Molecular crystal, solid electrolyte, and power storage device

Abstract

A molecular crystal includes: an organic molecule having an oxygen atom and at least one type of atom selected from the group consisting of nitrogen atoms, sulfur atoms and phosphorus atoms; and an alkali metal salt. The molecular crystal has a melting point of at least 80°C.

Description

Molecular crystals, solid electrolytes and energy storage devices

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims priority to Japanese Patent Application No. 2023-202025, filed on November 29, 2023, the entire contents of which are incorporated herein by reference. The present disclosure relates to molecular crystals, solid electrolytes, and electricity storage devices.

[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] For example, solid electrolytes used in on-board energy storage devices are required to have high heat resistance so that they do not melt even at high temperatures (e.g., 80°C or higher) in order to maintain their solid electrolyte state (i.e., solid state) under various environments. However, conventional crystalline organic solid electrolytes have insufficient heat resistance and may melt at high temperatures. In this case, the electrolyte cannot maintain its solid state in the energy storage device, raising concerns that it may leak from the device or cause short circuits inside the battery due to overcharging or overdischarging.

[0007] The present disclosure has been made in view of the above circumstances, and has as one object to provide a molecular crystal that exhibits high ionic conductivity while maintaining a solid state at high temperatures, and another object to provide a solid electrolyte that exhibits high ionic conductivity while maintaining a solid state at high temperatures, and an electricity storage device including the solid electrolyte.

[0008] As a result of intensive research aimed at solving the above-mentioned problems, the present inventors have found that a molecular crystal containing a specific organic molecule and an alkali metal salt as structural units in a crystal lattice and having a melting point equal to or higher than a predetermined temperature exhibits high ionic conductivity while maintaining a solid state at high temperatures. The present disclosure provides the following molecular crystal, solid electrolyte, and electricity storage device.

[0009] [1] A molecular crystal comprising an organic molecule having at least one atom selected from the group consisting of nitrogen atoms, sulfur atoms, and phosphorus atoms, and an oxygen atom, and an alkali metal salt, and having a melting point of 80° C. or higher. [2] The organic molecule has a polar group containing at least one atom selected from the group consisting of nitrogen atoms, sulfur atoms, and phosphorus atoms, and an oxygen atom. [3] The molecular crystal according to [1] or [2], wherein the organic molecule comprises a compound having two or more amide groups. [4] An ionic conductivity at 25° C. of 1.0×10 －６ [5] The molecular crystal according to any one of [1] to [3], having an ionic conductivity of 1.0 × 10 S / cm or more at −20° C. －８[1] The molecular crystal of any one of [1] to [4], wherein the alkali metal salt comprises an imide-based alkali metal salt. [2] A solid electrolyte comprising the molecular crystal of any one of [1] to [6]. [3] An electricity storage device comprising the solid electrolyte of [2].

[0010] According to the present disclosure, it is possible to obtain molecular crystals and solid electrolytes that exhibit high ionic conductivity while maintaining a solid state at high temperatures. Furthermore, by using the solid electrolyte of the present disclosure as an electrolyte for an electricity storage device such as a secondary battery or a capacitor, it is possible to obtain an electricity storage device that combines the safety ensured by the solidification of the electrolyte with ionic conductivity.

[0011] FIG. 1 is a schematic diagram of a press die used to prepare the sample pellets.

[0012] The molecular crystal, solid electrolyte, and electricity storage device of the present disclosure will be described in detail below.

[0013] Molecular Crystal: A molecular crystal is a substance in which structural units are regularly arranged in a crystal lattice. The molecular crystal of the present disclosure (hereinafter also referred to as "the present molecular crystal") is a crystalline organic substance containing, as structural units of the crystal, an organic molecule (hereinafter also referred to as "organic molecule (M)") having at least one atom selected from the group consisting of nitrogen atoms, sulfur atoms, and phosphorus atoms, and an oxygen atom, and an alkali metal salt. The present molecular crystal has ion conduction paths due to the regular arrangement of structural units, thereby exhibiting excellent ion conductivity.

