WO2025182754A1 - Composite electrolyte, method for producing same, and power storage device - Google Patents

Abstract

This composite electrolyte contains an inorganic solid electrolyte, a polymer, and an alkali metal salt. The polymer has a structure represented by formula (1), and the alkali metal salt is included in an amount of 5-250 mol% with respect to the total amount of ester groups in the polymer. In formula (1), R represents a hydrogen atom or an alkyl group. X and Y are the same or different and represent a hydrogen atom, a hydroxyl group, or an alkyl group. n represents an integer of 1 or more, and m represents an integer of 0-10.

Description

Composite electrolyte, method for producing the same, and electricity storage device

CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to Japanese Patent Application No. 2024-27699, filed February 27, 2024, the entire contents of which are incorporated herein by reference.
The present disclosure relates to a composite electrolyte, a method for producing the same, and an electricity storage device.

A variety of energy storage devices have been put into 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 to replace the rare metal lithium.

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 two electrodes via the electrolyte. Traditionally, non-aqueous electrolytes have been used primarily as electrolytes. However, because non-aqueous electrolytes contain flammable organic solvents, there are concerns about electrolyte leakage and short circuits inside the battery due to overcharging and over-discharging.

In response to this, non-flammable inorganic solid electrolytes made from inorganic materials have been proposed as an alternative to non-aqueous electrolyte solutions. However, while inorganic solid electrolytes are safe, they have the disadvantage that, because they are manufactured by compacting powder, gaps are likely to form at the interfaces with the positive and negative electrodes, or at the interfaces between particles of the inorganic solid electrolyte material, resulting in high interfacial resistance.

In recent years, in order to achieve both improved safety and reduced interfacial resistance, it has been proposed to use a composite electrolyte containing an inorganic solid electrolyte and a non-aqueous electrolyte solution as the electrolyte for lithium-ion secondary batteries (see, for example, Patent Document 1). Patent Document 1 discloses that an inorganic solid electrolyte is formed by sintering an inorganic oxide to produce a sintered body having a dense portion and a porous portion, and that the sintered body is then immersed in an electrolyte solution such as an organic electrolyte solution or an ionic liquid and evacuated, thereby filling the porous portions of the sintered body with the electrolyte solution to obtain a composite electrolyte.

Patent Document 2 also discloses a lithium ion conductive composite material containing lithium ion conductive particles, which are inorganic solid electrolytes, a polymer such as polyethylene oxide, and an alkali metal salt. Patent Document 2 states that this composite material has reduced interfacial resistance between the polymer and the particles in terms of lithium ion conductivity.

JP 2017-111890 A Special Publication No. 2019-519885

When manufacturing inorganic solid electrolytes, for example, oxide-based solid electrolytes generally achieve high ionic conductivity by compacting the powder and then sintering it at high temperatures (e.g., temperatures exceeding 1,000°C) to improve the bonding between particles and between the electrodes and electrolyte and reduce interfacial resistance. However, sintering is an energy-intensive process, and industrialization requires the introduction of large-area sintering equipment. Therefore, when the inorganic solid electrolyte to be filled with electrolyte solution is a sintered body, as in Patent Document 1, there are concerns that the production of the composite electrolyte will require a lot of energy and that practical application and industrialization will be difficult. Sulfide-based solid electrolytes also generally require pressing at high pressures (e.g., pressures exceeding 10 MPa), which has the disadvantage of significant process constraints. Furthermore, when manufacturing electricity storage devices, good formability of the solid electrolyte layer is also required.

The inventors' investigation of the lithium ion conductive composite material described in Patent Document 2 revealed that the reduction in interfacial resistance was insufficient, and there is room for further improvement in ion conductivity.

This disclosure has been made in light of these circumstances, and one of its objectives is to provide a composite electrolyte of an inorganic solid electrolyte and an organic electrolyte that exhibits high ionic conductivity and good formability even without sintering during the production of the composite electrolyte.

As a result of extensive research to solve the above problems, the inventors have discovered that by using a specific substance as the polymer in a composite electrolyte of an inorganic solid electrolyte and an organic electrolyte, it is possible to obtain a composite electrolyte that exhibits good ionic conductivity. Specifically, this disclosure provides the following composite electrolyte, a method for producing the same, and an electricity storage device.

[1] A composite electrolyte comprising an inorganic solid electrolyte, a polymer, and an alkali metal salt, wherein the polymer has a structure represented by the following formula (1), and the alkali metal salt is contained in an amount of 5 mol % to 250 mol % based on the total amount of ester groups in the polymer:
[In formula (1), R represents a hydrogen atom or an alkyl group; X and Y are the same or different and represent a hydrogen atom, a hydroxyl group, or an alkyl group; n represents an integer of 1 or more; and m represents an integer of 0 to 10.]
[2] The composite electrolyte according to [1], wherein the inorganic solid electrolyte contains an oxide having a NASICON-type crystal structure.
[3] The composite electrolyte according to [1] or [2], wherein the inorganic solid electrolyte is in a particulate form.
[4] The composite electrolyte according to [3], which is a non-sintered body of a mixture of the inorganic solid electrolyte, the polymer, and the alkali metal salt.
[5] The composite electrolyte according to any one of [1] to [4], wherein the inorganic solid electrolyte comprises a solid electrolyte represented by the general formula: Li _1+2a+b+cd M1 _a M2 _b M3 _2-a-b Si _c W _d P _3-cd O ₁₂ (wherein M1 contains an element that can be a divalent cation, M2 contains an element that can be a trivalent cation, and M3 contains at least one element selected from Ti and Zr, and a≧0, b>0, c>0, and d≧0 are satisfied).
[6] The composite electrolyte according to any one of [1] to [5], wherein the content of the inorganic solid electrolyte is 20% by mass or more and 95% by mass or less.
[7] The composite electrolyte according to any one of [1] to [6], wherein the alkali metal salt includes lithium bis(fluorosulfonyl)imide and/or lithium bis(trifluoromethanesulfonyl)imide.
[8] A method for producing the composite electrolyte according to any one of [1] to [7], comprising a step of mixing the inorganic solid electrolyte in particulate form, the polymer, and the alkali metal salt.
[9] An electricity storage device comprising the composite electrolyte according to any one of [1] to [7].

According to the present disclosure, a composite electrolyte that exhibits high ionic conductivity and has good moldability can be obtained without the need for a sintering process during the production of the composite electrolyte. Furthermore, by using the composite electrolyte of the present disclosure as the electrolyte for an electricity storage device such as a secondary battery or capacitor, it is possible to obtain an electricity storage device that combines high ionic conductivity with the safety ensured by the solidification of the electrolyte.

FIG. 1 is a schematic diagram of a press die used to prepare the sample pellets of Comparative Example 3.

The composite electrolyte and electricity storage device disclosed herein are described in detail below.

<Composite electrolyte>
The composite electrolyte of the present disclosure contains an inorganic solid electrolyte, a polymer, and an alkali metal salt. Each component contained in the composite electrolyte of the present disclosure will be described in detail below. Note that, unless otherwise specified, each component may be contained alone or in combination of two or more.

<Inorganic solid electrolyte>
The inorganic solid electrolyte contained in the composite electrolyte of the present disclosure is not particularly limited as long as it is an inorganic solid exhibiting ion conductivity. As the inorganic solid electrolyte, for example, at least one of an oxide-based solid electrolyte, a sulfide-based solid electrolyte, and a chloride-based solid electrolyte can be used. Of these, an oxide-based solid electrolyte or a sulfide-based solid electrolyte is preferred in terms of exhibiting high ion conductivity, and an oxide-based solid electrolyte is more preferred in terms of higher safety in the atmosphere.

