Molecular crystal, solid electrolyte, and power storage device

Abstract

The present invention provides a molecular crystal comprising an organic molecule and an alkali metal salt, wherein the organic molecule has at least one atom selected from nitrogen, sulfur and phosphorus, and an oxygen atom, and the molecular crystal has a melting point of 80°C or higher.

Description

Technical Field

[0001] [Cross-reference to related applications]

[0002] This application claims priority based on Japanese Patent Application No. 2023-202025, filed on November 29, 2023, the entirety of which is incorporated herein by reference.

[0003] This invention relates to molecular crystals, solid electrolytes, and energy storage devices. Background Technology

[0004] As energy storage devices, various rechargeable batteries, such as nickel-metal hydride batteries and lithium-ion batteries, as well as double-layer capacitors, have been put into practical use. Among them, lithium-ion batteries are widely used due to their high energy density and high battery capacity. In addition, in recent years, sodium-ion batteries, potassium-ion batteries, and magnesium-ion batteries, which use lithium as a substitute for lithium, a rare metal, have attracted attention as alternatives to lithium-ion batteries.

[0005] Lithium-ion secondary batteries, widely used as energy storage devices, have a negative electrode, a positive electrode, and an electrolyte. They are charged and discharged by allowing lithium ions to move between the two electrodes via the electrolyte. Historically, organic electrolytes have been the primary type used. However, in recent years, as a technology to eliminate the risks of electrolyte leakage, overcharging, and over-discharging causing internal short circuits, the use of solid or gel-like electrolytes made of inorganic or organic materials has been proposed to replace organic electrolytes.

[0006] Organic solid electrolytes have the advantage of excellent adhesion to electrodes due to the moderate softness and high formability of organic materials; however, on the other hand, they tend to have lower ionic conductivity compared to inorganic solid electrolytes. Therefore, in order to improve practicality, various studies have been conducted on the development of new materials constituting organic solid electrolytes (for example, see Non-Patent Document 1). In Non-Patent Document 1, a crystalline organic solid electrolyte is disclosed by forming a soft solid crystal containing lithium chloride and isoquinoline.

[0007] Existing technical documents

[0008] Non-patent literature

[0009] Non-patent literature 1: Ionics, 2018, Vol. 24, pp. 343-349 Summary of the Invention

[0010] The technical problem that the invention aims to solve

[0011] For example, for solid electrolytes used in automotive energy storage devices, high heat resistance is required to maintain the state of the substance as a solid electrolyte (i.e., a solid state) under various environments, even at high temperatures (e.g., above 80°C). However, conventional crystalline organic solid electrolytes have insufficient heat resistance and sometimes melt at high temperatures. In this case, it is worrying that the electrolyte cannot remain in a solid state in the energy storage device, and the risk of internal short circuits in the battery caused by electrolyte leakage from the device, overcharging, and over-discharging cannot be eliminated.

[0012] The present invention was made in view of the above circumstances, and one of its objectives is to provide a molecular crystal that maintains a solid state at high temperatures and exhibits high ionic conductivity. Another objective is to provide a solid electrolyte that maintains a solid state at high temperatures and exhibits high ionic conductivity, and an energy storage device comprising the solid electrolyte.

[0013] Technical solutions for solving technical problems

[0014] To solve the aforementioned technical problems, the inventors conducted in-depth research and discovered that molecular crystals containing specific organic molecules and alkali metal salts as structural units in the crystal lattice, and with melting points above a specified temperature, maintain a solid state at high temperatures while exhibiting high ionic conductivity. According to the present invention, the following molecular crystals, solid electrolytes, and energy storage devices are provided.

[0015] [1] A molecular crystal comprising an organic molecule and an alkali metal salt, wherein the organic molecule has at least one atom selected from nitrogen, sulfur and phosphorus, and an oxygen atom, and the molecular crystal has a melting point of 80°C or higher.

[0016] [2] According to the molecular crystal described in [1], the organic molecule has a polar group, which includes at least one atom selected from nitrogen, sulfur and phosphorus, and an oxygen atom.

[0017] [3] According to the molecular crystal described in [1] or [2], wherein the organic molecule comprises a compound having two or more amide groups.

[0018] [4] The molecular crystal described in any one of [1] to [3], wherein the molecular crystal has an ionic conductivity of 1.0 × 10⁻⁶ at 25 °C. -6 S / cm or higher.

[0019] [5] The molecular crystal described in any one of [1] to [4], wherein the molecular crystal has an ionic conductivity of 1.0 × 10⁻⁶ at -20 °C. -8 S / cm or higher.

