.

[0065] <<Production and Evaluation of Molecular Crystals>> [Example 1] 1. Production of Molecular Crystals In a glove box under an argon atmosphere, 0.1379 g of succinonitrile (manufactured by Tokyo Chemical Industry Co., Ltd.) as an organic molecule and 0.1624 g of lithium tetrafluoroborate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as an alkali metal salt were weighed out so that the molar ratio was 1:1, and mechanically mixed using a mortar at room temperature (25 ° C.) for 5 minutes to obtain a mixture (referred to as mixture M) (mechanical mixing step). The melting point of mixture M was measured using a differential scanning calorimeter (manufactured by TA Instruments, DSC250, measurement atmosphere: nitrogen atmosphere) at a heating rate of 10 ° C. per minute, and a large endothermic peak (melting point) was observed at 77 ° C. Mixture M was transferred to a vial and stirred with a magnetic stirrer at a set temperature of 90 ° C. for 1 hour, and it was confirmed that it was uniformly dissolved (heating step). The resulting melt was then allowed to cool to room temperature to give a white solid.

[0066] 2. Evaluation (Confirmation of Crystallinity by Powder X-ray Diffraction Measurement) The obtained white solid was subjected to powder X-ray diffraction measurement using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker AXS). CuKα was used as the X-ray source, and the applied voltage was 40 kV and the current was 40 mA. The measurement range was 2θ = 5-60 deg, the scanning speed was 2.3 deg / min, and the step angle was 0.02 deg. If an X-ray diffraction spectrum having a sharp peak at an angle within the measurement range is observed, it indicates that the measured sample (solid) is crystalline. Therefore, if an X-ray diffraction spectrum having a sharp peak at an angle within the measurement range and different from those of the organic molecule and alkali metal salt used as raw materials was obtained, it was determined that a molecular crystal composed of an organic molecule and an alkali metal salt had been produced. In addition, when the organic molecules used as raw materials are liquid at room temperature, it is determined that molecular crystals composed of organic molecules and alkali metal salts have been produced when an X-ray diffraction spectrum having a sharp peak in the angle measurement range and different from that of alkali metal salts is observed. Powder X-ray diffraction measurements were performed on the white solid obtained as described above, and an X-ray diffraction spectrum having a sharp peak in the angle measurement range and different from that of the raw materials was observed. This confirmed that the obtained white solid is crystalline, i.e., a molecular crystal composed of organic molecules and alkali metal salts.

[0067] (Measurement of Ionic Conductivity) Using a press die 10 consisting of a die set 11, an upper punch 12, and a lower punch 13 (see FIG. 1), a sample pellet was prepared in a glove box under an argon atmosphere. First, the die set 11 containing 0.0200 g of white solid (molecular crystal) was placed on the lower punch 13, and the upper punch 12 was placed on the die set 11. The white solid was compressed for 1 minute at a pressure of 10 MPa using a hydraulic press to obtain a circular sample pellet with a diameter of 1 cm and a thickness of 200 μm. Next, the sample pellet was sealed in an all-solid-state battery evaluation cell (KP-SolidCell, manufactured by Hosen Co., Ltd.) under an argon atmosphere in a glove box. Ionic conductivity measurements were performed in the atmosphere using the all-solid-state battery evaluation cell containing the sample pellet. To measure ionic conductivity, first, the resistance value was measured by AC impedance measurement while the sealed cell 20 containing the sample pellet was heat-treated in a thermostatic chamber at 50°C (corresponding to the heat treatment temperature in Table 1). The heat treatment was terminated when the resistance value became constant. Next, the resistance values ​​at 25°C and -20°C were measured by AC impedance measurement. Using the obtained resistance values, the ionic conductivity (σ) was calculated according to the following mathematical formula (1). The AC impedance measurement for calculating the ionic conductivity was carried out after the cell was kept at the measurement temperature in a thermostatic bath for 2 hours. σ = L / (R × S) (1) (In mathematical formula (1), σ is the ionic conductivity (unit: S / cm), R is the resistance (unit: Ω), and S is the cross-sectional area of ​​the molecular crystal at the time of measurement (unit: cm ２ ), and L represents the distance between the electrodes (unit: cm). The ionic conductivities of the obtained molecular crystals at 25°C and -20°C were 6.6 × 10 －５ S / cm, 2.7×10 －８ S / cm.