The crystalline structure of the inorganic solid electrolyte is not particularly limited. Examples of crystalline structures that inorganic solid electrolytes have include a NASICON structure, a LISICON structure, a perovskite structure, and a garnet structure.

Specific examples of these solid electrolytes having a NASICON structure include Li _1+x Al _x Ti _2-x (PO ₄ ) ₃ (x≧0, also called "LTP" or "LATP"), Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ (x≧0, also called "LGP" or "LAGP"), LiZr ₂ (PO ₄ ) ₃ (also called "LZP"), and NASICON Na ₃ Zr ₂ Si ₂ PO ₁₂ (also referred to as "NZSP"), and oxides in which some of the elements constituting these compounds are substituted with various elements (for example, B, Na, Al, Si, Ca, Ga, Ge, Sc, Fe, Sr, In, Ti, Hf, Sn, V, Nb, Ta, Sb, Bi, W, lanthanoid elements, etc.).

Examples of solid electrolytes having a LISICON structure include Li ₁₄ ZnGe ₄ O ₁₆ and oxides in which some of the elements constituting the compound are substituted with the various elements mentioned above.
Examples of solid electrolytes having a perovskite structure include Li _0.35 La _0.55 TiO ₃ and oxides in which some of the elements constituting this compound are substituted with the various elements mentioned above.
Examples of solid electrolytes having a garnet structure include Li ₇ La ₃ Zr ₂ O ₁₂ and Li ₅ La ₃ Nb ₂ O ₁₂ , and oxides in which some of the elements constituting these compounds are substituted with the above-mentioned various elements.

One embodiment of the composite electrolyte disclosed herein is an inorganic-organic composite electrolyte comprising an inorganic solid electrolyte having a NASICON-type crystal structure and an organic electrolyte, where the organic electrolyte comprises a polymer and an alkali metal salt. Unlike layered structures, the NASICON-type crystal structure provides three-dimensional mobility for alkali metal ions, and zirconium (Zr) is stable even under high voltages, making it useful as a solid electrolyte for high operating voltages. In the composite electrolyte disclosed herein, an oxide having a NASICON-type crystal structure can preferably be used as the inorganic solid electrolyte. The crystal structure of the solid electrolyte can be determined from the diffraction profile obtained by powder X-ray diffraction measurement.

When an oxide having a NASICON-type crystal structure is used as the inorganic solid electrolyte contained in the composite electrolyte, a preferred example of the oxide is the oxide (x1) shown below.
Oxide (x1): A solid electrolyte represented by the general formula Li _1+2a+b+c-d M1 _a M2 _b M3 _2-a-b Si _c W _d P _3-c-d O ₁₂ (wherein M1 contains an element that can form a divalent cation, M2 contains an element that can form a trivalent cation, and M3 contains at least one element selected from Ti and Zr, and a≧0, b>0, c>0, and d≧0 are satisfied).

Regarding oxide (x1): The oxide (x1) is a solid electrolyte in which, in an oxide having a basic skeleton of LiZr ₂ (PO ₄ ) ₃ or LiTi ₂ (PO ₄ ) ₃ , at least a portion of Zr or Ti is substituted with M2 (including an element that becomes a trivalent cation) and optionally with M1 (including an element that becomes a divalent cation), and a portion of P is substituted with Si and optionally with W.

In the oxide (x1), examples of M1 include elements that form divalent cations among Group 2 elements, Group 12 elements, and transition elements (Groups 3 to 11 elements), as well as Sn and Pd.

M2 may be an element that forms a trivalent cation among Group 3 elements, Group 13 elements, and transition elements (Groups 3 to 11 elements), such as Sb, Bi, or Fe. Preferred Group 13 elements are Al, B (boron), or In.

M3 may contain at least one element selected from the group consisting of Ti and Zr. That is, M3 may be Ti or Zr, or may further contain, together with Ti and/or Zr, an element other than Ti and Zr that forms a tetravalent cation. Examples of elements that form a tetravalent cation other than Ti and Zr include Group 14 elements, transition elements (Groups 3 to 11 elements) that form a tetravalent cation, and Te. The Group 14 element is preferably Si.

In the general formula of the oxide (x1), a, b, c, and d are not particularly limited as long as they satisfy a ≥ 0, b > 0, c > 0, and d ≥ 0. For example, when a = 0, b > 0, c > 0, and d = 0, the oxide (x1) is represented by "Li _1+b+c M2 _b M3 _2-b Si _c P _3-c O ₁₂ ".

In more detail regarding a, b, c, and d in the general formula of oxide (x1), a is, for example, 0.5 or less, and may be 0.4 or less. In terms of making it difficult for impurity phases to form and thereby obtaining an inorganic solid electrolyte exhibiting high ionic conductivity, it is preferable that a≦0.3, more preferably a≦0.15, and even more preferably a≦0.1. Furthermore, when a>0, the lower limit of a is preferably a≧0.01, and more preferably a≧0.03.

b is, for example, 2.0 or less, and may be 1.9 or less. It is preferable that b≦1.9, and more preferably b≦1.85, since this favors α-phase formation and thereby allows for the production of an inorganic solid electrolyte with higher ionic conductivity. Furthermore, the lower limit of b is preferably b≧0.01, and more preferably b≧0.02.

c is, for example, 1.8 or less, and may be 1.5 or less. In order to obtain an inorganic solid electrolyte exhibiting higher ionic conductivity, it is preferable that c≦1.2, more preferably c≦1.0, and even more preferably c≦0.95. Regarding the lower limit of c, it is preferable that c≧0.01, and more preferably c≧0.03.

d is, for example, 0.5 or less, and may be 0.3 or less. Since impurity phases are less likely to form, and an inorganic solid electrolyte exhibiting high ionic conductivity can be obtained, it is preferable that d≦0.2 be satisfied, and it is more preferable that d≦0.1 be satisfied. Furthermore, when d>0, the lower limit of d is preferably d≧0.01, and it is more preferable that d≧0.02 be satisfied.

Incidentally, zirconium phosphate oxides with a NASICON-type crystal structure can have four phase structures: α, α', β, and β'. Of these, the α phase has an isotropic crystal structure and therefore has the highest Li-ion conductivity.

In the above general formula representing oxide (x1), the stoichiometric ratio of O is 12, but the stoichiometric ratio of O does not have to be strictly 12 as long as charge neutrality of the oxide as a whole can be maintained. In other words, the O content in oxide (x1) may be less than 12 or greater than 12, as long as charge neutrality of the oxide (x1) as a whole can be maintained. For example, oxide (x1) may be represented by Li _1+2a+b+c-d M1 _a M2 _b M3 _2-a-b Si _c W _d P _3-c-d O _12±β (where 0≦β≦1 is satisfied and M1, M2, M3, a, b, c, and d are the same as M1, M2, M3, a, b, c, and d in the above general formula for oxide (x1)), as long as charge neutrality of the oxide (x1) as a whole is maintained.

The method for producing oxide (x1) is not particularly limited. Oxide (x1) can be produced, for example, by weighing and mixing raw materials so as to satisfy the stoichiometric ratio of the composition represented by the general formula above (mixing step), and then firing the resulting mixture (firing step).

As raw materials for oxide (x1), it is possible to use the Li supply component, M1 supply component, M2 supply component, M3 supply component, Si supply component, W supply component, and P supply component that correspond to the elements required to obtain the desired oxide (x1). For example, when obtaining oxide (x1) where a = 0, b > 0, c > 0, and d = 0, the Li supply component, M2 supply component, M3 supply component, Si supply component, and P supply component are used as raw materials.