[0020] [6] The molecular crystal described in any one of [1] to [5], wherein the alkali metal salt comprises an imide-based alkali metal salt.

[0021] [7] A solid electrolyte containing a molecular crystal described in any one of [1] to [6].

[0022] [8] An energy storage device having the solid electrolyte described in [7].

[0023] Invention Effects

[0024] According to the present invention, molecular crystals and solid electrolytes that maintain a solid state at high temperatures and exhibit high ionic conductivity can be obtained. Furthermore, by using the solid electrolyte of the present invention as the electrolyte for energy storage devices such as secondary batteries and capacitors, energy storage devices that combine the safety ensured by the solidification of the electrolyte with high ionic conductivity can be obtained. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the mold used in the preparation of sample particles. Detailed Implementation

[0026] The molecular crystal, solid electrolyte, and energy storage device of the present invention will be described in detail below.

[0027] Molecular Crystals

[0028] Molecular crystals are substances formed by the regular arrangement of structural units in a crystal lattice. The molecular crystal of the present invention (hereinafter also referred to as "this molecular crystal") is a crystalline organic compound comprising organic molecules (hereinafter also referred to as "organic molecules (M)") and alkali metal salts as structural units of the crystal, wherein the organic molecules have at least one atom selected from nitrogen, sulfur, and phosphorus atoms and an oxygen atom. This molecular crystal has ion conduction pathways through the regular arrangement of structural units, thereby exhibiting excellent ion conductivity.

[0029] The ratio of organic molecules (M) to alkali metal salts in this molecular crystal is not particularly limited. Regarding the ratio of organic molecules (M) to alkali metal salts in the molecular crystal, the amount of organic molecules (M) can be 0.1 to 10 moles relative to 1 mole of alkali metal salt. From the viewpoint of improving ionic conductivity, the ratio of organic molecules (M) to alkali metal salts in the molecular crystal is preferably 0.2 to 5.0 moles relative to 1 mole of alkali metal salt, more preferably 0.25 to 4.0 moles, and even more preferably 0.5 to 2.0 moles.

[0030] The melting point of this molecular crystal is 80°C or higher. If the melting point of the molecular crystal is lower than 80°C, its heat resistance is insufficient, for example, there are concerns about its applicability to automotive applications. From the viewpoint of being suitable for applications requiring heat resistance, the melting point of this 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 upper limit to the melting point of this molecular crystal. For example, the melting point of this molecular crystal is 300°C or lower. From the viewpoint of further improving the ionic conductivity of this molecular crystal, the melting point of this 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 at atmospheric pressure, obtained by differential scanning calorimetry (DSC). Details of the measurement method are as described in the examples below.

[0031] It should be noted that the melting point of this molecular crystal only needs to be above 80°C, while the melting point of one or both of the organic molecules (M) and alkali metal salts constituting this molecular crystal can be below 80°C.

[0032] There are no particular limitations on the organic molecules (M) and alkali metal salts constituting this molecular crystal, as long as they can produce molecular crystals with a melting point above 80°C. The following provides a detailed description of the organic molecules (M) and alkali metal salts constituting this molecular crystal.

[0033] <Organic Molecules (M)>

[0034] The organic molecule (M) has at least one atom selected from nitrogen, sulfur, and phosphorus (hereinafter also referred to as "specific heteroatoms"), and an oxygen atom. It is believed that by having specific heteroatoms and oxygen atoms in the organic molecules constituting the molecular crystal, the polarity of the organic molecules is increased, thereby suppressing the volatilization and sublimation of organic molecules during the manufacture of the molecular crystal, resulting in a molecular crystal with low compositional inhomogeneity. Furthermore, it is believed that by suppressing the compositional inhomogeneity of the molecular crystal, a uniform ion conduction pathway is formed within the molecular crystal, thereby obtaining a molecular crystal exhibiting excellent ion conductivity.

[0035] The number of specific heteroatoms and oxygen atoms per molecule of an organic molecule (M) is one or more. From the viewpoint of obtaining a molecular crystal with excellent ionic conductivity, the number of specific heteroatoms in the organic molecule (M) is preferably two or more. Similarly, for the same reason, the number of oxygen atoms in the organic molecule (M) is preferably two or more. From the viewpoint of further improving the polarity and ionic conductivity of the organic molecule (M), the organic molecule (M) preferably has a total of three or more specific heteroatoms and oxygen atoms per molecule, more preferably four or more.