[0014] The ratio of the organic molecule (M) to the alkali metal salt contained in the molecular crystal is not particularly limited. The ratio of the organic molecule (M) to the alkali metal salt in the molecular crystal may be 0.1 to 10 moles of the organic molecule (M) per mole of the alkali metal salt. From the viewpoint of increasing ionic conductivity, the ratio of the organic molecule (M) to the alkali metal salt in the molecular crystal is preferably 0.2 to 5.0 moles, more preferably 0.25 to 4.0 moles, and even more preferably 0.5 to 2.0 moles, of the organic molecule (M) per mole of the alkali metal salt.

[0015] The molecular crystal has a melting point of 80°C or higher. If the melting point of the molecular crystal is lower than 80°C, the heat resistance may be insufficient, making it difficult to apply the molecular crystal to, for example, in-vehicle applications. From the viewpoint of making the molecular crystal applicable to applications requiring heat resistance, the melting point of the molecular crystal is preferably 82°C or higher, more preferably 85°C or higher, even more preferably 87°C or higher, and even more preferably 90°C or higher. There is no particular limitation on the upper limit of the melting point of the molecular crystal. The melting point of the molecular crystal is, for example, 300°C or lower. In order to further increase the ionic conductivity of the molecular crystal, the melting point of the molecular crystal is preferably 250°C or lower, more preferably 200°C or lower, and even more preferably 150°C or lower. In this specification, the melting point of the molecular crystal is a value under atmospheric pressure, obtained by differential scanning calorimetry (DSC). Details of the measurement method follow the method described in the Examples below.

[0016] The melting point of the present molecular crystal is not limited to 80°C or higher, and the melting point of either or both of the organic molecule (M) and the alkali metal salt constituting the present molecular crystal may be lower than 80°C.

[0017] The organic molecule (M) and alkali metal salt constituting the present molecular crystal are not particularly limited as long as they can produce a molecular crystal having a melting point of 80° C. or higher. The organic molecule (M) and alkali metal salt constituting the present molecular crystal are described in detail below.

[0018] <Organic Molecule (M)> The organic molecule (M) has at least one atom selected from the group consisting of nitrogen, sulfur, and phosphorus atoms (hereinafter also referred to as the "specific heteroatom"), and an oxygen atom. The specific heteroatom and oxygen atom in the organic molecules that make up the molecular crystal increase the polarity of the organic molecules, which is thought to suppress volatilization and sublimation of the organic molecules during production of the molecular crystal, resulting in a molecular crystal with little variation in composition. Furthermore, the suppression of variation in the molecular crystal composition is thought to form uniform ion conduction paths in the molecular crystal, thereby resulting in a molecular crystal that exhibits excellent ion conductivity.

[0019] The number of specific heteroatoms and oxygen atoms in one molecule of the organic molecule (M) may each be one or more. From the viewpoint of obtaining a molecular crystal with excellent ion conductivity, the number of specific heteroatoms in the organic molecule (M) is preferably two or more. For the same reason, the number of oxygen atoms in the organic molecule (M) is preferably two or more. From the viewpoint of further increasing the polarity of the organic molecule (M) and improving ion conductivity, the organic molecule (M) preferably has a total of three or more specific heteroatoms and oxygen atoms in one molecule, and more preferably has a total of four or more specific heteroatoms and oxygen atoms.

[0020] In order to obtain a molecular crystal with excellent ion conductivity, it is preferable that the organic molecule (M) has a functional group containing a specific heteroatom and an oxygen atom (hereinafter, simply referred to as a "polar group") in the molecule. Examples of the polar group include an amide group (-CO-NR １ R ２ , -NR １ -CO-R ３ ), phosphate group (-O-P(=O)(-OR １ ) ２ ), a phosphonic acid group (-P(=O)(-OR １ ) ２ ), sulfonic acid group, sulfonyl group, sulfonyloxy group, etc. １ and R ２ are each independently a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group, an aryl group, or an aralkyl group, and R ３ is an alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group having 1 to 6 carbon atoms. １ may be the same or different.

[0021] From the viewpoint of the ionic conductivity of the molecular crystal and the availability of the material, the organic molecule (M) preferably has an amide group, a phosphonic acid group, or a sulfonyloxy group as a polar group, and more preferably has an amide group.