The Li supply component, M1 supply component, M2 supply component, M3 supply component, Si supply component, W supply component, and P supply component for obtaining oxide (x1) can be, for example, carbonates, bicarbonates, sulfates, sulfites, nitrates, nitrites, phosphates, acetates, citrates, ammonium salts, oxides, hydroxides, chlorides, sulfides, etc., of these metal elements. Note that these supply components may be compounds in which one type of supply component contains two or more elements selected from Li, M1, M2, M3, Si, W, and P. When oxide (x1) contains Zr, zirconium phosphate-based compounds can be preferably used as the P supply component, and layered zirconium phosphate is particularly preferred.

When producing oxide (x1), the raw materials may be mixed by dry mixing or by wet mixing using a liquid. By using wet mixing, the density of the inorganic solid electrolyte after firing can be increased compared to dry mixing. The ionic conductivity of the resulting inorganic solid electrolyte can also be relatively improved. Water, various organic solvents, and mixtures of these can be used as the liquid for wet mixing.

In the firing step, the mixture obtained in the mixing step may be fired without being shaped, or may be shaped and then fired. The firing temperature is not limited, but can be, for example, 900°C or higher, preferably 1,000°C or higher, more preferably 1,150°C or higher, and even more preferably 1,200°C or higher. The upper limit of the firing temperature can be, for example, 1,500°C or lower, preferably 1,400°C or lower, and more preferably 1,350°C or lower. During firing, the temperature can be increased in stages from a temperature lower than the firing temperature, and finally maintained at the temperature required for firing.

In addition, oxide (x1) exhibits high Li ion conductivity. Therefore, oxide (x1) is suitable as a material for producing a composite electrolyte for an electricity storage device in which the ion-conducting carrier is lithium ions.

The inorganic solid electrolyte obtained in the firing step is preferably pulverized by any method to form particles, and then used to produce a composite electrolyte. The inorganic solid electrolyte can be pulverized using a pulverizer such as a ball mill, bead mill, or blender. Alternatively, a composite electrolyte can be produced by granulating powdered inorganic solid electrolyte and using the granulated material as a particulate inorganic solid electrolyte.

The average particle size of the inorganic solid electrolyte used to produce the composite electrolyte is preferably 0.01 μm or more and 20 μm or less in volume-based median particle size measured in an aqueous medium. When the average particle size of the inorganic solid electrolyte is within the above range, a composite electrolyte can be obtained that exhibits high ionic conductivity while improving handleability. From the standpoint of handleability, the average particle size of the inorganic solid electrolyte is more preferably 0.05 μm or more in volume-based median diameter, even more preferably 0.1 μm or more, and even more preferably 0.2 μm or more. Furthermore, from the standpoint of improving the ionic conductivity of the composite electrolyte, the average particle size of the inorganic solid electrolyte is more preferably 15 μm or less in volume-based median diameter, even more preferably 10 μm or less, and even more preferably 5 μm or less. The average particle size of the inorganic solid electrolyte is a value measured by laser diffraction/scattering particle size measurement.

From the viewpoint of ensuring the formability of the composite electrolyte and obtaining a composite electrolyte that exhibits high ionic conductivity, the content of the inorganic solid electrolyte in the composite electrolyte is preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 30% by mass or more, even more preferably 35% by mass or more, and even more preferably 40% by mass or more, based on the total amount of the composite electrolyte. Furthermore, from the viewpoint of ensuring the flexibility of the composite electrolyte and improving its handleability, the upper limit of the content of the inorganic solid electrolyte is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less, based on the total amount of the composite electrolyte.

<Polymer>
The composite electrolyte of the present disclosure contains a polymer (hereinafter also referred to as a "polyester-based polymer") having a structure represented by the following formula (1): The polyester-based polymer exhibits ionic conductivity when mixed with an alkali metal salt.
[In formula (1), R represents a hydrogen atom or an alkyl group; X and Y are the same or different and represent a hydrogen atom, a hydroxyl group, or an alkyl group; n represents an integer of 1 or more; and m represents an integer of 0 to 10.]

In the above formula (1), R is preferably a hydrogen atom or a methyl group from the viewpoint of availability of raw materials and ease of synthesis of the polyester polymer.
m is preferably an integer of 0 to 8, more preferably an integer of 0 to 6, and more preferably an integer of 0 to 4, from the viewpoint of availability of raw materials.

Monomers used to introduce the repeating unit (-CO-CHR-(CH ₂ ) _m -O-) in the above formula (1) into the polymer include lactones and lactides. Specific examples of these include lactones such as β-propiolactone, γ-butyrolactone, β-butyrolactone, pivalolactone, δ-valerolactone, and ε-caprolactone. Lactides include glycolide obtained by dehydration condensation of two molecules of glycolic acid, dilactide obtained by dehydration condensation of two molecules of lactic acid, and tetramethyl glycolide. When the above lactides are used as the monomer, two repeating units in the above formula (1) are introduced into the polymer per molecule of lactide.

In terms of obtaining a composite electrolyte with superior ionic conductivity, it is preferable that the monomer constituting the polyester polymer is at least one selected from the group consisting of gamma-butyrolactone, delta-valerolactone, epsilon-caprolactone, and dilactide.

In the above formula (1), n can be set appropriately depending on the desired molecular weight of the polyester polymer. n is, for example, 10 to 1,500, preferably 20 to 1,200, more preferably 30 to 1,000, and even more preferably 30 to 800.

The polyester polymer may further contain repeating units different from those in formula (1) above (hereinafter also referred to as "other repeating units"), as long as the effects of the present invention are not impaired. Examples of other repeating units include dioxepanone, ethylene oxalate, dioxanone, γ-nonalactone, γ-decalactone, γ-undecalactone, cyclopentadecanolide, and cyclohexedecanolide. In order to prevent a decrease in the ionic conductivity of the composite electrolyte, the proportion of other repeating units in the polyester polymer is preferably 5 mol % or less, more preferably 2 mol % or less, even more preferably 0.5 mol % or less, and particularly preferably 0.1 mol % or less, relative to the total repeating units of the polyester polymer.

The polymer chain form of the polyester polymer is not particularly limited, and may be linear or branched. When the polyester polymer is branched, crystallization of the polyester polymer is suppressed, facilitating molecular motion of the polymer. Therefore, the polyester polymer is preferably a star polymer having a core portion and three or more branched chains (arm portions) extending from the core portion.

The terminal structure of the polyester polymer may be a hydroxyl group or a carboxyl group derived from the monomer. Alternatively, the terminal of the polyester polymer may be modified using a hydroxyl group or a carboxyl group present at the polymer end. By modifying the terminal hydroxyl group or terminal carboxyl group of the polyester polymer, the heat resistance (durability) of the polyester polymer can be improved and the crystallinity of the polyester polymer can be reduced. When X and Y in the above formula (1) are alkyl groups, the alkyl group is preferably a linear or branched alkyl group having 3 to 20 carbon atoms, from the viewpoint of improving the heat resistance of the polyester polymer and promoting a reduction in crystallinity.

The method for producing the polyester polymer is not particularly limited, and it can be produced using any conventionally known method as appropriate. In terms of being able to produce the polyester polymer represented by formula (1) above simply and inexpensively, it is preferable to produce the polyester polymer by ring-opening polymerization using at least one monomer selected from the group consisting of the lactones and lactides described above.