[0036] From the perspective of obtaining molecular crystals with superior ionic conductivity, organic molecules (M) preferably have functional groups (hereinafter also simply referred to as "polar groups") containing specific heteroatoms and oxygen atoms within the molecule. Examples of polar groups include, for instance, amide groups (-CO-NR). 1 R 2 -NR 1 -CO-R 3 ), phosphate group (-OP (=O) (-OR) 1 2) Phosphonic acid group (-P (=O) (-OR) 1 2) Sulfonic acid group, sulfonyl group, sulfonyloxy group, etc. Here, R... 1 and R 2 R is independently composed of hydrogen atoms or alkyl, cycloalkyl, aryl, or aralkyl groups having 1 to 6 carbon atoms. 3 It is an alkyl, cycloalkyl, aryl, or aralkyl group having 1 to 6 carbon atoms. Two Rs in one functional group. 1 They can be the same or different.

[0037] From the viewpoint of the ionic conductivity of molecular crystals and the ease of obtaining materials, organic molecules (M) preferably have amide groups, phosphonic acid groups or sulfonyloxy groups as polar groups, and more preferably have amide groups.

[0038] When using a compound having polar groups containing specific heteroatoms and oxygen atoms as the organic molecule (M), the number of polar groups within one molecule of the compound is not particularly limited, as long as it is one or more. From the viewpoint of promoting the crystallization of the mixture of organic molecule (M) and alkali metal salt, the number of polar groups within one molecule of organic molecule (M) is preferably 1 to 10, more preferably 2 to 6, and even more preferably 2 to 4.

[0039] From the viewpoint of increasing the polarity of organic molecules (M) and thereby obtaining molecular crystals with excellent ion conductivity, organic molecules (M) preferably include one or more compounds selected from compounds having two or more amide groups per molecule, compounds having two or more phosphonic acid groups per molecule, and compounds having one or more sulfonyloxy groups per molecule, and more preferably compounds having two or more amide groups per molecule.

[0040] The structure of the organic molecule (M) is not particularly limited. The organic molecule (M) can be a molecule composed of a chain structure or a molecule with a ring structure. The molecular weight of the organic molecule (M) is, for example, 600 or less. From the viewpoint of promoting the crystallization of the organic molecule (M) with the alkali metal salt, the molecular weight of the organic molecule (M) is preferably 500 or less, more preferably 450 or less. Regarding the lower limit of the molecular weight of the organic molecule (M), it is, for example, 30 or more, preferably 50 or more, more preferably 80 or more, and even more preferably 90 or more.

[0041] Specific examples of organic molecules (M), and compounds having nitrogen and oxygen atoms, include 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'-ethylenediaminetetra(methylenephosphonic acid), etc.

[0042] Examples of compounds containing sulfur and oxygen atoms include 1,3-propanesulfonyl lactone and 1,4-butanesulfonyl lactone.

[0043] Examples of compounds containing both phosphorus and oxygen atoms include tetraethyl ethylene diphosphonate and N,N,N',N'-ethylenediaminetetra(methylenephosphonic acid). It should be noted that N,N,N',N'-ethylenediaminetetra(methylenephosphonic acid) is both a compound containing a nitrogen atom as a specific heteroatom and a compound containing a phosphorus atom. As an organic molecule (M), it can be used alone or in combination with two or more alkali metal salts, provided it can crystallize together.

[0044] Alkali metal salts

[0045] Alkali metal salts are any salts that produce alkali metal ions; there are no particular limitations. Examples of alkali metal salts include lithium salts, sodium salts, and potassium salts.

[0046] Specific examples of alkali metal salts include Li₂CO₃, LiBr, LiCl, LiI, LiSCN, LiBF₄, LiAsF₆, LiClO₄, CH₃COOLi, CF₃COOLi, LiCF₃SO₃, LiPF₆, LiC(CF₃SO₂)₃, and lithium bis(fluorosulfonyl)imide (Li + (FSO2)2N - ), Lithium bis(trifluoromethanesulfonyl)imide (Li + (CF3SO2)2N - Lithium salts such as (fluorosulfonyl) and (trifluoromethanesulfonyl)imide lithium; and salts of these lithium salts with anions of alkali metals other than lithium (e.g., sodium, potassium, etc.). Considering high ionic dissociation and the ability to further improve the ionic conductivity of the molecular crystal, lithium salts or sodium salts are preferred, with lithium salts being more preferred.

[0047] In order to further improve the ionic conductivity of the molecular crystal, the alkali metal salt constituting the molecular crystal preferably includes an imide-based alkali metal salt. From the perspective of high ionic dissociation, imide-based lithium salts are preferred among the imide-based alkali metal salts. Among the imide-based 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 are more preferred, and lithium bis(fluorosulfonyl)imide is even more preferred.