[0068] [Examples 2 to 7] White solids were obtained by performing the mechanical mixing and heating steps in the same manner as in Example 1, except that the type and amount of raw materials, the heating temperature of mixture M in the heating step, and the heat treatment temperature for ionic conductivity measurement were changed as shown in Table 1. Powder X-ray diffraction measurements were performed on the white solids obtained in each Example in the same manner as in Example 1. As a result, X-ray diffraction spectra with sharp peaks in the angle range measured and different from those of the raw materials were observed. It was therefore confirmed that all of the white solids were molecular crystals composed of organic molecules and alkali metal salts. Furthermore, ionic conductivity measurements at 25°C and -20°C were performed using the white solids (molecular crystals) of each Example in the same manner as in Example 1. The melting point measurement results of mixture M obtained by the mechanical mixing step, as well as the ionic conductivity measurement results, are shown in Table 1.

[0069] Comparative Example 1 In a glove box under an argon atmosphere, 0.1397 g of succinonitrile (Tokyo Chemical Industry Co., Ltd.) as an organic molecule and 0.1624 g of lithium tetrafluoroborate (Fujifilm Wako Pure Chemical Industries, Ltd.) as an alkali metal salt were weighed out to a molar ratio of 1:1 and gently mixed with a medicine spoon at room temperature (25°C) without mechanical manipulation. Next, the mixture was transferred to a vial and stirred for 1 hour using a magnetic stirrer set at 90°C, and uniform dissolution was confirmed (heating step). The resulting molten material was then allowed to cool to room temperature, yielding a white solid (molecular crystal). The ionic conductivity of the resulting molecular crystal was measured at 25°C and -20°C in the same manner as in Example 1, with a heat treatment temperature of 50°C. The ionic conductivity measurement results are shown in Table 1.

[0070] Comparative Example 2 In a glove box under an argon atmosphere, 0.1391 g of succinonitrile (Tokyo Chemical Industry Co., Ltd.) as an organic molecule and 0.1621 g of lithium tetrafluoroborate (Fujifilm Wako Pure Chemical Industries, Ltd.) as an alkali metal salt were weighed out to a molar ratio of 1:1 and mechanically mixed in a mortar at room temperature (25°C) for 5 minutes (mechanical mixing step), yielding a white solid (molecular crystal). The melting point of the molecular crystal obtained by mechanical mixing of the raw materials was measured by DSC in the same manner as in Example 1 and was found to be 77°C. The ionic conductivity of the molecular crystal obtained by mechanical mixing of the raw materials was measured at 25°C and -20°C in the same manner as in Example 1, except for a heat treatment temperature of 50°C. The results of the ionic conductivity measurements are shown in Table 1. In Comparative Example 2, the resistance of the measured sample was significantly high in the Nyquist plot at -20°C, making it impossible to read the resistance value and therefore impossible to calculate the ionic conductivity. Therefore, in Table 1, the column for the measurement results of ionic conductivity at −20° C. is marked as “unreadable” (the same applies to Comparative Example 3).