In the above-mentioned ring-opening polymerization, for example, the desired polyester polymer can be obtained by charging the monomers and, if necessary, a solvent into a reactor, adding an initiator, and polymerizing. The charging method for the various raw materials, including the monomers, may be a batch-type initial lump-sum charging in which all raw materials are charged at once, a semi-continuous charging in which at least some of the raw materials are continuously fed into the reactor, or a continuous polymerization method in which all raw materials are continuously fed while the produced resin is continuously withdrawn from the reactor.

As the initiator, monoalcohols or polyhydric alcohols are preferably used, as they allow the desired polymer to be easily obtained. The monoalcohol is preferably an alkyl alcohol, such as methanol, ethanol, propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, 2-methyl-2-propanol, 1-pentanol, 2-pentanol, and 2,2-dimethyl-1-propanol. Specific examples of polyhydric alcohols include trihydric alcohols such as glycerin, trimethylolethane, trimethylolpropane, tris(2-hydroxyethyl)isocyanurate, hexanetriol, octanetriol, and decanetriol; tetrahydric alcohols such as ditrimethylolethane, ditrimethylolpropane, diglycerin, and pentaerythritol; pentahydric alcohols such as tritrimethylolethane, tritrimethylolpropane, and triglycerin; hexahydric or higher alcohols such as polytrimethylolethane, polytrimethylolpropane, polyglycerin, dipentaerythritol, tripentaerythritol, sorbitol, and polypentaerythritol; and alkylene oxide adducts of trihydric or higher alcohols.

When producing a polyester polymer, the amount of initiator used is, for example, 0.01 to 15 parts by mass, and preferably 0.02 to 10 parts by mass, per 100 parts by mass of the total amount of monomers used in the polymerization.

The ring-opening polymerization is preferably carried out in the presence of a catalyst to ensure efficient reaction. Conventional acid catalysts, base catalysts, or metal catalysts can be used as appropriate as the catalyst. Specific examples of acid catalysts include sulfonic acid, methanesulfonic acid, trifluoroacetic acid, 10-camphorsulfonic acid, phosphoric acid, phosphoric acid monoesters (methyl phosphate, ethyl phosphate, octyl phosphate, phenyl phosphate, etc.), phosphoric acid diesters (dimethyl phosphate, diethyl phosphate, dibutyl phosphate, diphenyl phosphate, etc.), phosphorous acid, phosphorous acid esters, tin tetrachloride, phosphorus pentafluoride, and boron trifluoride complexes.

Specific examples of base catalysts include hydroxides such as sodium hydroxide and potassium hydroxide; tertiary amine compounds such as tetrabutylammonium bromide, tetrabutylammonium chloride, tetramethylammonium bromide, tetramethylammonium chloride, 1,8-diazabicyclo[5,4,0]-7-undecene, and 1,4-diazabicyclo[2.2,2]octane; phosphorus compounds such as ethylphosphine, phenylphosphine, dimethylphosphine, diphenylphosphine, triphenylphosphine, and tributylphosphine; imidazole compounds such as 2-phenylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, and 2-ethyl-4-methylimidazole; and the like.

Specific examples of metal catalysts include metal salts of tin, zinc, lead, titanium, aluminum, iron, zirconium, etc. From the standpoint of reactivity, tin catalysts are particularly preferred. Examples of tin catalysts include tin(II) 2-ethylhexanoate, tin(II) acetate, tin(IV) acetate, tin(II) chloride, dibutyltin diacetate, dibutyltin dilaurate, dioctyltin(IV) diacetate, and tin(II) trifluoromethanesulfonate.

When producing polyester polymers, the amount of catalyst used is, for example, 0.01 to 20 parts by mass, and preferably 0.05 to 10 parts by mass, per 100 parts by mass of the total amount of monomers used in polymerization.

When a solvent is used in the reaction, an organic solvent is preferably used as the solvent. Examples of organic solvents include aromatic hydrocarbons such as benzene, toluene, and xylene; aliphatic hydrocarbons such as hexane and heptane; esters such as ethyl acetate and butyl acetate; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; and ethers such as propylene glycol monomethyl ether. One or more solvents can be used. The amount of solvent used is, for example, 10 to 1,500 parts by mass, and preferably 20 to 1,000 parts by mass, per 100 parts by mass of the total amount of monomers used in the polymerization.

The polymerization temperature and polymerization time are not particularly limited and may be set as appropriate. From the perspective of increasing the reaction rate while suppressing side reactions, the polymerization temperature is, for example, within the range of 0°C to 120°C, and preferably within the range of 5°C to 100°C. The polymerization time is, for example, 1 to 150 hours, and preferably 5 to 100 hours. The pressure during polymerization may be any pressure that can maintain the polymerization temperature. From the perspective of suppressing a decrease in the degree of polymerization, the reaction is preferably carried out under dry air (for example, under conditions where the dew point at atmospheric pressure is -40°C or less). Alternatively, the reaction may be carried out under dry nitrogen or dry argon instead of dry air. The polymerization reaction is preferably carried out with stirring within the reactor.

By carrying out ring-opening polymerization of lactones or lactides using a monoalcohol or polyalcohol as an initiator, a polyester polymer having hydroxyl groups at its terminals can be obtained. The polyester polymer obtained in this manner may be mixed with an inorganic solid electrolyte and an alkali metal salt in its original state (i.e., with terminal hydroxyl groups). Alternatively, the terminal hydroxyl groups of the polyester polymer obtained by the above polymerization reaction may be reacted with a compound (modifier) having a reactive functional group capable of reacting with hydroxyl groups to introduce a structure derived from the modifier into the terminal of the polyester polymer, and the resulting polymer may then be mixed with an inorganic solid electrolyte and an alkali metal salt. The reaction between the polyester polymer having terminal hydroxyl groups and the modifier can be carried out, for example, in an appropriate solvent, using a catalyst as necessary.

When a polyester polymer is produced by solution polymerization, the polyester polymer dissolved in a solvent can be isolated by known desolvation methods such as reprecipitation, or by drying methods such as heat treatment. Polyester polymers may also be produced by bulk polymerization or other methods without using a solvent.

The glass transition temperature (Tg) of the polyester polymer is, for example, 65°C or lower. From the viewpoint of obtaining a composite electrolyte with higher ionic conductivity, the glass transition temperature of the polyester polymer is preferably 50°C or lower, more preferably 20°C or lower, preferably 0°C or lower, more preferably -20°C or lower, even more preferably -30°C or lower, even more preferably -40°C or lower, and even more preferably -50°C or lower. There are no particular restrictions on the lower limit of the glass transition temperature of the polyester polymer, and it is, for example, -80°C or higher. Note that in this specification, the glass transition temperature of the polyester polymer is a value determined by differential scanning calorimetry (DSC).

From the viewpoint of obtaining a composite electrolyte that has high strength and high ionic conductivity and from the viewpoint of improving the formability of the composite electrolyte, the number average molecular weight (Mn) of the polyester polymer is preferably 1,000 or more, more preferably 3,000 or more, even more preferably 5,000 or more, even more preferably 8,000 or more, even more preferably 10,000 or more, even more preferably 15,000 or more, and particularly preferably 20,000 or more. The upper limit of Mn of the polyester polymer is preferably 100,000 or less, more preferably 80,000 or less, and even more preferably 50,000 or less, from the viewpoint of ensuring appropriate fluidity of the polyester polymer and flexibility of the composite electrolyte obtained from the inorganic solid electrolyte, polyester polymer, and alkali metal salt.