[0048] 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. Regarding the lower limit of the molecular weight of the alkali metal salt, it 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 can be used alone, or two or more can be used in combination.

[0049] The melting point of alkali metal salts at 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 alkali metal salts; for example, it can be below 300°C or below 250°C.

[0050] It should be noted that the molecular crystal may further include structural units different from those of the organic molecule (M) and the alkali metal salt (e.g., organic molecules having specific heteroatoms but no oxygen atoms) without impairing the effects of the present invention. However, considering the ionic conductivity and ease of manufacture of the molecular crystal, it is preferable that the molecular crystal is composed of an organic molecule (M) and an alkali metal salt.

[0051] This molecular crystal exhibits high ionic conductivity not only at room temperature (25°C), but also at temperatures below room temperature (e.g., below 0°C) and above room temperature (e.g., above 80°C). Specifically, for a solid electrolyte with a thickness of approximately 200 μm (more specifically, 200 ± 20 μm), the preferred ionic conductivity, measured by AC impedance spectroscopy at 25°C, is 1.0 × 10⁻⁶. -6 S / cm or higher. From the viewpoint of obtaining a high-performance energy storage device, an ionic conductivity of 1.5 × 10⁻⁶ is more preferable under the same conditions. -6 S / cm or higher, more preferably 2.0 × 10⁻⁶ -6 S / cm or higher. It should be noted that the details of the method for measuring ionic conductivity are as described in the examples below.

[0052] Furthermore, for solid electrolytes with a thickness of approximately 200 μm (specifically, 200 ± 20 μm), the preferred ionic conductivity, measured using AC impedance spectroscopy at -20 °C, is 1.0 × 10⁻⁶.-8 S / cm or higher, more preferably 4.0×10 -8 S / cm or higher, more preferably 1.0×10 -7 S / cm or higher.

[0053] Furthermore, for solid electrolytes with a thickness of approximately 200 μm (specifically, 200 ± 20 μm), the preferred ionic conductivity, measured by AC impedance spectroscopy at 80 °C, is 1.0 × 10⁻⁶. -5 S / cm or higher, more preferably 5.0 × 10 -5 S / cm or higher, more preferably 1.0×10 -4 S / cm or higher.

[0054] <Methods for Manufacturing Molecular Crystals>

[0055] This molecular crystal can be manufactured using organic molecules (M) and alkali metal salts as raw materials. The manufacturing method of this molecular crystal is not particularly limited. This molecular crystal can be manufactured, for example, by the following methods: a method comprising a step of mixing organic molecules (M) and alkali metal salts while applying mechanical energy (hereinafter also referred to as the "mechanical mixing step") (hereinafter also referred to as the "first manufacturing method"); a method comprising a step of dissolving organic molecules (M) and alkali metal salts in a solvent (hereinafter also referred to as the "dissolution step") and a step of removing the solvent from the solution formed by dissolving organic molecules (M) and alkali metal salts in the solvent (hereinafter also referred to as the "removal step") (hereinafter also referred to as the "second manufacturing method").

[0056] (First manufacturing method)

[0057] Mechanical mixing process

[0058] In a mechanical mixing process, organic molecules (M) and alkali metal salts are mixed simultaneously by applying mechanical energy to the substance through means such as impact, shearing, compression, and friction (hereinafter also referred to as "mechanical mixing"). Preferably, this results in a change in the physicochemical properties of the organic molecules (M), the alkali metal salt, or both.

[0059] The ratio of organic molecule (M) to alkali metal salt can be set according to the types of organic molecule (M) and alkali metal salt, and is not particularly limited. For example, the ratio of organic molecule (M) to alkali metal salt can be set to 0.1 to 10 moles relative to 1 mole of alkali metal salt. From the viewpoint of obtaining molecular crystals exhibiting excellent ionic conductivity, the ratio of organic molecule (M) to alkali metal salt is preferably set to 0.2 to 5.0 moles relative to 1 mole of alkali metal salt, more preferably 0.25 to 4.0 moles, and even more preferably 0.5 to 2.0 moles.

[0060] There are no particular limitations on the method used to mix organic molecules (M) with alkali metal salts while applying mechanical energy. In the mechanical mixing process, the mixing of organic molecules (M) with alkali metal salts can be performed using various equipment such as ball mills, bead mills, blenders, homogenizers, pulverizers, homogenizing mixers, and disperser-type mixers. Furthermore, in the case of small-scale manufacturing of molecular crystals, the mechanical mixing of organic molecules (M) with alkali metal salts can also be performed using a mortar and pestle. The mechanical mixing of organic molecules (M) with alkali metal salts can be performed dry or wet. Further, the mechanical mixing of organic molecules (M) with alkali metal salts can be performed at room temperature or at low temperature. For example, when the melting point of the organic molecules (M) is below room temperature, liquid organic molecules can be mechanically mixed with alkali metal salts. Alternatively, solid organic molecules can be mechanically mixed with alkali metal salts at a temperature lower than the melting point of the organic molecules (M).