[0022] When a compound having a polar group containing a specific heteroatom and an oxygen atom is used as the organic molecule (M), the number of polar groups in one molecule of the compound is not particularly limited, as long as it is at least 1. From the viewpoint of promoting crystallization of a mixture of the organic molecule (M) and an alkali metal salt, the number of polar groups in one molecule of the organic molecule (M) is preferably 1 to 10, more preferably 2 to 6, and even more preferably 2 to 4.

[0023] From the viewpoint of increasing the polarity of the organic molecule (M) and thereby obtaining a molecular crystal with excellent ionic conductivity, the organic molecule (M) preferably contains one or more compounds selected from the group consisting of a compound having two or more amide groups in one molecule, a compound having two or more phosphonic acid groups in one molecule, and a compound having one or more sulfonyloxy groups in one molecule, and more preferably contains a compound having two or more amide groups in one molecule.

[0024] The structure of the organic molecule (M) is not particularly limited. The organic molecule (M) may be a molecule having a chain structure or a cyclic structure. The molecular weight of the organic molecule (M) is, for example, 600 or less. From the viewpoint of promoting crystallization of the organic molecule (M) and the alkali metal salt, the molecular weight of the organic molecule (M) is preferably 500 or less, and more preferably 450 or less. The lower limit of the molecular weight of the organic molecule (M) is, for example, 30 or more, preferably 50 or more, more preferably 80 or more, and even more preferably 90 or more.

[0025] Specific examples of the organic molecule (M) include compounds having a nitrogen atom and an oxygen atom, such as malonamide, succinamide, glutaramide, adipamide, N,N,N',N'-tetramethylmalonamide, N,N,N',N'-tetraethylmalonamide, N,N,N',N'-tetramethylsuccinamide, terephthalamide, N,N'-diacetylethylenediamine, and N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid).

[0026] Examples of compounds having a sulfur atom and an oxygen atom include 1,3-propane sultone and 1,4-butane sultone.

[0027] Examples of compounds having a phosphorus atom and an oxygen atom include tetraethyl ethylenediphosphonate and N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid). N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid) is a compound having a nitrogen atom as a specific heteroatom, and also a compound having a phosphorus atom. As the organic molecule (M), one type may be used alone, or two or more types may be used in combination, as long as they cause crystallization together with an alkali metal salt.

[0028] <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.

[0029] Specific examples of alkali metal salts include LiCO ３ , LiBr, LiCl, LiI, LiSCN, LiBF ４ , LiAsF ６ , LiClO ４ , C.H. ３ COOLi, CF ３ COOLi, 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, in that they have high ionic dissociation properties and can further increase the ionic conductivity of the present molecular crystal.

[0030] In terms of being able to further increase the ionic conductivity of the present molecular crystal, the alkali metal salt constituting the present molecular crystal preferably includes an imide-based alkali metal salt. In terms of high ionic dissociation, the imide-based alkali metal salt is preferably an imide-based lithium salt, and among the imide-based lithium salts, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, or lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide is particularly preferred, lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide is more preferred, and lithium bis(fluorosulfonyl)imide is even more preferred.

[0031] 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.

[0032] The melting point of the alkali metal salt under atmospheric pressure 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.

[0033] The present molecular crystal may further contain a structural unit different from the organic molecule (M) and the alkali metal salt (e.g., an organic molecule having a specific heteroatom and no oxygen atom) within a range that does not impair the effects of the present disclosure. However, in consideration of the ionic conductivity and ease of production of the present molecular crystal, the present molecular crystal is preferably a molecular crystal composed of the organic molecule (M) and the alkali metal salt.

[0034] This molecular crystal exhibits high ionic conductivity not only at room temperature (25°C) but also at temperatures lower than room temperature (for example, 0°C or lower) and higher than room temperature (for example, 80°C or higher). Specifically, for a solid electrolyte having a thickness of about 200 μm (specifically, 200±20 μm), the ionic conductivity measured at 25°C using an AC impedance method is 1.0×10 －６ From the viewpoint of obtaining an electricity storage device with excellent performance, the ionic conductivity under the same conditions is preferably 1.5×10 S / cm or more. －６ S / cm or more, and more preferably 2.0×10 －６ 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.