The preferred range of Mn for polyester polymers can be set by appropriately combining the upper and lower limits of the preferred ranges of Mn described above. The range of Mn for polyester polymers is preferably 1,000 or more and 100,000 or less, more preferably 3,000 or more and 80,000 or less, and even more preferably 5,000 or more and 50,000 or less.

Furthermore, the weight average molecular weight (Mw) of the polyester polymer is preferably 1,500 or more, more preferably 4,000 or more, even more preferably 7,500 or more, even more preferably 10,000 or more, even more preferably 15,000 or more, even more preferably 20,000 or more, and particularly preferably 26,000 or more. The upper limit of the Mw of the polyester polymer is preferably 150,000 or less, more preferably 100,000 or less, and even more preferably 80,000 or less. The Mw range of the polyester polymer is preferably 1,500 or more and 150,000 or less, more preferably 4,000 or more and 100,000 or less, and even more preferably 7,500 or more and 80,000 or less.

For polyester polymers, the molecular weight distribution (Mw/Mn), expressed as the ratio of Mw to Mn, is preferably 4.5 or less, more preferably 4.0 or less, even more preferably 3.5 or less, even more preferably 3.0 or less, even more preferably 2.5 or less, and even more preferably 2.0 or less, from the viewpoint of obtaining a composite electrolyte exhibiting good ionic conductivity. There is no particular restriction on the lower limit of Mw/Mn for polyester polymers, and it is 1.0 or more. In this specification, the Mw and Mn of the polymer are values calculated in terms of standard polystyrene obtained using gel permeation chromatography (GPC).

The content of the polyester polymer in the composite electrolyte is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, based on the total amount of the composite electrolyte, from the viewpoint of reducing interfacial resistance and obtaining a composite electrolyte that exhibits high ionic conductivity. Furthermore, the upper limit of the content of the polyester polymer is preferably 60% by mass or less, more preferably 50% by mass or less, even more preferably 40% by mass or less, and even more preferably 30% by mass or less, based on the total amount of the composite electrolyte, from the viewpoint of ensuring the formability and handleability of the composite electrolyte.

<Alkali metal salt>
The alkali metal salt is not particularly limited as long as it generates alkali metal ions, and examples of the alkali metal salt include lithium salt, sodium salt, and potassium salt.

Specific examples of alkali metal salts include lithium salts such as _Li2CO3 , _LiBr , LiCl, LiI, _LiSCN , LiBF4, _LiAsF6 , _{LiClO4, CH3COOLi , CF3COOLi} , _LiCF3SO3 , _LiPF6 , LiC( _CF3SO2 ) ₃ , lithium bis(fluorosulfonyl)imide (Li ⁺ ( _FSO2 ) _{2N- )} , lithium bis(trifluoromethanesulfonyl)imide (Li ⁺ ( _CF3SO2 ⁾ _{2N- )} , and lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide; and salts of the anions _of these lithium salts with ^alkali metals other than lithium (for example, sodium, potassium, etc.). Among these, lithium salts and sodium salts are preferred because they have high ion dissociation properties and can further increase the ionic conductivity of the composite electrolyte of the present disclosure. Furthermore, it is preferable that the type of alkali metal ion contained in the inorganic solid electrolyte is the same as the type of alkali metal ion contained in the alkali metal salt.

In order to further increase the ionic conductivity of the composite electrolyte of the present disclosure, the alkali metal salt contained in the composite electrolyte of the present disclosure preferably includes an imide-based alkali metal salt. Among these, imide-based lithium salts are preferred because of their high ionic dissociability. Among imide-based lithium salts, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, or lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide are particularly preferred, with lithium bis(fluorosulfonyl)imide and/or lithium bis(trifluoromethanesulfonyl)imide being more preferred, and lithium bis(fluorosulfonyl)imide being even more preferred.

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.

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.

The content of alkali metal salt in the composite electrolyte is 5 mol% or more and 250 mol% or less, based on the total amount of ester groups in the polyester polymer (100 mol%). If the content of alkali metal salt is less than 5 mol% based on the total amount of ester groups in the polyester polymer, the ion source in the composite electrolyte will be insufficient, making it impossible to obtain a composite electrolyte that exhibits good ionic conductivity. Furthermore, if the content of alkali metal salt in the composite electrolyte exceeds 250 mol% based on the total amount of ester groups in the polyester polymer, the alkali metal salt will not be able to dissolve sufficiently in the polyester polymer, and the ionic conductivity of the resulting composite electrolyte will tend not to be sufficiently high.

From the viewpoint of obtaining a composite electrolyte with superior ionic conductivity, the content of alkali metal salt in the composite electrolyte is preferably 7 mol% or more, more preferably 20 mol% or more, and even more preferably 40 mol% or more, relative to the total amount of ester groups in the polyester polymer. Furthermore, the content of alkali metal salt in the composite electrolyte is preferably 220 mol% or less, more preferably 150 mol% or less, and even more preferably 120 mol% or less, relative to the total amount of ester groups in the polyester polymer.

The composite electrolyte of the present disclosure may further contain components (hereinafter also referred to as "other components") that are different from the inorganic solid electrolyte, polyester polymer, and alkali metal salt, as long as the effects of the present invention are not impaired. Examples of other components include the following:

Surface Modifiers A surface modifier may be used in the production of a composite electrolyte to uniformly disperse the inorganic solid electrolyte in the composite electrolyte and thereby obtain a composite electrolyte exhibiting high ionic conductivity. In particular, when a particulate inorganic solid electrolyte is used, aggregation of the inorganic solid electrolyte is likely to occur when the particulate inorganic solid electrolyte is mixed with a polyester-based polymer. In view of this, treating the particulate inorganic solid electrolyte with a surface modifier and then mixing the surface-modified particulate inorganic solid electrolyte with a polyester-based polymer is effective in suppressing aggregation of the inorganic solid electrolyte. As such a surface modifier, a substance exhibiting affinity for both the inorganic solid electrolyte and the polyester-based polymer is preferred, and for example, a known silane coupling agent can be used.

Specific examples of surface modifiers include silane coupling agents having one or more functional groups such as epoxy groups, (meth)acryloyl groups, amino groups, vinyl groups, thiol groups, isocyanate groups, and blocked isocyanate groups. Of these, silane coupling agents having amino groups are preferred because they have a higher affinity with polyester polymers and are highly effective in improving the uniform dispersion of inorganic solid electrolytes.

Specific examples of silane coupling agents having an amino group include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, N-(3-cyclohexylamino)propyltrimethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylidene)propylamine, and N-phenyl-3-aminopropyltrimethoxysilane.

When the inorganic solid electrolyte is treated with a surface modifier before use, the amount of surface modifier used is preferably 0.5 mass% or more, more preferably 1 mass% or more, and even more preferably 2 mass% or more, relative to the total amount of inorganic solid electrolyte. Furthermore, the amount of surface modifier used is preferably 25 mass% or less, more preferably 20 mass% or less, and even more preferably 15 mass% or less, relative to the total amount of inorganic solid electrolyte.

Other components include, in addition to those mentioned above, antioxidants, colorants, etc. The content of these components may be set appropriately as long as it does not impair the effects of the present invention.

<Method of manufacturing composite electrolyte>
The composite electrolyte of the present disclosure can be produced by mixing an inorganic solid electrolyte, a polyester-based polymer, and an alkali metal salt, and molding the mixture as needed. This method allows a composite electrolyte exhibiting high ionic conductivity to be obtained without the need for a step of sintering the inorganic solid electrolyte or a composition containing the inorganic solid electrolyte during the production of the composite electrolyte. In particular, when producing the composite electrolyte, it is preferable to use a particle aggregate (i.e., powder) of the inorganic solid electrolyte, and to use a method including a step of mixing the particulate inorganic solid electrolyte with the polyester-based polymer and the alkali metal salt (hereinafter also referred to as the "mixing step").