[0061] It should be noted that the mechanical mixing of organic molecules (M) and alkali metal salts can be carried out while heating is present. However, in order to suppress the volatilization and sublimation of organic molecules (M), the heating in the mechanical mixing process can be carried out at a lower temperature than the heating temperature in any subsequent heating process. Specifically, the mechanical mixing of organic molecules (M) and alkali metal salts is carried out, for example, at a temperature below 50°C. The temperature at which organic molecules (M) and alkali metal salts are mechanically mixed is preferably below 40°C, more preferably below 35°C. The time for mechanical mixing depends on the amount and type of organic molecules (M) and alkali metal salts used, but can be, for example, 1 to 60 minutes, or 1 to 30 minutes.

[0062] In the case of manufacturing molecular crystals using the first manufacturing method, the first manufacturing 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 alkali metal salts) (hereinafter also referred to as the "heating step"). This heating step enables the organic molecules (M) and alkali metal salts in the mixture to become more homogeneous, further improving the ionic conductivity of the molecular crystal. In particular, by combining the mechanical mixing step with the heating step, the mechanical mixing induces intermolecular interactions between the alkali metal ions derived from the alkali metal salt and the organic molecules (M), resulting in the suppression of volatilization and sublimation of the organic molecules (M) during heat treatment. This provides the advantage of easily obtaining molecular crystals with the desired composition.

[0063] Heating process

[0064] In the heating process, there are no particular limitations on the temperature and method of heating the mixture. The heating temperature of the mixture can be appropriately set according to the type of organic molecules (M) and alkali metal salts used in the raw materials. From the viewpoint of making the organic molecules (M) and alkali metal salts in the mixture more homogeneous, 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 sufficient to achieve the homogenization of the organic molecules (M) and alkali metal salts in the mixture. The heating time is, for example, 1 minute to 3 hours, preferably 3 minutes to 2 hours. In addition, the heat treatment can usually be carried out under normal pressure, but it can also be carried out under pressure or depressurization.

[0065] When heating the mixture, it is preferable to further homogenize the organic molecules (M) and the alkali metal salt by simultaneously stirring the mixture. The stirring method is not particularly limited; examples include magnetic stirrers, stirring rods, mixers with impellers, and external circulation mixers. Alternatively, mechanical operations that generate greater shear forces, such as homogenizers, dispersion mixers, and homogenizers, can also be used for stirring.

[0066] After heating a mixture containing organic molecules (M) and an alkali metal salt, the heated mixture can be crystallized by lowering its temperature to obtain the target molecular crystal. When lowering the temperature of the heated mixture, it can be cooled slowly (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 thermostat at a temperature lower than room temperature (e.g., below 15°C) for a short time). From the viewpoint of ideally enabling crystallization of the heated mixture, it is preferable to lower the temperature of the mixture slowly. The fact that the product obtained by this manufacturing method is a molecular crystal containing organic molecules (M) and an alkali metal salt can be detected by performing powder X-ray diffraction analysis of the product. Details of the analysis method are as described in the examples below.

[0067] (Second manufacturing method)

[0068] Dissolving process

[0069] When the organic molecule (M) has a high melting point (e.g., above 100°C), the second manufacturing method is preferred as a method for manufacturing the molecular crystal. An organic solvent is preferably used as the solvent for dissolving the organic molecule (M) and the alkali metal salt. This organic solvent is preferably one that can dissolve both the organic molecule (M) and the alkali metal salt, and whose boiling point is lower than the individual boiling points of the organic molecule (M) and the alkali metal salt; 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 the solvent, it is preferable to achieve homogenization of the organic molecule (M) and the alkali metal salt by simultaneously stirring. The stirring method is not particularly limited; for example, the stirring method exemplified in the first manufacturing method can be appropriately used.

[0070] • Removal process

[0071] Regarding methods for removing solvent from solutions containing organic molecules (M) and alkali metal salts dissolved in a solvent, there are no particular limitations as long as the solvent can be removed from the solution. From the perspective of simple and efficient solvent removal, heat treatment is preferred. The heat treatment can be carried out at atmospheric pressure, or under pressure or reduced pressure. When a solvent with a high boiling point is used in the dissolution process, heat treatment under reduced pressure is preferred to suppress the decomposition of the organic molecules (M) and alkali metal salts.