[0035] 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 1.0×10 －８ S / cm or more, and －８ S / cm or more, and more preferably 1.0×10 －７ It is more preferable that the viscosity is 25 S / cm or more.

[0036] Furthermore, the ionic conductivity of a solid electrolyte having a thickness of about 200 μm (specifically, 200±20 μm) measured at 80° C. using an AC impedance method is 1.0×10 －５ S / cm or more, and preferably 5.0×10 －５ S / cm or more, and more preferably 1.0×10 －４ It is more preferable that the viscosity is 25 S / cm or more.

[0037] <Method for Producing Molecular Crystal> The present molecular crystal can be produced using an organic molecule (M) and an alkali metal salt as raw materials. The method for producing the present molecular crystal is not particularly limited. The present molecular crystal can be produced, for example, by a method (hereinafter also referred to as a "first production method") including a step of mixing the organic molecule (M) and an alkali metal salt while applying mechanical energy (hereinafter also referred to as a "mechanical mixing step"); a method (hereinafter also referred to as a "second production method") including a step of dissolving the organic molecule (M) and the alkali metal salt in a solvent (hereinafter also referred to as a "dissolving step"), and a step of removing the solvent from the solution in which the organic molecule (M) and the alkali metal salt are dissolved in the solvent (hereinafter also referred to as a "removing step"); or the like.

[0038] (First Production Method) Mechanical Mixing Step In the mechanical mixing step, the organic molecules (M) and the alkali metal salt are mixed while applying mechanical energy to a substance containing the organic molecules (M) and the alkali metal salt by means of impact, shear, compression, friction, etc. (hereinafter also referred to as "mechanical mixing") This preferably causes changes in the physicochemical properties of the organic molecules (M), the alkali metal salt, or both.

[0039] The compounding ratio of the organic molecule (M) to the alkali metal salt is not particularly limited and may be set depending on the types of the organic molecule (M) and the alkali metal salt. The compounding ratio of the organic molecule (M) to the alkali metal salt may be, for example, 0.1 to 10 moles of the organic molecule (M) per mole of the alkali metal salt. The compounding ratio of the organic molecule (M) to the alkali metal salt is preferably 0.2 to 5.0 moles, more preferably 0.25 to 4.0 moles, and even more preferably 0.5 to 2.0 moles, of the organic molecule (M) per mole of the alkali metal salt, in order to obtain a molecular crystal exhibiting excellent ionic conductivity.

[0040] The method for mixing the organic molecule (M) and the alkali metal salt while applying mechanical energy is not particularly limited. In the mechanical mixing step, the organic molecule (M) 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 organic molecule (M) and the alkali metal salt can also be mechanically mixed using a mortar and pestle. The mechanical mixing of the organic molecule (M) and the alkali metal salt can be performed in a dry or wet manner. Furthermore, the mechanical mixing of the organic molecule (M) and the alkali metal salt can be performed at room temperature or at a low temperature. For example, when the melting point of the organic molecule (M) is lower than room temperature, the organic molecule in a liquid state can be mechanically mixed with the alkali metal salt. Furthermore, the organic molecule in a solid state can be mechanically mixed with the alkali metal salt at a temperature lower than the melting point of the organic molecule (M).

[0041] The organic molecules (M) and the alkali metal salt may be mechanically mixed while being heated. However, in order to suppress volatilization or sublimation of the organic molecules (M), the heating in the mechanical mixing step is preferably carried out at a temperature lower than the heating temperature in the heating step optionally carried out after the mechanical mixing step. Specifically, in the mechanical mixing step, the organic molecules (M) and the alkali metal salt are mechanically mixed at, for example, 50°C or lower. The temperature when mechanically mixing the organic molecules (M) and the alkali metal salt is preferably 40°C or lower, more preferably 35°C or lower. The time for mechanical mixing varies depending on the amount and type of the organic molecules (M) and the alkali metal salt used, but may be, for example, 1 to 60 minutes, or 1 to 30 minutes.