In the mixing step for producing the composite electrolyte, the inorganic solid electrolyte, polyester polymer, and alkali metal salt need only be mixed uniformly, and the method for doing so is not particularly limited. For example, the inorganic solid electrolyte, polyester polymer, and alkali metal salt may be mixed simultaneously in a container, or they may be mixed sequentially in a container. Alternatively, the inorganic solid electrolyte may be added after mixing the polyester polymer and alkali metal salt. From the perspective of ensuring uniform dispersion of the inorganic solid electrolyte, it is preferable to mix the inorganic solid electrolyte and surface modifier, optionally adding a solvent (hereinafter also referred to as "dispersion solvent"), to obtain an inorganic solid electrolyte surface-modified with the surface modifier, and then mix the surface-modified inorganic solid electrolyte with the polyester polymer and alkali metal salt.

When a dispersion solvent is used when mixing the inorganic solid electrolyte and the surface modifier, the dispersion solvent is not particularly limited as long as it does not react with the inorganic solid electrolyte. Specific examples of dispersion solvents include tetrahydrofuran, acetone, tert-butyl methyl ether, diethyl ether, 1,4-dioxane, acetonitrile, ethyl acetate, and N-methyl-2-pyrrolidone. A single dispersion solvent may be used, or two or more may be combined. The amount of dispersion solvent used may be set appropriately, but from the perspective of improving the dispersibility of the inorganic solid electrolyte when mixed with the organic electrolyte, it may be, for example, 50 to 2000 parts by mass per 100 parts by mass of the total amount of inorganic solid electrolyte used to produce the composite electrolyte.

When mixing two or more of the inorganic solid electrolyte, polyester polymer, and alkali metal salt, it is preferable to do so while stirring. The stirring method is not particularly limited, and examples include various methods using a planetary mixer, magnetic stirrer, stirring rod, a stirrer with stirring blades (three-one motor), external circulation stirring, etc. Stirring may also be performed while mechanical mixing is performed using a homomixer, disperser-type mixer, homogenizer, etc.

If the mixture obtained in the mixing step contains a solvent, it is preferable to remove the solvent from the mixture (solvent removal step). There are no particular restrictions on the method for removing the solvent, and any known desolvation method can be used as appropriate. For example, desolvation can be performed by heating, natural drying, air blowing, or reduced pressure. Desolvation can also be performed by combining two or more of these methods as appropriate. When removing the solvent by heating, the heating temperature can be set appropriately depending on the type of solvent, but can be, for example, 30 to 100°C, and preferably 35 to 65°C. The heating time is, for example, 30 minutes to 24 hours. The heating process can be performed under atmospheric pressure or under reduced pressure. From the perspective of removing the solvent from the mixture at as low a temperature as possible, it is recommended to perform the desolvation process under reduced pressure at a heating temperature of 60°C or less.

When a mixture containing an inorganic solid electrolyte, a polyester polymer, and an alkali metal salt is molded to obtain a molded body, the molding method is not particularly limited, and any known molding method can be used as appropriate. Specific examples of molding methods include extrusion molding, injection molding, pressure molding, slip casting, mold cast molding, and tape casting. The shape of the molded body is not particularly limited, and can be set appropriately depending on the application and shape of the electricity storage device. The shape of the molded body is, for example, rectangular or circular.

A composite electrolyte can be obtained by the above-mentioned mixing process, or by the mixing process and solvent removal process. The composite electrolyte obtained in this manner exhibits high ionic conductivity. Therefore, by using the composite electrolyte of the present disclosure as an electrolyte material for an electricity storage device, an electricity storage device with high ionic conductivity can be obtained.

Specifically, the ionic conductivity of a molded article made of the composite electrolyte of the present disclosure, measured at 25°C using an AC impedance method, is preferably 1 x 10 ^-6 S/cm or more. From the viewpoint of obtaining an electricity storage device with excellent performance, the ionic conductivity under the same conditions is more preferably 1 x 10 ^-5 S/cm or more, even more preferably 1 x 10 ^-4 S/cm or more, and even more preferably 1 x 10 ^-3 S/cm or more. Details of the method for measuring the ionic conductivity follow the method described in the Examples below.

While inorganic solid electrolytes are non-flammable and highly safe, they suffer from high grain boundary resistance, which hinders further improvements in ionic conductivity. To overcome these drawbacks, inorganic solid electrolytes are typically manufactured by compacting the powder and then heating it to high temperatures (e.g., temperatures above 1,000°C) (sintering), thereby improving the bonding between particles and between the electrode and electrolyte. However, considering the installation of sintering equipment and the energy consumption, incorporating a sintering process is not necessarily suitable for industrialization or practical use. Furthermore, if high-temperature sintering is required in the production of all-solid-state batteries, there is a concern that co-sintering of the positive and negative electrodes with the solid electrolyte will have to be avoided in order to avoid damage to the positive and negative electrodes. Thus, from the perspectives of energy conservation, industrialization, and practical use, manufacturing methods for solid electrolytes that include a sintering process pose significant technical barriers.

Here, the polyester polymer represented by the above formula (1) exhibits ionic conductivity in the presence of an alkali metal salt, and assumes a liquid or liquid-like state when containing an alkali metal salt. The inventors focused on this property of the polyester polymer represented by the above formula (1) and attempted to use the polyester polymer represented by the above formula (1) as a binder in a composite electrolyte of an inorganic solid electrolyte and an organic electrolyte. They found that a mixture of an inorganic solid electrolyte, a polyester polymer represented by the above formula (1), and an alkali metal salt exhibits high ionic conductivity even without sintering. In other words, one preferred embodiment of the composite electrolyte of the present disclosure is a non-sintered body of a mixture of an inorganic solid electrolyte, a polyester polymer, and an alkali metal salt.

The reason for these results is thought to be that the gaps in the inorganic solid electrolyte are filled with a mixture of polyester polymer and alkali metal salt, which firmly binds the inorganic solid electrolyte particles together and makes it easier to form ion conduction paths in the composite electrolyte. Furthermore, the polyester polymer containing alkali metal salt has a moderate viscosity, which makes it easier for the polyester polymer to remain in the gaps in the inorganic solid electrolyte, and this is also thought to have contributed to the formation of ion conduction paths. However, these are merely speculations and do not limit the present invention in any way.

<Electricity storage device>
The power storage device of the present disclosure (hereinafter also referred to as "the device") includes the composite 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.

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 including an electrode layer consisting of a positive electrode layer and a negative electrode layer, and a solid electrolyte layer. The solid electrolyte layer is disposed between the positive electrode layer and the negative electrode layer so that the solid electrolyte layer is in contact with the electrode layer. The materials constituting the positive electrode layer and the negative electrode layer are not particularly limited, and may be appropriately selected from materials known as electrode materials for lithium-ion secondary batteries. For example, the positive electrode layer may be configured to include a positive electrode current collector and a positive electrode mixture layer. The positive electrode current collector may be a metal foil such as aluminum or stainless steel. The positive electrode mixture layer is a layer containing a positive electrode active material and is disposed on the surface of the positive electrode current collector. Examples of the positive electrode active material include metal oxides having a layered rock salt, spinel, or olivine crystal structure. The negative electrode layer may be configured to include a negative electrode current collector and a negative electrode mixture layer. The negative electrode current collector may be a metal foil such as copper foil or lithium foil. The negative electrode mixture layer is a layer containing a negative electrode active material, and is disposed on the surface of the negative electrode current collector. Examples of the negative electrode active material include metallic lithium, graphite, Li ₄ Ti ₅ O ₁₂ , silicon monoxide, and silicon.