[0072] It should be noted that the method for manufacturing this molecular crystal is not limited to the method described above. For example, in the first manufacturing method described above, the molecular crystal can also be manufactured by heating and mixing organic molecules (M) with alkali metal salts without mechanical mixing.

[0073] <<Solid Electrolytes>>

[0074] The solid electrolyte of the present invention contains the aforementioned molecular crystal. The solid electrolyte of the present invention 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, casting, die casting, and tape casting can be used. Among these methods, from the perspective of minimizing the porosity of the obtained solid electrolyte and facilitating the formation of ion conduction pathways, pressure molding is preferred. The shape of the molded body is not particularly limited and can be appropriately set according to the shape of the energy storage device to which it is applied. The shape of the molded body is, for example, rectangular or circular.

[0075] Without impairing the effects of the present invention, the solid electrolyte of the present invention may further comprise other components besides the molecular crystal. Examples of other components include molecular crystals different from the molecular crystal, polymeric solid electrolytes, polymeric gel electrolytes, inorganic electrolytes (e.g., ceramic electrolytes, glass electrolytes, etc.), binders, etc.

[0076] In particular, from the viewpoint of obtaining a solid electrolyte with excellent ion conductivity at high temperatures, the content of the intrinsic 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 relative to the total amount of the solid electrolyte.

[0077] The solid electrolyte of the present invention maintains a solid state not only at room temperature but also at high temperatures above 80°C, and exhibits high ionic conductivity. Therefore, by using the solid electrolyte of the present invention as the electrolyte for energy storage devices, it is possible to obtain energy storage devices with high ionic conductivity that can be applied to applications intended for use at high temperatures (e.g., automotive applications).

[0078] <<Electronic Storage Devices>>

[0079] The energy storage device of the present invention (hereinafter also referred to as "the device") includes the solid electrolyte of the present invention. Specific embodiments of the device include secondary batteries and capacitors. When the device is a secondary battery, one embodiment is an all-solid-state battery; from the viewpoint of excellent ion conductivity, a lithium-ion secondary battery is preferred.

[0080] The following describes an all-solid-state lithium-ion secondary battery as one form of this device. A lithium-ion secondary battery is a laminate comprising electrodes consisting of a positive electrode and a negative electrode, and a solid electrolyte, wherein the solid electrolyte is disposed between the positive and negative electrodes in a manner that connects the solid electrolyte to the electrodes. The materials constituting the positive and negative electrodes are not particularly limited, and can be appropriately selected from materials known as electrode materials for lithium-ion secondary batteries. For example, aluminum or stainless steel foil can be used as the positive electrode current collector. Copper or lithium foil can be used as the negative electrode current collector.

[0081] In the lithium-ion secondary battery of the present invention, the solid electrolyte is formed using a molecular crystal comprising an organic molecule (M) and an alkali metal salt. The thickness of the solid electrolyte is not particularly limited and can be appropriately set according to the application of the secondary battery. For example, the thickness of the solid electrolyte is 5 μm to 500 μm.

[0082] There are no particular limitations on the method for manufacturing lithium-ion secondary batteries, and known methods can be appropriately adopted depending on the battery structure. For example, a molded body formed using molecular crystals can be annealed, and a solid electrolyte obtained therefrom can be sandwiched between a positive electrode and a negative electrode to create a laminate having a positive electrode, a solid electrolyte, and a negative electrode. Alternatively, an electrolyte material containing molecular crystals can be housed in a container by sandwiching positive and negative electrodes, and the container can be annealed to create a laminate having a positive electrode, a solid electrolyte, and a negative electrode. The laminate having a positive electrode, a solid electrolyte, and a negative electrode is usually housed in a casing and used as a secondary battery.

[0083] It should be noted that this device is not limited to the above-described configuration where the charge carriers for ion conduction are lithium ions. For example, it could also be a secondary battery using other ions such as sodium ions as charge carriers. Furthermore, this device could also be a capacitor. One type of capacitor can be configured as follows: it includes a positive electrode, a negative electrode, and a solid electrolyte, with the solid electrolyte disposed between the positive and negative electrodes in a manner that connects the solid electrolyte to the electrodes.

[0084] Energy storage devices incorporating this solid electrolyte can be used for a wide variety of applications. Specifically, they can be used as power sources in various mobile devices such as mobile phones, personal computers, smartphones, game consoles, and wearable devices; various mobile vehicles such as electric vehicles, hybrid vehicles, robots, and drones; and various electrical and electronic devices such as digital cameras, camcorders, music players, power tools, and home appliances.