[0071] Comparative Example 3 In a glove box under an argon atmosphere, 0.1149 g of succinamide (Tokyo Chemical Industry Co., Ltd.) as an organic molecule and 0.1851 g of lithium bis(fluorosulfonyl)imide (Kanto Chemical Co., Ltd.) as an alkali metal salt were weighed out to a molar ratio of 1:1 and mechanically mixed using a mortar at room temperature (25°C) for 5 minutes (mechanical mixing step), yielding a white solid (molecular crystal). The melting point of the molecular crystal obtained by mechanical mixing of the raw materials was measured by DSC in the same manner as in Example 1, and was found to be 97°C. The ionic conductivity of the molecular crystal obtained by mechanical mixing of the raw materials was measured at 25°C and -20°C in the same manner as in Example 1, with the heat treatment temperature set to 80°C. The results of the ionic conductivity measurements are shown in Table 1. In Comparative Example 3, the resistance value of the measured sample was significantly high in the Nyquist plot at -20°C, making it impossible to read the resistance value and therefore impossible to calculate the ionic conductivity.

[0072]

[0073] The details of the compounds used in Table 1 are as follows: SN: succinonitrile (manufactured by Tokyo Chemical Industry Co., Ltd.) SL: tetrahydrothiophene 1,1-dioxide (manufactured by Tokyo Chemical Industry Co., Ltd.) DMS: dimethyl sulfone (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) 3SL: 3-sulfolene (manufactured by Tokyo Chemical Industry Co., Ltd.) AdN: adiponitrile (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) LiBF ４ : Lithium tetrafluoroborate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) LiFSI: Lithium bis(fluorosulfonyl)imide (manufactured by Kanto Chemical Co., Ltd.) SA: Succinamide (manufactured by Tokyo Chemical Industry Co., Ltd.)

[0074] <Evaluation Results> As is clear from the results of Examples 1 to 7, the molecular crystals obtained by the present production method, which includes a mechanical mixing step and a heating step, exhibited high ionic conductivity at both room temperature (25°C) and low temperature (-20°C). Among these, Examples 2 to 7, which used imide-based alkali metal salts as the alkali metal salts, exhibited high ionic conductivity, as shown in LiBF ４ Furthermore, Examples 2 to 4 and 7, which used the molecule represented by formula (1) or succinamide as the (A) organic molecule, showed higher ionic conductivity than Examples 1, 5, and 6, which used the molecule represented by formula (2).

[0075] In contrast, Comparative Example 1, which contained (A) organic molecules and alkali metal salts as constituent units but was only heated without mechanical mixing, and Comparative Examples 2 and 3, which were not heated after mechanical mixing, showed lower ionic conductivities at 25°C and -20°C than Examples 1 to 7.

[0076] From the above results, it has become clear that (A) by using a method for producing a molecular crystal having organic molecules and alkali metal salts as constituent units, which includes a mechanical mixing step and a heating step, the resulting molecular crystal exhibits high ionic conductivity at room temperature and is less likely to decrease in ionic conductivity even at low temperatures.

[0077] The present invention is not limited to the above-described embodiments, and encompasses various modifications and equivalent modifications within the scope of the spirit of the present invention. Therefore, in light of the above teachings, various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are to be understood as falling within the scope and spirit of the present invention.

[0078] 10... Press mold

Claims

1. A method for producing molecular crystals, comprising: a step of mixing an organic molecule having at least one atom selected from the group consisting of a sulfur atom, an oxygen atom, a nitrogen atom, and a phosphorus atom with an alkali metal salt while applying mechanical energy to obtain a mixture; and a step of heating the mixture.

2. The method for producing molecular crystals according to claim 1, wherein the mixture is heated to a temperature equal to or higher than the melting point of the mixture.

3. The method for producing a molecular crystal according to claim 1, wherein the total number of carbon atoms per molecule of said organic molecule is 6 or less.

4. The method for producing a molecular crystal according to claim 1, wherein the organic molecule is at least one selected from the group consisting of sulfones, nitriles and amides.

5. The method for producing a molecular crystal according to claim 1, wherein the alkali metal salt comprises an imide-type alkali metal salt.

6. A method for producing a solid electrolyte, comprising the steps of: producing a solid electrolyte using the molecular crystal obtained by the method according to any one of claims 1 to 5.

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