In the lithium-ion secondary battery disclosed herein, the solid electrolyte layer is formed from a composite electrolyte containing an inorganic solid electrolyte, a polyester polymer, and an alkali metal salt. The thickness of the solid electrolyte layer is not particularly limited and can be set appropriately depending on the application of the secondary battery, etc. The thickness of the solid electrolyte layer is, for example, 5 to 5,000 μm. From the perspective of making the thickness of the solid electrolyte layer as thin as possible and achieving a smaller, lighter, and higher-capacity all-solid-state secondary battery, the thickness of the solid electrolyte layer is preferably 50 μm or less, and more preferably 20 μm or less.

The method for manufacturing the solid electrolyte layer and 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 comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer may be manufactured by sandwiching a solid electrolyte layer obtained by molding the composite electrolyte of the present disclosure between a positive electrode layer and a negative electrode layer, and preferably applying pressure for bonding. Alternatively, a laminate comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer may be manufactured by placing the unmolded composite electrolyte between the positive electrode layer and the negative electrode layer in a container, and then applying pressure to the container, preferably for bonding. A laminate comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer is typically housed in a case and used as a secondary battery.

This device is not limited to the above configuration in which the ion-conducting carrier is lithium ions, but may also 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 capacitor is one that includes an anode body, a cathode body, and a solid electrolyte, with the solid electrolyte disposed between the anode body and the cathode body so that the solid electrolyte is in contact with the electrode.

The power storage device comprising the composite electrolyte of the present disclosure can be used in a variety of applications. Specifically, it can be used 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 and hybrid vehicles, robots, and drones; and various electrical and electronic devices such as digital cameras, video cameras, music players, power tools, and home appliances.

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

[Molecular weight measurement]
The molecular weight of the polymer was determined using a gel permeation chromatography (hereinafter also referred to as "GPC") apparatus according to the following procedure.
A sample solution was obtained by dissolving 4 mg of the polymer in 4 mL of tetrahydrofuran. The obtained sample solution was filtered through a polytetrafluoroethylene membrane filter, and 100 μL of the solution was injected into a GPC apparatus to measure the weight-average molecular weight and number-average molecular weight (hereinafter also referred to as "Mw" and "Mn", respectively).
Column: Tosoh TSKgel Super Multipore HZ-M x 4 Temperature: 40°C
Eluent: tetrahydrofuran Detector: differential refractometer Flow rate: 600 μL/min
Standard material: polystyrene

[Glass transition temperature measurement]
The glass transition temperature of the polymer was determined using a differential scanning calorimeter (hereinafter also referred to as "DSC") according to the following procedure.
5 mg of the polymer sealed in an aluminum pan was cooled to −80° C., and then the temperature was swept up to 100° C. at a rate of 10° C./min to obtain a heat flux curve. The glass transition temperature was determined from the intersection of the baseline of the heat flux curve and the tangent at the inflection point.
Model: TA Instruments DSC250
Measurement atmosphere: Nitrogen

1. Polymer Synthesis [Synthesis Example 1] Synthesis of Polymer A
100 parts of ε-caprolactone, 0.33 parts of 1-butanol, 0.17 parts of tin(II) 2-ethylhexanoate, and a stirrer were placed in a test tube and stirred at 95°C for 96 hours in dry air (dew point -60°C or lower). The ε-caprolactone and 1-butanol used were dehydrated using molecular sieves. The polymer was precipitated by pouring the polymerization solution into a large amount of isopropanol. The precipitate was recovered by filtration under reduced pressure and dried in vacuo to obtain Polymer A. The molecular weight of Polymer A was measured using GPC, and was found to be Mn 18,700 and Mw 26,800.

[Synthesis Examples 2 to 4] Synthesis of Polymers B to D Polymers B to D were obtained by the same procedure as in Synthesis Example 1, except that the types and amounts of raw materials charged into the test tubes were changed as shown in Table 1. Here, in Synthesis Examples 3 and 4, toluene was used as the solvent during charging. The amount of toluene charged (parts by mass) is shown in Table 1. Furthermore, for each polymer, Mn and Mw were measured using GPC in the same manner as in Synthesis Example 1.

Details of the compounds shown in Table 1 are given below.
CL: ε-caprolactone (manufactured by Tokyo Chemical Industry Co., Ltd.)
DLLA: DL-lactide (manufactured by Tokyo Chemical Industry Co., Ltd.)
BuOH: 1-butanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
Sn(Oct) ₂ : tin(II) 2-ethylhexanoate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
Toluene: Toluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

2. Preparation and Evaluation of Composite Electrolyte [Example 1]
(1) Preparation of Composite Electrolyte CPE-1 81 parts of LICGC PW- ₀₁ (main crystalline phase _LiAlTiSiP012 ( _{Li -} substituted NASICON type), D50 0.4 _μm ) [manufactured by Ohara Corporation] (hereinafter also referred to as "LICGC"), 8 parts of 3-aminopropyltriethoxysilane, and 800 parts of tetrahydrofuran _were added to a vessel and stirred at room temperature for 5 minutes at 2,000 rpm using a mixer (manufactured by Thinky Corporation, model ARE-310). Subsequently, 100 parts of Polymer A and 90 parts of lithium bis(fluorosulfonyl)imide [manufactured by Nippon Shokubai Co., Ltd.] were added and stirred at room temperature for 30 minutes at 2,000 rpm using a mixer, to obtain Composition CA. Composition CA was cast into a silicon cup and dried under vacuum at 40° C. for 12 hours to obtain a composite electrolyte CPE-1.

(2) Measurement of Ionic Conductivity The ionic conductivity of the composite electrolyte CPE-1 was measured using an AC impedance method according to the following procedure.
The composite electrolyte CPE-1 was sandwiched between two stainless steel plates, pressurized at 370 MPa, and then punched out with a punch to form a pellet of 10 mm diameter and 0.65 mm thickness. Metallic lithium foil formed into a disk shape with a diameter of 8 mm was placed on both sides of the pellet, and the pellet with metallic lithium foil on both sides was sandwiched between two stainless steel plates, and the impedance between the metallic lithium foils was measured at 25 ° C. During the measurement, an alternating current was applied between the electrodes, and the ionic conductivity was calculated from the real impedance intercept of the resulting Cole-Cole plot. All of the above operations were performed in a glove box with a dew point of -80 ° C or less.
The ionic conductivity (σ) was calculated by the following formula (1).
σ=L/(R×S) (1)
(In formula (1), σ represents ionic conductivity (unit: S/cm), R represents real impedance intercept (unit: Ω), S represents the cross-sectional area of the metallic lithium foil at the time of measurement (unit: cm ² ), and L represents the distance between the stainless steel plates (unit: cm).)
The conditions for measuring the impedance are as follows:
Measuring equipment: Biologic VMP-300
Applied voltage: 100 mV
Frequency: 10mHz to 7MHz
The measurement results are shown in Table 2.

(3) Formability of Composite Electrolyte The formability of the composite electrolyte was evaluated based on the ease of forming the sample pellets and the self-supporting ability of the sample pellets. The evaluation was based on the following criteria.
○: Can be easily molded into any shape, and sample pellets have sufficient self-supporting properties. △: Can be molded into any shape, but self-supporting properties are insufficient, such as deformation due to its own weight. ×: Difficult to mold into any shape. The evaluation results are shown in Table 2.