[0042] When producing a molecular crystal by the first production method, the first production method preferably further includes a step of heating the mixture obtained by the mechanical mixing step (i.e., a mixture of organic molecules (M) and an alkali metal salt) (hereinafter also referred to as the "heating step"). This heating step can make the organic molecules (M) and the alkali metal salt in the mixture more uniform, thereby further increasing the ionic conductivity of the molecular crystal. In particular, a method that combines the mechanical mixing step and the heating step is thought to generate intermolecular interactions between the alkali metal ions derived from the alkali metal salt and the organic molecules (M) due to the mechanical mixing, which makes it possible to suppress volatilization or sublimation of the organic molecules (M) during heat treatment. This has the advantage of making it easier to obtain a molecular crystal of the desired composition.

[0043] Heating Step In the heating step, the temperature and method for heating the mixture are not particularly limited. The heating temperature of the mixture may be appropriately set depending on the types of organic molecules (M) and alkali metal salt used as raw materials. From the viewpoint of making the organic molecules (M) and alkali metal salt in the mixture more uniform, the heating temperature is, for example, 40°C or higher, preferably 50°C or higher, and more preferably 60°C or higher. The heating time is not particularly limited as long as it is long enough to homogenize the organic molecules (M) and alkali metal salt in the mixture. The heating time is, for example, 1 minute to 3 hours, and preferably 3 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.

[0044] When the mixture is heated, it is preferable to heat the mixture while stirring it, thereby further homogenizing the organic molecule (M) 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.

[0045] After heating a mixture containing an organic molecule (M) 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., by 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). From the viewpoint of favorable crystallization of the heated mixture, gradually lowering the temperature of the mixture is preferable. The fact that the product obtained by this production method is a molecular crystal containing an organic molecule (M) and an alkali metal salt can be confirmed by powder X-ray diffraction measurement of the product. Details of the measurement method follow those described in the Examples below.

[0046] (Second Manufacturing Method) - Dissolution Step When the melting point of the organic molecule (M) is high (e.g., 100°C or higher), it is preferable to apply the second manufacturing method as a method for manufacturing the molecular crystal. An organic solvent can be preferably used as a solvent for dissolving the organic molecule (M) and the alkali metal salt. The organic solvent is preferably a solvent that can dissolve the organic molecule (M) and the alkali metal salt and has a boiling point lower than the boiling points of the organic molecule (M) and the alkali metal salt, and more preferably a solvent with a relatively low dielectric constant. Examples of such organic solvents include dimethyl carbonate, tetrahydrofuran, and ethyl acetate. When dissolving the organic molecule (M) and the alkali metal salt in a solvent, it is preferable to stir the mixture to homogenize the organic molecule (M) and the alkali metal salt. The stirring method is not particularly limited, and the stirring methods exemplified in the first manufacturing method can be used as appropriate.

[0047] Removal Step The method for removing the solvent from the solution in which the organic molecule (M) and the alkali metal salt are dissolved is not particularly limited as long as it can remove the solvent from the solution. As a method for removing the solvent, heat treatment is preferred because it can remove the solvent simply and efficiently. The heat treatment may be carried out under normal pressure, but may also be carried out under increased pressure or reduced pressure. When a solvent with a high boiling point is used in the dissolving step, it is preferable to carry out the heat treatment under reduced pressure in order to suppress decomposition of the organic molecule (M) and the alkali metal salt.

[0048] The method for producing the present molecular crystal is not limited to the above. For example, the present molecular crystal may be produced by a method in which the organic molecule (M) and the alkali metal salt are heated and mixed without mechanical mixing in the first production method described above.

[0049] <Solid Electrolyte> The solid electrolyte of the present disclosure contains the above-described molecular crystal. The solid electrolyte of the present disclosure can be obtained by molding an electrolyte material containing the molecular crystal 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, it is preferable to obtain a molded body by pressure molding, since this can minimize the porosity of the obtained 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.

[0050] The solid electrolyte of the present disclosure may further contain components other than the present molecular crystal, as long as the effects of the present disclosure are not impaired. Examples of other components include molecular crystals other than the present molecular crystal, polymer solid electrolytes, polymer gel electrolytes, inorganic electrolytes (e.g., ceramic electrolytes, glass electrolytes, etc.), binders, etc.