[0085] Example

[0086] The present invention will now be described in detail based on embodiments. It should be noted that the present invention is not limited to these embodiments. Unless otherwise specified, "parts" and "%" refer to "parts by mass" and "% by mass," respectively.

[0087] <<Preparation and Evaluation of Molecular Crystals>>

[0088] [Example 1]

[0089] 1. Manufacturing of molecular crystals

[0090] 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 measured in a 1:1 molar ratio and mechanically mixed in a mortar at room temperature (25°C) for 15 minutes to obtain a mixture. The mixture obtained by mechanical mixing of the raw materials was then transferred to a tubular flask and stirred for 10 minutes using a magnetic stirrer set at 80°C. After cooling to room temperature, a white solid was obtained.

[0091] The melting point of the white solid was determined using a differential scanning calorimeter (TA Instruments, DSC250, under nitrogen atmosphere) at a heating rate of 10°C per minute. A large endothermic peak (melting point) was observed at 97°C.

[0092] 2. Evaluation

[0093] (Confirmation of crystalline state by powder X-ray diffraction)

[0094] The obtained white solid was subjected to powder X-ray diffraction (PXRD) using an X-ray diffraction apparatus (Bruker AXS, D8 ADVANCE). It should be noted that Cu Kα was used as the X-ray source, with an applied voltage of 40 kV and a current of 40 mA. The measurement range was set to 2θ = 5–55°, the scan rate to 2.3° / min, and the step angle to 0.02°. The presence of sharp peaks within the measurement range indicates that the sample (solid) is crystalline. Therefore, the formation of a molecular crystal composed of organic molecules and an alkali metal salt is determined by an X-ray diffraction pattern with sharp peaks within the measurement range that differ from both the organic molecules and the alkali metal salt used as raw materials.

[0095] The white solid obtained by the above method was subjected to powder X-ray diffraction analysis. The results showed that the X-ray diffraction pattern had sharp peaks within the measurement range and was different from that of the raw material. This confirms that the obtained white solid is crystalline, that is, a molecular crystal composed of organic molecules and alkali metal salts.

[0096] (Determination of ionic conductivity)

[0097] A die 10 consisting of a mold 11, an upper punch 12, and a lower punch 13 is used (see reference). Figure 1 Sample particles were prepared in a glove box under an argon atmosphere. First, a mold 11 containing 0.0200 g of white solid (molecular crystal) was placed on a lower punch 13, and an upper punch 12 was placed on the mold 11. The white solid was compressed for 1 minute using a hydraulic press at a pressure of 10 MPa to obtain spherical sample particles with a diameter of 1 cm. Then, the sample particles were sealed into an all-solid-state battery evaluation cell (manufactured by Hosen Co., Ltd., KP-SolidCell) in a glove box under an argon atmosphere.

[0098] The ionic conductivity of an all-solid-state battery evaluation unit containing sample particles was measured under atmospheric conditions. The ionic conductivity was measured as follows: First, the all-solid-state battery evaluation unit containing sample particles was heat-treated in a constant temperature bath at 80°C (corresponding to the heat treatment temperature in Table 1), while the resistance value was measured by AC impedance spectroscopy. The heat treatment was ended when the resistance value stabilized. Next, the resistance values ​​were measured at -20°C, 25°C, and 80°C by AC impedance spectroscopy. The ionic conductivity (σ) was calculated using the obtained resistance values ​​by the following mathematical formula (1). It should be noted that the AC impedance spectroscopy used to calculate the ionic conductivity was performed after the unit was kept at the measurement temperature in a constant temperature bath for 2 hours.

[0099] σ = L / (R×S) (1)

[0100] (In mathematical formula (1), σ represents ionic conductivity (unit: S / cm), R represents resistance (unit: Ω), and S represents the cross-sectional area of ​​the molecular crystal during measurement (unit: cm²). 2 L represents the distance between electrodes (unit: cm).

[0101] The obtained molecular crystals exhibited ionic conductivity of 3.4 × 10⁻⁶ at -20℃, 25℃, and 80℃, respectively. -5 S / cm, 8.1×10 -4 S / cm, 1.8×10 -3 S / cm. Furthermore, the obtained molecular crystals maintained a solid state even at 80°C.

[0102] [Examples 2-6]

[0103] The types and amounts of raw materials, and the heat treatment temperature for ionic conductivity measurement were changed as described in Table 1. Otherwise, the same procedures as in Example 1 were performed to obtain molecular crystals. Furthermore, differential scanning calorimetry and ionic conductivity measurements were performed on each molecular crystal at -20°C, 25°C, and 80°C. The results are shown in Table 1. Each molecular crystal maintained a solid state even at 80°C.