[Examples 2 to 14 and Comparative Example 1]
Composite electrolytes CPE-2 to CPE-15 were obtained in the same manner as in Example 1, except that the types and amounts of raw materials were changed as shown in Tables 2 and 3. The ionic conductivity of each of the composite electrolytes CPE-2 to CPE-15 was measured, and the formability of each composite electrolyte was evaluated, in the same manner as in Example 1. The results are shown in Tables 2 and 3.
[Comparative Examples 2 to 4]
Organic electrolytes PE-1 to PE-3 were obtained in the same manner as in Example 1, except that the types and amounts of raw materials were changed as shown in Table 3. Furthermore, the ionic conductivity of organic electrolytes PE-1 to PE-3 was measured and the moldability of each organic electrolyte was evaluated in the same manner as in Example 1, except that organic electrolytes PE-1 to PE-3 were used instead of composite electrolyte CPE-1. The results are shown in Table 3.

[Comparative Example 5]
A press die 10 consisting of a die set 11, an upper punch 12, and a lower punch 13 (see FIG. 1) was used to prepare a sample pellet. The die set 11 containing 0.1 g of LICGC was placed on the lower punch 13, and the upper punch 12 was placed on the die set 11. Compression was performed using a hydraulic press at a pressure of 340 MPa to obtain a circular inorganic solid electrolyte pellet IE-1 having a diameter of 10 mm and a thickness of 0.65 mm. Thereafter, the ionic conductivity was measured in the same manner as in Example 1, and the moldability of the inorganic solid electrolyte pellet IE-1 was evaluated. The results are shown in Table 3.

Details of the compounds shown in Tables 2 and 3 are given below.
LICGC: LICGC PW-01 (main crystalline phase: Li _1+x+y Al _x Ti _2-x Si _y P _3-y O ₁₂ (Li-substituted NASICON type), D50: 0.4 μm) [manufactured by Ohara Corporation]
AES: 3-aminopropyltriethoxysilane (manufactured by Tokyo Chemical Industry Co., Ltd.)
Polymers A to D: Polymers produced in Synthesis Examples 1 to 4 PEO: Polyethylene oxide (Mw 100,000) [Sigma-Aldrich]
LiFSI: Lithium bis(fluorosulfonyl)imide (manufactured by Nippon Shokubai Co., Ltd.)
THF: Tetrahydrofuran (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
AcCN: Acetonitrile (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

3. Evaluation Results The composite electrolytes of Examples 1 to 14 exhibited high ionic conductivities at 25°C. This result is thought to be due to the fact that the polyester polymer filled the spaces between the inorganic solid electrolyte particles, thereby contributing to a reduction in grain boundary resistance. Among these, the composite electrolytes produced using polymer B as the polymer (Examples 3 to 12) exhibited higher ionic conductivities at 25°C than the composite electrolytes produced using polymer A (Examples 1 and 2) with a molecular weight smaller than that of polymer B. Similarly, when the composite electrolyte produced using polymer D (Example 14) was compared with the composite electrolyte produced using polymer C (Example 13) with a molecular weight smaller than that of polymer D, the composite electrolyte of Example 14 exhibited higher ionic conductivities at 25°C. Furthermore, looking at the results of Examples in which the type of polymer and the blending amount of alkali metal salt were the same, a tendency was observed that the higher the mass proportion of the inorganic solid electrolyte, the more likely it was that a composite electrolyte with good moldability and sufficient self-supporting properties would be obtained (Example 2 compared to Example 1, and Examples 9 to 12 compared to Example 8). These results suggest that the larger the molecular weight of the polymer and the greater the mass fraction of the inorganic solid electrolyte, the easier it is to obtain a composite electrolyte with excellent ionic conductivity, good formability, and sufficient self-supporting properties. This is presumably because the polymer chains of a polymer with a large molecular weight are strongly entangled, and a small amount is enough to hold together the inorganic solid electrolyte powder.

In contrast, the composite electrolyte of Comparative Example 1, which was produced using PEO as the polymer, had low ionic conductivity at 25°C. This result is thought to be due to the insufficient effect of PEO in reducing grain boundary resistance. Furthermore, organic electrolytes produced using a polymer and an alkali metal salt without an inorganic solid electrolyte (Comparative Examples 2-4) had lower ionic conductivity at 25°C compared to composite electrolytes using the same type of polymer (Examples 1-2 compared to Comparative Example 2, Examples 3-12 compared to Comparative Example 3, and Example 13 compared to Comparative Example 4). These results demonstrate that while organic electrolytes composed of a polyester polymer and an alkali metal salt have low ionic conductivity, when combined with an inorganic solid electrolyte, they exhibit high ionic conductivity at room temperature (25°C). Furthermore, the inorganic solid electrolyte of Comparative Example 5, which was produced by compression molding inorganic solid electrolyte powder, had high grain boundary resistance, making it impossible to measure ionic conductivity. Furthermore, the inorganic solid electrolyte of Comparative Example 5 also had poor moldability.

These results demonstrate that by creating a composite electrolyte containing an inorganic solid electrolyte, a polyester polymer represented by formula (1) above, and an alkali metal salt, it is possible to sufficiently reduce interfacial resistance without performing a sintering process during the production of the composite electrolyte, and to obtain a composite electrolyte that exhibits high ionic conductivity.

The present invention is not limited to the above-described embodiments, and encompasses various modifications and modifications within the scope of equivalents, provided that they do not deviate from 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, or fewer than one, should be understood to fall within the scope and spirit of the present invention.

10... Press mold

Claims (9)

an inorganic solid electrolyte;
A polymer,
an alkali metal salt;
Contains
The polymer has a structure represented by the following formula (1):
A composite electrolyte comprising the alkali metal salt in an amount of 5 mol % to 250 mol % based on the total amount of ester groups in the polymer.
[In formula (1), R represents a hydrogen atom or an alkyl group; X and Y are the same or different and represent a hydrogen atom, a hydroxyl group, or an alkyl group; n represents an integer of 1 or more; and m represents an integer of 0 to 10.] The composite electrolyte described in claim 1, wherein the inorganic solid electrolyte includes an oxide having a NASICON-type crystal structure. The composite electrolyte according to claim 1, wherein the inorganic solid electrolyte is in particulate form. The composite electrolyte described in claim 3, which is a non-sintered body of a mixture of the inorganic solid electrolyte, the polymer, and the alkali metal salt. The composite electrolyte according to claim 1, wherein the inorganic solid electrolyte comprises a solid electrolyte represented by the general formula: Li _1+2a+b+c-d M1 _a M2 _b M3 _2-a-b Si _c W _d P _3-c-d O ₁₂ (wherein M1 contains an element that becomes a divalent cation, M2 contains an element that becomes a trivalent cation, and M3 contains at least one element of Ti and Zr, and a≧0, b>0, c>0, and d≧0 are satisfied). The composite electrolyte according to claim 1, wherein the content of the inorganic solid electrolyte is 20% by mass or more and 95% by mass or less. The composite electrolyte according to claim 1, wherein the alkali metal salt comprises lithium bis(fluorosulfonyl)imide and/or lithium bis(trifluoromethanesulfonyl)imide. A method for producing the composite electrolyte according to any one of claims 1 to 7, comprising:
A method for producing a composite electrolyte, comprising: mixing the particulate inorganic solid electrolyte, the polymer, and the alkali metal salt. An electricity storage device comprising the composite electrolyte described in any one of claims 1 to 7.