[0051] The content of the present molecular crystal 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, from the viewpoint of obtaining a solid electrolyte having excellent ionic conductivity, particularly at high temperatures.

[0052] The solid electrolyte of the present disclosure exhibits high ionic conductivity while maintaining a solid state not only at room temperature but also at high temperatures of 80° C. or higher. Therefore, by using the solid electrolyte of the present disclosure as the electrolyte of an electricity storage device, it is possible to obtain an electricity storage device that is applicable to applications expected to be used at high temperatures (for example, in-vehicle applications) and has high ionic conductivity.

[0053] <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. Specific embodiments of the device include a secondary battery, a capacitor, and the like. When the device is a secondary battery, one embodiment is an all-solid-state battery, and a lithium-ion secondary battery is preferred in terms of excellent ionic conductivity.

[0054] 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.

[0055] In the lithium-ion secondary battery of the present disclosure, the solid electrolyte is formed using a molecular crystal containing an organic molecule (M) 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] <<Production and Evaluation of Molecular Crystals>> [Example 1] 1. Production of Molecular Crystals In a glove box under an argon atmosphere, 38 parts of succinamide (manufactured by Tokyo Chemical Industry Co., Ltd.) as an organic molecule and 62 parts of lithium bis(fluorosulfonyl)imide (manufactured by Kanto Chemical Co., 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 15 minutes to obtain a mixture. Subsequently, the mixture obtained by mechanically mixing the raw materials was transferred to a vial and stirred for 10 minutes with a magnetic stirrer set at a temperature of 80 ° C. After that, it was allowed to cool to room temperature to obtain a white solid. The melting point of the white solid 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 97 ° C.

[0061] 2. Evaluation (Confirmation of Crystallinity by Powder X-ray Diffraction Measurement) Powder X-ray diffraction measurement was performed on the obtained white solid 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 = 5-55 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, this 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 that of the organic molecule and alkali metal salt used as raw materials is obtained, it can be determined that molecular crystals composed of organic molecules and alkali metal salts have been produced. Powder X-ray diffraction measurement was performed on the white solid obtained as described above, and an X-ray diffraction spectrum having a sharp peak at an angle within the measurement range and different from that of the raw material was observed. This confirmed that the obtained white solid was crystalline, that is, a molecular crystal composed of organic molecules and alkali metal salts.

[0062] (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 Figure 1), sample pellets were 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. Next, in a glove box under an argon atmosphere, the sample pellet was encapsulated in an all-solid-state battery evaluation cell (KP-SolidCell, manufactured by Hosen Co., Ltd.). Using the all-solid-state battery evaluation cell encapsulating the sample pellet, ionic conductivity was measured in the atmosphere. To measure ionic conductivity, the all-solid-state battery evaluation cell encapsulating the sample pellet was first heat-treated in a thermostatic chamber at 80°C (corresponding to the heat treatment temperature in Table 1), while measuring the resistance value by AC impedance measurement. The heat treatment was terminated when the resistance value became constant. Next, resistance values ​​at -20°C, 25°C, and 80°C were measured by AC impedance measurement. Using the obtained resistance values, ionic conductivity (σ) was calculated according to the following formula (1). The AC impedance measurement for calculating 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 formula (1), σ is ionic conductivity (unit: S / cm), R is 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 −20° C., 25° C., and 80° C. were 3.4×10 －５ S / cm, 8.1×10 －４ S / cm, 1.8×10 －３ The molecular crystals obtained maintained a solid state even at a temperature of 80°C.

[0063] Examples 2 to 6 Molecular crystals were obtained in the same manner as in Example 1, except that the types and amounts of raw materials and the heat treatment temperature for ionic conductivity measurement were changed as shown in Table 1. Differential scanning calorimetry and ionic conductivity measurements at −20° C., 25° C., and 80° C. were also performed on each molecular crystal. The results are shown in Table 1. Each molecular crystal maintained a solid state even at a temperature of 80° C.