[0104] [Comparative Example 1]

[0105] In a glove box under an argon atmosphere, 30 parts of succinate (manufactured by Tokyo Chemical Industry Co., Ltd.) as an organic molecule and 70 parts of lithium bis(trifluoromethanesulfonyl)imide (manufactured by Fujifilm and Kazumitsu Chemical Co., Ltd., trade name: lithium bis(trifluoromethanesulfonyl)imide) as an alkali metal salt were measured in a molar ratio of 3:2 and mechanically mixed at room temperature (25°C) using a mortar 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 the result was 55°C. In addition, for the molecular crystal obtained by mechanically mixing the raw materials, the heat treatment temperature for measuring ionic conductivity was set to 40°C. Otherwise, ionic conductivity measurements were performed at -20°C, 25°C, and 80°C, in the same manner as in Example 1. The results are shown in Table 1. In Comparative Example 1, the molecular crystal melted at 80°C, and its performance as a solid electrolyte could not be evaluated.

[0106] [Table 1]

[0107]

[0108] The details of the compounds listed in Table 1 are shown below.

[0109] •SA: Succinamide [Manufactured by Tokyo Chemical Industry Co., Ltd.]

[0110] • MA: Malondiamide [Manufactured by Tokyo Chemical Industry Co., Ltd.]

[0111] •TMMA: N,N,N′,N′-Tetramethylmalonamide [Manufactured by Tokyo Chemical Industry Co., Ltd.]

[0112] • TPA: Terephthalamide [Manufactured by Tokyo Chemical Industry Co., Ltd.]

[0113] •DAED: N,N'-Diacetylethylenediamine [Manufactured by Tokyo Chemical Industry Co., Ltd.]

[0114] •SN: Butadionitrile [Manufactured by Tokyo Chemical Industry Co., Ltd.]

[0115] • LiFSI: Lithium bis(fluorosulfonyl)imide [Manufactured by Kanto Chemical Co., Ltd.]

[0116] • LiTFSI: Lithium Bis(trifluoromethanesulfonyl)imide [Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: Lithium Bis(trifluoromethanesulfonyl)imide]

[0117] <<Evaluation Results>>

[0118] The results from Examples 1-6 show that molecular crystals containing organic molecules with specific heteroatoms and oxygen atoms as structural units and alkali metal salts, with melting points above 80°C, exhibit high ionic conductivity at both room temperature (25°C) and high temperature (80°C). Furthermore, the ionic conductivity of the molecular crystals from Examples 1-6 at low temperature (-20°C) is 3.5 × 10⁻⁶. -8 With a S / cm or higher, it exhibits sufficiently high ionic conductivity even at low temperatures.

[0119] In contrast, in Comparative Example 1, which used molecular crystals with a melting point of 55°C to prepare a solid electrolyte, the solid electrolyte melted at a high temperature (80°C) and failed to maintain a solid state at high temperatures.

[0120] The results above show that molecular crystals containing organic molecules with specific heteroatoms and oxygen atoms and alkali metal salts as structural units, and with melting points above 80°C, maintain a solid state at high temperatures and exhibit high ionic conductivity.

[0121] This invention is not limited to the embodiments described above, and includes various modifications and equivalent variations without departing from the spirit of the invention. Therefore, it should be understood that, in accordance with the above teachings, various combinations, forms, and other combinations and forms including only one element, its superordinate concept, or its subordinate concept also fall within the scope and conception of this invention.

[0122] Explanation of reference numerals in the attached figures

[0123] 10… Press mold.

Claims

1. A molecular crystal, characterized in that, Include: An organic molecule having at least one atom selected from nitrogen, sulfur, and phosphorus, and an oxygen atom; and Alkali metal salts, The melting point of the molecular crystal is above 80°C.

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

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

4. The molecular crystal according to claim 1, wherein, The molecular crystal has an ionic conductivity of 1.0 × 10⁻⁶ at 25 °C. -6 S / cm or higher.

5. The molecular crystal according to claim 1, wherein, The molecular crystal has an ionic conductivity of 1.0 × 10⁻⁶ at -20°C. -8 S / cm or higher.

6. The molecular crystal according to claim 1, wherein, The alkali metal salts include imide-based alkali metal salts.

7. A solid electrolyte, characterized in that, The molecular crystal contained in any one of claims 1 to 6.

8. An energy storage device, characterized in that, It possesses the solid electrolyte as described in claim 7.

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