[0064] Comparative Example 1 In a glove box under an argon atmosphere, 30 parts of succinonitrile (manufactured by Tokyo Chemical Industry Co., Ltd.) as an organic molecule and 70 parts of lithium bis(trifluoromethanesulfonyl)imide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: bis(trifluoromethanesulfonyl)imide lithium) as an alkali metal salt were weighed out so that the molar ratio was 3:2, and mechanically mixed using a mortar at room temperature (25°C) to obtain a white solid (molecular crystal). The melting point of the molecular crystal obtained by mechanically mixing the raw materials was determined by differential scanning calorimetry and was found to be 55°C. In addition, the ionic conductivity of the molecular crystal obtained by mechanically mixing the raw materials was measured at -20°C, 25°C, and 80°C in the same manner as in Example 1, except that the heat treatment temperature for the ionic conductivity measurement was 40°C. The results are shown in Table 1. In Comparative Example 1, the molecular crystal melted at 80°C, making it impossible to evaluate its performance as a solid electrolyte.

[0065]

[0066] Details of the compounds listed in Table 1 are shown below. SA: succinamide [manufactured by Tokyo Chemical Industry Co., Ltd.] MA: malonamide [manufactured by Tokyo Chemical Industry Co., Ltd.] TMMA: N,N,N',N'-tetramethylmalonamide [manufactured by Tokyo Chemical Industry Co., Ltd.] TPA: terephthalamide [manufactured by Tokyo Chemical Industry Co., Ltd.] DAED: N,N'-diacetylethylenediamine [manufactured by Tokyo Chemical Industry Co., Ltd.] SN: succinonitrile [manufactured by Tokyo Chemical Industry Co., Ltd.] LiFSI: lithium bis(fluorosulfonyl)imide [manufactured by Kanto Chemical Co., Ltd.] LiTFSI: lithium bis(trifluoromethanesulfonyl)imide [manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: bis(trifluoromethanesulfonyl)imide lithium]

[0067] <Evaluation Results> As is clear from the results of Examples 1 to 6, molecular crystals containing organic molecules having specific heteroatoms and oxygen atoms and alkali metal salts as structural units and having melting points of 80°C or higher exhibited high ionic conductivity at both room temperature (25°C) and high temperature (80°C). In addition, the ionic conductivity of the molecular crystals of Examples 1 to 6 at low temperature (-20°C) was 3.5 x 10 －８ S / cm or more, showing sufficiently high ionic conductivity even at low temperatures.

[0068] In contrast, in Comparative Example 1, in which the solid electrolyte was prepared using molecular crystals with a melting point of 55° C., the solid electrolyte melted at a high temperature (80° C.) and could not maintain a solid state at high temperatures.

[0069] From the above results, it has become clear that a molecular crystal containing an organic molecule having a specific heteroatom and an oxygen atom and an alkali metal salt as constituent units and having a melting point of 80°C or higher exhibits high ionic conductivity while maintaining a solid state at high temperatures.

[0070] 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.

[0071] 10... Press mold

Claims

1. A molecular crystal comprising: an organic molecule having at least one atom selected from the group consisting of nitrogen atoms, sulfur atoms, and phosphorus atoms, and an oxygen atom; and an alkali metal salt; and having a melting point of 80°C or higher.

2. The molecular crystal according to claim 1, wherein the organic molecule has a polar group containing an oxygen atom and at least one atom selected from the group consisting of a nitrogen atom, a sulfur atom and a phosphorus atom.

3. The molecular crystal according to claim 1, wherein the organic molecule comprises a compound having two or more amide groups.

4. Ion conductivity at 25°C is 1.0 x 10 －６ The molecular crystal according to claim 1, wherein the molecular crystal has a viscosity of 1.0 S / cm or more.

5. Ion conductivity at -20°C is 1.0 x 10 －８ The molecular crystal according to claim 1, wherein the molecular crystal has a viscosity of 1.0 S / cm or more.

6. The molecular crystal of claim 1, wherein the alkali metal salt comprises an imide-based alkali metal salt.

7. A solid electrolyte comprising the molecular crystal according to any one of claims 1 to 6.

8. An electricity storage device comprising the solid electrolyte according to claim 7.

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